JPH09306014A - Condensing optical system for recording and/or reproducing of optical information recording medium and object lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical system for recording and/or reproducing which enables the recording and/or reproducing of a DVD (digital video disk) and enables the recording and/or reproducing of optical information recording media having different disk thicknesses and the reproducing of a CD-R with one optical pickup device. SOLUTION: This condensing optical system is composed of a coupling lens 13 for converting the divergent light from a laser beam source to convergent light so as to condense the luminous flux from this laser beam source as a light spot via a transparent substrate onto the information recording surface of the high-density optical information recording medium and an object lens 16 for condensing this convergent light onto the information recording surface. The wavelength o the light source, defined as λ, and the numerical aperture of the information recording surface side of the object lens 16, defined an NA1, are so set at to attain 670nm<=λ<=700nm, 0.61<=NA1<=0.65.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical condensing optical system for an optical disc which records and / or reproduces optical information by irradiating an optical information recording medium (hereinafter also referred to as an optical disc) with a light beam such as a laser beam. In particular, the present invention relates to a condensing optical system for recording and / or reproducing a high-density optical disc, and an objective lens used in such a condensing optical system.

[0002]

2. Description of the Related Art A conventional technique according to the present invention will be described.

In recent years, with the practical use of a short wavelength red semiconductor laser, a DVD (digital video disc) which is a high density optical information recording medium having an optical disc size comparable to that of a conventional CD (compact disc) and having a larger capacity. Is being developed. In this DVD type optical disk (hereinafter simply referred to as DVD), the numerical aperture NA on the optical disk side of the pickup objective lens is set to 0.6 when a short wavelength semiconductor laser of 635 nm or 650 nm is used.
Also, the thickness of the disk substrate is 0.6 mm, which is half that of conventional CDs.
And Further, the track pitch is 0.74 μm, the shortest pit length is 0.4 μm, and the track pitch of CD is 1.6 μm.
m and the minimum pit length is 0.83 μm or less.

FIG. 1 shows an example of a condensing optical system for reproducing a DVD.

In FIG. 1, the light beam emitted from the laser light source 1 passes through the hologram beam splitter 2 and enters the collimator lens 3, becomes a parallel light beam, and is limited by the diaphragm 5 to a predetermined light beam and enters the objective lens 6. The objective lens 6 forms a non-aberration light spot on the information recording surface 8 through a substrate (disk substrate, here t = 0.6 mm) 7 having a predetermined thickness when a parallel light beam is incident.

The light flux modulated and reflected by the information pits on the information recording surface 8 returns to the hologram beam splitter 2 via the objective lens 6 and the collimator lens 3, where it is separated from the optical path from the laser light source 1, Detector 9
Incident on. This photodetector 9 is a PIN photodiode which is multi-divided. Each element outputs a current proportional to the intensity of the incident light flux, and this current is sent to a detection circuit system not shown in the figure. , A focus error signal and a track error signal are generated. The objective lens 6 is controlled in a focusing direction and a tracking direction by a two-dimensional actuator (not shown) composed of a magnetic circuit and a coil based on the focus error signal and the track error signal, so that the light spot position is always on the information track. Match.

By the way, 635 nm used in DVD
Or a short wavelength red semiconductor laser of 650 nm is a CD
Is difficult to manufacture as compared with a semiconductor laser having a wavelength λ = 780 nm used for reproducing.

Further, the shorter the wavelength λ, the more difficult the semiconductor laser becomes in terms of life, power, astigmatism and the like.

Further, in the optical element used, the influence of the optical surface shape error on the wavefront aberration is inversely proportional to the wavelength λ, so that the wavelength of the semiconductor laser is preferably as long as possible.

On the other hand, in an optical disc device capable of reproducing a DVD, a conventional low density optical information recording medium such as a CD or a CD-R type optical disc which has become popular recently (hereinafter referred to as CD-R) is used.
It is hoped that the software assets can be regenerated.

In the optical pickup device used in the optical disc device capable of reproducing such a DVD, a large NA (for example, NA 0.6) is used in order to reduce the light spot condensed by the objective lens 6. When the thickness of the substrate 7 placed in the condensed light flux deviates from a predetermined thickness (0.6 mm), large spherical aberration is generated.

