OPTICAL INFORMATION RECORDING MEDIUM AND METHOD OF READING THE SAME INFORMATION

Abstract: The optical information recording medium according to the present invention is capable of recording information at an extremely high density and reading the information at a high speed by providing at least one cell having a diffraction grating on a planar substrate; dividing the cell into n ( n is 2 or a greater integer) regions so that each of the divided cells displays one set of binary data; providing light receiving elements the number of which corresponds to that n by which the cell is divided, for use in reading the information; applying one beam of light to the cell; and receiving by the light receiving elements the refracted light from the divided cells, whereby the data are reproduced.

Description: Technical Field The present invention relates to an optical information recording medium from which information is optically reproduced and an information reading method for the same and, more particularly, to an optical information recording medium which uses diffraction gratings as information recording elements so that information can be recorded at a very high density and from which a plurality of pieces of information are read at once simultaneously so that information can be read at a higher speed, and an information reading method for the same. Generally, optical recording has advantages in that, e.g., a recording medium and a head do not contact each other when compared to magnetic recording, and high-density recording can be performed. Various types of media are known as optical recording media for optical recording, e.g., a read-only medium, a write-once medium, and an erasable programmable medium. Optical recording media of these types have already been in practical use in various fields in the forms of, e.g., an optical recording disk (e.g., a CD, a CD-ROM, a CD-I, a laser disk, write-once and programmable disks, and a magneto-optical disk) and an optical card (e.g., a ROM card, write-once and programmable cards, and a magneto-optical card). In a conventional read-only or write-once optical recording disk, a laser beam generated by a light source, e.g., a semiconductor laser, is focused to form a spot having a diameter of 1 to 2 µm on the disk surface, and the surface state of the corresponding portion on the disk surface is detected by utilizing light reflected by the disk surface. More specifically, when a pit is present, light is randomly reflected to reduce the intensity of the reflected light. When a pit is not present, light is reflected by a mirror surface. Data is read by utilizing a difference (intensity of light) between these two states. In this case, however, a decrease in spot diameter of the laser light beam is limited to as small as about 1 to 2 µm, and it is impossible to decrease the spot diameter to be extremely smaller than this. Accordingly, the information amount of the conventional optical recording disk is limited by the fact that one pit on the disk represents one piece of information and by the limited spot diameter of the laser light beam. As a result, information recording at a high density exceeding a certain limit cannot be performed. In US-A-5 032 033 a recording medium is disclosed comprising a cell which contains a plurality of diffraction gratings disposed on a planar substrate. The recording density and the plurality of pieces of information to be read simultaneously can be improved over the disclosure of US-A-5 032 003. It is the object of the present invention to provide an optical information recording medium which uses diffraction gratings as information recording elements so that information can be recorded at a very high density, from which a plurality of pieces of information are read at once simultaneously so that information can be read at a higher speed, and which can be produced in mass production with a good formability at a low cost, and an information reading method for the same. The object of the invention is solved by an optical information recording medium as being defined by claim 1 and by a method of reading the medium as defined by claim 6. In order to achieve the object of the present invention, in an optical information recording medium according to the present invention, which is formed by disposing at least one cell comprising small diffraction gratings on a surface of a planar substrate, a ratio of a line width to a grating pitch of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on first original data used for manufacturing a main body of the optical information recording medium, thereby recording first data. A ratio of a line width to a grating pitch of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on first original data used for manufacturing a main body of the optical information recording medium, thereby recording first data, and a grating pitch of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on second original data used for manufacturing the main body of the optical information recording medium, thereby recording second data. A ratio of a line width to a grating pitch of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on first original data used for manufacturing a main body of the optical information recording medium, thereby recording first data, and an azimuth angle of diffraction of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on third original data used for manufacturing the main body of the optical information recording medium, thereby recording third data. A ratio of a line width to a grating pitch of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on first original data used for manufacturing a main body of the optical information recording medium, thereby recording first data, a grating pitch of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on second original data used for manufacturing the main body of the optical information recording medium, thereby recording second data, and an azimuth angle of diffraction of the