Soliton Kerr Frequency Combs Generated in Integrated Photonic Chips

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.

Dynamics and Applications of Dissipative Kerr Solitons

Maxim Karpov

Optical frequency combs are optical sources, which spectrum consists of a series of equally spaced narrowband frequencies. They made an outstanding leap forward in the accuracy of optical frequency metrology and became an attractive tool for numerous applications including optical atomic clocks, fast telecommunications, astronomy, molecular spectroscopy, and microwave and optical waveform synthesis. Discovered in 2007, microresonator-based frequency combs (also Kerr combs) have become a breakthrough in the field by enabling the optical comb generation from a continuous-wave laser via nonlinear parametric frequency conversion effects enhanced within a high-quality microresonator. Kerr combs attracted significant attention due to their ability to operate in a soliton regime when self-sustaining optical pulses â dissipative Kerr solitons - are formed in the microresonator relying on a double balance between the dispersion and nonlinearity of the system as well as cavity losses and the gain from the driving laser. Dissipative Kerr solitons allow access to broadband, coherent optical combs with large repetition rates from microwave to terahertz domains, which can be generated from chip-scale microresonators. Due to compactness and unprecedented performance, such soliton-based Kerr combs represent a promising solution for a variety of real-world optical comb applications, which has been demonstrated over the last four years. In this thesis, several aspects of dissipative Kerr soliton dynamics are investigated in integrated silicon nitride microresonators. The results include the first experimental study of the Raman-induced self-frequency shift in dissipative Kerr solitons, the discovery and explanation of the soliton switching phenomenon, which enables controllable successive elimination of soliton pulses from a microresonator, the experimental observation of breathing soliton states as well as the demonstration of collectively-ordered soliton ensembles â perfect soliton crystals. The results are universal across other soliton generating platforms. Apart from elucidating basic dynamical properties of dissipative Kerr solitons, they contribute to the understanding of soliton behavior in the presence of high-order nonlinear, dispersion and thermal effects in real systems. Besides the study of the soliton dynamics, probing and manipulation techniques for dissipative Kerr solitons are developed. They enable deterministic soliton switching and controllable access to application-relevant single soliton states. The techniques also allow for non-destructive monitoring of key soliton parameters and controllable soliton state translations in the parameter space of the driven microresonator system. The developed understanding and control of soliton states are used to demonstrate dissipative Kerr solitons operating at 1 um wavelength and covering the biological imaging window. Furthermore, in collaboration with KIT soliton-based combs generated in silicon nitride microresonators are employed for massively parallel optical coherent communications and ultrafast optical ranging, where the record performance of DKS states in both applications has been demonstrated. Lastly, a rack-mountable standalone system for the DKS generation, which can be readily used outside of the laboratory environment, is developed, tested and is employed in first experiments on optical circuit switching for data centers and all-optical convolution neural networks.

EPFL2020

Raman Self-Frequency Shift of Dissipative Kerr Solitons in an Optical Microresonator

Victor Brasch, Michael Wolfgang Geiselmann, Hairun Guo, Maxim Karpov, Tobias Kippenberg, Arne Kordts, Martin Hubert Peter Pfeiffer, Michail Zervas

The formation of temporal dissipative Kerr solitons in microresonators driven by a continuous-wave laser enables the generation of coherent, broadband, and spectrally smooth optical frequency combs as well as femtosecond pulse sources with compact form factors. Here we report the observation of a Raman-induced soliton self-frequency shift for a microresonator dissipative Kerr soliton also referred to as the frequency-locked Raman soliton. In amorphous silicon nitride microresonator-based single soliton states the Raman effect manifests itself by a spectrum that is sech(2) in shape and whose center is spectrally redshifted from the continuous wave pump laser. The shift is theoretically described by the first-order shock term of the material's Raman response, and we infer a Raman shock time of similar to 20 fs for amorphous silicon nitride. Moreover, we observe that the Raman-induced frequency shift can lead to a cancellation or overcompensation of the soliton recoil caused by the formation of a coherent dispersive wave. The observations are in agreement with numerical simulations based on the Lugiato-Lefever equation with a Raman shock term. Our results contribute to the understanding of Kerr frequency combs in the soliton regime, enable one to substantially improve the accuracy of modeling, and are relevant to the understanding of the fundamental timing jitter of microresonator solitons.

American Physical Society2016

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

Dynamics and Applications of Dissipative Kerr Solitons

Maxim Karpov

EPFL2020