U.S. patent application number 09/826931 was filed with the patent office on 2001-08-16 for polarization insensitive grating in a planar channel optical waveguide and method to achieve the same.
Invention is credited to Albert, Jacques, Bilodeau, Francois, Hill, Kenneth O., Johnson, Derwyn C., Mihailov, Stephen J..
Application Number | 20010014200 09/826931 |
Document ID | / |
Family ID | 23669720 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014200 |
Kind Code |
A1 |
Albert, Jacques ; et
al. |
August 16, 2001 |
Polarization insensitive grating in a planar channel optical
waveguide and method to achieve the same
Abstract
A method is disclosed wherein optical structures such as Bragg
gratings can be written into planar waveguides disposed on a
substrate. By employing the method of this invention, a
polarization insensitive device can be made. Such relatively thin
planar waveguides disposed on a substrate are inherently
polarization sensitive and the structure itself is birefringent. By
writing structures in the waveguides or by simply writing a
refractive index change in the waveguide by limiting the beam width
at the waveguide layer to less than or equal to the thickness of
the waveguiding layers on the substrate, in combination with
conventional wider beam writing techniques, polarization
sensitivity can be lessened or obviated.
Inventors: |
Albert, Jacques; (Hull,
CA) ; Bilodeau, Francois; (Nepean, CA) ; Hill,
Kenneth O.; (Kanata, CA) ; Johnson, Derwyn C.;
(Nepean, CA) ; Mihailov, Stephen J.; (Ottawa,
CA) |
Correspondence
Address: |
FREEDMAN & ASSOCIATES
117 CENTREPOINTE DRIVE
SUITE 350
NEPEAN, ONTARIO
K2G 5X3
CA
|
Family ID: |
23669720 |
Appl. No.: |
09/826931 |
Filed: |
April 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09826931 |
Apr 6, 2001 |
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09421234 |
Oct 20, 1999 |
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6256435 |
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Current U.S.
Class: |
385/129 ;
430/290 |
Current CPC
Class: |
G02B 6/02109 20130101;
G02B 6/124 20130101; G02B 6/02147 20130101 |
Class at
Publication: |
385/129 ;
430/290 |
International
Class: |
G02B 006/10 |
Claims
What is claimed is:
1. A method of inducing a region of modified refractive index in a
planar waveguide device comprising the steps of: providing a planar
waveguide comprised of layers affixed to a substrate layer, wherein
at least one of (a) an optical property, (b) density, and (c)
thermal coefficient of expansion of the substrate differs from that
of the planar waveguide layers, the planar waveguide layers being
substantially thinner than the substrate layer, the planar
waveguide layers having a composite thickness of t.sub.1 .mu.m;
and, irradiating the waveguide with a narrow beam of light and
ensuring that the beam of light incident upon the planar waveguide
is restricted to a width no greater than t.sub.1 .mu.m as the beam
of light impinges upon the planar waveguide.
2. A method as defined in claim 1, further comprising the step of
irradiating the waveguide with a wide beam having a width
substantially greater than t.sub.1 .mu.m.
3. A method as defined in claim 2 wherein the waveguide is
irradiated with the narrow and the wide beams simultaneously.
4. A method as defined in claim 3 wherein the narrow beam and the
wide beam comprise a single beam of light having an intensity which
varies radially in a predetermined manner.
5. A method of providing an optical structure in a planar waveguide
device comprising the steps of: providing a layered structure
having a composite thickness t.sub.1 which includes a thin
waveguiding core layer surrounded by cladding layers, the core
layer having a thickness nt.sub.1 where n<1.0, the layered
structure being affixed to a substrate of thickness greater than
mt.sub.1 where m>5; and, irradiating a portion of the
waveguiding core layer with a beam of light for a sufficient
duration and with a sufficient intensity to permanently change the
refractive index of regions within the waveguiding core layer of
the portion, the beam having a spot size of less than t.sub.1.
