Study on high power single-frequency fiber amplifier

掺镱的高功率双包层光纤激光器的研究和应用是目前国内外激光研究领域的热点之一。同传统的固体激光器相比，光纤激光器无论在效率、体积以及光束质量等方面，均占有明显的优势。其中高功率单频单纵模高光束质量的激光源研究对目前和潜在的应用尤为重要，如相干合束、重力波探测、自由空间光通信、测距、激光雷达以及非线性频率转换等领域。本文首先对100W量级单频线偏振光纤放大器进行了实验上的研究，其次对高功率单频光纤放大器中SBS进行了理论和实验研究，最后对获得的高功率单频光纤放大器在光束相干合成系统中应用进行了相关探索。 第一章介绍了采用种子源主振荡－放大(MOPA)实现高功率单频线偏振光纤激光输出的技术，以及高功率单频线偏振光纤放大器的国内外新进展。另外综述了高功率单频光纤放大器在重力波探测、光纤激光倍频以及光束相干合成中的应用。 第二章首先分析了光纤中SBS产生机理，分析了抑制SBS的方法。从含有SBS的光纤放大器速率方程出发，研究光纤内的温度梯度对SBS的影响。采取最佳的冷却措施，能大幅提高泵浦功率的阈值，提升放大器的性能。在单频单模线偏振光纤放大器中的受激布里渊散射实验中，研究了保偏光纤放大器中的受激布里渊散射的光谱变化特性，并在光纤放大器前后两端观察到多级次的stokes谱线和多级的anti-stokes谱线。 第三章先以泵浦光空间耦合方式实验研究单频线偏振光纤放大器。在泵浦光功率为30W是获得17.3W的输出。在泵浦功率约190W时，以空间耦合方式获得了128W单频输出功率。随后进行了全光纤化的高功单频线偏振放大实验。以Nufern 的10/130μm和20/400μm为增益介质，分别获得12.7W和43.8W的单频输出。最后对高功率光纤放大器工程化的关键技术进行了分析。 第四章先采用主动相位控制进行了主动相干合成实验研究。分别进行2路、4路和6路10W量级相干合成实验，分析了光束的远场图样及相干合成的效果。随后采用基于空间滤波光反馈的光纤激光被动相位锁定技术进行被动相干合成实验研究，建立了二维4路300W量级高功率光纤放大器阵列相干合成系统，实现了最高1062W的稳定相干耦合激光。The technique and applications of high power Yb-doped double-cladding fiber laser is one of the research focuses in international research fields at present. Comparing with traditional solid lasers, fiber laser has distinguished superiority in efficiency, volume, beam quality and so on. Especially, the high power single-frequency fiber amplifier is very important for the present and potential application including coherent beam combination, gravitational wave detection, free space optical communications, range finding, lidar, and parametric wavelength conversion in resonant cavities。 In this dissertation, experimental study on high power single-frequency fiber amplifier on 100W level is presented firstly. Then the stimulated Brillouin scattering (SBS) in the single-frequency linearly polarized ytterbium-doped fiber amplifier was theoretically and experimentally studied. Finally the application of high power single-frequency fiber amplifier in the system of coherent beam combination was explored. In the first chapter, the scheme of high power single-frequency linearly polarized fiber amplifier including the master-oscillator power amplifier (MOPA) was demonstrated, and it was emphasis on the latest progresses and development trends of high power single-frequency linearly polarized fiber amplifier. On the other hand, the applications of high power single-frequency linearly polarized fiber amplifier in gravitational wave detection, parametric wavelength conversion in resonant cavities frequency doubling and coherent beam combination were reviewed. In the second chapter, the influence of temperature distribution on stimulated Brillouin Scattering in high power single-frequency fiber amplifier is studied, based on the rate equations combining with SBS. The results indicate that temperature gradients along a fiber can broaden the SBS gain profile and thereby suppress SBS. The threshold of pump power and the output power of amplifier can be increased significantly by cooling fibers around the output port of amplifier actively. Finally，we report experimental results for the stimulated Brillouin scattering (SBS) in the single-frequency, single-mode, linearly polarized ytterbium-doped fiber amplifier. The evolution of SBS spectra was investigated. Multiorder stokes and anti-stokes scattering can be observed in the forward and backward directions of the fiber amplifier. In the third chapter, a high power single-frequency fiber amplifier with master oscillator power amplifier (MOPA) system using free space coupling technique was constructed. The system can generates up to 17.3-W and 128-W power of amplified output when 30W and 190W launched pump light was coupled respectively. Then, 12.7W and 43.8W single-frequency amplified output was obtained from a all-fiber master oscillator power amplifier (MOPA) system with the gain fiber of 10/130μm and 20/400μm, respectively. Finally, the key techniques of high power single-frequency fiber amplifier are analyzed. In the fourth charter, Experimental system of active coherent beam combination is built with six 10W-level fiber amplifiers. Two，four and six fiber amplifiers are coherently combined respectively, and the interference patterns in far field were analyzed. Then the experimental system of the passive coherent beam combination with four 300W-level high power fiber amplifiers was presented, the high power fiber amplifiers were passive phase locked with ring cavity. The interference patterns at different output power are observed. The maximum coherent output power of the fiber array is up to 1062W by multi-stage amplification