JP2004326984A - Optical head and its objective lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head and its objective lens that can provide excellent pickup characteristics even when focusing a first and a second laser beams different in frequencies with each other through an objective lens having a diffraction structure on a first and a second recording surfaces which have transparent protective layers different in thickness with each other. <P>SOLUTION: When setting the height H of the fine concentric level difference 30 constituting the center diffraction grating 35 in the center refraction surface area 33, the objective lens 3 of this optical head correlates the pit areas S<SB>1</SB>, S<SB>2</SB>of the first optical recording medium 41 and the second optical recording medium 42 with a first dimension h<SB>1</SB>corresponding to the wavelength of the first laser beam L1 and a second dimension h<SB>2</SB>corresponding to the wavelength of the second laser beam L2 so as to satisfy the formula H=(S<SB>1</SB>×h<SB>2</SB>+S<SB>2</SB>×h<SB>1</SB>)/(S<SB>1</SB>+S<SB>2</SB>). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical head device for reproducing and recording optical recording media such as CDs and DVDs having different substrate thicknesses using laser beams having different wavelengths. More specifically, the present invention relates to an objective lens suitable for use in such an optical head device.
[0002]
[Prior art]
As optical recording media, those having different thicknesses and different recording densities of a transparent protective layer for protecting a recording surface, such as CDs and DVDs, are known. For reproduction and recording of CDs (including CD-Rs), wavelengths are required. A first laser light source that emits a first laser light of 785 nm is used, and a second laser light source that emits a second laser light of a wavelength of 655 nm is used for DVD reproduction.
[0003]
Here, in an optical head device for recording and reproducing information on and from an optical recording medium, a configuration is used in which a common objective lens is used to converge a laser beam on the recording surface of a CD and a DVD for miniaturization and compactness. Stuff has been proposed.
[0004]
However, in the case of a CD, the thickness of the transparent protective layer for protecting the recording surface is 1.2 mm, and in the case of the DVD, the thickness of the transparent protective layer is 0.6 mm, which is thinner than the CD, and the recording density is higher than that of the CD. . Therefore, as an objective lens, a diffraction grating consisting of concentric fine steps is formed on a lens surface having a single refractive power, and the incident light beam is diffracted by this diffraction grating, so that the thickness of the transparent protective layer varies. Japanese Patent Application Laid-Open No. H11-163873 proposes that a good focus can be obtained on the recording surfaces of two types of optical recording media.
[0005]
Here, the height of the step forming the diffraction grating needs to be 2π of the phase of the laser beam, but the diffraction grating includes a first laser beam having a wavelength of 785 nm for CD reproduction and recording and a DVD reproduction. The second laser light having a wavelength of 655 nm is incident. Therefore, the refractive index of the objective lens when the first laser light is incident, the refractive index of the objective lens when the second laser light is incident, the wavelength of the first laser light, and the refractive index of the second laser light Let n be the wavelength₁, N₂, Λ₁, And λ₂And the height of the step of the diffraction grating is
h₁= Λ₁/ (N₁-1)
h₂= Λ₂/ (N₂-1)
H required by₁Or h₂Is set to The height of the step of the diffraction grating is (h₁+ H₂) / 2.
[0006]
[Patent Document 1]
JP 2000-81566 A
[0007]
[Problems to be solved by the invention]
However, emphasizing the second laser beam, the height of the step of the diffraction grating is increased by h.₂In this case, the S-curve characteristics (focusing error signal) are as shown in FIG. 5A, and a good resolution is obtained for DVD, but no S-curve is obtained for CD. In addition, the jitter indicating the reproduction performance is a good value for a DVD, but is a value of a required specification limit level even if a CD can be reproduced.
[0008]
On the other hand, emphasizing the first laser light, the height of the step of the diffraction grating is set to h.₁5B, the S-curve characteristics are as shown in FIG. 5B. A good resolution can be obtained for a CD, but the amplitude of the S-curve is low for a DVD. If the S-curve amplitude of the DVD is low, there is a problem that focus servo cannot be performed at the time of recording / reproducing on a two-layer disc. In addition, there is a problem that the focus servo of a bad disk such as a disk (fingerprint disk) to which a fingerprint is attached comes off. The jitter is also a good value for a CD, but exceeds the required specification limit for a DVD, and is at a level at which reproduction is impossible.
[0009]
In addition, the height of the step of the diffraction grating is set to (h₁+ H₂) / 2, the S-curve characteristics are as shown in FIG. 5 (c). With such an S-curve, almost satisfactory reproduction performance of a CD equivalent to that of FIG. 5 (b) is obtained. However, DVDs that require a higher resolution than CDs have poorer reproduction performance.
