Single-mode fiber self-adaptive coupling method in turbulent environment

Abstract

The application discloses a single-mode optical fiber adaptive coupling method in a turbulent environment, and relates to the fields of single-mode optical fiber coupling, automatic control, fiber lasers and fiber imaging. The application analyzes the relationship between the coupling efficiency and the offset distance, simplifies the relationship curve into a Gaussian model, and is used for subsequent adaptive updating of the closed-loop gain rate. In addition, the gradient direction generated randomly in the traditional control method is changed into all-directional disturbance, and the gradient direction accurately and quickly approaching the target value is calculated. By projecting the two-dimensional micro-disturbance into one-dimensional space, using the least square method, the step length and the gradient direction required for iteration are automatically solved, so that the focused light spot is quickly captured and tracked. Compared with the traditional control method, the application guarantees the convergence speed of the algorithm, effectively improves the convergence stability, makes the system have a wider effective correction area, and improves the closed-loop control bandwidth range of the disturbance correction.

Description

TECHNICAL FIELD

[0001] The present application relates to the fields of single-mode fiber coupling, automatic control, fiber laser, fiber imaging, and the like, and in particular to a single-mode fiber adaptive coupling method in a turbulent environment, which has an important application prospect in free space laser communication. BACKGROUND

[0002] As a new type of communication technology, free space laser communication (FSOC) has the advantages of high transmission rate, unlimited bandwidth, good security performance, and has been widely used in inter-satellite, satellite-ground communication and other occasions. In the process of continuous development of free space optical communication system, more and more mature optical fiber communication technology is combined, which also makes the coupling of space laser to single-mode fiber a key problem in the field of FSOC. In the process of fiber coupling, the coupling efficiency directly affects the communication bit error rate and determines the key factors such as communication transmission distance. The traditional method of optimizing optical lens design and fiber manufacturing to improve coupling efficiency can only improve the static coupling efficiency, and the actual FSOC coupling also needs to cope with the dynamic error problem caused by atmospheric turbulence and mechanical vibration of the carrier platform. In addition to optimizing the structure of the coupling device to achieve the ability to resist dynamic disturbance, the control method of the system to realize fiber coupling is also worth paying attention to.

[0003] At present, the common control methods of fiber coupling include hill climbing method, nutation algorithm, stochastic parallel gradient descent (SPGD) algorithm, etc. The first two methods take a long time and have insufficient correction and tracking ability for dynamic disturbance. SPGD algorithm is currently the most widely used, especially in adaptive optics systems without wavefront sensors, coherent synthesis, and fiber coupling, but this method can only achieve good results when the relative position difference between the fiber and the spot is small, and the compensation ability for fiber coupling efficiency under the vibration of the receiving platform and atmospheric turbulence disturbance is limited. At the same time, the space optical coupling system is very concerned about the influence of the received signal light power on the bit error rate of the communication system, and the bit error rate often depends on the minimum value of the coupling power, so stable coupling is more important than high average coupling to a certain extent. In addition, the fiber coupling algorithm cannot be automated at present, and many parameters need to be adjusted manually by experienced engineers, which greatly limits the communication time and application scenarios, especially in the atmospheric turbulence environment, there is light flicker, even if it is manually adjusted, it is also difficult to find the optimal control parameters. Therefore, it is crucial to improve the effective range of the algorithm, while maintaining the convergence speed, convergence range and stability of the algorithm after convergence, and to realize the fully automated operation of the algorithm in the flickering environment. The current control methods cannot take into account these characteristics.

[0004] In summary, there is an urgent need to find a stable, convergent, disturbance-resistant, and large-effective-range fully automatic coupling algorithm to meet the control requirements of fiber optic coupling in practical dynamic application scenarios. Summary of the Invention

[0005] The technical problem this invention aims to solve is to overcome the shortcomings of traditional control algorithms in applications where spatial light is coupled to single-mode fiber, such as poor closed-loop stability, low effective correction bandwidth, and low automation. Furthermore, it aims to overcome the problem of convergence failure due to the inability to obtain gradient information when the fiber end face and the light spot are significantly offset. Based on these findings, an adaptive coupling method for single-mode fiber in turbulent environments is proposed.

