Abstract : The dynamics of semiconductor lasers exposed to optical feedback is the subject of this thesis. We have analyzed the instability leading to the so-called Low Frequency Fluctuations (LFF) appearing in the laser output intensity at moderate-to-strong feedback levels.
We have investigated the dynamical nature of the LFF by means of non-standard measurements. We show that the LFF are a consequence of two joint elements: the existence of a saddle node-Andronov bifurcation and the noise present in the system which may anticipate the bifurcation. We have demonstrated experimentally that, according to this type of bifurcation, a semiconductor laser with optical feedback responds to perturbations as an excitable medium. This provides the first experimental evidence of excitability in an optical system. Moreover, varying the noise level in the system we have observed, for the first time, Coherence Resonance.
We have also obtained strong indications about the physical mechanism at the origin of the LFF-instability. The analysis of the optical spectrum time-resolved over sub-nanosecond time-scales has showed that this instability is generated by the interaction among different modes of the laser.
Finally, we have explored the configuration where the optical feedback is frequency selective, describing the characteristics of the system while tuning the frequency of the feedback.
The dynamics induced by optical feedback were investigated also in Vertical-Cavity Surface-Emitting Lasers (VCSELs). VCSELs are intrinsically single-longitudinal-mode devices because of the large frequency separation of the cavity resonances. In spite of this, even in VCSELs we have observed a parameter region characterized by low frequency fluctuations in the intensity output. We have showed that, in this case, the instability is related to the interaction between the two polarization modes of the VCSEL.