[0010]
Further, the refracting surface of the objective lens is divided into a center side and an outer peripheral side, and the height of the step is set to (h₁+ H₂) / 2 is formed on the center side diffraction grating, and the height of the step is h on the outer peripheral side.₂A diffraction grating formed by the center side diffraction grating is used for reproducing a CD using the first laser beam, and a center diffraction grating is used for reproducing a DVD using the second laser beam. In addition, the light diffracted by the outer diffraction grating may be used. In this case, the step height of the outer peripheral side diffraction grating is conventionally set to h in order to increase the efficiency of the second laser light.₂However, in this case, the phase of the diffracted light in the second laser light is shifted between the center-side diffraction grating and the outer-peripheral-side diffraction grating, so that the wavefront aberration becomes worse and the transmittance of the entire lens becomes worse. all right.
[0011]
In view of the above problems, it is an object of the present invention to provide first and second laser beams having different thicknesses of transparent protective layers through an objective lens having a diffractive lens structure. It is an object of the present invention to provide an optical head device and an objective lens for the optical head device which can obtain good pickup characteristics even when the light is focused on the recording surface of the optical recording medium.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the present invention focuses a first laser beam emitted from a first laser light source on a recording surface of a first optical recording medium via an objective lens, and A second laser beam having a shorter wavelength than the first laser light, emitted from a second laser light source, to a recording surface of a second optical recording medium having a transparent protective layer thinner than the transparent protective layer of the first optical recording medium. In the optical head device for condensing the laser light through the objective lens, the refraction surface of the objective lens surrounds a center-side refraction surface region centered on the optical axis and an outside of the center-side refraction surface region. And a central diffraction grating formed of a plurality of concentric fine steps over the entire area of the central refractive surface region, and using the first laser light source. Recording and reproduction of the first optical recording medium The recording / reproducing of the second optical recording medium using the second laser light source using the diffracted light beam obtained through the center side refraction surface region includes the light beam passing through the outer periphery side refraction surface region. And a diffracted light beam obtained through the center-side refraction surface region, the height of the step of the center-side diffraction grating, the wavelength λ₁And the wavelength λ when the first laser beam of₂H and n are the refractive indices of the central refraction surface area when the second laser light is incident, respectively.₁, And n₂Where, the height H of the step of the center-side diffraction grating is
h₂  <H <(h₁+ H₂) / 2
Where h₁= Λ₁/ (N₁-1)
h₂= Λ₂/ (N₂-1)
Is set so as to satisfy the following.
[0013]
In the present invention, focusing on the fact that the second optical recording medium and the second laser beam are for high-density recording, the height of the step of the center-side diffraction grating is made to correspond to the first laser beam. Dimension h₁And a dimension h corresponding to the second laser beam.₂And the dimension h corresponding to the second laser beam₂Is set to a value close to. That is, the height of the step is set by performing weighting corresponding to the difficulty of the reproducing / recording operation in view of the size of the pit size and the size of the beam spot on the optical recording medium. For this reason, in the second optical recording medium, a sufficient S-shaped curve amplitude can be obtained, good pickup characteristics can be obtained in both the first and second optical recording media, and the disc (to which the fingerprint is attached) can be obtained. Good recording / reproducing performance can be obtained without deviating the focus servo even for a bad disk such as a fingerprint disk. Also, the height of the step of the center-side diffraction grating is set to the dimension h.₁And dimension h₂The dimension h corresponding to the second laser light having a shorter wavelength than the intermediate value of₂Therefore, even if the wavelength of the second laser beam becomes slightly longer due to the temperature rise, the step height H at the center temperature is made to correspond to the wavelength closer to the second laser beam. Therefore, good pickup characteristics can be obtained in the second optical recording medium.
[0014]
In the present invention, the area of a pit recorded or reproduced by the first laser beam with respect to the recording surface of the first optical recording medium and the second pit with respect to the recording surface of the second optical recording medium. The area of the pit recorded or reproduced by the laser light of₁, And S₂Where, the height H of the step of the center-side diffraction grating is
H = (S₁× h₂+ S₂× h₁) / (S₁+ S₂)
Is preferably set to satisfy the following.
[0015]
In the present invention, the diameter of a beam spot formed by the first laser light with respect to the recording surface of the first optical recording medium, and the second diameter with respect to the recording surface of the second optical recording medium. Let the diameter of the beam spot formed by the laser light be φ₁, And φ₂Where, the height H of the step of the center-side diffraction grating is
H = (φ₁ ²× h₂+ Φ₂ ²× h₁) / (Φ₁ ²+ Φ₂ ²)
Is preferably set to satisfy the following.
