CN101809661B - Objective lens for optical pickup devices and optical pickup device - Google Patents
Objective lens for optical pickup devices and optical pickup device Download PDFInfo
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
- G02B27/4283—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive element with major temperature dependent properties
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/18—Diffraction gratings
- G02B5/1876—Diffractive Fresnel lenses; Zone plates; Kinoforms
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1353—Diffractive elements, e.g. holograms or gratings
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1372—Lenses
- G11B7/1374—Objective lenses
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B2007/0003—Recording, reproducing or erasing systems characterised by the structure or type of the carrier
- G11B2007/0006—Recording, reproducing or erasing systems characterised by the structure or type of the carrier adapted for scanning different types of carrier, e.g. CD & DVD
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Abstract
An objective lens for optical pickup devices for focusing the diffracted light produced by an optical path difference imparting structure to form a spot on an information recording surface of an optical information recording medium. The objective lens enables reduction of the variation of the diffraction efficiency due to a variation of the wavelength used. The objective lens is characterized in that the wavelength at which the diffraction efficiency of basic structures forming an optical path difference imparting structure of the objective lens takes on a maximum value is adjusted depending on the basic structures, thereby the total diffraction efficiency is improved, and the variation of the diffraction efficiency due to a variation of the wavelength used can be reduced. An optical pickup device using such an objective lens is also provided.
Description
Technical Field
The present invention relates to an objective lens for an optical pickup device that records and/or reproduces information (also referred to as "recording/reproducing" in the present specification) by condensing a short-wavelength light beam as a spot on an information recording surface of an optical information recording medium, and an optical pickup device that is provided with the objective lens for an optical pickup device.
Background
As an objective lens for an optical pickup device, a diffraction type objective lens having a diffraction structure on an optical surface has been put to practical use. For example, the objective lenses for optical pickup devices can be used interchangeably with both DVDs and CDs, and spherical aberration caused by the difference in substrate thickness between DVDs and CDs is corrected by the difference in diffraction caused by the difference in wavelength used.
In recent years, as a light source for reproducing information recorded on an optical information recording medium and recording information on the optical information recording medium in an optical pickup device, a laser light source used has been made shorter in wavelength, and for example, a laser beam having a wavelength of 400 to 420nm such as a blue-violet semiconductor laser or a blue SHG laser for performing infrared semiconductor laser wavelength conversion by utilizing a nonlinear optical effect has been put into practical use. When such a blue-violet laser light source is used, 15 to 20GB of information can be recorded on an optical information recording medium having a diameter of 12cm by using an objective lens having the same Numerical Aperture (NA) as that of a DVD (digital versatile disc), and 23 to 25GB of information can be recorded on an optical information recording medium having a diameter of 12cm by increasing the NA of the objective lens to 0.85. In the following description, an optical information recording medium and an optical disk using a blue-violet laser light source are collectively referred to as a "high-density optical information recording medium".
In the high-density optical information recording medium using the NA0.85 objective lens, since the coma aberration generated by the tilt (skew) of the optical information recording medium is increased, there is a case where the protective layer is designed to be thinner than that of the DVD (0.1 mm relative to 0.6mm of the DVD) to reduce the amount of the coma aberration generated by the tilt. However, if information can be recorded and reproduced only on and from the high-density optical information recording medium, the value of the optical information recording medium as a product of a reproducing/recording machine (optical information recording/reproducing apparatus) is insufficient. Considering the fact that DVDs and CDs (compact discs) on which various information is recorded are now on the market, it is not sufficient to record/reproduce information only on/from a high-density optical information recording medium, and it is essential to improve the commercial value of an optical information recording medium for a high-density optical information recording medium, for example, to record/reproduce information on/from DVDs and CDs which users have collected in the same manner. In light of such background, an optical pickup device mounted in an optical information recording medium player/recorder for a high-density optical information recording medium is desired to have the following performance: information can be recorded/reproduced appropriately on/from any of a high-density optical information recording medium, a DVD, and a CD while maintaining compatibility. Therefore, in an objective lens for an optical pickup device that can be used interchangeably with both a high-density optical information recording medium and a DVD/CD, an objective lens that can be used interchangeably with each other is used to correct spherical aberration caused by a difference in substrate thickness between the high-density optical information recording medium and the DVD/CD by using a diffraction effect that is different between the use wavelengths. Not only for the above-mentioned interchangeable applications, but also many diffraction type optical elements for color correction and spherical aberration correction accompanying temperature change have been proposed.
As such an optical path difference imparting structure in the diffraction type optical element, a flame type, a step type, a binary type, and the like having a saw-toothed cross-sectional shape are known. In addition, the optical pickup apparatus is not limited to one type of optical path difference providing structure, and for example, a plurality of types of optical path difference providing structures may be superimposed on the same region of one optical surface of the objective lens, a plurality of types of optical path difference providing structures may be provided on different regions of one optical surface of the objective lens, or an optical path difference providing structure may be provided on both the collimator lens and the objective lens.
The diffraction efficiency of such a diffraction optical element is determined by the optical path difference imparting structure and the wavelength used. However, the diffraction efficiency tends to vary from the design reference value (hereinafter referred to as wavelength-dependent variation of diffraction efficiency) due to a variation in the use wavelength or a variation in the refractive index associated with the change in the use wavelength. In particular, when the higher order diffraction light is used, the wavelength dependent variation of the diffraction efficiency is larger than when the lower order diffraction light is used. For example, in a diffraction type interchangeable objective lens commonly used for a blue-violet laser for a high-density optical information recording medium, a red laser for DVD, and an infrared laser for CD, aberration is often corrected for the blue-violet laser by a higher-order diffracted light of 2 orders or more. In this case, the wavelength-dependent variation in diffraction efficiency increases.
In a general optical pickup device, although the intensity of light emitted from a semiconductor laser is monitored and feedback control is performed to obtain a spot having an intensity optimum for information recording/reproduction, a change in diffraction efficiency of an objective lens is not monitored, and if the change is large, the intensity of the spot suitable for information recording/reproduction may not be obtained. On the other hand, in an optical pickup device using a diffraction type optical element, the following patent documents disclose techniques for obtaining a proper diffraction efficiency.
Patent document 1: japanese laid-open patent publication No. 2001-93179
Patent document 2: japanese unexamined patent publication Hei 10-133104
Disclosure of Invention
Problems to be solved by the invention
The diffraction optical element for an optical pickup device using a blue-violet laser beam and a red laser beam disclosed in patent document 1 is designed to improve the diffraction efficiency in both cases by using a diffracted light having a higher diffraction order than that of the red laser beam in the case of the blue-violet laser beam, but there is no description about a wavelength-dependent variation technique for reducing the diffraction efficiency. Patent document 2 discloses a method of designing a phase structure cross-sectional shape in consideration of diffraction efficiency, and discloses a lens or the like having substantially uniform diffraction efficiency over the entire optical surface, but similarly there is no description about a wavelength-dependent variation technique for reducing diffraction efficiency.
In view of the problems of the prior art described above, an object of the present invention is to: an objective lens for an optical pickup device for condensing diffracted light generated by an optical path difference imparting structure as a spot on an information recording surface of an optical information recording medium, wherein the objective lens is capable of suppressing a wavelength-dependent variation in diffraction efficiency to a small value, and an optical pickup device using the objective lens; in particular, it relates to a diffraction type interchangeable objective lens using blue-violet laser light, red laser light and infrared laser light, and provides an objective lens with small wavelength dependent variation of diffraction efficiency, and an optical pickup device using the objective lens.
Means for solving the problems
In order to solve the above problems, embodiment 1 of the present invention is an objective lens for an optical pickup device for condensing a light beam having a predetermined wavelength λ 1 satisfying 390nm ≦ λ 1 ≦ 420nm on an information recording surface of an optical information recording medium to perform information recording/reproduction,
the objective lens for an optical pickup device is characterized in that a plurality of basic structures for providing an optical path difference structure are provided on the surface of an optical surface, and the following formula is satisfied:
4nm≤|λα-λβ|≤60nm (1)
(λβ-λ1)×(λα-λ1)＜0 (2)，
wherein,
λ α: a wavelength at which the diffraction efficiency of a base structure is maximum within a wavelength range of λ 1 ± 50nm
λ β: the wavelength at which the diffraction efficiency of the further basic structure is maximal in the wavelength range λ 1 ± 50 nm.
For example, when a plurality of basic structures formed on an objective lens are used, the functions of the basic structures are different from each other, and therefore, the wavelength (peak value) at which the diffraction efficiency is maximized may be deviated from the used wavelength to some extent. As a result, the total diffraction efficiency of the light beam with the wavelength λ 1 passing through a plurality of base structures may be greatly reduced with respect to wavelength variation. Therefore, the intensity of the light beam emitted from the light source monitored in front of the objective lens in the optical pickup device does not correspond to the intensity of the spot actually condensed on the information recording surface of the optical information recording medium, and there is a possibility that it is difficult to perform appropriate information recording and/or reproduction. However, according to the present invention, when the wavelength λ 1 passes through a plurality of infrastructures, wavelength-dependent variation of the total diffraction efficiency can be suppressed.
When the wavelength λ α at which the diffraction efficiency of a certain basic structure is the maximum and the wavelength λ β at which the diffraction efficiency of another basic structure is the maximum are both smaller (λ 1 > λ α, λ β) or both larger (λ 1 < λ α, λ β) than the wavelength λ 1, the total diffraction efficiency of the light beam having the wavelength λ 1 passing through the plurality of basic structures significantly varies with respect to the wavelength variation. When the formula (2) is satisfied, the wavelength λ 1 can more effectively suppress the wavelength-dependent variation of the total diffraction efficiency when passing through a plurality of infrastructure structures. Further, by satisfying both conditional expressions (1) and (2), it is possible to prevent a large decrease in the value of the diffraction efficiency at λ 1 while suppressing wavelength-dependent variation in the diffraction efficiency, and to maintain a sufficient diffraction efficiency at λ 1.
An objective lens for an optical pickup apparatus according to embodiment 2 is characterized in that, in embodiment 1, at least some of the plurality of base structures are formed to be overlapped on a predetermined region of the optical surface.
An objective lens for an optical pickup apparatus according to embodiment 3 is characterized in that, in embodiment 1 or 2, at least some of the plurality of basic structures are formed on different regions of the optical surface. The different regions of the optical surface include the following: separately formed on an optical surface on a light source side or an optical surface on an optical information recording medium side; are formed on the optical surface on the light source side or the optical surface on the optical information recording medium side.
The objective lens for an optical pickup apparatus according to embodiment 4 is characterized in that, in any of embodiments 1 to 3, the objective lens condenses the light beam with the wavelength λ 1 on the information recording surface of the optical information recording medium with the protective substrate thickness t1 to perform information recording/reproduction, and condenses the light beam with the wavelength λ 2 (> λ 1) on the information recording surface of the optical information recording medium with the protective substrate thickness t2 (> t1) to perform information recording/reproduction.
An objective lens for an optical pickup apparatus according to embodiment 5 is characterized in that, in any of embodiments 1 to 3, the objective lens condenses the light beam with the wavelength λ 1 on the information recording surface of the optical information recording medium with the protective substrate thickness t1 to perform information recording/reproduction, condenses the light beam with the wavelength λ 2 (> λ 1) on the information recording surface of the optical information recording medium with the protective substrate thickness t2 (> t1) to perform information recording/reproduction, and condenses the light beam with the wavelength λ 3 (> λ 2) on the information recording surface of the optical information recording medium with the protective substrate thickness t3 (> t2) to perform information recording/reproduction.
