  We demonstrate protected single-soliton formation and operation in a Kerr
microresonator using a phase-modulated pump laser. Phase modulation gives rise
to spatially varying effective loss and detuning parameters, which in turn lead
to an operation regime in which multi-soliton degeneracy is lifted and a single
soliton is the only observable behavior. Direct excitation of single solitons
is indicated by observed reversal of the characteristic 'soliton step.' Phase
modulation also enables precise control of the soliton pulse train's
properties, and measured dynamics agree closely with simulations. We show that
the technique can be extended to high repetition-frequency Kerr solitons
through subharmonic phase modulation. These results facilitate straightforward
generation and control of Kerr-soliton microcombs for integrated photonics
systems.
