Path for significant SBS suppression in fiber amplifiers

Abstract

In recent years, a key approach for SBS suppression in fiber amplifiers for high power single frequency fiber lasers is based on designs of transverse acoustic properties in fibers. Although this approach provides some SBS suppressions, our new analysis considering the often omitted leaky acoustic modes demonstrates that it is fundamentally limited to SBS threshold increase of less than ten-fold in the most ideal case. We have established an acoustic mode solver based on simultaneous longitudinal and shear acoustic wave equations and rigorous boundary conditions. This enables us not only the capability of studying designs of arbitrary acoustic velocity profiles, but also finding leaky acoustic modes with their waveguide losses. In this work, we will demonstrate the applicability of acoustic modes and impact of leaky acoustic modes in SBS in fibers, especially those with transverse acoustic velocity profiles designed to suppress SBS. With consideration of the leaky acoustic modes, we have found that design of transverse acoustic velocity profiles only has a very limited SBS suppression in fibers with even the most optimized acoustic anti-guides. Longitudinal acoustic property profiling, on the other hand, can potentially provide well over two-orders of magnitude SBS suppression.