Effects of Precipitation on Propagation at 0.63, 3.5, and 10.6 Microns

It is well known that optical waves suffer much attenuation in propagation through precipitation in the atmosphere. Penetration of light through a dense fog is much more difficult than through a heavy shower. One would also expect similar phenomena for near infrared waves. However, quantitative data are necessary in order to evaluate the potential use of lasers for short range communication through the atmosphere. Attenuation by precipitation is the dominant factor in determining the feasibility of open-air laser communication systems. The interesting fluctuations of laser beams caused by turbulence in the clear atmosphere are not discussed here. 723 724 T H E BELL SYSTEM TECHNICAL JOURNAL, M A Y - J U N E 1968 The gas lasers which provide significant amounts of power within the infrared windows of the clear atmosphere are 3.5n and 10.6/r, 3.39fi is not suitable because it is attenuated by the small amount of methane in the air. 1 The wavelengths of visible gas lasers with suitable output power are 0.49/x and 0.63/x. Since the attenuation of optical waves in a fog is largely the result of scattering by numerous small drops, it is not likely t h a t transmission of 0.49/x and 0.63^. through a fog would differ appreciably. The literature reveals t h a t the attenuation of blue light (~0.49/A) through a light fog is somewhat greater than t h a t of red light (^0.63/x), while their attenuations in a dense fog differ little from one another. 2 In this study, 0.63/A is the visible wavelength used.