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Breakthrough switching speed

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Researchers think they have opened the door to terabit per second Ethernet links using multiplexed 10Gbit/s data streams and small chalcogenide demux chips to demultiplex the 10 gig streams.

An optical network cable can have lasers pumping multiple 10Gbit/s into different colours of the light spectrum and squirting them in parallel down the cable. The receiving and recombination of these streams is a problem at output rates higher than 40Gbits according to a research paper (pdf), 'Breakthrough switching speed with an all-optical chalcogenide glass chip: 640 Gbit/s demultiplexing', published in Optics Express, Vol. 17, Issue 4, on February 16th.

The paper, by a group of researchers from Australia, China and Denmark, describes how injecting multiple 10gig data streams into optical cables is not a problem using existing optical technology (electro-optic modulator per stream) and optical time-division multiplexing (OTDM). The obstacle has been recombining those separate data streams at the end of the link and doing it fast enough.

Until now the re-combination has been carried out using photo-detectors that can operate up to 40 Gbit/s or so. That limits us to just four 10gig streams. Achieving higher data rates this way means we have to send more parallel data streams down the cable and demultiplex - switch or recombine them - into one data stream faster still. Japanese researchers using optical switching reached an impressive upper limit of 640Gbit/s in 1998.

Each second, the Japanese resarchers' kit performed this task; 640 billion bits of data, spread across 64 separate 10Gbit/s data streams, were received, demultiplexed, and sent on in the right order. The demultiplexer has a 640 billionth of a second to deal with each bit. The photonic devices used needed wave guides, one per data stream, to carry out the demultiplexing and these were tens of metres long, making the technology quite impractical.

This latest reseach uses wave guides just 5cm long by making them from chalcogenide glass chips with switching speeds measured in femto seconds, a billionth of a millionth of a second, or a quadrillionth.

Chalcogens are a group of elements in the periodic table of elements that have a particular electron configuration determining their behaviour in chemical reactions. The group consists of oxygen, sulfur, selenium, tellurium, polonium and ununhexium. Sulfide, selenide and telluride are termed the heavier chalgogens and compounds of them with other elements are called chalcogenides.

Australia's Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS) worked with high-speed optical networking researchers at the Technical University of Denmark to develop the chalcogenide chips needed. The particular material chosen was arsenic trisulfide (AS2S3) which has a very high non-linearity property, meaning that the light pulses in an incoming data stream can be detected in a very much shorter length of wave guide material.

The researchers' paper states: "the high non-linearity enables compact components with the potential for monolithic integration of multiple functionalities on a single-chip. ... The size reduction to the cm range is the critical step that will allow the fabrication of complex multichannel devices with a high degree of functionality on a single chip operating at Tbit/s rates at practical power levels. ... these results confirm the enormous potential of chalcogenide-based waveguides for ultrafast optical signal processing."

They believe their technology can be extended to demultiplex 100 10Gbit/s data streams and so achieve a terabit Ethernet capability. This would require a monolithic chip with separate waveguides per data stream. Commercialisation of such technology is, of course, if it takes place at all, many years away. ®

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