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Polished metal mirror surfaces have the surface smoothness, and low scatter normally specified for UV optics.

Electroless Nickel coating is amorphous, so there are no crystallites, or grain boundaries on the mirror surfaces. Laser induced damage, especially with pulsed lasers, is initiated at grain boundaries by "Thermoelastic Damage". Optical coatings such as gold coating, or enhanced silver coating made on electroless nickel are "epitaxial", so the reflective coating is also free of crystalline structure.

Laser Beam Products have taken the techniques used in semiconductor polishing, and developed them for the chemical polishing of metal mirrors for lasers. The material removal is entirely chemical, there is no mechanical abrasive action, leaving a flaw free, ultra smooth mirror surface, of sub nanometer surface roughness.

Because no abrasive is used, there are no embedded particles in the surface to act as absorbing sites.

POLISHING

Laser optics require exceptional surface quality for low scatter, and high laser damage threshold. A polished laser mirror surface quality is far superior to anything even the very best diamond machining can offer. Where surface quality is important, many diamond machined optics have to be "post polished" to improve the surface quality and remove the low level chatter, and flycutting patterns on the surface.

With a polished mirror there are no cutting arcs, target patterns or chatter marks that are so easily visible on the surface of a diamond machined mirror.

Flycutting arcs from diamond machining of Copper mirror

These unwanted surface faults can initiate laser damage, and cause diffraction of the laser beam. In some cases the diffraction and scatter of diamond machined optics is so great, a red alignment beam can be is completely lost after reflecting off a few mirrors.

Often a "target pattern" is visible on the surface of diamond machined mirror as seen on this surface roughness measurement.

Sometimes on diamond turned concave or convex mirrors a spike of metal can be left in the centre of the spherical mirror due to poor proof of centre during machining.

Many optical designers and specifiers are aware of the importance of surface quality and specify surface roughness as Ra (an average roughness) or Rrms (a root mean square). By both measurements Laser Beam Products offer the smoothest optical mirror surfaces available.