Laser induced damage in single crystal calcium fluoride
thesisposted on 07.12.2012 by Ian Laidler
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
The thesis describes the establishment of a laser damage facility in the ultra violet. The laser is a pulsed excimer laser (25 n sec) capable of an output energy of 1 Joule at wavelengths of 193, 245, 308, 337 and 351 nm. The problems involved in making reliable damage threshold measurements are addressed, such as energy calibration, beam attenuation, damage detection and temporal and spatial beam profiling. A computer controlled frame store and video system enabling single shot spatial profiling and peak fluence measurements to be performed is described. Such a system is essential if reliable results are to be obtained from lasers whose spatial output cannot be described by a Gaussian. Using the damage facility, work has been performed on single crystal Caf2 laser windows ascertaining the bulk and surface damage thresholds as a function of crystal purity, surface finishing and polishing procedures. The results of a T.E.M. study give an insight into the fundamental damage mechanism of this material to be electron avalanche breakdown. The role of transient primary defects in the bulk breakdown of CaF2 has been investigated using dye laser probe techniques, and indicate that the presence of U.V. generated self trapped excitons, acting as sinks for the conduction band electrons, greatly enhances the damage threshold. Work on coatings using a LIMA (Laser-induced ion mass analyser) is reported and the potential of this machine as a laser damage diagnostic tool is explored.