331 / 2019-02-26 15:28:44

Simulated Coherent X-ray Imaging by XFEL Source

XFEL; coherence; x-ray imaging; simulation

全体主题 > Science and Technology based on XFEL

There are three prominent properties of the XFEL over other x-ray sources, which are femtoseconds pulse duration, high pulse brightness, and nearly full coherence, make the XFEL source extraordinary for researches. The first two properties contribute to a large amount of excellent measurements on ultra-fast processes, while the last one promotes feasibility of various coherent imaging techniques.
The effect of coherence, in the case of free space propagation, mainly depends on the propagation distance, and thus different imaging techniques can be classified according to the separation between the sample and detector (compared to the sample scale). For long range propagation, the superposition of full coherent lights results a image quite different from the images of non-coherent and inadequately coherent sources. However, all operating and scheduled XFEL facilities can not generate completely coherent pulses. So the images from a real pulse and an ideal pulse is compared, for several concise free space imaging setups. The real pulse comes from a simulated result of the European XFEL.
The coherent diffraction imaging (CDI) is investigated. It is originally proposed to utilize full coherent source although realized by a partially coherent synchrotron radiation source. The request for high coherence are partially drawn from its long detecting distance, which is far beyond the sample scale and approximates infinity. This long propagation distance lead to the recorded image containing frequency information of the sample, but not a point-to point image in space. Thus the comparison between images of real and ideal pulses are carried out through reconstruction procedures.
Unlike far-field CDI, phase contrast imaging (PCI) uses near-field image. With a short propagation distance, only interference from adjacent areas is significant, and partially coherent light is enough to form edge-enhanced image. Little distinction is expected from images of the real pulse and ideal pulse, but the retrieved phases are more susceptible to coherence and carefully compared.
The absorption contrast imaging, which commonly use non-coherent source, can only be adaptive in extremely short distance, which is too short to place a detector, for XFEL source. Only absorption contrast measuring with lens and mirrors is practicable.