Uncoated copper mirrors are popular in high power CO2 laser welding systems. Having no coating at all, there is almost nothing to “go wrong” with the mirror and they can withstand very high laser powers.
Unfortunately Copper reacts with the atmosphere and uncoated Copper mirrors will lose reflectivity over time. Exactly how long it takes and the amount of tarnish and corrosion is dependent on the local conditions such as the humidity and the local air quality. A similar problem happens with Copper printed circuit boards where they can become difficult to solder.
Many laser users clean the copper mirror surface with a super fine abrasive polish, but it’s very easy to inadvertently remove copper from the centre of the mirror . This creates a considerably concave mirror out of what was once a flat mirror. The contour map above shows such a mirror with a regular concave surface. Each circle corresponds to a drop or “sag”of 0.3 microns, so this mirror is nearly 4 microns concave in the centre. That is about 10 times out of specification and indeed this mirror did turn the laser beam elliptical and astigmatic, just as a genuine weakly concave mirror would.
This will result in an oval shaped focussed spot giving uneven performance depending on if it’s the minor or major axis of the oval that is parallel to the direction of travel. It is possible to form what appears to be a circular spot with an astigmatic beam. Either side of this “circle of least confusion” though, the spot is elliptical.