  Dissipative soliton Kerr frequency combs in microresonators have recently
been demonstrated with self-injection locking. They have the advantage of
turnkey deterministic comb generation, and also simplify dark soliton
generation in the normal dispersion regime. Here, the formation process of dark
solitons 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
position so that a wide range of dark soliton states can be accessed. Moreover,
soliton duty cycle is readily controlled by the feedback phase. Direct imaging
of the dark soliton 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.
