Polychromatic light interferometer for high-accuracy positioning

Superposition of different-wavelength, multimode- and single-mode laser diode beams is investigated to locate the fringe of zero interference order in a Twyman-Green interferometer. The possibility of central fringe detection using three multimode laser diodes or one single-mode together with a pair of multimode laser diodes is shown. If a single-mode laser diode is applied, a simultaneous fringe-counting technique for displacement measurement is available. The influence of the angle between wavefronts entering the interferometer is analyzed. A repeatability of about 5 nm is shown for surface position determination when using three-beam source.