Infinite polarization tracking system and method for optical polarization state

Abstract

The present invention belongs to the field of optical communications technology, and more specifically, relates to an infinite polarization tracking system and method for the polarization state of light. The method comprises the following steps: a two-stage phase modulator receives a beam of polarized light from an optical transmitter and adjusts the polarization state of the polarized light to output signal light, wherein the signal light includes a first signal light and a second signal light; an extraction module extracts the optical power in the X and Y polarization directions of the signal light and converts it into an electrical signal; a processing module receives the electrical signal and processes it to obtain the difference in optical power in the X and Y polarization directions; the processing module processes the electrical signal using a dither algorithm to obtain the Stokes vector of the current signal light and obtains a control voltage signal based on the Stokes vector; and a two-stage phase modulator adjusts the polarization state of the polarized light based on the control voltage signal, so that the output signal light approaches a circle with S1 = 0 on the Poincare sphere. The present invention achieves stable polarization state of light through a two-stage phase modulation structure.

Description

Technical Field

[0001] The present invention belongs to the field of optical communication technology, and more specifically, relates to an infinite polarization tracking system and method for optical polarization states. Background Art

[0002] Coherent optical communication systems have been widely adopted due to their advantages, including high sensitivity, long relay distances, large communication capacity, and multiple modulation methods. Coherent optical communication primarily utilizes coherent modulation and heterodyne detection techniques. Coherent modulation uses the transmitted signal to change the frequency, phase, and amplitude of the optical carrier, requiring the optical signal to have a defined frequency and phase. Heterodyne detection utilizes a beam of local oscillator light at the receiving end to coherently couple the signal light with the local oscillator light in an optical coupler, generating a beat frequency signal whose frequency, phase, and amplitude vary in the same manner as the signal light. Coherent reception is sensitive to the polarization of light. Therefore, in coherent optical communication systems, the frequency and phase of the local oscillator light signal must be stable, and the polarization state of the local oscillator light signal must be consistent with that of the received light signal. However, during communication, the signal light is transmitted through the optical fiber. Movement, vibration, and temperature changes in the optical fiber can cause disturbances in the polarization state of the signal light, necessitating real-time tracking, stabilization, and control of the polarization state of the optical signal.

[0003] Existing SHCD (self-differential coherent detection) systems primarily transmit a modulated signal and a tone replica, serving as a local oscillator (LO) signal, from a transmitter to a receiver for coherent reception. This minimizes the effects of LO optical phase noise and eliminates frequency offset, allowing the use of large-linewidth uncooled lasers while simplifying digital signal processing algorithms. However, like traditional coherent optical communication systems, SHCD suffers from polarization sensitivity during coherent reception. Therefore, an automatic polarization tracking scheme is required to compensate for random polarization variations in the received LO signal.

[0004] To this end, Chinese patent CN112485930A discloses a control method and system for achieving polarization stabilization. The method uses a rapid positioning algorithm to quickly adjust the output polarization state of any input light after passing through a polarization controller to near any specified target polarization state on the Poincaré sphere. The random gradient descent algorithm is then used to further stabilize the output polarization state to the target polarization state, thereby stabilizing any polarization state to any set target polarization state. However, the method requires adjustment through a polarization controller composed of five-stage equivalent phase difference adjustable angle fixed wave plates. When the voltage applied to the first three wave plates reaches the limit, the voltage of the first three wave plates needs to be retracted to the intermediate voltage to facilitate further voltage adjustment. Such a solution is too complicated and cannot achieve infinite tracking of the light polarization state. The adjustment control speed is inevitably affected, resulting in poor control effect. Summary of the Invention

[0005] In order to overcome at least one of the above-mentioned defects in the prior art, the present invention provides an infinite polarization tracking system and method for the polarization state of light, which can achieve infinite tracking control of the polarization state of light through a two-stage phase modulator.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A method for infinite polarization tracking of the polarization state of light is provided, comprising the following steps:

