New techniques for determining vibrational temperatures, dissociation, and gain limitations in CW CO<inf>2</inf>lasers

Abstract

We have developed an accurate method of determining vibrational temperatures and populations in CO 2 laser discharges. Our technique involves the use of both the regular 00 ° 1 and sequence 00 ° 2 laser transitions as probes of a CO 2 laser amplifier. We have been able to separately investigate the quantitative effects of gas heating, dissociation, and ν 3 mode excitation efficiency on the small-signal gain in typical CW CO 2 lasers. In general we find that the maximum gain attained in a typical flowing gas CW CO 2 laser is limited by dissociation of CO 2 at high discharge currents. To investigate the more fundamental limitations on the gain, we used a short discharge tube with fast flow rates. Contrary to many previous results, we find that thermal effects play a somewhat secondary role in the discharge dynamics, and that the lower laser level populations are small under all discharge conditions. Our results show that the chief limitation on gain in CW CO 2 lasers is the of the ν 3 mode vibrational temperature T 3 at high discharge currents. This saturation effect is observed for a wide range of gas mixtures and pressures, and has been studied in detail. Gain coefficients as high as 3.3 percent/cm have been obtained in a conventional 1-cm bore CW discharge tube. We also report preliminary results of an experiment which uses a tunable diode laser to measure gain on a large variety of transitions in a CO 2 discharge. The diode laser measurements give a striking confirmation of the results described above, and provide the first direct experimental evidence that a Boltzmann distribution exists in the vibrational modes of discharge excited CO 2 .