WO2011077647A1 - 光学ヘッド、光ディスク装置、情報処理装置及び対物レンズ - Google Patents
光学ヘッド、光ディスク装置、情報処理装置及び対物レンズ Download PDFInfo
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- WO2011077647A1 WO2011077647A1 PCT/JP2010/007084 JP2010007084W WO2011077647A1 WO 2011077647 A1 WO2011077647 A1 WO 2011077647A1 JP 2010007084 W JP2010007084 W JP 2010007084W WO 2011077647 A1 WO2011077647 A1 WO 2011077647A1
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- objective lens
- spherical aberration
- information recording
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- wavelength
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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
- 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
- G11B7/1392—Means for controlling the beam wavefront, e.g. for correction of aberration
- G11B7/13922—Means for controlling the beam wavefront, e.g. for correction of aberration passive
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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
- G11B2007/0009—Recording, reproducing or erasing systems characterised by the structure or type of the carrier for carriers having data stored in three dimensions, e.g. volume storage
- G11B2007/0013—Recording, reproducing or erasing systems characterised by the structure or type of the carrier for carriers having data stored in three dimensions, e.g. volume storage for carriers having multiple discrete layers
Definitions
- the present invention relates to an optical head that records or reproduces information on an information recording medium such as an optical disk, an optical disk device including the optical head, an information processing device including the optical disk device, and an objective lens used in the optical head.
- an information recording medium such as an optical disk
- an optical disk device including the optical head
- an information processing device including the optical disk device
- an objective lens used in the optical head is about.
- Blu-ray Disc which is a high-density and large-capacity optical information recording medium (hereinafter also referred to as optical disc), is the same size as Compact Disc (CD) and DVD. It has been put into practical use.
- the BD uses an blue-violet laser light source that emits a blue-violet laser beam having a wavelength of about 405 nm and an objective lens having a numerical aperture (NA) of about 0.85, and an information recording layer having a light transmission layer thickness of about 100 ⁇ m.
- the BD is practically used as a single-layer disc having one information recording surface and a two-layer disc having two information recording surfaces, and the recording capacity of one layer is about 25 GB.
- the laser beam emitted from the light source is accompanied by a sudden change in the laser beam emission power associated with switching between recording and reproduction.
- the wavelength of light changes.
- the refractive index of the objective lens changes, and a position shift (defocus) of the focus point of the objective lens occurs. This defocus due to wavelength change is called axial chromatic aberration.
- Patent Document 1 discloses an optical head using an optical element having a diffractive structure or an objective lens having a diffractive structure as shown in FIG. 26 in order to suppress axial chromatic aberration.
- FIG. 26 is a diagram schematically showing the shape of a conventional objective lens.
- concentric diffractive structures are formed, which give the diffracted light a (positive) power component of a convex lens.
- n-order diffracted light that forms a light spot for recording or reproduction is generated.
- unnecessary diffracted light of adjacent orders for example, n + 1 order, n + 2 order, n-1 order, n-2 order, etc.
- the unnecessary diffracted light forms a light spot at a position different from the n-th order diffracted light, and is an information recording surface other than the information recording surface to be recorded or reproduced and the information recording surface to be recorded or reproduced. Or reflected from the surface of the optical disk.
- Patent Document 2 discloses an optical head that defines the position of a light spot by unnecessary diffracted light so that the unnecessary diffracted light causes a large defocus on the light receiving element.
- FIG. 27 is a diagram showing a schematic configuration of a conventional multilayer optical disc.
- Table 1 below is a table showing the distance between the surface of the optical disc and the information recording surface in the conventional multilayer optical disc.
- the distance t0 between the surface 161 of the optical disk and the information recording surface L0 is 100 ⁇ m
- the distance t1 between the surface 161 of the optical disk and the information recording surface L1 is 84 ⁇ m
- the surface 161 and the information recording surface of the optical disk is 50 ⁇ m.
- the surface 161 of the optical disk and the other information recording surfaces L0 to L3 are recorded during recording or reproduction on the information recording surfaces L0 to L3.
- the values that can be taken are widely distributed in the range of ⁇ 50 ⁇ m to +100 ⁇ m. Therefore, in an optical head using an optical element having a diffractive structure or an objective lens having a diffractive structure, no matter how the position of the light spot of unnecessary diffracted light is defined, the thickness of the light transmission layer varies. In view of (for example, ⁇ 5 ⁇ m), it may be unavoidable that unnecessary diffracted light is condensed on the light receiving element.
- the present invention has been made in view of such problems, and an optical head, an optical disc apparatus, and an information processing capable of recording or reproducing information on an information recording medium having a plurality of information recording surfaces.
- An object is to provide an apparatus and an objective lens.
- An optical head is an optical head that records or reproduces information on an information recording medium having a plurality of information recording surfaces, and includes a light source that emits laser light, and an annular diffraction structure. And an objective lens that converges the n-th order (n is a natural number) diffracted light generated by diffracting the laser light onto a predetermined information recording surface of the information recording medium, and is reflected by the predetermined information recording surface And a photodetector that receives the laser beam, and the diffractive structure adds a positive power component and a spherical aberration component to the nth-order diffracted light.
- the light source emits laser light.
- the objective lens has an annular diffractive structure, and converges n-th order (n is a natural number) diffracted light generated by diffracting laser light onto a predetermined information recording surface of the information recording medium.
- the photodetector receives the laser beam reflected by a predetermined information recording surface.
- the diffractive structure adds a positive power component and a spherical aberration component to the nth-order diffracted light.
- a positive power component is added to the nth-order diffracted light
- axial chromatic aberration can be corrected and a spherical aberration component is added to the nth-order diffracted light.
- the diffraction stray light reflected by another information recording surface other than the information recording surface is not condensed at one point by the spherical aberration component, and interference between the signal light and the diffraction stray light can be reduced, and a plurality of information recording surfaces are provided.
- Information can be recorded or reproduced favorably on the information recording medium.
- Embodiment 1 is a diagram showing a schematic configuration of a multilayer optical disc according to Embodiment 1 of the present invention. It is a figure which shows typically schematic structure of the objective lens actuator in Embodiment 1 of this invention. It is a figure which shows typically schematic structure of the collimating lens actuator in Embodiment 1 of this invention.
- A is a figure which shows the emitted light when a collimating lens exists in a reference position
- B is a figure which shows the emitted light when a collimating lens moves to the light source side
- C It is a figure which shows the emitted light when a collimating lens moves to the objective lens side.
- FIG. 1 shows typically the shape of the objective lens in Embodiment 1 of this invention. It is a schematic diagram which shows a mode that a blue-violet laser beam is converged on the information recording surface L2 of a multilayer optical disk using the objective lens of Embodiment 1 of this invention.
- (A) to (C) are diagrams showing a state of a focused spot on the light receiving element in the first embodiment of the present invention. In the first embodiment of the present invention, when the condensing spot is formed on the light receiving element without converging the + secondary light on the information recording surface, the + 1st order light and the + secondary light converge on the multilayer optical disc. It is a schematic diagram.
- Embodiment 1 of this invention it is a figure which shows the relationship between the spherical aberration characteristic by a diffractive lens structure, and the 3rd spherical aberration and 4th spherical aberration which generate
- Embodiment 1 of this invention it is a figure which shows the relationship between the spherical aberration characteristic by a diffractive lens structure, and the 5th spherical aberration which generate
- Embodiment 1 of this invention it is a figure which shows the relationship between the spherical aberration characteristic by a diffractive lens structure, and the spherical aberration amount (total spherical aberration) which should be corrected which generate
- the longitudinal chromatic aberration characteristic is 0.1 ⁇ m / nm
- the minimum pitch of the annular pattern of the diffractive lens structure, the temperature change, and the individual difference of the light source wavelength are corrected.
- FIG. 14 is a diagram showing the relationship between the minimum pitch of the annular pattern of the diffractive lens structure and the total spherical aberration when the allowable range of individual differences in the light source wavelength is expanded with respect to FIG. It is a figure for demonstrating the specific Example of the objective lens in Embodiment 1 of this invention.
- FIG. 3 is a diagram illustrating an optical path in an objective lens having a diffractive structure according to Example 1.
- 5 is a graph showing longitudinal aberration (spherical aberration) when parallel light is incident on the objective lens in the objective lens having the diffractive structure of Example 1.
- FIG. 6 is a diagram illustrating an optical path in an objective lens having a diffractive structure according to Example 2.
- FIG. 3 is a diagram illustrating an optical path in an objective lens having a diffractive structure according to Example 1.
- FIG. 6 is a graph showing longitudinal aberration (spherical aberration) when parallel light enters the objective lens in the objective lens having the structure of Example 2.
- 6 is a diagram illustrating an optical path in an objective lens having a diffractive structure according to Example 3.
- FIG. 6 is a graph showing longitudinal aberration (spherical aberration) when parallel light enters the objective lens in the objective lens having the diffractive structure of Example 3.
- It is a figure which shows schematic structure of the optical disk apparatus in Embodiment 2 of this invention.
- It is a figure which shows schematic structure of the computer in Embodiment 3 of this invention.
- It is a figure which shows schematic structure of the optical disk player in Embodiment 4 of this invention.
- It is a figure which shows schematic structure of the optical disk recorder in Embodiment 5 of this invention.
- It is a figure which shows typically the shape of the conventional objective lens.
- It is a figure which shows schematic structure of the conventional multilayer optical disk.
- FIG. 1 is a diagram showing a schematic configuration of an optical head according to Embodiment 1 of the present invention.
- FIG. 2 is a diagram showing a schematic configuration of the multilayer optical disc according to Embodiment 1 of the present invention.
- an optical head 40 includes a blue-violet laser light source 1, a polarizing beam splitter 3, a collimating lens 4, a rising mirror 5, a quarter wavelength plate 6, an objective lens 8, an objective lens actuator 9, a collimating lens actuator 14, and detection.
- a hologram 21, a detection lens 22, a light receiving element 23, and a front monitor sensor 24 are provided.
- the multilayer optical disc 60 includes four information recording surfaces L0 to L3 having a light transmission layer thickness t0 to t3 of 100 ⁇ m to 50 ⁇ m.
- the thickness t0 of the light transmission layer on the information recording surface L0 is, for example, 100 ⁇ m
- the thickness t1 of the light transmission layer on the information recording surface L1 for example, is 83 ⁇ m
- the thickness t3 of the light transmission layer of the information recording surface L3 is, for example, 50 ⁇ m.
- the light transmission layer represents a layer from the information recording surface to the surface (light incident surface) 61 of the multilayer optical disc 60. Therefore, the thickness of the light transmission layer on the information recording surface represents the distance from the information recording surface to the surface 61 of the multilayer optical disc 60.