6. A method of providing an optical structure in a planar waveguide
device comprising the steps of: providing a layered structure
having a substrate layer of a thickness t.sub.s and a substantially
thinner photosensitive waveguiding core layer surrounded by a
cladding having a combined thickness of t.sub.1; and, irradiating a
portion of the waveguiding core layer with a beam of light for a
sufficient duration and with a sufficient intensity to permanently
change the refractive index of regions within the waveguiding core
layer of the portion, the beam having a diameter upon the planar
waveguide device wherein 95% if its power is confined to an area of
less than or equal to t.sub.1.
7. A grating fabricated by the method of claim 6 wherein the
irradiating beam of light has periodic or aperiodic variations
along a longitudinal axis of the waveguiding core layer.
8. A grating fabricated by the method of claim 1 wherein the narrow
beam of light has periodic or aperiodic variations along a
longitudinal axis of the waveguiding core layer.
9. A grating fabricated by the method of claim 1 wherein the
irradiating beam of light has periodic or aperiodic variations
along a longitudinal axis of the waveguiding core layer.
10. A method of providing an optical structure in a planar
waveguide device comprising the steps of: providing a layered
structure having a substrate layer of a thickness t.sub.s and a
substantially thinner waveguiding core layer surrounded by a
cladding having a combined thickness of t.sub.1; and, irradiating a
portion of the waveguiding core layer with a beam of light for a
sufficient duration and with a sufficient intensity to permanently
change the refractive index of regions within the waveguiding core
layer of the portion, the beam having a diameter having a
non-uniform intensity pattern that varies radially, the light
energy impinging upon an area of a dimension t.sub.1 or less over
the waveguide core being substantially different from the light
energy impinging upon other areas of the waveguide layers.
Description
FIELD OF THE INVENTION
[0001] This invention relates to planar optical circuits and more
particularly to a method for lessening unwanted polarization
dependence within planar waveguides of such circuits.
BACKGROUND OF THE INVENTION
[0002] The invention is directed to a method for modifying the
refractive index of planar optical waveguides by ultraviolet light
irradiation, including but not restricted to forming an optical
structure such as a Bragg or long period grating. More
particularly, the novel method includes steps, which can minimize
birefringence effects normally associated with writing such
structures in multi-layer devices supported by a relatively thick
substrate.
[0003] The sensitivity of optical waveguide fibers to light of
certain wavelength and intensity has been known since the late
1970's. It was found that the loss characteristic and refractive
index of a waveguide fiber could be permanently changed by exposing
the waveguide to light of a given wavelength and intensity. A
publication which describes the effect and how it may be used is,
"Light-sensitive optical fibers and planar waveguides", Kashyap et
al., BT Techno., 1, Vol. 11, No. 2, April 1993. The publication
discusses the making of light-induced reflection gratings, page
150, section 2.1, and notes that the amount of refractive index
change increases as light wavelength is reduced from 600 nm to 240
nm, where the photosensitivity of the waveguide appears to peak,
with the notable exception of irradiation using strong 193 nm light
where the photosensitivity can be very large (as demonstrated by
Malo et al. Electron. Lett. Vol. 31, p.879 (1995)).
[0004] In "Bragg grating formation and germanosilicate fiber
photosensitivity", SPIE V. 1516, Intn'l Workshop on Photoinduced
Self-Organization Effects In Optical Fiber, Meltz et al., 1991, the
mechanism and magnitude of photosensitivity is discussed (page 185,
first paragraph, section 1.). This publication also discusses an
interferometric technique of writing gratings (pp. 185-6, section
2.) At page 189, first paragraph, a measurement of induced
birefringence is presented. See also FIG. 6 of that
publication.
[0005] Another publication, "Characterization of UV-induced
birefringence in photosensitive Ge-doped silica optical fibers",
Erdogan et al., J. Opt. Soc. Am. B/V.11, No. 10, October 1994,
notes the dependence of induced birefringence on the orientation of
the polarization direction of the light incident upon the waveguide
fiber. In particular, data presented in the publication shows that
the induced birefringence is greatest when the polarization
direction is oriented perpendicular to the long axis of the fiber
and least when the polarization direction is parallel to the long
axis of the fiber. See FIG. 3a. and FIG. 4. of the publication.
[0006] The Erdogan et al. publication points out that the induced
birefringence polarization anisotropy can be used to make such
devices, "as polarization mode converters and rocking filters",
page 2100, first paragraph. However, in devices using resonant
propagation, "the birefringence can result in substantial
polarization dependence of resonant grating properties, such as
reflectivity", page 2100, first paragraph.
