Nonlinear processes and laser damage induced by a temporally modulated nanosecond UV laser beam. Part I: stimulated Brillouin scattering

Abstract

Phase modulation is currently applied on pump laser beams propagating in thick optical elements to avoid optical damage induced by Brillouin backscattering. The experimental measurements show that adding a 2 GHz amplitude modulation to such a phase-modulated laser beam can compromise this optical preservation by increasing the Brillouin backscattering signal. These results are confirmed and explained with a theoretical analysis, developed in the perturbative regime, and with the numerical results obtained by solving three-wave coupling equations. Particularly, we show that when the frequency of the amplitude modulation is a harmonic of the phase modulation frequency, the Brillouin gain value is increased and can exceed the one obtained without amplitude modulation.