U.S. patent application number 14/550645 was filed with the patent office on 2015-03-19 for system and method for an optical phase shifter.
The applicant listed for this patent is FutureWei Technologies, Inc.. Invention is credited to Bryce Dorin, Winnie N. Ye.
Application Number | 20150078702 14/550645 |
Document ID | / |
Family ID | 51932925 |
Filed Date | 2015-03-19 |
United States Patent
Application |
20150078702 |
Kind Code |
A1 |
Dorin; Bryce ; et
al. |
March 19, 2015 |
System and Method for an Optical Phase Shifter
Abstract
In one embodiment, an optical phase shifter includes a first
waveguide phase shifter and a second waveguide phase shifter. The
optical phase shifter also includes a first polarization rotator
optically coupled between the first waveguide phase shifter and the
second waveguide phase shifter, where the first waveguide phase
shifter, second waveguide phase shifter, and first polarization
rotator are integrated on a single substrate.
Inventors: |
Dorin; Bryce; (Ottawa,
CA) ; Ye; Winnie N.; (Ottawa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FutureWei Technologies, Inc. |
Plano |
TX |
US |
|
|
Family ID: |
51932925 |
Appl. No.: |
14/550645 |
Filed: |
November 21, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13955449 |
Jul 31, 2013 |
8923660 |
|
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14550645 |
|
|
|
|
61827400 |
May 24, 2013 |
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Current U.S.
Class: |
385/3 ; 385/1;
385/11 |
Current CPC
Class: |
G02F 2201/16 20130101;
G02F 1/025 20130101; G02F 2001/212 20130101; G02F 1/0147 20130101;
G02F 1/225 20130101; G02B 27/286 20130101; G02B 6/2766 20130101;
G02F 1/0136 20130101; G02F 2203/06 20130101; G02F 1/2257
20130101 |
Class at
Publication: |
385/3 ; 385/11;
385/1 |
International
Class: |
G02B 6/27 20060101
G02B006/27; G02F 1/225 20060101 G02F001/225; G02F 1/01 20060101
G02F001/01 |
Claims
1. An optical phase shifter comprising: a first waveguide phase
shifter; a second waveguide phase shifter; and a first polarization
rotator optically coupled between the first waveguide phase shifter
and the second waveguide phase shifter, wherein the first waveguide
phase shifter, second waveguide phase shifter, and first
polarization rotator are integrated on a single substrate.
2. The optical phase shifter of claim 1, wherein the single
substrate is silicon-on-insulator (SOI).
3. The optical phase shifter of claim 1, wherein the first
waveguide phase shifter is a passive phase shifter.
4. The optical phase shifter of claim 1, wherein the first
waveguide phase shifter is an active phase shifter.
5. The optical phase shifter of claim 4, wherein the first
waveguide phase shifter is a thermo-optical phase shifter.
6. The optical phase shifter of claim 4, wherein the first
waveguide phase shifter is an electro-optical phase shifter.
7. The optical phase shifter of claim 1, wherein the first
waveguide phase shifter comprises lithium niobate.
8. The optical phase shifter of claim 1, further comprising a
second polarization rotator optically coupled to the second
waveguide phase shifter.
9. The optical phase shifter of claim 1, wherein the first
polarization rotator comprises an asymmetrical waveguide.
10. The optical phase shifter of claim 1, wherein the first
polarization rotator comprises lithium niobate.
11. A method comprising: phase shifting a first optical signal to
produce a first phase shifted optical signal by a phase shifter;
rotating a first polarization of the first phase shifted optical
signal to produce a first rotated optical signal; and phase
shifting the first rotated optical signal to produce a second phase
shifted optical signal, wherein phase shifting the first optical
signal, rotating the first polarization of the first phase shifted
optical signal, and phase shifting the first rotated optical
signals are performed on a photonic integrated circuit (PIC).
12. The method of claim 11, wherein rotating the first polarization
of the first phase shifted optical signal comprises rotating the
polarization of the first phase shifted optical signal by ninety
degrees.
13. The method of claim 11, further comprising rotating a second
polarization of the second phase shifted optical signal.
