The development and performance of a temperature optical fibre sensor is presented in the article. It is based on fluorescence lifetime measurement, exploiting a simple digital signal processing scheme for on-line fluorescence lifetime determination. Experimental implementation of the sensor was preceded by a numerical ray tracing Monte Carlo optimisation of probe collection efficiency. It was shown that one-fibre probes are a preferred alternative from a collection efficiency perspective. Two- and multiple fibre probes can be optimised by proper construction to achieve several times higher efficiency than previously reported probes. Model and modified digital lock-in signal processing scheme were experimentally confirmed by a set of measurements, using alexandrite crystal as a sensing element, with HeNe laser source for excitation. Accuracy of ±0.3 K was achieved in the 20÷90 °C temperature range with almost basic equipment.

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