  We study the generation of biphoton modally entangled states in such
waveguide structures that lead to spectrum broadening. The process of
spontaneous parametric down conversion (SPDC) has been considered for the
biphoton generation. The main subject of our study is the dependence of the
biphoton spectra on the structure spatial characteristics. The representation
of the core structures as definite assembles of nonlinear layers allows us to
find analytical description for biphoton spectrum. We show that chirped
biphotons with discrete as well as continuous spectra can be generated
depending on the waveguide structure. The conditions for spectrum
discretization are obtained analytically. It is shown that the biphoton
entangled states can be controlled by varying the parameters of the waveguide
structure.
