  We show how a chaotic system can be locked to emit a periodic waveform
belonging to its chaotic attractor. We numerically demonstrate our idea in a
system composed of a semiconductor laser driven to chaos by optical feedback
from a short external cavity. The clue is the injection of an appropriate
periodic signal that modulates the phase and amplitude of the intra-cavity
radiation, a chaotic analogy of conventional mode-locking. The result is a time
process that manifests a chaotic signature embedded in a long-scale periodic
train.
