分析了kW级光纤激光器实现单模激光输出的模式控制方式。采用主振荡功率放大方式，实现了工作波长1.08 μm、最大输出功率1.05 kW的全光纤单模激光输出。对激光光谱和光束质量随激光功率的变化等输出特性进行了研究，结果表明：随着激光功率的增大中心波长和光束质量无变化，但谱宽逐渐展宽。分析了激光光谱展宽的原因，认为随着激光功率的增大，光栅纤芯的折射率变化增大，引起本振级的谱宽展宽，而本振级因谱宽展宽引入的噪声在放大级被放大，再加上放大级本身引入的自发辐射，共同造成放大级的谱宽展宽。对光束质量变差的原因进行了分析，认为光纤弯曲导致原先在纤芯中传输的激光部分被泄露到包层中，从而使光束质量变差。

The method of controlling mode with which a single mode laser output is achieved under high power is analyzed. Using master-oscillator power amplifier (MOPA), the continuous and single mode laser of 1.08 μm wavelength, 1.05 kW maximum output power is obtained. The laser spectrum and beam quality as functions of laser power are studied. The results indicate that the center wavelength and beam quality remain unchanged, and the spectrum width broadens with the increase of laser power. The reason for the broadening of laser spectrum width is that as laser power becomes higher, the change of refractive index of fiber grating core increases, which induces the spectrum broadening of master oscillator stage. Also, the noise induced by the spectrum broadening of master oscillator stage is amplified in amplifier stage, which combined with spontaneous radiation induced by amplifier makes the spectrum width of amplifier stage broaden. Moreover, the reason that the beam quality worsens is that part of the laser transmitting in the fiber core leaks to inner cladding due to fiber bending.