"From picoseconds to attoseconds" to achieve attosecond time jitter synchronization for picosecond lasers
High Precision Time Synchronization Technology
The development of ultrashort pulsed lasers has enabled mankind to probe and manipulate the material world within extremely short time scales, and the combination of two or even more ultrashort pulsed lasers has enriched this capability in more dimensions. High-precision time synchronization is a key technology to realize the cooperative work of ultrashort-pulse lasers. Among the laser synchronization techniques, Balanced Optical Cross-Correlation (BOC) and laser interferometry play an important role in precisely controlling the time synchronization and phase synchronization of multiple laser sources. They provide critical support for high-precision, multi-pulse synthesis and stable output of laser systems.
Balanced optical inter-correlation techniques typically rely on generating sum-frequency signals by mixing two signals (e.g., two laser pulses) fed into a nonlinear medium. These generated signals are then sent to a balanced detector, which determines the delay between the two input signals by measuring the difference in intensity of the output signals. The laser interference technique obtains information about the phase of the laser by analyzing the interference pattern of the laser beam and is used to control and synchronize multiple laser beams. This technique plays an extremely important role in laser synchronization, especially where precise control of the relative position and phase of the laser beams is required.
Picosecond Synchronization for Picosecond Lasers
Recently, the State Key Laboratory of Strong-Field Laser Physics of Shanghai Institute of Optical Machinery (SIOEM) has achieved the attosecond synchronization of picosecond laser pulses based on the independently constructed time synchronization system. Arsec science is an important branch of ultrafast optics and laser science, whose core goal is to detect and manipulate ultrafast phenomena such as the motion of electrons, which provides a new perspective for understanding the fundamental laws of the material world. For example, in chemical reactions, the breaking and reorganization of molecular bonds is determined by the ultrafast motion of electrons, and the attosecond time scale offers the possibility of directly observing and manipulating these processes. The attosecond (10-18 seconds) is currently the shortest unit on the time scale that humans can precisely manipulate, and the realization of this ultra-high precision time control cannot be achieved without the support of laser time synchronization technology. Since picosecond (10-12 s) laser pulses are an important basic light source for many attosecond science experiments, how to correct the time jitter of picosecond lasers to the attosecond level is the basis for ensuring the usefulness of attosecond science.
Results are published in High Power Laser Science and Engineering 2024, Issue 6 (Hongyang Li, Keyang Liu, Ye Tian, Liwei Song, "Long-term stable timing fluctuation correction for a picosecond laser with attosecond-level accuracy," High Power Laser Sci. Eng. 12, 06000e89 (2024)).
Figure 1 Schematic of picosecond laser synchronization
The research team further developed the laser synchronization technology to measure and provide real-time feedback on the picosecond laser with high-precision time jitter, which controls the system's time jitter in the attosecond-level range and improves the reliability of the laser system during long-time operation. The experimental setup is shown in Fig. 1. The research team utilized the multi-pass cavity (MPC) pulse compression technique, balanced optical inter-correlation technique, and near-field interferometry for time jitter measurement, and developed an analysis and control system for real-time correction of time jitter. Limiting to the gain bandwidth of the Yb:YAG crystal, the output pulse width of solid-state lasers utilizing this crystal is usually in the order of several hundred femtoseconds or even picoseconds, and the compression of 0.8 ps to 95 fs by using the MPC improves the measurement accuracy of the BOC from 14.57 mV/fs to 52.5 mV/fs, and the time jitter is pre-corrected by the BOC to 1.12 fs at this measurement accuracy, and the result is as shown in Fig. 2. Based on this, the phase fluctuation was compensated using an interferometry-based feedback loop to 189 as (λ/18) RMS, and the results are shown in Fig. 3.
Fig. 2 (a) Schematic of the noncommutative BOC, (b) Intercalation curves for pulse widths of 0.8 ps and 95 fs. (c) Timing drift with feedback off (gray line) and on (black line, red line)
Fig. 3 (a) Schematic of the intensity distribution of the interference fringes with (red line) and without (black line) phase drift (inset shows the interference pattern), (b) BOC (gray line) and BOC with interference on at the same time (red line) timing jitter correction results
Summary and Outlook
The related study provides the possibility of basic scientific research on the attosecond time scale, which is of great scientific value for the development of attosecond-resolution imaging, ultrafast dynamics detection, and pump-probe experiments. In the future, the measurement accuracy and system stability will be further improved in higher power and more complex multi-laser pulse systems.
