Microresonator-based solitons for massively parallel coherent optical communications

Solitons are waveforms that preserve their shape while propagating, as a result of a balance of dispersion and nonlinearity(1,2). Soliton-based data transmission schemes were investigated in the 1980s and showed promise as a way of overcoming the limitations imposed by dispersion of optical fibres. However, these approaches were later abandoned in favour of wavelength-division multiplexing schemes, which are easier to implement and offer improved scalability to higher data rates. Here we show that solitons could make a comeback in optical communications, not as a competitor but as a key element of massively parallel wavelength-division multiplexing. Instead of encoding data on the soliton pulse train itself, we use continuous-wave tones of the associated frequency comb as carriers for communication. Dissipative Kerr solitons (DKSs)(3,4) (solitons that rely on a double balance of parametric gain and cavity loss, as well as dispersion and nonlinearity) are generated as continuously circulating pulses in an integrated silicon nitride microresonator(5) via four-photon interactions mediated by the Kerr nonlinearity, leading to low-noise, spectrally smooth, broadband optical frequency combs(6). We use two interleaved DKS frequency combs to transmit a data stream of more than 50 terabits per second on 179 individual optical carriers that span the entire telecommunication C and L bands (centred around infrared telecommunication wavelengths of 1.55 micrometres). We also demonstrate coherent detection of a wavelength-division multiplexing data stream by using a pair of DKS frequency combs-one as a multi-wavelength light source at the transmitter and the other as the corresponding local oscillator at the receiver. This approach exploits the scalability of microresonator-based DKS frequency comb sources for massively parallel optical communications at both the transmitter and the receiver. Our results demonstrate the potential of these sources to replace the arrays of continuous-wave lasers that are currently used in highspeed communications. In combination with advanced spatial multiplexing schemes(7,8) and highly integrated silicon photonic circuits(9), DKS frequency combs could bring chip-scale petabit-per-second transceivers into reach.

Soliton Kerr Frequency Combs Generated in Integrated Photonic Chips

Victor Brasch

In 2007 a new method to generate optical frequency combs was discovered. In contrast to conventional frequency combs which are generated from pulsed laser sources, these Kerr frequency combs (also known as microresonator frequency combs) are generated entirely by nonlinear effects from a single, strong continuous wave pump laser that is coupled to a microresonator. Within the first few years the field of Kerr frequency combs was able to achieve many milestone results such as fully stabilized spectra, demonstrations of several applications and octave spanning spectra. However, in particular broad Kerr frequency comb spectra showed low coherence. The reason for this was the emergence of noise intrinsic to the process behind the generation of these early Kerr frequency combs. Within the first year of my PhD this situation changed radically with the discovery of temporal dissipative Kerr solitons in crystalline microresonators in our group. These states feature a pulsed output that originates from the nonlinear dynamics inside the microresonator which enables broadband and coherent Kerr frequency combs. The dissipative Kerr solitons inside the microresonator represent a pulse shape that maintains itself indefinitely while propagating around the resonator. This is possible because all the losses that occur are compensated by the continuous wave pump laser. The deformation of the waveform that is usually caused by the dispersion of the cavity on the other hand is compensated by the nonlinearity of the medium. My work shows for the first time that these dissipative Kerr soliton states do also exist in integrated microresonator platforms. The microresonator platform that was used here are planar silicon nitride waveguide resonators with silicon dioxide cladding. In order to allow for soliton generation the fabrication of the integrated microresonators had to be optimized. Once the desired soliton states were demonstrated in the integrated microresonators an experimental protocol to stabilize these otherwise instable soliton states was developed. Thanks to this protocol it was possible to investigate key properties of the soliton states. In particular it was possible to observe a phenomenon known as dispersive wave or soliton Cherenkov radiation in microresonators for the first time and to show that the dissipative Kerr soliton states remain stable and coherent in the presence of the dispersive wave. In addition the dispersive wave broadens the optical spectrum substantially. The resulting gain in optical bandwidth allowed for the self-referencing of the Kerr frequency comb without using conventional broadening techniques. In an independent experiment the full phase stabilization with respect to an optical reference was also demonstrated and verified. Several additional effects were discovered which had substantial influence on the soliton dynamics inside the microresonator and which had not been observed before in microresonators such as the Raman frequency shift, a shift of the dispersive wave and the interaction of multiple solitons inside the cavity. In summary my work opens the door towards coherent Kerr frequency combs with large bandwidth from integrated photonic chips. Such frequency combs can find use in different applications such as optical data transmission, which we also demonstrated in a collaboration with a research group from the Karlsruhe Institute of Technology.

