Automatic polarization compensation

Abstract

Transmitters, receivers, systems and methods for compensating for polarization drift caused by a communications channel are provided. One or more polarization reference signal(s) or sets of polarization reference signals may be transmitted in combination with a polarized communications signal. The combination may be via wavelength or time multiplexing. Polarization measurements of the one or more polarization reference signals or sets of polarization reference signals and subsequent heterodyne detections thereof may be used for compensation of the polarization drift of the combined signals which are received. The polarization compensation may be for a State of Polarization (SOP) of a polarized communications signal such as any one polarization basis. The polarization compensation may be for complete polarization channel compensation such as for three different polarization basis which are mutually unbiased.

Description

AUTOMATIC POLARIZATION COMPENSATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 723,964 filed on November 22, 2024, which is incorporated by reference herein in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under DE-AC05-00OR22725 awarded by U.S. Department of Energy. The Government has certain rights to this invention.FIELD OF THE DISCLOSURE

[0003] This disclosure relates to communications systems transmitting a polarized communications signal over a channel and more specifically to automatic polarization compensation of a polarization drift caused by the channel.BACKGROUND

[0004] Known communications systems transmit a polarized communications signal over a communications channel. The communications channel may be free-space or fiber-based communications channel. The polarized communications signal may be a classical signal or a quantum signal. In some cases, information may be encoded or embedded in a specific polarization or in a polarization entangled state in a case of a quantum signal. However, communications channels cause a transformation or drift in the transmitted signal’s polarization.

[0005] The transformation is different depending on the location of the communications channel in an environment such as via local stress-induced birefringence. Stabilizing the polarization state typically uses a known reference polarization state transmitted through the communications channels and at least partially measured at the output of the communications channels, e.g., receiver, and using the measurement results to compensate for the polarization change.

[0006] Different applications may require different kinds of stability. For example, some applications only need the transmitted power through a polarizer to be stable, where others need the state of polarization (SOP) of a certain signal to be stable. These applications typically include classical communications. However, other applications such as for quantumcommunication require an understanding of a complete polarization transformation by the communications channel for the polarization of the polarized communications signal to be stable. This is because a polarization entangled state can be represented in any polarization basis.

[0007] Techniques used for polarization stabilizing for one mode, such as SOP may not work for a complete polarization stabilization because of rotation sensitivity. For example, when using one reference polarization state (e.g., one polarization reference signal) to stabilize a certain SOP. the measurement is insensitive to polarization rotations on a Poincare sphere about that measured SOP. Thus, for a complete polarization stabilization multiple reference polarization states may be used.

[0008] However, increasing the number of polarization references signals may impact the performance of a communications system. For example, for a quantum communications system, the laser used to generate the reference polarization states near the dim quantum signal will likely produce a significant amount of spontaneous Raman scattering at the wavelength of the quantum signal and especially where the communications channel is fiber-based and the channel length is relatively long. One solution would be to separate the polarization reference signal(s) and the quantum signal by several hundreds of nanometers, however, due to polarization-mode dispersion (PMD) having such as spacing is untenable.

[0009] Additionally, combining polarization reference signal(s) and the quantum signal and separating them at a receiving node adds insertion loss on the quantum signal reducing the entanglement distribution rate.SUMMARY

[0010] Accordingly, disclosed is a method comprising receiving, over an optical communications channel, a first set of polarization reference signals and a second set of polarization reference signals. The first set and second set of polarization reference signals are wavelength-multiplexed with a polarized communications signal having a wavelength (w). The first set of polarization reference signals is wavelength-shifted up relative to the wavelength (w) by an offset +AA and the second set of polarization reference signals is wavelength-shifted down relative to the wavelength (w) by the offset -Ad. The first and second sets of polarization reference signals each have a first polarization reference signal and a second polarization reference signal where one of the first polarization reference signal and the second polarizationreference signal in each set is frequency- shifted frequency with respect to the other by a first RF- offset X. The first polarization reference signal in each set is polarized in a first one of a set of three different polarization basis when launched into the optical communications channel. The second polarization reference signal in each set is polarized in a second one of a set of three different polarization basis when launched into the optical communications channel, where the first one and second one are different. The method further comprises measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal in each set of polarization reference signals that was polarized in the first one of a set of three different polarization basis to produce a first control signal, controlling, based on the first control signal, the polarizations of the wavelength-multiplexed signals received, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal in each set of polarization reference signals that was polarized in the first one of a set of three different polarization basis to produce a second control signal, controlling, based on the second control signal, the polarizations of the wavelength-multiplexed signals received, by using the polarization-compensation module with a rotation axis about the second one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal in each set of polarization reference signals that was polarized in the second one of a set of three different polarization basis to produce a third control signal, and controlling, based on the third control signal, the polarizations of the wavelength-multiplex signals received, by using the polarization-compensation module with a rotation axis about the first one of a set of three different polarization basis.

[0011] Also disclosed is an optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), wavelength-multiplexed signals comprising a first set of polarization reference signals and a second set of polarization reference signals and a polarized communications signal having a wavelength (w). The first set of polarization reference signals is wavelength-shifted up relative to the wavelength (w) by an offset +AA and the second set of polarization reference signals is wavelength- shifted down relative to the wavelength (w) by the offset -AA. The first and second sets of polarization reference signals each have a first polarization reference signal and a second polarization reference signal where one of the firstpolarization reference signal and the second polarization reference signal in each set is frequency- shifted frequency with respect to the other by a first RF-offset X. The first polarization reference signal in each set is polarized in a first one of a set of three different polarization basis when launched into the optical communications channel. The second polarization reference signal in each set is polarized in a second one of a set of three different polarization basis when launched into the optical communications channel, where the first one and second one are different. The RX comprises a polarization-compensation module, a wavelength demultiplexer, first and second polarizationOmeasurement modules, first and second local oscillator (LO) modules, first and second heterodyne detection (HD) modules) and a polarization-feedback module. The polarization-compensation module is configured to receive the wavelength- multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the wavelength-multiplexed signals about the three different polarization basis. The wavelength demultiplexer is configured to separate the polarized communications signal from the first set of polarization reference signals and the second set of polarization reference signals. The first polarization-measurement module is configured to measure a polarization of the first and second polarization reference signals in each set of polarization reference signals wavelength- multiplexed with the polarized communications signal in the second one of a set of three different polarization basis and issue a first measurement signal. The second polarizationmeasurement module is configured to measure a polarization of the first and second polarization reference signals in each set of polarization reference signal wavelength-multiplexed with the polarized communications signal in a third one of a set of three different polarization basis and issue a second measurement signal. The first LO module is configured to emit a first LO signal that is frequency offset from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals which is unshifted, by a second RF-offset Yl. The second LO module is configured to emit a second LO signal that is frequency offset from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals which is unshifted, by a third RF-offset Y2 different from YL The first HD module is configured to combine the first measurement signal with the first LO signal and combine the first measurement signal with thesecond LO signal and produce a first detector signal having portions corresponding to detections from the first polarization reference signal in each set of polarization reference signals and detections from the second polarization reference signal in each set of polarization reference signals. The second HD module is configured to combine the second measurement signal with the first LO signal and combine the second measurement signal with the second LO signal and produce a second detector signal having portions corresponding to detections from the first polarization reference signal in each set of polarization reference signals and detections from the second polarization reference signal in each set of polarization reference signals. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on an average of respective portions of the first detector signal and the second detector signal obtained by filtering.

[0012] Also disclosed is a system comprising at least one RX with compensation based on sets of polarization reference signals and at least one TX optically coupled with a respective RX through the optical communications channel, Each TX comprises first and second light sources, first and second arrays of optical filters, a reference signal generating module and a wavelength division multiplexing element. The first light source is configured to emit light at a wavelength which is up shifted relative to a respective polarized communications signal by the offset +AA. The second light source is configured to emit light at a wavelength which is down shifted relative to the respective polarized communications signal by the offset -AT. The first array of optical filters is configured to filter the light emitted by the first light source and the second array of optical filters configured to filter the light emitted by the second light source. The reference signal generating module is configured to produce from the light from the first light source the first set of polarization reference signals and from the light from the second light source the second set of polarization reference signals. The WDM element is configured to multiplex the first and second sets of polarization reference signals with an obtained respective polarized communications signal. Each TX launches, into the communications channel, the wavelength- multiplexed signals.

[0013] Also disclosed is a method for receiving, over an optical communications channel, wavelength-multiplexed signals comprising first and second polarization reference signals where one is shifted relative to the other by an RF-offset X. When launched into the optical communications channel, the first polarization reference signal is polarized in a first one of a set of three different polarization basis and the second polarization reference signal is polarized in a second one of a set of three different polarization basis. The first one and second one are different. The method further comprising measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal that was polarized in the first one of a set of three different polarization basis set to produce a first control signal, controlling, based on the first control signal, the polarizations of the wavelength-multiplexed signals which also includes a polarized communications signal, by using a polarizationcompensation module with a rotational axis about a third one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal polarized in the first one of a set of three different polarization basis to produce a second control signal, controlling, based on the second control signal, the polarizations of the wavelength-multiplexed signals, which also includes the polarized communications signal by using the polarization-compensation module with a rotational axis about the second one of a set of three different polarization basis; measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal polarized in the second one of a set of three different polarization basis to produce a third control signal, and controlling, based on the third control signal, the polarizations of the wavelength- multiplexed signals which also includes the polarized communications signal by using the polarization-compensation module with rotational axis about the first one of a set of three different polarization basis. The first polarization reference signal and the second polarization reference signal are wavelength- shifted relative to the polarized communications signal by an offset AT.

[0014] Also disclosed is an optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), wavelength-multiplexed signals comprising a polarized communications signal, a first polarization reference signal, and a second polarization reference signal. The first and second polarization reference signals each are wavelength- shifted relative to the polarized communications signal by an offset Ad, and where one is shifted relative to the other by an RF-offset X. When launched by the TX into the optical communications channel, the first polarized reference signal is polarized in a first one of a set of three different basis and the second polarization reference signal is polarized in a second one of set of three different polarization basis. The first one and second one are different. The RX comprises a polarizationcompensation module, a wavelength demultiplexer, first and second polarization-measurement modules, a local oscillator (LO) module, first and second heterodyne detection (HD) modules and a polarization-feedback module. The polarization-compensation module is configured to receive the wavelength-multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the wavelength-multiplexed signals about the three different polarization basis. The wavelength demultiplexer is configured to separate the polarized communications signal from the wavelength-multiplexed first and second polarization reference signals. The first polarization-measurement module is configured to measure a polarization of the wavelength- multiplexed first and second polarization reference signals in the second polarization basis and issue a first measurement signal. The second polarization-measurement module is configured to measure a polarization of the wavelength-multiplexed first and second polarization reference signals in a third polarization basis and issue a second measurement signal. The LO module is configured to emit an LO signal that is frequency offset relative to the first polarization reference signal or the second polarization reference signal which is not shifted, by a second RF-offset Y. The first HD module is configured to combine the first measurement signal and the LO signal and produce a first detector signal having a portion corresponding to detections associated with the first polarization reference signal and a portion corresponding to detections associated with the second polarization reference signal. The second HD module is configured to combine the second measurement signal and the LO signal and produce a second detector signal having a portion corresponding to detections associated with the first polarization reference signal and a portion corresponding to detections associated with the second polarization reference signal. Thepolarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on respective portions of the first detector signal and the second detector signal obtained by filtering.

[0015] Also disclosed is a system comprising at least one RX with compensation based on first and second polarization reference signals which are wavelength multiplexed with a polarized communications signal and at least one TX optically coupled with a respective RX through the optical communications channel. Each TX comprises a light source, an array of optical filters, a reference signal generating module and a wavelength division multiplexing element. The light source is configured to emit a light at a wavelength which is shifted relative to a respective polarized communications signal by the offset AA. The array of optical filters is configured to filter the light emitted. The reference signal generating module is configured to produce from the light the first polarization reference signal and the second polarization reference signal. The WDM element is configured to multiplex the first and second polarization reference signals with an obtained respective polarized communications signal. Each TX launches, into the optical communications channel, the wavelength-multiplexed signals.

[0016] Also disclosed is a method comprising receiving, over an optical communications channel, time-multiplexed signals comprising first and second polarization reference signals where one is shifted relative to the other by an RF-offset X, and a polarized communications signal. When launched into the optical communications channel, the first polarization reference signal is polarized in a first one of a set of three different polarization basis and the second polarization reference signal is polarized in a second one of a set of three different polarization basis. The first one and the second one are different. The method further comprises demultiplexing the time-multiplexed signals based on time and sending the polarized communications signal for further processing. The method further comprises measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal that was polarized in the first one of a set of three different polarization basis set to produce a first control signal, controlling, based on the first control signal, the polarizations of the time-multiplexed signals, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal polarizedin the first one of a set of three different polarization basis to produce a second control signal, controlling, based on the second control signal, the polarizations of the time-multiplexed signals, by using the polarization-compensation module with a rotational axis about the second one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal polarized in the second one of a set of three different polarization basis to produce a third control signal, and controlling, based on the third control signal, the polarizations of the time-multiplexed signals which also includes the polarized communications signal by using the polarization-compensation module with rotational axis about the first one of a set of three different polarization basis.

[0017] Also disclosed is an optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), time-multiplexed signals comprising a polarized communications signal, a first polarization reference signal, and a second polarization reference signal. The first and second polarization reference signals each are offset in time relative to the polarized communications signal when combined. One polarization reference signal is shifted relative to the other by an RF-offset X. When launched by the TX into the optical communications channel, the first polarized reference signal is polarized in a first one of a set of three different basis and the second polarization reference signal is polarized in a second one of set of three different polarization basis. The first one and the second one are different. The RX comprises a polarization-compensation module, a time demultiplexer, first and second polarization-measurement modules, a local oscillator (LO) module, first and second heterodyne detection (HD) modules and a polarization-feedback module. The polarization-compensation module is configured to receive the time-multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the time-multiplexed signals about the three different polarization basis. The time demultiplexer is configured to separate the polarized communications signal from the time-multiplexed first and second polarization reference signals. The first polarization-measurement module is configured to measure a polarization of the time- multiplexed first and second polarization reference signals in a second polarization basis and issue a first measurement signal. The second polarization-measurement module is configured to measure a polarization of the time-multiplexed first and second polarization reference signals in a third polarization basis and issue a second measurement signal. The LO module is configuredto emit an LO signal that is frequency offset relative to the first polarization reference signal or the second polarization reference signal which is not shifted, by a second RF-offset Y. The first HD module is configured to combine the first measurement signal and the LO signal, and produce a first detector signal having portions corresponding to detections associated with the first polarization reference signal and corresponding to detections associated with the second polarization reference signal. The second HD module is configured to combine the second measurement signal and the LO signal and produce a second detector signal having portions corresponding to detections associated with the first polarization reference signal and corresponding to detections associated with the second polarization reference signal. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on respective portions of the first detector signal and the second detector signal obtained by filtering.

[0018] Also disclosed is a system comprising at least one RX with compensation based on first and second polarization reference signals which are time multiplexed with a polarized communications signal and at least one TX optically coupled with a respective RX through the optical communications channel. Each TX comprises a pulsed light source, a reference signal generating module and a time division multiplexing element. The pulsed light source is configured to emit pulsed light. The reference signal generating module is configured to produce from the light the first polarization reference signal and the second polarization reference signal. The TDM element is configured to multiplex the first and second polarization reference signals with an obtained respective polarized communications signal. Each TX launches, into the optical communications channel, the time-multiplexed signals.

[0019] Also disclosed is an optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), multiplexed signals comprising a polarized communications signal and a polarization reference signal. When launched by the TX into the optical communications channel, the polarization reference signal is polarized in a first one of a set of three different basis. The RX comprises polarization-compensation module, a demultiplexer, first and second polarization-measurement modules, a local oscillator (LO) module, first and second heterodyne detection (HD) modules, and a polarization-feedback module. The polarization-compensation module is configured to receive the multiplexed signalsand compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the multiplexed signals about the other two bases of three different polarization basis. The demultiplexer is configured to separate the polarized communications signal from the multiplexed polarization reference signal. The first polarization-measurement module is configured to measure a polarization of the polarization reference signal in a second polarization basis and issue a first measurement. The second polarization-measurement module is configured to measure a polarization of the polarization reference signal in a third polarization basis and issue a second measurement. The LO module is configured to emit an LO signal that is frequency offset relative to the polarization reference signal. The first HD module is configured to combine the first measurement signal and the LO signal and produce a first detector signal. The second HD module is configured to combine the second measurement signal and the LO signal and produce a second detector signal. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on the first detector signal and the second detector signal.

[0020] Also disclosed is a system comprising a RX with compensation based on a polarization reference signal which is multiplexed with a polarized communications signal and an optical transmitter(s) (TX) optically coupled with the RX through the optical communications channel. The TX comprises a light source configured to emit a light, a reference signal generating module configured to produce from the light the polarization reference signal and a multiplexing element configured to multiplex the polarization reference signal with an obtained respective polarized communications signal, wherein each TX launches, into the communications channel, the multiplexed signalsBRIEF DESCRIPTION OF THE DRAWINGS

[0021] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0022] Fig. 1 illustrates a block diagram of a communications system with automatic polarization compensation in accordance with aspects of the disclosure;

[0023] Fig. 2 illustrates a block diagram of a transmitter for providing polarization reference signals in combination with a polarized communications signal in accordance with aspects of the disclosure;

[0024] Fig. 3 illustrates a block diagram of a receiving node for automatically compensating for a transformation of a polarization on a polarized communications signal in accordance with aspects of the disclosure;

[0025] Fig. 4 illustrates an example of a flow for compensating for the transformation of a polarization on a polarized communications signal in accordance with aspects of the disclosure;

[0026] Fig. 5 illustrates a Poincare sphere which is plotted in a Stokes Vector Cartesian basis with a set of three different polarization bases;

[0027] Fig. 6 illustrates an example of a transmitter and receiver in accordance with aspects of the disclosure;

[0028] Fig. 7 illustrates an example of a flow for calibrating the receiving node for automatically compensating for a transformation of a polarization of a polarized communications signal in accordance with aspects of the disclosure;

[0029] Fig. 8 illustrates an example of one or more components which may be selected for a polarization-compensation module in a receiving node in accordance with aspects of the disclosure;

[0030] Figs. 9A-9C illustrate a simulation in a Poincare sphere where Fig. 9A illustrates an uncontrolled polarization transformation using a multi-coordinated random walk on a reference signal of T / 360 (0.5°) step sizes controlling a phase and angle of several variable waveplates, Fig. 9B illustrates the same reference signal and a test signal having a V polarization where the polarization is compensated based on measurements in two polarization bases on the reference signal, and Fig. 9C illustrates the same reference signal, the same test signal and another reference signal which is launched at an A polarization where the polarization of the test signal is compensated based on measurements in two polarization bases of one of the reference signals and one polarization basis of the other;

[0031] Figs. 10A and 10B illustrate a comparison of a simulation of the outputs of feedback modules based on measurements of the two references signals with and without slope sign in error, where Fig. 10A illustrates without and Fig. 10B illustrates with;

[0032] Figs. 11 A- 11 D illustrate a simulation in a Poincare sphere where Fig. 1 1 A illustrates an uncontrolled polarization transformation using a multi-coordinated random walk on a reference signal of TT / 360 (0.5°) step sizes controlling a phase and angle of several variable waveplates, the reference signal launched at L polarization, Fig. 11B illustrates the same reference signal, another reference signal (launched at A) and a test signal (launched at V) and where the polarization is compensated based on three measurements in two polarization bases on the two reference signals without slope sign in error, Fig. 11C illustrates the same reference signal, another reference signal (launched at A) and a test signal (launched at V) and where the polarization is compensated based on three measurements in two polarization bases on the two reference signals with slope sign in error, and Fig. 11D the same reference signal, another reference signal (launched at A) and a test signal (launched at V) and where the polarization is compensated based on three measurements in two polarization basis on the two reference signals with slope sign in error having a different random walk (TT / 5000 (0.036°) step sizes;

[0033] Figs. 12A-12F illustrate results from different C-band classical test signals with automatic polarization compensation in accordance with aspects of the disclosure and without, where Fig. 12A illustrates the test signals after 1 Hz induced polarization transformation without the automatic polarization compensation and through a disabled automatic polarization compensation, Fig. 12B illustrates the test signals after 10 mHz induced polarization transformation without the automatic polarization compensation and with the automatic polarization compensation with a 1 Hz integration bandwidth in proportional-integral-derivative (PID) controller, Fig. 12C illustrates the test signals after 100 mHz induced polarization transformation without the automatic polarization compensation and with the automatic polarization compensation with a 1 Hz integration bandwidth in PID controller, Fig. 12D illustrates the test signals after 100 mHz induced polarization transformation without the automatic polarization compensation and with the automatic polarization compensation with a 1 kHz integration bandwidth in PID controller, Fig. 12E illustrates the test signals after 1 Hz induced polarization transformation without the automatic polarization compensation and with the automatic polarization compensation with a 1 kHz integration bandwidth in PID controller, and Fig. 12F illustrates the test signals after 10 Hz induced polarization transformation without the automatic polarization compensation and with the automatic polarization compensation with a 1 kHz integration bandwidth in PID controller;

[0034] Fig. 13A and 13B illustrate results of characterization of a channel used for the transmission of an L-band photon of a polarization entanglement, where a L-band transmitter with a polarization reference signal was transmitted through a fiber-based channel to a receiving node with the automatic polarization compensation and a corresponding C-band photon was measured by a polarization analyzer at the output of the transmitter, where Fig. 13A illustrates a relative process fidelities (RPF) for Cn and Cn+1 and Fig. 13B illustrates a RPF for Cl and Cn;

[0035] Figs. 14A and 14B illustrate a fidelity of two-qubit states for a polarization entanglement where both receiving nodes have the automatic polarization compensation in accordance with aspects of the disclosure and both transmitter transmitted polarization reference signals and one of the photons pairs (L-band transmitter transmitted the L-band photons and the C-band transmitter transmitted the C-band photons) via a fiber-based channel, where Fig. 14A illustrates the fidelities for successive estimates pnand pn+iand Fig. 14B illustrates the fidelities between the first estimate pi and successive states p,i

[0036] Fig. 15 illustrates a block diagram of another transmitter for providing polarization reference signals (multiple sets) in combination with a polarized communications signal in accordance with aspects of the disclosure;

[0037] Fig. 16 illustrates a block diagram of another receiving node having polarization compensation in accordance with aspects of the disclosure;

[0038] Figs. 17A-17I illustrate simulation results showing comparison between compensation techniques and uncorrected polarization drift at different offsets of polarization reference signals and polarized communications signal;

[0039] Fig. 18 illustrates a block diagram of another transmitter for providing polarization reference signals in combination with a polarized communication signal in accordance with aspects of the disclosure, in Fig. 18, the combination is via time-multiplexing;

[0040] Fig. 19 illustrates a block diagram of another receiving node having polarization compensation in accordance with aspects of the disclosure;

[0041] Fig. 20 illustrates a block diagram of another transmitter for providing a polarization reference signal in combination with a polarized communication signal in accordance with aspects of the disclosure; and

[0042] Fig. 21 illustrates a block diagram of another receiving node having polarization compensation in accordance with aspects of the disclosure.DETAILED DESCRIPTION

[0043] Aspects of the disclosure provide systems (including at least one transmitter and receiving node) and methods for different compensations for polarization transformation (also referred to herein as “drift”) caused by the communications channel (referred to herein as Automatic Polarization Compensation or APC). The different polarization compensations may be based on the application. The different polarization compensations include a State of Polarization (SOP) of a polarized communications signal to be compensated, e.g., any one polarization basis and complete polarization channel compensation, e.g., in any polarization basis. For example, the complete polarization channel compensation may be used in quantum communication where the polarized communications signal is polarization entangled signal in any basis.

[0044] For purposes of the description, a basis may include circularly polarized (circular basis): Left-handed (L) and Right-handed (R), diagonal basis: Diagonal (D) and anti-diagonal (A) and linear polarized (Linear basis): Horizontal (H) and Vertical (V).

[0045] The number of transmitters and receiving nodes in the system may also depend on the application. For example, for classical communication, there may be one transmitter and receiving node. However, for quantum communication, there may be two transmitters and two receiving nodes.

[0046] In accordance with aspects of the disclosure, the transmitter(s) transmits polarization reference signal(s) along with a polarized communications signal. The number of polarization reference signal(s) may be based on the type of polarization compensation. For example, for a SOP polarization compensation, one polarization reference signal may be transmitted. For a complete polarization channel compensation, two polarization reference signals may be used where each have a different polarization basis. In some aspects, even for the SOP polarization compensation, the transmitter may transmit two polarization reference signals, however, the receiving node may ignore one of them.

[0047] In an aspect of the disclosure, each transmitter may combine the polarization reference signal(s) with the polarized communications signal by multiplexing. In some aspects, the multiplexing may be wavelength multiplexing such as using a wavelength division multiplexing element. Wavelength multiplexing allows for fast polarization compensation with 100% up-time.

[0048] Tn other aspects, the multiplexing may be time multiplexing such as by using time division multiplexing (TDM) element. The TDM element may be a passive element or an active element. Advantageously, by using time multiplexing, the polarization reference signal(s) may have the same wavelength as the polarized communications signal and therefore, the compensation is not impacted by polarization-mode dispersion (PMD) in signals propagating in the communications channel.

[0049] In some aspects of the disclosure, the transmitter may transmit multiple sets of polarization reference signals which may be wavelength multiplexed with the polarized communications signal. In this aspect, one of the sets may have a wavelength above the polarized communications signal and the other may have a wavelength below the polarized communications signal. Advantageously, as will be described later, the use of multiple sets of polarization reference signals reduces the impact of PMD.

