Photothermal measurement of absorption and wavefront deformations in UV optics

A measurement system for quantitative registration of transient and irreversible lens effects in DUV optics induced by absorbed UV laser radiation was developed. It is based on a strongly improved Hartmann-Shack wavefront sensor with an extreme sensitivity of ~λ/10,000 rms @ 193nm, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted through the laser-irradiated site of a sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test laser is distorted into a convex or concave lens, dependent on sign and magnitude of index change and expansion. The observed wavefront distortion yields a quantitative measure of the absorption losses in the sample. Results for fused silica and CaF2 are presented.