  Nonreciprocal phenomena in the normal state are well established and key to
many commercial applications. In contrast, superconducting analogs, such as the
superconducting diode effect (SDE), are only starting to be experimentally
explored and pose significant challenges to their theoretical understanding. In
this work we put forth a phenomenological picture of the SDE based on the
generalized Ginzburg-Landau free energy, which includes a Lifshitz invariant as
the hallmark of noncentrosymmetric helical phase of the finite-momentum Cooper
pairs. We reveal that such a Lifshitz invariant drives the SDE in
quasi-two-dimensional systems in an applied magnetic field and cannot be
removed by a gauge transformation, due to the inherently inhomogeneous magnetic
response. For a thin film, the SDE scales with the square of its thickness and
nonlinearly with the strength of the in-plane magnetic field. We derive an
explicit formula that relates the SDE at small magnetic fields to the strength
of Rashba spin-orbit coupling, g-factor, and Fermi energy. For a
noncentrosymmetric Josephson junction, we self-consistently obtain generalized
anharmonic current-phase relation which support the SDE. The transparency of
our approach, which agrees well with experimentally-measured SDE, offers an
important method to study nonreciprocal phenomena, central to superconducting
spintronics and topological superconductivity.