- An intermediate layer is disposed between the information recording surfaces, a cover layer is disposed on the light incident surface side of the information recording surface closest to the surface 61 of the multilayer optical disc 60, and information furthest away from the surface 61 of the multilayer optical disc 60.
- a substrate is disposed on the recording surface opposite to the light incident surface.
- the optical head 40 records or reproduces information on a multilayer optical disc (information recording medium) 60 having a plurality of information recording surfaces each having a different thickness of the light transmission layer.
- the blue-violet laser light source 1 emits blue-violet laser light having a wavelength of about 405 nm.
- the objective lens 8 has a ring-shaped diffraction structure, and converges n-th order (n is a natural number) diffracted light generated by diffracting blue-violet laser light onto a predetermined information recording surface of the multilayer optical disc 60.
- the light receiving element (light detector) 23 receives the blue-violet laser beam reflected by a predetermined information recording surface.
- the collimating lens (coupling lens) 4 is disposed between the blue-violet laser light source 1 and the objective lens 8.
- the collimating lens actuator (spherical aberration correcting unit) 14 corrects the spherical aberration generated according to the distance from the light incident surface of the multilayer optical disc 60 to the information recording surface by moving the collimating lens 4 in the optical axis direction.
- the blue-violet laser light having a wavelength of about 405 nm emitted from the blue-violet laser light source 1 enters the polarization beam splitter 3 as S-polarized light.
- the blue-violet laser light reflected by the polarization beam splitter 3 is converted into substantially parallel light by the collimator lens 4 and enters the rising mirror 5.
- a part of the blue-violet laser light incident on the rising mirror 5 is reflected in the direction of the quarter-wave plate 6.
- the other part of the laser light incident on the rising mirror 5 passes through the rising mirror 5 and enters the front monitor sensor 24. Based on the output of the front monitor sensor 24, the output of the blue-violet laser light source 1 is controlled. On the other hand, the blue-violet laser light reflected by the rising mirror 5 is converted into circularly polarized light by the quarter-wave plate 6 and then light spotted on any of the information recording surfaces L0 to L3 of the multilayer optical disk 60 by the objective lens 8. As converged.
- the blue-violet laser beam reflected by the predetermined information recording surface of the multilayer optical disk 60 is transmitted again through the objective lens 8, converted into linearly polarized light different from the forward path by the quarter wavelength plate 6, and then reflected by the rising mirror 5. Is done.
- the blue-violet laser light reflected by the rising mirror 5 passes through the collimating lens 4 and then enters the polarization beam splitter 3 as P-polarized light.
- the blue-violet laser light transmitted through the polarization beam splitter 3 is guided to the light receiving element 23 via the detection hologram 21 and the detection lens 22.
- the blue-violet laser light detected by the light receiving element 23 is photoelectrically converted.
- a signal generated by the photoelectric conversion is calculated by a control unit (not shown), a focus error signal for following the surface shake of the multilayer optical disc 60, a tracking error signal for following the eccentricity of the multilayer optical disc 60, A reproduction signal is generated.
- the focus error signal for following the surface blur of the multilayer optical disc 60 is a so-called astigmatism method in which a condensing spot given astigmatism by the detection lens 22 is detected by a four-divided light receiving pattern in the light receiving element 23, etc. Is detected.
- the tracking error signal for following the eccentricity of the multilayer optical disc 60 detects the 0th order light and the 1st order diffracted light generated when passing through the detection hologram 21 in a predetermined light receiving area of the light receiving element 23. Is generated by As a result, the tracking error signal varies when the groove position, width, and depth of the information track formed on the multilayer optical disc 60 vary, and information is recorded on the information track, resulting in a change in reflectance. It is possible to suppress fluctuations in the tracking error signal.
- the detection of the focus error signal and the tracking error signal is not limited to these detection methods.
- the tracking error signal is a so-called differential signal using a main beam and a sub beam generated by a diffraction grating.
- a push-pull method (DPP method) or the like can be used.
- FIG. 3 is a diagram schematically showing a schematic configuration of the objective lens actuator according to Embodiment 1 of the present invention.
- the plurality of suspension wires 9a support an objective lens holder (movable part) 9b.
- the objective lens actuator 9 moves the objective lens 8 in the biaxial direction (focus direction FCD and tracking direction TD) based on the focus error signal and the tracking error signal so that the light spot follows the information track of the rotating multilayer optical disc 60.
- the objective lens actuator 9 may have a structure capable of tilting the objective lens 8 in the radial direction RD of the multilayer optical disc 60 in addition to the displacement in the focus direction FCD and the tracking direction TD.
- the collimating lens actuator in the first embodiment will be described.
- the collimating lens 4 can be moved in the optical axis direction of the collimating lens 4 by a collimating lens actuator 14.
- FIG. 4 is a diagram schematically showing a schematic configuration of the collimating lens actuator 14 according to Embodiment 1 of the present invention.
- the collimating lens actuator 14 includes a stepping motor 72, a screw shaft 73, a main shaft 74, a sub shaft 75, and a lens holder 76.
- the stepping motor 72 is driven and the screw shaft 73 is rotated, the lens holder 76 that holds the collimating lens 4 moves the collimating lens 4 along the main shaft 74 and the sub shaft 75 in the optical axis direction AD.
- FIG. 5A is a diagram illustrating the emitted light when the collimating lens is at the reference position
- FIG. 5B is a diagram illustrating the emitted light when the collimating lens is moved to the light source side
- FIG. 5C is a diagram illustrating emitted light when the collimating lens moves to the objective lens side.
- the collimating lens 4 when the collimating lens 4 is at the reference position, the light emitted from the collimating lens 4 becomes substantially parallel light.
- FIG. 5B by moving the collimating lens 4 from the reference position to the light source side, the light emitted from the collimating lens 4 becomes divergent light, and the light transmission layer of the multilayer optical disc 60 becomes thick. It is possible to correct spherical aberration that occurs in such a case.
- the collimating lens 4 by moving the collimating lens 4 from the reference position to the objective lens side, the light emitted from the collimating lens 4 becomes convergent light, and the light transmission layer of the multilayer optical disc 60 becomes thin. It is possible to correct spherical aberration that occurs in some cases. That is, in the multilayer optical disc 60 having a plurality of information recording surfaces, the spherical aberration can be corrected by moving the collimating lens 4 according to the thickness of the light transmission layer of each information recording surface.
- the configuration of the collimating lens actuator 14 that moves the collimating lens 4 in the optical axis direction is not limited to the configuration using the stepping motor 72 as shown in FIG. 4, for example, by driving a magnetic circuit or a piezoelectric element. Any configuration such as an actuator may be used. In the configuration using the stepping motor 72, it is not necessary to monitor the position of the collimating lens 4 in the optical axis direction, and the system can be simplified. On the other hand, an actuator driven by a magnetic circuit or a piezoelectric element has a small driving portion and is suitable for downsizing an optical head.
- the objective lens 8 is designed as follows, for example.
- the design light transmission layer thickness represents the thickness of the light transmission layer at which spherical aberration is minimized (substantially zero) when parallel light enters the objective lens 8.
- Design wavelength 405 nm
- Design temperature 40 ° C
- Designed light transmission layer thickness 75 ⁇ m
- Focal length 1.3mm
- the objective lens 8 is a single lens made of resin. Therefore, the specific gravity is smaller than that of a glass objective lens, and the burden on the objective lens actuator 9 that performs focus servo or tracking servo can be reduced. Moreover, it is possible to mass-produce with high accuracy by injection molding, which is suitable for cost reduction.
- the objective lens 8 has a design light transmission layer thickness of 75 ⁇ m. Therefore, when the blue-violet laser light is focused on the information recording surface L0 having a light transmission layer thickness of 100 ⁇ m and the information recording surface L1 having a light transmission layer thickness of 83 ⁇ m, the collimating lens 4 is moved to the light source side. Then, divergent light is made incident on the objective lens 8. Thereby, the spherical aberration which is generated when the thickness of the light transmission layer is deviated from the design light transmission layer thickness is corrected.
- the collimating lens 4 is moved to the objective lens side.
- convergent light is made incident on the objective lens 8.
- FIG. 6 is a diagram schematically showing the shape of the objective lens according to Embodiment 1 of the present invention.
- the objective lens 8 has a first surface (incident surface) 81 on which blue-violet laser light is incident and a second surface (emitted surface) 82 from which blue-violet laser light is emitted.
- the objective lens 8 has an annular diffractive lens structure centered on the optical axis on the first surface (incident surface) 81.
- This diffractive lens structure has a step ⁇ in the optical axis direction at the boundary of each annular zone, and in the blue-violet laser light having a wavelength of 405 nm from the blue-violet laser light source 1, the diffraction efficiency of the + 1st order light (+ 1st order diffracted light) is the maximum. It is comprised so that.
- diffraction stray light Since the diffraction efficiencies of diffraction orders other than the + 1st order light, such as 0th order light, ⁇ 1st order light, and ⁇ 2nd order light, cannot be made zero, these lights are considered as unnecessary light called diffraction stray light. Become. Such diffracted stray light is converged at a position different from the + first-order light on the optical axis to form a light spot.
- the diffracted light having the power component of the convex lens is such that when the wavelength of the blue-violet laser light emitted from the blue-violet laser light source 1 is shifted to the long wavelength side, the diffraction angle on the diffraction surface becomes large and the convex power Becomes stronger.
- the wavelength of the blue-violet laser light emitted from the blue-violet laser light source 1 is shifted to the short wavelength side, the diffraction angle on the diffraction surface becomes small and the convex power becomes weak.
- the axial chromatic aberration characteristic of the objective lens 8 of the first embodiment is 0.1 ⁇ m / nm. That is, the defocus amount for a wavelength change of 1 nm is 0.1 ⁇ m.
- the output power changes between reproduction and recording.
- the permissible defocus amount can be somewhat increased by giving a predetermined electrical offset immediately before switching between reproduction and recording. Therefore, if the longitudinal chromatic aberration characteristic is 0.15 ⁇ m / nm or less, there is often no substantial problem. On the other hand, 0.05 ⁇ m / nm is sufficient for the longitudinal chromatic aberration characteristics even in consideration of various error factors.
- the axial chromatic aberration characteristic is less than 0.05 ⁇ m / nm, in order to increase the (positive) power component of the convex lens due to the diffractive lens structure, the pitch of the annular pattern decreases, and the diffraction efficiency decreases sharply. To do.
- the axial chromatic aberration characteristic of the optical head 40 for recording information on the multilayer optical disc 60 is 0.05 ⁇ m / nm or more and 0.15 ⁇ m / nm or less.
