Towards optical quality yttrium aluminum garnet (YAG) fibers: recent efforts at AFRL/RX

Abstract

Traditional silica fibers currently are unlikely to be able to sustain the powers needed for future Air Force applications. The low thermal conductivity of silica makes it difficult to control thermal gradients within the fibers resulting in failure or degradation in beam quality. While some of these problems can be ameliorated by using longer fibers, this results in problems with nonlinear effects such as stimulated Raman and Brillouin scattering (SRS and SBS). Yttrium aluminum garnet (Y3Al5O12, YAG) has the potential for overcoming these problems due to 1) higher thermal conductivity, 2) reduced thermal lensing, and 3) higher SBS threshold. Polycrystalline YAG has been demonstrated to be a highly efficient and economical laser host material in slab form. Polycrystalline YAG can be doped more uniformly and at higher levels than single-crystals with no dopant loss by zone refinement, has higher fracture toughness than single-crystals, and supports higher power densities. Despite the anticipated advantages, polycrystalline YAG has never been demonstrated in high-power fiber lasers. The development and characterization of YAG fibers for high energy laser applications is the primary goal of our research. Recent results in the processing of optical quality polycrystalline YAG fibers will be presented and discussed.