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Abstract
In this paper, we describe the first demonstration of an optical fiber sensor based on two cascaded architectures of the Mach–Zehnder interferometer (MZI) with an up-down-tapered (UDT) hetero-core fiber structure for simultaneous pH measurement at two different spatial locations. The two fiber structures, namely structures I and II, were fabricated by sandwiching a 45 mm and 60 mm long piece of no-core fiber (NCF) between two single-mode fibers (SMFs), respectively. By inserting a down-taper between two adjacent up-tapers in the NCF section using the over-fusion splicing technique, the UDT hetero-core fiber structure was achieved. The down-taper works as an optical attenuator, while the two up-tapers each function as a fiber splitter/combiner. By cascading the two sensor structures I and II, two distinct interference dips were obtained, resulting in dual sensing points (so-called sensing points I and II). According to experimental results, it was shown that the proposed sensing points I and II are pH-sensitive, and exhibit sensitivities of 1.0428 nm/pH and ${-}{1.7857}\;{\rm nm/pH}$ for simultaneous measurement for each point as the pH ranged from 8 to 14 and from 1 to 7 pH, respectively. The obtained results show that the proposed dual point pH sensor has the potential to be used for the simultaneous detection of pH parameters in any environment and at various places.
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