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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.

We propose a vector mode conversion approach based on asymmetric fiber Bragg gratings (AFBGs) written in step-index fiber and vortex fiber, respectively. The mode coupling properties of AFBGs are numerically investigated. Compared to step-index fiber, the large mode separation in the vortex fiber is beneficial to extracting the desired vector mode at specific wavelengths. In addition, the polarization of incident light and the attenuation coefficient of index change distribution of the AFBG play critical roles in the mode coupling process. The proposed AFBG provides an efficient method to realize high-order vector mode conversion, and it shows great potential for orbital angular momentum multiplexing and fiber lasers with vortex beam output.

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