U.S. patent number RE37,809 [Application Number 09/053,422] was granted by the patent office on 2002-07-30 for laser with electrically-controlled grating reflector.
This patent grant is currently assigned to Gemfire Corporation. Invention is credited to William K. Bischel, Michael J. Brinkman, David A. G. Deacon, Simon J. Field.
United States Patent |
RE37,809 |
Deacon , et al. |
July 30, 2002 |
Laser with electrically-controlled grating reflector
Abstract
One or more lasers are combined with optical energy transfer
devices and energy guiding devices which use an electric field for
control. The optical energy transfer devices may form gratings,
mirrors, lenses and the like using a class of poled structures in
solid material. The poled structures may be combined with waveguide
structures. Electric fields applied to the poled structures control
routing, reflection and refraction of optical energy. Adjustable
tunability is obtained by a poled structure which produces a
spatial gradient in a variable index of refraction along an axis in
the presence of a variable electric field.
Inventors: |
Deacon; David A. G. (Los Altos,
CA), Field; Simon J. (Palo Alto, CA), Brinkman; Michael
J. (Redwood City, CA), Bischel; William K. (Menlo Park,
CA) |
Assignee: |
Gemfire Corporation (Palo Alto,
CA)
|
Family
ID: |
23173763 |
Appl.
No.: |
09/053,422 |
Filed: |
April 1, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
303801 |
Sep 9, 1994 |
05504772 |
Apr 2, 1996 |
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Current U.S.
Class: |
372/102 |
Current CPC
Class: |
H01S
3/108 (20130101); H01S 5/4062 (20130101); H01S
3/063 (20130101); H01S 5/141 (20130101); H01S
3/1055 (20130101); G02F 2201/30 (20130101); H01S
3/0092 (20130101); H01S 3/0635 (20130101); H01S
3/109 (20130101); G02F 2201/307 (20130101) |
Current International
Class: |
H01S
3/108 (20060101); H01S 3/1055 (20060101); H01S
3/105 (20060101); H01S 5/00 (20060101); H01S
5/14 (20060101); H01S 3/06 (20060101); H01S
3/109 (20060101); H01S 3/063 (20060101); H01S
5/06 (20060101); H01S 003/08 () |
Field of
Search: |
;372/6,12,37,102
;385/8,10,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2 684 772 |
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Jun 1993 |
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FR |
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1 516 427 |
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Jul 1978 |
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GB |
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1 557 484 |
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Dec 1979 |
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GB |
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2-269323 |
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Nov 1990 |
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JP |
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6-67234 |
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Mar 1994 |
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JP |
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6-110024 |
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Apr 1994 |
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JP |
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Other References
RA. Becker et al, "Electrooptical switching in thin film waveguides
for a computer communication bus", Applied Optics, vol. 18, No. 19,
Oct. 1, 1979.* .
Blistanov et al., "Modulation and conversion of light in lithium
niobate crystals with a regular domain structure," Sov. J. Quantum
Electron., 16(12): 1678-1679 (Dec. 1986). .
Risk et al., "Distributed-Bragg-reflection properties of segmented
KTP waveguides," Optics Letters 18(4):272-274 (Feb. 1993). .
Shinozaki et al. "Self-quasi-phase-matched second-harmonic
generation in the proton-exchanged LiNbO.sub.3 optical waveguide
with periodically domain-inverted regions," Appl. Phys. Lett.,
59(5):510-512 (Jul. 1991). .
Yamada et al., "First-order quasi-phase matched LiNbO.sub.3
waveguide periodically poled by applying an external field for
efficient blue second-harmonic generation," Appl. Phys Lett.,
62(5): 435-436 (Feb. 1993)..
|
Primary Examiner: Lee; John D.
Attorney, Agent or Firm: Townsend and Townsend and Crew
Allen; Kenneth R.
Claims
What is claimed is:
1. A laser comprising: a solid material for passing optical energy;
at least a first electrically-conductive material forming a first
electrode, said first electrode confronting said solid material and
bridging at least two elements of an electrically-controllable
grating structure in said solid material; an optical amplifier
disposed along an optical axis, said optical axis traversing said
solid material, said at least two elements being disposed
transverse of said optical axis; and an optical coupling means
disposed between said solid material and said optical amplifier
along said optical axis.
2. The device according to claim 1 further including: an optical
reflector disposed along said optical axis, said optical amplifier
being disposed between said optical reflector and said solid
dielectric material.
3. The device according to claim 1 further including: an optical
reflector disposed along said optical axis, said optical amplifier
being disposed between said optical reflector and said solid
dielectric material; waveguide means along said optical axis in
said solid dielectric material, wherein said optical amplifier is a
semiconductor diode, wherein said optical coupling has butt
coupling and antireflective means disposed between said
semiconductor diode and said waveguide; and wherein said at least
two elements form a feedback mirror capable of producing laser
oscillation.
4. The device according to claim 1 further including: an optical
reflector disposed along said optical axis, said optical amplifier
being disposed between said optical reflector and said solid
dielectric material; a modulation controller for modulating said
electric field creating means; said optical coupling being
antireflective to inhibit laser oscillation in absence of said
electric field; and wherein said grating is a field-controlled
feedback mirror for producing laser oscillation in proportion to
the strength of said electric field.
5. The device according to claim 4 for amplitude modulation of an
optical signal, wherein said grating comprises alternates of said
first type of said elements with a second type of said elements,
said second type being a poled structure having a reverse sense to
said first type of said elements and wherein average optical
distance across said first type of elements is substantially equal
to average optical distance across said second type of elements
along said optical axis.
6. The device according to claim 4 for frequency modulation of an
optical signal, wherein said grating comprises alternating said
first type of said elements with a second type of said elements,
said second type being a poled structure having a reverse sense to
said first type of said elements and wherein average optical
distance across said first type of elements differs from average
optical distance across said second type of elements along said
optical axis.
7. The device according to claim 1 for mode locking optical energy,
further including: an optical reflector disposed along said optical
axis, said optical amplifier being disposed between said optical
reflector and said solid dielectric material; and a mode locker
operated at a frequency that is a multiple of a frequency which is
the reciprocal of the round-trip optical transit time between said
grating and said optical reflector.
8. The device according to claim 1 wherein said at least two
elements are formed from a plurality of types of domains having a
plurality of electro-optic coefficients, and further including: an
optical reflector disposed along said optical axis, said optical
amplifier being disposed between said optical reflector and said
solid dielectric material; wherein said optical coupling means is
antireflective to inhibit laser oscillation in absence of optical
feedback from said at least two elements; and wherein the sum over
all domain types of the product, for each domain type, of said
electro-optic coefficient times the average distance across the
domain type along said optical axis, differs from zero.
9. The device according to claim 1, for nonlinear conversion of
optical energy, further including: an optical reflector disposed
along said optical axis, said optical amplifier being disposed
between said optical reflector and said solid dielectric material;
wherein said solid dielectric material further includes a pattern
of differing domains transverse to said optical axis, at least a
first type of said domains being an optically nonlinear structure
and forming a plurality of components alternating with a second
type of said domains, said pattern being phase matched to interact
between three optical waves of at least two different frequencies,
wherein a linear combination of the values of the frequencies of
said three optical waves is substantially zero to generate at least
one optical output beam.
10. The device according to claim 9, wherein said elements and said
components together form a combined structure with both reflecting
and nonlinear optical properties.
11. The device according to claim 9 wherein said optically
nonlinear structure is a frequency doubler.
12. The device according to claim 9 wherein said optically
nonlinear structure is a frequency mixer.
13. The device according to claim 9 wherein said optically
nonlinear structure is an optical parametric oscillator frequency
doubler.
14. The device according to claim 1, wherein said grating comprises
alternates of said first types of said elements and second types of
said elements which are spaced at at least two different
periods.
15. The device according to claim 1 further including: an optical
reflector disposed along said optical axis, said optical amplifier
being disposed between said optical reflector and said solid
dielectric material; and at least one electro-optically active
region in said solid material transverse of said optical axis and
having an electrode adjacent said active region for inducing an
electric field.
16. The device according to claim 15 wherein said active region
defines an optical focussing device.
17. The device according to claim 15 wherein said active region has
a reflective interface at a skew with said optical axis and forms a
reflecting grating for diverting optical energy.
18. The device according to claim 15 wherein said active region is
a variable dispersion, electrically-controllable waveguide segment
along said optical path.
19. A laser comprising: a solid material for passing optical
energy; an input waveguide in said solid material; a base reflector
disposed along an optical axis; a plurality of output waveguides
encountering said input waveguide at intersection regions along
said input waveguide; a plurality of electrically-switchable beam
redirectors disposed along said input waveguide at said
intersection regions, each one of said electrically-switchable beam
redirectors comprising a first electrically-conductive material
forming a first electrode, said first electrode confronting said
solid material and bridging at least one electrically-active
element in said solid material; a plurality of gratings disposed
along said output waveguides defining electrically-selectable
retroreflectors further defining cavities between said base
reflectors and said gratings; an optical amplifier disposed along
an optical axis, said optical axis traversing said solid material;
and an optical coupling means disposed between said solid material
and said optical amplifier along said optical axis.
20. The laser according to claim 19 wherein said
electrically-switchable beam redirectors are total internal
reflectors.
21. The laser according to claim 19 wherein said
electrically-switchable beam redirectors are switchable minors.
22. The laser according to claim 19 wherein said gratings have
differing periods in order to support selectable-frequency
operation..Iadd.
23. A laser comprising a gain medium and a cavity, wherein one end
of the cavity is defined by a grating comprising an electro-optic
material and means for applying an electric field to the
electro-optic material, wherein the electro-optic material has a
first portion forming a refractive index grating and comprising an
alternating pattern of discrete, substantially non-overlapping
first and second regions, the first regions having a first
refractive index in the absence of an electric field and the second
regions having a second refractive index in the absence of an
electric field, the second refractive index being different to the
first refractive index..Iaddend..Iadd.
24. A laser according to claim 23, wherein the gain medium is a
semiconductor laser diode..Iaddend..Iadd.
25. A laser according to claim 23, further comprising means for
superimposing an alternating voltage on a steady voltage for
producing an amplitude modulated electric field at the electrically
controllable grating so as to wavelength modulate the
laser..Iaddend..Iadd.
26. A wavelength division multiplexed light source comprising an
array of lasers a claimed in claim 23..Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to lasers in connection with optical devices
for controlling optical beams using electric field control. In
particular, the invention relates to lasers in connection with
devices constructed with poled structures, including periodically
poled structures, and electrodes which permit controlled
propagation of optical energy in the presence of controlled
electric fields applied between electrodes.
The invention is especially applicable to the fields of laser
control, communications, flat panel displays, scanning devices and
recording and reproduction devices.
Interactions with energy beams such as optical or acoustic beams
can be controlled by means of applied electric fields in
electro-optic (EO) or piezoelectric materials. An electrically
controlled spatial pattern of beam interaction is desired in a
whole class of switched or modulated devices. Patterned responses
can be achieved in uniform substrates using the electro-optic or
piezoelectric effect by patterning the electric field. However,
Maxwell's equations for the electric field prevent sharp field
variations from extending over a large range. Some materials can be
poled, which means their electro-optical and/or piezoelectric
response can be oriented in response to some outside influence. In
these materials, is possible to create sharp spatial variations in
EO coefficient over potentially large ranges. By combining slowly
varying electric fields with sharply varying (poled) material, new
types of patterned structures can be fabricated and used.
Polable EO materials have an additional degree of freedom which
must be controlled, as compared to fixed EO crystals. Usually, the
substrate must be poled into a uniformly aligned state before any
macroscopic EO response can be observed. Uniformly poled substrates
have been fabricated both from base materials where the molecules
initially have no order, and from base materials where the
molecules spontaneously align with each other locally, but only
within randomly oriented microscopic domains. An example of the
first type of material is the nonlinear polymer. Examples of the
second type of material are sintered piezoelectric materials such
as lead zirconate titanate (PZT), liquid crystals, and crystalline
ferroelectric materials such as lithium niobate (LiNbO.sub.3).
Nonlinear polymer poling is described in .diamond-solid. E. Van
Tomme, P. P. Van Daele, R. G. Baets, P. E. Lagasse, "Integrated
optic devices based on nonlinear optical polymers", IEEE JQE 27
778, 1991. PZT poling is described for example in .diamond-solid.
U.S. Pat. No. 4,410,823, October 1983, Miller et al, "Surface
acoustic wave device employing reflectors". (Liquid crystal poling
is described in standard references, such as S. Chandrasekhar,
Liquid Crystals, Second Edition (1992), Cambridge University Press,
Cambridge.) Ferroelectric crystal poling is described in
.diamond-solid. U.S. Pat. No. 5,036,220 July 1991, Byer et al.,
"Nonlinear optical radiation generator and method of controlling
regions of ferroelectric polarization domains in solid state
bodies".
Examples of poled EO devices include: .diamond-solid. the beam
diffractor in a polymer layer with interdigitated electrodes of S.
Ura, R. Ohyama, T. Suhara, and H. Nishihara, "Electro-optic
functional waveguide using new polymer p-NAn-PVA for integrated
photonic devices," Jpn. J. Appl. Phys., 31, 1378 (1992) [UOS92];
.diamond-solid. the beam modulator in a polymer layer with planar
electrodes of U.S. Pat. No. 5,157,541 October 1992, Schildkraut et
al. "Optical article for reflection modulation"; .diamond-solid.
the total internal reflection beam reflector in a lithium niobate
waveguide with an electrode pair of H. Naitoh, K. Muto, T.
Nakayama, "Mirror-type optical branch and switch", Appl. Opt. 17,
101-104 (1978); .diamond-solid. the 2.times.2 waveguide switch in
lithium niobate with two electrodes of M. Papuchon, Am. Roy,
"Electrically active optical bifurcation: BOA", Appl. Phys. Lett.
31, 266-267 (1977); and .diamond-solid. the wye junction beam
router in a lithium niobate waveguide with three electrodes of H.
Sasaki and I. Anderson, "Theoretical and experimental studies on
active y-junctions in optical waveguides", IEEE Journ. Quant.
Elect. QE14, 883-892 (1978).
These devices use uniformly poled material with varied electrode
and optical structures. Many of the advantages of patterned poled
devices have not been recognized. For example, in the book by
.diamond-solid. H. Nishihara, M. Haruna, T. Suhara, Optical
Integrated Circuits, McGraw-Hill, New York (1989) [NHS89], many
electro-optical devices activated by various electrode patterns are
described, but all of these devices are fabricated on a uniformly
poled substrate. The same is true of another review article,
.diamond-solid. T. Suhara and H. Nishihara, "Integrated optics
components and devices using periodic structures," IEEE J. Quantum
Electron., QE-22, 845, (1986) [TH86], which describes the general
characteristics of grating coupled devices without recognizing the
advantages of a poled grating as opposed to an electrode
grating.
In selected instances in the literature, certain advantages of
patterned poled substrates have been pointed out. .diamond-solid. A
surface acoustic wave reflector with an array of domain reversals
in a piezoelectric ceramic (but no electrodes) is described in U.S.
Pat. No. 4,410,823, Miller et al.; .diamond-solid. A beam steerer
with triangular domain reversed regions in LiTaO.sub.3 is described
in Q. Chen, Y. Chiu, D. N. Lambeth, T. E. Schlesinger, D. D.
Stancil, "Thin film electro-optic beam deflector using domain
reversal in LiTaO.sub.3 ", CTuN63, CLEO'93 Conference Proceedings,
pp 196 et. seq., Optical Society of America. .diamond-solid. A
Mach-Zehnder modulator with domain reversals to compensate phase
differences between microwave and optical beams is described in
U.S. Pat. No. 5,278,924, January 1994, Schaffner, "Periodic domain
reversal electro-optic modulator". .diamond-solid. A Mach-Zehnder
electric field sensor with one domain reversed region in an
electro-optic substrate is described in U.S. Pat. No. 5,267,336,
November 1993, Sriram et al., "Electro-optical sensor for detecting
electric fields".
Use of patterned poled structures offers efficiency advantages in
beam control (including generation, modulation, redirection,
focussing, filtration, conversion, analysis, detection, and
isolation) with applications in laser control; communications; data
storage; and display. What is needed in these areas are adjustable
methods for beam control with high efficiency. Due to the sharp
domain transitions, higher efficiency devices can generally be
obtained using pattern poled substrates to create the high
frequency variations; the electrodes are needed to excite the
patterned poled substrate, not to create the high frequency
variations.
The poling process in polymers is quite different from that of
crystals, and results in poorly defined domain boundaries. In
crystals, there are a discrete number of (usually two) poling
directions which are stable, and poling a local region consists of
flipping atoms between these alternative states. Poled regions are
fully aligned, and sharp boundaries exist between oppositely
aligned domains. In poled polymers, any molecule can be oriented in
any direction regardless of the poling direction. The poling
process produces only an average component of alignment within a
random distribution of individual molecules. In polymers, the
poling (and the related EO coefficients) therefore have a
continuous variation in strength and orientation. The sharp domain
boundaries obtained in crystals are absent. This has a profound
influence on the efficiency of certain types of poled device in
polymers. Since the poling strength and direction in polymers
follows the strength and direction of the local applied electric
field, it is not possible to obtain poling features with spatial
dimensions any sharper than permitted by Maxwell's equations. In
polymers, there is very little advantage to be obtained from
spatially patterning the poled regions instead of the
electrodes.
In devices based on optical polymers, poling is required to create
an electro-optical response. The poling is done by applying a
voltage to electrodes fabricated on the device (in the presence of
heat). The entire polymer film may be poled with a uniform
electrode, after which the electrodes are spatially patterned for
the desired functionality. The EO performance of the device will
not change much if the poling is accomplished with the patterned
electrodes, since the active region within reach of the electric
field is still poled almost as well. The choice of whether to pole
the whole layer or just the region under the electrodes is mainly
by convenience in fabrication. Examples of polymer EO devices where
the poling is spatially patterned outside the active region of the
device are .diamond-solid. the switched waveguides of U.S. Pat. No.
4,867,516, September 1989, Baken et al., "Electro-optically induced
optical waveguide, and active devices comprising such a waveguide",
and .diamond-solid. U.S. Pat. No. 5,103,492, April 1992, Ticknor et
al., "Electro-optic channel switch". None of these devices have the
electrodes traverse multiple boundaries of a patterned poled
structure.
The poling process also changes the index of refraction ellipsoid
in polymers. This fact has some desirable consequences, such as
making possible waveguides fabricated by poling a stripe of polable
polymer as described in .diamond-solid. J. I. Thackara, G. F.
Lipscomb, M. A. Stiller, A. J. Ticknor, and R. Lytel, "Poled
electro-optic waveguide formation in thin-film organic media,"
Appl. Phys. Lett., 52, 1031 (1988) [TLS88] and in .diamond-solid.
U.S. Pat. Nos. 5,006,285, April 1991, and 5,007,696, April 1991,
Thackara et al. "Electro-optic channel waveguide". However, it
leaves a problem in that poled polymer boundaries are lossy in
their unexcited state (they scatter, diffract and refract). Devices
in which a light beam crosses poled polymer boundaries have the
problem that although transparency may be achieved, the poled
polymer must be activated electrically to produce a uniform index
of refraction. Poled crystalline devices do not have this problem
because poling does not change their index of refraction.
A solution to the problem of lack of transverse spatial definition
in poled polymers was proposed in .diamond-solid. U.S. Pat. No.
5,016,959 May 1991, Diemeer, "Electro-optical component and method
for making the same", who describe a total internal reflection
(TIR) waveguide switch in which the entire polymer film is poled,
but the electro-optic coefficient of selected regions is destroyed
by irradiation, creating unpoled regions with sharp spatial
boundaries. While the underlying molecules in these unpoled
irradiated regions remain aligned, they no longer have any
electro-optic response. This approach is useful in creating sharp
poled-unpoled domain boundaries in polymer films. It has the
disadvantage that it cannot produce reverse poled domains so its
efficiency is considerably reduced compared to the equivalent
crystal poling technique.
In nonlinear frequency conversion devices, domains of different
polarity are typically periodically poled into a nonlinear optic
material, but not excited by an electric field. The poled structure
periodically changes along the axis of the beam to allow net energy
conversion despite a phase difference that accumulates between the
two beams. This process is known as quasi-phasematching, and has
been demonstrated in ferroelectrics [U.S. Pat. No. 5,036,220, Byer
et al.] such as lithium niobate, KTP, and lithium tantalate, as
well as in polymers, as described in .diamond-solid. U.S. Pat. No.
4,865,406 September 1989, Khanarian et al, "Frequency doubling
polymeric waveguide". Electrodes are not typically used in these
devices, since the phasematching occurs in the absence of an
electric field. Generalized frequency conversion in polymers is
described in .diamond-solid. U.S. Pat. No. 5,061,028 October 1991,
Khanarian et al, "Polymeric waveguides with bidirectional poling
for radiation phase matching", as well as TE-TM modulation.
Khanarian et al. used patterned electrodes in both patents to pole
the polymer film; the attendant loss in sharpness of the spatial
pattern becomes a severe problem where more complex electrode
structures are needed such as in the latter patent.
Devices are known employing periodic structures which use electric
fields to control gratings in order to control propagating fields.
A diffraction grating modulator is shown in .diamond-solid. U.S.
Pat. No. 4,006,963, February 1977, Baues et al. "Controllable,
electro-optical grating coupler". This structure is fabricated by
removing material periodically in an electro-optic substrate to
form a permanent grating. By exciting the substrate
electro-optically, the fixed index grating has a greater or lesser
effect, producing some tuning. This structure does not contain
poled regions. The drawbacks of the Baues structure are the same as
for the polymer film; the grating cannot be made transparent
without the application of a very strong field.
The current technology for an EO switchable grating is shown in
FIG. 1 (Prior Art). In this structure, periodically patterned
electrodes serve as the elements that define the grating. The
underlying material does not have a patterned poled structure, as
hereinafter explained. An input beam 12 is coupled into a
electro-optically active material 2 which contains an electrically
controllable grating 6. When the voltage source 10 to the grating
electrodes is off, the input beam continues to propagate through
the material to form the output beam 16. When the
grating-controlling voltage source is switched on, an index
modulation grating is produced in the material, and a portion of
the input beam is coupled into a reflected output beam 14. The
material has an electro-optically active poled region 4 with a
single domain, with the same polarity throughout the poled
structure. A first electrode 6 is interdigitated with a second
electrode 7 on a common surface 18 of the substrate. When a voltage
is applied between the electrodes, the vertical component of
electric field along the path of the beam 12 alternately has
opposite sign, creating alternate positive and negative index
changes to form a grating. The strength of the grating is
controlled by the voltage source connected between the two
electrodes by two conductors 8.
A second general problem with the existing art of EO and
piezoelectric devices using uniform substrates and patterned
electrodes is that the pattern of the excited electric field decays
rapidly with distance away from the electrodes. The pattern is
essentially washed out at a distance from the electrodes equal to
the pattern features size. This problem is aggravated in the case
of a grating because of the very small feature size. Prior art
gratings formed by interdigitated electrodes produce a modulated
effect only in a shallow surface layer. EO structures interact
weakly with waveguides whose dimension is larger than the feature
size. While longer grating periods may be used in higher order
interaction devices, the lack of sharp definition described above
again seriously limits efficiency. The minimum grating period for
efficient interaction with current technology is about 10 microns.
What is needed is a way to maintain the efficiency of EO devices
based on small structures, despite a high aspect ratio (i.e. the
ratio of the width of the optical beam to the feature size).
Switchable patterned structures are needed which persist throughout
the width of waveguides and even large unguided beams.
In bulk material, gratings may be formed by holographic exposure
and acoustic excitation. Holographic exposure is very difficult,
and storage materials such as SBN are not yet developed to a
commercial state. Acoustic excitation is very expensive to
implement and to power, and requires additional components such as
soft mounts and impedance matched damping structures. Other methods
form surface gratings, including deposition techniques, material
removal techniques and material modification techniques (such as
indiffusion, outdiffusion, and ion exchange). What is needed is an
approach capable of a large enough aspect ratio to produce bulk
interaction structures, preferably with feature control at an
accessible surface.
While the EO material can in principle by any electro-optically
active material, liquid crystals are a special case and have
limited applicability. A light modulator based on diffraction from
an adjustable pattern of aligned liquid crystal domains is
described in .diamond-solid. U.S. Pat. No. 5,182,665, January 1993,
O'Callaghan et al., "Diffractive light modulator". A light
modulator based on total internal reflection modulated by liquid
crystal domain formation is described in, .diamond-solid. U.S. Pat.
No. 4,813,771 March 1989, Handschy et al., "Electro-optic switching
devices using ferroelectric liquid crystals". In all of these
devices, the domains must physically appear or disappear to produce
the desired effect. The orientation of the molecules in the liquid
crystal device changes in response to an applied field, producing a
patterned structure which interacts with light. However, liquid
crystals have important drawbacks. They are of course liquid and
more difficult to package, and they have a limited temperature
range and more complex fabrication process than solid state
devices. High aspect ratio structures cannot be made because of the
decay of the exciting field pattern with distance. The molecular
orientation relaxes as soon as the field is turned off, and
re-establishing the pattern takes a long time, so fast switching is
not possible.
The structures which switch light from waveguide to waveguide in
the prior art have a high insertion loss or large channel spacing
which render them unsuitable for large routing structures. A large
switching structure must have switching elements with insertion
loss low enough to permit light to propagate through the structure.
If a waveguide has 100 switches, for example, the switches must
have less than about 0.03 dB insertion loss. In the prior art this
is not possible. R. A. Becker and W. S. C. Chang, "Electro-optical
switching in thin film waveguides for a computer communications
bus", Appl. Opt. 18, 3296 (1979), demonstrate a multimode crossing
waveguide array structure coupled via interdigitated electro-optic
grating switches. This switch has an inherently high insertion loss
(0.4 dB) and poor switching efficiency (.about.10%). U.S. Pat. No.
5,040,864, 8/1991, J. H. Hong, "Optical Crosspoint Switch Module",
discloses a planar waveguide structure which may in principle have
a low insertion loss, but which requires very large crossing
junctions for efficient switching, and is therefore incapable of
producing a high density switching array.
In summary, the prior art has shortcomings in several areas: 1)
large aspect ratios of controllable patterns are needed for
efficient interaction with bulk waves or small patterns; 2) sharp
domain transitions are needed for efficiency in higher order
interactions; 3) transparency of domain structures is needed at
zero applied field for proper unpowered operation; and 4) low
insertion loss is required for arrays of switches. Poled structures
contained in the above and other structures have not been fully
utilized heretofore to realize practical devices.
SUMMARY OF THE INVENTION
According to the invention, one or more lasers are combined with
optical energy transfer devices and energy guiding devices which
use an electric field for control. The optical energy transfer
devices may form gratings, mirrors, lenses and the like using a
class of poled structures in solid material. The poled structures
may be combined with waveguide structures. Electric fields applied
to the poled structures control routing, reflection and refraction
of optical energy. Adjustable tunability is obtained by a poled
structure which produces a spatial gradient in a variable index of
refraction along an axis in the presence of a variable electric
field.
In one embodiment, the present invention is a switchable grating
which consists of a poled material with an alternating domain
structure of specific period. When an electric field is applied
across the periodic structure, a Bragg grating is formed by the
electro-optic effect, reflecting optical radiation with a certain
bandwidth around a center wavelength. The grating may be used by
itself, or in combination with other gratings to form integrated
structures in a ferroelectric crystal. Specifically of interest is
an integrated structure in which one or more optical waveguides
interact with one or more periodic structures to form a wavelength
selective integrated optical modulator, switch, or feedback
element.
The invention will be better understood upon reference to the
following detailed description in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a modulator with interdigitated electrodes, according to
the prior art.
FIG. 2 is a generalized embodiment of the switched grating for
interacting with bulk optical beams, according to the
invention.
FIG. 3 is an embodiment of a waveguide retroreflector using the
switched grating.
FIG. 4 is an embodiment of an electrode configuration for the
retroreflecting device with three electrodes disposed on the same
face of the crystal.
FIG. 5 is an embodiment of an electrode configuration for the same
device, in which two electrodes are disposed on the same face of
the crystal.
FIG. 6 is an embodiment of an electrode configuration for the
device, in which three electrodes with tapered separation are
disposed on the same face of the crystal.
FIG. 7 is a tee embodiment of a poled crossing waveguide
coupler.
FIG. 8 is an x embodiment of a poled crossing waveguide
coupler.
FIG. 9 is an embodiment of a poled waveguide output coupler, with
output out of the plane of the waveguide.
FIG. 10 is an embodiment of a parallel waveguide poled directional
coupler.
FIG. 11 is a top view schematic diagram of the an x crossing
waveguide coupler with illustrations of alternative input and
output mode profiles.
FIG. 12 is an embodiment of an x crossing waveguide coupler with
tapered coupling region geometry excited with a tapered electrode
gap.
FIG. 13 is an embodiment of an x crossing waveguide coupler with
generalized coupling region geometry and electrode pattern.
FIG. 14 is a bulk optics embodiment of a tunable-frequency poled
electro-optic retroreflector.
FIG. 15 is a waveguide embodiment of a tunable-frequency poled
electro-optic retroreflector.
FIG. 16 is a bulk optics embodiment of a tunable-frequency
electro-optic retroreflector with electro-optic cladding and
independent excitation of poled grating and cladding.
FIG. 17 is a waveguide embodiment of a multiple frequency poled
electro-optic retroreflector.
FIG. 18 is an illustration of a phase shifted poled grating.
FIG. 19 is an embodiment of a multiple period grating
reflector.
FIG. 20 is an illustration of the frequency response curves of two
devices with multiple periodicity and different free spectral
range.
FIG. 21 is an embodiment of a twin grating tunable reflector.
FIG. 22 is a schematic illustration of an integrated etalon
consisting of twin gratings with adjustable optical path
length.
FIG. 23 is an embodiment of a dual grating switchable wye junction
with phase shifter.
FIG. 24 is an embodiment of a poled waveguide mode converter.
FIG. 25 is an embodiment of a waveguide router using the waveguide
mode converter.
FIG. 26 is an embodiment of a switchable parallel waveguide
resonator.
FIG. 27 is an embodiment of a three-arm waveguide etalon.
FIG. 28 is an embodiment of a ring waveguide etalon.
FIG. 29A is an embodiment of a modulator/attenuator with
controllable poled mid-structure.
FIG. 29B is an embodiment of an adjustable lens structure.
FIG. 30 is an embodiment of a poled total internal reflecting (TIR)
waveguide switch with switched poled waveguide stub.
FIG. 31 is an embodiment of a dual TIR waveguide switch.
FIG. 32 is an embodiment of a TIR electrically switched beam
director with switched unpoled waveguide stub.
FIG. 33 is an embodiment of a two position poled waveguide router
without TIR.
FIG. 34 is an embodiment of an array of poled TIR switches with a
50% switch packing density.
FIG. 35 is an embodiment of an array of poled TIR switches with a
100% switch density.
FIG. 36 is an embodiment of a dual waveguide structure for high
density packing architectures with permanent turning mirror and
asymmetric loss crossing region.
FIG. 37 is an embodiment of a switched waveguide array with TIR
switches.
FIG. 38 is an embodiment of a switched waveguide array with grating
switches.
FIG. 39A is an embodiment of an m.times.m communications switch
array with system control lines.
FIG. 39B is an embodiment of a 3.times.3 switch array with WDM
capability.
FIG. 40 is an embodiment of a two dimensional switching array with
pixel elements.
FIG. 41 is an embodiment of a one dimensional switching array with
pixel elements coupled to data tracks.
FIG. 42 is an embodiment of a switchable spectrum analyzer using
selectable grating reflector sections and a detector array.
FIG. 43 is an illustration of a poled acoustic multilayer
interferometric structure.
FIG. 44 is an illustration of a poled acoustic transducer.
FIG. 45 is an embodiment of a tuned coherent detector of
multi-frequency light waves.
FIG. 46 is an embodiment of a low loss switchable waveguide
splitter using a single poled region.
FIG. 47 is an embodiment of a low loss switchable waveguide
splitter using multiple poled regions.
FIG. 48 is an illustration of the key design elements for a
1.times.3 waveguide splitter.
FIG. 49 is a multiple layer stack of active waveguide devices shown
as an adjustable phased array modulator.
FIG. 50 is an embodiment of an adjustable waveguide attentuator of
the prior art.
FIG. 51 is an embodiment of a multiple poled segment adjustable
waveguide attenuator.
FIG. 52 is an embodiment of a structure with widened bandwidth
using an angle-broadened poled grating.
FIG. 53 is an embodiment of a structure with widened bandwidth
using a curved waveguide.
FIG. 54 is an embodiment of an electrically controllable poled
lens.
FIG. 55 is an embodiment of a laser feedback device using a
periodically poled reflector.
