A temperature-insensitive refractive index sensor based on in-line Mach–Zehnder interferometer with micro spindle structures and photonic crystal fibers

A high-sensitivity refractive index (RI) sensor is proposed and designed, which is an in-line Mach–Zehnder interferometer (IMZI) with a photonic crystal fiber (PCF) and multilevel micro spindle structures fabricated by the arc-discharging and tapering method. To improve the sensitivity, a tapered multimode fiber (MMF) is inserted into the PCF. The performance of RI and temperature detection has been analyzed and discussed. The maximum RI sensitivity of −2958.875 nm/RIU is achieved by detecting the NaCl solution. In addition, the maximum temperature sensitivity is 6.4 pm/${}^{\text{o}}\text{C}$ in the range of 25 ${}^{\text{o}}\text{C}$–65 ${}^{\text{o}}\text{C}$, and the maximum measurement error changes caused by temperature is −0.0170‰/${}^{\text{o}}\text{C}$. It shows that the sensor is very sensitive to RI yet insensitive to temperature.