For this device the operating wavelength is 205 μm

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light. For this device, the operating wavelength is 2.05 μm, and the polarization extinction ratio is about 1000:1. To avoid any substrate absorption effects and substrate defect issues, the coating was deposited onto infrasil that was polished in-house at Advanced Thin Films. The optic was tested for LDT at Spica Technologies, at the operating wavelength of 2.05 μm with a 100 ns pulse width, 25 kHz repetition rate, p-polarization (transmission), and 23 μm spot size. The part was irradiated up to 32.5 J/cm 2 , with no observable damage in 20 sites. The outstanding performance of this relatively complex coating is a good indicator of the capability for IBS coatings at 2 μm. Figure 1. Measured coating performance for a 2.05 μm 45° Thin Film Polarizer, deposited onto a Advanced Thin Film- polished infrasil substrate. The lower limit for the measured damage threshold is >32.5 J/cm 2 (100 ns, 25 kHz). The optic was not damaged at all during testing, so the LDT exceeds the capability of the test laser. The next test for the coating in Figure 1 was to evaluate the fundamental absorption in the coating. Recently, these types of precision absorption measurements have become more widely available due to the commercialization of Photothermal Common-path Interferometry, or PCI 14 . However, it is still very uncommon for these measurements to be performed in the 2-3 μm spectral region. Typically, the high power pump laser used for the measurement has a wavelength around 1μm, where the majority of very high power coatings are produced and high power CW lasers are readily available. There is no fundamental reason why the PCI system must operate at 1μm, and in fact we were able to use a system with a 2μm pump laser, at Stanford University. The measured data for the coating of Figure 1 is shown in Figure 2 below. The details of this photothermal response curve as a function of axial distance from the coating plane are described elsewhere 14 . For the purposes of this paper, it is sufficient to point out that the measured absorption for the coated surface is 11.2 ppm. This is an extremely low value for a transmitting coating, and would generally be considered to be very good for a 1μm coating or a high finesse mirror. While pure absorption is not always a good
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Advanced Thin Films | 5733 Central Avenue, Boulder, CO 80301 | Ph: 303-815-1545 | indicator of whether a coating will have a high pulsed LDT, it is very representative of the performance for a CW situation. Figure 2. PCI Absorption measurement for the polarizer coating of Figure 1. The pump laser wavelength was 2.004 μm, and the absorption of the coated surface is only 11.2 ppm. The scan shows the absorption as a function of axial distance away from the coated surface, which is identified with an arrow. The second IBS coating tested in the 3 μm region involved a coating for 2.94 μm, designed as a 45° HR for an Er:YAG laser, with a transmission band from 635-655 nm for an alignment laser. This is a relatively standard coating for Er:YAG lasers operating at 2.94 μm, and used single crystal sapphire as the substrate. The measured spectral trace for
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  • Fall '19
  • LDT

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