pith. sign in

arxiv: 2512.21093 · v3 · submitted 2025-12-24 · ⚛️ physics.optics

Dual-comb spectroscopy for the characterization of laboratory flames

Bernat Frangi , Laura Monroy , Aldo Moreno-Oyervides , Oscar E. Bonilla-Manrique , Mariano Rubio-Rubio , Mario Sanchez-Sanz , Pedro Mart\'in-Mateos This is my paper

Pith reviewed 2026-05-16 19:51 UTC · model grok-4.3

classification ⚛️ physics.optics
keywords dual-comb spectroscopycombustion diagnosticsmethane detectionmid-infraredflame characterizationelectro-optical combsMcKenna burner
0
0 comments X

The pith

A dual-comb spectroscopy system using electro-optical combs measures unburned methane in laboratory flames at a detection limit of 1.1 ppm for a 1 m path.

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

The paper develops an electro-optical dual-comb spectroscopy setup at 3427.43 nm that combines difference frequency generation with differential detection to quantify methane in a McKenna burner flame. This yields calibration-free readings that map spatial concentration changes across the combustion zone and resolve fast instabilities such as pulsations and leakage under lean conditions. Readers would care because precise in-situ flame data supports better control of efficiency and emissions in combustion systems. The temporal resolution reveals dynamic behavior that conventional tools often miss.

Core claim

Operating at a center wavelength of 3427.43 nm, the dual-comb system based on electro-optical frequency comb generators and difference frequency generation uses differential detection to provide precise, calibration-free measurements of unburned methane concentrations in the flame, with an estimated detection limit of 1.1 ppm over 1 m path length, resolving spatial concentration gradients and identifying dynamic instabilities including self-sustained pulsations and fuel leakage under fuel-lean conditions.

What carries the argument

Electro-optical dual-comb architecture with difference frequency generation and differential detection for mid-infrared absorption measurements of methane in flames.

If this is right

  • The method supplies non-invasive, calibration-free values of unburned methane inside the combustion region.
  • Spatial gradients in concentration can be mapped directly across the flame.
  • High-speed sampling identifies instabilities such as pulsations and leakage in real time.
  • The electro-optical design supports flexible adaptation for other laboratory combustion studies.

Where Pith is reading between the lines

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

  • Wavelength tuning could extend the same platform to other combustion species without major redesign.
  • Real-time instability data might feed into active control loops for practical burners.
  • Longer optical paths in larger systems would further improve the effective detection limit.

Load-bearing premise

Differential detection removes background effects and interferences from other species accurately enough for calibration-free methane readings in the hot, complex flame environment.

What would settle it

A side-by-side test against an independent calibrated sensor that shows systematic offsets in methane concentration, or no response when fuel flow rate is deliberately changed, would disprove the claimed detection limit and interference rejection.

Figures

Figures reproduced from arXiv: 2512.21093 by Aldo Moreno-Oyervides, Bernat Frangi, Laura Monroy, Mariano Rubio-Rubio, Mario Sanchez-Sanz, Oscar E. Bonilla-Manrique, Pedro Mart\'in-Mateos.

Figure 1
Figure 1. Figure 1: Experimental setup for dual-comb generation and flame characterization. TEC: temperature controller, EDFA: erbium￾doped fiber amplifier, AOM: acousto-optic modulator, PM: phase￾modulator, PPLN: DFG module, PD: photodetector [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Translation platform integrating the optical components and mid-infrared photodetectors for flame measurements. The laser propagation path is indicated for visualization purposes. The dual-comb signal is subsequently amplified by an EDFA and frequency down-converted to the mid-infrared region (MIR) using a DFG process in a periodically poled lithium niobate (PPLN) crystal (NTT Electronics WD-3440- 000-A-B-… view at source ↗
Figure 3
Figure 3. Figure 3: Peak absorption for the CH4 spectral features between 3390 nm and 3440 nm with largest absorption as a function of (a) temperature for a concentration of 0.01 VMR and (b) concentration for a temperature of 1200 K. Simulations were done in both cases using HITEMP for a pressure of 1 atm and a path length of 7 cm. 3/9 [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Fitted CH4 concentration (a) and its standard deviation (b) on the simulated 0.01 VMR measurements for the 308 studied comb configurations. Each point corresponds to the average or the standard deviation, respectively, of 100 simulated measurements and fittings. The insets show only the most viable comb configurations and the legend is given by ranges. 4/9 [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Evolution of CH4 concentration in a 5-second continuous measurement at the burner base (z = 1 mm) for an equivalence ratio of γ = 0.7. The chosen 13-tooth configuration was used and the complete measurement was analyzed in small intervals of 0.025 s. The inset shows the corresponding FFT, where the zero-frequency component has been removed. The CH4 concentration was then determined by fitting a theoretical… view at source ↗
Figure 6
Figure 6. Figure 6: Measured temperature (a) and CH4 concentration (b) as a function of distance above the burner base for different values of γ. Temperature was measured using a thermocouple and used as an input to the fitting algorithm. Concentration is an average of 3 measurements, each obtained using either the 13-tooth or the 21-tooth comb configuration. Solid lines represent the trend. The inset shows a typical measured… view at source ↗
read the original abstract

