Patent Application
20070263521
Embodiments of the present invention will be described below with reference to the drawings.
The optical disk apparatus 1 is entirely controlled by a control unit 2. When the optical disk apparatus 1 receives a replay instruction from an external device (not shown) with the optical disk 100 being loaded, the control unit 2 controls a driving unit 3 and a signal processing unit 4 to read out information stored on the optical disk 100.
According to the control of the control unit 2, the driving unit 3 rotates the optical disk 100 at a predetermined rotational speed by a spindle motor 5, moves an optical pickup 7 coarsely in a tracking direction, which is the radial direction of the optical disk 100, by a sled motor 6, and moves an objective lens 9 finely in a focusing direction, which is the direction toward or away from the optical disk 100, and in the tracking direction by a two-axis actuator 8.
At the same time, the signal processing unit 4 instructs the optical pickup 7 to emit a predetermined light beam from the via the objective lens 9 to a desired track on the optical disk 100, generates a replay signal on the basis of the result of detecting the light beam reflected at the optical disk 100, and sends the replay signal to an external device (not shown) via the control unit 2.
More specifically, the objective lens 9 of the optical pickup 7 focuses a light beam having a wavelength of approximately 405 nm, which corresponds to the BD system, at a recording layer of the optical disk 100 to be accessed (hereinafter this recording layer is referred to as a “focused recording layer”). The light beam reflected at the focused recording layer includes a recording signal component (hereinafter the reflected light beam is referred to as a “signal light beam”). The signal light beam is converged by the objective lens 9 and is photoelectrically converted to generate various signals that are supplied to the driving unit 3 and the signal processing unit 4.
The driving unit 3 drives the two-axis actuator 8 on the basis of focus error signals and tracking error signals from the optical pickup 7. The signal processing unit 4 carries out predetermined signal processing on the replay signal sent from the optical pickup 7 and then outputs the processed replay signal to an external unit via the control unit 2.
As illustrated in
The polarizing beam splitter 13 allows the light beam from the collimating lens 12 to be transmitted through depending on the polarization direction of the light beam. The light beam transmitted through the polarizing beam splitter 13 is incident on a spherical-aberration compensation element 14. As the spherical-aberration compensation element 14, a liquid crystal phase plate such as that described in the following document may be employed: M. Iwasaki, M. Ogasawara, and S. Ohtaki, “A New Liquid Crystal Panel for Spherical Aberration Compensation,” Technical Digest of Optical Data Storage Topical Meeting, Santa Fe, pp. 103 (2001).
The spherical-aberration compensation element 14 constituted of such a liquid crystal phase panel includes concentric electrodes 14a, 14b, and 14c having different diameters, as illustrated in
Thus, the control unit 2 (refer to
The light beam whose aberration has been compensated for by the spherical-aberration compensation element 14 of the optical pickup 7 is converted from a liner polarized light beam into a circularly polarized light beam by a quarter-wave plate 15. Then, circularly polarized light beam is gathered by the objective lens 9 having a numerical aperture (NA) of 0.85 and is incident on the recording layer of the optical disk 100.
The light beam reflected at the recording layer of the optical disk 100 (hereinafter referred to as a “reflection light beam”) is received by the objective lens 9, is linearly polarized by the quarter-wave plate 15 in a polarization direction orthogonal to the polarization direction of the light beam before being reflected, and is incident on the polarizing beam splitter 13. Due to the polarization direction, the linearly polarized reflection light beam is reflected at the polarizing beam splitter 13 in a right angle and enters a light-receiving system 16.
In the light-receiving system 16, the polarization direction of the reflected reflection light beam emitted from the polarizing beam splitter 13 is adjusted by a half-wave plate 17 so that the reflection light beam is transmitted through a polarizing beam splitter 18. The transmitted reflection light beam is circularly polarized again at the quarter-wave plate 19, is gathered by a focusing lens 20 having a NA of approximately 0.1, and is incident on a stray-light removing element 30.
The stray-light removing element 30 has a reflection surface 30a on one side. The reflection surface 30a is provided at a position that matches the focal point of the reflection light beam focused by the focusing lens 20. In this way, substantially 100% of the focused reflection light beam is reflected at the reflection surface 30a of the stray-light removing element 30, is converted into a diffused light beam, and is incident on the focusing lens 20. At this time, as described in detail below, the polarization direction of the stray light components included in the reflection light beam is changed by the stray-light removing element 30, and only the signal light beam that has been reflected at the focused recording layer is guided to a detection light path through a focusing lens 21, a cylindrical lens 22, and a light-receiving element 23.
