U.S. patent number RE41,204 [Application Number 11/440,783] was granted by the patent office on 2010-04-06 for tunable bragg grating and devices employing the same.
Invention is credited to Ravinder Jain, Balaji Srinivasan.
United States Patent |
RE41,204 |
Jain , et al. |
April 6, 2010 |
Tunable bragg grating and devices employing the same
Abstract
The present invention relates generally to electro-optically
active waveguide segments, and more particularly to the use of a
selective voltage input to control the phase, frequency and/or
amplitude of a propagating wave in the waveguide. Particular device
structures and methods of manufacturing are described herein.
Inventors: |
Jain; Ravinder (Albuquerque,
NM), Srinivasan; Balaji (Mandaveli, Chennai, IN) |
Family
ID: |
22117340 |
Appl.
No.: |
11/440,783 |
Filed: |
May 24, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11084868 |
Mar 18, 2005 |
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09246125 |
Feb 8, 1999 |
6221565 |
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60074040 |
Feb 9, 1998 |
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Reissue of: |
09768048 |
Jan 24, 2001 |
06534248 |
Mar 18, 2003 |
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Current U.S.
Class: |
430/321; 385/40;
385/123; 385/10; 216/37; 216/24; 430/320; 385/8 |
Current CPC
Class: |
G02F
1/011 (20130101); G02B 6/29355 (20130101); G02F
1/0115 (20130101); G02B 6/29322 (20130101); G02B
6/29353 (20130101); G02B 6/2746 (20130101); G02F
2201/307 (20130101); G02F 2203/62 (20130101); G02F
2203/585 (20130101); G02B 6/4207 (20130101); G02F
1/215 (20210101); G02F 1/212 (20210101); G02B
6/274 (20130101); G02B 6/022 (20130101); G02F
2203/055 (20130101); G02F 2202/06 (20130101); H01S
3/0675 (20130101); H01S 3/105 (20130101); G02F
1/3558 (20130101); G02F 2202/07 (20130101); G02F
2203/07 (20130101); G02B 6/02195 (20130101) |
Current International
Class: |
G02B
6/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 794 450 |
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Sep 1997 |
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EP |
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794450 |
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Sep 1997 |
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EP |
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2 190 741 |
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Nov 1987 |
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GB |
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WO 90/08970 |
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Aug 1990 |
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WO |
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98/57230 |
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Dec 1998 |
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WO |
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Other References
Iocco et al. "Bragg grating fast tunable filter", Electron. Lett.,
vol. 33(25) pp. 2147-2148 (Nov. 1997). cited by examiner .
Fujiwara et al. UV exzcited poling and electrically tunable bragg
gratings in germanisilicate fiber, Optical fiber Communications
'95, Technical Digest, Post Conference Ed. pp. 347-350 (Mar. 1995).
cited by examiner.
|
Primary Examiner: Angebranndt; Martin J
Attorney, Agent or Firm: Donohue; Michael J. Davis Wright
Tremaine LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
.Iadd.More than one reissue application has been filed for the
reissue U.S. Pat. No. 6,534,248 B2. Reissue application Ser. No.
11/084,868, filed on Mar. 18, 2005 is now abandoned. The reissue
applications are application Ser. No. 11/440,781, filed on May 24,
2006; Ser. No. 11/440,782, filed on May 24, 2006, Ser. No.
11/440,783, filed on May 24, 2006, all of which are divisional
reissues of U.S. Pat. No. 6,534,248 B2..Iaddend.
The present application is a division of co-pending U.S. patent
application No. 09/246,125, entitled "Tunable Bragg Gratings and
Devices Employing the Same," now U.S. Pat. No. 6,221,565, filed
Feb. 8, 1999, which in turn claims the prioruy of U.S. Provisional
Application No. 60/074,040, entitled "Advanced Electro-Optic Poled
Waveguide Devices and New Lightwave System Applications Thereof"
filed Feb. 9, 1998, now abandoned. The entire disclosure and
contents of the above-mentioned applications are hereby
incorporated by reference.
This application refers to the following U.S. Patents. The first is
U.S. Pat. No. 5,617,449, entitled "Technique for Fabrication of a
Poled ElectroOptic Fiber Segment," issued Apr. 1, 1997. The second
is U.S. Pat. No. 5,830,622, entitled "Optical Grating," issued Nov.
3, 1998. Both of these applications are hereby incorporated by
reference.
This invention is made with government support under AFSOR grant
number F49620-96-1-0079, awarded by the United States Department of
Defense. The government may have certain rights in this invention.
Claims
What is claimed is:
.[.1. A method for manufacturing an electro-optic waveguide segment
having a core and a cladding; the method comprising the steps of:
forming a Bragg grating in said waveguide segment; removing a first
selective portion of said cladding above at least a portion of said
Bragg grating to form a first recess within said cladding, said
first selective portion having an outer dimension which is less
than an unmodified section of said cladding; depositing a first
electrically conductive material covering at least part of said
first selective portion and in direct contact with a deepest
portion of said first recess, thereby forming a first electrode,
wherein there are substantially no air gaps between said first
electrode and said first recess; removing a second selective
portion of said cladding to form a second recess within said
cladding, said second selective portion having an outer dimension
which is less than an unmodified section of said cladding, said
first and second selected portions not contacting one-another;
depositing a second electrically conductive material covering at
least part of said second selective portion and in direct contact
with a deepest portion of said of said second recess, thereby
forming a second electrode, wherein there are substantially no air
gaps between said second electrode and said second recess; and
poling said waveguide segment using at least a electric field
applied to either said first or second electrode to induce an
non-linearity in said waveguide segment..].
.[.2. The method recited in claim 1, wherein said first recess is
substantially filled with said first electrically conductive
material..].
.[.3. The method recited in claim 2, wherein said first
electrically conductive material is optically transparent..].
.[.4. The method recited in claim 1, wherein said second recess is
substantially filled with a second electrically conductive
material..].
.[.5. The method recited in claim 4, wherein said second
electrically conductive material is optically transparent..].
.[.6. The method recited in claim 1 wherein said poling step is
performed using ultraviolet light injected into said waveguide
segment in combination with said electric field..].
.[.7. The method recited in claim 1, wherein said poling step is
performed using said electrical field in combination with heating
said waveguide segment..].
.[.8. A method for manufacturing an electro-optic waveguide segment
having a core and a cladding; the method comprising the steps of:
forming a Bragg grating in said waveguide segment; removing a first
selective portion of said cladding above at least a portion of said
Bragg grating to form a first recess within said cladding, said
first selective portion having an outer dimension which is less
than an unmodified section of said cladding; depositing a first
electrically conductive material covering at least part of said
first selective portion and in direct contact with a deepest
portion of said first recess, thereby forming a first electrode,
wherein there are substantially no air gaps between said first
electrode and said first recess; removing a second selective
portion of said cladding to form a second recess within said
cladding, said second selective portion having an outer dimension
which is less than an unmodified section of said cladding, said
first and second selected portions not contacting one-another;
depositing a second electrically conductive material covering at
least part of said second selective portion and in direct contact
with a deepest portion of said second recess, thereby forming a
second electrode, wherein there are substantially no air gaps
between said second electrode and said second recess; and poling
said waveguide segment using ultraviolet light injected into said
waveguide segment to induce an non-linearity in said
waveguide..].
.[.9. A method for manufacturing an electro-optic waveguide segment
having a core and a cladding; the method comprising the steps of:
forming a Bragg grating in said waveguide segment; removing a first
selective portion of said cladding above at least a portion of said
Bragg grating to form a first recess within said cladding, said
first selective portion having an outer dimension which is less
than an unmodified section of said cladding; depositing a first
electrically conductive material covering at least part of said
first selective portion and in direct contact with a deepest
portion of said first recess, thereby forming a first electrode,
wherein there are substantially no air gaps between said first
electrode and said first recess; removing a second selective
portion of said cladding to form a second recess within said
cladding, said second selective portion having an outer dimension
which is less than an unmodified section of said cladding, said
first and second selected portions not contacting one-another;
depositing a second electrically conductive material covering at
least part of said second selective portion and in direct contact
with a deepest portion of said second recess; and poling said
waveguide segment by heating said waveguide segment in combination
with ultraviolet light injected into said waveguide segment to
induce an non-linearity in said waveguide segment..].
