JPH11174318A - Objective lens and optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate and reduce chromatic aberration. SOLUTION: This objective lens is equipped with a 1st lens group 26 disposed to be positioned on an object point side in a state where an aspherical surface is formed at least on one surface through which an optical axis passes, and a 2nd lens group 28 disposed in a state where its optical axis is aligned with that of the 1st lens group 26. Numerical aperture NA by the 1st and the 2nd lens groups 26 and 28 is set to >=0.65. Then, a hologram 30 is provided on the 2nd surface of the rear lens 27 of the 1st lens group 26 positioned nearer to the object point side than the surface, which is positioned nearest to an image point side, of the 2nd lens group 28.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens and an optical pickup device for recording and / or reproducing information signals on an optical recording medium such as an optical disk, a magneto-optical disk and an optical memory card.

[0002]

2. Description of the Related Art For example, an optical pickup device for reproducing an information signal from an optical disk is known. This type of optical pickup device includes an objective lens that focuses a laser beam on a signal recording surface of an optical disc.

In recent years, there has been a demand for higher capacity of information signals of optical discs while advanced information has been advanced. In order to increase the surface density of an optical disc, it is necessary to reduce the spot diameter of a laser beam. To reduce the spot diameter, it is necessary to shorten the wavelength of the laser beam and increase the numerical aperture NA of the objective lens.

When the numerical aperture NA is increased to 0.6 or more, it is difficult to mold a single lens aspherical objective lens when the numerical aperture NA is increased to 0.6 or more. Was difficult. Therefore, in order to reduce the curvature, it is necessary to use a material having a high refractive index as a lens material. However, even when formed using a material having a high refractive index, the numerical aperture NA of a single lens objective lens is limited to about 0.7.

Therefore, in order to achieve high NA while dispersing the power of the lens and reducing the curvature, an objective lens unit having two lenses or two lens groups provided with their optical axes coincident with each other. Has been proposed.

[0006]

By the way, when an optical pickup device reproduces an optical disk, a reproduced signal contains noise. As a factor of the noise of the reproduction signal of the optical disk, the thickness of the light transmitting layer on the signal reading surface side of the disk may vary. For example, the wavelength is 650 nm,
In an optical system in which the aperture ratio NA of the objective lens unit is 0.85, the wavefront aberration (spherical aberration) is 0.032λ rms due to the change in the thickness of the light transmission layer of the optical disk by 5 μm.
Will occur. Therefore, in order to suppress the aberration generated due to the variation in the thickness of the light transmission layer of the optical disc within an allowable range, it becomes necessary to increase the dimensional accuracy of the optical disc as the numerical aperture NA increases.

However, increasing the dimensional accuracy of the optical disk may increase the manufacturing cost of the optical disk manufacturing process. For this reason, it is necessary to provide aberration correction means in the optical pickup device in order to avoid an increase in the manufacturing cost of the optical disk.

In the optical pickup device, when the objective lens is a single lens aspherical lens, it is difficult to correct aberrations due to variations in the thickness of the light transmitting layer of the optical disk. Therefore, the optical pickup device is configured to include an objective lens unit having two or more lens groups as an objective lens, and corrects aberration by changing characteristics such as a refractive index and a distance of a part of the objective lens unit. It is necessary to provide aberration correction means.

Another factor of the noise of the reproduction signal of the optical disk is a mode hop due to the fluctuation of the output of the laser light emitted from the light source. When the wavelength of the laser light changes, the focal position is displaced, so that a focusing error occurs. Particularly, in a system or a magneto-optical system that performs verification at the time of recording an information signal, a mode hop is likely to occur because switching between reproduction and recording is frequently performed. Since the depth of focus is proportional to the wavelength and inversely proportional to the square of the numerical aperture NA, it becomes smaller as the wavelength becomes shorter and the NA becomes higher. As a result, the permissible value of defocus becomes smaller, and the precision of the focusing servo must be increased.

Since the wavelength of a semiconductor laser has temperature dependence, the wavelength shifts to a longer wavelength side as the temperature increases. The wavelength of the semiconductor laser changes by about ± 5 nm within the range of 20 ° C. to 60 ° C. as the actual use temperature. With respect to this wavelength variation, a lens having an Abbe number of about 60 has a problem that the displacement of the focal position reaches several μm. The problem of the short focal length and the displacement amount of the large focal position becomes a serious problem unless chromatic aberration is corrected.

