Article recently published in Electro Optics magazine (issue 257, October 2015)
As our customers are starting to work with visible light as well as the more traditional infrared, optics are increasingly required to perform well over a broader range of wavelengths. This means we need to look at how the mid-spatial frequency surface roughness affects the reflection of light in addition to the Ra surface roughness value and scratch-dig specifications.
A lot of applications are becoming more and more broadband and the days of having a mirror specified just for the infrared (IR) are becoming numbered; people want a multifunctional mirror that will work not just in the IR but perhaps for the visible as well.
What we are finding is that although the final application of the mirror might be for an IR application such as CO2 laser cutting (where surface roughness values in the range of tens of nanometres are often adequate) it may not be possible to test or align the part because these techniques often involve the use of visible light. For example you might be using terahertz or CO2 lasers – long wavelength applications where the quality of the mirrors doesn’t matter too much. But you have to align it or test it and that generally requires some sort of visible laser or visible technique. Then there will be scatter and diffraction of the visible light and although this might not matter when it is installed on the customer’s premises, if you can’t line it up in the factory then you are a bit stuck.
This is something that people overlook. We’ve had customers who think the mirrors don’t need to be that high a quality for their terahertz application, but then when they try to align their equipment with a visible laser they can’t do it.
Metal mirrors used for CO2 and terahertz applications are often produced by Single Point Diamond Turning (SPDT) whereby a flat, spheric, aspheric or even a freeform reflective surface is machined directly onto the mirror.
However, a simple surface roughness value and/or a scratch-dig specification have been found to be inadequate for SPDT metal mirrors used with visible or near infrared radiation. Using white light interferometric testing, we produced a cross section profile of a typical SPDT surface. Several families of grooves could be observed with different spacings and amplitudes; this significantly reduces the amount of specularly reflected visible light.
So although the SPDT parabolic mirror had a good surface roughness value of Ra = 5nm, when the mirror surface was analysed by spatial frequency a regularly repeating set of grooves could be seen with a spatial frequency in the order of 50–100 lines per mm. This repetitive mid spatial frequency surface roughness rendered the mirror unusable at 1um wavelength and produced a large amount of scattered and diffracted light at 633nm.
As a result of our chemical polishing, the mid spatial frequencies of the surface roughness of the same mirror were reduced considerably.
If you want good quality reflection from a mirror surface, then surface roughness Ra doesn’t tell the whole story. It’s the roughness and the important mid-spatial frequencies content which control the quality you get. So simply having a surface roughness specification of Ra = 5nm is just not sufficient.