In recent years, free space optical communication has gained significant importance owing to its unique features: large bandwidth, license-free spectrum, high data rate, easy and quick deployability, less power and low mass requirements. FSO communication uses the optical carrier in the near infrared band to establish either terrestrial links within the Earth's atmosphere or inter-satellite or deep space links or ground-to-satellite or satellite-to-ground links. However, despite the great potential of FSO communication, its performance is limited by the adverse effects viz., absorption, scattering, and turbulence of the atmospheric channel. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of the paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. The latter part of the paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers i.e., link, network or transport layer to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of the optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high-capacity and low-cost backhaul solutions.

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Optical Communication in Space: Challenges and Mitigation Techniques

Depuis les annees 50, la recuperation de la phase d’un faisceau optique diffracte par un objet quelconque, est un sujet important dans plusieurs domaines scientifiques, comme la microscopie, l’astronomie et bien d’autres. Generalement, les methodes qui le permettent se divisent en deux grandes categories : les methodes interferometriques et les methodes iteratives basees sur la propagation du faisceau. L’interet de ces dernieres, reside dans le fait qu’elles sont moins sensibles au bruit, et leur implementation experimentale est plus simple. Aussi, le developpement des techniques informatiques a rendu cette approche plus rapide et plus interessante. Cependant, meme si l’efficacite de ces methodes a ete demontree dans plusieurs domaines, leur utilisation est restee limitee a cause de certaines exigences sur les conditions experimentales, et a la non-convergence de leur algorithme vers une solution unique dans un grand nombre de cas. Ceci est encore plus vrai pour les objets dits "objets complexes", possedant une amplitude et une phase, ce qui reduit fortement leur champ d’application. Afin de surmonter ces problemes de convergence, diverses strategies experimentales ont ete developpees. Elles ont toutes comme principe d’introduire de nouvelles contraintes bien connues dans le plan de l’objet. Cela permet d’augmenter le nombre de spectres acquis, et donc accroitre et diversifier les sources d’informations sur l’objet de base, ce qui va aider l’algorithme iteratif a converger plus rapidement vers une solution finale et unique. Comme exemple de ces strategies experimentales, on peut acquerir plusieurs spectres provenant de differentes zones de l’objet, ou moduler la longueur d’onde du faisceau incident, ou meme enregistrer les spectres dans des plans paralleles, connectes entre eux par la transformee de Fresnel. Dans ce contexte, le present travail vise a demontrer experimentalement une technique connue sous SSPR (Spread Spectrum Phase Retrieval), propose en 2007 par Zhang, tout en lui introduisant un certain nombre de modifications, afin de la rendre plus pratique. L’idee consiste a moduler le front d’onde de l’objet par M phases aleatoires, generees avec un modulateur spatial de lumiere a base de cristaux liquides (LC-SLM), puis enregistrer dans le plan de Fourier les M spectres correspondants. Ces M spectres seront ensuite utilises dans un algorithme iteratif permettant de remonter au front d’onde de l’objet initial, en simulant la propagation du front d’onde entre les deux espaces, spatial et frequentiel.La premiere partie de cette these comporte une etude detaillee sur les modulateurs spatiaux de lumiere, afin de pouvoir choisir le mieux adapte a notre application. Une fois que le modulateur a base de cristaux liquides (LC-SLM) est selectionne, on presentera ses caracteristiques techniques, ainsi que les tests et les etapes de calibrations necessaires pour assurer son fonctionnement lineaire et optimal. Ensuite, on montrera plusieurs types d’applications possibles avec ce composant, et dans divers domaines scientifiques, comme l’holographie, la microscopie, l’optique adaptative ainsi que les methodes interferometriques permettant de reconstruire la phase d’un faisceau lumineux. Dans la deuxieme grande partie, on concentre notre travail autour de la methode iterative SSPR. On montrera comment on peut rendre l’application de cette methode plus simple en utilisant un modulateur spatial de lumiere a base de cristaux liquides, et en travaillant dans le plan de Fourier a la place du plan de Fresnel. Cependant, apres avoir applique experimentalement cette methode, on remarque que les resultats obtenus sont tres mauvais par rapport aux resultats des simulations. On effectue donc, une etude detaillee concernant les sources de bruits pouvant etre responsable de la degradation de la qualite des reconstructions obtenues. [...]

