Photonic chip based optical frequency comb using soliton induced Cherenkov radiation (1410.8598v2)

Abstract: By continuous wave pumping of a dispersion engineered, planar silicon nitride microresonator, continuously circulating, sub-30fs short temporal dissipative solitons are generated, that correspond to pulses of 6 optical cycles and constitute a coherent optical frequency comb in the spectral domain. Emission of soliton induced Cherenkov radiation caused by higher order dispersion broadens the spectral bandwidth to 2/3 of an octave, sufficient for self referencing, in excellent agreement with recent theoretical predictions and the broadest coherent microresonator frequency comb generated to date. In a further step, this frequency comb is fully phase stabilized. The ability to preserve coherence over a broad spectral bandwidth using soliton induced Cherenkov radiation marks a critical milestone in the development of planar optical frequency combs, enabling on one hand application in e.g. coherent communications, broadband dual comb spectroscopy and Raman spectral imaging, while on the other hand significantly relaxing dispersion requirements for broadband microresonator frequency combs and providing a path for their generation in the visible and UV. Our results underscore the utility and effectiveness of planar microresonator frequency comb technology, that offers the potential to make frequency metrology accessible beyond specialized laboratories.

• The paper demonstrates harnessing soliton-induced Cherenkov radiation by engineering dispersion in SiN microresonators, enabling sub-30 fs dissipative Kerr soliton generation.
• It achieves an unprecedented 2/3-octave coherent spectrum, establishing the broadest microresonator frequency comb to date for self-referencing.
• The paper reports a relative accuracy of 3 × 10^15 compared to a fiber laser comb, underscoring its precision and stability for advanced applications.

Photonic Chip Based Optical Frequency Comb Using Soliton Induced Cherenkov Radiation

The paper presents a significant advancement in the field of optical frequency comb technology through the development of a photonic chip-based optical frequency comb utilizing soliton-induced Cherenkov radiation in silicon nitride (SiN) microresonators. This paper addresses the long-standing challenge of achieving broad spectral bandwidth and coherence simultaneously in microresonator frequency combs.

The authors successfully generate sub-30 fs short temporal dissipative Kerr solitons within a planar silicon nitride microresonator, leading to a coherent microresonator frequency comb with an unprecedented spectral bandwidth of 2/3 of an octave, suitable for self-referencing. These advancements immediately suggest broader implications for the integration of such microresonators into various applications beyond specialized laboratories, making frequency metrology more accessible.

Key Contributions and Findings

1. Harnessing Cherenkov Radiation: The researchers have effectively accessed soliton-induced Cherenkov radiation in an optical microresonator for the first time. By engineering the dispersion properties of the planar silicon nitride microresonator, they facilitated the generation of short temporal solitons, which in turn emitted Cherenkov radiation to broaden the spectral bandwidth significantly.
2. Spectral Bandwidth and Coherence: They achieved a coherent spectrum extending over 2/3 of an octave, which marks it as the broadest coherent microresonator frequency comb generated to date. This bandwidth is significant, as it allows for self-referencing without additional spectral broadening techniques.
3. High Accuracy and Stability: The overall relative accuracy of the generated comb with respect to a reference fiber laser frequency comb was measured to be 3 × 10^15. This underlines not only the coherence but also the precision achievable with the described method.
4. Stable Soliton States: By developing a robust laser tuning technique, the authors ensured the stable generation of soliton states which are crucial for practical applications. The solitons remained coherent and stable for extended periods, offering potential for various technological implementations.
5. Practical Applications and Future Prospects: The research opens pathways for using these microresonator combs in coherent communications, broadband dual-comb spectroscopy, and Raman spectral imaging. It also alleviates dispersion requirements for broadband applications, presenting a route for extending these combs into visible and ultraviolet spectra.

Implications

The demonstrated capability of soliton-induced Cherenkov radiation in expanding the spectral bandwidth represents a critical milestone toward widely applicable, chip-scale optical frequency combs. The findings suggest that this approach is a transformative step for integrated nonlinear photonics, propelling developments in fields such as frequency metrology, telecommunications, and beyond. The demonstrated ability to achieve octave-spanning frequency combs on-chip is particularly promising for the future realization of fully integrated photonic solutions that bridge radiofrequency and optical domains.

It is anticipated that future research will explore enhanced dispersion designs to achieve even broader bandwidths, the integration of these combs with other photonic components for a complete system-on-chip solution, and the deployment in real-world applications, such as astrophysical instruments and high-precision timekeeping devices. The interplay of theory and experimental verification shown in this paper will guide future explorations in optimizing optical frequency comb characteristics for diverse applications.