High power laser beams propagating in the atmosphere are subjected to a variety of effects, the most important of which are absorption, scattering, turbulence induced beam spreading and wander, thermal blooming, and gas breakdown. In this paper simplified models are used to show how the various atmospheric effects interrelate and impact on the best laser choice for high power applications through their dependence on the laser wavelength and temporal mode (e.g., cw or pulsed) of operation. Results for sea level propagation at seven common laser wavelengths varying from 0.34 microm to 10.6 microm. are presented that show the mid-ir wavelengths to be favored for typical turbulence and aerosol scattering conditions. At the longer 10.6-microm CO(2) laser wavelength thermal blooming is dominant due to stronger molecular absorption, while at the shorter wavelengths turbulence induced beam spreading and aerosol absorption and scattering effects become important and tend to limit the increase in irradiance expected on the basis of diffraction effects alone.

High power laser propagation.

Atmospheric absorption of 10.6-μ radiation can either heat or cool the air, depending upon atmospheric conditions. Absorption by CO2 is essentially from the (100) to the (001) states. The depleted (100) state is rapidly replenished by energy transfer from translation, cooling the atmosphere. The (001) state slowly transfers energy through the N2 back to translation, eventually heating the atmosphere. Cooling increases the density and index of refraction, and the resulting gradient tends to focus a gaussian beam. This partially offsets the usual heating effects and associated ray divergence.

Effects of 10.6-mu Laser Induced Air Chemistry on the Atmospheric Refractive Index.

Thermal blooming of laser beams in liquids and gases has been studied using geometrical optics The intensity profiles, light rays, and fluid density changes are determined as functions of time and of the initial beam profile Short and long time behavior are derived The effects of thermal conduction and viscosity are included Special beam profiles such as a uniformly illuminated aperture, a parabolic profile, and a Gaussian beam are treated The on-axis beam intensity as given by the theory is compared to the data proved by Kenemuth et al and is shown to fit within the experimental error

Thermal blooming of laser beams in fluids.

Thermal blooming on laser propagation due to the presence of aerosol particles is considered. Emphasis is placed on the derivation of an effective absorption coefficient αeff(t), where αeff(t) corresponds to that part of aerosol absorption which heats up the air only. Becuase of the small but nonvanishing heat capacity of the aerosol particles, air is not instantly heated resulting in a small time delay in thermal blooming.

Effective absorption for thermal blooming due to aerosols

Appropriate correction of the initial phase of a laser beam is shown by numerical computation to be capable of appreciably reducing thermal blooming for a cw beam.

Phase compensation for thermal blooming.

Theoretical evaluation of the effects of absorption of a laser beam at 10.6 μ has been investigated using the techniques of geometric optics. The interaction is nonlinear because the refractive index depends, through the mechanism of absorption, upon the irradiance distribution in the propagating wave. Atmospheric absorption at 10.6 μ is caused by CO2 and H2O in the atmosphere. Associated with absorption by CO2 and transverse flow caused by atmospheric winds or beam motion are vibrational relaxation effects, which can either heat or cool the atmosphere. If the atmosphere is cooled, the beam is self-focused. The velocity-altitude dependences of the heating and cooling regimes are defined. The amount of cooling increases with increasing altitude and decreasing relative humidity. Numerical results for the irradiance distribution in each regime are presented. At low altitudes the initially circular contours of the constant irradiance have been deformed into crescent-shaped contours and the beam has deflected into the wind. At high altitudes where significant cooling has occurred, the contours of constant irradiance are oval shaped with significant enhancement of the peak irradiance.

M. Camac

Papers

Effects of Absorption at 10.6 μ on Laser-Beam Transmission