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Polymer Optical Fiber Liquid Level Sensor: A Review

Published in : IEEE Sensors Journal (Volume: 22, Issue: 2, January 2022)
Authors : Runjie He, Chuanxin Teng, Santosh Kumar, Carlos Marques, Rui Min
Summary Contributed by:  Rui Min (Author)

Efficient liquid level measurements are crucial for industrial processes with high safety and accuracy requirements. Traditional liquid level sensors' application range is severely limited by their large size and non-electromagnetic interference. In recent years, polymer optical fiber has shown promising potential for its use in liquid level sensors.

The versatility of optical fiber allows it to meet a wide range of sensing requirements. Polymer materials for manufacturing optical sensors are chosen based on their properties like moisture absorption, elastic modulus, and chemical stability. The most frequently used materials in the manufacturing of POF include silicone, polydimethylsiloxane (PDMS), perfluorinated polymer (CYTOP), and polymethyl methacrylate (PMMA). The POF material's flexibility allows customized production of POFs to fulfill specific requirements of different applications.

An optical fiber sensor can be intrinsic or extrinsic. The intrinsic optical fiber sensor measures the change in its physical properties to detect some specific property of the signal passing through it. The extrinsic sensor uses optical fiber as a transmission medium with an external sensor device at its end.

Extrinsic POF liquid level sensors can be classified based on their measurement principles, such as intensity modulation and wavelength modulation. In an intensity modulation-based liquid level sensor, except for the extrinsic POF sensor, the liquid level is mainly measured by an evanescent field. The principle of the evanescent field is that when the external information changes, the optical transmission in the POF fails to meet the total internal reflection condition, and the extra propagation loss will be excited. Thus, external information gets measured as the change in optical intensity.

The bending and polishing processing of POFs, such as race-track type, V-shaped polished, D-shaped polished, or twisted taper type, helps improve the accuracy of the liquid level sensor. These bent and polished POF-based liquid level sensors are simple, low-cost, and easy to produce. The intensity of the light source may affect the accuracy of intensity-modulated POF sensors. Optical fiber coupling can improve accuracy.

The wavelength modulation-based liquid level sensors are based on fiber Bragg gratings (FBG) and special structure gratings, like chirped FBG. The basic idea behind FBG sensing is to create a narrow-band filter by regularly altering the optical fiber's refractive index (RI) along the axial direction to make it sensitive to light meeting a particular wavelength condition (Bragg condition). Here, the shift in wavelength is used to measure or sense the external environment. The fabrication of a CFBG, an FBG superposition of many distinct Bragg wavelengths, involves modulating the axial RI of the grating or changing the grating period linearly. The wavelength modulation-based POF sensors' excellent precision and fabrication flexibility make them the perfect option for a wide range of applications. POF performs well in liquid level sensing applications.

The innumerable advantages like low cost, light weight, electromagnetic immunity, chemical corrosion resistance, good bandwidth, and simple installation make POFs a perfect candidate for liquid level sensing and measurement. In areas like multi-parameter and real-time measurements, with better precision and smaller size, a more targeted study is required.

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