  We clarify theoretically that the topological ring-cavity (TRC) modes
propagating along the interface between two honeycomb-type photonic crystals
distinct in topology can be exploited for achieving stable single-mode lasing,
with the maximal intensity larger than a whispering-gallery-mode counterpart by
order of magnitude. Especially, we show that the TRC modes located at the bulk
bandgap center benefit maximally from the gain profile since they are most
concentrated and uniform along the ring cavity, and that, inheriting from the
Dirac-like dispersion of topological interface states, they are separated in
frequency from each other and from other photonic modes, both favoring
intrinsically single-mode lasing. A TRC mode running in a specific direction
with desired orbital angular momentum can be stimulated selectively by
injecting circularly polarized light. The TRC laser proposed in the present
work can be fabricated by means of advanced semiconductor nanotechnologies,
which generates chiral laser beams ideal for novel photonic functions.
