  We study the vacuum-induced degradation of high-finesse optical cavities with
mirror coatings composed of SiO$_2$-Ta$_{2}$O$_{5}$ dielectric stacks, and
present methods to protect these coatings and to recover their initial quality
factor. For separate coatings with reflectivities centered at 370 nm and 422
nm, a vacuum-induced continuous increase in optical loss occurs if the
surface-layer coating is made of Ta$_{2}$O$_{5}$, while it does not occur if it
is made of SiO$_2$. The incurred optical loss can be reversed by filling the
vacuum chamber with oxygen at atmospheric pressure, and the recovery rate can
be strongly accelerated by continuous laser illumination at 422 nm. Both the
degradation and the recovery processes depend strongly on temperature. We find
that a 1 nm-thick layer of SiO$_2$ passivating the Ta$_{2}$O$_{5}$ surface
layer is sufficient to reduce the degradation rate by more than a factor of 10,
strongly supporting surface oxygen depletion as the primary degradation
mechanism.
