An Optical Liquid-Level Sensor Based on D-Shape Fiber Modal Interferometer

A novel coreless-D-shape-coreless (CDC) fiber structure for liquid-level sensing is proposed and experimentally investigated. The D-shape fiber with cladding partially etched is sensitive to the ambient refractive index change and the CDC structure transmission dip wavelength shifts related to the length of D-shape fiber immersed in the liquid, so liquid level is measured by monitoring the dip wavelength. The sensitivities for three different liquids with the RIs of 1.333, 1.355, and 1.377 are 191.89, 208.11, and 213.80 pm/mm, respectively. The temperature influence is also studied. The sensor exhibits a low-temperature cross-sensitivity of -0.128 mm/°C. The proposed modal interferometer based on the D-shape fiber with high sensitivity, low cost, and easy fabrication is suitable to liquid-level sensing applications.