WO2011148832A1

WO2011148832A1 - Objective lens for optical pickup device, optical pickup device, and optical information read and write device

Info

Publication number: WO2011148832A1
Application number: PCT/JP2011/061366
Authority: WO
Inventors: 井上寿志; 中村健太郎
Original assignee: コニカミノルタオプト株式会社
Priority date: 2010-05-24
Filing date: 2011-05-18
Publication date: 2011-12-01
Also published as: CN102906815A; JPWO2011148832A1

Abstract

Disclosed is an objective lens which is suitable for slim optical pickup devices that ensure compatibility between two types of optical discs. The objective lens has a reduced diameter, an effective diameter φ1(mm) that satisfies Equation (1) below when a first optical disc is used, and a focal length of f(mm). The objective lens can be adapted to extend a working distance WD2(mm) for use with a second optical disc to satisfy Equation (2) below by imparting negative paraxial diffraction power using diffracted beams of light of the same order produced in a first optical-path difference imparting structure. Furthermore, both the first and a second optical-path difference imparting structures have no diffraction structure superimposed thereon, thus being simplified in the shape of optical planes to facilitate manufacturing. Furthermore, using diffracted beams of light of the same order produced in the first optical-path difference imparting structure allows for maintaining in a preferred condition spherical aberration which may be caused by variations in wavelength of a light source or variations in environmental temperature. 1.7≤φ1≤2.9 (1) 0.10≤WD2/f≤0.42 (2)

Description

Objective lens for optical pickup device, optical pickup device and optical information recording / reproducing device

The present invention relates to an optical pickup device, an objective lens, and an optical information recording / reproducing apparatus capable of recording and / or reproducing (recording / reproducing) information interchangeably with different types of optical discs.

In recent years, in an optical pickup device, a laser light source used as a light source for reproducing information recorded on an optical disc and recording information on the optical disc has been shortened. For example, a wavelength 390 such as a blue-violet semiconductor laser is used. A laser light source of ˜420 nm has been put into practical use. When these blue-violet laser light sources are used, it is possible to record 15 to 20 GB of information on an optical disk having a diameter of 12 cm when an objective lens having the same numerical aperture (NA) as that of a DVD (digital versatile disk) is used. When the NA of the objective optical element is increased to 0.85, 23 to 25 GB of information can be recorded on an optical disk having a diameter of 12 cm.

BD (Blu-ray Disc) is an example of an optical disc that uses an NA 0.85 objective lens as described above. Since the coma generated due to the tilt (skew) of the optical disk increases, the BD has a thinner protective substrate (0.1 mm with respect to 0.6 mm of DVD) than the case of the DVD cage, and is caused by skew. The amount of coma is reduced.

By the way, simply saying that information can be recorded / reproduced appropriately with respect to a BD cannot be said to be sufficient as a product of an optical disc player / recorder (optical information recording / reproducing apparatus). In light of the fact that DVDs that record a wide variety of information are currently being sold, it is not only possible to record / reproduce information with respect to BDs. For example, DVDs owned by users are equally appropriate. In addition, making it possible to record / reproduce information leads to an increase in commercial value as an optical disc player / recorder for BD. From such a background, an optical pickup device mounted on an optical disc player / recorder for BD may have a performance capable of appropriately recording / reproducing information while maintaining compatibility with both BD and DVD. desired.

As a method for appropriately recording / reproducing information while maintaining compatibility with BD and DVD, the recording density of an optical disc that records / reproduces information between the optical system for BD and the optical system for DVD Although a method of selectively switching according to the above is conceivable, a plurality of optical systems are required, which is disadvantageous for miniaturization and increases the cost.

Accordingly, in order to simplify the configuration of the optical pickup device and to reduce the cost, the optical pickup device can be used by sharing the optical system for BD and the optical system for DVD in a compatible optical pickup device. It is preferable to reduce the number of optical components constituting the lens as much as possible. And, it is most advantageous to simplify the configuration of the optical pickup device and to reduce the cost to make the objective lens arranged facing the optical disc in common. In order to obtain a common objective lens for a plurality of types of optical disks having different recording / reproducing wavelengths, it is desirable to form a diffractive structure having a wavelength dependency of spherical aberration in the objective lens.

Here, in Patent Document 1, information can be recorded / reproduced to be compatible with BD and DVD using an objective lens in which a phase structure is formed in the central region and the peripheral region is aspherical. An optical pickup device is described.

Japanese Patent No. 4404092

By the way, in an optical pickup device capable of recording / reproducing information on a BD or DVD, a notebook type PC is used instead of a relatively thick type called a so-called half-height mounted on a stationary recorder or the like that has been conventionally used. In addition, a relatively thin optical pickup device called a so-called slim type that is mounted on the back of a thin TV or the like has been developed. In the slim type optical pickup device, it is necessary to reduce the effective diameter and focal length of the objective lens to be compact as compared with the conventional half-height type.

Here, in the three types of embodiments disclosed in Patent Document 1, the effective diameter of the objective lens is 3 mm, and the working distance when using the DVD is 0.284 mm to 0.330 mm. φ3 mm is an objective lens of a size often used in the above-described half-height type. However, if the effective diameter is simply reduced so that the objective lens of Patent Document 1 can be mounted on a slim type optical pickup device, the working distance when using a DVD is correspondingly shortened, and the rotating optical disc has warpage, etc. There is a risk of causing interference with the objective lens. Moreover, the objective lens on which the phase structure disclosed in Patent Document 1 is superimposed has a complicated optical surface shape, and there is a problem that it is difficult to manufacture particularly for a small-diameter objective lens. However, in the objective lens in which the superposition of the phase structure is stopped and only the single structure shown in Patent Document 1 is used, when the objective lens is a plastic lens, the spherical aberration is good when a temperature change occurs. There is a problem that it cannot be corrected.

The present invention is intended to solve the above-mentioned problems, and in the case where two different optical disks, BD and DVD, are used interchangeably, a sufficient working distance can be ensured particularly for a DVD while having a small diameter. An objective lens for an optical pickup device, an optical pickup device, and an optical information recording / reproducing device, which are easy to manufacture and have stable performance even when temperature changes, but do not greatly deteriorate wavelength characteristics. For the purpose.

