Research On Phase Control Of Fiber Laser Coherent Beam Combination

Fiber laser has obtained rapid development in recent years,and been more and more widely used in the fields of laser communication,industrial processing,military national defense and so on because of its advantage such as compact structure,convenient heat management and good output beam quality.However,the output power of single fiber laser is limited due to the nonlinear effect,mode instability,thermal effect and other factors.Fiber laser coherent combination is an effective method to realize high power laser output with high beam quality,and phase control is the key technology to realize effective coherent combination.In fiber laser coherent combining system,phase control includs the control of piston phase error,tilt-tip error and optical path difference.This paper focuses on the control algorithm of fiber laser coherent combination system.The main contents of this paper include the following aspects:1.The principle of the algorithm including hill climbing method,stochastic parallel gradient descent(SPGD)algorithm,and dither techonology is analysed,and the simulation platform using for the algorithm research of the piston and tilt-tip phase error control is designed.The effect of various parameters on the tilt-tip error control using single-frequency dither techonology is discussed based on this simulation platform,and the bandwidth of tilt-tip error control using single-frequency dither techonology is analysed by simulation,concluded that the tilt-tip phase error control bandwidth is greater than 100 Hz for random tilt-tip noise with variance less than 25 ?rad in three fiber laser coherent beam combination system.2.Proposing the idea that applied single-frequency dither techonology to tilt-tip phase error adaptive control.The single-frequency dither techonology can effectively utilize the high order resonance frequency of the control device,which can enhance the tilt control bandwidth.A fiber laser array coherent combination experimental platform is builded,and the circuit system of tilt-tip phase error control using single-frequency dither techonology is designed and implemented based on the field programmable gate array(FPGA).The corresponding verification experiments are carried out,and the tilt phase difference between two beams is compensated.When the tilt-tip control is start-up,two separate spots effectively coincide,and the light intensity measured by photoelectric detector increases from 0.48 V to 0.79 V,the fringe contrast is about 26.4%,which increased to 77.7% after applying phase-locked control.3.The circuit of phase-locking using the single-frequency dither techonology is designed based on FPGA,uasing which has achieved good experimental results in the experiment of seven fiber laser coherent array combination.The normalized power in the bucket is increased from 0.22 to 0.94 when the phase-locking close loop,and the residual is less than ?/20.The fringe contrast is increased from 26.4% to 77.7% after applying phase locked control.4 Cascaed phase-locking is proposed in order to weaken the effect of the control number on the control bandwidth in coherent combination of large array fiber laser.Coherent combination experiment of 8×2 fiber laser beams is demonstrated,the first degree phase-locked control use SPGD algorithm,the phase residuals is less than ?/28;the second degree phase control is based on single-frequency dithering algorithm,the residual is less than ?/24.5.A all fiber optical path difference adaptive control algorithm is designed based on hill-climbing algorithm,and successfully applied to femtosecond laser coherent polarization combination system.This method can realize completable optical path difference compensation for range of ?60?,which proves that this optical path difference adaptive control algorithm can be used in femtosecond laser coherent optical polarization combination system to correction the slow drift of the optical path difference.