Optical Approach Delivers Fast, Scalable Data Transmission

Microresonator soliton frequency combs have been shown to considerably increase the performance of wavelength division multiplexing (WDM) techniques in optical communications. The technology could be used to develop efficient, highly scalable communication systems.

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

WDM enables transmission of ultrahigh data rates by using a multitude of independent data channels on a single waveguide, with the information encoded on different wavelengths of light. Frequency combs are produced by continuously circulating optical solitons — waveforms that preserve their shape during propagation — in silicon-based microresonators.

Rather than encoding data on the soliton pulse train itself, researchers used continuous-wave tones of the associated frequency comb as carriers for communication. Dissipative Kerr solitons (DKSs) were generated as continuously circulating pulses in an integrated silicon nitride microresonator via four-photon interactions mediated by the Kerr nonlinearity, leading to low-noise, spectrally smooth, broadband optical frequency combs.

The research team, from Karlsruhe Institute of Technology (KIT) and Ecole Polytechnique Fédérale de Lausanne (EPFL), used two interleaved DKS frequency combs to transmit a data stream of more than 50 terabits per second on 179 individual optical carriers spanning the entire telecommunication C and L bands, over a distance of 75 kilometers.

“This is equivalent to more than five billion phone calls or more than two million HD TV channels,” said KIT professor Christian Koos.

According to Koos, this is the highest data rate ever achieved using a frequency comb source in chip format.

The team also demonstrated coherent detection of a WDM data stream by using a pair of DKS frequency combs, one as a multiwavelength light source at the transmitter and the other as the corresponding local oscillator at the receiver.

This optical chip is carrying a multitude of silicon nitride microresonators. Courtesy of J.N. Kemal/ P. Marin Palomo/KIT.

Low-loss optical silicon nitride microresonators, within which solitons circulate continuously, were used in the experiments because they can be integrated easily into compact communication systems.

The soliton was formed through nonlinear optical processes, occurring as a result of the high intensity of the light field in the microresonator. The microresonator was pumped through a continuous-wave laser from which, by means of the soliton, hundreds of new equidistant laser lines were generated.

“Our soliton comb sources are ideally suited for data transmission and can be produced in large quantities at low costs on compact microchips,” said EPFL professor Tobias Kippenberg.

The researchers’ approach exploits the scalability of microresonator-based DKS frequency comb sources for massively parallel optical communications at both the transmitter and the receiver, enabling efficient and compact high-capacity optical communication systems. Results demonstrate the potential of these sources to replace the arrays of continuous-wave lasers that are currently used in high-speed communications. In combination with advanced spatial multiplexing schemes and highly integrated silicon photonic circuits, DKS frequency combs could bring chip-scale petabit-per-second transceivers within reach.

The comb sources are currently being brought to application by a spin-off of EPFL.

The research was published in Nature (doi:10.1038/nature22387).