Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2001-11-26
2003-10-28
Juba, John (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S001000, C359S008000, C359S022000, C359S025000, C430S001000, C428S064400, C428S064900
Reexamination Certificate
active
06639700
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application(s) No(s). P2000-360262 filed Nov. 27, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hologram recording medium for recording an information signal in the form of optical phase information. The present invention also relates to a hologram recording/reconstructing apparatus and method for recording and/or reconstructing an information signal in the form of optical phase information.
2. Description of the Related Art
A conventional hologram recording medium is formed of, e.g., a film or a disk. A reference light and an object light, which are each given as a coherent laser beam, are irradiated to a hologram recording medium, whereupon the hologram recording medium records an information signal, which is to be recorded, as an interference pattern of the reference light and the object light, i.e., as phase information of the reference light and the object light. The object light contains an information signal in the form of optical phase information, and is provided as a laser beam reflected from an object or a laser beam modulated by a spatial modulator. Also, a hologram is reconstructed from a hologram recording medium when it is irradiated by the reference light or a reconstruction light that is phase conjugate to the reference light.
There are many reports related to hologram recording/reconstructing apparatuses using disk-shaped hologram recording mediums. For example, U.S. Pat. No. 5,671,073 proposes a method of multiplex-recording holograms with rotation of a disk-shaped hologram recording medium by using a spherical wave as a reference light (hereinafter referred to as “shift multiplex recording”).
Recently, one example of hologram recording mediums obtained by machining a crystal into a disk-like shape has also been reported (Tao Shiquan et al. “Multi-track storage of 10,000 holograms in a disk-type photorefractive crystal,” SPIE Vol. 3864, pp. 270 (1999)).
Another conventional method of recording holograms in a hologram recording medium is one for multiplex-recording holograms based on changes of the incident angle of each of a reference light and/or an object light with respect to the hologram recording medium (hereinafter referred to as “angle multiplex recording”).
In the angle multiplex recording, the incident angle of each of a reference light and/or an object light upon a hologram recording medium must be changed using a beam deflector or the like.
To perform the angle multiplex recording on a hologram recording medium, various methods using a beam deflector are known for changing the incident angles of a reference light and/or an object light.
Methods for changing the incident angle of each of a reference light and/or an object light by a beam deflector include one of mechanically controlling the incident angle by using a galvanometric mirror or the like, and another one of electrically controlling the incident angle by using an acousto-optical deflector (hereinafter referred to as an “AOD”), an electro-optical deflector (hereinafter referred to as an “EOD”), or the like.
Assuming here that the aperture width of a beam deflector is D, the aperture shape factor is a (1.22 for a circular shape and 1 for a rectangular shape), the wavelength of a reference light and/or an object light irradiated to a hologram recording medium is &lgr;, and the angle amplitude of the reference light and/or the object light is &phgr;, the number N of resolving points of a beam deflector is given by the following formula 1:
N
=
φ
D
a
λ
(
1
)
From the formula 1, it is understood that the number N of resolving points increases as the aperture width D has a larger value and the angle amplitude &phgr; of the reference light and/or the object light has a larger value. Because the product of incident height and incident angle of the reference light and/or the object light at each plane is constant based on the Lagrange-Helmholtz relationship, the number N of resolving points remains the same even when beam shaping optical systems are disposed before and after the beam deflector.
Still another method for changing the incident angle of each of a reference light and/or an object light irradiated to a hologram recording medium is one of deflecting a beam with a wedge-shaped prism. In other words, it is known that the direction in which a reference light and/or an object light propagates can be changed in small amount and can be adjusted in small angle by rotating a wedge-shaped prism. Accordingly, the reference light and/or the object light can be deflected in any desired direction. Then, by setting an apical angle of the wedge-shaped prism to a smaller value, a deflection angle due to the prism rotation can be reduced. This method is therefore effective in adjusting the direction, in which the reference light and/or the object light propagates, in small angle.
New methods for hologram multiplex recording have also been proposed recently. One example of those methods is the so-called peristrophic multiplex recording described below in detail (Kevin Curtis et al. “Method for holographic storage using peristrophic multiplexing,” 19, Opt. Lett. 993 (1994) and A. Pu et al. “High density holographic storage in thin film,” SPIE Vol. 2338, Optical Data Storage (1994), 69).
In the peristrophic multiplex recording, as shown in
FIG. 33
, multiplex recording of holograms is realized by rotating a reference light
41
in the direction of an arrow R
2
along a conical surface with its apex defined by a part of a disk-shaped hologram recording medium
40
.
Also, in the peristrophic multiplex recording, the degree of multiplexity can be further increased by changing the incident angle of the reference light
41
upon the hologram recording medium
40
in the direction of an arrow R
3
so that the above-mentioned angle multiplex recording of holograms is realized in the direction of radius vector in a combined manner.
Assuming here that the Bragg angle is d&thgr;, the wavelength is &lgr;, the thickness of the hologram recording medium
40
is t, the incident angle of the reference light
41
upon the hologram recording medium
40
is &thgr;
R
, and the incident angle of an object light
42
upon the hologram recording medium
40
is &thgr;
S
, the Bragg angle is d&thgr; in the peristrophic multiplex recording is given by the following formula 2:
d
θ
=
2
λ
t
cos
θ
s
sin
θ
R
(
sin
θ
R
+
sin
θ
s
)
(
2
)
Additionally, in usual angle multiplex recording of holograms, the Bragg angle d&thgr;′ is given by the following equation 3 wherein the refractive index of the hologram recording medium
40
is n:
d
θ
′
=
λ
·
cos
θ
s
n
·
t
·
sin
(
θ
R
+
θ
s
)
(
3
)
As conventional methods for reconstructing holograms from a hologram recording medium, there is known a phase conjugate reconstructing method which employs, as a reconstruction light, a phase conjugate light having the same phase as a reference light but propagating in an opposite direction to the reference light. A method for generating a phase conjugate light to perform the phase conjugate reconstruction is practiced, for example, by splitting an irradiated reference light with a beam splitter and creating an optical path separate from the reference light irradiated for recording holograms.
When recording and/or reconstructing holograms using the conventional hologram recording mediums described above, however, the number N of resolving points obtained with the methods using an AOD and an EOD as beam deflectors are about 1000 or several tens, respectively. Those methods have limitations in further increasing the degree of multip
Boutsikaris Leo
Juba John
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
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