Micro-Spherical D-Shaped No Core Fiber-Based MZI Sensor for RI Sensing With Temperature Compensation

A fiber sensor based on a D-shaped no core fiber (DNCF) composite spherical structure cascaded with fiber Bragg grating (FBG) is proposed and experimentally demonstrated. This sensor is utilized for measuring refractive index (RI) and temperature. The spherical structure can excite high-order modes. To investigate the enhanced sensitivity of the spherical structure on the DNCF, the RI sensing performance is compared between a single-sphere cascaded structure and a double-sphere cascaded structure. FBG is insensitive to RI changes but sensitive to temperature changes; therefore, temperature compensation is achieved using cascaded FBG. The experimental results demonstrate that the proposed sensor exhibits excellent linearity. Within the RI range of 1.430-1.440, the sensitivity of the double-sphere-DNCF sensor with a remaining thickness of 67.2~\mu m is 1196.543 nm/RIU. Meanwhile, the temperature sensitivity is 0.0116 and 0.0108 nm/°C in the range of 25 °C-70 °C. The sensor is a low-cost sensing solution. This cost-effective approach, coupled with the simplicity of the sensor design and fabrication, makes it a viable and economical option for various applications in industries such as biochemistry, food processing, and medical diagnostics.