It’s not widely appreciated how easy it is to distort a laser mirror with mechanical forces that can be generated from inadvertently over-tightening mounts, or water fittings. Although our experience is mostly with metal mirrors such as copper or aluminium, we know glass mirrors are just as sensitive to distortion. Sometimes the distortion is elastic, so the surface figure is restored when the mechanical force is removed. Often though the distortion can be permanent. We recently repaired a water cooled mirror for a customer that was very thick ( 33mm ) and looked to be in good condition.
By reflecting a high mode quality visible laser beam from the mirror at 45 degrees incidence we could see the distortion the mirror introduced into the reflected beam profile. Using our phase shifting interferometer we could resolve a 2-3mm diameter convex ‘pip’, perhaps just a micron or so high, in the centre of the mirror. This is where the surface had been ‘punched through’ by the mounting screw in the rear of the mirror.
There was 16 mm thickness of solid copper between the bottom of the screw hole and the mirror face. Despite this though, it was still possible for the mechanical force from tightening of the mounting screw to permanently distort the mirror face.
The hot spot created in the reflected beam by the mirror distortion when used with a CO2 laser would have likely caused damage to other optics, such as the focus lens.