1. Field of the Invention
The present invention relates to a hologram memory device.
2. Discussion of the Related Art
Though discs, tapes, and the like are used widely as information storing medium currently, as amount of the information becomes greater, a requirement for a new storing medium becomes greater due to the slow access time and limited capacity of current storing medium. In order to meet this requirement, different information storing medium are under development, particularly the hologram memory device attracts attention because it is favorable in view of large capacity and fast access time.
The operation principle of a conventional hologram memory device will be explained referring to FIGS. 1a and 1b.
Upon incident of an objective light 1 and a first reference light 2 to the hologram memory cell 3 in recording, a first interference pattern 4 caused by the interference of the two lights 1 and 2 is recorded in the hologram memory cell 3. When only the first reference light 2 is directed to the hologram memory cell 3 without direction of the first objective light 1 in reproduction, the first objective light 1 is reproduced according to the hologram theory, to display a first reproduction light 5. In this instance, since the first reproduction light 5 carries all the information of the first objective light 1 recorded in the hologram memory cell 3, the hologram memory cell 3 can be used as the information recording medium.
The hologram memory device is featured in that it can overlap more than two informations in a single cell in storage of the informations. That is, when a second reference light 12 is directed to the hologram memory cell 3 having a particular information recorded therein according to the first objective light 1 and the first reference light 2, at an angle different from the first reference light 2, and a second objective light 11 having an information different from the first objective light 1 is directed thereto at an angle identical to the first objective light 1 in recording, the second reference light 12 and the second objective light 11 makes interference, a second interference pattern 14 is recorded in the hologram memory cell 3 overlapped with the first interference pattern 4 recorded therein. In reproduction thereafter, if the second reference light 12 is only directed to the hologram memory cell 3, a second reproduction light 15 having the same information as the second objective light 11 is displayed. Thus, employing such a principle, different informations can be stored in the same cell in overlap.
The conventional hologram device having the aforementioned operation principle will be explained.
FIG. 2 illustrates a first exemplary conventional hologram memory device, provided with a laser beam source 20, a beam expander 21 for expanding a size of the laser beam emitted from the laser beam source 20, a beam splitter 24 for splitting the beam passed through the beam expander into an objective beam 22 and a reference beam 23, a beam scanner 25 for deflecting and directing an incident beam to a desired direction, a beam cut-off 26 for allowing an incident beam to pass or cutting-off the incident beam, a beam modulator 27 for adjusting a spatial beam distribution of the incident beam, a focusing lens 28 for focusing the incident light onto a point, a hologram memory cell 29 for recording and reproducing optical information, and an optical detector 30 for reading an optical information reproduced from the hologram memory cell and converting into an electrical signal.
The steps of process of the aforementioned hologram memory device for recording the information will be explained.
First, for the first recording, when the beam emitted from the laser beam source 20 is incident to the beam splitter 24 through the beam expander 21, the beam splitter 24 splits the beam into an objective beam 22 and a reference beam. Then, the beam cut-off 26 disposed to allow transmission of the objective beam 22 allows to pass the objective beam 22 to provide the beam to the beam modulator 27. In this instance, the objective beam 22 is modulated of its optical beam distribution at passing the beam modulator 27 into a first information beam 22a having a beam distribution corresponding to a information to be recorded, which is then provided to the hologram memory cell 29. On the other hand, the reference beam 23 is reflected or refracted at a certain angle by the beam scanner 25, to provide a first reference beam 23a. The first reference beam 23a is directed to the focusing lens 28 and refracted to incident to the hologram memory cell 29 at a specific angle q1. Consequently, the first information beam 22a and the first reference beam 23a form an interference pattern in the hologram memory cell 29, to record the interference pattern therein. In the steps of process for conducting a second recording of information in the hologram memory cell 29 having the first recording of information made therein overlapped with the first recording, when the objective beam 22 is provided to the beam modulator 27 through the beam cut-off 26, the objective beam 22 is modulated of its beam distribution into a second information beam 22b having a beam distribution corresponding to the information to be recorded, which is then provided to the hologram memory cell 29. In this instance, the second information beam, having an information different from the first information beam 22a used in the first recording, is adapted to be directed to the hologram memory cell 29 at the same position and angle as the first information beam 22a. On the other hand, the reference beam 23 is directed to the beam scanner 25. The beam scanner 25, adjusted to directed a beam in a different direction for overlapped recording of an information, produces a second reference beam 23b travelling in a direction different from the first reference beam 23a. The second reference beam 23b is focused by the focusing lens 28 onto the hologram memory cell 29. In this instance, though the position of the second reference beam 23b incident to the hologram memory cell 29 is the same with the first reference beam 23a, the incident angle q2 of the second reference beam 23b is different from the incident angle q1 of the first reference beam 23a. Accordingly, the second reference beam 23b and the second information beam 22b make an interference pattern, to record the interference pattern in the hologram memory cell 29. Further, alikely, by changing the incident angle of the reference beam 23 as well as the beam distribution of the objective beam 22 incident to the hologram memory cell 29, another informations can be recorded overlapped one over the other continuously.
