JP2011198446A - Optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pickup device which is capable of reducing the movement amount of a coupling lens suitable as a slim type thanks to small size and low cost, and capable of recording and reproducing information on/from an optical disk having multilayered information recording surfaces.SOLUTION: Focus jump for selecting one of the plurality of information recording surfaces is performed by moving a group of positive lenses L2 of the coupling lens CL in an optical axis direction. A spherical aberration which increases with a change of environmental temperature or a wavelength variation of a light source is corrected by driving a liquid crystal element LCD.

Description

  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 commercialization of BDs having the same. 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. In other information recording surfaces having different transparent 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 “focus jump” in this specification.

Japanese Patent No. 4144663

  However, in order to record / reproduce information on, for example, an optical disc having three or more layers of information recording surfaces by the optical pickup device described in Patent Document 1, one of the information recording surfaces is selected. In this case, a long moving distance of the coupling lens is required. When the moving distance of the coupling lens is increased, the optical path length from the light source to the objective lens is increased, and there is a problem that the optical pickup device cannot be reduced in size, for example. In addition, there is a problem that a large actuator for driving the coupling lens is required and the cost is increased.

  In particular, in an optical pickup device capable of recording / reproducing information with respect to a BD having an information recording surface of three layers or more, a relatively thick type called a so-called half height, which is conventionally mounted on a stationary recorder or the like, has a cup. While a relatively large space for moving the ring lens can be secured, a relatively thin optical pickup device called a so-called slim type mounted on the back of a notebook PC or thin TV has a space for moving the coupling lens. There is a problem that it cannot be secured sufficiently.

  Further, in general, in an optical pickup device, when information is recorded / reproduced with respect to an optical disc, the objective lens is tilted along the radial direction and / or the tangential direction of the optical disc (in this specification, The coma generated by the lens tilt) can cancel the coma generated by the warp or tilt of the optical disc (referred to as disc tilt in this specification). Therefore, if the amount of coma generated when the lens is tilted is small, the amount of lens tilt required to correct the coma due to disc tilt increases, so it is necessary to ensure a sufficiently large dynamic range of the lens tilt. As a result, problems such as an increase in the size of the optical pickup device and an increase in power consumption of the actuator occur. However, in the optical pickup device for BD, when recording / reproducing information on the information recording surface L0 (100 μm) having the thicker transparent substrate, the coupling lens is moved in the optical axis direction. As a result, the divergent light beam enters the objective lens, so that the coma aberration amount when the lens is tilted is smaller than when the parallel light beam is incident. As described above, in the optical pickup device corresponding to the BD having three or more layers, there is a possibility that the coma aberration amount when the lens is tilted becomes small, and the coma aberration due to the disc tilt cannot be corrected satisfactorily. There is. In addition, when an objective lens made of a plastic material is used to achieve a high NA, spherical aberration is generated in a beam spot due to a temperature change (referred to as temperature aberration in this specification). For example, a plastic having a focal length of 1.41 mm The amount of change in spherical aberration due to a 30 ° C. change in the objective lens made of material is about 100 mλrms, which exceeds the Marshall limit value of 70 mλrms. Since the conventional DVD has an NA of about 0.60 to 0.67, the amount of spherical aberration generated due to temperature change is relatively small and it is not necessary to correct this spherical aberration. This is because the aberration is proportional to the fourth power of NA, and the amount of spherical aberration generated due to a temperature change increases. In addition, when a semiconductor laser is used as a light source, the oscillation wavelength may vary due to mode hopping or the like, but the spherical aberration may increase due to such wavelength variation. Therefore, in a BD optical pickup device equipped with a plastic objective lens, it is necessary to correct temperature aberration and wavelength aberration by moving the coupling lens in the optical axis direction. Therefore, when the environmental temperature becomes high or the oscillation wavelength fluctuates while recording / reproducing information with respect to the information recording surface L0 using the plastic objective lens, Since the degree of divergence of incident light to the objective lens is further increased, the amount of coma aberration when the lens is tilted is further reduced, and the problem that coma aberration due to the disk tilt cannot be corrected well becomes greater.

  The present invention has been made in consideration of the above-mentioned problems, and can reduce the amount of movement of the coupling lens, is compact and low-cost, and is suitable as a slim type optical pickup device, and has a multilayer information recording surface. An object of the present invention is to provide an optical pickup device capable of recording / reproducing information on / from an optical disc.

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, a liquid crystal element, and an objective lens, and a distance from a light incident surface. Select one of the information recording surfaces in the optical disc having three or more information recording surfaces (thicknesses of transparent substrates) in the thickness direction, and select the light flux of wavelength λ1 emitted from the light source by the objective lens An optical pickup device that records and / or reproduces information by focusing on the recorded information recording surface,
The coupling lens has at least one moving lens that is movable in the optical axis direction, and moves the moving lens in the optical axis direction between the light source and the objective lens, so that Select one of the information recording surfaces,
The liquid crystal element is driven to correct at least part of spherical aberration on an information recording surface for recording and / or reproducing information and / or part of spherical aberration generated in the objective lens,
The objective lens is a single lens made of plastic,
The image side numerical aperture (NA) of the objective lens is 0.8 or more and 0.95 or less,
When the maximum transparent substrate thickness among the transparent substrate thicknesses is T _MAX (mm), the spherical aberration is minimum at room temperature (25 ± 3 ° C.) and the cover glass thickness T (mm) satisfying the expression (1). An optical pickup device characterized in that the magnification M at the time satisfies the expression (2).
T _MAX · 0.70 ≤ T ≤ T _MAX · 1.1 (1)
−0.003 ≦ M ≦ 0.003 (2)

  Thin optical pickup devices, so-called slim type optical pickup devices, have relatively severe restrictions on the moving space of the coupling lens compared to thick optical pickup devices, so-called half-height types. However, since a fixed element such as a polarizing beam splitter or a half mirror is generally arranged between the coupling lens and the light source, it is difficult to move the coupling lens greatly to the light source side. However, there is a relatively large space on the objective lens side. Therefore, as a result of earnest research, the present inventor has set the cover glass thickness T so as to satisfy the expression (1), so that the amount of movement of the coupling lens toward the light source side with respect to the origin is set to the objective lens side with respect to the origin. It has been found that the amount of movement of the coupling lens toward can be made smaller. As a result, while avoiding interference between the coupling lens and the fixed element, the total movement amount of the coupling lens is secured, and information recording is performed by selecting one of information recording surfaces of three or more layers. / It was made possible to play.

