WO2012070438A1

WO2012070438A1 - Objective lens for optical pickup device and optical pickup device

Info

Publication number: WO2012070438A1
Application number: PCT/JP2011/076360
Authority: WO
Inventors: 立山清乃; 忍菅
Original assignee: コニカミノルタオプト株式会社
Priority date: 2010-11-26
Filing date: 2011-11-16
Publication date: 2012-05-31
Also published as: JPWO2012070438A1

Abstract

Provided are an optical pickup device and a coupling lens with which it is possible to carry out recording and playing of information on an optical disc having multi-layered information recording faces while being both compact and inexpensive. Given an objective lens such as satisfies formula (1), satisfying formula (2) allows alleviating an occurrence of high-order spherical aberration in all information recording faces when concentrating light. Furthermore, satisfying formula (3) reduces a risk that a beam will be limited at a lateral face of an optical disc, allowing maintaining a spot diameter even when a divergent beam enters the objective lens. F ≤ 1.8mm (1); TMIN*1.2 ≤ T ≤ TMAX*0.86 and TMIN*1.2 < TMAX*0.86 (2); 1.1 ≤ A2/B2 ≤ 1.6 (3); where f is the focal length of the objective lens with respect to the beam; T is the transparent substrate thickness of the optical disc such that third-order spherical aberration |SA3| ≤ 0.05λrms upon parallel beam entry; TMIN is the minimum transparent substrate thickness among the transparent substrate thicknesses of the optical disc; TMAX is the maximum transparent substrate thickness among the transparent substrate thicknesses of the optical disc; A2 is the optical surface diameter of the optical disc-side of the objective lens; and B2 is the effective diameter of the optical disc-side of the objective lens.

Description

Objective lens for optical pickup device and optical pickup device

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

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 is known. As an example, an optical disc that records and reproduces information with specifications of NA 0.85 and light source wavelength 405 nm, so-called Blu-rayｒａDisc (hereinafter referred to as BD), DVD (NA 0.6, light source wavelength 650 nm, storage) Information of 25 GB per layer can be recorded on an optical disk having a diameter of 12 cm which is the same size as a capacity of 4.7 GB.

Incidentally, in contrast to a BD having a one-layer information recording surface, a BD having a two-layer information recording surface has recently been developed and is already on the market. Here, in Patent Document 1, the magnification is changed by moving a coupling lens including two lenses arranged between a light source and an objective lens in the optical axis direction, for example, two layers (or four layers, etc.). There has been disclosed an optical pickup device that can select any one of information recording surfaces and collect a light beam with suppressed aberration.

JP 2005-129204 A JP 2004-335080 A

By the way, the optical surface of the objective lens is a region through which the light beam from the light source passes so that it can be collected. In general, the margin is set to be slightly larger than the effective diameter in order to guarantee the performance. However, a predetermined value is determined for the margin by experience, simulation, or the like. However, the present inventor has found that an objective lens for an optical disk having an information recording surface of three or more layers may not be able to form a good condensing spot with a margin that is conventionally considered appropriate. . The reason will be described.

FIG. 1 is a schematic diagram of a condensing optical system of an optical pickup device capable of selecting / recording / reproducing information by selecting an information recording surface of an optical disc OD having three or more layers of information recording surfaces. Here, the objective lens OBJ has a higher-order spherical surface when the focused spot is formed on the information recording surface RL3 having the thickest substrate thickness and when the focused spot is formed on the information recording surface RL1 having the smallest substrate thickness. The design center disk thickness is set so that the aberration becomes small in a balanced manner. That is, if the design center disk thickness is too thin, the higher-order spherical aberration at the time of condensing the information recording surface RL3 having the largest substrate thickness becomes too large. If the design center disk thickness is too thick, the information recording with the smallest substrate thickness is performed. Since the higher-order spherical aberration at the time of condensing on the surface RL1 becomes too large, the design center disk thickness is determined so as to balance it. Here, the design center disk thickness means the substrate thickness of the information recording surface on which an appropriate focused spot is formed when a parallel light beam is incident on the objective lens.

