WO2011052469A1

WO2011052469A1 - Light pickup device

Info

Publication number: WO2011052469A1
Application number: PCT/JP2010/068570
Authority: WO
Inventors: 徹木村; 雄樹小野
Original assignee: コニカミノルタオプト株式会社
Priority date: 2009-10-28
Filing date: 2010-10-21
Publication date: 2011-05-05
Also published as: JPWO2011052469A1

Abstract

Disclosed is a light pickup device that, while being compact and low-cost, can record/replay information to/from an optical disc having a multi-layered information recording surface. An objective lens is formed from a glass material having a smaller variation in refractive index corresponding to thermal variation compared to, for example, plastic, can suppress an increase in spherical aberration even if the ambient temperature varies, and consequently can suppress the amount of movement of a coupling lens to a low level. Furthermore, of a positive lens group and a negative lens group that configure the aforementioned coupling lens, the information recording surface in the aforementioned optical disc to/from which information is recorded and/or replayed is caused to be selected by moving at least one of the lenses of the aforementioned positive lens group in the direction of the light axis, and thus the amount of movement of the aforementioned coupling lens is suppressed to a yet lower level. As a result, the length of the light path from the light source to the aforementioned objective lens is suppressed, and it is possible to aim to reduce the cost and size of the light pickup device.

Description

Optical pickup device

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

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

By the way, many of the conventional BDs have an information recording surface of one layer or two layers, but due to the market demand for storing larger data on one BD, the information recording surface of three layers or more. Research is also progressing on BDs that have. However, since the NA of the luminous flux when recording and / or reproducing information is as large as 0.85, a BD having a plurality of information recording surfaces is supposed to give a minimum spherical aberration to one information recording surface. As a result, spherical aberration increases on other information recording surfaces with different substrate thicknesses, making it impossible to appropriately record / reproduce information. 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, Patent Document 1 discloses a luminous flux in which a magnification is changed by moving a coupling lens arranged between a light source and an objective lens in the optical axis direction, and aberration is suppressed with respect to a selected information recording surface. An optical pickup device capable of condensing light is disclosed.

Japanese Patent No. 4144663

However, if the optical pickup device described in the above-mentioned Patent Document 1 is used to record / reproduce information on, for example, an optical disc having three or more layers of information recording surfaces, a long moving distance of the coupling lens is required. Become. In addition, when a plastic objective lens is used, since the refractive index change with respect to the temperature change is relatively large, spherical aberration is likely to increase due to environmental temperature fluctuations, but when this is corrected by movement of the coupling lens, In addition, a longer moving distance 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 a thin optical pickup device that is required to be downsized, there is a restriction that the optical path length from the light source to the objective lens cannot be increased, so that it is difficult to cope with a BD having three or more layers of information recording surfaces. The problem becomes more obvious.

The present invention has been made in consideration of the above-mentioned problems, and provides an optical pickup device capable of recording / reproducing information with respect to an optical disc having a multilayer information recording surface while being compact and low in cost. The purpose is to provide.

The optical pickup device according to claim 1 is an optical pickup device that selects and records information and / or reproduces information by selecting any information recording surface in an optical disk having three or more information recording surfaces in the thickness direction. There,
A light source that emits a light beam having a wavelength of λ1 (390 nm <λ1 <415 nm), an objective lens that focuses the light beam on an information recording surface of an optical disc, and a positive light source disposed between the light source and the objective lens. A coupling lens composed of a positive lens group having a refractive power and a negative lens group having a negative refractive power,
The image side numerical aperture (NA) of the objective lens is 0.8 or more,
The objective lens is a single ball, formed from a glass material,
Information is recorded and / or reproduced by causing the light beam emitted from the light source to enter the objective lens via the coupling lens and condensing it on the selected information recording surface of the optical disc by the objective lens. Is supposed to do
When changing an information recording surface on which information is to be recorded and / or reproduced on the optical disc from one information recording surface to another information recording surface, at least one lens of the positive lens group is moved in the optical axis direction. Features.

According to the present invention, since the objective lens is formed of a glass material that has a smaller refractive index change with respect to a temperature change than, for example, plastic, an increase in spherical aberration can be effectively suppressed even when the environmental temperature changes. Thus, it is not necessary to correct spherical aberration that increases with changes in temperature, whereby the amount of movement of the coupling lens can be kept small.

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

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

In order to make the above compatible, in the optical pickup device of the present invention, the coupling lens has a two-group configuration including a positive lens group and a negative lens group, and at least one lens of the positive lens group moves in the optical axis direction. It was made to make it composition.

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.

The optimum value of the magnification is maintained at −0.1, the coupling lenses are configured in two groups, and at least one lens is moved in the optical axis direction, thereby reducing the amount of movement of the coupling lens required at the time of focus jump. Can be small.

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, then the coupling lens The system power P _C and the focal length f _C of the entire coupling lens system are expressed by the following equation (1).

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 ) (1)
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 (2).

M = −f 2 _O / f _C (2)
Considering the space for arranging an optical element such as a polarizing 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. From the above, based on the condition that the optimum value as described above exists in the value determined by the expression (2), the entire focal length range of the coupling lens for the optical pickup device for BD is a predetermined value. The focal length f _C of the entire coupling lens system cannot be reduced excessively considering only the amount of movement of the coupling lens required at the time of focus jump.

