Discussions and experiments about sampling rate dependency in piecewise parabolic phase modulation for stimulated Brillouin scattering suppression in fiber amplifier

Piecewise parabolic phase modulation, in which the optical spectrum linewidth is equal to the product of its chirp rate and period, is believed to be promising in SBS suppression, as it can yield a nearly flat-top optical spectrum. However, no kilowatt-level laser output has been achieved with this scheme because of its high sampling rate dependency on arbitrary waveform generators. In this paper, its high dependency on high sampling rate is theoretically discussed based on fast Fourier transform. We propose and experimentally validate two approaches to mitigate the dependence on sampling rate: extrema clamping, in which the local maxima and minima of the waveform are reassigned, and increasing the modulation depth as a means of further relaxing the sampling rate requirement. We established an amplifier system which includes a piecewise parabola phase-modulated single-frequency fiber laser, a two-stage pre-amplifier, and a counter-pumping main stage, subsequently. Finally, a 7.56-GHz piecewise parabola phase-modulated, 1750-W MOPA fiber amplifier, is experimentally achieved.