This research article in Nature Photonics presents a novel Vernier dual-microcomb system for precise optical frequency division, a crucial step towards miniaturised optical atomic clocks. The authors demonstrate a chip-scale platform using two interconnected microresonators to divide the frequency of a highly stable 871 nm laser down to the radio frequency range with only two feedback loops. Their approach overcomes limitations of previous microcomb-based systems by enabling easier carrier-envelope offset detection and alignment with atomic transitions. Furthermore, they introduce an interferometric noise suppression technique to enhance the stability of the resulting radio frequency clock output, showcasing performance comparable to established optical references and paving the way for integrated, high-performance atomic clocks.
Nature Photonics Paper Link: Vernier microcombs for integrated optical atomic clocks
Let us dive deeper and later listen to a more focused audio summary of this research.
Vernier Microcombs for Integrated Optical Atomic Clocks
Executive Summary:
This paper presents a significant advancement towards chip-scale optical atomic clocks by demonstrating a novel optical frequency division (OFD) scheme using a Vernier dual-microcomb system. The researchers successfully frequency divided an ultranarrow-linewidth continuous-wave laser at 871 nm down to an ~235 MHz radio frequency (RF) output, a crucial step in linking high-frequency optical atomic transitions to the more easily measurable RF domain. The dual-comb approach overcomes limitations of single microcomb systems, particularly regarding self-referencing and spectral alignment with atomic transitions. Furthermore, the implementation includes an innovative noise suppression technique to enhance the stability of the RF clock output, achieving performance comparable to the optical reference. This work paves the way for the development of compact, mass-manufacturable, high-performance optical atomic clocks suitable for deployment beyond laboratory settings.
The Vernier dual-microcomb scheme presents several advantages over traditional single microcomb systems for optical frequency division (OFD) in atomic clocks. These advantages primarily address the challenges associated with self-referencing and spectral alignment that limit the effectiveness of single microcomb approaches.
Challenges with Single Microcomb Systems:
Traditional single microcomb-based OFD for optical atomic clocks faces significant hurdles:
Difficult Self-Referencing: Self-referencing a microcomb, a crucial step for establishing an absolute frequency reference, typically requires an octave-spanning comb with terahertz repetition rates. This leads to a carrier-envelope offset frequency (fCEO) in the range of ±500 GHz, which is difficult to design due to microresonator fabrication imperfections and challenging for direct electronic detection.
Challenging Spectral Alignment: Spectrally aligning the sparsely spaced modes of a terahertz repetition rate microcomb with the narrow linewidth of atomic clock transitions with sufficient signal-to-noise ratio is also a significant difficulty.
Limitations of Previous Demonstrations: Earlier demonstrations of microcomb-based OFD with atom-referenced lasers often relied on low repetition rate combs or hybrid systems, frequently using thermal vapours as atomic references, which offer limited frequency stability compared to trapped atoms.
Advantages of the Vernier Dual-Microcomb Scheme:
The Vernier dual-microcomb scheme overcomes these limitations by employing a pair of microcombs with slightly different, yet still high (~THz), repetition rates. This approach offers several key advantages:
Simplified fCEO Detection: The Vernier scheme enables shifting the ultrahigh-frequency (~100 GHz) carrier-envelope offset beat down to frequencies where detection is possible. By beating the two combs against each other, both the large repetition rates and the fCEO can be detected without requiring an octave-spanning comb with an extremely high and difficult-to-manage fCEO.
Enhanced Flexibility in Spectral Alignment: The dual-comb system provides greater freedom in selecting comb lines from either or both combs for sum-frequency generation (SFG). This flexibility aids in reaching a much wider range of wavelengths, including those relevant to specific atomic clock transitions like the 871 nm laser used in this work, which is intended for frequency doubling to the ¹⁷¹Yb⁺ clock transition at 435.5 nm.
Potential for Wide Applicability: The Vernier dual-microcomb OFD platform is useful for detecting high-frequency fCEO and potentially for performing OFD on a variety of atomic species through a dual-comb sum-frequency process. For example, SFG between the pump laser and a comb line can cover a broad range of wavelengths suitable for other atomic clock transitions.
