JP4893314B2 - Optical pickup device - Google Patents

Description

  The present invention relates to an optical pickup device, and more particularly to an optical pickup device that includes a diffractive optical element between an objective lens and a photodetector and realizes good recording or reproduction on a multilayer optical disc as well as a single-layer optical disc.

  In general, an optical disk, which is a disk-shaped optical recording medium, records information signals such as video information, audio information, and computer data on a transparent substrate in a spiral or concentric pattern on a high density, and It is often used because a desired track can be accessed at a high speed when a recorded track is reproduced.

  For example, CD (Compact Disc) and DVD (Digital Versatile Disc) are already on the market as this type of optical disc, and recently, two types of high-density optical recording media with higher density are in circulation. That is, BD (Blu-ray Disc) and HD-DVD (High Definition DVD).

  In addition, in order to increase the capacity of each optical disc, development and standardization of a multilayer optical disc having a signal recording surface in multiple layers is progressing. DVD-ROM and DVD-R single-sided dual-layer optical discs are already commercially available, and DVD-RW, BD, and HD-DVD are also being standardized for single-sided dual-layer optical discs. In addition, recent conferences have announced the development of BD 4-layer discs and 8-layer optical discs.

  By the way, in order to record or reproduce the single-sided double-layer optical disc described above, a DVD reproduction type pickup apparatus detects a tracking error signal (DPD signal) by using a one-beam DPD method (phase difference tracking method). . With respect to DVD-R and DVD-RW, which are recordable optical disks, the DPD signal can be detected after recording the signal, but cannot be detected before the signal is recorded. Therefore, the tracking error signal (DPP signal) is detected by using the three-beam DPP method (differential push-pull method) for the DVD recording type optical disc.

  In an HD-DVD that is recorded or reproduced using a blue laser light source having a wavelength of about 405 nm, the same tracking error signal detection as that of the DVD is used for each of the reproduction type and the recording type. On the other hand, in the BD, which is another standard for recording or reproducing using a blue laser light source having a wavelength of about 405 nm, the DPD signal can be detected by detecting the tracking error signal of the BD-ROM. In this state, detection of the DPD signal is not guaranteed as a standard. Therefore, it is common to detect the DPP signal in both the recording type and the reproducing type.

  Here, when the DPP method is used for a single-sided dual-layer optical disc, reflected light from another layer that has not been reproduced (defocused layer) enters the photodetector as crosstalk light and is reproduced. There is a problem in that coherent noise is generated by overlapping with the signal detection light of the layer. The behavior of this coherent noise is determined by the optical path difference between two interfering lights, that is, the optical path difference due to the interlayer distance of the optical disk. Furthermore, it behaves extremely complicated under the influence of fluctuations in the interlayer distance depending on the position of the optical disc, the tilt state of the optical disc, and the guide track (groove) for recording being reproduced.

  When the DPP signal is used, the light amount of the sub beam is set smaller than that of the main beam in order to prevent erroneous recording due to the sub beam at the time of recording and to use light efficiently. For this reason, the above problem arises because the sub beam is more greatly affected by the crosstalk light from the defocused layer than the main light.

  The influence of the crosstalk light has been sometimes caused by a double-layer optical disc of DVD, but it has been reported that a serious problem that the recording and reproduction cannot be performed more remarkably occurs in the BD system (for example, Non-patent document 1). The reason is as follows. First, the interlayer distance is narrower than that of DVD, the optical path difference is reduced, and the size of crosstalk light is increased. Secondly, in the blue laser light source having a wavelength of 405 nm, the laser coherence is improved. Third, in the BD, the numerical aperture (NA) of the objective lens is increased, so that crosstalk light also returns to the photodetector more greatly.

  Therefore, as a method for solving these problems, a tracking error signal detection method using a PP method (push-pull method) using a one-beam method has been proposed (for example, see Non-Patent Document 2).

  11 is a configuration diagram of an example of a detection system of a conventional optical pickup device described in Non-Patent Document 2, and FIG. 12 is a diagram of a HOE pattern (far field pattern) used in the detection system of the conventional optical pickup device described in Non-Patent Document 2. FIG. 13 is a diagram showing an example, and FIG. 13 is a diagram showing an example of the spread of crosstalk light in the conventional optical pickup device described in Non-Patent Document 2.

  The operation of the conventional optical pickup apparatus until the reflected light reflected from the optical disk returns to the PD (light detector) and is received will be briefly described with reference to FIGS. In FIG. 11, in detecting the focus error signal, the reflected light from the optical disk is transmitted by the hologram optical element (HOE) 101 without diffracting action, becomes convergent light through the lens 102, and passes through the cylindrical lens 103 to be astigmatic. A signal given aberration and received by a focusing PD (light detector) 104 is used.

  On the other hand, for the detection of the tracking error signal, the reflected light from the optical disk is diffracted by the HOE 101, passes through the lens 102 and becomes convergent light, and the astigmatism caused by the HOE 101 is canceled by the cylindrical lens 103, and tracking PD (light detection) The signal received by the device 105 is used.

  FIG. 12 shows a pattern of the HOE 101, and the HOE 101 is divided into five areas A, B, C, D, and X. FIG. 13 shows the state of crosstalk light in the focusing PD 104 and tracking PD 105. The light passing through the region A of the HOE 101 by the reflected light from the reproducing layer is light receiving part 105A of the PD 105, the light passing through the region B of the HOE 101 is light passing through the light receiving part 105B of the PD 105, and the region C of the HOE 101 is PD 105. The light that passes through the region D of the light receiving unit 105C and the HOE 101 forms a spot at the light receiving unit 105D of the PD 105. Note that crosstalk light from a layer that is not reproduced (other layer) is a gray portion.

  Of crosstalk light from other layers, light that is not diffracted by the HOE 101 (0th order diffracted light) is applied to the focusing PD 104 but is not applied to the tracking PD 105. Since the light is not diffracted, a spot can be formed at the same position regardless of the region of the HOE 101.

