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Polarization switching hides data

17 Jun 2002

US scientists develop a fiber-optic setup that conceals information in polarization state fluctuations.

Polarization-encoded output from a fiber ring laser could be used to secure fiber-optic communication, say US physicists from Georgia Institute of Technology and the University of Maryland (Physical Review Letters 88 097903).

Gregory VanWiggeren and Rajarshi Roy's novel scheme has an advantage over similar schemes that use either the amplitude or frequency of the light to encode information. This is because, by using irregular fluctuations in polarization instead, there is no direct correspondence between the detected polarization-state and the message itself.

None of these methods has yet been used in secured fiber-optic lines, but Roy told Optics.org that the setup uses all off-the-shelf components and could, in principle at least, be applied to existing fiber if further developed.

In the experimental setup, the transmitter ring is an erbium-doped fiber amplifier with a polarization controller (PC). The PC consists of loops of fiber that can be twisted to alter their net birefringence. An adjacent phase modulator can then control the net birefringence in the fiber ring.

After travelling through a standard communications fiber, the light enters a receiver that has two different branches. Light in the first branch passes through a polarizer and then onto a photodiode, while in the second branch there is a polarization controller and a sequence of waveplates that allows conversion of any input polarization state into any output polarization state.

After sending a message through the system, Roy and VanWiggeren found that the signals received at the two detectors bore no resemblence to the original sent message. But, after subtracting one received signal from the other, the original message was recovered.

"The technique uses chaotic waveforms to carry information, which conceal information rather than encrypt it," said Roy. "It is totally compatible with conventional encryption and provides a further level of concealment."

Roy concedes that, like any other "secure" communication method, the new scheme is not perfect: "The question is whether the message can be recovered [by hackers] in a time frame that compromises its value," he points out.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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