Patent Application
20060280098
The present invention claims priority under 35 U.S.C. §119 to Japanese Application No. 2005-152912 filed May 25, 2005, which is incorporated herein by reference.
An embodiment of the present invention may relate to a diffraction element in which a groove part and a protruded part are alternately arranged, and may relate to a device for use with an optical disk device provided with the diffraction element.
Various structures have been proposed to detect a signal from an optical disk. Even when various structures are utilized, an optical disk device commonly includes a laser light source, a photo-detector, and an optical system that structures a forward path for guiding a laser beam emitted from the laser light source to an optical disk and a return path for guiding a return light beam reflected by the optical disk to the photo-detector. Further, various diffraction elements are used for the optical disk device.
For example, the following technique has been disclosed. In order to obtain a tracking error signal by a DPP (Differential Push-Pull) method or the like, a main beam comprised of a 0-order (zero-order) light beam and sub-beams comprised of diffracted light beams are generated from a light beam emitted from a laser light source by a diffraction element. A diffraction element is used as an above-mentioned diffraction element in which groove parts are formed in an area smaller than the cross sectional area of a light beam and the offset of tracking is canceled by using a diffracted light beam and a light beam which is not diffracted (see, for example, Japanese Patent Laid-Open No. Hei 10-162383)
However, in the case of the diffraction element which is described in the above-mentioned prior art, a large difference in the phase of the main beam occurs between an area provided with groove parts and a flat portion which is not provided with the groove parts, and thus the occurrence of aberration is not prevented.
In view of the problems described above, an embodiment of the present invention may advantageously provide a diffraction element which is capable of canceling the offset of tracking by using a diffracted light beam and a light beam which is not diffracted and which is capable of preventing the generation of an astigmatism, and which provides a device for use with an optical disk in which the diffraction element is used.
Thus, according to an embodiment of the present invention, there may be provided a diffraction element including a grating region where a plurality of groove parts and a plurality of protruded parts are alternately formed on the surface of the diffraction element, and also flat regions are formed where the surface of the diffraction element is formed to be a flat face. The flat region is formed at the height of “nλ” from a center position in the depth direction of the groove part (wherein “λ” is the wavelength of an incident light beam to the diffraction element and “n” is an integer number) and the flat regions are formed on both sides of the grating region in the longitudinal direction of the groove part.
The diffraction element to which the present invention is applied can be used in a device for use with an optical disk. The device may include a laser light source, a photo-detector, an optical system which structures a forward path for guiding a laser beam emitted from the laser light source to the optical disk and a return path for guiding a return light beam reflected by the optical disk to a photo-detector, and a diffraction element which is disposed in the forward path or the return path of the optical system between the laser light source and the optical disk. Further, the cross sectional area of the light beam emitted from the laser light source on the diffraction element is set to be larger than the grating region.
In accordance with an embodiment, the center position in the depth direction of the groove part is set to be the same height position in the longitudinal direction of the groove part. According to the structure described above, the generation of astigmatism due to the diffraction element can be prevented.
In accordance with an embodiment, respective center positions in the depth direction of a plurality of groove parts are set to be the same height position. According to the structure described above, the generation of astigmatism due to the diffraction element can be prevented. Therefore, it is preferable that the center position in the depth direction of the groove part is set to be the same height position in the longitudinal direction of the groove part and the respective center positions in the depth direction of a plurality of the groove parts are set to be the same height position. In this case, the flat region may be formed at the same height position as the center position in the depth direction of the groove part.
In accordance with an embodiment, stepped portions formed as boundaries between the grating region and the flat regions formed on both sides of the grating region are formed in parallel. According to the structure described above, the diffraction element may be disposed such that the grating region formed in the direction perpendicular to the longitudinal direction of the groove part is extended to the outside of the cross sectional area of the light beam on the diffraction element. Therefore, when the diffraction element is to be mounted on an optical disk device, adjustment in the direction perpendicular to the longitudinal direction of the groove part can be roughly performed and workability of mounting work can be improved.
Thus, the diffraction element may include a grating region where a plurality of groove parts and a plurality of protruded parts are alternately formed on the surface of the diffraction element, and flat regions where the surface of the diffraction element is formed to be a flat face and which are formed on both sides of the grating region in the longitudinal direction of the groove part. Further, the flat region is formed at a height of “nλ” from the center position in the depth direction of the groove part (wherein “λ” is the wavelength of an incident light beam to the diffraction element and “n” is an integer number). Therefore, for example, when the diffraction element to which the present invention is applied is used in the optical system of an optical disk device, and when the diffraction element is disposed to be used as a three-beam generating element such that the grating region is located over a region which is smaller than the cross sectional area of a light beam, the offset of tracking can be canceled by using a diffracted light beam and a light beam which is not diffracted, and generation of astigmatism in the diffraction element can be prevented.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.
Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:
FIGS. 3(a), 3(b), 3(c), 3(d) and 3(e) are explanatory views showing variations of the light intensity distribution of 0-order light beam before and after the transmission of the diffraction element which is used in the first embodiment.
In
The photo-detector 3 is used to generate a focusing error signal and a tracking error signal when information is recorded or reproduced by detecting the return light beam reflected by the optical recording disk 10. The focusing error signal and the tracking error signal is fed back to an objective lens drive device 7.
The optical recording disk 10 is, for example, a DVD-RAM (Digital Versatile Disk Random Access Memory). In a DVD-RAM, lands and grooves with wobble (undulation) are alternately formed in a concentric manner (not shown) and both the lands and the grooves are used as a track in which a pit is formed. A signal obtained from the wobble is used for pull-in of a clock.
In the optical disk device 1 in accordance with the first embodiment, a diffraction element 8 comprised of a grating or a hologram element is disposed between the semiconductor laser 2 and the beam splitter 41 for generating a sub-beam comprised of −1st-order diffracted light beam, a main beam comprised of 0-order light beam, and a sub-beam comprised of +1st-order diffracted light beam from the laser beam emitted from the semiconductor laser 2. Therefore, reproduction of information can be performed by the main beam comprised of 0-order light beam which is converged on a track of the optical disk 10 through the objective lens 44 and the return light beam which is detected by the photo-detector 3. Further, recording of information can be performed by the main beam comprised of 0-order light beam which is converged on a track of the optical recording disk 10 through the objective lens 44. In addition, a sub-beam comprised of −1st-order diffracted light beam and a sub-beam comprised of +1st-order diffracted light beam are converged through the objective lens 44 at positions interposing the spot of the main beam in the tangential direction of the track of the optical disk 10. A tracking error signal can be obtained by detecting the return light beam with the photo-detector 3 and by utilizing a Differential Push Pull (DDP) method or the like.
As shown in FIGS. 2(a), 2(b) and 2(c), in the diffraction element 8 which is used in the optical disk device 1 in accordance with the first embodiment, a grating region 86 where a plurality of groove parts 81 and a plurality of protruded parts 82 are alternately arranged is formed on the center portion 86 of the diffraction element 8. Flat regions 87, 88 whose faces are formed to be flat faces are formed on both sides of the grating region 86.
In accordance with an embodiment, all groove parts 81 are formed such that each depth dimension “d” between the bottom part 810 of the groove part 81 and the upper faces 820 of protruded parts 82 on both sides of the groove part 81 is formed to be the same depth in the longitudinal direction (shown by the arrow “L) of the groove part 81. Further, the depths of a plurality of the groove parts 81 are respectively set to be the same depth (See FIGS. 2(b) and 2(c). Therefore, the center position (shown by the alternate long and short dash line “C” in
Thus, the diffraction element in accordance with the present invention may include a grating region 86 where a plurality of groove parts 81 and a plurality of protruded parts 82 are alternately formed on the surface of the diffraction element 8, and flat regions (87, 88) are formed where the surface of the diffraction element is formed to be a flat face and which are formed on both sides of the grating region 86 in the longitudinal direction of the groove part. Further, the flat regions (87, 88) are formed at a height of “nλ” from the center position C in the depth direction of the groove part (wherein “λ” is the wavelength of an incident light beam to the diffraction element and “n” is an integer number). Therefore, for example, when the diffraction element 8 to which the present invention is applied is used in the optical system for use with an optical disk device, and when the diffraction element is disposed to be used as a three-beam generating element such that the grating region 86 is located over a region which is smaller than the cross sectional area of a light beam, the offset of tracking can be canceled by using a diffracted light beam and a light beam which is not diffracted, and generation of astigmatism in the diffraction element can be prevented.
In accordance with this embodiment, the width dimension of the groove part 81 and the width dimension of the protruded part 82 in the diffraction element 8 are the same as each other and all the duty ratio of the grating is 50:50. Further, in this embodiment, stepped portions SP are formed as boundaries between the grating region 86 and the flat regions 87, 88 formed on both sides of the grating region 86 are formed in parallel as shown in
In the diffraction element 8 structured as described above, the grating region 86 is formed in a striped shape (see stripe-like repeating sections 82, 81, 82, 81, 82 in
FIGS. 3(a), 3(b), 3(c), 3(d) and 3(e) are explanatory views showing variations of a light intensity distribution of 0-order light beam before and after the transmission of the diffraction element which is used in the first embodiment.
