Coating
Hard Gold Coating
Electroplated hard gold coating has excellent and consistent reflectivity in the infrared and good reflectivity in the visible, which allows the easy use of visible alignment lasers. Unlike vacuum deposited hard gold coatings, our electroplated gold never peels, flakes or delaminates.
Polarisation changes to the laser beam on reflection are virtually zero, so for metal cutting and other polarisation sensitive applications, gold mirrors are the most reliable way of maintaining polarisation. Because reflectivity is almost independent of angle of incidence, gold is an excellent choice of coating for scanner mirrors or for mirrors used at varying angles of incidence – collimator mirrors for example.
Electroplated hard gold coating is in intimate thermal contact with the mirror substrate so localised burns, dust and scratches have little effect on the mirror’s performance – ideal for high power carbon dioxide laser cutting.
Electroplated gold is widely used with Er:YAG lasers, CO2 lasers, QCLs, OPOs and many other applications in the infrared from 1-300 microns wavelength, including terahertz imaging.
Benefits of hard gold coating
Our hard gold coating covers all the mirror surfaces – front sides and rear – so there are no discontinuities that can initiate delamination of the coating. Cassegrain reflectors and Herriot cells use our gold coated metal mirrors because the through holes in the mirror are also gold coated. The coating will withstand typical medical sterilisation procedures.
Chemically deposited hard gold coating has a huge laser damage threshold. With a copper mirror substrate, gold coated mirrors will withstand 40 Kilowatts – see laser damage resistance of CO2 laser optics.
There is a strong technical and economic case for using gold mirrors as beam delivery optics. A survey of used MaxR coated mirrors used for beam delivery has shown that most mirrors regardless of vendor, even if 99.8% reflective when new, are typically 98% reflective.
MaxR Coating
MaxR high reflectivity coatings for CO2 lasers are made on our copper or aluminium mirrors. There are situations e.g. laser gain media, when even tiny losses are a problem (such as cavity fold mirrors) and an enhanced (or MaxR) coating is essential.
Our MaxR coated mirrors are optimised for use as CO2 laser cavity mirrors BUT can also be used as beam delivery mirrors outside the laser resonator, such as in flying optics systems or as “table mirrors”.
Benefits of MaxR Coating
Our MaxR coating is particularly rugged and customers tell us it is the most durable mirror coating they have used. It withstands multiple cleaning and is very adherent. Our MaxR coating offers excellent resistance to the darkening effect of UV radiation generated in gas laser discharges, making it an excellent coating for cavity optics such as in CO2 lasers. These mirrors have a long operating life and a long shelf life.
The sides, edges and rear of the mirror are covered with electroless nickel, preventing it reacting with the laser gas and preventing the substrate from diffusing into the coating over a period – a problem we have seen with competitor’s copper mirrors.
MaxR coating is available on our copper substrate mirrors. Internal water cooling is recommended for resonator mirrors and water cooled copper mirrors are the product of choice for high power fold optics.
A series of thin film, high and low refractive index materials are deposited onto the polished substrate in a vacuum coating chamber. This can enhance the reflectivity at the specified wavelength and angle of incidence to greater than 99.8%. Such high levels of reflectance means that sophisticated calorimetric measurement is necessary to ensure specifications are met. We can do this in-house.
We offer different enhanced metal coatings for standard wavelengths. This coating design is highly optimised for CO2 laser lines, 10um – 11um, other wavelengths such as telecom or 1 micron will need coatings optimised for that spectral region.
UV/Visible
Mirror coatings are designed to achieve the maximum reflectivity over the widest spectrum of wavelengths. There is always a trade-off, in that the wider the range of operating wavelengths, the lower the overall reflectance.
Protected aluminium mirror coating offers excellent reflectivity in the UV and visible, >85%R from 200-700mm wavelength. The protective over-layer improves resistance to scratching and improves durability whilst cleaning. Above 700nm wavelength, gold or silver coatings should be used.
This protected aluminium coating can be deposited on to our copper or aluminium mirror substrates. The substrates are electroless nickel plated and super polished for extreme surface smoothness and scatter reduction. Polish quality is an important feature at shorter wavelengths. Diamond machined mirrors will have a level of scatter and diffraction from surface artefacts that is often not acceptable.
Uses for protected aluminium coating
Choose this coating for visible and near UV multispectral applications such as spectroscopy, microscopy, imaging, medical and scientific instrumentation or where electrical discharges generate destructive UV radiation.
Our protected aluminium coated mirrors can be manufactured with mounting features built-in, such as tapped holes and flanges. This reduces OEMs manufacturing costs and can simplify replacement.
Visible/IR
Protected Silver
Protected Silver mirror coating offers excellent reflectivity in the visible and near IR, approximately 99%R from 500-5um wavelength. The protective outer layer reduces silver’s tendency to tarnish and improves resistance to scratching whilst cleaning. This coating also withstands high temperatures and has a long shelf-life.
Silver coatings are especially useful in the 750nm – 1um region where aluminium coatings absorb. Silver does have poor reflectivity in the UV though.
Uses for protected silver coating
This coating can be deposited on to our copper or aluminium mirror substrates. These have been electroless nickel plated and super polished for extreme surface smoothness and scatter reduction.
Choose this coating for visible and near infrared multispectral applications such as spectroscopy, imaging, medical and scientific instrumentation, visual alignment and telecoms.
Protected silver coated mirrors can be manufactured with mounting features built-in such as tapped holes and flanges, to reduce manufacturing costs and simplify replacement.
Reflectivity
Protected silver coatings provide excellent reflectivity in the visible and infrared:
>98% @ 0.5-0.8 microns
>98% @ 2-10 microns
Other mirror coatings
We can supply mirrors with a variety of optical coatings including Gold, MaxR, Protected Aluminium, Protected Silver and thin film coating optimised for HeNe laser.
Tips for specifying mirror coatings
- Specify the angle of incidence, wavelength(s) and polarisation
- Very thin mirrors may distort due to tensile and compressive forces in the thin film coating layers
- A small perimeter area will be uncoated
- High reflectivity e.g. 99.8%+ can only be achieved over a small spectrum
- Thin Film modelling software can allow a coating to be designed and characterised
- High reflectivity e.g. 99.8%+ can only be achieved over a small spectrum
- Vacuum coating is a batch process. The price is fixed regardless of the quantity per batch. Small numbers can be quite expensive
