JP5487769B2 - Optical pickup device - Google Patents

Description

  The present invention relates to an optical pickup device used for recording and reproducing information signals on an optical disc.

  In recent years, due to the explosive increase in the amount of information accompanying the spread of terrestrial digital broadcasting and the like, large capacity optical discs are required. Following CD (Compact Disk) and DVD (Digital Versatile Disk), larger capacity BDs are required. (Blu-ray Disk) is becoming popular. In addition, for the purpose of further increasing the capacity, research and development of a multilayer disk in which recording layers are stacked and the capacity is increased several times as much as the number of stacked layers are being actively conducted.

  2. Description of the Related Art A recording / reproducing apparatus that records and reproduces information signals on an optical disc is provided with an optical pickup device that irradiates the optical disc with laser light. In the optical pickup device, the light beam emitted from the laser light source is condensed on the recording layer of the optical disk by the objective lens, and the light beam reflected by the recording layer is incident on the light receiving element.

  When recording or reproducing an information signal with respect to a multi-layer disc by an optical pickup device, a reflected light beam from a recording layer other than the target layer which is a recording layer on which recording or reproduction is performed becomes crosstalk light, which is the target. The signal may fluctuate due to interference with the signal light that is a reflected light beam from the layer, and the servo signal may be deteriorated. In order to avoid this inconvenience, the dual-layer discs that are currently popular mainly use one servo signal beam and devise hologram division.

  In an optical pickup apparatus for BD using an objective lens having a high NA (numerical aperture), means for correcting spherical aberration due to a difference in spacer thickness between recording layers of BD is required. There is a limit to the amount of correction, and considering the simplification and miniaturization of the overall configuration of the optical pickup device, it is desirable to reduce the amount of correction of spherical aberration even in a multilayer disk. For this reason, in an optical disc having three or more recording layers, it is necessary to narrow the interval between the recording layers with respect to the two-layer disc. However, if the interval between the recording layers is narrowed, the defocus difference between the signal light and the crosstalk light becomes small. Therefore, it is difficult to avoid the influence of the crosstalk light by using one beam and dividing the hologram as described above. Become.

  Therefore, it has been proposed to install a dedicated element for removing crosstalk light in the optical pickup device. For example, in the optical pickup device disclosed in Patent Document 1, an element having a mask region that diffracts or blocks only a region overlapping with a light receiving element of crosstalk light, and a light beam reflected by an optical disk is received by the light receiving element. It is arranged in the optical path of the return path until it is incident on.

  In Patent Document 2, a pinhole that allows only signal light to pass through is provided in the vicinity of the focus of the signal light in the expander optical system provided in the optical path of the return path, or the crosstalk light is crossed at the focal position of the crosstalk light. An optical pickup device in which a mask for shielding only talk light is arranged is disclosed.

  Further, in the optical pickup device described in Patent Document 3, a prism 100 as shown in FIG. 14 is provided near the focal point of the signal light in the optical system in which the signal light is focused in the expander optical system provided in the optical path of the return path. It is installed. The prism 100 includes a transmission film 101 and absorption films 102 and 103 parallel to the optical axis direction. The transmissive film 101 is disposed in a region including the signal light focal point 104 and transmits light. The absorption films 102 and 103 are disposed adjacent to both sides of the transmission film 101 and absorb light.

  In the optical pickup device described in Patent Document 3 in which such a prism 100 is installed, the crosstalk light is focused on the absorption films 102 and 103, and the crosstalk light is absorbed by the absorption films 102 and 103. Is removed.

JP 2005-203090 A Special table 2007-511023 gazette JP 2007-141357 A

  In the above-mentioned Patent Document 1, since the mask area is arranged at a position where there is no difference in the light beam diameter between the signal light and the crosstalk light in the substantially parallel light beam, the mask area becomes larger than the light beam diameter of the signal light. , The loss of signal light increases. Such a loss of signal light is particularly noticeable when the recording layer that reflects the crosstalk light is close to the recording layer that reflects the signal light, and thus becomes larger as the recording layer of the optical disc becomes multilayered.

  Further, in Patent Document 2, when crosstalk light is removed by providing a pinhole that allows only signal light to pass through, coherent noise cannot be removed. When a mask that blocks only the crosstalk light is arranged, 2 (N-1) masks are required for the N-layer disk because the mask is arranged at the focal point of the crosstalk light.

  For this reason, especially in the case of dealing with an optical disk having three or more layers, the configuration of an element including a mask becomes complicated. Further, as the number of optical discs increases, the total thickness between the recording layer (surface layer) on the outermost surface and the recording layer (lowermost layer) on the innermost side of the optical disc becomes thicker. Since the focal point of the crosstalk light from the lower layer and the focal point of the crosstalk light from the surface layer when the lowermost layer is the target layer are separated from the focal position of the signal light in the expander optical system, the entire element including the mask is large. Become.

