Combs of light accelerate communication

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A miniaturised optical frequency comb device, capable of transmitting data streams at several terabits per second over hundreds of kilometres, has been demonstrated, proving for the first time that optical frequency comb sources are suited for data transmission. The research, carried out by scientists from the Karlsruhe Institute of Technology (KIT) in Germany and the Swiss École Polytechnique Fédérale de Lausanne (EPFL), was published in Nature Photonics, and may help to accelerate data transmission in large computing centres and worldwide communication networks.

The amount of data generated and transmitted worldwide is growing continuously. With the help of light, data can be transmitted rapidly and efficiently. Optical communication is based on glass fibres, through which optical signals can be transmitted over large distances with hardly any losses. So-called wavelength division multiplexing (WDM) techniques allow for the transmission of several data channels independently of each other on a single optical fibre, thereby enabling extremely high data rates.

However, scalability of such systems is limited, as presently an individual laser is required for each transmission channel. In addition, it is difficult to stabilise the wavelengths of these lasers, which requires additional spectral guard bands between the data channels to prevent crosstalk.

In the study presented in Nature Photonics, the scientists of KIT, together with their EPFL colleagues, applied a miniaturised frequency comb as optical source. They reached a data rate of 1.44Tb/s and the data was transmitted over a distance of 300km. This corresponds to a data volume of more than 100 million telephone calls or up to 500,000 high-definition (HD) videos. For the first time, the study shows that miniaturised optical frequency comb sources are suited for coherent data transmission in the terabit range.

Optical frequency combs consist of many densely spaced spectral lines, the distances of which are identical and exactly known. So far, frequency combs have been used mainly for highly precise optical atomic clocks or optical rulers measuring optical frequencies with utmost precision. However, conventional frequency comb sources are bulky and costly devices and hence not very well suited for use in data transmission. Moreover, spacing of the spectral lines in conventional frequency combs often is too small and does not correspond to the channel spacing used in optical communications, which is typically larger than 20GHz.

In their joint experiment, the researchers of KIT and the EPFL have now demonstrated that integrated optical frequency comb sources with large line spacings can be realised on photonic chips and applied for the transmission of large data volumes. For this purpose, they use an optical microresonator made of silicon nitride, into which laser light is coupled via a waveguide and stored for a long time.

‘Due to the high light intensity in the resonator, the so-called Kerr effect can be exploited to produce a multitude of spectral lines from a single continuous-wave laser beam, hence forming a frequency comb,’ explained Jörg Pfeifle, who performed the transmission experiment at KIT. Kerr combs are characterised by a large optical bandwidth and can feature line spacings that perfectly meet the requirements of data transmission. The underlying microresonators are produced with the help of complex nanofabrication methods by the EPFL Center of Micronanotechnology. ‘We are among the few university research groups that are able to produce such samples,’ commented Kippenberg.

‘The use of Kerr combs might revolutionise communication within data centres, where highly compact transmission systems of high capacity are required most urgently,’ said Christian Koos, who coordinates the work under a Starting Independent Researcher Grant funded by the European Research Council (ERC). ‘We are just at the beginning. In the experiment presented, we only use 20 lines of the frequency comb. This may certainly be increased. New experiments are planned.’