Reflective Volume Bragg Grating (RBG) recorded in photo-thermo-refractive glass offers notable advantages, including high efficiency, narrow bandwidth, and multiplexing capabilities, making them ideal for applications in dense wavelength division multiplexing (DWDM) and demultiplexing. However, the side lobes caused by sudden changes in coupling strength at both ends of the grating always lead to interchannel interference, which evidently affects the effectiveness of multiplexing and demultiplexing. In this work, a theoretical model of the apodization for RBG, which based on Kogelnik's coupled wave theory and F-matrix theory. It is simulated and analyzed the effects of three apodization functions including cosine, gaussian, and hyperbolic secant functions is established. The simulation results indicate that the side lobes of the RBG with apodization is significantly reduced and the peak efficiency is determined by the total refractive index modulation. This work provides a theoretical design basis and parameters optimizing method for the development of apodized RBG.
The pulse compressor has evolved to be a core module of 100 Petawatt (PW) or Exawatt laser facilities; however, the diffraction efficiency bandwidth, laser-induced damage threshold (LIDT), and aperture of its currently deployed gratings strongly restrict the ultra-intense and ultrashort pulse compression. Maximizing the energy-loading capability of gratings is a worldwide challenge in the high-peak-power laser field. Here, for 10’s femtosecond-PW pulse compression, an ultra-broadband gold grating was developed to dramatically broaden the high diffraction efficiency bandwidth from 100–200 nm to 400 nm. Moreover, a core mechanism was elucidated whereby the high diffraction efficiency combined with the deep penetration effect of electrons under high-energy laser irradiation can tap the LIDT potential of metal grating. Accordingly, the mixed metal grating was invented and demonstrated experimentally with superior performance in terms of diffraction efficiency bandwidth and LIDT compared with normal gold gratings. In addition, for picosecond-PW pulse compression, a multilayer dielectric grating (MDG) design paradigm was proposed. Importantly, TM-polarized MDGs had the superiority of a high LIDT owing to the low electric field intensity. Furthermore, a novel grating with a LIDT 3.5 times higher than the conventional gratings installed in NIF-ARC and SG-II was obtained by taking advantage of TM polarization and a small incident angle. These results make a pioneering technical reserve to facilitate future 100 PW-class ultrafast laser systems.
Meter-scale optical films have become core components in large-size optical systems. Improving film thickness uniformity is a crucial issue to ensure optical properties. Ag films are commonly used as high-reflection mirrors for optical systems. Here a meter-scale magnetron sputtering equipment has been independently developed for coating Ag films. Two strategies were performed to correct the film thickness uniformity in the horizontal direction and vertical directions respectively. Specifically, the film thickness uniformity at the horizontal position was guaranteed by scanning the target parallel to the horizontal direction. Meanwhile, a mask was designed to optimize vertical film thickness uniformity. The film thickness, fitted by X-ray reflectometry (XRR), non-uniformity is less than 3% both in the 1800 mm range of the horizontal direction and the 1100 mm range of the vertical direction. This study has brought new insight into the development of meter-scale thin film optics.
Compressing high-energy laser pulses to a single cycle will break the current limitation of super-scale projects and facilitate future Exawatt lasers. However, the lack of ultra-broadband gratings capable of supporting single-cycle pulse stretching and compression is a core problem that cannot be overcome. Recently, we solved this problem and developed gratings with the broadest bandwidth (broader than 400 nm) that can support near-single-cycle laser pulses (about 4 fs). A 200 mm (Length) ultra-broadband grating is being fabricated supporting a single-cycle terawatt-level prototype. Meanwhile, the 1600 mm (Length) fabrication campaign is underway and will be completed in 2024–2025 at SIOM, facilitating the single-cycle Exawatt lasers.
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