Patent Application
20080002555
The present invention contains subject matter related to Japanese Patent Application JP 2006-157634 filed in the Japanese Patent Office on Jun. 6, 2006, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical pickup and an optical disc apparatus, and in particular relates to an optical pickup and an optical disc apparatus preferably corresponding to an optical disc with a plurality of recording layers.
2. Description of the Related Art
In order to increase the recording capacity of an optical disc, a multi-layered optical disc made by stacking a plurality of recording layers has been proposed. When a signal is recorded on and reproduced from such a multi-layered optical disc, a light beam condensed by an objective lens of the optical pickup is focused on a target recording layer.
When information is recorded on and reproduced from the multi-layered optical disc, it is necessary to regulate the power of a light beam in accordance with the position of a target recording layer and to correct the spherical aberration of the light beam corresponding to the thickness of a cover layer, which differs depending on the position of the target recording layer.
Recently, in order to further increase the recording capacity, Blu-ray Disc™ (referred to as BD below) including blue-violet semiconductor laser with a wavelength of about 405 nm and an objective lens with a numerical aperture of 0.85 has been put to practical use. Then, a multi-formatted optical disc apparatus has been developed in that in addition to conventional DVDs (digital versatile discs) and CDs (compact discs), the BD can be used.
In such an optical disc apparatus, it is necessary to quickly determine the number of layers of a mounted optical disc. Thus, an optical disc apparatus has been proposed in that the light (i.e., stray light) reflected from positions other than an in-focus recording layer, on which a light beam is focused, is received on an independent photo detector for detecting stray light, and the number of layers is determined based on the amount of the detected stray light (see Japanese Patent Laid-Open No. 2006-31773, for example).
However, in the optical disc apparatus mentioned above, the stray light becomes incident in a photo detector for detecting a signal together with the focused beam reflected from the in-focus recording layer so as to deteriorate the quality of the detected signal, while the focused beam enters the photo detector for detecting stray light so as to deteriorate accuracies in determining the number of layers.
The present invention has been made in view of such problems, and it is desirable to propose an optical pickup and an optical disc apparatus capable of securely determining the kind of a multi-layered optical disc.
According to an embodiment of the present invention, there is provided an optical pickup configured to irradiate an optical disc having a plurality of recording layers with a light beam to receive a reflected light beam reflected from the recoding layer of the optical disc, in which the optical pickup includes an objective lens configured to condense the light beam emitted from a light source onto an in-focus recording layer of the optical disc and to receive the reflected light beam; a condenser lens configured to condense the reflected light beam received by the objective lens; a polarization optical element configured to include boundary surfaces positioned backward and forward a focal point of focused light condensed by the condenser lens, the focused light being reflected by the in-focus recording layer in the reflected light beam on a plane including the optical axis of the reflected light beam condensed by the condenser lens, and spaced from the focal point by a predetermined distance so as to change the polarization direction of stray light included in the reflection light beam by reflecting only the stray light in the reflected light beam reflected from a non in-focus recording layer by the boundary surfaces; a polarization beam splitter configured to separate the stray light from the focused light based on the polarization direction by emitting the reflected light beam emitted from the polarization optical element therein; a photo detector for detecting stray light having a plurality of light receiving regions for detecting the amount of the stray light separated by the polarization beam splitter; and a signal processor for determining the kind of the optical disc on the basis of the amounts of the stray light respectively detected in the plurality of light receiving regions.
The polarization optical element changes the polarization direction of only the stray light, and the stray light is separated from the focused light by the polarization beam splitter, so that by emitting only the stray light to the photo detector for detecting stray light, the kind determination of the optical disc can be securely executed based on the amount of the stray light.