diffraction gratings constituting the diffraction grating cell is appropriately changed based on third original data used for manufacturing the main body of the optical information recording medium, thereby recording third data. An intensity of diffracted light component is coded based on the following equation as a ratio of the line width to the grating pitch of the diffraction gratings: η = ( 2 π ) 2 sin 2 ( 2π λ r cos ) sin( π d ℓ) 2 where η is the diffraction efficiency (a value of 0 to 1), r is the depth of the diffraction gratings, ℓ is the line width, d is the grating pitch,  is the angle of incidence of reproduced illumination light, and λ is the wavelength of the reproduction illumination light. Either a reflection type cell or a transmission type cell is used as the cell. According to an information reading method for an optical information recording medium of the second aspect of the present invention, at least one light-receiving element is disposed, one incident light beam is caused to be incident on the diffraction grating cell, and a diffracted light component emerging from the diffraction grating cell is received by the light-receiving element, so that the first data is reproduced as a change in intensity of the diffracted light component. A plurality of light-receiving elements are disposed, one incident light beam is caused to be incident on the diffraction grating cell, and diffracted light components emerging from the diffraction grating cell are received by the light-receiving elements, so that the first data is reproduced as a change in intensity of the diffracted light component and the second data is reproduced as a change in diffraction angle of the diffracted light component. A plurality of light-receiving elements are disposed, one incident light beam is caused to be incident on the diffraction grating cell, and diffracted light components emerging from the diffraction grating cell are received by the light-receiving elements, so that the first data is reproduced as a change in intensity of the diffracted light component and the third data is reproduced as a change in exit direction of the diffracted light component. A plurality of light-receiving elements are disposed, one incident light beam is caused to be incident on the diffraction grating cell, and diffracted light components emerging from the diffraction grating cell are received by the light-receiving elements, so that the first data is reproduced as a change in intensity of the diffracted light component, the second data is reproduced as a change in diffraction angle of the diffracted light component, and the third data is reproduced as a change in exit direction of the diffracted light component. Beam-like light is caused to be incident as the incident light. Brief Description of the Drawings FIG. 1 is a plan view showing an optical information recording medium as the embodiment according to the present invention; FIG. 2 includes plan views showing arrangements of one cell applied to the optical information recording medium according to the present invention; FIG. 3 is a graph showing an example of the relationship between the ratio of the line width of diffraction gratings constituting the cell of the optical information recording medium, to the grating pitch, and a diffraction efficiency; FIG. 4 is a sectional view showing the arrangement of rectangular diffraction gratings constituting the cell of the optical information recording medium; FIG. 5 is a plan view showing the arrangement of the optical information recording medium applied to a card-like base material; FIG. 6 is a schematic view showing the arrangement of an optical system for realizing an information reading method for the optical information recording medium; FIG. 7 is a schematic view showing the state of an information reading method for an optical information recording medium as another embodiment according to the present invention. Best Mode of Carrying Out the Invention The gist of the present invention resides in the following respects. Namely, when diffraction gratings are used as the information recording elements of an optical information recording medium, data is recorded by appropriately changing the ratio of the line width to the grating pitch of the diffraction gratings constituting the diffraction grating cell, so that the diffraction efficiency of the diffraction grating is controlled, that is, the intensity of the diffracted light component is changed during data reproduction, so that data can be read, thereby realizing an increase in density of data recording. Also, control of the diffraction efficiency is performed independently of recording information by using the grating pitch and the grating azimuth angle, and control of the diffraction efficiency and information recording are combined as required to record independent data, thereby realizing a further increase in density of data recording. FIG. 1 is a plan view showing the arrangement of an reflection type optical information recording medium according to the present invention. More specifically, as shown in FIG. 1, the optical information recording medium of this embodiment is constituted by disposing, on the surface of a planar substrate 31, a plurality of cells 32, 33, and 34 comprising small diffraction gratings through a reflecting layer (not shown). The cell 32 records first data by appropriately changing the ratio of the line width to the grating pitch of the diffraction gratings constituting the cell 32 based on the first original data used for manufacturing this optical information recording medium. The cell 33 records second data by appropriately changing the grating pitch of the diffraction gratings constituting the diffraction grating cell 33 based on the second original data used for manufacturing this optical information recording medium. The cell 34 records third data by appropriately changing the azimuth angle of the diffraction gratings constituting the cell 34 based on -the third original data used