  We study the theory of linearly chirped biphoton wave-packets produced in two
basic quasi-phase-matching configurations: chirped photonic-like crystals and
aperiodically poled crystals. The novelty is that these structures are
considered as definite assembles of nonlinear layers that leads to detailed
description of spontaneous parametric down-conversion (SPDC) processes through
the discrete Gauss sums. We demonstrate that biphoton spectra for chirped
photonic crystals involving a small number of layers consist from definite
well-resolved spectral lines. We also discuss the forming of broadband spectra
of signal (idler) waves in SPDC for both configurations as number of layers
increases as well as in dependence of chirping parameters .
