Thin fiber-based Mach–Zehnder interferometric sensor for measurement of liquid level, refractive index, temperature, and axial strain

An all-fiber Mach-Zehnder interferometric sensor capable of measuring liquid level, refractive index (RI), temperature, and axial strain is proposed and experimentally demonstrated. The proposed sensor is based on a fiber ball-thin fiber (TF)-core-offset structure sandwiched between two standard single-mode fibers. The variations of ambient liquid level, RI, temperature, and axial strain cause the change of phase difference between the cladding modes and the core mode, which leads to the shift of interference spectrum. The wavelength shifts of three resonant dips in the transmission spectrum are used to investigate the sensing characteristics of the sensor. Experimental results show that the sensor with TF length of 20 mm exhibits high RI and liquid-level sensitivities of $ - {131.7092}\;{\rm nm/RIU}$-131.7092nm/RIU and $ - {120.7}\;{\rm pm/mm}$-120.7pm/mm at a wavelength of 1589.5 nm. Meanwhile, the sensor is insensitive to temperature and axial strain, and the maximum sensitivities are 0.0390 nm/°C and $ - {4}.{84}\; \times \;{{10}^{ - 4}}\;{\rm nm}/\unicode{x00B5} \varepsilon $-4.84×10-4nm/µε, respectively. In addition, the sensor shows superiority in measuring multiple parameters simultaneously.