Polarization locking device and method, optical communication equipment and optical chip

Abstract

The invention provides a polarization locking device and method, optical communication equipment and an optical chip. The polarization locking device comprises a polarization beam splitter used for performing orthogonal decomposition on mixed light to obtain a first light beam and a second light beam; the first light beam comprises N pieces of first polarized light with different wavelengths, and the second light beam comprises N pieces of second polarized light with different wavelengths; the polarization adjusting structure group comprises at least one polarization adjusting structure, and the polarization adjusting structure comprises N micro-ring resonators and N phase shifters; each micro-ring resonator is used for selecting the first polarized light with the corresponding wavelength for gating and outputting the gated light to the corresponding phase shifter for phase modulation; the phase shifter outputs the phase-modulated light to the multiplexer for mixing, and the mixed light is output to the coupler; and the coupler couples the N first polarized light after phase modulation and the N second polarized light with different wavelengths to obtain modulated mixed light. According to the invention, N input lights with different wavelengths can be respectively locked to a specific stable polarization state.

Description

Technical Field

[0001] The present invention relates to the technical field of wavelength division multiplexing coherent communication networks, and in particular to a polarization locking device, method, optical communication equipment and optical chip. Background Art

[0002] During the propagation of light in optical fiber, the polarization state is easily affected by external environmental factors. In the fields of quantum communication, coherent optical communication, medical imaging, etc., it is crucial to obtain a single stable polarized light. The process of converting an arbitrarily changing polarization state into a stable polarization state is called polarization state locking, which is usually achieved by adaptive polarization control devices. However, the locking goal of the current adaptive polarization controller is to achieve stable output of a single polarization state, and it is difficult to achieve stable output of any polarization state of incident light of different wavelengths.

[0003] Since the optical fiber birefringence effect has a strong wavelength dependence, the corresponding transmission matrix is ​​different when light of different wavelengths passes through the optical fiber, and the polarization rotation caused is also different; and the difference in polarization effect caused by wavelength has a cumulative effect. When two beams of light with a small wavelength difference pass through a longer optical fiber path, the polarization changes produced may be very different. When a polarization controller is used in a multi-wavelength system, such as a wavelength division multiplexing optical communication system, it is difficult to simultaneously perform polarization tracking and locking for polarized light of different wavelengths. If a polarization control system is designed separately for each wavelength of polarized light, this method not only occupies a large volume, but also causes a waste of resources. Therefore, how to achieve stable output of arbitrary polarization states of incident light of multiple different wavelengths is a technical problem that urgently needs to be solved. Summary of the invention

[0004] The object of the present invention is to provide a polarization locking device, method, optical communication equipment and optical chip capable of respectively locking N input lights of different wavelengths to specific polarization states.

[0005] The present invention provides a polarization locking device, comprising a polarization beam splitter and a polarization adjustment structure group; The polarization beam splitter is used to perform orthogonal decomposition on the mixed light to obtain a first beam of light and a second beam of light, and output the first beam of light to the polarization adjustment structure group; the mixed light includes N lights of different wavelengths, the first beam of light includes N first polarized lights of different wavelengths, the second beam of light includes N second polarized lights of different wavelengths, the polarization direction of the first polarized light and the polarization direction of the second polarized light are perpendicular to each other and perpendicular to the propagation direction of the mixed light; N is an integer greater than 1; The polarization adjustment structure group includes at least one polarization adjustment structure, and the polarization adjustment structure includes a multiplexer, a coupler, N microring resonators and N phase shifters; the N microring resonators correspond to the N phase shifters one by one; each of the microring resonators is used to select the first polarized light of the corresponding wavelength from the first beam of light for gating, and output the selected light to the corresponding phase shifter for phase modulation; The phase shifter outputs the phase-modulated light to the multiplexer; The multiplexer mixes the N phase-modulated first polarized lights and outputs the mixed light to the coupler; The coupler couples N phase-modulated first polarized lights and N second polarized lights of different wavelengths to obtain modulated mixed light.

[0006] Further, a mixer and a demultiplexer group are included; One end of the mixer is connected to the polarization adjustment structure group, and is used to convert part of the mixed light output by the polarization adjustment structure group to obtain optical signals of six different output channels; The other end of the mixer is connected to a demultiplexer group, which includes six first demultiplexers. The six first demultiplexers are respectively used to decouple the optical signals of the six different output channels to obtain optical signals of six different output channels corresponding to each wavelength.

[0007] Further, it also includes a balanced photodetector and a drive control circuit; The balanced photodetector is used to obtain three difference optical signals based on the six optical signals output by the demultiplexer group to obtain three Stokes parameters; the six optical signals are divided into three pairs of optical signals, and a difference operation is performed on each pair of optical signals to obtain a corresponding difference signal; Determine the current polarization state corresponding to each wavelength based on the three Stokes parameters; The drive control circuit is used to obtain a feedback value based on the current polarization state to update the drive voltage signal of the phase shifter corresponding to each wavelength; the feedback value is the square of the distance between the current polarization state and a preset target polarization state.

