The dissertation thesis deals with a free space optical links, especially is focused directly to the transmission optical beams. The influence of environmental conditions like atmospheric turbulence, buildings movement and vibration is described. As a possibility to reduce these undesirable influences, the generation of the transmission beam by the help of the two-mode optical fiber is proposed. A possibility to drive the shape of the resulting transmission beam by means of the change of power ration of individual modes in fiber is described. The coherence features of the proposed beam are analyzed considering the possibility to reduce the influence of atmospheric turbulence. In the thesis there are introduced beam quality parameters that enable to evaluate the quality of the optical beam for the free space optical link. These parameters give the relationship between the shape of the optical beam and the power balance of the optical link and the statistical evaluation of the optical link.

Most modern clock and data recovery circuits (CDR) are based on analog blocks that need to be redesigned whenever the technology process is to be changed. On the other hand, CDR based blind oversampling architecture (BO-CDR) can be completely designed in a digital process which makes its migration very simple. The main disadvantages of the BO-CDR that are usually mentioned in a literature are complexity of its digital circuitry and finite phase resolution resulting in larger jitter sensitivity and higher error rate. This thesis will show that those problems can be solved by designing a new algorithm of BO-CDR and subsequent optimization. For this task an FPGA was selected as simulation and verification platform. This enables to change parameters of the optimized circuit in real time while measuring on real links (unlike a simulation using inaccurate link models). The output of this optimization is a new BO-CDR algorithm with heavily reduced complexity and very low error rate. A new FPGA-based method of jitter measurement was developed (primary for CDR analysis), which enables a quick link characterization without using probing or additional equipment. The new method requires only a minimum usage of FPGA resources. Finally, new measurement equipment was developed to measure bit error distribution on FSO links to be able to develop a suitable error correction scheme based on ARQ protocol.

Au cours de la dernière décennie, les communications optiques en espace libre (FSO) ont pris de l’ampleur dans les deux domaines académiques et industriels. L’importance de FSO s’appuie sur la possibilité de faire un système de transmission économique et écologique avec un débit élevé et sans licence à l’opposition des systèmes de transmission radiofréquences (RF). Dans la plupart des travaux antécédents sur les systèmes multi-émetteurs, seulement les canaux décorrélés ont été considérés. Un canal décorrélé nécessite un espace suffisant entre les émetteurs. Cette condition devient difficile et non-réalisable dans certaines applications. Pour cette raison, nos études se focalisent sur les performances des codes à répétition RC (Repitition Codes) et les codes OSTBC (Orthogonal Space-Time Block Codes) dans des canaux log-normaux corrélés en utilisant une modulation d’intensité et une détection directe (IM/DD). En addition, les effets des différentes conditions météorologiques sur le taux d’erreur moyen (ABER) sont étudiés. Les systèmes FSO à multi-entrées/ multi-sorties MIMO (Multiple-Input Multiple-Output) avec une modulation SSK (Space Shift Keying) ont été abordés. Les résultats obtenus montrent que la SSK est supérieure aux RC avec une modulation d’impulsion (Multiple Pulse Amplitude Modulation) pour toute efficacité spectrale égale ou supérieure à 4 bit/s/Hz. Nous avons aussi analysé les performances d’un système à sauts multiples (Multi-Hop) et des relais à transmission directe (forward relays). Nos simulations montrent que le système ainsi considéré est efficace pour atténuer les effets météorologiques et les pertes géométriques dans les systèmes de communication FSO. Nous avons montré qu’un tel système avec plusieurs entrées et une sortie (MISO, i.e. multiple-input single-output) à sauts multiples est supérieur à un système MISO avec un lien direct (direct link) avec une forte atténuation. Pour satisfaire la demande croissante des réseaux de communication à débits élevés, la communauté scientifique s'intéresse de plus en plus aux systèmes FSO avec des relais full-duplex (FD). Pour ces derniers systèmes, nous avons étudié la probabilité d'erreur moyenne (ABER) et nous avons analysé leurs performances. En considérant des différentes conditions de transmission, les performances de relais FD ont été comparées à celles d'un système avec un lien direct ou des relais half-duplex. Les résultats obtenus montrent que les relais FD ont le minimum ABER. En conséquence, les résultats obtenus dans cette thèse sont très prometteurs pour la prochaine génération de FSO. / Free-space optical (FSO) communication has been the subject of ongoing research activities and commercial attention in the past few years. Such attention is driven by the promise of high data rate, license-free operation, and cheap and ecological friendly means of communications alternative to congested radio frequency communications. In most previous work considering multiple transmitters, uncorrelated channel conditions have been considered. An uncorrelated channel requires sufficient spacing between transmitters. However, this can be difficult and may not be always feasible in some applications. Thereby, this thesis studies repetition codes (RCs) and orthogonal space-time block codes performance in correlated log-normal FSO channels using intensity modulation and direct detection (IM/DD). Furthermore, the effect of different weather conditions on the average bit error rate (ABER) performance of the FSO links is studied. Multiple-input multiple-output (MIMO) FSO communication systems using space shift keying (SSK) modulation have been also analyzed. Obtained results show that SSK is a potential technique for spectral efficiencies equal or greater than 4 bits/s/Hz as compared to RCs with multiple pulse amplitude modulations. The performance analysis of a multi-hop decode and forward relays for FSO communication system using IM/DD is also considered in this thesis. It is shown that multi-hop is an efficient technique to mitigate atmospheric turbulence and different weather attenuation effects and geometric losses in FSO communication systems. Our simulation results show that multiple-input single-output (MISO) multi-hop FSO systems are superior to direct link and MISO systems over links exhibiting high attenuation. Meeting the growing demand for higher data rates communication networks, a system with full-duplex (FD) relays is considered. For such a system, the outage probability and the ABER performance are analyzed under different turbulence conditions, misalignment error and path loss effects. FD relays are compared with the direct link and half-duplex relays. Obtained results show that FD relays have the lowest ABER and the outage probability as compared to the two other systems. Finally, the obtained results in this thesis are very promising towards the next generation of FSO systems.

