U.S. patent application number 15/225742 was filed with the patent office on 2018-12-27 for polarization demultiplexing of optical signals.
The applicant listed for this patent is FINISAR CORPORATION. Invention is credited to Ilya LYUBOMIRSKY.
Application Number | 20180375589 15/225742 |
Document ID | / |
Family ID | 52995607 |
Filed Date | 2018-12-27 |
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United States Patent
Application |
20180375589 |
Kind Code |
A9 |
LYUBOMIRSKY; Ilya |
December 27, 2018 |
POLARIZATION DEMULTIPLEXING OF OPTICAL SIGNALS
Abstract
An example embodiment includes optical receiver that includes a
polarization beam splitter (PBS), a polarization controller, and a
forward error correction (FEC). The PBS is configured to split a
received optical signal having an unknown polarization state into
two orthogonal polarizations (x'-polarization and y'-polarization).
The polarization controller includes no more than two couplers and
no more than two phase shifters per wavelength channel of the
x'-polarization and the y'-polarization. The polarization
controller is configured to demultiplex the x'-polarization and the
y'-polarization into a first demultiplexed signal having an first
polarization on which a data signal is modulated and a second
demultiplexed signal having a second, orthogonal polarization on
which a pilot carrier oscillator signal is encoded. The FEC decoder
module is configured to correct a burst of errors resulting from
resetting one of the phase shifters based on error correction code
(ECC) data encoded in the data signal.
Inventors: |
LYUBOMIRSKY; Ilya;
(Pleasanton, CA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
FINISAR CORPORATION |
Sunnyvale |
CA |
US |
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Prior
Publication: |
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Document Identifier |
Publication Date |
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US 20170033873 A1 |
February 2, 2017 |
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Family ID: |
52995607 |
Appl. No.: |
15/225742 |
Filed: |
August 1, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14527349 |
Oct 29, 2014 |
9407376 |
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15225742 |
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61897147 |
Oct 29, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 10/614 20130101;
H04B 10/616 20130101; H04B 10/6151 20130101; H04J 14/06 20130101;
H04B 10/6166 20130101; H04J 14/02 20130101 |
International
Class: |
H04B 10/61 20060101
H04B010/61; H04J 14/06 20060101 H04J014/06; H04J 14/02 20060101
H04J014/02 |
Claims
1. An optical receiver comprising: a polarization beam splitter
(PBS) configured to split a received optical signal having an
unknown polarization state into two orthogonal polarizations that
include an x'-polarization and a y'-polarization; a polarization
controller having non-endless polarization tracking, the
polarization controller including only two couplers and only two
phase shifters per channel, a first phase shifter of the two phase
shifters being configured to reset when a first phase rotation
angle exceeds a range of about 0 to about 2.pi., and a second phase
shifter of the two phase shifters being configured to maintain a
second phase rotation angle between about 0 and about .pi.; and a
forward error correction (FEC) decoder module configured to correct
a burst of errors resulting from resetting one of the two phase
shifters based on error correction code (ECC) data encoded in the
received optical signal.
2. The optical receiver of claim 1, further comprising a photonic
integrated circuit (PIC) that includes the PBS and the polarization
controller that is constructed of silicon photonics or indium
phosphide.
3. The optical receiver of claim 1, wherein the polarization
controller includes an optical network that includes the two
couplers and the first phase shifter or the second phase
shifter.
4. The optical receiver of claim 1, further comprising: an optical
90-degree hybrid; and one or more p-i-n photodetectors, wherein:
the polarization controller is configured to demultiplex the
x'-polarization and the y'-polarization into a first demultiplexed
signal having an first and a second demultiplexed signal having a
second polarization that is orthogonal to the first polarization,
and the optical 90-degree hybrid receives the first demultiplexed
signal and the second demultiplexed signal from the polarization
controller.
5. The optical receiver of claim 4, wherein: a data signal is
modulated on the first demultiplexed signal; and a pilot carrier
oscillator signal is encoded on the second demultiplexed
signal.
6. The optical receiver of claim 5, wherein the data signal
modulated on the first demultiplexed signal includes data modulated
using single polarization quadrature phase shift keying (QPSK).
7. The optical receiver of claim 1, further comprising one or more
semiconductor optical amplifiers configured to amplify at least one
of the x'-polarization or the y'-polarization.
8. The optical receiver of claim 1, wherein the first phase shifter
is configured rotate one of the x'-polarization or the
y'-polarization by a first rotation matrix: ( - j .phi. 1 2 0 0 j
.phi. 1 2 ) ; ##EQU00006## in which: e represents Euler's number; j
represents the imaginary number; and .PHI..sub.1 represents the
first phase rotation angle.
9. The optical receiver of claim 8, wherein: a first of the two
couplers is configured to receive a phase-shifted signal output by
the first phase shifter and the other of the x'-polarization or the
y'-polarization and output a third signal and a fourth signal; and
each of the third signal and the fourth signal include a portion of
the phase-shifted signal and a portion the x'-polarization or the
y'-polarization that is received by the first of the two
couplers.
10. The optical receiver of claim 9, wherein the second phase
shifter and one or both of the two couplers are configured to
rotate the phase-shifted signal output by the first phase shifter
according to a second rotation matrix: ( cos ( .phi. 2 2 ) - j sin
( .phi. 2 2 ) - j sin ( .phi. 2 2 ) cos ( .phi. 2 2 ) ) ;
##EQU00007## in which .PHI..sub.2 represents the second phase
rotation angle.
11. The optical receiver of claim 1, further comprising: a first
optical demultiplexer between the PBS and the first phase shifter,
wherein the first optical demultiplexer is configured to separate
the x'-polarization or the y'-polarization output from the PBS into
two or more wavelength channels; a first optical multiplexer
between the first phase shifter and a first coupler of the two
couplers, the first optical multiplexer being configured to
multiplex a phase-shifted wavelength channel output from the first
phase shifter with one or more other phase-shifted wavelength
channels; a second optical demultiplexer between the first coupler
and the second phase shifter, wherein the second optical
demultiplexer is configured to separate a first intermediate signal
output from the first coupler into the two or more wavelength
channels; and a second optical multiplexer between the second phase
shifter and a second coupler of the two couplers, the second
optical multiplexer being configured to multiplex a phase-shifted
wavelength channel output from the second phase shifter with one or
more other phase-shifted wavelength channels.
12. A polarization controller that is configured to have a
non-endless polarization tracking and to receive optical signal
having an unknown polarization state, the polarization controller
comprising: only two phase shifters per channel, a first phase
shifter of the two phase shifters being configured to reset when a
first phase rotation angle exceeds a range of about 0 to about
2.pi. and a second phase shifter of the two phase shifters being
configured to maintain a second phase rotation angle between about
0 and about .pi.; and only two couplers, wherein: the received
optical signal includes two orthogonal polarizations that include
an x'-polarization and a y'-polarization; and the polarization
controller is configured to: demultiplex the x'-polarization and
the y'-polarization into a first demultiplexed signal having a
first polarization and a second demultiplexed signal having a
second polarization that is orthogonal to the first polarization;
and allow communication of a burst of errors from the polarization
controller that result from resetting the first phase shifter.
