Influence of a transverse hypersonic inhomogeneity on the spectrum formed as a result of stimulated Brillouin scattering in single-mode fiber waveguides

Quartz glass fiber waveguides suffer not only from a transverse optical inhomogeneity but also from a transverse inhomogeneity of a hypersonic wave responsible for stimulated Brillouin scattering (STBS). Experiments on such waveguides revealed a considerable broadening of the STBS spectrum (compared with a homogeneous nonwaveguide medium) in the case of a single-mode waveguide characterized by antiwaveguide propagation of hypersound, i.e., when the velocity of hypersound in the core was higher than the velocity in the cladding. The width of the STBS spectrum depended weakly on the duration of the pump pulses and was governed by the nature of the transverse hypersonic inhomogeneity. The best agreement between the experimental results on the width of the spectrum and theoretical calculations was obtained for a waveguide with a central dip in the refractive index profile. Determination not only of the width but also of the profile of the STBS line yielded more detailed information on the refractive index profile.