Liquid-Level Sensing Based on Periodic Evanescent Field Absorption from a Multimode Optical Fibre

The working of a robust, long dynamic range, quasicontinuous fibre-optic liquid-level sensor is demonstrated in this communication. The sensor principle is based on periodic evanescent light wave absorption when rising level of liquid gradually replaces the surrounding air medium from a periodically uncladded step-index multimode optical fibre which would cause local variation in normalized frequency parameter (V) of the fibre. With our proposed sensor design, liquidlevel variation as small as 2.5 cm can be measured with high accuracy and repeatability, which can be further enhanced by reducing the spacing between two adjacent regions of the fibre. Owing to its simplicity and robustness in operation, we envision that the proposed sensing technique could emerge as an alternative to the existing optical-based liquid-level sensors.