Following on from a blog article we published in December 2012, we now have more data about why our chemically polished mirrors perform better than those made by diamond machining.
We had a number of visitors to our stand at the Laser World of Photonics Show in Munich last year talking about this very subject. This photo shows you clearly the surface roughness of a diamond machined mirror that results in scatter and diffraction.
The surface roughness of such a mirror may well be measured as quite good, in this case we were told Ra = 5nm. A single descriptor for surface roughness (e.g., Ra, Rq, Rz,) can be misleading in evaluating surface roughness though. Two surfaces, one polished and one diamond turned, can yield near identical roughness values, yet behave entirely differently.
The repetitive nature of diamond tool marks can make a diamond turned surface with a specified surface roughness value unacceptable for visible, and certainly UV & X-ray wavelengths. A polished surface (with ‘random’ surface imperfections, but an identical roughness value) would be perfectly adequate. Hence many diamond turned or SPDT optics for visible and UV applications are often post-polished.
If coated, these SPDT artefacts will cause rapid changes in coating thickness. With high power lasers, the sharp edges or “cusps” on the diamond turned surface will add a shear stress to the coating and the coating may fail. Our proprietary polishing technique is very effective in removing the mid spatial frequency surface roughness and this is a significant improvement for visible optics.
If you want more information our newsletter of June 2014 has the surface roughness values and can be found on our website. You can also visit our web page that tells you more about our custom lapping and polishing services for laser and non-laser applications.