  Further, in the optical pickup device disclosed in Patent Document 3, if the multi-layer disc is to be used, the cross-talk light is separated from the focal position of the signal light in the multi-layer disc as described above. Need to expand to. This increases the size of the prism 100 and makes it difficult to manufacture the prism 100.

  The present invention has been made in view of the above, and an object of the present invention is to provide an optical pickup device that can efficiently remove crosstalk light even with respect to a multilayer disk with a simple configuration.

In order to achieve the above object, an optical pickup device according to the present invention includes an objective lens that makes a light beam incident on an optical disk having three or more recording layers, and a reflected light beam that is a light beam reflected by the optical disk. A light receiving element that is focused through an optical lens and then incident through a second condenser lens, and the signal light that is a reflected light beam from a recording layer on which an information signal is recorded or reproduced on the optical disc. The reflected light beam passing through the first condenser lens is disposed at a position of the signal light focal point that is a focal point of the first condenser lens so that a reflection surface is substantially perpendicular to the optical axis of the reflected light flux. The information mirror is recorded or reproduced by reducing the amount of light passing through a predetermined area including the reflecting mirror and the reflecting surface of the mirror and including the optical axis of the reflected light beam. And a mask to reduce the multiple crosstalk light is reflected light beam from the recording layer other than the recording layer being made, the reflective surface is a position of the signal light focus and the plurality of cross is disposed at a position different from the plurality of crosstalk light focus a focal by the first condenser lens talk light, said mask is a position different from the signal light focus point, and the plurality of cross Of the talk light focus, between the first crosstalk light focus closest to the signal light focus and the second crosstalk light focus farthest from the signal light focus, the first and second crosses. It is disposed at a position different from the talk light focus, and is disposed at a position where there is a difference in beam diameter between the signal light and the plurality of crosstalk lights.

  According to the present invention, it is possible to provide an optical pickup device that can efficiently remove crosstalk light even for a multilayer disk with a simple configuration.

1 is a schematic configuration diagram of an optical pickup device according to an embodiment of the present invention. It is a schematic block diagram which shows an example of the optical pick-up apparatus which has an expander optical system. It is a figure which shows the focus in the expander optical system in case crosstalk light and signal light are mixed. It is sectional drawing which shows schematic structure of the optical filter part in the optical pick-up apparatus shown in FIG. It is a figure which shows the crosstalk light which generate | occur | produces in a 2 layer disc. It is a figure which shows the positional relationship of the mask in a optical filter part, the focus of crosstalk light, and the focus of signal light. It is a figure for demonstrating the method of calculating the magnitude | size equivalent to the radius of a mask, and the distance of a mask and the focus of signal light. It is a figure which shows the structural example which provided the mask in the quarter wavelength plate. It is a figure which shows the other structural example which provided the mask in the quarter wavelength plate. It is a figure which shows a mode that the light reception cell of the light receiving element and crosstalk light in an Example overlap. It is a figure which shows the cross-section of the optical disk which the optical pick-up apparatus based on an example assumed correspondence. It is a figure which shows the crosstalk light generate | occur | produced with the optical disk shown in FIG. It is a figure which shows the positional relationship of the focus of the signal light in the expander optical system in an Example, a mask, and the focus of crosstalk light. It is a perspective view which shows an example of the prism which removes crosstalk light with the conventional optical pick-up apparatus.

  Embodiments of the present invention will be described below with reference to the drawings. Throughout the drawings, the same or equivalent parts and components are denoted by the same or equivalent reference numerals, and the description thereof is omitted or simplified.

  FIG. 1 is a schematic configuration diagram of an optical pickup device according to an embodiment of the present invention. As shown in FIG. 1, the optical pickup device 1 according to the present embodiment includes a light source 2, a polarizing beam splitter 3, a condenser lens 4, a quarter-wave plate 5, a rising mirror 6, An objective lens 7, a quarter-wave plate 8, an optical filter unit 9, a mirror 10, a condenser lens 11, and a light receiving element 12 are provided.

  In the optical pickup device 1, the laser light emitted from the light source 2 is reflected by the optical disc 13, which is a multilayer disc having a plurality of recording layers, and finally received by the light receiving element 12. The light path from the light source 2 until it enters the optical disk 13 is defined as the forward path, and the light path from the light reflected from the optical disk 13 to the light receiving element 12 is defined as the return path.

  As shown in FIG. 1, a polarizing beam splitter 3, a condenser lens 4, a quarter-wave plate 5, a rising mirror 6, and an objective lens 7 are sequentially arranged on the forward path from the light source 2 to the optical disk 13. .

  The light source 2 is composed of a semiconductor laser or the like, and emits laser light having a predetermined wavelength toward one light transmitting surface 3 a of the four light transmitting surfaces 3 a to 3 d of the polarization beam splitter 3.

