This dissertation is primarily devoted to optical communications systems. These involve the transmission of visible and infrared radiation through space, the atmosphere, along optical fibres, and if absorption allows, under water. The visible and infrared radiation used is generated by lasers and light emitting diodes. The lasers involve gas discharge phenomena such as electronic, ionic and molecular collisions leading to atomic and molecular exitation and ionization as well as processes like elactronic and ionic diffusion and recombination. The prime example is the Co2 laser. Of more importance in the context of optical communication is the Nd : YAG laser. This depends for its operation on a gas discharge through a high pressure gas, usually xenon in a flash lamp which is the pumping source. Solid state lasers which are of major importance in fibre optic communications systems also involve excitation and recombination processes, as do Light Emitting Diodes (LED'S) and semiconductor lasers.