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 fiber tapers converging at a 40 mm-long micro-fiber are used as transducers. The tapers are designed to provide the propagation of the two main cylindrical modes in the micro-fiber avoiding higher order modes or modes with other symmetries. The liquid level is calculated in real time from the measurement of the frequency 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 of the most responsive parameter, which is the width of the micro-fiber. Phase sensibilities of 3.7 rad/mm are experimentally obtained and values as high as of the micro-fiber 11.4 rad/mm are theoretically predicted. The strong dependence of the spatial frequency with the width of the micro-fiber has been used to multiplex three sensors in series in this domain. The maximum detected crosstalk between sensors is 0.2 rad/mm.