Secure Chaotic Transmission on a Free-Space Optics Data Link

In this paper, we numerically demonstrate secure data transmission, using synchronized ldquotwinrdquo semiconductor lasers working in the chaotic regime, which represent the transmitter and receiver of a cryptographic scheme, compatible with free-space optics technology for line-of-sight communication links. Chaotic dynamics and synchronization are obtained by current injection into the laser pair of a common, chaotic driving-signal. Results of simulations are reported for the configuration in which the chaotic driving-current is obtained by photodetection of the emission of a third laser (driver), chaotic by delayed optical feedback in a short cavity scheme, selected with different parameters with respect to the laser pair. The emissions of the synchronized, matched lasers are highly correlated, whereas their correlation with the driver is low. The digital message modulates the pumping current of the transmitter. Message recovery is performed by subtracting the chaos, locally generated by the synchronized receiver laser, from the signal obtained by photodetection (at the receiver side) of the chaos-masked message transmitted in free space. Simulations have been performed with the Lang-Kobayashi model, keeping into account both attenuation of the optical signal in a line-of-sight configuration, and noise. Security has been investigated and demonstrated by considering the effect, on synchronization and message recovery, of the parameter mismatch between transmitter and receiver.