FIG. 56 is an embodiment of a laser feedback device using a
periodically poled waveguide reflector.
FIG. 57 an embodiment of a laser feedback device using multiple
switched feedback gratings.
FIG. 58 is an embodiment of a wavelength-tuned adjustable focussing
system.
DESCRIPTION OF SPECIFIC EMBODIMENTS
FIGS. 55-58 are particularly relevant to the claims of this
application. Structures and methods employed in connection with the
claim invention are explained in the remainder of the
specification. Referring to FIG. 2, there is shown a generalized
embodiment of a device 11 used in the present invention, which is a
patterned poled dielectric device. Essentially, this device is an
electrically-controllable stacked dielectric optical energy
redirector, or more succinctly, an electrically-switchable mirror.
In a preferred embodiment, the invention is a bulk optical
reflector in a ferroelectric crystal 20 of lithium niobate. The
electrically-controlled switching element is a poled grating 22,
which consists of alternating poled domains of two types 36 and
38.
A domain, which may be of any shape or size, is a physical region
within which certain material properties are approximately
constant. A poled domain is a region in a material in which the
molecular groups have a directionality and these groups are
substantially aligned (or are partially aligned) in, or near, a
direction called the poling direction. There are many types of
domains including domains of aligned atomic structures in different
directions, domains of aligned molecules or atomic structures with
various modified parameters such as the nonlinear activity or the
electro-optic coefficient, domains of atomic structures with no
preferred direction, domains defined by regions activated by
different electrodes, poled regions in which the poling direction
varies systematically across the region such as occurs in the case
of polymers and fused silica poled with localized electrodes,
domains of randomly oriented molecules, and by extension, a random
domain structure: domains of sub-domains which are randomly poled
within the domain. A poled structure is a set of individual
domains. A patterned poled region is a region in a material in
which the domains within the region have been poled according to a
spatial pattern, with more than one domain type. There may be a
systematic offset between the poled pattern and the imposed pattern
used during the poling process, depending on the nature of this
process. The boundaries of the pattern may also be somewhat
irregular and not follow the imposed pattern perfectly,
particularly if the poling process is not under complete control.
The device is described as a patterned poled dielectric because an
electric field is applied in controlling the device, so the
material must be a dielectric in order to withstand the required
field without damage. Typically, the poling process is also
accomplished using an electric field, which the material must also
withstand. In general, we mean by dielectric the capability of the
material to withstand the minimum electric fields needed for the
application.
In operation, an optical input beam 40 is incident on and through
the crystal, along an optical axis. The optical axis is normal to
the phase front of the beam and is defined by the mean location of
the propagating beam across its intensity profile at the phase
front. The optical axis is straight in a uniform material, but may
bend in several situations including curved waveguides, nonuniform
media, and in reflective or diffractive structures. The input beam
40 preferably has a sufficiently small spot size 21 throughout the
crystal length so that it is not apertured by the crystal, causing
undesirable power loss and mode conversion. In a bulk-interaction
device such as is shown in FIG. 2, the domains 36 and 38 must
penetrate a sufficient distance through the substrate 20 so that
they overlap at least a portion of the input beam 40. The grating
22 lies transverse of the input beam 40. This means the planes 34
of the grating 22 are transverse of the axis of the input beam 40.
For two lines (or a line and a plane, or two planes) to be
transverse of each other we mean that they are not parallel. Since
the grating is transverse of the beam 40, the beam passes through
at least a portion of the structure of the grating 22.
The optical beam 40 is derived from an optical frequency source
(not shown) and has a wavelength such that the beam is not
substantially absorbed in the crystal, and such that the
photorefractive effect does not distort the beam significantly. The
optical frequency source means may include one or more optical
exciters capable of supplying sufficient brightness within the
wavelength acceptance of the grating reflector 22 to produce a
useful switched output beam 44. The output beam may be coupled to
other elements on the same substrate, or it may be coupled to
external devices, in which case the output surface through which
beam 44 emerges is preferably antireflection coated. The
antireflection coating may be a multilayer dielectric coating, a
single quarter wave layer of a material with almost the appropriate
index of refraction, or a sol-gel coating. The exciter may be any
light source including a laser, a light emitting diode, an arm
lamp, a discharge, or even a filament, provided that the desired
spectral brightness is achieved. The desired spectral brightness
may be supplied directly from one or more exciters, indirectly from
one or more frequency converted (doubled, mixed, or parametrically
amplified) exciters, or in combination with several of the above
alternatives. Absorption effects will limit the wavelength to the
range from about 400 to 4000 nm. The effect of the photorefractive
phenomenon varies with the configuration, the wavelength, dopants,
and the poling structure, and we assume here that it has been
brought under control so that any beam distortion remains within
acceptable limits.
The grating 22 is formed or defined by the boundary 34 between
alternating domains of two different types. The first type of
domain 36 has a different electro-optic (E-O) coefficient than the
second type of domain 38, so that a uniform electric field applied
between the electrodes 24 and 26 results in different changes in
the index of refraction in the two types of domains. Because the
index of refraction changes the phase velocity of the wave, there
is art impedance mismatch between the regions of different index or
phase velocity. It is advantageous to accomplish such an index
change with material in which the regions 36 have a reverse sense
relative to the poling direction of the other domain type 38 and
the original wafer 20, as shown by the poling sense arrows 39, 41.
By reverse sense we mean the poling direction is opposite to some
reference direction. (An alternative realization of the field
controllable grating is in an irradiated masked polymer film which
has its E-O coefficient destroyed inside or outside the regions
36.) A uniform electric field applied to the structure 22 produces
a modulated index of retraction. The pattern of index modulation
adds to the pre-existing index of refraction distribution; the
simplest configuration has no index modulation in the absence of
the applied electric field, and develops an index grating linearly
in response to the applied field. A period 48 for the grating 22 is
the distance between two domain boundaries entirely including a
region corresponding to each domain type.
An alternative realization of the index of refraction grating is
obtained by applying a strain field to the poled regions. The
photoelastic response of the material produces different index of
refraction changes in the different poled regions. The strain field
may be applied permanently by, for example, laying down a film on
top of the substrate at a high temperature and then cooling to room
temperature. A concentration of strain may be achieved by etching
away a stripe of the film, for example.
The poled elements 36 and 38 alternate across the grating 22 with
no space between them. If additional domain types are available,
more complicated patterns of alternation are possible with domains
separated by variable distances of the different domain types. For
some applications, the grating 22 is a uniformly periodic grating
as shown in FIG. 2 so that the domain types contained in one period
along the length of the grating 22 are reproduced in the other
periods. For other applications, it is advantageous to modify the
period to obtain advantages such as multiple spectral peaks or a
broader spectral bandwidth. By grating we mean an array of
distinguishable structures, including all possible variations of
geometry and periodicity.
A periodic index grating is capable of supplying virtual photons in
an interaction between optical beams. This means the grating
structure is capable of supplying momentum, but not energy, to the
interaction. For an interaction to proceed, both energy and
momentum must be conserved, and the grating is useful when a
momentum increment is required to simultaneously satisfy the two
conservation relations. The grating periodicity defines the
momentum which is available to the interaction. The grating
strength determines the "intensity" of the virtual photon beam. The
number of periods in the section of the grating traversed by the
optical beam determines the bandwidth of the virtual photon momenta
which are available. Because of the bandwidth limitation, the
interaction can only proceed within a specific range (or ranges) of
optical frequencies. Grating devices are therefore inherently
frequency selective, and typically operate around a nominal
wavelength.
For example, in a simple reflection process at an angle, as
illustrative in FIG. 2, the photons of the input beam 40 have the
same optical frequency as the photons of the output beams 44 and
42, so energy conservation is observed. However, the momentum of
the photons in input beam 40 and diverted output beam 44 are not
the same; for the reflection process to occur, the change in
momentum must be supplied by the grating 22 as illustrated by the
vector diagram 43 associated with FIG. 2. The grating 22 supplies a
virtual (with momentum but no energy) photon to the interaction to
enable the conservation of momentum. The momentum vector associated
with the i.sup.th mode, k.sub.i =2.pi.n.sub.i /.lambda..sub.i, is
equal to the product of 2.pi. times the effective index n.sub.i for
that mode divided by the wavelength .lambda..sub.i for that wave,
and it points in the direction of propagation. The magnitude of the
momentum vector is also called the propagation constant. In the
case of a single period grating, the momentum vector k.sub.g
=2.pi./.LAMBDA. points perpendicular to the grating surfaces, and
it can have any wavelength value A which is present in the Fourier
transform of the grating. The optical spacing (the width of the
grating lines and spaces) associated with the propagation constant
k.sub.g of a 50% duty cycle grating is therefore .LAMBDA./2. The
frequency of interaction may also be tuned by adjusting for example
the index of refraction of the optical beams, or the grating period
by thermal expansion or other means. Depending on how a given
device is implemented, an index structure may have a spectrum of
wavelengths and vector directions which can be contributed to the
interaction. Also, multiple virtual photons may be contributed to
an interaction in a so-called "higher order" grating interaction. A
"higher order" grating is one which has a period which is related
to the required period for momentum conservation by division by an
integer. The required momentum virtual photon is obtained from the
harmonics of the "higher order" grating. The condition that
momentum be conserved by the process is commonly called the Bragg
condition, so the gratings of this invention am Bragg gratings, and
the incidence angle on the gratings is the Bragg angle for the
in-band or resonant frequency component. This dual conservation of
energy and momentum is required for any energy beam interaction,
whether the energy beam is optical, microwave, acoustic, or any
other wavelike energy form consisting of a time-variable energy
field. Only the implementation of the grating may change, to
produce an impedance modulation for the different forms of energy
so that the pattern of the structure can couple with the wavelike
energy form.
In FIG. 2, the index grating functions as a frequency-selective
optical energy router or reflector. A beam of a characteristic
frequency within the interaction bandwidth (capable of interacting
with one or more of the virtual photons) is known as an in-band
beam, while energy beams of other frequencies are known as
out-of-band beam. The grating 22 has a frequency bandwidth which
corresponds to the full width at half maximum of the reflection
efficiency of the grating as a function of optical frequency. When
the index grating is present (the grating is "on"), a beam having
an optical frequency within the bandwidth of the grating is
reflected from the grating at the angle 46 around a normal 47 to
the grating structure. An out-of-band beam transmits through the
crystal along the same optical axis and in the same direction as
the input beam, forming part of the transmitted output beam 42. An
electric field applied in the region including the grating controls
the strength of the index modulation (which can also be thought of
as the intensity of the virtual photons), adjusting the ratio of
the power in the transmitted output beam 42 to that in the
reflected output beam 44.
For a weak retro reflecting grating (which does not substantially
deplete the input beam), the full width half maximum bandwidth
.DELTA..lambda. is given by ##EQU1##
where .lambda.=vacuum wavelength of the input beam, n=index of
refraction of the beam, and L=length of the grating. For highly
reflecting gratings, the effective length is smaller than the total
length of the grating, increasing the bandwidth.
The two types of domains may exhibit an index difference before an
electric field is applied. In this case, a permanent index grating
accompanies the poled switchable index grating. As the electric
field is applied, the net modulation in the index of refraction
(the grating strength) may be increased or decreased, depending on
the polarity. The "grating off" situation (index grating value near
zero) is then achieved at a specific value of applied field. The
grating can then be turned "on" by applying any other field
strength. If the polarity of the applied field is reversed, for
example, an index grating is produced with twice the strength of
the original permanent grating.
The poled grating structure of our invention has two major
advantages over the prior art. First, the poled domain structures
can have very sharp boundaries, providing a strong Fourier
coefficient at virtual photon momenta which are multiples of the
momentum corresponding to the basic grating period. This is very
useful in cases where it is impractical to perform lithography with
the required small feature size. Second, strong index modulation
gratings can be made even if the optical mode dimension is large
compared to the grating period. This is not possible in a uniformly
poled substrate excited by patterned electrodes, because the
electric field modulation decays exponentially with distance away
from the plane of the electrode array, losing most of the
modulation within a distance equal to the grating period. The
poling process can create poled features with an extremely high
aspect ratio, or the ratio of depth of the domain to its width.
Using an electric field poling technique, aspect ratios in excess
of 250:1 have been fabricated. Because we use essentially uniform
electrodes, we get good electrostatic penetration; with deep domain
walls, good modulation is available across the entire beam.
The grating may also be a two-dimensional array of index changes,
in which case the grating has periodicities in two dimensions. The
virtual photon contributed by the grating can then contribute
momentum in two dimensions. This might be useful, for example, in
an application with several output beams from a single grating.
In the preferred embodiment, the ferroelectric crystal is a
commercially-available, z-cut, lithium niobate single-crystal
wafer. Other cuts, including x-, y-, and angle-cuts can also be
used, depending on the poling method and the desired orientation of
the poled domains. The fabrication steps include primarily poling
and electrode fabrication. Prior to processing, the crystal is
cleaned (for example by oxygen plasma ashing) to remove all
hydrocarbons and other contaminants remaining from the polishing
and handling processes. To control the poling, a mask and
processing electrodes are used to create a pattern of applied
electric field at the surface of and through the wafer, as
described in U.S. patent application Ser. No. 08/239,799 filed May
9, 1994. The poling pattern is adjusted to produce the poled domain
inversion in regions 36 during the application of the poling field.
In brief, a silica layer several microns thick is deposited on
the+z surface 23 of the wafer 20. This film is thinned or removed
over the regions 36 where domain inversion is desired, a liquid
electrode or deposited metal film is used to make a good
equipotential surface over the patterned silica, and an electric
field exceeding approximately 24 kV/mm is applied with the+z
surface 23 at a higher potential than the-z surface 25. Using this
technique, ferroelectric crystals of lithium niobate have been
poled to create patterns of two domain types which are of reverse
polarity (domain inversion). The magnitude of the electro-optic
coefficient for the two types of domains is identical, although
with a reverse polarity.
In addition to the preferred technique, domain inversion has been
achieved in ferroelectrics using in-diffusion, ion-exchange, and
alternate electric field poling techniques. Domain formation by
thermally-enhanced in-diffusion has been demonstrated in lithium
niobate, using titanium. The triangular shape of the inverted
region limits the interaction efficiency for small domain size,
however, and is useful mainly in waveguide devices with long
periods. Patterned poling via ion exchange has been demonstrated in
KTP in a salt bath containing rubidium and barium ions, in which
the potassium ions in the crystal were exchanged for the rubidium
ions. Electric field poling using alternate techniques to the
preferred one have also been demonstrated in both lithium niobate
and lithium tantalate. Potentially, all solid ferroelectric
materials, including KTP and barium titanate, can be poled by
electric field domain-inversion techniques. (Solid means holding
its structure for a certain period of time, such as cooled fluids,
glasses, crosslinked polymers, etc.)
Gratings with different characteristics are generated by the
different techniques. Electric field poling aligns the domains in
the crystal without producing an intrinsic change in the index of
refraction, while the ion-exchange and diffusion techniques do
create a index change in the poled regions. A permanent index
grating accompanies the switchable poled grating when these latter
methods are used.
In general, there are two types of differing domains, at least the
first type of which is poled. Although only two types of domains
are required, more complex switchable grating structures can be
fabricated with additional types of domains. The second domain type
may be reverse poled, unpoled, or poled at another angle, and it
may be distinguished by possessing a distinct electrical activity
coefficient, (e.g., the electro-optic or the piezo-optic
coefficient). For example, it may in some applications be cost
effective to fabricate the device from unpoled lithium niobate
wafers, in which case the substrate wafer is comprised of multiple
randomly oriented domains. The poled domains will have a uniform
orientation while the orientation in the other domains will be
random. The performance of the device will be affected by the
details of the random pattern, depending on the type of device. As
another example, the second domains may be oriented perpendicular
to the first or at another angle, and the difference in the
electrical response can still produce a useful electronically
controlled structure. The poled domains may also be formed in a
material which was previously unpoled and randomly oriented on a
molecular scale, such as in fused silica or polymers. The poling
process orients the structure of the material to form the first
domain type, while the second domain type consists of the unpoled
or randomly oriented regions in the material.
In an alternate technique, the poled structure can be formed by
selectively changing or destroying the electrical activity
coefficient in regions corresponding to the second domain type. The
orientation of the atomic structures in these regions does not need
to be altered: if the electrical activity is changed in the second
domain region, the domains are different. For example in nonlinear
polymers, the electro-optic coefficient may be disabled by
irradiation, producing regions of electrical activity where the
irradiation is masked off. A similar effect has been demonstrated
in lithium niobate, where proton exchange destroys the nonlinear
coefficient. Modification of the electro-optic coefficient can also
be achieved by optical radiation, electron bombardment, and/or ion
bombardment in many other materials, including most nonlinear
materials such as KTP and lithium tantalate.
In lithium niobate, an applied field E.sub.3 along the z axis of
the crystal induces a change in the extraordinary index of
refraction .delta.n.sub.e which is given by ##EQU2##
where r.sub.33 is the appropriate electro-optic nonlinear optical
coefficient. Because r.sub.33 is the largest nonlinear constant in
lithium niobate, it is best to use the change in the extraordinary
index in practical devices. (The nonlinear constant r.sub.13 which
produces a change in the ordinary index of refraction due to an
applied E.sub.3, is a factor of 3.6 smiler than r.sub.33.) To use
the change in the extraordinary index, the light waves must be
polarized along the z axis of the material. In a z-cut crystal,
this polarization is called TM. (In TE polarization, the electric
vector lies in the plane of the crystal surface. The only other
significant nonlinear coefficient is r.sub.15, which couples TE and
TM waves upon the application of an electric field E.sub.1 or
E.sub.2.)
Because the index change induced in the poled structures is quite
small (with an applied field of 10 V/.mu.m along the z axis of a
lithium niobate substrate, the index change .delta.n.sub.e is only
1.6.times.10.sup.3), the grating reflector of FIG. 2 has a strong
angular dependence. The Brewster angle for a weak index change is
45.degree., so the gratings will totally transmit any TE polarized
wave when the planes of the grating are disposed at and angle of
45.degree. with respect to the phase front of the light beam. The
device may therefore be used as a polarizer. The reflected beam
will always be essentially polarized at 45.degree. incidence. If
the reflection coefficient for the TM wave is high, which can be
arranged with enough grating periods and a high applied field, the
extinction ratio of the polarizer can also be very high in the
forward direction. At normal incidence, of course, there is no
difference in reflection between the two polarizations due to this
effect (although there are differences due to other effects such as
the different electro-optic coefficients described above). A total
internal reflection device operating at grazing incidence is far
from Brewster's angle and has little difference in reflection due
to this effect.
The wafer material can be any polable solid dielectric material,
including ferroelectrics, polymer films, and some amorphous
materials such as fused silica which can also be poled for
producing many useful devices according to the invention. The poled
material may also be a thin film deposited on a substrate of a
second material. Many of the polable thin film, such as fused
silica, lithium niobate, potassium niobate, barium titanate, zinc
oxide, II-VI materials, and various polymers, have been
successfully deposited on a substrate. A wide variety of substrates
have been used, including MgO, silicon, gallium arsenide, lithium
niobate, and various glasses, including quartz and fused silica.
For the domains to be electronically switchable, they must consist
of electro-optic materials, which are materials having an index
change induced by an applied electric field.
After the poling step, the liquid electrode material and silica
masking film are preferably removed. Referring again to FIG. 2, a
first electrode 24 and a second electrode 26 confront the
dielectric material in order to provide a means to create the
electric field which controls the grating. (Confronting a material
means placed close to the material but not necessarily touching,
approximately aligned to the surface of the material but not
necessarily with a constant gap dimension, and includes situations
with additional material of varying dimensions placed on top of the
material.) The electrodes 24 and 26, consisting of an
electrically-conductive material, are preferably laid out on
opposing surfaces of the crystal in a spatially delimited manner
using standard deposition techniques. These electrodes are referred
to as being on opposing planes even though the surfaces may be
curved and/or non-parallel as part of a larger geometry. The
electrodes may be formed by any material that provides sufficient
transport of electrical charge to achieve an adequate field
strength to activate the poled grating in a time consistent with
the application. For example, the electrodes could alternatively
consist of metals such as aluminum, gold, titanium, chromium, etc.,
conductive paint, epoxy semiconducting material, or optically
transparent materials such as oxides of indium and tin, and liquid
conductors such as salt solutions. They may also confront the
surfaces 23 and 25 with a gap filled with air, an optically
transparent buffer layer, and/or other material. Only one electrode
is required since a potential voltage difference can be created
between that electrode and any potential reference such as an
exterior ground plane, a second electrode, or multiple electrodes.
The electrodes are the electric field creating means because the
application of a voltage to an electrode establishes an electric
field pattern which is determined by the electrode. A voltage and
current supply is of course also needed. The electrodes are placed
so that the control electric field is applied through the active
volume of the invention, which may consist of a pattern poled
region or a grating.
In the case of metallic electrodes, it may be best to incorporate a
coating deposited below the electrode, to reduce the optical loss
which occurs when a portion of the guided wave mode extends to the
metallic electrode. The coating should be thin enough to maintain
high electric field at the surface in the case of multiple
electrodes mounted on the same surface, but thick enough to reduce
the optical loss. Mother coating is also useful above the
electrodes to reduce the probability of breakdown.
A voltage control source 32 (or potential source) provides the
electrical potential to drive the electrodes through connections 30
to activate the grating. The activated electrodes are polarized
relative to each other according to the polarity of the applied
voltage. The voltage of the source produces a large enough electric
field through the poled regions to switch a significant amount of
light into the switched output beam 44. The voltage of the source
is variable to provide a means to control the ratio of power in the
two output beams. Substantially all of the input beam may be
reflected with a long grating if the electric field is sufficiently
high, forming an electrically activated mirror. For lower electric
fields, the grating forms a partial reflector. The voltage control
source may be a battery, an electrical transformer, a gas powered
generator, or any other type of controllable source of electrical
current and potential. The control means 32 may also incorporate a
controller which generates a time dependent voltage, and which
supplies the current to change the voltage on the electrodes 24 and
26 at the frequencies required by the application. The control
means 32 may also have multiple outputs capable of controlling
multiple devices, and which might be sequenced temporally according
to some pattern. The source 32 may have control inputs for manual
or electronic control of its function by computer or by another
instrument.
In order to avoid unnecessary repetition, it should be understood
that the variations described in reference to FIG. 2 apply to the
embodiments described below, and that the variations described in
reference to the figures below also apply to FIG. 2.
Referring now to FIG. 3, a guided-wave embodiment of the present
invention is shown. Specifically, this embodiment is an
electrically-controlled, frequency-selective waveguide
retroreflector. All of the optical beam in this device are confined
in two dimensions by an optical waveguide 64, which traverses one
surface of the polable dielectric material that forms the substrate
60 of the device 61.
A waveguide is any structure which permits the propagation of a
wave throughout its length despite diffractive effects, and
possibly curvature of the guide structure. An optical waveguide is
defined by an extended region of increased index of refraction
relative to the surrounding medium. The strength of the guiding, or
the confinement, of the wave depends on the wavelength, the index
difference and the guide width. Stronger confinement leads
generally to narrower modes. A waveguide may support multiple
optical modes or only a single mode, depending on the strength of
the confinement. In general, an optical mode is distinguished by
its electromagnetic field geometry in two dimensions, by its
polarization state, and by its wavelength. The polarization state
of a wave guided in a birefringent material or an asymmetric
waveguide is typically linear polarized. However, the general
polarization state may contain a component of nonparallel
polarization as well as elliptical and unpolarized components,
particularly if the wave has a large bandwidth. If the index of
refraction difference is small enough (e.g. .DELTA.n=0.003) and the
dimension of the guide is narrow enough (e.g. W=4.mu.m), the guide
will only confine a single transverse mode (the lowest order mode)
over a range of wavelengths. If the waveguide is implemented on the
surface of a substrate so that there is an asymmetry in the index
of refraction above and below the waveguide, there is a cutoff
value in index difference or waveguide width below which no mode is
confined. A waveguide may be implemented in a substrate (e.g. by
indiffusion), on a substrate (e.g. by etching away the surrounding
regions, or by applying a coating and etching away all but a strip
to define the waveguide), inside a substrate (e.g. by contacting or
bonding several processed substrate layers together). In all cases,
we speak of the waveguide as traversing the substrate. The optical
mode which propagates in the waveguide has a transverse dimension
which is related to all of the confinement parameters, not just the
waveguide width.
The substrate is preferably a single crystal of lithium niobate,
forming a chip which has two opposing faces 63 and 65 which are
separated by the thickness of the wafer. The opposing faces need
not be parallel or even flat. The waveguide is preferably formed by
a well-established technique such as annealed proton exchange (APE)
on face 63. Alternatively, ions other than protons may also be
indiffused or ion exchanged into the substrate material. The APE
waveguide increases the crystal extraordinary refractive index,
forming a waveguide for light polarized along the z-axis. For a
z-cut crystal, this corresponds to a TM polarized mode. Waveguides
formed by alternate techniques, such as titanium indiffusion in
lithium niobate, may support both the TM and TE polarization.
Preferably, the waveguide is designed to support only a single
lowest order transverse mode, eliminating the complexities
associated with higher order modes. The higher order transverse
modes have different propagation constants than the lowest order
mode, and higher scattering loss, which can be problems in some
applications. However, multimode waveguides might be preferred for
some applications, such as for high power propagation.
One alternative configuration is to excite the grating by applying
pressure rather than by directly applying an electric field. The
effect of an applied pressure is indirectly the same: by the
piezoelectric effect, the applied stress produces an electric
field, which in turn changes the index of refraction of the
domains. However, no sustaining energy need be applied to maintain
the stress if the structure is compressed mechanically, for
example. This alternative, like the others mentioned herein, apply
also to the other similar realizations of the invention described
below.
Once the waveguide dimensions are determined, a photomask for the
waveguide is generated and the pattern is transferred to a masking
material on the substrate, by one of many well known lithographic
processes. The mask material may be SiO.sub.2, tantalum or other
metals, or other acid resisting materials. To fabricate an APE
waveguide, the masked substrate material is immersed in molten
benzoic acid to exchange protons from the acid for lithium ions in
the crystal. The resulting step index waveguide may then be
annealed for several hours at around 300.degree. C. to diffuse the
protons deeper into the crystal and create a low-loss waveguide
with high electrical activity coefficients.
In addition to in-diffusion and ion exchange two-dimensional
waveguides, planar and two dimensional ridge or strip-loaded
waveguides can be formed. Planar waveguides may be formed by
depositing the electrically active material on a substrate of lower
index. Deposition techniques for waveguide fabrication are
well-known and include liquid phase epitaxy (LPE), molecular beam
epitaxy (MBE), flame hydrolysis, spinning, and sputtering. Ridge
waveguides can be formed from these planar guides by using
processes such as lift-off, wet etch, or dry etch such as reactive
ion etching (RIE). Planar guides can also be used in the present
invention, particularly in devices using a variable angle of
diffraction off the grating.
The grating 62 in this embodiment is disposed normal to the optical
waveguide 64 which traverses the substrate. The grating is composed
of a first type 66 and second type 68 of domain, which do not
necessarily extend through the substrate. For example, when the
active material is poled using indiffusion or ion exchange, the
inverted domains 66 typically extend to a finite depth in the
material. The partial domains may also be formed when the poling is
achieved by destroying the electrical activity of the material (or
reducing the electro-optic activity) by a technique such as ion
bombardment or UV irradiation.
The optical input beam 80 is incident on and is coupled into the
waveguide. Coupling refers to the process of transferring power
from one region into another across some kind of generalized
boundary such as across an interface, or between two parallel or
angled waveguides, or between a planar guide and a stripe guide, or
between single mode and multimode waveguides, etc. When the grating
is on, a portion of the input beam is coupled back into a
retroreflected output beam 82. While the retroreflection of the
grating need not be perfect, i.e. the grating may reflect the light
to within a few degrees of the reverse direction, the waveguide
captures most of this light and forms a perfectly retroreflected
beam. The imperfection of the retroreflection results in a coupling
loss of the retroreflected beam into the waveguide 64. When the
grating is off (when the controlling electrical field is adjusted
to the "off" position in which the index grating has a minimum
value near zero, typically at zero field), the input beam continues
to propagate in the same direction through the waveguide to form a
transmitted output beam 84. As in the bulk device, the strength of
the grating can be varied with the voltage source 76 to control the
ratio of the power in the two output beams.
A first electrode 70 and second 72 electrode confront opposing
faces of the dielectric material 60. The substrate is a dielectric
because it is capable of withstanding an applied electric field
without damage, but it need not be a perfect insulator as long as
the current flow does not adversely affect the performance of the
device. The electrodes may be formed of any electrically conducting
material. There must also be a means for creating an electric field
through the dielectric material using the first electrode
structure.
The electrodes bridge at least two of the elements of the first
type of poled structure that forms the grating. This means the
electric field produced by the electrodes penetrates into at least
the two elements. Thus, these elements can be activated by the
field. Two wires 74 preferably connect the voltage control source
76 to the two electrodes to provide an electric field in the region
formed by the intersection of the waveguide 64 and the poled
structure 62. The wires may be formed from any material and in any
geometry with sufficient conductivity at the operating frequency to
allow charging the electrodes as desired for the application. The
wires may be round, flat, coaxial cables, or integrated lead
pattern conductors, and they may be resistors, capacitors,
semiconductors, or leaky insulators.
Alternately, the electrodes can be arranged in any manner that
allows an electric field to be applied across the electrically
active material. For example, the electrodes may be interspersed in
different layers on a substrate, with the active material between
the electrodes. This configuration enables high electric fields to
be produced with low voltages, and is particularly useful for
amorphous active materials, such as silica and some polymers, which
can be deposited over the electrode material.
The poled structure 62 is preferably deeper than the waveguide so
that the intersection between the waveguide 64 and the poled
structure 62 has the transverse dimensions of the mode in the
waveguide and the longitudinal dimensions of the grating.
FIGS. 4, 5 and 6 show alternate electrode configurations in which
the electrodes are disposed on a common face of the dielectric
material 189. These configurations are especially useful for
embodiments of the present invention that use a waveguide 180 to
guide an optical beam, since the same-surface electrode
configurations permit high electric fields at low voltage. These
electrode structures are of particular interest for low voltage
control of the grating 182 because of the proximity of the
electrodes to the section of the waveguide which traverses the
grating. In the electrode configuration 186 depicted in FIG. 4, the
first electrode 170 and second electrode 172 confront the
dielectric material on the same surface. These electrodes are
referred to as being on a common plane even though the surface may
be curved as part of a larger geometry. The first electrode is
placed above a portion of the waveguide that contains several
grating elements, each of which consists of alternate regions of a
first type of domain 184 and a second type of domain 185. The
second electrode is positioned around the first electrode. The
distance between the electrodes along the waveguide is
approximately constant along the axis of the waveguide for cases
where a uniform field along the axis of the waveguide is desired.
The electrode spacing may also be varied to taper the field
strength, as shown schematically in the device 188 of FIG. 6. A
voltage source 174 connected between the two electrodes disposed as
shown in FIG. 4, is capable of generating electric fields between
the electrodes. The electric field vectors 176 have their largest
component perpendicular to the surface of the material, in the
region of the electrically-active waveguide. For a z-cut
ferroelectric crystal such as lithium niobate, this electric field
structure activates the largest electro-optic coefficient r.sub.33,
creating a change in index for a TM polarized optical beam. For an
applied electric field of 10 V/.mu.m and an optical beam with a
wavelength of 1.5 .mu.m in lithium niobate, the strength of a first
order grating is 40 cm.sup.-1.
A means 178 for contacting the electrodes to a voltage source is
required for each of the electrode configurations. To form this
means, an electrically conducting material, such as a wire, is
electrically contacted between the electrodes on the device and the
terminals of the potential source. In all electrode configurations,
each electrode typically has a section, or pad, or contact, to
which the wire is contacted. The pads are preferably of large
enough size to reduce placement tolerances on the electrical
contact means for easier bonding. The wire can then be contacted to
the pads using a technique such as wire bonding by ultrasonic
waves, heating, or conductive epoxy. Alternately, a spring-loaded
conductor plate can be placed in direct contact with the electrode
to make the required electrical connection to the voltage source.
In the figures, the electrodes are typically large enough and
function as the contact pads by themselves.
Another realization 187 of the same-surface electrode structure is
shown in FIG. 5, wherein the first electrode 171 and second
electrode 173 are placed on either side of the optical waveguide.
When an electric potential is applied across the two electrodes
positioned in this manner, the electric field vectors 177 have
their largest component parallel to the substrate surface. For a
z-cut ferroelectric crystal, the electro-optic coefficient that
creates a change in index for a TM polarized optical wave and the
applied electric field is r.sub.13. For an applied electric field
of 10 V/.mu.m and an optical beam with a wavelength of 1.5 .mu.m in
lithium niobate, the first order grating coupling constant is 12
cm.sup.31.sub.1.
