Communicating with Light Polarization
A new and novel way of communicating over fiber optics is being developed by physicists supported by the Office of Naval Research. Rather than using the amplitude and frequency of electromagnetic waves, they're using the polarization of the wave to carry the signal. Such a method offers a novel and elegant method of secure communication over fiber optic lines.
Electromagnetic waves, like light and radio waves, have amplitude (wave height), frequency (how often the wave crests each second), and polarization (the plane in which the wave moves). Changes in amplitude and frequency have long been used to carry information (AM radio uses changes in the amplitude of radio waves; FM radio uses changes in their frequency), but polarization has not been so thoroughly explored.
ONR-supported physicists Gregory VanWiggeren (Georgia Tech) and Rajarshi Roy (University of Maryland) have demonstrated an ingenious method to communicate through fiber optics by using dynamically fluctuating states of light polarization. Unlike previous methods, the state of the light's polarization is not directly used to encode data. Instead the message (encoded as binary data of the sort used by digital systems) modulates a special kind of laser light. Van Wiggeren and Roy used an erbium-doped fiber ring laser. The erbium amplifies the optical signal, and the ring laser transmits the message. In a ring laser the coherent laser light moves in a ring-shaped path, but the light can also be split from the ring to be transmitted through a fiber optic cable.
The nonlinearities of the optic fiber produce dynamical chaotic variations in the polarization, and the signal is input as a modulation of this naturally occurring chaos. The signal can be kept small relative to the background light amplitude. The light beam is then split, with part of it going through a communications channel to a receiver. The receiver breaks the transmitted signal into two parts. One of these is delayed by about 239 nanoseconds, the time it takes the signal to circulate once around the ring laser. The light received directly is compared, by measuring polarizations, to the time delayed light. Then the chaotic variations are subtracted, which leaves only the signal behind. Variations in stress and temperature on the communications would be equally subtracted out.
"This is quite a clever method, which hides the signal in noise," says ONR science officer Mike Shlesinger, who oversees the research. "It provides a definite advantage over direct encoding of polarization, leaving an eavesdropper only chaotic static, and no means to extract the signal."