Measurements of small wave-front distortions using a three-wave lateral shearing interferometer

The solution of the parabolic equation for radiation diffracted from a periodic hexagonal structure is analysed theoretically. It is shown that diffraction does not prevent high-precision measurements of wave-front distortions using a three-beam lateral shearing interferometer. The proposed measuring technique is based on the recording of interference-pattern distortions at arbitrary distances from a beam replicator. The results of numerical calculations supporting this technique are presented. The optical quality of an etalon plate is measured in an aperture of diameter 12 cm with a high degree of precision. By considering the example of a large-aperture KDP crystal, it is shown that the application of this method makes it possible to synthesise from individual measurements the wave front in an aperture considerably larger than the test beam aperture.