Spherical concave mirror measurement by phase-shifting point diffraction interferometer with two optical fibers

A phase-shifting point diffraction interferometer (PS/PDI) with two optical fibers has been developed. This system consists of high numerical aperture optics and it forms an interferogram on a CCD without using imaging optics. The positions of the fiber cores were determined by an iterative method for analyzing interferograms. The surface figure of a test mirror was obtained from the measured amplitude and phase by digital holography. The surface figure of a spherical concave mirror was measured at two different axial orientations of the mirror. The difference between the two results had a peak-to-valley value of 2 nm and a root-mean-square (rms) value of 0.85 nm. When the moiré pattern, which is main cause for the difference, is eliminated, the measurement accuracy has an rms=0.15 nm. This procedure can also be used to measure aspherical mirrors. The measurement accuracy when measuring an aspherical mirror is expected to be the same as that when measuring a spherical mirror.