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Laser induced damage in single crystal calcium fluoride

By Ian Laidler


A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.The thesis describes the establishment of a laser damage facility\ud in the ultra violet. The laser is a pulsed excimer laser (25 n sec)\ud capable of an output energy of 1 Joule at wavelengths of 193, 245,\ud 308, 337 and 351 nm. The problems involved in making reliable\ud damage threshold measurements are addressed, such as energy calibration,\ud beam attenuation, damage detection and temporal and spatial beam\ud profiling. A computer controlled frame store and video system\ud enabling single shot spatial profiling and peak fluence measurements\ud to be performed is described. Such a system is essential if reliable\ud results are to be obtained from lasers whose spatial output cannot\ud be described by a Gaussian.\ud Using the damage facility, work has been performed on single crystal\ud Caf2 laser windows ascertaining the bulk and surface damage thresholds\ud as a function of crystal purity, surface finishing and polishing\ud procedures. The results of a T.E.M. study give an insight into the\ud fundamental damage mechanism of this material to be electron avalanche\ud breakdown. The role of transient primary defects in the bulk breakdown\ud of CaF2 has been investigated using dye laser probe techniques, and\ud indicate that the presence of U.V. generated self trapped excitons,\ud acting as sinks for the conduction band electrons, greatly enhances\ud the damage threshold.\ud Work on coatings using a LIMA (Laser-induced ion mass analyser)\ud is reported and the potential of this machine as a laser damage\ud diagnostic tool is explored

Publisher: © Ian Laidler
Year: 1988
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