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Absolute measurement of the fiber-to-waveguide distance during a coupling process by low-coherence interferometry

During a fiber-to-waveguide connection process, it can be important to determine accurately the absolute distance between the end of the fiber and the edge of the optical waveguide without visual control. We investigate here the use of a low-coherence optical source whose light, transmitted by the fiber, is launched into the waveguide in order to control the alignment. The end faces of the waveguide and of the fiber form a Fabry-Perot cavity for the transmitted light beam. The optical transmission versus the distance exhibits a fringe pattern whose visibility is related to the coherence properties of the broadband source. We deduce the absolute distance from visibility measurements, we assess the accuracy of the method, and the results are compared with the direct distance measurements.