Propagation of 632.8 nm and 4.67 μm Laser beams in a turbulent flow containing CO2 and H2O at high temperatures

Laser based missile defence systems (DIRCM) are being increasingly employed on aircraft. In certain circumstances the laser must pass through the exhaust gases of the aircraft engine. In order to predict the degree of divergence and dispersion of the laser, an understanding of the exhaust gas structure and its influence is required. Specifically the effect of parameters such as temperature, carbon dioxide, turbulence intensity and length scales as well as the laser beam wavelength and beam diameter. A parametric study under laboratory controlled conditions was undertaken to examine these effects. The results of beam propagation through high temperature turbulent flows and for various CO2 and H2O concentrations are presented in this paper for wavelengths 632.8 nm and 4.67 μm. It was found that the beam displacement showed an approximate inverse square relationship to temperature. At high combustion temperatures the 632.8 nm beam was significantly broken-up and dispersed. Displacement of both beams appeared to be asymptotic above 600 °C. Carbon dioxide absorption effects were found not to significantly influence the beam displacement at the wavelengths and temperatures studied. Quantifying these effects at high temperatures will assist with the development of a parametrically based laser beam propagation model. © 2008 SPIE.