Multiplexing Soliton-Combs in Optical Microresonators

Maxim Karpov, Tobias Kippenberg, Erwan Guillaume Albert Lucas, Arslan Sajid Raja
SPIE-INT SOC OPTICAL ENGINEERING, 2019
Conference paper

Abstract

Nonlinear wave mixing in optical microresonators new perospects for compact optical frequency combs with many promising applications. We demonstrate simultaneous generation of multiple frequency combs from a single optical microresonator and a single continuous-wave laser. Similar to space-division multiplexing, we generate several dissipative Kerr soliton states circulating solitonic pulses driven by a continuous-wave laser in different spatial (or polarization) modes of a MgF2 microresonator. Up to three distinct combs are produced simultaneously, featuring excellent mutual coherence and substantial repetition rate differences, useful for fast acquisition and efficient rejection of soliton intermodulation products. This method could enable the deployment of dual-and triple-comb-based methods to applications where they remained impractical with current technology.

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

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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

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.

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Optical-frequency combs, that is spectra of equidistant coherent optical lines, have revolutionized the precision measurements of time and frequency. In 2007 a new method to generate optical frequency combs was discovered. In contrast to conventional generation methods based on pulsed laser sources, these 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.

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