Coherence effects from visible light to X-rays

The development of the concept of coherence of light and its application for experiments in the XUV range are reviewed. Coherence was originally defined by the visibility of interference fringes, and the main effort of early work was to produce coherency from non-coherent, chaotic or thermal sources. With the invention of the laser, fully coherent light became available, leading to further clarification of the coherence concept. The laser offered the first possibility to observe easily interference effects between different, independent light sources. By providing many photons in a single mode, non-linear effects, such as the generation of higher harmonics or sum and difference frequencies became possible. The generation of non-classical fields, i.e. fields which are only allowed by quantum theory but have no analogue in classical physics, is a more recent development. Synchrotron sources in the XUV region are now reaching the brightness which visible light had at the beginning of the sixties and the first soft X-ray lasers are available. Brightness is now sufficient to prepare spatially coherent X-ray beam with sufficient intensity for holography or the observation of coherency effects in the fluctuations of the light intensity. Non-linear effects might become observable by mixing synchrotron radiation with laser light. © 1994.