# Research on Key Technologies of Quantum-Safe Metro-Optimized Optical Transport Networks
## 3 Results  
To validate the proposed method, an integrated system combining time perturbation with DS-QKD to achieve OSU optical physical-layer security has been developed, as illustrated in Figure 6. The manufacturers and models are listed in Table 3. This DS-QKD system employs the decoy-state BB84 protocol using polarization coding to generate a seed key via the quantum and protocol channels. The design considerations stem from the fact that these systems are highly sensitive to degradation and cannot amplify single photons, which necessitates a low-temperature environment. Consequently, a single G.652 standard single-mode fiber (SMF) is utilized as the quantum channel, spanning a total length of 100 km without any optical parametric amplifiers. The quantum channel exhibits linear loss (attenuation) of 10 dB over 100 km. The secure seed key is fed into the FPGA at both the transmitter and receiver ends. The FPGA then generates a running key to control the scrambling and descrambling processes. Key synchronization between the scrambler and descrambler is achieved through a dedicated synchronization channel. Subsequently, the M-OTN device's output OSU optical signal is scrambled using an FPC array. Moreover, the decoy BB84 protocol signal for key generation, the synchronization signal for key alignment, and the scrambled optical signal are multiplexed with the classical channel via a WDM and transmitted over the same 100 km single-mode fiber at wavelengths of 1546.92 nm, 1548.51 nm, and 1550.12 nm, respectively. In the classical channel, when the transmission distances are 50 km and 100 km, the optical signal losses are approximately 10 dB and 20 dB, respectively. To mitigate power loss due to scrambling/descrambling and fiber transmission, two erbium-doped fiber amplifiers (EDFAs) are employed. The noise superimposed on the optical signal by the EDFAs is minimal and can be neglected. Additionally, DCF is used to address dispersion issues over the 100 km single-mode fiber. After scrambling/descrambling, the bit error rate (BER) and eye diagram can be measured using an Ethernet tester (MTS 5800, VIAVI Solutions Inc., Chandler, AZ, USA) and an optical oscilloscope (TEKTRONIX DSA8200, TEKTRONIX Inc., Beaverton, OR, USA), respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Device** & **Manufacturer** & **Model** \\ \hline M-OTN & CETC, Guilin, China & Design with XILINX 7V690T \\ Scrambler/descrambler & CETC, Guilin, China & Design with XILINX 7K325T \\ DS-QKD & QuantumCTek, Hefei, China & QKD-POL1250-S \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Device manufacturers and models. The eye diagram of the OSU electrical signal, as shown in Figure 7, is open and clear. The experimental results of OSU optical signal scrambling and descrambling are depicted in Figure 8. Figure 8a,b show the unscrambled and scrambled signals, respectively. Due to the use of an FPC array for scrambling the overlapping bits, the scrambled signal is significantly different from the original 10.709 Gbps non-return-to-zero (NRZ) data-modulated optical signal. This indicates that the scrambler completely disrupted the temporal position relationship between different bits, rendering the OSU optical signal noisy and not digitizable. The eye diagrams after descrambling, as shown in Figure 8c, reveal an open eye diagram when using matched key descrambling. Conversely, when using unmatched key descrambling, the eye diagram remains closed, and the outline does not exhibit any numeric features of \"0\" or \"1\", as depicted in Figure 8d. Therefore, an eavesdropper cannot simultaneously acquire the scrambling and descrambling strategies, as well as the synchronization key, making it impossible to extract OSU data via optoelectronic transformation. Figure 6: Experimental setup for real-time OSU optical signal time scrambling/descrambling and key transmission via DS-QKD.