JP2008117473A - Objective lens optical system, luminous flux dividing element, and optical pickup optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens optical system, a luminous flux dividing element, and an optical pickup system which can perform recording and reproducing on a disk in which luminous flux of the same wavelength is used and thickness is different while securing working distance. <P>SOLUTION: An objective lens optical system 100 has a luminous flux dividing element 1 and an objective lens 2, converges luminous flux of wavelength λ on an information recording plane of a first optical recording medium having a transparent substrate of thickness t1, and converges luminous flux of wavelength λ having a transparent substrate of thickness t2 (t1<t2) on an information recording plane of a second optical recording medium 202. One plane of the luminous flux dividing element 1 is divided into a plurality of regions. In the plurality of regions, luminous flux transmitted respectively are converged on either information recording plane respectively out of the first and the second optical recording media. The region of the second optical recording medium has negative power. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an objective lens optical system, a beam splitting element, and an optical pickup optical system capable of recording or reproducing information with respect to optical recording media having a plurality of different thicknesses.

  Conventionally, compatible optical disc apparatuses have been proposed that can play back together optical discs of different types such as CD (compact disc: including CD such as CD-R) and DVD (digital versatile disc). A CD or DVD (hereinafter collectively referred to as an optical disk) uses a transparent substrate, and an information recording surface is provided on one surface of the transparent substrate. The optical disk has a structure in which two transparent substrates are bonded together with their information recording surfaces facing each other, or such a transparent substrate is a transparent protective substrate, and the information recording surface of the transparent substrate is a protective substrate. It is configured to be pasted so as to face each other.

  When reproducing the information signal stored in the optical disk having such a configuration, it is necessary to focus the laser beam from the light source on the information recording surface of the optical disk via a transparent substrate by the optical disk device. The laser beam has a wavelength of about 780 nm and a NA of 0.45 to 0.53 in CD, and the DVD has a wavelength of about 650 nm and a NA of 0.60 to 0.67. In addition, the thickness of the transparent substrate used in the CD is 1.2 mm, whereas the thickness of the transparent substrate used in the DVD is 0.6 mm, which depends on the type of optical disc (difference in laser beam wavelength). Accordingly, the thickness of the transparent substrate provided with the information recording surface is different. In a compatible optical disk device that reproduces optical disks of different types, it is necessary to focus the laser beam on the information recording surface even if the thickness of the transparent substrate varies depending on the type of optical disk.

  As such a compatible optical disc device, an objective lens is provided for each type of optical disc in the pickup, and the objective lens is exchanged according to the type of the optical disc to be used, or a pickup is provided for each type of optical disc. It is conceivable to change the pickup depending on the type. However, in order to realize cost reduction and downsizing of the apparatus, it is desirable that the same lens can be used for any kind of optical disk as the objective lens.

  As a representative example of such an objective lens, there is one described in Patent Document 1. The objective lens described in this document is divided into three or more annular lens surfaces in the radial direction, and every other annular lens surface and the other annular lens surfaces have different refractive powers. ing. Then, with respect to the laser beam of the same wavelength, every other annular lens surface condenses the laser beam on the information recording surface of an optical disk (DVD) on a thin transparent substrate (0.6 mm), for example. Every other annular lens surface focuses the laser beam on the information recording surface of an optical disk (CD) with a thick transparent substrate (1.2 mm), for example.

  Another representative example is described in Patent Document 2. This document uses a short wavelength (635 nm or 650 nm) laser beam for a thin transparent substrate DVD and a long wavelength (780 nm) laser beam for a thick transparent substrate CD. An optical disc apparatus is disclosed. This optical disc apparatus has an objective lens commonly used for these laser beams. In this objective lens, a diffractive lens structure is formed in which minute steps of an annular zone are densely provided on one surface of a refractive lens having a positive power. Such a diffractive lens structure condenses the diffracted light of the short wavelength laser beam on the DVD of the thin transparent substrate and the diffracted light of the long wavelength laser beam on the information recording surface of the CD of the thick transparent substrate. Designed. Each diffracted light is designed to collect the same order of diffracted light on the information recording surface. The reason for using a laser beam with a short wavelength for DVD is that the recording density of DVD is higher than that of CD, and it is therefore necessary to narrow down the beam spot. As is well known, the size of the light spot is proportional to the wavelength and inversely proportional to the numerical aperture NA.

Another representative example is described in Patent Document 3. In this document, a short wavelength 680 nm laser beam is used for a thin 0.6 mm transparent substrate, and a long wavelength (780 nm) laser is used for a thick 1.2 mm transparent substrate CD. An objective lens of an optical disc apparatus using a beam is disclosed. In this objective lens, the lens surface is divided into a plurality of ring-shaped areas, and each area is focused on an optical disk having any wavelength and substrate thickness.
Japanese Patent Laid-Open No. 9-145995 JP 2000-81666 A Japanese Patent Laid-Open No. 7-302437

  As one of the new optical disk apparatuses proposed in recent years, an optical disk apparatus compatible with Blu-ray and HDDVD using a blue laser having a wavelength of about 405 nm for improving the recording density has been proposed. Blu-ray has an NA of 0.85 for a light beam with a wavelength of 405 to 408 nm, and the thickness of the transparent substrate is 0.075 to 0.1 mm considering both a double-layer optical disk and a single-layer optical disk. In HDDVD, the NA for a light beam having a wavelength of 405 to 408 nm is 0.65, and the thickness of the transparent substrate is 0.6 mm. Accordingly, there is a need in the future for an apparatus that is compatible with two types of optical disks having different thicknesses that are recorded and reproduced by a laser beam having the same wavelength.

