Damage Mechanisms in Optical Materials For High-Power, Short-Wavelength Laser Systems
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Richard F. Haglund, jr.* Jne Center for Atomic and Molecular Physics at Surfaces Vanderbilt University
Damage to optical materials under intense photon irradiation has always been a major problem in the design and operation of high-energy and high-average-power lasers. In short-wavelength lasers, operating at visible and ultraviolet wavelengths, the problem appears to be especially acute; presently attainable damage thresholds seriously compromise the engineering design of laser windows and mirrors, pulsed power trains and oscillator-amplifier systems architecture. Given the present interest in ultraviolet excimer lasers and in short-pulse, high-power free-electron lasers operating at visible and shorter wavelengths, the "optical damage problem" poses a scientific and technological challenge of significantdimensions. The solution of this problem even has significant implications outside the realm of lasers, for example, in large space-borne systems (such as the Hubble Telescope) exposed to intense ultraviolet radiation. The dimensions of the problem are illustrated by the Large-Aperture kryptonfluoride laser amplifier Module (LAM) shown schematically in Figure 1. This device, now operating at the Los Alamos National Laboratory, is typical of current and planned large excimer lasers for fusion applications. The LAM has an active volume of some 2 m , and optical surfaces (resona-
tor mirror and windows) exceeding 1 m 2 in size; the fabrication of these optical elements was the most expensive and timeconsuming single item in the construction of the laser. During laser operation, a population inversion in an Ar-Kr-F2 mix ture is created through electron-beam excitation of the laser gas by two 400 kA beams of 650 keV electrons from a cold cathode discharge. The electron trajectories in the gas are constrained by a 4 kG magnetic field transverse to the optical axis produced by a pair of large Helmholtzcoils. The mirror and window have fused-silica or Pyrex substrates, typically coated with multilayer dielectric thin films of SiC>2, AI2O3, or other metal halides or metal oxides. The design fluence of the LAM is 1 J/cm i n a 5 - n s p u l s e ; t h e significance of this flux level can be gauged by observing that, for a KrF laser (248 nm), 1 J/cm 2 corresponds to some 104 photons per surface site on the irradiated optical elements! In electron-beam-pumped lasers, bremsstrahlung x-rays produced by the deceleration of near-megavolt pump-beam electrons, as well as scattered or diffusing low-energy electrons, also irradiate nearby optical surfaces. The measured electron fluxat the LAM window surfaces is about 30 mA/cm 2 , while the total radiation dose is approximately 2 rad/cm 2 per shot. Moreover, MIRROR (HR COATINGS)
there are many low-energy ion species in the laser plasma — including oxygen, argon and fluorine—known to be efficient at producing reactive-ion etching of optical materials, particularly in the presence of UV radiation. Hence, erosion and damage to material surfaces in the high-power laser environment may r
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