The measured spectral trace for this coating is shown

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sapphire as the substrate. The measured spectral trace for this coating is shown in Figure 4. The medical industry is one of the biggest users of Er:YAG lasers, and it is often preferred to have a ‘long pulse’ output rather than a Q-switched output (Figure 5). This means that the lasers are operating with pulse widths of a few hundred microseconds rather than tens of nanoseconds. So, the corresponding peak power density is lower, but the integrated pulse energy is quite high. Testing of the mirror in Figure 4 under these conditions indicates a pulsed LDT of 1500 J/cm 2 , 10 Hz, 100-on-1 test, ISO 11254 procedure. While comparison of the nanosecond LDT data and the microsecond LDT data is not clear-cut, this is considered an excellent result, especially in conjunction with the inherent insensitivity to humidity. 3 μm PULSED LASER DAMAGE THRESHOLD IBS coatings have a clear advantage in the 3 μm region, as their high density makes them completely impervious to water absorption. This is in stark contrast to the quite porous evaporated coatings that routinely absorb/desorb water, making both their spectral features and stress sensitive to humidity. Water has a maximum absorption around 3 μm, which makes it useful for numerous medical applications but simultaneously provides a challenge for high LDT optical coatings. Our first example IBS coating around 3 μm is a 0° mirror designed for extremely high reflectivity (>99.99%) at 2.8 μm, using a suprasil 3000 substrate. This coating was tested at IPG Photonics using an Er:YSGG laser operating at 2.78 μm with 32 ns pulse width, 7 Hz repetition rate, and a Gaussian laser profile as shown in Figure 3. Testing results indicate that the LDT is >10J/cm 2 , limited by the available pulse energy for the Er:YSGG laser. Once again, this represents a very good result, showing a 2.8μm pulsed LDT comparable to that achieved with coatings in the 1μm region. Figure 3. (top) Measured temporal profile of the Er:YSGG laser used for LDT testing of a 99.99% mirror at 2.8 μm. (bottom) Measured spatial profile. 0 2 4 6 8 10 12 14 0 0.5 1 1.5 2 2.5 3 Distance [mm] Absorption [ppm] α = 11.2 ppm Pulse duration Δτ FWHM =32 ns time, ns -60 -40 -20 0 20 40 60 sidnal, a.u. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Beam spot for mirror #1 mm 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Δ P 0 1 2 3 4 5 6 7 8 9 10 11
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Advanced Thin Films | 5733 Central Avenue, Boulder, CO 80301 | Ph: 303-815-1545 | The final relevant category of testing is the continuous wave (CW) regime, and as with some of the other tests it can be challenging to obtain a laser that has sufficient power for estimating LDT. However, due to the lack of available data in this region, even placing limits on the LDT is useful. For this test, the 99.99% HR mirror at 2.8 μm was used, in conjunction with a CW tunable Cr:ZnSe laser. Table 1 shows the results, at several wavelengths within the Cr:ZnSe tuning range. In each case, the optic was not damaged, and the maximum laser power density was reached.
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  • Fall '19
  • LDT

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