The HOEs (click for an HOE tutorial)
in our systems are
produced on a glass plate coated with a film of dichromated gelatin emulsion by exposing it to two mutually coherent laser
beams, referred to as object and reference beams. The object beam emanates from a pinhole placed on the normal center line
of the plate (see a in the above figure). These spherical wave fronts interfere in the gelatin with plane waves of a collimated reference beam,
forming molecular cross-links in the film in proportion to the light intensity due to the absorption of the light by the
dye. The interference fringes are registered in the film as variations in hardness and index of refraction. The
photo-sensitive dichromate is then removed from the gelatin during post-exposure chemical processing, and the
resulting hologram is relatively free of absorption. The HOE is then dried and hermetically sealed with a glass cover
cemented to the substrate and sealed around the edges. When the completed HOE is illuminated with the photo-conjugate to the
construction reference wave, it reconstructs the conjugate of the original object beam, thus forming a real image of the
original point source (see b in figure 1 above).
In lidar applications, backscattered laser light acts as the reconstruction beam
and is focused by the HOE.
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