Modeling lasers may be realized with different levels of sophistication. Rigorously it requires a full quantum treatment but many laser dynamical properties may be captured by semiclassical or even purely classical approaches. In this book we deliberately chose the simplest point of view, i.e. purely classical equations, and try to extract analytically as much information as possible. The basic framework of our approach is provided by the rate equations . In their simplest version, they apply to an idealized active system consisting of only two energy levels coupled to a reservoir. They were introduced as soon as the laser was discovered to explain (regular or irregular, damped or undamped) intensity spikes commonly seen with solid state lasers (for a historical review, see the introduction in). These rate equations are discussed and sometimes derived from a semiclassical theory in textbooks on lasers. They capture the essential features of the response of a single-mode laser and they may be modified to account for specific effects such as the modulation of a parameter or optical feedback. The most basic processes involved in laser operation are schematically represented in Figure 1.1. N 1 and N 2 denote the number of atoms in the ground and excited levels, respectively. The process of light–matter interaction is restricted to stimulated emission and absorption.
Modeling lasers may be realized with different levels of sophistication. Rigorously it requires a full quantum treatment but many laser dynamical properties may be captured by semiclassical or even purely classical approaches. In this book we deliberately chose the simplest point of view, i.e. purely classical equations, and try to extract analytically as much information as possible. The basic framework of our approach is provided by the rate equations . In their simplest version, they apply to an idealized active system consisting of only two energy levels coupled to a reservoir. They were introduced as soon as the laser was discovered to explain (regular or irregular, damped or undamped) intensity spikes commonly seen with solid state lasers (for a historical review, see the introduction in). These rate equations are discussed and sometimes derived from a semiclassical theory in textbooks on lasers. They capture the essential features of the response of a single-mode laser and they may be modified to account for specific effects such as the modulation of a parameter or optical feedback. The most basic processes involved in laser operation are schematically represented in Figure 1.1. N 1 and N 2 denote the number of atoms in the ground and excited levels, respectively. The process of light–matter interaction is restricted to stimulated emission and absorption.
In Chapter 1, we introduced the standard rate equations (SRE) for a laser containing two-level atoms between which stimulated emission is possible. But real lasers exhibit much more complicated energy level schemes. Throughout this chapter, we consider several models in which the light–matter interaction is described by population equations for all the levels involved in the laser operation. Although they are mathematically more complicated, we shall investigate these equations in the same way as in Chapter 1, i.e. by formulating dimensionless equations and by analyzing the stability properties of the steady states. The basic ingredients of the SRE model used up to here, namely pumping and relaxation processes, play key roles in the efficiency of lasers. But already during the pioneering days of the laser, it appeared important to investigate three- and four-level models in order to obtain more reliable information on quantities such as the power conversion efficiency or the response time. The common extensions of the two-level SRE typically consider an open two-level system, or three- or four-level systems, depending on the nature of the active medium. Restriction to as few as four levels is again a crude simplification of the complex population dynamics occurring in most lasers. But it is rather surprising to see how good these simple kinetic models are. As we demonstrate by studying specific examples, the solution of the three- or four-level rate equations differs only slightly from the solution of the SRE.
