A two-dimensional noncontact optical fiber water droplet sensor that uses the fluorescence from a laser-diode-pumped Tm3+:YAG and that uses a single fiber bundle for the probe fiber has been constructed. The sensor is based on absorption spectroscopy but has the significant advantage that the residual pump light works for the reference signal. The water depth can be evaluated from the minimum intensity ratio between the fluorescence signal and the reference signal when the probe is scanned in plane. The sensor can also estimate both the size and the position of a water droplet. A high degree of discrimination of a water droplet from other perturbations such as oil and surface irregularities has been demonstrated. An approximate calculation in a hemispherical model of a water droplet was carried out and is shown to be in good agreement with experimental results.
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