Compared with conventional optical fiber lasers, linearly polarized fiber lasers are widely used in coherent detection, coherent combination, polarization combination, and nonlinear frequency transformation. Therefore, linearly polarized fiber lasers have received special attention in recent years. Although the power of linearly polarized fiber lasers has been significantly improved in recent years, its power level is still far from that of randomly polarized fiber lasers. Because of the polarization characteristics of the linearly polarized fiber laser, its nonlinear effect and thermal effect are different from those of conventional lasers, and polarization deterioration needs to be considered additionally. Therefore, the spectrum, polarization, and beam quality of the output laser need to be strictly controlled in the process of boosting power. In recent years, the power boosting process of linearly polarized fiber lasers mainly includes two types of system structures. One based on an oscillator stage and amplifier stage is called FOL-MOPA, and the other based on a phase-modulated single-frequency laser and amplifier stage is called SFL-MOPA. The output power of these two configurations has reached more than 3 kW and even more than 4 kW for SFL-MOPA. In the process of power breakthrough, a series of problems that will lead to the deterioration of output parameters should be dealt with. In recent years, various research teams have accumulated many effective methods.

Firstly, in terms of spectral control and spectral linewidth control, stimulated Brillouin scattering (SBS) suppression and stimulated Raman scattering (SRS) suppression are mainly considered in linearly polarized fiber lasers. According to the difference in system structures, the nonlinear problems in SFL-MOPA and FOL-MOPA structures are different. Because there is no phase control in the FOL-MOPA structure, the spectral linewidth in the amplifier stage will be naturally broadened by self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), so the SBS effect will hardly occur unless the linewidth is too narrow. Since the SBS effect is weakened, and SRS gain has obvious polarization-dependent characteristics, the SRS effect is particularly prominent in the FOL-MOPA structure. To solve this problem, various teams have explored methods such as controlling seed source structure to suppress Raman noise, utilizing Raman filtering instruments, adopting a backward pumping scheme, and designing gain fiber, which can suppress the SRS effect to some extent. In addition, the SFL-MOPA structure suppresses the linewidth broadening of the amplifier stage through phase modulation, which results in a serious SBS process. In order to suppress this process, designing seed source modulation signals, adjusting the number of modulation stages, injecting composite signals, and inventing special optical fibers can be effective.

Secondly, in view of the polarization deterioration in the amplification process of linearly polarized fiber lasers, there have been intermittent studies since the invention of polarization-maintaining fibers. Although polarization-maintaining fibers have natural high birefringence, they are limited by fabrication and environment. Under the interference of internal and external factors such as temperature, external force, and nonlinearity, a decline in polarization-maintaining ability is inevitable. In addition, its fast axis and slow axis can both support the operation of linearly polarized lasers, so polarization mode selection is needed. Linearly polarized fiber laser seed source is the basis for realizing high-power linearly polarized fiber lasers. For the FOL-MOPA structure, the main schemes include winding gain fiber to select modes in slow axis and vertical splicing of gain fiber and fiber grating. For the SFL-MOPA structure, the technology of realizing an mW-level seed source with high polarization extinction ratio (PER) is mature at present. In fact, the focus of polarization control is on the amplifier stage. Because of the large heat generated by the amplifier stage, the birefringence of polarization-maintaining fibers will decrease, resulting in intensified mode coupling, so reducing heat generation is a major control method. In addition, bending mode selection, active polarization control, fiber polarizers, or special polarization-maintaining fibers are also proposed to deal with polarization deterioration.

Finally, linearly polarized fiber lasers will suffer from more serious mode deterioration and transverse mode instability (TMI) effects. The suppression methods are the same as that of conventional fiber lasers in both system configurations. The main methods include increasing high-order mode loss, using a bidirectional pumping scheme to disperse heat, improving seed power with high beam quality, using a special pumping source, etc. However, the research on the TMI effect is still controversial at present, so the related theoretical and experimental research needs to be improved.

Apart from polarization control, the spectral control and mode control schemes of linearly polarized fiber lasers are similar to those of conventional fiber lasers. In fact, all the methods mentioned in this paper are used in conventional fiber lasers, but not all the technologies used in conventional fiber lasers are applied to linearly polarized fiber lasers, so linearly polarized fiber lasers have a broad exploration space. In terms of polarization control, the current research is mainly limited by fiber structure and fiber control. If the research idea can be extended to the optical fiber device and system structure, more discoveries can be made.