Wearable respiratory sensor based on Mach-Zehnder interferometer in seven-core fiber

In this paper, a wearable respiration sensor based on single-mode–gourd-shaped–seven-core–gourd-shaped–single-mode fiber structure is proposed and experimentally demonstrated for the first time. Attribute to the gourd-shaped structures between the seven-core fiber (SCF) and the single-mode fiber (SMF), the excitation of the supermode as well as the mode coupling in SCF are enhanced, and the sensor probe has good mechanical properties to ensure long-term respiration measurement. The static sensing experiment reveal that the sensor has a maximum sensitivity of −7.039 dB/m−1 in the curvature range of 0–4.66 m−1 with a low temperature cross-sensitivity of −0.03 m−1/°C. The sensor probe is embedded in a flexible textile band and the respiratory response of the sensor is experimentally achieved for different humans, different parts of the body (abdomen, chest, back), different states (motionless, exercising), and irregular respiration. In addition, repeatability and stability tests are carried out, and instability of the sensor is 0.003 Hz in 3 months. The sensor has advantages of high sensitivity, comfortable wearing, good stability, more importantly, the fixed position could be free on the upper body, indicating great potential in real-time respiration monitoring.