JP2012243340A - Optical pickup device, optical disc device, and information reproducing method - Google Patents

Abstract

An object of the present invention is to reduce the influence of reflected light that causes crosstalk from adjacent layers in a multi-layer optical disc, and at the same time to maintain the downsizing of an optical pickup.
A first divided wavelength plate 731 and a second divided wavelength plate 741 are provided before and after the focal position of a condensing lens system with respect to one recording layer of a multilayer optical disc 501 to detect the second divided wavelength plate and the detection. The analyzer 730 is provided between the detector 52 and the non-polarized diffraction grating 732 is provided in the vicinity of the analyzer and the first divided wave plate, thereby reducing the influence of stray light from other layers.
[Selection] Figure 1

Description

  The present invention relates to an optical pickup device, an optical disk device, and an information reproducing method, and more particularly to a reading optical system of the optical pickup device.

  The recording capacity of one layer of the optical disk greatly depends on the wavelength of the semiconductor laser to be used and the numerical aperture (NA) of the objective lens. As the wavelength of the semiconductor laser is shorter or as the NA is larger, the recording density can be increased and the capacity per layer can be increased. The main component of the optical disk drive currently on the market is a DVD (Digital Versatile Disc) drive that uses red light with a wavelength of around 650 nm and an objective lens of NA 0.6. An optical disk drive using a blue-violet semiconductor laser having a wavelength of around 405 nm as a light source and using an objective lens with NA of 0.85 has been developed, and its share in the market is increasing. As a method of further increasing the recording density achieved at present, it is conceivable to shorten the wavelength used, but it is expected that it is difficult to develop a semiconductor laser in the ultraviolet region shorter than the blue-violet color. Further, regarding the increase in NA of the objective lens, since the limit of the NA of the objective lens in air is 1, it is difficult to improve the recording density due to the NA of the objective lens.

  In such a situation, two layers are implemented as a method for increasing the capacity of one optical disk. Non-Patent Document 1 introduces the technology of a two-layer phase change disk. When the two-layer optical disk is irradiated with laser light, adjacent layers are simultaneously irradiated, so that crosstalk between layers becomes a problem. In order to reduce this problem, the layer spacing is increased. Since the laser light is condensed and the layers other than the target layer (the layer) are shifted from the condensing position of the laser light, crosstalk can be reduced.

  On the other hand, when the layer spacing is increased, spherical aberration becomes a problem. The recording layer is embedded in a polycarbonate different from the refractive index of air, and the spherical aberration varies depending on the depth from the disk surface. The objective lens is designed so that its spherical aberration is smaller than that of a specific layer. When the focal point of the laser beam is moved to another layer, the distance from the surface of the focal point is different, so that spherical aberration occurs. . This aberration can be corrected by placing an expander lens optical system or a liquid crystal element, which is usually composed of two lenses, in front of the objective lens. That is, the aberration can be corrected by changing the distance between the two lenses or the phase of the liquid crystal element. However, it is difficult to correct a large spherical aberration in consideration of realizing a compensation range of the liquid crystal element or a lens moving mechanism in a small optical disk drive device. Accordingly, the thickness of the entire multilayer is limited, and the multilayer spacing becomes narrow in a multilayer optical disc having a large number of layers. For this reason, interlayer crosstalk remains in a multilayer disk having a narrow layer spacing.

  In order to reduce the above-described crosstalk, according to Patent Document 1, when the reflected light from the multilayer optical disk is collected by the lens, the collection position of the reflected light from the target layer and the adjacent layer is on the optical axis. Take advantage of different things. Two divisional wave plates composed of a plus quarter wave plate and a minus quarter wave plate are used, and the division directions are aligned and arranged before and after. It arrange | positions so that the condensing position of reflected light may come between both division | segmentation wavelength plates from the target layer. On the other hand, the condensing position of the reflected light from other than the target layer that becomes stray light comes to be outside the region sandwiched between the two divided wavelength plates. By adopting such an arrangement, it is possible to change the deflection direction of the reflected light from the target layer and the reflected light from the other layers after passing through both wavelength plates. In order to extract only the reflected light from the target layer, an analyzer may be installed after both wave plates. As a result, crosstalk from other layers can be reduced.