- the objective lens reduces the change in the convergence position of the blue-violet laser light generated with the wavelength change of the blue-violet laser light emitted from the blue-violet laser light source 1, and the unit of blue-violet laser light emitted from the blue-violet laser light source 1
- the amount of change D [ ⁇ m / nm] of the convergence position of the blue-violet laser light generated with the wavelength change satisfies 0.05 [ ⁇ m / nm] ⁇ D ⁇ 0.15 [ ⁇ m / nm].
- the objective lens 8 having this diffractive lens structure is divided into a diffractive lens and a base refracting lens excluding the diffractive lens
- the positive axial chromatic aberration of the base refracting lens is expressed by the diffractive lens. Is corrected by negative axial chromatic aberration.
- the objective lens 8 having the diffractive lens structure according to the first embodiment has the virtual lens. It can be considered as a cemented lens (synthetic lens) of a typical refractive lens and a base refractive lens.
- the relationship between the focal lengths (power distribution) of the two refractive lenses can be uniquely determined by an arbitrary combined focal length and axial chromatic aberration characteristic conditions.
- the focal length of the diffractive lens is 74 mm, and the focal length of the base refractive lens is 1.319 mm.
- the ratio of the reciprocal of the focal length is about 0.02.
- the focal length of the diffractive lens is 303 mm, and the focal length of the base refractive lens is 1.304 mm.
- the ratio of power (reciprocal of focal length) is 0.004.
- h represents the height from the optical axis
- n represents the diffraction order
- P 2 , P 4 , P 6 ,..., P 2k represent the coefficients
- ⁇ represents the laser beam Represents the wavelength.
- a spherical aberration component is further superimposed on the power component of the diffracted light.
- the spherical aberration component added by the diffractive lens structure is a spherical aberration that changes in a direction in which the spherical aberration is under-corrected (under) when the wavelength of the blue-violet laser light emitted from the blue-violet laser light source 1 is shifted to the long wavelength side. Has characteristics.
- FIG. 7 is a schematic diagram showing how the blue-violet laser beam is converged on the information recording surface L2 of the multilayer optical disc 60 using the objective lens 8 of the first embodiment.
- the forward + 1st order light is reflected by the information recording surface L2, and then further diffracted by the diffractive lens structure formed on the first surface 81 of the objective lens 8 to generate + 1st order light.
- This + 1st order light is referred to as “return + 1st order light”.
- the primary light of this return path is condensed on the light receiving element 23 and becomes signal light.
- the objective lens 8 adds the power component of the convex lens to the + 1st order light by the diffractive lens structure.
- the position FB where the 0th-order light converges is farther from the objective lens 8 than the position FA where the + 1st-order light converges.
- the + secondary light has a large convex lens power
- the position FC where the + secondary light converges is closer to the objective lens 8 than the position FA where the + secondary light converges.
- the power component of the convex lens added by the diffractive lens structure of the objective lens 8 is uniquely determined by the axial chromatic aberration performance of the objective lens 8. Therefore, the interval between the position where the + 1st order light converges and the position where the 0th order light and the + secondary light converge is uniquely determined.
- the zero-order light converges in the vicinity of the information recording surface L1 on the back side when viewed from the disc surface 61 with respect to the information recording surface L2, and from the disc surface 61 with respect to the information recording surface L2.
- the + secondary light may converge near the information recording surface L3 on the near side as viewed.
- the 0th-order light converged in the vicinity of the information recording surface L1 (the 0th-order light in the forward path) is reflected by the information recording surface L1 and then enters the objective lens 8.
- the first surface 81 of the objective lens 8 generates the 0th-order light on the return path, and the generated 0th-order light on the return path is condensed on the light receiving element 23.
- + secondary light converged in the vicinity of the information recording surface L3 (outgoing + secondary light) is reflected by the information recording surface L3 and then enters the objective lens 8.
- the first surface 81 of the objective lens 8 generates + secondary light on the return path, and the generated + secondary light on the return path is condensed on the light receiving element 23.
- the diffracted stray light reflected by the information recording surface (or the surface of the multilayer optical disk) different from the information recording surface to be recorded or reproduced is condensed on the light receiving element 23 and interferes with the signal light,
- the information signal is deteriorated or an offset is generated in the servo signal (focus error signal or tracking error signal).
- a spherical aberration component is further superimposed on the power component of the diffracted light. Therefore, spherical aberration components remain in the zeroth-order light on the return path and the + second-order light (diffracted stray light) on the return path that is collected on the light receiving element 23.
- FIGS. 8A to 8C are diagrams showing the state of the focused spot on the light receiving element in the first embodiment of the present invention.
- the condensing spot is given astigmatism in the 45-degree direction by the detection lens 22, and is enlarged by defocusing.
- FIG. 8A shows a condensing spot (signal light) formed by reflecting the + first-order light in the forward path on the information recording surface L2 and condensing the + first-order light in the return path on the light receiving element.
- FIG. 8B shows a case where the spherical aberration component is not superimposed on the diffracted light, that is, in the conventional objective lens having only the power component, the 0th-order light in the forward path is reflected by the information recording surface L1, and the 0th-order light in the return path.
- FIG. 3 is a diagram showing a condensing spot (diffraction stray light) formed by condensing on the light receiving element.
- FIG. 8C shows a collection formed by reflecting the 0th-order light of the forward path on the information recording surface L1 and condensing the 0th-order light of the return path on the light receiving element in the objective lens 8 of the first embodiment. It is a figure which shows a light spot (diffraction stray light).
- the diffracted stray light in the case of using a conventional objective lens forms a condensed spot of almost the same size as the signal light (FIG. 8A) depending on the defocused state.
- the quality of the information signal or the servo signal is deteriorated due to the interference between the diffraction stray light and the signal light.
- the diffracted stray light of the objective lens 8 according to the first embodiment is not condensed at one point due to the spherical aberration component in any defocused state. Therefore, interference between the diffraction stray light and the signal light (FIG. 8A) can be greatly reduced, and the quality of the information signal or servo signal is improved.
- a condensing spot may be formed on the light receiving element 23 through different diffraction orders in the forward path and the return path.
- FIG. 9 shows that in the first embodiment of the present invention, when the + secondary light is not converged on the information recording surface and a condensing spot is formed on the light receiving element, the + 1st order light and the + secondary light converge on the multilayer optical disc. It is a schematic diagram which shows a mode that it does.
- the + secondary light converges further on the surface 61 side of the disc than the information recording surface L3, and the + secondary light in the forward path is not converged on any information recording surface. Then, after being reflected by the information recording surface L3, the first-order 81 of the objective lens 8 generates + 1st order light in the return path.
- the first-order 81 of the objective lens 8 generates + 1st order light in the return path.
- the conventional objective lens uses the forward + secondary light and the backward + 1st light.
- the diffracted stray light collected on the light receiving element 23 by (or the + 1st order light and the + secondary light on the return path) is collected with the same size as the signal light (the + 1st order light on the forward path and the + 1st order light on the return path). A light spot is formed. Therefore, the quality of the information signal or the servo signal is deteriorated due to the interference.
- the diffracted stray light of the objective lens 8 according to the first embodiment is not collected at one point due to the spherical aberration component, so that the interference with the signal light is reduced as described above. Obviously you can do that.
- the multilayer optical disc has a large number of information recording surfaces, and the information recording surface interval is also small. For this reason, it is extremely difficult to specify the power component of the diffractive lens so that all the diffracted stray light reflected by each information recording surface is not collected on the light receiving element and to achieve both the power component and the longitudinal chromatic aberration performance. .
- the optical head 40 according to the first embodiment in which interference between the signal light and the diffracted stray light is significantly reduced by superimposing a spherical aberration component on the power component of the diffracted light, records information on the multilayer optical disk 60. Or it is very suitable for reproduction.
- the spherical aberration component added by the diffractive lens structure has spherical aberration when the wavelength of the blue-violet laser light emitted from the blue-violet laser light source 1 is shifted to the long wavelength side.
- the present invention is not limited to such a spherical aberration characteristic.
- the spherical aberration component added by the diffractive lens structure changes in a direction in which the spherical aberration becomes overcorrected when the wavelength of the blue-violet laser light emitted from the blue-violet laser light source 1 is shifted to the long wavelength side. It may have such spherical aberration characteristics. Also in this case, it is clear that the interference between the signal light and the diffraction stray light can be greatly reduced.
- the spherical aberration component added by the diffractive lens structure is spherical when the wavelength of the blue-violet laser light emitted from the blue-violet laser light source 1 is shifted to the long wavelength side.
- Spherical aberration characteristics such that the aberration changes in the direction of undercorrection (under), and the change in the spherical aberration of the resin-made refractive lens that becomes overcorrected (over) due to temperature rise, the blue-violet laser due to temperature rise It can be canceled by the change in spherical aberration that occurs with the wavelength shift of the light source 1.
- spherical aberration that occurs with temperature changes is overcorrected due to changes in the refractive index of the glass material when the wavelength of the laser light emitted from the light source is shifted to the longer wavelength side due to temperature rise.
- the spherical aberration is undercorrected (under) when the wavelength of the laser light emitted from the light source is shifted to the long wavelength side due to the temperature rise due to the diffractive lens structure. It has spherical aberration characteristics that change in direction.
- the objective lens 8 of the first embodiment is designed so that spherical aberration does not occur with respect to wavelength change.
- the spherical aberration component added by the diffractive structure satisfies the following expression (2).
- ⁇ represents a spherical aberration that occurs due to a change in the diffraction angle due to the diffractive structure with a change in the unit wavelength of the laser light emitted from the light source
- ⁇ represents a change in the unit wavelength of the laser light emitted from the light source.
- the spherical aberration generated by the change in the refractive index of the objective lens is represented.
- ⁇ represents a spherical aberration caused by a change in the diffraction angle by the diffractive lens structure in accordance with a change in the unit wavelength of the laser light emitted from the light source
- ⁇ represents a change in the unit wavelength of the laser light emitted from the light source. Accordingly, the spherical aberration generated by the change in the refractive index of the objective lens is represented, and ⁇ and ⁇ are opposite in polarity.
- the movable range of the collimating lens 4 must be secured so that the following first to fifth spherical aberrations can be corrected.
- the optical head 40 includes a first spherical aberration that occurs when the light transmission layer thickness of the information recording surface deviates from the design light transmission layer thickness, a second spherical aberration that remains in the objective lens and other optical elements, This is caused by the third spherical aberration that occurs when the wavelength of the laser light emitted from the light source shifts due to the temperature change, the fourth spherical aberration that occurs due to the refractive index change of the glass material due to the temperature change, and the individual difference of the light source wavelength.
- the fifth spherical aberration is corrected.
- the first spherical aberration is based on the standard of the multilayer optical disc 60, that is, the difference in the light transmission layer thickness between the information recording surface L0 having the largest light transmission layer thickness and the information recording surface L3 having the smallest light transmission layer thickness. (50 ⁇ m) and the thickness variation (for example, ⁇ 5 ⁇ m) of the light transmission layer on each information recording surface is uniquely determined.
