Stimulated Brillouin scattering threshold in double-clad fibers with various cladding shapes

Abstract

We present a detailed numerical investigation about the stimulated Brillouin scattering (SBS) threshold power of double-clad optical fibers having different inner-cladding shapes. Generally, fibers with broken symmetry inner-cladding are usually employed in fiber lasers and amplifier in order to enhance pump light absorption by the doped core [1]. The acoustic response of such geometries is not known because their numerical characterizations are complicated. Even more difficulties occur when the fiber possess an acoustic antiwaveguide core since in that case the Brillouin response is often ruled by very high-order modes [2].We have recently developed a numerical tool which does not rely on any kind of symmetry of the fiber structure and is able to compute all the supported acoustic modes [3] in any arbitrary geometry. Here, we apply this tool to investigate the effect of the cladding shape of fiber on the SBS threshold power.In order to highlight the impact of the geometry, we have kept fixed the inner cladding area so that the amount of material and the number of modes stays the same. Our reference fiber is a circularly symmetric fiber with an inner cladding area fixed to 7088 µm 2 corresponding to a 95 µm diameter cladding. Its 5-µm core is a step index with a 0.2 NA behaving as an acoustic waveguide, i.e. a fiber for which the acoustic velocity is higher in the inner cladding than in the core. All the other fibers considered have an acoustic anti-waveguiding core with the two values of the acoustic velocity being reversed.