WO2013005672A1

WO2013005672A1 - Optical pickup device

Info

Publication number: WO2013005672A1
Application number: PCT/JP2012/066702
Authority: WO
Inventors: 木村　徹; 田中秀樹
Original assignee: コニカミノルタアドバンストレイヤー株式会社
Priority date: 2011-07-07
Filing date: 2012-06-29
Publication date: 2013-01-10

Abstract

Provided is an optical pickup device that utilizes an antireflection film so as to be able to form an optimal focused spot even when a light beam with a relatively large divergence angle is incident on an objective lens at the time of a focus jump. In the objective lens on which the antireflection film is formed, if the transmittance (Tc) of the central region of a circle formed within the 10% range from the center of the effective diameter of a first surface and the transmittance (Tp) of an annular peripheral region formed outside the 90% range of the effective diameter of the first surface are set to meet equation (1), the rim intensity (perimeter intensity) of the spotlight is increased such that a so-called super-resolution phenomenon occurs, allowing the diameter of the focused spot to be reduced. Consequently, an optimal focused spot can be formed even in the event when a light beam with a relatively large divergence angle is incident on the objective lens. Thus, the recording/reproducing characteristics of the optical pickup device can be improved. 1.0<Tp/Tc<1.3 (1)

Description

Optical pickup device

The present invention relates to an optical pickup device capable of recording and / or reproducing information with respect to an optical disc having three or more information recording surfaces in the thickness direction.

There is known a high-density optical disk system capable of recording and / or reproducing information (hereinafter, “recording and / or reproduction” is referred to as “recording / reproduction”) using a blue-violet semiconductor laser having a wavelength of about 400 nm. As an example, an optical disc for recording / reproducing information with specifications of NA 0.85 and light source wavelength 405 nm, so-called Blu-ray Disc (hereinafter referred to as BD), DVD (NA 0.6, light source wavelength 650 nm, storage capacity 4. It is possible to record 25 GB of information per layer on an optical disk having a diameter of 12 cm, which is the same size as 7 GB).

By the way, many of the conventional BDs have an information recording surface of one layer or two layers, but due to the market demand for storing larger data on one BD, the information recording surface of three layers or more. Research is also progressing with the aim of commercializing BDs having, and some have been commercialized.
However, since the NA of the luminous flux when recording / reproducing information is as large as 0.85, in a BD having a plurality of information recording surfaces, a minimum spherical aberration is imparted to one information recording surface. However, on other information recording surfaces with different substrate thicknesses, there is a problem that spherical aberration increases and information cannot be properly recorded / reproduced. The problem of spherical aberration becomes more apparent as the number of information recording surfaces increases (that is, as the distance between the information recording surface having the smallest distance from the surface and the information recording surface having the largest distance from the surface increases).

On the other hand, in Patent Document 1, the magnification of the objective lens is changed by moving a coupling lens arranged between the light source and the objective lens in the optical axis direction, and the selected information recording surface is tertiary. An optical pickup device capable of condensing a light beam with reduced spherical aberration is disclosed. The operation of changing the information recording surface on which information is to be recorded / reproduced from one information recording surface to another information recording surface may be referred to as “interlayer focus jump” in this specification.

Japanese Patent No. 4144663

By the way, in the conventional optical pickup device disclosed in Patent Document 1, it is generated by an interlayer focus jump by moving the coupling lens in the optical axis direction and changing the magnification of the objective lens at the time of recording / reproducing of the double-layer disc. Although the spherical aberration is corrected, the NA changes when the magnification of the objective lens is changed. Therefore, the condensing spot size on the information recording surface changes according to the change in the NA. There is a risk of deterioration. More specifically, when recording / reproducing is performed on the L0 layer, which is the information recording surface having the longest distance from the surface of the optical disc, divergent light is incident on the objective lens, so that the reference state (the objective lens magnification is zero). NA) becomes smaller than that in the above state), so that the diameter of the focused spot is increased. As a result, the possibility of adversely affecting recording / reproducing characteristics such as jitter increases. The change in the focused spot diameter of the objective lens tends to become more prominent as the change in magnification is larger, that is, as the amount of spherical aberration generated by the interlayer focus jump is larger.

Furthermore, an optical disc having an information recording surface of three or more layers is called BD-XL. In BD-XL, the amount of spherical aberration generated by an interlayer focus jump is larger than that of a two-layer disc, so that spherical aberration is corrected. There is a characteristic that the magnification change of the objective lens becomes larger. Therefore, it can be said that the above-described recording / reproduction characteristic deterioration due to the change in the condensed spot diameter becomes more prominent than the conventional BD pickup that does not support BD-XL. In addition, the coupling lens may be displaced in the direction of the optical axis in order to correct the spherical aberration that occurs when the temperature changes, but this makes the change in magnification of the objective lens larger, and the above problem becomes more apparent. Will be.

The present invention has been made in consideration of the above-described problems, and uses an anti-reflection film to achieve optimum focusing even when a light beam having a relatively large divergence angle is incident on an objective lens during a focus jump. An object of the present invention is to provide an optical pickup device capable of forming a spot.

The optical pickup device according to claim 1 includes a light source that emits a light beam having a wavelength λ1 (390 nm <λ1 <415 nm), a coupling lens, and an objective lens, and the coupling lens is displaced in an optical axis direction. By selecting one of the information recording surfaces in the optical disc having three or more information recording surfaces in the thickness direction that are different from each other in distance (transparent substrate thickness) from the light beam incident surface, the wavelength λ1 emitted from the light source is selected. An optical pickup device that records and / or reproduces information by condensing the light beam on the selected information recording surface by the objective lens,
The objective lens is a single lens having an image-side numerical aperture (NA) of 0.8 or more and less than 0.95, and an antireflection film is formed at least on the first surface on the light source side,
In the objective lens on which the antireflection film is formed, the transmittance Tc in a central region inside 10% of the image-side numerical aperture NA and the transmittance in a peripheral region outside 90% of the image-side numerical aperture NA. Tp satisfies the expression (1).
1.0 <Tp / Tc <1.3 (1)

According to the present invention, in the objective lens on which the antireflection film is formed, the transmittance Tc in the central region inside 10% of the image-side numerical aperture NA and the outer side of 90% of the image-side numerical aperture NA. If the transmittance Tp in the peripheral region of the lens satisfies the expression (1), the rim intensity (the intensity of light passing through the peripheral portion with respect to the intensity of light passing near the optical axis of the objective lens) increases. A so-called super-resolution phenomenon occurs, and the diameter of the focused spot can be reduced. As a result, even when a light beam having a relatively large divergence angle is incident on the objective lens during the focus jump, an optimum condensing spot can be formed, so that the recording / reproducing characteristics of the optical pickup device can be improved. More preferably, the following expression is satisfied.
1.05 <Tp / Tc <1.25 (1 ′)

An optical pickup device according to a second aspect is the optical pickup device according to the first aspect, wherein the antireflection film applied to the first surface has a wavelength λ0 (nm) at which the reflectance with respect to the normal incident light is minimized, The wavelength λ1 (nm) satisfies the formula (2).
1.30 <λ0 / λ1 <1.90 (2)

By exceeding the lower limit of the value of equation (2), the transmittance of the peripheral region can be more reliably reduced and the rim strength can be increased. On the other hand, by preventing the value of the expression (2) from exceeding the upper limit, it is possible to prevent the transmittance of the entire effective diameter from being reduced and to prevent the light utilization efficiency from being lowered. That is, by satisfying the condition of the expression (2), the transmittance of the peripheral region is larger than that of the central region, and the rim strength can be appropriately increased. More preferably, the following expression is satisfied.
1.40 <λ0 / λ1 <1.85 (2 ′)

According to a third aspect of the present invention, in the optical pickup device according to the first or second aspect, the transmittance of the wavelength prevention film continuously changes within an effective diameter according to the height from the optical axis. It is characterized by.

If there is a difference in transmittance between the central region and the peripheral region, the transmittance of the central region and the peripheral region may be kept constant, and the transmittance between the central region and the peripheral region may be linear. However, in such a case, it is necessary to apply an antireflection film for each region, which complicates the film formation process and increases the manufacturing cost. On the other hand, by setting the transmittance characteristics so as to continuously change within the effective diameter according to the height from the optical axis, there is no need to apply an antireflection film for each region, and the film formation process of the antireflection film Therefore, the manufacturing cost of the objective lens can be reduced. It is preferable that the transmittance of the antireflection film continuously changes in a curved line from the region closest to the optical axis to the peripheral region.

According to a fourth aspect of the present invention, in the optical pickup device according to any one of the first to third aspects, the objective lens is made of resin.

When the objective lens is made of resin for cost reduction and weight reduction, the amount of change in the refractive index at the time of temperature change is larger than that of glass and so on, and spherical aberration deterioration that occurs at the time of temperature change cannot be ignored. Therefore, in addition to correcting the spherical aberration caused by the interlayer focus jump, the magnification of the objective lens is changed more greatly in order to correct the spherical aberration deterioration at the time of temperature change. In such a case, it can be said that the recording / reproduction characteristic deterioration due to the above-mentioned change in the diameter of the focused spot is further manifested. In particular, when recording / reproducing is performed on the information recording surface L0 layer having the longest distance from the surface, when the objective lens becomes high temperature, strong divergent light is incident on the objective lens. Becomes larger and the possibility of adversely affecting the jitter becomes larger. On the other hand, according to the present invention, the anti-reflection film satisfying the expression (1) is applied so that the focused spot diameter does not become too large even when a strong divergent light beam is incident on the objective lens. Is possible. That is, when the objective lens is made of resin, the problem of the present invention is increased. However, even in such a case, the present invention can solve the problem, and it can be said that the effect of the present invention becomes more remarkable.

According to a fifth aspect of the present invention, in the optical pickup device according to the fourth aspect of the present invention, an optical path for correcting a spherical aberration that occurs when a temperature change occurs in the objective lens on the optical surface of the objective lens. A difference providing structure is formed.

When the objective lens is provided with an optical path difference providing structure for correcting spherical aberration that occurs when a temperature change occurs, there is a problem that the transmittance is reduced due to a shape error of the optical path difference providing structure or a molding transfer defect. Such a problem becomes more apparent around the effective diameter where the diffraction pitch tends to be small. On the other hand, it is possible to compensate for such a decrease in transmittance by applying an antireflection film satisfying the expression (1). Furthermore, by providing such an optical path difference providing structure, the amount of spherical aberration generated when the temperature of the objective lens changes can be kept small, so by displacing the coupling lens in the optical axis direction, The change in magnification when spherical aberration correction is performed is reduced, and the change in the focused spot diameter can be suppressed by a synergistic effect.

According to a sixth aspect of the present invention, in the optical pickup device according to any of the first to fifth aspects, the optical path difference providing structure is formed on the optical surface on the light source side and extends along the optical axis. In the optical path difference providing structure, a negative step structure in which the step surface faces away from the optical axis is provided near the optical axis in the optical path difference providing structure. A positive step structure in which the step surface faces the optical axis side is provided at a position farther from the optical axis than the negative step structure, and the step surface in the negative step structure is relative to the optical axis. It is parallel, and the step surface in the positive step structure is non-parallel to the optical axis.

When correcting the temperature characteristics using the optical path difference providing structure, the positive / negative switching type in which the direction of the step changes from negative to positive according to the direction from the lens center to the periphery ensures a larger diffraction pitch than the type in which the direction of the step does not change. As a result, it is possible to obtain a lens that has a small influence on the reduction in transmittance due to manufacturing errors during molding and molding transfer defects.

The invention according to claim 6 will be described with reference to the drawings. In an optical path difference providing structure in which a step surface extending along the optical axis and an annular refracting surface are alternately connected, a negative surface with the step surface facing away from the optical axis is located near the optical axis. There is a type in which a positive step structure with a step surface facing the optical axis is provided at a position farther from the optical axis than the negative step structure.

By the way, when manufacturing a mold for molding an objective lens having such a type of optical path difference providing structure, a minute shape corresponding to the optical path difference providing structure is formed using a sharp tool called a sword tip tool. It is common. As shown in FIGS. 1 and 2, the sword tip bite BT has a rake face SP bordered by a first ridge line E1 intersecting at an acute angle and a second ridge line E2 closer to the axis MA than the first ridge line E1. ing.

First, according to the processing mode of the mold shown in FIG. 1A, the second edge E2 of the sword tip tool BT is set so that the mold material M is parallel to the axis MA and the mold material M is centered on the axis MA. As shown by the arrows, the blade tip BT is moved from the peripheral side so as to approach the axis MA and is cut while being displaced stepwise in the optical axis direction in synchronization therewith. At this time, in the mold material M, the step wall M2 facing away from the axis MA is cut by the second ridge line E2, and thus extends in parallel with the axis MA. On the other hand, since the step wall M1 facing the axis MA side is cut by the first ridge line E1, it is inclined with respect to the axis line by an angle equal to the crossing angle of the sword tip tool BT. Become.

When the objective lens is molded using the mold processed in the processing mode shown in FIG. 1A, the shape of the objective lens OBJ shown in FIG. 1B is obtained, and this objective lens OBJ is located near the optical axis. Is provided with a negative step structure NSS in which the step surface ST1 formed by the step wall M1 faces away from the optical axis OA, and at a position farther from the optical axis than the negative step structure NSS, the step wall M2 is provided. Has a light path difference providing structure provided with a positive step structure PSS in which the step surface ST2 formed by the step faces the optical axis side.

When collimated light is incident on the objective lens OBJ shown in FIG. 1B, the light beam incident on the step surface ST1 tilted with respect to the optical axis OA is not used for forming a condensed spot. On the other hand, among the light rays incident on the ring-shaped surface, the light rays that pass through the step surface ST2 are also not used for the formation of the condensed spot. This is called ray vignetting.

On the other hand, according to the mold machining mode shown in FIG. 2 (a), the first edge E1 of the sword tip bite BT is set so that the mold material M is parallel to the axis MA and the mold material M is centered on the axis MA. As shown by the arrows, the blade tip BT is moved from the peripheral side so as to approach the axis MA and is cut while being displaced stepwise in the optical axis direction in synchronization therewith. At this time, in the mold material M, the step wall M2 facing the opposite side to the axis MA is cut by the second ridge line E2, so that it tilts with respect to the axis MA, but faces the axis MA side. About the level | step difference wall M1, since it cuts with the 1st ridgeline E1, it becomes parallel with respect to the axis line MA. In addition, there is an attempt of a machining mode in which both the step walls M1 and M2 are parallel by rotating the sword tip tool BT during cutting, but at the present time, since the difficulty of die machining increases dramatically, the manufacturing cost There are concerns about the impact on

When the objective lens is molded using the mold processed according to the processing mode shown in FIG. 2A, the shape of the objective lens OBJ shown in FIG. 2B is obtained. This objective lens OBJ is also located near the optical axis. Is provided with a negative step structure NSS in which the step surface ST1 formed by the step wall M1 faces away from the optical axis OA, and at a position farther from the optical axis than the negative step structure NSS, the step wall M2 is provided. Has a light path difference providing structure provided with a positive step structure PSS in which the step surface ST2 formed by the step faces the optical axis side. That is, the step surface ST1 in the negative step structure NSS is parallel to the optical axis OA, and the step surface ST2 in the positive step structure PSS is non-parallel to the optical axis OA.

