# Research on Key Technologies of Quantum-Safe Metro-Optimized Optical Transport Networks
## 2 Methods
### Time-Domain Scrambling/Descrambling Combined with DS-QKD  
The operational mechanism of the real-time OSU optical signal time-domain scrambling system integrated with the DS-QKD system is illustrated in Figure 4. This system establishes a symmetric key-based scrambling/descrambling framework, involving both the DS-QKD transmitter and receiver. The key exchange occurs via DS-QKD, facilitating periodic and random renewal of the shared seed key between two authorized users, which are known exclusively to the transmitter and receiver. On the transmitting side, the DS-QKD transmitter sends quantum-generated random keys over the quantum channel to the receiver without transmitting any accompanying message data. The scrambler employs the running key, generated by field-programmable gate arrays (FPGAs), to scramble the OSU optical signal in real time. Given that the M-OTN OTU2 interface's line rate is approximately 10.709 Gbps, the FPGAs used for scrambling and descrambling are XILINX 7K325T devices, which support a maximum SerDes (GTX) line rate of 12.0 Gbps. On the receiving side, the DS-QKD receiver captures the quantum bits from the transmitter and derives the seed key through the protocol channel. The descrambler then applies the identical running key used by the transmitter to restore the scrambled OSU optical signal. Synchronization between the transmitter and receiver for scrambling and descrambling is maintained via the FPGA synchronization channel. It should be noted that all communications within the protocol, synchronization, and data channels consist of conventional signals, allowing these channels to be multiplexed using a wavelength-division multiplexer (WDM) and transmitted over the classical communication channel. Typically, at the start of transmission, the DS-QKD transmitter and receiver exchange and store a quantum key. The FPGA within the transmitter generates the running key based on the seed key and synchronization marker. The OSU optical signal entering the scrambler undergoes real-time scrambling by dynamically adjusting physical parameters according to the running key. Simultaneously, the synchronization marker, transmitted via the synchronization channel, ensures the alignment of the scrambling and descrambling initiation signals. At the receiver, the same running key derived from the seed key is used to demodulate the received signal, triggered by the synchronization marker. This dynamic key approach significantly enhances security, as an eavesdropper must not only decipher the ultra-high-speed scrambled signal but also intercept the key before the next update cycle. Figure 3: Allocating OPU PBs to different OSU services. Fiber Bragg Grating (FBG) and FPCs are two common optical resonant structures. FBG is sensitive to temperature and strain, while FPCs are sensitive to cavity length changes. Given the difficulty of controlling the temperature of FBG, FPCs are modularized to control cavity length more effectively. The time-domain scrambling of the OSU optical signal is achieved through FPCs, as illustrated in Figure 5a. When an OSU optical signal enters a single-fiber delay, it undergoes multiple reflections between the dual reflective surfaces of the cavity (Figure 5b). Each scrambling/descrambling controller comprises 12 FPCs. While increasing the number of FPCs enhances the scrambling effect, it also introduces significant dispersion, necessitating adjustment using dispersion compensation fiber (DCF). This reflection mechanism splits the collimated light into multiple sub-beams through the cavity structure, with each beam acquiring distinct transmission delays. The delayed sub-beams are subsequently focused through a lens assembly into the output fiber. The temporal characteristics of the composite output signal are determined by the coherent superposition of these differentially delayed optical components.