国家同步辐射实验室光谱辐射标准和计量光束线(U27)的SGM分支是专门为光学元件性能测试和探测器定标而建造的.为了能够精确测量光学元件在极紫外和软X射线波段的性能,必须充分抑制高次谐波提高光谱纯度.对于已经建成的光束线,要改变光学设计和现有结构来抑制高次谐波是困难的,最简单且有效的方法是用不同材料的滤片来抑制不同波段的高次谐波.为了研究高次谐波的抑制效果,可将840 1/mm透射光栅放在U27光束线SGM分支的出射狭缝后面色散出射光,用探测器做角度扫描记录下信号强度曲线,然后分析得到高次谐波的含量和分布.本文分别研究了不同厚度的Al(200、400和600 nm)、Si3N4/Mo/Si,Si3N4/Mo/Si/Mo/Si多层膜滤片(100/50/200 nm,100/50/150/150/250 nm)和Al/Mg/Al滤片对13～43 nm光谱高次谐波的抑制效果.研究结果显示,400 nm厚的Al滤片适合于17～33 nm光谱高次谐波的抑制,在保证探测器信号强度的条件下,高次谐波信号强度占探测器信号强度的比例＜2%,经探测器量子效率修正后,高次谐波比例＜0.6%.Si3N4/Mo/Si/Mo/Si多层膜滤片可以有效地抑制13～19 nm的高次谐波,Al/Mg/Al滤片对30～43 nm的高次谐波有很好的抑制作用.这一结果为光学元件的透射率、反射率和探测器精确定标奠定了基础.%The Spherical Grating Monochromator(SGM)branch of Spectral Radiation Standard and Metrology(U27)beamline is specially built for measurement of the properties of optical elements and detectors.In order to accurately measure the performance of optical elements,higher-order harmonics must be suppressed efficiently.For the existing beamline where the design cannot readily be altered,the simplest method is to use transmission filters to suppress higher-order harmonics.With a 840 1/mm transmission grating used behind the exit of SGM in U27 beamline,the exit beam can be dispersed and the contributions of the different orders can be analyzed.Results of higher-order suppression by 200 nm,400 nm,600 nm thickness Al filters,100/50/200 nm,100/50/150/150/250 nm thickness Si3 N4/Mo/Si,Si3N4/Mo/Si/Mo/Si multilayers and Al/Mg/Al filter in the region of 13～43 nm show that when the thickness of Al filter is 400 nm,and the wavelength is between 17 nm and 33 nm,the contributions of higher orders to the detector signal intensity are less than 2% ,and the detector intensity is strong enough when the beam on.After being corrected by quantum efficiency of the detector,higher order contributions are less than 0.6%.Si3N4/Mo/Si/Mo/Si filter can be efficiently used to suppress higher-order harmonics in the region of 13～19 nm,and Al/Mg/Al filter can be used to suppress higher-order harmonics in the region of 30～43 nm efficiently.These are important for the accurate calibration of absolute reflectivities of multilayer and detector etc.
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