Incidentally, when each information recording surface is selected, the coupling lens CL is moved to change the divergence angle or the convergence angle of the light beam incident on the objective lens OBJ. Then, when the information recording surface RL3 having the thickest substrate is condensed, as shown by the dotted line in FIG. 1, a divergent light beam needs to be incident on the objective lens OBJ. On the other hand, at the time of condensing the information recording surface RL1 having the thinnest substrate thickness, as shown by a one-dot chain line in FIG. When the convergent light beam is incident, there is no particular problem. However, when the divergent light beam is incident, the surface S2 on the optical disk side of the objective lens OBJ has an area where the light beam is emitted further outside the effective diameter. In addition, the surface S1 of the light source of the objective lens OBJ has the same problem although not as much as the surface S2.

In particular, recently, there is a strong demand to use a BD having an information recording surface of three or more layers in a relatively thin optical pickup device called a so-called slim type that is mounted on the back of a notebook PC or a thin television. In such a slim type optical pickup device, an objective lens having a short focal length is frequently used, and the divergence angle of light incident on the objective lens at the time of information recording surface recording / reproduction having such a thick substrate as the objective lens. Therefore, there is a strong tendency that a region where light is emitted on the surface S2 on the optical disc side is widened.

In Patent Document 2, it is described that an optimum preform diameter r with respect to R of the concave surface of the mold is specified in mold press molding. Here, in all of the examples of Patent Document 2, the effective diameter and optical surface diameter of the surface S2 on the optical disk side are described, but since the substrate thickness is 0.1 mm or 0.0875 mm, the single-layer disk or 2 Obviously, it is not intended to be used for an optical disk having three or more layers, and naturally the problem of the spread of the emission area on the surface S2 side when diverging light is incident is considered. I can say no.

The present invention has been made in consideration of the above-mentioned problems, and is for 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. An objective lens and an optical pickup device are provided.

The objective lens for an optical pickup device according to claim 1 records 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. Therefore, a light source that emits a light beam having a wavelength λ1 (390 nm <λ1 <415 nm), an objective lens that collects the light beam on an information recording surface of an optical disc, and the light source and the objective lens are disposed. An objective lens for an optical pickup device having a coupled lens,
The objective lens is a single lens and satisfies the following expression.
f ≦ 1.8mm (1)
TMIN × 1.2 ≦ T ≦ TMAX × 0.86 and TMIN × 1.2 <TMAX × 0.86 (2)
1.1 ≤ A2 / B2 ≤ 1.6 (3)
However,
f: Focal length of the objective lens with respect to the luminous flux T: Transparent substrate thickness of the optical disk in which third-order spherical aberration | SA3 | ≦ 0.05λrms when parallel light is incident
TMIN: the smallest transparent substrate thickness among the transparent substrate thicknesses on the optical disc TMAX: the largest transparent substrate thickness among the transparent substrate thicknesses on the optical disc
A2: Optical surface diameter on the optical disc side of the objective lens
B2: Effective diameter of the objective lens on the optical disk side

According to the present invention, in the objective lens satisfying the expression (1), by satisfying the expression (2), it is possible to suppress the occurrence of higher-order spherical aberration when condensing on all information recording surfaces. Further, by satisfying the expression (3), even if divergent light is incident on the objective lens, the possibility that the light beam is vignetted on the side surface of the optical disk is reduced, and the spot diameter can be maintained.

More specifically, an objective lens having a focal length f satisfying the expression (1) is suitable for a so-called slim type optical pickup device. In addition, it is preferable that f is 0.9 mm or more. Further, in the formula (2), when T exceeds TMAX × 0.86, when converging on the information recording surface having the thinnest substrate thickness, the convergence angle of the convergent light beam incident on the objective lens is reduced. In the case of this coupling lens, it must be moved largely to the light source side. The slim type pickup does not have enough space around the light source for compactness, and if the coupling lens moves greatly, there is a possibility of causing interference with other parts. On the other hand, when T is less than TMIN × 1.2, when condensing on the information recording surface with the thickest substrate thickness, the divergence angle of the divergent light beam incident on the objective lens becomes too large, and coma aberration during tracking increases. At the same time, even if the low-order spherical aberration is corrected, the high-order spherical aberration remains, which leads to deterioration of the optical characteristics.

