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‘Combs of light’ accelerate communications

16 Apr 2014

Researchers exploit optical frequency comb sources on silicon chip for terabit data transmission.

Miniaturized optical frequency comb sources allow for transmission of data streams of several terabits per second over hundreds of kilometers – this has now been demonstrated by researchers at Karlsruhe Institute of Technology, Germany, and the Swiss École Polytechnique Fédérale de Lausanne in work recently published in Nature Photonics.

The team says that these results "may contribute to accelerating data transmission in large computing centers and worldwide communication networks."

Wavelength division multiplexing allows transmission of several data channels on one optical fiber, thereby enabling extremely high data rates. However, scalability of such systems is limited, as presently an individual laser is required for each channel and it is difficult to stabilize the wavelengths to prevent crosstalk.

The researchers applied a miniaturized frequency comb as optical source, achieving a data rate of 1.44 Tbits/s over a distance of 300km. This corresponds to a data volume equivalent to more than 100 million simultaneous phone calls or 500,000 high-definition videos. For the first time, say the KIT and EPFL scientists, this result shows that miniaturized optical frequency comb sources are suited for coherent data transmission in the terabit range.

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.

Optical microresonator

The German-Swiss teams have now demonstrated that integrated optical frequency comb sources with large line spacings can be realized 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.

Jörg Pfeifle, who performed the transmission experiment at KIT, explained, “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.”

This method to generate these so-called Kerr frequency combs was discovered by Tobias Kippenberg, at EPFL, in 2007. So-called "Kerr combs" are characterized 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,” comments Kippenberg. Work at EPFL was funded by the Swiss program NCCR Nanotera and the European Space Agency ESA.

KIT scientists are the first to use such Kerr frequency combs for high-speed data transmission. “The use of Kerr combs might revolutionize communication within data centers, where highly compact transmission systems of high capacity are required most urgently,” said Christian Koos, project coordinator at KIT.

About the Author

Matthew Peach is a contributing editor to optics.org.

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