reaches a constant value α (1 − | S$_{11}$ | 2 − | S$_{12}$ | $^{2}$)( | S$_{11}$ | 2 + | S$_{21}$ | $^{2}$). For z larger than the characteristic decay length l$_{d}$ = 1 /σ , the correlation P ( δ ; z ) vanishes at the dip δ = 0, i.e. when signal and idler wave packets are tempo- rally overlapped, according to the analysis of Sec.III.B. model for the scalar wave equation, in the paraxial and quasi-monochromatic approximations, has been adopted. As for input beam excitation in a coherent state the clas- sical picture of light propagation is retrieved, quantum interference effects with no classical counterpart have been highlighted for photon number state excitation of the waveguide structure. In particular, the tendency of photon pairs to bunch when decaying into the con- tinuum has been predicted. Such an effect, which is similar to the two-photon Hong-Ou-Mandel interference in a beam splitter [18], may be observed as a dip in photon coincidence measurements. 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