Photo-thermal measurement of absorptance losses, temperature induced wavefront deformation and compaction in DUV-optics

Abstract

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 upon a strongly improved Hartmann-Shack wavefront sensor with an extreme sensitivity of approximately lambda/10000 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, depending on sign and magnitude of index change and expansion. This transient wavefront distortion yields a quantitative measure of the absorption losses in the sample. In the case of fused silica, an additional permanent change indicates irreversible material compaction. Results for both fused silica and CaF(2) are presented and compared.