Guided propagation of light

Since the mid-1990s, fiber optic transmission has become hegemonic in telecom operators' networks: it has a monopoly in the field of long-distance, high-speed links, and is playing an increasingly important, but not exclusive, role in access networks.

Instead of electrical signals, light waves are used to carry information - now mainly data for Internet traffic - along thin glass cylinders known as optical fibers. Since the late 1990s, transoceanic cables have been exclusively optical cables.

Optical fibers have the advantage of being able to transmit information at much higher speeds than previously used media (coaxial cables, radio): in 2015, the most efficient transmission systems can carry up to 800 Gbit/s, or around ten million telephone channels on a single fiber. This performance explains why optical fiber has become the preferred choice for long-distance, high-capacity networks.

It also plays a growing role, shared with other technologies, in access networks, for connecting operators' customers. Compared with copper cable, whose possibilities have been extraordinarily enhanced by ADSL, they offer the possibility of even higher data rates, and therefore increasingly wideband Internet access.

As research progressed, signal attenuation along optical fibers decreased extraordinarily. As early as 1966, the possibility of using fiber as a transmission medium could be envisaged, given the progress that could reasonably be expected. It would take many more years of effort to master glass chemistry and achieve losses compatible with transmission use. The first commercial fiber installed in the United States in 1977 lost half as much light over one kilometer, but by 1978, attenuation of 0.2 dB/km at a wavelength of 1,550 nm had been achieved by a single-mode fiber manufactured in Japan. It should be noted that this value, close to the theoretical minimum, has not been lowered since.

In this article, we first explain the principle of guiding light through a refractive medium, then describe how optical fibers work and how they are used in communications networks. Finally, we give a brief overview of other applications for optical fibers.

For further details, please refer to the articles in the Electronics/Automatics (Photonic Optics) treatise cited in the bibliography.