Abstract

There has been a recent growth of interest in dual-moded (DM) optical fibers for a variety of nonlinear switching and modulation schemes.^1-3 We report here what we believe to be the first observation of forward-stimulated Brillouin scatter (FSBS) between the LP₀₁ and LP₁₁ modes of a dual-mode (DM) fiber. Unlike normal SBS, forwhich the threshold power depends inversely on the laser linewidth, FSBS is easily observable with multimode light. This is because the pump (L) and Brillouin (B) waves propagate along exactly the same optical path and hence can maintain mutual coherence over kilometer lengths of fiber. Conservation of momentum in both cases is given by k_L⁰¹ = k_B¹¹ + K(Ω) and k_L¹¹ = k_B⁰¹ − K(Ω), where k_L^ij = ωn_ij/c, k_B^ij = (ω−Ω)n_ij/c, and K(Ω) = Ω/|v_a| = 2π/Λ, where v_a and Λ are the acoustic phase velocity and wavelength; the other parameters have their usual meanings. In FSBS the direction of the phonon wave depends on whether the frequency downconversion is LP₀₁ → LP₁₁ or vice versa (Fig. 1). In the DM fiber used, conventional SBS at 5145 nm yielded a shift of 32.4 GHz, which, because Λ = 0.176μm gives a phonon velocity v_a = 5702 m/s, in close agreement with v_ext = 5760 for silica. An Ar⁺ laser running single-mode at 5145 nm was used to deliver light to 500 m of DM fiber (Fig. 2). To verify true single-frequency operation, part of the laser output was split off and monitored directly at a photodiode and confocal Fabry-Perot. A shutter system allowed for independent monitoring of the light emerging from the fiber. After careful adjustment of the launching conditions, the frequency spectrum in Fig. 3 was obtained at the photodiode. A clear frequency component is apparent at 16.6 MHz; no such signal was present in the laser output. In FSBS intermodal beating excites a flexural wave with A equal to the beat period L_b, which couples the LP₀₁ and LP₁₁ modes by microbending. For our fiber parameters, L_b = 0.17 mm at 5145 m/s. At Ω/2π = 16.6 MHz, a fiber diameter d = 125 μm and a shear velocity v_t = 3764 m/s, Ωd/4πv_t = 0.274, corresponding⁴ to a flexural wave velocity v_af ≈ 0.5v_ext = 2880 m/s. The product of L_b and Ω/2π gives an independent estimate of v_af = 2822 m/s; the excellent agreement confirms that FSBS is taking place. For single-frequency excitation, the Brillouin gains are of the same order for SBS⁵ and FSBS; the ~10⁵× smaller modal overlap in FSBS is compensated for by an ~10⁵× smaller spontaneous acoustic absorption per acoustic wave length. The measured thresholds were ~45 mW/μm² for SBS and ~5 mW/μm² for FSBS.

© 1990 Optical Society of America