Abstract
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.
Highlights
Optical fiber interferometric refractive index (RI) sensors such as in-fiber Mach-Zehnder, Michelson, Fabry-Pérot and Sagnac refractometers are attracting increasing interest in the context of chemical and biochemical sensing applications due to their high sensitivity, well established immunity to electromagnetic interference, small size and their ability to be remotely operated [1,2,3,4]
The simulation results show results show that the cladding mode expands to a higher order with the length of the that the cladding mode expands to a higher order with the length of the cavity increasing, but thecavity increasing, the increase oftocavity length to a greater loss and deterioration of the increase of cavitybut length leads a greater lossalso andleads deterioration of the interference spectrum, which is shown as Figure
Light from a broad band source (BBS) injects into the sensor from its one-end spliced single mode optical fibers (SMF), and the transmission spectrum of the sensor is sent to an optical spectrum analyzer (OSA) from a spliced SMF
Summary
Optical fiber interferometric refractive index (RI) sensors such as in-fiber Mach-Zehnder, Michelson, Fabry-Pérot and Sagnac refractometers are attracting increasing interest in the context of chemical and biochemical sensing applications due to their high sensitivity, well established immunity to electromagnetic interference, small size and their ability to be remotely operated [1,2,3,4] In these existing refractometers, cores mismatch sensing structures are usually used, and the resolution of this kind of sensor is generally 10−3 ~10−4 in the range of 1.3 to 1.4 RI and 10−4 ~10−5 in the range of 1.4 to 1.5 RI according to either low and high concentrations of aqueous solution, respectively [1,2,5]. The maximum sensitivity corresponds to 5.6 × 10−5 RI resolution
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