An objective lens for an optical pickup apparatus according to embodiment 6 is characterized in that, in embodiment 5, the plurality of basic structures include a1 st basic structure and a2 nd basic structure, the 1 st basic structure is a structure in which the amount of r-th (r is an integer) diffracted light of the wavelength λ 1 light beam passing through the 1 st basic structure is larger than the amount of diffracted light of any other order, the amount of s-th (s is an integer) diffracted light of the wavelength λ 2 light beam is larger than the amount of diffracted light of any other order, and the amount of t-th (t is an integer) diffracted light of the wavelength λ 3 light beam is larger than the amount of diffracted light of any other order, and the 2 nd basic structure is a structure in which the amount of u-th (u is an integer) diffracted light of the wavelength λ 1 light beam passing through the 2 nd basic structure is larger than the amount of diffracted light of any other order, the amount of v-th (v is an integer) diffracted light of the wavelength λ 2 light beam is larger than the amount of any, And an optical path difference providing structure in which the amount of light diffracted in the w-th order (w is an integer) of the light beam having the wavelength λ 3 is larger than the amount of light diffracted in any other order.
An objective lens for an optical pickup apparatus according to embodiment 7 is characterized in that, in embodiment 6, the plurality of basic structures include a3 rd basic structure in addition to the 1 st basic structure and the 2 nd basic structure, and the 3 rd basic structure is an optical path difference providing structure in which an amount of x-order (x is an integer) diffracted light of the wavelength λ 1 light beam passing through the 3 rd basic structure is larger than an amount of diffracted light of any other order, an amount of y-order (y is an integer) diffracted light of the wavelength λ 2 light beam is larger than an amount of diffracted light of any other order, and an amount of z-order (z is an integer) diffracted light of the wavelength λ 3 light beam is larger than an amount of diffracted light of any other order.
An objective lens for an optical pickup apparatus according to embodiment 8 is characterized in that, in embodiment 7, r is 0, s is 0, t is ± 1, u is 2, v is 1, w is 1, x is 10, y is 6, and z is 5.
An objective lens for an optical pickup apparatus according to embodiment 9 is characterized in that in any of embodiments 5 to 8, at least one of the plurality of basic structures is a structure for correcting spherical aberration caused by a thickness of a protective substrate for an optical information recording medium in accordance with a difference between the wavelength λ 1 and the wavelength λ 2.
An objective lens for an optical pickup apparatus according to embodiment 10 is characterized in that in any of embodiments 5 to 9, at least one of the plurality of basic structures is a structure for correcting spherical aberration caused by a thickness of a protective substrate for an optical information recording medium in accordance with a difference between the wavelength λ 1 and the wavelength λ 3.
An objective lens for an optical pickup apparatus according to embodiment 11 is characterized in that in any of embodiments 5 to 8, at least one of the plurality of basic structures is a structure for correcting spherical aberration caused by a thickness of a protective substrate for an optical information recording medium in accordance with a difference between the wavelength λ 1 and a wavelength other than the wavelength λ 1, and another of the plurality of basic structures is a structure for correcting a change in spherical aberration caused by a temperature change when recording/reproducing an optical information recording medium by a light beam having the wavelength λ 1.
An objective lens for an optical pickup apparatus according to embodiment 12 is characterized in that, in any one of embodiments 1 to 10, at least one of the plurality of basic structures is a structure for correcting a change in spherical aberration caused by a temperature change when recording/reproducing an optical information recording medium by the light beam having the wavelength λ 1.
An objective lens for an optical pickup apparatus according to embodiment 13 is characterized in that, in any of embodiments 1 to 12, the basic structure in which the diffraction efficiency at the wavelength λ α is maximized and the basic structure in which the diffraction efficiency at the wavelength λ β is maximized are both provided with steps that give an optical path difference of 4 wavelengths or more of the wavelength λ 1.
The effect of the present invention is more remarkable by the above configuration because the wavelength-dependent variation of the diffraction efficiency is particularly large when the base structure is provided with a step giving an optical path difference of 4 wavelengths or more of the wavelength λ 1.
The optical pickup device according to embodiment 14 is characterized by including a light source for emitting a light beam having a wavelength λ 1 and an objective lens according to any one of embodiments 1 to 13.
An optical pickup apparatus according to embodiment 15 is characterized in that, in embodiment 14, a monitoring unit is provided for monitoring the intensity of the light beam emitted from the light source before the light beam enters the objective lens.
The optical pickup device of the present invention is provided with a first light source. The second light source may be provided in addition to the first light source, and the third light source may be further provided. The optical pickup apparatus of the present invention includes a condensing optical system for condensing a first light beam on an information recording surface of a1 st optical information recording medium. When a second light source is provided, the condensing optical system condenses a second light beam on the information recording surface of the 2 nd optical information recording medium. When a third light source is provided, the condensing optical system condenses a third light beam on the information recording surface of the 3 rd optical information recording medium. The optical pickup device of the present invention includes a light receiving element for receiving light reflected from the information recording surface of the 1 st optical information recording medium. The optical information recording medium may further include a light receiving element for receiving light reflected from the information recording surface of the 2 nd or 3 rd optical information recording medium.
The 1 st optical information recording medium has a protective substrate with a thickness t1 and an information recording surface. The 2 nd optical information recording medium has a protective substrate with a thickness of t2(t 1. ltoreq. t2) and an information recording surface. The 3 rd optical information recording medium has a protective substrate with a thickness of t3(t2 < t3) and an information recording surface. Preferably, the 1 st optical information recording medium is a high-density optical information recording medium, the 2 nd optical information recording medium is a DVD, and the 3 rd optical information recording medium is a CD, but not limited thereto. The 1 st, 2 nd or 3 rd optical information recording medium may be a multilayer optical information recording medium having a multilayer information recording surface.
In the present specification, an optical information recording medium (e.g., BD: Blu-ray Disc) having a protective substrate thickness of about 0.1mm, which is used for recording/reproducing information with an objective lens having an NA of 0.85, is exemplified as the high-density optical information recording medium. Examples of other high-density optical information recording media include optical information recording media (for example, HD DVD: HD for short) having a protective substrate thickness of about 0.6mm, which record/reproduce information with an objective lens having NA0.65 to 0.67. The high-density optical information recording medium also includes an optical information recording medium having a protective film (in this specification, the protective substrate also includes a protective film) with a thickness of several to several tens of nm on the information recording surface, and also includes an optical information recording medium in which a protective substrate is not formed. Further, the high-density optical information recording medium includes an optical magnetic disk using a blue-violet semiconductor laser and a blue-violet SHG laser as a light source for information recording/reproduction. In the present specification, the term "DVD" is a generic term for DVD-series optical information recording media that record/reproduce information with an objective lens having an NA of about 0.60 to 0.67 and protect a substrate having a thickness of about 0.6mm, and includes DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW and the like. In the present specification, CD is a generic name of CD-series optical information recording media for recording/reproducing information with an objective lens having an NA of about 0.45 to 0.51 and protecting a substrate having a thickness of about 1.2mm, and includes CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW and the like. The recording density of the high-density optical information recording medium is highest, and then decreases in the order of DVD and CD.
The thicknesses t1, t2, t3 of the protecting substrates preferably satisfy the following conditional formulae (6), (7), (8), but are not limited thereto:
t1 is more than or equal to 0.0750mm and less than or equal to 0.125mm or t1 is more than or equal to 0.5mm and less than or equal to 0.7mm (6)
0.5mm≤t2≤0.7mm (7)
1.0mm≤t3≤1.3mm (8)。
In this 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. Preferably, the first wavelength λ 1 of the first light flux emitted from the first light source, the second wavelength λ 2 of the second light flux emitted from the second light source (λ 2 > λ 1), and the third wavelength λ 3 of the third light flux emitted from the third light source (λ 3 > λ 2) satisfy the following conditional expressions (9) and (10):
1.5×λ1＜λ2＜1.7×λ1 (9)
1.9×λ1＜λ3＜2.1×λ1 (10)。
when a BD, a HD, a DVD, and a CD are used as the 1 st optical information recording medium, the 2 nd optical information recording medium, and the 3 rd optical information recording medium, respectively, the first wavelength λ 1 of the first light source is 390nm to 420 nm. The second wavelength λ 2 of the second light source is preferably 570nm to 680nm, more preferably 630nm to 670nm, and the third wavelength λ 3 of the third light source is preferably 750nm to 880nm, more preferably 760nm to 820 nm.
At least 2 light sources among the first light source, the second light source, and the third light source may be unitized. The unitization means that, for example, the first light source and the second light source are fixedly housed in the 1-card.
As the light receiving element, a photodetector such as a photodiode is preferably used. The light reflected on the information recording surface of the optical information recording medium is incident on the light receiving element, and a read signal of the information recorded on each optical information recording medium can be obtained by using the output signal. Further, a change in the shape of a spot on the light receiving element and a change in the amount of light due to a change in position are detected, focus detection and track detection are performed, and the objective lens can be moved for focusing and tracking based on the detection. The light receiving element may be composed of a plurality of photodetectors. The light receiving element may be provided with a main photodetector and a sub photodetector. For example, the following light receiving elements may be used: two pairs of 2 photodetectors are provided on both sides of a photodetector that receives main light for information recording and reproduction, and sub light for tracking adjustment is received by the 2 pairs of photodetectors. The light receiving element may have a plurality of light receiving elements corresponding to the respective light sources.
Preferably, the optical pickup device includes a monitoring unit for monitoring the intensity of the light beam emitted from the light source before the light beam enters the objective lens. Such a monitoring unit can monitor the intensity of the light beam emitted from the light source, but does not detect the intensity of the light beam after passing through the objective optical element, and therefore does not detect a variation in diffraction efficiency in the optical path difference imparting structure such as the base structure. Therefore, the effect of the present invention is more remarkable in the light beam device having the monitoring unit.
The condenser optical system is provided with an objective lens. The condenser optical system may have only an objective lens, or may be provided with a coupling lens such as a collimator lens in addition to the objective lens. The coupling lens means a single lens or a lens group that changes a beam divergence angle disposed between the objective lens and the light source. The collimator lens is a kind of coupling lens, and is a lens that converts light incident on the collimator lens into parallel light and emits the parallel light. The condenser optical system may include an optical element such as a diffractive optical element for dividing a light beam emitted from the light source into a main beam for information recording and reproduction and two sub-beams for tracking and the like. In the present specification, the objective lens is an optical system which is disposed at a position facing an optical information recording medium in an optical pickup device and has a function of condensing a light beam emitted from a light source on an information recording surface of the optical information recording medium. Preferably, the objective lens is an optical system disposed at a position facing the optical information recording medium in the optical pickup device, and having a function of condensing the light beam emitted from the light source on the information recording surface of the optical information recording medium, and the optical system is capable of being displaced at least in an optical axis direction integrally by the actuator. The objective lens may be composed of two or more plural lenses, or may be a single lens, and a single lens is preferable. Further, the objective lens may be a glass lens or a plastic lens, or may be a hybrid lens in which a retardation providing structure or the like is provided on the glass lens with a photocurable resin or the like, but even if the objective lens is a plastic lens and a retardation providing structure for correcting spherical aberration accompanying temperature change is provided, the wavelength variation of diffraction efficiency can be reduced according to the present invention, so that the effect of the present invention is more remarkable when the objective lens is a plastic lens. When the objective lens includes a plurality of lenses, a glass lens and a plastic lens may be used in combination. When the objective lens includes a plurality of lenses, the objective lens may be a combination of a flat optical element having an optical path difference providing structure of a basic structure and an aspherical lens (which may or may not have an optical path difference providing structure). It is also preferable that the refractive surface of the objective lens is aspherical. It is also preferable that the base surface of the optical path difference providing structure of the objective lens is aspherical.
