  This paper presents a novel approach to wave propagation inside the
Fabry-Perot framework. It states that the time-averaged Poynting vector modulus
could be nonequivalent with the squared-field amplitude modulus. This fact
permits the introduction of a new kind of nonlinear medium whose nonlinearity
is proportional to the time-averaged Poynting vector modulus. Its transmittance
is calculated and found to differ with that obtained for the Kerr medium, whose
nonlinearity is proportional to the squared-field amplitude modulus. The latter
emphasizes the nonequivalence of these magnitudes. A space-time symmetry
analysis shows that the Poynting nonlinearity should be only possible in
noncentrosymmetric materials.

Optics