Strong-field four-wave mixing in an optically-pumped methyl fluoride laser

Continuously tunable coherent radiation in the 9.8-10.1 μm region has been obtained by pumping ¹²CH₃F and ¹³CH₃F with a 10-atmosphere CO₂ laser. Pulse energies up to 2.5 mJ were observed. The experimental conditions were similar to those used for optically-pumped Raman FIR lasers, and simultaneous emission of tunable FIR radiation was also observed. Under some circumstances, a fixed-frequency mid-infrared component was also present. A detailed theoretical analysis of the RFWM process that produces the mid-infrared emission is presented. It is based on a six-level density matrix model. The importance of FIR cascade and refilling transitions, as well as double-Raman transitions, is demonstrated. Contributions to the MIR gain resulting from both degenerate and nondegenerate parametric processes are analyzed. The pressure dependence of the MIR emission was studied, both theoretically and experimentally, and the possibility of pressure switching between tunable and fixed-frequency operating modes is discussed