JPH1196581A - Objective lens and optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an objective lens that the changeover of openings due to thicknesses of substrates of optical record mediums (optical disks) is unnecessary. SOLUTION: This lens is the objective lens of an optical pickup which converges luminous fluxes from light sources on recording surfaces of optical record mediums 7, 8 whose thicknesses of substrates are different and it is provided with a diaphragm 6a controlling diameters of luminous fluxes to be made incident on the objective lens or to be emitted from the lens 6 and diameters of luminous fluxes controlled by the diaphragm plane are the same regardless of kinds (for example, thicknesses of substrates are 0.6 mm and 1.2 mm) of the optical recording mediums 7, 8. Thus, since it is not needed to change diaphragm diameters of the objective lens 6 by the difference of the thicknesses of the substrates of the optical record mediums, the objective lens that the changeover of openings due to the thicknesses of the substrates is not necessary is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an objective lens and an optical pickup using the objective lens.

[0002]

2. Description of the Related Art Conventionally, an optical pickup of an optical disk drive for recording, reproducing, or erasing information by condensing a light beam from a light source such as a semiconductor laser on a recording surface of an optical recording medium by an objective lens through a transparent substrate has been conventionally used. Widely known. FIG. 12 shows a configuration of a conventional general optical pickup.

In FIG. 12, a semiconductor laser (LD)
The linearly polarized light emitted from the light 101 is converted into substantially parallel light by a collimating lens (CL) 102, passes through a polarizing beam splitter (PBS) 103, passes through a λ / 4 plate 104, becomes circularly polarized, and is deflected by a polarizing prism (CL). 4) Optical path at 105)
The light is deflected by 5 degrees, enters the objective lens (OL) 106, and is condensed as a minute light spot on a recording surface of an optical disk 107 as an optical recording medium via a transparent substrate. The reproduction, recording, or erasing of information is performed by the light spot.

The light reflected from the recording surface of the optical disk 107 becomes circularly polarized in the opposite direction to the outward path, and the objective lens 106
Again, the light is converted into substantially parallel light, deflected by the deflecting prism 105, passed through the λ / 4 plate 104, converted into linearly polarized light orthogonal to the outward path, reflected by the polarization beam splitter 103, and converged by the condenser lens 108. The light is received by a light receiving element 109 serving as a photodetector. Then, an information signal and a servo signal (track servo signal, focus servo signal) are detected from the output of the light receiving element 109.

[0005] In recent years, there has been a strong demand for large capacity optical discs, and accordingly, shorter wavelength light sources and higher N
The transition to A is in progress. In general, the spot diameter of a light spot focused on the recording surface of an optical disk is proportional to the wavelength λ of the light source and inversely proportional to the numerical aperture NA of the objective lens. This spot shape is deteriorated by coma aberration generated by tilting the optical disk, and the coma aberration is proportional to the cube of the numerical aperture NA of the objective lens and is proportional to the substrate thickness of the optical disk.
Therefore, the standard recording density of the conventional optical recording medium, C
The substrate thickness of the DVD of high recording density and large capacity is 0.6 mm, while the substrate thickness of D is 1.2 mm.
The thickness has become.

[0006] Some conventional optical discs have a strong wavelength dependency (for example, the reflectivity and recording power of the optical disc have a strong wavelength dependency), and a short wavelength light source for high density recording. Then, there is a problem that the conventional optical disc cannot be reproduced or recorded. For this reason,
Large-capacity optical disks such as DVDs and conventional CDs
In order to realize compatibility of standard type optical discs such as the above, it is necessary to have two kinds of light sources of a short wavelength and a different wavelength (specifically, a light source wavelength for DVD: 6
For example, JP-A-8-55363 discloses two types of light sources having different wavelengths for the purpose of recording / reproducing on / from optical discs having different substrate thicknesses or corresponding wavelengths. An optical head with the same is disclosed.