for manufacturing this optical information recording medium. In the cell 32, regarding the ratio of the line width to the grating pitch of the diffraction grating, the intensity of the diffracted light component is coded based on the following equation: η = ( 2 π ) 2 sin 2 ( 2π λ r cos ) sin( π d ℓ) 2 where η is the diffraction efficiency (a value of 0 to 1), r is the depth of the diffraction grating, ℓ is the line width, d is the grating pitch,  is the azimuth angle of incidence of reproduced illumination light, and λ is the wavelength of the reproduction illumination light. Note that this equation is valid only for a surface relief type rectangular diffraction grating having a small depth. FIG. 2 includes plan views showing arrangements of the diffraction gratings constituting the cell 32 of the above cells. Referring to FIG. 2, when the line width of the diffraction grating becomes 1/2 the grating pitch, i.e., when line width : grating pitch = 1 : 2, the diffraction efficiency becomes maximum, as shown in FIG. 3. When the difference between the line width and this value becomes large, the diffraction efficiency become lowered. FIG. 3 is obtained based on the above equation. Although recessed portions are expressed as lines in FIG. 2, projecting portions may be regarded as lines instead. FIG. 2 shows examples in which the diffraction gratings are formed by a three-dimensional pattern. However, the diffraction grating may be formed in accordance with any method, i.e., a method of changing the light transmittance, reflectance, or the phase (to form the diffraction grating with a three-dimensional pattern correspond to change the phase between the recessed portions and the projecting portions). An example is a method of forming the diffraction grating by changing the light transmittance or reflectance includes a method of expressing the diffraction grating by changing the density (to express portions corresponding to the recessed or projecting portions in FIG. 2 in black (light absorption or light shielding) and white (reflection or transmittance)). Another example of forming the diffraction grating by changing the phase includes a method of forming the grating with layers of two media having different refractive indices (to form portions corresponding to the recessed and projecting portions in FIG. 9 with different media). FIG. 4 is a sectional view showing the arrangement of a rectangular diffraction grating formed with a three-dimensional surface pattern. As shown in FIG. 4, when the optical information recording medium of this embodiment is formed with the three-dimensional pattern, the depthwise directions can be sufficiently expressed by binary expression. Accordingly, this optical information recording medium can be manufactured also by using a binary device, e.g., an electron beam lithography system, to have a good formability in accordance with simple duplicating process without requiring depth control and the like. FIG. 5 is a plan view showing an example of a case wherein the optical information recording medium of this embodiment is applied to a card-like base material. As shown in FIG. 5, since the thickness of the optical information recording medium of this embodiment can be greatly decreased, even when, e.g., this optical information recording medium is adhered to a card-like base material, no problem arises concerning the thickness. In the optical information recording medium of this embodiment having the above arrangement, the ratio of the line width to the grating pitch of the diffraction gratings constituting the cell 32 is approximately changed based on the first original data used for manufacturing the main body of the optical information recording medium, thereby recording the first data. Therefore, data reproduction can be performed by changing the intensity of the diffracted light component, so that information can be recorded at a very high density. Since the multivalue is expressed by the ratio of the line width to the grating pitch of the diffraction gratings constituting the cell 32, the distribution of the diffraction grating becomes uniform, and the formability in embossing becomes good. Since data recording is not related to the depth of the diffraction grating, this optical information recording medium can be manufactured by an apparatus, e.g., an electron beam lithography system capable of binary expression, which has a fine-processing capability. Control of the depth in the duplicate can be easily performed. Since the optical information recording medium is of the surface relief type, it can be duplicated easily by, e.g., embossing, thereby realizing mass production at a low cost. More specifically, since the diffraction grating is formed to be spatially uniform, it has a good formability in the duplicating process. Since it suffices if the diffraction grating has a uniform depth, conditions for duplication become moderate, thereby facilitating duplication. From the foregoing, a multivalue expression of a stable (high-reliability) optical information recording medium can be realized while facilitating duplication. In addition to the above characteristics concerning the diffracted light intensity, in the optical information recording medium of this embodiment, the second data is recorded by appropriately changing the grating pitch of the diffraction gratings constituting the cell 33 based on the second original data used for manufacturing the optical information recording medium main body. Thus, data reproduction by changing the diffraction angle of the diffracted light component can be performed, thereby enabling higher-density information recording. In addition to the above characteristics concerning the diffracted light intensity, in the optical information recording medium of this embodiment, the third data is recorded by appropriately changing the azimuth angle of the diffraction gratings constituting the cell 34 based on the third original data used for manufacturing the optical information recording