The objective lens according to claim 1 includes a first light source that emits a first light beam having a first wavelength λ1 (nm) (390 ≦ λ1 ≦ 415), and a second wavelength λ2 (nm) (630 ≦ λ2 ≦ 670). A second light source that emits the second light flux, and records and / or reproduces information of a BD having a protective substrate with a thickness of t1 using the first light flux, and uses the second light flux. An objective lens used in an optical pickup device for recording and / or reproducing information of a DVD having a protective substrate having a thickness t2 (t1 <t2),
The objective lens is a single ball,
The optical surface of the objective lens has a central region and a peripheral region around the central region,
The central region has a first optical path difference providing structure,
The objective lens condenses the first light flux that passes through the central region so that information can be recorded and / or reproduced on the information recording surface of the BD, and the second light flux that passes through the central region. Are collected so that information can be recorded and / or reproduced on the information recording surface of the DVD,
The objective lens condenses the first light flux passing through the peripheral area so that information can be recorded and / or reproduced on the information recording surface of the BD, and the second light flux passes through the peripheral area. Is not condensed so that information can be recorded and / or reproduced on the information recording surface of the DVD,
The first optical path difference providing structure makes the Nth-order diffracted light quantity of the first light beam that has passed through the first optical path difference providing structure larger than any other order diffracted light quantity, and passes through the first optical path difference providing structure. The Nth-order diffracted light amount of the second light flux is larger than any other order diffracted light amount,
The following formula is satisfied.
1.7 ≦ φ1 ≦ 2.9 (1)
0.10 ≦ WD2 / f ≦ 0.42 (2)
However,
φ1: Effective diameter of the objective lens when using the BD (mm)
WD2: Working distance of the objective lens when using the DVD (mm)
f: Focal length (mm) of the objective lens in the first light flux

As a result of earnest research, the inventor has the diffraction order of the diffracted light of the first light flux most frequently generated in the first optical path difference providing structure, even when the effective diameter is small enough to satisfy the formula (1), By making the diffraction order of the diffracted light of the second light beam the same order, it becomes possible to give a negative paraxial power. As a result, while achieving compatibility between BD and DVD, and even when using DVD And found that working distance can be increased. The working distance of the objective lens of the present invention satisfies the formula (2), and is suitable for a so-called slim type optical pickup device. Further, in the present invention, the first optical path difference providing structure does not have to be a structure in which a plurality of optical path difference providing structures are superimposed, that is, the objective lens can be easily manufactured with a relatively simple optical surface shape. By using the same-order diffracted light generated in the one-optical path difference providing structure, spherical aberration when the wavelength is changed to a long wavelength can be made under, and the environmental temperature is changed even if the objective lens is made of plastic. It was found that the spherical aberration that sometimes occurs can be maintained well. Further, the present inventor has also found out that the wavelength characteristic is not greatly deteriorated while the temperature characteristic is improved as described above.

Explaining the significance of the upper and lower limits of the conditional expression, if the value of the expression (1) is less than the upper limit, the objective lens is suitable for a so-called slim type optical pickup device, while the value of the expression (1) is more than the lower limit. Thus, it is possible to prevent the working distance when using the second optical disk from becoming too short. Further, if the value of the expression (2) is not more than the upper limit, the pitch of the first optical path difference providing structure does not become too small, so that the objective lens can be easily manufactured, and the axial chromatic aberration can be prevented from becoming too large. . On the other hand, if the value of the expression (2) is equal to or greater than the lower limit, the working distance when using the second optical disk can be secured, so that the possibility of interference between the optical disk and the objective lens can be reduced. *

The objective lens according to claim 2 is characterized in that, in the invention according to claim 1, the first optical path difference providing structure is not a structure in which a plurality of optical path difference providing structures are superimposed. According to the present invention, it is possible to obtain an objective lens having a simple shape that is easy to manufacture.

According to a third aspect of the present invention, there is provided the objective lens according to the second aspect, wherein the first optical path difference providing structure comprises only a blaze type structure.

The objective lens according to claim 4 is characterized in that | N | = 1 in the invention according to any one of claims 1 to 3. According to the present invention, since the height of the step of the first optical path difference providing structure can be reduced, an objective lens that is easy to manufacture can be obtained, and fluctuations in diffraction efficiency during wavelength fluctuations can be suppressed to a low level.

The objective lens described in claim 5 is characterized in that, in the invention described in claim 4, N = + 1. The first-order diffracted light amount of the first light beam that has passed through the first optical path difference providing structure is made larger than any other order of diffracted light amount, and the first-order diffraction of the second light beam that has passed through the first optical path difference providing structure. This is because the highest diffraction efficiency can be obtained when the amount of light is larger than any other order of the amount of diffracted light. According to the examination result of the present inventor, the first-order diffracted light quantity of the first light beam that has passed through the first optical path difference providing structure is made larger than any other order diffracted light quantity and passed through the first optical path difference providing structure. When the first-order diffracted light amount of the second light beam is larger than any other order diffracted light amount, the diffraction efficiency of the first-order diffracted light beam of the first light beam is 89.54%, and the first-order diffracted light beam of the second light beam The diffraction efficiency of the diffracted light was 78.17%. On the other hand, the second-order diffracted light amount of the first light beam that has passed through the first optical path difference providing structure is made larger than any other order of diffracted light amount, and the second-order secondary light beam that has passed through the first optical path difference providing structure. When the design is changed to the first optical path difference providing structure in which the diffracted light quantity of the first light beam is larger than any other order diffracted light quantity, the diffraction efficiency of the second-order diffracted light of the first light flux is 76.17%. The diffraction efficiency of the second-order diffracted light was 47.21%. It can be seen that the diffraction efficiency decreases as the diffraction order increases.

The objective lens described in claim 6 is characterized in that, in the invention described in any one of claims 1 to 5, the following expression is satisfied.
0.9 · λ1 / (n-1) ≦ d ≦ 2.2 · λ1 / (n-1) (3)
However,
d: Level difference in the optical axis direction of the first optical path difference providing structure (nm)
n: Refractive index of the objective lens at the first wavelength λ1 According to the present invention, since the height of the step of the first optical path difference providing structure can be reduced, an objective lens that is easy to manufacture can be obtained and diffraction at the time of wavelength variation Variations in efficiency can be kept low.

According to a seventh aspect of the present invention, in the invention according to any one of the first to sixth aspects, the step closest to the optical axis of the first optical path difference providing structure faces a direction opposite to the optical axis. It is characterized by. According to the present invention, since the value of the paraxial power can be made negative, the working distance can be extended even when the DVD is used while achieving the two compatibility of BD and DVD.

An objective lens according to an eighth aspect is characterized in that, in the invention according to any one of the first to seventh aspects, the value of the paraxial power at the second wavelength λ2 of the first optical path difference providing structure is negative. And

The objective lens described in claim 9 is characterized in that, in the invention described in claim 8, the following expression is satisfied.
−0.44 ≦ P ₀ * f ≦ −0.06 (4)
However,
P ₀ : Paraxial power at the second wavelength λ2 of the first optical path difference providing structure

If the value of the expression (4) is less than the upper limit, the pitch of the first optical path difference providing structure does not become too small, so that the objective lens can be easily manufactured, and the axial chromatic aberration can be prevented from becoming too large. On the other hand, if the value of the expression (4) is equal to or greater than the lower limit, the working distance when using the second optical disk can be secured, so that the possibility of interference between the optical disk and the objective lens can be reduced.