  However, in the above-mentioned Patent Document 1, there is a description about DVD and CD, but there is no description about Blu-ray and HDDVD. If the wavelength of the laser used is short, the amount of wavefront aberration for the same ray aberration (mm) increases in inverse proportion to the wavelength, and if the NA is large, for example, the third-order spherical aberration increases in proportion to the fourth power of NA. Aberration correction becomes more difficult.

  As described above, the same is true for Blu-ray and HDDVD whose wavelength and NA are different from those of DVD and CD, that is, Blu-ray and HDDVD that require a shorter wavelength and a larger NA than DVD and CD described in Patent Document 1 above. It is difficult to achieve the desired light spot shape by focusing with the objective lens, the objective lens optical system or the optical pickup optical system, using the technique described in Patent Document 1.

  Further, in Patent Document 2, since diffracted light by the diffractive lens structure is used, it is not possible to deal with transparent substrates having different thicknesses unless the light beams have different wavelengths, and transparent substrates having the same or almost the same wavelength and different thicknesses. The technique of Patent Document 2 cannot be used. Patent Document 3 does not disclose a technique for making two types of optical disks having different thicknesses recorded and reproduced by a light beam having the same wavelength compatible.

  The applicant of the present application has proposed an objective lens optical system that solves such problems of the conventional example in a prior application (Japanese Patent Application No. 2006-13411). However, in this prior application, since the phase plate does not have power, the focal lengths of the Blu-ray area and the HDDVD area are equal. At this time, since HDDVD has a larger transparent substrate thickness than Blu-ray, the working distance of HDDVD is inevitably shorter. Therefore, there is a problem that the optical head contacts the disk when recording / reproducing HDDVD. It was.

  The present invention has been made to solve such a problem, and an objective lens optical capable of recording / reproducing optical recording media having different thicknesses using light beams having the same wavelength while ensuring a working distance. An object of the present invention is to provide a system, a beam splitting element, and an optical pickup optical system.

  An objective lens optical system according to the present invention includes a light beam splitting element and an objective lens, focuses a light beam having a wavelength λ on an information recording surface of a first optical recording medium having a transparent substrate having a thickness t1, and , An objective lens optical system for condensing a light beam having a wavelength λ on an information recording surface of a second optical recording medium having a transparent substrate having a thickness t2 (t1 <t2), and at least one surface of the light beam splitting element is A first optical recording medium region for condensing the transmitted light beam on the information recording surface of the first optical recording medium via the objective lens; and the second light beam transmitted through the objective lens. The optical recording medium is divided into second optical recording medium areas for focusing on the information recording surface, and the second optical recording medium area has negative power.

  Here, it is preferable that the second optical recording medium region has a radius of curvature that forms a concave lens surface. Further, the first optical recording medium area may have a negative power smaller than that of the second optical recording medium area. The second optical recording medium region may have a diffractive structure.

  The first optical recording medium region is preferably a flat surface. Furthermore, any one of the plurality of first optical recording medium regions is formed at a height different from the other regions in the optical axis direction, and the difference in height is mλ (m is an integer). You may make it be.

  Further, in the light beam splitting element, it is preferable that the surface opposite to the surface divided into the plurality of regions is a flat surface.

  In a preferred embodiment, the plurality of regions formed on one surface of the light beam splitting element are split concentrically around the optical axis. The light source having the wavelength λ is a blue laser, and 0.075 mm <t1 <0.125 mm and 0.575 mm <t2 <0.625 mm. Furthermore, the focal length with respect to the second optical recording medium is longer than the focal length with respect to the first optical recording medium. In particular, it is preferable that the plurality of regions are composed of 5 or more.

  Further, it is desirable that the lens surface of the objective lens is designed so that aberration correction is performed on the first optical recording medium. Here, “aberration correction” means that the RMS wavefront aberration is 0.070 λrms or less, and more preferably 0.040 λrms or less so that the aberration of the entire optical pickup is 0.070 λrms or less. Furthermore, it is desirable that the NA corresponding to the first optical recording medium is larger than the NA corresponding to the second optical recording medium.

  Further, it is desirable that a working distance between the objective lens and the second optical recording medium is 0.3 mm or more.

