Method and system for correcting polarization state of a space-borne laser signal in orbit

Abstract

This invention provides a method and system for on-orbit polarization state correction of spaceborne laser signals. First, the rotation direction and pulse parameters required for switching between left- and right-hand circular polarization states are calibrated on the ground, from zero position to the default polarization state. During on-orbit operation, the motor is driven back to zero or polarity switching is performed based on the ground calibration parameters to resolve the issue of unclear rotation direction caused by command errors. To address environmental drift and accumulated errors, the characteristic of a laser corner bevel prism to reverse the rotation direction of the returning light is utilized, combined with the physical properties of orthogonally linearly polarized light from a polarization beam splitter, to fine-tune the rotating motor and monitor the energy received by the detector in real time. When the received energy reaches its peak value, it is determined to be the optimal polarization state under the current environment, achieving closed-loop circularity optimization and performance correction. This method eliminates dependence on interstellar links and effectively solves the problems of on-orbit parameter drift and multi-parameter coupling debugging of open-loop motors.

Description

Technical Field

[0001] This invention relates to the field of spaceborne laser communication, and more specifically, to a method and system for on-orbit polarization state correction of spaceborne laser signals. Background Technology

[0002] With the rapid development of aerospace communication technology, traditional microwave communication, limited by frequency bandwidth and electromagnetic compatibility, can no longer meet the demands of massive data transmission. Inter-satellite laser communication technology has gradually become mainstream. Among these, laser terminals with polarization state switching capabilities improve signal isolation by transmitting and receiving different polarization states, which is an important way to improve the reliability of spaceborne products. Currently, spaceborne laser terminals mainly achieve polarization state changes by using an open-loop rotating motor to drive a quarter-wave plate. While open-loop motors have advantages such as simple control interfaces and miniaturization, they lack real-time feedback. In practical applications, the direct switching method of pulse count calibration from the ground often leads to deterioration of polarization state roundness due to environmental changes (such as mechanical vibration and vacuum weightlessness) and cumulative errors from multiple switching, affecting link margin. Furthermore, in cases of misoperation or unclear state handover, it is difficult to confirm the on-orbit rotation direction, increasing the difficulty of link establishment.

[0003] While corrections can currently be made using the already acquired and tracked counterpart satellite, this method relies on a stable link established between the two satellites. However, in the initial stages of acquisition, the polarization state is highly coupled with parameters such as pointing accuracy and optical axis consistency, making debugging extremely difficult. In the existing technological field, there is still a lack of relevant patents or papers supporting on-orbit correction and circularity optimization for open-loop polarization state switching. Existing research, such as patent CN112039601B, proposes an inter-satellite self-zero-difference coherent optical radio frequency communication method that achieves detection through polarization modulation, but it does not address on-orbit correction. The 2024 paper "Low-loss Dynamic Polarization Controller Based on Silicon-based Optoelectronic Chips" in the *Acta Physica Sinica* achieved a high extinction ratio, but did not propose an on-orbit setting method.

[0004] Furthermore, the 2007 paper "Study on the Effect of Quarter-Wave Plate on Polarized Light Using Jones Matrix" in the *Journal of Hengyang Normal University* explored the principle of waveplate's effect on polarization state. The 2015 master's thesis from Beijing Institute of Technology, "Research on Quarter-Wave Plate Method for Measuring Birefringence," analyzed the influence of rotational positioning accuracy on measurement error. The 2008 paper "A New Method for Accurately Determining the Axis Orientation of Quarter-Wave Plate in the Optical Path" in the *Acta Optica Sinica* achieved precise waveplate installation using the least squares method and a feedback loop control system. Although the aforementioned literature and the 2011 master's thesis from Changchun University of Technology, "Application of Adaptive Signal Processing Based on Laser Polarization State," studied polarization state control from theoretical or ground-based experimental perspectives, none of them provided an effective solution for addressing the problem of on-orbit polarization state parameter drift and correction in spaceborne laser terminals with open-loop rotating motor configurations. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method and system for on-orbit polarization state correction of spaceborne laser signals.

[0006] According to one aspect of the present invention, an on-orbit polarization state correction method for a spaceborne laser signal includes:

[0007] Step S1: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch from zero position to the default polarization state; Step S2: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch polarity between left-handed and right-handed circular polarization states; Step S3: If polarization state correction is required during on-orbit operation, perform at least one of the following actions: Action A: According to the parameters specified in step S1, rotate the drive motor from the zero position to the default polarization state position; Action B: According to the parameters specified in step S2, the drive motor is rotated to achieve the switching of the specified polarization state; Action C: Based on the characteristic of the laser corner bevel prism to reverse the rotation direction of the circular polarization state of the original path return light, and the transmission and reflection beam splitting characteristics of the polarization beam splitter for two orthogonally linearly polarized beams, the rotating motor is finely adjusted and the light reflection energy received by the detector is monitored. When the energy reaches the maximum value, it is determined to be the optimal polarization state performance.

[0008] Preferably, the open-loop rotating waveplate motor is equipped with a quarter-wave plate, and the effective rotational stroke of the open-loop rotating waveplate motor is 350 degrees. The polarization state changes as the quarter-wave plate rotates within the effective rotational stroke. The zero-position region is the remaining 10-degree non-rotatable region outside the effective rotational stroke.