The objective lens described in claim 10 is characterized in that, in the invention described in any one of claims 1 to 9, the following expression is satisfied.
0.75 ≦ dx / f ≦ 1.70 (5)
However,
dx: axial thickness of the objective lens

If the value of the expression (5) is less than or equal to the upper limit, spherical aberration deterioration with respect to environmental temperature changes can be suppressed, the pitch of the optical path difference providing structure does not become too small, and the objective lens is easy to manufacture. A working distance when using an optical disc can be secured. On the other hand, if the value of the expression (5) is equal to or more than the lower limit, the light source side optical surface and the optical disc side optical surface of the objective lens caused by manufacturing errors even when using a short wavelength, high NA optical disc such as BD. It is possible to prevent deterioration of the optical characteristics due to the optical axis decentration of the lens, and to prevent the astigmatism from increasing, and furthermore, the edge thickness of the optical surface of the objective lens does not become too thin. Can flow smoothly and molding becomes easy.

The objective lens according to claim 11 is the objective lens according to any one of claims 1 to 10, wherein the peripheral region has a second optical path difference providing structure, and the second optical path difference providing structure is the second optical path difference structure. The fifth-order diffracted light amount of the first light beam that has passed through the optical path difference providing structure is made larger than any other order of diffracted light amount, and the third-order diffracted light amount of the second light beam that has passed through the second optical path difference providing structure is changed. The diffracted light quantity of any order is made larger. When the first optical path difference providing structure of the present invention is used, the present inventor generates the fifth-order diffracted light of the first light beam and the third-order diffracted light of the second light beam in the second optical path difference-providing structure, thereby generating a DVD. The present inventors have found that the flare state can be improved and an aperture stop effect can be provided.

The objective lens according to a twelfth aspect is the invention according to any one of the first to eleventh aspects, wherein the second light flux passing through the central region and the second light flux passing through the peripheral region. The distance Δ in the optical axis direction with respect to the light condensing position is 0.005 mm or more.

Taking the longitudinal spherical aberration diagram of FIG. 1 as an example, the optical axis direction from the condensing position of the second light beam BM1 that has passed through the central region to the closest condensing position of the second light beam BM2 that has passed through the peripheral region If Δ is 0.005 mm or more, the two do not cover each other, and the aperture limiting function can be effectively provided.

The objective lens according to claim 13 is characterized in that, in the invention according to any one of claims 1 to 12, the second optical path difference providing structure comprises only a blaze structure. Thereby, the diffraction efficiency in a reference wavelength can be maintained high.

An optical pickup device according to a fourteenth aspect includes the objective lens according to any one of the first to thirteenth aspects.

An optical pickup device according to a fifteenth aspect is the invention according to the fourteenth aspect, which is a slim type.

An optical information recording / reproducing apparatus according to a sixteenth aspect includes the optical pickup apparatus according to the fourteenth or fifteenth aspect.

The optical pickup device according to the present invention has a first light source and a second light source. Furthermore, the optical pickup device of the present invention has a condensing optical system for condensing the first light beam on the information recording surface of the BD and condensing the second light beam on the information recording surface of the DVD. The optical pickup device of the present invention includes a light receiving element that receives a reflected light beam from an information recording surface of a BD or DVD.

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 (t1) of the protective substrate is 0.05. It is a generic name for BD series optical discs of about 0.125 mm and includes BD having only a single information recording layer, BD having two or more information recording layers, and the like. Further, in the present specification, DVD means that information is recorded / reproduced by a light beam having a wavelength of about 630 to 670 nm and an objective lens having an NA of about 0.60 to 0.67, and the thickness of the protective substrate is 0.5. It is a generic term for DVD series optical discs of about 0.7 mm and includes DVD-ROM, DVD-Video, DVD- Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, and the like. As for the recording density, the recording density of BD is the highest, and then the order of DVD is lower.

In the present specification, the first light source and the second 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 flux emitted from the first light source, and the second wavelength λ2 (λ2> λ1) of the second light flux emitted from the second light source, λ1 is 390 nm or more and 415 nm or less, λ2 is not less than 630 nm and not more than 670 nm.

Also, at least two of the first light source and the second 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 an objective lens. The condensing optical system preferably has a coupling lens such as a collimator in addition to the objective lens. The coupling lens is a single lens or a lens group that is disposed between the objective lens and the light source and changes the divergence angle of the light beam. The collimator is a type of coupling lens, and is a lens that emits light incident on the collimator as parallel light. In this specification, the objective lens refers to an optical system that is disposed at a position facing the optical disk in the optical pickup device and has a function of condensing the light beam emitted from the light source onto the information recording surface of the optical disk. The objective lens is preferably an objective lens composed of a single convex lens. The objective lens may be a glass lens or a plastic lens, or an optical path difference providing structure is provided on the glass lens with a photo-curing resin, a UV-curing resin, or a thermosetting resin. A hybrid lens may also be used. When the objective lens has a plurality of lenses, a glass lens and a plastic lens may be mixed and used. 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, 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 a glass lens is the linear expansion coefficient a. 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 a, cracks are likely to occur when the temperature is lowered. The linear expansion coefficient a of the glass material is preferably 200 (10E-7 / K) or less, and more preferably 120 or less.

As the plastic, cycloolefin resin is preferably used, and specifically, ZEONEX manufactured by Nippon Zeon Co., Ltd., APEL manufactured by Mitsui Chemicals, 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 is described below. At least one optical surface of the objective lens has a central region and a peripheral region around the central region. The central region is preferably a region including the optical axis of the objective lens, but a minute region including the optical axis is used as an unused region or a special purpose region, and the surroundings are defined as a central region (also referred to as a central region). Also good. The central region and the peripheral region are preferably provided on the same optical surface. As shown in FIG. 2, the central region CN and the peripheral region OT are preferably provided concentrically around the optical axis on the same optical surface. The central region and the peripheral region are preferably adjacent to each other, but there may be a slight gap between them. A first optical path difference providing structure is provided in the central region. It is preferable that a second optical path difference providing structure is provided in the peripheral region.

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

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

The first optical path difference providing structure is preferably provided in a region of 70% or more of the area of the central region of the objective lens, and more preferably 90% or more. More preferably, the first optical path difference providing structure is provided on the entire surface of the central region. The second optical path difference providing structure is preferably provided in a region of 70% or more of the area of the peripheral region of the objective lens, and more preferably 90% or more. More preferably, the second optical path difference providing structure is provided on the entire surface of the peripheral region.