  The light beam splitting element according to the present invention condenses the light beam having the wavelength λ on the information recording surface of the first optical recording medium having the transparent substrate having the thickness t1, and the light beam having the wavelength λ has the thickness t2 (t1 <T2) A light beam splitting element provided on the incident side of the objective lens for focusing on the information recording surface of the second optical recording medium having the transparent substrate, wherein at least one surface transmits the transmitted light beam to the objective lens. A first optical recording medium region for condensing the first optical recording medium on the information recording surface of the first optical recording medium, and the transmitted light beam on the information recording surface of the second optical recording medium via the objective lens. Are divided into second optical recording medium areas for focusing, and the second optical recording medium areas have negative power.

  Here, it is preferable that the second optical recording medium region has a radius of curvature that forms a concave lens surface. Further, the first optical recording medium area may have a negative power smaller than that of the second optical recording medium area. The second optical recording medium region may have a diffractive structure.

  The first optical recording medium region is preferably a flat surface. Furthermore, any one of the plurality of first optical recording medium regions is formed at a height different from the other regions in the optical axis direction, and the difference in height is mλ (m is an integer). You may make it be.

  Further, in the light beam splitting element, it is preferable that the surface opposite to the surface divided into the plurality of regions is a flat surface.

  In a preferred embodiment, the plurality of regions formed on one surface of the light beam splitting element are split concentrically around the optical axis.

  The light source having the wavelength λ is a blue laser, and 0.075 mm <t1 <0.125 mm and 0.575 mm <t2 <0.625 mm. Furthermore, the focal length with respect to the second optical recording medium is longer than the focal length with respect to the first optical recording medium. In particular, it is preferable that the plurality of regions are composed of 5 or more.

  An optical pickup optical system according to the present invention includes a light beam splitting element and an objective lens, focuses a light beam having a wavelength λ on an information recording surface of a first optical recording medium having a transparent substrate having a thickness t1, and , An optical pickup optical system that collects a light beam having a wavelength λ on an information recording surface of a second optical recording medium having a transparent substrate having a thickness t2 (t1 <t2), and at least one surface of the light beam splitting element is A first optical recording medium region for condensing the transmitted light beam on the information recording surface of the first optical recording medium via the objective lens; and the second light beam transmitted through the objective lens. Are divided into second optical recording medium areas for focusing on the information recording surface of the optical recording medium, and the second optical recording medium area has a negative power.

  According to the present invention, there is provided an objective lens optical system, a light beam splitting element, and an optical pickup optical system capable of recording and reproducing optical recording media having different thicknesses using a light beam having the same wavelength while ensuring a working distance. Can be provided.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings.

  FIG. 13 shows the specifications of the Blu-ray and HDDVD of the objective lens optical system according to this embodiment. As shown in FIG. 13, Blu-ray and HDDVD have the same wavelength, but different substrate thicknesses. Therefore, when an objective lens optimized for one optical disk is used for the other optical disk, Spherical aberration due to the difference in substrate thickness occurs, and good recording / reproduction cannot be realized. In the objective lens optical system according to the present embodiment, a light beam splitting element is provided in order to suppress the occurrence of such spherical aberration.

  The configuration of the objective lens optical system according to the present embodiment will be described with reference to FIG. The objective lens optical system 100 according to the present embodiment is configured by fixing the light beam splitting element 1 and the objective lens 2 in the lens barrel 3 with an ultraviolet curable adhesive or the like. Here, the lens barrel 3 is made of plastic, for example.

  The incident surface of the light beam splitting element 1 is divided into a plurality of concentric regions around the optical axis. The plurality of regions are preferably 5 or more. In addition, the area | region here is distinguished in shape from the surface shape differing or the level | step difference being provided between adjacent area | regions.

  Since each area has a surface shape that corrects aberrations for either Blu-ray or HDDVD, compatibility of Blu-ray and HDDVD can be realized without providing a plurality of lenses corresponding to each disk. As a result, it is not necessary to provide a mechanism for switching the optical path in the optical pickup, and the optical pickup can be reduced in size and cost.

  More specifically, as shown in FIG. 1, the light beam splitting element 1 is divided into two types of concentric areas, a Blu-ray area 11 and an HDDVD area 12, and the Blu-ray area 11 and HDDVD area are divided. The regions 12 are alternately arranged outward from the optical axis. The outermost peripheral region of the light beam splitting element 1 is necessarily the Blu-ray region 11 having a large NA. The Blu-ray region 11 of the light beam splitting element 1 according to the present embodiment is a flat surface, and the HDDVD region 12 has a radius of curvature that forms a concave lens surface as a whole. Here, the Blu-ray area 11 may not be a flat surface but a convex lens surface. Furthermore, the Blu-ray area 11 may be a concave lens surface having a negative power as long as it is smaller than the negative power of the HDDVD area 12. The exit surface of the beam splitter 1 is a flat surface.

