  We experimentally realize an optical fiber ring resonator that includes a
tapered section with subwavelength-diameter waist. In this section, the guided
light exhibits a significant evanescent field which allows for efficient
interfacing with optical emitters. A commercial tunable fiber beam splitter
provides simple and robust coupling to the resonator. Key parameters of the
resonator such as its out-coupling rate, free spectral range, and birefringence
can be adjusted. Thanks to the low taper- and coupling-losses, the resonator
exhibits an unloaded finesse of F=75+/-1, sufficient for reaching the regime of
strong coupling for emitters placed in the evanescent field. The system is
ideally suited for trapping ensembles of laser-cooled atoms along the nanofiber
section. Based on measured parameters, we estimate that the system can serve as
a platform for optical multimode strong coupling experiments. Finally, we
discuss the possibilities of using the resonator for applications based on
chiral quantum optics.