[0016]
In the present invention, when a constant of a value of 2.6 to 4.0 is k, the height H of the step of the center-side diffraction grating is represented by the following equation.
H = (k × h₂+ 1 × h₁) / (K + 1)
Is preferably set to satisfy the following.
[0017]
In the present invention, the outer peripheral side refraction surface region has a refracting power substantially suitable for the second laser beam to form the second beam spot on a recording surface of the second optical recording medium. Is preferably formed. In order to form a diffraction grating in the outer refractive surface area, it is necessary to form minute steps at a narrow pitch. However, the second laser beam is focused on the recording surface of the second optical recording medium by the refracting power. In this case, it is not necessary to form fine steps at a narrow pitch in the outer-side refractive surface area. Further, the efficiency loss due to the step portion can be eliminated.
[0018]
Further, in the present invention, an outer peripheral diffraction grating formed of a plurality of concentric fine steps is formed over the entire outer peripheral refraction surface region, and the second laser light source is used. In the recording / reproducing of the optical recording medium of No. 2, a configuration using a diffracted light beam obtained through the center-side refractive surface region and a diffracted light beam obtained through the outer-side refractive surface region may be adopted. In such a case, it is preferable that the height of the step constituting the outer peripheral diffraction grating is set to the same height H as the step height of the central diffraction grating. The height of the step of the center-side diffraction grating is H, and the height of the step of the outer-side diffraction grating is h.₂In the configuration described above, the phase shift between the center-side diffraction grating and the outer-peripheral-side diffraction grating deteriorates the wavefront aberration. Is eliminated, the wavefront aberration is improved, and the transmittance of the entire lens is improved.
[0019]
Next, the present invention relates to an objective lens used for an optical head device, and the objective lens of the present invention is provided with the refractive surface having the above-described configuration.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an optical head device including an objective lens to which the present invention is applied will be described with reference to the drawings.
[0021]
[Embodiment 1]
(overall structure)
FIG. 1 is a schematic configuration diagram mainly showing an optical system of the optical head device of the present embodiment.
[0022]
In FIG. 1, an optical head device 1 of the present embodiment reproduces and records information on a plurality of types of optical recording media 4 having different substrate thicknesses and recording densities, such as CDs, CD-Rs, and DVDs. For this purpose, a first laser light source 11 that emits a first laser L1 having a center wavelength of 785 nm used for recording and reproduction of a CD and a second laser L2 having a wavelength of 655 nm used for reproduction of a DVD are used. Two laser light sources 12 are provided. Each laser beam is guided to the optical recording medium 4 via the common light condensing optical system Lo, and return light of each laser beam reflected by the optical recording medium 4 is guided to the common light receiving element 25.
[0023]
The condensing optical system Lo includes a first beam splitter that causes the first laser L1 to travel straight and reflects the second laser L2 so that both lights coincide with the system optical axis L (the optical axis of the objective lens). 21, a second beam splitter 22 that passes lasers L1 and L2 that travel along the system optical axis L, and a collimator lens 23 that converts the laser beams L1 and L2 that have passed through the second beam splitter 22 into parallel light. And an objective lens 3 for forming a beam spot of the laser beams L1 and L2 emitted from the collimating lens 23 on the recording surface of the optical recording medium 4.
[0024]
In the optical head device 1 configured as described above, the beam spot of the first laser beam L1 is placed on the recording surface of the CD 41 by the objective lens 3 with respect to the recording surface 41a of the CD 41 as the first optical recording medium 4. It is formed. A beam spot of the second laser beam L2 is formed on the recording surface of the DVD 42 by the objective lens 3 on the recording surface 42a of the DVD 42 as the second optical recording medium 4.
[0025]
The first and second laser beams L1 and L2 condensed on the optical recording medium 4 (CD41, DVD42) in this manner are reflected by the optical recording medium 4 and then transmitted through the common condensing optical system Lo. The light returns to the second beam splitter 22 and converges on the common light receiving element 25. Then, information reproduction or the like of the optical recording medium 4 (CD41, DVD42) is performed based on the signal detected by the common light receiving element 25.
[0026]
(Structure of objective lens)
The configuration of the objective lens 3 of the present example will be described in detail with reference to FIGS. 2A, 2B, 2C, and 2D are a plan view, a cross-sectional view, and a partially enlarged cross-sectional view of a center-side refraction surface region centered on the optical axis, respectively, showing the objective lens 3. FIG. 4 is a partially enlarged cross-sectional view showing a step formed in a center-side refraction surface region. FIG. 3 is an explanatory diagram showing the convergence state of laser light of each wavelength by the objective lens 3.
