U.S. patent application number 16/675375 was filed with the patent office on 2021-05-06 for coherent receiver with mixed signal processing.
The applicant listed for this patent is Google LLC. Invention is credited to Hong Liu, Xiang Zhou.
Application Number | 20210135762 16/675375 |
Document ID | / |
Family ID | 1000005535262 |
Filed Date | 2021-05-06 |
United States Patent
Application |
20210135762 |
Kind Code |
A1 |
Zhou; Xiang ; et
al. |
May 6, 2021 |
COHERENT RECEIVER WITH MIXED SIGNAL PROCESSING
Abstract
A receiver system is provided for receiving a coherent Pulse
Amplitude Modulation (PAM) encoded signal. The receiver system may
include an optical polarization component configured to modulate a
polarization of the received coherent PAM encoded signal. The
receiver system may further include a digital signal processor
(DSP) configured to perform polarization recovery between the
received coherent PAM encoded signal and the LO signal using a
first control loop, and to perform phase recovery between the
received coherent PAM encoded signal and the LO signal using a
second control loop.
Inventors: |
Zhou; Xiang; (Sunnyvale,
CA) ; Liu; Hong; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Google LLC |
Mountain View |
CA |
US |
|
|
Family ID: |
1000005535262 |
Appl. No.: |
16/675375 |
Filed: |
November 6, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 14/06 20130101;
H04B 10/5561 20130101; H04B 10/25073 20130101; H04B 10/541
20130101; H04B 10/524 20130101 |
International
Class: |
H04B 10/556 20060101
H04B010/556; H04B 10/54 20060101 H04B010/54; H04B 10/524 20060101
H04B010/524; H04B 10/2507 20060101 H04B010/2507; H04J 14/06
20060101 H04J014/06 |
Claims
1. A receiver system, comprising: an optical polarization component
configured to modulate a polarization of a received coherent Pulse
Amplitude Modulation (PAM) encoded signal; and a digital signal
processor (DSP) configured to: perform polarization recovery
between the received coherent PAM encoded signal and a local
oscillator (LO) signal using a first control loop perform phase
recovery between the received coherent PAM encoded signal and the
LO signal using a second control loop; and a low-speed circuit
configured to detect an average power of a polarized portion of the
received coherent PAM encoded signal, wherein the DSP is configured
to determine the polarization adjustments by minimizing the average
power of the polarized portion.
2. The receiver system of claim 1, wherein the DSP is configured to
determine polarization adjustments to the received coherent PAM
encoded signal in an electrical domain and the optical polarization
component is configured to apply the polarization adjustments to
the received coherent PAM encoded signal in an optical domain.
3. The receiver system of claim 2, wherein the DSP is configured to
determine the polarization adjustments by maximizing a beating
signal power between the LO signal and a polarized portion of the
received coherent PAM encoded signal with encoded data.
4. (canceled)
5. The receiver system of claim 1, wherein the optical polarization
component is an optical polarization controller for receiving a
single polarization or dual polarization coherent PAM encoded
signal, or an optical demultiplexer for receiving a
dual-polarization coherent PAM encoded signal.
6. A receiver system, comprising: an optical polarization component
configured to modulate a polarization of a received coherent Pulse
Amplitude Modulation (PAM) encoded signal; and a digital signal
processor (DSP) configured to: perform polarization recovery
between the received coherent PAM encoded signal and a local
oscillator (LO) signal using a first control loop perform phase
recovery between the received coherent PAM encoded signal and the
LO signal using a second control loop; and one or more optical
phase modulators, wherein the DSP is configured to determine phase
adjustments to the LO in an electrical domain and the one or more
optical phase modulators are configured to apply the phase
adjustments to the LO signal in an optical domain.
7. The receiver system of claim 6, wherein the DSP is configured to
determine the phase adjustments based on known pilot data symbols
inserted in the received coherent PAM encoded signal.
8. The receiver system of claim 6, wherein the DSP is configured to
determine frequency adjustments to the LO signal in an electrical
domain and the one or more optical phase modulators are configured
to apply the frequency adjustments to the LO signal in an optical
domain.
9. The receiver system of claim 1, wherein the DSP is configured to
determine frequency adjustments to the LO in an electrical domain
and the LO is configured to apply the frequency adjustments to the
LO signal in an optical domain.
10. The receiver system of claim 1, further comprising: a
wavelength demultiplexer configured to receive coherent PAM encoded
signal that includes a plurality of wavelengths.
11. The receiver system of claim 1, wherein the first control loop
is a polarization feedback control loop, and the second control
loop is a phase feedback control loop.
12. The receiver system of claim 1, wherein the first control loop
and the second control loop are part of a same control loop for
joint polarization and phase recovery.
13. The receiver system of claim 12, wherein the control loop for
joint polarization and phase recovery is configured to introduce a
45.degree. polarization angle between the received signal and the
LO.
14. The receiver system of claim 12, further comprising: a joint
polarization-diverse and phase-diverse coherent detection
circuitry, wherein, the polarization-diverse coherent detection
circuitry and the phase-diverse coherent detection circuitry share
one or more common circuit elements.
15. The receiver system of claim 1, further comprising: a two-mode
switch, wherein, when the two-mode switch is set in a first mode,
the two-mode switch is configured as an optical coupler (OC) for
receiving coherent PAM encoded signal, and when the two-mode switch
is set in a second mode, the two-mode switch is configured as a
switch for receiving an intensity modulation direct-detect (IM-DD)
PAM encoded signal.
16. (canceled)
17. (canceled)
18. (canceled)
19. (canceled)
20. (canceled)
21. The receiver system of claim 6, wherein the first control loop
is a polarization feedback control loop, and the second control
loop is a phase feedback control loop.
22. The receiver system of claim 6, wherein the first control loop
and the second control loop are part of a same control loop for
joint polarization and phase recovery.
23. The receiver system of claim 6, further comprising: a
wavelength demultiplexer configured to receive coherent PAM encoded
signal that includes a plurality of wavelengths.
24. A receiver system, comprising: an optical polarization
component configured to modulate a polarization of a received
coherent Pulse Amplitude Modulation (PAM) encoded signal; and a
digital signal processor (DSP) configured to: perform polarization
recovery between the received coherent PAM encoded signal and a
local oscillator (LO) signal using a first control loop; and
perform phase recovery between the received coherent PAM encoded
signal and the LO signal using a second control loop; and wherein
the first control loop or second control loop for joint
polarization and phase recovery is configured to introduce a
45.degree. polarization angle between the received signal and the
LO.
25. The receiver system of claim 24 wherein the first control loop
is a polarization feedback control loop, and the second control
loop is a phase feedback control loop.
26. The receiver system of claim 24 wherein the first control loop
and the second control loop are part of a same control loop for
joint polarization and phase recovery.
Description
BACKGROUND
[0001] In an optical network, a transmitter system may modulate
light signals to encode data, and a receiver system may detect and
decode the modulated light signals to recover the data. An
Intensity-Modulation/Direct-Detection (IM-DD) Pulse Amplitude
Modulation (PAM) system modulates intensity of a carrier signal to
encode data. In contrast, a coherent PAM system modulates amplitude
of the optical field of a carrier signal to encode data. A coherent
Quadrature Amplitude Modulation (QAM) system modulates the optical
field of two carrier signals to encode data, such as either or both
amplitude and/or phase of the two carrier signals.
[0002] In long-haul and metro networks, QAM systems are widely
used. However, for low-power datacenter-reach optical
interconnects, QAM technology has several challenges. First, QAM
transceivers may not be compatible with IM-DD PAM transceivers, but
interoperability is often required in datacenters, as datacenter
networks are often upgraded block by block. Second, QAM systems may
use more power than PAM systems since QAM transceivers may require
higher bias voltage for modulation, and additional components such
as additional Digital Signal Processing (DSP) functional blocks may
be needed for polarization and phase recovery. Third, to perform
phase modulation, QAM systems may have stricter requirements on the
lasers and modulators used, such as speed, bandwidth, extinction
ratio etc.
[0003] Existing polarization-diverse and phase-diverse receiver
systems can receive and decode coherent encoded signals, such as
coherent QAM or coherent PAM encoded signals. Since properties of a
signal can change during transmission, the receiver system needs to
perform recovery of these properties once the signal is received.
For example, a received coherent PAM encoded signal may be split by
a Polarizing Beam Splitter (PBS) into two beams with orthogonal
polarizations, such as the transverse electric (TE) mode polarized
light beam and the transverse magnetic (TM) mode polarized light
beam. A 90.degree. polarization rotator may be built into the PBS
to convert the TM mode polarized light beam into the TE mode
polarized such that the two outputs of the PBS are both TE-mode
polarized light beams. For convenience, through this document, the
principal polarization axes of the PBS are denoted as x-axis and
y-axis. Further, the original TE-mode polarized signal is denoted
as "x-polarized beam," while the TE-mode polarized signal converted
from the original TM mode signal is denoted as "y-polarized
beam."
[0004] Recovery of polarization and phase information is then
performed on each of the polarized beams by reference to a signal
from a Local Oscillator (LO) with known properties. In this regard,
an Optical Coupler (OC) splits the LO signal into two beams, and
interferences may then be performed between each polarized coherent
PAM encoded beam and one of the two LO beams by two respective
hybrids. For instance, an x-polarized 90.degree. hybrid may perform
interference between the x-polarized beam and a first LO beam, and
a y-polarized 90.degree. hybrid may perform interference between
the y-polarized beam and a second LO beam. Since properties of the
LO are known, the resultant wave from each interference may be
analyzed to determine properties of each polarized coherent PAM
encoded beam.
[0005] As a result of the interference, the two hybrids may output
four signals--two phases for each of the two polarizations. These
four signals are then respectively passed through four
photodetectors (PD) that convert optical signals to electrical
signals, and four transimpedance amplifiers (TIA) that amplify the
electrical signals. In some examples, each of the four PDs may be a
pair of balanced PDs configured to cancel common-node noises. The
amplified electrical signals are then respectively passed through
four analog to digital converters (ADC) and converted into digital
signals. Once the four separated signals are converted from optical
into electrical by the PDs/TIAs, and digitized by the ADCs, the
Digital Signal Processor (DSP) may extract amplitude and phase
information in the electrical domain to decode and recover the
data.
[0006] Although a coherent PAM system may use a conventional
polarization- and phase-diverse digital coherent receiver system,
such a complex receiver system may be inefficient for recovering
coherent PAM encoded signals. Further, the conventional
polarization-diverse and phase-diverse receiver system is not
compatible with an IM-DD PAM transmitter system.
