Balloon-Shaped Optical Fiber Humidity Sensor Based on PVA Coating for Respiratory Monitoring

A polyvinyl alcohol (PVA)-coated optical fiber humidity sensor for respiratory monitoring is proposed. The humidity sensor forms a fiber Mach–Zehnder interferometer (MZI) by bending the single-mode fiber (SMF) coated with PVA. The refractive index of PVA coatings varies with changes in relative humidity (RH), causing phase changes in higher-order modes and resulting in shifts in the transmission spectrum. The sensor exhibits excellent dynamic humidity response performance (92.8 ms for response time and 63.6 ms for recovery time), realizing a humidity sensitivity of −1.927 nm/%RH within the humidity range of 86.1% to 92.2%. Compared to the balloon-shaped fiber optic sensor based on polydimethylsiloxane (PDMS) coating previously proposed by our research group, the PVA coating facilitates easier surface composite on the fiber, exhibits faster response speed, and its humidity response range is more suitable for respiratory monitoring. Ultimately, the sensor was encapsulated within a mask to enable human respiration monitoring functionality.