- the second spherical aberration is determined by the aberration standard of the optical element used in the optical head 40 (for example, 0 ⁇ 20 m ⁇ for an objective lens).
- the third spherical aberration and the fourth spherical aberration are determined by the temperature compensation range to be considered in the optical head 40 and the wavelength shift amount when the temperature of the blue-violet laser light source 1 changes.
- the operating temperature range of the objective lens 8 is 10 to 70 ° C., that is, ⁇ 30 ° C. with respect to the design temperature of 40 ° C.
- the wavelength change of the blue-violet laser light source 1 due to the temperature change is 0.06 nm / ° C., and the wavelength change corresponding to the operating temperature range of ⁇ 30 ° C. is 1.8 nm.
- FIG. 10 is a diagram showing the relationship between the spherical aberration characteristic due to the diffractive lens structure and the third spherical aberration and the fourth spherical aberration that occur due to a temperature change in the first embodiment of the present invention.
- FIG. 10 shows the third spherical aberration and the fourth spherical aberration that occur due to a temperature change of a maximum of ⁇ 30 ° C. under the above-described conditions.
- the vertical axis represents spherical aberration [m ⁇ ]
- the horizontal axis represents spherical aberration characteristics.
- a white square point indicates the third spherical aberration
- a white rhombus point indicates the fourth spherical aberration
- a black rhombus point indicates the sum of the third spherical aberration and the fourth spherical aberration (temperature spherical surface). Aberration).
- the third spherical aberration and the fourth spherical surface become smaller as the spherical aberration when the wavelength of the incident light is shifted to the longer wavelength side becomes less corrected (under) (as it goes to the left in FIG. 10).
- Aberration is reduced.
- the spherical aberration (temperature spherical aberration) generated by the temperature change which is the sum of the third spherical aberration and the fourth spherical aberration, is also reduced.
- point A in FIG. Become. That is, spherical aberration does not occur due to temperature changes.
- FIG. 11 is a diagram showing the relationship between the spherical aberration characteristic due to the diffractive lens structure and the fifth spherical aberration caused by the individual difference of the light source wavelength in the first embodiment of the present invention.
- the individual difference of the light source wavelength with respect to the design wavelength of 405 nm is, for example, ⁇ 5 nm.
- FIG. 11 shows the fifth spherical aberration that occurs when the light source wavelength changes from 400 nm to 410 nm.
- the vertical axis represents spherical aberration [m ⁇ ]
- the horizontal axis represents spherical aberration characteristics.
- the black triangular point indicates the fifth spherical aberration.
- the fifth spherical aberration decreases as the spherical aberration when the wavelength of the incident light is shifted to the longer wavelength side becomes under-corrected (under).
- the value of the fifth spherical aberration also changes depending on the spherical aberration characteristic of the diffractive lens structure.
- the spherical aberration amount to be corrected for determining the movable range of the collimating lens 4 is the spherical aberration of FIG. 10, that is, the temperature spherical aberration (third spherical aberration + fourth spherical aberration) and the light source of FIG. It is determined by the sum of absolute values with the fifth spherical aberration that occurs due to individual differences in wavelength.
- FIG. 12 shows the relationship between the spherical aberration characteristic due to the diffractive lens structure and the amount of spherical aberration to be corrected (total spherical aberration) caused by individual differences in temperature change and light source wavelength in the first embodiment of the present invention.
- FIG. FIG. 12 shows the third spherical aberration and the fourth spherical aberration that occur due to a temperature change of a maximum of ⁇ 30 ° C., and the fifth spherical aberration that occurs due to an individual difference of the light source wavelength of a maximum of ⁇ 5 nm.
- the vertical axis represents spherical aberration [m ⁇ ]
- the horizontal axis represents spherical aberration characteristics.
- the black triangle point indicates the fifth spherical aberration
- the black rhombus point indicates the sum (temperature spherical aberration) of the third spherical aberration and the fourth spherical aberration
- the white circle point indicates the third spherical aberration. The sum (total spherical aberration) of the aberration, the fourth spherical aberration, and the fifth spherical aberration is shown.
- the total spherical aberration decreases as the spherical aberration when the wavelength of the incident light is shifted to the longer wavelength side becomes less corrected (under) (as it goes to the left in FIG. 12). Almost no decrease at the middle point B as a boundary.
- the absolute value of the temperature spherical aberration (third spherical aberration + fourth spherical aberration) cancels out the absolute value of the fifth spherical aberration caused by individual differences in the light source wavelength. Is due to.
- the movable collimating lens 4 is movable.
- the range does not decrease.
- the objective lens 8 of the first embodiment adds the power component of the convex lens to the + 1st order light by the diffractive lens structure. For this reason, if the spherical aberration characteristic when the wavelength of the incident light is shifted to the longer wavelength side is further undercorrected (under), the pitch of the ring-shaped pattern becomes smaller. As a result, there arises a problem that the processing difficulty of the mold is increased and the transferability of the injection molding is lowered, and the diffraction efficiency is further lowered due to the narrow pitch.
- FIG. 13 shows the occurrence of the minimum pitch of the annular pattern of the diffractive lens structure, the temperature change, and the individual difference of the light source wavelength when the longitudinal chromatic aberration characteristic is 0.1 ⁇ m / nm in the first embodiment of the present invention It is a figure which shows the relationship with the spherical-aberration amount (total spherical aberration) which should be corrected.
- FIG. 13 shows the third spherical aberration and the fourth spherical aberration that are generated by a temperature change of maximum ⁇ 30 ° C., and the fifth spherical aberration that is generated by an individual difference of the light source wavelength of maximum ⁇ 5 nm. Yes.
- FIG. 13 shows the occurrence of the minimum pitch of the annular pattern of the diffractive lens structure, the temperature change, and the individual difference of the light source wavelength when the longitudinal chromatic aberration characteristic is 0.1 ⁇ m / nm in the first embodiment of the present invention It is a figure which shows the relationship
- the vertical axis represents spherical aberration [m ⁇ ]
- the horizontal axis represents the minimum pitch [ ⁇ m] of the diffractive lens structure.
- the black triangle point indicates the fifth spherical aberration
- the black rhombus point indicates the sum (temperature spherical aberration) of the third spherical aberration and the fourth spherical aberration
- the white circle point indicates the third spherical aberration. The sum (total spherical aberration) of the aberration, the fourth spherical aberration, and the fifth spherical aberration is shown.
- the objective lens 8 of the first embodiment is designed so that spherical aberration does not occur with respect to wavelength change. That is, the objective lens 8 according to the first embodiment has a spherical aberration characteristic at point C in FIG. At this time, the amount of spherical aberration to be corrected (total spherical aberration) to be corrected due to the temperature change of maximum ⁇ 30 ° C. and the individual difference of the light source wavelength of maximum ⁇ 5 nm is almost the minimum value.
- the minimum pitch is also 5 ⁇ m. Therefore, it is possible to suppress a decrease in transfer efficiency of injection molding and a decrease in diffraction efficiency due to a narrow pitch.
- FIG. 14 is a graph showing the relationship between the minimum pitch of the zonal pattern of the diffractive lens structure and the total spherical aberration when the allowable range of individual differences in the light source wavelength is expanded with respect to FIG.
- the allowable range of individual differences in the light source wavelength is expanded to a maximum of ⁇ 7 nm (ie, 398 nm to 412 nm).
- FIG. 14 shows the third spherical aberration and the fourth spherical aberration that are generated by a temperature change of a maximum of ⁇ 30 ° C., and the fifth spherical aberration that is generated by an individual difference of the light source wavelength of a maximum of ⁇ 7 nm.
- FIG. 14 shows the third spherical aberration and the fourth spherical aberration that are generated by a temperature change of a maximum of ⁇ 30 ° C., and the fifth spherical aberration that is generated by an individual difference of the light source wavelength of a maximum of ⁇
- the vertical axis represents spherical aberration [m ⁇ ]
- the horizontal axis represents the minimum pitch [ ⁇ m] of the diffractive lens structure.
- the black triangle point indicates the fifth spherical aberration
- the black rhombus point indicates the sum (temperature spherical aberration) of the third spherical aberration and the fourth spherical aberration
- the white circle point indicates the third spherical aberration. The sum (total spherical aberration) of the aberration, the fourth spherical aberration, and the fifth spherical aberration is shown.
- the total spherical aberration at the point C ′ where the minimum pitch is 5 ⁇ m is equal to the total spherical aberration at the point C in FIG.
- the collimating lens 4 In addition, in the optical head for a multi-layer optical disc in which the amount of spherical aberration generated in proportion to the amount of deviation of the objective lens from the optimum light transmission layer thickness increases as the distance between the information recording surfaces increases, the collimating lens 4
- the objective lens 8 according to the first embodiment that can realize the compact optical head 40 is very suitable.
- the optical head 40 using the objective lens 8 of the first embodiment generates a predetermined amount of spherical aberration as the temperature changes. Therefore, a temperature change in the optical head 40 is detected using a temperature sensor or the like, and for example, the amplitude of an information signal or a servo signal is maximized, or a predetermined index value (jitter or the like) in the information signal is minimized. As described above, it is preferable to correct the spherical aberration caused by the temperature change by moving the collimating lens 4.
- the spherical aberration that does not cause the spherical aberration with respect to the wavelength change.
- a spherical aberration component is superimposed on the diffracted light so as to obtain characteristics.
- the optical head for recording or reproducing information with respect to the multilayer optical disk has been described.
- the present invention is not limited to such an optical head.
- the present invention can be applied to a compatible optical head that records or reproduces information on at least one of BD, DVD, and CD in addition to a multilayer optical disk.
- FIG. 15 is a diagram for describing a specific example of the objective lens according to the first embodiment of the present invention.
- the objective lens 8 is made of a resin material and includes a first surface 81 made of a diffractive surface and a second surface 82 made of an aspheric surface.
- the diffractive surface is divided into a plurality of annular zones in the radial direction from the optical axis, and has a step parallel to the optical axis between adjacent regions.
- the step has a depth that gives a phase difference that is an integral multiple of the wavelength of the laser light in the material refractive index corresponding to the design wavelength and design temperature.
- the zone width (the distance in the radial direction between a certain step and the step closest to the step) is configured to monotonously narrow toward the periphery from the optical axis of the lens.
- the diffractive surface has a convex power and has a function of correcting defocus and third-order spherical aberration that occur when the wavelength of the laser beam changes.
- the light beam 2 transmitted through the diffractive surface is transmitted through the aspherical surface of the second surface 82 and is well focused on the information recording surface of the multilayer optical disc 60.
