Rami Arieli: "The Laser Adventure" Chapter 6 Page 2.
The active medium
The material used as the active medium determines:
  1. Laser Wavelength.
  2. Preferred pumping method.
  3. Order of magnitude of the laser output.
  4. The efficiency of the laser system.
We saw that the two basic requirements for laser action are:
  1. Population Inversion between the upper and lower laser energy levels.
  2. The active medium must be transparent to the output wavelength.
The active medium determines most of the laser properties, and that is why the laser name is derived from the name of the active medium.

This chapter is constructed as a data base, with information on the different laser types.
Each laser is classified according to the classification schemes described earlier.