 crests each second), and polarization (the plane in which the wave moves). Changes in amplitude and frequency have long been used to carry information. For example, AM radio uses changes in the amplitude of radio waves, whereas FM radio uses changes in frequency. Yet wave polarization has not been SO thoroughly explored. Office of Naval Research-supported physicists Gregory Van Wiggeren from the Georgia Institute of Technology, and Rajarshi Roy, from the 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 com- munications 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, with the time-delayed light. The chaotic variations are then subtracted, which leaves only the signal behind. Variations in stress and temperature on the communications would be equally subtracted out. LIGHT EMITTERS "This is quite a clever method, which hides the signal in noise," says Office of Naval 