We report the highest average Stokes power using a single frequency methane filled Hollow Core Fiber (HCF) Raman amplifier to the best of our knowledge. HCFs guide light within their core and have high thresholds for detrimental nonlinearities. Gas filled HCF amplifiers use the long interaction lengths of the HCF to lase in hard-to-access wavelength bands with narrow linewidths. Results were obtained using a methane or deuterium filled HCF to convert 1.06 μm, nanosecond-scale, 0.52 mJ pulses to ≈1.55 μm. Optical-to-optical efficiency decreased at high pressures which analysis indicates is due to secondary Raman shifts. Measurements and simulations relating to the power scaling effort of the methane or deuterium filled HCF Raman amplifiers are presented.
KEYWORDS: Gas lasers, Optical fibers, Mid-IR, Fiber lasers, Signal attenuation, Resolution enhancement technologies, Molecules, Data modeling, Energy transfer, Molecular energy transfer
The output of solid core fiber lasers is constrained in the mid-infrared due to the absorption properties of silica. Optically pumped gas lasers can reach the mid-infrared but require long path lengths for interaction between the pump light and gain medium. Optically pumped gas lasers where the gain medium is contained in a hollow-core optical fiber may provide a robust and compact platform that combines advantages of fiber and optically-pumped gas lasers. Experimental demonstrations of gas-filled-fiber lasers have been reported. The energy output of a molecular gas laser operating in a hollow-core optical fiber is computationally modeled using rate equations. The rate equations include terms for various physical processes including molecular self-collisions, molecular collisions with the fiber walls, and fiber attenuation. The rate equations are solved for a time-dependent, one-dimensional fiber model with an acetylene gain medium that lases along rotation-vibrational transitions. The energy output and losses are computed for multiple configurations. Model correspondence with reported experiments is shown. The computed energy losses due to backwards propagating light, fiber losses, and molecular collisions are applied to pulsed, continuous wave, and synchronously pumped gas lasers operating in hollow-core optical fibers. Energy losses due to molecular collisions are used to estimate heating in the gain medium.
The physics of high brightness, high-power lasers present a variety of challenges with respect to simulation. The Air Force Research Laboratory is developing high-fidelity models for Yb-doped, Tm-doped, and Raman fiber amplifiers, hollow-core optical fiber gas lasers, and diode pumped alkali lasers. The approach to simulation and the physics specific to each laser technology are described, along with highlights of results, and relevant modeling considerations and limitations.
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