Forward Brillouin scattering in few-mode fiber towards sensing

Forward Brillouin scattering is a nonlinear interaction between co-propagating optical waves and mechanical cladding modes. The effect has emerged as a new modality for fiber sensing of media beyond the cladding boundaries, where guided light cannot reach. Forward Brillouin scattering has been mostly examined over to single-mode fibers. Here we extend the study of forward Brillouin scattering to the few optical modes regime. We predict and observe the stimulation of new classes of mechanical cladding modes, with first-order and fourth-order azimuthal symmetries, which are inaccessible in single-mode fibers. The frequencies range of the process is extended to 1.8 GHz. We also report the stimulation of mechanical cladding modes above their cut-off frequencies, where they are not entirely transverse. Lastly, we demonstrate the detection of liquid outside the cladding of the few-mode fiber using the mechanical modes of first-order azimuthal symmetry, reached herein for the first time.