Rami Arieli: "The Laser Adventure" Chapter 6.2.3 page 3
Tuning the Laser Wavelength
An example of such tuning element can be seen in figure 6.17, which show a prism inside the optical cavity.
Figure 6.17: Choosing a single wavelength in a tunable laser with a prism.

The dispersion of the prism cause each wavelength to bend at different angle, and only one wavelength will continue to move back and forth within the optical cavity.
Moving the prism enable selecting the desired wavelength.

The pump bands are: