The increase of continuous wave laser power is an important topic in various industrial and defense applications. One of the important limitation is due to optical coatings. In order to study this absorption, it is of prime importance to measure and determine the origin of this absorption. We have developed a LIT system (LIT) to perform low-absorption measurement at 1.07 μm. A multipass setup was realized and calibrated with a sensitivity of a few ppm and a ten times better accuracy is demonstrated. Then, this instrument was used to study single layers made with different materials and deposited by PIAD and multilayer components.
Laser-induced contamination (LIC) degrades the performance of optical components and can result in optical losses or even laser-induced damage. LIC deposit formation limits reliable operation of high repetition rate industrial lasers. In this work, we investigate LIC growth on dielectric oxide thin films in air environment irradiated by MHz sub-ps laser at 515 nm. We study the LIC growth dynamic in dependence on thin film deposition method, thin film material and thin film thickness.
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.
The increase of continuous wave laser power is an important topic in various new industrial and defence applications. One of the important limitation is due to the thin film component absorption (intrinsic and defects-related) that induces beam distortions and eventually laser-induced damages. In order to study this absorption, it is of prime importance to accurately measure low absorption levels and to determine the origin of this absorption. In this work, we present the use of Lock-In Thermography (LIT) to absorption measurement. This technique relies on the use of a pump laser beam at 1 μm that is modulated at low frequency and an infrared camera that images the thin-film sample that is being heated. By applying a lock-in treatment on thermal images, we show that we can obtain an image of the temperature increase over the optical component with low noise level. A LIT setup with a sensitivity of a few ppm and a ten times better accuracy is demonstrated. We also show that this setup can be used to make mappings of local absorption and can easily reveal local defects with absorption that can be one order of magnitude higher that intrinsic one. This setup is finally implemented to make measurements on different single layer thin-films. Layers made with different materials (Nb2O5, SiO2, TiO2, HfO2) and deposited by plasma ion assisted deposition or plasma-assisted reactive magnetron sputtering are studied. We explore also the effect of annealing on these dense coatings. Finally, we investigate how these intrinsic absorption levels can be used to investigate the absorption of multilayers structures.
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