Scientists hit record-breaking 26Tbps by laser

By Jack Clark

Researchers have used a single laser to transmit data at a 26 terabits per second over an optical fiber cable, a data-transmission breakthrough that promises to come in useful for cloud computing and 3D TV transmissions.

The transmission is the biggest volume of data ever carried by a laser beam, according to the group of scientists, led by Germany's Karlsruhe Institute of Technology. With the demonstration, which sent the equivalent of 200,000 high-resolution images across 50km in one second, the researchers said they had broken their own record of 10Tbps, set in 2010.

"To the best of our knowledge, this is the largest line rate ever encoded onto a single light source," the researchers said in an announcement on Monday.
The data-transmission breakthrough was achieved by using a single laser to create a swathe of pulses — dubbed 'frequency combs' — which were separated by a wavelength of 12.5GHz. These combs were then magnified into 325 colour channels via 'inverse fast Fourier transform', then sent down a 50km cable. Once it reached the other end, an optical fast Fourier transform encoded it back into data.

A Fourier transform is, in essence, a mathematical method for taking any complicated time signal and breaking it down into its basic frequencies. The reverse applies for an inverse Fourier transform.
To the best of our knowledge, this is the largest line rate ever encoded onto a single light source.

The technology, described in Nature Photonics on Sunday, could one day be worked into a single system-on-a-chip (SoC) for super-fast transmission of data at distances of 50 to 100km, or even at greater distances with amplification across the fiber length.

It also could eventually be used for bandwidth-hungry services such as cloud computing, 3D HD television and virtual-reality applications, the researchers suggested.
The news is part of an overall shift in data-transmission technology, as companies seek to break past the limits of encoding data in electrons by putting the data into light instead.

The Thunderbolt technology that sits at the heart of Apple's latest range of products is the result of Intel's own research in this area through its Light Peak silicon photonics technology. IBM has also been working on its own CMOS Integrated Silicon Nanophotonics technology. Unlike Intel, IBM has not yet managed to place a laser onto the same die as the processor.

The chief reason for the record-breaking speed is that the researchers "did not choose a receiver where we had to cope with 325 highly stabilised local lasers, but rather employed an optical fast Fourier transform to separate out the low-bitrate tributaries", said Wolfgang Freude, who took part in the research.
"What would be usually done numerically, we do on the optical side at the speed of light, so there is no electronic bottleneck," he added.

The data is encoded in real-time via two synchronised field-programmable gate array (FPGA) boards, then broken up and transmitted. It is then received, analysed by an optical Fourier processor, sent to a modulation analyser and sampled by a 20GHz oscilloscope.

The resulting low-bitrate tributaries — the 325 distinct frequencies of light — are then electronically processed using standard Orthogonal Frequency Division Multiplexing (OFDM) algorithms.
The 26Tbps technique differs from the IBM and Intel projects in that it can transmit data across longer distances, according to Freude.

"What IBM and Intel are interested in is communication on-chip or between chips," he said. "What we are interested in is [transmission] over distances — without amplification — of between 50 and 100km. Then the amplifier comes in and we have the possibility of transatlantic connection."