  Any one of the plurality of Blu-ray regions 11 may be formed at a different height from the other regions in the optical axis direction. That is, the plurality of Blu-ray areas 11 may have different plate thicknesses. For example, in the HDDVD area 12 having a concave lens surface, the height in the light beam direction increases as it approaches the outside, so that the Blu-ray area 11 also increases in height in the light beam direction as it approaches the outside. Try to reduce the level difference. However, the height difference (plate thickness difference) needs to be mλ (m is an integer) so as not to cause a phase shift. By adopting such a configuration, the difference in height in the light beam direction between the Blu-ray area 11 and the HDDVD area, that is, the level difference can be reduced, so that the mold can be easily manufactured and the moldability is good. Become. Here, even if the plate thickness of the Blu-ray region 11 is changed by mλ, if the wavelength variation of the incident light flux or the refractive index fluctuation of the light beam splitting element 1 occurs due to the change of the environmental temperature, the portion changed by mλ is changed. Aberrations can occur. From the viewpoint of suppressing the occurrence of such aberration, it is also possible to reduce the amount of the step by performing parallel movement in the light beam direction without changing the thickness of the light beam splitting element 1 in the Blu-ray region 11.

  Although the Blu-ray area 11 and the HDDVD area 12 are alternately arranged here, they may not be alternately. As described above, the regions are distinguished in shape. For example, it is possible to provide a configuration for correcting chromatic aberration in each region. Further, the Blu-ray area 11 and the HDDVD area 12 may not be concentric but may be divided radially around the optical axis. In addition, since one surface (incident surface or exit surface) of the light beam splitting element 1 is a flat surface, it is not necessary to consider the eccentricity of the light incident surface and the light exit surface of the light beam splitting element 1, which is advantageous from the viewpoint of manufacturing. It becomes. In addition, since one surface of the light beam splitting element 1 is largely shifted in the light condensing position, it is preferable that the surface of the light beam splitting element 1 is composed only of the exclusive area of the Blu-ray area 11 and the HDDVD area 12. It does not prevent the area from being provided.

  The light beam splitting element 1 can be made of glass or plastic, but is preferably plastic from the viewpoint of moldability in each region.

  The objective lens 2 is subjected to aberration correction for a Blu-ray disc that is a disc having a thin substrate thickness. In other words, the objective lens 2 is a Blu-ray disc dedicated lens. Since the lens for exclusive use of Blu-ray Disc has a large NA, it is advantageous compared to the case of using a lens for exclusive use of HDDVD in that a dedicated region for exclusive use of Blu-ray can be provided in an outer region where it is difficult to correct aberrations.

  The objective lens 2 can be made of, for example, glass or plastic, but glass having an advantage that a change in refractive index when temperature changes is small and an amount of generated aberration is small is more preferable. The objective lens 2 is preferably made of a glass having an Abbe number νd> 40 or more, in which the change in refractive index when the laser wavelength changes is small and the amount of generated aberration is small.

  The behavior of the light beam incident on the objective lens optical system 100 shown in FIG. 1 will be described. A light beam (laser light) emitted from a light source (not shown) enters the objective lens optical system 100 via a beam splitter and a collimator lens. The light incident on the objective lens optical system 100 is substantially parallel light.

  When the Blu-ray disc 201 is placed at a predetermined position and reproduced and recorded, the light flux from the light source enters both the Blu-ray area 11 and the HDDVD area 12. The light beam incident on the Blu-ray area 11 is incident on the objective lens 2 without being refracted because the Blu-ray area 11 is flat. As described above, since the aberration of the objective lens 2 is corrected with respect to the Blu-ray disc 201, the light beam incident on the objective lens 2 is condensed on the information recording surface of the Blu-ray disc 201.

  On the other hand, when the Blu-ray disc 201 is reproduced and recorded, the light beam incident on the HDDVD area 12 is converted into divergent light because the HDDVD area 12 is a concave lens surface, and then enters the objective lens 2. Since the light beam incident on the objective lens 2 is divergent light, it generates a large spherical aberration and is not condensed on the information recording surface of the Blu-ray disc 201. Therefore, only the light that has been emitted from the light source and incident on the objective lens optical system 100 and that has passed through the Blu-ray area 11 is condensed on the information recording surface of the Blu-ray disc 201.

  Next, when the HDDVD disc 202 is placed at a predetermined position and reproduced and recorded, the light flux from the light source enters both the Blu-ray area 11 and the HDDVD area 12. The light beam incident on the HDDVD region 12 is incident on the objective lens 2 after being converted into divergent light because the HDDVD region 12 is a concave lens surface. Since the light beam incident on the objective lens 2 is divergent light and has a long focal distance, it can be condensed on the information recording surface of the HDDVD disc 202 with a thick substrate while maintaining a long working distance. It becomes.

  On the other hand, when the HDDVD disc 202 is reproduced and recorded, the light beam incident on the Blu-ray area 11 enters the objective lens 2 without being refracted because the Blu-ray area 11 is flat. As described above, since the objective lens 2 is aberration-corrected with respect to the Blu-ray disc 201 and is not corrected with respect to the HDDVD disc 202 having a different thickness, the light beam incident on the objective lens 2 is reflected on the HDDVD disc 202. The light is not condensed on the information recording surface. Therefore, only light that has passed through the HDDVD region 12 out of the light flux emitted from the light source and incident on the objective lens optical system 100 is condensed on the information recording surface of the HDDVD disc 202.