  On the other hand, among crosstalk light reflected from other layers, the spot position of light diffracted by the HOE 101 (diffracted light) differs depending on the region through which the HOE 101 passes. The diffracted light of the crosstalk light that has passed through the region X is positioned on the left and right of the focusing PD 104 and does not reach the tracking PD 105. Further, the diffracted light of the crosstalk light that has passed through the regions A, B, C, and D is positioned around the tracking PD 105 and does not reach the tracking PD 105. The tracking error signal APP is obtained when TA is a signal obtained by the light receiving unit 105A, TB is a signal obtained by the light receiving unit 105B, TC is a signal obtained by the light receiving unit 105C, and TD is a signal obtained by the light receiving unit 105D. Is calculated by the following formula.

APP = (TC-TD) -k (TA-TB) (1)
In the above formula, k is a constant.

  As another method for solving the problem of crosstalk light, a tracking error signal detection method using a PP method using a one-beam method has been proposed as in Non-Patent Document 2 (for example, see Non-Patent Document 3). FIG. 14 is a structural diagram of an example of a liquid crystal element used in the detection system of the conventional optical pickup device described in Non-Patent Document 3, and FIG. 15 is a diagram showing a beam collected in the light receiving cell of the conventional optical pickup device described in Non-Patent Document 3. It is the figure which showed the state which shines.

  The operation of the conventional optical pickup device described in Non-Patent Document 3 until the reflected light reflected by the optical disk is detected using FIGS. 14 and 15 will be briefly described. The non-polarized HOE 111, the liquid crystal active rotator 112, and the polarized HOE 113 are formed in this order from the side shown.

  The non-polarized HOE 111 has a lens effect, generates ± first-order diffracted light, and detects a focus error signal. The liquid crystal active rotator 112 changes the polarization direction according to voltage on (ON) and off (OFF). The polarization HOE 113 includes a recording pattern 113A and a reproduction pattern 113B.

  When a recording type tracking error signal is detected, as shown in FIG. 15A, the voltage is turned off by the liquid crystal active rotator 112 and the polarization direction is changed to a direction perpendicular to the incident polarization direction. Then, the pattern 113A for recording type is made to act on the polarization HOE 113, and the tracking error signal is detected using the PP method similar to Non-Patent Document 2. The central region E of the recording pattern 113A is excluded from the calculation of the tracking error signal to reduce the influence of crosstalk light from other layers.

  On the other hand, when detecting a regenerative tracking error signal, as shown in FIG. 15B, the voltage is turned on by the liquid crystal active rotator 112 to make the incident polarization direction and the outgoing polarization direction the same. Then, the reproduction-type pattern 113B of the polarization HOE 113 is applied, and the tracking error signal is detected using the DPD method.

Alexander van der Lee, et.al., "Drive consideration for multilayer discs", ISOM06 Technical Digest P.30 Mo-C-5 Kousei SANO, et.al., “Novel One-Beam Tracking Detection Method for Dual-Layer Blu-ray Discs”, Japanese Journal of Applied Physics Vol. 45, No. 2B, 2006, pp. 1174-1177 (Fig. 4 FIG. 6 and FIG. 7) Noriaki Nishi et.al., “Novel One-Beam Detection Method with Changeable Multi Division Patterns”, Proc. Of SPIE Vol.6282,62821H-1 (Figs. 4 and 5)

  According to the conventional optical pickup device described in Non-Patent Document 2 described with reference to FIGS. 11 to 13, the tracking error signal is stabilized so that the tracking PD 105 is not affected by the crosstalk light. However, since the focus PD 104, the tracking PD 105, and the detection beam are divided into two and the signal obtained from the focus PD 104 cannot be used for the detection of the main signal, the signal output is reduced, and the S / N of the signal detection is reduced. Decreases. In particular, this is a significant problem when performing high-speed playback.

  In addition, according to the conventional optical pickup device described in Non-Patent Document 3 described with reference to FIGS. 14 and 15, it is possible to detect the recording type and the reproduction type tracking error signal while reducing the influence of the crosstalk light. However, since there are many expensive parts such as the polarization HOE 113 of the two patterns 113A and 113B and the liquid crystal active rotator 112, there is a problem that the entire apparatus is expensive.

  The present invention has been made in view of the above points, and an object of the present invention is to provide an optical pickup device that has a good signal S / N and can be realized at low cost when recording or reproducing a multilayer optical disk.

In order to achieve the above object, a first invention includes a light source that emits light, an objective lens that collects light emitted from the light source on an optical disc to form a spot, and an optical disc that passes through the objective lens. A diffractive optical element that diffracts reflected light and emits it in a predetermined direction, a photodetector that receives and photoelectrically converts the light emitted from the diffractive optical element, and an optical disk that emits light from the light source and travels toward the objective lens An optical path separation element that separates the reflected light that is reflected from the light, passes through the objective lens, and travels toward the photodetector ,
The diffractive optical element has a first dividing line that divides the reflected light beam in a direction perpendicular to the direction of the track when the track of the optical disk is projected, and a direction opposite to the direction parallel to the direction of the track. Are divided into at least six divided areas by the second and third dividing lines that divide the reflected light beam in approximately 45 directions, and each of the six divided areas changes the direction by diffracting incident reflected light. The reflected light is given astigmatism by at least two or more of the divided regions of the diffractive optical element, or by a cylindrical lens,
The photodetector has at least six light receiving cells that individually receive the first-order diffracted light as signal detection light emitted through the six divided regions of the diffractive optical element, and each spot received by the six light receiving cells. Is arranged at a position where the circle of least confusion is approximately halfway between the two focal lines generated by the diffracted light to which astigmatism is applied, and includes the same direction as the track direction in at least six divided regions. The two light receiving cells that receive the first-order diffracted light emitted through the two divided regions are each divided into two by a dividing line in a direction perpendicular to the track direction ,
The six light receiving cells are, when the optical disc is a multilayer optical disc having a plurality of recording layers in the thickness direction, reflected by the recording layer to be reproduced and diffracted by the diffractive optical element among the plurality of recording layers. The crosstalk light, which is the first-order diffracted light, is not irradiated on any of the six light receiving cells and is reflected by the recording layer that is not reproduced and diffracted by the diffractive optical element. Each of the light receiving cells is arranged at a position where it is not irradiated .

