Transmission systems on optical fibers

Compared with other existing transmission media, fiber offers virtually constant attenuation over an enormous frequency range (several thousand gigahertz), and thus offers the advantage of gigantic bandwidths, making it possible to transmit the very high digital data rates (several terabits/second) demanded by the proliferation of services and the growing need for image transmission . It also became clear very quickly that, compared with coaxial cable systems of equivalent capacity, optical systems enabled a significant reduction in the distance between repeater-regenerators, from a few kilometers to several tens of kilometers. From 1978 onwards, systems working at an optical wavelength of 0.8 μm were installed, delivering data rates of between 50 and 100 Mbit/s, with a repeater spacing of 10 km, i.e. around three times greater than systems using coaxial cable of equivalent capacity.

The second generation of fiber optic transmission systems, which appeared in the 1980s, is a direct result of the development of single-mode fiber and semiconductor lasers at 1.3 μm, the wavelength at which chromatic dispersion (i.e., the distortion induced in signals by propagation) is minimal. Data rates in excess of 1 Gbit/s, with repeater spacing of several tens of kilometers, are then achieved. The ranges of these systems are limited by fiber losses, 0.5 dB/km in the best case, and the idea then arises of developing sources emitting at a wavelength of 1.55 μm, for which attenuation is minimal. However, this gain is destroyed by the effect of chromatic dispersion, as not all wavelengths propagate at the same speed. This chromatic dispersion of the fiber material is much more pronounced than at 1.3 μm, and this is where the limitation in bandwidth, and therefore throughput, comes from. Simultaneous progress on both single-mode lasers and the transmission medium (dispersion-shifted fibers) provided solutions to these problems, and the first systems working at 1.55 μm appeared in the late 1980s, with throughput in excess of 2 Gbit/s.

Fiber amplifiers, which appeared in the late 1980s and rapidly became industrial products, were to bring about a considerable upheaval in the field of fiber optic communications: inserted in the transmission line, they compensate for the attenuation of the fiber and therefore increase the range of transmission systems, at the cost of adding noise ...