[0006] The technical solution adopted by this invention to solve its technical problem is:

[0007] To address the aforementioned pressing technical problems, a single-mode fiber adaptive coupling method (Least Squares Iterative algorithm, LSI) for turbulent environments is proposed. Compared to the traditional SPGD algorithm, the LSI algorithm adjusts both the gradient direction and gain rate (step size), and achieves "blinding" optimization by combining fiber optic two-dimensional imaging scanning technology. This results in higher stability, a wider effective convergence range, and a higher frequency effective correction bandwidth. The implementation steps are as follows:

[0008] An adaptive coupling method for single-mode optical fibers in turbulent environments is proposed. Before implementing this method, a spatial optical coupling experimental platform is first built, and the selection and parameter design of components including a laser, a single-mode laser-emitting fiber, a laser-emitting collimating lens, an optical fiber coupling device, and a controller are completed. This is achieved through the following steps:

[0009] Step S110, calibration scan, includes: inputting the scan voltage range, number of scan points, and scan interval time; the controller outputs the corresponding voltage signal to drive the fiber end face to complete the scan on the two-dimensional plane; the scan obtains a one-to-one correspondence between different voltage positions and performance indicators, i.e. Solve for the parameters required for the Gaussian model: the standard deviation of the Gaussian model. ;in, It is the performance metric at the i-th scan position. It is the control voltage corresponding to the X-axis direction at the i-th scan position. It is the control voltage corresponding to the Y-axis direction at the i-th scan position;

[0010] Step S120: Set the basic parameters of the system closed-loop control, including: the disturbance rate of small disturbances. Closed-loop frequency f, and initial voltage ,in and are the control voltages of the X-axis and Y-axis before the closed loop starts, respectively;

[0011] In step S130, when the closed loop starts, the photoelectric detector converts the optical signal into an electrical signal after detecting the optical signal, and inputs the electrical signal into the AD converter, and the controller reads the coupling light energy value at this time as the initial performance index for control;

[0012] In step S140, gradient acquisition: the controller executes the LSI algorithm, and according to the input disturbance rate , applies omnidirectional disturbance, obtains the corresponding coupling light energy size when the positive and negative disturbance voltages are loaded in different directions, calculates the current optimal gradient direction, and projects the performance index corresponding to the disturbance in the gradient direction;

[0013] In step S150, gain rate, that is, iteration step length update: according to the Gaussian model, a system of over-determined equations is constructed, and the least square method is used to update the gain rate , wherein, is the calculated gain rate, is a positive integer from 1 to 4, is the performance index corresponding to the four omnidirectional disturbances, is the projection of the four omnidirectional disturbances in the accurate gradient direction, is the initial performance index obtained in step S130, is the standard deviation of the Gaussian model calculated in step S110;

[0014] In step S160, at this time, the direction and step length of the gradient descent are determined, and the driving voltage of the position of the fiber end face is updated, and the fiber end face is quickly shifted to the corresponding iteration point;

[0015] In step S170, steps S130-S160 are repeated, so that the coupling efficiency of the system converges to the optimal value after multiple iterations and can resist disturbances and other conditions in the closed loop process to maintain stability.

[0016] The beneficial effects of the present application compared with the prior art are:

[0017] The method based on the Gaussian model is adopted to adjust the gain rate, so that the gain rate can be adaptively updated according to the collected light energy, the convergence speed of the algorithm is ensured, the convergence stability, the effective convergence range and the effective correction bandwidth are improved, the interference in the closed loop process can be quickly corrected, and the algorithm will not fall into a local extreme value or cannot converge with the increase of the iteration number or the disturbance number.

[0018] The application utilizes two-dimensional scanning imaging technology of the optical fiber in solving Gaussian model parameters, visualizes the current coupling state without referring to additional equipment, breaks the traditional SPGD 'blind optimization' feature, additionally adds image information, and improves system usability.

[0019] The application realizes automatic operation of single-mode optical fiber coupling, can adaptively solve iteration steps even in a turbulent environment with light intensity flickering, improves unmanned operation capability of the system, widens application scenarios, and improves use convenience.

[0020] In addition, the application realizes full-automatic coupling without parameter adjustment, does not need to introduce additional monitoring equipment or real-time debugging by technical personnel, greatly increases the usability of the system, and has important application prospects in space optical communication. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 For the application principle diagram of a single-mode optical fiber adaptive coupling method in a turbulent environment, the simulated coupling efficiency curve, and the specific experimental principle diagram of the AFC optical fiber coupling system under the wavelength of incident laser light of 1550nm, wherein (a) is a basic principle diagram of single-mode optical fiber coupling, (b) is a curve of coupling efficiency changing with offset under the condition that the wavelength of laser light is 1550nm, the mode field radius of single-mode optical fiber is 5um, the focal length of the coupling lens is 47.4mm, and the clear aperture is 20mm, and (c) is a schematic diagram of a spatial light coupling experiment platform based on an adaptive optical fiber coupler.

[0022] Figure 2 For the algorithm flowchart of a single-mode optical fiber adaptive coupling method in a turbulent environment, it contains the calibration scanning, accurate gradient direction measurement, and LSI iteration correction process.

