This book is intended as a general introduction to an art in applied physics, that of the generation of light by stimulated emission of radiation. Being an introduction to a rather advanced subject, the exposition is based on knowledge and experience borrowed from physics and related sciences.

When writing or selecting such a book, it is importunt to form a clear idea about the expected degree of preparation of the person who is going to use it. This book is written for a person trained in physics at least to the extent that is expected of an engineering student graduating from one of the better schools of engineering in the United States. Specifically this means completion of two years of physics at the university level, including the study of some modern physics. The exposition of the subject is held at a level that ought not to present many difficulties to a senior majoring in physics or chemistry. I made it a point to define all new concepts and quantities in terms of those that are familiar from elementary physics. I hope Ihat the book will be of some use to more advanced colleagues also, but it is not written for specialists in the laser field.

Some of the phenomena on which lasers are based do not fit into the framework of classical physics. More precisely said, they occur because the laws of classical physics are not suitable for the description of phenomena on the atomic and molecular scale. These must be described in terms of quantum theory, which is inevitably necessary for the full comprehension of lasers. The purpose of this book, however, is not to teach quantum mechanics, not even in an applied form. Therefore the results needed from quantum theory are stated concisely, without justification, and are used to the minimum extent necessary. Whenever possible, a classical, or semiclassical argument is given. This procedure is followed because I believe that a quantum mechanical argument presents a serious obstacle to an engineering-oriented reader and that even a reader oriented toward theoretical physics will find it easier to penetrate the quantum theoretical calculations after learning to know the subject from a more pedestrian point of view.

The selection of subject material was guided by the thought that this book ought to be general; that is, it should include exposition of all laser types. For the theoretical subjects the chosen level of presentation served as a limiting factor. An additional limiting factor was my own ability to digest material of ever-growing variety and quantity at a rate fast enough so that the parts of the manuscript that were written first would not become hopelessly obsolete at the end. Having decided to include the description of all laser types, I drew the line and abstained from entering into the discussion of any applications.

This is neither a scientific monograph nor an engineering handbook. There are many good monographs and review articles in the laser field; I bring them to the reader's attention at the proper places. To a lesser extent I also point out sources of engineering data. But I have tried to avoid cluttering the text with material that would detract from the main purpose of the book, the concise exposition of laser principles and techniques.

The selection of references was made on the basis of their usefulness to the reader. To be consistent with the educational purpose of the book, I had to resist the temptation to use the references as a means for erecting a monument to the pioneers in the laser field. I omitted citation of many outstanding contributions because there are more convenient ways of leading the reader to the information they contain. A reader interested in the historical priorities or in the original observations published will quickly find his way to the original sources by consulting the intermediate references I cite. I tried to compensate for the economy of references by inserting short historical summaries about the evolution of different aspects of the subject. The work of gathering material for the book was finished around the end of 1968; therefore very few items that appeared later found their way into the book.

Since this volume is offered to the public as the second edition of a much smaller book of conspicuously different content, an explanation about its evolution is in order. The first edition of Lasers was written during the spring of 1962. It was completed under pressure to produce, as quickly as possible, a brief comprehensive technical description of lasers at a time when no such book was available. While that book was being printed the visible helium-neon laser and the semiconductor injection lasers made their appearance. Discoveries followed one another at such a pace that in 1963 I recognized that my book was obsolete and decided to rewrite it. This time I worked more leisurely and more carefully. In about two years time I developed the subject more systemati- cally, and in 1965 I finished a new book that included the important laser discoveries made in 1963 and 1964. This was published under the title Introduction to Laser Physics. It seemed at that time that it is still feasible und worthwhile to attempt a tabulation of all reported laser lines, and that it is desirable to present a selective author- and discovery-oriented bibliography in an introductory book. I straddled the fence between an introductory book and a monograph, and I included in Introduction to Laser Physics a chapter on nonlinear effects and one on laser applications. Further expansion of the sub- ject matter and shifts in emphasis that took place in the technology between 1964 and 1967 suggested that drastic revisions are again necessary in order to keep my writings up-to-date. By 1967 a number of monographs appeared on lasers. They had been written mostly for the use of specialists. It seemed to me that there was still room for a general, student-oriented book. With this thought in mind, I revised the content of the book not only by adding new material but also by omitting subjects that seem less relevant now than they appeared earlier. The topics omitted include the theory of linewidth, which was a carry-over from masers, the Basov-Krokhin calculations pertaining to the operation of a gas laser with excitation transfer, und many details concerning four-levei rare earth lasers which failed to attain practical significance. I also omitted the discussion of nonlinear effects and laser applications. On the other hand, the discussion of cavity modes has been expanded and an exposition of the Lamb dip and of laser amplification has been included in terms of a new and elementary formalism. Other new subjects are the semiconductor lasers excited by means other than carrier injection, the fluid lasers developed after the chelate types, and, most of all, the powerful gas lasers of ionic and molecular type. together with the chemical laser types. The inclusion of the last groups of lasers required considerable expansion of the general expository material on spectroscopy.

Not only was the subject matter revised but the text of the subject matter that was retained was carefully scrutinized from the point of view of accuracy and clarity. I devoted a greater effort than before to the problem of presentation, and whenever I could do so I replaced complicated arguments found in the literature by more elementary ones.

Finally, I must answer the question, why do I call this volume Lasers? I do so because this title best describes its contents and because its writing was the logical continuation of the work I began in 1962 with the first edition.

The first edition of this book was written while I was a member of the technical staff of Hughes Research Laboratories. For its preparation I had the full support of my colleagues and the laboratory management.

Although I left the laboratories a long time ago and wrote the present edition entirely during my tenure at San Fernando Valley State College, I continued to receive assistance from my former colleagues at Hughes in the form of technical information, advice, and criticism. The laboratory management permitted me to use drawings and photographs that belong to the company, including many items prepared since the appearance of the first edition. In expressing my thanks for the assistance I have received from Hughes Research Laboratories, I wish to make it clear that the responsibility for the material in the book is entirely my own.

In addition to Hughes, I am indebted to Bell Telephone Laboratories; Edgerton, Germeshausen and Grier; Lincoln Laboratories of Massachusetts Institute of Technology; and Philips Research Laboratories (Eindhoven) for furnishing photographs of their apparatus and giving permission for their use. I wish to thank the American Institute of Physics for permitting the reproduction of numerous figures published in its journals.

My colleagues, Samuel Cunningham, Victor Evtuhov, Robert Hellwarth, Bemard Soffer, and Harvey Winston read various parts of the manuscript and offered numerous constructive suggestions. Mr. 0. T. Sylvest, my student assistant, helped to improve the accuracy and readability of the work. To all these I express my heartfelt thanks.

My wife, Birgit Lengyel, prepared many of the illustrations, typed the entire manuscript, and assisted in the proofreading. Her participation, her unfailing support in all aspects of this enterprise, and her good natured endurance of a preoccupied, bookwriting husband is most gratefully appreciated.

BELA A. LENGYEL

Pacific Palisades, California

January 1971