When parallel light is incident on the objective lens OBJ shown in FIG. 2B, the step surface ST1 is parallel to the optical axis OA, so that the light beam does not substantially enter the step surface ST1. No vignetting will occur. That is, only the light rays incident from the stepped surface ST2 are not used for forming the condensing spot, which is apparent when compared with FIG. 1B, but the objective lens of FIG. 2B is hatched. It can be said that it is excellent from the viewpoint of light utilization efficiency. That is, by selecting the objective lens shown in FIG. 2B, it is possible to obtain a pickup device equipped with an objective lens that has a low mold processing difficulty, a low manufacturing cost, and a sufficiently high transmittance. On the other hand, the lens as shown in FIG. 2 (b) tends to have a low transmittance in a region away from the optical axis, which increases the problem of increasing the diameter of the focused spot. For example, since the problem can be solved, the difficulty of mold processing is low, the manufacturing cost is suppressed, the transmittance is sufficiently high, and the condensing spot can be prevented from becoming large.

According to a seventh aspect of the present invention, there is provided the optical pickup device according to any one of the fourth to sixth aspects, wherein the coupling lens corrects a spherical aberration that occurs when a temperature change occurs in the objective lens. It is characterized by being displaced in the optical axis direction.

As a result, the amount of displacement of the coupling lens in the optical axis direction is increased as compared with the interlayer focus jump, and the effect of the present invention is further exhibited.

An optical pickup device according to an eighth aspect of the present invention is the optical pickup device according to any one of the first to seventh aspects, wherein the objective lens material has a refractive index smaller than 1.58 with respect to the wavelength λ1, and the first surface. The antireflective film applied to the film has 2 to 4 layers.

Any resin material can be selected as long as the refractive index with respect to the wavelength λ1 is smaller than 1.58. In addition, when the number of antireflection films on the first surface of the objective lens is two or more, a sufficient antireflection effect can be obtained, and a decrease in the overall transmittance can be prevented. On the other hand, when the antireflection film on the first surface having a large inclination angle of the aspherical surface is made to be four layers or less, it is possible to prevent an increase in film thickness error and to maintain a stable film thickness during mass production. . Therefore, the antireflection film applied to the first surface is preferably 2 to 4 layers. The antireflection film applied to the first surface is preferably three layers or four layers, and most preferably three layers.

The optical pickup device according to the present invention has at least one light source (first light source). Of course, a plurality of types of light sources may be provided so as to support a plurality of types of optical disks. Furthermore, the optical pickup device of the present invention has a condensing optical system for condensing at least the first light flux from the first light source on the information recording surface of the first optical disc. In the optical pickup apparatus that can handle a plurality of types of optical disks, the condensing optical system condenses the second light beam on the information recording surface of the second optical disk, and the third light beam on the information recording surface of the third optical disk. You may make it condense. The optical pickup device of the present invention includes a light receiving element that receives at least a reflected light beam from the information recording surface of the first optical disc. In an optical pickup device that can handle a plurality of types of optical disks, the light receiving element receives a reflected light beam from the information recording surface of the second optical disk and receives a reflected light beam from the information recording surface of the third optical disk. Also good. In this specification, “object side” means the light source side, and “image side” means the optical disk side.

The first optical disc has a protective substrate having a thickness t1 and an information recording surface. The second optical disc has a protective substrate having a thickness t2 (t1 <t2) and an information recording surface. The third optical disc has a protective substrate having a thickness t3 (t2 <t3) and an information recording surface. The first optical disc is preferably a BD, the second optical disc is a DVD, and the third optical disc is preferably a CD, but is not limited thereto.

The first optical disc has three or more information recording surfaces stacked in the thickness direction. In other words, the first optical disc has three or more information recording surfaces in the thickness direction that have different distances from the light incident surface of the optical disc to the information recording surface (this is referred to as “transparent substrate thickness” in this specification). It is. Of course, you may have four or more information recording surfaces. The second optical disc and the third optical disc may also have a plurality of information recording surfaces. The “maximum transparent substrate thickness” means the transparent substrate thickness of the information recording surface farthest from the light incident surface of the optical disc among the plurality of information recording surfaces, and the “minimum transparent substrate thickness” means the optical disc. The thickness of the transparent substrate on the information recording surface closest to the incident surface of the light beam in FIG. In addition, it is preferable that the transparent substrate thickness of all the information recording surfaces is 0.03 mm or more and 0.125 mm or less. The difference between the maximum transparent substrate thickness and the minimum transparent substrate thickness is preferably 0.025 mm or more.

Therefore, the optical pickup device selects one of the plurality of information recording surfaces of the first optical disc, and condenses the light beam emitted from the light source onto the selected information recording surface by the objective lens. By doing so, information is recorded and / or reproduced.

In this specification, BD means that information is recorded / reproduced by a light beam having a wavelength of about 390 to 415 nm and an objective lens having an NA of about 0.8 to 0.9, and the thickness of the protective substrate is 0.05 to 0.00 mm. A generic term for BD series optical discs of about 125 mm, including BD having only a single information recording surface, BD-XL having four information recording surfaces, and BD having three or more information recording surfaces. However, the optical pickup device of the present invention is compatible with a BD having an information recording surface of at least three layers. Further, in this specification, 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. Including DVD-ROM, DVD-Video, DVD- Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, and the like. In this specification, CD is a general term for CD series optical discs in which information is recorded / reproduced by an objective lens having an NA of about 0.45 to 0.51 and the thickness of the protective substrate is about 1.2 mm. Including CD-ROM, 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 addition, regarding the thicknesses t1, t2, and t3 of the protective substrate, it is preferable to satisfy the following conditional expressions (3), (4), and (5), but is not limited thereto. The thickness of the protective substrate referred to here is the thickness of the protective substrate provided on the surface of the optical disk. That is, the thickness of the protective substrate from the optical disc surface to the information recording surface closest to the surface.

0.030mm ≦ t1 ≦ 0.125mm (3)
0.5mm ≤ t2 ≤ 0.7mm (4)
1.0 mm ≤ t3 ≤ 1.3 mm (5)

In the present specification, the first light source, the second light source, and the third 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 second wavelength λ2 (λ2> λ1) of the second light beam emitted from the second light source, and the third of the third light beam emitted from the third light source. The wavelength λ3 (λ3> λ2) preferably satisfies the following conditional expressions (6) and (7).
1.5 · λ1 <λ2 <1.7 · λ1 (6)
1.8 · λ1 <λ3 <2.0 · λ1 (7)

When BD, DVD, and CD are used as the first optical disc, the second optical disc, and the third optical disc, respectively, the first wavelength λ1 of the first light source is preferably 350 nm or more and 440 nm or less, more preferably 390 nm. Longer and shorter than 415 nm, the second wavelength λ2 of the second light source is preferably 570 nm or more and 680 nm or less, more preferably 630 nm or more and 670 nm or less, and the third wavelength λ3 of the third light source is preferably 750 nm. As mentioned above, it is 880 nm or less, More preferably, it is 760 nm or more and 820 nm or less.

Also, at least two of the first light source, the second light source, and the third 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. In addition to the light source, a light receiving element to be described later may be packaged.

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 has a coupling lens and an objective lens. The coupling lens is a lens group that is disposed between the objective lens and the light source and changes the divergence angle of the light beam. The collimator is a kind of coupling lens, and is a coupling lens that emits an incident light beam as parallel light or substantially parallel light. The coupling lens may be composed of only a positive lens group or may have a positive lens group and a negative lens group. The positive lens group has at least one positive lens. The positive lens group may include only one positive lens or may include a plurality of lenses. When the negative lens group is included, the negative lens group includes at least one negative lens. The negative lens group may include only one negative lens or may include a plurality of lenses. Examples of a preferable coupling lens include only a single positive lens or a combination of a single positive lens and a single negative lens.

In the present specification, a lens that is movable in the optical axis direction in the coupling lens may be referred to as a “movable lens”. Further, in this specification, “movement amount of the coupling lens” is used in the same meaning as “movement amount of the movable lens”.

By the way, when performing the focus jump, as a method of reducing the amount of movement of the coupling lens, among the lens groups constituting the coupling lens, the power of the lens group moved in the optical axis direction is increased (that is, in the optical axis direction). It is conceivable to shorten the focal length of the lens group that is moved to (1). This is because the amount of movement of the lens group moved in the optical axis direction decreases as the power of the lens group increases (that is, as the focal length of the lens group decreases). However, when the coupling lens has a group configuration, if the focal length of the lens group moved in the optical axis direction (that is, equal to the focal length of the coupling lens) is shortened, the spot condensed by the objective lens becomes an ellipse. There is a risk that the recording and / or reproduction of information on the BD may be hindered. The reason for this will be described below.

Generally, since a light beam emitted from a semiconductor laser used as a light source of an optical pickup device has an elliptical shape, the light quantity distribution in the major axis direction and the minor axis direction of the ellipse is different. If the focal length of the coupling lens becomes too short, the asymmetry of the light amount distribution taken in by the coupling lens becomes remarkable, so that the spot condensed by the objective lens becomes elliptical, and information recording on the BD and / or There is a risk that playback will be hindered. Therefore, when the coupling lens has a one-group configuration, it is difficult to reduce both the amount of movement of the coupling lens required at the time of focus jump and the symmetry of the light amount distribution captured by the coupling lens.

In order to achieve both of the above, the coupling lens has a two-group configuration including a positive lens group and a negative lens group, and at least one lens in the positive lens group is moved in the optical axis direction, thereby It is preferable to select whether to collect light on the information recording surface.

In order to simplify the description, it is assumed that the coupling lens is a two-group thin lens system composed of a positive lens and a negative lens, and the positive lens is moved along the optical axis direction during focus jump. If the power of the positive lens is P _P , the focal length of the positive lens is f _P , the power of the negative lens is P _N , the focal length of the negative lens is f _N , and the distance between the positive lens and the negative lens is L, all coupling lenses The system power P _C and the focal length f _C of the entire coupling lens system are expressed by the following equation (8).
P _C = P _P + P _N -L · P _P · P _N
P _C = 1 / f _C
P _C = 1 / f _P + 1 / f _N −L / (f _P · f _N ) (8)

Here, when the focal length of the objective lens is f 2 _O , the magnification M of the condensing optical system composed of the coupling lens and the objective lens is expressed by the following equation (9).
M = −f _O / f _C (9)

In order to improve the symmetry of the distribution of the amount of light captured by the coupling lens and make the shape of the spot collected by the objective lens circular, it is optical with respect to the ellipticity of the light beam emitted from the semiconductor laser used as the light source. It is necessary to set the system magnification M to an optimum value. In the BD optical pickup device, the optimum value of the magnification of the condensing optical system is about -0.1. In consideration of a space for arranging an optical element such as a polarization beam splitter arranged between the light source and the coupling lens, the focal length f _C of the entire coupling lens system cannot be extremely shortened. Furthermore, when recording and / or reproducing information on the BD, the distance between the objective lens and the BD (also referred to as a working distance) is not too short, and in order to reduce the thickness of the optical pickup device, optimal range of the focal length f _O of the lens naturally determined. As described above, from equation (9), as the coupling lens for the optical pickup device for BD, the focal length range of the entire system needs to be a certain predetermined range, and the movement of the coupling lens required at the time of focus jump The focal length f _C of the entire coupling lens system cannot be reduced excessively considering only the amount.

Here, in order to keep the movement amount at the time of focus jump small, the power P _{P of the} positive lens is increased, and further, the power P of the negative lens is set so that the focal length f _{C of the} entire coupling lens system is not too short. _It is preferable to increase the absolute value of _N (see equation (8)).

As described above, in the coupling lens composed of the two lens groups of the positive lens group and the negative lens group, the movement amount of the positive lens group required at the time of focus jump is reduced by moving the positive lens group in the optical axis direction. In addition, it is possible to achieve both the symmetry of the light amount distribution captured by the coupling lens.

Further, the arrangement of the positive lens group and the negative lens group may be arranged in the order of the negative lens group and the positive lens group from the light source side, or may be arranged in the order of the positive lens group and the negative lens group from the light source side. good. The preferred arrangement is the former.

From the above, from the viewpoint of reducing the movement amount of the coupling lens, one of the preferred examples of the coupling lens in the optical pickup device is a combination of one positive lens and one negative lens, and the negative lens from the light source side, They are arranged in the order of positive lenses. However, the present invention is not limited to this, and from the viewpoint of simplifying the configuration of the coupling lens as much as possible, there can be an option of a single positive lens coupling lens.

For the above reasons, at least one lens (preferably a positive lens) of the positive lens group is movable in the optical axis direction in order to correct spherical aberration occurring on the selected information recording surface of the first optical disk. It is preferable that For example, when recording and / or reproducing on one information recording surface of the first optical disk and then recording and / or reproducing on another information recording surface of the first optical disk, the positive lens group of the coupling lens group Spherical aberration that occurs at the time of focus jump to a different information recording surface of the first optical disk by moving at least one lens in the optical axis direction, changing the divergence of the light beam, and changing the magnification of the objective lens Correct.

3 (a) to 3 (d) are diagrams showing the examination results of each case conducted by the present inventors. The inventor has made a plurality of information recordings using an objective lens made of plastic, having a focal length f = 1.18 mm, an optical surface being an aspherical surface or a diffractive surface, and an image-side numerical aperture of 0.85. In the first optical disc (BD) having a surface, when the maximum spherical aberration difference A generated when the optimum focused spot is formed on each of the information recording surfaces that are separated as much as possible, and when the environmental temperature changes by ± 30 ° C. The maximum spherical aberration B that occurred and the maximum spherical aberration C that occurred when the wavelength of the light source changed by ± 5 nm were determined. This is represented by the bar graph of FIG. Such spherical aberration can be corrected by moving the coupling lens in the optical axis direction and changing the magnification of the objective lens. However, if the same coupling lens is used, the total amount of spherical aberration is the amount of movement of the coupling lens. It is equivalent to.

Here, as shown in FIGS. 3A and 3B, when an optical disk having two information recording surfaces is used, the amount of spherical aberration is obtained regardless of whether the optical surface is an aspherical refractive surface or a diffractive surface. Is about 410 to 430 mλ, and the amount of movement of the coupling lens is relatively small. On the other hand, as shown in FIG. 3C, when an optical disk having four information recording surfaces is used, the total amount of spherical aberration is 680 mλ in the case of an objective lens having an aspherical refractive surface. The amount of movement is required to be about 1.5 times that required when an optical disc having two information recording surfaces is used. Further, as shown in FIG. 3 (d), when an optical disk having four information recording surfaces is used in an objective lens having a diffractive optical surface, spherical aberration that occurs due to a temperature change is an effect of the diffractive surface. However, as a result, the spherical aberration generated with the change in wavelength increases, and as a result, the total amount of spherical aberration becomes 660 mλ, and the amount of movement of the coupling lens is 2 on the information recording surface. Similarly, about 1.5 times as much is required as compared with the case of using one optical disk.

