The invention of the solid state ruby laser (acronym for light amplification by stimulated emission of radiation) in May 1960 [1] and the He-Ne gas laser [2] 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][4][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 [6] using atmospheric optical channels were carried out in the early 1960s. These experiments showed the feasibility of modulating a coherent optical carrier wave at very high frequencies. However, the high cost of development for all the necessary components, and the limitations imposed on the atmospheric channel by rain, fog, snow and dust make such high-speed systems economically unattractive. However, numerous developments of free-space optical channel systems operating at baseband frequencies were in progress for earth-to-space communications [7,8].It soon became apparent that some form of optical waveguide was required. By 1963, bundles of several hundred glass fibres were already being used for small-scale Optical CDMA Networks: Principles, Analysis and Applications, First Edition. Hooshang Ghafouri-Shiraz and M. Massoud Karbassian.
