1.College of Electronic Information and Automation, Civil Aviation University of China, Tianjin 300300, China 2.National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61875045) and the Natural Science Foundation of Tianjin, China (Grant No. 17JCYBJC18200).
Received Date:30 August 2018
Accepted Date:19 November 2018
Available Online:01 January 2019
Published Online:20 January 2019
Abstract:In this paper, a double-pass amplification experiment of a Ne-like Ar C line 69.8 nm laser is established. The 45-cmlong capillary is used as the discharge load to obtain a double-pass amplification output of a Ne-like Ar C line 69.8 nm laser. Under the same initial experimental conditions that the initial pressure is 15.4 Pa and the main pulse current amplitude is 13.5 kA, the laser pulse intensity and the full width at half maximum (FWHM) of the laser pulse of the single-pass amplification output and the double-pass amplification output are measured by a vacuum X-ray diode (XRD) behind a vacuum ultraviolet (VUV) monochromator (Acton VSN-515) which is used to disperse the extreme ultraviolet (EUV) emission. And then the laser beam divergence of single-pass amplification output and double-pass amplification output are also measured by a space-resolving flat-field EUV spectrograph combined with an EUV CCD (Andor Newton DO920P-BN). The amplitude of the double-pass amplification laser output is 9 times larger than that of single-pass amplification output, and the FWHM of the double-pass amplification laser pulse is nearly 2.4 ns. While the laser beam divergence angle of the double-pass amplification output is 6.6 times wider than that of single-pass amplification output. By comparing the single-pass amplification and double-pass amplification output experimental results, the gain duration of the gain medium in the double-pass amplification and the radial distribution characteristics of the gain medium are analyzed by using the calculation formula of the double-pass amplification laser intensity. The gain duration is more than 4 ns, during this time the gain coefficient decreases at 1.6 ns. And the gain coefficient is the smallest at 2.8 ns, meanwhile the intensity of the single-pass amplification laser is maximum, and the gain medium is in the gain saturation state. So this result indicates that the minimum gain coefficient at this moment is due to the gain saturation effect. Using a similar calculation method to analyze the spatial distribution of gain coefficients, the gain on the plasma axis is larger than that off the plasma axis. These results lay a foundation for the subsequent establishment of resonant cavity and the multi-pass amplification experiment of capillary discharge Ne-like Ar laser. Keywords:capillary discharge/ 69.8 nm laser/ double-pass amplification/ gain medium
表1单程放大与双程放大尖峰位置处激光强度 Table1.The peak position laser intensity of single-pass amplification and double-pass amplification.
在激光尖峰对应位置处, 用双程放大激光强度减去单程放大激光强度, 得到(Id?Is), 然后除以${I_{\text{s}}}^\prime $并求对数值, 再除以毛细管长度l, 就得到了激光尖峰位置对应的增益系数(图6). 其中数据点为激光尖峰位置对应的增益系数, 在中间峰值处的增益系数最小, 而两侧激光尖峰所对应的增益系数相对较高. 图 6 69.8 nm激光峰值处的增益系数在空间上的分布情况 Figure6. Gain coefficient as a function of angle in the spatial distribution for 69.8 nm laser peak.
图 1 双程放大实验反射镜位置示意图
图 2 单程放大和双程放大输出脉冲波形 (a)单程放大输出; (b)双程放大输出
图 3 毛细管径向上CCD图像 (a)单程放大强度图像; (b)双程放大强度图像
图 4 毛细管径向上激光束散角 (a) 单程放大的束散角; (b) 双程放大的束散角
图 5 69.8 nm激光的增益系数随时间的变化
图 6 69.8 nm激光峰值处的增益系数在空间上的分布情况