[0007] The Erdogan, et al., data shows that even in the
configuration where the polarization direction is along the long
axis of the waveguide, some birefringence is still induced in the
waveguide. Comparing the curves of FIG. 3a. and FIG. 4., the
non-polarization dependent induced birefringence is a factor in the
range of about 4 to 12 smaller than the polarization dependent
induced birefringence. However, even this smaller amount of
birefringence is undesirable. A more versatile and effective
grating would result from a writing method which produces a grating
having minimal birefringence.
[0008] Notwithstanding, polarization effects or sensitivity from
irradiating waveguides in multilayer structures exhibited as a
result of disposing a relatively thin waveguide comprised of an
assembly of material layers supporting low loss light propagation
deposited on a thick substrate comprised of a material having
different characteristics from those of the deposited layers, is
significantly more evident and problematic than the effects and
causes described by Edrogan et al. It is this polarization
sensitivity caused by mismatching and thickness differences in
layered material that is addressed by this invention.
[0009] Planar optical circuits, often termed planar lightwave
circuits (PLCs) are well known and for particular applications some
of which include optical gratings formed therein, such as Bragg
gratings, or long period gratings. Since most signals propagating
through optical fiber have an indeterminate polarization state, it
is preferred that the gratings through which these signals
propagate, be substantially polarization insensitive. J. Albert et
al., the applicants have disclosed in a paper entitled
"Polarization-independent strong Bragg gratings in Planar Lightwave
Circuits" Electron Lett. 34, 485-486 (1998), a method of lessening
the polarization dependence or "polarization sensitivity" of planar
waveguides having Bragg gratings formed therein. By using an
intense ArF excimer laser a refractive index change is produced and
is birefringent. This birefringence is large enough and of the
proper sign to compensate the inherent birefringence exhibited in
most PLCs.
[0010] Notwithstanding, in the instant invention the birefringence
can be controlled independently of the size of the index change. An
instance where this control is particularly useful is in the path
length trimming of a Mach-Zehnder interferometer that is initially
polarization independent. Of course it is preferred that the
trimming be nonbirefringent to maintain the polarization
independence of the device. However, this invention can be used in
other phased array devices, or arrayed wave guides (AWGs),
requiring similar polarization insensitively in the arms of the
AWG.
[0011] Planar waveguides usually have different propagation
constants for TE (transverse electric) and TM (transverse magnetic)
waveguide modes and therefore are known to be polarization
sensitve. Stated more simply, the response of these waveguides
differs for orthogonally polarized light beams. For wavelength
multi/demultiplexers, this difference in propagation constants
results in a wavelength shift in the spectral response peak or the
passband of each wavelength channel. This wavelength shift is
sensitive to the design of the planar waveguide, and can be as
large as 3 nm. As WDM systems are being designed towards smaller
and smaller channel spacing (from 1.6 nm to 0.8 nm or even less in
the future), even a small polarization dependent wavelength shift
(e.g. 0.3.about.0.4 nm) is of concern.
[0012] Quite surprisingly, the inventors of the instant application
have discovered that the size of the beam, relative to the size of
the waveguide in which a grating is to be photo-induced, greatly
affects the polarization dependence of the grating or structure
being written into the waveguide. For example, photo-induced
birefringence occurs when irradiating a planar waveguide as
described with a beam of suitable intensity having a spot size that
is substantially greater than the width of the waveguide region. In
some instances this lo birefringence offsets or compensates for the
birefringence present in the planar waveguide prior to irradiation.
However, most often, when writing an optical structure by
photoinducing a refractive index change in the waveguide using
current techniques, the amount of photo-induced birefringence
cannot be accurately controlled; achieving as a desired refractive
index change An does not always occur at the point where
irradiation of the waveguide induces a birefringence that yeilds a
substantially polarization insensitive device. However, by
utilizing conventional techniques of irradiating with a beam sized
larger than the waveguide width in combination with irradiating the
waveguide with a smaller beam spot size less than or equal to the
width of the waveguide channel, improved control over the
polarization sensitivity of the device can be afforded. In fact, a
polarization insensitive device can be manufactured. This technique
is not only limited to writing gratings such as Bragg and long
period gratings, but can be used to induce an index change to
realize many other possible structures.