  In Patent Document 2, one divided wavelength plate divided into three is used, and reflected light from other layers is removed. The optical pickup optical system shown in FIG. 4 is shown. In this optical system, a divided wavelength plate 70 is added as compared with a normal optical pickup optical system. Laser light emitted from the semiconductor laser 101 is converted into a circular collimated light beam by the collimating lens 403 and the triangular prism 102. The collimated beam passes through the polarization beam splitter 103, is converted into circularly polarized light by the λ / 4 plate 104, and is narrowed down to the multilayer disk 501 (here, a two-layer disk is shown) by the objective lens 404. The read target layer is 511, and the position of the minimum spot of the laser beam is on 511. Reflected light 83 is also generated from the adjacent layer 512 and becomes stray light that causes crosstalk. Reflected light from the multilayer disk, including stray light, returns to the objective lens 404 and is converted into linearly polarized light in a direction orthogonal to the original polarization direction by the λ / 4 plate 104. Therefore, the reflected light 83 that has passed through the λ / 4 plate 104 is reflected by the polarizing beam splitter 103 and travels toward the condenser lens 405.

  Reference numeral 70 denotes a λ / 2 plate in which a half region is set in a predetermined optical axis direction, a divided wavelength plate in which the other half is a non-polarized region, and 43 denotes a reflecting mirror. The polarization direction of the reflected light from the read target layer that has exited from the divided wavelength plate 70 is transmitted to a different region of the divided wavelength plate 70 and thus is converted to the orthogonal direction, but the polarization direction of the reflected light from the adjacent layer is divided. It does not change because it passes through the same region of the wave plate. Of the reflected light that has returned to the condenser lens 405, the light from the adjacent layer is reflected by the polarization beam splitter 103 because the polarization direction has not changed as described above. On the other hand, the reflected light from the reading target layer is transmitted through the polarization beam splitter 103 because the polarization direction is rotated by 90 degrees. Therefore, only the reflected light from the reading target layer passes through the detection lens 406. The light transmitted through the detection lens 406 is detected by the detector 52, and a tracking error signal or the like is generated by the electric circuit 53. This method is suitable for a differential push-pull method that uses three beams to obtain a tracking error signal.

JP 2006-344344 JP 2007-310926

Jpn.J.Appl.Phys.Vol.42 (2003) pp.956-960

  In Patent Documents 1 and 2 described above, in order to take measures against interlayer crosstalk, the operation of condensing the reflected light from the optical disk once onto the divided wavelength plate and returning it to the shape of the beam before the light collection increases the optical path. Therefore, it is necessary to add a new lens, and the size of the apparatus cannot be avoided. Specifically, as shown in FIG. 4, an optical path is added below the polarizing beam splitter 103, and a condensing lens 405, a divided wavelength plate 70, and a reflecting plate 43 are newly added. For this reason, the optical pickup becomes larger by the amount of the optical system for preventing crosstalk.

  The first and second divided wavelength plates are provided before and after the focal position of the condenser lens system with respect to one recording layer, and the analyzer is provided between the second divided wavelength plate and the detector.

  Specifically, the first divided wave plate is between the focal position for the reflected light from the one recording layer and the focal position for the reflected light from the nearest layer deeper than the one recording layer. The second divided wave plate is installed between a focal position for the reflected light from the one recording layer and a focal position for the reflected light from the nearest layer that is shallower than the one recording layer. Is done. In the detector, the zero-order transmitted light and diffracted light of the non-polarized diffraction grating that has passed through the analyzer are detected.

  Then, a non-polarized diffraction grating is installed in the vicinity of the first divided wavelength plate on the condenser lens side to detect the diffracted light and obtain a focus servo signal.

  By using the polarization action of the first divided wave plate, the second divided wave plate, and the analyzer, the influence of stray light from other layers can be reduced, and the cross when the recording / reproducing operation of the multilayer disk is performed. Talk is also reduced. Further, since the optical element for obtaining the focus servo signal is installed in the condensed light flux, the optical pickup can be made small.

The figure which shows the optical pick-up by this invention. The figure which shows the irradiation state on the 1st division | segmentation wavelength plate of the divided beam. The figure which shows the irradiation state on the 2nd division | segmentation wavelength plate of the divided beam. The figure which shows the structure of the conventional optical pick-up which uses a division | segmentation wavelength plate. The figure which shows the effect | action of this invention. The figure which shows the polarization direction of the reflected light from the said layer after a 1st division | segmentation wavelength plate and permeation | transmission. The figure which shows the polarization direction of the reflected light from the said 2nd division | segmentation wavelength plate and the said layer after permeation | transmission. The figure which shows the polarization direction of the reflected light from the other layer after a 1st division | segmentation wavelength plate and permeation | transmission. The figure which shows the polarization direction of the reflected light from the other layer after a 2nd division | segmentation wavelength plate and permeation | transmission. The figure which shows the polarization direction of the reflected 2nd divided wavelength plate and the reflected light from the said layer. The figure which shows the polarization direction of the reflected light from the inverted 2nd division | segmentation wavelength plate and other layers. The figure which shows the diffraction by a non-polarization diffraction grating. The figure which shows the division | segmentation state of a non-polarization diffraction grating. The figure which shows the shape of a detector. The figure which shows the cross section of the optical element which integrated the non-polarization diffraction grating, the 1st division | segmentation wavelength plate, the 2nd division | segmentation wavelength plate, and the analyzer. The figure which shows the outline of a signal processing circuit. The figure which shows a 2nd Example. The figure which shows the light beam splitting optical system using two biprisms. The figure which shows the light beam splitting optical system using two division | segmentation diffraction gratings. The figure which shows the cross section of a diffraction grating. The figure which shows the cross section of a diffraction grating. The figure which shows the light beam splitting optical system using two parallel plates.

