{"id":"f041c532-f7d1-4b0d-8e68-9fec6680f241","arxiv_id":"2507.00233","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"A harmonically modelocked erbium fiber laser with a free-space subcavity achieves a fully stabilized 1 GHz frequency comb with performance comparable to conventional fiber combs.","lead":"This paper demonstrates a fiber laser that produces a fully stabilized optical frequency comb at 1 GHz repetition rate using harmonic modelocking, where multiple pulses circulate in the gain cavity but a short subcavity maintains coherence. The design brings mature erbium fiber technology to high repetition rates, which could benefit spectroscopy, astronomy, and photonic communications.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The claimed 1e-20 Allan deviation at 1 h is hard to reconcile with the 3-hour frequency record in Fig S4, which stays within 15 mHz of the mean; unstated detrending or an inconsistent analysis appears necessary.","rationale":"The reader's weakest assumption concerned pulse independence and supermode noise. That concern is largely addressed by the paper's clean RF beat spectra, absence of supermode sidebands, and out-of-loop phase noise of 0.36 rad, so it is not the most load-bearing issue. My concern is instead a concrete internal inconsistency in the main quantitative evidence: the 1e-20 Allan deviation at 1 h does not appear compatible with the 15 mHz range of the 3-hour frequency record in Fig. S4 unless an unstated drift removal was performed. This matters because the central claim of matching conventional comb precision rests on the long-term stability metric, not just on the sub-0.1 Hz linewidth. The verdict should remain conditional rather than reject: the design demonstration and the short-term coherence data are credible, but the headline long-term stability number needs either a corrected analysis or a disclosed detrending procedure before it can be cited.","tokens_in":11886,"tokens_out":9489,"duration_ms":116363,"concrete_test":"Obtain the raw phase-counter record behind Fig. S4 and recompute the overlapping Allan deviation with the stated 0.5 s averaging and the full 3 h span, without any detrending. If the tau = 3600 s point does not reach 1e-20, the headline long-term stability claim is unsupported. Also check whether a linear or quadratic drift was removed before plotting Fig. 3; such removal must be disclosed and justified, because a drifting offset would invalidate the claimed averaging-down behavior.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The quantitative headline of Section 4 is the optical Allan deviation in Fig. 3, averaging down from 1e-17 at 1 s to 1e-20 at 1 h. At the 1565 nm optical frequency (~191 THz), 1e-20 corresponds to roughly 2 microhertz. The supporting data in Supplement S4 (Fig. S4) show the out-of-loop frequency difference staying within about 15 mHz of its average over 3 hours, i.e. a fractional range of about 8e-17. Unless a linear drift or another trend was removed before computing Fig. 3, these two statements are in tension. For white frequency noise, the 1 h Allan deviation from a 0.5 s record with a few mHz scatter would be at least tens of microhertz, or about 1e-19, not 1e-20. If the 15 mHz range is instead dominated by slow drift, the 1 h Allan deviation would be even larger. The authors' own caveat that the results 'may be measurement limited' does not resolve this; it makes the lack of a detailed analysis more consequential. Since no raw counter data or analysis code are provided, the central claim of long-term stability at the 1e-20 level is not independently verifiable and, as written, appears internally inconsistent with Fig. S4.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper reports a harmonically modelocked Er-fiber comb in which a short free-space subcavity sets a 1.04 GHz repetition rate while a long NALM fiber gain cavity supplies gain and modelocking, so that multiple pulses circulate in the gain cavity. The authors fully stabilize the CEO and repetition-rate degrees of freedom and characterize the laser with spectra, RIN, in-loop and out-of-loop optical beats (0.23, 0.48, and 0.36 rad integrated phase noise; out-of-loop linewidth below 0.1 Hz), and an optical Allan deviation reported to average from 1e-17 at 1 s to 1e-20 at 1 h. They also describe a numerical model predicting reduced timing jitter at low beamsplitter reflectivity and demonstrate two design variants, a fiberized subcavity and an empty V-shaped subcavity.","tokens_in":12160,"tokens_out":12797,"duration_ms":146370,"significance":"If the long-term stability result survives scrutiny, this design extends mature Er-fiber comb technology to GHz repetition rates while retaining conventional comb performance and offers repetition-rate tuning over more than an order of magnitude. The separation of the repetition-rate-defining subcavity from the gain medium is an elegant form of harmonic modelocking, and the beamsplitter rejection port is a plausible mechanism for supermode suppression. The numerical model is clearly presented and makes falsifiable predictions, and the experimental characterization includes out-of-loop verification rather than only in-loop locks. The main weaknesses are that the headline Allan deviation is not supported by the raw data shown, and the supermode and jitter benefits are not quantified