[0007]

As described above, in an optical pickup for a DVD which realizes a large capacity with high-density recording, a conventional CD-based disc is affected by spherical aberration due to a difference in substrate thickness of the optical disc. There is a problem that reproduction cannot be performed well. Therefore, various technologies corresponding to two substrate thicknesses as disclosed in JP-A-8-55363 have been proposed.

[0008] In particular, an optical pickup for DVD is used for CD-R.
In the case of performing recording or reproduction of, two light sources are required as described above. At this time, the spherical aberration is corrected by changing the incident light to the objective lens to an appropriate divergent light,
Correspondence to substrate thickness is achieved. In this case, depending on the design, there is a lens that can form a good light spot even when a light beam is incident on the entire effective diameter of the objective lens. However, the numerical aperture NA of the objective lens on the optical disk side is too large for a CD ( (For example, about NA = 0.57), there is a problem that the optical disk becomes weak against the tilt of the optical disk. Therefore, it is necessary to switch the aperture of the objective lens. Conventionally, this switching of the aperture has been performed by using a wavelength-selective diffraction grating or phase film, but the diffraction grating has poor light use efficiency, and the phase film has a structure as shown in the example of FIG. There is a problem that a phase difference occurs (a step occurs on the wavefront after passing through the phase film), and the light beam cannot be focused to the diffraction limit.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides an objective lens which does not need to switch the aperture depending on the substrate thickness of an optical disc. It is another object of the present invention to provide an optical pickup supporting two wavelengths.

[0010]

In order to achieve the above object, an objective lens according to the first aspect of the present invention focuses a light beam from a light source on a recording surface of an optical recording medium having a different substrate thickness from a transparent substrate side, thereby obtaining information. An objective lens of an optical pickup that performs recording, reproduction, or erasing of the optical recording medium. The objective lens includes a diaphragm that restricts the diameter of a light beam incident on the object lens or emitted from the object lens. It is characterized by being the same regardless of the type (substrate thickness).

According to a second aspect of the present invention, in addition to the configuration of the first aspect, the optical recording medium having the first substrate thickness condenses well with a numerical aperture NA1, and the second substrate thickness is reduced. The optical recording medium has a good numerical aperture NA2, and the first light source side of the objective lens with respect to the optical recording medium of each substrate thickness.
It is characterized in that the positions of the outermost light beams on the surfaces are equal.

Further, in the objective lens according to the third aspect, in addition to the configuration according to the first or second aspect, the optical recording medium having the first substrate thickness with the focal length f1 has a luminous flux (parallel light beam) at the object distance infinity. ), A light beam (divergent light) emitted from a light source at a finite object distance and passed through a stop surface having an aperture radius of f1 × NA1 on an optical recording medium having a second substrate thickness. In this case, the light is condensed favorably at a numerical aperture NA2.

According to a fourth aspect of the present invention, in addition to the configuration of the first aspect, an optical recording medium having a first substrate thickness converges light well with a numerical aperture NA1, and the second substrate thickness is reduced. The optical recording medium has a good numerical aperture NA2, and the position of the outermost light beam on the surface of the objective lens closest to the optical recording medium is equal to the optical recording medium of each substrate thickness. .

The objective lens according to claim 5 is the objective lens according to claim 2 or 3 or 4, wherein the numerical aperture NA is
1 satisfies 0.55 ≦ NA1 ≦ 0.65, and the numerical aperture NA2 satisfies NA2 ≦ 0.55.

According to a sixth aspect of the present invention, in the objective lens according to any one of the first to fifth aspects, the diaphragm for restricting a light beam is an aperture having a circular opening.

According to a seventh aspect of the present invention, in the objective lens according to any one of the first to fifth aspects, the stop for limiting a light flux is formed by applying a coating film that does not transmit light to the objective lens. It is characterized by having.

An eighth aspect of the objective lens according to the first aspect is characterized in that the first surface on the light source side is convex and both surfaces are aspherical.

An optical pickup according to a ninth aspect of the present invention is an optical disk drive for recording, reproducing or erasing information by condensing a light beam from a light source onto a recording surface of an optical recording medium having a different substrate thickness by an objective lens from a transparent substrate side. Wherein the objective lens according to any one of claims 1 to 8 is used as the objective lens.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail based on illustrated embodiments.