SUMMARY
[0007] The present disclosure provides for receiver system
comprising an optical polarization component and a digital signal
processor (DSP). The optical polarization component may be
configured to modulate a polarization of a received coherent Pulse
Amplitude Modulation (PAM) encoded signal. The DSP may be
configured to perform polarization recovery between the received
coherent PAM encoded signal and a local oscillator (LO) signal
using a first control loop; and to perform phase recovery between
the received coherent PAM encoded signal and the LO signal using a
second control loop.
[0008] The DSP may be configured to determine polarization
adjustments to the received coherent PAM encoded signal in an
electrical domain and the optical polarization component may be
configured to apply the polarization adjustments to the received
coherent PAM encoded signal in an optical domain. The DSP may be
configured to determine the polarization adjustments by maximizing
a beating signal power between the LO signal and a polarized
portion of the received coherent PAM encoded signal with encoded
data. The receiver system may further comprise a low-speed circuit
configured to detect an average power of a polarized portion of the
received coherent PAM encoded signal, wherein the DSP may be
configured to determine the polarization adjustments by minimizing
the average power of the polarized portion.
[0009] The optical polarization component may be an optical
polarization controller for receiving a single polarization or dual
polarization coherent PAM encoded signal, or an optical
demultiplexer for receiving a dual-polarization coherent PAM
encoded signal.
[0010] The receiver system may further comprise one or more optical
phase modulators, wherein the DSP may be configured to determine
phase adjustments to the LO in an electrical domain and the one or
more optical phase modulators may be configured to apply the phase
adjustments to the LO signal in an optical domain. The DSP may be
configured to determine the phase adjustments based on known pilot
data symbols inserted in the received coherent PAM encoded signal.
The DSP may be configured to determine frequency adjustments to the
LO signal in an electrical domain and the one or more optical phase
modulators may be configured to apply the frequency adjustments to
the LO signal in an optical domain.
[0011] The DSP may be configured to determine frequency adjustments
to the LO in an electrical domain and the LO may be configured to
apply the frequency adjustments to the LO signal in an optical
domain.
[0012] The receiver system may further comprise a wavelength
demultiplexer configured to receive coherent PAM encoded signal
that includes a plurality of wavelengths.
[0013] The first control loop may be a polarization feedback
control loop, and the second control loop may be a phase feedback
control loop.
[0014] The first control loop and the second control loop may be
part of a same control loop for joint polarization and phase
recovery. The control loop for joint polarization and phase
recovery may be configured to introduce a 45.degree. polarization
angle between the received signal and the LO. The receiver system
may further comprise a joint polarization-diverse and phase-diverse
coherent detection circuitry, wherein, the polarization-diverse
coherent detection circuitry and the phase-diverse coherent
detection circuitry may share one or more common circuit
elements.
[0015] The receiver system may further comprise a two-mode switch,
wherein, when the two-mode switch is set in a first mode, the
two-mode switch may be configured as an optical coupler (OC) for
receiving coherent PAM encoded signal, and when the two-mode switch
is set in a second mode, the two-mode switch may be configured as a
switch for receiving an intensity modulation direct-detect (IM-DD)
PAM encoded signal.
[0016] The present disclosure further provides for a receiver
system comprising a polarization beam splitter, two hybrids, an
analog equalizer, and a DSP. The polarization beam splitter may be
configured to split a received coherent PAM encoded signal into two
polarized PAM encoded signals. The two hybrids may be configured to
combine each of the two polarized PAM encoded signals with a LO
signal. The analog equalizer may be configured to perform
polarization recovery between the received coherent PAM encoded
signal and the LO signal, and perform phase recovery between the
received coherent PAM encoded signal and the LO signal. The DSP may
be configured to provide equalizer coefficients to the analog
equalizer for polarization and phase recovery. The analog equalizer
may be a 4.times.2 multi-input multi-output equalizer.
[0017] The present disclosure still further provides for receiving,
by a receiver system, a coherent PAM encoded signal; receiving, by
the receiver system, a LO signal; performing, by the receiver
system using mixed signal processing, polarization recovery between
the received coherent PAM encoded signal and the LO signal using a
first control loop; and performing, by the receiver system using
mixed signal processing, phase recovery between the received
coherent PAM encoded signal and the LO signal using a second
control loop.
[0018] The mixed signal processing may include processing in an
optical domain and processing in an electrical domain. The mixed
signal processing may include processing in an analog domain and
processing in a digital domain.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIGS. 1A and 1B show example receiver systems with mixed
electrical and optical signal processing for single polarization
signals in accordance with aspects of the disclosure.
[0020] FIGS. 2A and 2B show example receiver systems with mixed
electrical and optical signal processing for dual polarization
signals in accordance with aspects of the disclosure.
[0021] FIGS. 3A and 3B show example receiver systems with mixed
electrical and optical signal processing for signals with multiple
wavelengths in accordance with aspects of the disclosure.
[0022] FIGS. 4A and 4B show example receiver system with mixed
electrical and optical signal processing with joint
polarization-diverse and phase-diverse coherent detection in
accordance with aspects of the disclosure.
[0023] FIGS. 5A, 5B, and 5C show example 1.times.2 optical
polarization controllers that can be used to enable joint
polarization-diverse and phase-diverse coherent detection in
accordance with aspects of the disclosure.
[0024] FIG. 6 shows an example receiver system with mixed analog
and digital signal processing in accordance with aspects of the
disclosure.
[0025] FIGS. 7A and 7B show example transmission systems with mixed
signal processing in accordance with aspects of the disclosure.
[0026] FIG. 8 is a block diagram showing an example receiver system
in accordance with aspects of the disclosure.
[0027] FIG. 9 is a flow diagram illustrating an example method in
accordance with aspects of the disclosure.
DETAILED DESCRIPTION
Overview
[0028] The technology relates generally to a coherent receiver
system with mixed signal processing. A PAM system may be used for
datacenter-reach optical interconnects. In terms of
interoperability, a coherent PAM transmitter and an IM-DD PAM
transmitter may both use the same type of modulators, such as
Mach-Zehnder Modulators (MZM). A receiver system is provided with
mixed signal processing. For instance, the receiver system may
include an optical polarization component configured to modulate a
polarization of the received coherent PAM encoded signal. The
receiver system may further include a digital signal processor
(DSP) configured to perform polarization recovery between the
received coherent PAM encoded signal and the LO signal using a
first control circuitry or loop, and to perform phase recovery
between the received coherent PAM encoded signal and the LO signal
using a second control circuitry or loop.
[0029] With respect to polarization recovery, polarization
adjustments to be made may be determined by the DSP in the
electrical domain, and the optical polarization controller may make
these polarization adjustments to the received coherent PAM encoded
signal in the optical domain. For example, the DSP may determine
polarization adjustments in a feedback control loop by maximizing a
beating signal power between the LO and a polarized portion of the
coherent PAM encoded signal with encoded data. Alternatively, the
DSP may determine polarization adjustments in a feedback control
loop by minimizing power of a polarized portion without encoded
data.
[0030] With respect to phase recovery, phase adjustments to be made
may be determined by the DSP in the electrical domain, and one or
more optical phase modulator may make these phase adjustments to
the LO signal in the optical domain. For example, the DSP may
determine phase adjustments in a feedback control loop based on
known pilot data symbols inserted in the coherent PAM encoded
signal.
[0031] In some instances, instead of independent polarization and
phase recovery control loops or circuitry, polarization and phase
recovery may be performed using a single control loop or circuitry
through a joint polarization-diverse and phase-diverse coherent
detection method. In this regard, the optical polarization
component may be implemented using a 1.times.2 optical polarization
controller. The 1.times.2 optical polarization controller may be
configured to apply polarization adjustments to the coherent PAM
encoded signal to introduce a 45.degree. of polarization angle
between the adjusted PAM encoded signal and the LO. Interference
may be performed between the adjusted coherent PAM encoded signal
and two phase-diverse LO signals. The DSP may then analyze the
resulting beams and known pilot data symbols to determine
adjustments for polarization and phase recovery.
[0032] The receiver system may be configured with additional and/or
alternative features for receiving different types of PAM encoded
signals. For instance, to receive a dual-polarization coherent PAM
encoded signal, an optical polarization demultiplexer may be used
as the optical polarization component to separate the two encoded
polarizations. As another example, to receive a PAM encoded signal
with multiple wavelengths, a wavelength demultiplexer may be
provided to separate the multiple wavelengths into different beams
for separate analysis.
[0033] Further, the receiver system may be configured to receive an
intensity modulation direct-detect (IM-DD) PAM encoded signal as
well as a coherent PAM encoded signal. In this regard, a two-mode
switch may be provided such that, when the two-mode switch is set
in a first mode, the two-mode switch is configured as an optical
coupler (OC) for receiving a coherent PAM encoded signal. In
contrast, when the two-mode switch is set in a second mode, the
two-mode switch is configured as a switch for receiving an IM-DD
PAM encoded signal.
[0034] Additionally or alternatively, a coherent receiver system
may be configured with mixed analog and digital processing. For
instance, the receiver system may include a polarization beam
splitter (PBS) configured to split a PAM encoded signal into two
polarized PAM encoded signals, and two hybrids configured to
combine each of the two polarized PAM encoded signals with a local
oscillator (LO) signal. The receiver system may further include an
analog equalizer and a DSP configured to perform polarization and
phase recovery between the received coherent PAM encoded signal and
the LO signal.
[0035] In another aspect, a PAM transmission system may be provided
with interoperability between coherent PAM and IM-DD PAM
transmission. In this regard, the PAM transmission may include a
transmitter system and a receiver system. When set in a first mode,
the transmitter system may be configured to transmit coherent PAM
encoded data, and the receiver system may be configured to receive
the coherent PAM encoded data. When set in a second mode, the
transmitter system may be configured to transmit IM-DD PAM encoded
data, and the receiver system may be configured to receive the
IM-DD PAM encoded data. In this regard, a two-mode switch may be
provided in both the transmitter system and the receiver system to
allow the change between the coherent PAM and IM-DD PAM modes.
[0036] The technology provides power efficient receiver systems in
datacenters. The technology provides receiver systems that are
compatible with both coherent PAM and IM-DD PAM transmission
systems, which increases design flexibility of a datacenter.
