Exact cascading nonlinearity in quasi-phase-matched quadratic media

The evolution of light pulses and beams in a quasi-phase-matched (QPM) quadratic medium is usually described by considering only the spatial harmonic of the QPM grating that minimizes the residual phase-mismatch. I show that, for strongly phase-mismatched interactions (the cascading regime), several harmonics need to be accounted for in order to obtain the correct value of the effective cubic nonlinearity, of which I find a simple analytical expression. I discuss the effects of the higher order harmonics of the grating on solitary wave propagation.