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)).