When the objective lens is a glass lens, a glass material having a glass transition point Tg of 400 ℃ or lower is preferably used. By using a glass material having a glass transition point Tg of 400 ℃ or lower, molding at a relatively low temperature is possible, and the life of the mold can be prolonged. Examples of the glass material having a low glass transition point Tg include K-PG325 and K-PG375 (both product names) manufactured by Takara optical glass, Kaisha.
However, since the glass lens is generally heavier than the resin lens, if the objective lens is a glass lens, the weight thereof increases, and the load on the actuator for driving the objective lens increases. Therefore, when the objective lens is a glass lens, a glass material having a small specific gravity is preferably used. Specifically, the specific gravity is preferably 3.0 or less, more preferably 2.8 or less.
When the objective lens is a plastic lens, a cyclic olefin resin material is preferably used, and the following resin materials are more preferably used among cyclic olefins: a refractive index at 25 deg.C to 405nm in the range of 1.53 to 1.60, and a refractive index change dN/dT (deg.C) to 405nm with temperature change in the temperature range of-5 deg.C to 70 deg.C-1) at-20X 10-5to-5X 10-5(more preferably-10X 10-5to-8X 10-5) Within the range. When the objective lens is a plastic lens, it is preferable that the coupling lens is also a plastic lens.
The objective lens is explained below. The objective lens has various basic structures for providing an optical path difference structure on an optical surface. Here, the basic structure is an optical path difference providing structure provided for providing a predetermined function to an optical surface, and means an optical path difference providing structure in which the amount of light diffracted by the 1 st beam passing through the basic structure for a number of times (a is an integer) is larger than the amount of light diffracted by any other number of times. In addition, when the value of a is equal in a certain base structure and another base structure, and the wavelengths at which the diffraction efficiency is maximum in the wavelength λ 1 ± 50nm are equal, the 2 base structures are the same base structure. On the other hand, if the a values are different or the wavelengths at which the diffraction efficiency is maximum in the wavelength range λ 1 ± 50nm are different, the 2 basic structure is a heterogeneous basic structure. The term "having a plurality of types of infrastructures" means that at least 2 types of different infrastructures are provided.
When the objective lens is used in an optical pickup device having 3 light sources, i.e., a1 st light source, a2 nd light source and a3 rd light source, the basic structure is an optical path difference providing structure provided for providing a predetermined function to an optical surface, and the optical path difference providing structure is such that the amount of light diffracted in the 1 st beam (a is an integer) is larger than the amount of light diffracted in any other order, the amount of light diffracted in the 2 nd beam (b is an integer) is larger than the amount of light diffracted in any other order, and the amount of light diffracted in the 3 rd beam (c is an integer) is larger than the amount of light diffracted in any other order. In this case, when the values of a, b, and c are equal in a certain base structure and another base structure, and the wavelengths at which the diffraction efficiency is maximum in the wavelength range λ 1 ± 50nm are equal, the 2 base structures are the same base structure. On the contrary, if at least one of the values of a, b and c is different or the wavelength at which the diffraction efficiency is maximum in the wavelength range of λ 1 ± 50nm is different, the 2 basic structure is a heterogeneous basic structure. The term "having a plurality of types of infrastructures" means that at least 2 types of different infrastructures are provided.
When the objective lens used in the optical pickup apparatus including 3 light sources has a plurality of basic structures, and the plurality of basic structures include the 1 st basic structure and the 2 nd basic structure, the following expression can be made: the 1 st basic structure is an optical path difference imparting structure which makes the light quantity of the 1 st light beam passing through the 1 st basic structure be greater than the light quantity of the 1 st light beam diffracted by any other times (r is an integer), the light quantity of the 2 nd light beam diffracted by s times (s is an integer) be greater than the light quantity of the 2 nd light beam diffracted by any other times, and the light quantity of the 3 rd light beam diffracted by t times (t is an integer) be greater than the light quantity of the 3 rd light beam diffracted by any other times; the 2 nd basic structure is a structure for giving an optical path difference in which the amount of light diffracted by the 1 st beam passing through the 2 nd basic structure in u times (u is an integer) is larger than the amount of light diffracted by any other number of times, the amount of light diffracted by the 2 nd beam in v times (v is an integer) is larger than the amount of light diffracted by any other number of times, and the amount of light diffracted by the 3 rd beam in w times (w is an integer) is larger than the amount of light diffracted by any other number of times. At this time, 1) at least one value of r, s, t and u, v, w is different, or 2) r, s, t and u, v, w are equal, the wavelength at which the diffraction efficiency is maximum in the range of wavelength λ 1 ± 50nm is different in the 1 st basic structure and the 2 nd basic structure. Also, the plurality of infrastructures may include a3 rd infrastructure in addition to the 1 st and 2 nd infrastructures. Can be expressed as: the 3 rd basic structure is an optical path difference imparting structure in which the amount of light diffracted by the 1 st beam (x is an integer) is larger than the amount of light diffracted by any other number of times, the amount of light diffracted by the 2 nd beam (y is an integer) is larger than the amount of light diffracted by any other number of times, and the amount of light diffracted by the 3 rd beam (z is an integer) is larger than the amount of light diffracted by any other number of times. At this time, 1) at least one value of r, s, t and x, y, z is different, or 2) at least one value of r, s, t and x, y, z is equal, the wavelength at which the diffraction efficiency is maximum in the range of wavelength λ 1 ± 50nm is different between the 1 st basic structure and the 3 rd basic structure.
The optical path difference imparting structure referred to in the present specification is a generic term of a structure for imparting 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 imparting structure includes a diffraction structure. The optical path difference providing structure preferably has a plurality of steps. The step adds an optical path difference and/or a phase difference to an incident beam. The optical path difference added by the optical path difference adding structure may be an integral multiple of the wavelength of the incident light beam or a non-integral 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.
The predetermined function of the base structure includes, for example, a function of correcting spherical aberration caused by the thickness of the protective substrate for an optical information recording medium in accordance with the difference between the wavelength λ 1 and a wavelength other than the wavelength λ 1. The method specifically comprises the following steps: correcting spherical aberration caused by the thickness of the protective substrate for the optical information recording medium in accordance with the difference between the wavelength λ 1 and the wavelength λ 2; a function of correcting spherical aberration caused by the thickness of the protective substrate for an optical information recording medium in accordance with the difference between the wavelength λ 1 and the wavelength λ 3. Another example of the function is to correct a change in spherical aberration caused by a temperature change when recording/reproducing an optical information recording medium with a light beam having a wavelength λ 1.
These base structures satisfy the following formulas (1) and (2):
4nm≤|λα-λβ|≤60nm (1)
(λβ-λ1)×(λα-λ1)＜0 (2)
wherein,
λ α: a wavelength at which the diffraction efficiency of a base structure is maximum within a wavelength range of λ 1 + -50 nm
λ β: the diffraction efficiency of the further basic structure is the wavelength at which the maximum is found in the wavelength range λ 1 ± 50 nm.
Both the basic structure having the maximum diffraction efficiency at the wavelength λ α and the basic structure having the maximum diffraction efficiency at the wavelength λ β are preferably provided with steps that can provide an optical path length difference of 4 wavelengths or more at the wavelength λ 1. The effect of the present invention is more remarkable with this structure because the wavelength-dependent variation of diffraction efficiency is extremely large when the basic structure is provided with a step capable of giving an optical path difference of 4 wavelengths or more of the wavelength λ 1. Therefore, a base structure in which the diffraction effect is maximized at the wavelength λ α and a base structure in which the diffraction effect is maximized at the wavelength λ β are preferable, and the base structures are base structures B, E, F described below and the like.
When the basic structure having the maximum diffraction efficiency at the wavelength λ α or the basic structure having the maximum diffraction efficiency at the wavelength λ β is configured such that the amount of light diffracted in the 1 st order (a is an integer) is larger than the amount of light diffracted in any other order, the amount of light diffracted in the 2 nd order (b is an integer) is larger than the amount of light diffracted in any other order, and the amount of light diffracted in the 3 rd order (c is an integer) is larger than the amount of light diffracted in any other order, one of the preferable modes is to configure the basic structure in which at least one of a, b, and c is a positive integer, and at least one of a, b, and c is a negative integer.
As the wavelength at which the diffraction efficiency of a certain basic structure is maximized, the wavelength of the light beam irradiated to the basic structure is gradually changed, and the wavelength at which the spot intensity is maximized and the wavelength at which the spot light amount is maximized can be regarded as the wavelength at which the diffraction efficiency of a certain basic structure is maximized.
Further, since the diffraction efficiency depends on the zone depth of the substructure, the diffraction efficiency of the substructure for each wavelength can be set as appropriate for the application of the optical pickup device. For example, in the case of an optical pickup device that records and reproduces information on a BD and only reproduces information on a DVD or CD, it is preferable that the diffraction efficiency of the infrastructure be regarded as the 1 st beam. On the other hand, in the case of an optical pickup device that reproduces only a BD and records and reproduces DVDs and CDs, it is preferable that the diffraction efficiency of the infrastructure of the central area described later be regarded as the 2 nd and 3 rd light beams, and the diffraction efficiency of the infrastructure of the peripheral area described later be regarded as the 2 nd light beam.
At least some of the plurality of basic structures may be formed to overlap a predetermined region of the surface of the objective optical surface, or all of the plurality of basic structures may be formed to overlap a predetermined region of the surface of the objective optical surface. The meaning of "overlapping" is literally understood to overlap. In the present specification, a case where a certain basic structure and another basic structure are provided on different optical surfaces, and a case where a certain basic structure and another basic structure are provided on the same optical surface but are provided in different regions without any overlapping region are not included in the present specification. In addition, the plurality of base structures may be formed at least on different regions of the surface of the objective optical surface, or all of the plurality of base structures may be formed on different regions of the surface of the objective optical surface without any overlapping portions. For example, the plurality of types of base structures may be formed in a row on either one of the optical surface of the objective lens on the light source side or the optical information recording medium side, or one of the plurality of types of base structures may be formed on the optical surface on the light source side and the other may be formed on the optical surface on the optical information recording medium side, without overlapping the plurality of types of base structures.
Preferably, the basic structure of the optical path difference providing structure has a plurality of concentric annular zones around the optical axis. The base structure may take various cross-sectional shapes (cross-sectional shapes including optical axis planes). The most general cross-sectional shape of the base structure is a flame-type cross-sectional shape including the optical axis, as shown in fig. 3(a) and (b). The flame-type shape is, as shown in FIGS. 3(a) and (b), a cross-sectional shape of the optical axis of the optical element including the base structure is a zigzag shape, that is, the base structure is not perpendicular to the base surface nor parallel to the base surface but has an inclined surface. The shape may be a repeated shape of a stepped structure as shown in fig. 3(c), or a binary shape as shown in fig. 3 (d). The step-like shape is a shape in which the cross section of the optical axis of the optical element including the basic structure is stepped, that is, the basic structure has only a plane parallel to the base plane and a plane parallel to the optical axis, and has no plane inclined to the base plane, and the length in the optical axis direction gradually changes as going forward in the base plane direction. For example, when a is 0, b is 1, and c is 0, the 1 st base structure has a stepped shape as shown in fig. 3 (c).
Preferably, the base structure is a structure in which a unit shape is periodically repeated. The term "unit shape is repeated periodically" as used herein includes, of course, a shape in which the same shape is repeated at the same period. The unit shape of 1 unit of the cycle is regularly a shape in which the cycle gradually increases or gradually decreases, and is also included in the "unit shape periodically repeats".
