Performance Optimization Study Of Fiber-laser Coherent Combining System Using A Hardware-based SPGD-algorithm Control Platform

Coherent beam combining(CBC) of fiber lasers based on stochastic parallel gradient descend(SPGD) algorithm is an effective way to obtain high-brightness and high-quality laser beams. The key to realize a CBC system is to correct the phase errors between the beamlets. The phase control accuracy and the control bandwidth, which depend on the algorithm’s feature, the employed control platform and the phase control device, are two major parameters for the evaluation of the CBC system’s correcting capabilities. This paper is focused on the performance optimization problem for a fiber-laser CBC system using SPGD algorithm. The main contexts include two parts, one is the implementation of a hardware control platform based on the field-programmable logic gate array(FPGA), and another is the resonant performance optimization of the adaptive fiber-optics collimator(AFOC).Firstly, a FPGA-based hardware control platform is developed for the actual application of a fiber-laser CBC system, which permits the coherent control of at most 21-channel laser beams. The system performances, regarding to the phase control accuracy and the control bandwidth, of this platform is analyzed. The results show that the theoretical control bandwidth of phase-locking is about 1.37 k Hz, while the theoretical bandwidth of tip/tilt control is 9 Hz.Then, the biquad digital filter technique is employed to optimize the resonance characteristics of the AFOC device for the first time. The relevant theoretical analysis and simulation about this kind of filter are completed, and the results show that the first-order resonant peak of AFOC is suppressed and the control bandwidth is effectively increased. To integrate the step-profile damping structure with the biquad digital filter technique, a resonant optimization solution for the homemade AFOC device is established, and the experimental result shows that the control bandwidth of the AFOC increases from 1 k Hz up to 2.5 k Hz.Furthermore, the whole actual CBC control system is divided into several modules, and the analysis, design, programing and debugging of each module are completed respectively, with the integration of the homemade hardware platform and the biquad digital filter technique. These control modules include data acquisition, pseudo-random number generation, SPGD control iteration, voltage output synchronization, biquad digital filter, parameter configuration, information displaying and data exchanging, and the phase control and tip/tilt control functions can be achieved with the combination of these modules.Finally, a fiber-laser CBC system of seven-channel beams based on the hardware control platform is established. Under the indoor weak turbulence conditions, the hardware-based experiments of phase-locking and tip/tilt control are completed, and the system performances are analyzed through the parameters of power-in-bucket(PIB), phase control accuracy and the control bandwidth. Experimental results of phase-locking show that the PIB value increases from 0.32 to 1.20 after phase locking achieved, the phase locking control accuracy is 1/19λ, and control bandwidth of phase-locking is 1 k Hz. Experimental results of tip/tilt control show that the PIB value increases from 0.45 to 0.68 after the closed-loop control, and the bandwidth of tip/tilt control is about 6 Hz.This paper provides the hardware supports and technical groundwork to the following study on actual application and performance optimization problem about fiber laser CBC system based SPGD algorithm.