FIG. 2 is a diagram showing the relationship between the substrate thickness and the wavefront aberration.

This will be described with reference to FIG. 2. An objective lens optimized under the conditions of NA 0.6, laser light wavelength 635 nm emitted from a laser light source, substrate thickness 0.6 mm, and substrate refractive index 1.58. If the thickness of the
The aberration increases by about 0.01 λrms for every mm displacement.
If the substrate thickness is shifted by ± 0.07 mm, the aberration becomes 0.07 λrms, and reaches the Marechal's limit value which is a standard for performing normal reading.

FIG. 3 is a block diagram showing an example of an optical system for reproducing both DVD and CD. The difference from the optical system of FIG. 1 is that when reproducing a CD having a 1.2 mm thick substrate instead of a DVD having a 0.6 mm thick substrate, the 1.2 mm thick substrate is used in the actuator section. The objective lens 61 and the diaphragm 51, which are designed to obtain an optical spot without aberration, are switched for reproduction.

By the way, since the CD has an aluminum thin film coating on the information recording surface, the CD-R has a high reflectance for the light of the wavelength of 635 nm, but the CD-R reflects the light of the wavelength of 635 nm as shown in FIG. The rate is low. FIG.
It is a characteristic view which shows the reflectance of a CD-R disk, and the dependence of a wavelength. Also, the pit depth of the CD-R disc is 78.
Since the depth is optimal when reading with light having a wavelength of 0 nm, a better signal can be obtained by reading with light having a wavelength close to the wavelength of 780 nm.

Therefore, in order to reproduce both the DVD and the CD-R disc in the best performance condition, two optical pickup devices, which are optimal for the DVD and the CD-R disc, are provided in one optical disc device. Need to be installed. However, when two optical pickup devices are attached, it becomes difficult to make the optical disc device compact,
For example, in Japanese Patent Laid-Open No. 8-55363, 635n
A method using two light sources of a semiconductor laser having a wavelength of m and a semiconductor laser having a wavelength of 780 nm has been proposed.

[0017]

As described above, the DVD
The short wavelength red semiconductor laser of 635 nm or 650 nm used in the CD is a wavelength λ = 7 used for reproducing a CD.
It is more difficult to manufacture than a semiconductor laser of 80 nm, and the shorter the wavelength λ, the more difficult it is in terms of life, power, astigmatism, etc. Since the influence is inversely proportional to the wavelength λ, it is desirable to be able to read a DVD by using a semiconductor laser having a wavelength as long as possible.

In addition, a single optical disk device can be used for DVD and C
In order to make the D-R reproducible device, two optical pickup devices are used, or a method of using two light sources having different wavelengths makes the optical pickup device and the optical disc device compact and low cost. Have difficulty.

The present invention has been made to solve the above problems. That is, the first object is to provide a condensing optical system for recording and / or reproducing an optical information recording medium that can read a DVD using a semiconductor laser having a wavelength as long as possible. One optical pickup device enables recording and / or reproducing of DVD and CD-R, and an object thereof is to provide a condensing optical system for recording and / or reproducing of an optical information recording medium having a simple structure and compact. Is.

[0020]

The object of the present invention is achieved by adopting the following constitution.

1. Configuration of the first invention (Claim 1): Divergence from the laser light source so that the light flux from the laser light source is condensed as a light spot on the information recording surface of the high density optical information recording medium through the transparent substrate. A coupling lens for converting light into convergent light and an objective lens for converging the convergent light on the information recording surface, and the wavelength of the light source is λ,
When the numerical aperture of the objective lens on the information recording surface side is NA1, 670 nm ≤ λ ≤ 700 nm 0.61 ≤ NA1 ≤ 0.65 for recording and / or reproduction of an optical information recording medium Focusing optics.

