Experimental and theoretical analysis of QKD signals in a 'QKD+WDM' coexistence network

Quantum key distribution (QKD), the most mature quantum cryptographic approach, provides a means to distribute unconditional secure cryptographic keys by encoding key bits on single photons. The QKD systems have been designed to operate on a dedicated point-to-point fiber links. However, the continued deployment of sophisticated optical networks makes it important for QKD signal to coexist on the same network with wavelength division multiplexed (WDM) signals. This coexistence would be cost effective, as well as provide QKD signals with the management techniques that already exist in such networks. However, there exist several technological challenges to overcome in order to make QKD and WDM coexistence practical in optical networks. This dissertation explores some of the physical phenomena behind these challenges and ways of mitigating their deleterious effects. These phenomena are: anti-Stokes Raman scattering into the 1310nm QKD band from 1550nm WDM signals, spontaneous emission (into the QKD band) from lasers and amplifiers deployed to maintain WDM signals, and XPM (cross phase modulation) imposed upon the QKD signals by the more powerful WDM signals.;In the first part of the dissertation, a filtering architecture is characterized, and used to experimentally determine the channel spacing required between the QKD and WDM channels in order to mitigate the effect of anti-Stokes noise on the QKD signals. A second study is a theoretical analysis of errors induced on a phase-based QKD system from XPM caused by classical WDM signals propagating on the same fiber. For the worst-case interaction scenario, it is shown that XPM is not a significant impairment in coexistence networks, where QKD and WDM signals share the same telecommunications-grade fiber when the wavelength separation between the signals is greater than 160nm for WDM systems transmitting up to 64 channels at typical power levels. Finally, an amplifier bypass architecture is designed and built to bypass a QKD signal in a coexistence network with mid-span amplified link. Experiments are performed to show good QKD signal isolation with the bypass, and impairment-free transmission of the existing WDM channels. The results of this dissertation show that QKD signals can coexist with WDM signals in today's optical networks.