  Hereinafter, an optical pickup device or an optical disk drive device to which the present invention is applied will be described with reference to the drawings.

  FIG. 1 shows an outline of the entire optical system of the first embodiment. The laser light from the semiconductor laser light source 101 is converted into a circular collimated light beam by the collimating lens 403. The collimated beam passes through the polarizing beam splitter 103 and the λ / 4 plate 104 and is converted into circularly polarized light. The circularly polarized beam is narrowed down to the multilayer disk 501 by the objective lens 404. The target layer in the multilayer disc is 511. The reflected light from the multilayer disk returns to the objective lens 404, passes through the λ / 4 plate 104, and is converted into linearly polarized light in a direction orthogonal to the original polarization direction. The polarizing beam splitter 103 reflects the light in this polarization direction, so that the beam is reflected and enters the condenser lens 402. Convergent light is emitted from the condenser lens 402, but before the minimum spot is formed, the split non-polarization diffraction grating 732 and the first split wave plate 731 are installed. After the minimum spot is formed, the second divided wave plate 741 and the analyzer 730 are placed. The light transmitted through the analyzer 730 is detected by the detector 52, and a data signal, a focus error signal, and a tracking error signal are generated by the electronic circuit 53 based on the electric signal.

  The first and second divided wave plates 731 and 741 and the analyzer 730 are involved in the removal of stray light from other layers, and the symmetry of the focused beam is used. FIG. 5 shows a state in which the reflected light from the multilayer disk is incident on the first and second divided wave plates 731 and 741, and the action of stray light removal will be described by an example using a λ / 2 plate as the divided wave plate. To do. The polarization direction of the reflected light incident on the condenser lens 402 is the horizontal direction (lateral direction). A light ray 811 (solid line) is reflected light from the layer and is collected by the condenser lens 402. The first and second divided wave plates 731 and 741 can be wave plates that are divided into two in the same direction with respect to the optical axis, and the divided wave plates 731 and 741 are installed before and after the condensing point. The shape of the first divided wave plate 731 is shown in FIG. The first division wavelength plate 731 is composed of a λ / 2 plate 711 and a non-polarization portion 712 that does not change the polarization state. Here, the optical axis of the optical system is adjusted so that the beam spot passes through both the regions 711 and 712 across the dividing line (the boundary line between the regions 711 and 712) of the first divided wavelength plate. The same applies to the second divided wave plate 741 described below. The directions of the dividing lines of the first and second divided wave plates are substantially the same.

  The light irradiated on the left half of the condenser lens 402 irradiates the non-polarization region 712 of the first divided wave plate 731 in the shape of a semicircular 8112, but the polarization direction after transmission does not change. The arrow 61 shown below the non-polarization region in FIG. 6 represents the polarization direction, and the polarization direction of the arrow 61 represents a direction equivalent to the polarization direction of the incident light to the condenser lens 402. The light irradiated on the right half of the condenser lens 402 irradiates the λ / 2 plate 711 in a semicircular 8111 beam shape. Since the optical axis of the λ / 2 plate is set to change the horizontal polarization of 61 in the vertical direction on the paper surface of FIG. 6, the polarization direction of the light of the semicircular 8111 beam shape is transmitted through the polarizing plate 711. Thereafter, the image is converted in the direction indicated by the arrow 62. The reflected light from the layer that has passed through the first divided wave plate 731 passes through the minimum spot surface 521 and then irradiates the second divided wave plate 741 in FIG. Since the semicircular beam 8111 in FIG. 6 passes through the focal point, it becomes an 8113 semicircular beam on the λ / 2 plate 722 on the second divided wavelength plate 741, and irradiates the left side. On the other hand, the semicircular beam indicated by 8112 in FIG. 6 irradiates the right side on the second divided wave plate in FIG. Since 721 is an unpolarized region, the polarization direction is the polarization direction represented by 61 and does not change even after the beam represented by 8114 has passed through the region 721. In summary, the polarization direction of the reflected light from the layer does not change after passing through the first and second divided wave plates.

