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REVIEW 2 major objections 4 minor 29 references

Versatile High-Power Monolithic All-Glass Fiber Amplifier for Pulsed Signals with a Wide Range of Repetition Rates

T0 review · 2 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read A single monolithic all-glass fiber amplifier delivers 2 MW peak power at 1 MHz, 625 W at 20 MHz, and 645 W at 1 GHz.

desk verdict A real engineering advance in monolithic tapered-fiber amplifiers, but the 2 MW peak-power headline rests on an assumed pulse duration that is never measured. read the letter →

arxiv 2412.19604 v1 pith:K6B2KTMZ submitted 2024-12-27 physics.optics

Hossein Fathi , Ebrahim Aghayari , Andrey Grishchenko , Amit Yadav , Edik Rafailov , Regina Gumenyuk , Valery Filippov This is my paper · ORCID
classification physics.optics
keywords fiberamplifiertapereddouble-cladspunmonolithicall-glasspicosecondpulseshighpeakpoweraveragepolarizationmaintenance
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper reports a single-stage fiber amplifier built from a spun tapered double-clad fiber that directly amplifies narrow-linewidth picosecond pulses from a few tens of milliwatts to hundreds of watts of average power and megawatt-level peak power, without any free-space optics. Using the same amplifier module, the authors demonstrate 50 ps pulses reaching over 2 MW peak power at 1 MHz, 625 W average power at 20 MHz, and 20 ps pulses reaching 645 W average power at 1 GHz, with near-diffraction-limited beam quality at high average power and a high degree of linear polarization throughout. The significance is that one alignment-free, monolithic design can cover a wide range of repetition rates and power levels, overcoming the usual trade-off between nonlinear effects and transverse mode instabilities.

What carries the argument

The central object is the spun tapered double-clad fiber (sT-DCF), whose core and cladding diameters grow along its length from 8.3/75/90 µm to 90/814/977 µm over about 6.7 m. Tapering enlarges the mode area gradually, keeping the mode content stable and raising the thresholds for stimulated Raman scattering and other nonlinear effects, while spinning the preform during drawing reduces intrinsic birefringence to roughly $10^{-8}$, making the polarization state insensitive to heating. The fiber is pumped from both ends in a monolithic configuration, with a backward 976 nm pump of up to 840 W and a forward 915 nm pump of 18 W, enabling alignment-free high-power operation.

What would settle it

Measure the output pulse duration at the maximum power of each system using a background-free autocorrelator or a fast streak camera; if the pulses broaden substantially (e.g., from 50 ps to 100 ps), the true peak power would be a factor of two or more below the stated 2 MW, 625 kW, and 32 kW values.

Watch

Extended reading notes

Core claim

A monolithic all-glass spun tapered double-clad fiber (sT-DCF) amplifier can amplify narrow-linewidth picosecond pulses in a single stage across three distinct repetition-rate regimes while maintaining high beam quality and polarization stability. The 1 MHz system produced 50 ps pulses with over 2 MW peak power (155 W average power) at a slope efficiency of 59% and a degree of polarization of 70%; the 20 MHz system produced 50 ps pulses with 625 W average power at 76.6% slope efficiency and 88.3% DOP; the 1 GHz system produced 20 ps pulses with 645 W average power at 78.6% slope efficiency and 87.6% DOP. The design eliminates free-space pump coupling by splicing a pump combiner directly to the fiber, removing alignment dependence and internal back reflections.

Load-bearing premise

The claimed peak powers assume that the seed pulse duration (50 ps or 20 ps) stays unchanged through high-power amplification, because only average power and spectra were measured at maximum output, with no direct autocorrelation or temporal characterization.

Editorial extensions

If this is right

  • The same monolithic sT-DCF amplifier can serve applications across a wide range of repetition rates, from high-peak-power regimes at 1 MHz to high-average-power regimes at 1 GHz, without reconfiguration.
  • Because the 20 MHz and 1 GHz systems were limited only by available pump power, adding more pump diodes should scale average power beyond 625 W and 645 W while retaining beam quality and polarization.
  • Eliminating free-space pump alignment improves reliability and compactness, making the amplifier attractive for industrial and scientific laser systems where alignment drift causes downtime.
  • A single amplifier stage can replace multi-stage or free-space amplifier chains for picosecond pulses, reducing system complexity and footprint.
  • The demonstrated combination of high average power, high peak power, and polarization maintenance is directly relevant to material processing, high-harmonic generation, and burst-mode laser systems.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The technique could be combined with post-compression or coherent beam combining to further boost peak power, since the amplifier already delivers a clean, near-diffraction-limited beam at high average power.
  • The polarization instability at high peak power (attributed to polarization modulation instability) may be mitigated by slightly increasing fiber birefringence, which could extend the peak-power ceiling at low repetition rates.
  • If the pumped power is scaled further, the same fiber design may reach kilowatt-class average power in the 20 MHz and 1 GHz regimes, with the main remaining questions being thermal handling and SRS onset.
  • The monolithic all-glass approach could transfer to other rare-earth-doped fibers or wavelengths, potentially enabling high-power pulsed amplifiers beyond the 1 µm ytterbium band.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. The manuscript reports an experimental study of a monolithic all-glass spun tapered double-clad fiber (sT-DCF) amplifier in a master oscillator-power amplifier configuration. Three seed systems are amplified: 50 ps pulses at 1 MHz to 155 W average power with a claimed peak power above 2 MW, 50 ps pulses at 20 MHz to 625 W average power, and 20 ps pulses at 1 GHz to 645 W average power. For each case the paper reports output power scaling, optical spectra, ASE and Raman levels, beam quality M2, and degree of polarization, and it attributes the 1 MHz performance limits mainly to ASE and polarization modulation instability.

