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Broadband at Light Speed

Tue, 04/07/2009 - 5:35am
Alfred Poor, Contributing Editor

Alfred Poor

Netflix is rapidly changing its movie rental business from sending polycarbonate discs through the mail to streaming bits across the Internet. Skype typically has about 15 million people making voice and video calls over the Internet at any given time. And more than 5 million people watch more than 5 million hours of free online video coverage of the NCAA Division I Men’s Basketball Championship “March Madness” tournament online in 2008, with even more expected to watch this year. For a communications network that originally was designed to distribute text messages between defense contractors, the Internet has held up pretty well under the demands for increasing bandwidth.

Researchers have helped keep hardware one step ahead of this demand. Wavelength division multiplexing (WDM) helped increase capacity for fiber optic data transmission. We now can have 200 channels or more, each operating at 10 to 40 Gigabits per second (Gb/s), but this alone is not enough.

The key is to get each of these wavelength channels to operate at higher data rates. Optical time division demultiplexing (OTDM) provides way to achieve this, but channel rates of 160 Gb/s and higher are too fast for electrical circuitry. Instead, a nonlinear optical solution must be found to separate the various data streams quickly enough.

Japanese researchers have demonstrated a way to do this using optical fibers, but this requires lengths on the order of 50 meters which does not make for a particularly space-efficient solution. Research at the Center for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS) in Australia has resulted in the development of chalcogenide glass planar waveguides. This is glass that uses sulfer, selenium, or tellurium. The resulting material has non-resonant linearity and ultra-fast response times. An optical chip made from this glass can contain long, serpentine wave guides; the tight bends have extremely low light loss due to a high refractive index of the glass.

In a paper published by the Optical Society of America’s journal Optics Express (http://www.opticsinfobase.org/DirectPDFAccess/B4789B1A-BDB9-137E-C64FF214978A08BD_176267.pdf), Danish scientists working in collaboration with CUDOS report the development of a waveguide chip that is just 5 cm long. (For the metric-impaired, that’s about two inches.) This chip is able to demultiplex a 640 Gb/s optical data stream — which is about 16 times faster than conventional electronic devices — into separate 10  Gb/s data channels. The optical-only device was able to produce excellent performance with an average power loss of just 2 dB and no errors.

How fast is this data stream? If you figure that a typical DVD movie takes up three Gigabytes (GB) of data, then you could transmit the contents of 25 movies in just one second with a 640 Gb/s bandwidth. According to the researchers, this technology has the potential to process data streams at speeds of one  Terabit per second (Tb/s), and could be used to make Terabit Ethernet network connections a reality. It also opens the door to other possible optical signal processing tasks. And it could play an important role in helping the Internet keep pace with the data demands of companies and consumers around the world. And the potential uses of this device are not limited to just network communications; it is an ultra-fast chip that could be combined with other devices for other functions as well. For example, this could be a key component in a high-speed computer that uses light instead of electricity.

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