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19 August 1996 Large-area damage testing of optics
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The damage threshold specifications for the National Ignition Facility will include a mixture of standard small- area tests and new large-area tests. During our studies of laser damage and conditioning processes of various materials we have found that some damage morphologies are fairly small and this damage does not grow with further illumination. This type of damage might not be detrimental to the laser performance. We should therefore assume that some damage can be allowed on the optics, but decide on a maximum damage morphology limit. A new specification of damage threshold termed 'functional damage threshold' was derived. Further correlation of damage size and type to system performance must be determined in order to use this measurement, but it is clear that it will be a large factor in the optics performance specifications. Large-area tests have verified that small area testing is not always sufficient when the optic in question has defect-initiated damage. This was evident for example on sputtered polarizer and mirror coatings where the defect density was low enough that the features could be missed by standard small-area testing. For some materials, the scale-length at which damage non- uniformities occur will effect the comparison of small-area and large-area tests. An example of this was the sub- aperture tests on KD*P crystals on the Beamlet test station. The tests verified the large-area damage threshold to be similar to that found when testing a small-area. Implying that for this KD*P material, the dominate damage mechanism is of sufficiently small scale-length that small-area testing is capable of determining the threshold. The Beamlet test station experiments also demonstrated the use of on- line laser conditioning to increase the crystals damage threshold.
© (1996) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Lynn Matthew Sheehan, Mark R. Kozlowski, Christopher J. Stolz, Francois Y. Genin, Michael J. Runkel, Sheldon Schwartz, and Jean Hue "Large-area damage testing of optics", Proc. SPIE 2775, Specification, Production, and Testing of Optical Components and Systems, (19 August 1996);

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