Investigation of Window Materials for Repetitively Pulsed CO2 Lasers

Abstract

A need exists to develop infrared window materials for use on repetitively pulsed CO2 lasers. Data obtained on single pulse damage criteria at 10.6 μm may not be sufficient to determine the response of materials under repetitive illumination. The optical, mechanical and physical properties of all potential transmissive optical materials at 10.6 μm were collected and compared. From this list, 19 optical materials were selected for detailed analysis using a set of baseline laser parameters. This analysis included pressure induced optical distortion, fracture criteria and thermal effects from laser energy absorption. A set of figures of merit were developed for axial and planar stress due to temperature profiles in the material and optical distortion (thermal lensing). Maps were developed to demonstrate the influence of the assumed laser parameters on these figures of merit. It was shown that no one optical material met all figures of merit. The three best materials based on this analysis are CVD ZnSe, GaAs and Polytran NaCl. Samples of these materials were fabricated and tested on a REP-pulsed CO2 laser at fluences between 15–25 J/cm2 with pulse trains up to 20 pulses at 20 Hz.