13. The optical receiver of claim 12, further comprising: a
polarization beam splitter (PBS) that is configured to split the
received optical signal into the x'-polarization and the
y'-polarization; a first optical demultiplexer between the PBS and
the first phase shifter, wherein the first optical demultiplexer is
configured to separate the x'-polarization or the y'-polarization
output from the PBS into two or more wavelength channels; a first
optical multiplexer between the first phase shifter and a first
coupler of the two couplers, the first optical multiplexer being
configured to multiplex a phase-shifted wavelength channel output
from the first phase shifter with one or more other phase-shifted
wavelength channels; a second optical demultiplexer between the
first coupler and the second phase shifter, wherein the second
optical demultiplexer is configured to separate a first
intermediate signal output from the first coupler into the two or
more wavelength channels; and a second optical multiplexer between
the second phase shifter and a second coupler of the two couplers,
the second optical multiplexer being configured to multiplex a
phase-shifted wavelength channel output from the second phase
shifter with one or more other phase-shifted wavelength
channels.
14. The polarization controller of claim 12, further comprising: a
polarization beam splitter (PBS) that is configured to split the
received optical signal into the x'-polarization and the
y'-polarization; a first waveguide that connects to the PBS, a
second waveguide that connects to the PBS; and a polarization
rotator that is positioned between the PBS and a first of the two
couplers.
15. The polarization controller of claim 12, further comprising: a
polarization beam splitter (PBS) that is configured to split the
received optical signal into the x'-polarization and the
y'-polarization; a first semiconductor optical amplifier (SOA) that
is positioned between the PBS and the first phase shifter; a
polarization rotator that is positioned between the PBS and a first
of the two couplers; and a second SOA that is positioned between
the polarization rotator and the first of the two couplers.
16. The polarization controller of claim 12, wherein the first
phase shifter is configured rotate one of the x'-polarization or
the y'-polarization by a first rotation matrix: ( - j .phi. 1 2 0 0
j .phi. 1 2 ) ; ##EQU00008## in which: e represents Euler's number;
j represents the imaginary number; and .PHI..sub.1 represents the
first phase rotation angle.
17. The polarization controller of claim 16, wherein: a first of
the two couplers is configured to receive a phase-shifted signal
output by the first phase shifter and the other of the
x'-polarization or the y'-polarization and output a third signal
and a fourth signal; and each of the third signal and the fourth
signal include a portion of the phase-shifted signal and a portion
the x'-polarization or the y'-polarization that is received by the
first of the two couplers.
18. The polarization controller of claim 17, wherein the second
phase shifter and one or both of the two couplers are configured to
rotate the phase-shifted signal output by the first phase shifter
according to a second rotation matrix: ( cos ( .phi. 2 2 ) - j sin
( .phi. 2 2 ) - j sin ( .phi. 2 2 ) cos ( .phi. 2 2 ) ) ;
##EQU00009## in which .PHI..sub.2 represents the second phase
rotation angle.
19. The polarization controller of claim 12, wherein: one or both
of the two phase shifters include a phase modulator; and one or
both of the couplers includes a 2.times.2, 50/50 splitter.
20. The polarization controller of claim 12, wherein at least some
portion of the polarization controller is a photonic integrated
circuit (PIC) constructed of silicon photonics or indium phosphide.
Description
RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
14/527,349, filed Oct. 29, 2014, titled POLARIZATION DEMULTIPLEXING
OF OPTICAL SIGNALS, which claims priority to and the benefit of
U.S. Provisional Application No. 61/897,147, both are incorporated
herein by reference in their entirety.
FIELD
[0002] The embodiments discussed herein are related to polarization
demultiplexing of optical signals. In particular, some embodiments
relate to polarization demultiplexing in pilot carrier single
polarization quadrature phase shift keying optical signals.
SUMMARY
[0003] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below.
This Summary is not intended to identify key features of the
claimed subject matter, nor is it intended to be used as an aid in
determining the scope of the claimed subject matter.
[0004] An example embodiment includes optical receiver. The optical
receiver includes a polarization beam splitter (PBS), a
polarization controller, and a forward error correction (FEC)
module. The PBS is configured to split a received optical signal
having an unknown polarization state into two orthogonal
polarizations (x'-polarization and y'-polarization). The
polarization controller includes no more than two couplers and no
more than two phase shifters per wavelength channel of the
x'-polarization and the y'-polarization. The polarization
controller is configured to demultiplex the x'-polarization and the
y'-polarization into a first demultiplexed signal having an first
polarization on which a data signal is modulated and a second
demultiplexed signal having a second polarization that is
orthogonal to the first polarization on which a pilot carrier
oscillator signal is encoded. The FEC decoder module is configured
to correct a burst of errors resulting from resetting one of the
phase shifters based on error correction code (ECC) data encoded in
the data signal.
[0005] Another example embodiment includes an optical communication
link. The optical communication link includes an optical
transmitter, an optical receiver, and a fiber. The optical
transmitter includes an FEC encoder module, a modulator, a laser, a
pilot encoder module, and a polarization beam combiner. The FEC
encoder module is configured to encode ECC data on a data signal.
The modulator is configured to modulate the data signal onto a
first polarization of an optical signal. The pilot encoder module
is configured to encode a pilot carrier oscillator signal onto a
second polarization of the optical signal. The optical receiver
includes a PBS, a polarization controller, and an FEC decoder
module. The PBS is configured to split a received optical signal
having an unknown polarization state into two orthogonal
polarizations (x'-polarization and y'-polarization). The
polarization controller includes no more than two couplers and no
more than two phase shifters. The FEC decoder module is configured
to correct a burst of errors resulting from resetting one of the
phase shifters based on the ECC data encoded in the data signal.
The fiber optically couples the optical transmitter to the optical
receiver.
[0006] Another example embodiment includes method of polarization
demultiplexing. The method includes splitting a received optical
signal having an unknown polarization state into two orthogonal
polarizations (x'-polarization and y'-polarization). The method
includes phase shifting either the x'-polarization or the
y'-polarization according to a first rotation angle. The method
includes generating a third signal and a fourth signal, each of the
third signal and the fourth signal being a combination of a phase
shifted first signal and the other of the x'-polarization or the
y'-polarization. The method includes phase shifting the third
signal according to a second rotation angle. The method includes
generating a first demultiplexed signal and a second demultiplexed
signal, each of the first demultiplexed signal and the second
demultiplexed signal including a combination of a phase shifted
third signal and the fourth signal. The method includes outputting
the first demultiplexed signal and the second demultiplexed
signal.
[0007] The object and advantages of the embodiments will be
realized and achieved at least by the elements, features, and
combinations particularly pointed out in the claims.
[0008] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are not restrictive of the invention, as
claimed.
BACKGROUND
[0009] Dense wavelength division multiplexing (DWDM) may be used to
increase bandwidth in optical communication links. In systems
implementing DWDM, multiple optical signals may be combined and
transmitted on the same optical fiber simultaneously. Each of the
optical signals has different wavelengths. In effect, one fiber is
transformed into multiple virtual fibers. Communication networks
that implement DWDM networks can carry different types of traffic
at different speeds.
[0010] An example of a communication network that implements DWDM
may be referred to as a metro DWDM communication network. The metro
DWDM communication network may be installed to serve cities or
metropolitan areas. The metro DWDM communication network may
communicate data hundreds of kilometers.