  In this invention, the reflected light from the optical disk obtained by one beam is incident on a diffractive optical element having at least six divided regions, is diffracted in each divided region, changes its direction, and has astigmatism. And at least six light receiving cells of the photodetector respectively receive the light, thereby generating a tracking error signal and a focus error signal by the astigmatism method from the output signals of the at least six light receiving cells. .

Here, in the second invention, each divided region of the diffractive optical element in the first invention has a lens function, and each divided region of the diffractive optical element and each light receiving cell of the photodetector corresponding thereto. and correcting the difference in distance between.

  According to a third aspect of the present invention, each of the divided regions of the diffractive optical element according to the first invention has a lens function, and each divided region of the diffractive optical element and each light receiving cell of the photodetector corresponding thereto And a difference between the imaging magnification from the light source to the optical disc and the imaging magnification from the optical disc to the photodetector are corrected.

According to a fourth aspect of the present invention, in order to achieve the above object, two divided areas including the same direction as the track direction are divided into the first divided area and the second divided area among the six divided areas of the diffractive optical element. When the remaining four divided areas are divided into imaginary lines that pass in the same direction as the track direction and pass through the center of the reflected light, the two divided areas belonging to one same side are defined as the divided areas. When the divided area and the fourth divided area are used, and the other two divided areas belonging to the same side are the fifth divided area and the sixth divided area, the third divided area and the fourth divided area receiving the output signals a and B of the two light receiving cells of the photodetector for receiving diffracted first-order diffracted light, respectively, the fifth divided regions and the sixth primary diffracted light diffracted by the division regions of the respective Output signals C and C of the other two light receiving cells of the photodetector By using the D, following equation
PP = (A + B)-(C + D)
And tracking error signal generation means for generating a tracking error signal PP. In the present invention, the tracking error signal can be generated by the push-pull method.

In order to achieve the above object, according to a fifth aspect of the present invention, two divided areas including the same direction as the track direction among the six divided areas of the diffractive optical element are divided into the first divided area and the second divided area. When the remaining four divided areas are divided into imaginary lines that pass in the same direction as the track direction and pass through the center of the reflected light, the two divided areas belonging to one same side are defined as the divided areas. When the divided area and the fourth divided area are used, and the other two divided areas belonging to the same side are the fifth divided area and the sixth divided area, the photodetector has the first, second, and second divided areas. 3, 4, comprises first, second, third, fourth, fifth and sixth light receiving cells individually receiving the first-order diffracted light diffracted respectively by the fifth and sixth split areas, In addition, the first light receiving cell and the second light receiving cell are divided in a direction orthogonal to the track direction. In divided into two parts, respectively,
Third divisional regions and the third and the output signals A and B of the fourth light receiving cells fourth divided regions by diffracted first-order diffracted light received respectively, the fifth divided regions and the sixth divided areas fifth and sixth output signals C and D of the light receiving cells, each divided into two in which the first and second same direction as the direction of the light receiving cell track for receiving the first-order diffracted light diffracted respectively by When divided by virtual lines, the output signals E1 and F1 of the two divided light receiving cell portions of the first and second light receiving cells belonging to one same side, and the first and second light receiving cells belonging to the other same side Using the output signals E2 and F2 of the remaining two divided light receiving cell sections, the following expression APP = (A + B) − (C + D) −k {(E1 + F1) − (E2 + F2)}
(Where k is a constant)
And a tracking error signal generating means for generating a tracking error signal APP. In the present invention, the tracking error signal can be generated by the new push-pull method.

In order to achieve the above object, according to a sixth aspect of the present invention, two divided regions including the same direction as the track direction among the six divided regions of the diffractive optical element are divided into the first divided region and the second divided region. when the divided region, the photodetector, the first divided region and a second first-order diffracted light are being respectively applied astigmatism diffracted by the division regions, or the first divided region and a second divided region is diffracted respectively by the and first has at least a light receiving cell and the second light receiving cells individually receiving the first-order diffracted light granted astigmatism by the cylindrical lens, the a first light receiving cell The two light receiving cells are each divided into two by dividing lines in a direction perpendicular to the track direction,
When the first and second light receiving cells are divided by virtual lines in the same direction as the track direction, the output signals E1 and F1 of the two divided light receiving cell portions of the first and second light receiving cells belonging to one same side And the output signals E2 and F2 of the remaining two divided light receiving cells of the first and second light receiving cells belonging to the other side, the following equation FE = (E1 + F2) − (E2 + F1)
And a focus error signal generating means for generating a focus error signal FE.

  According to the present invention, the fourth to sixth diffractive optical elements through which a light beam that interferes with the zero-order light and the first-order light generated by diffracting at the tracking groove of the recording optical disk or the pit in the track direction of the reproducing optical disk pass. Focus using the output signals of the first light receiving cell and the second light receiving cell that individually receive the diffracted light that has been provided with astigmatism by the first divided region and the second divided region other than the first divided region. An error signal FE can be generated.

  According to the first invention, the reflected light from the optical disk obtained by one beam is incident on a diffractive optical element having at least six divided regions, and diffraction is provided with astigmatism according to each divided region. Output light of at least six light receiving cells by emitting light or adding astigmatism to the diffracted light diffracted in each divided region and receiving them by at least six light receiving cells of the photodetector. A tracking error signal can be generated from the signal, and particularly when the optical disc is a multilayer optical disc, the at least six light receiving cells are arranged so as not to receive crosstalk light from other recording layers other than the recording layer to be reproduced. Therefore, it is possible to generate a stable tracking error signal with a good signal S / N ratio.