(Embodiment 1) FIG. 1 is an explanatory view showing the structure of an objective lens according to an embodiment of the present invention. FIG. 1 (a) shows a state where a light beam is focused on an optical disk 7 such as a DVD having a thin substrate. ,
(B) shows a state in which a light beam is focused on an optical disk 8 such as a CD having a large substrate thickness. As shown in FIG. 1A, the objective lens 6 of the present invention has an aperture (aperture) 6a. When a general DVD objective lens is used as a lens, the thickness of the substrate of the optical disk 7 is 0.6 mm. The surface shape is optimized so that aberration is minimized. For example, the objective lens has a focal length f1 = 3 mm and a numerical aperture NA1 = 0.6, which is optimized for parallel light having an object distance of infinity. When this DVD objective lens is used, if a parallel light is focused on an optical disk 8 such as a CD having a substrate thickness of 1.2 mm, a favorable light spot cannot be obtained due to the influence of spherical aberration. However, as shown in FIG. 1B, the spherical aberration due to the difference in the substrate thickness can be corrected by diverging the light to the objective lens 6 through the stop 6a with the light source placed at an appropriate finite distance.

At this time, when the light is condensed on the optical disk 8 having a substrate thickness of 1.2 mm at, for example, NA = 0.5, the effective radius of the incident light beam (radius of the stop) is f1 × NA1 in most cases.
Less than. In other words, if the light beam enters the entire effective diameter of the objective lens 6, the light is condensed at an NA larger than the desired NA. In most cases, the NA is about 0.57. For an optical disk 8 having a substrate thickness of 1.2 mm, N
A = 0.57 is a large value, and the deterioration of the light spot due to the tilt is large and is not appropriate. Therefore, the aperture 6a
It is necessary to limit the opening to an appropriate size.

On the other hand, when the divergent light beam enters the entire effective diameter of the objective lens 6, that is, when the radius of the diaphragm 6a is f1
At the time of × NA1, if the numerical aperture on the optical disk side is a desired numerical aperture, for example, 0.55 or less, it is considered that the tilt of the optical disk 8 having the substrate thickness of 1.2 mm is within the allowable range,
No aperture restriction is required. From this, the numerical aperture NA2 for the optical disk 8 having a large substrate thickness is NA2 ≦ 0.55
It is good to The distance between the aperture 6a and the objective lens 6 at this time is preferably as short as possible, and there is no problem even if they are in contact with each other.

FIGS. 2A and 2B show a specific example of the shape of the objective lens 6 and the light condensing state on two types of optical discs 7 and 8 having different substrate thicknesses. In the objective lens 6 shown in FIG. 2, the first surface on the light source side is a convex surface, and both surfaces are expressed by the following formula (1).
Is an aspherical lens represented by the following aspherical formula. Each surface coefficient of the aspherical formula (1) is as shown in Table 1 below. The objective lens 6 has a light source wavelength: λ = 650 nm, a substrate thickness t = 0.6 mm, a focal length f = 3.0 mm, and a numerical aperture NA = 0.6. The refractive index of the glass material forming the objective lens 6 is n ₆₅₀ for light having a wavelength λ = 650 nm.
= 1.586, n ₇₈₅ for light of wavelength λ = 785 nm
= 1.582. The center thickness is 1.744 mm
It is.

[0024]

(Equation 1)

[0025]

[Table 1]

Next, the positions of the outermost rays on each surface when the y-axis is taken in the radial direction of the lens are shown in Table 2 below. Table 2
OBJ of the surface No. is the object plane, IMG is the image plane,
STOP indicates a stop surface. ANG indicates the outgoing angle of the outermost ray from each surface. In an actual system, the recording surface and the image surface are the same surface (this is the case in the figure).

