Sensitivity-Enhanced RI Sensor Based on Cascaded Microfiber Mach-Zehnder Interferometers Near the Dispersion Turning Point With Vernier Effect

A highly sensitive fiber-optic refractive index (RI) sensor with low-temperature cross-sensitivity is proposed and demonstrated. The RI sensitivity of Mach-Zehnder interferometer (MZI), which is generated by the optical path difference (OPD) between HE11 and HE12 modes in tapered microfiber, can be theoretically improved to \pm \infty by operating at the dispersion turning point (DTP), but limited by the narrow free spectral range (FSR) and fragile sensor structure as the waist diameter is less than 3~\mu \text{m} at DTP. To solve the problem of fragile sensor structure and difficulty in micromachining of traditional tapered microfiber working at the DTP, the Vernier effect is introduced by cascading two microfiber MZIs with larger diameters of about 8~\mu \text{m} . The RI sensitivity can be amplified 20\times reaching 46467 nm/RIU with temperature cross-sensitivity only - 1.21\times 10^{-4} RIU/°C in the aqueous solution. Moreover, the magnification factor M can be adjusted desirably as the RI sensitivity and FSR of single MZI can be adjusted by changing the waist diameter and the waist length; then, the RI sensitivity can be improved further exceeding 106 nm/RIU, which is the highest of all the fiber-optic RI sensors to the best of our knowledge. The proposed RI sensor with ultrahigh sensitivity, and robust and reliable structure shows great significance for biochemical trace detection, such as slight RI variations caused by small biological molecules or extremely low concentration biological samples.