Thermal laser induced damage in optical coatings due to an incident pulse train

The temperature distribution of a slightly absorbing optical thin film atop a glass substrate subjected to a laser pulse train of pulse duration 1ns is derived from the classical heat equation. The critical fluence defined by the point at which thermal damage occurs is derived and the dependence of laser induced damage threshold (LIDT) on repetition rate, pulse duration, wavelength, thermal properties, beam and optic dimensions is discussed. A comparison is made between the theoretical LIDT and well known experimentally observed scaling laws for both Gaussian and ”Top-Hat” pulse profiles. It is found that the ratio of beam to optic diameter is an important parameter in LIDT determination. Larger substrates are found to have a lower LIDT and it is suggested that LIDT follows an inverse scaling rule with respect to repetition rate.