[0009] Preferably, the pulse parameter corresponding to the total stroke of the open-loop rotating waveplate motor from one end to the other end is 350 degrees; within the total stroke, the quarter-waveplate generates 4 left-handed circular polarization states and 4 right-handed circular polarization states as the motor rotates; the default polarization state position is selected from the angular position corresponding to the left-handed or right-handed circular polarization state that is far from the motor limit position and conforms to the initial use state of the product on the track.

[0010] Preferably, step S1 specifically includes: Select one end limit position of the open-loop rotating waveplate motor as the zero position, calibrate the pulse parameter A required for the rotating motor to rotate from the zero position to the default circular polarization state, and record the rotation direction A at the same time; The pulse parameter B and rotation direction B are calibrated to rotate from the default circular polarization state to the zero position. The pulse parameter B causes the motor to remain at the zero position for 1 to 2 seconds.

[0011] 5. The method according to claim 1, wherein step 2 specifically includes: When the waveplate is rotated so that the polarization direction of the incident light makes a 45-degree angle with the fast / slow axis of the quarter-wave plate, a circularly polarized state is obtained, where the incident light from the quarter-wave plate is linearly polarized. By rotating a quarter-wave plate by a fixed number of pulses using forward and reverse motors, the switching between left-handed and right-handed circular polarization states of the laser can be achieved. Specifically, when the rotation of the waveplate makes the angle between the polarization direction of its incident light and the fast / slow axis of the quarter-wave plate 45 degrees, a circular polarization state can be obtained, and the incident light of the quarter-wave plate is linearly polarized. The rotation pulse parameters that still meet the polarization state roundness index requirements after multiple consecutive switching between two circular polarization states are selected.

[0012] Preferably, step S3 specifically includes: When the on-orbit polarization state is unknown, first rotate the rotating waveplate motor to the zero position through a pulse, and then execute action A; Determine if the current polarization state is the desired target polarization state. If yes, execute action C; otherwise, execute action B, and then execute action C. When the on-orbit polarization state is known, determine whether the current polarization state is the desired target polarization state. If so, determine whether action C has been completed. If action C has been completed, apply it directly. If action C has not been completed, execute action C. If not, then execute action B, and then execute action C.

[0013] Preferably, in action C, the laser corner bevel prism's characteristic of reversing the circular polarization direction of the returning light, and the polarization beam splitter's transmission and reflection characteristics for two orthogonally linearly polarized beams, are as follows: The laser emission branch emits initial linearly polarized light, which is incident on a 45° polarization beam splitter. After reflection, the S-polarized component passes through a quarter-wave plate, is reflected by a cornerstone prism, and then passes back through a quarter-wave plate. The returning light passes through the PBS again, while the P-polarized component is transmitted into the energy detector.

[0014] Preferably, in action C, the calculation process for determining the optimal polarization state performance is as follows: The reflection axis and transmission axis of the polarizing beam splitter PBS are defined to be orthogonal to each other, wherein the reflection axis of the PBS is the S-polarization direction and the transmission axis of the polarizing beam splitter PBS is the P-polarization direction. The initial linearly polarized light emitted by the laser emission branch is The angle between its polarization direction and the transmission axis of the PBS is . ; When the initially linearly polarized light is incident on the polarizing beam splitter PBS, its electric vector is decomposed in the PBS coordinate system as follows: P component: along the direction of transmitted light from PBS; S component: along the PBS reflection axis; The expression for the S-component of reflection is:

[0015] in, Let represent the electric field amplitude of the initially linearly polarized incident light, and Es represent the amplitude of the S-polarization component reflected by the polarizing beam splitter PBS. This indicates the angle between the polarization direction of the initially linearly polarized incident light and the transmission axis of the PBS; The S-component is incident on a quarter-wave plate. Let Φ be the angle between the fast axis of the quarter-wave plate and the transmission axis of the PBS. By rotating by -Φ, the electric field of the S-component is projected onto the fast and slow axis coordinate system of the wave plate. The electric field of the S-component before it is incident on the wave plate is decomposed along the fast and slow axes as follows:

[0016] Where Φ represents the angle between the fast axis and the transmission axis of the waveplate. This represents the electric field of the S component before it passes through the waveplate. This represents the rotation matrix from the PBS to the waveplate coordinate system; The quarter-wave plate introduces a phase delay of π / 2 on the fast and slow axes. The electric field of the S component after passing through the wave plate is obtained by the Jones matrix:

[0017] in, This represents the Jones matrix of a quarter-wave plate. This represents the electric field of the S component after passing through the waveplate; Will Rotate Φ back to the PBS coordinate system, and the polarization state output to the pyramid is as follows:

[0018] in, This represents the electric field vector in the PBS coordinate system after the S-component passes through a quarter-wave plate. The matrix in the formula above has two rows and one column; the first row represents the first row of the matrix, which contains the S-component; the second row represents the second row of the matrix, which contains the P-component. This represents the rotation matrix from the waveplate coordinate system to the PBS coordinate system; The S-component, after being reflected by a pyramidal prism, is equivalent to mirror reflection. By reversing the left and right circular polarizations, i.e. changing the polarization direction, the electric field after reflection is:

[0019] in, This represents the electric field after the S component is reflected by the pyramid; The S-component, passing backward through the quarter-wave plate, is projected again onto the fast and slow axes of the wave plate, adding a delay:

[0020] in, This represents the electric field of the S component returning to the PBS after passing through the quarter-wave plate in the reverse direction; Neglecting the fixed coefficients, the remaining key term is calculated to be sin(2Φ):

[0021] The P-polarization component of the S-component returned by PBS transmission detection is: for:

[0022] in, This represents the electric field amplitude of the returned S component that can pass through the PBS and reach the detector P polarization component. The light intensity energy received by the detector is proportional to The square of:

[0023] in, Indicates and The initial light intensity is proportional to the square of the value. This indicates the intensity of the light signal measured by the detector. By gradually fine-tuning the rotating motor in both directions, Φ is adjusted to find the position of the motor rotation corresponding to the maximum received energy, thus achieving optimization of the polarization state roundness index.