- the amount of third-order spherical aberration generated on the diffractive surface is almost the same as the amount of third-order spherical aberration generated on the base lens other than the diffractive surface when the wavelength is changed. It is designed so that two third-order spherical aberrations occur in opposite directions. Therefore, the third-order spherical aberration hardly occurs with respect to the wavelength change in the entire lens.
- the base lens refers to a lens composed of only an aspheric surface remaining after removing the diffractive structure from a surface where the diffractive structure is arranged in a certain diffractive lens.
- an objective lens having a diffractive structure shown in three types of numerical examples will be described.
- the axial chromatic aberration is kept small, but the direction and amount of the spherical aberration generated by the unit wavelength change are different.
- the amount of the third-order spherical aberration generated at the time of the temperature change is different. Is different.
- Tables 2 to 6 are shown in Tables 2 to 6 below.
- Table 2 is a table showing the specifications of the objective lens in Example 1
- Table 3 is a table showing the surface shapes of the objective lens and the multilayer optical disc in Example 1
- Table 4 shows the wavelength and wavelength in Example 1.
- FIG. 5 is a table showing the refractive indexes of the objective lens and the light transmission layer with respect to temperature
- Table 5 is a table showing the aspherical coefficient and phase function of the first surface of the objective lens in Example 1
- Table 6 is an example.
- 2 is a table showing an aspherical coefficient and a phase function of the second surface of the objective lens in FIG.
- the surface numbers in Table 3 indicate the surfaces through which the laser beam passes in order, the surface number “0” indicates the emission point, and the surface number “1” indicates the incident surface (first surface) of the objective lens.
- the surface number “2” represents the exit surface (second surface) of the objective lens, the surface number “3” represents the surface of the multilayer optical disc, and the surface number “4” represents the information recording of the multilayer optical disc. Represents a surface.
- the feature of the objective lens of Example 1 is that spherical aberration occurs on the over side when the wavelength of the laser light is shifted to the long wavelength side. Since the spherical aberration that occurs when the temperature of the base lens rises is over, the spherical aberration that occurs when the temperature changes, including the wavelength change, is added, and is greatly generated on the over side.
- the minimum pitch of the diffractive structure is relatively large.
- Table 7 is a table showing the specifications of the objective lens in Example 2
- Table 8 is a table showing the surface shapes of the objective lens and multilayer optical disc in Example 2
- Table 9 is the wavelength and wavelength in Example 2.
- FIG. 10 is a table showing the refractive indices of the objective lens and the light transmission layer with respect to temperature
- Table 10 is a table showing the aspherical coefficient and phase function of the first surface of the objective lens in Example 2
- Table 11 is an example.
- 2 is a table showing an aspheric coefficient and a phase function of the second surface of the objective lens in FIG.
- the surface numbers in Table 8 indicate the surfaces through which the laser light passes in order, the surface number “0” indicates the light emitting point, and the surface number “1” indicates the incident surface (first surface) of the objective lens.
- the surface number “2” represents the exit surface (second surface) of the objective lens, the surface number “3” represents the surface of the multilayer optical disc, and the surface number “4” represents the information recording of the multilayer optical disc. Represents a surface.
- the feature of the objective lens of Example 2 is that the spherical aberration generated when the wavelength of the laser beam is shifted is very small.
- the minimum pitch of the diffractive structure is medium.
- Table 12 is a table showing the specifications of the objective lens in Example 3
- Table 13 is a table showing the surface shapes of the objective lens and the multilayer optical disc in Example 3
- Table 14 is the wavelength and wavelength in Example 3.
- FIG. 15 is a table showing the refractive indexes of the objective lens and the light transmission layer with respect to temperature
- Table 15 is a table showing the aspherical coefficient and phase function of the first surface of the objective lens in Example 3
- Table 16 is an example.
- 3 is a table showing an aspherical coefficient and a phase function of the second surface of the objective lens in FIG.
- the surface numbers in Table 13 indicate the surfaces through which the laser beam passes in order, the surface number “0” indicates the light emitting point, and the surface number “1” indicates the incident surface (first surface) of the objective lens.
- the surface number “2” represents the exit surface (second surface) of the objective lens, the surface number “3” represents the surface of the multilayer optical disc, and the surface number “4” represents the information recording of the multilayer optical disc. Represents a surface.
- the feature of the objective lens of Example 3 is that spherical aberration occurs on the under side when the wavelength of the laser light is shifted to the long wavelength side. Since the spherical aberration caused by the temperature change of the base lens is on the over side, a part of the spherical aberration caused by the temperature change including the wavelength change is canceled out.
- the minimum pitch of the diffractive structure is relatively small.
- the first surface of the objective lens is divided into a plurality of regions in a ring shape from the optical axis toward the radial direction.
- a step parallel to the optical axis is provided between regions adjacent to each other.
- the step has a depth that gives a phase difference that is an integral multiple of the wavelength of the laser light in the material refractive index corresponding to the design wavelength and design temperature.
- the depth of the step is an integral multiple of ⁇ / (nd-1).
- ⁇ represents the wavelength of the laser beam
- nd represents the material refractive index.
- the wavefront transmitted through the diffractive lens of the design wavelength and the design temperature is continuously connected, and the wavefront has no aberration.
- the material refractive index is lowered, so that the optical path difference due to the step is reduced. Therefore, in the wavefront shape of the spherical aberration that occurs when the temperature rises, the wavefront becomes discontinuous due to the change in the optical path difference of the phase step.
- the wavefront has a shape with reduced wavefront shape undulation, and the spherical aberration as a whole is reduced.
- the optical path difference given by diffraction becomes smaller.
- chromatic aberration of an objective lens made of an aspheric surface without a step structure causes over spherical aberration when the temperature rises.
- the spherical aberration that occurs when the temperature changes is significantly reduced by the above-described action as compared with a normal aspheric lens. Therefore, by using the configuration of the objective lens according to the present embodiment, even when a resin material having excellent mass productivity is used, aberrations that occur when temperature changes are reduced, and favorable information recording or reproduction is possible.
- the diffractive surface has a convex power
- the spherical aberration generated when the wavelength of the laser beam is shifted together with the longitudinal chromatic aberration is corrected.
- this is not preferable because the ring width of the diffractive structure is narrowed, the diffraction efficiency is lowered, and aberration deterioration due to wavelength change becomes severe.
- the amount of under spherical aberration generated only by diffraction is substantially the same as the amount of over spherical aberration generated by the base lens. That is, it is preferable that the absolute value of the spherical aberration generated by the unit wavelength change in the entire diffractive lens is small.
- the objective lens of Example 2 is selected. It is preferable.
- Such an objective lens according to the present embodiment has the following configuration.
- the objective lens is a single lens made of resin, and has an annular diffractive structure on at least one surface of the objective lens.
- the diffractive structure has a convex power, the zone width of the diffractive structure monotonously decreases from the center to the periphery of the objective lens, and the phase difference between the center and the periphery of the objective lens is n of the wavelength ⁇ of the laser light. Is double.
- the objective lens satisfies the following expressions (4) to (8).
- NA represents the numerical aperture
- f represents the focal length
- ⁇ CA represents the focal position change amount (axial chromatic aberration) per unit wavelength change of the objective lens
- ⁇ CA0 represents the zero next time of the objective lens.
- the axial chromatic aberration of the folded light is represented
- ⁇ SA ( ⁇ ) represents the amount of third-order spherical aberration generated per unit wavelength change of the objective lens.
- the objective lens satisfies the following formula (9).
- ⁇ SA (t) / f
- ⁇ SA (t) represents a third-order spherical aberration generation amount per unit temperature change.
- the objective lens satisfies the following formula (10).
- ⁇ n 0.9 ⁇ 10 ⁇ 5 (10)
- ⁇ n represents the refractive index change rate per unit temperature change of the material constituting the objective lens.
- the objective lens satisfies the following expression (11).
- ⁇ d represents the dispersion value of the material constituting the objective lens.
- the objective lens of the present embodiment will be described more specifically with reference to construction data, aberration diagrams, and the like.
- the surface to which the aspheric coefficient is given is an aspherical refractive optical surface or a surface having a refractive action equivalent to an aspherical surface (for example, a diffractive surface). It is defined by the following formula (12) representing the aspheric surface shape.
- X represents the distance from the tangent plane of the aspherical vertex of the aspherical point whose height from the optical axis is h
- h represents the height from the optical axis
- K j represents the conic constant of the lens j-th surface
- a j, n represents the n-th order of the lens j-th surface.
- Represents an aspherical constant, j 1, 2, 3, 4,.
- phase difference caused by the diffractive structure added to the optical surface is given by the following equation (13).
- ⁇ (h) represents a phase function
- h represents a height from the optical axis
- P j m
- j 1, 2, 3, 4...
- the first surface 81 of the objective lens 8 is composed of a diffractive surface
- the second surface 82 is composed of an aspheric surface.
- the design wavelength is 405 nm
- the design temperature is 40 ° C.
- the focal length is 1.3 mm
- the numerical aperture (NA) is 0.86
- the light transmission layer thickness of the multilayer optical disc is 0.0875 mm.
- the first surface 81 of the objective lens 8 is divided into a total of 76 regions, each represented by a different aspheric surface. Further, the phase step between the regions has a depth corresponding to a phase difference of 1 time with respect to the design wavelength.
- FIG. 16 is a diagram showing an optical path in the objective lens 8 having the diffractive structure of Example 1.
- FIG. 17 is a graph showing longitudinal aberration (spherical aberration) when parallel light is incident on the objective lens 8 in the objective lens 8 having the diffractive structure of the first embodiment.
- the solid line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 405 nm, which is the design center wavelength
- the broken line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 404 nm
- the alternate long and short dash line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 406 nm.
- Example 1 when the wavelength of the laser light is shifted to the long wavelength side, over-side spherical aberration occurs.
- Table 17 is a table showing the amount of the third-order spherical aberration SA3 that occurs when the temperature and wavelength change in the objective lens of Example 1.
- the plus is the under side (undercorrection) and the minus is the over side (overcorrection).
- the third-order spherical aberration SA3 is generated at about -34 m ⁇ . Further, when the temperature changes by + 40 ° C., the third-order spherical aberration SA3 is generated by ⁇ 152 m ⁇ . Further, when the temperature changes by + 40 ° C. and the wavelength of the laser beam changes by +2 nm from the design center, the third-order spherical aberration SA3 is generated by ⁇ 185 m ⁇ .
- the wavelength change rate with respect to the temperature of the laser beam is set to +0.05 nm / ° C.
- the minimum diffraction pitch of the objective lens of Example 1 is 10 ⁇ m, which is larger than the objective lenses of Examples 2 and 3 described below.
