Freeform characterization based on nanostructured diffraction gratings.

The in-line characterization of freeform optical elements during the production cycle is challenging. Recently, we presented a compact sensor setup for the characterization of the wavefront generated by freeform optical elements in transmission. The sensor is based on a common-path interferometer consisting of diffractive components and Fourier filtering being adapted to the subsequent numerical post processing. Additionally, it offers several degrees of freedom for enlarging the measurement range of the wavefront gradients. In this contribution, we propose an advanced sensor setup for the measurement of wavefronts generated by freeform elements in reflection. The main advantage is the uni-axial illumination of the test object and the measuring system without the need for conventional beamsplitters. Due to this uni-axial arrangement, the main challenge is to avoid the effect of stray light and back reflections on the measurement signal-to-noise ratio. This is achieved by implementing a highly absorbing amplitude grating based on nanostructured silicon. We demonstrate the experimentally realized measurement system and compare its performance to a commercial Shack-Hartmann sensor.