[0023] Figure 3 For the application of a single-mode optical fiber adaptive coupling method in a turbulent environment in an initial large disturbance deviation experiment (Example 1), the experimental results of the SPGD algorithm and the LSI algorithm of the application are compared, wherein (a) and (d) are the complete coupling power corresponding performance index value curves, including the data in the open loop stage, the large initial deviation stage, the closed loop stage, and the open loop stage again, (b) and (e) show the detailed data from the large initial deviation stage to the closed loop stage, (c) and (f) are the control voltages of the AFC in the control process using the LSI algorithm and the SPGD algorithm respectively.

[0024] Figure 4This is a comparison of the experimental results of the SPGD algorithm and the LSI algorithm of the present invention in a variable frequency dynamic disturbance experiment (Example 2) of the single-mode fiber adaptive coupling method under turbulent environment of the present invention. Figures (a) to (d) are the corresponding performance index curves collected by the photodetector of the LSI algorithm of the present invention, and (e) to (f) are the corresponding performance index curves collected by the photodetector of the SPGD algorithm. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and two embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0026] This invention provides an adaptive coupling method for single-mode optical fibers in turbulent environments. Before implementing this method, a spatial optical coupling experimental platform is first built, and the selection and parameter design of components including a laser, a laser-emitting single-mode fiber, a laser-emitting collimating lens, an optical fiber coupling device, and a controller are completed. The optical fiber coupling device also needs to complete the design of parameters such as the mode field radius of the coupling and receiving single-mode fiber and the focal length and aperture of the coupling lens.

[0027] Lasers, single-mode fibers, and collimators are used as the transmitters in optical coupling experiments. Their parameters can be set according to actual communication needs. The laser output wavelength can be common bands such as 1550nm, 1064nm, and 808nm.

[0028] The optical fiber coupling device is not limited, as long as it has a single-mode optical fiber for coupling and receiving, and a two-dimensional adjustable driver, such as an adaptive fiber coupler (AFC) or a fast reflector.

[0029] The controller on which the control algorithm is based can be an integration of various high-precision analog-to-digital (Analog to Digital, AD) and digital-to-analog (DA) chips with real-time signal processing chips such as FPGA (Field Programmable Gate Array) and DSP (Digital Signal Processor). Alternatively, a regular PC (Personal Computer) and its matching data acquisition card can be used to achieve closed-loop control.

[0030] like Figure 2 As shown, the method includes the following steps:

[0031] Step S110, calibration scan, includes: inputting the scan voltage range, number of scan points, and scan interval time; the controller outputs the corresponding voltage signal to drive the fiber end face to complete the scan on the two-dimensional plane; the scan obtains a one-to-one correspondence between different voltage positions and performance indicators, i.e. , the parameters required by the Gaussian model are solved: Gaussian model standard deviation . Wherein, is the performance index at the i-th scanning position, is the control voltage corresponding to the X-axis direction at the i-th scanning position, is the control voltage corresponding to the Y-axis direction at the i-th scanning position. This step is performed because: when the spatial laser beam enters the coupling lens, an Airy diffraction pattern will be formed on the lens focal plane, and according to the mode field matching principle, the light power distribution obtained by coupling is similar to a Gaussian distribution. Static scanning calibration in the room needs to be completed before the experiment, and subsequent scanning is not required to complete closed-loop control. In the case of static undisturbance, the algorithm calibration is first performed. First, the optical path is aligned so that the outgoing parallel light is focused to the center of the fiber end face through the coupling lens. Then, according to the allowed voltage range of the device, the upper and lower limits of the scanning, the scanning accuracy, etc. are set, so as to complete the two-dimensional scanning calibration. The performance index J obtained by scanning is used for imaging, and the required parameters of the model are calculated.

[0032] The scanning voltage range, scanning accuracy, scanning frequency, etc. can be designed according to the performance of the system. The actual scanning mode is not limited, and in the embodiment of the present application, “bow” scanning is adopted, and “Z” scanning, “back” scanning, etc. can be adopted.

[0033] In step S120, the basic parameters of the system closed-loop control are set, including: the disturbance rate of the small disturbance , the closed-loop frequency f, and the initial voltage . Wherein and are the control voltages of the X-axis and the Y-axis before the closed loop starts, respectively.

[0034] Wherein, the disturbance rate can be fixed according to the static scanning result, such as the disturbance of the focused light spot at the center of the fiber end face, and the coupling efficiency after the disturbance is preferably 90% of the optimal coupling efficiency.

[0035] The closed-loop frequency f is set, which can be selected according to the lower one of the tolerable closed-loop frequency of the fiber coupling device (such as the resonance frequency of the fast mirror) and the executable closed-loop frequency of the controller (such as the calculation frequency of the PC).

[0036] In step S130, when the closed loop starts, the photodetector converts the optical signal into an electrical signal after detecting the optical signal, and inputs the electrical signal into the AD converter. The controller reads the coupling light energy value at this time as the initial performance index of the control. The purpose of the closed-loop process is to maximize the coupling light energy, and at this time the corresponding coupling efficiency reaches the highest.