  Next, the change in the polarization direction of the reflected light from the adjacent layer will be described. A light ray 814 (dotted line) in FIG. 5 is reflected light from an adjacent layer that is deeper than the layer and has the smallest layer interval. A minimum spot is formed at 54 positions close to. Reflected light from a deeper layer forms a minimum spot closer to the condenser lens 402 than 54, and the minimum spot position does not approach the first divided wavelength plate 731. Further, the minimum spot position of the reflected light from a layer shallower than the layer is located beyond the position of the second divided wave plate 741. For these reasons, the minimum spot position of the reflected light from a layer other than the layer does not enter between the first divided wave plate 731 and the second divided wave plate 741. For this reason, the light beam does not cross the optical axis between the first divided wave plate and the second divided wave plate. That is, the beams incident on the left and right regions of the first divided wave plate are directly incident on the left and right regions of the second divided wave plate. The description will be made using reflected light from an adjacent layer that is deeper than the layer and has the smallest layer spacing, but the reflected light from other layers is also similar from the first divided wave plate 731 and the second composite wave plate 741. Affected. 831 of FIG. 8 and 741 of FIG. 9 are the 1st division | segmentation wavelength plate and the 2nd division | segmentation wavelength plate, and are the same as what was shown in FIG. 6 and FIG. 7, respectively. The right region of the first divided wavelength plate 731 is a λ / 2 plate 711, and the polarization direction of light rotates to a polarization direction of 62 in the semicircular region 8141. The light of this portion is an 8144 semicircular irradiation region in the second divided wavelength plate of FIG. 9, but since this region 721 is a non-polarization region, the polarization direction does not change. Therefore, the polarization direction after passing through the second divided wave plate is the vertical direction indicated by 62. On the other hand, since the polarization direction of the beam 8142 incident on the left side of the first divided wave plate 731 in FIG. 8 is the non-polarization region, the polarization state 61 of the incident light in the lateral direction continues. This beam is incident on the 722 λ / 2 plate of the second divided wavelength plate 741 of FIG. 9 in a semicircular state of 8143, and the polarization direction becomes the vertical direction indicated by 62. In summary, the polarization direction of the reflected light from other than the layer is 90 ° rotated after being transmitted through the first and second divided wavelength plates to become the vertical direction.

  An analyzer 730 is installed after the second divided wave plate 741. The role of this analyzer is to transmit only the reflected light from the layer to the detector 52. Since the polarization direction of the reflected light from the layer is the horizontal direction and the polarization direction of the reflected light from other than the layer is the vertical direction, if the analyzer is set to transmit the polarized light in the horizontal direction, the layer Only the reflected light from can be detected by the detector, and the influence of the reflected light from other layers can be reduced.

  When the second divided wavelength plate is reversed left and right, the polarization direction of the reflected light from the layer is in the vertical direction, and the reflected light from the other layer is in the horizontal direction. Therefore, if the analyzer is set so that the vertically polarized light can pass, only the reflected light from the layer can be guided to the photodetector. The polarization direction of the reflected light from the layer after passing through the first divided wave plate 731 is already shown in FIG. This polarization direction is incident on the second divided wave plate 741 in FIG. The beam represented by 8111 in FIG. 6 becomes the beam 8115 in FIG. 10, and the polarization direction does not change in the vertical state. Further, the beam represented by 8112 in FIG. 6 becomes the beam 8116 in FIG. 10, and the polarization direction changes in the vertical direction. Therefore, the polarization direction of the reflected light from the layer is the vertical direction. The reflected light from other than the layer has the polarization direction of the outgoing light from the first divided wave plate as shown in FIG. 8, and the light in this polarization direction is incident on the second divided wave plate shown in FIG. To do. The beam 8142 in FIG. 8 becomes the beam 8146 in FIG. 11, and the beam 8141 in FIG. 8 becomes the beam 8145 in FIG. The beam of 8146 is in the horizontal direction without changing the polarization direction, and the beam of 8145 is turned in the horizontal direction by rotating the polarization direction by 90 degrees. Therefore, the polarization direction of the reflected light from other than the layer as a whole becomes the horizontal direction. Accordingly, when the analyzer 730 shown in the drawing is used that transmits the vertical polarization direction, only the reflected light from the layer can be detected by the detector.

  Table 1 shows possible combinations of the first divided wave plate, the second divided wave plate, and the analyzer.

