Optical Multiple-Input Multiple-Output (MIMO) Demultiplexer

Abstract

To provide a multi-stage optical MIMO demultiplexer.SOLUTION: An optical polarization demultiplexer 200 is formed by: a polarization splitter and rotator (PBSR) 202; two 50 / 50 couplers 204 and 206; and two phase shifters 208 and 210. The two phase shifters 208 and 210 are controlled by another control signals Φ1212 and Φ2214. Each of the two phase shifters 208 and 210 is a differential phase shifter. The phase shifter 208 is mounted as an interferometer having two individual phase shift elements 208a and 208b that adjust an optical phase to one direction with one arm of an interferometer, and adjust an optical phase to an opposite direction with the other arm. The phase shifter 210 also has the same structure.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] TECHNICAL FIELD This disclosure relates generally to optical demultiplexers. [Background technology]

[0002] In optical communication systems, multiplexing techniques (such as polarization-division multiplexing (PDM)) allow different signals to be multiplexed on different channels (e.g., different polarization modes of the same carrier frequency) and transmitted simultaneously through a single fiber, improving communication capacity and / or photon efficiency. However, a challenge with using PDM is that polarization modes tend to undergo random and unpredictable rotations and losses as they propagate through optical communication systems due to, for example, stresses (bending and twisting) in the glass fiber, ambient temperature changes, or other non-ideal properties of the communication system. As a result, signals with different polarization modes will mix when they are received. In such a scenario, signals must be separated at the receiver via multiple-input-multiple-output (MIMO) demultiplexing. Summary of the Invention [Means for solving the problem]

[0003] Implementations of the present disclosure are generally directed to optical demultiplexers, such as optical polarization demultiplexers.

[0004] One general aspect includes a MIMO optical coupler including a pair of MIMO inputs configured to input light into a first pair of optical transmission lines; a first optical phase shifter configured to apply a first relative phase shift between the first pair of optical transmission lines; a first 2x2 optical coupler configured to combine the first pair of optical transmission lines and output a second pair of optical transmission lines; a second optical phase shifter configured to apply a second relative phase shift between the second pair of optical transmission lines; a second 2x2 optical coupler configured to combine and output a third pair of optical transmission lines; a third optical phase shifter configured to apply a third relative phase shift between the third pair of optical transmission lines; a third 2x2 optical coupler configured to combine the third pair of optical transmission lines and output a fourth pair of optical transmission lines; and a 2x2 optical multiple-input multiple-output (MIMO) demultiplexer including a pair of MIMO outputs configured to output the fourth pair of optical transmission lines. Other embodiments of this aspect include corresponding computer systems, devices, and computer programs stored on one or more computer storage devices, each configured to perform the operations of the method.

[0005] Implementations may include one or more of the following features: a 2x2 optical MIMO demultiplexer, wherein the first optical phase shifter is configured to apply a first relative phase shift value that is a binary number; a 2x2 optical MIMO demultiplexer, wherein the first relative phase shift value is a binary number between c+π / 2 and c-π / 2 (c is a real number); a 2x2 optical MIMO demultiplexer, wherein the second optical phase shifter is configured to apply a second relative phase shift value within a finite range that includes -nπ and +nπ (n is an integer); a 2x2 optical MIMO demultiplexer, wherein the second optical phase shifter is configured for analog operation within the range (-nπ, +nπ); a 2x2 optical MIMO demultiplexer, wherein the third optical phase shifter is configured to apply a third relative phase shift value within a finite range determined by the value of the first relative phase shift. a 2x2 optical MIMO demultiplexer, wherein the third optical phase shifter is configured to operate between 0 and +nπ (n is an integer) based on the value of the first relative phase shift being c-π / 2, and to operate between -nπ and 0 based on the value of the first relative phase shift being c+π / 2; a 2x2 optical MIMO demultiplexer, wherein the third optical phase shifter is configured for analog operation within the range (0, +nπ) or the range (-nπ, 0); a 2x2 optical MIMO demultiplexer further including at least one processor and at least one memory storing instructions for performing operations for controlling values ​​of the first relative phase shift, the second relative phase shift, and the third relative phase shift based on being executed by the at least one processor; and a 2x2 optical MIMO demultiplexer further including a first optical attenuator configured to apply a first relative attenuation between the first pair of optical transmission lines. and a second optical attenuator configured to apply a second relative attenuation between a third pair of optical transmission lines. The second optical phase shifter, the third optical phase shifter, and the first optical phase shifter have a phase shift range of 2π or less. Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

[0006] Another general aspect includes an optical multiple-input multiple-output (MIMO) receiver including: an input port configured to receive input light; means for performing adaptive 2x2 optical MIMO polarization demultiplexing on the input light using three stages of optical phase shifting to output a first optical signal and a second optical signal; and at least one optical detector configured to detect the first optical signal and the second optical signal. Other embodiments of this aspect include corresponding computer systems, devices, and computer programs stored on one or more computer storage devices, each configured to perform the operations of the method.

[0007] Implementations may include one or more of the following features: An optical MIMO receiver in which each stage of three-stage optical phase shifting has a range of 2π or less. Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

[0008] Another general aspect includes a method for performing 2x2 optical multiple-input multiple-output (MIMO) demultiplexing, the method including: receiving light into a first pair of optical transmission lines via a pair of MIMO inputs; controlling a first optical phase shifter to apply a first relative phase shift between the first pair of optical transmission lines; combining the first pair of optical transmission lines with a first 2x2 optical coupler to output a second pair of optical transmission lines; and applying a second relative phase shift between the second pair of optical transmission lines.

[0013] The method includes controlling a second optical phase shifter to apply a third relative phase shift between the first and second optical transmission lines; coupling the second pair of optical transmission lines with a second 2x2 optical coupler to output a third pair of optical transmission lines; controlling a third optical phase shifter to apply a third relative phase shift between the third pair of optical transmission lines; coupling the third pair of optical transmission lines with a third 2x2 optical coupler to output a fourth pair of optical transmission lines; and outputting the fourth pair of optical transmission lines via a pair of MIMO outputs. Other embodiments of this aspect include corresponding computer systems, devices, and computer programs stored on one or more computer storage devices, each configured to perform the operations of the method.

[0009] Implementations may include one or more of the following features: controlling the first optical phase shifter includes applying a first relative phase shift value that is a binary number; the first phase shift value is a binary number between c+π / 2 and c-π / 2 (c is a real number); controlling the second optical phase shifter includes applying a second relative phase shift value within a finite range that includes -nπ and +nπ (n is an integer); controlling the second optical phase shifter is performed by an analog operation within the range (-nπ, +nπ); controlling the third optical phase shifter includes applying a third relative phase shift value within a finite range determined by the value of the first relative phase shift; controlling the third optical phase shifter further includes controlling the third optical phase shifter to operate between 0 and +nπ (n is an integer) based on the first relative phase shift value being c-π / 2, and to operate between -nπ and 0 based on the first relative phase shift value being c+π / 2. The method further includes: detecting a first reference signal on a first polarization channel at a first MIMO output of a pair of MIMO outputs; detecting a second reference signal on a second polarization channel at a second MIMO output of the pair of MIMO outputs; determining an error amount measured from the first reference signal and the second reference signal; and controlling at least one of the first optical phase shifter, the second optical phase shifter, or the third optical phase shifter based on the error amount measured from the first reference signal and the second reference signal. ... detecting a second reference signal on a second polarization channel at a second MIMO output of the pair of MIMO outputs; determining an error amount measured from the first reference signal and the second reference signal; and controlling at least one of the first optical phase shifter, the second optical phase shifter, or the third optical phase shifter based on the error amount measured from the first reference signal and the second reference signal. The method further includes: detecting a second reference signal on a second polarization channel at a second MIMO output of the pair of MIMO outputs; detecting a second reference signal on Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

[0010] Another general aspect includes an optical multiple-input multiple-output (MIMO) receiver configured to perform optical MIMO polarization demultiplexing. The optical MIMO receiver includes an input port configured to receive input light; a polarizing beam splitter rotator (PBSR) configured to split the input light into a pair of optical transmission paths; an optical phase shifter configured to apply a relative phase shift between the pair of optical transmission paths; a 2x2 optical coupler configured to couple the pair of optical transmission paths; and a controller configured to control the optical phase shifter using binary control having two operating states for applying the relative phase shift between the pair of optical transmission paths. Other embodiments of this aspect include corresponding computer systems, devices, and computer programs stored on one or more computer storage devices, each configured to perform the operations of the method.

[0011] Another general aspect includes an optical multiple-input multiple-output (MIMO) receiver configured to perform optical MIMO polarization demultiplexing. The optical MIMO receiver includes an input port configured to receive input light; a polarizing beam splitter rotator (PBSR) configured to split the input light into a pair of optical transmission paths; an optical attenuator configured to apply a relative attenuation between the pair of optical transmission paths; a 2x2 optical coupler configured to couple the pair of optical transmission paths; and a controller configured to control the relative attenuation applied by the optical attenuator. Other embodiments of this aspect include corresponding computer systems, devices, and computer programs stored on one or more computer storage devices, each configured to perform the operations of the method.

[0012] Implementations may include one or more of the following features: an optical MIMO receiver, wherein the optical attenuator is a differential optical attenuator configured to (i) apply a first attenuation to one of a pair of optical transmission lines and (ii) apply a second attenuation to the other of the pair of optical transmission lines, the first attenuation and the second attenuation being equal in decibel magnitude and opposite in sign; an optical MIMO receiver, further including a second optical attenuator configured to apply a second relative attenuation between a second pair of optical transmission lines, the second pair of optical transmission lines including light attenuated by the optical attenuator, and the controller further configured to control the second relative attenuation applied by the second optical attenuator. Implementations of the described techniques may include hardware, methods or processes, or computer software on a computer-accessible medium.

[0013] In some implementations, the techniques described herein for optical MIMO polarization demultiplexing can be applied to general 2x2 optical MIMO demultiplexing. For example, in some implementations, the techniques described herein can be implemented separately from or without PBSR.

[0014] The details of one or more implementations of the subject matter of this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the detailed description, the drawings, and the claims. [Brief explanation of the drawings]

[0015] [Figure 1] 1A and 1B are diagrams illustrating an example of a dual polarization communication system using coherent detection and direct detection (IMDD).

[0016] [Figure 2] FIG. 2 is a diagram showing an example of an optical polarization demultiplexer having two control signals.