[0037] The incident laser wavelength should meet the response band requirements of the photodetector; the coupled signal needs to be split, and the specific splitting ratio (e.g., 95:5) can be designed according to the actual communication requirements and the rated power of the detector.

[0038] The function of a photodetector is to achieve a linear conversion between optical power and voltage signals. Its operating wavelength range should cover the wavelength of the signal laser beam. Photodiodes, phototransistors, avalanche diodes, photomultiplier tubes, or other photodetectors can be used, as long as they can achieve a linear conversion between optical and voltage signals. The type of controller is not limited. For applications with low bandwidth requirements, a standard PC can be used as the controller, while for applications with high bandwidth requirements, real-time signal processing chips such as FPGAs and DSPs can be used.

[0039] S140, Gradient Acquisition: The controller executes the LSI algorithm based on the input perturbation rate. The magnitude of the coupled light energy is obtained by applying an omnidirectional perturbation and obtaining the corresponding magnitude of the coupled light energy when positive and negative perturbation voltages are applied in different directions. The current optimal gradient direction is calculated, and the performance index corresponding to the perturbation is projected onto the gradient direction.

[0040] The rules for applying omnidirectional perturbation voltage are as follows: First, positive and negative perturbation voltages are applied only in the X-axis direction, while no perturbation voltage is applied in the Y-axis direction. The magnitude of the perturbation voltage is determined by the perturbation rate. Decision; the detector obtains the corresponding performance indicators at this time. ,in, This represents the performance metric obtained by applying a positive perturbation voltage in the X-axis direction. This represents the function for obtaining performance metrics. This represents the control voltage along the X-axis in iteration t-1, where t is the current iteration number. This represents the magnitude of the disturbance voltage along the X-axis. Representative at The control voltage in the Y-axis direction during the next iteration. The first indicator represents the performance obtained when a negative perturbation voltage is applied in the X-axis direction; the second indicator represents the performance obtained when positive and negative perturbation voltages are applied only in the Y-axis direction, with no perturbation voltage applied in the X-axis direction. ,in, and These represent the performance metrics obtained by applying positive and negative perturbation voltages in the Y-axis direction, respectively. This represents the magnitude of the perturbation voltage along the Y-axis. The gradient direction is calculated based on the coupled light energy corresponding to the voltage when positive and negative perturbation voltages are applied in different directions. The calculation rules are as follows: The X-axis gradient is... The gradient along the Y-axis is wherein, and respectively represent the performance index obtained by applying positive and negative perturbation voltage in the X-axis direction, and respectively represent the performance index obtained by applying positive and negative perturbation voltage in the Y-axis direction, is a very small quantity to prevent the denominator from being zero. The voltage corresponding to the small perturbation is projected in the direction of the accurate gradient to obtain the scalar of the fourth perturbation voltage in the projection direction: , , , wherein, is the perturbation rate, and are the calculated X-axis gradient and Y-axis gradient respectively.

[0041] S150, gain rate, i.e. iteration step length update: according to the Gaussian model, an overdetermined equation set is constructed, so that the gain rate is updated using the least square method wherein, is the calculated gain rate, is a positive integer from 1 to 4, is the performance index corresponding to the fourth omnidirectional perturbation, is the projection of the fourth omnidirectional perturbation in the direction of the accurate gradient, is the initial performance index obtained in step S130, is the standard deviation of the Gaussian model calculated in step S110.

[0042] Step S160, at this point, the direction and step length of the gradient descent are determined, the driving voltage of the position of the fiber end face is updated, and the fiber end face is quickly shifted to the corresponding iteration point.

[0043] wherein, the voltage update rule is: wherein, t is the current iteration number, represents the control voltage in the X-axis direction of the tthiteration, represents the control voltage in the X-axis direction of the (t-1)thiteration, represents the gain rate, and are the X-axis gradient and Y-axis gradient respectively, represents the control voltage in the Y-axis direction of the tthiteration, represents the control voltage in the Y-axis direction of the (t-1)thiteration.

[0044] Step S170, repeat steps S130-S160, so that the coupling efficiency of the system converges to the optimal value after multiple iterations and can resist disturbances in the closed loop process to maintain stability.

[0045] The basic principle of the present application is that as shown in (a) of FIG. 1, the coupling efficiency is defined as the ratio of the power coupled into the fiber to the power of the incident light at the pupil plane of the coupling lens. According to the Parseval theorem, the power of the focused spot at the fiber end face S0 is equivalent to the power of the incident light at the incident pupil plane S1, so the coupling efficiency formula can be expressed as: Figure 1

[0046]

[0047] wherein, represents the coupling efficiency, represents the power of the light coupled into the single-mode fiber, represents the power of the focused spot, is the incident light field at the focal plane S0 of the coupling lens, is the complex conjugate of is the light field transmitted in the single-mode fiber, is the coordinate of the X-axis and the Y-axis in the rectangular coordinate system, and in the polar coordinate can be expressed as:

[0048]

[0049] wherein is the circular constant, represents the imaginary unit, is the focal length of the coupling lens, and D is the aperture of the coupling lens, is the wavelength, is the distance from any point on the focal plane to the center of the Airy disk, is the wave number is the zero-order first kind Bessel function.