Further, assuming that the laser light emitted from the semiconductor laser has a spread of 1 nm in terms of the FWHM of the wavelength of the semiconductor laser, even if a low-dispersion glass lens having an Abbe number of about 80 is used, for example, the laser beam has a wavelength of 0 nm. .007-
A spherical aberration of about 0.01 λ rms occurs. A shift in the wavelength of the laser beam caused by a change in temperature causes a shift in the focal position and chromatic aberration, but these can be canceled by aberration correction means or focusing servo if they have a predetermined accuracy or speed. However, chromatic aberration caused by the spread of the wavelength of laser light emitted from a semiconductor laser cannot be corrected without using an achromatic lens system.

When achromatism is performed by a conventional refraction lens, the Abbe number of glass or optical plastic material is distributed between about 20 and 70. Chromatic aberration is corrected by using a convex lens and a concave lens of Flint. However, since two types of lens materials are required, there is a disadvantage that the manufacturing cost is increased.

Further, since a lens formed of an optical plastic material has a hygroscopic property, there is also a problem that a time constant is long in a process of absorbing and discharging moisture and a relatively large spherical aberration is generated.

Accordingly, an object of the present invention is to provide an objective lens capable of correcting chromatic aberration and an optical pickup device provided with the objective lens.

[0015]

In order to achieve the above-mentioned object, an objective lens according to the present invention has a first lens unit and a second lens unit having a numerical aperture NA of 0.65 or more, and a second lens unit. An optical element that diffracts incident light is provided on at least one surface located closer to the object point than the surface located closest to the image point of the group.

Further, in the optical pickup device according to the present invention, the numerical aperture NA of the first and second lens groups is 0.1.
An optical element that diffracts incident light is provided on at least one surface of the second lens group that is closer to the object point than the surface that is closest to the image point of the second lens group.

In the objective lens having the high numerical aperture configured as described above, the achromatic condition of the chromatic aberration which is a problem is generally derived by the following equation. If _two thin lenses having the focal lengths f ₁ and f ₂ are arranged at an interval t in the optical axis direction, the focal length f of the composite lens is given by Expression (1).

[0018]

(Equation 1)

Taking the total derivative of the equation (1), the equation (2) is obtained.

[0020]

(Equation 2)

The focal length of the thin lens can be obtained by formula (3) if the radii of curvature of the thin lenses are r ₁ and r ₂ .

[0022]

(Equation 3)

In the equation (3), t in the second term on the right side is the thickness of the lens. If this thickness t is ignored, f (n−
The value of 1) does not depend on the wavelength. When the equation (3) is totally differentiated, the equation (4) is obtained.

[0024]

(Equation 4)

The Abbe number is given by equation (5).

[0026]

(Equation 5)

From equations (4) and (5), equation (6) is obtained.

[0028]

(Equation 6)

From the equations (2) and (6), the achromatic condition of the thin lens is obtained. If t = 0, ν ₂ / ν ₁ = −
Since f ₁ / f ₂ , equation (7) is obtained.

[0030]

(Equation 7)

Equation (7) is the achromatic condition for a thin lens. That is, the conditions are such as an achromatized laminated lens which is formed by closely contacting a convex lens of crown glass and a concave lens of flint glass.

In the case where two thin lenses are provided at a predetermined distance from each other, the achromatizing condition is similarly obtained from formulas (2) and (6) as formula (8).

[0033]

(Equation 8)

As is apparent from the equation (8), even if the same material is used, achromaticity can be realized if the condition of t = 0.5 (f ₁ + f ₂ ) is satisfied. By satisfying this condition, the distance between the front lens and the rear lens increases, so that the size of the objective lens unit in the optical axis direction increases.

When this condition is applied to the equation (1), f _{t = tc} = 2f _{t = 0} , and the focal length is doubled as compared with the case of t = 0. It is difficult to increase. Therefore, equation (1) also indicates that the focal length moves in the long direction by increasing the distance of the second lens group.

