Coherent Combining Of MOPA With Multi-dithering Technique

Coherent beam combining of medium power lasers or amplifiers plays an important part in obtaining high power laser with good beam quality. The master oscillator power amplifier (MOPA) scheme is believed a promising way to realize the coherent beam combination. At present, three methods are used to realize the MOPA scheme, which are heterodyne detection technique, stochastic parallel gradient descent algorithm (SPGD algorithm) and multi-dithering technique.In the dissertation, multi-dithering technique is studied theoretically and experimentally, and the coherent beam combinations for two and three fiber amplifiers are firstly demonstrated at home. The main results are as follows.1. The feasibility of coherent beam combining using multi-dithering technique is demonstrated according to wave optics principle. The influence of systemic parameters, i.e. transition time, integration time, feedback coefficient and the number of laser beams, on the results of coherent beam combining is studied in detail. In addition, the controlling model is proposed for coherent beam combination using multi-dithering technique according to automatic control system theory, and the transport function is obtained. On this base, the stability, speed of response and other properties of the control system are analyzed.2. The phase fluctuations of the output lasers caused by fiber amplifiers and environment are measured with multi-dithering. It is found that the dominant phase noises are at frequencies below one kilohertz. Then the experiments for coherent beam combination of two and three beams are performed respectively. In the experiment, the output power of each beam is about 1W. When the control loop is open, the far-field pattern is dynamic and blurred, and the coherence for two beams combination is 0.19 and for three beams is 0.12. When the control loop is closed, the far-field pattern is stable and clear, and the coherences for two and three beams are improved to 0.93 and 0.81 respectively. These results show that phase noises of the output lasers are weakened by the control system and the high coherence output laser is obtained.3. On the base of the studies of theories and experiments, the difficulties of combination in scaling to a large number of beams are pointed out. However, a new way (grouped coherent beam combination) is proposed to overcome the difficulties, which is demonstrated by computer simulation.The results and analysis of the dissertation demonstrate that coherent beam combining system using multi-dithering technique may finally leads to a practical fiber laser system with high brightness laser output.