Ordinary glass cube beam splitters are constructed with a dielectric or hybrid coating on the hypotenuse of a right angle prism which is then cemented to another right angle prism. These fail in high energy applications because the cement absorbs enough energy to cause problems.
For higher power applications, the two prisms are optically contacted so that there is no cement to fail.
I have read that for even higher power applications, the prisms are air spaced.
Are these air spaced prisms constructed by coating one prism, and bringing it close to, but not touching, the second prism? If so, why do these have a higher damage threshold than optically contacted prisms? Is the spacing chosen to be large enough to avoid frustrated total internal reflection?
Or, when people refer to air spaced prisms do they mean that the mechanism is frustrated total internal reflection, and there is no coating on either prism?