[0013] In summary it is now possible to irradiate a planar
waveguide as described heretofore, with a beam having a narrow or
reduced width substantially equal or less than the width of the
waveguide core in combination with a beam of a substantially
greater width to obviate polarization senstivity.
[0014] It is therefore, an object of this invention, to provide a
method for writing Bragg gratings and other optical structures
while independently controlling the amount of birefringence induced
by the irradiation. An important but not exclusive use of the
invention is to substantially lessen or eliminate polarization
dependence at a wavelength of interest normally associated with
such structures disposed in planar waveguides.
[0015] It is a further object of the invention, to provide a
photo-induced Bragg gratings having little or essentially no
polarization dependence at a wavelength of interest.
[0016] The following definitions may be helpful for the
understanding of this specification.
[0017] An optical waveguide grating is a periodic cyclic or a
periodic variation in refractive index of the waveguide along the
long axis of the waveguide.
[0018] Photo-sensitivity is an interaction between certain glass
compositions and selected light wavelengths wherein incident light
permanently changes the refractive index or the loss
characteristics of the irradiated glass.
[0019] Side writing is a technique for forming a grating in an
optical waveguide fiber wherein light is caused to form a periodic
series of alternating light and dark fringes along the long axis of
the waveguide. An example of such a periodic series is an
interference pattern formed on the side of a waveguide fiber and
along a portion of the long axis of a waveguide fiber. The periodic
light intensity pattern, produced by the light interference,
induces a periodic change in refractive index along a portion of
the long axis of the waveguide fiber.
[0020] A planar lightwave circuit (PLC) is a waveguide having a
core with a substantially square or rectangular cross section and
having a cladding material of a lower refractive index surrounding
the core, the whole assembly being deposited and adhering to a
substrate whose thickness is substantially larger than the
thickness of the waveguide layers.
[0021] PLCs are generally more polarization dependent than optical
fibres having substantially cylindrically symmetric cross sections
with a core centered along the axis of symmetry.
[0022] PLCs suffer both from form birefringence due to the planar
geometry and from material birefringence associated with
fabrication processes where non equal thermo-elastic coefficient
between the substrate and waveguide lead to residual strains.
[0023] It is an object of this invention to provide a planar
waveguide structure having a grating or other wavelength dependent
structure disposed therein, wherein the polarization sensitivity of
the function of the structure is controlled to a desired value. A
particular important example being that the structure be
substantially polarization insensitive.
SUMMARY OF THE INVENTION
[0024] In accordance with the invention, there is provided, a
method of inducing a region of modified refractive index in a
planar waveguide device comprising the steps of: providing a planar
waveguide comprised of layers affixed to a substrate layer, wherein
at least one of
[0025] (a) an optical property,
[0026] (b) density, and
[0027] (c) thermal coefficient of expansion of the substrate
differs from that of the planar waveguide layers, the planar
waveguide layers being substantially thinner than the substrate
layer, the planar waveguide layers having a composite thickness of
t.sub.1 .mu.m; and, irradiating the waveguide with a narrow beam of
light and ensuring that the beam of light incident upon the planar
waveguide is restricted to a width no greater than t.sub.1 .mu.m as
the beam of light impinges upon the planar waveguide.
[0028] In accordance with the invention there is further provided,
a method of providing an optical structure in a planar waveguide
device comprising the steps of:
[0029] providing a layered structure having a composite thickness
t.sub.1 which includes a thin waveguiding core layer surrounded by
cladding layers, the core layer having a thickness nt.sub.1 where
n<1.0, the layered structure being affixed to a substrate of
thickness greater than mt.sub.1 where m>5; and, irradiating a
portion of the waveguiding core layer with a beam of light for a
sufficient duration and with a sufficient intensity to permanently
change the refractive index of regions within the waveguiding core
layer of the portion, the beam having a spot size of less than
t.sub.1.
