The cavity of this laser is plane concave. Consequently, the radius of the beam at the plane mirror is equivalent to the laser beam waist. Beam divergence is the angle at which the beam transversely spreads. This is inversely related to the waist. Refer to figure 5.
TEM modes are intensity patterns that can be views, if looking at the cross section of a beam.
These modes are comparable to the natural modes of a standing wave. By adjusting the cavity length certain modes can be isolated. The laser, two mirrors, and telescope are located on a 4'x 8' optical table. The Fabry Perot interferometer is placed some distance from the table. Refer to figure 6.