Polarization maintaining, narrow linewidth fiber laser with> 1kW output power using a novel dual sine and noise modulation for enhanced SBS suppression

Stimulated Brillouin scattering (SBS) is the lowest threshold nonlinear effect that limits power scaling in narrow linewidth continuous wave fiber lasers. While several SBS mitigation techniques exist, optical linewidth broadening through external phase modulation has become the predominant method for SBS suppression. Arbitrary waveform generators (AWG) and pseudo-random binary sequence (PRBS) modulation schemes provide enhanced control of lineshape to effectively mitigate SBS but are expensive and difficult to implement. White noise (WNS) phase modulation is simple and easy to implement, but the resulting line shape is non-ideal and has a slow roll-off. Thus, the SBS enhancement obtained experimentally with WNS broadening is reasonably lower than the theoretical value. We attribute this to the increased SBS seeding due to the overlap between the WNS broadened signal and the Brillouin gain spectrum. A modulation scheme that is implemented easily and provides adequate line shape control would be of great advantage from a practical and engineering point of view. In this work, we propose a simple, yet powerful modulation technique to synthesize a line shape to have fast roll-off and improved flatness by incorporating dual sine and noise modulation. An in-house built kW-class, polarization maintaining, multi-stage Ytterbium-doped fiber amplifier is used to quantify the SBS enhancement of the proposed modulation scheme. We experimentally compare the results to that of pure noise broadened modulation at similar RMS linewidths and demonstrate over 2.3x enhancement in SBS limited output power at ~7.3GHz and <1kW SBS unlimited output power at ~10.4GHz in a fully polarization maintained system.