Characteristics of laser beam polarisation

Properties of laser beam polarisation:

An extremely important feature or property of a laser beam polarization is its state of polarization. It identifies the direction, which is always at right angles to the direction of propagation, in which the electric field is vibrating. This vibration is done only in one direction along the direction of the beam path (linear polarisation) or it can be complex.

In the furthermore case, the most common behaviours are those of elliptical and circular polarisation. With circular polarization, the electric field changes its orientation within the wavelength by 360°; with elliptical polarization, the rate of change is the same, but this time the magnitude of the field is also different.

We can simply determine the polarization states by observing the optics in the laser source but the optics can also affect it by its transmission path. It is common for the beam the emerges for a carbon dioxide laser to be linearly polarised.

Its importance

The importance of polarisation is the processing of materials depending on the way the metals reflect incident radiation. Without going into theoretical details, reflectance is minimized (and consequently light absorption is maximized) when the electric field orientation of a linearly polarized beam is parallel to the plane of incidence (the plane in which the incident beam, reflected beam and contains the surface normal to). It is highly important to get the light sources for optical fibre and perform the correct custom optical mirror mount to understand the properties of laser light polarization.

While cutting the metals we can observe the main effect of the polarization, where the value of the actual angle of incidence can be high, since the beam hits and gets absorbed by the inclined cut surface during the process and not by the workpiece’s original face.

At these high angles, the difference in reflection between orthogonal linearly polarized beams can be up to 80% and manifests as differences in cut quality and speed.

During the welding, it is quite difficult to observe the effect since the beam gets connected by an intermediate absorption mechanism and it does not connect with the surface directly.

Again, with laser surfacing, the angle of incidence does not deviate from the original zero degrees and the difference in polarization-dependent reflectance is not apparent.

Although it is quite difficult to get the direction of polarization in order to follow the plane of incidence when changing the processing direction, a compromise solution is sought, especially for laser cutting.