Brillouin Lasing in a Silica Microsphere With Allowance for Rayleigh Scattering

We demonstrate cascade Brillouin lasing in a silica microsphere with 11-GHz spectral spacing between neighboring orders. Using unidirectional pump, up to three Brillouin cascades are attained in both forward and backward spectra due to Rayleigh scattering. We show experimentally and numerically that for weak and strong intermode coupling, the system behavior differs. For weak coupling, first-order Brillouin waves are mainly generated in the backward direction while second-order waves are mainly observed in the forward one. For strong coupling, powers for each order are the same in both directions.