Spatial-frequency multiplexing of high-sensitivity liquid level sensors based on multimode interference micro-fibers

Abstract

This paper shows a new fiber optic sensor multiplexed system for liquid level sensing. Biconical fibertapers converging at a 40 mm-long micro-fiber are used as transducers. The tapers are designed to providethe propagation of the two main cylindrical modes in the micro-fiber avoiding higher order modes ormodes with other symmetries. The liquid level is calculated in real time from the measurement of thefrequency and phase components of the spectral interference pattern of the submerged micro-fiber.The system is fully characterized by theoretical simulations in terms of the sensitivity as a function ofthe most responsive parameter, which is the width of the micro-fiber. Phase sensibilities of 3.7 rad/mmare experimentally obtained and values as high as 11.4 rad/mm are theoretically predicted. The strongdependence of the spatial frequency with the width of the micro-fiber has been used to multiplex threesensors in series in this domain. The maximum detected crosstalk between sensors is 0.2 rad/mm.