Further, by using mixed optical and electrical processing, and/or
mixed electrical and digital processing, components with high power
consumption may be reduced. Additionally, by using a PAM system
that does not perform phase modulation, requirements on optical
equipment, such as lasers, may be relaxed, which may further
increase power efficiency and design flexibility.
Example Systems
[0037] An efficient coherent PAM receiver system may be provided
with mixed signal processing capabilities. As mentioned above, to
receive and decode a one-dimensional signal, such as a coherent PAM
encoded signal, a polarization-diverse and phase-diverse receiver
system may be inefficient. First, a coherent PAM encoded signal
uses only one carrier signal and do not include phase modulation,
thus it is unnecessary to separate multiple phases. Further, where
the coherent PAM encoded signal includes only a single
polarization, it is also unnecessary to separate multiple
polarizations. Accordingly, FIGS. 1A-4 show example receiver
systems that use mixed optical and electrical signal processing.
FIG. 6 shows example receiver system with mixed analog and digital
processing. FIGS. 1A-3B show receiver systems where polarization
and phase recovery are performed using two independent control
circuitries or loops, while FIGS. 4A and 4B shows a receiver system
where polarization and phase recovery are performed using a single
control circuitry or loop through a joint polarization-diverse and
phase-diverse coherent detection method. Each of these receiver
systems may include one or more frequency control circuitry or
loops for coarse and/or fine adjustments. FIGS. 5A, 5B, and 5C show
example implementations of an optical polarization component that
can be used in the joint polarization-diverse and phase-diverse
coherent detection method.
[0038] Referring to FIG. 1A, an example coherent PAM receiver
system 100A for receiving a single polarization coherent PAM
encoded signal is shown. The receiver system 100A may receive an
incoming signal from a transmitter system (not shown). For
instance, a transmitter system may have generated a coherent
carrier signal, and encoded with the carrier signal with data using
a data modulator. For PAM encoding, the data modulator may be a
high-speed optical modulator, such as a Mach-Zehnder Modulator
(MZM). The data modulator may encode data by converting high-speed
electrical data signal into high-speed optical data signal and add
to the carrier signal. In this example, data is encoded in only one
polarization, thus resulting in a single polarization coherent PAM
encoded signal. Although the transverse electric (TE) mode
polarized light beam, denoted as x-polarization, is shown in FIG.
1A as the polarization with encoded data, in other examples data
may be encoded in any other polarization. The coherent PAM encoded
signal may then be transmitted by the transmitter system to the
receiver system 100A, such as through one or more optical fibers.
While in transmission, various properties of the coherent PAM
encoded signal, such as polarization, phase, and/or frequency, may
change randomly.
[0039] To recover properties of the received single polarization
coherent PAM encoded signal that may have changed during
transmission, the receiver system 100A may also receive a signal
from a Local Oscillator (LO) 120 as reference. An optical coupler
(OC) 130 mixes the received coherent PAM encoded signal and the LO
signal. For example, an optical polarization component, such as an
optical polarization controller 110, may initially select
x-polarized component of the coherent PAM encoded signal for
passing through the OC 130, and an optical phase modulator 140 may
initially allow the LO signal to pass through the OC 130 without
modification to the phase or frequency. During the recovery
process, the optical polarization controller 110 and the optical
phase modulator 140 may change the polarization, frequency, and/or
phase of the signals via feedback control loops with one or more
processors, such as coherent PAM digital signal processor (DSP)
180, as described further below.
[0040] To be analyzed by the DSP 180, which is in the electrical
domain, the mixed LO signal and the received single polarization
coherent PAM encoded signal are converted into electrical and/or
digital domains. For instance as shown, the mixed LO and coherent
PAM encoded signal are passed through a photodetector (PD) 150 for
converting from optical to electrical domain. In some instances the
PD 150 may be a pair of balanced PDs configured to cancel
common-mode noises. For example, the mixed signal may be used as
input to each of the two PDs (shown as 2 solid lines), and the
output of the two PDs may be combined into one electrical signal
that cancels out the common-mode noises (shown as 1 dashed line).
The electrical signal may be amplified by the transimpedance
amplifier (TIA) 160. The amplified electrical signal may then be
digitized by an analog to digital converter (ADC) 170. The
digitized signal may be passed to the coherent PAM digital signal
processor (DSP) 180 for analyses. As such, the configuration of
receiver system 100A reduces the number of PD, TIA, and ADCs
required by a factor of four as compared to a conventional
polarization-diverse and phase-diverse receiver system.
[0041] One or more feedback control loops may be used between the
DSP 180 and the optical polarization controller 110, the LO 120,
and/or the optical phase modulator 140 in order to match properties
of the LO signal and the received single polarization coherent PAM
encoded signal. The DSP 180 may be one or more processors of any of
a number of types, such as processors 820 of FIG. 8. The DSP 180
may determine adjustments to be made in the electrical domain,
while the optical polarization controller 110, the LO 120, and/or
the optical phase modulator 140 may make these adjustments in the
optical domain. By analyzing effects of the adjustments, the DSP
180 may determine further adjustments to be made, until the DSP 180
determines that the properties of the LO signal sufficiently match
the properties of the coherent PAM encoded signal. These feedback
control loops are thus implemented by mixed optical and electrical
components. The feedback control loops may be used to perform
polarization, phase, and/or frequency recovery.
[0042] With respect to polarization recovery, a polarization
feedback control loop 192 between the DSP 180 and the optical
polarization controller 110 may be used. Polarization adjustments
to be made may be determined by the DSP 180 in the electrical
domain, and the optical polarization controller 110 may make these
polarization adjustments to the received coherent PAM encoded
signal in the optical domain. The optical polarization controller
110 may be an optical signal processor configured to change
polarization of a coherent PAM encoded signal. As an example, the
optical polarization controller 110 may be implemented using
interferometers with variable phase shifters. The optical
polarization controller 110 shown in FIG. 1A is a 1.times.1 optical
polarization controller (1 input, 1 output). The DSP 180 may
determine polarization adjustments to be made by analyzing beating
signal power between the LO signal and the received single
polarization coherent PAM encoded signal. Since the maximum
achievable beating signal power between two signals may be achieved
when the two signals have the same polarization, the DSP 180 may
determine the polarization adjustments to be made by maximizing
beating signal power between the LO signal and the x-polarized
coherent PAM encoded signal. For instance, initially the DSP 180
may receive the LO signal and the x-polarized coherent PAM encoded
signal, and determine an initial beating signal power. The DSP 180
may increase the polarization control coefficients, for example the
required phase shifter control voltages, by a predetermined amount
and instruct the optical polarization controller 110, via
polarization feedback control loop 192, to adjust polarization of
the coherent PAM encoded signal by the predetermined amount. Since
the maximum beating signal power may not be known by the receiver
system at the starting stage, multiple back and forth adjustments
are usually needed until adjustments in either direction would
decrease the power. After the adjustment, the DSP 180 may continue
to receive LO signal and x-polarized coherent PAM encoded signal,
and determine beating signal power. If the DSP 180 determines that,
after the adjustment, the beating signal power between the LO
signal and the x-polarized coherent PAM encoded signal increased
due to the increased polarization control coefficients, the DSP 180
may further increase the polarization control coefficient, and the
optical polarization controller 110 may apply this adjustment.
Conversely, if the DSP 180 determines that the beating signal power
between the LO signal and the x-polarized coherent PAM encoded
signal decreased due to the increased polarization control
coefficient, the DSP 180 may decrease the polarization control
coefficient, and the optical polarization controller 110 may apply
this adjustment. Further adjustments may be determined and applied
until the beating signal power is maximized, for example to a
predetermined threshold.
[0043] With respect to carrier frequency and/or phase recovery, one
or more frequency and/or phase feedback control loops may be used
between the DSP 180 and the LO 120 and/or optical phase modulator
140. Phase and/or frequency adjustments to be made may be
determined by the DSP 180 in the electrical domain, and implemented
by the LO 120 and/or the optical phase modulator 140 in the optical
domain. In comparison to the high-speed data modulator, such as an
MZM, that encodes data in the transmitter system, the optical phase
modulator 140 in the receiver system 100A may be a low-speed
optical modulator. Since laser phase changes for the carrier signal
is much slower than the high-speed coherent PAM encoded signal, the
required bandwidth for the optical phase modulator 140 may also be
significantly lower than the required bandwidth for the data
modulator in the transmitter system. By using a low-speed,
low-bandwidth modulator as the optical phase modulator 140, the
receiver system 100A may thus reduce power consumption.
[0044] The DSP 180 may determine the frequency adjustments to be
made using fast Fourier transforms (FFT). FFT may also be a
low-speed operation since laser frequency for the carrier signal
typically changes very slowly compared to data rate of the coherent
PAM encoded signal. As such, the DSP 180 may reduce power
consumption by performing the low-speed operation, as compared to
the high-speed operations involving the hybrids of receiver system
100. Based on the FFT, frequency offset between LO signal and the
coherent PAM encoded signal may be estimated. Frequency adjustments
may then be determined based on the frequency offset and applied
via feedback control loop 194 and/or 196.
[0045] For instance, initially the DSP 180 may receive the LO
signal and the coherent PAM encoded signal, and use FFT to
determine an initial frequency offset between the LO signal and the
coherent PAM encoded signal. The DSP 180 may increase the frequency
control coefficient, for example the required temperature control
voltage or laser drive current for the LO, by a predetermined
amount and instruct the LO 120, via frequency feedback control loop
194, to adjust frequency based on the predetermined amount.
Additionally or alternatively, the DSP 180 may instruct the optical
phase modulator 140 via frequency/phase feedback control loop 196,
to adjust frequency based on the predetermined amount. For example,
coarse frequency adjustments may be made via frequency feedback
control loop 194, while fine frequency adjustments may be made via
feedback control loop 196. After the adjustment, the DSP 180 may
continue to receive LO signal and coherent PAM encoded signal and
use FFT to determine frequency offset. If the DSP 180 determines
that, after the adjustment, the frequency offset between the LO
signal and the single polarization coherent PAM encoded signal
decreased due to the increased frequency control coefficient, the
DSP 180 may further increase the frequency control coefficient, and
the LO 120 and/or optical phase modulator 140 may apply this
adjustment. Conversely, if the DSP 180 determines that the
frequency offset between the LO signal and the single polarization
coherent PAM encoded signal increased due to the increased
frequency control coefficient, the DSP 180 may decrease the
frequency control coefficient, and the LO 120 and/or optical phase
modulator 140 may apply this adjustment. Further adjustments may be
determined and applied until the frequency offset is minimized, for
example to a predetermined threshold.
