jtm) achieved with Ag overcoated metal mirrors is quoted (18) at 0.9938. Therefore, low-pressure cooling will be sufficient for the primary mirror. Because of the very large power densities incident upon the secondary mirror, high-pressure high-flow-rate cooling will be necessary. Oxygen-free, high- conductivity (OFHC) copper mirrors are the optimum choice under these conditions due to their high damage threshold. An enhanced-reflectivity dielectric coating (Optical Coating Laboratory, Inc.) is employed to minimize the absorbed power. Preliminary specifications of the transmitting optical system are given in Table 1. Sensing and correcting the optical figure of the large mirror is necessary to permit near diffraction limited performance of the optical transmitter and to affect maximum laser power interception by the receptor. Within the limitation imposed by the optical round-trip duration (0.285 sec), active alteration of the beam phase can be employed to correct for defocusing effects caused by atmospheric turbulence and thermal blooming. Only those physical mechanisms having characteristic time scales in excess of the roundtrip duration are subject to compensation. A number of coherent optical adaptive techniques (COAT) have been designed to optimize the laser power delivered to a target for a wide variety of scenarios. The approach of Berggren and Lenertz (15) is perhaps the simplest and most effective; a coherent source is located at the receptor and an interferometer, located at the transmitter, then measures the reverse beam as focussed by the large primary mirror providing the correction signals for focus and figure control. The primary mirror will probably be an assembly of semi-rigid faceplates rather than a large thin plate or membrane because of the necessity for active cooling. System considerations for implementation of this approach were briefly reviewed in Ref. (15). Atmospheric Transmission For the various line wavelengths associated with CO and CO2 lasers in the infrared, attenuation via molecular absorption is of primary importance (20). Molecular scattering is only significant for visible wavelength lasers and is completely negligible for CO and CO2 lasers. Aerosol scattering and absorption are less significant for CO and CO2 laser wavelengths propagating in clear air. Under hazy or overcast conditions, aerosol attenuation becomes significant, especially at lower altitudes. There is evidence, however, that multi-megawatt infrared lasers may be capable of hole-burning in various types of light clouds or fog (21). The effects of different meteorological conditions, viz., different aerosol distributions, are considered in the results which follow. The calculational procedure employs two standard-atmosphere models (Midlatitude Summer and Winter) (22). Molecular absorption coefficients for each laser line transition are calculated for the various atmospheric layers using the High Resolution Absorption Coefficient Code (HIRACC) (23). Absorption parameters of the atmospheric species required by HIRACC are obtained from the AFGL line parameter tape (24). The most recent version (October, 1978) of this line parameter listing was acquired for these calculations. The meaningful calculation of atmospheric molecular absorption requires an accuracy in the precision of each laser line wavenumber to <0.01 cm '. Considerable care has been exercised to obtain the best measurements of line frequencies (25-27). The two aerosol models used here are for clear and hazy meteorological conditions (28), corresponding to 23-km and 5- km-horizontal visibility at sea level and a wavelength of 0.55 pm. Results of the calculations are given in Tables 2, 3, and 4. Transmission efficien-
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