Apodized fiber Bragg grating filters for efficient QKD channel separation in QKD-over-DWDM systems

This study theoretically investigates the optimal filter configuration for separating a quantum key distribution (QKD) channel from classical C-band communication channels in a discrete-variable (DV) QKD-over-dense wavelength division multiplexing (DWDM) communication system with an O-band QKD channel. Three filter configurations are analyzed and compared in terms of quantum bit error rate (QBER), secret key rate (SKR), and fiber link distance: two cascaded 1310-nm bandpass filters, a 1310-nm uniform fiber Bragg grating (FBG) with a 1310-nm circulator, and a 1310-nm apodized FBG with a 1310-nm circulator. The findings reveal that the optical filter configuration incorporating a Gaussian apodized FBG exhibits the best performance with an isolation of ~ 93 dB between 1310 nm and 1550 nm wavelengths. This optimal performance can be attributed to the Gaussian apodization profile, which effectively suppresses spectral side-lobes to enhance channel isolation while maintaining a lower insertion loss compared to the cascaded bandpass filter configuration. For 50 classical WDM channels, the QKD fiber link distance is 35.4 km with a Gaussian apodized FBG, while a configuration incorporating two cascaded 1310-nm bandpass filters yields only 18.3 km. Finally, the impact of adopting a raised-cosine apodized FBG to further increase filter channel isolation is examined. Despite a substantial increase in channel isolation by using a raised-cosine apodized FBG filter, QKD fiber link distance yields negligible improvement for this system with a QKD channel in O-band, although this filter may be highly useful for QKD channels in C- or L-bands.