Accuracy of scanning interrogation of FBG sensors

František Urban Jr – Radek Helán – František Urban Sr

   Progress in the fibre optic technology spurred the development of optical fibre-based sensors. A successful fibre optics-based element for sensorial systems is fibre Bragg grating. It allows to read the measured value by the spectral shift of light, which is the variable with a good potential for accurate measurement. Various types of the interrogation of the light spectrum reflected from the measuring grating are discussed in this article and the work concentrates on the spectrum scanning that gives the good chances to diminish measuring errors by a proper choice of parameters. The focal point is the accuracy of establishing the correct value of the maximum reflectance wavelength. The significance of the noise in the photoelectric scanning signal for the measuring accuracy is analysed and the crucial factor is found in the minimizing the reflectance spectral width of the measuring grating and using the spectrum of the scanning light with the same or smaller spectral width. An important aspect is the relation of the maximal reflectance and the wavelength width of the measuring grating reflectance spectrum. The article also shows the potentiality of the apodized gratings for improving the scanning interrogation. A specific index apodization profile was analysed and its advantages are discussed.

Keywords: fibre Bragg grating, interrogation, reflectance spectrum linewidth, apodized grating

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