Digital signal processing for fiber-optic thermometers

A digital signal processing scheme for measurement of exponentially decaying signals, such as those found in fluorescence lifetime-based fiber-optic sensors, is discussed. To measure the time constants of the exponential decay, the system uses modified digital phase-sensitive-detection with the phase locked to a fixed value and the modulation period tracking the measured lifetime. A key feature of this system is its ability to compensate for correlated and uncorrelated offsets of the decay signal and to work with very low signal-to-noise ratio (SNR=3). The test results give a typical resolution of 0.05% for slow decay (λ>500 μs) and of 0.1% for fast decay. The system nonlinearity, after the correction for the electronic time constant, is 0.1%. Such a system has been applied to measuring the fluorescence decay time of a chromium-doped YAG crystal used as a sensing element of a fiber-optic thermometer. The calibration of the thermometer has shown a temperature resolution of 0.1°C from -25°C to 500°C