medium main body. Thus, data reproduction by changing the exit direction of the diffracted light component can be performed, thereby enabling higher-density information recording. An information reading method for the optical information recording medium of this embodiment having the above arrangement will be described. FIG. 6 is a schematic view showing the arrangement of an optical system for realizing an information reading method for the optical information recording medium (reflection type) of this embodiment. Only one cell on the substrate 1 will be considered. Referring to FIG. 6, beam-like incident light 37 can be vertically incident on a substrate 31, on which a cell 35 recording data is formed, from a light source 36, e.g., a laser light beam source. A plurality of light-receiving elements 38 are circularly disposed, as shown in FIG. 6, thereby receiving diffracted light components 39 and 310 from the cell 35. When information on the optical information recording medium is to be read, the beam-like incident light 37 from the light source 36 is caused to be vertically incident on the cell 35. When the incident light 37 is vertically incident on the cell 35, diffracted light components of ±1st order emerge with a symmetrical positional relationship. A case will be considered wherein only +1st-order diffracted light component 39 is detected by a light-receiving element 38 indicated as a solid element in FIG. 6 and no light is detected by other light-receiving elements 38. In this case, the grating interval and the azimuth angle of grating of the diffraction grating of a cell in question can be obtained from the position of the light-receiving element 38 indicated as the solid element. The ratio of the line width to the grating pitch of the diffraction grating of the cell can be obtained from the light intensity detected by the light-receiving element 38. More specifically, when the grating pitch of the diffraction grating is smaller than that of the diffraction grating shown in FIG. 6, the diffracted light component is detected by a light-receiving element 38 at an outer side which is located in the same circumferential direction in accordance with the following equation. The diffracted light component obtained by the diffraction grating is expressed by the following equation: mλ = d(sinα + sinβ) where λ is the wavelength of the illumination light (incident light), d is the grating pitch, a is the angle of incidence of the illumination light, and β is the exit angle of mth-order diffracted light component. Usually, a diffracted light component having an m = 1st order, i.e., +1st-order diffracted light component is considered. When the azimuth angle of the diffraction grating is changed, the diffracted light component is detected by a light-receiving element 38 having the same distance in the direction of diameter but different position on the circumferential direction accordingly. When the ratio of the line width to the grating pitch of the diffraction grating is changed, the light intensity of the diffracted light component detected by the light-receiving element 38 is changed accordingly. Therefore, in the optical information recording medium of this embodiment, first to third three different types of independent pieces of information can be recorded only by a cell on which reproduction illumination light (described as incident light 7 in FIG. 6) is incident. In addition, since each of these three types of information can record binary data or data having three or more values, high-density recording can be performed. Furthermore, when the incident light 37 is scanned or the substrate 31 is moved, large-capacity information can be read at a speed a plurality of times the conventional speed even if the scanning speed or rotating speed is the same as the conventional one. FIG. 7 is a schematic view showing the state of an information reading method for an optical information recording medium on which data is recorded by changing only the line width and the grating pitch. The same portions as in FIG. 6 are denoted by the same reference numerals. Note that FIG. 7 shows a transmission type optical information recording medium. Referring to FIG. 7, upon incidence of incident light (parallel light) 37 having a certain wavelength, cells having different grating pitches emerge diffracted light components in different directions, as in diffracted light components 311 and 312. Since ratios of the line widths to the grating pitches of the diffraction gratings are different, the light intensities of the diffracted light components 311 and 312 are also different. The diffraction angles and light intensities of the diffracted light components 311 and 312 can be obtained by detecting the diffracted light components 311 and 312 by light-receiving elements 38, and original information can be reproduced from the obtained diffraction angles and light intensities. As described above, in the information reading method for the optical information recording medium of this embodiment, one cell causes diffracted light components to emerge in a plurality of directions simultaneously upon reception of one incident light 37. Since a plurality of data can be simultaneously reproduced, information reading at a very high speed can be realized. The number of exit directions of the diffracted light components can be increased by increasing the spot diameter of the incident light 37 and the number of cells, so that a larger number of pieces of information can be read simultaneously. Then, the spot diameter need not be decreased, unlike in the conventional case, and a lens system and the like for decreasing the spot diameter can be omitted or simplified. Furthermore, when a plurality of light beams corresponding to the respective cells are incident on the cells as the incident light 37, the diffracted light component emerging from each cell is also a beam. Then, the light can be easily received by the light-receiving elements, and a read error can be eliminated. In