  The polarization beam splitter 3 has a cubic or prismatic shape, transmits a forward light beam incident on the light transmission surface 3a from the light source 2, and directs the light transmission surface 3b facing the light transmission surface 3a toward the condenser lens 4. And inject. The polarization beam splitter 3 reflects the returning light beam incident on the light transmitting surface 3b from the condensing lens 4 by the polarizing film 3a and emits the light from the light transmitting surface 3c perpendicular to the light transmitting surfaces 3a and 3b. After being reflected at 10, the light beam in the return path that is incident again from the light-transmitting surface 3 c is transmitted through the quarter-wave plate 8 and is emitted from the light-transmitting surface 3 d facing the light-transmitting surface 3 c.

  In the present embodiment, the light source 2 emits an S-polarized light beam, and the polarizing film 3a transmits an S-polarized light beam and reflects a P-polarized light beam.

  The condensing lens 4 converts the forward light beam that has passed through the polarizing beam splitter 3 into substantially parallel light and makes it incident on the quarter-wave plate 5, and polarizes the return light beam incident from the quarter-wave plate 5. The light is condensed on the reflection surface 10 a of the mirror 10 via the beam splitter 3.

  The quarter-wave plate 5 converts the forward light beam from S-polarized light to circularly-polarized light, and converts the return light beam from circularly-polarized light to P-polarized light. The rising mirror 6 bends the optical path of the outgoing light beam from the quarter-wave plate 5 to be incident on the objective lens 7, and bends the optical path of the returning light beam that is transmitted through the objective lens 7 and is divided into 4 minutes. The first wavelength plate 5 is made incident.

  The quarter-wave plate 8, the optical filter unit 9, and the mirror 10 are sequentially arranged on the light transmitting surface 3 c side of the polarizing beam splitter 3.

  The quarter-wave plate 8 converts the P-polarized return light beam incident from the polarization beam splitter 3 into circularly-polarized light, and converts the P-polarized return light beam incident after being reflected by the mirror 10 into S-polarized light. Convert.

  The optical filter unit 9 and the mirror 10 are integrated, the reflecting surface 10a of the mirror 10 is substantially perpendicular to the optical axis of the light beam on the return path, and the recording layer on which the optical disk 13 is recorded or reproduced. The focal point O of the signal light, which is a reflected light beam from the target layer, by the condenser lens 4 is disposed on the reflecting surface 10a.

  The optical filter unit 9 filters the crosstalk light included in the light flux on the return path. The mirror 10 reflects the return light beam incident from the quarter-wave plate 8 substantially perpendicularly to the reflecting surface 10a.

  The condenser lens 11 and the light receiving element 12 are sequentially arranged on the light transmitting surface 3 d side of the polarization beam splitter 3.

  The condensing lens 11 makes the return light beam that has passed through the polarization beam splitter 3 converged light and enters the light receiving element 12. The light receiving element 12 converts the incident light into an electric signal and outputs the electric signal to an external arithmetic processing circuit.

  In the optical pickup device 1, the condenser lenses 4 and 11 constitute an expander optical system having one focal point O in the middle.

  Here, the expander optical system generally refers to an optical system that receives parallel light and emits parallel light, such as the condenser lenses 24 and 25 of the optical pickup device 20 shown in FIG.

  In the optical pickup device 20, the S-polarized light beam emitted from the light source 21 passes through the polarization beam splitter 22, becomes substantially parallel light by the condenser lens 23, and becomes convergent light by the condenser lens 24 and is collected at the focal point O. After being emitted, it becomes divergent light again and becomes substantially parallel light by the condenser lens 25. The light beam that has passed through the condensing lens 25 passes through the quarter-wave plate 26 and is converted into circularly polarized light, is incident on the objective lens 28 via the rising mirror 27, and is incident on the target layer of the optical disk 30 by the objective lens 28. And is reflected by the optical disc 30.

  The light beam reflected by the optical disk 30 passes through the objective lens 28, enters the quarter-wave plate 26 through the rising mirror 27, is converted to P-polarized light, and is condensed at the focal point O ′ by the condenser lens 25. After that, it becomes divergent light again and enters the condenser lens 24, and becomes substantially parallel light by the condenser lens 24. The light beam that has passed through the condenser lens 24 becomes convergent light by the condenser lens 23, is reflected by the polarizing film 22 a of the polarizing beam splitter 22, and enters the light receiving element 29.

  In the present embodiment, it is assumed that the magnification of the expander optical system is 1, the condensing lenses 23 and 24 in FIG. 2 are shared, and the condensing lens 11 in FIG. The condenser lens 4 to the condenser lens 11 are referred to as an expander optical system as described above.

  FIG. 3 is a diagram illustrating a state near the focal point O on the return path in the expander optical system when crosstalk light and signal light are mixed. Here, the signal light is a reflected light beam from a target layer that is a recording layer on which an information signal is recorded or reproduced on the optical disc 30, and the crosstalk light is a reflected light beam from a recording layer other than the target layer. is there.

