High Sensitivity Refractive Index Sensor Based on Highly Overcoupled Tapered Fiber-Optic Couplers

In this paper, a simple and compact fiber-optic sensor based on an overcoupled tapered fiber coupler is studied. The coupler is fabricated to be operated well beyond the initial coupling cycles, where the rapid exchange of energy between outputs ports enable the fabrication of a highly sensitive device. The suitability and sensitivity of the proposed scheme is demonstrated by measuring refractive index (RI) variations of sugar concentrations in water. The device presents a linear response in terms of power transmission or wavelength shift versus RI changes. The best achieved sensitivity is 0.442 units of normalized transmission per unit of sugar concentration, with a noise detection limit of 0.003 weight percentage of sugar concentration (wt %). From this result the minimum detectable RI change is estimated as $5 \times 10^{\mathrm {-6}}$ RI unit (RIU). The sensor can be also wavelength-encoded, exhibiting a sensitivity of 2171 nm/RIU, maintaining a linear response in a large range of RI. These experimental results are within the best results reported in the framework of fiber couplers and modal interferometer-based RI sensors.