Central fringe identification by phase quadrature interferometric technique and tunable laser-diode

When the optical path difference is not zero, the phase of the interferogram is strongly influenced by the wavelength variation of the light source. Under this condition, the phase variation can be estimated with a phase quadrature interferometric technique. And the central fringe of the interferogram can be identified as the, phase variation remains unchanged. Based on this fact, an improved method to identify the central fringe is presented. It has some merits, such as simple optical setup, easier operation in real time, inexpensive, ability to judge which arm of interferometer is longer, etc. The feasibility is demonstrated with 10 nm resolution. (C) 2001 Elsevier Science B.V. All rights reserved.