  Driven Kerr nonlinear optical resonators can sustain localized structures
known as dissipative Kerr cavity solitons, which have recently attracted
significant attention as the temporal counterparts of microresonator optical
frequency combs. Whilst conventional wisdom asserts that bright cavity solitons
can only exist in the region of anomalous dispersion, recent theoretical
studies have predicted that higher-order dispersion can fundamentally alter the
situation, enabling bright localized structures even under conditions of normal
dispersion driving. Here we demonstrate a flexible optical fibre ring resonator
platform that offers unprecedented control over dispersion conditions, and we
report on the first experimental observations of bright localized structures
that are fundamentally enabled by higher-order dispersion. In broad agreement
with past theoretical predictions, we find that several distinct bright
structures can co-exist for the same parameters, and we observe experimental
evidence of their collapsed snaking bifurcation structure. In addition to
enabling direct experimental verifications of a number of theoretical
predictions, we show that the ability to judiciously control the dispersion
conditions offers a novel route for ultrashort pulse generation: the bright
structures observed in our work correspond to pulses of light as short as
220~fs -- the record for a passive all-fibre ring resonator. We envisage that
our work will stimulate further fundamental studies on the impact of
higher-order dispersion on Kerr cavity dynamics, as well as guide the
development of novel ultrashort pulse sources and dispersion-engineered
microresonator frequency combs.
