{"id":"986c9693-b559-4cce-bad3-a583af8169f2","arxiv_id":"2505.00895","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Space-time wave packets produced on-axis spectra that red-shift or blue-shift with propagation over 100 to 200 meters in open air, and that can also be tuned at a fixed position by changing an internal pulse parameter.","lead":"Space-time wave packets, a class of engineered laser pulses, were used to make the color of light on the beam axis shift with distance over a 200 meter outdoor laser range. The work suggests that a future LIDAR or remote sensor could measure target distance by reading the wavelength of scattered light.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Wavelength-dependent alignment of the 200 µm fiber is uncontrolled; a lateral offset could mimic the reported axial spectral shifts.","rationale":"The reader's weakest_assumption identifies the same concern I find load-bearing: the uncharacterized alignment of the multimode fiber. I evaluated alternative concerns—the Lmax scaling in the δθ→0 regime, the adequacy of the propagation-invariant control, and the parameter notation typos—but none is as directly fatal to the central claim. The control in Fig. 5(a) does not establish that the fiber is on-axis because the propagation-invariant STWP's full spectrum is transversely uniform in wavelength composition; only the encoded STWPs are wavelength-dispersed in the transverse dimension, making λc sensitive to lateral offset. The paper provides no error bars, no repeated trials, no alignment tolerance, and no description of how x=0 is determined at each z, so the specific λc values in Fig. 5(b–d) are unverified. A lateral scan at one z would quantify this sensitivity and settle the issue. I therefore agree with the reader's CONDITIONAL verdict; the result is plausible but not yet sufficiently supported.","tokens_in":7935,"tokens_out":6595,"duration_ms":68330,"concrete_test":"Perform a lateral scan of the 200 µm multimode fiber across the STWP at a fixed axial position (z ≈ 150 m) for the red-shifting encoded STWP, recording λc at lateral offsets from −5 mm to +5 mm in 100 µm steps. If |dλc/dx| is large enough that a plausible pointing drift of ±1 mm shifts λc by more than ~2 nm (the designed axial shift is ~0.2 nm/m over 50 m, i.e., ~10 nm), then the reported λc values at different z are not robust to alignment error, and the axial encoding observation would need to be repeated with quantified alignment control.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that the on-axis spectrum of a spectrally encoded STWP shifts with propagation distance, with λc = 1053/1065/1075 nm (red-shifting) at z = 100/150/200 m and λc = 1073/1067/1055 nm (blue-shifting). The measurements rely entirely on a 200 µm multimode fiber mounted on a tripod and re-aligned at each z. For a spectrally encoded STWP, each wavelength λ is associated with a different spatial frequency kx(λ) and hence a different transverse profile; the on-axis spectrum I(λ,z) is defined only at the transverse coordinate x = 0. The paper reports no procedure for establishing x=0, no alignment tolerance, no repeated acquisitions, and no error budget for cart positioning or pointing jitter. If the fiber is laterally offset by δx, the recorded spectrum samples a different wavelength slice, shifting λc. The control in Fig. 5(a) does not mitigate this: for a propagation-invariant STWP the full source spectrum is present across the transverse profile, so a small offset still records the full spectrum; only the encoded STWPs have a wavelength-dependent spatial profile that makes λc sensitive to offset. A systematic z-dependent lateral drift of the fiber or beam would therefore produce exactly the monotonic λc trends in Fig. 5(b,c) even in the absence of axial spectral encoding. The same applies to Fig. 5(d), where the fiber position is fixed but the internal parameter δθ is changed; if the transverse profile changes with δθ, a fixed offset can produce an apparent λc shift. Thus the empirical precondition for the LIDAR/ranging claim is not currently supported.