[0024] According to another aspect of the present invention, an on-orbit polarization state correction system for a spaceborne laser signal includes: Module M1: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch from zero position to the default polarization state; Module M2: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch polarity between left-handed and right-handed circular polarization states; Module M3: During on-orbit operation, if polarization state correction is required, perform at least one of the following actions: Action A: According to the parameters calibrated by module M1, rotate the drive motor from the zero position to the default polarization state position; Action B: Based on the parameters calibrated by module M2, drive the motor to rotate to achieve the switching of the specified polarization state; Action C: Based on the characteristic of the laser corner bevel prism to reverse the rotation direction of the circular polarization state of the original path return light, and the transmission and reflection beam splitting characteristics of the polarization beam splitter for two orthogonally linearly polarized beams, the rotating motor is finely adjusted and the light reflection energy received by the detector is monitored. When the energy reaches the maximum value, it is determined to be the optimal polarization state performance.

[0025] Preferably, the open-loop rotating waveplate motor is equipped with a quarter-wave plate, and the effective rotational stroke of the open-loop rotating waveplate motor is 350 degrees. The polarization state changes as the quarter-wave plate rotates within the effective rotational stroke. The zero-position region is the remaining 10-degree non-rotatable region outside the effective rotational stroke.

[0026] Compared with the prior art, the present invention has the following beneficial effects: This invention presents a calibration method for ground-based rotary motors, starting with zero-position calibration and multiple polarization state switching parameter assessments. During on-orbit self-closed-loop calibration, parameters are fine-tuned to obtain the strongest optical feedback energy. When the on-orbit polarization state is uncertain, it can accurately adjust to the desired polarization state direction, while simultaneously achieving high-precision optimization of polarization state roundness after on-orbit polarization state switching. This method features a simple interface, a highly reliable open-loop rotary motor, and requires no additional hardware or software design. It offers advantages such as simple engineering implementation, convenient on-orbit application, and miniaturized product design, providing a reference and basis for subsequent parameter optimization of polarization state switching in on-orbit laser communication. Attached Figure Description

[0027] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the on-orbit polarization state correction method for spaceborne laser signals according to the present invention; Figure 2 A schematic diagram illustrating the principle of adjusting the polarization state using a quarter-wave plate; Figure 3 This is a schematic diagram of the combination of the rotary motor and the quarter-wave plate of the present invention; Figure 4 This is a measured diagram of the polarization state of the output light from the rotary motor of the present invention. Figure 5 This is a schematic diagram of the optical path in the self-calibration mode of the present invention. Detailed Implementation

[0028] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0029] Example 1: This embodiment provides an on-orbit correction method for the polarization state of a spaceborne laser signal. This method can effectively solve the problem of accurately adjusting the on-orbit polarization state of a spaceborne laser from the current uncertain polarization state to the required polarization state, and at the same time solve the problem of the polarization state roundness gradually deteriorating after multiple on-orbit polarization state switching, thus achieving high-precision tuning of polarization state roundness.

[0030] The correction method of the present invention is as follows: 1) Ground calibration to obtain the adjustment direction and motor parameters from the zero position of the rotating motor to the default circular polarization state. Linearly polarized spatial light is converted into circularly polarized light after passing through a quarter-wave plate inside the rotating motor. The rotation of the quarter-wave plate can change the polarization state and its circularity. Since the motor rotates nearly 360°, the default polarization state changes as the motor rotates once: "left-handed circular polarization state 1 → linear polarization state → right-handed circular polarization state 2 → linear polarization state → left-handed circular polarization state 3 → linear polarization state → right-handed circular polarization state 4 → linear polarization state" or "right-handed circular polarization state 1 → linear polarization state → left-handed circular polarization state 2 → linear polarization state → right-handed circular polarization state 3 → linear polarization state → left-handed circular polarization state 4 → linear polarization state". The second or third circular polarization state, which is farthest from the motor's zero position, is preferentially selected as the default polarization state. Since the rotation angles corresponding to the pulse parameters of the same motor rotating in the forward and reverse directions are different, and the rotation angles corresponding to the rotation pulse parameters of different motors are also different, the polarization state characteristics of the output light are observed in real time while the motor rotates from the zero position during the ground calibration of each product. This confirms the motor rotation direction and rotation pulse parameters from the zero position of the motor to the default polarization state of the laser signal.

[0031] 2) Ground calibration obtains the rotation direction and motor parameters of the rotary motor from the default circular polarization state to another circular polarization state. During ground calibration, the polarization characteristics of the output light are observed in real time while the motor is rotated to confirm the motor rotation direction and rotation pulse parameters from the default circular polarization state of the laser signal to another circular polarization state. Since the error accumulates after multiple switching of the open-loop control polarization state, to ensure that the rotation pulse parameters are selected reasonably, if the polarization state circularity index requirement is still met after multiple switching between the two circular polarization states, then the rotation pulse parameter is selected as the calibration value.