[0050] Fig. 1 illustrates an example of a communication systems with APC in accordance with aspects of the disclosure. The example illustrated may be for quantum communication where entangled photons are delivered to a pair of receiving nodes (e.g., First Receiver 20A and Second Receiver 20B). The receiving nodes may be in different facilities. Since the entangled photons have different frequencies (which may be correlated), different polarization references need to be used. Thus, each transmitter 10A, 10B includes a portion of the APC. The transmitters 10 A, 10B may be co-located in the same facility. The communications channels 15A may be the same or a different type of communications channel. For example, both communications channels may be fiber-based, free-space or one may be fiber-based and the other may be free-space. The fiberbased channel may be a single-mode fiber. The transmitters 10A and 10B may be provided with the entangled photons (e.g., the polarized communications signal) from a separate quantum source. An example of a quantum source is illustrated in Fig. 7 of U.S. Provisional application 63 / 723.964, which is incorporated by reference. However, other quantum sources of entangled pairs of photons may be used. In some aspects, the first transmitter 10A may be connected to an output port of a wavelength-selective switch) (WSS) (or a pulse shaper which is used as both a WSS and can apply a wavelength-dependent phase shift) and the second transmitter may be connected to another output port of the WSS.

[0051] As described above, in other aspects, there may be one transmitter (e.g., first transmitter 10A) and one receiver (e.g., first receiver 20A) in the communications system, such as forclassical communication. In this case, the polarized communications signal may be generated by the first transmitter 10A or obtained from another source.

[0052] Fig. 2 illustrates a block diagram of a transmitter (e.g., first transmitter 10A and / or second transmitter 10B) in accordance with aspects of the disclosure. The transmitters 10A, 10B comprise a light source 100, a pedestal filter 102, a beam splitter 104, first and second reference signal generating modules 106A, 106B, another beam splitter 108 (combiner), an attenuation module 110 and a wavelength division multiplexing element (e.g. WDM 112). Additionally, the transmitters 10A, 10B may include calibration optics 109. An example of the transmitter in accordance with aspects of the disclosure is shown in Fig. 6.

[0053] The light source 100 may be a laser. In an aspect of the disclosure, the laser is a continuous laser. The laser is power and frequency- stabilized. In some aspects, the power of the laser may be measured to confirm the power stabilization to avoid overcompensation. In a case where the power is not stable, there may be a perceived polarization change (which is actually a power change). The measured power may be used to normalize the measurement at the receiverside.

[0054] The laser may be a tunable laser to a target wavelength / frequency. In an aspect of the disclosure, the target wavelength is relatively close to (but not overlapping) the wavelength / frequency of the polarized communications signal to reduce the impact of PMD on the APC (e.g., Ab). The offset may be determined by wavelength or frequency. For example, in some aspects, the frequency offset may be 200 GHz. Thus, the laser may be tuned based on the polarized communications signal. In some aspects, the transmitters 10 A, 10B may comprise a processor (not shown) configured to tune the laser based on the wavelength / frequency of the polarized communications signal. The offset may be set in advance, and the processor may apply the set offset to tune the laser. As noted above, since the wavelengths / frequencies of the polarized communications signal in two transmitters is different, the laser in each transmitter 10 A, 10B will also be tuned to a different wavelength / frequency. In an aspect of the disclosure, the same offset may be used in both transmitters 10A, 10B.

[0055] In order to minimize the wavelength / frequency overlap between the polarization references signal(s) and the polarized communications signal, the transmitter 10 A, 10B includes the pedestal filter 102. The pedestal filter 102 may include one or more optical filter components. For example, wavelength division multiplexing elements may be used as a filter.The elements may be dense wavelength division multiplexers (DWDMs). In some aspects, each DWDM may have a 100-GHz passband. As shown in Fig. 6, four DWDMs 102-1, 102-2, 102-3, and 102-4 are used. In this example, each has the same passband. However, the disclosure is not limited to the same passband or having four DWDMs. The through (pass) port of the DWDM (e.g., 102-1) is connected to the subsequent DWDM (e.g., 102-2). The drop-port may be directed to a light dump. In Fig. 6. “Cxx” refers to the channel number. Each DWDM may be connected to the subsequent DWDM via a polarization maintaining fiber (PMF).

[0056] To generate the two polarization reference signals, the light is split into two paths using a beam splitter 104. The beam splitter 104 may be connected to the last element in the pedestal filter 102 via a PMF. In some aspects, the beam splitter 104 is a 50 / 50 beam splitter to maintain the power substantially equal in each path. The output of the beam splitter 104 is connected to the first reference signal generating module 106 A and the second reference signal generating module 106B. In an aspect of the disclosure, each polarization reference signal is in a different polarization basis. When two polarization reference signals are used, there are two polarization bases, where they are mutually unbiased. Any two polarization bases may be used. In the example illustrated in Fig. 6, one polarization basis is V / H (linear) where “V” is used, and the other polarization is R / L (circular) where “R” is used. However, the disclosure is not limited to these bases. In an aspect of the disclosure, the two polarization reference signals are frequency shifted relative to each other. For example, the first reference signal generating module 106A comprises an acoustic-optic modulator (AOM) 106A-2 driven by a RF source 106A-1. The frequency shift is to enable two polarization reference signals to be easily separated (such as by a filter). The light source 100 may have a frequency drift and a linewidth. The frequency shift (“X”) may be larger than twice the frequency drift times the linewidth. In some aspects, the frequency shift X may be 200MHz. In other aspects, the frequency shift may be greater. The frequency of the RF source 106A-1 is the same as the frequency shift. In the above example, the frequency of the RF source 106A-1 is 200MHz. The AOM 106A-2 may be connected to the beam splitter 104 via a PMF. The first reference signal generating module 106 A may also comprise optics. For example, the first reference signal generating module 106A may comprise a lens 106A-3 that collimates the light. The first reference signal generating module 106 A may also comprise one or more light rotating elements. The type and number of light rotating elements may be based on the target polarization basis. For example, in the example illustrated inFig. 6, the target polarization is “V’ for the first reference signal generating module 106A and there is no need for a light rotating element. In this case, the first reference generating module 106A comprises a polarizer 106A-4 which outputs light at a specific polarization (e.g., first polarization reference signal). The lens 106A-3 may be connected to the AOM 106A-2 via a PMF and the polarizer 106A-4 via free space.

[0057] The second reference signal generating module 106B may comprise an attenuator 106B- 1. The attenuator 106B-1 may be needed to balance the power with the other path since the light in the other path is being modulated by the AOM 106A-2. The second reference signal generating module 106B may also comprise a lens 106B-2 to collimate the light. The lens 106B- 2 may be followed by a polarizer 106B-3 which outputs light at a specific polarization. In the example illustrated in Fig. 6, since the target polarization is “R”, the second reference signal generating modules 106B comprises two light rotating elements. In this case, the light rotating elements are a quarter waveplate 106B-4 and a half waveplate 106B-5. The quarter waveplate 106B-4 converts the linear polarized light to circular polarized light and the half waveplate 106B-5 rotates the circular polarized light to a specific polarization (e.g., second polarization reference signal).

[0058] The light output from the first and second reference signal generating modules 106 A, 106B is mixed and combined by the beam splitter (combiner) 108. Once again, the beam splitter108 may be 50 / 50. Since the goal is to have one of the polarization references signals in this case “R” after being combined, the transmitters 10A, 10B may comprise calibration optics 109. The calibration optics 109 are used to change the orientations of the quarter waveplate 106B-4 and a half waveplate 106B-5 such that the light output of the beam splitter 108 (after combination) achieves the target polarization, e.g., R. The beam splitter 108 may cause a phase shift which may affect the polarization. The beam splitter 108 sends half of the input light to the attenuation module 110 and the other half may be sent to a light absorbing beam dump. In other aspects, the other half may be simply sent to another output port of the beam splitter 108, which is not connected.

[0059] The calibration optics 109 may only be used when configuring the orientations of the quarter waveplate 106B-4 and a half waveplate 106B-5. In some aspects, the calibration optics109 may be removed once the orientations are set. In other aspects, the calibration optics may be rotated in and out of the light path as needed such as to recalibrate if there is a need or desire tochange the basis for the polarization reference signal. The calibration optics 109 may include a quarter waveplate 109-1 and a polarizer 109-2. Light at the output of the polarizer 109-2 may be picked off and the polarization measured. A processor may rotate the quarter waveplate 106B-4 and a half waveplate 106B-5 as needed based on the detection. In other aspects, the quarter waveplate 106N-4 and the half waveplate 106B-5 may be manually rotated as needed.

[0060] The attenuation module 110 is configured to power-stabilize the first and second polarization reference signals and reduce the power, e.g. to generate the two dim polarization reference signals. In the example illustrated in Fig. 6, the attenuation module 110 comprises a noise eater 110-1 and an attenuator 110-2. The NE 110-1 acts as a variable optical attenuator. In a case where there is a power variation, the NE 110-1 can output a fixed power level. A NE 110-1 may be configured for different wavelengths / frequencies. For example, an NE from Thorlabs EVOA1550A may be used in a 1250 nm to 1625 nm range with a calibration. This NE may be adjusted to achieve a target output power. The NE 110-1 may be connected to the attenuator 110-2 via a SMF. It is noted that the path may include an input lens into the NE 110-1. The NE 110-1 may be connected to the beam splitter 108 also via a SMF. The attenuator 110-2 may also be a variable attenuator such as VOA50-APC also available from Thorlabs. The combination reduces the power level to a target such that the first and second polarization reference signals have a minimal impact on the polarized communications signal. For example, in some aspect, the power level of the first and second polarization reference signals may be between -10 dBm to -100 dBm.

[0061] The first and second polarization reference signals are combined with the polarized communications signals. In some aspects, the combination uses a WDM element 112. In the example illustrated in Fig. 6, the WDM element 112 is a DWDM. Once again, Cxx is the channel for the polarization reference signal(s) and the polarized communications signal (e.g., quantum signal such as one of photons in the entangled pairs) is input to the express port. Each transmitter 10 A, 10B launches the respective first and second polarization reference signals and the respective polarized communications signal into the communications channel 15 A, 15B.

[0062] Fig. 3 illustrates a block diagram of a receiving node (e.g., receivers 20A, 20B) with the APC in accordance with aspects of the disclosure. Each receiver 20A, 20B has the APC. Each receiver 20A, 20B comprises a polarization-compensation module 150, a demultiplexing module 152, polarization-measurement modules (first and second) 154A, 154B, a LO module 156, abeam splitter 168, heterodyne modules (first and second) 158 A, 158B, power splitters 160A, 160B, RF signal processing modules 162A, 162B, 162C (collectively RF signal processing module), digital filter modules 164A, 164B, 164C (collectively Digital filter module), feedback modules 166A, 166B, 166C (collectively feedback module), and LO feedback module 170. Additionally, the receivers 20A, 20B comprise a calibration module 172.

[0063] The polarization-compensation module 150 is connected to the communications channel (collectively referred to herein as 15). After the polarized communications signal and the first and second polarization reference signals propagate through the communication channels they are received by the polarization-compensation module 150. The polarization-compensation module 150 is configured to rotate the polarization of the polarized communications signal (and the first and second polarization reference signals) under the control of the feedback modules 166A, 166B, 166C. This control is based on polarization measurements on the first and second polarization reference signals.

[0064] In an aspect of the disclosure, the polarization-compensation module 150 may comprise a plurality of variable waveplates (VWP). For example, the VWP may be piezo fiber squeezers (FS) 150-1 such as illustrated in Fig. 6. Advantageously, the FSs 150-1 have a relatively high- bandwidth and a low fiber connection insertion loss. For example, a FS 150-1 has a bandwidth of DC to about 20 kHz and an insertion loss of 0.01 dB. The polarization-compensation module 150 is not limited to FSs and other polarization rotation means may be used such as free-space or waveguide EOMs or liquid-crystal variable retarders. Fig. 8 illustrates a table of alternate polarization rotation means which may be used in the polarization-compensation module 150 in accordance with aspects of the disclosure. The type of polarization rotation means used may be application specific. In some aspects of the disclosure, a combination of different polarization rotation means may be used and connected in series. For example, one polarization rotation means may be used for a faster compensation or correction and another polarization rotation means may be used for a slower compensation or correction. For instance, fiber squeezers may be cascaded with a faster lithium niobate modulator for enhanced control and to enable control over larger phase shifts since the lithium niobate phase range is limited.

[0065] A change in the phase of a VWP, rotates the polarization of light along a great circle of the rotation axis being the polarization that are eigenstates of the slow / fast axes of the VWP. For example, changing the phase of a VWP with the slow / fast axes in the H / V basis will take a Dinput polarization to R, then A and L and back to D. Accordingly, a VWP in the H / V basis and another one in the D / A basis will both transform R input polarization but the great circles are practically orthogonal to one another at R in the Poincare sphere representation of polarization as shown in Fig. 5. Since the H / V-basis VWP moves R along the D / A basis (as measured by Stokes S2 = D - A), measuring R in the D / A basis provides information pertinent to the H / V-basis VWP. Similarly, since the D / A-basis VWP moves R along the H / V basis (as measured by Stokes 51 = H - V), measuring R in the H / V basis provides information pertinent to the D / A basis VWP and these two measurements are approximately orthogonal on the Poincare sphere in the neighborhood of R. Given an R-reference polarization, a D / A-basis measurement after propagating through the communications channel 15 (and polarization-compensation module 150) can be used to adjust the H / V-basis VWP to constrain polarization after the polarizationcompensation module 150. Simultaneously, a H / V-basis measurement on the R-reference polarization can be used to adjust the D / A-basis VWP to further constrain the polarization after the polarization-compensation module 150. Similarly, measuring for an V or H polarization reference in the H / V basis or the D / A basis to control an R / L basis VWP would constrain polarization after the polarization-compensation module 150.

[0066] Thus as described herein to control polarization using a VWP with rotational axis about basis 1, requires a measurement in basis 2 on a reference signal with polarization in basis 3, where basis 1, 2, and 3 can be any set of 3 mutually unbiased bases picked without duplication, i.e., basis 1, 2, and 3 are all different for a given control loop.

[0067] In the example illustrated in Fig. 6, the FS 150-1 comprises four different fiber squeezers. Two in the H / V basis and two in the D / A basis which alternate. For example, channel 1 fiber squeezer (Ch-1) is in the H / V basis, Ch-2 is in the D / A basis, Ch-3 is the H / V basis, and Ch-4 is the D / A basis. In an aspect of the disclosure, in order to obtain a fiber squeezer associated with the R / L, the Ch-3 fiber squeezer may be configured as a quarter waveplate to rotate the Ch-4 squeezer (which is natively in the D / A basis) into the R / L.

[0068] Since in this aspect of the disclosure, the first and second polarization reference signals are wavelength multiplexed with the polarized communications signal, the receivers 20A, 20B have a demultiplexing module 152 to separate the first and second polarization reference signals from the polarized communications signal. The demultiplexing module 152 may comprise one or more optical-add-drop multiplexers (OADM) 152-1, 152-2. The first OADM 152-1 separatesthe first and second polarization reference signals from the polarized communications signal. In an aspect of the disclosure, the OADM may have a 100-GHz channel. Once again, in Fig. 6, the Cxx is the channel used for the first and second polarization reference signals. The channel is different for the different receivers 20A, 20B. The second OADM 152-2 may be used for additional filtering. In another aspect, a DWDM 152-3 may also be used for additional filtering on the polarized communications signal. The polarized communications signal may be sent to a quantum receiver for further processing, e.g., polarization projections.

[0069] The first and second polarization reference signals output from the OADM 152-1 may be focused using a lens 500 onto a beam splitter 502 (not shown in Fig. 3). The beam splitter 502 may be a 50 / 50 beam splitter to have substantially the same power into the first polarizationmeasurement module 154A and the second polarization-measurement module 154B. SMF may be used for the connections between the polarization-compensation module 150 and the lens 500 (via the OADM 152-1). In an aspect of the disclosure, to reduce additional drift within the receivers 20A, 20B, the SMF between the polarization-compensation module 150 and the lens 500 may have passive temperature stabilization. In some aspects, the passive temperature stabilization may include foam-insulation around the SMF. Additionally, a housing of the receivers 20A, 20B may include venting. In other aspects, the receivers 20A, 20B may include active temperature stabilization. For example, the receivers 20A, 20B may include a temperature sensor in the optical path between the polarization-compensation module 150 and the lens 500, a heater / thermal cooling and a processor. The processor may control the heater / thermal cooling based on the temperature detected by the temperature sensor.

[0070] Each polarization-measurement module 154A, 154B may comprise a half waveplate 154A-1, 154B-1, a quarter waveplate 154A-2, 154B-2 and a polarizer 154A-3, 154B-3. Each polarization-measurement module 154A, 154B may also comprise a lens. 154A-4, 154B-4. The half waveplate 154A-1, 154B-1 and a quarter waveplate 154A-2, 154B-2 may be motorized and the orientation set for the respective measurement during a calibration phase.

[0071] The output of the first polarization-measurement module 154A is directed to a first heterodyne module 158A and the output of the second polarization-measurement module 154B is directed to a second heterodyne module 158B via PMFs.

[0072] The LO module 156 is configured to generate the Local Oscillators (LO) for the first and second heterodyne modules 154A, 154B. The LO module 156 comprises a frequency stabilizedlaser. Tn some aspects, the frequency stabilized laser in the LO module 156 is identical to the laser used as the source for the first and second polarization reference signals. In an aspect of the disclosure, the LO has a frequency offset Y from the polarization reference signal which is not frequency shifted and in the example in Fig. 6, the R polarization reference signal. In some aspects of the disclosure, the frequency offset may be 70MHz. The output of the LO module 156 is split using a beam splitter 168 to supply the LO to the first heterodyne module 158 A and the second heterodyne module 158B.

[0073] The first and second heterodyne modules 158 A, 158B each have a pair of fast diodes 158A-2, 158A-3, 158B-2, 158B-3 and a balanced detector 158A-4, 158B-4. The respective LO and the respective measurement signal are mixed in a beam splitter 158A-1, 158B-1 and propagated to the photodiodes. Advantageously, by using heterodyne detection ultra-high- bandwidth (up to GHz) and ultra-sensitive detection near the single-photon level (per mode) may be achieved. Moreover, the LO module 156 and light source 100 for the first and second polarization reference signals do not need to be phase coherent. The laser used for the LO used in heterodyne detection may be a bright mW laser (continuous) which enables the use of conventional fast photodiodes 158A-2, 158A-3, 158B-2, 158B3. Using fast photodiodes 158A-2, 158A-3, 158B-2, 158B-3 and high-speed electronic amplifiers 503A, 503B enables a balanced detector 158A-4, 158B-4 with GHz bandwidth. Moreover, because heterodyne detection can detect down to about one input photon per mode, using a narrowband local oscillator enables the detection of signals with total power below -100 dBm where the spontaneous Raman scattering generated from such a signal is negligible in most instances and the cross-talk can most likely be reduced to tolerable, even negligible, levels with reduced demultiplexing filtering. Thus, the heterodyne detection also allows for using the dim polarization reference signals described above. Additionally, the use of the high-speed detection in the first and second heterodyne modules 158A, 158B enables the use of certain polarization rotation means in the polarizationcompensation module 150 including the FSs. The first and second heterodyne modules 158A, 158B output photocurrents. These photocurrents go through the amplifiers 503 A, 503B (transimpedance amplifiers) to provide an amplified voltage (v) proportional to photocurrent (i) where the gain is GTIA (f).

[0074] The LO feedback module 170 is configured to maintain the fixed offset Y. There may be frequency drift in the LO. As an input for the LO feedback module 170 some of the power fromthe first heterodyne module 158A is supplied via a power splitter(s) 160A. In Fig. 6, the power splitters are represented as 1 x 2. The input to the first power splitter is the amplified signal from the first heterodyne module 158 A, the power splitter divides the power and sends a portion for RF signal processing (module 162 A) and a portion for feedback. An additional power splitter may be used to supply a portion to oscillator 170-2 and for filtering. The amount of power of the detection used for the LO feedback module 170 may be a relatively small portion allowing the majority to be used for the main control.

[0075] The LO feedback module 170 comprises a processor such as a microcontroller (MC 170- 1) in Fig, 6, an oscillator 170-2, a power supply (V 170-3) for the amplifier 170-4 and a pair of filters 170-5, 170-6. The filters 170-5, 170-6 are configured to isolate lower frequency peaks to examine the relative frequencies of the LO and laser used as the source for the first and second polarization reference signals. For example, LPF 170-5 may have a passband less than 200 MHz and BPF 170-6 may have a passband between 10-200 MHz. The output of the filters 170-5, 170- 6 is amplified.

[0076] The oscilloscope 170-2 is used to count zero-voltage threshold crossings to approximately determine the frequency. The processor (e.g., MC 170-1) may fine tune the LO module 156 based on the determined frequency. In an aspect of the disclosure, the processor may implement a proportional-integral-derivative (PID) control loop based on the determined frequency and a setpoint (e.g., Y). The laser used as the source and the laser used in the LO module 156, when initially set to the same wavelength is within a few hundred MHz of each other which is within the balanced detectors’ electrical bandwidth. The feedback maintains the offset that matches a predetermined fraction of the balanced detectors electrical bandwidth B. In some aspects, the frequency offset of the first and second reference signals X added with the offset Y is equal to or less than B so the signals are within the balanced detector’s bandwidth. The balanced detectors electrical bandwidth may be about 500 MHz. It is noted that the first and second heterodyne modules 158 A, 158B may have different balanced detector’s bandwidths.

[0077] To initially set the LO frequency (and offset), the light source 100 is turned ON, and the LO module 156 is turned ON. The frequency of the LO is adjusted so that when the LO is reduced in frequency, the heterodyne signal gets bigger. The frequency of the LOs should be less than the lowest detection peak in the output from the first heterodyne module 106 A.

[0078] Another power splitter 160B is connected to the output of the second heterodyne module 158B. This power splitter 160B is used to route the detections from the measurement for two feedback modules (166B, 166C). In other words, the measurements in the D polarization are used for two feedback modules.

[0079] Each RF signal processing module 162A, 162B and 162C comprises one or more filters. For example. RF signal processing module 162A may comprise a bandpass filter 162A-1. This bandpass filter 162A-1 may be transmissive to electrical signals near frequency Y. For example, the BPF 162A-1 may have a passband between 10-200 MHz. RF signal processing module 162B may comprise a bandpass filter 162B-1. This bandpass filter 162B-1 may be transmissive to electrical signals near frequency Y. For example, the BPF 162B-1 may have a passband between 10-200 MHz. RF signal processing module 162C may comprise a high pass filter to transmit electrical signals near X + Y. For example, the HPF may have a passband of greater than 200MHz.

[0080] Each RF signal processing module 162 A, 162B and 162C may also comprise an amplifier 162A-2, 162B-2, 162C-2 and power source (V) 162A-3, 162B-3, 162C-3 therefore. The power source 162A-3, 162B-3, 162C-3 supplies power to the respective amplifier.

[0081] In an aspect of the disclosure, the RF signal processing modules 162A, 162B and 162C may also comprise another filter. For example, RF signal processing modules 162A, 162B may comprise a low pass filter 162A-4, 162B-4. These low pass filters 162A-4, 162B-4 may have a passband less than 200MHz. The RF signal processing module 162C may comprise a band pass filter 162C-4. The BPF 162C-4 may have a passband between 200-400MHz. Thus, each filtered heterodyne detection output is amplified and filtered again to improve the SNR. RF signal processing module 162A, 162B and 162C may also comprise a power detector 162A-5, 162B-5 and 162C-5. Each power detectors 162A-5, 162B-5 and 162C-5 is supplied with a power source “V” 162A-6. 162B-6, 162C-6. In some aspects of the disclosure, the power detectors 162A-5, 162B-5 and 162C-5 may output the power in units of decibels, which uses a 10 log 10() formula. In other aspects, the power detectors 162A-5, 162B-5 and 162C-5 are linear power detectors.

[0082] The output of the power detector 162A-5, 162B-5 and 162C-5 is based on the measurements. The filters in the respective path 162A-1, 162A-4, 162B-4, 162C-1 and 162C-4 effectively enable the power detectors to each focus on one of the polarization reference signals for each measurement by significantly attenuating the other. The output of the power detector162A-5, 162B-5 and 162C-5 is an analog voltage. For example, a D measurement of the polarization reference signal R is used as the input to the feedback module 166B (H) and a D measurement of the polarization reference signal V is used as the input to feedback module 166C (R) and a V measurement of the polarization reference signal R is used as the input to the feedback module 166 A (D).

[0083] Each receiver 20A, 20B also has digital filter modules 164A, 164B, 164C. Each digital filter module 164A, 164B, 164C converts the analog detection into a digital value for further processing by sampling. In an aspect of the disclosure, each digital filter module 164A, 164B, 164C acts as a Butterworth low-pass filter. In some aspects, the Butterworth low-pass filter may be used such as an 8thorder 10kHz low pass filter. The Butterworth low-pass filter may limit the bandwidth of the feedback.

[0084] Each receiver 20A, 20B also has feedback modules 166A, 166B and 166C. Each feedback module 166 A, 166B, 166C is communicatively coupled to the first and second heterodyne modules 154A, 154B (such as via the RF signal processing modules 162A, 162B, 162C and the digital filter modules 164A, 164B, 164C). Each feedback module 166A, 166B and 166C comprise a processor. The processor is configured to control the polarization-compensation module 150 based on a respective detection. For example, in some aspects, each processor may implement its own feedback algorithm such as a PID control. The processor may be included in an integrated and multi-purpose device such as a MokuiGo available from Liquid Instruments. The respective digital filter reduces noise in detections from the respective heterodyne modules 158A, 158B.

