Short wavelength Free-Electron Lasers (FELs) are among the newest light sources available to scientists to probe a wide range of phenomena, with chemical, physical and biological applications, using soft and hard X-rays. These sources include the currently most powerful hard X-ray light sources in the world and are characterized by extremely high powers and high transverse coherence, but the first FELs had, and many still have, reduced longitudinal coherence. Now it is possible to achieve good longitudinal coherence (narrow bandwidth in the frequency domain) and here we discuss and illustrate a range of experiments utilizing this property, and their underlying physics. The primary applications are those which require high resolution (for example resonant experiments), or temporal coherence (for example coherent control experiments). The currently available light sources extend the vast range of laboratory laser techniques to short wavelengths.

短波自由电子激光（FEL）是科学家可利用的最新光源之一，可以使用软X射线和硬X射线探测化学，物理和生物应用中的各种现象。这些光源包括世界上目前最强大的硬X射线光源，并且具有极高的功率和很高的横向相干性，但是最初的FEL且至今仍有许多纵向相干性降低。现在可以实现良好的纵向相干性（频域中的窄带宽），在此我们讨论和说明利用该性质及其基础物理原理进行的一系列实验。主要应用是那些需要高分辨率（例如共振实验）或时间相干（例如相干控制实验）的应用。