[0032] 3) In-orbit rotation to zero (limit) position followed by fixed rotation direction and pulse parameters enables rapid recovery of the default (or commonly used) circular polarization state of the laser signal. Switching between left-hand and right-hand circular polarization states is achieved through fixed rotation direction and pulse parameters in-orbit rotation.

[0033] 4) The polarization state switching method described above can accurately switch to the circularly polarized state required for the application. However, due to factors such as the mechanical vibration of the active phase during the launch phase of the spaceborne product and the difference between the on-orbit vacuum environment and the ground calibration environment, there is a deviation between the number of rotation pulses of the rotating motor and the actual rotation angle, resulting in a deterioration in the circularity index of the polarization state after switching. The receiving and receiving paths of the spaceborne laser product are designed with a common optical path from the quarter-wave plate to the optical antenna port. After the spatially linearly polarized light of the laser signal transmission branch is converted into circularly polarized light through the quarter-wave plate, it is reflected back to the receiving branch by the cornerstone reflector configured in the product. At this time, the spatially circularly polarized light of the receiving branch is converted back into linearly polarized light after passing through the quarter-wave plate. The better the circularity index of the spatially circularly polarized light, the more energy the receiving branch acquires, which is reflected in the corresponding received energy telemetry. On-orbit, by gradually fine-tuning the number of pulses of the rotating motor from two rotation directions, the maximum value of the received energy is found, thereby achieving polarization state circularity optimization.

[0034] The specific optical path is as follows: the laser emission branch emits initial linearly polarized light, which is incident on a 45° polarizing beam splitter (PBS). After reflection, the S-polarized component passes through a quarter-wave plate, converting the linearly polarized light into left-hand (right-hand) circularly polarized light. After reflection by a cornerstone prism, equivalent to mirror reflection, the left-hand (right-hand) circularly polarized light is converted into right-hand (left-hand) circularly polarized light. After passing through the quarter-wave plate again in the opposite direction, it reverts to P-polarized light. The returning light passes through the PBS again, and the P-polarized component is transmitted into the energy detector. The energy detector detects the strongest light intensity when the circularity of the left-hand (right-hand) circularly polarized light passing through the quarter-wave plate is optimal.

[0035] 5) By rotating the motor from zero position to the default circular polarization state, switching between left and right circular polarization states, and fine-tuning the rotating motor, the optimal combination of polarization states is found, thereby obtaining accurate and precise circular polarization state of the on-orbit laser signal.

[0036] Example 2 This embodiment provides an on-orbit polarization state correction method for spaceborne laser signals, which is used for switching and correcting different polarization states on-orbit, enabling the on-orbit laser terminal to accurately adjust from an unknown laser polarization state to the required polarization state, while also optimizing the on-orbit polarization state roundness index.

[0037] refer to Figure 1 The on-orbit polarization state correction steps include: ① The rotary motor returns to the zero position (i.e., the limit position), and rotates from the zero position to the ground-calibrated fixed direction for the corresponding number of pulses to switch to the default polarization state; ② The polarization state is switched by rotating to the ground-calibrated fixed direction for the corresponding number of pulses; ③ The spaceborne laser sets a self-closed loop, and the rotary motor is rotated in both forward and reverse directions to fine-tune the number of pulses until the loop receives the strongest light energy; The above steps are used individually or in combination according to the laser polarization state of the current spaceborne laser terminal product and the polarization state requirements required for use, in order to achieve the purpose of meeting the polarization state requirements for use.

[0038] refer to Figure 2 The principle behind adjusting the polarization state using a quarter-wave plate is that when light passes through the plate, the two orthogonal polarization components along the fast and slow axes generate a phase difference. The thickness of the quarter-wave plate is designed to introduce a phase difference of π / 2 for a specific wavelength. When linearly polarized light is incident, its vibration direction forms an angle θ with the fast axis of the wave plate. When the angle θ equals 45°, the amplitudes of the two components along the fast and slow axes are equal, and their superposition forms circularly polarized light (left-handed or right-handed, depending on the direction of the wave plate).

[0039] refer to Figure 3 A quarter-wave plate is assembled in a rotating motor and can rotate with the motor. At this time, the angle θ between the incident linearly polarized light and the fast axis of the wave plate changes accordingly. When the angle θ rotates to 45°, circularly polarized light is formed. When the angle θ is equal to 0° or 90°, it is linearly polarized.

[0040] refer to Figure 4 As the paddle motor rotates, the angle θ changes, and the left-hand circular polarization state, linear polarization state, and right-hand circular polarization state are obtained by actual measurement.

[0041] Since the motor's rotation range is close to 360°, and a switch between left-handed and right-handed circular polarization states can be achieved every 90°, a single rotation of the motor will result in two left-handed circular polarization states and two right-handed circular polarization states. The polarization state changes during one rotation as follows: "Left-handed circular polarization state 1 → Linear polarization state → Right-handed circular polarization state 2 → Linear polarization state → Left-handed circular polarization state 3 → Linear polarization state → Right-handed circular polarization state 4 → Linear polarization state" or "Right-handed circular polarization state 1 → Linear polarization state → Left-handed circular polarization state 2 → Linear polarization state → Right-handed circular polarization state 3 → Linear polarization state → Left-handed circular polarization state 4". The second or third circular polarization state, furthest from the motor's zero position, is preferentially selected as the default polarization state and another commonly used polarization state.