  Due to the arrangement of the mirror 10 as described above, the signal light is collected at the focal point O on the reflection surface 10 a of the mirror 10. On the other hand, the crosstalk light is focused on a focal point P at a position different from the focal point O because the reflection position on the optical disc 13 is defocused. That is, the expander optical system plays a role of spatially separating the focus O of the signal light and the focus P of the crosstalk light.

  Here, when the distance between the focal points O and P is Δ and the defocus amount (air conversion) of the optical disk 13 is δ, the following (Equation 1) is established in the paraxial region. However, the ratio Me is the ratio between the focal length of the condenser lens 4 and the focal length of the objective lens 7.

2 × δ × Me ² = Δ (Formula 1)
FIG. 4 is a cross-sectional view illustrating a schematic configuration of the optical filter unit 9. As shown in FIG. 4, the optical filter unit 9 is formed in a rectangular parallelepiped shape with a material such as glass that transmits light, and one of the opposing planes 9 a and 9 b that is substantially perpendicular to the optical axis of the light flux in the return path. The mirror 10 is formed by mirror processing. The optical filter unit 9 is disposed such that the planes 9a and 9b and the reflection surface 10a of the mirror 10 are substantially perpendicular to the optical axis of the incident light beam on the return path, and the focal point O is positioned on the reflection surface 10a.

  On the other flat surface 9 b of the optical filter unit 9, a mask 14 for reducing the amount of light passing through a predetermined region including the optical axis of the light flux on the return path is disposed. As shown in FIG. 3, since the focal point O of the signal light and the focal point P of the crosstalk light are spatially separated, the mask 14 is located at a place where there is a difference in the beam diameter between the signal light and the crosstalk light. Will be placed.

  The mask 14 has a transmittance of 0%, or a transmittance that attenuates the crosstalk light to the extent that there is no problem even if it enters the light receiving element 12. The mask 14 includes, for example, a binary diffraction grating whose depth is set to the maximum diffraction efficiency, a mirror structure using a metal thin film, a diffusion structure by ground glass processing, and a refractive structure processed into a wedge shape, but is not limited thereto. However, care must be taken so that stray light generated by diffraction, refraction, reflection, diffusion, etc. in the mask 14 does not enter the light receiving element 12.

  The distance dm (in air) between the focal point O and the mask 14 in the optical axis direction of the incident return light beam, and the size hm corresponding to the radius of the mask 14 set the light receiving cell of the light receiving element 12 to the position of the focal point O. It is determined based on the effective size he of the light receiving element, which is a size corresponding to the radius when projected, the defocus amount of crosstalk light that can be generated by the optical disc 13, and the like.

  The light receiving element effective size he shown in FIG. 4 will be described. The light receiving element 12 has a light receiving cell (not shown) for detecting the amount of light, and its size is represented by he ′ corresponding to a radius around the optical axis. When the ratio of the distance from the condensing lens 11 to the focal point O and the distance from the light receiving element 12 to the condensing lens 11 is a magnification M, the light receiving element effective size he in the expander optical system is as follows (Formula 2): It is represented by In other words, the light receiving element effective size he is equivalent to the effective size when the light receiving cell size he ′ is projected onto the focal point O in the expander optical system.

he = M × he ′ (Formula 2)
Next, the range of the defocus amount of crosstalk light that can be generated by the optical disc 13 will be described. FIG. 5 is a diagram showing crosstalk light when the optical disc 13 is a two-layer disc having two recording layers of the L0 layer and the L1 layer.

  In FIG. 5, the recording layer on the back side of the optical disc 13 is the L0 layer, the recording layer on the front side is the L1 layer, and the interlayer spacing is t0. FIG. 5 (a) shows the L0 layer, and FIG. 5 (b) shows the L1 layer. Signal light and crosstalk light in the case of a target layer are shown.

  As shown in FIG. 5A, the crosstalk light from the L1 layer when the L0 layer is the target layer is defocused from the L0 layer to the surface side (minus side) of the optical disc 13, and the defocus amount δ = t0 / n. Here, n is the refractive index of the optical disc 13.

  Further, as shown in FIG. 5B, the crosstalk light from the L0 layer when the L1 layer is the target layer is defocused from the L1 layer to the back side (plus side) of the optical disc 13, and the defocused light is defocused. The focus amount δ = t0 / n.

  When the optical disc 13 is a multi-layer disc having three or more layers, there can be a plurality of crosstalk lights as shown in FIGS. 5A and 5B. Therefore, the defocus amount δ of the crosstalk light in the optical disc 13 Is expressed as the following (Equation 3). The range of the amount of defocus is the same on the front side (minus side) and the back side (plus side) of the optical disc 13.

tmin / n ≦ δ ≦ tmax / n (Formula 3)
here,
tmin: Minimum value of the distance in the thickness direction of the optical disk 13 between the reflection position of the crosstalk light and the reflection position of the signal light in the optical disk 13 tmax: The reflection position of the crosstalk light in the optical disk 13 and the signal light This is the maximum value of the distance in the thickness direction of the optical disc 13 between the reflection position.

