DIGITAL OPTICAL-FIBER POINT SENSOR FOR HIGH-TEMPERATURE MEASUREMENT

The development and performance of a high-temperature optical fiber point sensor based on fluorescence decay time of a Chromium-doped material are reported, The device exploits a novel digital signal processing scheme for decay time measurement, It is based on a modified phase-sensitive-detection technique with the phase locked to a fixed value and the modulation frequency tracking the measured decay time, The sensor was calibrated in the temperature range from -25 to 500 degrees C with a 0.1 degrees C resolution, Remarkable features are the system immunity to fluorescent intensity losses and the long-term stability, showing a maximum drift of 0.3 degrees C over more than 600 h

Two-point temperature sensor using an optical fiber-switching scheme

A two-point optical fiber temperature sensor which makes use of a multimode 1 X 2 fiber switch has been developed. It exploits the fluorescence decay of a Nd-doped glass as temperature sensing mechanism. The fluorescence responses from the Nd:glass probes are alternatively routed, via the optical switch, to a DSP which converts the fluorescence decay time into a modulation frequency proportional to it. The system, which demonstrates the feasibility of multi-point temperature sensing with a decay time-based approach, could be easily expanded to multiple- sensor configuration by means of a binary-tree scheme. The probes were tested in the temperature range 0 degree(s)C to 250 degree(s)C. The results of the tests and the calibration of one probe are reported

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°

AN OVEN TO TEST FLUORESCENT-DECAY TEMPERATURE SENSORS

This paper describes the design, construction and testing of a special oven developed by us to characterize fluorescent-decay temperature sensors. The oven is designed to hold sensitive materials of different type, shape and dimensions, and is equipped with a fibre-optic system to convey the pumping and the emission radiation to and from the sensitive specimen. The results obtained over the temperature range -20 to 200-degrees-C show an oven temperature stability of +/- 0.03-degrees-C (at 3-sigma level) and a temperature uniformity of +/- 0.03-degrees-C over the specimen. A transducer based on an Nd:glass phosphor, excited by a laser diode, is calibrated against a platinum resistance thermometer in order to assess the oven performance. A transducer resolution (sensor plus electronics) of +/- 0.2-degrees-C for temperatures between -20 and 100-degrees-C results

Resistance comparisons at nanovolt levels using an isolating current ratio generator

NRC publication: Ye