Closed-loop Interrogation Techniques for Temperature Measurement using Fibre Bragg Gratings

Abstract

Closed-loop schemes which maintain the amplitude of the reflected light from the FBG sensors at a constant level are proposed to implement interrogators for FBG temperature sensors. Two systems are presented: a very low-cost system where a thermoelectric cooler is used to tune a DFB laser, and a more complex multipoint system with a broadband laser where n tracking FBGs located inside the interrogator are tuned by n thermoelectric modules. The variation of the laser wavelength with temperature (or the variation of the tracking FBGs with the temperature) is used to perform the measurements. Since the values of temperature in these devices are uniquely associated with the sensing FBG reflected wavelength, if the temperature behaviour of the FBG sensor is known, by measuring the temperature in the laser (or in the tracking FBGs) it is possible to accurately calculate the temperature in the FBG sensors. A single-point sensor prototype was constructed to validate the technique and a very high resolution of ±0.08 m • C was measured in a 100 • C temperature range.