  In the above (Equation 3), tmin and tmax are values determined by the interval between the recording layers of the optical disc 13, and n is a value determined by the material of the spacer between the recording layers (such as polycarbonate). The range of the defocus amount δ of the crosstalk light that can be generated is determined by the design value of the optical disc 13.

  The crosstalk light defocused to the plus side is filtered by the mask 14 when entering the optical filter unit 9 and the mirror 10. The crosstalk light defocused to the minus side is filtered by the mask 14 when it is emitted from the optical filter unit 9 after being reflected by the reflecting surface 10 a of the mirror 10.

  Next, how to determine the size hm corresponding to the radius of the mask 14 and the distance dm between the mask 14 and the focal point O will be described with reference to FIGS.

  FIG. 6 is a diagram showing a positional relationship between the mask 14 in the optical filter unit 9 and the focal point P and the focal point O of the crosstalk light defocused to the plus side filtered by the mask 14, and FIG. FIG. 6B shows a state in which the focal point P is closest to the focal point O, FIG. 6B shows a state in which the focal point P is closest to the focal point O and a state farthest from the focal point O, and FIG. It is a figure which shows the state where the focus P is the farthest from the focus O. The size hm corresponding to the radius of the mask 14 is made smaller than the effective light receiving element size he.

  FIG. 6A shows a state in which when the focal point P becomes closer to the focal point O, the crosstalk light beam leaks from the outside of the mask 14 to the light receiving cell of the light receiving element 12. This corresponds to crosstalk light having the smallest defocus amount among talk light. At this time, the position of the mask 14 is farther from the focal point O of the signal light than the focal point P of the crosstalk light. For this reason, compared with the case where the mask 14 is arrange | positioned in the position of the focus P, it becomes advantageous with respect to crosstalk light of a small defocus amount.

  On the other hand, FIG. 6C shows a state in which a point on the outermost periphery of the light receiving cell of the light receiving element 12 projected onto the focal point O coincides with an extension of a straight line connecting the focal point P and a point on the outer periphery of the mask 14. It is. When the focal point P is further away from the focal point O, the light flux leaks from the outside of the mask 14 to the light receiving cell of the light receiving element 12. This state corresponds to the crosstalk light having the largest defocus amount among the crosstalk light filtered by the mask 14. At this time, the position of the mask 14 is closer to the focal point O of the signal light than the focal point P of the crosstalk light. For this reason, compared with the case where the mask 14 is arranged at the position of the focal point P, the optical filter unit 9 can be made small, which is advantageous for simplification of the configuration of the optical pickup device 1.

  As described above, the range of the distance Δ between the focal point P and the focal point O of the crosstalk light that can be completely filtered by the mask 14 is set to minimize Δ1 shown in FIG. 7A and maximize Δ2 shown in FIG. To do. FIG. 7A shows a state in which the distance between the focal point P and the focal point O of the crosstalk light having the smallest defocus amount is minimized when all the generated crosstalk light can be completely filtered by the mask 14. The schematic diagram shown in FIG. 7B shows the maximum distance between the focal point P and the focal point O of the crosstalk light having the largest defocus amount when all the possible crosstalk light can be completely filtered by the mask 14. It is a schematic diagram which shows the state which becomes.

  Δ1 in FIG. 7A is expressed by the following (formula 4) from the similar relationship between ΔPOQ and ΔPSR.

Δ1 = he × dm / (he + hm) (Formula 4)
Δ2 in FIG. 7B is expressed by the following (formula 5) from the similar relationship between ΔPOQ and ΔRTQ.

Δ2 = he × dm / (he−hm) (Formula 5)
Here, the distance Δmin between the focal point P and the focal point O of the crosstalk light having the smallest defocus amount among the generated crosstalk light, and the focal point of the crosstalk light having the largest defocus amount among the possible crosstalk light. Assuming that the distance Δmax between P and the focal point O, Δmin and Δmax are expressed by the following (Expression 6) and (Expression 7) from (Expression 1) and (Expression 3).

Δmin = 2 × (tmin) × Me ² / n (Expression 6)
Δmax = 2 × (tmax) × Me ² / n (Expression 7)
In order to filter all generated crosstalk light with the mask 14, it is necessary to satisfy Δmin ≧ Δ1 and Δmax ≦ Δ2. The following (Expression 8) and (Expression 9) are obtained from the relations of (Expression 4) to (Expression 7) and Δmin ≧ Δ1 and Δmax ≦ Δ2.

2 × (tmin) × Me ² / n ≧ he × dm / (he + hm) (Equation 8)
2 × (tmax) × Me ² / n ≦ he × dm / (he−hm) (Equation 9)
The light flux on the return path passes through the mask 14 and then enters the reflecting surface 10a of the mirror 10 substantially perpendicularly and is reflected, travels in the same optical path in the opposite direction, and passes through the mask 14 again. Therefore, the mask 14 that satisfies the above (Expression 8) and (Expression 9) can filter both the crosstalk light defocused to the plus side and the crosstalk light defocused to the minus side.

