Kotov M. Diffraction methods of optical wavefront research

Methods of investigation of the optical wavefront based on the phenomenon of diffraction are considered. The features of the application of the Shack-Hartmann sensor for a wave with random phase fluctuations have been analyzed: the influence of the speckle field on the accuracy of the wavefront aberration measurement is experimentally investigated and the use of holography techniques to reduce such influence is observed. It is shown that speckle size in the observation plane depends on the statistical parameters of a diffuser, not only on aperture size: the correlation radius in the observation plane reduces if the initial correlation radius reduces or aperture size increases. The correlation function also defines the root mean square deviation of the focused spots: centroid estimation error increases when the speckles decrease. The application of a holographic lenslet array in Shack-Hartmann sensor to the measurements of wavefront aberrations in a speckle field by means of iterative recording algorithm has been offered. The deformation of thin steel plate with a rough surface was investigated. Three holographic lenslet arrays were recorded for different state of the sample. Subsidiary control is realized by the holographic interferometry in real time. The theoretical analysis of the Talbot effect in beams with aberrations is carried out by the methods of computer simulation and physical experiment. The influence of the spatial parameters of a grating (aperture size, period and profile) has been investigated. It is shown that the gratings with narrow angular spectrum (for example, with the sinusoidal transmission) are the most suitable for wavefront sensing. It is shown that in the case of full-aperture illuminating wavefront reconstruction on small dimension array gives accurate results agreeing with the Shack-Hartmann sensor measurements. The grating image is getting worse when the illuminating aperture is reduced closely to the reconstruction area. In particular, two side rows and columns, next to each aperture border, should be excluded from the analysis. The adaptive Talbot wavefront sensor with an extended measuring range and high angular sensitivity is proposed; it is based on the adaptation of the parameters of the diffraction grating of the sensor to the curvature of the test wavefront. The iterative adapting of the grating in the Talbot sensor considerably expands the measurement range for the second order aberrations, keeping the angular sensitivity constant. In the work TFT liquid crystal spatial light modulator is used as a periodical binary amplitude grating. The experiment shows that the adaptive Talbot sensor is able to detect the small local slopes of the wavefront on the background of the static second order aberrations.