[0042] When calibrating laser terminal products in a ground-based laboratory, a rotating motor is used to make the motor rotate from zero position to a specified direction. At the same time, the change in the polarization state of the output light is observed after the rotation stops, thereby obtaining the rotation direction and rotation pulse parameters of the motor corresponding to the polarization state of the default laser signal from the zero position of the motor.

[0043] When calibrating laser terminal products in a ground-based laboratory, a rotary motor is used to repeatedly switch between the most recent polarization state 2 (3) and polarization state 3 (2). The change in the polarization state of the output light after stopping is observed by combining the number of rotation pulses and the rotation direction. Since the error of open-loop control polarization state switching increases after multiple switching, in order to ensure that the rotation pulse parameters are selected reasonably, if the circularity of the polarization state is still better than the index requirement after multiple switching between left-hand and right-hand circular polarization states, then the rotation pulse number and rotation direction of the motor are confirmed to be the parameter calibration values ​​required for left-hand and right-hand circular polarization state switching.

[0044] Because the rotation angles corresponding to the pulse parameters for forward and reverse rotation of the same motor differ, and the rotation angles corresponding to the rotation pulse parameters of different motors also differ, each product's different switching methods need to be calibrated individually in a laboratory environment. For example, the polarization state switching parameters calibrated during ground testing of a certain spaceborne product are shown in the table below. The left and right circular polarization state switching functions are normal, and after multiple switching (>10 times), the measured circularity of the polarization state is better than 0.95.

[0045]

[0046] refer to Figure 5 In spaceborne laser products, the light-receiving path from the quarter-wave plate to the optical antenna port is designed as a common optical path. The initial linearly polarized light from the laser emission branch is reflected by a 45° polarization beam splitter (PBS) (the initial linear polarization direction makes an angle of 45° with the transmission axis of the PBS). After reflection, it passes through the quarter-wave plate (the fast axis of the wave plate makes an angle θ with the reflection axis of the PBS), and after reflection by the optical antenna and a pyramidal mirror, it passes back through the quarter-wave plate. The returning light then passes through the PBS again and is transmitted into the energy detector. When θ = 45°, the energy of the returning light transmitted into the energy detector is the strongest, which is reflected in the corresponding received energy telemetry. When θ = 45°, the light between the waveplate motor and the pyramidal mirror is circularly polarized. During on-orbit polarization state correction, the maximum received energy is found by finely adjusting the angle of the rotary motor in both rotation directions, thus achieving polarization state circularity optimization. For example, a certain spaceborne product uses this method for polarization state optimization. First, it rotates forward a small number of pulses (200 or 100 pulses, depending on the calibration accuracy). It then performs multiple forward rotations for fine-tuning until the received energy significantly decreases. Next, it reverses the direction for a small number of pulses (consistent with the number of pulses in a single forward fine-tuning). After multiple reverse fine-tunings, it observes that the received energy telemetry gradually strengthens and then significantly weakens. The adjustment pulses and corresponding coupled energy telemetry values ​​are recorded. Then, it analyzes the number of rotations required to reach the maximum capacity value and the telemetry value corresponding to the maximum coupled energy. Finally, it observes the telemetry stopping at the position of the maximum coupled energy by performing multiple motor fine-tuning rotations. This process can be repeated multiple times.

[0047] refer to Figure 1 After the spaceborne product has undergone active mechanical vibration during the launch phase, its polarization state roundness may be better than that of the product after a slight change due to vibration. By setting a self-closed loop for the spaceborne laser in step ③ and finely adjusting the number of pulses in the forward and reverse rotation of the rotating motor to the point where the light energy received by the loop is strongest, the polarization state roundness index can be optimized.

[0048] When the on-orbit polarization state is switched according to usage requirements, the accurate and rapid switching between left-hand and right-hand circular polarization states is achieved through step ②. Due to the difference between the rotation pulse number of the rotating motor and the actual rotation angle, there is a deviation between the ground laboratory environment and the on-orbit vacuum, weightlessness and other environments. After multiple switching, the circularity index of the polarization state will deteriorate due to the accumulation of deviation. At this time, the polarization state circularity index can be optimized through step ③.

[0049] When switching between left-hand and right-hand circular polarization states in orbit, if the current polarization state cannot be identified due to misoperation, step ① can quickly establish the default polarization state, step ② can accurately switch to the required polarization state, and step ③ can optimize the circularity index of the polarization state, thereby achieving precise adjustment of the polarization state.

[0050] Example 3 A method for on-orbit polarization state correction of spaceborne laser signals, comprising: Step S1: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch from zero position to the default polarization state; Step S2: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch polarity between left-handed and right-handed circular polarization states; Step S3: If polarization state correction is required during on-orbit operation, perform at least one of the following actions: Action A: According to the parameters specified in step S1, rotate the drive motor from the zero position to the default polarization state position; Action B: According to the parameters specified in step S2, the drive motor is rotated to achieve the switching of the specified polarization state; Action C: Based on the characteristic of the laser corner bevel prism to reverse the rotation direction of the circular polarization state of the original path return light, and the transmission and reflection beam splitting characteristics of the polarization beam splitter for two orthogonally linearly polarized beams, the rotating motor is finely adjusted and the light reflection energy received by the detector is monitored. When the energy reaches the maximum value, it is determined to be the optimal polarization state performance.

