Compact breath monitoring based on helical intermediate-period fiber grating

Tiny and soft breath sensors offer unique capabilities in a continuous and long-term recording of vital physiological parameters for human health monitoring. Here, we propose an all fiber-optic breath sensor based on helical intermediate-period fiber grating (E-HIPFG) written in an elliptic core polarization-maintaining fiber. This E-HIPFG-based breath sensor shows ultrafast response time and recovery time, which are 34.46 ms and 41.34 ms, respectively. The response time and recovery time are three orders faster than the corresponding values of conventional optical fiber-based breath sensors with humidity-sensitive material. In addition, this sensor does not combine with any additional coating materials or fiber architectures, increasing the stability in a continuous and long-term detection for breath monitoring. The E-HIPFG sensor is insensitive to external influences, such as temperature, torsion and strain, endowing the possibility of stable detection in the moving body. Particularly, the temperature sensitivity of the sensor is 2.75 pm/degrees C, which is two orders lower than the corresponding value of previous reports. Due to its considerable stability, quick response, and insensitivity to external influences, the E-HIPFG-based breath sensor offers significant potential for long-term and compact breathing monitoring devices.