[0027]

[Table 2]

In the objective lens of the present embodiment, the radius of the stop is 1.8 mm (NA1 × f1), which is the same both when the substrate thickness is 0.6 mm and when the substrate thickness is 1.2 mm (* mark in Table 2). ), When t = 1.2 mm and λ = 785 nm, the angle of collection of the outermost ray is 32 degrees, which is NA = 0.
Equivalent to 53. At this time, t = 0.6 mm, λ = 65
The wavefront of 0 nm is 0.011 (λ), t = 1.2 mm, λ
The wavefront at = 785 nm is 0.021 (λ), which is sufficient for obtaining a diffraction-limited light spot.

(Embodiment 2) FIG. 3 is an explanatory view of the structure of an objective lens showing another embodiment of the present invention. FIG. 3 (a) shows an optical disk 7 (t = 0.6 mm) such as a DVD having a thin substrate. (B) shows a state in which the light beam is condensed and the shape of the stop.
Is an optical disk 8 having a thick substrate such as a CD (t = 1.2 m
m) shows a state where the light beam is condensed. As shown in FIG. 3, on the first surface of the objective lens 6 on the light source side, light is not transmitted through a lens such as a reflection film or a light absorption film in a region having a radius r1 = f1 × NA1 or more from the lens optical axis c. Membrane 6b
To form a stop having an aperture radius of r1 = f1 × NA1, so that the same effect as the stop of the first embodiment can be obtained.

FIGS. 4 (a) and 4 (b) show a specific example of the shape of the objective lens 6 of this embodiment and the state of light focusing on two types of optical disks 7, 8 having different substrate thicknesses. The objective lens 6 shown in FIG. 4 is an aspheric lens in which the first surface on the light source side is a convex surface and both surfaces are represented by the aspheric expression shown in the above formula (1). The surface coefficients are as shown in Table 3 below. The objective lens 6 has a light source wavelength of λ = 650 n.
m, focal length f = 3.0 for substrate thickness t = 0.6 mm
mm and the numerical aperture NA = 0.6. The refractive index of the glass material forming the objective lens 6 is n ₆₅₀ = 1.586 for light having a wavelength of λ = 650 nm, and n ₇₈₅ = 1.582 for light having a wavelength of λ = 785 nm. The center thickness is 2.3.
50 mm. A stop having an opening radius r1 = f1 × NA1 made of the coating film 6b shown in FIG. 3 is formed on the first surface of the objective lens shown in FIG.

[0031]

[Table 3]

Next, Table 4 below shows the position of the outermost ray on each surface when the y axis is taken in the radial direction of the lens. Table 4
OBJ of the surface No. is the object plane, IMG is the image plane,
STOP indicates a stop surface. ANG indicates the outgoing angle of the outermost ray from each surface. In an actual system, the recording surface and the image surface are the same surface (this is the case in the figure).

[0033]

[Table 4]

In the objective lens of the present embodiment, the radius of the stop is 1.8 mm (NA1 × f1), which is the same both when the substrate thickness is 0.6 mm and when the substrate thickness is 1.2 mm (* mark in Table 4). ), Where the stop is placed on the first surface of the lens. As shown in FIG. 3, the aperture is replaced with a light-shielding film such as a reflection film (or an absorption film) coated on the lens surface. When t = 1.2 mm and λ = 785 nm, the converging angle of the outermost ray is 31.5 degrees.
= 0.52. At this time, t = 0.6 mm, λ
= 650 nm wavefront is 0.014 (λ), t = 1.2 m
The wavefront at m, λ = 785 nm is 0.020 (λ), which is sufficient for obtaining a diffraction-limited light spot.

(Embodiment 3) FIG. 5 is an explanatory view of the structure of an objective lens showing another embodiment of the present invention. FIG. 5 (a) shows an optical disk 7 (t = 0.6 mm) such as a DVD having a thin substrate. (B) shows a state in which the light beam is condensed and the shape of the stop.
Is an optical disk 8 having a thick substrate such as a CD (t = 1.2 m
m) shows a state where the light beam is condensed. As shown in FIG. 5, the most disk-side surface of the objective lens 6 (second surface)
A film 6c that does not transmit light into the lens, such as a reflection film or a light absorption film, in a region having a radius of r2 or more from the lens optical axis.
Is coated to form an aperture having an opening radius r2. At this time, the objective lens 6 is arranged such that a light beam within a radius r2 of a desired NA (DVD) is applied to the optical disks 7 and 8 having different substrate thicknesses on the surface (second surface) closest to the disk.
Is set to NA = 0.6, and to CD-R, NA = 0.5).

