JP2013206496A - Optical pickup device and objective - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pickup device that can suppress NA change of an objective which is a problem in making the capacity of an optical disk larger, and suitably records and reproduces information, and an objective used therefor.SOLUTION: In an optical pickup device that records and reproduces information by converging light on an information recording layer of a first optical disk having an interval of 0.05 mm or more between an information recording layer closest to a luminous flux incidence surface and an information recording layer farthest from the luminous flux incidence surface, an objective OBJ is a plastic-made single lens of 0.87 or more and less than 0.95 in image-side numerical aperture (NA) satisfies a condition (2) of -0.35≥D/f≥-0.90, where D[mm] is the distance (represented with a negative value) from a plane in contact with a light source-side plane peak of the objective OBJ to an aperture limit position of an aperture limit stop AP and f[mm] is the focal length of the objective OBJ at a wavelength λ1.

Description

  The present invention relates to an optical pickup device and an objective lens capable of recording / reproducing information with respect to an optical disc having a plurality of information recording layers.

In recent years, an optical pickup device for recording / reproducing information on / from an optical disc has been rapidly developed and used in various fields. In such an optical pickup device, as the amount of information handled increases, the capacity of the optical disk has been increased by shortening the wavelength of the light source and increasing the NA of the objective lens. As is well known, since the spot size collected by the objective lens is proportional to (wavelength λ / NA), the spot size and the capacity of the optical disc have an inversely proportional relationship. Hereinafter, the specifications of optical disks (used wavelength, NA, capacity per sheet) are listed.
CD: 780 nm, NA 0.45, 0.65 GB
DVD: 650 nm, NA 0.60, 4.7 GB
Blu-ray Disc (BD): 405 nm, NA 0.85, 25 GB

  In addition, DVDs and BDs also realize so-called multilayer optical disks in which the recording capacity per optical disk is increased by laminating a plurality of information recording layers in the thickness direction. As an example of a multi-layer type optical disc, BDXL standard optical discs having a recording capacity of 100 GB per sheet by laminating three information recording layers having a recording capacity of 33.3 GB are put on the market.

  However, as the amount of information to be recorded further increases in the future, it is expected that the capacity of an optical disk having excellent long-term storability and durability will be further increased as compared with hard disk drives and flash memories. However, in the ultraviolet region having a wavelength shorter than that of the blue-violet laser beam currently used in BD, there is a fact that the absorptance of the optical material used for the optical disc and the optical pickup device is large. Therefore, in order to use ultraviolet light as a light source wavelength, it faces a problem that it is necessary to develop an inexpensive optical material that does not absorb ultraviolet light. Considering that it is extremely difficult to overcome such a problem with the current technology, it is considered that the realization of the short wavelength of the light source, which is one of the means for increasing the capacity, is low. Therefore, in order to achieve further increase in capacity of the optical disk, it can be said that it is effective to increase the number of optical disks and increase the NA of the objective lens.

  Among them, regarding the multilayering of optical disks, in a system with an objective lens NA of 0.85 and a light source wavelength of 405 nm, recording is 512 GB on an optical disk having a diameter of 12 cm, which is the same size as BD by the structure of 32 GB per layer and 16 recording layers. Non-patent document 1 describes an example of an optical disk that has achieved capacity. Further, with regard to increasing the NA of the objective lens, Patent Document 1 describes an example of an objective lens having an NA greater than 0.85.

IEICE Technical Report, vol. 110, no. 438, MR2010-57, pp. 1-6, March 2011

Japanese Patent No. 4817036

  By the way, the selection of the information recording layer for recording / reproducing information is performed by moving the objective lens in the optical axis direction (referred to as focusing) and spherical aberration caused by the difference in thickness from the light incident surface of the optical disk to the information recording layer. In order to correct this, the coupling lens or the like disposed between the light source and the objective lens is moved in the optical axis direction, and the magnification of the objective lens is changed. However, if the magnification of the optical system is changed, the divergence angle or convergence angle of the light beam incident on the objective lens changes, and the NA of the objective lens, which should be essentially constant, may change. The change in NA causes a change in spot diameter on the surface of the information recording layer, which may cause a reading error.

  The present invention has been made in consideration of the above-described problems, and an optical pickup capable of suppressing NA change of an objective lens, which is a problem in promoting an increase in capacity of an optical disk, and appropriately recording / reproducing information. An object is to provide an apparatus and an objective lens used therefor.

The optical pickup device according to claim 1 includes a first light source that emits a light beam having a wavelength λ1 (390 nm <λ1 <415 nm), a spherical aberration correction element, an aperture limiting element, and an objective lens. A light beam having a wavelength λ1 emitted from one light source is incident on the objective lens through the spherical aberration correction element and the aperture limiting element, and has a plurality of information recording layers stacked in the optical axis direction. An optical pickup device for recording / reproducing information by focusing light on the information recording layer of the first optical disc having a distance of 0.05 mm or more between the information recording layer closest to the surface and the information recording layer farthest from the light incident surface Because
When the information recording layer for recording / reproducing information is changed from one to the other, the magnification of the objective lens is changed by the spherical aberration correction element, which is caused by the difference in thickness from the light incident surface to the information recording layer. The objective lens is a single lens, and the aperture limiting element is disposed on the optical disc side with respect to the plane contacting the light source side surface apex of the objective lens. It is characterized by satisfying the formula.
0.87 ≦ NA ≦ 0.95 (1)
−0.35 ≧ D / f ≧ −0.90 (2)
However,
NA: Numerical aperture D [mm] of the objective lens: Distance in the optical axis direction from the plane contacting the light source side surface apex of the objective lens to the aperture limiting position of the aperture limiting element (represented by a negative value)
f [mm]: focal length of the objective lens at the wavelength λ1

  In the conventional optical pickup device, the number of information recording layers is as few as 2 to 4 layers, and the change in magnification for dealing with a plurality of layers is 0 or very small. It did not turn. However, it has been found that when the number of information recording layers is increased and the NA is higher than 0.85, the magnification change when selecting the information recording layer is increased. The present inventor has found that in such an optical pickup device, the problem of NA change accompanying change in magnification becomes particularly significant. More specific description will be given below.

