Nonlinear stimulated Brillouin scattering in a single-mode optical fiber
  We predict and experimentally observe a nonlinear variant of stimulated Brillouin scattering in a single-mode fiber that arises from consideration of higher-order processes for phonon generation. The effect manifests itself at high laser excitation as the appearance of Stokes gain for a detuning equal to half of the conventional Brillouin frequency in the fiber, with no accompanying anti-Stokes absorption at the opposite detuning, and requires counter-propagating pump beams for phase-matching. We believe that this could be a new nonlinear optical effect that has not been observed before. 
K. Kieu, D. Churin, E. M. Wright, R. A. Norwood, and N. Peyghambarian
physics.optics