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Performance Assessment of a fast Temperature Sensing System Based on bare FBGs and fast Spectrum Analyzer
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1  politecnico di torino

Abstract: Fiber Bragg Gratings (FBGs) are one of the most widespread types of fiber sensors for measuring temperature and deformations because of their robustness and sensitivity, combined with the possibility of multiplexing to form complex multipoint sensing systems. Traditional interrogation techniques - especially in the case of multiple sensors - are based on broadband sources and spectrum analyzers; this solution, although effective, presents however two weaknesses: resolution (at least for not too expensive devices) and time necessary for the spectral scanning. Recently fast spectrometers have been introduced in the market, but their resolution is still limited to about 100 pm, which corresponds to about 10 °C when used for temperature sensing.The paper presents a fast and high resolution temperature sensing system based on such fast spectrum analyzers and critically assesses its performance. The resolution has been improved by using a spectral fitting method with Gaussian functions and a resolution of 0.18 °C has been achieved. On the other hand, the time response depends not only on the interrogation system but also on the sensor itself so, in the proposed setup, bare FBG sensors are used instead of more common FBG temperature sensors for their reduced footprint that allows point measurements and very small time constant, comparable with thermocouples. The temperature behavior of bare FBGs , however, is typically not provided with the required accuracy by FBG manufacturer; moreover they exhibit a strong cross-sensitivity with strain, so a proper preliminary characterization is necessary before their practical application. This has been carried out with an environmental chamber and an acquisition and processing system specifically developed to manage fast spectrum analyzers. Given its performance, the potential applications of the proposed sensing system are mainly in the medical field where a non-metallic temperature probe with small dimensions and fast response is required, like in measuring temperature during thermal ablation of cells.
Keywords: fast temperature measurement, fiber optical sensors, fiber bragg gratings.