Research On High-speed Control Technology Of Fiber Laser Arrays

Fiber laser has the advantages of higher pump efficiency,good heat dissipation,good beam quality and compact structure,therefore,it has been widely used in the filed of superlaser and communication.In such fields,fiber laser array with modular structure can achieve laser beam with high power and high beam quality,as well as ensure the stable and efficient coupling of space laser to single mode fiber,so it is an important research object in the field of superlaser and communication.Recent years,with the development of the new adaptive optical devices(such as adaptive fiber optical collimator,AFOC),the application area of fiber laser array gradually expand,and it begin to have the ability to cope with actual atmospheric turbulence.High-speed control of fiber laser array means longer atmospheric transmission distance and higher combined beam power in the field of superlaser,it also means lower communication bit error rate in the field of laser communication.However,due to the resonance and response delay of AFOC,the control bandwidth of the array is limited.In addition,the control scale of the fiber laser array will also develop to more than 100,which complicates the design of a real-time algorithm controller.In this paper,the high bandwidth control of AFOC and the design of a real-time controller are studied.The main contents are as follows:In the first section,the control bandwidth of fiber laser array is analyzed.The control bandwidth formula of stochastic parallel gradient descent(SPGD)algorithm is derived;The frequency response characteristics of AFOC is tested,and the corresponding transfer function model is fitted.The influence of the AFOC's inherent response delay and resonance on the SPGD algorithm is analyzed by simulation,and the necessity of the delay compensation and resonance suppression is proved.In the second section,an improved SPGD algorithm named PD-SPGD algorithm is designed to compensate for the AFOC's inherent response delay.The working principle of PD-SPGD is introduced.Single mode fiber adaptive coupling experiment based on adaptive fiber coupler(AFC)is carried out in the laboratory,the experimental results shows that PD-SPGD could reduce the convergence time from 6.3 ms to 2.5 ms and increases the correction bandwidth of sinusoidal dynamic angular jitter by more than one time.Adaptive single mode fiber coupling experiment under 520 m atmospheric turbulence is carried out,the results shows that the system with PD-SPGD algorithm could effectively suppress the atmospheric turbulence,and further reduce the bit error rate in laser communication.In the third section,active resonance suppression method based on biquad filters is proposed.The basic principle of this method is introduced,and its effect is simulated.The digital realization of the filters is introduced.Testing experiment is conducted to verify the effectiveness of the filters,the results show that the AFOC's resonance could be almost completely compensated,and the response oscillation in time domain is significantly suppressed.In the fourth section,modular SPGD hardware control platform for large-scale fiber laser array is designed.The scheme and the detail implementation of the control platform are given.The performance of the platform is analyzed,and the results show that the control precision,real-time performance,and the number of control channels can meet the requirements of the 100-unit fiber laser array,and it is easy to expand on a larger scale.In the fifth section,coherent beam combination experiment using a 7-unit fiber laser array is conducted to verify the effectiveness of the proposed PD-SPGD algorithm,biquad filters,and the modular hardware control platform.The experimental results show that the modular control platform can ensure stable phase-locking control and tip/tilt control.The biquad filters can effectively suppress the mean square error and dither range of the closed-loop performance metrics.On this basis,PD-SPGD can reduce the convergence time from 5.63 ms to 1.83 ms,and increase the control bandwidth by more than two times.In this paper,the theoretical model,numerical analysis,and experiments are studied around the high-speed control technology of fiber laser array.The high bandwidth control of AFOC and the design of a large-scale controller are proposed to solve the key issue in the fiber laser array.The results of this paper have great potential in subsequent researches.