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper reports an outdoor propagation experiment at the TISTEF laser range in which space-time wave packets (STWPs) are synthesized with a spatial light modulator and launched over distances of 100–200 m. The authors implement three configurations: a propagation-invariant STWP, a red-shifting axially encoded STWP, and a blue-shifting axially encoded STWP. On-axis spectra are captured with a 200 µm multimode fiber connected to a spectrometer with 1 nm resolution at z = 100, 150, and 200 m. They report a stable ~25 nm source spectrum for the propagation-invariant control, and ~5 nm FWHM encoded spectra with central wavelengths λc = 1053, 1065, and 1075 nm for the red-shifting case and λc = 1073, 1067, and 1055 nm for the blue-shifting case. In a fourth configuration, at fixed z ≈ 135 m, varying the spectral tilt angle δθ shifts λc from 1058 to 1064 to 1070 nm. The manuscript interprets these observations as a demonstration of axial spectral encoding over hundreds of meters in an open-air environment, with potential for spectral-measurement-based LIDAR ranging.","tokens_in":8266,"tokens_out":4313,"duration_ms":44920,"significance":"If the measurements are reliable, this is a significant demonstration: it extends axial spectral encoding from a laboratory proof-of-principle to an open-field environment, and the reported spectral shifts (22 nm over 100 m in the red-shifting case) are far larger than the 1 nm spectrometer resolution. The propagation-invariant control in Fig. 5(a) is a useful internal check, because it shows that the measurement chain does not itself produce large spectral drifts when the on-axis spectrum is expected to be z-independent. The paper does not fit free parameters to the displayed data: the amplitude-mask centers and widths are inherited from the earlier design theory in Ref. [45]. The principal weakness is the lack of experimental characterization of the measurement geometry: no error bars, no repeated acquisitions, and no alignment-tolerance study are reported. Because encoded STWPs have wavelength-dependent transverse profiles, this is not a cosmetic issue but a load-bearing assumption for the quoted λc values. If that assumption is validated with additional measurements, the result would be an important step toward STWP-based axial ranging.","major_comments":[{"comment":"The central claim depends on the recorded spectrum being the true on-axis (x = 0) spectrum. For the encoded STWPs, each wavelength is associated with a different spatial frequency, so the transverse profile is wavelength-dependent; a lateral displacement of the 200 µm fiber relative to the beam axis will therefore sample a different wavelength interval and can shift λc. The manuscript does not state how x = 0 was established, what alignment tolerance was maintained, or whether repeated acquisitions were made at each z. The control in Fig. 5(a) does not rule out such a bias, because the propagation-invariant STWP carries the full source spectrum across its transverse profile, making its recorded λc insensitive to small lateral offsets. Please provide an alignment procedure, offset-sensitivity measurements or transverse scans, and repeated measurements with error bars for λc at each z.","section":"Measurement configuration (Fig. 3) and Fig. 5"},{"comment":"The fixed-z result is subject to the same alignment sensitivity. Changing δθ changes the beam geometry and the wavelength-dependent transverse profile; if the fiber is not exactly on axis, a fixed lateral offset can produce apparent λc shifts even though the source spectrum is unchanged. The paper should show either measurements at multiple lateral positions for each δθ, or a procedure that recenters the fiber on the beam for each δθ, so that the reported monotonic trend is not an alignment artifact.","section":"Fig. 5(d), fixed-z tuning"},{"comment":"The quantitative claims of approximately linear shifts in λc with z require an error budget covering at least the distance determination by the range-finder, the spectral calibration, and the extraction of λc from spectra with FWHM ≈ 5 nm recorded at 1 nm resolution. No such budget is provided, and no error bars appear in Fig. 5. Given that the quoted shifts are large relative to the resolution, this is not fatal, but the absence of error bars and repeated points weakens the support for the specific λc values and for the proposed ranging application.","section":"Fig. 5 and extraction of λc"}],"minor_comments":[{"comment":"The last sentence, 'These results indicates the potential for using spectral measurements for ranging in LIDAR and sensing applications,' should read 'These results indicate...'