Optical Communication at Record-High Speed

Soliton frequency combs, generated in silicon nitride microresonators, are used for massively parallel data transmission via various frequency channels. (Source: J. N. Kemal, P. Marin-Palomo / KIT)

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.

Optical solitons are special wave packages that propa­gate without changing their shape. In optical communi­cations, solitons can be used for generating frequency combs with various spectral lines, which allow to realize parti­cularly efficient and compact high-capacity optical communi­cation systems. This was demon­strated recently by researchers from KIT’s Institute of Photonics and Quantum Elec­tronics (IPQ) and Institute of Micro­structure Tech­nology (IMT) together with researchers from EPFL’s Labora­tory of Photonics and Quantum Measure­ments (LPQM).

The researchers used silicon nitride micro­resonators that can easily be integrated into compact communi­cation 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 serve as high-preci­sion optical references for measure­ment of frequencies. Kerr frequency combs feature large optical bandwidths along with rather large line spacings, and are particularly well suited for data trans­mission. Each individual spectral line can be used for trans­mitting a separate data channel.

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

The comp­nents 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 was for the first time generated by the working group around Tobias Kippen­berg at EPFL in 2014. Explaining the advantages of the approach, Kippen­berg says, “Our soliton comb sources are ideally suited for data trans­mission and can be produced in large quantities at low costs on compact micro­chips.” The soliton forms through nonlinear optical processes occurring due to the high intensity of the light field in the micro­resonator. The microresonator is only pumped through a continuous-wave laser from which, by means of the soliton, hundreds of new equ­idistant laser lines are generated. The comb sources are currently being brought to appli­cation by a spin-off of EPFL.

The work shows that micro­resonator soliton frequency comb sources can consi­derably increase the per­formance of wavelength division multi­plexing (WDM) techniques in optical communi­cations. WDM allows to transmit ultra-high data rates by using a multi­tude of independent data channels on a single optical waveguide. To this end, the infor­mation is encoded on laser light of different wave­lengths. For coherent communi­cations, micro­resonator soliton frequency comb sources can be used not only at the trans­mitter, but also at the receiver side of WDM systems. 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. (Source: KIT)

Reference: P. Marin-Palomo et al.: Microresonator-based solitons for massively parallel coherent optical communications, Nature 546, 274 (2017); DOI: 10.1038/nature22387

Links: Inst. of Photonics and Quantum Electronics, Karlsruhe Institute for Technologie KIT, Karlsruhe, Germany • Laboratory of Photonics and Quantum Measurements (LPQM), École Polytechnique Fédérale de Lausanne EPFL, Switzerland

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