Significance. If the results hold, this is a significant engineering demonstration: a single-stage all-fiber amplifier with no free-space pump optics reaches 625 W and 645 W average power at high repetition rates and MW-level peak power at 1 MHz, with DOP between 70% and 88% and M2 between 1.28 and 2.0. The monolithic construction and the alignment-free kW-level pumping scheme are practical strengths. However, the headline peak-power value is not directly measured but computed from average power and an assumed pulse duration, and none of the quantitative results are accompanied by measurement uncertainties; these points weaken confidence in the exact advertised numbers.

major comments (2)
  1. [Amplification of 1 MHz laser; Table 1] The headline claim of over 2 MW peak power is not based on a direct measurement of the output pulse duration. The text states that the peak power was determined by considering only the signal region and excluding the ASE contribution from the total output power, but no autocorrelation, FROG, or streak-camera measurement is reported anywhere in the manuscript. The value is therefore computed as P_signal/(RR × τ) with τ taken as the 50 ps seed duration. Since Fig. 4c shows spectral broadening from 0.45 nm at 3 W to 0.67 nm at 155 W, nonlinear temporal reshaping cannot be ruled out; if the output FWHM were 70 ps rather than 50 ps, the peak power would drop to about 1.4 MW. The same issue applies to the 20 ps output label for the 1 GHz channel, although the peak power there is much lower. Please add a measured output pulse duration at maximum power, or explicitly relabel the peak-power and pulse-duration claims as estimates that assume unchanged seed duration.
  2. [Figures 4-6; Table 1] No measurement uncertainties or repeated-measurement statistics are provided for any of the reported quantities: average output power, slope efficiency, DOP, M2, spectral bandwidth, or ASE/Raman suppression ratios. DOP and M2 are also single-point characterizations taken at the maximum power only. Because the paper makes quantitative comparisons with previous demonstrations (for example, 1.26 MW peak power and 573 W average power) and reports fine distinctions such as 88.3% versus 87.6% DOP, error bars or multiple measurements are needed to support the precision implied by the claims.
minor comments (4)
  1. [Discussion, first paragraph] The sentence 'In the amplification of 20 ps pulses with 1 MHz RR' should read '50 ps pulses'; the 1 MHz channel is described elsewhere as 50 ps, and Table 1 consistently lists 50 ps for 1 MHz.
  2. [Author contributions] The contributions state 'A.G. and E.G. made the taper,' but E.G. is not an author of the manuscript; this is likely a typo for A.G. (Andrey Grishchenko) and should be corrected.
  3. [Amplification of 1 MHz laser; Fig. 4] The claim that 'fast growth of ASE and degradation of DOP limit further power scaling' is not directly documented: DOP is shown only at maximum output power, and no DOP-versus-power curve is presented. Either provide that curve or soften the claim to say that ASE growth was observed.
  4. [Captions of Figs. 4, 5, and 6] The phrase 'at different output power' should be 'at different output powers' in the captions of Figs. 4(e), 5(e), and 6(e).

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: experimental report with direct measurements; peak-power claim is arithmetic from measured average power and assumed pulse duration, not a fitted prediction.

full rationale

The paper is an experimental characterization of a monolithic all-glass spun tapered double-clad fiber amplifier. The main claims are measured quantities: average output power, slope efficiency, optical spectra, signal-to-ASE and signal-to-Raman ratios, degree of polarization, and M2 beam-quality factors. No parameter is fitted to a subset of data and then used to predict a closely related quantity. The only derived headline number, the 2 MW peak power at 1 MHz, is computed as P_signal divided by (repetition rate × pulse duration), as stated in the text: 'The peak power was determined by considering only the signal region, excluding the amplified spontaneous emission contribution from the total output power.' This is an arithmetic conversion, not a circular derivation, because the calculation does not presuppose the result it claims to establish. The pulse duration is taken from the seed laser specification (50 ps or 20 ps) and is not re-measured at full power; if the output pulses broadened significantly under amplification, the true peak power would be lower than claimed. That is a missing measurement and a correctness risk, but it is not circularity: no equation in the paper defines the output pulse duration in terms of the claimed peak power, nor is the peak power an input to any fit. Self-citations to prior work on tapered double-clad fibers and spun tapered fibers are used as background and design motivation, not as evidence for the specific performance numbers reported here. The amplifier's construction, pumping scheme, and measured output characteristics are independently presented, so the central claims do not reduce to a self-citation chain or to a definition. The paper is self-contained against external benchmarks in the sense that the reported powers, spectra, DOP, and M2 values are direct experimental observations. Accordingly, the circularity score is 0.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The scientific content is experimental, so this ledger captures the physical and fabrication assumptions inherited from prior tapered-fiber work rather than fitted parameters. The most fragile assumption is that pulse duration is unchanged, since it directly supports the reported peak powers. No new physical entities are introduced.

