Radiation propagation in materials

Until 1970, the technology for producing optical fibers with suitable linear attenuation for optical information transmission was not yet available. It was not uncommon to obtain attenuations of the order of 100 to 1,000 dB/km with conventional glass fibers.

The first conclusive work at this time was carried out by Corning Glass Works, which obtained fibers a few hundred meters long with attenuations of less than 20 dB/km, i.e. 99% loss per 1 km.

Mass production of optical fibers with an attenuation of 0.2 dB/km has been a given since the 1980s, i.e. 5% loss per 1 km.

Today, the technique of guided propagation in optical fibers is a growing and increasingly obvious field of interest, and many systems, both civil and military, use it as a means of transmitting information. Why is this so?

Compared with conventional information transmission systems, such as coaxial two-wire lines, optical guided transmission offers a number of advantages. These include the following physical and mechanical characteristics:

• compactness ;

• lightness, with a gain of a factor of 20 when comparing silica optical fibers and copper wires;

• abundance of raw materials, especially silica ;

• suppleness and good flexibility ;

• corrosion resistance ;

• good electrical insulation between transmitter and receiver.

Optical and electromagnetic characteristics such as :

• Moderate linear attenuation, of the order of 0.2 dB/km, allowing wide repeater spacing;

• carrier frequency of transmitted information equal to that of light ( $\approx 10{}^{+14}$ Hz), typically 10,000 times higher than the highest frequencies used in radiofrequency; and thus increased bandwidth potential;

• wavelength multiplexing is possible, with frequencies shifted from a few gigahertz to some 1,000 GHz, thereby increasing the amount of information transmitted;

• no crosstalk, and discreet transmission ;

• non-existent probability of error messages or false information ;

• very good insensitivity to severe radio-electric environments such as lightning or nuclear radiation.

The advantages of fiber optics, as briefly mentioned...