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 a semiconductor 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 effect not reported before and agrees with the recent theoretical predictions by Chialvo et al. (Phys. Rev. E, 65 (2002) 050902(R)).