  The principle of the present invention will be described with reference to FIG. FIG. 1A is an optical path diagram of the objective lens OL and the aperture limiting element AP shown as a comparative example, and FIG. 1B is an optical path diagram of the objective lens OL and the aperture limiting element AP shown as an example of the present invention. Yes, the left side is the light source side, and the right side is the optical disc side. In the comparative example of FIG. 1A, the aperture limiting element AP is arranged on the light source side with respect to the plane in contact with the surface vertex P of the objective lens OL. Therefore, when the numerical aperture when the parallel light beam indicated by the solid line is incident on the objective lens OL is NA0, the magnification of the objective lens is changed, and the numerical aperture when the divergent light indicated by the dotted line is incident is NA1 or It can be seen that the numerical aperture NA2 is NA1> NA0> NA2 when the magnification of the objective lens is changed and the convergent light indicated by the alternate long and short dash line is incident, and the numerical aperture is changed.

  On the other hand, according to the present invention, as shown in FIG. 1B, the aperture limiting element AP is disposed on the optical disc side with respect to the plane contacting the surface vertex P of the objective lens OL. When the numerical aperture when the parallel light beam indicated by the solid line is incident on the objective lens OL is NA0, the magnification of the objective lens is changed, and the numerical aperture when the divergent light indicated by the dotted line is incident is NA1 or the objective lens. The numerical aperture NA2 when the convergent light indicated by the alternate long and short dash line is incident is NA1≈NA0≈NA2, and the change in the numerical aperture can be suppressed.

  From the above, in order to suppress the change in the numerical aperture (NA) when selecting the information recording layer, a preferable result is obtained by the technical idea that the aperture limiting element is arranged on the optical disc side with respect to the plane contacting the surface vertex of the objective lens. It turns out that it is obtained. At this time, the present inventor has examined how much the aperture should be arranged on the optical disc side with respect to the surface vertex of the objective lens.

  As a result of such examination, when NA is larger than 0.87, spherical aberration generated when the light is condensed on an information recording layer different from a certain information recording layer becomes excessive, which is corrected by a variation in magnification. I found out that some ideas were needed.

  More specifically, for an objective lens with NA of 0.87 or more and NA of 0.95 or less, spherical aberration that occurs when the distance from the light incident surface of the optical disk is changed by 0.05 mm or more is corrected by changing the magnification. The change in NA was investigated using D / f as a parameter. (Note that D [mm] is the distance in the optical axis direction from the plane in contact with the apex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element, and the direction from the plane in contact with the apex of the objective lens toward the optical disc side. F [mm] is the focal length of the objective lens at the wavelength λ1.) As a result, if the D / f range is 0.35 or more and 0.90 or less, even if the magnification changes. It was found that the change in NA can be suppressed.

  Therefore, a relatively large change in magnification is required to correct spherical aberration so as to focus on the information recording layer of a multi-layer optical disk that satisfies the NA of equation (1) and the substrate thickness change is in the range of 0.05 mm or more. However, the NA change can be suppressed by satisfying the expression (2), and stable optical disc recording / reproduction can be performed.

According to a second aspect of the present invention, there is provided the optical pickup device according to the first aspect, wherein the following expression is satisfied.
0.90 ≦ NA ≦ 0.94 (1 ′)

According to a third aspect of the present invention, there is provided the optical pickup device according to the first or second aspect, wherein the following expression is satisfied.
−0.45 ≧ D / f ≧ −0.85 (2 ′)

  The objective lens of Claim 4 is used for the optical pick-up apparatus in any one of Claims 1-3, It is characterized by the above-mentioned.

  The optical pickup device according to the present invention includes a first light source that emits a light beam having a wavelength λ1 (390 nm <λ1 <415 nm).

  The first optical disk used in the optical pickup device of the present invention has a plurality of information recording layers stacked in the thickness direction (optical axis direction). In an optical disc having five or more information recording layers, the optical pickup device of the present invention can be used more suitably. More preferably, it has 10 or more information recording layers. In the first optical disc, the distance between the information recording layer closest to the light beam incident surface and the information recording layer farthest from the light beam incident surface is 0.05 mm or more. In addition to the information recording layer, the optical disc may have a reference layer (referred to as a reference layer) for controlling the position of the objective lens. The objective lens position control here refers to at least one of tracking and focusing. There may be only one reference layer or a plurality of reference layers, but it is preferable that the number of layers of the information recording layer is smaller than the number of layers of the information recording layer. It is preferable because the design of the apparatus becomes easy. When the optical disc has a reference layer, the optical pickup device of the present invention further includes a second light source having a wavelength different from that of the first light source, and records / reproduces a light beam having a wavelength λ1 emitted from the first light source. It is preferable that the light beam having the wavelength λ2 emitted from the second light source is condensed on the reference layer as a light beam for position control of the objective lens at the same time as the light beam is condensed on the information recording layer. In this case, it is preferable to have a second focusing means for focusing the light beam emitted from the second light source on the reference layer, and the second focusing means focuses the light beam emitted from the first light source on the information recording layer. It is preferable to be independent of the first focusing means. Further, when the numerical aperture NA required for recording / reproducing information on the optical disc is 0.87 or more and 0.95 or less, the effect of the present invention becomes more remarkable, and the NA is 0.90 or more. , 0.94 or less, it becomes even more prominent. The optical disk preferably has a protective substrate layer on the surface, and the thickness of the protective substrate layer is not particularly limited.

  Further, the optical pickup device of the present invention further includes a third light source having the same wavelength as the first light source but having a different emission power, and when reproducing information from the first optical disc, the first light source When a light beam having a wavelength λ1 emitted from the light source is condensed on the information recording layer of the objective lens and information is recorded on the first optical disc, the light beam having a wavelength λ1 emitted from the third light source is The light may be condensed on the information recording layer. In general, since optical discs require higher wavelength energy than information reproduction, in such a configuration, the third light source emits light with higher power than the first light source. .

  However, the optical pickup device of the present invention further includes a fourth light source that emits a light beam having a wavelength λ4 (λ1 <λ4) and / or a fifth light source that emits a light beam having a wavelength λ5 (λ4 <λ5). In addition to the optical disc, an optical pickup device compatible with at least one or all of BD, DVD, and CD may be used.

