Dependence of stimulated Brillouin scattering in multimode fibers on beam quality, pulse duration, and coherence length

We experimentally studied the threshold power for stimulated Brillouin scattering (SBS) in multimode optical fibers as a function of input beam product (a measure of the beam quality), using a Q-switched Nd: YAG laser producing short pulses with a coherence length of at least 8 m. Numerical simulations of the dependence of this threshold on pulse duration were performed to extrapolate the experimental data to the continuous-wave (CW) case. The CW SBS threshold was found to increase with increased beam product (decreased beam quality), with a threshold value of 1 kW, for example, for a beam product of 2.5-mm mrad at the input of a 100-mum core-diameter, 10-m-long fiber. When we compared our measured results wit published data for SBS in single-mode fibers recalculated to our core diameter, we found that our CW SBS power threshold values were two to four times higher. Using further numerical simulations, we extrapolated the SBS threshold for the long coherence length to the case of a short coherence length (of the order of centimeters) as in a high-power CW Nd:YAG laser for material processing. (C) 2003 Optical Society of America.