Alternately, for TE waveguides the active electro-optic
coefficients are switched for the two configurations. For an
electric field vector perpendicular to the surface of the chip, the
appropriate coefficient is r.sub.13, while for an electric field
vector parallel to the surface of the chip, the electro-optic
coefficient used is r.sub.33. Similar situations apply for x- or
y-cut crystals, or intermediate cuts.
As a further variation of the configuration of FIG. 5, the
electrodes are asymmetrically arranged so that one electrode
approximately covers the waveguide 180 and the other electrode is
displaced somewhat to the side. In this configuration, the strong
vertical field induced under the edges of the adjacent electrodes
is made to pass predominantly through the waveguide region under
one of the electrodes.
In FIG. 6, the electrodes 175 and 179 have a separation from the
center electrode 181 which is tapered. When a voltage is applied
across these electrodes, this configuration produces a tapered
field strength, with the strong field towards the right and the
weaker field towards the left. By "tapered" we mean that any
parameter has a generalized spatial variation from one value to
another without specifying whether the variation is linear or even
monotonic; the parameter may be a gap, a width, a density, an
index, a thickness, a duty cycle, etc. The index changes induced in
the poled domains towards the left of the waveguide 180 are
therefore weaker than the index changes induced towards the right.
This might be useful, for example, to obtain a very narrow
bandwidth total reflector where it is needed to extend the length
of the interaction region. In non-normal incidence angle devices,
such as shown in FIG. 7 and FIG. 8, the taper might be useful to
optimize the coupling of a specific input mode into a specific
output mode.
In all electrode configurations, the voltage applied can range from
a constant value to a rapidly varying or pulsed signal, and can be
applied with either polarity applied between the electrodes. The
value of the voltage is chosen to avoid catastrophic damage to the
electrically-active material and surrounding materials in a given
application.
When a constant electric field is applied across materials such as
lithium niobate, charge accumulation at the electrodes can cause DC
drift of the electric field strength with time. The charges can be
dispersed by occasionally alternating the polarity of the voltage
source, so that the electric field strength returns to its full
value. If the time averaged electric field is close to zero, the
net charge drift will also be close to zero. For applications
sensitive to such drift, care should be taken to minimize the
photorefractive sensitivity of the material, such as by
in-diffusion of MgO, and operation is preferably arranged without a
DC field.
Surface layers are useful for preventing electric field breakdown
and lossy optical contact with the electrodes. Losses are
particularly important for waveguide devices, since the beam
travels at or near the surface, while breakdown is most critical
when electrodes of opposite polarity are placed on the same
surface. This concern applies to the poling of the active material
as well as to the electro-optic switching. The largest vector
component of the electric field between two same-surface electrodes
is parallel to the surface of the material. Both the breakdown
problem and the optical loss problem can be considerably reduced by
depositing a layer of optically transparent material with a high
dielectric strength between the guiding region and the electrodes.
Silicon dioxide is one good example of such a material. Since there
is also a potential for breakdown in the air above and along the
surfaces between the electrodes, a similar layer of the
high-dielectric-strength material can be deposited on top of the
electrodes.
FIG. 7 and FIG. 8 show two embodiments of a electrically-controlled
frequency-selective waveguide coupler. In FIG. 7, a pair of
two-dimensional waveguides traverse one face of a dielectric
material, and intersect at an angle 118 to make a tee, forming a
three-port device. A grating 100, consisting of a first type 104
and second type 102 of domains, is disposed at an angle to the two
guides in the intersection region between them (the volume jointly
occupied by the optical modes in the two waveguides). The peak
index change in the intersection region is preferably equal to the
peak index change in the waveguides. This is done if the
fabrication of the tee structure is accomplished in one step (be it
by indiffusion, ion exchange, etching, etc.). In the alternative
approach of laying down two waveguides in subsequent steps, which
is most convenient in the crossing waveguide geometry of FIG. 8,
the peak index change in the intersection region is twice the index
change in the waveguides, which is not needed. As always, the
periodicity and angle of the grating is chosen such that the
reflection process is phase matched by the momentum of a virtual
photon within the bandwidth of the grating. For optimal coupling
between an m-band input beam in the first waveguide and an output
beam 114 in the second waveguide 108, the angle of incidence of the
input beam is equal to the angle of diffraction off the grating. In
this case, the bisector of the angle between the two guides is
normal to the domain boundaries of the grating in the plane of the
waveguide.
An input beam 112 is incident on and is coupled into the first
waveguide 106. A first electrode 120 and second electrode 122 are
laid out on the same face of the dielectric material so that an
electric field is created in the intersection region between the
waveguides, when a voltage source 124 connected to the two
electrodes by conductors 126 is turned on. The electric field
controls the strength of the grating in the intersection region via
the electro-optic effect, coupling the in-band beam from the first
waveguide into the second waveguide to form a reflected output beam
114. With the grating turned off, the input beam continues to
propagate predominantly down the first waveguide segment to form a
transmitted output beam 116 with very little loss. Alternately,
counter-propagating beams can be used in the waveguide so that the
input beam enters though the second waveguide 108, and is switched
into the output waveguide 106 by interacting with the grating.
In single mode systems, the grating strength is preferably
spatially distributed in a nonuniform manner so that a lowest order
Gaussian mode entering waveguide 106 is coupled into the lowest
order Gaussian mode of waveguide 108. The grating strength can be
modulated by adjusting the geometry of the electrode, by adjusting
the gaps between the electrodes, and by adjusting the duty cycle of
the grating. The bandwidth of the grating may also be enhanced by
one of a number of well known techniques such as chirping, phase
shifting, and the use of multiple period structures.
The size of the coupling region is limited, in the geometry of
FIGS. 7 and 8 by the size of the intersection region between the
guides where their modes overlap. To obtain a high net interaction
strength for a given electric field strength, it is desirable to
increase the size of the waveguides to produce a larger
intersection. However, large waveguides are multimode, which may
not be desirable for some applications. If adiabatic expansions and
contractions are used, the advantages of both a large intersection
region and single mode waveguides can be obtained simultaneously.
The input waveguide 106 begins as a narrow waveguide and is
increased in width adiabatically as the intersection region is
approached. The output waveguide 108 has a large width at the
intersection to capture most of the reflected light, and it is
tapered down in width adiabatically to a narrow waveguide. The idea
of adiabatic tapering of an input and/or an output waveguide can be
applied to many of the interactions described herein.
Referring to FIG. 8, the two waveguides 136 and 138 intersect at an
angle 158 to make an x intersection, forming a four-port device.
This device is a particularly versatile waveguide switch, since two
switching operations occur simultaneously (beam 142 into beams 146
and 148, and beam 144 into beams 148 and 146). The grating 130,
consisting of a first type 134 and second type 132 of domains, is
disposed at an angle to the two guides in the intersection region
between them. The angle of the grating is preferably chosen such
that the bisector of the angle between the two guides is normal to
the domain boundaries of the grating, in the plane of the
waveguide.
A first input beam 142 is incident on and is coupled into the first
waveguide 136 and a second input beam 144 is coupled into the
second waveguide 138. A first electrode 150 and second electrode
152 are laid out on the dielectric material so that an electric
field is created in the intersection region between the waveguides,
when a voltage source 154 connected between the two electrodes is
turned on. The electric field controls the strength of the index
grating in the intersection region through the electro-optic
effect. When the grating is on, a portion of the in-band component
of the first input beam is coupled from the first waveguide to the
second waveguide to form a first output beam 146. At the same time,
a portion of the in-band component of the second input beam from
the second waveguide is coupled into the first waveguide to form
the second output beam 148. In addition, the out-of-band components
of the two beams, and any unswitched components of the in-band
beams, continue to propagate down their respective waveguides to
form additional portions of the appropriate output beams. Thus, for
two beams with multiple optical frequency components, a single
frequency component in the two input beams can be switched between
the two output beams.
The waveguide may only be a segment, in which case it is connected
to other optical components located either off the substrate, or
integrated onto the same substrate. For example, the waveguide
segment could be connected to pump lasers, optical fibers, crossing
waveguides, other switchable gratings, mirror devices, and other
elements. An array of crossing waveguide switches would comprise an
optical switching network.
In FIG. 9, a further embodiment of the waveguide coupling switch is
shown. The domain walls of the grating are now disposed at a
non-normal angle to the surface 157 of the crystal 158, so that the
input beam 159 in waveguide 160 is reflected out of the plane of
the crystal to form a reflected output beam 161. As before, an
unreflected beam continues to propagate through the waveguide to
form a transmitted output beam 162. An optically transparent first
electrode 163, which can consist of indium tin oxide, is disposed
on one face of the dielectric material 158, over a portion of the
grating that crosses the waveguide. A second electrode structure
164, which may be optically absorbing, is disposed on the material.
As in all cases described in this disclosure, the second electrode
may be arranged in one of many alternate configurations:
surrounding the first electrode as in FIG. 7, on opposite sides of
the material 158 as on FIG. 2, tapered similar to the configuration
shown in FIG. 6. The electrodes are connected with two wires 156 to
a voltage source 154, which controls the power splitting ratio of
the in-band beam between the transmitted beam 162 and the reflected
beam 161. Alternately, the electrode configuration could be as
shown in FIG. 5, in which case both electrodes may be opaque.
Referring again to FIG. 9, the domain walls are preferably formed
by electric field poling of a ferroelectric crystal which is cut at
an angle to the z-axis 165. Since the electric field poled domains
travel preferentially down the z axis, poling an angle-cut crystal
by this technique results in domain boundaries parallel to the z
axis, at the same angle to the surface. The angle 166 of the cut of
the crystal is preferably 45.degree. so that light propagating in
the plane of the crystal may be reflected out of the substrate
normal to the surface of the material (any angle may be used). The
domains shown in FIG. 9 are planar, but can also be configured in
more general configurations. A planar grating will produce a flat
output phase front from a flat input phase front. If the device
shown is used as a bulk reflector without the waveguide, a
collimated input beam will produce a collimated output beam. The
device is useful as a bulk reflector for example if a beam is
incident from outside the device, or if the waveguide is brought to
an end within the device with some distance between the end of the
waveguide and the poled reflector. In some cases, however, it may
be desirable to produce a curved output phase front from a
collimated beam, as in the case of some applications requiring
focussing, such as reading data from a disk. By patterning a set of
curved domains on the upper surface of the substrate illustrated in
FIG. 9, a set of curved domains may be poled into the bulk of the
material since the domain inversion propagates preferentially along
the z axis. A concave (or convex) set of domains may therefore be
formed which create a cylindrical lens when excited by a field.
Wedges and more complicated volume structures oriented at an angle
to the surface may be formed by the same process.
In an alternate method, a z-cut crystal can be used as the
substrate if the poling technique causes the domain boundaries to
propagate at an angle to the z-axis. For example, titanium (Ti)
in-diffusion in a z-cut crystal of lithium niobate produces
triangular domains that would be appropriate for reflecting the
beam out of the surface of the crystal. The angle of the domains
formed by in-diffusion with respect to the surface is typically
about 30.degree., so that an input beam incident on the grating
will be reflected out of the surface at an angle of about
60.degree. to the surface of the crystal. The output beam may then
extracted with a prism, or from the rear surface (which may be
polished at an angle) after a total internal reflection from the
top surface.
The electrode structure shown excites both an E.sub.3 component,
and either an E.sub.1 or an E.sub.2 component. A TM polarized input
wave 159 experiences an index change which is a combination of the
extraordinary and the ordinary index changes.
In FIG. 10 there is shown an embodiment of a switchable waveguide
directional coupler. A first waveguide 204 is substantially
parallel to a second waveguide 206, over a certain length. While
the beams propagate adjacent each other and in a similar direction,
their central axes are displaced. The central axes are never
brought coaxial so that the waveguides do not intersect. However,
the waveguide segments are in close proximity in a location defined
by the length of the coupler, so that the transverse profiles of
the optical modes of the two waveguides overlap to a large or small
extent. The propagation of the two modes is then at least
evanescently coupled (which means the exponential tails overlap).
The evanescent portion of the mode field is the exponentially
decaying portion outside the high index region of the waveguide.
The propagation constant associated with a mode of each of the two
waveguides is determined by k=2.pi.n.sub.eff /.lambda. in the
direction of propagation. The effective index n.sub.eff is the
ratio of the speed of light in a vacuum to the group velocity of
propagation, which varies according to the mode in the waveguide.
The value of n.sub.eff is determined by the overlap of the mode
profile with the guided wave structure.
Preferably, the width of the two waveguides, and thus the
propagation constants of the modes in the two waveguides, are
different, so that coupling between the modes is not phasematched
when the grating is off. (The index of refraction profiles of the
two waveguides may also be adjusted to create different propagation
constants.) With the grating off, any input beam 210 in the first
waveguide will continue to propagate in that waveguide to form a
transmitted output beam 214 exiting the first waveguide 204. When
the grating is on, the grating makes up the difference in the
propagation constants of the two waveguides so that coupling
between the two modes is phasematched, and an in-band output beam
212 exits the second waveguide 206. To optimize the coupling, the
grating period A is chosen so that the magnitude of the difference
of the propagation constants in the two waveguides is equal to the
grating constant (within an error tolerance). The propagation
constants of the two waveguides may alternately be chosen to be
equal, so that coupling between the two waveguides occurs when the
grating is off. In this case, turning the grating on reduces the
coupling between the two guides.
The strength of the grating determines a coupling constant, which
defines the level of coupling between the two waveguides. Along the
length of the interaction region of the two waveguides, the power
transfers sinusoidally back and forth between the guides, so that
coupling initially occurs from the first waveguide to the second,
and then back to the first waveguide. The distance between two
locations where the power is maximized in a given waveguide mode is
known as the beat length of the coupled waveguides. The beat length
depends on the strength of the grating.
A first electrode 220 and second electrode 222 are positioned on
the material surface to create an electric field across the grating
region 202 when a voltage is applied between the two electrodes. A
voltage source 226 is connected to the two electrodes with an
electrically conductive material 224. The strength of the grating,
and thus the beat length between the two waveguides, is controlled
by the voltage applied across the grating.
The propagation constants of the two guides are strongly dependent
on wavelength. Since the momentum of the virtual photon is
essentially or dominantly fixed (i.e. determined by parameters
which are not varied in an application), power is transferred to
the second waveguide only in the vicinity of a single frequency
with a frequency bandwidth depending on the length of the coupling
region. Depending on the grating strength, an adjustable portion of
the in-band input beam exits the second waveguide as the coupled
output beam 212, while the out-of-band portion of the input beam
exits the first waveguide as the transmitted output beam 214 along
with the remainder of the in-band beam.
The coupling between the two modes can be controlled
electro-optically by several means, including changing the strength
of the coupling between the modes, increasing the overlap of the
modes, or changing the effective index of one of the waveguides.
Electro-optically controlled coupling, described above, is the
preferable method. In order to couple efficiently between the modes
in the two waveguides, the input beam is forward-scattered, which
requires the smallest grating period.
The coupling grating can alternatively be implemented as a
combination of permanent and switched gratings as described above
in conjunction with FIG. 2. Here we give a detailed example of how
this can be done. After forming the desired periodic domains, the
substrate can be chemically etched to form a relief grating with
exactly the same period as the poled structure. For the preferred
material of lithium niobate, the etch can be accomplished without
any further masking steps, since the different types of domains
etch at different rates. For example, hydrofluoric acid (HF) causes
the -z domains of lithium niobate to etch significantly
(>100.times.) faster than the +z domains. Thus by immersing the
z-cut crystals in a 50% HF solution, the regions consisting of the
first type of domain are etched while the regions consisting of the
second type of domain essentially remain unetched. This procedure
produces a permanent coupling grating which can be used on its own
to produce coupling between the two waveguides. After the
electrodes are applied, the poled grating can be excited to produce
an additive index of refraction grating which is superimposed on
that of the etched substrate. The etch depth may be controlled so
that the effective index change induced by the permanent etched
grating can be partially or wholly compensated by the
electro-optically induced grating when the electrodes are excited
at one polarity, while the index grating is doubled at the other
excitation polarity. A push-pull grating is thereby produced
whereby the grating can be switched between an inactive state and a
strongly active state.
An etched grating is also useful when the etched region is filled
with an electro-optical material, such as a polymer or an optically
transparent liquid crystal, with a high electro-optic coefficient
and an index close to that of the substrate. Preferably, the filled
etched region extends down into the optical beam. When a voltage is
applied across the filled etched region, the index of the filler
material is also varied around that of the rest of the
waveguide.
Alternately, the overlap of the modes in the two waveguides can be
electro-optically modified. For example, the region between the two
waveguides could have its refractive index raised. This reduces the
confinement of the waveguides, and spreads the spatial extent of
the individual modes towards each other, increasing the overlap. To
implement this approach, the region between the two waveguides may
be reverse poled with respect to the polarity of the substrate
traversed by the waveguides. If the electrode extends across both
the waveguides and the intermediate region, an applied voltage will
increase the index of the area between the waveguides while
decreasing the index within the two waveguides. The resulting
reduction in mode confinement thus increaes the overlap and the
coupling between the two modes. Care must be taken not to induce
undesirable reflections or mode coupling loss in the waveguides,
which might occur at the edge of the poled region. These losses can
be minimized, for example, by tapering the geometry of the poled
regions or of the electrodes so that any mode change occurs
adiabatically along the waveguide, minimizing reflections. Art
adiabatic change means a very slow change compared to an
equilibrium maintaining process which occurs at a definite rate. In
this case, it means the change is slow compared to the rate of
energy redistribution which occurs due to diffraction within the
waveguide and which maintains the light in the mode characteristic
to the waveguide.
A third means to change the coupling between the two waveguides is
to change the effective index of one of the waveguides relative to
the other. Thus, the propagation constant of the guide is changed,
which is turn alters the phasematching condition. This effect may
be maximized by poling one of the waveguides so that its
electro-optic coefficient has the opposite sign from that of the
other waveguide. In this case, the coupling grating may be a
permanent or a switched grating. A first electrode covers both
waveguides and the region between them, while a second electrode
may be disposed on both sides of the first electrode. An electric
field applied between the two electrodes causes the propagation
constant of one waveguide to increase, and that of the other
waveguide to decrease, thus maximizing the difference in
propagation constants. The grating coupling process is maximally
efficient only at a particular difference in propagation constants.
By tuning the applied voltage, the phasematching may be adjusted as
desired. This effect can be used to create a wavelength tunable
filter.
The parallel waveguides shown in FIG. 10 may be non-parallel, and
the waveguides may not even be straight. If it is desired, for
instance, to spatially modify the interaction strength between the
waveguides, this end can be accomplished by spatially adjusting the
separation between the guides. These modifications may also, of
course, be applied to the subsequent embodiments of parallel
waveguide couplers described herein.
Referring to FIGS. 12 and 13 there are shown alternate embodiments
of the crossing waveguide coupler for controlling the profile of
the reflected beam. In each embodiment, the area covered by the
grating does not extend entirely across the intersection region of
the two waveguides. The motivation for these grating structures is
best understood with reference to FIG. 11. Depending on how it is
configured, the power coupling structure 282 may distort the
spatial profile of the mode 284 it couples into the output
waveguide. A power coupler which is uniform in space and which
uniformly covers the entire intersection region 280 between two
waveguides disposed at a large angle to each other such as
90.degree. will produce an output beam profile such as assymmetric
profile 286. The power in the input beam decreases as it passes
through the power coupling structure or grating. In the case of a
fight angle intersection, the near field profile of the reflected
beam matches the monotonically decreasing power in the input beam.
The disadvantage with the nonsymmetric profile 286 lies in single
mode structures where only a fraction of the coupled power will
remain in the waveguide. Much of the power will be lost from the
guide.
For single mode devices, a structure is needed which couples power
into the Gaussian-like spatial configuration 288 of the lowest
order mode of the output waveguide. To accomplish this goal, the
region 282 must be extended out into the evanescent tails of the
guided modes, and the net interaction must be modulated, either
geometrically or by spatially adjusting the local strength of the
power coupling grating. FIGS. 12 and 13 show ways to accomplish
this end with geometrical arrangements of gratings. It is also
possible to accomplish this end by spatially modulating the "duty
cycle" of the grating within the power coupling region 282, by
changing the order of the grating in selected regions, and in the
case of electrically controlled coupling, by tapering the strength
of the applied electric fields (by adjusting electrode spacing as
illustrated in FIG. 6, or by adjusting the electrode duty cycle in
the case of grating electrode structures). The duty cycle of a
grating means the fraction of each period which is occupied by a
given domain type; the duty cycle may vary with position.
In FIG. 12, a device 300 with a modified grating structure is
shown, in which the grating area 310 covers part, but not all of
the rectangular intersection region of the two normal guides 316
and 318. With the grating unactivated, the input beam 302 passes
through guide 316 undeflected to exit as output beam 308. The
dimensions of the intersection region match the widths 304 and 305
of the two waveguides. The presence of a small region of power
coupling structure at any point in the intersection region will
result in local coupling between a given transverse segment of the
beam profile in an input waveguide into a given transverse segment
of the beam profile in an output waveguide. The reflected beam
profile is constructed from the propagated sum of these
phased-coupled contributions. The grating region 310 depicted is
triangular in shape, with the points of the triangle 311,312, and
313. The shape of the grating region can be modified from the
triangular, and the local grating strength can be modulated. The
exact shape of the grating region which optimizes single mode
coupling characteristic between the waveguides can be calculated
with an established waveguide propagation technique, such as the
beam propagation method.
A further embodiment of a single-mode coupling grating device 340
is shown in FIG. 13. The grating region 350 is a double convex
shape, with one point at comer 351 common with waveguides 346 and
348 and beams 330 and 342, and the other point on opposite comer
352, common with both waveguides and beams 342 and 332. This
structure has the advantage of reflecting most of the power in the
middle of the beam, where the optical intensity is the highest, and
thus better couples the power between the lowest order modes in the
two waveguides 346 and 348. The optimal shape of the grating region
again depends on the coupling constant of the grating.
Referring to FIGS. 12 and 13, a first electrode 320 is disposed on
the same surface of the substrate as the waveguide, over the
grating region, and a second electrode 322 is disposed on the same
surface around the first electrode. The distance between the two
electrodes may be constant as illustrated in FIG. 13, or it may be
tapered as illustrated in one dimension in FIG. 12. A voltage
control source 324 is connected with two wires 326 to the two
electrodes. An electric field can thus be applied through the
grating region to activate one of the electro-optic coefficients
and change the coupling between the input beam and the output
beam.
For purposes of illustration, FIG. 12 also shows a tapered input
waveguide segment 287 and a tapered output segment 289. An input
beam 285 expands adiabatically through the tapered segment 287 to
increase the intersection area and thereby increase the total
reflection from grating 310. The grating is capable of reflecting
the now-expanded beam 285 toward the output beam 308. If desired,
the output waveguide may also contain a tapered segment 289 to
reduce the width of the output beam. (Alternatively, the output
beam may be kept wide if desired for later beam switching
interactions.)
The grating may extend beyond the intersection region of the two
waveguides. A grating extended along the input waveguide enables
residual transmitted light after the intersection region to be
removed from the waveguide, typically into radiation modes. The
extended grating minimizes crosstalk between optical channels in
switching arrays, in which an individual waveguide may have more
than one signal channel propagating along its length.
Specifically contemplated by the invention is a means for tuning
the grating. Several embodiments in which tuning is achieved are
shown in FIGS. 14-17. Referring to FIG. 14, there is a bulk optical
device 400 in which the strength and center wavelength of a normal
incidence reflection grating are controlled by a single voltage
source 426. This device consists of a patterned poled grating
region 410, which is electro-optically activated by two electrodes
420 and 422 on opposing surfaces of the material and connected to
426 by conductors 424. The strength and the center frequency of the
grating are tuned simultaneously by applying a single voltage
between the two electrodes of the device. The average refractive
index of the grating changes with the applied electric field,
causing a change in the center wavelength of the grating that is
proportional to the electric field. The average index is calculated
over a single period of the grating in a periodic grating, by
summing the weighted index changes in the various types of domains.
The weighting factor is the physical length 416 and 418 of each
domain type, along the optical path of the input beam 404. The
condition for frequency tuning is that the weighted sum must not
equal zero so that the average index changes as a result of the
electric field.
The product of the index of refraction and the physical distance
traversed by an optical beam is known as the optical distance. (The
index of refraction is replaced by the effective index of
refraction for waveguide devices.) A 50% duty cycle is obtained in
a grating with two types of domain if the average optical distance
across the two types of domains is substantially equal
(approximately equal within the error range determined by the needs
of the application). The average is taken over many subsequent
domains to allow for the possibility of a chirped, nonperiodic, or
other more general type of grating. In general the domains may have
different indices of refraction as well as different electro-optic
coefficients. The general condition for tuning is expressed in
terms of the physical distance travelled in the different types of
domains. For each domain, the total optical phase advance is given
by the optical distance travelled (times 2.pi./.lambda.). However,
the change in the phase advance is given by the product of the
applied electric field, the appropriate electro-optic coefficient,
and the physical distance (times 2.pi./.lambda.). The average
change in index of refraction experienced by the wave is equal to
sum of the changes in phase advance in all domains traversed by the
optical wave within a section of the material of length 1 (times
.lambda./2.pi.1). This change in average index determines the
change in the peak interaction wavelength according to
.delta..lambda./.lambda.=.delta.n/n. The grating strength is
changed simultaneously with the wavelength in this structure, but
such simultaneous change may be undesirable. The structure may be
designed so that the operating point about which timing is
accomplished maintains a sufficiently high grating strength for the
application across the entire wavelength tuning range. Or, a
separate tuning structure maybe used as is described below in
reference to FIGS. 16 and 17.
The change in the average refractive index can be achieved by many
different means. One alternative is that of randomly
non-electro-optically active domains 414 alternating with
electro-optically active domains 412. The electro-optically active
regions are poled domains, while the non-electro-optically active
domains may be randomly poled or unpoled or radiation-disabled.
Thus, the electric field causes an average increase in the index
.DELTA.n.sub.avg across the grating. In the poled-random
configuration of FIG. 14, .DELTA.n.sub.avg is equal to the product
of the index change in the active domains 412 times the duty cycle.
The duty cycle is equal to the length 418 divided by the sum of the
lengths 418 and 416. The tunability that can be achieved using this
technique is .lambda..delta.n.sub.avg /n in a poled-random
structure, where .lambda. is the optical wavelength, and n is the
original (effective) index of the material. Assuming a wavelength
of 1.55 .mu.m and a 10 V/.mu.m electric field in lithium niobate,
the tuning range for a 50% duty cycle structure is 1.1 nm.
When the input beam 404 is within the bandwidth of the grating, the
grating couples the beam into a retroreflecting output beam 402;
otherwise the input beam forms a transmitted output beam 406.
Contrast this behavior with that of a 50% duty cycle grating where
the two domain types have the same electro-optic coefficients but
opposite polarity, as in the case of domain inversion. In this
latter case, there is no change in the average index of refraction
since the change in index of the first domain type cancels with the
change in index of the other domain type. A 50% duty cycle domain
reversal grating does not tone its center frequency.
An alternate means to achieve an average effective index change in
domain reversed gratings is to use a non-50% duty cycle for the
poled domain area with unequal lengths 416.noteq.418. The
tunability that can be obtained using this technique is
(2D-1).DELTA.n.lambda./n, where D is the duty cycle of the largest
domain type (D>0.5). For example, with a 75% duty cycle, a
wavelength .lambda. of 1.55 .mu.m, and a 10 V/.mu.m electric field
in lithium niobate, the tuning range is 0.54 nm. The domain
reversed grating is also stronger than a grating in which the
second domain type is not electro-optically active.
In FIG. 15, a waveguide device 440 using the same average index
effect is shown. In this case, the average effective index of the
waveguide 442 in the rating region 450 changes with the applied
electric field, causing a change in the center wavelength of the
grating. A voltage control source 466 is used to apply an electric
field between a first electrode 460 and second electrode 462, which
are preferably placed on the same surface of the material. The
average effective index can be achieved by a variety of geometries,
including non-electro-optically active domains or a domain reversal
grating with a non-50% duty cycle. When the input beam 445 is
within the bandwidth of the grating, the grating couples the beam
into a retroreflecting output beam 444; otherwise the input beam
forms a transmitted output beam 446.
A means to enhance the tunability of a grating in a waveguide
device 480 is to overlay a second electro-optic material 482 on the
waveguide to form a cladding, as shown in FIG. 16. The cladding
should be transparent to the wave propagating in the waveguide and
it should be electric field-sensitive to enable adjustable
modification of its index of refraction. The average effective
index is determined partly by the index of refraction of the
cladding. The second material may have a higher electro-optic
coefficient than the substrate. Liquid crystals and polymers are
good examples of materials which can be used as cladding. The index
of the cladding is preferably close to that of the guiding region
so that a large portion of the guided beam propagates in the
cladding.
For this embodiment, a first electrode 502 is surrounded by a
second electrode 504 on the substrate, for applying an electric
field across the poled grating 490. Preferably, the electrodes are
placed below the cladding, directly on the substrate. If the first
electrode 502 is positioned directly above the waveguide 484 as
shown in FIG. 16, it must be made of an optically transparent
material. The electrodes may also be disposed to either side of the
waveguide 484, in which case they need not be transparent. A third
electrode 506 is positioned on top of the cladding, above the
waveguide and the first electrode. For this embodiment, the center
wavelength and strength of the grating are separately controllable.
The grating strength is controlled by a first voltage source 510,
connected by two wires 513, 514 to the first and second electrodes,
while the center wavelength of the grating is controlled by a
second voltage source 512, connected between the first and third
electrodes with two wires 514 and 515. In an alternate electrode
configuration, only two electrodes are used, both of which are
preferably positioned on top of the cladding material so that their
induced field penetrates both the cladding material above the
grating, and the grating structure itself. A single voltage source
then controls both the center wavelength and the grating strength,
but not independently.
The amount of tunability that can be achieved with an
electro-optically active cladding depends on what portion of the
guided beam propagates in the cladding. If the two indices are
relatively close so that 10% of the beam propagates in the
cladding, then the average change in the effective index of the
guided mode is equal to 10% of the change in index of the cladding.
For a cladding index change of 0.1, the tunability is on the order
of 7 nm.
FIG. 17 shows an embodiment of a discretely tunable grating device
520, which consists of several individually controllable gratings
530, 532, 534. The gratings in series with all gratings in the path
of the input beam 522, and forward 523 and reflected 524 beam. Each
individual grating in the structure may also be continuously
tunable over a small range. Each grating in FIG. 17 has a first
electrode 542 and a second electrode 544, which are connected to a
voltage controlling network 552 with wires. The gratings can be
switched on one at a time, so that only one wavelength in a small
passband will be reflected at a time, or multiple gratings can be
switched on simultaneously to create a programmable optical filter,
with a center wavelength and bandwidth which are separately
controlled. The gratings themselves may be implemented with the
variations described above, including the possibility of multiple
periods in each grating.
The structure can be realized either in the bulk or as a waveguide
device. In the latter case, an optical waveguide 528 is fabricated
on the substrate so that the waveguide intersects the poled
gratings. The poled domains 536 may extend only through the
waveguide and do not necessarily extend all the way through the
material. Both electrodes are preferably (for higher field
strength) deposited on the same face of the substrate as the
waveguide. The second electrodes of all the gratings may be
connected as shown to minimize the number of electrical
connections.
Alternately, the individually-addressable grating structure can be
a bulk device, in which case the waveguide 528 is omitted, and the
poled regions 530, 532 and 534 are optimally fabricated with
sufficient depth to overlap with the propagating optical mode. The
two electrodes for controlling each grating are then optimally
positioned on opposing faces of the material to optimize the field
penetration, as shown for example in FIG. 2 for a single grating.
Cross excitation between adjacent gratings caused by fringing of
the electric fields between the electrodes can be minimized by
separating the grating-electrode groups by an amount comparable to
the substrate thickness, or by adding interspersed fixed-potential
electrodes.
An alternate means for tuning the grating is to vary the
temperature of the active material. The tuning occurs because of
two effects: thermal expansion and the thermo-optic effect. For
different materials, either one of these two effects may dominate
thermally induced tuning. In lithium niobate, the larger effect is
thermal expansion, for which the largest (a-axis) expansion
coefficient .DELTA.L/L is +14.times.10.sup.-60 C..sup.-1, while the
thermo-optic coefficient for the ordinary axis .DELTA.n.sub.0 /n is
+5.6.times.10.sup.-60 C.sup.-1. For a temperature range of
100.degree. C., the combination of these two effects gives a total
wavelength tuning range of 2.6 nm.
For many purposes, it is desirable to create poled gratings with a
generalized frequency content. Multiple interaction peaks may be
desired for example, or simply a broadened bandwidth of
interaction. To accomplish this end, some way is needed to
determine the pattern of poled region boundaries which corresponds
to a given mathematical function containing the desired
frequencies. FIG. 18 illustrates the results of the process in the
case of a single frequency containing arbitrary phase shifts.
