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

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.

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

Photonic Damascene process for integrated high Q microresonator based nonlinear photonic devices

Martin Hubert Peter Pfeiffer

Chip-based optical waveguides formed by a silicon nitride (Si3N4) core and a silicon dioxide (SiO2) cladding are an important platform for low loss waveguides and nonlinear optics. High Q microresonators based on this material system have become a standard platformfor Kerr frequency comb generation. After their discovery in 2007, Kerr frequency combs have attracted significant interest due to their novel properties, featuring repetition rates from the microwave to the terahertz range, and the potential to realize mass-producible, compact frequency comb systems. The excitation of dissipative Kerr solitons (DKS) has quickly become the preferred operational state, allowing generation of fully coherent frequency combs with designable properties. However, the design and fabrication of microresonators that allow for DKS excitation has remained challenging. In this thesis, a novel fabrication process, the photonic Damascene process, is introduced that solves problems of previous fabrication processes. Inverting the process order and depositing the Si3N4 thin film onto a pre-patterned substrate allows for wafer-scale, crack-free fabrication of low loss photonic waveguides. Through process optimization, high dimensional accuracy similar to existing process schemes and good process stability is achieved. This enables a significant advance in the sample design and understanding of the important design elements required for microresonators with excellent linear and nonlinear performance. Especially, the design of the coupling between the microresonator and the bus waveguide is found to be critical. Together, the advances in design and fabrication allow for the demonstration of octave spanning DKS frequency combs in microresonators with 1-THz free spectral range. Finally, a systematic study of the loss processes in the photonic waveguides is presented and allows for the first time a quantification of the scattering and absorption loss rates. The ultra-smooth sidewalls of waveguides fabricated with a novel reflow process lead to dominant absorption losses. Material analysis techniques reveal for the first time the critical role of transition metal impurities, found to be present in significant amounts, for integrated photonic waveguides. The findings of this thesis provide the technological basis for design and fabrication of chipscale microresonator based Kerr frequency comb generators. In the future it will enable custom-designed and application-specific nonlinear waveguide devices that drive the further proliferation of Kerr comb technology.

EPFL2018

Multiplexing Soliton-Combs in Optical Microresonators

Maxim Karpov, Tobias Kippenberg, Erwan Guillaume Albert Lucas, Arslan Sajid Raja

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.