  According to the first invention, the function of the cylindrical lens can be incorporated into the diffractive optical element by emitting the diffracted light with astigmatism in each divided region of the diffractive optical element. As a result, the number of parts can be reduced, and the number of light receiving cells on the photodetector can be reduced compared to the conventional non-patent document 2, so that the circuit scale is small, the noise is small, and the cost is low. Since it is a 1-beam system, there is no light loss due to the grating as compared with the 3-beam system, no grating adjustment is required, and it is possible to easily cope with optical disks with different track pitches and optical systems with different systems.

  Further, according to the second invention, the lens power is given to each divided region of the diffractive optical element, whereby the degree of freedom can be given to the arrangement of the light receiving cells. According to the third invention, the imaging magnification is increased. Therefore, the arrangement of the light source and the light detector does not need to be conjugate, so that the arrangement can be given a degree of freedom.

  According to the fourth invention, the tracking error signal PP can be generated by a general push-pull method, and according to the fifth invention, the tracking error signal APP can be generated by a new push-pull method. Since no tracking error signal APP is offset at the recording boundary, stable tracking can be applied. The influence of overlapping of diffracted light at the center of the pupil can be reduced by the divided area of the diffractive optical element and the calculation method. Furthermore, a general DPD signal can be obtained as a tracking error signal as well as an APP signal.

  Furthermore, according to the sixth invention, a diffractive optical element through which light of interference between the zeroth order light and the first order light diffracted by a tracking groove of a recordable optical disc or a pit in the track direction of a reproduction type optical disc passes. Astigmatism is imparted to the light passing through the first divided region and the second divided region other than the third to sixth divided regions, and the first light receiving cell and the second light receiving cell individually receive each diffracted light. Since the focus error signal FE is generated using the output signal of the light receiving cell, there is little change in the amount of light when traversing the tracking groove, etc., and fluctuation of the focus error signal can be reduced. A focus error signal can be generated by the astigmatism method having high affinity with the processor (FEP).

  Next, an embodiment of the present invention will be described in detail with reference to FIGS. FIG. 1 is an overall configuration diagram of an embodiment of an optical pickup device according to the present invention. In FIG. 1, an optical pickup device 1 uses two laser recording layers having a substrate thickness t of 0.075 mm and 0.1 mm using laser light LS from a laser light source 2 that emits laser light LS having a wavelength λ of 405 nm. This is a device for recording or reproducing a Blu-Ray standard single-sided dual-layer optical disc (BD) 11 having a signal surface 11B.

  In the present embodiment, the objective lens 7 is optimally designed to focus the laser light at the middle of the signal surfaces of the two recording layers, that is, 0.0875 mm. By moving the collimator lens 4 in the optical path direction, When recording or reproducing the first recording layer of t = 0.075 mm, slightly converged light, and when recording or reproducing the second recording layer of t = 0.1 mm, recording or reproducing as slightly divergent light Can play. Here, a case where the recording signal of the first recording layer having the substrate thickness t = 0.075 mm is reproduced will be described.

  The laser light LS emitted from the laser light source 2 is p-polarized linearly polarized light, and is transmitted through the polarization-selective dielectric multilayer film 3A (p-polarized light: transmissive, s-polarized light: reflected) of the polarizing beam splitter (optical path separating element) 3. Thereafter, the light is slightly converged by the collimator lens 4, turned 90 ° by the reflective film 5 A of the rising mirror 5, and given a phase difference of λ / 4 by the quarter-wave plate 6, becomes circularly polarized light and becomes objective The light enters the lens 7. The beam obtained by focusing with the objective lens 7 is made incident from the beam incident surface 11A of the optical disk 11, and collected with good aberration on the signal surface 11B of the first recording layer having the substrate thickness t = 0.075 mm. Light is recorded or reproduced.

  Then, the laser light (reflected light) LT reflected by the signal surface 11B of the optical disk 11 is incident on the objective lens 7 again, becomes slightly divergent light by the objective lens 11, and enters the quarter wavelength plate 6. A phase difference of λ / 4 is given, and the light is converted into s-polarized linearly polarized light, turned 90 ° by the reflecting film 5 A of the rising mirror 5, converged by the collimator lens 4, and incident on the polarizing beam splitter 3.

  The s-polarized reflected light incident on the polarizing beam splitter 3 is reflected by the polarizing dielectric multilayer film 3A, incident on the diffractive optical element 8, diffracted by the HOE surface 8Z, and given astigmatism by the cylindrical lens 9. After being received, the light is received by the six divided light receiving cells 10A to 10F described later of the photodetector 10. The photodetector 10 photoelectrically converts the light received by the light receiving cells 10A to 10F, and then calculates a tracking error signal, a focus error signal, and a main data signal according to predetermined arithmetic expressions described later for each electric signal. calculate.

  Next, a detection system that is a main part of the present embodiment will be described with reference to FIGS. 2 is a diagram showing in detail the detection system of the optical pickup device 1, FIG. 3 is a diagram showing the divided regions 8A to 8F of the HOE surface 8Z, and FIG. 4 is a light receiving cell 10A to 10F of the photodetector 10. It is the figure which showed arrangement | positioning. In FIG. 2, the light beam reflected by the polarization beam splitter 3 and incident on the diffractive optical element 8 is diffracted by a hologram pattern composed of six divided regions 8 </ b> A to 8 </ b> F on the HOE surface 8 </ b> Z of the diffractive optical element 8.

  As shown in FIG. 3, the above six divided areas are divided into lines 8k that divide a circular light incident area into two in the direction (radial direction) perpendicular to the direction of the track when the track of the optical disk 11 is projected. Is included. The dividing lines 8i and 8j are inserted in the direction of ± 45 ° with respect to the dividing line 8k through the center of the circular light incident area. By these three dividing lines 8k, 8i, and 8j, the circular light incident area is divided into six divided areas 8A to 8F, and each of the divided areas has a hologram pattern having a different diffraction structure. The diffractive structure is formed in a blazed shape or stepped shape, and is designed so that the intensity of the first-order diffracted light is 70% or more, and six first-order diffracted lights corresponding to the six divided regions 8A to 8F are generated. In the state where there is no lens shift or lens tilt at the objective lens 7, the light flux center of the reflected light from the optical disk 11 passes through almost the center of the circular light incident area.

