Optical: systems and elements – Holographic system or element – For producing or reconstructing images from multiple holograms
Reexamination Certificate
2001-12-10
2003-07-22
Juba, John (Department: 2372)
Optical: systems and elements
Holographic system or element
For producing or reconstructing images from multiple holograms
C359S030000, C359S031000, C369S094000
Reexamination Certificate
active
06597478
ABSTRACT:
Priority is claimed to Patent Application Numbers 2000-86369, filed in the Republic of Korea on Dec. 29, 2000, herein incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase-conjugate holographic data storage device using a multifocal lens, and more particularly, to a phase-conjugate holographic data storage device capable of enhancing data recording and reproducing characteristics and data recording density by using a multifocal lens.
2. Description of the Related Art
Holography is a technique for reproducing the original form of optical signals, that is, a technique for recording a certain pattern resulting from an interference phenomenon between an object beam reflected by an object and a reference beam traveling straight at a different angle by taking advantage of the characteristics of a laser beam and then reproducing a three dimensional image of the object using the recorded pattern.
In holography, a laser beam having a uniform mono-waveform is divided into two beams using a beam splitter such as a semitransparent mirror. One of the two beams is flashed on an object so as to generate an object beam and then, the object beam is caused to interfere with the other beam, thereby creating an interference pattern. After that, the interference pattern is recorded in a storage medium. In reproducing a three dimensional image of the object using the interference pattern, a reference beam is transmitted through the storage medium.
Specifically, a holographic data storage device is used for recording data by transforming optical signals differently modulated depending on types of data into an object beam and reproducing the data. The object beam interferes with a reference beam so as to make an interference pattern. The interference pattern is recorded in a solid formed of material which reacts according to the intensity of the interference pattern (a storage medium) and the interference pattern recorded in the storage medium is called a hologram. If the reference beam is applied to such a hologram, the original form of the object beam used in the recording is reconstructed. Here, the hologram data recorded in the storage medium can be read by using only the reference beam used in recording because a reference beam having a different wavelength or phase is not diffracted by the hologram recorded in the storage medium but just passes through the hologram. Such a holographic technique has been applied to advertising, marketing, industry, exhibitions, packaging or decoration, and new technical applications are being developed.
A typical phase-conjugate holographic data storage device will be described with reference to
FIGS. 1A and 1B
.
FIG. 1A
is a diagram illustrating the data recording principle of a typical phase-conjugate holographic data storage device and
FIG. 1B
is a diagram illustrating the data reproducing principle of a typical phase-conjugate holographic data storage device.
Referring to
FIG. 1A
, a focusing lens
11
is placed in the path of an object beam
15
. A data recording medium
13
is placed at a focal spot into which the object beam
15
converges. A spatial light modulator (SLM)
12
is placed between the focusing lens
11
and the data recording medium
13
on the incidence path of the object beam
15
and is closer to the focusing lens
11
. The SLM
12
modulates an optical signal spatially depending on the type of data, and the focusing lens
11
reduces the space band width of the modulated optical signal.
The data storage principle of the phase-conjugate holographic data storage device is as follows. As shown in
FIG. 1A
, in recording data, the object beam
15
passes through the focusing lens
11
and then is modulated by the SLM
12
. The modulated object beam
15
is connected to the storage recording medium
13
and focused by the focusing lens
11
and thus, its space band width is reduced. At this time, a reference beam
16
a
interferes with the object beam
15
and the interference pattern is recorded in the data recording medium
13
. As shown in
FIG. 1B
, in reading data, a reference beam
16
b
is applied to the data recording medium
13
. As a result, the interference pattern recorded in the data recording medium
13
is reconstructed, output and then sensed by an image sensor
14
. In such a mechanism of recording and reproducing data, if a monofocal lens is used in focusing an object beam including data, as is the case in the prior art, light intensity, that is, a direct current (DC) term increases significantly in the middle of the Fourier plane at which the object beam is focused. Thus, the characteristics of recording and reproducing data are deteriorated.
Methods of solving this problem which have been used in the prior art, will be described with reference to
FIGS. 2 and 3
. Referring to
FIG. 2
, a data recording medium
23
is arranged a predetermined distance apart from the focal plane of a focusing lens
21
. An object beam
24
is spatially modulated by a SLM
22
and is focused by the focusing lens
21
. After that, the object beam
24
and a reference beam
25
a
interfere with each other, thereby creating an interference pattern. The interference pattern is recorded in the data recording medium
23
. In this case shown in
FIG. 2
, the data recording medium
23
is arranged at a distance of several millimeters to several centimeters from the focal spot of the object beam
24
induced by the focusing lens
21
. As a result, the focal plane on which the object beam
24
passing through the focusing lens
21
and the SLM
22
is focused is not included in an area in which the interference phenomenon occurs and thus, the size of a signal beam in recording data increases, so that the light intensity in the middle of the Fourier plane, that is, the DC term cannot affect the characteristics of recording and reproducing data.
In the case of reproducing data recorded in the data recording medium
23
, the reference beam
25
a
is applied to the data recording medium
23
. As a result, a phase-conjugate signal beam is reproduced and then is detected by an image sensor (not shown).
FIG. 3
is a diagram of a conventional phase-conjugate holographic data storage device in which a phase mask
34
is additionally installed between a SLM
32
and a data recording medium
33
. Referring to
FIG. 3
, the phase mask
34
is placed on an optical path along which an object beam
35
is spatially modulated by the SLM
32
and is focused at a focal plane. As a result, in the case of recording data, the object beam
35
sequentially passing through the focusing lens
31
, the SLM
32
and the phase mask
34
interferes with a reference beam
36
a
, thereby generating an interference pattern. The interference pattern is recorded in the data recording medium
33
. If the object beam passes through the phase mask
34
, the DC term occurring in the middle of the Fourier plane is weakened. In the case of reproducing data, the phase mask
34
is not necessary.
However, in the case of the holographic data storage device of
FIG. 2
, focusing of the object beam
24
and the interference phenomenon induced by the object beam
24
and the reference beam
25
b
occur at different locations. Accordingly, the holographic data storage device has a problem in that the size of a spot to be recorded becomes greater than the size of the spot on the Fourier plane and thus, data recording density becomes smaller than that of the holographic data storage devices of
FIGS. 1A and 1B
. In the case of the phase-conjugate holographic data storage device shown in
FIG. 3
, it is required to install the phase mask
34
specially designed between the SLM
32
and the data recording medium
33
and thus, the manufacturing cost of the data storage device increases. In addition, the phase-conjugate holographic data storage device has another problem in that the phase mask
34
should be aligned very precisely.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to
Kim Ji-deog
Lee Hong-seok
Lee Suk-han
Boutsikaris Leo
Burns Doane Swecker & Mathis L.L.P.
Juba John
Samsung Electronics Co,. Ltd.
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