Two-mode elliptical core optical fiber sensors for strain and temperature measurement

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Abstract
We propose two sensing methods for strain and temperature measurement based on conventional elliptical core (e core), two-mode fiber strain sensors, that combine the properties of a two-mode, e core fiber sensor and (i) a bimetallic strip or (ii) refractive index changes in a liquid filled hollow core fiber. The output from a conventional two-mode, e core fiber sensor is (i) projected upon a multimode fiber attached to a bimetallic strip or (ii) projected from an angular endface onto the endface of a multimode fiber. The multimode fiber acts as a spatial filter and picks up part of the two-lobe pattern incident upon its core. A measurement of the differential phase variation gives an indication of the strain. The modulation depth of the output signal is shown to be a faithful indication of the temperature in both cases. Strain sensitivities typical of two-mode fiber sensors (0.033 degrees phase shift mu strain-1 cm-1, minimum detectable strains of 0.17 mu m m-1 for 1 m sensing length) have been obtained with a temperature compensation accuracy of +or-1 degrees C over a 75 degrees C range. A theoretical model has been developed and experimental results are shown to match closely with theoretical predictions.