Low numerical aperature large-mode-area neodymium-doped fibers fabricated by SPCVD and ASD for laser operation near 920nm

We present the fabrication by Surface Plasma Chemical Vapor Deposition (SPCVD) and All Solution Doping (ASD) of step-index Nd-doped fibers with a 30/125μm (core/cladding diameters) geometry and a low numerical aperture (NA) near 0.05. A phospho-alumino-silicate (SiO₂-Al₂O₃-P₂O₅-Nd₂O₃) core composition was used to reduce the formation of Nd³⁺ ions clusters while keeping a low refractive index through the peculiar AlPO₄ chemical complex. Operated in CW laser regime, a 0.053 NA fiber generated up to 17W of output power at 921nm, limited by the available pump power at 808nm (51W), yielding a 37% power conversion efficiency. The profile of the output beam for a bend diameter of ~12cm is gaussian and nearly diffraction limited (M² ~1.1). This is in good agreement with the large discrepancy, in terms of calculated bending losses, between the fundamental LP₀₁ and the higher order modes.