[0008] Further, the polarization adjustment structure group includes a first polarization adjustment structure, a second polarization adjustment structure, a third polarization adjustment structure and a fourth polarization adjustment structure, the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are connected in series in sequence, the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are respectively the polarization adjustment structures, and the phase shifter in the polarization adjustment structure is used to increase or decrease the phase of the input light to obtain the target phase; The drive control circuit is used to obtain a first initial phase, a second initial phase, a third initial phase, a fourth initial phase and a boundary factor, use the first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor as inputs of a feedback function, and use a four-dimensional gradient descent algorithm to perform p iterations, output a target feedback value, and update the drive voltage signal of the phase shifter in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure based on the target feedback value; The first initial phase is the phase of the selected light when it enters the phase shifter in the first polarization adjustment structure, the second initial phase is the phase of the selected light when it enters the phase shifter in the second polarization adjustment structure, the third initial phase is the phase of the selected light when it enters the phase shifter in the third polarization adjustment structure, and the fourth initial phase is the phase of the selected light when it enters the phase shifter in the fourth polarization adjustment structure; The boundary factor is determined based on a phase change boundary, a step size of a four-dimensional gradient descent algorithm, and a phase of any polarization adjustment structure in the polarization adjustment structure group; The target feedback value is 2 calculated by the pth iteration. 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector, and the target polarization state is the preset output polarization state; each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase.

[0009] Furthermore, the calculation formula of the boundary factor is: * ; in, is the boundary factor. When the phase change boundary is set to hour, is the maximum phase shift, is the minimum phase shift, is the step size of the four-dimensional gradient descent algorithm, which is equal to the phase offset, is the phase shift of any polarization adjustment structure in the polarization adjustment structure group at the p-th iteration, where p represents the number of iterations, and m represents any polarization adjustment structure in the polarization adjustment structure group.

[0010] Furthermore, the phase shifter is a lithium niobate electro-optic phase shifter or a silicon-based thermo-optic phase shifter.

[0011] Furthermore, it also includes a polarization beam combiner and a second demultiplexer; One end of the polarization beam combiner is connected to the polarization adjustment structure group, and is used for orthogonally coupling the modulated mixed light output by the polarization adjustment structure group into polarized light; The other end of the polarization beam combiner is connected to the second demultiplexer, and the second demultiplexer is used to decouple the polarized light output by the polarization beam combiner to obtain stable polarized light of different wavelengths.

[0012] The present invention also provides a polarization locking method, which is applied to a polarization locking device, and the method comprises: The mixed light is orthogonally decomposed to obtain a first light beam and a second light beam; the mixed light comprises N lights of different wavelengths, the first light beam comprises N first polarized lights of different wavelengths, the second light beam comprises N second polarized lights of different wavelengths, the polarization directions of the first polarized lights and the second polarized lights are perpendicular to each other and perpendicular to the propagation direction of the mixed light; N is an integer greater than 1; Selecting a first polarized light of a corresponding wavelength from the first light beam for gating, and performing phase modulation on the gated light to obtain N phase-modulated first polarized lights; Mixing the N phase-modulated first polarized lights; The mixed N phase-modulated first polarized lights and N second polarized lights with different wavelengths are coupled to obtain modulated mixed light.

[0013] Further, the polarization adjustment structure group includes a first polarization adjustment structure, a second polarization adjustment structure, a third polarization adjustment structure and a fourth polarization adjustment structure, the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are sequentially connected in series, the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are respectively the polarization adjustment structures, and the phase shifter in the polarization adjustment structure is used to increase or decrease the phase of the input light to obtain the target phase; the polarization locking device also includes a balanced photodetector and a drive control circuit, and the method also includes: Acquire a first initial phase, a second initial phase, a third initial phase, and a fourth initial phase, wherein the first initial phase is the phase of the selected light when it enters the phase shifter in the first polarization adjustment structure, the second initial phase is the phase of the selected light when it enters the phase shifter in the second polarization adjustment structure, the third initial phase is the phase of the selected light when it enters the phase shifter in the third polarization adjustment structure, and the fourth initial phase is the phase of the selected light when it enters the phase shifter in the fourth polarization adjustment structure; Acquire a boundary factor, where the boundary factor is determined based on a phase change boundary, a step size of a four-dimensional gradient descent algorithm, and a phase of any polarization adjustment structure in the polarization adjustment structure group; The first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor are used as inputs of the feedback function, and the four-dimensional gradient descent algorithm is used for p iterations to output a target feedback value; the target feedback value is 2 obtained by calculating the pth iteration 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector, and the target polarization state is the preset output polarization state; each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase; The target feedback value is output to the driving control circuit to update the driving voltage signals of the phase shifters in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure, and the fourth polarization adjustment structure.

[0014] The present invention also provides an optical communication device, comprising a polarization locking device.

[0015] The present invention also provides an optical chip, comprising a polarization locking device.

[0016] Technical effect: The present invention can realize the simultaneous polarization locking of multiple input lights of different wavelengths. Multiple input lights of different wavelengths are input at the same time, and after locking, polarized light with a stable polarization state at each wavelength is output.

