Research On Self-mixing Frequency-shifted Velocity Measurement Based On Distributed Feedback Fiber Laser

Laser self-mixing interferometry,as a sort of non-contact optical sensing measurement technology,has been widely concerned by experts in various fields,such as aerospace,biomedical,autopilot,natural disaster prevention,etc.Compared with the traditional dual-beam interferometry scheme,laser self-mixing technology has the advantages of simple structure,easy collimation of optical path,high sensitivity and independent distance of coherent length of light source,which helps realize the longdistance measurement of non-cooperative target physical quantity.In particular,incorporating external cavity frequency shift technology into laser self-mixing measurement technology changes the interference intensity and frequency of both feedback light and intracavity laser,which further expand the application field of the laser self-mixing measurement scheme.In the last few years,the research of laser self-mixing interferometry based on external cavity shift frequency feedback technology mainly focuses on semiconductor laser and microchip laser.However,the frequency shift light feedback effect of fiber laser is still lack of systematic research.Hence,this paper proposes an all-fiber frequencyshifted self-mixing velocimetry system using a distributed feedback(DFB)fiber laser with low phase noise,narrow line width and short cavity length as the light source.A pair of fiber-coupled acousto-optic modulators are used in the external cavity to enhance the frequency of the measured target feedback light,which is able to meet the requirements of high precision real-time measurement of target velocity in complex environment.The main work and innovations of this paper are as follows:1.This paper introduces the research status of laser self-mixed interferometry based on frequency shift feedback technology and the composition of DFB fiber laser.Combined with the theoretical model of three-mirror cavity and the theory of frequency shift feedback,the model of all-fiber laser self-mixing velocimetry is established,and the simulation results provide the theoretical basis for subsequent experiments.2.The self-mixed velocimetry system of DFB fiber laser based on frequency shift feedback technology is built,and the dynamic responses of DFB fiber laser to the optical feedback with different frequency shifts are studied.The experimental results show that by changing the frequency shift of the external cavity,the interaction intensity between the feedback light and the gain medium can be effectively enhanced.When the feedback optical frequency shift is resonant with the laser relaxation frequency,the self-mixing modulation effect of the laser is the largest.And the minimum optical feedback intensity that can be measured by this interferometer is 5.12 f W.3.The problems related to the self-mixing frequency shift velocimetry system based on DFB fiber laser are studied.The experimental results indicates that the measurement error is less than 0.5%.The optical path structure is optimized to eliminate the interference of the backscattered light inside the fiber to the laser,and the real-time measurement of the target velocity at 40 km away is realized.Moreover,by modulating the acousto-optic modulator to generate a pulse signal,the target speed and distance can be measured simultaneously.