That is, as in the present invention, when T is equal to or greater than the lower limit of the expression (2), it is possible to suppress the occurrence of higher-order spherical aberration even when the light beam is condensed on the information recording surface with the thickest substrate thickness. When the value is equal to or less than the upper limit of the expression (2), it is possible to suppress the occurrence of high-order spherical aberration even when the light beam is condensed on the information recording surface having the thinnest substrate. Further, when the expression (2) is satisfied, the coupling lens does not come too close to the light source even when the light beam is condensed on the information recording surface with the thickest substrate, and is suitable for a slim type optical pickup device.

In equation (3), if A2 / B2 falls below the lower limit, the light beam emitted from the optical disk side surface of the objective lens passes outside the optical surface due to problems in the processing accuracy of the objective lens and assembly errors in the pickup. The demand for NA increases and the required NA cannot be satisfied. In particular, when a light beam is incident obliquely with respect to the optical axis, such a problem may occur remarkably. On the other hand, if A2 / B2 exceeds the upper limit of the expression (3), the outer diameter of the objective lens becomes too large to be used particularly in a slim type optical pickup device. The space of the end surface provided around the surface cannot be secured, and it becomes difficult to check using the reflected light from the end surface when the objective lens is attached to the optical pickup device.

In other words, by setting T to be equal to or less than the upper limit of the equation (2), the divergence angle is suppressed, and by setting A2 / B2 to be equal to or greater than the lower limit of the equation (3), In addition, the outer diameter of the objective lens can be reduced while suppressing the vignetting of light on the surface on the optical disc side, although the information can be collected with high precision so that information can be recorded / reproduced. It is. On the other hand, by setting A2 / B2 to be equal to or less than the upper limit of the expression (3), it is possible to secure the space of the end surface provided around the optical surface of the objective lens. It can be attached with high accuracy.

The optical pickup device according to claim 2 is characterized in that, in the invention according to claim 1, the objective lens is made of glass. If the objective lens is made of glass, it has excellent heat resistance and light resistance. In particular, aberrations that occur when the temperature changes are reduced, and there is no need to perform magnification correction due to temperature changes. Therefore, the amount of movement of the coupling lens can be reduced, and the enlargement of the emission area on the side surface of the optical disk can be suppressed.

According to a third aspect of the present invention, there is provided the optical pickup device according to the first or second aspect, wherein the following expression is satisfied.
1.15 <A2 / B2 <1.3 (5A)

By making A2 / B2 larger than the lower limit of the formula (5A), it is possible to secure a sufficient optical surface diameter with respect to the effective diameter in the glass objective lens, and to guarantee the performance within the effective diameter. On the other hand, by making A2 / B2 smaller than the upper limit of the formula (5A), the outer diameter of the objective lens can be reduced while securing a wider end face. As described above, the glass objective lens can reduce the amount of movement of the coupling lens as much as it is not necessary to perform magnification correction due to temperature change, so that the emission area on the optical surface on the optical disk side can be kept small. Therefore, the upper limit can be kept small as in the equation (5A).

In this specification, the “optical surface” of the objective lens is a surface having an optical function of condensing a light beam, and generally refers to a region inside the end surface. The “end surface” of the objective lens is generally a surface orthogonal to the optical axis. When the objective lens is assembled to the optical pickup device, the light beam is emitted from the inspection device and the reflected light from the end surface is used. Often used for positioning. The optical surface and the end surface may be in contact with each other, but a small boundary portion may be provided between them. The “effective diameter” of an objective lens is an area in an optical surface through which a light beam used for recording / reproduction passes when a parallel light beam is incident on the objective lens. , An area in the optical plane through which the parallel light flux that has passed through the inside of the aperture limiting element passes. Therefore, the “effective diameter on the light source side” is the inner diameter of the stop.

