M squared

The beam quality factor is a dimensionless quantity that characterizes a laser beam, the larger the worse the beam is to be focused, ie the greater is the smallest possible spot size.

Definition

The beam quality is the divergence angle of the laser beam on a real compared to the divergence angle of an ideal Gaussian beam having the same diameter at the beam waist:

A Gaussian beam has, by definition, the beam quality in real laser beams is always greater than one, eg is a typical value for the beam quality of a single-mode laser

The beam quality factor is often referred to loosely as " beam quality ". This is however physically impossible, strictly speaking, the entropy of the radiation is a measure of the quality of a radiation field. Instead of the beam quality also two other sizes are sometimes used:

• The beam parameter product of the wavelength
• The beam quality, also called Strahlpropagations or K - factor:

Typical values ​​and practical

Typical M2 values ​​in high-power lasers are:

When comparing the focusing ability of these lasers, however, is still different wavelength considered ( see above beam parameter product and beam quality ).

In particular, in laser material processing with laser cutting, laser welding and laser marking the beam quality has a great impact on the working result: during laser welding results in a high M2 value (and thus a large divergence angle or focus diameter) to a broader and less deep seam and the laser marking to a less sharp image.