Most laser material processing is sensitive to the polarisation of the laser beam. Laser Beam Products offer a range of components and assemblies that control or manipulative the polarisation of a laser.

Zero Phase Shift Mirrors

Most lasers emit light with a known accurate polaristion, often circular polarisation in the case of metal cutting and welding lasers. Since light undergoes a change of polarisation on reflection, every mirror following the laser can degrade the polarisation state. Ideally this should be eliminated as far as possible.

Degradation of the state of polarisation is often measured as “phase shift” ( an unwanted change in the electrical field of the laser beam). Solid metal mirrors have one of the lowest polarisation effects known, less than 0.5 degrees of phase shift. Other vendors offer mirrors with phase shifts of up to 2 degrees, yet still misleadingly call these “zero phase shift”

Phase shifts as little as 4 degrees can be seen to reduce the edge quality of laser cut metal, and this can happen with just two poorly specified mirrors. Where more than two mirrors are used in a laser beam delivery system, solid metal mirrors are the best choice.

Phase Shifting Mirrors

Sometimes called “polarisers” “ECQ’s” “retarders” or “wave plates”.

This type of mirror has a deliberate and large phase shift. To turn linearly polarised light into circularly polarised light, a phase shift of exactly 90 degrees is needed.

Some years ago coating limitations meant it was only possible to achieve a full 90 degrees of phase shift with two mirrors each of 45 degrees phase shift. In modern times just one mirror with a sophisticated coating can give 90 degrees of phase shift. (90 degrees of phase shift is often called a ¼ wave shift since 90 degrees is ¼ of a 360 degree cycle).

Polarisation Locking Mirrors

Laser resonators rely on small differences in the reflectivity of fold mirrors to different polarisation states to give a stable linearly polarised output. In some cases the small reflection differences of the fold mirrors are not enough to keep the polarisaton from fluctuating. With dielectric coatings designed to give large differences in the reflection of S polarisation compared to P polarisation a truly stable “locked” polarisation is achieved.