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In a comprehensive survey of the bulk-damage problem in laser glass, Erlan Bliss of the Air Force Cambridge Research Laboratory stressed the importance of studying the dependence of damage thresholds on several critical parameters, such as pulse duration, temperature, wavelength, and focusing configuration, as a means of identifying which of several competing processes were responsible for the observed damage. The mechanisms discussed were absorption and subsequent heating which lead to thermal fracture, plasma formation arising from multiphoton ionization and avalanche breakdown, and stimulated Brillouin scattering leading to rupture by the acoustical wave. Each of these processes exhibits a characteristic dependence on pulse duration, although that alone does not uniquely characterize the damage mechanism. In addition, each of these processes is greatly enhanced by the presence of self-focusing or self-trapping, which can lead to a local energy density far greater than the average taken over the total beam area. When self-trapping occurs, damage can result from any of several mechanisms. Which mechanism prevails under this condition is also a sensitive function of several experimental parameters, including pulse duration, focusing configuration, and others. For each mechanism, the damage threshold decreases with decreasing pulse duration to some critical value, and then stays constant, or even rises again, as the pulse length is decreased farther. The over-all envelope of these damage-threshold curves, taking the various mechanisms into account, exhibits a gradual increase with increasing pulse length. Lieutenant Bliss urged that the dependence of damage on several variables be given close consideration in the design and interpretation of damage experiments. The question of the nature of the focus in the case of electrostrictive trapping was considered theoretically by Edwin L. Kerr of Perkin-Elmer Corp. He stressed the dependence of the damage threshold on beam radius for a fixed pulse length (about 50 ns), in contrast to the approach of the preceding paper. The most significant feature of the self-trapped focus was the existence of multiple foci of increasing intensity formed along the axis of the incident beam, with the actual points of maximum intensity moving rapidly along the axis toward the laser during the course of the laser pulse. Experimental results in various glasses were compared to the computed values, and good agreement was obtained. Only one damage mechanism—elastic rupture of the material—was considered in detail in this paper, but the author pointed out the possibility that the enhanced electric field strength at the foci could lead to breakdown as well. Two other papers considered the question of self-focusing as a factor in the onset of damage. Fred Quelle of the Office of Naval Research, reviewed the theory of self-focusing, both due to thermally induced index changes and electrostriction. He concluded that the energy density which can be propagated in glass without damage occurring is least (lowest threshold) for times such as microseconds, assuming that self-trapping is a necessary precursor to damage. J. Davit of Compagnie Générale d'Electricité (CGE) discussed the formation of filamentary damage tracks and presented experimental data for pulse lengths down to 2 ns. This filamentary mechanism of damage competes with surface damage, so experimental conditions had to be maintained in which the filamentary threshold was lower than the surface-damage threshold. When this was done, fairly consistent results were obtained, with thresholds for filamentary damage in the vicinity of 10 to 20 J/cm2, in much the same range of values as at 30 ns. In addition, with a pulse length of 30 ps (picoseconds), the threshold for filament formation was found to be 10 J/cm2. Picosecond pulse-damage experiments are considered quite tentative at present, however.
Glass, Alexander J.
Guenther, Arthur H.