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Nicolas Védrenne

Publications and source records attributed to Nicolas Védrenne.

4 recordsLinked to original sources

RAMA: A new agile AO bench for the telescope FEELINGS

The ground-based observation of extended objects such as satellites suffer from severe atmospheric propagation constraints. Specifically, high tracking velocities and low-elevation lines of sight generate non-stationary turbulence alongside strong scintillation. Developing robust wavefront control strategies is therefore critical to maintain stable observations. In this context, we present RAMA, an adaptive optics (AO) testbench deployed on ONERA's 60\,cm FEELINGS telescope. Designed as a pathfinder for future systems like the PROVIDENCE ground station, RAMA evaluates a visible, non-modulated Pyramid Wavefront Sensor (PWFS). The hardware baseline also includes two pupil-conjugated deformable mirrors (DM97 and DM192) driven by the DAO Real-Time Computer (RTC). Inheriting the modular and evolving philosophy of the PAPYRUS project, the bench provides a flexible environment to test new components on-sky and allows for direct comparisons between classical controllers and advanced, data-driven strategies. By implementing Convolutional Neural Networks (CNN) for phase reconstruction and Reinforcement Learning (RL) for loop control, RAMA aims to overcome the specific limitations associated with scintillation and extended-object observations. This paper details the opto-mechanical design, numerical simulations of the bench expected wavefont control performance, preliminary laboratory closed-loop results and the first on-sky optical coupling with the telescope.

astro-ph.IM↗

Space-Ground Coherent Optical links: Ground Receiver Performance With Adaptive Optics and Digital Phase-Locked Loop

In the framework of optical high data rate satellite-to-ground links, we investigate the performance of a coherent receiver which combines an adaptive optics system and a digital carrier synchronization technique. We propose a digital carrier synchronization system based on a phase-locked loop and use end-to-end simulations to characterize the impact of the atmospheric turbulence in the case of a representative low Earth orbit satellite-to-ground transmission link. The results show that classical digital phase-locked loop techniques can be a reliable solution to accurately compensate for the Doppler frequency shift between the incoming signal and the local oscillator at the ground station, in the presence of a preliminary coarse frequency estimation. Adaptive optics compensation enables the loop to lock after a few milliseconds and to accurately track the phase fluctuations. The bit-error-rate performance is not degraded by the digital loop but suffers from the residual signal amplitude fluctuations.

eess.SP↗

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↗

Cn2 profile from Shack-Hartmann data with CO-SLIDAR data processing

Cn2 profile monitoring usually makes use of wavefront slope correlations or of scintillation pattern correlations. Wavefront slope correlations provide sensitivity to layers close to the receiving plane. In addition, scintillation correlations allow a better sensitivity to high turbulence layers. Wavefront slope and scintillation correlations are therefore complementary. Slopes and scintillation being recorded simultaneously with a Shack-Hartmann wavefront sensor (SHWFS), we propose here to exploit their correlation to retrieve the Cn2 profile. The measurement method named COupled SLodar scIDAR (CO-SLIDAR) uses correlations of SHWFS data from two separated stars. A maximum-likelihood method is developed to estimate precisely the positions and intensities corresponding to each SHWFS spot, which are used as inputs for CO-SLIDAR. First results are presented using SHWFS real data from a binary star.

astro-ph.IM↗