- the first surface 81 of the objective lens 8 is composed of a diffractive surface, and the second surface 82 is composed of an aspheric surface.
- the design wavelength is 405 nm
- the design temperature is 40 ° C.
- the focal length is 1.3 mm
- the numerical aperture (NA) is 0.86
- the light transmission layer thickness of the multilayer optical disc is 0.0875 mm.
- the first surface 81 of the objective lens 8 is divided into a total of 74 regions, each represented by a different aspheric surface. Further, the phase step between the regions has a depth corresponding to a phase difference of 1 time with respect to the design wavelength.
- FIG. 18 is a diagram illustrating an optical path in the objective lens 8 having the diffractive structure according to the second embodiment.
- FIG. 19 is a graph showing longitudinal aberration (spherical aberration) when parallel light is incident on the objective lens 8 in the objective lens 8 having the structure of the second embodiment.
- the solid line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 405 nm, which is the design center wavelength
- the broken line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 404 nm
- the alternate long and short dash line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 406 nm.
- the amount of spherical aberration that occurs when the wavelength of the laser beam is shifted is small.
- Table 18 is a table showing the amount of the third-order spherical aberration SA3 that occurs when the temperature and wavelength change in the objective lens of Example 2.
- the third-order spherical aberration SA3 occurs about ⁇ 1 m ⁇ . Further, when the temperature changes by + 40 ° C., the third-order spherical aberration SA3 occurs at ⁇ 103 m ⁇ . Further, when the temperature changes by + 40 ° C. and the wavelength of the laser beam changes by +2 nm from the design center, the third-order spherical aberration SA3 is generated by ⁇ 103 m ⁇ .
- the minimum diffraction pitch of the objective lens of Example 2 is 5 ⁇ m, which is moderate as compared with the objective lenses of Examples 1 and 3.
- the first surface 81 of the objective lens 8 is composed of a diffractive surface, and the second surface 82 is composed of an aspheric surface.
- the design wavelength is 405 nm
- the design temperature is 40 ° C.
- the focal length is 1.3 mm
- the numerical aperture (NA) is 0.86
- the light transmission layer thickness of the multilayer optical disc is 0.0875 mm.
- the first surface 81 of the objective lens 8 is divided into a total of 79 regions, each represented by a different aspheric surface. Further, the phase step between the regions has a depth corresponding to a phase difference of 1 time with respect to the design wavelength.
- FIG. 20 is a diagram showing an optical path in the objective lens 8 having the diffractive structure of the third embodiment.
- FIG. 21 is a graph showing longitudinal aberration (spherical aberration) when parallel light is incident on the objective lens 8 in the objective lens 8 having the diffractive structure of Example 3.
- the solid line shows the longitudinal aberration of the objective lens when the wavelength of the laser beam is 405 nm, which is the design center wavelength
- the broken line shows the longitudinal aberration of the objective lens when the wavelength of the laser beam is 404 nm
- the alternate long and short dash line indicates the longitudinal aberration of the objective lens when the wavelength of the laser light is 406 nm.
- Example 3 when the wavelength of the laser beam is shifted to the longer wavelength side, an under-side spherical aberration occurs.
- Table 19 is a table showing the amount of third-order spherical aberration SA3 that occurs when the temperature and wavelength change in the objective lens of Example 3.
- the third-order spherical aberration SA3 is generated about +21 m ⁇ . Further, when the temperature changes by + 40 ° C., the third-order spherical aberration SA3 is generated by ⁇ 69 m ⁇ . Further, when the temperature changes by + 40 ° C. and the wavelength of the laser beam changes by +2 nm from the design center, the third-order spherical aberration SA3 is generated by ⁇ 48 m ⁇ .
- the minimum diffraction pitch of the objective lens of Example 3 is 3 ⁇ m, which is smaller than the objective lenses of Examples 1 and 2.
- the diffracted stray light reflected by another information recording surface other than the predetermined information recording surface is not condensed at one point by the spherical aberration component, and interference between the signal light and the diffracted stray light can be reduced, and light transmission Information can be recorded or reproduced satisfactorily for a multilayer optical disc having a plurality of information recording surfaces with different layer thicknesses.
- the multilayer optical disc has four information recording surfaces.
- the present invention is not particularly limited to this, and may be a multilayer optical disc having two information recording surfaces. Even when the multilayer optical disc has two information recording surfaces, the same effect as described above can be obtained.
- the multilayer optical disc may have three information recording surfaces, and may have five or more information recording surfaces. When the multilayer optical disk has three or more information recording surfaces, a higher effect can be obtained.
- FIG. 22 is a diagram showing a schematic configuration of the optical disc apparatus according to Embodiment 2 of the present invention.
- the optical disc apparatus 50 includes an optical disc drive unit 51, a control unit 52, and an optical head 53 therein.
- the optical disk drive unit 51 drives the multilayer optical disk 60 to rotate.
- the optical head 53 is the optical head described in the first embodiment.
- the control unit 52 controls the driving of the optical disc driving unit 51 and the optical head 53 and performs signal processing of control signals and information signals photoelectrically converted by the optical head 53.
- the control unit 52 has a function of interfacing information signals between the outside and the inside of the optical disc device 50.
- the optical disk device 50 according to the second embodiment is equipped with the optical head described in the first embodiment, it is suitable for recording or reproducing information on a multilayer optical disk, and can realize a more compact configuration.
- FIG. 23 is a diagram showing a schematic configuration of a computer according to the third embodiment of the present invention.
- a computer 500 reads out from the optical disc device 50 according to the second embodiment, an input device 501 such as a keyboard, a mouse, or a touch panel for inputting information, and information input from the input device 501 and the optical disc device 50.
- An arithmetic unit 502 such as a central processing unit (CPU) that performs an operation based on the information, and an output of a cathode ray tube or a liquid crystal display device that displays information such as a result calculated by the arithmetic device 502 or a printer that prints the information
- the computer 500 corresponds to an example of an information processing device
- the arithmetic device 502 corresponds to an example of an information processing unit.
- the computer 500 includes the optical disc device 50 according to the second embodiment, it is suitable for recording or reproducing information on a multilayer optical disc and can realize a more compact configuration.
- FIG. 24 is a diagram showing a schematic configuration of the optical disc player according to Embodiment 4 of the present invention.
- the optical disc player 600 includes the optical disc device 50 according to the second embodiment and a decoder 601 that converts an information signal obtained from the optical disc device 50 into an image signal.
- the optical disc player 600 can be used as a car navigation system by adding a position sensor such as GPS and a central processing unit (CPU).
- the optical disc player 600 may also include a display device 602 such as a liquid crystal monitor.
- the optical disc player 600 corresponds to an example of an information processing apparatus
- the decoder 601 corresponds to an example of an information processing unit.
- the optical disc player 600 includes the optical disc device 50 according to the second embodiment, it is suitable for recording or reproducing information on a multilayer optical disc, and can realize a more compact configuration.
- FIG. 25 is a diagram showing a schematic configuration of the optical disc recorder according to Embodiment 5 of the present invention.
- the optical disc recorder 700 includes the optical disc device 50 according to the second embodiment and an encoder 701 that converts image information into an information signal to be recorded on the optical disc by the optical disc device 50. Desirably, a recorded image can also be reproduced by providing a decoder 702 that converts an information signal obtained from the optical disk device 50 into image information.
- the optical disk recorder 700 may include an output device 703 such as a cathode ray tube or a liquid crystal display device that displays information or a printer that prints information.
- the optical disc recorder 700 corresponds to an example of an information processing apparatus
- the encoder 701 and the decoder 702 correspond to an example of an information processing unit.
- the optical disc recorder 700 includes the optical disc device 50 according to the second embodiment, it is suitable for recording or reproducing information on a multilayer optical disc and can realize a more compact configuration.
- An optical head is an optical head that records or reproduces information on an information recording medium having a plurality of information recording surfaces, and includes a light source that emits laser light, and an annular diffraction structure. And an objective lens that converges the n-th order (n is a natural number) diffracted light generated by diffracting the laser light onto a predetermined information recording surface of the information recording medium, and is reflected by the predetermined information recording surface And a photodetector that receives the laser beam, and the diffractive structure adds a positive power component and a spherical aberration component to the nth-order diffracted light.
- the light source emits laser light.
- the objective lens has an annular diffractive structure, and converges n-th order (n is a natural number) diffracted light generated by diffracting laser light onto a predetermined information recording surface of the information recording medium.
- the photodetector receives the laser beam reflected by a predetermined information recording surface.
- the diffractive structure adds a positive power component and a spherical aberration component to the nth-order diffracted light.
- a positive power component is added to the nth-order diffracted light, axial chromatic aberration can be corrected, and a spherical aberration component is added to the nth-order diffracted light, so that it is not a predetermined information recording surface.
- the diffraction stray light reflected by the other information recording surface is not condensed at one point by the spherical aberration component, and interference between the signal light and the diffraction stray light can be reduced, and the information recording medium having a plurality of information recording surfaces can be obtained.
- information can be recorded or reproduced favorably.
- the spherical aberration component added by the diffractive structure is in a direction in which the spherical aberration is insufficiently corrected when the wavelength of the laser beam emitted from the light source is shifted to the long wavelength side. It preferably has a varying spherical aberration characteristic.
- the spherical aberration component added by the diffractive structure has a spherical aberration characteristic that changes in a direction in which the spherical aberration is insufficiently corrected when the wavelength of the laser beam emitted from the light source is shifted to the long wavelength side. Therefore, the change of the spherical aberration of the objective lens that is overcorrected due to the temperature rise can be canceled by the change of the spherical aberration caused by the wavelength shift of the light source due to the temperature rise.
- the information recording medium has three or more information recording surfaces, and the spherical aberration component added by the diffraction structure satisfies the following expression (14).
- SA1 represents spherical aberration that occurs due to a change in diffraction angle by the diffractive structure in accordance with a unit wavelength change of the laser light emitted from the light source
- SA2 represents the laser light emitted from the light source.
- the information recording medium has three or more information recording surfaces, and the spherical aberration component added by the diffractive structure satisfies the above equation (14), so that the laser light emitted from the light source
- the spherical aberration that occurs due to the change in the diffraction angle due to the diffraction structure due to the unit wavelength change and the spherical aberration that occurs due to the change in the refractive index of the objective lens due to the unit wavelength change of the laser light emitted from the light source cancel each other.
- Spherical aberration that occurs when the wavelength changes can be reduced.
- the spherical aberration component added by the diffractive structure satisfies the following expression (15).
- SA1 represents spherical aberration that occurs due to a change in diffraction angle by the diffractive structure in accordance with a unit wavelength change of the laser light emitted from the light source
- SA2 represents the laser light emitted from the light source.