However, if the objective lens is made of glass and the optical surface is an aspherical refracting surface, the spherical aberration B (= 140 mλ) due to the environmental temperature change becomes almost zero, so the amount of movement of the coupling lens is smaller (FIG. 3 ( c), the spherical aberration is equivalent to the correction amount of 540 mλ. Furthermore, if the objective lens is made of glass and the optical surface is a diffractive surface that corrects spherical aberration that occurs when the wavelength varies, in addition to spherical aberration B caused by environmental temperature changes, spherical aberration C caused by wavelength fluctuations of the light source due to the function of the diffractive surface. Therefore, the amount of movement of the coupling lens is smaller (corresponding to the correction amount of the spherical aberration of 500 mλ in FIG. 3C). That is, in order to reduce the amount of movement of the coupling lens, the objective lens is preferably made of a glass material. However, even if the objective lens is improved in this way, the amount of movement of the coupling lens when the optical disk having two information recording surfaces is used is smaller than that of the coupling lens when the optical disk having four information recording surfaces is used. Since the amount of movement is still about twice, it is preferable to further devise in order to suppress the amount of movement of the coupling lens. The same applies to the amount of movement of the coupling lens when using an optical disc having three information recording surfaces or five or more information recording surfaces. On the other hand, the amount of movement of the coupling lens can be further reduced by breaking the sine condition of the objective lens.

In the above examination, as an optical disc having two information recording surfaces (an information recording surface having a smaller distance from the light beam incident surface of the optical disc is RL1, an information recording surface having a larger distance from the light beam incident surface of the optical disc is RL2), an optical disc in which the distance from the light incident surface of the optical disc to RL1 is 75 μm and the distance from the light incident surface of the optical disc to RL2 is 100 μm. Further, as an optical disk having four information recording surfaces (assuming that the information recording surface having the smallest distance from the light beam incident surface of the optical disk is RL1, and the information recording surface having the largest distance from the light beam incident surface of the optical disk is RL4), An optical disk was assumed in which the distance from the light beam incident surface of the optical disk to RL1 was 50 μm and the distance from the light beam incident surface of the optical disk to RL4 was 100 μm.

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. The objective lens is a single plastic lens or glass lens. Preferably, the objective lens is a single convex lens. The objective lens may be composed of only a refractive surface or may have an optical path difference providing structure. In addition, the hybrid lens which provided the optical path difference providing structure with the photocurable resin, UV curable resin, or thermosetting resin etc. on the glass lens may be sufficient. The objective lens preferably has a refractive surface that is aspheric.
In the objective lens, the base surface on which the optical path difference providing structure is provided is preferably an aspherical surface.

If the objective lens is a glass lens, as described with reference to FIG. 3, it is not necessary to move the coupling lens in order to correct the spherical aberration caused by the temperature change. This is preferable because it can be reduced and the optical pickup device can be downsized.

When the objective lens is a glass lens, it is preferable to use a glass material having a glass transition point Tg of 500 ° C. or lower, more preferably 400 ° C. or lower. By using a glass material having a glass transition point Tg of 500 ° C. or lower, molding at a relatively low temperature is possible, so that the life of the mold can be extended. Examples of such a glass material having a low glass transition point Tg include K-PG325 and K-PG375 (both product names) manufactured by Sumita Optical Glass Co., Ltd.

In addition, a physical property value that is important when molding a glass lens is the linear expansion coefficient α. Even if a material having a Tg of 400 ° C. or lower is selected, the temperature difference from room temperature is still large compared to the resin material. When lens molding is performed using a glass material having a large linear expansion coefficient α, cracks are likely to occur when the temperature is lowered. The linear expansion coefficient α of the glass material is preferably 200 (10E-7 / K) or less, more preferably 120 or less.

By the way, since the specific gravity of a glass lens is generally larger than that of a plastic lens, if the objective lens is a glass lens, the weight increases and a load is imposed on the actuator that drives the objective lens. Therefore, when the objective lens is a glass lens, it is preferable to use a glass material having a small specific gravity. Specifically, the specific gravity is preferably 4.0 or less, more preferably the specific gravity is 3.0 or less.

When the objective lens is a plastic lens, it is preferable to use an alicyclic hydrocarbon polymer material such as a cyclic olefin resin material. The resin material has a refractive index within a range of 1.54 to 1.60 at a temperature of 25 ° C. with respect to a wavelength of 405 nm, and a wavelength of 405 nm associated with a temperature change within a temperature range of −5 ° C. to 70 ° C. The refractive index change rate dN / dT (° C. ⁻¹ ) is -20 × 10 ^{−5 to} −5 × 10 ⁻⁵ (more preferably −10 × 10 ^{−5 to} −8 × 10 ⁻⁵ ). It is more preferable to use a certain resin material. When the objective lens is a plastic lens, the coupling lens is preferably a plastic lens.

Some preferred examples of the alicyclic hydrocarbon polymer are shown below.

A first preferred example is a polymer block [A] containing a repeating unit [1] represented by the following formula (1), a repeating unit [1] represented by the following formula (1) and the following formula ( 2) and / or polymer block [B] containing the repeating unit [3] represented by the following formula (3), and the repeating unit in the block [A] It consists of a block copolymer in which the relationship between the molar fraction a (mol%) of [1] and the molar fraction b (mol%) of the repeating unit [1] in the block [B] is a> b. It is a resin composition.

(Wherein R ¹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and R ^{2 to} R ¹² each independently represents a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, a hydroxyl group, a carbon number of 1 ˜20 alkoxy groups or halogen groups.)

(In the formula, R ¹³ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.)

(Wherein R ¹⁴ and R ¹⁵ each independently represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.)

Next, the second preferred example is obtained by addition polymerization of a monomer composition comprising at least an α-olefin having 2 to 20 carbon atoms and a cyclic olefin represented by the following general formula (4). Polymer (B) obtained by addition polymerization of polymer (A) and a monomer composition comprising an α-olefin having 2 to 20 carbon atoms and a cyclic olefin represented by the following general formula (5) ).

[Wherein n is 0 or 1, m is 0 or an integer of 1 or more, q is 0 or 1, and R ¹ to R ¹⁸ , R ^a and R ^b are each independently a hydrogen atom, A halogen atom or a hydrocarbon group, R ¹⁵ to R ¹⁸ may be bonded to each other to form a monocycle or polycycle, and the monocycle or polycycle in parentheses may have a double bond Alternatively, R ¹⁵ and R ¹⁶ , or R ¹⁷ and R ¹⁸ may form an alkylidene group. ]

[Wherein, R ¹⁹ to R ²⁶ each independently represents a hydrogen atom, a halogen atom or a hydrocarbon group. ]

In order to add further performance to the resin material, the following additives may be added.

(Stabilizer)
It is preferable to add at least one stabilizer selected from a phenol stabilizer, a hindered amine stabilizer, a phosphorus stabilizer, and a sulfur stabilizer. By suitably selecting and adding these stabilizers, for example, it is possible to more highly suppress the white turbidity and the optical characteristic fluctuations such as the refractive index fluctuations when continuously irradiated with light having a short wavelength of 405 nm. .

As the preferred phenol-based stabilizer, conventionally known ones can be used. For example, 2-t-butyl-6- (3-t-butyl-2-hydroxy-5-methylbenzyl) -4-methylphenyl acrylate, 2 , 4-di-t-amyl-6- (1- (3,5-di-t-amyl-2-hydroxyphenyl) ethyl) phenyl acrylate and the like, and JP-A Nos. 63-179953 and 1-168643. Acrylate compounds described in Japanese Patent Publication No. 1; octadecyl-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 2,2′-methylene-bis (4-methyl-6-tert-butylphenol) ), 1,1,3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 1,3,5-trimethyl-2,4,6-tris ( , 5-di-tert-butyl-4-hydroxybenzyl) benzene, tetrakis (methylene-3- (3 ′, 5′-di-tert-butyl-4′-hydroxyphenylpropionate)) methane [ie pentaerythris Limethyl-tetrakis (3- (3,5-di-tert-butyl-4-hydroxyphenylpropionate))], triethylene glycol bis (3- (3-tert-butyl-4-hydroxy-5-methylphenyl) ) Propionate) and other alkyl-substituted phenolic compounds; 6- (4-hydroxy-3,5-di-t-butylanilino) -2,4-bisoctylthio-1,3,5-triazine, 4-bisoctylthio -1,3,5-triazine, 2-octylthio-4,6-bis- (3,5-di-t-butyl-4-oxyanilino) -1,3,5- Triazine group-containing phenol compounds such as triazine; and the like.

Preferred hindered amine stabilizers include bis (2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis (2,2,6,6-tetramethyl-4-piperidyl) succinate, bis ( 1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, bis (N-octoxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis (N-benzyloxy-2, 2,6,6-tetramethyl-4-piperidyl) sebacate, bis (N-cyclohexyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis (1,2,2,6,6) -Pentamethyl-4-piperidyl) 2- (3,5-di-t-butyl-4-hydroxybenzyl) -2-butylmalonate, bis (1-acryloyl-2,2, , 6-Tetramethyl-4-piperidyl) 2,2-bis (3,5-di-t-butyl-4-hydroxybenzyl) -2-butylmalonate, bis (1,2,2,6,6- Pentamethyl-4-piperidyl) decandioate, 2,2,6,6-tetramethyl-4-piperidyl methacrylate, 4- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxy] -1- [2- (3- (3,5-di-t-butyl-4-hydroxyphenyl) propionyloxy) ethyl] -2,2,6,6-tetramethylpiperidine, 2-methyl-2- ( 2,2,6,6-tetramethyl-4-piperidyl) amino-N- (2,2,6,6-tetramethyl-4-piperidyl) propionamide, tetrakis (2,2,6,6-tetramethyl -4-pi Lysyl) 1,2,3,4-butane tetracarboxylate, tetrakis (1,2,2,6,6-pentamethyl-4-piperidyl) 1,2,3,4-butane tetracarboxylate, and the like.

Further, the preferable phosphorus stabilizer is not particularly limited as long as it is a substance usually used in the general resin industry. For example, triphenyl phosphite, diphenylisodecyl phosphite, phenyl diisodecyl phosphite, tris (nonyl). Phenyl) phosphite, tris (dinonylphenyl) phosphite, tris (2,4-di-t-butylphenyl) phosphite, 10- (3,5-di-t-butyl-4-hydroxybenzyl) -9 Monophosphite compounds such as 1,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; 4,4′-butylidene-bis (3-methyl-6-tert-butylphenyl-di-tridecyl) Phosphite), 4,4 ′ isopropylidene-bis (phenyl-di-alkyl (C12-C15)) Fight) and the like diphosphite compounds such as. Among these, monophosphite compounds are preferable, and tris (nonylphenyl) phosphite, tris (dinonylphenyl) phosphite, tris (2,4-di-t-butylphenyl) phosphite and the like are particularly preferable.

Preferred sulfur stabilizers include, for example, dilauryl 3,3-thiodipropionate, dimyristyl 3,3′-thiodipropionate, distearyl 3,3-thiodipropionate, lauryl stearyl 3,3- Thiodipropionate, pentaerythritol-tetrakis- (β-lauryl-thio) -propionate, 3,9-bis (2-dodecylthioethyl) -2,4,8,10-tetraoxaspiro [5,5] undecane Etc.

The amount of each of these stabilizers is appropriately selected within a range not to impair the purpose of the present invention, but is usually 0.01 to 2 parts by mass with respect to 100 parts by mass of the alicyclic hydrocarbon-based copolymer, The amount is preferably 0.01 to 1 part by mass.

(Surfactant)
A surfactant is a compound having a hydrophilic group and a hydrophobic group in the same molecule. The surfactant can prevent white turbidity of the resin composition by adjusting the rate of moisture adhesion to the resin surface and the rate of moisture evaporation from the surface.

Specific examples of the hydrophilic group of the surfactant include a hydroxy group, a hydroxyalkyl group having 1 or more carbon atoms, a hydroxyl group, a carbonyl group, an ester group, an amino group, an amide group, an ammonium salt, a thiol, a sulfonate, A phosphate, a polyalkylene glycol group, etc. are mentioned. Here, the amino group may be primary, secondary, or tertiary. Specific examples of the hydrophobic group of the surfactant include an alkyl group having 6 or more carbon atoms, a silyl group having an alkyl group having 6 or more carbon atoms, and a fluoroalkyl group having 6 or more carbon atoms. Here, the alkyl group having 6 or more carbon atoms may have an aromatic ring as a substituent. Specific examples of the alkyl group include hexyl, heptyl, octyl, nonyl, decyl, undecenyl, dodecyl, tridecyl, tetradecyl, myristyl, stearyl, lauryl, palmityl, cyclohexyl and the like. Examples of the aromatic ring include a phenyl group. The surfactant only needs to have at least one hydrophilic group and hydrophobic group as described above in the same molecule, and may have two or more groups.

More specifically, examples of such a surfactant include myristyl diethanolamine, 2-hydroxyethyl-2-hydroxydodecylamine, 2-hydroxyethyl-2-hydroxytridecylamine, 2-hydroxyethyl-2- Hydroxytetradecylamine, pentaerythritol monostearate, pentaerythritol distearate, pentaerythritol tristearate, di-2-hydroxyethyl-2-hydroxydodecylamine, alkyl (8-18 carbon atoms) benzyldimethylammonium chloride, ethylene Examples thereof include bisalkyl (carbon number 8 to 18) amide, stearyl diethanolamide, lauryl diethanolamide, myristyl diethanolamide, palmityl diethanolamide, and the like. Among these, amine compounds or amide compounds having a hydroxyalkyl group are preferably used. In the present invention, two or more of these compounds may be used in combination.

From the viewpoint of effectively suppressing the white turbidity of the molded product accompanying fluctuations in temperature and humidity and maintaining the light transmittance of the molded product high, the surfactant is added to 100 parts by mass of the alicyclic hydrocarbon-based polymer. On the other hand, it is preferable to add 0.01 to 10 parts by mass. The addition amount of the surfactant is more preferably 0.05 to 5 parts by mass, still more preferably 0.3 to 3 parts by mass with respect to 100 parts by mass of the alicyclic hydrocarbon-based polymer.

(Plasticizer)
The plasticizer is added as necessary to adjust the melt index of the copolymer.

Plasticizers include bis (2-ethylhexyl) adipate, bis (2-butoxyethyl) adipate, bis (2-ethylhexyl) azelate, dipropylene glycol dibenzoate, tri-n-butyl citrate, tricitrate citrate -N-butylacetyl, epoxidized soybean oil, 2-ethylhexyl epoxidized tall oil, chlorinated paraffin, tri-2-ethylhexyl phosphate, tricresyl phosphate, t-butylphenyl phosphate, tri-2-ethylhexyl phosphate Diphenyl, dibutyl phthalate, diisohexyl phthalate, diheptyl phthalate, dinonyl phthalate, diundecyl phthalate, di-2-ethylhexyl phthalate, diisononyl phthalate, diisodecyl phthalate, ditridecyl phthalate, butyl benzyl phthalate, dicyclyl phthalate Known materials such as hexyl, di-2-ethylhexyl sebacate, tri-2-ethylhexyl trimellitic acid, Santizer 278, Paraplex G40, Drapex 334F, Plastolein 9720, Mesamol, DNODP-610, HB-40, etc. are applicable. . The selection of the plasticizer and the addition amount are appropriately performed under the condition that the permeability of the copolymer and the resistance to environmental changes are not impaired.

As these resins, cycloolefin resins are preferably used. Specifically, ZEONEX manufactured by Nippon Zeon Co., Ltd., APEL manufactured by Mitsui Chemicals, Inc., TOPAS® ADVANCED® POLYMERS manufactured by TOPAS, and JSR manufactured by ARTON are preferable. Take as an example.

Further, the Abbe number of the material constituting the objective lens is preferably 50 or more.

