Change in Focal Length of ZnSe Lens During Focusing CO₂ Laser Beam

Thermally Indnced Optical Distortion in Lens at High Power Laser Level (Report 2)

Abstract

Change in focus position and focus power density caused by thermally induced optical distortion were analyzed theoretically and experimentally by physical optics and measurement using a beam measurement system, respectively, when high power CO₂ laser beam having Gaussian intensity profile is focused by edge-cooled ZnSe lens. A simple equation was also derived to predict steady focus change on the basis of geometrical optics. Results obtained are summarized as follows:
(1) Focus position was determined at a precision of ±0.2 mm by means of the beam measurement system using a slit probe and a pyroelectric detector, even when power ripple of incident laser beam is as high as 30%.
(2) Focus change is predicted on the basis of the physical optics and the thermal conduction theory, when absorptivity of the lens and intensity distribution of incident beam are known.
(3) At absorption powers less than 5W, original power density can be almost recovered by adjusting focus position of lens having thermally induced optical distortion. However, at absorption powers higher than 10W, power density cannot be recovered by focus adjustment any more.
(4) Thermally induced optical distortion makes the slope of power density along optical axis smaller at lens side, which is opposite to the case of spherical aberration, and also focus depth deeper.