Linearly polarized fiber lasers have a wide range of applications in areas such as beam combination and nonlinear frequency conversion. Mode instability, nonlinear effects, and other polarization-dependent factors, however, limit the enhancement in the fiber laser output power and thereby hinder the increase in the output power of linearly polarized lasers. In recent years, theoretical breakthroughs and technological advances in the laser fiber fabrication process alongside nonlinear effect/mode instability effect mitigation methods, and laser cavity design have prompted a rapid progress for high power linearly polarized fiber lasers, resulting in a continuous improvement in their overall performance. This paper aims to present the research results and development of linearly polarized fiber lasers globally from the aspects of laser linewidth, operation waveband, operation regime, and emerging power-scaling methods. An outlook on the development trend of linearly polarized fiber lasers is also discussed.

First, general progress in linearly polarized fiber lasers with different linewidths, that is, single-frequency, narrow-linewidth, and conventional fiber lasers, alongside superfluorescent fiber sources, and supercontinuum fiber sources, is summarized and reviewed. Second, linearly polarized fiber lasers operating at other wavebands are reviewed, including the Er-doped fiber laser at ~1.5 μm, Tm-doped fiber laser at ~2 μm, Yb-doped fiber laser at the long waveband (1.1?1.2 μm), Nd-doped fiber laser at ~0.9 μm, Raman fiber laser at 1.1?1.2 μm, and fiber lasers with manipulated output wavelength (central wavelength tunability, multiwavelength operation, and sweeping wavelength). Third, the research progress in pulsed linearly polarized fiber lasers with pulse durations ranging from nanosecond, picosecond to femtosecond is introduced. Then, the polarization dependence of nonlinear effects and mode instability is summarized, and mitigation methods of these two effects are briefly introduced. Finally, based on the aforementioned progress, some emerging techniques for further power scaling of linearly polarized fiber lasers are discussed, including, but not limited to, employment of low-quantum-defect fiber lasers schemes, exploitation of new laser materials such as single-crystal fibers, and adaptation of the coherent beam combination method.

Linearly polarized fiber lasers have made rapid progress in multiple types of linewidths, multiwaveband operation, and pulsed laser outputs with different durations, thus opening up the potential to not only further improve the performance in laser processing, coherent detection, and other fields, but also enable new application fields in the generation of mid-infrared lasers, visible/ultraviolet light, and structured light generation. However, compared with their randomly polarized fiber laser counterparts, linearly polarized fiber lasers still present a large gap in spectral coverage range and output power. Therefore, one of the future directions for linearly polarized fiber laser research is the development of high-performance polarization-maintaining fibers and high-quality multi-waveband fiber devices. Another potential focus is the continuous improvement in the output power and performance of the laser, including the employment of low-quantum defect schemes, new gain media, and coherent beam combination technology, to meet the application requirements. Lastly, new laser bands can be explored to broaden the spectral range of linearly polarized lasers, such as the direct generation of linearly polarized visible lasers in an oscillator scheme and the generation of a wider range of tunable linearly polarized lasers based on nonlinear effects, to meet diversified application requirements. A series of scientific and engineering problems must be addressed, including the comprehensive suppression of mode instability and nonlinear effects, polarization state evolution, control and compensation, and broad-spectrum polarizability measurements. Addressing the relevant physical and technical problems will not only promote the performance of linearly polarized fiber lasers to a new level but also has great significance for the development of laser science and is expected to be promoted to other new applications.