Here, in the optical pickup apparatus of the present invention, in order to reduce the movement amount at the time of focus jump, to increase the power P _P of the positive lens, further, the focal length f _C of the coupling lens system is too short no way to increase the absolute value of the power P _N of the negative lens ((see 1)).

As described above, in the optical pickup device according to the present invention, it is possible to reduce both the amount of movement of the positive lens group required at the time of focus jump and the symmetry of the light amount distribution taken in by the coupling lens.

Compared with a coupling lens with only a positive lens group, the negative lens group is added to the positive lens group, thereby reducing the focal length of the entire coupling lens system while maintaining the NA of the entire coupling lens system. Therefore, the optical pickup device can be downsized.

Furthermore, the NA of the entire coupling lens system can be reduced in a state where the focal length of the entire coupling lens system is smaller than the focal length of the coupling lens of only the positive lens group, so that the asymmetry of the light quantity distribution captured by the coupling lens Can be relaxed.

As a result, even in an optical pickup device that has a relatively large amount of movement of the coupling lens, which is an optical disc having three or more layers, the optical path length from the light source to the objective lens is suppressed, and the optical pickup device can be reduced in size and size. Cost can be reduced.

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

An optical pickup device according to a second aspect of the present invention is the optical pickup device according to the first aspect of the present invention, in a state where a parallel light beam is incident on the objective lens, between 60% and 90% of the effective radius of the objective lens, The coma aberration correction state is set such that the sine condition violation amount of the objective lens has a positive maximum value, and the sine condition violation amount monotonously decreases in the peripheral portion.

In a state where a parallel light beam is incident on the objective lens, the sine condition violation amount of the objective lens has a positive maximum value between 60% and 90% of the effective radius of the objective lens, and in the peripheral part By using an objective lens in which the coma aberration correction state is set so that the sine condition violation amount monotonously decreases, it becomes possible to increase the amount of change in spherical aberration when the magnification of the objective lens fluctuates, By pulling, it is possible to reduce the amount of movement of the coupling lens.

Note that “the state in which a parallel light beam is incident on the objective lens” means that the position of the movable lens of the coupling lens is optimized so that the light beam emitted from the coupling lens and directed to the objective lens becomes a parallel light beam. It is synonymous with that.

Further, by using the above objective lens, a higher-order spherical aberration larger than the third-order in the state where the third-order spherical aberration generated at the time of focus jump is corrected by the movement of the coupling lens (in this specification, “focus” It is also possible to suppress “residual aberration at the time of jump”.

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

Furthermore, since the maximum value of the sine condition violation amount exists on the negative side in addition to the positive side, the amount of coma aberration generated when the optical surfaces of the objective lens are decentered parallel to the direction perpendicular to the optical axis (this In the specification, the amount of coma generated (sometimes referred to as “surface shift sensitivity”) can be reduced, and the spherical aberration generated when the lens thickness on the optical axis deviates from the design value. Since the generation amount (in this specification, the generation amount of such spherical aberration may be referred to as “axial thickness error sensitivity”) can be reduced, an objective lens that is easy to manufacture can be obtained.

In order to exert such effects more effectively, the sine condition violation amount has a negative maximum value between 10% to 50% of the effective radius of the objective lens, and the sine condition violation amount from negative to positive in the periphery. The off-axis coma aberration correction state so that the sine condition violation amount has a positive maximum value between 60% and 90% of the effective radius, and the sine condition violation amount decreases monotonously at the periphery. Is preferably set.

The objective lens used in the optical pickup device of the present invention may be set in the shape of the sine condition violation amount, giving priority to reducing the amount of movement of the coupling lens, and the residual aberration at the time of focus jump. The shape of the sine condition violation amount may be set with priority on keeping it small, or the shape of the sine condition violation amount is set with priority on reducing the surface shift sensitivity and on-axis thickness error sensitivity. It may be. Of course, the shape of the sine condition violation amount may be set so as to balance the three points.

According to a third aspect of the present invention, there is provided the optical pickup device according to the first aspect of the present invention, in a state where a parallel light beam is incident on the objective lens, between 40% and 80% of the effective radius of the objective lens. The coma aberration correction state is set so that the sine condition violation amount of the objective lens has a negative maximum value, and the sine condition violation amount monotonously decreases in the periphery.

Since the maximum value of the sine condition violation amount exists on the negative side, the surface shift sensitivity of the objective lens can be kept small, and the on-axis thickness error sensitivity can be reduced, so that an objective lens that is easy to manufacture can be obtained. it can. Accordingly, the occurrence of aberration based on the manufacturing error of the objective lens can be prevented, so that it is possible to suppress the movement distance of the coupling lens from being extended based on the manufacturing error.

According to a fourth aspect of the present invention, there is provided the optical pickup device according to the first aspect, wherein the objective lens can be tilted along a radial direction and / or a tangential direction of the optical disc. The optical disc against third-order coma aberration CM (LT) generated when the objective lens is tilted in a state where information is recorded and / or reproduced with respect to the information recording surface having the longest distance from the light beam incident surface. The absolute value of the ratio of the third-order coma aberration CM (DT) that occurs when the same is tilted by the same amount is 0.4 or more.

The “state of recording and / or reproducing information on the information recording surface having the longest distance from the light incident surface of the optical disk” means that the information recording surface having the longest distance from the light incident surface is This is synonymous with optimizing the position of the movable lens of the coupling lens so that the third-order spherical aberration generated when the spot focused by the objective lens is focused is corrected.

