And al 2 o 3 the higher deposition energy compared to

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, and Al 2 O 3 . The higher deposition energy, compared to thermal evaporative techniques, provides essentially bulk packing density, low scatter, and insensitivity to absorption of chemicals such as water or other contaminants. The process is extremely well controlled, allowing deposition of hundreds of thin film layers with full computer control. The near-UV through near-infrared is considered the typical range for these coatings, providing some of the highest LDT values in the industry. For example, Advanced Thin Films produces 1064 nm High Reflector coatings with damage thresholds of 48 Joules/cm 2 (for 20 nS pulses at 20 Hz, ~industry standard test conditions). The high transparency region for these and other materials extends well past the near-infrared, however, making them suitable for coatings at 2 μm and longer. While this is generally
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Advanced Thin Films | 5733 Central Avenue, Boulder, CO 80301 | Ph: 303-815-1545 | known within the optical coating community, there is nevertheless a lack of data that quantitatively details this extended range of high power IBS coating performance. This is one reason why optical designers have historically chosen lower performing electron beam evaporated coatings in this region. One way to quantify coating material performance is to measure the transparency over a wide spectral range. While this is generally useful and a good starting point, it is not always a good indicator that the material will be suitable for high LDT coatings. So, this work instead concentrates directly on LDT data, and coating absorption at the parts-per-million level, for the 2-3 μm spectral range. The transparency window mentioned above is also critical for the substrates onto which coatings are deposited. In addition to the majority of laser materials mentioned above (including fibers), other substrates suitable for high power IBS coatings in the 2-3 μm region are: infrasil, suprasil, undoped YAG, sapphire, silicon, silicon carbide, and germanium. Coating adhesion is extremely good, and several Advanced Thin Films-coated optics with 2 μm AR coatings operate at cryogenic temperatures. 2 μm PULSED LASER DAMAGE THRESHOLD The first example involves a specific coating at 2 μm, and the most appropriate damage test involves a coating that transmits some or all of the light. This is because it is generally accepted that Anti-Reflective (AR) coatings have lower LDT than High Reflector (HR) coatings. Another factor that plays into LDT is the number of layers in the coating. A simple AR coating has relatively few layers, so a more robust damage test involves a coating with many more layers such as a non-Brewster angle Thin Film Polarizer (TFP). The measured spectral performance for such a coating is shown in Figure 1. Due to the longer wavelength, it should be noted that the overall coating thickness for this device is essentially twice that for the comparable optic at 1 μm. The curves in Figure 1 indicate the coating performance for both p-polarized and s-polarized light. For this device, the operating wavelength is 2.05
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  • Fall '19
  • LDT

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