Sensitivity Enhanced and Noises Suppressed Double-Fish-Tail-Shaped Fiber Bragg Grating Sensor for Heart Rate Monitoring

Abstract

Wearable fiber Bragg grating (FBG) sensors offer attractive advantages for physiological monitoring. However, the weak strain generated by respiration and cardiac activity and the susceptibility to laser intensity noise and motion-induced disturbances remain major challenges. In this work, we propose and experimentally validate a flexible double-fish-tail structural FBG sensor employing differential intensity demodulation technology, which intrinsically supports dual-parameter sensing of respiratory rate and heart rate via frequency-domain separation. Wearable experiments demonstrate stable detection of both respiratory and cardiac signals under respective dedicated test conditions, verifying the dual-physiological-modality sensing capability of the device. A flexible polydimethylsiloxane (PDMS)-encapsulated double-fish-tail structure is designed to enhance strain transfer from thoracic deformation to the embedded FBG. First, finite element analysis is employed to optimize the sensor geometry, and a 60 mm sensing structure is selected based on its superior average strain response and sensor installation. The optimized sensor exhibits a strain sensitivity of 3.73 pm/με, more than doubling the sensitivity compared to the case where the grating is eccentrically mounted under the same double-fish-tail structure. Finite element simulations show that, compared to conventional rectangular packaging structures, this double-fish-tail geometry can achieve a strain amplification factor of up to 5.76 times. To improve the detection of weak physiological signals, an intensity demodulation scheme incorporating a reference channel is developed to suppress relative intensity noise (RIN) and dynamic common-mode disturbances. Under perfect optical path matching, the differential configuration effectively suppresses laser-induced intensity fluctuations and fiber-jitter-induced disturbances, reducing the noise standard deviation to below 0.071 mV, corresponding to an equivalent wavelength noise of 0.018 pm. Wearable experiments demonstrate stable detection of both respiratory and cardiac signals. A 5 min respiratory measurement yields a dominant frequency of 0.21 Hz, corresponding to 12.6 breaths/min, while repeated 18 s heartbeat measurements produce characteristic frequencies of 1.38–1.46 Hz, corresponding to heart rates of 82.8–87.6 beats/min. The proposed sensor combines enhanced strain sensitivity, low-noise high-speed demodulation, and flexible wearable integration, providing a promising approach for real-time physiological monitoring.