  We investigate synchronization and subsequently communication using chaotic
rare-earth-doped fiber ring lasers, represented by a physically realistic
model. The lasers are coupled by transmitting a fraction c of the circulating
electric field in the transmitter and injecting it into the optical cavity of
the receiver. We then analyze a coupling strategy which relies on modulation of
the intensity of the light alone. This avoids problems associated with the
polarization and phase of the laser light.
  We study synchronization as a function of the coupling strength and see
excellent convergence, even with small coupling constants. We prove that in an
open-loop configuration (c=1) synchronization is guaranteed due to the
particular structure of our equations and of the injection method we use for
these coupled laser systems. We also analyze the generalized synchronization of
these model lasers when there is parameter mismatch between the transmitter and
the receiver.
  We then address communicating information between the transmitter and
receiver lasers by modulation of the chaotic optical waveform.
  Finally we comment on the cryptographic setting of our algorithms, especially
the open-loop strategy at c=1, and hope this may lead others to perform the
cryptographic analyses to determine which, if any, of the communications
strategies we investigate are secure.
