  Dissipative soliton Kerr frequency combs in microresonators have recently
been demonstrated with the self-injection locking process. They have the
advantage of turnkey deterministic comb generation and simplifying dark soliton
generation in the normal dispersion regime. Here, the formation process of dark
pulses triggered by self-injection locking is studied by regarding them as a
pair of domain walls that connect domains having different intracavity powers.
The self-injection locking mechanism allows the domain walls to self-regulate
their position so that a wide range of dark comb states can be accessed, and
the duty cycle is controlled by the feedback phase. Direct imaging of the dark
pulse shape using the electro-optic sampling technique is used to verify the
theory. The results provide new physical insights as well as a new operational
modality for this important class of nonlinear waves.
