Light beams propagating dominantly in one direction.

In most cases, a Laser emits Light in the form of a laser beam. This means that the light dominantly propagates in a certain direction, typically with most of the light power concentrated to a small area of the order of square millimeter. Laser beams are often approaching Gaussian beams, where the transverse profile of the Intensity of the beam can be described with a Gaussian function, the width of which varies along the propagation direction. This variation of beam size can be very small for beams with large width or very FAST for tightly focused beams.