What is laser conditioning? A review focused on dielectric multilayers

Laser conditioning is a phenomenon which has been much explored during the last 15 years. Many optical components for high power lasers have shown enhancement of their laser-induced damage threshold after under-threshold pre-irradiation. The improvement brought by laser conditioning is thought to be necessary in the building of new NIF and LMJ lasers, especially for dielectric multilayers and KDP crystals. However, we lack of a complete physical explanation for the observations of conditioning. In particular, we don't know the effect of most physical parameters like pulselength, wavelength, repetition frequency of the laser shot. In this tentative review, we first gather results relevant to the general problem of laser conditioning, especially on dielectric multilayers irradiated with nanosecond pulselength. We find necessary to come back to the basic notion of damage and threshold. As a definition basis, we establish that laser conditioning is a damage (irreversible modification) with minor optical (functional) consequence and no continuing damaging with successive testing. This way of presenting the phenomenon contains a non-threshold description of optical damage. We finally explore some specific questions, that are technologically important for LMJ and NIF construction. Particularly, is laser conditioning necessary once we admit the occurrence of small damages, that is with a functional definition of damage threshold ?.