  Typical microresonators exhibit a large frequency spacing between resonances
and a limited tunability. This impedes their use in a large class of
applications which require a resonance of the microcavity to coincide with a
predetermined frequency. Here, we experimentally overcome this limitation with
highly prolate-shaped whispering-gallery-mode "bottle microresonators"
fabricated from standard optical glass fibers. Our resonators combine an
ultra-high quality factor of 360 million, a small mode volume, and near
lossless fibre coupling, characteristic of whispering-gallery-mode resonators,
with a simple and customizable mode structure enabling full tunability.