Optical spectroscopy, in particular dual-comb (DC) spectroscopy, is a critical, non-invasive tool for combustion diagnostics, offering high precision and calibration-free advantages. However, its implementation remains challenging, especially in the mid-infrared region. This work presents the development of a robust DC spectroscopic system based on electro-optical (EO) frequency comb generators and difference frequency generation (DFG), specifically designed for the characterization of laboratory flames. Operating at a center wavelength of 3427.43 nm, the system utilizes a differential detection strategy to enable precise, calibration-free measurements of unburned methane ($\mathrm{CH_{4}}$) concentrations in a McKenna burner. The experimental results demonstrate an estimated detection limit of 1.1 ppm for a 1 m path length and effectively resolve spatial concentration gradients across the combustion region. Furthermore, the system's high temporal resolution allowed for the identification of dynamic combustion instabilities, including self-sustained pulsations and fuel leakage under fuel-lean conditions. These findings validate the proposed EO architecture as a flexible and highly sensitive tool for advanced flame characterization.

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 describes the development of an electro-optical dual-comb spectroscopy system using difference frequency generation, operating at a center wavelength of 3427.43 nm, for non-invasive characterization of laboratory flames in a McKenna burner. It employs a differential detection strategy to perform calibration-free measurements of unburned methane (CH4) concentrations, reporting an estimated detection limit of 1.1 ppm for a 1 m path length, spatial resolution of concentration gradients, and high-temporal-resolution observations of combustion instabilities including self-sustained pulsations and fuel leakage under fuel-lean conditions.

Significance. If the experimental claims are substantiated with adequate validation, the work demonstrates a practical mid-infrared dual-comb architecture that could advance combustion diagnostics by combining high sensitivity, temporal resolution, and flexibility for mapping species in complex flame environments. The EO-based approach may lower barriers to implementing such systems compared to traditional mode-locked laser sources.

major comments (2)
  1. [Abstract] Abstract: The estimated 1.1 ppm detection limit for CH4 is stated without accompanying details on the noise floor, averaging time, path-length specifics, or uncertainty analysis (e.g., no error bars or validation data), which is load-bearing for the quantitative performance claim.
  2. [Differential detection description] The section describing the differential detection strategy: The central claim that this approach enables precise, calibration-free CH4 measurements by fully canceling interferences from H2O, CO2, temperature-induced shifts/broadening, and scattering lacks quantitative demonstration, such as residual spectra after subtraction, chosen line parameters, or validation against known overlaps in the 3.4 µm region under high-temperature combustion conditions.
minor comments (2)
  1. [Figures] Figure captions should explicitly state the experimental conditions (e.g., burner flow rates, averaging times) to allow readers to assess reproducibility.
  2. [Discussion] Consider adding a brief comparison table of the reported detection limit against prior mid-IR dual-comb or absorption spectroscopy results in combustion environments.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thoughtful and detailed review of our manuscript. The comments have helped us strengthen the presentation of our quantitative claims and the supporting evidence for the differential detection approach. We have revised the abstract and the relevant sections of the manuscript accordingly, adding the requested details and demonstrations while preserving the original scientific content.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The estimated 1.1 ppm detection limit for CH4 is stated without accompanying details on the noise floor, averaging time, path-length specifics, or uncertainty analysis (e.g., no error bars or validation data), which is load-bearing for the quantitative performance claim.

