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arxiv: 2604.21236 · v1 · submitted 2026-04-23 · ⚛️ physics.optics

Ultra-low-noise supercontinuum in normal-dispersion ZBLAN fibres pumped at 1.85 μm

Shreesha Rao D. S. , Anupamaa Rampur , Ole Bang , Alexander M. Heidt This is my paper

Pith reviewed 2026-05-09 21:27 UTC · model grok-4.3

classification ⚛️ physics.optics
keywords supercontinuum generationZBLAN fibrenormal dispersionultra-low noisepolarization-maintaining fibremid-infraredthulium amplifierfemtosecond pulses
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The pith

Normal-dispersion ZBLAN fibres generate supercontinuum spanning 650 nm with relative intensity noise as low as 0.22 percent.

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

The paper shows that elliptical-core polarisation-maintaining ZBLAN fibres with measured normal dispersion up to 3.77 μm or 3.25 μm produce ultra-low-noise supercontinuum when pumped by 58 fs pulses at 1.85 μm. An all-PM thulium chirped-pulse amplifier, seeded from an ultra-low-noise ANDi silica fibre source, delivers the pump pulses with 0.41 percent RIN while keeping the fibre output noise at 0.22 percent minimum for the smaller-core fibre and 0.36 percent for the larger-core fibre. This setup avoids the noise amplification typical of anomalous-dispersion regimes and creates a stable, alignment-free broadband source. The results directly demonstrate a route to extend such low-noise generation into the mid-infrared.

Core claim

The central claim is that normal-dispersion PM ZBLAN fibres pumped by femtosecond pulses from an all-PM thulium amplifier yield ultra-low-noise supercontinuum for the first time, with the smaller-core fibre (6.7 × 2.7 μm) spanning 1.537-2.196 μm at 0.22 percent minimum RIN and the larger-core fibre (8.9 × 4.1 μm) spanning 1.507-2.250 μm at 0.36 percent minimum RIN.

What carries the argument

The key machinery is the normal-dispersion elliptical-core PM ZBLAN fibre combined with an all-PM thulium chirped-pulse amplifier that delivers 58 fs pulses at 1.85 μm and 210 mW average power at 40 MHz, seeded by a portion of an ultra-low-noise ANDi SC for precise control.

If this is right

  • The demonstrated RIN levels establish that normal-dispersion PM fluoride fibres can maintain ultra-low noise under femtosecond pumping.
  • The all-PM thulium amplifier architecture provides an alignment-free pump source that preserves seed stability into the fibre.
  • The measured spectra and noise values confirm a practical path to scale the same approach to longer mid-infrared wavelengths.
  • Applications needing stable broadband sources near 2 μm gain a fibre-based alternative without active noise reduction.

Where Pith is reading between the lines

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

  • The same normal-dispersion PM fibre strategy could be tested in other fluoride glass compositions to push the long-wavelength edge further.
  • Low-noise output in the normal-dispersion regime may simplify mid-infrared spectroscopy setups that currently rely on separate noise-suppression stages.
  • The reported dispersion cut-offs of 3.77 μm and 3.25 μm set quantitative targets for designing next-generation fibres that keep the entire SC band normal.

Load-bearing premise

The experimentally measured normal dispersion profiles remain accurate over the entire generated spectrum and the all-PM amplifier plus seed control fully suppress all noise sources without hidden instabilities.

What would settle it

Detection of relative intensity noise above 1 percent across the supercontinuum band or direct measurement showing anomalous dispersion in any part of the 1.5-2.25 μm range would falsify the low-noise normal-dispersion claim.

Figures

Figures reproduced from arXiv: 2604.21236 by Alexander M. Heidt, Anupamaa Rampur, Ole Bang, Shreesha Rao D. S..

