Laser communication stable transmission method based on dynamic compensation of emission optical axis

Abstract

The invention provides a laser communication stable transmission method based on dynamic compensation of a transmitting optical axis, which comprises the following steps: firstly initializing variables, then establishing a link to track communication, entering SPGD circulation in the tracking process, carrying out positive and negative disturbance on an angle of a transmitting end, collecting optical power data by a receiving end, obtaining a corresponding evaluation function, counting optical power fluctuation, and adopting an improved SPGD iterative formula to determine the dynamic compensation of the transmitting optical axis. And calculating a transmitting end optical axis pointing angle of next iteration, controlling transmitting optical axis compensation according to the pointing angle, then judging whether convergence or reaching a preset maximum period number is carried out or not, if not, continuing iteration, and if so, ending circulation. In the autonomous calibration process, normal work of a communication link is kept, rapid information feedback and processing are achieved through the continuously existing communication link, the response speed and stability of the system are improved, convergence is conducted by combining an SPGD algorithm and adopting small signal jitter, accidental link breakage can be avoided to the maximum extent, and cooperative dynamic compensation of the optical axes of receiving and transmitting is achieved; and the convergence speed is improved by 50%.

Description

Technical Field

[0001] This invention relates to the field of laser communication technology, and in particular to a stable laser communication transmission method based on dynamic compensation of the emission optical axis. Background Technology

[0002] Laser communication, with its advantages of high bandwidth, strong anti-interference capability, low power consumption, and narrow beam stealth, has become a key technology for high-speed wireless communication between satellites, between space and ground, and on the ground. However, during free-space transmission, laser links are highly susceptible to various dynamic disturbances, including platform micro-vibrations, thermal deformation, beam drift and spread caused by atmospheric turbulence, and errors in attitude control systems. These disturbances can cause the transmitting optical axis to shift from the center of the receiving field of view, and in severe cases, even lead to link interruption.

[0003] To maintain a stable communication link, high-precision alignment and real-time compensation of the transmit and receive axes are essential. Currently, the industry mainly employs two typical technical approaches for transmit and receive coaxial calibration: one is a passive reflection calibration method based on a corner cube retroreflector; the other is an active alignment algorithm based on nutation scanning. However, both of these methods have shortcomings in high-dynamic, high-feedback laser communication scenarios.

[0004] Passive reflection calibration based on cornerstone prisms is widely used for initial optical axis alignment or static calibration because cornerstone prisms have the characteristic that incident light returns along its original path. A typical approach involves installing a cornerstone at the receiver and emitting a calibration beam from the transmitter. The beam is reflected back to the transmitter by the cornerstone, and the optical axis deviation is inferred by detecting the position of the echo spot. Cornerstone calibration usually requires interrupting the normal communication link and switching to a dedicated calibration mode, thus it is often used for initial calibration before link establishment. This method leads to interruption of service data transmission, making it unsuitable for bidirectional communication or applications requiring continuous link establishment. Furthermore, this method generally only calibrates unidirectional deviation at the transmitter and cannot achieve true coaxial closed-loop compensation for transmission and reception.

[0005] Nutrated scanning-based active alignment algorithms acquire changes in receiver optical power by applying periodic circular or spiral perturbations (i.e., "nutation") around the estimated optical axis. This method requires multiple nutations, and each nutation cycle involves multiple samplings, resulting in longer calibration times. Furthermore, traditional nutation methods have a fixed number of circular sampling points, and their iterative step direction often deviates from the true gradient direction, potentially leading to oscillating convergence. Moreover, traditional nutation methods only use received optical power as an evaluation function, neglecting the dynamic response delay or nonlinear errors of the receiver tracking system, resulting in inaccurate received optical power monitoring and calibration errors.

[0006] In summary, existing calibration techniques based on pyramids or nutation are insufficient to meet the requirements for constructing continuous, stable, and high-precision laser communication links under high-dynamic platforms. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the above-mentioned technologies and provide a stable laser communication transmission method based on dynamic compensation of the emitted optical axis.