As shown in FIGS. 3(a), 3(b) and 3(d), in the optical disk device 1 in accordance with the first embodiment, the light quantity distribution of the laser light beam when the diffraction element 8 is shown cut in a direction perpendicular to the groove part 81 of the diffraction element 8 does not indicate a large variation between before and after the transmission through the diffraction element 8. However, as shown in FIGS. 3(a), 3(c) and 3(e), the light quantity distribution of the laser light beam when the diffraction element 8 is cut in a direction parallel to the groove part 81 of the diffraction element 8 indicates a large variation before and after the transmission through the diffraction element 8. In other words, in this diffraction element 8, the ±1st-order diffraction efficiencies in the grating region 86 in the longitudinal direction of the groove part 81 are high but the ±1st-order diffraction efficiencies in both the flat regions 87, 88 are zero. Therefore, the optical intensity of the zero-order light beam emitted through the grating region 86 decreases largely but the optical intensity of the zero-order light beam emitted through both the side regions 87, 88 does not decrease. Accordingly, the peak shape of the zero-order light beam becomes to be the shape in which, although the light quantity decreases largely in the grating region, the level of the lower slope portion is raised as shown by the arrow “B” in
Further, in the first embodiment, all the duty ratio of the grating in the diffraction element 8 may be set to be 50:50 and thus the generation of high-order diffraction light beams can be restrained. Therefore, when sub-beams are converged on the optical disk 10, both the spot diameters of +1st-order sub-spot and −1st-order sub-spot are enlarged in comparison with the conventional example (see FIGS. 4(a) and 4(b). Accordingly, since the tolerance of positional accuracy between a track and the sub-beams becomes wider, when the optical disk device 1 is manufactured, working efficiency can be improved. Moreover, even when an optical disk 10 with a different track pitch is used, a tracking error signal can be appropriately obtained.
In addition, even when the diffraction element 8 is used as a three-beam generating element by arranging the diffraction element 8 such that the grating region 86 and the flat regions 87, 88 are located within a region which is smaller than the cross sectional area of a light beam as described in this embodiment, the flat regions 87, 88 are formed at the same height as the center position in the depth direction of the groove part 81. Therefore, the generation of astigmatism in the diffraction element 8 can be surely prevented and the diffraction element 8 can be made thinner.
As shown in FIGS. 5(a), 5(b) and FIGS. 6(a), 6(b), also in an optical disk device 1 in accordance with the second and third embodiments, similarly to the first embodiment, a grating region 86 where a plurality of groove parts and a plurality of protruded parts are alternately arranged is formed at the center portion of the diffraction element 8. Further, flat regions 87, 88 whose face is formed to be a flat face are formed on both sides of the grating region 86.
The groove parts 81 in the second and third embodiments are respectively formed such that the depth dimension “d” between the upper faces 820 of the protruded parts 82 on the both sides of the groove part 81 and the bottom part 810 of the groove part 81 is formed to be the same depth in the longitudinal direction of the groove part 81 (shown by the arrow “L”). Further, the depths of adjacent groove parts 81 are formed to be the same depth as each other. Therefore, the center position (shown by the alternate long and short dash line “C” in
In addition, in the grating region of the diffraction element 8, the width dimension of the groove part 81 and the width dimension of the protruded part 82 are equal to each other and thus all the duty ratio of the grating is 50:50. However, in the diffraction element 8 of the second embodiment, stepped portions SP are formed as a boundary between the grating region 86 and the flat regions 87, 88 are formed in an elliptical shape as shown in
In addition, in the first, the second and the third embodiments described above, the flat regions 87, 88 are formed to be the same height as the center position in the depth direction of the groove part 81. In other words, the flat regions 87, 88 are formed to be the height which corresponds to the case of “n=0” when the height of the flat regions 87, 88 from the center position in the depth direction of the groove part 81 is set to be (nλ). However, the flat regions 87, 88 may also be formed at a height that is different from the center position in the depth direction of the groove part 81. In other words, the flat regions 87, 88 may be formed at a height position which is lower or higher by the integral number of times of the wavelength (λ) of the incident light beam from the center position in the depth direction of the groove part 81.
Also, in accordance with an embodiment, stepped portions formed as boundaries between the grating region and the flat regions may be formed on both sides of the grating region and are formed in parallel. According to the structure described above, the diffraction element may be disposed such that the grating region formed in the direction perpendicular to the longitudinal direction of the groove part is extended to the outside of the cross sectional area of the light beam on the diffraction element. Therefore, when the diffraction element is to be mounted on an optical disk device, adjustment in the direction perpendicular to the longitudinal direction of the groove part can be roughly performed and workability of mounting work can be improved.
While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.
The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-152912 | May 2005 | JP | national |
