Laser beam quality is a critical parameter in any laser device. The M2 factor is a dimensionless value assigned to different laser beams used to identify beam quality in a quantitative way. It is calculated using the waist width and divergence of a laser beam. The higher the M2 value, the worse the beam quality. The ideal M2 value is 1, which is the value for a Gaussian beam that has a bell-shaped peak along its central optical axis. However, the Gaussian beam is a theoretical idealized beam that is not achievable in reality.

The M2 factor is defined by how tightly a laser beam’s waist can be focused. A laser with an M2 factor of 1 is considered a perfect Gaussian TEM00 beam. A Gaussian beam is an ideal conical-shaped beam. This isn't possible from actual devices, though, so the goal is to get as close to that value as possible. Power intensity per unit area of a beam is usually more widely distributed, so the beam can’t reach perfect performance. 

The M2 factor is also known as the beam propagation factor, beam propagation ratio, or beam quality factor. All terms refer to the same dimensionless number that gives a quantitative indication of the quality of a laser beam. 

The M2 factor works by measuring the waist width and divergence angle of a laser. These parameters are then used to determine the M2 factor using the equation detailed later in this article. By identifying the M2 factor of the laser, one can estimate how well its power output is applied to the target. The higher the M2 value the worse the beam quality.