September 28, 2009

27 Times Faster Optical Communication While Still Keeping Power Low and Energy Efficient

This silicon chip is patterned with waveguides that split optical signals and combine them with laser light to speed data rates.
Credit: Alexander Gaeta

Cornell University researches have developed an advancedment in optical communcation that is faster and uses less power.
MIT Technology Review: The new device could also be a critical step in the development of practical optical chips. Electronics have an upper limit of about 100 gigahertz. Optical chips could make computers run faster without generating waste heat, but because of the nature of light--photons don't like to interact--it takes a lot of energy to create speedy optical signals. The new ultrafast modulator gets around this problem because it can compress data encoded with conventional equipment to ultrahigh speeds. The Cornell device is called a "time telescope." While an ordinary lens changes the spatial form of a light wave, a time lens stretches it out or compresses it over time

New Scientist magazine has coverage

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."

Nature Photonics Abstract: Ultrafast waveform compression using a time-domain telescope

Photonic systems provide access to extremely large bandwidths, which can approach a petahertz. Unfortunately, full utilization of this bandwidth is not achievable using standard electro-optical technologies, and higher (>100 GHz) performance requires all-optical processing with nonlinear-optical elements. A solution to the implementation of these elements in robust, compact and efficient systems is emerging in photonic integrated circuits, as evidenced by their recent application in various ultrahigh-bandwidth instruments. These devices enable the characterization of extremely complex signals by linking the high-speed optical domain with slower speed electronics. Here, we extend the application of these devices beyond characterization and demonstrate an instrument that generates complex and rapidly updateable ultrafast optical waveforms. We generate waveforms with 1.5-ps minimum features by compressing lower-bandwidth replicas created with a 10 GHz electro-optic modulator. In effect, our device allows for ultrahigh-speed direct 270 GHz modulation using relatively low speed devices and represents a new class of ultrafast waveform generators.

MIT Technology Review has coverage

Researchers at Cornell University have developed a simple silicon device for speeding up optical data. The device incorporates a silicon chip called a "time lens," lengths of optical fiber, and a laser. It splits up a data stream encoded at 10 gigabits per second, puts it back together, and outputs the same data at 270 gigabits per second. Speeding up optical data transmission usually requires a lot of energy and bulky, expensive optics. The new system is energy efficient and is integrated on a compact silicon chip. It could be used to move vast quantities of data at fast speeds over the Internet or on optical chips inside computers.

Most of today's telecommunications data is encoded at a rate of 10 gigabits per second. As engineers have tried to expand to greater bandwidths, they've come up against a problem. "As you get to very high data rates, there are no easy ways of encoding the data," says Alexander Gaeta.

Here's how the Cornell system works. First, a signal is encoded on laser light using a conventional modulator. The light signal is then coupled into the Cornell chip through an optical-fiber coil, which carries it onto a nanoscale-patterned silicon waveguide. Just as a guitar chord is made up of notes from different strings, the signal is made up of different frequencies of light. While on the chip, the signal interacts with light from a laser, causing it to split into these component frequencies. The light travels through another length of cable onto another nanoscale-patterned silicon waveguide, where it interacts with light from the same laser. In the process, the signal is put back together, but with its phase altered. It then leaves the chip by means of another length of optical fiber, at a rate of 270 gigabits per second.

The physics are complex, but the net effect, says Bergman, is to "take a stream of bits that are kind of slow and make them go much faster." The time telescope transmits more data in less time, and does so in an energy-efficient manner, because the only power required is that needed to run the laser.

The Cornell device is one of a series of recent breakthroughs in silicon photonics. "Silicon is this amazing electronic material, and for a long time it was viewed as being a so-so optical material," says Gaeta. Over the past five years, researchers have been overturning this notion. In 2005, researchers at Intel made the first silicon laser; subsequently, other optical components, including modulators--devices for encoding information on light waves--have been made from the material.

The Gaeta group has a lot of publications and is doing a lot of work to advance optical communication
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