When the base structure has a flame-type shape, the base structure has a repeating zigzag shape of a unit shape. The same zigzag shape may be repeated as shown in fig. 3(a), or the zigzag shape may be gradually increased or decreased as it goes forward in the base surface direction as shown in fig. 3 (b). Further, a combination of a shape in which the size of the saw-toothed shape gradually increases and a shape in which the size of the saw-toothed shape gradually decreases may be used. However, when the size of the saw-tooth shape gradually changes, it is preferable that the size of the saw-tooth shape in the optical axis direction (or the direction of the light beam passing therethrough) hardly changes. In the flame type shape, the length of 1 sawtooth shape in the optical axis direction (the length in the light ray direction passing through the sawtooth shape may be used) is referred to as the pitch depth, and the length of 1 sawtooth shape in the base surface direction is referred to as the pitch width. The following shapes are also possible: the step of the flame shape is reversed toward the optical axis (center) in a certain region, and the step of the flame shape is reversed toward the optical axis (center) in another region, and a transition region necessary for switching the orientation of the step of the flame shape is provided therebetween. The transition region is a region corresponding to an extreme point of the optical path difference function when the optical path difference added by the basic structure of the optical path difference adding structure is expressed by the optical path difference function. When the optical path difference function is at an extreme point, the slope of the optical path difference function becomes small, the zone pitch can be widened, and the reduction in diffraction efficiency due to the shape error of the optical path difference imparting structure can be suppressed.
When the foundation structure has a step-like shape, the unit shape is a repeated shape of the step-like shape. The shape may be a repeated shape of a step shape like a plurality of layers (for example, 4 and 5 layers) shown in fig. 3 (c). Further, the size of the steps may be gradually increased or decreased as the steps are advanced in the direction of the base surface, but the length in the optical axis direction (or the direction of the light passing therethrough) is preferably almost constant.
When the base structure has a binary shape, the binary shape may be a shape in which the size of the binary gradually increases and the size of the binary gradually decreases as the base surface moves forward, but it is preferable that the length of the light passing through the base structure in the direction is almost constant. For example, when a is 0, b is 0, and c is ± 1, the base structure has a binary shape as shown in fig. 3 (d).
In the case of a configuration in which a plurality of types of base structures are stacked, it is preferable that a flame-type shape trace of the base structure is left, that is, it is preferable that a structure for imparting an optical path difference is formed by stacking the base structures, and a base surface on which the optical path difference imparting structure is provided has an inclined surface, not a right angle nor a parallel surface. By forming such a shape, it is possible to preferably prevent the reduction or disappearance of the optical function intended to be provided (for example, the improvement of the temperature characteristic, the improvement of the wavelength characteristic, the diffraction of only the specific wavelength, or the like) in the base structure, and to exhibit the optical function intended also in the superimposed optical path difference providing structure.
Among the plurality of base structures, at least 2 base structures of the flame type base structure having a larger pitch width (or period width) and the flame type base structure having a smaller pitch width (or period width) than the flame type base structure are preferably such that, when the 2 base structures are overlapped, at least one of the step positions (faces at almost right angles to the base face) of the base structure having the larger pitch width (or period width) does not coincide with the step position of the base structure having the smaller pitch width (or period width). More preferably, more than half of the step position of the large base structure does not coincide with the step position of the small base structure. In other words, it is preferable that the period of the large substructure is not equal to the integral multiple of the period of the small substructure, and the positions are separated from each other by steps. Such overlapping is preferable because the flame-shaped trace can be left.
In addition, it is preferable that at least one optical surface of the objective lens has a central region and a peripheral region around the central region. More preferably, at least one optical surface of the objective lens has a peripheral-most region around the peripheral region. By providing the outermost region, it is possible to more suitably perform recording and/or reproduction of an optical information recording medium having a high NA. The central region is preferably a region including the optical axis of the objective lens, but may be a region not including the central region. Preferably, the central region, the peripheral region and the most peripheral region are provided on the same optical surface. Preferably, as shown in fig. 4, the central region CN, the peripheral region MD, and the most peripheral region OT are provided on the same optical surface in concentric circles around the optical axis. Preferably, a base structure for an optical path difference providing structure is provided in the central region of the objective lens, and a base structure for an optical path difference providing structure is also provided in the peripheral region. When there is the outermost region, the outermost region may be a refractive surface, or a basic structure of an optical path difference providing structure may be provided in the outermost region. Preferably, the central region, the peripheral region and the most peripheral region are adjacent to each other, but may have a slight gap therebetween.
The objective lens preferably condenses the first, second, and third light fluxes having passed through the central region of the objective lens, respectively, into condensed spots. Preferably, the objective lens condenses the first light beam passing through the central region of the objective lens on the information recording surface of the 1 st optical information recording medium so as to enable information recording and/or reproduction. The objective lens condenses the second light beam passing through the central region of the objective lens on the information recording surface of the 2 nd optical information recording medium so as to be capable of recording and/or reproducing information. The objective lens condenses the third light beam passing through the central region of the objective lens on the information recording surface of the 3 rd optical information recording medium so as to be capable of recording and/or reproducing information. When the protective substrate thickness t1 of the 1 st optical information recording medium and the protective substrate thickness t2 of the 2 nd optical information recording medium are different, it is preferable that at least one substructure is provided in the central region, and spherical aberration caused by the difference between the protective substrate thickness t1 of the 1 st optical information recording medium and the protective substrate thickness t2 of the 2 nd optical information recording medium and/or spherical aberration caused by the difference between the wavelengths of the first light beam and the second light beam are corrected for the first light beam and the second light beam passing through the substructure. It is preferable that at least one base structure is provided in the central region, and spherical aberration caused by the difference between the protective substrate thickness t1 of the 1 st optical information recording medium and the protective substrate thickness t3 of the 3 rd optical information recording medium and/or spherical aberration caused by the difference between the wavelengths of the first light beam and the third light beam are corrected for the first light beam and the third light beam passing through the base structure.
Among the spots formed by the third light beam passing through the central region of the objective lens, the spot having the largest amount of light is the first best focus, and the spot having the second largest amount of light is the second best focus. That is, in the third beam passing through the central area, the diffracted light having the largest light amount forms the first best focus, and the diffracted light having the next largest light amount forms the second best focus. Preferably, the smallest spot diameter is the first best focus, and the second smallest spot diameter is the second best focus.
Preferably, the spot formed by the third light beam at the first best focus is used for recording and/or reproducing the 3 rd optical information recording medium, and the spot formed by the third light beam at the second best focus is not used for recording and/or reproducing the 3 rd optical information recording medium, but the spot formed by the third light beam at the first best focus may be used for recording and/or reproducing the 3 rd optical information recording medium, and the spot formed by the third light beam at the second best focus may be used for recording and/or reproducing the 3 rd optical information recording medium. When the first optical path difference providing structure is provided on the surface of the objective lens on the light source side, the second best focus is preferably closer to the objective lens than the first best focus.
The first best focus and the second best focus satisfy the following formula (3):
0.05≤L/f≤0.35 (3)
wherein, f [ mm ]: a focal length L [ mm ] of an objective lens of the third light beam when the third light beam passing through the first optical path difference providing structure is the third light beam forming the first best focus: a distance between the first best focus and the second best focus.
More preferably satisfies the following formula (3)':
0.10≤L/f≤0.25 (3)’
more preferably, the following formula (3) ":
0.11≤L/f≤0.24 (3)”。
l is preferably 0.18mm to 0.63 mm. And preferably f is 1.8mm to 3.0 mm.
By satisfying the lower limits of the expressions (3), (3)' and (3) ", it is possible to prevent unnecessary light which is not used in recording and/or reproducing the 3 rd optical information recording medium in the third beam from adversely affecting the light receiving element for tracking at the time of recording and/or reproducing the 3 rd optical information recording medium, and to maintain good tracking performance at the time of recording and/or reproducing the 3 rd optical information recording medium. By satisfying the upper limits of the respective equations (3), (3)' and (3) ", the pitch of the infrastructure that determines the distance L between the first optimum focus and the second optimum focus can be relaxed.
Further, a plurality of types of base structures may be provided in a central region of the objective lens in an overlapping manner, or the central region may be divided into a plurality of regions, and different base structures may be provided in the respective regions.
The objective lens condenses the first and second light fluxes having passed through the peripheral region of the objective lens, respectively, so as to form condensed spots. Preferably, the objective lens condenses the first light beam passing through the peripheral region of the objective lens on the information recording surface of the 1 st optical information recording medium so as to enable information recording and/or reproduction. The objective lens condenses the second light beam passing through the peripheral region of the objective lens on the information recording surface of the 2 nd optical information recording medium so as to be capable of recording and/or reproducing information. When the protective substrate thickness t1 of the 1 st optical information recording medium and the protective substrate thickness t2 of the 2 nd optical information recording medium are different, it is preferable that at least one substructure provided in the peripheral region is used, and spherical aberration caused by the difference between the protective substrate thickness t1 of the 1 st optical information recording medium and the protective substrate thickness t2 of the 2 nd optical information recording medium and/or spherical aberration caused by the difference between the wavelengths of the first light beam and the second light beam are corrected for the first light beam and the second light beam passing through the substructure.
Preferably, the third light beam passing through the peripheral region is not used for recording and/or reproducing the 3 rd optical information recording medium. It is preferable that the third light beam passing through the peripheral region does not contribute to formation of a condensed spot on the information recording surface of the 3 rd optical information recording medium. That is, it is preferable that flare is formed on the information recording surface of the 3 rd optical information recording medium by the third light beam passing through the peripheral region where at least one kind of the basic structure is provided on the objective lens. Preferably, the third light flux passing through the objective lens has, in order from the optical axis (or the spot center portion) to the outside, a spot center portion having a high light intensity density, a spot middle portion having a lower light intensity density than the spot center portion, and a spot peripheral portion having a higher light intensity density than the spot middle portion and a lower light intensity density than the spot center portion, on the spot formed on the information recording surface of the 3 rd optical information recording medium. The spot center portion is used for recording and/or reproducing information on/from the optical information recording medium, and the spot intermediate portion and the spot peripheral portion are not used for recording and/or reproducing information on/from the optical information recording medium. The periphery of the spot is called flare. That is, the third light beam passing through the peripheral region having at least one basic structure of the objective lens forms a spot peripheral portion on the information recording surface of the 3 rd optical information recording medium. The spot or spot of the third light beam is preferably the spot at the first best focus. Preferably, the spot formed on the information recording surface of the 2 nd optical information recording medium by the second light beam passing through the objective lens also has a spot center portion, a spot intermediate portion, and a spot peripheral portion.
When the objective lens has the outermost peripheral region, the objective lens condenses the first light beam passing through the outermost peripheral region of the objective lens on the information recording surface of the 1 st optical information recording medium so as to enable information recording and/or reproduction. Preferably, the first light beam transmitted through the peripheral area is corrected for spherical aberration during recording and/or reproduction of the 1 st optical information recording medium.
In a preferred embodiment, the second light beam transmitted through the peripheral area is not used for recording and/or reproducing the 2 nd optical information recording medium, and the third light beam transmitted through the peripheral area is not used for recording and/or reproducing the 3 rd optical information recording medium. Preferably, the second light flux and the third light flux passing through the peripheral area do not contribute to formation of the condensed spots on the information recording surfaces of the 2 nd optical information recording medium and the 3 rd optical information recording medium, respectively. That is, when the objective lens has the peripheral area, it is preferable that the third light flux passing through the peripheral area of the objective lens forms flare on the information recording surface of the 3 rd optical information recording medium. In other words, it is preferable that the third light beam passing through the peripheral area of the objective lens forms a spot peripheral portion on the information recording surface of the 3 rd optical information recording medium. When the objective lens has the peripheral area, it is preferable that the second light beam passing through the peripheral area of the objective lens forms flare on the information recording surface of the 2 nd optical information recording medium. In other words, it is preferable that the second light beam passing through the peripheral area of the objective lens forms a spot peripheral portion on the information recording surface of the 2 nd optical information recording medium.