  In the BD, information is recorded / reproduced by an objective lens having an NA of 0.85, and the thickness of the protective substrate is about 0.1 mm. DVD is a general term for DVD-series optical discs in which information is recorded / reproduced by an objective lens having an NA of about 0.60 to 0.67, and the thickness of the protective substrate is about 0.6 mm. DVD-ROM, DVD -Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, etc. CD is a general term for CD optical discs in which information is recorded / reproduced by an objective lens having an NA of about 0.45 to 0.53, and the thickness of the protective substrate is about 1.2 mm. CD-Audio, CD-Video, CD-R, CD-RW and the like. As for the recording density, the recording density of BD is the highest, followed by the order of DVD and CD.

  In the present specification, the light sources such as the first light source to the fifth light source are preferably laser light sources. As the laser light source, a semiconductor laser, a silicon laser, or the like can be preferably used. The first wavelength λ1 of the first light beam emitted from the first light source, the fourth wavelength λ4 (λ4> λ1) of the fourth light beam emitted from the fourth light source, and the fifth wavelength of the fifth light beam emitted from the fifth light source. The wavelength λ5 (λ5> λ4) preferably satisfies the following conditional expression.

1.5 × λ1 <λ4 <1.7 × λ1
1.9 × λ1 <λ5 <2.1 × λ1

  The first wavelength λ1 of the first light source is preferably 350 nm or more and 440 nm or less, more preferably 390 nm or more and 415 nm or less, and the fourth wavelength λ4 of the fourth light source is preferably 570 nm or more and 680 nm or less. More preferably, the wavelength is 630 nm or more and 670 nm or less, and when the fifth light source is provided, the fifth wavelength λ5 of the fifth light source is preferably 750 nm or more and 880 nm or less, and more preferably 760 nm or more and 820 nm or less. Moreover, when it has a 2nd light source, it is preferable that a 2nd light source and a 4th light source are common.

  Further, at least two of the first light source, the second light source, the fourth light source, and the fifth light source may be unitized. The unitization means that the first light source and the second light source are fixedly housed in one package, for example. However, the unitization is not limited to this, and the two light sources are fixed so that the aberration cannot be corrected. Is widely included. In addition to the light source, a light receiving element to be described later may be packaged.

  In this specification, “recording / reproducing information” means performing at least one of recording and reproducing information. Further, in this specification, the “light-incident surface of the optical disc” refers to the surface of the optical disc that faces the objective lens when recording / reproducing information.

  The optical pickup device may include a light receiving element that receives a light beam reflected from the optical disk. As the light receiving element, a photodetector such as a photodiode is preferably used. Light reflected on the information recording surface of the optical disc enters the light receiving element, and a read signal of information recorded on each optical disc is obtained using the output signal. Furthermore, it detects the change in the light amount due to the spot shape change and position change on the light receiving element, performs focus detection and track detection, and based on this detection, the objective lens can be moved for focusing and tracking I can do it. The light receiving element may comprise a plurality of photodetectors. The light receiving element may have a main photodetector and a sub photodetector. For example, two sub photodetectors are provided on both sides of a photodetector that receives main light used for recording and reproducing information, and the sub light for tracking adjustment is received by the two sub photodetectors. It is good also as a simple light receiving element. The light receiving element may have a plurality of light receiving elements corresponding to the respective light sources.

  The condensing optical system used for the optical pickup device has an objective lens. The condensing optical system may include only the objective lens, but may include a coupling lens such as a collimator lens in addition to the objective lens. The coupling lens is a single lens or a lens group that is disposed between the objective lens and the light source and changes the divergence angle of the light beam. The collimating lens is a kind of coupling lens, and is a lens that emits light incident on the collimating lens as parallel light. Further, the condensing optical system has an optical element such as a diffractive optical element that divides the light beam emitted from the light source into a main light beam used for recording and reproducing information and two sub light beams used for tracking and the like. May be. In this specification, the objective lens refers to an optical system that is disposed at a position facing the optical disk in the optical pickup device and has a function of condensing the light beam emitted from the light source onto the information recording surface of the optical disk. Preferably, the objective lens is an optical system which is disposed at a position facing the optical disk in the optical pickup device and has a function of condensing the light beam emitted from the light source on the information recording surface of the optical disk, and further includes an actuator An optical system that can be integrally displaced at least in the optical axis direction.

  The objective lens is a single objective lens. The objective lens preferably has a refractive surface that is aspheric.

The refractive index n of the objective lens material preferably satisfies the following formula.
1.59 ≦ n ≦ 1.64 (3)

  When n is 1.59 or more, the tilt angle of the light source side optical surface does not become too large, so that the processing accuracy of the mold can be improved and the objective lens is strong against parallel decentering of the optical surface. On the other hand, when n is 1.64 or less, the peripheral portion of the optical surface on the light source side can suppress the amount of protrusion on the optical information recording medium side smaller than the vicinity of the optical axis. Therefore, the chromatic aberration of the objective lens can be kept small.

The Abbe number refractive index μd of the objective lens material preferably satisfies the following expression.
50 ≦ μd ≦ 65 (4)
If μd is 50 or more, the chromatic aberration of the objective lens is small and the objective lens is strong against mode hopping of the semiconductor laser. On the other hand, when μd is 65 or less, since the refractive index does not become too small, it is easy to form the shape of the optical surface on the light source side, and the range of selection of optical materials is widened.

  The objective lens may be a glass lens, a plastic lens, or a hybrid lens in which an optical path difference providing structure or the like is provided on a glass lens with a photocurable resin or the like.

  When the objective lens is a glass lens, the influence of temperature change in the optical pickup device can be reduced.

When the objective lens is a plastic lens, it is preferable to use a cyclic olefin-based resin material. Among the cyclic olefin-based materials, the refractive index at a temperature of 25 ° C. with respect to a wavelength of 405 nm is 1.52 to 1.60. The refractive index change rate dN / dT (° C. ⁻¹ ) is −20 × 10 ^{−5 to} −5 × 10 ^{− with} respect to the wavelength of 405 nm accompanying the temperature change within the range of −5 ° C. to 70 ° C. ^It is more preferable to use a resin material within a range of ⁵ (more preferably, −10 × 10 ^{−5 to} −8 × 10 ⁻⁵ ). When the objective lens is a plastic lens, the coupling lens is preferably a plastic lens.

