Introduction

First of all, we will describe the effects of optical feedback (that is, the phenomenon of “self-mixing ») by a simplified study of the laser emission of the diode, first without an object present, then with an object present in the trajectory of the light beam. The simplicity of this study rests in the fact that the energy provided by the spontaneous emission of photons will be neglected and, as a result, optical noise will be generated by the random and subtle nature of this type of emission. This frequently-employed simplification leads to a sufficiently precise description of the phenomenon of “self-mixing”, but does not allow for the introduction of the notion of the light beam's temporal coherence, which is the origin of the limited range of the metrological applications of this phenomenon.

Secondly, we will show how to take advantage of weak optical feedback for measuring speed, displacement, and distances.