Ghost Resonance in a Semiconductor Laser with Optical Feedback

We show both experimentally and numerically a ghost resonance in the sudden power dropouts
exhibited by asemiconductor laser subject to optical feedback driven by two simultaneous weak
periodic signals. The small signal modulation conspires with the complex internal dynamics of the
system to produce a resonance at a ghost frequency, i.e. a frequency that is not present in the
driving signals. This is an eminently nonlinear efect not reported before and agrees with the recent
theoretical predictions by Chialvo et al. [Phys. Rev. E 56 , 050902(R), 2002].