Haas Laser Technologies, Inc is one of the world’s leading suppliers of metal mirrors for a wide variety of industrial laser applications. Advanced production techniques and specialized precious metal deposition processes have enabled the company to develop a wide variety of sizes along with convex, concave, cylindrical and aspheric curvatures. Internal water cooling is also available.
Copper is the prime metal substrate employed for mirrors used in high-power C.W. and pulsed infra-red lasers. Its excellent thermal properties give long life-times and provide reliable performance in industrial applications.
Bare copper, like most bulk metals, has a grain structure in the surface due to its natural crystalinity. This grain structure shows the grain boundaries which are subject to a thermoelastic mechanism failure when under irradiation known as “slip banding”. Under this condition, grain boundaries on the surface slip, increasing the localized absorption, swiftly leading to catastrophic melting, cratering, roughening, and destruction of the optic. This happens way before the mirror damages.
“Slip banding” is the limiting damage mechanism in mirror substrates and why the material should be chosen for it’s thermoelastic properties and not it’s thermal properties. It is the thermoelastic weakness in the grain structure of crystalline materials that leads to optic damage.
“The use of microdispersed polycrystalline materials between the reflective layer and the metal base significantly improves the stability of metal optics…” (Prokohorov “ Laser Heating of Metals”, 1990). Prokohorov’s theory of damage to optical materials from “slip banding” has been widely demonstrated to show that the limiting factor is the crystalline surface structure. If the surface structure contains no grain boundaries, the limitation imposed on the mirror by slip banding is removed, and the mirror is free to work at much higher powers.
Haas Laser Technologies, Inc’s use of amorphous nickel plating, gives a surface layer free from grain boundaries and crystalline features and hence no possibility of damage by “slip banding”. This enables the highest possible damage thresholds for pulsed and CW lasers. The amorphous nickel is then gold plated giving an exceptionally smooth surface which never peels or flakes even when burnt or scratched. There is no danger of immediate catastrophic failure as with nearly all other mirrors. Mirrors can therefore be replaced when convenient.
In addition, gold is totally inert to all aqueous and organtic liquids and gases over long periods of exposure. Mirrors can be stored for many years without loss in performance. Bare copper absorbs sulfur and chlorine like a sponge causing tarnishing and some loss of reflectance in industrial environments.
Metals can be difficult to polish. Soft metals such as copper can be particularly problematic in achieving good surface form and polish quality. The very best surface form and finish are still only achievable with traditional polishing techniques.
A secondary method to polishing is diamond turning. Diamond turned mirrors exhibit lower damage thresholds, have rougher surfaces, and have more scatter. In addition, they tend to project “Target Patterns” and show ripple marks common with diamond machining.
Talysurf measurements at the National Physical Laboratory, and electron Microscopy measurements at several universities have shown conforming surface roughness results on Haas Laser Technologies, Inc’s polished optics to be between 5-10 Angstroms RMS. This can be compared to the very best diamond turned surfaces of between 50 – 100 Angstroms.
CW LASER DAMAGE
Copper baser mirrors have long been the mirror of choice for high power CW and pulsed lasers. Although widely studied, values for the Laser Induced Damage Threshold (LIDT) have been inconsistent, and a variety of damage mechanisms have been postulated.
Reliable damage thresholds have been established in conjunction with the University of Surrey, Lazer Zentrum Hannover, and several research institutions over a two-year period to investigate the physical processes involved.
The most useful and fundamental result is that LIDT in CW lasers is related to the diameter of the irradiated area and the power.
LIDT = P/D (P = laser power in watts, D = spot size in mm)
To establish whether the LIDT value is exceeded in a specific system, one needs simply to know the maximum power used, and divide by the beams diameter.
The LIDT for Haas Laser Technologies, Inc’s Cu/Ni/Au optics is 4000 W/mm.
Some comparisons are: Fresh bare copper 2500 W/mm
Gold coated Silicon 600-800 W/mm
The value of 4000W/mm as LIDT for a Haas Laser Technologies, Inc’s Cu/Ni/Au type mirror has been obtained with a wide variety of spot sizes and powers. The relationship is remarkably linear, allowing LIDT’s to be accurate for any combination of laser power and beam diameter.
PULSED LASER DAMAGE
From a theoretical standpoint it is difficult to account for pulsed laser damage to bare copper mirrors to occur strictly from heating effects. Even taking into account non-linearity in the physical constants of the metal, catastrophic damage by heating and melting is not attributable as a result of the intrinsic absorption of the copper substrate.
One obvious explanation of pulsed laser damage is that defects such as scratches and digs from polishing that act as damage initiation sites. This can be seen in variations in pulsed LIDT values.
Haas Laser Technologies, Inc’s chemical polishing enables mirror surfaces to be free from residual features like scratches and digs. This process produces a typical surface roughness 5 Angstroms. In addition, there are no repetitive spatial features such as turning lines like in diamond turned optics which introduce beam scatter.
Pulsed damage thresholds of copper mirrors are measured to be typically between
6.4 – 9.6 J/cm2. Silicon is also measured to be within the same range. Haas Laser Technologies, Inc’s Cu/Ni/Au mirrors have been independently measured to be
46.7 J/cm2. This is 10 times higher than copper and silicon.
*Based on a 80ns laser pulse
Gold is totally inert to all aqueous and organic liquids over long periods of exposure.
This means that there is no loss in reflectance over time as compared to copper and silicon based mirrors. Typical reflectance for Cu/Ni/Au mirrors are as follows:
10.6um (CO2) >99.0%.
1.064nm (Nd:YAG) >98.5%
632.8nm (HeNe) >90%
(Goldmax enhanced coatings are also available for internal resonator optics which provide a reflectivity of >99.8%)
Chemically deposited gold coatings have almost unmeasurable phase shift. Therefore, many mirrors can be used in a system without making the polarization elliptical. Gold coated mirrors can be used with any wavelength or angle of incidence. Hence, there is no need to choose a mirror optimized for a very narrow set of conditions.
INTERFEROMETRY AND TESTING
There is a long history and evolution of mirror testing techniques starting from the manufacture of telescopes and astronomical reflectors. We, at Haas Laser Technologies, Inc have adapted and modified these techniques for modern day laser mirrors.
In the early stages of manufacture, a spherometer is used which can determine accuracies of 1 fringe over a 150mm diameter. A Fizeau interferometer in used on finished mirrors to image the wave front distortion in a double pass configuration and to measure the radius of curvature directly. Calibration results using the Fizeau test along with Ronchi and Foucault tests are compared to that of independent tests performed by the National Physical Laboratory London.
These are a few benefits of Cu/Ni/Au mirrors.
- High resistance to high power industrial lasers.
- No effect on beam divergence or mode.
- No effect on beam polarization.
- Excellent reflectivity.
- Rugged to withstand cleaning, debris, and fumes.
Comparison between gold plated copper –vs- silicon mirrors:
Specification Copper Base Silicon Base
Damage Threshold 4000 W/mm 800 W/mm
Phase Shift <0.5 Deg. 2-6 Deg.
Reflectivity 99% 99.2+
Coating Adhesion Never peels or flakes Fails Catastrophically
Wavelength/ Incidence Not Important Needs to be specified
Environmental Inert to all chemicals Sensitive to environment