Next, the sine condition of the objective lens will be described. The objective lens for the optical pickup device according to the present invention may or may not satisfy the sine condition.

As shown in FIG. 4, the sine condition is h when a light beam having a height h ₁ from the optical axis is incident on the lens in parallel with the optical axis, and when the emission angle when the light beam is emitted from the lens is U. ₁ / sinU satisfies a certain value. If this is a constant value regardless of the height from the height h ₁ from the optical axis, the sine condition is satisfied and the lateral magnification of each ray within the effective diameter can be regarded as constant. This sine condition is a calculated value on the axis, but is effective in correcting off-axis lateral magnification error (ie off-axis coma).

On the other hand, when h ₁ / sin U does not become a constant value, OSC = h ₁ / sin U−f is defined as the sine condition violation amount. FIG. 5 is a graph showing the sine condition violation amount in the objective lens on the horizontal axis and the height from the optical axis on the vertical axis. In the case of an objective lens that satisfies the sine condition, the graph coincides with the vertical axis, but in the case of an objective lens that does not satisfy the sine condition, the graph moves away from the vertical axis to the positive side and / or the negative side as shown in FIG. It becomes. For an objective lens that does not satisfy the sine condition, if the sine condition is satisfied near the optical axis and effective diameter, the sine condition violation amount always has a maximum value. Here, the maximum value on the positive side of the sine condition violation amount is OSCmax, and the maximum value on the negative side is OSCmin.

The objective lens having the characteristics shown in FIG. 5A is an example in which the sine condition violation amount has one negative maximum value OSCmin and does not have a positive maximum value OSCmax. According to such an objective lens, since the surface shift sensitivity is small and the on-axis thickness error sensitivity is small, it is easy to manufacture. On the other hand, as the coupling lens moves, the higher-order spherical aberration increases and the magnification changes. It has the characteristic that the change in spherical aberration due to is small. Therefore, when the coupling lens is moved to select an information recording surface in an optical disc having three or more layers, there is a possibility that the necessary movement amount increases.

On the other hand, the objective lens having the characteristics shown in FIGS. 5B and 5C has a magnification M (−0.003 ≦ M ≦ 0.003) and 70 to 90% of the effective radius of the objective lens. The sine condition violation amount has at least one maximum value OSCmax on the positive side. (Preferably only one) As shown in FIGS. 5B and 5C, the violating sine condition has a positive maximum value OSCmax between 70% and 90% of the effective radius of the objective lens. According to the objective lens, the high-order spherical aberration generated with the movement of the coupling lens is reduced, and the change of the spherical aberration due to the change in magnification is large. For this reason, when the coupling lens is moved, a necessary movement amount can be reduced.

In the example of FIG. 5B, the sine condition violation amount has one negative maximum value on the optical axis side than the positive maximum value. In the example of FIG. 5C, the sine condition violation amount has only a positive maximum value and does not have a negative maximum value. In both the example of FIG. 5B and the example of FIG. 5C, the sine condition violation amount monotonously decreases in the peripheral portion from the maximum value.

In the above-mentioned magnification M as shown in FIG. 5B, the sine condition violation amount has a positive maximum value and the sine condition violation amount has a negative maximum value between 70% and 90% of the effective radius. (Characteristic 1) The residual higher-order spherical aberration at the time of focus jump can be reduced, (Characteristic 2) the amount of movement of the coupling lens at the time of focus jump can be reduced, and (Characteristic 3) the substrate thickness In addition to being able to further suppress the reduction in lens tilt sensitivity even when the environmental temperature becomes high during recording / reproducing information on the thicker information recording surface (Characteristic 4) It is possible to suppress the amount of aberration that occurs when two opposing optical surfaces are shifted in the direction perpendicular to the optical axis due to manufacturing errors. (Characteristic 5) The lens thickness on the optical axis depends on the manufacturing error in the optical axis direction. Occurrence of aberration in case of deviation Since it is possible to also suppress, it is possible to provide a more easily manufacturable objective lens.

On the other hand, in the magnification M as shown in FIG. 5C, the sine condition violation amount has a positive maximum value and the sine condition violation amount has a negative maximum value between 70% and 90% of the effective radius. When there is no value, (Characteristic 1) the residual higher-order spherical aberration at the time of focus jump can be further reduced, (Characteristic 2) the amount of movement of the coupling lens at the time of focus jump can be further reduced, and ( Characteristic 3) Even when the environmental temperature becomes high during the recording / reproducing of information on the information recording surface having the larger substrate thickness, it is possible to further suppress the reduction of the lens tilt sensitivity.

In order to further suppress higher-order spherical aberration, the third-order spherical aberration generated in the objective lens due to the change in the divergence / convergence of incident light and the third-order spherical aberration generated during the focus jump are generated. It is preferable to set the positive maximum value of the sine condition so as to be almost similar to the change of the spherical aberration and the higher order spherical aberration.

The objective lens may be set in a shape that violates the sine condition, giving priority to reducing the amount of movement of the coupling lens, or giving priority to minimizing residual aberration during focus jump. The shape of the condition violation amount may be set.

In this way, the objective lens does not satisfy the sine condition, and in order to meet (Characteristic 1), (Characteristic 2), and (Characteristic 3) described later, the transparent substrate thickness of the optical disc (information recording / reproduction from the surface) The thickness of the transparent substrate having a thickness satisfying the formula (10) at room temperature (25 ± 3 ° C.), where T _MAX (mm) is the maximum transparent substrate thickness. At T (mm), the magnification M at which the spherical aberration is minimized satisfies the expression (11), and at the magnification M, the sine condition violation amount is a positive maximum between 70% and 90% of the effective radius. It is preferable to have a value.
T _MAX × 0.85 ≦ T ≦ T _MAX × 1.1 (10)
−0.003 ≦ M ≦ 0.003 (11)

Examples of preferable characteristics in an objective lens suitable for a BD having three or more layers include the following three.
(Characteristic 1) Residual higher order spherical aberration at the time of focus jump is small.
(Characteristic 2) The amount of movement of the coupling lens when performing a focus jump is small.
(Characteristic 3) The tilt sensitivity of the objective lens when information is recorded / reproduced with respect to the information recording surface having the larger substrate thickness is not too small. In particular, when a plastic objective lens is used, the lens tilt sensitivity is small when the environmental temperature becomes high during the recording / reproducing of information on the information recording surface having the larger substrate thickness. It is necessary not to become too much.
An objective lens that is preferable from the viewpoint of satisfying all the characteristics (Characteristic 1) to (Characteristic 3) will be described in detail below.

(Characteristic 1) In the magnification M satisfying the expression (11), the sine condition violation amount has a positive maximum value between 70% and 90% of the effective radius, so that the high at the time of focus jump This is preferable because secondary spherical aberration can be effectively suppressed.

(Characteristic 2) In order to reduce the amount of movement of the coupling lens when performing the focus jump, it is necessary to increase the amount of change in spherical aberration with respect to the change in magnification. Higher-order spherical aberration at the time of focus jump is effective by setting the sine condition violation amount to a positive maximum value between 70% and 90% of the effective radius at the magnification M satisfying equation (11). In addition, it is possible to increase the amount of change in the third-order spherical aberration with respect to the change in magnification.

(Characteristic 3) Some optical pickup devices that record / reproduce information with respect to a two-layer BD have a plastic objective lens mounted thereon, and the objective lens has a thicker substrate thickness for information recording. Spherical aberration by combining a substrate thickness of 87.5 μm between the surface L0 (100 μm) and the thinner information recording surface L1 (75 μm) with a magnification of zero (corresponding to the case where a parallel light beam is incident) Is designed to be minimal. In the plastic objective lens designed in this manner, the amount of coma generated when the lens is tilted is minimized because the environmental temperature is high during the recording / reproducing of information on the information recording surface L0. In this state, the amount of third-order coma aberration due to tilt when the objective lens performs tracking (referred to as lens shift in this specification) is defined as CM (LT). Conversely, a plastic objective lens for BD having three or more layers can withstand practical use if it is designed so that the minimum amount of coma generated when the lens is tilted is greater than CM (LT). It can be said. When information is recorded / reproduced with respect to the information recording surface L0, the amount of coma aberration generated by a plastic objective lens for a two-layer BD with a lens tilt of 0.5 degrees at a high temperature (55 degrees) CM (LT) is about 0.02 λrms, and the ratio of the third-order coma aberration CM (DT) generated when the optical disk is tilted by the same amount in the same state and CM (LT) is about 0.36. Become. As a result of studying a plastic objective lens suitable for BD having three or more layers with these values as target values, spherical aberration is minimized at a normal temperature (25 ± 3 ° C.) and a magnification satisfying the expression (11). It has been found that the target value of CM (LT) is met by setting the spherical aberration correction state so that the design substrate thickness at that time is equal to or greater than the lower limit of the equation (10).

In the present specification, the “transparent substrate thickness T” is a transparent substrate thickness (also referred to as a design substrate thickness) used as a reference when comparing the characteristics of the objective lens. the _MIN ~ T _MAX is intended to be distinguished.

Furthermore, it is preferable that the objective lens satisfies the following conditional expression (12).
T _MAX × 0.85 ≦ T ≦ T _MAX × 1.0 (12)

By the spherical aberration is not larger than T _MAX design board thickness is corrected to zero, the light beam incident on the objective lens when recording / reproducing information for the information recording surface of the transparent towards the substrate thickness is thin An increase in the degree of convergence can be further prevented. Therefore, when information is recorded / reproduced on the information recording surface with the thinner transparent substrate, it is possible to further prevent the occurrence of coma aberration when the objective lens is shifted. In a BD having three or more layers, where the maximum difference in the thickness of the transparent substrate on the information recording surface is larger than that on the two-layer BD, the light beam incident on the objective lens when information is recorded / reproduced on the information recording surface having the thinnest transparent substrate thickness Since the degree of convergence of the lens becomes too large and the lens shift characteristic is likely to be deteriorated, such an objective lens can solve a larger problem unique to such a BD having three or more layers. That is, when the transparent substrate thickness T satisfies the upper limit of the expression (12), the degree of convergence of the light beam incident on the objective lens becomes too large when information is recorded / reproduced on the information recording surface with the thinnest transparent substrate thickness. This is further preferable, and as a result, the lens shift characteristics can be further improved, and the residual higher-order spherical aberration when the focus jump is made to the information recording surface having the thinnest transparent substrate can be further reduced.

On the other hand, in another objective lens design example, when the magnification is M and the substrate thickness of the optical disk is T, one of the combinations (M, T) that minimizes the third-order spherical aberration is M = 0, T = Tcen (μm),
In another combination, when the value of the third order coma aberration per unit angle with respect to the objective lens tilt is LT and the value of the third order coma aberration per unit angle with respect to the optical disc tilt is DT, It is M = Mcmc and T = Tcmc (μm) that minimize LT│-│DT││, and it is preferable that the following expression is satisfied.
0.5 · T0 ≦ Tcen ≦ 0.85 · T0 (13)
Tcen <Tcmc (14)
Mccm <0 (15)
However, T0: The maximum substrate thickness (μm) among the substrate thicknesses of the optical disk

By designing the objective lens so that the design substrate thickness is small so as to satisfy the expression (13), it is possible to provide an objective lens suitable for the thinned optical pickup device, but the objective lens tilt sensitivity is lowered. On the other hand, if the objective lens tilt sensitivity is increased by breaking the sine condition, the coma aberration correction amount with respect to the tilt amount of the objective lens can be increased. it can.

In the case of an objective lens that satisfies the sine condition, generally, when M = 0 and the design substrate thickness Tcen, the absolute value of the coma aberration LT generated when the objective lens is tilted by a unit angle and the unit angle. The absolute value of the coma aberration DT generated when the optical disk is tilted substantially coincides (| LT | ≈ | DT |). However, when the sine condition is not satisfied, (| LT | ≠ | DT |). On the other hand, as the combination (M, T) that minimizes the third-order spherical aberration, when M = Mcmc and T = Tcmc, if || LT |-| DT || Tcmc, that is, if the expression (14) is satisfied, the sine condition is not satisfied. Thereby, the above-mentioned effect can be acquired. In this specification, the term “minimum” includes a value in the range of + 10% with respect to the actual minimum value. However, the transparent substrate thickness may be intermediate.

According to the above objective lens, the optical pickup is designed to satisfy the conditional expressions (13) and (14), while having the same objective lens tilt characteristic as the conventional lens (currently used lens). An objective lens that contributes to miniaturization of the apparatus can be realized.

The objective lens may be dedicated to BD or may be compatible with BD / DVD / CD. Hereinafter, an objective lens for BD / DVD / CD compatibility will be described. *

At least one optical surface of the objective lens is distinguished from the “peripheral region” in the claims by the central region, the intermediate region around the central region, and the peripheral region around the intermediate region (hereinafter referred to as a dedicated region) ) At least. The central region is preferably a region including the optical axis of the objective lens. However, a minute region including the optical axis may be an unused region or a special purpose region, and the periphery thereof may be a central region. The central region, the intermediate region, and the dedicated region are preferably provided on the same optical surface. As shown in FIG. 6, it is preferable that the central region CN, the intermediate region MD, and the dedicated region OT are provided concentrically around the optical axis on the same optical surface. In addition, a first optical path difference providing structure is provided in the central area of the objective lens, and a second optical path difference providing structure is provided in the intermediate area. The dedicated area may be a refracting surface, or a third optical path difference providing structure may be provided in the dedicated area. The central region, intermediate region, and dedicated region are preferably adjacent to each other, but there may be a slight gap between them.

The central area of the objective lens can be said to be a BD / DVD / CD shared area used for recording / reproducing BD, DVD and CD. That is, the objective lens condenses the first light flux passing through the central area so that information can be recorded / reproduced on the information recording surface of the BD, and the second light flux passing through the central area is recorded as information recording on the DVD. The light is condensed so that information can be recorded and / or reproduced on the surface, and the third light flux passing through the central region is condensed so that information can be recorded / reproduced on the information recording surface of the CD. In addition, the first optical path difference providing structure provided in the central region has the BD protective substrate thickness t1 and the DVD protective substrate thickness with respect to the first and second light fluxes passing through the first optical path difference providing structure. It is preferable to correct spherical aberration generated due to the difference in thickness t2 / spherical aberration generated due to the difference in wavelength between the first light beam and the second light beam. Further, the first optical path difference providing structure is different from the thickness t1 of the BD protective substrate and the thickness t3 of the CD protective substrate with respect to the first and third light fluxes that have passed through the first optical path difference providing structure. It is preferable to correct the spherical aberration caused by the difference in the wavelength of the first light beam and the third light beam.

