Flat surface measurements on fiber point diffraction interferometer

We present a two-step program for point-by-point flat surface measurements with a fiber point diffraction interferometer (FPDI). The point diffraction wavefront reflected by a flat mirror under test is an aberrated spherical wavefront carrying the surface information of the flat mirror. The aberrated spherical wavefront interferes with a reference point diffraction wavefront through a plate beamsplitter (BS). The aberrations of the plate BS are also measured by the FPDI method. The figure of the flat mirror can be evaluated point by point after correcting the aberrations of the plate BS. This method makes use of the nearly perfect point diffraction wavefront, thus it can assure accurate flatness measurement on the optic under test. This research extends to the field of FPDI applications, and provides a new route for the high precision measurement of flat optics.