A microresonator frequency comb optical clock (1309.3525v1)

Abstract: Optical-frequency combs enable measurement precision at the 20th digit, and accuracy entirely commensurate with their reference oscillator. A new direction in experiments is the creation of ultracompact frequency combs by way of nonlinear parametric optics in microresonators. We refer to these as microcombs, and here we report a silicon-chip-based microcomb optical clock that phase-coherently converts an optical-frequency reference to a microwave signal. A low-noise comb spectrum with 25 THz span is generated with a 2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium frequency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clocks output is the electronically countable 33 GHz microcomb line spacing, which features an absolute stability better than the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of tools needed for interfacing microcombs to state-of-the-art optical clocks.

• The paper introduces a novel microcomb optical clock that stabilizes a 25 THz comb spectrum to rubidium transitions by phase-locking two teeth 108 modes apart.
• It employs parametric seeding in a 2 mm silica disk microresonator to generate a low-noise, equidistant comb with a 33 GHz line spacing.
• The demonstrated clock achieves an average frequency stability of 10^-12 over 10,000 seconds, highlighting its potential for compact, high-precision metrology.

Overview of "A Microresonator Frequency Comb Optical Clock"

The paper "A Microresonator Frequency Comb Optical Clock" presents a significant advancement in the domain of precision measurement technologies, particularly focusing on the development of a microresonator-based frequency comb optical clock. This research addresses the integration of ultracompact microcombs with optical clock technologies, proposing a novel architecture that allows for the stabilization of a microcomb to atomic rubidium (Rb) transitions, thereby effectively converting an optical frequency reference to an electronically countable microwave signal.

Key Findings and Methodology

The paper demonstrates the generation of a low-noise microcomb spectrum with a significant span of 25 THz, achieved via a 2 mm diameter silica disk microresonator. This endeavors into the field of microcombs, a type of frequency comb generated through nonlinear parametric oscillation and four-wave mixing driven by a continuous wave (CW) laser. Notably, microcombs present unique advantages, such as large comb-mode spacings in the tens of GHz range and reduced size and power consumption due to their monolithic construction.

The microcomb in this paper is electronically stabilized to Rb reference lasers with a frequency spacing of 3.5 THz. This stabilization is realized by phase-locking two specific teeth of the comb, spaced 108 modes apart, to these optical transitions. The resulting optical clock features a line spacing of 33 GHz, with an absolute stability surpassing that of the Rb references by a factor corresponding to the division ratio of 108.

A key element of this research is using parametric seeding to inhibit the formation of partially nonequidistant subcombs within the spectrum, ensuring the robustness and accuracy essential for potential high-performance applications. This technique permits the deterministic suppression of subcombs and guarantees the equidistance of the generated microcomb lines.

Numerical Results and Implications

One of the pivotal results from the reported paper is that the average frequency stability of the clock, as characterized by its Allan deviation, reaches 10^-12 over a measurement period of 10,000 seconds. Such stability is crucial for applications where long-term precision is needed, and it underscores the efficacy of optical frequency division achieved with the microcomb technology.

Despite the current limitations imposed by the Rb reference lasers' noise and systematic drifts, the approach outlined has potential utilities extending beyond present-day rubidium-based references. The methodology delineated in this paper promises a path towards fully integrated chip-scale photonic and atomic systems, offering significant enhancements in compactness and performance compared to traditional frequency comb systems.

Future Directions

The research sets a foundation for future endeavors concentrating on increasing the power of the generated optical waveforms and achieving self-referencing capabilities. This would entail broadening the spectral span further and refining the microcomb's phase coherence across an entire optical octave. Moreover, the architecture has demonstrated the capacity to support atomic references of higher orders of magnitude performance, which could be pivotal in evolving microwave photonics and metrology.

In summary, this paper provides a robust framework for integrating microresonator-based technology with high-precision optical clocks, promising to enhance measurement precision across various scientific domains. Future advancements in this field could see the deployment of highly stable and compact optical clocks, with significant applications in precision spectroscopy, telecommunications, and frequency synthesis.