FIGS. 6 (a) and 6 (b) show a specific example of the shape of the objective lens 6 of the present embodiment and the state of light focusing on two types of optical disks 7, 8 having different substrate thicknesses. The objective lens 6 shown in FIG. 6 is an aspheric lens in which the first surface on the light source side is a convex surface and both surfaces are represented by the aspheric surface expression shown in the above formula (1). The surface coefficients are as shown in Table 5 below. The objective lens 6 has a light source wavelength of λ = 650 n.
m, focal length f = 3.0 for substrate thickness t = 0.6 mm
mm and the numerical aperture NA = 0.6. The refractive index of the glass material forming the objective lens 6 is n ₆₅₀ = 1.586 for light having a wavelength of λ = 650 nm, and n ₇₈₅ = 1.582 for light having a wavelength of λ = 785 nm. The center thickness is 2.4.
78 mm. A stop having an opening radius r2 made of the coating film 6c shown in FIG. 5 is formed on the surface (second surface) closest to the disk of the objective lens shown in FIG.

[0037]

[Table 5]

Next, Table 6 below shows the positions of the outermost rays on each surface when the y-axis is taken in the radial direction of the lens. In Table 6, OBJ of the surface No. is the object plane, IMG is the image plane, ST
OP indicates a stop surface. ANG indicates the outgoing angle of the outermost ray from each surface. In an actual system, the recording surface and the image surface are the same surface (this is the case in the figure).

[0039]

[Table 6]

In the objective lens according to the present embodiment, the radius of the stop is 1.43 mm and the radius is 1.2 even when the substrate thickness is 0.6 mm.
mm is the same (marked with * in Table 6), and this is the case where the stop is placed on the second surface of the lens. As shown in FIG. 5, the aperture is replaced with a light-shielding film such as a reflection film (or an absorption film) coated on the lens surface. t
= 1.2 mm, λ = 785 nm, the angle of collection of the outermost ray is 33.3 degrees, which corresponds to NA = 0.55. At this time, t = 0.6 mm, λ = 650 nm
Is 0.004 (λ), t = 1.2 mm, λ = 78
The wavefront of 5 nm is 0.004 (λ), which is sufficient for obtaining a diffraction-limited light spot.

(Embodiment 4) FIG. 7 is a schematic structural view of an optical pickup showing another embodiment of the present invention. In FIG.
Reference numerals 11 and 12 denote first and second semiconductor lasers (LD).
And the wavelengths of the two LDs are different, for example, LD11
Is 650 nm and LD12 is 785 nm. Light emitted from the first semiconductor laser 11 passes through a first hologram (HOE) 21, is converted into substantially parallel light by a first coupling lens (CL) 31, and passes through a beam splitter (BS) 4. The light is deflected by a deflecting prism (DP) 5, enters an objective lens (OL) 6, and is condensed through a substrate on a recording surface of a first optical recording medium 7 (for example, an optical disk for DVD having a substrate thickness of 0.6 mm). To form a minute light spot. The light emitted from the second semiconductor laser 12 passes through a second hologram (HOE) 22, is coupled as divergent light by a second coupling lens (CL) 32, and is split into a beam splitter (B).
S) The light is deflected by 4 and further deflected by a deflecting prism (DP) 5 to be incident on an objective lens (OL) 6 for recording on a second optical recording medium 8 (for example, a CD optical disk having a substrate thickness of 1.2 mm). The light is condensed through the substrate on the surface to form a minute light spot.

