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17 Nov 2005
Chaotic optical communications is demonstrated on a commercial fibre network at gigabit/s data rates.
Chaos, a well-known nonlinear phenomenon which governs the behaviour of everything from the weather to the turbulent flow of liquids, has now been harnessed to protect the privacy of optical networks.
In this week’s issue of Nature (Vol.437 pp.343), a team of European scientists report “chaos-based optical communications” at a data rate of up 2.4 Gbit/s over a commercial optical network in Greece. Although the idea of such chaotic encryption has been around since the early 1990s this is the first time that is has been demonstrated in the field.
The experiment took place over a transmission distance of up to 120 km in the Athens' metropolitan area network. It was performed as part of a European Commission funded IST project called OCCULT that involves researchers from Greece, France, Italy, Spain, Germany and the UK (see project partners).
The experiment made use of a matched pair of laser diodes (one at the transmitter and one at the receiver) that were driven by optical nonlinear feedback so that they entered chaotic operation and were synchronized to each other. Digital message data was then embedded or hidden within the transmitted chaotic optical carrier signal so that it was unrecognizable to an eavesdropper.
To decode the digital message data at the receiver is simply a question of subtraction. The locally-generated chaotic carrier at the receiver is simply subtracted from the incoming signal which contains both the chaotic carrier and the message. As the two chaotic carriers are synchronized and a perfect replica of each other they are cancelled out and the message is retrieved.
According to Claudio Mirasso, the project co-ordinator from the Universitat de les Illes Balears in Palma de Mallorca, Spain, the chaos approach is attractive because it is compatible with both installed optical fibre and the popular transmission technique of wavelength division multiplexing (WDM).
“Concerning the bit rate, the major limitation we had was with the 3 dB-bandwidth of our lasers which was close to 3 -- 5 GHz. Lasers with high bandwidths would allow us to reach higher bit rates and 10 Gbit/s could be easily expected,” Mirasso told Optics.org.
“Concerning the security level, we have preliminary results that suggest that the security can be high but we have not quantified it yet. This is the main task we have for the future -- to define, test and calibrate the security that our system can offer.”
project partners:
University of Athens, Greece
University of Franche-Comte, France
University of Pavia, Italy
Universitat de les Illes Balears, Spain
Instituto de Fisica de Cantabria, Spain
Universitat Politecnica de Catalunya, Spain
Consejo Superior de Investigaciones Cientificas, Spain
University of Wales at Bangor, UK
Darmstadt University of Technology, Germany
Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
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