What causes CO2 laser optics to heat ?

Around 1995 Laser Beam Products partnered with some commercial companies and universities (University of Surrey and the Laser Zentrum Hanover) to investigate the mechanism of CW CO2 laser induced damage to mirrors and lenses. The results were published at the Laser Damage Conference held in Boulder Colorado, but that paper seems almost impossible to find on the internet. I’ll post the abstract that was published by SPIE at the time at the end of this blog

There were many interesting findings, one was that laser induced damage to optics is often the result of external contamination. So what materials should be kept away from CO2 laser mirrors and lenses exactly?.

Well one day I used our CO2 laser calorimeter to measure the excess absorption from potential materials that laser cutting optics could be exposed to such as swarf, plastic fumes, vacuum oil etc. It only took me a few hours, and I would have liked to have turned it into a decent bit of scientific work, however  the results were interesting, and you can read them on our website Contamination of CO2 laser optics. Lets just say……. “Beware Heat Transfer Paste !”.

Abstract of SPIE publication at Laser Induced Damage in Optical Materials Conference

The measurement of intrinsic laser induced damage thresholds (LIDT) in optical components for continuous wave (CW) CO2 radiation has been investigated. A combination of analytical and numerical models showed that the temperature rise is mainly determined by the surface absorption in transmissive as well as reflective components, and is proportional to the ratio of power to linear dimension (P/d) of the irradiated spot rather than to the conventional power/area (P/d2) parameter. The former ratio therefore represents the correct power scaling law for LIDT measurement in CW laser systems. The precise time domain within which this law holds is a function of spot diameter. This prediction has been confirmed by experimental LIDT tests on well characterized uncoated ZnSe substrates and copper mirrors, and on coated ZnSe windows and copper mirrors. Measured P/d values, though lower than predicted by modelling are considerably higher than those inferred from the technical literature, and show that transmissive components may be used at much higher powers than are at present believed. The results indicate that surface absorption occurs primarily in the sub-surface processing layer. This has been shown by transmission electron microscopy and spectroscopic ellipsometry to be a few hundred nm in depth.

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