Holographic optical element
Encyclopedia
H.O.E stands for Holographic optical element and it is a hologram that 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. HOEs 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 beam
s. The object beam emanates from a pinhole placed on the normal center line of the plate wave fronts interfere in the gelatin
with plane wave
s 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
, in particular, for optical memory discs and diode lasers. Diode lasers have a very large angle of divergence, which can be compensated by the installment of HOE focusing elements. The simplest HOE is a Fresnel zone plate which can act as a focusing lens. If the HOE is illuminated by a reference beam of at the same angle of incidence
as the recording, it can act as a classical optical element. Allows you to see holographic images using heat radiant imagery.
Reference beam
A reference beam is a laser beam used to read and write holograms. It is one of two laser beams used to create a hologram. In order to read a hologram out, some aspects of the reference beam must be reproduced exactly as when it was used to write the hologram...
s. The object beam emanates from a pinhole placed on the normal center line of the plate wave fronts interfere in the gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
with plane wave
Plane wave
In the physics of wave propagation, a plane wave is a constant-frequency wave whose wavefronts are infinite parallel planes of constant peak-to-peak amplitude normal to the phase velocity vector....
s of a collimated
Collimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...
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
Substrate
Substrate may mean:*Substrate , Natural stone, masonry surface, ceramic and porcelain tiles*Substrate , the material used in the bottom of an aquarium*Substrate , the material used in the bottom of a vivarium or terrarium...
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
Point source
A point source is a localised, relatively small source of something.Point source may also refer to:*Point source , a localised source of pollution**Point source water pollution, water pollution with a localized source...
Uses
HOEs are most important in optoelectronicsOptoelectronics
Optoelectronics is the study and application of electronic devices that source, detect and control light, usually considered a sub-field of photonics. In this context, light often includes invisible forms of radiation such as gamma rays, X-rays, ultraviolet and infrared, in addition to visible light...
, in particular, for optical memory discs and diode lasers. Diode lasers have a very large angle of divergence, which can be compensated by the installment of HOE focusing elements. The simplest HOE is a Fresnel zone plate which can act as a focusing lens. If the HOE is illuminated by a reference beam of at the same angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
as the recording, it can act as a classical optical element. Allows you to see holographic images using heat radiant imagery.