In the light-receiving system 16, the signal light beam reflected at the reflection surface 30a of the stray-light removing element 30 is substantially collimated at the focusing lens 20. Then, the substantially collimated signal light beam is a linearly polarized at a quarter-wave plate 19 in a direction orthogonal to the polarization direction of the signal light beam before being reflected at the reflection surface 30a. The linearly polarized signal light beam is incident on the polarizing beam splitter 18. Due to the polarization direction, the signal light beam is reflected in a right angle at the polarizing beam splitter 18, which is a polarizing optical element, and forms an image on the light-receiving element 23 via the focusing lens 21 and the cylindrical lens 22. Then, the light-receiving element 23 generates various detection signals corresponding to the intensity of the received signal light beam.
Next, computation carried out on the various detection signals generated at the light-receiving element 23 will be described. Here, an astigmatism method is employed as a method of obtaining a focal-point error signal FES, and a phase difference method is employed as a method of obtaining a track error signal TES. However, instead, various other methods may be employed; for example, a knife edge method or a spot size method may be employed as a method of detecting a focal-point error signal, and a push-pull method a three-beam method, or differential push-pull method may be employed as a method detecting a track error signal.
As illustrated in
Accordingly, by carrying out the following computation on the signals A to D, a focal-point error signal FES that has a so-called S-shaped waveform in which the signal level is zero when focused but is positive (+) or negative (−) when not focused.
FES=(A+C)−(B+D) (1)
The optical disk apparatus 1 according to this embodiment supports a three-layer BD-ROM disk, which is a multiple layer information recording medium. The optical disk apparatus 1 generates a track error signal TES corresponding to a play-only optical disk, such as the BD-ROM disk, that has predetermined information pit rows by employing the phase difference method applying the following expression.
TES=φ(A+C)−φ(B+D) (2)
Here, +represents the operator of the signal phase. A playback signal RFS is generated by adding the output signals A to D from the light-receiving regions 23a to 23d by applying the following expression.
RFS=A+B+C+D (3)
Next, removal of a stray light component from a reflection light beam by the stray-light removing element 30 according to an embodiment of the present invention will be described in detail.
The small cubic prisms 31a and 31b are bonded together with an optical material, such as an adhesive or a dielectric thin film that is transparent to the wavelength of the laser beam or a metal thin film that has an absorbing property. In this way, a boundary surface 30b consisting the above-mentioned optical material is formed between the small cubic prisms 31a and 31b. The index of refraction of the optical material constituting the boundary surface 30b is represented by n1.
The cubic small cubic prism 31c is bonded to the small cubic prisms 31a and 31b with an optical material, such as an adhesive or a dielectric thin film that is transparent to the wavelength of the laser beam or a metal thin film that has an absorbing property. For bonding the small cubic prism 31c, an optical material having an index of refraction n2 most similar to the index of refraction ng of the three small cubic prisms 31a, 31b, and 31c is selected so as to suppress the reflectance of the transmitted light beam.
As described above, the reflection surface 30a on one side of the stray-light removing element 30 is provided at a position that matches the focal point of the reflection light beam focused by the focusing lens 20. The boundary surface 30b is positioned on a plane including the optical axis of the reflection light beam.
According to this embodiment, the NA of the objective lens 9 is set to 0.85; the NA of the focusing lens 21 is set to 0.1; and the three signal layers of the BD-ROM disk are referred to as an L0 layer, an L1 layer, and an L2 layer, from the layer farthest from the objective lens 9.
As described above, a focused light beam reflected at the L1 layer, which is the focused layer (hereinafter this light beam is referred to as a “signal light beam”), is substantially collimated by the objective lens 9 and is focused at the reflection surface 30a of the stray-light removing element 30 by the focusing lens 20. Then, the signal light beam is reflected at the reflection surface 30a of the stray-light removing element 30, is diffused, and is incident on the focusing lens 20.
Since the signal light beam transmitted through the inside of the stray-light removing element 30 is focused at the reflection surface 30a, the signal light beam does not contact the boundary surface 30b interposed between the small cubic prisms 31a and 31b at any point in the optical path approaching the reflection surface 30a and the optical path moving away from the reflection surface 30a. Thus, the boundary surface 30b does not have any effect on the signal light beam focused at the reflection surface 30a. In addition, since the boundary surface 30b is provided away from the reflection surface 30a by a distance equal to the thickness of the small cubic prism 31c, even when the optical axis of the signal light is displaced, the boundary surface 30b does not have any effect on the signal light beam.
