Hybrid structured fiber-optic Fabry–Pérot interferometer for simultaneous bicarbonate and temperature measurements

We present a temperature-compensated Fabry-Pérot interferometer (FPI) fiber-optic bicarbonate concentration sensor based on the Vernier effect. The proposed sensor is fabricated by splicing a section of single-mode fiber (SMF) at the misalignment of the lead-in SMF so that the core of the lead-in SMF is completely exposed, and then splicing a section of multimode fiber (MMF) to align it with the lead-in SMF. This sensor consists of a concatenated structure of FPI1 and FPI2. Vernier effect can be acquired in the presence of close free spectral ranges (FSRs) of FPI1 and FPI2 in water, and the sensor sensitivity could be significantly enhanced accordingly. The fiber-optic sensor FPI shows a concentration sensitivity up to 95.92 nm/M. In addition, the FPI1 is very sensitive to sample concentration because of its open cavity. The concentration sensitivity of FPI1 is 10.189 nm/M while FPI2 is insensitive to concentration variation because of its silicon cavity. Due to the thermo-optic and thermal expansion effects of the open cavity, the fiber-optic FPI sensor shows a temperature sensitivity of 1.217 nm/℃. Specifically, FPI1 exhibits a temperature sensitivity of −0.114 nm/℃ due to the thermo-optic effect of the open cavity liquid, while FPI2 exhibits a temperature sensitivity of 0.051 nm/℃ because of the thermal expansion effect of FPI2. Consequently, the proposed fiber-optic FPI sensor enables dual-parameter measurement of bicarbonate concentration and temperature based on the wavelength interrogation of the interferometric transmission dips by using the transfer matrix method. This approach solves the temperature cross sensitivity issue caused by high thermo-optic coefficient of water sample, which promises in-situ monitoring of ocean carbon sources and sinks.