.[.10. A method for manufacturing an electro-optic waveguide
segment having a core and a cladding; the method comprising the
steps of: removing a first selective portion of said cladding to
form a first recess within said cladding, said first selective
portion having an outer dimension which is less than an unmodified
section of said cladding; removing a second selective portion of
said cladding to form a second recess within said cladding, said
second selective portion having an outer dimension which is less
than an unmodified section of said cladding, said first and second
selected portions not contacting one another; depositing a first
electrically conductive material within said first recess and in
direct contact with a deepest portion of said first recess, thereby
forming a first electrode, wherein there are substantially no air
gaps between said first electrode and said first recess; depositing
a second electrically conductive material within said second recess
and in direct contact with a deepest portion of said second recess,
thereby forming a second electrode, wherein there are substantially
no air gaps between said second electrode and said second recess;
and poling said waveguide segment using at least a electric field
applied to said first and second selective portions to induce a
non-linearity in said waveguide segment..].
.[.11. The method for manufacturing recited in claim 10, further
comprising the step of forming a Bragg grating in said waveguide
segment..].
.Iadd.12. An optical filter, comprising: a first optical fiber
comprising a core and a cladding configured to receive a first
input light signal comprising a plurality of wavelengths; a second
optical fiber comprising a core and a cladding configured to
generate an output light signal; a first polarization beam-splitter
coupled to the first and second optical fibers and configured to
convert the first input light signal in the first optical fiber to
a first linear polarization state and to convert the first input
light signal in the second optical fiber to a second linear
polarization state orthogonal to the first linear polarization
state; a first electro-optic waveguide disposed in the first
optical fiber, and configured to convert the light signal having
the first linear polarization state to a light signal having a
circular polarization state having a predetermined handedness; a
second electro-optic waveguide disposed in the second optical
fiber, and configured to convert the light signal having the second
linear polarization state to a light signal having a circular
polarization state having a predetermined handedness; and a first
and a second electro-optic tunable Bragg gratings disposed in the
first and second optical fibers, respectively, and configured to
reflect selected wavelengths of the light signals having circular
polarization states and to reverse the handedness of the circular
polarization states, the first and second electro-optic tunable
Bragg gratings each comprising: a first recess within the cladding
of its respective optical fiber; a second recess within the
cladding of its respective optical fiber, the first and second
recesses not contacting one another; a first electrically
conductive material deposited in the first recess by vapor
deposition or sputtering to form a first electrode; a second
electrically conductive material deposited in the second recess by
vapor deposition or sputtering to form a second electrode; and a
Bragg grating; wherein the electro-optic waveguides are configured
to receive the light signals having reverse-handedness circular
polarization states and to convert the circular polarization states
back to orthogonal linear polarization states and thereby generate
an output signal, having the selected wavelengths, on the second
optical fiber..Iaddend.
.Iadd.13. The filter of claim 12, further comprising: a second
polarization beam-splitter coupled to the first and second optical
fibers and configured to receive an input light signal at a
selected wavelength in the first optical fiber and to convert the
light signal at the selected wavelength to the first linear
polarization state in the first optical fiber and to convert the
input light signal to the second linear polarization state in the
second optical fiber; a third electro-optic waveguide disposed in
the first optical fiber, intermediate the second polarization
beam-splitter and the first electro-optic tunable Bragg grating and
configured to convert the light signal having the first linear
polarization state to a light signal having a circular polarization
state; and a fourth electro-optic waveguide disposed in the second
optical fiber intermediate the second polarization beam-splitter
and the second electro-optic tunable Bragg grating and configured
to convert the light signal having the second linear polarization
state to a light signal having a circular polarization state, to
thereby add the light signal at the selected wavelength to the
input light signal..Iaddend.
.Iadd.14. The filter of claim 12, wherein the first and second
electro-optical tunable Bragg grating disposed in the first and
second optical fibers, respectively, configured to shift a light
spectra of light signals passed through the first and second
electro-optical tunable Bragg grating, respectively..Iaddend.
.Iadd.15. The filter of claim 12 wherein the first and second
tunable electro-optical electro-optic waveguides are poled
electro-optic waveguides..Iaddend.
.Iadd.16. The filter of claim 12 wherein the first polarization
beam-splitter is configured to convert the input light signal to
two orthogonal linear polarization states comprising a vertical
polarization and a horizontal polarization,
respectively..Iaddend.
.Iadd.17. The filter of claim 12 wherein the input light signal in
the first optical fiber is randomly polarized..Iaddend.
.Iadd.18. An optical filtering method, comprising: converting a
polarization state of a first input light signal to a first linear
polarization state in a first optical fiber, the first optical
fiber comprising a core and a cladding; converting the polarization
state of the first input light signal to a second linear
polarization state being orthogonal to the first linear
polarization state in a second optical fiber, the second optical
fiber comprising a core and a cladding; converting the light from
the first input signal having the first linear polarization state
in the first optical fiber to light from the first input signal
having a circular polarization state having a predetermined
handedness; converting the light from the first input signal having
the second linear polarization state in the second optical fiber to
light from the first input signal having a circular polarization
state having a predetermined handedness; using first and second
electro-optical tunable Bragg grating disposed in the first and
second optical fibers, respectively, to reflect the light signals
of selected wavelengths in the light of the first input signal and
to reverse the handedness of the circular polarization states, the
first and second electro-optic tunable Bragg grating each
comprising: a first recess within the cladding of its respective
optical fiber; a second recess within the cladding of its
respective optical fiber, the first and second recesses not
contacting one another; a first electrically conductive material
deposited in the first recess by vapor deposition or sputtering to
form a first electrode; a second electrically conductive material
deposited in the second recess by vapor deposition or sputtering to
form a second electrode; and a Bragg grating; and receiving the
light signals having reverse-handedness circular polarization
states and converting the circular polarization states back to
orthogonal linear polarization states and thereby generating an
output signal, having the selected wavelengths, on the second
optical fiber..Iaddend.
.Iadd.19. The method of claim 18, further comprising receiving a
second input light signal having an additional selected wavelength;
converting the polarization state of the second light signal at the
additional selected wavelength to first and second orthogonal
linear polarization states in the first and second optical fibers,
respectively, and converting the light from the second input signal
having the first and second orthogonal linear polarization states
to light signals having circular polarization states, respectively,
to thereby add the light from the second input signal at the
additional selected wavelength to the first input light
signal..Iaddend.
.Iadd.20. The method of claim 18, further comprising using the
first and second tunable electro-optical Bragg grating disposed in
the first and second optical fibers, respectively, to shift a light
spectra of light signals passed through the first and second
electro-optical tunable Bragg grating, respectively..Iaddend.
.Iadd.21. The method of claim 18 wherein converting the
polarization state of the first input light signal to the first and
second orthogonal linear polarization states is performed by a
polarization beamsplitter disposed along the first and second
optical fibers..Iaddend.
.Iadd.22. The method of claim 18 wherein converting the light
signals having the two orthogonal linear polarization states in the
first and second optical fibers to light signals having circular
polarization states is performed by first and second electro-optic
waveguides disposed in the first and second optical fibers,
respectively..Iaddend.
.Iadd.23. The method of claim 22 wherein receiving the light
signals having reverse-handedness circular polarization states and
converting back to orthogonal linear polarization states is
performed by the first and second electro-optic
waveguides..Iaddend.
.Iadd.24. The method of claim 22 wherein the first and second
electro-optic waveguides are poled electro-optic
waveguides..Iaddend.
.Iadd.25. The method of claim 18 wherein converting the
polarization state of the first input light signal to two
orthogonal linear polarization states comprises converting a random
polarization of the first input light signal to two orthogonal
linear polarization states comprising a vertical polarization and a
horizontal polarization in first and second optical fibers,
respectively..Iaddend.
.Iadd.26. The method of claim 18 wherein the polarization state of
the first input light signal is random polarization and converting
the polarization state of the first input light signal to the first
linear polarization state in the first optical fiber comprises
converting the random polarization state of the first input light
signal to the first linear polarization state in the first optical
fiber..Iaddend.
.Iadd.27. An optical filtering method, comprising: splitting an
input light signal having a random polarization into a first light
signal having a linear polarized state on a first fiber and a
second light signal having a linear polarized state on a second
fiber, the first and second light signals having orthogonal linear
polarization states; converting the light signals having the first
and second orthogonal linear polarization states to light signals
having circular polarization states with a predetermined
handedness; reflecting light signals of selected wavelengths having
circular polarization states using a tunable electro-optic Bragg
grating to generate reflected light signals having reverse
predetermined handedness circular polarization states, the tunable
electro-optic Bragg grating comprising: an optic fiber comprising a
core and a cladding; a first recess within the cladding of the
optical fiber; a second recess within the cladding of the optical
fiber, the first and second recesses not contacting one another; a
first electrically conductive material deposited in the first
recess by vapor deposition or sputtering to form a first electrode;
a second electrically conductive material deposited in the second
recess by vapor deposition or sputtering to form a second
electrode; and a Bragg grating; converting the reflected light
signals of selected wavelengths having reverse predetermined
handedness circular polarization states back to orthogonal linear
polarization states; and combining the reflected light signals of
selected wavelengths with orthogonal linear polarization states
thereby generating an output signal having the selected
wavelengths..Iaddend.