[0017] [Figure 3] FIG. 3 is a diagram illustrating an example of an optical polarization demultiplexer according to an embodiment of the present disclosure.

[0018] [Figure 4] FIG. 4 is a diagram illustrating another example of an optical polarization demultiplexer according to an embodiment of the present disclosure.

[0019] [Figure 5] 5A and 5B are diagrams illustrating an example of a transmitter configured to transmit a reference signal (eg, a pilot tone).

[0020] [Figure 6] FIG. 6 illustrates an example demultiplexer configured to receive a reference signal (eg, a pilot tone) for generating feedback information, according to an embodiment of the present disclosure.

[0021] [Figure 7] 7A and 7B illustrate example demultiplexers configured to receive and process reference signals (eg, pilot tones) according to embodiments of the present disclosure.

[0022] [Figure 8] FIG. 8 is a flowchart illustrating an example of controlling an optical polarization demultiplexer according to an embodiment of the present disclosure.

[0023] [Figure 9] FIG. 9 is a flowchart illustrating an example of controlling a relative phase shift value in an optical polarization demultiplexer according to an embodiment of the present disclosure.

[0024] [Figure 10] FIG. 10 is a flowchart illustrating an example of controlling the relative attenuation value in an optical polarization demultiplexer according to an embodiment of the present disclosure.

[0025] [Figure 11] FIG. 11 is a diagram illustrating an example of a simulation result of the optical polarization demultiplexer according to the embodiment of the present disclosure.

[0026] [Figure 12] FIG. 12 illustrates an example of a computing system that can be used to implement one or more components of a system that performs adaptive control of an optical polarization demultiplexer. DETAILED DESCRIPTION OF THE INVENTION

[0027] (Detailed explanation) Disclosed herein are systems and techniques that provide a novel multi-stage optical MIMO demultiplexer (e.g., an optical MIMO polarization demultiplexer) that can achieve significantly improved efficiency and speed with lower data loss rates. This is achieved through a novel implementation that enables the "endless" characteristic of adaptive demultiplexing without requiring a reset that interrupts data reception. In some embodiments, this is achieved through an adaptive three-stage phase-shift demultiplexer structure, where the phase shifter in the first stage is controlled to apply a binary value, and the phase shifters in the second and third stages are controlled to operate over a finite range (e.g., a continuous range) of phase shift values. The control of the phase shifts in the three stages is tailored to accommodate random and unpredictable rotations and losses of received polarization without requiring a reset of the phase shift that interrupts signal reception, resulting in a characteristic referred to as "endless" operation of the demultiplexer.

[0028] Multi-polarization sensing is generally challenging because the polarization state of an optical waveform tends to drift as it passes through a communication system (e.g., due to random changes in birefringence in a fiber transmission line). In long-distance systems, this random polarization drift gradually accumulates without bound. In optical communication systems that use polarization division multiplexing (PDM) to transmit different signals in two polarization modes, random and unknown polarization drift makes it difficult for the receiver to accurately detect the proper orientation of the two polarization modes, resulting in the mixing of different signals at the receiver (sometimes called "crosstalk"). Thus, even if a signal is transmitted in one polarization mode, the signal may actually be received in both polarization modes at the receiver. In addition to polarization drift, other nonidealities, such as polarization-dependent loss (PDL), which amplifies or attenuates different polarization modes differently, can degrade performance in optical communication systems.

[0029] To compensate for polarization drift and other nonidealities, multi-polarized receivers require adaptive MIMO demultiplexing to separate and demix the signals transmitted in the two polarization modes. Such MIMO demultiplexing can be performed in the optical domain using optical phase shifters or in the electronics domain using digital signal processing (DSP). Optical MIMO demultiplexing offers several advantages over DSP-based MIMO demultiplexing. For example, optical demultiplexing can reduce power consumption, complexity, and sensitivity to symbol rate. On the other hand, DSP-based demultiplexing typically requires high power consumption and can be prohibitively complex for systems with high symbol rates or large mode counts.

[0030] Furthermore, optical polarization demultiplexing can be combined with intensity modulation and direct detection (IMDD) transmission methods, such as pulse amplitude modulation (PAM), which transmit information only as the square of the optical field. This is because optical demultiplexing can be performed before optical detection is performed on the light using optical elements that separate the two polarization modes of light. On the other hand, the nonlinearity of optical direct detection in IMDD results in information loss that cannot be recovered by DSP techniques alone, so DSP-based polarization demultiplexing cannot be used in combination with IMDD. Instead, DSP-based demultiplexing typically requires coherent reception. In such systems, the two polarization modes of light are first separated by coherent detection, and then the full field of each polarization is detected, allowing the DSP to perform processing on the received signal in both polarization modes. An example of this distinction is described below with reference to Figures 1A and 1B.

[0031] 1A and 1B illustrate exemplary dual-polarization communication systems 100 and 150 utilizing coherent and direct detection (IMDD), respectively. Transmitters (102 and 152) perform polarization division multiplexing by first splitting laser input light (104 and 154) into two optical paths that are directed to two modulators: a first modulator (106 and 156) and a second modulator (108 and 158). The first modulator (106 and 156) modulates the light in one optical path with a first data stream x or X (110 and 160), and the second modulator (108 and 128) modulates the light in the other optical path with a second data stream y or Y (112 and 162). In the coherent case, x and y are complex numbers representing the optical field, whereas in IMDD, X and Y are real numbers representing the optical power. In this disclosure, lowercase letters represent complex numbers (field) and uppercase letters represent real numbers (power). Two modulated optical waveforms, one modulated with x (X) and the other with y (Y), are combined in a polarizing beam splitter / rotator (PBSR) (114 and 164), which converts one optical waveform into an orthogonal polarization. After the PBSR, the two optical waveforms carrying x (X) and y (Y) coexist in the same optical transmission path but with orthogonal polarizations.

[0032] This dual-polarized (DP) optical waveform passes through fiber links (116 and 166). As the DP waveform passes through the fiber, the polarization of the two waveforms may change due to various unknown birefringence and twists within the fiber. If the fiber links (116 and 166) do not have significant polarization-dependent loss (PDL), the two polarizations remain orthogonal. For example, x (X) may change from linear horizontal polarization to right-handed circular polarization, which means that y (Y) changes from linear vertical polarization to left-handed circular polarization. However, the presence of PDL reduces the polarization orthogonality of the DP optical waveform, complicating demultiplexing of x (X) and y (Y).

[0033] At the receivers (118 and 168), the DP waveform enters the PBSRs (120 and 162), which split the DP waveform into two waveforms, h and v, with orthogonal polarizations. Due to nonidealities in optical communication systems, the PBSR outputs, h and v, are linear orthogonal combinations of x and y, respectively. (More precisely, the received signals are noisy versions of x and y due to additive noise in the system, but for this discussion we will assume a noiseless scenario.) In particular, h is a linear combination of x and y, and similarly, v is a linear combination of x and y. For example, h = (xy) / sqrt(2) and v = (x + y) / sqrt(2). The goal of MIMO demultiplexing is to extract the original signals x and y from the received h and v. This can be done via DSP-based demultiplexing (as in Figure 1A for coherent detection) or optical demultiplexing (as in Figure 1B for direct detection / IMDD).

[0034] In the coherent case of Figure 1A, there is a local oscillator (LO) laser (122) where h and v are interfered at two optical 90-degree hybrids (124 and 126). The output waveform from hybrid 126 is photodetected by photodetector 128, and the resulting electrical signal is fed to a digital signal processor (DSP) (130), which performs demultiplexing to separate signals x' and y' using MIMO signal processing. Thus, in the coherent reception example of Figure 1A, both the magnitude and phase of the optical field are detected by receiver 118, so MIMO demultiplexing must be performed by DSP 130 after detection by photodetector 128.

[0035] In contrast, in the IMDD reception example of Figure 1B, receiver 168 detects optical power. This nonlinearity results in information loss. Therefore, IMDD systems require MIMO demultiplexing using optical systems before optical detection. This is because IMDD loses optical phase information during optical detection, and no amount of electrical signal processing can always recover x and y. In the example of Figure 1B, h and v are linear orthogonal combinations of x and y, respectively. Therefore, using IMDD to directly detect h and v without first performing demultiplexing can result in a loss of basic information. However, if h and v are optically demultiplexed into x and y before optical detection, there is no information loss.

[0036] For two orthogonal polarization states, a PDM optical communication system can be represented as a 2x2 optical multiple-input multiple-output (MIMO) channel. Therefore, optical transmission can be modeled as a 2x2 matrix F. Matrix F is a transfer function that describes the effects of polarization and chromatic dispersion on communication from the transmitter to the receiver. For example, matrix F can model the effects of the fiber connecting the transmitter and receiver, as well as the effects of the optical components in the transmitter and receiver themselves. For purposes of this disclosure, matrix F will be referred to as the "channel matrix F," with the understanding that "channel" can represent various effects of the optical communication system, such as the fiber transmission line and transmitter and / or receiver components.

[0037]

number

[0038] To estimate the original signals x and y from the received signals h and v, an optical demultiplexer D is applied at the receiver to generate estimates x′ and y′.

[0039]

number

[0040] And if x' = ax and y' = bx ('a' and 'b' are complex constants), the receiver will successfully demultiplex the polarizations.

[0041] Consider the simple case of a lossless system (where the optical channel matrix F is unitary), which approximately applies to most short optical fiber links. In such a scenario, fiber losses, especially fiber polarization-dependent loss (PDL), can be neglected. The channel matrix F can then be characterized by four real numbers. Because the receiver only needs to achieve x′ = ax and y′ = bx for successful demultiplexing, the demultiplexing matrix D can be characterized by two real numbers. Therefore, in a lossless scenario, the four real numbers in the channel matrix F can be expressed as only two independently controlled real parameters to be compensated by the demultiplexing matrix D.

[0042] Therefore, for a unitary system (lossless scenario), an optical demultiplexer (i.e., matrix D above) requires a theoretical minimum of at least two phase control signals to reverse the effects of the channel matrix F and demultiplex. An example of a two-stage demultiplexer is described below with reference to Figure 2. However, using only two phase control signals in a demultiplexer poses a problem: demultiplexing requires an infinite range of phase shifts to achieve "endless" performance, which cannot be achieved using practical phase shifters. Instead, practical phase shifters have a finite limit to the range of phase shifts. Therefore, when demultiplexing optical signals that have passed through randomly varying phase distortions in a fiber, the phase shifters in a two-stage demultiplexer may reach the boundaries of their practical range and require a "reset," which can cause interruptions and / or delays in data reception. An example of this problem is described below with reference to Figure 2.