[0008] Therefore, the present invention provides a stable laser communication transmission method based on dynamic compensation of the emitted optical axis, comprising the following steps: S1. Initialize variables; S2. After establishing and tracking communication, enter the SPGD loop; S3. Perform positive and negative angle perturbations at the transmitting end, collect optical power data at the receiving end, and obtain the corresponding evaluation function; S4. Statistically analyze the optical power fluctuations and use the improved SPGD iterative formula to calculate the pointing angle of the transmitting end optical axis for the next iteration. S5. Based on the pointing angle, control the compensation of the emitted optical axis, and then determine whether it converges or reaches the preset maximum number of cycles. If the iteration fails to converge or the maximum number of cycles is not reached, return to step S3 and continue iterating; if the iteration converges or the maximum number of cycles is reached, end the loop.

[0009] Preferably, in step S4, the improved SPGD iterative formula is: ; in, This represents the pointing angle of the i-th path in the k-th iteration; This represents the pointing angle of the i-th path in the previous iteration; This represents the initial learning rate of SPGD; This represents the learning rate adjustment factor, which is calculated by statistically analyzing the degree of optical power fluctuation. This represents the perturbation amount of the pointing angle of the i-th path in the k-th iteration; This represents the change in the evaluation function. .

[0010] Preferably, the evaluation metrics for the statistical optical power fluctuation algorithm include the peak-to-peak value of the received optical power and the standard deviation of the received optical power.

[0011] Preferably, the algorithm for evaluating the statistical optical power fluctuation of the peak-to-peak received optical power is as follows: .

[0012] Preferably, the algorithm for evaluating the statistical optical power fluctuation as the standard deviation of received optical power is as follows: .

[0013] Preferably, before starting step S1, an algorithm for the received optical power in an FSO system with coaxial misalignment is derived theoretically. This received optical power algorithm provides theoretical support for the practical application of the improved SPGD iterative formula. The algorithm is as follows:

[0014] in, Indicates the received optical power; This indicates the coaxial error at the transmitting end; This indicates the tracking residual at the transmitter. This indicates the coaxial error at the receiving end; This indicates that the receiver is tracking the residual. Indicates the entrance pupil light power; Indicates the transmittance of the receiving system; This indicates the fiber coupling efficiency.

[0015] Preferably, the algorithm for fiber coupling efficiency is as follows:

[0016] in, This indicates the offset of the focused spot center relative to the fiber core; This represents the normalized parameter, which is obtained by calibrating the laser communication terminal.

[0017] Preferably, the offset of the focused spot center relative to the fiber core is calculated using the following formula:

[0018] in, This indicates the focal length of the focusing lens.

[0019] Preferably, a CCD camera is set at the receiving end to monitor the optical axis angle of the receiving end, so as to accurately calculate the entrance pupil light power and provide data feedback for adjusting the optical axis pointing angle of the transmitting end.

[0020] Preferably, the angular disturbance at the transmitting end is a small-signal disturbance.

[0021] The beneficial effects of the present invention are as follows: The present invention provides a stable laser communication transmission method based on dynamic compensation of the emission optical axis, which has the following beneficial effects.

[0022] (1) By introducing the statistical optical power fluctuation level as a learning rate adjustment factor, the improved SPGD algorithm can increase the convergence speed by 50%. (2) Using CCD camera image feedback can not only improve the accuracy of received optical power, but also improve the accuracy of transmitter axis calibration; (3) During the self-calibration process, the communication link is kept working normally. By utilizing the continuous communication link, rapid information feedback and processing can be achieved, further improving the system's response speed and stability. At the same time, combined with the SPGD algorithm, small signal random jitter is used for convergence, which can avoid accidental link loss to the greatest extent, ensure the stability of the communication link, and realize dynamic compensation of the receiving and transmitting axes. This truly achieves "communicating, calibrating, and establishing a stable link" without interrupting communication. Attached Figure Description

[0023] Figure 1 This is a flowchart of a laser communication stable transmission method based on dynamic compensation of the emitted optical axis; Figure 2 These are simulation diagrams of transmitter compensation for the traditional SPGD algorithm, the Adam SPGD algorithm, and the improved SPGD algorithm. Figure 3 This is a schematic diagram of the optical path system of a laser communication receiver. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments to aid in understanding its content. Unless otherwise specified, the methods used in this invention are conventional methods; the raw materials and apparatus used, unless otherwise specified, are conventional commercially available products.

[0025] This invention provides a stable laser communication transmission method based on dynamic compensation of the emitted optical axis, which is described in detail in the following aspects.

[0026] The establishment of a physical model.

