Semiconductor Laser Self-mixing Speckle Interference Theory And Its Applications

When a portion of light emitted from a laser source is reflected or scattered back into the laser cavity by an external target, the reflected light will mix with the light inside the cavity, causing a modulation of the laser output power. This phenomenon is called as self-mixing effect, in which the reflected light carries some information of the external target. The self-mixing effect 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.When a coherent light is scattered by the rough surface, the scatted light forms a granular intensity distribution known as speckle. Speckles are coherent superposition of the light scatted by the rough surface. Therefore, speckles carrier some information of the rough surface.Combining the principles of self-mixing interference and speckle metrology, we introduce speckle into laser diode. When laser diode illuminates the rough surface moving transversally across the light beam, the feedback light forms the random speckle due to the random pattern of the target surface, causing random fluctuations in the laser output, which is called as self-mixing speckle interference. The PD packaged in the laser diode can detect the signal of the self-mixing speckle interference and determine the motion state of the target.Firstly, we study self-mixing interference. Lang-Kobayashi rate equations are used to analyze the output properties of laser diode under the condition of week feedback. The simulation calculations and experiment results are presented to verify the dynamics of the laser diode.Secondly, we focus on self-mixing speckle interference. Lang-Kobayashi rate equations are modified to obtain the output characteristics of laser diode under variable feedback conditions. The influences of optical feedback and external cavity length on the laser output are studied, and the relation between fluctuations of speckle signal and the transverse velocity of rough surface is discussed. We experimented to verify. The simulation calculations and experiment results are also given. The self-mixing speckle interference provides for prospective new velocity sensor applications.