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Introduction
Microresonator-based frequency combs are valuable due to their high coherence, flexible spacing, and broad bandwidth, enabling applications in various fields. Chromatic dispersion is crucial in determining microcomb dynamics. In anomalous dispersion (AD), microcombs form bright localized dissipative structures (LDS) or dissipative Kerr solitons (DKSs). Normal dispersion (ND) leads to dark LDS and switching waves (SWs). Recent studies focused on near-zero-dispersion solitons, where higher-order dispersion significantly influences dynamics. Theoretical predictions suggested bright solitons in ND regimes due to third-order dispersion (TOD) and were experimentally confirmed. However, the existence of a corresponding structure in the anomalous dispersion regime remained unexplored. This work aims to investigate Kerr comb generation in the near-zero anomalous-dispersion regime using a highly nonlinear fiber Fabry-Perot (F-P) microcavity and pulsed pumping to reduce power demands and thermal effects.
Literature Review
The introduction extensively reviews existing literature on microcomb dynamics, highlighting the role of dispersion in shaping Kerr comb behavior. It discusses the formation of dissipative Kerr solitons (DKSs) in anomalous dispersion and switching waves (SWs) in normal dispersion. The review focuses on recent theoretical and experimental works exploring soliton dynamics near the zero-dispersion regime, particularly emphasizing the influence of higher-order dispersion, specifically third-order dispersion (TOD), on soliton stability and structure. The lack of systematic study, especially experimental investigation, of near-zero-dispersion solitons in the anomalous dispersion regime is highlighted as the motivation for the current research.
Methodology
The study employed both theoretical simulations and experimental techniques. The theoretical analysis utilized the normalized Lugiato-Lefever Equation for Fabry-Pérot cavity (FP-LLE) to simulate microcomb evolution under pulsed pumping in the near-zero anomalous-dispersion regime. The simulations incorporated parameters for GVD and TOD to investigate their impact on soliton formation and stability. The experimental setup involved a highly nonlinear fiber-based Fabry-Perot microresonator with ultra-small anomalous GVD, precisely measured using a dual-cavity method and fiber Mach-Zehnder interferometry. A pulsed pumping scheme was employed, and the generated microcombs were characterized using an optical spectrum analyzer (OSA) and an electronic spectrum analyzer (ESA). Beatnote measurements were performed to analyze the repetition rate. Desynchronization frequency was introduced to compensate for the group-velocity shift caused by TOD. The experiment involved tuning the central wavelength of the driving pulse across the resonance to explore various microcomb states.
Key Findings
The simulations predicted the existence and evolution of anomalous-dispersion based near-zero-dispersion solitons (AD-NZDS), appearing as stable multi-soliton clusters. The number of bound solitons in the cluster decreased with increasing detuning. The experimental results confirmed the existence of both broadband modulational instability (MI) combs and AD-NZDS. The broadband MI combs exhibited a wide spectral span (2/3 octave, >84 THz), high conversion efficiency, and a large number of comb lines (>8400). The AD-NZDS showed a local repetition rate up to 8.6 THz, an individual pulse duration <100 fs, a spectral span >32 THz, and >3200 comb lines. The phase noise of the AD-NZDS was found to be significantly lower compared to the broadband MI state. The characteristics of both states were thoroughly analyzed in both the time and frequency domains. The transition between the broadband MI state and the AD-NZDS state is also experimentally demonstrated.
Discussion
The findings demonstrate the successful generation and characterization of two distinct microcomb states: broadband MI combs and AD-NZDS, in a near-zero anomalous-dispersion regime. The two types of combs offer complementary advantages for different applications. The broadband MI combs are well-suited for high-power applications due to their high conversion efficiency and wide spectral range. In contrast, AD-NZDS are advantageous for applications requiring low phase noise and ultra-high repetition rates, such as optical computing, light sensing, communication, and spectroscopy. The experimental results confirm the theoretical predictions, highlighting the importance of higher-order dispersion in shaping the dynamics of Kerr combs near the zero-dispersion point. This work extends the understanding of Kerr cavity soliton dynamics and provides a flexible strategy for selecting the desired operating state based on the application.
Conclusion
This work provides experimental and theoretical evidence for the generation of both broadband MI combs and AD-NZDS in a near-zero anomalous-dispersion regime. The different characteristics of these two comb states offer a flexible approach to selecting the optimal state for specific applications. The research expands the understanding of Kerr microcomb dynamics and could inspire further investigations into dispersion-controlled microresonators and broadband coherent comb devices. Future work could explore optimization strategies for enhanced performance and broader applications.
Limitations
The study primarily focused on a specific type of fiber-based Fabry-Perot microresonator. Generalizability to other microresonator platforms may require further investigation. The study used a specific pulsed pumping scheme; the impact of different pumping schemes on comb generation warrants further research. The analysis of noise characteristics could be further expanded to explore different noise sources and their impact on the stability and performance of the generated combs.
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