[0043] FIG. 2 illustrates an example of an optical polarization demultiplexer 200 with two control signals. The demultiplexer 200 consists of a polarization splitter rotator (PBSR) 202, two 50 / 50 couplers 204 and 206, and two phase shifters 208 and 210 (e.g., differential phase shifters). The two phase shifters 208 and 210 are controlled by separate control signals φ1 (212) and φ2 (214). In the example of FIG. 2, each of the phase shifters 208 and 210 is a differential phase shifter. For example, the phase shifter 208 is implemented as an interferometer with two separate phase-shifting elements (208a and 208b) that adjust the optical phase in one direction in one arm of the interferometer and in the opposite direction in the other arm. A similar structure is shown for the phase shifter 210. Alternatively, in some embodiments, each of the phase shifters 208 and 210 can be implemented as a non-differential phase shifter with only one phase shift element in a single arm. The differential implementation shown in FIG. 2 has several advantages over a non-differential implementation. For example, a differential implementation has the advantage of requiring less range per phase shifter. Furthermore, for thermo-optic phase shifters, a differential phase shifter has the advantages of halving worst-case power consumption compared to a single-phase phase shifter, as well as constant total power consumption and reduced thermal transients. For purposes of this disclosure, a differential phase shifter (e.g., phase shifter 208) is considered a single phase shifter, even though it is implemented with two phase shifters (e.g., phase shifter elements 208a and 208b), with the understanding that it has one control signal (e.g., φ1, 212).

[0044] With this structure, demultiplexer 200 can be represented by a matrix D (using Mueller notation for polarization).

[0045]

number

[0046] However, as mentioned above, the demultiplexer 200 configuration in Figure 2 suffers from a major problem: to achieve "endless" performance, demultiplexing requires an infinite range of phase shifts for φ1 (212). In practical systems, this means that as the demultiplexer 200 demultiplexes signals received over randomly varying fiber, the phase-shift control φ1 (212) eventually reaches the limits of its practical range. For example, if the phase shifters 208 and 210 are implemented as thermo-optic phase shifters, there are practical limits on the amount of input current. If the randomly drifting phase caused by channel F requires that φ1 be continuously increased, the input limit for φ1 necessitates that at some point the phase shifter 208 must be decreased by 2π (the so-called "reset"). However, signal reception must be interrupted during this reset, potentially resulting in data loss and potentially significant error bursts in high-speed communications.

[0047] To address this issue, demultiplexers can implement two or more stages of phase shifters. However, increasing the number of phase-shifting stages (as in the lossless scenario using a unitary demultiplexer) increases the complexity of the algorithm and control, slowing down the control speed of the large number of phase-shifting variables. Furthermore, it can be difficult to ensure that, for any given input, the phase-shifting control is not "trapped" into a specific state during operation (and cannot escape from the trapped state unless the phase shifter exceeds its limit). In addition, more complex control systems may face an increased risk of converging to a local state that is not the desired (e.g., suboptimal) multiplexing behavior. This complexity and uncertainty can make the design of dual-polarization IMDD systems challenging.

[0048] Furthermore, the presence of polarization-dependent loss (PDL) can complicate the challenge. PDL refers to two orthogonal polarizations that are attenuated differently, resulting in a non-unitary channel matrix F. While PDL can be negligible in fibers, PDL can be significant in discrete devices such as amplifiers and wavelength division multiplexers. Designing a non-unitary optical demultiplexer is difficult. In general, a non-unitary demultiplexer can be characterized by four real quantities, and the theoretical minimum control set consists of two optical phase shifters and two optical attenuators.

[0049] Disclosed herein for lossless scenarios without PDL is an implementation that achieves the "endless" property of optical MIMO polarization demultiplexing using as few as three stages of finite-range phase shifting, an example of which is described below with reference to Figure 3. Additionally, for PDL scenarios, disclosed herein is an implementation that achieves the "endless" property using as few as three stages of finite-range phase shifting and two stages of optical attenuation, an example of which is described below with reference to Figure 4.

[0050] 3 is a diagram illustrating an example of an optical polarization demultiplexer 300 according to an embodiment of the present disclosure. The demultiplexer 300 can be implemented as part of a direct detection receiver (e.g., receiver 168 of FIG. 1). In some embodiments, the demultiplexer 300 is implemented via integrated photonics, which can reduce cost compared to bulk optics.

[0051] Demultiplexer 300 includes three stages of phase shifting (302, 304, and 306). Each stage is controlled by a phase shift control signal. For example, first stage 302 is controlled by a first control signal 308, second stage 304 is controlled by a second control signal 310, and third stage 306 is controlled by a third control signal 312. Each control signal controls the amount of phase shifting implemented in the respective phase shift stage.

[0052] 3, each stage includes a phase shifter operating on a pair of optical transmission lines and a 2x2 coupler. For example, first stage 302 includes a pair of transmission lines 314 and 316, optical phase shift elements 318 and 320 (together forming a differential phase shifter), and a 2x2 coupler 322. Similarly, second stage 304 includes a pair of transmission lines 324 and 326, optical phase shift elements 328 and 330 (together forming a differential phase shifter), and a 2x2 coupler 332. Finally, third stage 306 includes a pair of transmission lines 334 and 336, optical phase shift elements 338 and 340 (together forming a differential phase shifter), and a 2x2 coupler 342.

[0053] While the example of Figure 3 shows a differential implementation of the phase shifter, some embodiments may use a non-differential implementation with only one optical phase shift element (in one transmission line) in a stage. Throughout this disclosure, the phase difference between two optical transmission lines (in a stage) will be referred to simply as "φ," regardless of whether the phase shift is implemented by a differential phase shifter (i.e., each phase shift element of a differential pair designed to shift by ±φ / 2, as shown in the example of Figure 3) or a non-differential phase shifter (which shifts the phase of light in only one transmission line by an amount of ±φ relative to the light in the other transmission line). Thus, the term "phase shifter" can apply to differential or non-differential phase shifters.

[0054] Phase shifters can be thermo-optical (thermo-optic phase shifter, TOPS), electro-optical (electro-optic phase shifter, EOPS), or other types. TOPS generally have the slowest response speed, but can be made faster by covering them with metal and / or shortening the distance to the heat sink. Power consumption in TOPS can be reduced by having the optical path pass through a heated region multiple times. EOPS can be operated, for example, by current injection, carrier depletion, or the Pockels effect. Each phase shifter can consist of multiple sections, including a fast-response, high-power phase shifter section and a slower, lower-power phase shifter section.

[0055] The 2x2 coupler can be implemented, for example, by a directional coupler, a multimode interference coupler, or an adiabatic coupler.

[0056] As described above, the three stages (302, 304, 306) of the demultiplexer 300 are coordinated within specific ranges or operating values ​​to enable the demultiplexer 300 to achieve the "endless" demultiplexing characteristic without requiring resetting of any of the phase shifters. In particular, in the example of FIG. 3, the first control signal φ1 of the first stage 302 is a digital signal having a value of either -π / 2 or +π / 2. The second control signal φ2 of the second stage 304 is an analog or digital signal operating over a continuous or discrete set of values ​​between -π and +π. The third control signal φ3 of the third stage 306 is an analog or digital signal operating over a continuous or discrete set of values ​​within a range dependent on the first control signal φ1, i.e., operating between 0 and +π when φ1 is -π / 2 and operating between -π and 0 when φ1 is +π / 2.

[0057] During operation of the demultiplexer 300, light passing through the fiber first enters a splitter, such as a PBSR 346, which splits the input light into two optical transmission paths 314 and 316. The PBSR splits the input light into two polarizations and rotates one polarization so that both outputs of the PBSR have the same polarization. Thus, optical transmission path 314 contains light that was in one polarization when it entered the PBSR, and optical transmission path 316 contains light that was in the orthogonal polarization when it entered the PBSR, but once in optical transmission paths 314 and 316, the light in both optical transmission paths 314 and 316 has the same polarization. While the example in FIG. 3 shows a splitter implemented by a PBSR 346, other types of splitters can be used, including passive integrated optical devices such as polarization splitting grating couplers (PSGCs).

[0058] The split input light enters two optical transmission lines 314 and 316 of the first stage 302 and undergoes a relative phase shift via phase shift elements 318 and 320, such that the light in one optical transmission line is phase shifted by an amount φ1 relative to the light in the other optical transmission line. This relative phase shift φ1 is controlled by control signal 308. The phase-shifted light in the two optical transmission lines then enters a 2x2 coupler 322, which combines the relatively phase-shifted light. This process is repeated via the second stage 304 and the third stage 306, each undergoing a different phase shift controlled by control signals φ2 (310) and φ3 (312).

[0059] Controller 344 controls the relative phase shift amounts of the three stages 302, 304, and 306 via control signals 308, 310, and 312. In a closed-loop feedback scenario, this control can be based on feedback information 348, which is, for example, a measurement of error in the received signal. Specific algorithms used by controller 344 to control and adjust control signals 308, 310, and 312 are described below with reference to FIGS. 8-10. While FIG. 3 shows controller 344 as part of demultiplexer 300, in some embodiments, controller 344 may be implemented separately in the receiver (as a separate component of receiver 168 in FIG. 1).

[0060] As described above, demultiplexer 300 compensates for random birefringence changes that rotate the polarization of light, caused by distortions introduced by optical communication systems. In addition to compensating for phase shifts, demultiplexers can also be designed to compensate for other non-idealities, such as polarization-dependent loss (PDL). While PDL may be negligible in most short optical fiber links, as fiber lengths increase, PDL can have a greater impact on the proper reception of optical signals.

[0061] In a polarization-dependent loss (PDL) scenario, the amount of loss experienced by each of the two polarization modes of light may be different. For example, the loss in the transverse magnetic (TM) mode may be greater or less than the loss in the transverse electric (TE) mode. This results in a non-unitary channel matrix F. In this case, demultiplexing by phase shift control alone may not be sufficient to completely separate the signals containing the two mixed polarization modes. Instead, a combination of optical phase shifters and optical attenuators is implemented in the demultiplexer, as described below with reference to Figure 4. In general, PDL can be caused by the fiber line itself or by other elements of the communication system, such as fiber connectors, isolators, amplifiers, splitters, fiber couplers, and PBSRs.

