The polarization-dependent transverse mode properties in a few-mode nonpolarization-maintaining (non-PM) fiber amplifier with PM fiber laser seeding are investigated. In addition, the effect of spectral broadening on degeneration of the degree of polarization (DOP) is studied. The experimental results show that a high DOP can be achieved if the output laser of the non-PM fiber amplifier has good beam quality and a narrow linewidth without any broadening. A kilowatt (kW) fiber laser system with PM fiber seed and a non-PM fiber amplifier is demonstrated with the DOP at around 90% by controlling the beam quality and spectrum broadening. With the active polarization control technique, the system can achieve a kW linearly polarized output with 6.5 GHz linewidth. The $M^2$ is around 1.1, and the polarization extinction ratio is about 14.5 dB. Unfortunately, the random self-pulsing issue arose when the output power exceeded 1093 W. We believe that a higher power of linearly polarized laser output based on adaptive polarization control in a non-PM fiber amplifier will be obtained if the self-pulsing issue is solved.

As a major limitation for power scaling of high power narrow linewidth fiber master oscillator power amplifiers (MOPAs), Stimulated Brillouin Scattering (SBS) induced self-pulsing in polarization maintaining (PM) fiber amplifiers is well characterized and analyzed in this paper by comparing different white noise signal (WNS) phase-modulated modes in experiments. It is found that the self-pulsing effect is not observed in the PM-amplifier with single-frequency laser seed injection, and cascaded WNS modulation provides superior self-pulsing suppression than single WNS modulation with similar output linewidth. Moreover, the experimental results indicate that the self-pulsing threshold can hardly be predicted only by the output linewidth or the defined SBS threshold in a WNS phase modulated fiber amplifier system. As self-pulsing is originated from the spectral spikes in WNS modulated system, we theoretically analyzed characteristics of these spikes in different phase-modulation modes. It indicates the spectral peak intensity can be reduced by cascaded modulation, for which self-pulsing can be suppressed. The theoretical predictions agree well with the experimental results. At the same time, in order to suppress the mode instability effect, a plum blossom shaped bending mode selection device is used in this high-power narrow linewidth fiber amplifier system. Finally, a 32 GHz cascaded WNSs modulated, over than 2.5 kW linearly polarized all-fiber amplifier with a slope efficiency of 86.7% is demonstrated. The polarization extinction ratio (PER) is measured larger than 14 dB and the beam quality factor M2 maintains lower than 1.3 in the power scaling process.

We present a reliable and all-fiberized single-polarization, high-order mode fiber laser. The experimental setup employed polarization-maintaining ytterbium-doped fibers and a combination of different fiber Bragg gratings to achieve high mode purity and stable output. The system achieved a maximum output power of 3.8 W, a polarization extinction ratio (PER) of 96.7%, and a mode purity of 95.32% for the LP₁₁ mode. Furthermore, the laser demonstrated notable stability; during a 120 min stability test, the standard deviation of the output power was measured at 0.15%, while the standard deviation of the polarization extinction ratio (PER) was 0.07%. This work offers a reliable solution for the generation of stable, high-purity LP₁₁ mode lasers.