experimentally.","major_comments":[{"comment":"The headline long-term stability claim is not supported by the presented data. The 3-hour out-of-loop record in Fig. S4 stays within roughly 15 mHz of its mean, and the text states that noncommon-fiber phase drifts were reduced to 'below 1 Hz'; at 191 THz the claimed 1e-20 at 1 h corresponds to about 2 microhertz, which is orders of magnitude below both numbers. Please provide the raw counter data, the exact Allan deviation computation (including any detrending, dead-time, or averaging procedure), and an explicit measurement-noise floor for the out-of-loop path, separating comb stability from environmental phase shifts.","section":"Section 4, Fig. 3, and Supplement S4"},{"comment":"Clean harmonic modelocking is inferred from RF spectra at 30 kHz resolution, but no quantitative supermode-suppression measurement is reported, and the predicted timing-jitter reduction at low beamsplitter reflectivity is not experimentally verified. Since the paper claims noise filtering and potential jitter improvements as advantages, please add a quantitative supermode characterization (e.g., sideband-to-carrier ratio or a phase-noise spectrum spanning the gain-cavity FSR) and explicitly distinguish the simulated jitter prediction from measured performance.","section":"Section 3 and Supplement S2"}],"minor_comments":[{"comment":"The RIN peak near 300 Hz is left unexplained; a sentence on its origin would help the reader assess whether it is a fundamental or technical noise source.","section":"Fig. 1c"},{"comment":"The statement that the CEO frequency is highly dependent on the relative cavity matching would be more convincing with a quantitative plot of CEO frequency versus cavity-length offset; the current actuator-assignment explanation is qualitative.","section":"Section 2"},{"comment":"The model compresses the temporal window to 0.1-1 ns; please justify why this compression does not alter the simulated nonlinear interaction and state the numerical grid and round-trip counts used.","section":"Supplement S2"},{"comment":"Please specify the integration bandwidth for the phase-noise values and add confidence intervals or error bars to the Allan deviation, particularly at the longest averaging times.","section":"Fig. 3 and Fig. 2 captions"},{"comment":"The paper does not include a data-availability statement; providing the raw counter records and analysis script would materially strengthen reproducibility.","section":"General"}],"recommendation":"major_revision","confidential_remarks":"The experimental architecture is compelling and likely of interest to the journal's readership. The central obstacle is the long-term Allan deviation claim, which appears inconsistent with the displayed raw data unless additional analysis is supplied. I believe this is fixable within a revision, and I would not reject on the current evidence."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe headline: this is a credible demonstration of a 1 GHz erbium fiber comb that gets its high repetition rate from a free-space subcavity coupled to a NALM, rather than from a short gain cavity. That architecture is genuinely new as far as I know, and the paper shows it can be fully CEO-stabilized and repetition-rate-stabilized with the same actuator count as a conventional comb. The out-of-loop comparison against a reference comb, with a beat linewidth below 0.1 Hz and integrated phase noise of 0.36 rad, is strong evidence of clean coherent operation.\n\nWhat the paper does well: the locking scheme is thoughtful – using the CEO beat as a proxy for cavity matching, with the pump current and fiber stretcher on the gain cavity, and the EOM plus PZT on the subcavity. That avoids the crosstalk problem of a separate PDH lock between the two cavities. The RF spectra are clean, showing no supermode noise peaks. The design variants (fiberized subcavity and empty V-shaped cavity) suggest the architecture is practical and extendable.\n\nThe soft spot is the long-term stability claim. The paper reports an optical Allan deviation of 1e-20 at 1 hour. At 191 THz that is about 2 microhertz. But the supplemental frequency record (Fig S4) shows the out-of-loop beat staying within 15 mHz of its average over 3 hours. For white frequency noise, that scatter implies an Allan deviation at 1 hour of at least tens of microhertz, roughly 1e-19 – an order of magnitude higher than claimed. Either a trend was removed without being disclosed, or the two plots are inconsistent. The authors' comment that the results 'may be measurement limited' does not resolve this; it makes the missing analysis more consequential. The lack of raw counter data or analysis code makes it impossible to check.\n\nA lesser concern: the claim that multiple pulses can be treated as independent relies on a stated assumption, not a demonstration. The paper's own jitter-reduction model is presented as a prediction, which is fine, but it is not validated by the experiment.