Investigation On Fiber Interferometer Based Non-Invasive Physiological Parameters Sensing Technology

With the development of the human society and modern economy,demand for material living,especially physical health is growing higher and higher.And it is crucial and significant to realize long-term physiological monitoring,including heart rate,breath rate,blood pressure,etc.As investigated before,long-term monitoring of physiological signals shows great value in diseases’ point of care testing or evaluation,such as cardiac or cardiovascular system health,respiratory health,sleep monitoring,etc.However,currently traditional physiological signals monitoring devices are basically invasive type,cuff types or wearable types.Thus,people’s using experiences are not very good,and also there’re risks of cross infection.Therefore,it is hard to achieve long-term and real time physiological monitoring.The main contents and work in the article is briefly summarized as follows:Firstly,compared with conventional compact miniaturized modal interferometer,a long-fiber based modal in-line interferometer structure is proposed,constructed with multimode fiber or few mode fiber sandwiched by single mode fibers(SMFs).Radial offset fusion technique is employed to stimulate the high order non-circular symmetric mode field.The SMF acts lead in and lead out role in the in-line modal interference bewteen fundamental mode and high order modes.Accoridng to the numerical simulations,the radial offset value shows great impact on the high order mode excitation,which consequently influces the physiologicl parameters monitoring performances.The experiment results show that the actual optimal radial offset value is basically consistent with the simulated result,where the fabricated sensor depicts relatively high cardiac and breath monitoring sensitivity and the equilibrium between the high excitation coefficient and low splicing loss achieves a good state.During monitoring,the optical intensity is directed detected by a photodetector and wavelet decomposition is utilized to extract respiratory and cardiac signal,achieving noninvasive physiological monitoring,including heart rate,breath rate.It can also realize activity monitoring such as on bed,off bed and body movement.Secondly,to overcome the signal distortion or fading phenomenon in fiber MachZehnder interferometer(MZI),a thin piezoelectric sheet assisted cost-effective phase demodulator is proposed,which is mainly based on phase generator carrier(PGC).In addition,two materials with distinct different Young’s modulus are involved to separately mold the two arms of the MZI,realizing under mattress physiological monitoring with high robustness and high sensitivity.We firstly theoretically analyse the prposed sensing system’s phase demodulation methodology and the relative sensing sensitivity for the physiological parameters monitoring.Moreover,the driving frequency and driving amplitude is investigated in detail to acquire the optimal driving parameters.As for physiological monitoring performance investigation,both static and dynamic monitoring experiments are addressed to verify the monitoring performances.Finally,a cardiopulmonary coupling based supported vector machine model is established to discriminate the wake and sleep state.Besides,principle component analysis,traditional fast Fourier transform,lomb-scargle method are further analyzed to investigate the classification accuracy.Lastly,considering the signal fading issue and the quiescent differential phase drift phenomenon in MZI which is induced by the low frequency phase noise from from surroundings,human breath,etc.,an all-optical homodyne demodulation technique based on tunable laser and closed-loop control system is proposed.We firstly theoretically analyse the real-time all-optical control mechanism for the quadrature point of the interferometer,which is mainly realized by tunning the wavelength of laser to dynamically compensate the mismatched phase difference and consequently realize homodyne detection with high stablility.The MZI’s quiescent operating point is successfully maintained in quadrature point through continuously tuning the output wavelength of the laser source such that a high fidelity ballistocardiogram(BCG)signal obtained.In the experiments,it is verified that the obtained BCG signal shows high fidelity and high repeatability by dynamically tuning the wavelength of the laser to maintain the quadrature point.In addtition,through multi-subject monitoring and consistency checking,the heart rate monitoring accuracy is confirmed with high accuracy.Moreover,according to the obtained high fidelity BCG signal,the relation between the blood pressure and the features extracted from the BCG signal is investigated,which provides another feasible approach in future cuff-less and noninvasive blood pressure monitoring.