Four types of copper-based mirror are offered by ULO Optics and described in subsequent data
sheets 11.1, 11.2, 11.3, 11.4.
Copper is an attractive substrate material for mirrors in CO2 laser systems due to its high thermal
All copper used in the manufacture of these mirrors is high-specification, oxygen-free.
Technical section 11 deals with zero-phase Cu mirror types, and
a fifth class of Cu mirror, for ‘phase retardation’, is described in technical data section 12.
Dimensions / tolerances
Copper mirrors can be fabricated in sizes from 5.0mm diameter up to 250mm diameter, and in
thicknesses from 3mm upwards (to suit the mirror diameter and type of cooling required).
Metric diameters and thicknesses are treated as standard, and are available from stock or with speedy
delivery. Imperial (inch) sizes are usually made-to-order only, and carry a small surcharge.
Long-radius mirrors are available and lists of relevant radii are given in the individual product data
Dimensional tolerances are:
Diameters : +0/-0.15mm
Thicknesses : +/-0.15mm
Parallelism : ETV within 0.05mm
The four product classes are described below:
Data section 11.1, NiCu mirrors
Nickel-copper (NiCu) mirrors consist of a copper substrate, precision-lapped to the required surface
form, nickle-plated using an electroless process. The thin nickel layer is optically polished to a high
standard of accuracy and cosmetic finish.
The mirror is then hard gold plated. Of all types of Cu mirror, the NiCu type is the most commonly
Data section 11.3, ‘Goldmax’ mirrors
The ‘Goldmax’ range are enhanced-coated mirrors. The manufacturing process is similar to that for
NiCu mirrors up to the final stage. A dielectric stack is vacuum-deposited onto the surface, giving
high reflectivity for both of ‘S’ and ‘P’ polarizations, (R>=99.6%).
An added advantage of this coating type is that it is of the ‘zero-phase-shift’ type, so that, if these
mirrors are used external to a laser cavity (in the beam delivery system), the state of polarization of
the laser remains unaffected.
A wide range of mirror types is available, and low-volume ‘specials’ are attractively priced.
Data section 11.4, ‘Supermax’ mirrors
‘Supermax’ coated mirrors are diamond-machined to the required surface form, and enhanced reflectivity
coated. Supermax mirrors have the highest reflectivity (lowest absorption) of all the Cu
mirror types, with R=99.9% being typical.
These mirrors have an excellent ‘zero-phase’ characteristic when used at 45° incidence, and a high
durability coating which allows for easy cleaning.
The production process is suitable for volume production, and small quantities of ‘specials’ are
uneconomic. Hence, Supermax mirrors are available in a limited number of sizes (the standard size
list below does not apply to Supermax mirrors).
The standard sizes are detailed in table 11.01 below:
Water cooling is via two rear tapped M6 holes. If requested, water cooled mirrors will be supplied complete
with pipe fittings to suit 6/4 metric piping.
Cu mirrors are available with the following custom features:
* Rectangular or square shapes.
* Central or offset holes.
* Holes at 45 degrees to the surface.
* Convex radii.
* Arrays of holes.
As part of the manufacturing process, these mirrors will often have a rear, central tapped hole. This
tapped hole should not be used to retain the mirror, since, in general, this procedure may cause severe
deformation of the mirror surface.
Laser Damage Thresholds
A substantial volume of experimental work shows that the best way of expressing the laser-induceddamage-
threshold (LIDT) for pulsed radiation is in an intensity related form, such as:
‘megawatts/sq mm’, or similar, for a given pulse length.
Recent experimental work on continuous-wave (cw) LIDT has shown that it is a different form of
expression, called ‘P/d’, which is consistant for changing laser powers for cw irradiation.
The individual data sections will express pulsed LIDT in the form ‘megawatts/sq mm’, and cw LIDT
in the form ‘W/mm’ where the linear dimension ‘d’ refers to a 1/e2 beam diameter on the test surface.
LIDT values apply to clean, unstressed components.
For any particular product, the cw LIDT expressed in P/d units is constant with changing laser
power. The relationship between P/d units and now obsolete (for cw) intensity-based units is shown
in table 11.02, where the tabulated results are ‘kW/cm2’.
Example: A product has a cw LIDT of 1000W/mm. If used with a 500W laser it will damage at a
beam diameter of 0.5mm