[0051] Preferably, the open-loop rotating waveplate motor is equipped with a quarter-wave plate, and the effective rotational stroke of the open-loop rotating waveplate motor is 350 degrees. The polarization state changes as the quarter-wave plate rotates within the effective rotational stroke. The zero-position region is the remaining 10-degree non-rotatable region outside the effective rotational stroke.

[0052] Preferably, the pulse parameter corresponding to the total stroke of the open-loop rotating waveplate motor from one end to the other end is 350 degrees; within the total stroke, the quarter-waveplate generates 4 left-handed circular polarization states and 4 right-handed circular polarization states as the motor rotates; the default polarization state position is selected from the angular position corresponding to the left-handed or right-handed circular polarization state that is far from the motor limit position and conforms to the initial use state of the product on the track.

[0053] Preferably, step S1 specifically includes: Select one end limit position of the open-loop rotating waveplate motor as the zero position, calibrate the pulse parameter A required for the rotating motor to rotate from the zero position to the default circular polarization state, and record the rotation direction A at the same time; The pulse parameter B and rotation direction B are calibrated to rotate from the default circular polarization state to the zero position. The pulse parameter B causes the motor to remain at the zero position for 1 to 2 seconds.

[0054] 5. The method according to claim 1, wherein step 2 specifically includes: When the waveplate is rotated so that the polarization direction of the incident light makes a 45-degree angle with the fast / slow axis of the quarter-wave plate, a circularly polarized state is obtained, where the incident light from the quarter-wave plate is linearly polarized. By rotating a quarter-wave plate by a fixed number of pulses using forward and reverse motors, the switching between left-handed and right-handed circular polarization states of the laser can be achieved. Specifically, when the rotation of the waveplate makes the angle between the polarization direction of its incident light and the fast / slow axis of the quarter-wave plate 45 degrees, a circular polarization state can be obtained, and the incident light of the quarter-wave plate is linearly polarized. The rotation pulse parameters that still meet the polarization state roundness index requirements after multiple consecutive switching between two circular polarization states are selected.

[0055] Preferably, step S3 specifically includes: When the on-orbit polarization state is unknown, first rotate the rotating waveplate motor to the zero position through a pulse, and then execute action A; Determine if the current polarization state is the desired target polarization state. If yes, execute action C; otherwise, execute action B, and then execute action C. When the on-orbit polarization state is known, determine whether the current polarization state is the desired target polarization state. If so, determine whether action C has been completed. If action C has been completed, apply it directly. If action C has not been completed, execute action C. If not, then execute action B, and then execute action C.

[0056] Preferably, in action C, the laser corner bevel prism's characteristic of reversing the circular polarization direction of the returning light, and the polarization beam splitter's transmission and reflection characteristics for two orthogonally linearly polarized beams, are as follows: The laser emission branch emits initial linearly polarized light, which is incident on a 45° polarization beam splitter. After reflection, the S-polarized component passes through a quarter-wave plate, is reflected by a cornerstone prism, and then passes back through a quarter-wave plate. The returning light passes through the PBS again, while the P-polarized component is transmitted into the energy detector.

[0057] Preferably, in action C, the calculation process for determining the optimal polarization state performance is as follows: The reflection axis (S-polarization direction) and transmission axis (P-polarization direction) of the polarization beam splitter (PBS) are defined to be orthogonal to each other; the initial linearly polarized light emitted by the laser emission branch is... The angle between its polarization direction and the transmission axis (P direction) of the PBS is... .

[0058] When the initially linearly polarized light is incident on the PBS, its electric vector is decomposed in the PBS coordinate system as follows: P component (transmission): along the direction of transmitted light from PBS; S-component (reflection): along the PBS reflection axis; The expression for the S-polarization component of reflection is:

[0059] : The electric field amplitude of the initially linearly polarized light; Es: Amplitude of the S component reflected by PBS; The angle between the polarization direction of the initial linearly polarized light and the transmission axis (P direction) of the PBS; The S-component (reflected) light is incident on a quarter-wave plate; let Φ be the angle between the fast axis of the quarter-wave plate and the transmission axis of the PBS, and rotate by -Φ to project the electric field onto the fast and slow axis coordinate system (EWPC) of the wave plate. The electric field incident on the wave plate is decomposed along the fast and slow axes as follows:

[0060] Φ: Angle between the fast axis and the P-axis of the waveplate; The S-component passes through the electric field before the waveplate (fast and slow axis system). A quarter-wave plate introduces a phase delay of π / 2 on the fast and slow axes. The electric field after passing through the wave plate is obtained using the Jones matrix:

[0061] : Quarter-wave plate Jones matrix; : The electric field of the S component at the point after passing through the waveplate (fast and slow axis system); To make it easier to understand, Rotate Φ back to the PBS coordinate system, and the polarization state output to the pyramid is as follows.