\n\nThis paper deserves a serious referee. The architecture is novel and the short-term coherence measurements are convincing. The referee should ask for the Allan deviation calculation to be reconciled with the raw frequency record, and for the detrending or averaging procedure to be stated explicitly. I'd engage with it.","headline":"A credible new architecture for GHz fiber combs, but the long-term Allan deviation claim needs reconciliation with the supplemental data.","tokens_in":12681,"tokens_out":3784,"would_cite":true,"duration_ms":41427,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["42.55.Wd","42.62.Eh","42.60.Fc"],"model":"deepseek-v4-flash","headline":"A fiber laser with many pulses in its cavity can make a fully stabilized 1 GHz frequency comb.","keywords":["frequency comb","harmonic modelocking","fiber laser","erbium","GHz repetition rate","carrier-envelope offset","supermode noise","optical Allan deviation"],"falsifier":"Measure the out-of-loop beat between the GHz comb and a reference comb with a phase-noise analyzer spanning offset frequencies around the gain-cavity free spectral range, about 104 MHz; clean harmonic modelocking predicts no supermode sidebands there, while any residual supermode noise would appear as peaks at those offsets.","tokens_in":11706,"feed_emoji":"💡","tokens_out":6049,"duration_ms":59289,"temperature":0.7,"pith_summary":"This paper claims that a fiber frequency comb can run at gigahertz repetition rates with many pulses circulating in the laser cavity at once, without the noise that has historically plagued harmonic modelocking. The authors demonstrate a fully stabilized erbium fiber comb at 1.04 GHz whose out-of-loop beat with a reference comb has a linewidth below 0.1 Hz and whose optical Allan deviation averages down from $10^{-17}$ at one second to $10^{-20}$ in an hour. The point is to separate the repetition-rate-determining cavity from the gain medium, so mature fiber-laser technology can reach high repetition rates that previously required solid-state lasers, external filtering cavities, or microresonators. If correct, the result turns repetition rate into a freely selectable design parameter and opens a route to lower timing jitter through the filtering intrinsic to the two-cavity design.","feed_headline":"Harmonic modelocking produces a stabilized 1 GHz fiber comb","feed_subtitle":"Out-of-loop beat below 0.1 Hz and Allan deviation to 10^-20 match conventional comb performance.","key_machinery":"The key mechanism is the two-cavity design: a short free-space subcavity, roughly 15 cm long and setting the approximately 1 GHz repetition rate, coupled by a beamsplitter to a much longer nonlinear amplifying loop mirror (NALM) fiber gain cavity with a fundamental rate near 104 MHz. Harmonic modelocking means the gain-cavity round-trip time is an integer multiple (here 10) of the subcavity round-trip time, so returning amplified pulses reinforce the subcavity pulse at the beamsplitter. The interference at the beamsplitter is a highly selective filter: light that does not match the subcavity in wavelength, phase, or timing is rejected rather than left in the cavity, which is what suppresses supermode noise and, in the numerical model, reduces quantum-noise-driven timing jitter. The subcavity acts as a low-noise pulse reservoir, and the whole arrangement is described as pulsed self-injection locking, coupling the laser field to a delayed version of itself.","core_discovery":"The central discovery is that harmonic modelocking, in which several pulses circulate in the gain cavity, can be made to behave like a single-pulse comb by adding a short subcavity that governs the repetition rate and interferometrically filters the returning amplified pulses. The subcavity holds a single pulse, so adjacent output pulses are coherent; the longer NALM fiber cavity provides gain and modelocking without its length constraining the repetition rate. The beamsplitter that couples the two cavities acts as a highly selective filter that rejects amplified light not matching the subcavity in wavelength, phase, or timing, which suppresses supermode noise. The paper shows that with the CEO frequency locked via pump current and fiber stretcher and the optical beat locked via subcavity actuators, the resulting comb is fully stabilized and its out-of-loop comparison with a conventional 200 MHz fiber comb shows a beat linewidth below 0.1 Hz and an Allan deviation reaching $10^{-20}$ in an hour. The supplement's numerical model indicates that the same filtering should reduce fundamental timing jitter in proportion to the beamsplitter reflectivity, with a tenfold jitter reduction at 10 percent reflectivity.","pith_inferences":["The paper's separation of optical cavity and gain medium suggests a general design principle: any short optical resonator can be turned into the repetition-rate-defining element of a modelocked laser without paying the power penalty of external cavity locking.","Because the interference filter rejects noise at a rejection port, the design might combine with balanced detection at that port to provide an in-loop error signal with a reduced noise floor, potentially improving the achievable comb linewidth beyond what was demonstrated.","The claim that timing jitter scales with beamsplitter reflectivity is testable at lower reflectivities; a 10 percent reflectivity run should show a clear jitter reduction compared to a standard NALM comb, confirming the noise-filtering picture."],"forward_implications":["Fiber comb technology can operate at repetition rates from the fundamental up to at least the 12th harmonic, roughly 1.2 GHz, simply by changing the subcavity length, giving more than an order of magnitude of range with a single laser.","A dual comb for spectroscopy could be made by