Thermal noise in mirror optical coatings may not only limit the sensitivity of future gravitational-wave
detectors in their most sensitive frequency band but is also a major impediment for experiments that aim to
reach the standard quantum limit or cool mechanical systems to their quantum ground state. We present the
design and experimental characterization of a highly reflecting mirror without any optical coating. This
coating-free mirror is based on total internal reflection and Brewster-angle coupling. In order to characterize its
performance, the coating-free mirror was incorporated into a triangular ring cavity together with a high quality
conventional mirror. The finesse of this cavity was measured using an amplitude transfer function to be about
F 4000. This finesse corresponds to a reflectivity of the coating-free mirror of about R 99.89%. In addition,
the dependence of the reflectivity on rotation was mapped out