  High-finesse microcavities offer a platform for compact, high-precision
sensing by employing high-reflectivity, low-loss mirrors to create effective
optical path lengths that are orders of magnitude larger than the device
geometry. Here, we investigate the radiation hardness of Fabry-Perot
microcavities formed from dielectric mirrors deposited on the tips of optical
fibers. The microcavities are irradiated under both conventional (~0.1 Gy/s)
and ultrahigh (FLASH, ~20 Gy/s) radiotherapy dose rates. Within our measurement
sensitivity of ~40 ppm loss, we observe no degradation in the mirror absorption
after irradiation with over 300 Gy accumulated dose. This result highlights the
excellent radiation hardness of the dielectric mirrors forming the cavities,
enabling new optics-based, real-time, in-vivo, tissue-equivalent radiation
dosimeters with ~10 micron spatial resolution (our motivation), as well as
other applications in high-radiation environments.