  Further, when a plastic objective lens is used, it is necessary to correct spherical aberration that increases due to the change in refractive index according to environmental temperature changes and spherical aberration that increases according to wavelength fluctuations of the light source. In addition to spherical aberration caused by selecting one of the information recording surfaces, in order to correct such spherical aberration, a relatively long amount of movement of the coupling lens is required, which increases the size of the optical pickup device. Invite. On the other hand, according to the present invention, by using a liquid crystal element, at least a part of spherical aberration on the information recording surface for recording and / or reproducing information and / or spherical aberration generated in the objective lens. Since at least a part can be corrected, the amount of movement of the coupling lens can be suppressed as a result, which is suitable for a slim type optical pickup device, for example.

  In this specification, the “transparent substrate thickness” is the distance from the light beam incident surface of the optical disc to the information recording surface. In an optical disc having a plurality of information recording surfaces in the thickness direction, each information recording surface is transparent. The substrate thickness will be different from each other. In general, an objective lens for an optical pickup is combined with a cover glass having a predetermined thickness, and the correction state of the spherical aberration is determined so that the spherical aberration is minimized (the thickness of the cover glass is determined as the design cover glass). Also called thickness). The design cover glass thickness may be the same as or different from the transparent substrate thickness of any information recording surface of the optical disc. When the thickness of the cover glass changes, the characteristics of the objective lens also change. Therefore, when discussing the characteristics of the objective lens for the optical pickup, it is necessary to consider the cover glass thickness as a set. Therefore, in this specification, when describing the characteristics of the objective lens, the term “cover glass” is used to distinguish it from the “transparent substrate” of the optical disk. (Note that although the term “cover glass” is used, the cover glass thickness is not limited to glass, but a resin may be added.)

  According to a second aspect of the present invention, there is provided the optical pickup device according to the first aspect, wherein the liquid crystal element is driven to correct spherical aberration generated in the objective lens due to a temperature change. Thereby, the spherical aberration which increases when the refractive index changes according to the environmental temperature change can be corrected by the liquid crystal element.

  According to a third aspect of the present invention, in the optical pickup device according to the first or second aspect, the liquid crystal element is driven to correct a spherical aberration generated in the objective lens due to a wavelength change. To do. Thereby, the spherical aberration which increases according to the wavelength variation of the light source can be corrected by the liquid crystal element.

  According to a fourth aspect of the present invention, there is provided the optical pickup device according to the second or third aspect of the invention, wherein the coupling lens is arranged so as to change the information recording surface for recording and / or reproducing information from a light beam incident surface. The moving lens is moved in the direction of the optical axis in order to correct spherical aberration caused by a change in the distance (including the substrate thickness of the optical disk) to the information recording surface.

An optical pickup device according to a fifth aspect of the present invention is the optical pickup device according to any one of the first to fourth aspects, wherein the maximum transparent substrate thickness among the transparent substrate thicknesses is T _MAX (mm). 3) and a cover glass thickness T (mm) satisfying the expression (1A), the magnification M at which the spherical aberration is minimized satisfies the expression (2).
T _MAX・ 0.80 ≦ T ≦ T _MAX・ 1.1 (1A)
−0.003 ≦ M ≦ 0.003 (2)

  In addition, as described above, the pickup device for BD corresponding to three or more layers has a too small tilt sensitivity of the objective lens when recording / reproducing information on the information recording surface having the thicker transparent substrate. It is necessary not to. In particular, when a plastic objective lens is used, the lens tilt sensitivity when the environmental temperature becomes high during recording / reproduction of information on the information recording surface with the thicker transparent substrate is high. It is necessary not to become too small. By satisfying the expression (1 ′), it is possible to prevent the tilt sensitivity of the objective lens from becoming too small when recording / reproducing information on the information recording surface having the thicker transparent substrate.

An optical pickup device according to a sixth aspect is characterized in that, in the invention according to any one of the first to fifth aspects, a focal length f (mm) of the objective lens at the wavelength λ1 satisfies the following expression. To do.
1.0 ≦ f ≦ 1.45 (3)

  An optical pickup device according to a seventh aspect is the invention according to any one of the first to sixth aspects, wherein the sine condition violation amount is a positive maximum between 70% and 90% of the effective radius at the magnification M. The sine condition violation amount does not have a negative maximum value.

  With such a configuration, the residual higher-order spherical aberration at the time of focus jump can be made smaller, the amount of movement of the coupling lens at the time of focus jump can be made smaller, and information on the thicker transparent substrate thickness The lens tilt sensitivity can be reduced even during the recording / reproduction of information on the recording surface, and the lens tilt sensitivity can be reduced even if the objective lens is made of plastic and the ambient temperature is high. It is possible to further suppress the reduction of the above.

  An optical pickup device according to an eighth aspect is the invention according to any one of the first to sixth aspects, wherein the sine condition violation amount is a positive maximum between 70% to 90% of the effective radius at the magnification M. Further, the sine condition violation amount has a negative maximum value.

  With such a configuration, the residual higher-order spherical aberration at the time of focus jump can be reduced, the amount of movement of the coupling lens at the time of focus jump can be reduced, and the information recording surface with the thicker transparent substrate The lens tilt sensitivity can be reduced even during recording / reproduction of information, and even when the objective lens is made of plastic and the ambient temperature becomes high, the lens tilt sensitivity is reduced. In addition, the amount of aberration that occurs when two opposing optical surfaces shift in the direction perpendicular to the optical axis due to manufacturing errors can be suppressed, and a lens on the optical axis can be suppressed. Since it is possible to suppress the amount of aberration generated when the thickness is shifted in the optical axis direction due to manufacturing errors, it is possible to provide an objective lens that is easier to manufacture.