By using the equation (6), the C-line (656.725 nm) of the thin single lens is converted to the F-line (58
9.2938 nm) can be estimated. For example, in an optical system having a numerical aperture NA of 0.85, an entrance pupil diameter of 4.0 mm, and a wavelength of 650 nm, the focal length f is 2.353 mm. For example, if the Abbe number is 3
When the material of 6.3 is used, a defocus of 65 μm occurs in a wavelength from red to blue. This corresponds to a defocus of about 4 μm per 10 nm. On the other hand, the depth of focus Z ₀ is given by equation (9).

[0037]

(Equation 9)

The above-mentioned depth of focus Z ₀ is symmetrical with respect to the front and back of the best image plane, and is twice as large as this depth of focus Z _0.
Z ₀ (hereinafter, referred to as DOOF (Depth of Focus))
Is the desired value.

When the DOF in the above-described optical system is calculated, it is 0.9 μm, which corresponds to 20% of the focal point shift due to wavelength fluctuation. The axial chromatic aberration in the case of a single lens has been described above, but the same applies to the case of a two-group lens.

Next, the case of three groups of thin lenses will be described. A first lens group, a second lens group,
As the third lens group, the distance between the first lens group and the second lens group is 0, and the distance between the second lens group and the third lens group is t.

The focal length f of the entire optical system can be expressed by equation (10).

[0042]

(Equation 10)

When the equation (10) is fully differentiated, the following equation (11) is obtained.
It becomes an expression.

[0044]

[Equation 11]

Equation (11) is modified using equation (6). For simplicity, the first lens group and the third
If the Abbe number of the lens groups to be equally [nu _r,
Equation (12) is obtained.

[0046]

(Equation 12)

In order to suppress chromatic aberration, the left side of equation (12) may be set to 0, and equation (13) is obtained.

[0048]

(Equation 13)

Here, the first lens group is a rear lens, the second lens group is a diffractive lens, and the third lens group is a front lens. Assuming that the Abbe number of the diffractive lens is ν ₂ , the Abbe number of the diffractive lens is given by Expression (14).

[0050]

[Equation 14]

First, assuming that t = 0, equation (13) becomes
(15)

[0052]

(Equation 15)

That is, a relational expression of the focal length f of the optical system, the Abbe number of the lens material, and the focal length of the diffractive lens is obtained.

As an optical plastic material for forming a lens, for example, polymethyl methacrylate (PMM)
A), styrene acrylonitrile (SAN), polycarbonate (PC), thermosetting plastic, polystyrene (PS) and the like. Among them, PS, PC
Is not suitable for a lens requiring high precision because of its large birefringence. Abbe number of general plastic material is
It is in the range of 31-58. To achromatize, the focal length of the diffractive lens should be 10 to 18 times the focal length of the entire optical system.
You can double it. In the case of an optical glass material, the Abbe number can be selected more freely. However, when a diffractive lens is not used, the focus given by the equation (6) is required unless a material having an Abbe number of 70 or more is used. Depth is ± 5n
45 defocus caused by the wavelength fluctuation of the laser light of m
%.

Therefore, when a material having an Abbe number of 70 or less is used, it is not necessary to positively achromatize using a diffraction lens. Therefore, it is necessary to achromatize a lens formed of a material having an Abbe number of 70 or less using a diffractive lens. Since the minimum Abbe number of the material forming the lens is about 21, from the condition of equation (15), the focal length of the diffractive lens and the focal length of the entire optical system are:
Equation (16) needs to be satisfied.

[0056]

(Equation 16)

[0057]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a specific embodiment of the present invention will be described with reference to the drawings. The optical pickup device 1 has a diameter of 120 mm.
The recording is performed on a high recording density disk 5 (hereinafter, referred to as an optical disk 5) having an overall thickness of 1.2 mm by bonding an optical disk having a disk substrate thickness of 0.1 mm and a disk reinforcing plate. And / or used for reproduction. That is, the optical disc 5 has a reflection surface, which is a signal recording surface, formed at a position 0.1 mm inward in the thickness direction from the surface of one signal reading surface.

Optical system 10 provided in optical pickup device 1
Although not shown, a light source that emits laser light having a short wavelength of 670 nm or less, a collimator lens that converts the laser light emitted from the light source into parallel light, and a three-beam And a diffraction grating for splitting light. The light source has a wavelength of 670 nm or less, for example, 650 n.
It has a semiconductor laser that emits about m laser light.