The optical pickup device according to claim 4 is characterized in that, in the invention according to claim 1, the objective lens is made of plastic. By making the objective lens made of plastic, it can be made light and inexpensive. The workability of the diffractive structure is excellent, and BD, DVD, and CD can be interchanged using diffraction.

An optical pickup device according to a fifth aspect of the invention according to the fourth aspect of the invention satisfies the following expression.
1.2 <A2 / B2 <1.4 (3 ')

By making A2 / B2 larger than the lower limit of the expression (3 ′), it is possible to secure a more sufficient optical surface diameter with respect to the effective diameter and to guarantee the performance within the effective diameter. On the other hand, by making A2 / B2 smaller than the upper limit of the expression (3 '), the outer diameter of the objective lens can be reduced while securing a wider end face. When the objective lens is made of plastic, the refractive index change due to temperature change tends to be larger than that of glass.Therefore, when focusing on the information recording surface with the thickest substrate at a high temperature, the divergence angle is larger. A large divergent light beam needs to be incident. Therefore, when the objective lens is made of plastic, it is desirable to satisfy the expression (3 ′).

An optical pickup device according to a sixth aspect of the invention is characterized in that, in the invention according to any one of the first to fifth aspects, the following expression is satisfied.
1.0 ≤ A1 / B1 ≤ 1.1 (4)
However,
A1: Optical surface diameter on the light source side of the objective lens
B1: Effective diameter of the objective lens on the light source side

Since the optical surface on the light source side of the objective lens is closer to the diaphragm than the optical surface on the optical disk side, the influence at the time of incidence of the convergent light beam or divergent light beam is small, and it can be said that the A1 / B1 involvement is relatively small. However, a better objective lens can be obtained by designing the optical surface on the light source side together with the design of the optical surface on the optical disc side. More specifically, by setting A1 / B1 to be equal to or greater than the lower limit of the expression (4), it is possible to secure a further sufficient optical surface diameter with respect to the effective diameter and to guarantee the performance within the effective diameter. By setting A1 / B1 to be equal to or less than the upper limit of the formula (4), the outer diameter of the objective lens can be reduced. In an objective lens having a high NA, the optical surface on the light source side has a large expected angle of the surface and is often difficult to mold. However, this makes it possible to reduce the expected angle and improve manufacturability.

An optical pickup device according to a seventh aspect includes the objective lens according to any one of the first to sixth aspects and a coupling lens, and the optical disk is moved by moving the coupling lens in an optical axis direction. One of the information recording surfaces is selected.

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 (5), (6), and (7), 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 (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. 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. A light receiving element may be used. 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 that is arranged between the objective lens and the light source and changes the divergence angle of the light beam. The coupling lens may be composed of a single positive lens or may have a positive lens and a negative lens. In the case of having a plurality of lenses, the positive lens has at least one positive lens. The positive lens may be a single positive lens or may have a plurality of lenses. The negative lens has at least one negative lens. The negative lens may be a single negative lens or a plurality of lenses. The positive lens and the negative lens may be arranged in the order of the negative lens and the positive lens from the light source side, or may be arranged in the order of the positive lens and the negative lens from the light source side.

In order to correct the spherical aberration generated on the selected information recording surface of the first optical disc, the coupling lens (a negative lens or a positive lens when a positive lens and a negative lens are provided) can be moved in the optical axis direction. ing. For example, when recording and / or reproducing on one information recording surface of the first optical disc and then recording and / or reproducing on another information recording surface of the first optical disc, the coupling lens moves in the optical axis direction. Then, by changing the divergence of the light beam and changing the magnification of the objective lens, the spherical aberration generated at the time of focus jump to a different information recording surface of the first optical disc is corrected.

