Study On Real Time Velocity Measurement With Self-mixing Speckle From Fiber Laser

When light out from laser illuminates the rough surface of the measured target, part of light reflected or scattered reenter the laser cavity, mixing with the original one. Then the laser output power is modulated, and self-mixed speckle can be detected. The amplitude and the frequency of speckle is influenced on the different distance between measured target and the laser fan-out, the dissimilar roughness of measured target, and the distinct velocity the target moved at. Studing objects' property by self-mixing speckle is one hotspot in recent years. If our target of certain roughness is chosen, and the distance detected is fixed, changing the velocity of target, analyzing the self-mixing speckle output, a relationship between speckle and velocity can be obtained. Based on this, a velocity measured system of self-mixed speckle will be built.In this paper, the basic theory of a ring cavity laser with1550nm erbium-doped fiber (EDF) is reviewed firstly. Grounded on this, two new fiber lasers are showed: the equivalent Fabry-Perot cavity fiber laser and the Q cavity fiber laser. The theory about the new cavities is studied, and the power expression of output laser is gained. Secondly, two velocity measured systems of different EDF laser are built. Self-mixed speckle signal which is collected from two systems is processed by two ways--energy density of speckle frequency spectrum and the number of box-counting (NBC). In our study, the linear dependence between energy density of speckle frequency spectrum and velocity is received. The index relationship between NBC and velocity is obtained. According to the experiential formula, a program is wrote by labview, where the signal is fastly collected and processed, and the target velocity is showed real time. Thirdly, to improve the measured precision, a programme of adaptive noise caceller is wrote by matlab to filter. Finally, some melioration is taken for our velocity measured system by additional program:the velocity measured real time can be saved automatically; the value of velocity is inspected real time, and if it exceeds the bound set at the beginning the system will give an alarm; the measured system can be controlled remotely by Ethernet.