Low-Pump-Power, Low-Phase-Noise, and Microwave to Millimeter-wave Repetition Rate Operation in Microcombs

Abstract: Microresonator-based frequency combs (microcombs or Kerr-combs) can potentially miniaturize the numerous applications of conventional frequency combs. A priority is the realization of broad-band (ideally octave spanning) spectra at detectable repetition rates for comb self referencing. However, access to these rates involves pumping larger mode volumes and hence higher threshold powers. Moreover, threshold power sets both the scale for power per comb tooth and also the optical pump. Along these lines, it is shown that a class of resonators having surface-loss-limited Q factors can operate over a wide range of repetition rates with minimal variation in threshold power. A new, surface-loss-limited resonator illustrates the idea. Comb generation on mode spacings ranging from 2.6 GHz to 220 GHz with overall low threshold power (as low as 1 mW) is demonstrated. A record number of comb lines for a microcomb (around 1900) is also observed with pump power of 200 mW. The ability to engineer a wide range of repetition rates with these devices is also used to investigate a recently observed mechanism in microcombs associated with dispersion of subcomb offset frequencies. We observe high-coherence, phase-locking in cases where these offset frequencies are small enough so as to be tuned into coincidence. In these cases, a record-low microcomb phase noise is reported at a level comparable to an open-loop, high-performance microwave oscillator.

An Examination of Microcomb Innovations: Low-Pump-Power and Multi-Rate Operation

Microresonator-based frequency combs, commonly known as microcombs or Kerr-combs, present formidable potential for miniaturizing applications traditionally dominated by conventional frequency combs. The work discussed in this paper introduces significant advancements within this field, particularly emphasizing low-pump-power operation at diverse repetition rates, maintaining minimal phase noise.

Key Contributions and Findings

The authors introduce a novel class of silica-based resonators, fabricated on a silicon chip, characterized by ultra-high Q factors reaching up to 875 million. One of the core achievements in this work is demonstrating microcomb operation across an unprecedented span of repetition rates, from 2.6 GHz to 220 GHz. Despite these variations in rate, threshold power remained less than 5 mW, showcasing the advantageous impact of surface-loss-limited Q scaling on pump efficacy.

The research details an experiment leveraging surface-loss-limited resonators that help decouple the typical dependence of threshold power from the free spectral range (FSR). This approach mitigates the adverse effects that typically arise as the microcomb transitions from millimeter-wave to microwave repetition rates. By maintaining low threshold power across varying repetition rates, the authors achieve dense comb lines, ultimately generating around 1900 lines with a 200 mW pump power in a 33 GHz FSR device.

Substantial attention is given to exploring high coherence phase locking in microcombs. The paper identifies conditions enabling the tuning of subcomb offset frequencies into alignment, yielding significantly low phase noise akin to open-loop high-performance microwave oscillators.

Implications and Future Directions

The impact of achieving low-power, broad-span microcomb operation extends across several scientific domains. The ability to access wide-ranging microwave repetition rates is imperative for optical clocks, astronomical spectral calibration, and line-by-line pulse shaping. Furthermore, the lithographic precision offered by the resonators facilitates potential integration with waveguides and other optical devices, advancing efforts to create entirely integrated optical systems.

Practical advances anticipated include the pursuit of optimizing the system for octave span operation with reduced repetition rates. As material and dispersion properties continue to be refined, it suggests an evolution toward more stable and scalable microcomb systems compatible with existing photonic architectures.

In summary, the exploration and results depicted in this paper constitute a robust foundation for future developments in microcomb technology. Through surface-loss-limited scaling and careful engineering of resonator parameters, significant strides have been made toward integrating versatile and efficient frequency combs into compact, chip-scale devices, fostering progress across the fields of metrology, telecommunications, and beyond.