Referring now to FIG. 18, optical phase shifts 564 and 565 can be
incorporated at one or more positions along a sinusoidal function
560 to modify its wavelength structure. The mean level of the
function is given by the straight line 561. Also shown is the
corresponding squared wave function 562 with identical phase
shifts, as can be achieved by a typical poling process. To achieve
the translation of the continuous function into the square wave
function, the regions 570 where the curve 560 exceed the average
561 sine wave corresponds to one type of domain, while the regions
572 where the curve 560 falls below 561 corresponds to a second
type of domain. The Fourier transform of the square wave curve 562
will have the same frequency components as the transform of the
sinusoidal function 560 in the low frequency range below the
harmonics of the sine wave frequency. This approach works for any
type of generalized frequency distribution as long as the bandwidth
does not exceed a small fraction of the carrier frequency.
A phase shifted grating may be implemented in any of the devices
described herein such as in FIG. 2 for example, where the location
of the domain walls 34 in the grating 22 can be determined by the
pattern 562 of FIG. 18 rather than a periodic function. The phase
shifted pattern can be controlled with a poling mask incorporating
the desired pattern.
Arbitrary multiple period gratings can be specified using a similar
technique. Each period present in the grating is represented in a
Fourier series (or integral) by a corresponding sign wave of the
desired amplitude. All waves are added together to form a resultant
wave. The positive portion of the resultant wave corresponds to one
type of domain, while the negative portion corresponds to the
second type of domain. The number of superimposed gratings can in
principle be scaled up to any number, limited in practice by the
minimum attainable feature size.
FIG. 19 shows an alternate way of fabricating a superimposed
multiple-period grating device 580. A two grating waveguide
structure is depicted, with a switchable single period poled
grating 582, and a permanent relief grating 584 interacting with a
single beam in a waveguide. A coating 588 is shown deposited on top
of the relief grating to reduce the loss which occurs when the
evanescent tail of the guided wave mode overlaps with the metallic
electrode. This coating is an important design optimization element
for all of the elements described herein, and should be applied
between each electrode structure and adjacent optical waveguides. A
coating is also useful above the electrodes in all of the elements
described herein to reduce the probability of break-down.
The electrically controllable gratings in the superperiod structure
are switched by a single pair of electrodes 602 and 604, connected
by wires 606 to a voltage control source 608. The first electrode
602 is preferably centered over the waveguide, while the second
electrode 604 runs parallel to the first, on either side of the
waveguide. The device depicted is a waveguide device, with a
waveguide 586 confining the input beam 590, as well as the
transmitted output beam 592 and the reflected output beam 594.
The multiple period grating structure can be configured in many
ways. For example multiple independent peaks in the frequency
spectrum can be useful as a multiple frequency feedback mirror. Two
operations (e.g.., phasematching and reflection) can be achieved in
a single grating which incorporates the proper two periods for
enabling the processes. As a final example, the grating can be
fabricated with the phase and amplitude of its components adjusted
for equal effect on the two polarization modes, making a
polarization insensitive component.
Another useful modification of a periodic structure is a chirped
period. Along the length of the grating structure, the period can
be gradually increased or decreased, so that the center wavelength
varies from one end of the grating to the other. Thus, the
wavelength bandwidth of the grating is broadened over that of a
constant period grating. The chirping across the grating is not
necessarily linear: many different wavelength reflection profiles
in frequency space (e.g.., square wave, Lorentzian) can be
achieved, depending on the variation in the chirp rate. As
described above, the duty cycle and/or the strength of the exciting
electric field can also be spatially adjusted to modify the
strength of different portions of the chirped grated. The duty
cycle of the grating can be controlled by the mask as desired. The
electric field strength can be controlled by adjusting the
separation of the electrodes as shown for instance in FIG. 6.
A wide spectrum tunable device can be realized in a structure
containing two separate gratings which have a multiple peak
structure, as shown in FIGS. 21 and 22. FIG. 20 demonstrates the
basic principle of these devices and depicts the multipeak comb
transmission (or reflection) profiles 620 and 622 as a function of
the optical frequency for two such gratings. The first grating
profile 620 has transmission peaks separated by a first period 626,
while the second grating profile 622 has peaks separated by a
second period 624 that is slightly different from the first. The
key idea is for the device to operate only at a frequency
determined by the overlap of peaks from both curves (frequency
v.sub.1). Tuning is achieved by tuning the comb of transmission
peaks of the gratings with respect to each other. Different
transmission peaks in the two combs will overlap each other in
various ranges of the relative frequency shift, so that the net
transmission of the combined gratings jumps discretely over a much
wider wavelength range than can be achieved with only thermal or
electro-optic tuning. In the example of FIG. 20 where the peak
separations differ by 10%, if the frequency of the first grating is
increased by 10% of the frequency separation 626, the next higher
frequency peaks will superimpose, resulting in an effective
frequency shift ten times larger than the tuning amount.
In FIG. 21, a guided wave embodiment of the device is shown, in
which two gratings 650 and 652 are placed over a single waveguide
642. An input beam 644 is partially reflected into beam 643 and
transmitted as beam 645. A first electrode 666 and second electrode
668 are positioned around the first grating 650 so that a first
voltage source 6652 connected to the electrodes activates that
grating. A third electrode 664 is positioned, along with the second
electrode, around the second grating 652. The second grating is
controlled by a second voltage source 660 connected to the second
and third electrodes. In the preferred embodiment, each grating is
a multiple peak structure as described in FIG. 20, and the device
forms a frequency-hop-tuned reflector. According to the curves of
FIG. 20, the gratings are configured as broadband reflectors,
reflecting essentially all the incident radiation frequencies
except a comb of equally spaced frequencies where the transmission
is high. The cascaded gratings will therefore reflect all
frequencies in the frequency range illustrated in FIG. 20, except
where the two transmission peaks overlap at v.sub.1. Provided that
the reflections of the two gratings are arranged to add in phase in
the reflected beam 643, the transmitted spectrum will be
essentially equal to the product of the two transmission curves 620
and 622. When the center frequency of one of the gratings is tuned,
the single transmission peak at v.sub.1 will hop to the next
adjacent peak, and then the next, and so on. Such a structure is
particularly useful as an electrically tuned receiver in, for
example, a wavelength-division-multiplexed (WDM) communication
system. The receiver can be configured to detect only incoming
light in a specific band, while being insensitive to light at other
frequencies.
As seen above, a grating structure can be shifted by about 0.5 nm,
assuming a 10 V/.mu.m field in a domain inverted grating with duty
cycle of 75%. This continuous tuning range can be used to produce
discontinuous tuning in the structure 640 across perhaps 100 bands
in a 50 nm range, if the width of the individual frequency peaks
628 are narrower than about 1/100+L th of the frequency
separation.
Note that if the frequency of the input light is known to lie only
within the transmission bands of curve 620 in FIG. 20, for example,
the device can be realized with only a single grating structure
with the transmission spectrum of curve 622, using essentially the
Moire effect. By tuning the center frequency of the spectrum 622,
any one of the desired bands can be selected while reflecting the
rest. The tee structure of FIG. 7 is then particularly interesting
in this context: the input beam 112 containing multiple frequency
components is then split by the grating structure 100 (configured
for tuning as described herein) into a single transmitted beam 116
which can be detected or otherwise processed, and a reflected beam
114 which contains all the other frequency components. The power
contained in beam 114 is not lost, but can be routed to other nodes
in a communications network, for example.
Other variations can be formed of this basic structure, wherein,
for example, the spectra of FIG. 20 are the reflection curves of
the individual gratings instead of the transmission curves. In this
case, the structure acts as an etalon when the frequencies of the
reflection peaks align with each other, with reflectivity according
to the relative phase of the reflected waves. Otherwise, the net
reflection of the compound structure is essentially the sum of the
reflection curves of the two individual structures.
It maybe important to optimize the relative phase of the two
reflections by adjusting the optical path length 653 between the
two gratings. The relative phase can be controlled by using an
electro-optic structure (as shown for example in FIG. 22) between
the two grating entrances 654 and 655 to adjust the optical path
length 653. For a lithium niobate crystal and an input wavelength
of 1.5 .mu.m, an activated distance between the gratings of at
least 250 .mu.m is required to adjust the relative phase between
the two beam of up to .+-..pi., (using a z-axis applied field of 10
V/.mu.m). The strength of one of the gratings (but not its
frequency) may optionally be controlled via a field applied at its
electrode if the grating is not designed for tuning (its average
index of refraction is configured to be independent of the applied
voltage). If both gratings are tuned together, narrow range
continuous tuning results. As an alternative or supplement to
electronic excitation, the phase of the two reflections and the
peak wavelengths of the gratings can all be varied together through
thermal or mechanical control of the chip.
FIG. 22 shows schematically two grating reflectors 633 and 634
separated by a phase shifter section 635 and forming an integrated
etalon 640 having a characteristic free spectral range (FSR). (The
structure 630 is essentially the same as that of FIG. 21, with the
addition of the phase shifter section, which consists of electrodes
capable of actuating a region of electro-optic material traversed
by the waveguide 636.) For simplicity, we consider the case of
uniform single-period gratings, but the individual gratings may
generally be more complex structures. The gratings may be fixed or
electronically actuatable. The reflections off the two gratings can
be made to add in phase for a beam at a reference frequency by
adjusting the voltage applied to the phase shifter section 635. A
beam at a second frequency will also add in phase if the
frequencies of the two beams are separated by a multiple of the
FSR. Since the FSR is inversely proportional to the optical path
length between the two gratings, choice of the path length
determines the density of the reflection peak structure of the
etalon device. As an example, two short high reflecting gratings
separated by 200 .mu.m in lithium niobate can have grating
reflection peaks separated by a multiple of 1 nm. The multiple peak
structures 620 or 622 described in FIG. 20 can each by implemented
as an integrated etalon.
A dual grating wye junction embodiment is shown in FIG 23, in which
the two gratings 690 and 692 extend across two separate waveguides
682 and 684. In general a wye junction has an input and multiple
output waveguides which may lie in a plane or in a volume. The two
waveguides are connected to the first waveguide 686 with a wye
junction 688. The power in the optical input beam 691 is split
between the second waveguide 682 and third waveguide 684 so that
approximately 50% of the input beam 691 is incident on each of the
gratings. The two gratings may have a simple reflection structure,
or they may have a series of high reflection peaks. The gratings
may be permanent, or they may be electronically adjustable, in
which case electrodes 694 and 696 are provided for exciting the
gratings. A common electrode 698 is then provided across the wafer
(or alternately on the same surface as the waveguides, adjacent to
the other electrodes similarly to FIG. 21).
The relative optical path length of the two branches of the
waveguide can be adjusted by the electrode 689 which is disposed on
one waveguide over a region of electro-optic activity. By adjusting
the voltage on the phase adjusting electrode 689, the two
reverse-propagating reflected beams may be adjusted to have the
same phase when they meet at the wye junction. The reflected modes
superpose and form a wave front profile which may have a phase
discontinuity in the center, depending on the relative phase of the
two waves. As the combined wave propagated, the spatial
concentration of the optical mode in the region of the guide is
strongly affected by the phase shift. If they have the same phase,
the profile forms a symmetric mode which couples efficiently into
the lowest order mode of the input waveguide to form the
retroreflected output beam 693. Two reflected beams which add
out-of-phase at the wye junction will have very low coupling into
any symmetric mode (such as the lowest order mode) of waveguide
686. If the waveguide 686 is single mode, this reflected energy
will be rejected from the waveguide. Thus, by adjusting the optical
path length of one of the arms of the wye with the electrode 689,
the reflection can be rapidly adjusted from almost 100% to a value
very close to zero. Furthermore, if the gratings are implemented as
electronically tunable reflectors in one of the tunable
configurations described herein, the modulated reflection property
can be shifted into different regions of the spectrum.
Referring to FIG. 24, there is shown a switchable waveguide mode
converter 720 using a poled grating 722. The waveguide 730
preferably supports both an input mode and an output mode, which
may be two transverse modes or two modes of polarization (e.g. TE
and TM). The two modes in the waveguide typically have different
propagation constants, which are determined by the effective
indices of the modes. The grating 722 is excited electrically by
electrodes 740 and 742, coupled to the source of electrical
potential 744 by the connections 746. The grating period .LAMBDA.
(724) is chosen so that the magnitude of the difference of the
propagation constants in the two waveguides is equal to the grating
constant 2.pi.n/.LAMBDA.. When the grating is on, the grating makes
up the difference in the propagation constants of the two
waveguides so that coupling between the two modes is phasematched.
The grating strength and the device interaction length in the
grating should be set to optimize the flow of power from the input
mode into the output mode. The net rate of power conversion from
one mode into the other is determined by the strength of the
electro-optic coefficient (r.sub.51 in lithium niobate) and by the
strength of the electric field.
For two transverse modes, the coupling depends on the spatial
overlap of the two modes in the presence of the grating structure,
and on the strength of the grating. The two modes may be orthogonal
by symmetry, so that even if the modes are phasematched, there will
be no conversion in a symmetric structure. In this case, the
phasematching structure itself can be made asymmetric to eliminate
the problem. In the preferred embodiment of FIG. 24, the asymmetry
can be introduced via the electric fields which excite the poled
structure. The vertical component of the electric field reverses
sign midway between the two electrodes 740 and 742. It is best to
center the electrodes on the waveguide to optimize mode conversion
between transverse modes of different symmetry. The reverse is true
when coupling transverse modes of the same symmetry: now the
phasematching structure should be made symmetric to optimize the
conversion. Several alternative approaches can also be used. A
three electrode structure has a symmetric vertical component of the
electric field and an asymmetric horizontal field. The horizontal
field can be used in conjunction with one of the
horizontally-coupled electro-optic coefficients to couple modes of
differnt symmetry. Or, the poled structure may have a phase
reversal plane that essentially bisects the waveguide, in which
case a symmetric component of the electric field can be used to
couple modes of differnt symmetry (vertical field in the case of
three electrodes, horizontal field in the case of two).
Since the propagation constants of the two modes are strongly
dependent on wavelength, the beat length of their interaction also
depends on the wavelength. Thus, for a given length of the coupling
region between the two modes, the power coupled into the second
mode is frequency-sensitive. The coupling has a frequency bandwidth
associated with it. For a given grating strength, a portion of the
in-band input beam is coupled into the output mode which exits as
the coupled output beam, while the remainder of the input beam
exits the first waveguide as the transmitted output beam.
The structure shown in FIG. 24 can also be used to couple between
TE and TM polarized modes. The electro-optic coefficient r.sub.51
enables coupling between the two orthogonal polarizations in a
lithium niobate crystal, for example. As before, the period of the
grating is chosen so that the grating constant is equal to the
difference in propagation constants between the two modes. The
interaction length is chosen to optimize the power transfer.
A waveguide, such as a titanium-indiffused waveguide which supports
both TE and TM modes, is used in applications where both
polarizations can enter or leave the converter. A waveguide such as
a proton exchanged waveguide which supports only one polarization
(TM in z-cut lithium niobate substrates or TE in x- or y-cut) can
be useful in applications where only a single polarization is
desired. Such a one-polarization waveguide can act as a very
effective filter for the other polarization. The wrong polarization
component will rapidly disperse away from the waveguide due to
diffraction, leaving only the guided polarization in the waveguide.
For example, the proton exchanged output waveguide 731 may act to
guide only the input polarization or only the output polarization,
as desired. This device can be used as an optical modulator with
excellent transmission and extinction if the grating coupling is
strong, and the interaction length and electric field are selected
correctly. A modulator configured with a proton exchanged waveguide
will transmit essentially all of the correctly polarized input
light, and produce very low transmission of light which is coupled
into the perpendicular polarized mode. Alternately, the input
waveguide may be titanium-indiffused to accept either polarization
at the input. The index profiles that form the waveguides for the
two beams are preferably similar so that the profiles of the TE and
TM modes overlap well, and the coupling efficiency is
maximized.
To activate the r.sub.51 coefficient, an electric field is applied
along the Y or the X axis of the crystal. The electrode
configuration that will achieve the appropriate field direction
depends on the cut of the crystal. For a z-cut crystal with a
waveguide oriented along the x axis, the first electrode and second
electrode can be placed on either side of the waveguide.
Alternately, for a y-cut crystal with a waveguide oriented along
the x axis, the first electrode can be placed directly over the
waveguide, with the second electrode on either side of the
waveguide, parallel to the first electrode.
Since the poled domains in the grating 722 can be made to extend
through a bulk substrate (such as 0.5 mm thick or more), the
structure of FIG. 24 is also useful for a controllable bulk
polarization converter. In this case the waveguide 730 is
unnecessary, and the electrodes are optimally configured on either
side of a thin bulk slab of poled material.
Referring to FIG. 25, there is shown a switched beam director 700
incorporating a wye power splitter 702 and a transverse mode
converter 704. The mode converter works in a similar way to the
transverse mode converter described above in relation to FIG. 24.
The grating structure 706 phase matches energy conversion from the
lowest order (symmetric) mode incident in waveguide 708 into the
next higher order (antisymmetric) mode of the waveguide. The length
and strength of the interaction region where the waveguide and the
grating structure overlap are chosen to convert approximately half
of the input single symmetric mode power into a higher order
antisymmetric mode. Furthermore, the optical path length between
the grating mode converter section 704 and the wye splitter 702 is
chosen so that the phase of the two modes adds constructively at
one of the branches 712 of the wye and destructively at the other
branch 713. The result is that the power is routed primarily into
the waveguide 712 with the constructive interference, with very
little power leakage into the other waveguide 713. In this
condition, any reverse propagating power in the guide 713 is
essentially excluded from coupling into a reverse propagating mode
in the guide 708 after the mode coupler 704. The device forms an
efficient power router in the forward direction and an isolating
structure in the reverse direction.
By adjusting the optical path length between the grating mode
converter section 704 and the wye splitter 702, it is possible to
switch the output power from guide 712 to guide 713. This is done
by adjusting the relative optical path length for the lowest order
mode and the higher order mode so that the two modes slip phase by
.pi. relative to each other, now producing constructive
interference in the guide 713 and destructive interference in the
guide 712. The relative path length adjustment can be achieved in
the path length adjustment section 705 by exciting the electrode
pair 711 and 709 with the voltage source 714, changing the index of
refraction under the electrode 711 via the electro-optic effect in
the substrate 703, which is preferably lithium niobate (but may be
any electro-optic material with transparency for the waves such as
lithium tantalate, KTP, GaAs, InP, AgGaS.sub.2, crystalline quartz,
etc.). The propagation distance of the waveguide 708 under the
electrode 711 is selected, along with the excitation voltage, to
enable changing the relative phase of the two modes by at least the
desired amount.
The grating 706 may be a permanent grating fabricated by any of the
techniques known in the art. However, to optimize the functioning
of the device, it is desirable to have almost exactly equal power
in the symmetric and the antisymmetric modes. It is difficult to
achieve sufficient control in existing fabrication techniques to
achieve this goal, and it is therefore desirable to have some
adjustment in the grating strength. This adjustability can be
achieved with the use of at least some poled grating sections,
excited by the electrodes 709 and 710, which are driven by the
power supply 715, and which can be used by themselves to accomplish
the desired mode conversion, or to adjust the strength of a
combined poled-permanent grating.
The input waveguide 708 is best implemented as a single mode
waveguide incorporating a (preferably adiabatic) taper 701 to
permit guiding of the two modes between the transverse mode coupler
704 and the wye splitter 702. The waveguides 712 and 713 are both
preferably single mode. While any order modes may be used in the
device as long as their symmetry is opposite, it is most desirable
for interconnection purposes to work with the lowest order mode at
the input and output legs. The intermediate excited mode is less
critical, and could be, for instance, a higher order antisymmetric
mode.
FIG. 26 shows a parallel waveguide switchable resonator 750 in
which an input waveguide 752 is coupled to a parallel waveguide 754
along an interaction region 753. Grating reflectors 755 and 756 are
disposed across the waveguide 754 in such a way as to retroreflect
light propagating in the guide. The pair of separated reflectors
and the waveguide 754 form an integrated etalon coupled to the
input waveguide 752. The length of the coupling region 753 and the
separation of the parallel waveguides in the coupling region are
chosen so that a certain desired fraction T of the input beam 757
is coupled into the waveguide 754. The light coupled into the
etalon structure 754, 755, and 756 resonates between the reflectors
755 and 756, and couples out into two principal output channels:
the forward propagating wave 759 and the reverse propagating wave
758 in waveguide 752. The same fraction T of the power circulating
in the etalon couples into each of the two output channels 758 and
759.
As for any etalon, the integrated etalon has a frequency acceptance
structure comprised of multiple peaks in frequency space with width
dependent on the loss of the resonator, and separation equal to the
free spectral range. If the optical frequency of the input beam 757
matches one of these resonant frequencies, the power circulating in
the etalon will build up to a value P.sub.circ determined by
P.sub.circ =P.sub.inc T/(T+.GAMMA./2).sup.2 where P.sub.inc is the
incident power 757 in the waveguide 752, .GAMMA. is the loss of the
etalon not including the output coupling into the forward
propagating wave 759 and the reverse propagating wave 758 in
waveguide 752, and we have assumed weak coupling and low loss. The
output coupled wave from the etalon which propagates in the reverse
direction in waveguide 752 forms the reflected wave 758. The
reflected power in beam 758 in equal to P.sub.ref =P.sub.inc
/(1+.GAMMA./2T).sup.2 on the peak of the resonance. When
T>.GAMMA./2, essentially all of the incident power is reflected.
The output coupled wave from the etalon which propagates in the
forward direction in waveguide 752 is out of phase (on a cavity
resonance) with the uncoupled portion of the input wave 757, and
the two beams destructively interfere, producing a low amplitude
output beam 759. Because the two beams have unequal amplitude, the
residual power P.sub.trans =P.sub.inc /(1+2T/.GAMMA.).sup.2 in the
output beam 759 in not quite zero, but it can be very close. If the
coupling T is made very large compared to the loss F of the etalon,
the transmission of the device is greatly suppressed (by 26 dB if
T=10.GAMMA.). This structure then acts as a very low loss reflector
at a comb of frequencies separated by the FSR.
The device can be switched by changing the optical path length
between the two reflectors 755 and 756. Electrodes 761 and 762 are
disposed to produce an electric field through the waveguide 754
between the mirrors 755 and 756. The electrodes are excited with a
voltage source 763, changing the effective index of the substrate
under the electrode 761 via the electro-optic effect, thereby
changing the optical path length between the mirrors and shifting
the resonances of the integrated etalon. If the resonances are
shifted by more than either the width of the resonances or the
frequency bandwidth of the incident beam, the reflection will drop
to zero, and the transmission will rise to essentially 100% as the
circulating power within the etalon is suppressed approximately
P.sub.inc T/4.
The gratings 755 and 756 may be permanent gratings, or they may be
poled gratings excited by electrodes as shown in previous diagrams
and discussed above. If the grating 756 is a poled grating, the
device may also be switched by switching it off. With grating 756
off, i.e. not reflecting, the loss to the incident wave 757 is
equal to the coupling constant T, but now the comb structure is
eliminated instead of just being frequency shifted as by the
electrode 761. The difference in switching function between these
two modes of operation may be significant with for example a
broadband input signal where it is necessary to switch off the
reflection rather than just change its frequency. For a single
frequency input beam, the reflection can be switched equally well
by changing the path length with electrode 761 or by spoiling the Q
of the resonator by switching off the mirror 756. However, if the
reflectivity of the mirror 756 is retained and only the frequency
spectrum of the etalon is shifted with the electrode 761, other
frequency components of a broadband input wave would be reflected,
and this might be highly undesirable in some applications.
The power P.sub.circ which builds up in the etalon can be quite
large if T and .GAMMA. are small, and can be useful in applications
such as second harmonic generation, for example. In this
application, a quasi-phasematched (QPM) periodic poled structure in
a section of the lithium niobate substrate is incorporated into the
resonator between, say, the mirror 756 and the interaction region
753, or possibly within the interaction region itself. One of the
resonant frequencies of the etalon is then tuned to coincide with
the phasematching frequency for the QPM frequency doubler. The
power buildup which occurs enhances the frequency conversion
efficiency of the device as the square of the buildup factor
P.sub.circ /P.sub.inc. The high reflection which occurs at this
frequency can also be used to injection lock the pump laser to the
desired frequency if the FSR is large enough that the other
resonant modes are not injection locked simultaneously. The linear
integrated etalon geometry described above in reference to FIGS. 21
and 22 can also be used to accomplish the same purposes.
To optimize the power building up in the etalon between the
reflectors 755 and 756, the losses in the resonator must be
minimized. The coupling of FIG. 26 cannot be "impedance matched",
in analogy to the process known in the art of bulk buildup
cavities, where the input coupling into the resonator is adjusted
to cancel by destructive interference the portion of the incident
beam which is not coupled into the cavity. This is the condition of
the etalon transmission interference peak. As described above, what
happens in the integrated structure is that the transmitted beam
can be nearly cancelled while the power builds up in the coupled
resonator, but a strong reflected wave emerges. The reflected wave
may be eliminated in a ring waveguide structure, as is illustrated
in FIGS. 27 and 28.
An output 751 proportional to the power circulating within the
etalon may be taken through the grating 756, if desired, or
alternately through the grating 755.
In FIG. 27, a three-arm etalon 760 is shown with an input waveguide
752, a parallel waveguide coupling region 753, a ring resonator
formed by three waveguide segments 764, 765, and 766, three grating
reflectors 767, 768, and 769. The optical path length adjustment
section formed between the electrodes 761 and 762 is optional. The
grating reflector 767 is disposed to optimally reflect the power
arriving from waveguide 764 into the waveguide 765. In a single
mode system, the spatial configuration of the grating (and its
electrodes if any) is designed to couple from the lowest order mode
of waveguide 764 into the lowest order mode of 765. The gratings
768 and 769 are similarly configured to optimize the power flow
from waveguide 765 into waveguide 766, and then into waveguide 764
again, forming a Fabry-Perot resonator with a determinate optical
path length, FSR, optical loss coefficient, and coupling T with the
input waveguide 752. Now, impedance matching is possible, and is
accomplished when the coupling coefficient T equals the total
round-trip loss coefficient of the resonator less the output
coupling loss, principally in the coupling region 753. If a phase
matched frequency doubler is disposed within the resonator, the
converted power out of the fundamental frequency beam circulating
in the resonator does count as one of the losses in the total
round-trip loss.
If an input beam 757 is incident on the device with a frequency
equal to one of the resonances of the three-arm etalon, power will
couple across the parallel waveguide interaction region into the
etalon and build up to a circulating power of P.sub.circ =P.sub.inc
T/(T+.GAMMA.).sup.2. Because of the ring structure, the power will
circulate primarily in one direction, from waveguide 764 to
waveguides 765, 766, and back to 764. There is now only a single
output coupled wave from the etalon onto the waveguide 752, and it
propagates in the forward direction. The output coupled wave
interferes destructively with the reminder of the input wave 757,
forming a weak transmitted wave 759. The transmitted power
P.sub.trans in the output beam 759 is given by P.sub.trans
=P.sub.inc (1-.GAMMA./T).sup.2 /(1+.GAMMA./T).sup.2, and can be
brought to zero if .GAMMA.=T, which is the impedance matched
condition. In this case, all the incident power flows into the
resonator. In the impedance matched condition, the two beams have
equal amplitude and the transmitted power drops to zero. There is
essentially no reflected power in beam 758 except for reflections
from discontinuities in the waveguide 752, which can be minimized
by good design.
The grating 767 or any of the other gratings n-lay be configured as
a switchable grating, in which case the quality Q of the etalon may
be spoiled by turning off the grating, eliminating the comb
structure entirely but leaving some optical loss due to power
coupled into the waveguide 764. An output beam 751 may be taken in
transmission through the grating 768, and/or through the gratings
767 or 769.
FIG. 28 shows a ring waveguide etalon 770. As before, the input
waveguide 752 is coupled to a waveguide 772 in a parallel
interaction region 753. The interaction region 753 includes a
grating in FIG. 28 (although it is not required) to emphasize that
grating coupling is a useful option in the etalon geometry of FIGS.
26, 27, and 28. The waveguide 772 follows a curved closed path
(with any geometry including potentially multiple loops with
crossings), feeding a portion of the power emerging from section
753 back into the interaction region 753. As before, electrodes 761
and 773 are supplied to allow the optical path length, and hence
the FSR to be adjusted, although in this case they are shown
disposed on the same face of the substrate. A straight section 771
is provided where certain critical functional components may be
fabricated, according to the application of the etalon structure.
If the etalon device 770 is used for frequency doubling, it would
be advantageous to insert the frequency doubling structure into a
straight section such as 771 of the ring, but provision must be
made to couple the frequency converted light out of the ring
waveguide.
The functioning of the device 770 is otherwise similar to that of
the device 760. While the device 760 may consume less surface area
on a substrate, the device 770 may have lower optical loss in the
etalon, particularly if the diameter is one cm or larger.
The devices 760 and 770 can function as buildup cavities for
frequency doubling in which the feedback into the optical source is
minimal. They can also switch the transmission of a given frequency
without retroreflection, which is useful in applications including
optical communications.
In WDM communications, many communications channels separated by
their optical wavelength may be carried on the same optical fiber.
To detect a channel, the light in the desired wavelength region
must first be separated from the remaining channels which are
routed to other destinations. This separation function is performed
by a channel dropping filter. A channel dropping filter is a
communications device which is used in a wavelength division
multiplexed (WDM) environment. It is desired to multiplex several
channels across a single transmission fiber by carrying the
channels on different wavelengths. A critical component in such a
system is a channel dropping filter which allows the extraction of
a single channel for routing or detection purposes. The ideal
filter will extract essentially all of the light in a channel with
good extinction ratio, so that the same wavelength may be used
later in the network without undesirable crosstalk. It must have
very low insertion losses for the out-of-band components because
multiple channel dropping filters may be installed on any given
line. Preferably, it should be switchable so that a channel may be
dropped at a destination location, and after the communication is
finished the channel may continue past that location to another
destination. The inverse of the channel dropping filter is the
channel adding filter which adds a channel to a fiber without
significantly affecting the power propagating in the other
channels. Transmission and reflection filters have been analyzed in
detail [HL91, KHO87]. Several of the above structures may be used
for channel dropping filters, including the devices described in
reference to FIGS. 7, 10, 26, 27 and 28.
The grating coupled waveguide tee of FIG. 7 is a channel dropping
filter with low loss for the out-of-band components. With prior art
gratings, this configuration has difficulty with crosstalk, since
achieving 99.9% outcoupling for the in-band component requires a
very long gating. The coupling strength of our periodic poled
gratings is significantly increased over the prior art, due to the
ability to use higher order gratings with sharp interfaces which
extend entirely across the waveguide. Whereas the prior art is
limited to shallow waveguides to optimize the overlap between the
necessarily shallow grating and the waveguide, we are able to use
the lower loss waveguide configuration with essentially equal depth
and width because our grating structure extends entirely across the
depth of the waveguide. This structure can also be used as a
channel adding filter.
The device of FIG. 10, if the grating is configured as described in
Haus et al. "Narrow band optical channel dropping filters" J.
Lightwave Technol. 10, 57-62 (1992), is also a channel dropping
filter. Our contribution in this case is only the poled grating
coupling technique, which enables strong coupling between the
waveguides in a short distance, and which relieves fabrication
difficulties in permitting efficient higher order gratings to be
produced.
The devices 750, 760 and 770 can be used as channel dropping
filters by tuning a resonance of the etalon to the frequency of the
channel to be extracted from the input waveguide 752. If the
integrated etalon is nearly impedance matched, essentially all the
power at the resonant frequency is transferred into the etalon. In
the ring geometries of FIGS. 27 and 28, the transmitted and
reflected powers in the waveguide 752 can be reduced to any desired
level, minimizing crosstalk. The light corresponding to the desired
channel is completely extracted (dropped) from the input waveguide,
leaving neither reflections or transmissions. In the linear
geometry of FIG. 26, some light is lost to reflection, which does
not significantly reduce the detection efficiency, but which may
cause crosstalk problems in a communications network. The signal
carried by the light can be detected by placing a detector over a
waveguide segment of the etalon and coupled to the light in the
waveguide. Or, the detector can be coupled to one of the output
waveguides such as 754 in FIG. 26, 764, 765, or 766 in FIG. 27, and
794 is FIG. 28. In the case of the device 760, the outcoupling can
be accomplished by adjusting the reflection of one of the resonator
grating reflectors 767, 768 or 769 so that a small portion of the
circulating power is coupled out into the continuation segments of
the waveguides as shown for output beam 751. Those continuation
waveguide segments may also be connected to ports of other devices,
which may be either discrete devices or integrated on the same
substrate. In the case of the device 770, a parallel waveguide
output coupler (with or without grating) may be placed in the
straight section 771 of the ring. Although only a fraction of the
circulating power may be outcoupled at these ports, the total
outcoupled power may be very close to 100% of the channel power
entering the waveguide 752 due to the buildup which occurs in the
etalon. Output coupling is shown with an adjacent waveguide 794,
producing the output beam 751.
