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arxiv: 2605.15998 · v1 · submitted 2026-05-15 · ⚛️ physics.optics

Massively Degenerate Coherent Perfect Absorption in Gradient-Index Fibers

Helmut H\"orner , \c{S}ahin K. \"Ozdemir , Stefan Rotter This is my paper

Pith reviewed 2026-05-19 19:08 UTC · model grok-4.3

classification ⚛️ physics.optics
keywords coherent perfect absorptiongradient-index fibermultimode absorptionself-imagingGRIN fiberoptical interferencewaveguide absorber
0
0 comments X p. Extension

The pith

A gradient-index fiber uses its self-imaging property to absorb arbitrary multimode light fields with reflectivities below one percent.

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

The paper shows how a standard GRIN fiber can function as a compact multimode coherent perfect absorber. It replaces large free-space setups by letting the fiber's periodic self-imaging rephase many spatial modes at once for any input state. Standard GRIN profiles already optimized for low intermodal dispersion turn out to support this near-degenerate rephasing. The result is highly efficient absorption across a wide field of view, with reflectivities dropping well below one percent under realistic conditions. This gives a monolithic, scalable platform for multimode absorption in fiber and integrated systems.

Core claim

Using the self-imaging property of the GRIN fiber, the bulky free-space architecture of previous coherent perfect absorbers is replaced by a monolithic waveguiding platform that supports near-degenerate rephasing of many spatial modes. Standard GRIN profiles optimized for minimal intermodal dispersion enable highly efficient absorption of complex multimode fields, with field-of-view reflectivities well below 1% for realistic parameters.

What carries the argument

The self-imaging property of the gradient-index fiber, which produces periodic refocusing that allows many spatial modes to rephase near-degenerately for arbitrary multimode inputs.

If this is right

  • The approach replaces free-space CPA setups with a compact fiber platform.
  • Standard low-dispersion GRIN profiles already suffice for high-efficiency multimode absorption.
  • This enables scalable multimode absorption in fiber-based and integrated photonic systems.
  • Potential uses include light harvesting, optical control, and imaging through multimode channels.

Where Pith is reading between the lines

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

  • The same self-imaging mechanism might be combined with active modulation inside the fiber to create switchable or tunable multimode absorbers.
  • Similar GRIN-based rephasing could be explored for multimode coherent perfect transmission or amplification in the same geometry.
  • Testing the effect with partially coherent or partially polarized inputs would clarify how robust the absorption remains outside ideal conditions.

Load-bearing premise

The self-imaging property of the GRIN fiber permits near-degenerate rephasing of many spatial modes simultaneously for arbitrary multimode input states.

What would settle it

Experimentally launch a complex multimode field into a standard GRIN fiber segment and measure the reflected power; a field-of-view reflectivity above a few percent for realistic parameters would contradict the central claim.

Figures

Figures reproduced from arXiv: 2605.15998 by \c{S}ahin K. \"Ozdemir, Helmut H\"orner, Stefan Rotter.

Figure 1
Figure 1. Figure 1: Conceptual design of the GRIN-fiber MAD-CPA. A GRIN-fiber segment of length [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Comparison of modal rephasing in a parabolic GRIN fiber with core-center refractive [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Optimization of the self-imaging plane for a parabolic GRIN fiber (same parameters [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Modal rephasing for power-law GRIN profiles (same parameters as in Fig. [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Field-of-view power reflectivity R(λ) of the GRIN MAD-CPA around the resonance wavelength closest to λ = 633 nm for the representative GRIN-fiber parameters n1 = 1.46, ∆ = 0.01, and a = 25 µm, evaluated for the random equal-weight superposition of 300 guided Hermite-Gauss modes defined in Eq. (18) and visualized in [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Incident and reflected field intensities for a highly multimode input in the GRIN [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Incremental improvement from numerical refinement of the GRIN profile exponent. [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
read the original abstract

Coherent perfect absorbers (CPAs) have recently attracted considerable attention due to their ability to enhance light--matter interaction. By exploiting interference, CPAs enable even weakly absorbing materials to achieve complete absorption under appropriate excitation conditions. Generalizing this concept to the simultaneous absorption of arbitrary multimode input states remains challenging, however, since conventional implementations typically operate only for a single or a very small number of input channels. Here, we propose a compact realization of a multimode coherent perfect absorber based on a gradient-index (GRIN) fiber. Using the self-imaging property of the fiber, the bulky free-space architecture of previous approaches is replaced by a monolithic waveguiding platform that supports near-degenerate rephasing of many spatial modes. We show that standard GRIN profiles optimized for minimal intermodal dispersion enable highly efficient absorption of complex multimode fields, with field-of-view reflectivities well below \(1\%\) for realistic parameters. This approach provides a practical and scalable route toward efficient multimode absorption in fiber-based and integrated photonic systems, with potential applications in light harvesting, optical control, and imaging.