.Iadd.28. The method of claim 27 wherein converting the light
signals having the first and second orthogonal linear polarization
states to light signals having circular polarization states used a
poled electro-optic waveguide..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electro-optically active
waveguide segments, and more particularly to the use of a selective
voltage input to control the phase, frequency and/or amplitude of a
propagating wave in the waveguide.
2. Description of the Prior Art
Modulation and switching of optical signals are basic functions in
an optical communication system. Through modulation, the
information to be communicated is expressed in one or more
parameters of a light signal, such as the amplitude, the
polarization, the phase or frequency of the field, or of the
magnitude or spatial distribution of the power and/or intensity.
Through switching, the light signal may be routed through a network
of optical nodes and connections. Optical connections are mostly
realized with the help of glass fibers.
Moreover, standard glass fibers are not polarization-retaining,
whereas many optical devices, such as receivers, switches, and
modulators, are polarization sensitive. With a view towards low
cost mass production, precisely integrated embodiments are of great
importance for large-scale introduction such as, e.g., in optical
communication and/or distribution networks with large numbers of
connections. Therefore, a cheap, stable and electrically
controllable polarization control devices are needed to satisfy a
long felt need in the communications industry.
Several devices have been disclosed that provide phase control. For
example, U.S. Pat. No. 5,239,407, by Brueck et al, discloses the
use of thermal poling in a waveguide to establish a preferred
non-linearity. One significant drawback to this approach is that
this non-linearity may only be achieved along one axis.
Additionally, the non-linearity is generated only on one side of
the sample, e.g., the positive biased side of the sample and
therefore does not effect a wavefront in a consistent manner across
the wavefront. Finally, to avoid breakdown, a significant portion
of the cladding layer is removed and thus the optical loss in this
device is significant.
Fujiwara et al. discloses, in an article entitled "UV Excited
Poling and Electrically Tunable Bragg Gratings in a Germanosilicate
Fiber," two innovations: 1) the use of an ultraviolet (UV) beam in
combination with an applied electric field to produce poling; and
2) the use of two internal electrodes for applying a voltage across
a Bragg grating. Their technique, however, has a number of
drawbacks. Specifically, the fiber is drawn from a preform with two
holes for electrode wires that are to be inserted following the
fiber drawing. This wire insertion is a difficult manufacturing
step. To avoid breakdown, one wire is inserted from each end of the
fiber. This means that the modulation frequency is limited to low
values since a high-speed travelling wave geometry is not possible.
Furthermore, splicing to either end of the fiber is not possible
because of the electrodes. Thus, discrete optical system alignment
for coupling into the fibers would be necessary and would negate
any benefit from an electro-optically active fiber segment.
Additionally, the use of UV poling as taught by Fujiwara et al. has
the added disadvantage of requiring custom fabrication of a preform
that are relatively difficult to fabricate.
Finally, U.S. Pat. No. 5,617,499, by Brueck et al., discloses a
poled electro-optic fiber segment as illustrated in FIG. 1 of the
present application. The device comprises a first electrical
contact 1, a fiber core 2, a cladding 3, and a second electrical
contact 4. As may be seen, a significant portion of cladding 3 has
been removed. Thus, the optical loss in this device may be
significant. Additionally, the modulation frequency is limited to
low values since a high-speed travelling wave geometry is
significantly restricted. Finally, the Brueck et al. device is
limited to temperature/electric poling because access to core 2 via
cladding 3 is impaired by first electrical contact 1 and the bottom
electrical contact 4 making it difficult to perform UV poling in
such a structure. Further, poling using elevated temperatures may
significantly degrade the reflectivity of a Bragg grating disposed
in the poled waveguide.
Therefore, there needs to be an easily manufactured poled
electro-optic fiber segment which allows for poling by: 1) voltage,
2) ultraviolet radiation, 3) thermal heating, or 4) any combination
of the above.
The use of tunable Bragg gratings is known in the prior art. These
gratings have been controlled by thermal tuning, piezoelectric
tuning, Fabry-Perot tuning, and refractive index tuning in bulk
semiconductors. Each of these methods of tuning has disadvantages
associated with them. In particular, the speed of tuning is in the
range of a few milliseconds for thermal tuning and a few .mu.sec
for piezoelectric tuning. Additionally, such methods have the
disadvantage of applying stress on an optical fiber that can reduce
the strength of the fiber over a period of time. Finally, these
methods tend to be very bulky. On the other hand, integration of
bulk non-linear materials such as lithium niabate to a fiber
segment is lossy, expensive and is not desirable. Therefore, other
methods of tuning a Bragg grating are needed.
The first optical grating or so-called Bragg grating was made in
1978 using the standing wave pattern originating from two
counter-propagating beams in a Ge-doped core optical fiber. Since
that time, techniques have been developed to exploit the
photosensitivity of germanosilicate fibers, the photosensitivity
being established by the bleaching of oxygen deficient centers by
UV light to create the regions of differing refractive index. The
refractive index change, which is induced by the UV light, arises
from the creation of polarizable color centers and structural
rearrangement of the glass network.
While these gratings have evolved, an efficient control system for
this type of grating has not yet been demonstrated. Several
attempts to develop such as control system have fallen short of the
goal of a commercially usable device. For example, U.S. Pat. No.
5,617,499, by Brueck et al, discloses the use of a Bragg grating in
a poled device, but provides no teaching of control. Additionally,
Fujiwara et al. discloses the use of a Bragg grating in their
device, but provides no teaching of control. Unfortunately, both
these devices have the drawbacks discussed above.
Therefore, there needs to be an easily manufactured electro-optic
fiber segment having a tunable Bragg Grating which reduces the
possibly of breakdown during poling while achieving a smaller
dimmension between electrodes without producing a high loss for the
mode of propagation in the core region or without degrading the
Bragg grating.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
poled electro-optic waveguide segment which allows for poling by:
1) voltage, 2) ultraviolet radiation, 3) thermal heating, or 4) any
combination of the above.
It is yet another object to provide a means for allowing
polarization control of a signal in a waveguide by a selective
voltage input.
It is a further object to provide an electro-optic fiber segment
having a tunable Bragg grating which reduces the possibly of
breakdown during poling while achieving a smaller dimmension
between electrodes without producing a high ion for the mode of
propagation in the core region or without degrading the Bragg
grating.
It is yet another object to provide a tunable Bragg grating which
does not increase signal loss due to attenuation in the cladding
layer.
It is yet another object to provide optical systems which utilize
tunable Bragg gratings and tunable bioinfringent devices.
In all of the above embodiments, it is an object to provide an
electro-optic waveguide segment that may be easily attached to
conventional waveguides.
Finally, it is an object of the invention to provide an
electro-optic waveguide which may be poled by using an external
electrode which is disposed in a recess in the cladding
material.
Other objects and features of the present invention will be
apparent from the following detailed description of the preferred
embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in conjunction with the
accompanying drawings, in which:
FIG. 1 is a perspective view of a prior art electro-optic waveguide
segment;
FIG. 2 is a cross sectional view of a electro-optic waveguide
segment constructed in accordance with a preferred embodiment of
the invention;
FIG. 3 is a cross sectional view of a electro-optic waveguide
segment constructed in accordance with an alternate embodiment of
the invention;
FIG. 4 is an exploded detail view of the recess of FIGS. 2, 3, and
5;
FIG. 5 is a cross sectional view of a electro-optic waveguide
segment constructed in accordance with another alternate embodiment
of the invention;
FIG. 6 is a cross sectional view of a electro-optic waveguide
segment constructed in accordance with yet another alternate
embodiment of the invention;
FIG. 7 is a graph of transmitted intensity v. wavelength for the
electro-optic waveguide illustrated in FIG. 6 and constructed in
accordance with an embodiment of the invention;
FIG. 8 is a cross sectional view of a modulator utilizing the
electro-optic waveguide illustrated in FIG. 6;
FIG. 9 is a graph of intensity v. wavelength for the modulator
illustrated in FIG. 8;
FIG. 10 is block diagram of a Michelson interferometer utilizing
the electro-optic waveguide illustrated in FIG. 6;
FIG. 11 is a block diagram of a frequency modulator utilizing the
electro-optic waveguide illustrated in FIG. 6;
FIG. 12 is a block diagram of a wavelength add-drop multiplexer or
filter constructed in accordance with a preferred embodiment of the
invention and utilizing the electro-optic waveguides illustrated in
FIGS. 2, 3, and 5;
FIG. 13 is a block diagram of a high-speed 1.times.2 switch
constructed in accordance with a preferred embodiment of the
invention and utilizing the electro-optic waveguides illustrated in
FIGS. 2, 3, and 5;
FIG. 14 is a block diagram of an amplitude modulator constructed in
accordance with a preferred embodiment of the invention and
utilizing the electro-optic waveguides illustrated in FIGS. 2, 3,
and 5;
FIG. 15 is a block diagram of a reflection isolator for a polarized
source constructed in accordance with a preferred embodiment of the
invention and utilizing the electro-optic waveguides illustrated in
FIGS. 2, 3, and 5; and
FIG. 16 is a block diagram of an alternate embodiment of the
isolator illustrated in FIG. 15, but having an unpolarized
source.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Before a substantive discussion of the preferred embodiment may
begin, it is essential to define several key terms.