Simplified System Architecture: The demonstrated OFD of an 871 nm laser down to an RF output was achieved using only two feedback servos. This simplification, enabled by the unique characteristics of the Vernier scheme, contributes to the potential development of a low size, weight, and power (SWaP) package.
Compact Footprint and Improved Manufacturing: The use of high-repetition-rate microcombs fabricated in silicon nitride (SiN) allows for a significantly smaller footprint compared to systems using lower repetition rate microcombs, which can potentially lead to a higher manufacturing yield in mass production.
Effective Noise Suppression: The Vernier dual-microcomb scheme allows for an innovative open-loop electronic mixing technique to suppress interferometric noise arising from phase perturbations in the fibre leads. By detecting dual-comb beat notes on both sides of the pump frequency and electronically mixing them, correlated noise components are largely cancelled, leading to a significant improvement in the fractional frequency instability of the RF clock output, comparable to the optical reference.
In summary, the Vernier dual-microcomb scheme overcomes key limitations of single microcomb systems for optical frequency division in atomic clocks by simplifying fCEO detection, enhancing spectral flexibility, potentially expanding the range of applicable atomic species, and enabling effective noise suppression, all while promising a more compact and manufacturable platform.
Further Notes and Details:
The Vernier dual-microcomb system directly addresses several key limitations that have hindered the use of microcombs in optical frequency division (OFD) for atomic clocks, as highlighted in the sources. These limitations and how the Vernier system tackles them are:
Difficulty in Achieving Self-Referencing: Traditional single microcomb systems often require an octave span and terahertz repetition rates for self-referencing, leading to a carrier-envelope offset frequency (fCEO) in the range of ±500 GHz. This high fCEO is difficult to design due to microresonator fabrication imperfections and challenging for electronic detection.
The Vernier dual-microcomb scheme overcomes this by shifting the ultrahigh-frequency (~100 GHz) carrier-envelope offset beat down to frequencies where detection is possible. By using two combs with slightly different repetition rates, the system can detect both the large repetition rates and the fCEO through heterodyne beating, without needing an extremely high fCEO from a single octave-spanning comb. The experiment demonstrated the detection of fxCEO, which ideally removes contributions from both combs' fCEO.
Challenges in Spectral Alignment with Atomic Transitions: The coarse spacing of terahertz microcombs in single systems makes it difficult to spectrally align a comb mode with specific narrow linewidth atomic clock transitions with sufficient signal-to-noise ratio.
The Vernier scheme provides substantial advantages over single broadband comb schemes through the "freedom of picking comb lines from either or both of the combs for sum-frequency generation (SFG) to aid in reaching a much greater variety of wavelengths". This flexibility allows the researchers to reach the 871 nm laser frequency, which is relevant for potential frequency doubling to the ¹⁷¹Yb⁺ clock transition. The system performed a novel f-2f process via SFG using lines from both combs and also used SFG between the pump and a Vernier comb line to relate the combs to the 871 nm LO laser.
Limitations of Previous Microcomb-Based OFD Demonstrations: Earlier efforts often relied on low repetition rate combs or hybrid systems, frequently using thermal vapours with limited frequency stabilities compared to cooled or trapped atoms.
The Vernier dual-microcomb system, in contrast, successfully frequency divided an ultranarrow-linewidth continuous-wave laser at 871 nm (a proxy for a high-stability atomic reference) down to an ~235 MHz RF output. The system was designed with the ¹⁷¹Yb⁺ clock transition in mind, which supports better frequency stability than thermal atomic references. Furthermore, the achieved fractional frequency instability of the RF clock output essentially overlayed with that of the optical reference, demonstrating successful transfer of stability.
Complexity and Number of Servo Locks: Traditional approaches for stabilising microcombs in OFD systems might require more complex setups and a greater number of feedback loops.
The Vernier dual-microcomb system achieved OFD using only two feedback servos by stabilising the fxCEO and fVernier beats. This simplified architecture, which avoids the need for a third servo to stabilise the offset frequency, is a significant advantage towards developing a low size, weight, and power package.