  Here, the working distance (working distance) from the objective lens 2 to the surface of the disc 200 is more disadvantageous for HDDVD than for Blu-ray because the substrate is thicker. In particular, when the objective lens optical system is applied to a thin drive, since the focal length of the objective lens optical system is shortened, a collision between the objective lens 2 and the disk 200 may occur, so it is important to ensure a working distance. Therefore, in the objective lens optical system 100 according to the present embodiment, the HDDVD region 12 is configured with a radius of curvature that becomes a concave lens surface so as to increase the focal length of the HDDVD disc 202 having a large substrate thickness, thereby ensuring a working distance of HDDVD. It was decided.

  Note that the greater the difference in focal length here, the farther the condensing position of the light flux from the Blu-ray area 11 and the HDDVD area 12, the less the influence of unnecessary light, which is advantageous for the formation of a light spot.

  Here, in order to realize a high recording density, the Blu-ray disc 201 and the HDDVD disc 202 are also studied and put into practical use as a two-layer type disc having two information recording surfaces. The distance between the layers in such a two-layer type disc is 25 ± 5 μm or 20 ± 5 μm.

  As shown in FIG. 2, in the present embodiment, it is assumed that the Blu-ray disc 201 has a first layer 1 and a second layer 2. At this time, when recording / reproducing is performed on the layer 1 of the Blu-ray disc 201, the light beam transmitted through the Blu-ray area 11 is collected on the layer 1, but the light beam transmitted through the HDDVD area 12 is collected on the layer 2. there's a possibility that. If the light beam is mistakenly collected on the layer 2 of the Blu-ray disc 201, there is a risk of erroneous writing, erroneous erasing, or a focus servo error. Therefore, in the present embodiment, when recording / reproducing is performed on the layer 1 of the Blu-ray disc 201, a lens surface that does not collect the light beam transmitted through the HDDVD region 12 on the layer 2 is provided. Has.

  Further, the HDDVD region 12 has a lens surface that prevents the light beam transmitted through the HDDVD region 12 from being condensed on the layer 1 when recording / reproducing is performed on the layer 2 of the Blu-ray disc 201.

  Similarly, assume that the HDDVD disc 202 has a first layer 1 and a second layer 2 as shown in FIG. At this time, when recording / reproduction is performed on layer 1 of the HDDVD disc 202, the light beam transmitted through the HDDVD region 12 is condensed on the layer 1, but the light beam transmitted through the Blu-ray region 11 is collected on the layer 2. there's a possibility that. If the light flux is mistakenly collected on layer 2 of the HDDVD disc 202, there is a risk of erroneous writing, erroneous erasure, or a focus servo error. Therefore, in the present embodiment, when recording / reproducing on the layer 1 of the HDDVD disc 202, a lens surface that does not collect the light beam transmitted through the Blu-ray area 11 on the layer 2 is provided. Has.

  Further, the Blu-ray area 11 has a lens surface that prevents the light beam transmitted through the Blu-ray area 11 from being condensed on the layer 1 when recording / reproducing is performed on the layer 2 of the HDDVD disc 202.

  As described above, the light beam splitting element 1 according to the present embodiment is optically designed so as to realize good recording / reproduction even for a two-layer disc.

  FIG. 12 shows a modification of the light beam splitter 1. In the example shown in FIG. 12A, a Blu-ray area 11a and an HDDVD area 12a are provided on the incident surface of the light beam splitting element 1, and a Blu-ray area 11b and an HDDVD area 12b are also provided on the exit surface. At this time, the HDDVD area 12b has a radius of curvature that becomes a concave lens surface, and the Blu-ray area 11b is a flat surface. As in the example shown in FIG. 12A, when the lens surface is provided in a part of the area on both the entrance surface and the exit surface, the field angle characteristics are improved as compared with the case where the lens surface is provided only on one surface. To do.

  In the example shown in FIG. 12B, the incident surface is a flat surface, and the Blu-ray region 11b and the HDDVD region 12b are provided only on the output surface. At this time, the Blu-ray area 11b is a flat surface, and the HDDVD area 12b has a radius of curvature that becomes a concave lens surface.

  Also with the configuration shown in FIG. 12, it is possible to perform recording / reproduction with respect to optical recording media having different thicknesses using light beams having the same wavelength.

  In the above example, at least a part of one surface of the light beam splitting element 1 has a radius of curvature that becomes a concave lens surface. The power may be generated.

  In the above example, the design is based on the assumption that parallel light is incident on the light beam splitter 1. However, the present invention is not limited to this, and divergent light or convergent light is incident on the light beam splitter 1 so as to be output. The surface shapes of the light beam splitting element 1 and the objective lens 2 may be designed so that the incident light is condensed by the objective lens 2.

Example.
FIG. 14 shows the design result of the objective lens optical system of this example, and FIG. 15 shows the lens data. Further, FIG. 16 shows the surface shape data of the objective lens of this embodiment, FIG. 17 shows the surface shape data of the light beam splitting element, and FIG. 18 shows the position of the region in the light beam splitting element. As shown in FIG. 14, the focal length of Blu-ray is f = 1.765 mm, the focal length of HDDVD is f = 1.840 mm, and the focal length of HDDVD is longer than that of Blu-ray. In this embodiment, the curvature radius of the HDDVD area is set to be a concave lens surface, and the focal length is made longer than Blu-ray. As a result, a working distance can be secured more than when the HDDVD area is formed of a flat surface and a convex lens surface.