2. Constitution of the second invention (claim 6): A light beam from a laser light source is condensed as a light spot on the information recording surface of a high density optical information recording medium through a transparent substrate, and information is recorded on the information recording surface. A recording and / or reproducing condensing optical system of an optical information recording medium for recording and / or reproducing information on the information recording surface, wherein the wavelength of the light source is λ, and the information of the condensing optical system is When the numerical aperture on the recording surface side is NA1, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, the thickness of the transparent substrate of the high density optical information recording medium is t1, and the high density optical information recording is The transparent substrate of the low-density optical information recording medium having an information recording density lower than that of the medium has a thickness of t2, and the condensing optical system for obtaining a light spot that enables recording and / or reproduction of the low-density optical information recording medium. NA2 is the required numerical aperture on the optical information recording medium side. In this case, the wavefront aberration through the transparent substrate having the thickness t1 is 0.05λrms or less, and the light flux of (1/2) NA2 is overcorrected as compared with the light flux of NA2. Converging optical system for recording and / or reproducing of an optical information recording medium (however, t1 <
t2, NA1> NA2).

3. Structure of the third invention (invention of claim 7): A light beam from a laser light source is condensed as a light spot on the information recording surface of a high-density optical information recording medium through a transparent substrate, and is formed on the information recording surface. A condensing optical system for recording and / or reproducing an optical information recording medium for recording information and / or reproducing information on the information recording surface, wherein the wavelength of the light source is λ, The numerical aperture on the information recording surface side is N
When A1 is set, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, the thickness of the transparent substrate of the high-density optical information recording medium is t1, and the information recording density is higher than that of the high-density optical information recording medium. The thickness of the transparent substrate of the low-density optical information recording medium is t2, and the optical information recording medium side of the condensing optical system for obtaining a light spot enabling recording and / or reproduction of the low-density optical information recording medium is required. When the numerical aperture is NA2, the wavefront aberration when passing through a transparent substrate having a thickness t1 is 0.05 λrms or less, and an interference fringe of a light spot in a focused state is obtained by an interferometer through the transparent substrate having a thickness t1. 2. The optical information recording medium according to claim 1, wherein the optical information recording medium has a wavefront aberration that results in interference fringes having a V-shaped bent portion near a numerical aperture NA3 when standing upright so as to be substantially linear. Record And / or reproduction focusing optical system (where, t1 <t2, (1 /
2) NA2 <NA3 <NA1).

4. Structure of a fourth invention (claim 8): A light flux from a laser light source is condensed as a light spot through a transparent substrate by a coupling lens and an objective lens on an information recording surface of a high density optical information recording medium, A condensing optical system for recording and / or reproducing of an optical information recording medium for recording information on an information recording surface and / or reproducing information on the information recording surface, wherein the wavelength of the light source is λ, When the numerical aperture of the objective lens on the information recording surface side is NA1, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, and the thickness of the transparent substrate of the high density optical information recording medium is t1, The thickness of the transparent substrate of the low-density optical information recording medium whose information recording density is lower than that of the high-density optical information recording medium is t2, and the above-mentioned optical spot for recording and / or reproducing the low-density optical information recording medium is obtained. Objective lens light When the required numerical aperture on the information recording medium side is NA2, the numerical aperture on the optical information recording medium side of the objective lens is NA1, and the recording and / or recording on the high density optical information recording medium is performed at a magnification m1 of the objective lens alone.
Alternatively, the recording and / or reproduction of the high-density optical information recording medium is performed with the numerical aperture of the objective lens on the optical information recording medium side being NA2 and the magnification m2 of the objective lens alone (m1> m2). A condensing optical system for recording and / or reproducing of an optical information recording medium characterized by the above.

[5] A fifth aspect of the invention (claim 9): a light flux from a laser light source is condensed as a light spot through a transparent substrate by a coupling lens and an objective lens on an information recording surface of a high density optical information recording medium, A condensing optical system for recording and / or reproducing of an optical information recording medium for recording information on an information recording surface and / or reproducing information on the information recording surface, wherein the wavelength of the light source is λ, When the numerical aperture of the objective lens on the information recording surface side is NA1, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, and the thickness of the transparent substrate of the high density optical information recording medium is t1, The thickness of the transparent substrate of the low-density optical information recording medium whose information recording density is lower than that of the high-density optical information recording medium is t2, and the above-mentioned optical spot for recording and / or reproducing the low-density optical information recording medium is obtained. Objective lens light When the required numerical aperture on the information recording medium side is NA2, the numerical aperture on the optical information recording medium side of the condensing optical system is NA1, and the position on the optical axis of the coupling lens is the first position. Recording and / or reproducing is performed on the information recording medium, the numerical aperture of the objective lens on the optical information recording medium side is NA2, and the position on the optical axis of the coupling lens is farther from the first position than the first position. A recording and / or reproducing condensing optical system for the optical information recording medium, wherein recording and / or reproducing of the high density optical information recording medium is performed at a second position.

