= Stationary intensity under free paraxial propagation
{title2=$\langle|E(x,z)|^2\rangle=\langle|E(0,z)|^2\rangle$}
The free <parabolic wave equation> multiplies each transverse <Fourier mode> by $e^{-i\nu^2x/(2k)}$, of unit modulus. A statistically stationary input therefore keeps its uncentered spectral measure and mean intensity. A pure phase screen of unit incident amplitude has total mean intensity one even when its coherent intensity is smaller. This statement does not require a Gaussian joint phase distribution; second-order stationarity and free paraxial propagation suffice.
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