[0085] In an aspect of the disclosure, the PID control also comprises the use of “a slope sign in error”. The slope sign in error is based on a sign change of the slope of measurement is consecutive measurement, e.g., of the Stokes measurements. In particular, as the polarization drifts around the Poincare sphere as shown in Fig. 5, the location of the drift may result in a change in sign and result in a discontinuous “jump”. While this may not be a significant issue with relatively small changes, e.g., drifts, when the change is larger, the input error may be modified to include the slope sign in error. For example, the input error enon iteration n may bemodified from en= (Mn- T), where Mnis the current measurement and T is the target setpoint, to

[0086] where Cnis the PID output correction applied after PID iteration n using Mnand Sgn() is the sign function. This slope sign provides linear control with information about the slope of the Stokes measurements as a function of the polarization-compensation module 150 (e.g., VWP) applied retardance using a finite difference to approximate the derivative.

[0087] This control combines a linear control with a twist, e.g., a non-linear control but retains some of the benefits of linear control found with the PID, namely, it retains most of the reliable continuous deterministic control. This enables control over the entire Poincare Sphere. While using the slope sign in error may be noisier because it may confuse a signal noise for a slope change, it may provide the complete polarization channel control.

[0088] The first and second polarization-measurement modules 154A, 154B effectively provide “partial” Stokes measurements on the first and second polarization reference signals. They are “partial” because only one polarization in the set is measured, however, it is interpreted as the full Stokes measurement assuming that the power does not fluctuate as noted above.

[0089] The outputs from the processors (e.g., PID control) may be converted back into an analog signal to control the polarization-compensation module 150.

[0090] The calibration module 172 is used for calibrating the optics in the first and second polarization-measurement modules 154A, 154B such as the quarter and half waveplates 154A-1, 154A-2, 154B-1, 154B-2 and in a case where the polarization-compensation module 150 comprises VWPs, the channels.

[0091] The calibration module 172 comprises circulators 172-5. 175-6 in the optical path between the first polarization-measurement module 154A and the first heterodyne module 158A and the second polarization-measurement module 154B and the second heterodyne module 158B. The calibration module 172 may also comprise a patch panel (PP) 172-7. During normal operation (non-calibration), the circulators 172-5, 175-6 are configured to allow the measurements from the first and second polarization-measurement modules 154A, 154B to proceed to the first and second heterodyne modules 158A, 158B, respectively. However, during calibration, the circulators 172-5, 175-6 are configured to back propagate the LO from the LOmodule 156 through the first and second polarization-measurement modules 154A, 154B and the polarization-compensation module 150 via the PP 172-7.

[0092] The calibration module 172 may also comprise a human interface device (HMI) 172-1, a detector 172-2, a polarizer 172-3 and a polarization controller 172-4. The detector 172-2 and HMI 172-1 are used to evaluate whether the adjustments made to the quarter and half waveplates 154A-1, 154A-2, 154B-1, 154B-2 and certain VWPs meet a target criterion. The polarization controller 172-4 sets the basis for the detection by the detector 172-2 via the polarizer 172-3. Since the polarizer 172-3 only lets light at a specific polarization pass, the polarizer 172-3 enables a detection of a change in polarization in the set polarization basis (by the polarization controller 172-4 based on a change in the power output from the polarizer 172-3.

[0093] In some aspects of the disclosure, the feedback modules 166A, 166B and 166C may also be used in calibration. For example, hardware within the feedback modules may be used to generate a modulation signal for the calibration, e.g., a waveform generator. Additionally, when the polarization-compensation module 150 includes VWPs, there are four of them, an additional waveform generator is needed (there are three feedback modules). For example, the Moku:Go which may be used for the PIDs in the feedback modules 166A, 166B and 166C may be used as a waveform generator to generate a sine wave as the polarization modulation signal. For example, the polarization modulation signal may be a 10Hz 4Vpk-pk, 2-V offset sine wave.However, the polarization modulation signal is not limited to the above example. Additionally, in a case where a Moku-Go is used as the digital filter (e.g., 164C-1), the Moku-Go may be used as the waveform generator for the additional channel.

[0094] Fig. 7 illustrates an example of the calibration process. The calibration process may be performed at initial set up. Additionally, the calibration process may be periodically executed. The SMF between the polarization-compensation module 150 and the first and second polarization-measurement modules 154A, 154B affects the measurement, in which case, periodic calibration, may be needed, especially if there is a temperature change. Between the beam splitter 502 and the lens 154A-4, 154B-4 is free space. This enables a longer period between calibration than if fibers were used.

[0095] At S700, the PP 172-7 is controlled to back propagate the LO. The PP is used as a switchboard to achieve sufficient isolation. Once the LO is back propagated, a polarization modulation signal is generated for channel-4 (which as noted above is the VWP for the D / Abasis) at S702. For example, the Moku:Go may be used to generate the polarization modulation signal. While the polarization modulation signal is being generated, the detector 172-2 is monitored for a polarization modulation of the back propagated LO, e.g., peak-to-peak variation via the fiber polarizer 172-3. The detector 172-2 acts a power meter. The output may be viewable on the HID 172-1. Alternatively, the peak-peak variation of the power-meter analog output may be analyzed by an oscilloscope. In some aspects, in a case where the Moku:Go is used in the PID such as for Ch-3, the oscilloscope feature of the Moku:Go may be used. To reveal the maximum peak-peak variation present, a polarization controller 172-4, before the fiber polarizer 172-3, may be adjusted as needed to set the polarization basis.

[0096] Additionally, at S702, the quarter and half waveplates 154B-1, 154B-2 may be calibrated for the second polarization-measurement module 154B. For example, to minimize the peak-peak variation the quarter and half waveplates 154B-1, 154B-2 are rotated. In an aspect of the disclosure, the rotation may be iterative between the quarter and half wave plates 154B-1, 154B- 2 to reach the local minima. The rotation may be manual. Alternatively, a user may cause a processor to rotate the respective waveplates 154B-1, 154B-2.

[0097] At S704, the channel 3 VWP is calibrated (which as noted above may be in the H / V basis). This calibration makes the VWP act as a quarter waveplate to rotate channel 4 (D / A basis) into the R / L basis. In this case, the polarization modulation signal generated on channel 4 (such as from 164C-1) is stopped and the polarization modulation signal is generated on channel 2 (such as from 166A-1). Here the target is a phase shift of 7t / 2 or -idl to maximize the modulation, e.g., local maximum. The channel 3 VWP is adjusted to achieve the target. Once again, prior to or during the generation of the polarization modulation signal, the polarization controller 172-4 may be used to set the polarization basis for detection.

[0098] At S706, the quarter and half waveplates 154B-1, 154B-2 may be recalibrated for the second polarization-measurement module 154B to minimize the peak-peak variation from the modulation on channel 2. S702 is to prepare for S704. The polarization controller 172-4 may also be used prior to or during the detection. However, once S704 is executed, the waveplates should be rotated again. A similar iterative process may be used for the rotation of the quarter and half waveplates 154B-1, 154B-2.

[0099] At S708, the quarter and half waveplates 154A-1. 154A-2 may be calibrated for the first polarization-measurement module 154A to minimize the peak-peak variation from themodulation on channel 1. Tn this case, the polarization modulation signal generated on channel 2 (such as from 166A-1) is stopped and the polarization modulation signal is generated on channel 1 (such as from 166B-1). Similarly, the polarization controller 172-4 may be used to set the polarization basis for the detection. A similar iterative process may be used for the rotation of the quarter and half waveplates 154A-1, 154A-2 to minimize the peak-peak variation.

[0100] In other aspects, the calibration could be completely automated if desired using an automated optical switch with sufficient isolation and a script reprogramming the Moku:Go, querying for the peak-peak variation, and adjusting the quarter and half wave-plates using the algorithms described above.

[0101] The quarter and half waveplates 154A-1, 154B-1, 154A-2, 154B-2 are able to be calibrated within a few tenths of a degree using the motorized waveplates. The calibration should be stable for the duration of the polarization control. In order to maintain the calibration, the SMF may be relatively short and coiled to a small space for physical securing from movement.

[0102] After S708, the LO may be redirected back to serve as the LO for the first and second heterodyne modules 158 A, 158B. At S710, the setpoints (target for the error) and signs for slope sign in error are determined. These parameters are determined for each control loop, e.g., each feedback module 166A, 166B, and 166C (such as the PID control). In some aspects, a different slope sign in error may be used for different feedback modules 166A, 166B and 166C. For example, in some aspects for channel 1, e.g., which is the H / V basis (e.g., PID-H 166B-1) and channel 2, e.g., which is the D / A basis (e.g., PID-D 166A-1) may have the same sign and channel 4, e.g., which is the R / L basis (e.g., PID-R 166C-1), the slope sign in error may be different. However, other combinations may be used. The sign choice affects the stabilized location on the Poincare sphere (see Fig. 5).

[0103] To calibrate the setpoints, a polarization controller 172-4 may be adjusted to find the maximum power received by each RF power detector 162A-5, 162B-5, 162C-5 (which may be the minimum voltage due to a negative response slope). In an aspect of the disclosure, the ideal setpoint is 50% of the maximum signal power (e.g., the overlap of R onto H). The setpoints determine the location where the polarization reference signals are stabilized on the Poincare sphere. The setting of these setpoints effectively stabilizes the polarization reference signals to different places on the Poincare sphere and thereby stabilized the polarization channel tosomewhat different transformations. Also, in an aspect of the disclosure, the setting considers that due to shot noise, a 3-dB reduction in the heterodyne signal peak(s) will result in less than a 3-dB total power change; the actual reduction is dictated by the SNR between the shot noise within the filtering bandwidths and the heterodyne signal peak(s).The slope sign in error and setpoint are saved in each feedback module 166A, 166B, 166C.

[0104] At S712, the gains for each feedback module 166 A, 166B, 166C are determined. The loop gains may be determined based on one or more factors. In general, the derivative may not be used due to its propensity to cause instability and provide minor improvements for this sort of signal tracking, e.g., Kd. In an aspect of the disclosure, the integral term (Ki) provides the primary control with the proportional (Kp) term adding small faster corrections. To achieve minimal error while stabilizing the polarized communications signal, the proportional and integral gains (Ki and Kp) can be turned up until self-oscillation occurs then dialed back until somewhat lower than self-oscillation occurs.

[0105] Additionally, due to imperfect laser frequency stability, the PID may be tuned to be slower and averaged over the power-variation of a frequency drift. For example, the PID proportional gain may equal 1 dB and integral corner frequency may be 1 Hz. This enables precise control. Additionally, these gains may be used for in-ground fiber. The integral corner frequency may be higher in a case where there is a faster polarization drift. For example, an integral comer frequency of 1 kHz may be used. Moreover, if there is drift at frequencies near the edge of the PID bandwidth that can cause oscillation from 180° phase lag. To help dampen that oscillation, the integral terms may have slightly different bandwidth for each loop, e.g., 3 Hz, 2 Hz, and 1 Hz or 1 kHz, 800 Hz, and 600 Hz to dampen oscillation. The integration bandwidth should be higher than the correction bandwidth, e.g., expected drift.

[0106] At S714, the LO module 156 is tuned to the target offset, e.g., Y, as described above.

[0107] Fig. 4 illustrates a flow for the APC. At 300, the polarization reference signals are prepared. For example, since the polarization reference signals are a function of the polarized communications signal, e.g., such that the wavelength may be different, at S300, either a processor which controls the light source or a user manually tunes the light source (e.g., laser 100) to a target wavelength which is close to the polarized communications signal and the laser emits light at the tuned frequency. This light is filtered by the pedestal filter 102, split into two paths and the first and second polarization reference signals are generated in the respectivemodules 106A, 106B, where the two polarization reference signals are frequency shifted with respect to each other. The two polarization reference signals are subsequently combined and attenuated to be dim polarization reference signals. The polarized communications signal is acquired at 302 and multiplexed such as using WDM 112. For example, the polarized communications signals may be a quantum signal (entangled photon(s)). The polarization reference signals and the polarized communications signal are launched and propagate through the communication channel 15. The polarization reference signals (first and second) and the polarized communications signal are received by the polarization-compensation module 150 where the same are corrected based on the control from the feedback modules 166A, 166B and 166C at 304. The correction is based on a prior cycle of measurements. The first and second polarization reference signals and the polarized communications signal are demultiplexed at 306 (demultiplexing module 152) which separates the first and second polarization reference signals from the polarized communications signal. The polarized communications signal is sent for further processing and the first and second polarization reference signals are sent for measurements. At 308, the polarizations and frequencies of the first and second polarization reference signals are measured using the first and second polarization-measurement modules 154A, 154B and the first and second heterodyne modules 158A, 158B. At 310, the detection is RF signal processed including selecting certain RF frequency bandwidths, apply amplifications, filter noise and measure the RF power. The measured RF power is further filtered and converted to a digital signal via the digital signal process 312 which samples and low-pass filters the measured RF power over time, which effectively integrates the measured power. At S314, the PID control (which is an example of the feedback modules) takes the digital signal processing output and controls a polarization correction, closing the feedback loop, e.g., three PID controls for the three control loops.

[0108] A computer simulation of the APC described herein was executed to show the compensation difference between using one polarization reference signal and two polarization reference signals. Figs. 9A-9C illustrate the results of the simulation on a Poincare sphere. Fig. 9A illustrates a single polarization reference signal with no compensation. The polarization reference signal starts out as having an L polarization. A polarization drift was simulated using a multi-coordinate random walk of 7t / 360° step sizes controlling the phase and angle of several VWPs. Since there is no compensation, there is a clear drift. Fig. 9B illustrates a polarizationreference signal (the same one which started out as a L polarization). A simulated test signal starts out a V polarization. The test signal was corrected (controlled) by a 2-channel PID using H / V and D / A measurements on the single polarization reference signal. As can be seen, the polarization of the polarization reference signal was maintained relatively stable as compared with Fig. 9A and the uncontrolled drift. However, the two measurements on one polarization reference signal do not achieve the complete polarization channel stability for any input polarization. There is ambiguity for the rotations along the great circle with the R rotational axis which is seen in the curve for the test signal. In contrast, as illustrated in Fig. 9C, there is minimal drift in the test signal when two polarization reference signals are used. In Fig. 9C, the two polarization reference signals one starting at an L polarization (Reference 1) and one starting at an A polarization (Reference 2) and the test signal started at a V polarization as shown. The V polarization test signal was corrected (control) using a 3-channels PID with H / V and D / A basis measurements on Reference 1 and H / V basis measurement on Reference 2. All three (Reference 1, Reference 2 and Test signal) experienced minimal drift. This suggests that for a complete polarization channel stability, such as for quantum applications, two polarization reference signals should be used, however, for SOP, one polarization reference signal may be used, such as described herein.

[0109] Figs. 10A and 10B illustrate additional simulation results showing the difference in the PID outputs without slope sign in error and with the slope sign in error. The same two polarization reference signals were used. Here a random walk bias of 7t / 5000 step size was used. The outputs from three PIDs are shown. PID_HV is based on the measurement of Reference 1 in the H / V basis, PID_DA is based on the measure of Reference 1 in the D / A basis and PID_HV2 is based on the measurement of Reference 2 in the H / V basis. Fig. 10A illustrates a range limitation of about it due to a sign change of the slope of the Stokes measurement as a function of the VWP applied retardance as the polarization would drift around the sphere which is reflected by the nearly discontinuous jumps in Fig. 10A. The range limitation may not be an issue for a small drift range, but if wrapping around the Poincare sphere is expected then, in accordance with aspects of the disclosure, the range is extended using the slope sign in error. As illustrated in Fig. 10B, there are no discontinuous jumps when using the slope sign in error.

[0110] Figs. 11A-1 ID also illustrate the effect of the slope sign in error. Figs. 11A-1 ID illustrate the results in the Poincare sphere as opposed to the PID outputs. Fig. 11 A illustrates polarizationreference signal (Reference 1 ) which started at an L-polarization having a simulated uncontrolled polarization drift. The simulation used a multi-coordinate biased random walk of 7t / 360 step sizes controlling the phase and angle of several VWPs. Fig. 11B illustrates two polarization reference signals (Reference 1, starting at L polarization and Reference 2, starting at A polarization) and a test signal starting at a V polarization. The polarization is corrected (controlled) by 3-channel PID using measurements on Reference 1 in both the H / V basis and the D / A basis and a measurement of Reference 2 in the H / V basis with the same simulated drift. In Fig. 11B no slope sign in error was used. Fig. 11C illustrates the same Reference signals and Test Signal based on the same measurements but with slope sign in error in the PIDs. Fig. 1 ID illustrates the same Reference signals and Test signal based on the same measurements and using slope sign in error in the PIDs, but the simulated drift was using random walk with smaller K / 5000 step sizes. Figs. 11C and 1 ID illustrate the improvement in the polarization control using the slope sign in error verses without. Fig. 1 ID illustrates that the error may be further reduced with a smaller random walk step size, which demonstrates that if the Stokes measurement’s SNR and the precision in the polarization-compensation module 150 is good, then a precise control may be achieved by the APC.

[0111] The impact of APC in accordance with aspects of the disclosure was tested on a classical communications signal (e.g., polarized communications signal). In the test, the classical communications signal was a C-band signal. A polarization drift was induced using an external fiber-squeezer device before the receiver with APC. A test signal laser with wavelength 1559.75 nm was multiplexed with the C-band APC using reference wavelength 1558.2 nm and the two signals propagated together through a 3.5-km fiber. After the signals propagated through the communications channel, controlled polarization drift was induced using an fiber-squeezer device, then the test signal was split 50 / 50 using a fiber fused coupler where half went to a polarizer to be detected by a power meter (Thorlabs PM101A, S154C) while the other half went through the receiver with APC in accordance with aspects of the disclosure for correction and then went to a polarizer to be detected by a power meter (Thorlabs PM101A, S154C). Both power meter analog outputs were then recorded with data acquisition hardware (Liquid Instruments Moku:Go).

[0112] Fig. 12A provides calibration of a characteristic oscillation range for both test signals (polarized communications signa) (with the APC off (disabled) in this case). After engaging theAPC to correct for the induced polarization drift, where the PTDs have a 1 Hz integration bandwidth and the induced polarization drift was 10 mHz, Fig. 12B illustrates a substantial difference in the power. The orange is with the APC and the blue is without. The APC with 1-Hz PID integration bandwidth tracks a 10-mHz polarization drift without issue. It is noted that there is slightly increased polarization noise on the APC-corrected test signal compared to the non- APC test signal. This increased polarization noise is somewhat due to the inherent nature of close-loop control which will necessarily have at least slightly higher noise compared to a stable uncontrolled signal. Additionally, there is excess polarization noise due to the imperfect frequency stabilization. With a 1-Hz PID integration bandwidth, most of the residual frequency- induced power variation is averaged out but not all is averaged out leading to increased polarization noise. This is more pronounced in the C-band testing because the lasers used for the reference and laser used for LO were not manufacture calibrated for no frequency drift. However, in accordance with aspects of the disclosure, the lasers may be calibrated for minimal frequency drift.

[0113] The induced polarization drift increased to 100 mHz while the integration bandwidth was maintained. Fig. 12C shows a difference in the tracking. 1-Hz PID integration bandwidth is not able to fully track at 100-mHz variation though there is still significant correction applied compared to the uncorrected signal. Orange is APC corrected where the blue is uncorrected.

[0114] The PID integration bandwidth was increased to 1 kHz and different induced polarization drifts were applied. In Fig. 12D, the induced polarization drift was 100 mHz; in Fig. 12E, the induced polarization drift was 1 Hz; and in Fig. 12, the induced polarization drift was 10 Hz. Figs. 12B-12F demonstrate that for complete tracking, the drift should be much less than the PID integration time. For example, the expected drift should be about 100 times less than the integration bandwidth (the integrator gain is variable with the frequency, lower at higher frequencies). The 1-Hz control bandwidth may be sufficient in most cases for few kilometer lengths of in-ground fiber but if higher- speed drift is expected, the integration bandwidth can be increased, at the cost of transferring more of the frequency-induced power variation into polarization noise onto the polarized communications signal.

[0115] Another test was done to confirm the polarization stabilization in the communications channel 15 for a quantum signal (such as entangled photons). To characterize the APC using entanglement-assisted process tomography, a 100-GHz channel from the entanglement sourcewas transmitted, centered on 1579.6 nm, through L-band APC transmitter (with references at 1581.2 nm) then through a 5-km spool heated by variable electronics waste heat (computer fan) followed by the APC receiver and then to the polarization analyzer for tomography measurement. The other corresponding entangled photons (a 100-GHz channel, centered on 1559.8 nm) were measured by the polarization analyzer directly after an WSS as a reference. The set up is shown in Fig. 7 of U.S. Provisional Application No. 63 / 723,964, which is incorporated by reference.

[0116] Entanglement-assisted process tomography is a special case of ancillary assisted process tomography.

[0117] A series of tomographies with the APC enabled were measured. For tomographies measured with the APC enabled, the relative process fidelity (RPT) between Cn and Cn+1 are shown in Fig. 13A and between Cl and Cn are shown in Fig. 13B. The RPTs were calculated using a generalized Ulhmann fidelity. C is a Choi matrix.

[0118] For the first two-qubit tomography measured in a series, a reference tomography was used to estimate state imperfections. Using that reference tomography, the second measured tomography was analyzed providing the first estimated process.

[0119] RPT stabilized through both comparisons due to the action of the APC stabilizing the complete polarization channel. For successive tomographies, the measured average relative process fidelity is 0.96 +- 0.01 measured over 4.5 hrs. Moreover, the measured average relative process fidelity between Cl and Cn is 0.94 +- 0.03. This decrease (increase) in fidelity average (standard deviation) and the slight tilt in Fig. 13B may be attributed to calibration drift from a passively temperature-stabilized measurement system and there may be a contribution from polarization-mode dispersion as well. Comparing the calibrations done in successive days, shows about 3 - 5° shift in the optimal waveplate angles from which a few percent error due to the calibration drift is estimated. The rest is likely due to polarization-mode dispersion. This suggested that for longer distributions, the calibration process may need to be repeated.Additionally, active temperature stabilization may be used.

[0120] Another test was done to confirm the polarization stabilization for a quantum communications system with two quantum receivers in different locations.

[0121] A WSS in the entablement source was configured to transmit a 100-GHz channel from the entanglement source, centered on 1579.6 nm, to a L-band APC transmitter (with references at1581.2 nm) and corresponding entangled photons (another 100-GHz channel, centered on 1559.8 nm) to a C-band APC transmitter (with references at 1558.2 nm). The L-band APC transmitter and the C-band APC transmitter are examples of transmitters 10A, 10B. Each is then distributed across the network (communications channel) receiving the APC correction after transmission. The L-band APC transmitter and the L-band APC receiver were 3.2 km away from each other and connected via SML and the C-band APC transmitter and the C-band APC receiver was 3.5 km away from each other and connected via SML. The C-band photons were then transmitter 1.3 km to another quantum hub for single-photon detection.

[0122] After enabling each APC, a series of two-qubit polarization tomographies were repeatedly measured one after another and analyzed. The APC operated continuously — correcting the polarization for over 30 hours — until one of the K- range limits was hit then the APC relocked to another fringe. In this test, the slope sign in error was not used. During this period, 95 successive two-qubit tomographies (each taking about 16-17 minutes) were collected. In Pigs. 12A, 12B, the relative generalized Ulhmann fidelity for successive estimated two-qubit states and the relative fidelity between the first estimated state and successive states are shown. The integration time per measurement setting was 15s using a 660ps coincidence window, collecting an average of 310 coincidence counts over 36-setting tomography. Due to the low coincidence rate, there is noticeable noise from statistical noise. Despite that, Pig. 12A illustrates a flat trend over the >30-hour period with an average relative fidelity of 0.96 +- 0.02 between successive estimated states implying the APC managed the stability on the order of two tomographies throughout this period. There may have been a drift on the time scale of two tomographies but not necessarily large enough to be seen in the graph with the statistical noise present. Thus, Pig. 12A demonstrates that two independent APC have inherent stabilization on the time scale of a half hour to over 30 hours.

[0123] Pig. 12B illustrates the relative fidelity between the first estimate state and successive states. On this time scale (>30 hrs), there was a significant polarization drift corrected by the APC outputs. The average relative fidelity was 0.94 +- 0.03. The decrease (increase) in average (standard deviation) of relative fidelity can be seen graphically by the slight trend down in Fig. 12B. There may also be an increase in standard deviation over time, but it is uncertain whether that is due to increased sample numbers or an actual change in the standard deviation over time.Overall, the fidelity was held stably by two independent APC for entanglement distribution for over 30 continuous hours without interruption.

[0124] In other aspects of the disclosure, multiple sets of polarization references signals may be sent along with the polarized communications signal to minimize the adverse effects of the PMD. Above, the adverse effects of PMD were minimized by using a small offset between the polarization reference signals and the polarized communications signal. In accordance with other aspects of the disclosure, one set of polarization reference signals may be used which is below, in wavelength, of the polarized communications signal and another set of polarization reference signals may be used which is above, in wavelength, of the polarized communications signal. In accordance with this aspect, the wavelength of the polarized communications signal may be centered with respect to the sets of polarization reference signals, e.g., the offset straddle the polarized communications signal. The multiple sets of polarization reference signals may be used in application specific situations such as where the communications channel is long (e.g., long SMF) or in a case where the bandwidth prohibits the small offset as described above. The use of multiple sets of polarization reference signals enables the wavelength offset between the polarization reference signals and the polarized communications signal to be larger. For example, in a case where the polarized communication signal is at 1550nm, the sets of polarization reference signals may be +-5 nm from the signal.