An example of the infrastructure is described below. For example, the base structure a is an optical path difference imparting structure in which the amount of light diffracted by the first beam passing through the base structure a for 2 th order is larger than the amount of light diffracted by any other number of times, the amount of light diffracted by the second beam for 1 st order is larger than the amount of light diffracted by any other number of times, and the amount of light diffracted by the third beam for 1 st order is larger than the amount of light diffracted by any other number of times. Preferably, the base structure a is an optical path difference providing structure in which the first and third light fluxes emitted through the base structure a with substantially regular wave surfaces and the second light flux emitted through the base structure a with irregular wave surfaces are emitted. Further, the base structure a is preferably a structure in which a difference in optical path length between diffraction angles of the second light beam passing through the base structure a and the first light beam and the third light beam is given. Further, the step amount (pitch depth) in the optical axis direction of the infrastructure a is preferably a step amount capable of giving an optical path difference of approximately 2 wavelengths for the 1 st beam, approximately 1.2 wavelengths for the 2 nd beam, and approximately 1 wavelength for the 3 rd beam, respectively.
As another example of the basic structure, the basic structure B is a structure for giving an optical path difference that the 0 th order (transmitted light) diffracted light quantity of the first light flux passing through the basic structure B is larger than the light quantity of any other order, the 0 th order (transmitted light) diffracted light quantity of the second light flux is larger than the light quantity of any other order, and the ± 1 st order diffracted light quantity of the third light flux is larger than the light quantity of any other order. Preferably, the base structure B is an optical path difference providing structure in which the first and second light fluxes transmitted through the base structure B are emitted with substantially regular wave surfaces and the third light flux transmitted through the base structure B is emitted with irregular wave surfaces. In addition, the base structure B is preferably a structure in which the diffraction angle of the third light beam passing through the base structure B is different from the diffraction angles of the first light beam and the second light beam, and the optical path difference is given. Preferably, the step amount in the optical axis direction of the infrastructure B is a step amount capable of giving an optical path difference of approximately 5 wavelengths of the 1 st wavelength, an optical path difference of approximately 3 wavelengths of the 2 nd wavelength, and an optical path difference of approximately 2.5 wavelengths of the 3 rd wavelength to the 1 st beam. Further, the shape of the base structure B is preferably a binary shape as shown in fig. 3 (d).
The basic structure C is an optical path difference imparting structure for making the light quantity of the first beam diffracted at 1 st order greater than the light quantity of the first beam diffracted at any other order, the light quantity of the second beam diffracted at 1 st order greater than the light quantity of the second beam diffracted at any other order, and the light quantity of the third beam diffracted at 1 st order greater than the light quantity of the third beam diffracted at any other order. Preferably, the step amount in the optical axis direction of the basic structure C is a step amount capable of giving an optical path difference of approximately 1 wavelength for the 1 st light beam, approximately 0.6 wavelength for the 2 nd light beam, and approximately 0.5 wavelength for the 3 rd light beam, respectively.
The basic structure D is an optical path difference imparting structure for making the light quantity of the first beam passing through the basic structure D in the 3 rd order diffraction greater than that of any other order, the light quantity of the second beam in the 2 nd order diffraction greater than that of any other order, and the light quantity of the third beam in the 2 nd order diffraction greater than that of any other order. Preferably, the step amount in the optical axis direction of the basic structure D is a step amount capable of giving an optical path difference of substantially 3 wavelengths for the 1 st light beam, an optical path difference of substantially 1.9 wavelengths for the 2 nd light beam, and an optical path difference of substantially 1.6 wavelengths for the 3 rd light beam, to the 1 st light beam.
The basic structure E is an optical path difference imparting structure for making the light quantity of the first light beam passing through the basic structure E diffracted for 10 times larger than the light quantity of the first light beam diffracted for any other times, the light quantity of the second light beam diffracted for 6 times larger than the light quantity of the second light beam diffracted for any other times, and the light quantity of the third light beam diffracted for 5 times larger than the light quantity of the third light beam diffracted for any other times. Preferably, the step amount in the optical axis direction of the basic structure E is a step amount capable of giving an optical path difference of about 10 wavelengths for the 1 st light beam, about 6 wavelengths for the 2 nd light beam, and about 5 wavelengths for the 3 rd light beam, about 1 wavelength.
The basic structure F is an optical path difference imparting structure for making the light quantity of the first beam passing through the basic structure F in the 5 th order diffraction larger than that of any other order, the light quantity of the second beam in the 3 rd order diffraction larger than that of any other order, and the light quantities of the third beam in the 3 rd order diffraction and the 2 nd order diffraction larger than that of any other order. And the quantity of diffracted light of the 3 rd order diffracted light of the third light beam is preferably a little larger than that of the 2 nd order diffracted light. Preferably, the step amount in the optical axis direction of the basic structure F is a step amount capable of giving an optical path difference of approximately 5 wavelengths for the 1 st light beam, approximately 3 wavelengths for the 2 nd light beam, and approximately 2.5 wavelengths for the 3 rd light beam, respectively.
The basic structure G is an optical path difference imparting structure in which the amount of light diffracted in the 2 nd order of the first light beam passing through the basic structure G is larger than the amount of light diffracted in any other order, the amount of light diffracted in the 1 st order of the second light beam is larger than the amount of light diffracted in any other order, and the amount of light diffracted in the 1 st order of the third light beam is larger than the amount of light diffracted in any other order. Preferably, the step amount in the optical axis direction of the basic structure G is a step amount capable of giving an optical path difference of approximately 2 wavelengths of the 1 st wavelength to the 1 st beam, approximately 1.2 wavelengths of the 2 nd wavelength to the 2 nd beam, and approximately 1 wavelength of the 3 rd wavelength to the 3 rd beam.
The base structures E, F, and G have a function of making the spherical aberration insufficient when the temperature rises and the wavelengths of the first light source, the second light source, and the third light source are elongated, so that excessive spherical aberration caused by a decrease in the refractive index of the plastic when the temperature rises can be compensated for, and good spherical aberration can be obtained. The step depth of the base structures F and G may be shallower than that of the base structure E. Further, it is preferable that the base structure E, the base structure F, and the base structure G are provided on different mother aspherical surfaces (base surfaces) from the base structures a, B, C, and D. The base structures E, F and G are preferably provided on a mother aspherical surface (base surface) set so that the base structures E, F and G do not affect the direction of the incident beam as much as possible while giving the above optical path difference to the incident beam. Further, the base structure E, the base structure F, and the base structure G preferably have the following structures: the optical element extends into the optical element with moving away from the optical axis in the direction perpendicular to the optical axis, and extends outward with moving away from the optical axis with a certain boundary. (that is, a structure which gradually becomes deeper and then becomes shallower at a certain place is preferable.)
When the objective lens is a plastic lens, one preferable embodiment is a superposed structure in which at least two basic structures are superposed on each other in the central region. Further, one preferable embodiment is a triple-overlapped structure in which three basic structures are overlapped. As a specific preferred embodiment, a triple-overlapped structure is mentioned in which, for example, a base structure E, a base structure F, or a base structure G is overlapped with a base structure a and a base structure B. More preferably, the base structure A and the base structure B are superposed on each other to form a base structure E. In addition, when the amount of light diffracted in the 1 st order (a is an integer) is larger than the amount of light diffracted in any other order, the amount of light diffracted in the 2 nd order (b is an integer) is larger than the amount of light diffracted in any other order, and the amount of light diffracted in the 3 rd order (c is an integer) is larger than the amount of light diffracted in any other order, a preferable example is a superimposed structure in which at least one of a, b, and c is a positive integer and at least one is a negative integer is provided in the central region, and the basic structure E, the basic structure F, or the basic structure G is superimposed.
When the objective lens is a plastic lens, the base structure E, the base structure F, or the base structure G may be superimposed on any one of the base structures a, C, and D in the peripheral region. A superposed configuration of the base structure a and the base structure F is preferred. In addition, when the amount of light diffracted in the 1 st order (a is an integer) is larger than the amount of light diffracted in any other order, the amount of light diffracted in the 2 nd order (b is an integer) is larger than the amount of light diffracted in any other order, and the amount of light diffracted in the 3 rd order (c is an integer) is larger than the amount of light diffracted in any other order, a preferable example is a superimposed structure in which at least one of a, b, and c is a positive integer and at least one is a negative integer is formed in the peripheral region, and the basic structure E, the basic structure F, or the basic structure G is superimposed.
When the objective lens is a plastic lens, the most peripheral region preferably has at least one of the base structure E, the base structure F, and the base structure G. Preferably a structure having a base structure F.
And the objective lens is a glass lens, preferably having the most peripheral region which is a refractive surface.
The numerical aperture on the image side of the objective lens necessary for reproducing and/or recording information from/on the 1 st optical information recording medium is NA1, the numerical aperture on the image side of the objective lens necessary for reproducing and/or recording information from/on the 2 nd optical information recording medium is NA2(NA 1. gtoreq. NA2), and the numerical aperture on the image side of the objective lens necessary for reproducing and/or recording information from/on the 3 rd optical information recording medium is NA3(NA2 > NA 3). Preferably, NA1 is 0.8 to 0.9, or 0.55 to 0.7. Particularly preferably NA1 is 0.85. Preferably, NA2 is 0.55 to 0.7. Particularly preferably, NA2 is 0.60. Preferably, NA3 is 0.4 to 0.55. Particularly preferred is a NA3 of 0.45 or 0.53.
The boundary between the central region and the peripheral region of the objective lens is preferably formed in a portion corresponding to a range of 0.9NA3 or more and 1.2NA3 or less (more preferably, 0.95NA3 or more and 1.15NA3 or less) when the third beam is used. More preferably, the boundary between the central region and the peripheral region of the objective lens is formed in a portion corresponding to NA 3. The boundary between the peripheral region and the most peripheral region of the objective lens is preferably formed in a portion corresponding to a range of 0.9NA2 or more and 1.2NA2 or less (more preferably, 0.95NA2 or more and 1.15NA2 or less) when the second light beam is used. More preferably, the boundary between the peripheral region and the most peripheral region of the objective lens is formed in a portion corresponding to NA 2. The boundary outside the outermost periphery of the objective lens is preferably formed in a portion corresponding to a range of 0.9NA1 or more and 1.2NA1 or less (more preferably 0.95NA1 or more and 1.15NA1 or less) when the first light beam is used. More preferably, the boundary outside the outermost periphery of the objective lens is formed in a portion corresponding to NA 1.
In addition, the light use efficiency of any two of the first to third light fluxes may be set to 80% or more, and the light use efficiency of the remaining one light flux may be set to 30% or more and 80% or less. The light utilization efficiency of the remaining one beam may be 40% or more and 70% or less. In this case, the light beam having a light use efficiency of 30% to 80% (or 40% to 70%) is preferably the third light beam.
The light use efficiency as described herein is calculated by a/B when the light amount in the airy spot of the condensed spot formed on the information recording surface of the optical information recording medium is a, the light amount in the airy spot of the condensed spot formed on the information recording surface of the optical information recording medium is B, and the light amount in the airy spot of the condensed spot formed on the information recording surface of the optical information recording medium is a, and the objective lens is formed of the same material and has the same focal length, on-axis thickness, numerical aperture, and wavefront aberration without the formation of the first, second, and third optical path difference providing structures. Here, airy spot means a spot-focusing circle having a radius r 'represented by r' 0.61 · λ/NA.
