APPARATUS FOR AND METHOD OF PRODUCING A POPULATION INVERSION IN A FLOWING GASEOUS LASER MEDIUM

摘要

1308472 Lasers AVCO CORP 8 May 1970 22458/70 Heading H1C In order to provide population inversion in CO 2 gas the gas is heated and then expanded through a nozzle. The arrangement is based on the fact that the polyatomic CO 2 , has several modes of vibration and the times required for equilibrium with translation and rotation of the modes can be quite different and this difference in vibrational relaxation times of separate modes allows the production of a complete population inversion between vibrational levels of different modes. After heating, the polyatomic gas is expanded through a supersonic nozzle such that the flow time in the nozzle is short compared to the effective relaxation time of the atoms in an upper lasing level (asymmetric mode of vibration) and long compared to the effective relaxation time of atoms in a lower lasing level (symmetric mode of vibration). As the CO 2 gas passes from subsonic to supersonic velocity in the nozzle the temperature and density drops rapidly and since the upper level vibrational energy is not able to follow this rapid temperature change it remains at a high level whereas the lower level energy remains close to equilibrium, i.e. the downstream temperature and population inversion results N 2 /CO in accordance with the invention is added to the CO 2 to lengthen the effective relaxation time for the upper lasing level and H 2 O vapour to decrease the effective relaxation time of the lower level. In the master oscillatoramplifier system shown in Fig. 9, CO 2 gas together with suitable auxiliary gases is heated in chamber 13 and then expanded to supersonic velocities through nozzle 14, Figs. 12, 13 (not shown), into chamber 15. Input beam 16 from N 2 /CO 2 oscillator 12 amplified in an intermediate amplifier, not shown, is passed through the population inverted amplifier 11 via windows 17, 18 to obtain amplification. A folded mirror arrangement may be used to increase the path length in the amplifier 11. In an alternative arrangement, Fig. 10 (not shown), cyanogen C 2 N 2 and a mixture of oxygen, nitrogen and hydrogen are fed into a combustion chamber where they are burned to give a mixture of N 2 /CO, CO 2 and H 2 O in complete equilibrium. The burned gases are then expanded through a supersonic nozzle into a divergent chamber where the CO 2 exists in a state of population inversion. Windows may be provided in the walls of the divergent chamber, as in Fig. 9, if the arrangement is to operate as an amplifier. If the arrangement is to operate as an oscillator the windows are replaced by mirrors, the output mirror being in the form of an optically polished copper surface provided with a plurality of apertures for energy extraction. Operating parameters of the device are discussed in the Specification.