[0125] Fig. 15 illustrates a block diagram of a transmitter 10A’, 10B’ having the APC in accordance with aspects of the disclosure where multiple sets of polarization reference signals are transmitted. Transmitter 10A’, 10B’ differ from transmitter 10A, 10B in that transmitter 10A’. 10B’ have two light sources 100A, 100B, which are distinct. Like the light source 100, the first light source 100A and the second light source 100B may be a frequency stabilized laser. Each frequency stabilized laser may be tunable. For example, the first light source 100A may be tunable to a wavelength W. which is above, the wavelength of the polarized communications signal and the second light source 100B may be tunable to a wavelength -W, which is below, the wavelength of the polarized communications signal. In transmitter 10A’, 10B’, the noise eater(s) 1500A, 1500B are separate from the attenuation module (e.g., 110’). This is because, each light source 100 A, 100B may have a different power variation and the NE 1500 A, 1500B provide the variable power attenuation separately for each light source 100 A, 100B.

[0126] Each light source 100A, 100B is also separately filtered such as by using a pedestal filter 102A, 102B. Similar to above, each pedestal filter 102A, 102B may have multiple wavelength division multiplexing elements such as DWDMs. In some aspects, each DWDM may have a 100-GHz passband. The channel used for the different light sources 100A, 100B may be different. For example, for light source 100A, the channel may be Czz and for light source 100B, the channel may be Cxx. Light from both light sources 100 A, 100B (after filtering) is guided to a beam splitter 104, which separates the light into two paths for the first reference signal generating module 106 A and the second reference signal generating module 106B, respectively.

[0127] In an aspect of the disclosure, the first reference signal generating module 106A and the second reference signal generating module 106B may comprise the same components as described above. Another beam splitter 108 (combiner) combines the light output from the first reference signal generating module 106 A and the second reference signal generating module 106B also in a similar manner as described above. Transmitters 10A’, 10B’ may also comprise calibration optics 109 to ensure that the polarization reference signals, within each set are generally orthogonal.

[0128] The generated sets of polarization reference signals are focused by a lens into an attenuation module 110’. The attenuation module 110’ differs from attenuation module 110 in that the NE is removed from the attenuation module 110’. The attenuation module 110’ includes an attenuator configured to reduce the power level of the sets of polarization reference signals to a target such that the first and second polarization reference signals (in each set) have a minimal impact on the polarized communications signal. The attenuator may be the same as described above. Although, the attenuation may be less since the offset may be greater.

[0129] The sets of polarization reference signals are combined with the polarized communications signals. In some aspects, the combination uses a WDM element 112. For example, the WDM element 112 may be a DWDM such as described above. However, the DWDM may be connected in reverse. For example, in Fig. 6, the obtained polarized communications signal is input to the express port of the DWDM and the first and second polarization reference signals are supplied to a channel port (Cxx). In this aspect of the disclosure, the obtained polarized communications signal is input to a channel port (Cyy), and the sets of polarization reference signals are input to the express port.

[0130] Transmitters 10A’, 10B’ launch the combined polarized communications signal and the sets of polarization reference signals to the communications channel 15.

[0131] Fig. 16 illustrates a block diagram of a receiving node (e.g., receivers 20A’, 20B’) having the APC in accordance with this embodiment where multiple sets of polarization reference signals are transmitted. The receivers 20A’, 20B’ differ from receivers 20A, 20B in that receivers 20 A’, 20B; have two LO modules (first LO module 156A and a second LO module 156B) and two LO feedback modules 170A, 170B. Both the first LO module 156A and the second LO module 156B generate an LO used in both the first and second heterodyne modules 158A, 158B.

[0132] The first LO module 156 A and the second LO module 156B each comprise a frequency stabilized laser. In some aspects, the frequency stabilized laser in the first LO module 156 A and the second LO module 156B is the same as the laser used as the source for the sets of polarization reference signals. In an aspect of the disclosure, the first LO (output from the first LO module 156A) has a frequency offset Y1 from the polarization reference signal which is not frequency shifted in a first set of the sets of polarization reference signals. In some aspects of the disclosure, the frequency offset may be 70MHz. In an aspect of the disclosure, the second LO (output from the second LO module 156B) has a frequency offset Y2 from the polarization reference signal which is not frequency shifted in a second set of the sets of polarization reference signals. Y2 is different from Yl. In some aspects, the difference between Y2 and Y 1 may be between 20MHz and 40MHz such that the two LOs may be distinguished in subsequent filtering and processing. To initially set the two LOs frequencies (and offsets), the first light source 100A and the second light source 100B is turned ON. and the first LO module 156A is turned ON (with the second LO module 156B turned OFF). The frequency of the LO is adjusted so that when the LO is reduced in frequency, the heterodyne signal gets bigger. The frequency of the LOs should be less than the lowest detection peak in the output from the first heterodyne module 106A. With the first LO module 156A still being ON, the second LO module 156B is turned ON. A similar adjustment to the LO frequency is made, e.g. lower in frequency than sets of polarization reference signals (and at the same time offset from the LO generated by the first LO module 156A).

[0133] The output of the first LO module 156A and the second LO module 156B is split using a beam splitter 168 to supply the LOs to the first heterodyne module 158A and the second heterodyne module 158B.

[0134] To minimize the frequency drift in the LOs, each has LO feedback modules 170A, 170B. Receivers 20A’, 20B’ may have an extra power splitter 160C connected to power splitter 160A to generate the two inputs for the feedback.

[0135] The input in each path may be filtered. For example, each LO feedback module 170A, 170B may comprise a pair of Band Pass filters (BPF). The passband for the BPFs is based on the separation between the two LOs. For example, the BPFs in the first LO feedback module 170A are configured to allow detections related to the first LO (output from the first LO module 156 A) and reject detections related to the second LO (output from the second LO module 158B). The BPFs in the second LO feedback module 170B are configured to allow detections related to the second LO (output from the second LO module 156B) and reject detections related to the first LO (output from the first LO module 158A). For example, in the case where the difference between Y1 and Y2 is between 20-40 MHz, then the passband may be +- 20 MHz, +-10 MHz or +-5MHz. Each LO feedback module 170 A, 170B may comprise an amplifier connected to the output of the BPFs. Similar to above, each LO feedback module 170A, 170B counts zerovoltage threshold crossings to approximately determine the frequency. This may be performed by an oscilloscope. Each LO feedback module 170A, 170B further comprises a processor, such as a microcontroller that polls the oscilloscope for the determined frequency. Based on the determined frequency, the processor controls the respective LO module 156A, 156B. In some aspects of the disclosure, the control may use a PID, where the setpoint is the initial offset (Y 1 or Y2) and the error is the difference between the determined frequency and the initial offset.

[0136] Additionally, the filters in the RF signal processing modules 162A’, 162B’, 162C’ may be different from the filters in the RF signal processing modules 162A, 162B, 162C described above due to the different sets of polarization reference signals (and wavelengths thereof). For example, in RF signal processing modules 162A’, 162B’, two band pass filters (BPFs) may be used instead of one BPF and one low pass filter (LPF) in the RF signal processing modules 162A, 162B. In RF signal processing module 163’, two band pass filters (BPFs) may be used instead of one BPF and one high pass filter (HPF). For example, the BPFs in RF signal processing modules 162A’,162B’ may have a passband of 10-200MHz and the BPFs in RF signal processing module 163’ may have a passband of 200-400MHz.

[0137] In an aspect of the disclosure, the calibration module 172’ may include an additional processor configured to tune one of the LO modules (the first LO module 156 A or the second LO module 156B) to emit light at the average of the two LO wavelengths. This enables the LO emitted during calibration to match a signal wavelength. Other than the averaging, the calibration method and components are the same as described above. The additional processor may be the microprocessor used in one of the first or second LO feedback modules 170A, 170B.

[0138] In other aspects, both LO modules (first LO module 156A and the second LO module 156B) may be used in the calibration.

[0139] In an aspect of the disclosure, the detection signals (from the sets of polarization reference signals) may be averaged. The averaging minimizes the effect of the PMD on the correction. For example, in an aspect of the disclosure, the feedback modules described above may be modified to average the detection signals. In a case where each loop includes a PID control as the feedback modules 166A’, 166B, 166C’, respectively, the PID may add a digital gain to implement the average prior to calculating the error.

[0140] The first heterodyne module 158 A outputs a detection signal which is a sum of measurements mixed with the first LO (having a first frequency, e.g., Yl) and measurements mixed with the second LO (having a second frequency, e.g. Y2). The summed detection signal will have mixed frequencies Yl and Y2). The measurements are in a polarization basis.

[0141] The second heterodyne module 158B outputs a first detection signal which is a sum of measurements mixed with the first LO (having a first frequency, e.g., Y 1) and measurements mixed with the second LO (having a second frequency, e.g. Y2). The measurements are in different polarization basis. The measurements are with respect to frequency unshifted polarization reference signal in each set. The summed detection signal will have mixed frequencies Yl and Y2).

[0142] The second heterodyne module 158B also outputs a second detection signal which is a sum of measurements mixed with the first LO (having a first frequency, e.g., Yl) and measurements mixed with the second LO (having a second frequency, e.g. Y2). Here the measurements may be based on the frequency shifted polarization reference signal in each set(X). These detection signals from the second heterodyne module 158B are separated by respective BPFs in the RF signal processing modules 162B’, 162C’

[0143] After respective filtering in each RF signal processing module 162A’, 162B’, 162C’, power of the respective summed detection signal is determined by a power detector, in a similar manner as described above. Each digital filter module 164A, 164B, 164C converts the analog detection (power detected of the summed detection signal) into a digital value for further processing by sampling such as using Butterworth low-pass filter.

[0144] The feedback modules 166A’, 166B’, 166C’ divide the detected power of the summed detection signal (digital value) by two to complete the average. This is treated as the current measurement. In an aspect of the disclosure, where the feedback modules 166A’, 166B’, 166C’ implement a PID control, the current measurement is subtracted from the setpoint. In some aspects, the slope sign in error as described above may be used.

[0145] The feedback modules 166A’, 166B’, 166C’ control the polarization-compensation module 150 in a similar manner as described above. For example, the PID control may issue a signal to circuitry in the polarization-compensation module 150 to rotate the polarized communications signal (and the sets of reference signals).

[0146] The receivers 20 A’, 20B’ may include a similar demultiplexing module 152 as described above. However, in some aspects, the elements may be connected differently. For example, as described above, the OADM 152-1 may be connected such that the polarized communications signal is output from the OUT port and the first and second polarization reference signals are output from a channel port (Cxx) (DROP port), in receivers 20 A’, 20B’, the sets of polarization reference signals may be output from the OUT port and the polarized communications signal may be output from the channel port (Cyy) (DROP port). OADM 152-2 may be similarly connected.

[0147] The other components of receivers 20A’, 20B’ are the same and configured similar to above.

[0148] Figs. 17A-17I illustrate the results of a simulation based on a SMF which is 5 km long. Each figure (Fig. 17A-17I) is a Poincare sphere representation of the polarization as a result of the simulation. The simulated PMD was 0.04ps / km1 / 2. Several polarization-dependent phase shifts interspersed with polarization rotations driven by pseudo-random sampling were applied. Figs. 17A, 17D, and 17G illustrate uncorrected drift. Fig. 17B illustrates polarizationcompensation with first and second polarization reference signals such as described in Figs 2-3, at 1 nm offset. Fig. 17C illustrates polarization compensation with two sets of polarization reference signals straddling a polarized communications signal at 1 nm offset. Fig.l7F illustrates polarization compensation with two sets of polarization reference signals straddling a polarized communications signal at 4 nm offset. Fig. 17E illustrates polarization compensation with first and second polarization reference signals such as described in Figs. 2 and 3, at 4 nm offset. Fig. 17H illustrates polarization compensation with first and second polarization reference signals such as described in Figs. 2 and 3, at 8 nm offset. Fig. 171 illustrates polarization compensation with two sets of polarization reference signals straddling a polarized communications signal at 8 nm offset.

[0149] As shown, as the offset from the polarized communications signal increases, the improvement by using the straddle increases. For example, at a 1 nm offset, two sets of polarization reference signals straddling a polarized communications signal do not make a significant improvement from only using the first and second polarization reference signals. At an 8 nm offset (Fig. 17H v. Fig. 171), there is a significant improvement in the compensation. Although, the overall compensation when 8 nm is used v. 4 nm is worse.

[0150] In other aspects, additional sets on both sides of the polarized communications signal may be used.

[0151] In other aspects of the disclosure, instead of combining the polarization reference signal(s) with a polarized communications signal using wavelength division multiplexing (such as by using a wavelength offset), the polarization reference signal(s) are combined with a polarized communications signal using time division multiplexing. Advantageously, this allows for the wavelength of the polarization reference signal(s) to be the same as the wavelength of the polarized communications signal. This enables polarization compensation without any effects from PMD.

[0152] Fig. 18 illustrates a block diagram of a transmitter 10A”, 10B” having the APC in accordance with aspects of the disclosure where the polarization reference signal(s) are time multiplexed with a polarized communications signal for launching into a communications channel 15. Transmitter 10A”, 10B” differ from transmitters 10A, 10B (and 10A’, 10B’) in that transmitter 10A”, 10B” have a pulsed light source 100” instead of a continuous light source 100, 100A, 100B. In some aspects, the pulsed light source 100” may be a tunable pulsed laser.Similar to above, the laser (pulsed) may be frequency stabilized. The tunable pulsed laser may be set to a wavelength of a polarized communications signal. In some aspects, the source of the polarized communications signal will transmit a signal indicating the wavelength of the polarized communications signal to a processor which controls the tunable pulsed laser. The processor then controls the tunable pulsed laser to emit pulses at the wavelength.

[0153] In an aspect of the disclosure, since the polarized communications signal is time multiplexed with the polarization reference signal(s) the timing of the emission of the pulses from the laser and the polarized communications signal are coordinated. The clocks of the processor which controls the pulsed light source 100” and the clock used to generate the polarized communications signal are synchronized. In some aspects of the disclosure, a signal synchronization clock may be used. A master, such as the source of the polarized communications signal may issue a signal synchronization clock to the transmitters 10A”.10B”, and specifically to a processor which controls the light source 100”. As will be described later, the master may also transfer a signal synchronization clock to the receivers 20A”, 20B” also to coordinate the timing between the transmitters 10A”, 10B” and the receivers 20A”, 20B”, e.g., the clocks are synchronized.

[0154] Other clock synchronization methods may also be used. For example, global positioning system (GPS) signals may be received by each device (source, transmitters 10A”, 10B” and 20A”, 20B”) for synchronization.

[0155] Additionally, to assure that the polarization reference signal(s) do not overlap in time with the polarized communications signal at the time division multiplexing element 112’, the signal synchronization clock may include a trigger for generating the pulse(s) from the light source 100”. In some aspects, the source outputs the trigger signal with a time offset from the timing of the polarized communications signal. The time offset may be based on the distance between the source of the polarized communications signal and the TDM 112” and the distance between the light source 100” and the TDM 112” to account for the delays in the propagation of the light through the fibers and free space. This enables the polarized communications signal and the polarization reference signal(s) to have separate time slots in the communications channel 15. In response to the receipt of the trigger signal, the processor controls the light source 100” to emit the pulse.

[0156] In other aspects of the disclosure, instead of a trigger signal being included in the signal synchronization clock, the source may include a timing of the polarized communications signal (e.g., assigned time slot for the polarized communications signal) or the assigned time slot for the polarization reference signal(s). In response to receipt of this timing signal (time slot information), the processor may determine the timing for controlling the light source 100” to be offset from the assigned time slot for the polarized communications signal (or the time slot assigned to the polarization reference signal(s)) based on the distance between the light source 100” and the TDM 112” and control the light source 100” to emit the pulse(s) based on the determination.

[0157] In a case where the polarized communications signal is periodically transmitted and / or transmitted in multiple time slots, one timing signal may include the timing information for the multiple time slots or multiple timing signals may be used, each having the timing associated with a time slot.

[0158] Since the polarization reference signal(s) and the polarized communications signal are time multiplexed, transmitters 10A”, 10B” need not have the pedestal filter to remove overlap as the wavelength overlap in this aspect is intentional. However, in some aspects, there may be an optical filter between the light source 100” and the beam splitter 104 to remove noise.

[0159] Transmitters 10A”, 10B” also comprise first reference signal generating module 106A and the second reference signal generating module 106B and another beam splitter 108 (combiner).

[0160] In an aspect of the disclosure, the first reference signal generating module 106 A and the second reference signal generating module 106B may comprise the same components as described above. Another beam splitter 108 (combiner) combines the light output from the first reference signal generating module 106 A and the second reference signal generating module 106B also in a similar manner as described above. Transmitters 10 A’, 10B’ may also comprise calibration optics 109 to ensure that the polarization reference signals have target polarizations.

[0161] Transmitters 10A”, 10B” may also comprise an attenuation module 110. In some aspects, the attenuation module 110 may comprise a NE and an attenuator such as described above. The NE is configured to output a fixed power level regardless of a variation in the power level of the input to the NE (light source 100”). The attenuator may be configured to reduce the power level of the polarization reference signal(s) to a target such that the polarization referencesignal(s) have a minimal impact on the polarized communications signal. The amount of attenuation may be less since the polarized communications signal is offset in time from the polarization reference signal(s).

[0162] Transmitters 10A”, 10B” also comprises the TDM element (TDM 112”). In an aspect of the disclosure, the TDM element may be a passive element or an active element. For example, the passive element may be a beam splitter such as a fused fiber coupler. A portion of the fused fiber coupler may be connected in the optical path of the polarization reference signal(s) and a portion of the fused fiber coupler may be connected in the optical path of the polarized communications signal. A SMF may connect the attenuation module 110 to the fused fiber coupler. Similarly, a SMF may connect the source of the polarized communications signal to the fused fiber coupler. The output of the fused fiber coupler may be connected to the communications channel 15. In a case where the channel is free space, the output of the fused fiber coupler may be collimated prior to being launched into free space.

[0163] The active element may be an optical switch. For example, the optical switch may be MEMS based. In some aspects, the optical switch may be controlled by a processor to control the input (e.g., path for the polarization reference signal(s) or the path for the polarized communications signal). In this aspect of the disclosure, the processor would also receive the signal synchronization clock such that its clock may be synchronized to the master (e.g., source) and the processor controlling the light source 100”. In some aspects of the disclosure, this processor may also receive time slot information for the polarization reference signal(s) and / or the polarized communications signal from either or both the source of polarized communications signal and the processor controlling the light source 100” (not shown in Fig. 18).

[0164] The processor controls the optical switch based on the received information (e.g., time slots).

[0165] In some aspects, the processor which controls the light source 100” may be the same processor which controls the optical switch.

[0166] The switch is not limited to being a MEMS and other optical switches may be used such as electro-optic switches.

[0167] Transmitters 10A”, 10B” launch the combined polarized communications signal and the polarization reference signal(s) to the communications channel 15. When there are two polarization reference signals a first polarization reference signal may be output by the firstreference signal generating module 106A and a second polarization reference signal may be output by the second reference signal generating module 106B. Similar to above, the first polarization reference signal may be in a mutually unbiased basis with respect to the second polarization reference signal.

[0168] Fig. 19 illustrates a block diagram of a receiving node (e.g., receivers 20A”, 20B”) having the APC in accordance with these aspects where the polarization reference signal(s) are combined with the polarized communications signal via TDM.

[0169] Each receiver 20A”, 20B” comprises a polarization-compensation module 150, a demultiplexing module 152”, polarization-measurement modules (first and second) 154A, 154B, a LO module 156, a beam splitter 168, heterodyne modules (first and second) 158A, 158B, power splitters 160 A, 160B, RF signal processing modules 162 A, 162B, 162C (collectively RF signal processing module), digital filter modules 164A, 164B, 164C (collectively Digital filter module), feedback modules 166A, 166B, 166C (collectively feedback module), and LO feedback module 170 (and a calibration module 172).

[0170] The receivers 20 A”, 20B” differ from receivers 20 A, 20B (and receivers 20A’, 20B’) in the demultiplexing module 152”. For example, the receivers 20A, 20B (and receivers 20A’, 20B’) used WDM elements (such as the OADMs), but in accordance with aspects of the disclosure, the receivers 20A”, 20B” have a TDM element for demultiplexing. The TDM element may also be an active or passive element. For example, a similar optical switch may be used in the receivers 20A”, 20B”. In accordance with this aspect, the master (such as the source of the polarized communications signal may share the signal synchronization clock with the demultiplexing module 152” (and specifically with a processor controlling the optical switch). This way, the clock in the processor controlling the optical switch is synchronized with the clock in the processor (at the source) and the processor controlling the light source 100”.

[0171] Additionally, as noted above, the signal synchronization clock may comprise a trigger signal (time) for the light source 100” or the time slot for the polarized communications signal and / or the polarization reference signal(s). This timing information may also be shared with the demultiplexing module 152” (and specifically with a processor controlling the optical switch). The processor may control the optical switch based on the timing information (received) to separate the polarized communication signal from the polarization reference signal(s). The optical switch sends the polarized communications signal for further processing. The opticalswitch sends the polarization references signal(s) for measurement by the first and second polarization-measurement modules 154A, 154B.

[0172] When the optical switch is used in the demultiplexing module 152”, the other components in the receivers 20A”, 20B” operate in a similar manner to described above with respect to Fig. 3 (and the example illustrated in Fig. 6).

[0173] In other aspects, a beam splitter, such as a fused fiber beam splitter similar to described above may be used. One output the fused fiber beam splitter may be connected to the first and second polarization-measurement modules 154A, 154B (such as via another beam splitter, e.g., 502) via a lens (e.g., 500). The connection may be via a SMF. The other output may be connected to additional filtering. Since the first and second polarization reference signals may remain in the other output, the first and second polarization reference signals may be separated from the polarized communications signal using a time-domain filter 1900C prior to further processing of the polarized communications signal. The time-domain filter 1900C may be an optical switch and route the first and second polarization reference signals in a different path from the polarized communications signal. For example, the first and second polarization reference signals may be routed to another receiving node and multiplexed with another polarized communications signal. The another polarized communications signal may be generated by the receiving node. For example, a transmitter may transmit the first and second polarization references signals with a quantum signal (one of the photons of entangled photon pairs). The other photons of the entangled photon pairs) may be sent to a second receiving node. A receiver may receive the first and second polarization references signals and the quantum signal. The receiver uses the first and second references signals as described above for compensation and executes processing on the quantum signal. Since part of the first and second polarization reference signal can be separately routed from the quantum signal by the timedomain filter 1900C, this part may be recombined with the processed quantum signal and sent to the second receiver.

[0174] In this case, the same first and second polarization reference signals may be shared amongst multiple receiving nodes. Similar to above, the master (such as the source of the polarized communications signal may share the signal synchronization clock with a processor controlling the time-domain filter 1900C (e.g., optical switch). This way, the clock in the processor controlling the optical switch is synchronized with the clock in the processor (at thesource) and the processor controlling the light source 100”. Any timing information also included in the signal synchronization clock, e.g., trigger information, may also be shared and used to control the optical switch.

[0175] When a beam splitter (e.g., fused fiber beam splitter) is used in the demultiplexing module 152”, the receivers 20A”, 20B” may further comprise time-domain filters 1900A, 1900B at the output of the first and second heterodyne modules 158A, 158B. The time-domain filters 1900A, 1900B may be configured to remove any detection associated with the polarized communications signal in a case where the polarized communications signal is detected / measured by the combination of the first and second heterodyne modules 158 A, 158B and first and second polarization-measurement modules 154A, 154B. The time-domain filters 1900 A, 1900B may not be needed if the polarized communications signal is outside the detector’ s bandwidth and is low enough in power to not contribute a significant amount of its own shot noise. In a case where the polarized communications signal is a quantum signal such as a single photon, the time-domain filters 1900 A, 1900B may not be needed as it will not impact the measurement.

[0176] In some aspects of the disclosure, the time-domain filters 1900A, 1900B may be electrical switches which are controlled based on time. Each electrical switch may OPEN to isolate the detection results obtained from the first and second heterodyne modules 158 A, 158B from the RF signal processing modules 162A, 162B, 162C and downstream processing or to CLOSE to allow the detection results to be received by the RF signal processing modules 162A, 162B, 162C and downstream processing.

[0177] The electrical switches may be controlled by a processor. In an aspect of the disclosure, this processor may also receive the signal synchronization clock from the master (such as the source of the polarized communications signal). Additionally, as noted above, the signal synchronization clock may comprise a trigger signal (time) for the light source 100” or the time slot for the polarized communications signal and / or the polarization reference signal(s). This timing information may also be shared with the processor to control the electrical switch. The processor may control the switch based on the timing information (received) to isolate any potential detection of the polarized communications signal.

[0178] Thus, the feedback modules 166A, 166B, 166C do not receive any detection results based on the polarized communications signal.

[0179] Since the polarization reference signal (s) may be interleaved between the polarized communications signal, detections from polarization reference signal(s) at time (Tl) may be used by the feedback modules 166A, 166B, 166C to control the polarization-compensation module 150 to correct (control) the polarization of a subsequent received polarized communications signal (such as time T2 later than Tl) and later polarization reference signal(s) such as at time (T3. which is later than both Tl and T2).

[0180] The RF signal processing modules 162A, 162B, 162C, the digital filter modules 164A, 164B, 164C, the feedback modules 166A, 166B, 166C and the polarization-compensation module 150 are the same as described above with respect to Fig. 3 and function in a similar manner.

[0181] In an aspect of the disclosure, the LO module 156 used in the receivers 20A”, 20B” in a case where time multiplexing is used, may emit pulses. In this aspect, the pulses may be synchronized in time with the pulses of the polarization reference signals. For example, a processor which controls the LO module 156 may receive the signal synchronization clock from the master (such as the source of the polarized communications signal) and use information thereon to synchronize the pulses. For example, in a case where the signal synchronization clock includes timing information for the polarization reference signals, the processor may control the LO module 156 to emit a pulse at a time where both reach the first and second heterodyne detection modules 154A, 154B at the same time.

