Research On Multi-user Human Respiratory Rates Detection Based On Optical Fiber Carbon Dioxide Concentration Sensor

As an important human physiological signal,respiratory signal is of great significance to human health detection.The detection of respiratory signals has also been the focus of attention.At the same time,the fiber-loop ring-down cavity technology has been widely used in the fields of gas concentration detection and current detection because of its insensitivity to light source fluctuations and its use in distributed detection.However,the combination of fiber-loop ring-down cavity technology and carbon dioxide concentration sensing for respiratory signal detection has not been studied in detail.The main innovations and research contents of this paper are as follows:（1）Based on the absorption line data in the HITRAN database,the absorption line diagrams of N₂,O₂,CO₂ and H₂O in the exhaled gas are studied,and the absorption line selected based on the spectral absorption carbon dioxide concentration detection was determined to be located at 1572.34 nm.However,there is a lack of data on the absorption line of CO₂ under low temperature conditions and the temperature sensitivity problem that may exists in the detection of CO₂ concentration based on the principle of light absorption.The empirical formula of carbon dioxide gas absorption coefficient and temperature dependence in the range of 250⁵00K is obtained by formula derivation and MATLAB simulation fitting for the problems above.The reliability of the empirical formula is compared and analyzed,and the comparison result with average error of 3.305% is obtained.（2）The effects of coupler insertion loss and coupling ratio in a ring-down cavity and fiber attenuation factor on the output light intensity are simulated and analyzed.Then an absorption air chamber structure is designed based on the traditional White-type air chamber structure.The key parameters in the air chamber are optimized,and the air chamber is combined with the fiber-loop ring-down cavity structure to design an optical fiber CO₂ concentration sensor.The sensor is modeledby MATLAB.Through simulation experiments,the simulation data of the ring-down time of the sensing system under different CO₂ concentrations is obtained.Four sensing ranges is obtained: 1²0ppm,300¹000ppm,30000⁵0000ppm and50¹00%（500000¹000000ppm）.The range of 30,000⁵0000ppm is the target range of the sensor designed in this paper.In this interval,the simulated linearity of the Optical fiber CO₂ concentration sensor designed in this paper is 0.9942,the theoretical detection resolution is 57.47 ppm,and it is 91.53 ppm higher than the detection resolution of the same type of respiratory CO₂ concentration detection sensor in the literature.（3）On the basis of the above two points,this paper designs a multi-user respiratory frequency detection system based on optical fiber CO₂ sensor for the problem that traditional direct detection method cannot achieve "one-to-many" detection.This system realizes the respiratory rate detections of multiple users simultaneously by only one device,which improves the efficiency of the detection system.The modeling of the dual-channel respiratory signal detection system is simulated by MATLAB,which verifies the feasibility of this paper’s multi-user respiratory frequency detection system.And the maximum number of signal channels of the multi-user respiratory frequency detection system designed in this paper is theoretically analyzed.