Modified Mach–Zehnder interferometer for spatial coherence measurement

Spatial coherence of light sources is usually obtained by using the classical Youngs interferometer. Despite the original experiment has been improved in successive works, some drawbacks still remain. In addition, several pairs of points must be used to characterize the totally the 2D complex-coherence degree of the source. In this work, an alternative based on a modified Mach-Zehnder interferometer which includes a pair of lenses is presented. With this modified Mach-Zehnder interferometer, we are able to measure the spatial coherence length between any two points of the laser beam section simultaneously. The set-up does not have any movable part, which makes it robust and portable. To test it, the two-dimensional spatial coherence of a high-speed laser with two cavities has been measured for different pulse energy values. We observe from the experimental measurements that the complex degree of coherence changes with the selected output energy. Both laser cavities seem to have similar complex coherence degree for maximum energy although it is not symmetrical. Thus, this analysis will allow us to determine the best configuration of the double cavity laser for interferometric applications. Furthermore, the proposed approach can be applied to any other light sources.