The emergence and rapid development of ultra-high intensity, ultrashort pulse lasers have provided unprecedented extreme physical conditions and brand-new experimental means for human beings, and have become the latest frontier of international laser science and technology as well as the focus area of competition. Pulse compression grating is the core component in the ultra-high intensity ultra-short laser device, and the aperture of the grating determines the upper limit of the laser output power. Domestic and foreign development of fine-beam scanning exposure, static interference field transmission exposure, exposure splicing and mechanical scribing and other methods, do not have the bidirectional meter-scale grating preparation capabilities.
Fig. 1 Full-frequency error results of Φ300mm off-axis parabolic mirror exposure system: (a) Low-frequency surface shape error of off-axis mirror measured by 4-inch Zygo interferometer. (b) Pictures of the mid-frequency error and light field distribution obtained after filtering according to the model; (c) High-frequency error obtained by using a Zygo white-light profiler with a 20x lens and a photograph of the grating mask measured by a microscope. (d) 1D power spectral density curve.
Shanghai Institute of Optical Machinery (SIOM) has proposed an innovative scheme to fabricate meter-scale pulsed compression gratings using a large-diameter off-axis reflective exposure system. The core of the program is to use high-precision off-axis parabolic mirrors to form two parallel beams of light to construct a uniform exposure light field on a large scale, and the light field uniformity is mainly determined by the surface error of the off-axis parabolic mirrors, especially in the middle and high frequency errors. Due to the lack of a quantitative evaluation system of manufacturing errors on light field uniformity and the related high precision machining process with consistent convergence of errors across the frequency range, there is still no successful precedent.
Based on the free light field diffraction theory, the team has established a mapping model between the frequency band error on the surface of reflection-exposed off-axis parabolic mirrors and the homogeneity of the exposure light field, established a quantitative index system for the frequency band error of the mirror surface shape, and then put forward an innovative processing technology for the unanimous convergence of the full-frequency band error of the exposure mirror. According to the index evaluation system determined by the model, the medium- and high-frequency errors of the exposure mirrors should be better than 0.65 nm and 0.5 nm, respectively, and therefore, an off-axis reflective exposure system of Φ300 mm was fabricated by adopting the above processing technology. In this system, the RMS of the mirror was suppressed to 0.586 nm and 0.462 nm, and the periodic error and the regular stripe error were completely eliminated. Finally, a multilayer dielectric film (MLD) diffraction grating with a size of 200 mm×150 mm was successfully fabricated using this exposure system, with an average diffraction efficiency of 98.1% at the -1 level and a diffraction wavefront PV better than 0.3 wavelengths.
This research for the manufacture of large aperture diffraction grating provides a new way for the subsequent development of 100 pat-watt high-power laser device required for the meter-level pulse compression grating to lay the technical foundation.
Figure 2 Diffraction wavefront and efficiency distribution of 200mm×150mm MLD grating: (a) -1 level diffraction wavefront. (b) 0-level diffraction wavefront. (c) +1-level diffraction wavefront. (d) Diffraction efficiency of the MLD grating at 1740 l/mm, with uniform diffraction efficiency within the effective aperture at 1053 nm (Ave=98.1%, σ=0.3%, Max=98.6%). (e) A physical image of the MLD grating using the reflection exposure method.