  As shown in FIG. 14, in Blu-ray and HDDVD, the effective diameter of the lens is different because the image-side numerical aperture NA is different from 0.85 and 0.65, respectively. Since compatibility with conventional DVDs, CDs, and the like has different wavelengths, it has been possible to switch the effective diameter according to the wavelength by using an aperture limiting element used in a wavelength selection filter or the like.

  However, since the present invention is intended for compatibility at the same wavelength, a wavelength selection filter or the like cannot be used. Therefore, in this embodiment, when the Blu-ray area outside the effective diameter of HDDVD is used in HDDVD, it is shaped so as to generate spherical aberration, and the defocus effect by shifting the focal length of Blu-ray and HDDVD is used. The structure has a role of restricting the aperture by generating a large wavefront aberration. As a result, the effective diameter can be switched without separately providing an aperture limiting element in the optical pickup.

・・・・・式（１） Further, the surface shape data shown in FIG. 16 is expressed by the following equation.

・ ・ ・ ・ ・ Formula (1)

In equation (1), the distance (sag amount) from the tangential plane on the optical axis of the aspheric surface at the coordinate point on the aspheric surface where the height from the optical axis is h is Zj (h), and the light of the aspheric surface The curvature (1 / curvature radius) on the axis is C, the conic coefficient is K, the aspherical coefficients from the 4th to the 16th order are A ₄ , A ₆ , A ₈ , A ₁₀ , A ₁₂ , A ₁₄ , A, respectively. ₁₆ is set. FIGS. 4 and 5 show light spot diagrams on the Blu-ray and HDDVD of the objective lens optical system of the present embodiment. The 1 / e ² spot diameter is 0.360 μm for Blu-ray, 0.465 μm for HDDVD, the side lobe is 2.43% for Blu-ray, and 2.61% for HDDVD.

The theoretical value of the 1 / e ² spot diameter for Blu-ray and HDDVD is 0.39 μm or less for Blu-ray and 0.51 μm or less for HDDVD. Therefore, the objective lens optical system of this example has a light spot that is not problematic in practical use. You can see that it is obtained. In the present invention, the 1 / e ² spot diameter and the side lobe can be controlled by changing the surface shape and the number of regions of each region of the beam splitter.

  Next, in order to confirm the adaptability of the objective lens optical system according to the present example to the two-layer disc, the dependence of the light spot intensity on the disc thickness is shown in FIGS. This is because when recording / reproduction is performed on a certain layer, recording / reproduction is performed at positions of +15 to +35 μm (FIGS. 6 and 8) and −15 to −35 μm (FIGS. 7 and 9) from the layer on which recording / reproduction is performed. It is the figure which showed what intensity | strength light spot is formed when the light intensity in the layer currently performed is made into 100%. If the intensity is high, the light beam is condensed on another layer, which causes erroneous writing, erroneous erasure, and focus servo error. In order to compare with the results of this example, the results of comparative examples described later are also shown. Here, in the figure, the data is shown for the region outside +15 μm or −15 μm because the distance between the first layer and the second layer is 25 ± 5 μm, so in the region of +15 μm to −15 μm, This is because a light spot that affects other layers is not formed. As can be seen from the figure, the light condensing to the other layer is controlled by using the objective lens optical system of the present embodiment. Therefore, it can be seen that good recording / reproduction can be realized even for a two-layer disc by using the objective lens optical system of the present invention.

  FIG. 24A shows a ray diagram when recording / reproduction is performed on the layer 1 of the HDDVD using the objective lens optical system according to the present example. As shown in the figure, the light beam that has passed through the HDDVD region 12 and entered through the objective lens 2 is focused on the layer 1, while passing through the Blu-ray region 11 and entered through the objective lens 2. It can be seen that the luminous flux collected is concentrated at a position far away from the layer 2. Therefore, by using the objective lens optical system of the present invention, good recording / reproduction can be realized even for a two-layer disc.

  Whether or not the light beam is condensed at a predetermined position can also be confirmed from the sum signal at the detector. When the light beam is condensed at the defocus position within the above range, the intensity of the sum signal increases at the condensing position. Therefore, it can be determined that light collection is suppressed when the sum signal intensity in the predetermined range is suppressed to one fifth or less, preferably one tenth or less of the sum signal intensity on the information recording surface.

  Further, the inter-surface distance of surface number 7 in FIG. 15 shows the working distance in the example, which was 0.623 mm for Blu-ray and 0.466 mm for HDDVD. This value satisfies “0.3 mm or more”, which is a desirable working distance condition for solving the problem of contact between the optical head and the disk. On the other hand, in the comparative example to be described later, as shown in FIG. 20, although it was 0.4614 mm for Blu-ray, it was 0.1453 mm for HDDVD, and the condition was not satisfied.

