Study On Key Methods Of Signal Reconstruction And Demodulation In Laser Self-mixing Interference

In recent years,with the rapid development of aerospace field,the research on vibration measurement technology has become increasingly urgent,and the non-contact,high-efficiency and high-precision detection of weak vibration of spacecraft motion pair is of great significance in the aerospace field.The laser self-mixing interferometry is more and more favored by researchers because of its high measurement accuracy,compact structure,low cost,and incomparable self-collimation with the traditional two-path interferometry system.Laser self-mixing interferometry(SMI)is a kind of measurement method based on weak coupling phenomenon of laser diode.The emergent beam of the laser is reflected or scattered by the surface of the object and then returns into the laser cavity again,after mixing with the light in the cavity,it modulates the power output of the laser and forms SMI.It is a major problem in SMI field to study a universal,efficient and accurate SMI vibration reconstruction method.The main research object of this paper is low-frequency micro-nano target vibration reconstruction measurement.The purpose of this paper is to explore the vibration information extraction method based on semiconductor laser SMI technology,aiming at solving the problems of poor universality,low reconstruction efficiency and large measurement error of reconstruction algorithms in this field.The main research contents are as follows:In this paper,the Fabry-Pérot(F-P)compound cavity is taken as a simplified model,and the mechanism of SMI effect of semiconductor laser is studied,which lays a theoretical foundation for the next study of laser SMI vibration measurement.Firstly,aiming at the steep shift of SMI signal under moderate feedback condition,and the displacement reconstruction can `t be performed by conventional methods,a multiple Hilbert transform phase unwrapping algorithm suitable for different optical feedback level mechanisms is proposed.Through the secondary envelope extraction algorithm,the moderate optical feedback horizontal interference fringes with large feedback level parameter C are moved,so that the fringe depolarization of SMI is realized,and the multiple Hilbert transform phase unwrapping algorithm is applicable to different optical feedback levels.The experimental results show that when C value is 2.5,the absolute error is 156 nm under the condition of moderate feedback level.The new algorithm solves the problem of displacement reconstruction of SMI signal under the condition of moderate feedback.Secondly,a fast algorithm for local extremum detection is proposed to solve the problem of large time delay of traditional SMI reconstruction algorithm under weak feedback level.In this method,the upper and lower envelopes of SMI signals are extracted by envelope spectrum algorithm,and the window f unction for judging the direction of interference fringes is obtained by determining the position of reverse point.Then,local extremum detection algorithm is used to obtain the local maxima containing the correct jumping point direction;Then,the vibration displacement of the reconstructed object is obtained by cubic spline interpolation based on the coordinates of local extreme points.Experimental results show that the time of local extremum micro-vibration reconstruction algorithm is 1/3 of that of integral reconstruction algorithm,and the absolute error of the reconstruction is less than 0.1 ?m for the reconstruction displacement of the target whose amplitude is less than 10.0 ?m.The local extremum detection algorithm makes up for the shortcoming that the jump point detection algorithm can not detect the jump points under weak feedback level,and has high signal processing efficiency.Finally,aiming at the problem that weak SMI signals with low SNR can`t be demodulated by conventional methods,a quatrature demodulation method based on double soft phase-locked technology is proposed.Firstly,the correspondence relationship between the first harmonic and the second harmonic of the modulated SMI signal spectrum,and the first and second order of SMI signal expanded by Bessel function of the first kind is established.Then,two reference frequenc y signals are constructed based on the double soft phase-locked method,and the first harmonic and second harmonic terms of the modulated signal spectrum are extracted;Then,the vibration displacement of the target is calculated by the arctangent operation.The soft phase-locked measurement method only extracts the signals in the specified frequency band centered on the modulation frequency,and can effectively filter out other frequency components,so this method can demodulate weak SMI signals with low SNR.Experimental results show that,the reconstruction absolute error of the double soft phase-locked demodulation algorithm is less than 10 nm for the vibration of the target peak-peak displacement amplitude less than 4.0 ?m.