.","section":"Abstract"},{"comment":"In the Fig. 5(a) caption, 'δ θ = ×10−4' is missing the coefficient, and in Section 3 the text 'δ θ=2×10−4,×10−4, and 0.5×10−4' also appears to be missing the coefficient '1×10−4'. The units (degrees) should be stated consistently with the definition of δθ.","section":"Fig. 5 caption and Section 3"},{"comment":"The expression r(λ,x) = ±kx(λ)x · rect[(x ± xo(λ))/Wo(λ)] would benefit from parentheses around the phase term; as written, '±kx(λ)x·rect' is ambiguous.","section":"Fig. 2 and the expression for r(λ,x)"},{"comment":"The manuscript contains two identical 'DATA AVAILABILITY' sections; one should be removed.","section":"Data availability"},{"comment":"'This indicates the surprisingly utility of SLMs' should be 'the surprising utility' or 'the surprisingly high utility'.","section":"Section 3, last paragraph"}],"recommendation":"major_revision","confidential_remarks":"I do not see a novelty-disclosure issue: Ref. [45] is the group's own design theory, and this paper is an extended propagation test of that design. The main request is for additional experimental characterization rather than new theory. Given that the data availability statement says the data are not public, I would encourage the editor to request the underlying spectra as supplemental material for this claims-driven result."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: the paper does something real — it takes axial spectral encoding, previously shown at lab scale [45], out to 200 m in open air, and it adds a fixed-z tuning knob via delta_theta. The measured shifts are large (up to 22 nm) relative to the 1 nm spectrometer resolution, and the propagation-invariant control is stable across 100–200 m. That is a genuine step for the STWP and remote-sensing community, though not field-ready LIDAR.\n\nThe soft spot is exactly the one in the stress-test note, and I think it lands. The on-axis claim rests on a 200 µm multimode fiber manually aligned to the central lobe at each z. For an encoded STWP, the transverse profile is wavelength-dependent, so a lateral offset samples a different wavelength slice. The paper gives no procedure for establishing x=0, no alignment tolerance, no repeated acquisitions, and no error budget for cart position or pointing jitter. The control in Fig. 5(a) does not close this gap because a propagation-invariant STWP carries the full spectrum across its profile, so an offset leaves lambda_c unchanged; only the encoded packets are sensitive. A systematic z-dependent offset of the fiber or beam could therefore produce exactly the monotonic lambda_c trends in Fig. 5(b,c), and the same logic applies to Fig. 5(d). I do not think the result is necessarily wrong — the shifts are large and the spectral shapes look plausible — but the empirical precondition for the LIDAR claim is not currently supported.\n\nMinor issues: the Fig. 5(a) caption has a missing value ('delta_theta = ×10^-4'), there are a few typos ('results indicates', 'surprisingly utility', duplicate Data Availability section), and the data are not public. These are easy fixes.\n\nThe citation pattern is heavily self-referential, but that is legitimate here because this group built the underlying theory and the short-range proof of principle; no fitting to the displayed data is evident.\n\nWho is this for? STWP researchers and people working on spectral ranging. It is a solid engineering-style demonstration that deserves a serious referee, not a desk reject. The referee should ask for an alignment-tolerance study, repeated measurements or error bars, and preferably a control that deliberately scans lateral offset to bound the systematic. My recommendation: peer review, with the understanding that revision should address the alignment question head-on.","headline":"A credible 200 m demonstration of axial spectral encoding that is undermined by an uncontrolled alignment assumption; worth refereeing, but the central figure needs an error budget before the ranging claim can stand.","tokens_in":8794,"tokens_out":2706,"would_cite":false,"duration_ms":29858,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Space-time wave packets can be programmed so their on-axis color shifts predictably along 200 m of open air.","keywords":["space-time wave packets","axial spectral encoding","propagation-invariant beams","spectral ranging","LIDAR","open-air laser range","spectral tilt angle","spatial light modulator"],"falsifier":"Repeat the $z=100$, 150, and 200 m measurements several times at the same cart positions without moving the fiber, and also translate the collecting fiber laterally by a small fraction of the central-lobe width at one fixed $z$; if the recorded central wavelength varies by more than the roughly 1-nm spectrometer resolution at fixed position, or if a lateral offset comparable to the fiber diameter reverses the apparent red/blue trend, then the claimed axial encoding is not cleanly separated from alignment artifacts.","tokens_in":7755,"feed_emoji":"🌈","tokens_out":11142,"duration_ms":99620,"temperature":0.7,"pith_summary":"The paper claims that specially structured light pulses called space-time wave packets can be