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 generated by the optical path difference between HE ₁₁ and HE ₁₂ modes in tapered microfiber, can be theoretically improved to ±∞ 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 um at DTP. To solve the problem of fragile sensor structure and difficulty in micromachining of traditional tapered microfiber working at the DTP, Vernier effect is introduced by cascading two microfiber Mach-Zehnder interferometers (MZIs) with larger diameters of about 8 um. The RI sensitivity can be amplified 20 times reaching 46467 nm/RIU with temperature cross-sensitivity only -1.21×10 ^-4 RIU/°C in 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 10 ⁶ 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, 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.