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The rapid development of both terrestrial and undersea optical fibre networks has paved the way for a global communication network. Highly efficient semiconductor injection lasers have played a leading role in facing the challenges of the information era. In this chapter, before discussing the operating principle of the semiconductor distributed feedback (DFB) laser diode (LD), general concepts with regard to the principles of lasers will first be presented. | 2 Principles of Distributed Feedback Semiconductor Laser Diodes Coupled Wave Theory 2.1 INTRODUCTION The rapid development of both terrestrial and undersea optical fibre networks has paved the way for a global communication network. Highly efficient semiconductor injection lasers have played a leading role in facing the challenges of the information era. In this chapter before discussing the operating principle of the semiconductor distributed feedback DFB laser diode LD general concepts with regard to the principles of lasers will first be presented. In section 2.2.1 general absorption and emission of radiation will be discussed with the help of a simple two-level system. In order for any travelling wave to be amplified along a two-level system the condition of population inversion has to be satisfied and the detail of this will be presented in section 2.2.2. Due to the dispersive nature of the material any amplification will be accompanied by a finite change of phase. In section 2.2.3 such dispersive properties of atomic transitions will be discussed. In semiconductor lasers rather than two discrete energy levels electrons jump between two energy bands which consist of a finite number of energy levels closely packed together. Following the Fermi-Dirac distribution function population inversion in semiconductor lasers will be explained in section 2.3.1. Even though the population inversion condition is satisfied it is still necessary to form an optical resonator within the laser structure. In section 2.3.2 the simplest Fabry-Perot FP etalon which consists of two partially reflecting mirrors facing one another will be investigated. A brief historical development of semiconductor lasers will be reviewed in section 2.3.3. The improvements in both the lateral and transverse carrier confinements will be highlighted. In semiconductor lasers energy comes in the form of external current injection and it is important to understand how the injection current can affect the .
