Spatial phase-shifting polarization point-piffraction interferometer for wavefront measurement

Wavefront measurement is widely used in the field of optical manufacturing, military, astronomy, medical treatment, etc., and it reflects the performance of the optical system through evaluating aberrations. Relevant studies have been carried out by many researchers. Among them, point-diffraction interferometer and spatial phase-shifting interferometer are two significant instruments for the wavefront measurement. Point-diffraction interferometer is a simple self-referencing configuration with high precision, and spatial phase-shifting interferometer can be used in the vibration environment or for measuring the dynamic object. Owing to these advantages, they have been widely used in the field of wavefront measurement. In this paper, to realize the combination of these two techniques, we propose a new method of fabricating a polarization point-diffraction plate. Through laser drilling technology, we fabricate a pinhole at a micron level on a wire grid polarizer with a period and depth of 140 nm and 100 nm respectively, and fabricate a polarization point-diffraction plate. We analyze the principle of laser drilling, the orthogonally polarized reference beam and test beam generation mechanism of the polarization point-diffraction plate. The principle of spatial phase-shifting interferometer is deduced by adopting Stokes vector and mueller matrix. Combining with the spatial phase-shifting system with beam splitter, a spatial phase-shifting polarization point-diffraction interferometer is built. In the experimental apparatus, the diameter of the pinhole on the polarization point-diffraction plate is 10.2 m, the beam splitter is a chessboard phase grating whose period, duty cycle, and etched depth are 34 m, 0.5, and 577 nm respectively, and the phase-shifting component is a 22 wave-plate array which is glued together with a 1/4 wave-plate, a 1/2 wave-plate, a 3/4 wave-plate and a full wave-plate; the four fast-axes of the wave-plates are all along the horizontal direction. The spatial phase-shifting polarization point-diffraction interferometer is used to measure the transmitted wavefront through a collimating lens with a focal length of 550 mm and F#10 which are used on a collimator. The measured peak-to-valley value, root-mean-square value and Zernike fitting coefficients are in good agreement with those obtained by SID4 wavefront sensor made by Phasics corporation in France, which verifies the reliability of the measuring results obtained by spatial phase-shifting polarization point-diffraction interferometer. The spatial phase-shifting polarization point-diffraction interferometer introduces spatial phase-shifting technology into traditional point-diffraction interferometer, thereby achieving real-time wavefront measurement with high resolution and precision, and also improving the immunity to vibration, air turbulence, etc.