title: Closed-loop phase stepping in a calibrated fiber-optic fringe projector for shape measurement.
abstract: Active homodyne feedback control can be used to stabilize an interferometer against unwanted phase drifts introduced by, for example, temperature gradients. The technique is commonly used in fiber-optic sensors to maintain the fiber at its most sensitive (quadrature) position. We describe an extension of the technique to introduce stabilized, pi/2-rad phase steps in a full-field interferometer. The technique was implemented in a single-mode, fiber-optic interference fringe projector used for shape measurement and can be easily applied to other fiber- or bulk-optic interferometers, for example, speckle pattern and holographic interferometers. Fresnel reflections from the distal fiber ends undergo a double pass in the fibers and interfere at the fourth port of a directional coupler. The interference intensity (and hence phase) is maintained at quadrature by feedback control to a phase modulator in one of the fiber arms. Stepping between quadrature positions (separated by pi rad for light undergoing a double pass) introduces stabilized phase steps in the projected fringes (separated by pi/2 rad for a single pass). A root-mean-square phase stability of 0.61 mrad in a 50-Hz bandwidth and phase step accuracy of 1.17 mrad were measured.
authors.affiliation: Department of Mechanical and Chemical Engineering, Heriot-Watt University, Edinburgh, United Kingdom. a.moore@hw.ac.uk
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Moore
authors.forename: Roy
authors.initials: R
authors.lastname: McBride
authors.forename: James S
authors.initials: JS
authors.lastname: Barton
authors.forename: Julian D C
authors.initials: JD
authors.lastname: Jones