We demonstrate an all-fiber narrow-linewidth amplifier employing a bidirectional pump scheme and cascaded white-noise-source phase-modulated seed laser. The stimulated Raman scattering effect in the amplifier is investigated by substituting different types of seed lasers. The influence of pump distributions and seed injection power on mode instability (MI) in the amplifier is also experimentally investigated. As a result, a 3 dB linewidth of 0.175 nm and a beam quality of M 2 ≈ 1.5 are obtained at the output of ~3 kW, without observation of MI and stimulated Brillouin scattering effect. With the further increase of pump power, MI occurs as the output exceeds 3.17 kW, along with beam quality degradation. Optical efficiency decreases to 71.5% at the ultimate output of 3.5 kW. Therefore MI becomes the main limitation to further power scaling.

As a major limitation for power scaling of high power narrow linewidth fiber master oscillator power amplifiers (MOPAs), Stimulated Brillouin Scattering (SBS) induced selfpulsing 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 M 2 maintains lower than 1.3 in the power scaling process.

In this paper, we demonstrate a high-power, narrow-linewidth, polarization-maintaining fiber amplifier with near-diffraction-limited beam quality. By optimizing the phase modulation signal, a nearly top-hat-shaped spectrum was generated for self-pulsing suppressing. That results in doubling the self-pulsing threshold we got from conventional white noise signal phase modulation with the same optical linewidth. Based on an optimized signal and a high power, polarization-maintaining, counter-pumped fiber amplifier, we obtain a 3.25 kW narrow-linewidth linearly polarized laser output with a linewidth of ∼ 20 G H z , the polarization extinction ratio is about 15 dB, and the M 2 is less than 1.22 at the maximum output power. To the best of our knowledge, this is the first demonstration of a narrow-linewidth, linear polarization, all-fiber amplifier with 3.25 kW laser output.

In this work, we realize a high-power, narrow linewidth, linearly polarized laser output based on a compact all-fiber polarization maintaining (PM) master oscillator power amplifier. The seed is a fiber oscillator laser (FOL) with few-longitudinal-mode for spectral broadening suppression. The spectral broadening characteristics in the FOL seed and in the PM-amplifier with different pumping schemes are studied experimentally and theoretically. The effect of different types of fibers in the amplifier on spectral broadening is also analysed theoretically. Finally, we demonstrate a narrow linewidth linearly polarized all-fiber amplifier operating at the maximum output power of 3.08 kW with a 3 dB linewidth of 0.2 nm. The polarization extinction ratio is measured to be larger than 93.5% and the M 2 maintains lower than 1.45 in the power scaling process. To the best of our knowledge, this is the highest demonstrated output power for narrow linewidth linearly polarized all-fiber lasers with the charactristics of low cost and a compact structure.

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