[0062] FIG. 4 illustrates an example of an optical polarization demultiplexer 400 according to an embodiment of the present disclosure. The demultiplexer 400 can be implemented as part of a direct detection receiver (e.g., receiver 168 of FIG. 1). In some embodiments, the demultiplexer 400 is implemented via integrated photonics, which can reduce cost compared to bulk optics. The demultiplexer 400 provides both relative attenuation control and relative phase shift control between the two polarization modes to compensate for PDL in the received optical waveform.

[0063] The demultiplexer 400 includes three stages (402, 404, and 406) of relative phase shift control and / or optical attenuation control. Each stage is controlled by one or more control signals. For example, the first stage 402 is controlled by a first attenuation control signal 408 and a first phase shift control signal 410. The second stage 404 is controlled by a second phase shift control signal 412. The third stage 406 is controlled by a second attenuation control signal 414 and a third phase shift control signal 416. Each control signal controls the amount of phase shift or optical attenuation implemented in the respective stage.

[0064] 4, the first stage 402 includes first and second optical transmission lines 418 and 420, first and second optical attenuators 422 and 424 (together forming a differential attenuator), first and second phase shift elements 426 and 428 (together forming a differential phase shifter), and a 2x2 coupler 430. Similarly, the second stage 404 includes first and second optical transmission lines 432 and 434, first and second phase shift elements 436 and 438 (together forming a differential phase shifter), and a 2x2 coupler 440. Finally, the third stage 406 includes first and second optical transmission lines 442 and 444, first and second optical attenuators 446 and 448 (together forming a differential attenuator), first and second phase shift elements 450 and 452 (together forming a differential phase shifter), and a 2x2 coupler 454.

[0065] While the example of FIG. 4 shows a differential implementation of the optical attenuator and optical phase shifter, some embodiments may use a non-differential implementation having only one optical attenuator (in one optical transmission line) and one phase shift element (in one optical transmission line) in one stage. Throughout this disclosure, the relative optical attenuation between two optical transmission lines is referred to simply as “a,” regardless of whether the attenuation is implemented with differential attenuators (i.e., each attenuator in the differential pair is designed to attenuate light by + / −a / 2, as shown in the example of FIG. 4 ) or a single optical attenuator (which attenuates light in only one optical transmission line by an amount + / −a relative to the light in the other optical transmission line). The attenuation “a” of an optical attenuator represents any suitable measure of attenuation, such as exponential loss, where the actual effect on light transmission is exponential in “a” (e.g., a field is multiplied by exp{−a / 2} when passing through an optical attenuator labeled a / 2, just as it is multiplied by exp{−iφ / 2} when passing through a phase shifter labeled φ / 2).

[0066] Similarly, the relative phase difference between two optical transmission lines is simply referred to as "φ", whether the relative phase shift is implemented by a differential phase shifter (i.e., each phase shift element of a differential pair designed to shift by + / -φ / 2, as shown in the example of Figure 3) or by a non-differential phase shifter (which shifts the phase of the light in only one optical transmission line by an amount of + / -φ relative to the light in the other optical transmission line).

[0067] As described above, the three stages of the demultiplexer 400 are coordinated within specific ranges or operating values ​​to enable the demultiplexer 400 to achieve the "endless" demultiplexing characteristic without requiring resetting of any of the phase shifters. Regarding phase shift control, in the example of FIG. 4, the first phase shift control signal φ1 (410) of the first stage 402 is a digital signal having a value of either -π / 2 or +π / 2. The second phase shift control signal φ2 (412) of the second stage 404 is an analog or digital signal operating over a continuous or discrete set of values ​​between -π and +π. The third phase shift control signal φ3 (416) of the third stage 406 is an analog or digital signal operating over a continuous or discrete set of values ​​within a range that depends on the first control signal φ1 (410), i.e., operating between 0 and +π when φ1 is -π / 2 and operating between -π and 0 when φ1 is +π / 2. For attenuation control, the first attenuation control signal a1 (408) and the second attenuation control signal a2 (414) each operate over a continuous or discrete set of values ​​within a range. For example, the range may be (-3, +3). For example, the range may be (-1, +1). As yet another example, the range may be (-0.6, +0.6), which corresponds to approximately -5.2 dB to +5.2 dB. Other suitable ranges may be used.

[0068] During operation of the demultiplexer 400, light passing through the fiber first enters a splitter, such as a PBSR 458, which splits the input light into two optical transmission paths 418 and 420. While the example in FIG. 4 shows a splitter implemented by a PBSR 458, other types of splitters can be used, including passive optical integrated devices such as a polarization splitting grating coupler (PSGC). The split input light enters the two optical transmission paths 418 and 420 of the first stage 402 and undergoes relative attenuation via optical attenuators 422 and 424, such that the light in one optical transmission path is attenuated relative to the light in the other optical transmission path. This relative attenuation, a1, is controlled by an attenuation control signal 408.

[0069] The relatively attenuated light in the two optical transmission lines then undergoes a relative phase shift via phase shift elements 426 and 428 (forming a differential phase shifter) such that the phase of the light in one optical transmission line is shifted relative to the phase of the light in the other optical transmission line. The amount of this relative phase shift φ1 is controlled by control signal 410. The phase-shifted light in the two optical transmission lines then enters 2x2 coupler 430, which combines the relatively phase-shifted light. This process continues through second stage 404 and third stage 406, such that the two polarizations of light undergo relative phase shifts and / or relative attenuation controlled by phase control signals 412 and 416, and attenuation control signal 414.

[0070] Controller 456 controls the relative attenuation and relative phase shift of the different stages 402, 404, and 406 via control signals 408, 410, 412, 414, and 416. By controlling both the relative attenuation and relative phase shift between the two polarizations of light, demultiplexer 400 can compensate for both random phase shift and PDL (non-unitary channel matrix F). In a closed-loop feedback scenario, this control can be based on feedback information 460, for example, a measurement of error in the received signal. Specific algorithms used by controller 456 to control and adjust control signals 408, 410, 412, 414, and 416 are described below with reference to FIGS. 8-10. While FIG. 4 illustrates controller 456 as part of demultiplexer 400, in some embodiments, controller 456 may be implemented separately in the receiver (as a separate component of receiver 168 in FIG. 1).

[0071] In general, the control (e.g., by controller 344 in FIG. 3 or 456 in FIG. 4) is designed to reduce the amount of crosstalk between signals received in the two polarization modes of the optical waveform. In a feedback control scenario, the controller can adjust the control based on feedback information (e.g., feedback 348 in FIG. 3 and feedback 460 in FIG. 4). The feedback information can include, for example, a measurement of an error in the received waveform. The controller can be designed to adjust the control signal to reduce the measured error. The error measurement can be implemented in various ways. For example, the error measurement can reflect the amount of crosstalk between signals in the two polarization modes of the light.

[0072] To measure the amount of crosstalk, in some embodiments, a communication system may utilize a reference signal (e.g., a pilot tone or signal) that is transmitted in addition to the information-carrying signal. The reference signal has waveform characteristics that are known to both the transmitter and the receiver, allowing the receiver to estimate and compensate for the random effects of the communication channel.

[0073] 5A and 5B illustrate example transmitters 500 and 520 according to embodiments of the present disclosure configured to transmit reference signals (e.g., pilot tones). The transmitter 500 of FIG. 5A transmits pilot tones 502(A) and 504(B) in respective optical polarization modes of the laser input. In some embodiments, the pilot tones 502 and 504 are low-frequency tones and can have different tone frequencies for the two polarizations. For example, the first pilot tone 502 can be transmitted at a frequency of 1 MHz, and the second pilot tone 504 can be transmitted at a frequency of 2 MHz. The modulation depth of the pilot tones 502 and 504 is a percentage of the signal average power; for example, the modulation depth of the pilot tones 502 and 504 can be 2% of the signal average power.

[0074] In the example of Figure 5A, pilot tones 502(A) and 504(B) are added to electrical signals 506(X) and 508(Y), respectively, before modulating the laser input of each waveguide. For example, pilot tones 502(A) and 504(B) can be applied by digitally adding the tones to a digital-to-analog converter (DAC) output. Alternatively, pilot tones 502(A) and 504(B) can be applied by analogically adding them to the inputs of the drivers of modulators 510 and 512, or internal to the drivers of modulators 510 and 512, or to the outputs of the drivers of modulators 510 and 512.

[0075] Figure 5B shows an example of a transmitter 520 showing further details of the modulation and pilot tones. In this example, pilot tones 522(A) and 524(B) are applied by adding them in an analog manner to modulated signals 526(X) and 528(Y), respectively, at the outputs of drivers 534 and 536 of modulators 530 and 532. In the example of Figure 5B, modulators 530 and 532 are shown implemented as Mach-Zehnder interferometer (MZI) modulators, although other suitable optical modulators may be used.

[0076] Thus, in transmitters 500 and 520 in Figures 5A and 5B, pilot tones A and B are added to input signals X and Y, respectively, and then combined in PBSRs 514 and 538 for transmission over fiber. Specifically, pilot tone A and signal X are transmitted in one optical polarization mode, while pilot tone B and signal Y are transmitted in the other optical polarization mode. The combined optical PDM waveform propagates through the communication system toward the receiver, where various nonidealities within the system cause random and unpredictable rotational drift of the two polarization modes and polarization-dependent loss (PDL). These nonidealities affect both the pilot tones and the signals propagating in each polarization mode. Because the pilot tones (A and B) are known, the receiver can measure the deviation (or error) of the received pilot tones compared to the original pilot tones (A and B), which allows the receiver to estimate the error in signals X and Y themselves. Based on the error estimate, the receiver can then compensate for the polarization drift and PDL, more accurately recovering signals X and Y.

[0077] Examples of receiver structures for detecting pilot tones and measuring errors in received pilot tones are described below with reference to Figures 6, 7A, and 7B. Examples of using such error measurements in feedback control of relative phase shift and / or relative attenuation are described below with reference to Figures 9 and 10.

[0078] 6 illustrates an example demultiplexer 600 configured to receive pilot tones to generate feedback information, according to an embodiment of the present disclosure. In the receiver 600, received waveforms 614 and 616 of two polarizations of received light are processed by a pilot tone detector 602 to detect the power of the received pilot tone in each polarization mode 614 and 616. The pilot tone detector 602 then provides one or more pilot tone measurements 604 as feedback information (e.g., as feedback 348 in FIG. 3 and feedback 460 in FIG. 4) to a controller 606. The controller 606 uses these pilot tone measurements 604 to adapt control signals (608, 610, 612) that apply relative phase shifts and / or relative attenuation to the received optical signals.

