Observation of Fresnel diffraction in a two-beam laser interferometer

A displacement-angle interferometer capable of ${10}^{\mathrm{-}6}$ resolution in fringe division was developed for the precise measurement of the silicon (220) lattice spacing by x-ray and optical interferometry. With a view to achieving ${10}^{\mathrm{-}8}$ measurement uncertainty, the interference pattern was studied by the Fresnel (Gaussian) scalar approximation of the free-space propagation of interfering beams. Imperfect alignment and diffraction phenomena having been identified, and subsequently experimentally proved, as important error sources, remedial steps were identified and taken with consequent improvement of the experiment accuracy. The investigation brought into light theoretical and experimental evidences of corrections to the interference phase which were overlooked in previous analyses.