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

Area of Science:

• Quantum Information Science
• Optical Communications Engineering
• Photonics

Background:

• Quantum key distribution (QKD) systems face challenges integrating with existing dense wavelength division multiplexing (DWDM) infrastructure.
• Separating quantum channels from classical communication channels is crucial for secure QKD transmission.

Purpose of the Study:

• To theoretically investigate and compare filter configurations for optimal separation of O-band QKD channels from C-band classical channels in a DWDM system.
• To evaluate filter performance based on quantum bit error rate (QBER), secret key rate (SKR), and achievable fiber link distance.

Main Methods:

• Analysis of three filter configurations: cascaded 1310-nm bandpass filters, uniform 1310-nm fiber Bragg grating (FBG) with circulator, and apodized 1310-nm FBG with circulator.
• Theoretical comparison based on channel isolation, insertion loss, QBER, SKR, and QKD fiber link distance for 50 classical WDM channels.

Main Results:

• The Gaussian apodized FBG with a circulator demonstrated superior performance, achieving ~93 dB isolation between 1310 nm and 1550 nm.
• This configuration extended the QKD fiber link distance to 35.4 km, significantly outperforming cascaded bandpass filters (18.3 km).
• Raised-cosine apodized FBG offered increased isolation but negligible improvement in link distance for O-band QKD channels.

Conclusions:

• Gaussian apodized FBGs are optimal for enhancing channel isolation and extending QKD fiber link distance in DWDM systems with O-band QKD channels.
• Apodization profiles in FBGs effectively suppress side-lobes, improving performance over simpler filter designs.
• While beneficial for C- or L-band QKD, raised-cosine FBGs show limited additional advantage for O-band systems in this context.