4. Optically Active Defects

Optical elements used for lasers are imperfect. Even a super polished glass surfaces hide thousands of defects upon and beyond the top layer. Defects are various infringements such as cracks, scratches, digs, or impurities that causes optical element to break faster or at much lower fluencies than it was initially intended to be. Most of the defects are created during optical element manufacturing processes such as grinding, polishing, cleaning or coating deposition. Defected area might trail several hundreds of microns beneath the top layer. Normally desirable defect-free bulk material is covered by several machining effected zones [see fig. for details]. Each surface and sub-surface defects are important considering laser-induced damage possibility. However, it is a complex task to determine the damage threshold limiting process.

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Fig. 1 Schematic illustration of defect’s layers on the optical material.

Firstly, grinding or lapping initiates deformation of the surface and creates plenty of micro cracks due to friction between material and abrasives. Later polishing conceals these defects with an amorphous or microcrystalline structure, called the “Beilby” layer that might contain various impurities such as polishing slurry remains, leftover polishing material and so on. Finally there are oxide layer and adsorbed gases or water vapor layer. These are formed due surface interaction with outer environment. Later defects created due cleaning and coating processes take place.

Cooperating in various projects “Lidaris” team has gain a valuable information on LIDT limiting processes research. So, if you are interesting to find a weak link in your production line, you are welcome to ask about available testing and measurement plan.