[0182] As described above, certain applications may use different polarization compensations including complete polarization channel compensation (such as quantum applications) and SOP compensation (such as classical applications).

[0183] Fig. 20 illustrates a block diagram for a transmitter 10A’” having the APC in accordance with aspects of the disclosure for SOP compensation. Here, only one polarization reference signal may be transmitted. Additionally, in this aspect of the disclosure, one transmitter 10A’” and one receiver node (e.g., 20A’”) may be included in the communications system. For example, the transmitter 10A’” may transmit a classical signal to receiver 20A’”. Since communication here may be classical and there is no entanglement, there may not be a need for pairs of transmitters and receiving nodes.

[0184] The transmitter 10A’” comprises a light source 100, a pedestal filter 102, a reference signal generating module 2000, an attenuation module 110 and a WDM element (WDM 112).The light source 100 may be a laser. Since the polarization reference signal and a polarized communications signal may be combined via wavelength multiplexing, the laser may be a continuous laser. In other aspects, the polarization reference signal and a polarized communications signal may be combined via time multiplexing (not shown in Fig. 20) and the laser may be a pulsed laser. The laser is power and frequency-stabilized. The laser may be a tunable laser to a target wavelength / frequency. In an aspect of the disclosure, when wavelength multiplexing is used, the target wavelength is relatively close to (but not overlapping) the wavelength / frequency of the polarized communications signal to reduce the impact of PMD on the APC (e.g., A8). The offset may be determined by wavelength or frequency. For example, in some aspects, the frequency offset may be 200 Ghz. Thus, the laser may be tuned based on the polarized communications signal. A processor (not shown) may be configured to tune the laser based on the wavelength / frequency of the polarized communications signal. The offset may be set in advance, and the processor may apply the set offset to tune the laser.

[0185] The pedestal filter 102 minimizes the wavelength / frequency overlap between the polarization references signal and the polarized communications signal. The pedestal filter 102 may comprise similar DWDM elements as described above. Since only one polarization reference signal is used in this aspect of the disclosure, the beam splitter 104 (and the additional beam splitter and combiner may be omitted). The pedestal filter 102 is connected to the light source 100 via a PMF.

[0186] The polarization reference signal is produced by the reference signal generating module 2000. The reference signal generating module 2000 is connected to the pedestal filter 102 via a PMF. The reference signal generating module 2000 may comprise a lens to collimate the light and a polarizer to output the light at a specific polarization. In some aspects of the disclosure, the reference signal generating module 200 may also comprise light rotating element(s). The light rotating elements may be based on the specific basis for the polarization reference signal. The polarization reference signal may be in one of three different polarization bases (linear, circular or diagonal basis). The light rotating element(s) may be a quarter waveplate and / or a half waveplate. In some aspects of the disclosure, the reference signal generating module 2000 may be configured to provide any of the three different polarization bases (one at a time) by making different rotations of the light using the light rotating element(s). For example, the transmitter 10A’” may comprise a processor configured to rotate one or more of the light rotating element(s)as needed to produce a polarization reference signal (in any one of the different polarization bases). In this aspect of the disclosure, the source of the polarized communications signal may notify the processor of what basis should be used for the polarization reference signal.

[0187] In other aspects, the specific basis of the polarization reference signal may be set in advance and maintained.

[0188] The components within the reference signal generating module 2000 may be in optical communication via free space.

[0189] The polarization reference signal may be focused into the attenuation module 110 using a lens. The attenuation module 110 may be similar to described above with respect to Fig. 3. The attenuation module 110 may comprise a NE and attenuator. The NE can output a fixed power level regardless of the power variation at the input. The NE may be configured for different wavelengths / frequencies. The attenuator in combination with the NE reduces the power level to a target such that the polarization reference signal has a minimal impact on the polarized communications signal (e.g., dim polarization reference signal). However, even though the polarization reference signal is “dim”, since the receiver 20A’” uses heterodyne detection, the polarization is able to be detected.

[0190] The attenuation module 110 may be connected to the WDM 112 via a SMF. The WDM 112 combines the polarization reference signal with the polarized communications signal for transmission over the communications channel 15.

[0191] The transmitter 10A’” launches the combined polarization reference signal and the polarized communications signal into the communications channel 15.

[0192] Fig. 21 illustrates a block diagram of a receiving node (e.g., receiver 20A’”) with the APC in accordance with aspects of the disclosure. Since there is only one polarization reference signal, one of the RF signal processing modules, digital filter modules, and feedback modules may be removed. This is because making two measurements in two different polarization bases on a polarization reference signal (in a third polarization basis) enables feedback in two different polarization bases. The specific basis of the removed modules is based on the polarization basis of the polarization reference signal. For example, when the polarization reference signal has a polarization basis of R, the modules associated with the feedback for the R basis may be omitted, (feedback for the H and D bases remain). For purposes of the description. Fig. 21 illustrates RFsignal processing modules 162A, 162B, digital filter modules 164A, 164B and feedback modules 166A, 166B. However, the removed modules may be the “A” or “B” modules.

[0193] Similarly, the polarization-compensation module 150”’ may have one or more channels removed. For example, as described above, the polarization-compensation module may comprise four fiber squeezers to rotate the polarization in the three different polarization bases. In this aspect of the disclosure, the polarization-compensation module 150”’ may rotate the polarization in two of the three polarization bases. Once again, the bases depend on the polarization basis of the polarization reference signal. Thus, in this aspect of the disclosed one or more of the fiber squeezers may be removed. For example, in a case where the feedback is for the H / V and D / A bases, the polarization-compensation module 150”’ may include two FS one for the H / V basis and another for the D / A basis. However, in a case where the feedback is for H / V and R / L, there may be three FS where one of the FS is used as a quarter waveplate. In a case where the polarization of the polarization reference signal changes (e.g., on-demand), during a calibration process, the polarization-compensation module 150’” may be configured as needed (e.g., FS calibrated) and the waveplates in the first and second polarization measurement modules 154A, 154B may be rotated to define the measurement basis.

[0194] A similar calibration process as described in Fig. 7 may be used and implemented by the calibration module 172’”. The calibration module 172’” may be similar as described above, however one wave generator may be omitted.

[0195] In some aspects, the power splitter 160B may be omitted.

[0196] Additionally, since there is only one polarization reference signal (and not another one frequency- shifted), the filtering in the RF signal processing modules 162A, 162B may be different. For example, certain filtering and passbands were included in order to differentiate detections / measurements from the different polarization reference signals, which are not needed. In some aspects, each RF signal processing modules 162A, 162B may include a single band pass filter (BPF). The BPF in each RF signal processing module 162A, 162B here, may have the same passband, such as 10-200 MHz.

[0197] The LO module 156 provides the LO for the first and second heterodyne module. In some aspects, the LO may be frequency offset from the polarization reference signal. A similar frequency offset, such as 70MHz may be used.

[0198] The first heterodyne module 158A produces a first detection signal based on the LO and a measurement of the polarization reference signal in a basis. The second heterodyne module 158B produces a second detection signal based on the LO and a measurement of the same polarization refence signal in a different basis. The bases of measurement are different from the basis of the polarization reference signal. Similar to above, the first detection signal may be used as input for the LO feedback module 170. The LO feedback module 170 may only have a bandpass filter (BPF) (the low pass filter may be omitted). The passband may be +-20 MHz.

[0199] The first detection signal is also input to the RF signal processing module 162A. The second detection signal is input to RF signal processing module 162B. In this aspect, the first and second detection signals may have the same frequency.

[0200] Each RF signal processing module 162 A, 162B detects the power of the respective detection signal (after amplification and filtering). The respective digital filter module 164A, 164B receives the detected power and converts into a digital value and the respective feedback module 166A, 166B controls the polarization-compensation module 150’” based on the digital value. Similar to above, in some aspects, the feedback modules 166A, 166B may implement a PID control (including slope sign in error).

[0201] In some aspects, instead of combining the polarization reference signal (one) with the polarized communications signal via WDM, the combination may be via TDM in a similar manner as described above and the receiver 20A’” may comprise TDM elements instead of WDM elements.

[0202] In other aspects, the transmitter may generate two polarization reference signals, however, the receiver may ignore one of the two polarization reference signals such as in the feedback modules.

[0203] In some aspects of the disclosure, the polarization reference signal or the first and second polarization reference signals or the sets of polarization reference signals as described herein may be shared between multiple receivers. For example, after the demultiplexing module 152 (such as wavelength demultiplexing, e.g., OADM) and prior to lens 500, a beam splitter may be positioned in the optical path to divide the light of the reference signal, e.g., such as 50 / 50. Half may be sent to the local measurements (first and second detection modules 154A, 154B in the receiver) and half may be sent to another receiving node for processing. The half which is sent to another receiving node, may be multiplexed with another polarized communications signal.

[0204] In accordance with a first aspect, a method comprising receiving, over an optical communications channel, a first set of polarization reference signals and a second set of polarization reference signals. The first set and second set are wavelength-multiplexed with a polarized communications signal having a wavelength (w). The first set of polarization reference signals are wavelength- shifted up relative to the wavelength (w) by an offset +AA and the second set of polarization reference signals are wavelength- shifted down relative to the wavelength (w) by the offset -AT. The first and second sets of polarization reference signals each have a first polarization reference signal and a second polarization reference signal where one of the first polarization reference signal and the second polarization reference signal in each set is frequency-shifted frequency with respect to the other by a first RF-offset X. The first polarization reference signal in each set is polarized in a first one of a set of three different polarization basis when launched into the optical communications channel. The second polarization reference signal in each set is polarized in a second one of a set of three different polarization basis when launched into the optical communications channel. The first one and second one are different. The method further comprises measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal in each set of polarization reference signals that was polarized in the first one of a set of three different polarization basis to produce a first control signal, controlling, based on the first control signal, the polarizations of the wavelength-multiplexed signals received, by using a polarizationcompensation module with a rotational axis about a third one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal in each set of polarization reference signals that was polarized in the first one of a set of three different polarization basis to produce a second control signal, controlling, based on the second control signal, the polarizations of the wavelength-multiplexed signals received, by using the polarization-compensation module with a rotation axis about the second one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal in each set of polarization reference signals that was polarized in the second one of a set of three different polarization basis to produce a third control signal and controlling, based on the third control signal, the polarizations of the wavelength-multiplexsignals received, by using the polarization-compensation module with a rotation axis about the first one of a set of three different polarization basis.

[0205] In accordance with a second aspect, the method according to the first aspect, wherein the offset +- AA is less than 4 nm.

[0206] In accordance with a third aspect, the method according to the first aspect or the second aspect, wherein the first control signal is based on an average of the measuring of the polarization of the first polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis, the second control signal is based on the average of the of the measuring of the first polarization reference signal in each set of polarization reference signals in third one of a set of three different basis, and the third control signal is based on the average of the measuring of the second polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis.

[0207] In accordance with a fourth aspect, the method according to the first aspect, the second aspect or the third aspect, where the method further comprises controlling a first frequency stabilized laser to produce light at the offset +AA relative to the polarized communications signal and controlling a second frequency stabilized laser to produce light at the offset -AA relative to the polarized communications signal.

[0208] In accordance with a fifth aspect, the method according to the fourth aspect, where the method further comprises pedestal filtering the light from each of the first frequency stabilized laser and the second frequency stabilized laser.

[0209] In accordance with a sixth aspect, the method according to the fourth aspect or the fifth aspect, where the method further comprises variably attenuating the light from each of the first frequency stabilized laser and the second frequency stabilized laser to produce a target power level for each.

[0210] In accordance with a seventh aspect, the method according to any one of the first to sixth aspects, where the method further comprises detecting the measuring in the third one of a set of three different polarization basis. The detecting comprises generating a first Local oscillator (LO) having a second RF-offset Y 1 from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals and generating a second (LO) having a third RF-offset Y2,different from Yl , from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals and performing heterodyne detection by mixing the first LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and mixing the second LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and mixing the first LO with the measurement of the second polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and mixing the second LO and the measurement of the second polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and detecting results thereof.

[0211] In accordance with an eighth aspect, the method according to the seventh aspect, where the method further comprises filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal in each set of polarization reference signals, filtering using a second set of filters the detecting results to obtain a second filtered detection result to isolate detection from the second polarization reference signal in each set of polarization reference signals, and detecting a power of the first filtered detection result and a power of the second filtered detection result.

[0212] In accordance with a ninth aspect, the method according to the eighth aspect, wherein the second control signal is based on the power of the first filtered detection result and the third control signal is based on the power of the second filtered detection result.

[0213] In accordance with a tenth aspect, the method according to any one of the first to sixth aspects, where the method further comprises detecting the measuring in the second one of a set of three different polarization basis. The detecting comprises generating a first LO having a second RF-offset Yl from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set polarization reference signals and generating a second LO having a third RF-offset Y2, different from Yl, from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals and performing heterodyne detection by mixing the first LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the second one of a set of threedifferent polarization basis and mixing the second LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and mixing the first LO with the measurement of the second polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and mixing the second LO and the measurement of the second polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and detecting results thereof.

[0214] In accordance with an eleventh aspect, the method according to the tenth aspect, where the method further comprises filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal in each set of polarization reference signals and detecting a power of the first filtered detection result. The first control signal is based on the power of the first filtered detection result.

[0215] In accordance with a twelfth aspect, the method according to the ninth aspect or the eleventh aspect, wherein the first control signal, the second control signal and the third control signal are generated using proportional integral derivative (PID) control based on an error signal determined from the power and a setpoint.

[0216] In accordance with a thirteenth aspect, the method according to the twelfth aspect, where the method further comprises modifying the error signal using a predefined sign function based on previous corrections and measurements.

[0217] In accordance with a fourteenth aspect, the method according to any one of the first to thirteenth aspects, wherein the three different polarization basis are mutually unbiased bases.

[0218] In accordance with a fifteenth aspect, the method according to any one of the first to fourteenth aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0219] In accordance with a sixteenth aspect, an optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), wavelength-multiplexed signals comprising a first set of polarization reference signals and a second set of polarization reference signals and a polarized communications signal having a wavelength (w). The first set of polarization reference signals are wavelength- shifted up relative to the wavelength (w) by an offset +AA and the second set of polarization reference signals are wavelength- shifted down relative to the wavelength (w) by the offset -AT. The first and second sets of polarization reference signals each have a first polarization reference signal and a second polarizationreference signal where one of the first polarization reference signal and the second polarization reference signal in each set is frequency- shifted frequency with respect to the other by a first RF- offset X. The first polarization reference signal in each set is polarized in a first one of a set of three different polarization basis when launched into the optical communications channel and the second polarization reference signal in each set is polarized in a second one of a set of three different polarization basis when launched into the optical communications channel. The first one and second one are different. The RX comprises a polarization-compensation module, a wavelength demultiplexer, first and second polarization-measurement modules, first and second LO modules, first and second heterodyne detection (HD) modules and a polarization-feedback module. The polarization-compensation module is configured to receive the wavelength- multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the wavelength-multiplexed signals about the three different polarization basis. The wavelength demultiplexer is configured to separate the polarized communications signal from the first set of polarization reference signals and the second set of polarization reference signals. The first polarization-measurement module is configured to measure a polarization of the first and second polarization reference signals in each set of polarization reference signals wavelength- multiplexed with the polarized communications signal in the second one of a set of three different polarization basis and issue a first measurement signal. The second polarizationmeasurement module is configured to measure a polarization of the first and second polarization reference signals in each set of polarization reference signal wavelength-multiplexed with the polarized communications signal in a third one of a set of three different polarization basis and issue a second measurement signal. The first LO module is configured to emit a first LO signal that is frequency offset from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals which is unshifted, by a second RF-offset Yl. The second LO module is configured to emit a second LO signal that is frequency offset from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals which is unshifted, by a third RF-offset Y2 different from YL The first HD module is configured to combine the first measurement signal with the first LO signal and combine the first measurement signal with thesecond LO signal and produce a first detector signal having portions corresponding to detections from the first polarization reference signal in each set of polarization reference signals and detections from the second polarization reference signal in each set of polarization reference signals. The second HD module is configured to combine the second measurement signal with the first LO signal and combine the second measurement signal with the second LO signal and produce a second detector signal having portions corresponding to detections from the first polarization reference signal in each set of polarization reference signals and detections from the second polarization reference signal in each set of polarization reference signals. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on an average of respective portions of the first detector signal and the second detector signal obtained by filtering.

[0220] In accordance with a seventeenth aspect, the RX according to the sixteenth aspect, wherein the portion of the first detector signal corresponding to detections from to the first polarization reference signal in each set of polarization reference signals is used to control the polarization-feedback module in the third one of a set of three different polarization basis, the portion of the second detector signal corresponding to detections from the first polarization reference signal in each set of polarization reference signals is used to control the polarizationfeedback module in the second one of a set of three different polarization basis, and the portion of the second detector signal correspond to detections from the second polarization reference signal in each set of polarization reference signals is used to control the polarization-feedback module in the first one of a set of three different polarization basis.

[0221] In accordance with an eighteenth aspect, the RX according to the sixteenth aspect or the seventeenth aspect, where the RX further comprises an RF-signal processing module configured to filter the first detection signal to isolate the portion corresponding to detections from the first polarization reference signal in each set of polarization reference signals from the portion corresponding to detections from the second polarization reference signal in each set of polarization reference signals and output a first power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal, filter the second detection signal to isolate the portion corresponding to detections from the first polarization reference signal in each set of polarization reference signals from the portioncorresponding to detections from the second polarization reference signal in each set of polarization reference signals and output a second power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal, and filter the second detection signal to isolate the portion corresponding to detections from the second polarization reference signal in each set of polarization reference signals from the portion corresponding to detections from the first polarization reference signal in each set of polarization reference signals and output a third power signal corresponding to a power level of the portion corresponding to detections from the second polarization reference signal.

[0222] In accordance with a nineteenth aspect, the RX according to the eighteenth aspect, where the RX further comprises a digital filtering module configured to low-pass filter the first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

[0223] In accordance with a twentieth aspect, the RX according to any one of the seventeenth to nineteenth aspects, wherein the polarization-feedback module comprises: a first PID controller configured to selectively control, based on the portion of the first detector signal corresponding to detections from the first polarization reference signal in each set of polarization reference signals, first rotations of the wavelength-multiplexed signals’ polarizations about the third one of a set of three different polarization basis, a second PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the first polarization reference signal in each set of polarization reference signals, second rotations of the wavelength-multiplexed signals’ polarizations about the second one of a set of three different polarization basis, and a third PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the second polarization reference signal in each set of polarization reference signals, third rotations of the wavelength- multiplexed signals’ polarizations about the first one of a set of three different polarization basis.

[0224] In accordance with a twenty-first aspect, the RX according to the twentieth aspect, wherein the first PID controller, the second PID controller and the third PID controller are configured to modify an input error signal using a predefined sign function based on previous corrections and measurements, the input error signal is a difference between a current measurement and a target setpoint

[0225] In accordance with a twenty-second aspect, the RX according to any one of the sixteenth to twenty-first aspects, wherein the polarization-feedback module comprises an array of fiber squeezers.

[0226] In accordance with a twenty-third aspect, the RX according to the twenty-second aspect, wherein the array of fiber squeezers comprises a first fiber squeezer for the second one of a set of three different polarization basis, a second fiber squeezer for the third one of a set of three different polarization basis, and a third fiber squeezer and a fourth fiber squeezer collectively calibrated for the first one of a set of three different polarization basis.

[0227] In accordance with a twenty-fourth aspect, the RX according to the twenty-third aspect, wherein the third fiber squeezer is configured as a quarter waveplate.

[0228] In accordance with a twenty-fifth aspect, the RX according to any one of the sixteenth to twenty-first aspects, wherein the polarization-feedback module comprises a plurality of sub- polarization-feedback modules including a first polarization-feedback module and a second polarization-feedback module where the first polarization-feedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

[0229] In accordance with a twenty- sixth aspect, the RX according to the twenty-fifth aspect, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM). The EOM is selected from a free space EOM or a waveguide EOM.

[0230] In accordance with a twenty- seventh aspect, the RX according to any one of the twenty- second to twenty-sixth aspects, wherein the first polarization-measurement module and the second polarization-measurement module comprise a motorized half-waveplate and motorized quarter waveplate and a polarizer.

[0231] In accordance with a twenty-eighth aspect, the RX according to the twenty- seventh aspect, where the RX further comprises a calibration module configured to back propagate either the first LO signal or the second LO signal having an average of the two wavelengths through each of the first polarization-measurement module and the second polarization-measurement module, a detector configured to detect the back propagated LO signal, and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

[0232] In accordance with a twenty-ninth aspect, the RX according to any one of the sixteenth to twenty-eighth aspects, where the RX further comprises an LO-feedback module configured to cause the first and second LO modules to stabilize, based at least in part on the first detector signal, the second RF-offset Fl at which the first LO module emits the first LO signal and the third RF-offset Y2 at which second LO module emits the second LO signal.

[0233] In accordance with a thirtieth aspect, the RX according to any one of the sixteenth to twenty-ninth aspects, where the RX further comprises a beam splitter configured to divide the first LO signal and the second LO signal to be supplied to each of the first HD module and the second HD module.

[0234] In accordance with a thirty-first aspect, the RX according to any one of the sixteenth to thirtieth aspects, wherein the three different polarization basis are mutually unbiased bases.

[0235] In accordance with a thirty-second aspect, the RX according to any one of the sixteenth to thirty-first aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0236] In accordance with a thirty-third aspect, a system comprising at least one RX of any one of the sixteenth to thirty-second aspects, and at least one TX optically coupled with a respective RX through the optical communications channel. Each TX comprises a first light source configured to emit light at a wavelength which is up shifted relative to a respective polarized communications signal by the offset +AA, a second light source configured to emit light at a wavelength which is down shifted relative to the respective polarized communications signal by the offset -AA, a first array of optical filters configured to filter the light emitted by the first light source and a second array of optical filters configured to filter the light emitted by the second light source, a reference signal generating module configured to produce from the light from the first light source the first set of polarization reference signals and from the light from the second light source the second set of polarization reference signals, and a wavelength division multiplexing element configured to multiplex the first and second sets of polarization reference signals with an obtained respective polarized communications signal. Each TX launches, into the optical communications channel, the wavelength-multiplexed signals.

[0237] In accordance with a thirty-fourth aspect, the system according to the thirty-third aspect, wherein each TX further comprises a first noise eater (NE) and a second NE. The first NE is configured to output a fixed power of light based on the light input from the first light source and the second NE is configured to output a fixed power of light based on light input from the second light source.

[0238] In accordance with a thirty-fifth aspect, the system according to the thirty-fourth aspect, wherein the reference signal generating module comprises a beam splitter configured to divide the light from each of the first and second light sources into a first light and a second light and a first reference signal generating module for providing the first polarization reference signal in each set of polarization reference signals and a second reference signal generating module for providing the second polarization reference signal in each set of polarization reference signals. The first reference signal generating module comprises a first lens, a first polarizer, a quarter waveplate, and a half waveplate. The second reference signal generating module comprises an RF-driven acousto-optic modulator (AOM) configured to frequency shift the second light to cause the first RF-offset X of the second polarization reference signal in each set relative to the first polarization reference signal, a second lens, and a second polarizer.

[0239] In accordance with a thirty-sixth aspect, the system according to the thirty-fifth aspect, wherein each TX further comprises an attenuation module configured to attenuate the first and second sets of polarization reference signals output from the first reference signal generating module and the second reference signal generating module, respectively, which is combined upstream of the attenuation module.

[0240] In accordance with a thirty-seventh aspect, the system according to any one of the thirty- third to thirty-sixth aspects, wherein the wavelength division multiplexing element is a dense wavelength division element (DWDM) configured to wavelength-multiplex the polarized communications signal, the first and second sets of polarization reference signals.

[0241] In accordance with a thirty-eighth aspect, the system according to any one of the thirty- third to thirty-seventh aspects, wherein respective polarized communications signals are a quantum signal.

[0242] In accordance with a thirty-ninth aspect, the system according to any one of the thirty- third to thirty-eighth aspects, wherein the first and second sets of polarization references signals is at a power level between -10 dBm to -100 dBm when launched.

[0243] In accordance with a fortieth aspect, a method comprising receiving, over an optical communications channel, wavelength-multiplexed signals comprising first and second polarization reference signals where one is shifted relative to the other by an RF-offset X. When launched into the optical communications channel, the first polarization reference signal is polarized in a first one of a set of three different polarization basis and the second polarization reference signal is polarized in a second one of a set of three different polarization basis. The first one and the second one are different. The method further comprises measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal that was polarized in the first one of a set of three different polarization basis set to produce a first control signal, controlling, based on the first control signal, the polarizations of the wavelength-multiplexed signals which also includes a polarized communications signal, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal polarized in the first one of a set of three different polarization basis to produce a second control signal, controlling, based on the second control signal, the polarizations of the wavelength-multiplexed signals, which also includes the polarized communications signal by using the polarization-compensation module with a rotational axis about the second one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal polarized in the second one of a set of three different polarization basis to produce a third control signal, and controlling, based on the third control signal, the polarizations of the wavelength-multiplexed signals which also includes the polarized communications signal by using the polarization-compensation module with rotational axis about the first one of a set of three different polarization basis. The first polarization reference signal and the second polarization reference signal are wavelength-shifted relative to the polarized communications signal by an offset AA.

[0244] In accordance with a forty-first aspect, the method according to the fortieth aspect, where the method further comprises controlling a frequency stabilized laser to produce light at the offset AA relative to the polarized communications signal.