[0046] The DSP 180 may determine phase adjustments to be made using
pilot data symbols. Pilot symbols are special data symbols which
are known to a receiver system. Pilot symbols may be inserted by a
transmitter system into the regular data symbols of an optical
signal, such as the coherent PAM encoded signal, through
time-division multiplexing. For example, the pilot symbols may be
inserted in fixed time slots. For instance, initially a maximum
achievable pilot symbol signal level, which represents the closest
phase match between LO 120 and received coherent PAM encoded
signal, may be measured and calibrated. This may be performed using
training data with random LO phase change through the optical phase
modulator 140. Once the maximum achievable pilot symbol signal
level is calibrated, known pilot data symbols may be sent
periodically by a transmitter system via the coherent PAM encoded
signal to receiver system 100A. Based on the pilot symbol signal
level of these known pilot data symbols, phase offset between LO
signal and the coherent PAM encoded signal may be estimated, based
on which adjustments may be determined and applied in phase
feedback control loop 196.
[0047] For instance, initially the DSP 180 may receive the LO
signal and pilot symbols encoded in the coherent PAM encoded
signal, and determine an initial phase offset between the LO signal
and the coherent PAM encoded signal based on the pilot symbol
signal level. The DSP 180 may then increase the phase control
coefficient, for example the required drive voltage for the optical
Phase Modulator 140, by a predetermined amount and instruct the
optical phase modulator 140, via phase feedback control loop 196,
to adjust phase by the predetermined amount. After the adjustment,
the DSP 180 may continue to receive LO signal and pilot symbols in
the coherent PAM encoded signal, and determine phase offset based
on the pilot symbol signal level. If the DSP 180 determines that,
after the adjustment, the phase offset between the LO signal and
the single polarization coherent PAM encoded signal decreased due
to the increased phase control coefficient, the DSP may further
increase the phase control coefficient, and the LO 120 and/or
optical phase modulator 240 may apply this adjustment. Conversely,
if the DSP 180 determines that the phase offset between the LO
signal and the coherent PAM encoded signal increased due to the
increased the phase control coefficient, the DSP may decrease the
phase control coefficient, and the LO 120 and/or optical phase
modulator 140 may apply this adjustment. Further adjustments may be
determined and applied until the phase offset is minimized, for
example to a predetermined threshold.
[0048] The receiver system 100A provide efficient recovery of
encoded data in a number of ways. For instance as shown, one OC 130
is used instead of two hybrids in the conventional
polarization-diverse and phase-diverse receiver. The number of PDs,
TIAs, and ADCs are also reduced by a factor of four as compared to
the conventional polarization-diverse and phase-diverse receiver.
Further, instead of processing all polarization and phase
information in the electrical domain, which may require high-speed
operations, adjustments are made in the optical domain by optical
components, such as the optical polarization controller 110,
optical phase modulator 140, and LO 120, which may perform
low-speed operations. Optical components such as optical
polarization controller 110 and optical phase modulator 140 may
also increase sensitivity of the receiver system without increasing
processing power of the DSP 180. Additionally, the feedback control
loops 192, 194, 196 may perform recovery of multiple properties
simultaneously.
[0049] Alternatively or additionally, polarization recovery may be
performed by monitoring the average optical power in a polarization
state that is orthogonal to the signal polarization state. For
instance, FIG. 1B shows another example coherent PAM receiver
system 100B for receiving a single polarization coherent PAM
encoded signal. The receiver system 100B is configured with similar
components as the receiver system 100A of FIG. 1A, and is labeled
as such. However, additional circuit elements are provided in
receiver system 100B to measure optical power in a polarization
that is orthogonal to the polarization of the encoded signal. For
example as shown, the optical polarization controller 110 may
select the TE-mode polarized signal denoted as x-polarized beam
with the encoded data as output passing through the OC 130, and the
TE-mode polarized signal converted from transverse magnetic (TM)
mode signal denoted as y-polarized beam as output passing through a
circuit including a set of PD 152, TIA 162, and ADC 172, and then
to the DSP 180. The optical polarization controller 110 shown in
FIG. 1B is a 1.times.2 optical polarization controller (1 input, 2
outputs).
[0050] To perform polarization recovery, the DSP 180 may maximize
the average optical power in the x-polarized output by minimizing
the average optical power in the y-polarized output in a
polarization feedback control loop 198. For instance, initially the
DSP 180 may measure an average optical power of the y-polarized
output from the optical polarization controller 110. The DSP 180
may then increase the polarization control coefficient by a
predetermined amount and instruct the optical polarization
controller 110, via polarization feedback control loop 198, to
adjust polarization of the coherent PAM encoded signal by the
predetermined amount. After the adjustment, the DSP 180 may
continue to receive y-polarized output from the optical
polarization controller 110, and measure its average optical power.
If the DSP 180 determines that, after the adjustment, the average
optical power of the y-polarized output decreased due to the
increased polarization control coefficient, the DSP 180 may further
decrease the polarization control coefficient, and the optical
polarization controller 110 may apply the adjustment. Conversely,
if the DSP 180 determines that the average optical power of the
y-polarized output increased due to the increased polarization
control coefficient, the DSP 180 may decrease the polarization
control coefficient, and the optical polarization controller 110
may apply the adjustment. Further adjustments may be determined and
applied until the average optical power in the y-polarization is
minimized, for example to a predetermined threshold.
[0051] The receiver system 100B provide efficient recovery of
encoded data in a number of ways. For instance, one or more of the
PD 152, TIA 162, and ADC 172 may be low-speed components, since
only average optical power, not encoded data, is analyzed. By
performing polarization recovery using components having a
different polarization, polarization recovery for receiver system
100B may be decoupled from frequency and phase recovery. As such,
the receiver system 100B may perform polarization recovery before,
during, and/or after frequency and/or phase recovery, which
provides additional flexibility.
[0052] In some instances, to increase data capacity, PAM encoded
data may be added to two polarizations, which results in a
dual-polarization coherent PAM encoded signal. In this regard, FIG.
2A shows an example coherent PAM receiver system 200A for receiving
a dual-polarization coherent PAM encoded signal. As shown, incoming
signal may be received by the receiver system 200A, which may be
modulated by a transmitter system in both x- and y-polarizations
with PAM encoded data. Although x- and y-polarizations are shown in
FIG. 2A as the polarization with encoded data, in other examples
data may be encoded in any other polarizations. The receiver system
200A is configured with similar components as the receiver system
100A of FIG. 1A, and is labeled as such. For example, similar
components, such as OCs, PDs, TIAs, ADCs, LO, DSP, optical phase
modulators, are also used in receiver system 200A. However, since
the dual-polarization coherent PAM encoded signal includes data
encoded to two different polarizations that carry two independent
data signals, receiver system 200A includes some differences from
the receiver system 100A.
[0053] For instance, an optical polarization component, such as a
1.times.2 optical polarization controller, may be provided to
separate the received dual-polarization coherent PAM encoded signal
into two polarized beams. In the example shown in FIG. 2A, an
optical polarization demultiplexer 210 is used, which is a
1.times.2 optical controller where the two outputs are aligned to
the two polarization multiplexed, orthogonal signals. The optical
polarization demultiplexer 210 may initially separate the received
dual-polarization coherent PAM encoded signal into an x-polarized
beam and a y-polarized beam, and perform recovery using feedback
control loops as described further below. These two polarized beams
may each be analyzed by reference to a signal from an LO 220. As
such, OC 234 may also separate the LO signal into two LO beams.
Then, OC 230 mixes the x-polarized beam and the first LO beam, and
OC 232 mixes the y-polarized beam and the second LO beam. The two
mixed beams are then respectively converted from optical to
electrical domains by respective PDs 250, 252, amplified by
respective TIAs 260, 262, and digitized by respective ADCs 270,
272. One or more feedback control loops may be used between the DSP
280, optical polarization demultiplexer 210, LO 220, optical phase
modulators 240, 242 to perform polarization, frequency, and/or
phase recovery.
[0054] With respect to polarization recovery, a low-frequency RF
dither signal may be used. The low-frequency RF dither signal may
be added by the transmitter system (not shown) to either the
x-polarized encoded data signal or the y-polarized encoded data
signal to be transmitted. Since the RF dither signal was only added
to one of the two polarizations, by maximizing the RF pilot tone of
the dither signal in one polarized beam, and/or minimizing the RF
pilot tone of the dither signal in the other polarized beam, the
receiver system 200A may separate the two encoded data signals. For
example, where the dither signal was added by the transmitter
system to the x-polarized signal, then the receiver system 200A may
perform polarization recovery by maximizing the RF pilot tone of
the dither signal in the x-polarized beam; where the dither signal
was added by the transmitter system to the y-polarized signal, then
the receiver system 200A may perform polarization recovery by
maximizing the RF pilot tone of the dither signal in the
y-polarized beam.
[0055] Polarization adjustments may be determined and applied in a
polarization feedback control loop 292 based on analyzing the pilot
tone of the RF dither signal. For instance, the DSP 280 may
initially determine the RF pilot tone of the dither signal in the
x-polarized beam. The DSP 280 may then increase the polarization
control coefficient by a predetermined amount and instruct the
optical polarization demultiplexer 210, via polarization feedback
control loop 292, to adjust polarization of the dual-polarization
coherent PAM encoded signal by the predetermined amount. After the
adjustment, the DSP 280 may continue to receive the
dual-polarization coherent PAM encoded signal. If the DSP 280
determines that, after the adjustment, the RF pilot tone of the
dither signal in the x-polarized signal increased due to the
increased polarization control coefficient, the DSP 280 may further
increase the polarization control coefficient, and the optical
polarization demultiplexer 210 may apply this adjustment.
Conversely, if the DSP 280 determines that the RF pilot tone of the
dither signal in the x-polarized signal decreased due to the
increased polarization control coefficient, the DSP 280 may
decrease the polarization control coefficient, and the optical
polarization demultiplexer 210 may apply this adjustment.
Eventually, the RF pilot tone of the dither signal in the
x-polarized signal may be maximized, for example to a predetermined
threshold. Additionally or alternatively, the DSP 280 may determine
adjustments to minimize the RF pilot tone of the dither signal in
the y-polarized signal in the polarization feedback control loop
292, which may also be applied by the optical polarization
demultiplexer 210.
