光源尺寸和光谱带宽对波带板成像的影响

Influence of source size and spectral bandwidth on the imaging of a zone plate

X射线菲涅耳波带板成像能实现亚微米空间分辨能力,有可能应用于激光等离子体或聚变靶的高分辨X射线成像诊断.之前的数值模拟研究表明,成像分辨能力受光源尺寸、入射光或成像光谱带宽的影响.本文报道在632.8 nm为中心波长的可见光波段,对波带板成像的数值模拟和原理性验证实验.数值模拟表明:随着扩展光源尺寸增加,视场中央分辨能力基本不变,而像对比度下降;随着成像的光谱带宽的增加,视场中央分辨能力与像对比度同时下降.实验证实了数值模拟的结论,且实验与数值模拟结果的定量比较也符合得较好.

Direct X-ray imaging by a Fresnel zone plate(FZP) has achieved a spatial resolution of 10 nm on a synchrotron beamline. It may be used to obtain submicron-resolution X-ray images of laser-plasma sources or fusion targets. However, none of previous imaging experiments with laser-plasma kilo-elelctron-volt X-ray sources shows such a high resolution. In comparison with the FZP imaging on a synchrotron, we consider a case of imaging an extended object with a laser-plasma X-ray source that the illumination monochromaticity is lower and the field of view larger. Our simulations show that the spatial resolution is affected by both the object size and the spectral bandwidth of the source, which can explain the previous experiments. We conclude that by using a 100-zone FZP to image an object with up to 700 μm in size, a spatial resolution better than 1 μm can be realized by using X-rays of several kilo-electron volts and a spectral bandwidth just less than 3%. In this paper,we report a proof-of-principle study in simulation and experiment in an optical range centered at 632.8 nm. The simulation is performed with the same method as that previously used for X-ray imaging but with a 100-zone FZP working in the optical range. Simulations show that with the increase of the object size, the field-of-view contrast is degraded, but the spatial resolution is nearly unchanged. With the increase of the spectral bandwidth for the illumination, both the contrast and the resolution are degraded. In the experiments, different spectral bandwidths are realized by band-pass filters and different object sizes by an adjustable aperture. The experimental results are confirmed to be in agreement with the simulations. These results reveal that given a satisfied spectral bandwidth of laser-plasma X rays, the FZP imaging will be a promising approach to 1 μm or higher resolution X-ray imaging of a 1-mm-size object.