The ring geometries excel in terms of extension ratio (which is
high when the light separation efficiency is high) and low
crosstalk because they can be adjusted to have almost total
transfer of power into the etalon. All of the etalon devices can be
designed with very low insertion loss for the out-of-band beams.
All of the devices of FIGS. 26-28 are switchable by means of the
phase shifting electrodes 761, and 762 (and 763 in FIG. 28).
As described before, the optical path length may be adjusted using
electrode 761 to shift the frequency of the integrated etalon
resonances. The desired channel may be selected this way directly.
Or, multiple channels may be selected by this technique using the
approach described above in reference to FIGS. 20, 21, and 22; if
the FSR of the etalon is selected to be slightly different from the
channel separation, the Moire effect is used to select widely
spaced channels with a minimum of continuous tuning. (A good choice
is to make the FSR equal to the channel spacing plus a few times
the frequency width obtained when convolving the channel bandwidth
with the etalon resonance bandwidth).
As a variation on the structures 750, 760, and 770, the coupling
region 753 may be implemented as a grating-assisted coupler as
described above in reference to FIG. 10. This has the advantage, in
the poled-grating implementation, that the coupling fraction T can
be adjusted. Particularly for the ring resonator designs 760 and
770, an adjustable coupling is useful to achieve impedance
matching. As a further variation, the electrodes may be implemented
on the same face of the substrate, as described above to obtain
lower voltage excitation.
The structures of FIGS. 27 and 28 may also be used as efficient
channel adding filters if the signal to be added to the output beam
759 is brought in on the waveguide 766, for example, or if it is
coupled into the straight section 771 via the waveguide 794. These
input interactions will preferably be impedance matched.
Referring now to FIG. 29A, there is shown a waveguide
modulator/attenuator 800 using a poled segment 806. The function of
the poled segment 806 is to (switchably) collect the light emitted
from an input waveguide segment 802 and launch it into an output
waveguide segment 804 when switched on. In this device, an input
light beam 820 is coupled into the input waveguide 802. A poled
segment 806 is positioned between the input segment ad the output
waveguide segment 804. The input and output waveguide segments are
preferably permanent waveguides which may be fabricated by any of
the standard techniques including indiffusion and ion exchange. The
segment 806 is preferably a reverse poled region within a uniformly
poled substrate so that there is essentially no difference in index
of refraction and hence no waveguiding effect when the electric
field is off. The segment 806 is a waveguide segment as shown in
FIG. 29A. (It may alternatively be configured in several
geometrically different ways such as a positive lens structure, a
negative lens structure, or a compound structure for relaying light
between many such elements: see FIG. 29B.) The segment 806 is
turned on by applying an electric field through the segment. The
electric field changes the index of refraction of the poled segment
and surrounding regions. Because the segment 806 is poled
differently (preferably reverse poled) from the substrate material,
the index of the segment can be raised relative to the surrounding
material by applying the correct field polarity, forming a
waveguide. The index inside the boundary of the waveguide may be
increased, or the index at and outside the boundary may be
depressed. When the poled segment is on, a continuous waveguide is
formed, joining the input and output segments. This is achieved by
butting the waveguides together, aligning them to the same axis,
and adjusting the width of the poled segment so that its transverse
mode profile optimally matches the mode profile of the input and
output waveguides 802 and 804.
With the poled segment off, the input beam is not confined in the
poled region, so that the beam expands substantially by diffraction
before it gets to the output waveguide segment. If the separation
of the input and output waveguide segments is much grater than the
Rayleigh range of the unguided beam, so that the beam expands to a
dimension much larger than that of the output waveguide, only a
small portion of the input beam will be coupled into the output
waveguide segment to form the output beam 822. By adjusting the
length of the segment 806 relative to the Rayleigh range, the
amount of power transmitted in the off condition can be reduced to
the desired degree.
The location of the ends of the poled segment 806 are adjusted
relative to the locations of the ends of the input and output
waveguides to minimize the loss caused by the discontinuity.
Because the permanent waveguides have a diffuse boundary, the poled
waveguide has a discrete boundary, and the index change in the
switched segment adds to the pre-existing index, it is desirable to
leave a small gap on the order of half the diffusion length between
the lithographically defined boundary of the waveguides 802 and
804, and the ends of the poled segment 806. To further reduce the
reflection and other loss at the junction between waveguides 802
and 806, it is also advantageous to taper the onset of the index
change in the segment 806 by either making the exciting electrode
810 slightly shorter than the segment 806 or by tapering the
electrode width near its end, in both cases taking advantage of the
reduction of the electric field by the fringing effect.
One distinguishing aspect of this configuration is that the
reflected power can be minimized in both the on and the off
conditions. With the switch off, the reflection is dominated by the
residual reflection at the end 803 of the waveguide 802. This
reflection may be minimized by tapering thee reduction of the index
difference along the length of the waveguide. The reflection from
the end 805 of the waveguide 804 is suppressed by the square of the
"off" transmission. In the "on" condition, the reflection is
minimized by also tapering the index difference of the structure
806 along the direction of propagation, creating a smooth boundary
rather than a sharp interface.
The boundaries of the excited poled region confine the beam
laterally when they are activated because of the increase in the
index of refraction within the boundaries. If the depth of the
poled region equals the depth of the waveguides 802 and 804, the
beam is also confined in the vertical direction by the poled
segment boundaries. However, it is difficult to control the depth
of the poling in a z-cut lithium niobate wafer. It is easiest to
pole a deep domain, and take one of several alternative measures to
obtain confinement in the vertical dimension. The preferred
approach is to arrange the electrodes so that the amplitude of the
electric field falls off in the vertical dimension. This is
achieved by the same-side electrode configuration shown in FIG.
29A, but not with electrodes placed on opposite sides of the
substrate. The penetration depth of the electric fields can be
reduced by narrowing the gap between the two electrodes and by
reducing the width of the overall electrode structure. In addition
or as an alternative, a weak permanent waveguide can be fabricated
in the volume between the input and output waveguides, which is
insufficient to convey much energy by itself, but which in
combination with the index elevation produced in he poled segment
806 can optimally confine the light in two dimensions to convey
essentially all the light into the output waveguide 804. This can
be done, for example, by adjusting the permanent index change
(relative to the substrate) within the segment to about 0.6 of the
index change in the waveguides 802 and 804. If the "on" index
change in the segment 806 is adjusted to about 0.5 of the same
value, the combined index change is sufficient to achieve
reasonable guiding while the permanent index change is
insufficient. In the "on" condition, the mode is confined in both
transverse dimensions even though the switched index change
produced in the poled region may be considerably deeper than the
desired waveguide dimension: the effective depth of the "on"
waveguide is mainly determined by the permanent index change. The
weak waveguide may be fabricated in a second masking step, or it
may be fabricated in the same masking step with a narrower mask
segment defining the weaker waveguide segment.
As a related alternative, the region between the input and output
waveguides may be a planar waveguide, in which case the propagating
mode can at minimum diffract in one dimension. Switching on a poled
section will in this case add the needed transverse confinement
despite having a deeper index change than the planar waveguide.
Since in both cases the confinement of the waveguide in the two
dimensions is achieved by two independent techniques, switchable
waveguides of essentially any aspect ratio (the ratio of the
waveguide width to depth) can be formed. Both the planar and
channel waveguides can be fabricated by the same technique, which
is preferably the annealed proton exchange process. Separate proton
exchange steps may be used to define the planar guide and channel
waveguide. The waveguide fabrication process is completed by
annealing, during which the index changes are diffused down to the
desired depth, and the optical activity of the material is
restored. Preferably, the two sets of guides are annealed for the
same length of time, although one set can be made deeper by
partially annealing before the second proton exchange step is
performed.
An important alternative is to use a full, uniform permanent
waveguide traversing the poled segment 806, and to use the
electrically excited segment to turn off the guiding. In this case,
the polarity of the field is chosen to depress the index in the
poled region, and the depth of the poled region can be very large
(in fact this has some advantage in terms of mode dispersal). This
type of switched waveguide is normally on (i.e. transmitting), and
requires the application of an electric field to switch it off.
There are advantages to both normally-on and normally-off switch
configurations in terms of their behavior during a power failure,
so it is important that this invention is capable of providing both
modes. To switch the waveguiding off in the segment 806, an index
change is desired which is approximately equal and opposite to the
index change induced in the permanent waveguide. The effect of the
variation with depth of the electric field on the "off" state is
quite small because it is sufficient to suppress the majority of
the waveguide in order to strongly disperse the light.
Confinement can be achieved in both dimensions without the need of
a planar waveguide, by a finite-depth poling technique. Several
poling techniques (such as for example titanium-indiffusion in
lithium niobate and lithium tantalate and ion exchange in KTP),
produce poling to a finite depth, which can potentially be
optimized to form a poled channel waveguide with a particular
depth. These techniques, however, produce an index change along
with the poling, forming somewhat of a permanent waveguide
depending on the processing parameters. Depending on the strength
of this index change, the poled waveguide segment may be fabricated
in either the "normally on" or the "normally off"
configuration.
Preferably, the electric field is created in the poled region by
applying a voltage across two electrodes, which are laid out on the
same face of the crystal as the polled waveguide segment. A first
electrode 810 is laid out over the poled region, while the second
electrode 812 is placed in proximity to one or more sides of the
first electrode. For a z-cut crystal, this configuration activates
the d.sub.33 electro-optic coefficient of the substrate. A voltage
source 816 is electrically connected via two wires 814 to the
electrodes to provide the driving voltage for the device. This
device can be used as a digital or nonlinear analog modulator. A
full-on voltage is defined to be the voltage at which the loss
across the poled region is the lowest. The off voltage is defined
as that voltage which reduces the coupling to the output waveguide
segment to the desired extent. By continuously varying the voltage
between the on and the off voltages, the device can be used as
either an analog modulator or a variable attenuator.
In an alternative structure, the structure 806 forms a switched
curved waveguide, which again aligns with the input 802 and output
804 waveguides. The mode of such a structure is called a
"whispering gallery" mode in the extreme case where the curvature
is small and the mode confinement on the inside edge becomes
independent of the inside waveguide edge. For larger curvatures,
the mode is a modified whispering gallery mode where some
confinement is provided by the inside edge of the waveguide. The
poled structure provides an advantage in addition to the
switchability, namely that the sharp index of refraction transition
on its outside wall greatly improves the confinement of the
modified whispering gallery mode which propagates in the curved
waveguide. The input and output waveguides need not be coaxial or
parallel in this case, potentially increasing the forward isolation
in the switched-off condition. If the input and output waveguides
are arranged along axes at an angle to each other, the structure
806 may be a curved waveguide segment with a single radius of
curvature or a tapered radius of curvature, used to optimally
couple power between them when the curved waveguide structure 806
is turned on.
FIG. 29B shows an alternative structure 801 which is a switched
lens modulator/attenuator in which the prismatic structure of
segment 806 is modified into a lenslike structure in which the
product of the local optical path length and the local (signed)
index change is reduced quadratically with transverse distance away
from the axis of the guides 802 and 804. The lenslike structure is
placed such that it concentrates or refocuses the beam 821 emerging
from the end 803 of the input waveguide 802 into the end 805 of the
output waveguide 804. The optical wave is allowed to diffract away
from the end 803, and passes through the lenslike structure 807.
Note that in this structure, multiple elements may be placed
adjacent each other, increasing the net focussing effect. The index
of refraction within the regions 807 is increased to obtain a
focussing effect. If the surrounding region is poled in a reverse
direction to the regions 807, or if the electro-optic coefficient
of the surrounding region is otherwise opposite to that of the
regions 807, the spaces between the lenses also act as focussing
regions. (The negative lens shape formed by the regions between the
lenses 807, excited to a lower index value, acts as a converging
lens structure.) The electrode 810 is placed over the structure 806
with electrodes 812 being placed outside the structure but adjacent
the electrode 810 with a gap as desired. When the electrodes are
not actuated, the beam continues to diverge, and very little power
is refocussed into the waveguide end 805. When the switch is on,
the beam is refocussed, and a fraction of the power continues
through the guide 804. Vertical confinement is needed for efficient
power collection in the on state, while it is undesirable in the
off state. Vertical confinement may be provided as needed by, for
example, providing a uniform planar waveguide 835 across the entire
surface on which the structures are patterned. Vertical confinement
may also be provided by the lenslike structure 806 if it is poled
deep into the substrate, and the electric field reduction as a
function of depth is tailored to collect and refocus the energy
back to the waveguide end 805. The structure of FIG. 29B may of
course also be used in other contexts which may not have one or
both waveguides 802 and 804.
Referring to FIG. 30, there is shown a poled total internal
reflecting (TIR) optical energy redirector 830 using a poled
waveguide segment. This figure illustrates both a poled TIR
reflector for high switched reflection combined with a poled
waveguide segment for low insertion loss. An input waveguide 832
extends entirely across the device. A poled region 836 extends
across the waveguide at an angle 848, forming a TIR interface for
the beam propagating in the guide when the poled region is
electro-optically activated. A portion of the poled region also
forms a poled waveguide segment 837 that is connected to an output
waveguide segment 834. The poled waveguide segment and the output
waveguide segment are both laid out at twice the angle 848 with
respect to the input waveguide. A voltage source 846 provides the
electrical activation for the switch, and is connected to it
through two wires 844.
The poled region 836 is defined by six vertical faces according to
the diagram, with one face traversing the waveguide 832 at a
shallow angle 848 equal to the TIR angle and less than the critical
angle for total internal reflection for a desired electrode
excitation. This face is the TIR reflecting interface. The next
three consecutive vertical faces of the poled region enclose a
projection outside of the waveguide 832. The projection is a
switchable waveguide segment. The placement of the next two
vertical faces is not critical, and may follow the waveguide
boundaries and cross it at 90.degree..
The domains (836 and the region of the substrate outside 836) are
characterized by a quiescent index of refraction distribution,
which is the spatial distribution of the index in the absence of
applied electric field. When an exciting electric field
distribution is applied through the domains, they will have an
excited index of refraction distribution which is different from
the respective quiescent distribution. The excited distribution
will also have a range according to the accessible range of the
applied electric field. The advantage of juxtaposing two domain
types near one another is that the electric response may be
opposite in the two domains, producing a transition with double the
change in index across the region of juxtaposition. In the case of
index or refraction changes, the transition forms a reflection
boundary with larger reflection than would be attained with a
single domain type.
When the switch is on, an input beam 851 that is coupled into the
waveguide reflects off the TIR interface, propagates down the poled
waveguide segment, and passes into the output waveguide segment 834
to form a deflected output beam 854. When the switch is off, the
input beam propagates through the poled interface and continues
through the input waveguide to form an undeflected output beam 852.
Because the index change at the TIR interface is low, the
reflection in the off state is very low. Because the permanent
waveguide segment 834 is separated by several mode exponential
decay lengths from the guide 832, the power lost due to scatter as
the beam passes by the switching region is also extremely low. An
"off" switch is essentially invisible to the waveguide, producing
extremely low loss in the input guide. The additional loss of the
switched region in the off state compared to an equal length of
unperturbed waveguide is called the insertion loss. Low insertion
loss is especially desirable when the input waveguide is a bus with
many poled switches.
The angle .theta. (848) of the poled interface with respect to the
input waveguide must be less than the maximum or critical TIR angle
.theta..sub.c, as derived from Snell's law: ##EQU3##
where .theta.=TIR angle (between the waveguide and the poling
interface), n=index of refraction of waveguiding region, and
.DELTA.n=electro-optic change in index on each side of poling
boundary Since the index change occurs on each side of the poling
boundary with opposite sign, the effective index change is
2.DELTA.n. This expression assumes slowly varying (adiabatic)
changes in the index away from the boundary. Due to the doubling in
the effective index change, the maximum switching angle that can be
achieved with a poled TIR switch is increased by 2 over the prior
art switches with a pair of electrodes and no poled interface. This
is a very significant increase since it increases the maximum
packing density of switch arrays which can be achieved using a TIR
switch.
The critical angle .theta..sub.c depends on the polarization of the
input beam because the index change .DELTA.n depends on the
polarization. In z-cut lithium niobate, for example, with a
vertical field E.sub.3, the TM wave is sensitive to the change in
the extraordinary index of refraction through the r.sub.33 and the
TE wave to the change in the ordinary index through r.sub.13. Since
r.sub.33 >r.sub.13, it is far easier to switch TM waves. Use of
annealed proton exchanged waveguides is very convenient because
they guide only waves polarized in the z-direction. In x-cut
y-propagating (or y-cut x-propagating) lithium niobate, on the
other hand, the TE wave has the higher change in index. Note that
in this case, the electrode configuration must be changed to
produce a field component in the z direction in the plane of the
substrate, instead of in the vertical direction.
The design angle for actual TIR switches must be chosen after
optimizing several factors. The mode to be switched includes two
angular distributions (in the waveguide fabrication plane and out
of the plane) which can be different if the widths of the waveguide
in the two planes are different. The angular content .delta..phi.
of the mode in a given plane covers approximately
.delta..phi.=.+-..lambda./.pi.w.sub.o where w.sub.o is the
1/e.sup.2 mode waist in that plane. We wish most of the light to be
reflected at the TIR interface, so the angle of incidence must be
less than the critical angle .theta..sub.c by approximately the
angular content .delta..phi. in the plane of the switched
waveguides. The angular content .delta..phi. is inversely related
to the waist size, but so is the packing density which we wish to
optimize. The angular content of the mode in the direction out of
the plane of the waveguides also must be taken into account because
it also contributes to the effective incidence angle, although in a
geometrically more complex way.
An alternative way of producing a TIR switch is with a strain field
instead of or in addition to the electric field. The strain field
is most conveniently implemented in a permanent fashion; the
electric field is most useful for producing changes in the
reflection. An oriented strain field applied at a domain boundary
produces different changes in the index of refraction, via the
photoelastic effect, in the two domains, resulting in an index of
refraction interface. As mentioned above in reference to FIG. 2,
the strain field may be produced by heating the sample to a high
temperature, depositing a film with a different coefficient of
thermal expansion, and cooling to room temperature. A pattern
applied to the film by etching away regions such as strips will
produce a strain field about the gap in the film. This strain field
can then be used to actuate an index of refraction difference at
domain boundaries. If the applied film is a dielectric an electric
field may be applied through it to the poled regions provided that
the deposition of electrodes does not undesirably change the strain
field. The film is preferably a film with low optical absorption so
that it can be contacted directly to the substrate instead of being
spaced by a buffer layer.
The poled region includes a portion of the input waveguide and has
an interface normal to the propagation axis of the waveguide. The
portion of the input waveguide that contains the TIR interface
crossing defines the length of the switch:
where .theta. is previously defined, ##EQU4## L=length of the
switch measured along the input waveguide, and W=width of the
waveguide
Thus, in order to minimize the size of the switch, the width of the
waveguide must be made as small as possible. For space-critical
applications, it is preferable that the waveguide segments be
single mode. As a numerical example, if the width of the
single-mode waveguide is 4 .mu.m, the maximum index change .DELTA.n
is 0.0015, and the index of refraction is 2.16, then the TIR angle
.theta. is 3.degree. and the length of the switch L is 76
.mu.m.
The poled waveguide segment forms an angle with respect to the
input guide equal to 2.theta., which is the deflection angle of the
TIR interface. In order to efficiently modematch the beam
reflecting off the TIR interface into the poled waveguide segment,
the poled segment should have nearly the same transverse mode
profile as the input waveguide. Efficient mode matching can be
achieved by selecting the proper combination of width and index
difference of the poled waveguide. The poled waveguide segment
intersects the input waveguide along the latter half of the side of
the waveguide occupied by the switch interface. The exact
dimensions and placement of the waveguide are determined to
optimally match the near field mode profile emerging from the total
internal reflection process to the mode of the waveguide in terms
of direction of propagation and transverse profile. The same is
true of the match between the poled waveguide segment and the
permanent waveguide segment 834, similarly to what was described
above in reference to FIG. 29A.
The permanent waveguide segment is essentially a continuation of
the poled waveguide segment. The length of the poled segment
depends on optimizing losses in the input waveguide and the
switched waveguide. In order to avoid scattering interaction
between the undeflected beam in the input waveguide when the switch
is off, the permanent waveguide segment must be separated by some
distance (at least an optical wavelength) from the input guide. For
a bus waveguide with many switches, the loss in the input guide
must be reduced to a value related to the inverse of the number of
switches. The modal profile of a beam in the input guide extends a
certain distance beyond the indiffused edge of the guide, where it
decays exponentially. If the permanent segment is separated from
the input guide by several of these exponential decay constants,
the loss can be reduced to an acceptable level for a bus
waveguide.
The length of the poled segment affects the loss in the reflected
beam as well. The poled waveguide segment may have higher losses
per unit length than an indiffused waveguide, due to higher wall
roughness. In addition, there are the above mentioned mode
conversion losses at each end of the waveguide, which are minimized
by optimally matching the mode profiles. If the poled segment is
short (on the order of the Rayleigh range of the beam), the
transmitting beam does not substantially convert into the mode of
the poled segment, thus reducing the coupling losses. The optimal
length of the poled segment depends on the relative loss that is
tolerable in beams in the input waveguide and the switched
waveguide.
As in the case of the waveguide segment modulator/attenuator shown
in FIG. 29A, there is a need for vertical confinement of the mode
in the switched waveguide segment 837. The same options described
there can be implemented here. Shown in FIG. 30 is a planar
waveguide 835 which confines the beam in a plane parallel to the
surface of the substrate. Since the planar waveguide is uniform,
its presence does not affect the loss of the waveguide switch
junction in its off state. In place of the planar waveguide, or in
some combination, the other alternatives may also be implemented,
including tailoring the depth of the electric fields to obtain
vertical confinement, using short depth poling, using a partial
waveguide which is augmented by the field induced index change, and
using a full permanent waveguide which is turned off by a field
activated poled region. The latter two alternatives have the
disadvantage that the loss to the beam through waveguide 832 is
higher due to the adjacent index discontinuities.
Horizontal confinement is also an issue in optimizing the switching
region. If high switched efficiency is desired, it is preferable to
have a large TIR reflection angle. The left half of the input wave
851 reflects first off the interface 838, forming the fight half of
the reflected wave. However, after reflection, the fight half of
the reflected wave is unconfined in the transverse dimension until
it arrives into the waveguide segment 837. During its unconfined
passage, it will expand by diffraction, reducing the fraction of
the beam power which couples into the output waveguide 834. This
effect degrades the efficiency of the switch in its on position.
However, the mean unguided distance is limited to approximately the
waveguide width divided by four times the sine of the angle 848.
The right half of the input wave remains confined after it passes
the waveguide segment 837 until its reflection off the interface
838 because of the permanent index change due to the fight hand
side of the waveguide 832. It then matches well into the output
waveguide 834. Both portions of the input beam 851 suffer an
undesired reflection from the side of the waveguide 832 after
reflecting from the TIR surface 838. Since this surface is at the
same angle to the axis of propagation of the beam as the surface
838 was, but with only a fraction of the index difference, there
only be a partial, not a total, reflection from this surface which
also adds to the loss of the switch.
The electrode design is a critical aspect of this switch, in order
to optimize the efficiency of the reflector and minimize the loss
of the waveguide. Preferably, two electrodes are used to activate
the switch. A first electrode 840 is placed over the TIR interface
838, while a second electrode 842 is placed alongside the first
electrode, adjacent to that interface. The main parameters for
optimization are the separation of the two electrodes and the
distance between the edge of the first electrode and the poling
boundary, which may or may not overlap. The spacing between the two
electrodes influences the voltage required to activate the device,
as well as the width of the electric field pattern which penetrates
the substrate and produces the index change profile. Electrodes
that are spaced further apart require higher voltages, but create
an electric field that extends deeper into the substrate than
closely spaced electrodes.
The electric field penetration depth is critical to obtaining a
large net reflection. Because the fields get weaker the farther
they are away from the electrodes, the induced index change at the
poling boundary also drops with depth, as does the TIR angle. At a
certain depth called the effective depth, the index change becomes
insufficient to maintain total reflection for the central ray of
the optical beam at the angle of the switch structure. Since the
reflection drops rapidly with index change at values below the
minimum TIR value, the TIR mirror essentially stops functioning at
this depth. For high net reflection into the guides 837 and 834,
the device design should be adjusted to create an effective depth
below the majority of the field profile in the guide 832.
The second important operating parameter influenced by the
electrode design is the penetration of the evanescent fields of the
reflecting wave beyond the TIR interface 838. Although no power may
be transmitted beyond the TIR interface in the "on" condition, the
electromagnetic fields penetrate the TIR surface by a distance on
the order of a wavelength. There will also be spatial dependence of
the applied electric field beyond the TIR surface, the field
strength being reduced (and in fact inverted) in regions closer to
the other electrode 842. The index change is therefore reduced
beyond the TIR interface. Care must be taken that the evanescent
fields decay to a negligible value before substantial variation in
the field occurs, or power will leak through the TIR interface.
Optimally, the first electrode will overlap the poling interface by
a distance chosen for maximum index change and for sufficient
constancy of field beyond the interface 838.
The first electrode also extends across the poled waveguide segment
836, and possibly into adjacent areas. The shapes of the two
electrodes exciting this region 836 are determined by optimizing
the power flow through the waveguide segment and into the permanent
waveguide 834. Other electrode structures can be used to modify the
strength of the electric field in the poled region. If, for
example, the second electrode is extended around the first to form
a U shape, the electric field under the first electrode is
increased on the average, but it forms somewhat of a two-lobed
waveguide, which may not provide an ideal index profile.
The TIR switch is an optical energy router and can also be used as
a modulator. If the voltage source is continuously variable, then
the modulator is analog, with a nonlinear relationship between
voltage and reflectivity. As the applied voltage is increased, the
depth of the total reflecting interface is increased, producing a
continuously adjustable reflection out of the wave 851 into the
wave 854. The modulator can be used in reflection or transmission
mode, depending on whether the transmission should go to zero or
100% when the voltage is removed. For special nonlinear
applications, the nonlinearity of the reflection and transmission
coefficients as a function of voltage, such as where the receiver
is logarithmically sensitive to the level of the signal, might be
useful.
FIG. 31 shows a TIR switch with two TIR reflectors. If it is
desired to increase the angle between the output waveguide 834 and
the input waveguide 832, a second TIR interface 839 may be added.
The angle between the input waveguide 832 and the output waveguide
834 is doubled relative to that of FIG. 30, and may be doubled
again and again by adding additional TIR interfaces. The interface
839 is created at an angle 849 relative to the interface 838 equal
to twice the angle 848. (Subsequent TIR interfaces, if any are
added, should be added at the same angle 838 relative to the
previous TIR interface.) The switched waveguide portion 837 of FIG.
30 is no longer required since the dual TIR mirror structure brings
the light so far away from the input waveguide 832 that the
permanent waveguide 834 may be butted directly against the end of
the poled region 836 without contributing significant loss to the
waveguide 832. Again, vertical confinement is provided in the poled
segment 836. The poled segment 836 and the output waveguide 834 are
configured and aligned so that the field profile propagating in the
chain of TIR and waveguide segments optimally matches the local
lowest order mode field profile of the input waveguide 832. After
the TIR reflectors, the deflected beam is matched into a permanent
waveguide 834 to form the output beam 854 when the switch is
on.
The shape of the inside boundary of the poled region outside the
input waveguide 832 is defined by the reflection of the input
waveguide through the TIR mirrors, one after the other. This
definition of the inside boundary achieves optimum guiding of the
inside edge of the waveguide mode while it is reflecting from the
two TIR mirrors.
FIG. 32 shows a TIR switched beam director with a poled TIR switch
831 with an electrically switched waveguide segment. In this
structure, the region 836 is reverse poled, lies behind the
interface 838, and is excited as before by a pair of electrodes 840
and 842, which are activated by voltage source 846 and connected
via conductors 844. The polarity of the excitation is again
selected to produce a negative index change coming from the
direction of the input beam 851. When the switch is on, the beam is
reflected off the TIR interface 838 towards the permanent waveguide
834, but unlike in FIG. 30, there is no poled waveguide segment
joining them. Instead, the electrode 842 is extended over the
intermediate region between the input waveguide and the output
waveguide 834. A coupling waveguide segment may be formed by
applying an electric field to a region between a lateral boundary
of the segment of the input waveguide 832 containing the TIR
reflection boundary and an input boundary of the output waveguide
834. The three dimensional distribution of the electric fields is
determined, as always, by the shape of the electrodes and Maxwell's
equations. The electric fields produced by that electrode produce a
positive index change through the electro-optic effect, providing
the desired switched waveguiding section. As described elsewhere,
this waveguide segment is also configured and aligned to optimize
the coupling of the input mode 851 into the output mode 854. As an
alternative in this and any of the TIR switch implementations, the
output waveguide may originate at the input waveguide with
negligible gap. This alternative has higher insertion loss in the
switch off (straight through) configuration, but it has a simpler
structure.
Referring to FIG. 33, there is shown a two position waveguide
router using a poled segment, which is not based on total internal
reflection. The poled region 866 forms an electrically excitable
waveguide segment which crosses the input waveguide 862 at a small
angle. When the field is applied, the index in the segment 866 is
increased, while the index in the adjacent region in the input
waveguide is decreased. Thus, the input beam 880 is at least
partially coupled into the poled waveguide segment. When the switch
is off, the input beam continues to propagate through the input
guide to form an unswitched output beam 882. The small angle may be
tapered adiabatically, forming a low loss waveguide bend, if it is
desired to switch all or most of the input light into the output
guide 864 to exit the device as the switched output beam 884.
At least two electrodes are used to apply an electric field across
the poled region to activate the waveguide. A first electrode 870
is positioned above the poled waveguide segment, while a second
electrode 872 is positioned adjacent to the first electrode. The
second electrode 872 is adjacent to the first electrode and may be
placed on both sides of the poled waveguide segment, in order to
achieve a high power splitting ratio. As before, the electrodes are
excited by the power supply 846 through conductors 844, and a
planar waveguide 835, or the electric field falloff with depth, or
one of the other approaches described herein is used to obtain
vertical confinement for the switched propagating modes.
Referring to FIG. 34, several poled TIR switches are placed side by
side to form an array 900. The poled regions 912 and 914 forming
the TIR interfaces are placed one after the other along the
waveguide 910. Each poled region has the same crystal orientation,
with the z axis of the crystal in the regions 912 and 914 reversed
relative to that of the reminder of the crystal. The other aspects
and many variations of this configuration have been described above
in reference to FIG. 30.
Each of the switches are individually activated using a
multi-output voltage control source 926, which is connected to the
electrodes with wires 928. When all switches are off, the input
beam 902 propagates down the input waveguide 910 to form an
unswitched output beam 904 with negligible loss. If the first
switch is on, then the input beam reflects off the first TIR
interface to form a first reflected output beam 908 in waveguide
916. If the first switch is off and the second on, the input beam
reflects off the second TIR interface to form a second reflected
output beam 906 in waveguide 918, and so on for the subsequent
switches. This multiple switch structure can be extended to n
switches.
An electrode is laid out over each TIR interface as described
above. One or more of the electrodes 920, 922, and 924 serve as the
cathode for one switch and the anode for another. For example, a
voltage is applied between the second electrode 922 and the first
and third electrodes 920 and 924 to activate the second switch
forming the output beam 906. An electrode 922 that acts as both an
anode and a cathode should preferably extend adjacent to the TIR
interface of the prior poled segment 912 while also covering the
TIR interface of one poled region 914 and one waveguide segment of
one poled region 914. Only a portion of the structure is shown,
with two complete poled segments 912 and 914 and one complete
electrode 922. This structure can be replicated into n switches by
aligning duplicate complete electrodes and poled segments.
In order to avoid crosstalk in the channels, the voltage on the
electrodes may be applied in such a manner that the input beam does
not see any electro-optic index changes until it enters the region
of the activated switch. For example, to activate the TIR interface
of the second poled region 914, a voltage may be applied between
electrodes 922 and 924, keeping the same potential on electrodes
920 and 922 and prior electrodes.
Although the total length of the poled regions is longer than L,
the distance occupied along the waveguide by a given region is
equal to L by definition. A linear array of TIR switches with a
100% packing density would therefore have new poled regions
starting every distance L. This is called 100% packing density
because at this density the adjacent regions just barely touch each
other at the inside corner of the poled region in the waveguide.
Having adjacent regions touch each other is disadvantageous because
some of the light guided in the previous poled structure can leak
out into the next poled structure where the structures touch.
