  In recent years, the topic of negative frequencies has resurfaced in optics
motivated by the optical analogue of Hawking radiation. We discuss the physical
meaning of negative frequencies and the conditions under which they are
relevant. We review how negative frequencies are treated in current pulse
propagation models based on the electric field and the analytic signal. We
focus on experimentally measured signals predicted by the conservation of
negative comoving frequency in the nonlinear polarization terms to advance
these concepts. We propose a new formalism called the double analytic signal
which clearly separates negative frequencies from positive ones. Additionally,
we reduce this new formalism to the analytic signal to prove their equivalence.
Throughout the paper, we present numerical solutions of the unidirectional
pulse propagation equation to illustrate the electric field, analytic signal,
and double analytic signal formalisms and to highlight their differences.