  The objective lens may have an optical path difference providing structure for correcting a spherical aberration caused by a change in refractive index with respect to a temperature change or a spherical aberration caused by a change in wavelength of the light source. The optical path difference providing structure referred to in this specification is a general term for structures that add an optical path difference to an incident light beam. The optical path difference providing structure also includes a phase difference providing structure for providing a phase difference. The phase difference providing structure includes a diffractive structure. The optical path difference providing structure has a step, preferably a plurality of steps. This step adds an optical path difference / phase difference to the incident light flux. The optical path difference added by the optical path difference providing structure may be an integer multiple of the wavelength of the incident light beam or a non-integer multiple of the wavelength of the incident light beam. The steps may be arranged with a periodic interval in the direction perpendicular to the optical axis, or may be arranged with a non-periodic interval in the direction perpendicular to the optical axis. Preferably, the optical path difference providing structure is a diffractive structure.

  As the sine condition violation amount of the objective lens, the sine condition violation amount has a positive maximum value between 50% and 90% of the effective aperture radius h, and the sine condition violation amount monotonously decreases in the peripheral part. It is desirable to do. As a result, it is possible to appropriately correct the change in the divergence and convergence of incident light without leaving high-order spherical aberration caused by the change in the position of the information recording layer for recording / reproducing information. Here, “high-order spherical aberration” refers to spherical aberration of the fifth or higher order. When the light beam having the wavelength λ1 is incident on the objective lens at the design magnification, it is preferable that the sine condition violation amount monotonously decreases in the peripheral portion from the position where the sine condition violation amount has a positive maximum value. “Monotonically decreasing” means that it continues to decrease from a maximum value within a certain range. Preferably, when the light beam having the wavelength λ1 is incident on the objective lens at the design magnification, the sine condition violation amount has a positive maximum value between 60% and 90% of the effective aperture radius h.

  Further, as the shape of the S2 surface (optical disc side surface) of the objective lens, it is sufficient if the inclination is continuous, and when the S2 surface shape is continuous, the continuous S2 surface shape is the same as that of the lens surface. It includes a shape in which a section is translated in the optical axis direction. FIG. 2 is a cross-sectional view of an objective lens in which the S2 surface has an inflection point but no concave portion, and FIG. 3 is a cross-sectional view of the objective lens in which the S2 surface has an inflection point and a concave portion.

  Referring to FIGS. 2 and 3, having such a surface shape of S2 surface 12a or 12b has the following effects. That is, a normal objective lens is a biconvex lens having convex surfaces on both sides, or a meniscus lens composed of a convex lens and a concave lens. Each lens has its characteristics. When the distance from the center of the S2 surface and the distance to the disk 30 is the working distance (WD), the meniscus lens has a concave S2 surface, so the working distance compared to the biconvex lens. Will be shorter. On the other hand, the off-axis field angle characteristic shows that the meniscus lens is better than the convex lens because both surfaces are curved in the same direction.

  On the other hand, the objective lens 1a or 1b can satisfy both characteristics of the meniscus lens and the biconvex lens because the concave portion is formed on the outer peripheral side and the convex portion is formed in the central portion. In other words, the working distance is long, and the angle-of-view characteristic can be improved as the off-axis characteristic while satisfying the on-axis characteristic.

  In other words, in the objective lens 1a shown in FIG. 2, the sag amount gradually increases from the radial center h0 of the S2 surface 12a toward the outer periphery 13a, and the sag amount almost changes after a radius of radius h3 in FIG. do not do. In the objective lens 1b shown in FIG. 3, the sag amount gradually increases from the radial center h0 of the S2 surface 12b toward the outer periphery 13b, and when reaching a certain radius, in this example, the radius h4, the outer periphery 13b Until then, the amount of sag gradually becomes smaller. From the position where the increase / decrease amount of the sag amount changes, the inner diameter has a characteristic of a biconvex lens, and the outer diameter can have the characteristics of a meniscus lens.

  In other words, since the central part of the objective lens is a biconvex lens, the working distance can be lengthened, and since it does not have a very large radius of curvature, it is possible to improve the angle of view characteristics while being a biconvex lens. is there. Then, by providing the meniscus feature on the outer diameter side (outer peripheral portion of the S2 surface) from the position where the increase / decrease amount of the sag amount changes, it is possible to obtain a favorable angle-of-view characteristic that is a feature of the meniscus lens. Further, since the portion corresponding to the meniscus lens is formed in the outer peripheral portion instead of the lens central portion, the working distance is not shortened. In this way, the objective lenses 1a and 1b are long by incorporating the features of the meniscus lens by forming a substantially flat or concave portion in the outer peripheral portion and forming the convex portion in the inner peripheral portion and incorporating the features of the biconvex lens. A working distance can be secured, and an angle of view characteristic as well as an on-axis characteristic can be improved. From the above viewpoint, it is preferable that the radii h1 to h4 in FIGS. 2 and 3 are all present in the region through which the laser beam passes.

  Further, in the objective lens 1a shown in FIG. 2, the meniscus shape is formed from the position of the radius h3 where the increase / decrease amount of the sag changes to the outer diameter, but the objective lens 1b shown in FIG. 3 has a minimum value k. Extreme meniscus shape. By adopting such a shape, the working distance can be further increased, and the angle of view characteristics can be further improved as on-axis characteristics and off-axis characteristics.

  On the other hand, the effective optical surface diameter of the S1 surface (light source side surface) of the objective lens is preferably φ2.0 mm or more and φ3.0 mm or less. When the effective diameter is φ2, Omm or more, the working distance of the objective lens can be secured sufficiently, and the risk of collision with the optical disk can be reduced. In addition, since the amount of image height coma generated when the magnification of the objective lens changes can be suppressed, the characteristics of the optical pickup device are improved. On the other hand, when the effective diameter is φ3.0 mm or less, the optical system can be made compact, which is advantageous for downsizing the pickup device.

  It should be noted that information recording / reproduction of an information recording layer (referred to as layer A) of an optical disc having a plurality of information recording layers is performed, and then information recording / reproduction of another information recording layer (referred to as layer B) is performed. In this case, spherical aberration due to the difference between the thickness from the light beam incident surface of the optical disk to the layer A and the thickness from the light beam incident surface of the optical disk to the surface layer B occurs. Therefore, a spherical aberration correction element that corrects the spherical aberration caused by the difference in thickness from the light beam incident surface of the optical disk to the information recording layer in different information recording layers is required.