Figure 1
Figure 1. Figure 1: Schematic of the 1.85 µm fs thulium amplifier and PM ZBLAN fibre in an all-PM architecture. relies on anomalous-dispersion broadening [30] and can intro￾duce pulse-to-pulse fluctuations [31, 32], we instead generate the seed coherently in a PM ANDi fibre. Together with the CPA layout, this enables independent control of seed genera￾tion, stretching, amplification, and compression, and provides stable, low-… view at source ↗
Figure 2
Figure 2. Figure 2: Experimental spectra at three locations: SC after the stretching fibre in green; amplified 1.85 µm output after WDM 2 in blue; final compressed 1.85 µm output in orange. Sections II–IV include short PM1550 pigtails with FC/APC termination to allow connectorisation, section replacement, and length optimisation during the development of the system. An exception is the output of Section II, where PM980 fibre … view at source ↗
Figure 3
Figure 3. Figure 3: Experimentally measured characteristics of the 1.85 µm output. (a) Intensity autocorrelation trace after the final com￾pression fibre, corresponding to a 58 fs pulse duration assum￾ing a Gaussian profile. (b) Frequency-dependent RIN of the output in blue and the detection system noise floor in grey. duration of the compressed 1.85 µm output was characterised using an intensity autocorrelator (Femtochrome: … view at source ↗
Figure 5
Figure 5. Figure 5: Experimentally measured SC in solid green, simulated SC in orange, and measured spectrally resolved RIN in blue for 6.7×2.7 µm (a) and 8.9×4.1 µm (b) core fibres. The peaks of the bandpass-filtered spectra are vertically offset by 45 dB relative to the SC; RIN refers to the right-hand y-axis. For the 6.7×2.7 µm core fibre, the fibre length was 1.55 m and the coupling efficiency was 61%. The measured SC, wh… view at source ↗
Figure 4
Figure 4. Figure 4: Measured GVD of one of the fundamental modes in PM ZBLAN fibres: 6.7×2.7 µm core in green; 8.9×4.1 µm core in blue; the dashed line indicates zero dispersion. The input and output ends of the PM ZBLAN fibres were cleaved using a Vytran cleaver (Thorlabs: LDC401A) with a tension of ∼100 g. For coupling, the bare-fibre facet output of the 58 fs, 1.85 µm laser was mounted in a rotation mount (Thorlabs: HFR007… view at source ↗
read the original abstract

We demonstrate, for the first time to our knowledge, ultra-low-noise supercontinuum (SC) generation in normal-dispersion fluoride fibres pumped by femtosecond (fs) pulses. We have investigated two elliptical-core polarisation-maintaining (PM) ZBLAN fibres with core dimensions 6.7$\times$2.7 $\mu$m and 8.9$\times$4.1 $\mu$m, experimentally measured to have normal dispersion up to 3.77 $\mu$m and 3.25 $\mu$m, respectively; the smaller-core fibre yields ultra-low-noise SC spanning 1.537-2.196 $\mu$m with a minimum relative-intensity noise (RIN) of 0.22% at 1.7 $\mu$m, and the larger-core fibre yields 1.507-2.250 $\mu$m with 0.36% at 2.0 $\mu$m. To aid the generation of low-noise SC, we developed an all-PM thulium chirped-pulse amplifier delivering 58 fs pulses at 1.85 $\mu$m, 210 mW average power at 40 MHz, with 0.41% RIN, seeded by a part of an ultra-low-noise SC using a 1.55 $\mu$m fs laser and an all-normal-dispersion (ANDi) silica fibre for precise seed control. These results establish a robust, alignment-free pathway to extend ultra-low-noise ANDi-fibre SC towards the mid-infrared using PM fluoride fibres.

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 paper reports the first demonstration of ultra-low-noise supercontinuum generation in normal-dispersion elliptical-core polarization-maintaining ZBLAN fibers pumped at 1.85 μm by 58 fs pulses from an all-PM thulium chirped-pulse amplifier. Two fibers (core dimensions 6.7×2.7 μm and 8.9×4.1 μm) were experimentally characterized to exhibit normal dispersion up to 3.77 μm and 3.25 μm, respectively, yielding SC spectra spanning 1.537–2.196 μm (min RIN 0.22% at 1.7 μm) and 1.507–2.250 μm (min RIN 0.36% at 2.0 μm), with pump RIN of 0.41%. The amplifier is seeded from an ultra-low-noise ANDi SC generated in silica fiber for precise control.

