  We report on microtube ring resonators with quantum wells embedded as an
optically active material. Optical modes are observed over a broad energy
range. Their properties strongly depend on the exact geometry of the microtube
along its axis. In particular we observe (i) preferential emission of light on
the inside edge of the microtube and (ii) confinement of light also in
direction of the tube axis by an axially varying geometry which is explained in
an expanded waveguide model.