We have noted above that the comer which touches the preceding
poled region is formed by two vertical faces of the poled region
whose placement is not critical. By moving these faces so that the
width of the poled region is thinned on the side of this inside
comer, it can be arranged that the regions no longer touch each
other, reducing the leak of optical energy. For example, the inside
comer can be moved to the middle of the waveguide by halving the
length of the face which traverses the waveguide at 90.degree.. The
face which used to parallel the waveguide now parallels the TIR
interface, and becomes a critically positioned surface. We call the
poled regions with this geometry "dense packed" poled regions.
(There are other ways the objective of minimizing the light leak
may be accomplished, such as adding a seventh vertical face between
the two noncritical faces, but the alternative just described has
another advantage in dense packing.)
FIG. 35 shows a configuration wherein the linear density of
switches is be doubled by using the dense packed geometry for the
poled region and reversing the polarity of the adjacent poled
regions. The interfaces of the poled regions transverse to the
waveguide are now identical but for a translation along the axis of
the waveguide. The poled regions will therefore stack solidly along
the waveguide, doubling the switch density. In fact, only the
reverse poled region is fully spatially defined, since the other
region has the same poling direction as the substrate (in the
optimal case where the substrate is fully poled). Two regions 952
and 954 of reverse poling are shown in FIG. 35. The TIR interfaces
can be thought of as the first face or the input face and the
second face or the output face of the poled region where unswitched
light travelling in the waveguide 950 potentially enters or leaves,
respectively, the unexcited poled region.
The TIR interface for the output beam 946 is formed between the
poled substrate and the first (input) face of the reverse poled
region 952, and is excited by electrode 966. The TIR interface for
the output beam 947 is formed between the second (output) face of
the reverse poled region 952 and the poled substrate, and is
excited by electrode 967. The TIR interface for the output beam 948
is formed between the poled substrate and the first face of the
reverse poled region 954, and is excited by electrode 968. The TIR
interface for the output beam 949 is formed between the second face
of the reverse poled region 954 and the poled substrate, and is
excited by electrode 969. The electrodes extend above the
respective TIR interfaces, and along the switched waveguide
segments which connect to the permanent output waveguides 956, 957,
958, and 959. Preferably, one or more of the electrodes 966, 967,
968,969 and 970 serve as the cathode for one switch and the anode
for another. Each electrode therefore extends parallel to and along
the full length of the TIR interface of the previous switch.
Each of the switches is individually switchable by applying
electric fields with voltage source 926 via conductors 928. When
all switches are off, the input beam 942 propagates down the bus
waveguide 950 to form an unswitched output beam 944. When the first
switch is on, the input beam reflects off its respective TIR
interface to couple into the first output waveguide segment 956 to
form a first reflected output beam 946. For the subsequent
switches, the input beam reflect off the respective subsequent TIR
interface to couple into a waveguide segment 957, 958, or 959 to
form a reflected output beam 947, 948, or 949. The voltage on the
electrodes is typically set so that there is no optical
interference from adjacent switches: all preceding switches are
off. This can be accomplished for example by maintaining all the
preceding electrodes at the same potential as the switched
electrode. This multiple switch structure can be extended to n
switches.
It is desirable to extend the upstream end of the dense packed
poled regions significantly beyond the edge of the input waveguide
950, maintaining the angle of the vertical surfaces with respect to
the waveguide. This extension captures the full exponential tail of
the input waveguide mode, and pushes the remaining noncritically
positioned surface of the extended dense packed poled region out of
the waveguide 950, thereby diminishing the optical loss. (Upstream
and downstream are defined in relation to the direction of
propagation of the input beam 942.)
If the switched waveguide segment of the poled regions is designed
as described above in reference to FIG. 30, the separation of the
output waveguides becomes equal to their width in the highest
density packing, so that they merge into a planar waveguide. While
a planar output waveguide may be useful for some applications, the
output waveguides may be separated using a second poled TIR
interface within each switch. The use of two TIR interfaces in a
switch has been described in reference to FIG. 31. Note that in the
case of FIG. 35, the geometry of the poled region is slightly
different to accomplish the stacking. The "output waveguide"
section of the extended dense packed poled regions is rotated about
the end of the first TIR interface to an angle 3.theta. relative to
the input waveguide 942, maintaining the parallelism of its faces.
This "output waveguide" section therefore becomes a second TIR
reflector segment.
The width of the second TIR reflector segment is about 50% larger
than the input waveguide. The mode propagating in the second TIR
reflector segment is unconfined on its inner side for a distance of
about 2W/sin.theta. where W has been defined as the waveguide
width. Any diffraction which occurs on this side will result in
reduced power coupling into the output waveguides 956-959. It is
desirable to keep this distance less than about a Rayleigh range.
In the case of a 4 .mu.m wide waveguide operating at a TIR angle of
4.5.degree., the total unconfined distance is about 100 .mu.m,
which is approximately equal to the Rayleigh range for a blue beam.
One solution to optimizing the performance of an array of such
switches lies in adding a permanent reduction of index of
refraction (without degrading the electrooptic coefficient) in a
strategic location within the second TIR reflector segment. This
strategic location is the zone bounded by the inside wall of the
extended dense packed poled region, and by the inside wall of the
poled region 836 as defined in reference to FIG. 31. The permanent
index of refraction reduction defines a permanent waveguide
boundary at the optimal location for confinement of the mode as it
is reflecting from the two successive TIR mirrors. The added index
reduction tapers to zero as it approaches the input waveguide, and
the loss added to the input waveguide can be reduced sufficiently
by truncating the index reduction region sufficiently far from the
guide. The index reduction also does not interfere with the TIR
function of the previous TIR interface (indeed, it helps).
Thus, the switched beam reflects from two consecutive TIR
interfaces, doubling the total deflection angle of the switch to
4.theta.. By doubling the output angle, space is now made available
for output waveguides of width equal to the input waveguide, with a
separation equal to their width in the densest configuration.
The output waveguides connect to the poled region in FIG. 35 at the
final corner of the second TIR reflector, at an angle .theta.
relative to the second TIR interface and optimally aligned to
collect the light reflecting off the second TIR interface.
Preferably, the two TIR reflectors for a given switch are connected
without an intervening waveguiding segment. This minimizes the path
length that the deflected beam must travel in the poled waveguide,
which may have a higher loss than a permanent channel waveguide due
to wall roughness and asymmetry.
In an alternate poling boundary structure, the boundary between two
adjacent poled regions may be a curved TIR structure. The mode of
such a structure is again a whispering gallery mode, possibly
modified by some confinement on the inside boundary of the
waveguide. The radius of curvature of the poling boundary is made
small enough so the whispering gallery mode matches well with the
waveguide mode for large power coupling between the two types of
guide, yet large enough for practical total internal reflection to
occur for the distribution of angles within the mode.
FIG. 36 shows a dual crossing waveguide structure 980 for higher
packing density. This structure incorporates two innovations: an
asymmetric loss waveguide cross 997, and 90.degree. mirrors 976 and
977. The density is increased with the addition of a second input
waveguide 982 parallel to the first input waveguide 984, on the
same surface of the substrate 981, effectively doubling the packing
density. The switching elements 983 and 985 have been illustrated
schematically as one of the variants of the poled TIR switch
described above, but can alternatively be any integrated optic
switch described in the literature, so we do not describe the
switch in detail here or in the FIG. 36. (The switches may also be
implemented in alternate ways described herein such as the grating
switches described in reference to FIG. 7, the coupler described in
reference to FIG. 10, the splitter described in reference to FIG.
25, and the guiding switch described in reference to FIG. 33.)
A first input beam 992 propagates down the first waveguide, while a
second input beam 994 propagates down the second waveguide. The two
beams may originate from distinct sources or from the same optical
source via an active or passive splitter. When the corresponding
switch is off, the input beam 992 and 994 propagate through to form
the undeflected output beam 993 and 995, respectively. If the
corresponding switch is on, the first input beam 994 is deflected
into the output beam 996, while the second input beam 992 is
deflected into the output beam 998.
In the asymmetric waveguide cross 997, two waveguides cross each
other with the index of refraction profiles adjusted to minimize
the loss in one guide at the expense of somewhat higher loss in the
other. The crossing guides are laid out at a large angle with
respect to each other (herein illustrated at 90.degree.), in order
to minimize the crossing loss. Referring to the geometry of FIG.
36, the second deflected beam 998 crosses over the first waveguide
984 (in this case so that the switched output light beams can
propagate in parallel output waveguides 986 and 988). The waveguide
988 is broken at the crossing point with the waveguide 984, leaving
the gaps 990 and 991. This is done to minimize the loss in the
waveguide 984, producing an asymmetric loss structure with higher
loss in waveguide 988 than in waveguide 984 in the crossing region.
For later convenience, we say that the asymmetric cross "points"
along the waveguide with lower loss. The asymmetric cross 997
points along the waveguide 984. If the gaps 990 and 991 are wider
than several exponential decay lengths for the mode in the guide
984, the cross structure will provide essentially no additional
loss to the waveguide 984. A large number of asymmetric cross
structures may then be sequenced pointing along the waveguide 984
to produce a low loss waveguide crossing many others. The gaps 990
and 991 will produce some reflection and scatter to the beam 998
propagating in the broken waveguide 988, and the width of the gap
may be minimized subject to the combined constraints of desired low
loss in the two waveguides. To minimize the optical loss from the
beam 998 propagating in the waveguide 988 at the cross structure,
the index profile transverse to the axis of propagation of the
guide may be modulated or tapered along the axis of the guide. The
goal is to maintain very low loss in the waveguide 984 while
minimizing the loss in 988. This purpose is achieved if the index
of refraction change in the regions adjacent to the guide 984 is
small and slowly varying compared to the index of refraction change
of the waveguide 984 itself. (All index of refraction changes
referred to are relative to the substrate.)
The loss in the second waveguide has two components: one due to
reflection from the index discontinuities, and one due to
diffractive spreading. The reflection loss is determined by the
magnitude of the index change in the waveguides, and by its taper
profile at the ends and sides of the waveguides. For example, if
the index change at the core of the waveguides is the same in both
at .DELTA.n=0.003, the net reflection loss at the four interfaces
will be less than 5%, neglecting corrections due to the exact index
profiles which can reduce the reflection. The diffractive loss is
even lower because the gap width is typically much less that the
free space Rayleigh range. If, for example, the narrowest mode
dimension is the depth, at 2 .mu.m, then the Rayleigh range is 55
.mu.m, assuming an index in the material of 2.2 and a wavelength of
0.5 .mu.m. The diffractive loss at each gap is less than 1%,
assuming a 3 .mu.m wide gap. If the waveguide depth is 4 .mu.m, the
diffractive loss is substantially smaller. The diffractive loss may
be minimized by increasing the dimensions of the waveguide relative
to the gap size.
In general, the "gaps" 990 have an index of refraction distribution
adjacent the crossing region. This index of refraction distribution
is defined relative to the index of refraction of the substrate.
The index of refraction in the gaps may taper from a value equal to
the index of refraction distribution of the waveguide 988 to
another value adjacent the crossing region. The important part of
the crossing region is the volume within which propagates the
optical mode of the waveguide 984. To minimize the loss in the
waveguide 984, the index of refraction adjacent the crossing region
in this important part is much smaller than the index of refraction
distribution within the waveguide 984.
The crossed waveguide geometry with asymmetric optical loss may be
combined in many geometric variations. For example, three or more
input waveguides may be used with multiple crossing points where
switched output waveguides traverse input waveguides. The selection
of preferred waveguides, preferred in the sense of having its loss
minimized at the crossing point, can be also done in many ways. We
have discussed an example in which the preferred guides are
parallel. However, in a more complex system, there may be preferred
guides which cross each other as well as crossing unpreferred
guides. The selection of how to accomplish the crossings of the
preferred guides depends on the application. The waveguide crossing
structures in a device may be any combination of asymmetric loss
crossings and symmetric loss crossings where the gap widths are
zero.
For switches that deflect the beam at a small angle (such as a TIR
switch), additional beam turning means such as 976 and 977 may be
provided, in order to achieve the desired large angle of
intersection at the waveguide cross. The beam turning means 976 and
977 is preferably a vertical micromirror, and is installed at a
fixed position. Each micromirror is formed by removing the
substrate material within its volume, leaving a flat vertical
surface (preferably with low roughness) adjacent to the waveguide
and oriented at such an angle so as to direct the reflected light
optimally down the output waveguide 986 or 988. The micromirrors
may be fabricated using conventional processing techniques,
including laser ablution with, for example, a high power excimer
laser or ion beam etching, both of which might define the mirror
geometry with the aid of a mask. The volume may be filled with a
low index, low loss material such as aluminum oxide or silica to
prevent contamination of the mirror surface, and to maintain the
total internal reflection property of the mirror.
The angle of the micromirror relative to an input of one of the
waveguides is preferably adjusted to provide total internal
reflection. The thickness of the micromirror volume in the
direction normal to its reflective surface is preferably much
greater than a wavelength of light in order to minimize leakage
through the micromirror volume of the evanescent tail of the
reflected light wave. The angle relative to the other waveguide is
adjusted so that the mean propagation direction of the reflected
beam is parallel to the central axis of the other waveguide. The
location of the micromirror is adjusted to optimize the coupling of
the light from one waveguide to the other. The location of the
mirror in the junction region is preferably adjusted so that the
"centers of gravity" of the two beam profiles illuminating the
mirror surface are at the same place. The length of the mirror
transverse of the incident and reflected beams is greater than
about twice the width of the waveguide to reflect essentially the
entire mode, including the exponentially decreasing intensity in
the beam tails. Light input from one of the waveguide modes
diffracts through the waveguide junction region to the micromirror,
reflects, and diffracts back through the waveguide junction region
at the reflected angle before coupling into the output waveguide
mode. The junction region between the two waveguides in the
vicinity of the mirror is optimally kept small compared to the
Rayleigh range of the unconfined beam, which can be accomplished
with waveguides having widths in the 2 to 5 micron range.
The structure of FIG. 36 makes possible a large interdigitated
array of switched light distribution waveguides. The entire
structure 980 may be replicated many times along a pair of input
waveguides, producing a set of interleaved output waveguides with a
simple pattern of alternating parentage (in this context, parentage
means deriving optical power from a specific "parent" input
waveguide). Each input waveguide may be connected to a large number
of output waveguides as long as the switching elements have a very
low insertion loss, as is the case for the elements listed above
and described herein. Because of the asymmetric cross structures,
adding more input waveguides above the others (with additional
switches, micromirrors, asymmetric waveguide crosses, and
interleaved output waveguides) does not significantly increase the
loss of the lower input waveguides or affect their ability to
distribute light over a long distance to many output waveguides. It
will increase moderately the optical source power required for each
additional input waveguide in order to deliver the same power to
the end of their respective output waveguides. As many input
waveguides as desired may be used in parallel to distribute a
potentially large total power of light. Their output waveguides may
be interleaved in many alternative patterns using the approach of
FIG. 36. The same result may be achieved using grating reflectors
in the place of the TIR switches. If the grating reflectors are
oriented at a large angle to the input waveguides, the micromirrors
are also no longer needed.
The structure described in the previous paragraph is a one-to-many
architecture in that it has one output per switch with a
multiplicity of switches per input. There is no way to connect many
inputs into the same output. What is needed is a many-to-one
architecture. The many-to-many configuration is then obtained by
combining the one-to-many and the many-to-one configurations.
FIG. 37 shows an array 1060 of waveguides with TIR switches
arranged in a many-to-one configuration. In the structure shown,
two Input waveguides 1072 and 1074 switch two input beams 1062 and
1064 into an output beam 1070 in one output waveguide 1076. The
input TIR switches 1090 and 1092, and the output switches 1094 and
1096 have been described before in reference to FIGS. 30-32 and 36,
so they are shown only schematically, leaving off many elements
(such as the electrodes, the contacts, the power supply, the
controller, the vertical confinement means, the depth of the poled
regions, the type of output waveguide confinement) for clarity. The
input TIR switch is arranged with the beam propagating in a forward
sense as described in reference to FIG. 36, while the output TIR
switch is arranged with the beam propagating in a reverse sense.
The switches 1090 and 1092 are switched at substantially the same
time, as are switches 1094 and 1096, because both are required to
accomplish injection of power into the output waveguide 1076. As
described in reference to FIG. 36, when the switches 1090 or 1094
are on, a fraction of the beams 1062 and 1064 are switched,
respectively, into the waveguide 1078 or 1084. The remainder of the
input beams propagates along the extension of the input waveguide
into an output path as beams 1066 or 1068, which may be used in
some other component or brought into a beam dump for absorption or
scatter out of the system. Micromirrors 1082 and 1088 are provided
to reflect the beams from waveguides 1078 and 1084 into the
waveguides 1080 or 1086, respectively. In their on condition, TIR
switches 1092 or 1096 receive the beams propagating in the
waveguides 1080 or 1086, respectively, forming the output beam
1070. If it is desired to switch the beam 1062 into the output beam
1070, clearly the switch 1096 and all subsequent switches must be
off. (It would otherwise reflect much of the desired beam out of
the waveguide 1076.) A similar constraint applies for all the other
switched beams in multiple switch arrays.
The substrate 1098 is processed as described herein to produce the
structures illustrated. When the switches 1090 or 1094 are off, the
input beam propagates through the switching regions 1090 or 1094
with negligible loss, traverse the waveguide 1076 (in an asymmetric
cross if desired), and emerge as output beams 1066 or 1068,
respectively, possibly for use as inputs to additional
switches.
Additional input waveguides may also be provided, coupling into the
waveguide 1076 (or not coupled, as desired), in a modified
repetition of this structure in the direction of the output beam
1070. Additional output waveguides may also be provided, coupled if
desired to the input waveguides 1072 and/or 1074, in a modified
repetition of this structure in the direction of the beams 1066 and
1068.
FIG. 38 shows an array 1210 of grating reflectors in a many-to-many
configuration. In the structure shown, two input waveguides 1222
and 1224 switch two input beams 1212 and 1214 into two output beam
1220 and 1221 in two output waveguides 1226 and 1228 which abut or
encounter the input waveguides. The grating switches 1230, 1232,
1234, and 1236 containing the gratings 1238, 1240, 1244, and 1246
have been described before in reference to FIGS. 7, 8, 12, and 13,
so they are shown only schematically, leaving off many elements
(such as the electrodes, the contacts, the power supply, the
controller, the vertical confinement means, the depth of the poled
regions, the tapering of the poled regions or electrode spacing)
for clarity. When the switches 1230 or 1232 are on, a fraction of
the beam 1212 is switched into the output beams 1220 or 1221,
respectively. The remainder of the input beam propagates along a
continuation of the input waveguide into an output path as beam
1250, which may be used in some other component or brought into a
beam dump for absorption or scatter out of the system. When the
switches 1234 or 1236 are on, a fraction of the beam 1214 is
switched into the output beams 1220 or 1221, respectively. The
remainder of the input beam propagates along a continuation of the
input waveguide into an output path as beam 1252, which may be used
in some other component or brought into a beam dump for absorption
or scatter out of the system. It should be understood that the
structures admit to bidirectional propagation.
The substrate 1248 is processed as described herein to produce the
structures illustrated. When the switches are off, the input beams
propagate through the switching regions (in which the waveguides
may be configured as an asymmetric cross if desired), and emerge as
output beams 1250 and 1252, respectively, possibly for use as
inputs to additional switches. The waveguides may cross each other
in simple large-angle junctions as shown, or the junctions may be
asymmetric crosses, which do not substantially affect the placement
of the gratings 1238, 1240, 1244, and 1246. Note that the gratings
may in fact be parts of a single large grating which covers the
substrate and which is only activated in the regions of the
different switches by the desired electrodes. If the gratings are
constructed from poled domains, for example, this allows the entire
substrate to be poled for the gratings, which may be simpler in
production. Alternatively, the gratings could be arranged in
stripes or other groupings.
Additional input waveguides may also be provided, coupling into the
waveguides 1226 or 1228 (or not coupled, as desired), in a modified
repetition of this structure in the direction of the output beams
1220 and 1221. Additional output waveguides may also be provided,
coupled if desired to the input waveguides 1222 and/or 1224, in a
modified repetition of this structure in the direction of the beams
1250 and 1252.
FIG. 39A shows schematically an example application of the
alternative switch arrays in the n.times.n communications routing
application. In this application, the optical power in n input
optical channels is to be routed to n output optical channels with
minimal loss and minimal crosstalk. A controller sets up an
addressable path between one channel and another. A simple square
array is formed by repeating the structure of FIG. 38 until n
inputs are arrayed on the left and n outputs are arrayed on the
bottom, with switches at all n.sup.2 of the waveguide
intersections. The intersection angle may be any convenient angle.
In this structure, the switching of any channel into any other is
accomplished by activating one of the switches. The light beams
cross each other at the waveguide crosses with a small amount of
crosstalk which can be reduced by optimizing the waveguide
geometry. This structure is capable of independent one-to-one
connections between any input and any output. Note also that the
connections are bidirectional so that a communications channel can
be used equally well, and in fact simultaneously, in both
directions. The switches are shown as implemented with gratings for
specificity, but they may be implemented with dual FIR switches as
described in reference to FIG. 37 by replicating the structure of
FIG. 37 forming the n.times.n inputs and outputs, or with any other
optical switching technique now known or yet to be discovered. Note
that in the case of the TIR switches, the optical data path does
not pass through the vertex of the intersection between the input
and the output waveguide. Instead, it passes through another
waveguide near the intersection. According to the specific geometry
of the switch, the input and output waveguides may intersect at a
large angle as shown in FIGS. 37, 38 and 39, or at an oblique
angle. The fixed reflectors 1088 and 1082 in the dual-TIR switching
geometry may not be required in the case of the obliquely
intersecting waveguides.
In this simple square geometry with n parallel input waveguides,
there will be one input waveguide which can be connected into the
closest output waveguide with a single switch, forming a best case
connection with lowest loss. At the other extreme, there will be
one waveguide which must traverse 2(n-1) waveguide crosses to be
switched into the farthest output waveguide. This worst case
connection will have much higher loss then the best case
connection. To reduce the maximum insertion loss of the switch
array structure, asymmetric cross junctions may be used as
described in reference to FIG. 36. The loss of the worst case
connection will be best helped with every waveguide cross it
traverses being an asymmetric cross pointing in the direction of
propagation of the light along either the input or the output
waveguide. This structure is clearly not generalizable to the inner
waveguides because use of asymmetric switches in the intermediate
junctions will help some switching paths at the expense of others.
What is needed is an algorithm for selecting the optimal direction
for the asymmetric crosses. A good way to dispose the asymmetric
crosses is for roughly half of the crosses to point in each
direction. Observe that the n(n-1) crosses on the upper left of the
diagonal (but not including the diagonal) are predominantly used to
distribute energy to the right. These crosses therefore should
point along the direction of the input waveguides, while the
crosses on the lower right should point in the direction of the
output waveguides. In a bidirectional structure, the crosses on the
diagonal should be simple symmetric crosses, herein called the
simple diagonal arrangement of the asymmetric crosses. Other
arrangements may be used according to different usage patterns, but
this is a good general purpose arrangement.
A n.times.m (where n>m) arrangement will permit full
connectivity only between m "input" lines and m "output" lines.
Here, "input" and "output" are only used for identification
purposes since all lines are bidirectional. The additional n-m
"system" lines may be useful for system control in both monitoring
and broadcast functions. If line A wishes connection to line B, for
example, it would send system requests for that function until
answered. Line m+3, for example, might be dedicated to scanning all
the "input" lines for system requests. (To provide a similar line
to monitor the "output" lines, a larger matrix of lines is
required, such as the n.times.n matrix shown in FIG. 39A where m
lines are dedicated to users in a sub group of m.times.m lines. A
line such as line n-2 may then be used to monitor the "output"
lines.) In monitoring, the system will turn on successive gratings
corresponding to the "input" or "output" lines, and detect whether
the line is active. Some power will be switched into the monitor
detector by the successively switched-on gratings in line with the
monitor detector if any one of the monitored lines is active. An
active line will have an activated reflector connecting it to
another selected line. However, the activated reflector will leak
some power through to form a beam which can be detected by the
monitor detector. When the monitor detector connected to line m+3
in this example switches on the switch 1255 (drawn as a grating
switch for specificity) and receives the request from line A, the
control system will have to check whether line B is busy. When the
connection is made to line n-2 through switch 1253, the residual
beam which leaks past the line B connection switch will alert the
system that line B is active. If no activity is sensed, a system
request can be sent to both lines A and B (possibly through the
same monitor line if it has multiplexed send/receive capability, or
possibly through a separate system line), and the switch 1254 can
be closed to establish the connection.
The broadcast function is not feasible from lines within the basic
m.times.m switching block which is used for one-to-one connections,
because even partially turning on the required row of switches
corresponding to all the outputs from a given input would interfere
with some of the already established and potentially active
communications connections between other channels. Broadcast is
best accomplished from the system lines which are "outside" of the
m.times.m switching block illustrated in FIG. 39A. (The "inside
corner" of the geometry is the best case waveguide connection with
the lowest loss, between lines 1 on the "input" side and line 1 on
the "output" side.) Line C is shown to be actively connected to
most or all of the "output" lines in FIG. 39A by means of gratings
1256 as an example of broadcast. The switches 1256 on line C must
be only partially turned on so that sufficient power is delivered
to each "output" line. A similar protocol may be used to prevent
collisions between channels in the case of broadcast as in the case
of simple communications connection. Broadcast connections would
only be set up with inactive channels, and the system can group
channels together and/or wait for individual channels to permit
broadcast to them.
To increase switching efficiency, the waveguides may be large
multimode waveguides, which in the case of a single mode
communications network will be connected to the single mode input
and output ports 1 through m with adiabatic expanders described
elsewhere herein.
The entire structure described above in reference to FIG. 39A is
useful as an asynchronous transfer mode (ATM) switch, or in any
point-to-point switched communications application. One useful
variation of the structure is for multiple wavelength operation in
a WDM network. Wavelength selective optical switches can be
implemented as described herein by using poled grating switches, or
by using tunable fixed gratings which tune into and out of a
specified communications band. In a WDM network, the desire is to
switch a specific wavelength between channels without affecting
other wavelengths which may be travelling (bidirectionally) in the
same channel. With a tunable switch which can select a frequency of
reflection while essentially transmitting the other set of
frequencies in the WDM spectrum, the simple geometry of FIG. 39A is
appropriate. However, if a switched grating is used which has a
single frequency of operation, separate connection paths are
necessary for each wavelength.
FIG. 39B shows a switched WDM communications network 1260 with
separate paths for each frequency used in the network. This example
is for a two frequency WDM network, but may be generalized to any
number of frequencies of communication. Three "input" waveguides
1276 are shown in
FIG. 39B connected to three ports 1a, 2a, and 3a, and three output
waveguides 1276 are shown connected to three ports 1b, 2b, and 3b.
The waveguides form nine intersections. At each intersection, there
are three additional optical paths connecting each "input" and each
"output". The additional paths are identical in this example, and
consist of three types. The first type 1266 of optical path
consists of a pair of fixed frequency switched reflectors both
capable of reflecting the first one of the two signal frequency
bands of the WDM system. The reflectors are preferably gratings
transverse of the "input" and the "output" waveguides associated
with the intersection, and reflect power in the first frequency
band between the corresponding waveguide and an additional
waveguide segment connecting the two gratings. The second type 1268
of optical path consists of a second pair of fixed frequency
switched reflectors both capable of reflecting the second one of
the two signal frequency bands of the WDM system. Again, the
reflectors are preferably gratings placed transverse of the
respective waveguides and reflect power in the second frequency
band between the corresponding waveguides and an additional
waveguide segment connecting the second two gratings. The third
type 1270 of optical path consists of a pair of frequency
independent switched reflectors beth capable of reflecting beth
signal frequency bands of the WDM system. This third type of
optical path may be implemented as the pair of TIR reflectors
connected by waveguides and fixed mirror (described in reference to
FIG. 37).
In this case, ports 1a, 2a, 1b, and 2b plus the associated
waveguides 1276, 1277 form a 2.times.2 switching network capable of
switching two frequency channels simultaneously between any "input"
port and any "output" port. System control ports 3a and 3b with
associated waveguides 1276, 1277 provide monitoring and system
communication functions. If the first frequency of the WDM system
is desired to be switched between port 2a and 1b, for example, the
two switches associated with the optical path of type 1266 at the
intersection of the waveguides connecting to ports 2a and 1b are
turned on, routing optical power at the first frequency between
ports 2a and 1b through the waveguide connecting the two switches.
If all frequencies associated with a given port are to be routed
into another port, the switches and optical path of type 1270 are
turned on at the intersection corresponding to the two ports. The
optical paths 1270 are really superfluous in a 2.times.2 network
because to switch both WDM frequencies between any two channels,
both corresponding paths 1266 and 1268 may be activated. However,
in a high order communications network with many WDM frequencies, a
single all-frequency connection is desirable since it will have the
lowest loss.
FIG. 40 shows a two dimensional one-to-many routing structure. A
first row of waveguide routing switches connects optical power from
an input waveguide into columns of pixel waveguides. Again, no
details of the switches are shown; they are shown schematically
only as gratings, but may be implemented in several different ways.
A two dimensional array of "pixel" switches routes power out of the
pixel waveguides at "pixel locations". (What happens to this power
at the pixel locations depends on the application.) Two levels of
switching are used to reach all the pixels. This structure may be
used for display, to actuate or control processes or devices, or to
read certain types of dam. In the latter ease the direction of the
power flow is reversed, and the device operates as a many-to-one
routing structure.
An input optical beam 1342 propagates in an input waveguide 1352
and is coupled into one of many pixel waveguides 1354 by one of a
two dimensional array 1356 of switching elements. The switching
elements 1364 may be implemented as grating switches as described
above in reference to FIGS. 7, 8, 12, 13, and 38, or they may be
TIR switches as described in reference to FIGS. 30-32 and 37, or
they may be any other switchable element. The beam 1344 is shown
being switched by switch element 1358 into a pixel waveguide
whereupon it is switched for a second time by switch element 1360,
forming beam 1346 which propagates into the pixel element 1362. The
pixel elements 1366 may be separated from the waveguides 1354 by
waveguide segments as shown, or they may abut the waveguides at a
short distance so that little of the switched light passes by the
pixel elements.
In the case of the display application, the pixel elements may be
for producing emission of the light 1346 out of the plane of the
substrate 1348. The pixel elements may then be roughened patches on
the surface of the substrate 1348, or angled micromirrors, or
roughened angled micromirrors for light diffusion, or
phosphor-filled pits, or other means of producing visible light. In
the case of the display, the input beam 1342 may contain several
colors, in which case the waveguides are capable of guiding all of
the colors and the switches are capable of coupling all of the
colors. The waveguide switches are scanned in a sequence to produce
the image of the display. A grating switch is implemented as a
multiple period grating, but the TIR switch needs little
modification for this purpose. The waveguides, if single mode, must
effectively guide the shortest wavelength beam. The input beam 1342
is preferably modulated externally (including all its color
components) so that the switching elements are simple on-off
devices. Note that a single row electrode may be disposed across
the columns of waveguides to actuate a row of pixel switches if the
pixel elements are arranged in a more-or-less straight line and are
connectable electrically along a row.
In the case of a projection display, a additional lens structure is
required to collect the light emitted by all the pixels in the
array and refocus them on a screen at a (large) distance from the
lens. The lens should preferably have a good off axis performance
so that the focal plane is reasonably flat at the screen, and it
should have a large enough numerical aperture (NA) to collect most
of the light emitted by the pixel array. It would be advantageous
to couple a lens array to the pixel structure to reduce the
divergence of the beams produced by the individual lenses, reducing
the (costly) NA requirement on the projection lens. Another way to
achieve this is again to taper the waveguides to the largest
possible size at the pixel. It is relatively easy to taper the
pixels to a large transverse size, but difficult to obtain a very
deep waveguide. Large pixels may be made by coupling a wide
waveguide with a long grating coupler.
The light distributed in the routing structure may also be used to
activate processes, as for example in the case of a DNA reader or
an allergy reader, or a protein reader. In each of these specific
cases, a separate array of DNA or allergens or proteins is prepared
with fluorescent tags which can be excited by the light. One type
of molecule or one preparation of molecules may be arranged for
excitation over each pixel. The light is scanned electronically
among the different pixels, and the speed and order of the scanning
may be determined according to the results. The fluorescence may be
collected for detection by an external lens and detector. However,
for some applications, it is advantageous for the pixel (and its
lens) and waveguide structure itself to collect and guide the
emitted radiation to an optical energy detection means as well as
to control the emission of the source light. Depending on the
desired light illumination and collection geometry, the lens may be
a collimating lens, a refocussing lens, or even, conceivably a lens
to produce a diverging beam. A collimating lens is separated from
the end of the waveguide by the focal length of the lens so that
the transmitted (and collected) beam is essentially parallel.
