= Two-crystal second-harmonic interference
{title2=$I_{2\omega}=4\alpha^2I_0^2L^2\cos^2(\Delta k_gd/2)$}
Two identical phase-matched crystals generate coherent harmonic amplitudes. In a dispersive gap, the harmonic from the first crystal accumulates a different phase from twice the transmitted pump phase. Their interference gives the displayed undepleted-pump intensity in the stated equal-coupling amplitude normalization. Depletion changes the fringe shape; in the ideal lossless identical-crystal model, an opposite relative phase can coherently undo the first conversion.
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