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Definition of a laser beam

As a tool, the laser beam is characterised by certain properties. Fundamental properties of laser light are its monochromatic nature (one wavelength) and directionality (narrow divergence). In contrast to the beam from a normal light source, laser beams can be very sharply focused. In combination with a laser beam's high power density, a beam of such intensity and limited local focus can be generated that material can be melted or vaporised.

In laser material processing, this is employed in laser beam cutting or welding, for example. Since the laser beam can do this work virtually without contact, it does not deteriorate during the process as does a drill or milling head. The laser beam is directed by the beam generator (laser device) via beam steering to the processing optics using a mirror or optical fibre technology. The processing optic concentrates the laser beam and and focuses it, with focal lengths adjusted to the particular process. The typical properties of the focused laser beam are described by the minimum achievable beam diameter (focus) and the beam caustic. The caustic represents the path of the beam diameter coming from the processing optic to the focus and after it.

A significant parameter for laser material processing is so-called depth of focus, defined as Rayleigh length. It states the distance from the focus at which the area of the beam cross-section has doubled. If, for example, Rayleigh length and sheet metal thickness are adjusted to each other when laser beam cutting, straight cutting edges can be produced. If the sheet metal thickness is too great, the edges become increasingly more rounded.

The laser beam as a tool can thus be defined as follows:

  • The power of the laser beam describes the "strength" or "force" of the tool.
  • The beam distribution on the workpiece surface or in the focal plane describes the "shape of the tool head".
  • The dimensions of the laser beam on the workpiece surface or in the focal plane describe the "sharpness" of the tool, or the "tool width".
  • The Rayleigh length of the focus describes the usable "tool length".

It can be seen from the above that analysis of the laser beam tool by beam diagnostics must primarily be directed at the 4 named beam properties in order to describe or analyse the interaction of the laser beam with the tool during the process, and where applicable identify sources of error.