Kilohertz dynamic Fourier filter for synthetic-aperture binary hologram reconstruction

Abstract

Synthetic-aperture holography is a powerful tool for off-axis holographic imaging because it can fill the entire illumination aperture of the object under observation and provide super resolution. Normally, in off-axis holography, only the small portion of the Fourier domain containing the real hologram is filtered from the zeroth order and virtual hologram and the space where the real hologram does not fill is left null. However, in synthetic-aperture holography, a change in direction of the illumination beam changes the portion of Fourier space the hologram records and, through angular multiplexing or multiple acquisitions of these holograms, the entire Fourier space of the hologram can be filled. This is a straightforward task in holographic imaging as the filters are applied digitally. Here, we present a method based on a synthetic aperture for generating large field-of-view binary holograms using a digital micromirror device (DMD). Because of the relatively large pixel size (7.56 μm) of the DMD, only a small wavelength-dependent angular distribution (~3 deg.) for each sub hologram can be achieved. However, by changing the DMD illumination direction, a much larger sweep of Fourier space can be filled. Difficulties arise in filtering the real holographic information from the zeroth-order term and conjugate hologram as these pieces of unwanted information shift in the Fourier domain with the illumination direction. A static pinhole filter can no longer remove unwanted components. A dynamic Fourier filter was implemented based on a DMD with 4 kHz switching speed which was placed in the far-field of the hologram-producing DMD.