Recently, the Department of High Power Laser Component Technology and Engineering of Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences (SIPM, CAS) has made new progress in the research of the index system of the optical components of the pulsed compression grating double-beam static interferometric holographic exposure system and the process of controlling the homogeneity of the exposure light field. For the first time, the research has established a quantitative evaluation system for the uniformity of reflection exposure optical field, and successfully realized the application verification in the small-diameter reflection exposure system. The research results are summarized as "Specifications and control of spatial frequency errors of components in two-beam laser static holographic exposure for pulse compression grating fabric". The related research results were published in High Power Laser Science and Engineering under the title of "Specifications and control of spatial frequency errors of components in two-beam laser static holographic exposure for pulse compression grating fabrication".
The emergence and rapid development of ultra-high intensity, ultra-short pulse lasers have provided unprecedented extreme physical conditions and new experimental means for human beings, and have become the latest frontier of international laser science and technology and the focus of competition. Pulse compression grating is the core component of ultra-high intensity and ultra-short laser device, and the aperture of the grating determines the upper limit of 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.
Shanghai Institute of Optical Machinery (SIOM) has proposed an innovative scheme to fabricate meter-scale pulse compression gratings using a large-caliber off-axis reflective exposure system. The core of the program is the use of high-precision off-axis parabolic mirrors to form two parallel beams of light to construct a wide range of uniform exposure light field, and the light field uniformity is mainly determined by the off-axis parabolic mirror surface error, especially in the high frequency error. 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 mid- 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-scale pulse compression grating laid the technical foundation.
Fig. 1 Full-frequency error results of Φ300mm off-axis parabolic mirror exposure system: (a) Low-frequency face shape error of the off-axis mirror measured by using a 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.
Fig. 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.
