Coherence effects in off-axis holography: simulations and experiment

In holography the aim is to recover the complex amplitude of a wavefield. This can be accomplished by allowing the wavefield of interest to interfere with a known reference wave in a given plane and recording the resulting intensity pattern. This intensity pattern is in fact a hologram whose distribution can be mathematically described using the well-known interference equation. This equation contains several terms: the DC terms, as well as the so-called real and twin images. Separating the real image from these other terms generally requires an extra processing step. Sometimes this can be done by temporal phase shifting—where the reference wave is phase stepped by a known amount between captures. Alternatively, off-axis holography can be used, which allows one to measure the complex amplitude of an optical wavefield in a single shot. This off-axis approach is very effective, particularly when a long-coherence-length laser source is used. However, in some optical configurations when a short-coherence-length laser is used, one observes experimentally a reduction in fringe contrast—a contrast reduction that is spatially dependent. In this manuscript we examine this question in more detail by developing a theoretical model to describe this effect. Experimental results are also provided.