Passive Fiber Optic Quasi-Distributed Sensor Network for Water Level Monitoring at Discrete Points in a Reservoir With Address Multiplexing

Introduction. Monitoring water levels at discrete points in reservoirs is crucial for various industries such as nuclear fuel and energy, chemical, and agriculture. Fiber-optic sensors (FOS) have become essential for remote water level sensing over the past twenty-five years. However, the drawback of quasi-distributed multiplexed FOS networks lies in the use of expensive multiplexing technologies, typically wave technology, as well as interrogation. These technologies require costly and temperature-sensitive components like wideband light sources for the entire C-band and ordered waveguide gratings, for instance, fiber Bragg gratings with different central wavelengths. This study aims at addressing these challenges by designing a passive multiplexed quasi-distributed fiber-optic sensor network for water level monitoring in reservoirs. This network offers improved metrological and functional characteristics while enabling inexpensive address multiplexing and interrogation of FOSs based on addressable fiber Bragg structures (AFBS) using radio-photonic technologies.

Methods. Fresnel-type water pumps generate reflected radiation whose power varies depending on its location in water or air. Each FOS channel contains an AFBS with a common central wavelength and a unique address frequency formed by two symmetrical transparency windows with different spacings, achieved by introducing two phase π-shifts. The output of the channel combiner connects to a photodetector via a fiber-optic backbone, and the information is sent to a radio-photonic interrogator. This interrogator allocates the power of the received signal at a specific address frequency, allowing determination of whether the FOS is in water or air. The address of the FOS is determined by the AFBS's address frequency in its measuring channel.

Conclusion. A simple, fully optical quasi-distributed sensor network for monitoring water levels at discrete points, based on the address multiplexing method, is proposed. This system was demonstrated for monitoring a supply tank for preparing reagent solutions for wastewater treatment plants. Simulation and experimental studies confirmed the feasibility of this system. Statistical analysis of data obtained during a twenty-day test cycle showed a relative error in level measurement of 0.3% and an absolute measurement error of 2 mm. These results are comparable to standard continuous electronic level gauges operating on pressure and microwave radars.