Wavelength dependence of fiber channel reciprocity for bidirectional physical-layer key distribution

To share the reciprocity of a single fiber channel in physical-layer secure key distribution (PLSKD), the bidirectional same wavelength (B-SW) transmission system is generally applied. However, the elastic Rayleigh backscattering noise severely degrades key performance after long fiber transmission. Here, the feasibility of a bidirectional wavelength division multiplexing (B-WDM) transmission in a single fiber for high-speed long-range PLSKD is verified, where the wavelength dependence of fiber channel reciprocity is evaluated. PLSKD based on B-WDM in one standard 100 GHz WDM channel is demonstrated, offering a key generation rate of 4.8 Gbps over a 200 km fiber link, providing a 200% increment in terms of key generation rate (KGR) compared with B-SW transmission.