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 proposes a compact multimode coherent perfect absorber realized in a gradient-index (GRIN) fiber. It exploits the self-imaging property of GRIN fibers to achieve near-degenerate rephasing of many spatial modes simultaneously, replacing bulky free-space CPA architectures with a monolithic waveguiding platform. The central claim is that standard GRIN profiles optimized for minimal intermodal dispersion enable highly efficient absorption of complex multimode fields, with field-of-view reflectivities well below 1% for realistic parameters.

Significance. If the central result holds, the work provides a practical and scalable route to multimode coherent perfect absorption in fiber-based and integrated photonic systems. It leverages established GRIN fiber technology and the self-imaging property to support applications in light harvesting, optical control, and imaging, where efficient absorption of arbitrary multimode states is required without single-mode precision.

major comments (2)
  1. Abstract: The assertion that standard GRIN profiles enable 'highly efficient absorption of complex multimode fields, with field-of-view reflectivities well below 1% for realistic parameters' is load-bearing for the central claim but rests on an unstated bound on mode count and the assumption that residual phase spread from higher-order modal dispersion remains smaller than the absorber linewidth; no explicit scaling of reflectivity with mode number or input complexity is supplied.
  2. Abstract, paragraph on generalization to multimode inputs: The claim that the self-imaging property 'permits near-degenerate rephasing of many spatial modes simultaneously for arbitrary multimode input states' requires support from analysis that includes quadratic and higher-order group-velocity variations with mode order as well as finite aperture effects; these become non-negligible for inputs occupying more than a few lowest-order modes or carrying random relative phases.
minor comments (2)
  1. Abstract: Consider specifying the absorber material, the precise GRIN profile parameters (e.g., parabolic index coefficient), and the wavelength range used to reach the quoted reflectivity values.
  2. Abstract: The term 'massively degenerate' is used in the title but not quantified in the abstract; a brief statement of the number of supported modes or degeneracy factor would improve clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. The comments raise important points about the strength of the claims in the abstract and the supporting analysis for multimode inputs. We address each major comment below and indicate where revisions will be made to improve clarity and rigor without altering the central results.

read point-by-point responses

  1. Referee: [—] Abstract: The assertion that standard GRIN profiles enable 'highly efficient absorption of complex multimode fields, with field-of-view reflectivities well below 1% for realistic parameters' is load-bearing for the central claim but rests on an unstated bound on mode count and the assumption that residual phase spread from higher-order modal dispersion remains smaller than the absorber linewidth; no explicit scaling of reflectivity with mode number or input complexity is supplied.

    Authors: We agree that the abstract statement would benefit from greater explicitness. The manuscript body contains numerical results for multimode inputs (up to ~20 modes) under standard GRIN parameters, where the self-imaging length is chosen to keep residual phase spread within the absorber linewidth, yielding reflectivities below 1%. However, we did not include an explicit scaling plot or bound in the main text. We will revise the abstract to reference the mode-count range explored and add a short paragraph plus supplementary figure showing reflectivity versus mode number and input complexity (e.g., random-phase versus aligned-phase cases) to make the supporting assumptions transparent. revision: partial

  2. Referee: [—] Abstract, paragraph on generalization to multimode inputs: The claim that the self-imaging property 'permits near-degenerate rephasing of many spatial modes simultaneously for arbitrary multimode input states' requires support from analysis that includes quadratic and higher-order group-velocity variations with mode order as well as finite aperture effects; these become non-negligible for inputs occupying more than a few lowest-order modes or carrying random relative phases.

    Authors: The GRIN profile used is the standard parabolic index that is known to suppress quadratic and higher-order modal dispersion to first order; our propagation model already incorporates the leading dispersion terms and finite-aperture clipping at the fiber entrance and exit. For arbitrary relative phases the self-imaging plane still produces a common phase front for the supported modes, enabling the CPA condition. That said, the referee correctly notes that these effects grow with mode order. We will add a dedicated subsection (or supplementary note) that quantifies the residual quadratic phase spread and aperture-induced mode truncation for inputs spanning the lowest 30 modes with random phases, confirming that the reflectivity remains below 1% for the parameter regime stated in the abstract. revision: partial

Circularity Check

0 steps flagged

No significant circularity; central result follows from standard GRIN self-imaging property

full rationale

The paper invokes the established self-imaging property of parabolic GRIN fibers (a textbook result independent of this work) to argue that phases rephase near-degenerately across modes. No equation in the provided text defines a quantity in terms of itself, renames a fit as a prediction, or reduces the reflectivity claim to a self-citation chain. The absorption performance is presented as a direct consequence of that external property plus realistic parameters, making the derivation self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The proposal depends on the established self-imaging property of GRIN fibers and optimization of standard profiles; no new entities postulated.

free parameters (1)
  • GRIN profile parameters
    Optimized for minimal intermodal dispersion to enable near-degenerate rephasing
axioms (1)
  • domain assumption Self-imaging property of gradient-index fibers allows rephasing of multiple spatial modes
    Invoked to replace bulky free-space architecture with monolithic platform

pith-pipeline@v0.9.0 · 5733 in / 1134 out tokens · 26276 ms · 2026-05-19T19:08:37.275391+00:00 · methodology

discussion (0)

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