As shown in FIG. 1, the optical system 10 includes a polarizing beam splitter 13 that transmits a laser beam emitted from a light source and reflects a laser beam reflected from an optical disk 5, and converts a linearly polarized light into a circularly polarized light. And a two-group objective lens unit 15 that focuses the laser light passing through the quarter-wave plate 14 on the signal recording surface of the optical disk 5.

As shown in FIG. 1, the optical system 10 includes a condenser lens 16 and a multi-lens 17 for condensing the reflected laser light from the optical disk 5 reflected by the polarization beam splitter 13, And a photodetector 18 for receiving the reflected laser light from the signal recording surface.

[0061] Then, the optical pickup apparatus 1, as shown in FIG. 1, the two groups bobbin 20 on which an objective lens unit 15 is provided, 1 in the arrow a ₁ direction and the arrow a ₂ direction in the drawing the bobbin 20, FIG. and an electromagnetic driving mechanism 21 that moves in two directions perpendicular to each other in the arrow b ₁ direction and the arrow b ₂ direction shown in 1.

Although not shown, the bobbin 20 is slidable in the axial direction of the support shaft provided in parallel with the optical axis direction of the second-group objective lens unit 15 and in the direction around the support shaft. It is supported rotatably.

The bobbin 20 is slid in the axial direction of the support shaft by being driven and displaced by the electromagnetic drive mechanism 21, and is further slid in the direction around the support shaft. That is, when the bobbin 20 is slid and displaced in the axial direction of the support shaft, the second-group objective lens unit 15 moves in the first direction parallel to the optical axis thereof in the directions indicated by arrows a ₁ and a
_The optical disc 5 is driven and displaced in _two directions to perform focusing control, and the bobbin 20 is rotationally displaced around the axis of the support shaft. it is driven and displaced in FIG. 1 in the arrow b ₁ direction and the arrow b ₂ direction in the tracking control for the optical disk 5 is performed.

As shown in FIG. 1, the electromagnetic drive mechanism 21 for driving and displacing the bobbin 20 includes a focusing coil 22 and a tracking coil 23 disposed on the bobbin 20, and the focusing coil 22 and the tracking coil 23. Focusing magnet 24 and tracking magnet 25 disposed opposite to 23
It is comprised including.

In the electromagnetic drive mechanism 21, the bobbin 20 is driven and displaced in the axial direction of the support shaft by supplying a focusing error signal to the focusing coil 22, and a tracking error signal is supplied to the tracking coil 23. Thereby, the bobbin 20 is rotated and displaced in the direction around the support shaft. In the optical pickup device 1, a so-called astigmatism method (astigma method) is used as a focusing error detection method, and a so-called three spot (three beam) method is used as a tracking error detection method. This astigmatism method
The reflected laser light from the optical disk 5 is detected by a photodetector having a detection area divided into four parts via, for example, a cylindrical lens, and the sum and / or difference of the detection outputs obtained from each detection area is obtained, thereby recording the signal of the laser light. In this case, a focusing error signal, which is a defocus component with respect to the surface, is obtained. In the three-spot method, one laser beam emitted from a light source is divided into one main laser beam and two sub-laser beams using a diffraction grating or the like, and the laser beam is irradiated to the center of a recording track. Two sub laser beams are irradiated before and after the main laser beam. The reflected laser light of the sub-laser light irradiated before and after the main laser light is detected by two photodetectors, and the difference between the detection outputs obtained from the respective photodetectors is obtained, whereby the deviation component of the main laser light from the recording track is obtained. A tracking error signal is obtained.

Although not shown, the optical pickup device 1 is provided on the bobbin 20 with another optical disk corresponding to another optical disk, as necessary, for example, when reproducing another optical disk having a specification different from that of the optical disk 5. A configuration in which an objective lens included in the optical system is provided may be employed.

As shown in FIGS. 1 and 2, the two-group objective lens section 15 which is a main part of the present invention is disposed so as to be located on the object point side to form a first lens group 26. A first lens 27 (hereinafter, referred to as a rear lens 27), and a second lens 29 (hereinafter, referred to as a second lens group 28) which is disposed so that the rear lens 27 has the same optical axis as the rear lens 27. , Referred to as a front lens 29).

