Characterization of an optical fiber thermometer using Tm3+:: YAG crystal, based on the fluorescence lifetime approach

A practically applicable optical fiber thermometer was designed and tested, based on the detection of the decay of the fluorescence in a miniature Tm3+:YAG crystal, directly coupled to a silica transmission fiber and excited by light from a periodically modulated 785 nm laser diode. This work represents a novel development from prior work using silica fibers or other doped crystals and addresses effectively a wide temperature range. The modulation period was varied proportionally to the fluorescence lifetime of the crystal and was used to enable the fluorescence decay rate to be determined, and thus the probe temperature. In the system design, a phased-locked-detection (PLD) scheme was used to process the fluorescence signal and a background radiation compensation circuit was included to prevent the saturation of the signal amplifier at high temperature. Calibration of the system was carried out from room temperature to 800 degreesC and long-term tests have shown that the fiber thermometer could work over in a satisfactory way the temperature range from room temperature to 800 degreesC. Details of the probe construction and the results of tests and evaluation are presented. (C) 2003 Elsevier B.V. All rights reserved.