[0027] In free space, a laser beam can typically be approximated as a fundamental Gaussian beam. Its complex amplitude... radial coordinates Transmission distance Related:

[0028] in, This represents the initial amplitude of the light field; The waist radius; The radius of the beam; The radius of curvature of the wavefront; represents Guyi phase shift; i represents imaginary number; k represents wave number.

[0029] Furthermore, , where λ is the wavelength.

[0030] Furthermore, , where λ is the wavelength.

[0031] Furthermore, Where Z0 is the Rayleigh range.

[0032] For a transmission distance of The radius of the entrance pupil of the receiver is In an FSO system with ideal alignment, the total optical power received by the receiver can be obtained by integrating the Gaussian intensity into the optical power within the entrance pupil. get: in, This represents the polar coordinate angle within the receiving aperture.

[0033] The received light is focused by the optical system and coupled into a single-mode fiber (SMF). Ideally, the focused light field at the fiber end face... The optical field distribution is an Airy disk, while the optical field distribution propagating in an SMF is approximately a Gaussian beam. Therefore, the relationship between the coupling efficiency of a single-mode fiber and the optical system can be simplified as follows:

[0034] in, This represents the focused light field at the end face of the optical fiber. This represents the light field distribution in the SMF.

[0035] Furthermore, ,in, It is the diameter of the focusing lens. It is the focal length of the lens. It is the mode field radius of a single-mode fiber.

[0036] Since coupling efficiency is significantly affected by alignment accuracy, mismatch at the micro-arc level will significantly reduce coupling efficiency. When tracking residuals exist at the receiver... and receiving coaxial error This causes the center of the focused spot to shift relative to the fiber core. At this point, the beam distribution in the single-mode fiber (SMF) follows the Nakagami-Rice distribution, as follows:

[0037] in, This is a zero-order modified Bessel function. In this case, the coupling efficiency... It can be represented as an offset. The function.

[0038] For an FSO receiving system with defined optical parameters, an approximate calculation can be performed using a normalized Gaussian coupling efficiency model, as follows: in, This represents the normalized Gaussian coupling efficiency. This represents the normalization parameter, which is obtained by calibrating the laser communication terminal. It is the only parameter in this normalized Gaussian model that needs to be solved based on the system parameters.

[0039] When there is a coaxial error between the transmitter and receiver. and the tracking residual at the transmitter When, the equivalent pointing error is This causes the beam center to shift laterally on the receiving plane. At this point, the deviation of the light spot center from the entrance pupil center will also affect the optical power received by the FSO system. Therefore, considering the error effects at both the receiving and transmitting ends, the received optical power in the FSO system... It can be represented as: in, It is the transmittance of the receiving system.

[0040] The above analysis shows that the received optical power is directly proportional to the entrance pupil optical power. To improve the received optical power, in addition to improving the fiber coupling efficiency, it is also necessary to increase the entrance pupil optical power. Based on the analytical expression for the entrance pupil optical power under transmit / receive coaxial misalignment derived in this application, the transmit / receive coaxial calibration algorithm can be better applied.

[0041] II. Adaptive Compensation Algorithm.

[0042] The stochastic parallel gradient descent algorithm (SPGD) is suitable for model-free multivariable control systems. By selecting appropriate parameters and evaluation functions, its convergence can be guaranteed, making it particularly suitable for dynamic compensation of the emitted optical axis.

[0043] For a coaxial calibration system, using This represents the system's evaluation function, i.e., the received optical power. Let be the pointing angle of the i-th path in the k-th iteration. This represents the pointing angle perturbation of the i-th path in the k-th iteration. The pointing angle in the previous iteration... Add and subtract random angular disturbances respectively. The evaluation function under the k-th iteration can then be calculated. .

[0044] Calculate the change in the evaluation function, i.e. .

[0045] The pointing angle of the i-th path can be calculated using the iterative formula of the SPGD algorithm:

[0046] Based on the characteristics of Gaussian beams, the magnitude of the entrance pupil power fluctuation is related to the coaxiality of the transmitter and receiver. Therefore, during the SPGD iteration process, the magnitude of the entrance pupil power fluctuation at the receiver can reflect the magnitude of the coaxiality deviation. Thus, the iterative formula for the improved SPGD algorithm is: Defined as a learning rate adjustment factor, it is calculated by statistically analyzing the fluctuation of received optical power. The specific formula can use either the peak-to-peak value of the received optical power or the standard deviation of the received optical power as the evaluation metric. It should be noted that all algorithms for statistically analyzing fluctuation can be used to statistically analyze the fluctuation of received optical power in this application. This embodiment only presents a few relatively simple statistical metrics, as follows.

