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.
Highlights
Fiber Bragg Grating (FBG) sensors are the most widely used fiber optical sensors for temperature, strain and displacement measurements in civil, chemical and biochemical engineering [1,2,3] because of their well mastered and repeatable fabrication technology, combined with high sensitivity, light weight and electromagnetic immunity
Another advantage of FBG is that the measurement signal is encoded into the wavelength; so it is very robust to intensity noise, a property highly appreciable in conditions that require long fiber spans to remote the measurement system or when sharp bends in the fiber are necessary to reach the measurement point [4]
Most recent commercial FBG interrogators for in-field sensing applications mainly adopt Charge-Coupled Device (CCD) spectrometers [5], which provide quite a fast response, but present a CCD resolution limited to about 100 pm, a value 50 times larger than what would be desirable
Summary
Fiber Bragg Grating (FBG) sensors are the most widely used fiber optical sensors for temperature, strain and displacement measurements in civil, chemical and biochemical engineering [1,2,3] because of their well mastered and repeatable fabrication technology, combined with high sensitivity, light weight and electromagnetic immunity. Measurement signal wavelength encoding, represents a drawback because it makes the sensor interrogation system more complex: in a standard FBG the peak wavelength shifts by about 10 pm for a 1 °C temperature variation; so, a wavelength resolution in the order of the picometer is required to obtain a temperature resolution comparable to that of other commonly used sensors, such as thermocouples and electronic devices. Such a spectral resolution, is well below the capabilities of typical reasonably low-cost and portable spectral measurement systems (here bench-top lab equipment is not considered), which have a resolution limited to hundreds of picometers. In this paper we propose a temperature sensing system able to overcome these limitations by using a bare FBG as the sensing component, and a spectral fitting procedure to improve the resolution of standard spectrometers without increasing the scanning time
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