Rapid advancements in high-energy ultrafast lasers and free electron lasers have made it possible to obtain extreme physical conditions in the laboratory, which lays the foundation for investigating the interaction between light and matter and probing ultrafast dynamic processes. High temporal resolution is a prerequisite for realizing the value of these large-scale facilities. Here, we propose a new method that has the potential to enable the various subsystems of large scientific facilities to work together well, and the measurement accuracy and synchronization precision of timing jitter are greatly improved by combining a balanced optical cross-correlator (BOC) with near-field interferometry technology. Initially, we compressed a 0.8 ps laser pulse to 95 fs, which not only improved the measurement accuracy by 3.6 times but also increased the BOC synchronization precision from 8.3 fs root-mean-square (RMS) to 1.12 fs RMS. Subsequently, we successfully compensated the phase drift between the laser pulses to 189 as RMS by using the BOC for pre-correction and near-field interferometry technology for fine compensation. This method realizes the measurement and correction of the timing jitter of ps-level lasers with as-level accuracy, and has the potential to promote ultrafast dynamics detection and pump–probe experiments.

This paper presents a detailed technical overview of the femtosecond precision timing and synchronization systems implemented at the Shanghai high repetition rate XFEL and extreme light facility (SHINE). These systems are designed to deliver stabilized optical references to multiple receiver clients, ensuring high-precision synchronization between the optical master oscillator (OMO) and optical/RF subsystems. The core components include an OMO, fiber length stabilizers and laser-to-laser synchronization modules that achieve femtosecond-level accuracy. Our discussion extends to the various subsystems that comprise the synchronization infrastructure, including the OMO, fiber length stabilizer and advanced phase detection techniques. Finally, we highlight ongoing research and development efforts aimed at enhancing the functionality and efficiency of these systems, thereby contributing to the advancement of X-ray free-electron laser technology and its applications in scientific research.

An advanced deformable Kirkpatrick–Baez (K-B) mirror system was developed, equipped with high-speed piezoelectric actuators, and designed to induce beam decoherence and significantly enhance the quality of X-ray imaging by minimizing undesirable speckles in synchrotron radiation or free-electron laser facilities. Each individual mirror is engineered with 36 independent piezoelectric actuators that operate in a randomized manner, orchestrating the mirror surface to oscillate at a high frequency up to 100 kHz. Through in situ imaging single-slit diffraction measurement, it has been demonstrated that this high-frequency-vibration mirror system is pivotal in disrupting the coherent nature, thereby diminishing speckle formation. The impact of the K-B mirror system is profound, with the capability to reduce the image contrast to as low as 0.04, signifying a substantial reduction in speckle visibility. Moreover, the coherence of the X-ray beam is significantly lowered from an initial value exceeding 80% to 13%.

This study investigates the influence of seismic activities on the optical synchronization system of the European X-ray Free-Electron Laser. We analyze the controller input/output data of phase-locked loops in length-stabilized links, focusing on the response to earthquakes, ocean-generated microseism and civilization noise. By comparing the controller data with external data, we were able to identify disturbances and their effects on the control signals. Our results show that seismic events influence the stability of the phase-locked loops. Even earthquakes that are approximately 5000 km away cause remarkable fluctuations in the in-loop control signals. Ocean-generated microseism in particular has an enormous influence on the in-loop control signals due to its constant presence. The optical synchronization system is so highly sensitive that it can even identify vibrations caused by civilization, such as road traffic or major events like concerts or sport events. The phase-locked loops manage to eliminate more than 99% of the existing interference.

We presented an attosecond-precision timing detector based on linear optics. The minimum measurement floor is 1×10–10 fs2/Hz with only 1 mW input optical power. With this novel technique, the residual dispersion of a 5.2 km fiber link is characterized and precisely compensated. Finally, a comprehensive feedback model has been developed to analyze the noise coupling in a long-distance link stabilization system. The simulation results demonstrate an out-of-loop jitter of merely 359 as, integrated at [1 Hz, 1 MHz], at 1 mW input power per photodetector of our timing detector. Remarkably, the system is capable of maintaining sub-femtosecond precision even at optical power levels as low as 240 nW (for a 5.2 km link length), or link lengths as long as 20 km (with 1 μW optical power), respectively.

Measurements of the bunch arrival times at the European X-ray free-electron laser show noise contributions in the spectral range between 0.05 and 0.5 Hz with peak-to-peak jitter of up to 25 fs. Correlation with distributed acoustic sensing measurements confirms the seismic origin. The seismic noise in this frequency band is known to be ocean-generated microseism. Both primary and secondary ocean-generated microseisms were identified using seismometers and a numerical ocean wave model. Whereas secondary microseism has a strong impact on the bunch arrival time, primary microseism has no notable effect. Rayleigh waves cause the effect, while Love waves have minimal impact. In the presented cases, the noise originates from the North Atlantic and/or the North Sea. The amplitude of the noise depends on the local weather conditions and is much stronger in winter. Ocean-generated microseism is a significant bottleneck that must be addressed to achieve femtosecond bunch arrival time stability in the sub-Hz regime.