The progress made in fiber-optic communications technology over the past ten years or so has served to free transmission systems from the twin constraints of loss and band with, bringing wide-ranging reductions in transmission costs, a dramatic increase in transmission capacity, stimulating a revolution in the quality and architecture of communications networks and services.

The essential elements for optical communication systems are semiconductor laser, optical fiber, photodiode, cross connect switches, erbium doped fiber amplifiers and wavelength-Division multiplexers respectively. The realization of optical amplification technology has a wide range of possible uses. Not only it is applicable to trunk transmission lines, optical subscriber networks and virtually all communications systems, but in providing simple compensation for the losses produced in optical circuits, for instance, optical switches and signal processors.

In this tutorial we describe the properties of some realized erbium doped fiber amplifiers, design, simulation, realization and measurement of low noise broadband optical receivers in thin film and integrated circuit technology. The dense wavelength multiplexing and its technology problems are described. The currently available systems of 4, 8, 16 and 32 wave length with a total capacity of 320 GBPS per fiber are presented. Finally systems under development make a data rate of 5.12 Gbps per fiber are presented. Finally systems under development make a data rate of 5.12 Tbit/s and a repeater spacing of 320 km possible. Systems for handling 320 different wavelengths are only a year away. A short look on coherent optical fiber systems shows the difficulties of their application in the near future to replace optical amplifier.