High sensitivity liquid level sensor based on dual side-hole fiber Mach–Zehnder interferometer

A novel highly sensitive liquid-level sensor based on an asymmetric dual side-hole fiber (DSHF) using the core mode as the sensing mode and the cladding mode as the reference mode is described. The side-hole located close to the fiber core is opened by polishing and used to access the core mode as the sensing mode, the side hole located furthest from fiber core being used to tightly confine the cladding mode as the reference mode. The small core size of the DSHF further enhances the sensitivity. Pure-water-level sensitivities of 1.496 nm/mm, 2.026 nm/mm and 4.019 nm/mm are demonstrated experimentally when the distance between the DSHF core and its proximal (opened) side hole are 1.75μm, 1.6μm and 1.1μm, respectively. The sensor also exhibits low-temperature cross-sensitivity of ∼0.013 mm/°C, ∼0.011 mm/°C and ∼0.006 mm/°C. The fabrication process in this case is not sensitive to the polishing process and its uniformity is determined mainly by the quality of the DSHF itself, which makes it possible to ensure repeatable fabrication. Such reliability of fabrication lends itself well to good reproducibility for potential multiple sensor unit manufacture.