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 of the objective lens in a state where information is recorded and / or reproduced on the optical disc. It varies depending on the magnification. 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. .

Therefore, in the optical pickup device of the present invention, the third-order coma aberration generated when the objective lens is tilted in a state where information is recorded and / or reproduced with respect to the information recording surface having the longest distance from the light incident surface. Information with the longest distance from the light incident surface so that the absolute value of the ratio of the third-order coma aberration CM (DT) generated when the optical disk is tilted by the same amount with respect to CM (LT) is 0.4 or more. The amount of violation of the sine condition of the objective lens in a state where information was recorded and / or reproduced on the recording surface was set. 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.

In order to further exert such effects, it is preferable that the absolute value of the ratio of DT to LT is 0.5 or more.

In order to further exert 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.

One is to set the amount of violation of the sine condition of the objective lens so that the amount of violation of the sine condition of the objective lens is on the plus side when a parallel light beam is incident on the objective lens.

Another one occurs when the spot focused by the objective lens is focused on the information recording surface with the shortest distance from the light beam incident surface in a state where a parallel light beam is incident on the objective lens. The absolute value of the spherical aberration generated when the spot focused by the objective lens is focused on the information recording surface having the longest distance from the light incident surface is smaller than the absolute value of the spherical aberration. Thus, the correction state of the spherical aberration of the objective lens is set.

This is because the position of the movable lens in a state where a parallel light beam is incident on the objective lens is T0, and the position of the movable lens in a state where information is recorded and / or reproduced on the information recording surface having the longest distance from the light beam incident surface. Is T1, and T2 is the position of the movable lens in a state where information is recorded and / or reproduced with respect to the information recording surface with the shortest distance from the light incident surface. It is.

| T1-T0 | <| T2-T0 | (3)
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.

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

The first optical disc has three or more information recording surfaces stacked in the thickness direction. 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.

In this specification, BD means that information is recorded / reproduced by a light beam having a wavelength of about 390 to 415 nm and an objective lens having an NA of about 0.8 to 0.9, and the thickness of the protective substrate is 0.05 to 0.00 mm. A 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 preferable to be able to cope 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 protective substrate is about 0.6 mm. Including DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, and the like. In this specification, CD is a general term for CD series optical discs in which information is recorded / reproduced by an objective lens having an NA of about 0.45 to 0.51 and the thickness of the protective substrate is about 1.2 mm. Including CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW and the like. As for the recording density, the recording density of BD is the highest, followed by the order of DVD and CD.

In addition, regarding the thicknesses t1, t2, and t3 of the protective substrate, it is preferable to satisfy the following conditional expressions (4), (5), and (6), but is not limited thereto. The thickness of the protective substrate referred to here is the thickness of the protective substrate provided on the surface of the optical disk. That is, the thickness of the protective substrate from the optical disc surface to the information recording surface closest to the surface.

0.050 mm ≦ t1 ≦ 0.125 mm (4)
0.5mm ≦ t2 ≦ 0.7mm (5)
1.0mm ≦ t3 ≦ 1.3mm (6)
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 (7) and (8).

1.5 · λ1 <λ2 <1.7 · λ1 (7)
1.8 · λ1 <λ3 <2.0 · λ1 (8)
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. 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 415 nm or less. It is 750 nm or more and 880 nm or less, More preferably, it is 760 nm or more and 820 nm or less.

Also, at least two of the first light source, the second light source, and the third light source may be unitized. The unitization means that the first light source and the second light source are fixedly housed in one package, for example. In addition to the light source, a light receiving element to be described later may be packaged.

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

The condensing optical system has a coupling lens and an objective lens. The coupling lens is a lens group that is disposed between the objective lens and the light source and changes the divergence angle of the light beam. The coupling lens has 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. The negative lens group has at least one negative lens. The negative lens group may include only one negative lens or may include a plurality of lenses. An example of a preferable coupling lens is a combination of one positive lens and one negative 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. In the former, the light from the light source is incident on the positive lens with the divergence angle enlarged by the negative lens, and in the latter, the light from the light source is incident on the negative lens after being condensed by the positive lens. From the viewpoint of the light diameter, the former increases from the negative lens to the positive lens, and the latter increases when the positive lens is incident and decreases toward the negative lens. Therefore, from the viewpoint of reducing the light diameter in order to reduce the size of the coupling lens, the preferred arrangement is the former.

As described above, the optimum example of the coupling lens in the optical pickup device of the present invention is a combination of one positive lens and one negative lens, and is arranged in order of the negative lens and the positive lens from the light source side. .

