Laser Self-mixing Interference Phase Modulation Technology Studies

The principle of self-mixing interference (SMI) is a new measuring technique which is applied in interferometry since 1980's. The light emitted from the laser is reflected or scattered from the external target, a portion of the laser output which contains the information of the target back into the laser cavity and mix with the original light in the laser cavity forming the self-mixing effect, and modulate the change of output power. The self-mixing interference comes from the optical feedback effect. People always tried to eliminate the optical feedback effect previously. Gradually people began to make use of it actively to measure some physical quantities, thus self-mixing interference technology was brought forth. Compared to the traditional interferences, SMI includes a simple, single-axis optical arrangement that requires minimal optical components. So it has received more and more attention, and has been widely used for displacement, velocity and vibrancy measurement and ranging.In order to improve accuracy of the self-mixing interference, some measurement methods are introduced into the SMI system. Two methods have mainly been used now: injection current modulation and external cavity modulation. Because of the disadvantages of the two methods, measurement accuracy can not meet the need of application. Combined the basic theories of self-mixing interferometry with phase modulation, and introduced the electro-optic crystal (EOC) into SMI with external laser cavity modulation, we can realize effective phase modulation for a self-mixing interferomence signal. So the self-mixing interferometer based on these theories is simple, compact and especially potential in instrument miniaturization and integration.In the article, we introduce optical and electro-optical properties of the EOC, and simulate the EOC, electrode and electro-optic effect. From the principles of self-mixing interferometry and phase modulation using electro-optic crystals, the article put an electro-optic modulator in the external cavity and analyzes sinusoidal phase modulation using electro-optic crystal in self-mixing interference. It is proved that implementing phase modulation to laser beams using electro-optic crystals can obtain good measurement accuracy over a wide frequency rage(20Hz～100KHz). Combined optics with micro micro-mechanical processing technic, we analyze the beam path, system arrangement and fine adjustment of the self-mixing interferometer, and form micro-interferometer based on phase modulation in self-mixing interferometry. This model is the micromation, integration and applicability of the phase modulation in self-mixing interferometry, and shows good adaptability in resolution of a few nanometers.