EPFL2016

Ultralow-power chip-based soliton microcombs for photonic integration

Botao Du, Bahareh Ghadiani, Hairun Guo, Maxim Karpov, Tobias Kippenberg, Junqiu Liu, Martin Hubert Peter Pfeiffer, Arslan Sajid Raja, Michail Zervas

The generation of dissipative Kerr solitons in optical microresonators has provided a route to compact frequency combs of high repetition rate, which have already been employed for optical frequency synthesizers, ultrafast ranging, coherent telecommunication, and dual-comb spectroscopy. Silicon nitride (Si3N4) microresonators are promising for photonic integrated soliton microcombs. Yet to date, soliton formation in Si3N4 microresonators at electronically detectable repetition rates, typically less than 100 GHz, is hindered by the requirement of external power amplifiers, due to the low quality (Q) factors, as well as by thermal effects that necessitate the use of frequency agile lasers to aces the soliton state. These requirements complicate future photonic integration, heterogeneous or hybrid, of soliton microcomb devices based on Si3N4 microresonators with other active or passive components. Here, using the photonic Damascene reflow process, we demonstrate ultralow-power single-soliton formation in high- Q (Q(0) > 15 x 10(6))Si3N4 microresonators with 9.8 mW input power (6.2 mW in the waveguide) for devices of electronically detectable, 99-GHz repetition rate. We show that solitons can be accessed via simple, slow laser piezo tuning, in many resonances in the same sample. These power levels are compatible with current silicon-photonics-based lasers for full photonic integration of soliton microcombs, at repetition rates suitable for applications such as ultrafast ranging and coherent communication. Our results show the technological readiness of Si3N4 optical waveguides for future all-on-chip soliton microcomb devices. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OPTICAL SOC AMER2018

Self-referenced photonic chip soliton Kerr frequency comb

Victor Brasch, Michael Wolfgang Geiselmann, John David Jost, Tobias Kippenberg, Erwan Guillaume Albert Lucas

Self-referencing turns pulsed laser systems into self-referenced frequency combs. Such frequency combs allow counting of optical frequencies and have a wide range of applications. The required optical bandwidth to implement self-referencing is typically obtained via nonlinear broadening in optical fibers. Recent advances in the field of Kerr frequency combs have provided a path toward the development of compact frequency comb sources that provide broadband frequency combs, exhibit microwave repetition rates and are compatible with on-chip photonic integration. These devices have the potential to significantly expand the use of frequency combs. Yet to date, self-referencing of such Kerr frequency combs has only been attained by applying conventional, fiber-based broadening techniques. Here we demonstrate external broadening-free self-referencing of a Kerr frequency comb. An optical spectrum spanning two-thirds of an octave is directly synthesized from a continuous wave laser-driven silicon nitride microresonator using temporal dissipative Kerr soliton formation and soliton Cherenkov radiation. Using this coherent bandwidth and two continuous wave transfer lasers in a 2f-3f self-referencing scheme, we are able to detect the offset frequency of the soliton Kerr frequency comb. By stabilizing the repetition rate to a radio frequency reference, the self-referenced frequency comb is used to count and track the continuous wave pump laser's frequency. This work demonstrates the principal ability of soliton Kerr frequency combs to provide microwave-to-optical clockworks on a chip.

2017

Soliton Kerr Frequency Combs Generated in Integrated Photonic Chips

Victor Brasch

EPFL2016