In order to correct spherical aberration generated on the selected information recording surface of the first optical disc, at least one lens (preferably positive lens) of the positive lens group is movable in the optical axis direction. 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 an optical disc (BD) having a surface, the maximum spherical aberration difference A that occurs when an optimum focused spot is formed on each information recording surface that is the maximum distance, and the maximum that occurs when the environmental temperature changes by ± 30 ° C. And the maximum spherical aberration C that occurs when the wavelength of the light source changes by ± 5 nm. 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 B (= 140 mλ) due to the environmental temperature change becomes almost zero, and therefore 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. Furthermore, if the objective lens is made of glass and the optical surface is a diffractive surface that corrects spherical aberration that occurs when the wavelength varies, in addition to spherical aberration B caused by environmental temperature changes, spherical aberration C caused by wavelength fluctuations of the light source due to the function of the diffractive surface. Therefore, the amount of movement of the coupling lens is smaller (corresponding to the correction amount of the spherical aberration of 500 mλ in FIG. 1C). However, even if the objective lens is improved in this way, the amount of movement of the coupling lens when the optical disk having two information recording surfaces is used is smaller than that of the coupling lens when the optical disk having four information recording surfaces is used. Since the amount of movement is still about twice, further ingenuity is required to suppress the amount of movement of the coupling lens. The same applies to the amount of movement of the coupling lens when using an optical disc having three information recording surfaces or five or more information recording surfaces.

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.

The following describes the relationship between the configuration of the coupling lens and the amount of movement. FIG. 2 shows a coupling lens CL1 including only a positive lens group (one in the figure) L1, and a coupling lens CL2 including a positive lens group (one in the figure) L2 and a negative lens group (one in the figure) L3. FIG. Here, the focal length of the coupling lens CL1 is equal to the combined focal length of the coupling lens CL2, which is f. Therefore, the focal length f2 of the positive lens unit L2 of the coupling lens CL2 is shorter than the focal length f of the positive lens unit L1 of the coupling lens CL1.

Here, the amount of movement of the coupling lens when giving the same magnification change is inversely proportional to the power of the moving lens (= 1 / focal length). Therefore, when the positive lens unit L1 of the coupling lens CL1 and the positive lens unit L2 of the coupling lens CL2 are moved to give the same magnification change, (1 / f) <(1 / f2). , (Movement amount of coupling lens CL1)> (movement amount of coupling lens CL2). In other words, the movement amount of the positive lens unit L2 of the coupling lens CL2 of the two lenses of the positive lens and the negative lens is less than the movement of the coupling lens CL1 of one lens. .

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

When the objective lens is a glass lens, as described with reference to FIG. 1, it is not necessary to move the coupling lens in order to correct the spherical aberration caused by the temperature change. This is preferable because it can be reduced and the optical pickup device can be downsized.

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

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

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

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

Next, preferable conditions for the sine condition of the objective lens will be described. As shown in FIG. 3, 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 light beam is emitted from the lens at an emission angle U. ₁ / sinU satisfies a certain value. When 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 light 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. 4 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 corresponds to 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.

4A is an example in which the sine condition violation amount has one local maximum value OSCmin on the negative side. (Examples in which the sine condition violation amount has one negative maximum OSCmin are not only those shown by the solid line in FIG. 4A, but also the sine condition violation amount from the middle as shown by the dotted line. Such an objective lens may be an example.) According to such an objective lens, since the surface shift sensitivity is small and the on-axis thickness error sensitivity is small, the manufacturing is easy. Further, even when information is recorded and / or reproduced on the information recording surface having the longest distance from the light incident surface, the third-order coma aberration CM (DT) generated when the optical disc is tilted by the same amount is also obtained. In addition to canceling with third-order coma aberration (CT) that occurs when the objective lens is tilted, changes in spot diameter can be suppressed regardless of the recording / reproducing layer. Usually, in order to cancel DT with CT for the information recording surface having the longest distance from the light incident surface, the sine condition violation amount needs to be positive, but if there is no negative maximum value, the marginal ray This is because the sine condition violation amount becomes a large positive value and the spot diameter becomes large. In particular, when the sine condition violation amount of the objective lens has one negative maximum value, in BD, the design substrate thickness that minimizes spherical aberration when a parallel light beam is incident is 0.087 mm or less, 0 It is also possible to use an objective lens designed with a thinner design substrate thickness, such as 0.05 mm or more. On the other hand, as the coupling lens moves, higher-order spherical aberration increases, and the change in spherical aberration due to the change in magnification 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 FIG. 4C has one local maximum value OSCmax whose sine condition violation amount is between 60% and 90% of the effective radius of the objective lens. This is an example in which the sine condition violation amount monotonously decreases in the peripheral portion, but does not have a negative maximum value. According to such an objective lens, the high-order spherical aberration generated with the movement of the coupling lens is reduced, and the change of the spherical aberration due to the change in magnification is large. When the coupling lens is moved to select the surface, the necessary movement amount can be reduced. In particular, in the BD, in an objective lens designed with a thin design substrate thickness such that the design substrate thickness that minimizes spherical aberration when a parallel light beam is incident is 0.15 mm or less and 0.0875 mm or more. preferable.

Furthermore, the objective lens having the characteristics shown in FIG. 4B has a maximum value OSCmax where the sine condition violation amount is between 60% and 90% of the effective radius of the objective lens and 10% of the effective radius of the objective lens. Between the maximum value OSCmax, the sine condition violation amount decreases monotonously at the periphery, and the sine condition violation amount on the optical axis side from the maximum value OSCmin. This is an example of monotonously decreasing. This example is a balance between the characteristics shown in FIG. 4A and the characteristics shown in FIG. 4C. Therefore, the information recording surface of an optical disk having three or more layers is ensured while ensuring manufacturability. Therefore, when the coupling lens is moved, a necessary movement amount can be reduced.

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 0.75 or more and 0.9 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.

Moreover, it is preferable that the objective lens satisfies the following conditional expression (9).

