Bio-Chemical Sensors Based on Excessively Tilted Fiber Grating

Excessively tilted fiber gratings (ExTFGs) are a type of special optical fiber grating device different from traditional fiber Bragg gratings, long period fiber gratings, and tilted fiber Bragg gratings. Due to the excessively tilted fiber fringe structure in the fiber core, ExTFGs could couple the light of the core mode into the high-order forward-propagating cladding modes, which would split into two sets of polarization dependent modes resulting in dual-peak resonances in the transmission spectrum. ExTFGs have the properties of the high refractive index sensitivity and low thermal crosstalk, which makes them very suitable for biochemical sensing applications. This paper will review the development of ExTFGs in terms of the mode coupling behavior, spectra characteristic, especially the refractive index sensitivity enhancement, biochemical modification methods of the sensor, and their applications in the bio-chemical sensing area, including pondus hydrogenii (pH) heavy metal ions, humidity, glucose, and immune sensing for various animal virus and biomarkers. Moreover, several composite sensing structures based on ExTFGs will be summarized.