Light diffusing fibers for liquid level sensing

In this work, we demonstrated the possibility realize a continuous measurement of liquid level based on light diffusing fibers (LDFs). The sensor consists of two parallel LDFs coupled together. The illuminating fiber is connected to a laser diode. The light scattered into the liquid is then coupled, always by the scattering, to the detection fiber and delivered to a detector. By setting a working wavelength that is strongly absorbed by the liquid, the power coupled between the fibers depends on liquid level. The sensor is made by a polymeric beam of Polyvinyl chloride (PVC), on which two LDF (3M Fibrance) with a diffusion length of 1m have been glued side by side at a distance of 1.5mm. The fiber has a core diameter of 170 μm, a low-index polymeric cladding with a diameter of 230 μm, and loose tube PVC jacket with an outer diameter of 900 μm. As light source a 1550nm fiber coupled laser diode is used. At this wavelength, water, employed as the test liquid, exhibits a strong absorption (=1210 m-1). A high sensitivity photodetector connected to a data acquisition module (DAQ) is used for measuring the detection fiber output power at different liquid levels The optical coupling phenomena between the fibers could be modelled by coupled power equations. Co-propagation and counter-propagation coupling configurations have been analyzed and experimentally validated. The measurements results are in good agreement with the theory, and demonstrate that both configurations could be used for liquid level sensing. The counter-propagation configuration exhibits a nonlinear response as function of the liquid level, while the co-propagation coupling configuration response is linear simplifying the calibration procedure. In the co-propagation configuration, the resolution ranges from ±8mm at low liquid level up to ±2mm at high liquid level over a 1m length measurement range.