Collimating lenses are most useful if there is a large volume of
material to be traversed by the interrogating light beam. A
refocussing lens is separated from the end of the waveguide by the
object distance, the inverse of which is related to the difference
between the inverse of the image distance and the inverse of the
focal length, where the image distance is the distance from the
lens to the desired image beam spot. The refocussing lens is used
if it is desired to concentrate the sample into a small spot and to
illuminate and/or read it from a waveguide. A diverging beam is
created by a lens separated by less than its focal length from the
end of the waveguide. The output beam from a simple lens is not
necessarily round if the divergences of the wave approaching the
lens are different in the two planes. The simplest way to make a
beam round (for minimum spot area after refocussing) is to start
with a round beam at the end of the waveguide, which may be
accomplished by design in the waveguide, or by tapering the
waveguide. The lens preferably has the appropriate numerical
aperture to admit the entire wave from the waveguide and focus it
to a diffraction limited spot or collimated beam according to the
application.
The pixel element 1362 may be any of the elements mentioned above
in this case, and it may be associated directly with the material
to be activated, or indirectly as by alignment with an external
plate to which the material has been conjugated. Each pixel element
may contain a lens aligned as described above so that a switch
array may be coupled with a lens array with the image beam spots in
a substantially common plane of focus. (Substantially common, in
this case, means within a Rayleigh range or so of the true plane of
focus, which may be quite distorted due to aberration. Use of a
type of reflector instead of a diffuser in the pixel element 1362
is preferred if the routing structure is also used to detect the
fluorescent emission: the reflector couples the emission back into
the waveguide whence it came. This coupling is maintained for as
long as the switches for a given pixel are activated. If desired,
the light source may be switched off prior to switching to another
pixel element in order to resolve the decay of the emission.
Used as a data reader, the sense of the light propagation is
reversed from that illustrated in FIG. 40. Light from a device
containing data is collected at the pixel elements and coupled into
the routing waveguide structure which guides it back out the input
waveguide 1352. Connected to the waveguide 1352 is a detector to
read the data. The detector may be simultaneously connected to the
waveguide via a beamsplitter between the waveguide 1352 and the
light source used for illumination of the data media. The pixel
elements 1366 (or simply "pixels") are preferably coupled with the
data spots via lenses to collect the light routed through the
structure 1350 and direct it to the data medium. The lens coupling
also serves for collecting reflected or otherwise emitted light
from the data medium and refocussing it on the end of the waveguide
coupled to the pixel element. The data may be in a target volume,
in which case the lens may be configured to collimate the light
beam 1346. The data may be on a target surface, in which case the
different pixel elements may correspond to different tracks on the
rotating disk of a magneto-optical data storage surface, for
example, or of a CD. The lens is configured to refocus the light
from the pixel to the data spot in a diffraction limited way. By
associating the different pixels with different tracks,
track-to-track switching may be accomplished electronically with
essentially no delay time.
The different pixels may also be coupled to different planes on the
data medium. This is useful for reading data which have been
recorded in multiple planes on the medium, to increase total
storage capacity. Switching between the planes may also be
accomplished electronically by switching among pixels coupled to
the different planes.
In addition, several different pixel elements may be focussed to
locations separated by a fraction of the track separation
transverse of (preferably normal to) a given track. When the track
wanders, positive tracking may be accomplished electronically by
switching between pixels, instead of mechanically. A sensor and
electronics is needed to detect track wander, and a controller for
switching to the desired pixels. The signal strength or the signal
to noise ratio (SNR) may be detected in the different channels to
determine the preferred (best aligned) channel. If the switches
along the waveguide 1352 are configured as 4-way crosses instead of
3-way, with the fourth leg emerging at the edge of the substrate, a
detector array 1368 may be placed in registration with the fourth
legs, with individual detectors 1367 individually aligned with the
columns for detecting the return power from each column. The
optimal reflectivity for the gratings which lie along the waveguide
1352 is approximately 50% if the detectors 1367 are used, in order
to maximize the return power from the data media on the detector
array 1368. If a single beamsplitter is disposed in the waveguide
1352 upstream of the router structure, its optimal reflection is
also 50%.
Note that partial excitation of the different pixels can be
achieved by partial excitation of the switches along either the
input waveguide or the pixel waveguides. The switching elements
1364 can be adjusted by means of the applied electric field to vary
their reflection coefficient. Some of the beam may be transmitted
through the desired partially-excited switches for use in a second
pixel simultaneously. Multiple pixel excitation is of particular
interest in the case of track wander correction, since multiple
detectors may also be configured in the router 1350. For example,
if three different pixels on three different columns of the routing
structure 1350 are to be simultaneously excited their corresponding
pixel column switches will need to be partially excited. A
computation is required of the controller to determine the
appropriate excitation of the multiple switches. Neglecting losses
at the switches, to produce equal intensities on their respective
detectors for optimal SNR, the first switch corresponding to the
first pixel column should be excited to reflect about 3/16 of the
incident light, the second switch corresponding to the second pixel
column should be excited to reflect about 1/4 of the remaining
light which has passed through the first switch, and the last
switch corresponding to the third pixel column should be excited to
reflect about 1/2 of the remaining light which has passed through
the previous two switches. About 15% of the incident beam is
reflected into each detector, assuming 100% reflection from the
medium and 100% light collection efficiencies. This result is quite
good compared with the optimal 25% of the beam which is received on
a single detector in the case of a single pixel (optimum switch
excitation=50% reflectivity). Indeed, more total photons are
collected with three beams than with only one. Electronic tracking
will result in cheaper, faster, and more reliable data read/write
devices.
Any combination of these approaches (electronic track switching,
electronic data plane switching, and electronic tracking) may be
taken to increase the performance of a data storage device. A means
is also needed to accomplish variable focussing electronically,
potentially removing all mechanical motion (except for rotation of
the media) from the drive. As described below in reference to FIG.
54, electronically variable focussing may be accomplished with a
zone-plate lens by changing the wavelength of the light beam
1342.
As drawn, the routing structure of FIG. 40 is a polarizing
structure, with the 90.degree. grating switches reflecting only the
TM mode. As a result, only beamsplitting based on intensity can be
used. It would be quite advantageous to be able to use polarizing
beamsplitters because this would result in a factor of four
increase in the signal strength for a given light intensity.
However, a switching structure capable of transporting and then
separating the two polarizations is required. Although the
polarization dependence of the TIR switches may be made negligible
at a sufficiently grazing TIR angle (well below the angle for total
internal reflection for the TE mode), there is a packing density
penalty in using very low angle switching geometries.
FIG. 41 shows a linear array of strongly polarization dependent
switches arranged as a data reader 1370. The switches are excited
with a beam 1342 which is TM polarized and highly reflected in the
activated switch 1372. Waveguides 1376 and 1378 such as titanium
indiffused waveguides in lithium niobate are used which guide both
polarizations. The pixel elements are implemented as micromirrors
1374 combined with integrated lenses 1380 and data spots e.g. 1382
arranged in tracks 1384 on a disk 1386 rotating about the axis
1388. The orthogonally polarized light which is reflected from
birefringent data spots (or separators) on the data track is
collected by the lens 1380, refocussed back to the waveguide 1378,
and reflected by the micromirror back into the plane of the guides
with TE polarization. Because the TE mode is both polarized at
Brewster's angle for the grating and has different propagation
constant not phase matched for reflection, it propagates through
the switch without reflection into the detector 1367 of the
detector array 1368. (Alternately, if the switch is a TIR switch,
the reflectivity is much less for the TE wave than the TM wave, and
a large portion of the TE wave transmits through the switch an
impinges on the detector.) If another switch 1373 is actuated
instead of the switch 1372, the beam will propagate to a different
pixel 1375 and be focussed according to the alignment of the pixel
1375 and its microlens 1381 either into another data track, or to
another data plane, or to the same track but with a transverse
deviation of a fraction of a track width (according to whether the
pixel 1375 is for track switching, data plane switching, or
tracking control).
Many variations are apparent on the structures described in
reference to FIGS. 40 and 41, such as that any of the switches in
the router may be oriented differently to change directions of
optical propagation in the plane, that multiple types of switches
may be used in a single device, and that higher levels of switching
may be added. Additional variations are too numerous to
mention.
FIG. 42 shows a switchable integrated spectrum analyzer 930. The
input beam 921 enters the input waveguide 923 which stops after a
certain distance. The input beam 921 may be propagating in another
waveguide or it may be a free space beam which is preferably
aligned and mode matched to optimize the power into the waveguide
923. The device 930 is provided with a planar waveguide 835 which
constrains propagation within the plane. The light beam 927
emerging from the end of the input waveguide diverges in one plane
within the planar waveguide until it passes through the integrated
lens element 925. The integrated lens has an elevated index of
refraction relative to the planar waveguide within a boundary
defining an optical thickness that reduces approximately
quadratically from the optical axis. (Or if it has a depressed
index, the optical thickness increases approximately
quadratically.) The lens may be fabricated by masked indiffusion or
ion exchange, or it may be a reverse poled segment excited by
electrodes.
The lens 925 collimates the light beam which then passes to at
least one of three grating sections 929, 931, and 933. The gratings
are formed from individual cells, each cell being a domain, the
domains being distinguished from the background material and
separated by varying amounts according to the application. The
cells have a permanent or adjustable index of refraction difference
from the substrate, and different cells may be of different domain
types. The permanent domain types include, for example, indiffused
regions, ion exchanged regions, etched regions, radiation bombarded
regions, and in general, regions formed by any type of index of
refraction modifying process. The grating sections may be
fabricated by etching, ion exchange, or indiffusion, in which case
the gratings are permanent, but they are shown in the preferred
embodiment fabricated from poled domains. Electrodes 932, 934, and
936 are used to individually excite the gratings in combination
with the common electrode 938. The common electrode 938 may be
placed on the opposite side of the substrate as shown for
simplicity, or surrounding the electrodes 932, 934, and 936 for low
voltage excitation.
The cells in an individual grating may be arranged in alternate
ways to form the desired periodicity in the desired direction to
supply virtual photons with the required momenta. They may be
arranged in rows to define certain planes with a virtual photon
momentum normal to the planes with momentum defined by the spacing
of the rows. In this case, there will also be virtual photons with
momentum along the planes with momentum defined by the spacing of
the cells in the rows. To phasematch retroreflection, the momentum
of the virtual photon is exactly twice the momentum of the incident
photons, and is directed in the opposite direction. Any other
reflection process has a smaller momentum and is directed
transverse of the incident axis. The period .LAMBDA. of the row
spacing is therefore fractionally related to the incident
wavelength .lambda. in that .LAMBDA. is a fraction of the quantity
.lambda./2n.sub.eff. In a general case, the cells may be separated
by a distribution of distances which varies with position through
the grating so that the virtual photon momentum along any axis of
incidence is determined by the spatial frequency spectrum
(determined through the Fourier transform) of the cell distribution
along that axis.
At least one of the gratings 929, 931, or 933 is turned on by
adjusting the potential state of the corresponding electrode. In
FIG. 42, grating 929 is shown activated. The activated grating
contributes virtual photons to the incident photons, phase matching
the scattering process into an output direction forming a plurality
of output beams 935 and 937 with different wavelengths, the output
beam being separated in angle according to their wavelength. The
output beams from the activated grating 929 pass through the lens
939 which refocusses the output beams onto a detector array 941.
The detector array is a group of sensors disposed to receive a
portion of the output beams for detection, and are preferably
bonded to an edge of the device 930 as shown. However, if it is
desired to integrate the device 930 onto a larger substrate, it may
not be desirable to have an edge of the substrate in this location.
In this case, other beam extraction methods (such as vertical
deflecting mirrors) can be used to deflect a portion of the beams
935 and 937 into the detector array. The sensing means is placed
approximately within about one Rayleigh range of the focal plane of
the output lens 939. In this position, the input beam angles are
mapped into output beam positions. Since the gratings map input
wavelength into output beam angles, a collimated input beam results
in different input wavelengths being mapped into different
positions in the focal plane, with spatial resolution of the
wavelength spectrum depending on the characteristics of the
grating. The detected power as a function of the location of the
detector in the array 941 is related to the frequency power
spectrum of the input beam 921. The device 930 is therefore a
spectrum analyzer. It is also a multichannel detector if the input
beam is divided into channels occupying several displaced frequency
channels, and the device is configured to disperse the channels
into predetermined detectors or groups of detectors.
By switching on different gratings, the device can be reconfigured
to function in different frequency ranges. For example, if grating
931 or 933 is activated, the dispersed light is focussed by lens
939 onto either a different detector array 943 or a different
portion of an extended detector array 941. The frequency range of
the gratings is determined by the angle of the grating to the beam,
and the periodicities of the grating. Grating 931 is shown to have
a shallower angle to the beam so that a higher optical frequency
range is selected when it is activated. Grating 933 has multiple
periodicities transverse to each other so that multiple overlapping
frequency ranges can be selected. Multiple frequencies may be
mapped into poled region boundaries as described above in reference
to FIG. 18. The poled elements of the grating 933 may be arranged
generally in planes oriented normal to the two principle virtual
photon momentum directions. The phasing of the planes is determined
by the process for transcribing the component frequencies of the
desired grating into domain boundaries. However, the general
grating may have momentum components in all directions, in which
case the resulting domain boundaries may not organize into planes
except possibly in a principal direction.
A transmitted beam 913 is refocussed by integrated lens 907 into an
output waveguide segment 909 to form the output beam 911 which
contains at least a portion of the out of band portions of the
input beam 921 which did not interact with the gratings.
A useful variation of the switched range spectrum analyzer combines
elements of FIGS. 42 and 30-35. The basic idea stems from the fact
that the spectral range of a grating can be shifted by changing its
angle, or equivalently the source point. In this variation, a
waveguide routing structure is used to allow the source point to be
switched. Waveguide switches are placed on the input waveguide 923
(and possibly on the emanating waveguides) at one or more
locations, producing an array of parallel source waveguides among
which the input light beam 921 is switchable. The waveguides all
end in the same plane, preferably the focal plane of the input lens
925. The remainder of the spectrum analyzer remains the same,
although with multiple inputs it may not be necessary to have the
additional gratings 931 and 933. The separation of the multiple
switched input waveguides is adjusted according to the application
to achieve the desired switchable spectral ranges for the analyzer
930.
FIG. 43 shows a poled acoustic multilayer interferometric structure
953. The incident acoustic wave 972 may be a bulk or a surface
acoustic wave. A poled structure is fabricated in the region 955 of
a piezoelectric substrate 965, containing two types of domains 963
and 964. It is known (e.g. U.S. Pat. No. 4,410,823 Miller et al.)
that polarity reversals result in partial acoustic wave reflection.
The reflection into beam 973 and the transmission into beam 961 is
affected by the spacing of the interfaces between the poled
regions. If high reflection and low transmission is desired,
adjacent interfaces should be spaced by a distance equal to an
integral multiple of half an acoustic wavelength. If high
transmission is required through a structure, with low reflection,
the spacing should be equal to a quarter of an acoustic wavelength
plus any integral multiple of half a wavelength. By applying an
appropriate number of poled regions near an interface where the
acoustic impedance changes, an antireflection (AR) structure can be
fabricated provided that the phases of the reflected waves are
chosen to be out of phase with and the same amplitude as the
reflected wave from the interface.
FIG. 44 shows a poled bulk acoustic transducer 971. An input
acoustic beam 972 is incident on a poled region of a piezoelectric
substrate 965 containing a pair of electrodes 974 and 975. The
poled region contains two types of domains 963 and 964 which are
optimally reversed domains. The electric field induced by the
acoustic wave in each of the poled regions can be selected to be
identical by reversing the poling direction every half acoustic
wavelength. In this case, a single electrode may be used to pick up
the induced voltage instead of the prior art interdigitated
electrodes. The electrodes 974 and 975 are used to detect the
presence of the input wave 972. The output voltage, tapped by
conductors 979 and seen in the electronic controller 978, varies
sinusoidally (for a narrowband input) as a function of time with an
amplitude related to the amplitude of the acoustic wave. As
discussed above, if the poled interface spacing is a half
wavelength, the structure also acts as a high reflector, which may
not be desirable in a given implementation. This characteristic may
be eliminated by spacing the interfaces alternately at one quarter
wavelength and three quarters of a wavelength as shown in FIG. 44.
In this case, the structure is an antireflection coating,
eliminating the undesired reflection. Since almost the entire
acoustic wave penetrates into the poled structure, where its energy
can be almost totally absorbed into the detection electronics, this
structure 971 is an efficient tuned detector of acoustic energy.
The bandwidth of the structure is inversely related to the number
of acoustic periods that fit within the poled structure covered by
the electrodes. The efficiency is related to the acoustic path
length under the electrodes. The bandwidth and the efficiency of
the detector are therefore related, and can be adjusted by changing
the size of the detection region.
The structure 971 can also be used as an acoustic generator,
essentially by running the process in reverse. A time dependent
electrical signal is applied between the two electrodes at the
frequency of the acoustic wave it is desired to excite. The
piezoelectric coefficient of the substrate produces a periodic
strain at the frequency of the acoustic wave, and a pair of waves
are generated, one 961 propagating in the forward direction and one
973 in the reverse direction. A high efficiency unidirectional
generator can be made if it is desired to generate only a single
wave, by combining the devices 953 and 971, with 953 being
configured as a total reflector for the undesired wave. If the
total reflector is oriented at 90.degree. to the undesired wave and
the phase of the reflected wave is chosen to be in phase with the
desired wave, the two waves will emerge in a single direction as
essentially a single wave.
A variation of the structure of FIG. 44 is a strain-actuated
optical interaction device. In this device, the poled regions 964
and 963 are actuated by a strain field, producing a change in the
index of refraction through the photoelastic effect. Now the
structure 975 is a strain-inducing pad which may be deposited onto
the substrate material 965 at an elevated temperature so that the
different coefficients of thermal expansion of the film and the
substrate create a strain field at room temperature. The mechanical
strain field, working through the photoelastic tensor, produces
index changes in the substrate which change from domain to domain,
again producing a substrate with patterned index of refraction
which can be used as described elsewhere herein. Electric fields
using the electro-optic effect can be combined with the
photoelastic effect provided that the deposition process of the
electrodes do not undesirably affect the desired strain field.
The structure 890 of FIG. 45 is a tuned coherent detector of pairs
of light waves. It is tuned in the sense that it will only sense
frequency differences between light waves within a certain
bandwidth about a desired central "resonant" frequency difference.
In the simplest case, the device is configured with equal
separations between interdigitated electrodes 885 and 886 which
form a periodic structure with period .LAMBDA.. At a given instant,
the two input frequencies present in the input beam 887 produce an
interference pattern of electric fields within the waveguide 888
with a spatial period which depends on the optical frequency
difference and the index of refraction of the substrate 889 at the
optical frequency. At a frequency difference where the spatial
period of the interference pattern equals the period .LAMBDA., the
electrode structure is on resonance, and the electrodes will be
excited to a potential difference due to the induced displacement
charge at the top of the waveguide.
The frequency response characteristic is related to a sinc.sup.2
function with resonant frequency determined by the optical
frequency difference at which two optical waves slip phase by 2.pi.
in a poled grating period. The buffer layer 891 is required to
minimize the loss to the propagating optical waves when the
electrode structure is laid down. It does not substantially reduce
the strength of the induced potential if its thickness is much
smaller than the period .LAMBDA.. The interference pattern has a
low frequency component which oscillates at the frequency
difference between the two light waves. The electronic signal which
is picked up by the electronic controller 978 via leads 979
therefore also oscillates at the difference frequency. The
amplitude of the electronic signal is large at the resonance
difference frequency, and falls off at other difference frequencies
according to the bandwidth of the device, which is related to the
inverse of the number of beat periods contained within the
interdigitated electrode structure.
The interdigitated electrodes may alternately be configured with
multiple frequency components so that there are several resonant
frequencies, or so that the bandwidth of the response is modified.
Note also that the device may be sensitive to multiple orders. If
the electrodes are narrow compared to a half period, there will be
a significant response at the odd harmonics of the resonant
difference frequency. By shifting the fingers relative to each
other so that there is asymmetry along the axis of the waveguide, a
responsivity can be created to the even harmonics. This higher
order response can only be improved at the expense of lowering the
first order response. It can be minimized by centering the
electrodes relative to each other, and by increasing their width.
Finally, the waveguide 888 is not strictly necessary. It may be
omitted, but the detected waves should be brought very close to the
electrodes to optimize the signal pickup.
FIG. 46 shows a low loss switchable waveguide splitter 780. This
device has a permanent wye waveguide splitter 774 consisting of an
input waveguide segment widening into a wye junction and branching
into two output waveguide segments 775 and 776 which are both
optical path possibilities for light incident in the input segment.
The widths and index profiles of the input and output segments are
preferably equal. The splitter 780 also has a poled structure 778
which has an electro-optic coefficient within the region of the wye
splitter 774. The poled region 778 may be a thin layer near the top
of the substrate, which may have multiple layers, or it may extend
throughout the substrate. The remainder of the substrate may be
poled or unpoled. A pair of planar electrodes 777 and 779 are
disposed adjacent to each other over the waveguides, with one
electrode 777 covering a portion of one output waveguide 775, and
the other electrode 779 covering a portion of the other output
waveguide 776. The electrodes are planar only to the extent that
this optimizes fabrication convenience and function: if the surface
they are applied to is flat or curved, they conform. The edge 781
of the electrode 777 crosses the waveguide 775 at a very shallow
angle, and forms a smooth continuation of the inside edge of the
waveguide 776 at the wye junction. Likewise, the edge 783 of the
electrode 779 crosses the waveguide 776 at a very shallow angle,
and forms a smooth continuation of the inside edge of the waveguide
775 at the wye junction. When the electrodes are excited relative
to each other with one polarity, the index of refraction under the
electrode 777 is depressed and the index under the electrode 779 is
increased. As a result, an excited region under the electrode edge
781 forms a waveguide boundary, steering the input beam 789 almost
entirely into the output beam 784 with very little power leakage
into the alternate output beam 782. The increased index under the
electrode 779 aids in steering the optical energy away from the
boundary 781. When the opposite polarity is applied between the
electrodes, the input beam is steered almost entirely into the
other output beam 782. If no voltage is applied, the input power is
evenly divided into the two output ports if the structure is made
symmetric. This structure is therefore a 3 dB splitter which can be
electrically switched as a beam director into one of two directions
with low loss.
The electrodes 777 and 779 are tapered away from the wye structure
774 at the input to the structure forming a gradual approach of the
lower index region towards the waveguide to minimize optical
losses. The smoothing effect of the electrostatic field
distribution produces a very smooth index of refraction transition
under both electrodes. The edge of the electrodes which crosses the
output waveguides far from the wye branching region is preferably
arranged at 90.degree. to the waveguide to minimize losses.
The wye splitter may be arranged in an symmetric way to produce a
splitting ratio different from 3 dB with the fields off. This can
be done by increasing the deviation angle for one of the waveguides
and/or decreasing the angle for the other. The switching function
operates almost as well with an asymmetric structure as with a
symmetric structure, provided that a sufficiently large electric
field is applied with the electrodes. The extinction ratio (the
ratio between the power in the switched-on waveguide and the power
in the switched-off waveguide) can remain very large despite a
large asymmetry. However, the optical losses will be somewhat
different in the two legs of an asymmetric switchable waveguide
splitter. The device 780 may, therefore, be configured as a
splitter with any desired splitting ratio, and still be switched
with good efficiency and high extinction ratio.
This device may be cascaded to allow switching among more than two
output waveguides. If, for instance, the output waveguide 775 is
connected to the input of a second device similar to 780, its power
may be passively or actively switched into an additional pair of
waveguides. Sixteen switched output lines may be accomplished with
four sets of one, two, four, and eight switches similar to 780. The
power division ratio among these lines may be configured to be
equal in the unswitched state, or any other power division ratio.
When the switches are activated, a single output waveguide may be
turned on, a single output waveguide may be turned off, or any
combination of output waveguides may be turned on and off.
The direction of propagation of the light in the device may be
reversed. In this case, an input on either one of the output ports
775 and 776 can be switched to emerge from the input port. In the
absence of an applied voltage, the power at each output port is
coupled into the input port with a given attenuation (3 dB in the
case of a symmetric device). When the field is switched on, power
in the "on" waveguide is connected into the input port with very
low loss, while the power in the "off" waveguide is very
effectively diffracted away from the input waveguide. The "off"
waveguide is essentially isolated from the input port.
Alternatively, a mirror image device may be connected back-to-back
with the switch 780 so that the input waveguides join together,
forming a 2.times.2 switch or router. An input on either pair of
waveguide ports may be switched into either waveguide of the other
port pair. Again, cascading is possible, producing an n.times.n
switch/router.
FIG. 47 shows an alternative realization 790 of a switchable
waveguide splitter using multiple poled regions. In this
configuration, the switched index difference along the boundaries
of the waveguides in the wye region is enhanced, thereby confining
better the optical mode into a narrower region, and reducing the
residual coupling into the switched-off output waveguide. Two poled
regions 785 and 786 are disposed on each side of the input
waveguide 774 along the wye splitting region. The poled regions
have boundaries 787 and 788 which cross the output waveguides 775
and 776 at a very shallow angle, and which form a smooth
continuation of the inside edges of the waveguides 776 and 775 at
the wye junction. The boundaries of the poled regions taper slowly
away from the input waveguide to allow a slow onset of the
electrically excited index change, and they cross the output
waveguides at a large distance from the wye junction where the
electric field is substantially reduced, in order to reduce the
optical loss. Electrodes 791 and 792 are disposed substantially
over the poled regions 785 and 786.
A potential difference is applied to the electrodes, exciting an
electric field in an electrostatic pattern throughout the volume
between and around them. The electric field penetrates the poled
regions and the surrounding regions, inducing a corresponding
pattern of optical index changes. The local optical index change is
related to the product of the local electric field direction and
the local electrooptic coefficient. The poled regions are
preferably surrounded by regions of opposite polarity, in which
case their electro-optic coefficient is of opposite sign to that of
the surrounding regions. At the interfaces 787 and 788 there is a
sharp change in the index of refraction. On one side of the
waveguide, the index is reduced at the interface, producing a
guiding tendency away from the low index region. The opposite is
true of the other side. If the applied electric field is large
enough, the interface with the reduced index forms a waveguide
boundary. Since the guiding interface connects smoothly as an
extension of the inside boundary of the output waveguide across
from the poled region, the input light beam 789 is guided into that
output waveguide. The light leak is low into the switched-off
waveguide if the curvature of the guiding boundary is gradual.
There is low loss at the input, because the poled regions approach
the waveguide slowly. There is low loss at the wye junction,
because the portions of the poled regions which extend beyond the
junction depress the guiding effect of the switched-off output
waveguide, and enhance the guiding of the switched-on output
waveguide.
As an alternative, the poled regions could be surrounded by unpoled
material. There is still an abrupt change in the index at the
interfaces 787 and 788 so the device still functions, but the index
change is only half the value obtained when the poled regions are
surrounded with reverse poled material, so the applied field must
be higher. The alternatives described before also apply to this
device.
FIG. 48 shows the key design elements of a 1.times.3 switch. The
design elements illustrated here show how to transform the device
780 of FIG. 46 into a 1.times.3 switch with a single poled region
and patterned electrodes. The device contains a permanent branching
waveguide with the desired number n (n=three) of output branches.
The waveguide passes through a poled region which extends deeper
than the waveguides (for good extinction ratio) and significantly
beyond the junction region where the waveguides have become
separated by a large amount (such as three times their width).
Several zones are defined by the waveguide boundaries, by their
smooth extensions back into the boundaries of the input waveguide,
and by normal boundaries across the output waveguides at a distance
significantly beyond the junction region. There are (n.sup.2
+2n-2)/2 zones so defined. It is useful to extend the outermost
zone beyond the outside of the outermost waveguide as shown to
taper the input. A separate electrode is placed over each of the
regions with a small gap between all electrodes, but sufficient gap
to avoid electrical breakdown when excited.
To operate the device, electric fields are independently applied to
the zones with polarity determined by whether or not the
corresponding zone is confined within the desired waveguide. For
example, the five zones of FIG. 48 are excited according to Table
1. As before, the magnitude of the electric field is adjusted to
produce a good guiding boundary along the edges of adjacent zones
excited at different polarities.
TABLE I Electrode Number Top Middle Bottom 1 + - - 2 + + - 3 - + -
4 - + + 5 - - +
Alternatively, the design elements of FIG. 48 also show how to
transform the device 790 of FIG. 47 into a 1.times.3 switch with
multiple poled regions. The device again contains a permanent
branching waveguide with the desired number n (n=three) of output
branches. Again, several zones are defined by the waveguide
boundaries, by their smooth extensions back into the boundaries of
the input waveguide, and by boundaries which cross the output
waveguides at a distance significantly beyond the junction region.
Again, it is useful to extend the outermost zone beyond the outside
of the outermost waveguide as shown, in order to taper the input.
Each zone is poled in the opposite direction to neighboring zones
with a common zone boundary. Zones with the same poling direction
may share at most a vertex. Preferably, the input waveguide region
is poled oppositely to the innermost zones (i.e. the zones closest
to the input waveguide). In FIG. 48 the innermost zones are
labelled zones 2 and 4. This zone-based polarity selection
procedure results in only zones 2 and 4 being reverse poled, while
zones 1, 3, and 5, which are the output waveguide zones, are poled
positive (in the same direction as the surrounding region, if the
surrounding region is poled). If four output waveguides are used,
there are nine zones, six of which are reverse poled, including all
of the output waveguide zones. The splitter implementations which
have an even number of output waveguides, therefore, have some
advantage because only the even splitters have their output
waveguide zones poled opposite to a potential substrate poling,
with the attendant advantage of increased confinement at the final
division point and higher transmission for the "on" states and
better reverse isolation in the "off" states. A separate electrode
is placed over each of the regions.
To operate the device, electric fields are independently applied to
the zones, but now the rule for the polarity is different. The
polarity is determined by two factors: whether or not the
corresponding zone is contained within the desired waveguide, and
the polarity of the poled region underneath. For example, if a
positive polarity applied to a positively poled region produces an
increase in the index of refraction, the following selection rules
are followed: if a zone is poled positive, the electrical
excitation polarity is selected to be positive if the zone is
inside the desired waveguide and negative if the zone is outside;
if a zone is reverse poled (negative), the polarity is selected to
be negative if the zone is inside the desired waveguide, and
positive if the zone is outside. In Table II are shown the optimal
poling direction of the zones for the n=3 case with three output
ports as shown in FIG. 48. The design of 1.times.n and n.times.n
switches is derived by induction from the descriptions of the FIGS.
46, 47 and 48.
TABLE II Zone Poling Direction Top Middle Bottom 1 - - + + 2 + + +
- 3 - + - + 4 + - + + 5 - + + -
The planar components described herein may be stacked into multiple
layer three dimensional structures containing electro-optically
controlled devices and waveguide components. Stacks or
three-dimensional constructions of planar waveguides and switches
are fabricated by alternately layering or depositing
electro-optically active, polable thin films, preferably polymers,
and buffer isolation layers, which may be either insulating or
electrically conducting. Advantages of stacked structures include
better crosstalk isolation due to more widely spaced waveguide
elements. Higher power handling capability is also achieved because
more optical power can be distributed among the layers. Individual
layers can be used if desired to distribute individual wavelengths
in a display device.
Once deposited on a suitable substrate, poling of the active
optical waveguide/switching layer is done using the techniques
heretofore described. A buffer layer of lower index is necessary to
isolate one active layer from adjacent layers, and is designed to
establish the desired guiding in the dimension normal to the plane.
Buffer layers of SiO.sub.2, for example, may be used. Next comes a
ground plane which can be fabricated from a metallic layer since it
is isolated from the optically active layers, followed by a thick
buffer layer. The buffer layers must also be capable of
withstanding the applied voltages between the different layers of
electrodes and ground planes. In polymers, a large area may be
poled, and desired regions selectively de-poled by UV irradiation
techniques as previously described in order to create waveguide
features, even after a transparent buffer layer, such as SiO.sub.2
has been applied. Or, poling can be performed electrically. With
polymers, de-poling one layer by UV irradiation will not affect the
layer behind it because of the shielding provided by the underlying
metallic ground plane. Metal electrodes and conductive paths may
then be laid down by standard masking and coating techniques,
followed by another insulating buffer layer, and the next active
layer. The buffer layer should be planarized to minimize the loss
in the subsequent active optical waveguide/switching layer. This
process of adding layers may be repeated as often as desired for a
given device.
A variation in fabrication technique for making activation paths
and electrodes for the poled device stacks is to coat the
electro-optic layer with an insulating layer that is subsequently
doped or infused to produce electrically conductive patterns within
the buffer layer using standard lithographic masking techniques.
