In this paper, an all-fiberized and narrow-linewidth 5 kW power-level fiber amplifier is presented. The laser is achieved based on the master oscillator power amplification configuration, in which the phase-modulated single-frequency laser is applied as the seed laser and a bidirectional pumping configuration is applied in the power amplifier. The stimulated Brillouin scattering, stimulated Raman scattering, and transverse mode instability effects are all effectively suppressed in the experiment. Consequently, the output power is scaled up to 4.92 kW with a slope efficiency of as high as approximately 80%. The 3-dB spectral width is about 0.59 nm, and the beam quality is measured to be M²∼1.22 at maximum output power. Furthermore, we have also conducted a detailed spectral analysis on the spectral width of the signal laser, which reveals that the spectral wing broadening phenomenon could lead to the obvious decrease of the spectral purity at certain output power. Overall, this work could provide a reference for obtaining and optimizing high-power narrow-linewidth fiber lasers.

High-power fiber lasers have experienced a dramatic development over the last decade. Further increasing the output power needs an upscaling of the fiber mode area, while maintaining a single-mode output. Here, we propose an all-solid anti-resonant fiber (ARF) structure, which ensures single-mode operation in broadband by resonantly coupling higher-order modes into the cladding. A series of fibers with core sizes ranging from 40 to 100 μm are proposed exhibiting maximum mode area exceeding 5000 μm². Numerical simulations show this resonant coupling scheme provides a higher-order mode (mainly TE₀₁, TM₀₁, and HE₂₁) suppression ratio of more than 20 dB, while keeping the fundamental mode loss lower than 1 dB/m. The proposed structure also exhibits high tolerance for core index depression.

We address the power scaling issue in end-pumped laser rod amplifiers by studying, experimentally and numerically, the magnitude of thermal lensing in a high-energy diode-pumped Yb:YAG crystal. The spatio-temporal temperature profile of the gain medium and the focal length of the induced thermal lens are determined numerically. The influence of the repetition rate and pumping power on the temperature distribution is analyzed. Experimental measurements covered repetition rates between 1 and 10 Hz and up to 4 kW pumping power.

为了控制重频放大器的热致波前畸变，设计并加工了均匀冷却的背面水冷激活镜激光放大器，对放大器的热畸变特性开展了实验研究，实验发现在泵浦功率密度较高即重复频率达到10 Hz，平均功率密度达到200 W/cm²时，放大器的热畸变既影响远场分布又对近场产生显著的调制。近场的调制会给放大器带来较大的损伤风险。为了消除热畸变对近场的调制，首先对泵浦强度分布进行了匀化，然后对介质进行了边缘热平衡控制，消除了热畸变引起的近场调制。通过对上述因素的控制，采用水冷激活镜构型的四程放大器实现了在10 Hz频率下良好运行。在没有进行主动补偿的情况下，实现了远场焦斑优于5倍衍射极限的输出。