Metrology for laser damage

High-power lasers are used in a wide range of applications in industry, science, medicine and defense. One of the technological obstacles to the development of high-power or high-energy laser sources is damage to optical components under irradiation. Indeed, when a high laser flux passes through an optical component or is reflected on a mirror, reversible effects can be observed, such as non-linear effects or heating, which can cause stress and deformation. If the quantity of light is increased (either by increasing the intensity or by confining the beam), irreversible effects can occur: melting, vaporization, cracking, shattering, craters, delamination... altering the optical function of the component, or even rendering it unusable. These permanent changes to the material are referred to as "laser damage". The disadvantage of this phenomenon is that it affects the service life of optical components, as well as the maintenance costs of laser chains. It can also lead to serious safety problems. Understanding the physical phenomena involved in laser parameters, and measuring these effects, is therefore of major importance when it comes to designing a laser system and operating it under optimum conditions of reliability, safety and cost-effectiveness.

The problem of flux resistance has been studied since the invention of the laser, and there is an impressive database on the subject. . In this article, we propose a synthetic approach to this subject, necessarily limited, but which will enable the engineer or researcher confronted with the problem to familiarize himself with the concepts related to the field, the physical phenomena involved and the way in which these effects can be quantified and the measurements related to its application.

First, we'll explain the various physical mechanisms that can lead to the destruction of an optical component subjected to laser flux, a necessary step in understanding and interpreting the measurements. We will then describe the methods used to measure resistance to laser flux, and discuss their interpretation and the influence of laser parameters on damage. Finally, we will briefly present the materials and manufacturing processes specific to laser flux-resistant components.