‘Time telescope’ could boost fibre-optic communication

From NewScientist:

A “telescope” that can magnify time could dramatically increase the amount of data that can be sent through fibre optic cables, speeding up broadband internet and other long-distance communications.

It isn’t possible to speed up the flashes of light that stream through the global network of optical fibres at around 200 million metres per second. But more information can be squeezed into each burst of light, says Mark Foster at Cornell University in Ithaca, New York, using what he and his colleague Alexander Gaeta call a “time telescope” fitted with “time lenses”.

Time lenses

“A time lens is essentially like an optical lens,” says Foster. An optical lens can deflect a light beam into a much smaller area of space; a time lens deflects a section of a light beam into a smaller chunk of time.

The Cornell team made their time lenses using a silicon waveguide that can channel light. An information-carrying pulse made from a series of small laser bursts signalling digital 1s and 0s travels through an optical fibre and into the waveguide. As it enters, it is combined with another laser pulse from an infrared laser. The infrared pulse vibrates the atoms of the waveguide, which in turn shifts the frequencies of the data-carrying pulse before it exits the waveguide and passes into an optical fibre beyond.

“The front of the [data-carrying] pulse is shifted down in frequency and the end is shifted up in frequency within the silicon waveguide,” says Foster. Because the speed of light passing through a medium depends on its frequency, the front of the pulse is slowed down while its rear speeds up. At the time lens’s focal point the rear of the pulse catches up with the front, producing a fleeting image with a spectrum encoding the entire light pulse.

Magnified data

The Cornell team compressed a light pulse carrying 24 bits of data in this way. They used a second time lens to convert the compressed image back into a 24-bit light pulse like the one they started with. The second lens was more powerful than the first, however, so the second 24-bit pulse was 1/27th the length of the one that went in: the pulse duration shrank from 2.5 nanoseconds to 92 picoseconds, but no information was lost. The two lenses work together like the two lenses of a simple telescope or microscope.

A similar device could be used to compress the data passing through the packet-based optical networks that underlie global communications, says Foster. “We would be able to send 27 times as much information on the same wavelength channel.”

Small lag

Optical routers can’t read data from such tightly compressed light pulses, however, so a second time telescope would be needed to decompress data at the receiving end of a link, says Foster.

That would introduce a lag in the data stream, he says. “But the lag is pretty small: it would take the signal approximately a millisecond to travel through the device. Even for a voice conversation it would be too small to be noticeable.”

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