We regularly make metal laser mirrors with one or more through holes in them for our customers. They are easy to make from metals, and with our own unique polishing techniques we can keep the surface form accuracy right up to the hole.
In addition, our gold coating uniformly covers the sides and internal surfaces of through holes, so all surfaces are coated with gold.
The hole in this mirror was used to pass a visible laser beam through. A visible laser diode was placed behind the mirror, and the beam passed through the central hole. The laser diode is protected from back reflections and scattered radiation by the mirror face.
We have also made mirrors with slots at the rear to accept a solid state laser bar that injects light into a resonant cavity, and mirrors with through holes for in line monitoring applications.
Mulitipass cells for chemical sensing, such as Herriott cells and White cells, often use Gold coated mirrors depending on the gas being sensed.