Seminars Archive

Measuring the two-dimensional, time-resolved spatial coherence: The Heterodyne Speckle Coherence Mapping.

Abstract
The classical apparatus for measuring the transverse coherence is undoubtedly the Young's double pinhole interferometer. However, as it allows to probe only one length scale at a time (the distance between the pinholes) and in one direction only (the line adjoining the pinholes), the method is rather inefficient for a complete two-dimensional coherence mapping. Though in the last decade a number of alternative methods has been developed and operated at the soft and hard X-rays, the matter of a straightforward and accurate measure of the 2D coherence function of quasi-coherent radiation is still open. While most of the techniques use ad-hoc engineered test plates (e.g. gratings, apertures, diffracting wires, redundant arrays etc) to produce the reference wavefront, we developed a statistical approach where the reference wavefront is provided by a huge number of scattering particles in aqueous solution. The advantages respect to more conventional approaches are striking. The method is capable to provide one-shot 2D measures of the modulus of the Complex Coherence Function, being thus a good candidate for the time-resolved measures. In addition, it is completely free of a-priory assumptions and is endowed of the data finesse typical of statistical methods. Last, it is extremely sensitive and versatile, potentially opening the way to a number of still unexplored applications. Results obtained with synchrotron and FEL radiation will be reported, showing some ancillary applications to X-Ray instrumentation diagnostics.