[0017] The present invention can lock the polarization state at any position while achieving control continuity, avoid resetting operations while achieving continuous polarization tracking, and has strong robustness. High-speed polarization tracking and locking can be achieved only by improving device performance and reducing loop delay. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic structural diagram of a polarization locking device according to an embodiment; Figure 2 is a schematic structural diagram of a polarization beam splitter according to an embodiment; Figure 3 This is a schematic diagram of the structure of a dual-bus micro-ring resonator according to an embodiment; Figure 4 is a resonant power distribution diagram of a dual-bus microring resonator according to an embodiment; Figure 5is a diagram of detuned power distribution of a dual-bus microring resonator according to an embodiment; Figure 6 is a transmission response diagram of a dual-bus microring resonator according to an embodiment; Figure 7 is a schematic structural diagram of a 90° mixer according to an embodiment; Figure 8 is a schematic diagram of an arrayed waveguide grating structure according to an embodiment; Fig. 9 is a schematic diagram of a polarization locking method in another embodiment; Fig.10 is a logic block diagram of a four-dimensional gradient descent algorithm in another embodiment; Fig.11 is a schematic diagram of the rotation of the polarization state around the equator of the Poincare sphere; Fig.12 is a schematic diagram of the first polarization state output after polarization locking; Fig.13 is the phase change diagram during polarization locking; Fig.14 It is a schematic diagram of the rotation of the polarization state around the 90° and 270° meridians of the Poincare sphere; Fig.15 This is a schematic diagram of the first polarization state output after polarization tracking; Fig.16 This is a schematic diagram of the application scenario of the polarization locking device in the wavelength division multiplexing optical communication system. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0020] In addition to solving the technical problems in the background technology part, the present invention also solves a minor technical problem, that is, the continuity of polarization control leads to the technical problem that the device is difficult to reset. In order to solve the problem of continuity of control, the existing algorithms introduce higher complexity, find the optimal solution by matrix operation, or achieve the continuity of control by inverting the phase under the premise of introducing 3dB loss. However, these methods have too much calculation and are difficult to achieve high-speed negative feedback loop control, or need to rely on precise phase boundary position calibration, which is easily affected by environmental factors and has poor robustness. Therefore, how to use limited boundaries to achieve continuous polarization locking is also a technical problem that needs to be solved by the present invention.

[0021] like Figure 1 As shown, an embodiment of the present invention provides a polarization locking device, including a polarization beam splitter 1 and a polarization adjustment structure group.

[0022] The polarization beam splitter 1 is used to orthogonally decompose the mixed light to obtain the first beam and the second beam, and output the first beam to the polarization adjustment structure group; the mixed light includes N lights of different wavelengths, the first beam includes N first polarized lights of different wavelengths, the second beam includes N second polarized lights of different wavelengths, the polarization direction of the first polarized light and the polarization direction of the second polarized light are perpendicular to each other and perpendicular to the propagation direction of the mixed light; N is an integer greater than 1. Wherein, , … Represents different wavelengths of light; , … They respectively represent the micro-ring radii of N micro-ring resonators, and n is a positive integer.

[0023] In some embodiments, Figure 2 As shown, the polarization beam splitter 1 mainly uses an asymmetric directional coupler to make the effective refractive index of the TM mode (Transverse Magnetic mode) of one waveguide equal to the effective refractive index of the TE mode (Transverse Electric Mode, which refers to the wave in which the electric field component is perpendicular to the propagation direction during the propagation of electromagnetic waves) of another waveguide, and the two meet the phase matching condition, and the TM mode is converted into the TE mode in the other waveguide. The operating wavelength exceeds 80nm, can cover the C band and L band, cover a wide spectral range, and can support higher data transmission rates and larger communication capacity.

[0024] The polarization adjustment structure group includes at least one polarization adjustment structure 2, which includes a multiplexer 21, a coupler 22, N microring resonators 23 and N phase shifters 24; the N microring resonators 23 correspond to the N phase shifters 24 one by one; each microring resonator 23 is used to select the first polarized light of the corresponding wavelength from the first beam of light for selection, and output the selected light to the corresponding phase shifter 24 for phase modulation.

[0025] The N microring resonators 23 correspond one-to-one to the N phase shifters 24, realizing functions such as multi-channel gating, wavelength routing and multiplexing. The one-to-one connection method can avoid the complicated alignment process, simplify the operation steps and improve the overall stability of the system.

[0026] In some embodiments, Figure 3 As shown, the microring resonator 23 is a dual-bus microring resonator structure, and its basic structure is composed of two straight waveguides and one ring waveguide. The input light is input from the input end 901 of the straight waveguide, and then coupled into the ring waveguide through the straight waveguide. The light of a specific wavelength will resonate in the ring waveguide, and finally output from the download end 902 of one of the two straight waveguides, achieving the effect of wavelength selection; the light that cannot form resonance will be output from the output end 903 of one of the two straight waveguides, where represents the microannulus radius.

[0027] Therefore, the first polarized light incident into the microring resonator 23 needs to satisfy the formula: , where k is the integer multiple of the light wave circling in the microring, is the wavelength of the first polarized light input, is the effective refractive index, is the microring radius of the microring resonator, and n is a positive integer.

[0028] The resonant power distribution of the dual-bus microring resonator is shown in Figure 4 As shown; the detuned power distribution of the dual-bus microring resonator is shown in Figure 5 As shown; the transmission response of the dual-bus microring resonator is shown in Figure 6 As shown, 903 represents the output end, and 902 represents the download end. Therefore, by controlling the microring radius of the microring resonator, the wavelength discrimination can reach 0.4nm, which means that the microring resonator can distinguish different wavelengths very finely.

[0029] In some embodiments, the microring resonator 23 is often used as a filter, dispersion compensator, modulator and other devices in an optical system, and has the characteristics of low cost, simple structure, high integration, and low loss.

[0030] The phase shifter 24 outputs the phase-modulated light to the multiplexer 21 .