[0008] Step 1: The secondary phase modulator receives a beam of polarized light from an optical transmitter and adjusts the polarization state of the polarized light to output signal light, where the signal light includes a first signal light and a second signal light;

[0009] Step 2: The extraction module extracts the optical power of the signal light in the X and Y polarization directions and converts it into an electrical signal;

[0010] Step 3: The processing module receives the electrical signal and processes it to obtain the difference in optical power between the X and Y polarization directions;

[0011] Step 4: The processing module processes the optical power difference using a dither algorithm to obtain the Stokes vector of the current signal light and obtains a control voltage signal based on the Stokes vector.

[0012] Step 5: The secondary phase modulator adjusts the polarization state of the polarized light according to the control voltage signal, so that the output signal light tends to the circle with S1=0 on the Poincare sphere.

[0013] In this solution, the optical power of the signal light in the X and Y polarization directions is directly extracted. The corresponding Stokes vector is calculated through the dither algorithm. The Stokes vector is used to describe the polarization state of the signal light. The control voltage signal can be obtained from the Stokes vector. In this way, the two-stage phase modulator can adjust the polarization state of the polarized light according to the control voltage signal to achieve polarization stabilization.

[0014] Preferably, the above step 2 specifically includes:

[0015] The first signal light and the second signal light are respectively split by two power splitters to obtain a first light beam and a second light beam;

[0016] The two first light beams are received by two photodetectors respectively and converted into electrical signals. The second light beam is combined by a polarization beam combiner and continues to propagate along the original light path.

[0017] Preferably, in the above step 4, the difference in optical power is processed by a dither algorithm to obtain the Stokes vector of the current signal light, and the control voltage is obtained according to the Stokes parameter vector, which specifically includes: extracting the Stokes S1 component of the current polarization state of the signal light;

[0018] The Stokes S2 component is estimated based on the Stokes S1 component using the dither algorithm;

[0019] The rotation angle of the current polarization state around the S3 axis to S1=0 is calculated based on the Stokes S1 and S2 components, and the corresponding control voltage signal is obtained based on the rotation angle.

[0020] Preferably, in step 4 above, estimating the Stokes S2 component based on the Stokes S1 component using the dither algorithm specifically includes:

[0021] Set the perturbation matrix Vb, where Vb contains Ld values;

[0022] Multiply each value in Vb by delta and add them to the second voltage control stage of the two-stage phase modulator in sequence to obtain the perturbed feedback value ΔS1; where delta is the perturbation coefficient;

[0023] The value of the Stokes S2 component is calculated based on the feedback value ΔS1. The specific formula is:

[0024] T1=sum(ΔS1*Vb) / delta / Ld,

[0025] θ=acos(min(1,-T1)),

[0026]

[0027] Wherein, θ is the ellipticity; ΔS11, ΔS12, ΔS13, and ΔS14 are feedback values obtained by applying respective values in sequence; and T1 is the feedback value obtained by the two-stage phase modulator after perturbation.

[0028] Preferably, in step 4, a perturbation matrix Va is set simultaneously, where Va contains Ld values, and when each value in Vb is multiplied by delta and then added to the second voltage control stage of the two-stage phase modulator in sequence, each value in Va is multiplied by delta and then added to the first voltage control stage of the two-stage phase modulator in sequence.

[0029] Preferably, when the rotation angle is 2π or 0, the processing module inputs a reverse voltage to the second voltage control stage in the two-stage phase modulator.

[0030] This solution also provides a system for infinite tracking of optical polarization states, comprising an optical transmitter, a secondary phase modulator, an extraction module, and a processing module connected in sequence. The optical transmitter emits polarized light, which is then phase- and polarization-state adjusted by the secondary phase modulator and then output as signal light. The extraction module extracts part of the power of the signal light and converts it into an electrical signal. The processing module processes the electrical signal to obtain a control voltage signal, converts the control voltage signal into an electrical signal, and inputs the signal into the secondary phase modulator. The secondary phase modulator adjusts the polarization state of the polarized light according to the control voltage signal so that the power values of the signal light in the X and Y polarization directions are equal.