14. The method of claim 11, further comprising: splitting an input
optical signal to produce the first optical signal and a second
optical signal; transmitting the second optical signal to produce a
transmitted optical signal; and combining the transmitted optical
signal and the second phase shifted optical signal.
15. The method of claim 14, wherein transmitting the second optical
signal comprises: phase shifting the second optical signal to
produce a third phase shifted optical signal; rotating a second
polarization of the third phase shifted optical signal to produce a
second rotated optical signal; and phase shifting the second
rotated optical signal to produce the transmitted optical
signal.
16. A Mach-Zehnder interferometer comprising: a first optical
coupler; a first optical leg coupled to the first optical coupler,
wherein the first optical leg comprises a first waveguide phase
shifter, a second waveguide phase shifter, and a first polarization
rotator coupled between the first waveguide phase shifter and the
second waveguide phase shifter; a second optical leg coupled to the
first optical coupler; and a second optical coupler optically
coupled to the first optical leg and the second optical leg,
wherein the first optical leg and the second optical leg are
integrated on a single substrate.
17. The Mach-Zehnder interferometer of claim 16, wherein the first
optical leg further comprises a second polarization rotator
optically coupled to the second waveguide phase shifter.
18. The Mach-Zehnder interferometer of claim 16, wherein the second
optical leg comprises: a third waveguide phase shifter; a fourth
waveguide phase shifter; and a second polarization rotator coupled
between the third waveguide phase shifter and the fourth waveguide
phase shifter.
19. The Mach-Zehnder interferometer of claim 18, wherein the first
phase shifter is an active phase shifter and the third waveguide
phase shifter is a passive phase shifter.
20. The Mach-Zehnder interferometer of claim 16, wherein the
Mach-Zehnder interferometer has a push-pull configuration.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/955,449 filed on Jul. 31, 2013, and
entitled "System and Method for an Optical Phase Shifter," which
claims priority to U.S. Provisional Application Serial No.
61/827,400 filed on May 24, 2013, and entitled "Polarization
Independent Waveguide Optical Phase Shifter," both of which are
hereby incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a system and method for
photonics, and, in particular, to a system and method for an
optical phase shifter.
BACKGROUND
[0003] In some photonic devices, shifting the phase of an optical
signal is desirable. Optical phase shifting may be used in optical
modulators, switches, sensors, multiplexers, demultiplexers, and
other devices. When light propagates through a media, it travels an
optical path length that depends on the effective index of
refraction of the media. The optical phase may be adjusted when
light propagates through a media having a desired optical path
length to adjust the optical phase.
[0004] Optical devices may be integrated in a photonic integrated
circuit (PIC) containing optical waveguides. Optical waveguides are
light conduits that contain a slab, strip, or cylinder of a
dielectric material surrounded by another dielectric material
having a lower refractive index. The light propagates along, and is
confined to, the higher refractive index material through total
internal reflection. In a PIC, the core may be silicon, surrounded
by a lower refractive index material, such as silicon dioxide,
silicon nitride, silicon oxynitride, and/or air. The waveguides may
be a single mode or multi-mode waveguide. In an example, a PIC
operates at a telecommunications wavelength, such as 1550 nm or
1310 nm. The light may be coupled into, out of, or between optical
waveguides. In a PIC, multiple photonic functions are integrated on
a substrate, such as silicon-on-insulator (SOI). PICs are used for
optical communications, and for other applications, such as
biomedical application sand photonic computing. PICs may provide
increased functionality, while being compact, and enabling higher
performance than discrete optical devices.
SUMMARY
[0005] An embodiment optical phase shifter includes a first
waveguide phase shifter and a second waveguide phase shifter. The
optical phase shifter also includes a first polarization rotator
optically coupled between the first waveguide phase shifter and the
second waveguide phase shifter, where the first waveguide phase
shifter, second waveguide phase shifter, and first polarization
rotator are integrated on a single substrate.
[0006] An embodiment method includes phase shifting a first optical
signal to produce a first phase shifted optical signal by a phase
shifter and rotating a first polarization of the first phase
shifted optical signal to produce a first rotated optical signal.
