A PDMS Coating-Based Balloon-Shaped Fiber Optic Respiratory Monitoring Sensor

A respiratory monitoring sensor based on a balloon-shaped optical fiber is proposed. The sensor consists of a single-mode fiber (SMF) coated with polydimethylsiloxane (PDMS) bent into a balloon shape to form a fiber optic Mach–Zehnder interferometer. The sensor’s sensitivity to temperature enables monitoring of breathing status by recognizing the temperature changes that occur during human respiration. By adjusting the bending radius of the balloon-shaped SMF, high-order modes can be effectively excited to interfere with the core mode. Due to the high thermo-optic coefficient and thermal expansion coefficient of PDMS itself, the balloon-shaped fiber optic sensor can achieve temperature sensitivity. The experimental results show that the temperature sensitivity is −166.29 pm/°C in a temperature range of 30 °C to 60 °C. Finally, the proposed sensor was mounted into a respiratory mask to monitor different breathing states (normal, fast, slow, and oral–nasal breathing transitions) and breathing frequencies.