An ultra-thin silver-coated tilted fiber Bragg grating (TFBG) sensor with clear surface plasmon resonance (SPR) together with strong evanescent wave in transmission for "surface" and "bulk" surrounding refractive index (SRI) measurement is proposed and experimentally demonstrated. The thickness of the silver coating over the fiber surface is precisely controlled at 1216 nm (much thinner than 4050 nm for traditional SPR excitation). The transmission spectrum of the sensor provides a fine comb of narrowband resonances that overlap with the broader absorption of the surface plasmon and thus provide a unique tool to measure small shifts of the plasmon and identify the "surface" SRI changes with high accuracy. Meanwhile, the ultra-thin nanometric-coating permits part of high-order cladding modes to become leaky modes which have a large sensitivity to variations in the background solution for "bulk" SRI measurement. Experimental results show that above two resonances have an inverse amplitude responses to the SRI changing. Biological solutions (urine of rats with different concentration of Aquaporin) with different RI ranging from 1.3400 to 1.3408 were clearly discriminated in-situ by using the differential amplitude monitoring between "cut-off" cladding resonance and plasmonic resonance, with an amplitude variation sensitivity of 8100 dB/RIU and a limit of detection of 10-5 RIU.

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Keywords Nanometric coating, Optical fiber sensor, Surface plasmon resonance, Tilted fiber bragg grating
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Conference 24th International Conference on Optical Fibre Sensors, OFS 2015
Qiu, X. (Xuhui), Guo, T, Liu, F. (Fu), Guan, B.-O. (Bai-Ou), Tam, H.-Y. (Hwa-Yaw), & Albert, J. (2015). Ultra-Thin silver-coated tilted fiber grating for surface and bulk refractive index measurement. Presented at the 24th International Conference on Optical Fibre Sensors, OFS 2015. doi:10.1117/12.2194721