Infrared fiber optics may logically be divided into three broad categories: glass, crystalline, and hollow waveguides. These categories may be further subdivided based on the fiber material or structure or both, as shown in Fig. 1.3. An example of a typical fiber for each subdivision is given in the last row of Fig. 1.3. Another way of viewing IR fibers that is more convenient from the viewpoint of laser power delivery applications, is to ask for the best IR fibers for use at the two most popular IR laser wavelengths: the 10-µm CO2 laser and the 3-µm Er:YAG laser. The chart in Fig. 1.4 is a guide to the most useful fibers available today for delivering these two laser energies.