Lab-on-Fiber Humidity Sensor for Real-Time Respiratory Rate Monitoring

Respiratory rate (RR) monitoring has received widespread attention in medical field. Complex, expensive, and bulky commercial equipment limits the monitoring cost. To take full advantage of lab-on-fiber technologies, we present a novel, compact size lab-on-fiber breathing monitoring sensor based on a tapered no-core fiber (NCF) structure. The sensor is composed of humidity-sensitive polymer materials, single-mode fiber (SMF)–NCF-SMF structure, and monitors human RR by measuring exhaled humidity. The theoretical results prove that the self-imaging phenomenon can be induced by the changing of NCF diameter. The static relative humidity test results show a maximum relative humidity sensitivity of 0.0438 nm/%RH in the range of 35%RH–58%RH. Through the single-wavelength intensity fluctuation experiment, human respiratory information is collected. In the breathing process, a response time of 0.67 s can be achieved. The RR monitoring under different heart rates, breathing patterns, and human postures displays real-time tracking and high repeatability. Moreover, the compact size, low cost, and high sensitivity make our lab-on-fiber sensor more competitive in the field of medical treatment and our daily life.