In addition, the optical path difference providing structure referred to in this specification is a general term for structures that add an optical path difference to an incident light beam. The optical path difference providing structure also includes a phase difference providing structure for providing a phase difference. The phase difference providing structure includes a diffractive structure. The optical path difference providing structure of the present invention is preferably a diffractive structure. The optical path difference providing structure has a step, preferably a plurality of steps. This step adds an optical path difference and / or phase difference to the incident light flux. The optical path difference added by the optical path difference providing structure may be an integer multiple of the wavelength of the incident light beam or a non-integer multiple of the wavelength of the incident light beam. The steps may be arranged with a periodic interval in the direction perpendicular to the optical axis, or may be arranged with a non-periodic interval in the direction perpendicular to the optical axis. When the objective lens provided with the optical path difference providing structure is a single aspherical lens, the incident angle of the light flux to the objective lens differs depending on the height from the optical axis. Each will be slightly different. For example, when the objective lens is a single-lens aspherical convex lens, even if it is an optical path difference providing structure that provides the same optical path difference, generally the distance from the optical axis tends to increase.

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

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

As shown in FIGS. 3A and 3B, the blaze-type structure means that the cross-sectional shape including the optical axis of the optical element having the optical path difference providing structure is a sawtooth shape. In the example of FIG. 3, it is assumed that the upper side is the light source side and the lower side is the optical disc side, and the optical path difference providing structure is formed on a plane as a mother aspherical surface. In the blazed structure, the length in the direction perpendicular to the optical axis of one blaze unit is called a pitch P. (See FIGS. 3A and 3B) The length of the step in the direction parallel to the optical axis of the blaze is referred to as a step amount d. (See Fig. 3 (a))

In addition, as shown in FIGS. 3 (c) and 3 (d), the staircase structure has a cross-sectional shape including an optical axis of an optical element having an optical path difference providing structure (referred to as a staircase unit). ). In the present specification, “V level” means a ring-shaped surface (hereinafter also referred to as a terrace surface) corresponding to (or facing) the vertical direction of the optical axis in one step unit of the step structure. In other words, it is divided by V steps and divided into V ring zones. Particularly, a three-level or higher staircase structure has a small step and a large step.

For example, the optical path difference providing structure illustrated in FIG. 3C is referred to as a five-level staircase structure, and the optical path difference providing structure illustrated in FIG. 3D is referred to as a two-level staircase structure (also referred to as a binary structure). . A two-level staircase structure is described below. A plurality of annular zones including a plurality of concentric annular zones around the optical axis, and a plurality of annular zones including the optical axis of the objective lens have a plurality of stepped surfaces Pa and Pb extending in parallel to the optical axis, The light source side terrace surface Pc for connecting the light source side ends of the adjacent step surfaces Pa and Pb and the optical disk side terrace surface Pd for connecting the optical disk side ends of the adjacent step surfaces Pa and Pb are formed. The surface Pc and the optical disc side terrace surface Pd are alternately arranged along the direction intersecting the optical axis.

Also, in the staircase structure, the length of one staircase unit in the direction perpendicular to the optical axis is called a pitch P. (See FIGS. 3C and 3D) The length of the step in the direction parallel to the optical axis of the staircase is referred to as step amounts B1 and B2. In the case of a three-level or higher staircase structure, a large step amount B1 and a small step amount B2 exist. (See Fig. 3 (c))

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

When the optical path difference providing structure has a blazed structure, the sawtooth shape as a unit shape is repeated. As shown in FIG. 3 (a), the same sawtooth shape may be repeated, and as shown in FIG. 3 (b), the shape of the sawtooth shape gradually increases as it moves away from the optical axis. A shape in which the pitch becomes longer or a shape in which the pitch becomes shorter may be used. In addition, in some areas, the blazed structure has a step opposite to the optical axis (center) side, and in other areas, the blazed structure has a step toward the optical axis (center). It is good also as a shape in which the transition area | region required in order to switch the direction of the level | step difference of a blaze | braze type | mold structure is provided in the meantime. In addition, when it is set as the structure which switches the direction of the level | step difference of a blaze | braze type | mold in this way, it becomes possible to widen an annular zone pitch and it can suppress the transmittance | permeability fall by the manufacturing error of an optical path difference providing structure.

When the optical path difference providing structure has a staircase structure, there may be a shape in which a 5-level stair unit as shown in FIG. Furthermore, it may be a shape in which the pitch of the staircase unit gradually increases as it moves away from the optical axis, or a shape in which the pitch of the staircase unit gradually decreases.

The first optical path difference providing structure and the second optical path difference providing structure may be provided on different optical surfaces of the objective lens, respectively, but are preferably provided on the same optical surface. Providing them on the same optical surface is preferable because it makes it possible to reduce eccentricity errors during manufacturing. Moreover, it is preferable that the first optical path difference providing structure and the second optical path difference providing structure are provided on the surface on the light source side of the objective lens rather than the surface on the optical disc side of the objective lens. In other words, the first optical path difference providing structure and the second optical path difference providing structure are preferably provided on the optical surface having the smaller absolute value of the radius of curvature of the objective lens.

The first optical path difference providing structure is preferably a blazed structure. Moreover, it is preferable that the first optical path difference providing structure is composed of only one type of blaze type structure and other structures are not superimposed. In addition, the first optical path difference providing structure makes the Nth-order diffracted light amount of the first light flux that has passed through the first optical path difference providing structure larger than any other order diffracted light amount, and passed through the first optical path difference providing structure. The Nth-order diffracted light quantity of the second light beam is made larger than any other order diffracted light quantity. That is, the order of the diffracted light generated most in the first light flux that has passed through the first optical path difference providing structure is equal to the order of the diffracted light generated most in the second light flux that has passed through the first optical path difference providing structure. . The absolute value of N is preferably 1 from the viewpoints of high diffraction efficiency, ease of manufacture, small diffraction efficiency fluctuation at the time of wavelength fluctuation, and the like. Further, N is more preferably +1.

When | N | = 1, it is preferable that the step amount of the first optical path difference providing structure satisfies the following conditional expression.
0.9 · λ1 / (n-1) ≦ d ≦ 2.2 · λ1 / (n-1) (3)
Here, d is the step amount (nm) in the optical axis direction of the first optical path difference providing structure, and n represents the refractive index of the objective lens at the first wavelength λ1. If the objective lens provided with the optical path difference providing structure is a single aspherical convex lens, the incident angle of the light flux to the objective lens differs depending on the height from the optical axis, so that the optical path difference providing structure that gives the same optical path difference Even so, in general, as the distance from the optical axis increases, the step amount tends to increase. The reason why the upper limit is multiplied by 2.2 in the conditional expression (3) is because the increase in the level difference is taken into account. It is preferable that the conditional expression (3) is satisfied in all the steps of the first optical path difference providing structure.

