Intensity Profile Measurement of Facussed CO₂ Laser Beam by Acrylic Plastic

Intensity Distribution of High Power Laser Beam (Report 1)

Abstract

A method by which intensity distribution profiles of focussed CO₂ laser beam are determined from the shape of evaporated groove in acrylic plastic, polymethyl-methacrylate (PMMA), scanned across the beam has been developed. This is based on the fact that acrylic plastic is sublimated by focussed laser beam with negligible thermal conduction loss. In this paper, two essential thermal constants are determined; energy for evaporation H=3000 J/cm³, and threshold energy of evaporation G=6 J/cm². An equation is derived which correlates the beam intensity distribution curve to shape of the groove cross-section in PMMA scanned across the beam. An accurate intensity distribution has been experimentally shown to be obtained up to power density, at least, 10⁵5W/cm² unless the scanning speed v is smaller than a critical value given by
vmin=2W/√πa(2aH+G),
where 2a is 1/e-diameter of the beam, and W laser power.