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Browsing by Subject "Laser damage"

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    Contamination Induced Continuous-Wave Laser Damage of Optical Elements
    (2018-06) Brown, Andrew
    Continuous-Wave laser-induced optical breakdown affects anyone whose work requires tightly focused light, high power sources, or delicate materials. It often occurs unexpectedly and seemingly randomly at optical intensities far lower than those predicted by ultra-short pulse laser experiments. Further complicating the issue is that the majority of laser damage experiments use carefully controlled laboratory conditions with short-pulsed lasers focused to small spots on clean, pristine materials. Continuous-Wave laser damage is usually attributed to contamination, and occurs under radically different conditions. To determine the origin of contamination-induced breakdown, microparticle contaminated optics were stressed using a 17 kW continuous-wave laser. Contamination-induced breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. For both half-wave and high reflectivity coatings, damage thresholds were found to strongly follow the bandgap energy of the film. It is theorized that surface contamination heated by the laser thermally generates free carriers in the films. If the free carrier concentration exceeds a certain threshold, runaway absorption and breakdown will occur. A thermal model incorporating the particle absorption, interfacial heat transfer, and free carrier absorption was developed, and it explains the observed data. The bandgap of the film, the absorption and thermal contact of the contaminant, and the evaporation time of the particle, all determine whether a material can survive. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism. These findings strongly suggest that low bandgap materials are a liability in optics exposed to environmental contamination. Laser conditioning was examined as a means of preventing damage by removing contamination without initiating damage. Absorption measurements taken using photo thermal common-path interferometry show up to a 90% absorption reduction with conditioned samples. Regular laser conditioning at low irradiances can prolong the life of optics that must operate in difficult environmental conditions.
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    Fabrication and characterization of micromachined dielectric thin fi lms and temperature sensors using thermoluminescence
    (2013-03) Kim, Sangho Sam
    High-power laser technology has a number of applications, whether for the military (i.e., anti-missile weaponry) or for material processing, medical surgery, laser-induced nuclear fusion, and high-density data storage. However, external obstacles could cause a laser to problematically change its direction. Optical components such as mirrors already address this problem by deflecting a laser beam, but can be damaged easily due to the intensity of the laser. Therefore, this dissertation examines how to improve reliability of high power laser application systems by three signicant standards. First, we demonstrate that an atomic layer deposition (ALD) of Al2O3 can stabilize novel dielectric optical mirrors composed of SiO2 nanorods, whose porosity makes it attractive for use as a low refractive index material. Such a deposition can stabilize material properties in dry versus humid atmospheres, where both the refractive index and coefficient of thermal expansion (CTE) vary dramatically. This encapsulation ability is demonstrated in dielectric multilayers as a Distributed Bragg Reflector (DBR). Second, we show that the difference in hydroxyl signatures of micromachined dielectric membranes can make detection of optical materials' laser damage more accurate. This signature difference, appearing as the decrease in post-laser absorption peaks associated with hydroxyl groups (OH), is measured by Fourier transform infrared spectroscopy and corresponds to regions of high infuence from a Nd:YAG laser. This detection technique will be useful to determine the lifespan of the optical components used in a high power laser. Third, we found that heterogeneous thermoluminescent (TL) multilayers composed of LiF:Mg,Ti and CaF2:Dy with Kapton as an interlayer can enhance reconstruction of laser heating events through thermal gradients that penetrate deep into a material, thereby preserving memory of the temperature history of the surface. Using the finite-difference time-domain method (FDTD) and the first order kinetics model of TL, we estimate dynamic heat transfer and then populate the final luminescent intensity. A thermal contact conductance between the critical layers is also introduced to better simulate experimental results, thereby resolving dynamic temperatures by hundreds of milliseconds.