This textbook provides an accessible introduction to laser physics, exploring three key questions: What is special about laser light? What is the physics underlying its generation? And how can laser light be manipulated and used? The material is presented in a manner that helps the reader develop an understanding of the underlying physics principles and processes without getting bogged down in mathematical details, but there is enough mathematics for the understanding to be quantitative. The book begins by describing the special properties of laser light and focuses on the physics of light-matter interaction to understand how it acquires them. It builds upon Maxwells equations for light and the classical electron oscillator model for the atoms, augmented with a quantum mechanical picture of atomic energy levels, to provide an accurate physical understanding of light-matter interaction. This approach bypasses the need for a course in quantum theory. Derivations of key equations are included not only to help readers follow the development of the subject, but also to illustrate that to approximate, simplify, and get analytic results is integral to understanding any complex phenomenon. Subjects such as nonlinear pulse propagation and nonlinear optics are discussed, but only at a level that should help readers to get started in these fields. The choice of topics has been limited to atomic/molecular lasers to allow for their coverage in one semester-long course. Problems, references and illustrations are included throughout the book, making it an ideal textbook for advanced undergraduate and graduate physics and engineering students. Students do not require prior knowledge of physics much beyond an undergraduate-level introduction to electromagnetic theory, including Maxwell’s equations, polarization in atomic media and electromagnetic wave propagation. Key features: · Provides a qualitative and quantitative understanding of, and appreciation for, the special properties of laser light · Explores quantum statistics of laser light · Presents an analytical approach to pulse formation
The analysis in Chapter 3 used a phenomenological form of the permittivity to describe active materials. A proper understanding of optical amplification requires a quantum-mechanical approach for describing the interaction of light with atoms of an active medium [1]. However, even a relatively simple atom such as hydrogen or helium allows so many energy transitions that its full description is intractable even with modern computing machinery [2, 3]. The only solution is to look for idealized models that contain the most essential features of a realistic system. The semiclassical two-level-atom model has proven to be quite successful in this respect [4]. Even though a real atom has infinitely many energy levels, two energy levels whose energy difference nearly matches the photon energy suffice to understand the interaction dynamics when the atom interacts with nearly monochromatic radiation. Moreover, if the optical field contains a sufficiently large number of photons (> 100), it can be treated classically using a set of optical Bloch equations. In this chapter, we learn the underlying physical concepts behind the optical Bloch equations. We apply these equations in subsequent chapters to actual optical amplifiers and show that they can be solved analytically under certain conditions to provide a realistic description of optical amplifiers. It is essential to have a thorough understanding of the concept of a quantum state [5]. To effectively use the modern machinery of quantum mechanics, physical states need to be represented as vectors in so-called Hilbert space [6].
Preface. List of symbols. 1. Introduction. 2. Basic properties and types of lasers. 3. Laser resonators. 4. The intensity of laser light. Rate equations. 5. The basic equations of the semiclassical laser theory. 6. Applications of semiclassical theory. 7. Ultrashort pulses. 8. Instability hierachies of laser light. Chaos, and routes to chaos. 9. Optical bistability. 10. Quantum theory of the laser I. A first approach via quantum mechanical Langevin equations. Coherence, noise and photon-statistics. 11. Quantum theory of the laser II. A second approach via the density matrix equation and quantum classical correspondence. 12 A theoretical treatment of the two-photon laser. 13. The laser - trailblazer of synergetics. References. Subject index.
So far we have treated the energy levels of an atomic system as stable states with infinite lifetimes. The presence of an electromagnetic field changes this picture. Absorption of photons will excite atoms and ions to higher levels, while stimulated emission will make excited ones decay. Einstein (1917) showed, using purely statistical arguments, that an excited ion will decay through spontaneous emission in the absence of an electromagnetic field. This process can also be derived by using a rigorous quantum electrodynamic treatment, where the field is quantized. In this chapter we start by discussing transitions between atomic levels in a general sense, postulating the operators responsible for the decay. After the first few sections we will concentrate on the most important few multipoles or terms in the expansion of the electromagnetic field. We will introduce a number of different properties describing the transition, and eventually arrive at the measurable transition rate and, directly linked to it, the lifetime of an excited state. We will start, however, with the most basic property, the component strength. Our aim is to arrive at computationally useful expressions as soon as possible. For a more thorough treatment of the theory of radiative transitions we refer to the book by Sobelman (1972). For a brief review of the many factors affecting spectra of atoms, see Martin and Wiese (1996) .
The quantum world within an atom or molecule that once attracted explorations only by academic physicists now provides fertile sustenance for chemists seeking control of chemical reactions and for engineers developing ever smaller electronic devices or tools for processing information with greater security. Whereas the first pioneers could only discover the most elementary properties – the discrete energy levels that characterize the internal structures of atoms and the radiative transitions that link these structures to our external world – it is now possible to alter that structure at will, albeit briefly. Objective This monograph presents the physical principles that describe such deliberately crafted changes, namely how single atoms or molecules (or other simple quantum systems) are affected by coherent interactions, primarily laser light – a subject that has been regarded first as a part of quantum electronics and then quantum optics but is most generally described as coherent atomic excitation [Sho90]. This physics has relevance to such basic concerns as the detection and quantitative analysis of trace amounts of chemicals, the catalysis or control of chemical reactions, the alignment of molecules, and the processing of quantum information. The physics necessarily involves elementary quantum mechanics, but it has many associations to the classical dynamics that governs macroscopic objects – waves and particles. The mathematics that quantifies the changes is that of differential equations, specifically coupled ordinary differential equations (ODEs), whose parameters incorporate the controls of experimenters and whose solutions, appropriately interpreted, quantify the resulting changes.