  Thereafter, the light from the diffractive optical element 8 is given astigmatism by the cylindrical lens 9 in FIG. 2 and then condensed as a spot on the corresponding light receiving cells 10A to 10F of the photodetector 10. Here, the light receiving cells 10A to 10F are arranged as shown in FIG. 4 according to the diffraction directions of the divided regions 8A to 8F of the diffractive optical element 8, and the first-order diffracted light from the divided region 8A is received. Similarly, the first order diffracted light from the divided region 8B is received by the light receiving cell 10B, the first order diffracted light from the divided region 8C is received by the light receiving cell 10C, and the first order diffracted light from the divided region 8D is received by the light receiving cell. 10D, the first-order diffracted light from the divided region 8E is received by the light receiving cell 10E (10E1 and 10E2), and the first-order diffracted light from the divided region 8F is received by the light receiving cell 10F (10F1 and 10F2). . The center of the light incident area of the HOE surface 8Z and the center of the photodetector 10 are substantially aligned with the optical axis direction.

  Each spot on each of the light receiving cells 10A to 10F has a fan-shaped shape corresponding to the divided area, but is suitable when it is considered that there is no divided area and a circular hologram pattern is a circular spot of the least circle of confusion. It has second-order astigmatism and other aberrations. And it becomes a circle of minimum confusion almost halfway between two focal lines generated by third-order astigmatism. It is desirable that the third-order astigmatism is in the 45 ° direction due to the shape of the light receiving cell.

  That is, in the optical pickup device of the present embodiment, since the hologram pattern itself can have components such as aberrations and powers, the incident light is incident on the divided regions 8A to 8F of the HOE surface 8Z of the diffractive optical element 8. It is possible to integrate the function of the cylindrical lens with the hologram pattern so as to give astigmatism in the direction of approximately 45 °. Further, two divided areas 8E and 8F correspond to a focus error signal described later. When the function of the cylindrical lens is integrated with the hologram pattern, the two divided areas are at least astigmatized. Provide a function to add aberration. In consideration of the case where the optical pickup device is adjusted by a servo signal such as a focus error signal or a tracking error signal, it is desirable that all divided regions have astigmatism in the same direction. Then, the photodetector 10 is arranged at a position where each spot condensed on the light receiving cell of the photodetector 10 becomes a minimum circle of confusion between the two focal points generated by the diffracted light to which astigmatism is given. Is done.

  FIG. 5 shows an example of a spot at the light receiving cell in the light beam diffracted by one circular hologram pattern. FIG. 5A shows a case where the spot on the light receiving cell becomes a minimum circle of confusion when there is no divided region and a single hologram pattern is formed. This spot is free from coma and spherical aberration and has only a large third-order astigmatism in the 45 ° direction. This is determined by a combination of a hologram pattern and a cylindrical lens. Since the hologram pattern itself can have components such as various aberrations and powers, in this embodiment (FIG. 5), the third-order astigmatism in the 45 ° direction is provided so that the function of the cylindrical lens is integrated with the hologram pattern. ing.

  Further, each of the divided regions 8A to 8F of the HOE surface 8Z in FIG. 3 has a lens action (lens power). Here, the distances between the divided regions 8A to 8F of the HOE surface 8Z in FIG. 3 and the corresponding light receiving cells 10A to 10F in FIG. 4 are slightly different as is clear from the drawing. Therefore, it is desirable that the hologram patterns in the divided regions 8A to 8F have a minimum confusion circle with the same spot diameter on the light receiving cell with a slight difference in lens action (lens power).

  That is, as shown in FIG. 10, the distances 50a to 50f from the center position O of the light receiving cells 10A to 10F of the photodetector 10 vary slightly depending on the condensing position (particularly, it is difficult to make the distances 50e and 50f the same). ). Therefore, the difference in the distances 50a to 50f is corrected so that the light receiving cells 10A to 10F can be arranged in a planar shape with respect to the lens action (lens power) of each of the divided regions 8A to 8F of the HOE surface 8Z in FIG. .

  Further, the lens action (lens power) applied to each of the divided regions 8A to 8F of the HOE surface 8Z is arbitrary, and the position of the photodetector 10, that is, the focus of the detection system can be arbitrarily set by giving positive power and negative power. You can change the distance. Therefore, the magnification of the detection system can be made different regardless of the focal length of the collimator lens 4 that is restricted on the emission side. Therefore, in the present embodiment, with the lens action (lens power) applied to each of the divided regions 8A to 8F on the HOE surface 8Z, the imaging magnification from the laser light source 2 to the optical disc 11 and the optical disc 11 to the photodetector 10 are increased. The difference in imaging magnification is corrected.

  5B and 5C show spot shapes when the optical disk is defocused from FIG. 5A. By defocusing, the spot extends in the direction of ± 45 °, and a focus error signal can be obtained by the astigmatism method. By assigning a dividing line to the HOE surface 8Z in such a state, it becomes a fan-shaped spot as shown in FIG. 5D and can be condensed on each light receiving cell.

  FIG. 6 shows an S curve and a spot shape for obtaining a focus error signal. FIG. 6A shows the spot shape of the light receiving cells 10E1 and 10F2 when the optical disk 11 is close to the objective lens 7 and the S curve is maximized. At this time, no spots exist in the light receiving cells 10E2 and 10F1. FIG. 6B shows the spot shape of the light receiving cells 10E2 and 10F1 when the optical disk 11 is far from the objective lens 7 and the S curve is minimized. At this time, no spots exist in the light receiving cells 10E1 and 10F2.