  As described above, since the defocus amount of the crosstalk light that can be generated is determined from the design value of the optical disk 13, hm, within a range that satisfies (Expression 8) and (Expression 9), depending on the optical disk 13 to be used. dm is determined, and the optical filter unit 9 is configured.

  Note that, for a crosstalk light component having a very large defocus amount, the light beam may be sufficiently diffused and the amount of light may be weak. It is also conceivable to combine the optical filter unit 9 with other crosstalk light countermeasure methods. Therefore, it is possible to limit tmin and tmax as necessary.

  Further, the mask 14 may be designed to be a little larger to ensure a margin.

  Next, the operation of the optical pickup device 1 will be described.

  When the light source 2 emits laser light, the emitted S-polarized light beam passes through the polarization beam splitter 3 and becomes substantially parallel light by the condenser lens 4 and passes through the quarter-wave plate 5 and becomes circularly polarized light. The light is converted, enters the objective lens 7 through the rising mirror 6, and is focused on the target layer of the optical disk 13 by the objective lens 7.

  The light incident on the optical disc 13 is reflected by the optical disc 13. The reflected light flux on the return path includes signal light and crosstalk light.

  Of the light flux on the return path, the signal light flux passes through the objective lens 7, enters the quarter-wave plate 5 through the rising mirror 6, is converted to P-polarized light, and becomes convergent light by the condenser lens 4. After being reflected by the polarizing film 31 of the polarizing beam splitter 3 and converted into circularly polarized light by the quarter-wave plate 8, it passes through the optical filter unit 9 and is condensed at the focal point O on the reflecting surface 10 a of the mirror 10. Is done.

  The light flux of the signal light is reflected by the reflecting surface 10a of the mirror 10, becomes divergent light, passes again through the quarter-wave plate 8 and is converted to S-polarized light, and then passes through the polarization beam splitter 3. The light is converged by the condenser lens 11 and enters the light receiving element 12.

  Of the return beam, the crosstalk beam enters the optical filter unit 9 through the same path as the signal beam, passes through the mask 14 and is reflected on the reflection surface 10a of the mirror 10, and then enters. The same optical path travels in the opposite direction and passes through the mask 14 again. When the light beam of the crosstalk light enters the optical filter unit 9, the crosstalk light defocused to the plus side is filtered by the mask 14. Then, after the crosstalk light beam is reflected by the reflecting surface 10a of the mirror 10 and then passes through the mask 14, the crosstalk light defocused to the minus side is filtered. As a result, the crosstalk light is prevented from entering the light receiving cell of the light receiving element 12.

  As described above, according to the first embodiment, by providing the optical filter unit 9 having the mask 14 whose size and arrangement position are determined according to the design value of the optical disc 13, the optical disc 13 has three or more layers. Even in the case of a multi-layer disc, crosstalk light can be efficiently removed with a simple configuration. Crosstalk light focused on the objective lens 7 side relative to the focal point O of the signal light (crosstalk light defocused on the plus side) and crosstalk light focused on the light receiving element 12 side (defocused on the minus side) Crosstalk light) can be filtered with the same mask 14, which is advantageous for simplification and miniaturization of the optical filter unit 9.

  The optical filter unit 9 is easy to design even when it is compatible with filtering of crosstalk light with a small defocus amount, and the increase in size is suppressed even when it is compatible with filtering of crosstalk light with a large defocus amount.

  In addition, since the mask 14 is disposed at a location where there is a difference between the beam diameters of the signal light and the crosstalk light, the size of the mask 14 can be kept small, and loss of the signal light due to the mask 14 can be reduced.

  Note that the mask 14 is only required to be arranged at a distance of dm from the focal point O and having a size of hm corresponding to the radius, and the mask 14 is formed on the optical filter unit 9 integrated with the mirror 10 as shown in FIG. The configuration may not be provided.

  For example, as shown in FIG. 8, the mirror 10 may be a single unit and a mask 14 may be provided on the quarter-wave plate 8. Moreover, as shown in FIG. 9, the structure which integrated the mirror 10 and the quarter wavelength plate 8 which provided the mask 14 may be integrated, and although a illustration is abbreviate | omitted, the mirror 10 and the quarter wavelength plate are omitted. 8 and the mask 14 may be provided separately.

  Next, specific examples of the above-described embodiment of the present invention will be described.

  The optical pickup device of the present embodiment has the same overall configuration as the optical pickup device 1 shown in FIG. 1, and will be described with reference to FIG. However, in this embodiment, the optical pickup device 1 includes a diffraction element (not shown) that divides laser light from the light source 2 into three beams in the outward optical system, and the light receiving element 12 has three beams. The three-beam DPP (Differential Push Pull) method is applied to convert the light received by the three light receiving cells into electrical signals and detect the tracking error signal from the differential calculation. Shall.

