Significance Compared with traditional random polarization fiber lasers, some special applications (such as coherent beam combining and nonlinear frequency conversion) require fiber lasers with high power, a narrow linewidth, linear polarization, and good beam quality output. To fulfill this requirement, high- power narrow-linewidth linearly polarized fiber lasers have recently attracted extensive attention, and their output power has increased from 1 kW to 5 kW in the last decade. The output power of narrow linewidth linearly polarized fiber lasers used for coherent beam combining and nonlinear frequency conversion has also significantly improved. This paper reviews the recent development of high- power narrow-linewidth linearly polarized fiber lasers. Representative application results for high- power narrow-linewidth linearly polarized fiber lasers in coherent beam combining and nonlinear frequency conversion are also presented. Progress High- power narrow-linewidth linearly polarized fiber lasers are generally based on a master oscillator power amplifier (MOPA) structure. Several types of laser seeds have been used in these MOPAs, including single- frequency, phase- modulated single- frequency, and fiber- oscillator laser seeds. At present, a single- frequency linearly polarized laser has an output on approximately the kilowatt level, but the stimulated Brillouin scattering (SBS) effect seriously restricts further increases in output power. A linewidth on approximately the gigahertz scale can also meet the requirements for coherent beam combining and nonlinear frequency conversion. The SBS effect on the amplification process can be greatly suppressed by using the phase modulation technique to broaden the spectrum of a single- frequency laser, making it possible to achieve a higher output with a narrow-linewidth linearly polarized fiber laser. Currently, sine- wave, white noise signal (WNS), pseudo- random binary sequence (PRBS), and optimized signal phase modulation techniques are used in high- power narrow-linewidth linearly polarized fiber laser systems. At present, narrow-linewidth linearly polarized MOPAs based on these phase modulation methods have achieved laser outputs measured in kilowatts. Among them, the most representative is the realization of a 5 kW linearly polarized power output with a 10 GHz linewidth based on nonlinear optimized signal phase modulation, which fully verified its ability to obtain an output with higher power and a narrower linewidth. A MOPA based on a narrow linewidth fiber oscillator seed is also an effective method to achieve a high- power narrow-linewidth linearly polarized fiber laser output. Currently, this method can be used to achieve a narrow-linewidth linearly polarized power output with a maximum value of 4.6 kW and linewidth of 91 GHz. Narrow-linewidth linearly polarized MOPAs based on these different seed sources have different advantages and disadvantages. In general, a single- frequency or single- frequency phase- modulated seed source has a stable time domain, and the spectrum broadening is not obvious during amplification, which has a high stimulated Raman scattering (SRS) threshold. However, SBS is an important limiting factor. A narrow-linewidth linearly polarized MOPA based on a fiber- oscillator seed has a simple structure and low cost, along with a high SBS threshold. However, self- phase modulation (SPM) and four- wave mixing (FWM) cause a serious spectrum- broadening effect during the amplification process. At the same time, because of the time- domain instability of the multi- longitudinal mode seed, the SRS threshold during the amplification process is much lower than the theoretical expectation. This paper also presents representative results for the application of high- power narrow-linewidth linearly polarized fiber lasers in the fields of coherent beam combining and nonlinear frequency conversion. A 21.6 kW laser output has been achieved by using 19 narrow-linewidth linearly polarized fiber amplifiers for aperture- divided coherent beam combining. Recently, the maximum number of combining channels has also exceeded 1000. A 5.02 kW laser output has been achieved by using four narrow-linewidth linearly polarized fiber amplifiers for filled- aperture coherent beam combining. For nonlinear frequency conversion, a kilowatt continuous wave 532 nm green laser has been realized based on the frequency doubling technology of a high- power narrow-linewidth linearly polarized fiber laser. Conclusions and Prospects Although the output power of linearly polarized fiber lasers has reached 5 kW, there is still a significant difference in the maximum output power values of randomly polarized fiber lasers. In the future, based on the subsequent development of polarization- maintaining fiber materials, artificial intelligence, and other technologies, further improvements in the output performances of narrow-linewidth linearly polarized fiber lasers are expected to support future applications such as higher power coherent synthesis and nonlinear frequency conversion.