  Examples of such spherical aberration correction elements include a liquid crystal device that corrects spherical aberration of a light beam having a wavelength λ1, and at least one lens that is movable in the optical axis direction, as a device that takes in a divergent light beam from a light source and guides it to an objective lens. And a magnification conversion means for changing the magnification of the objective lens of the light beam having the wavelength λ1. Examples of the magnification conversion means include the above-described single ball coupling lens movable in the optical axis direction, a single ball collimator lens, and at least one positive lens and at least one negative lens arranged in a parallel beam. A two-group coupling lens that is movable in the optical axis direction, or a beam expander (beam shrinker) that has at least one positive lens and at least one negative lens and is movable in the optical axis direction. And the like).

  The correction range of the cover layer is preferably 0.10 mm to 0.25 mm when 0.87 ≦ NA <0.90, and preferably 0.05 mm to 0.15 mm when 0.90 ≦ NA ≦ 0.94. If the correction range is greater than or equal to the lower limit, the number of information recording layers can be increased, which is advantageous for increasing the capacity of the optical disk. Moreover, since the interlayer spacing can be sufficiently secured, the influence of interlayer crosstalk can be reduced. On the other hand, if the value is below the upper limit, the residual amount of higher-order spherical aberration can be reduced when the spherical aberration caused by the difference in the information recording layer thickness is corrected by the change in magnification, and the image height coma when the magnification is changed. Since the amount of aberration can be suppressed, the characteristics of the optical pickup device are improved.

  In addition, it is preferable to further include a liquid crystal aberration correction element as a means for correcting the image height frame. By providing a means for correcting the image height coma aberration when the magnification changes, the correction range of the cover layer can be set large even when the NA is increased, which is advantageous for increasing the capacity.

  Furthermore, it is preferable that the optical pickup device has first focusing means for focusing the light beam having the wavelength λ1 on any one of the information recording layers.

  The optical pickup device of the present invention has an aperture limiting element. Examples of the aperture limiting element include a commonly used donut-shaped aperture, and a case where the aperture of an actuator bobbin also serves as the aperture.

  As described in FIG. 1, the objective lens and the aperture limiting element may be separated. At that time, the relative distance between the objective lens and the aperture limiting element is preferably fixed. Further, an objective lens unit in which the objective lens and the aperture limiting element are integrated may be used.

As shown in FIG. 1B, the aperture limiting element is disposed closer to the optical disc than the vertex of the light source side surface of the objective lens. Moreover, the following formula | equation (2) is satisfy | filled.
−0.35 ≧ D / f ≧ −0.90 (2)
However, D [mm] represents the distance (negative value) in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element, and f [mm] represents the focal length of the objective lens at the wavelength λ1. Represents.

More preferably, the following expression (2 ′) is satisfied.
−0.45 ≧ D / f ≧ −0.85 (2 ′)

The objective lens preferably satisfies the following conditional expression (5).
1.20 <d / f <1.55 (5)
However, d [mm] represents the axial thickness of the objective lens, and f [mm] represents the focal length of the objective lens in the first light flux.

In order to ensure the thickness of the periphery of the objective lens, it is necessary to set the axial thickness larger than the focal length as the NA increases. If the value of equation (5) is above the lower limit, the thickness of the periphery of the objective lens can be secured. Therefore, when the objective lens is made of resin, the fluidity of the resin during molding is improved and the moldability is good. It becomes an objective lens. Furthermore, the strength of the edge can be secured sufficiently. On the other hand, if the value of the expression (5) is below the upper limit, the working distance can be sufficiently secured, and the risk of collision with the optical disk can be reduced. Also, since the effective diameter of the optical surface on the optical disk side can be ensured, the off-axis coma aberration can be corrected well.
Further, it is preferable to connect the light source side optical surface and the light source side edge surface in a shape different from that of the light source side optical surface because the position of the light source side edge surface can be arbitrarily set. For example, in the case of a glass objective lens, if the edge becomes thin, the edge easily breaks during handling, so that there is a problem that the axial thickness cannot be reduced very much (in other words, the working distance cannot be secured). Therefore, as shown in FIG. 33, when the light source side optical surface and the light source side edge surface are connected by a straight line, the edge thickness can be secured as compared with the case where the light source side optical surface is extended and directly connected to the light source side edge surface. Therefore, it is possible to reduce the axial thickness, which is preferable. The shape that connects the light source side optical surface and the light source side edge surface may be any shape that can arbitrarily set the position of the light source side edge surface, and is not limited to the linear shape shown in FIG.

  When the position of the light source side edge surface is set closer to the light source side optical surface than the position of the center of gravity of the objective lens, the vertical distance between the center of gravity of the objective lens and the magnetic neutral point (operation center point) of the actuator coil Therefore, it is easy to balance the entire objective lens driving unit, it is possible to suppress weight variation, reduce the weight and size of the counter balance, and improve the operation sensitivity of the driving unit. Further, the rigidity of the entire objective lens driving unit can be improved, the secondary resonance frequency can be increased, and the servo band can be easily secured. Furthermore, the rolling of the objective lens can be suppressed, and the stable operation of the drive unit can be ensured.

  The optical information recording / reproducing apparatus preferably has an optical disc drive apparatus having the optical pickup device described above.

  Here, the optical disk drive apparatus provided in the optical information recording / reproducing apparatus will be described. The optical disk drive apparatus can hold an optical disk mounted from the optical information recording / reproducing apparatus main body containing the optical pickup apparatus or the like. There are a system in which only the tray is taken out, and a system in which the optical disc drive apparatus main body in which the optical pickup device is stored is taken out to the outside.

  An optical information recording / reproducing apparatus using each of the above-described methods is generally equipped with the following components, but is not limited thereto. An optical pickup device housed in a housing or the like, a drive source of an optical pickup device such as a seek motor that moves the optical pickup device together with the housing toward the inner periphery or outer periphery of the optical disc, and the optical pickup device housing the inner periphery or outer periphery of the optical disc These include a transfer means of an optical pickup device having a guide rail or the like that guides toward the head, a spindle motor that rotates the optical disk, and the like.

