Study On Optimization Algorithm Based Adaptive Optics In Laser Phased Array

High power fiber laser phased array, which provides a promising way for high speed, high precise and high efficiency beam steering without mechanically movement, is an effective choice for the application of next generation of high power laser system. Compared with the microwave phased array, the wavelength (~1 micrometer) of the laser is much smaller than that (~1 millimeter to 1 meter) of microwave, the subsequently problems, such as the wave-front control of the single beam, phase locking of each element and turbulence compensation of the beam array, those are few considered in the microwave phased array but should be considered and solved in the fiber laser phased array. Optimization algorithm based adaptive optics (OABAO) without a wave-front sensor can avoid the problem of wave-front compensation which is encountered in the adaptive optics systems based on wave-front measurements when there is any error in the measurement of the distorted wave-front. The OABAO system is simple and compact. OABAO can be used both for wave-front correction of a single beam and active phase locking of coherently combined beam array, which is one of the most important techniques in the high power fiber laser phased array system. OABAO, including single beam adaptive optics and beam array adaptive optics are both investigated in theory, simulation and experiment in this thesis.First, OABAO system is investigated in detail after the introduction of the basic principle of the OABAO. The system model of array beam coherent combination using OABAO is build up and the convergence characteristic is proved. The similarities and differences between single beam and beam array adaptive optics are analyzed. It is shown that except the control object and the wave-front corrector, the optimization algorithm based single beam and beam array adaptive optics are almost the same in metric function, optimization algorithm and algorithmic controller. Therefore, the two types of adaptive optics can be investigated using the same method in theory and experiment. The performances of each module in optimization algorithm based array beam coherent combination are analyzed, and the modules used in experiment are selected and optimized. The relationship between control bandwidth, control precision and each module is investigated by using stochastic parallel gradient descent algorithm. Simulation software with graphics-user-interface used for modeling and simulation of optimization algorithm based beam cleanup and coherent beam combination in laser phased array is developed.Second, beam cleanup using OABAO is investigated in theory and experiment. In theory, three types of wave-front correct method, i.e. piston type only, global type based on Zernike polynomial, combination type which combines both piston and global type in adaptive optics are investigated. Beam cleanup technique of high-order mode laser using OABAO are proposed and investigated. With such technique, high power laser with higher brightness than fundamental mode laser would be realized. The influence of the intensity fluctuation on the convergence characteristic of the optimization algorithm is investigated in theory. To avoid the influence of the intensity fluctuation on beam cleanup system, beam cleanup technology based on normalized performance metric: power in the bucket is proposed. In experiment, algorithmic controller with an increased updating rate of 1 kHz for beam cleanup system is designed and manufactured. Using this controller, beam cleanup for fundamental mode and high-order mode lasers, as well as the influence of the intensity fluctuation on optimization algorithm based beam cleanup system are investigated experimentally. The influence of the wave-front aberration of a single beam on coherent beam combination of array beam is also investigated. Results show that with beam cleanup, coherent beam combination can be realized with a higher beam quality.Then, coherent beam combination using OABAO technique is investigated in theory and experiment. In theory, contrast studies on confocal launched and parallel launched coherent beam combination system are carried out, and results indicate there are some advantages in the confocal launched system than in the parallel launched system. Algorithmic controller with an updating rate of 40 kHz, control band about 250Hz and control precision ofλ/44 for 16 channel coherent beam combination is designed and manufactured. In experiment, four-channel 137W, two-channel 260W, and nine-channel 1.14 kW coherent beam combination are realized. The nine-channel 1.14 kW coherent beam combination is the highest power and largest array demonstration of coherent beam combination of all fiber amplifiers. Moreover, turbulence compensation of the coherently combined beam is studied, and OABAO technique using a beacon is proposed for turbulence compensation of coherently combined beam. Based on the beam cleanup, coherent beam combination and turbulence compensation techniques, an overall wave-front compensation laser phased array system based on OABAO is proposed, which may provide a reference for the development of the laser phased array system.At last, exploration investigation of high power laser coherent beam combination in laser phased array are carried out in the area from frequency domain (coherent beam combination of multi-wavelength laser), time domain (coherent beam combination of pulsed laser) and spatial domain (coherent beam combination of high-order mode laser) to phase control method. Simultaneous spectral and coherent beam combination technique is proposed and validated by various experiments. This technique can increase the total power to N times of the value in coherent beam combination of single-frequency laser, where N is the number of laser wavelength for spectral combination, which may become a promising way for high power coherent beam combination. Coherent beam combination using a phase modulated single-frequency laser (another type of multi-wavelength laser), coherent beam combination with hybrid phase locking in a master-oscillator power-amplifier configuration are presented in theory and realized in experiment. Coherent beam combination of pulsed fiber lasers with active phase control using optimization algorithm is presented in theory. Experimental demonstration of coherent beam combination of pulsed fiber laser is realized for the first time based on hybrid phase control. Coherent beam combination of high-order mode laser with wave-front compensation is presented, simulation results indicate that this technique can improve the power in the main lobe effectively, which also lead to a higher power than coherent beam combination of fundamental mode laser. The results of this dissertation lay a foundation to the uniform investigation of single beam adaptive optics and array beam coherent combination in laser phased array. The theoretical and experimental investigations of the OABAO, especially the related studies on the high power coherent beam combination may provide some references for the development of laser phased array system.