Respiration Rate Sensor Based on Fiber Cavity Attenuated Phase Shift Spectroscopy

TL;DR

The paper addresses the need for robust, real-time respiration-rate monitoring that resists baseline drift and motion artifacts. It introduces a wearable fiber-cavity CAPS sensor embedded in a chest belt, where breathing-induced strain and transverse pressure perturb a Fabry–Pérot cavity, producing CAPS phase changes that yield RR estimates. The system achieves an RMSE of about 0.91 breaths per minute over an RR range of 8–44 BrPM and scales across various postures, including sleep, demonstrated against a thoracic-impedance reference. This work offers a low-drift, motion-tolerant solution with potential for integrated optical-fiber sleep monitoring and home-based respiratory assessment.

Abstract

Respiratory rate (RR) is a vital sign with significant diagnostic value. Existing RR monitors often suffer from baseline drift over time, breaths can be occluded by limb or body movements, and many systems struggle to resolve shallow or extreme thoracic motion. Here, we propose a novel RR-monitoring sensor based on fiber-cavity attenuated phase-shift spectroscopy~(CAPS). The sensor comprises a fiber cavity whose portion is embedded into a flexible chest binder in a sinusoidal-like pattern, which the patient wears. Thoracic expansion and contraction during breathing modulate the cavity, and RR is extracted through CAPS measurements. Our sensor exhibits high reproducibility, strong sensitivity to strain and pressure induced by chest movements, inherent resistance to baseline drift, and the ability to detect body movements. The system achieves a root-mean-square error of 0.91 breaths per minute relative to the ground truth, across RR values ranging from 8 to 44 breaths per minute, when tested on multiple subjects in various postures, including sitting, supine, prone, and lateral positions. We anticipate that this work will contribute to the development of comprehensive optical fiber-based sleep monitoring systems.

Figures (9)

• Figure 1: Experimental schematics. A 1550 nm laser is simultaneously wavelength-modulated to scan the fiber cavity resonances and amplitude-modulated for CAPS measurements. Breathing-induced pressure and strain variations on the SMF-28 fiber modulate the cavity response, and the resulting CAPS signals are measured using a lock-in amplifier.
• Figure 2: Contraction and expansion of the thoracic cage during exhalation and inhalation, respectively. (Image adapted from guyton2016physiology)
• Figure 3: The Mindray monitor used as a reference device for respiration monitoring.
• Figure 4: A volunteer in different representative postures. (a) lying on the side, (b) lying supine, (c) sitting without back support, and (d) sitting with back support.
• Figure 5: Representative CAPS phase waveforms measured from subjects in different postures: (a) standing, (b) sitting with back support, (c) sitting without back support, (d) lying down. In (d), the subject is initially supine; after the first dotted line, the subject turns to one side; after the second dotted line, to the opposite side; and finally returns to the supine position.