Introduction to Optical Communications
The invention of the solid state ruby laser (acronym for light amplification by stimulated emission of radiation) in May 1960  and the He-Ne gas laser  in December 1960 has led to some wide-ranging and very significant scientific and technological progress. This so-called ‘discovery of the century’, followed by the first use of semiconductor lasers [3–5] in communications, heralded the start of optical communications. The laser provided a powerful coherent light source together with the possibility of modulation at high frequency, and this opened up a new portion of the electromagnetic spectrum with frequencies many times higher than those commonly available in radio communication systems. In addition the narrow beam divergence of the laser made enhanced free-space optical transmission a practical possibility.
Since optical frequencies are of the order of 100 THz, and information capacity increases directly with frequency bandwidth, the laser potentially offers a few order of magnitude increase in available bandwidth compared with microwave systems. Thus, by using only a small portion of the available frequency spectrum, a single laser could, in principle, carry millions of telephone conversations or TV channels.
With the potential of such wideband transmission capabilities in mind, a number of experiments  using atmospheric optical channels were carried out in the early 1960s. These ...