The method also includes phase shifting the first rotated optical
signal to produce a second phase shifted optical signal, where
phase shifting the first optical signal, rotating the first
polarization of the first phase shifted optical signal, and phase
shifting the first rotated optical signals are performed on a
photonic integrated circuit (PIC).
[0007] An embodiment Mach-Zehnder interferometer includes a first
optical coupler and a first optical leg coupled to the first
optical coupler. The first optical leg includes a first waveguide
phase shifter and a second waveguide phase shifter. The first
optical leg also includes a first polarization rotator coupled
between the first waveguide phase shifter and the second waveguide
phase shifter. Additionally, the Mach-Zehnder interferometer
includes a second optical leg coupled to the first optical coupler
and a second optical coupler optically coupled to the first optical
leg and the second optical leg, where the first optical leg and the
second optical leg are integrated on a single substrate.
[0008] The foregoing has outlined rather broadly the features of an
embodiment of the present invention in order that the detailed
description of the invention that follows may be better understood.
Additional features and advantages of embodiments of the invention
will be described hereinafter, which form the subject of the claims
of the invention. It should be appreciated by those skilled in the
art that the conception and specific embodiments disclosed may be
readily utilized as a basis for modifying or designing other
structures or processes for carrying out the same purposes of the
present invention. It should also be realized by those skilled in
the art that such equivalent constructions do not depart from the
spirit and scope of the invention as set forth in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawing, in
which:
[0010] FIG. 1 illustrates an embodiment optical phase shifter;
[0011] FIG. 2 illustrates another embodiment optical phase
shifter;
[0012] FIG. 3 illustrates a flowchart for an embodiment method of
optical phase shifting;
[0013] FIG. 4 illustrates an embodiment Mach-Zehnder
interferometer;
[0014] FIG. 5 illustrates another embodiment Mach-Zehnder
interferometer;
[0015] FIG. 6 illustrates an additional embodiment Mach-Zehnder
interferometer;
[0016] FIG. 7 illustrates another embodiment Mach-Zehnder
interferometer; and
[0017] FIG. 8 illustrates a flowchart for an embodiment method of
switching using a Mach-Zehnder interferometer.
[0018] Corresponding numerals and symbols in the different figures
generally refer to corresponding parts unless otherwise indicated.
The figures are drawn to clearly illustrate the relevant aspects of
the embodiments and are not necessarily drawn to scale.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0019] It should be understood at the outset that although an
illustrative implementation of one or more embodiments are provided
below, the disclosed systems and/or methods may be implemented
using any number of techniques, whether currently known or in
existence. The disclosure should in no way be limited to the
illustrative implementations, drawings, and techniques illustrated
below, including the exemplary designs and implementations
illustrated and described herein, but may be modified within the
scope of the appended claims along with their full scope of
equivalents.
[0020] An optical signal may be viewed as a propagating oscillating
electric field orthogonal to an oscillating magnetic field at an
optical frequency. The polarization of the optical field is
indicated by the direction of the electric field vector.
Propagating light may be decomposed into transverse electric (TE)
polarization and transverse magnetic (TM) polarization. For TE
polarized light, the electrical fields are orthogonal to the plane
of propagation. For TM polarized light, the magnetic field is
orthogonal to the direction of propagation.
[0021] Many optical components are affected by the polarization of
the optical signal. For example, polarization mode dispersion
(PMD), polarization dependent loss (PDL), and polarization
dependent wavelength characteristics (PDlambda) may occur,
especially when a highly birefringent material is used. Silicon
waveguides may have a high geometrical birefringence. Silicon is
useful for PICs because of its high index of refraction and its
compatibility with electronic integrated circuit fabrication
methods. In a birefringent material, the refractive index depends
on the polarization of an optical signal. The magnitude of a phase
shift depends on the confinement factor and effective index, which
differs for TE and TM modes in a waveguide. The phase shift is
given by:
.theta. = 2 .pi. .lamda. .GAMMA..DELTA. nL p , or ##EQU00001##
.theta. = 2 .pi. .lamda. n eff .DELTA. L p ##EQU00001.2##
where .lamda. is the wavelength, .GAMMA. is the confinement factor,
n.sub.eff is the effective refractive index for the polarization,
.DELTA.n is the refractive index change induced into the waveguide,
L.sub.p is the length of the device, and .DELTA.L.sub.p is the
change in length of the device.
