A novel harmonic mode-locked fiber laser based on nonlinear multimode interference (NL-MMI) in a microfiber-assisted ultrafast optical switch is proposed in this Letter. The microfiber-assisted ultrafast optical switch can be obtained by tapering the splicing point of the graded-index multimode fiber (GIMF) and single-mode fiber, which not only helps to shorten the self-imaging period in GIMF to relax the strict requirement of NL-MMI on the length of multimode fiber, but also improves the harmonic order. In the experiment, with the waist diameter of ∼15µm, the repetition rates of the fiber laser can be stably locked at 285 MHz, corresponding to the 16th-order harmonic mode-locking, with the pulse duration of 1.52 ps. Our results provide novel insight into the design of a high-repetition-rate laser and the application of microfibers in the mode-locking device.

The vector dynamics of solitons are crucial but easily neglected for realizing vortex solitons. In this Letter, we investigate the effect of vector dynamics on cylindrical vector beams (CVBs) implementation and propose a novel technical method to realize femtosecond CVBs based on vector-locked solitons, which are presented as group-velocity-locked vector solitons (GVLVSs) in the experiment. The outstanding vector properties of GVLVSs not only greatly improve the efficiency of solitons converted into CVBs and output power of CVBs (2.4 times and 4.1 times that of scalar solitons and vector change periodical solitons, with the purity of 97.2%), but also relax the obstacle of ultrafast CVBs from the fundamental frequency to the harmonic regime (up to 198 MHz) for the first time, to the best of our knowledge. This is the highest repetition rate reported for ultrafast CVBs based on passive mode-locking. The investigation of the influence of solitons vector dynamics evolution on the realization of CVBs provides guidance for the excellent performance of ultrafast CVBs.

设计了一台紧凑型高温激光二极管阵列侧面泵浦Nd：YAG脉冲激光器。通过半导体制冷器控制泵浦源工作温度在60℃，其发射中心波长为808 nm，谱线宽度为4 nm。模拟了泵浦源在40℃、50℃和60℃条件下60 s内的温度场分布。实验所用激光增益介质为Nd：YAG晶体，尺寸为φ 5 mm×50 mm，掺Nd³⁺摩尔浓度为1.0at.%。采用磷酸二氘钾晶体作为电光调Q开关，在泵浦源电脉宽250 μs，重复频率20 Hz、1 Hz条件下，获得最大能量为230 mJ、246 mJ的单脉冲输出，对应的脉冲宽度分别为8.4 ns、7.8 ns。光束发散角约为1.6 mrad。设计的Nd：YAG脉冲激光器总的电-光转换效率大于4.6%。

基于二维材料的非线性光开关是调Q光纤激光器的核心器件，二维材料光开关的浓度会直接影响其非线性光学吸收特性，从而改变脉冲的时域特性。因此，针对二维材料浓度对调Q光脉冲的影响进行了研究，并通过实验制作了基于不同浓度Ti: Bi₂Te₃可饱和吸收光开关，分析了Ti: Bi₂Te₃浓度对非线性光学吸收特性的影响，获得了调Q光脉冲的调制深度、脉冲宽度、重复频率和单脉冲能量随不同浓度Bi₂Te₃的变化关系。最终，针对谐振腔参数对Ti:Bi₂Te₃浓度进行优化，在泵浦功率为71 mW时，获得了中心波长为1 560 nm、脉冲宽度为8 μs，重复频率为14.2 kHz、平均输出功率2.15 mW、对应单脉冲能量为151.4 nJ的脉冲输出。