  Optimal wavelength assignment in dense-wavelength-division-multiplexing
(DWDM) systems that integrate both quantum and classical channels is studied.
In such systems, weak quantum key distribution (QKD) signals travel alongside
intense classical signals on the same fiber, where the former can be masked by
the background noise induced by the latter. Here, we investigate how optimal
wavelength assignment can mitigate this problem. We consider different DWDM
structures and various sources of crosstalk and propose several near-optimal
wavelength assignment methods that maximize the total secret key rate of the
QKD channels. Our numerical results show that the optimum wavelength assignment
pattern is commonly consisted of several interspersed quantum and classical
bands. Using our proposed techniques, the total secret key rate of quantum
channels can substantially be improved, as compared to conventional assignment
methods, in the noise dominated regimes. Alternatively, we can maximize the
number of QKD users supported under certain key rate constraints.