In addition, the polarization direction of the reflected light from the other layer and the layer after passing through the second divided wave plate is shown. 1) and 2) are methods using a λ / 2 plate, and the operation has already been described. 3) and 4) are methods using a λ / 4 plate, and the polarization directions of the two layers can be made different depending on the difference in the passing region between the layer and the other layer. It is assumed that the + λ / 4 plate and the −λ / 4 plate are converted into right circularly polarized light and left circularly polarized light, respectively, after transmission of linearly polarized light in the horizontal direction. In 3), the reflected light from the layer becomes right circularly polarized light after passing through the right + λ / 4 plate of the first divided wavelength plate, and then the -λ / 4 plate on the left side of the second divided wavelength plate. , The polarization state returns to the original state and becomes the linearly polarized light in the horizontal direction. The polarization state of the light that has passed through the left side of the first divided wave plate becomes left circularly polarized light, and then passes through the + λ / 4 plate on the right side of the second divided wave plate to become the original linearly polarized light in the lateral direction. Return. In summary, the polarization direction of the reflected light from the layer becomes the linearly polarized light in the lateral direction after passing through the second divided wave plate. The reflected light from the other layer becomes right circularly polarized light after passing through the right + λ / 4 plate of the first divided wave plate, and then passes through the + λ / 4 plate on the right side of the second divided wave plate, It becomes linearly polarized light in the vertical direction. In addition, the polarization state of the light that has passed through the left of the first divided wave plate is left circularly polarized light, and then passes through the left -λ / 4 plate of the second divided wave plate. Become. Accordingly, by installing an analyzer that transmits linearly polarized light in the horizontal direction, only the reflected light from the layer can be transmitted to the detector. In 4), the arrangement of the left and right λ / 4 plates of the first divided wave plate and the second divided wave plate is reversed, and after passing through the second divided wave plate, reflected light from other layers Is polarized in the horizontal direction, and the reflected light from the layer is polarized in the vertical direction. The analyzer in this case is set so as to transmit the linearly polarized light in the vertical direction, and only the reflected light from the layer reaches the detector.

  The left and right wave plates of the first divided wave plate have a role of converting light in a linear polarization state into polarization states that are orthogonal to each other. The second divided wave plate may be the same function as the first wave plate or may be inverted. Since the polarization direction of the reflected light from the other layer and the polarization direction of the reflected light from the relevant layer are orthogonal to each other, interlayer crosstalk from the other layer can be reduced by adjusting the analyzer to the polarization direction of the reflected light from the relevant layer. Can be removed. In the above description, the polarization state of the light incident on the condenser lens 402 is linearly polarized light in the horizontal direction. However, in the case of the vertical direction, only the vertical and horizontal directions in Table 1 are switched, and the effect of removing the crosstalk is not changed. In all of 1) to 4), even if the left and right sides of the first divided wave plate and the second divided wave plate are interchanged, the polarized light after passing through the second divided wave plate of the reflected light from the layer and the other layer Needless to say, there is no change in state and the effect does not change.

  In summary, by using the first divided wave plate and the second divided wave plate, the polarization direction of the reflected light from the other layer can be made orthogonal to the reflected light from the layer. Therefore, the crosstalk from the other layer can be reduced by blocking the reflected light from the other layer and transmitting the reflected light from the other layer by the analyzer. In this description, the polarized light of the reflected light from the disk is linearly polarized light. However, stray light can be removed even with circularly polarized light. However, an analyzer having selectivity for circularly polarized light is required.

Next, a method for obtaining a focus error signal will be described. As shown in FIG. 12, a non-polarization split diffraction grating 732 is installed in the vicinity of the split wave plate 731 to generate zero-order diffracted light 816 and plus first-order diffracted lights 817 and 818 that pass through without action. Here, “installed in the vicinity” means that the distance between the first divided wavelength plate and the non-polarized divided diffraction grating 732 is within about 0.3 mm. In FIG. 12, the non-polarization split diffraction grating 732 is installed between the split wavelength plate 731 and the condenser lens 402, but it may be installed between the split wavelength plate 731 and the minimum spot surface 521. However, the non-polarization division diffraction grating 732 is preferably installed between the division wavelength plate 731 and the condenser lens 402 in terms of manufacturing the element.
FIG. 13 shows a division state of the non-polarization division diffraction grating 732. The ball-shaped figure shown inside shows the irradiation state of the reflected light from the layer on the diffraction grating, and represents the positional relationship between the division position of the diffraction grating and the light beam. The portion similar to the seam of the ball shows the state of diffracted light when the disc has a guide groove. The diffraction grating is divided into three as a whole, and the plus first-order light at the diffraction gratings 733 and 734 is emitted as a light beam 818 in FIG. 12 where it does not overlap with the zero-order light 816. Similarly, the plus first-order light at the diffraction grating 735 is emitted to a place where it does not overlap with the other 816 and 818 as indicated by 817 in FIG. The diffraction direction of the plus first-order diffracted beams 817 and 818 is the direction of the dividing line of the dividing wave plate. Thereby, only the plus primary light of the reflected light from the layer reaches the detector, so that the stray light in the other layer can be removed by the focus error signal.

