Measurement Of Respiratory Displacement Based On Self-mixing Interference With Weak Light Feedback

The traditional contact physiological sensing technology represented by polysomnography(PSG)often brings discomfort to users.As a non-contact physiological sensing technology,laser self-mixing interference technology has the advantage of low cost and simple structure and high resolution.This paper studies the reconstruction of respiratory displacement curve based on laser self-mixing interference with weak light feedback.Firstly,the theoretical derivation and simulation study of laser self-mixing are carried out,and the change law of laser self-mixing waveform under different parameters is summarized.Then based on theoretical research,the hardware system of laser self-mixing interference is designed,and the upper limit of system speed is analyzed.Later,a speed and displacement reconstruction algorithm based on the frequency domain of the laser self-mixing signal is designed.The phase spectrum analysis of the all-phase FFT is utilized to calculate the first and second harmonics' phase of laser self-mixing signal,which can realize the discrimination of the target motion direction in the frequency domain under the weak light feedback.The accuracy of the discrimination is as high as 99.23%through the simulation and verification of the self-mixed signal.The self-mixed spectrum is corrected by the time-shift phase difference correction method based on all-phase FFT.It can be found in the simulation that compared with the traditional FFT,the relative error of the corrected velocity measurement was reduced from 3.35%to 1.1667×10-5%.After realizing the detection of the velocity direction and magnitude,the velocity curve was reconstructed and integrated to obtain the displacement curve.The speed and displacement curves are basically consistent with the original signals.Finally,by making use of the above-mentioned laser self-mixing hardware system and algorithm,the detection of thoracic cavity displacement,that is,the respiratory waveform,was implemented.The date of PSG S-2000 was used as the verification signal.Through the Bland-Altman diagram and the calculation of correlation coefficient,it shows that respiration detection of laser self-mixing interference exhibits a strong correlation and consistency with the traditional contact detection method.