  We report on the observation of multimode strong coupling of a small ensemble
of atoms interacting with the field of a 30-m long fiber resonator containing a
nanofiber section. The collective light--matter coupling strength exceeds the
free spectral range and the atoms couple to consecutive longitudinal resonator
modes. The measured transmission spectra of the coupled atom-resonator system
provide evidence of this regime, realized with a few hundred atoms with an
intrinsic single-atom cooperativity of 0.26. These results are the starting
point for studies in a new setting of light-matter interaction, with strong
quantum non-linearities and a new type of dynamics.
