Hybrid fiber-based time synchronization and vibration detection system.

We propose a hybrid fiber-based time synchronization and vibration detection system. The vibration is detected by exploring the idle light of the time synchronization system, i.e., the Rayleigh backscattering of the timing pulse disseminated in the fiber link. The addition of a sensing function does not affect the performance of time synchronization. In the multiuser experimental demonstration, time deviation results are 3.6 ps at τ = 1 s and 1.4 ps at τ = 104 s on the 40-km fiber link. Meanwhile, the hybrid system can accurately detect and locate vibrations occurring on the link. This method enables multiple functions of the optical fiber network without occupying extra optical channels. Moreover, it gives a possible solution for enhancing the security of the time synchronization network through vibration detection.

Indoor optical fiber eavesdropping approach and its avoidance.

The optical fiber network has become a worldwide infrastructure. In addition to the basic functions in telecommunication, its sensing ability has attracted more and more attention. In this paper, we discuss the risk of household fiber being used for eavesdropping and demonstrate its performance in the lab. Using a 3-meter tail fiber in front of the household optical modem, voices of normal human speech can be eavesdropped by a laser interferometer and recovered 1.1 km away. The detection distance limit and system noise are analyzed quantitatively. We also give some practical ways to prevent eavesdropping through household fiber.

Epicenter localization using forward-transmission laser interferometry.

Widely distributed optical fibers, together with phase-sensitive laser interferometry, can expand seismic detection methods and have great potential for epicenter localization. In this paper, we propose an integral response method based on a forward transmission scheme. It uses spectrum analysis and parameter fitting to localize the epicenter. With the given shape of the fiber ring, the integral phase changes of light propagating in the forward and reverse directions can be used to determine the direction, depth, distance of the epicenter, and seismic wave speed. For the noisy case with SNR = 20 dB, the simulation results show ultrahigh precision when epicenter distance is 200 km: the error of the orientation angle is ∼0.003°±0.002°, the error of the P-wave speed is ∼0.9 ± 1.2 m/s, the error of the epicenter depth is ∼9.5 ± 400 m, and the error of the epicenter distance is ∼200 ± 760 m.

Noise analysis of the fiber-based vibration detection system.

Detecting seismic events using a fiber-based CW laser interferometer attracts wide attention. To make the detection more effective, we analyze the system's noise level by setting up two vibration detection systems. By changing the fiber length (0∼100 km) and laser noise level, respectively, we detect the minor phase change caused by a 160 µm-fiber-length vibration. Furthermore, we use three indicators, Power Spectral Density, Background Noise Level, and Signal-to-Noise Ratio to analyze the noise level of the whole system. The relation between the system's background noise and corresponding detection result is carried out. This quantitative research can serve as a reference and help people to realize the most efficient vibration detection system.