The term "waveguide" is used in this application to mean any device
used to channel an optical signal, at any frequency. Specific
examples of waveguides include, but are not limited to: fiber-optic
waveguides, planar glass, as well as crystalline and semiconductor
waveguides.
The term "fiber optic cable" or "fiber optic waveguide" is used in
this application to mean any waveguide utilized to guide light
waves from one point to another. This definition specifically
includes both single mode and multi-mode fibers as well as any
waveguide having any cross-sectional shape. In addition, this term
also includes any waveguide whether doped or undoped.
The term "input voltage" is used in this application to mean any
voltage that is applied to the devices discussed below. In
particular embodiments, specific voltages are used. Examples of
these voltages include, but are not limited to: a DC voltage, an AC
voltage, and pulsed voltage.
The term "electrical contact" is used in this application to mean
any material having the property of electrical conductivity.
Examples of electrical contacts include, but are not limited to:
knife edges, cono-fusical projections, wires made with either
metallic, semi-metallic or semi-conducting materials, and
conventional planar and non-planar deposited materials.
The term "Bragg grating" is used in this application to mean a
structure containing alternating periodic segments of varying
periods of high and low refractive index material segments and/or
appropriately embedded phase shift segments at well defined
locations of the structure. A period is defined as one set of
adjacent high and low refractive material segments. It is
understood by this definition that the order of the high and low
material is irrelevant, only that there is a change in refractive
index between adjacent segments. While only uniform gratings are
illustrated, non-uniform gratings are also contemplated within the
scope of the invention.
The term "dopant" is used in this application to mean any
extraneous element or combination thereof which is added to a
material to enhance or suppress a characteristic of that material.
Examples of dopants include, but are not limited to: germanium
(Ge), hydrogen (H), sodium (Na), lithium (Li), lead (Pb), zirconium
(Zr), zinc (Zn), erbium (Er), praseodymium (Pr), thulium (Tm),
potassium (K) and calcium. The particular characteristics of
interest in this application are photosensitivity of the material
and/or non-linearity created due to poling.
The term "recess" or "groove" is used in this application to mean
any removal of material in the cladding. The recess or groove is
defined by a surface which is controlled by the variables R and
.alpha.. In a preferred embodiment, .alpha. would be between 0 and
<=180 degrees and R would be between 0 and +infinity, and from
-infinity to -r, i.e., having a concave or convex surface,
respectively. Additionally, r is defined as the closet radius of
curvature of a near circular waveguide core. It should be
appreciated that recesses or grooves may be part of a preform or
formed after waveguide creation. Additionally, recesses or grooves
may or may not extend the entire direction of propagation of the
waveguide.
Poled Electro-Optic Waveguide
With reference to the Figures, wherein like references characters
indicate like elements throughout the several views and, in
particular, with reference to FIG. 2, a electro-optic waveguide
segment 10 is illustrated. The electro-optic fiber segment 10
comprises a fiber-optic waveguide 12 having a core 14 and cladding
16. Disposed in contact with waveguide 12 is a first electrical
contact 18 and a second electrical contact 20 preferably on
opposite sides of waveguide 12.
It should be appreciated that any waveguide may be used in the
poled electro-optic Device. The following discussion addresses the
use of a fiber optic waveguide to illustrate how the teachings of
the present invention may be utilized in an optical system. The
teachings of this invention may be used with any of the waveguides
defined above.
In a preferred embodiment waveguide 12 is a D-shaped fiber optic
waveguide having a 2.times.4 .mu.m core and having a germanium
oxide (GeO.sub.2) dopant with a concentration of between 0% and
30%. Preferably, a 15% GeO.sub.2 doping would be utilized. Such a
fiber is commercially available from KVH Corporation. It should be
appreciated that while germanium is a preferred dopant, other
dopants such as (but not limited to) hydrogen, sodium, lithium,
lead, zirconium, and zinc may be utilized. The purpose of these
dopants is to increase the photosensitivity of core 14 of waveguide
12. Therefore, any dopant may be utilized in conjunction with the
teachings of this invention so long as the dopant has the property
of enhancing photosensitivity.
Additionally, other approaches to increasing photosensitivity may
be employed with the teachings of this invention. For example, the
fabrication of waveguide 12 maybe conducted in a reduced pressure
environment that has the effect of increasing photosensitivity of
waveguide 12. It should be appreciated that the above example is
merely illustrative of one way in which photosensitivity is
effected by fabrication of waveguide 12 and that any other
fabrication technique may be utilized with the teachings of this
invention.
As may be seen in the fully manufactured device illustrated in
FIGS. 5 and 6, a voltage is applied to second electrical contact
20. Contact 18 is in turn connected to ground and thus completes
the electrical circuit. The specific voltage that is applied is
critical in the operation of the device. Particular voltages and
their effect shall be described in conjunction with the device
descriptions provided below.
We shall first describe in general terms the manufacturing process
for electro-optic fiber segment 10. As discussed above, a
conventional waveguide 12 is utilized in this invention. This
waveguide preferably is doped with a material that will enhance
photosensitivity or is processed in a manner to allow for the same.
Examples of dopants include, but are not limited to: germanium
(Ge), hydrogen (H), sodium (Na), lithium (Li), lead (Pb), zirconium
(Zr), zinc (Zn), erbium (Er), praseodymium (Pr), thulium (Tm),
potassium (K) and calcium. These enhanced waveguides are
commercially available and therefore will not be described in
detail.
The first step is the formation of grooves 26 by the removal of
selective portions of cladding 16 of the preform before waveguide
12 is drawn. The main purpose of this step is to assure that
grooves 26 have an outer dimmension that is less than an unmodified
section of cladding 16. It should be appreciated that the more
cladding 16 material one is able to remove, the lower the voltage
required in the poling step. While this step has been described as
conducted in the preform stage, it should be appreciated that this
step may be performed even after the formation of waveguide 12.
Turning now to FIG. 4, an exploded view of groove 26 is provided.
As may be seen groove 26 has a surface 19 which is shown to be
smooth and tapered for clarity. It should be appreciated that
surface 19 may be abrupt and/or irregular without departing from
the scope of the invention. As illustrated groove 26 is provided
with a concave segment in contact with cladding 16 and has a
surface 19 which is defined by R and .alpha.. In a preferred
embodiment, .alpha. would be between 0 and <=180 degrees and R
would be between 0 and +infinity, and from -infinity to -r, i.e.,
having a concave or convex surface 19, respectively. Additionally,
r is defined as the closet radius of curvature of a near circular
waveguide core.
Next, a first electrically conductive material 20 may be deposited
within grooves 26, thereby forming a first electrode 20. A major
improvement of this invention is the use of cono-fusical
projections 22 and 24 with and without first constructing electrode
20. Examples of cono-fusical projections 22 and 24 include, but are
not limited to: knife edge, wires made with either metallic,
semi-metallic or semi-conducting materials, and conventional planar
and non-planar deposited materials. The use of these projections 22
and 24 allow for a much simplified manufacturing process. As maybe
seen in FIGS. 2 and 3, projections 22 and 24 are disposed in good
electrical contact with groove or removed region 26. In a preferred
embodiment, these projections would be in direct contact with the
bottom of groove 26. By utilizing such a structure, the voltage
that may be applied prior to breakdown may be significantly higher
than that of either Brueck et al. or Fujiwara et al.
The use of such projections solves a long-felt need in the industry
for a simple low-cost fabrication of waveguides 10, 10', 10'', and
10'''. This is because one is able to position projections 22 and
24 very close to core 14 without significantly degrading either the
structural integrity of waveguide segment 12 or the operational
characteristics of waveguide segment 12. Another important feature
of this approach is the ability to significantly reduce or
eliminate any air gaps or pockets (as occurs in prior art devices)
between projections 22,24 and cladding 16. These air gaps are a
significant factor in the voltage needed for polling of such
devices. Additionally, by using this approach, the modulation
frequency is no longer limited to low values as in Fujiwara et al.
since a high-speed travelling wave geometry is now possible.
Furthermore, splicing to either end of the fiber is quite easy
because of the electrodes would be disposed in groove 26. Thus,
discrete optical system alignment for coupling into the fibers is
no longer necessary. Finally, UV poling is possible with this
device since access to cladding 16 is not inhibited.