  FIG. 14 shows the shape of the photodetector 52. The photodetector 522 detects the plus primary light 818 of the diffraction gratings 733 and 734, and the photodetector 523 detects the plus primary light 817 of the diffraction grating 735. The detected beam shape is shown on each detector. The photodetector 524 detects the 0th-order diffracted light 816, and the inside is divided into 2 in the horizontal direction and 4 in the vertical direction, for a total of 8 divisions. The alphabetical symbol shown on each detector represents the output signal. The data signal RF is expressed as RF = A + B + C + D + E + F + G + H, and is the sum of all the light incident on the detector 524. The focus error signal FE is expressed as FE = (B + C + F + G) − (A + D + E + H) −m (I−J). m is a constant and is determined by the light quantity balance.

  Next, generation of a tracking signal will be described. A push-pull signal is used when reading from and writing to a multi-layer disc having guide grooves. When the tracking error signal at this time is TE, TE = {(B + C) − (G + F)} − k {(A + D) − (E + H)}. This method is called a compensated push-pull method, and no offset occurs in the signal even if the objective lens is moved for tracking. k is a constant determined in consideration of the light quantity ratio. When there is no guide groove, a DPD method (Differential Push-Pull method) is used. The tracking error signal DPD at this time is expressed as DPD = phase {(A + B), (H + G)}-phase {(C + D), (E + F)}. Phase {(A + B), (H + G)} represents the phase difference between the A + B signal and the H + G signal, and phase {(C + D), (E + F)} represents the signal phase difference as well.

  The focus position of the emitted light from the objective lens is controlled using the focus error signal and the tracking error signal. These signals are generated by the electric circuit 53 and drive an actuator for controlling the position of the objective lens 404.

Here, the size of each of 522, 524, and 523 is about 100 × 100 μm ² , and the overall size 52 is about 300 × 100 μm ² . This is almost the same as the size of the conventional three beam type.

  The non-polarized diffraction grating 732, the first divided wave plate 731, the second divided wave plate 741, and the analyzer 730 shown in FIG. 12 are individual elements, but as shown in FIG. An element 780 in which optical components are integrated can be formed by using a liquid crystal material or the like, which facilitates adjustment. FIG. 15 is a schematic view of the element cross section in the direction along the optical axis, and the reflected light of the disk is irradiated from below. 751 is a non-polarization diffraction grating, 752 is a first divided wave plate, 753 is a second divided wave plate, and 754 is an analyzer. Between these elements, glass members 791, 792, and 793 that transmit the used laser light are held. The divided shape of the divided wave plate may be divided into two as shown in FIGS.

  FIG. 16 shows a configuration example of the optical disc drive apparatus of the present embodiment. Circuits 211 to 214 are for recording data on the multilayer optical disc 501. Reference numeral 211 denotes an error correction encoding circuit, which adds an error correction code to data. A recording encoding circuit 212 modulates data by the 1-7PP method. A recording compensation circuit 213 generates a pulse for writing suitable for the mark length. Based on the generated pulse train, the semiconductor laser driving circuit 214 drives the semiconductor laser in the optical pickup 60 to modulate the laser light 80 emitted from the objective lens. A phase change film is formed on an optical disc 501 that is rotationally driven by a motor 502. The phase change film is heated by a laser beam and becomes an amorphous state when rapidly cooled, and becomes a crystalline state when slowly cooled. These two states have different reflectivities and can form a mark. In the writing state, high-frequency superposition that lowers the coherency of the laser light is not performed, so that the reflected light from the adjacent layer and the reflected light from the layer are likely to interfere with each other. For this reason, when measures for removing the stray light from other layers are not taken, there arises a problem that tracking is lost or data on adjacent tracks is erased.

  Circuits 221 to 226 are for reading data. Reference numeral 221 denotes an equalizer that improves the signal-to-noise ratio near the shortest mark length. This signal is input to the PLL circuit 222, and a clock is extracted. Further, the data signal processed by the equalizer is digitized by the A / D converter 223 at the timing of the extracted clock. Reference numeral 224 denotes a PRML (Partial Response Maximum Likelihood) signal processing circuit which performs Viterbi decoding. The recording / decoding circuit 225 performs decoding based on the 1-7PP modulation rule, and the error correction circuit 226 restores the data.

  FIG. 17 shows the optical system of the second embodiment. In this optical system, a light beam splitting element 107 is added to the optical system of Embodiment 1 (FIG. 1). This element has a characteristic that the reflected light from the layer is divided into two in the return path and the light intensity on the optical axis is eliminated. The dividing direction is substantially the same as the dividing direction of the divided wave plate. By this element, the reflected light from the layer that has become parallel rays is converted into two parallel rays that are parallel to the optical axis and do not pass through the optical axis. The two parallel rays are condensed by the condenser lens 402, pass through the non-polarized diffraction grating 732, the first and second divided wave plates 731 and 741, and the analyzer 730, and then detected by the detector 52.