Noise Issues: Interferometric noise from fibre leads can limit the stability of RF clock outputs in microcomb-based systems.
The Vernier dual-microcomb system incorporates an innovative open-loop electronic mixing technique to suppress this interferometric noise. By detecting dual-comb beat notes on both sides of the pump frequency (fclock+ and fclock-) and electronically mixing them, the correlated but opposite-signed frequency noise components are largely cancelled, leading to a substantially reduced frequency instability.
By addressing these limitations, the Vernier dual-microcomb scheme represents a significant advancement towards realising compact, mass-manufacturable, high-performance optical atomic clocks.
The Vernier dual-microcomb optical frequency division (OFD) scheme is a novel approach designed to overcome limitations of single microcomb systems for the development of chip-scale optical atomic clocks. It functions by using two broadband Kerr microcombs with slightly different terahertz repetition rates, both typically fabricated on a silicon nitride (SiN) platform and pumped by a shared continuous-wave laser at 1550 nm.
More focused audio summary:
A breakdown of the scheme:
Generation of Dual Microcombs: The system generates two octave-spanning or broadband terahertz repetition rate microcombs. One is referred to as the main comb and the other as the Vernier comb, and they have slightly different repetition rates (e.g., ~896 GHz and ~876 GHz). This difference in repetition rates (δfrep) is crucial for the Vernier effect.
Leveraging the Vernier Effect for fCEO Detection: The small difference in repetition rates allows for the detection of the carrier-envelope offset (fCEO) frequency at a lower, more manageable frequency. Instead of trying to detect a very high fCEO from a single terahertz-rate octave-spanning comb (which is challenging due to fabrication imperfections), the Vernier scheme generates beat notes (fVernier) that are sensitive to the difference in repetition rates and the fCEO of both combs. This enables "shifting an ultrahigh-frequency (~100 GHz) carrier-envelope offset beat down to frequencies where detection is possible". The system derives a signal fxCEO by electronically combining other beat notes, which ideally removes contributions from both combs’ fCEO and relates their repetition rates to the local oscillator (LO) laser.
Facilitating Spectral Access via Sum-Frequency Generation (SFG): The dual-comb architecture provides flexibility in reaching desired wavelengths through sum-frequency generation (SFG). Researchers can pick comb lines from either or both combs for SFG, aiding in accessing a wider range of wavelengths, including the 871 nm laser used in the demonstration. Key SFG processes include:
A novel f-2f process that sums one comb line from each comb at around 2 μm to generate a beat note (ff-2f) containing information about both combs' fCEO and repetition rates.
SFG between the 1550 nm pump laser and a Vernier comb line (around 2 μm) to create a product that is then beat against the 871 nm LO laser, generating fcomb-LO and linking the dual microcombs to the optical reference.
Optical Frequency Division (OFD) and Stabilisation: The scheme achieves OFD of an optical reference (in this case, an 871 nm laser phase-locked to a fibre comb) down to an RF frequency (around 235 MHz). This is accomplished by stabilising the repetition rates of the two microcombs to the LO laser using only two feedback servos. This is done by phase-locking the fxCEO and fVernier beat notes to RF synthesizers. Stabilising these beat notes ensures that the repetition rates of both combs, and consequently the frequency of the dual-comb beat (fclock), are locked to the stable optical reference.
Interferometric Noise Suppression: The system incorporates an innovative open-loop electronic mixing technique to suppress interferometric noise arising from phase fluctuations in the fibre leads. By detecting dual-comb beat notes on both sides of the pump frequency (fclock+ and fclock-) and electronically mixing them, the correlated but opposite-signed frequency noise components are largely cancelled, significantly improving the stability of the RF clock output.
The Vernier dual-microcomb OFD scheme offers several advantages over single microcomb approaches, addressing challenges related to self-referencing, spectral alignment with atomic transitions, complexity of stabilisation, and noise. By successfully demonstrating OFD of an 871 nm laser down to an RF output with high stability, this work represents a significant step towards the development of compact, mass-manufacturable optical atomic clocks. The potential for future on-chip integration of the dual combs and related components further enhances the prospects for widespread deployment of high-performance optical atomic clock technology.
