The direction of propagation of the beam is perpendicular to the reference plane. The beam distribution in space is described in relation to this reference plane.

The spatial properties of the laser beam is determined by the optical cavity, and especially by the standing wave pattern created inside (as described in chapter 4).

It is possible to control some beam properties by using additional optical elements inside the optical cavity.

Additional optical elements can be used outside the optical cavity, but they can not influence the basic nature of the laser modes. Thus, they can not change the limits of the beam divergence.