[0062] The electric field vector of the S component in the PBS coordinate system after passing through a quarter-wave plate (the first row is the S component, and the second row is the P component). After reflection by a pyramidal prism, it is equivalent to mirror reflection. By exchanging the left and right circular polarizations (i.e., changing the polarization direction), the electric field after reflection is:

[0063] The electric field after the S component is reflected by the pyramid The S-component, passing backward through the quarter-wave plate, is projected again onto the fast and slow axes of the wave plate, adding a delay:

[0064] The electric field of the S component returning to the PBS after passing through the quarter-wave plate in the reverse direction. Neglecting the fixed coefficients, the remaining key term is calculated to be sin(2Φ):

[0065] The P-polarization component of the returned light was detected by PBS transmission. for:

[0066] The electric field amplitude of the P-polarization component of the returned light that can pass through the PBS to reach the detector. The light intensity energy received by the detector is proportional to The square of:

[0067] :and The initial light intensity is proportional to the square of the light intensity. The intensity of the light signal measured by the detector.

[0068] Based on the signal energy and incident angle ultimately detected by the detector It is closely related to the waveplate angle Φ. When Φ = 45° When the maximum value is reached, the emitted light, after passing through the quarter-wave plate, becomes either left- or right-hand circularly polarized light with a high polarization circularity index. The process of gradually fine-tuning the rotating motor in both directions of rotation is equivalent to adjusting Φ. Therefore, the position of the motor rotation corresponding to the maximum received energy can be found, thus optimizing the polarization circularity index.

[0069] The present invention also provides an on-orbit polarization state correction system for spaceborne laser signals. The on-orbit polarization state correction system for spaceborne laser signals can be implemented by executing the process steps of the on-orbit polarization state correction method for spaceborne laser signals. That is, those skilled in the art can understand the on-orbit polarization state correction method for spaceborne laser signals as a preferred embodiment of the on-orbit polarization state correction system for spaceborne laser signals.

[0070] Those skilled in the art will understand that, besides implementing the system and its various devices, modules, and units provided by this invention in the form of purely computer-readable program code, the same functions can be achieved entirely through logical programming of the method steps, making the system and its various devices, modules, and units of this invention function in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system and its various devices, modules, and units provided by this invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; alternatively, the devices, modules, and units for implementing various functions can be considered as both software modules implementing the method and structures within the hardware component.

[0071] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A method for on-orbit polarization state correction of spaceborne laser signals, characterized in that, include: Step S1: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch from zero position to the default polarization state; Step S2: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch polarity between left-handed and right-handed circular polarization states; Step S3: If polarization state correction is required during on-orbit operation, perform at least one of the following actions: Action A: According to the parameters specified in step S1, rotate the drive motor from the zero position to the default polarization state position; Action B: According to the parameters specified in step S2, the drive motor is rotated to achieve the switching of the specified polarization state; Action C: Based on the characteristic of the laser corner bevel prism to reverse the rotation direction of the circular polarization state of the original path return light, and the transmission and reflection beam splitting characteristics of the polarization beam splitter for two orthogonally linearly polarized beams, the rotating motor is finely adjusted and the light reflection energy received by the detector is monitored. When the energy reaches the maximum value, it is determined to be the optimal polarization state performance.

2. The method according to claim 1, characterized in that, The open-loop rotating waveplate motor is equipped with a quarter-wave plate. The effective rotational stroke of the open-loop rotating waveplate motor is 350 degrees. The polarization state changes as the quarter-wave plate rotates within the effective rotational stroke. The zero-position region is the remaining 10-degree non-rotatable region outside the effective rotational stroke.

3. The method according to claim 1, characterized in that, The pulse parameter corresponding to the total stroke of the open-loop rotating waveplate motor from one end to the other end is 350 degrees. Within the total stroke, the quarter-waveplate generates four left-handed circular polarization states and four right-handed circular polarization states as the motor rotates. The default polarization state position is selected from the angular position corresponding to the left-handed or right-handed circular polarization state that is far from the motor limit and conforms to the initial use state of the product on the track.

4. The method according to claim 1, characterized in that, Step S1 specifically includes: Select one end limit position of the open-loop rotating waveplate motor as the zero position, calibrate the pulse parameter A required for the rotating motor to rotate from the zero position to the default circular polarization state, and record the rotation direction A at the same time; The pulse parameter B and rotation direction B are calibrated to rotate from the default circular polarization state to the zero position. The pulse parameter B causes the motor to remain at the zero position for 1 to 2 seconds.

5. The method according to claim 1, characterized in that, Step 2 specifically includes: When the waveplate is rotated so that the polarization direction of the incident light makes a 45-degree angle with the fast / slow axis of the quarter-wave plate, a circularly polarized state is obtained, where the incident light from the quarter-wave plate is linearly polarized. By rotating a quarter-wave plate by a fixed number of pulses using forward and reverse motors, the switching between left-handed and right-handed circular polarization states of the laser can be achieved. Specifically, when the rotation of the waveplate makes the angle between the polarization direction of its incident light and the fast / slow axis of the quarter-wave plate 45 degrees, a circular polarization state can be obtained, and the incident light of the quarter-wave plate is linearly polarized. The rotation pulse parameters that still meet the polarization state roundness index requirements after multiple consecutive switching between two circular polarization states are selected.

