Phase shifting interferograms processing for fiber point-diffraction interferometer

To characterize the spherical surface of extreme ultraviolet (EUV), the fiber phase shifting point-diffraction interferometer (FPS/PDI) has recently been designed and implemented. The diffraction wavefront from an optical fiber acts as the perfect referenced spherical wave and new-style interferometer configuration is used. The laboratory apparatus has been put up to measure spherical surface in experiment. The main content of this paper is the key technique of FPS/PDI about the method of interference image processing and the precision analysis for the interferometer. In experiment apparatus, a concave spherical surface is measured and a piezoelectric ceramics (PZT) is used for phase shifting. The wrap phase distribution is got by five-step phase shifting method, and then the unweighted least-squares phase unwrapping algorithm is optimized and used to obtain the unwrapped phase distribution. The error of spherical figure is derived from the fitting method of Zernike polynomials. The data processing result is analyzed in comparison with the measurement result of ZYGO interferometer. The repeat precision of the FPS/PDI is evaluated by multi-measurement. Finally, the major error sources are discussed and some optimized methods for the system are proposed. The results show that the interferometer has achieved worthy measurement precision and has great development potential.