At this time, the second coupling lens 32
The second coupling lens 32 is designed such that the divergent light from the lens corrects spherical aberration due to the difference in substrate thickness between the first and second optical recording media 7 and 8. Further, as shown in the first embodiment, the objective lens 6 is optimized for an optical recording medium 7 having a smaller substrate thickness for parallel light, and is adapted to an optical recording medium 8 having a larger substrate thickness. On the other hand, it is designed such that the aberration is corrected for the divergent light, and the NA2 on the optical disk side becomes a desired value when the light beam enters the entire effective diameter of the objective lens 6. That is, as the objective lens 6,
The objective lens described in any one of Embodiments 1 to 3 is used. The beam splitter (BS) 4 may be of a type that separates by wavelength or a type that separates by polarization.

The light reflected by the optical recording medium 7 (8) follows the original path, enters each of the holograms 21 and 22 and is diffracted, and the information signal and the servo signal (focus servo signal, track servo signal) Photodetector 91 for detection
(92).

As shown in FIG. 8, each hologram 21 (22) is composed of three areas A, B and C.
The light is diffracted in three directions by the three regions A, B, and C, and FIG.
And enters the four-divided photodetector 91 (92). Here, the light diffracted in the area A of the hologram 21 (22) is
The light enters the center of the regions E and F on the photodetector 91 (92),
Focus servo signal (= E-
F) is detected. The lights diffracted in the regions B and C of the hologram 21 (22) are respectively detected by the photodetectors 91 (9).
2) The light enters the upper G and H regions, and a track servo signal (= GH) is detected. The information signal is E,
It is detected from the sum total output of F, G, and H (may be a part).

In the optical pickup shown in FIG. 7, the holograms (HOEs) 21 and 22 may be constituted by polarizing holograms (polarizing diffraction gratings). A polarizing diffraction grating is an optical element that causes transmission or diffraction depending on the polarization state of light incident on a grating groove. This polarizing diffraction grating is described, for example, in Document 1 “Yuzo Ono“ Polarizing hologram optical element ”O
plus E A polarizing hologram using LiNbO ₃ as a material as disclosed in PP.86-90 ”, March 1991, is also disclosed in Reference 2“ Hideo Maeda, et al. “High-density dual grating for magneto-optical head”. Optics Vol.20 No.8 (August 1991
Moon) ”, a narrow pitch (1 of wavelength λ)
/ 2), or a deep groove diffraction grating.

At this time, the light sources LD11 and LD12
Λ / 4 plate is required between the optical recording medium. In particular, when the wavelengths of LD11 and LD12 are different as described above,
It is preferable to use one designed to be a λ / 4 plate for two wavelengths. The λ / 4 plate may be made of a birefringent material such as quartz, or may be made of a deposited film or the like. Further, the coupling lenses 31 and 32 can be shared. In this case, a coupling lens is provided between the beam splitter 4 and the objective lens 6.

(Embodiment 5) FIG. 10 is a structural explanatory view of an objective lens showing another embodiment of the present invention. The objective lens 6 according to the present embodiment is arranged such that the first optical disc 7 having a small substrate thickness is N
The outermost light beam condensed by A1 and the outermost light beam condensed by NA2 on the second optical disk 8 having a large substrate thickness are designed so that the positions passing through the second surface of the lens are equal. And like the objective lens shown in FIGS.
The first surface of the lens 6 is coated with a film 6b serving as an aperture.

(Embodiment 6) FIG. 11 is an explanatory view of the structure of an objective lens showing another embodiment of the present invention. In the objective lens 6 of the present embodiment, a stop (aperture) 6a is provided on the light source side, and a film 6c serving as a stop is provided on the second surface of the lens 6, similarly to the objective lens shown in FIGS. Is coated. In the embodiment shown in FIG. 11, only the optical disk 8 having a large substrate thickness is shown for the sake of clarity.