[0056] With respect to carrier frequency and/or phase recovery, one
or more frequency and/or phase feedback control loops may be used
between the DSP 280 and the LO 220 and/or optical phase modulator
240, 242. Phase and/or frequency adjustments to be made may be
determined by the DSP 280 in the electrical domain, and applied by
the LO 220 and/or the optical phase modulators 240, 242 in the
optical domain. For instance, frequency feedback control loop 294
may be configured similarly as frequency feedback control loop 194
of FIG. 1A. Likewise, frequency and/or phase feedback control loop
296 may be configured similarly as frequency and/or phase feedback
control loop 196 of FIG. 1A, except that the adjustments to the two
LO beams are applied respectively by the optical phase modulator
240 and the optical phase modulator 242.
[0057] The receiver system 200A provides efficient recovery of
dual-polarization encoded data. To receive data encoded to two
polarizations, the number of PDs, TIAs, and ADCs in receiver system
200A are reduced by a factor of two as compared to a
polarization-diverse and phase-diverse receiver system. Further,
instead of processing all polarization and phase information in the
electrical domain, which may require high-speed operations,
adjustments are made in the optical domain by optical components,
which may perform low-speed operations. Optical components such as
optical polarization demultiplexer 210 and optical phase modulators
240, 242 may also increase sensitivity of the receiver system
without increasing processing power of the DSP 280. Additionally,
the feedback control loops 292, 294, 296 may perform recovery of
multiple properties simultaneously.
[0058] In some instances, the pilot tone of RF dither signal used
for polarization described above may be monitored by an independent
low-speed circuitry. For instance, FIG. 2B shows another example
coherent PAM receiver system 200B for receiving a dual-polarization
coherent PAM encoded signal. The receiver system 200B is configured
with similar components as the receiver system 200A of FIG. 2A, and
is labeled as such. However, an optical tap 212 may be added to
divert some of the x-polarized beam to an additional set of PD 254,
TIA 264, and ADC 274. The diverted x-polarized beam may be used to
analyze pilot tone of RF dither signal inserted into the
x-polarized beam, based on which polarization may be adjusted as
described above. The set of PD 254, TIA 264, and ADC 274 may be
low-speed components, since only average optical power, not encoded
signal, is analyzed.
[0059] According to some examples, light from multiple lasers of
different frequencies may be PAM encoded, which results in a
coherent PAM encoded signal with multiple wavelengths, each
carrying an independent data signal. This may result in increased
data capacity. FIG. 3A shows an example coherent PAM receiver
system 300A for receiving a coherent PAM encoded signal with
multiple wavelengths. As shown, incoming signal may be received by
the receiver system 300A, which may include multiple wavelengths
each modulated by a transmitter system with PAM encoded data.
Although four wavelengths are shown in FIG. 3A, which is typical
when a coarse wavelength division multiplexing (CWDM) transceiver
is used, in other examples data may be encoded in any number of
wavelengths. The receiver system 300A is configured with similar
components as the receiver system 100A of FIG. 1A, and is labeled
as such. For example, similar components, such as OCs, PDs, TIAs,
ADCs, LO, DSP, optical phase modulators, are also used in receiver
system 300A. However, since the coherent PAM encoded signal
received by receiver system 300A includes multiple wavelengths
carrying multiple independent data signals, receiver system 300A
includes some differences from the receiver system 300A.
[0060] For instance as shown, the multiple wavelengths may be
separated by a demultiplexer into different beams, such as a coarse
wavelength division multiplexing (CWDM) demultiplexer (DEMUX) 312.
Further as shown, the optical polarization controller 310 may be
placed before the CWDM DEMUX 312 in the receiver system 300A. As
such, the single optical polarization demultiplexer 310 may change
polarization of light before the different wavelengths are
separated into different beams. This also allows a single
polarization feedback control loop 392 to be used. One or more
frequency and/or phase feedback control loops, such as frequency
feedback control loop 394 and frequency and/or phase feedback
control loop 396, may be used between the DSP 380, LO 320, and/or
optical phase modulator 340 to perform frequency and/or phase
recovery similarly as described for receiver system 100A. For ease
of illustration, OC 330, optical phase modulator 340, PD 350, TIA
360, and ADC 370 are only shown for one of the wavelengths.
However, in actual systems a separate set of OC, optical phase
modulator, PD, TIA, and ADC may be required for each wavelength of
the received PAM encoded signal.
[0061] The receiver system 300A is a more efficient
multi-wavelength receiver than a polarization-diverse and
phase-diverse receiver system. Although the receiver system 300A
requires 4 sets of OCs, optical phase modulators, PDs, TIAs, and
ADCs to receive the additional wavelengths, to receive a same
number of wavelengths, a conventional polarization-diverse and
phase-diverse receiver system would require 16 sets of PDs, TIAs,
and ADCs, and 8 hybrids. Further, instead of processing all
polarization and phase information in the electrical domain, which
may be high-speed operations, adjustments are made by the receiver
system 300A in the optical domain by optical components, which may
be low-speed operations. Optical components such as optical
polarization controller 310 and optical phase modulator 340 may
also increase sensitivity of the receiver system without increasing
processing power of the DSP 380. Additionally, the feedback control
loops 392, 394, 396 may perform recovery of multiple properties
simultaneously.
[0062] Further, polarization recovery for PAM encoded data with
multiple wavelengths may also be performed by analyzing signals
from a separate polarization. For instance, FIG. 3B shows another
example coherent PAM receiver system 300B for receiving a coherent
PAM encoded signal with multiple wavelengths. The receiver system
300B is configured with similar components as the receiver system
300A of FIG. 3A, and is labeled as such. However, referring to FIG.
3B, an optional circuit including PD 352, TIA 362, and ADC 372 may
be provided to measure optical power in a polarization that is
orthogonal to the polarization of the data signal. As such,
receiver system 300B may perform polarization recovery as described
for receiver system 100B. For example as shown, DSP 380 may be
configured to minimize optical power of y-polarized light by
instructing optical polarization controller 310 via feedback
control loop 398.
[0063] Although receiver systems 300A and 300B shown are configured
to receive single polarization coherent PAM encoded signals, the
receiver systems 300A and 300B may also be extended to receive
dual-polarization PAM encoded signals. For example, the optical
polarization controller 310 may be replaced with an optical
polarization demultiplexer for separating the two polarizations.
Additional optical phase modulators, OCs, PDs, TIAs, and ADCs may
be provided for the two polarizations. Recovery of phase and/or
frequency may also be performed using additional feedback control
loops for the two polarizations.
[0064] FIG. 4A shows another example receiver system 400A with
mixed optical and electrical signal processing through joint
polarization-diverse and phase-diverse detection. Referring to FIG.
4A, the receiver system 400A is configured with similar components
as the receiver system 200A of FIG. 2A, and is labeled as such. For
example, the receiver system 400A also includes an LO, a DSP, 3
OCs, and 2 sets of PDs, TIAs, ADCs. However, instead of an optical
polarization demultiplexer for polarization recovery and two
optical phase modulators for frequency and/or phase recovery as
shown in FIG. 2A, the receiver system 400A includes a 1.times.2
optical polarization controller 410 configured to introduce a
45.degree. polarization angle between the received signal after
polarization adjustment and the LO, and followed by a joint
polarization-diverse and phase-diverse detection circuitry. Thus,
only a single control loop is required to control the 1.times.2
polarization controller to enable joint polarization-diverse and
phase-diverse coherent detection. Final polarization and phase
recovery may be achieved in the electrical domain.
[0065] For instance as shown, the incoming dual-polarization
coherent PAM encoded signal may be represented by vectors A.sub.in
and B.sub.in, where A.sub.in and B.sub.in are two
orthogonally-polarized optical signals carrying independent data. A
1.times.2 optical polarization controller 410 may change the
polarization of the incoming dual-polarization coherent PAM encoded
signal A.sub.in and B.sub.in, to output A.sub.out and B.sub.out. In
this regard, the 1.times.2 optical polarization controller 410 may
adjust the polarization of A.sub.in and B.sub.in such that the
resulting A.sub.out and B.sub.out are still orthogonal to each
other, but the resulting A.sub.out is 45.degree. from a reference
axis and aligned with the LO polarization. In this regard the
reference axis may be one of the two principal axes of the PBS
labeled as x- and y-axis. For example, A.sub.out may be 45.degree.
from the y-axis in one direction, while B.sub.out may be 45.degree.
from the y-axis in an opposite direction, making A.sub.out
90.degree. from B.sub.out. As such, the 1.times.2 optical
polarization controller 410 may generate two outputs
X o .times. u .times. t = A o .times. u .times. t - .times. x + B o
.times. u .times. t - .times. x = 2 2 .times. A in - 2 2 .times. B
in .times. .times. and .times. .times. Y o .times. u .times. t = A
out_y + B out_y = 2 2 .times. A in + 2 2 .times. B in ,
##EQU00001##
where A.sub.out_x is x component of A.sub.out, A.sub.out_y is y
component of A.sub.out, B.sub.out_x is x component of B.sub.out and
B.sub.out_y is y component of B.sub.out.
[0066] Further as shown, the LO signal from LO 420 may be split by
OC 434 into two signals, LO1 and LO2. OC 434 may introduce a .pi./2
phase difference between LO1 and LO2. For example, a 3 dB OC that
separates a beam using a cross path and a parallel path may have a
.pi./2 phase difference between the two paths. Then, OC 430 may mix
X.sub.out with LO1, and generate two outputs
C x + = 1 2 .times. A in - 1 2 .times. B in + j .times. 1 2 .times.
E L , and .times. .times. C x - = 1 2 .times. jA in - 1 2 .times.
jB in + 1 2 .times. E L , ##EQU00002##
where E.sub.L denotes the complex optical field of LO1, and
jE.sub.L denotes the complex field of LO2. Likewise, OC 432 may mix
the Y.sub.out with LO2, and generate two outputs
c y + = 1 2 .times. A in + 1 2 .times. B in - 1 2 .times. E L , and
.times. .times. C y - = 1 2 .times. jA in + 1 2 .times. jB in + 1 2
.times. j .times. E L . ##EQU00003##
[0067] Outputs from OC 430 and OC 432 are then respectively passed
through PDs 450, 452 for optical to electrical conversion, TIAs
460, 462 for amplification, and ADCs 470, 472 for electrical to
digital conversion, resulting in two digitized electrical signals.
