Fringe-shift generators for characterizing interferometer system response

Two techniques have been developed for the generation of well characterized fringe patterns in displacement and velocity interferometers. In the first, a piezoelectric crystal placed at the interferometer target location is oscillated at resonance with applied ac voltages near 100 V. Quartz or lithium niobate crystals with resonant frequencies near 1 MHz produce surface displacements of 200–1000 nm, and peak surface velocities of 0.4–4.0 m/sec. The fringe-shift patterns produced in the interferometer using this technique can be used to test various aspects of the interferometer performance. The second technique employs a fused silica disk which is rotated in one leg of the interferometer. This rotation introduces a large variation ([inverted lazy s] 0.3 mm) in the optical path length of the interferometer leg during each revolution, giving a fringe shift of hundreds of fringes. This technique is especially useful for maximizing interferometer fringe contrast during setup of the instrument.