Introducing angular dispersion into a pulsed field associates each
frequency with a particular angle with respect to the propagation
axis. A perennial yet implicit assumption is that the propagation
angle is differentiable with respect to the frequency. Recent work on
space–time wave packets has shown that the existence of a frequency at
which the derivative of the propagation angle does not exist—which we
refer to as non-differentiable angular dispersion—allows for the
optical field to exhibit unique and useful characteristics that are
unattainable by endowing optical fields with conventional angular
dispersion. Because these novel, to the best of our knowledge,
features are retained in principle even when the specific
non-differentiable frequency is not part of the selected spectrum, the
question arises as to the impact of the proximity of the spectrum to
this frequency. We show here that operating in the vicinity of the
non-differentiable frequency is imperative to reduce the deleterious
impact of (1) errors in implementing the angular-dispersion profile
and (2) the spectral uncertainty intrinsic to finite-energy wave
packets in any realistic system. Non-differential angular dispersion
can then be viewed as a resource—quantified by a Schmidt number—that
is maximized in the vicinity of the non-differentiable frequency.
These results will be useful in designing novel phase-matching of
nonlinear interactions in dispersive media.