    Authors: We agree that the detection-limit statement in the abstract requires explicit supporting information. In the revised manuscript we have expanded the abstract to specify the measured noise floor (in absorbance units), the averaging time (1 s), confirmation of the 1 m path length, and the uncertainty analysis performed on repeated measurements. Error bars derived from these measurements are now shown in the relevant figure, and a brief description of the validation procedure has been added to the main text. revision: yes

  2. Referee: [Differential detection description] The section describing the differential detection strategy: The central claim that this approach enables precise, calibration-free CH4 measurements by fully canceling interferences from H2O, CO2, temperature-induced shifts/broadening, and scattering lacks quantitative demonstration, such as residual spectra after subtraction, chosen line parameters, or validation against known overlaps in the 3.4 µm region under high-temperature combustion conditions.

    Authors: We appreciate the referee’s request for quantitative evidence. The differential detection is designed to suppress common-mode absorption and scattering, but we acknowledge that the original manuscript provided only qualitative description. In the revision we have inserted residual spectra after subtraction, listed the specific HITRAN line parameters used for CH4, H2O and CO2 in the 3.4 µm window, and added a comparison of the measured residuals against simulated high-temperature spectra (accounting for temperature-induced broadening and shifts) to demonstrate the degree of cancellation achieved under combustion conditions. revision: yes

Circularity Check

0 steps flagged

No circularity: direct experimental measurements without derivations

full rationale

The paper reports experimental implementation and results from a dual-comb spectroscopy system for McKenna burner flame characterization. No mathematical derivations, fitted parameters presented as predictions, or load-bearing self-citations are present. The 1.1 ppm detection limit, spatial gradients, and dynamic instability observations are stated as direct measurement outcomes. The differential detection strategy at 3427.43 nm is described as enabling calibration-free CH4 measurements, but this is an experimental claim without any derivation chain that reduces to its own inputs by construction. The work is self-contained against external benchmarks as a system demonstration.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper is an experimental implementation relying on established optical principles rather than new theoretical derivations; no free parameters, ad-hoc axioms, or invented entities are introduced in the abstract.

axioms (1)
  • standard math Standard principles of electro-optical frequency comb generation, difference frequency generation, and differential absorption spectroscopy apply in the mid-infrared regime for methane detection.
    The system description assumes these established techniques function as expected without additional justification in the abstract.

pith-pipeline@v0.9.0 · 5512 in / 1290 out tokens · 25523 ms · 2026-05-16T19:51:41.464872+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

What do these tags mean?
matches
The paper's claim is directly supported by a theorem in the formal canon.
supports
The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends
The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses
The paper appears to rely on the theorem as machinery.
contradicts
The paper's claim conflicts with a theorem or certificate in the canon.
unclear
Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

Works this paper leans on

30 extracted references · 30 canonical work pages

  1. [1]

    M. E. Webber, J. Wang, S. T. Sanders, D. S. Baer, R. K. Hanson, In situ combustion measurements of CO, CO2, H2O and temperature using diode laser absorption sen- sors, Proceedings of the Combustion Institute 28 (1) (2000) 407–413. doi:10.1016/S0082-0784(00) 80237-4

    work page doi:10.1016/s0082-0784(00 2000

  2. [2]

    Zhang, Z

    W. Zhang, Z. Zhang, X. Han, C. Yuan, Y . Liu, L. Ma, W. Ren, On the determination of the standing oblique detonation wave in an engine combustor using laser ab- sorption spectroscopy of hydroxyl radical, Aerospace Science and Technology 152 (2024) 109344. doi: 10.1016/j.ast.2024.109344

    work page doi:10.1016/j.ast.2024.109344 2024

  3. [3]

    X. Chao, J. B. Jeffries, R. K. Hanson, Wavelength- modulation-spectroscopy for real-time, in situ NO de- tection in combustion gases with a 5.2 µm quantum- cascade laser, Applied Physics B 106 (4) (2012) 987–997. doi:10.1007/s00340-011-4839-y. 6/9

    work page doi:10.1007/s00340-011-4839-y 2012

  4. [4]

    G. Wang, R. Wang, W. Zhao, Precise temperature mea- surement through wavelength modulation heterodyne phase-sensitive dispersion spectroscopy, Photonics 12 (6) (2025).doi:10.3390/photonics12060537

    work page doi:10.3390/photonics12060537 2025

  5. [5]

    L. Ma, Z. Wang, K.-P. Cheong, H. Ning, W. Ren, Tem- perature and H2O sensing in laminar premixed flames using mid-infrared heterodyne phase-sensitive dispersion spectroscopy, Applied Physics B 124 (6) (2018) 117. doi:10.1007/s00340-018-6990-1

    work page doi:10.1007/s00340-018-6990-1 2018

  6. [6]

