Forward Brillouin Scattering Spectra in Coated Single-Mode Fibers

The analysis of forward stimulated Brillouin scattering is extended to standard single-mode fibers with coating layers. The cutoff frequencies, linewidths, and transverse profiles of the guided acoustic modes are modified by the presence of the coating layer and by its mechanical properties. The solutions are generally more complex than those of bare, uncoated fibers. Nevertheless, the boundary conditions can be brought into the form of matrix coefficients and solved to obtain the guided modes of the coated fibers. The forward stimulated Brillouin scattering of coated fibers is highly sensitive of submicron variations in the thickness of the coating layer, due to interference effects. Some modes are more sensitive to such variations than others. In contrast to bare fibers, the forward Brillouin scattering linewidths of coated fibers do not maintain a one-to-one correspondence with the mechanical impedance of media outside the coating. The linewidths of different modes, with the same substance outside the coating, may vary greatly. That property makes the forward Brillouin sensing of media outside the coating a more challenging task. Nevertheless, sensing is possible given careful pre-calibration.