High Power Fiber Laser Synthesis Of Coherence Study

The method of using the fused biconical tapered coupler to realize the coherentbeam combination is the passive coherent beam combination. Compared with the activecoherent beam combination, the system of the passive coherent beam combination issimple which doesn’t need a complex phase modulation. As the principle of the fusedbiconical tapered coupler is complex, the manufacturing process is not mature. Fewinstitutions take this method to achieve the coherent beam combination. So thedevelopment of the passive coherent beam combination is slow. However, because ofthe unique advantages of using the fused biconical tapered coupler in the structure, onceit is obtained a good result, the passive coherent beam combination will be the bestmethod using the easiest way to realize the coherent beam combination. So, it issignificant to set up the system in the laboratory and do the related research.In this dissertation, the system based on the fused biconical tapered coupler toachieve the coherent beam combination was researched theoretically and experimentally.The main contents are as follows:1. The theories of the passive coherent beam combination were introduced in detail.The optical field distribution of the coherent beam with the two fiber laser arrays andthree fiber laser arrays were analyzed respectively in theory. The output spectrum of thecoherent fiber lasers after the fused biconical coupler was simulated. The influence ofthe difference between two fiber gratings was analyzed.2. The fiber coupler was made by ourselves in the laboratory and the experimentsystem was set up, the coherent phenomena was observed through the fiber spectra. Itwas found that when the center wavelengths of the two fiber lasers were less than0.3nmand the core spacing of the two fiber lasers were close enough, the evanescent-wavecoupling was occurred, the fiber lasers were coherent. The spectra of the coherent fiberlasers was no longer a simple superposition of the two individual beam spectra, someof the supermode were restrained in the competition, some of the modes werestimulated.3. Based on the analysis of the existed beam quality evaluation method, a method to evaluate the coherent beam quality was presented. Then the influences of theevaluation coherent beam quality were analyzed in theory, including the number of thefiber laser array, the distance of the near-field arrangement. Usually, the fringe visibilitywere used to evaluate the degree of the coherence. The relationship between the degreeof the coherence and the fringe visibility was obtained theoretically.4. All-fiber laser with the output power of468W was made using domestic deviceswith the domestic fiber gratings. The characteristics of the single fiber laser wereanalyzed in theory and experiment, including the spectrum and the power stability. Theoutput power of768W was obtained using the domestic fiber.