While we have discussed placing projections 22,24 in direct contact
with cladding 16, it should be appreciated that cladding 16 may be
entirely removed and projections 22,24 may contact core 14.
Additionally, a contact 20,20 may be placed between projections
22,24 and core 14 as illustrated in FIGS. 2 and 3.
After projections 22,24 have been placed, waveguide segment 12 is
poled using at least a electric field applied to projections 22,24
to induce an non-linearity in waveguide segment 12. It should be
appreciated that by selecting the appropriate voltage, one may
generate a specific non-linearity. Examples of these voltages
include, but are not limited to: a DC voltage, an AC voltage, and
pulsed. In a preferred embodiment, a DC voltage having a magnitude
of 4 Kv was applied for 15 minutes.
One major improvement contemplated by this invention is the ability
to pole waveguide segment 12 by using an applied voltage alone.
It should be appreciated that while the invention contemplates the
use of only one input to pole waveguide 12, one may utilize the
well known approaches of either Brueck et al. or Fujiwara et al.
with the teachings of this invention. By this, applicant means that
the poling step may be performed using ultraviolet light injected
into cladding 16 in combination with said predetermined electric
field, as illustrated in FIGS. 2 and 3. It should be appreciated
that UV poling has produced the largest non-linearity, e.g.
r.sub.33=6.0 pm/V. Additionally, the poling step may be performed
using the electrical field in combination with heating waveguide
segment 12. Finally, all three inputs may be combined to
significantly reduce the magnitude and/or duration of the
predetermined electric field. It should be appreciated that while
FIGS. 2 and 3 illustrate only one combination, any of the above
combinations may be utilized with the teachings in these
figures.
It should be appreciated that while the preferred embodiment
utilizes projections 22,24, there are other methods of
accomplishing this step. For example, contacts 18,20 may be formed
before the poling step to allow an electrical interface to core
14.
The next step involves the placement of another electrode 18. It
should be appreciated that there are several ways in which this
step may be accomplished. The first method for accomplishing this
is to place waveguide segment 10' on a second electrically
conductive material 18 which is not in contact with electrode 20.
In this manner, an electrical circuit is formed.
The physical affixing of waveguide segment 10 to electrically
conductive material 18 may be accomplished in many ways well known
in the prior art and therefore will not be discussed in any detail.
After contacts 18 and 20 are positioned, waveguide 12 may be poled
as discussed above. It should be appreciated that the teachings of
the two embodiments may be combined in that one projection 20 or 22
may be used in conjunction with one or more contacts 18 and/or
20.
Finally, in one embodiment illustrated in FIGS. 3, 5 and 6,
conductive material 20 may also be transparent. This would allow
the ability to completely fill grooves 26 as illustrated in FIGS.
3, 5 and 6. This has a significant advantage in that it overcomes a
major problem with prior art devices. By having optical material
replaced in grooves 26, structural integrity of waveguide 12 is
maintained. Thus, making it easier to package. Additionally, high
modulation frequencies should be more obtainable for all of these
structures.
Now that we have discussed the manufacturing of electro-optic
waveguide segment 10, we will discuss its operation. Turning to
FIG. 5, electro-optic waveguide segment 10 has been processed as
described above. In particular, it has been poled to create a
non-linearity that behaves differently in two orthogonal axes.
When voltage is applied to poled waveguide section 10, a phase
shift created which is different for the two orthogonal
polarization modes of waveguide 10. By applying appropriate
voltages, one may introduce different phase shifts which scatter
the input polarization mode to any other polarization mode and
hence functions like a variable waveplate that is governed by the
equation below: .pi..times. .times..lamda..function. ##EQU00001##
where V.sub..pi. is the voltage required to introduce a .pi. phase
shift between the two orthogonal linear eigenmodes of waveguide 12.
Therefore, assuming r.sub.33=0.3 pm/V, .lamda..sub.0=1.55 .mu.m,
L=10 cm, one need D to equal 4 .mu.m for V.sub..pi..about.50 V. If
the input voltage is V.sub..pi./2 as illustrated in FIG. 5 and the
input signal is linearly polarized, one would generate a circularly
polarized output. It should be appreciated that this is merely one
example of a particular non-linearity induced by poling. It should
be appreciated that any desired phase effect may be generated by
utilizing the teachings of the present invention.
Next, we shall discuss how to apply the above teachings to a
waveguide segment having a tunable Bragg grating. It should be
appreciated that the above devices have been poled while the
devices discussed below may or may not be poled. In addition,
details of the general processing steps are provided.
Tunable Bragg Grating
With reference to the Figures, wherein like references characters
indicate like elements throughout the several views and, in
particular, with reference to FIG. 6, a electro-optic waveguide
segment having a tunable Bragg Grating 10''' is illustrated. The
tunable Bragg grating waveguide segment 10''' comprises a
fiber-optic waveguide 12 having a core 14 and cladding 16. Disposed
in contact with waveguide 12 is a first electrical contact 18 and a
second electrical contact 20 preferably on opposite sides of
waveguide 12. Segments of high index material 28 are formed within
core 14 by a process described below. As may be seen, there are
alternating segments of high index material 28 and unmodified core
30, any two forming a period in the Bragg grating. For convenience,
only a few periods have been illustrated, it should be appreciated
that any number of periods may be formed as described below.
Turning now to the construction of tunable Bragg grating waveguide
segment 10''', it should be appreciated that the teachings
associated with waveguide 10 may be used in conjunction with this
invention. The first step for manufacturing tunable Bragg grating
waveguide segment 10''' is the selective removal of at least a part
of cladding 16. FIGS. 2, 3, 5 and 6 illustrate how at least a
portion of cladding 16 is removed. While it might appear in FIG. 2
that the cladding 16 is completely removed in certain regions, this
is not the case. In fact, it is highly undesirable to completely
remove cladding 16 in the embodiment illustrated in FIG. 2 while
cladding 16 may be completely removed in the embodiment illustrated
in FIG. 6. Applicant has found that by reducing the thickness of
cladding 16, in select regions, so that the distance "D" between
proximal faces of contacts 18 and 20 is less than 5 .mu.m, one may
significantly reduce the input voltage to second electrical contact
20 and achieve the same tunability of tunable Bragg grating
waveguide segment 10'''. In the alternative, the voltage may be
increased and a larger shift may be achieved in tunable Bragg
grating waveguide segment 10'''. The voltage sensitivity of the
operational tunable Bragg grating waveguide segment 10''' is
described in detail below.
It should be appreciated that the term input voltage is quite
different that the voltage used for poling. The input voltage is
used for selective control of tunable Bragg grating waveguide
segment 10''' while the poling voltage is used to create a
non-linearity in tunable Bragg grating waveguide segment 10'''. As
discussed above, tunable Bragg grating waveguide segment 10''' does
not need to be poled to function and in certain instances, it is
undesirable to pole tunable Bragg grating waveguide segment
10'''.
While it is preferable to maintain the proximal surfaces of
cladding 16 in close proximity, it should be appreciated that this
removal step may be eliminated so long as one is willing to induce
a higher voltage into tunable Bragg grating waveguide segment
10'''. Additionally, applicant prefers to mechanically polish
waveguide 12, but it should be appreciated that any chemical
etching or other process means may be utilized to reduce the
thickness of cladding 16. Several examples of other methods
include, but are not limited to: chemical etching, ion beam
etching, and laser etching.
The next step in the manufacturing process is the formation of
Bragg gratings. This is performed in a conventional fashion by
ultra-violet (UV) irradiation, using a holographic fringe pattern
of two interfering beams. The uniform grating is constituted by
periodic, linearly spaced regions 28 and 30 of alternating high and
low refractive index material extending in the direction of light
propagation. Preferably, the grating as initially formed has a
total length of 5 mm and .about.1000 periods. It is contemplated
within the scope of the invention that the length and/or the number
of periods may be modified. It should also be appreciated that
Bragg gratings may be formed in core 14 and/or cladding 16.
Additionally, it is contemplated that other post-processing
operations may be performed to produce different structures,
including for example, structures in which one or more concomitant
regions of the grating are exposed to further, localized
irradiation and including structures in which region of high
refractive index 28 is irradiated to further increase the depth of
refractive index modulation in that region. While the above
discussion focuses on increasing the refractive index in region 28
with respect to region 30, it should be appreciated that the same
effect may be achieved by reducing the refractive index of region
28 with respect to region 30. Therefore, as long as the process
changes the relative refractive index between regions 28 and 30, a
Bragg grating will be formed. Thus, any process for increasing or
decreasing the refractive index is contemplated by this invention.
Examples of such processes include, but are not limited to: ion
etching, chemical etching, and ion-implantation.
