Abstract
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 M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> maintains lower than 1.3 in the power scaling process.
Highlights
High power linearly polarized all-fiber lasers or master oscillator power amplifiers (MOPAs) with narrow linewidth and near diffraction limited beam quality have important applications for nonlinear frequency conversion [1], spectral beam combining (SBC) [2]–[5] and coherent beam combining (CBC) [6]–[8]
We demonstrate a narrow linewidth linearly polarized all-fiber laser operating at maximum output power of 2.62 kW with a 3dB linewidth of 32 GHz by suppressing the self-pulsing effect
In recent years, researchers found self-pulsing has become a serious limitation for power scaling of narrow linewidth fiber amplifier based on white noise signal (WNS) phase-modulation
Summary
High power linearly polarized all-fiber lasers or MOPAs with narrow linewidth and near diffraction limited beam quality have important applications for nonlinear frequency conversion [1], spectral beam combining (SBC) [2]–[5] and coherent beam combining (CBC) [6]–[8]. Nonlinear effects (NLE) such as SBS [9], four-wave-mixing (FWM) [10], [11] and mode instability (MI) [12] effects have limited the power scaling of this type of narrow linewidth fiber laser systems. In 2008, Nufern company achieve a kilowatt linearly polarized laser output with a linewidth less than 10 GHz based on 25/400 μm LMA PM gain fiber and WNS phase modulation [27]. Ma et al based on three-level sinusoidal phase modulation and PM fiber amplifier, obtained a 1.89kW linearly polarized laser output with the linewidth of 45 GHz [20]. In 2019 [38], we reported a 1.5 kW PM fiber amplifier with 13 GHz linewidth and near diffraction-limited beam quality based on WNS phase modulation
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