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arxiv: 2511.20504 · v1 · submitted 2025-11-25 · ⚛️ physics.optics

Ultralow noise microwaves with free-running frequency combs and electrical feedforward

Takuma Nakamura , William Groman , Qing-Xin Ji , Oguzhan Kara , Benjamin Rudin , Anatoliy Savchenkov , Vladimir Iltchenko , Wei Zhang
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Andrey Matsko John E. Bowers Florian Emaury Kerry J. Vahala Scott A. Diddams Franklyn Quinlan
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Pith reviewed 2026-05-17 04:16 UTC · model grok-4.3

classification ⚛️ physics.optics
keywords frequency combsmicrowave generationphase noisefeedforward controlmicrocomboptical frequency divisiontiming jitter
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The pith

Electronic feedforward on free-running frequency combs produces 10 GHz microwaves with phase noise down to -153 dBc/Hz and femtosecond jitter.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that electronic feedforward can replace feedback loops for controlling optical frequency combs when generating microwaves. This change allows the combs to run freely without tight stabilization, while still achieving very low phase noise on the output signal. Experiments used both a solid-state laser and a microcomb to extract a 10 GHz carrier and cancel its noise electronically. The result is robust performance with no added high-frequency noise bumps, supporting simpler and more compact systems for applications like radar and timing.

Core claim

By using electrical feedforward noise cancelation on the 10 GHz carrier instead of feedback control of the comb, free-running frequency combs generate microwaves with phase noise as low as -153 dBc/Hz at offsets greater than 10 kHz, femtosecond timing jitter, and no servo-bump noise increase at high offsets. This holds for both a high-repetition-rate solid-state mode-locked laser and a microcomb, relaxing requirements on the comb source and enabling more manufacturable designs.

What carries the argument

Electrical feedforward noise cancelation applied to the 10 GHz carrier extracted from the optical frequency comb.

If this is right

  • The approach removes the need for stringent free-running noise performance and fast feedback dynamics in the comb source.
  • It eliminates the large noise increase at high offset frequencies that servo loops typically produce.
  • The method works with both solid-state mode-locked lasers and chip-scale microcombs.
  • It moves optical microwave generation closer to fully chip-scale, manufacturable implementations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The relaxed stability demands on the comb could allow integration with a wider range of existing photonic components for radar and navigation systems.
  • Future work might test whether the same feedforward technique scales to other carrier frequencies or higher repetition rates.
  • Combining this with on-chip detectors could reduce overall system size and power without sacrificing the reported noise levels.

Load-bearing premise

The electronic feedforward path can sense and cancel the comb phase noise over the relevant frequency offsets without adding its own noise, bandwidth limits, or residual errors.

What would settle it

A measurement showing phase noise well above -153 dBc/Hz at offsets above 10 kHz or reappearance of servo-bump noise when the feedforward system is applied to a free-running comb would falsify the claim.

Figures

Figures reproduced from arXiv: 2511.20504 by Anatoliy Savchenkov, Andrey Matsko, Benjamin Rudin, Florian Emaury, Franklyn Quinlan, John E. Bowers, Kerry J. Vahala, Oguzhan Kara, Qing-Xin Ji, Scott A. Diddams, Takuma Nakamura, Vladimir Iltchenko, Wei Zhang, William Groman.

Figure 2
Figure 2. Figure 2: For each comb source, the comb output was amplified and split into three branches. [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
read the original abstract

Optically generated microwave signals exhibit some of the lowest phase noise and timing jitter of any microwave-generating technology to date. The success of octave-spanning optical frequency combs in down-converting ultrastable optical frequency references has motivated the development of compact, robust and highly manufacturable optical systems that maintain the ultralow microwave phase noise of their tabletop counterparts. Two-point optical frequency division using chip-scale components and ~1 THz-spanning microcombs has been quite successful, but with stringent requirements on the comb source's free-running noise and feedback control dynamics. Here we introduce a major simplification of this architecture that replaces feedback control of the frequency comb in favor of electronic feedforward noise cancelation that significantly relaxes the comb requirements. Demonstrated with both a high repetition rate solid-state mode-locked laser and a microcomb, feedforward on a 10 GHz carrier results in more robust operation with phase noise as low as -153 dBc/Hz at offsets >10 kHz, femtosecond timing jitter, and elimination of the large "servo bump" noise increase at high offset frequency. The system's compatibility with a variety of highly manufacturable mode-locked laser designs and its resilience and straightforward implementation represents an important step forward towards a fully chip-scale implementation of optically generated microwaves, with applications in radar, sensing, and position, navigation and timing.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript introduces an electronic feedforward noise cancellation technique to generate ultralow-noise microwaves from free-running frequency combs, replacing traditional feedback stabilization. Demonstrated on both a high-repetition-rate solid-state mode-locked laser and a microcomb, the approach yields phase noise as low as -153 dBc/Hz at offsets >10 kHz on a 10 GHz carrier, femtosecond timing jitter, and removal of the servo-bump artifact, while relaxing requirements on the comb source.