designed so the color of light measured on their axis changes as they travel, and that this programmed color shift survives real propagation over 200 m in an outdoor laser range. Two configurations are demonstrated: one in which the on-axis spectrum red-shifts or blue-shifts along the beam path, and one in which the spectrum at a fixed position shifts by adjusting an internal parameter of the pulse without changing its total spectrum. The result matters because it suggests a ranging method: the distance to a target could be read directly from the wavelength of scattered light, without time-of-flight electronics. A sympathetic reading takes the reported central wavelengths at three distances as evidence that the encoding is deterministic, approximately linear, and stable enough for outdoor use.","feed_headline":"A beam whose color shifts with distance survives 200 m","feed_subtitle":"Space-time wave packets encode distance in wavelength, opening a spectral route to LIDAR.","key_machinery":"The mechanism is the spectral support of the STWP on the free-space light cone $k_x^2+k_z^2=(\\omega/c)^2$. A propagation-invariant STWP is the conic section of that cone with a tilted plane $\\omega-\\omega_o=(k_z-k_o)c\\tan\\theta$; each temporal frequency is tied to one spatial frequency, so the on-axis spectrum is the full source spectrum at every $z$. Axial spectral encoding changes the SLM reflectance to $r(\\lambda,x)=\\pm k_x(\\lambda)x\\,\\mathrm{rect}[(x\\pm x_o(\\lambda))/W_o(\\lambda)]$, where the mask center $x_o(\\lambda)$ and width $W_o(\\lambda)$ concentrate each wavelength into a selected spatial band; the relation $z(\\lambda_c)\\sim (k/k_x)x_o(\\lambda_c)$ turns that mask layout into an axial spectral schedule. The finite-energy propagation length is governed either by the spectral uncertainty $\\delta\\omega$ through $L_{\\mathrm{max}}\\sim \\delta\\omega/[c|1-\\cot\\theta|]$ or, when $\\delta\\theta\\to0$, by the aperture width through $L_{\\mathrm{max}}\\sim W\\Delta x/\\lambda_o$; the experiment operates in the latter regime, which is why an outdoor 200-m test meaningfully extends the earlier laboratory-scale demonstration.","core_discovery":"On the paper's own terms, the central result is that axial spectral encoding — a design in which each wavelength in an STWP is mapped to a different spatial-frequency band through an amplitude mask on a spatial light modulator — produces a narrow on-axis spectrum whose central wavelength is a designed function of propagation distance, and that this survives propagation over 200 m in an outdoor laser range. For the red-shifting encoding, the on-axis central wavelength moved from 1053 nm at $z=100$ m to 1065 nm at 150 m and 1075 nm at 200 m; for the blue-shifting counterpart it moved from 1073 to 1067 to 1055 nm over the same positions, while a propagation-invariant STWP kept the full 25-nm source spectrum stable at all three positions. In the second configuration, at fixed $z\\approx135$ m, changing the spectral tilt angle by $\\delta\\theta = 2\\times10^{-4}$, $1\\times10^{-4}$, and $5\\times10^{-5}$ degrees shifted the on-axis wavelength from 1058 to 1064 to 1070 nm, without spectral filtering. The approximately linear change of central wavelength with distance is the design target of the mask, and the 200-m survival of that schedule is the new experimental claim.","pith_inferences":["A direct extension the paper does not perform is closing the LIDAR loop: scatter the STWP from a diffusive target at a known $z$ and test whether the backscattered wavelength matches the forward-axis calibration; success would make spectral ranging a real measurement rather than a property of an unobstructed beam.","Because encoded STWPs carry different wavelengths in different transverse regions, a target offset from the axis may scatter a different color than the on-axis spectrum predicts; the paper does not characterize this transverse wavelength gradient, which is a likely source of ranging ambiguity in realistic scenes.","The reported slopes could be compared quantitatively with the slopes designed into the SLM masks; the paper reports only the measured wavelengths, so a mismatch between designed and measured $\\lambda_c(z)$ would separate mask-design accuracy from pointing and alignment errors.","The fixed-$z$ spectral tuning suggests a spectral dial that could interrogate a stationary target at multiple wavelengths without a tunable source; the authors do not discuss this application, but it follows directly from their second configuration."],"forward_implications":["If the axial spectral schedule survives outdoor propagation, ranging can in principle be