Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-08-02
2003-03-18
Hindi, Nabil (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06535472
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a holographic memory and to an optical information recording/reproducing apparatus utilizing the holographic memory, and more particularly to a hologram recording apparatus and a method therefor for recording a signal which are free from deterioration when it is reproduced later.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system which applies the principle of holography. In the following, a holographic memory system will be generally described with reference to FIG.
1
.
In
FIG. 1
, an encoder
25
converts digital data to be recorded in a holographic memory
1
to a light/dark dot pattern image on a plane, and rearranges the dot pattern image into a data array of, for example, 480 bits in the vertical direction and 640 bits in the horizontal direction (480×640) to generate sequence data in unit pages. This data is sent to a spatial light modulator (SLM)
15
, such as a transmission-type TFT liquid crystal display (LCD) panel, by way of example.
The spatial light modulator
15
, which has a modulation processing unit corresponding to the unit page composed of 480 bits in the vertical direction and 640 bits in the horizontal direction (480×640), optically modulates a light beam irradiated thereto to spatial light on/off signal in accordance with the unit page sequence data from the encoder
25
, and sends the modulated signal beam or signal light to a lens
16
. More specifically, the spatial light modulator
15
passes therethrough a signal beam corresponding to a logical value “1” in the unit page sequence data which is an electrical signal, and blocks the signal beam corresponding to a logical value “0” in the unit page sequence data to achieve photoelectric conversion in accordance with respective bit contents of the unit page data, thereby generating a signal beam which is modulated as signal light of the unit page sequence.
The signal light is incident on the holographic memory
1
through a lens
16
. In addition to the signal light, the holographic memory
1
is also irradiated with reference light at an incident angle &bgr; from a predetermined base line orthogonal to the optical axis of the beam of the signal light.
The signal light and the reference light interfere with each other in the holographic memory
1
to produce interference fringes which are stored in the holographic memory
1
as a refractive index grating or hologram to record the data. Also, the holographic memory
1
provides for three-dimensional data recording by entering the reference light thereto with a different incident angle &bgr; to record a plurality of two-dimensional planar data in an angle multiplex scheme.
For reproducing recorded data from the holographic memory
1
, the reference light only is directed into the holographic memory
1
at the same incident angle &bgr; as recording, toward the center of a region in which the signal light beam and the reference light beam intersect. In other words, unlike recording, the signal light is not directed. In this way, diffraction light from the interference fringes recorded in the holographic memory
1
is transmitted to a CCD (Charge Coupled Device)
20
, which functions as a photodetector, through a lens
19
. The CCD
20
converts light and dark of the incident light to the intensity of an electrical signal to produce an analog electrical signal having a level in accordance with the luminance of the incident light, which is output to a decoder
26
. The decoder
26
compares this analog signal with a predetermined amplitude value (slice level) to reproduce corresponding data “1” and “0”.
Since the holographic memory records data in two-dimensional planar data sequences as described above, the angle multiplex recording can be accomplished by changing the incident angle &bgr; of the reference light. Stated another way, a plurality of two-dimensional planes as recording units can be defined in the holographic memory by changing the incident angle &bgr; of the reference light, with the result that three-dimensional recording is enabled.
Conventionally, for a rewritable holographic memory
1
utilizing the photo-refractive effect, Fe-added lithium niobate (LiNbO
3
, or abbreviated as “LN”) single crystals are used as recording materials, while a wavelength of 532 nm, which is a second harmonic of an Nd:YAG laser, is used as recording light. In this conventional recording scheme (called the “conventional single-color recording scheme”), corresponding to interference fringes formed from signal light and reference light, which are recording light, electrons are excited from an Fe
2+
state to a conduction band in light regions of the interference fringes, undergo a photo-refractive process, and are finally trapped to an Fe
3+
state to complete the storage.
However, the conventional single-color recording scheme implies a problem that reproduction light gradually erases the recorded hologram when a signal is read from the hologram (which is so called reproduction deterioration). The medium has a sensitivity to light of one wavelength that is used at the time of recording and reproduction. In the single color hologram, recorded information is electrons trapped at the trap level (storage center) which is produced by Fe. That is, every time reproduction is performed, electrons are gradually excited to the conduction band from the trap level, thereby erasing the stored information. According to the conventional holographic memory, when signals are read from a hologram recorded there, reproduction light gradually erases the hologram, so that the reproduction deterioration occurs.
On the other hand, a two-color hologram scheme is known as a recording scheme which suffers from less reproduction deterioration.
The two-color hologram recording is characterized in that a hologram is recorded by simultaneously irradiating other light called “gate light” (at wavelength &lgr;
2
), in addition to recording light (reference light and signal light at wavelength &lgr;
1
) for forming the hologram. The gate light acts to develop a recording sensitivity at the wavelength (&lgr;
1
) of the recording light only during the irradiation of the gate light. Such a characteristic is based on carriers temporarily formed by the irradiated gate light at a relatively shallow energy state called an “intermediate excitation state” within a portion of the crystal irradiated with the gate light. The carriers at the intermediate excitation state are excited by the recording light (a spatial light/dark pattern corresponding to interference fringes formed by the reference light and the signal light), and finally accumulated in the form of a variable density distribution of the carriers corresponding to the interference fringes at a deep trap state. The latter process of the two-color hologram scheme, which is called the “photo-refractive effect,” is in principle the same process as the single-color hologram. For example, with the two-color hologram recording scheme using crystals which are processed to be reduced to LiNbO
3
with no additive component or with Fe added thereto, and have a composition close to the stoichiometry (abbreviated as “SLN”) (H. Guenther, R. M. Macfarlane, Y. Furukawa, K. Kitamura, R. Neurgaonkar; “Two-color holography in reduced near-stoichiometric lithium niobate”, Appl. Opt. Vol. 37, pp. 7611-7623 (1998)), the lifetime of carriers at the intermediate excitation state (metastable state) can be extended from microseconds to seconds, thereby making it possible to use a continuous oscillating laser having relatively small power for recording.
While the two-color hologram recording scheme requires a reduction of a recording material to increase the PR center density (bipolaron-polaron mechanism), this results in a lower density of Fe
3+
(trivalent) to degrade the transparency of the material itself. Also, since the light sensitivity is still insufficient for a practical level,
Furukawa Yasunori
Hatano Hideki
Kitamura Kenji
Lee Myeongkyu
Takekawa Shunji
Director General of National Institute for Research in Inorganic
Hindi Nabil
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