  The optical pickup device according to claim 9 is the invention according to any one of claims 1 to 8, wherein the coupling lens is a single lens, and the single lens is the moving lens. To do.

  An optical pickup device according to a tenth aspect is the invention according to any one of the first to eighth aspects, wherein the coupling lens has a two-group configuration of a positive lens group and a negative lens group, and the positive lens At least one lens in the group is the moving lens.

  According to the present invention, it is possible to further reduce the amount of movement of the moving lens, and it is possible to provide a more compact optical pickup device.

  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 transparent substrate having a thickness t1 and an information recording surface. The second optical disc has a transparent substrate having a thickness t2 (t1 <t2) and an information recording surface. The third optical disc has a transparent substrate having a thickness of 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 disk 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.

When the minimum transparent substrate thickness among the transparent substrate thicknesses is T _MIN and the maximum transparent substrate thickness among the transparent substrate thicknesses is T _MAX , it is preferable to satisfy the equation (4).
0.03 (mm) <T _MAX -T _MIN <0.06 (mm) (4)

  In an optical disk having an information recording surface of three or more layers that satisfies the formula (4), as described above, the amount of movement of the coupling lens when performing a focus jump becomes longer, and the coupling lens Although the problem regarding the movement amount becomes large, the present invention solves such a large problem.

  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 information recording / reproduction 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 transparent substrate is 0.05 to 0.00. A general term for BD series optical discs of about 125 mm, including a BD having only a single information recording surface, a BD having three or more information recording surfaces, etc. The optical pickup device of the present invention has at least three layers. It is possible to deal with a BD having the above information recording surface. 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 transparent 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 transparent substrate has a thickness of 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 transparent substrate, it is preferable to satisfy the following conditional expressions (5), (6), and (7), but is not limited thereto.

0.050 mm ≤ t1 ≤ 0.125 mm (5)
0.5mm ≤ t2 ≤ 0.7mm (6)
1.0 mm ≤ t3 ≤ 1.3 mm (7)

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 (8) and (9).
1.5 · λ1 <λ2 <1.7 · λ1 (8)
1.8 · λ1 <λ3 <2.0 · λ1 (9)

  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.

  Further, 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.

  A liquid crystal element includes a liquid crystal layer that causes a phase change with respect to a light beam that is transmitted by application of a voltage, opposing electrode layers for applying a voltage to the liquid crystal layer, and a power source that supplies a voltage to the electrode layer. In this configuration, at least one of the electrode layers facing each other is divided into a predetermined pattern. By applying a voltage to this electrode layer, the alignment state of the liquid crystal layer changes, and the transmitted light flux On the other hand, a predetermined phase is added, and thereby spherical aberration can be corrected.

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

  In general, 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 amount 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 (10).
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 ) (10)

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 (11).
M = −f _O / f _C (11)

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 (11), as the coupling lens for the optical pickup device for BD, the focal length range of the entire system needs to be a predetermined range, and the movement of the coupling lens necessary 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 (10)).

  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, the optimum example of the coupling lens in the optical pickup device is composed of a combination of one positive lens and one negative lens, and the negative lens and the positive lens from the light source side. It is preferable to arrange in this order. 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.

  FIG. 1 is a diagram showing the results of studies 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 a first optical disc (BD) having a surface, when the optimum spherical aberration difference AS generated when each optimum focusing spot is formed on the information recording surface that is farthest away, and when the environmental temperature changes by ± 30 ° C. The maximum spherical aberration BS that occurs and the maximum spherical aberration CS that occurs when the wavelength of the light source changes 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. 1A and 1B, 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. 1C, 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 refracting 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. Furthermore, as shown in FIG. 1D, when an optical disk having four information recording surfaces is used in an objective lens having a diffractive optical surface, spherical aberration that occurs as a result of the temperature change is caused by 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 BS (= 140 mλ) due to a change in environmental temperature is almost zero, so that the amount of movement of the coupling lens is smaller (FIG. 1 ( c), the spherical aberration is equivalent to the correction amount of 540 mλ. Further, when 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 CS 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. 1C). 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, it can be said that it is desirable that the objective lens is made of plastic in terms of cost. As is apparent from FIG. 1, the amount of movement of the coupling lens when using the optical disk having four information recording surfaces is still 2 as compared to the amount of movement of the coupling lens when using the optical disk having two information recording surfaces. Since it is about twice, it is preferable to further devise in order to suppress the amount of movement of the coupling lens, particularly when a plastic objective lens is used. 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. Therefore, in the present invention, it is possible to further reduce the amount of movement of the coupling lens 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. 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. 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. The optical surface on the light source side of the objective lens may be referred to as the optical surface on the object side, and the optical surface on the optical disk side may be referred to as the optical surface on the image side. In the objective lens, the absolute value of the radius of curvature of the optical surface on the light source side is preferably smaller than the absolute value of the radius of curvature of the optical surface on the image side.

As the plastic forming the objective lens, an alicyclic hydrocarbon polymer material such as a cyclic olefin resin material is preferably used. Further, 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.

(In the formula, 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, or 1 carbon atom. ˜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.)

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

  Next, a second preferred example is obtained by addition polymerization of an α-olefin having 2 to 20 carbon atoms and a monomer composition comprising 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 a preferable 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-t-butylphenyl) butane, 1,3,5-trimethyl-2,4,6-tris (3 5-di-t-butyl-4-hydroxybenzyl) benzene, tetrakis (methylene-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenylpropionate)) methane [ie pentaerythritol Methyl-tetrakis (3- (3,5-di-tert-butyl-4-hydroxyphenylpropionate))], triethylene glycol bis (3- (3-tert-butyl-4-hydroxy-5-methylphenyl) Alkyl-substituted phenolic compounds such as propionate); 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-to Triazine group-containing phenol compounds such as azine; 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 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-t-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-pipe Jill) 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) phos Aito) 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 blending amount of each of these stabilizers is appropriately selected within a range that does not 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, Preferably it is 0.01-1 mass part.