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 lens. The objective optical element may be a glass lens, a plastic lens, or a hybrid lens in which a diffractive structure or the like is provided on a glass lens with a photocurable resin or the like. The objective optical element preferably has a refractive surface that is aspheric. Further, when the objective lens is provided with an optical path difference providing structure, the base surface is preferably an aspherical surface.

If the objective lens is a glass lens, it is not necessary to move the coupling lens to correct spherical aberration caused by temperature changes, so the amount of movement of the coupling lens can be reduced, and the optical pickup device can be downsized. This is preferable because it is possible.

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.

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 weight increases and a load is imposed on the actuator that drives the objective lens. Therefore, when the objective lens is a glass lens, it is preferable to use a glass material having a small specific gravity. Specifically, the specific gravity is preferably 4.0 or less, more preferably the specific gravity is 3.0 or less.

In addition, one of the important physical properties when molding and manufacturing 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 larger than that of a plastic 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.

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

As the plastic, cycloolefin resin is preferably used. Specifically, ZEONEX manufactured by Nippon Zeon Co., Ltd., APEL manufactured by Mitsui Chemicals, Inc., TOPAS® ADVANCED® POLYMERS manufactured by TOPAS, JSR manufactured by ARTON, etc. are preferable examples. Can be mentioned.

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

The objective lens satisfies the following formula.
f ≦ 1.8mm (1)
TMIN × 1.2 ≦ T ≦ TMAX × 0.86 and TMIN × 1.2 <TMAX × 0.86 (2)
1.1 ≤ A2 / B2 ≤ 1.6 (3)
However,
f: Focal length of objective lens with respect to luminous flux T: Thickness of transparent substrate of optical disk satisfying third-order spherical aberration | SA3 | ≦ 0.05λrms when parallel light is incident (also referred to as design center disk thickness)
TMIN: the smallest transparent substrate thickness among the transparent substrate thicknesses on the optical disc TMAX: the largest transparent substrate thickness among the transparent substrate thicknesses on the optical disc
A2: Optical surface diameter on the optical disc side of the objective lens
B2: Effective diameter of the objective lens on the optical disc side

Furthermore, when the objective lens is made of glass, it is preferable that the following expression is satisfied.
1.15 ≤ A2 / B2 ≤ 1.25 (3 ')

On the other hand, when the objective lens is made of plastic, it is preferable that the following expression is satisfied.
1.2 <A2 / B2 <1.4 (3 ')

Furthermore, it is preferable that the following formula is satisfied.
1.0 ≤ A1 / B1 ≤ 1.1 (4)
However,
A1: Optical surface diameter on the light source side of the objective lens
B1: Effective diameter on the light source side of the objective lens

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 an objective lens satisfy | fills the following conditional expression (10).
0.9 ≦ d / f ≦ 1.5 (10)
Here, d represents the thickness (mm) on the optical axis of the objective lens, and f represents the focal length of the objective lens in the first light flux. Note that f is preferably 0.9 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 (10), it is possible to suppress the generation of astigmatism and the surface shift sensitivity.

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

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

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

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

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

According to the present invention, it is possible to provide an optical pickup device and a coupling lens that are 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 the schematic of the condensing optical system for demonstrating this invention. It is a figure which shows schematically the structure of optical pick-up apparatus PU1. It is a figure which shows the value of the said Example with a reference example, taking a focal distance on a horizontal axis and taking ratio of (effective diameter / optical surface diameter) on a vertical axis | shaft.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 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 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. 2 shows a BD-dedicated optical pickup device, 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 aperture AP, and the λ / 4 wavelength plate QWP, rising mirror MR, coupling CL having positive lens L2 having positive refractive power and negative lens L3 having negative refractive power, uniaxial actuator AC1 for moving only positive lens L2 in the optical axis direction, polarizing prism A PBS, a semiconductor laser LD that emits a laser beam (beam) of 405 nm, a sensor lens SL, and a light receiving element PD that receives reflected beams from the information recording surfaces RL1 to RL3 of the BD.