The first, second, and third light beams may be incident on the objective lens as parallel light, or may be incident on the objective lens as divergent light or convergent light. Preferably, the magnification m1 of the objective lens when the first light beam is incident on the objective lens satisfies the following formula (4):
-0.01＜m1＜0.01 (4)。
when the first light flux is incident on the objective lens as divergent light, it is preferable that the magnification m1 of the objective lens when the 1 st light flux is incident on the objective lens satisfies the following expression (4)':
-0.10＜m1＜0.00 (4)‘。
further, when the second light flux is incident on the objective lens as parallel light or substantially parallel light, it is preferable that the beam magnification m2 of the objective lens when the second light flux is incident on the objective lens satisfies the following formula (5):
-0.01＜m2＜0.01 (5)。
when the second light flux is incident on the objective lens as divergent light, it is preferable that the magnification m2 of the objective lens when the second light flux is incident on the objective lens satisfies the following expression (5'):
-0.10＜m2＜0.00 (5’)。
further, when the third light flux is incident on the objective lens as parallel light or substantially parallel light, it is preferable that the magnification m3 of the incident light flux when the third light flux is incident on the objective lens satisfies the following formula (6):
-0.01＜m3＜0.01 (6)。
when the third light flux is incident on the objective lens as divergent light, it is preferable that the magnification m3 of the objective lens when the third light flux is incident on the objective lens satisfies the following expression (6'):
-0.10＜m3＜0.00 (6‘)。
when the 3 rd optical information recording medium is used, the Working Distance (WD) of the objective lens is preferably 0.20mm to 1.5 mm. Preferably 0.3mm to 1.00 mm. When the 2 nd optical information recording medium is used, the WD of the objective lens is preferably 0.4mm to 0.7 mm. When the 1 st optical information recording medium is used, the WD of the objective lens is preferably 0.4mm to 0.9mm (preferably 0.6mm to 0.9mm when t1 < t 2).
The entrance pupil diameter of the objective lens is preferably set when the 1 st optical information recording medium is used
2.8mm or more
4.5mm or less.
An optical information recording/reproducing apparatus according to the present invention includes an optical information recording medium driving apparatus including the optical pickup device.
To explain an optical information recording medium drive device provided in an optical information recording/reproducing device, the optical information recording medium drive device is configured as follows: only a disk capable of supporting an optical information recording medium in a mounted state is taken out to the outside from an optical information recording/reproducing apparatus housing an optical pickup device and the like; the optical information recording medium drive device main body is taken out to the outside together with the optical pickup device and the like.
The optical information recording and reproducing device according to each of the above-described embodiments is generally provided with the following components, but is not limited thereto: an optical pickup device housed in a case or the like; an optical pickup drive source such as a tracking motor for moving the optical pickup together with the housing to the inner periphery or the outer periphery of the optical information recording medium; an optical pickup device transfer section including a guide rail for guiding the optical pickup device housing to the inner periphery or the outer periphery of the optical information recording medium; a spindle motor for rotationally driving the optical information recording medium.
In the former method, in addition to the above-described components, a disc capable of supporting an optical information recording medium in a mounted state, a loading mechanism for sliding the disc, and the like are provided; in the latter method, the tray and the loading mechanism are not provided, and the respective components are preferably provided on a base drawer that can be drawn out to the outside.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present invention, it is possible to provide an objective lens for an optical pickup apparatus, which can suppress a variation in diffraction efficiency due to a change in a used wavelength to be small, and an optical pickup apparatus using the objective lens, in relation to an objective lens for an optical pickup apparatus, which condenses diffracted light generated by an optical path difference imparting structure as a spot on an information recording surface of an optical information recording medium; in particular, it relates to a diffraction type interchangeable objective lens using a blue-violet laser beam, a red laser beam and an infrared laser beam, and it is possible to provide an objective lens having a small fluctuation in diffraction efficiency due to a change in wavelength, and an optical pickup device using the objective lens.
Drawings
FIG. 1: the optical pickup device of the present embodiment is schematically illustrated in cross section.
FIG. 2: the structure of FIG. 1 is a cross-sectional view in the direction of the arrows, taken along the line II-II.
FIG. 3: the cross-sectional view of the optical path difference providing structure of the objective lens of the embodiment of the invention.
FIG. 4: an exemplary schematic cross-sectional view of an objective OBJ of the present invention.
FIG. 5: the wavelength dependence (a) of the 1 st basic structure, the wavelength dependence (b) of the 2 nd basic structure, the wavelength dependence (c) of the 3 rd basic structure, and the total wavelength dependence (d) of the comparative example are shown with the diffraction efficiency on the vertical axis and the wavelength on the horizontal axis.
FIG. 6: the diffraction efficiency on the vertical axis and the wavelength on the horizontal axis are shown in the graph of the wave dependence (a) and the total wavelength dependence (b) of the 3 rd basic structure according to example 1.
FIG. 7: the diffraction efficiency on the vertical axis and the wavelength on the horizontal axis are shown in the graph, and the wave dependence (a) and the total wavelength dependence (b) of the 3 rd basic structure according to example 2 are shown.
FIG. 8: the diffraction efficiency on the vertical axis and the wavelength on the horizontal axis are shown in the graph, and the wave dependence (a) and the total wavelength dependence (b) of the 3 rd basic structure according to example 3 are shown.
Description of the symbols
1 outer cover
2 spectacle frame
3 collimating lens
3a lens unit
3b hollow cylindrical part
4-mirror
5 semiconductor laser
6 bluish violet is with polarization spectrometer
7 Power monitor
9 lambda/4 wave plate
10 objective lens
11 polarization spectrometer
12 servo lens
13 photo detector
14 driving device
152 laser 1 plug-in
16 coupling lens
Detailed Description
Preferred examples of the embodiments of the present invention will be described in further detail below with reference to the drawings. Fig. 1 is a schematic cross-sectional view of an optical pickup device according to the present embodiment. FIG. 2 is a cross-sectional view of the structure of FIG. 1 taken along the line II-II and viewed in the direction of the arrows. The optical pickup apparatus of the present embodiment can reproduce information from 3 kinds of optical information recording media, such as BD (or HD), DVD, and CD, but the present invention is also applicable to an optical pickup apparatus that reproduces information from 2 kinds of optical information recording media (including preferably a high-density optical information recording medium), and an optical pickup apparatus that records/reproduces information from 1 kind of optical information recording media (in this case, preferably a high-density optical information recording medium).
The objective lens 10 of the present embodiment has a plurality of basic structures for providing an optical path difference structure formed on the optical surface thereof, and these basic structures satisfy the following expression:
4nm≤|λα-λβ|≤60nm (1)
(λβ-λ1)×(λα-λ1)＜0 (2)，
wherein,
λ α: a wavelength at which the diffraction efficiency of a base structure is maximum within a wavelength range of λ 1 + -50 nm
λ β: the diffraction efficiency of the further basic structure is the wavelength at which the maximum is found in the wavelength range λ 1 ± 50 nm.
The objective lens 10 having the optical path difference providing structure is explained as follows. The objective lens 10 has a structure in which at least 2 basic structures (the 1 st basic structure and the 2 nd basic structure) are overlapped on an optical surface.
The 1 st basic structure is an optical path difference imparting structure in which the amount of light diffracted in the 1 st order (r is an integer) is larger than the amount of light diffracted in any other order, the amount of light diffracted in the 2 nd order (s is an integer) is larger than the amount of light diffracted in any other order, and the amount of light diffracted in the 3 rd order (t is an integer) is larger than the amount of light diffracted in any other order. The 2 nd basic structure is an optical path difference imparting structure in which the amount of light diffracted by the 1 st beam (u is an integer) is larger than the amount of light diffracted by any other number of times, the amount of light diffracted by the 2 nd beam (v is an integer) is larger than the amount of light diffracted by any other number of times, and the amount of light diffracted by the 3 rd beam (w is an integer) is larger than the amount of light diffracted by any other number of times.
The 3 rd chassis may be overlapped in addition to the 1 st and 2 nd chassis. The 3 rd basic structure is an optical path difference imparting structure in which the amount of light diffracted by the 1 st beam (x is an integer) is larger than the amount of light diffracted by any other number of times, the amount of light diffracted by the 2 nd beam (y is an integer) is larger than the amount of light diffracted by any other number of times, and the amount of light diffracted by the 3 rd beam (z is an integer) is larger than the amount of light diffracted by any other number of times.
Preferably, at least one of r, s, and t is not 0.1 or 2 of r, s, t may be 0. Preferably at least one of u, v, w is different from 0. More preferably, u, v and w are not 0. Preferably at least one of x, y, z is different from 0. More preferably, none of x, y and z is 0. It is also preferred that the following conditional expressions or, preferably, r ═ 0, s ═ 0, t ═ 1, u ═ 2, v ═ 1, w ═ 1, x ═ 10, y ═ 6, and z ═ 5 are satisfied.
r+s+t＜u+v+w＜x+y+z。
A preferred design method for designing, for example, the aspherical objective lens 10 will be described below. First, a reference aspherical surface is designed, and a structure in which a base structure having the largest pitch width and having set values of r, s, and t is placed as a1 st base structure is placed thereon. Next, as a2 nd base structure, a base structure having a pitch width that is the second largest than that of the 1 st base structure and having set values of u, v, and w is superimposed on each surface in each pitch width of the 1 st base structure. Further, as the 3 rd base structure, the base structures having the pitch widths larger than the 2 nd base structure are superimposed on the 1 st base structure and the 2 nd base structure on the respective surfaces within the pitch widths of the 1 st base structure, and the values of x, y, and z are set, respectively. When there are the 4 th and subsequent base structures, the above operation may be repeated. As described above, the base structures are preferably overlapped in order from the base structure having the wide pitch. Further, the 1 st foundation structure, the 2 nd foundation structure, and the 3 rd foundation structure may be designed separately, and these foundation structures may be finally superposed on the reference plane, but the former method is preferable.
The optical surface of the objective lens 10 has a central region, a peripheral region around the central region, and a most peripheral region around the peripheral region. The central region is the region containing the optical axis of the objective lens. As shown in fig. 4, the central region CN, the peripheral region MD, and the most peripheral region OT are provided on the same optical surface in concentric circles around the optical axis. The objective lens 10 has a structure in which the above-described base structure a, base structure B, and base structure E are superimposed on each other in the central region, and has a structure in which the base structure a and the base structure F are superimposed on each other in the peripheral region. In the present embodiment, the base structure F is also provided in the outermost peripheral region.
The base structure of the central region is provided over the entire surface of the central region. The base structure of the peripheral region is also provided over the entire surface of the peripheral region. The base structure of the outermost region is also provided over the entire surface of the outermost region.
The objective lens 10 condenses the 1 st, 2 nd and 3 rd light beams passing through the central region of the objective lens 10 into condensed spots, respectively. That is, the objective lens 10 condenses the 1 st light beam passing through the central area of the objective lens 10 on the information recording surface of the BD so as to be able to record and/or reproduce information. The objective lens 10 focuses the 2 nd beam passing through the central region of the objective lens 10 on the information recording surface of the DVD so as to be able to record and/or reproduce information. The objective lens 10 focuses the 3 rd light beam passing through the central area of the objective lens 10 on the information recording surface of the CD so as to be able to record and/or reproduce information. Since the protective substrate thickness t1 of the BD is different from the protective substrate thickness t2 of the DVD, at least one base structure (preferably, the base structure a) provided on the central region corrects spherical aberration generated due to the difference between the protective substrate thickness t1 of the BD and the protective substrate thickness t2 of the DVD and/or spherical aberration generated due to the difference between the wavelengths of the 1 st beam and the 2 nd beam with respect to the 1 st beam and the 2 nd beam passing through the base structure. At least one base structure (which may be the same as or different from the base structure described above) provided in the central region, and preferably the base structure B, corrects spherical aberration caused by the difference between the protective substrate thickness t1 of the BD and the protective substrate thickness t3 of the CD and/or spherical aberration caused by the difference between the wavelengths of the 1 st beam and the 3 rd beam with respect to the 1 st beam and the 3 rd beam passing through the base structure.
