Repeatability of pulsed laser irradiation parameters and their role in measurement of fatigue laser-induced damage threshold

When laser radiation interacts with optical material, at some specific fluence level, optical material is damaged. This fluence level is called laser induced damage threshold (LIDT) and depends on various factors including laser instabilities. Finding LIDT value for specific optical material is a key factor of using high power lasers in optical schemes successfully. Crucial importance is to accurately measure LIDT with methods that produce its high repeatability. International Standard Organization (ISO 21254) offers to use least square method for data approximation and LIDT calculation. Thus, the aim of this thesis is to investigate how various laser instabilities influence LIDT value determination using previously mention least square method. After investigating how well LIDT value is calculated using ISO 21254 recommendations, I have determined that least square method gives poor repeatability. To improve this method, four rules that set clear boundaries for choosing data that is included in linear regression calculation was offered. After conducting Monte Carlo simulations, it was shown that rule “Farthest – Closest” gives the most precise LIDT value (with 11,6% error on average) when instabilities are 1-5% and low defect density. That was confirmed by real experiments. Rule “Closest – Closest” demonstrated the best results when there are higher level of instabilities or defect density. It was determined that fluence fluctuation has the highest impact of all tested laser instabilities for precise LIDT value determination using aforementioned method. Not a single one of investigated least square method rules gave precise LIDT values universally. Seeking for higher LIDT determination repeatability, ISO 21254 standard recommendations should be set more specific when least square method should be applied and give recommendations about other statistical data approximation methods when it is not appropriate to use it