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Photothermal tomography of optical coatings based on surface thermal lensing technology

Surface thermal lensing (STL) technology was developed into an effective apparatus for the measurement and analysis of weak-absorption optical coatings. Previous work focused on the measurement of the whole sample and was based on the assumption that the film is very thin. In this paper, by changing the modulated frequency of the pump laser from 1 Hz to 101 kHz, we accurately controlled the thermal diffusion length and got both the amplitude and phase of the photothermal signal of different samples. Based on the comparison of these signals, some analysis of 1) the thermal conductivity measurement and 2) the depth localization of strongly absorbing layer was performed.