The two digitized electrical signals may be represented by
D.sub.x=A.sub.inE.sub.L sin (.phi..sub.x)-B.sub.inE.sub.L sin
(.phi..sub.y), and D.sub.y=A.sub.inE.sub.L cos
(.phi..sub.x)+B.sub.inE.sub.L cos (.phi..sub.y), where .phi..sub.x
is the relative carrier phase difference between the received
signal A.sub.out and the LO, while .phi..sub.y is the relative
carrier phase difference between the received signal B.sub.out and
the LO.
[0068] DSP 480 may determine the carrier phase differences using
pilot data symbol based carrier phase estimation algorithms. For
instance, two known PAM pilot data symbols, such as d1 and d2, may
be inserted into the two orthogonally-polarized incoming coherent
PAM encoded signal at different time slots. For example, d1 may be
inserted in one polarization channel, for example channel for
A.sub.in, at time slot n, while d2 may be inserted in another
polarization channel, for example channel for B.sub.in at time slot
n+1. The received digitized pilot data symbol of d1 may then be
given by D.sub.1(n)x=A.sub.inE.sub.L sin (.phi..sub.x), and
D.sub.1(n)y)=A.sub.inE.sub.L cos (.phi..sub.x). The DSP 480 can
then solve the two equations to extract .phi..sub.x. The digitized
pilot data symbol of d2 can be given by
D.sub.2(n+1)x=-B.sub.inE.sub.L sin(.phi..sub.y), and
D.sub.2(n+1)y=B.sub.inE.sub.L cos (.phi..sub.y). The DSP 480 can
then solve the two equations to extract .phi..sub.y.
[0069] Further, DSP 480 may also determine polarization information
of the incoming signals using the pilot data symbol. For instance,
once the values of .phi..sub.x and .PHI..sub.y are obtained,
A.sub.in and B.sub.in can be determined by the DSP 480 based on the
above relationships D.sub.x=A.sub.inE.sub.L sin
(.phi..sub.x)-B.sub.inE.sub.L sin (.phi..sub.y), and
D.sub.y=A.sub.inE.sub.L cos (.phi..sub.x)+B.sub.inE.sub.L cos
(.phi..sub.y). Then, DSP 480 may determine polarization states of
A.sub.out and B.sub.out based on A.sub.in and B.sub.in, and the
relationships above for the digitized pilot data symbols
D.sub.1(n)x, D.sub.1(n)y, D.sub.2 (n+1)x, D.sub.2 (n+1)y. Further,
since the current polarization adjustments applied by the 1.times.2
optical polarization controller 410 to the A.sub.in and B.sub.in
are also known, polarization states of A.sub.in and B.sub.in may be
deduced from the polarization of A.sub.out and B.sub.out. Since
polarization of the incoming signals may change randomly, dynamic
adjustments may need to be applied to maintain a 45.degree.
difference between A.sub.out and the reference axis.
[0070] As such, the current polarization states of the incoming
signal A.sub.in and B.sub.in are determined, the DSP 480 may
determine the appropriate adjustments to be made by the 1.times.2
optical polarization controller 410 via polarization control loop
492. For instance, DSP 480 may determine polarization offsets
between A.sub.in (and/or B.sub.in) and the LO signal, and determine
adjustments to be made to A.sub.in (and/or B.sub.in) to ensure that
A.sub.out have a 45.degree. offset from the reference y-axis, which
would typically be aligned with the polarization of the LO. The
purpose to have a 45.degree. polarization angle between A.sub.out
(and/or B.sub.out) and the LO is to make sure that the signal
components of A.sub.out and B.sub.out can be equally distributed
over the two output ports X.sub.out and Y.sub.out of the 1.times.2
polarization controller. As such, half of X.sub.out (or Y.sub.out)
comes from A.sub.out and another half comes from B.sub.out. As
shown by the formulae above, such a two-port equal signal
components distribution could enable joint polarization-diverse and
phase-diverse coherent detection, in which the polarization diverse
detection circuitry and the phase diverse detection circuitry are
combined into a single circuitry, which includes the 1.times.2
optical polarization controller 410, two optical couplers 430, 432,
two sets of PDs 450, 452, TIAs 460, 462 and ADCs 470, 472, as well
as the DSP 480. In contrast, with conventional polarization-diverse
and phase-diverse coherent detection, in addition to the DSP, two
hybrids plus four sets of PDs, TIAs and ADCs are needed.
[0071] Since relatively accurate incoming signal polarization and
phase information can be extracted from the pilot data symbol as
analyzed using the relationships above, only one polarization
adjustment by the 1.times.2 optical polarization controller 410 may
be needed for each change of incoming signal polarization through
the control loop 492. This could improve the response time of the
polarization controller as compared to the conventional trial and
error based feedback control algorithms, where typically multiple
polarization adjustments are needed for each polarization change.
Note that here the polarization control loop is not used to align
the received signal polarization to the LO in the optical domain
like the receiver systems shown in FIGS. 1A-3B. Instead, the
polarization control loop 492 is used to enable joint
polarization-diverse and phase-diverse coherent detection. For this
method, the final polarization and phase recovery are achieved in
the electrical domain by the DSP 480 using the relationships
described above. Furthermore, in the feedforward based polarization
and phase recovery algorithms, polarization and phase errors can be
estimated and removed from the current data symbols without
feedback delay, which may improve overall polarization and phase
tracking speed. Especially, the use of fast feedforward based phase
recovery algorithms could improve the laser phase noise
tolerance.
[0072] The receiver system 400A is a more efficient
multi-wavelength receiver than a polarization-diverse and
phase-diverse receiver system. For instance, the receiver system
400A requires 2 sets of PDs, TIAs, and ADCs, as compared to the 4
sets required by conventional polarization-diverse and
phase-diverse receiver system. The receiver system 400A also uses
only one optical polarization controller, instead of two hybrids in
the conventional polarization-diverse and phase-diverse receiver
system, or the two optical phase modulators in the receiver system
200A. Further, instead of processing all polarization and phase
information in the electrical domain, which may be high-speed
operations, adjustments are made by the receiver system 400A in the
optical domain by optical components, which may be low-speed
operations. Optical components such as the 1.times.2 optical
polarization controller 410 may also increase sensitivity of the
receiver system without increasing processing power of the DSP 480.
Additionally, the single control circuitry or loop allows recovery
of multiple properties, which may be performed simultaneously.
[0073] Further, polarization recovery for the example system with
joint polarization- and phase-diverse detection may also be
performed by analyzing signals from a separate polarization. For
instance, FIG. 4B shows another example coherent PAM receiver
system 400B with joint polarization- and phase-diverse detection.
The receiver system 400B is configured with similar components as
the receiver system 400A of FIG. 4A, and is labeled as such.
However, referring to FIG. 4B, an optional circuit including PD
454, TIA 464, and ADC 474 may be provided to measure pilot tone of
dither signals in the incoming signals. As such, receiver system
400B may perform polarization recovery similarly as described for
receiver system 200B. For example as shown, polarization offsets
may be monitored by inserting two low-speed dither signals into the
two orthogonally-polarized channels for A.sub.in and B.sub.in, and
the pilot tones may then be monitored by the DSP 480 using
low-speed components including optical tap 412, PD 454, TIA 464,
ADC 474. The introduction of this additional low-speed polarization
control loop enables independent polarization adjustment of the
1.times.2 polarization controller 410 without performing carrier
phase recovery and timing synchronization, which may simplify the
receiver control loop design.
[0074] Although FIGS. 4A and 4B show the example receiver systems
400A and 400B configured for receiving a dual-polarization coherent
PAM encoded signal with a single wavelength, features of the
receiver system may be extended for receiving single polarization
signals and signals with multiple wavelengths. For instance, for
receiving single polarization coherent PAM encoded signal, the
receiver system 400A may perform the same operations, but assuming
either A.sub.in or B.sub.in to be zero. For receiving a coherent
PAM encoded signal with multiple wavelengths, a wavelength
demultiplexer such as a CWDM demultiplexer may be added similarly
as shown in FIG. 3A.
[0075] FIGS. 5A, 5B, and 5C show example implementations for a
1.times.2 optical polarization controller. Referring to FIG. 5A,
1.times.2 optical polarization controller 500A includes a PBS 510
that splits the incoming dual-polarization coherent PAM encoded
signal including A.sub.in and B.sub.in into two beams X.sub.in and
Y.sub.in. Then, phase shifter 520 makes phase adjustments to one of
the split beams. For example as shown, phase of the beam X.sub.in
is changed by phase shifter 520 based on adjustments determined by
the DSP 480. The adjusted beam X.sub.out and unadjusted beam
Y.sub.in may then pass through OC 530 for further processing. Here
the received signal polarization is adjusted by changing the
relative phase between its X-component and Y-component. The
relative phase adjustment can be made by only adjusting the phase
for the X-component or by adjusting the phase for both the
X-component and the Y-component. Less voltage applied to the phase
shifter is required if phase adjustments are made to both the X-
and Y-components.
[0076] Referring to FIG. 5B, 1.times.2 optical polarization
controller 500B includes similar components as 1.times.2 optical
polarization controller 500A, and is labeled as such. However, the
1.times.2 optical polarization controller 500B further includes a
second phase shifter 540. The second phase shifter 540 may change
phase of the beam Y.sub.in based on adjustments determined by DSP
480. The adjusted beams X.sub.out and Y.sub.out may then pass
through OC 530 for further processing. For example as shown, a
second-stage 2.times.2 MZI with phase shifter 550 may be used. The
second stage 2.times.2 MZI 550 may be used to further reduce the
required control voltage for each phase shifter. It may also be
used to enable IM-DD PAM and coherent PAM two-mode operation as
described below, where all the signal components of A.sub.in (or
B.sub.in) are directed to one of the two output ports of the
1.times.2 polarization controller.
[0077] Referring to FIG. 5C, 1.times.2 optical polarization
controller 500C includes similar components as 1.times.2 optical
polarization controller 500B, and is labeled as such. However, the
1.times.2 optical polarization controller 500C is further
configured to receive a coherent PAM encoded signal with multiple
wavelengths. In this regard, one or more wavelength demultiplexers,
such as CWDM DEMUX 560 and CWDM DEMUX 562, are provided. As shown,
CWDM DEMUX 560 splits the various wavelengths in beam X.sub.in into
separate beams, and CWDM DEMUX 562 splits the various wavelengths
in beam Y.sub.in into separate beams. The multiple wavelengths may
share the same PBS 510 and CWDM DEMUX 560 and 562, but may not
share the same phase shifters, OC, and second-stage 2.times.2 MZI
with phase shifter. Thus, although not shown, a separate set of
phase shifters, OC, and second-stage 2.times.2 MZI with phase
shifter may be required for each wavelength received.
