Measurement of wavefront deformations and absorptance in high power laser optics

Beam guiding and focusing optics for high power lasers are subjected to thermal stress, leading to transient distortions of the transmitted wavefront. This 'thermal lens' effect is caused by the residual absorption of optical materials (glasses, mirrors, coatings), resulting in local changes of both refractive index and surface figure. Thus, time-dependent wavefront aberrations associated with the heating of the opto-mechanical system can lead to significant deviations of the beam parameters from targeted design and process values ('focus shift'). A measurement system for quantitative registration of this thermal lens effect was developed. It is based upon a Hartmann-Shack wavefront sensor with extreme sensitivity, which can record relative wavefront changes in the range of λ/10,000. Since the extent of deformation is directly proportional to the absorption, the technique can be employed for rapid assessment of the material quality. We present results from photo-thermal measurements on optical elements under 193nm irradiation. In addition, examples for assessment of complex optical systems for high power solid-state lasers are given, as e.g. F-theta objectives.