In the preferred embodiment, the Bragg grating is formed before
poling. This is the preferred process when there is: 1) no poling
2) poling by voltage alone, or 3) poling when thermal processes are
involved. This is not the case in all embodiments. For example, if
one is to utilize UV radiation in the poling process, it is
preferable to pole first and then form Bragg grating after the
poling step. For the rest of this discussion, we will assume that
the Bragg grating is formed before poling.
The above description has discussed one particular method of
forming Bragg gratings. It should be appreciated that any other
method know in the art may be used to form Bragg gratings in
conjunction with the teachings of this invention. Examples of such
methods include, but are not limited to: phase masks, etching and
redepositing of materials, holographic interfaces and prism
interfaces.
After the Bragg gratings are formed, contacts 18 and 20 are
deposited in the polished or removed regions 26. Any process for
depositing electrically conductive material may be utilized in this
step. Preferably, a vapor deposition process will be utilized, but
other processes such as, but not limited to: sputtering may be
substituted. In a preferred embodiment, contacts 18 and 20 would be
made of either gold, chromium, aluminum, and/or nickel. It should
be appreciated that contacts 18 and 20 may be of similar or
different material. Additionally and material may be used for
contacts 18 and/or 20 so long as the material has the property of
electrical conductivity. Examples of such materials include, but
are not limited to: gold, chromium, aluminum, palladium, and
nickel. Contacts 18 and 20 preferably have profiles that do not
exceed the removed region 26 of cladding 16, as shown by the dashed
line in FIG. 3. FIGS. 5 and 6 illustrates contacts 18 and 20 as
being at the same level as cladding 16. By having contacts 18 and
20 located substantially within removed regions 26, one is able to
construct a more durable waveguide 12. While this feature may seem
simple, it is counter intuitive in that having less material makes
a structure that is substantially more durable in use.
Additionally, the use of a cono-fusical contacts or projections 22
and/or 24, such as but not limited to a wire or finger projection,
may be used in conjunction with contacts 18 and/or 20. These
contacts or projections 22 and/or 24 are preferably biased to
engage contacts 18,20 or cladding 16 (if contacts 18,20 are not
present) by a spring or other well known biasing means. As
discussed above, there is no need for contacts 18,20 for the poling
process. These contacts 18 and 20 are preferably used in the
operational device.
It should be appreciated that by having contact 18 and/or 20 not
exceeding removed region or groove 26, two major improvements are
achieved. First, the alignment of cono-fusical contacts 22 and/or
24 is assisted by having a groove 26 to reside within. Second, and
more importantly, the breakdown caused by air gaps is significantly
reduced.
It should be appreciated that it is contemplated within the scope
of the invention that contacts 18 and 20 may have profiles that
exceed removed regions 26 as illustrated in FIG. 1. While this
embodiment would be less desirable, it would still provide
functionality to tunable Bragg grating waveguide segment 10''' and
is considered within the scope of the invention.
It should be appreciated that any embodiment for waveguide segments
10, 10', and 10'' may be used in conjunction with the teachings for
tunable Bragg grating waveguide segment 10'''.
In one embodiment, cladding 16 may be entirely removed or partially
removed as illustrated in FIGS. 2, 3 and 5. As may be seen surface
19 is illustrated as being gradually tapered. While a gradual taper
is the preferred embodiment, it should be appreciated that any
interface may exist between cladding 16 and contacts 18 and 20. For
example, FIG. 6 illustrates an abrupt edge or surface 19. In this
embodiment, the material for contacts 18 and 20 has the additional
feature of being optically transparent, i.e., allowing an optical
signal to be transmitted therethrough. Examples of materials for
optically transparent contacts 18 and 20 include, but are not
limited to: heavily doped semiconductor such as polysilicate,
indium, tin oxide, gallium arsenide, iridium phosphate, and
aluminum arsenide.
Returning to the description of the preferred embodiment, the next
step in the formation of tunable Bragg grating waveguide segment
10''' is optional. This step is the poling of tunable Bragg grating
waveguide segment 10'''. This process is conducted as discussed
above. In an embodiment, tunable Bragg grating waveguide segment
10''' would be poled by exposing waveguide 12 to a 260.degree. C.
environment for 15 minutes and applying a predetermined poled
voltage of 4 Kv. It should be appreciated that the particular time
and/or temperature may be varied and temperatures between 0.degree.
C. and 100.degree. C. as well as times between 0 and three years
are considered within the scope of the invention.
Finally, post-processing steps may be performed such as encasing at
least a portion of tunable Bragg grating waveguide segment 10''' in
a protective coating such as plastic or glass. It should be
appreciated that this step is optional and is only provided to
illustrate that further post-processing steps may be utilized in
conjunction with the teachings of this invention.
It should be appreciated that while a particular sequence has been
described above, the order of the steps may be modified so long as
the teaching of the invention is utilized. An example of such a
modification is the particular step at which the Bragg grating is
formed. The grating may be formed at any step in the manufacturing
process.
Now that the method for forming tunable Bragg grating waveguide
segment 10''' has been described, we will discuss how the device
functions. By applying a voltage (V.sub.t), the average refractive
index of tunable Bragg grating waveguide segment 10''' is modified.
This, in turn, allows tunable Bragg grating waveguide segment 10'''
to affect a wavefront by selectively reflecting and/or selectively
passing particular frequencies through tunable Bragg grating
waveguide segment 10'''. Thus, tunable Bragg grating waveguide
segment 10''' may be viewed as a selective gate for allowing
specific frequencies through while reflecting others. Now that the
general concept has been described, the discussion shall turn to
the mathematical relationships that allows for the selective
control of tunable Bragg grating waveguide segment 10'''.
The shift (.DELTA..lamda.) in the peak wavelength (.lamda..sub.B)
of tunable Bragg grating waveguide segment 10''' is due to a shift
in the average refractive index in tunable Bragg grating waveguide
segment 10'''. This may be defined as: .DELTA..times.
.times..lamda..lamda..times..DELTA..times.
.times..lamda..times..times. ##EQU00002## where r is the
appropriate electro-optic (e-o) coefficient in the poled waveguide,
n is the average refractive index of tunable Bragg grating
waveguide segment 10''', V.sub.t is the applied voltage, D is the
distance between proximal faces of electrodes 18 and 20.
The Bragg condition is defined as: .LAMBDA..lamda..times.
##EQU00003## where .LAMBDA. is the period of tunable Bragg grating
waveguide segment 10''', n.sub.m is the effective modal index, and
.lamda..sub.B is the Bragg wavelength.
As may be seen, by controlling the formation process, setting the
number of periods, and establishing the average refractive index of
tunable Bragg grating waveguide segment 10''' during fabrication,
one may design tunable Bragg grating waveguide segment 10''' to be
tunable over any desired wavelength range. Thus, by
pre-establishing the number of periods and the average refractive
index, the change in wavelength is controllable by one variable:
the magnitude of the applied voltage (V.sub.t). Additionally,
tuning speeds up to 10 GHz are capable, making such devices
attractive for communication system components such as, but not
limited to: reflection-type amplitude modulators.
This device has been fabricated and experimental results have been
generated which track closely to the expected results. What follows
is a discussion of a specific experiment and is provided by way of
an example. It should be appreciated that the following is merely
one example of the teachings of this invention and does not limit
this invention to the specific design constraints enumerated in the
example.
EXAMPLE 1
In this example, a D-shaped fiber optic waveguide 12, manufactured
by KVH Corp. and having a 2.times.4 .mu.m core diameter, was used
for Bragg grating fabrication. This fiber optic waveguide 12 had a
15% GeO.sub.2 concentration. Fiber optic waveguide 12 was processed
with the flat side of the D-shaped fiber polished to .about.8 .mu.m
from the core, with 1.5'' pigtails.
After polishing, fiber optic waveguide 12 was exposed to hydrogen
at 100 atm. and 50.degree. C. for 2 days to increase its
photosensitivity of waveguide 12. Next, the polished section of
fiber optic waveguide 12 was exposed to 193 nm radiation from an
ArF laser (20 mJ/pulse, 10 Hz) for 20 minutes through a phase mask
to form the Bragg grating. The Bragg grating formation was
monitored by coupling an edge emitting diode (ELED) source through
the fiber and observing the transmission spectrum on an optical
spectrum analyzer. This process produced a 13 dB reflective grating
observed at 1558 nm which degraded to 7 dB reflectivity upon
heating at 260.degree. C. for 12 hours; peak reflectivity shifted
to 1557 nm.
The fiber was then laid with the flat side down on a silicon (Si)
substrate or contact 18 and a 30 .mu.m polyamide layer was spin
coated around it. After hard-baking the sample at 180.degree. C.
for 15 hours, a gold electrode 20 was deposited over the tunable
Bragg grating 10. This device is illustrated in FIG. 6.
