Displacement Measurement Technology Fiber Laser Based On Self-mixing Interference

The laser self-mixing interference (SMI) is based on the external optical feedback effect. When a portion of light emitted from a laser source is reflected or scattered back into the laser cavity by an external target, the feedback light which carries information of the external target will mix with the light inside the cavity, causing a modulation of both the optical output power and lasing frequency. The self-mixing interferometric system has a lot of advantages such as inherent compactness, easy to collimate and high sensitivity. The SMI is able to achieve precision measurement of many physical quantities, such as velocity, displacement, distance and vibration, and it becomes a novel kind of powerful metrological tool.The basic theory of SMI based on the fiber laser has been studied. The principle of the linear cavity fiber laserf SMI effect has been demonstrated through the four mirror cavity model. The optical output power and frequency has been deduced and the behavior of a laser under different optical feedback conditions have been analyzed and simulated as well. The influence of the experimental parameters on the shape of the SMI signal has been discussed in detail. Furthermore, the theory of SMI in the fiber ring laser has been studied. And the measurement system based on the SMI in the fiber lasers have been developed to validate the theoretical analysis.The application of the fiber laser SMI in micro-displacement measurment is presented. In order to improve the displacement measurement accuracy of the interferometer, sinusoidal phase modulation method is introduced into the SMI system. The SMI signal is modulated by an electro-optic modulator or a fiber stretcher modulator in the external cavity. Two kinds of demodulation methods have been proposed. And the measurement principle based the fast Fourier transforms analysis and four-bucket integration technique has been demonstrated respectively. Error sources during the measurement and its influence on the measurement accuracy has been discussed and simulated. Experimentally, the fiber laser SMI system with phase modulation has been set up to reconstruct micro-displacement of the external target. The experimental results show that the displacement of target has been retrieved with high accuracy.In order to enlarge the measurement range of the interferometer, the synthetic-wavelength method and self-mixing interference technique has been combined for large-scale displacement measurement. The measurement principle has been demonstrated and the experiments have been carried out.