[0031] Preferably, it also includes two power splitters and a polarization combiner, the two power splitters are used to split the two signal lights output by the secondary phase modulator respectively to obtain two first light beams and two second light beams, and the polarization combiner is used to combine the two second light beams.

[0032] Preferably, the extraction module comprises two photodetectors, which respectively receive the two first light beams and convert them into electrical signals.

[0033] Preferably, the processing module includes an FPGA, and an analog-to-digital converter and a digital-to-analog converter electrically connected to the FPGA;

[0034] The analog-to-digital converter is also electrically connected to the photodetector and converts the electrical signal output by the photodetector into a digital signal and inputs it into the FPGA;

[0035] FPGA processes the digital signal to obtain a control voltage signal;

[0036] The digital-to-analog converter is also electrically connected to the secondary phase modulation structure, and converts the control voltage signal output by the FPGA into an electrical signal and inputs the electrical signal into the secondary phase modulator.

[0037] Compared with the prior art, the beneficial effects are:

[0038] The present invention directly extracts the optical power of the signal light in the X and Y polarization directions, calculates the corresponding Stokes vectors through a dither algorithm, uses the Stokes vectors to describe the polarization state of the signal light, and obtains a control voltage signal through the Stokes vectors. In this way, a two-stage phase modulator can adjust the polarization state of the polarized light according to the control voltage signal to achieve polarization stabilization. In addition, since the two-stage phase modulator has a boundary, when the control reaches the boundary, the voltage direction is changed, causing the voltage of the first voltage control stage in the two-stage phase modulator to change sharply, while the power of the signal light in the X and Y directions does not change, so that the two-stage phase modulator can achieve infinite tracking control of the polarization state of light. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 11 is a flow chart of a method for infinite tracking of light polarization states according to embodiment 1 of the present invention;

[0040] Figure 2 1 is a schematic structural block diagram of a system for infinite tracking of optical polarization states according to embodiment 2 of the present invention. DETAILED DESCRIPTION

[0041] The accompanying drawings are for illustrative purposes only and are not to be construed as limiting the present invention. To better illustrate the present embodiment, some components in the accompanying drawings may be omitted, enlarged, or reduced in size, and do not represent actual product dimensions. Those skilled in the art will appreciate that some well-known structures and their descriptions may be omitted from the accompanying drawings. The positional relationships depicted in the accompanying drawings are for illustrative purposes only and are not to be construed as limiting the present invention.

[0042] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "long", "short", etc. indicating the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0043] The technical solution of the present invention is further described in detail below through specific embodiments and in conjunction with the accompanying drawings:

[0044] Example 1:

[0045] like Figure 1 An embodiment of a method for infinitely tracking the polarization state of light is shown, which can be applied to a coherent optical communication system and includes the following steps:

[0046] Step 1: The secondary phase modulator receives a beam of polarized light from an optical transmitter and adjusts the polarization state of the polarized light to output signal light, where the signal light includes a first signal light and a second signal light;

[0047] Step 2: The extraction module extracts the optical power of the signal light in the X and Y polarization directions and converts it into an electrical signal;

[0048] Step 3: The processing module receives the electrical signal and processes it to obtain the difference in optical power between the X and Y polarization directions;

[0049] Step 4: The processing module processes the optical power difference using a dither algorithm to obtain the Stokes vector of the current signal light and obtains a control voltage signal based on the Stokes vector.

[0050] Step 5: The secondary phase modulator adjusts the polarization state of the polarized light according to the control voltage signal, so that the output signal light tends to the circle with S1=0 on the Poincare sphere.