  The light beam splitting element 107 for splitting the reflected light in the return path will be described. FIG. 18 is a diagram illustrating an example of a light beam splitting element that splits a beam using two biprisms. Parallel rays are incident on the first biprism 408, and parallel rays in the traveling direction that are symmetric at the same angle with respect to the optical axis are formed with the perpendicular of the optical axis as a dividing line. The second biprism 409 changes the traveling direction of parallel rays having an angle with respect to the optical axis to be parallel to the optical axis. By using two biprisms in this way, a normal beam can be converted into split parallel rays.

  FIG. 19 is a diagram illustrating an example of a light beam splitting optical system that performs parallel splitting using the transmission gratings 41 and 42. The gratings 41 and 42 are each composed of two regions having different directions of diffracted light by the grating. The gratings in the two regions have the same groove direction and the same groove pitch as the dividing line, and the generation of zero-order light. This is a sawtooth grating with a groove depth of 1 / (n-1). n is the refractive index of the grating and is assumed to be in the air. Even when the groove depth is an integral multiple of this, zero-order light is not generated. FIG. 20 shows the shape of the sawtooth of the grating 41. Since the sawtooth in the areas 410 and 411 are inverted from each other, incident light from below is diffracted in a direction symmetric to the optical axis. . FIG. 21 shows the sawtooth shape of the grating 42. Since the sawtooth shape at 421 and the shape at 410 and the sawtooth shape at 420 and the shape at 411 are the same, the two beams transmitted through the grating 41 and having an angle with respect to the optical axis are After passing through the grating 42, the light becomes parallel to the optical axis with a gap.

  FIG. 22 is a diagram illustrating an example of a light beam splitting optical system using parallel plates. The divided flat plate element 44 is composed of two parallel flat plates 441 and 442, and each parallel flat plate is inclined at the same angle with respect to the optical axis and is symmetric with respect to the optical axis. The ridge line formed by the joint portion of the two parallel flat plates is perpendicular to the optical axis, and the ridge line or valley line formed by the joint portion of the parallel flat plate is in the radial direction. Incident parallel light from above the plane of the paper is divided into two at the position of the valley line, and is incident on different parallel plates. If the parallel plate is made of transparent glass or plastic, the refractive index is larger than that of air, so that the light beam is directed away from the plane including the valley and the optical axis on the incident surface, and becomes a beam parallel to the optical axis on the output surface.

  FIGS. 2 and 3 show the irradiation state of the reflected light from the layer on the first and second divided wave plates 731 and 741. In both figures, the wave plate is irradiated in a half-moon state away from the division position. Since light is always accompanied by diffraction, light oozes around the half-moon state. If the light beam splitting element 107 is not used, this bleeding may lead to a decrease in the amount of RF signal light. When the light amount of the RF signal decreases, the relative noise amount of the RF signal increases, leading to erroneous reading of data. For example, as shown in FIG. 7, when the second divided wavelength plate is irradiated in a circular state in which the left and right polarization states are different, the left and right light oozes out to the periphery. For example, when the light of the layer 8113 starts to ooze into the right region 721, the polarization direction is reversed and the detector cannot reach the detector. Similarly, if the beam 8114 begins to bleed into the left region 722, the detector cannot be reached. For this reason, the light quantity of RF signal may fall. However, by using the light beam splitting element 107, as shown in FIG. 3, since the beam is separated from the center dividing line, the amount of light entering into the adjacent region by diffraction is reduced, so avoiding a decrease in the amount of RF signal light. Is possible.

  The stray light from the other layer also irradiates the second divided wave plate in a half moon shape, although the beam is not inverted between the first and second divided wave plates as in the layer. Also in this case, light oozes out near the periphery of the beam due to diffraction. When the beam splitting element is not used, the light that has oozed into the respective regions of the second composite wave plate enters the opposite regions and becomes stray light. However, by using the beam splitting element, each half-moon shaped irradiation area is separated from the dividing line at the center of the second split wave plate, so that the bleeding into the facing area due to diffraction is reduced, and detection is performed as a result. The stray light incident on the device can be further reduced.

  As described above, according to the present application, crosstalk due to reflected light from other layers mixed in the data signal itself can be reduced, so that the quality of the data signal can be improved.

  Furthermore, it is possible to reduce the fluctuation of the tracking signal and the displacement of the focus signal by the DPP method. When reading or writing a multi-layer optical disk, it is necessary to accurately control the focal position and tracking position of the laser beam with respect to the optical disk using an error signal, but when reflected light from an adjacent layer enters the detector as stray light, The tracking position and the focal position are out of order, and the data signal cannot be read accurately or the writing position cannot be determined accurately. In the present invention, these problems can be eliminated.

