The far infrared (~1 THz), infrared (~10 THz), and visible (~100 THz) portions of the electromagnetic spectrum have been utilized for many applications. Figure 3 shows that, for a dry atmosphere, the total zenith attenuation is only about 1 dB at 1 THz. This value of zenith attenuation is fairly constant for frequencies through visible light (100 THz). A ‘‘window’’ in the standard atmosphere exists around 890 GHz, the frequency of HCN masers, but the zenith attenuation at this frequency is still two orders of magnitude higher than that in the dry atmosphere (i. e., 100 dB versus 1 dB total zenith attenuation). For all three frequency ranges, the key attenuating elements in the atmosphere are aerosols, usually cloud and fog droplets. Thick fog (liquid water density « 0.5 g/m3) will cut visibility to 50 m (19). Most free-space laser links are therefore short-range (<1 km) unless extraordinary powers are contemplated, such as the ~1 MW chemical oxygen iodine laser (COIL) lasers planned for missile interception. At such huge power levels, though, the almost instantaneous heating process caused by the passage of the beam through the atmosphere will modify the constituents along the path, thereby
altering the transmission characteristics significantly.