performed by measuring a single on-axis wavelength rather than by timing a round trip.","The approximate linearity of $\\lambda_c(z)$ means a two-point calibration converts central wavelength to distance, a much simpler data product than a time-of-flight histogram.","The fixed-position tuning configuration offers a way to shift the wavelength returning from a target at a known range without touching the laser spectrum, potentially useful for wavelength-multiplexed sensing.","The demonstrated stability of the propagation-invariant STWP spectrum over 100–200 m acts as a control, separating the encoded spectral dynamics from path-averaged atmospheric effects.","SLM-based synthesis, rather than fixed high-resolution phase plates, is sufficient for the precision required, so the encoding can be reprogrammed electronically between configurations."],"supporting_citations":[{"why":"Introduces axial spectral encoding of STWPs and the laboratory-scale proof of principle that this paper extends to 200 m outdoors.","marker":"[45]"},{"why":"Establishes the aperture-limited propagation regime $L_{\\mathrm{max}}\\sim W\\Delta x/\\lambda_o$ at $\\delta\\theta\\to0$, the regime in which the 200-m test operates.","marker":"[35]"},{"why":"Provides the quantitative control over spectral tilt angle and spectral uncertainty that lets the encoded spectral supports be programmed on the SLM.","marker":"[22]"},{"why":"Earlier long-range propagation of broadband STWPs over 70 m, the baseline that the 200-m open-air measurement builds on.","marker":"[23]"},{"why":"Relates the spectral tilt angle $\\theta$ to the group velocity $\\tilde{v}=c\\tan\\theta$, used in the fixed-position tuning configuration.","marker":"[31]"},{"why":"Baseline synthesis of STWPs with transmissive phase plates, which the paper contrasts with the SLM-based synthesis used here.","marker":"[49]"},{"why":"Supplies the defining spatiotemporal-spectral structure of STWPs and the spectral-support framework on which axial spectral encoding is built.","marker":"[1]"}],"fun_headline_variants":["Spectrum shifts with distance over 200 m in wave packets","Color shifts with distance: wave packets encode range","Axial spectral encoding survives 200 m laser range","Distance written in wavelength: wave packet LIDAR"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing assumption is that the spectrum recorded by the tripod-mounted 200-µm multimode fiber at each distance is the true on-axis spectrum, with no wavelength-dependent alignment or pointing bias, so the measured central wavelengths reflect the designed encoding rather than the apparatus.","fun_headline_variants_meta":{"raw":{"variants":["Spectrum shifts with distance over 200 m in wave packets","Color shifts with distance: wave packets encode range","Axial spectral encoding survives 200 m laser range","Distance written in wavelength: wave packet LIDAR"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000623,"raw_usage":{"total_tokens":2871,"prompt_tokens":918,"completion_tokens":1953,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":534,"completion_tokens_details":{"reasoning_tokens":1889}},"tokens_in":534,"tokens_out":1953,"duration_ms":13001,"temperature":1.0,"reasoning_tokens":1889,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T04:32:24.333942+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Repeat the $z=100$, 150, and 200 m measurements several times at the same cart positions without moving the fiber, and also translate the collecting fiber laterally by a small fraction of the central-lobe width at one fixed $z$; if the recorded central wavelength varies by more than the roughly 1-nm spectrometer resolution at fixed position, or if a lateral offset comparable to the fiber diameter reverses the apparent red/blue trend, then the claimed axial encoding is not cleanly separated from alignment artifacts.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Introduces axial spectral encoding of STWPs and the laboratory-scale proof of principle that this paper extends to 200 m outdoors."},{"cited_title":"Yessenov, B","cited_arxiv_id":null,"evidence_quote":"Provides the quantitative control over spectral tilt angle and spectral uncertainty that lets the encoded spectral supports be programmed on the SLM."},{"cited_title":"Bhaduri, M","cited_arxiv_id":null,"evidence_quote":"Earlier long-range propagation of broadband STWPs over 70 m, the baseline that the 200-m open-air measurement builds on."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Baseline synthesis of STWPs with transmissive phase plates, which the paper contrasts with the SLM-based synthesis used here."}],"review_version":1}