Pressure-induced optical distortion in laser windows

Abstract

Theoretical studies are made of the effects of pressure on candidate materials for use as windows in high‐power infrared laser systems. When the window in a laser system is subjected to a uniform static pressure, the resulting deformation can distort the laser beam or fracture the window. Two aspects of this problem are considered: pressure‐induced optical distortion and the effect of including pressure in previously calculated figures of merit for thermally‐induced optical distortion. An expression for the window thickness required to prevent pressure‐induced optical distortion from halving the target intensity is derived, and values are tabulated for various materials. Comparing these values with the minimum thickness required to prevent material fracture indicates that the thickness of large‐diameter windows of strong materials is determined by the pressure‐induced optical distortion, whereas the thickness of small‐diameter windows of strong materials is determined by the pressure‐induced fracture. The optical distortion resulting from the static deformation of a window under its own weight is negligible with respect to that from a pressure of 7.3 psi under usual operating conditions. An expression obtained for the strength of a material at which the fracture and pressure‐induced optical‐distortion effects are equally important should be useful in material‐strengthening programs. In previous figures of merit, various materials were compared on the basis of a common fixed value of the window thickness. In many applications, where the window diameter and the pressure have specified values, new figures of merit are required in order to account for the greater thickness required for weaker materials. The new figures of merit, which are simply the values of intensity the window can transmit under specified conditions, change the preference of materials in a number of cases.