Accurate model and performance analysis of broadband pulsed amplification in picosecond petawatt laser system
Authors: Li Da-Wei , Wang Tao , Yin Xiao-Lei , Li Jia-Mei , Wang Li , Zhang Teng , Zhang Tian-Xiong , Cui Yong , Lu Xing-Qiang , Wang Li , Zhang Jie , Xu Guang ,   Joint Laboratory of High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China,   Shanghai Institute of Laser Plasma, China Academy of Engineering Physics, Shanghai 201800, China,   Changzhou Institute of Technology, School of Sciences, Changzhou 213032, China,   Shanghai Institute of Laser Technology, Shanghai 201800, China,   Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
Published: May 2021

In order to accurately analyze the broadband pulsed amplification performances of the domestic picosecond petawatt laser system, which uses large aperture N<sub>31</sub> or N<sub>41</sub> neodymium glass as gain medium, the broadband pulsed amplification model is improved by introducing the actual stimulated emission cross section (SECS) of neodymium glass. Comparing with the SECS under Gaussian approximation, the amplified pulsed spectrum gain narrowing effect with different SECSs are analyzed. It is found that in the actual SECS of N<sub>31</sub> neodymium glass laser, the gain-narrowing effect is enhanced, the output energy decreases, gain’s saturation effect weakens, system’s accumulated <i>B</i> integral augments, but the laser system turns insensitive to the center wavelength simultaneously. Based on the Shenguang II high energy picosecond petawatt laser system which uses N<sub>31</sub> neodymium glass, the spectral shape, center wavelength, and energy stability of amplified output pulse are simulated by using different SECSs. It is shown that the super-Gaussian spectral shape narrows more greatly than Gaussian spectral shape, the spectrum bandwidth narrows from 10 to about 3 nm with gain larger than 10<sup>7</sup>, and the accumulated <i>B</i> integral increases to 1.7. Additionally, the gain-narrowing effect makes the output spectrum (with 1054 nm of center wavelength) less affected by changing the inputted center wavelength from 1052 to 1056 nm, and the gain saturation effect can improve output energy stability to less than 2% (root mean square (RMS)) with about 3% (RMS) inputted energy stability, which are beneficial to the subsequent pulse compression and physical experiment. Based on the above analysis, a broadband pulsed amplified experiment is conducted by using Shenguang II petawatt laser system, the injected seed is about 10 nm (full width at half maximum (FWHM)) with 5 order super Gaussian shape at 1054-nm center wavelength, and 1.2 mJ with 3% (RMS) energy stability from optical parametric chirped pulse amplification. The amplified pulse with 1900 J at 1054.2 nm (3 nm FWHM) and stability < 2% (shot to shot) is achieved, and the spectral shapes and bandwidths after bar and disk amplifiers are measured, which are consistent with theoretical analysis results. The results can provide a necessary reference for constructing high energy broadband laser system and improving its performances in the future.