Flexible wearable Fabry-Perot interferometer optic-fiber sensor with double parallel structure: realizing real-time breath monitoring enhanced by the means of Vernier effects

This paper presents and demonstrates what we believe to be a novel dual-parallel structure Fabry-Perot interferometer (FPI) flexible wearable fiber optic sensor with enhanced sensitivity through the Vernier effect for real-time human breath monitoring. The Fabry-Perot (FP) structure of the sensor is composed of a single-mode fiber fused to a capillary coated with an agar film at the end. The two FPIs are used as the reference element and the relative humidity (RH) sensing element, respectively. The influence of the capillary length difference on the RH response is experimentally investigated. The sensor has the best RH response in the range of 60%RH to 100%RH with an RH sensitivity of 2.93 nm/%RH. Based on the temperature experimental data, a sensitivity matrix can be constructed and solved to eliminate the influence of temperature on the sensor, so that it can be better applied to human breath monitoring. Through vibration and heating tests, it is verified that the sensor can eliminate the influence of external interference factors. In the applied research on breath monitoring of nine subjects, the sensor shows rapid response and recovery times of 0.47 s and 0.65 s, respectively. Combined with the random forest machine learning algorithm, this sensor is able to effectively differentiate the breathing patterns (normal and deep breathing) of different individuals (gender, body types). With high repeatability, reliability, and validity of the monitoring of breathing patterns, the proposed monitoring system provides a new way of thinking for the development of flexible and wearable breathing sensors.