assumptions (4)
  • domain assumption Yb-doped fiber with the given core absorption, NA, and double-D cladding provides efficient gain when pumped at 976 nm and 915 nm.
    The amplifier model assumes standard Yb absorption and emission behavior; the paper does not derive gain from first principles.
  • domain assumption The seed pulse duration remains approximately 50 ps or 20 ps after amplification, allowing peak power to be inferred from average power and repetition rate.
    No output autocorrelation is reported; peak power values in Table 1 rest on unchanged pulse duration.
  • domain assumption Spinning the fiber reduces intrinsic birefringence to roughly 1e-8 and suppresses thermally induced polarization changes.
    This is taken from the authors' earlier references 20 and 21 and is used to explain the high DOP at high power.
  • domain assumption The tapered core keeps the mode content stable and raises nonlinear thresholds.
    Inherited from the T-DCF concept of reference 18; the paper does not model mode evolution.

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Cite this review

Pith. "Pith review of Versatile High-Power Monolithic All-Glass Fiber Amplifier for Pulsed Signals with a Wide Range of Repetition Rates." pith.science (2026). https://pith.science/paper/K6B2KTMZ

@misc{pith2026241219604,
  author       = {Pith},
  title        = {Pith review of: Versatile High-Power Monolithic All-Glass Fiber Amplifier for Pulsed Signals with a Wide Range of Repetition Rates},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/K6B2KTMZ}},
  note         = {Machine review of arXiv:2412.19604}
}
read the original abstract

This study presents a compact, high-power monolithic all-glass spun tapered double-clad fiber amplifier for single-stage amplification of narrow linewidth picosecond pulsed signals from a few tens of mW to several hundred Watts of average power and MW level of peak power, covering a wide range of repetition rates. The absence of free-space elements in the amplifier module enhances its overall reliability by omitting the dependency on the pump alignment and internal back reflections. The versatile all-glass amplifier module delivers 50 ps pulses with over 2 MW peak power at 1 MHz, 50 ps pulses with over 625 W average power at 20 MHz, and 20 ps pulses with over 645 W average power at 1 GHz, all exhibiting excellent spectral, spatial, and polarization characteristics. This monolithic all-glass ultra-large mode area fiber amplifier is verified as a robust solution for direct amplification of short pulses attaining high peak/average power laser systems with excellent spectral, spatial, and polarization characteristics.

Figures

Figures reproduced from arXiv: 2412.19604 by the authors.

Figure 1
Figure 1. Schematic of the experimental master oscillator-power amplifier (MOPA) architecture using monolithic all-glass sT-DCF. The components are labeled as follows: Amp, amplifier; ISO, Isolator; BPF, bandpass filter; ILP, in-line polarizer; LD, laser diode. A mode-locked external cavity semiconductor (ML SC) laser is used to efficiently generate 20 ps pulses at a 1 GHz repetition rate, meeting the rising demand for high-r… view at source ↗
Figure 2
Figure 2. Optical spectrum characterization of the MHz and GHz front-end laser system. (a) at 1 MHz repetition rate with 5 mW average output power, (b) at 20 MHz repetition rate with 100 mW average output power. (c) The optical spectrum of the 20 ps pulses of 1 GHz mode-locked semiconductor seed laser with 120 mW average output power. At the end of both the MHz and GHz F-E laser systems, a 10 W PM isolator protects the seed l… view at source ↗
Figure 3
Figure 3. Longitudinal profile of the active spun tapered-double clad fiber. Amplification of 1 MHz laser This section presents the amplification of 50 ps pulses at a 1 MHz repetition rate. The direct amplification of low repetition rates picosecond pulses is limited in standard constant core/clad ration fibers due to the onset of strong non-linear effects such as self-phase modulation and four-wave mixing, particularly at hi… view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Experimental characterization of the amplified 1 MHz pulsed signal. (a) The output power of the 1 MHz laser system versus the total launched pump power, showing the power scaling performance. (b) Degree of polarization measurements at the maximum output power. (c) and …
Figure 5
Figure 5. Figure 5: Experimental characterization of the amplified 20 MHz pulsed signal. (a) The output power of the 20 MHz laser system versus the total launched pump power, showing the power scaling performance. (b) Degree of polarization measurements at the maximum output power. (c) an…
Figure 6
Figure 6. Figure 6: Experimental characterization of the amplified 1 GHz laser system. (a) The output power of the 1 GHz laser system versus the total launched pump power, showing the power scaling performance. (b) Degree of polarization measurements at the maximum output power. (c) and (…

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Reference graph

Works this paper leans on

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