Although not shown, the second group objective lens section 15 includes a rear lens holder for holding the rear lens 27,
And a front lens holder for holding the front lens 29. Although not shown, the two-group objective lens unit 15
Has an electromagnetic drive means for moving the front lens 29 in the optical axis direction with respect to the rear lens 27 in order to correct spherical aberration.

Rear lens 27 of the second-group objective lens unit 15
Is formed of, for example, polymethyl methacrylate (PMMA), and the numerical aperture NA of the front lens 29 and the rear lens 27 is 0.65 or more, and is set to 0.85, for example. . The optical plastic material for forming the front lens 29 and the rear lens 27 is, for example, styrene acrylonitrile (SA) in addition to polymethyl methacrylate (PMMA).
N), polycarbonate (PC), thermosetting plastic, polystyrene (PS) and the like.

As shown in FIG. 2, the second group objective lens unit 15
Are assigned surface numbers in order from the object point side. Rear lens 2
As shown in FIG. 2, a hologram 30 that diffracts incident light to correct chromatic aberration is provided on the second surface 7. The configuration of the rear lens 27 and the front lens 29 is acrylic resin [aspherical surface-spherical surface + diffractive surface] -crown glass [spherical surface-plane]. The thickness of the light transmitting layer of the optical disk 5 is 0.1 mm, and the working distance (working distance) of the second lens unit 15 is 0.1 mm.

The aspherical shapes of the rear lens 27 and the front lens 29 of the second-group objective lens unit 15 are defined by the following aspherical expression.

[0072]

[Equation 17]

Here, X: distance from the tangent plane to the aspherical vertex of a point on the aspheric surface whose height from the optical axis is Y, Y: height from the optical axis, R: radius of curvature, k: conical constant , A, B, C, D:
Fourth-, sixth-, eighth-, and tenth-order aspherical coefficients Further, it is desirable that the hologram 30 has a blazed angle in terms of efficiency as a phase type. Similarly to a general hologram, the phase shift on each surface when two point light sources are located at infinity at the time of manufacture is represented by a polar coordinate polynomial on the substrate.

The optical path difference at the diffraction reference wavelength (hereinafter referred to as OP
Called D. ) Is given by equation (18). However, unit: m

[0075]

(Equation 18)

The actual lens shape is changed into a shape having a blazed angle or a step shape in order to cause diffraction.

With respect to the second group objective lens unit 15 of the optical pickup device 1 configured as described above, the wavelength of the laser beam emitted from the light source is 660 nm, 650 nm, and 640 nm.
An isometric plot of the wavefront aberration (spherical aberration) when nm is set will be described with reference to the drawings.

As shown in FIGS. 3 and 4, when the wavelength of the laser beam was 660 nm, the deviation from the focal position at the wavelength of 650 nm was 6 nm. Further, as shown in FIGS. 4 and 5, when the wavelength of the laser beam was 640 nm, the deviation from the focal position at the wavelength of 650 nm was 5 nm in the second lens unit 15.

Therefore, as shown in FIGS. 3, 4 and 5, the second-group objective lens unit 15 does not greatly shift the focal position even when the wavelength of the laser beam changes.
No defocus has occurred. The second-group objective lens unit 15 is a D.O. O. Since F is 900 nm, the chromatic aberration is corrected to a sufficiently small value.

As described above, the second group objective lens section 15 of the optical pickup device 1 can correct the chromatic aberration by providing the hologram 30 on the second surface of the rear lens 27.

In the second group objective lens section 15, the rear lens 27 can be formed of a resin material such as an optical plastic material. Therefore, in the second-group objective lens section 15, since the rear lens 27 is formed of a relatively inexpensive resin material, the manufacturing cost can be reduced and the weight can be reduced.

Further, another two-group objective lens unit 115 according to the present invention will be described with reference to FIG. As shown in FIG. 6, the two-group objective lens unit 115 is disposed on the object point side, and constitutes a first lens group 126, a rear lens 127, and the rear lens 127 and the optical axis. And a front lens 129 that constitutes a second lens group 128 that is disposed in such a manner that

As shown in FIG. 6, the second-group objective lens unit 11
The surface numbers are assigned in order from the object point side of No. 5. As shown in FIG. 6, a hologram 130 that diffracts incident light is provided on the third surface of the front lens 129 to correct chromatic aberration. The configuration of the rear lens 127 and the front lens 129 is [aspherical surface-aspherical surface]-[spherical surface + diffractive surface-spherical surface].
It has been. The thickness of the light transmitting layer of the optical disk 6 is 0.6 mm, and the working distance (working distance) of the second-group objective lens unit 115 is 0.75 mm.

