Abstract: This work is focused on generation of quantum-entangled photons through spontaneous parametric down-conversion in array of nonlinear waveguides. First we present short description of the process together with its Hamiltonian. Next we discuss possible methods of calculation of the output states. We mention first and second order of the perturbation theory and outline the solution using the Schmidt decomposition (but both methods are neglecting the time-ordering effects). Finally we calculate the first two nontrivial terms of Dyson series (this method already contains the time-ordering effects).