It’s the way of the world, technology makes products smaller and lighter. We can make copper mirrors as small as 5mm diameter and 1mm thick. When Gold coated they are finding their way into medical and dental laser systems based on Er:YAG lasers.
The high laser damage threshold is the key enabler, and the good reflectivity of Gold in the visible is useful when aligning articulated arms with a red or green laser. Of course the same mirror can be used with CO2 lasers, so a just one articulated arm can be used for all three wavelengths, 633nm, 2.94um, and 10.6um.
Not such a big issue for dental laser arms is the weight of the mirrors, but for other applications the solution of making copper mirrors thinner and thinner becomes futile, as the loss of rigidity and stiffness means the mirrors are difficult to manufacture, and almost impossible to mount. Aluminium mirrors are around 30% of the weight of copper ones, they offer good laser damage resistance, and can have exactly the same coatings made on them as copper mirrors do. We have a page on our web site dedicated to Aluminium mirrors.
A good rule of thumb is that a laser mirror should have a diameter to thickness ratio of 5:1, like our standard 50mm diameter 10mm thick parts. On request 12:1 is possible, such as our 3″ diameter 1/4″ thick mirrors, but please no thinner!