[0027]
2A, 2B, and 2C, the objective lens 3 of the present example is a positive lens on which the laser beams L1 and L2 emitted from the first laser light source 11 and the second laser light source 12 are incident. It is a convex lens provided with an incident-side refracting surface 31 having power and an exit-side refracting surface 32 for emitting a laser beam toward the optical recording medium 4.
[0028]
The incident-side refracting surface 31 includes a circular center-side refracting surface region 33 that includes the optical axis L concentrically around the optical axis L, and an outer-side refraction that annularly surrounds the outer periphery of the center-side refracting surface region 33. And the boundary portion between the center side refraction surface region 33 and the outer periphery side refraction surface region 34 is a position corresponding to NA = 0.45 to 0.55.
[0029]
A central diffraction grating 35 is formed by a plurality of concentric fine steps 30 over the entire central refractive surface region 33.
[0030]
The center side refraction surface region 33 of the objective lens 3 is a refraction surface region having a different refracting power from the outer peripheral side refraction surface region 34. The center side diffraction grating 35 formed in the center side refraction surface region 33 has a diffraction characteristic of forming a beam spot of the diffracted light beam of the first laser beam L1 passing through the region on the recording surface of the CD 41. . In addition, it has a diffraction characteristic of forming a beam spot of the diffracted light beam of the second laser beam L2 passing through the area on the recording surface of the DVD 42.
[0031]
In this example, the first-order diffracted light is used for both the diffracted light flux of the first laser light L1 and the diffracted light flux of the second laser light L2 by the center-side diffraction grating 35.
[0032]
On the other hand, the outer peripheral refraction surface area 34 of the objective lens 3 has a refracting power for forming a beam spot of a light beam portion of the second laser beam L2 passing through the area on the recording surface of the DVD 42. That is, in the outer peripheral side refraction surface area 34, grooves with a narrow pitch forming the diffraction grating are not formed. Therefore, it is easy to manufacture a mold for molding the objective lens 3. Further, since there is no stepped portion, there is no loss of light passing through the outer peripheral side refraction region 34, and the transmittance is high.
[0033]
In the optical head device 1 including the objective lens 3 having such a configuration, when reproducing information from the CD 41, only the first laser light source 11 is driven and the first laser light L1 is emitted. The beam spot B of the diffracted light beam component of the first laser light L1 generated by diffraction by the center-side diffraction grating 35 formed in the light beam component passing through the central refraction surface region 33 of the objective lens 3 (41) is formed on the recording surface of the CD 41 as shown by a dotted line in FIG. The light beam component of the first laser beam L1 passing through the outer peripheral refraction surface area 34 of the objective lens 3 is unnecessary light that is not necessary for reproduction, and is not converged as a beam spot on the recording surface of the CD 41.
[0034]
On the other hand, when reproducing information from the DVD 42, only the second laser light source 12 is driven to emit the second laser light L2. As shown by a solid line in FIG. 3, of the light beam components of the laser beam L2 that pass through the central refraction surface region 33 of the objective lens 3, the diffraction generated by being diffracted by the central diffraction grating 35 formed therein. A beam spot B (42) is formed on the recording surface of the DVD 42 by the light component and the light beam component of the second laser light L2 that passes through the outer refraction surface area 34 of the objective lens 3.
[0035]
(Configuration example 1 of center-side diffraction grating)
2A, 2B, 2C, and 2D, in the objective lens 3 used in the optical head device 1 of the present embodiment, the height of the plurality of steps 30 forming the center-side diffraction grating 35 is high. The wavelength λ₁Of the center side refraction surface region 33 when the first laser light L1₂The refractive indices of the center side refraction surface region 33 when the second laser light L2 is₁, And n₂Then, the height H of the step 30 of the center-side diffraction grating 35 is given by the following equation.
h₂  <H <(h₁+ H₂) / 2
Where h₁= Λ₁/ (N₁-1)
h₂= Λ₂/ (N₂-1)
Is set to meet.
[0036]
Here, the refractive index n (n₁= N₂= N), the wavelength λ of the first laser light L1₁, And the wavelength λ of the second laser light L2₂Is 1.54, 785 nm, and 655 nm, respectively, h₁, And h₂Are 1.45 μm and 1.21 μm, respectively, so the height H of the step 30 of the center-side diffraction grating 35 is
1.21 μm <H (μm) <1.33 μm
Is set to satisfy
[0037]
Further, in the present embodiment, the height H of the step 30 of the center-side diffraction grating 35 is set within the above range, the area of the pit formed by the first laser beam L1 with respect to the recording surface 41a of the CD 41, and the DVD 42 The area of the pit formed by the second laser beam L2 with respect to the recording surface 42a of₁, And S₂And the following equation
H = (S₁× h₂+ S₂× h₁) / (S₁+ S₂)
Is set to meet.