[0245] In accordance with a forty-second aspect, the method according to the fortieth aspect or the forty-first aspect, where the method further comprises pedestal filtering the light from the frequency stabilized laser.

[0246] In accordance with a forty-third aspect, the method according to the fortieth aspect, the forty-first aspect or the forty-second aspect, where the method further comprises detecting the measuring in the third one of a set of three different polarization basis. The detecting comprises generating a LO having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the third one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the third one of a set of three different polarization basis and detecting results thereof.

[0247] In accordance with a forty-fourth aspect, the method according to the forty-third aspect, where the method further comprises filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal, filtering using a second set of filters the detecting results to obtain a second filtered detection result to isolate detection from the second polarization reference signal and detecting a power of the first filtered detection result and a power of the second filtered detection result.

[0248] In accordance with a forty-fifth aspect, the method according to the forty-fourth aspect, wherein the second control signal is based on the power of the first filtered detection result and the third control signal is based on the power of second filtered detection result.

[0249] In accordance with a forty- sixth aspect, the method according to any one of the fortieth to forty-second aspects, where the method further comprises detecting the measuring in the second one of a set of three different polarization basis. The detecting comprises generating a LO having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the second one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the second one of a set of three different polarization basis and detecting results thereof.

[0250] In accordance with a forty-seventh aspect, the method according to the forty-sixth aspect, where the method further comprises filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal and detecting a power of the first filtered detection result. The first control signal is based on the power of the first filtered detection result.

[0251] In accordance with a forty-eighth aspect, the method according to the forty-fifth aspect or the forty-seventh aspect, wherein the first control signal, the second control signal and the third control signal are generated using PID control based on an error signal determined from the power and a setpoint.

[0252] In accordance with a forty-ninth aspect, the method according to the forty-eighth aspect, where the method further comprises modifying the error signal using a predefined sign function based on previous corrections and measurements.

[0253] In accordance with a fiftieth aspect, the method according to any one of the fortieth to forty-ninth aspects, wherein the three different polarization basis are mutually unbiased bases.

[0254] In accordance with a fifty-first aspect, the method according to any one of the fortieth to fiftieth aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0255] In accordance with a fifty-second aspect, a RX for receiving, over an optical communications channel from TX, wavelength-multiplexed signals comprising a polarized communications signal, a first polarization reference signal, and a second polarization reference signal. The first and second polarization reference signals each are wavelength-shifted relative to the polarized communications signal by an offset AT, and where one is shifted relative to the other by an RF-offset X. When launched by the TX into the optical communications channel, the first polarized reference signal is polarized in a first one of a set of three different basis and the second polarization reference signal is polarized in a second one of set of three different polarization basis. The first one and the second one are different. The RX comprises a polarization-compensation module, a wavelength demultiplexer, first and second polarization-measurement modules, LO module, first and second HD modules and a polarization-feedback module. The polarizationcompensation module is configured to receive the wavelength-multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the wavelength-multiplexed signals about the three different polarization basis. The wavelength demultiplexer is configured to separate the polarizedcommunications signal from the wavelength-multiplexed first and second polarization reference signals. The first polarization-measurement module is configured to measure a polarization of the wavelength-multiplexed first and second polarization reference signals in the second polarization basis and issue a first measurement signal. The second polarization-measurement module is configured to measure a polarization of the wavelength-multiplexed first and second polarization reference signals in a third polarization basis and issue a second measurement signal. The LO module is configured to emit an LO signal that is frequency offset relative to the first polarization reference signal or the second polarization reference signal which is not shifted, by a second RF- offset Y. The first HD module is configured to combine the first measurement signal and the LO signal and produce a first detector signal having a portion corresponding to detections associated with the first polarization reference signal and a portion corresponding to detections associated with the second polarization reference signal. The second HD module is configured to combine the second measurement signal and the LO signal and produce a second detector signal having a portion corresponding to detections associated with the first polarization reference signal and a portion corresponding to detections associated with the second polarization reference signal. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on respective portions of the first detector signal and the second detector signal obtained by filtering.

[0256] In accordance with a fifty-third aspect, the RX according to the fifty-second aspect, wherein the portion of the first detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the third one of a set of three different polarization basis, the portion of the second detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the second one of a set of three different polarization basis, and the portion of the second detector signal correspond to detections from the second polarization reference signal is used to control the polarization-feedback module in the first one of a set of three different polarization basis.

[0257] In accordance with a fifty-fourth aspect, the RX according to the fifty-first aspect or the fifty second aspect, where the RX further comprises an RF-signal processing module configured to filter the first detection signal to isolate the portion corresponding to detections from the firstpolarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a first power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal, filter the second detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a second power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal and filter the second detection signal to isolate the portion corresponding to detections from the second polarization reference signal from the portion corresponding to detections from the first polarization reference signal and output a third power signal corresponding to a power level of the portion corresponding to detections from the second polarization reference signal.

[0258] In accordance with a fifty-fifth aspect, the RX according to the fifth-fourth aspect, where the RX further comprises a digital filtering module configured to low-pass filter the first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

[0259] In accordance with a fifty- sixth aspect, the RX according to any one of the fifty-third to fifty-fifth aspects, wherein the polarization-feedback module comprise a first PID controller configured to selectively control, based on the portion of the first detector signal corresponding to detections from the first polarization reference signal, first rotations of the wavelength-multiplexed signals’ polarizations about the third one of a set of three different polarization basis, a second PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the first polarization reference signal, second rotations of the wavelength-multiplexed signals’ polarizations about the second one of a set of three different polarization basis, and a third PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the second polarization reference signal, third rotations of the wavelength-multiplexed signals’ polarizations about the first one of a set of three different polarization basis.

[0260] In accordance with a fifty-seventh aspect, the RX according to the fifty-sixth aspect, wherein the first PID controller, the second PID controller and the third PID controller are configured to modify an input error signal using a predefined sign function based on previous corrections and measurements. The input error signal is a difference between a current measurement and a target setpoint.

[0261] In accordance with a fifty-eighth aspect, the RX according to any one of the fifty- second to fifty- seventh aspects, wherein the polarization-feedback module comprises an array of fiber squeezers.

[0262] In accordance with a fifty-ninth aspect, the RX according to the fifty-eighth aspect, wherein the array of fiber squeezers comprises a first fiber squeezer for the second one of a set of three different polarization basis, a second fiber squeezer for the third one of a set of three different polarization basis, and a third fiber squeezer and a fourth fiber squeezer collectively calibrated for the first one of a set of three different polarization basis.

[0263] In accordance with a sixtieth aspect, the RX according to the fifty-ninth aspect, wherein the third fiber squeezer is configured as a quarter waveplate.

[0264] In accordance with a sixty-first aspect, the RX according to any one of the fifty-second to fifty-seventh aspects, wherein the polarization-feedback module comprises a plurality of subpolarization-feedback modules including a first polarization-feedback module and a second polarization-feedback module where the first polarization-feedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

[0265] In accordance with a sixty-second aspect, the RX according to the sixty-first aspect, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM). The EOM is selected from a free space EOM or a waveguide EOM.

[0266] In accordance with a sixty-third aspect, the RX according to any one of the fifty-eighth to sixty-second aspects, wherein the first polarization-measurement module and the second polarization-measurement module comprises a motorized half-waveplate and motorized quarter waveplate and a polarizer.

[0267] In accordance with a sixty-fourth aspect, the RX according to the sixty-third aspect, where the RX further comprises a calibration module configured to back propagate LO signal through each of the first polarization-measurement module and the second polarization-measurement module, a detector configured to detect the back propagated LO signal, and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

[0268] In accordance with a sixth-fifth aspect, the RX according to any one of the fifty-second to sixty-fourth aspects, where the RX further comprises an LO-feedback module configured to cause the LO module to stabilize, based at least in part on the first detector signal, the second RF-offset Y at which the LO module emits the LO signal.

[0269] In accordance with a sixty-sixth aspect, the RX according to any one of the fifty-second to sixty-fifth aspects, where the RX further comprises a beam splitter configured to divide the LO signal to be supplied to each of the first HD module and the second HD module.

[0270] In accordance with a sixty-seventh aspect, the RX according to any one of the fifty-second to sixty-sixth aspects, wherein the three different polarization basis are mutually unbiased bases.

[0271] In accordance with a sixty-eighth aspect, the RX according to any one of the fifty-second to sixty- seventh aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0272] In accordance with a sixty-ninth aspect, a system comprising at least one RX according to any one of the fifty-second to sixty-eighth aspects, and at least one TX optically coupled with a respective one of the RXs through the optical communications channel. Each TX comprises a light source configured to emit a light at a wavelength which is shifted relative to a respective polarized communications signal by the offset AT, an array of optical filters configured to filter the light emitted, a reference signal generating module configured to produce from the light the first polarization reference signal and the second polarization reference signal, and a wavelength division multiplexing element configured to multiplex the first and second polarization reference signals with an obtained respective polarized communications signal. Each TX launches, into the optical communications channel, the wavelength-multiplexed signals.

[0273] In accordance with a seventieth aspect, the system according to the sixty-ninth aspect, wherein the reference signal generating module comprises a beam splitter configured to divide the light into a first light and a second light and a first reference signal generating module and a second reference signal generating module. The first reference signal generating module comprises a first lens, a first polarizer, a quarter waveplate, and a half waveplate. The second reference signal generating module comprises an RF-driven acousto-optic modulator (AOM) configured to frequency shift the second light to cause the first RF-offset X of the second polarization reference signal relative to the first polarization reference signal, a second lens, and a second polarizer.

[0274] In accordance with a seventy-first aspect, the system according to the seventieth aspect, wherein each TX further comprises an attenuation module configured to attenuate the first polarization reference signal and the second polarization reference signal output from the first reference signal generating module and the second reference signal generating module, respectively, which are combined upstream of the attenuation module.

[0275] In accordance with a seventy-second aspect, the system according to any one of the sixtyninth to seventy-first aspects, wherein the wavelength division multiplexing element is a dense wavelength division element (DWDM) configured to wavelength-multiplex the polarized communications signal, the first polarization reference signal, and the second polarization reference signal.

[0276] In accordance with a seventy-third aspect, the system according to any one of the sixtyninth to seventh-second aspects, wherein respective polarized communications signals are a quantum signal.

[0277] In accordance with a seventy-fourth aspect, the system according to the seventy-third aspect, wherein the attenuation module comprises a noise eater and an attenuator, and wherein the attenuation module is configured to output the first polarization reference signal and the second polarization reference signal having a power level between -10 dBm to -100 dBm.

[0278] In accordance with a seventy-fifth aspect, a method comprising receiving, over an optical communications channel, time-multiplexed signals comprising first and second polarization reference signals where one is shifted relative to the other by an RF-offset X and a polarized communications signal. When launched into the optical communications channel, the first polarization reference signal is polarized in a first one of a set of three different polarization basisand the second polarization reference signal is polarized in a second one of a set of three different polarization basis. The first one and the second one are different. The method further comprises demultiplexing the time-multiplexed signals based on time, and sending the polarized communications signal for further processing. The method further comprises measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal that was polarized in the first one of a set of three different polarization basis set to produce a first control signal, controlling, based on the first control signal, the polarizations of the time-multiplexed signals, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal polarized in the first one of a set of three different polarization basis to produce a second control signal, controlling, based on the second control signal, the polarizations of the time-multiplexed signals, by using the polarization-compensation module with a rotational axis about the second one of a set of three different polarization basis, measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal polarized in the second one of a set of three different polarization basis to produce a third control signal, and controlling, based on the third control signal, the polarizations of the time-multiplexed signals which also includes the polarized communications signal by using the polarization-compensation module with rotational axis about the first one of a set of three different polarization basis.

[0279] In accordance with a seventy-sixth aspect, the method according to the seventy-fifth aspect, where the method further comprises receiving a signal synchronization clock and controlling a frequency stabilized pulsed laser to produce light which is a seed for the first and second polarization reference signals, which when combined with the polarized communications signal will be offset in time.

[0280] In accordance with a seventy-seventh aspect, the method according to the seventy-sixth aspect, where the method further comprises combining the first and second polarization reference signals with the polarized communications signal in time to have the offset.

[0281] In accordance with a seventy-eighth aspect, the method according to any one of the seventy-fifth to seventy-seventh aspects, where the method further comprises receiving a signal synchronization clock. The demultiplexing comprises using the signal synchronization clock to separate the polarized communications signal from the first and second polarization referencesignals.

[0282] In accordance with a seventy-ninth aspect, the method according to any one of the seventh-fifth to seventy-eighth aspects, where the method further comprises detecting the measuring in the third one of a set of three different polarization basis. The detecting comprises generating a LO having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the third one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the third one of a set of three different polarization basis and detecting results thereof.

[0283] In accordance with an eightieth aspect, the method according to the seventy-ninth aspect, where the method further comprises filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal, filtering using a second set of filters the detecting results to obtain a second filtered detection result to isolate detection from the second polarization reference signal and detecting a power of the first filtered detection result and a power of the second filtered detection result.

[0284] In accordance with an eighty-first aspect, the method according to the eightieth aspect, where the method further comprises electric filtering based on time, the detecting results to isolate any detections corresponding to the polarized communications signal.

[0285] In accordance with an eighty-second aspect, the method according to the eighty-first aspect, wherein the electric filtering comprises receiving a signal synchronization clock and filtering based on the signal synchronization clock received.

[0286] In accordance with an eighty-third aspect, the method according to any one of the eightieth to eighty-second aspects, wherein the second control signal is based on the power of the first filtered detection result and the third control signal is based on the power of second filtered detection result.

[0287] In accordance with an eighty-fourth aspect, the method according to any one of the seventy-fifth to seventy-eighth aspects, where the method further comprises detecting the measuring in the second one of a set of three different polarization basis. The detecting comprises generating a LO having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted and performingheterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the second one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the second one of a set of three different polarization basis and detecting results thereof.

[0288] In accordance with an eighty-fifth aspect, the method according to the eighty-fourth aspect, where the method further comprises filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal and detecting a power of the first filtered detection result. The first control signal is based on the power of the first filtered detection result.

[0289] In accordance with an eighty- sixth aspect, the method according to the eighty-fifth aspect, where the method further comprises electric filtering based on time, the detecting results prior to detecting the power to isolate any detections corresponding to the polarized communications signal.

[0290] In accordance with an eighty- seventh aspect, the method according to the eighty-sixth aspect, wherein the electric filtering comprises receiving a signal synchronization clock and filtering based on the signal synchronization clock received.

[0291] In accordance with an eighty-eighth aspect, the method according to any one of the eightieth to eighty-second aspects or the eighty-fourth to the eighty-sixth aspects, wherein the first control signal, the second control signal and the third control signal are generated using PID control based on an error signal determined from the power and a setpoint.

[0292] In accordance with an eighty-ninth aspect, the method according to the eighty-eighth aspect, where the method further comprises modifying the error signal using a predefined sign function based on previous corrections and measurements.

[0293] In accordance with a ninetieth aspect, the method according to any one of the seventyfifth to eighty-ninth aspects, wherein the three different polarization basis are mutually unbiased bases.

[0294] In accordance with a ninety-first aspect, the method according to any one of the seventyfifth to ninetieth aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0295] In accordance with a ninety- second aspect, a RX for receiving, over an optical communications channel from a TX, time-multiplexed signals comprising a polarizedcommunications signal, a first polarization reference signal, and a second polarization reference signal. The first and second polarization reference signals each are offset in time relative to the polarized communications signal when combined and where one polarization reference signal is shifted relative to the other by an RF-offset X. When launched by the TX into the optical communications channel, the first polarized reference signal is polarized in a first one of a set of three different basis and the second polarization reference signal is polarized in a second one of set of three different polarization basis. The first one and the second one are different. The RX comprises a polarization-compensation module, a time demultiplexer, first and second polarization-measurement modules, a LO module, first and second HD modules, and a polarization-feedback module. The polarization-compensation module is configured to receive the time-multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the time-multiplexed signals about the three different polarization basis. The time demultiplexer is configured to separate the polarized communications signal from the time-multiplexed first and second polarization reference signals. The first polarization-measurement module is configured to measure a polarization of the time-multiplexed first and second polarization reference signals in a second polarization basis and issue a first measurement signal. The second polarization-measurement module is configured to measure a polarization of the time- multiplexed first and second polarization reference signals in a third polarization basis and issue a second measurement signal. The LO module is configured to emit an LO signal that is frequency offset relative to the first polarization reference signal or the second polarization reference signal which is not shifted, by a second RF-offset Y. The first HD module is configured to combine the first measurement signal and the LO signal and produce a first detector signal having portions corresponding to detections associated with the first polarization reference signal and corresponding to detections associated with the second polarization reference signal. The second HD module is configured to combine the second measurement signal and the LO signal and produce a second detector signal having portions corresponding to detections associated with the first polarization reference signal and corresponding to detections associated with the second polarization reference signal. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarization-compensation module. The polarization-feedback module is configured to selectively control the polarization-compensationmodule based on respective portions of the first detector signal and the second detector signal obtained by filtering.

[0296] In accordance with a ninety-third aspect, the RX according to the ninety-second aspect, wherein the first and second polarization reference signals have the same wavelength as the polarized communications signal.

[0297] In accordance with a ninety-fourth aspect, the RX according to the ninety-second aspect or the ninety-third aspect, wherein the time demultiplexer comprises an optical switch and a processor.

[0298] In accordance with a ninety-fifth aspect, the RX according to the ninety-fourth aspect, wherein the processor is configured to receive a signal synchronization clock and control the optical switch based on the signal synchronization clock.

[0299] In accordance with a ninety- sixth aspect, the RX according to the ninety-fifth aspect, wherein the signal synchronization clock is received from a source of the polarized communications signal.

[0300] In accordance with a ninety- seventh aspect, the RX according to any one of the ninety- second to ninety-sixth aspects, wherein the portion of the first detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the third one of a set of three different polarization basis, the portion of the second detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the second one of a set of three different polarization basis, and the portion of the second detector signal correspond to detections from the second polarization reference signal is used to control the polarization-feedback module in the first one of a set of three different polarization basis.

[0301] In accordance with a ninety-eighth aspect, the RX according to the ninety- seventh aspect, where the RX further comprises an RF-signal processing module configured to filter the first detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a first power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal, filter the second detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarizationreference signal and output a second power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal and filter the second detection signal to isolate the portion corresponding to detections from the second polarization reference signal from the portion corresponding to detections from the first polarization reference signal and output a third power signal corresponding to a power level of the portion corresponding to detections from the second polarization reference signal.

[0302] In accordance with a ninety-nineth aspect, the RX according to the ninety-eighth aspect, where the RX further comprises an electric switch between the first and second HD modules and the RF-signal processing module, respectively. The electric switch is controlled by a processor to isolate any detection corresponding to the polarized communications signal from the RF-signal processing module and allow detections corresponding to the first and second polarization reference signals to reach the RF-signal processing module.

[0303] In accordance with a one hundredth aspect, the RX according to the ninety-ninth aspect, where the RX further comprises a digital filtering module configured to low-pass filter the first, second and third power signals and provide respective digital power signals to the polarizationfeedback module.

[0304] In accordance with a one hundred-and-first aspect, the RX according to any one of the ninety-seventh to one hundredth aspects, wherein the polarization-feedback module comprises a first PID controller configured to selectively control, based on the portion of the first detector signal corresponding to detections from the first polarization reference signal, first rotations of the wavelength-multiplexed signals’ polarizations about the third one of a set of three different polarization basis, a second PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the first polarization reference signal, second rotations of the wavelength-multiplexed signals’ polarizations about the second one of a set of three different polarization basis, and a third PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the second polarization reference signal, third rotations of the wavelength-multiplexed signals’ polarizations about the first one of a set of three different polarization basis.

[0305] In accordance with a one hundred-and-second aspect, the RX according to the one hundred-and-first aspect, wherein the first PID controller, the second PID controller and the third PID controller are configured to modify an input error signal using a predefined sign functionbased on previous corrections and measurements. The input error signal is a difference between a current measurement and a target setpoint.

[0306] In accordance with a one hundred-and-third aspect, the RX according to any one of the ninety-second to one hundred-and-second aspects, wherein the polarization-feedback module comprising an array of fiber squeezers.

[0307] In accordance with a one hundred-and-fourth aspect, the RX according to the one hundred-and-third aspect, wherein the array of fiber squeezers comprises a first fiber squeezer for the second one of a set of three different polarization basis, a second fiber squeezer for the third one of a set of three different polarization basis, and a third fiber squeezer and a fourth fiber squeezer collectively calibrated for the first one of a set of three different polarization basis.

[0308] In accordance with a one hundred-and-fifth aspect, the RX according to the one hundred- and-fourth aspect, wherein the third fiber squeezer is configured as a quarter waveplate.

[0309] In accordance with a one hundred-and-sixth aspect, the RX according to the ninety- second to one hundred-and-second aspects, wherein the polarization-feedback module comprises a plurality of sub-polarization-feedback modules including a first polarization-feedback module and a second polarization-feedback module where the first polarization-feedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

[0310] In accordance with a one hundred-and-seventh aspect, the RX according to the one hundred-and-sixth aspect, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM). The EOM is selected from a free space EOM or a waveguide EOM.

[0311] In accordance with a one hundred-and-eighth aspect, the RX according to any one of the one hundred-and-third to one hundred-and-seventh aspects, wherein the first polarizationmeasurement module and the second polarization-measurement module comprises a motorized half-waveplate and motorized quarter waveplate and a polarizer.

[0312] In accordance with a one hundred-and-ninth aspect, the RX according to the one hundred-and-eighth aspect, where the RX further comprises a calibration module configured to back propagate LO signal through each of the first polarization-measurement module and the second polarization-measurement module, a detector configured to detect the back propagatedLO signal and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

[0313] In accordance with a one hundred-and-tenth aspect, the RX according to any one of the ninety-second to one hundred-and-ninth aspects, where the RX further comprises an LO- feedback module configured to cause the LO module to stabilize, based at least in part on the first detector signal, the second RF-offset Y at which the LO module emits the LO signal.

[0314] In accordance with a one hundred-and-eleventh aspect, the RX according to any one of the ninety-second to one hundred-and-tenth aspects, where the RX further comprises a beam splitter configured to divide the LO signal to be supplied to each of the first HD module and the second HD module.

[0315] In accordance with a one hundred- and-twelfth aspect, the RX according to any one of the ninety-second to one hundred-and-eleventh aspects, wherein the three different polarization basis are mutually unbiased bases.

[0316] In accordance with a one hundred-and-thirteenth aspect, the RX according to any one of the ninety- second to one hundred- and-twelfth aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0317] In accordance with a one hundred-and-fourteenth aspect, a system comprising at least one RX according to any one of the ninety-second to the one-hundred-and-thirteenth aspects and at least one TX optically coupled with a respective one of the RXs through the optical communications channel. Each TX comprises a pulsed light source configured to emit pulsed light, a reference signal generating module configured to produce from the light the first polarization reference signal and the second polarization reference signal and a time division multiplexing element configured to multiplex the first and second polarization reference signals with an obtained respective polarized communications signal. Each TX launches, into the optical communications channel, the time-multiplexed signals.

[0318] In accordance with a one hundred-and-fifteenth aspect, the system according to the one hundred-and-fourteenth aspect, where the system further comprises a processor configured to receive a signal synchronization clock from a source of the obtained respective polarized communications signal and control a timing of an emission of pulsed light from the pulsed light source based on the signal synchronization clock.

[0319] In accordance with a one hundred-and-sixteenth aspect, the system according to the one hundred-and-fourteenth aspect or the one-hundred-and-fifteenth aspect, wherein the time division multiplexing element comprises a fused fiber coupler.

[0320] In accordance with a one hundred-and-seventeenth aspect, the system according to the one hundred-and-fourteenth aspect or the one hundred-and-fifteenth aspect, wherein the time divisional multiplexing element comprises an optical switch and a processor. The processor is configured to receive a signal synchronization clock and control the optical switch based on the signal synchronization clock such that the first and second polarization reference signals are offset in time with the polarized communications signal.

[0321] In accordance with a one hundred-and-eighteenth aspect, the system according to any one of the one hundred-and-fourteenth to one hundred-and-seventeenth aspects, wherein the reference signal generating module comprises a beam splitter configured to divide the pulsed light into a first light pulse and a second light pulse and a first reference signal generating module and a second reference signal generating module. The first reference signal generating module comprises a first lens, a first polarizer, a quarter waveplate and a half waveplate. The second reference signal generating module comprises an RF-driven acousto-optic modulator (AOM) configured to frequency shift the second light pulse to cause the first RF-offset X of the second polarization reference signal relative to the first polarization reference signal, a second lens and a second polarizer.

[0322] In accordance with a one hundred-and-nineteenth aspect, the system according to the one hundred-and-eighteenth aspect, wherein each TX further comprises an attenuation module configured to attenuate the first polarization reference signal and the second polarization reference signal output from the first reference signal generating module and the second reference signal generating module, respectively, which are combined upstream of the attenuation module.

[0323] In accordance with a one hundred-and-twentieth aspect, the system according to the one hundred-and-nineteenth aspect, wherein respective polarized communications signals are a quantum signal.

[0324] In accordance with a one hundred-and-twenty-first aspect, the system according to the one hundred-and-twentieth aspect, wherein the attenuation module comprises a noise eater and an attenuator. The attenuation module is configured to output the first polarization reference signal and the second polarization reference signal having a power level between -10 dBm to - 100 dBm.

