Beam Divergence

The BEAM DIVERGENCE ANGLE of a laser beam is a measure for how fast the beam expands far from the beam waist, i.e., in the so-called far field. Note that it is not a local property of a beam, for a certain position along its path, but a property of the beam as a whole. 

A low beam divergence can be important for applications such as pointing or free-space optical communications. Beams with very small divergence, with approximately constant beam radius over significant propagation distances, are called collimated beams; they can be generated from strongly divergence beams with beam collimators.

Some amount of divergence is unavoidable due to the general nature of waves (assuming that the light propagates in a homogeneous medium, not in a waveguide). That amount is larger for tightly focused beams. If a beam has a substantially larger beam divergence than physically possibly, it is said to have a poor beam quality. 

Measurement of Beam Divergence

For the measurement of beam divergence, one usually measures the beam caustic, the beam radius at different positions, using a beam profiler.

It is also possible to derive the beam divergence from the complex amplitude profile of the beam in a single plane

One may also simply measure the beam intensity profile at a location far away from the beam waist, where the beam radius is much larger than its value at the beam waist. The beam divergence angle may then be approximated by the measured beam radius divided by the distance from the beam waist.