“…The most widely used optical sensors are those based on the surface plasmon resonance (SPR) phenomenon [ 20 , 21 , 22 ], but many other optical transduction techniques have been proposed [ 23 , 24 ], such as fluorescence [ 25 ], evanescent fields [ 26 ], interferometric techniques [ 27 ], fiber Bragg gratings (FBGs) [ 28 , 29 , 30 , 31 , 32 ], long-period fiber gratings (LPFGs) [ 33 ], and intensity-based sensors [ 34 , 35 , 36 ].…”