Chapter 18: Free-space Optical Communication Links

Olivier Bouchet, Herv Sizun, France T l com R&D, Cesson-Sevigne and France T l com R&D,

Belfort , France

1 Introduction

Today the wireless numerical data transmission world market is based primarily on Hertzian type technologies. All the futurologies of large industrialised countries show some limit to this technology. They will not be able to absorb the growing development of the requirements in new transmission channels for cordless phone, networks data-processing, high definition television, communicating objects etc. Several techniques are available: microwaves , links at frequencies higher than 100 GHz and free-space optical communication links.

This last technique uses in free-space the modulation of an infrared laser beam to exchange binary data full duplex by the intermediary of a Transmitter/Receiver couple. Several factors determine the revival of this technique: absence of regulation, free licencing, easy, fast and inexpensive deployment, high data rates.

In this contribution we shall treat systems and propagation aspects, including restrictive effects. The aspect system relates to apparatuses laser safety, results of a survey of manufacturers as well as an analysis of their uses. The performance evaluation of such links passes by a knowledge of the atmospheric effects on the propagation in the used frequency spectrum. Fog, rain, and snow in particular constitute very restrictive elements in their use.