Due to their advantages of high efficiency, excellent beam quality, effective thermal dissipation, and easy integration, fiber lasers are widely used in industrial processing, medical treatment, military defense, and scientific research fields. With the advancement of technology, power scaling has emerged as a critical objective, with beam combining technology serving as an important approach. Consequently, this imposes more stringent demands on narrow-linewidth polarization-maintaining lasers used as sub-beams.

For narrow-linewidth lasers using fiber oscillators as seed sources, the stimulated Brillouin scattering (SBS) effect need not be considered. However, the SRS effect restricted further power enhancement. Derived from formula deductions, the SRS threshold was related to parameters including fiber mode field radius and Raman gain coefficient. Simulation results showed that the SRS threshold first decreased rapidly and then stabilized as fiber length increased (stabilizing at approximately 1.0 kW after 5 m), and rose with the expansion of the effective mode field radius, reaching about 7.5 kW at 20 μm. Therefore, adopting a short fiber length and a large-mode-area fiber structure could disperse optical energy density and shorten the interaction distance between light waves and the medium, thereby suppressing the SRS effect. Beam quality affected the focusing capability and processing accuracy of lasers. Small-core-diameter fibers could achieve fundamental mode output but limit laser power. High-power lasers required large-mode-area double-clad fibers, which, however, were prone to transmit high-order modes and thus degrade beam quality. By estimating the bending loss of the LP₀₁ mode through formulas, simulations revealed that the loss of all modes decreased as the fiber coiling radius increased. Moreover, under the same coiling radius, the loss of high-order modes (LP₁₁, LP₂₁) was much higher than that of the fundamental mode (LP₀₁). Therefore, coiling the fiber could suppress high-order modes, but excessively small coiling radii would increase the loss of the fundamental mode, so a reasonable coiling radius must be selected. The SeeFiberLaser software was used to simulate forward-pumped and backward-pumped amplifiers. In the case of forward pumping, the absorption of pump light was concentrated at the front end of the fiber, easily leading to gain saturation, with over 96% absorption observed at the 10 m position of the fiber. For backward pumping, the absorption of pump light was concentrated at the rear end of the fiber, resulting in more uniform gain distribution and less susceptibility to saturation, with an absorption efficiency exceeding 96%. Through B-integral analysis, the power integral in the fiber was smaller under backward pumping, which could suppress nonlinear effects.

A 2.5 kW narrow-linewidth polarization-maintaining fiber laser was constructed based on the simulations. The seed source adopted a fiber Bragg grating (FBG) resonant cavity structure with a 975 nm wavelength-locked semiconductor laser serving as the pump source. The pump light entered the polarization-maintaining narrow-linewidth fiber grating through a combiner. An isolator was installed between the seed source and the main amplification stage to protect the seed source, and a polarization-maintaining mode field adapter (MFA) was used to address the mode field mismatch. The main amplification stage employed backward pumping, with a 20 μm/400 μm double-clad polarization-maintaining ytterbium-doped fiber as the gain fiber (10 m in length), coiled in a racetrack configuration (with a diameter of 10 cm). The laser light was output through a polarization-maintaining fiber end cap after passing through a cladding light stripper (CLS). When the pump power reached 3.0 kW, the output power was 2.53 kW, with an optical-to-optical conversion efficiency of approximately 83.3%. The power curve showed linearity, indicating stable conversion efficiency. When the power exceeded 2.17 kW, the reflected light power increased rapidly. The 3 dB RMS linewidth of the seed source was 0.0242 nm (6.41 GHz), which broadened to 0.0860 nm (22.8 GHz) at full-power output. The central wavelength drifted slightly due to temperature changes. The M ² factors in the x and y directions were 1.344 and 1.348 respectively, approaching that of an ideal Gaussian beam, and the polarization extinction ratio was 16.38 dB.

Focusing on high-power narrow-linewidth polarization-maintaining fiber lasers, this study confirms through simulations that short fibers and large-mode-area structures can increase the SRS threshold, fiber coiling can optimize beam quality, and backward pumping can suppress nonlinear effects. Based on these findings, the constructed laser achieves an output power of 2.53 kW, with a beam quality of approximately 1.34, a 3 dB linewidth of 0.086 nm, and a polarization extinction ratio of 16.38 dB. It provides an ideal light source for applications such as beam combining and nonlinear frequency conversion.