Benefits of E-nickel
CO2 laser OEM's have long specified nickel plated copper mirrors for their cavity mirrors, especially in slab discharge lasers and sealed laser cavities. The reasons are:
- Inertness to the discharge laser gas - The hot laser gas can react with any exposed copper and undermine the coating at any borders/edges.
- Exposed copper reacts with the discharge, and can contaminate the laser gas.
Inertness to cooling water
Copper in the presence of water and in contact with dissimilar metals such as steel water fittings, aluminium mounts etc, can rapidly corrode.
A diamond machined (SPDT) copper mirror will have visible cutting arcs and regularly spaced chatter patterns on the face. Along with this is the exposed crystalline structure of the copper. The surface grains of copper are typically a few hundred microns in size, and give the mirror surface a crazed, or orange peel texture.
Laser damage occurs by the mechanism of thermo-elastic deformation, where these individual grains break away from each other (in the same way a crazy paving stone can break away from the rest). As Electroless nickel is amorphous, it has no crystalline grain structure, so polished surfaces are smoother and have much higher laser damage thresholds.A benefit of LBP's chemical polishing is the dramatic reduction in "mid spatial frequency surface roughness" ; this is the source of scatter and diffraction at visible wavlengths.
Ease of cleaning/preparing for coating
Bare copper mirrors react with the contamination in the environment, and will easily stain. Physically touching the soft copper will cause scratches. Electroless nickel coated mirror substrates will remain unchanged for months, and although we do vacuum pack mirrors, it’s not strictly necessary. Additionally Enickel can be cleaned with solvent/tissue (we prefer cotton wool) quite robustly without scratching.
Diffusion Barrier Different metals such as Gold, Copper and Silver in intimate contact can rapidly diffuse into each other, creating a "solid solution". This can happen over weeks or months, and degrades the mirror refelctivity. An Electroless Nickel layer is a very effective "diffusion barrier". We have sample mirrors that are more than 20 years old, still in perfect conditions. Unlike other metal mirrors, LBP mirrors perform at elevated temperatures reliably.