Unifying frequency combs in active and passive cavities: CW driving of temporal solitons in ring lasers

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.