Laser-light transmission through the atmosphere

Abstract

Transparency of the atmosphere for laser radiation is one of the most important parameters in the laser monitoring equations, whose solutions enable extraction of quantitative information on the profiles of atmospheric gases and particles. The energy loss of a laser beam propa-gating through the atmosphere is mainly the result, as a rule, of the following simultaneously-acting phenomena: 1) molecular absorption; 2) molecular scattering; 3) particulate scattering. In this connection we shall now consider these phenomena, paying special attention to their quantitative descriptions; and will subsequently select the most effective laser wavelengths from the standpoint of energy losses under various meteorological conditions. The data given in this chapter refer to a wide range of wavelengths, including ultraviolet, visible, and infrared spectra of the electromagnetic wave scale. We are, however, mainly concerned with the region from approximately 0.2 to 20 lam, since beyond these limits optical radiation is nearly completely absorbed by small thick-nesses of the atmosphere. At short wavelengths, such absorption is due to oxygen and ozone, while at longer wavelengths, to water vapor. 3.1 Molecular Absorption 3.1.1 Basic Definitions Attenuation Coefficient. The attenuation coefficient a(v) of a medium for radiation of wavenumber v is a proportionality factor in Bouguer's law 1 characterizing transmission properties of the medium. Its units are conventionally cm-1 (linear attenuation coefficient), or cmZ/cm 3 (volume extinction coefficient); the former terminology preferred for transmission studies, and the latter for the remote sounding of specific volumes of the atmosphere. Written in differential form, Bouguer's law becomes, for a plane wave propagating along the z direction, ~/I(v) = -z(v)~(v, z)ctz,