Interferometric characterisation of laser induced thermal lenses in optical materials

Even low absorptions of high power laser beams in optical materials induce significant inhomogeneous temperature changes within the beampath of the laser. Thermal expansion and the temperature dependence of the refractive index change the optical properties of the optical elements, which can be described by adding a "thermal lens" to the system. It's focus shift and aberrations usually disturb the performance of the optical system, and the detailed knowledge of the temperature field allows to estimate the optical performance change and helps to select the best material and coatings. An interferometer test setup is used to determine the optical path length changes in cubic test samples, which are placed into the beam path of a high power laser. The optical axes of the interferometer and the high power laser are perpendicular to each other. The 2-dimensional wavefront map shows the gradient index profile that emerges around the laser beam and enables to discriminate between surface and volume absorption effects. Results of measurements with different test samples will be presented.