According to the embodiment of the present invention, there is provided an optical disc apparatus configured to irradiate an optical disc having a plurality of recording layers with a light beam to receive a reflected light beam reflected from the recoding layer of the optical disc, in which the optical disc apparatus includes an objective lens configured to condense the light beam emitted from a light source onto an in-focus recording layer of the optical disc and to receive the reflected light beam; a condenser lens configured to condense the reflected light beam received by the objective lens; a polarization optical element configured to include boundary surfaces positioned backward and forward the focal point of focused light condensed by the condenser lens, the focused light being reflected by the in-focus recording layer in the reflected light beam on a plane including the optical axis of the reflected light beam condensed by the condenser lens, and spaced from the focal point by a predetermined distance so as to change the polarization direction of stray light included in the reflection light beam by reflecting only the stray light in the reflected light beam reflected from a non in-focus recording layer by the boundary surfaces; a polarization beam splitter configured to separate the stray light from the focused light based on the polarization direction by emitting the reflected light beam emitted from the polarization optical element therein; a photo detector for detecting stray light having a plurality of light receiving regions for detecting the amount of the stray light separated by the polarization beam splitter; and a signal processor for determining the kind of the optical disc on the basis of the amounts of the stray light respectively detected in the plurality of light receiving regions.
The polarization optical element changes the polarization direction of only the stray light, and the stray light is separated from the focused light by the polarization beam splitter, so that by emitting only the stray light to the photo detector for detecting stray light, the kind determination of the optical disc can be securely executed based on the amount of the stray light.
According to the embodiment of the present invention, an optical pickup and an optical disc apparatus are achieved in that a polarization optical element changes the polarization direction of only stray light included in a reflected light beam, and the stray light is separated from focused light by a polarization beam splitter, so that by emitting only the stray light to a photo detector for detecting stray light, the kind determination of the optical disc can be securely executed based on the amount of the stray light.
An embodiment of the present invention will be described below in detail with reference to the drawings.
(1-1) The Whole Configuration of Optical Disc Apparatus
Referring to
The optical disc apparatus 1 is totally controlled by a control unit 2. When the control unit 2 receives reproducing instructions from an outside instrument (not shown) in a state that the optical disc 100 is mounted thereon, the control unit 2 instructs a drive unit 3 and a signal processor 4 to read out information stored in the optical disc 100.
In practice, under the control of the control unit 2, the drive unit 3 rotates the optical disc 100 at a desired rotational speed with a spindle motor 5; largely moves an optical pickup 7 in a tracking direction, which is the radial direction of the optical disc 100, with a sled motor 6; and further finely moves an objective lens 9 in two directions of a focusing direction and the tracking direction, which are directions moving the objective lens 9 close to and separating from the optical disc 100, with a two-axis actuator 8.
Simultaneously, the signal processor 4 irradiates a desired track of the optical disc 100 with a predetermined light beam from the objective lens 9 using the optical pickup 7 so as to produce a reproducing signal based on the detected reflection light. Then, the reproducing signal is fed to the outside instrument (not shown) via the control unit 2.
Namely, the optical pickup 7 condenses a light beam with a wavelength corresponding to the kind of the mounted optical disc using an objective lens unit 9 so as to radiate an access target recording layer by focusing the light beam thereon (this recording layer is referred to as an in-focus recoding layer). Simultaneously, the light beam, including a recording signal component (referred to as a signal light beam) reflected from the in-focus recoding layer, is received by the objective lens unit 9 so as to produce various detection signals by photo-electric conversion for supplying them to the signal processor 4.
The drive unit 3 drives the two-axis actuator 8 on the basis of a focus error signal and a tracking error signal supplied from the signal processor 4. The signal processor 4 also executes predetermined signal processing on a reproducing signal supplied from the optical pickup 7 so as to outside output the reproducing signal via the control unit 2.
(1-2) Configuration of Optical Pickup
As shown in
The polarization beam splitter 13 passes the light beam from the collimator lens 12 therethrough corresponding to the polarization direction of the light beam so as to emit the light beam to a spherical-aberration correcting element 14. This spherical-aberration correcting element 14 may include a liquid crystal phase plate like described in “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 correcting element 14 made of such a liquid crystal phase plate, as shown in
Hence, the control unit 2 (
Then, the optical pickup 7 converts the light beam corrected in aberration by the spherical-aberration correcting element 14 into circular polarized light from linear polarized light with a quarter undulation plate 15, and further condenses the light beam with the objective lens 9 with a numerical aperture (NA) of 0.85 so as to irradiate the recording layer of the optical disc 100 with the light beam.
