Effect of a CO_2 laser pulse on transmission through fog at visible and IR wavelengths

Abstract

The extinction coefficients of laboratory generated fog at 0.63 and 10.6 μm are monitored during and after passage of a coaxial CO2 laser pulse of 2-J/cm2 fluence. Pulse passage causes a slight decrease in extinction at 10.6 μm and a marked increase in this quantity at 0.63 μm. This effect is consistent with the significant reduction in fog droplet size, caused by absorption of energy from the pulse. The data are analyzed to provide the time dependence of particle size following pulse passage, and the inferred particle growth rate is consistent with the mechanism of recondensation, onto droplets which survive the pulse passage, of water vapor driven from the fog droplets by absorption of pulse energy. For any aerosol whose particle size is significantly altered by laser pulse passage, the effect of the pulse on light extinction is determined by initial aerosol particle size and index of refraction as well as the wavelength of the light to be transmitted.