Chip-scale optical atomic clocks are considered advantageous for several key reasons, as detailed in the sources:
Compactness and Low Size, Weight, and Power (SWaP): Current state-of-the-art optical atomic clocks, such as optical lattice and ion trap clocks, are bulky and have volumes on the order of many litres. Miniaturising these clocks to achieve low size, weight, and power (SWaP) is a critical challenge. Microcombs are highlighted as "an essential part of future chip-scale optical atomic clocks" in achieving this miniaturisation. The use of high-repetition-rate microcombs on a SiN platform, as demonstrated in the paper, allows for a "significantly smaller footprint" compared to systems using lower repetition rate combs.
Potential for Widespread Deployment: The bulkiness of current high-performance optical clocks hinders their widespread deployment beyond laboratory settings. Chip-scale optical atomic clocks, being compact and potentially mass-manufacturable, could enable the deployment of high-performance timing technology in various applications that were previously limited by the size and complexity of traditional systems.
Applications in Diverse Fields: Optical atomic clocks offer exceptional long-term frequency stability, making them valuable for numerous fields. Miniaturised versions would be suitable for "timing systems, fundamental physics tests, and geodesy", as well as "navigation, telecommunications". The potential for chronometric levelling and geodesy is also mentioned.
Enhanced Robustness and Reduced Sensitivity to Environmental Perturbations: Future full on-chip integration of dual combs and other components is envisioned. Such integrated systems would be "less sensitive to environmental perturbations" due to the elimination of many optical fibres currently used in non-integrated setups.
Potential for Mass Manufacturing and Reduced Costs: Kerr microcombs are seen as "mass-manufacturable, compact alternatives to bulk frequency combs". The smaller footprint of high-repetition-rate microcomb structures could lead to a "higher manufacturing yield in mass production" and potentially lower costs compared to hand-assembled, large-volume systems. The all-planar geometry used in the demonstrated system also avoids the complexity of coupling to suspended whispering gallery mode resonators, further aiding manufacturability.
In essence, chip-scale optical atomic clocks promise to bring the high precision and stability of optical frequency standards to a wider range of applications by significantly reducing their size, weight, power consumption, and cost, while also improving their robustness.
The researchers implemented an innovative open-loop electronic mixing technique to suppress excess noise observed in the initial RF clock output. This noise was attributed to time-varying phase perturbations in the fibre leads that bring light in and out of the individual microrings in the dual-comb system. These phase fluctuations, denoted as ϕ₁(t) and ϕ₂(t) for the two arms of the interferometer, introduce frequency noise on the generated clock signals.
A breakdown of the noise suppression technique:
Origin of the Noise: The dual-comb system generates an RF clock signal (fclock) by heterodyning sidebands of the two microcombs. The light travelling through the fibre leads to and from the microresonators experiences time-varying phase shifts. These phase fluctuations (ϕ₁(t) and ϕ₂(t)) are translated into frequency noise on the detected beat notes.
Detection of Dual Clock Signals: The key insight is that the first sidebands on either side of the pump frequency (fclock+ and fclock-) carry frequency noise with equal amplitude but opposite signs. This arises because the frequency fluctuations are proportional to the derivative of the time-varying phases (ϕ′(t)).
fclock+ is derived from a higher-frequency sideband and is influenced by the phase derivatives.
fclock- is derived from a lower-frequency sideband and is influenced by phase derivatives with an opposite sign.
Electronic Mixing for Noise Cancellation: To suppress this noise, the researchers detected fclock+ and fclock- on separate photodetectors. The resulting heterodyne beats were then electronically mixed.
Ideally, by summing fclock+ and fclock-, the correlated but opposite-signed frequency noise components (δfN) largely cancel out, leaving a signal proportional to twice the difference in the repetition rates (2(frep1 − frep2)), which is the desired clock frequency without the interferometric noise.
Practical Implementation: In practice, to avoid issues with second harmonics from the mixer, frequency-divided versions of the two signals (fclock+/168 and fclock-/168) were used. One of the divided clock signals (fclock-) was also frequency-upshifted by mixing with a 40 MHz signal (derived from a GPS-disciplined oscillator) and filtered before being mixed with the other divided clock signal (fclock+). The resulting noise-suppressed RF clock output was at ~235 MHz.
