Complex External Optical Feedback Under The Conditions Of Laser Self-mixing Interference: Experimental Observations And Theoretical Analysis

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 interference, in which the reflected light carries some information of the external target. The self-mixing interference comes from the optical feed-back 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.We analyze the self-mixing interference effect in the LD with multiple external reflectors and present the analysis based on the compound cavity effect in lasers. Each external cavity is considered to be an optical thin film, and the reflectance ratio can be obtained from the theory of the thin film optics. The general expression of the threshold gain and frequency in the compound laser cavity are deduced and some simulation analysis is made at different conditions. The simulating and experimental results demonstrate the validity of the principle. It can be used to analyze the fiber-couplsd self-mixing system; the simulation is accordant with experimental results.We also analyze the self-mixing interference in DFB (Distribute Feedback) semiconductor lasers. The general expression of the threshold gain and frequency in the DFB lasers was deduced by using the coupled wave function. Numerical simulations indicate that, for specific coupling coefficient and cavity length value, the DFB lasers; are more sensitive to optical feedback than the Fabry-Perot semiconductor laser. These results show that high-accuracy self-mixing sensors can be obtained by using the DFB lasers.At last we have built a self-mixing interference experiment system, which includes the use of the semiconductor laser driver and temperature control and the control of the PZT. We also do some self-mixing experiments by using this experiment system. Some experimental results are presented.