Detailed presentation by themes
(4) Applications in optical secret communications
Optical secret communication is an example of application of the research carried out in complex photonics. The concept is presented in figure 1. A message signal is concealed inside a chaotic carrier wave generated by a laser used for emission. The signal composed of the original message and the chaos mixed together is then sent to a receiving laser via a channel. In the receiver, the same chaos as the one used for concealing the message in the emitter can be regenerated by using synchronization techniques. The chaos retrieved by synchronization in the receiver can be used to extract the original message from the transmitted signal (chaos+message) by subtraction of this same chaos content. In this process, the quality of the synchronization has a major impact on the quality of the decoded message.
Figure 1: Optical secret communication with laser chaos
In the method described here, the possibility to share the same chaos carrier waveform by chaos synchronization between distant users is an essential point. As mentioned before, in order to perform successful chaos synchronization, the emitter and the receiver must have the same physical structure and parameter values. In consequence, in this transmission method, an index quantifying an allowable limit of error in the synchronization due to a gap in the parameter values is defined. More precisely, in a configuration in which the range of the parameter quantifying the synchronization feasibility is short, if a third user aiming at eavesdropping the message taps the transmitted signal, his chances of synchronization will be low and the security of the transmission preserved. In optical secret communication using laser chaos, by contrast to hiding the message by means of encoding methods as traditionally done, the objective is to hide the existence of the message in a chaotic wave carrier like in technologies using digital watermarking.
Recently, systems using laser chaos for optical secret communications have been developed and experimental transmissions on commercial optical fiber networks with low bit error rates have been implemented. In addition, apparatus using photonic integrated circuits and error correction techniques specially designed for optical secret communications have been demonstrated.