[0031] The multiplexer 21 mixes the N phase-modulated first polarized lights and outputs the mixed lights to the coupler 22 .

[0032] The coupler 22 couples N phase-modulated first polarized lights and N second polarized lights of different wavelengths to obtain modulated mixed light.

[0033] The present invention can realize polarization locking of N lights of different wavelengths input simultaneously through a polarization adjustment structure, and output polarized light of a stable polarization state at each wavelength after locking.

[0034] In some embodiments, the coupler 22 is a coupler with a splitting ratio of 50:50, that is, the input optical signal is evenly distributed to the two output ports. Setting the splitting ratio to 50:50 can not only reduce cross-interference between different channels and improve signal purity, but also improve the signal-to-noise ratio of the system and reduce loss.

[0035] In some embodiments, a mixer 3 and a demultiplexer group 4 are also included.

[0036] One end of the mixer 3 is connected to the polarization adjustment structure group, and is used to convert part of the mixed light output by the polarization adjustment structure group to obtain optical signals of six different output channels.

[0037] The other end of the mixer 3 is connected to the demultiplexer group 4. The demultiplexer group 4 includes six first demultiplexers. The six first demultiplexers are respectively used to decouple the optical signals of six different output channels to obtain optical signals of six different output channels corresponding to each wavelength.

[0038] In some embodiments, the optical signals of six different output channels are respectively expressed as , , , , , ;in, ; ; ; ; ; ; in, represents the expected value in the first polarization direction (i.e., X direction), represents the conjugate expected value in the first polarization (i.e., X direction); represents the expected value in the second polarization (i.e., Y direction), It represents the conjugate expected value in the second polarization direction (ie, the Y direction).

[0039] In some embodiments, the mixer 3 is a 90° mixer, and the specific structure of the 90° mixer is as follows: Figure 7 shown. represents the electric field of the polarization component in the X direction, represents the electric field of the polarization component in the Y direction. The expression for solving the light intensity through the electric field is: . and They pass through a 2:1 coupler and then through the first MMI (Multi-Mode Interference) multiplied by the matrix of the transmission structure. The expression is: ,get and The light intensity expression of The phase shift and the second MMI are conjugated and expressed as: ,get and The light intensity expression of and Without MMI, represents the imaginary unit, Respectively represent the light intensity of the optical signal of six different output channels.

[0040] In some embodiments, a balanced photodetector 5 and a drive control circuit 6 are further included; The balanced photodetector 5 is used to obtain three difference optical signals based on the six optical signals output by the demultiplexer group 4, and obtain three Stokes parameters; the six optical signals are divided into three pairs of optical signals, and each pair of optical signals is subjected to difference operation to obtain a corresponding difference signal; In some embodiments, the three Stokes parameters are respectively , , It indicates that, , , , Represents the intensity difference between horizontal and vertical linear polarization; Indicates the intensity difference of ±45 degree linear polarization; represents the intensity difference between right-handed and left-handed circular polarization, and the Stokes parameter , , As a coordinate axis in three-dimensional space, we can construct a geometric model, which is called the Poincare sphere in polarization optics. Any polarization state can be represented as a point on the Poincare sphere. For two points on the sphere, the transformation can be achieved by rotating the rotation axis of the center of the circle by a certain angle. The output polarization state corresponding to each wavelength can be obtained as feedback input to control the drive circuit.

[0041] The unit vector of the rotation axis is denoted by , expressed in spherical coordinates as ,in, is the azimuth in spherical coordinates, is the elevation angle in spherical coordinates, Indicates transposing the objects within the brackets.

[0042] Assume the rotation angle is , then the rotation matrix R can be expressed as: ; in, , , , , , Represents the identity matrix.

[0043] The above R matrix contains 3 independent parameters , , , where the parameters , Used to describe the axis of rotation, parameters To describe the rotation angle, the rotation between any polarization states requires at least three degrees of freedom to describe. When the input and output polarization states are determined, and , , Within the specified period [0,2π], the rotation matrix R is unique. Therefore, no matter how the rotation matrix R is decomposed, it ultimately requires three independent degrees of freedom to describe it, and when the decomposition method is determined, the parameters describing these three degrees of freedom are also unique. However, when there are four or more degrees of freedom, there are infinite ways to implement the polarization state transformation operation of the rotation matrix R. Therefore, the four-level polarization controller can use redundant design to avoid the reset problem caused by the control parameters of the polarization tracking process falling into the local optimal solution.

[0044] The Poincaré Sphere is a geometric model used to describe polarization states. It was proposed by French mathematician and physicist Henri Poincaré. The Poincaré Sphere is a unit sphere that is used to represent the polarization state of electromagnetic waves and their phase changes. There are three types of polarization states represented by the Poincaré Sphere: Linear polarization: Located on the equator of the Poincaré Sphere, the angle represents the polarization direction. Circular polarization: Located at the north pole and south pole of the Poincaré Sphere, the north pole represents left-handed circular polarization, and the south pole represents right-handed circular polarization. Elliptical polarization: Located in the middle area of ​​the Poincaré Sphere, it is neither completely linear polarization nor circular polarization.

[0045] The current polarization state corresponding to each wavelength is determined based on the three Stokes parameters; Stokes parameters are a set of physical quantities used to describe the state of polarized light.

