  Optical frequency combs generated in normally dispersive Kerr microresonators
have been observed to correspond to dark temporal structures, and theoretically
explained as interlocked switching waves (also known as domain walls or
fronts). The time-domain dynamics that underpin the formation of this type of
frequency combs has however so far eluded direct experimental observation. Here
we use a closely related system - a synchronously driven optical fibre ring
resonator - to experimentally study the dynamics of deterministically excited
switching waves. We measure the switching wave velocities across broad
parameter regions, and observe clear signatures of interlocking behaviour
leading to the formation of persisting dark pulses. Our experimental findings
are in good agreement with simulations of the mean-field Lugiato-Lefever
equation, and strongly support the nature of normal-dispersion microresonator
frequency combs suggested in the literature.
