Programmable CGH on photochromic material using DMD

Abstract

Computer Generated Holograms (CGHs) are useful for wavefront shaping and complex optics testing, including aspherical and free-form optics. Today, CGHs are recorded directly with a laser or intermediates masks but allows only recording binary CGHs; binary CGHs are efficient but can reconstruct only pixilated images. We propose to use a Digital Micro-mirror Device (DMD) for writing binary CGHs as well as grayscale CGHs, able to reconstruct fulfilled images. DMD is actually studied at LAM, for generating programmable slit masks in multi-object spectrographs. It is composed of 2048x1080 individually controllable micro-mirrors, with a pitch of 13.68 μm. This is a real-time reconfigurable mask, perfect for recording CGHs. A first setup has been developed for hologram recording, where the DMD is enlightened with a collimated beam and illuminates a photosensible plate through an Offner relay, with a magnification of 1:1. Our set up resolution is 2-3 μm, leading to a CGH resolution equal to the DMD micro mirror size. In order to write and erase CGHs during test procedure or on request, we use a photochromic plate called PUR-GD71-50-ST developed at Politecnico di Milano. It is opaque at rest, and becomes transparent when it is illuminated with visible light, between 500 and 700 nm; then it can be erased by a UV flash. We choose to code the CGHs in equally spaced levels, so called stepped CGH. We recorded up to 1000x1000 pixels CGHs with a contrast greater than 50, knowing that the material is able to reach an ultimate contrast of 1000. A second bench has also been developed, dedicated to the reconstruction of the recorded images with a 632.8nm He-Ne laser beam. Very faithful reconstructions have been obtained. Thanks to our recording and reconstruction set-ups, we have been able to successfully record binary and stepped CGHs, and reconstruct them with a high fidelity, revealing the potential of this method for generating programmable/rewritable stepped CGHs on photochromic materials.