In order to establish a non-linearity in tunable Bragg grating
waveguide segment 10''', the 70 .mu.m thick fiber optic waveguide
12 was poled with 3.3 kV at 260.degree. C. for 15 minutes. The ELED
source was mechanically spliced to the D-shaped fiber pigtail
through a 30:70 coupler which enables observation of the reflection
spectrum of tunable Bragg grating 10. The other pigtail was
mechanically spliced to a 9 .mu.m core 1550 nm single mode fiber.
Each mechanical splice had a loss of .about.10 dB primarily due to
the mode mismatch of the two fibers and the difficulty in alignment
of the fiber cores.
The reflection spectrum was observed instead of the transmission
spectrum (as done conventionally) since the latter was noisy. FIG.
7 illustrates the spectral tuning performance of tunable Bragg
grating 10 as a function of the applied voltage. As may be seen,
.about.0.02 nm (2.5 GHz) tuning is observed for an applied voltage
of3 kV.
Thus, the operational device tracks closely with the expected
results.
Turning now to FIG. 5, an alternate embodiment of the invention is
illustrated. For clarity, like elements have been provided with
like reference numeral except that a double prime has been added to
each reference numeral where there is a slight difference in the
particular element in this embodiment. The following discussion
will focus on the differences between the elements of this
embodiment and that of the preferred embodiment.
As may be seen, a variable waveplate is illustrated as element
10''. The key difference between tunable Bragg grating waveguide
segment 10'''' is the lack of high and low index material 22,22'
which form Bragg gratings 10,10'. In all other respects, the
teachings for the embodiments for waveguide segments 10,10' are
applicable for tunable Bragg grating waveguide segment 10'''.
Another significant difference is that tunable Bragg grating
waveguide segment 10''' does not need to be poled.
Other Devices
Turning now to FIGS. 8 through 16, devices that utilize the
teachings of the present invention are disclosed. It should be
appreciated that these devices have novel features apart from the
inventions discussed above. In particular, devices that discuss
tunable Bragg gratings may use the novel Bragg gratings discussed
above or any other know tunable Bragg grating. In addition, devices
which discuss the use of electro-optic poled waveplates, may use
the novel variable waveplates discussed above or any other
waveplate known in the art. The key novelty, other than the
specific configuration, is the departure from the approach of
utilizing interfering wavefronts to selectively control signal
propagation, to a new approach of utilizing polarization control to
selectively control signal propagation.
Modulator
The first device, illustrated in FIG. 8, is a modulator 40 that
preferably utilizes the electro-optic waveguide 10''' illustrated
in FIG. 6. As may be seen, modulator 40 comprises a fiber segment
which is conventional in nature. An unmodulated input is inserted
into this fiber segment which then comes into contact with tunable
Bragg grating waveguide segment 10'''. Tunable Bragg grating
waveguide segment 10''' has been designed to reflect a particular
frequency and pass all other frequencies for a particular DC input
voltage V.sub.t as illustrated in FIG. 7. Instead of applying a DC
voltage, an AC input voltage V.sub.s is applied to Bragg grating
waveguide segment 10'''. This input voltage is illustrated in FIG.
9. As may be seen, FIG. 9 is a graph of wavelength v. intensity for
the modulator illustrated in FIG. 8. This AC input voltage causes
the reflected wavelength to shift as discussed above in the
operation of tunable Bragg grating waveguide segment 10'''. In
devices employing other tunable Bragg gratings, a suitable control
signal would be applied. Thus, two modulated outputs are generated,
the reflected output is reflected back to the input port. The
output port receives a modulated output which has a substantially
reduced reflected frequency component.
Michelson Interferometer
The second device, illustrated in FIG. 10, is a Michelson
interferometer 50 that preferably utilizes the electro-optic
waveguide 10''' illustrated in FIG. 6.
In long-haul WDM-based telecom and datacom systems, the individual
WDM channels undergo unequal amplification due to the non-flatness
in the gain spectrum of the erbium-doped fiber amplifier (EDFA) 52.
The amplified spontaneous emission (ASE) of optical amplifiers can
significantly degrade the performance of WDM systems by saturating
the gain of the EDFA. Variations in the gain within the bandwidth
of the erbium amplifier will be compensated for by applying
appropriate voltages to the different tunable Bragg grating
waveguide segment 10''' in arm 58 and detuning their reflection
spectra with respect to corresponding gratings 60 in the other arm
56 of Michelson interferometer 50.
The amplitude of the different input wavelengths after passing
through the EDFA will preferably be split into two equal parts by
the 50:50 coupler 54. These wavelengths will be reflected by
appropriately designed matched Bragg gratings 10''' and 60 written
in the two arms 56,58 of Michelson interferometer 50 of which one
arm 58 will consist of tunable Bragg grating waveguide segment
10'''. In an alternate embodiment, both arms 56,58 would utilize
tunable Bragg grating waveguide segment 10'''. For example,
conventional Bragg gratings 60 may be replaced by tunable Bragg
grating waveguide segment 10'''. This is illustrated in FIG. 10 by
dashed boxes over Bragg gratings 60.
Upon reflection, the different wavelengths from arms 56 and 58 will
interfere with each other and exit through output port 68.
Preferably, a 95:5 coupler 62 at output port 68 will tap off 5% of
the different wavelengths and feed them to an optical spectrum
analyzer 64 which detects any variation in the amplitude for the
different wavelengths and communicates it to the control signal
generator 66 which in turn controls the input voltage to tunable
Bragg grating waveguide segment 10'''.
Turning now to an example, suppose the amplitude of the wavelength
channel at .lamda..sub.1 is larger than that of the other channels,
control signal generator 66 will send an appropriate voltage to
tunable Bragg grating waveguide segment 10''' and cause a shift in
the reflection spectrum of a respective tunable Bragg grating
waveguide segment 10'''. Such a controlled mismatch in the
reflectivities in the two arms 56,58 of the interferometer 50
should enable very precise gain equalization. V.sub.t=5 V=>1 GHz
offset=>0.3 dB detuning.
The proposed device is novel enough that there is no conventional
devices to compare it with. However, the device that has a similar
functionality is a acousto-optic tunable filter; but such devices
are not as rugged as the electro-optic tunable filter described
above and also involves exciting undesirable higher order modes in
the fiber.
One possible drawback of the above Michelson interferometer-based
device is that the two arms of the interferometer have to maintain
a constant phase difference of .pi./2 for it to work without any
back reflection losses. This can be circumvented by using a
non-interferometric scheme described below. In this embodiment,
coupler 54 would be replaced with a polarization splitter. The
details of the polarization splitter are provided in conjunction
with the discussion of FIG. 12.
Frequency Modulators
The third device, illustrated in FIG. 11, is a modulator 70 that
preferably utilizes the electro-optic waveguide 10''' illustrated
in FIG. 6. Tunable Bragg grating waveguide segment 10''' (whose
bandwidth is much smaller compared to that of the input radiation)
is biased in such a way that its reflection spectrum corresponds to
the linear portion of the input radiation spectrum. Any change in
the input voltage Vs will alter the refractive index of tunable
Bragg grating waveguide segment 10''' which in turn shifts the
reflection spectrum of the Bragg grating disposed in tunable Bragg
grating waveguide segment 10'''. Thereby the amount of input
radiation that is reflected is varied. If the transmitted or
reflected radiation is monitored we will observe a modulated light
signal that resembles the modulating input voltage Vs waveform as
discussed in conjunction with FIGS. 8 and 9. A major advantage of
this scheme over conventional modulators is its simplicity.
Furthermore, the modulated output can be extracted either as the
transmitted or the reflected radiation due to this in-line
all-fiber device.
To construct a frequency modulator 70 from the teachings of
modulator 40, a broadband dichroic mirror 74 is introduced at the
input end of fiber 76. Radiation is launched into the fiber 76 to
form a laser cavity with mirror 74 at one end and tunable Bragg
grating waveguide segment 10''' at the other end. Tunable Bragg
grating waveguide segment 10''' is designed to give a narrowband
reflection spectrum at the flat portion of gain bandwidth of the
rare-earth-doped fiber 76. Hence the laser will produce radiation
only within the narrow band of frequencies defined by tunable Bragg
grating waveguide segment 10''' reflection spectrum.
Applying a modulating voltage to tunable Bragg grating waveguide
segment 10''' changes the local refractive index of the poled fiber
section which in turn shifts the reflection spectrum of tunable
Bragg grating waveguide segment 10''' and hence modulates the
output frequency of the fiber laser 70.
