arXiv subjects

Nicolas Barré

Publications and source records attributed to Nicolas Barré.

10 recordsLinked to original sources

arXiv2026

Accuracy assessment of scalar wave propagation methods for diffractive optics design: from thin elements to thick binary grating

We present a systematic accuracy assessment of the thin-element approximation (TEA), the beam propagation method (BPM), and the wave propagation method (WPM) for binary diffractive gratings, using the rigorous Fourier modal method (FMM) as a reference. Random binary gratings are generated over a range of spatial frequency cutoffs and thicknesses, and the transmitted field overlap between each scalar method and the reference is measured. The results are summarized as accuracy maps in the spatial frequency-thickness parameter space, revealing the domain of validity of each method and providing practical guidelines for the choice of forward model in diffractive optics inverse design pipelines.

physics.optics↗

arXiv2026

Maximally localized modes of a multimode fiber

This article presents an optimization method to find the most spatially concentrated basis of a multimode fiber, obtained by minimizing the sum of the spatial spreads of the individual modes over all unitary transformations of a given orthonormal mode set. The resulting modes are the optical analogue of maximally localized Wannier functions in solid-state physics. We apply the method to the Laguerre-Gaussian basis of a graded-index fiber for mode counts ranging from 6 to 55. In all cases, the modes spontaneously organize into concentric rings without any geometric constraint being imposed. The spot sizes and ellipticities evolve from one ring to the next in ways that geometric packing approaches cannot predict. For large mode counts, the optimizer finds solutions where neither the number of spots per ring nor the spots within a given ring follow a regular pattern, indicating that the fully symmetric arrangement is no longer a minimum of the spread functional. A constrained variant of the method enables the optimizer to target any prescribed bundle geometry while quantifying its localization cost, opening a route to physically grounded photonic lantern design.

physics.optics↗

arXiv2026

Inverse design of a spatial demultiplexer for free-space optical communications: direct optimization over turbulence statistics

Atmospheric turbulence severely limits the coupling of received optical wavefronts into single-mode fibers in satellite-to-ground free-space optical links. Spatial demultiplexing receivers address this challenge by distributing the incoming field across a bundle of single-mode fibers whose outputs are recombined coherently, relaxing the requirements on wavefront correction. In this work, we investigate the design of such receivers from two complementary angles. We first compare the power coupling statistics achieved by several modal bases and show that the spatial support of the modes matters far more than the specific choice of basis, questioning the relevance of mode-selective approaches for this application. We then present the inverse design of a compact two-plane refractive system optimized directly over an ensemble of turbulence realizations using stochastic gradient descent, with no constraint imposed on the input modal decomposition. The optimized system significantly improves over direct coupling into the fiber bundle, approaches the performance of an ideal modal projection, and remains competitive across a broad range of turbulence conditions.

physics.optics↗

arXiv2023

Sensorless wavefront correction in two-photon microscopy across different turbidity scales

Adaptive optics (AO) is a powerful tool to increase the imaging depth of multiphoton scanning microscopes. For highly scattering tissues, sensorless wavefront correction techniques exhibit robust performance and present a straight-forward implementation of AO. However, for many applications such as live-tissue imaging, the speed of aberration correction remains a critical bottleneck. Dynamic Adaptive Scattering compensation Holography (DASH) -- a fast-converging sensorless AO technique introduced recently for scatter compensation in nonlinear scanning microscopy -- addresses this issue. DASH has been targeted at highly turbid media, but to-date it has remained an open question how it performs for mild turbidity, where limitations imposed by phase-only wavefront shaping are expected to impede its convergence. In this work, we study the performance of DASH across different turbidity regimes, in simulation as well as experiments. We further provide a direct comparison between DASH and a novel, modified version of the Continuous Sequential Algorithm (CSA) which we call Amplified CSA (a-CSA).

physics.optics↗

arXiv2023

Polarization-Controlled Nonlinear Computer-Generated Holography

Dynamic phase-only beam shaping with a liquid crystal spatial light modulator is a powerful technique for tailoring the beam's intensity profile or wave front. While shaping and controlling the light field is a highly researched topic, dynamic nonlinear beam shaping has hardly been explored so far. One potential reason is that generating the second harmonic is a degenerate process as it mixes two fields at the same frequency. To overcome this problem, we propose the use of type II phase matching as a control mechanism to distinguish the two involved fields. We experimentally demonstrate that arbitrary intensity distributions can be shaped in the frequency-converted field at the same quality as for linear beam shaping and with conversion efficiencies similar to the case without beam shaping. We envision this technique as a milestone towards beam shaping beyond the physical limits of liquid crystal displays, i.e. facilitating dynamic phase-only beam shaping in the ultraviolet spectral range.