The steps of process for reproducing information from the hologram memory device will be explained.
The laser beam emitted from the laser beam source 20 is split into the objective beam 22 and the reference beam 23 during passing through the beam expander 21 and the beam splitter 24. The reference beam 23 is provided to the beam scanner 25, and, as the objective beam 22 will not be used in reproduction, the objective beam 22 is cut-off by putting the beam cut-off 26 into a cut-off state. Then, the beam scanner 25 is adjusted so that the reference beam 23 incident to the beam scanner 25 is reflected or refracted at a specific angle, which is then focused onto the hologram memory cell 29 by the focusing lens 28. If a reproduction of the information recorded at the first time is intended, the reference beam 23 is directed to the hologram memory cell 29 at an angle q1 the same with the first reference beam 23a. The first reference beam 23a produces a first reproduction beam 31a according to the hologram principle, which is then provided to the optical detector 30. In this instance, the first reproduction beam 31a is reproduced from the hologram memory cell 29 carrying the optical information of the first information beam 22a used in the first recording as it was. The optical detector 30 than generates an electrical signal corresponding to the optical information. In case reproduction of the information recorded for the second time is intended, when the beam scanner 25 is adjusted, to generate the second reference beam 23b to incident to the hologram memory cell 29 at an angle q2, a second reproduction beam 31b is produced. In case reproduction of another information is intended, alike the aforementioned fashion, when the beam scanner 25 is adjusted, to generate the second reference beam 23b to incident to the hologram memory cell 29, informations recorded in the hologram memory cell 29 at different angles can be reproduced. However, since the hologram memory device shown in FIG. 2 has only hologram memory cell 29 with a small capacity of information recordal, it has a limitation in recording a great amount of information to a certain extent. Accordingly, a large capacity hologram memory becomes necessary instead of the single memory cell.
FIG. 3 illustrates another exemplary conventional hologram memory device in which a hologram memory array is used in place of the single hologram memory cell.
Referring to FIG. 3, the hologram memory array 32 is fitted to a precision moving device for mechanical moving of the hologram memory array 32 in case of recording or reproduction of information in/from each cell. That is, when it is intended to record information in a cell of the hologram memory array 32, the precision moving device 33 is moved to bring the cell assigned to recorded the information to a designated position, and the information is recorded in the cell in the same fashion explained, before. And, when it is intended to reproduce the information recorded in a certain cell, the precision moving device 33 is moved such that the cell is moved to a position at which the information is recorded therein, and the information in the cell is reproduced in the same fashion explained before.
However, the aforementioned conventional hologram memory devices have the following problems.
First, for directing reference beams each having different direction of travel incident from the beam scanner to the same hologram memory cell, an expensive focusing lens is required.
Second, when the hologram memory array is applied for providing a large capacity of recording, the precision moving device, which is very expensive, should be provided for moving a desired hologram memory cell to a specific position, resulting in a high cost of the entire hologram memory device.
Third, the generally required several seconds of movement of the precision moving device for a precise movement of the hologram memory array is not suitable for a hologram memory device which should satisfy both fast reproduction and large capacity recording of information.