Conventional frequency modulation is done using a distributed
feedback (DFB) semiconductor laser diode which has the drawback of
chirping at high modulation frequencies (>100 MHz). Moreover,
such laser diodes are not capable of producing a good circular
spatial beam quality that is essential for efficient coupling into
single-mode fibers. The proposed device can be modulated up to GHz
frequencies and its all-fiber construction makes it attractive for
splicing with other single mode fibers with negligible insertion
loss.
Wavelength Add-Drop Multiplexer or Filter
The fourth device, illustrated in FIG. 12, is a wavelength add-drop
multiplexer or filter 80 that preferably utilizes poled
electro-optic waveguide 10, 10' and/or 10'' illustrated in FIGS. 2,
3, and 5.
In this scheme, polarization beamsplitter 84 splits the input
random polarization 82 into two orthogonal linear polarization
states. These linear polarization states are converted to circular
polarization states by poled electro-optic waveguide 10, 10' and/or
10'' below and denoted as e-o in FIG. 12. Upon reflection from the
Bragg gratings 60, the circular polarization changes its handedness
and are subsequently converted to linear polarization states
(orthogonal to the initial linear polarization state) upon passing
through poled electro-optic waveguide 10, 10' and/or 10''.
The polarization beamsplitter 84 works such that the vertical
polarization switches over to the cross-port and the horizontal
polarization goes to the through-port. Since the linear
polarization states become orthogonal with respect to the original
polarization state after being reflected by Bragg gratings 60, the
light exits through the output port as two linear orthogonal
polarizations.
In an alternate embodiment, tunable Bragg grating waveguide segment
10''' may be utilized in place of or in conjunction with Bragg
gratings 60. In this embodiment, tunable Bragg grating waveguide
segment 10''' may be configured to lie between two frequencies of
interest and then shifted to either of these frequencies by the
application of an appropriate input voltage. Thus, the device would
be capable of adding or dropping signals at two distinct
frequencies.
Low insertion loss (<1 dB), low cross-talk (<-30 dB), and
polarization insensitive add/drop multiplexers/filters (ADM) are
attractive for high bit-rate WDM-based datacom and telecom
applications. As discussed above, the random input polarization
state 82 is split into two linear components by polarization
beamsplitter 84. The linear polarizations are converted to circular
polarization preferably by poled tunable .lamda./4 plate (or poled
electro-optic waveguide 10, 10' and/or 10''). Upon reflection by
grating 60, the polarizations of the wavelength channel
corresponding to the Bragg wavelength (.lamda..sub.1) are flipped
to orthogonal states after passing through the tuned .lamda./4
plate. This channel then exits through port 2 which is denoted by
reference numeral 86. Another channel (also at .lamda..sub.1) can
be added through port 5, denoted as reference numeral 92, using the
same principle.
Bragg grating 60 in the above device is not perfectly matched to
the source wavelengths and quite often the grating spectrum has to
be shifted after the grating fabrication. Tunable Bragg grating
waveguide segment 10''' is an elegant solution to this problem.
Furthermore, it can also be used to match the reflection spectra of
the gratings written on the two arms of the device described
above.
If several wavelengths need to be simultaneously filtered among the
input wavelengths, several matched tunable Bragg grating waveguide
segment 10''' (that are resonant at those particular wavelengths)
can be written on the two arms 88,90 of the above device 80. This
feature is unique to the polarization manipulation-based scheme
used in our device.
In conventional Mach Zehnder interferometer-based wavelength
add-drop filters, the two arms have to be phase-matched; i.e., the
light propagating in the two arms should maintain a constant phase
difference between them. This is difficult to achieve in practice,
and is affected by changes in temperature and strain of the fiber.
The proposed device is much more attractive compared to Mach
Zehnder interferometer-based filters because of the
non-interferometric scheme (its operation is not phase-sensitive)
used.
High-Speed Switch
The fifth device, illustrated in FIG. 13, is a high-speed 1.times.2
switch 100 that preferably utilizes poled electro-optic waveguide
10, 10' and/or 10'' illustrated in FIGS. 2, 3, and 5.
Random input polarization 82 is split in to two linear components
by polarization beamsplitter 84. Polarization beamsplitter 84 is
designed in such a way that the vertical polarization mode goes
through in the same input fiber 106 whereas the horizontal
polarization mode switches over to the coupled fiber 102,104. If no
voltage is applied to poled electro-optic waveguide 10, 10' and/or
10'' (denoted as e-o in FIG. 13), the vertical polarization mode
goes through to port 7, the horizontal polarization mode couples
back to the original input fiber and hence both the linearly
polarized modes exit through port 7.
If a voltage of V.sub..pi. is applied to poled electro-optic
waveguide 10, 10' and/or 10'', it acts as a half-wave-plate and
rotates the polarization mode in the two arms 88,90 to its
orthogonal linear polarization modes. The original vertical
polarization becomes horizontal polarization and couples into port
8. Similarly the original horizontal polarization in the other arm
becomes vertical polarization and also exits through port 8.
The major advantage in the above device is the non-interferometric
scheme employed which makes the operation of the switch relatively
insensitive to environmental (such as temperature and strain)
variations and any relative phase changes between the two arms
88,90 of the device 100.
Amplitude Modulator
The sixth device, illustrated in FIG. 14, is an amplitude modulator
110 that preferably utilizes poled electro-optic waveguide 10, 10'
and/or 10'' illustrated in FIGS. 2, 3, and 5.
The principle of operation of this device is similar to that of the
1.times.2 switch 100 described above except that the voltage
applied to the poled section in this case is a continuous waveform
instead of the discrete DC levels employed in the above device. In
contrast to the 1.times.2 switch 100 where the linear polarization
states in the two arms 88,90 are switched between two discrete
states, the variable waveplate continuously changes the
polarization state in the two arms into elliptical, circular, and
linear polarization states. These polarization states are resolved
by the polarization beamsplitter 84 at the output end to distribute
the light in the two arms at port 5 and 6 between ports 7 and 8.
Thus, if we monitor one of the output ports (7 or 8) we will detect
modulated light whose waveform resembles that of the voltage
applied to poled electro-optic waveguide 10, 10' and/or 10''.
The major advantage in the above device is the non-interferometric
scheme employed which makes the operation of the switch relatively
insensitive to environmental variations. Moreover, a DC bias
voltage is not required as for the case of a Mach-Zehnder
interferometer based amplitude modulator conventionally used.
Reflection Isolators
The seventh and eight devices, illustrated in FIGS. 15 and 16, are
reflection isolators 120,120' that preferably utilizes poled
electro-optic waveguide 10, 10' and/or 10'' illustrated in FIGS. 2,
3, and 5.
The polarization beamsplitter 84 is designed to pass the input
linearly polarized (vertically polarized in the case illustrated
above) laser radiation and cross-couple the orthogonal
polarization. The poled electro-optic waveguide 10, 10' and/or 10''
(denoted as e-o in FIGS. 15 and 16) is configured to act as a
quarter-wave-plate which converts the linear polarization into
circularly polarized light.
Upon reflection from external components 102, the circular
polarization reverses its handedness (to attain the orthogonal
polarization state) and after passing through poled electro-optic
waveguide 10, 10' and/or 10'' becomes horizontally polarized. The
polarization beamsplitter 84 subsequently cross-couples this
polarization so that it exits through port 2. Thus any reflected
radiation is prevented from reaching the source 122 or in other
words, the polarized source 122 is "isolated" from any reflections
from external components.
If the radiation from the laser source 122' is not linearly
polarized but has a definite polarization (circular or elliptical),
then we can introduce a poled electro-optic waveguide 10, 10'
and/or 10'' at the input end of the isolator device 120' and
convert the input polarization to linear polarization of desired
orientation. In case the input polarization is random, the isolator
will still perform the function described above but will involve an
additional insertion loss of up to 3 dB.
There is no comparable all-fiber isolator that has been
demonstrated to date. The conventional device is a bulk Faraday
isolator which involves coupling light out of the fiber and then
back into the fiber and thereby introduces an insertion loss of
>1 dB.
It will be apparent to those skilled in the art that the present
invention has many uses and applications and may be practiced other
than were specifically described herein. There are a host of
applications for the invention, some of which are listed here: 1)
integrated Mach-Zender interferometers and other switching
structures for high speed switching; 2) A/D conversion, 3)
waveguide crossbar switches, 4) electronically tunable filters for
communication multiplexers/demutiplexers; 5) waveguide optical
parametric oscillators; waveguide sum and difference frequency
generation; and 6) second harmonic generation in waveguides with
counter-propagating fundamental frequency beams.
Although the present invention has been fully described in
conjunction with the preferred embodiment thereof with reference to
the accompanying drawings, it is to be understood that various
changes and modifications may be apparent to those skilled in the
art. Such changes and modifications are to be understood as
included within the scope of the present invention as defined by
the appended claims, unless they depart therefrom.
* * * * *