[0325] In accordance with a one hundred-and-twenty-second aspect, an RX for receiving, over an optical communications channel from TX, multiplexed signals comprising a polarized communications signal and a polarization reference signal. When launched by the TX into the optical communications channel, the polarization reference signal is polarized in a first one of a set of three different basis. The RX comprises a polarization-compensation module, a demultiplexer, first and second polarization-measurement modules, a LO module, first and second HD modules, and a polarization-feedback module. The polarization-compensation module is configured to receive the multiplexed signals and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the multiplexed signals about the other two bases of three different polarization basis. The demultiplexer is configured to separate the polarized communications signal from the multiplexed polarization reference signal. The first polarization-measurement module is configured to measure a polarization of the polarization reference signal in a second polarization basis and issue a first measurement. The second polarization-measurement module is configured to measure a polarization of the polarization reference signal in a third polarization basis and issue a second measurement. The LO module is configured to emit an LO signal that is frequency offset relative to the polarization reference signal. The first HD module is configured to combine the first measurement signal and the LO signal and produce a first detector signal. The second HD module is configured to combine the second measurement signal and the LO signal and produce a second detector signal. The polarization-feedback module is communicatively coupled with the first and second HD modules and the polarizationcompensation module. The polarization-feedback module is configured to selectively control the polarization-compensation module based on the first detector signal and the second detector signal.

[0326] In accordance with a one hundred-and-twenty-third aspect, the RX according to the one hundred-and-twenty-second aspect, wherein the polarized communications signal and the polarization reference signal are wavelength multiplexed. The polarization reference signal and the polarized communications signal have a different wavelength

[0327] In accordance with a one hundred-and-twenty-fourth aspect, the RX according to the one hundred-and-twenty-second aspect, wherein the polarized communications signal and the polarization reference signal are time multiplexed. The polarization reference signal and the polarized communication signal have the same wavelength

[0328] In accordance with a one hundred-and-twenty-fifth aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-twenty-fourth aspects, wherein the first detector signal is used to control the polarization-feedback module in the third one of a set of three different polarization basis and the second detector signal is used to control the polarization-feedback module in the second one of a set of three different polarization basis.

[0329] In accordance with a one hundred-and-twenty-sixth aspect, the RX according to the one hundred-and-twenty-fifth aspect, where the RX further comprises a digital filtering module configured to low-pass filter first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

[0330] In accordance with a one hundred- and-twenty- seventh aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-twenty-sixth aspects, wherein the polarization-feedback module comprises a first PID controller configured to selectively control, based on the first detector signal first rotations of the multiplexed signal’s polarization about the third one of a set of three different polarization basis and a second PID controller configured to selectively control, based on the second detector signal, second rotations of the multiplexed signals’ polarizations about the second one of a set of three different polarization basis.

[0331] In accordance with a one hundred-and-twenty-eighth aspect, the RX according to the one hundred-and-twenty-seventh aspect, wherein the first PID controller and the second PID control are configured to modify an input error signal using a predefined sign function based on previous corrections and measurements. The input error signal is a difference between a current measurement and a target setpoint.

[0332] In accordance with a one hundred-and-twenty-ninth aspect, the RX according to any one of the one hundred-and-twenty- second to one hundred-and-twenty-eighth aspects, wherein the polarization-feedback module comprises an array of fiber squeezers.

[0333] In accordance with a one hundred-and-thirtieth aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-twenty-eighth aspects, wherein the polarization-feedback module comprises a plurality of sub-polarization-feedback modules including a first polarization-feedback module and a second polarization-feedback module where the first polarization-feedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

[0334] In accordance with a one hundred-and-thirty-first aspect, the RX according to the one hundred-and-thirtieth aspect, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM). The EOM is selected from a free space EOM or a waveguide EOM.

[0335] In accordance with a one hundred-and-thirty-second aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-thirty-first aspects, wherein the first polarization-measurement module and the second polarization-measurement module comprise a motorized half-waveplate and motorized quarter waveplate and a polarizer.

[0336] In accordance with a one hundred-and-thirty-third aspect, the RX according to the one hundred-and-thirty-second aspect, where the RX further comprises a calibration module configured to back propagate LO signal through each of the first polarization-measurement module and the second polarization-measurement module, a detector configured to detect the back propagated LO signal, and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

[0337] In accordance with a one hundred-and-thirty-fourth aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-thirty-third aspects, where the RX further comprises an LO-feedback module configured to cause the LO module to stabilize, based at least in part on the first detector signal, the second RF-offset Y at which the LO module emits the LO signal.

[0338] In accordance with a one hundred-and-thirty-fifth aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-thirty-fourth aspects, where the RX further comprises a beam splitter configured to divide the LO signal to be supplied to each of the first HD module and the second HD module.

[0339] In accordance with a one hundred-and-thirty-sixth aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-thirty-fifth aspects, wherein the three different polarization basis are mutually unbiased bases.

[0340] In accordance with a one hundred-and-thirty-seventh aspect, the RX according to any one of the one hundred-and-twenty-second to one hundred-and-thirty-sixth aspects, wherein the three different polarization basis are R / L, H / V, and D / A bases.

[0341] In accordance with a one hundred- and- thirty-eighth aspect, a system comprising a RX according to any one of the one hundred-and-twenty-second to one hundred-and-thirty-seventh aspects and a TX optically coupled with the RX through the optical communications channel. The TX comprises a light source configured to emit a light, a reference signal generating module configured to produce from the light the polarization reference signal, and a multiplexing element configured to multiplex the polarization reference signal with an obtained respective polarized communications signal. Each TX launches, into the communications channel, the multiplexed signals.

[0342] In accordance with a one hundred-and-thirty-nineth aspect, the system according to the one hundred-and-thirty-eighth aspect, wherein when the multiplexing is wavelength multiplexing, the TX further comprises an array of optical filters between the light source and the reference signal generating module.

[0343] In accordance with a one hundred-and-fortieth aspect, the system according to the one hundred-and-thirty-nineth aspect, wherein the multiplexing element is a dense wavelength division modulator (DWDM) configured to wavelength-multiplex the polarized communications signal, and the polarization reference signal.

[0344] References in the specification to “one aspect”, “certain aspects”, “some aspects” or “an aspect”, indicate that the aspect(s) described may include a particular feature or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect.

[0345] Aspects of the present disclosure may be implemented and run on a general -purpose computer or special-purpose computer system. The computer system may be any type known or will be known systems and may include a hardware processor, memory device, a storage device, input / output devices, internal buses, and / or a communications interface for communicating with other computer systems in conjunction with communication hardware and software, etc.

[0346] Various aspects of the present disclosure may be embodied as a program, software, or computer instructions embodied or stored in a computer or machine-usable or readable medium, or a group of media which causes the computer or machine to perform the steps of the method when executed on the computer, processor, and / or machine. A program storage device readable by a machine, e.g., a computer-readable medium, tangibly embodying a program of instructions executable by the machine to perform various functionalities and methods described in the present disclosure is also provided, e.g., a computer program product.

[0347] The computer-readable medium could be a computer-readable storage device or a computer-readable signal medium. A computer-readable storage device may be, for example, a magnetic, optical, electronic, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing; however, the computer-readable storage device is not limited to these examples except a computer-readable storage device excludes computer-readable signal medium. Additional examples of the computer-readable storage device can include: a portable computer diskette, a hard disk, a magnetic storage device, a portable compact disc read-only memory (CD-ROM), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical storage device, or any appropriate combination of the foregoing; however, the computer- readable storage device is also not limited to these examples. Any tangible medium that can contain, or store, a program for use by or in connection with an instruction execution system, apparatus, or device could be a computer-readable storage device.

[0348] A computer-readable signal medium may include a propagated data signal with computer-readable program code embodied therein, such as, but not limited to, in baseband or as part of a carrier wave. A propagated signal may take any of a plurality of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer- readable signal medium may be any computer-readable medium (exclusive of computer-readable storage device) that can communicate, propagate, or transport a program for use by or inconnection with a system, apparatus, or device. Program code embodied on a computer-readable signal medium may be transmitted using any appropriate medium, including but not limited to wireless, wired, optical fiber cable. RF, etc., or any suitable combination of the foregoing.

[0349] The terms “computer system” and “network” as may be used in the present application may include a variety of combinations of fixed and / or portable computer hardware, software, peripherals, mobile, and storage devices. The computer system may include a plurality of individual components that are networked or otherwise linked to perform collaboratively or may include one or more stand-alone components. The hardware and software components of the computer system of the present application may include and may be included within fixed and portable devices such as mobile phone, tablet, smartphone, desktop, laptop, and / or server. A module may be a component of a device, software, program, or system that implements some “functionality”, which can be embodied as software, hardware, firmware, electronic circuitry, or etc.

[0350] As used herein, the term “processor” may include a single core processor, a multi-core processor, multiple processors located in a single device, or multiple processors in wired or wireless communication with each other and distributed over a network of devices, the Internet, or the cloud. Accordingly, as used herein, functions, features or instructions performed or configured to be performed by a “processor”, may include the performance of the functions, features or instructions by a single core processor, may include performance of the functions, features or instructions collectively or collaboratively by multiple cores of a multi-core processor, or may include performance of the functions, features or instructions collectively or collaboratively by multiple processors, where each processor or core is not required to perform every function, feature or instruction individually. For example, multiple processors may allow load balancing. As used herein, the term “processor” may be replaced with the term “circuit”. The term “processor” may refer to, be part of. or include processor hardware (shared, dedicated, or group) that executes code and memory hardware (shared, dedicated, or group) that stores code executed by the processor.

[0351] In the description and claims herein, the term “about” indicates that the value listed may be somewhat altered, as long as the alteration does not result in nonconformance of the process or device. For example, for some elements the term “about” can refer to a variation of ±0.1%, for other elements, the term “about” can refer to a variation of ±1% or ±10%, or any point therein.For example, the term about when used for a measurement in mm, may include + / 0.1 , 0.2, 0.3, etc., where the difference between the stated number may be larger when the state number is larger. For example, about 1.5 may include 1.2-1.8, where about 20. may include 19.0-21.0.

[0352] Reference herein to any numerical range expressly includes each numerical value (including fractional numbers and whole numbers) encompassed by that range. To illustrate, reference herein to a range of “at least 50” or “at least about 50” includes whole numbers of 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, etc., and fractional numbers 50.1, 50.2 50.3, 50.4, 50.5, 50.6, 50.7, 50.8, 50.9, etc. In a further illustration, reference herein to a range of “less than 50” or “less than about 50” includes whole numbers 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, etc., and fractional numbers 49.9, 49.8, 49.7, 49.6, 49.5, 49.4, 49.3, 49.2, 49.1, 49.0, etc.

[0353] The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting the scope of the disclosure and is not intended to be exhaustive. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure.

Claims

What is claimed is:

1. A method comprising : receiving, over an optical communications channel, a first set of polarization reference signals and a second set of polarization reference signals, the first set and second set being wavelength-multiplexed with a polarized communications signal having a wavelength (w), the first set of polarization reference signals being wavelength-shifted up relative to the wavelength (w) by an offset +AA, the second set of polarization reference signals being wavelength-shifted down relative to the wavelength (w) by the offset -Ad., the first and second sets of polarization reference signals each have a first polarization reference signal and a second polarization reference signal where one of the first polarization reference signal and the second polarization reference signal in each set is frequency-shifted frequency with respect to the other by a first RF-offset X, and the first polarization reference signal in each set is polarized in a first one of a set of three different polarization basis when launched into the optical communications channel and the second polarization reference signal in each set is polarized in a second one of a set of three different polarization basis when launched into the optical communications channel, where the first one and second one are different; measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal in each set of polarization reference signals that was polarized in the first one of a set of three different polarization basis to produce a first control signal; controlling, based on the first control signal, the polarizations of the wavelength- multiplexed signals received, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis; measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal in each set of polarization reference signals that was polarized in the first one of a set of three different polarization basis to produce a second control signal; controlling, based on the second control signal, the polarizations of the wavelength-multiplexed signals received, by using the polarization-compensation module with a rotation axis about the second one of a set of three different polarization basis;measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal in each set of polarization reference signals that was polarized in the second one of a set of three different polarization basis to produce a third control signal; and controlling, based on the third control signal, the polarizations of the wavelengthmultiplex signals received, by using the polarization-compensation module with a rotation axis about the first one of a set of three different polarization basis.

2. The method of claim 1, wherein the offset +- AA is less than 4 nm.

3. The method of claim 1, wherein the first control signal is based on an average of the measuring of the polarization of the first polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis, the second control signal is based on the average of the of the measuring of the first polarization reference signal in each set of polarization reference signals in third one of a set of three different basis, and the third control signal is based on the average of the measuring of the second polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis.

4. The method of any one of claims 1 to 3, further comprising controlling a first frequency stabilized laser to produce light at the offset +AA relative to the polarized communications signal and controlling a second frequency stabilized laser to produce light at the offset -AA relative to the polarized communications signal.

5. The method of claim 4, further comprising: pedestal filtering the light from each of the first frequency stabilized laser and the second frequency stabilized laser.

6. The method of claim 4, further comprising: variably attenuating the light from each of the first frequency stabilized laser and the second frequency stabilized laser to produce a target power level for each.

7. The method of claim 3, further comprising: detecting the measuring in the third one of a set of three different polarization basis, the detecting comprising: generating a first Local oscillator (LO) having a second RF-offset Y 1 from either the first polarization reference signal in each set of polarization reference signals or the secondpolarization reference signal in each set of polarization reference signals and generating a second (LO) having a third RF-offset Y2, different from Yl, from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals; and performing heterodyne detection by mixing the first LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and mixing the second LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and mixing the first LO with the measurement of the second polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and mixing the second LO and the measurement of the second polarization reference signal in each set of polarization reference signals in the third one of a set of three different polarization basis and detecting results thereof.

8. The method of claim 7, further comprising: filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal in each set of polarization reference signals; filtering using a second set of filters the detecting results to obtain a second filtered detection result to isolate detection from the second polarization reference signal in each set of polarization reference signals; and detecting a power of the first filtered detection result and a power of the second filtered detection result.

9. The method of claim 8, wherein the second control signal is based on the power of the first filtered detection result and the third control signal is based on the power of the second filtered detection result.

10. The method of claim 3, further comprising: detecting the measuring in the second one of a set of three different polarization basis, the detecting comprising: generating a first Local oscillator (LO) having a second RF-offset Y 1 from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set polarization reference signals and generating a second(LO) having a third RF-offset Y2, different from Y 1 , from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals; and performing heterodyne detection by mixing the first LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and mixing the second LO with the measurement of the first polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and mixing the first LO with the measurement of the second polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and mixing the second LO and the measurement of the second polarization reference signal in each set of polarization reference signals in the second one of a set of three different polarization basis and detecting results thereof.

11. The method of claim 10, further comprising: filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal in each set of polarization reference signals, detecting a power of the first filtered detection result, wherein the first control signal is based the power of the first filtered detection result.

12. The method of claim 9 or claim 11, wherein the first control signal, the second control signal and the third control signal are generated using proportional integral derivative (PID) control based on an error signal determined from the power and a setpoint.

13. The method of claim 12, further comprising: modifying the error signal using a predefined sign function based on previous corrections and measurements.

14. The method of any one of claims 1 to 13, wherein the three different polarization basis are mutually unbiased bases.

15. The method of claim 14, wherein the three different polarization basis are R / L, H / V, and D / A bases.

16. An optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), wavelength-multiplexed signals comprising a first set of polarization reference signals and a second set of polarization reference signals and a polarized communications signal having a wavelength (w), the first set of polarization reference signalsbeing wavelength- shifted up relative to the wavelength (w) by an offset +AA, the second set of polarization reference signals being wavelength- shifted down relative to the wavelength (w) by the offset -AA, the first and second sets of polarization reference signals each have a first polarization reference signal and a second polarization reference signal where one of the first polarization reference signal and the second polarization reference signal in each set is frequency- shifted frequency with respect to the other by a first RF-offset X, and the first polarization reference signal in each set is polarized in a first one of a set of three different polarization basis when launched into the optical communications channel and the second polarization reference signal in each set is polarized in a second one of a set of three different polarization basis when launched into the optical communications channel, where the first one and second one are different, the RX comprising: a polarization-compensation module configured to receive the wavelength- multiplexed signals, and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the wavelength-multiplexed signals about the three different polarization basis; a wavelength demultiplexer configured to separate the polarized communications signal from the first set of polarization reference signals and the second set of polarization reference signals; a first polarization-measurement module configured to measure a polarization of the first and second polarization reference signals in each set of polarization reference signals wavelength-multiplexed with the polarized communications signal in the second one of a set of three different polarization basis and issue a first measurement signal; a second polarization-measurement module configured to measure a polarization of the first and second polarization reference signals in each set of polarization reference signal wavelength-multiplexed with the polarized communications signal in a third one of a set of three different polarization basis and issue a second measurement signal; a first local oscillator (LO) module configured to emit a first LO signal that is frequency offset from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals which is unshifted, by a second RF-offset Yl;a second LO module configured to emit a second LO signal that is frequency offset from either the first polarization reference signal in each set of polarization reference signals or the second polarization reference signal in each set of polarization reference signals which is unshifted, by a third RF-offset Y2 different from Yl; a first heterodyne detection (HD) module configured to combine the first measurement signal with the first LO signal and combine the first measurement signal with the second LO signal and produce a first detector signal having portions corresponding to detections from the first polarization reference signal in each set of polarization reference signals and detections from the second polarization reference signal in each set of polarization reference signals; a second HD module configured to combine the second measurement signal with the first LO signal and combine the second measurement signal with the second LO signal and produce a second detector signal having portions corresponding to detections from the first polarization reference signal in each set of polarization reference signals and detections from the second polarization reference signal in each set of polarization reference signals; and a polarization-feedback module communicatively coupled with the first and second HD modules and the polarization-compensation module, the polarization-feedback module configured to selectively control the polarization-compensation module based on an average of respective portions of the first detector signal and the second detector signal obtained by filtering.

17. The RX of claim 16, wherein the portion of the first detector signal corresponding to detections from to the first polarization reference signal in each set of polarization reference signals is used to control the polarization-feedback module in the third one of a set of three different polarization basis, the portion of the second detector signal corresponding to detections from the first polarization reference signal in each set of polarization reference signals is used to control the polarization-feedback module in the second one of a set of three different polarization basis, the portion of the second detector signal correspond to detections from the second polarization reference signal in each set of polarization reference signals is used to control the polarization-feedback module in the first one of a set of three different polarization basis.

18. The RX of claim 16, further comprising: an RF-signal processing module configured to:filter the first detection signal to isolate the portion corresponding to detections from the first polarization reference signal in each set of polarization reference signals from the portion corresponding to detections from the second polarization reference signal in each set of polarization reference signals and output a first power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal; filter the second detection signal to isolate the portion corresponding to detections from the first polarization reference signal in each set of polarization reference signals from the portion corresponding to detections from the second polarization reference signal in each set of polarization reference signals and output a second power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal; and filter the second detection signal to isolate the portion corresponding to detections from the second polarization reference signal in each set of polarization reference signals from the portion corresponding to detections from the first polarization reference signal in each set of polarization reference signals and output a third power signal corresponding to a power level of the portion corresponding to detections from the second polarization reference signal.

19. The RX of claim 18, further comprising: a digital filtering module configured to low-pass filter the first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

20. The RX of claim 19, wherein the polarization-feedback module comprises: a first proportional-integral-derivative (PID) controller configured to selectively control, based on the portion of the first detector signal corresponding to detections from the first polarization reference signal in each set of polarization reference signals, first rotations of the wavelength-multiplexed signals’ polarizations about the third one of a set of three different polarization basis; a second PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the first polarization reference signal in each set of polarization reference signals, second rotations of the wavelength-multiplexed signals’ polarizations about the second one of a set of three different polarization basis; anda third PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the second polarization reference signal in each set of polarization reference signals, third rotations of the wavelength-multiplexed signals’ polarizations about the first one of a set of three different polarization basis.

21. The RX of claim 20, wherein the first PID controller, the second PID controller and the third PID controller are configured to modify an input error signal using a predefined sign function based on previous corrections and measurements, the input error signal is a difference between a current measurement and a target setpoint.

22. The RX of any one of claims 16 to 21, wherein the polarization-feedback module comprises an array of fiber squeezers.

23. The RX of claim 22, wherein the array of fiber squeezers comprises a first fiber squeezer for the second one of a set of three different polarization basis, a second fiber squeezer for the third one of a set of three different polarization basis, and a third fiber squeezer and a fourth fiber squeezer collectively calibrated for the first one of a set of three different polarization basis.

24. The RX of claim 23, wherein the third fiber squeezer is configured as a quarter waveplate.

25. The RX of any one of claims 16 to 21, wherein the polarization-feedback module comprises a plurality of sub-polarization-feedback modules including a first polarizationfeedback module and a second polarization-feedback module where the first polarizationfeedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

26. The RX of claim 25, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM), the EOM being selected from a free space EOM or a waveguide EOM.

27. The RX of claim 22, wherein the first polarization-measurement module and the second polarization-measurement module comprises a motorized half-waveplate and motorized quarter waveplate and a polarizer.

28. The RX of claim 27, further comprising: a calibration module configured to back propagate either the first LO signal or the second LO signal having an average of the two wavelengths through each of the first polarization-measurement module and the second polarization-measurement module;a detector configured to detect the back propagated LO signal; and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

29. The RX of any one of claims 16 to 28, further comprising: an LO-feedback module configured to cause the first and second LO modules to stabilize, based at least in part on the first detector signal, the second RF-offset Fl at which the first LO module emits the first LO signal and the third RF-offset Y2 at which second LO module emits the second LO signal.

30. The RX of any one of claims 16 to 29, further comprising: a beam splitter configured to divide the first LO signal and the second LO signal to be supplied to each of the first HD module and the second HD module.

31. The RX of any one of claims 16 to 30, wherein the three different polarization basis are mutually unbiased bases.

32. The RX of claim 31, wherein the three different polarization basis are R / L, H / V, and D / A bases.

33. A system comprising: at least one RX of any one of claims 16 to 32; and at least one optical transmitter (TX) optically coupled with a respective RX through the optical communications channel, each TX comprising: a first light source configured to emit light at a wavelength which is up shifted relative to a respective polarized communications signal by the offset +AA; a second light source configured to emit light at a wavelength which is down shifted relative to the respective polarized communications signal by the offset -Ad.; a first array of optical filters configured to filter the light emitted by the first light source and a second array of optical filters configured to filter the light emitted by the second light source; a reference signal generating module configured to produce from the light from the first light source the first set of polarization reference signals and from the light from the second light source the second set of polarization reference signals; anda wavelength division multiplexing element configured to multiplex the first and second sets of polarization reference signals with an obtained respective polarized communications signal, wherein each TX launches, into the optical communications channel, the wavelength- multiplexed signals.

34. The system of claim 33, wherein each TX further comprises a first noise eater (NE) and a second NE, the first NE configured to output a fixed power of light based on the light input from the first light source and the second NE configured to output a fixed power of light based on light input from the second light source.

35. The system of claim 34, wherein the reference signal generating module comprises: a beam splitter configured to divide the light from each of the first and second light sources into a first light and a second light; and a first reference signal generating module for providing the first polarization reference signal in each set of polarization reference signals and a second reference signal generating module for providing the second polarization reference signal in each set of polarization reference signals, the first reference signal generating module comprising: a first lens; a first polarizer; a quarter waveplate; and a half waveplate, the second reference signal generating module comprising: an RF-driven acousto-optic modulator (AOM) configured to frequency shift the second light to cause the first RF-offset X of the second polarization reference signal in each set relative to the first polarization reference signal; a second lens; and a second polarizer.

36. The system of claim 35. wherein each TX further comprises an attenuation module configured to attenuate the first and second sets of polarization reference signals output from the first reference signal generating module and the second reference signal generating module, respectively, which is combined upstream of the attenuation module.

37. The system of any one of claims 33 to 36, wherein the wavelength division multiplexing element is a dense wavelength division element (DWDM) configured to wavelength-multiplex the polarized communications signal, the first and second sets of polarization reference signals.

38. The system of any one of claims 33 to 37, wherein respective polarized communications signals are a quantum signal.

39. The system of claim 38. wherein the first and second sets of polarization references signals having a power level between -10 dBm to -100 dBm when launched.

40. A method comprising: receiving, over an optical communications channel, wavelength-multiplexed signals comprising first and second polarization reference signals where one is shifted relative to the other by an RF-offset X, wherein, when launched into the optical communications channel, the first polarization reference signal is polarized in a first one of a set of three different polarization basis and the second polarization reference signal is polarized in a second one of a set of three different polarization basis, where the first one and second one are different; measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal that was polarized in the first one of a set of three different polarization basis set to produce a first control signal; controlling, based on the first control signal, the polarizations of the wavelength- multiplexed signals which also includes a polarized communications signal, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis; measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal polarized in the first one of a set of three different polarization basis to produce a second control signal; controlling, based on the second control signal, the polarizations of the wavelength- multiplexed signals, which also includes the polarized communications signal by using the polarization-compensation module with a rotational axis about the second one of a set of three different polarization basis; measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal polarized in the second one of a set of three different polarization basis to produce a third control signal; andcontrolling, based on the third control signal, the polarizations of the wavelength- multiplexed signals which also includes the polarized communications signal by using the polarization-compensation module with rotational axis about the first one of a set of three different polarization basis, wherein the first polarization reference signal and the second polarization reference signal are wavelength- shifted relative to the polarized communications signal by an offset AA.

41. The method of claim 40, further comprising: controlling a frequency stabilized laser to produce light at the offset AA relative to the polarized communications signal.

42. The method of claim 40 or claim 41, further comprising pedestal filtering the light from the frequency stabilized laser.

43. The method of any one of claims 40 to 42, further comprising: detecting the measuring in the third one of a set of three different polarization basis, the detecting comprising: generating a Local oscillator (LO) having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted; and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the third one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the third one of a set of three different polarization basis and detecting results thereof.

