Precise and secure time synchronization between two remote sites is necessary in many practical scenarios. Recently, the two-way fiber time transfer (TWFTT) system has become an attractive option for high-precision time synchronization. However, TWFTT is implemented based on symmetric link assumption, and such assumption may be maliciously utilized to cause significant time asynchronization to TWFTT. In this paper, we propose two novel controllable asymmetry attack schemes for TWFTT, namely, the link asymmetry attack and the attenuation asymmetry attack. We theoretically analyze the controllability and the effectiveness of these two attacks. Experimental results show that our link asymmetry attack can introduce 2.49 ns time offset per asymmetric link length (in meters), and our attenuation asymmetry attack can introduce at most 36.6 times time offset (compared to normal situation) when the attenuation is 45.3%. Our work can provide instructive insights for future studies of protecting TWFTT from delay asymmetry attacks.

Two-way fiber-optic time transfer (TWFTT) is a promising precise time synchronization technique with sub-nanosecond stability. However, the asymmetric delay attack is a severe threat, which can deteriorate the performance of the TWFTT system. In this paper, a clock model-based scheme is used to defend the sub-nanosecond asymmetric delay attack. For the scheme, a security threshold is set according to a two-state clock model, and the estimated frequency difference is excluded from the measured time difference to detect the sub-nanosecond asymmetric delay attack. Systematic detection and mitigation scheme for asymmetric delay attack is developed in this paper. Theoretical simulation and experimental demonstration are implemented to explore the feasibility of the method. A TWFTT system of time stability with 24.5 ps, 3.98 ps, and 2.95 ps at average time 1 s, 10 s, and 100 s is shown under sub-nanosecond asymmetric time delay attack experimentally for the first time. The proposed method provides a promising secure sub-nanosecond precise time synchronization technique against asymmetric delay attacks.

Fiber-delay measurement is one of the key fundamental technologies in numerous fields. Here, we propose and experimentally demonstrate a high-precision and concise optical time delay measurement system based on the technique of linear optical sampling, reaching the precision better than 100 fs under averaging. The use of only two optical frequency combs without locking the carrier-envelope-offset frequency greatly simplifies the structure of the time-delay measurement system. We also experimentally investigate the current limitations on the precision of the system. The timing jitter noises of two sources are mainly non-common mode and are both restricted to the frequency sources. Our results indicate that the proposed device can measure fiber length fluctuations below 10 µm, paving the way for further analyses of the external disturbances on the fiber link.

We demonstrate a high-precision, high robustness frequency offset locking method,which made the frequency offset between mode-locked laser and continuous-wave laser below less than 3 Hz.The coarse frequency lock control is realized by the feedback control of PZT with electrical delay line as the reference. The fine frequency compensation is realized by feed-forward control of an acousto-optic modulator. The fractional frequency instability was ${7.4}\times {10}^{-10}$ for an averaging time of 1 s, $3.3\times {10}^{-8}$ for an averaging time of 10 000 s when the narrow linewidth laser is free-running. In this experiment, the fractional frequency instability can be achieved at $1.1\times {10}^{-15}$ for an averaging time of 1 s, at $3.6\times {10}^{-18}$ for an averaging time of 10 000 s when the system is fine frequency locked. Compared with the unlocked laser, the fractional frequency instability can be improved about 5–6 orders of magnitude. This work lays the foundation for simple structure, high robustness and high precision laser frequency control situation, such as quantum precision measurement and optical lattice clocks.

优化样品结构参数、磁场退火热处理、串并联桥式设计、施加电流热效应、以及额外偏置磁场等是调控TMR磁传感性能的常用方法。借助这些方法可以提高TMR磁传感的灵敏度、抗噪声指数、线性度和线性磁场范围等关键性能参数。其中，通过改变TMR磁传感单元的钉扎层、自由层以及势垒层材料和厚度等样品结构参数能够改变交换偏置场，进而提升TMR磁传感性能参数。本文通过微磁学仿真和实验测量发现，通过改变自由层CoFeB/Ru/NiFe/IrMn中的交换耦合作用，可以调制TMR自由层的交换偏置场大小和提升TMR磁传感单元的性能。当逐步增强IrMn钉扎效果时，TMR磁传感单元的线性磁场范围随之增大，但是磁场灵敏度降低；进而发现在±0.5倍自由层（主要是CoFeB层）磁矩变化范围内TMR磁传感器的线性度最佳。