  Then, by calculating the focus error signal FE while changing the defocus, a good S curve characteristic can be obtained as shown in FIG. In FIG. 6C, the horizontal axis indicates the defocus amount (one scale of 4 μm), and the vertical axis indicates the level of the focus error signal. When there is no defocus at all, fan-shaped spots are uniformly formed in the light receiving cells 10E1, 10E2, 10F1, and 10F2.

FIG. 7 is a circuit diagram showing an example of an arithmetic circuit for calculating the focus error signal FE and the tracking error signal. In the figure, by using the signal E1 of the light receiving cell 10E1, the signal E2 of the light receiving cell 10E2, the signal F1 of the light receiving cell 10F1, and the signal F2 of the light receiving cell 10F2, the adders 26 and 27 and the subtractor 34 make FE = (E1 + F2) -(E2 + F1)
The focus error signal FE is calculated by the following formula. The characteristic of the focus error signal FE shows the S curve in FIG. Since the astigmatism method can be used for focus error detection in this way, the affinity for the conventional FEP is high.

  Next, a method for calculating the tracking error signal will be described. Four divided regions (the castles 8A, 8B, 8C, FIG. 3) of the diffractive optical element 8 through which the light beams of the 0th order light and the 1st order light that are diffracted when the grooves and pits of the optical disk are reproduced are passed. 8D), each of the output signals of the light receiving cells 10A, 10B, 10C, and 10D in FIG. 4 is denoted as A, B, C, and D, respectively. The tracking error signal PP by the push-pull method can be calculated by the adders 21 and 22 and the subtracter 25 in FIG.

PP = (A + B)-(C + D)
8A shows a case where a Blu-ray standard optical disc (BD) is reproduced, and FIG. 8B shows a case where a HD-DVD standard optical disc is reproduced, which is reflected from the signal surface of the optical disc on the HOE surface 8Z. The spot shape including the diffracted light thus produced is shown. The reflected light from the optical disc has a portion where the primary light diffracted by the track grooves and pits (hereinafter referred to as tracks) overlaps the non-diffracted zero-order light (FIGS. 8A and 8B). Part interferes. It is known that a push-pull signal can be obtained by the difference in the left and right light intensity of this overlapping portion (interfering portion).

  The size of the overlapping portion of the diffracted light is determined by the standard of the optical disc, and FIGS. 8A and 8B are different in the overlapping ratio of the first-order light and the zero-order light. The overlapping ratio of the first-order light and the zero-order light is uniquely determined by the numerical aperture (NA) of the objective lens 7, the wavelength of the recording / reproducing laser beam, and the track pitch on the optical disk. In FIG. When the HOE plane 8Z is divided by dividing lines 8i and 8j in the direction of ° (FIGS. 3 and 8), the dividing lines 8k and 8j in FIG. 8A in the case of BD have a margin of about 7% with respect to the spot radius, In the case of HD-DVD, the dividing lines 8k and 8j in FIG. 8B only have a margin of about 14% with respect to the spot radius.

  Therefore, the overlapping portion of the diffracted light does not cover the divided regions 8E and 8F used in the calculation of the focus error signal FE, and the change in the light amount when traversing the track hardly occurs. Does not occur. On the contrary, since the push-pull signal PP does not include a portion where the diffracted light does not overlap (particularly near the center of the light beam) more than necessary, the influence of the diffracted light having a cycle twice the track pitch can be eliminated. The division lines 8k and 8j are most preferably ± 45 °, but it is less affected when the overlapping portions of the diffracted light are slightly applied to the divided regions 8E and 8F, so that ± 45 ± 5 ° can be sufficiently handled.

By the way, it is known that the tracking error signal PP is offset when the lens is shifted or the disc is tilted in the radial direction. Therefore, using the adders 21 to 24, the subtracters 25, 31, 33, and the multiplier 32 shown in FIG. 7, the following expression APP = (A + B) − (C + D) −k {(E1 + F1) − (E2 + F2)} (4 )
By calculating the tracking signal error APP according to the above, it is possible to reduce not only the above-described offset but also the offset generated at the boundary of the recording mark.

  That is, the offset can be reduced by correcting with the DC component obtained by taking the difference between the signal E2 and the signal F2 from the sum of the signal E1 and the signal F1. Note that the multiplication coefficient k (multiplication coefficient by the multiplier 32 shown in FIG. 7) in the equation (4) corrects the lens shift and the offset at the recording boundary so that the occurrence of the offset is reduced even if there is a disturbance or the like. Is optimized.

  FIG. 9 is a diagram showing the state of crosstalk light on the photodetector 10 when reproducing the signal surface of the first recording layer having the substrate thickness t = 0.075 mm. The first-order diffracted light reflected from the signal surface 11B of the first recording layer to be reproduced at t = 0.075 mm of the optical disk 11 and diffracted by the HOE surface 8Z of the diffractive optical element 8 is shown in FIG. Light is condensed on the light receiving cells 10A to 10F as indicated by 40a to 40f, but light diffracted by other orders on the HOE surface 8Z (including 0th order diffracted light) becomes crosstalk light. It is the 0th-order diffracted light and the -1st-order diffracted light that form spots near the light receiving cells 10A to 10F. Since higher order light goes away from the light receiving cell, there is no problem.

  First, the 0th-order diffracted light from the signal surface 11B of the first recording layer to be reproduced forms a small-diameter circular spot at the center position of the light receiving cells 10A to 10F as shown by 41 in FIG. Therefore, if the light receiving cells 10A to 10F are slightly separated from the center position of the light receiving surface of the photodetector 10, there is no influence of the 0th order diffracted light. Next, the −1st order diffracted light from the signal surface 11B of the first recording layer to be reproduced is symmetric with respect to the center position and the track direction of the light receiving cells 10A to 10F as indicated by 42a, 42b and 42c in FIG. Therefore, depending on the arrangement in the track direction, it is possible to prevent the -1st order diffracted light from being applied to the light receiving cells 10A to 10F.

