and (100) levels, respectively, and AvD is the full line width of the transition. Predicted and experimental gain values are evaluated at 2=10.56 /zm corresponding to the P(16) transition. At this wavelength t106 has a value of 5.38 s [28], The mean-free-path (mfp) of the pumping photon before it is absorbed, 2mfp, is defined as the reciprocal of the absorption coefficient. The zmfp for the Doppler- broadened case at line center of the 4.3 /zm transition is given in Ref. [15] for various P-branch transitions as a function of total pressure. At a total pressure of 1.0 Torr, 2mfp is about 1 mm. Noting that the absorption coefficient is exactly half of its maximum value, the optical mean-free-path at half-bandwidth is then 2 mm. The significant parameter is the ratio of zmfp to the characteristic dimension of the container. If this is unity or greater, the medium may be considered optically thin. 3.3 Gain Measurement and Discussion To demonstrate the blackbody pumping concept, an experiment was performed using the apparatus described in Ref. [15] (see also Refs [26] and [27], in which solar pumping is simulated by a graphite oven heated by a Globar helix. An evacuated cylindrical aluminum chamber contains the blackbody radiator at one end and a cantilever-mounted 5 mm i.d. sapphire tube at the other end. The oven is mounted on rails and can be moved over the sapphire tube which contains the gas mixture. The sapphire container has a built-in mirror at its free end and a sodium chloride Brewster window at the supported end. A conventional CW 10.6 /zm CO2 probe laser, operating on the P(16) line at 10.56 /zm, is used to obtain measurements. The optical apparatus shown in Fig. 5 is arranged so that the probe beam is chopped and sent through a germanium beam splitter. Only the signal beam traverses the tube. It is reflected by the mirror of the tube, and its variation under the blackbody radiation is detected and monitored simultaneously with the reference beam on a dualbeam oscilloscope. In addition, another signal from a switch which monitors the oven's position is recorded on the same oscilloscope. Through calibration measurements, both the active optical path length and FOO AT t'lirxo ntr/l rvnri rtTArA I I Cl It
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