[0046] The driving control circuit 6 is used to obtain a feedback value based on the current polarization state to update the driving voltage signal of the phase shifter 24 corresponding to each wavelength.

[0047] In some embodiments, the polarization adjustment structure group includes a first polarization adjustment structure, a second polarization adjustment structure, a third polarization adjustment structure and a fourth polarization adjustment structure, which are connected in series in sequence. The first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are polarization adjustment structures 2 respectively, and the phase shifter 24 in the polarization adjustment structure 2 is used to increase or decrease the phase of the input light to obtain the target phase.

[0048] The driving control circuit 6 is used to obtain the first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor, and use the first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor as inputs of the feedback function, and use a four-dimensional gradient descent algorithm to perform p iterations, output a target feedback value, and update the driving voltage signal of the phase shifter 24 in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure based on the target feedback value.

[0049] Among them, the first initial phase is the phase of the light after being selected when it enters the phase shifter 24 in the first polarization adjustment structure, the second initial phase is the phase of the light after being selected when it enters the phase shifter 24 in the second polarization adjustment structure, the third initial phase is the phase of the light after being selected when it enters the phase shifter 24 in the third polarization adjustment structure, and the fourth initial phase is the phase of the light after being selected when it enters the phase shifter 24 in the fourth polarization adjustment structure.

[0050] The boundary factor is determined based on a phase change boundary, a step size of a four-dimensional gradient descent algorithm, and a phase of any polarization adjustment structure in the polarization adjustment structure group.

[0051] The target feedback value is calculated at the pth iteration. 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector 5, and the target polarization state is the preset output polarization state; each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase.

[0052] Furthermore, the boundary factor is calculated as * ; in, is the boundary factor. When the phase change boundary is set to hour, is the maximum phase shift, is the minimum phase shift, is the step size of the four-dimensional gradient descent algorithm, which is equal to the phase offset, is the phase shift of any polarization adjustment structure in the polarization adjustment structure group at the p-th iteration, where p represents the number of iterations, and m represents any polarization adjustment structure in the polarization adjustment structure group.

[0053] The above four-dimensional gradient descent algorithm can realize continuous polarization control without reset.

[0054] In some embodiments, the phase shifter 24 is a lithium niobate electro-optic phase shifter or a silicon-based thermo-optic phase shifter.

[0055] In some embodiments, a polarization beam combiner 7 and a second demultiplexer 8 are further included.

[0056] One end of the polarization beam combiner 7 is connected to the polarization adjustment structure group, and is used for orthogonally coupling the modulated mixed light output by the polarization adjustment structure group into polarized light.

[0057] The other end of the polarization beam combiner 7 is connected to the second demultiplexer 8 , and the second demultiplexer 8 is used to decouple the polarized light output by the polarization beam combiner 7 to obtain stable polarized light of different wavelengths.

[0058] The continuous change of polarization state in two orthogonal directions can be locked by limited phase modulation, which can prove the effectiveness of the four-dimensional gradient descent algorithm. At the same time, the algorithm has no precise requirements on the boundary position, so it is less affected by environmental factors and has strong robustness.

[0059] In some embodiments, the first demultiplexer and the second demultiplexer are implemented by arrayed waveguide gratings, such as Figure 8 As shown in the figure, it is mainly composed of two Rowland circles and a series of waveguides of different lengths. Light of different wavelengths enters the Rowland circle on the left from the same port and transmits freely in it. When entering the waveguide array, due to the difference in waveguide length, light of different wavelengths will accumulate different phase differences, and finally pass through the Rowland circle on the right and be transmitted to different channels. The two Rowland circle areas can be regarded as flat waveguides. The length difference between adjacent waveguides satisfies the grating equation. The insertion loss of the arrayed waveguide grating is generally around 2-3dB, and the crosstalk is around -20dB. The insertion loss of 2-3dB means that the signal is less lost during transmission, which is conducive to long-distance transmission. The crosstalk level of -20dB means that the interference between different channels is small, which helps to maintain the clarity of the signal and the performance of the system. The arrayed waveguide grating can not only provide good channel uniformity, so that the output power and bandwidth of each wavelength are very close, but also has high wavelength selectivity, which can accurately separate and combine different wavelengths, and can also increase or decrease the number of channels as needed.

[0060] like Fig. 9 As shown, another embodiment provides a polarization locking method, which is applied to a polarization locking device. The polarization locking method includes: Step 100: orthogonally decompose the mixed light to obtain a first beam of light and a second beam of light; the mixed light includes lights of N different wavelengths, the first beam of light includes first polarized lights of N different wavelengths, the second beam of light includes second polarized lights of N different wavelengths, the polarization direction of the first polarized light and the polarization direction of the second polarized light are perpendicular to each other and perpendicular to the propagation direction of the mixed light; N is an integer greater than 1.

[0061] Step 200: Select a first polarized light of a corresponding wavelength from the first light beam for gating, and perform phase modulation on the gated light to obtain N phase-modulated first polarized lights.

[0062] Step 300: Mix N phase-modulated first polarized lights.

[0063] Step 400: coupling the mixed N phase-modulated first polarized lights and N second polarized lights of different wavelengths to obtain modulated mixed light.

