Photothermal microscopy characterization of multiphoton annealing of defects in thin-film coatings for high-power lasers

Abstract

A focus error method photothermal microscope was designed for the simultaneous annealing and characterization ofdefects in thin film multilayer coatings for high power lasers. The technique relies in the detection of the thermal lensinduced by the local absorption of a light power focused laser. A 10W CW laser at 1.06mm wavelength was used as apump and a HeNe laser at 632.8nm as a probe. A 4 quadrant detector and specifically designed astigmatic optic is usedto determine the defocusing of the transmitted probe beam at the modulation frequency of the pump. The instrumentscans the surface and detects the evolution of the absorptance with time with sensitivity below 0.1ppm. The pump beamfocus determines the spatial resolution of the instrument and the probe beam size, much larger than the pump, has tomatch the modulation frequency that yields a thermal diffusion distance of the order of the probe beam in onemodulation period. The detailed design of the instrument will be presented showing the design parameters that should beconsidered for an adequate sensitivity. The sensitivity of the system is better than 0.1ppm and allows the realization ofspatial sweeps and even measurements of the evolution of absorption as a function of time. These capabilities allow thelocation of defects and their characterization.