[0030] In accordance with the invention, a method of providing an
optical structure in a planar waveguide device comprises the steps
of:
[0031] providing a layered structure having a substrate layer of a
thickness t.sub.s and a substantially thinner waveguiding core
layer surrounded by a cladding having a combined thickness of
t.sub.1; and,
[0032] irradiating a portion of the waveguiding core layer with a
beam of light for a sufficient duration and with a sufficient
intensity to permanently change the refractive index of regions
within the waveguiding core layer of the portion, the beam having a
diameter upon the planar waveguide device wherein 95% if its power
is confined to an area of less than or equal to t.sub.1.
[0033] In accordance with yet another aspect of the invention,
there is provided, a method of providing an optical structure in a
planar waveguide device comprising the steps of: providing a
layered structure having a substrate layer of a thickness t.sub.s
and a substantially thinner waveguiding core layer surrounded by a
cladding having a combined thickness of t.sub.1; and,
[0034] irradiating a portion of the waveguiding core layer with a
beam of light for a sufficient duration and with a sufficient
intensity to permanently change the refractive index of regions
within the waveguiding core layer of the portion, the beam having a
diameter having a non-uniform intensity pattern that varies
radially, the light energy impinging upon an area of a dimension
t.sub.1 or less over the waveguide core being substantially
different from the light energy impinging upon other areas of the
waveguide layers.
[0035] Conveniently, by employing the techniques of limiting the
beam width impinging a waveguide disposed on a planar substrate in
the manner that will be described, polarization sensitivity can be
lessened or obviated and controlled.
[0036] Hence, the method of this invention can be useful in
controlling the amount of birefringence present in a planar
waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] Exemplary embodiments of the invention will now be described
in conjunction with the drawings in which:
[0038] FIG. 1 is a perspective view of a planar optical waveguide
device in accordance with an embodiment of the invention, having
refractive index grating disposed within a region of the waveguide
core;
[0039] FIG. 2 is a cross-sectional view of the planar optical
waveguide device shown in FIG. 1;
[0040] FIG. 3a is a graph illustrating Bragg wavelength (nm) versus
laser irradiation time (s) for a laser spot width of 100 .mu.m
illustrating the relationship between TE and TM modes;
[0041] FIG. 3b is a graph illustrating Bragg wavelength (nm) versus
laser irradiation time(s) for a laser spot width of 15 .mu.m
illustrating the relationship between TE and TM modes; and,
[0042] FIG. 4a is a graph illustrating Power (dB) versus wavelength
(nm) for Bragg gratings written with UV-induced birefringence
ON;
[0043] FIG. 4b is a graph illustrating Power (dB) versus wavelength
(nm) for Bragg gratings written with UV-induced birefringence
OFF;
[0044] FIG. 5 is a graph of UV-induced birefringence versus laser
irradiation time; and,
[0045] FIG. 6 is a diagram illustrating the irradiation of a
waveguide region in accordance with an aspect of this
invention.
DETAILED DESCRIPTION
[0046] Referring now to FIG. 1 a planar waveguide device in the
form of a PLC is shown having a substrate layer 10 a cladding layer
12 and a core layer 14 having a higher refractive index than the
surrounding cladding layer 12. Disposed within a region of the core
layer 14 is a grating having a refractive index variation that may
be periodic or aperiodic. The writing of a permanent grating in a
waveguide can be done through the use of a variety of well known
techniques, such as by using a phase mask, an amplitude mask, or by
using multiple beam interference techniques.
[0047] FIG. 2 illustrates a cross-section of the device shown in
FIG. 1, perpendicular to the axis of the propagation of the
waveguide; hence, the section labeled as "a" is an end-view of the
longitudinal core section. Apart from the grating (not illustrated
in this figure), there is no refractive index variation along the
propagation axis, or varying on a scale much longer than the
wavelength of the guided light, such as adiabatic tapers or bends
with radii or at least 0.1 mm. In the exemplary embodiment shown,
all lines are straight and intersect at right angles, however the
waveguides can in practice have many different shapes, and can have
varying cross-sectional dimensions. In FIG. 2 the waveguide "a" is
a light transmissive core layer where most of the light launched
therein is confined by the relative refractive difference between
"a" and the cladding materials "b" to "i" surrounding the core
layer. The structure is fabricated on supporting substrate "j"
which is planar and which is substantially thicker; Thickness "T"
is smaller than width "W", both quantities being significantly
larger than any of the core cross-section dimensions. This
invention provides a remedy for the problems of high polarization
sensitivity resulting from material and thickness differences
between the core and supporting substrate and conventional grating
writing practices.
