Theory of forward stimulated Brillouin scattering in dual-mode single-core fibers

Recently, stimulated Brillouin scattering in a forward direction (FSBS) in a dual-mode single-core optical fiber was reported by the authors (see Electron Lett., vol.26 p.1195-6, 1990). Frequency shifts on the order of 17 MHz were seen in fiber supporting LP₀₁ and LP ₁₁ modes at 514.5 nm. The phenomenon is examined in detail, and the governing differential equations of the three-wave parametric process, (involving laser pump, Brillouin signal, and acoustic flexural-wave phonon) are derived and solved. FSBS is possible because although the overlap integral between a flexural fiber mode and the light is small, the phonon lifetime is much longer than in conventional stimulated Brillouin scattering (SBS). FSBS may also be the first example of a nonlinear effect which is enhanced by increasing the optical model area at constant pump power