  In addition to these components, the former method is provided with a tray that can be held in a state in which an optical disk is mounted and a loading mechanism for sliding the tray, and the latter method has no tray and loading mechanism. It is preferable that each component is provided in a drawer corresponding to a chassis that can be pulled out to the outside.

  According to the present invention, it is possible to provide an optical pickup device capable of suppressing NA change of an objective lens, which is a problem in promoting an increase in capacity of an optical disc, and appropriately recording / reproducing information, and an objective lens used therefor. it can.

It is a figure which shows the relationship between the position of an aperture stop, and NA. It is a figure which shows the cross-sectional shape by the side of S2 of an objective lens. It is a figure which shows the cross-sectional shape by the side of S2 of an objective lens. It is a figure which shows schematically the structure of optical pick-up apparatus PU1. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of a comparative example. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens and aperture stop of a comparative example. It is a figure which shows the relative relationship of an objective lens and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 1. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 1, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 2. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 2, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 3. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 3, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 4. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 4, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 5. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 5, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 6. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 6, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 7. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 7, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 8. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 8, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 9. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 9, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 10. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 10, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 11. It is a figure which shows the variation | change_quantity of NA according to the change of the relative position of the objective lens of Example 11, and an aperture stop. It is a figure which shows the spherical aberration and sine condition violation amount of the objective lens of Example 12. FIG. 18 is a diagram illustrating the amount of change in NA according to a change in the relative position between the objective lens and the aperture stop in Example 12. It is a figure which shows the relationship between NA and D / f. It is sectional drawing of the objective lens which connected the light source side optical surface and the light source side edge surface with the straight line.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 shows that information is appropriately recorded on an OD that is an optical disc having three information recording surfaces RL1 to RL3 (referred to as RL1, RL2, and RL3 in ascending order of distance from the light incident surface of the optical disc) in the thickness direction. FIG. 2 is a diagram schematically showing a configuration of an optical pickup device PU1 of the present embodiment that can perform reproduction. In the optical disc OD, the distance between the information recording layer closest to the light beam incident surface and the information recording layer farthest from the light beam incident surface is 0.05 mm or more. The present invention is not limited to the present embodiment. For example, although FIG. 4 shows an optical pickup device dedicated to OD, an optical pickup device compatible with other optical disks can be obtained by separately arranging an objective lens compatible with BD / DVD / CD. Moreover, it is good also as four or more information recording surfaces instead of three information recording surfaces (it is also called an information recording layer).

  The optical pickup device PU1 moves the objective lens OBJ, the objective lens OBJ in the focusing direction and the tracking direction, and tilts in the radial direction and / or tangential direction of the optical disc, the λ / 4 wavelength plate QWP, the aperture A diaphragm CL (aperture limiting element) AP, a rising mirror MR, a coupling CL having a positive lens L2 having a positive refractive power and a negative lens L3 having a negative refractive power, and moving only the positive lens L2 in the optical axis direction 1 It includes a shaft actuator AC1, a polarizing prism PBS, a semiconductor laser LD that emits a laser beam (beam) of 405 nm, a sensor lens SL, and a light receiving element PD that receives reflected beams from the information recording surfaces RL1 to RL3 of the BD.

  In the present embodiment, the coupling lens CL is disposed between the polarizing prism PBS and the λ / 4 wavelength plate QWP. The semiconductor laser LD is arranged in the order of the negative lens L3 and the positive lens L2. However, the semiconductor laser LD may be arranged in the order of the positive lens L2 and the negative lens L3. The negative lens L3 is movable in the optical axis direction, and the positive lens L2 is fixed to the optical pickup device. The spherical aberration caused by the difference in thickness from the light incident surface to the information recording surface can be corrected by moving the coupling lens CL as a spherical aberration correction element in the optical axis direction. The coupling lens CL operates as an optical pickup device. Sometimes, it is arranged at the same number of fixed positions (here, three) as the number of information recording surfaces.

Here, the objective lens OBJ is a single lens made of plastic having an image-side numerical aperture (NA) of 0.87 or more and less than 0.95, and satisfies the following formula.
−0.35 ≧ D / f ≧ −0.90 (2)
However,
D [mm]: Distance in the optical axis direction from the vertex of the light source side surface of the objective lens OBJ to the aperture limiting position of the aperture limiting aperture AP (expressed as a negative value)
f [mm]: focal length of objective lens OBJ at wavelength λ1

  First, a case where recording / reproduction is performed on the first information recording surface RL1 of the OD will be described. In such a case, the positive lens L2 of the coupling lens CL is moved to the position of the solid line by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens L3 of the collimator lens CL, and the divergence angle is increased. After passing through L2 to be a weakly convergent light beam, it is reflected by the rising mirror MR, converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, the diameter of the light beam is regulated by the aperture stop AP, and the objective lens A spot formed on the first information recording surface RL1 by the OBJ through the transparent substrate PL1 having the first thickness as shown by a solid line.

  The reflected light beam modulated by the information pits on the first information recording surface RL1 is again transmitted through the objective lens OBJ and the aperture stop AP, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and is launched. The light is reflected by the mirror MR, passes through the positive lens L2 and the negative lens L3 of the collimating lens CL, becomes a convergent light beam, is reflected by the polarizing prism PBS, and then converges on the light receiving surface of the light receiving element PD by the sensor lens SL. . Then, using the output signal of the light receiving element PD, the information recorded on the first information recording surface RL1 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.

  Next, a case where recording / reproduction is performed on the second information recording surface RL2 of the OD will be described. In such a case, the positive lens L2 of the coupling lens CL is moved to the position of the alternate long and short dash line by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens L3 of the collimator lens CL, and the divergence angle is increased. After passing through L2 to be a substantially parallel light beam, it is reflected by the rising mirror MR, converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, the light beam diameter is regulated by the aperture stop AP, and the objective lens A spot formed on the second information recording surface RL2 by the OBJ through the transparent substrate PL2 having a second thickness (thicker than the first thickness) as shown by a one-dot chain line.

