Lasers Fundamentals and Applications
Lasers: Fundamentals and Applications, serves as a vital textbook to accompany undergraduate and graduate courses on lasers and their applications. Ever since their invention in 1960, lasers have assumed tremendous importance in the fields of science, eng
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Semiconductor Lasers
13.1 Introduction Semiconductor-based light sources such as light-emitting diodes (LED) and laser diodes have revolutionized the application of photonic components in science, engineering, and technology. They have become ubiquitous components and are found in most places, be it markets where they are used as scanners for products, at home where they are found in CD and DVD readers or laser printers, in communication systems as sources, etc. Unlike the lasers discussed earlier, laser diodes are based on semiconductors such as gallium arsenide (GaAs), gallium indium arsenide (GaInAs), gallium nitride (GaN), etc. They cover the range of wavelengths from the blue region to the infrared. As compared to other laser systems, semiconductor lasers have some very attractive characteristics: they are very small in size, can be directly modulated by varying the drive current, are very efficient converters of electrical energy to light, can be designed to emit a broad range of wavelengths, etc. In this chapter, we will discuss the basic principle of operation of semiconductor laser diodes and some of their important properties that lead to their widespread applications.
13.2 Some Basics of Semiconductors The primary difference between electrons in semiconductors and other laser media is that in semiconductors, all the electrons occupy and share the entire volume of the crystal, while in the case of other laser systems such as neodymium:YAG laser and ruby laser, the lasing atoms are spaced far apart and the electrons are localized to their respective ions with very little interaction with other ions. Thus in a semiconductor, the quantum mechanical wave functions of all electrons overlap with each other and according to Pauli exclusion principle cannot occupy the same quantum state. Thus each electron in the crystal must be associated with a unique quantum state. The atoms comprising the semiconductor when isolated have the same electron configuration. Thus electrons belonging to different atoms may be in the same K. Thyagarajan, A. Ghatak, Lasers, Graduate Texts in Physics, C Springer Science+Business Media, LLC 2010 DOI 10.1007/978-1-4419-6442-7_13,
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13 Semiconductor Lasers
Fig. 13.1 Schematic diagram showing energy band diagram in a solid; each horizontal line corresponds to an energy level and filled circles represent electrons occupying the levels
Conduction band
Valence band
energy state. However, when the atoms are brought close together to form the solid, interactions among the atoms lead to a splitting of the energy levels and this leads to the formation of energy bands which are separated by forbidden regions of energy. Figure 13.1 shows a schematic diagram in which each energy level is represented by a horizontal line; in each band formed by a group of energy levels, there are as many sublevels as there are atoms in the crystal. Since the number of atoms is very large, within each band, the allowed energy values are almost continuous. The highest energy band in a solid that is c
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