Laser beams are often close to Gaussian beams, where the transverse profile of the optical intensity can be described with a Gaussian function, the width of which varies along the propagation direction.
Generally, laser beams exhibit a high degree of spatial coherence, which is related to a high beam quality. As a result, one obtains good focusability and the potential to form collimated beams with very low beam divergence.
When a laser beam hits some object (for example a workpiece in laser material processing), the arriving optical intensity is also called the irradiance.
The propagation of Gaussian beams can be calculated with a set of relatively simple equations. In cases with non-ideal beam quality, one can use a generalized set of equations which also involves the so-called beam quality factor M2. In this case, the equations cannot predict the detailed evolution of beam profile, but only of the beam radius based on the second moment of the intensity profile (D4σ method).