0.9 ≦ d / f ≦ 1.5 (9)
However, d represents the thickness (mm) on the optical axis of the objective lens, and f represents the focal length of the objective lens in the first light flux. Note that f is preferably 1.0 mm or more and 1.8 mm or less.

In the case of an objective lens corresponding to an optical disk with a short wavelength and high NA such as BD, if the ratio of the thickness on the optical axis to the focal length of the objective lens becomes too large, an off-axis light beam enters the objective lens. In this case, astigmatism tends to occur, and a working distance cannot be secured. On the other hand, if the ratio of the thickness on the optical axis to the focal length of the objective lens becomes too small, there arises a problem that the surface shift sensitivity increases. By satisfying conditional expression (9), it is possible to suppress astigmatism and surface shift sensitivity.

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

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

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

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

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

According to the present invention, it is possible to provide an optical pickup device capable of recording / reproducing information with respect to an optical disc having a multilayer information recording surface while being compact and low in cost.

It is a figure which compares and shows each spherical aberration based on the examination result which this inventor performed. It is sectional drawing of coupling lens CL1 of a comparative example, and coupling lens CL2 concerning an example of this invention. 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. It is a graph which shows the sine condition violation amount of an Example.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 5 shows that information is appropriately recorded on a BD that is an optical disc having three information recording surfaces RL1 to RL3 in the thickness direction (referred to as RL1, RL2, and RL3 in order of increasing distance from the light incident surface of the optical disc). FIG. 2 is a diagram schematically showing a configuration of an optical pickup device PU1 of the present embodiment that can perform reproduction. Such an optical pickup device PU1 can be mounted on an optical information recording / reproducing device. The present invention is not limited to the present embodiment.

The optical pickup device PU1 moves the objective lens OBJ, the objective lens OBJ in the focusing direction and the tracking direction, and tilts in the radial direction and / or tangential direction of the optical disc, the λ / 4 wavelength plate QWP, Coupling CL having a positive lens unit L2 composed of one positive lens having a refractive power and a negative lens unit L3 composed of one negative lens having a negative refractive power, only the positive lens unit L2 in the optical axis direction. A uniaxial actuator AC1 to be moved, a polarizing prism PBS, a semiconductor laser LD that emits a laser beam (beam) of 405 nm, a sensor lens SL, and a light receiving element PD that receives reflected beams from the information recording surfaces RL1 to RL3 of the BD. . In the present embodiment, the coupling lens CL is disposed between the polarizing prism PBS and the λ / 4 wavelength plate QWP. In the present embodiment, the objective lens OBJ is a single ball made of glass, but two or more optical elements may be used in combination.

First, a case where recording / reproduction is performed on the first information recording surface RL1 of the BD will be described. In such a case, the positive lens group L2 of the coupling lens CL is moved to the position of the solid line by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens group L3 of the collimator lens CL, and the divergence angle is increased. After passing through the lens unit L2 to be a weakly convergent light beam, it is converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the first thickness is obtained by the objective lens OBJ. Through the protective substrate PL1, the spot is formed on the first information recording surface RL1 as shown by the solid line.

The reflected light beam modulated by the information pits on the first information recording surface RL1 is again transmitted through the objective lens OBJ and the diaphragm, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens group L2 and the negative lens group L3 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged on the light receiving surface of the light receiving element PD by the sensor lens SL. Then, using the output signal of the light receiving element PD, the information recorded on the first information recording surface RL1 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.

Next, a case where recording / reproduction is performed on the second information recording surface RL2 of the BD will be described. In such a case, the positive lens group L2 of the coupling lens CL is moved to the position of the alternate long and short dash line by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens group L3 of the collimator lens CL, and the divergence angle is increased. After passing through the lens unit L2 to be a substantially parallel light beam, it is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the objective lens OBJ has the second thickness. This is a spot formed on the second information recording surface RL2 as shown by the alternate long and short dash line through the protective substrate PL2 having a thickness (thicker than the first thickness).

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

Next, a case where recording / reproduction is performed on the third information recording surface RL3 of the BD will be described. In such a case, the positive lens group L2 of the coupling lens CL is moved to the dotted line position by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens group L3 of the collimator lens CL, and the divergence angle is increased. After passing through the lens unit L2 to be a weak divergent light beam, it is converted from linearly polarized light into circularly polarized light by the λ / 4 wave plate QWP, the diameter of the light beam is regulated by a diaphragm (not shown), and the third thickness is obtained by the objective lens OBJ. This is a spot formed on the third information recording surface RL3 as shown by the dotted line through the protective substrate PL3 (thicker than the second thickness).

The reflected light beam modulated by the information pits on the third information recording surface RL3 is again transmitted through the objective lens OBJ and the diaphragm, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens group L2 and the negative lens group L3 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged on the light receiving surface of the light receiving element PD by the sensor lens SL. Then, using the output signal of the light receiving element PD, the information recorded on the third information recording surface RL3 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.
(Second Embodiment)
FIG. 6 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 beam incident surface of the optical disc) in the thickness direction. FIG. 6 is a diagram schematically showing a configuration of an optical pickup device PU2 according to a second embodiment capable of performing reproduction. Such an optical pickup device PU2 can be mounted on an optical information recording / reproducing device. The present invention is not limited to the present embodiment.

