Optical: systems and elements – Holographic system or element – For producing or reconstructing images from multiple holograms
Reexamination Certificate
1998-02-10
2001-04-24
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
For producing or reconstructing images from multiple holograms
C359S019000, C359S022000, C359S034000, C365S216000, C369S112040
Reexamination Certificate
active
06222651
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to holograms in general, and in particular to volume phase holograms.
BACKGROUND OF THE INVENTION
Holograms are recordings in or on photosensitive plates of light intensity patterns created by the interference of two beams of mutually coherent light. Typically, one of the beams is produced by light waves from an object wave and the other beam is produced by light waves from a reference source. The light intensity patterns contain information on both the phase and amplitude of the light waves. This information is in coded form, and the hologram itself bears no resemblance to the object. Instead, the intensity patterns that form the hologram are in essence “fringes” that run through the thickness of the photosensitive plate. After processing, when the hologram is illuminated by the reference beam, light is diffracted from the hologram such that the object beam is reconstructed, thereby generating a wavefront that makes it appear as though the light had originated from the object and thus in essence, creating a three-dimensional image of the object.
There are two major categories of holograms: transmissive and reflective. Transmission holograms are created, in essence, by two wavefronts incident upon the photosensitive plate from the same side. On the other hand, reflection holograms are created by two wavefronts incident upon the photosensitive plate from opposite sides. In transmission holograms, the interference fringes recorded are roughly perpendicular to the photosensitive plate surface, somewhat like the slats of a venetian blind, whereas in reflection holograms, the interference fringes recorded are more nearly parallel with the surface of the plate, like the pages of a book. These two categories are further divided into two physical types of holograms: surface relief holograms and volume phase holograms.
Volume phase holograms work using the same principle as Bragg volume gratings. Bragg volume gratings are made up of multiple layers of material with different refractive indices. In volume phase holograms, the surfaces or “fringes” of these layers which have different refractive indices are created by two plane waves or two waves and are also referred to as “Bragg planes.” In essence, volume phase holograms behave as if they consisted of multiple overlapped stacks of Bragg planes.
The direction and wavelength of light that is reflected from a single Bragg grating, and thus a volume phase hologram, depends upon how the layers are tipped and the distance between the layers. An efficient Bragg grating reflects almost all of the light rays that satisfy Bragg's Law, and lets light rays that do not satisfy the law pass through. Bragg's Law states that n&lgr;=2d cos &thgr;, where n is an integer typically 1, &lgr; is the wavelength of the light ray, d is the distance between the Bragg planes, and &thgr; is the angle, known as a “Bragg angle,” between the light ray and the Bragg plane normal vector. A range of different wavelengths can satisfy Bragg's Law for a given grating. As determined by the equation, each wavelength of the visible spectrum has a different angle.
A problem with Bragg gratings and thus volume phase holograms, is that Bragg gratings are three-dimensional. Therefore, for a given wavelength &lgr;, any Bragg grating can be illuminated within a wide arc of the Bragg angle &thgr; centered on the Bragg plane normal. This effect produces an undesirable result when the hologram is illuminated, because light incident on the hologram from other directions, also referred to as ambient light, illuminates the hologram and causes a distortion or a lack of clarity in the desired object image. Therefore, there is a need for a holographic system wherein the acceptable angles of illumination are narrowed.
SUMMARY OF THE INVENTION
The present invention is a holographic resonant system for narrowing the range of illumination angles so that the system is only illuminated from a chosen angle, thus making the system insensitive to ambient light. The holographic resonant system includes a single layer of photosensitive material in which a plurality of volume phase holograms is superimposed. A number of parameters and an orientation for the volume phase holograms are calculated such that the holographic resonant system produces a desired behavior. The desired behavior is determined by selecting an angle of incidence, &thgr;
in
, from a single direction and a wavelength, &lgr;
playback
, such that only light beams from angle &thgr;
in
having a wavelength &lgr;
playback
illuminate the holographic resonant system. In addition, an angle of reflection &thgr;
out
is selected such that light beams also having the wavelength &lgr;
playback
are reflected from the holographic resonant system at angle &thgr;
out
. The parameters and orientation of the volume phase holograms need not be identical.
A first volume phase hologram selects incoming light from an upper surface of the holographic resonant system having angle of incidence &thgr;
in
and wavelength &lgr;
playback
and then internally diffracts it to a second volume phase hologram. The second volume phase hologram accepts the diffracted light from the first hologram and internally diffracts it a second time to form an image that is reflected from the upper surface of the holographic resonant system at angle &thgr;
out
having wavelength &lgr;
playback
.
In accordance with other aspects of the invention, the light beam diffracted from the first volume phase hologram and the internal light beam incident upon the second volume phase hologram, preferably, lie in both the XY and XZ planes.
In accordance with still other aspects of this invention, instead of the light beam diffracted from the first volume phase hologram and the internal light beam incident upon the second volume phase hologram lying in the both the XY and XZ planes, they lie in only the XZ plane.
In accordance with further other aspects of the invention, the internal light beam diffracted from the first volume phase hologram and the internal light beam incident upon the second volume phase hologram are, preferably, collinear and parallel to the upper surface of the holographic resonant system.
In accordance with further aspects of this invention, instead of the internal light beam diffracted from the first volume phase hologram and the internal light beam incident upon the second volume phase hologram being collinear, they are substantially collinear. Further, instead of the internal light beam diffracted from the first volume phase hologram and the internal light beam incident upon the second volume phase hologram being parallel to the upper surface of the holographic resonant system, they are not parallel to the upper surface of the holographic resonant system.
In accordance with yet further aspects of this invention, instead of including a single layer of photosensitive material having superimposed volume phase holograms, the holographic resonant system includes a plurality of layers of photosensitive material, each layer containing a volume phase hologram, where a first volume phase hologram in a first layer diffracts the incident light into a second hologram in a second layer which then diffracts the light to produce an image reflected from the upper surface of the holographic resonant system.
In accordance with yet other further aspects of this invention, instead of the first volume phase hologram diffracting the incident light directly to the second volume phase hologram, the first volume phase hologram diffracts the incident light beam to either an upper or lower surface of the holographic resonant system, where the diffracted light beam is then totally internally reflected from either the upper or lower surface. The second volume phase hologram then accepts the reflected light beam and further diffracts the light beam to produce an image reflected from the upper surface of the holographic resonant system.
In accordance with yet still other further aspects of this invention, the holographic
Christensen O'Connor Johnson & Kindness PLLC
Curtis Craig
Spyrou Cassandra
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