Abstract

The development of efficient (low pump power) optical amplifiers and sources based on rare-earth doped or other active fibers depends critically on the optical gain of the fiber, especially when three-level or low gain (e.g., nonlinear) processes are involved. In optically end-pumped fibers, the gain depends on optical energy confinement provided by the fiber and consequently on the fiber N.A. and core dimension. These dependences were investigated experimentally and theoretically to establish the range of fiber parameters which optimizes the gain. In Nd:SiO₂ fibers from different manufacturers, we have measured an optical gain ranging from 0.15 to 0.4 dB/mW. These data were analyzed with a model of gain in fibers accounting for energy confinement through pump and signal mode spatial overlap. It shows that the observed gain differences are largely due to differences in the fiber index profiles as opposed to differences in doping levels. This suggests that the fiber gain can be optimized by selecting the core size and N. A. in a well-defined range, predicted theoretically and supported by gain measurements. As much as a 2–4 gain increase (and equivalent reduction in the required pump power) over what we have observed may be expected from careful fiber design.

© 1988 Optical Society of America