  We study the spectral properties of Spontaneous Parametric Down Conversion in
a periodically poled waveguided structure of KTP crystal pumped by ultra-short
pulses. Our theoretical analysis reveals a strongly multimode and asymmetric
structure of the two-photon spectral amplitude for type-II SPDC. Experimental
evidence, based on Hong-Ou-Mandel polarization interference with narrowband
filtering, confirms this result.