[0050] The alignment error between the fiber end face and the focused spot will cause a decrease in the coupling efficiency. This positional mismatch can be caused by the incident light field with tilt aberration, or by the inaccurate positioning of the fiber end face. Because the incident angle deviation is equivalent to the shift of the fiber end face, the beam transmission in the single-mode fiber follows the Nakagami-Rice distribution rule, which can be expressed as:

[0051]

[0052] wherein, is the shift distance of the fiber end face from the center of the focused spot, and k is the wave number is the zero-order modified Bessel function, is the mode field radius of the single-mode fiber. ​​​​​​​

[0053] Combining the above equation, the coupling efficiency can be simplified to:

[0054] .

[0055] This invention is based on an adaptive fiber coupler (AFC). Single-mode fiber mode field radius The focal length and aperture of the coupling lens are respectively and The simulation yielded a curve showing the coupling efficiency as a function of offset, as shown below. Figure 1 As shown in (b), the curve approximates a Gaussian distribution, and each offset distance corresponds to a unique coupling efficiency value. The coupling efficiency model is approximated using a Gaussian model:

[0056] ,

[0057] in, It is the coupling efficiency approximated by the Gaussian model. It is the offset distance between the fiber end face and the center of the focused spot. These are the parameters of the Gaussian model.

[0058] The parameters of the Gaussian model can be calculated by obtaining the true coupling efficiency curve through static scanning, and then taking the intermediate value for solution:

[0059] ,

[0060] in, It is the offset distance between the fiber end face and the center of the focused spot at half the point of maximum coupling efficiency obtained from the scan. It is half of the maximum coupling efficiency obtained by scanning. This represents the logarithmic operation with the natural constant as the base.

[0061] This invention proposes obtaining accurate gradient directions through omnidirectional perturbation and achieving a two-dimensional to one-dimensional solution transformation through perturbation projection, resulting in accurate gradient directions. for:

[0062] ,

[0063] in, It is the direction without a minimal gradient. Represents the 2-norm. It is a very small quantity to prevent the denominator from being 0.

[0064] The one-dimensional projection of a two-dimensional minute perturbation onto the precise gradient direction can be expressed as:

[0065] ,

[0066] ,

[0067] ,

[0068] ,

[0069] wherein, , , and represent the projection of the positive X-axis disturbance, the negative X-axis disturbance, the positive Y-axis disturbance, and the negative Y-axis disturbance in the accurate gradient direction, respectively, is the disturbance rate, is the calculated accurate gradient direction.

[0070] Then, based on the Gaussian model, an overdetermined equation set is constructed:

[0071] ,

[0072] wherein i = 0, 1, 2, 3, 4, 5; JM is the coupled optical power under the optimal coupling efficiency in the current iteration period, and is an unknown number that needs to be offset when solving the equation.

[0073] Taking the logarithm operation on both sides of the above equation, we obtain:

[0074] ,

[0075] wherein j = 1, 2, 3, 4.

[0076] Subtracting the above two equations, JM is eliminated, and we obtain:

[0077] ,

[0078] Among them, only is the unknown number to be solved, so it is an overdetermined equation set, and a function H can be constructed to solve it by least squares:

[0079] ,

[0080] The derivative of H needs to be 0:

[0081] .

[0082] Therefore, using least squares iteration, the distance between the current fiber position and the ideal fiber position (i.e., the position corresponding to the maximum coupling efficiency) can be quickly calculated as the gain rate By combining the calculated accurate gradient direction with the updated driving voltage of the fiber end face, adaptive capture and tracking of the focused spot of a spatial laser can be achieved on the single-mode fiber end face.

[0083] ,

[0084] Where t is the current iteration number, This represents the control voltage vectors along the X and Y axes in the t-th iteration. This represents the control voltage vectors in the X-axis and Y-axis directions during the (t-1)th iteration.

[0085] The following two examples illustrate how to implement this invention. Both examples are based on a space optical coupling platform with an adaptive fiber coupler, such as... Figure 1 As shown in (c), the platform consists of a 1550nm laser, collimator, fast reflector for interference generation, coupling lens, single-mode fiber, optical power meter, controller, photodetector, and high-voltage amplifier. The fiber optic coupling device responsible for the overall coupling function includes a coupling lens (focal length 47.4mm, aperture 20mm) and an AFC with a single-mode fiber (mode field radius 5). The system includes a 1×2 fiber optic beam splitter, a photodetector (InGaAs photodiode, model C12485-210), a dual-channel high-voltage amplifier, a PC controller, and an AD conversion data acquisition card (NI PCI-6221). It is worth noting that this platform uses an AFC (Automatic Coupling Function) as the dynamic coupling device. However, in practice, the dynamic coupling device can be a fast reflector or other dynamic coupling equipment, as long as it has two-dimensional control capabilities; other requirements are not specified.