As described above, since the stray light is incident on the focusing lens 20 as a diffused light beam, the focal position of the focusing lens 20 is positioned behind the reflection surface 30a of the stray-light removing element 30. Thus, the stray light reflected at the reflection surface 30a is emitted from the stray-light removing element 30 after contacting the boundary surface 30b. At this time, the stray light is reflected, transmitted or absorbed at the boundary surface 30b.
In this case, since the absorption at the boundary surface 30b composed of a metal thin film is great, most of the incident light is not transmitted through the boundary surface 30b but is reflected and absorbed.
More specifically, when the incident angle of the light beam to the boundary surface 30b is small, the light reflected at the boundary surface 30b is reflected at the polarizing beam splitter 18 and is incident on the light-receiving element 23. However, as the incident angle becomes greater, the polarization direction of the reflected light changes due to phase rotation, and the amount of light transmitted through the polarizing beam splitter 18 increases while the amount of light incident on the light-receiving element 23 decreases. In particular, when the reflection angle is 85° or greater, the amount of light guided to the light-receiving element 23 is significantly reduced.
In this case, different from the case in which the boundary surface 30b is composed of a metal thin film (
According to this embodiment, the numerical aperture of the focusing lens 20 is 0.1. Under this condition, the angle of the outermost light beam to the optical axis is approximately 6°. The angle of a light beam is 4° or smaller inside the stray-light removing element 30 due to refraction caused by air and the boundary surface of the optical material. Therefore, a light beam is incident on the boundary surface 30b of the stray-light removing element 30 at 86° or greater. Accordingly, as the calculation results shown in
Stray light generated at the L2 layer forward of the L1 layer, which is the focused layer, has been described above. Similarly, when the L0 layer is the focused layer and stray light is generated at the L1 layer or L2 layer forward of the L0 layer, the stray light, after being reflected at the reflection surface 30a of the stray-light removing element 30, is reflected at the boundary surface 30b so as to change the polarization direction of the stray-light or absorbs the reflected stray light at the boundary surface 30b. In this way, the stray light is prevented from being incident on the light-receiving element 23.
Similarly, stray light generated at a recording layer behind the focused recording layer is prevented from being incident on the light-receiving element 23 by reflecting, transmitting, or absorbing the stray light at the boundary surface 30b of the stray-light removing element 30.
As described above, since the stray light is incident on the focusing lens 20 as a converged light beam, the focal position of the focusing lens 20 is positioned forward of the reflection surface 30a of the stray-light removing element 30. Thus, the stray light incident on the removing element 30 contacts the boundary surface 30b before reaching the reflection surface 30a. In this case also, the stray-light removing element 30 prevents the stray light from being incident on the light-receiving element 23 by reflecting at the boundary surface 30b the stray light that has not yet reached the reflection surface 30a so as to change the polarization direction of the stray light or by absorbing the stray light, in accordance with the properties illustrated in
According to the above-described structure, in the optical pickup 7, the reflection light beam that has been reflected at the optical disk 100 is reflected at the reflection surface 30a of the stray-light removing element 30, is emitted to the polarizing beam splitter 18, is reflected at the polarizing beam splitter 18, and is incident on the light-receiving element 23.
The boundary surface 30b of the stray-light removing element 30 is provided away from the reflection surface 30a by a predetermined distance on the optical axis of the reflection light beam. When stray light generated by a light beam reflected at an unfocused recording layer of the optical disk 100 is focused by the focusing lens 20, the focal point of the stray light is positioned forward of or behind the reflection surface 30a. Therefore, the stray light is incident on the boundary surface 30b before or after being reflected at the reflection surface 30a.
The stray-light removing element 30 prevents stray-light from being incident on the light-receiving element 23 by transmitting the stray light through the polarizing beam splitter 18 as a result of changing the polarization direction of the stray light reflected at the boundary surface 30b and by absorbing the stray light at the boundary surface 30b.
According to the above-described structure, stray light that is incident on the light-receiving element 23 can be reliably removed by emitting only the stray light components of the reflection light beam to the boundary surface 30b of the stray-light removing element 30.
The above-described optical disk apparatus 1 according to an embodiment of the present invention supports the optical disk 100 having three recording layers. However, the present invention is not limited, and instead, an optical disk apparatus that supports an optical disk having a plurality of recording layers, e.g., two or four recording layers, is included in the scope of the present invention.
