Method for Measuring Small Optical Absorption Coefficients with Use of a Shack-Hartmann Wave-Front Detector

We present a method for measuring absorption at the 1 x 10^-5 cm^-1 level in high-quality optical materials. Using a ShackHartmann wave-front detector, thermal lensing in these materials may be measured. Then, the absorption coefficient may be estimated by fitting the observed deformation to a thermal lensing model based on the temperature dependences of the refractive index and the thermal expansion coefficient. For a particular sample of fused silica, the absorption coefficient was determined to be 1.8 +/- 0.4 x 10^-5 cm^-1 . Obtaining this result requires a resolution in the optical path length better than +/-0.1 nm.