[0048] In preferred embodiments of this invention, the substrate
material provides mechanical strength to the waveguide layer
typically made of a different material. The deposition of the
glasses on the substrate can be performed in any of several ways;
what is important is that glass layer(s) adhere to the substrate.
In the instance where the deposited glass and the glass substrate
material were nominally the same (e.g. silica), they would still be
considered "different" materials in the sense that the substrate
has been prepared in bulk form and as a result has different
physical properties (density, thermal expansion coefficient, etc .
. . ) than the light guiding layers. Analysis reveals in order to
control the amount of birefringence of the light irradiated region
of the waveguide, at least some of the irradiation must be carried
out in such way that the width of the beam irradiating the
waveguide region should be less than beam widths typically used in
conventional waveguide writing techniques. In order to determine a
suitable range of beam widths that will control the polarization
sensitivity, the critical beam width where one changes the sign of
the induced birefringence is related to the thickness of the
deposited glass layers. To determine a preferred range, the
following criterion is utilized: UV-induced birefringence is "ON"
or present when the laser spot width is equal to or larger than
twice the total deposited layers thickness and "OFF" or when the
spot width is equal to or less than the total deposited layers
thickness.
[0049] Controlling the beam width can be achieved in a number of
ways: A preferred method is by use of an aperture or amplitude mask
disposed in the beam path before the last substantially focusing
lens used to concentrate the laser light on the sample in the
system Turning now to FIG. 6 a light beam is shown directed toward
a waveguide chip 40, and more particularly to within a region
confined to a width t.sub.1 over the core of the waveguide chip
which is equal to the combined thickness of the layers upon the
substrate. An aperture 44 and lens 45 combination conveniently
provide a means of confining the beam to have a desired diameter
upon the waveguide region. When w.sub.1 the beam width, is less
than t.sub.1 the light induced refractive index change is
substantially non-birefringent.
[0050] It is possible to control the beam width using non-phase
mask based methods such as those that generate the interfering UV
beams. The use of a blocking mask disposed adjacent to the surface
of the waveguide device is less preferred and is considered to be
unpractical due to the high laser power density at that location.
It is also undesirable to use the magnifying power of a lens
because one needs independent control of beam spot size and power
density.
[0051] Turning now to FIGS. 3a, and 3b graphs are shown,
illustrating the effects of using a laser beam having spot width of
100 .mu.m and 15 .mu.m respectively. FIG. 5 illustrates the UV
induced birefringence from the linear fits shown in FIGS. 3a and
3b. What becomes evident is that the slope of UV induced
birefringence is opposite for the two beam spot sizes indicated.
The result of utilizing this phenomenon is illustrated in FIGS. 4a
dn 4b where when UV induced birefringence is "ON" the device is
polarization insensitive.
[0052] Of course numerous other embodiments may be envisaged,
without departing from the spirit and scope of the invention. For
example, although the preferred embodiment of the invention two
beams of light are used, one after the other, in alternative
embodiment a single beam having a step-wise non-uniform energy
distribution that varies radially in, for example two discrete
steps between a first region confined to substantially over the
core and as second region confined to the cladding region adjacent
to the core is practicable although somewhat more difficult to
implement. This can be achieved by using an intensity mask to vary
the intensity radially in this manner. In all instances, what is
desirable is to control the ratio between the light energy
impinging upon the core (or composite thickness layers upon the
substrate), and the light energy impinging the region adjacent to
the core.
[0053] Another embodiment of this invention relates to varying the
refractive index over a length of the waveguide in order to lessen
or obviate the inherent polarization sensitivity of the waveguide
region itself. Such applications are useful for, example in
trimming Mach Zehnder devices.
[0054] References to the beam width or spot size throughout this
disclosure shall be construed as the dimension in the direction
substantially perpendicular to the waveguide axis.
* * * * *