Comparative example.
FIG. 19 shows the design result of the objective lens optical system of this comparative example, and FIG. 20 shows the lens data. FIG. 21 shows the surface shape data of the objective lens of this comparative example, FIG. 22 shows the surface shape data of the light beam splitting element, and FIG. 23 shows the position of the region in the light beam splitting element. As shown in FIG. 19, unlike the example, the comparative example has the same focal length for both Blu-ray and HDDVD.

  The surface shape of the beam splitting element has the same radius of curvature in the Blu-ray and HDDVD regions, and only the aspheric coefficient of the HDDVD region is used, and only correction of spherical aberration caused by the substrate thickness difference between Blu-ray and HDDVD is considered.

FIGS. 10 and 11 show light spot diagrams in Blu-ray and HDDVD in the objective lens optical system of the comparative example. The 1 / e ² spot diameter is 0.377 μm for Blu-ray and 0.515 μm for HDDVD. In addition, the side lobe is 1.88% for Blu-ray and 1.76% for HDDVD, which is a satisfactory result.

  As described above, the disc thickness dependence of the light spot intensity is shown in FIGS. A light spot having an intensity of about 4% is formed in the vicinity of -20 μm in Blu-ray, and an intensity of about 8% is formed in the vicinity of 15 μm in HDDVD. . Therefore, in the case of the objective lens optical system of the comparative example, erroneous writing and erroneous erasure with a double-layer disc, an error with a focus servo, etc. are likely to occur, and there is a possibility that good recording and reproduction cannot be performed with respect to the dual-layer disc. .

  FIG. 24B shows a ray diagram when recording / reproduction is performed on layer 1 of the HDDVD using the objective lens optical system according to this comparative example. As shown in the figure, the light beam that has passed through the HDDVD region 12 and entered through the objective lens 2 is focused on the layer 1, while passing through the Blu-ray region 11 and entered through the objective lens 2. It can be seen that the collected light beam is also collected in the vicinity of layer 2. For this reason, in the case of the objective lens optical system of the comparative example, erroneous writing and erroneous erasure with a double-layer disc, an error with a focus servo, etc. are likely to occur, and there is a possibility that good recording and reproduction cannot be performed with respect to the dual-layer disc. There is.

It is a schematic block diagram of the objective lens optical system concerning this Embodiment. It is a figure for demonstrating the problem at the time of recording / reproducing with respect to a double layer disc using the objective-lens optical system concerning this Embodiment. It is a figure for demonstrating the problem at the time of recording / reproducing with respect to a double layer disc using the objective-lens optical system concerning this Embodiment. It is a light spot figure at the time of irradiating the light beam for Blu-ray discs with respect to the objective-lens optical system concerning a present Example. It is a light spot figure at the time of irradiating the light beam for HDDVD with respect to the objective-lens optical system concerning a present Example. It is a graph which shows the disc thickness dependence of the light intensity at the time of irradiating the light beam for Blu-ray discs with respect to the objective-lens optical system concerning a present Example. It is a graph which shows the disc thickness dependence of the light intensity at the time of irradiating the light beam for Blu-ray discs with respect to the objective-lens optical system concerning a present Example. It is a graph which shows the disc thickness dependence of the light intensity at the time of irradiating the light beam for HDDVD with respect to the objective-lens optical system concerning a present Example. It is a graph which shows the disc thickness dependence of the light intensity at the time of irradiating the light beam for HDDVD with respect to the objective-lens optical system concerning a present Example. It is a graph which shows the disc thickness dependence of the light intensity at the time of irradiating the light beam for Blu-ray discs with respect to the objective-lens optical system concerning a comparative example. It is a graph which shows the disc thickness dependence of the light intensity at the time of irradiating the light beam for HDDVD with respect to the objective-lens optical system concerning a comparative example. It is a partial enlarged view of the light beam splitting element concerning this Embodiment. It is a comparison table of Blu-ray and HDDVD specifications. It is a table | surface which shows the design data of the objective-lens optical system concerning a present Example. It is a table | surface which shows the lens data of the objective-lens optical system concerning a present Example. It is a table | surface which shows the surface shape data of the objective lens concerning a present Example. It is a table | surface which shows the surface shape data of the light beam splitting element concerning a present Example. It is a table | surface which shows the area | region position in the light beam splitting element concerning a present Example. It is a table | surface which shows the design data of the objective-lens optical system concerning this comparative example. It is a table | surface which shows the lens data of the objective-lens optical system concerning this comparative example. It is a table | surface which shows the surface shape data of the objective lens concerning this comparative example. It is a table | surface which shows the surface shape data of the light beam splitting element concerning this comparative example. It is a table | surface which shows the area | region position in the light beam splitting element concerning this comparative example. It is a light ray figure in the light beam splitting element concerning a present Example and this comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light beam splitting element 2 Objective lens 3 Lens barrel 11 Blu-ray area 12 HDDVD area 100 Objective lens optical system 200 Disc 201 Blu-ray disc 202 HDDVD disc

Claims (26)