- the spherical aberration component added by the diffractive structure satisfies the above equation (15), and thus is generated by a change in the diffraction angle due to the diffractive structure accompanying a change in the unit wavelength of the laser light emitted from the light source.
- Spherical aberration and spherical aberration that occurs due to the change in the refractive index of the objective lens accompanying the change in the unit wavelength of the laser light emitted from the light source are substantially offset, and spherical aberration that occurs when the wavelength changes can be reduced. it can.
- the objective lens reduces a change in a convergence position of the laser light generated in accordance with a wavelength change of the laser light emitted from the light source, and the laser emitted from the light source. It is preferable that the change amount D [ ⁇ m / nm] of the convergence position of the laser beam generated with the change of the unit wavelength of the light satisfies the following expression (16).
- the axial chromatic aberration correction function of the objective lens reduces the change in the convergence position of the laser light that occurs with the change in the wavelength of the laser light emitted from the light source. Further, the amount of change D [ ⁇ m / nm] of the convergence position of the laser light generated with the unit wavelength change of the laser light emitted from the light source satisfies the above equation (16).
- h represents the height from the optical axis
- n represents the diffraction order
- P 2 , P 4 , P 6 ,..., P 2k represent the coefficients
- ⁇ represents the laser beam. Represents the wavelength of.
- the power phi D of the lens by a diffractive structure the power phi R of the base of the refractive lens, excluding the diffractive structure in the objective lens, so satisfying the above equation (17), the recording power and reproducing power It is possible to satisfactorily correct the change in the convergence position caused by the wavelength variation of the laser light emitted from the light source at the time of switching to or when the ambient temperature changes.
- the information is transmitted from the light incident surface of the information recording medium by moving the coupling lens disposed between the light source and the objective lens and the coupling lens in the optical axis direction. It is preferable to further include a spherical aberration correction unit that corrects a spherical aberration generated according to the length of the distance to the recording surface.
- the coupling lens is disposed between the light source and the objective lens, and the spherical aberration correction unit moves the coupling lens in the optical axis direction, thereby moving information from the light incident surface of the information recording medium.
- Spherical aberration that occurs according to the length of the distance to the recording surface is corrected. Therefore, it is possible to correct spherical aberration that occurs according to the length of the distance from the light incident surface of the information recording medium to the information recording surface.
- the objective lens is a single lens made of resin.
- the objective lens is a single resin lens, the specific gravity is smaller than that of the glass objective lens, and the burden on the objective lens actuator that performs focus servo or tracking servo can be reduced. Moreover, it is possible to mass-produce with high accuracy by injection molding, which is suitable for cost reduction.
- the minimum pitch of the diffractive structure of the objective lens is 5 ⁇ m or more.
- the minimum pitch of the diffractive structure of the objective lens is 5 ⁇ m or more, it is possible to suppress a decrease in transferability of injection molding and a decrease in diffraction efficiency due to a narrow pitch.
- An optical disc device includes any one of the above optical heads, a drive unit for rotationally driving an information recording medium, and a control unit for controlling the optical head and the drive unit. Prepare. According to this configuration, the optical head described above can be applied to an optical disc apparatus.
- An information processing apparatus includes the above-described optical disc device and an information processing unit that processes information recorded on the optical disc device and / or information reproduced from the optical disc device. According to this configuration, an optical disc device including the above-described optical head can be applied to an information processing device.
- An objective lens according to another aspect of the present invention is an objective lens provided in an optical head for recording or reproducing information on an information recording medium having a plurality of information recording surfaces, and is generated by diffracting laser light.
- a positive power component is added to the nth-order diffracted light, axial chromatic aberration can be corrected, and a spherical aberration component is added to the nth-order diffracted light, so that it is not a predetermined information recording surface.
- the diffracted stray light reflected by the other information recording surface is not condensed at one point by the spherical aberration component, and interference between the signal light and the diffracted stray light can be reduced.
- the spherical aberration component added by the diffractive structure has a spherical aberration characteristic that changes in a direction in which the spherical aberration is insufficiently corrected when the wavelength of the laser beam is shifted to the long wavelength side. It is preferable to have.
- the spherical aberration component added by the diffractive structure has a spherical aberration characteristic that changes in a direction in which the spherical aberration is insufficiently corrected when the wavelength of the laser beam is shifted to the longer wavelength side. Therefore, the change of the spherical aberration of the objective lens that is overcorrected can be canceled by the change of the spherical aberration that occurs due to the wavelength shift of the light source due to the temperature rise.
- the information recording medium has three information recording surfaces, and the spherical aberration component added by the diffraction structure satisfies the following expression (18).
- SA1 represents spherical aberration that occurs due to a change in diffraction angle by the diffractive structure in accordance with a unit wavelength change of the laser light emitted from the light source
- SA2 represents the laser light emitted from the light source.
- the information recording medium has three information recording surfaces, and the spherical aberration component added by the diffractive structure satisfies the above equation (18), so the unit wavelength of the laser light emitted from the light source Along with the change, the spherical aberration caused by the change in the diffraction angle by the diffractive structure and the spherical aberration caused by the change in the refractive index of the objective lens accompanying the change in the unit wavelength of the laser light emitted from the light source are canceled out. It is possible to reduce spherical aberration that occurs at the time of change.
- the spherical aberration component added by the diffractive structure satisfies the following expression (19).
- SA1 represents spherical aberration that occurs due to a change in diffraction angle by the diffractive structure in accordance with a unit wavelength change of the laser light emitted from the light source
- SA2 represents the laser light emitted from the light source.
- the spherical aberration component added by the diffractive structure satisfies the above equation (19), and therefore occurs due to a change in the diffraction angle by the diffractive structure accompanying a change in the unit wavelength of the laser light emitted from the light source.
- Spherical aberration and spherical aberration that occurs due to the change in the refractive index of the objective lens accompanying the change in the unit wavelength of the laser light emitted from the light source are substantially offset, and spherical aberration that occurs when the wavelength changes can be reduced. it can.
- the objective lens reduces the change in the convergence position of the laser light that occurs with a change in the wavelength of the laser light, and the laser that occurs with a change in the unit wavelength of the laser light.
- the change amount D [ ⁇ m / nm] of the light convergence position preferably satisfies the following expression (20).
- the axial chromatic aberration correction function reduces the change in the convergence position of the laser light that occurs with the change in the wavelength of the laser light. Further, the amount of change D [ ⁇ m / nm] of the convergence position of the laser light that is generated along with the unit wavelength change of the laser light satisfies the above equation (20).
- h represents the height from the optical axis
- n represents the diffraction order
- P 2 , P 4 , P 6 ,..., P 2k represent the coefficients
- ⁇ represents the wavelength.
- the power phi D of the lens by a diffractive structure the power phi R of the base of the refractive lens, excluding the diffractive structure in the objective lens, so satisfying the above expression (21), the recording power and reproducing power It is possible to satisfactorily correct the change in the convergence position caused by the wavelength variation of the laser light emitted from the light source at the time of switching to or when the ambient temperature changes.
- An objective lens according to another aspect of the present invention is an objective lens provided in an optical head for recording or reproducing information with respect to an information recording medium having a plurality of information recording surfaces, and the objective lens is made of a resin.
- a single lens having an annular diffractive structure on at least one surface of the objective lens, the diffractive structure having a convex power, and the annular width of the diffractive structure is monotonous from the center to the periphery of the objective lens
- the phase difference between the center and the periphery of the objective lens is n times the wavelength ⁇ of the laser beam, and the objective lens satisfies the following equations (22) to (26).
- NA the numerical aperture
- f the focal length
- ⁇ CA axial chromatic aberration per unit wavelength change of the objective lens
- ⁇ CA0 the axis of the 0th-order diffracted light of the objective lens.
- the chromatic aberration is represented
- ⁇ SA ( ⁇ ) represents a third-order spherical aberration generation amount per unit wavelength change of the objective lens.
- the objective lens is a single lens made of resin, and has an annular diffraction structure on at least one surface of the objective lens.
- the diffractive structure has a convex power, the zone width of the diffractive structure monotonously decreases from the center to the periphery of the objective lens, and the phase difference between the center and the periphery of the objective lens is n of the wavelength ⁇ of the laser light. Is double.
- the objective lens satisfies the above expressions (22) to (26).
- axial chromatic aberration can be corrected, and spherical aberration that occurs when the wavelength is shifted can be reduced, and information can be recorded or recorded on an information recording medium having a plurality of information recording surfaces. Can be played.
- the objective lens satisfies the following expression (27).
- ⁇ SA (t) / f
- the objective lens can correct axial chromatic aberration by satisfying the above equation (27), and can reduce spherical aberration that occurs when the wavelength is shifted.
- Information can be recorded or reproduced favorably on an information recording medium having the information recording surface.
- the objective lens satisfies the following expression (28).
- ⁇ n 0.9 ⁇ 10 ⁇ 5 (28)
- ⁇ n represents a refractive index change rate per unit temperature change of the material constituting the objective lens.
- the objective lens can correct axial chromatic aberration by satisfying the above expression (28), and can reduce spherical aberration that occurs when the wavelength is shifted.
- Information can be recorded or reproduced favorably on an information recording medium having the information recording surface.
- the objective lens satisfies the following expression (29).
- ⁇ d represents a dispersion value of the material constituting the objective lens.
- the objective lens can correct axial chromatic aberration by satisfying the above expression (29), and can reduce spherical aberration that occurs when the wavelength is shifted.
- Information can be recorded or reproduced favorably on an information recording medium having the information recording surface.
- the optical head, the optical disc apparatus, the information processing apparatus, and the objective lens according to the present invention can record or reproduce information favorably on an information recording medium having a plurality of information recording surfaces. It is suitable for an optical head that records or reproduces information, an optical disk device that includes the optical head, an information processing device that includes the optical disk device, and an objective lens that is used in the optical head.