[0064] In some embodiments, the polarization adjustment structure group includes a first polarization adjustment structure, a second polarization adjustment structure, a third polarization adjustment structure and a fourth polarization adjustment structure, the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are connected in series in sequence, the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are polarization adjustment structures respectively, and the phase shifter in the polarization adjustment structure is used to increase or decrease the phase of the input light to obtain the target phase; the polarization locking device also includes a balanced photodetector and a drive control circuit. The polarization locking method also includes: A first initial phase, a second initial phase, a third initial phase and a fourth initial phase are obtained, wherein the first initial phase is the phase of the light after being selected when it enters the phase shifter in the first polarization adjustment structure, the second initial phase is the phase of the light after being selected when it enters the phase shifter in the second polarization adjustment structure, the third initial phase is the phase of the light after being selected when it enters the phase shifter in the third polarization adjustment structure, and the fourth initial phase is the phase of the light after being selected when it enters the phase shifter in the fourth polarization adjustment structure.

[0065] A boundary factor is obtained, where the boundary factor is determined based on a phase change boundary, a step size of a four-dimensional gradient descent algorithm, and a phase of any polarization adjustment structure in the polarization adjustment structure group.

[0066] The first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor are used as the input of the feedback function, and the four-dimensional gradient descent algorithm is used for p iterations to output the target feedback value; the target feedback value is 2 calculated by the pth iteration 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector, and the target polarization state is the preset output polarization state; each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase.

[0067] The target feedback value is output to the driving control circuit to update the driving voltage signal of the phase shifter in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure.

[0068] In some embodiments, Fig.10 The following is the logic block diagram of the four-dimensional gradient descent algorithm. First, the first initial phase is obtained. , the second initial phase , the third initial phase With the fourth initial phase , the first initial phase is the phase of the selected light when it enters the phase shifter in the first polarization adjustment structure, and the second initial phase is the phase of the selected light when it enters the phase shifter in the second polarization adjustment structure, and the third initial phase is the phase of the selected light when it enters the phase shifter in the third polarization adjustment structure, and the fourth initial position It is the phase of the selected light when it enters the phase shifter in the fourth polarization adjustment structure.

[0069] Obtain the boundary factor b, which is determined based on the phase change boundary and the step size of the four-dimensional gradient descent algorithm; when the phase change boundary of the phase shifter is set to , the boundary factor b is defined as * ; in, is the boundary factor. When the phase change boundary is set to hour, is the maximum phase shift, is the minimum phase shift, is the step size of the four-dimensional gradient descent algorithm, which is equal to the phase offset, is the phase shift of any polarization adjustment structure in the polarization adjustment structure group at the pth iteration, where p represents the number of iterations and m represents any polarization adjustment structure in the polarization adjustment structure group. Preferably, in this embodiment, the value range of m is 1-4.

[0070] The first initial phase , the second initial phase , the third initial phase With the fourth initial phase and the boundary factor b as the input of the feedback function, and the four-dimensional gradient descent algorithm is used for p iterations to output the target feedback value; the target feedback value is 2 calculated at the pth iteration 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector, and the target polarization state is the preset output polarization state; when the output polarization state is set to ( , , ) polarization state, the feedback function is positioned as FB=( ) 2 +( ) 2 +( ) 2 , where FB is the feedback function, , , To balance the Stokes parameters in the current cycle obtained by the feedback of the photodetector, the parameters , , Represent different output polarization state components. For the phase of the phase shifter in each polarization adjustment structure, there are two change directions: increasing and decreasing in each algorithm cycle. Therefore, the four-level phase adjustment structure has a total of 2 4 =16 change directions.

[0071] Each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase.

[0072] when When reaching the boundary, the step length The phase increment caused by this gradually becomes ineffective and loses its effect on the feedback function FB, causing the algorithm to actively choose a convergence path away from the boundary direction. Due to the existence of redundant degrees of freedom, there are infinitely many convergence paths, so there must be a path direction away from the phase boundary. After introducing the boundary factor b, the 16 directions are "tested" by marking the direction i corresponding to the minimum value of the feedback function FB. Direction i is the direction that satisfies the fastest convergence speed and is relatively far from the phase boundary. According to direction i, the phase , , , Apply a step size greater than that used in the pth iteration and enter the p+1th iteration loop, for example Fig.10 The step size of the reference polarization state is , the next iteration step size for the reference polarization state is 2 . , , , They respectively represent the phase of the phase shifter in the first polarization adjustment structure, the phase of the phase shifter in the second polarization adjustment structure, the phase of the phase shifter in the third polarization adjustment structure and the phase of the phase shifter in the fourth polarization adjustment structure at the p-th iteration.

[0073] The target feedback value is output to the driving control circuit to update the driving voltage signal of the phase shifter in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure.

[0074] Due to the redundant design of the four-level structure relative to the two-level structure, there must be more than one change direction that makes the feedback function FB show a convergence trend. In the pth cycle, the 16 change directions are "tested" in turn, and the feedback function corresponding to each change direction is recorded. , =1, 2, ...16, and introduce the boundary factor b, so that the phase change near the boundary is more inclined to change in the direction away from the boundary, while ensuring that the phase changes within the specified boundary. Among them, only one iteration is performed in each cycle, break means end; Min() means taking the minimum value.

[0075] This four-level polarization adjustment structure provides four independent control degrees of freedom, which means that there are multiple paths to reach the same polarization state. When the system falls into a local optimal solution during the optimization process, this local optimal solution can be bypassed by changing other control parameters to find the global optimal solution. Redundant design can be used to avoid the reset problem caused by the control parameters falling into the local optimal solution during polarization tracking, and the boundary factor b is added to achieve continuous polarization locking within a limited boundary.

