B: Basics of Laser Systems

Abstract

In ophthalmology, the unique optical properties of lasers are used in many diagnostic (Chapters 5–7) and therapeutic (Chapter 10) methods. Because of this special role, we added this separate chapter about the theoretical and technical basics of laser systems to ensure a common knowledge base for all readers. This chapter serves as a quick introduc-tion and reference and does not intend to cover all details about laser physics. In 1960, the first laser 1) was built by Theodore Maiman (1927–2007) based on pi-oneering work by Charles Townes (born 1915) and Arthur Schawlow (1921–1999). In those days, nobody was really aware of the extraordinary importance of this spe-cial kind of light source. Today, lasers are considered to be one of the paramount achievements of the twentieth century. Their applications range from information and medical technology to mechanical processing, metrology, and chemical sens-ing. The success story of lasers actually dates back to the development of quantum mechanics in the early twentieth century. In particular, Albert Einstein's (1879– 1955) publications on light–matter interaction [1, 2] and Nils Bohr's (1885–1962) atom model paved the way for this unique device. These models take into account the observation that the energy of an atom (or a molecule) cannot be continuously altered. Only certain discrete energy states are " allowed " . For example, an atom may transit from a discrete lower state j1i with energy E 1 to an excited state j2i with energy E 2 by absorbing light which carries exactly the difference energy E D E 2 E 1 (Figure B.1a). The transition also works the other way round in that the excited atom " falls back " (is depleted/de-excited) to a lower state, whereupon the energy difference is released by emission of light (Figure B.1b,c). To describe this behavior, Einstein proposed a simple formalism which we will use in the following discussions.