NORMAL ACOUSTIC MODES AND BRILLOUIN-SCATTERING IN SINGLE-MODE OPTICAL FIBERS

Brillouin scattering in single-mode optical fibers has been investigated both theoretically and experimentally for a 180°scattering geometry and experimentally for a 90°scattering angle. The acoustic normal modes were derived and numerically evaluated for a cylindrical step-index fiber with both an infinite and a finite cladding. It was found that the phonon population which contributes to the Brillouin back-scattering spectrum is both narrower in range and shifted in frequency when compared to the corresponding case in the bulk medium. Back-scattering experiments were performed which yielded the intrinsic phonon lifetime for the longitudinally polarized excitations which are responsible for stimulated Brillouin scattering in the core of a 1.5-m (radius) fiber. Brillouin spectra were also observed at a scattering angle of 90°, and the widths of the spectral components were interpreted in terms of nonconservation of wave vector along the radial direction in the acousto-optic interaction. The consequences of these results to stimulated Brillouin scattering in optical fibers are discussed. © 1979 The American Physical Society.