The blazed wavelength λB of the first optical path difference providing structure (theoretically, the wavelength at which the diffraction efficiency is 100% in the first optical path difference providing structure) is preferably larger than λ1 and smaller than λ2. More preferably, it is 470 nm or more and 550 nm or less. More preferably, they are 480 nm or more and 530 nm or less.

The second optical path difference providing structure is preferably a blazed structure. Moreover, it is preferable that the second optical path difference providing structure is composed of only one type of blaze type structure and other structures are not superimposed. In addition, the second optical path difference providing structure makes the fifth-order diffracted light quantity of the first light flux that has passed through the second optical path difference providing structure larger than any other order diffracted light quantity, and passed through the second optical path difference providing structure. It is preferable that the third-order diffracted light quantity of the second light beam is larger than any other order diffracted light quantity. In particular, when | N | of the first optical path difference providing structure is 1, it is preferable to employ such a second optical path difference providing structure because proper flare can be produced when using a DVD.

In order to perform good flare out when using a DVD, the distance Δ in the optical axis direction between the condensing position of the second light beam that has passed through the central region and the condensing position of the second light beam that has passed through the peripheral region is It is preferable that it is 0.005 mm or more.

The first optical path difference providing structure preferably has a negative paraxial power at the second wavelength λ2. Particularly preferably, the following formula is satisfied.
−0.44 ≦ P ₀ * f ≦ −0.06 (4)
However,
P ₀ : Power of the first optical path difference providing structure f: Focal length of the objective lens It is more preferable that the following expression is satisfied.
−0.44 ≦ P ₀ * f ≦ −0.14 (4 ′)

In order to increase the working distance when using a DVD with a thick substrate, it is preferable that the first optical path difference providing structure has a negative paraxial power (also referred to as power in this specification) with respect to the second light flux. . Here, “having paraxial power” means that C ₁ h ² is not 0 when the optical path difference function of the first optical path difference providing structure is expressed by the following equation ( ² ). The paraxial power P in the diffractive structure can be generally expressed by the following equation. “Having negative paraxial power” means that this value is negative. Where C ₁ is the optical path difference function coefficient, m is the diffraction order, λ 2 is the wavelength of the second light source used in the optical pickup device, and λ _B is the blazed wavelength of the first optical path difference providing structure (its diffraction structure) The wavelength at which the diffraction efficiency is 100% in FIG.
P = −2 × m × (λ2 / λ _B ) × C ₁ (8)

When the first optical path difference providing structure has a negative paraxial power with respect to the second light flux, at least the step closest to the optical axis of the first optical path difference providing structure is directed in a direction opposite to the optical axis. Is preferred. “The step is directed in the direction opposite to the optical axis” means a state as shown in FIG. FIG. 23A shows a state in which the step is directed in the direction of the optical axis. Preferably, at least a half step in the direction perpendicular to the optical axis from the optical axis to the boundary between the central region and the peripheral region and the step existing between the optical axes are directed in the opposite direction to the optical axis. It is.

For example, when the first optical path difference providing structure is near the optical axis, the step is opposite to the optical axis, but is switched halfway, and near the peripheral region, the first optical path difference providing structure has a step on the optical axis. The shape may be suitable. However, preferably, all the steps of the first optical path difference providing structure provided in the central region are directed in a direction opposite to the optical axis.

The numerical aperture on the image side of the objective lens necessary for reproducing / recording information on the first optical disc is NA1, and the numerical aperture on the image side of the objective lens necessary for reproducing / recording information on the second optical disc. Is NA2 (NA1> NA2). 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.

The boundary between the central region and the peripheral region of the objective lens is 0.9 · NA 2 or more and 1.2 · NA 2 or less (more preferably 0.95 · NA 2 or more, 1.15 · NA 2) when the second light flux is used. It is preferably formed in a portion corresponding to the following range. More preferably, the boundary between the central region and the peripheral region of the objective lens is formed in a portion corresponding to NA2.

The objective lens preferably satisfies the following conditional expression (4).
0.75 ≦ dx / f ≦ 1.70 (5)
Here, dx represents the thickness (mm) on the optical axis of the objective lens, and f represents the focal length (mm) of the objective lens in the first light flux.

If the value of the expression (5) is less than or equal to the upper limit, spherical aberration deterioration with respect to environmental temperature changes can be suppressed, the pitch of the optical path difference providing structure does not become too small, and the objective lens is easy to manufacture. A working distance of the optical disc can be secured. On the other hand, if the value of the expression (5) is equal to or greater than the lower limit, the deterioration of the optical characteristics due to the optical axis decentering of the light source side optical surface and the optical disc side optical surface of the objective lens caused by the manufacturing error does not become excessive. Since the edge thickness of the optical surface of the objective lens does not become too thin, the material can be smoothly flowed in injection molding or the like, and molding becomes easy.
It is more preferable to satisfy the following formula.
0.90 ≦ dx / f ≦ 1.41 (5 ′)

Furthermore, when the optical disk having a short wavelength and a high NA such as BD is used, the objective lens is likely to generate astigmatism and decentered coma, but the conditional expression (5) occurs. By satisfying the above, it is possible to suppress the generation of astigmatism and decentration coma.

In addition, satisfying conditional expression (5) results in a thick objective lens with a thick on-axis objective lens, so that the working distance during DVD recording / playback tends to be short. By providing the objective lens with the first optical path difference providing structure of the invention, the working distance in DVD recording / reproduction can be sufficiently ensured, so that the effect of the present invention becomes more remarkable.

