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 × 10-5 cm-1 level in high-quality optical materials. Using a Shack-Hartmann 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 × 10-5 cm-1. Obtaining this result requires a resolution in the optical path length better than +/-0.1 nm.