  Solitons are self-reinforcing localized wave packets arising from a balance
of linear and nonlinear effects. This definition encompasses the interplay of
nonlinear gain and loss, leading to the concept of dissipative solitons that
has been instrumental in understanding the wide variety of mode locking
phenomena in ultrafast optics. To date, most studies have involved the group
velocity dispersion as a key ingredient for soliton generation. Here, we report
on a novel kind of soliton, both theoretically and experimentally, which builds
up in spectrally confined cavities when dispersion is practically absent.
Precisely, the interplay between the Kerr nonlinearity and spectral filtering
results in an infinite hierarchy of eigenfunctions which, combined with optical
gain, allow for the generation of stable dispersion-less dissipative solitons
in a previously uncharted regime. When the filter order tends to be infinite,
we find an unexpected link between dissipative and conservative solitons, in
the form of Nyquist-pulse-like solitons endowed with an ultra-flat spectrum. In
contrast to the dispersion-enabled solitons, these dispersion-less Nyquist
solitons build on a fully confined spectrum and their energy scaling is not
constrained by the pulse duration. This study broadens the fundamental scope of
dissipative soliton physics and opens new avenues for engineering optical
solitons endowed with superior temporal and spectral features.
