Marie-Thérèse Velluet

NOISE PROPAGATION IN WAVE-FRONT SENSING WITH PHASE DIVERSITY

The phase diversity technique is studied as a wave-front sensor to be implemented with widely extended sources. The wave-front phase expanded on the Zernike polynomials is estimated from a pair of images (in focus and out of focus) by use of a maximum-likelihood approach. The propagation of the photon noise in the images on the estimated phase is derived from a theoretical analysis. The covariance matrix of the phase estimator is calculated, and the optimal distance between the observation planes that minimizes the noise propagation is determined. The phase error is inversely proportional to the number of photons in the images. The noise variance on the Zernike polynomials increases with the order of the polynomial. These results are confirmed with both numerical and experimental validations. The influence of the spectral bandwidth on the phase estimator is also studied with simulations.

First results of wavefront sensing on SOTA

For satellite to ground laser links, atmospheric turbulence is a major cause of impairments. The induced phase perturbations along the propagation path cause beam scintillation in the receiver plane and they can also severely compromise the coupling of the flux into a receiver of limited size. To address these impairments, dedicated mitigation strategies must be developed. This requires accurate understanding of the perturbation origin. Beam propagation models have demonstrated their ability to reproduce statistical characteristics of optical perturbations on a satellite to ground laser link for elevations as low as 20°. For smaller elevations, measurements performed on stars illustrated the limits of analytical approaches and the interest for end-to-end models. We report here the first propagation channel measurements performed on a LEO microsatellite with a Shack-Hartmann wavefront sensor (WFS). The laser beam at 976 nm provided by SOTA optical terminal have been analyzed with a Shack- Hartmann wavefront sensor located at Coudé focus of the French ground station (1,55 m MéO telescope) in July 2015. Wavefront characteristics and scintillation patterns recorded with the WFS are analyzed and compared to atmospheric turbulence perturbations model fed with in situ measurements of atmospheric parameters retrieved from GDIMM.

Mitigation of atmospheric effects by adaptive optics for free-space optical communications

Data-rates of long-range free-space optical communication links are deteriorated by atmospheric turbulence which causes power in the bucket fluctuations. In order to compensate for those effects the use of adaptive optics is envisioned. Different solutions have been proposed for the correction. We study here the performances of several compensation methods, encompassing both amplitude and phase and phase-only precompensation. In the case of phase-only precompensation we studied two system designs, one which is dedicated to symmetrical communication systems and the other to dissymmetric systems. In the dissymmetric case we studied two ways of driving the deformable mirror: the use of a Shack-Hartmann wavefront sensor and a model-free phase modulation. For each compensation architecture simulation results covered weak, moderate and strong turbulence conditions.

Experimental demonstration of the full-wave iterative compensation in free space optical communications

Long-range free space optical communications suffer from atmospheric turbulence effects. To mitigate them, a bidirectional full-wave compensation technique seems promising. We present an experimental implementation and characterization of this concept on a laboratory breadboard. Experimental results confirm former numerical results for similar propagation conditions. The effects of measurement and control errors are analyzed by numerical modeling.

Towards experimental validation of full-wave precompensation for laser telecommunications

We designed an optical bench to demonstrate full-wave precompensation for laser telecommunications. This technique requires a device performing time reversed waves. We propose and characterize a solution to realize such a function.

Automatic alignment system for telescopes

From the local wavefront slope measurement given by a Shack Hartmann wavefront sensor (SH), it is possible to evaluate the decentering aberrations introduced by optical system misalignments. These slopes can be expressed as a function of the centered system aberrations and of the optical axis displacements in the image and pupil planes. After calibration of the wavefront sensor, the system alignment can be, for small aberrations, automatically controlled. The validity of the concept was proved on a representative observation system.