  The reflected light beam modulated by the information pits on the second information recording surface RL2 is again transmitted through the objective lens OBJ and the aperture stop AP, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. The light is reflected by the mirror MR, passes through the positive lens L2 and the negative lens L3 of the collimating lens CL, becomes a convergent light beam, is reflected by the polarizing prism PBS, and then converges on the light receiving surface of the light receiving element PD by the sensor lens SL. . Then, using the output signal of the light receiving element PD, the information recorded on the second information recording surface RL2 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.

  Next, a case where recording / reproduction is performed on the third information recording surface RL3 of the OD will be described. In such a case, the positive lens L2 of the coupling lens CL is moved to the dotted line position by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens L3 of the collimator lens CL, and the divergence angle is increased. After passing through L2 to be a weak divergent light beam, it is reflected by the rising mirror MR, converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, and its light beam diameter is regulated by the aperture stop AP. The spot is formed on the third information recording surface RL3 by the OBJ through the transparent substrate PL3 having a third thickness (thicker than the second thickness) as indicated by a dotted line.

  The reflected light beam modulated by the information pits on the third information recording surface RL3 is again transmitted through the objective lens OBJ and the aperture stop AP, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and then started up. The light is reflected by the mirror MR, passes through the positive lens L2 and the negative lens L3 of the collimating lens CL, becomes a convergent light beam, is reflected by the polarizing prism PBS, and then converges on the light receiving surface of the light receiving element PD by the sensor lens SL. . Then, using the output signal of the light receiving element PD, the information recorded on the third information recording surface RL3 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.

<Example>
Next, examples of objective lenses that can be used in the above-described embodiments will be described below in comparison with comparative examples. In the following table, r is the radius of curvature [mm] of each surface, d is the distance between each surface [mm], N (λ) is the refractive index of each surface at wavelength λ1, and νd is the Abbe number of each surface. Yes. In the following (including the lens data in the table), a power of 10 (for example, 2.5 × 10 ⁻³ ) is expressed using E (for example, 2.5 × E-03). The optical surface of the objective lens is formed as an aspherical surface that is axisymmetric about the optical axis, each of which is defined by an equation in which the coefficient shown in Table 1 is substituted into Equation (1). Here, the wavelength λ1 is represented as λ.

[Equation 1: Aspheric expression]
^{z = (y 2 / R)} / [1 + √ {1- (K + 1) (y / R) 2}] + A 4 y 4 + A 6 y 6 + A 8 y 8 + A 10 y 10 + A 12 y 12 + A 14 y 14 + A ₁₆ y ¹⁶ + A ₁₈ y ¹⁸ + A ₂₀ y ²⁰
However,
z: Aspherical shape (distance in the direction along the optical axis from the apex of the aspherical surface)
y: distance from the optical axis R: radius of curvature K: conic coefficient _{A 4, A 6, A 8} , A 10, A 12, A 14, A 16, A 18, A 20: aspherical coefficients

(Comparative example)
Table 1 shows lens data of the comparative example. FIG. 5 shows the spherical aberration and the sine condition violation amount of the objective lens of the comparative example. FIG. 6 shows the amount of change in NA according to the change in the relative position of the objective lens and aperture stop of the comparative example. Here, the horizontal axis represents the thickness of the protective substrate layer, and the vertical axis represents the protective substrate layer. The NA of the state in which the spherical aberration when the thickness is changed is corrected by the change in magnification is taken, and the position of the aperture stop is changed at three places for comparison. In the comparative example, NA = 0.85, D / f = −0.35. Here, as shown in FIG. 7, when the aperture stop is in the plane in contact with the surface apex of the objective lens OBJ, it is assumed to be 0 mm, and then expressed negatively toward the optical disk side and positive toward the light source side. To do. As shown in FIG. 5, the sine condition violation amount is small. Further, as shown in FIG. 6, when the position of the aperture stop is located 0.50 mm from the plane in contact with the surface vertex of the objective lens, the change in NA becomes the smallest.

Example 1
Table 2 shows lens data of Example 1. FIG. 8 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 9 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.56. The amount of violation of the sine condition takes a positive maximum value around 0.4 when the effective opening radius h is 1, and decreases monotonously before and after that. In addition, as shown in FIG. 8, when the position of the aperture stop is positioned 0.75 mm from the plane contacting the surface vertex of the objective lens, the change in NA is the smallest.

(Example 2)
Table 3 shows lens data of Example 2. FIG. 10 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 11 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.45. When the effective opening radius h is 1, the sine condition violation amount takes a positive maximum value in the vicinity of 0.25, and decreases monotonously before and after that. Also, as shown in FIG. 10, when the aperture stop is positioned 0.6 mm from the plane in contact with the surface apex of the objective lens, the change in NA is the smallest.

(Example 3)
Table 4 shows lens data of Example 3. FIG. 12 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 13 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.56. The sine condition violation amount takes a positive maximum value around 0.15 when the effective opening radius h is 1, and decreases monotonously before and after that. Also, as shown in FIG. 12, when the position of the aperture stop is located 0.75 mm from the plane contacting the surface apex of the objective lens, the change in NA is the smallest.

Example 4
Table 5 shows lens data of Example 4. FIG. 14 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 15 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.45. The sine condition violation amount takes a positive maximum value in the vicinity of 0.2 when the effective opening radius h is 1, and decreases monotonously before and after that. Further, as shown in FIG. 14, when the position of the aperture stop is located on the 0.60 mm optical disk side from the plane in contact with the surface vertex of the objective lens, the change in NA becomes the smallest.

(Example 5)
Table 6 shows lens data of Example 5. FIG. 16 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 17 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.92, D / f = −0.58. The amount of violation of the sine condition takes a positive maximum value around 0.4 when the effective opening radius h is 1, and decreases monotonously before and after that. Further, as shown in FIG. 16, when the position of the aperture stop is located 0.75 mm from the plane contacting the surface vertex of the objective lens, the change in NA becomes the smallest.

(Example 6)
Table 7 shows lens data of Example 6. FIG. 18 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 19 shows the amount of change in NA according to the change in the relative position of the objective lens and aperture stop in this example. In this embodiment, NA = 0.93, D / f = −0.65. The amount of violation of the sine condition takes a positive maximum value around 0.4 when the effective opening radius h is 1, and decreases monotonously before and after that. Also, as shown in FIG. 18, when the aperture stop is positioned 1.05 mm from the plane in contact with the surface apex of the objective lens, the change in NA is the smallest.

