Determination of lasing-level populations in a longwave $Q$-switched $СО_2$-gasdynamic laser

The feasibility has been shown of diagnostics of vibrational-level populations in the active medium of a coupled-mode $СО_2$-gasdynamic laser using the energy characteristics of pulsed operation in a Q-switched regime. The expressions have been derived for the determination of the initial population difference of lasing levels from the data measured. Experiments were performed on the $03^10—10^00$ ($\lambda$ = 18.4 μm) and $04^20—03^30$ ($\lambda$ = 17.2 μm) transitions in a $СO_2 —Аr$ gas mixture. An advantage of the method consists in that the use of the $Q$-switched regime allows one to exclude the interaction of vibrational transitions during lasing.