In the present embodiment, the coupling lens CL is disposed between the polarizing prism PBS and the λ / 4 wavelength plate QWP. The semiconductor laser LD is arranged in the order of the negative lens L3 and the positive lens L2. However, the semiconductor laser LD may be arranged in the order of the positive lens L2 and the negative lens L3. The positive lens L2 is movable in the optical axis direction, and the negative lens L3 is fixed to the optical pickup device.

Referring to FIG. 1, a single objective lens OBJ has an optical surface OP1 formed on a light source side surface S1, and an end surface EP1 formed around the optical surface OP1. The objective lens OBJ has an optical surface OP2 formed on the surface S2 on the optical disc side, and an end surface EP2 formed around the optical surface OP2. The end surface EP2 is orthogonal to the optical axis X, and when the objective lens OBJ is assembled to the optical pickup device PU1, the inspection light is irradiated from the inspection device (not shown) toward the end surface EP2, and the reflected light thereof. Positioning is performed by receiving light. There may be a case where a flange is further provided outside the end faces EP1 and EP2.

The objective lens OBJ satisfies the following formula.
f ≦ 1.8mm (1)
TMIN × 1.2 ≦ T ≦ TMAX × 0.86 and TMIN × 1.2 <TMAX × 0.86 (2)
1.1 ≤ A2 / B2 ≤ 1.6 (3)
1.0 ≤ A1 / B1 ≤ 1.1 (4)
However,
f: Focal length of the objective lens OBJ with respect to the luminous flux T: Transparent substrate thickness of the optical disk with third-order spherical aberration | SA3 | ≦ 0.05λrms when parallel light is incident
TMIN: the smallest transparent substrate thickness of the transparent substrate thickness in the optical disk (here, the substrate thickness of the information recording surface RL1)
TMAX: The maximum transparent substrate thickness among the transparent substrate thicknesses in the optical disk (here, the substrate thickness of the information recording surface RL3)
A2: Optical surface diameter on the optical disc side in the objective lens OBJ
B2: Effective diameter on the optical disc side in the objective lens OBJ
A1: Optical surface diameter on the light source side in the objective lens OBJ
B1: Effective diameter on the light source side of the objective lens OBJ = inner diameter of the aperture AP

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 L2 of the coupling lens CL is moved to the position of the alternate long and short dash line by the uniaxial actuator AC1. Here, the divergent beam of the beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD is transmitted through the polarizing prism PBS, passes through the negative lens L3 of the collimator lens CL, and the divergence angle is increased. After passing through L2 to be a weakly convergent light beam, it is reflected by the rising mirror MR, converted from linearly polarized light to circularly polarized light by the λ / 4 wave plate QWP, the diameter of the light beam is regulated by the stop AP, and the objective lens OBJ As a result, a spot is formed on the first information recording surface RL1 through the transparent substrate PL1 having the first thickness, as indicated by a one-dot chain line.

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

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

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

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

The reflected light beam modulated by the information pits on the third information recording surface RL3 is again transmitted through the objective lens OBJ and the diaphragm, and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP, and the rising mirror MR , And passes through the positive lens L2 and the negative lens L3 of the collimator lens CL to be a convergent light beam. After being 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.

(Example)
Next, examples of coupling lenses and objective lenses that can be used in the above-described embodiment will be described below. Here, the design wavelength is 405 nm, ri in the following table is the radius of curvature, d is the position in the optical axis direction between the surfaces, and n is the refractive index of each surface at the design wavelength of 405 nm. In the following (including the lens data in the table), a power of 10 (for example, 2.5 × 10 ⁻³ ) is expressed by using E (for example, 2.5 × E-3). The optical surface of the objective lens is formed as an aspherical surface that is axisymmetric about the optical axis, each of which is defined by a mathematical formula obtained by substituting the coefficient shown in Table 1 into Formula 1.

Here, X (h) is an axis in the optical axis direction (the light traveling direction is positive), κ is a conical coefficient, A _i is an aspheric coefficient, h is a height from the optical axis, and r is a paraxial curvature. Radius.