physics.optics↗

arXiv2022

Direct laser written aperiodic photonic volume elements for complex light shaping with high efficiency: inverse design and fabrication

Light plays the central role in many applications. The key to unlocking its versatility lies in shaping it into the most appropriate form for the task at hand. Specifically tailored refractive index modifications, directly manufactured inside glass using a short pulsed laser, enable an almost arbitrary control of the light flow. However, the stringent requirements for quantitative knowledge of these modifications, as well as for fabrication precision, have so far prevented the fabrication of light-efficient aperiodic photonic volume elements (APVEs). Here we present a powerful approach to the design and manufacturing of light-efficient APVEs. We optimize application-specific 3D arrangements of hundred thousands of microscopic voxels and manufacture them using femtosecond direct laser writing inside millimeter-sized glass volumes. We experimentally achieve unprecedented diffraction efficiencies up to 80%, which is enabled by precise voxel characterization and adaptive optics during fabrication. We demonstrate APVEs with various functionalities, including a spatial mode converter and combined intensity shaping and wavelength-multiplexing. Our elements can be freely designed and are efficient, compact and robust. Our approach is not limited to borosilicate glass, but is potentially extendable to other substrates, including birefringent and nonlinear materials, giving a preview of even broader functionalities including polarization modulation and dynamic elements.

physics.optics↗

arXiv2021

Inverse design of gradient-index volume multimode converters

Graded-index optical elements are capable of shaping light precisely and in very specific ways. While classical freeform optics uses only a two-dimensional domain such as the surface of a lens, recent technological advances in laser manufacturing offer promising prospects for the realization of arbitrary three-dimensional graded-index volumes, i.e. transparent dielectric substrates with voxel-wise modified refractive index distributions. Such elements would be able to perform complex light transformations on compact scales. Here we present an algorithmic approach for computing 3D graded-index devices, which utilizes numerical beam propagation and error reduction based on gradient descent. We present solutions for millimeter-sized elements addressing important tasks in photonics: a mode sorter, a photonic lantern and a multimode intensity beam shaper. We further discuss suitable cost functions for all designs to be used in the algorithm. The 3D graded-index designs are spatially smooth and require a relatively small refractive index range in the order of $10^{-2}$, which is within the reach of direct laser writing manufacturing processes such as two-photon polymerization.

physics.optics↗

arXiv2021

Holographic beam shaping of partially coherent light

We present an algorithm for holographic shaping of partially coherent light, bridging the gap between traditional coherent and geometric optical approaches. The description of partially coherent light relies on a mode expansion formalism, with possibly thousands of individual modes, and the inverse-design optimization algorithm is based on gradient descent with the help of algorithmic differentiation. We demonstrate numerically and experimentally that an optical system consisting of two phase patterns can potentially achieve any intensity profile transformation with good accuracy.

physics.optics↗

arXiv2018

Beam shaping algorithm with optimization parameters

In this article, we present a general mode-conversion algorithm allowing to build an optical system composed of an alternation of phase masks and free propagations. The originality of our approach lies in the introduction of free parameters that can be used to perform an optimization regarding some properties of interest. Here, we apply this algorithm to overlap balancing, for the conversion of an important number of modes with limited phase resources. Moreover, we discuss about the convergence and the limitations of such an algorithm, notably concerning the loss of unitarity that tends to occur in a practical implementation.

physics.comp-ph↗

arXiv2010

Sawja: Static Analysis Workshop for Java

Static analysis is a powerful technique for automatic verification of programs but raises major engineering challenges when developing a full-fledged analyzer for a realistic language such as Java. This paper describes the Sawja library: a static analysis framework fully compliant with Java 6 which provides OCaml modules for efficiently manipulating Java bytecode programs. We present the main features of the library, including (i) efficient functional data-structures for representing program with implicit sharing and lazy parsing, (ii) an intermediate stack-less representation, and (iii) fast computation and manipulation of complete programs.

cs.PL↗