Parallel waveguide fiber Bragg gratings – Used for detecting human respiratory rate, trunk status, and pulse

The indicators of respiratory rate (RespRate), heart rate (HR), and arterial pulse waveform (APW) directly reflect human health levels. Therefore, it is crucial to monitor these indicators. This article introduces a parallel waveguide fiber Bragg grating (PWFBG) sensor capable of measuring respiratory rate and either trunk bending or pulse, and estimating HR based on the measured pulse. Traditional measurement methods require separate sensors for each parameter, leading to increased complexity and cost. We used femtosecond (fs) laser to inscribe a coupling waveguide in a single-mode fiber (SMF), and then inscribed two Fiber Bragg Gratings (FBGs) with different center wavelengths in the fiber core and waveguide respectively, to create the PWFBG structure. To reduce demodulation costs, we made the two FBGs corresponding to the sensor into filters. By using the edge filtering method, we achieved intensity demodulation of the sensor. Finally, the PWFBG structure is encapsulated in polydimethylsiloxane (PDMS) and fixed on the human chest or neck to measure respiratory rate and the degree of trunk bending or pulse. Experimental measurements show that the proposed sensor structure is highly sensitive to human respiration and pulse, and can also assess the degree of trunk bending. In tests, volunteers’ heart rates were 84 bpm at rest, 12 bpm after exercise, and 104 bpm after resting, with results comparing favorably with commercial instruments, falling within the ± 1.96 SD range, demonstrating the sensor’s accuracy. Additionally, the core FBG and waveguide FBG have similar temperature responses (17 pm/℃ and 18.1 pm/℃ respectively), allowing the core FBG to compensate for the temperature changes affecting the waveguide FBG. This sensor has significant potential for health monitoring, health management, and biological research, especially in high magnetic field environments like magnetic resonance imaging (MRI).