[0038]
Here, the minimum pit length (width of the pit in the track direction) formed on the recording surface 41a of the CD 41 is about 0.8 μm, whereas the minimum pit length (width of the pit formed on the recording surface 42a of the DVD 42) is about 0.8 μm. The width of the pit in the track direction is about 0.4 μm, and the track pitch of the CD 41 is 1.6 μm, while the track pitch of the DVD 42 is 0.74 μm. Therefore, the area S of the pit formed on the recording surface 41a of the CD 41₁And the area S of a pit formed on the recording surface 42a of the DVD 42₂Is about 4: 1.
[0039]
Therefore, in the present embodiment, the height H of the step 30 of the center-side diffraction grating 35 is expressed by the following equation.
H = (4 × h₂+ 1 × h₁) / 5
And set to 1.26 μm.
[0040]
If the objective lens 3 in which the step 30 having the height H is formed in the center-side refraction surface region 33 is used, the height H of the step 30 of the center-side diffraction grating 35 corresponds to the first laser beam L1. Dimension h₁And a dimension h corresponding to the second laser beam L2.₂And the dimension h corresponding to the second laser beam L2₂Since the value is set to a value close to the S curve, the S curve characteristic when the objective lens 3 is used is different from the S curve characteristic shown in FIG. 5A and the S curve characteristic shown in FIG. Represented in the middle. Therefore, a sufficient S-curve amplitude can be obtained for both the CD 41 and the DVD 42. In particular, in the DVD 42, a good focus servo can be obtained even for a fingerprint disk, and a good pickup characteristic can be obtained.
[0041]
Moreover, based on the ratio of the area of the pit, the dimension h corresponding to the second laser beam L2 is determined.₂And the height H of the step 30 is set by performing appropriate weighting corresponding to the difficulty of the reproducing / recording operation, so that the DVD 42 can obtain good wavefront aberration and transmittance, Even with the CD 41, wavelength aberration and transmittance within the allowable range can be obtained. Therefore, good pickup characteristics can be obtained for both the CD 41 and the DVD 42.
[0042]
In addition, the height H of the step 30 of the center side diffraction grating 35 is set to the dimension h.₁And dimension h₂The dimension h corresponding to the second laser light L2 having a shorter wavelength than the intermediate value of₂Therefore, even if the wavelength of the second laser beam L2 becomes slightly longer due to the temperature rise, the step height H at the center temperature is made to correspond to the wavelength closer to the second laser beam L2. Therefore, good pickup characteristics can be obtained in the DVD 42.
[0043]
(Configuration example 2 of center diffraction grating)
2A, 2B, 2C, and 2D, the objective lens 3 used in the optical head device 1 according to the present embodiment also has a center-side diffraction grating in this example, similarly to the configuration example 1. 35, the height of the plurality of steps 30 and the wavelength λ₁Of the center side refraction surface region 33 when the first laser light L1₂The refractive indices of the center side refraction surface region 33 when the second laser light L2 is₁, And n₂Then, the height H of the step 30 of the center-side diffraction grating 35 is given by the following equation.
h₂  <H <(h₁+ H₂) / 2
Where h₁= Λ₁/ (N₁-1)
h₂= Λ₂/ (N₂-1)
Is set to meet.
[0044]
Here, the refractive index n (n₁= N₂= N), the wavelength λ of the first laser light L1₁, And the wavelength λ of the second laser light L2₂Are 1.54, 785 nm, and 655 nm, respectively, h₁, And h₂Are 1.45 μm and 1.21 μm, respectively, so the height H of the step 30 of the center-side diffraction grating 35 is
1.21 μm <H (μm) <1.33 μm
Is set to satisfy
[0045]
In the present embodiment, the height H of the step 30 of the center-side diffraction grating 35 is the diameter of the beam spot formed by the first laser light L1 on the recording surface 41a of the CD 41, and The diameter of the beam spot formed by the second laser beam L2 on the recording surface 42a of the DVD 42 is φ₁, And φ₂Then, the height H of the step 30 of the center-side diffraction grating 35 is given by the following equation.
H = (φ₁ ²× h₂+ Φ₂ ²× h₁) / (Φ₁ ²+ Φ₂ ²)
Is set to meet.
[0046]
Here, since the diameter of the beam spot is represented by (constant × wavelength ÷ aperture ratio (NA) of the lens), for example, the aperture ratio for the first laser beam L1 is 0.45, and the aperture ratio for the second laser beam L2 is When the aperture ratio is 0.60, φ₁And φ₂Is 1.6: 1, so φ₁ ²And φ₂ ²Will be about 2.6: 1.
[0047]
Therefore, in the present embodiment, the height H of the step 30 of the center-side diffraction grating 35 is expressed by the following equation.