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Abstract
Description
図1は、本発明の実施の形態1における光学ヘッドの概略構成を示す図である。図2は、本発明の実施の形態1における多層光ディスクの概略構成を示す図である。
設計温度 :40℃
設計光透過層厚 :75μm
焦点距離 :1.3mm
開口数(NA) :0.85
作動距離 :0.3mm
屈折率(nd) :1.51
アッベ数(νd):57
Φ(h)=P2×h2+P4×h4+P6×h6+・・・+P2k×h2k(ただし、kは自然数)
で表され、
回折レンズ構造によるレンズのパワーφDが、
φD=-(2×P2×n×λ)
で表されるとき、回折レンズ構造によるレンズのパワーφDと、対物レンズ8において回折レンズ構造を除いたベースの屈折レンズのパワーφRとは、下記の(1)式を満たすことが望ましい。
ここで、hは、光軸からの高さを表し、nは、回折次数を表し、P2,P4,P6,・・・,P2kは、係数を表し、λは、レーザ光の波長を表す。
β=+3.9[mλ/℃]
α+β=+4.2[mλ/℃]
NA>0.8・・・(5)
n/f>30・・・(6)
ΔCA≦ΔCA0/2[μm/nm]・・・(7)
|ΔSA(λ)/f|<0.003[λ/(nm・mm)]・・・(8)
ここで、ΔSA(t)は、単位温度変化当たりの3次の球面収差発生量を表す。
ここで、Δnは、対物レンズを構成する材料の単位温度変化当たりの屈折率変化率を表す。
ここで、νdは、対物レンズを構成する材料の分散値を表す。
図22は、本発明の実施の形態2における光ディスク装置の概略構成を示す図である。
図23は、本発明の実施の形態3におけるコンピュータの概略構成を示す図である。
図24は、本発明の実施の形態4における光ディスクプレーヤの概略構成を示す図である。
図25は、本発明の実施の形態5における光ディスクレコーダの概略構成を示す図である。
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表す。
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表し、SA1とSA2とは逆極性である。
ここで、hは、光軸からの高さを表し、nは、回折次数を表し、P2,P4,P6,・・・,P2kは、係数を表し、λは、前記レーザ光の波長を表す。
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表す。
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表し、SA1とSA2とは逆極性である。
ここで、hは、光軸からの高さを表し、nは、回折次数を表し、P2,P4,P6,・・・,P2kは、係数を表し、λは、波長を表す。
NA>0.8・・・(23)
n/f>30・・・(24)
ΔCA≦ΔCA0/2[μm/nm]・・・(25)
|ΔSA(λ)/f|<0.003[λ/(nm・mm)]・・・(26)
ここで、NAは、開口数を表し、fは、焦点距離を表し、ΔCAは、前記対物レンズの単位波長変化当たりの軸上色収差を表し、ΔCA0は、前記対物レンズの0次回折光の軸上色収差を表し、ΔSA(λ)は、前記対物レンズの単位波長変化当たりの3次の球面収差発生量を表す。
ここで、ΔSA(t)は、単位温度変化当たりの3次の球面収差発生量を表す。
ここで、Δnは、前記対物レンズを構成する材料の単位温度変化当たりの屈折率変化率を表す。
ここで、νdは、前記対物レンズを構成する材料の分散値を表す。
Claims (21)
- 複数の情報記録面を有する情報記録媒体に対して、情報を記録又は再生する光学ヘッドであって、
レーザ光を出射する光源と、
輪帯状の回折構造を有し、前記レーザ光を回折させて生成したn次(nは自然数)の回折光を、前記情報記録媒体の所定の情報記録面に収束させる対物レンズと、
前記所定の情報記録面で反射した前記レーザ光を受光する光検出器とを備え、
前記回折構造は、前記n次の回折光に、正のパワー成分及び球面収差成分を付加することを特徴とする光学ヘッド。 - 前記回折構造が付加する前記球面収差成分は、前記光源から出射される前記レーザ光の波長が長波長側にシフトした場合に、球面収差が補正不足となる方向に変化する球面収差特性を有することを特徴とする請求項1記載の光学ヘッド。
- 前記情報記録媒体は、3つ以上の情報記録面を有し、
前記回折構造が付加する前記球面収差成分は、下記の(1)式を満たすことを特徴とする請求項1又は2記載の光学ヘッド。
SA1≒-SA2・・・(1)
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表す。 - 前記回折構造が付加する前記球面収差成分は、下記の(2)式を満たすことを特徴とする請求項1~3のいずれかに記載の光学ヘッド。
0.8×|SA1|≦|SA2|≦1.2×|SA1|・・・(2)
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表し、SA1とSA2とは逆極性である。 - 前記対物レンズは、前記光源から出射される前記レーザ光の波長変化に伴って発生する前記レーザ光の収束位置の変化を低減し、
前記光源から出射される前記レーザ光の単位波長変化に伴って発生する前記レーザ光の収束位置の変化量D[μm/nm]は、下記の(3)式を満たすことを特徴とする請求項1~4のいずれかに記載の光学ヘッド。
0.05[μm/nm]≦D≦0.15[μm/nm]・・・(3) - 前記回折構造による光路長の付加量を表す光路差関数Φ(h)が、
Φ(h)=P2×h2+P4×h4+P6×h6+・・・+P2k×h2k(ただし、kは自然数)
で表され、
前記回折構造によるレンズのパワーφDが、
φD=-(2×P2×n×λ)
で表されるとき、
前記回折構造によるレンズのパワーφDと、前記対物レンズにおいて前記回折構造を除いたベースの屈折レンズのパワーφRとは、下記の(4)式を満たすことを特徴とする請求項1~5のいずれかに記載の光学ヘッド。
0.004≦φD/φR≦0.020・・・(4)
ここで、hは、光軸からの高さを表し、nは、回折次数を表し、P2,P4,P6,・・・,P2kは、係数を表し、λは、前記レーザ光の波長を表す。 - 前記光源と前記対物レンズとの間に配置されるカップリングレンズと、
前記カップリングレンズを光軸方向に移動させることにより、前記情報記録媒体の光入射面から情報記録面までの距離の長さに応じて発生する球面収差を補正する球面収差補正部とをさらに備えることを特徴とする請求項1~6のいずれかに記載の光学ヘッド。 - 前記対物レンズは、樹脂製の単レンズであることを特徴とする請求項1~7のいずれかに記載の光学ヘッド。
- 前記対物レンズの前記回折構造の最小ピッチが5μm以上であることを特徴とする請求項1~8のいずれかに記載の光学ヘッド。
- 請求項1~9のいずれかに記載の光学ヘッドと、
情報記録媒体を回転駆動するための駆動部と、
前記光学ヘッド及び前記駆動部を制御する制御部とを備えることを特徴とする光ディスク装置。 - 請求項10記載の光ディスク装置と、
前記光ディスク装置に記録する情報及び/又は前記光ディスク装置から再生された情報を処理する情報処理部とを備えることを特徴とする情報処理装置。 - 複数の情報記録面を有する情報記録媒体に対して、情報を記録又は再生する光学ヘッドに備わる対物レンズであって、
レーザ光を回折させて生成したn次(nは自然数)の回折光を、前記情報記録媒体の所定の情報記録面に収束させる輪帯状の回折構造を備え、
前記回折構造は、前記n次の回折光に、正のパワー成分及び球面収差成分を付加することを特徴とする対物レンズ。 - 前記回折構造が付加する前記球面収差成分は、前記レーザ光の波長が長波長側にシフトした場合に、球面収差が補正不足となる方向に変化する球面収差特性を有することを特徴とする、請求項12記載の対物レンズ。
- 前記情報記録媒体は、3つの情報記録面を有し、
前記回折構造が付加する前記球面収差成分は、下記の(5)式を満たすことを特徴とする請求項12又は13記載の対物レンズ。
SA1≒-SA2・・・(5)
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表す。 - 前記回折構造が付加する前記球面収差成分は、下記の(6)式を満たすことを特徴とする請求項12~14のいずれかに記載の対物レンズ。
0.8×|SA1|≦|SA2|≦1.2×|SA1|・・・(6)
ここで、SA1は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記回折構造による回折角の変化によって発生する球面収差を表し、SA2は、前記光源から出射される前記レーザ光の単位波長変化に伴い、前記対物レンズの屈折率の変化によって発生する球面収差を表し、SA1とSA2とは逆極性である。 - 前記対物レンズは、前記レーザ光の波長変化に伴って発生する前記レーザ光の収束位置の変化を低減し、
前記レーザ光の単位波長変化に伴って発生する前記レーザ光の収束位置の変化量D[μm/nm]は、下記の(7)式を満たすことを特徴とする請求項12~15のいずれかに記載の対物レンズ。
0.05[μm/nm]≦D≦0.15[μm/nm]・・・(7) - 前記回折構造による光路長の付加量を表す光路差関数Φ(h)が、
Φ(h)=P2×h2+P4×h4+P6×h6+・・・+P2k×h2k(ただし、kは自然数)
で表され、
前記回折構造によるレンズのパワーφDが、
φD=-(2×P2×n×λ)
で表されるとき、
前記回折構造によるレンズのパワーφDと、前記回折構造を除いたベースの屈折レンズのパワーφRとは、下記の(8)式を満たすことを特徴とする請求項12~16のいずれかに記載の対物レンズ。
0.004≦φD/φR≦0.020・・・(8)
ここで、hは、光軸からの高さを表し、nは、回折次数を表し、P2,P4,P6,・・・,P2kは、係数を表し、λは、波長を表す。 - 複数の情報記録面を有する情報記録媒体に対して、情報を記録又は再生する光学ヘッドに備わる対物レンズであって、
前記対物レンズは、樹脂からなる単レンズであり、
前記対物レンズの少なくとも一面に輪帯状の回折構造を備え、
前記回折構造は、凸のパワーを有し、
前記回折構造の輪帯幅は、前記対物レンズの中心から周辺にかけて単調に減少し、
前記対物レンズの中心と周辺との位相差は、レーザ光の波長λのn倍であり、
前記対物レンズは、下記の(9)~(13)式を満たすことを特徴とする対物レンズ。
λ<450[nm]・・・(9)
NA>0.8・・・(10)
n/f>30・・・(11)
ΔCA≦ΔCA0/2[μm/nm]・・・(12)
|ΔSA(λ)/f|<0.003[λ/(nm・mm)]・・・(13)
ここで、NAは、開口数を表し、fは、焦点距離を表し、ΔCAは、前記対物レンズの単位波長変化当たりの軸上色収差を表し、ΔCA0は、前記対物レンズの0次回折光の軸上色収差を表し、ΔSA(λ)は、前記対物レンズの単位波長変化当たりの3次の球面収差発生量を表す。 - 前記対物レンズは、下記の(14)式を満たすことを特徴とする請求項18記載の対物レンズ。
|ΔSA(t)/f|<0.003[λ/(℃・mm)]・・・(14)
ここで、ΔSA(t)は、単位温度変化当たりの3次の球面収差発生量を表す。 - 前記対物レンズは、下記の(15)式を満たすことを特徴とする請求項18又は19記載の対物レンズ。
Δn>0.9×10-5・・・(15)
ここで、Δnは、前記対物レンズを構成する材料の単位温度変化当たりの屈折率変化率を表す。 - 前記対物レンズは、下記の(16)式を満たすことを特徴とする請求項18~20のいずれかに記載の対物レンズ。
νd<70・・・(16)
ここで、νdは、前記対物レンズを構成する材料の分散値を表す。
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WO2013047202A1 (ja) * | 2011-09-30 | 2013-04-04 | コニカミノルタアドバンストレイヤー株式会社 | 対物レンズ及び光ピックアップ装置 |
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