Thermal runaway in germanium laser windows

The absorption coefficient of 50-Omega cm germanium was measured over the 400-2000 cm(-1) wavenumber region for temperatures from 300 K to 450 K. Lattice absorption is negligible for wavenumbers greater than 900 cm(-1); at a wavelength of 10.6 microm the absorption is due entirely to free carrier effects. The thermal runaway characteristics of germanium windows on a CO(2) laser, operating at 10.6 microm, were studied. For each cooling mechanism used there was a well-defined critical value of power density (P(c)), below which the germanium temperature stabilized and above which runaway occurred. The highest value of P(c), obtained was 88 W cm(-2), by cooling the germanium below ambient.