H = (2.6 × h₂+ 1 × h₁) /3.6
And set to 1.28 μm.
[0048]
If the objective lens 3 in which the step 30 having the height H is formed in the center-side refraction surface region 33 is used, the height H of the step 30 of the center-side diffraction grating 35 corresponds to the first laser beam L1. Dimension h₁And a dimension h corresponding to the second laser beam L2.₂And the dimension h corresponding to the second laser beam L2₂Since the value is set to a value close to the S curve, the S curve characteristic when the objective lens 3 is used is different from the S curve characteristic shown in FIG. 5A and the S curve characteristic shown in FIG. Represented in the middle. Therefore, a sufficient S-curve amplitude can be obtained in both the CD 41 and the DVD 42, and good pickup characteristics can be obtained.
[0049]
Moreover, based on the ratio of the beam spot areas, the dimension h corresponding to the second laser beam L2 is determined.₂And the height H of the step 30 is set by performing appropriate weighting corresponding to the difficulty of the reproducing / recording operation, so that good pickup characteristics can be obtained in both the CD 41 and the DVD 42. Can be.
[0050]
In addition, the height H of the step 30 of the center side diffraction grating 35 is set to the dimension h.₁And dimension h₂The dimension h corresponding to the second laser light L2 having a shorter wavelength than the intermediate value of₂Therefore, even if the wavelength of the second laser beam L2 becomes slightly longer due to the temperature rise, the step height H at the center temperature is made to correspond to the wavelength closer to the second laser beam L2. Therefore, good pickup characteristics can be obtained in the DVD 42.
[0051]
(Configuration example 3 of center side diffraction grating)
2A, 2B, 2C, and 2D, the objective lens 3 used in the optical head device 1 according to the present embodiment employs the center-side diffraction grating 35 as in the first and second configuration examples. The height and wavelength λ of the plurality of steps 30 to be constituted₁Of the center side refraction surface region 33 when the first laser light L1₂The refractive indices of the center side refraction surface region 33 when the second laser light L2 is₁, And n₂Then, the height H of the step 30 of the center-side diffraction grating 35 is given by the following equation.
h₂  <H <(h₁+ H₂) / 2
Where h₁= Λ₁/ (N₁-1)
h₂= Λ₂/ (N₂-1)
Is set to meet.
[0052]
Here, the refractive index n (n₁= N₂= N), the wavelength λ of the first laser light L1₁, And the wavelength λ of the second laser light L2₂Are 1.54, 785 nm, and 655 nm, respectively, h₁, And h₂Are 1.45 μm and 1.21 μm, respectively, so the height H of the step 30 of the center-side diffraction grating 35 is
1.21 μm <H (μm) <1.33 μm
Is set to meet.
[0053]
In the present embodiment, the height H of the step 30 of the center-side diffraction grating 35 is determined in consideration of both the configuration example 1 and the configuration example 2.
H = (k × h₂+ 1 × h₁) / (K + 1)
Constant k = 2.6 to 4.0
Is set to meet.
[0054]
Here, if k = 3, the height H of the step 30 of the center-side diffraction grating 35 is set to 1.27 μm.
[0055]
Also in the case where the objective lens 3 in which the step 30 having the height H is formed in the center-side refraction surface region 33 is used, a sufficient S-curve amplitude is obtained in both the CD 41 and the DVD 42 as in the configuration examples 1 and 2. And good pickup characteristics can be obtained.
[0056]
(Another example of the objective lens 3)
In the objective lens 3 described above, by setting the refracting power of the outer peripheral refraction surface area 34, a beam spot based on the light beam component of the second laser light beam passing through the area is formed on the recording surface of the DVD 42. . However, a diffraction grating can also be formed in the outer-side refraction surface region 34 so that the beam spot of the diffracted light beam component of the second laser beam L2 by the diffraction grating is formed on the recording surface of the DVD 42. .
[0057]
FIG. 4A is a plan view showing an example of an objective lens in which a diffraction grating is also formed on the outer-side refraction surface region, and FIG. 4B is a cross-sectional view thereof, and FIGS. 4C and 4D. Is a partially enlarged sectional view of each of them.
[0058]
As shown in these figures, the objective lens 3A of the present example is a refracting surface having a positive power on which the laser beams L1 and L2 emitted from the first laser light source 11 and the second laser light source 12 are incident. It is a convex lens having an incident side refraction surface 31A and an exit side refraction surface 32A for emitting a laser beam toward an optical recording medium.