(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 surfactants 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. It is preferable to add 0.01-10 mass parts with respect to it. The addition amount of the surfactant is more preferably 0.05 to 5 parts by mass, and 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, tri-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, butylbenzyl phthalate, dicyclophthalate Xylyl, di-2-ethylhexyl sebacate, tri-2-ethylhexyl trimellitic acid, Santizer 278, Paraplex G40, Drapex 334F, Plastolein 9720, Mesamoll, 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, and specifically, ZEONEX manufactured by Nippon Zeon, APEL manufactured by Mitsui Chemicals, TOPAS manufactured by TOPAS ADVANCED POLYMERS, ARTON manufactured by JSR, and the like are preferable. Take as an example.

  Moreover, it is preferable that the Abbe number of the material which comprises an objective lens is 50 or more.

When the objective lens is a plastic single lens having an image-side numerical aperture (NA) of 0.8 or more and 0.95 or less, it is the largest transparent substrate thickness (at the deepest position) among the transparent substrate thicknesses of the optical disk. When the distance between the information recording surface and the surface of the optical disk is T _MAX (mm), the surface is spherical at room temperature (25 ± 3 ° C.) and at the cover glass thickness T (mm) satisfying the following expression (1). It is preferable that the magnification M when the aberration is minimized satisfies the expression (2).
T _MAX × 0.70 ≦ T ≦ T _MAX × 1.1 (1)
−0.003 ≦ M ≦ 0.003 (2)

  Also, regarding the coma generated when the lens is tilted, the target value to be satisfied by the objective lens for BD of three or more layers is examined, and the information recording surface (that is, the transparent substrate thickness is the farthest from the light incident surface). Third-order coma aberration CM (DT) and CM (LT) generated when the optical disc is tilted when the environmental temperature becomes high during recording / reproduction of information on the thickest information recording surface) The ratio was set to about 0.36. As described above, the value of this ratio is the same as the information recording surface L0 (100 μm) on the thicker transparent substrate in the optical pickup device equipped with an objective lens that records / reproduces information with respect to the two-layer BD. ), When the environmental temperature becomes high during the recording / reproduction of information, the third-order coma aberration CM (DT) when the objective lens is tilted, and the CM (LT) when the optical disk is tilted Is equal to the ratio.

  As a result of studying an objective lens suitable for three or more layers of BDs using these values as target values, the present inventors have found that spherical aberration does not occur at a normal temperature (25 ± 3 ° C.) and a magnification satisfying the expression (2). It has been found that the target value of CM (LT) is satisfied by setting the correction state of the spherical aberration so that the cover glass thickness T when it becomes the minimum is equal to or greater than the lower limit of the equation (1). The CM (LT) can be increased as the cover glass thickness T increases. However, when the cover glass thickness T exceeds the upper limit of the formula (1), information is recorded on the information recording surface with the thinnest transparent substrate. / Remaining higher order spherical aberration when the focus shifts to the information recording surface with the thinnest transparent substrate thickness due to excessive convergence of the light beam incident on the objective lens during reproduction or poor lens shift characteristics This is not preferable because of the problem of increasing the size. Further, by satisfying the expression (1), it is possible to reduce the amount of movement of the coupling lens toward the light source with respect to the origin relative to the amount of movement of the coupling lens toward the objective lens with respect to the origin. Therefore, it is particularly suitable for an optical pickup device such as a slim type.

More preferably, the following conditional expression (1 ′) is satisfied.
T _MAX × 0.85 ≦ T ≦ T _MAX × 1.1 (1 ′)

  When the cover glass thickness T satisfies the upper limit of the expression (1 ′), 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. 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.

More preferably, the following conditional expression (1 ″) is satisfied. At this time, it is particularly preferable that M = 0.
T _MAX × 0.85 ≦ T ≦ T _MAX × 1.0 (1 ″)

The position where the parallel light beam is emitted from the coupling lens to the objective lens side is defined as the origin of the moving lens. At this time, by satisfying the conditional expression (1), the maximum moving distance of the moving lens from the origin toward the light source can be made smaller than the maximum moving distance of the coupling lens from the origin toward the objective lens. With this, in a slim type optical pickup device having a limited space, it is possible to select one of information recording surfaces of three or more layers while avoiding interference between the coupling lens and the element fixed on the light source side. Information can be recorded / reproduced. When the light source side from the origin is (−) and the optical disk side from the origin is (+), the position of the moving lens when the information recording surface farthest from the light beam incident surface of the optical disk is selected and the light beam is condensed is A (<0), the position of the moving lens when focusing the light beam by selecting the information recording surface closest to the incident surface of the optical disc is B (> 0), and further satisfies the following equation: Is preferred.
−10 ≦ B / A ≦ −1.5 (12)

Alternatively, by satisfying conditional expression (1), it is possible to move the moving lens only from the origin to the objective lens side without moving the moving lens from the origin to the light source side. This enables slim type optical pickup devices with limited space to select one of three or more information recording surfaces while further avoiding interference between the coupling lens and the element fixed on the light source side. Thus, information can be recorded / reproduced. When the light source side from the origin is (−) and the optical disk side from the origin is (+), the position of the moving lens when the information recording surface farthest from the light beam incident surface of the optical disk is selected and the light beam is condensed is A (> 0), the position of the moving lens when focusing the light beam by selecting the information recording surface closest to the incident surface of the optical disc is B (> 0), and further satisfies the following equation: Is preferred.
5 ≦ B / A ≦ 15 (13)

Next, preferable conditions for the sine condition of the objective lens will be described. As shown in FIG. 2, the sine condition is h when a light beam having a height h ₁ from the optical axis is incident on the lens parallel to 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. 3 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 matches 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. 3A is an example in which the amount of violation of the sine condition 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. 3B and 3C has a sine condition violation amount on the positive side between 70% and 90% of the effective radius of the objective lens at the magnification M described above. At least one local maximum value OSCmax (preferably only one). According to the objective lens as shown in FIGS. 3B and 3C, the sine condition violation amount has a positive maximum value OSCmax between 70% and 90% of the effective radius of the objective lens. Since the higher-order spherical aberration that occurs with the movement of the ring lens is reduced and the change in spherical aberration due to the change in magnification is large, the coupling lens is moved to select the information recording surface in an optical disc with three or more layers. In this case, the necessary movement amount can be reduced.