The optical pickup device PU2 differs from the above-described optical pickup device PU1 only in the arrangement of the coupling lens CL. Specifically, the positive lens group L2 of the coupling lens CL is disposed between the polarizing prism PBS and the objective lens OBJ, and the negative lens group L3 is disposed between the blue-violet semiconductor laser LD and the polarizing prism PBS. Yes. About another structure, it is the same as that of optical pick-up apparatus PU1.

First, a case where recording / reproduction is performed on the first information recording surface RL1 of the BD will be described. In such a case, the positive lens group L2 of the coupling lens CL is moved to the position of the solid line by the uniaxial actuator AC1. Here, the divergent light beam of the light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD passes through the negative lens group L3 of the collimator lens CL, increases the divergence angle, passes through the polarizing prism PBS, and is further positive. After passing through the lens unit L2 to be a weakly convergent light beam, it is converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the first thickness is obtained by the objective lens OBJ. Through the protective substrate PL1, the spot is formed on the first information recording surface RL1 as shown by the solid line.

The reflected light beam modulated by the information pits on the first information recording surface RL1 is again transmitted through the objective lens OBJ and the diaphragm, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens unit L2 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged on the light receiving surface of the light receiving element PD by the sensor lens SL. Then, using the output signal of the light receiving element PD, the information recorded on the first information recording surface RL1 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.

Next, a case where recording / reproduction is performed on the second information recording surface RL2 of the BD will be described. In such a case, the positive lens group L2 of the coupling lens CL is moved to the position of the alternate long and short dash line by the uniaxial actuator AC1. Here, the divergent light beam of the light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD passes through the negative lens group L3 of the collimator lens CL, increases the divergence angle, passes through the polarizing prism PBS, and is further positive. After passing through the lens unit L2 to be a substantially parallel light beam, it is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, the light beam diameter is regulated by a diaphragm (not shown), and the objective lens OBJ has the second thickness. This is a spot formed on the second information recording surface RL2 as shown by the alternate long and short dash line through the protective substrate PL2 having a thickness (thicker than the first thickness).

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

Next, a case where recording / reproduction is performed on the third information recording surface RL3 of the BD will be described. In such a case, the positive lens group L2 of the coupling lens CL is moved to the dotted line position by the uniaxial actuator AC1. Here, the divergent light beam of the light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD passes through the negative lens group L3 of the collimator lens CL, increases the divergence angle, passes through the polarizing prism PBS, and is further positive. After passing through the lens unit L2 to be a weak divergent light beam, it is converted from linearly polarized light into circularly polarized light by the λ / 4 wave plate QWP, the diameter of the light beam is regulated by a diaphragm (not shown), and the third thickness is obtained by the objective lens OBJ. This is a spot formed on the third information recording surface RL3 as shown by the dotted line through the protective substrate PL3 (thicker than the second thickness).

The reflected light beam modulated by the information pits on the third information recording surface RL3 is again transmitted through the objective lens OBJ and the aperture, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. After passing through the positive lens unit L2 to be a convergent light beam and reflected by the polarizing prism PBS, it is converged on the light receiving surface of the light receiving element PD by the sensor lens SL. Then, using the output signal of the light receiving element PD, the information recorded on the third information recording surface RL3 can be read by focusing or tracking the objective lens OBJ by the triaxial actuator AC2.

In the above embodiment, the negative lens group in the coupling lens may be disposed on the objective lens side, and the positive lens group may be disposed on the light source side.

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

Furthermore, in the above embodiment, the focal length of the objective lens OBJ is preferably in the range of 1.0 mm to 1.8 mm. Thereby, an optical pickup device suitable for a thin optical pickup device that is required to be downsized can be obtained.

Next, examples of a condensing optical system that can be used in the above-described embodiment will be described below. The focal length of the objective lens of the condensing optical system is 1.41 mm, the focal length of the positive lens of the coupling lens is 8 mm, and the focal length of the entire coupling lens system is 14.1 mm. The magnification of the system is -0.1. Further, the design wavelength of the condensing optical system is 405 nm, ri in the table below is the radius of curvature, di is the position in the optical axis direction from the i-th surface to the i + 1-th surface, and ndi is each of the d-line (587.6 nm). The refractive index of the surface, n405i represents the refractive index of each surface at the design wavelength of 405 nm, and νd represents the Abbe number in the d-line. Further, the objective lens in the example is made of glass and SK5 manufactured by HOYA Corporation is used. In the following (including the lens data in the table), a power of 10 (for example, 2.5 × 10 ⁻³ ) is represented by using E (for example, 2.5 × E−3). The optical surface of the objective lens is formed as an aspherical surface that is axisymmetric about the optical axis, each of which is defined by an equation in which the coefficient shown in Table 1 is substituted into Equation (1).

Here, X (h) is an axis in the optical axis direction (the light traveling direction is positive), κ is a conical coefficient, A _i is an aspherical coefficient, h is a height from the optical axis, and r is a paraxial curvature. Radius.
Example 1
Tables 1 and 2 show the lens data of Example 1. This embodiment is composed of a two-group coupling lens having a negative lens and a positive lens, and an objective lens. The objective lens in this example has a spherical aberration and a sine condition violation amount (with a combination of a protective substrate thickness of 0.075 mm and a magnification of zero (corresponding to the parallel light beam entering the objective lens) ( The shape of the optical surface is determined so that (see FIG. 7) is corrected satisfactorily. Further, the sine condition violation amount of the objective lens in this state is shown in FIG.