44. The method of claim 43, further comprising: filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal; filtering using a second set of filters the detecting results to obtain a second filtered detection result to isolate detection from the second polarization reference signal; and detecting a power of the first filtered detection result and a power of the second filtered detection result.

45. The method of claim 44, wherein the second control signal is based on the power of the first filtered detection result and the third control signal is based on the power of second filtered detection result.

46. The method of any one of claims 40 to 42, further comprising: detecting the measuring in the second one of a set of three different polarization basis, the detecting comprising: generating a Local oscillator (LO) having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted; and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the second one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the second one of a set of three different polarization basis and detecting results thereof.

47. The method of claim 46, further comprising: filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal, and detecting a power of the first filtered detection result, wherein the first control signal is based the power of the first filtered detection result.

48. The method of claim 45 or claim 47, wherein the first control signal, the second control signal and the third control signal are generated using proportional integral derivative (PID) control based on an error signal determined from the power and a setpoint.

49. The method of claim 48, further comprising: modifying the error signal using a predefined sign function based on previous corrections and measurements.

50. The method of any one of claims 40 to 49, wherein the three different polarization basis are mutually unbiased bases.

51. The method of claim 50, wherein the three different polarization basis are R / L, H / V, and D / A bases.

52. An optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), wavelength-multiplexed signals comprising a polarized communications signal, a first polarization reference signal, and a second polarization reference signal, wherein the first and second polarization reference signals each are wavelength-shifted relative to the polarized communications signal by an offset A2. and where one is shifted relative to the other by an RF-offset X, wherein when launched by the TX into the opticalcommunications channel, the first polarized reference signal is polarized in a first one of a set of three different basis and the second polarization reference signal is polarized in a second one of set of three different polarization basis, where the first one and second one are different, the RX comprising: a polarization-compensation module configured to receive the wavelength- multiplexed signals, and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the wavelength-multiplexed signals about the three different polarization basis; a wavelength demultiplexer configured to separate the polarized communications signal from the wavelength-multiplexed first and second polarization reference signals; a first polarization-measurement module configured to measure a polarization of the wavelength-multiplexed first and second polarization reference signals in the second polarization basis and issue a first measurement signal; a second polarization-measurement module configured to measure a polarization of the wavelength-multiplexed first and second polarization reference signals in a third polarization basis and issue a second measurement signal; a local oscillator (LO) module configured to emit an LO signal that is frequency offset relative to the first polarization reference signal or the second polarization reference signal which is not shifted, by a second RF-offset Y a first heterodyne detection (HD) module configured to combine the first measurement signal and the LO signal, and produce a first detector signal having a portion corresponding to detections associated with the first polarization reference signal and a portion corresponding to detections associated with the second polarization reference signal; a second HD module configured to combine the second measurement signal and the LO signal, and produce a second detector signal having a portion corresponding to detections associated with the first polarization reference signal and a portion corresponding to detections associated with the second polarization reference signal; and a polarization-feedback module communicatively coupled with the first and second HD modules and the polarization-compensation module, the polarization-feedback module configured to selectively control the polarization-compensation module based onrespective portions of the first detector signal and the second detector signal obtained by filtering.

53. The RX of claim 52, wherein the portion of the first detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the third one of a set of three different polarization basis, the portion of the second detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the second one of a set of three different polarization basis, the portion of the second detector signal correspond to detections from the second polarization reference signal is used to control the polarization-feedback module in the first one of a set of three different polarization basis.

54. The RX of claim 52 or claim 53, further comprising: an RF-signal processing module configured to: filter the first detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a first power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal; filter the second detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a second power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal; and filter the second detection signal to isolate the portion corresponding to detections from the second polarization reference signal from the portion corresponding to detections from the first polarization reference signal and output a third power signal corresponding to a power level of the portion corresponding to detections from the second polarization reference signal.

55. The RX of claim 54, further comprising: a digital filtering module configured to low-pass filter the first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

56. The RX of any one of claims 53 to 55, wherein the polarization-feedback module comprises: a first proportional-integral-derivative (PID) controller configured to selectively control, based on the portion of the first detector signal corresponding to detections from the first polarization reference signal, first rotations of the wavelength-multiplexed signals’ polarizations about the third one of a set of three different polarization basis; a second PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the first polarization reference signal, second rotations of the wavelength-multiplexed signals’ polarizations about the second one of a set of three different polarization basis; and a third PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the second polarization reference signal, third rotations of the wavelength-multiplexed signals’ polarizations about the first one of a set of three different polarization basis.

57. The RX of claim 56, wherein the first PID controller, the second PID controller and the third PID controller are configured to modify an input error signal using a predefined sign function based on previous corrections and measurements, the input error signal is a difference between a current measurement and a target setpoint.

58. The RX of any one of claims 52 to 57, wherein the polarization-feedback module comprising an array of fiber squeezers.

59. The RX of claim 58, wherein the array of fiber squeezers comprises a first fiber squeezer for the second one of a set of three different polarization basis, a second fiber squeezer for the third one of a set of three different polarization basis, and a third fiber squeezer and a fourth fiber squeezer collectively calibrated for the first one of a set of three different polarization basis.

60. The RX of claim 59, wherein the third fiber squeezer is configured as a quarter waveplate.

61. The RX of any one of claims 52 to 57, wherein the polarization-feedback module comprises a plurality of sub-polarization-feedback modules including a first polarizationfeedback module and a second polarization-feedback module where the first polarizationfeedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

62. The RX of claim 61 , wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM), the EOM being selected from a free space EOM or a waveguide EOM.

63. The RX of any one of claims 58 to 62, wherein the first polarization-measurement module and the second polarization-measurement module comprises a motorized half-waveplate and motorized quarter waveplate and a polarizer.

64. The RX of claim 63, further comprising: a calibration module configured to back propagate LO signal through each of the first polarization-measurement module and the second polarization-measurement module; a detector configured to detect the back propagated LO signal; and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

65. The RX of any one of claims 52 to 64, further comprising: an LO-feedback module configured to cause the LO module to stabilize, based at least in part on the first detector signal, the second RF-offset Y at which the LO module emits the LO signal.

66. The RX of any one of claims 52 to 65, further comprising: a beam splitter configured to divide the LO signal to be supplied to each of the first HD module and the second HD module.

67. The RX of any one of claims 52 to 66, wherein the three different polarization basis are mutually unbiased bases.

68. The RX of claim 67, wherein the three different polarization basis are R / L, H / V, and D / A bases.

69. A system comprising: at least one RX of any one of claims 52 to 68; and at least one optical transmitter(s) (TX) optically coupled with a respective one of the RXs through the optical communications channel, each TX comprising: a light source configured to emit a light at a wavelength which is shifted relative to a respective polarized communications signal by the offset AA; an array of optical fdters configured to filter the light emitted;a reference signal generating module configured to produce from the light the first polarization reference signal and the second polarization reference signal; and a wavelength division multiplexing element configured to multiplex the first and second polarization reference signals with an obtained respective polarized communications signal, wherein each TX launches, into the optical communications channel, the wavelength- multiplexed signals.

70. The system of claim 69, wherein the reference signal generating module comprises: a beam splitter configured to divide the light into a first light and a second light; and a first reference signal generating module and a second reference signal generating module, the first reference signal generating module comprising: a first lens; a first polarizer; a quarter waveplate; and a half waveplate, the second reference signal generating module comprising: an RF-driven acousto-optic modulator (AOM) configured to frequency shift the second light to cause the first RF-offset X of the second polarization reference signal relative to the first polarization reference signal; a second lens; and a second polarizer.

71. The system of claim 70, wherein each TX further comprises an attenuation module configured to attenuate the first polarization reference signal and the second polarization reference signal output from the first reference signal generating module and the second reference signal generating module, respectively, which are combined upstream of the attenuation module.

72. The system of any one of claims 69 to 71, wherein the wavelength division multiplexing element is a dense wavelength division element (DWDM) configured to wavelength-multiplex the polarized communications signal, the first polarization reference signal, and the second polarization reference signal.

73. The system of claim 72, wherein respective polarized communications signals are a quantum signal.

74. The system of claim 73. wherein the attenuation module comprises a noise eater and an attenuator, and wherein the attenuation module is configured to output the first polarization reference signal and the second polarization reference signal having a power level between -10 dBm to -100 dBm.

75. A method comprising : receiving, over an optical communications channel, time-multiplexed signals comprising first and second polarization reference signals where one is shifted relative to the other by an RF-offset X, and a polarized communications signal, wherein, when launched into the optical communications channel, the first polarization reference signal is polarized in a first one of a set of three different polarization basis and the second polarization reference signal is polarized in a second one of a set of three different polarization basis, where the first one and second one are different, and demultiplexing the time-multiplexed signals based on time, and sending the polarized communications signal for further processing; measuring polarization in the second one of a set of three different polarization basis of the first polarization reference signal that was polarized in the first one of a set of three different polarization basis set to produce a first control signal; controlling, based on the first control signal, the polarizations of the time-multiplexed signals, by using a polarization-compensation module with a rotational axis about a third one of a set of three different polarization basis; measuring polarization in the third one of a set of three different polarization basis of the first polarization reference signal polarized in the first one of a set of three different polarization basis to produce a second control signal; controlling, based on the second control signal, the polarizations of the time-multiplexed signals, by using the polarization-compensation module with a rotational axis about the second one of a set of three different polarization basis; measuring polarization in the third one of a set of three different polarization basis of the second polarization reference signal polarized in the second one of a set of three different polarization basis to produce a third control signal; and controlling, based on the third control signal, the polarizations of the time-multiplexedsignals which also includes the polarized communications signal by using the polarizationcompensation module with rotational axis about the first one of a set of three different polarization basis.

76. The method of claim 75, further comprising: receiving a signal synchronization clock and controlling a frequency stabilized pulsed laser to produce light which is a seed for the first and second polarization reference signals, which when combined with the polarized communications signal will be offset in time.

77. The method of claim 76, further comprising: combining the first and second polarization reference signals with the polarized communications signal in time to have the offset.

78. The method of any one of claims 75 to 77, further comprising: receiving a signal synchronization clock, and wherein the demultiplexing comprises using the signal synchronization clock to separate the polarized communications signal from the first and second polarization reference signals.

79. The method of any one of claims 75 to 77, further comprising: detecting the measuring in the third one of a set of three different polarization basis, the detecting comprising: generating a Local oscillator (LO) having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted; and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the third one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the third one of a set of three different polarization basis and detecting results thereof.

80. The method of claim 79, further comprising: filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal; filtering using a second set of filters the detecting results to obtain a second filtered detection result to isolate detection from the second polarization reference signal; and detecting a power of the first filtered detection result and a power of the second filtered detection result.

81. The method of claim 80, further comprising: electric filtering based on time, the detecting results to isolate any detections corresponding to the polarized communications signal.

82. The method of claim 81, wherein the electric filtering comprises receiving a signal synchronization clock and filtering based on the signal synchronization clock received.

83. The method of any one of claims 80 to 82, wherein the second control signal is based on the power of the first filtered detection result and the third control signal is based on the power of second filtered detection result.

84. The method of any one of claims 75 to 78, further comprising: detecting the measuring in the second one of a set of three different polarization basis, the detecting comprising: generating a Local oscillator (LO) having a second RF-offset Y from either the first polarization reference signal or the second polarization reference signal which is not shifted; and performing heterodyne detection by mixing the LO with the measurement of the first polarization reference signal in the second one of a set of three different polarization basis and mixing the LO with the measurement of the second polarization reference signal in the second one of a set of three different polarization basis and detecting results thereof.

85. The method of claim 84, further comprising: filtering using a first set of filters the detecting results to obtain a first filtered detection result to isolate detection from the first polarization reference signal, and detecting a power of the first filtered detection result, wherein the first control signal is based the power of the first filtered detection result.

86. The method of claim 85, further comprising: electric filtering based on time, the detecting results prior to detecting the power to isolate any detections corresponding to the polarized communications signal.

87. The method of claim 86, wherein the electric filtering comprises receiving a signal synchronization clock and filtering based on the signal synchronization clock received.

88. The method of any one of claims 80 to 82 or claims 84 to 86, wherein the first control signal, the second control signal and the third control signal are generated using proportionalintegral derivative (PID) control based on an error signal determined from the power and a setpoint.

89. The method of claim 88, further comprising: modifying the error signal using a predefined sign function based on previous corrections and measurements.

90. The method of any one of claims 75 to 89, wherein the three different polarization basis are mutually unbiased bases.

91. The method of claim 90, wherein the three different polarization basis are R / L, H / V, and D / A bases.

92. An optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), time-multiplexed signals comprising a polarized communications signal, a first polarization reference signal, and a second polarization reference signal, wherein the first and second polarization reference signals each are offset in time relative to the polarized communications signal when combined, and where one polarization reference signal is shifted relative to the other by an RF-offset X, wherein when launched by the TX into the optical communications channel, the first polarized reference signal is polarized in a first one of a set of three different basis and the second polarization reference signal is polarized in a second one of set of three different polarization basis, where the first one and second one are different, the RX comprising: a polarization-compensation module configured to receive the time-multiplexed signals, and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the time-multiplexed signals about the three different polarization basis; a time demultiplexer configured to separate the polarized communications signal from the time-multiplexed first and second polarization reference signals; a first polarization-measurement module configured to measure a polarization of the time-multiplexed first and second polarization reference signals in a second polarization basis and issue a first measurement signal; a second polarization-measurement module configured to measure a polarization of the time-multiplexed first and second polarization reference signals in a third polarization basis and issue a second measurement signal;a local oscillator (LO) module configured to emit an LO signal that is frequency offset relative to the first polarization reference signal or the second polarization reference signal which is not shifted, by a second RF-offset Y a first heterodyne detection (HD) module configured to combine the first measurement signal and the LO signal, and produce a first detector signal having portions corresponding to detections associated with the first polarization reference signal and corresponding to detections associated with the second polarization reference signal; a second HD module configured to combine the second measurement signal and the LO signal, and produce a second detector signal having portions corresponding to detections associated with the first polarization reference signal and corresponding to detections associated with the second polarization reference signal; and a polarization-feedback module communicatively coupled with the first and second HD modules and the polarization-compensation module, the polarization-feedback module configured to selectively control the polarization-compensation module based on respective portions of the first detector signal and the second detector signal obtained by filtering.

93. The RX of claim 92, wherein the first and second polarization reference signals have the same wavelength as the polarized communications signal.

94. The RX of claim 92 or claim 93, wherein the time demultiplexer comprises an optical switch and a processor.

95. The RX of claim 94, wherein the processor is configured to receive a signal synchronization clock and control the optical switch based on the signal synchronization clock.

96. The RX of claim 95, wherein the signal synchronization clock is received from a source of the polarized communications signal.

97. The RX of any one of claims 92 to 96, wherein the portion of the first detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the third one of a set of three different polarization basis, the portion of the second detector signal corresponding to detections from the first polarization reference signal is used to control the polarization-feedback module in the second one of a set of three different polarization basis, the portion of the second detector signal correspond todetections from the second polarization reference signal is used to control the polarizationfeedback module in the first one of a set of three different polarization basis.

98. The RX of claim 97. further comprising: an RF-signal processing module configured to: filter the first detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a first power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal; filter the second detection signal to isolate the portion corresponding to detections from the first polarization reference signal from the portion corresponding to detections from the second polarization reference signal and output a second power signal corresponding to a power level of the portion corresponding to detections from the first polarization reference signal; and filter the second detection signal to isolate the portion corresponding to detections from the second polarization reference signal from the portion corresponding to detections from the first polarization reference signal and output a third power signal corresponding to a power level of the portion corresponding to detections from the second polarization reference signal.

99. The RX of claim 98, further comprising: an electric switch between the first and second HD modules and the RF-signal processing module, respectively, the electric switch being controlled by a processor to isolate any detection corresponding to the polarized communications signal from the RF-signal processing module and allow detections corresponding to the first and second polarization reference signals to reach the RF-signal processing module.

100. The RX of claim 99, further comprising: a digital filtering module configured to low-pass filter the first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

101. The RX of any one of claims 97 to 100, wherein the polarization-feedback module comprises:a first proportional-integral-derivative (PID) controller configured to selectively control, based on the portion of the first detector signal corresponding to detections from the first polarization reference signal, first rotations of the wavelength-multiplexed signals’ polarizations about the third one of a set of three different polarization basis; a second PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the first polarization reference signal, second rotations of the wavelength-multiplexed signals’ polarizations about the second one of a set of three different polarization basis; and a third PID controller configured to selectively control, based on the portion of the second detector signal corresponding to detections from the second polarization reference signal, third rotations of the wavelength-multiplexed signals’ polarizations about the first one of a set of three different polarization basis.

102. The RX of claim 101, wherein the first PID controller, the second PID controller and the third PID controller are configured to modify an input error signal using a predefined sign function based on previous corrections and measurements, the input error signal is a difference between a current measurement and a target setpoint.

103. The RX of any one of claims 92 to 102, wherein the polarization-feedback module comprising an array of fiber squeezers.

104. The RX of claim 103, wherein the array of fiber squeezers comprises a first fiber squeezer for the second one of a set of three different polarization basis, a second fiber squeezer for the third one of a set of three different polarization basis, and a third fiber squeezer and a fourth fiber squeezer collectively calibrated for the first one of a set of three different polarization basis.

105. The RX of claim 104, wherein the third fiber squeezer is configured as a quarter waveplate.

106. The RX of any one of claims 92 to 102, wherein the polarization-feedback module comprises a plurality of sub-polarization-feedback modules including a first polarizationfeedback module and a second polarization-feedback module where the first polarizationfeedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

107. The RX of claim 106, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM), the EOM being selected from a free space EOM or a waveguide EOM.

108. The RX of any one of claims 103 to 107, wherein the first polarization-measurement module and the second polarization-measurement module comprises a motorized half-waveplate and motorized quarter waveplate and a polarizer.

109. The RX of claim 108, further comprising: a calibration module configured to back propagate LO signal through each of the first polarization-measurement module and the second polarization-measurement module; a detector configured to detect the back propagated LO signal; and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

110. The RX of any one of claims 92 to 109, further comprising: an LO-feedback module configured to cause the LO module to stabilize, based at least in part on the first detector signal, the second RF-offset Y at which the LO module emits the LO signal.

111. The RX of any one of claims 92 to 110, further comprising: a beam splitter configured to divide the LO signal to be supplied to each of the first HD module and the second HD module.

112. The RX of any one of claims 92 to 111, wherein the three different polarization basis are mutually unbiased bases.

113. The RX of claim 112, wherein the three different polarization basis are R / L, H / V, and D / A bases.

114. A system comprising : at least one RX of any one of claims 92 to 113; and at least one optical transmitter(s) (TX) optically coupled with a respective one of the RXs through the optical communications channel, each TX comprising: a pulsed light source configured to emit pulsed light; a reference signal generating module configured to produce from the light the first polarization reference signal and the second polarization reference signal; anda time division multiplexing element configured to multiplex the first and second polarization reference signals with an obtained respective polarized communications signal, wherein each TX launches, into the optical communications channel, the time- multiplexed signals.

115. The system of claim 114, further comprising: a processor configured to receive a signal synchronization clock from a source of the obtained respective polarized communications signal and control a timing of an emission of pulsed light from the pulsed light source based on the signal synchronization clock.

116. The system of claim 114 or claim 115, wherein the time division multiplexing element comprises a fused fiber coupler.

117. The system of claim 114 or claim 115, wherein the time divisional multiplexing element comprises an optical switch and a processor, wherein the processor is configured to receive a signal synchronization clock and control the optical switch based on the signal synchronization clock such that the first and second polarization reference signals are offset in time with the polarized communications signal.

118. The system of any one of claims 114 to 117, wherein the reference signal generating module comprises: a beam splitter configured to divide the pulsed light into a first light pulse and a second light pulse; and a first reference signal generating module and a second reference signal generating module, the first reference signal generating module comprising: a first lens; a first polarizer; a quarter waveplate; and a half waveplate, the second reference signal generating module comprising: an RF-driven acousto-optic modulator (AOM) configured to frequency shift the second light pulse to cause the first RF-offset X of the second polarization reference signal relative to the first polarization reference signal; a second lens; anda second polarizer.

119. The system of claim 118, wherein each TX further comprises: an attenuation module configured to attenuate the first polarization reference signal and the second polarization reference signal output from the first reference signal generating module and the second reference signal generating module, respectively, which are combined upstream of the attenuation module.

120. The system of claim 119, wherein respective polarized communications signals are a quantum signal.

121. The system of claim 120, wherein the attenuation module comprises a noise eater and an attenuator, and wherein the attenuation module is configured to output the first polarization reference signal and the second polarization reference signal having a power level between -10 dBm to -100 dBm.

122. An optical receiver (RX) for receiving, over an optical communications channel from an optical transmitter (TX), multiplexed signals comprising a polarized communications signal and a polarization reference signal, wherein when launched by the TX into the optical communications channel, the polarization reference signal is polarized in a first one of a set of three different basis, the RX comprising: a polarization-compensation module configured to receive the multiplexed signals, and compensate for variations in their polarizations caused by propagation through the optical communications channel by selectively rotating polarizations of the multiplexed signals about the other two bases of three different polarization basis; a demultiplexer configured to separate the polarized communications signal from the multiplexed polarization reference signal; a first polarization-measurement module configured to measure a polarization of the polarization reference signal in a second polarization basis and issue a first measurement signal; a second polarization-measurement module configured to measure a polarization of the polarization reference signal in a third polarization basis and issue a second measurement signal; a local oscillator (LO) module configured to emit an LO signal that is frequency offset relative to the polarization reference signal;a first heterodyne detection (HD) module configured to combine the first measurement signal and the LO signal, and produce a first detector signal; a second HD module configured to combine the second measurement signal and the LO signal, and produce a second detector signal; and a polarization-feedback module communicatively coupled with the first and second HD modules and the polarization-compensation module, the polarization-feedback module configured to selectively control the polarization-compensation module based on the first detector signal and the second detector signal.

123. The RX of claim 122, wherein the polarized communications signal and the polarization reference signal are wavelength multiplexed, and where the polarization reference signal and the polarized communications signal have a different wavelength.

124. The RX of claim 122, wherein the polarized communications signal and the polarization reference signal are time multiplexed, and where the polarization reference signal and the polarized communication signal have the same wavelength.

125. The RX of any one of claims 122 to 124, wherein the first detector signal is used to control the polarization-feedback module in the third one of a set of three different polarization basis and the second detector signal is used to control the polarization-feedback module in the second one of a set of three different polarization basis.

126. The RX of claim 125, further comprising: a digital filtering module configured to low-pass filter first, second and third power signals and provide respective digital power signals to the polarization-feedback module.

127. The RX of any one of claims 122 to 126, wherein the polarization-feedback module comprises: a first proportional-integral-derivative (PID) controller configured to selectively control, based on the first detector signal first rotations of the multiplexed signal’s polarization about the third one of a set of three different polarization basis; and a second PID controller configured to selectively control, based on the second detector signal, second rotations of the multiplexed signals’ polarizations about the second one of a set of three different polarization basis.

128. The RX of claim 127, wherein the first PID controller and the second PID control are configured to modify an input error signal using a predefined sign function based on previouscorrections and measurements, the input error signal is a difference between a current measurement and a target setpoint.

129. The RX of any one of claims 122 to 128, wherein the polarization-feedback module comprises an array of fiber squeezers.

130. The RX of any one of claims 122 to 128, wherein the polarization-feedback module comprises a plurality of sub-polarization-feedback modules including a first polarizationfeedback module and a second polarization-feedback module where the first polarizationfeedback module and the second polarization-feedback module have different bandwidth and / or different retardance and / or insertion loss.

131. The RX of claim 130, wherein the first polarization-feedback module comprises an array of fiber squeezers and the second polarization-feedback module comprises an electro-optic modulator (EOM), the EOM being selected from a free space EOM or a waveguide EOM.

132. The RX of any one of claims 122 to 131, wherein the first polarization-measurement module and the second polarization-measurement module comprises a motorized half-waveplate and motorized quarter waveplate and a polarizer.

133. The RX of claim 132, further comprising: a calibration module configured to back propagate LO signal through each of the first polarization-measurement module and the second polarization-measurement module; a detector configured to detect the back propagated LO signal; and a processor configured to rotate at least one of the motorized quarter waveplate or half waveplate based on the detection and adjust the polarization-feedback module.

134. The RX of any one of claims 122 to 133, further comprising: an LO-feedback module configured to cause the LO module to stabilize, based at least in part on the first detector signal, the second RF-offset Y at which the LO module emits the LO signal.

135. The RX of any one of claims 122 to 134, further comprising: a beam splitter configured to divide the LO signal to be supplied to each of the first HD module and the second HD module.

136. The RX of any one of claims 122 to 135, wherein the three different polarization basis are mutually unbiased bases.

137. The RX of claim 136, wherein the three different polarization basis are R / L, H / V, and D / A bases.

138. A system comprising: a RX of any one of claims 122 to 137; and an optical transmitter(s) (TX) optically coupled with the RX through the optical communications channel, the TX comprising: a light source configured to emit a light; a reference signal generating module configured to produce from the light the polarization reference signal; and a multiplexing element configured to multiplex the polarization reference signal with an obtained respective polarized communications signal, wherein each TX launches, into the communications channel, the multiplexed signals.

139. The system of claim 138, wherein when the multiplexing is wavelength multiplexing, the TX further comprises an array of optical filters between the light source and the reference signal generating module.

140. The system of claim 139, wherein the multiplexing element is a dense wavelength division modulator (DWDM) configured to wavelength-multiplex the polarized communications signal, and the polarization reference signal.

Images