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Absolute distance measurements using point-diffracted spherical waves

We describe a novel interferometric scheme that attempts to measure the absolute distance of a three-dimensionally moving target with respect to a reference station. The target transmits two orthogonal pairs of lateral-shearing interferograms generated using four near-perfect spherical waves emitted from single-mode fibers by means of point diffraction. The interferograms are modulated with varying amounts of phase shifting and monitored by the reference station holding a 2-D array of photodetectors. Captured interferograms are analyzed by use of a modified version of the Rimmer-Wyant technique and reconstructed as the spherical wavefront emitted form the center of four fibers in the target. The absolute distance of the target is then determined by fitting the reconstructed wavefront into Zernike polynomials. Experimental results are discussed to verify that the proposed method is capable of measuring the absolute distance along not only the optical axis, but also out of the axis.