[0022] Large silicon waveguides may be polarization agnostic.
However, such large waveguides have a large bend radius, leading to
a low density of components. Also, such specially designed
waveguides may be extremely sensitive to wavelength, dimensional
parameter variations, and material parameter variations, so
production may be problematic. It is desirable to use very fine
waveguides for a high density PIC with a large refractive index
contrast between the core and the cladding. This facilitates very
small device sizes, but has a high birefringence.
[0023] When both TE and TM polarizations normally exist in an
optical waveguide, a polarization diversity approach may be used.
Polarization splitters split the optical signal to two separate
paths based on polarization, with TE polarized light propagating
along one path and TM polarized light propagating along the other
path. Processing is applied to both paths in separate circuits to
obtain similar effects. The outputs of the separate circuits are
then combined. However, this approach leads to the device size more
than doubling. Also, such networks may be susceptible to
temperature gradients between the separate circuits.
[0024] In another example, the orientation of the TE and TM
polarizations is exchanged at the midpoint of a semiconductor
waveguide section. A gap is introduced into the waveguide at the
midpoint, which leads to additional insertion losses. A discrete
polarization rotating component is inserted into the gap for
rotating both polarization orientations by ninety degrees. For
example, a thin polyimide half wave-plate may be inserted into the
gap. Alternatively, a polarization splitting grating coupler is
inserted into the gap. Also, the assembly, with a micron tolerance,
is costly.
[0025] The phase of an optical signal may be adjusted by an optical
phase shifter which adjusts the optical phase by propagating the
optical signal along a desired optical path length. A phase shifter
may be used for a variety of optical components. For example, phase
modulators, intensity modulators, photonic switches, multiplexers,
arrayed waveguide gratings, and demultiplexers may include an
optical phase shifter.
[0026] FIG. 1 illustrates optical phase shifter 100. Initially, an
optical signal enters optical phase shifter 100 at waveguide phase
shifter 102. The entering optical signal contains both a TE
polarized component and a TM polarization component. The optical
path length of waveguide phase shifter 102 is approximately equal
to half of the average length for the desired phase shift for the
TE polarization and the TM polarization. For example, waveguide
phase shifter 102 may be from about 100 .mu.m to about 10 mm in
length. In one example, waveguide phase shifter 102 is passive. In
another example, waveguide phase shifter 102 is active, and the
optical path length may be adjusted by applying a voltage, current,
stress, and/or heat. The TE polarization mode and the TM
polarization mode experience different phase shifts in the passive
waveguide phase shifter 102, because waveguide phase shifter 102 is
birefringent. The TE polarization mode and the TM polarization mode
experience different phase shifts in the active waveguide phase
shifter 102, because waveguide phase shifter 102 has different
confinement factors for the different polarizations. In one
example, waveguide phase shifter 102 is made of silicon. In other
examples, waveguide phase shifter 102 is made of InP, or other
birefringent materials, such as InGaAsP and AlGaAs.
[0027] The optical output of waveguide phase shifter 102 proceeds
to polarization rotator 104, which rotates the polarization by
ninety degrees. Thus, the TE polarization is converted to a TM
polarization, and the TM polarization is converted to a TE
polarization. Waveguide phase shifter 102 and polarization rotator
104 are integrated on a single substrate.
[0028] After the polarization has been rotated, the optical signal
proceeds to waveguide phase shifter 106. Waveguide phase shifter
106 is similar to waveguide phase shifter 102. Thus, the optical
path length and phase shift experienced by the TE polarization mode
and the TM polarization mode are similar. In one example, the phase
shift for waveguide phase shifter 102 is within .pi./16 of that of
waveguide phase shifter 106. In other examples the phase shift of
waveguide phase shifter 102 and waveguide phase shifter 106 are
within .pi./8, .pi./24, .pi./32, .pi./48, or .pi./64. The original
TE polarized light passes through waveguide phase shifter 102 as TE
polarized and through waveguide phase shifter 106 as TM polarized
light. Conversely, the original TM polarized light passes through
waveguide phase shifter 102 as TM polarized and through waveguide
phase shifter 106 as TE polarized light. Waveguide phase shifter
102, polarization rotator 104, and waveguide phase shifter 106 are
integrated on a single substrate.
