  Whispering-gallery mode (WGM) resonators combine small optical mode volumes
with narrow resonance linewidths, making them exciting platforms for a variety
of applications. Here we report a flying WGM microlaser, realized by optically
trapping a dye-doped microparticle within a liquid-filled hollow-core photonic
crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed
excitation laser whose wavelength matches the absorption band of the dye. The
laser emits into core-guided modes that can be detected at the endfaces of the
HC-PCF. Using radiation forces, the microlaser can be freely propelled along
the HC-PCF over multi-cm distances - orders of magnitude further than in
previous experiments where tweezers and fiber traps were used. The system can
be used to measure temperature with high spatial resolution, by exploiting the
temperature-dependent frequency shift of the lasing modes, and also for precise
delivery of light to remote locations.
