Classifying lasers into major types is a useful step between the previous general discussion of lasers and the specifics of the rest of this book. There are many potential criteria for classifying lasers, but the two most useful ones are the type of active medium and the way in which it is excited.

This approach follows the usual practice in the laser world, where most devices are groups as gas, solid-state, or semiconductor lasers. A few important lasers are exceptions. From a practical standpoint, the most important is the tunable dye laser described in Chap. 17, in which the active medium is an organic dye dissolved in a liquid solvent. To match the categories with the usual states of matter, dye lasers can be called liquid lasers. This is good enough for most practical purposes, although a few other liquid lasers have been demonstrated, and a few dyes can lase in vapor form or solid hosts. Two other exceptions are free-electron and x-ray lasers, for reasons described below.

Gas lasers can be excited in many different ways, producing devices that differ greatly. For that reason, it is useful to break them down by excitation mechanism. However, for other lasers the nature of the medium limits excitation mechanisms so much that such a breakdown is not useful.