2 μm低重复频率高峰值功率高光束质量激光在中长波光参量非线性频率变换等领域具有较为广阔的应用前景。采用L型谐振腔结构，使用42 W的掺Tm光纤激光器泵浦Ho: YLF晶体。基于磷酸钛氧铷（RTP）电光调Q技术，实现了重复频率50 Hz、脉冲宽度18 ns、脉冲能量13.5 mJ、峰值功率0.75 MW的2.05 μm Ho: YLF固体激光输出。光束的水平方向和竖直方向M²因子分别为1.4和1.1。该Ho: YLF固体激光器采用光纤激光器泵浦，具有结构紧凑的特点，为更高能量的Ho激光输出奠定了基础。

Based on the Nd-doped single-mode fiber as the gain medium, an all-fiber 12th harmonic mode-locked (HML) laser operating at the 0.9 µm waveband was obtained for the first time, to the best of our knowledge. A mandrel with a diameter of 10 mm was employed to introduce bending losses to suppress mode competition at 1.06 µm, which resulted in a suppression ratio of up to 54 dB. The 1st–12th order HML pulses with the tunable repetition rate of 494.62 kHz–5.94 MHz were obtained in the mode-locked laser with a center wavelength of ∼904nm. In addition, the laser has an extremely low threshold pump power of 88 mW. To the best of our knowledge, this is the first time that an HML pulse has been achieved in a 0.9 µm Nd-doped single-mode all-fiber mode-locked laser with the advantages of low cost, simple structure, and compactness, which could be an ideal light source for two-photon microscopy.

In two-photon microscopy, low illumination powers on samples and a high signal-to-noise ratio (SNR) of the excitation laser are highly desired for alleviating the problems of photobleaching and phototoxicity, as well as providing clean backgrounds for images. However, the high-repetition-rate Ti:sapphire laser and the low-SNR Raman-shift lasers fall short of meeting these demands, especially when used for deep penetrations. Here, we demonstrate a 937-nm laser frequency-doubled from an all-fiber mode-locked laser at 1.8 μm with a low repetition rate of ∼9 MHz and a high SNR of 74 dB. We showcase two-photon excitations with low illumination powers on multiple types of biological tissues, including fluorescence imaging of mouse brain neurons labeled with green and yellow fluorescence proteins (GFP and YFP), DiI-stained and GFP-labeled blood vessels, Alexa Fluor 488/568-stained mouse kidney, and second-harmonic-generation imaging of the mouse skull, leg, and tail. We achieve a penetration depth in mouse brain tissues up to 620 μm with an illumination power as low as ∼10 mW, and, even for the DiI dye with an extremely low excitation efficiency of 3.3%, the penetration depth is still up to 530 μm, indicating that the low-repetition-rate source works efficiently for a wide range of dyes with a fixed excitation wavelength. The low-repetition-rate and high-SNR excitation source holds great potential for biological investigations, such as in vivo deep-tissue imaging.