Digital signal processing for fluorescence-based fiber-optic temperature sensor

The monitoring of the fluorescence lifetime of selected materials has been one of the more successful schemes in optical fiber temperature sensing. In principle, both the rise of the fluorescent signal at constant excitation and its decay are described by a first-order exponential, where the time constant T is a function of the temperature. However few corrections must be introduced to account for instrumental contributions resulting in a baseline offset B, noise and radiation leakage from the excitation source into the detection channel. These corrections can be better applied by Means of a digital signal processing circuit. The system uses modified digital phase sensitive detection with phase locked to a fixed value and the modulation period tracking the measured lifetime. It can compensate for correlated and uncoffelated offsets of the decay signal and to work with very low signal-to-noise ratio. The test results give a typical resolution of 0.1% for decay. Such a system has been applied to measuring the fluorescence decay time of a chromium-doped YAG crystal used as s sensing element of a optical fiber thermometer The calibration of the thermometer has shown a temperature resolution of 0.1degreesC from 0degreesC to 100 degreesC.