In this work, we propose a method to discriminate between upper and lower side material removal during double side polishing of fused silica parts. It consists in engraving cone-shaped craters on the two sides, then measuring the profile of each crater before/after a polishing run. The comparison of the profiles leads to the thickness removed on each side during the run. The craters have been engraved using a CO2 laser and their profiles measured thanks to a nano-scratcher. We have evaluated that this method can determine material removal with an accuracy of about 1μm, is insensitive to a part repositioning error under the tip of about 35μm, and has a repeatability of 0.5μm. Finally, we have been able to measure effective removal differences of 2μm between the two sides.
KEYWORDS: Laser ablation, Carbon dioxide lasers, Optical testing, Near field optics, High power lasers, Phase measurement, Visualization, Diffraction, Geometrical optics
The Laser MégaJoule (LMJ) is a French high power laser that requires thousands of large optical components. For all
those optics, scratches, digs and other defects are severely specified. Indeed, diffraction of the laser beam by such
defects can lead to dangerous “hot spots” on downstream optics. With the help of a near-field measurement setup, we
make quantitative comparison between simulated and measured near-fields of reference objects (such as circular phase
steps). This leads to a better understanding which parameters impact the diffracted field. In this paper, we proposed to
study two types of reference objects: phase disks and phase rings. We actually made these objects by CO2 laser ablation.
The experimental setup to observe the diffracted intensity by these objects will be described and calibrated. Comparisons
between simulations and measurements of the light propagation through these objects show that we are able to predict
the light behavior based on complete phase measurement of these objects.
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