The following examples are objective lenses for BD having four layers of information recording surfaces, and the transparent substrate thickness of each layer is 0.535 mm, 0.07 mm, 0.0875 mm, and 0.1 mm. Since Examples 1 to 3 have a smaller T than Examples 4 to 6, light is incident to the outside of the effective diameter when recording and reproducing the thickest layer. In addition, in Examples 1 to 3, since a plastic material is used, when the temperature changes (high temperature), the refractive index decreases, so that the light beam enters more outward. The optical surface diameter of Examples 1 to 3 is set to be larger than that of Examples 4 to 6 so that the light beam is not kicked.

(Examples 1 to 3)
Table 1 shows lens data of Examples 1 to 3. The objective lens of the present example has a focal length f = 1.14 mm, is made of plastic, and has an aspheric surface obtained by substituting the aspheric coefficient in Table 1 into Equation 1, but A1 / B1, The value of A2 / B2 is changed in Examples 1 to 3. In Example 2, since the effective diameter is equal to the optical surface diameter on the optical surface on the light source side, the lens surface has a diaphragm function.

(Examples 4 to 6)
Table 2 shows lens data of Examples 4 to 6. The objective lens of this example has a focal length f = 1.14 mm, is made of glass, and has an aspherical surface obtained by substituting the aspherical coefficient in Table 2 into equation (1). The value of A2 / B2 is changed in Examples 4-6. In Example 4, since the effective diameter is equal to the optical surface diameter on the optical surface on the light source side, the lens surface has a diaphragm function.

Table 3 summarizes numerical values corresponding to conditional expressions (2) to (4) for each example. FIG. 3 is a graph showing the values of the above-described embodiment, with the horizontal axis representing the focal length and the vertical axis representing the ratio of (effective diameter / optical surface diameter).

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

OBJ Objective lens PU1 Optical pickup device LD Blue-violet semiconductor laser AC1 1-axis actuator AC2 3-axis actuator AP Aperture PBS Polarizing prism CL Coupling lens L2 Positive lens L3 Negative lens 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

In order to record and / or reproduce information by selecting any information recording surface in an optical disc having three or more information recording surfaces in the thickness direction, a light beam having a wavelength λ1 (390 nm <λ1 <415 nm) is used. An objective lens for an optical pickup device, comprising: an emitted light source; an objective lens for condensing the luminous flux on an information recording surface of an optical disc; and a coupling lens disposed between the light source and the objective lens Because
The objective lens is a single lens and satisfies the following expression.
f ≦ 1.8mm (1)
TMIN × 1.2 ≦ T ≦ TMAX × 0.86 and TMIN × 1.2 <TMAX × 0.86 (2)
1.1 ≤ A2 / B2 ≤ 1.6 (3)
However,
f: Focal length of the objective lens with respect to the luminous flux T: Transparent substrate thickness of the optical disk in which third-order spherical aberration | SA3 | ≦ 0.05λrms when parallel light is incident
TMIN: the smallest transparent substrate thickness among the transparent substrate thicknesses on the optical disc TMAX: the largest transparent substrate thickness among the transparent substrate thicknesses on the optical disc
A2: Optical surface diameter on the optical disc side of the objective lens
B2: Effective diameter of the objective lens on the optical disk side
The objective lens according to claim 1, wherein the objective lens is made of glass.
The objective lens according to claim 2, wherein the following expression is satisfied.
1.15 <A2 / B2 <1.3 (5A)
The objective lens according to claim 1, wherein the objective lens is made of plastic.
The objective lens according to claim 4, wherein the following expression is satisfied.
1.2 <A2 / B2 <1.4 (3 ')
The objective lens according to claim 1, wherein the following expression is satisfied.
1.0 ≤ A1 / B1 ≤ 1.1 (4)
However,
A1: Optical surface diameter on the light source side of the objective lens
B1: Effective diameter of the objective lens on the light source side
The objective lens according to claim 1, and a coupling lens, wherein the information recording surface of the optical disc is selected by moving the coupling lens in the optical axis direction. An optical pickup device characterized by that.