Microresonator Kerr Frequency Combs with High Conversion Efficiency
The paper conducted by Xue et al. addresses the challenge of achieving high conversion efficiency in microresonator-based Kerr frequency combs. The research presents a significant advancement by establishing a conversion efficiency of approximately 30% in microcombs within the fiber telecommunication band. This is particularly notable given that prior to this work, microcombs were typically limited to conversion efficiencies of only a few percent. Consequently, these findings have substantial implications for applications requiring high power per comb line, including optical communications and RF photonics.
Microresonator-based optical frequency combs (microcombs) have garnered considerable interest due to their low power consumption and potential for integration at the chip level. Despite these promising attributes, their deployment in practical applications has been hindered by low conversion efficiencies, a parameter crucial for numerous applications like fiber optics and RF filtering.
The researchers focused on dark-pulse mode-locking within the normal dispersion region to achieve the high conversion efficiency reported. The paper elucidates the underlying reasons for the efficiency improvement, chiefly attributing it to effective coupling conditions for the pump and the duty cycle of localized time-domain structures. The investigation also discerns a correspondence between the duty cycle and conversion efficiency, a relationship that provides a useful framework for enhancing comb outputs.
Xue et al. provide a detailed theoretical and numerical analysis, complemented by experimental results, to reinforce their findings about efficient comb generation. By addressing both phase-locked bright soliton and dark-pulse comb states, they demonstrate numerically that dark pulses exhibit higher conversion efficiencies due to larger effective detuning and broader time-domain pulse widths, compared to bright solitons. Their insights suggest that combs with wider bandwidths, which typically correlate with reduced time-domain efficiency in bright solitons, do not suffer such degradation in dark-pulse mode-locking states.
A significant aspect of this paper is the focus on effective coupling strategies. The authors clarify how different coupling and detuning parameters influence the conversion efficiency. Furthermore, they explore the implications of intracavity power distributions and evaluate how they affect the overall power balance between pump and newly generated frequencies.
The practical outcome of the research is evidenced by experimental data showing over 30% conversion efficiency and a correspondingly high on-chip comb power of 209 mW, not including the pump. This high level of efficiency is facilitated by microresonator designs enabling effective critical coupling and favorable cavity dynamics, as exhibited by the examination of two distinct microrings (rings 1 and 2) under varying coupling conditions.
The experimental observations, along with the theoretical analyses presented, offer valuable guidelines for future explorations aiming at novel, high-efficiency microcomb states. Such achievements are likely to propel advancements in telecommunications and other fields dependent on high-quality optical frequency combs. This work also raises possibilities for further optimizing microcomb performance by developing innovative resonator designs and exploring dynamic coupling strategies. Overall, this research contributes substantially to the ongoing efforts to enhance microcomb efficiency and broadens the application potential of this technology in integrated photonics and beyond.