[0059]
The incident side refraction surface 31A is divided into a circular center side refraction surface region 33A including the optical axis L, and an annular outer periphery side refraction surface region 34A concentrically surrounding the center side refraction surface region 33A. . In the center side refraction surface region 33A, a center side diffraction grating 35A composed of concentric fine steps 30 is formed over the entire region. Further, an outer-side diffraction grating 36 formed of concentric fine steps 30 is also formed in the outer-side refraction surface area 34A over the entire area.
[0060]
The objective lens 3A of the present example forms a beam spot on the recording surface 41a of the CD 41 by a light beam component of the first laser beam L1 emitted during recording or reproduction of the CD 41 and passing through the central refraction surface region 33A. . More specifically, the beam spot B (41) is formed on the recording surface of the CD 41 by the diffracted light beam component generated by the diffraction effect of the center side diffraction grating 35A formed in the center side refraction surface region 33A among the light beam components. It is formed.
[0061]
However, of the first laser beam L1, a light beam component passing through the outer peripheral side refraction surface region 34A is an unnecessary light component that does not contribute to recording or reproduction, and in this example, is formed in the outer peripheral side refraction surface region 34A. Due to the diffraction effect of the outer peripheral side diffraction grating 36, the light is diffracted so as not to converge on the beam spot forming position on the recording surface of the CD 41.
[0062]
In addition, the objective lens 3A of this example forms a beam spot on the recording surface 42a of the DVD 42 by the second laser light L2 emitted during reproduction of the DVD 42. That is, of the light beam components of the second laser beam that pass through the center-side refraction surface region 33A, the diffracted light components generated by the diffraction effect of the center-side diffraction grating 35A formed in the center-side refraction surface region 33A , A beam spot is formed on the recording surface of the DVD 42. At the same time, of the light beam components of the second laser light L2 that pass through the outer peripheral side refraction surface region 34A, the diffracted light beam components generated by the diffraction effect of the outer peripheral side diffraction grating 36 formed in the relevant region are also used. , A beam spot is formed at the same position on the recording surface of the DVD 42.
[0063]
In the present example, the first-order diffracted light of the first laser light L1 or the second laser light L2 is used for both recording and reproduction of the CD 41 and reproduction of the DVD.
[0064]
Also in the objective lens 3A having such a configuration, the height H of the step 30 of the center-side diffraction grating 35A is set to, for example, 1.28 μm as described in the configuration examples 1, 2, and 3.
[0065]
Further, in the objective lens 3A of this example, the outer peripheral side diffraction grating 36 is also formed in the outer peripheral side refraction surface region 34A, but the outer peripheral side diffraction grating 36 is the outer peripheral side portion of the first laser beam L1. The height of the step 30 of the outer diffraction grating 36 is also the same as the height H of the central refraction surface region 33A, for example, 1 so that the unnecessary light components of .28 μm. Therefore, in the second laser light L2, the phases on the center side and the outer peripheral side can be matched, so that good wavefront aberration can be obtained and the transmittance can be improved. Therefore, better reproduction performance can be obtained than when the light beam component passing through the outer peripheral side is not diffracted.
[0066]
(Other forms)
The present invention is also applicable to a lens other than the objective lens 3 in the optical head device 1 through which a laser beam of a different wavelength passes, for example, a collimator lens 23 or the like.
[0067]
【The invention's effect】
As described above, in the present invention, focusing on the fact that the second optical recording medium and the second laser beam are for high-density recording, the height of the step of the center-side diffraction grating is set to the first level. The value is set to a value closer to the dimension corresponding to the second laser light than to the middle between the dimension corresponding to the laser light and the dimension corresponding to the second laser light. That is, the height of the step is set by performing weighting corresponding to the difficulty of the reproducing / recording operation in view of the size of the pit size and the size of the beam spot on the optical recording medium. For this reason, since the S-curve amplitude of the second optical recording medium is sufficiently large, the focus servo is not deviated even with a fingerprint disk. Also, good focus servo can be performed with a two-layer disc, and good pickup characteristics can be obtained in both the first and second optical recording media. Further, since the height of the step of the center-side diffraction grating is set to a value close to the dimension corresponding to the second laser light, even if the wavelength of the second laser light becomes slightly longer due to the temperature rise. In addition, good pickup characteristics can be obtained in the second optical recording medium.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram mainly showing an optical system of an optical head device to which the present invention is applied.
2A, 2B, 2C, and 2D are a plan view, a cross-sectional view, and a partially enlarged cross-section of a center-side refracting surface region centered on an optical axis, respectively, of the objective lens shown in FIG. FIG. 3 is a diagram and a partially enlarged cross-sectional view showing a step formed in a center-side refraction surface region.
