Abstract
In this paper, we report the demonstration of an optical fiber sensor based on the cascaded up-down-tapered (UDT) hetero-core fiber structures in a sequence for simultaneous refractive index (RI) measurement at dual spatial locations for the first time. The two hetero-core fiber structures, namely sensor structures A and B, were fabricated by sandwiching a section of no-core fiber (NCF) with lengths of 45 and 60 mm between two standard single-mode fibers (SMF), respectively. The UDT hetero-core fiber structure was formed by introducing a down taper between two adjacent up tapers in the NCF section by using the over-fusion splicing method. The dual up tapers respectively serve as a fiber splitter/combiner, and the down taper acts as an optical attenuator. The proposed dual point sensing head (sensing point A and sensing point B) was engineered by cascading the two sensor structures A and B to generate two separate interference dips and achieve wavelength division multiplexing. Experimental results show that for the dual point sensing structure, sensing point A and sensing point B are sensitive to RI independently and simultaneously, and exhibit sensitivities of 99 nm/RIU and 93.7 nm/RIU for simultaneous measurement of RI ranging from 1.33 to 1.38 for each point, respectively. During measurement, there is no mutual interference to each sensing head. The results point out that the proposed dual point sensor can be potentially applied for the simultaneous detection of RI in any environment and at different locations.