This master’s thesis deals with the impact of atmospheric turbulence on phase changes of a free space optical signal. This problematic is investigated by the interferometric method. A part of the thesis is focused on the phenomenon of atmospheric turbulence. Then the physical effect of interference is discussed and optical interferometers are described. The experimental part of the thesis is focused on measurement of phase shift of optical signal by interferometric method. The result of the thesis is to determine how phase shift of an optical beam depends on the strength of turbulence.

[pt] Este trabalho apresenta o estudo de comunicação óptica quântica no infravermelho, utilizando um sistema híbrido fibra-óptica – espaço-livre, como prova de princípio para o estabelecimento de chaves secretas a fim de utilização em criptografia do tipo one-time pad. Ao modular a polarização da luz de um laser em polarizações ortogonais, podem-se codificar os bits clássicos 1 e 0 em cada uma dessas polarizações, sendo detectadas por detectores contadores de fótons únicos, e, assim, utilizar o canal quântico para transmissão dos bits quânticos entre dois interlocutores, utilizandoos para o estabelecimento da chave criptográfica, que pode ser usada em qualquer tipo de informação a ser transmitida por um canal clássico ou quântico. Ao realizar a transmissão em espaço-livre, sujeita a variações climáticas, como temperatura atmosférica, luz solar, presença de nuvens, chuva e vento, foi também estudada a influência destes fenômenos na qualidade da transmissão e dos dados obtidos. Os resultados experimentais demonstraram consistência com a teoria e com outros trabalhos publicados na área até esta data com relação às taxas de erro de bit quântico e também à taxa de transmissão de bits. As taxas de erro obtidas, por estarem abaixo do limiar teórico para segurança da informação em comunicação quântica, provam, ainda, a possibilidade de estabelecimento de chave secreta para criptografia através do uso de distribuição quântica das chaves (QKD). Os resultados também apresentaram boa qualidade da informação recuperada após a descriptografia. / [en] This work presents the study of optical quantum communication in the infrared region, using a hybrid optical-fiber – free-space system, as proof of principle for the agreement on secret keys by two parties for use in one-time pad encryption. By modulating the polarization of laser light into orthogonal polarizations, one can encode the classic bits 1 and 0 in each of these polarizations, being detected by single photon counter detectors, and can use the quantum channel to transmit the quantum bits between two interlocutors. It is then possible to use those bits to establish the cryptographic key, which can be used in any type of information to be transmitted by a classic or quantum channel. While carrying out transmission in free space optics, subject to climatic variations, such as atmospheric temperature, sunlight, presence of clouds and rain, and the presence of wind, the influence of these phenomena on the quality of transmission and on the data obtained was also studied. The experimental results showed consistency with the theory and with other works published to date with regard to quantum bit error rates and to the bit rate. The error rates obtained, being below the theoretical threshold for information security in quantum communication, further proves the possibility of establishing a secret key for encryption through the use of quantum key distribution (QKD). It also presented good quality on the information recovered after decryption.