Making light work of data speeds

Every time you talk on a telephone, type on a keyboard or download a film from the internet, tiny devices translate your real-world inputs into digital signals. At the heart of these devices are circuits known as analogue-to-digital converters (ADCs). And every year, technology companies gradually improve their effectiveness, speeding up communications around the world. Now research into light particles - known as photons - could help communications technology take a giant leap forward. Anatoly Khilo, an assistant professor of microsystems engineering at the Masdar Institute in Abu Dhabi, is researching the use of photons to dramatically improve ADC performance - and thereby revolutionise information technology. "This will mean faster internet speed, which will also lead to less cost to support the demanding needs of future internet applications," he said. "At present, if you want to increase the bandwidth, you can always install more fibre optics into the ground, but this is an expensive way to go. "With this new type of ADC, you can just use an existing fibre and send more data over it." Some of the simplest analogue-to-digital converters are those found in telephone receivers. When you talk into a microphone, your voice is transformed into an analogue waveform. An ADC then samples that continuous signal, breaking it down into chunks that are discrete in both time and amplitude. That "digital" wave is now rendered into 1s and 0s, which can be operated on by microprocessors and transferred over an electronic network. "There's two dimensions to the process," says Michael Perrott, a professor of microsystems engineering at Masdar Institute, who was involved at an early stage of the research project. "The finer it parses up the amplitude, the better the resolution of the ADC. The faster it samples it, the higher the speed or bandwidth of the ADC." Analogue-to-digital converters, he says, are "literally everywhere", and improving rapidly. "The speed of transistors improves every year but we're starting to reach limits in the traditional CMOS (complementary metal-oxide-semiconductor) technology that has been the mainstay of our industry for decades," he added. "If we want to keep increasing speeds, we need to look for disruptive technology solutions. "Integrated photonics offers a compelling approach to achieve greatly increased bandwidth, and therefore speed, within future integrated systems." However, the demand for these ultra-fast ADCs is not in telephone receivers but in the many connections between fibre-optic cables. Indeed, a fibre-optic junction needs to sample a light wave 40 billion times a second, while digitising a voice only requires 40,000 samples a second. A key bottleneck to such fast sampling is "jitter" - small variations in the sample times of the analogue-to-digital converter. The timing of existing high-performance ADCs is already extremely accurate, to within 60-80 femtoseconds, or millionths of a billionth of a second. But when the data is travelling as fast as it is in a fibre-optic cable, even such a minuscule inaccuracy can cause a bottleneck. So faster ADCs are needed to meet the world's ever-increasing demand for internet bandwidth. Dr Khilo has found a way, using integrated photonics technology miniaturised to fit on a silicon chip and sample analogue signals with little or no jitter. The project has already attracted interest from the military and funding from the US Defence Advanced Research Projects Agency, because of its potential use in radar devices. "People are interested in detecting a signal at high frequency with high accuracy," he said. "But it has many other applications. "Fibre-optics is one, but it can also be used in improving mobile phone reception or in medical applications, to do high resolution imaging." His recent paper in the journal Optics Express has been heavily downloaded and he has also been approached by the technology corporation Fujitsu. "Lots of people are interested to see where this will go and what impact it will have." And the Masdar Institute group are not alone in pushing the field. Companies such as IBM and Intel are also looking into chip-based photonics. "On-chip optical interconnects could bring current circuit architectures to the next level," said Dr Marcus Dahlem, an assistant professor at the Masdar Institute, who also worked on the project. From TheNational