[0086] The laser output light passes through a collimator, reaches a fast-reflecting mirror, and is then reflected into the coupling lens of the fiber optic coupler, converging onto a single-mode fiber on the focal plane. The single-mode fiber is connected to a 1×2 fiber beam splitter, splitting the coupled light into two parts. 5% of the energy enters a photodetector. During algorithm optimization, the voltage value of the photodetector is used as a closed-loop performance metric. The remaining 95% of the energy enters an optical power meter to display the real-time optical power coupled into the fiber.

[0087] Example 1:

[0088] Example 1 is an experiment on initial large perturbation deviation, which compares the effective convergence range of the two algorithms.

[0089] Before the experiment begins, the LSI algorithm requires static scanning calibration. Afterward, it can directly enter the closed-loop process without repeating the scan. The optical path platform is set up, and the position of the coupling fiber is adjusted to maximize coupling efficiency, ensuring a single scan is completed under undisturbed conditions. The scanned image is used as initial calibration data to calculate the Gaussian model parameters.

[0090] Step 1) The signal generator generates a sinusoidal signal with an amplitude of 100mV and a frequency of 1Hz, which is continuously applied to the fast-reflecting mirror to simulate dynamic disturbances such as turbulence or platform vibration. The controller applies a set of large bias voltages (-4V, -4V) to the AFC to simulate large initial deviations that may occur in real-world application scenarios.

[0091] Step 2) The controller executes the LSI algorithm of this invention and updates the gain rate using the performance index J value of the current coupling power. (When executing the SPGD algorithm, the perturbation rate) With gain rate It always remains consistent with the initial settings and requires no updates.

[0092] Step 3) Based on gain rate and perturbation rate Calculate the required voltage The controller outputs the control voltage to the high-voltage amplifier, driving the fiber end face to quickly shift to the corresponding iteration point.

[0093] Step 4) Repeat steps 2) to 3) until the coupling efficiency of the system converges to the optimal value after multiple iterations or the required number of iterations is reached, then the process ends.

[0094] Step 5) Switch to the SPGD algorithm and repeat steps 1) to 4).

[0095] The experimental results are attached. Figure 3 As shown. Figure 3 (a) and (d) show the complete curves of the coupling power corresponding to the performance index values, including the data in the open-loop stage, the large initial deviation stage, the closed-loop stage, and the re-open-loop stage. (b) and (e) show the detailed data from the large initial deviation stage to the closed-loop stage. (c) and (f) show the control voltage of AFC during the control process using the LSI algorithm and the SPGD algorithm, respectively.

[0096] from Figure 3 As can be clearly seen in (a) and (d), when faced with a bias voltage as large as (-4V, -4V), it means that the fiber endface is far from the focal point of the coupled spot. At this point, the detectable optical power is very low, and the performance data is close to 0. The extremely low coupling energy makes it impossible for the SPGD algorithm to achieve effective control. (f) shows that the control voltage calculated by SPGD fluctuates almost around the initial bias voltage value, making it impossible for the fiber endface to locate the spot. (b) to (c) show that the control voltage calculated by the LSI algorithm of this invention can quickly explore a large range after the closed loop is established, allowing the fiber endface to quickly locate the spot and move it to its center. Furthermore, this invention can effectively correct the sinusoidal disturbance introduced by the fast-reflecting mirror after convergence. Figure 3The control voltage of (c) and the closed-loop performance index curve of (a) in the figure can be observed, the application has good ability to track the sinusoidal disturbance: in the closed-loop, the performance index curve is basically stable, and the fluctuation of the coupling light energy is obviously smaller than that in the open-loop period; after re-opening the loop, the static large offset is effectively corrected, but the sinusoidal disturbance still exists, so the performance index curve also fluctuates near the peak. In summary, the effective range of the application is very wide, and it has good ability to offset dynamic disturbances.

[0097] Example 2:

[0098] Example 2 is a variable frequency dynamic disturbance experiment, which compares the effective correction bandwidth of two algorithms. The optical path is built the same as in Example 1, and the experimental parameters set by the control algorithm are also consistent with Example 1. The main difference between the two is that the frequency of the sinusoidal disturbance applied in the optical path is changed, as follows:

[0099] Step 1) The signal generator generates a sinusoidal signal with an amplitude of 300mV and a frequency of 5Hz, which continuously acts on the fast mirror to simulate dynamic disturbances such as turbulence or platform vibration.