[0078] Instead of or in addition to mixed optical and electrical
processing, a coherent PAM receiver may be provided with mixed
analog and digital processing. For instance, FIG. 6 shows an
example coherent PAM receiver with mixed analog and digital
processing for receiving dual-polarization coherent PAM encoded
signal. As shown, the receiver system 600 includes some similar
components as a polarization-diverse and phase-diverse, such as PBS
610, LO 620, OC 630, two hybrids 640, 642, four sets of PDs 650,
652, 654, 656, and TIAs 660, 662, 664, 666, and DSP 680. However,
to perform mixed analog and digital processing, receiver system 600
includes some differences from a polarization-diverse and
phase-diverse receiver system.
[0079] For instance, the receiver system 600 further includes an
analog equalizer 690 for performing both polarization and carrier
phase recovery. As an example, the analog equalizer 690 may be a
4.times.2 analog multiple-input-multiple-output (MIMO) equalizer.
To perform polarization and carrier phase recovery, the analog
equalizer 690 may receive four inputs from the four respective
PDs/TIAs, which may include detected in-phase component in
x-polarization (Ix), detected quadrature component in
x-polarization (Qx), detected in-phase component in y-polarization
(Iy), and quadrature component in y-polarization (Qy). Note that
although PAM signal has no quadrature components, the carrier has
quadrature components. Unless the carrier phase is removed and the
signal and LO polarization is aligned prior to coherent mixing as
shown in FIG. 1A-3B, polarization-diverse and phase-diverse
coherent detection circuitry is needed. Based on these inputs, the
analog equalizer 590 may generate analog outputs to the DSP 680.
For instance, the analog equalizer 590 may generate two
polarization and phase recovered real-valued signals, Ix_output and
Iy_output, where Ix_output denotes the PAM signal carried over the
X-polarization channel while Iy_output denotes the PAM signal
carried over the Y-polarization channel. For example, Ix_output and
Iy_output may be generated based on the relationships
Ix_output=C.sub.1Ix+C.sub.2Qx+C.sub.3Iy+C.sub.4Qy, and
Iy_output=D.sub.1Ix+D.sub.2Qx+D.sub.3Iy+D.sub.4Qy, where C.sub.n
and D.sub.m are equalizer coefficients. The outputs may be
converted into digital outputs by ADCs 670 and 672 before being
passed to DSP 680.
[0080] The coefficients to be used by the analog equalizer 690 may
be determined by the DSP 680 using one or more control loops, such
as feedback control loop 692. In this regard, the DSP 680 may make
various determinations in the digital domain, while the analog
equalizer 690 may make various determinations in the analog domain.
For instance, initially the analog equalizer 690 may detect the Ix,
Qx, Iy and Qy, and calculate Ix_output and Iy_output using
predetermined coefficients. The analog equalizer 690 may then send
the Ix_output and Iy_output to the DSP 680. Based on the Ix_output
and the Iy_output, the DSP 580 may determine new coefficients to be
used by the analog equalizer 690, and instruct the analog equalizer
690 via feedback control loop 692. The analog equalizer
coefficients may be obtained by the DSP 680 by comparing the two
outputs of the analog equalizer 690 with the expected PAM signal
level distributions using classic gradient descent based algorithms
such as the well known least mean square (LMS) algorithm. The
analog equalizer 690 may then continue to detect the Ix, Qx, Iy and
Qy, and use the new coefficients to calculate another set of
Ix_output and Iy_output, which are again sent to the DSP 680.
Further coefficients may be determined by the DSP 680 and used by
the analog equalizer 690 until the calculated mean square error
through the LMS algorithm is below a predetermined threshold or
reach its minimum. Thus, the feedback control loops 692 enables
both polarization and phase recovery, which may also be
simultaneous.
[0081] The receiver system 600 provide efficient recovery of
encoded data using mixed analog and digital processing. For
instance, by using the 4.times.2 analog MIMO equalizer, the
required number of ADCs are reduced by a factor of two as compared
to the receiver system 100, which results in power saving. The
receiver system 600 with mixed analog and digital signal processing
may also be extended for receiving a coherent PAM encoded signal
with multiple wavelengths. For instance, a mixed analog and digital
processing receiver for 4 wavelengths may require 8 hybrids and 16
sets of PDs and TIAs, but only 8 ADCs, unlike a
polarization-diverse and phase-diverse receiver system that would
require 16 ADCs for receiving 4 wavelengths. Thus, the receiver
system 600 is also a more efficient multi-wavelength receiver.
[0082] Although the example receiver systems described above
include either mixed optical/electrical signal processing, or mixed
analog/digital signal processing, other combinations may be used.
For example, a receiver system may include mixed optical and analog
signal processing, where functions of the DSP is implemented using
analog circuits, which may further reduce power consumption. As
another example, a receiver system may include mixed
optical/analog/digital signal processing, where some functions of
the DSP are moved to analog circuits while others are performed
digitally, which may also reduce power consumption of the DSP.
[0083] Although the example receiver systems described above are
shown to process coherent PAM encoded signals, each of the example
receiver systems may be further configured with compatibility to
IM-DD PAM encoded signals. In this regard, the receiver systems
described above may be provided with two modes--a first mode for
receiving coherent PAM encoded signals, and a second mode for
receiving IM-DD PAM encoded signals. For instance, if signals are
received from a coherent PAM transmitter system, the receiver
system may be configured in the first mode; and if signals are
received from an IM-DD PAM transmitter system, the receiver system
may be configured in the second mode. FIGS. 7A and 7B show an
example transmission system where the receiver system performs
mixed signal processing. In FIG. 7A, the transmission system 700A
is configured to communicate using a coherent PAM encoded signal.
In FIG. 7B, the transmission system 700B is configured to
communicate using an IM-DD PAM encoded signal.
[0084] Referring to FIG. 7A, the coherent PAM transmission system
700A includes a coherent PAM transmitter system 710 and a coherent
PAM receiver system 720. Although four lasers with four wavelengths
are shown in FIG. 7A, in other examples data may be encoded in any
number of wavelengths. For ease of illustration, operations
involving only one wavelength is shown (as lines and arrows), but
it should be understood that analogous operations may be performed
for each of the wavelengths. Further, components such as 2-mode
switches 740 and 770, MZM 750, optical phase modulator 340, CWDM
DEMUX 312, CWDM MUX 760, PD 350, TIA 360, and ADC 370 are only
shown for one of the wavelengths. However, a separate set of 2-mode
switches, MZM, optical phase modulator, CWDM MUX and DEMUX, PD,
TIA, and ADC may be required for each wavelength of the received
PAM encoded signal. Still further, control loops for recovery of
polarization, phase, and/or frequency are also omitted.
[0085] The coherent PAM transmitter system 710 may encode data on
coherent carrier light for transmission. For example, the coherent
PAM transmitter system 710 may include one or more lasers, such as
lasers 732, 734, 736, 738 shown, which may emit light of different
four wavelengths. The emitted light from each laser may be passed
through a switch, such as 2-mode switch 740. In this regard, the
2-mode switch 740 may be set to a first mode for transmitting
coherent PAM signals. In this first mode, the 2-mode switch
essentially acts as an OC, which allows part of the emitted light
to be PAM encoded by a data modulator, such as an MZM 750, while
also diverts part of the emitted light to the receiver system 720
to be used as an LO signal. The 2-mode switch may be implemented
using a silicon photonics based Mach-Zehnder interferometer. For
example, the 2-mode switch 740 may be a 2.times.2 Mach-Zehnder
interferometer (MZI), in which the operation mode can be switched
by adjusting the relative phase between the two paths of the MZI.
Further as shown, the encoded light from the different lasers may
then be combined by a wavelength multiplexer, such as a CWDM MUX
760. The coherent PAM encoded signal with multiple wavelengths may
then be transmitted to the coherent PAM receiver system 720, for
example via one or more optical fibers.
[0086] The coherent PAM receiver system 720 may then receive the
coherent PAM encoded signal with multiple wavelengths from the
transmitter system 710. In this example, the coherent PAM receiver
system 720 is configured with similar components as the receiver
system 300A of FIG. 3A, and is labeled as such. However, to provide
backward interoperability, the coherent PAM receiver system 720
includes some differences from the receiver system 300A. For
instance, OC 330 of receiver system 300A is replaced with 2-mode
switch 770. To receive coherent PAM encoded signal, the 2-mode
switch 770 may be set in a first mode, which essentially acts as
the OC 330 of receiver system 300A. Like the 2-mode switch 740, the
2-mode switch 770 may also be implemented using a silicon photonics
based Mach-Zehnder interferometer, such as a 2.times.2 MZI. Thus in
the first mode, the receiver system 720 is configured the same way
as the receiver system 300A of FIG. 3A.
[0087] The coherent PAM transmission system 700A may be
re-configured into an IM-DD PAM transmission system. For instance,
referring to FIG. 7B, the IM-DD PAM transmission system 700B
includes an IM-DD PAM transmitter system 712 and an IM-DD PAM
receiver system 722. As shown, the IM-DD PAM transmitter system 712
is configured with similar components as the coherent PAM
transmitter system 710 of FIG. 7A, and the IM-DD PAM receiver
system 722 is configured with similar components as the coherent
PAM receiver system 720 of FIG. 7A. Further, to ensure
compatibility, the number of lasers in the IM-DD PAM transmission
system 700B remains the same as the coherent PAM transmission
system 700A. However, to perform IM-DD PAM transmission, IM-DD PAM
transmission system 700B includes some differences from the
coherent PAM transmission system 700A.
[0088] For instance, in the IM-DD PAM transmitter system 712, the
2-mode switch 740 is set to a second mode. Since only intensity is
modulated for IM-DD PAM, there is no need for an LO signal as
reference. As such, in the second mode the 2-mode switch 740 may
direct all laser power from a laser to the signal path of the IM-DD
PAM receiver system 722. Thus as shown, all laser power from the
laser 732 is directed by the 2-mode switch 740 to be modulated by a
data modulator, which may also be an MZM 750. Thus, to change from
coherent PAM transmission to IM-DD PAM transmission, all that is
required is to change the mode for 2-mode switch 740 and the
modulation performed by the MZM 750.
