Role of the modal composition of pump in the multi-peak Brillouin gain spectrum in a few-mode fiber

We study stimulated Brillouin scattering (SBS) in a few-mode graded-index fiber in both the intermodal and intramodal configurations. A multi-peak Brillouin gain spectrum (BGS) is observed in both types of SBS interactions owing to the presence of higher-order acoustic modes, whose contribution to the Brillouin gain is found to depend on the modal composition of the pump beam. The numerically simulated results match well with the experimentally observed multi-peak BGS in both the intramodal and intermodal cases. The linewidth of the individual BGS peaks are extracted from a multi-Lorentzian fit to the data and are found to be significantly different for different modal compositions of the pump beam.