Transmission systems over optical fiber

Compared to other existing transmission media, fiber has a nearly constant attenuation over a large frequency range (several thousands of gigahertz) and offers the advantage of huge bandwidth, for now consider the transmission of digital flow very large (several terabit / second) required by the multiplication of services and the increased need for transmission of images. Very fast too, it is apparent that the optical system, compared to coaxial cable systems of equivalent capacity, a significant gain on the distance between repeaters-regenerators, passing a few kilometers to several tens of kilometers. From 1978 installed systems were working at the optical wavelength of 0.8 m, delivering a throughput of between 50 and 100 Mbit / s with repeater spacing of 10 km, ie approximately three times more than coaxial cable systems of equivalent capacity.

The second generation of transmission systems over optical fiber, which emerged in the 1980s, follows directly from the development of single mode fiber and semiconductor laser at 1.3 m, wavelength for which the chromatic dispersion (ie the distortion on the signals induced by the spread) is minimal. Data rates exceeding 1 Gbit / s with repeater spacing of several tens of kilometers, are met. The scope of these systems are limited by the losses of the fiber, 0.5 dB / km at best, and the idea appears to develop sources emitting at a wavelength of 1.55 m for which l mitigation is minimal. However, this gain is destroyed by the effect of chromatic dispersion, all wavelengths can not propagate at the same speed. The chromatic dispersion of the material of the fiber is much higher than 1.3 m and it is that comes when the limitation of bandwidth and therefore the flow. Simultaneous progress on both the lasers emitting a single mode on the medium of transmission (dispersion shifted fiber) will bring solutions to these problems and the first systems working at 1.55 m appear in the late 1980s, with a throughput greater than 2 Gbit / s.

Emerged in the late 1980s and became very rapid industrial products, fiber amplifiers will bring a revolution in the field of optical fiber communications: inserted into the transmission line, they can compensate for the attenuation of the fiber and thus increase the range of transmission systems, at the cost of added noise. Used as preamplifiers, they increase the sensitivity of optical receivers. Finally, their huge bandwidth (30 nm and even more today) allows consideration of the amplification of multiple optical carriers in the spectrum juxtaposed, forming what is called a multiplex. Thus was born the concept of multiplexing wavelength (WDM Wavelength Division Multiplexing), each fiber carrying a multiplex of N channels is equivalent in capacity to N fibers each carrying a channel, and it is easily conceivable that this approach can potentially d increase the capacity of a network of very large without changing its physical infrastructure. Systems using this technique, mostly with a flow rate of 2.5 Gbit / s per channel, are now being installed in all major global players in their transport networks to cope with traffic growth expected in the coming years. Systems with N × 10 Gbit / s are offered by manufacturers and installed and the shift to multiplex large numbers of channels and (or) high capacity per channel will likely continue in the coming years, to address the need for capacity growth experienced by transport networks such as metropolitan networks.

Finally, the optical transmission can now achieve a quality (in terms of error rates) much higher than earlier systems, particularly microwave.

The optical fiber is also used in video communications networks to transmit a multiplex of sub-carrier electric intensity modulate a carrier perspective. Each of these sub-carrier, which corresponds to a television channel, is itself so analog modulated (frequency modulation, amplitude modulation single sideband) or digital (phase modulation, amplitude modulation on two quadrature carriers ...).