A light beam splitting element and an objective lens are provided, a light beam having a wavelength λ is condensed on an information recording surface of a first optical recording medium having a transparent substrate having a thickness t1, and a light beam having a wavelength λ is formed with a thickness t2 ( An objective lens optical system for focusing on the information recording surface of the second optical recording medium having a transparent substrate with t1 <t2),
At least one surface of the light beam splitting element has a first optical recording medium region for condensing the transmitted light beam on the information recording surface of the first optical recording medium via the objective lens, and the transmitted light beam. Divided into second optical recording medium areas for focusing on the information recording surface of the second optical recording medium via the objective lens;
The second optical recording medium region is an objective lens optical system having negative power.   The objective lens optical system according to claim 1, wherein the second optical recording medium region has a radius of curvature that forms a concave lens surface.   3. The objective lens optical system according to claim 1, wherein the first optical recording medium area has a negative power smaller than that of the second optical recording medium area.   2. The objective lens optical system according to claim 1, wherein the second optical recording medium region has a diffractive structure.   The objective lens optical system according to claim 1, wherein the first optical recording medium region is a flat surface.   One of the plurality of first optical recording medium regions is formed at a height different from the other regions in the optical axis direction, and the difference in height is mλ (m is an integer). 6. The objective lens optical system according to claim 5, wherein:   7. The objective lens optical system according to claim 1, wherein in the light beam splitting element, a surface opposite to the surface divided into a plurality of regions is a flat surface.   The objective lens optical system according to claim 1, wherein the plurality of regions formed on one surface of the light beam splitting element are concentrically divided with the optical axis as a center.   9. The objective lens according to claim 1, wherein the light source having the wavelength λ is a blue laser and satisfies 0.075 mm <t1 <0.125 mm and 0.575 mm <t2 <0.625 mm. Optical system.   9. The objective lens optical system according to claim 1, wherein a focal length with respect to the second optical recording medium is longer than a focal length with respect to the first optical recording medium.   The objective lens optical system according to any one of claims 1 to 10, wherein the plurality of regions includes five or more.   The objective lens optical system according to claim 1, wherein the lens surface of the objective lens is designed so that aberration correction is performed with respect to the first optical recording medium.   13. The objective lens optical system according to claim 12, wherein an NA corresponding to the first optical recording medium is larger than an NA corresponding to the second optical recording medium.   The objective lens optical system according to claim 1, wherein a working distance between the objective lens and the second optical recording medium is 0.3 mm or more. The light beam having the wavelength λ is condensed on the information recording surface of the first optical recording medium having the transparent substrate having the thickness t1, and the light beam having the wavelength λ is the first having the transparent substrate having the thickness t2 (t1 <t2). A light beam splitting element provided on the incident side of the objective lens for focusing on the information recording surface of the optical recording medium,
At least one surface has a first optical recording medium region for condensing the transmitted light beam on the information recording surface of the first optical recording medium via the objective lens, and the transmitted light beam via the objective lens. Are divided into second optical recording medium areas for focusing on the information recording surface of the second optical recording medium,
The region for the second optical recording medium is a light beam splitting element having negative power.   16. The light beam splitting element according to claim 15, wherein the second optical recording medium region has a radius of curvature that forms a concave lens surface.   17. The light beam splitting element according to claim 15, wherein the first optical recording medium region has a negative power smaller than that of the second optical recording medium region.   16. The light beam splitting element according to claim 15, wherein the second optical recording medium region has a diffractive structure.   The light beam splitting element according to claim 15, wherein the area for the first optical recording medium is a plane.   One of the plurality of first optical recording medium regions is formed at a height different from the other regions in the optical axis direction, and the difference in height is mλ (m is an integer). 20. The light beam splitting element according to claim 19, wherein the light beam splitting element is provided.   21. The light beam splitting element according to claim 15, wherein the surface opposite to the surface divided into the plurality of regions is a flat surface.   The light beam splitting element according to any one of claims 15 to 21, wherein the plurality of regions formed on one surface of the light beam splitting element are split concentrically around the optical axis.   23. The beam splitting according to claim 15, wherein the light source having the wavelength [lambda] is a blue laser, and 0.075 mm <t1 <0.125 mm and 0.575 mm <t2 <0.625 mm. element.   The light beam splitting element according to any one of claims 15 to 23, wherein a focal length with respect to the second optical recording medium is longer than a focal length with respect to the first optical recording medium.   25. The light beam splitting element according to claim 15, wherein the plurality of regions includes five or more. A light beam splitting element and an objective lens are provided, a light beam having a wavelength λ is condensed on an information recording surface of a first optical recording medium having a transparent substrate having a thickness t1, and a light beam having a wavelength λ is formed with a thickness t2 ( An optical pickup optical system for condensing on an information recording surface of a second optical recording medium having a transparent substrate of t1 <t2),
At least one surface of the light beam splitting element has a first optical recording medium region for condensing the transmitted light beam on the information recording surface of the first optical recording medium via the objective lens, and the transmitted light beam. Divided into second optical recording medium areas for focusing on the information recording surface of the second optical recording medium via the objective lens;
The second optical recording medium area is an optical pickup optical system having negative power.

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