Analysis of the functional properties of a fiber-optic sensor for field monitoring of water quality

Abstract

The article examines the design principles and numerical modeling results of a fiber-optic sensor intended for online field monitoring of turbidity and changes in the optical properties of water samples. The authors justify the choice of a D-shaped geometry for the sensitive segment of a single-mode quartz fiber (initial structure: core ≈ 9 μm, cladding 125 μm, polished section ≈ 20 mm long with a cladding thickness of ≈ 3 μm), which significantly enhances the interaction of the evanescent field with the surrounding medium. For the experimental scenario, an IR laser with a wavelength of λ ≈ 980 nm and an output power of 5 mW was used. In the proposed model, the refractive index of the external medium was varied, corresponding to changes in salinity C in the range of 5–100 g/L. The dependences of the critical angle θ, the penetration depth δ of the evanescent field, and the relative output power P on the parameter C were investigated. The simulation results showed that with increasing salinity C and, accordingly, the refractive index n₂, the critical angle θ increases, which gradually disrupts the conditions of total internal reflection and leads to a significant decrease in the output signal. In particular, in the range C ≈ 0–100 g/L the relative power P decreases by approximately three orders of magnitude, with the most intense attenuation occurring within 0–40 g/L. The penetration depth δ exhibits a non-monotonic dependence with a local maximum near C ≈ 30–40 g/L, which determines a narrow region of increased sensor sensitivity. The sensor is highly sensitive to local (near-surface) changes in the environment due to the small value of δ. This makes it effective for detecting local anomalies, but at the same time limits its capability when it is necessary to assess volumetric properties of water. The results of the study confirm the suitability of the D-shaped fiber-optic design for highly sensitive local monitoring of turbidity and salinity in freshwater and slightly saline water samples, as well as the possibility of tuning the sensor to the working range of environmental parameters. Practical implementation requires well-considered engineering solutions for calibration, temperature compensation, protection of the sensitive segment, and integration with hardware-software systems to ensure stable operation under field conditions.