## 3 Experiment  
### Experiment and Analysis of Single Safety Optical Fiber Timing Equipment  
In this study, the SM2 algorithm was used to generate a pair of keys. A public key is distributed to the local site device, while a private key is distributed to the remote site device. Then, the data \(T_{LR}\) of the fiber-optic time transfer system are encrypted. Additionally, the data are sent to the remote site for decryption through the externally injected link. The decrypted data are compared with the time difference data transmitted to the remote site via the optical fiber link. If the data are the same, the time synchronization of the system is completed according to Formula (3). Finally, we used an evaluation device SR620 to test the performance of the synchronization. As long as the private key of the user at the remote site is not leaking, it is difficult for a third party to decrypt the content information even if it intercepts the ciphertext. The experiments were performed on a safety optical fiber system based on standard small form-factor pluggable (SFP) optical transceivers. The single-mode SFPs were operated at the wavelength of about 1550 nm. The local safety device and the remote safety device were connected by using two SFPs with high stability but different wavelengths linked to an optical circulator. In this study, two adjacent channels C42 and C43 were selected. The wavelength of the C42 channel was 1543.730 nm, and the wavelength of the C43 channel was 1542.956 nm. Additionally, the transmission rate was 1.25 Gb/s. The 1 PPS signal and 10 MHz signal input by the clock source and the time interval information \(T_{LR}\) were encrypted and encoded using the local site equipment. Then, they were modulated to the local SFP by means of intensity modulation. After the laser was connected to the local circulator, the data were sent to the safety equipment at the remote site through 50 km of optical fiber. At that same time, the security device at the remote site passed through the circulator, connected to a photodetector. The photodetector detected the signal from the local terminal. The remote site converted and divided the signal into two paths. A 10 MHz carrier signal was obtained through a carrier recovery module. The signal was purified through a phase-locked loop (PLL) to be used as a clock source of a time-keeping module in the safety equipment at the remote site. At the same time, the purified and stable 1 PPS time signal was output. The other path demodulated the 1 PPS signal and the encrypted information transmitted from the local site. The encrypted information was decrypted  
Figure 4: Flowchart of alarm for optical fiber time transmission safety equipment. using a decryption unit. Two channels of 1 PPS signals were calculated via the TIC module to obtain \(T_{RL}\). The decrypted data were calculated via the operation control unit by using Formula (3) to obtain a comparison result. Additionally, time-delay compensation was performed using the delay phase control unit. In this way, the system realized the two-way time transfer of the optical fiber link. Additionally, the remote site output a synchronous 1 PPS signal and a 10 MHz signal. In order to verify the feasibility of the algorithm, we used an external injection to transfer the data encrypted by the SM2 algorithm to the decryption unit of the remote security device through the external connection line. The decrypted data were compared with the data transfer through the optical fiber link. If the decrypted data were consistent, the time synchronization was completed. A time interval counter SR620 was used to collect data. It can observe the synchronization effect of the optical fiber time transfer system before and after the security encryption unit is destroyed. Once ciphertext is tampered with, the device immediately finds it and sends an alarm. The data for a total of 3000 s were collected, as shown in Figure 5. During the safe operation of 1500 s, the data of SR620 were observed.