Optical fibers for telecommunications

An optical fiber is a dielectric guide that conducts light over long distances. The vast majority of fibers in use are rotationally symmetrical around their axis and made of isotropic materials (glass). Our aim is to present the fundamental properties of these fibers for their application in telecommunications, i.e. their properties with regard to the attenuation and distortion undergone by signals during propagation. But we will also look at new structures that have appeared in recent years, such as microstructured fibers, in which the condition of isotropy of the material is no longer respected.

The idea of transporting optical signals over long distances on a fiber was launched in 1966, but it would take years to master the manufacturing processes and control the composition of materials, which has a decisive influence on losses. It will then be possible to achieve attenuations low enough to allow signals to be transmitted over distances long enough to be of practical interest and make optical technology competitive. Starting from 1,000 dB/km in 1960, attenuation fell to 20 dB/km in 1975, then to 0.2 dB/km in 1984.

Compared with other existing transmission media, optical fiber offers low, virtually constant attenuation over an enormous frequency range, and thus the advantage of gigantic bandwidths, making it possible to transmit very high digital data rates. But fiber isn't just a perfect attenuator: the variation in refractive index as a function of wavelength is the main cause of chromatic dispersion, which distorts transmitted signals. The greater the distance, and the greater the bandwidth of the transmitted signals, the more this linear effect becomes apparent. As long as the attenuation of the fibers was sufficiently high that the signal had to be regenerated before it was significantly distorted, dispersion was neglected. With the reduction of losses and the emergence of very high-capacity systems, chromatic dispersion has become a fundamental effect.

Fiber amplifiers have made it possible to inject high powers into fibers and compensate for propagation losses; the counterpart is the appearance of non-linear effects, which are also a source of signal degradation, but can also be used positively under certain conditions to compensate for the influence of chromatic dispersion. In general, linear and non-linear effects interact and cannot be isolated and treated separately.

Fiber optics are therefore a complex propagation medium, whose effect on a signal can only be predicted using simulation software: many laboratories have developed such tools.