This book is motivated by the very favorable reception given to the previous editions as well as by the considerable range of new developments in the laser ?eld since the publication of the third edition in 1989. These new developmentsinclude, among others, Quantum-Well and Multiple-Quantum Well lasers, diode-pumpedsolid-state lasers, new concepts for both stable and unstable resonators, femtosecond lasers, ultra-high-brightness lasers etc. The basic aim of the book has remained the same, namely to providea broad and uni?ed description of laser behavior at the simplest level which is compatible with a correct physical understanding. The book is thereforeintendedas a text-bookfor a senior-levelor ?rst-year graduatecourse and/or as a reference book. This edition corrects several errors introduced in the previous edition. The most relevant additions or changes to since the third edition can be summarized as follows: 1. A much-more detailed description of Ampli?ed Spontaneous Emission has been given [Chapt. 2] and a novel simpli?ed treatment of this phenomenon both for homogeneousor inhomogeneouslines has been introduced [Appendix C]. 2. A major fraction of a chapter [Chapt.
3] is dedicated to the interaction of radiation with semiconductor media, either in a bulk form or in a quantum-con?ned structure (quantum-well, quantum-wire and quantum dot). 3. A moderntheory of stable and unstable resonatorsis introduced,where a more ext- sive use is made of the ABCD matrix formalism and where the most recent topics of dynamically stable resonators as well as unstable resonators, with mirrors having Gaussian or super-Gaussian transverse re?ectivity pro?les, are considered [Chapt. 5].