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A novel technique which uses the interference of diffraction patterns to establish the mechanical alignment of butted optical fibers is described, and its application to mode-spot concentricity-error measurement is demonstrated. Complete spatial alignment to within 0.2 μm of relative offset and about 7-min of arc of relative tilt is achieved. In conjunction with a straightforward optical transmission loss apparatus, the method, when applied to mode-spot concentricity-error measurements, provides good agreement with alternative techniques. Accuracies are estimated and improvements suggested.