6. Structure of a sixth invention (claim 10): Recording and / or recording of an optical information recording medium, wherein a light beam from a laser light source is condensed as a light spot on a information recording surface of a high density optical information recording medium via a transparent substrate. In the reproducing objective lens, when the wavelength of the light source is λ, the numerical aperture on the information recording surface side during use is NA1, and the lateral magnification is m, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0. 65, and the recording and / or recording of the optical information recording medium is characterized in that the wavefront aberration when passing through the transparent substrate with the convergent light incident in the range of 0.03 ≦ m is within the Marechal limit. Objective lens for reproduction.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a recording and / or reproducing condensing optical system of an optical information recording medium according to the present invention will be described with reference to FIGS.

FIG. 5 is an optical path diagram showing the basic structure of an example of the recording and / or reproducing condensing optical system of the optical information recording medium of the present invention.

In FIG. 5, 13 is a coupling lens composed of a positive single lens, 16 is an objective lens, 17 is a transparent substrate of an optical disk, and 18 is a recording surface of the optical disk.
The divergent light beam emitted from the light source (not shown) is the objective lens 1
After being converted into convergent light by a coupling lens 13 arranged close to 6, the light enters the objective lens 16 and the transparent substrate 1
It is condensed on the recording surface 18 via 7.

Here, the reason why the convergent light is made incident on the objective lens 16 in this example, and desirable conditions of the optical system composed of the objective lens 16 and the coupling lens 13 will be described.

In order to reproduce information on a high density optical information recording medium such as a DVD, the size of the light spot focused on the recording surface is smaller than that of a low density optical information recording medium such as a CD. And need to. Therefore, in the DVD, in order to obtain a small light spot, a short wavelength red semiconductor laser is usually used and the numerical aperture of the objective lens on the optical disk side is set to 0.60. 670n such a DVD
In order to reproduce using a semiconductor laser having a wavelength of m to 700 nm, the numerical aperture on the optical disk side is 0.61 to 0.65.
It is necessary to use an objective lens with a large diameter.

Letting NA∞ (hereinafter referred to as reduced NA) be the numerical aperture converted to infinite light incidence for a finite conjugate type objective lens having a lateral magnification m and a numerical aperture NA on the image side (optical disk side), NA∞ = ( 1-m) .NA ... When the conversion NA increases, the influence of environmental changes such as lens design, difficulty in maintaining performance, and temperature characteristics increases. Therefore, when the NA on the image side during use is determined by the information recording medium for recording and / or reproducing, the converted NA can be reduced by making m positive, that is, by making the convergent light incident. Therefore, the refractive power shared by the objective lens can be reduced.

Further, the wavefront aberration of the objective lens itself when the lateral magnification m of the convergent light is in the range of 0.03≤m (1) is passed through the transparent substrate of the optical information recording medium to minimize the wavefront aberration. Also, by setting the distance within the Marechal limit, the deterioration of aberrations when the optical axis of the coupling lens and the optical axis of the objective lens are decentered is reduced, and a desirable configuration as a converging optical system for recording / reproducing of an optical information recording medium is obtained. Become.

By making the objective lens movable at least in the optical axis direction, the weight of the movable portion can be reduced, and the recording surface of the optical information recording medium can be focused with a small amount of movement.

Further, when the NA is further increased, the occurrence of spherical aberration due to the change in object-image distance due to the shake of the disc or the like and the change in temperature becomes large. To cope with this,
Not only the objective lens, but also the light source and the coupling lens can be moved independently or integrally with the objective lens to perform focusing by moving.

The lateral magnification m of the objective lens itself is further 0.03≤m≤0.23 ... (1 ') (1-m) .NA≤0.63 ... (2) It is desirable to satisfy.