Significance. If the experimental claims are substantiated, the work offers a meaningful simplification of two-point optical frequency division architectures by eliminating the need for tight feedback loops on the comb. This could enable more robust, manufacturable chip-scale microwave sources for radar, sensing, and PNT applications. The dual demonstration with a conventional solid-state laser and a microcomb provides evidence of broader applicability.

major comments (2)
  1. [§III] §III (Experimental Results) and associated figures: the reported phase-noise floor of -153 dBc/Hz and femtosecond timing jitter are presented without error bars, measurement bandwidth details, data-exclusion criteria, or a separate characterization of the electronic feedforward chain's additive noise. These omissions make it impossible to assess whether the claimed cancellation depth is limited by the comb or by the electronics.
  2. [§II, §IV] §II (Principle of Operation) and §IV (Microcomb Implementation): the central claim that feedforward relaxes comb requirements rests on the assumption that the electronic transfer function exactly inverts the comb's phase fluctuations across the relevant offset range. No measured transfer-function calibration, loop-delay quantification, or residual-error spectrum after cancellation is supplied, leaving open the possibility that frequency-dependent mismatch or amplifier noise sets the observed floor, especially for the higher-noise microcomb case.
minor comments (2)
  1. [Abstract] The abstract states 'femtosecond timing jitter' without specifying the integration bandwidth or the exact rms value; this should be quantified in the main text or a table.
  2. [Figure captions] Figure captions for the phase-noise spectra should explicitly note the resolution bandwidth, averaging, and whether the traces include the electronic noise floor measurement.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments, which have helped us identify areas where the experimental characterization can be strengthened. We address each major comment below and will incorporate the requested details into the revised manuscript.

read point-by-point responses

  1. Referee: [§III] §III (Experimental Results) and associated figures: the reported phase-noise floor of -153 dBc/Hz and femtosecond timing jitter are presented without error bars, measurement bandwidth details, data-exclusion criteria, or a separate characterization of the electronic feedforward chain's additive noise. These omissions make it impossible to assess whether the claimed cancellation depth is limited by the comb or by the electronics.

    Authors: We agree that explicit quantification of measurement uncertainties and the additive noise of the feedforward electronics is necessary for a complete assessment. In the revised manuscript we will add error bars derived from repeated acquisitions, state the precise measurement bandwidth and integration limits used for the femtosecond jitter values, clarify any data-selection criteria, and include a dedicated characterization of the electronic chain's phase-noise contribution measured independently of the comb. These additions will allow readers to determine the dominant noise source at the observed floor. revision: yes

  2. Referee: [§II, §IV] §II (Principle of Operation) and §IV (Microcomb Implementation): the central claim that feedforward relaxes comb requirements rests on the assumption that the electronic transfer function exactly inverts the comb's phase fluctuations across the relevant offset range. No measured transfer-function calibration, loop-delay quantification, or residual-error spectrum after cancellation is supplied, leaving open the possibility that frequency-dependent mismatch or amplifier noise sets the observed floor, especially for the higher-noise microcomb case.

    Authors: We acknowledge that direct verification of the inversion accuracy strengthens the central claim. The revised manuscript will include a measured magnitude and phase response of the feedforward transfer function over the relevant offset frequencies, a quantification of the total loop delay, and the residual phase-error spectrum after cancellation for both the solid-state laser and microcomb cases. These data will demonstrate that mismatch and amplifier noise do not set the observed floor within the bandwidth of interest. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental demonstration with direct measurements

full rationale

The paper is an experimental report on feedforward noise cancellation applied to mode-locked lasers and microcombs. It presents measured phase noise spectra, timing jitter values, and comparisons between feedback and feedforward configurations. No derivations, first-principles calculations, fitted parameters, or equations are described that reduce the claimed performance metrics to prior inputs by construction. Central results rely on laboratory measurements rather than any self-referential chain, self-citation load-bearing premise, or ansatz smuggled through prior work. The work is self-contained against external benchmarks of phase noise performance.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental demonstration paper; no free parameters, domain axioms, or invented entities are introduced or required to support the central claim in the abstract.

pith-pipeline@v0.9.0 · 5601 in / 1172 out tokens · 73311 ms · 2026-05-17T04:16:46.759004+00:00 · methodology

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Reference graph

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