  We experimentally demonstrate a ring geometry all-fiber cavity system for
cavity quantum electrodynamics with an ensemble of cold atoms. The fiber cavity
contains a nanofiber section which mediates atom-light interactions through an
evanescent field. We observe well-resolved, vacuum Rabi splitting of the cavity
transmission spectrum in the weak driving limit due to a collective enhancement
of the coupling rate by the ensemble of atoms within the evanescent field, and
we present a simple theoretical model to describe this. In addition, we
demonstrate a method to control and stabilize the resonant frequency of the
cavity by utilizing the thermal properties of the nanofiber.