In FIG. 11, the light beam passing through the first stop 6a is condensed by the objective lens 6, but the peripheral light having a large NA and causing large aberration is shielded by the stop (film 6c) located on the second and subsequent surfaces. Then, only the light beam narrowed down by NA2 is directed to the optical disk 8. At this time, in the case of an optical disk having a small substrate thickness, the light beam passing through the stop 6a is condensed by the objective lens 6, and the light beam passing through the optical axis side end (point P) of the film 6c is condensed exactly at NA1. Designed to be. The fourth point is the same point (point P in FIG. 11) where the outermost ray of the condensed light beam is on the second surface (disk side) of the objective lens as described above.
And an objective lens and a stop that converge at NA1 for an optical disk with a thin substrate thickness and converge at NA2 for an optical disk with a large substrate thickness (corresponding to the objective lens in FIGS. 5 and 6). ).

The objective lens 6 shown in FIGS. 10 and 11 is also an aspheric lens having a convex first surface and both aspheric surfaces, as in the first to third embodiments. 10 and 11 can also be used in the optical pickup shown in FIG.

[0051]

As described above, claim 1 or claim 2
According to the invention described in (3), it is not necessary to change the aperture diameter of the objective lens due to a difference in the substrate thickness of the optical recording medium (optical disk). An element and a mechanism for switching the aperture are not required, and a simple and low-cost optical system can be configured.

According to the fourth aspect of the present invention, it is not necessary to change the aperture diameter of the objective lens due to the difference in the substrate thickness of the optical recording medium (optical disk). This eliminates the need for a conventional aperture switching element or mechanism, and can provide a simple and low-cost optical system. In addition, since the light flux is not restricted on the lens incident side, only the inner peripheral light that can easily correct aberration can be used for an optical disk having a large substrate thickness (FIG. 11), which is advantageous in that the lens can be easily designed. There is.

According to the invention set forth in claim 5, claim 2 is provided.
In addition to the effects of 3 or 4, the effect of coma aberration is small even on an optical disk having a large substrate thickness or an optical disk having a small substrate thickness, and good recording and reproduction can be performed when used in an optical pickup. Become.

According to the invention set forth in claim 6, according to claim 1,
In addition to the effects of any one of the above, the aperture can be configured with a simple and inexpensive aperture. Further, since the opening as an aperture can be integrally formed in the lens mounting portion, the configuration can be simplified.

According to the invention of claim 7, according to claim 1,
In addition to the effects of any one of the above, the functional part equivalent to the aperture is provided on the objective lens with a coating film formed by vapor deposition or the like, so that an independent aperture (parts) is not required, and a simple configuration as an optical system. Becomes possible.

According to the invention described in claim 8, according to claim 1
In addition to the effects of any one of the above, the first surface on the light source side is convex and both surfaces are aspherical, so that spherical aberration can be favorably corrected.

According to the ninth aspect of the present invention, the first aspect is provided.
By using the objective lens described in any one of the above, the number of components is reduced, and an inexpensive and simple optical pickup having a two-substrate thickness and two wavelengths can be provided.

[Brief description of the drawings]

FIGS. 1A and 1B are explanatory diagrams of a configuration of an objective lens according to an embodiment of the present invention. FIG. 1A shows a state in which a light beam is focused on an optical disk such as a DVD having a small substrate thickness, and FIG. FIG. 3 is a diagram illustrating a state where a light beam is condensed on an optical disk having a large substrate thickness.

FIG. 2 is a diagram showing a specific example of the shape of an objective lens having the configuration shown in FIG. 1 and a light focusing state on two types of optical disks having different substrate thicknesses.

FIG. 3 is an explanatory view of a configuration of an objective lens showing another embodiment of the present invention. FIG. 3 (a) shows a state in which a light beam is condensed on an optical disk such as a DVD having a thin substrate and the shape of an aperture. FIG. 4B is a diagram showing a state where a light beam is focused on an optical disk such as a CD having a thick substrate.

FIG. 4 is a diagram showing a specific example of the shape of the objective lens having the configuration shown in FIG. 3 and a light focusing state on two types of optical discs having different substrate thicknesses.