[0011] In communication networks implementing DWDM such as the
metro DWDM communication network, chromatic dispersion may occur.
Chromatic dispersion may result in pulse broadening and an increase
in bit errors, for instance. Chromatic dispersion may result from
the physical properties of the optical fibers and the optical
signals and may act to effectively slow the feasible baud rate of
optical signals.
[0012] Optical polarization multiplexing can be used to double the
data capacity of each wavelength channel or to transmit pilot
carrier oscillator signals to aid in detection. In systems
implementing optical polarization multiplexing, optical signal
state of polarization (SOP) may be rotated by the fiber
birefringence, which may require some form of polarization control
or demultiplexing at the receiver. Polarization demultiplexing is
accomplished using digital signal processing multi-input
multi-output (DSP MIMO) processing in digital coherent receivers.
One type of receiver that performs the polarization demultiplexing
is referred to as digital coherent receivers. The digital coherent
receivers use a local oscillator laser, and the DSP MIMO. However,
the local oscillator laser increases the cost of the digital
coherent receivers and the DSP MIMO processing increases the power
dissipation of the transceiver. An efficient scheme for
polarization control/demultiplexing in the optical domain may
enable single polarization coherent receivers based on pilot
carrier with lower cost and lower power use.
[0013] The subject matter claimed herein is not limited to
embodiments that solve any disadvantages or that operate only in
environments such as those described above. Rather, this background
is only provided to illustrate one example technology area where
some embodiments described herein may be practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Example embodiments will be described and explained with
additional specificity and detail through the use of the
accompanying drawings in which:
[0015] FIG. 1 illustrates a block diagram of an example
communication link, in which some embodiments described herein may
be implemented;
[0016] FIG. 2A illustrates a block diagram of example polarization
controllers that may be implemented in the communication link of
FIG. 1;
[0017] FIG. 2B illustrates block diagram of another example
polarization controllers that may be implemented in the
communication link of FIG. 1;
[0018] FIG. 3 illustrates a block diagram of an example wavelength
division multiplex polarization controller; and
[0019] FIG. 4 is a flowchart of an example method of polarization
demultiplexing,
[0020] all in accordance with at least one embodiment described
herein.
DESCRIPTION OF SOME EXAMPLE EMBODIMENTS
[0021] Conventional understanding of polarization controllers is
that a system may include endless polarization control and
redundant phase shifters to eliminate errors during the reset of
phase shifters. For example, a polarization controller including
endless polarization control is discussed in Noe et al., Endless
Polarization Control Systems for Coherent Optics, J. OF LIGHTWAVE
TECH., Vol. 6, No. 7, July 1988, pp. 1199-1208, which is herein
incorporated by reference in its entirety. However, the redundant
phase shifters may increase the insertion loss of the system and
controlling the redundant phase shifters may increase complexity of
a control system.
[0022] An example embodiment disclosed herein includes an optical
receiver that breaks with this conventional understanding. The
optical receiver includes a polarization controller with only two
phase shifters or only two phase shifters per wavelength channel.
The polarization controllers have non-endless polarization control,
but can nevertheless provide reliable communications by taking
advantage of forward error correction (FEC) coding and/or decoding.
When one of the two phase shifters resets, a resulting burst of
errors is corrected by an FEC decoding module. The optical receiver
simplifies the optical polarization demultiplexing and reduces
insertion losses when compared to polarization controllers with
endless polarization control. This and other embodiments are
described herein with reference to the accompanying drawings.
[0023] FIG. 1 illustrates a block diagram of an example optical
communication link (link) 100, in which some embodiments may be
implemented. The link 100 depicts a single wavelength channel that
may be configured to communicate data-carrying optical signals
(optical signals).
[0024] The link 100 depicted in FIG. 1 includes an optical
transmitter (transmitter) 102 optically coupled to an optical
receiver (receiver) 104 via a single mode fiber (SMF) 106. The
transmitter 102 may be configured to generate the optical signal.
In particular the transmitter 102 may be configured to modulate a
data signal on a first polarization of the optical signal and to
encode a pilot carrier oscillator signal (pilot carrier signal) on
a second polarization of the optical signal.
[0025] The transmitter 102 depicted in FIG. 1 includes an example
of a pilot carrier single polarization quadrature phase-shift
keying (PCSP QPSK) transmitter. Accordingly, the transmitter 102 is
configured to generate and transmit the optical signal on which
QPSK data is modulated on the first polarization and a pilot
carrier signal is encoded on the second polarization.
[0026] In some embodiments, the transmitter 102 may include a
higher order quadrature amplitude modulation (QAM) transmitter
(e.g., 8 QAM, 16 QAM, 32 QAM, etc.) or configured for another
carrier modulation format including, for example, amplitude-shift
keying (ASK), phase-shift keying (PSK), frequency-shift keying
(FSK), minimum-shift keying (MSK), Gaussian MSK (GMSK),
continuous-phase FSK (CPFSK), multiple FSK (MFSK), or another
modulation format.
[0027] In some QPSK systems, to receive and interpret data from
optical signals, a local oscillator, such as a local oscillator
laser, may be included at a receiver (e.g., the receiver 104). The
local oscillator is used to mix with the optical signal for
coherent detection of the QPSK data in the optical signal. In the
link 100, instead of the local oscillator being included in the
receiver 104, a pilot carrier signal may be encoded in one of two
polarizations of the optical signal. In particular, the pilot
carrier signal may be encoded in the optical signal at the
transmitter 102.
[0028] For example, in the link 100, the transmitter 102 includes a
laser 108. The laser 108 may be configured to generate a continuous
wave (CW) optical signal, which may be output by the laser 108,
which is represented in FIG. 1 at 162. Some examples of the laser
108 might include an external cavity laser or a distributed
feedback (DFB) laser.
[0029] The CW optical signal 162 may be split at a beam splitter
160 into an x-polarization 112 and a y-polarization 110. The
x-polarization 112 and the y-polarization 110 may be defined
according to a coordinate system of the transmitter 102, however,
the designation as "x" and "y" are not necessarily meaningful other
than the implication that the x-polarization 112 is substantially
orthogonal to the y-polarization 110.
[0030] A CW pilot carrier signal may be encoded on the
y-polarization 110 by a pilot encoder module 164. The
x-polarization 112 may be communicated to an in-phase and
quadrature (IQ) modulator 114 (in FIG. 1 "IQ"). At the IQ modulator
114, two independent electrical tributaries 150A and 150B (in FIG.
1, "I" and "Q") of non-return-to-zero (NRZ) data are modulated into
a QPSK optical signal on the x-polarization 112 at the IQ modulator
114. In some embodiments, the QPSK signal may include a symbol rate
of about 28 gigabaud (GBaud) or any other suitable symbol rate.
[0031] The transmitter 102 may also include an FEC encoder module
140. The FEC encoder module 140 may be configured to encode
error-correcting code (ECC) data into one or both of the
tributaries 150A and/or 150B of the NRZ data. The ECC data may be
used for error correction of the optical signal representative of
the NRZ data at the receiver 104.