[0029] After waveguide phase shifter 106, there may be another
ninety degree polarization rotator (not pictured). This additional
phase rotator restores the optical signal to its original
polarization state. The TE polarized light is converted to TM
polarized light by the first polarization rotator, and back to TE
polarized light by the second polarization rotator. Conversely, the
TM polarized light is converted to TE polarized light by the first
polarization rotator, and back to TM polarized light by the second
polarization rotator.
[0030] FIG. 2 illustrates optical phase shifter 110. Optical phase
shifter 110 is disposed on substrate 118, and may be a part of a
PIC. Optical phase shifter 110 may be fabricated on
silicon-on-insulator (SOI), where the waveguide layer is fabricated
in the top silicon layer.
[0031] An input optical signal enters at waveguide phase shifter
112. Waveguide phase shifter 112 phase shifts the input optical
signal by approximately half the desired phase shift for the
average of the TE and TM polarizations. The TE and TM polarizations
are phase shifted by different amounts. In one example, waveguide
phase shifter 112 is a thermo-optical phase shifter. In another
example, waveguide phase shifter 112 is an electro-optical phase
shifter. Alternatively, waveguide phase shifter 112 is a passive
phase shifter. A passive waveguide phase shifter may require a
slightly longer length of the waveguide than that of the input and
output, or may rely on a stressed cladding for index modification.
Examples of materials that may be used for active phase shifters
include doped silicon, a heater on silicon, and lithium
niobate.
[0032] Then, the phase shifted light is polarization rotated by
ninety degrees by polarization rotator 114. Thus, the TE
polarization is transformed into a TM polarization, and the TM
polarization is transformed to a TE polarization. Polarization
rotator 114 is made of a highly birefringent material. The
polarization rotator may be made from a birefringent crystal like
lithium niobate. Alternatively, the polarization rotator is made
from an asymmetrical waveguide, for example composed of silicon, or
from an asymmetrical coupler.
[0033] Finally, the polarization rotated light proceeds to
waveguide phase shifter 116, which is similar to waveguide phase
shifter 112. After passing through waveguide phase shifter 112,
polarization rotator 114, and waveguide phase shifter 116, both
polarizations experience a similar total phase shift.
[0034] FIG. 3 illustrates flowchart 250 for a method of phase
shifting an optical signal. The TE polarization and the TM
polarization are phase shifted by approximately the same amount.
Initially, in step 252, the optical input is received. The optical
input may be received from another portion of a PIC. Alternatively,
the optical signal is received from an external component or
another source. The input optical signal has a TE polarization mode
and a TM polarization mode.
[0035] Then, in step 254, the light is phase shifted. For example,
the light is phase shifted by approximately half of the average
total desired phase shift for the TE polarization and the TM
polarization. The phase shifting may be performed by an active or
passive phase shifter.
[0036] Next, in step 256, the polarization of the phase shifted
light is rotated by ninety degrees. The TE polarization is
converted to a TM polarization, and the TM polarization is
converted to a TE polarization.
[0037] The polarization rotated light is then phase shifted, in
step 258, by a phase shifter similar to the one used in step 254.
Thus, both polarizations of light are phase shifted by the same
total amount, because they experience one phase shift as TE
polarized light and the other similar phase shift as TM polarized
light.
[0038] Optionally, in step 260, the light is again polarization
rotated by ninety degrees. This restores the light to its original
polarization, for applications that need the original
polarization.
[0039] Finally, in step 262, the output light is transmitted. This
may be done, for example, to another part of a PIC, another optical
device, or externally coupled.
[0040] An embodiment optical phase shifter may improve polarization
dependent loss performance. Also, an embodiment reduces the total
insertion loss, facilitating the construction of large optical
switches. An embodiment may lower thermal dependence due to
inherent thermal compensation. In an embodiment, manufacturability
is improved. For example, an embodiment may be fabricated on a
wafer scale, for example in a complementary metal oxide
semiconductor (CMOS) silicon wafer environment. Additionally, in an
embodiment, there is high power efficiency due to a reduced
component count. An embodiment facilitates increased flexibility
and scope of applications for PICs, because both TE and TM
polarizations are processed in the same optical circuit.
