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Communication is a process in which messages, ideas and information can be exchanged between two individuals. From the early days when languages were developed, the methods people use to communicate have experienced a dramatic evolution. Nowadays, rapid transmission of information over long distances and instant access to various information sources have become conspicuous and important features of our society. | 1 An Introduction to Optical Communication Systems 1.1 INTRODUCTION Communication is a process in which messages ideas and information can be exchanged between two individuals. From the early days when languages were developed the methods people use to communicate have experienced a dramatic evolution. Nowadays rapid transmission of information over long distances and instant access to various information sources have become conspicuous and important features of our society. The rapidly growing information era has been augmented by a global network of optical fibre 1 . By offering an enormous transmission bandwidth of about 1014Hz and a low signal attenuation the low-cost glass-based single-mode optical fibre SMF provides an ideal transmission medium. In order that information can be carried along the SMF information at the transmitter side is first converted into a stream of coherent photons. Using a specially designed semiconductor junction diode with heavy doping concentration semiconductor lasers have been used to provide the reliable optical source required in fibrebased lightwave communication. With its miniature size compatible to the SMF the semiconductor laser diode has played a crucial role in the success of optical fibre communication systems. This chapter has been organised as follows in section 1.2 the historical progress of optical communication is presented. Before exploring the characteristics of semiconductor lasers various configurations of optical fibre-based communication systems are discussed in section 1.3. Depending on the type of detection method used both direct and coherent detection schemes are discussed. Based upon the characteristics of coherent optical communication systems the performance requirements of semiconductor lasers are presented at the end of the chapter. In particular the significance of having an optical source that oscillates at a single frequency whilst having a narrow spectral linewidth is reviewed. 1.2 HISTORICAL .
