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An interferometric method for measurement of displacement based on a 3 × 3 fused fibre-optic directional coupler is described. The target position within one interference fringe is obtained with high resolution from phase-shifted signals generated in photodiodes attached to the input ports of two couplers. With the help of the spiral Lissajous representation of the same signals the degree of coherence and then the order of the interference fringe are determined. A resolution of around 50 pm over the range of 15 µm has been achieved with a superluminescent diode as a source.
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Tomić et al. (2002) studied this question.
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