  In the example of FIG. 3B, the sine condition violation amount has one negative maximum value on the optical axis side than the positive maximum value. Further, in the example of FIG. 3C, 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. 3B and the example of FIG. 3C, the sine condition violation amount monotonously decreases in the peripheral portion from the maximum value.

  In the magnification M as shown in FIG. 3B, 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. In this case, the residual higher-order spherical aberration at the time of focus jump can be reduced, the amount of movement of the coupling lens at the time of focus jump can be reduced, and information can be recorded on the information recording surface with the thicker transparent substrate. / In addition to being able to suppress the reduction in lens tilt sensitivity even when the ambient temperature becomes high during playback, the two opposing optical surfaces shift in the direction perpendicular to the optical axis due to manufacturing errors. The amount of aberration that occurs when the lens thickness on the optical axis shifts in the direction of the optical axis due to manufacturing errors can also be suppressed. Easy-to-use objective It is possible to provide a's.

  On the other hand, in the magnification M as shown in FIG. 3C, 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. If not, the residual higher-order spherical aberration at the time of focus jump can be further reduced, the amount of movement of the coupling lens at the time of focus jump can be further reduced, and the information recording surface with the thicker transparent substrate can be used. On the other hand, even when the environmental temperature becomes high during the recording / reproducing of information, 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.

Further, the cover glass thickness when the spherical aberration is minimized at the normal temperature (25 ± 3 ° C.) and the magnification M satisfying the above-mentioned expression (2) is T (mm), and the normal temperature (25 ± 3 ° C.). When the focal length of the wavelength λ1 is f (mm), the change rate ΔSA3 / (ΔM × f) of the third-order spherical aberration with respect to the magnification change of the objective lens at normal temperature (25 ± 3 ° C.) and the cover glass thickness T It is preferable that (λrms / mm) satisfies the formula (14).
21 <| ΔSA3 / (ΔM × f) | <25 (14)

  By defining the rate of change of the third-order spherical aberration with respect to the magnification change of the objective lens so as to satisfy the equation (14), both the suppression of the residual higher-order spherical aberration during the focus jump and the movement amount of the coupling lens are compatible. It becomes possible to do.

More preferably, the following conditional expression (14 ′) is satisfied.
21.5 <| ΔSA3 / (ΔM × f) | <24.5 (14 ′)

Further, the third-order spherical aberration ΔSA3 and the fifth-order spherical aberration ΔSA5 that occur when the magnification of the objective lens is changed at normal temperature (25 ± 3 ° C.) and the cover glass thickness T satisfy the expression (15). preferable.
4.2 <ΔSA3 / ΔSA5 <5.2 (15)

  By satisfying the equation (15), the ratio of the change of the third-order spherical aberration and the fifth-order spherical aberration when the magnification is changed is the ratio of the third-order spherical aberration and the fifth-order spherical aberration when the cover glass thickness is changed. Therefore, it is possible to achieve both the suppression of the residual higher-order spherical aberration during the focus jump and the suppression of the movement amount of the coupling lens.

More preferably, the following conditional expression (15 ′) is satisfied.
4.3 <ΔSA3 / ΔSA5 <4.9 (15 ′)

Further, the fifth-order coma aberration CM5 (occurred when an oblique light beam having a half angle of view of 1 degree is incident on the objective lens at normal temperature (25 ± 3 ° C.), the above-mentioned transparent substrate thickness T, and magnification M. (λrms) preferably satisfies the expression (16).
0.02 <| CM5 | <0.05 (16)

  Conditional expression (16) sets conditions for achieving both suppression of the residual high-order spherical aberration at the time of focus jump and suppression of the movement amount of the coupling lens from another viewpoint. By satisfying the expression (16) at the magnification M satisfying the expression (2), it is possible to satisfy both the suppression of the residual higher-order spherical aberration at the time of the focus jump and the suppression of the movement amount of the coupling lens.

Third-order coma aberration CM3 (λrms) generated when an oblique light beam having a half angle of view of 1 degree is incident on the objective lens at normal temperature (25 ± 3 ° C.), cover glass thickness, and magnification M. ) Satisfies the expression (17).
0 ≦ | CM3 | <0.02 (17)

  By satisfying conditional expression (17), it is possible to prevent the lens tilt sensitivity from becoming too small even when information is recorded / reproduced with respect to the information recording surface having the larger transparent substrate thickness. Further, even if the objective lens is made of plastic, the lens tilt sensitivity is small even when the environmental temperature becomes high during the recording / reproducing of information on the information recording surface having the thicker transparent substrate. Since it can prevent becoming too much, it is preferable.

Further, when the objective lens has a positive maximum value of the sine condition violation amount as OSC _MAX (mm) and the focal length of the wavelength λ1 at room temperature (25 ± 3 ° C.) as f (mm), the expression (18) It is preferable to satisfy.
0.003 <OSC _MAX /f<0.022 (18)

  If the coma aberration correction state when the oblique light beam is incident so that the sine condition violation amount is larger than the lower limit of the equation (18), the higher-order spherical aberration at the time of focus jump is not undercorrected, and the sine condition violation Setting the coma aberration correction state when the oblique light beam is incident so that the amount is smaller than the upper limit of the equation (18), the higher-order spherical aberration is not overcorrected. Can be suppressed.

More preferably, the following conditional expression (18 ′) is satisfied.
0.003 <OSC _MAX /f<0.015 (18 ′)

  When recording and / or reproducing information with respect to an optical disc, it is possible to tilt the objective lens along the radial direction and / or tangential direction of the optical disc. It is possible to cancel the coma generated by the tilt of the objective lens (referred to as lens tilt in the present specification) by the coma generated by the tilting of the objective lens (referred to as disc tilt in the specification). Reproduction can be performed stably.

  Here, if the coma generated by the lens tilt is too small relative to the coma generated by the disc tilt, the amount of lens tilt required to correct the coma generated by the disc tilt will increase. Or the objective lens collides with the optical disk when the lens is tilted.