    R. Wang, L. Xu, High-precision temperature measure- ment using frequency-division multiplexing laser dis- persion spectroscopy for dynamic combustion monitor- ing, IEEE Transactions on Instrumentation and Mea- surement 73 (2024) 1–8. doi:10.1109/TIM.2024. 3385845

    work page doi:10.1109/tim.2024 2024

  7. [7]

    Coddington, N

    I. Coddington, N. Newbury, W. Swann, Dual-comb spec- troscopy, Optica 3 (4) (2016) 414–426.doi:10.1364/ OPTICA.3.000414

    work page 2016

  8. [8]

    D. Yun, W. B. Sabin, S. C. Coburn, N. Hoghooghi, J. J. France, M. A. Hagenmaier, K. M. Rice, J. M. Don- bar, G. B. Rieker, Thermometry and velocimetry in a ramjet using dual comb spectroscopy of the O2 A- band, Opt. Express 31 (25) (2023) 42571–42580. doi: 10.1364/OE.507647

    work page doi:10.1364/oe.507647 2023

  9. [9]

    Takeshi, R

    N. Takeshi, R. Uchiyama, T. Takahoshi, F.-L. Hong, Y . Nakajima, Integration log rotational-state distribution thermometry for multi-component gas temperature mea- surement using dual-comb spectroscopy, J. Opt. Soc. Am. B 42 (12) (2025) 2875–2882.doi:10.1364/JOSAB. 576771

    work page doi:10.1364/josab 2025

  10. [10]

    K. Xu, L. Ma, J. Chen, X. Zhao, Q. Wang, R. Kan, Z. Zheng, W. Ren, Dual-comb spectroscopy for laminar premixed flames with a free-running fiber laser, Combus- tion Science and Technology 194 (12) (2022) 2523–2538. doi:10.1080/00102202.2021.1879796

    work page doi:10.1080/00102202.2021.1879796 2022

  11. [11]

    Schroeder, R

    P. Schroeder, R. Wright, S. Coburn, B. Sodergren, K. Cos- sel, S. Droste, G. Truong, E. Baumann, F. Giorgetta, I. Coddington, N. Newbury, G. Rieker, Dual frequency comb laser absorption spectroscopy in a 16 MW gas tur- bine exhaust, Proceedings of the Combustion Institute 36 (3) (2017) 4565–4573. doi:10.1016/j.proci. 2016.06.032

    work page doi:10.1016/j.proci 2017

  12. [12]

    D. A. Long, A. J. Fleisher, K. O. Douglass, S. E. Maxwell, K. Bielska, J. T. Hodges, D. F. Plusquellic, Multihetero- dyne spectroscopy with optical frequency combs gen- erated from a continuous-wave laser, Opt. Lett. 39 (9) (2014) 2688–2690.doi:10.1364/OL.39.002688

    work page doi:10.1364/ol.39.002688 2014

  13. [13]

    K. Beha, D. C. Cole, P. Del’Haye, A. Coillet, S. A. Did- dams, S. B. Papp, Electronic synthesis of light, Optica 4 (4) (2017) 406–411. doi:10.1364/OPTICA.4. 000406

    work page doi:10.1364/optica.4 2017

  14. [15]

    Millot, S

    G. Millot, S. Pitois, M. Yan, T. Hovhannisyan, A. Ben- dahmane, T. W. Hänsch, N. Picqué, Frequency-agile dual- comb spectroscopy, Nature Photonics 10 (1) (2016) 27– 30.doi:10.1038/nphoton.2015.250

    work page doi:10.1038/nphoton.2015.250 2016

  15. [16]

    Martín-Mateos, F

    P. Martín-Mateos, F. U. Khan, O. E. Bonilla-Manrique, Direct hyperspectral dual-comb imaging, Optica 7 (3) (2020) 199–202.doi:10.1364/OPTICA.382887

    work page doi:10.1364/optica.382887 2020

  16. [17]

    F. U. Khan, A. Moreno-Oyervides, O. E. Bonilla- Manrique, P. Martín-Mateos, Sub-GHz optical resolu- tion mid-infrared hyperspectral imaging with dual-comb, Optics and Lasers in Engineering 170 (2023) 107799. doi:10.1016/j.optlaseng.2023.107799

    work page doi:10.1016/j.optlaseng.2023.107799 2023

  17. [18]

