arXiv subjects

Vincent Michau

Publications and source records attributed to Vincent Michau.

7 recordsLinked to original sources

arXiv2026

The vanishing distance: a practical range boundary for dynamic wavefront shaping

Wavefront shaping (WFS) is a powerful modern method to control light propagation through scattering media, with applications ranging from biomedical imaging via cryptography to free-space optical communication. While WFS has been highly successful in static or slowly evolving media, its practical relevance in dynamic and extended scattering environments remains difficult to assess. In particular, no simple criterion currently identifies the propagation distance beyond which the residual ballistic component is no longer distinguishable from the scattered background. Here, we introduce the vanishing distance, defined as the propagation length where the ballistic power equals that carried by a single diffuse mode. Unlike the transport mean free path, which characterizes angular randomization of the scattered field, the vanishing distance identifies the loss of the ballistic channel on a modal-power basis. Beyond this distance, transmission or imaging can no longer rely on the ballistic component and must instead exploit the scattered field. We present a simple semi-analytical model, validated against Monte Carlo simulations and experiments in both monodisperse and heterogeneous scattering media, allowing the vanishing distance and the associated minimum source power to be estimated. Across representative scattering environments, the transition is found to occur typically after about 30-40 scattering mean free paths. These results provide a simple guideline for assessing the practical relevance of dynamic wavefront shaping.

physics.optics↗

arXiv2026

Analysis of a compact interferometric imager

The advent of photonic integrated circuits (PICs) will allow the replacement of the large aperture of an optical telescope by a dense array of small apertures combined interferometically. The light coming from aperture pairs can be combined by a PIC in order to extract interferogram characteristics known as complex visibilities, from which the observed object can then be reconstructed. In such a compact interferometric imager, the optical components dedicated to image formation in a regular telescope are no longer necessary. In particular, such a concept is relevant for space missions where weight and size are critical. In this communication, we study such an instrument concept, focusing on signal-to-noise considerations. We recall the design basis for the field and the spatial resolution, and we show that the spectral resolution must be no less than the field to resolution ratio. Then, we analyze the signal-to-noise ratio of this concept, assuming that each spatial frequency is recorded only once, and compare the signal-to-noise ratio with that of a monolithic telescope. We perform the comparison in Fourier space for an identical number of recorded photons. We show that the noise propagation of the interferometric imager is identical to that of a monolithic telescope that would have a flat Modulation Transfer Function with a level roughly given by the ratio of the small apertures' diameter to the maximum baseline. We conclude that the noise propagation in low and medium spatial frequencies is unfavorable for the interferometric imager.

physics.optics↗

arXiv2026

Nanosecond wavefront shaping to focus through agitated turbid media

Multiple scattering rapidly scrambles optical fields in fog, snow and turbid water, causing op- timized wavefront corrections to become obsolete on microsecond timescales. Although wavefront shaping enables focusing through static scattering layers, closed-loop control in dynamically evolving media has remained experimentally challenging because the correction bandwidth must approach the intrinsic decorrelation rate of the speckle. Here, we demonstrate closed-loop wavefront shaping with 32 independent degrees of freedom in an agitated turbid medium exhibiting sub-microsecond decorrelation. The medium thickness exceeds the transport mean free path, meaning the far-field speckle autocorrelation is limited to a diffraction-sized grain. Despite the microsecond decorrelation and this multiple-scattering regime, stable focusing is maintained as the correction cycle approaches the intrinsic dynamics of the medium. These results establish an experimentally accessible regime for coherent wave control in rapidly evolving complex media.

physics.optics↗

arXiv2024

A Silicon Photonic 32-Input Coherent Combiner for Turbulence Mitigation in Free Space Optics Links

A photonic integrated circuit (PIC) for the coherent combination of 32 input optical signals into a single output fiber is reported. The PIC was fabricated using a low-loss thick silicon-on-insulator (SOI) process and packaged with 32 input and 1 output fibers. The basic building block is a 2x2 Mach-Zehnder interferometer (MZI) with an external (to the MZI branches) and an internal thermal phase shifter, and a bandwidth in excess of 80 kHz. The PIC monolithically integrates 31 MZIs and 31 germanium photodetectors, and is suitable in principle for turbulence mitigation in LEO-ground and horizontal free space optics links. Improvements to the device for the coherent combination of 64 inputs and for the reduction of insertion losses are also discussed

physics.optics↗

arXiv2018

James Webb Space Telescope Optical Simulation Testbed V: Wide-field phase retrieval assessment

The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a hardware simulator for wavefront sensing and control designed to produce JWST-like images. A model of the JWST three mirror anas- tigmat is realized with three lenses in the form of a Cooke triplet, which provides JWST-like optical quality over a field equivalent to a NIRCam module. An Iris AO hexagonally segmented mirror stands in for the JWST primary. This setup successfully produces images extremely similar to expected JWST in-flight point spread functions (PSFs), and NIRCam images from cryotesting, in terms of the PSF morphology and sampling relative to the diffraction limit. The segmentation of the primary mirror into subapertures introduces complexity into wavefront sensing and control (WFS&C) of large space based telescopes like JWST. JOST provides a platform for independent analysis of WFS&C scenarios for both commissioning and maintenance activities on such ob- servatories. We present an update of the current status of the testbed including both single field and wide-field alignment results. We assess the optical quality of JOST over a wide field of view to inform the future imple- mentation of different wavefront sensing algorithms including the currently implemented Linearized Algorithm for Phase Diversity (LAPD). JOST complements other work at the Makidon Laboratory at the Space Telescope Science Institute, including the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed, that investigates coronagraphy for segmented aperture telescopes. Beyond JWST we intend to use JOST for WFS&C studies for future large segmented space telescopes such as LUVOIR.

astro-ph.IM↗

arXiv2014

Laser beam complex amplitude measurement by phase diversity

The control of the optical quality of a laser beam requires a complex amplitude measurement able to deal with strong modulus variations and potentially highly perturbed wavefronts. The method proposed here consists in an extension of phase diversity to complex amplitude measurements that is effective for highly perturbed beams. Named CAMELOT for Complex Amplitude MEasurement by a Likelihood Optimization Tool, it relies on the acquisition and processing of few images of the beam section taken along the optical path. The complex amplitude of the beam is retrieved from the images by the minimization of a Maximum a Posteriori error metric between the images and a model of the beam propagation. The analytical formalism of the method and its experimental validation are presented. The modulus of the beam is compared to a measurement of the beam profile, the phase of the beam is compared to a conventional phase diversity estimate. The precision of the experimental measurements is investigated by numerical simulations.

physics.optics↗

arXiv2012

Accurate measurement of Cn2 profile with Shack-Hartmann data

The precise reconstruction of the turbulent volume is a key point in the development of new-generation Adaptive Optics systems. We propose a new Cn2 profilometry method named CO-SLIDAR (COupled Slope and scIntillation Detection And Ranging), that uses correlations of slopes and scintillation indexes recorded on a Shack-Hartmann from two separated stars. CO-SLIDAR leads to an accurate Cn2 retrieval for both low and high altitude layers. Here, we present an end-to-end simulation of the Cn2 profile measurement. Two Shack-Hartmann geometries are considered. The detection noises are taken into account and a method to subtract the bias is proposed. Results are compared to Cn2 profiles obtained from correlations of slopes only or correlations of scintillation indexes only.

astro-ph.IM↗