  The coma generated by the lens tilt changes depending on the sine condition violation amount of the objective lens, and the sine condition violation amount depends on the magnification of the objective lens in a state where information is recorded / reproduced with respect to the optical disc. It changes depending on. Specifically, in an objective lens in which the sine condition violation amount is corrected when a parallel light beam is incident on the objective lens, the sine condition violation amount is changed to the negative side when a divergent light beam is incident on the objective lens. Therefore, the amount of coma generated by the lens tilt is reduced. The amount of coma aberration decreases as the divergence of the light beam incident on the objective lens increases.

  In the BD optical pickup device, the divergence of the light beam incident on the objective lens is maximized when information is recorded and / or reproduced on the information recording surface having the longest distance from the light beam incident surface. Furthermore, in the case of an objective lens made of a plastic material, the degree of divergence of the light flux is further increased in order to correct the spherical aberration caused by the change in the environmental temperature.

Therefore, at a high temperature (55 ± 3 ° C.), the cover glass thickness equal to the maximum transparent substrate thickness T _MAX is non-parallel to the objective lens so that the third-order spherical aberration of the focused spot by the objective lens is corrected. Absolute ratio of the third-order coma aberration CM (DT) generated when the cover glass is tilted by the same amount with respect to the third-order coma aberration CM (LT) generated when the objective lens is tilted in a state where the light beam is incident. It is preferable to set the sine condition violation amount of the objective lens in a state where information is recorded and / or reproduced on the information recording surface having the longest distance from the light incident surface so that the value is 0.3 or more. As a result, even when information is recorded and / or reproduced on the information recording surface having the longest distance from the light incident surface, coma aberration due to the disc tilt can be favorably corrected by the lens tilt. Good recording / reproducing characteristics can be obtained for all the information recording surfaces included.

That is, by satisfying the following conditional expression (19), it is possible to prevent the lens tilt sensitivity from becoming too small even when information is recorded / reproduced on the information recording surface having the thicker transparent substrate. it can. Further, even if the objective lens is made of plastic, the lens tilt sensitivity is small even when the environmental temperature becomes high during the recording / reproducing of information on the information recording surface having the thicker transparent substrate. Since it can prevent becoming too much, it is preferable.
0.3 ≦ | CM (LT) / CM (DT) | ≦ 0.8 (19)

In addition, Preferably, it is satisfy | filling the following conditional expressions (19 ').
0.35 ≦ | CM (LT) / CM (DT) | ≦ 0.8 (19 ′)

  In order to further exhibit such an effect, it is more preferable to set the sine condition violation amount and spherical aberration correction state of the objective lens as described below.

In a state where a light beam having a magnification M satisfying the expression (2) is incident on the objective lens, the absolute value of the spherical aberration of the spot collected through the cover glass thickness equal to the maximum transparent substrate thickness T _MAX is: It is preferable to set the spherical aberration correction state of the objective lens so that the spherical aberration of the spot collected through the cover glass thickness equal to the minimum transparent substrate thickness T _MIN becomes smaller.

This is the optical pickup device, the position of the movable lens in a state in which the light flux of the magnification M to the objective lens is incident T0, the recording and / or reproducing of the transparent substrate thickness information to the information recording surface which is T _MAX Assuming that the position of the movable lens in the state of performing T1 is T1, and the position of the movable lens in the state of recording and / or reproducing information on the information recording surface having the transparent substrate thickness _TMIN is T2, the following (20) It is synonymous with the expression being established.
| T1-T0 | <| T2-T0 | (20)

Furthermore, a non-parallel light beam is made incident on the objective lens so that the third-order spherical aberration of the focused spot by the objective lens is corrected at room temperature (25 ± 3 ° C.) and the maximum transparent substrate thickness T _MAX . With respect to the objective lens so that the third-order spherical aberration of the focused spot by the objective lens is corrected at the magnification M1 in the above state, normal temperature (25 ± 3 ° C.), and the minimum transparent substrate thickness T _MIN . It is preferable that the magnification M2 in a state where a parallel light beam is incident satisfies the expression (21).
0 ≦ M1 / M2 <0.92 (21)

In order to prevent the lens tilt sensitivity from becoming too small when recording / reproducing information on the information recording surface with the thicker transparent substrate, T is not an intermediate point between T _MAX and T _MIN. , Closer to T _MAX is preferable. Formula (21) defines the preferable range from the viewpoint of magnification.

More preferably, the following conditional expression (21 ′) is satisfied.
0 ≦ M1 / M2 <0.8 (21 ′)

Further, from the viewpoint of the lens shape, the refractive index N of the objective lens with respect to the wavelength λ1 at normal temperature (25 ± 3 ° C.) and the inclination angle θ (degree) in the outermost effective diameter of the optical surface on the light source side (object side) are ( It is preferable to satisfy the formula (22).
−59.8 × N + 162 <θ <−59.8 × N + 166 (22)

Further, from the viewpoint of the lens shape, the refractive index of the objective lens with respect to the wavelength λ1 at normal temperature (25 ± 3 ° C.) is N, and the first derivative X ′ of the aspherical deformation amount X (h) (mm) of the optical surface on the optical disk side. When the radius height at which (h) switches from negative to positive is H (mm), the equation (23) is satisfied. However, the aspherical deformation amount X (h) is defined by the distance in the optical axis direction from the plane contacting the surface vertex of the optical surface on the optical disk side to the aspherical surface, and negative when the plane is deformed from the plane to the light source side. The case of deformation from the plane to the optical disk side is positive, and H is a relative value when the effective radius is 1.
−2.8 × N + 5.1 <H <−2.8 × N + 5.4 (23)

  The objective lens used in the optical pickup device of the present invention may be an objective lens that substantially satisfies the sine condition.

  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.

The objective lens preferably satisfies the following conditional expression (24).
0.9 ≦ d / f ≦ 1.5 (24)
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.

  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 (24), it is possible to suppress the generation of astigmatism and the surface shift sensitivity.