[0047] The algorithm for evaluating the statistical optical power fluctuation of the peak-to-peak received optical power is as follows: .

[0048] The algorithm for evaluating the statistical optical power fluctuation, which is the standard deviation of received optical power, is as follows: .

[0049] It is important to note that the statistics on the fluctuation of received optical power are based on long-term observations of the received optical power and do not affect the overall SPGD process. The statistics on received optical power fluctuations target the received optical power over a specific period. After each perturbation in the SPGD algorithm, the receiver needs to perform an optical power count, which is equivalent to a change at the transmitter, yielding two types of data: an evaluation function and the fluctuation level. These fluctuation levels are then used for statistical analysis. This application typically applies a certain degree of filtering to the evaluation function (received optical power) required for SPGD.

[0050] III. Operating Procedures.

[0051] like Figure 1 As shown, a method for stable laser communication transmission based on dynamic compensation of the emitted optical axis includes the following steps: S1. Initialize the parameter variables in the FSO system. The variables to be initialized include random angle perturbation, learning rate (gain coefficient), and learning rate adjustment factor. S2, establish chain tracking communication, and enter the SPGD loop during the tracking process; S3. Perform positive angle perturbation at the transmitting end, collect optical power data at the receiving end, and obtain the evaluation function J+. S4. Perform negative perturbation of the transmitter angle, collect optical power data at the receiver, and obtain the evaluation function J-. In this embodiment, the optical power data acquisition at the receiving end in steps S3 and S4 is performed using an APD. All angular disturbances at the transmitting end are small-signal disturbances.

[0052] S5. Statistically analyze optical power fluctuations and use the improved SPGD iterative formula to calculate the pointing angle of the transmitting end optical axis for the next iteration. S6. Based on the pointing angle, control the compensation of the emitted optical axis, and then determine whether it converges or reaches the preset maximum number of cycles; If the iteration fails to converge or the maximum number of cycles is not reached, return to step S3 and continue iterating; if the iteration converges or the maximum number of cycles is reached, end the loop.

[0053] IV. Examples

[0054] To demonstrate the effectiveness of the proposed laser communication stable transmission method based on dynamic compensation of the transmit optical axis, simulations were conducted to compare the performance differences between the traditional SPGD algorithm, the Adam algorithm, and the proposed method. The system parameters are set as follows: Table 1 Laser communication link model parameters

[0055] The parameters of the traditional SPGD algorithm, the Adam SPGD algorithm, and the improved SPGD algorithm are shown in Table 2.

[0056] Table 2 Algorithm parameters in the model

[0057] Through simulation, this application compares the traditional SPGD algorithm, the Adam SPGD algorithm, and the improved SPGD algorithm, and finds that the proposed method can achieve the fastest convergence speed. Figure 2 As shown.

[0058] from Figure 2 As can be seen, the traditional SPGD algorithm has a slow initial convergence speed, requiring multiple iterations before entering a fast convergence phase. This is mainly because the algorithm has a fixed learning rate, which prevents the compensation amount from increasing in the initial stage. Later, the learning rate is too large, resulting in the compensation amount not decreasing quickly in a single iteration. In other words, the compensation is insufficient when the initial deviation is large, and over-compensation occurs in steady state, leading to a slow convergence speed and instability after convergence. The Adam SPGD algorithm has a smooth convergence curve and is stable after convergence, but its convergence speed is the slowest. The improved SPGD algorithm introduces the statistical optical power fluctuation as a learning rate adjustment factor, thus achieving fast convergence in the early stage and quickly adjusting the coaxial deviation of the transmitter and receiver. Later, the angle adjustment tends to be stable with smaller fluctuations, thereby achieving fast convergence and improving the convergence speed by more than 50%.

[0059] This invention focuses on realizing a real-time dynamic collaborative compensation mechanism for the optical axes of the transmitter and receiver during closed-loop tracking, in order to overcome the pointing deviation of the transmitted optical axis caused by factors such as platform vibration, atmospheric turbulence, thermal deformation, mechanical drift and attitude disturbance. It can be applied to high dynamic platforms, such as low-orbit satellites, UAVs, and high-speed mobile ground terminals.