[0100] Step 2) The controller executes the LSI algorithm of the application, which updates the gain rate based on the Gaussian model using the current coupling energy performance index J value (The disturbance rate and the gain rate always remain consistent with the initial set value and do not need to be updated).

[0101] Step 3) Calculate the required voltage based on the gain rate and the disturbance rate The controller outputs this control voltage to the high-voltage amplifier to drive the fiber end face to quickly shift to the corresponding iteration point.

[0102] Step 4) Repeat steps 2)~3) to make the coupling efficiency of the system converge to the optimal value after multiple iterations or reach the iteration requirement to end the closed loop and return to the open loop stage again.

[0103] Step 5) Use the SPGD algorithm, repeat steps 2)~4).

[0104] Step 6) Change the frequency of the sinusoidal signal generated by the signal generator, increase the frequency by 5Hz each time, and increase the frequency to 20Hz at the highest, to simulate dynamic disturbances such as turbulence or platform vibration at different frequencies. Repeat steps 2)~5).

[0105] Figure 4 The experimental results are shown in the following figures:The curves of (a)-(d) in the figure are the corresponding performance index value curves collected by the photodetector of the LSI algorithm of the application, including the data in the open loop stage, the closed loop stage and the second open loop stage. It can be observed that the closed loop correction effect gradually weakens with the increase of the disturbance frequency. This is due to the limitation of the closed loop iteration rate. In actual application scenarios, the closed loop delay can be reduced to increase the closed loop iteration frequency. The upper limit of the frequency is generally determined by the dynamic characteristics of the controlled device (such as the first-order resonant frequency of the AFC / fast mirror). In the four experiments of the LSI, the average value of the performance index J from the photodetector is obviously higher in the closed loop stage than in the open loop stage, and the fluctuation range is smaller than that in the open loop stage. The minimum value of J is also higher than the minimum value in the open loop stage. After the second open loop, the algorithm corrects a certain amount of initial deviation, and the average value of the performance index J is higher than that in the first open loop. However, the sinusoidal disturbance still exists, and the fluctuation is still more violent than that in the closed loop stage.

[0106] Figure 4 The curves of (e)-(f) in the figure are the corresponding performance index value curves collected by the photodetector of the SPGD algorithm, also including the data in the open loop stage, the closed loop stage and the second open loop stage. When the SPGD algorithm is controlled, effective correction can only be completed when the sinusoidal disturbance frequency is 5 Hz. The average value of the performance index J in the closed loop stage is higher than that in the open loop stage and the second open loop stage. Figure 4 When the sinusoidal disturbance frequency is 10 Hz, compared with (b), the performance index J value from the photodetector fluctuates violently, and the minimum value is lower than the minimum value in the second open loop stage, which means that good dynamic tracking is not achieved. When the sinusoidal disturbance frequency is more than 10 Hz, the SPGD algorithm control has failed, the performance index J value curve diverges, and it cannot be stabilized within a certain range. At this time, the violent shaking caused by the failure of the SPGD algorithm control will cause damage to the device, and the test needs to be stopped. It can be considered that under the same delay and the same disturbance, the effective correction bandwidth of the SPGD algorithm in the embodiment is close to 10 Hz, while the effective correction bandwidth of the LSI algorithm is greater than 20 Hz, which is twice that of the SPGD algorithm. This shows that the LSI algorithm of the application has a higher effective correction bandwidth.

[0107] At this point, the application has completed the detailed description of a single-mode fiber adaptive coupling method in a turbulent environment. In some examples, well-known methods, structures and techniques are not described in detail in order not to obscure the understanding of the present specification. Although the application is described in terms of a limited number of embodiments, those skilled in the art, with the benefit of the above description, will appreciate that other embodiments can be conceived within the scope of the application described herein. In addition, it should be noted that the language used in the specification is mainly selected for readability and teaching purposes, rather than for the purpose of interpreting or limiting the subject matter of the application.

