Optical Fiber Water Droplet Sensor Using Absorption of Fluorescence of Tm(3 +):YAG

A two-dimensional noncontact optical fiber water droplet sensorthat uses the fluorescence from a laser-diode-pumpedTm(3+):YAG and that uses a single fiber bundle for the probefiber has been constructed. The sensor is based on absorptionspectroscopy but has the significant advantage that the residual pumplight works for the reference signal. The water depth can beevaluated from the minimum intensity ratio between the fluorescencesignal and the reference signal when the probe is scanned inplane. The sensor can also estimate both the size and the positionof a water droplet. A high degree of discrimination of a waterdroplet from other perturbations such as oil and surface irregularitieshas been demonstrated. An approximate calculation in ahemispherical model of a water droplet was carried out and is shown tobe in good agreement with experimental results.