Optical communications : design and validation

Decades before its arrival at subscribers' homes, fiber optics has established itself in the shadow of the Internet, to the point of becoming its strongest pillar. But around this fiber must be built an optical telecommunications system with an often complex architecture.

This article focuses on "terrestrial" systems. Terrestrial systems are the communication arteries that link the major cities of a given country or continent, over distances of tens to thousands of kilometers. The optical signals propagating on these systems must be regenerated regularly (typically 80 km) at regeneration sites, and can then be routed to their destination at optical routing sites.

We strongly encourage the reader to take a look at the introductory article [E 7 079] . In what follows, we will assume that the reader is familiar with the conventional vocabulary of optical telecommunications and its basic concepts.

Here, we take the opposite approach to the previous article. Rather than breaking down the system into its elementary bricks, our intention is to provide the reader with the keys to building a high-performance system from these bricks. We will show that the design of complex systems is not limited to the juxtaposition of their constituent elements, and that the complex interactions between these bricks are responsible for most of the innovations that have marked the history of optical telecommunications. In this respect, we'd like to take a closer look at the astonishing results achieved by the simultaneous combination of five new bricks, all of which had previously been dismissed for the lack of benefit they brought when taken in isolation. Today, these five bricks are referred to as coherent technologies.

We review the physical effects that occur during the propagation of an optical wave in a fiber. Our motivation is less to detail their fundamental origin, than to anticipate the nature and amplitude of the distortions they generate as a function of system parameters. In particular, we describe these distortions as a function of fiber type. We then describe a generic method for determining the operating points of the system that maximize its range, balancing linear and non-linear effects. Of course, these operating points depend on the modulation technique employed, the main variants of which are then described.

Finally, we need to validate our design choices through experiments, which will first be carried...