Threshold of the second Stokes scattering in a fiber stimulated Brillouin scattering beam combiner

Second order stimulated Brillouin scattering in optical fiber was investigated experimentally and theoretically with an eye toward determining the operational dynamic range of SBS fiber beam combiners. Experimentally, we found that the threshold for the second Stokes scattering is almost twenty times that for the first Stokes scattering. This dramatic increase in the second Stokes threshold is largely due to the decreasing L_eff with increasing pump power as seen by the second Stokes beam. The increase in threshold was verified by using an angle cleave at the entrance facet of the fiber to suppress the internal reflection of the first Stokes beam. At maximum pump power available, more than twice the original second Stokes threshold, no second Stokes beam was detected. Theoretically, we modeled the transmitted power in the pump, Stokes, and second order Stokes beams resulting in good fits to the experimental data. Finally, we predict the second order Stokes threshold in the absence of a reflected Stokes signal to be more than 80 times the first order threshold. In all cases, we find that the second order SBS not to be a significant limiting factor in the operational dynamic range of SBS fiber beam combiners