[0079] While FIG. 6 illustrates controller 606 and pilot tone detector 602 as part of demultiplexer 600, in some embodiments, controller 606 and / or pilot tone detector 602 may be implemented separately in a receiver (as another component of receiver 168 of FIG. 1). Additionally, while FIG. 6 illustrates controller 606 and pilot tone detector 602 as separate modules, in some embodiments, controller 606 and pilot tone detector 602 may be implemented by an integrated circuit without being separated into separate modules. Additionally, while the example of FIG. 6 illustrates a scenario in which only relative phase shift is adapted via control signals 608, 610, and 612 (e.g., as in demultiplexer 300 of FIG. 3), these techniques may also be applied to adapt both relative phase shift and relative attenuation (e.g., as in demultiplexer 400 of FIG. 4).

[0080] In the example of FIG. 6, assume that a first pilot tone (A) is transmitted in a first polarization mode (say X) and a second pilot tone (B) is transmitted in a second polarization mode (say Y). At the receiver, it is desirable for the received polarization modes (H, V) to satisfy H = X and V = Y. However, as the optical waveform passes through the communication system, polarization drift and PDL can cause the two polarization modes carrying the two pilot tones (A and B) to undergo random and unpredictable rotations. Therefore, upon receiving these randomly rotated polarization modes, when the demultiplexer 600 attempts to detect pilot tones A and B, it may actually detect a mutual mixing of pilot tones A and B in each polarization mode H and V.

[0081] To estimate this reciprocal mixing effect, the receiver can detect the power of each pilot tone (A and B) in each of the two polarization modes (H and V). For example, in FIG. 6, pilot tone detector 602 detects the power of tone A (P HA ), the power of tone B in polarization mode H (P HB ), the power of tone A in polarization mode V (P VA ), and the power of tone B in polarization mode V (P VB Four different quantities can be detected: P HB and P VA represents the amount of crosstalk between pilot tones A and B in the two polarization modes H and V.

[0082] The controller 610 then calculates an error signal based on these received pilot tone components to estimate the amount of crosstalk between the two polarization modes caused by non-idealities in the communication system. For example, in some embodiments, the error can be calculated as follows:

[0083]

number

[0084] However, other measures of error can be used to estimate the amount of crosstalk between the pilot tones (A and B) in the two polarization modes (H and V). In general, the error measure is P HB and / or P VA should increase as the value of increases. The error measure provides an estimate of how well the controller 606 adapts the control signals (e.g., 608, 610, and 612) to adjust the relative phase shift and / or relative attenuation between the two polarization modes H and V to compensate for random polarization drift and PDL. The controller 606 can therefore use this error measurement in a feedback control loop to dynamically adjust the control signals (e.g., 608, 610, and 612) to further reduce the error. Details of an exemplary feedback algorithm are described below with reference to FIGS. 9 and 10.

[0085] Pilot tones A and B can be detected from the received waveform at various points in the reception process, examples of which are described below with reference to Figures 7A and 7B.

[0086] 7A and 7B illustrate different exemplary embodiments of demultiplexers according to embodiments of the present disclosure configured to receive and process pilot tones. Specifically, FIGS. 7A and 7B illustrate examples of detecting pilot tones (A and B) at different points in the reception process. As discussed above, the power of each pilot tone A and B in each of the two polarization modes (H and V) must be detected. In the example demultiplexer 700 of FIG. 7A, the pilot tones (A and B) are detected from the received waveform at the outputs of transimpedance amplifiers (TIAs) 704 and 706. Alternatively, as shown in the example demultiplexer 720 of FIG. 7B, the pilot tones (A and B) are detected from the received waveform in the optical domain (specifically, at the outputs of separate photodiodes 724 and 726 that are coupled to the received optical signal via an optical coupler).

[0087] In both the examples of Figures 7A and 7B, the various received powers of the pilot tone components can be detected by using Fourier transform techniques, such as multiplying the received signals by sines and / or cosines at the pilot tone frequencies and summing the results or filtering the results with a narrowband electrical filter.

[0088] An example of using error measurements in feedback control of relative phase shift and / or relative attenuation will now be described with reference to Figures 8-10. The control system functions to minimize the measured error in the received optical waveform. When the error is minimized, each of the PDM signals is received in its respective polarization mode with minimal crosstalk (e.g., signal X is received in polarization mode H and signal Y is received in polarization mode V).

[0089] 8 is a flow chart illustrating an example method 800 for controlling an optical polarization demultiplexer according to an embodiment of the present disclosure. Method 800 can be used to control relative phase shifts in a demultiplexer, such as demultiplexer 300 of FIG. 3.

[0090] In step 802, light is received into a first pair of optical transmission lines (314, 316) via a pair of MIMO inputs. In step 804, a first optical phase shifter (e.g., a differential phase shifter formed by 318 and 320) is controlled to apply a first relative phase shift between the first pair of optical transmission lines (314, 316). In some embodiments, the first optical phase shifter can be controlled in a binary manner, for example, between values ​​(c+π / 2) and (c−π / 2), where “c” is a real number reflecting the offset. This control can be based on feedback information (e.g., using pilot tones).

[0091] In step 806, the first pair of optical transmission lines (314, 316) is coupled to a first 2x2 optical coupler (322) to output a second pair of optical transmission lines (324, 326).

[0092] In step 808, a second optical phase shifter (e.g., a differential phase shifter formed by 328 and 330) is controlled to apply a second relative phase shift between the second pair of optical transmission lines (324, 326). In some embodiments, the second optical phase shifter can be controlled within a finite range of values ​​including -nπ and +nπ, where "n" is an integer. For example, this can be by analog operation within the range (-nπ, +nπ). This control can be based on feedback information (e.g., using a pilot tone).

[0093] In step 810, the second pair of optical transmission lines (324, 326) is coupled to a second 2x2 optical coupler (332) to output a third pair of optical transmission lines (334, 336).

[0094] In step 812, a third optical phase shifter (e.g., a differential phase shifter formed by 338 and 340) is controlled to apply a third relative phase shift between the third pair of optical transmission lines (334, 336). In some embodiments, the third optical phase shifter can be controlled within a finite range that depends on the value of the first relative phase shift. For example, as described above, the third optical phase shifter can be controlled to operate between 0 and +nπ when the first relative phase shift is equal to (c-π / 2), and between -nπ and 0 when the first relative phase shift is equal to (c+π / 2), where "n" is an integer. This can be performed by analog operation within the ranges (0, +nπ) and (-nπ, 0). This control can be based on feedback information (e.g., using pilot tones).

[0095] In step 814, the third pair of optical transmission lines (334, 336) is coupled to a third 2x2 optical coupler (342) to output a fourth pair of optical transmission lines (350, 352). In step 816, the fourth pair of optical transmission lines (350, 352) is output via a pair of MIMO outputs.

[0096] Although the exemplary method 800 of Figure 8 shows a particular order of steps, one or more of these steps may be performed in a different order. For example, the control of the first, second, and third optical phase shifters may be performed in a different order. A specific example of controlling and adjusting three phase shifters is described with reference to Figure 9.

[0097] 9 is a flowchart illustrating an example method 900 for controlling relative phase shift values ​​in an optical polarization demultiplexer according to an embodiment of the present disclosure. Method 900 illustrates a particular way to adjust first, second, and third phase shifters to achieve the “endless” property of optical MIMO polarization demultiplexing using only three stages of finite-range phase shifting (for a lossless scenario with no PDL). For illustrative purposes, the description of method 900 is provided with reference to demultiplexer 300 of FIG. 3.

[0098] Method 900 is an iterative process that adapts relative phase shift control signals 308, 310, and 312 to gradually reduce a measured feedback error (eg, feedback 348 in FIG. 3, or feedback 604 in FIG. 6).

[0099] In step 902, at the start of an iteration, the demultiplexer initializes the relative phase shift values ​​of the three control signals 308, 310, and 312. For example, in some embodiments, the first control signal φ1 (308) is a binary (digital) value and is initially set to either −π / 2 or +π / 2. The second control signal φ2 (310) is a continuous (analog) or discrete (digital) value and is initially set to any value between −π and +π. The third control signal φ3 (312) is also a continuous (analog) or discrete (digital) value and is set to any value between 0 and +π if the first control signal φ1 (308) was set to −π / 2; otherwise, the third control signal φ3 (312) is set to any value between −π and 0 if the first control signal φ1 (308) was set to +π / 2. This relationship between the third control signal φ3 (312) and the first control signal φ1 (308) is maintained throughout the control process of the method 900.

[0100] In step 904, the third control signal φ3 (312) is adjusted (within its current range) to reduce the measured error in the feedback (e.g., feedback 348 in FIG. 3). The adjustment of the third control signal φ3 (312) can be performed by an optimization or pseudo-optimization algorithm (e.g., a gradient descent algorithm) that attempts to minimize or reduce the measured error. For example, the adjustment of the third control signal φ3 (312) can be performed by searching within a local neighborhood of the current value of the third control signal φ3 (312) to find a new value that reduces the measured error. As a specific example, consider adjusting the third control signal φ3 (312) in steps of + / −Δφ3 to find a value that reduces the measured error. The step size Δφ3 can be dynamically adjusted with each iteration. If the value of the third control signal φ3 (312) is within Δφ3 of the boundary of its range (i.e., within Δφ3 of either 0, +π, or −π), the third control signal φ3 (312) remains unchanged. If not, the third control signal φ3 (312) is first increased by Δφ3 and the resulting error in the feedback 348 is measured. Next, the third control signal f3 (312) is decreased by 2Δφ3 (i.e., decreased from its original value by Δφ3) and the resulting error in the feedback 348 is again measured. The value of the third control signal φ3 (312) with the reduced error is assigned as the new adjusted value of the third control signal φ3 (312).

