Research On Active Phase Control Algorithm For Fiber Laser Coherent Combining

Fiber laser with high conversion efficiency,good beam quality and compact system structure,has broad applications in laser processing,medical and defense.However,due to the limitations of the nonlinear effects of fiber gain medium and the thermal effects of device,the output power of a single fiber laser is limited.Coherent beam combining(CBC)technology is one of the more effective ways to obtain high beam quality and high power laser output.The fast and accurate control technology of beam phase is one of the key technologies to realize fiber laser coherent synthesis.Research on phase control can further promote the engineering application of coherent synthesis technology.In this paper,the CBC system based on the MOPA scheme is used as the research framework,and the active phase control algorithm of CBC is the main research object.The SPGD algorithm is studied in depth,combined with the optimization algorithm in deep learning to improve it,and the effectiveness of the SPGD algorithm and its improved algorithm in the coherent synthesis system is verified by simulation and experiment.The work of this paper provides theoretical guidance for the design and algorithm selection of CBC systems.The main research contents of the article are as follows:Firstly,the CBC technology and active phase control technology are reviewed and analyzed.The domestic and foreign research status of beam combining technology is introduced,and the CBC and ICBC are analyzed and compared.The active phase control method is introduced in detail,and the control methods are compared and analyzed,and the active phase control algorithm is determined as the key research object of this paper.Secondly,by establishing a theoretical model of CBC system,the effects of system parameters such as fill factor,phase error,intensity error,and polarization error on the synthesis effect are quantitatively analyzed.The influence of the turbulent atmosphere on the synthesis effect is analyzed by calculating the transmission characteristics(average intensity and scintillation effect)of the coherent synthesis array beam in the turbulent atmosphere.Secondly,the theoretical model of CBC using SPGD algorithm is established.Thefeasibility of CBC using SPGD algorithm was verified by static simulation.Through dynamic simulation,the influence of the phase noise amplitude and frequency on the synthesis effect was studied,and the relationship between the control bandwidth and the number of beams and disturbance amplitude was analyzed.In addition,the rotating phase screen is used to simulate atmospheric turbulence,and the influence of turbulent atmosphere on the correction effect of CBC system is studied.The simulation results indicate as the Greenwood frequency of the atmosphere increases,the synthesis effect of CBC system becomes worse.The faster the algorithm iteration speed,the better correction effect on the turbulent atmosphere.Thirdly,from the aspect of gain coefficient,a SPGD algorithm with adaptive gain coefficient,Ada SPGD algorithm,is proposed.The influence of the adaptive gain adjustment factor on the SPGD algorithm is studied by the static and dynamic simulation of the CBC using Ada SPGD algorithm.The simulation results show that the adaptive gain adjustment method proposed in this paper can effectively improve the convergence speed of the SPGD algorithm.From the aspect of gradient estimation,combined with the idea of momentum in physics,a SPGD algorithm with momentum term,Mom SPGD algorithm,is proposed.The feasibility of CBC using Mom SPGD algorithm is verified by static and dynamic numerical simulation,and the influence of momentum term on SPGD algorithm is analyzed.The momentum method proposed in this paper can improve the convergence effect and robustness of SPGD algorithm.Further,combining the adaptive gain adjustment method and the momentum method,a comprehensive improved algorithm,Adm SPGD algorithm,is proposed.The static and dynamic simulation results demonstrate that the Adm SPGD algorithm can improve the convergence speed and correction effect of the CBC,and the correction effect of the CBC using Adm SPGD algorithm is more significant for the turbulent atmosphere.Fourthly,The mathematical model of TIL coherent synthesis system was established,and the effectiveness of SPGD algorithm and Adm SPGD algorithm in the system was verified by numerical simulation;the effects of target characteristics on the closed-loop control effect of the system were analyzed.In order to verify the simulation results,A two-unit and multi-channel fiber lasercoherent synthesis experimental platform was built.The experimental results demonstrate that the SPGD algorithm and its improved algorithm can be applied to the coherent synthesis system,and the improved SPGD algorithm can improve the convergence speed and correction effect of CBC system.The phase control algorithm of the system can be effectively implemented under the non-cooperative expansion target.Finally,summarize the paper.The trend of CBC system and active phase control algorithm are analyzed.