The size of a holographic data-storage system could be reduced by a proposed design modification that calls for replacement and repositioning of some optical components and the use of some of the components to perform dual functions. The modification would enable a substantial decrease in length with a small increase in width, yielding an overall decrease in volume.
The upper part of the figure schematically depicts a typical three-dimensional holographic data-storage system. A laser and a polarizing beam splitter are used to generate reference and image light beams, which are coherent with each other. These beams are made to interfere with each other in a holographic storage medium (e.g., doped LiNbO3). In the simplest case, the reference and image beams enter the storage medium perpendicularly to each other. The reference beam is not modulated on its way to the storage medium. However, the image beam is expanded, then the desired image is impressed on the beam during passage through a transmissive spatial light modulator. The image beam is then directed through a first Fourier-transform lens into the storage medium. During retrieval of a stored image, the image beam is blocked, and a second Fourier-transform lens projects the image onto a charged-coupled-device (CCD) camera.
The lower part of the figure illustrates this system as modified according to the proposal. Among other changes, one of the lenses would be eliminated and the second beam-expanding lens would also serve as the first Fourier-transform lens. During recording, the image beam would be modulated with the desired image and reflected back through this lens by a reflective spatial light modulator (instead of by a transmissive one as before). A second polarizing beam splitter would be placed between the two beam-expanding lenses; with the chosen combination of polarizations, this beam splitter would pass the expanding (rightward-propagating) beam but would reflect the modulated (leftward-propagating) image beam downward into the holographic storage medium. As before, the reference and image beams would enter the holographic storage medium at right angles to each other during recording. As before, a second Fourier-transform lens would be used during retrieval of a stored image. In this case, a conjugate image would be formed and would be reflected rightward onto a CCD.
This work was done by Kevin Heim of Caltech for NASA's Jet Propulsion Laboratory.
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Reducing the volume of a holographic data-storage system
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