The objective lens of the present invention satisfies the following conditional expressions (1) and (2).
1.7 ≦ φ1 ≦ 2.9 (1)
0.10 ≦ WD2 / f ≦ 0.42 (2)
Where φ1 represents the effective diameter (mm) of the objective lens when using BD, WD2 represents the working distance (mm) of the objective lens when using DVD, and f is the focal length of the objective lens in the first light flux. (Mm) Furthermore, it is more preferable when the following formula is satisfied.
1.7 ≦ φ1 ≦ 2.4 (1 ′)
0.10 ≦ WD2 / f ≦ 0.32 (2 ′)

In particular, when both the first light beam and the second light beam are incident on the objective lens as substantially parallel light (the imaging magnification of the objective lens is about −0.01 to 0.01) or parallel light, the above conditional expression (2 It is preferable to satisfy '). More preferably, the following expression is satisfied.
0.18 ≦ WD2 / f ≦ 0.24 (2 ″)

The first light beam and the second light beam may be incident on the objective lens as parallel light, or may be incident on the objective lens as divergent light or convergent light. Even during tracking, in order to prevent coma from occurring, it is preferable that all of the first light flux and the second light flux be incident on the objective lens as parallel light or substantially parallel light. By using the first optical path difference providing structure of the present invention, all of the first light beam and the second light beam can be incident on the objective lens as parallel light or substantially parallel light, and thus the effect of the present invention is more remarkable. It becomes. When the first light beam becomes parallel light or substantially parallel light, it is preferable that the imaging magnification m1 of the objective lens when the first light beam is incident on the objective lens satisfy the following formula (9).
-0.01 <m1 <0.01 (9)

In addition, when the second light beam is incident on the objective lens as parallel light or substantially parallel light, the imaging magnification m2 of the objective lens when the second light beam is incident on the objective lens satisfies the following formula (10). Is preferred.
-0.01 <m2 <0.01 (10)

On the other hand, when the second light beam is incident on the objective lens as diverging light, the imaging magnification m2 of the objective lens when the second light beam is incident on the objective lens preferably satisfies the following expression (10) ′. .
−0.025 <m2 ≦ −0.01 (10) ′

WD of the objective optical element when using the second optical disc is preferably 0.2 mm or more and 0.55 mm or less. Furthermore, the WD of the objective optical element when using the first optical disk is preferably 0.25 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. The optical pickup device is preferably a slim type. The slim type refers to an optical pickup device having a height H = 8 mm or less (the outline is schematically shown by a dotted line in FIG. 4).

In addition, since the main purpose of watching video is on television, in many cases it is expected that the frequency of use of BD or DVD recording video is high, but in many cases only audio is recorded. The frequency of using existing CDs is not considered high. Therefore, in an optical pickup device included in an optical disk drive built in a television, a CD can be excluded and the application can be limited to two compatibility of BD and DVD. Since it is not necessary to consider the compatibility of the CD with the thickest substrate, the problem of working distance is reduced, and it becomes possible to use a smaller-diameter objective lens. In particular, liquid crystal televisions, plasma televisions, FED ( Field emission display) This is suitable for a thin optical pickup device for an optical disk drive incorporated in a thin television such as a television, LED television or organic EL television.

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 objective lens for an optical pickup device, an optical pickup device, and an optical information recording / reproducing device that can suppress the occurrence of an error signal or the like when three different optical disks are used interchangeably. .

It is a figure which shows an example of a longitudinal spherical aberration figure. It is the figure which looked at the single objective lens OL concerning this Embodiment in the optical axis direction. It is an axial direction sectional drawing for demonstrating the example of an optical path difference providing structure, (a), (b) shows a blaze | braze type | mold structure, (c), (d) shows a staircase type | mold structure. It is a figure which shows schematically the structure of optical pick-up apparatus PU1 of this Embodiment which can record and / or reproduce | regenerate information appropriately with respect to BD, DVD, and CD which are different optical disks. FIG. 4 is a longitudinal spherical aberration diagram when the BD of Example 1 is used. FIG. 4 is a longitudinal spherical aberration diagram when the DVD of Example 1 is used. FIG. 6 is a longitudinal spherical aberration diagram when the BD of Example 2 is used. FIG. 6 is a longitudinal spherical aberration diagram when the DVD of Example 2 is used. FIG. 5 is a longitudinal spherical aberration diagram when the BD of Example 3 is used. FIG. 6 is a longitudinal spherical aberration diagram when the DVD of Example 3 is used. FIG. 6 is a longitudinal spherical aberration diagram when the BD of Example 4 is used. FIG. 6 is a longitudinal spherical aberration diagram when the DVD of Example 4 is used. FIG. 6 is a longitudinal spherical aberration diagram when the BD of Example 5 is used. FIG. 6 is a longitudinal spherical aberration diagram when using the DVD of Example 5. FIG. 12 is a longitudinal spherical aberration diagram when the BD of Example 6 is used. FIG. 12 is a longitudinal spherical aberration diagram when the DVD of Example 6 is used. FIG. 12 is a longitudinal spherical aberration diagram when the BD of Example 7 is used. FIG. 10 is a longitudinal spherical aberration diagram when the DVD of Example 7 is used. FIG. 12 is a longitudinal spherical aberration diagram when the BD of Example 8 is used. FIG. 10 is a longitudinal spherical aberration diagram when using the DVD of Example 8. FIG. 12 is a longitudinal spherical aberration diagram when the BD of Example 9 is used. FIG. 12 is a longitudinal spherical aberration diagram when the BD of Example 10 is used. It is a figure for demonstrating the direction of an optical path difference providing structure, and the state (a) in which the level | step difference has faced the direction of the optical axis, and the state (b) which has faced the opposite direction are shown.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 is a diagram schematically showing a configuration of the optical pickup apparatus PU1 of the present embodiment that can appropriately record and / or reproduce information on BD and DVD, which are different optical disks. The optical pickup device PU1 is a slim type and can be mounted on an optical information recording / reproducing device. Here, the first optical disc is a BD, the second optical disc is a DVD, and the third optical disc is a CD. The present invention is not limited to the present embodiment.

The optical pickup device PU1 emits light when recording / reproducing information with respect to the objective lens OL, the λ / 4 wavelength plate QWP, the rising mirror M, the collimating lens COL, the polarization beam splitter BS, and the dichroic prisms DP and BD. A first semiconductor laser LD1 (first light source) that emits a laser beam (first beam) having a wavelength λ1 = 405 nm, and a laser beam (wavelength λ2 = 660 nm) that is emitted when information is recorded / reproduced on / from a DVD. A second semiconductor laser LD2 (second light source) emitting a second light beam), a sensor lens SEN, a light receiving element PD as a photodetector, and the like.

The first optical path difference providing structure formed in the central region of the single objective lens OL is not a superposition structure, and the first-order diffracted light amount of the first light beam that has passed through the first optical path difference providing structure is of any other order. The first order diffracted light amount of the second light flux that has passed through the first optical path difference providing structure is made larger than the diffracted light amount of any other order. Further, the second optical path difference providing structure formed in the peripheral region of the objective lens OL makes the fifth-order diffracted light quantity of the first light beam that has passed through the second optical path difference providing structure larger than any other order diffracted light quantity. The third-order diffracted light quantity of the second light flux that has passed through the second optical path difference providing structure is made larger than any other order diffracted light quantity, but is not limited to this combination of diffraction orders. Furthermore, the following expression is satisfied.
1.7 ≦ φ1 ≦ 2.9 (1)
0.10 ≦ WD2 / f ≦ 0.40 (2)
However,
φ1: Effective diameter when using BD (mm)
WD2: Working distance when using DVD (mm)
f: Focal length (mm) of the objective lens OL in the first light flux

The divergent light beam of the first light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD1 passes through the dichroic prism DP, passes through the polarization beam splitter BS, and then passes through the collimating lens COL as shown by the solid line. It becomes parallel light, is reflected by the rising mirror M, is converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, the diameter of the light beam is regulated by a diaphragm (not shown), and is incident on the objective lens OL. Here, the light beam condensed by the central region, the intermediate region, and the peripheral region of the objective lens OL becomes a spot formed on the information recording surface RL1 of the BD through the protective substrate PL1 having a thickness of 0.1 mm. .

