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Fiber optic refractive index (RI) sensors have attracted considerable attention for chemical and biochemical monitoring applications over the past few years due to their useful characteristics such as compact size, high-resolution detection, excellent aging characteristics, ability to operate in chemically hazardous environments, and immunity to electromagnetic noise. Many researchers have tried to enhance the effectiveness of optical fiber RI sensors by improving sensitivity 1, enhancing resolution 2, simplifying fabrication techniques 3, dropping cost 4, increasing the robustness of sensor structure 5, and reducing insertion loss 6.

Gratings and interferometers are the two main configurations studied for fiber optic RI sensing 7. Although the long period gratings (LPGs) are one of the broadly used RI sensors 8-12, writing gratings are usually expensive and function only in limited wavelength bands due to the phase matching phenomenon of fiber gratings. In-fiber interferometers such as Mach-Zehnder interferometer (MZI), Michelson interferometer (MI), and Fabry-Perot interferometer (FPI) have been introduced as alternative and viable approaches for RI sensing 13. Also, the combination of interferometers and gratings has been reported in the literature; for instance, MZI has been constructed based on a pair of LPGs to increase RI sensitivity further 14, 15.

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Recently, MZI based RI sensing has gained considerable attention due to its enhanced sensitivity and fabrication simplicity. Alternative configurations for MZI sensors can be achieved utilizing various fiber types (such as multimode 4, 13, 14, microfiber 15, or photonic crystal fiber (PCF) 13, 16, 17) or fabrication techniques (such as surface plasmon resonance (SPR) 2, 18, 19, core mismatch 13, 20, 21, and tapering 13, 22). For example, concatenation of single-mode abrupt taper and core-offset section is proposed to form an MZI 23. The sensor showed a relatively low RI sensitivity of 28.2 nm/RIU for a sensor length of 30 mm and its fabrication involved complex steps. In 2015, Zhao et al. 24 reported a 30 mm long all-fiber MZI-based RI sensor by splicing an SMF stub between two SMFs with small core offset at two splicing points. The fabricated sensor showed the RI sensitivity of 78.7 nm/RIU in the range of 1.333 to 1.374 24. Although the fabrication process was simple and cost-effective, the reported RI sensitivity was not yet very high. Another MZI sensor for RI measurement based on sandwiching and core-mismatched splicing of an SMF between two short sections of thin-core fibers was proposed by Rong et al. 20. The maximum RI sensitivity of 159 nm/RIU for water-based solutions with an RI close to 1.33 was reported. In 2015, a PCF taper-based MZI for RI sensing was presented with RI sensitivity of 51.902 nm/RIU by Wu et al. 22. The MZI sensor was fabricated by splicing a stub of PCF between two SMFs followed by tapering of the PCF. Such MZI configuration maybe not feasible for many monitoring applications due to its weak mechanical strength and the use of expensive fiber. An inexpensive and simple to fabricate RI sensor with RI sensitivity of 158.4 nm/RIU based on cascaded two SMF tapers was demonstrated by Wang et al. in 2016 25. The fabrication of an MZI sensor for RI measurement from a long tapered single mode fiber was proposed by Yadav et al. 26. The resultant sensor exhibited an ultra-high RI sensitivity of about 1500 nm/RIU in the limited RI range of 1.3325 to 1.3377 which was used in protein concentration detection.

In this work, we present an ultra-high sensitivity, simple to fabricate, low cost, and mechanically robust in-line MZI based RI sensor constructed by tapering an SMF. A customized flame-based tapering machine was used to achieve sharp taper transitions and a uniform long taper waist that creates the MZI effect in an SMF. For a specific taper waist length, the dependence of sensor’s RI sensitivity on taper waist diameter was investigated. In the RI range of 1.333 to 1.38, the refractive index sensitivities of 203 nm/RIU, 230 nm/RIU, 250 nm/RIU, 292 nm/RIU, and 415 nm/RIU were achieved for sensors with taper waist diameters of 62 µm, 51.5 µm, 49 µm, 40 µm, and 35.5, respectively. A maximum RI sensitivity of 4234 nm/RIU was attained in the RI range of 1.4204 to 1.4408 for taper waist diameter 35.5 µm and taper waist length of 19.8 mm.

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