[0032] The x-polarization 112, which includes the QPSK signal, may
exit the IQ modulator 114 and may be recombined with the
y-polarization 110 at a polarization beam combiner 116. The optical
signal including the y-polarization 110 and the x-polarization 112
may then be communicated via the SMF 106 to the receiver 104.
[0033] Before the optical signal is received and/or processed by
the receiver 104, the polarization state of the optical signal may
be altered. For example, birefringence in the SMF 106 may alter the
polarization state of the optical signal as the optical signal
propagates through the SMF 106. In some circumstances, a model of
fiber birefringence may be represented by an example fiber
birefringence expression:
U = ( cos ( .theta. 2 ) - jr 1 sin ( .theta. 2 ) - ( r 3 + jr 2 )
sin ( .theta. 2 ) - ( r 3 + jr 2 ) sin ( .theta. 2 ) cos ( .theta.
2 ) - jr 1 sin ( .theta. 2 ) ) ##EQU00001##
In the birefringence expression, U is a 2.times.2 matrix
representing the birefringence experienced in a fiber. The
parameter j represents the imaginary number. The parameters
r.sub.1, r.sub.2, and r.sub.3 represent components of a unit Stokes
vector r. The parameter .theta. represents a rotation angle about
the unit Stokes vector. Accordingly, the optical signal received by
the receiver may include an unknown polarization state.
[0034] The receiver 104 and/or one or more components included
therein may be configured to perform a polarization demultiplexing
of the optical signal received at the receiver 104 having an
unknown polarization state.
[0035] The receiver 104 includes a polarization beam splitter (PBS)
120. An example of the PBS 120 may include a grating. The PBS 120
may be optically coupled to the SMF 106. The optical signal having
the unknown polarization state exiting the SMF 106 may be separated
by the PBS 120 into two orthogonal polarizations. The orthogonal
polarizations may include an x'-polarization 166A and a
y'-polarization 166B. The x'-polarization 166A and the
y'-polarization 166B may be defined according to a coordinate
system defined in a reference frame of the receiver 104.
[0036] If, hypothetically, there is no birefringence in the SMF
106, then the x'-polarization 166A and the y'-polarization 166B
exiting the PBS 120 may match the transmitted x-polarization 112
and y-polarization 110 in the reference frame of the transmitter.
However, due to birefringence of the SMF 106, the x'-polarization
166A and the y'-polarization 166B output of PBS 120 may include
some mixtures of the transmitted polarization states (e.g., the
y-polarization 110 and the x-polarization 112) after being rotated
by the fiber birefringence (e.g. matrix U above).
[0037] Accordingly, the polarization demultiplexing performed by
the receiver 104 or components therein may generally receive the
x'-polarization 166A and the y'-polarization 166B and generate a
first demultiplexed signal 168A and a second demultiplexed signal
168B (generally, demultiplexed signal 168 or demultiplexed signals
168). The first demultiplexed signal 168A and the second
demultiplexed signal 168B may be substantially similar the
transmitted x-polarization 112 including encoded QPSK signal and
y-polarization 110 including the pilot carrier signal. As mentioned
above, the pilot carrier signal may be used in place of a local
oscillator implemented in digital coherent receivers.
[0038] The receiver 104 includes a polarization controller 118
configured to receive the x'-polarization 166A and the
y'-polarization 166B exiting the PBS 120. The polarization
controller 118 may include an optical network 138 and one or more
phase shifters 126A and 126B (generally, phase shifter 126 or phase
shifters 126). In the depicted embodiment, the optical network 138
includes a second phase shifter 126B and a coupling 124 such as a
50/50 splitter. The optical network 138 and the one or more phase
shifters 126 may demultiplex the x'-polarization 166A and the
y'-polarization 166B exiting the PBS 120. The polarization
controller 118 may then output demultiplexed signals 168.
[0039] For example, the polarization controller 118 may include a
first phase shifter 126A and the optical network 138. The
polarization controller 118 may accordingly include the optical
network 138 and the first phase shifter 126A that act as two stages
of polarization rotators for polarization demultiplexing. Thus, the
polarization controller 118 may be configured to not have endless
polarization tracking. Stated another way, the polarization
controller 118 has non-endless polarization tracking.
[0040] Additionally, the polarization controller 118 may not
include redundant phase shifters. For example, in some embodiments,
the polarization controller 118 may include only two phase shifters
126A and 126B. In these and other embodiments, one of the two phase
shifters 126A or 126B may be configured to reset and the other of
the two phase shifters 126A or 126B may be configured to not reset.
When one of the phase shifters 126A or 126B resets, a burst of
errors may be communicated through the polarization controller
118.
[0041] By reducing the number of phase shifters 126, the complexity
of the polarization controller 118 and an associated system
configured to control the phase shifters 126 (e.g., controller 252
of FIGS. 2A and 2B) may be reduced. For example, by including two
phase shifters 126, the associated system may not require a phase
unwinding algorithm. The reduction in phase shifters 126 may also
reduce the insertion losses when compared to polarization
controllers including four phase shifters and/or endless
polarization tracking.
[0042] In some embodiments, the PBS 120, the polarization
controller 118, or some portions thereof may be implemented as a
photonic integrated circuit (PIC) 122. The PIC 122 may be
constructed using silicon photonics, indium phosphide, or any other
suitable materials.
[0043] The demultiplexed signals 168 output by the polarization
controller 118 may be communicated to an optical 90-degree hybrid
(90-degree hybrid) 128. The 90-degree hybrid 128 may communicate
in-phase (I) optical signals and quadrature (Q) optical signals to
one or more p-i-n photodetectors 130A-130D (generally, PD 130 or
PDs 130). The PDs 130 may be organized into balanced pairs in some
embodiments. The PDs 130 may convert the optical signals to
electrical signals, which may be processed by a digital signal
processing (DSP) module 132.
[0044] The DSP module 132 may be followed by an FEC decoder module
134. The FEC decoder module 134 may be configured to correct bursts
of errors during phase resets of the phase shifters 126 (e.g., the
first phase shifter 126A). The FEC decoder module 134 may use any
suitable FEC code designed for burst error correction capability.
In an example of the receiver 104, each phase reset may be about 10
to about 100 times slower than the symbol (or baud) rate. The FEC
decoder module 134 may correct bursts of errors during phase
resets.
[0045] Accordingly, in some embodiments of the receiver 104, the
polarization controller 118 is configured to have a non-endless
tracking. When the phase shifter 126 (e.g., the first phase shifter
126A) resets, a burst of errors may result. The burst of errors is
corrected by the FEC decoder module 134. Thus, in these and other
embodiments, the receiver 104 may be simplified by omitting
components such as multiple other phase shifters, but may still
sufficiently communicate the data encoded in the optical
signal.