According to the above-described embodiment, the reflection surface 30a is formed at one side of the stray-light removing element 30 and reflects substantially 100% of the incident light so as to guide the reflected light to the light-receiving element 23. However, the present invention is not limited, and instead, the reflection surface 30a may be a half mirror in which part of the incident light is reflected at the reflection surface 30a and the remaining light is transmitted through the reflection surface 30a and a light-receiving element may be provided to light detected light transmitted through the reflection surface 30a.
According to the above-described embodiment, the boundary surface 30b is formed of a metal thin film, a dielectric thin film, or an adhesive layer. However, the present invention is not limited, and instead, the boundary surface 30b may be formed of a bumpy pattern that scatters light or a diffraction grating having high-order diffraction.
According to the above-described embodiment, the reflection light beam from the optical disk 100 is reflected at the reflection surface 30a of the stray-light removing element 30 and is incident on the light-receiving element 23, and the signal light beam is focused at the reflection surface 30a. In this way, stray light focused at a point in front of or behind the reflection surface 30a is reflected at the boundary surface 30b of the stray-light removing element 30 so as to change the phase or is absorbed, and the stray light incident on the light-receiving element 23 is removed. However, the present invention is not limited, and instead, the reflection light beam may be focused and boundaries may be provided in front of and behind the focal point of the reflection light beam.
A reflection light beam diffused at the center area of the stray-light removing element 40 is collimated by the aperture limiting unit 37 and is incident on the focusing lens 21 through a quarter-wave plate 38 and an analyzer 39 in this order. The focusing lens 21 focuses the reflection light beam and forms an image on the light-receiving element 23 via the cylindrical lens 22. The light-receiving element 23 generates various detection signals depending on the intensity of the received reflection light beam.
At this time, stray light focused at a point in front of or behind the center area of the stray-light removing element 40 is reflected at the boundary surface 40a or 40b so as to change the phase or is absorbed. In this way, similar to the above-described stray-light removing element 30, the stray-light removing element 40 prevents stray light from being incident on the light-receiving element 23.
By using an optical material, such as an adhesive or dielectric thin film transparent to the wavelength of the laser beam or such as a metal thin film having an absorbing property, the small cubic prism 41a and the small cubic prism 41b are bonded together, and the small cubic prism 41d and the small cubic prism 41e are bonded together. In this way, a boundary surface 40a is formed by the above-mentioned optical material between the small cubic prisms 41a and 41b, and a boundary surface 40b is formed by the above-mentioned optical material between the small cubic prisms 41d and 41e. The index of refraction of the optical material constituting the boundary surfaces 40a and 40b is represented by n1.
The small cubic prism 41c is bonded to the small cubic prisms 41a and 41b and to the small cubic prisms 41d and 41e with an optical material, such as an adhesive or dielectric thin film transparent to the wavelength of the laser beam or such as a metal thin film having an absorbing property. To bond the small cubic prism 41c, an optical material having an index of refraction n2 most similar to the index of refraction ng of the three small cubic prisms 41a, 41b, 41d, and 41e is selected so as to suppress the reflectance of the transmitted light beam.
As described above, the center of the small cubic prism 41c at the center area of the stray-light removing element 40 is provided at a position that matches the focal point of the reflection light beam focused by the focusing lens 20. The boundary surfaces 40a and 40b are positioned at points in front of and behind the focal point on a plane including the optical axis of the reflection light beam.
In this way, stray light from a forward recording layer whose focal point is positioned behind the center area of the small cubic prism 41c is reflected, transmitted, or absorbed at the boundary surface 40b, whereas stray light from a rearward recording layer whose focal point is positioned forward of the center area of the small cubic prism 41c is reflected, transmitted, or absorbed at the boundary surface 40a.
The boundaries 40a and 40b, similar to the boundary surface 30b of the stray-light removing element 30, reflect stray light that is incident at a great angle on the boundaries 40a and 40b and change the phase of the stray light so that the stray light is not transmitted through the analyzer 39. In this way, stray light can be prevented from being incident on the light-receiving element 23.
In the above, a case in which the stray-light removing element 30 is employed to the optical pickup 7 of the optical disk apparatus 1 according to an embodiment of the present invention has been described. However, stray-light removing elements having various other structures may be included in the scope of the present invention. In other words, the stray-light removing element 30 does not necessarily have to be installed in the optical pickup 7, and the optical pickup 7 does not necessarily have to be installed in the optical disk apparatus 1.
It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2006-134192 | May 2006 | JP | national |