[0101] In step 906, the second control signal φ2 (310) is adjusted to reduce the measured error. The adjustment of the second control signal φ2 (310) can be performed by an optimization or pseudo-optimization algorithm (e.g., a gradient descent algorithm) that attempts to minimize or reduce the measured error. For example, the adjustment of the second control signal φ2 (310) can be performed by searching within a local neighborhood of the current value of the second control signal φ2 (310) to find a new value that reduces the measured error. As a specific example, consider adjusting the second control signal φ2 (310) in steps of + / −Δφ2 to find a value that reduces the measured error. The step size Δφ2 can be dynamically adjusted at each iteration. For example, in some embodiments, the step size Δφ2 is calculated by multiplying the value sin (of the third control signal 312) by Δφ2. 2 In the search process of step 906, the second control signal φ2 (310) is first increased by Δφ2 and the resulting error in the feedback 348 is measured. Next, the second control signal φ2 (310) is decreased by 2Δφ2 (i.e., decreased from its original value by Δφ2), and the resulting error in the feedback 348 is again measured. The value of the second control signal φ2 (310) with the reduced error is denoted (for purposes of this description) as φ2′.

[0102] In step 908, the demultiplexer determines whether the value φ2′<−π (i.e., outside the lower limit). If so, in step 910, a new adjusted value of the second control signal φ2 (310) is set to −2π−φ2′. Further, in step 912, the values ​​of the first control signal φ1 (308) and the third control signal φ3 (312) are inverted. That is, if the value of the first control signal (308) is φ1=−π / 2 (meaning the third control signal 312 is within the range of 0 and +π), then a value of π is simultaneously added to the first control signal φ1 (308) and subtracted from the third control signal φ3 (312). Alternatively, if the value of the first control signal (308) is φ1 = +π / 2 (meaning that the third control signal 312 is between -π and 0), then the value of π is simultaneously subtracted from the first control signal φ1 (308) and added to the third control signal φ3 (312). During this simultaneous addition and subtraction, the control loop must be paused. In some embodiments, the simultaneous addition and subtraction of π may be performed sequentially (e.g., adjusting the first control signal φ1 (308) and then adjusting the third control signal φ3 (312), or vice versa). Nevertheless, the procedure for adjusting the first control signal φ1 (308) and the third control signal φ3 (312) described above must be performed quickly to avoid long downtimes and control delays in the control system.

[0103] If step 908 determines that φ2′ is not outside the lower bound, then the demultiplexer checks whether φ2′>+π (i.e., outside the upper bound) in step 914. If so, the new adjusted value of the second control signal φ2 (310) is set to +2π-φ2′ in step 916. Additionally, in step 912 (as described above), the values ​​of the first control signal φ1 (308) and the third control signal φ3 (312) are inverted.

[0104] If, in step 914, it is determined that φ2' is not outside the upper limit (meaning that φ2' is within the range of -π to +π), then in step 918, the new adjusted value of the second control signal φ2 (310) is set to φ2'. In this case, the first control signal φ1 (308) and the third control signal φ3 (312) are not inverted. Then, the process returns to step 904 and the next iteration of adjusting the control signals is performed.

[0105] The control process of method 900 can achieve "endless" demultiplexing operation without requiring a reset or interruption of data reception. This characteristic is made possible by the second stage phase shift (304 in FIG. 3) acting as a pass-through when the second control signal φ2 (310) reaches either of the boundary points (+π or -π). At this time, when the second control signal φ2 (310) is at the boundary point of its range, π is simultaneously added or subtracted from the first control signal φ1 (308) and the third control signal φ3 (312) (as described above in step 912). In this way, "endless" polarization demultiplexing operation is achieved without requiring a reset or interruption of data reception.

[0106] 9 shows a particular order of steps, one or more of these steps may be performed in a different order. For example, steps 908 and 914, i.e., checking whether the second control signal φ2 (310) is within the lower and upper limits of the range -π to +π, may be reversed.

[0107] Furthermore, the specific numerical ranges described in step 902 can be modified. For example, the possible values ​​of the first control signal φ1 (308) can have a fixed offset so that they are shifted binary values ​​of (-π / 2+c) or (+π / 2+c). The possible values ​​of the second control signal φ2 (310) can be shifted by an integer multiple of 2π, as long as the boundary points of the range allow for the pass-through characteristics described above. Also, the possible values ​​of the third control signal φ3 (312) can be shifted by an integer multiple of 2π.

[0108] 10 is a flowchart illustrating an example of a method 1000 for controlling relative attenuation values ​​in an optical polarization demultiplexer according to an embodiment of the present disclosure. Method 1000 can be used to control relative attenuation signals, such as relative attenuation control signals a1 (408) and a2 (414) in demultiplexer 400 of FIG. 4. For illustrative purposes, a description of method 1000 will be provided with reference to demultiplexer 400 of FIG. 4.

[0109] Although the exemplary method 1000 illustrates control of both relative attenuation control signals a1 (408) and a2 (414), in some scenarios, only one of the signals is implemented. For example, in some embodiments, only the first control signal a1 is implemented. This may be appropriate, for example, in scenarios where PDL levels are moderate (e.g., scenarios where the only source of PDL is at the receiver and not the fiber transmission line itself). Furthermore, if the PDL value is not expected to change significantly over time, the control value a1 can be set once at the beginning of operation (e.g., in the factory) and left unchanged.

[0110] Alternatively, as shown in method 1000, both optical attenuation control signals a1 and a2 can be adjusted (e.g., continuously) using, for example, a variable optical attenuator (VOA), which may be appropriate, for example, in scenarios where PDL levels are more important (e.g., scenarios where PDL occurs both at the receiver and in the fiber transmission line).

[0111] In general, the relative attenuation signals a1 (408) and a2 (414) can be controlled using an optimization or pseudo-optimization process designed to reduce or minimize a measured error in the feedback (e.g., feedback 460 in FIG. 4 or feedback 604 in FIG. 6). For example, in some embodiments, the relative attenuation control signals a1 (408) and a2 (414) can be controlled simultaneously through joint optimization. As another example, shown in method 1000 of FIG. 10, an iterative process can be implemented to adapt the relative attenuation control signals a1 (408) and a2 (414) to gradually reduce the measured feedback error.

[0112] In step 1002, at the beginning of an iteration, the demultiplexer initializes the two VOA control signals a1 (408) and a2 (414) to initial values, for example, zero values.

[0113] In step 1004, the first VOA control signal a1 (408) is adjusted (within its tolerance range, such as -3 to +3) to reduce the measured error in the feedback. The adjustment of the first VOA control signal a1 (408) can be performed by an optimization or pseudo-optimization algorithm (e.g., a gradient descent algorithm) that attempts to minimize or reduce the measured error. For example, the adjustment of the first VOA control signal a1 (408) can be performed by searching within a local neighborhood of the current value of the first VOA control signal a1 (408) to find a new value that reduces the measured error. As a specific example, consider adjusting the first VOA control signal a1 (408) in steps of + / - Δa1 to find a value that reduces the measured error. The step size Δa1 can be dynamically adjusted at each iteration. The first VOA control signal a1 (408) is first increased by Δa1, and the resulting error in the feedback 460 is measured. Next, the first VOA control signal a1 (408) is decreased by 2Δa1 (i.e., decreased by Δa1 from its original value) and the error resulting from the feedback 460 is again measured. The value of the first VOA control signal a1 (408) with the reduced error is assigned as the new adjusted value of the first VOA control signal a1 (408).

[0114] In step 1006, the second VOA control signal a2 (414) is adjusted (within its tolerance range, such as -3 to +3) to reduce the measured error. The adjustment of the second VOA control signal a2 (414) can be performed by an optimization or pseudo-optimization algorithm (e.g., a gradient descent algorithm) that attempts to minimize or reduce the measured error. For example, the adjustment of the second VOA control signal a2 (414) can be performed by searching within a local neighborhood of the current value of the second VOA control signal a2 (414) to find a new value that reduces the measured error. As a specific example, consider adjusting the second VOA control signal a2 (414) in steps of + / - Δa2 to find a value that reduces the measured error. The step size Δa2 can be dynamically adjusted at each iteration. For example, in some embodiments, the step size Δa2 is calculated by multiplying the value sin (of the first VOA control signal 408) by Δa2. 2 (a1) may be configured to increase as (a1) becomes smaller (or vice versa). In the search process of step 1006, the second VOA control signal a2 (414) is first increased by Δa2, and the error resulting from the feedback 460 is measured. Next, the second VOA control signal a2 (414) is decreased by 2Δa2 (i.e., decreased from its original value by Δa2), and the error resulting from the feedback 460 is again measured. The value of the second VOA control signal a2 (414) with the reduced error is assigned as the new adjusted value of the second VOA control signal a2 (414). Then, returning to step 1004, the next iteration of adjusting the control signal is performed.

[0115] Although the example method 1000 of Figure 10 shows a particular order of steps, one or more of these steps may be performed in a different order. For example, steps 1004 and 1006 may be reversed. Furthermore, specific numerical ranges may also be changed. For example, the range of values ​​from -3 to +3 for the first and second VOA control signals may be changed to a different range of values.

[0116] In some embodiments, the techniques described herein for optical MIMO polarization demultiplexing can be applied to general 2x2 optical MIMO demultiplexing. For example, in some embodiments, the techniques described herein can be implemented separately from or without PBSR.

[0117] 11 is a diagram illustrating an example of a simulation result showing the operation of a PDM MIMO demultiplexer according to an embodiment of the present disclosure. In the simulation result of FIG. 11, the light input to the demultiplexer (the pseudo-light received from the fiber transmission line) is continuously and randomly polarization scrambled. The demultiplexer (e.g., demultiplexer 300 of FIG. 3) is then controlled to continuously demultiplex the received signal.

[0118] Graph 1102 shows an example of how the three control signals φ1 (308), φ2 (310), and φ3 (312) change over time as they are adjusted by the control algorithm. Graph 1100 shows an example of the resulting amount of crosstalk, or the error "e" mentioned above.

[0119] 12 illustrates an example of a computing system 1200 that can be used to implement one or more components of a system that performs adaptive control of an optical polarization demultiplexer. The computing system 1200 can be used to implement the techniques described herein. For example, one or more portions of a controller (e.g., controller 344 of FIG. 3 , controller 456 of FIG. 4 , controller 606 of FIG. 6 ) and / or a pilot tone detector (e.g., pilot tone detector 602 of FIG. 6 ) can be implemented by components of the computing system 1200 described herein.

[0120] Computing system 1200 is intended to represent a variety of systems, including digital computers such as laptops, desktops, workstations, personal digital assistants, servers, blade servers, mainframes, and other suitable computers. The components, their connections and relationships, and functionality shown herein are merely examples and are not intended to be limiting.

