Hopf amplification of frequency-shifted optical feedback

The dynamical response of a laser to a frequency-shifted optical feedback (heterodyne optical feedback) is investigated both theoretically and experimentally. Compared to a conventional heterodyne method, the intensity modulation induced by the coherent interaction between the laser electric field and the frequency-shifted reinjected electric field can be several orders of magnitude higher. This method permits high sensitive interferometry and hence imaging. We call this laser detection technique: Laser optical feedback imaging (LOFI). By adding to the heterodyne feedback a small amount of homodyne optical feedback (i.e., feedback with the same optical frequency as the laser optical frequency), the laser can reach a Hopf instability domain. By working close to this instability, we are able to amplify still further the LOFI modulation induced by the heterodyne optical feedback.