We present a method for measuring absorption at the 1 × 10<sup>−5</sup> cm<sup>−1</sup> 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<sup>−5</sup> cm<sup>−1</sup>. Obtaining this result requires a resolution in the optical path length better than ±0.1 nm.
© 2003 Optical Society of America
Sanichiro Yoshida, David H. Reitze, David B. Tanner, and Justin D. Mansell, "Method for Measuring Small Optical Absorption Coefficients with Use of a Shack–Hartmann Wave-Front Detector," Appl. Opt. 42, 4835-4840 (2003)