Fibre position effects on the operation of opto-pair fibre displacement sensors

In this article, two optical-fibre probes used in displacement measurements are reported. In one arrangement, two optical fibres with protective buffer jackets are placed together, while in the second arrangement, the protective buffer jackets are removed and the two fibres are placed with the claddings in contact, with a concomitant smaller centre–centre distance. As the core and cladding diameters are the same in both probes, the effect of the centre–centre distance on double-fibre designs can be investigated. The reflected output powers are recorded for the probes as a function of the axial distance from a reflective surface. A theoretical model is also implemented, which considers the effect of centre-to-centre separation of the optical fibres on the sensor performance, and the experimental results are compared with the theoretical predictions. The computed response curve includes a dead region where there is no response to displacement, an active region with an increasing reflection power with increasing displacement and a decaying region in which the reflected power decreases with increase in displacement. The operational performance criteria of the two probes are also investigated. The hysteresis, reproducibility and time stability of the probes are investigated. The hysteresis is approximately 1%, reproducibility approximately 1.1% and time stability is better than 1.5% (600 s time scale) in both probes.