Do not tell anyone
Transporting confidential information securely over long distances is a constant challenge in telecommunications. Chaotic fluctuations in the light intensity of lasers can be used to mask information carried over fiber optic cables. In the 1990s, researchers at the Georgia Institute of Technology learned to control the chaotic fluctuations in light intensity that certain laser systems produce. Four years later, they succeeded in synchronizing two chaotically working lasers. At the time, they suggested using these effects for possible applications in telecommunications.
Now, in Science, February 20, 1998, Rajarshi Roy and his collaborator Gregory D. VanWiggeren describe the use of the chaotic fluctuations to encode information to be transmitted from one laser to another over fiber optic cables.
This allows chaotic carrier signals to hide confidential messages transmitted over existing fiber optic networks. The work also shows that information can be recovered from irregular or noisy signals.
"The system we've developed allows us to mix information with the chaos, transmit it, and then separate it from the chaos," said Roy, chair of the Georgia Institute of Technology's School of Physics. "In a traditional digital signal, the message is seen immediately. In our system, however, we encode the digital information into the chaos; if someone intercepts the message, it's not obvious to them."
In the experimental system, a stable diode laser produces a useful signal of square waves. This is then processed with an erbium-doped fiber amplifier (EDFA) and introduced into a chaotic signal. This in turn originates from an erbium-doped ring laser, the type now commonly found in the communications industry.
The resulting combined signal, a mix of the actual message and the chaotic carrier, then travels down an optical fiber to a second EDFA that is nearly identical to the first. During reception, this generates chaotic fluctuations that are synchronous with those of the transmission laser. The chaos portion of the signal, which is measured by a digital oscilloscope, is then subtracted from the combined signal and passed through a low-pass filter. Finally, the low-pass filter restores the original message, and the recipient can read it.
The diode lasers and erbium-doped ring lasers used in the experiment work with a wavelength of around 1.53 micrometers, which is ideal for transmission in fiber optic cables.
The EDFA systems for sending and receiving must be similar, if not identical, according to Roy. Only then does the chaotic coding and decoding work. Signal timing, laser state and phase, and other factors must be carefully balanced in both systems. Therefore, someone who intercepts the message with a similar laser cannot decrypt it without knowing the relevant parameters.
Other researchers have previously used chaos to mask information in mixed electronic and optoelectronic systems. But in the new work, messages are transported for the first time in a purely optical system using chaos. This happens around 100 times faster than electronic systems, and that is why the optical system is so attractive for modern communication systems. While the Science section describes sending signals at 10 megabits per second, Roy and VanWiggeren have meanwhile transmitted random bits of information at up to 150 megabits per second. According to the researchers, data can theoretically be sent infinitely fast, although in practice there is a limit to the capabilities of the detection equipment.
Before the chaotic system is practical, researchers need to further develop the demonstrated techniques and ensure that they work with longer fiber lengths. Optical transmission networks can introduce distortions that can interfere with the chaotic fluctuations and hamper signal recovery.
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