Open versus Closed Loop Performance of Synchronized Chaotic External-Cavity Semiconductor Lasers

We numerically study the synchronization or entrainment of two
unidirectional coupled single-mode semiconductor lasers in a
master-slave configuration. The emitter laser is an
external-cavity laser subject to optical feedback that operates in
a chaotic regime. The receiver can either operate at a chaotic
regime similar to the emitter (closed loop configuration) or
without optical feedback and consequently under CW when it is
uncoupled (open loop configuration). We compute the degree of
synchronization of the two lasers as a function of the
emitter-receiver coupling constant, the feedback rate of the
receiver and the detuning. We find that the closed loop scheme
has, in general, a larger region of synchronization when compared
with the open loop. We also study the possibility of message
encoding and decoding in the both open and closed loops and their
robustness against parameters mismatch. Finally we compute the
time it takes the system to recover the synchronization or
entrainment state when the coupling between the two subsystems is
lost. We find that this time is much larger in the closed loop
than in the open one.