Phase-Shifting Laser Interferometry for Measurement of Surface Form Error

I describe wedge phase-shifting (WPS) and polarization phase-shifting (PPS) Fizeau interferometers (FIs) for the measurement of surface form error of plane surfaces. In WPS, a pair of identical fused silica wedge plates, which together form a variable thickness parallel plate, is used in the FI cavity for introducing phase shifts. For PPS, two different methods are discussed. In the first method, a quarter-wave plate (QWP) is placed in the FI cavity for producing test and reference beams with orthogonal linear polarizations. Another QWP transforms the test and reference beams to opposite circular polarizations of which the components selected by a linear polarizer interfere. Polarization phase shifts are introduced by varying the angular orientation of the linear polarizer. In the second method, a Sagnac interferometer (SI)-based polarization phase-shifting FI is described. A polarization phase-shifting SI is used to generate laterally separated virtual point sources at the back focal plane of a telescope objective for producing tilted plane-wave components with orthogonal linear polarizations. The orthogonal linear polarization components reflected from the reference and the test surfaces are utilized for introducing polarization phase shift between the reference and test waves. The method for elimination of instrumental aberration is discussed.