[0076] In summary, this is an iterative optimization method that determines the most favorable direction by performing a fine search in a small range, and then taking larger steps in that direction to quickly converge to the optimal solution.

[0077] like Fig.11 As shown, assuming that the phase change range is [0, 2π], the initial circular polarization state undergoes a continuous phase rotation, which is displayed as a circle rotating around S1 on the Poincare sphere. The purpose of the continuous phase rotation is to artificially simulate the boundary problem caused by the continuous polarization rotation in the same direction. The dynamically polarized light after scrambling enters the algorithm loop, and the polarization tracking target is set to the first polarization state (X-line polarization state).

[0078] like Fig.12 The output state diagram of the first polarization state (X-line polarization state) after polarization locking is shown as follows: Fig.13 The figure shows the phase change during polarization locking. During the entire control process, there is no boundary "lost lock" problem, indicating that the four-level gradient descent algorithm is reliable. , , , Respectively represent the phases of the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure. Fig.14 As shown in , the scrambling mode is changed to apply an azimuth angle change in the same direction to the initial 45° linear polarization state, which is reflected as a continuously rotating ring around S3 on the Poincare sphere. Fig.15Shown is the output state diagram of the first polarization state (X-line polarization state) after polarization tracking.

[0079] The continuous change of polarization state in two orthogonal directions can be locked by limited phase modulation, which can prove the effectiveness of the four-dimensional gradient descent algorithm. At the same time, the algorithm has no requirements on the boundary position, so it is less affected by environmental factors and has strong robustness.

[0080] Another embodiment provides an optical communication device, comprising the polarization locking device mentioned in the above embodiment.

[0081] Another embodiment provides an optical chip, comprising the polarization locking device mentioned in the above embodiment.

[0082] like Fig.16 As shown, another embodiment provides an application scenario of the above-mentioned multi-wavelength polarization locking device in a wavelength division multiplexing optical communication system.

[0083] Polarization: refers to the phenomenon that the vibration vector (including direction and amplitude) is asymmetric in a plane perpendicular to the propagation direction of a transverse wave (such as light).

[0084] Unpolarized light and polarized light: In a plane perpendicular to the propagation direction of the light, if the vibration vector of light can point to all directions and the amplitude is the same in all directions, then this light is called unpolarized light, otherwise this light is called polarized light.

[0085] Although the embodiments of the present invention are described in detail above, it is obvious to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations are within the scope and spirit of the present invention as described in the claims. Moreover, the present invention described herein may have other embodiments and may be implemented or realized in a variety of ways.

Claims

1. A polarization locking device, characterized in that: It includes a polarization beam splitter and a polarization adjustment structure group; The polarization beam splitter is used to perform orthogonal decomposition on the mixed light to obtain a first beam of light and a second beam of light, and output the first beam of light to the polarization adjustment structure group; the mixed light includes N lights of different wavelengths, the first beam of light includes N first polarized lights of different wavelengths, the second beam of light includes N second polarized lights of different wavelengths, the polarization direction of the first polarized light and the polarization direction of the second polarized light are perpendicular to each other and perpendicular to the propagation direction of the mixed light; N is an integer greater than 1; The polarization adjustment structure group includes at least one polarization adjustment structure, and the polarization adjustment structure includes a multiplexer, a coupler, N microring resonators and N phase shifters; The N microring resonators correspond to the N phase shifters one by one; each of the microring resonators is used to select the first polarized light of the corresponding wavelength from the first beam of light for gating, and output the gated light to the corresponding phase shifter for phase modulation; The phase shifter outputs the phase-modulated light to the multiplexer; The multiplexer mixes the N phase-modulated first polarized lights and outputs the mixed light to the coupler; The coupler couples N phase-modulated first polarized lights and N second polarized lights of different wavelengths to obtain modulated mixed light.

2. The device according to claim 1, characterized in that Also included are mixer and demultiplexer banks; One end of the mixer is connected to the polarization adjustment structure group, and is used to convert part of the mixed light output by the polarization adjustment structure group to obtain optical signals of six different output channels; The other end of the mixer is connected to a demultiplexer group, which includes six first demultiplexers. The six first demultiplexers are respectively used to decouple the optical signals of the six different output channels to obtain optical signals of six different output channels corresponding to each wavelength.

3. The device according to claim 2, characterized in that Also included are a balanced photodetector and a drive control circuit; The balanced photodetector is used to obtain three difference optical signals based on the six optical signals output by the demultiplexer group to obtain three Stokes parameters; the six optical signals are divided into three pairs of optical signals, and a difference operation is performed on each pair of optical signals to obtain a corresponding difference signal; Determine the current polarization state corresponding to each wavelength based on the three Stokes parameters; The drive control circuit is used to obtain a feedback value based on the current polarization state to update the drive voltage signal of the phase shifter corresponding to each wavelength; The feedback value is the square of the distance between the current polarization state and the preset target polarization state.