Table 1 shows a state where information is recorded and / or reproduced on the information recording surface (RL1) having a distance of 0.05 mm from the light incident surface in this embodiment. When recording and / or reproducing information with respect to RL1, the positive lens is moved toward the objective lens so that the light beam emitted from the positive lens becomes a weak convergent light beam, and from the light beam incident surface to the information recording surface. The third-order spherical aberration associated with the change in the distance is corrected.

On the other hand, Table 2 shows a state where information is recorded and / or reproduced on the information recording surface (RL3) having a distance of 0.1 mm from the light incident surface in the present embodiment. When recording and / or reproducing information on an information recording surface whose distance from the light beam incident surface is 0.1 mm, the light beam emitted from the positive lens is weak by moving the positive lens to the light source side. A divergent light beam is used to correct third-order spherical aberration associated with a change in the distance from the light beam incident surface to the information recording surface.
(Example 2)
Tables 3 and 4 show lens data of Example 2. This embodiment is composed of a two-group coupling lens having a negative lens and a positive lens, and an objective lens. The coupling lens in this example is the same as that in Example 1 described above. In the objective lens in this example, spherical aberration is satisfactorily corrected by a combination of a protective substrate thickness of 0.075 mm and a magnification of zero (corresponding to a parallel light beam entering the objective lens). The coma aberration correction state is set so that the sine condition violation amount of the objective lens has a positive maximum value between 60% and 90% of the effective radius, and the sine condition violation amount decreases monotonously at the periphery. Has been. Also, the sine condition violation amount of the objective lens in this state is shown in FIG.

Table 3 shows a state where information is recorded and / or reproduced on the information recording surface (RL1) whose distance from the light incident surface in the present embodiment is 0.05 mm. When recording and / or reproducing information with respect to RL1, the positive lens is moved toward the objective lens so that the light beam emitted from the positive lens becomes a weak convergent light beam, and from the light beam incident surface to the information recording surface. The third-order spherical aberration associated with the change in the distance is corrected.

On the other hand, Table 4 shows a state where information is recorded and / or reproduced on the information recording surface (RL3) having a distance of 0.1 mm from the light incident surface in the present embodiment. When recording and / or reproducing information on an information recording surface whose distance from the light beam incident surface is 0.1 mm, the light beam emitted from the positive lens is weak by moving the positive lens to the light source side. A divergent light beam is used to correct third-order spherical aberration associated with a change in the distance from the light beam incident surface to the information recording surface.
(Example 3)
Tables 5 and 6 show the lens data of Example 3. This embodiment is composed of a two-group coupling lens having a negative lens and a positive lens, and an objective lens. The coupling lens in this example is the same as that in Example 1 described above. The objective lens in this example has a spherical aberration and a sine condition violation amount in a combination of a protective substrate thickness of 0.090 mm and a magnification of zero (corresponding to a parallel light beam entering the objective lens). The optical surface shape is determined so that (see FIG. 7) is corrected satisfactorily. Further, the sine condition violation amount of the objective lens in this state is shown in FIG.

Table 5 shows a state where information is recorded and / or reproduced on the information recording surface (RL1) having a distance of 0.05 mm from the light incident surface in this comparative example. When recording and / or reproducing information with respect to RL1, the positive lens is moved toward the objective lens so that the light beam emitted from the positive lens becomes a weak convergent light beam, and from the light beam incident surface to the information recording surface. The third-order spherical aberration associated with the change in the distance is corrected.

On the other hand, Table 6 shows a state where information is recorded and / or reproduced on the information recording surface (RL3) having a distance of 0.1 mm from the light incident surface in this embodiment. When recording and / or reproducing information on an information recording surface whose distance from the light beam incident surface is 0.1 mm, the light beam emitted from the positive lens is weak by moving the positive lens to the light source side. A divergent light beam is used to correct third-order spherical aberration associated with a change in the distance from the light beam incident surface to the information recording surface.

Table 7 shows the amount of movement of the positive lens when the information is recorded and / or reproduced with respect to RL1 in Example 1, Example 2, and Example 3 (the zero point of the amount of movement is relative to the objective lens). The position of the positive lens when a parallel light beam is incident, “+” when the positive lens moves toward the objective lens with respect to the zero point, and “−” when the positive lens moves in the direction closer to the light source The absolute value of the third-order spherical aberration (λrms) and fifth-order spherical aberration (λrms) with the positive lens position optimized, and a light beam tilted by 0.5 degrees with respect to the objective lens. The absolute value of the third-order coma aberration (λrms) generated in the objective lens and the absolute value of the fifth-order coma aberration (λrms), and the absolute value of the third-order coma aberration (λrms) generated when the objective lens is tilted by 0.5 degrees. Value and absolute value of fifth-order coma aberration (λrms), optical data Indicating the absolute value and the absolute value of the fifth-order coma aberration ([lambda] rms) of the third-order coma aberration generated by tilting the disk 0.5 degrees ([lambda] rms).

