Theoretical Analysis of an In-line Thin-core Fiber Based Refractometer

A highly sensitive liquid refractive index (RI) sensor with a single-mode fiber-multimode fiber-tapered thin-core fiber-single-mode fiber (SMTTS) structure can be simulated and the method is the beam propagation method (BPM). The modal perturbation between core and cladding mode happened because of modal mismatch and the transmission spectrum is calculated by BPM. Liquid RI and spectral dip wavelength shift for demodulating RI values is obtained and their relationship is approximately linear. To make a comparison, transmissions of single mode fiber-thin-core fiber-single mode fiber (STS) as well as single mode fiber-multimode fiber-thin-core fiber-single mode fiber (SMTS) are also calculated. It is significantly shown that the interference is improved by the segment of multimode fiber (MMF). And the tapered thin-core fiber (TCF) has the functions not only further enhance the fringe visibility, but also increase the RI sensitivity, which is higher with the thinner thin-core taper diameter. The average RI sensitivity would reach up to −116.7nm/RIU within the RI range of 1.33 to 1.39 if the taper diameter is $53.5\ \mu\mathrm{m}$. This is much higher than the value of SMTS structure. And in the practice application, it could be a slimmer taper in the SMTTS structure to improve the sensitivity just achieving by a simple fabrication process. All process could be finished using fusion splicer, cutter and hydrogen flame.