Effects of undesired external reflections on a laser diode feedback interferometer for measurement of displacements without ambiguity

When a small fraction of the power emitted from a single frequency laser is allowed to reenter the laser cavity, as in the case of a remote surface either reflective or diffusive illuminated by the laser spot, an injection modulation of the cavity field is generated, both in amplitude and frequency. By means of this technique known as laser diode feedback interferometry is possible to recover the displacement of the remote surface without ambiguity. When an additional external undesired (spurious) reflection appears, the stability conditions of the system can be modified and its performances for measurement of displacements can be degraded. In this work we present a detailed theoretical analysis for a semiconductor laser in an external cavity in presence of an undesired external reflection. The limit of the stable and bistability operating regime, useful for displacement measurement are determined as a function of the undesired external cavity parameters.