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Soliton combs enable ‘record’ optical data rate of 55 Tbit/s

14 Jun 2017

Described as “an important step towards highly efficient chip-scale transceivers for petabit networks”.

Researchers at Karlsruhe Institute of Technology (KIT) and École Polytechnique Fédérale de Lausanne (EPFL) have set a new record for optical data transmission. The team exploits optical solitons circulating in silicon nitride microresonators to generate broadband optical frequency combs. Two such superimposed frequency combs enable massive parallel data transmission on 179 wavelength channels at a data rate of more than 50 terabits per second.

The achievement has just been published in Nature (DOI: 10.1038/nature22387).

Optical solitons are special waves that propagate through a fiber without changing their shape. In optical communications, solitons can generate frequency combs with various spectral lines, which enable efficient and compact high-capacity optical communication systems. This very high rate effect was demonstrated recently by the KIT and EPFL researchers.

The team used silicon nitride microresonators, which can easily be integrated into compact communications systems. Within these resonators, solitons circulate continuously, thus generating broadband optical frequency combs. Such frequency combs, for which John Hall and Theodor W. Hänsch were awarded the Nobel Prize in Physics in 2005, consist of a multitude of spectral lines, which are aligned on a regular equidistant grid.

Traditionally, frequency combs act as high-precision optical references for precise measurement of frequencies. So-called Kerr frequency combs feature large optical bandwidths with large line spacings, and these are well suited for data transmission. Each individual spectral line can be used for transmitting a separate data channel.

’55 terabits per second’

In their experiments, the researchers used two interleaved frequency combs to transmit data on 179 individual optical carriers, which completely cover the optical telecommunication C and L bands and allow a transmission of data in a rate of 55 terabits per second over a distance of 75 km.

“This is equivalent to transmitting more than five billion phone calls or more than two million HD TV channels over a single fiber. It is the highest data rate ever reached using a frequency comb source in chip format,” said Christian Koos, professor at KIT’s IPQ and IMT. Prof. Koos received a Starting Independent Researcher Grant of the European Research Council for his research on optical frequency combs.

The team says that these components have the potential to reduce the energy consumption of the light source in communication systems drastically.

The basis of the researchers’ work are low-loss optical silicon nitride microresonators. In these, the soliton state, described in the Nature article, was for the first time generated by the working group around Professor Tobias Kippenberg at EPFL in 2014.

Combs ‘in development’

Prof. Kippenberg commented, “Our soliton comb sources are ideally suited for data transmission and can be produced in large quantities at low cost on compact microchips.” The soliton forms through so-called nonlinear optical processes occurring due to the high intensity of the light field in the microresonator. The comb sources are currently being brought to application by a spin-off of EPFL.

The Nature article explains that microresonator soliton frequency comb sources can considerably increase the performance of wavelength division multiplexing techniques in optical communications. The comb sources dramatically increase scalability of the respective systems and enable highly parallel coherent data transmission with light. According to Christian Koos, this is “an important step towards highly efficient chip-scale transceivers for future petabit networks”.

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