Optical communications: introduction and performance assessment

Decades before it was connected directly to subscribers, fiber optics had taken root in the shadow of the Internet, to the point of becoming its strongest pillar. But around this fiber must be built optical telecommunications systems of often complex architecture, which form the arteries that link the major cities of a single country or continent, from a few dozen to a few thousand kilometers apart. The optical signals they carry must be regularly regenerated at regeneration sites, and can then be routed to their destination at optical routing sites.

This topic is covered in two complementary files: the present article [E 7 079] and the following one [E 7 081] "Design and validation". We begin by defining the vocabulary that will enable readers new to the subject to acquire the most basic notions. We break down transmission systems into their main components.

We review the various ways of multiplexing multiple signals, exploiting time, wavelength, polarization or spatial mode of propagation. We then review the operating principles and characteristics of optical amplifiers, whether based on stimulated emission in an erbium-doped fiber or Raman-stimulated emission, or both simultaneously.

We then review detection techniques, with particular emphasis on the one that has revolutionized the field of optical telecommunications since the end of the 2010s, namely coherent detection. We devote the final part of the article to performance evaluation criteria, discussing the areas of application, advantages and limitations of each. Some criteria are better suited to the characterization of individual components, others to the characterization of the complete system.