ABSTRACT
The zero path difference of the system (i.e. when the two dihedrons are exactly at the same height) was determined before the experiment, using an optical source, with a very good accuracy (~ 1 jum). The path difference between the two interfering halfbeams was then varied from shot to shot by accurately translating one of the two dihedrons vertically. Dihedrons were used rather than simple flat mirrors (as previously in [13]) to ensure that the conditions of spatial coherence are kept constant while changing the path difference. The spatial coherence of the X-ray laser beam is here given by the large distance (3.5 meters) between the source and the interferometer. Figure 3 shows a set of interferograms which were obtained for the zero path difference (top image) and for increasing values of it, up to 1.39 mm (i.e. 10 X). One can see that the visibility of the fringes gradually decreases while increasing the path difference A.