[0089] Likewise, in the IM-DD PAM receiver system 722, the 2-mode
switch 770 is set to a second mode. Since data was only encoded by
intensity for IM-DD PAM, there is no need to perform polarization,
frequency, and/or phase recovery with a LO signal. As such, in the
second mode the 2-mode switch 770 may act as a switch that receives
all laser power for each wavelength. For instance, the received
laser power for one wavelength may then be converted from optical
to electrical domain by PD 350, amplified by TIA 360, digitized by
ADC 370, and then analyzed and decoded by DSP 380. Further, since
only total intensity is measured, there is no need for canceling
common-mode noise. Thus, where a pair of balanced PDs are used, in
the second mode the 2-mode switch 770 may direct all received
optical power for each wavelength to one of the PDs.
[0090] Although the transmission systems 700A and 700B are shown
with receiver systems 720 and 722 configured similarly as receiver
system 300A, the receiver systems may alternatively configured as
any of the receiver systems described in FIGS. 1A-4B, and 6. For
example, to receive dual-polarization coherent PAM encoded signal,
the optical polarization controller 310 may be replaced with an
optical polarization demultiplexer as in receiver system 200A or
200B, or by a 2.times.1 optical polarization controller as in
receiver system 400A or 400B. Additional optical phase modulators,
OCs, PDs, TIAs, and ADCs may be provided for the two polarizations.
As another example, the receiver system in transmission systems
700A and 700B may be configured with mixed analog and digital
processing, such as shown in FIG. 6.
[0091] FIG. 8 illustrates an example block diagram of some
components in a receiver system, such as receiver system 100A,
100B, 200A, 200B, 300A, 300B, 400A, 400B, or 600. It should not be
considered as limiting the scope of the disclosure or usefulness of
the features described herein. In this example, the receiver system
is shown with one or more computing devices 810. The computing
devices 810 contains one or more processors 820, memory 830 and
other components typically present in general purpose computing
devices. Memory 830 of the computing devices 810 can store
information accessible by the one or more processors 820, including
instructions 834 that can be executed by the one or more processors
820.
[0092] Memory 830 can also include data 832 that can be retrieved,
manipulated or stored by the processor. The memory can be of any
non-transitory type capable of storing information accessible by
the processor, such as a hard-drive, memory card, ROM, RAM, DVD,
CD-ROM, write-capable, and read-only memories. For example, the
data 832 may include parameters, thresholds, and other values for
performing polarization, frequency, and/or phase recovery.
[0093] Data 832 may be retrieved, stored, or modified by the one or
more processors 820 in accordance with the instructions 834. For
instance, although the subject matter described herein is not
limited by any particular data structure, the data can be stored in
computer registers, in a relational database as a table having many
different fields and records, or XML documents. The data can also
be formatted in any computing device-readable format such as, but
not limited to, binary values, ASCII or Unicode. Moreover, the data
can comprise any information sufficient to identify the relevant
information, such as numbers, descriptive text, propriety codes,
pointers, references to data stored in other memories such as at
other network locations, or information that is used by a function
to calculate the relevant data.
[0094] The instructions 834 can be any set of instructions to be
executed directly, such as machine code, or indirectly, such as
scripts, by the one or more processors. In that regard, the terms
"instructions," "application," "steps," and "programs" can be used
interchangeably herein. The instructions can be stored in object
code format for direct processing by a processor, or in any other
computing device language including scripts or collections of
independent source code modules that are interpreted on demand or
compiled in advance. For instance, the instructions 834 may include
functions or methods for performing polarization, frequency, and/or
phase recovery.
[0095] The one or more processors 820 can be any conventional
processors, such as a commercially available CPU. Alternatively,
the processors can be dedicated components such as an application
specific integrated circuit ("ASIC") or other hardware-based
processor. For example, DSP 180, 280, 380, 480, and 680 may be
configured as the one or more processors 820, with access to memory
such as data and instructions. Although not necessary, one or more
of the computing devices 810 may include specialized hardware
components to perform specific computing processes.
[0096] Although FIG. 8 functionally illustrates the processor,
memory, and other elements of computing devices 810 as being within
the same block, the processor, computer, computing device, or
memory can actually comprise multiple processors, computers,
computing devices, or memories that may or may not be stored within
the same physical housing. For example, the memory can be a hard
drive or other storage media located in housings different from
that of the computing devices 810. Accordingly, references to a
processor, computer, computing device, or memory will be understood
to include references to a collection of processors, computers,
computing devices, or memories that may or may not operate in
parallel. For example, the computing devices 810 may include server
computing devices operating as a load-balanced server farm,
distributed system, etc. Yet further, although some functions
described below are indicated as taking place on a single computing
device having a single processor, various aspects of the subject
matter described herein can be implemented by a plurality of
computing devices, for example, communicating information over a
network.
[0097] The computing devices 810 may be capable of directly and
indirectly communicating with other transmitter systems and/or
receiver systems over a network. Computing devices in a network,
such as computing devices 810, may be interconnected using various
protocols and systems, such that computing devices in the network
can be part of the Internet, World Wide Web, specific intranets,
wide area networks, or local networks. Computing devices in the
network can utilize standard communication protocols, such as
Ethernet, WiFi and HTTP, protocols that are proprietary to one or
more companies, and various combinations of the foregoing. Although
certain advantages are obtained when information is transmitted or
received as noted above, other aspects of the subject matter
described herein are not limited to any particular manner of
transmission of information.
Example Methods
[0098] Further to example systems described above, example methods
are now described. Such methods may be performed using the systems
described above, modifications thereof, or any of a variety of
systems having different configurations. It should be understood
that the operations involved in the following methods need not be
performed in the precise order described. Rather, various
operations may be handled in a different order or simultaneously,
and operations may be added or omitted.
[0099] FIG. 9 shows an example flow diagram 800 for receiving
coherent PAM encoded signal. Flow diagram 900 may be performed by a
receiver system, such as any of the receiver systems shown in FIGS.
1A-4, and 6. Processors in the receiver system, such as processors
820 of FIG. 8, may receive data and make various determinations as
shown in the flow diagram 900.
[0100] Referring to FIG. 9, at block 910, a coherent Pulse
Amplitude Modulation (PAM) encoded signal may be received. For
example, the coherent PAM encoded signal may be a single
polarization signal, such as shown in FIGS. 1A, 1B, 3A, and 3B. As
another example, the coherent PAM encoded signal may be a
dual-polarization signal, such as shown in FIGS. 2A, 2B, 4, and 6.
As still another example, the coherent PAM encoded signal may
include multiple wavelengths, such as shown in FIGS. 3A and 3B.
[0101] At block 920, a local oscillator (LO) signal is received.
For instance, for single polarization PAM encoded signal with a
single wavelength as shown in FIGS. 1A and 1B, the LO signal may be
received and mixed with one PAM signal. In contrast, for
dual-polarization PAM encoded signal such as shown in FIG. 2A, 2B,
4, 6, or for PAM encoded signal with multiple wavelengths such as
shown in FIGS. 3A and 3B, the LO signal received may be split into
multiple beams before being combining with respective PAM encoded
beams of different polarization and/or wavelengths.
[0102] At block 930, polarization recovery between the received
coherent PAM encoded signal and the LO signal is performed using a
first control loop using mixed signal processing. For instance as
described in relation to FIGS. 1A and 3A, the polarization feedback
control loop may be performed by analyzing beating signal between
LO and a polarized portion of the received coherent PAM encoded
signal with encoded data. Alternatively as described in relation to
FIGS. 1B and 3B, the polarization feedback control loop may be
performed by minimizing optical power of a polarized portion of the
received coherent PAM encoded signal without encoded data. In
another example described in relation to FIGS. 2A and 2B,
polarization feedback control loop may be performed by minimizing
pilot tone of a dither signal. As another alternative shown in
FIGS. 4A and 4B, the polarization control loop may include a
1.times.2 polarization controller followed by a joint
polarization-diverse and phase-diverse coherent detection
circuitry. The 1.times.2 polarization controller may be configured
to introduce a 45.degree. polarization angle between the received
signal after polarization adjustment and the LO, and the final
polarization recovery may be achieved in the electrical domain.
This is different from the polarization recovery methods described
in FIGS. 1A-3B, where polarization controller/demultiplexer is
introduced to align the received signal polarization to the LO such
that polarization recovery can be achieved in the optical domain
while electrical processing is only used to determine how to adjust
the polarization controller/demultiplexer.
[0103] As still another example described in relation to FIG. 6,
polarization recovery may be performed with mixed analog and
digital signal processing via a feedback control loop between an
analog equalizer 690 and a DSP 680.
[0104] At block 940, phase recovery between the received coherent
PAM encoded signal and the LO signal using a second control loop
using mixed signal processing. For example as described in relation
to FIGS. 1A, 1B, 2A, 2B, 3A, 3B, the phase feedback control loop
may be performed by analyzing pilot data symbols. As an
alternative, a feedforward control loop may be used to perform
phase recovery as shown in FIGS. 4A and 4B. Further as described in
relation to FIG. 6, phase recovery may be performed with mixed
analog and digital signal processing via a feedback control loop
between an analog equalizer 690 and a DSP 680. Additionally, for
instance as described in relation to FIGS. 1A, 1B, 2A, 2B, 2A, and
3B, a frequency feedback control loop may be performed by analyzing
FFT between the LO signal and the coherent PAM encoded signal.
[0105] The technology provides power efficient receiver systems in
datacenters. The technology provides receiver systems that are
compatible with both coherent PAM and IM-DD PAM transmission
systems, which increases design flexibility of a datacenter.
Further, by using mixed optical and electrical processing, and/or
mixed electrical and digital processing, components with high power
consumption may be reduced. Additionally, by using a PAM system
that does not perform phase modulation, requirements on optical
equipment, such as lasers, may be relaxed, which may further
increase power efficiency and design flexibility.
[0106] Unless otherwise stated, the foregoing alternative examples
are not mutually exclusive, but may be implemented in various
combinations to achieve unique advantages. As these and other
variations and combinations of the features discussed above can be
utilized without departing from the subject matter defined by the
claims, the foregoing description of the embodiments should be
taken by way of illustration rather than by way of limitation of
the subject matter defined by the claims. In addition, the
provision of the examples described herein, as well as clauses
phrased as "such as," "including" and the like, should not be
interpreted as limiting the subject matter of the claims to the
specific examples; rather, the examples are intended to illustrate
only one of many possible embodiments. Further, the same reference
numbers in different drawings can identify the same or similar
elements.
* * * * *