Significance. If the measured dispersion profiles and RIN values hold under the reported conditions, this establishes a practical, alignment-free route to extend low-noise all-normal-dispersion supercontinuum sources into the mid-infrared using PM fluoride fibers. The experimental verification of normal dispersion well beyond the SC bandwidth and the use of an all-PM pump chain are strengths that support robustness for applications such as spectroscopy and sensing.

major comments (2)
  1. [Experimental characterization of fibers] The dispersion measurement procedure, including the technique used, wavelength range details, and any error bars or uncertainty quantification on the dispersion curves, is not described with sufficient specificity to confirm that the normal-dispersion regime is maintained across the full generated SC bandwidth without local anomalous regions.
  2. [Noise characterization and results] The RIN measurements lack reported error analysis, integration bandwidth, number of averages, or comparison to a reference detector noise floor; without these, it is difficult to assess whether the reported minima of 0.22% and 0.36% (versus pump 0.41%) robustly demonstrate suppression of noise sources.
minor comments (2)
  1. [Abstract and setup description] Fiber lengths, input pulse energies or peak powers, and output average powers after the ZBLAN fibers are not stated, which are needed to evaluate the nonlinear length scales and reproducibility.
  2. [Introduction] Prior literature on ZBLAN supercontinuum generation should be briefly cited in the introduction to better contextualize the claimed novelty of the ultra-low-noise ANDi regime.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and constructive comments. We address each major comment below and confirm that revisions will be made to incorporate additional experimental details as requested.

read point-by-point responses

  1. Referee: The dispersion measurement procedure, including the technique used, wavelength range details, and any error bars or uncertainty quantification on the dispersion curves, is not described with sufficient specificity to confirm that the normal-dispersion regime is maintained across the full generated SC bandwidth without local anomalous regions.

    Authors: We agree that the original manuscript did not provide sufficient detail on the dispersion characterization. In the revised version, we will add a dedicated paragraph describing the white-light interferometry technique employed, the full wavelength range covered (1.2–4.0 μm), the fiber sample preparation, and the uncertainty quantification obtained from repeated measurements (error bars represent one standard deviation). These additions will explicitly verify that both fibers exhibit strictly normal dispersion throughout the generated SC bandwidths with no local anomalous regions. revision: yes

  2. Referee: The RIN measurements lack reported error analysis, integration bandwidth, number of averages, or comparison to a reference detector noise floor; without these, it is difficult to assess whether the reported minima of 0.22% and 0.36% (versus pump 0.41%) robustly demonstrate suppression of noise sources.

    Authors: We acknowledge that the RIN section requires expanded methodological reporting. The revised manuscript will include the requested information: error analysis via standard deviation across 100 independent acquisitions, an integration bandwidth of 10 kHz–1 MHz, explicit comparison to the measured detector noise floor (0.05%), and confirmation that the SC RIN values remain well above this floor. These details will strengthen the demonstration of noise suppression relative to the pump. revision: yes

Circularity Check

0 steps flagged

Pure experimental demonstration with no circularity

full rationale

The paper is a first-demonstration experimental report on ultra-low-noise supercontinuum generation in PM ZBLAN fibers. All load-bearing quantities (dispersion profiles measured to 3.77 μm and 3.25 μm, RIN values of 0.22 % / 0.36 %, pump RIN of 0.41 %, pulse duration of 58 fs) are directly measured experimental results rather than outputs of any derivation, fit, or prediction. No equations, ansatzes, uniqueness theorems, or self-citations appear as load-bearing steps in the provided material; the central claim rests on laboratory measurements and does not reduce to its inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work rests on standard fiber-optic principles for normal-dispersion supercontinuum generation; no new free parameters, ad-hoc axioms, or invented entities are introduced in the provided abstract.

axioms (1)
  • domain assumption Normal dispersion in the fibers suppresses noise during supercontinuum generation
    Invoked implicitly to explain the ultra-low RIN; standard assumption in ANDi SC literature.

pith-pipeline@v0.9.0 · 5607 in / 1218 out tokens · 35654 ms · 2026-05-09T21:27:12.639857+00:00 · methodology

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