All-fiber-optic evanescent liquid level and leak sensor

Summary form only given. Evanescent fiber-optic sensors based on the principle of total-internal-reflection are an attractive class of intensity-modulated sensors that provide simple and effective techniques for measurements and control of various physical and biomedical quantities. In this report, we present the experimental realisation of a simple all-fiber-optic evanescent sensor for liquid level and leak detection. The principal sensor arrangement includes an intensity-modulated on-off-switching scheme based on the frustrated-total-internal reflection effect caused by the refractive-index change of the surrounding medium and the occurrence of an evanescent field at the fiber-liquid boundary. The key optical element in the scheme is a single multimode fiber used in the specific multifunctional regime where it serves to transmit both the forward laser emission to the investigated medium and the useful signal backreflected from the fiber tip that is the sensing fiber part. To maximize the sensor sensitivity, we use a specially shaped tip in two modifications, angled (porro-prism) and retroreflecting (corner-cube) profile. When the fiber tip does not hit the liquid level, a maximum backreflected signal is detected due to the total-internal-reflection. However, when the fiber tip goes into the liquid medium, the tip becomes "transparent" for the laser emission because of the frustrated-total-internal-reflection and the backreflectance signal decreases to zero. Thus, the combination of the specific fiber features provides a high signal-to-noise ratio and consequently, a high accuracy of liquid level and leak measurement.