[0041] FIG. 4 illustrates Mach-Zehnder interferometer (MZI) 190
containing a polarization insensitive phase shifter. A Mach-Zehnder
interferometer may be used for switching in telecommunications, for
example for high speed dense wavelength division multiplexing
(DWDM). Incoming light enters input 191 or input 193, and proceeds
to coupler 192, where it is split. Half of the optical signal is
coupled to leg 194 and half of the optical signal is coupled to leg
196. The optical signals from legs 194 and 196 are combined by
coupler 206, where it is output in output 208 or output 209. The
output depends on the relative optical path lengths of leg 196 and
leg 194. When the optical path lengths are the same, or have a
difference in phase shift of a multiple of 2n, between leg 194 and
leg 196, there is complete constructive interference in leg 209.
However, if the path lengths have a relative phase shift of -.pi.,
.pi., 3.pi., etc., there is complete destructive interference in
leg 209. For intermediate relative phase shifts, there is an
intermediate interference. If the optical path lengths are varied,
for example by introducing a variable phase shift into one or both
legs, Mach-Zehnder interferometer 190 may be used as an optical
switch. Mach-Zehnder interferometer 190 is integrated on a single
substrate, for example on a PIC.
[0042] Leg 194 of Mach-Zehnder interferometer 190 contains a
polarization insensitive phase shifter. An optical signal
propagating in leg 194 initially is phase shifted by waveguide
phase shifter 198, which shifts the optical phase by approximately
half of the total desired phase shift. Then, the polarization is
rotated by ninety degrees by polarization rotator 200. Next, the
optical signal is phase shifted by waveguide phase shifter 202,
similar to waveguide phase shifter 198. The TE and TM polarizations
are phase shifted by the same amount, because both polarizations
experience one phase shift as TE polarized light and the other
phase shift as TM polarized light. Finally, the polarization is
phase shifted by ninety degrees by polarization rotator 204. The
second polarization rotator returns the optical output to its
original polarization. This may be used if coupler 192 and coupler
206 are polarization sensitive. If the polarizations experienced by
coupler 192 and coupler 206 are different, and they are
polarization sensitive, the coupling effects will be different, and
there will be noise, preventing complete destructive interference
and complete constructive interference. To switch Mach-Zehnder
interferometer 190, waveguide phase shifter 198 and waveguide phase
shifter 202 may be adjusted, for example by applying a current,
voltage, stressed cladding, or heat, to alter the phase shift
between the optical signal propagating along leg 194 and the
optical signal propagating along leg 196.
[0043] FIG. 5 illustrates Mach-Zehnder interferometer 240, a
mirrored Mach-Zehnder interferometer containing a polarization
insensitive phase shifter in both legs. Mach-Zehnder interferometer
240 is integrated on a single substrate, such as an SOI substrate.
As with Mach-Zehnder interferometer 190, an optical signal enters
Mach-Zehnder interferometer 240 at input 212 or 214, and is split
by coupler 216. Half the light is coupled to leg 242, and half the
light is coupled to leg 244. The optical signals from the two legs
are combined by coupler 222, and output to output 224 and/or output
226. Leg 242 contains waveguide phase shifter 228, polarization
rotator 230, waveguide phase shifter 232, and polarization rotator
246. Leg 244 contains waveguide phase shifter 234, polarization
rotator 236, waveguide phase shifter 238, and polarization rotator
248. One leg may contain active phase shifters while the other leg
contains passive phase shifters, both legs may contain active phase
shifters, or both legs may contain passive phase shifters.
Incorporating phase shifters in both legs helps the legs have
similar losses. Different losses in the two legs may lead to more
crosstalk and may prevent complete constructive interference and
complete destructive interference.
