Flexible wearable fiber optic sensor for real-time human breath monitoring based on Fabry-Perot interferometer with agar film

A flexible wearable sensor based on the Fabry-Perot (FP) structure for human breath monitoring is proposed and demonstrated. This sensor is constructed by fusing a single-mode optical fiber to a small section of capillary and coating the end with agar film. In high relative humidity (RH) environments, the agar film absorbs moisture from the air, which leads to a change in the length of the FP cavity to cause a shift in the interference spectrum. The sensor has a sensitivity of 0.95 nm/%RH for testing RH in the range of 60%RH to 85%RH. The temperature sensitivity of the sensor varies nonlinearly from 30°C to 90°C, averaging 0.34 nm/°C over the range of 30°C to 55°C and 0.06 nm/°C over the range of 60°C to 90°C. The temperature and RH crosstalk problems can be solved by using cascaded fiber Bragg grating followed by matrix operations to monitor temperature and RH parameters simultaneously. In addition, the sensor has a fast response time of 0.88 s and a recovery time of 1.17 s for human breathing. The normal and deep breathing of nine subjects are monitored using the sensor, demonstrating that the sensor is able to recognize normal and deep breathing accurately. The success of this sensor proves that it is feasible to develop sensors with good resistance to vibration interference, good repeatability and high stability for human breath monitoring, which is of great significance for research on human breath monitoring.