Highly humidity sensitive Fabry-Perot interferometer sensor based on a liquid-solid microcavity for breath monitoring

A novel highly humidity-sensitive with a fast response time Fabry-Perot sensor based on a sealed liquid glycerol microcavity is proposed and experimental lly applied for breath monitoring. The interference cavity structure is composed of single-mode fiber and hollow-core fiber. The hollow-core fiber is filled with glycerol and ultraviolet adhesive through capillary action to form a liquid–solid microcavity. The porous nature of the ultraviolet adhesive enables the liquid glycerol to detect external humidity. With the increase of humidity, the high-water absorption glycerol and ultraviolet adhesive absorb water and expand, the effective cavity length of the FP cavity changes and then affecting affects the sensitivity of the sensor. The results show that the humidity sensitivity of the sensor reaches 1.06917 nm/%RH. Due to the high thermo-expansion coefficient of glycerol and ultraviolet adhesive, the sensor also has high-temperature sensitivity, reaching up to 0.73857 nm/°C. Temperature and humidity parameters can be monitored simultaneously by cascading fiber Bragg grating, it is possible to monitor temperature and humidity parameters simultaneously. What's more, the sensor has fast response time (180 ms ∼ 230 ms). We applied the sensor to human respiration monitoring, and tested the response of the sensor under fast breathing and slow breathing. Compared with the gas–solid cavity structure of the optical fiber Fabry-Perot humidity sensor, the proposed sensor has the advantages of high sensitivity, simple manufacturing, and good robustness.