(Example 7)
Table 8 shows lens data of Example 7. FIG. 20 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 21 shows the amount of change in NA according to the change in the relative position of the objective lens and aperture stop in this example. In this embodiment, NA = 0.94, D / f = −0.85. The sine condition violation amount takes a positive maximum value in the vicinity of 0.38 when the effective opening radius h is 1, and monotonously decreases before and after that. Also, as shown in FIG. 20, when the aperture stop is positioned at the 0.90 mm optical disk side from the plane in contact with the surface apex of the objective lens, the change in NA becomes the smallest.

(Example 8)
Table 9 shows lens data of Example 8. FIG. 22 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 23 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.875 and D / f = −0.40. The sine condition violation amount takes a positive maximum value in the vicinity of 0.38 when the effective opening radius h is 1, and monotonously decreases before and after that. Also, as shown in FIG. 22, when the aperture stop is positioned 0.55 mm from the plane contacting the surface vertex of the objective lens, the change in NA is minimized.

Example 9
Table 10 shows lens data of Example 9. FIG. 24 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 25 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.64. The sine condition violation amount takes a positive maximum value in the vicinity of 0.82 when the effective opening radius h is 1, and decreases monotonously before and after that. Also, as shown in FIG. 24, when the position of the aperture stop is positioned on the 0.85 mm optical disc side from the plane in contact with the surface vertex of the objective lens, the change in NA becomes the smallest.

(Example 10)
Table 11 shows lens data of Example 10. FIG. 26 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 27 shows the amount of change in NA according to the change in the relative positions of the objective lens and aperture stop in this example. In this embodiment, NA = 0.875 and D / f = −0.40. The sine condition violation amount has a maximum value in the vicinity of 0.8 when the effective opening radius h is 1. Also, as shown in FIG. 26, when the aperture stop is positioned 0.55 mm from the plane in contact with the surface vertex of the objective lens, the change in NA is the smallest.

(Example 11)
Table 12 shows lens data of Example 11. FIG. 28 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 29 shows the amount of change in NA according to the change in the relative positions of the objective lens and the aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.45. The sine condition violation amount takes a positive maximum value around 0.42 when the effective opening radius h is 1, and monotonously decreases before and after that. Further, as shown in FIG. 28, when the aperture stop is positioned 0.66 mm from the plane in contact with the surface apex of the objective lens, the change in NA is the smallest.

(Example 12)
Table 13 shows lens data of Example 9. FIG. 30 shows the spherical aberration and the sine condition violation amount of the objective lens of this example. FIG. 31 shows the amount of change in NA according to the change in the relative position of the objective lens and aperture stop in this example. In this embodiment, NA = 0.90 and D / f = −0.56. The sine condition violation amount takes a positive maximum value in the vicinity of 0.38 when the effective opening radius h is 1, and monotonously decreases before and after that. Further, as shown in FIG. 30, when the position of the aperture stop is located 0.75 mm from the plane contacting the surface vertex of the objective lens, the change in NA becomes the smallest.

  Table 14 shows the values of the respective examples. The objective lenses of the comparative example and Examples 1 to 12 are designed to be aspheric so that spherical aberration is corrected by a combination of zero magnification and a design value of the protective layer thickness. The protective layer thickness correction range in Table 14 refers to the distance between the information recording layer closest to the light beam incident surface of the optical disc and the information recording layer farthest from the light beam incident surface.

  FIG. 32 shows the relationship between NA and D / f. FIG. 32 shows that there is a correlation between NA and D / f. That is, the absolute value of D / f is small when NA is near 0.87, and the absolute value of D / f is large when NA is near 0.95. In the comparative example, NA is 0.85 and D / f is -0.35. Therefore, under the condition of 0.87 ≦ NA ≦ 0.95, the range of −0.35 ≧ D / f ≧ −0.90 is optimal.

  The present invention is not limited to the embodiments described in the specification, and other embodiments and modifications are apparent to those skilled in the art from the embodiments and ideas described in the present specification. It is. The description and examples are for illustrative purposes only, and the scope of the invention is indicated by the following claims.

OBJ Objective lens PU1 Optical pickup device LD Blue-violet semiconductor laser AC1 1-axis actuator AC2 3-axis actuator BS Polarizing beam splitter PBS Polarizing prism CL Coupling lens MR Rising mirror L2 Positive lens group L3 Negative lens group QWP λ / 4 wavelength plate PL1 ~ PL3 Protection board RL1 ~ RL3 Information recording surface SL Sensor lens

Claims (4)

A first light source that emits a light beam having a wavelength λ1 (390 nm <λ1 <415 nm), a spherical aberration correction element, an aperture limiting element, and an objective lens, and the light beam having a wavelength λ1 emitted from the first light source. An information recording layer and a light beam incident surface that are closest to the light beam incident surface and have a plurality of information recording layers that are made to enter the objective lens through the spherical aberration correction element and the aperture limiting element and are stacked in the optical axis direction. An optical pickup device for recording / reproducing information by focusing on the information recording layer of the first optical disc whose distance from the farthest information recording layer is 0.05 mm or more,
When the information recording layer for recording / reproducing information is changed from one to the other, the magnification of the objective lens is changed by the spherical aberration correction element, which is caused by the difference in thickness from the light incident surface to the information recording layer. The objective lens is a single lens, and the aperture limiting element is disposed on the optical disc side with respect to the plane contacting the light source side surface apex of the objective lens. An optical pickup device satisfying the formula:
0.87 ≦ NA ≦ 0.95 (1)
−0.35 ≧ D / f ≧ −0.90 (2)
However,
NA: Numerical aperture D [mm] of the objective lens: Distance in the optical axis direction from the plane contacting the light source side surface apex of the objective lens to the aperture limiting position of the aperture limiting element (represented by a negative value)
f [mm]: focal length of the objective lens at the wavelength λ1 The optical pickup device according to claim 1, wherein the following expression is satisfied.
0.90 ≦ NA ≦ 0.94 (1 ′) The optical pickup device according to claim 1, wherein the following expression is satisfied.
−0.45 ≧ D / f ≧ −0.85 (2 ′)   An objective lens that is used in the optical pickup device according to claim 1.