Nonlinear FM-AM conversion due to stimulated Brillouin scattering

We report an effect potentially harmful occurring in regenerative amplifiers due to stimulated Brillouin scattering (SBS). Most high energy laser facilities use phase-modulated pulses to prevent the transverse SBS effect in large optical components and to smooth the focal spot on target. However, this kind of pulse format may undergo a detrimental effect known as frequency modulation to amplitude modulation (FM-AM) conversion in the presence of spectral distortions. In the present letter, we show experimentally and numerically, that SBS can also potentially be created in the regenerative amplifier located in the front-end. In this scenario, some of the side bands of the pulse reflected by regen end-cavity mirror may act as a seed for SBS in an optical component, if the pulse spectrum contains frequency components exactly separated by the Brillouin frequency shift. This self-seeded SBS induces amplitude modulation with a nonlinear dependence that may be detrimental during down-stream propagation. However, we show that a careful choice of the modulation frequencies can mitigate this effect.