Optical chaos cryptography
Security of information in public or opened transmission channels is a key challenge in modern telecommunication networks. Nowadays, an ever growing proportion of data do require a very high level of confidentiality while they are transmitted. Ensuring such a confidentiality is the purpose of cryptography. Optical chaos cryptography is by far the most important application of laser chaos. Nowadays, security in optical-fiber network is ensured by public key cryptosystems, which are softwares whose guarantee of security is computational complexity. On the other hand, optical chaos cryptography relies on the synchronization of two chaotic laser beams. Typically, an information-bearing signal is encrypted within the noise-like output of a chaotic emitter, while a synchronous receiver recognizes the chaotic component and extracts it to reveal the originally encrypted signal. Within that framework, it can therefore be said that encryption relies on the unpredictability of chaotic oscillations, while decryption relies on their determinism. At the FEMTO-ST institute, an original architecture based on opto-electronic feedback has been developed, and enabled the generation of a wide variety of chaotic behaviors for dynamic variables like wavelength, intensity, and phase.
A milestone in chaos cryptography research is the successfull field experiment reported in 2005 , consisting in chaos encryption and transmission at 1 Gb/s over a network of 120 km of optical fiber in the metropolitan area of Athens (displayed in the figure). Prior to that experience, investigations had been led on the synchronization of optical chaos generated with current-modulated lasers (see refs.  and ), external-cavity lasers , and electro-optical (opto-electronic) feedback systems . Research is also undertaken on the bit error-rate performance of these optical chaos cryptosystem  and on the detrimental effect of fiber dispersion .