Laser Amplifiers

Abstract

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. Light transmitted through matter in thermal equilibrium is attenuated rather than amplified. This is because absorption by the large population of atoms in the lower energy level is more prevalent than stimulated emission by the smaller population of atoms in the upper level. An essential ingredient for achieving laser amplification is the presence of a greater number of atoms in the upper energy level than in the lower level, which is clearly a nonequilibrium situation. Achieving such a population inversion requires a source of power to excite (pump) the atoms into the higher energy level. Although the presentation throughout this chapter is couched in terms of “atoms” and “atomic levels,” these appelations are to be more broadly understood as “active medium” and “laser energy levels,” respectively. The properties of an ideal (optical or electronic) coherent amplifier are displayed schematically. Real coherent amplifiers deliver a gain and phase shift that are frequency dependent, typically in the manner illustrated here. The gain and phase shift constitute the amplifier's transfer function. For a sufficiently high input amplitude, furthermore, real amplifiers may exhibit saturation, a form of nonlinear behavior in which the output amplitude fails to increase in proportion to the input amplitude. Saturation introduces harmonic components into the output, provided that the amplifier bandwidth is sufficiently broad to pass them. Real amplifiers also introduce noise, so that a randomly fluctuating component is always present at the output, regardless of the input. An amplifier may therefore be characterized by the following features: ▪ gain; ▪ bandwidth; ▪ phase shift; ▪ power source; ▪ nonlinearity and gain saturation, and ▪ noise We proceed to discuss these characteristics in turn in this chapter. The theory of laser amplification is developed, leading to expressions for the amplifier gain, spectral bandwidth, and phase shift. The mechanisms by which an amplifier power source can achieve a population inversion are examined. Gain saturation and noise in the amplification process, are discussed respectively. This chapter relies on material presented in Chap. 12.