Measuring residual absorption in highly transparent optical materials using tunable sources. On calibration, spectroscopy and cross-validation

Photothermal Common-path interferometry (PCI) is considered as one of the most sensitive techniques for the measurement of residual absorption in bulk materials and coatings. At Fraunhofer IPM, the setup is combined with continuous-wave optical parametric oscillators (OPOs) as widely tunable pump sources for measurements in the visible, near infrared and middle infrared. This offers new possibilities for the calibration of the absolute absorption scale and for the determination of spectral signatures in the material's absorption. Cross-validation is achieved for bulk data by combinig the PCI results with data from a photoacoustic spectrometer and from loss measurements in whispering-gallery-resonators made from high-purity materials.