[0046] In some embodiments, the link 100 may represent one of
multiple wavelength channels that may be multiplexed onto a fiber
in a wavelength division multiplex (WDM) system such as a dense
wavelength division multiplex (DWDM) system. For example, an
implementation of the link 100 may include a metro DWDM system. The
metro DWDM system may be configured to communicate the optical
signals hundreds of kilometers (km). In embodiments of the link 100
that are implemented in a metro DWDM system, one or more optical
amplifiers may be included to compensate for the fiber transmission
losses. For instance, the metro DWDM system may communicate the
optical signals about 80 to about 100 km in a single un-amplified
link or multiple spans of about 80 to about 100 km may be traversed
with optical amplifiers in each span. Multiplexing multiple
wavelength channels in the DWDM system may be performed by DWDM
equipment such as transmission/multiplexing equipment,
reconfigurable optical add-drop multiplexer (ROADM) or other
suitable equipment, along the link 100. In some embodiments, the
link 100 may also be configured to reduce chromatic dispersion of
the optical signals, which may develop as the optical signals are
communicated along the link 100. In these and other embodiments,
the link 100 may include optical dispersion compensating fibers,
optical dispersion compensating filters. Some additional details of
a WDM system are provided with reference to FIG. 3.
[0047] FIGS. 2A and 2B illustrate block diagrams of example
polarization controllers 200A and 200B that may be implemented in
the link 100 of FIG. 1. Specifically, in some embodiments, the
polarization controllers 200A or 200B may correspond to the
polarization controller 118 discussed with reference to FIG. 1. The
polarization controllers 200A and 200B may be configured to reduce
the insertion losses associated with demultiplexing an optical
signal 250 having an unknown polarization state as compared to
polarization controllers with endless polarization control and/or
redundant phase shifters. The optical signal 250 may correspond to
optical signal generated by the transmitter 102 of FIG. 1 and
communicated via the SMF 106 of FIG. 1, for example.
[0048] The optical signal 250 may be communicated to a PBS 202. The
PBS 202 may include a 2-D grating coupling, for instance, or any
other suitable beam splitter. The PBS 202 may be coupled to a first
waveguide 204 and to a second waveguide 206. The PBS 202 may output
orthogonal polarizations (x'-polarization and y'-polarization). The
first and second signals may be substantially similar to the
x'-polarization 166A and the y'-polarization 166B described with
reference to FIG. 1.
[0049] The PBS 202 may output the x'-polarization and the
y'-polarization to the first waveguide 204 and to the second
waveguide 206. For example, the x'-polarization may be output to
the first waveguide 204 and the y'-polarization may be output to
the second waveguide 206 or vice versa.
[0050] In some embodiments, the x'-polarization or the
y'-polarization may include a transverse electric polarization and
the other of the x'-polarization or the y'-polarization may include
a transverse magnetic polarization. In these and other embodiments,
the polarization having the transverse magnetic polarization may be
communicated to a polarization rotator (PR) 208 that may rotate the
polarization to a transverse electric polarization. In some
embodiments, both the x'-polarization and the y'-polarization may
have transverse electric polarizations at the first and second
waveguides 204 and 206.
[0051] The x'-polarization and the y'-polarization may be
represented as [X' Y'], which may be referred to as a received
signal vector. An x-polarization (e.g., 112 of FIG. 1) and a
y-polarization (e.g., 110 of FIG. 1) included in a transmitted
optical signal and defined in relationship to a coordinate system
of a transmitter may be represented as [X Y], which may be referred
to as a transmitted signal vector.
[0052] Accordingly, a goal of the polarization controllers 200A and
200B is to demultiplex the received signal vector [X' Y'] into the
transmitted signal vector [X Y]. Demultiplexing the received signal
vector [X' Y'] may be accomplished in two stages. A first stage may
include applying a differential phase shift using a first phase
shifter 210. A second stage may include using an optical network
222 including two couplers 212 and 220, which may include 50/50
splitters or another coupler/mixer, and a second phase shifter 218.
The polarization controllers 200A and 200B are configured to
perform a first polarization rotation controlled at least partially
by the first phase shifter 210 and the second polarization rotation
controlled at least partially by the second phase shifter 218.
[0053] The action of the first phase shifter 210 may include
rotation the received signal vector [X' Y'] and/or the
x'-polarization or the y'-polarization on the first waveguide 204
according to a first rotation matrix:
( - j .phi. 1 2 0 0 j .phi. 1 2 ) ##EQU00002##
[0054] In the first rotation matrix, e represents Euler's number
(i.e., 2.71 . . . ). The parameter j represents the imaginary
number (i.e., j.sup.2=-1). The variable .PHI..sub.1 represents a
first phase rotation angle. The first phase rotation angle
.PHI..sub.1 may be controlled and varied by a controller 252.
Generally, the first phase rotation angle .PHI..sub.1 may be reset
when the phase rotation angle .PHI..sub.1 exceeds a range of about
0 to about 2.pi..
[0055] For example, when one end (e.g., 0 or 2.pi.) of the range is
exceeded, the first phase shifter 210 may be reset to an opposite
end of the range. For instance, when the first phase shifter 210
exceeds the range by increasing above 2.pi., the first phase
shifter 210 may be reset to 0 and when the first phase shifter 210
exceeds the range by decreasing below 0, the first phase shifter
may be reset to 2.pi.. During the reset time period, a burst of
errors may result. The burst of errors may result, for instance,
due to improperly demultiplexed signals. As discussed with
reference to FIG. 1, the burst of errors may be corrected using an
FEC decoder module such as the FEC decoder module 134 of FIG.
1.
[0056] In some embodiments, the first phase shifter 210 may include
a phase modulator. The phase modulator may be configured to reset
at a rate of about ten to about one hundred symbol periods, which
may minimize the burst of errors resulting from resetting the phase
modulator. Again, the burst of errors resulting from resetting the
phase modulator may be corrected using the FEC decoder module.
[0057] In some embodiments, a first polarization rotation may be
implemented by applying a differential phase shift in the
waveguides 204 and 206 to achieve the first rotation matrix shown
above. For example, in some embodiments the first polarization
rotator may include a structure as described in Moller, Lothar, WDM
Polarization Controller in PLC Technology, IEEE PHOTONICS
TECHNOLOGY LETTERS, Vol. 13, No. 6, June 2001, which is
incorporated herein by reference in its entirety.
[0058] Rotatation the received signal vector [X' Y'] and/or the
x'-polarization or the y'-polarization on the first waveguide 204
according to a first rotation matrix may generate a phase-shifted
first signal. The phase-shifted first signal may exit the first
phase shifter 210 and enter a first coupler 212, which may be
included in the optical network 222. Additionally, the
x'-polarization or the y'-polarization on the second waveguide 206
or exiting PR 208 (or amplifier 228 discussed below) may enter the
first coupler 212.
[0059] A vector representing the phase-shifted first signal and the
x'-polarization or the y'-polarization received by the first
coupler 212 may be represented as a first output vector [X'' Y''].
The first coupler 212 may include a 2.times.2, 50/50 splitter.
Accordingly, the first coupler 212 may receive the phase-shifted
first signal and the x'-polarization or the y'-polarization
received by the first coupler 212 and may output a third signal and
a fourth signal. The third signal and the fourth signal may include
some combination phase-shifted first signal and the x'-polarization
or the y'-polarization received by the first coupler 212. For
instance, in embodiments in which the first coupler 212 includes
the 2.times.2, 50/50 splitter each of the third signal and the
fourth signal may include 50% of the phase-shifted first signal and
50% the x'-polarization or the y'-polarization received by the
first coupler 212. The third signal may be output to a third
waveguide 214 and the fourth signal may be output to a fourth
waveguide 216.
