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/ o C in the range of 25 o C –65 o C , and the maximum measurement error changes caused by temperature is −0.0170‰/ o C . It shows that the sensor is very sensitive to RI yet insensitive to temperature. • An in-line Mach-Zehnder interferometer is proposed. It uses multilevel micro spindle structures, photonic crystal fibers and tapered multimode fiber structure to improve sensitivity and extinction ratio. • The sensor’s refractive index sensitivity reaches to −2958.875 nm/RIU in the range of 1.33–1.334. The extremely high sensitivity for refractive index is able to detect the subtle change of liquid’s refractive index. • In the range of 25 °C–65 °C, the maximum temperature sensitivity is 6.4 pm/°C. It shows that the sensor is insensitive to temperature which can avoid the crosstalk between the detection of refractive index and temperature.