5A and 5B are explanatory diagrams illustrating a configuration of an objective lens according to another embodiment of the present invention. FIG. 5A illustrates a state in which a light beam is condensed on an optical disk such as a DVD having a thin substrate and the shape of an aperture. FIG. 4B is a diagram showing a state where a light beam is focused on an optical disk such as a CD having a thick substrate.

6 is a diagram showing a specific example of the shape of the objective lens having the configuration shown in FIG. 5 and a light focusing state on two types of optical discs having different substrate thicknesses.

FIG. 7 is a schematic configuration diagram of an optical pickup showing another embodiment of the present invention.

8 is a plan view showing a configuration example of a hologram of the optical pickup shown in FIG.

9 is a plan view showing a configuration example of a photodetector of the optical pickup shown in FIG.

FIG. 10 is an explanatory diagram of a configuration of an objective lens showing another embodiment of the present invention.

FIG. 11 is a structural explanatory view of an objective lens showing another embodiment of the present invention.

FIG. 12 is a schematic configuration diagram of an optical pickup showing an example of a conventional technique.

FIG. 13 is an explanatory diagram of a phase film used for switching the aperture of a conventional objective lens.

[Explanation of symbols]

 4: Beam splitter (BS) 5: Deflection prism (DP) 6: Objective lens (OL) 6a: Aperture (aperture) 6b: Coating film (aperture provided on the first surface of the lens) 6c: Coating film (a lens 7: optical disk with thin substrate (optical recording medium) 8: optical disk with thick substrate (optical recording medium) 11, 12: semiconductor laser (LD) 21, 22: hologram (HOE) 31, 32: Coupling lens (CL) 91, 92: Photodetector

Claims (9)

[Claims] An objective lens of an optical pickup for recording, reproducing or erasing information by condensing a light beam from a light source on a recording surface of an optical recording medium having a different substrate thickness from a transparent substrate side. An objective lens comprising a stop for limiting the diameter of a light beam emitted from the objective lens, wherein the light beam diameter restricted by the stop surface is the same regardless of the type (substrate thickness) of the optical recording medium. 2. The objective lens according to claim 1, wherein:
An optical recording medium having a substrate thickness of is well focused at a numerical aperture NA1, and an optical recording medium having a second substrate thickness has a numerical aperture of N
An objective lens wherein the outermost light beam is converged satisfactorily at A2 and the position of the outermost light beam on the first surface on the light source side of the objective lens is equal to the optical recording medium of each substrate thickness. 3. An objective lens according to claim 1, wherein the optical recording medium having a first substrate thickness at a focal length f1 has a numerical aperture NA for a light beam (parallel light) at an infinite object distance.
In the optical recording medium having the second substrate thickness, the light is emitted from the light source at a finite object distance and the aperture radius is f1 × N.
The numerical aperture N for the light flux (divergent light) that has passed through the aperture surface of A1
An objective lens characterized in that it condenses well in A2. 4. The objective lens according to claim 1, wherein
An optical recording medium having a substrate thickness of is well focused at a numerical aperture NA1, and an optical recording medium having a second substrate thickness has a numerical aperture of N
An objective lens, wherein the position of the outermost light beam on the surface of the objective lens closest to the optical recording medium is equal to that of the optical recording medium of each substrate thickness, which is well focused at A2. 5. The objective lens according to claim 2, wherein the numerical aperture NA1 is 0.55 ≦ NA1 ≦ 0.65, and the numerical aperture NA2 is NA2 ≦ 0.55. Objective lens. 6. The objective lens according to claim 1, wherein the aperture for restricting a light beam is an aperture having a circular opening. 7. The objective lens according to claim 1, wherein the stop for restricting a light beam is formed by applying a coating film that does not transmit light to the objective lens. . 8. The objective lens according to claim 1, wherein the first surface on the light source side is convex and both surfaces are aspherical. 9. A light beam from a light source is focused by an objective lens onto a recording surface of an optical recording medium having a different substrate thickness from a transparent substrate side,
An optical pickup for an optical disc drive for recording, reproducing or erasing information, wherein the objective lens according to any one of claims 1 to 8 is used as the objective lens.