[0060] The third signal may be communicated to a second phase
shifter 218 which may be configured to apply a phase shift. The
phase-shifted third signal exiting the second phase shifter 218 and
the fourth signal on the fourth waveguide 216 may be communicated
to a second coupler 220. The second coupler 220 may output a fifth
signal and a sixth signal that are some combination of the
phase-shifted third signal and the fourth signal. For instance, in
some embodiments, the second coupler 220 may include a 2.times.2,
50/50 splitter. Accordingly, the second coupler 220 may receive the
phase-shifted third signal and the fourth signal and may output the
fifth signal and the sixth signal, each including 50% of the
phase-shifted third signal and 50% of the fourth signal. The fifth
and the sixth signals represent demultiplexed polarizations of the
optical signal 250 that are substantially similar to the
transmitted signal vector [X Y].
[0061] A result of the optical network 222 (i.e., the combination
of the first coupler 212, the second phase shifter 218, and the
second coupler 220) may include a rotation the first output vector
[X'' Y''] according to a second rotation matrix:
( cos ( .phi. 2 2 ) - j sin ( .phi. 2 2 ) - j sin ( .phi. 2 2 ) cos
( .phi. 2 2 ) ) ##EQU00003##
As already discussed, the j represents the imaginary number (i.e.,
j.sup.2=-1). The parameter .PHI..sub.2 represents a second phase
rotation angle.
[0062] The second phase rotation angle .PHI..sub.2 may be
controlled and varied by the controller 252. In some embodiments,
the second rotation angle .PHI..sub.2 may be kept in a range of
about 0 to about .pi.. Accordingly, in these and other embodiments,
the second phase shifter 218 may not be configured to reset. The
second phase shifter 218 may include a thermal-optic phase shifter
or a phase modulator.
[0063] With combined reference to FIGS. 1-2B, the demultiplexed
polarizations of the optical signal 250 (e.g., the fifth signal and
the sixth signal that exit the second coupler 220) may include or
be substantially equivalent to the x-polarization 112 and the
y-polarization 110. Specifically, the fifth signal may represent
the x-polarization 112, which may include a data signal such as the
QPSK data modulated thereon. Additionally, the sixth signal may
represent the y-polarization 110, which may include the pilot
carrier signal encoded thereon. In an example embodiment, the fifth
signal and the sixth signal may be communicated to, e.g., the
90-degree hybrid 128 of FIG. 1 for further processing as already
described above such that the data encoded on the optical signals
250 may be received and processed.
[0064] In some embodiments, the second phase shift, which may be
performed by the optical network 222, may be applied in a
differential fashion to both waveguides 214 and 216 to achieve the
second rotation matrix shown above.
[0065] In some embodiments, the polarization controllers 200A and
200B may include only two phase shifters 210 and 218 (as opposed to
more than two) and only two couplers 212 and 220 (as opposed to
more than two). The two phase shifters 210 and 218 and the two
couplers 212 and 220 may be sufficient to transform any
polarization state of the x'-polarization and/or the
y'-polarization to any other polarization state. This configuration
is specifically and explicitly included in some embodiments. This
configuration may result in a burst of errors during phase resets
which are corrected using the FEC decoder module 134 as described
above.
[0066] In some embodiments, the polarization controller 118 may be
constructed using one or more of bulk optics and PIC. For example,
the phase shifters 210 and 218, the couplers 212 and 220, the PBS
202, the PR 208, the waveguides 204, 206, 214, and 216 or some
combination thereof may be included in a PIC. The PIC may be
constructed using silicon photonic technologies, for instance.
Additionally or alternatively, one or more of the PBS 202, the PR
208, and the waveguides 206 and 204 may be constructed of bulk
optics.
[0067] With reference to FIG. 2B, a second polarization controller
200B may be substantially similar to the first polarization
controller 200A as described herein. In addition, the second
polarization controller 200B, may include a first semiconductor
optical amplifier (SOA) 226 between the PBS 202 and the first phase
shifter 210. Additionally or alternatively, the second polarization
controller 200B may include a second SOA 228 between the PR 208 and
the first coupler 212. The first and second SOAs 226 and 228 may be
configured to amplify the x'-polarization and the y'-polarization.
For example, before the x'-polarization and/or the y'-polarization
enters the first phase shifter 210, the first SOA 226 may amplify
the x'-polarization and/or the y'-polarization. Likewise, before
the x'-polarization and/or the y'-polarization enters the first
coupler 212, the second SOA 228 may amplify the x'-polarization
and/or the y'-polarization.
[0068] Additionally, in the second polarization controller 200B,
the first and second SOAs 226 and 228, the first and second phase
shifters 210 and 218, the first and second couplers 212 and 202 and
waveguides therebetween may be included in a PIC 230. The PIC 230
may be constructed using indium phosphide (InP) or other suitable
material(s). Additionally, with combined reference to FIGS. 1 and
2B, in some embodiments, the PIC 230 may include the 90-degree
hybrid 128 and/or the PDs 130. In these and other embodiments, the
PBS 202 and the PR 208 may be constructed of bulk optics. In some
embodiments of the second polarization controller 200B, the first
and the second phase shifters 210 and 218 may include phase
modulators. The phase modulators may have similar functions as the
phase modulators described with respect to FIG. 2A.
[0069] FIG. 3 illustrates a block diagram of an example WDM
polarization controller 300. The WDM polarization controller 300
may be included in and/or be suitable for polarization control in a
WDM system (not shown). The WDM system may include multiple links
similar to the link 100 of FIG. 1. In addition to components
included in FIG. 1, the WDM system may include a multiplexer
configured to multiplex multiple optical signals (e.g., the optical
signals of FIG. 1) having differing wavelengths into a WDM signal
350. The WDM signal 350 is communicated along a SMF (e.g., the SMF
106 of FIG. 1) or a multi-mode fiber (MMF). Additionally, the WDM
system may include a demultiplexer configured to separate the WDM
signal 350 into multiple wavelength channels. After being separated
by the demultiplexer, the data (e.g., QPSK data) on each wavelength
channel may be received and processed. An example WDM system in
which the WDM polarization controller 300 may be implemented may be
a DWDM system or a metro DWDM system having one or more DWDM
components.
[0070] In some embodiments, the WDM signal 350 may include one or
more wavelength channels. The wavelength channels may each have a
data signal (e.g., QPSK data) modulated on a first polarization and
a pilot carrier signal encoded on a second polarization. The first
polarization and the second polarization may be defined according
to a coordinate system of a transmitter, similar to the
x-polarization 112 and y-polarization 110 discussed elsewhere
herein. The WDM signal 350, when received at the WDM polarization
controller 300 may include an unknown polarization state.
[0071] The WDM polarization controller 300 may be configured to
receive the WDM signal 350 and perform a polarization
demultiplexing of each of the wavelength channels such that the
data signal of each of the wavelength channels may be interpreted
using the pilot carrier signal at a WDM receiver (not shown, but
similar to the receiver 104 of FIG. 1). In the WDM polarization
controller 300, the polarization control may be non-endless, thus
bursts of errors may occur during reset of one or more wavelength
parallel phase shifters (described below). The bursts of errors may
be corrected using an FEC decoder module (e.g., the FEC decoder
module 134 of FIG. 1).
