Self-mixing interference in a diode laser: experimental observations and theoretical analysis

The experimental results of an investigation of self-mixing effects or backscatter modulation in diode lasers coupled with a simple theoretical analysis are presented. The laser is used to send light, either in free space or through an optical fiber, to a movable target from which the optical backscatter is detected and fed back into the laser. In the experiment three significant conclusions are drawn: (1) self-mixing interference is not dependent on the coherence length of the laser, (2) the interference is not dependent on the use of a single-mode or multimode laser as the source, and (3) the interference is independent of the type of fiber employed, i.e., whether it is single mode or multimode. A comparison of this kind of interference with that in a conventional interferometer shows that self-mixing interference has the same phase sensitivity as that of the conventional arrangement, the modulation depth of the interference is comparable with that of a conventional interferometer, and the direction of the phase movement can be obtained from the interference signal. The above factors have implications for the optical sensing of a wide range of physical parameters. Several applications of the method are discussed that highlight the significant advantages of simplicity, compactness, and robustness as well as the self-aligning and self-detecting abilities of fiber-based self-mixing interferometry when compared with the use of conventional interference methods.