If the upper limit of conditional expression (1 ') is exceeded, the size (diameter) of the coupling lens in the direction perpendicular to the optical axis becomes large, and if the lower limit is exceeded, the error in the case of a high NA, especially the objective lens The spherical aberration due to the refractive index error increases.

If the upper limit of conditional expression (2) is exceeded, the thickness of the objective lens becomes thicker, so that it becomes necessary to enlarge the entire optical system in order to secure the necessary working distance.

When the objective lens is made of resin, it is desirable that the lateral magnification m further satisfies 0.03 ≦ m ≦ 0.125 (1 ″).

When the upper limit of conditional expression (1 ″) is exceeded, spherical aberration in the case where the objective lens is moved in the optical axis direction for focusing when a change in the object-image distance due to a shake of the optical information recording medium or the like occurs. If the lower limit is exceeded, the error in the case of a high NA, especially the amount of spherical aberration due to the refractive index error of the objective lens, increases.

Particularly, the resin material has a large change in the refractive index due to the temperature change. Therefore, in the case of a resin material, the temperature change is ΔT,
Assuming that the refractive index change due to temperature change is Δn, and Δn / ΔT = α, α is almost constant and a negative value from 0 ° C. to about 60 ° C. for the same material.

The wavefront aberration (spherical aberration) change ΔWT with respect to the refractive index change Δn is proportional to the fourth power of the converted NA, and is also proportional to the focal lengths f and Δn. That is, ΔWT = β · (NA∞) ⁴ · f · Δn ... Where β is a proportional coefficient.

By substituting the equation into the equation, ΔWT = β · {NA · (1-m)} ⁴ · f · α · ΔT .... From the equation, by making m positive, the temperature change of It can be seen that the influence becomes smaller as the fourth power of m.

Therefore, by satisfying the conditional expression (1 ″) and the conditional expression (2), the compact recording / reproducing converging optical system of the optical information recording medium is made of a lightweight resin at a low cost. This can be achieved with a manufactured objective lens.

By forming the coupling lens 13 with one or more spherical lens systems, the coupling lens can be manufactured by the same manufacturing method as a conventional collimator.

However, since the coupling lens has a function of converting the divergent light emitted from the light source into the convergent light, the coupling lens has a larger refractive power than the conventional collimator, and the light amount of the light source is large. If you try to capture it, you will have to take a large NA on the light source side. Therefore, the number of lenses to be used increases with only the spherical system. Therefore, it is desirable to introduce at least one aspheric surface to correct spherical aberration.

When the objective lens is made of resin, the change of the spherical aberration due to the change of the refractive index with respect to the temperature change of the refractive index can be reduced by making the convergent light incident on the objective lens, but the positive refraction constituting the coupling lens is reduced. By making at least one lens having power made of resin, it is possible to further correct the spherical aberration change of the entire optical system due to the change of the refractive index with respect to the temperature change.

This is because when the temperature rises by ΔT (0 <Δ
T) The refractive index change Δnc of the coupling lens becomes negative (Δnc <0). For this reason, the refractive power of the coupling lens becomes small, and the luminous flux emitted from the coupling lens has a smaller convergence degree than before the temperature rise. Therefore, the lateral magnification m of the objective lens itself changes in a decreasing direction (Δm <0).
I do.

Magnification m that minimizes the wavefront aberration of the objective lens
On the other hand, when Δm changes in the negative direction, the spherical aberration moves under. The refractive index change Δn of the objective lens itself is
When the temperature rises, the refractive index decreases, so Δn <0, and the spherical aberration moves excessively at this time.

Therefore, the influence on the spherical aberration due to the change in the lateral magnification of the objective lens due to the change in the refractive index of the coupling lens and the influence due to the change in the refractive index of the objective lens itself are canceled out, so that the coupling lens is made positive. By using a lens made of resin having a refractive power, the influence of temperature change can be further reduced.

Further, the correction effect is smaller than that in the conventional collimator and resin single-lens objective lens structure in which at least one of the collimator lenses is made of resin having a positive refractive power. large. This is because, even if the NA on the light source side is the same as that of the collimator, the coupling lens has a negative magnification, so the conversion NA of the coupling lens becomes large, and the absolute value of the magnification change Δm of the objective lens itself becomes large. That's why.

