The 2,000 kilometers of fiber-optic cable stacked in Gabriel Charlet’s lab in the Alcatel-Lucent Bell research facility in Nozay, France, are a reminder of a record-breaking achievement: in 2009 Charlet smashed the world high-speed long-distance record for fiber-optic communications, reaching a transmission rate of 7.2 terabits per second over a single fiber 7,040 kilometers long. That’s around five times as fast as existing commercial systems–the equivalent of transmitting more than 6,000 movie-length DVDs in a minute.
Charlet reinvigorated a field. The data-carrying capacity of the cables that form the backbone of the global telecommunications network had improved little in recent years: as other researchers tried to boost transmission rates, microscopic imperfections in the cables introduced distortions that could not be compensated for. These researchers were encoding digital data by varying the intensity of a pulse of light. For example, high intensity would represent a 1 and low intensity would represent a 0. At high data rates over long distances, the imperfections blurred the distinction between intensity levels, meaning that at distances over 7,000 kilometers, around 1.2 terabits per second was the limit of reliable communication.
To solve the problem, Charlet perfected a system that uses the polarization and phase of a pulse of light, rather than its intensity, to encode data. Errors induced by imperfections are far less problematic thanks to the development of a new receiver that detects the whole electrical field of the signal, rather than just its intensity. As a bonus, each pulse of light can now encode four bits of data instead of just one, because different polarizations can be used to indicate different bit values.
Drawing on Charlet’s research, Alcatel-Lucent recently launched a new generation of commercial equipment that transmits data at 3.2 terabits per second over distances of up to 7,000 kilometers (the speeds are slower than Charlet’s record because of the limitations of current chip designs; the next generation will use specially made chips). The next time you watch a video on YouTube, it may have been piped to you with Charlet’s help. –David Cohen