Table 8 shows the amount of movement of the positive lens when the information is recorded and / or reproduced with respect to the RL 3 in Example 1, Example 2 and Example 3 (the zero point of the amount of movement is the object lens). On the other hand, the position of the positive lens when a parallel light beam is incident is defined as “+” when the positive lens moves toward the objective lens with respect to the zero point, and “+” when the positive lens moves toward the light source. -"), The absolute value of the third-order spherical aberration (λrms) and fifth-order spherical aberration (λrms) with the positive lens position optimized, and a light beam inclined by 0.5 degrees with respect to the objective lens is incident The absolute value of the third-order coma aberration (λrms) and the absolute value of the fifth-order coma aberration (λrms) generated by the objective lens, and the third-order coma aberration (λrms) generated when the objective lens is tilted by 0.5 degrees. And the absolute value of the fifth-order coma aberration (λrms) Indicating the absolute value of the absolute value of the third-order coma aberration ([lambda] rms) that occurs by tilting the optical disc 0.5 degrees and fifth-order coma aberration ([lambda] rms).

First, in Example 1, the movement amount of the positive lens in the coupling lens composed of the positive lens and the negative lens is smaller than that in the case where the coupling lens is a single collimator lens. ing. Further, as can be seen from FIG. 7, Table 7, and Table 8, the objective lens of the condensing optical system of Example 2 has an effective radius of 6 in the state in which a parallel light beam is incident on the objective lens. Between 90% and 90%, the coma aberration correction state is set so that the sine condition violation amount of the objective lens has a positive maximum value, and the sine condition violation amount monotonously decreases in the peripheral area. From the condensing optical system of Example 1, it can be seen that the moving amount of the positive lens can be further reduced by about 10%. Further, the fifth-order spherical aberration (residual aberration at the time of focus jump) with the position of the positive lens optimized is approximately 0.01λrms in the first embodiment, but is substantially zero in the second embodiment. Thus, it can be seen that Example 2 can be further improved in terms of fifth-order spherical aberration.

As can be seen from Tables 7 and 8, the objective lens of the condensing optical system of Example 3 has a protective substrate thickness of 0.090 mm when a parallel light beam is incident on the objective lens. Since the shape of the optical surface is determined so that the spherical aberration is minimized, the absolute value of the movement amount of the positive lens in the state in which information is recorded and / or reproduced with respect to RL1 is set with respect to RL3. Thus, the absolute value of the movement amount of the positive lens in a state where information is recorded and / or reproduced is reduced. Thus, when recording and / or reproducing information with respect to RL3, the third-order coma generated when the optical disk is tilted by the same amount with respect to the third-order coma aberration (LT) generated when the objective lens is tilted. The absolute value of the ratio of aberration CM (DT) could be increased to 0.73. As a result, the amount of tilt of the objective lens required to correct coma caused by warpage or tilt of the optical disk can be reduced. On the other hand, in the condensing optical system of Example 1, since the absolute value of the ratio of DT to LT is 0.36, in Example 3, in order to correct coma aberration generated due to the warp or tilt of the optical disk. It can be seen that this is a more preferable example in that the amount of tilt of the objective lens necessary for the above can be reduced.

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

OBJ Objective lens PU1 Optical pickup device PU2 Optical pickup device LD Blue-violet semiconductor laser AC1 Single-axis actuator AC2 Three-axis actuator PBS Polarizing prism CL Coupling lens L2 Positive lens group L3 Negative lens group PL1 First protective substrate PL2 Second protection Substrate PL3 Third protective substrate RL1 First information recording surface RL2 Second information recording surface RL3 Third information recording surface QWP λ / 4 wavelength plate

Claims

An optical pickup device that selects and records information and / or reproduces information by selecting any information recording surface in an optical disc having three or more information recording surfaces in the thickness direction,
A light source that emits a light beam having a wavelength of λ1 (390 nm <λ1 <415 nm), an objective lens that focuses the light beam on an information recording surface of an optical disc, and a positive light source disposed between the light source and the objective lens. A coupling lens composed of a positive lens group having a refractive power and a negative lens group having a negative refractive power,
The image side numerical aperture (NA) of the objective lens is 0.8 or more,
The objective lens is a single ball, formed from a glass material,
Information is recorded and / or reproduced by causing the light beam emitted from the light source to enter the objective lens via the coupling lens and condensing it on the selected information recording surface of the optical disc by the objective lens. Is supposed to do
When changing an information recording surface on which information is to be recorded and / or reproduced on the optical disc from one information recording surface to another information recording surface, at least one lens of the positive lens group is moved in the optical axis direction. A characteristic optical pickup device.
In a state where a parallel light beam is incident on the objective lens, the sine condition violation amount of the objective lens has a positive maximum value between 60% and 90% of the effective radius of the objective lens, and the peripheral portion The optical pickup device according to claim 1, wherein the coma aberration correction state is set so that the sine condition violation amount decreases monotonously.
In a state where a parallel light beam is incident on the objective lens, the sine condition violation amount of the objective lens has a negative maximum value between 40% to 80% of the effective radius of the objective lens, and the peripheral portion The optical pickup device according to claim 1, wherein the coma aberration correction state is set so that the sine condition violation amount decreases monotonously.
When recording and / or reproducing information on the optical disc, the objective lens can be tilted along the radial direction and / or the tangential direction of the optical disc, and the light flux incident on the optical disc The same amount of the optical disc with respect to the third-order coma aberration CM (LT) generated when the objective lens is tilted in a state where information is recorded and / or reproduced with respect to the information recording surface having the longest distance from the surface. 2. The optical pickup device according to claim 1, wherein an absolute value of a ratio of third-order coma aberration CM (DT) generated when tilted is 0.4 or more.