Mode interaction aided excitation of dark solitons in microresonators constructed of normal dispersion waveguides (1404.2865v5)

Abstract: Kerr frequency combs from microresonators are now extensively investigated as a potentially portable technology for a variety of applications. Most studies employ anomalous dispersion microresonators that support modulational instability for comb initiation, and mode-locking transitions resulting in coherent bright soliton-like pulse generation have been reported. However, some experiments show comb generation in normal dispersion microresonators; simulations suggest the formation of dark pulse temporal profiles. Excitation of dark pulse solutions is difficult due to the lack of modulational instability in the effective blue-detuned pumping region; an excitation pathway has been demonstrated neither in experiment nor in simulation. Here we report experiments in which dark pulse combs are formed by mode-interaction-aided excitation; for the first time, a mode-locking transition is observed in the normal dispersion regime. The excitation pathway proposed is also supported by simulations.

• The paper demonstrates a deterministic pathway to excite dark solitons in SiN microresonators via mode interactions and thermal tuning.
• It employs line-by-line pulse shaping and modified Lugiato-Lefever simulations to characterize soliton dynamics under normal dispersion conditions.
• The findings pave the way for simplified Kerr frequency comb technology with potential applications in broadband optical communications.

Insights into Mode Interaction Aided Excitation of Dark Solitons in Normal Dispersion Microresonators

The paper focuses on the excitation of dark solitons within microresonators that operate under normal dispersion conditions, achieved through mode-interaction. Using silicon nitride (SiN) microrings, the researchers have advanced the understanding of Kerr frequency combs by demonstrating, for the first time, mode-locking transitions in this dispersion regime. The work provides significant insights into the mechanisms of dark soliton generation, an area that has remained largely theoretical until now.

The paper addresses the complexities in generating dark solitons in normal dispersion waveguides, where traditional modulational instability methods are ineffective. The research team proposed a deterministic excitation pathway for these solitons, facilitated by mode interactions, as opposed to the stochastic processes often associated with bright soliton generation. This facilitated method benefitted from the integration of an Au microheater creating a thermally tunable microresonator, providing stable control over resonance frequencies.

Experiments involved manipulating the microheater's voltage to adjust the resonance frequency towards the pump’s wavelength, and through this precision tuning, it was possible to observe transitions to a coherent, low-noise comb state indicating the formation of dark solitons. The researchers employed line-by-line pulse shaping techniques for time-domain characterization of the dark solitons, which were otherwise challenging to capture using conventional methodologies.

Theoretical analysis and simulations provided context for the experimental observations. Simulations based on the Lugiato-Lefever equation, modified to include mode interaction effects, were conducted to model the dynamics within the microresonator, confirming experimental findings. These simulations were key in understanding the deterministic pathways that allow for stable dark soliton formations, even in regimes traditionally assumed to be thermodynamically unstable.

This research holds notable implications for the field of optical communications and photonics, where the ability to engineer dark solitons in normal dispersion conditions could lead to wider bandwidth frequency combs suitable for various applications. Furthermore, the proven stability of the dark soliton combs under normal dispersion challenges the requisite for complex microresonator designs which aim for anomalous dispersion. This could significantly impact the design flexibility for devices operating across different spectral regions, notably the visible spectrum where achieving anomalous dispersion is challenging due to material constraints.

The methodological advancements and the robust nature of mode-interaction-induced dark soliton generation suggest a promising avenue for future developments. As optical microresonators continue evolving, it becomes plausible to anticipate the expansion of operational wavelengths and the simplified integration of Kerr comb technology into portable and versatile photonic systems.

By delicately balancing the factors of mode interaction and cavity design, this work sets the stage for forthcoming research that could further exploit these properties for practical fiber-optic and telecommunication solutions. The significant repeatability and reduced complexity showcased in this paper position dark solitons as a potentially game-changing subject for further inquiry in the photonics research community.