the above description, note that the azimuth angle of the diffraction grating indicates the angle through which the diffraction grating is pivoted about the normal to the surface of the substrate as the pivot axis, that the diffraction angle of the diffracted light component indicates the angle at which the diffracted light component emerges with respect to the normal to the surface of the substrate, and that the exit direction of the diffracted light component indicates the pivoting direction about the normal to the surface of the substrate as the pivot axis. FIG. 6 exemplifies a case wherein a reflection type cell is used as the cell, and FIG. 7 of the other embodiment exemplifies a case wherein a transmission type cell is used as the cell. However, the present invention is not limited to these. It is possible to use a transmission type cell as the cell in FIG. 6, and a reflection type cell as the cell in FIG. 14 of the other embodiment. When a transmission type cell is used as the cell, a reflecting layer need not be provided when the diffraction grating cell is formed on the substrate. Therefore, the manufacturing step of the optical information recording medium can be simplified accordingly. Both embodiments exemplify cases wherein an optical information recording medium is formed by forming a plurality of cells comprising small diffraction gratings on the surface of a planar substrate. However, the present invention is not limited to this. The present invention can similarly be applied to a case wherein an optical information recording medium is formed by forming at least one cell on the surface of a planar substrate. Both embodiments exemplify cases wherein an optical information recording medium records the first to third, i.e., three types of data. However, the present invention is not limited to this. It suffices if an optical information recording medium records at least the first data. FIG. 6 exemplifies a case wherein beam-like light is caused to be incident as the incident light, and FIG. 7 of the other embodiment exemplifies a case wherein parallel light is caused to be incident as the incident light. However, the present invention is not limited to these. Parallel light may be caused to be incident as the incident light in FIG. 6, and beam-like light may be caused to be incident as the incident light in FIG. 7 of the other embodiment. Both embodiments exemplify cases wherein the first to third, i.e., three types of data are recorded by different cells. However, the present invention is not limited to this, and these data may be recorded simultaneously by one cell. Both embodiments exemplify cases wherein a plurality of light-receiving elements are used. However, the present invention is not limited to this. When only the intensity of the diffracted light component is to be read, it suffices if at least one light-receiving element is provided.

Claims: An optical information recording medium formed by disposing at least one cell (35; 32, 33, 34) comprising small diffraction gratings on a surface of a planar substrate (31), characterized in that the ratio of a line width to the grating pitch of said diffraction gratings corresponds to first original data. An optical information recording medium according to claim 1, characterized in that the grating pitch of said diffraction gratings correspond to second original data. An optical information recording medium according to claim 1 or 2, characterized in that the azimuth angle of said diffraction gratings correspond to third original data. An optical information recording medium according to any one of claims 1 to 3, characterized in that an intensity of a diffracted light component (310) is coded based on the following equation as a ratio of the line width to the grating pitch of said diffractior gratings: η = ( 2 π ) 2 sin 2 ( 2π λ r cos ) sin( π d ℓ) 2 where η is a diffraction efficiency (a value of 0 to 1), r is a depth of the diffraction gratings, ℓ is the line width, d is the grating pitch,  is an angle of incidence of reproduction illumination light (37), and λ is a wavelength of the reproduction illumination light (37). An optical information recording medium according to any one of claims 1 to 4, characterized in that either a reflection type cell or a transmission type cell is used as said cell. A method of reading information from an optical information recording medium formed by disposing at least one cell (35; 32, 33, 34) comprising small diffraction gratings on a surface of a planar substrate (31), the ratio of the line width to the grating pitch of said diffraction gratings corresponding to first data, wherein at least one light-receiving element (38) is disposed, and one incident light beam (37) is caused to be incident on said diffraction grating cell (35), and a diffracted light component (310, 311, 312) emerging from said diffraction grating cell is received by said light-receiving element (38), so that the first data is reproduced as a change in intensity of the diffracted light component. A method of reading information from an optical information recording medium according to claim 6, wherein second data is reproduced as a change in diffraction angle of the diffracted light component (310, 31, 312). A method of reading information from an optical information recording medium according to claim 6 or 7, wherein third data is reproduced as a change in exit direction of the diffracted light component (310, 311, 312). An information reading method for an optical information recording medium according to any one of claims 6 to 8, characterized in that beam-like light is caused to be incident as the incident light (37).

Inventors: IWATA FUJIO, TAKAHASHI SUSUMU, TODA TOSHIKI, IWATA, FUJIO, TAKAHASHI, SUSUMU, IWATA, FUJIO GREEN-HOUSE-IWATA A 2-2-9, KASUGA, TAKAHASHI, SUSUMU SHINMATSUDO-HEIGHTS 601
Applicants: TOPPAN PRINTING CO LTD, TOPPAN PRINTING CO., LTD.
Application year: 1993
Main classification: G11B7
IPC codes: B41M5, G11B7, G06K19, G02B5, B42D15, G06K7