Speckle Interferometry Using Fiber Optic Phase Stepping

A system employing closed-loop phase-stepping is used to measure the out-of-plane deformation of a diffusely reflecting object. Optical fibers are used to provide reference and object beam illumination for a standard two-beam speckle interferom-eter, providing set-up flexibility and ease of alignment. Piezoelectric fiber-stretchers and a phase-measurement/servo system are used to provide highly accurate phase steps. Intensity data is captured with a charge-injection-device camera, and is converted into a phase map using a desktop computer. The closed-loop phase-stepping system provides 90° phase steps which are accurate to 0.02°, greatly improving this system relative to open-loop interferometers. The sys-tem is demonstrated on a speckle interferometer, measuring the rigid-body translation of a diffusely reflecting object with an accuracy of -±100, or roughly ±15 nm. This accuracy is achieved without the use of a pneumatically mounted optics table.