The intermediate area of the objective lens is used for BD / DVD recording / reproduction and can be said to be a BD / DVD shared area not used for CD recording / reproduction. That is, the objective lens condenses the first light flux passing through the intermediate area so that information can be recorded / reproduced on the information recording surface of the BD, and the second light flux passing through the intermediate area is recorded as information recording on the DVD. Light is collected so that information can be recorded / reproduced on the surface. On the other hand, the third light flux passing through the intermediate region is not condensed so that information can be recorded / reproduced on the information recording surface of the CD. The third light flux passing through the intermediate region of the objective lens preferably forms a flare on the information recording surface of the CD. As shown in FIG. 7, in the spot formed on the information recording surface of the CD by the third light beam that has passed through the objective lens, the spot center having a high light amount density in the order from the optical axis side (or the spot center) to the outside. It is preferable to have a portion SCN, a spot intermediate portion SMD whose light intensity density is lower than that of the spot central portion, and a spot peripheral portion SOT whose light intensity density is higher than that of the spot intermediate portion and lower than that of the spot central portion. The center portion of the spot is used for recording / reproducing information on the optical disc, and the middle portion of the spot and the peripheral portion of the spot are not used for recording / reproducing information on the optical disc. In the above, this spot peripheral part is called flare. However, there is no spot middle part around the center part of the spot and there is a spot peripheral part, that is, even when a light spot is formed thinly around the condensing spot, the spot peripheral part may be called a flare. Good. In other words, it can be said that the third light flux that has passed through the intermediate region of the objective lens preferably forms a spot peripheral portion on the information recording surface of the CD.

The dedicated area of the objective lens is used for BD recording / playback, and can be said to be a BD dedicated area not used for DVD / CD recording / playback. That is, the objective lens condenses the first light beam passing through the dedicated area so that information can be recorded / reproduced on the information recording surface of the BD. On the other hand, the second light beam passing through the dedicated area is not condensed so that information can be recorded / reproduced on the information recording surface of the DVD, and the third light beam passing through the dedicated area is used as the information recording surface of the CD. Do not collect light so that information can be recorded / reproduced on top. It is preferable that the second light flux and the third light flux that pass through the dedicated area of the objective lens form a flare on the information recording surface of the DVD and CD. That is, it is preferable that the second light flux and the third light flux that have passed through the dedicated area of the objective lens form a spot peripheral portion on the information recording surface of DVD and CD.

In the case of having the first optical path difference providing structure, it is preferably provided in a region of 70% or more of the area of the central region of the objective lens, and more preferably 90% or more. More preferably, the first optical path difference providing structure is provided on the entire surface of the central region. When it has a 2nd optical path difference providing structure, it is preferable to provide in the area | region 70% or more of the area of the intermediate area | region of an objective lens, and 90% or more is more preferable. More preferably, the second optical path difference providing structure is provided on the entire surface of the intermediate region. When the dedicated region has the third optical path difference providing structure, the third optical path difference providing structure is preferably provided in a region of 70% or more of the area of the dedicated region of the objective lens, and more preferably 90% or more. More preferably, the third optical path difference providing structure is provided on the entire surface of the dedicated region.

In addition, 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 of the present invention is preferably a diffractive structure. The optical path difference providing structure has a step, preferably a plurality of steps. This step adds an optical path difference and / or 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. When the objective lens provided with the optical path difference providing structure is a single aspherical lens, the incident angle of the light flux to the objective lens differs depending on the height from the optical axis. Each will be slightly different. For example, when the objective lens is a single-lens aspherical convex lens, even if it is an optical path difference providing structure that provides the same optical path difference, generally the distance from the optical axis tends to increase.

In addition, the diffractive structure referred to in this specification is a general term for structures that have a step and have a function of converging or diverging a light beam by diffraction. For example, a plurality of unit shapes are arranged around the optical axis, and a light beam is incident on each unit shape, and the wavefront of the transmitted light is shifted between adjacent annular zones, resulting in new It includes a structure that converges or diverges light by forming a simple wavefront. The diffractive structure preferably has a plurality of steps, and 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. In addition, when the objective lens provided with the diffractive structure is a single aspherical lens, the incident angle of the light beam to the objective lens differs depending on the height from the optical axis, so the step amount of the diffractive structure is slightly different for each annular zone. It will be. For example, when the objective lens is a single aspherical convex lens, even if it is a diffractive structure that generates diffracted light of the same diffraction order, generally, the distance from the optical axis tends to increase.

Incidentally, it is preferable that the optical path difference providing structure has a plurality of concentric annular zones with the optical axis as the center. In addition, the optical path difference providing structure can generally have various cross-sectional shapes (cross-sectional shapes on the plane including the optical axis), and the cross-sectional shapes including the optical axis are roughly classified into a blazed structure and a staircase structure.

As shown in FIGS. 8A and 8B, the blaze type structure means that the cross-sectional shape including the optical axis of the optical element having the optical path difference providing structure is a sawtooth shape. In the example of FIG. 8, it is assumed that the upper side is the light source side and the lower side is the optical disc side, and the optical path difference providing structure is formed on a plane as a mother aspherical surface. In the blazed structure, the length in the direction perpendicular to the optical axis of one blaze unit is called a pitch P. (See FIGS. 8A and 8B) The length of the step in the direction parallel to the optical axis of the blaze is referred to as a step amount B. (See Fig. 8 (a))

In addition, as shown in FIGS. 8C and 8D, the staircase structure has a cross-sectional shape including an optical axis of an optical element having an optical path difference providing structure (referred to as a staircase unit). ). In the present specification, “V level” means a ring-shaped surface (hereinafter also referred to as a terrace surface) corresponding to (or facing) the vertical direction of the optical axis in one step unit of the step structure. In other words, it is divided by V steps and divided into V ring zones. Particularly, a three-level or higher staircase structure has a small step and a large step. For example, the optical path difference providing structure illustrated in FIG. 8C is referred to as a five-level step structure, and the optical path difference providing structure illustrated in FIG. 8D is referred to as a two-level step structure (also referred to as a binary structure). .

The optical path difference providing structure is preferably a structure in which a certain unit shape is periodically repeated. 「“ The unit shape is periodically repeated ”here naturally includes shapes in which the same shape is repeated in the same cycle. In addition, the unit shape that is one unit of the cycle has regularity, and the shape in which the cycle gradually increases or decreases gradually is also included in the “unit shape is periodically repeated”. Suppose that

When the optical path difference providing structure has a blazed structure, the sawtooth shape as a unit shape is repeated. As shown in FIG. 8 (a), the same sawtooth shape may be repeated, or as shown in FIG. 8 (b), the shape of the sawtooth shape gradually increases as it moves away from the optical axis. A shape in which the pitch becomes longer or a shape in which the pitch becomes shorter may be used. In addition, in some areas, the blazed structure has a step opposite to the optical axis (center) side, and in other areas, the blazed structure has a step toward the optical axis (center). It is good also as a shape in which the transition area | region required in order to switch the direction of the level | step difference of a blaze | braze type | mold structure is provided in the meantime. In addition, when it is set as the structure which switches the direction of the level | step difference of a blaze | braze type | mold in this way, it becomes possible to widen an annular zone pitch and it can suppress the transmittance | permeability fall by the manufacturing error of an optical path difference providing structure.

In addition, when the first optical path difference providing structure and the second optical path difference providing structure are provided, they may be provided on different optical surfaces of the objective lens, but are preferably provided on the same optical surface. Furthermore, also when providing a 3rd optical path difference providing structure, it is preferable to provide in the same optical surface as a 1st optical path difference providing structure and a 2nd optical path difference providing structure. Providing them on the same optical surface is preferable because it makes it possible to reduce eccentricity errors during manufacturing. Moreover, when providing the 1st optical path difference providing structure, the 2nd optical path difference providing structure, and the 3rd optical path difference providing structure, it is preferable to provide in the light source side surface of an objective lens rather than the surface at the side of the optical disk of an objective lens. In other words, the first optical path difference providing structure, the second optical path difference providing structure, and the third optical path difference providing structure are preferably provided on the optical surface having the smaller absolute value of the radius of curvature of the objective lens. It is also conceivable to provide the first basic structure and the second basic structure on different optical surfaces without overlapping. Similarly, the third basic structure and the fourth basic structure may be provided on different optical surfaces without overlapping.

Next, a case where the first optical path difference providing structure is provided in the central region will be described. The first optical path difference providing structure is preferably a structure in which at least the first basic structure and the second basic structure are overlapped, but is not limited thereto. The first optical path difference providing structure is preferably a structure in which only the first basic structure and the second basic structure are overlapped.

The first basic structure is preferably a blazed structure. In addition, the first basic structure makes the first-order diffracted light quantity of the first light beam that has passed through the first basic structure larger than any other order of diffracted light quantity, and the first-order diffracted light quantity that has passed through the first basic structure. Is preferably larger than any other order of diffracted light, and the first order diffracted light of the third light beam having passed through the first basic structure is preferably larger than any other order of diffracted light. This is called a (1/1/1) structure. In particular, if the first-order diffracted light that is low order is generated, the step amount of the first basic structure does not become too large, so that the manufacture is facilitated, and the light quantity loss due to the manufacturing error can be suppressed, and the wavelength It is preferable because the diffraction efficiency fluctuation at the time of fluctuation can be reduced.

In such a case, it is preferable that the first basic structure provided at least in the vicinity of the optical axis in the central region has a step in a direction opposite to the optical axis. “The step is directed in the direction opposite to the optical axis” means a state as shown in FIG. In addition, the first basic structure provided “at least in the vicinity of the optical axis of the central region” refers to a step at least closest to the optical axis among steps of the (1/1/1) structure. Preferably, at least a (1/1/1) structure step existing between the optical axis and the half-position in the direction orthogonal to the optical axis from the optical axis to the boundary between the central region and the intermediate region is the optical axis. Is pointing in the opposite direction.

For example, in the first basic structure provided near the middle region of the central region, the step may be directed in the direction of the optical axis. That is, as shown in FIG. 10B, the step is directed in the opposite direction to the optical axis when the first foundation structure is near the optical axis, but is switched halfway, and the step of the first foundation structure is near the intermediate region. It is good also as a shape which faces the direction of an optical axis. However, it is preferable that all the steps of the first basic structure provided in the central region are directed in a direction opposite to the optical axis.

In this way, the direction of the step of the first basic structure in which the diffraction order of the first light beam is the first order is directed in the direction opposite to the optical axis, so that the three types of optical disks of BD / DVD / CD can be used interchangeably. Even with a thick objective lens having a large axial thickness, a sufficient working distance can be secured when the CD is used.

The first basic structure is the first basic structure from the viewpoint of securing a sufficient working distance when using a CD even in a thick objective lens having a thick on-axis thickness, which is used for compatibility with three types of optical disks of BD / DVD / CD. It is preferable to have a negative paraxial power with respect to the luminous flux. Here, “having negative paraxial power” means that B ₂ h ² > 0 when the optical path difference function of the first basic structure is expressed by the following equation ( ² ).

Also, the second basic structure is preferably a blazed structure. In the second basic structure, the second-order diffracted light amount of the first light beam that has passed through the second basic structure is made larger than the diffracted light amount of any other order, and the first-order diffraction of the second light beam that has passed through the first basic structure. It is preferable that the amount of light is made larger than any other order of diffracted light, and the first order diffracted light of the third light beam that has passed through the first basic structure is made larger than any other order of diffracted light. This is called a (2/1/1) structure. In particular, if low-order second-order diffracted light or first-order diffracted light is generated, the step amount of the second basic structure does not become too large, which facilitates manufacturing and suppresses light loss caused by manufacturing errors. This is preferable because it can reduce the diffraction efficiency fluctuation at the time of wavelength fluctuation.

Further, it is preferable that the step of the second basic structure provided at least in the vicinity of the optical axis in the central region is directed in the direction of the optical axis. “The step is directed in the direction of the optical axis” means a state as shown in FIG. The second basic structure provided “at least in the vicinity of the optical axis of the central region” refers to a step at least closest to the optical axis among steps of the (2/1/1) structure. Preferably, at least a half of the optical axis orthogonal direction from the optical axis to the boundary between the central region and the intermediate region and the (2/1/1) structure step existing between the optical axis and the optical axis direction It is suitable.

For example, in the second basic structure provided near the middle region of the central region, the step may be directed in a direction opposite to the optical axis. That is, as shown in FIG. 10A, the step is directed in the direction of the optical axis when the second foundation structure is in the vicinity of the optical axis, but is switched halfway, and the step of the second foundation structure is at the optical axis in the vicinity of the intermediate region. It is good also as a shape which faces the reverse direction. However, preferably, the second basic structure provided in the central region is that all the steps are directed in the direction of the optical axis. However, in the present invention, the structure shown in FIG. 10B is more effective.

When the first optical path difference providing structure is formed by superimposing the first basic structure having the (1/1/1) structure and the second basic structure having the (2/1/1) structure, the height of the step is extremely high. Can be lowered. Therefore, it is possible to further reduce manufacturing errors, further reduce the light amount loss, and further suppress the change in diffraction efficiency when the wavelength changes.

Furthermore, at least near the optical axis of the central region, the first basic structure in which the step is directed in the direction opposite to the optical axis, and at least near the optical axis of the central region, the step is directed in the direction of the optical axis. By superimposing two foundation structures, the height of the step after superposition is higher than when superimposing the steps so that the first and second foundation structures have the same step direction. As a result, it is possible to further suppress the light amount loss due to manufacturing errors and the like, and to further suppress the fluctuation of the diffraction efficiency at the time of wavelength fluctuation.

Further, not only can the three types of optical discs of BD / DVD / CD be compatible, but also the light usage efficiency that can maintain high light usage efficiency for any of the three types of optical discs of BD / DVD / CD. It is preferable to provide a balanced objective lens. For example, it is preferable to provide an objective lens that has a diffraction efficiency of 80% or more for the wavelength λ1, a diffraction efficiency of 60% or more for the wavelength λ2, and a diffraction efficiency of 50% or more for the wavelength λ3. Furthermore, it is more preferable to provide an objective lens that has a diffraction efficiency of 80% or more for the wavelength λ1, a diffraction efficiency of 70% or more for the wavelength λ2, and a diffraction efficiency of 60% or more for the wavelength λ3. In addition, by orienting the step of the first basic structure in the direction opposite to the optical axis, it is easier to change the aberration in the direction of under (undercorrection) when the wavelength changes to the long wavelength side. .

Viewpoint of the shape and step amount of the first optical path difference providing structure after the first basic structure in which the step is opposite to the optical axis and the second basic structure in which the step is directed toward the optical axis are overlapped Therefore, the first optical path difference providing structure in which the first basic structure having the (1/1/1) structure and the second basic structure having the (2/1/1) structure are overlapped is expressed as follows. be able to. The first optical path difference providing structure provided at least in the vicinity of the optical axis of the central region has both a step facing in the opposite direction to the optical axis and a step facing in the direction of the optical axis. It is preferable that the step amount d11 of the step facing the direction opposite to the axis and the step amount d12 of the step facing the direction of the optical axis satisfy the following conditional expressions (16) and (17). More preferably, the following conditional expressions (16) and (17) are satisfied in all the regions of the central region. If the objective lens provided with the optical path difference providing structure is a single aspherical convex lens, the incident angle of the light flux to the objective lens differs depending on the height from the optical axis, so that the optical path difference providing structure that gives the same optical path difference Even so, in general, as the distance from the optical axis increases, the step amount tends to increase. In the following conditional expression, the upper limit is multiplied by 1.5 because the increase in the level difference is taken into account. Here, n represents the refractive index of the objective lens at the first wavelength λ1.
0.6 · (λ1 / (n−1)) <d11 <1.5 · (λ1 / (n−1)) (16) 0.6 · (λ1 / (n−1)) <d12 <1. 5. (2λ1 / (n-1)) (17)

The first optical path difference providing structure provided “at least in the vicinity of the optical axis of the central region” includes at least a step facing in a direction opposite to the optical axis closest to the optical axis and an optical axis closest to the optical axis. An optical path difference providing structure having both of the steps facing the direction of. Preferably, the optical path difference providing structure has a step existing between at least a half position in the direction orthogonal to the optical axis from the optical axis to the boundary between the central region and the intermediate region.

