Multipass Lock-In Thermography for optical coating absorption measurements

The development of high power lasers implies specific needs of optics with very low absorption to avoid detrimental thermal effects up to damage of optical components, and associated metrology of these optics. In this work, we discuss on our recent developments of a thermographic-based technique to measure absorption: specifically, the Lock-In Thermography (LIT) technique applied in a multipass configuration to increase the effective power on the tested samples. We will present the system developed at the Institut Fresnel that is based on 1.5 kW Ytterbium fiber, that allows to submit samples to 5 kW effective power and measure absorption levels down to the ppm on optical coatings.