A Reliable Anti-Interference Temperature Sensor Based on a Four-Hole Microstructure Optical Fiber Mach–Zehnder Interferometer

In this paper, an in-line Mach-Zehnder interferometer (ILMZI)-based temperature sensor of a sandwich-like composite structure was proposed and experimentally demonstrated. Using simple cleaving and splicing operations, ILMZI was formed by compactly sandwiching a section of four-hole microstructure optical fiber (FHMF) of ultra-short length between two segments of multi-mode fibers and single-mode fibers in turn. Thanks to the large thermo-optic coefficient difference between the silica and air modes in FHMF, the sensor achieves a comparable sensitivity of 67.29 pm/°C over a wide dynamic range of 20.8-100.4 °C. Because the sensing area length is only a few hundred microns, the long-term stability and anti-interference of the sensor is greatly enhanced. In addition, the large airhole design confines the propagation mode in silica medium by total internal reflection, reducing the interaction between the liquid sample and the sensing element, making the sensor inert to the ambient refractive index. The maximum response deviation of the sensor under micro-vibration disturbances is 3.51×10-4nm/°C, and the maximum cross-sensitivity under strain disturbances is only 0.0018 °C/με. The proposed sensor is excellent in performance, easy in fabrication, simple in structure and low in cost, which can be expected to be used in practical temperature sensing applications without any additional packaging.