The reflected light beam modulated by the information pits on the information recording surface RL1 is transmitted again through the objective lens OL and a diaphragm (not shown), and then converted from circularly polarized light to linearly polarized light by the λ / 4 wavelength plate QWP. , Reflected by the collimating lens COL, reflected by the polarization beam splitter BS, and converged on the light receiving surface of the light receiving element PD via the sensor lens SEN. The information recorded on the BD can be read by using the output signal of the light receiving element PD to focus or track the objective lens OL by the biaxial actuator AC1. Here, when the wavelength fluctuation occurs in the first light flux or when recording / reproducing of a BD having a plurality of information recording layers, the spherical aberration generated due to the wavelength fluctuation or different information recording layers is changed in magnification. Correction can be made by changing the divergence angle or convergence angle of the light beam incident on the objective optical element OL by changing the collimating lens COL as means in the optical axis direction.

The divergent light beam of the second light beam (λ2 = 660 nm) emitted from the semiconductor laser LD2 is reflected by the dichroic prism DP, passes through the polarization beam splitter BS and the collimator lens COL, and is raised by the rising mirror M as shown by the dotted line. The light is reflected, converted from linearly polarized light to circularly polarized light by the λ / 4 wavelength plate QWP, and enters the objective lens OL. Here, the light beam condensed by the central region and the intermediate region of the objective lens OL (the light beam that has passed through the peripheral region is flared and forms a spot peripheral part) is passed through the protective substrate PL2 having a thickness of 0.6 mm. Thus, the spot is formed on the information recording surface RL2 of the DVD and forms the center of the spot.

The reflected light beam modulated by the information pits on the information recording surface RL2 is again transmitted through the objective lens OL, converted from circularly polarized light to linearly polarized light by the λ / 4 wave plate QWP, reflected by the rising mirror M, and collimated. The light beam is converged by the lens COL, reflected by the polarization beam splitter BS, and converged on the light receiving surface of the light receiving element PD via the sensor lens SEN. And the information recorded on DVD can be read using the output signal of light receiving element PD.

(Example)
Examples that can be used in the above-described embodiment will be described below. In a table (including lens data), a power of 10 (for example, 2.5 × 10 ⁻³ ) may be expressed using E (for example, 2.5 × E−3). The optical surface of the objective lens is formed as an aspherical surface that is symmetric about the optical axis and is defined by a mathematical formula in which the coefficients shown in Table 1 are substituted 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, Ai is an aspherical coefficient, h is a height from the optical axis, and r is a paraxial radius of curvature. It is.

Further, in the case of the embodiment using the diffractive structure, the optical path difference given to the light flux of each wavelength by the diffractive structure is defined by an equation in which the coefficient shown in the table is substituted into the optical path difference function of Formula 2. .

Here, λ is the wavelength of the incident light beam (also referred to as the used wavelength), λ _B is the design wavelength (called a blazed wavelength in the case of a blazed diffraction structure), dor is the diffraction order, and C _i is a coefficient of the optical path difference function.

Example 1
Table 1 shows lens data of the objective lens of Example 1. FIG. 5 shows a spherical aberration diagram of the objective lens of Example 1 when using the BD. As shown in FIG. 5, the spherical aberration is good in the BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.41.

FIG. 6 is a longitudinal spherical aberration diagram of the objective lens of Example 1 when using a DVD. The horizontal axis is positive in the direction away from the objective lens. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more makes the fifth-order diffracted light quantity of the first light beam that has passed through the second optical path difference providing structure larger than any other order diffracted light quantity. This is a structure in which the third-order diffracted light quantity of the second light beam that has passed through the two-optical path difference providing structure is larger than any other order diffracted light quantity (hereinafter referred to as a (5/3) structure). In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

When the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 6, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency of 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

(Example 2)
Table 2 shows lens data of the objective lens of Example 2. FIG. 7 shows a spherical aberration diagram of the objective lens of Example 2 when using the BD. As shown in FIG. 7, spherical aberration is good in BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.39.

FIG. 8 is a longitudinal spherical aberration diagram of the objective lens of Example 2 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

When the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 8, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency of 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

(Example 3)
Table 3 shows lens data of the objective lens of Example 3. FIG. 9 shows a spherical aberration diagram of the objective lens of Example 3 when using the BD. As shown in FIG. 9, spherical aberration is good in BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.25.

FIG. 10 is a longitudinal spherical aberration diagram of the objective lens of Example 3 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

Assuming that the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 10, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency of 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

Example 4
Table 4 shows lens data of the objective lens of Example 4. FIG. 11 shows a spherical aberration diagram of the objective lens of Example 4 when using the BD. As shown in FIG. 11, spherical aberration is good in BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.23.

FIG. 12 is a longitudinal spherical aberration diagram of the objective lens of Example 4 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

When the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 12, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency 99 of the second light beam that has passed through the peripheral region. It can be seen that a good flare is formed because the third-order diffracted light of .49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

(Example 5)
Table 5 shows lens data of the objective lens of Example 5. FIG. 13 shows a spherical aberration diagram of the objective lens of Example 5 when using the BD. As shown in FIG. 13, spherical aberration is good in BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this example, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.17.

FIG. 14 is a longitudinal spherical aberration diagram of the objective lens of Example 5 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

Assuming that the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 14, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency of 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

(Example 6)
Table 6 shows lens data of the objective lens of Example 6. FIG. 15 shows a spherical aberration diagram of the objective lens of Example 6 when using the BD. As shown in FIG. 15, the spherical aberration is good in the BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.18.

FIG. 16 is a longitudinal spherical aberration diagram of the objective lens of Example 6 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

If the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 16, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

(Example 7)
Table 7 shows lens data of the objective lens of Example 7. FIG. 17 shows a spherical aberration diagram of the objective lens of Example 7 when using the BD. As shown in FIG. 17, spherical aberration is good in BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.12.

