Investigation of Mode Coupling in Normal Dispersion Silicon Nitride Microresonators for Kerr Frequency Comb Generation (1405.6225v1)

Abstract: Kerr frequency combs generated from microresonators are the subject of intense study. Most research employs microresonators with anomalous dispersion, for which modulation instability is believed to play a key role in initiation of the comb. Comb generation in normal dispersion microresonators has also been reported but is less well understood. Here we report a detailed investigation of few-moded, normal dispersion silicon nitride microresonators, showing that mode coupling can strongly modify the local dispersion, even changing its sign. We demonstrate a link between mode coupling and initiation of comb generation by showing experimentally, for the first time to our knowledge, pinning of one of the initial comb sidebands near a mode crossing frequency. Associated with this route to comb formation, we observe direct generation of coherent, bandwidth-limited pulses at repetition rates down to 75 GHz, without the need to first pass through a chaotic state.

• The paper demonstrates that mode coupling in silicon nitride microresonators reverses local dispersion, enabling coherent and low-noise Kerr frequency combs.
• Experimental and simulation results reveal that mode coupling 'pins' the initial sideband near mode crossings, producing Type I combs with a free spectral range of just below 75GHz.
• These findings suggest that leveraging mode coupling can simplify microresonator designs and enhance performance for photonic applications such as optical clocks and spectroscopy.

Mode Coupling in Normal Dispersion Silicon Nitride Microresonators: Implications for Kerr Frequency Comb Generation

The research paper under discussion explores the intricate phenomena of mode coupling within normal dispersion silicon nitride microresonators and its effects on Kerr frequency comb generation. Silicon nitride microresonators are widely recognized for their application in creating Kerr frequency combs, although the majority of research has historically concentrated on anomalous dispersion regimes. Here, the authors dissect the role of mode coupling, presenting this as a potentially crucial element in the comb generation process, even in the domain of normal dispersion.

The paper presents a comprehensive investigation demonstrating that mode coupling substantially alters local dispersion properties in silicon nitride microresonators. This modification can lead to the reversal of dispersion signs, a phenomenon unexplored in extent until now. Experimentation, complemented by simulations, establishes a correlation between mode coupling and the initiation of frequency comb generation, highlighting the fundamental role of mode interactions.

Notably in this paper, the authors accomplished the generation of coherent, bandwidth-limited pulses with repetition rates as low as 75GHz through normal dispersion microresonators, circumventing the chaotic states traditionally associated with comb formation processes. This result is pivotal as it asserts the efficiency of mode coupling as a standalone mechanism in comb generation.

Key Findings and Numerical Results

• The paper recorded "Type I" comb generation, also known as natively mode-spaced (NMS) combs, in a microresonator with a free spectral range (FSR) just below 75 GHz. These combs display initial sidebands spaced by one FSR from the pump, with substantial coherence and minimal noise.
• Mode coupling significantly affects local dispersion, at times altering its sign, thereby impacting comb generation mechanisms.
• Experimental findings reveal that the initial sideband comb is "pinned" near a mode crossing frequency, a clear indication of mode coupling's role, as observed with the sideband remaining steady despite substantial pump wavelength variations.

Implications

This research underscores the significance of mode coupling not merely as a supplementary mechanism but as a primary facet capable of facilitating Kerr comb generation in normal dispersion regimes. The work implies that exploiting mode coupling could lead to simpler and more coherent comb generation processes, particularly valuable in applications requiring low-noise and coherent outputs.

The findings may influence the design and operational parameters of microresonators utilized in a variety of applications such as photonic communications, optical clocks, and spectroscopy. Additionally, the altered role of dispersion, achieved through mode coupling, could translate into advancements in nonlinear photonic devices less constrained by typical dispersion limitations.

Theoretical and Practical Speculations for Future Research

The detailed elucidation of mode coupling effects paves the way for further theoretical exploration of the underlying physics, potentially involving refined models that incorporate missing non-linear interactions. Given the paper's verification of mode coupling effects through experimental data, future work might aim to develop new comb generation schemes that optimize mode interactions for enhanced performance across different dispersion contexts.

Practically, implementing these findings could lead to innovative microresonator designs which improve the efficiency of Kerr frequency combs in normal dispersion regimes. Future investigations could consider exploring even smaller FSR resonators and understanding the interplay of mode coupling in variable material systems beyond silicon nitride, perhaps extending to polymer-based or hybrid photonic systems.

In summary, this research exemplifies a paradigm shift in understanding mode coupling's capacity to enable frequency comb generation within normal dispersion silicon nitride microresonators, providing actionable insights for both theoretical innovation and practical photonic applications.