[0060] This invention uses the SPGD algorithm for optical axis compensation on the transmitting side to achieve high stability of the laser communication link. To ensure the accuracy of the compensation, a single-mode fiber coupling model is also applied at the receiving end, using the image feedback mechanism of a CCD camera for receiving-side compensation.

[0061] like Figure 3 As shown, a CCD camera is set at the receiving end to monitor the optical axis angle of the receiving end, which facilitates accurate calculation of the entrance pupil light power and provides high-precision data feedback for adjusting the optical axis pointing angle of the transmitting end.

[0062] Specifically, since the CCD camera can monitor the angle of the receiving axis, the Gaussian coupling efficiency can be calculated using formula (5), and then the actual entrance pupil power can be calculated using formula (6), thereby improving the accuracy of the transmitting axis calibration.

[0063] In the description of this invention, it should be understood that the terms "left", "right", "up", "down", "top", "bottom", "front", "back", "inner", "outer", "back", "middle", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0064] However, the above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the present invention. Therefore, any substitution of equivalent components or equivalent changes and modifications made in accordance with the scope of protection of the present invention should still fall within the scope of the claims of the present invention.

Claims

1. A stable transmission method for laser communication based on dynamic compensation of emission optical axis, characterized in that, The method comprises the following steps: S1, initializing variables; S2, after chain establishment and tracking communication, entering an SPGD loop; S3, performing positive and negative perturbation of a transmitting end angle, collecting optical power data at a receiving end, and obtaining a corresponding evaluation function; S4, counting optical power fluctuation, and calculating a pointing angle of the optical axis of the transmitting end for next iteration by using an improved SPGD iteration formula; S5, controlling compensation of the transmitting optical axis according to the pointing angle, and then judging whether convergence is achieved or a preset maximum cycle number is reached; If convergence is not achieved or the maximum cycle number is not reached, returning to step S3 and continuing iteration; if convergence is achieved or the maximum cycle number is reached, ending the loop. 2.The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 1, wherein, In step S4, the improved SPGD iteration formula is as follows: ； wherein, represents the pointing angle of the i-th path of the k-th iteration; represents the pointing angle of the i-th path of the previous iteration; represents the initial learning rate of SPGD; represents the learning rate adjustment factor, which is calculated by the statistical light power fluctuation degree; represents the pointing angle perturbation of the i-th path of the k-th iteration; represents the evaluation function change amount, .

3. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 2, characterized in that, Evaluation indexes of the statistical optical power fluctuation algorithm include a peak-to-peak value of received optical power and a standard deviation of received optical power.

4. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 3, characterized in that, The algorithm of the statistical optical power fluctuation degree with the evaluation index of the peak-to-peak value of received optical power is as follows: 。 5. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 3, characterized in that, The algorithm of the statistical optical power fluctuation degree with the evaluation index of the standard deviation of received optical power is as follows: 。 6. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 1, characterized in that, Before step S1, an algorithm of received optical power in a FSO system with coaxial deviation is obtained by theoretical derivation, which provides theoretical support for actual application of the improved SPGD iteration formula, and the algorithm is as follows: ； wherein, represents received optical power; represents launch coaxial error; represents launch tracking residual; represents received coaxial error; represents received tracking residual; represents entrance pupil optical power; represents received system throughput; represents fiber coupling efficiency.

7. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 6, characterized in that, An algorithm of fiber coupling efficiency is as follows: ； wherein, represents an offset amount of the focused light spot center with respect to the optical fiber core; represents a normalization parameter, which is obtained by calibrating the laser communication terminal. 8.The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 7, characterized in that, A calculation of the offset of the focused light spot center relative to the fiber core is as follows: ； wherein denotes the focal length of the focusing lens.

9. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 1, characterized in that, A CCD camera is arranged at the receiving end to monitor the angle of the optical axis of the receiving end, so as to accurately calculate the in-pupil optical power and provide data feedback for adjustment of the pointing angle of the optical axis of the transmitting end.

10. The stable transmission method of laser communication based on dynamic compensation of emitting optical axis according to claim 1, characterized in that, The perturbation of the transmitting end angle is small signal perturbation.

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