polarization duplexing in a shared subcavity with birefringence, providing an ultrastable repetition-rate difference.","The filtering intrinsic to the beamsplitter should reduce fundamental timing jitter compared to a standard modelocked laser, with the reduction growing as the beamsplitter reflectivity is lowered.","An empty V-shaped subcavity can serve as a passively stable internal reference, potentially bringing some of the stability of high-finesse reference cavities directly into the oscillator.","The approach extends to microresonator gain loops, where harmonic modelocking could remove the continuous-wave background and improve pumping efficiency."],"supporting_citations":[{"why":"Supplies the nonlinear amplifying loop mirror mechanism used for gain and modelocking in the two-cavity design.","marker":"[22]"},{"why":"Provides the conventional all-polarization-maintaining Er fiber comb baseline and the standard method of CEO control via pump power.","marker":"[23]"},{"why":"Establishes harmonic modelocking as a long-studied technique with a known supermode-noise problem that the paper addresses.","marker":"[11]"},{"why":"Shows the prior art of supermode suppression with an intracavity Fabry-Perot etalon, compared against the rejection-port filtering of the new design.","marker":"[14]"},{"why":"Documents residual supermode traces even with Pound-Drever-Hall stabilization, the comparison point for the clean RF spectra shown here.","marker":"[15]"},{"why":"Provides measured comb properties of harmonically modelocked lasers, used to frame the coherence requirements for a clean frequency comb.","marker":"[16]"},{"why":"Represents a competing GHz fiber comb approach based on nested fiber ring resonators, which the subcavity design seeks to improve upon.","marker":"[19]"},{"why":"Supplies the quantum-noise model used in the supplement to estimate fundamental timing jitter and its reduction at low beamsplitter reflectivity.","marker":"[S1]"},{"why":"Supports the claim that amplified spontaneous emission noise is added once per gain-cavity round trip, justifying the jitter-reduction calculation.","marker":"[S2]"}],"fun_headline_variants":["Harmonic modelocking yields stable 1 GHz fiber comb","Multi-pulse fiber laser comb stabilized to 1 GHz","Subcavity filter tames multi-pulse comb to 1 GHz","Fiber comb hits 1 GHz with harmonic modelocking","Stable 1 GHz comb from harmonic fiber laser"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The design assumes that multiple pulses circulating in the gain cavity can be treated as independent, with each pulse experiencing the same nonlinear effects it would if it were the only pulse, provided the pulse energy is held constant; if pulse interactions or gain competition instead couple the pulses, harmonic modelocking would reintroduce supermode noise and the claimed comb quality would not hold.","fun_headline_variants_meta":{"raw":{"variants":["Harmonic modelocking yields stable 1 GHz fiber comb","Multi-pulse fiber laser comb stabilized to 1 GHz","Subcavity filter tames multi-pulse comb to 1 GHz","Fiber comb hits 1 GHz with harmonic modelocking","Stable 1 GHz comb from harmonic fiber laser"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000211,"raw_usage":{"total_tokens":1408,"prompt_tokens":931,"completion_tokens":477,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":547,"completion_tokens_details":{"reasoning_tokens":393}},"tokens_in":547,"tokens_out":477,"duration_ms":5734,"temperature":1.0,"reasoning_tokens":393,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T21:20:51.991583+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the out-of-loop beat between the GHz comb and a reference comb with a phase-noise analyzer spanning offset frequencies around the gain-cavity free spectral range, about 104 MHz; clean harmonic modelocking predicts no supermode sidebands there, while any residual supermode noise would appear as peaks at those offsets.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the nonlinear amplifying loop mirror mechanism used for gain and modelocking in the two-cavity design."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the conventional all-polarization-maintaining Er fiber comb baseline and the standard method of CEO control via pump power."},{"cited_title":"Becker, D","cited_arxiv_id":null,"evidence_quote":"Establishes harmonic modelocking as a long-studied technique with a known supermode-noise problem that the paper addresses."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows the prior art of supermode suppression with an intracavity Fabry-Perot etalon, compared against the rejection-port filtering of the new design."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Documents residual supermode traces even with Pound-Drever-Hall stabilization, the comparison point for the clean RF spectra shown here."},{"cited_title":"Hasegawa, Phase structure of harmonically mode-locked optical frequency combs, Optics Express 32, 8092--8100 (2024)","cited_arxiv_id":null,"evidence_quote":"Provides measured comb properties of harmonically modelocked lasers, used to frame the coherence requirements for a clean frequency comb."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Represents a competing GHz fiber comb approach based on nested fiber ring resonators, which the subcavity design seeks to improve upon."}],"review_version":1}