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A coherent optical amplifier is a device that increases the amplitude of an optical field while maintaining its phase. If the optical field at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. The output amplitude is | Fundamentals of Photonics Bahaa E. A. Saleh Malvin Carl Teich Copyright 1991 John Wiley Sons Inc. ISBNs 0-471-83965-5 Hardback 0-471-2-1374-8 Electronic CHAPTER LASER AMPLIFIERS THE LASER AMPLIFIER A. Amplifier Gain B. Amplifier Phase Shift AMPLIFIER POWER SOURCE A. Rate Equations B. Four- and Three-Level Pumping Schemes C. Examples of Laser Amplifiers AMPLIFIER NONLINEARITY AND GAIN SATURATION A. Gain Coefficient B. Gain C. Gain of Inhomogeneously Broadened Amplifiers AMPLIFIER NOISE Charles H. Townes born 1915 Nikolai G. Basov born 1922 Aleksandr M. Prokhorov bom 1916 Townes Basov and Prokhorov developed the principle of Zight amplification by the stimulated emission of radiation laser . They received the Nobel Prize in 1964. 460 A coherent optical amplifier is a device that increases the amplitude of an optical field while maintaining its phase. If the optical field at the input to such an amplifier is monochromatic the output will also be monochromatic with the same frequency. The output amplitude is increased relative to the input while the phase is unchanged or shifted by a fixed amount. In contrast an amplifier that increases the intensity of an optical wave without preserving the phase is called an incoherent optical amplifier. This chapter is concerned with coherent optical amplifiers. Such amplifiers are important for various applications examples include the amplification of weak optical pulses such as those that have traveled through a long length of optical fiber and the production of highly intense optical pulses such as those required for laser-fusion applications. Furthermore it is important to understand the principles underlying the operation of optical amplifiers as a prelude to the discussion of optical oscillators lasers in Chap. 14. The underlying principle for achieving the coherent amplification of light is light amplification by the stimulated emission of radiation known by its acronym as the LASER process. Stimulated .