[0051] Step 2 in this embodiment specifically includes:

[0052] The first signal light and the second signal light are respectively split by two power splitters to obtain a first light beam and a second light beam. It should be understood that the first signal light and the second signal light are polarized lights emitted by the same laser and then split by the power splitters. This facilitates the subsequent extraction of optical power in two different directions, X and Y. In a specific embodiment, in order not to interfere with information transmission in a communication system, the power ratio of the first light beam to the second light beam is 10:90. Since the first light beam and the second light beam are identical except for the optical power, the entire polarized light can be controlled by subsequent tracking processing using only the polarization state information of the first light beam.

[0053] Two optical detectors receive the two first light beams and convert them into electrical signals. The second light beam is then combined by a polarization beam combiner and continues along its original optical path. After converting the optical signals of the first light beams into electrical signals, the electrical signals can be accurately processed.

[0054] In step 4 of this embodiment, the optical power difference is processed by the dither algorithm to obtain the Stokes vector of the current signal light, and the control voltage is obtained according to the Stokes parameter vector. Specifically, the following steps are performed:

[0055] Extract the Stokes S1 component of the current polarization state of the signal light;

[0056] The Stokes S2 component is estimated based on the Stokes S1 component using the dither algorithm;

[0057] The rotation angle of the current polarization state around the S3 axis to S1=0 is calculated based on the Stokes S1 and S2 components, and the corresponding control voltage signal is obtained based on the rotation angle.

[0058] In step 4 of this embodiment, the Stokes S2 component is estimated based on the Stokes S1 component using the dither algorithm, which specifically includes:

[0059] Set the perturbation matrix Vb, where Vb contains Ld values;

[0060] Multiply each value in Vb by delta and add them to the second voltage control stage of the two-stage phase modulator in sequence to obtain the perturbed feedback value ΔS1; where delta is the perturbation coefficient, and its value is adjustable;

[0061] The value of the Stokes S2 component is calculated based on the feedback value ΔS1. The specific formula is:

[0062] T1=sum(ΔS1*Vb) / delta / Ld,

[0063] θ=acos(min(1,-T1)),

[0064]

[0065] Among them, θ is the ellipticity; ΔS11, ΔS12, ΔS13, and ΔS14 are the feedback values obtained by applying each value in sequence; T1 is the feedback value obtained by the two-stage phase modulator after perturbation. The first voltage control and the second voltage control stage of the two-stage phase modulator in this embodiment are to make the polarization state of light rotate around the S1 and S3 axes in the Stokes space by corresponding angles, and the second voltage control stage rotates the polarization state of light around the S3 axis by a certain angle and locks it on the great circle of S1=0. Since the processing module can only extract the Stokes S1 component of the current polarization state of the signal light, it is impossible to determine the specific rotation angle of the current polarization state from the S1=0 plane. The control of the polarization state of light requires knowing the S1 component and the S2 component of the current polarization state at the same time to determine the specific rotation angle. Therefore, it is necessary to estimate the S2 component based on the known S1 component. The specific angle of the current polarization state rotated around the S3 axis to the S1=0 plane can be determined through the S1 component and the S2 component, thereby obtaining the control voltage of the second voltage control stage, so that different polarization states can be directly rotated and locked to the S1=0 plane under the drive of the control voltage, thereby achieving the stability of the polarization state of light.

[0066] In this embodiment, a perturbation matrix Va is set simultaneously in step 4, where Va includes Ld values. When each value in Vb is multiplied by delta and then sequentially added to the second voltage control stage of the two-stage phase modulator, each value in Va is multiplied by delta and then sequentially added to the first voltage control stage of the two-stage phase modulator.

[0067] Because the adjustment range of the first and second voltage control stages of the two-stage phase modulator is limited and cannot be reset, that is, the control of the two-stage phase modulator has boundary conditions, in order to achieve infinite tracking control of the two-stage phase modulator, in this embodiment, when the rotation angle is 2π or 0, the processing module inputs a reverse voltage to the second-stage voltage control stage in the two-stage phase modulator. At this time, the first-stage voltage control structure in the two-stage phase modulator rotates around the S1 axis by an angle of π or -π, and the voltage changes sharply, but does not cause significant jitter or change in the power in the X and Y polarization directions. The polarization state does not jump out of the current plane of S1=0, thereby achieving infinite tracking control of the polarization state of light.

