Fig. 1. Flowchart of sensor preparation and fiber structure. (a) Flowchart of sensor preparation; (b) BTT structure and its size; (c) G652D SMF end face and its size

Fig. 2. Principle of sensor light transmission. (a) Sensor light transmission; (b) schematic diagram of BTT curvature sensor made based on MZI

Fig. 3. Test results of sensor with different SMF lengths. (a) Optical spectra; (b) frequency spectra

Fig. 4. Simulation of light transmission and energy distribution for the BTT-down taper-BTT sensor

Fig. 5. Curvature sensing experimental setup diagram

Fig. 6. Variation in transmission spectrum with curvature of different sensors. (a) Sensor A; (b) sensor B; (c) sensor C

Fig. 7. Linear fitted results of the characteristic wavelengths for different sensors. (a) Sensor A; (b) sensor B; (c) sensor C

Fig. 8. Repeatability curvature tests for sensors A, B, and C. (a) Sensor A; (b) sensor B; (c) sensor C

Fig. 9. Variation in sensitivity of the BTT curvature sensor with pulling cone length

Fig. 10. Temperature sensing experiment of sensor C. (a) Temperature sensing device; (b) temperature response when temperature is 30‒150 °C; (c) transmission spectra when temperature is 30‒150 °C