Claims

1. A single-mode fiber adaptive coupling method under turbulent conditions, characterized in that, Before implementing this method, a spatial optical coupling experimental platform is first built, and the selection and parameter design of components including the laser, single-mode laser-emitting fiber, laser-emitting collimating lens, fiber coupling device, and controller are completed. This is achieved through the following steps: Step S110, calibration scan, includes: inputting the scan voltage range, number of scan points, and scan interval time; the controller outputs the corresponding voltage signal to drive the fiber end face to complete the scan on the two-dimensional plane; the scan obtains a one-to-one correspondence between different voltage positions and performance indicators, i.e. Solve for the parameters required for the Gaussian model: the standard deviation of the Gaussian model. ;in, It is the performance metric at the i-th scan position. It is the control voltage corresponding to the X-axis direction at the i-th scan position. It is the control voltage corresponding to the Y-axis direction at the i-th scan position; Step S120: Set the basic parameters of the system closed-loop control, including: the disturbance rate of small disturbances. Closed-loop frequency f, and initial voltage ,in, and These are the control voltages for the X and Y axes before the closed loop begins; Step S130: When the closed loop starts, the photodetector detects the light signal and converts it into an electrical signal, which is then input to the AD converter. The controller reads the coupled light energy value at this time as the initial performance index of the control. Step S140, Gradient Acquisition: The controller executes the LSI algorithm based on the input perturbation rate. The magnitude of the coupled light energy is obtained by applying an omnidirectional perturbation and obtaining the corresponding magnitude of the coupled light energy when positive and negative perturbation voltages are applied in different directions. The current optimal gradient direction is calculated, and the performance index corresponding to the perturbation is projected onto the gradient direction. Step S150, Gain Rate Update (i.e., Iteration Step Size Update): Construct an overdetermined system of equations based on the Gaussian model, and then update the gain rate using the least squares method. ,in, That is, the calculated gain rate. It is a positive integer from 1 to 4. These are the performance indicators corresponding to the four omnidirectional perturbations. It is the projection of each of the four omnidirectional perturbations onto the accurate gradient direction. These are the initial performance metrics obtained in step S130. It is the standard deviation of the Gaussian model calculated in step S110; Step S160: Now that the direction and step size of gradient descent are determined, update the driving voltage at the current position of the fiber end face and drive the fiber end face to quickly shift to the corresponding iteration point. Step S170, repeat steps S130-S160, so that the coupling efficiency of the system converges to the optimal value after multiple iterations and can resist the disturbance in the closed-loop process to maintain stability.

2. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that: The laser, single-mode fiber, and collimating lens are used as the transmitter in the optical coupling experiment. The laser output wavelengths include 1550nm, 1064nm, and 808nm.

3. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that: The fiber optic coupling device includes an adaptive fiber optic coupler and a fast-reflecting mirror.

4. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that: The controller is an integration of a high-precision analog-to-digital converter chip, a digital-to-analog converter chip, and an FPGA or DSP, or it can use a regular PC and a matching data acquisition card to achieve closed-loop control.

5. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that: In step S110, the scanning methods include "bow" shaped scanning, "Z" shaped scanning, and "return" shaped scanning.

6. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that: Disturbance rate The settings are fixed based on the static scan results, and the focused spot is disturbed at the center of the fiber end face. The optimal coupling efficiency after disturbance should be 90% of the optimal coupling efficiency; the closed-loop frequency f should be selected based on the lower of the closed-loop frequency that the fiber optic coupling device can withstand and the closed-loop frequency that the controller can execute.

7. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that: The photodetector is used to achieve linear conversion of optical power to voltage signal. Its operating wavelength range covers the optical wavelength of the signal laser beam, including any one of photodiode, phototransistor, avalanche photodiode, and photomultiplier tube.

8. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 1, characterized in that, Step S140 includes the following process for applying omnidirectional perturbation voltage: First, positive and negative perturbation voltages are applied only in the X-axis direction, while no perturbation voltage is applied in the Y-axis direction. The detector then obtains the corresponding performance indicators. ,in, This represents the performance metric obtained by applying a positive perturbation voltage in the X-axis direction. This represents the function for obtaining performance metrics. This represents the control voltage along the X-axis in iteration t-1, where t is the current iteration number. This represents the magnitude of the disturbance voltage along the X-axis. Representative at The control voltage in the Y-axis direction during the next iteration. The first indicator represents the performance obtained when a negative perturbation voltage is applied in the X-axis direction; the second indicator represents the performance obtained when positive and negative perturbation voltages are applied only in the Y-axis direction, with no perturbation voltage applied in the X-axis direction. ,in, and These represent the performance metrics obtained by applying positive and negative perturbation voltages in the Y-axis direction, respectively. This represents the magnitude of the disturbance voltage in the Y-axis direction.

9. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 8, characterized in that, Step S140 includes: calculating the accurate gradient direction: based on , , , The gradient magnitudes on the X and Y axes are calculated, with the X-axis gradient being... The gradient along the Y-axis is ,in, To prevent the denominator from being 0; Projecting the voltage corresponding to the small perturbation onto the exact gradient direction yields the scalar of the fourth perturbation voltage in the projection direction: , , , .

10. The single-mode fiber adaptive coupling method under turbulent conditions according to claim 9, characterized in that, In step S160, the voltage update rule is as follows: Where t is the current iteration number, This represents the control voltage along the X-axis in the t-th iteration. This represents the control voltage along the X-axis in the (t-1)th iteration. Represents the gain rate. and These are the X-axis gradient and the Y-axis gradient, respectively. This represents the control voltage along the Y-axis in the t-th iteration. This represents the control voltage along the Y-axis during the (t-1)th iteration.

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