TelAztec (United States)

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 report demonstration of efficient continuous-wave lasing from chromium-doped zinc selenide using anti-reflection microstructures (ARMs) in place of thin-film AR coatings or Brewster angle cavity geometries. ARM textures are more resistant to laser-induced damage than coatings, exhibit low-loss, wide angular acceptance, broad wavelength effectiveness, and are not susceptible to water absorption. Slope-efficiencies of 68% were achieved, which compares favorably to the thin-film control samples at 58% for the same cavity. ARMs hold promise for near-term power scaling and wavelength agility of transition-metal-ion doped II-VI lasers.

Anti-reflecting (AR) surface relief microstructures (ARMs) are being developed as a replacement for thin-film AR coatings in laser-based systems to improve light transmission, power handling, operational bandwidth, and system reliability. Because ARMs textures have the potential to be replicated using simple embossing methods, the performance advantage and robustness of ARMs can be extended to moldable mid-infrared transmitting materials such as chalcogenide optical fibers. In this work, the optical performance of mid-infrared transparencies incorporating ARMs textures replicated from a master template has been modeled, and multiple master stamping tools have been fabricated in materials such as diamond, silicon carbide, nickel, silicon, and sapphire. Images from ARMs texture embossing trials using arsenic sulfide and arsenic selenide (AMTIR2) glasses, and fluoride glasses such as ZBLAN and indium fluoride provided by IRPhotonics, show excellent pattern transfer and fidelity. Transmission measurements of ARMs textures stamped into arsenic sulfide and arsenic selenide windows show broadband infrared performance equivalent to ARMs textured windows processed by direct patterning and etch methods. A system for molding ARMs textures directly into the end facets of multi-mode mid-infrared transmitting fibers is yielding promising initial results.

The image contrast and clarity recorded by backside illuminated HgCdTe focal plane arrays (FPAs) is strongly dependent on minimizing signal loss and detector noise caused by scattered and reflected light from the FPA window and imaging optics. Thin film anti-reflection (AR) treatments based on stacks of thin-film materials have been exclusively used to minimize substrate reflections for this application. The performance and lifetime of these thin-film AR coatings is limited, and can be inadequate for some space based applications due to the damage produced in the coatings by radiation exposure and extreme temperature variations. A new type of high performance AR treatment for HgCdTe FPAs promising very wide bandwidth operation and increased lifetime in high radiation environments is under development as part of the MDA's Space Tracking and Surveillance System, or STSS. Based on surface relief microstructures fabricated directly in the FPA window, the new textured AR treatment replaces thin-film coatings, eliminating inherent coating limitations such as stress, thermal expansion mismatch, adhesion, radiation hardness, and low laser damage thresholds. Progress on the design, fabrication, and space qualification of AR microstructures for staring format HgCdTe FPA windows, is reported here. Transmission data for FPA windows containing AR microstructures is presented, demonstrating a reduction of reflected light loss from 21% for an untreated window down to an average of less than 1% over a six micron wide spectral range in the long wave infrared region (7-13mm). The potential for AR microstructures to perform over even wider bandwidths such as the important dual-band infrared region (3-12mm), has been demonstrated. Such high AR performance is coupled with nearly un-measurable scattered light losses as recorded by sensitive instruments operated by NIST. Initial proton radiation exposure and thermal cycling tests show no damage to the microstructures and no degradation of the AR performance. Interferometer measurements of the surface flatness of FPA windows incorporating AR microstructures indicate no change from the initial surface flatness, a result that is a significant improvement over thin-film AR coatings, and one that has great potential for large format FPA fabrication.