[0044] FIG. 6 illustrates Mach-Zehnder interferometer 150, a
mirrored Mach-Zehnder interferometer. An optical signal enters at
input 122, and is split by coupler 124. The light is split into two
legs. In one leg, the light passes through waveguide phase shifter
126, polarization rotator 130, waveguide phase shifter 134, and
polarization rotator 152. In the other leg, light passes through
waveguide phase shifter 128, polarization rotator 132, waveguide
phase shifter 136, and polarization rotator 154. The optical
signals are then combined by coupler 138 to output 140. In one
example, Mach-Zehnder interferometer 150 is a push-pull
configuration, where the light experiences a .pi./2 phase shift in
one leg and a -.pi./2 phase shift in the other leg, leading to
complete destructive interference. As long as the phase difference
between the two legs is odd multiples of .pi. (e.g., .+-..pi.,
.+-.3.pi., etc.).
[0045] FIG. 7 illustrates Mach-Zehnder interferometer 120, a
mirrored Mach-Zehnder interferometer, where each leg contains only
one polarization rotator. Mach-Zehnder interferometer 120 may be
used when coupler 124 and coupler 138 are polarization insensitive.
An optical signal enters in input 122, and is split by coupler 124.
One leg contains waveguide phase shifter 126, polarization rotator
130, and waveguide phase shifter 134, while the other leg contains
waveguide phase shifter 128, polarization rotator 132, and
waveguide phase shifter 136. The optical signals from the two legs
are combined in coupler 138 to output 140.
[0046] FIG. 8 illustrates flowchart 160 for a method of switching
optical signals using a Mach-Zehnder interferometer. Initially, in
step 162, an optical input signal is received. The optical input
signal may be received from another portion of a PIC, another
optical component, or from an external source.
[0047] Then, in step 164, the optical input signal is split. One
portion of the optical input signal goes to a first leg of a
Mach-Zehnder interferometer, and proceeds to step 166. The other
portion of the optical input signal goes to a second leg of the
Mach-Zehnder interferometer, and proceeds to step 174. In one
example, only one leg contains a phase shifter. In another example,
both legs contain phase shifters.
[0048] In step 166 and step 174, the light in the two legs
experiences a phase shift. The phase shift in the two legs may be
the same, or it may be different. The phase shift in one or both
legs may be adjustable. A phase shift may be adjusted by applying a
voltage, current, stressed cladding, or heat to the phase
shifter.
[0049] Next, in step 168 and step 176, the polarizations of the
optical signals are in both legs are rotated by ninety degrees,
exchanging the TE polarization and the TM polarization.
[0050] After rotating the polarizations, the optical signals are
phase shifted again in step 170 and step 178. The phase shift
achieved by step 170 is very close to the phase shift achieved by
step 166 Likewise, the phase shift achieved by step 178 is very
close to the phase shift achieved by step 174. Thus, the TE and TM
polarizations are phase shifted by the same amount.
[0051] Optionally, the polarizations of the optical signals are
rotated by an additional ninety degrees in step 172 and step 180 to
return the light to its original polarization, so the polarization
at the input coupler is the same as the polarization at the output
coupler.
[0052] The optical signals from the two legs are combined in step
182. Depending on the relative phase shifts between optical signals
propagating along the two legs, there may be complete destructive
interference, complete constructive interference, or an
intermediate amount of interference.
[0053] Finally, in step 184, the optical output is transmitted, for
example to another portion of a PIC, another optical component, or
externally.
[0054] While several embodiments have been provided in the present
disclosure, it should be understood that the disclosed systems and
methods might be embodied in many other specific forms without
departing from the spirit or scope of the present disclosure. The
present examples are to be considered as illustrative and not
restrictive, and the intention is not to be limited to the details
given herein. For example, the various elements or components may
be combined or integrated in another system or certain features may
be omitted, or not implemented.
[0055] In addition, techniques, systems, subsystems, and methods
described and illustrated in the various embodiments as discrete or
separate may be combined or integrated with other systems, modules,
techniques, or methods without departing from the scope of the
present disclosure. Other items shown or discussed as coupled or
directly coupled or communicating with each other may be indirectly
coupled or communicating through some interface, device, or
intermediate component whether electrically, mechanically, or
otherwise. Other examples of changes, substitutions, and
alterations are ascertainable by one skilled in the art and could
be made without departing from the spirit and scope disclosed
herein.
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