[0068] This embodiment directly extracts the optical power of the signal light in the X and Y polarization directions, calculates the corresponding Stokes vectors using a dither algorithm, and uses the Stokes vectors to describe the polarization state of the signal light. A control voltage signal can be derived from the Stokes vectors. This allows the secondary phase modulator to adjust the polarization state of the polarized light based on the control voltage signal, achieving polarization stabilization. Furthermore, because the secondary phase modulator has a boundary, when the control reaches the boundary, the voltage direction is changed, causing the voltage of the first voltage control stage in the secondary phase modulator to change dramatically. However, the power of the signal light in the X and Y directions remains unchanged, allowing the secondary phase modulator to achieve infinite tracking control of the polarization state of light.

[0069] Example 2:

[0070] like Figure 2 FIG2 shows an embodiment of a system for infinitely tracking the polarization state of light, which is used to implement the infinitely tracking polarization state of light in Example 1. The system includes an optical transmitter, a secondary phase modulator, an extraction module, and a processing module connected in sequence. The optical transmitter emits polarized light, which is then phase- and polarization-adjusted by the secondary phase modulator and output as signal light. The extraction module extracts part of the power of the signal light and converts it into an electrical signal. The processing module processes the electrical signal to obtain a control voltage signal, converts the control voltage signal into an electrical signal, and inputs the electrical signal into the secondary phase modulator. The secondary phase modulator adjusts the polarization state of the polarized light according to the control voltage signal so that the power values of the signal light in the X and Y polarization directions are equal.

[0071] In this embodiment, polarized light emitted by the optical transmitter is also split by a polarization beam splitter (PBS), and the split light beams are input into a secondary phase modulation structure for polarization state adjustment.

[0072] This embodiment also includes two power splitters and a polarization beam combiner (PBC). The two power splitters are used to split the two signal light paths output by the secondary phase modulator, generating two first beams and two second beams, respectively. The polarization beam combiner is used to combine the two second beams. After the signal light is split, it is extracted by the optical detector without significantly affecting the original optical path.

[0073] The extraction module in this embodiment includes two photodetectors (PDs), which respectively receive the two first light beams and convert them into electrical signals.

[0074] The processing module in this embodiment includes an FPGA, and an analog-to-digital converter (AD) and a digital-to-analog converter (DA) both electrically connected to the FPGA;

[0075] The analog-to-digital converter is also electrically connected to the photodetector and converts the electrical signal output by the photodetector into a digital signal and inputs it into the FPGA;

[0076] FPGA processes the digital signal to obtain the control voltage signal (V1, V2);

[0077] The digital-to-analog converter is also electrically connected to the secondary phase modulation structure, and converts the control voltage signal output by the FPGA into an electrical signal and inputs the electrical signal into the secondary phase modulator.

[0078] The present invention is described with reference to the flowcharts or block diagrams of the methods, devices (systems), and computer program products of the embodiments of the present application. It should be understood that each process or block in the flowchart or block diagram, as well as the combination of processes or blocks in the flowchart or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart or block diagram. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0079] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

