External optics are needed to focus and direct laser beams and to manipulate light in other ways that let it be put to practical use. The field of optics is a science in itself and can legitimately be considered to include laser technology as well as other diverse areas. This chapter cannot hope to tell everything about optics, but it can give an overview of the types of optics often used with laser systems. Readers seeking more detail can select from a number of handbooks (e.g., Department of Defense, 1962; Driscoll, 1978; Malacara, 1988; Melles Griot, 1988; Wolfe and Zissis, 1985) and textbooks (e.g., Hecht, 1987; M ller, 1988; Smith, 1990; Smith and Thomson, 1988).

Optical elements perform many tasks. Their simplest functions are the refraction and reflection of light, actions we normally associate with lenses and mirrors. In optical systems, other functions also can be important, such as selectively transmitting certain wavelengths of light, altering the polarization components of light, or breaking light up into its spectral components. These are generally considered passive optical functions because the components require no special control or input power to perform them. There are also active components which operate under active control or require input energy (other than light) to perform their function; examples include modulators which alter the intensity of a transmitted laser beam and scanners which move a beam.

This chapter concentrates more on passive optics because they are the most widely used. The principles of active optical devices can be extremely...