For example, when λ1 is 390 to 415 nm (0.390 to 0.415 μm) and n is 1.54 to 1.60, the above conditional expression can be expressed as follows.

0.39 μm <d11 <1.15 μm (18)
0.39 μm <d12 <2.31 μm (19)

Further, as a method of overlapping the first foundation structure and the second foundation structure, the shape of the foundation structure is finely adjusted so that the positions of all the steps of the second foundation structure and the positions of the steps of the first foundation structure are matched. Alternatively, it is preferable to finely adjust the shape of the foundation structure so that the positions of all the steps of the first foundation structure and the positions of the steps of the second foundation structure are matched.

As described above, when the positions of all the steps of the second foundation structure are matched with the positions of the steps of the first foundation structure, d11 and d12 of the first optical path difference providing structure are the following conditional expressions (20) and (21 ) Is preferably satisfied. More preferably, the following conditional expressions (20) and (21) are satisfied in all the regions of the central region.
0.6 · (λ1 / (n-1)) <d11 <1.5 · (λ1 / (n-1)) (20) 0.6 · (λ1 / (n-1)) <d12 <1. 5. (λ1 / (n-1)) (21)

0.39 μm <d11 <1.15 μm (22)
0.39 μm <d12 <1.15 μm (23)

More preferably, the following conditional expressions (24) and (25) are preferably satisfied. More preferably, the following conditional expressions (24) and (25) are satisfied in all the regions of the central region.
0.9 · (λ1 / (n-1)) <d11 <1.5 · (λ1 / (n-1)) (24) 0.9 · (λ1 / (n-1)) <d12 <1. 5. (λ1 / (n-1)) (25)

0.59 μm <d11 <1.15 μm (26)
0.59 μm <d12 <1.15 μm (27)

Further, in the first basic structure having the (1/1/1) structure, when the wavelength of the incident light beam is changed to be longer, the spherical aberration is changed in the undercorrection direction (under), and (2 / In the second basic structure having the 1/1) structure, when the wavelength of the incident light beam is changed to be longer, it is preferable that the spherical aberration is changed in the undercorrection direction (under). With such a configuration, when the refractive index of the objective lens changes due to an increase in the temperature of the optical pickup device, the refractive index of the objective lens is also utilized by utilizing the fact that the wavelength of the light source increases due to the increase in the environmental temperature. It is possible to correct a change in spherical aberration due to a change in the rate and form an appropriate focused spot on the information recording surface of each optical disc. Thereby, even if the objective lens is made of plastic, it is possible to provide an objective lens that can maintain stable performance even when the temperature changes.

It is preferable that the paraxial power of the first foundation structure is larger than that of the second foundation structure. That is, it is preferable that the average pitch of the first foundation structure is smaller than the average pitch of the second foundation structure. Thereby, a working distance in the CD can be secured even in an objective lens having a large axial thickness, which is a BD / DVD / CD compatible objective lens. Furthermore, the chromatic aberration is reduced, a good light spot is formed even when the light source has a high frequency superposition, and the problem of stray light when the optical disk has a plurality of information recording surfaces is reduced. In order to improve the temperature characteristics and the wavelength characteristics at the time, in the first optical path difference providing structure, one ring zone closest to the optical axis of the second basic structure has two or more ring zones of the first basic structure. It is preferable that 6 (particularly preferably 2 to 3) are included. In this case, the “ring zone” closest to the optical axis of the second foundation structure is described, but in practice, it is usually a “circle” including the optical axis. Accordingly, the “annular zone closest to the optical axis” mentioned here includes a circular shape. Further, in one ring zone of the second foundation structure closest to the intermediate region, 1 to 5 ring zones of the first foundation structure (particularly preferably 2 to 3 rings) are included in one ring zone of the second foundation structure. ) Is included.

As shown in FIG. 11D, when the first basic structure and the second basic structure are directly overlapped, a part may protrude as shown by a dotted line, but the width of the protruding part is 5 μm or less. If it is narrow, the projecting portion is shifted in parallel along the optical axis, and eliminating the projecting portion has no significant effect, so that one annular zone of the second foundation structure can have a plurality of the first foundation structure. The zonal is just like that (see the solid line). Therefore, in the example of FIG. 11D, it is assumed that three annular zones of the first foundation structure are on one annular zone of the second foundation structure. When the first foundation structure and the second foundation structure are superimposed as they are, a dent may be eliminated in the same manner even when a dent having a width of 5 μm or less is generated.

Here, when Δλ (nm) is the amount of change in the first wavelength and ΔWD (μm) is the chromatic aberration of the objective lens caused by the change Δλ in the first wavelength, it is preferable that the following expression is satisfied.
0.3 (μm / nm) ≦ ΔWD / Δλ ≦ 0.6 (μm / nm) (28)

In order to obtain such a configuration, as described above, in the first optical path difference providing structure, two annular zones N1 of the first basic structure are equal to one annular zone closest to the optical axis of the second basic structure. It is preferable to include ˜6 (particularly preferably 2 to 3). By setting the chromatic aberration to the above-described range, the optical disc has a plurality of information recording surfaces while ensuring a working distance in the CD even in an objective lens having a large axial thickness, which is a BD / DVD / CD compatible objective lens. This is preferable because the problem of stray light can be reduced and the temperature and wavelength characteristics can be improved when using a DVD. In addition, the number N2 of the first foundation structure annular zones superimposed on one annular zone closest to the intermediate region in the second foundation structure is preferably equal to or smaller than N1, for example, 1 to 5 overlapping zones. It should be done.

The first basic structure preferably has a positive diffractive power, so that a working distance when using a CD can be secured even for an objective lens having a large axial thickness such as an objective lens for BD / DVD / CD. Further, the second basic structure preferably has a negative diffraction power. As described above, since both the first basic structure and the second basic structure have diffraction power, when using an optical disk having a plurality of information recording surfaces, unnecessary light reflected by the information recording surface which is not a recording / reproducing object is required light. It is preferable because it can be further away from the center.

The first best focus position where the light intensity of the spot formed by the third light flux is the strongest by the third light flux passing through the first optical path difference providing structure, and the second strongest light intensity of the spot formed by the third light flux. It is preferable that the best focus position satisfies the following conditional expression (29). Here, the best focus position refers to a position where the beam waist becomes a minimum within a certain defocus range. The first best focus position is the best focus position of the necessary light used for CD recording / reproduction, and the second best focus position is the best of the luminous flux having the largest light quantity among the unnecessary light that is not used for CD recording / reproduction. The focus position.
0.05 ≦ L / f13 ≦ 0.35 (29)
However, f13 [mm] indicates the focal length of the third light flux that passes through the first optical path difference providing structure and forms the first best focus, and L [mm] indicates the first best focus and the second best focus. Refers to the distance between.

More preferably, the following conditional expression (29) ′ is satisfied.
0.10 ≦ L / f13 ≦ 0.25 (29) ′

Several preferred examples of the first optical path difference providing structure described above are shown in FIGS. 11 (a), (b), and (c). Although FIG. 11 shows the first optical path difference providing structure ODS1 as a flat plate for convenience, it may be provided on a single aspherical convex lens. The first basic structure BS1 which is a (1/1/1) diffraction structure is overlapped with the second basic structure BS2 which is a (2/1/1) diffraction structure. In FIG. 11 (a), the step of the second foundation structure BS2 faces the direction of the optical axis OA, and the step of the first foundation structure BS1 faces the direction opposite to the optical axis OA. Furthermore, it can be seen that the positions of all the steps of the second foundation structure BS2 are aligned with the positions of the steps of the first foundation structure BS1. Next, in FIG. 11B, the step of the second foundation structure BS2 faces the direction of the optical axis OA, and the step of the first foundation structure BS1 also faces the direction of the optical axis OA. Furthermore, it can be seen that the positions of all the steps of the second foundation structure BS2 are aligned with the positions of the steps of the first foundation structure BS1. Next, in FIG. 11C, the step of the first basic structure BS1 faces in the direction opposite to the optical axis OA, and the step of the second basic structure BS2 also faces in the direction opposite to the optical axis OA. . Furthermore, it can be seen that the positions of all the steps of the second foundation structure BS2 are aligned with the positions of the steps of the first foundation structure BS1.

Furthermore, when the total number of annular zones in the central region is N and the focal length of the first light flux of the objective lens is f (mm), it is preferable that the following expression is satisfied. As a result, it is possible to prevent the working distance of the CD from becoming too short and to suppress the workability from being lowered due to the ring zone pitch becoming too small. Note that the number of steps substantially parallel to the optical axis in the central region may be regarded as the total number of annular zones in the central region.
160 (mm) ≤ N · f ≤ 210 (mm) (30)

As described above, the preferable structure of the first optical path difference providing structure has been described focusing on the structure in which the blaze type (1/1/1) structure and the blaze type (2/1/1) structure are superimposed. Examples of other first optical path difference providing structures include the following.

For example, (A) in the case of only a single blazed structure, (B) in the case of a single staircase structure, (C) in the case of superimposing a plurality of blazed structures, (D) a blazed structure and a staircase structure For example. As a preferable example of (A), a first optical path difference providing structure composed only of a blazed (2/1/1) structure or a (1/1/1) structure alone while utilizing the magnification difference of the objective lens. The 1st optical path difference providing structure which becomes becomes. As a preferable example of (B), a first optical path difference providing structure consisting only of a (1 / −3 / −4) structure which is a 7-level staircase structure or a 7-level staircase structure (1 / −) 2 / -3) a first optical path difference providing structure consisting only of the structure, a first optical path difference providing structure consisting only of a (1 / -1 / -2) structure which is a 6-level stepped structure, and the like. As a preferable example of (C), in addition to the first optical path difference providing structure in which the blaze type (2/1/1) structure and the blaze type (1/1/1) structure are superimposed, the blaze type (2 / 1/1) structure and a blaze-type (1/0/0) structure may be mentioned as a first optical path difference providing structure. As a preferable example of (D), a first optical path difference providing structure in which a blaze type (2/1/1) structure and a (1/0/0) structure that is a four-level stepped structure are superimposed, There is a first optical path difference providing structure in which a blaze type (2/1/1) structure and a (0/0/1) structure that is a two-level stepped structure are superimposed.

Next, the case where the second optical path difference providing structure is provided in the intermediate region will be described. The second optical path difference providing structure is preferably a structure in which at least two basic structures of the third basic structure and the fourth basic structure are overlapped, but is not limited thereto.

It is preferable that both the third basic structure and the fourth basic structure are blazed structures. Further, the third basic structure makes the first-order diffracted light amount of the first light beam that has passed through the third basic structure larger than any other order of diffracted light amount, and the first-order diffracted light amount of the second light beam that has passed through the third basic structure. Is preferably larger than any other order of diffracted light. In addition, it is preferable that the first-order diffracted light amount of the third light flux that has passed through the third basic structure is larger than any other order diffracted light amount. In addition, the fourth foundation structure makes the second-order diffracted light amount of the first light beam that has passed through the fourth foundation structure larger than any other order of diffracted light amount, and the first-order of the second light beam that has passed through the fourth foundation structure. Is preferably larger than any other order of diffracted light. In addition, it is preferable that the first-order diffracted light amount of the third light flux that has passed through the fourth basic structure is larger than any other order diffracted light amount. Thereby, in the second optical path difference providing structure in which at least the third basic structure and the fourth basic structure are overlapped, the amount of step in the optical axis direction can be reduced, thereby suppressing the decrease in diffraction efficiency at the time of wavelength variation. Further, the orders of the diffracted light having the highest light intensity in the first basic structure and the third basic structure are matched, and the orders of the diffracted light having the highest light intensity in the second basic structure and the fourth basic structure are matched. Therefore, the spherical aberration can be made continuous even when the temperature and the wavelength change for the light flux passing through the central region and the intermediate region, and as a result, the occurrence of higher order aberrations can be suppressed.

The second optical path difference providing structure may be a structure in which the fifth basic structure is overlapped in addition to the third and fourth basic structures. However, in order to simplify the structure and suppress a decrease in light utilization efficiency due to manufacturing errors. The second optical path difference providing structure preferably includes only the third basic structure and the fourth basic structure.

At this time, the fifth basic structure makes the 0th-order diffracted light quantity of the first light flux that has passed through the fifth basic structure larger than any other order diffracted light quantity, and the second light flux that has passed through the fifth basic structure. The 0th-order diffracted light amount is made larger than any other order diffracted light amount, and the G-th order diffracted light amount of the third light flux that has passed through the fifth basic structure is made larger than any other order diffracted light amount. It is preferable. By superimposing such a fifth basic structure, a phase shift occurs between the central region and the intermediate region without adversely affecting the first light beam and the second light beam passing through the intermediate region of the objective lens. Accordingly, it is possible to easily give only the third light flux an effect of forming a flare at a position far from the light spot on the information recording surface of the CD.

Preferably, G is ± 1. When G is ± 1, the fifth basic structure is preferably a two-level staircase structure (also referred to as a binary structure) as shown in FIG.

Further, the third-order diffracted light amount of the first light beam that has passed through the third basic structure is made larger than any other order of diffracted light amount, and the second-order diffracted light amount of the second light beam that has passed through the third basic structure is changed to other values. The diffraction light quantity of any order is made larger (also referred to as (3/2) structure), the second-order diffraction light quantity of the first light beam that has passed through the fourth basic structure is made larger than any other order of diffraction light quantity, The first-order diffracted light amount of the second light beam that has passed through the four basic structures may be made larger than any other order diffracted light amount (also referred to as a (2/1) structure). With such a configuration, the diffraction efficiency in BD can be further increased.

In addition, even when the 3rd foundation structure and the 4th foundation structure are the combination of the (1/1) structure and the (2/1) structure, it is the combination of the (3/2) structure and the (2/1) structure. Even in this case, the third basic structure provided at least in the middle region at the position closest to the central region has the step in the direction opposite to the optical axis, and at least in the middle region at the position closest to the central region. As for the 4th foundation structure provided, it is preferred that the level | step difference has faced the direction of an optical axis. More preferably, the steps of all the third foundation structures in the intermediate region are directed in the direction opposite to the optical axis, and the steps of all the fourth foundation structures in the intermediate region are directed in the direction of the optical axis. is there.

In the third basic structure, when the wavelength of the incident light beam changes so as to become longer, the spherical aberration changes in an undercorrected (under) direction, and in the fourth basic structure, the wavelength of the incident light beam becomes longer. If it changes, the spherical aberration may change in the direction of under-correction (under).

With such a configuration, even in the second optical path difference providing structure, when the refractive index of the objective lens changes due to the temperature increase of the optical pickup device, the wavelength of the light source also increases due to the increase in the environmental temperature. Is used to correct the deterioration of the spherical aberration due to the change in the refractive index of the objective lens, so that a more appropriate condensing spot can be formed on the information recording surface of each optical disc when the environmental temperature changes.

On the other hand, when one of the third basic structure and the fourth basic structure is changed so that the wavelength of the incident light beam becomes longer, the spherical aberration changes in the undercorrection (under) direction, and the incident light is incident on the other side. When the wavelength of the luminous flux to be changed is changed to be longer, the spherical aberration may be changed in the overcorrection (over) direction.