52: Detector, 53: Signal processing circuit, 101: Semiconductor laser, 103: Polarizing beam splitter, 104: λ / 4 wavelength plate, 107: Light beam splitter, 402: Condensing lens, 404: Objective lens, 501: Multilayer disc, 730: analyzer, 731: first divided wave plate, 732: non-polarized diffraction grating, 741: second divided wave plate

Claims (13)

An optical pickup device used for a multilayer medium,
A light source;
A condenser lens for condensing the reflected light reflected from the recording layer of the multilayer medium;
A detector for detecting light that has passed through the condenser lens;
Between the condensing lens and the detector, a first split wave plate, a non-polarization split diffraction grating provided near the first split wave plate, a second split wave plate, and an analyzer An optical pickup device comprising: The non-polarization split diffraction grating is installed between the first split wave plate and the condenser lens,
The first divided wavelength plate is installed between the focal position of the recording layer and the focal position of the first adjacent recording layer on the side opposite to the condenser lens adjacent to the recording layer,
The second divided wave plate is installed between a focal position of the recording layer and a focal position of a second adjacent recording layer on the condenser lens side adjacent to the recording layer. The optical pickup device according to claim 1. The first divided wave plate is divided into two regions, and the polarization states of transmitted light from the two regions are orthogonal to each other,
The second divided wave plate is divided into two regions, and has a wave plate region that is the same as the first divided wave plate or an inverted region.
2. The optical pickup device according to claim 1, wherein the analyzer selectively transmits light having a predetermined polarization direction that has passed through the second divided wave plate. 3.   2. The optical pickup device according to claim 1, wherein directions of dividing lines of the first and second divided wave plates substantially coincide with each other.   4. The optical pickup device according to claim 3, wherein the orthogonal state of the polarization state is an orthogonal state of linearly polarized light.   4. The optical pickup device according to claim 3, wherein the orthogonal state of the polarization state is an orthogonal state of circularly polarized light. The non-polarization split diffraction grating is
The reflected light is transmitted as zero-order light, and is divided into first plus first-order diffracted light in a region including the center of the beam and second plus first-order diffracted light in a region including the periphery of the beam. And the diffraction directions of the first plus first-order diffracted light and the second plus first-order diffracted light substantially coincide with the direction of the dividing line of the second divided wave plate,
2. The detector according to claim 1, wherein a region for detecting the first plus first-order diffracted light and a region for detecting the second plus first-order diffracted light are provided separately from each other. Optical pickup device. 8. The non-polarized split diffraction grating, the first split wave plate, the second split wave plate, and the analyzer are arranged as an integrated element. Optical pickup device. Furthermore, it has an element that divides the reflected light into two so as to provide a dark part at the center,
8. The optical pickup apparatus according to claim 7, wherein a direction of dividing the element to be divided substantially coincides with a direction of a dividing line of the second divided wave plate.   The optical pickup device according to claim 9, wherein the element to be divided is any one of two biprisms, two gratings, and two parallel plates. A light source;
A condenser lens for condensing the reflected light reflected from the recording layer of the multilayer medium;
A detector for detecting light that has passed through the condenser lens;
Between the condensing lens and the detector, a non-polarization split diffraction grating, a first split wave plate, a second split wave plate, and an analyzer,
A drive circuit for driving a light beam from the light source;
An optical disc apparatus comprising: a decoding circuit that decodes a signal detected by the detector. The non-polarization split diffraction grating is provided in the vicinity of the first split wave plate,
The first divided wavelength plate is installed between the focal position of the recording layer and the focal position of the first adjacent recording layer on the side opposite to the condenser lens adjacent to the recording layer,
The second divided wave plate is installed between a focal position of the recording layer and a focal position of a second adjacent recording layer on the condenser lens side adjacent to the recording layer. The optical disk device according to claim 11. Irradiating the first recording layer of the multilayer medium with light from a light source;
The reflected light reflected from the first recording layer is
A condensing lens that condenses the reflected light from the first recording layer;
Non-polarized split diffraction grating,
A first divided wavelength disposed between the focal position of the first recording layer and the focal position of the first adjacent recording layer on the side opposite to the condenser lens adjacent to the first recording layer. Board,
A second divided wavelength plate installed between the focal position of the first recording layer and the focal position of the second adjacent recording layer on the condenser lens side adjacent to the recording layer;
An analyzer that transmits light of a predetermined polarization direction;
And enter the photodetector,
From the signal detected by the photodetector, focus servo is performed,
An information reproducing method, wherein information is reproduced by decoding a signal recorded on the first recording layer.

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