The cavity in a common He-Ne laser uses a semi confocal optical cavity.
It is composed of one planar mirror, which reflects about 98% of the light striking it, and a second concave mirror reflecting 100%. This concave mirror has a focal length equal to the length of the cavity (see figure 6.2).

Figure 6.2: The Cavity Structure of He-Ne Laser

 This arrangement of the mirrors causes the radiation to be an almost parallel beam (see chapter 4.4).
The importance of Brewster windows is explained in chapter 7.5.