Characterization and optimization of the gain in Nd-doped single-mode fibers

A model of gain in four-level laser fibers involving modal overlaps is presented, together with measurements of the gain near 1060 nm in Nd-doped glass fibers from different manufacturers. The measured fiber gain ranged from 0.2 to 0.42 dB/mW of absorbed pump power. Using their model, the authors show that these differences are quantitatively explained by differences in the degree of confinement provided by the fibers, i.e. by differences in the core size and numerical aperature (NA). This analysis suggests a stimulated emission cross section of 1.0×10^-20 cm² for this transition in a silica-based host. Based on this model, a gain as large as 1 dB/mW, or over twice as much as reported to date, is expected with proper selection of the core dimension and NA