Furthermore, the optical pickup 7 receives the light beam reflected from the recording layer of the optical disc 100 with the objective lens 9, and the light beam is converted into a linear polarized beam with a polarizing direction perpendicular to that in the approaching route by the quarter undulation plate 15 so as to enter the polarization beam splitter 13 again. The reflected light beam is reflected at a right angle by the polarization beam splitter 13 based on the polarizing direction so as to enter a received ray system 16.
A condenser lens 17 in the received ray system 16 condenses the reflected light beam into the center of a polarization optical element 18. The reflected light beam, which is convergent light, incident in the polarization optical element 18 is converted into diffused light at the center of the polarization optical element 18 so as to emit from the polarization optical element 18. At this time, the polarization optical element 18 changes the polarization direction of only the stray light component included in the reflected light beam, as will be described later in detail.
The reflected light beam emitted from the polarization optical element 18 is collimated by a lens 19 so as to enter a polarization beam splitter 20. The polarization beam splitter 20 separates the focused light component from the stray light component included in the reflected light beam based on the respective polarization directions. That is, the polarization beam splitter 20 makes the focused light component included in the reflected light beam proceed straight based on its polarization direction, while makes the stray light component, which is changed in its polarization direction by the polarization optical element 18, reflect at a right angle and enter a condenser lens 24 based on its polarization direction.
The focused light proceeding straight through the polarization beam splitter 20 is condensed by a condenser lens 21 and is focused on a photo detector for detecting a signal 23 via a cylindrical lens 22. Then, the photo detector for detecting a signal 23 produces various detecting signals in accordance with the amount of received focused light so as to feed them to the signal processor 4 (
The signal processor 4 produces a reproducing signal, a focus error signal, a tracking error signal, and a spherical aberration correcting signal, based on the various detecting signals supplied from the photo detector for detecting a signal 23 so as to output the reproducing signal to an external instrument via the control unit, and to output the focus error signal, the tracking error signal, and the spherical aberration correcting signal to the drive unit 3 (
On the other hand, the stray light reflected from the polarization beam splitter 20 is condensed by the condenser lens 24 and is focused on a photo detector for detecting stray light 25. Then, the photo detector for detecting stray light 25 produces a stray light detecting signal in accordance with the amount of stray light so as to supply it to the signal processor 4 (
The signal processor 4 determines the number of layers of the optical disc 100 based on the stray light detecting signal supplied from the photo detector for detecting stray light 25 so as to inform the control unit 2 of the number of layers of the optical disc 100. Then, the control unit 2 regulates the laser power of the optical pickup 7 and the spherical aberration correction value in accordance with the number of layers of the optical disc 100.
Next, the computation processing on the various detection signals produced in the photo detector for detecting a signal 23 will be described. Means for obtaining a focal-point error signal FES herein employs an astigmatic method and means for obtaining a tracking error signal TES herein employs a phase contrast method. Alternatively, it is obvious that other methods, such as a knife-edge method and a spot-size method, may incorporate a focal-point error signal method and various methods, such as a push-pull method, a three-beam method, and a differential push-pull method, may incorporate a tracking error signal detecting method.
As shown in
Hence, by computing the signals A to D according to the following equation (1), a focal-point error signal FES can be produced that exhibits a so-called S-shaped waveform in which the level is zero during focusing and the level changes in ± directions during non focusing:
FES=(A+C)−(B+D) (1).