  Further, the light reflected from the signal surface 11B of the second recording layer that is not reproduced and having a substrate thickness t = 0.1 mm of the optical disk 11 and diffracted by the HOE surface 8Z also becomes crosstalk light. Since these crosstalk lights greatly spread according to the magnification, the influence of the first-order diffracted light is the main, and it is sufficient to consider even the zero-order diffracted light. First, first-order diffracted light (first-order diffracted light of crosstalk light) reflected by the second recording layer (other layer) that is not reproduced and diffracted by the HOE surface 8Z is divided into six divided regions 8A to 8D of the diffractive optical element 8. It spreads at the central angle of each fan shape of 8F, and is condensed on the light receiving surface of the photodetector 10 in the fan shape shown by 44a, 44b, 44c, 44d, 44e, 44f in FIG. The first-order diffracted light of the crosstalk light is mainly arranged in the radial direction and can be prevented from being applied to the light receiving cells 10A to 10F.

  Further, the zero-order diffracted light (the zero-order diffracted light of the crosstalk light) reflected by the other layer and diffracted by the HOE surface 8Z is centered at the center position of the light receiving cells 10A to 10F, as indicated by 43 in FIG. The circular spot having a diameter substantially larger than that of the spot 41 is formed and condensed. However, if the arrangement in the radial direction and the track direction is made appropriate, the light receiving cells 10A to 10F are not covered. It is possible.

  Therefore, by arranging the divided regions 8A to 8F and the light receiving cells 10A to 10F as shown in FIGS. 3 and 4 as described above, the crosstalk light does not overlap the light receiving cells 10A to 10F. Stable recording or reproduction is possible. In addition, it is possible to perform stable recording or reproduction by equally offsetting all the light receiving cells by applying the zero-order diffracted light of the crosstalk light to all the light receiving cells 10A to 10F.

  As shown in FIG. 7, the main signal RF is sent from the light receiving cells 10A to 10F represented by the following equations by the adders 21 to 24, 28, 29, and 30 to the signals A to D, E1, E2, F1, and F2. Calculated as the sum.

RF = A + B + C + D + E1 + E2 + F1 + F2 (5)
In addition, tracking error signals can be detected by the DPD method in correspondence with a reproduction type optical disc such as a BD-ROM. That is, the sum of the signals A and C (A + C) and the sum of the signals B and D (B + D) may be compared, and the tracking error signal is determined by the APP signal based on the new push-pull method and the DPD signal based on the DPD method. This is a compatible configuration.

  Furthermore, as shown in FIG. 4, in the present embodiment, the number of light receiving cells is small, so that the circuit scale is small, the cost is low, and noise can be suppressed, and no grating is required as shown in FIG. Therefore, there is an advantage that it can be easily applied to optical disks having different track pitches.

  As described above, according to the optical pickup device of the present embodiment, the dividing line in the direction perpendicular to the track direction (radial direction) is used when recording or reproducing not only a single layer optical disc but also a multilayer optical disc. Divided into 8 divisions and further divided into two division lines 8i and 8j that form a direction of approximately ± 45 ° with the division line 8k, thereby dividing into at least 6 divisional areas and at least 8 corresponding light receiving elements. By providing the cells 10A to 10F, recording or reproduction can be favorably performed.

  In the above embodiment, the configuration of the optical pickup device corresponding to one type of optical disk using only one blue laser light source 10 having a wavelength of 405 nm is shown. However, the present invention is limited to this embodiment. Rather than an optical pickup device that supports both BD and HD-DVD using the same blue laser light source, an optical pickup device that supports DVD using a red laser light source with a wavelength of 650 nm, or a plurality of optical disks including a CD It can also be applied to an optical pickup device corresponding to the above.

It is the figure which showed the whole structure of one Embodiment of the optical pick-up apparatus of this invention. It is the figure which showed the structure of one Embodiment of the detection system of the optical pick-up apparatus of this invention. It is the figure which showed an example of the division | segmentation pattern of the diffractive optical element in FIG. It is the figure which showed an example of the pattern of the light reception cell on the photodetector in FIG. It is the figure which showed an example of the spot shape on the light reception cell shown in FIG. It is the figure which showed the example of the focus error signal in this invention apparatus, and the spot shape on the light reception cell at that time. It is a block diagram of an example of the arithmetic circuit of the signal output of the optical pick-up apparatus of this invention. It is a spot shape including the overlapping part of the 0th order light and the 1st order light diffracted by the track in the diffractive optical element in the device of the present invention. It is the figure which showed the state on the photodetector of the crosstalk light from the other layer of the single-sided double layer optical disk in this invention apparatus. It is a figure explaining an example of the correction method of the difference of the distance of each division area of the diffraction type optical element in this invention apparatus, and each light reception cell of the photodetector corresponding to it. It is a block diagram of an example of the detection system of the conventional optical pick-up apparatus. It is a figure which shows an example of the HOE pattern (far field pattern) used for the detection system of the conventional optical pick-up apparatus. It is a figure which shows an example of the breadth of the crosstalk light of the conventional optical pick-up apparatus. It is a structural diagram of an example of a liquid crystal element used for a detection system of a conventional optical pickup device. It is a figure which shows an example of the state which a beam concentrates on the light reception cell of the conventional optical pick-up apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical pick-up apparatus 2 Laser light source 3 Polarizing beam splitter 3A Polarization selective dielectric multilayer film 4 Collimator lens 5 Standing mirror 5A Reflective film 6 1/4 wavelength plate 7 Objective lens 8 Diffractive optical element 8A-8F Division | segmentation area | region 8Z HOE Surface 8k, 8i, 8j Dividing line 9 Cylindrical lens 10 Photodetector 10A to 10F, 10E1, 10E2, 10F1, 10F2 Light receiving cell 11 Single-sided dual-layer optical disk 11A Beam incident surface 11B Signal surface LS Emission light LT Reflected light

Claims (6)