In one of the third basic structure and the fourth basic structure, when the wavelength of the incident light beam is changed so as to become longer, the spherical aberration changes in the undercorrection (under) direction. If the spherical aberration is changed in the overcorrection direction when the wavelength is changed to be longer, when the first light beam is condensed on the information recording surface of the BD as the entire objective lens, the first aberration is changed. The amount of change of the third-order spherical aberration when the wavelength of one light beam changes by +5 nm can be set to −30 mλrms to +50 mλrms, which is preferable. When the first light beam is condensed on the information recording surface of the BD as the entire objective lens, the amount of change in the third-order spherical aberration when the wavelength of the first light beam changes by +5 nm is -10 mλrms or more and +10 mλrms or less. More preferably. When the first light beam is condensed on the information recording surface of the BD as the entire objective lens, the amount of change in the fifth-order spherical aberration when the wavelength of the first light beam changes by +5 nm is -20 mλrms or more and 20 mλrms or less. It is preferable that More preferably, it is −10 mλrms or more and +10 mλrms or less.

With such a configuration, in either one of the third basic structure and the fourth basic structure, if the wavelength of the incident light beam is changed to be longer, the spherical aberration changes in the overcorrection direction. Even if the two-optical path difference providing structure is composed of only the third and fourth basic structures, flare can be easily produced when using the CD. Accordingly, flare out when using a CD can be performed with a simple second optical path difference providing structure, so that a decrease in light utilization efficiency due to a shadow effect is suppressed, and a decrease in light utilization efficiency due to manufacturing errors is also suppressed. As a result, the light utilization efficiency can be improved. This reduces the effect of correcting the temperature characteristics when using BD in the intermediate area, but the temperature characteristics are too poor because both the first basic structure and the second basic structure in the central area are insufficiently corrected at long wavelengths. This can be prevented, and the wavelength characteristic correction effect when using the BD can be increased. In addition, when the DVD is used, both the temperature characteristic and wavelength characteristic of the DVD can be improved.

When the wavelength of the incident light beam is changed to be longer in the fourth basic structure, the spherical aberration is changed in an undercorrected (under) direction, and the wavelength of the incident light beam is longer in the third basic structure. In this case, it is preferable that the spherical aberration changes in the overcorrected (over) direction because the flare can be easily moved farther when the CD is used.

Further, in order to improve the wavelength characteristics when using the DVD, in the second optical path difference providing structure, the ring zone of the third basic structure is 1 to 3 in one ring zone closest to the central region of the fourth basic structure. It is preferable that the number (particularly preferably 2 to 3) is included. More preferably, in the second optical path difference providing structure, 1 to 5 (particularly preferably 2 to 3) ring zones of the third foundation structure are provided for one ring zone closest to the dedicated area of the fourth foundation structure. It is included.

When providing the third optical path difference providing structure in the peripheral structure, it is possible to provide an arbitrary optical path difference providing structure. The third optical path difference providing structure preferably has a sixth basic structure. In the sixth basic structure, the P-order diffracted light amount of the first light beam that has passed through the sixth basic structure is made larger than any other order diffracted light amount, and the Q-order diffraction of the second light beam that has passed through the sixth basic structure. The light quantity is made larger than any other order of diffracted light quantity, and the R-order diffracted light quantity of the third light flux that has passed through the sixth basic structure is made larger than any other order of diffracted light quantity. Note that P is preferably 5 or less in order to suppress fluctuations in diffraction efficiency during wavelength fluctuations.

Here, FIG. 12 shows a schematic diagram of a preferable objective lens. It is the figure which showed the upper half from the optical axis among the cross sections of the objective lens containing optical axis OA. Note that FIG. 12 is a schematic diagram to the last, and is not a drawing showing an accurate length ratio or the like based on the embodiment.

12 has a central area CN, an intermediate area MD, and a dedicated area OT. A first optical path difference providing structure ODS1 is provided in the central area, a second optical path difference providing structure ODS2 is provided in the intermediate area, and a third optical path difference providing structure is provided in the dedicated area.

The first optical path difference providing structure ODS1 in FIG. 12 has a (2/1/1) blazed structure and a second basic structure BS2 whose level difference faces the optical axis, and (1/1/1). The blazed structure has a structure in which a first basic structure BS1 with a step facing away from the optical axis is superimposed. In FIG. 12, the second foundation structure BS2 has three annular zones, and four annular zones of the first foundation structure BS1 are included on the annular zone (circular shape) closest to the optical axis in the second foundation structure BS2. ing. In addition, two annular zones of the first foundation structure BS1 are included in one annular zone closest to the intermediate region in the second foundation structure BS2.

The second optical path difference providing structure ODS2 in FIG. 12 is a (2/1/1) blazed structure in which a step is directed toward the optical axis and a (1/1/1) fourth basic structure BS4. The blazed structure has a structure in which a third basic structure BS3 having a level difference opposite to the optical axis is superimposed. In FIG. 12, 4th foundation structure BS4 is a 3 ring zone, and 3 ring zones of 3rd foundation structure BS3 are contained on the ring zone nearest to the center area | region in 4th foundation structure BS4. Further, one ring zone of the third foundation structure BS3 is included in one ring zone closest to the dedicated area in the fourth foundation structure BS4.

The third optical path difference providing structure ODS3 in FIG. 12 is a (2/1/1) blaze structure, and is composed only of the sixth basic structure BS6 in which the step is directed toward the optical axis.

The numerical aperture on the image side of the objective lens necessary for reproducing / recording information on the first optical disc is NA1, and the numerical aperture on the image side of the objective lens necessary for reproducing / recording information on the second optical disc. Is NA2 (NA1> NA2), and the image-side numerical aperture of the objective lens necessary for reproducing / recording information on the third optical disk is NA3 (NA2> NA3). NA1 is preferably 0.8 or more and 0.95 or less, and more preferably 0.8 or more and 0.9 or less. In particular, NA1 is preferably 0.85. NA2 is preferably 0.55 or more and 0.7 or less. In particular, NA2 is preferably 0.60 or 0.65. NA3 is preferably 0.4 or more and 0.55 or less. In particular, NA3 is preferably 0.45 or 0.53.

Whether the objective lens is a BD-dedicated objective lens or a BD / DVD / CD compatible objective lens, it is preferable that the following conditional expression (31) is satisfied.
0.9 ≦ d / f ≦ 1.5 (31)
Here, d represents the thickness (mm) on the optical axis of the objective lens, and f represents the focal length of the objective lens in the first light flux. Note that f is preferably 1.0 mm or more and 1.8 mm or less.

In the case of an objective lens corresponding to an optical disk with a short wavelength and high NA such as BD, if the ratio of the thickness on the optical axis to the focal length of the objective lens becomes too large, an off-axis light beam enters the objective lens. In this case, astigmatism tends to occur, and a working distance cannot be secured. On the other hand, if the ratio of the thickness on the optical axis to the focal length of the objective lens becomes too small, there arises a problem that the surface shift sensitivity increases. By satisfying conditional expression (31), it is possible to suppress the generation of astigmatism and the surface shift sensitivity.

Also, the working distance of the objective lens when using the first optical disk is preferably 0.15 mm or more and 1.0 mm or less.

An optical information recording / reproducing apparatus according to the present invention includes an optical disc drive apparatus having the above-described optical pickup apparatus.

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.

The optical information recording / reproducing apparatus using each method described above 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, there is provided an optical pickup device that can form an optimum focused spot even when a light beam having a relatively large divergence angle is incident on an objective lens at the time of focus jump using an antireflection film. Can do.

(A) is a figure which shows the aspect which processes the metal mold | die which shape | molds the objective lens which has a certain type of optical path difference providing structure, (b) is sectional drawing of the objective lens shape | molded by this metal mold | die. is there. (A) is a figure which shows the aspect which processes the metal mold | die which shape | molds the objective lens which has another type of optical path difference providing structure, (b) is sectional drawing of the objective lens shape | molded by this metal mold | die. is there. It is a figure which compares and shows each spherical aberration based on the examination result of each case (a)-(d) which the inventor performed. It is a figure for demonstrating a sine condition. It is a figure which shows the example (a)-(c) of sine condition dissatisfied amount. It is the figure which looked at the single objective lens OBJ concerning this Embodiment in the optical axis direction. It is a figure which shows the state which forms the spot which the 3rd light beam which passed the objective lens forms on the information recording surface of a 3rd optical disk. It is an axial direction sectional view showing an example of an optical path difference grant structure, (a) and (b) show an example of a blaze type structure, and (c) and (d) show an example of a step type structure. (A) shows a state in which the step is directed in the direction of the optical axis, and (b) is a diagram showing a state in which the step is directed in a direction opposite to the optical axis. (A) shows a shape in which the step is in the direction of the optical axis in the vicinity of the optical axis, but changes in the middle, and in the vicinity of the intermediate region, the step is in the direction opposite to the optical axis. FIG. 4 is a diagram showing a shape in which a step is directed in the opposite direction to the optical axis in the vicinity of the axis, but is switched in the middle, and the step is directed toward the optical axis in the vicinity of the intermediate region. It is a conceptual diagram of a 1st optical path difference providing structure, (a), (b), (c) shows the preferable example of a 1st optical path difference providing structure, (d) is a 1st foundation structure and a 2nd foundation structure, It is a figure which shows the example which superimposed. It is a schematic diagram of a preferable objective lens. It is a figure which shows schematically the structure of optical pick-up apparatus PU1 only for BD. It is a figure which shows roughly the structure of optical pick-up apparatus PU2 of this Embodiment which can record and / or reproduce | regenerate information appropriately with respect to BD, DVD, and CD which are different optical disks. 5 is a graph showing the pupil radius on the vertical axis and the spherical aberration and the sine condition violation amount on the horizontal axis for Example 1. FIG. In Example 2, the vertical axis represents the pupil radius, and the horizontal axis represents the spherical aberration and the sine condition violation amount. In Example 3, the vertical axis represents the pupil radius, and the horizontal axis represents the spherical aberration and the sine condition violation amount. In Example 4, the vertical axis represents the pupil radius and the horizontal axis represents the spherical aberration and the sine condition violation amount. In Example 5, the vertical axis represents the pupil radius, and the horizontal axis represents the spherical aberration and the sine condition violation amount. It is a graph which shows the relationship between a pupil radius (an effective diameter shall be 100%) and the transmittance | permeability in the 1st design example of an antireflection film. It is a graph which shows the relationship between a pupil radius (an effective diameter shall be 100%) and the transmittance | permeability in the 2nd design example of an antireflection film.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 13 shows information recording appropriately on a BD that is an optical disc having three information recording surfaces RL1 to RL3 (referred to as RL1, RL2, and RL3 in ascending order of the 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. Such an optical pickup device PU1 can be mounted on an optical information recording / reproducing device. The present invention is not limited to the present embodiment. For example, FIG. 13 shows a BD-dedicated optical pickup device, but by using two BD-dedicated objective lenses OBJ and DVD / CD-compatible objective lenses, light for BD / DVD / CD compatibility is used. A pickup device can also be used. Further, in order to adapt to BDXL, it can be applied to an optical disc having four information recording surfaces in the thickness direction.

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, Coupling CL having a positive lens unit L2 composed of one positive lens having a refractive power and a negative lens unit L3 composed of one negative lens having a negative refractive power, only the positive lens unit L2 in the optical axis direction. A uniaxial actuator AC1 to be moved, 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. In this embodiment, the objective lens OBJ does not have to be provided with an optical path difference providing structure. However, for example, when an optical path difference providing structure is provided in order to correct a spherical aberration due to a temperature change, the objective lens OBJ has a blaze shape as shown in FIG. It is desirable to provide a type as shown in (b). Further, the antireflection film provided on the first surface (light source side) of the objective lens OBJ has 2 to 4 layers, and the antireflection film provided on the second surface (optical disk side) has 2 to 4 layers. The number of layers of the antireflection film is preferably 3 or 4 on both the first surface and the second surface, and most preferably 3 layers.

The objective lens OBJ is a single lens made of plastic or glass, and has an antireflection film formed on at least the first surface S1 on the light source side. The transmittance Tc in a circular central region inside 10% of the image-side numerical aperture NA of the first surface S1 and the transmittance in a ring-shaped peripheral region outside of 90% of the image-side numerical aperture NA of the first surface S1. Tp satisfies the equation (1).
1.0 <Tp / Tc <1.3 (1)

First, a case where recording / reproduction is performed on the first information recording surface RL1 of the BD will be described. In such a case, the positive lens group 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 group L3 of the collimator lens CL, and the divergence angle is increased. After passing through the lens unit L2 to be a weakly convergent light beam, it is converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the first thickness is obtained by the objective lens OBJ. Through the protective substrate PL1, the spot is formed on the first information recording surface RL1 as shown by the 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 diaphragm, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens group L2 and the negative lens group L3 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged 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 BD will be described. In such a case, the positive lens group 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 group L3 of the collimator lens CL, and the divergence angle is increased. After passing through the lens unit L2 to be a substantially parallel light beam, it is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the second thickness is obtained by the objective lens OBJ. This is a spot formed on the second information recording surface RL2 as shown by the alternate long and short dash line through the protective substrate PL2 having a thickness (thicker than the first thickness).

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, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens group L2 and the negative lens group L3 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged 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 BD will be described. In such a case, the positive lens group 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 group L3 of the collimator lens CL, and the divergence angle is increased. After passing through the lens unit L2 to be a weak divergent light beam, it is converted from linearly polarized light into circularly polarized light by the λ / 4 wave plate QWP, the diameter of the light beam is regulated by a diaphragm (not shown), and the third thickness is obtained by the objective lens OBJ. This is a spot formed on the third information recording surface RL3 as shown by the dotted line through the protective substrate PL3 (thicker than the second thickness).

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, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens group L2 and the negative lens group L3 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged 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.

In the above embodiment, in order to correct coma caused by warping or tilting of the optical disc when information is recorded and / or reproduced on the optical disc, the objective lens OBJ is attached by the triaxial actuator AC2. Tilt along the radial direction and / or tangential direction of the optical disc. As a result, it is possible to stably record and / or reproduce information on the warped optical disc, and to maintain a good spot quality on the information recording surface even when the optical disc is tilted during rotation.

(Second Embodiment)
FIG. 14 shows BD, DVD, and CD, which are optical discs 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. It is a figure which shows roughly the structure of optical pick-up apparatus PU2 of this Embodiment which can record / reproduce information appropriately with respect to this. Such an optical pickup device PU2 can be mounted on an optical information recording / reproducing device. The present invention is not limited to the present embodiment. Further, in order to adapt to BDXL, it can be applied to an optical disc having four information recording surfaces in the thickness direction.

The optical pickup device PU2 emits light when recording / reproducing information with respect to the objective lens OBJ, the λ / 4 wavelength plate QWP, the collimating lens COL, the polarization beam splitter BS, and the dichroic prisms DP and BD, and has a wavelength of λ1 = 405 nm. A first semiconductor laser LD1 (first light source) that emits a laser beam (first beam) and a laser beam (second beam) that is emitted when recording / reproducing information on a DVD and has a wavelength λ2 = 660 nm. The second semiconductor laser LD2 (second light source) that emits and the third semiconductor laser LD3 that emits a laser beam (third beam) having a wavelength λ3 = 785 nm emitted when information is recorded / reproduced with respect to the CD are integrated. A laser unit LDP, a sensor lens SEN, a light receiving element PD as a photodetector, and the like.