4. The device according to claim 3, characterized in that The polarization adjustment structure group includes a first polarization adjustment structure, a second polarization adjustment structure, a third polarization adjustment structure and a fourth polarization adjustment structure, wherein the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are sequentially connected in series, and the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are respectively the polarization adjustment structures, and the phase shifter in the polarization adjustment structure is used to increase or decrease the phase of the input light to obtain the target phase; The drive control circuit is used to obtain a first initial phase, a second initial phase, a third initial phase, a fourth initial phase and a boundary factor, use the first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor as inputs of a feedback function, and use a four-dimensional gradient descent algorithm to perform p iterations, output a target feedback value, and update the drive voltage signal of the phase shifter in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure based on the target feedback value; The first initial phase is the phase of the selected light when it enters the phase shifter in the first polarization adjustment structure, the second initial phase is the phase of the selected light when it enters the phase shifter in the second polarization adjustment structure, the third initial phase is the phase of the selected light when it enters the phase shifter in the third polarization adjustment structure, and the fourth initial phase is the phase of the selected light when it enters the phase shifter in the fourth polarization adjustment structure; The boundary factor is determined based on a phase change boundary, a step size of a four-dimensional gradient descent algorithm, and a phase of any polarization adjustment structure in the polarization adjustment structure group; The target feedback value is 2 calculated by the pth iteration. 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector, and the target polarization state is the preset output polarization state; each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase.

5. The device according to claim 4, characterized in that The calculation formula of the boundary factor is: * in, is the boundary factor. When the phase change boundary is set to hour, is the maximum phase shift, is the minimum phase shift, is the step size of the four-dimensional gradient descent algorithm, which is equal to the phase offset, is the phase shift of any polarization adjustment structure in the polarization adjustment structure group at the p-th iteration, where p represents the number of iterations, and m represents any polarization adjustment structure in the polarization adjustment structure group.

6. The device according to claim 1, characterized in that The phase shifter is a lithium niobate electro-optic phase shifter or a silicon-based thermo-optic phase shifter.

7. The device according to claim 2, characterized in that Also included is a polarization beam combiner and a second demultiplexer; One end of the polarization beam combiner is connected to the polarization adjustment structure group, and is used for orthogonally coupling the modulated mixed light output by the polarization adjustment structure group into polarized light; The other end of the polarization beam combiner is connected to the second demultiplexer, and the second demultiplexer is used to decouple the polarized light output by the polarization beam combiner to obtain stable polarized light of different wavelengths.

8. A polarization locking method, characterized in that: Applied to the polarization locking device according to any one of claims 1 to 7, the method comprising: The mixed light is orthogonally decomposed to obtain a first light beam and a second light beam; the mixed light comprises N lights of different wavelengths, the first light beam comprises N first polarized lights of different wavelengths, the second light beam comprises N second polarized lights of different wavelengths, the polarization directions of the first polarized lights and the second polarized lights are perpendicular to each other and perpendicular to the propagation direction of the mixed light; N is an integer greater than 1; Selecting a first polarized light of a corresponding wavelength from the first light beam for gating, and performing phase modulation on the gated light to obtain N phase-modulated first polarized lights; Mixing the N phase-modulated first polarized lights; The mixed N phase-modulated first polarized lights and N second polarized lights with different wavelengths are coupled to obtain modulated mixed light.

9. The method according to claim 8, characterized in that The polarization adjustment structure group includes a first polarization adjustment structure, a second polarization adjustment structure, a third polarization adjustment structure and a fourth polarization adjustment structure, wherein the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are sequentially connected in series, and the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure and the fourth polarization adjustment structure are respectively the polarization adjustment structures, and the phase shifter in the polarization adjustment structure is used to increase or decrease the phase of the input light to obtain the target phase; The polarization locking device further includes a balanced photodetector and a drive control circuit, and the method further includes: Acquire a first initial phase, a second initial phase, a third initial phase, and a fourth initial phase, wherein the first initial phase is the phase of the selected light when it enters the phase shifter in the first polarization adjustment structure, the second initial phase is the phase of the selected light when it enters the phase shifter in the second polarization adjustment structure, the third initial phase is the phase of the selected light when it enters the phase shifter in the third polarization adjustment structure, and the fourth initial phase is the phase of the selected light when it enters the phase shifter in the fourth polarization adjustment structure; Acquire a boundary factor, where the boundary factor is determined based on a phase change boundary, a step size of a four-dimensional gradient descent algorithm, and a phase of any polarization adjustment structure in the polarization adjustment structure group; The first initial phase, the second initial phase, the third initial phase, the fourth initial phase and the boundary factor are used as inputs of the feedback function, and the four-dimensional gradient descent algorithm is used for p iterations to output a target feedback value; the target feedback value is 2 obtained by calculating the pth iteration 4 The minimum value among the function values; p is a positive integer; the feedback function is the square of the distance between the current polarization state and the target polarization state, the current polarization state is the Stokes parameter in the current cycle obtained by feedback from the balanced photodetector, and the target polarization state is the preset output polarization state; each iteration, based on 2 4 The reference polarization state is the polarization state after increasing or decreasing the step size of the light of the first initial phase, the second initial phase, the third initial phase, and the fourth initial phase; The target feedback value is output to the driving control circuit to update the driving voltage signals of the phase shifters in the first polarization adjustment structure, the second polarization adjustment structure, the third polarization adjustment structure, and the fourth polarization adjustment structure.

10. An optical communication device, characterized in that: Comprising the polarization locking device as claimed in any one of claims 1 to 7.

11. An optical chip, characterized in that: Comprising the polarization locking device as claimed in any one of claims 1 to 7.

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