Multi-wavelength pinhole point diffraction interferometry for optics metrology with interferometric intensity based phase retrieval method

In this paper, we propose a multi-wavelength pinhole point diffraction interferometry (MPPDI) and a two-step phase extraction method based on the interferometric intensity information. MPPDI is mainly used to extend the dynamic measurement range of traditional pinhole point diffraction interferometry (PPDI) to realize the measurement of large-aperture and high-order aspheric surface. Spherical/aspherical optics will be test under three different wavelengths, and the corresponding interferograms are recoded by 3CMOS detector at the same moment. By combining different wavelengths, MPPDI can obtain a synthetic wavelength with a larger measuring range and greater flexibility. In interferometry, the choice of the wavelength of the light source has a great influence on the measurement range and accuracy. In MPPDI, we can choose different wavelength combinations to select the most suitable synthetic wavelength according to the asphericity of the tested mirror. To enhance efficiency and reduce the impact of air disturbances during phase shifts, we propose a new phase extraction method based on interferometric intensity to interpret the phase map of surface information. The background intensity of diffraction wavefront is firstly pre-recorded by 3CMOS, then phase-shift interferograms of test mirror is acquired with two-step piezoelectric ceramic driver (PZT) movement. From the analysis, it can be found that fringe modulation has tiny influence on the interferograms intensity. Therefore, based on global intensity distribution of fringe pattern and grayscale information, phase map can be retrieval using proposed two-step processing method (only single phase-shift). The two-step phase extraction method can reduce the number of phase shifts from 9 to 3, which improves the efficiency and reduces errors due to multiple phase shifts. MPPDI and two-step phase extraction we proposed have a larger measurement while reducing the number of phase shifts and avoiding error accumulation due to too many phase shifts. Suitable for large-aperture and high-order aspheric surface measurement.