The pulse width, temporal contrast and focal spot are critical parameters to characterize the performance of petawatt-scale femtosecond laser systems. In this study, based on the Shenguang II femtosecond petawatt laser facility, we systematically investigate the influence of spatial-temporal coupling on chirped pulses during the optical parametric amplification, parametric fluorescence in the frontend-small-signal amplification, wavefront distortion in high-energy amplification and spatial dispersion induced by large-aperture optical components – all affecting the pulse width and temporal contrast. Through comprehensive optimization, we achieve a focused spot approaching the diffraction limit of approximately 4 μm alongside a remarkable temporal contrast exceeding 1011 at 10 ps before the pulse peak under the full performance of pulse energy higher than 30 J and pulse width of 20–30 fs. This in-depth work provides insight toward the ideal outputs for future femtosecond petawatt lasers.
Abstract We present a finite-difference time-domain framework for modeling femtosecond (fs) to sub-picosecond (ps) laser interactions in hybrid multilayer optical coatings. A total-field/scattered-field formulation injects obliquely incident, transversely Gaussian pulses without prescribing a full initial field profile, enabling stable simulations beyond the sub-100-fs regime. The material response incorporates Keldysh photoionization and impact ionization described by a multiple-rate equation model, accurately capturing carrier multiplication in wide-bandgap dielectrics. The generated carriers are coupled to a Drude model whose collision frequency evolves in time through the electron temperature, providing a unified description of the optical response in both dielectric and metallic layers. The framework is applied to a hybrid SiO 2 /HfO 2 /Au grating irradiated by a 1057 nm, 500 fs pulse. Simulated carrier density and absorbed energy density predict laser-induced damage thresholds in very good agreement with the experiment. The approach is general and applicable to pulse durations from 10 fs to 1 ps.
A spot analysis system based on an improved YOLOv8-seg model was developed for high-power laser facilities to meet the urgent need for fast and precise positional control of optical components throughout the entire laser chain. The system addresses the inefficiency of traditional manual or semi-automatic methods when aligning numerous beamlines. An enhanced segmentation model (mean average precision@0.5 = 0.901) was constructed to recognize and extract various spot types. A multi-reference center detection mechanism was designed, achieving a positioning error of 1.9 pixels and a success rate of 98.7%, thereby improving centering accuracy and robustness in complex imaging scenarios. The system automates spot center location, morphological analysis and offset calculation with efficient processing speed to satisfy the real-time alignment demand of the facility. It outputs pixel/μm-level offset values, adjustment step size, step count and direction as structured decision parameters, providing actionable inputs for the closed-loop control of motorized actuators in rapid laser alignment.
We investigated laser-driven shock propagation in low-density foams through coordinated experiments and simulations. At the Vulcan laser facility, a nanosecond laser pulse launched a shock from a plastic ablator–titanium pusher assembly into a 0.1 g cm–3 trimethylolpropane triacrylate foam, diagnosed by side-on, time-resolved X-ray radiography using a picosecond Cu backlighter. This enabled direct tracking of the pusher motion and the shock front. Comparison with one-dimensional and two-dimensional hydrodynamic simulations (MULTI and FLASH) shows good agreement when assuming an effective on-target intensity of (3–5) × 1013 W cm–2, lower than nominal laser estimates, underscoring the influence of coupling losses. An analytical model based on impedance-matched shock relations connects the ablation pressure, pusher dynamics and transmitted shock strength. Overall, the combined results indicate that the foam behaves as an effectively homogeneous medium under these conditions, supporting modelling assumptions for foam-based fusion targets and establishing a validated platform for future shock-propagation studies in low-density materials.
The optics roughness in Öffner stretchers has been widely identified as a major cause of temporal contrast degradation at tens of picoseconds before the main pulse. However, we find that amplifier-induced noise may become the decisive factor for temporal contrast deterioration at a similar time window with the increase of the B-integral in chirped-pulse amplification systems. Even employing a high-precision convex mirror with root-mean-square roughness values better than 0.2 nm, the experimental measurements show that amplifier-induced noise generates asymmetric profiles under nonlinear effects – exhibiting an obvious pre-pedestal growth proportional to the accumulated B-integral. Analogous to post-to-pre-pulse conversion dynamics, the numerical simulations demonstrate the impact of the B-integral on the pre-pedestal, especially when the pulse peak power scales from the terawatt level to the petawatt level.
