A closed loop phase control system using an all-fiber optical configuration has been developed for use in phase stepping interferometry. This system drives the relative phase of two interfering beams through a sequence of π/2 rad increments so that the initial relative phase of these beams can be determined. This phase stepping system uses optical fibers to provide spatially uniform phase steps from a flexible, easily aligned optical configuration. In addition, this system uses phase feed back to eliminate phase modulator errors and to compensate for phase drifts caused by environmental disturbances.
Carolyn R. Mercer and Glenn Beheim, "Fiber optic phase stepping system for interferometry," Appl. Opt. 30, 729-734 (1991)