What is laser beam polarisation and is it important?

An important feature of any laser beam is its state of polarisation. This identifies the direction, which is always at right angles to the direction of propagation, in which the electric field is vibrating. This vibration can be simple, having only one direction along the beam path (linear polarisation) or it can be complex. In the latter case, there are many possibilities, but the most commonly encountered behaviours are those of circular and elliptical polarisation. With circular polarisation, the electric field changes its orientation by 360° within one wavelength; with elliptical polarisation the rate of change is the same, but this time the magnitude of the field varies as well. Such polarisation states are determined principally by the optics in the laser source but can also be affected by optics in the transmission path. It is usual for the beam emerging from a CO₂ gas laser to be linearly polarised. That from a Nd:YAG laser is usually completely randomly polarised.

The significance of polarisation in materials processing is related to the way metals reflect incident radiation. Without going into theoretical descriptions, reflections are minimised (and consequently light absorption is at a maximum) when a linearly polarised beam has its electric field orientation set parallel to the plane of incidence (the plane which includes the incident beam, the reflected beam and the normal to the surface). The main impact of polarisation effects can be seen when cutting metals, where the actual angle of incidence can be high (70° to 80°), since the beam strikes and is absorbed by the inclined cut surface during the process and not by the original face of the workpiece. At these high angles the difference in reflection between orthogonal linearly polarised beams can be as much as 80% and manifests itself as a difference in cut quality and speed.

In welding operations, the effect is not normally noticed since the beam couples by an intermediate absorption mechanism and does not interact with the surface directly. Again, with laser surfacing, the angle of incidence does not deviate from the original zero degrees and the difference in polarisation-dependent reflectivities is not apparent. Because it is difficult to get the polarisation direction to follow the plane of incidence when changing processing direction, a compromise solution is often sought particularly for laser cutting. This involves modifying the laser beam to induce circular polarisation, such that the radiation impacting on the target metal changes its polarisation very rapidly (at the frequency of the laser radiation), thereby producing an acceptable time-averaged level of absorption.

This is achieved for CO₂ lasers by the introduction of what are known as phase retarding mirrors into the beam delivery systems. Either two 1/8 wave phase retarder can be used, or a single ¹/ ₄ wave phase retarder. The orientation of these mirrors is important to the direction of the polarisation vector in the incident laser beam. The mirrors must be set to reflect the beam at 45° to the plane of the polarisation vector or the phase retarder will not work correctly.