An optical fiber interferometric refractometer for alcohol gas concentration and low refractive index (RI) solution (with 1.33–1.38 RI range) measurement is theoretically and experimentally demonstrated. The refractometer is based on a single-mode thin-core single-mode (STS) interferometric structure. By embedding a suitably sized air cavity at the splicing point, high-order cladding modes are successfully excited, which makes the sensor more suitable for low RI solution measurement. The effect of the air cavity’s diameter on the sensitivity of alcohol gas concentration was analyzed experimentally, which proved that RI sensitivity will increase with an enlarged diameter of the air cavity. On this basis, the air cavity is filled with graphene in order to improve the sensitivity of the sensor; and the measured sensitivity of the alcohol gas concentration is −1206.1 pm/%. Finally, the characteristics of the single-cavity structure, graphene-filled structure and double-cavity structure sensors are demonstrated, and the linear RI sensitivities are −54.593 nm/RIU (refractive index unit), −85.561 nm/RIU and 359.77 nm/RIU, respectively. Moreover, these sensor structures have the advantages of being compact and easily prepared.
Graphene's optical absorption coefficient increases linearly with the number of layers making it more effective in the construction of optical tuning graphene-based devices. Refractive index (RI) is one of the important optical parameters of the graphene for accurately describing its optical characteristics and further applications. In view of the RI research of the multilayer graphene is lacking and existing measurement methods are complicated. Optical power tuning RI of multilayer graphene is investigated using a simple measurement and no temperature cross sensitivity all optical fiber sensing structure. Optical power tuning RI characteristics of multilayer graphene are studied by tuning the introducing broad band light power from 0.57 mW to 22.7 mW. Different thickness graphene coating shows different tuning efficiency. At 4.86-µm thickness, a 3.433-nm Bragg wavelength shift is obtained with 156.2-pm/mW wavelength versus optical power tuning sensitivity corresponding to 3.25×10 3 RI change and 0.154 URI/W (URI, unit of RI) RI optical power tuning efficiency.
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