In this case, since the coupling lens has a large NA on the light source side and has a negative magnification, it is desirable to use an aspherical surface as described above.

Further, by using a single lens aspherical lens made of resin as the coupling lens, it is possible to obtain the required performance at a low cost. From the imaging magnification of the coupling lens,
It is desirable that at least the surface on the objective lens side is an aspheric surface.

Further, the lateral magnification mc of the coupling lens
Becomes smaller, it becomes necessary to make both surfaces aspherical in order to satisfactorily correct spherical aberration. For this, a known finite conjugate type objective lens design / production technology can be applied.

Converging light is made incident on the objective single lens,
By satisfying conditional expression (1), the NA can be increased without increasing the lens thickness of the objective single lens, and the influence of a change in the refractive index and the like is reduced. This is because, as shown in the above equation, the converted NA becomes smaller by setting 0 <m (convergent light incidence).

In the objective single lens, at least the convergent light incident side is made aspherical to correct spherical aberration while keeping the sine condition, and the wavefront aberration can be kept within the Marechal limit.

Further, since the objective lens itself can maintain its performance independently by keeping the wavefront aberration within the Marechal limit when the convergent light is incident in the range of the conditional expression (1) of the lateral magnification m, the objective lens can independently maintain its performance. It becomes easy to combine with the means for converting the divergent light into the convergent light, and the error sensitivity of the arrangement including the eccentricity can be reduced.

The objective single lens described above corrects the aberration with respect to the imaginary light source and keeps the wavefront aberration within the Marechal limit, whereby the combination with the means for converting the divergent light from the light source into the convergent light becomes easy, and the application The lens has a wide range. Although the imaginary light source is virtual, it is practically equivalent to converging the incident light beam at one point at the diffraction limited spot.

As described above, by constructing the convergent light to be incident on the objective lens 16, not only the glass material but also the large-diameter objective lens having the numerical aperture on the optical disk side of about 0.61 to 0.65 is used. It can be obtained also by using a resin material, and it becomes possible to record and reproduce information on a high density optical information recording medium such as a DVD by using a semiconductor laser having a wavelength of 670 nm to 700 nm.

Next, optical information recording capable of recording and / or reproducing on both optical information recording media such as high density optical information recording media such as DVD and low density optical information recording media such as CD and CD-R. The present invention, which describes a condensing optical system for recording and / or reproducing a medium, relates to a high-density optical disc (DVD) having a transparent substrate thickness of 0.6 mm and an optical disc (CD, CD) having a conventional transparent substrate thickness of 1.2 mm. It was invented for the purpose of performing recording and / or reproducing on an optical information recording medium such as a CD-R) with a single optical system. In the case of the conventional CD, the wavelength of the laser light source was 0.78 μm, and the required numerical aperture of the objective lens on the optical information recording medium side was about 0.45.
In addition, the wavelength is short for high-density DVD applications,
The required numerical aperture is large. Specifically, the wavelength is 0.635 μm to 0.65 μm, and the required numerical aperture is 0.6.
It is a degree. As described above, according to the present invention, a single optical system is used for DV.
Since it corresponds to D, CD, and CD-R, the wavelength of the laser light source is the same for DVD, CD, and CD-R.
A semiconductor laser with a wavelength of 0 nm to 700 nm is used.

As described above, by using the semiconductor laser having a wavelength of 670 nm to 700 nm, it is possible to obtain reflected light sufficient for reproducing information even in the CD-R, as can be understood from FIG.

Next, with reference to FIG. 6, recording and / or reproduction on both optical information recording media such as high density optical information recording media such as DVD and low density optical information recording media such as CD and CD-R. A first embodiment of a condensing optical system for recording and / or reproducing on an optical information recording medium capable of performing the above will be described.