The optical disc apparatus 1 according to the embodiment corresponds to a three-layered BD-ROM disc as a multi-layered information recording medium. From a reproduction-only optical disc having information pit columns formed in advance like the BD-ROM disc, a tracking error signal TES is produced by the phase contrast method according to the following equation (2):
TES=φ(A+C)−φ(B+D) (2),
where φ denotes an operator of a signal phase.
The reproducing signal RFS is also produced by adding the output signals A to D of the entire light-receiving regions 23a to 23d according to the following equation (3):
FES=A+B+C+D (3).
Then, the configuration of the polarization optical element 18 and the separation of stray light from focused light will be described in detail.
The small prisms 18a and 18b and the small prisms 18d and 18e are respectively bonded together with an optical material, such as an adhesive transparent to the wavelength of laser light, a dielectric thin film, or a metallic thin film having absorbency, therebetween. Thereby, between the small prisms 18a and 18b and between the small prisms 18d and 18e, boundary surfaces 18x and 18y made of the above-mentioned optical material are formed, respectively. The refractive index of the optical material forming the boundary surfaces 18x and 18y is designated by n1.
The small prism 18c is bonded to the small prisms 18a and 18b and to the small prisms 18d and 18e with the optical material, such as the adhesive transparent to the wavelength of laser light, the dielectric thin film, or the metallic thin film having absorbency, therebetween. This optical material suppresses the reflection index during transmission by selecting its refractive index n2 as close to the refractive index ng of the five small prisms 18a to 18e as possible.
As described above, the polarization optical element 18 is positioned so that the center of the small prism 18c agrees with the focal point of the reflected light beam condensed by the condenser lens 17 while the boundary surfaces 18x and 18y are positioned backward and forward the focal point of the reflected light beam on a plane including the optical axis of the reflected light beam.
According to the embodiment, the NA of the objective lens 9 is 0.85; the NA of the condenser lens 17 is 0.1; and signal layers of the three-layered BD-ROM disc are sequentially called as an L0 layer, an L1 layer, and an L2 layer from the side remote from the objective lens. In
As described above, the light beam reflected by the L1 layer i.e., the focused light, is substantially collimated by the objective lens 9, and after being condensed at the center of the polarization optical element 18, the focused light beam is converted into diffused light.
The focused light beam at this time, as shown in the solid lines of
Whereas, the stray light comes in contact with the boundary surface 18x or 18y during passing through the polarization optical element 18. Referring to
As described above, since this stray light enters the condenser lens 17 as the convergent light, its focal point due to the condenser lens 17 is located at a position nearer than the center of the polarization optical element 18. Thereby, the stray light incident in the polarization optical element 18 is emitted from the polarization optical element 18 after once contacting with the boundary surface 18x, and at this time, the boundary surface 18x reflects, transmits, or absorbs the stray light.
Although not shown in
Since the absorption due to the boundary surface 18x or 18y made of a metallic thin film is large in this case, the light transmitting through the boundary surface 18x or 18y scarcely exists and the reflection and the absorption are mainly generated.
That is, when the incident light angle in the boundary surface 18x or 18y is small, the light reflected from the boundary surface 18x or 18y passes through the polarization beam splitter 20 so as to enter the photo detector for detecting a signal 23. Whereas, as the incident light angle increases, the phase shift is generated in the reflected light to change the polarization direction, so that the amount of light reflected by the polarization beam splitter 20 and entering the photo detector for detecting stray light 25 increases while the amount of light entering the photo detector for detecting a signal 23 decreases. In particular, when the reflection angle is 85° or more, almost whole quantity of the light enters the photo detector for detecting stray light 25.
On the other hand,
In this case, differently from the case where the boundary surfaces 18x and 18y are made of a metallic thin film (
According to the embodiment, the numerical aperture of the condenser lens 17 is 0.1. Under this condition, the angle between the most outside light beam and the optical axis is about 6°, and the angle within the polarization optical element 18 is 4° or less because of the light refraction on the boundary plane between air and the optical material. Hence, the incident angle of the light beam in the boundary surfaces 18x and 18y of the polarization optical element 18 becomes 86° or more, so that according to the calculated results shown in
The stray light generated in the L0 layer on the deeper side and in the L2 layer on the nearer side when the L1 layer is the in-focus layer has been described as above. However, the stray light generated in the L1 layer and in the L2 layer on the nearer side when the L0 layer is the in-focus layer as well as the stray light generated in the L1 layer and in the L0 layer on the deeper side when the L2 layer is the in-focus layer can be separated from the focused light in the same way.