FIG. 18 is a longitudinal spherical aberration diagram of the objective lens of Example 7 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

Assuming that the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 18, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency of 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

(Example 8)
Table 8 shows lens data of the objective lens of Example 8. FIG. 19 shows a spherical aberration diagram of the objective lens of Example 8 when using the BD. As shown in FIG. 19, spherical aberration is good in BD. The spherical aberration is good even when the environmental temperature change of + 30 ° C. occurs, and the spherical aberration is not greatly deteriorated even when the light source wavelength fluctuation of +5 nm occurs. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.11.

FIG. 20 is a longitudinal spherical aberration diagram of the objective lens of Example 8 when using a DVD. The second optical path difference providing structure in the peripheral region having a numerical aperture NA of 0.6 or more is a (5/3) structure. In the figure, the vertical axis of the graph represents the radius of the optical surface of the objective lens as 1, and m represents the diffraction order. Further, the graph shows a diffraction order of 0 order or higher and a diffraction efficiency of 1% or higher.

Assuming that the second optical path difference providing structure is a (5/3) structure, as shown in FIG. 20, the first-order diffracted light of the second light beam that has passed through the central region has a diffraction efficiency of 99. It can be seen that a good flare is formed because the third-order diffracted light of 49% is separated from the condensing position in the optical axis direction, and diffracted light of other orders is hardly generated.

Example 9
Table 9 shows lens data of the objective lens of Example 9. FIG. 21 shows a spherical aberration diagram of the objective lens of Example 9 when using the BD. As shown in FIG. 21, spherical aberration is good in BD. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.05.

(Example 10)
Table 10 shows lens data of the objective lens of Example 10. In addition, FIG. 22 shows a spherical aberration diagram of the objective lens of Example 10 when using the BD. As shown in FIG. 22, spherical aberration is good in BD. In this embodiment, the paraxial power when the second light flux passes through the first optical path difference providing structure is −0.05.

Table 11 summarizes the numerical values that are characteristic of Examples 1 to 10. Incidentally, regarding the expression (3), in this embodiment, 0.892 μm <d <1.508 μm is satisfied.

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.

AC1 Biaxial actuator BS Polarizing beam splitter CN Central region COL Collimating lens DP Dichroic prism LD1 First semiconductor laser or blue-violet semiconductor laser LD2 Second semiconductor laser LD3 Third semiconductor laser LDP Laser unit M Rising mirror OL Objective lens OT Peripheral region PD light receiving element PL1 protective substrate PL2 protective substrate PL3 protective substrate PU1 optical pickup device QWP λ / 4 wave plate RL1 information recording surface RL2 information recording surface RL3 information recording surface SEN sensor lens

Claims

A first light source that emits a first light beam with a first wavelength λ1 (nm) (390 ≦ λ1 ≦ 415) and a second light source that emits a second light beam with a second wavelength λ2 (nm) (630 ≦ λ2 ≦ 670) And recording and / or reproducing information on a BD having a protective substrate having a thickness of t1 using the first light flux, and having a thickness of t2 (t1 <t2) using the second light flux. An objective lens used in an optical pickup device for recording and / or reproducing information of a DVD having a protective substrate,
The objective lens is a single ball,
The optical surface of the objective lens has a central region and a peripheral region around the central region,
The central region has a first optical path difference providing structure,
The objective lens condenses the first light flux that passes through the central region so that information can be recorded and / or reproduced on the information recording surface of the BD, and the second light flux that passes through the central region. Are collected so that information can be recorded and / or reproduced on the information recording surface of the DVD,
The objective lens condenses the first light flux passing through the peripheral area so that information can be recorded and / or reproduced on the information recording surface of the BD, and the second light flux passes through the peripheral area. Is not condensed so that information can be recorded and / or reproduced on the information recording surface of the DVD,
The first optical path difference providing structure makes the Nth-order diffracted light quantity of the first light beam that has passed through the first optical path difference providing structure larger than any other order diffracted light quantity, and passes through the first optical path difference providing structure. The Nth-order diffracted light amount of the second light flux is larger than any other order diffracted light amount,
An objective lens characterized by satisfying the following expression:
1.7 ≦ φ1 ≦ 2.9 (1)
0.10 ≦ WD2 / f ≦ 0.42 (2)
However,
φ1: Effective diameter of the objective lens when using the BD (mm)
WD2: Working distance of the objective lens when using the DVD (mm)
f: Focal length (mm) of the objective lens in the first light flux
The objective lens according to claim 1, wherein the first optical path difference providing structure is not a structure in which a plurality of optical path difference providing structures are superimposed.
3. The objective lens according to claim 2, wherein the first optical path difference providing structure comprises only a blaze type structure.
The objective lens according to claim 1, wherein | N | = 1.
The objective lens according to claim 4, wherein N = + 1.
The objective lens according to claim 1, wherein the following expression is satisfied.
0.9 · λ1 / (n-1) ≦ d ≦ 2.2 · λ1 / (n-1) (3)
However,
d: Level difference in the optical axis direction of the first optical path difference providing structure (nm)
n: Refractive index of the objective lens at the first wavelength λ1
The objective lens according to any one of claims 1 to 6, wherein a step closest to the optical axis of the first optical path difference providing structure is directed in a direction opposite to the optical axis.
The objective lens according to any one of claims 1 to 7, wherein the first optical path difference providing structure has a negative paraxial power value at the second wavelength λ2.
The objective lens according to claim 8, wherein the following expression is satisfied.
−0.44 ≦ P 0 * f ≦ −0.06 (4)
However,
P 0 : Paraxial power at the second wavelength λ2 of the first optical path difference providing structure
The objective lens according to any one of claims 1 to 9, wherein the following expression is satisfied.
0.75 ≦ dx / f ≦ 1.70 (5)
However,
dx: axial thickness of the objective lens
The peripheral region has a second optical path difference providing structure that does not overlap the optical path difference providing structure, and the second optical path difference providing structure is the fifth-order diffracted light quantity of the first light beam that has passed through the second optical path difference providing structure. Is made larger than any other order of the diffracted light amount, and the third order diffracted light amount of the second light beam that has passed through the second optical path difference providing structure is made larger than any other order of diffracted light amount. Item 11. The objective lens according to any one of Items 1 to 10.
A distance Δ in the optical axis direction between the condensing position of the second light flux that has passed through the central area and the condensing position of the second light flux that has passed through the peripheral area is 0.005 mm or more. The objective lens according to claim 1.
The objective lens according to any one of claims 1 to 12, wherein the second optical path difference providing structure comprises only a blaze type structure.
An optical pickup device comprising the objective lens according to any one of claims 1 to 13.
15. The optical pickup device according to claim 14, which is a slim type.
An optical information recording / reproducing device comprising the optical pickup device according to claim 14.