Fibre Bragg grating (FBG) sensors, for which the working principle and main features are presented further below in this paper, can be made by the deposition of a hygroscopic swelling material on the fibre. In this work, we report novel relative humidity sensors realized by functionalising fibre Bragg gratings with chitosan, a moisture-sensitive biopolymer never used before for this kind of fibre optic sensor. A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others.
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