  The constants of the optical system are: NA of the objective lens 7 is 0.85, and the focal lengths of the objective lens 7 and the condenser lenses 11 and 4 are 1.8 mm, 9 mm, and 18 mm, respectively. The magnification Me = 10, which is the ratio of the focal lengths of the two. The magnification M = 1, which is the ratio of the distance from the condenser lens 11 to the focal point O and the distance from the light receiving element 12 to the condenser lens 11. The light source 2 emits laser light having a wavelength of 405 nm corresponding to BD.

  FIG. 10 shows how the light receiving cells 121 to 123 of the light receiving element 12 overlap with the crosstalk light. In the present embodiment, since the three-beam DPP method is applied as described above, the cell range is different in the TAN direction that is the direction along the recording track of the optical disc 13 and the RAD direction perpendicular thereto.

  Therefore, in order to minimize the loss of signal light, the mask 14 has a rectangular shape that is long in the TAN direction. As shown in FIG. 10, the size of the projection of the mask 14 on the light receiving element 2 is 300 μm in the TAN direction and 100 μm in the RAD direction including the margin, and he_tan ′ = 150 μm as the distance corresponding to the radius from the optical axis. , He_rad ′ = 50 μm. Here, “_tan” is a symbol indicating the TAN direction and “_rad” is a symbol indicating the RAD direction, and will be omitted when discussing the TAN direction and the RAD direction together.

  In this embodiment, since M = 1, the light receiving element effective size he_tan in the TAN direction and the light receiving element effective size he_rad in the RAD direction at the focal point O in the expander optical system are the same as those on the light receiving element 12. Thus, he_tan = he_tan ′ = 150 μm and he_rad = he_rad ′ = 50 μm.

  FIG. 11 shows a cross-sectional structure of an optical disc 13 that is a three-layer disc assumed to be supported by the optical pickup device 1 according to the present embodiment. As shown in FIG. 11, the lowermost layer, the intermediate layer, and the outermost layer of the optical disc 13 are the L0 layer, the L1 layer, and the L2 layer, respectively, and the spacer thickness t0 = 15 μm between the L0 layer and the L1 layer, The thickness of the spacer between the L2 layer was t1 = 10 μm, and the refractive index n of the optical disk 13 was 1.6.

  Next, crosstalk light that can be generated in the optical disc 13 will be described with reference to FIG.

  When the L0 layer is the target layer, as shown in FIG. 12A, crosstalk lights C1 and C2 reflected by the L1 layer and the L2 layer are generated. When the L1 layer is the target layer, as shown in FIG. 12B, crosstalk lights C3 and C4 reflected by the L0 layer and the L2 layer are generated. When the L2 layer is the target layer, as shown in FIG. 12C, crosstalk lights C5 and C6 reflected by the L0 layer and the L2 layer are generated.

Table 1 summarizes the distance td (μm) in the thickness direction of the optical disc 13 between the reflection positions of the crosstalk lights C1 to C6 and the reflection positions of the signal lights in FIGS.

  In Table 1, the crosstalk layer indicates a recording layer in which crosstalk light is reflected. Further, a value corresponding to the crosstalk light reflected by the recording layer on the back side of the optical disc 13 with respect to the target layer is added with a plus sign, and a value corresponding to the crosstalk light reflected by the recording layer on the surface side is added with a minus sign. Is written. For example, for the crosstalk light C1 reflected by the L1 layer when the target layer is the L0 layer, td = t0 = 15 μm, and the reflection position of the crosstalk light C1 is closer to the surface side of the optical disc 13 than the target layer. Therefore, in Table 1, it is written as -15.

  From Table 1, in the present embodiment, the values of tmax and tmin in the above (Equation 3) are as follows.

tmax = 25 μm
tmin = 10 μm
Here, tmax corresponds to the crosstalk light C2 and C5, and tmin corresponds to the crosstalk light C4 and C6, respectively.

  The design value of the mask 14 can be obtained by substituting the values obtained above into (Equation 8) and (Equation 9). In this embodiment, since masks having different lengths are used in the RAD direction and the TAN direction, the following (Expression 10), (Expression 11), and TAN obtained from (Expression 8) and (Expression 9) in the RAD direction. The direction is designed by the following (Expression 12) and (Expression 13) obtained from (Expression 8) and (Expression 9).

2 × 10 × 10 ² /1.6≧50×dm/(50+hm_rad) (Equation 10)
2 × 25 × 10 ² /1.6≦50×dm/(50−hm_rad) (Equation 11)
2 × 10 × 10 ² /1.6≧150×dm/(150+hm_tan) (Formula 12)
2 × 25 × 10 ² /1.6≦150×dm/(150−hm_tan) (Formula 13)
In this example, hm_rad = 23 μm, hm_tan = 69 μm, and dm = 1.8 mm were selected as values satisfying the ranges of (Expression 10) to (Expression 13).