Then, the configuration of the photo detector for detecting stray light 25 and the method for determining the number of layers of the optical disc 100 by the photo detector for detecting stray light 25 will be described.
In the photo detector for detecting stray light 25, the light receiving plane is positioned at a position optically equivalent to that of the light receiving plane of the photo detector for detecting a signal 23. That is, when the focused light is assumed to be reflected by the polarization beam splitter 20 to enter the photo detector for detecting stray light 25, as shown by the solid lines of
As described above, since the focused light does not enter the photo detector for detecting stray light 25, if the incident light cannot be detected by the photo detector for detecting stray light 25, the mounted optical disc 100 is determined to be a monolayer optical disc.
The size of the stray light spot formed on the light receiving plane of the photo detector for detecting stray light 25 substantially changes in proportion to the space between a non in-focus layer and an in-focus layer, so that the layer space and the number of layers of the optical disc 100 can be determined based on the spot size and the received light amount on the light receiving plane of the photo detector for detecting stray light 25.
As shown in
The light receiving region 25aa of the photo detector for detecting stray light 25 is positioned so that its center substantially agrees with the center of the stray light condensed by the condenser lens 24. The light receiving regions 25bb1 and 25bb2 are point-symmetrically arranged with each other about the center of the light receiving region 25aa. Furthermore, the light receiving regions 25cc1 and 25cc2 are arranged outside the light receiving regions 25bb1 and 25bb2, respectively, as well as point-symmetrically with each other about the center of the light receiving region 25aa. Thereby, the light receiving regions 25aa, 25bb1, 25bb2, 25cc1, and 25cc2 are linearly arranged along the straight line passing the center of the stray light condensed by the condenser lens 24.
In this case, since the light amount received by the respective light receiving regions 25aa, 25bb1, 25bb2, 25cc1, and 25cc2 is substantially the same, if the following equation (4) is satisfied, the optical disc 100 is determined to be a two-layered optical disc.
AA=BB1=BB2=CC1=CC2>0 (4)
On the other hand,
In this case, the spot SP1 and the spot SP2 enter the light receiving regions 25aa, 25bb1, and 25bb2 while only the spot SP2 enters the light receiving regions 25cc1 and 25cc2, so that if the following equation (5) is satisfied, the optical disc 100 is determined to be a three-layered optical disc.
AA=BB1=BB2>CC1=CC2>0 (5)
In this case, the spot SP1, the spot SP2, and the spot SP3 enter the light receiving region 25aa, the spot SP2 and the spot SP3 enter the light receiving regions 25bb1 and 25bb2, and only the spot SP3 enters the light receiving regions 25cc1 and 25cc2, so that if the following equation (6) is satisfied, the optical disc 100 is determined to be a four-layered optical disc.
AA>BB1=BB2>CC1=CC2>0 (6)
Since when the optical disc 100 is a monolayer optical disc, the stray light is not generated, if the following equation (7) is satisfied, the optical disc 100 is determined to be a monolayer optical disc.
AA=BB1=BB2=CC1=CC2=0 (7)
When surface reflected light reflected from the surface of the optical disc 100 and other unnecessary light enter the photo detector for detecting stray light 25, an appropriate threshold value t may be set in consideration of the amount of the unnecessary light entering the light receiving regions 25aa, 25bb1, 25bb2, 25cc1, and 25cc2.
That is, if the following equation (4′) is satisfied, the optical disc 100 is determined to be a two-layered optical disc.
