  Nonlinearity and sharp transmission spectra of random 1D nonlinear layered
structures are combined together to produce photon pairs with extremely narrow
spectral bandwidths. Indistinguishable photons in a pair are nearly
unentangled. Also two-photon states with coincident frequencies can be
conveniently generated in these structures if photon pairs generated into a
certain range of emission angles are superposed. If two photons are emitted
into two different resonant peaks, the ratio of their spectral bandwidths may
differ considerably from one and two photons remain nearly unentangled.
