  Frequency combs have become a prominent research area in optics. Of
particular interest as integrated comb technology are chip-scale sources, such
as semiconductor lasers and microresonators, which consist of resonators
embedding a nonlinear medium either with or without population inversion. Such
active and passive cavities were so far treated distinctly. Here we propose a
formal unification by introducing a general equation that describes both types
of cavities. The equation also captures the physics of a hybrid device - a
semiconductor ring laser with an external optical drive - in which we show the
existence of temporal solitons, previously identified only in microresonators,
thanks to symmetry breaking and self-localization phenomena typical of
spatially-extended dissipative systems.
