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15 June 2011
German scientists have set a new record for the most data transmitted using a single laser – 26 terabits per second.  The technique, described in the journal Nature Photonics, could provide new way of meeting the demands of high-bandwidth communication over long distances while also keeping down the energy bill.
Achieving terabit-per-second optical transmission rates usually requires multiple lasers or time-interleaved data streams. Juerg Leuthold and co-workers from the Karlsruhe Institute of Technology in Germany used a single laser to generate 325 optical frequencies within a narrow band of wavelengths and encoded data on each frequency using orthogonal frequency division multiplexing (OFDM), which allows the channels to overlap in frequency without interfering with each other.
OFDM is a popular technique being explored for the next-generation of high-speed optical systems, and in one sense it is nothing new – it has been used in wireless systems for years.  The novel part of the experiment lay in the use of an optical, rather than electronic, solution for encoding and decoding the data.   Electronic chips can’t hope to keep up with 26Tbps line rates, so the researchers build a decoder using a cascade of delay interferometers with subsequent time gates which implements all-optical fast Fourier transformation (FFT) in real time. 
Using a single laser could significantly reduce the bill of materials and energy consumption for long-distance optical transmission.  However, the set-up reported by the Karlsruhe scientists is only the first step and needs considerable development – other technologies have shown higher overall capacities and spectral efficiencies (the capacity for a given amount of frequency spectrum). 
By Pauline Rigby