HARLIE

Holographic Airborne Rotating Lidar Instrument Experiment

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Holographic Optical Elements (HOEs)

An HOE is a hologram (click for a holography tutorial) of a point source and acts as a lens or a mirror having optical power, i.e., the ability to focus light.
 

The hologram consists of a diffraction pattern rendered as a surface relief, or a thin film containing an index modulation throughout the thickness of the film. For our purposes, holograms can be divided into two categories: reflection holograms, in which incident and diffracted light are on the same side of the HOE, and transmission holograms, in which incident and diffracted light are on opposite sides.
 

 Reflection HOE construction and reconstruction. a) Holographic film
is exposed to mutually coherent ligh from a point source (right hand side of HOE - straight in) and a collimaed reference 
beam (left hand side - 45 degrees from horizontal), b) the point source is reconstructed by illuminating the HOE with the 
conjugate to the reference beam (-45 degrees from horizontal on right hand side), reflecting to a point on the right hand side (0
degrees from horizontal). This can be done with atmospheric backscatter.
Figure 1. Reflection HOE construction and reconstruction. a) Holographic film is exposed to mutually coherent light from a point object source and a collimated reference beam, b) the point source is reconstructed by illuminating the HOE with the conjugate to the reference beam, which can be atmospheric backscatter.

 
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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