    R. J. Hargreaves, I. E. Gordon, M. Rey, A. V . Nikitin, V . G. Tyuterev, R. V . Kochanov, L. S. Rothman, An accurate, extensive, and practical line list of methane for the HITEMP database, The Astrophysical Journal Supplement Series 247 (2) (2020) 55.doi:10.3847/ 1538-4365/ab7a1a

    work page 2020

  18. [19]

    Pannier, C

    E. Pannier, C. O. Laux, RADIS: A nonequilibrium line-by-line radiative code for CO2 and HITRAN-like database species, Journal of Quantitative Spectroscopy and Radiative Transfer 222-223 (2019) 12–25. doi: 10.1016/j.jqsrt.2018.09.027

    work page doi:10.1016/j.jqsrt.2018.09.027 2019

  19. [20]

    Frangi, Dual-Comb Toolkit v1.0.1 [software], Zenodo (Dec

    B. Frangi, Dual-Comb Toolkit v1.0.1 [software], Zenodo (Dec. 2025).doi:10.5281/zenodo.18040729

    work page doi:10.5281/zenodo.18040729 2025

  20. [21]

    J. A. Nelder, R. Mead, A simplex method for function minimization, The Computer Journal 7 (4) (1965) 308– 313.doi:10.1093/comjnl/7.4.308

    work page doi:10.1093/comjnl/7.4.308 1965

  21. [22]

    Masset, R

    V . Mislavskii, N. Pestovskii, S. Tskhai, B. Kichatov, V . Gubernov, V . Bykov, U. Maas, Diffusive-thermal pul- sations of burner stabilized methane-air flames, Combus- tion and Flame 234 (2021) 111638. doi:10.1016/j. combustflame.2021.111638

    work page doi:10.1016/j 2021

  22. [23]

    X. Nie, S. Zhang, S. Wang, Pulsation of burner-stabilized CH4-O2 flames moderated by CO2 addition – frequen- cies, modes and regime diagrams (2025). arXiv:2507. 10905,doi:10.48550/arXiv.2507.10905

    work page doi:10.48550/arxiv.2507.10905 2025

  23. [24]

    Schenzle, R

    A. Schenzle, R. G. DeV oe, R. G. Brewer, Phase- modulation laser spectroscopy, Physical Review A 25 (5) (1982) 2606–2621. doi:10.1103/PhysRevA.25. 2606

    work page doi:10.1103/physreva.25 1982

  24. [25]

    Gordon, L

    I. Gordon, L. Rothman, R. Hargreaves, R. Hashemi, E. Karlovets, F. Skinner, E. Conway, C. Hill, R. Kochanov, Y . Tan, P. Wcisło, A. Finenko, K. Nelson, P. Bernath, M. Birk, V . Boudon, A. Campargue, K. Chance, A. Coustenis, B. Drouin, J. Flaud, R. Gamache, J. Hodges, D. Jacquemart, E. Mlawer, A. Nikitin, V . Perevalov, M. Rotger, J. Tennyson, G. Toon, H. ...

    work page doi:10.1016/j.jqsrt.2021 2022

  25. [26]

    A measurement of the 3427.43 nm absorption line at room temperature was taken using a 30-teeth dual comb

    work page

  26. [27]

    The same line was simulated at the same conditions using the HITRAN database [25], appropriate for room- temperature measurements

    work page

  27. [28]

    The average standard deviation between the measure- ment and the HITRAN simulation was found to be 0.014

    work page

  28. [29]

    For a simulation of N teeth, the average standard devia- tion was scaled according to [7]: σ(N) =0.014· N 30 = N 2143 .(A.4)

    work page

  29. [30]

    The values of 1/|J k(Ω)| 2 were obtained for each tooth, where Jk(Ω) is the k-th Bessel function evaluated at the modulation intensity Ω, and the average of all teeth, 1/J2, was computed

    work page

  30. [31]

    Two sequential effects were modeled to account for instabilities in the laser’s central wave- length

    The standard deviation σk of the k-th tooth was obtained as: σk = σ· 1/|J k(Ω)| 2 1/J2 .(A.5) The following additional considerations were made to simulate instrumental effects: Laser wavelength instability. Two sequential effects were modeled to account for instabilities in the laser’s central wave- length. (i) Shot-to-shot jitter: A random frequency off...

    work page