Optical response of high-power laser windows-ultrashort pulse regime

The passage of a high-power laser pulse through a material window can result in optical distortion that significantly reduces the far-field irradiance. Optical distortion due to pulses having durations on the order of the acoustic wave propagation time through the thickness of the window is a result of heating by the absorbed laser energy and thermally induced plane-strain stress waves. For a given laser pulse duration and axially symmetric irradiance distribution, expressions are derived for the evolution of the window's aberation function. It is shown that, in this pulse regime, there is no stress induced birefringence and the evolution of the beam profile is such that the Gaussian focus always propagates towards the window aperture. Expressions are derived for the threshold for fracturing. It is shown that in the case of KBr fracturing occurs before lensing whereas for CdTe, Ge, and TI-1173 glass, lensing occurs before fracturing.