In addition, the 1 st best focus and the 2 nd best focus are formed by the 3 rd light beam passing through the central region of the objective lens 10.
In the present embodiment, the spot formed by the 3 rd beam at the 1 st best focus is used for recording and/or reproducing the CD, and the spot formed by the 3 rd beam at the 2 nd best focus is not used for recording and/or reproducing the CD.
The objective lens 10 condenses the 1 st light flux and the 2 nd light flux passing through the peripheral region of the objective lens 10 into condensed spots, respectively. That is, the objective lens 10 condenses the 1 st light beam passing through the peripheral area of the objective lens 10 on the information recording surface of the BD so as to be able to record and/or reproduce information. The objective lens 10 focuses the 2 nd light beam passing through the peripheral region of the objective lens 10 on the information recording surface of the DVD so as to be able to record and/or reproduce information. Since the protective substrate thickness t1 of the BD is different from the protective substrate thickness t2 of the DVD, it is preferable that at least one base structure (preferably, the base structure a) provided in the peripheral region correct spherical aberration generated due to the difference between the protective substrate thickness t1 of the BD and the protective substrate thickness t2 of the DVD and/or spherical aberration generated due to the difference between the wavelengths of the 1 st beam and the 2 nd beam with respect to the 1 st beam and the 2 nd beam passing through the base structure.
The 3 rd light beam passing through the peripheral area is not used for recording and/or reproducing the CD, and forms flare on the information recording surface of the CD.
The objective lens 10 focuses the 1 st light beam passing through the outermost peripheral area of the objective lens 10 on the information recording surface of the BD so as to be able to record and/or reproduce information.
The 2 nd light beam passing through the outermost peripheral area is not used for recording and/or reproducing a DVD, and forms flare on the information recording surface of the DVD, and the 3 rd light beam passing through the outermost peripheral area is not used for recording and/or reproducing a CD, and forms flare on the information recording surface of the CD.
The boundary between the central region and the peripheral region of the objective lens 10 is formed in a portion corresponding to NA 3. A boundary between the peripheral region and the most peripheral region of the objective lens 10 is formed in a portion corresponding to NA 2. The boundary outside the outermost periphery of the objective lens 10 is formed in a portion corresponding to NA 1.
Next, the operation of the optical pickup device according to the present embodiment will be described. In fig. 2, when information is reproduced from a BD, the 1 st semiconductor laser 5 is caused to emit light, and a laser beam having a wavelength λ 1 of 405nm or so emitted is reflected by the polarization beam splitter 6 for violet and further reflected by the mirror 4, but a part of the laser beam passes through the mirror 4 and is detected by the power monitor 7 of the monitoring unit, and the intensity of the laser beam is monitored to adjust the intensity of the emitted light of the 1 st semiconductor laser 5 via a drive circuit, not shown. The light beam reflected by the mirror 4 passes through the collimator lens 3 and the λ/4 plate 9, and is condensed on the information recording surface of the BD via the objective lens 10.
The light beam reflected from the BD information recording surface passes through the objective lens 10, the λ/4 wave plate 9, and the collimator lens 3, is reflected by the mirror 4, passes through the polarization spectrometer 6 for violet and the polarization spectrometer 11, enters the photodetector 13 via the servo lens 12, and can be reproduced from the BD by its output signal.
The shape change and intensity distribution change of the light spot on the light detector 13 are detected, and focus detection and track detection are performed. Based on the detection, the objective lens 10 and the bobbin are integrally focused and driven by the actuator 14, and the light beam emitted from the 1 st semiconductor laser 5 is formed on the information recording surface of the BD.
When information is reproduced from a DVD, the 2 nd semiconductor laser beam in the 2 nd laser 1 package 15 is caused to emit light, and the emitted laser beam having a wavelength λ 2 of about 660nm passes through the CD diffraction grating 16, is reflected by the polarization spectrometer 11, passes through the blue-violet polarization spectrometer 6, and is further reflected by the mirror 4, but a part of the laser beam passes through the mirror 4 and is detected by the power monitor 7, and the intensity of the emitted light of the 2 nd semiconductor laser beam is monitored to adjust the intensity of the emitted light of the 2 nd semiconductor laser beam via a drive circuit, not shown. The light beam reflected by the mirror 4 passes through the collimator lens 3 and the λ/4 wave plate 9, and is condensed on the information recording surface of the DVD via the objective lens 10.
The light beam reflected from the DVD information recording surface passes through the objective lens 10, the λ/4 wave plate 9, and the collimator lens 3, is reflected by the mirror 4, passes through the polarization spectrometer 6 for violet and the polarization spectrometer 11, enters the photodetector 13 through the servo lens 12, and can be reproduced from the DVD by its output signal.
The shape change and intensity distribution change of the light spot on the light detector 13 are detected, and focus detection and track detection are performed. Based on this detection, the objective lens 10 and the bobbin are integrally focused and tracking-driven by the actuator 14, and the light beam emitted from the 2 nd semiconductor laser beam is formed on the information recording surface of the DVD.
When information is reproduced from a CD, the 3 rd semiconductor laser in the 2 nd laser 1 package 15 is caused to emit light, and a laser beam having a wavelength λ 3 of 785nm or so emitted is incident on the CD diffraction grating 16, thereby generating ± 1 st order diffracted light for a tracking signal. The laser beam is reflected by the polarization spectrometer 11, passes through the polarization spectrometer for blue-violet 6, and is further reflected by the mirror 4, but a part of the laser beam passes through the mirror 4 and is detected by the power monitor 7, and the intensity thereof is monitored to adjust the emission light intensity of the 3 rd semiconductor laser beam via a drive circuit, not shown. The light beam reflected by the mirror 4 passes through the collimator lens 3 and the λ/4 plate 9, and is condensed on the information recording surface of the CD via the objective lens 10.
The light beam reflected from the CD information recording surface passes through the objective lens 10, the λ/4 wave plate 9, and the collimator lens 3, is reflected by the mirror 4, passes through the polarization spectrometer 6 for violet and the polarization spectrometer 11, enters the photodetector 13 through the servo lens 12, and can be reproduced from the CD by its output signal.
The shape change and intensity distribution change of the light spot on the light detector 13 are detected, and focus detection and track detection are performed. Based on this detection, the objective lens 10 and the bobbin are integrally focused and tracking-driven by the actuator 14, and the light beam emitted from the 3 rd semiconductor laser beam is formed on the information recording surface of the CD.
Examples
The present inventors examined examples for a plurality of basic structures overlapping in the central region of the objective lens 10 used in the above-described embodiments, as compared with comparative examples as described below. The central region has a structure in which the above-described base structure B (hereinafter, also referred to as "1 st base structure"), base structure a (hereinafter, also referred to as "2 nd base structure"), and base structure E (hereinafter, also referred to as "3 rd base structure") overlap. Since the 1 st and 3 rd basic structures are step amounts giving an optical path difference of 4 wavelengths or more of the wavelength λ 1, the wavelength at which the diffraction efficiency of the 1 st basic structure is the maximum is λ α, and the wavelength at which the diffraction efficiency of the 3 rd basic structure is the maximum is λ β. As shown in fig. 5(a), the 1 st basic structure is designed such that the wavelength (λ α) at which the diffraction efficiency is maximized is 392.5nm, as shown in fig. 5(b), the 2 nd basic structure is designed such that the wavelength at which the diffraction efficiency is maximized is 395nm, and as shown in fig. 5(c), the 3 rd basic structure is designed such that the wavelength (λ β) at which the diffraction efficiency is maximized is 405nm, and thus | λ α - λ β |, becomes 12.5 nm. Since λ 1 is 405nm, the value of (λ β - λ 1) × (λ α - λ 1) is 0, and this embodiment is a comparative example out of the scope of the present invention. FIG. 5(a) is a calculation result of the 1 st base structure in which the wavelength (flame wavelength) at which the diffraction efficiency is maximum is 392.5 nm. FIG. 5(b) is a calculation result of the 2 nd basic structure when the wavelength (flame wavelength) at which the diffraction efficiency is maximum is 395 nm. FIG. 5(c) is a calculation result of the 3 rd basic structure when the wavelength (flame wavelength) at which the diffraction efficiency is maximum is 405 nm. The 1 st optical path difference of the 1 st basic structure, the 2 nd basic structure, and the 3 rd basic structure gives the diffraction efficiency of the whole structure, and is approximately the diffraction efficiency of the 1 st basic structure, the 2 nd basic structure, and the 3 rd basic structure after the respective diffraction efficiencies are mutually mounted, so the wavelength dependence is as shown in fig. 5 (d). As is clear from these figures, the 1 st basic structure and the 3 rd basic structure having a deep step structure have a large wavelength dependence, and therefore the 1 st optical path difference has a steep wavelength dependence on the entire structure. In particular, in this example, when the wavelength is shifted to a wavelength longer than the use wavelength, the decrease in diffraction efficiency increases. Since the average value (λ 1) of the emission wavelengths of the blue-violet semiconductor laser light is about 405nm, the 1 st optical path difference with respect to the wavelength change of 1nm changes by 4point in the diffraction efficiency of the entire structure. Here, even if the wavelength at which the diffraction efficiency of the 1 st infrastructure is maximized and the wavelength shift effect at which the diffraction efficiency of the 2 nd infrastructure is maximized are made very thin, the wavelength at which the diffraction efficiency of the 3 rd infrastructure is maximized is shifted.
In example 1, the 1 st basic structure and the 2 nd basic structure are used in common, and as shown in fig. 6(a), the 3 rd basic structure is designed such that the wavelength (λ β) at which the diffraction efficiency is maximum is 410nm, and in this case, | λ α - λ β |, becomes 17.5nm, and the value of (λ β - λ 1) × (λ α - λ 1) becomes-62.5 and less than 0, which is an example of the present invention. FIG. 6(a) is a calculation result of the 3 rd basic structure in which the wavelength (flame wavelength) at which the diffraction efficiency is maximum is 410 nm. The total diffraction efficiency of the 1 st optical path difference-imparting structure in which the above-described 3 basic structures are superimposed is a value obtained by mutually mounting the diffraction efficiency of the 1 st basic structure shown in fig. 5(a), the diffraction efficiency of the 2 nd basic structure shown in fig. 5(b), and the diffraction efficiency of the 3 rd basic structure shown in fig. 6(a), as shown in fig. 6 (b). Therefore, the wavelength at which the total diffraction efficiency of the 1 st optical path difference imparting structure in which the 3 basic structures are stacked is the maximum is 402nm as shown in fig. 6(b), and the fluctuation of the diffraction efficiency at 405nm of the blue-violet semiconductor laser wavelength with respect to the wavelength change of 1nm is recovered to 2 point. In example 2, the same basic structure 1 and basic structure 2 are used in common, and as shown in fig. 7(a), the basic structure 3 is designed so that the wavelength (λ β) at which the diffraction efficiency is maximized is 412.5nm, and in this case, | λ α - λ β |, 20nm, and the value of (λ β - λ 1) × (λ α - λ 1) is-93.75 smaller than 0, which is an example of the present invention. FIG. 7(a) is a calculation result of the 3 rd basic structure in which the wavelength (flame wavelength) at which the diffraction efficiency is maximum is 412.5 nm. The total diffraction efficiency of the 1 st optical path difference-imparting structure obtained by superimposing them is a value obtained by mutually mounting the diffraction efficiency of the 1 st basic structure shown in fig. 5(a), the diffraction efficiency of the 2 nd basic structure shown in fig. 5(b), and the diffraction efficiency of the 3 rd basic structure shown in fig. 7(a), as shown in fig. 7 (b). Therefore, the wavelength at which the total diffraction efficiency of the 1 st optical path difference imparting structure in which the 3 basic structures are stacked is maximum is 404nm as shown in fig. 7(b), and the fluctuation of the diffraction efficiency at 405nm of the blue-violet semiconductor laser wavelength with respect to the wavelength change of 1nm is recovered to 1 point. In example 3, the same basic structure 1 and basic structure 2 are used in common, and as shown in fig. 8(a), the basic structure 3 is designed so that the wavelength (λ β) at which the diffraction efficiency is maximized is 415nm, and in this case, | λ α - λ β |, 22.5nm, and the value of (λ β - λ 1) × (λ α - λ 1) is-125 smaller than 0, which is an example of the present invention. FIG. 8(a) is a calculation result of the 3 rd basic structure in which the wavelength (flame wavelength) at which the diffraction efficiency is maximum is 415 nm. The total diffraction efficiency of the 1 st optical path difference-imparting structure obtained by superimposing them is a value obtained by mutually mounting the diffraction efficiency of the 1 st basic structure shown in fig. 5(a), the diffraction efficiency of the 2 nd basic structure shown in fig. 5(b), and the diffraction efficiency of the 3 rd basic structure shown in fig. 8(a), as shown in fig. 8 (b). Therefore, the wavelength at which the total diffraction efficiency of the 1 st optical path difference imparting structure in which the 3 fundamental structures are superposed is the maximum is 405nm as shown in fig. 8(b), and the variation in diffraction efficiency with respect to the wavelength change of 1nm at 405nm of the wavelength of the blue-violet semiconductor laser is almost 0. Regarding the diffraction efficiency, each infrastructure generates a plurality of diffracted lights different in the diffraction order, but the diffraction efficiency of the light of the specific diffraction order in which the spot is formed on the optical disk is calculated. Similarly, although a plurality of diffracted lights are generated in the optical path difference providing structure in which a plurality of base structures are stacked, the diffraction efficiency is calculated only for a specific diffracted light which forms a spot on the optical disk.