[0121] Computing system 1200 includes processor 1202, memory 1204, storage device 1206, a high-speed interface 1208 connecting to memory 1204 and multiple high-speed expansion ports 1210, and a low-speed interface 1212 connecting to low-speed expansion port 1214 and storage device 1206. Each of processor 1202, memory 1204, storage device 1206, high-speed interface 1208, high-speed expansion port 1210, and low-speed interface 1212 are interconnected using various buses, which may be mounted on a common motherboard or in other manners as desired. Processor 1202 can process instructions for execution within computing system 1200, including instructions stored in memory 1204 or storage device 1206, and display graphical information for a GUI on an external input / output device, such as a display 1216 coupled to high-speed interface 1208. In other embodiments, multiple processors and / or multiple buses may be used, along with multiple memories and various types of memories, as desired. Additionally, multiple computing devices may be connected, each providing a portion of the operations (e.g., as a server bank, a fleet of blade servers, or a multi-processor system). In some embodiments, processor 1202 is a single-threaded processor. In some embodiments, processor 1202 is a multi-threaded processor. In some embodiments, processor 1202 is a quantum computer.

[0122] Memory 1204 stores information within computing system 1200. In some embodiments, memory 1204 is one or more volatile memory units. In some embodiments, memory 1204 is one or more non-volatile memory units. Memory 1204 may also be another form of computer-readable medium, such as a magnetic disk or optical disk.

[0123] Storage device 1206 can provide mass storage for computing system 1200. In some embodiments, storage device 1206 can be or include a computer-readable medium such as a floppy disk drive, a hard disk drive, an optical disk drive, or a tape drive, a flash memory or other similar solid-state memory device, or an array of devices, including devices in a storage area network or other configuration. Instructions can be stored on an information medium. When executed by one or more processing units (e.g., processor 1202), the instructions perform one or more methods, such as those described above. The instructions can be stored by one or more storage devices, such as a computer-readable or machine-readable medium (e.g., memory 1204, storage device 1206, or memory on processor 1202). High-speed interface 1208 manages bandwidth-intensive operations of computing system 1200, while low-speed interface 1212 manages less bandwidth-intensive operations. This allocation of functionality is merely an example. In some embodiments, high-speed interface 1208 is coupled to memory 1204, display 1216 (e.g., via a graphics processor or accelerator), and high-speed expansion port 1210, which can accept various expansion cards (not shown). In this embodiment, low-speed interface 1212 is coupled to storage device 1206 and low-speed expansion port 1214. Low-speed expansion port 1214, which can include various communication ports (e.g., USB, Bluetooth, Ethernet, wireless Ethernet), may be coupled to one or more input / output devices, such as a keyboard, pointing device, scanner, or networking device (e.g., via a network adapter), such as a switch or router.

[0124] The computing system 1200, as shown, may be implemented in several different forms. For example, it may be implemented as a standard server 1220 or multiple such servers. It may also be implemented in a personal computer such as a laptop computer 1222. It may also be implemented as part of a rack server system 1224.

[0125] The term "system," as used in this disclosure, may encompass all apparatus, devices, and machines for processing data, including, by way of example, a programmable processor, a computer, or multiple processors or computers. In addition to hardware, a processing system may include code that establishes an execution environment for the computer program, such as code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or one or more combinations thereof.

[0126] A computer program (also called a program, software, software application, script, executable logic, or code) can be written in any form of programming language, including compiled or interpreted, or declarative or procedural, and can be deployed in any form, such as a standalone program or as modules, components, subroutines, or other units suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored as part of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program, or in multiple cooperating files (e.g., files storing one or more modules, subprograms, or portions of code). A computer program can also be deployed to run on a single computer or on multiple computers located at a single site or distributed across multiple sites and interconnected by a communications network.

[0127] Computer-readable media suitable for storing computer program instructions and data include all forms of non-volatile or volatile memory, media, and memory devices, including, by way of example, semiconductor memory devices such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks or removable disks or magnetic tape; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and memory may be supplemented by, or incorporated in, special purpose logic circuitry. A server may be a general-purpose computer, a custom-built special-purpose electronic device, or a combination of both.

[0128] An implementation may include a back-end component (e.g., a data server), or a middleware component (e.g., an application server), or a front-end component (e.g., a client computer having a graphical user interface or a web browser through which a user can interact with an implementation of the subject matter described in this specification), or any combination of one or more such back-end, middleware, or front-end components. The components of the system may be interconnected by any form or medium of digital data communication (e.g., a communication network). Examples of communication networks include a local area network (LAN) and a wide area network (WAN) (e.g., the Internet).

[0129] The described functions can be implemented in digital electronic circuitry, or computer hardware, firmware, software, or combinations thereof. The apparatus can also be implemented in a computer program product tangibly embodied in an information carrier, such as a machine-readable storage device, for execution by a programmable processor, and the method steps can be performed by the programmable processor executing a program of instructions to manipulate input data and generate output, thereby performing the functions of the described implementation. The described functions can advantageously be implemented in one or more computer programs executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions used, directly or indirectly, in a computer to perform a particular activity or bring about a particular result. Computer programs can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including stand-alone programs or modules, components, subroutines, or other units suitable for use in a computing environment.

[0130] While this disclosure contains many specific implementation details, these should not be construed as limitations on the scope of the invention or the claims, but rather as descriptions of features specific to particular implementations of a particular invention. Certain features described in this disclosure in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Furthermore, while features may be described above as working in a particular combination, and even initially claimed as such, one or more features from a claimed combination may, in some cases, be deleted from the combination, such that the claimed combination is directed to a subcombination or variations of the subcombination.

[0131] Similarly, although the figures depict acts in a particular order, this should not be understood as requiring such acts to be performed in the particular order shown, or sequentially, or that all of the acts depicted be performed, to achieve desirable results. The present application provides the following aspects of the invention. (Aspect 1) a pair of MIMO inputs configured to input light into the first pair of optical transmission paths; a first optical phase shifter configured to apply a first relative phase shift between the first pair of optical transmission lines; a first 2x2 optical coupler configured to couple the first pair of optical transmission lines and output a second pair of optical transmission lines; a second optical phase shifter configured to apply a second relative phase shift between the second pair of optical transmission lines; a second 2x2 optical coupler configured to couple the second pair of optical transmission lines and output a third pair of optical transmission lines; a third optical phase shifter configured to apply a third relative phase shift between the third pair of optical transmission lines; a third 2x2 optical coupler configured to combine the third pair of optical transmission lines and output a fourth pair of optical transmission lines; and a pair of MIMO outputs configured to output the fourth pair of optical transmission paths; 1. A 2x2 optical multiple-input multiple-output (MIMO) demultiplexer comprising: (Aspect 2) 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein the first optical phase shifter is configured to apply the first relative phase shift value, which is binary. (Aspect 3) 3. The 2x2 optical MIMO demultiplexer of aspect 2, wherein the value of the first relative phase shift is binary between c+π / 2 and c−π / 2 (c is a real number). (Aspect 4) 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein the second optical phase shifter is configured to apply a value of the second relative phase shift within a finite range that includes −nπ and +nπ (n is an integer). (Aspect 5) 5. The 2x2 optical MIMO demultiplexer of claim 4, wherein the second optical phase shifter is configured for analog operation within the range (-nπ, +nπ). (Aspect 6) 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein the third optical phase shifter is configured to apply a value of the third relative phase shift within a finite range determined by a value of the first relative phase shift. (Aspect 7) A 2x2 optical MIMO demultiplexer as described in aspect 6, wherein the third optical phase shifter is configured to operate between 0 and +nπ (n is an integer) based on the value of the first relative phase shift being c-π / 2 (c is a real number), and is configured to operate between -nπ and 0 based on the value of the first relative phase shift being c+π / 2. (Aspect 8) 7. The 2x2 optical MIMO demultiplexer of claim 6, wherein the third optical phase shifter is configured for analog operation within the range (0, +nπ) or the range (-nπ, 0). (Aspect 9) At least one processor; and at least one memory storing instructions that, when executed by the at least one processor, perform operations to control values ​​of the first relative phase shift, the second relative phase shift, and the third relative phase shift; 2. The 2x2 optical MIMO demultiplexer of embodiment 1, further comprising: (Aspect 10) 2. The 2x2 optical MIMO demultiplexer of embodiment 1, further comprising: a first optical attenuator configured to apply a first relative attenuation between the first pair of optical transmission lines. (Aspect 11) 11. The 2x2 optical MIMO demultiplexer of embodiment 10, further comprising a second optical attenuator configured to apply a second relative attenuation between the third pair of optical transmission lines. (Aspect 12) 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein each of the first optical phase shifter, the second optical phase shifter, and the third optical phase shifter has a phase shift range of 2π or less. (Aspect 13) an input port configured to receive input light; means for performing adaptive 2×2 optical MIMO polarization demultiplexing on the input light using three-stage optical phase shifting, and outputting a first optical signal and a second optical signal; and at least one optical detector configured to detect the first optical signal and the second optical signal; 1. An optical multiple-input multiple-output (MIMO) receiver comprising: (Aspect 14) 1. A method for performing 2x2 optical multiple-input multiple-output (MIMO) demultiplexing, the method comprising: receiving light via a pair of MIMO inputs onto a first pair of optical transmission paths; controlling a first optical phase shifter to apply a first relative phase shift between the first pair of optical transmission lines; coupling the first pair of optical transmission lines with a first 2x2 optical coupler to output a second pair of optical transmission lines; controlling a second optical phase shifter to apply a second relative phase shift between the second pair of optical transmission lines; coupling the second pair of optical transmission lines with a second 2x2 optical coupler to output a third pair of optical transmission lines; controlling a third optical phase shifter to apply a third relative phase shift between the third pair of optical transmission lines; coupling the third pair of optical transmission lines with a third 2x2 optical coupler to output a fourth pair of optical transmission lines; and outputting the fourth pair of optical transmission lines via a pair of MIMO outputs; The method comprising: (Aspect 15) 15. The method of embodiment 14, wherein controlling the first optical phase shifter includes applying the first relative phase shift value that is binary. (Aspect 16) 16. The method of embodiment 15, wherein the value of the first phase shift is binary between c+π / 2 and c−π / 2, where c is a real number. (Aspect 17) 15. The method of claim 14, wherein controlling the second optical phase shifter includes applying a value of the second relative phase shift within a finite range that includes −nπ and +nπ (n is an integer). (Aspect 18) 18. The method of embodiment 17, wherein controlling the second optical phase shifter is performed by an analog operation in the range (-nπ, +nπ). (Aspect 19) 15. The method of claim 14, wherein controlling the third optical phase shifter includes applying a value of the third relative phase shift within a finite range determined by a value of the first relative phase shift. (Aspect 20) Controlling the third optical phase shifter controlling the third optical phase shifter to operate between 0 and +nπ (n is an integer) based on the value of the first relative phase shift being c-π / 2 (c is a real number), and to operate between -nπ and 0 based on the value of the first relative phase shift being c+π / 2; 20. The method of embodiment 19, further comprising: (Aspect 21) A method as described in aspect 19, wherein controlling the third optical phase shifter is performed by an analog operation within the range (0, +nπ) or within the range (-nπ, 0) depending on the value of the first relative phase shift. (Aspect 22) detecting a first reference signal on a first polarization channel at a first MIMO output of the pair of MIMO outputs; detecting a second reference signal on a second polarization channel at a second MIMO output of the pair of MIMO outputs; determining an amount of error in the demultiplexing measured from the first reference signal and the second reference signal; and controlling at least one of the first optical phase shifter, the second optical phase shifter, or the third optical phase shifter based on the error amount measured from the first reference signal and the second reference signal; 15. The method of embodiment 14, further comprising: (Aspect 23) A method according to aspect 14, wherein the 2x2 optical MIMO demultiplexing is performed endlessly during operation without resetting any of the first optical phase shifter, the second optical phase shifter, or the third optical phase shifter. (Aspect 24) 1. An optical multiple-input multiple-output (MIMO) receiver configured to perform optical MIMO polarization demultiplexing, comprising: an input port configured to receive input light; a polarizing beam splitter rotator (PBSR) configured to split the input light into a pair of optical transmission paths; an optical phase shifter configured to apply a relative phase shift between the pair of optical transmission lines; a 2x2 optical coupler configured to couple the pair of optical transmission lines; and a controller configured to control the optical phase shifter using a binary control having two operating states for a relative phase shift applied between the pair of optical transmission lines; The optical multiple-input multiple-output (MIMO) receiver comprises: (Aspect 25) 1. An optical multiple-input multiple-output (MIMO) receiver configured to perform optical MIMO polarization demultiplexing, comprising: an input port configured to receive input light; a polarizing beam splitter rotator (PBSR) configured to split the input light into a pair of optical transmission paths; an optical attenuator configured to apply a relative attenuation between the pair of optical transmission lines; a 2x2 optical coupler configured to couple the pair of optical transmission lines; and a controller configured to control the relative attenuation applied by the optical attenuator; The optical multiple-input multiple-output (MIMO) receiver comprises: (Aspect 26) 26. The optical multiple-input multiple-output (MIMO) receiver of claim 25, wherein the optical attenuator is a differential optical attenuator configured to (i) apply a first attenuation to one of the pair of optical transmission lines and (ii) apply a second attenuation to the other of the pair of optical transmission lines, the first attenuation and the second attenuation having equal decibel magnitudes and opposite decibel signs. (Aspect 27) a second optical attenuator configured to apply a second relative attenuation between a second pair of optical transmission lines; the second pair of optical transmission paths includes light attenuated by the optical attenuator; 26. The optical multiple-input multiple-output (MIMO) receiver of claim 25, wherein the controller is further configured to control the second relative attenuation applied by the second optical attenuator.