[0080] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A method for infinite tracking of light polarization states, characterized in that: The following steps are involved: Step 1: The secondary phase modulator receives a beam of polarized light emitted from an optical transmitter and adjusts the polarization state of the polarized light to output signal light, wherein the signal light includes a first signal light and a second signal light; Step 2: The extraction module extracts the optical power of the signal light in the X and Y polarization directions and converts it into an electrical signal. Specifically, the extraction module splits the first signal light and the second signal light by two power splitters to obtain a first light beam and a second light beam respectively. The two first light beams are received by two photodetectors respectively and converted into electrical signals, and the second light beam is combined by a polarization beam combiner and continues to propagate along the original optical path; Step 3: The processing module receives the electrical signal and processes it to obtain the difference in optical power between the X and Y polarization directions; Step 4: The processing module processes the optical power difference using a dither algorithm to obtain the Stokes vector of the current signal light, and obtains a control voltage signal according to the Stokes vector; The step 4 of processing the optical power difference by a dither algorithm to obtain the Stokes vector of the current signal light and obtaining the control voltage according to the Stokes parameter vector specifically includes: Extracting the Stokes S1 component of the current polarization state of the signal light; The Stokes S2 component is estimated based on the Stokes S1 component using a dither algorithm, which specifically includes: Set the perturbation matrix Vb, where Vb contains numerical values; Multiply each value in Vb by delta and add it to the second voltage control stage of the two-stage phase modulator in sequence to obtain the feedback value after perturbation. ; where delta is the perturbation coefficient; According to the feedback value Calculate the value of the Stokes S2 component. The specific formula is: ， ， ； Where θ is the ellipticity; are the feedback values obtained by applying each value in sequence; T1 is the feedback value obtained by the secondary phase modulator after perturbation; The rotation angle of the current polarization state around the S3 axis to S1=0 is calculated according to the Stokes S1 component and the Stokes S2 component, and a corresponding control voltage signal is obtained according to the rotation angle; Step 5: The secondary phase modulator adjusts the polarization state of the polarized light according to the control voltage signal, so that the output signal light tends to the circle with S1=0 on the Poincare sphere, thereby achieving a stable polarization state of the light.

2. The method for infinite tracking of polarization states of light according to claim 1, wherein: In step 4, the perturbation matrix Va is set simultaneously, where Va contains When each value in Vb is multiplied by delta and then sequentially added to the second voltage control stage of the two-stage phase modulator, each value in Va is multiplied by delta and then sequentially added to the first voltage control stage of the two-stage phase modulator.

3. The method for infinite tracking of polarization states of light according to claim 2, wherein: When the rotation angle is 2π or 0, the processing module inputs a reverse voltage to the second voltage control stage in the two-stage phase modulator.

4. A system for implementing the method for infinite tracking of light polarization states according to any one of claims 1 to 3, characterized in that: The invention comprises an optical transmitter, a secondary phase modulator, an extraction module and a processing module connected in sequence. The optical transmitter emits polarized light, which is phase- and polarization-state-adjusted by the secondary phase modulator and then outputs signal light. The extraction module extracts part of the power of the signal light and converts it into an electrical signal. The processing module processes the electrical signal to obtain a control voltage signal, converts the control voltage signal into an electrical signal and inputs it into the secondary phase modulator. The secondary phase modulator adjusts the polarization state of the polarized light according to the control voltage signal, so that the signal light tends to the ring with S1=0 on the Poincare sphere.

5. The system according to claim 4, characterized in that It also includes two power splitters and a polarization combiner. The two power splitters are respectively used to split the two signal lights output by the secondary phase modulator to obtain two first light beams and two second light beams. The polarization combiner is used to combine the two second light beams.

6. The system according to claim 5, characterized in that The extraction module includes two light detectors, which respectively receive the two first light beams and convert them into electrical signals.

7. The system according to claim 6, characterized in that The processing module includes an FPGA, and an analog-to-digital converter and a digital-to-analog converter both electrically connected to the FPGA; The analog-to-digital converter is also electrically connected to the photodetector, and converts the electrical signal output by the photodetector into a digital signal and inputs the digital signal into the FPGA; The FPGA processes the digital signal to obtain a control voltage signal; The digital-to-analog converter is also electrically connected to the secondary phase modulator, and converts the control voltage signal output by the FPGA into an electrical signal and inputs the electrical signal into the secondary phase modulator.

Images