Damage of Optical Elements

Abstract

Optically induced damage to the laser medium and to components of the laser system generally determine the limit of useful performance of solid-state lasers. The performance of a laser system in terms of pump efficiency, gain, nonlinear conversion , brightness, and beam quality increases with power density of the beam. Power density is fairly independent of the size of the laser, output power scales mostly with beam diameter rather than beam intensity. As a consequence, it is desirable to operate all lasers at the highest beam intensity consistent with reliable operation, which, in turn, is determined by the optical-damage threshold of the various components. Therefore, an understanding of the mechanisms which cause radiation damage to optical components and a knowledge of the damage threshold of the materials employed in a laser are of great importance to the engineer who is designing a laser system. Damage may occur either internally or at the surface of an optical component owing to a number of intrinsic and extrinsic factors. Intrinsic processes that limit the optical strength of materials include linear absorption, color-center formation, and a variety of nonlinear processes such as self-focusing, multiphoton absorption, and electron avalanche breakdown. Extrinsic factors include impurities, material defects (voids, dislocations, etc.), and in particular surface scratches and digs and surface contamination. In general, damage threshold is determined by the condition of the optical surfaces and the quality of optical coatings rather than by the bulk material. The problem of surface damage is compounded because most laser systems contain many surfaces. There are different ways of defining damage threshold. One might define damage as the physical appearance of a defect in the material or by a degradation in the output performance of the laser system. From the standpoint of the user of the laser system, the performance deterioration is of more importance than the physical appearance of a defect in the material. Often the occurrence of a small blemish or defect does not alter system performance. In this case it is important to determine whether the defect remains constant or increases with time. Quantitative measurements of the damage threshold of an optical component should be performed following a procedure specified by the International Standards Organization [11.1-3]. A test sample is irradiated with a well-characterized laser at different sites. The fluence levels are chosen so that at the higher fluence a high probability of damage exists, whereas at the lower levels the probability of damage is low. The percentage of failures is plotted vs. fluence. A least-squares linear fit to this data is calculated.