Infrared thermometry and interferential microscopy for analysis of crater formation at the surface of fused silica under CO2 laser irradiation

Abstract

In situ spatial and temporal temperature measurements of a fused silica surface heated by a 10.6 lm CO 2 laser were performed using an infrared camera. These measurements were derived from heat flux emission of the fused silica. High temperature measurements-in the range 400-2500 K-were performed at the surface of a semi-transparent media with a high spatial resolution. Particular attention was given to the experimental conception and to the calibration of the infrared device. Moreover, both conventional and interferential microscopes were used to characterize the silica surfaces after CO 2 laser irradiation. By associating these results with thermal camera measurements we identified the major surface temperature levels of silica transformation when heated during 250 ms. Surface deformation of silica is observed for temperatures <2000 K. This is consistent with other recent work using CO 2 laser heating. At higher temperatures, matter ejection, as deduced from microscope observations, occurs at temperatures that are still much lower than the standard boiling point. Such evaporation is described by a thermodynamical approach, and calculations show very good agreement with experiment. V

Evaluation of the fused silica thermal conductivity by comparing infrared thermometry measurements with two-dimensional simulations

“…Such behavior is expected, because strong silica ejection disturbs the IR measurement. 11 Since our simulation does not take matter ejection into account, its results are not expected to be valid for T > 2100 K. Analogous fused silica behavior has been recently reported at similar temperatures. 15 Since several different thermal conductivities have been published, we show a comparison with our resulting thermal conductivity in Fig.…”

“…To measure the temperature at their surface during CO 2 laser heating, IR thermography has been used, with the same operating condition as reported by Robin and co-workers. 11 Our CO 2 laser operates at 10.6 lm and presents a quasi-Gaussian shape with beam diameter of 700 lm at 1/e 2 . For the study presented here the laser pulse duration was fixed at 1 s, and its power, which is constant throughout the pulse, was varied between 3 W and 9 W on the sample's front surface.…”

Relevance of Carbon Dioxide Laser to Remove Scratches on Large Fused Silica Polished Optics

“…The form of the cracks visible in zone 1, independently of the finishing, indicates that this is a typical trailing indent scratch if we refer to the categorization made by Suratwala et al After the first heating by CO 2 laser (zone 2), we distinguish a central region with significant matter removal and a surrounding area where silica has been distorted by viscous flow, densification, or tensile surface forces . Then, we see obviously that even after the second heating (zone 3), the imprint of the initial scratch is still present although it was indiscernible with DMS characterization as mentioned in paragraph 4.…”

“…In that way, several works have been carried out to optimize the process . As, for example, thermal analysis of the laser silica interaction under CO 2 laser irradiation has been widely investigated with both experiments and simulations . A schema of the general succession of steps from laser irradiation to surface cooling is represented in Figure .…”

Removal of scratches on fused silica optics by using a CO_2 laser

“…The beam shape is Gaussian and the diameter is given at 1/e 2 . More details about the experimental arrangement can be found in reference [25].…”