Incorporating the electrodes into the buffer layer would serve to
minimize the thickness of the stacked device.
Hybridized devices consisting of different electro-optically active
materials could be used to ameliorate fabrication complexities. For
example, the first electro-optically active layer containing
waveguide devices could be fabricated in a LiNbO.sub.3 substrate,
which would also serve as the support substrate. Next a buffer
layer and a layer of electrodes for the lithium niobate devices are
deposited. Two insulating buffer layers sandwiching a conducting
plane would then be coated onto the device prior to depositing the
next active layer which could be a polable polymer. Subsequent
layers are built up, poled and patterned as described earlier. The
conducting planes in between buffer layers may serve both as
electrodes to permit area poling of each polymer layer and to
shield previous layers from the poling process.
Stacked waveguide arrays may be used, for example, as steering
devices for free space beam manipulation. Electrically activated
and individually addressable waveguide elements stacked closely
together, and aligned with a source array form a controllable
phased array for emitting optical radiation. The relative phases of
the beams can be adjusted by varying the voltages on the poled
zones as described previously. By adjusting these phases in a
linear ramp, the emitted light from an array of waveguides can be
swept in direction rapidly within the plane of the array. A linear
array of devices on a plane can therefore sweep within the plane
only. However, when poled waveguide array planes are vertically
integrated into a three dimensional bulk device, optical beams
emanating from the device may be directed in two dimensions.
An extension of this concept is the mode control of multimode laser
bar arrays using a stack of waveguide grating reflectors. The
waveguide stack is dimensionally matched to butt-couple to a laser
diode array. By controlling the phase of the individual elements,
the emission mode pattern of a multi element laser bar can be
controlled. In devices where single mode waveguide confinement is
not necessary, multimode or bulk arrays may also be stacked, for
example, to increase the power handling capacity of a switched
poled device.
FIG. 49 illustrates an embodiment of the phase array waveguide
stack section 1630 with only a single column of waveguides
illustrated for clarity. Optical radiation 1640 enters the stack
1630 through waveguides 1638 which have been fabricated in an
electro-optically active thin film 1650, such as a polable polymer.
Here the input beams 1640 are shown staggered to represent beams of
identical wavelength, but with different phases. Light travels
along the waveguides 1638 in which they encounter poled regions
1634 within which the index of refraction may be modified
electronically using the techniques described herein. Beams 1642
represent the output of the phased array after each light wave has
been individually phase adjusted to produce output component beams
that are aligned in phase.
Many other input and output wave scenarios are possible. For
instance, a single mode laser beam with a flat phase wavefront
could illuminate an area of waveguide elements, which would then
impose arbitrary phase delays across the spatial mode of the beam,
thereby allowing the beam to be electronically steered in free
space. Directional beam control devices using this method would be
much faster and more compact than current mechanical or A-O
devices. Using optical-to-electrical pickup devices described
herein or known in the art, phase differences or the presence of
multiple frequency components may be sensed within or external to
the stacked device in order to provide instantaneous information
for a feedback loop.
The device segment 1630 represented here is constructed on a
substrate 1632, such as SiO.sub.2, by alternately depositing
electrodes, buffer layers, and polable material in the following
manner. A broad area planar electrode 1654, composed of an opaque
metallic film or transparent conductive material such as
indium-tin-oxide, is deposited, and followed by an electrically
insulating buffer layer 1652, such as SiO.sub.2, which also serves
as the lower boundary layer for the waveguide 1638 fabricated in
the next layer of polable material 1650. On top of the polable
layer 1650, another buffer layer 1652 is added to form an upper
waveguide bound before depositing the patterned electrode 1636 used
to activate the poled structures. Another buffer layer 1652 is then
added, this time to electrically insulate the patterned electrode
from the next layer, another broad area planar electrode 1654. The
patterned electrode 1636 is separated from one planar electrode
only by a thick buffer layer, and from the other by buffer layers
and the polable material. Since it is desired to apply fields
across the polable material, the electrical separation across the
polable material should be less than the separation across the
buffer layer only. The layering sequence between broad area
electrodes is repeated until the last layer of polable material
1650, after which only a buffer layer 1652, patterned electrode
1636, and optional final insulating layer 1652 need be added to
complete the stack. Electrical leads 1646 and 1648 are brought into
contact with electrodes 1636 and 1654, respectively, through
integration and bonding techniques known to the art, and connected
to voltage distribution control unit 1644.
The voltage control unit 1644 serves a dual purpose: to activate
the poled devices individually, and to isolate each from the
electric field used to control neighboring layers of active
elements. The unit 1644 would be in essence a collection of coupled
floating power supplies in which the voltages between electrodes
1636 and 1654 sandwiching an active layer may be controlled without
changing the voltage differences across any other active layer.
Region 1634 depicts a poled region with one or more domains, and
electrode 1636 depicts an unbroken or a segmented or patterned
region with one or more isolated elements. Waveguide stack 1630 is
described as a device for phase control, but stacks of waveguide
structures may include any number of combinations of poled devices
described herein, in series optically, or otherwise configured.
FIG. 50 shows a prior art adjustable attenuator 1400. An input
waveguide 1402 traverses an electro-optically active region of a
substrate 1404. An input optical beam 1406 propagates along the
input waveguide into an output waveguide 1408, forming the output
optical beam 1410. Electrodes 1412, 1414, and 1416 are disposed
over the waveguide so that when electrode 1414 is excited at a
given polarity (positive or negative) with respect to the two
electrodes 1412 and 1416, there is an induced change in the index
of refraction within the segment 1418 region of the waveguide under
and adjacent to the electrodes due to the electro-optic effect. The
electrode configuration is somewhat arbitrary and may be different
and more complex than shown in the prior art represented by FIG.
50, but the common factor which all the patterns have in common is
that overall, they reduce the index of refraction in the core when
excited to a voltage, and increase the index of the surrounding
regions.
In the absence of applied electric field, the loss of the waveguide
segments is low, determined primarily by scattering on roughness
along the waveguide walls. However, when the electric field is
applied, the loss can be increased to a very large value. The three
electrode pattern allows a negative index change within the
waveguide at the same time as a positive index change occurs
outside the waveguide, substantially flattening and broadening the
index profile. When the field is applied, the modified section of
the waveguide 1418 under the electrodes has a much wider lowest
order mode profile from the input 1402 and output 1408 sections of
the waveguide. As a result, mode coupling loss occurs both when the
input beam 1416 transitions into the section 1418 and when the
light in section 1418 couples back into the output waveguide 1408.
If the index changes are large enough, the lowest order mode goes
below cutoff, and the light emerging from the end of the waveguide
1402 diffracts almost freely into the substrate, resulting in a
large coupling loss at the beginning of the waveguide 1408.
When a given mode enters the modified section 1418 of the
waveguide, the overlap between its intensity profile and any mode
profile of the modified section 1418 is reduced by the change in
the index profile of the modified segment. If the segment 1418 is
multimode, several propagating modes and radiation modes will be
excited. If it is single mode, many radiation modes will be
excited. The combination of these modes then propagates to the far
end of the segment 1418 and couples into the output waveguide
section 1408, where only a fraction of the light couples back into
a mode of the waveguide to form the output beam 1410. By
controlling the voltage applied to the electrodes, the loss in the
device 1400 can be adjusted from very low to very high.
The maximum loss which can be obtained depends on the magnitude of
the index change, the size of the excited regions, their length,
and on whether the input and output waveguides are single mode or
multimode. In a variation of the geometry, only two electrodes
might be disposed over the waveguide segment 1418, decreasing the
index within the waveguide segment and increasing the index to one
side instead of on both sides. The function is again as an
attenuator, but the rejected radiation fields will tend to leave
the device towards the side of the increased index. This ability to
direct the lost radiation might be of advantage in some systems
where control of the rejected light is desired. An absorber may
also be placed downstream of the segment 1418, on one or both
sides, to prevent the rejected light from interfering with other
functions elsewhere in the system.
FIG. 51 shows a poled switched attenuator 1420. This device is an
improvement on the prior art device of FIG. 50 in that poled
regions are used to increase the definition of the index change and
increase the index discontinuity, thereby increasing the amount of
attenuation which can be obtained in a single stage. Regions 1422
and 1424 are electro-optically poled in a reverse direction from
the surrounding material. (As an alternative, the surrounding
material may be unpoled, or have no electro-optic coefficient, or
it may simply be poled differently from the regions 1422 and 1424.)
The central electrode 1426 covers both poled regions and
surrounding material. It is excited relative to the electrodes 1428
and 1430 to produce a change in index of refraction in the poled
regions 1422, 1424, and the surrounding material. The device 1420
operates in a similar way as described above in reference to the
device 1400. The applied voltage reduces and broadens the index
profile of the waveguide segment 1418, reducing the coupling
between the mode of the output waveguide 1408 and the modes excited
in the segment 1418 by the input beam 1406. In this configuration,
the change in the index profile is abrupt at the beginning of the
modified waveguide region 1418, and therefore the loss is larger.
The number and shape of the poled segments 1422 and 1424 can be
varied so long as the mode coupling with the excited waveguide
segment 1418 is different from the mode coupling with the unexcited
segment. The device may be configured with high loss in the
electrically unexcited condition, adjusting to low loss in the
electrically excited condition. In this case the electrically
excited regions and/or the poled regions form a portion of the
structure of the waveguide segment 1418. The waveguide segment 1418
may itself may be configured in many different ways, most notably
if it is absent entirely without excitation, in which case the
device is similar to the switched waveguide modulator of FIG.
29A.
As described above, these devices may be cascaded, in this case to
increase the maximum attenuation.
The devices of FIG. 50 and FIG. 51 can also be operated as a
variable intensity localized ("point") light source. The light
propagating in waveguide 1402 is confined to follow the path of the
waveguide until a voltage is applied the electrode structure. When
the waveguiding effect is reduced or destroyed by changing the
index of refraction, part or all of the previously confined light
beam will now propagate according to free-space diffraction theory.
The diffracting beam will continue to propagate in the forward
direction while the beam area expands in two dimensions to be much
larger than the core of the waveguide 1408. At an appropriate
distance away from the electrode structure, the beam area can fill
a large fraction of the substrate aperture and appear to a viewer
as a point source of light emanating from a spatial location near
the electrode structure.
If desired, a one-dimensional localized source can also be
constructed with this technique. The waveguide segment 1418 in
FIGS. 50 and 51 can be embedded in a planar waveguide structure
fabricated using techniques known to the art, such that when an
appropriate voltage level is applied to the electrode structure,
the transverse confinement of the mode is destroyed while the
vertical confinement in the planar waveguide is not. Thus the beam
area would expand in one dimension, confining the light to a narrow
plane.
FIG. 52 shows a poled device 1500 with an angle broadened poled
grating. The method shown for broadening the bandwidth is an
alternative to the bandwidth modifying approaches described in
reference to FIG. 18 and elsewhere herein. A periodic structure
1500 is shown with poled regions 1502 which are preferably reverse
poled into a poled region of the substrate 1504. Other structures
such as waveguides and electrodes and additional gratings are
incorporated as desired. The domains 1502 cross the central axis of
propagation of the input beam 1508 with a pattern which may be
strictly periodic with a 50% duty cycle. The sides of the top
surfaces of the poled regions all align along lines drawn from an
alignment point 1506. The poled regions approximately reproduce
their surface shape some distance into the material. The result is
a poled structure with periodicity which changes linearly with the
transverse position in the poled substrate. An input beam 1508
which traverses the poled region may be a freely propagating
Gaussian beam (if the domains are deeply poled) or it may be
confined in a waveguide 1512. According to the function of the
grating, the input beam may be coupled into a filtered or frequency
converted output beam 1510, or into a retroreflected beam 1514. The
range of periodicities in the grating structure (and hence its
bandwidth) depends on the width of the beam and separation of the
point 1506 from the axis of the beam. By adjusting these
quantities, the bandwidth of the poled structure may be increased
substantially over the minimum value determined by the number of
first order periods which fit in the grating. There is a limit on
the maximum desirable angle for the poled boundaries, and therefore
the structure shown in FIG. 52 cannot be extended without limit.
However, a long interaction region can be obtained by cascading
several segments together. To maximize the coherence between the
segments, the periodicity of the domains along the central axis of
the beam should be unmodified at the joins between segments. There
will be at least one wedge shaped domain between segments.
Although increasing the bandwidth of the grating decreases the
interaction strength, it makes a device using that grating
significantly less sensitive to small frequency drifts. For
example, a frequency doubler device using an angle broadened
grating is more tolerant of temperature drifts. Another example
application is the channel dropping filter which tends to have
narrow bandwidth because of the strong gratings which must be used.
Use of an angle broadened grating enables a widened pass band to
accept high bandwidth communications signals. The angle broadened
grating can also be applied in the other grating configurations
discussed above.
There are alternatives for implementing the angle broadened grating
which do not follow the exact pattern described above. For example,
the relationship between the angle of the grating periods and
distance along the propagation axis might be more complex than
linear. A quadratic or exponential variation might be more
appropriate for some applications where the majority of the
interacting power exists at one end of the grating. The angle
broadening technique is also applicable to prior art types of
gratings such as indiffused, ion exchanged, and etched
gratings.
An alternative angle broadened device 1520 using a curved waveguide
is shown in FIG. 53. In this case, the poled regions 1522 have
parallel faces, and the angle of the faces are inclined only
relative to the local direction of propagation within the guide.
Again, the bandwidth is broadened by the different components of
the wave experiencing different Fourier components of the grating.
The curved waveguide has a higher loss than the straight waveguide,
but large curvatures are not required. Several sections as shown in
FIG. 53 may be concatenated, forming for example a sinuous
waveguide structure that waves back and forth around an essentially
straight line.
FIG. 54 shows a controllable poled lens 1530. Concentrically
arranged domains 1532, 1534, 1536, and 1538 are poled into an
electro-optic substrate 1540 with a reverse polarity from that of
the substrate. Transparent electrodes 1542 and 1544 are applied to
the two opposing surfaces of the device above and below the poled
regions. When an electric field is applied between the two
electrodes, the poled regions have their index of refraction either
increased or decreased according to the polarity. The geometry of
the poled regions is determined by the diffractive requirements of
focussing an optical wave of a given color. The separations between
the boundaries varies roughly quadratically with radius. If the
application requires focussing a plane wave to a round spot, for
example, the poled regions will be round (for equal focussing in
both planes), and separated by decreasing amounts as the diameter
of the poled region increases. The boundaries of the poled regions
are determined by the phase of a the desired outgoing wave relative
to the incoming wave at the surface of the lens structure. A poled
region boundary occurs every time the relative phase of the waves
changes by .pi.. For example, if the incoming wave is a plane wave
its phase is constant along the surface. If the outgoing wave is a
converging wave which will focus at a spot far from the surface, it
is essentially a spherical wave and the phase changes in that
spherical wave determine the boundaries. The lens 1530 is a phase
plate with adjustable phase delay according to the applied voltage,
and the domains occupy the Fresnel zones of the object.
To focus a plane wave of a given color, a voltage is applied which
is sufficient to phase retard (or advance) the plane wave by .pi..
Each different frequency has a different focal length defined by
the Fresnel zone structure of the poled lens 1530. Higher
frequencies have longer focal lengths. If it were not for
dispersion, every wavelength would optimally focus at the same
voltage. The voltage can be adjusted to compensate for the
dispersion in the substrate material 1540. If the voltage is
adjusted away from the optimal value, the amount of light which is
focussed to the spot is reduced because the phase of the light from
the different zones no longer add optimally. They will interfere
partially destructively, reducing the net intensity.
FIG. 55 shows a laser feedback device 1450. The laser source
consists of an amplifier region 1452, a rear reflector 1454, and a
low reflection output region 1456 which may be an antireflection
coated window, for example. While a conventional laser will have a
second high reflector, in this invention, the high reflector is
removed so that a grating feedback device can control the laser
oscillation. The reflection from the output region 1456 and the
coupler 1458 is low enough so that the laser does not lase without
additional feedback from an external source. The external feedback
source consists of an optical coupling system 1458 and a poled
material 1460 which reflects a beam from the optical amplifier when
excited by an electric field. Because the reflection spectrum of
the poled material 1460 may be very narrow in frequency space, it
may select a narrow region in which laser operation can occur about
the single frequency or frequencies which make up the grating
according to the distribution of grating periods. If the resonator
cavity is long enough so that the FSR is on the same order as the
width of the reflection spectrum, the combined device will
oscillate on a single longitudinal mode.
The means 1458 for coupling optical energy between the optical
amplifier and the material 1460 collects and refocuses the output
mode of the laser into the poled material. The coupler 1458 may
consist of many alternate realizations, including one or more of
the following components: high numerical aperture lenses, such as
GRIN (graded index), aspheric, diffractive, or multi-element
spherical lenses; tapered waveguides; proximity adjusters and
aligners in the case of butt coupling from waveguide to waveguide.
The surfaces of the coupler 1460 are preferably antireflection
coated. The AR coating may be a multilayer dielectric coating, or a
sol-gel coating, or a quarter wave layer of a material with the
appropriate index of refraction (geometric mean of the two adjacent
media). If the material is bulk pole, d, the optimal focus within
the material 1460 has a Rayleigh range approximately equal to the
length of the poled region. If the material has a waveguide for
confinement of the propagating beam, the optical coupling system
should optimally transform the laser mode into a mode profile at
the entrance of the waveguide which matches the desired mode of the
waveguide in terms of phase front angle, radii of curvature, and
transverse dimensions. The poled structure consists of at least two
types of domains 1461 and 1463 which are preferably oppositely
poled. The poled material has electrodes 1462 and 1464 which extend
across the poled region and which can be excited electrically by
the power supply 1466. When a voltage is applied to the electrodes,
the induced field in the material produces changes in the index of
refraction which vary spatially according to the poling direction
and the electric field strength. By inducing periodic structures in
the poling, electrically controllable periodic modulations in the
index of refraction can be induced.
The amplifier 1452 is provided with the necessary mounting and
excitation to produce an optical gain coefficient over an extended
region characterized by a central optical axis. The optical
bandwidth of the amplifier is limited according to the process
which gives rise to gain. The bandwidth is the width (typically the
3 dB full width) of the gain profile: the dependence of the gain as
a function of optical frequency. The semiconductor diode technology
(such as InGaAs, AlGaAs, AlGaInP, InGaAsP, ZnSe, GaN, InSb) is
advantageous for providing a large bandwidth, although without the
capability of supplying high power. The optical reflector 1454 is a
feedback mirror which can be a bulk mirror aligned and matched in
radius to the phase front of the mode to reflect the mode
propagating out the rear surface of the amplifier back into itself.
Or in the case of a waveguide amplifier (Nd:YAG, Er:YAG,
Nd:LiNbO.sub.3, Er:LiNbO.sub.3, and various combinations of rare
earth ions and crystalline or glassy hosts), it can be a facet of
the amplifier which is cleaved or polished normal to the waveguide.
If the resonator geometry is a ring, permitting unidirectional
propagation of light, the optical reflector is a multi element
structure composed of at least two elements to collect the light
reflected from the material 1460 which does not pass through the
amplifier, and to align and refocus this light back through the
amplifier to the material 1460 again with similar mode
characteristics as it had on previous passes.
Several beam interactions with the periodic poled material 1460 are
possible. If the periodicity is chosen to be a multiple of the
period required to retroreflect light within the gain profile of
the amplifier 1452, the device will function as a (higher order)
field controlled feedback mirror. The laser can be turned on when
the voltage 1466 is switched on, thereby creating retroreflection
within the bandwidth of the grating. The laser output can then be
amplitude modulated by modulating that voltage since the laser
oscillation varies in proportion to the strength of the electric
field. The modulation control means 1466 provides the voltage and
current required to establish the desired electric fields in the
material 1460 as a function of time. The laser can also be
modelocked by operating the modulation control means 1466 at a
frequency equal to a multiple of the round trip frequency for the
light between the material 1460 and the laser reflector 1454. Since
the reflectivity of the poled structure 1450 is modulated at the
same frequency, the light beam resonating between the two feedback
mirrors 1450 and 1454 tends to break up into one pulse (or more)
which of course bounces around with the round trip frequency. If
the frequency is a multiple (1.times., 2.times., 3.times., . . . )
of the round trip frequency, the reflectivity will be high each
time the pulse approaches the reflector 1450. At the higher
multiples, the reflectivity remains high for a shorter time so a
shorter pulse is produced, but some means may be needed to suppress
the additional pulses which tend to be formed at the other high
reflection times within the round trip optical transit time. The
additional pulses can be suppressed by also applying a component of
the signal at the round trip frequency to the reflector 1450, by
also modulating the amplifier 1452, or by other means including
conventional additional components. An optical output may be
extracted into the beam 1468 or 1469.
The laser is frequency stabilized by using the feedback device 1450
because periodic reflectors operate only at specific frequencies.
Incident frequencies outside the bandwidth of the poled structure
are not reflected. In a simple structure, the bandwidth is
determined by the inverse of the number of first order grating
periods which fit in the length of the poled region containing that
frequency component. In a more complicated structure with multiple
periods, the bandwidth is determined by the Fourier transform of
the poled structures along the bisector angle of the incident and
the reflected beam propagation directions. Because feedback is only
present over a limited frequency range, the output frequency of the
device 1450 can be much narrower than that of a free running laser
oscillator in which the poled structure has been replaced by a
simple mirror. If the bandwidth of the reflection is comparable to
the separation of the longitudinal modes of the extended cavity
formed by the reflector 1454 and the poled structure, the device
will operate in a single frequency mode.
The stabilization characteristic is particularly useful in the case
of the semiconductor diode laser, where the gain is very high and
broadband. With diode lasers, all undesired internal reflections
such as the reflection from the output region 1456 should
preferably be kept very low (such as below 10.sup.-3).
The electrodes 1462 and 1464 may be on the same side of the
substrate or on opposite sides, according to the field penetration
and drive voltage preferred for the application. The resonator
including the laser and the switched reflector may also be a ring
resonator instead of the linear resonator shown in FIG. 55.
Additional optical elements are required to form the ring resonator
as is known in the prior art, and the reflection from the poled
material 1460 is not at normal incidence. The periodicity and angle
of the grating must always be adjusted so that the virtual photon
added to the interaction produces momentum conservation between the
input and the output photon. This constraint determines both the
angle and the period of the poled grating.
FIG. 56 shows a laser feedback device 1470 with a waveguide. A
waveguide 1472 may be incorporated into the poled material 1460 to
confine the light beam for a long distance. This is particularly
useful in devices which require the interaction length to generate
a significant reflection, and in integrated devices where all light
is routed m waveguides. Waveguide lasers 1474 such as semiconductor
diode lasers or diode pumped solid state lasers may be butt-coupled
to the waveguide, as shown in FIG. 56, for rugged and efficient
operation. In butt coupling, the optical coupling system 1458 is
the AR coating on the surfaces 1475 and 1477 along with the
alignment and mounting structures necessary to maintain alignment.
The waveguides of the optical amplifier 1474 and the poled
substrate 1460 are aligned so that the optical field phase front
which emerges from the optical amplifier towards the substrate has
to an optimal extent the same angles, radii, and transverse
dimensions as the phase front of the mode which propagates in the
waveguide 1472. The separation of the two waveguides should be
within a Rayleigh range, and their deviation from coaxial alignment
should be less than a fraction of the transverse mode size. Either
one of the waveguides 1472 or the guide in the amplifier 1476 may
be tapered to optimize this overlap. In waveguide devices, it is
not necessary for the poled regions 1478 and 1480 to extend
entirely through the substrate 1460. Electrodes 1482, 1484, and
1486 are disposed over the poled region traversed by the waveguide
1472. When electrode 1484 is excited relative to electrodes 1482
and 1486, an index of refraction pattern is created in the
waveguide with structure determined essentially by the structure of
the poled substrate. This index pattern may act as a reflector as
described in reference to FIG. 55, and/or it may act as a coupler
to other waveguides as described above. An optical output may be
extracted from the device at either the through port 1488 or the
opposite end of the amplifier 1489.
A frequency doubler may be incorporated into the substrate 1460 if
the substrate material is a nonlinear optical material such as
lithium niobate, lithium tantalate, or KTP. The quasi phase matched
doubler may be incorporated as a part of the feedback grating
structure, prior to it, or after it. If the grating structure
incorporates multiple reflection frequencies, the optical
amplifiers 1452 or 1474 may be induced to oscillate at two or more
frequencies within their gain bandwidths. In this case, the
nonlinear frequency converter may be a sum frequency mixer instead
of a doubler, or several such devices may be cascaded to form
multiple frequency combinations of the multiple frequency
outputs.
The variations described above relative to the poled structure, its
excitation, and its mode of use can also be applied in combination
with the external optical amplifier. In particular, frequency
tunable lasers can be realized by combining the structures of FIGS.
55 and 56 with the tunable gratings of FIGS. 14 and 15,
respectively. As before, tuning is achieved by arranging the poled
grating structure so that the average index of refraction changes
with applied field. The frequency of operation of the optical
amplifier 1452 or 1474 is determined by the frequency of the
feedback from the poled structure 1460. The output frequency may
therefore be chirped and/or modulated by chirping and/or modulating
the average index of the poled structure. Changing the average
index changes the momentum vector of the light photons without
changing the momentum vector of the virtual photon contributed by
the grating. After the change in average index, the old reflection
frequency is no longer optimally phase matched for reflection; the
peak reflectivity has moved to a new frequency.
A frequency modulated (FM) laser may be constructed using the
configuration described in reference to FIGS. 55 and 56 with the
addition of changing the average index of refraction as described
in reference to FIGS. 14 and 15. By modulation, we mean changed as
a function of some parameter which is time in this case, as in
pulsed with a high or low duty cycle, sinusoidally varied, or
varied with any arbitrary temporal dependence. A control system may
be supplied to control the voltages and supply the currents needed
to apply the desired temporal variations in electric field.
Typically, the reflectivity of a grating required for optimal
feedback for a semiconductor laser is less than 10%. The remaining
light can be used for output. The laser can be forced to operate in
either the TM or TE polarization, depending on the confinement of
the optical beam in the waveguide on the grating chip, the
dispersion in the grating, and the relative gain of the two
polarizations in the gain element. Since the strength of the
grating is controllable, the reflectivity can be adjusted to
optimize the output coupling of the laser to maximize the output
power.
Similarly, the grating can be used to form reflectors of a passive
or buildup cavity. Since the coupling of a laser beam to a cavity
depends on the relative reflectivity of the input coupler compared
to the cavity losses, a variable reflectivity input coupler
provides a means to optimize this parameter, and thus impedance
match the resonator.
In a cavity, this invention can also be used for single pulse
switching, mode-locking, or cavity dumping, with little or no
chirping for lower power CW sources, such as semiconductor lasers.
In addition, the timing potential enables the laser to be used as a
source for communications, spectroscopy, and remote sensing.
FIG. 57 shows a selectable wavelength laser 1490 controlled by an
array of switches. The laser is preferably a diode laser 1474 with
waveguide 1476 butt coupled to a waveguide 1472 in a substrate
1461. The surfaces 1475 and 1477 are preferably AR coated so that
the optical amplifier 1474 will not lase from the reflectivity of
its own facets. The substrate may be any substrate capable of
supporting the switches 1492, which may be implemented in a variety
of ways including the TIR switches of FIGS. 30-32 and 34-35, the
grating switch structures of FIGS. 7-8 and 12-13, the couplers of
FIGS. 10 and 26-28, the splitters of FIGS. 23, 25, 33, and 46-48,
or any of the other optical waveguide switch structures now known
or yet to be discovered. The TIR switches 1492 have been
sufficiently described above that we indicate their presence only
schematically in this diagram. In their on position, these switches
reroute the optical energy from the amplifier down the one of the
waveguides 1494 that is associated with the switch.
A retroreflector array 1496 is disposed in the waveguides, here
shown as a grating. The grating reflects the incident light at a
specific frequency, and the laser lases within the bandwidth of the
grating. The grating elements are shown directed towards a somewhat
distant point 1498 so that the periods of the gratings farther from
the laser are progressively shorter. The reflection spectra of the
gratings is therefor essentially identical but shifted to shorter
and shorter wavelengths. By selecting the switch 1492 associated
with the desired grating period, the desired frequency of operation
of the laser can be selected. The optical frequency is determined
by the geometry, and linearly spaced optical wavelengths may be
obtained with a constant switch separation. If desired, any
wavelength spacing within the packing density capability of the
arrangement may be chosen. A large number of switches may be
disposed along the waveguide 1472 because of the low insertion loss
of the TIR switches and their high packing density. The tilted
output waveguides also pack together very compactly.
An output beam may be extracted from the rear surface of the
optical amplifier 1474 as beam 1489, or it may be extracted from
the waveguide 1472 as beam 1488 since the TIR switches will leak a
fraction of the laser light along the waveguide 1472. Many
alternative configurations such as are described in reference to
FIG. 56 are also relevant to this configuration. For instance, the
reflector array 1496 may consist of permanent gratings fabricated
by a large number of techniques, or switched gratings. It may
consist of a uniform grating structure in which the different
optical path lengths to the grating select a different FSR for the
laser cavity, producing single mode operation at a selectable array
of very narrowly spaced spectral peaks. It may even consist of an
array of permanent mirrors along the waveguides 1494 which might be
coated for high reflection or variable wavelength reflection.
Again, the differing separation of the mirrors provides the
opportunity to adjust the laser cavity path length switchably over
a large range.
A subset of the structure of FIG. 57 is a modulator using an
adjustable optical energy redirector 1492, and one of the feedback
reflectors 1496 in one of the waveguides 1494. If the waveguide
1472 is configured not to reflect, as by for example tapering its
width to zero following the redirector 1492, the laser again is
forced to lase with the feedback afforded by the reflector 1496. By
adjusting the amount of optical energy which is fed back through
the excitation of the redirector 1492, the laser output
characteristics can be controlled. The laser power may be modulated
in this way, in which case the reflector 1496 may be a fixed
grating or even a broadband fixed mirror. With the grating
reflector, there is the advantage of a fixed frequency so that the
laser power can be modulated deeply without frequency shifts,
producing almost pure amplitude modulation. If the redirector 1492
is modulated at an integer multiple of the cavity round trip time,
the device is a mode locker and produces a pulsed output. The round
trip time is the time taken by a coaxially aligned pulse to return
to its original position and direction within the cavity. By using
different switches the cavity length can be varied to vary the
pulse separation. By using two different switches simultaneously,
it is also possible to discriminate against the intermediate pulses
which tend to grow with mode locking frequencies at a high order
multiple of the round trip time. Frequency modulation can be
obtained by modulating the central frequency of the reflector. In
this case, the reflector 1496 should preferably be implemented as a
tunable grating such as is described in FIGS. 14-22.
FIG. 58 shows a wavelength tuned adjustable focussing system 1550.
The combination of a diffractive focussing element 1552 such as the
zone plate lens of FIG. 54 (or a fixed diffractive lens such as an
opaque or an etched zone plate) with a tunable light source 1554
offers important new capability in the field of data storage. When
a zone plate is combined with an adjustable frequency light source,
the distance to the focus is adjusted by tuning the light source.
This capability is useful for multilayer data storage devices where
the data is read from and written to data planes 1556 stacked at
various distances into the data storage medium 1558. If the
wavelength of the light source is tuned (as we have described above
by various means), the distance from the zone plate to the focus is
adjusted correspondingly. This change of focus by wavelength change
allows both instantaneous tracking of the optical data storage
surface of a distorted disk by analog wavelength change, and
selection of the desired data surface with random access to the
various stacked planes 1556 by discrete changes in the light
wavelength.
The frequency tunable laser of choice for driving the multiplane
data storage system is a laser system based on a semiconductor
diode laser 1560 with tuning based on feedback from an
electronically tunable grating 1562 such as we have described
above. The laser may also be frequency doubled in a quasi
phasematched section 1564, in which case use of an angle broadened
poled grating is the preferred method of rendering the acceptance
of the doubler broad enough to accept significant tuning of the
source laser. Lens system 1566 collimates and makes round the laser
output in preparation for the final focus in the zone plate lens
1552.
The lens system 1566 is not necessary since a zone plate can also
refocus a diverging beam. However, it is desirable to achieve a
round beam through the zone plate because this will produce the
smallest spot size and therefore the highest density data
reading/writing capability. The devices 1562, 1564, and 1566 may be
implemented in waveguides on the same substrate if desired, and in
combination with one of the out-of-plane reflectors described
above, can be integrated with zone plate lenses on the rear surface
of the substrate to achieve a small and lightweight unit capable of
rapid actuation in a data storage system.
The invention has now been explained with reference to specific
embodiments. Other embodiments will be apparent to those of
ordinary skill in the art. Therefore, it is not intended that the
invention be limited, except as indicated by the appended claims,
which form a part of this invention description.
* * * * *