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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&#x0025; is demonstrated. The polarization extinction ratio (PER) is measured larger than 14 dB and the beam quality factor M<sup>2</sup> maintains lower than 1.3 in the power scaling process.

2.5 kW Narrow Linewidth Linearly Polarized All-Fiber MOPA With Cascaded Phase-Modulation to Suppress SBS Induced Self-Pulsing