Fiber-optic Fabry-Pérot interferometers (FPI) can be applied as optical sensors, and excellent measurement sensitivity can be obtained by fine-tuning the interferometer design. In this work, we evaluate the ability of selected dielectric thin films to optimize the reflectivity of the Fabry-Pérot cavity. The spectral reflectance and transmittance of dielectric films made of titanium dioxide (TiO₂) and aluminum oxide (Al₂O₃) with thicknesses from 30 to 220 nm have been evaluated numerically and compared. TiO₂ films were found to be the most promising candidates for the tuning of FPI reflectivity. In order to verify and illustrate the results of modelling, TiO₂ films with the thickness of 80 nm have been deposited on the tip of a single-mode optical fiber by atomic layer deposition (ALD). The thickness, the structure, and the chemical properties of the films have been determined. The ability of the selected TiO₂ films to modify the reflectivity of the Fabry-Pérot cavity, to provide protection of the fibers from aggressive environments, and to create multi-cavity interferometric sensors in FPI has then been studied. The presented sensor exhibits an ability to measure refractive index in the range close to that of silica glass fiber, where sensors without reflective films do not work, as was demonstrated by the measurement of the refractive index of benzene. This opens up the prospects of applying the investigated sensor in biosensing, which we confirmed by measuring the refractive index of hemoglobin and glucose.

In this paper we investigated the response of a fiber-optic Fabry-Pérot sensing interferometer with thin ZnO layers deposited on the end faces of the optical fibers forming the cavity. Standard telecommunication single-mode optical fiber (SMF-28) segments were used with the thin ZnO layers deposited by Atomic Layer Deposition (ALD). Measurements were performed with the interferometer illuminated by two broadband sources operating at 1300 nm and 1550 nm. Reflected interference signal was acquired by an optical spectrum analyzer while the length of the air cavity was varied. Thickness of the ZnO layers used in the experiments was 50 nm, 100 nm, and 200 nm. Uncoated SMF-28 fiber was also used as a reference. Based on the results of measurements, the thickness of the ZnO layers and the length of the cavity were selected in order to achieve good visibility. Following, the interferometer was used to determine the refractive index of selected liquids.

Optical fiber sensors of refractive index play important role in analysis of biological and chemical samples. This work presents a theoretical investigation of a spectral response of fiber optic microsphere with zinc-oxide (ZnO) thin film deposited on the surface and evaluates the prospect of using such structure for refractive index sensing. Microsphere is fabricated by optical fiber tapering method on the base of a single mode fiber. A numerical model is described and simulation was conducted to assess the influence of the ZnO layer deposition on a reflected signal. The results indicate that ZnO film improves the performance in terms of a potential application in refractive index sensor. Full Text: PDF ReferencesY. Qian, Y. Zhao, Q. Wu, Y. Yang, Review of salinity measurement technology based on optical fiber sensor, Sensors and Actuators B: Chemical, 260, 86–105 (2018). CrossRef M. Jędrzejewska-Szczerska, Response of a New Low-Coherence Fabry-Pérot Sensor to Hematocrit Levels in Human Blood, Sensors, 14, 4, 6965–6976, (2014). CrossRef F. Sequeira et al., Refractive Index Sensing with D-Shaped Plastic Optical Fibers for Chemical and Biochemical Applications, Sensors, 16, 12, 2119, (2016). CrossRef M. Jędrzejewska-Szczerska et al., ALD thin ZnO layer as an active medium in a fiber-optic Fabry–Pérot interferometer, Sensors and Actuators A: Physical, 221, 88–94, (2015). CrossRef M. Hirsch, D. Majchrowicz, P. Wierzba, M. Weber, M. Bechelany, M. Jędrzejewska-Szczerska, Low-Coherence Interferometric Fiber-Optic Sensors with Potential Applications as Biosensors, Sensors, 17, 2, 261, (2017). CrossRef M. Hirsch, P. Wierzba, M. Jędrzejewska-Szczerska, Application of thin dielectric films in low coherence fiber-optic Fabry-Pérot sensing interferometers: comparative study, Proc. SPIE 10161, 101610D (2016). CrossRef J. Pluciński, K. Karpienko, Fiber optic Fabry-Pérot sensors: modeling versus measurements results, Proc. SPIE 10034, 100340H (2016). CrossRef F. Goldsmith, Quasioptical systems: Gaussian beam quasioptical propagation and applications. (Piscataway, NJ: IEEE Press 1998). CrossRef