Optical solitons are special wave packages that propagate without changing their shape. They are ubiquitous in nature, and occur in Plasma Physics, water waves to biological systems. While solitons also exist in optical fibers, discovered at Bell labs in the 80s, their technological use so far has been limited. While researchers studied their use for optical communication, eventually the approach was abandoned.
Now, a collaboration of a research group at KIT’s Institute of Photonics and Quantum Electronics (IPQ) and Institute of Microstructure Technology (IMT) with EPFL’s Laboratory of Photonics and Quantum Measurements (LPQM) have shown that solitons may experience a comeback: Instead of using a train of soliton pulses in an optical fiber, they generated continuously circulating optical solitons in compact silicon nitride optical microresonators. These continuously circulating solitons lead to broadband optical frequency combs. Two such superimposed frequency combs enabled massive parallel data transmission on 179 wavelength channels at a data rate of more than 50 terabits per second – a record for frequency combs. The work is published in Nature.
Optical frequency combs, for which John Hall and Theodor W. Hänsch were awarded the Nobel Prize in Physics in 2005, consist of a multitude of neighboring spectral lines, which are aligned on a regular equidistant grid. Traditionally, frequency combs serve as high-precision optical references for measurement of frequencies. The invention of so-called Kerr frequency combs, which are characterized by large optical bandwidths and by line spacings that are optimal for communications, make frequency combs equally well suited for data transmission. Each individual spectral line can be used for transmitting a data signal.
In their experiment, the researchers from KIT and EPFL used optical silicon nitride micro-resonators on a photonic chip that can easily be integrated into compact communication systems. For the communications demonstration, two interleaved frequency combs were used to transmit data on 179 individual optical carriers, which completely cover the optical telecommunication C and L bands and allow a transmission of data 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 components have the potential to reduce the energy consumption of the light source in communication systems drastically. The basis of the researchers’ work are solitons generated in low-loss optical silicon nitride micro-resonators. In these, an optical soliton state was generated for the first time by Kippenberg’s lab at EPFL in 2014. ”The soliton forms through nonlinear processes occurring due to the high intensity of the light field in the micro-resonator” explains Kippenberg. 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 silicon nitride integrated photonic chips are grown and fabricated in the Center for MicroNanotechnology (CMi) at EPFL. Meanwhile, a startup from LPQM, LiGenTec SA, is also offering access to these photonic integrated circuits to interested academic and industrial research laboratories.
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.
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