Effects of birefringence on Fizeau interferometry that uses polarization phase shifting technique

Interferometers that use different states of polarization for the reference and test beams can modulate the relative phase shift using polarization optics in the imaging system. This allows the interferometer to capture simultaneous images that have a fixed phase shift, which can be used for phase shifting interferometry. Since all measurements are made simultaneously, the interferometer is not sensitive to vibration. Fizeau interferometers of this type have advantage over Twyman-Green type systems because the optics are in the common path of both the reference and test wavefronts, therefore errors in these optics affect both wavefronts equally and do not limit the system accuracy. However, this is not strictly true for the polarization interferometer when both wavefronts are transmitted an optic that suffers from birefringence. If some of the components in the common path of the reference and test beams have residual birefringence, the two beams see different phases. Therefore, the interferometer is not strictly common path. As a result, an error can be introduced in the measurement. In this paper, we study the effect of birefringence on measurement accuracy when different polarization techniques are used in Fizeau interferometers. We demonstrate that measurement error is reduced dramatically for small amount of birefringence if the reference and test beams are circularly polarized rather than linearly polarized.