Coherent Combining Technology Of Master Oscillator Power Amplifier Fiber Arrays

Coherent beam combination of fiber laser arrays plays an important role in realizing high power, high radiance fiber laser systems. In this dissertation, coherent combining technique on fiber amplifier arrays using the master oscillator power amplifier (MOPA) configuration is studied experimentally and theoretically.In the dissertation, we arrive at the theoretical model of far-field intensity distribution of fiber amplifier arrays by Fraunhofer Diffraction theory, study the effects of the numbers of the laser arrays, fill factor, phase, partial coherence and polarization on far-field intensity patterns; Phase noise generated in ytterbium fiber amplifiers is theoretically investigated, which should be helpful in designing servo control systems; In the field of fiber laser coherent combining, phase control of ytterbium fiber amplifiers is the most difficult technology in the Master Oscillator Power Amplifier scheme. We design a high-frequency phase control eletronics by heterodyne detection principle. In the heterodyne detection control system, the frequency is shifted 40 MHz and the control precision is better thanλ/20, control-loop bandwidth is more than 10 kHz. The factors that affect control precision and closed-loop bandwidth are also analyzed, so that the performance of control eletronics can be further optimized and improved.Coherent combining technique of the master oscillator power amplifier configuration is experimentally investigated. First, we perform optical phase-noise measurements of a commercial 1 -W ytterbium fiber amplifier using our phase control eletronics, the dominant phase noises of the 1-W fiber amplifier are at frequencies below one kilohertz.Two methods to realize phase control of ytterbium fiber amplifiers are studied.a) "climbing hill". Using phase controll theory which is commonly used in adaptive optics, we propose "climbing hill" to realize phase control of ytterbium fiber amplifiers, which can automatically optimize the control voltage of phase modulator to keep the the output intensity of the system at the status of stable high brightness. The control precision is A/10 in the "climbing hill" control system. The coherent beam combining with two polarization maintaining ytterbium fibre amplifiers is experimentally realized.b) "heterodyne detection". In heterodyne detection control system, the detector detects the interferential signal of each amplifier and the modulated reference beam to fulfill the phase noise measurements of ytterbium fibre amplifiers. Heterodyne detection can realize real-time detection and correction of the phase change to ensure phase coherence and matching between all elements of fiber amplifier arrays. Coherent beam combination of three ytterbium fiber amplifiers is firstly realized by heterodyne detection method. in the experiments, the output power of each ytterbium fiber amplifier is not less than 1 W and the total output power of the three amplifiers is not less than 3 W. When the control loop is turned off, the far field interference pattern is dynamic and blurred, the visibility of the interferential fringes is 6%; while the control loop is turned on, the far field interference pattern turns to stable and clear, and the fringes visibility is 53%. This indicates the phase noises of fiber amplifiers are properly controlled, the fiber amplifiers are phase-locked and coherent beam combination of the three beams is achieved.In the MOPA configuration, each amplifier is independently locked to the reference beam, so the scalability of this system is good, high power fiber laser arrays can be obtained. Simultaneously, the MOPA configuration in this dissertation can be applied to coherent combination of solid-state lasers.The results of this dissertation lay a foundation to realize higher power beam combination of laser arrays and will be useful for the applications in future.