Segmented wave-front measurements by lateral shearing interferometry

The need for segmented wave front measurements has been rocketing for several years. The applications are various: thickness of metallic masks, diffracting elements, phasing of the primary segmented mirrors of telescopes, such as the Keck telescope, laser beam coherent recombination... Lateral shearing interferometers are common wave front sensors, used with success to test classical optical components. This technique does not require a reference wave, which is a major advantage. The lateral shearing interferometry has also proved successful to analyze segmented wave front; results of such a measurement by a diffraction-grating based interferometer are presented and analyzed. We dwell upon quadri-wave lateral shearing interferometers (QWLSI), which offer the possibility to characterize two-dimensionally the wave front, in a single measurement. This technique combines accuracy and qualities such as compactness and simplicity. Moreover, a chromatic regime of lateral shearing interferometers based on diffraction grating can be pointed out; this allows a two-color analysis to greatly extend the dynamic range. In the first parts we will present general considerations on QWLSI and segmented surface; then a technique to increase the dynamic range is investigated both theoretically and experimentally.