Key Distribution Using Scrambling of Digital Signals in Multimode Fiber

We propose and verify a scheme that leverages the reciprocity and multipath scattering effects of multimode fibers to use them as scrambling devices. Each of the legitimate users possesses a segment of multimode fiber to scramble digital signals. When the scrambled digital signals are transmitted to a remote end through the transmission link, they need to pass through the multimode fiber at the remote end for a second scrambling to ensure security. Since the scrambling effect on the signal only exists in the multimode fiber, channel interference is compensated for, significantly enhancing transmission distance while maintaining high correlation. Additionally, the device’s security stems from its intrinsic state, as devices in different locations and environments exhibit varying states. The results show that even if an eavesdropper obtains a device with similar parameters, they cannot achieve the same sequence as the legitimate end-users. Ultimately, we achieved a key distribution rate of $0.18 \mathrm{Gbit} / \mathrm{s}$ using two segments of multimode fiber to scramble signals over a 50 km single-mode fiber.