AA=BB1=BB2=CC1=CC2>t (4′)
If the following equation (5′) is satisfied, the optical disc 100 is determined to be a three-layered optical disc.
AA=BB1=BB2>CC1=CC2>t (5′)
If the following equation (6′) is satisfied, the optical disc 100 is determined to be a four-layered optical disc.
AA>BB1=BB2>CC1=CC2>t (6′)
If the following equation (7′) is satisfied, the optical disc 100 is determined to be a monolayer optical disc.
AA=BB1=BB2=CC1=CC2≦t (7′)
The signal processor 4 (
In the optical pickup 7 configured as above, the light beam reflected from the optical disc 100 is condensed by the condenser lens 17 so as to enter the polarization optical element 18.
The polarization optical element 18 is provided with the boundary surfaces 18x and 18y positioned backward and forward the focal point of the reflected light beam on a plane including the optical axis of the reflected light beam and spaced by a predetermined distance. The focal point of the stray light reflected by the non in-focus recording layer is positioned backward or forward the focal point of the focused light. Thereby, the focused light passes through the polarization optical element 18 without contacting with the boundary surface 18x or 18y whereas, the stray light comes in contact with the boundary surface 18x or 18y.
Thereby, the polarization optical element 18 reflects only the stray light included in the reflected light beam by the boundary surface 18x or 18y so as to change its polarization direction, so that in the subsequent stage of the polarization beam splitter 20, the stray light is separated from the focused light. Then, only the focused light is emitted to the photo detector for detecting a signal 23 while only the stray light is emitted to the photo detector for detecting stray light 25.
Then, the optical pickup 7 determines the number of layers of the optical disc 100 from the shape of the stray light spot formed on the light receiving plane of the photo detector for detecting stray light 25 on the basis of the stray light detection signals AA, BB1, BB2, CC1, and CC2 indicating the amount of the stray light received by the photo detector for detecting stray light 25.
By the configurations described above, the polarization optical element 18 changes the polarization direction of only the stray light component in the reflected light beam, so that the polarization beam splitter 20 separates the focused light from the stray light so as to emit only the stray light to the photo detector for detecting stray light 25. Thereby, the determination of the number of layers of the optical disc 100 based on the amount of the stray light can be executed more securely than in the related art.
According to the embodiment described above, the optical disc apparatus 1 corresponding to the optical disc 100 having four recording layers and incorporating the invention has been described. However, the present invention is not limited to the embodiment, so that the present invention may be widely incorporated in an optical disc apparatus corresponding to an optical disc having a plurality of recording layers, such as an optical disc having 2 or 3 recording layers and an optical disc having 5 or more recording layers.
According to the embodiment described above, the optical disc apparatus 1 corresponding to Blu-ray Disc™ and incorporating the invention has been described. However, the present invention is not limited to this, so that the present invention may be widely incorporated in various optical discs, such as DVD and CD.
According to the embodiment described above, the photo detector for detecting stray light 25 is provided with the five rectangular light receiving regions 25aa, 25bb1, 25bb2, 25cc1, and 25cc2 formed with the same area. However, the present invention is not limited to this, so that other various numbers of light receiving regions with other various shapes may be provided in the photo detector for detecting stray light 25.
For example,
On the other hand,
In the photo detector for detecting stray light 25 having such light receiving regions arranged in a concentric configuration, since the light receiving regions 25x, 25y, and 25z have respectively different light receiving areas, when determining the number of layers in the signal processor 4 (
Furthermore, according to the embodiment described above, the optical pickup of the optical disc apparatus 1 incorporating the invention has been described; the invention is not limited to this, so that other stray light removing elements configured in various ways may be incorporated in the invention. That is, a stray light removing element 30 may not be assembled in the optical pickup 7 and the optical pickup 7 may not be assembled in the optical disc apparatus 1.
The embodiments of the present invention may be broadly applied to an optical disc apparatus having a multi-layered optical disc.
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 |
|---|---|---|---|
| JP2006-157634 | Jun 2006 | JP | national |
