Faster internet with optical frequency combs

New Zealand researchers have created a crystal-based device that could enable the next generation of faster, more energy-efficient internet.

The breakthrough marks a milestone for The Dodd-Walls Centre for Quantum and Photonic Technologies — a virtual organisation gathering New Zealand’s top researchers working in the fields of light and quantum science, hosted by the University of Otago. The study was led by Dr Harald Schwefel and Dr Madhuri Kumari, and has been published in the journal Nature.

The internet is powered by lasers. Every email, phone call and website visit is encoded into data and sent around the world by laser light. In order to cram more data down a single optical fibre, the information is split into different frequencies of light that can be transmitted in parallel. With data capacity expected to double every year, the current infrastructure used to encode and process this data is reaching its limits.

Now, researchers have found a solution to this problem in the form of a device called a microresonator optical frequency comb, made out of a tiny disc of crystal. The device transforms a single colour of laser light into a rainbow of 160 different frequencies — each beam totally in sync with each other and perfectly stable. One such device could replace hundreds of power-consuming lasers currently used to encode and send data around the world.

“Lasers only emit one colour at a time,” said Dr Kumari. “What this means is that, if your application requires many different colours at once, you need many lasers. All of them cost money and consume energy. The idea of these new frequency combs is that you launch one colour into the microresonator [and] a whole range of new colours comes out.”

“It’s a really cool energy-saving scheme,” added Dr Schwefel. “It replaces a whole rack of lasers with [one] small energy-efficient device.”

The optical frequency combs are based on an unusual optical effect that happens when the intensity of light builds up to extremely high levels. You send a single colour of visible light into the crystal disc along with a microwave signal and, because the crystal disc is such high quality, the light and microwave radiation gets trapped inside. In most situations light never changes colour, but in this case the intensity becomes so high that the light and the microwave radiation start merging and making different colours.

Dr Schwefel expects the device to be incorporated into suboceanic landing stations — where all the information from land-based fibres is crammed into the few suboceanic fibres available — in less than a decade, perhaps within a few years.

“To develop the device for the telecommunications industry we will need to start working with major telecommunications companies,” he said. “We have started the process by collaborating with a New Zealand-based optical technology company.”

The internet is just one of the possible applications for the new optical frequency combs. Another use is high-precision spectroscopy — using laser light to study and identify the chemical composition, properties and structure of materials including diseases, explosives and chemicals. Dr Kumari’s next mission will be to explore this application amongst other possibilities.

“This is a very, very exciting project to be working on,” he said. “Optical frequency combs have literally revolutionised every field of applications they have touched. You can use them for vibrational spectroscopy, distance measurement, telecommunications. I’m looking forward to seeing how we can use ours.”

Image courtesy of the University of Otago.

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