Abstract
We proposed a refractive index (RI) sensor employing two twisted tapered plastic optical fibers (POFs). The tapered POFs were fabricated by a heating and drawing method and were twisted around each other to form a coupled structure. The sensor consisted of two input ports, a twisted region, and two output ports. The tapered POF could make the light couple from one POF to the other easily. The twisted tapered POFs could constitute a self-referencing sensor, and by monitoring the changes of the coupling ratio, the variations of the external medium RIs could be measured. The RI sensing performances for the sensors with different fiber diameters and twisted region lengths were studied. The sensitivities of 1700%/RIU and −3496%/RIU in the RI ranges of 1.37–1.41 and 1.41–1.44 were obtained, respectively. The sensor is a low-cost solution for liquid RI measurement, which has the features of simple structure and easy fabrication.
Highlights
Refractive index (RI) sensing plays an important role in many applications, such as biochemical analysis, food industry, environment monitoring, and so on
The tapered plastic optical fibers (POFs) were fabricated by a heating and drawing method
The results are shown in Figure 2; it was found that the transmission loss was less obvious between the twisted tapered active POF and the non-twisted one, but it decreased when the twisted region was surrounded by water or glycerin solution
Summary
Refractive index (RI) sensing plays an important role in many applications, such as biochemical analysis, food industry, environment monitoring, and so on. RI sensors based on tilted fiber Bragg gratings [1], long period fiber gratings [2], multi-mode interference [3], etc These sensors usually require complicated fabrication processes and employ wavelength modulation methods which require expensive demodulation devices in practical applications. Many coupled structured optical fibers sensors have been proposed, involving both glass optical fibers and POFs. References [16,17] presented a micro-glass fiber coupler for RI sensing, but the microfiber is very fragile, and the test equipment is expensive. Proposed a twisted ordinary commercial bare POF for liquid level detection, but the coupling efficiency of the ordinary bare fibers was very small, and the output signal was so weak that the signal-to-noise ratio was very low. The sensor could detect RIs higher than that of the fiber cladding and has the advantages of low cost and easy fabrication
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