The optical element such as a hologram or a diffractive lens for diffracting the incident light is provided at any position on the object point side with respect to the surface located on the most image point side of the second lens unit. You may be. Further, the second group objective lens unit of the optical pickup device according to the present invention may be configured to include an optical element such as a hologram that changes the phase of incident light and diffracts.

[0085]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments and comparative examples of the present invention will be described below with reference to the drawings and tables.

Embodiment 1 As shown in FIG. 2, surface numbers are assigned in order from the object point side of the second-group objective lens unit 15.

A design example of the second group objective lens unit 15 according to the present invention will be described below. Numerical aperture NA: 0.85, diffraction reference wavelength: 650 nm, design wavelength: 650 ± 15 nm, focal length: 2.34 mm

[0088]

[Table 1]

[0089]

[Table 2]

Embodiment 2 A design example of another two-group objective lens unit 115 according to the present invention will be described below. Numerical aperture NA: 0.66, diffraction reference wavelength: 650 nm, design wavelength: 650 ± 15 nm, focal length: 2.52 mm

[0091]

[Table 3]

[0092]

[Table 4]

The wavelength dependence of the focal position of the second group objective lens section 115 is shown below.

[0094]

[Table 5]

Comparative Example 1 Next, a design example of a two-group objective lens unit 215 as a comparative example will be described below. Numerical aperture NA: 0.79, diffraction reference wavelength:
650 nm, design wavelength: 650 ± 15 nm, focal length:
As shown in FIG. 7, a 2.34 mm two-group objective lens unit 215 is disposed on the object side to form a first lens group 226, and a rear lens 227. And a front lens 229 constituting a second lens group 228 arranged with the optical axis aligned.

Also, as shown in FIG. 7, surface numbers are assigned in order from the object point side to the image point side of the second group objective lens unit 215 of the comparative example, and the design example of this two group objective lens unit 215 is given. Is shown below.

[0097]

[Table 6]

[0098]

[Table 7]

An isometric plot of wavefront aberration when the wavelength of the laser beam emitted from the light source is 660 nm, 650 nm, and 640 nm for the two-group objective lens unit 215 configured as described above will be described with reference to the drawings. I do.

As shown in FIGS. 8, 9 and 10, defocus occurs in the second group objective lens section 215 when the wavelength changes. As shown in FIGS. 8 and 9, the second group objective lens unit 215 has a wavelength of the laser beam of 66.
In the case of 0 nm, the shift from the focal position at a wavelength of 650 nm was +790 nm. 9 and FIG.
As shown in (2), when the wavelength of the laser beam was 660 nm, the deviation from the focal position at the wavelength of 650 nm was -750 nm. That is,
The second-group objective lens unit 215 has a laser beam wavelength of ± 10 n
m, the focal position can be adjusted to a depth of focus of 1.
It is displaced 7 times.

Comparative Example 2 A design example of the second group objective lens unit 315, which is another comparative example, is shown below. Numerical aperture NA: 0.65, diffraction reference wavelength: 650 nm, design wavelength: 650 ± 15 nm, focal length: 2.52 mm As shown in FIG. 11, the two-group objective lens unit 315 is located on the object point side. A rear lens 327 that is provided to form a first lens group 326; and a front lens 329 that forms a second lens group 328 that is disposed so that the optical axis of the rear lens 327 coincides with the rear lens 327. ing.

As shown in FIG. 11, another comparative example 2
Surface numbers are assigned in order from the object point side to the image point side of the group objective lens unit 315, and the two-group objective lens unit 31
5 is shown below.

[0103]

[Table 8]

[0104]

[Table 9]

The wavelength dependence of the focal position of the second-group objective lens unit 315 is shown below.

[0106]

[Table 10]

As shown in Table 10, the second-group objective lens unit 3
In No. 15, an extremely large displacement of the focal position occurs as in the case of the above-described two-group objective lens unit 215.