FIG. 3 is an explanatory diagram showing a convergence state of first and second laser beams by the objective lens of FIG. 2;
4A and 4B are diagrams showing another example of the objective lens, wherein FIG. 4A is a plan view thereof, FIG. 4B is a sectional view thereof, and FIGS. 4C and 4D are partially enlarged sectional views thereof. It is.
FIGS. 5A, 5B, and 5C are graphs each showing an S-curve characteristic when a conventional objective lens is used.
[Explanation of symbols]
1 Optical head device
3, 3A objective lens
4 Optical recording media
11 First laser light source
12 Second laser light source
21 1st beam splitter
22 Second beam splitter
23 Collimating lens
24 grating
25 Common light receiving element
30 Concentric small steps
31, 31A Incident-side refraction surface
32, 32A Outgoing side refraction surface
33 Center refraction surface area
34, 34A Outer peripheral refraction surface area
35 Center diffraction grating
36 Outer side diffraction grating
41 CD (first optical recording medium)
41a Recording surface
42 DVD (second optical recording medium)
42a Recording surface
B (41) Beam spot
B (42) Beam spot
L Optical axis of objective lens (system optical axis)
Lo focusing optics
L1 First laser beam
L2 Second laser beam

Claims (7)

The first laser light emitted from the first laser light source is focused on the recording surface of the first optical recording medium via the objective lens, and is transparent to the transparent protective layer of the first optical recording medium. A second laser light emitted from a second laser light source and having a shorter wavelength than the first laser light is focused on the recording surface of the second optical recording medium having a thin protective layer via the objective lens. In the optical head device,
Dividing the refractive surface of the objective lens into a central refractive surface region centered on the optical axis thereof and an outer peripheral refractive surface region surrounding the outside of the central refractive surface region, Forming a central diffraction grating composed of a plurality of concentric fine steps over the entire area,
The recording and reproduction of the first optical recording medium performed using the first laser light source is performed using the second laser light source using a diffracted light beam obtained through the center-side refraction surface region. For recording and reproduction of the second optical recording medium, using a light beam that has passed through the outer-side refractive surface region and a diffracted light beam obtained through the center-side refractive surface region,
The height of the step of the center-side diffraction grating, the refractive index of the center-side refraction surface region when the first laser light of wavelength λ ₁ is incident, and the incidence of the second laser light of wavelength λ ₂ Assuming that the refractive index of the center-side refraction surface region at this time is H, n ₁ , and n ₂ , respectively, the height H of the step of the center-side diffraction grating is represented by the following equation: h ₂ <H <(h ₁ + _h 2) / 2
Where h ₁ = λ ₁ / (n ₁ −1)
h ₂ = λ ₂ / (n ₂ −1)
An optical head device set to satisfy the following. 2. The pit area recorded or reproduced by the first laser beam with respect to a recording surface of the first optical recording medium, and the pit area with respect to a recording surface of the second optical recording medium. Assuming that the areas of the pits recorded or reproduced by the laser light of _{No. 2} are S ₁ and S ₂ , respectively, the height H of the step of the center-side diffraction grating is expressed by the following equation: H = (S ₁ × h ₂₎ + S ₂ × h ₁ ) / (S ₁ + S ₂ )
An optical head device set to satisfy the following. 2. The device according to claim 1, wherein a diameter of a beam spot formed by the first laser beam with respect to a recording surface of the first optical recording medium, and a diameter of the second beam with respect to a recording surface of the second optical recording medium. each diameter of a beam spot formed by the laser beam, when the phi _1, and phi _2, the height H of the step of the center side diffraction grating, the following equation ^{_{H = (φ 1 2 × h}} 2 + φ ^{_{2 2 × h 1) / (}} φ 1 2 + φ 2 2)
An optical head device set to satisfy the following. In claim 1, assuming that a constant having a value of 2.6 to 4.0 is k, the height H of the step of the center-side diffraction grating is represented by the following equation: H = (k × h ₂ + 1 × h ₁ ) / (K + 1)
An optical head device set to satisfy the following. 5. The outer peripheral refraction surface region according to claim 1, wherein the outer peripheral side refraction surface region is suitable for the second laser beam to form the second beam spot on a recording surface of the second optical recording medium. An optical head device formed so as to have a high refractive power. 5. An outer peripheral diffraction grating comprising a plurality of concentric fine steps over the entire outer peripheral refraction surface region according to any one of claims 1 to 4,
When performing recording and reproduction of the second optical recording medium using the second laser light source, a diffracted light beam obtained through the center-side refractive surface region and a diffracted light beam obtained through the outer-side refractive surface region are obtained. Using the diffracted light beam
An optical head device, wherein the height of the step constituting the outer peripheral side diffraction grating is set to the same height H as the step height of the center side diffraction grating. An objective lens for an optical head device according to claim 1.

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