Numerical investigation of pulsed gas amplifiers operating in hollow-core optical fibers.

Optically pumped molecular gas amplifiers having a gain medium contained in a hollow-core optical fiber are investigated with numerical modeling to understand the primary physical processes that affect amplifier output and efficiency. A comparison of computational results with experimental measurements of incident pump, absorbed pump, and emitted mid-IR from a pulsed, acetylene-filled, hollow-core fiber amplifier [ Opt. Exp.25, 13351 (2017)] is used to explore the effects of various physical processes on pulsed amplifier operation. Single frequency, one-dimensional, time-dependent models are shown to align with experimentally measured lasing thresholds and ratios of absorbed pump to emitted laser energy but significantly over predict the amount of incident pump energy that is absorbed. A two-dimensional, time-dependent model that assumes Gaussian spectral and radial intensity profiles for the pump is developed and shows an improved ability to capture pump absorption. Results indicate that 1D, time-dependent models have utility in guiding experiments but the significant influence of the pump and laser propagation modes and the pump spectral characteristics on efficiency, threshold, and signal output must be explicitly included in high-fidelity high-power modeling.