Design And Research Of Self-mixing Speckle Velocity Measuring System Based On Fiber Laser

Laser self-mixing speckle technology is a new type technology of speckle measurement developed in recent years, it combines traditional speckle measurement with laser self-mixing interference and brings speckle phenomenon into self-mixing laser. It uses random speckle signal to modulate the light intensity, which could form the self-mixing speckle interference. The self-mixing speckle theoretical model roots in self-mixing laser interference, it refers to the output light beam of the laser incidence on the rough surface, after reflecting and scattering on the rough surface, part of the light returns to the laser’s resonant cavity. Because of the height and the reflectivity of each point on the rough surface changes randomly, the phase and the intensity of the feedback light returned to the laser cavity would change randomly too, when this part of the feedback light mixes with the original light in the cavity, it would modulate the light intensity in the cavity into a random speckle signal to form the external optical feedback effect, which will influence the frequency and intensity of the output beam. The mixing speckle signal carries the information about the surface of the reflector. Analyzing the mixing speckle signal in the outside resonant cavity made by the laser and the rough surface could determine the state of some objects such as surface roughness, movement speed, movement direction, etc.This article firstly introduced the origin and the development process of the self-mixing speckle laser interference and used the F-P cavity analytic method to build the theoretic model of the mixing speckle interference, deduced the laser dynamic’s numerical solution under the rough surface’s feedback and modified the numerical solution considering the actual situation, and numerically simulated the self-mixing speckle signal. Based on Erbium-doped fiber laser, we built the self-mixing speckle fiber laser velocity-measuring system and introduced the implementation and function of some main parts in the system, such as the formation of fiber laser, the speed control of target surface, the signal acquisition and processing, etc, in detail. Then, we used the completed system to finish the experiment of velocity measurement and used the stepper motor to control rotating platform as the experiment object and then measured it respectively under different rotational speed. Respectively we used autocorrelation, fractal analysis, average frequency demodulation method to analyze and process the acquisition of signal in the experiment to get the measurement speed of the turntable, and the errors between the actual speed and measuring speed were contrasted. At the same time, this experiment discussed the spectral filtering technology’s influence on the accuracy of measurement. A virtual instrument (VI) based on Labview is designed for signal capturing, spectrum adaptive filtering and velocity calculating in time, automatically.