Beam quality

The beam path of a real laser beam (green) has a larger divergence angle than that of an ideal Gaussian beam (red)

The beam quality , also known as the beam quality index , beam propagation or K factor , is a dimensionless quality feature of laser radiation . ${\ displaystyle K}$

In principle, it says something about how well a laser beam can be focused and how quickly it expands relative to its diameter when it propagates ( divergence , beam parameter product ).

The beam quality K is the reciprocal of the diffraction index M ² and therefore indicates the divergence angle of an ideal Gaussian beam compared to the divergence angle of a real laser beam with the same diameter at the beam waist: ${\ displaystyle \ varphi}$

{\ displaystyle K = {\ frac {1} {M ^ {2}}} = {\ frac {\ varphi _ {\ mathrm {Gauss}}} {\ varphi _ {\ mathrm {real}}}}}

for an ideal laser beam (basic mode , TEM 00) applies ${\ displaystyle M ^ {2} = 1 \ Rightarrow K = 1}$
with a real laser beam ${\ displaystyle M ^ {2}> 1 \ Rightarrow K <1.}$

A real laser beam is therefore more difficult to focus through a lens or focusing optics (e.g. a concave mirror ) than an ideal laser beam. This is expressed by the fact that with real rays only one area (focal point) is formed which cannot fall below certain dimensions at a certain distance. This means that the cross-section of a real laser beam - even a focused one - is always greater than zero. This results from the finite wavelength of the radiation and from the fact that real laser beams usually do not have the ideal power distribution, i.e. H. their power does not decrease equally in all directions towards the edge of the beam. The reason for this are transversal oscillation modes in the laser resonator , which leads to power peaks and troughs in the beam, which are also deviations in direction.

The divergence of a laser beam, i.e. H. the divergence of the beam during its propagation (beam propagation) is described by the numerical aperture or by angle specifications. Due to the resonator geometry, the angles can be different in both spatial directions, which means that the height and diameter or width of the focal spot differ from one another. This is particularly the case with laser diodes and diode lasers .

To z. B. to cover a large beam path in air in a processing machine without much divergence, a laser beam must have a low divergence.

Both the divergence and the focusability can be improved by widening the beam (see telescope ): with an arrangement of two lenses or mirrors, the cross-section of the beam is enlarged in favor of its parallelism . However, larger and more precise focusing lenses or mirrors are then required.

literature

Jürgen Eichler, Lothar Dünkel, Bernd Eppich: The beam quality of lasers - How do you determine the diffraction index and beam diameter in practice? In: Laser Technik Journal . tape 1 , no. 2 , October 2004, p. 63-66 , doi : 10.1002 / latj.200790019 .