Self-Temperature Compensated Fiber-Optic Liquid Level Sensor With Ultra-Long Linear Range Using Quasi-Single Modal Interferometer

In this paper, a thin-core fiber based in-line Mach-Zehnder interferometer is proposed and experimentally completed through symmetric core-offset splicing technique. The light field distribution of fundamental and cladding modes is investigated with varied core-offset and the state of quasi-single modal interference (QS-MI) is attained. The comprehensive tests under QS-MI state are then conducted and compared in term of liquid level sensing. Experimental results show that significant linearity- improvement is achieved in the range from 0 to 500 mm, and the responses of liquid level reach 101 pm/mm (short-range within 20-mm) and 41.8 pm/cm (long-range within 50-cm), respectively. Also owing to high response consistency, the temperature crosstalk is eliminated by means of self-differential compensation and the measured error is compressed within 0.68%. Furthermore,
$\sim 16$
-fold enhancement of detection limit at least is gained through building a fiber ring-cavity laser based active sensing system. It is the first time that the issues of linearity and temperature crosstalk are simultaneously overcome in the continuous long-range liquid level measurement. With the merits of low-cost, large-scale and ease of fabrication, our scheme is very practical and promising in the engineering and applications related to liquid level sensing.