With the increasing power of fiber lasers, single chirped and tilted fiber Bragg gratings (CTFBGs) cannot completely mitigate continuously enhanced system-excited stimulated Raman scattering (SRS). Although improving the loss rate of a single CTFBG or cascading multiple CTFBGs can provide better suppression of the stronger SRS, excessive insertion loss may cause significant attenuation of the output power. Confronting the challenge, we firstly present an SRS mitigation method based on a dual-structure fiber grating in this paper. The dual-structure fiber grating comprises a CTFBG and a fiber Bragg grating structure, which were designed and fabricated on a passive 25/400 double-clad fiber. To evaluate the performance of the grating, a 3 kW fiber master oscillator power amplifier laser is established. The experimental results demonstrate that the SRS mitigation rate of the grating is greater than 30 dB (99.9%), whereas the insertion loss is only approximately 3%, thus allowing for minimal deterioration of the output power. This solves the contradiction between high suppression rate and high insertion loss faced by CTFBGs, which in turn makes dual-structure fiber gratings particularly suitable for mitigating SRS in 3–5 kW high-power fiber lasers.

Four-channel off-axis holography is proposed to simultaneously understand the polarization states and the mode coefficients of linearly polarized (LP) modes in few-mode fiber. Far-field off-axis holograms in the four polarization directions of the fiber laser were acquired at the same moment through a four-channel holographic device. The weights, the relative phase differences, and the polarization parameters of the vector fiber laser mode can be solved simultaneously. The simulated and experimental mode analysis of the laser output by 1060-XP fiber with 6 LP modes at 632.8 nm is conducted, which shows that the similarity of the total intensity distribution of the laser before and after mode analysis is above 0.97. The mode polarization states, the mode weights, and the relative phase differences of the few-mode laser can be determined simultaneously in a single shot by four-channel off-axis holography.

Real-time monitoring of wavelength is important for high-speed wavelength phase-shifting interferometry. In this paper, a wavelength sensor based on a polarization-maintaining fiber interferometer with four-quadrant demodulation was proposed. We built the wavelength sensing system with resolution better than 0.005 pm and 0.1 ms sampling interval and measured the response time of the tuned wavelength at 35 ms in the phase-shifting process of a commercial wavelength phase-shifting free-space interferometer, as well as the wavelength drift velocity of 0.01 pm per second in the hysteresis process. The optical fiber wavelength sensor with four-quadrant demodulation provides a real-time wavelength sensing scheme for high-speed wavelength phase-shifting interferometers.

为了实现高功率光纤激光的窄线宽输出，研究了基于大模场三包层掺镱光纤（LMA-YTF）高功率窄线宽光纤激光的热效应和四波混频(FWM)效应。基于FWM效应模型，仿真分析了大模场三包层光纤(LMA-TCF)放大器光谱展宽的影响因素。建立了LMA-YTF的热分布模型，分析了大模场三包层光纤(LMA-YTF)中第二包层功率占比对光纤温度以及泵浦功率上限的影响，讨论了聚合物涂层导热系数和外部温度对光纤温度的影响，实验对比了不同反向合束器的泵浦功率上限，结果表明第二包层功率占比低的(6+1)×1合束器比(9+1)×1合束器拥有更高的泵浦功率上限。基于全国产化器件搭建了一台三包层光纤激光器，实现了输出功率6.7 kW、3 dB线宽为0.32 nm的激光输出。

在高功率光纤激光器反向泵浦信号合束器的制作过程中，经熔融拉锥后输出端的信号光纤纤芯变细，在与输入端信号光纤熔接时产生模场失配问题，造成反向泵浦信号合束器的信号光传输效率降低。针对这一问题，文中搭建了信号光纤熔接的芯径失配功率损耗模型，简析了光纤熔接时芯径失配与信号光功率损耗的关系。设计了一套泵浦信号反向合束器信号光功率损耗测试系统。提出了一种通过优化反向合束器信号光纤参数，提升反向泵浦信号合束器的信号光功率传输效率的方法，并通过预拉锥工艺，制作出一支25/400 （6+1）×1反向合束器，经测试，信号光传输效率优于98%，实验室使用该反向合束器搭建了一台MOPA结构光纤激光放大器，实现了3 kW稳定输出。

为了实现干涉仪出射准直波前的重构，提出了基于波长调谐移相的横向剪切干涉技术。干涉仪出射波前分别经楔板的前后表面反射，通过角锥棱镜返回后在干涉仪CCD上形成剪切干涉条纹。采用波长移相方法提取剪切干涉条纹的相位信息从而实现准直波前重构。分析相对剪切比对波面重构精度的影响，推导相对剪切比和其影响因素间的关系公式，给出波长移相中光程差常数分量的估算方法。测量干涉仪的三组出射波前，波前的峰谷值分别为3.22λ、2.10λ、0.83λ。该方法简化了传统测量干涉仪准直波前的横向剪切干涉装置，提高了测量精度，特别适合于测量波长移相干涉仪的出射波前。

High-power fiber-to-fiber coupling is extensively used in fiber laser applications, and its performance is determined by coupling efficiency. We demonstrate a novel method for alignment and monitoring efficiency by detecting backscattering power at the fiber end cap. The relationship between alignment error and backscattering power is determined by simulations and experiments. Through this method, a state-of-the-art kW-level fiber-to-fiber optic switch is developed (transmission efficiency >97%). It performs well for longer than 60 min. To the best of our knowledge, it is the first time to establish the mathematical model based on this method. Our results can provide guidance in high-power fiber-to-fiber coupling.