Therefore, according to the two-group objective lens units 15 and 115 of Embodiments 1 and 2, it is possible to correct chromatic aberration.

[0109]

As described above, according to the objective lens of the present invention, chromatic aberration can be corrected by providing an optical element that diffracts incident light.

Further, according to the optical pickup device of the present invention, chromatic aberration can be corrected by providing the objective lens portion with the optical element for diffracting incident light. Therefore, according to this optical pickup device, the reliability of the recording and / or reproducing operation of the information signal on the optical recording medium can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an optical system of an optical pickup device according to the present invention.

FIG. 2 is a sectional view showing an objective lens unit of the optical pickup device.

FIG. 3 is a schematic diagram illustrating wavefront aberration when the wavelength is 660 nm in the objective lens unit.

FIG. 4 is a schematic diagram showing wavefront aberration when the wavelength is 650 nm in the objective lens unit.

FIG. 5 is a schematic diagram illustrating wavefront aberration when the wavelength is 640 nm in the objective lens unit.

FIG. 6 is a sectional view showing another objective lens unit.

FIG. 7 is a cross-sectional view illustrating an objective lens unit of an optical pickup device as a comparative example.

FIG. 8 shows a comparative example in which the objective lens section has a wavelength of 66;
It is a schematic diagram which shows the wavefront aberration at 0 nm.

FIG. 9 shows a comparative example in which the objective lens section has a wavelength of 65;
It is a schematic diagram which shows the wavefront aberration at 0 nm.

FIG. 10 shows a comparative example in which the objective lens section has a wavelength of 6;
It is a schematic diagram which shows the wavefront aberration at the time of 40 nm.

FIG. 11 is a cross-sectional view illustrating an objective lens unit that is another comparative example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Optical pickup device, 5 optical disk, 15 2 group objective lens part, 26 1st lens group, 28 2nd lens group, 27 rear lens, 29 front lens, 30 hologram

Claims (11)

[Claims] An aspheric surface is formed on at least one surface through which an optical axis passes, and a first lens group disposed on an object point side coincides with the first lens group. The second to be arranged
The first and second lens groups have a numerical aperture NA of 0.1.
65 or more, and an optical element for diffracting incident light is provided on at least one surface of the second lens group located closer to the object point than the surface located closest to the image point. Objective lens. 2. The objective lens according to claim 1, wherein the focal length of the optical element is 7 to 39 times the focal length of the entire optical system. 3. The objective lens according to claim 1, wherein the first lens group has only a single lens formed of a resin material. 4. The lens of the first lens group, wherein both surfaces through which the optical axis passes are aspherical, and one surface has a hologram for diffracting incident light. The objective lens as described. 5. The objective lens according to claim 1, wherein the second lens group has only a single hemispherical single lens, and the image point side of the lens is a flat surface. 6. The apparatus according to claim 1, wherein the second lens group has only a single biconvex lens, and a radius of curvature on an image point side is larger than a radius of curvature on an object point side. The objective lens as described. 7. The apparatus according to claim 1, wherein the first lens group has a plurality of lenses, and one of the plurality of lenses has a hologram for correcting chromatic aberration. Objective lens. 8. The second lens group has first and second lenses whose optical axes coincide with each other, and the lens on the object point side has a hologram for correcting chromatic aberration. The objective lens according to claim 1. 9. The objective lens according to claim 1, wherein the second lens group has a flat surface or a spherical surface on the most image point side. 10. A first lens group having an aspheric surface formed on at least one surface through which an optical axis passes and disposed at an object point side, and having an optical axis coinciding with the first lens group. An objective lens unit having a second lens group disposed so as to be disposed; a lens holder holding the objective lens unit; and the lens holder being orthogonal to a first direction parallel to the optical axis and to the optical axis. Driving means for driving in the second directions, respectively, wherein the numerical aperture NA of the first and second lens groups is equal to 0.1.
65 or more, and an optical element for diffracting incident light is provided on at least one surface of the second lens group located closer to the object point than the surface located closest to the image point. Optical pickup device. 11. A method for recording and / or reproducing information signals on an optical recording medium having a thickness of 0.6 mm or less on a signal reading surface side for protecting a signal recording surface. The optical pickup device according to claim 10.