Laser-driven inertial confinement fusion ignition requires high-precision synchronization of multiple laser beams at the target. The common method of synchronization employs photodiodes and oscilloscopes to extract single-point temporal features of short pulse waveforms. However, the achievable time resolution is limited by the bandwidth of detection equipment, sampling rates of oscilloscopes, and signal jitter during acquisition and transmission. In addition, measurement efficiency is crucial for the maintenance of large-scale facilities with dozens to hundreds of laser beams. However, direct measurements using laser shots are limited by the thermal recovery time of the main amplifier and influence of residual lasers during harmonic conversion. This study proposes a synchronization measurement method based on a pulse-sequence alignment beam combined with a cross-correlation algorithm. This method reduces the uncertainties of single-point temporal feature extraction and bandwidth limitations by matching numerous temporal features using the cross-correlation algorithm. Moreover, the method is not affected by residual lasers and improves measurement efficiency as it does not require laser shots, which makes it particularly advantageous for achieving time synchronization of the laser beams split from the main amplifier output.

In this study, an alignment beam and a cross-correlation algorithm are employed. The alignment beam is coaxial with the laser output from the main amplifier and characterized by a nanosecond-scale pulse envelope consisting of picosecond-level sub-pulse sequences. These numerous sub-pulses provide sufficient temporal features for detailed data analysis. Photodiodes are installed at designated reference and target positions after beam splitting of the main amplifier output, and the temporal waveforms of the reference and target signals are recorded using a high-speed oscilloscope. The cross-correlation algorithm is then applied to extract the temporal intervals between the reference and each target signal. Since the reference and target signals originate from the alignment beam and have identical temporal features, the time relationships between each target signal can be obtained by comparing respective intervals with the reference signal. Finally, the method is successfully applied to precision time synchronization diagnostics in a high-power laser facility. Measurement efficiency and accuracy are verified through engineering practices.

This study proposes a synchronized measurement method based on an alignment beam source and a cross-correlation algorithm. The experimental results demonstrate that the cross-correlation algorithm is effective in extracting the temporal interval between pulse-sequence signals (Fig. 3) and exhibits robust performance in analyzing complex waveforms. The method is successfully applied to precision time synchronization diagnostics in the SG II upgrade facility. Online tests show that the synchronization measurement accuracy (root-mean-square, RMS) reaches 3.27 ps, which approaches the temporal resolution limit of the oscilloscope (Fig. 4). Engineering practices indicate that this method improves measurement efficiency by 86.7%, as synchronization measurements can be completed within approximately 2 min for each beam, compared to the 15 min required between shots when using low-energy laser.

This study proposes a time synchronization measurement method based on a pulse-sequence alignment beam combined with a cross-correlation algorithm. The cross-correlation algorithm is suitable for time synchronization measurement applications as it matches numerous temporal features of the alignment beam waveform. The method is successfully applied to time synchronization measurements at the target in a high-power laser facility, achieving a measurement accuracy (RMS) of 3.27 ps and demonstrating superior stability compared with the approach of extracting single-point temporal features within a single pulse. In the online synchronization measurement of 16 ns laser beams in the SG II upgrade facility, the measurement efficiency is improved by 42.5% based on the pulse-sequence alignment beam combined with a cross-correlation algorithm. This method optimizes the measurement of the upper and lower hemispherical optical paths after beam splitting from the main amplifier output, with total time of about 138 min, whereas measurements using low-energy laser shots require estimated total time of 240 min. The alignment beam method significantly enhances the efficiency of iterative synchronization maintenance for the laser facility. Finally, a synchronization adjustment accuracy (RMS) of 8.63 ps is achieved for the 16 ns laser beams in the SG II upgrade facility by applying the alignment beam method. The alignment beam method offers a new approach for time synchronization measurements across all optical paths in high-power laser facilities without laser shots or single-pulse waveforms.