Dye-doped optical fibre; fibre laser; microcavity; whispering gallery mode.
We report the observation of multimode laser operation at wavelengths corresponding to whispering-gallery modes from a freestanding microring cavity based on rhodamine B dye-doped PMMA hollow optical fibre. Cylindrical microcavities with diameters 155, 340 and 615 𝜇m were fabricated from a dye-doped hollow polymer optical fibre preform. An average mode spacing of 0.17 nm was observed for the 340 𝜇m cavity. This shows that the laser mode intensity distribution is concentrated on the outer edge of the cavity.