Fig. 3. Output spectrum of a cw, cryogenically cooled (77 °K) CO laser with line selection (11). cies were calculated for P- and R-branch midrotational transitions of isotopic CO2 (those lines capable of the largest discharge efficiency) and for the CO spectrum shown in Figure 3. Transmission efficiencies for CO-laser lines of importance are also listed individually in Tables 3 and 4. Most of the CO radiation absorption occurs at the lower altitudes due to water-vapor absorption, and it is highly inadvisable to place a CO-laser receptor site at an elevation less than 0.5 km. A significant improvement in CO-laser transmission efficiency is realized by high-altitude receptor operation, while for the 00°l—>02°0 R-branch lines of l2CIKO2, less improvement is to be gained, since the primary absorbing specie (natural CO2) is uniformly distributed in the atmosphere. For the CO laser lines, the transmission efficiency improves during the winter because of a decrease in humidity. At an elevation of 0.5 km, the yearly average transmission efficiency for the line-selected CO-laser spectrum is 84%. Mountaintop reception at an elevation of 3.5 km increases this value to 97%. These results are for receptor sites which are not subject to persistent overcast conditions. Hazy or overcast conditions have less of a degrading effect on transmission efficiency as the receptor-site elevation is increased. The yearly average transmission efficiency for the 9.114 /j.m ,2C'KO2-laser line to an elevation of 0.5 km is approximately 92% for clear air conditions. Mountaintop reception at an elevation of 3.5 km increases this value to about 98%. Transmission efficiencies for the CO2 laser lines were only computed using the Midlatitude Summer model since these conditions represent worst-case performance. Receptor Concepts A number of schemes for conversion of ir laser laser energy into electricity exist,
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