Packing more laser beams, data, into fibre optic strands

Internet data travels on a laser beam through a fibre optic cable as thin as a human hair. Marvelous as that is, it may not be enough – as the volume of data grows, some researchers are asking, why waste an entire fiber on just a single beam of light?

Professor David Miller says one fiber could theoretically transport perhaps as many as 100 different light beams, each carrying its own data stream. (Photo: Rod Searcey)

''In theory one fibre could transport perhaps as many as a hundred different beams, each carrying its own data stream of light flashing on and off,'' says David Miller, the W. M. Keck Professor of Electrical Engineering at Stanford. ''Our challenge is creating the optics to gather those beams, flow them through the fibre together and then separate out each data stream at the other end.''

Writing in Science, Miller reviewed the efforts of researchers at Stanford and other institutions to flow more light, and data, through these slender glass pipes to keep pace with the ever-increasing flow of digital data.

''This is still early days, but things are beginning to come together,'' Miller says. ''We now have mathematical proof of the theory, methodologies to make the optics we need and first designs that show this could be done.''

To help explain how more laser beams could travel on a single fibre, Miller likened waves of light to the motion of waves of water. With light the familiar, undulating motion occurs at such a miniscule scale that a strand of fiber as thin as a hair seems as huge as a river bed.

Just as many streams converge in a river, a number of laser beams would flow through the fibre. Inside the fibre these light waves would comingle, just like the water from separate streams.

''But now we can sort all this out,'' Miller says. ''We can distinguish each optical wave in this river of light and retrieve its unique data stream after it has traveled through the fiber.''

What makes this possible is a series of breakthroughs in the design and fabrication of optical structures that can combine and separate laser beams based on the shape of the wave they generate.

''We now know how to design those structures using efficient algorithms. Some of our approaches automate the designs and adapt them to changes in the fibre,'' Miller said. ''We have also proved mathematically that such designs can always be created for light beams in fibre.''

Several researchers have been central to this effort, including Miller's Stanford colleagues Shanhui Fan and Jelena Vuckovic.

Fan and Vuckovic, both professors of electrical engineering, have developed different computational approaches to automate the design of the necessary optical structures, and Vuckovic's group is now fabricating nanoscale device structures. Miller has contributed theoretical proofs, and self-designing and adaptive approaches.

The first practical uses for this technology are likely to occur at server farms that host cloud computing applications. At such installations, thousands of servers are linked together through traditional copper wires.

These servers share so much data so continuously, and consume so much electricity in the process, that energy costs and availability have become limiting factors in siting server farms.

Beaming photons through fibre requires so much less energy than pushing electrons through wires that lower electricity costs would justify the investment in optical transport.

''These optical ideas mean we can pack more data streams in less space with less energy, allowing us to keep on increasing our use of the Internet,'' says Miller.