Claims

1. a pair of MIMO inputs configured to input light into the first pair of optical transmission paths; a first optical phase shifter configured to apply a first relative phase shift between the first pair of optical transmission lines; a first 2x2 optical coupler configured to combine the first pair of optical transmission lines to output a second pair of optical transmission lines; a second optical phase shifter configured to apply a second relative phase shift between the second pair of optical transmission lines; a second 2x2 optical coupler configured to combine the second pair of optical transmission lines to output a third pair of optical transmission lines; a third optical phase shifter configured to apply a third relative phase shift between the third pair of optical transmission lines; a third 2x2 optical coupler configured to combine the third pair of optical transmission lines to output a fourth pair of optical transmission lines; a pair of MIMO outputs configured to output the fourth pair of optical transmission paths, the pair of MIMO outputs being outputs from the third 2x2 optical coupler as a pair of demultiplexed output optical signals; and controlling the first optical phase shifter to apply non-continuously varying values ​​of the first relative phase shift; controlling the second optical phase shifter to apply continuously varying values ​​of the second relative phase shift; and a controller configured to control the third optical phase shifter to apply continuously varying values ​​of the third relative phase shift; 1. A 2x2 optical multiple-input multiple-output (MIMO) demultiplexer comprising:

2. The 2x2 optical MIMO demultiplexer of claim 1 , wherein the controller is configured to control the first optical phase shifter to cause a value of the first relative phase shift to change binary.

3. 3. The 2x2 optical MIMO demultiplexer according to claim 2, wherein the value of the first relative phase shift is one of two values ​​between c+π / 2 and c−π / 2 (c is a real number).

4. 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein the controller is configured to control the second optical phase shifter so that the value of the second relative phase shift is continuous within a finite range that includes −nπ and +nπ (n is an integer).

5. 5. The 2x2 optical MIMO demultiplexer of claim 4, wherein the controller is configured to analogically control the second optical phase shifter so that a value of the second relative phase shift varies continuously within a range (-nπ, +nπ).

6. 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein the controller is configured to control the third optical phase shifter so that a value of the third relative phase shift varies within a finite range determined depending on a value of the first relative phase shift.

7. 7. The 2x2 optical MIMO demultiplexer of claim 6, wherein the controller is configured to control the third optical phase shifter so that a value of the third relative phase shift varies between 0 and +nπ (n is an integer) based on the value of the first relative phase shift being c−π / 2, and varies between −nπ and 0 based on the value of the first relative phase shift being c+π / 2 (c is a real number).

8. 7. The 2x2 optical MIMO demultiplexer of claim 6, wherein the controller is configured to analogically control the third optical phase shifter so that a value of the third relative phase shift varies continuously within a range (0, +nπ) or a range (-nπ, 0).

9. The controller at least one processor; and at least one memory storing instructions, when executed by the at least one processor, for the at least one processor to control the first optical phase shifter, the second optical phase shifter, and the third optical phase shifter; 2. The 2x2 optical MIMO demultiplexer of claim 1, comprising:

10. 10. The 2x2 optical MIMO demultiplexer of claim 1, further comprising a first optical attenuator configured to apply a first relative attenuation between the first pair of optical transmission lines.

11. 11. The 2x2 optical MIMO demultiplexer of claim 10, further comprising a second optical attenuator configured to apply a second relative attenuation between the third pair of optical transmission lines.

12. 2. The 2x2 optical MIMO demultiplexer of claim 1, wherein each of the first optical phase shifter, the second optical phase shifter, and the third optical phase shifter has a phase shift range of 2π or less.

13. an input port configured to receive input light; means for performing adaptive 2×2 optical MIMO polarization demultiplexing on the input light by using three-stage optical phase shifting, and outputting a first optical signal and a second optical signal, a first optical phase shifter controlled to apply a discontinuously varying first relative phase shift; a second optical phase shifter controlled to apply a continuously varying second relative phase shift; and a third optical phase shifter controlled to apply a continuously varying third relative phase shift; and at least one optical detector configured to detect the first optical signal and the second optical signal; 1. An optical multiple-input multiple-output (MIMO) receiver comprising:

14. 1. A method for performing 2x2 optical multiple-input multiple-output (MIMO) demultiplexing, the method comprising: receiving light via a pair of MIMO inputs onto a first pair of optical transmission paths; controlling a first optical phase shifter to apply a first relative phase shift between the first pair of optical transmission lines, wherein the first relative phase shift varies non-continuously; coupling the first pair of optical transmission lines with a first 2x2 optical coupler to output a second pair of optical transmission lines; controlling a second optical phase shifter to apply a second relative phase shift between the second pair of optical transmission lines, wherein the second relative phase shift is continuously varied; coupling the second pair of optical transmission lines with a second 2x2 optical coupler to output a third pair of optical transmission lines; controlling a third optical phase shifter to apply a third relative phase shift between the third pair of optical transmission lines, wherein the third relative phase shift is continuously varied; coupling the third pair of optical transmission lines with a third 2x2 optical coupler to output a fourth pair of optical transmission lines; and outputting the fourth pair of optical transmission lines via a pair of MIMO outputs, the pair of MIMO outputs being outputs from the third 2x2 optical coupler as a pair of demultiplexed output optical signals; The method comprising:

15. 15. The method of claim 14, wherein controlling the first optical phase shifter comprises applying a value of the first relative phase shift that varies non-continuously between two values.

16. 16. The method of claim 15, wherein the value of the first relative phase shift is one of two values ​​between c+π / 2 and c−π / 2, where c is a real number.

17. 15. The method of claim 14, wherein controlling the second optical phase shifter comprises applying values ​​of the second relative phase shift that are continuous within a finite range that includes −nπ and +nπ, where n is an integer.

18. 18. The method of claim 17, wherein controlling the second optical phase shifter is performed by analog operation within the range (-nπ, +nπ).

19. 15. The method of claim 14, wherein controlling the third optical phase shifter comprises applying a value of the third relative phase shift that varies within a finite range depending on a value of the first relative phase shift.

20. Controlling the third optical phase shifter controlling the third optical phase shifter to operate between 0 and +nπ (n is an integer and c is a real number) based on the value of the first relative phase shift being c−π / 2, and to operate between −nπ and 0 based on the value of the first relative phase shift being c+π / 2; 20. The method of claim 19, further comprising:

21. 20. The method of claim 19, wherein controlling the third optical phase shifter is performed by analog operation within the range (0, +nπ) or within the range (-nπ, 0) depending on the value of the first relative phase shift.

22. detecting a first reference signal on a first polarization channel at a first MIMO output of the pair of MIMO outputs; detecting a second reference signal on a second polarization channel at a second MIMO output of the pair of MIMO outputs; determining an amount of error in demultiplexing measured from the first reference signal and the second reference signal; and controlling at least one of the first optical phase shifter, the second optical phase shifter, or the third optical phase shifter based on the error amount measured from the first reference signal and the second reference signal; 15. The method of claim 14, further comprising:

23. 15. The method of claim 14, wherein the 2x2 optical MIMO demultiplexing is performed endlessly during operation without resetting any of the first optical phase shifter, the second optical phase shifter, or the third optical phase shifter.

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