6. The method according to claim 1, characterized in that, Step S3 specifically includes: When the on-orbit polarization state is unknown, first rotate the rotating waveplate motor to the zero position through a pulse, and then execute action A; Determine if the current polarization state is the desired target polarization state. If yes, execute action C; otherwise, execute action B, and then execute action C. When the on-orbit polarization state is known, determine whether the current polarization state is the desired target polarization state. If so, determine whether action C has been completed. If action C has been completed, apply it directly. If action C has not been completed, execute action C. If not, then execute action B, and then execute action C.

7. The method according to claim 1, characterized in that, In action C, the laser corner bevel prism reverses the rotation direction of the circularly polarized state of the returning light, and the polarizing beam splitter exhibits the transmission and reflection characteristics of two orthogonally linearly polarized beams, as detailed below: The laser emission branch emits initial linearly polarized light, which is incident on a 45° polarization beam splitter. After reflection, the S-polarized component passes through a quarter-wave plate, is reflected by a cornerstone prism, and then passes back through a quarter-wave plate. The returning light passes through the PBS again, while the P-polarized component is transmitted into the energy detector.

8. The method according to claim 1, characterized in that, In action C, the calculation process for determining the optimal polarization state performance is as follows: The reflection axis and transmission axis of the polarizing beam splitter PBS are defined to be orthogonal to each other, wherein the reflection axis of the PBS is the S-polarization direction and the transmission axis of the polarizing beam splitter PBS is the P-polarization direction. The initial linearly polarized light emitted by the laser emission branch is The angle between its polarization direction and the transmission axis of the PBS is . ; When the initially linearly polarized light is incident on the polarizing beam splitter PBS, its electric vector is decomposed in the PBS coordinate system as follows: P component: along the direction of transmitted light from PBS; S component: along the PBS reflection axis; The expression for the S-component of reflection is: in, Let represent the electric field amplitude of the initially linearly polarized incident light, and Es represent the amplitude of the S-polarization component reflected by the polarizing beam splitter PBS. This indicates the angle between the polarization direction of the initially linearly polarized incident light and the transmission axis of the PBS; The S-component is incident on a quarter-wave plate. Let Φ be the angle between the fast axis of the quarter-wave plate and the transmission axis of the PBS. By rotating by -Φ, the electric field of the S-component is projected onto the fast and slow axis coordinate system of the wave plate. The electric field of the S-component before it is incident on the wave plate is decomposed along the fast and slow axes as follows: Where Φ represents the angle between the fast axis and the transmission axis of the waveplate. This represents the electric field of the S component before it passes through the waveplate. This represents the rotation matrix from the PBS to the waveplate coordinate system; The quarter-wave plate introduces a phase delay of π / 2 on the fast and slow axes. The electric field of the S component after passing through the wave plate is obtained by the Jones matrix: in, This represents the Jones matrix of a quarter-wave plate. This represents the electric field of the S component after passing through the waveplate; Will Rotate Φ back to the PBS coordinate system, and the polarization state output to the pyramid is as follows: in, This represents the electric field vector in the PBS coordinate system after the S component passes through a quarter-wave plate. This represents the rotation matrix from the waveplate coordinate system to the PBS coordinate system; The S-component, after being reflected by a pyramidal prism, is equivalent to mirror reflection. By reversing the left and right circular polarizations, i.e. changing the polarization direction, the electric field after reflection is: in, This represents the electric field after the S component is reflected by the pyramid; The S-component, passing backward through the quarter-wave plate, is projected again onto the fast and slow axes of the wave plate, adding a delay: in, This represents the electric field of the S component returning to the PBS after passing through the quarter-wave plate in the reverse direction; Neglecting the fixed coefficients, the remaining key term is calculated to be sin(2Φ): The P-polarization component of the S-component returned by PBS transmission detection is: for: in, This represents the electric field amplitude of the returned S component that can pass through the PBS and reach the detector P polarization component. The light intensity energy received by the detector is proportional to The square of: in, Indicates and The initial light intensity is proportional to the square of the value. This indicates the intensity of the light signal measured by the detector. By gradually fine-tuning the rotating motor in both directions, Φ is adjusted to find the position of the motor rotation corresponding to the maximum received energy, thus achieving optimization of the polarization state roundness index.

9. A spaceborne laser signal on-orbit polarization state correction system, characterized in that, include: Module M1: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch from zero position to the default polarization state; Module M2: In a ground-based laboratory environment, calibrate the rotation direction and pulse parameters required for the open-loop rotating waveplate motor to switch polarity between left-handed and right-handed circular polarization states; Module M3: During on-orbit operation, if polarization state correction is required, perform at least one of the following actions: Action A: According to the parameters calibrated by module M1, rotate the drive motor from the zero position to the default polarization state position; Action B: Based on the parameters calibrated by module M2, drive the motor to rotate to achieve the switching of the specified polarization state; Action C: Based on the characteristic of the laser corner bevel prism to reverse the rotation direction of the circular polarization state of the original path return light, and the transmission and reflection beam splitting characteristics of the polarization beam splitter for two orthogonally linearly polarized beams, the rotating motor is finely adjusted and the light reflection energy received by the detector is monitored. When the energy reaches the maximum value, it is determined to be the optimal polarization state performance.

10. The method according to claim 1, characterized in that, The open-loop rotating waveplate motor is equipped with a quarter-wave plate. The effective rotational stroke of the open-loop rotating waveplate motor is 350 degrees. The polarization state changes as the quarter-wave plate rotates within the effective rotational stroke. The zero-position region is the remaining 10-degree non-rotatable region outside the effective rotational stroke.

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