Brillouin scattering induced by shear acoustic mode in a step-index fiber

We present the mechanism of backward Brillouin scattering induced by shear acoustic mode (SAM) in a step-index fiber. Unlike a longitudinal acoustic mode with negligible transverse displacement, a SAM has both considerable transverse and longitudinal displacements. During the light-sound coupling process, the fundamental and high-order SAMs can be guided and excited, ultimately generating a Brillouin gain spectrum with multipeak structure in a frequency range around 6 GHz. The interaction characteristics of the optical force with the displacement of all excited SAMs determine a partial cancellation effect, which is of great importance for the coupling coefficient of the optical-acoustic modes. The SAM-induced backward Brillouin scattering would provide a promising new approach for application such as multiparameter sensing. (C) 2020 Optical Society of America