  FIG. 13 is a diagram illustrating a positional relationship between the focal point O of the signal light, the mask 14, and the focal point of the crosstalk light in the expander optical system in the present embodiment. In FIG. 13, (Lsi, Lcj) indicates the position of the focal point of the crosstalk light from the Lj layer when the Li layer is the target layer (i ≠ j). A thick dotted line indicates signal light, and a thin dotted line indicates crosstalk light.

  Temporarily, if it is the structure which arrange | positions a mask in the focus of crosstalk light and removes crosstalk light, the range of (Ls2, Lc0)-(Ls2, Lc1) ((Ls1, Lc2)-(Ls0, Lc2)) All of them need masking. On the other hand, in this embodiment, since all the crosstalk light can be removed by arranging the mask only at one place in the application range of the mask 14 shown in FIG. 13, the configuration of the optical filter unit 9 is simplified. Miniaturization is possible.

  The optical filter unit 9 in this embodiment has the configuration shown in FIG. 4, and the thickness of the optical filter unit 9 (the distance between the mask 14 and the reflecting surface 10 a of the mirror 10) is the optical filter unit 9. In consideration of the refractive index of the material (such as glass), dm = 1.8 mm is satisfied.

  Further, the mask 14 is an aluminum thin film of 46 μm × 138 μm based on hm_rad = 23 μm and hm_tan = 69 μm, and the light incident on the mask 14 is reflected 100%. It is assumed that the reflected light does not reach the light receiving cell of the light receiving element 12. The mask 14 as described above can be easily realized by a known method, for example, etching is performed after forming a resist pattern by photolithography.

  As described above, in this embodiment, it is possible to prevent the crosstalk light in the three-layer disc from entering the light receiving element 12. Further, even with the three-beam DPP method, a stable crosstalk light removing action can be obtained.

  In this embodiment, a case where a three-layer disk is used is shown. However, for other multilayer disks, the values of tmin and tmax are obtained in the same manner as in this embodiment, and the mask 14 may be designed using these values. In an N-layer disc (N ≧ 2), when the outermost layer is LN-1 layer, the lowermost layer is L0 layer, and the spacer thickness ti is between Li layer and Li + 1 layer (0 ≦ i ≦ N-2), The values of tmin and tmax are calculated by the following (Expression 14) and (Expression 15). However, Min (ti) is the minimum value of ti.

tmin = Min (ti) (Expression 14)
tmax = Σ (ti) (Equation 15)

DESCRIPTION OF SYMBOLS 1 Optical pick-up apparatus 2 Light source 3 Polarizing beam splitter 4,11 Condensing lens 5,8 Quarter wave plate 6 Rising mirror 7 Objective lens 9 Optical filter part 10 Mirror
10a Reflecting surface 12 Light receiving element 13 Optical disc 14 Mask

Claims (2)

An objective lens for making a light beam incident on an optical disc having three or more recording layers;
A light receiving element that causes a reflected light beam, which is a light beam reflected by the optical disc, to be incident through a second condenser lens after being focused by the first condenser lens;
The reflection surface of the reflected light beam is reflected at the position of the signal light focus, which is the focal point of the first condenser lens, of the signal light that is the reflected light beam from the recording layer on which the information signal of the optical disk is recorded or reproduced. A mirror that is disposed substantially perpendicular to the optical axis and reflects the reflected light flux that has passed through the first condenser lens;
Other than the recording layer on which the information signal is recorded or reproduced on the optical disc by reducing the amount of light passing through a predetermined area including the optical axis of the reflected light beam, which is disposed substantially parallel to the reflecting surface of the mirror. A mask that reduces a plurality of crosstalk light beams reflected from each recording layer, and
The reflecting surface is a position of the signal light focus, and is disposed at a position different from the plurality of crosstalk light focus a focal by the first condenser lens of the plurality of crosstalk light,
The mask, the a position different from the signal light focus point, and wherein the plurality of crosstalk light focus, farthest from the signal light focus and the nearest first crosstalk light focus the signal light focus It is located between the second crosstalk light focus and at a position different from the first and second crosstalk light focal points, and has a beam diameter of the signal light and the plurality of crosstalk lights. An optical pickup device, wherein the optical pickup device is arranged at a position where there is a difference. The dm and hm satisfy the following (Expression 1) and (Expression 2), where dm is the distance between the signal light focus and the mask, and hm is the size corresponding to the radius of the mask. Item 4. The optical pickup device according to Item 1.
2 × (tmin) × Me ² / n ≧ he × dm / (he + hm) (Formula 1)
2 × (tmax) × Me ² / n ≦ he × dm / (he−hm) (Formula 2)
However,
Me: Ratio of the focal length of the first condenser lens and the focal length of the objective lens n: Refractive index of the optical disc he: Radius when the light receiving portion of the light receiving element is projected at the position of the signal light focal point Considerable size tmin: minimum value of distance in the thickness direction of the optical disc between the reflection position of the crosstalk light and the reflection position of the signal light in the optical disc tmax: the reflection position of the crosstalk light in the optical disc Value of the distance in the thickness direction of the optical disc between the reflection position of the signal light and the signal light

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