A study of the continuous wave (CW) laser induced damage threshold (LiDT) of fused silica and yttrium aluminum garnet (YAG) optics was conducted to further illustrate the enhanced survivability within high power laser systems of an anti-reflection (AR) treatment consisting of randomly distributed surface relief nanostructures (RAR). A series of three CW LiDT tests using the 1070nm wavelength, 16 KW fiber laser test bed at Penn State Electro-Optic Center (PSEOC) were designed and completed, with improvements in the testing protocol, areal coverage, and maximum exposure intensities implemented between test cycles. Initial results for accumulated power, stationary site exposures of RAR nano-textured optics showed no damage and low surface temperatures similar to the control optics with no AR treatment. In contrast, optics with thin-film AR coatings showed high surface temperatures consistent with absorption by the film layers. Surface discriminating absorption measurements made using the Photothermal Common-path Interferometry (PCI) method, showed zero added surface absorption for the RAR nanotextured optics, and absorption levels in the 2-5 part per million range for thin-film AR coated optics. In addition, the surface absorption of thin-film AR coatings was also found to have localized absorption spikes that are likely pre-cursors for damage. Subsequent CW LiDT testing protocol included raster scanning an increased intensity focused beam over the test optic surface where it was found that thin-film AR coated optics damaged at intensities in the 2 to 5 MW/cm2 range with surface temperatures over 250C during the long-duration exposures. Significantly, none of the 10 RAR nano-textured fused silica optics tested could be damaged up to the maximum system intensity of 15.5 MW/cm2, and surface temperatures remained low. YAG optics tested during the final cycle exhibited a similar result with RAR nano-textured surfaces surviving intensities over 3 times higher than thin-film AR coated surfaces. This result was correlated with PCI measurements that also show zero-added surface absorption for the RAR nano-textured YAG optics.
We describe the construction of elements of an optical materials property database. The database reports micromechanical properties (Young's modulus E, hardness H, fracture toughness Kc) for many optical glasses and crystals. Glass manufacturers included are Corning, Hoya, Schott, and Ohara. The materials included are many types of optical glasses and some optical crystals and polycrystals.
Peak-to-valley surface microroughness measurement data acquired from a white light interferometer are compared with data from the actual depth of subsurface damage (SSD) acquired destructively, in single crystalline optical materials (Si, CaF,, MgF2, LiNbO3, A1203) after deterministic microgrinding (DMG). The results demonstrate that SSD is always less than 1.4 times the peak-to-valley surface microroughness for these crystals regardless of crystallographic orientation. These results enable a maximum depth of SSD to be predicted non-invasively for these single crystal materials. The Center for Optics Manufacturing also has an extensive database comparing surface microroughness to SSD in optical glasses. This data will be presented, demonstrating the upper bound for SSD depth in optical glasses based on the surface microroughness. Interferometer settings and destructive techniques for physically determining SSD will be presented.