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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.
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Kerr combs' or
microcombs' are generated entirely via nonlinear frequency conversion in a microresonator pumped by a continuous-wave laser.
More recently, the discovery of dissipative soliton formation in these cavities has enabled the generation of low-noise comb states with reproducible spectral envelopes, required in applications.
Solitons are pulses of light which retain their shape as they circulate in the resonator, owing to the balance between counter-acting effects. On the one hand, the tendency of the pulse to spread due to anomalous group velocity dispersion is counteracted by the nonlinear self-phase modulation. On the other hand, the losses in the cavity are lifted by the nonlinear parametric gain provided by the driving laser. These states are robust attractors of the nonlinear cavity system under specific driving conditions.
In this thesis, the properties and dynamics of dissipative soliton states are studied experimentally in crystalline magnesium fluoride whispering gallery mode resonators. Several methods are developed to accurately determine and control the driving parameters as well as to improve the comb stability.
The observations provide an accurate verification of the Lugiato-Lefever equation commonly used to describe the system.
Furthermore, unexpected deviations from this canonical model are observed and accounted for with an enriched framework.
The improved fundamental understanding and control of the system is applied for the generation of an ultralow-noise microcomb driven with an ultra-stable laser. In combination with a novel transfer oscillator method, this comb is used to synthesize ultralow-noise microwaves via optical frequency division.
Lastly, a novel method for synthesizing multiple distinct frequency combs from a single resonator and with a single laser is devised. It relies on multiplexing solitons in different spatial modes of the microresonator. Up to three combs are generated simultaneously from a single device for the first time.Victor Brasch, Michael Wolfgang Geiselmann, Hairun Guo, Maxim Karpov, Tobias Kippenberg, Arne Kordts, Martin Hubert Peter Pfeiffer, Michail Zervas