A light source that emits light;
An objective lens for focusing the light emitted from the light source on an optical disc to form a spot;
A diffractive optical element that diffracts reflected light from the optical disk that has passed through the objective lens and emits the light in a predetermined direction;
A photodetector that receives and photoelectrically converts light emitted from the diffractive optical element;
An optical path separation element that separates the light emitted from the light source and traveling toward the objective lens and the reflected light reflected from the optical disk, transmitted through the objective lens, and directed toward the photodetector ;
With
The diffractive optical element is
A first dividing line that divides the luminous flux of the reflected light in a direction perpendicular to the direction of the track when the track of the optical disk is projected, and approximately 45 in a direction opposite to the direction parallel to the direction of the track. -Divided into at least six divided areas by the second and third dividing lines dividing the luminous flux of the reflected light in the direction, and the six divided areas each change the direction by diffracting incident reflected light. ,
The reflected light is given astigmatism by at least two or more of the divided regions of the diffractive optical element, or a cylindrical lens,
The photodetector is
Each spot received by the six light receiving cells has at least six light receiving cells that individually receive the first-order diffracted light as signal detection light emitted through the six divided regions of the diffractive optical element. 2 disposed at a position where the circle of least confusion is approximately halfway between two focal lines generated by diffracted light given astigmatism, and includes the same direction as the direction of the track in at least six of the divided regions. Two light receiving cells that receive the first-order diffracted light emitted through the two divided regions are each divided into two by a dividing line in a direction perpendicular to the track direction ,
When the optical disc is a multilayer optical disc having a plurality of recording layers in the thickness direction, the six light receiving cells are reflected by the recording layer to be reproduced and diffracted by the diffractive optical element. In addition, the first-order diffracted light, which is the 0th-order diffracted light and the −1st-order diffracted light, is not irradiated on any of the six light receiving cells and is reflected by the recording layer that is not reproduced and diffracted by the diffractive optical element. An optical pickup device, wherein a certain crosstalk light is arranged at a position where none of the six light receiving cells is irradiated. Each divided region of the diffractive optical element has a lens function, and corrects a difference in distance between each divided region of the diffractive optical element and each light receiving cell of the photodetector corresponding thereto. The optical pickup device according to claim 1.   Each divided region of the diffractive optical element has a lens action, corrects a difference in distance between each divided region of the diffractive optical element and each light receiving cell of the photodetector corresponding thereto, and The optical pickup device according to claim 1, wherein a difference between an imaging magnification from the light source to the optical disc and an imaging magnification from the optical disc to the photodetector is corrected. The two divided regions including the same direction as the direction of the track in the six split areas of the diffractive optical element and a first divided region and a second divided region, and the remaining four divided regions When divided by an imaginary line passing through the center of the reflected light in the same direction as the direction of the track, two divided regions belonging to the same side are defined as a third divided region and a fourth divided region. and, when the two divided regions that belong to the same side of the other and the fifth divided regions and sixth split areas, the first-order diffracted light diffracted by the third divided area and the fourth divided area the output signal a and B of the two light receiving cells of the photodetector for receiving each other of the optical detector for receiving the first-order diffracted light diffracted respectively by the fifth divided regions and sixth split areas Using the output signals C and D of the two light receiving cells Te, the following equation
PP = (A + B)-(C + D)
4. The optical pickup device according to claim 1, further comprising tracking error signal generation means for generating a tracking error signal PP. The two divided regions including the same direction as the direction of the track in the six split areas of the diffractive optical element and a first divided region and a second divided region, and the remaining four divided regions When divided by an imaginary line passing through the center of the reflected light in the same direction as the direction of the track, two divided regions belonging to the same side are defined as a third divided region and a fourth divided region. When the two divided regions belonging to the other same side are defined as a fifth divided region and a sixth divided region, the photodetector has the first, second, third, fourth, fifth, and the sixth first for individually receiving the first-order diffracted light diffracted respectively by divided areas, second, third, and a fourth, fifth and sixth light receiving cells, and the first light receiving The cell and the second light receiving cell are separated by a dividing line in a direction perpendicular to the track direction. Respectively divided into two parts,
The output signal A and B of the third divided area, and the third and fourth light receiving cells for receiving the first-order diffracted light diffracted respectively by the fourth divided regions, the divided regions of the fifth and sixth the fifth and sixth output signals C and D of the light receiving cell, the said first and second light receiving cells that are each divided into two tracks for receiving the first-order diffracted light diffracted respectively by divided areas Output signals E1 and F1 of the two divided light receiving cell portions of the first and second light receiving cells belonging to the same side, and the other belonging to the same side Using the output signals E2 and F2 of the remaining two divided light receiving cell portions of the first and second light receiving cells, the following expression APP = (A + B) − (C + D) −k {(E1 + F1) − (E2 + F2)}
(Where k is a constant)
4. The optical pickup device according to claim 1, further comprising tracking error signal generation means for generating a tracking error signal APP according to claim 1. When the two divided regions including the same direction as the direction of the track in the six divided region of the diffractive optical element and a first divided region and a second divided area, the light detector the first divided area and the primary diffracted light is respectively imparted astigmatism diffracted by the second divided area, or is diffracted respectively by the first divided region and a second divided region, the cylindrical lens by the first light receiving cell and the second light receiving cells the first-order diffraction light astigmatism imparted to receive individually and having at least a first light receiving cell and the second light receiving cells , Each of which is divided into two by a dividing line in a direction perpendicular to the direction of the track,
When the first and second light receiving cells are separated by a virtual line in the same direction as the track direction, the output signals of the two divided light receiving cell portions of the first and second light receiving cells belonging to the same one side Using E1 and F1 and the output signals E2 and F2 of the remaining two divided light receiving cell portions of the first and second light receiving cells belonging to the other side, the following expression FE = (E1 + F2) − (E2 + F1) )
6. The optical pickup device according to claim 1, further comprising a focus error signal generation unit configured to generate a focus error signal FE.