Introduction

Many of the characteristics of laser beams are determined by properties of their gain medium and by the loss and gain characteristics of the laser cavity. The previous chapter discussed factors that determine the wavelength and spectral bandwidth of laser beams, the characteristics of their longitudinal modes, gain requirements for steady-state oscillation, the ultimate power (or energy) of laser beams, the duration of a laser pulse when Q-switched or mode-locked, and so on. However, this wealth of information is insufficient for design applications where the spatial pattern of the energy delivery must be well defined. To illustrate this, recall that when a laser is used for illumination (such as in PLIF), a relatively uniform distribution of the energy may be required; for material processing, the beam energy may need to be concentrated into a narrow well-defined spot; and for holography or interferometry, the shape of the incident wavefronts may need to be geometrically simple. Furthermore, in all applications, the distribution of the energy passing through any optical element must be carefully controlled to prevent laser-induced damage by localized high-energy concentration. Popular belief has it that laser beams are always collimated and that their wavefronts are planar. But this is true only in the limit, when the beam diameter approaches infinity. Because of diffraction, the beam cannot remain collimated indefinitely when the diameter is finite; with the exception of a narrow range where the beam may be considered as nearly collimated, it must either converge or diverge.