Rami Arieli: "The Laser Adventure" Chapter 8 Section 2, page 1
8.2 Controlling the wavelength spectrum emitted from the laser
As seen in chapter 2, the active medium of the laser determines the wavelength (or wavelengths) emitted from the laser.

Figure 8.9 shows the wavelengths emitted from common lasers, and it can be seen that they are scattered all over the spectrum.

Figure 8.9: Wavelengths emitted from common lasers.

In chapter 5 it was explained that under the gain curve of the laser there are number of longitudinal laser modes, at equal distances from each other.

Some materials emit at several wavelengths, or a range of spectrum.

Some applications such as spectroscopy or holography require single wavelength, or even a single longitudinal mode (When a long coherence length is needed).

The method to obtain a single wavelength at the laser output is based on canceling all other wavelengths that has enough gain to create lasing.