Research On Self-mixing Interference Displacement Measurement Method Based On All-fiber Laser

In the laser application system,the light emitted from the laser is reflected or scattered when it hits the surface of an external object.Some of the light carrying the information of the external object is fed back into the laser resonant cavity and is mixed with the light inside the resonant cavity,causing the changes in the output power and frequency of the laser and forming an external light feedback effect.This is the socalled optical feedback self-mixing interference.Because of its many advantages,such as: simple structure,easy collimation and high sensitivity,it is widely used in the field of high-precision measurement.In this paper,self-mixing interference effect and displacement measurement of allfiber laser are studied.An all-fiber ring cavity fiber laser and all fiber linear cavity DFB fiber laser are designed,which are combined with the laser self-mixing interference theory,and the displacement measurement is carried out by the fringe counting method.Firstly,the fiber lasers with different resonator structures,laser self-mixing interference theory and displacement measurement principle are introduced.Then the self-mixing interference signal is simulated by MATLAB based on the theoretical analysis.The experimental verification is carried out by building an all-fiber self-mixing interference displacement measurement system based on different resonator structures.Finally,the acquired signals were filtered by algorithm and reconstructed by fringe counting method.And the error is analyzed.The measurement accuracy is further improved.The main work of this paper is as follows:(1)This paper proposes an all-fiber hybrid displacement sensing system based on a fiber ring laser,which combines the self-mixing interference with the intracavity spectroscopy.The dual-channel displacement measurement is realized by this experimental system.During the experiment,the spectral displacement of active cavity is measured mainly by using FBG as sensor.And the FBG is used as a filter to determine the output wavelength.Through monitoring the spectra of the ring fiber laser,the sensitivity of the displacement-measurement is about 90.14 pm/?m when the displacement applied on the FBG is changed from 0 to 35 ?m.Then,the characteristics of the SMI operating mode are studied by changing the vibration-frequency and peakto-peak amplitude of the object.When the frequency is changed from 10 Hz to 30 Hz and the peak-to-peak amplitude varies from 4 ?m to 8 ?m,the SMI signal is stable during the measurement.When the vibration-frequency is 30 Hz and the peak-to-peak amplitude is 4 ?m,the minimum error can be decreased to be 110 nm,whose relative error of displacement-measurement is 2.75%.(2)An all-fiber hybrid displacement sensing system based on linear cavity fiber laser is proposed.An ultra-short cavity fiber laser is built by using active DFB.The characteristics of self-mixing interference working mode are studied by changing the vibration frequency and peak-to-peak amplitude of the object.And the self-mixing interference displacement measurement based on narrow linewidth fiber laser is realized.The experimental results show that when the frequency is changed from 20 Hz to 50 Hz and the peak-to-peak amplitude varies from 4 ?m to 8 ?m,the SMI signal is stable during the measurement.When the vibration-frequency is 30 Hz and the peakto-peak amplitude is 4 ?m,the minimum error can be decreased to be 110 nm,whose relative error of displacement-measurement is 2.5%.