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Optique adaptative par modulateur spatial de lumière en microscopie et holographie

A free-space optical communication channel suffers degraded performance due to blurring and scintillation of the received signal caused by atmospheric turbulence. Adaptive optics (AO) improves the communication performance of such a channel by concentrating the received power on the detector. The degree of improvement with AO correction depends on the modulation format, and on the modulation order when pulse position modulation is utilized. Gains of up to 6 dB with AO have been experimentally validated in a laboratory test bed under simulated atmospheric conditions involving turbulence and background light. The fade statistics of the turbulent atmospheric channel have also been analyzed with and without AO correction.

Improved optical communications performance combining adaptive optics and pulse position modulation

The optical ground station (OGS), installed in the Teide Observatory since 1995, was built as part of ESA efforts in the research field of satellite optical communications to test laser telecommunication terminals on board of satellites in Low Earth Orbit and Geostationary Orbit As far as one side of the link is settled on the Earth, the laser beam (either on the uplink or on the downlink) has to bear with the atmospheric turbulence Within the framework of designing an adaptive optics system to improve the performance of the free-space optical communications at the OGS, turbulence conditions regarding uplink and downlink have been simulated within the object-oriented matlab adaptive optics toolbox as well as the possible utilization of a laser guide star to measure the wavefront in this context to cope with the angular difference in uplink and downlink beams due to the point-ahead angle required for the uplink Simulations have been carried out by reducing available atmospheric profiles regarding both night-time and day-time measurements and by having into account possible seasonal changes An AO proposal to reduce atmospheric aberrations and, therefore, ameliorate FSO links performance is presented and analyzed in this paper

Toward the uplink correction: application of adaptive optics techniques on free-space optical communications through the atmosphere

We present predicted and experimentally measured gains in communications performance of a laboratory-based, free-space optical communications system through the use of adaptive optics (AO). A commercially available avalanche photodiode detector (APD) is used in the receiver for signal detection. Background noise and atmospheric turbulence field conditions were simulated in the laboratory using an integration sphere and a specially designed heater. At an uncoded bit-error rate (BER) of 0.3, a 5- to 6-dB gain in the received signal power is shown when AO correction is applied in the presence of high background and turbulence. The data stream was a 100-megabits per second (Mbps) pseudo-random bit sequence (PRBS), on–off-keying (OOK)-modulated, 1064-nm laser pulse train. I. Introduction Free-space optical communication through atmospheric turbulence suffers due to distortion of the signal field distribution in the detector focal plane and the spreading of the signal intensity. These aberrations result in spot sizes in the focal plane that are several times diffraction limited for the aperture and require larger photodetectors to capture the required number of signal photons for the desired link performance. For uniformly distributed sky background intensity, the larger detector collects proportionally more background photons and results in degraded link performance. In general, larger-area detectors limit the data rate realizable due to the lower bandwidths and can contribute to temporal distortion of the received signal. By using adaptive optics (AO) techniques, the atmosphere-induced wavefront aberrations can be corrected, and near-diffraction-limited focused spot sizes can be realized. This would enable the use of smaller, high-speed detectors with better spatial discrimination against the unwanted background noise.

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Improved Optical Communications Performance Using Adaptive Optics with an Avalanche Photodiode Detector

The deep space optical communications subsystem offers a higher bandwidth communications link in smaller size, lower mass, and lower power consumption subsystem than does RF. To demonstrate the benefit of this technology to deep space communications NASA plans to launch an optical telecommunications package on the 2009 Mars Telecommunications orbiter spacecraft. Current performance goals are 30-Mbps from opposition, and 1-Mbps near conjunction (-3 degrees Sun-Earth-Probe angle). Yet, near conjunction the background noise from the day sky will degrade the performance of the optical link. Spectral and spatial filtering and higher modulation formats can mitigate the effects of background sky. Narrowband spectral filters can result in loss of link margin, and higher modulation formats require higher transmitted peak powers. In contrast, spatial filtering at the receiver has the potential of being lossless while providing the required sky background rejection. Adaptive optics techniques can correct wave front aberrations caused by atmospheric turbulence and enable near-diffraction-limited performance of the receiving telescope. Such performance facilitates spatial filtering, and allows the receiver field-of-view and hence the noise from the sky background to be reduced.

Keith Wilson

Papers

Daytime adaptive optics for deep space optical communications