Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2001-10-31
2004-03-16
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S011000, C359S033000
Reexamination Certificate
active
06707585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a holographic recording and reproducing apparatus and a holographic recording and reproducing method.
2. Description of the Related Art
A holographic memory system is known as a digital information recording system which applies the principle of holography utilizing a recording medium formed of a photorefractive material, i.e. so-called a holographic memory. In this information recording system, the recording information signals are recorded as changes in refractive index on the recording medium of a photorefractive crystal such as lithium niobate single crystals.
There is a conventional holographic recording and reproducing method utilizing the Fourier transform.
FIG. 1
shows a conventional 4f-based holographic recording and reproducing apparatus. A laser light beam
12
emitted from a laser light source
11
is split into a signal light beam
12
a
and a reference light beam
12
b
in a beam splitter
13
. The signal light beam
12
a
is expanded in its diameter by a beam expander
14
as collimated light, and then irradiated to a spatial light modulator (hereinafter abbreviated as “SLM”)
15
including a dot matrix panel such as a transmission-type TFT liquid crystal display (LCD) panel to which image data to be recorded are provided which are converted by an encoder as electric signals. Thus, the panel forms a bright and dark dot pattern on its plane corresponding to the image data. The signal light beam
12
a
is optically converted by the dot matrix panel
15
to include data signal components. The signal light beam
12
a
including dot pattern signal components passes through a Fourier transforming lens
16
which is positioned at a focal distance f apart from the SLM
15
. The Fourier transforming lens
16
performs Fourier transformation and then the signal light beam
12
a
including dot pattern signal components is converged into a recording medium
10
. On the other hand, the reference light beam
12
b
split in the beam splitter
13
is guided to the recording medium
10
by a fixed mirror
17
and a rotary mirror
17
a
, and intersects an optical path of the signal light beam
12
a
within the recording medium
10
to form a light interference pattern. The recording medium
10
made of a photorefractive crystal records the spatial intensity modulation represented by the light intensity of the light interference pattern as changes in refractive index corresponding to the data.
In the foregoing manner, the diffraction light from the image data illuminated by a coherent collimated light is focused through the Fourier transforming lens
16
and changed into a distribution on the focal plane, or Fourier plane. The distribution as a result of Fourier transformation is interfered with the coherent reference light to record an interference fringe thereof to the recording medium placed in the vicinity of the focal point. Ending the record of the first page, the rotary mirror
17
a
is rotated a predetermined amount and parallel moved in position a predetermined amount so that the incident angle of the recording reference light beam
12
b
on the recording medium
10
is changed to record the second page by the same procedure. In this way, the angle-multiplexed recording is carried out with sequential recording as the above.
In reproducing information, on the other hand, inverse Fourier transformation is carried out to reproduce a dot-pattern image. As shown in
FIG. 1
the optical path of the signal light beam
12
a
is cut off, for example, by the SLM
15
to illuminate only the recording reference light beam
12
b
to the recording medium
10
. In order to make incident the recording reference light beam
12
b
at the same angle as the recording reference light of upon recording the page to be reproduced, the rotary mirror
17
a
is changed and controlled in position and angle by the combination of mirror rotation and parallel movement. Reproductive light of the recorded interference pattern appears at an opposite side of the recording medium
10
to the side illuminated by the signal light beam
12
a
. If the reproduced light is guided to and inverse-Fourier-transformed by an inverse Fourier transforming lens
16
a
, the dot-pattern signal can be reproduced. Furthermore, if the inverse Fourier transforming lens
16
a
images the dot-pattern signal on an imaging device or photodetector
20
using a CCD (Charge Coupled Device) or CMOS sensor arranged in the focal point, and reconverted into an electric digital data signal and then sent to a decoder
25
, the original data is reproduced.
In this manner, the holographic memory system achieves a great capacity recording of information by multiplexedly recording a great deal of two dimensional data to a certain volume of the recording medium.
The holographic memory system using a photorefractive phenomenon for the recording principle. There is no threshold of recording and deterioration in date within the holographic memory. This matter implies that reference light irradiated to the medium for reproduction gradually erases the recorded hologram which is so called deterioration of reproduction. In the multiplexed recording of information carried out sequentially recording within a certain limited volume of the medium, the first recorded portion suffers from the deterioration of the following recorded portion due to the photorefractive phenomenon having no threshold. Thus the surplus recording is performed in counterbalance to such a deterioration so as to compensate the erased hologram portion. For this, the renewal of recording for so-called scheduling time is required as a so-called refresh operation. The refresh operation should be performed in one lump sum to the entire medium. Thus it is impossible that only one portion of the great quantity of multiplexedly recorded data is partly rewritten by a user as he likes without influencing the other portion in the medium.
In this way, a conventional holographic memory system has any nonvolatile storage and it is difficult to provide a rewritable storage. Further it is hard to partially rewrite an extremely minute portion of the great deal of data stored at a limited one volume region in the recording material because the rewriting of such a minute portion adversely influences the other portion storing the other data.
OBJECT AND SUMMARY OF THE INVENTION
Thus the present invention has been made in consideration of the above conditions, and an object of the present invention is to provide an apparatus of holographic recording and reproducing and a method therefor in which the user is allowed to rewrite the recorded data in the holographic memory.
According to the present invention, there is provided a holographic recording and reproducing apparatus for recording data on a recording medium and reproducing data from the recording medium, the recording medium being made of a photorefractive material, said apparatus comprising:
a support portion for detachably supporting the recording medium;
a recording-reference-light-beam-supplying-portion for supplying a coherent recording reference light beam having a first wavelength and propagating along a recording optical axis to a major surface of said recording medium;
a signal-light-beam-supplying-portion including a converging lens for converging a coherent signal light beam having the first wavelength which is modulated in accordance with image data, in an optical path into the recording medium such that said signal light beam intersects with the recording reference light beam to produce an optical interference pattern of refractive index with said reference and signal light beams within said recording medium;
a reproducing-reference-light-beam-supplying-portion for supplying into the recording medium a coherent reproducing reference light beam having the first wavelength and propagating in an opposite direction along said recording optical axis of the recording reference light beam to generate a phase conjugate wave from a refractive-index grating o
Lavarias Arnel C.
Nguyen Thong
Pioneer Corporation
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