All-fiber-optic sensor for liquid level measurement

An experimental realization of a simple all-fiber-optic sensor for liquid level measurement is demonstrated. It is an intensity-modulated on-off switching sensor whose operating principle is based on the frustrated-total-internal-reflection effect caused by the refractive-index change of the surrounding medium. The basic optical element in the scheme is a sensing single fiber which serves simultaneously to transmit both the forward laser emission to the investigated medium and the useful signal backreflected from the fiber tip that is the sensing element. In order to achieve a maximum sensor sensitivity, we use a specially shaped fiber tip in two profile variants: angled and retroreflecting. The experimental investigations of the sensor properties and the results obtained at the pure-water level measurement confirm the sensor potential in liquid level determination with a high accuracy, exceeding 10 mu m. [S0034-6748(99)01305-2].