[0072] The WDM polarization controller 300 may operate similarly to
the polarization controllers 200A and 200B of FIGS. 2A and 2B. For
example, a goal of the WDM polarization controller 300 is to
demultiplex an x'-polarization and a y'-polarization, which may be
defined in accordance with a coordinate system of a receiver, of
each of the wavelength channels into transmitted polarizations
x-polarization and y-polarization.
[0073] The WDM polarization controller 300 is configured to perform
a first polarization rotation of each of the wavelength channels
controlled by first wavelength selectable phase shifters 302A and a
second polarization rotation of each of the wavelength channels
controlled by the second wavelength selectable phase shifters 302B.
In some embodiments, the WDM polarization controller 300 may
implement differential phase shifting as discussed elsewhere
herein.
[0074] A difference between the polarization controllers 200A and
200B of FIGS. 2A and 2B and the WDM polarization controller 300 is
a substitution of wavelength selectable phase shifters 302A and
302B (generally, wavelength selectable phase shifter 302 or
wavelength selectable phase shifters 302) for the phase shifters
210 and 218 of FIGS. 2A and 2B. The wavelength selectable phase
shifters 302 may be configured to perform phase rotations similar
to that described with reference to the phase shifters 210 of FIGS.
2A and 2B or be included in an optical network 328 which may
perform a phase rotation along with a first coupler 312 and a
second coupler 320 similar to the optical network 222 of FIGS. 2A
and 2B. The wavelength selectable phase shifters 302 however, are
performed on each of the wavelength channels included in the WDM
signal 350.
[0075] Specifically, in the depicted WDM polarization controller
300 of FIG. 3, the WDM signal 350 having an unknown polarization
state may be separated into orthogonal polarizations at a PBS 322.
The orthogonal polarizations may include an x'-polarization and a
y'-polarization. Either the x'-polarization or the y'-polarization
may be communicated from the PBS 322 to a first optical
demultiplexer 306A of a first wavelength selectable phase shifter
302A. The first optical demultiplexer 306A may separate the
x'-polarization or the y'-polarization output from the PBS 322 into
wavelength channels (.lamda..sub.1-.lamda..sub.4 in FIG. 3). The
wavelength channels may be communicated through a first array of
parallel phase shifters 324. The first array of parallel phase
shifters 324 may include multiple parallel phase shifters
(individually labeled in FIG. 3 as 310A-310D, collectively referred
to as parallel phase shifters 310) that may each receive one of the
wavelength channels. Specifically, a first wavelength channel
.lamda..sub.1 may be communicated to a first of the parallel phase
shifters 310A, a second wavelength channel .lamda..sub.2 may be
communicated to a second of the parallel phase shifters 310B,
etc.
[0076] Each of the parallel phase shifters 310 may be substantially
similar to the first phase shifter 210 of the polarization
controllers 200A and 200B of FIGS. 2A and 2B. For example, each of
the parallel phase shifters 310A-310D may apply a differential
phase shift to each of the received wavelength channels. In some
embodiments, each of the parallel phase shifters 310 may rotate one
of the received wavelength channels by the first rotation matrix
described above. The rotation that results from the parallel phase
shifters 310 may be based on one or more phase rotation angles,
which may be controlled and/or varied by a controller (e.g., the
controller 252 of FIGS. 2A and 2B).
[0077] One or more of the phase rotation angles of the parallel
phase shifters 310 may be reset when the phase rotation angles
exceed a range of about 0 to about 2.pi.. For example, when one end
of a range of one of the phase rotation angles is exceeded, the
parallel phase shifters 310 may be reset to an opposite end of the
range. During the reset time period, a burst of errors may result.
The burst of errors may result, for instance, due to improperly
demultiplexed signals. As discussed with reference to FIG. 1, the
burst of errors may be corrected using an FEC decoder module such
as the FEC decoder module 134 of FIG. 1.
[0078] Rotation of the wavelength channels may generate
phase-shifted wavelength channels. The phase-shifted wavelength
channels may then be multiplexed by a first optical multiplexer
308A of the first wavelength selectable phase shifters 302A. The
multiplexed, phase-shifted wavelength channels may proceed to a
first coupler 312. The first coupler 312 may include a 2.times.2
50/50 splitter, for instance. The first coupler 312 may output two
intermediate signals. The two intermediate signals may include a
combination (e.g., 50/50) of the multiplexed, phase-shifted
wavelength channels with the x'-polarization or the y'-polarization
output from the PBS 322 that did not enter the first optical
demultiplexer 306A.
[0079] A first intermediate signal of the two intermediate signal
that are output from the first coupler 312 may be communicated to a
second demultiplexer 306B of the second wavelength selectable phase
shifters 302B. The second demultiplexer 306B may separate the first
intermediate signal output from the first coupler 312 into the
multiple wavelength channels (.lamda..sub.1-.lamda..sub.4 in FIG.
3). The wavelength channels may be communicated through a second
array of parallel phase shifters 326.
[0080] The second array of parallel phase shifters 326 may include
multiple parallel phase shifters (individually, labeled in FIG. 3
as 318A-318D, collectively referred to as second parallel phase
shifters 318) that may each receive one of the wavelength channels.
Specifically, a first wavelength channel .lamda..sub.1 may be
communicated to a first of the second parallel phase shifters 318A,
a second wavelength channel .lamda..sub.2 may be communicated to a
second of the second parallel phase shifters 318B, etc.
[0081] One or more of the second parallel phase shifters 318A-318D
may be substantially similar to the second phase shifter 218 of the
polarization controllers 200A and 200B of FIGS. 2A and 2B. Each of
the second parallel phase shifters 318A-318D may apply a phase
shift to one of the wavelength channels. Phase-shifted wavelength
channels output by the second parallel phase shifters 318A-318D may
be multiplexed by a second multiplexer 308B.
[0082] The phase-shifted, multiplexed signal exiting the second
wavelength selectable phase shifter 302B and a second intermediate
signal of the two intermediate signals exiting the first coupler
312 may be communicated to a second coupler 320. The second coupler
320 may receive the phase-shifted, multiplexed signal output from
the second multiplexer 308B and the second intermediate signal
output from the first coupler 312. The second coupler 320 may
output demultiplexed signals 352A and 352B. Each of the
demultiplexed signals 352A and 352B may include a combination of
the phase-shifted, multiplexed signal output from the second
multiplexer 308B and the second intermediate signal output from the
first coupler 312. For example, the second coupler 320 may include
a 2.times.2, 50/50 splitter. Accordingly, each of the demultiplexed
signals 352A and 352B may include 50% of the phase-shifted,
multiplexed signal and 50% of the second intermediate signal.
[0083] A result of the optical network 328 (i.e., the combination
of the first coupler 312, the second wavelength selectable phase
shifter 302B, and the second coupler 320) may include rotation of
each of the wavelength channels according to the second rotation
matrix described above. One or more second phase rotation angles
included in the second rotation matrix may be controlled and varied
by a controller (e.g., 252 of FIGS. 2A and 2B). In some
embodiments, the second rotation angle applied to each of the
wavelength channels may be kept in a range of about 0 to about
.pi.. The demultiplex signals 352A and 352B may include channel
wavelengths having polarizations that are substantially similar to
transmitted polarizations x-polarization and y-polarization on
which data signals may be modulated and a pilot carrier signal may
be encoded.
