Research On Micro-vibration Measurement Technique Based On Self-mixing Effect In Semiconductor Laser

Laser self-mixing interference effect occurs when a fraction of light back-reflected or back-scattered from external object re-enters the laser cavity and coherently mixes with the original laser field. Then output characteristics of the laser will be modulated. Compared with the traditional interference technique, self-mixing interference has just one interference channel, making the structure simple and compact. So it has attracted more and more attention nowadays. As one type of class B laser, semiconductor laser has high sensitivity to optical feedback and was firstly used in the sensing applications based on self-mixing interference effect. At present, self-mixing interference in semiconductor laser has been widely used in many measurement areas.In this paper, the background of laser self-mixing interference technique is introduced. The research progress of self-mixing interference both in theory and application is reviewed. The state of phase modulation and demodulation based on self-mixing interference is summarized. Focusing on micro-vibration measurement based on self-mixing effect in laser diode and phase modulation technique, this thesis mainly includes that:Based on three-mirror cavity model, self-mixing signals corresponding to sinusoidal vibrations with different frequencies and amplitudes are simulated. The corresponding self-mixing signals are observed in the experiments. It is verified that there is a linear relation between the vibration amplitude and the amplitude of self-mixing signal, for peak-to-peak value of vibration less than half of the laser wavelength. A laser diode self-mixing micro-vibration measurement system has been constructed based on the linear relation. Frequencies and amplitudes have been measured with different feedback surfaces and the experiment results are discussed.Phase modulation technique is introduced. The fast Fourier transform (FFT) phase demodulation principle based on external cavity length modulation of self-mixing interference is theoretically derived. According to the principle, the reconstructions of sinusoidal vibrations with different amplitudes are numerically simulated. The successful implementation of phase unwrapping process is deeply studied when vibration amplitude is more than λ/8. A preliminary experiment study of the self-mixing interference effect based on external cavity length modulation is performed. We use a pigtailed laser diode as light source, a circular piezoelectric ceramic (PZT) as phase modulator. The experiment results show that the method proposed has a good phase modulation effect, laying the foundation for further experimental research.