Ultra-sensitive Absorption Diagnostics of Thin Films for High-power Laser Interference Coatings

The goal of this project was to demonstrate a simple method to measure absorption loss in thin dielectrics films, in particular in multilayer coatings for high power lasers in which very low absorption losses are required. The absorption characterization method is a photo-thermal technique based on thermal lensing. The method measures the defocusing of an incident probe beam caused by a thermal lens generated by absorption of the incident pump beam. The simplicity and versatility of the method allow it to be incorporated into a deposition chamber to measure absorption loss in situ. Excellent results were obtained with both amplitude modulated and pulsed pump laser beams. Results obtained in this project demonstrated the method can measure absorption losses below 10 ppm at 1 μm wavelength. Future work consists of the development of a prototype suitable for incorporation into the deposition chamber. Such powerful diagnostics does not presently exist.