  The optical pickup device of the present invention can be applied to an objective lens having f of 1.0 mm or more and 2.4 mm or less, but the effect of the present invention is more remarkable in a slim type optical pickup device. Therefore, f of the objective lens in which the effect of the present invention becomes more remarkable is 1.0 mm or more and 1.45 mm or less. What becomes more conspicuous is 1.0 mm or more and 1.41 mm or less, and what becomes more conspicuous is 1.0 mm or more and 1.27 mm or less. The most prominent is 1.0 mm or more and 1.2 mm or less.

  Further, as an objective lens used in a slim type optical pickup device, it is preferable that the effective diameter of the light incident surface of the objective lens is 1.7 mm or more and 2.4 mm or less. More preferably, they are 1.7 mm or more and 2.2 mm or less, More preferably, they are 1.7 mm or more and 2.0 mm or less.

  Furthermore, as a coupling lens used in a slim type optical pickup device, the focal length of the first light flux is preferably 8.5 mm or more and 14.2 mm or less, more preferably 12 mm or more and 14 mm or less. is there.

  Further, 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 optical pickup device described above.

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

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

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

  In this specification, an optical pickup device having a thickness of 8 mm or less is defined as a slim type optical pickup device. As shown in FIG. 4, the thickness of the optical pickup device refers to the distance from the lower surface of the optical pickup body to the surface of the optical disk, and the working distance of the objective lens (the distance from the light beam exit surface of the objective lens to the optical disk surface). Shall be included. Note that the optical disk drive apparatus on which the slim type optical pickup apparatus is mounted preferably has a thickness of 13 mm or less, and examples thereof include an optical disk drive apparatus with a thickness of 12.7 mm or 9.5 mm.

  According to the present invention, there is provided an optical pickup device that is compact and low-cost, suitable as a slim type optical pickup device, and capable of recording / reproducing information with respect to an optical disc having a multilayer information recording surface. Can do.

It is a figure which compares and shows each spherical aberration based on the examination result which this inventor performed. It is a figure for demonstrating a sine condition. It is a figure which shows the example of sine condition dissatisfaction amount. It is a figure which shows schematically the structure of optical pick-up apparatus PU1. It is a figure which shows schematically the structure of optical pick-up apparatus PU2.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 shows that information is appropriately recorded on a BD that is an optical disc having three information recording surfaces RL1 to RL3 (referred to as RL1, RL2, and RL3 in order of increasing 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 is a slim type optical pickup device having a height H = 8 mm or less (outline is schematically shown by a dotted line). The present invention is not limited to the present embodiment. For example, FIG. 4 shows an optical pickup device dedicated to BD, but the objective lens OBJ is made compatible with BD / DVD / CD, or the objective lens for DVD / CD is separately arranged, so that the BD / DVD is used. / An optical pickup device compatible with CD can be used.

  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 raising mirror MR, a positive lens unit L2 composed of one positive lens having a positive refractive power, and a negative lens unit L3 composed of one negative lens having a negative refractive power, a positive lens unit L2 1-axis actuator AC1 that moves only in the optical axis direction, liquid crystal element LCD, polarizing prism PBS, semiconductor laser LD that emits a laser beam (beam) of 405 nm, sensor lens SL, and reflection from information recording surfaces RL1 to RL3 of BD A light receiving element PD that receives a light beam is included.

  In the present embodiment, the coupling lens CL is disposed between the polarizing prism PBS and the λ / 4 wavelength plate QWP. The amount of movement of the positive lens group L2 of the coupling lens CL is that the position where the luminous flux from the semiconductor laser LD becomes a parallel luminous flux when passing through the positive lens group L2 is the origin, and the direction from the origin to the semiconductor laser LD side. The maximum moving distance of the positive lens unit L2 is smaller than the maximum moving distance of the positive lens unit L2 from the origin toward the objective lens OBJ.

In the present embodiment, the objective lens OBJ is a single lens made of plastic, and when the maximum transparent substrate thickness among the transparent substrate thicknesses is T _MAX (mm), normal temperature (25 ± 3 ° C.) and ( In the cover glass thickness T (mm) satisfying the expression (1), the magnification M when the spherical aberration is minimized satisfies the expression (2).
T _MAX · 0.70 ≤ T ≤ T _MAX · 1.1 (1)
−0.003 ≦ M ≦ 0.003 (2)

Further, the focal length f (mm) at the wavelength λ1 of the objective lens OBJ satisfies the following expression.
1.0 ≦ f ≦ 1.45 (3)

  In the present embodiment, in principle, by moving the positive lens unit L2 of the coupling lens CL in the optical axis direction, a focus jump for selecting one of a plurality of information recording surfaces is performed, and a change in environmental temperature is performed. Further, spherical aberration that increases due to wavelength fluctuations of the light source is corrected by driving the liquid crystal element LCD. However, the liquid crystal element LCD may be driven to correct at least a part of the spherical aberration that occurs during the focus jump and / or at least a part of the spherical aberration that occurs in the objective lens OBJ. The liquid crystal element LCD may be disposed between the rising mirror MR and the objective lens OBJ.

  Further, when the spherical aberration is corrected by the liquid crystal element LCD and the optical axis of the objective lens OBJ and the optical axis of the liquid crystal element LCD are decentered in parallel, coma aberration occurs on the information recording surface, and recording / reproduction is performed. May adversely affect properties. Therefore, it is preferable that the liquid crystal element LCD and the objective lens OBJ are connected by a connecting means such as a lens frame and the relative positions thereof are fixed so as to perform tracking together.

  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 (B) 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 passes through the liquid crystal element LCD, is reflected by the rising mirror MR, is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, and is stopped by a diaphragm (not shown). The light beam diameter is regulated and becomes a spot formed on the first information recording surface RL1 by the objective lens OBJ through the transparent substrate PL1 having the first thickness 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 wave plate QWP, and is raised to the rising mirror MR. , Passes through the liquid crystal element LCD, passes through the positive lens group L2 and the negative lens group L3 of the collimating lens CL, and becomes a convergent light beam. After being reflected by the polarizing prism PBS, the light receiving element PD is received by the sensor lens SL. Converges on the light receiving surface. 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 (between B and A) 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 passes through the liquid crystal element LCD, is reflected by the rising mirror MR, is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, and is stopped by a diaphragm (not shown). The light beam diameter is regulated, and the objective lens OBJ is formed on the second information recording surface RL2 as shown by the one-dot chain line through the transparent substrate PL2 having the second thickness (thicker than the first thickness). Become a spot.