FIG. 6 is a diagram for explaining a state in which the light beam emitted from the objective lens 16 is condensed on the recording surface via the transparent substrate. 27 is a transparent substrate having a thickness of 0.6 mm and 278 is a substrate having a thickness of 0.6 mm. Recording surface of an optical disc having a transparent substrate, 2
8 is a transparent substrate having a thickness of 1.2 mm, 288 is a thickness of 1.2 m
6 shows a recording surface of an optical disc having a transparent substrate of m.
(A) shows that the light flux emitted from the lens 16 has a thickness of 0.
Recording surface 27 of an optical disk having a 6 mm transparent substrate 27
FIG. 6B shows a state in which the light is focused on the optical disc side, which is slightly closer to the optical disc side than the position where the optical disc having the transparent substrate 27 having a thickness of 0.6 mm is read by using the focus control actuator (not shown). To
The luminous flux emitted from the objective lens 16 has a thickness of 1.2 mm.
Recording surface 288 of optical information recording medium having transparent substrate 28
It shows the state where the light is focused on.

Next, a concrete example of the first embodiment (hereinafter,
Example 1) will be described in comparison with a conventional example.

In the conventional example and the first embodiment, s in the following table is the surface number of the optical system, r is the radius of curvature of each optical surface,
d represents the thickness or spacing between the optical surfaces, and n represents the refractive index of each optical system medium.

The surface number of the optical system is the coupling lens 13
The surface on the laser light source side is 1, the surface on the objective lens 16 side is 2,
The surface of the objective lens 16 facing the coupling lens 13 is 3,
The surface on the transparent substrate 27 (28) side is 4, the transparent substrate 27 (2)
8), the surface on the objective lens 16 side is 5, the recording surface 278 (28
The surface on the 8) side is set to 6.

In the conventional example and each example, when an aspherical surface is used for the lens surface and the optical surface, the aspherical shape is the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis, and the optical axis. When the traveling direction is positive, r is a paraxial radius of curvature, K is a conic coefficient, Aj is an aspherical surface coefficient, and Pj is an aspherical power,

[0068]

[Equation 1]

It is represented by

Further, in the conventional example, although the data of the collimator lens is not shown, by optimizing the design of the collimator lens, it is possible to make substantially parallel aberration-free light incident on the objective lens.

Further, the numerical data described in the conventional example are for the substrate thickness t1 = 0.6 mm. In the numerical data described in Example 1, the thickness or interval between the optical images when the substrate thickness t1 = 0.6 mm is d1, and the thickness between the optical images when the substrate thickness t2 = 1.2 mm or The distance is indicated by d2.
Also, the thickness of the transparent substrate of the second optical information recording medium of the conventional example is t2 = 1.20 mm as in the first embodiment.

Next, Tables 1 and 2 show conventional examples.

(Conventional example) Focal length f = 3.40 (mm) Optical information recording medium side numerical aperture NA1 = 0.60, NA2 =
0.37 Magnification m = 0 Light source wavelength λ = 635 (nm) Table 1 shows lens data of a conventional example.

[0074]

[Table 1]

Table 2 shows the conical coefficient of the s-th surface, the aspherical coefficient,
The power of the aspherical surface is shown.

[0076]

[Table 2]

Next, Table 3 and Table 4 show Example 1.
6 is a sectional view of an optical system of a coupling lens, an objective lens, and a substrate according to the first embodiment of the present invention, where (a) is a substrate thickness t1 and (b) is a substrate thickness t2. Is.

(Example 1) Focal length of coupling lens f _C = 16.0 Magnification of entire optical system m _T = -1 / 7 Focal length of objective lens f _O = 3.41 Magnification of objective lens m _O = +1/12 Optical information recording medium side numerical aperture NA1 = 0.63, NA2 =
0.39 Light source wavelength λ = 680 (nm) Table 3 shows lens data of Example 1.

[0079]

[Table 3]

Table 4 shows the conical coefficient, the aspherical surface coefficient, and the aspherical power of the s-th surface in the first embodiment.

[0081]

[Table 4]

FIGS. 7 (a) and 7 (a) are spherical aberration diagrams when the substrate thickness t1 = 0.6 mm in the conventional example and the example 1. FIG. Further, FIGS. 7B and 8B show spherical aberration diagrams when the substrate thickness t2 = 1.2 mm in the conventional example and the first example.

In the spherical aberration diagram at the substrate thickness t1, the conventional example (FIG. 7) is completely corrected, whereas
In the first embodiment, it can be seen that the correction is excessive (over) at the height near (1/2) NA2, and is almost completely corrected at the height near NA2. In the spherical aberration curve, the correction is insufficient (under) in the direction from (1/2) NA2 to the height of NA2.

