total active length, i.e. twice the length of the sapphire tube exposed to the oven, was 91 cm and that the gas temperature ranged from 50 to 100°C above room temperature during typical runs which usually lasted less than 5 s. For the gain measurements, a premixed CO2-He laser mixture is introduced into the sapphire tube which is kept at constant pressure while optical pumping takes place. A typical run measuring optical gain is shown in Fig. 6 for a 9:1 gas mixture of CO2:He at a pressure of 1 Torr with a 1500 K oven, a gas temperature of 400 K and with 80 mW of probe beam power. The upper trace is the reference beam ZR of the laser power and the bottom trace is the signal beam I after traversing the gain cell. After the oven reaches its operating temperature, the oscilloscope trace is turned on, a CO2-He gas mixture is introduced into the evacuated sapphire tube, and the oven is moved along the rails by its control rod. Absorption of the probe beam at 10.6 /zm by the mixture begins and continues until the oven travels about one-half of the tube length. Thereafter, the signal beam intensity increases due to the radiative pumping of the CO2-He mixture. It reaches a maximum value approximately 1 s after the oven covers the tube completely. The oven remains in this position for another 1 to 1.5 s; during this time the intensity of the signal beam decreases because of the increase in the translational temperature of the gas mixture in the tube. The oven is then pulled back out, taking about 3 s to completely uncover the tube. During this time, the mixture is at a higher temperature than when it was first introduced; hence, the signal beam now reaches its lowest value. Thereafter, as the tube cools, the signal beam gradually increases in intensity and reverts to its original level after the tube is evacuated. The difference between the peak and initial signal intensities corresponds to the optical gain. Using an optical gain coefficient of y=1.8x 10 3 cm 1 is obtained on this experiment for /=91 cm. The preliminary measurements showed that the observed gain coefficients were less, by a factor of about 1.7, than the calculated small signal gain coefficients [15]. It
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