As shown in FIG. 6, in the single objective lens OBJ according to the present embodiment, a central region CN including the optical axis on the aspherical optical surface on the light source side, an intermediate region MD disposed around the central region CN, Further, a dedicated region OT disposed around the periphery is formed concentrically with the optical axis as the center. Although not shown, the first optical path difference providing structure already described in detail is formed in the center region CN, and the second optical path difference providing structure already described in detail is formed in the intermediate region MD. In addition, a third optical path difference providing structure is formed in the dedicated region OT. In the present embodiment, the third optical path difference providing structure is a blazed diffractive structure. The objective lens of the present embodiment is a plastic lens. As shown in FIG. 11, the first optical path difference providing structure formed in the central region CN of the objective lens OBJ is a structure in which the first basic structure and the second basic structure are overlapped. The first-order diffracted light amount of the first light beam that has passed through the first basic structure is made larger than any other order of diffracted light amount, and the first-order diffracted light amount of the second light beam that has passed through the first basic structure is set to any other order. The first order diffracted light amount of the third light flux that has passed through the first basic structure is made larger than any other order diffracted light amount, and is provided at least near the optical axis of the central region CN. In the basic structure, the step is directed in the direction opposite to the optical axis, and in the second basic structure, the second-order diffracted light quantity of the first light beam that has passed through the second basic structure is greater than the diffracted light quantity of any other order. Of the second light flux that has passed through the second basic structure. The following diffracted light larger than the other diffracted light of any order, and larger than the third light flux of the first-order diffracted light of other diffraction light amount of any order which has passed through the second basic structure. The antireflection film provided on the first surface (light source side) of the objective lens OBJ has 2 to 4 layers, and the antireflection film provided on the second surface (optical disk side) has 5 to 9 layers. The number of antireflection films on the first surface is preferably 3 or 4, and most preferably 3 layers. Further, the number of antireflection films on the second surface is preferably 7 layers, 8 layers, or 9 layers, and most preferably 7 layers.

First, a case where recording / reproduction is performed on the first information recording surface RL1 of the BD will be described. In such a case, the coupling lens COL is moved to the first predetermined position by a single-axis actuator (not shown). Here, the divergent light beam of the light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD1 passes through the dichroic prism DP and the polarization prism BS, passes through the coupling lens COL, and becomes a weak convergent light beam. The first information recording surface is 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 a diaphragm (not shown), and the objective lens OBJ passes through the transparent substrate PL1 having the first thickness. It becomes a spot formed on RL1.

The reflected light flux modulated by the information pits on the first information recording surface RL1 is again transmitted through the objective lens OBJ and the aperture, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and coupled to the coupling lens COL. Is reflected by the polarizing prism BS, and then converged 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 BD will be described. In such a case, the coupling lens COL is moved to the second predetermined position by a uniaxial actuator (not shown). Here, the divergent light beam of the light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD1 passes through the dichroic prism DP and the polarization prism BS, passes through the coupling lens COL, and is converted into a parallel light beam. / 4 wavelength plate QWP converts linearly polarized light into circularly polarized light, the diameter of the light beam is regulated by a diaphragm (not shown), and the second information recording surface RL2 is passed through the transparent substrate PL2 having the second thickness by the objective lens OBJ. It becomes a spot formed on the top.

The reflected light flux modulated by the information pits on the second information recording surface RL2 is again transmitted through the objective lens OBJ and the stop, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and coupled to the coupling lens COL. Is reflected by the polarizing prism BS, and then converged 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 BD will be described. In such a case, the coupling lens COL is moved to a third predetermined position by a uniaxial actuator (not shown). Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD1 passes through the dichroic prism DP and the polarization prism BS, passes through the coupling lens COL, and becomes a weak divergent beam. The linearly polarized light is converted into circularly polarized light by the λ / 4 wavelength plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the third information recording surface is passed through the transparent substrate PL3 having the third thickness by the objective lens OBJ. It becomes a spot formed on RL3.

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, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. Is reflected by the polarizing prism BS, and then converged 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.

Furthermore, the case where information is recorded and / or reproduced on a DVD will be described. The divergent light beam of the second light beam (λ2 = 660 nm) emitted from the semiconductor laser LD2 of the laser unit LDP is reflected by the dichroic prism DP and passes through the polarization beam splitter BS and the collimating lens COL, as indicated by the dotted line. The / 4 wavelength plate QWP converts the linearly polarized light into circularly polarized light and enters the objective lens OBJ. Here, the light beam condensed by the central region and the intermediate region of the objective lens OBJ (the light beam that has passed through the dedicated region is flared and forms a spot peripheral portion) is recorded on the DVD through the protective substrate PL4. It becomes a spot formed on the surface RL4 and forms the center of the spot.

The reflected light beam modulated by the information pits on the information recording surface RL4 is transmitted again through the objective lens OBJ, converted from circularly polarized light to linearly polarized light by the λ / 4 wave plate QWP, and converted into a parallel light beam by the collimator lens COL. The light is reflected by the polarization beam splitter BS and converges on the light receiving surface of the light receiving element PD via the sensor lens SEN. And the information recorded on DVD can be read using the output signal of light receiving element PD.

Furthermore, the case of recording and / or reproducing information on a CD will be described. The divergent light beam of the third light beam (λ3 = 785 nm) emitted from the semiconductor laser LD3 of the laser unit LDP is reflected by the dichroic prism DP, as shown by the one-dot chain line, and passes through the polarization beam splitter BS and the collimating lens COL. The linearly polarized light is converted into circularly polarized light by the λ / 4 wavelength plate QWP, and is incident on the objective lens OBJ. Here, the light beam collected by the central region of the objective lens OBJ (the light beam that has passed through the intermediate region and the dedicated region is flared and forms a spot peripheral portion) is recorded on the CD information record through the protective substrate PL5. This is a spot formed on the surface RL5.

The reflected light flux modulated by the information pits on the information recording surface RL5 is again transmitted through the objective lens OBJ, converted from circularly polarized light to linearly polarized light by the λ / 4 wave plate QWP, and converted into a parallel light flux by the collimator lens COL. The light is reflected by the polarization beam splitter BS and converges on the light receiving surface of the light receiving element PD via the sensor lens SEN. And the information recorded on CD can be read using the output signal of light receiving element PD.

Next, examples of objective lenses that can be used in the above-described embodiment will be described below. The design wavelength of the objective lens is BD = 405 nm, DVD = 660 nm, CD = 785 nm, r in the table below is the radius of curvature (mm), d is the distance between surfaces (mm), and Nλ1 is the refractive index of each surface at the wavelength λ1. , Nλ2 represents the refractive index of each surface at wavelength λ2, Nλ3 represents the refractive index of each surface at wavelength λ3, and t represents the transparent substrate thickness (mm). In the following (including the lens data in the table), a power of 10 (for example, 2.5 × 10 ⁻³ ) is expressed by using E (for example, 2.5 × E-3). 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, X (h) is an axis in the optical axis direction (the light traveling direction is positive), κ is a conical coefficient, A _i is an aspheric coefficient, h is a height from the optical axis, and r is a paraxial curvature. Radius.

In the case of an embodiment having a diffractive structure, the optical path difference given to the light flux of each wavelength by the diffractive structure is defined by an equation obtained by substituting the coefficient shown in the table into the optical path difference function of Formula 2.

Here, h is the height from the optical axis, λ is the wavelength of the incident light beam, m is the diffraction order, and B _2i is the coefficient of the optical path difference function.

Example 1
Example 1 is a BD-dedicated resin objective lens composed only of a refractive surface. Table 1 shows lens data of Example 1. This example is a plastic objective lens corresponding to a multilayer BD (T _MAX = 0.1 mm, T _MIN = 0.05 mm) having three or more layers, and has a design substrate thickness T = 0.08 mm. FIG. 15 shows a graph of spherical aberration and sine condition violation amount of Example 1.

(Example 2)
Example 2 is a BD-dedicated resinous objective lens composed only of a refractive surface. Table 2 shows lens data of Example 2. This example is a plastic objective lens corresponding to a multilayer BD having three or more layers (T _MAX = 0.1 mm, T _MIN = 0.05 mm), and has a design substrate thickness T = 0.875 mm. FIG. 16 shows a graph of spherical aberration and sine condition violation amount of Example 2.

(Example 3)
Example 3 is a BD-dedicated resin objective lens having an optical path difference providing structure. Such an optical path difference providing structure is a type that is folded back halfway as shown in FIG. Table 3 shows lens data of Example 3. This example is a plastic objective lens compatible with a multilayer BD of three or more layers (T _MAX = 0.1 mm, T _MIN = 0.05 mm), and has a design substrate thickness T = 0.08 mm. FIG. 17 shows a graph of spherical aberration and sine condition violation amount of Example 3.

(Example 4)
Example 4 is a BD-dedicated resin objective lens having an optical path difference providing structure. Such an optical path difference providing structure is a type in which the step surface is always directed to the optical axis, as shown in FIG. Table 4 shows lens data of Example 4. This example is a plastic objective lens corresponding to a multilayer BD having three or more layers (T _MAX = 0.1 mm, T _MIN = 0.05 mm), and has a design substrate thickness T = 0.075 mm. FIG. 18 shows a graph of spherical aberration and sine condition violation amount of Example 4.

(Example 5)
Example 5 is a resin objective lens having a diffraction structure compatible with BD / DVD / CD. Table 5 shows lens data of Example 5. The present embodiment is a plastic objective lens compatible with a multilayer BD (T _MAX = 0.1 mm, T _MIN = 0.05 mm), DVD, and CD having three or more layers, and has a design substrate thickness T = 0.875 mm. did.

In Example 5, the objective lens is divided into three regions (a central region, an intermediate region, and a dedicated region) centered on the optical axis, and the first optical path difference providing structure is (2, 1, The optical path difference providing structure is formed by superimposing the first basic structure, which is the blazed diffraction structure (1, 1, 1), on the second basic structure, which is the blazed diffraction structure 1). In addition, the second optical path difference providing structure has a (1, 1, 1) blazed diffraction pattern in a fourth basic structure that is a (2, 1, 1) blazed diffraction structure in the entire intermediate region. The third basic structure, which is a structure, is overlapped, and an optical path difference providing structure in which a fifth basic structure, which is a binary diffraction structure of (0, 0, 1), is further overlapped. Furthermore, the third optical path difference providing structure is a (2, 1, 1) blazed diffractive structure in the entire exclusive region.

In the above embodiment, a design example of an antireflection film that can be applied to the optical surface on the light source side will be described. FIG. 20 is a graph showing the relationship between the pupil radius (effective diameter is 100%) and the transmittance in the first design example of the antireflection film. In the first design example, the transmittance gradually increases as the pupil radius increases from 0, the transmittance reaches the maximum value (97%) at the pupil radius of 65%, and then gradually decreases. That is, the transmittance continuously changes within the effective diameter according to the height from the optical axis.

FIG. 21 is a graph showing the relationship between the pupil radius (effective diameter is 100%) and the transmittance in the second design example of the antireflection film. In the first design example, the transmittance gradually increases as the pupil radius increases from 0, the transmittance reaches the maximum value (92%) at the pupil radius of 75%, and then gradually decreases. That is, the transmittance continuously changes within the effective diameter according to the height from the optical axis.

The design examples 1 and 2 of the antireflection film are both antireflection films formed on the objective lenses of Examples 1 to 5 described above. In each of Examples 1 to 5, a total of 10 examples can be considered: an example in which the antireflection film of design example 1 is provided, and an example in which the antireflection film of back diameter example 2 is provided. In both design examples 1 and 2, the first surface and the second surface have a three-layer structure. For design examples 1 and 2, the values shown in the claims are summarized in Table 6, and the specific numerical configuration of the antireflection film applied to each surface is shown in Table 7.
In Table 7, the first layer is a layer in contact with the lens surface, the second layer is a layer applied over the first layer, and the third layer is a layer applied over the second layer. As shown in Table 7, the antireflection film having a three-layer structure preferably has a structure in which a high refractive index second layer is sandwiched between a low refractive index first layer and a third layer. The high refractive index material is preferably an oxide of zirconium, and the low refractive index material is preferably an oxide of silicon, but is not limited thereto.

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 lenses PU1, PU2 Optical pickup device LD, LD1 Blue-violet semiconductor laser LD2 Red semiconductor laser LD3 Infrared semiconductor laser LDP Laser unit AC1 Single-axis actuator AC2 Three-axis actuator BS Polarizing beam splitter PBS Polarizing prism CL Coupling lens COL Collimating lens DP Dichroic prism L2 Positive lens group L3 Negative lens group QWP λ / 4 wavelength plates PL1 to PL5 Protection substrates RL1 to RL5 Information recording surface SEN Sensor lens SL Sensor lens

Claims

A light source that emits a light beam having a wavelength of λ1 (390 nm <λ1 <415 nm), a coupling lens, and an objective lens are provided, and the distance from the light beam incident surface (by shifting the coupling lens in the optical axis direction ( One of the information recording surfaces in the optical disc having three or more information recording surfaces in the thickness direction is selected from the transparent substrate thickness), and the light beam having the wavelength λ1 emitted from the light source is selected by the objective lens. An optical pickup device that records and / or reproduces information by focusing on an information recording surface,
The objective lens is a single lens having an image-side numerical aperture (NA) of 0.8 or more and less than 0.95, and an antireflection film is formed at least on the first surface on the light source side,
In the objective lens on which the antireflection film is formed, the transmittance Tc in a central region inside 10% of the image-side numerical aperture NA and the transmittance in a peripheral region outside 90% of the image-side numerical aperture NA. An optical pickup device, wherein Tp satisfies the expression (1).
1.0 <Tp / Tc <1.3 (1)
The antireflection film provided on the first surface is characterized in that the wavelength λ0 (nm) at which the reflectance with respect to normal incident light is minimum and the wavelength λ1 (nm) satisfy the expression (2). Item 4. The optical pickup device according to Item 1.
1.30 <λ0 / λ1 <1.90 (2)
The optical pickup device according to claim 1 or 2, wherein the transmittance of the antireflection film continuously changes within an effective diameter in accordance with a height from the optical axis.
4. The optical pickup device according to claim 1, wherein the objective lens is made of resin.
5. The light according to claim 4, wherein an optical path difference providing structure for correcting spherical aberration generated when a temperature change occurs in the objective lens is formed on the optical surface of the objective lens. Pickup device.
The optical path difference providing structure is formed on an optical surface on the light source side, and is formed by alternately connecting step surfaces extending along the optical axis and ring-shaped surfaces, and in the optical path difference providing structure, In the vicinity of the optical axis, a negative step structure is provided with the step surface facing away from the optical axis, and the step surface faces the optical axis side at a position farther from the optical axis than the negative step structure. A positive step structure is provided, the step surface in the negative step structure is parallel to the optical axis, and the step surface in the positive step structure is non-parallel to the optical axis. 6. The optical pickup device according to claim 5, wherein
7. The coupling lens according to claim 4, wherein the coupling lens is displaced in an optical axis direction in order to correct spherical aberration that occurs when a temperature change occurs in the objective lens. An optical pickup device according to claim 1.
8. The objective lens material according to claim 1, wherein a refractive index with respect to the wavelength λ1 is smaller than 1.58, and the antireflection film provided on the first surface has two to four layers. The optical pick-up apparatus in any one.