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Numerical and experimental investigation on laser damage threshold of highly reflective multilayer thin films

Ocak, Mustafa
The laser induced temperature distributions on the optical thin films are investigated in this study. Effects of optical design modifications on thermal performance of highly reflective (HR) multilayer thin films are analyzed. Firstly, a conventional 19 layer HR coating is selected as a reference and the laser induced temperature distribution is evaluated on it. Then, alternative HR designs are developed by employing non-quarter wave layers, over coat (OC) layers and two high index materials in the coating stacks. Temperature distributions for the design alternatives are then computed and compared with the conventional design. Herein, usage of non-quarter wave layers decreases the maximum temperature (at most 24.6%) and the highest interface temperature (at most 21.6%) at the top layer pairs of the alternative designs; however it causes higher absorption through inner layer pairs. Therefore, a new design method is proposed for which layer thicknesses are calculated using the developed MATLAB code to avoid high laser absorption. Results confirm that proposed design shows higher thermal performance by decreasing the internal and interface temperatures (at most 15.4%) compared to prior alternative designs. In the meantime, the conventional and alternative designs are fabricated for the laser induced damage threshold (LIDT) tests and damage morphology investigations. Herein damage types are identified and changes on damage characteristics are scrutinized. Results show that usage of modified layer thicknesses increased the LIDT by changing the damage characteristics and damage profiles due to decreased temperature rise by the low refractive index layers in accordance with the numerical simulations.