Performance Improvement: This noise suppression technique led to a substantial reduction in frequency instability of the RF clock output. The fractional frequency instability of the noise-suppressed clock essentially overlaid with that of the optical reference, demonstrating successful transfer of stability. The system achieved a fractional frequency instability of at least ~3 × 10⁻¹³/τ, reaching ~2 × 10⁻¹⁵ at 1,000 s averaging time.
Extraction of Interferometric Noise: The noise suppression scheme could also be modified to extract the differential-mode interferometric frequency noise directly by measuring the difference between fclock+ and fclock-.
This noise suppression technique was crucial in achieving high stability in the RF clock output and demonstrating the potential of the Vernier dual-microcomb OFD scheme for developing high-performance chip-scale optical atomic clocks. It allowed the system to effectively transfer the stability of the optical reference to the RF domain despite the presence of noise introduced by the fibre optics.
The development of integrated optical systems is a significant focus within the research presented in the sources, particularly in the context of miniaturising optical atomic clocks. The current Vernier dual-microcomb optical frequency division (OFD) system is largely fibre-based, but the ultimate goal is to create fully integrated chip-scale devices.
A discussion of integrated optical systems based on the provided information:
Motivation for Integration: The primary motivation for moving towards integrated optical systems is to achieve low size, weight, and power (SWaP) optical atomic clocks suitable for widespread deployment beyond laboratory settings. The current bulky nature of state-of-the-art optical lattice and ion trap clocks limits their applications. Microcombs fabricated on photonic chips are seen as a key enabler for this miniaturisation. Integrated systems are also expected to be less sensitive to environmental perturbations due to the elimination of many optical fibres used in current experimental setups.
Silicon Nitride (SiN) Platform: The researchers have already implemented their Vernier dual-microcomb scheme on a silicon nitride (SiN) platform. This is a crucial first step towards integration. The use of high-repetition-rate microcombs fabricated in SiN allows for a significantly smaller footprint compared to systems using lower repetition rate microcombs, potentially improving manufacturing yield. The all-planar geometry of SiN platforms also simplifies integration compared to other microresonator technologies.
Envisioned Future Integration: The briefing document and article outline a clear vision for future integration, including:
Integration of the dual combs onto a single SiN chip, along with on-chip thermal heaters for spectral alignment and microcomb feedback.
On-chip spectral filtering for the separation and routing of different wavelength bands.
Heterogeneous Integration: The sources highlight the exciting progress in heterogeneous integration of other materials onto the SiN platform, which is seen as crucial for achieving fully functional integrated systems. This includes:
III–V lasers for on-chip pumping of the microcombs.
Thin-film lithium niobate (PPLN) for on-chip second-order nonlinear frequency conversion processes like sum-frequency generation (SFG). The current experiment already utilises off-chip PPLN waveguides for crucial SFG steps.
Benefits of a Fully Integrated System: A fully integrated dual-comb system with on-chip pumping and nonlinear frequency conversion would offer several advantages:
Further reduction in size, weight, and power consumption.
Increased robustness and stability due to the reduced sensitivity to environmental perturbations by eliminating fibre connections.
Potential for mass manufacturing and lower costs.
Towards a Chip-Scale Optical Atomic Clock: The researchers believe that combining their integrated dual-microcomb system with advances in compact ion traps and optical lattices could one day lead to the development of a fully integrated high-performance optical atomic clock on a chip. Efforts towards integrated ion traps and optical lattices are noted as recent progress in this direction.
In summary, the development of integrated optical systems is a central theme in this research. The use of a SiN platform for the Vernier dual-microcombs is a significant step, and the envisioned future integration of lasers, nonlinear materials, and spectral management functionalities on the same chip holds immense promise for realising compact, stable, and mass-manufacturable optical atomic clocks.
Fearured Image: A DALLE Generated image based on a prompt highlighting silicon nitride optical chip with Vernier microcombs, and highlighting microfabrication details and precise light interactions.
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