[0084] The WDM polarization controller 300 of FIG. 3 includes four
parallel phase shifters 310 and 318 in each of the wavelength
selectable phase shifters 302. In some embodiments, the wavelength
selectable phase shifters 302 may include more than four or fewer
than four parallel phase shifters 310. These embodiments may be
implemented in WDM systems including more than four or fewer than
four wavelength channels.
[0085] FIG. 4 depicts a flowchart of an example method 400 of
polarization demultiplexing, in accordance with at least one
embodiment described herein. The polarization demultiplexing may be
performed on an optical signal such as the optical signal discussed
elsewhere herein. For example, the optical signal may include QPSK
data modulated on a first polarization and a pilot carrier signal
encoded on a second polarization. The first polarization and the
second polarization may be defined with respect to a transmitter
such as the x-polarization 112 and the y-polarization of FIG. 1.
The first polarization may be orthogonal to the first
polarization.
[0086] The method 400 may be performed in a communication link such
as the link 100 of FIG. 1. For example, the method 400 may be
performed in some embodiments by the receiver 104 or one or more
components included therein. The receiver 104 or one or more
components included therein may include non-transitory
computer-readable medium having stored thereon programming code or
instructions that are executable by a computing device to cause the
computing device to perform the method 400 or some portion thereof.
Additionally or alternatively, the receiver 104 may include a
processor that is configured to execute computer instructions to
cause a computing system to perform or control performance of the
method 400 or some portion thereof. Although illustrated as
discrete blocks, various blocks may be divided into additional
blocks, combined into fewer blocks, or eliminated, depending on the
desired implementation.
[0087] The method 400 may begin at block 402. At block 402, a
received optical signal having an unknown polarization state may be
split into two orthogonal polarizations (x'-polarization and
y'-polarization). For example a PBS may split the optical signal
having an unknown polarization state into the x'-polarization and
the y'-polarization. At block 404 a phase of either the
x'-polarization or the y'-polarization may be shifted according to
a first rotation angle. In some embodiments, shifting the phase of
the x'-polarization or the y'-polarization may include differential
phase shifting. In some embodiments, the shifting the phase of the
x'-polarization or the y'-polarization may include rotating the SOP
of the x'-polarization or the y'-polarization by a first rotation
matrix. The first rotation matrix may include:
( - j .phi. 1 2 0 0 j .phi. 1 2 ) ; ##EQU00004##
In the first rotation matrix, e represents Euler's number, j
represents the imaginary number; and .PHI..sub.1 represents the
first phase rotation angle.
[0088] At block 406, a third signal and a fourth signal may be
generated. In some embodiments one or both of the third signal and
the fourth signal may include a combination of a phase shifted
first signal and the other of the x'-polarization or the
y'-polarization. For instance if at block 404 x'-polarization is
phase shifted, then the third signal and the fourth signal may
include a combination of the phase shifted x'-polarization and the
y'-polarization. Alternatively, if at block 404 y'-polarization is
phase shifted, then the third signal and the fourth signal may
include a combination of the phase shifted y'-polarization and the
x'-polarization.
[0089] At block 408, a phase of the third signal may be shifted
according to a second rotation angle. In some embodiments, shifting
the phase of the third signal may include differential phase
shifting. In some embodiments, shifting the phase of the third
signal may include rotating the phase shifted first signal by a
second rotation matrix. The second rotation matrix may include:
( cos ( .phi. 2 2 ) - j sin ( .phi. 2 2 ) - j sin ( .phi. 2 2 ) cos
( .phi. 2 2 ) ) ##EQU00005##
In the second rotation matrix, e represents Euler's number, j
represents the imaginary number; and .PHI..sub.2 represents the
second phase rotation angle.
[0090] At block 410, a first demultiplexed signal and a second
demultiplexed signal may be generated. One or both of the first
demultiplexed signal and the second demultiplexed signal may
include a combination of a phase shifted third signal and the
fourth signal. At block 412, the first demultiplexed signal and the
second demultiplexed signal may be output. For example, the first
demultiplexed signal and the second demultiplexed signal may be
output to a 90 degree hybrid.
[0091] At block 414, in response to the first phase rotation angle
of a phase shifter exceeds a range of about 0 to about 2.pi., the
first phase rotation angle may be reset. At block 416 during the
resetting, a burst of errors may be allowed to result from
improperly demultiplexed signals. At block 418, the burst of errors
may be corrected using FEC processing.
[0092] One skilled in the art will appreciate that, for this and
other procedures and methods disclosed herein, the functions
performed in the processes and methods may be implemented in
differing order. Furthermore, the outlined steps and operations are
only provided as examples, and some of the steps and operations may
be optional, combined into fewer steps and operations, or expanded
into additional steps and operations without detracting from the
disclosed embodiments. For example, the method 400 may include
separating the x'-polarization or the y'-polarization into a
plurality of wavelength channels. In these and other embodiments,
the phase shifting the x'-polarization or the y'-polarization may
include phase shifting each of the plurality of wavelength
channels. Additionally or alternatively, the method 400 may include
separating the third signal into the plurality of wavelength
channels. In these and other embodiments, the phase shifting of the
third signal may include phase shifting each of the plurality of
wavelength channels.
[0093] The embodiments described herein may include the use of a
special purpose or general purpose computer including various
computer hardware or software modules, as discussed in greater
detail below.
[0094] Embodiments described herein may be implemented using
computer-readable media for carrying or having computer-executable
instructions or data structures stored thereon. Such
computer-readable media may be any available media that may be
accessed by a general purpose or special purpose computer. By way
of example, and not limitation, such computer-readable media may
comprise non-transitory computer-readable storage media including
RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic
disk storage or other magnetic storage devices, or any other
non-transitory storage medium which may be used to carry or store
desired program code means in the form of computer-executable
instructions or data structures and which may be accessed by a
general purpose or special purpose computer. Combinations of the
above should also be included within the scope of computer-readable
media.
[0095] Computer-executable instructions comprise, for example,
instructions and data which cause a general purpose computer,
special purpose computer, or special purpose processing device to
perform a certain function or group of functions. Although the
subject matter has been described in language specific to
structural features and/or methodological acts, it is to be
understood that the subject matter defined in the appended claims
is not necessarily limited to the specific features or acts
described above. Rather, the specific features and acts described
above are disclosed as example forms of implementing the
claims.
[0096] As used herein, the term "module" or "component" may refer
to software objects or routines that execute on the computing
system. The different components, modules, engines, and services
described herein may be implemented as objects or processes that
execute on the computing system (e.g., as separate threads). While
the system and methods described herein are preferably implemented
in software, implementations in hardware or a combination of
software and hardware are also possible and contemplated. In this
description, a "computing entity" may be any computing system as
previously defined herein, or any module or combination of
modulates running on a computing system.
[0097] All examples and conditional language recited herein are
intended for pedagogical objects to aid the reader in understanding
the invention and the concepts contributed by the inventor to
furthering the art, and are to be construed as being without
limitation to such specifically recited examples and conditions.
Although embodiments of the present inventions have been described
in detail, it should be understood that the various changes,
substitutions, and alterations could be made hereto without
departing from the spirit and scope of the invention.
* * * * *