Polarization point diffraction interferometry based on an even aspheric quarter-wave plate

The point diffraction interferometer (PDI), emerging spherical wave used as the reference wave, is one of potential wavefront testing tools with very high accurate. However, it is not feasible in the in situ measurement of the surface error in the polishing process. For the test mirror with low reflectivity, the poor fringe contrast would be formed. It could increase the phase noise, which can't be removed during phase retrieval, thus limiting the measurement accuracy. Therefore, the polarization point diffraction interferometry (PPDI) with even aspheric quarter-wave plate (EAQWP) is proposed for high-precision testing of the high-NA spherical mirror. The EAQWP is employed in the PPDI to transform the polarization states of the test and reference beams, then a good fringe contrast can be realized by adjusting the relative intensities of two interfering waves. Compared with the common quarter-wave plate (QWP), the substrate configuration of designed even aspheric surface reduces significantly the introduced wavefront distortion for large-NA measurement, at the same time, allows to use the same position and orientation of the EAQWP for testing mirrors with different NAs, which facilitates the system alignment in practical optical shop testing. Most importantly, the corresponding pose errors are corrected by computer-aided alignment with a difference restoration model. The viability of the new method and system are verified with an experiment for testing a spherical mirror with 0.04 reflectivity and 0.5 NA, and with a ZYGO interferometer match.