Research On Laser Self-mixing Interferometry And Its Application In Pipeline Leakage Detection

Laser self-mixing interferometry has the advantages of high precision,interference signal containing direction information,compact structure,easy alignment,non-contact and so on.In this paper,the vibration displacement measurement based on laser self-mixing interferometry is carried out.A method of measuring the vibration displacement is proposed,which combines multiple self-mixing interferometry and sinusoidal phase modulation,and can improve the measurement accuracy and meet the real-time measurement.Based on the method of selfmixing interference and sinusoidal phase modulation to measure the vibration displacement of external target objects,a vibration displacement measurement system based on multiple selfmixing interferometry is established,and the system is applied to pipeline leakage detection.Firstly,the theory of laser self-mixing interferometry is introduced,and the laser selfmixing interferometry is analyzed based on the three-mirror-pellet theory model of the selfmixing interferometry effect.The phase equation,power equation and the relation equation of phase and displacement are derived.The formation process of multiple self-mixing interferometry is analyzed in detail.Based on the theory of self-mixing interferometry,the theoretical model of multiple self-mixing interferometry is analyzed,and the phase equation,power equation and displacement equation of multiple self-mixing interferometry are derived.For different vibration amplitudes,the change of the number of fringes of the self-mixing interference signal and the number of fringes of the double self-mixing interference are tested.Secondly,the laser beam is phase-modulated by an electro-optic modulator placed in the external cavity of the laser.The multiple self-mixing interference techniques and sinusoidal phase modulation techniques are combined,and the phase quadrature demodulation method is used to demodulate the phase,so as to reconstruct the vibration displacement of the object.The numerical simulation and experimental results show that this method can effectively improve the vibration displacement measurement accuracy,and the measurement error is half.Finally,based on the advantages of the laser self-mixing interferometry system,such as the collimation and non-contact of the light path,and the sensitivity to low-frequency vibration,the multiple self-mixing interferometry vibration displacement measurement system proposed in this paper is applied to the pipeline leakage detection for real-time,which is used to measure the noise signal of the pipeline in real time,combined with the acoustic signals collected at the same time and at the same time,proposes spectral subtraction method to process the two signals.The experimental results show that this method can reduce the noise signal and enhance the leakage signal under the condition of small leakage.