  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 stop, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and the rising mirror MR , Passes through the liquid crystal element LCD, passes through the positive lens group L2 and the negative lens group L3 of the collimating lens CL, and becomes a convergent light beam. After being reflected by the polarizing prism PBS, the light receiving element PD is received by the sensor lens SL. Converges on the light receiving surface. 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 this case, the positive lens group L2 of the coupling lens CL is moved to the dotted line position (A) 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 passes through the liquid crystal element LCD, is reflected by the rising mirror MR, is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, and is stopped by a diaphragm (not shown). A spot formed on the third information recording surface RL3 as indicated by a dotted line through the transparent substrate PL3 having a third thickness (thicker than the second thickness) by the objective lens OBJ, with the light beam diameter being regulated. It becomes.

  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 diaphragm, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and the rising mirror MR , Passes through the liquid crystal element LCD, passes through the positive lens group L2 and the negative lens group L3 of the collimating lens CL, and becomes a convergent light beam. After being reflected by the polarizing prism PBS, the light receiving element PD is received by the sensor lens SL. Converges on the light receiving surface. 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.

  FIG. 5 shows that information is appropriately recorded on a BD that is an optical disk having three information recording surfaces RL1 to RL3 (referred to as RL1, RL2, and RL3 in order of increasing distance from the light beam incident surface of the optical disk) in the thickness direction. FIG. 6 is a diagram schematically showing a configuration of an optical pickup device PU2 of another embodiment capable of performing reproduction. Such an optical pickup device PU2 is also a slim type optical pickup device having a height H = 8 mm or less (outline is schematically shown by a dotted line).

  In the present embodiment, the only difference is that the coupling lens CL, which is a single lens, can be moved in the optical axis direction by the actuator AC1, and therefore the description of the other components is omitted. Also in the present embodiment, by moving the positive lens group L2 of the coupling lens CL in the optical axis direction, a focus jump for selecting one of a plurality of information recording surfaces is performed, and a change in environmental temperature or a wavelength of the light source The spherical aberration that increases due to the fluctuation is corrected by driving the liquid crystal element LCD.

  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 Blue-violet semiconductor laser AC1 1-axis actuator AC2 3-axis actuator PBS Polarizing prism CL Coupling lens L2 Positive lens group L3 Negative lens group LCD Liquid crystal element MR Rising mirror PL1 First transparent substrate PL2 Second transparent substrate PL3 Third transparent substrate RL1 First information recording surface RL2 Second information recording surface RL3 Third information recording surface QWP λ / 4 wavelength plate

Claims (10)

An information recording surface having a light source emitting a light beam having a wavelength λ1 (390 nm <λ1 <415 nm), a coupling lens, a liquid crystal element, and an objective lens, and having different distances (transparent substrate thicknesses) from the light beam incident surface. By selecting any one information recording surface in the optical disc having three or more in the thickness direction, and condensing the light beam having the wavelength λ1 emitted from the light source onto the selected information recording surface by the objective lens. An optical pickup device for recording and / or reproducing information,
The coupling lens has at least one moving lens that is movable in the optical axis direction, and moves the moving lens in the optical axis direction between the light source and the objective lens, so that Select one of the information recording surfaces,
The liquid crystal element is driven to correct a part of spherical aberration on an information recording surface for recording and / or reproducing information and / or at least a part of spherical aberration generated in the objective lens,
The objective lens is a single lens made of plastic,
The image side numerical aperture (NA) of the objective lens is 0.8 or more and 0.95 or less,
When the maximum transparent substrate thickness among the transparent substrate thicknesses is T _MAX (mm), the spherical aberration is minimum at room temperature (25 ± 3 ° C.) and the cover glass thickness T (mm) satisfying the expression (1). An optical pickup device characterized in that the magnification M at the time satisfies the expression (2).
T _MAX · 0.70 ≤ T ≤ T _MAX · 1.1 (1)
−0.003 ≦ M ≦ 0.003 (2)   The optical pickup device according to claim 1, wherein the liquid crystal element is driven to correct spherical aberration generated in the objective lens due to a temperature change.   3. The optical pickup device according to claim 1, wherein the liquid crystal element is driven to correct spherical aberration generated in the objective lens due to a wavelength change.   The coupling lens is a spherical surface generated with a change in the distance (including the substrate thickness of the optical disk) from the light incident surface to the information recording surface when changing the information recording surface on which information is recorded and / or reproduced. 4. The optical pickup device according to claim 2, wherein the moving lens is moved in the optical axis direction in order to correct aberration. When the maximum transparent substrate thickness among the transparent substrate thicknesses is T _MAX (mm), the spherical aberration is minimum at room temperature (25 ± 3 ° C.) and the cover glass thickness T (mm) satisfying the formula (1A). 5. The optical pickup device according to claim 1, wherein the magnification M satisfies the following expression (2).
T _MAX・ 0.80 ≦ T ≦ T _MAX・ 1.1 (1A)
−0.003 ≦ M ≦ 0.003 (2) 6. The optical pickup device according to claim 1, wherein a focal length f (mm) at the wavelength λ <b> 1 of the objective lens satisfies the following expression.
1.0 ≦ f ≦ 1.45 (3)   The objective lens is characterized in that the sine condition violation amount has a positive maximum value and the sine condition violation amount does not have a negative maximum value between 70% and 90% of the effective radius at the magnification M. The optical pickup device according to claim 1.   The objective lens has a positive maximum value for the sine condition violation amount between 70% and 90% of the effective radius at the magnification M, and further has a negative maximum value for the sine condition violation amount. An optical pickup device according to any one of claims 1 to 6.   The optical pickup device according to claim 1, wherein the coupling lens is a single lens, and the single lens is the moving lens.   9. The coupling lens according to claim 1, wherein the coupling lens has a two-group configuration of a positive lens group and a negative lens group, and at least one lens of the positive lens group is the moving lens. An optical pickup device according to claim 1.

Images