Light Speed
Faster optical networks from slower light

Context: Optical fibers cross cities and oceans and form the backbone of much of the world's high-speed data communications network. But the throughput of optical fiber is limited by how fast data can be switched across networks. In a conventional router, light from fibers must be converted into an electrical signal, switched to an appropriate cable, then converted back to light again; this process can slow the speed of information transfer by a factor of ten. A router that did not need to use an electrical signal would be inherently faster. But the electrical signals are necessary to hold data intact until the optical switch to the next cable is ready. A team of scientists from the Ecole Polytechnique Federale de Lausanne in Switzerland has demonstrated a novel way to slow down light in an optical fiber, so that switching from cable to cable can be coordinated with light signals instead of electricity.

Methods and Results: The speed at which light travels through different physical media is not constant; light interacts with the matter it passes through, and this interaction can slow it down ever so slightly. Kwang Yong Song and colleagues fired an intense laser down a standard telecommunications fiber, causing atoms in the fiber to vibrate. If an optical signal is sent from the fiber's other end, the light interacts with these moving atoms more than it would with unperturbed ones, and it slows down by tens of nanoseconds along several kilometers' length of fiber. Critically, the light's speed in the fiber can be easily modulated: the more intense the laser, the more the atoms move, and the more the optical signal is delayed.