From this, it is understood that, if the wavelength at which the diffraction efficiency of the 3 rd basic structure is maximized is designed to be longer than the wavelength at which the diffraction efficiency of the 1 st basic structure is maximized, the wavelength at which the total diffraction efficiency of the 1 st optical path difference imparting structure thus superimposed is maximized is also shifted to be longer, but it is known that if the wavelengths at which the diffraction efficiencies of the 1 st basic structure and the 3 rd basic structure are maximized are too far apart from each other, the peak value of the total diffraction efficiency is lowered. It is also preferable that the difference between the wavelengths at which the diffraction efficiencies of the 1 st and 3 rd basic structures become maximum is 4nm to 60 nm.
Similarly, the same effect can be obtained by designing the basic structures in the peripheral region to have a wavelength of 405nm, and by designing the basic structures B and F to have a larger wavelength and a smaller wavelength, respectively, at which the diffraction efficiency of the 1 st beam is the maximum.
Claims (15)
1. An objective lens for an optical pickup device for condensing a light beam having a predetermined wavelength λ 1 of 390nm or more and λ 1 or less and 420nm or less on an information recording surface of an optical information recording medium to record/reproduce information,
an objective lens for an optical pickup device, characterized in that a plurality of basic structures as optical path difference imparting structures are provided on an optical surface of the objective lens, and the following expression is satisfied:
4nm≤|λα-λβ|≤60nm (1)
(λβ-λ1)×(λα-λ1)＜0 (2)，
wherein,
λ α: a wavelength at which the diffraction efficiency of a certain infrastructure of the plurality of infrastructures is maximum within a wavelength λ 1 ± 50 nm;
λ β: a wavelength at which a diffraction efficiency of another of the plurality of base structures is maximum within a range of a wavelength λ 1 ± 50 nm;
the basic structure is an optical path difference providing structure provided for providing a predetermined function to an optical surface, and is an optical path difference providing structure for increasing the amount of a-th order diffracted light of a light beam passing through the basic structure to be larger than the amount of any other order diffracted light, wherein a is an integer.
2. The objective lens for an optical pickup apparatus according to claim 1, wherein at least some of the plurality of base structures are formed to be overlapped on a predetermined region of the surface of the optical surface.
3. The objective lens for an optical pickup apparatus according to claim 1 or 2, wherein at least some of the plurality of base structures are formed on different regions of the optical surface.
4. The objective lens for an optical pickup device according to claim 1 or 2, wherein the objective lens condenses the light beam having the wavelength λ 1 on the information recording surface of the optical information recording medium having a protective substrate thickness t1 for information recording/reproduction and condenses the light beam having the wavelength λ 2 on the information recording surface of the optical information recording medium having a protective substrate thickness t2 for information recording/reproduction, wherein λ 2 > λ 1, t2 ≧ t 1.
5. The objective lens for an optical pickup apparatus according to claim 1 or 2, wherein the objective lens condenses the light beam having the wavelength λ 1 on the information recording surface of the optical information recording medium having the protective substrate thickness t1 for information recording/reproduction, condenses the light beam having the wavelength λ 2 on the information recording surface of the optical information recording medium having the protective substrate thickness t2 for information recording/reproduction, and condenses the light beam having the wavelength λ 3 on the information recording surface of the optical information recording medium having the protective substrate thickness t3 for information recording/reproduction, wherein λ 2 > λ 1, t2 ≧ t1, λ 3 > λ 2, t3 > t 2.
6. The objective lens for an optical pickup device according to claim 5, wherein the plurality of basic structures include a1 st basic structure and a2 nd basic structure, the 1 st basic structure is a structure for giving an optical path difference in which an amount of light diffracted in r-th order of the light beam having the wavelength λ 1 passing through the 1 st basic structure is larger than an amount of light diffracted in any other order, an amount of light diffracted in s-th order of the light beam having the wavelength λ 2 is larger than an amount of light diffracted in any other order, an amount of light diffracted in t-th order of the light beam having the wavelength λ 3 is larger than an amount of light diffracted in any other order, and the 2 nd basic structure is a structure for giving an optical path difference in which an amount of light diffracted in u-th order of the light beam having the wavelength λ 1 passing through the 2 nd basic structure is larger than an amount of light diffracted in any other order, an amount of light diffracted in v-th order of the light beam having the wavelength λ 2 is larger than an amount of light diffracted in any other order, and wherein r, s, t, u, v and w are integers.
7. The objective lens for an optical pickup apparatus according to claim 6, wherein said plurality of basic structures include a3 rd basic structure in addition to said 1 st basic structure and said 2 nd basic structure, and said 3 rd basic structure is an optical path difference providing structure in which an amount of light of a light beam having a wavelength λ 1 which passes through said 3 rd basic structure is larger than an amount of light of any other order of diffraction, an amount of light of a light beam having a wavelength λ 2 which passes through said 3 rd basic structure is larger than an amount of light of any other order of diffraction, and an amount of light of a light beam having a wavelength λ 3 which passes through said 3 rd basic structure is larger than an amount of light of any other order of diffraction, wherein x, y, and z are integers.
8. The objective lens for an optical pickup device according to claim 7, wherein r is 0, s is 0, t is ± 1, u is 2, v is 1, w is 1, x is 10, y is 6, and z is 5.
9. The objective lens for an optical pickup apparatus according to claim 5, wherein at least one of the plurality of basic structures is a structure for correcting spherical aberration caused by a protective substrate thickness for an optical information recording medium, based on a difference between the wavelength λ 1 and the wavelength λ 2.
10. The objective lens for an optical pickup apparatus according to claim 5, wherein at least one of the plurality of basic structures is a structure for correcting spherical aberration caused by a protective substrate thickness for an optical information recording medium, based on a difference between the wavelength λ 1 and the wavelength λ 3.
11. The objective lens for an optical pickup apparatus according to claim 5, wherein at least one of the plurality of basic structures is a structure for correcting spherical aberration caused by a thickness of a protective substrate for an optical information recording medium based on a difference between the wavelength λ 1 and a wavelength other than the wavelength λ 1, and another of the plurality of basic structures other than the at least one basic structure is a structure for correcting a change in spherical aberration caused by a temperature change when recording/reproducing the optical information recording medium by the light beam having the wavelength λ 1.
12. The objective lens for an optical pickup apparatus according to claim 1 or 2, wherein at least one of the plurality of basic structures is a structure for correcting a change in spherical aberration caused by a change in temperature when recording/reproducing information on/from the optical information recording medium by the light beam having the wavelength λ 1.
13. The objective lens for an optical pickup device according to claim 1 or 2, wherein the basic structure having the maximum diffraction efficiency at the wavelength λ α and the basic structure having the maximum diffraction efficiency at the wavelength λ β each have a step for providing an optical path difference of 4 wavelengths or more with respect to the wavelength λ 1.
14. An optical pickup device comprising a light source for emitting a light beam having a wavelength λ 1 and an objective lens according to any one of claims 1 to 13.
15. The optical pickup device according to claim 14, comprising a monitoring unit for monitoring an intensity of the light beam emitted from the light source before the light beam enters the objective lens.
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| JP2007-251927 | 2007-09-27 | ||
| JP2007251927 | 2007-09-27 | ||
| PCT/JP2008/066726 WO2009041322A1 (en) | 2007-09-27 | 2008-09-17 | Objective lens for optical pickup devices and optical pickup device |
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| CN101809661B true CN101809661B (en) | 2012-07-11 |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1685407A (en) * | 2002-09-30 | 2005-10-19 | 柯尼卡美能达控股株式会社 | Optical element, objective optical element and optical pickup device |
| JP2006164498A (en) * | 2004-12-02 | 2006-06-22 | Konica Minolta Opto Inc | Objective optical system, optical pickup device, and optical information recording and reproducing apparatus |
| CN101034571B (en) * | 2006-03-07 | 2011-07-20 | 柯尼卡美能达精密光学株式会社 | Optical pickup apparatus, objective optical element and optical information recording reproducing apparatus |
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| CA2558035A1 (en) * | 2004-03-17 | 2005-09-22 | Sumitomo Electric Industries, Ltd. | Hologram color filter, method for fabricating the same, and color liquid crystal display comprising it |
| JP2007109801A (en) * | 2005-10-12 | 2007-04-26 | Sumitomo Electric Ind Ltd | Solid-state imaging apparatus and method of manufacturing same |
| JP4908838B2 (en) * | 2005-12-13 | 2012-04-04 | 富士通株式会社 | Multi-wavelength spectrometer |
| KR20080098644A (en) * | 2006-03-07 | 2008-11-11 | 코니카 미놀타 옵토 인코포레이티드 | Optical pickup device, objective optical element, and optical information recording/ reproducing device |
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- 2008-09-17 JP JP2009534293A patent/JP5408549B2/en not_active Expired - Fee Related
- 2008-09-17 CN CN2008801083930A patent/CN101809661B/en not_active Expired - Fee Related
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|---|---|---|---|---|
| CN1685407A (en) * | 2002-09-30 | 2005-10-19 | 柯尼卡美能达控股株式会社 | Optical element, objective optical element and optical pickup device |
| JP2006164498A (en) * | 2004-12-02 | 2006-06-22 | Konica Minolta Opto Inc | Objective optical system, optical pickup device, and optical information recording and reproducing apparatus |
| CN101034571B (en) * | 2006-03-07 | 2011-07-20 | 柯尼卡美能达精密光学株式会社 | Optical pickup apparatus, objective optical element and optical information recording reproducing apparatus |
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| JPWO2009041322A1 (en) | 2011-01-27 |
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| WO2009041322A1 (en) | 2009-04-02 |
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