When the interference fringes of the light spot in the focused state are observed using the interferometer through the transparent substrate having the substrate thickness t1 in the optical system of the first embodiment, the linearly observed interference fringes are observed. , As shown in FIG. 8C, interference having a V-shaped bent portion near a numerical aperture NA3 (NA3 = 0.27 in this example) that is larger than (1/2) · NA2 and smaller than NA1. It becomes a stripe. That is, the optical system of Example 1 has the wavefront aberration observed with such interference fringes.

NA3 is (1 / 2.NA2 <MA3 <(1/2). (NA1 + N
A2) is desirable.

In the first embodiment, the interference fringes are as shown in FIG.
In the above example, the tip of the portion bent in a V shape is rounded (having a curvature), but the spherical aberration is (1 /
2) At a height near NA2, as a correction over (over) than NA2, a portion corresponding to a V-shaped bent portion of the interference fringe has a V-shaped aberration that is observed as a pointed V-shape. Such an optical system or an optical system having a wavefront aberration observed by bending in a Z shape may be used.

In the spherical aberration diagram when the substrate thickness is t2, in the conventional example (FIG. 7), since the substrate thickness t1 is completely corrected, due to the influence of the excessive spherical aberration caused by the increase in the substrate thickness, Very large spherical aberration has occurred. In this embodiment, the substrate thickness t1
On the other hand, by correcting the light flux having the height of (1/2) NA2 excessively than the light flux having the height of NA2, the light flux having the height NA2 is lower than the light flux having the height (1/2) NA2. Therefore, it is possible to reduce the spherical aberration caused by the increase of the substrate thickness.

In the case of the CD method for an optical disc having a substrate thickness t2 = 1.2 mm, λ / NA = 1.73 (where λ
Is the light source wavelength (μm), NA is the numerical aperture on the optical information recording medium side)
Sufficient recording and / or reproducing performance can be obtained. λ = 0.
In the case of 78 μm, NA = 0.45 and λ = 0.68.
In the case of μm, NA = 0.39.

Therefore, when recording and / or reproducing at the substrate thickness t2, NA2 (when λ = 0.68 μm, NA =
NA of about 0.39) is inserted in the optical path.
A large spherical aberration occurring between 1 and NA2 can be removed, and good performance can be obtained. However, the N
Even if the light flux with the full aperture NA1 is passed through without using a diaphragm equivalent to A2, the light flux with NA2 or more becomes flare light due to the large spherical aberration. Therefore, the objective lens is moved along the optical axis to obtain the optimum focus position. (Best defocus position)
By selecting, it becomes possible to obtain a good imaging spot.

FIGS. 9 and 10 show spot shapes at the time of best defocus (each 0 μm) through the substrate thickness t1 in the conventional example and the first embodiment of the present invention. In the conventional example, since the spherical aberration is completely corrected through the substrate thickness t1, a good imaging spot can be obtained, but in the present invention, the spherical aberration having a height near (1/2) NA2 is overcorrected. Even with an optical system, a good imaging spot equivalent to that of the conventional example can be obtained.

FIG. 11 shows the first embodiment according to the present invention.
The spot shape when the defocus amount is about 9 μm through the substrate thickness t2 is shown. In any of these cases, by setting the defocus amount to about 9 μm, the light flux in the area with a large NA is condensed further rearward from the information recording surface to become flare light, and the influence on the image forming spot can be reduced. . Further, as in the present invention, the spherical aberration of the light flux having a height near (1/2) NA2 when passing through the substrate thickness t1 is overcorrected, and a spherical aberration curve is obtained from (1/2) NA2 to the height of NA2. By making the direction of undercorrection (under), it is possible to prevent the spherical aberration in the region of NA2 or less from becoming excessive when passing through the substrate thickness t2.
The image forming spot is sufficient for reproducing D and CD-R.

Next, it is possible to record and / or reproduce the optical information recording medium such as a high density optical information recording medium such as a DVD and a low density optical information recording medium such as a CD or a CD-R. Second condensing optical system for recording and / or reproducing medium
An example of the embodiment will be described with reference to FIGS. 12 and 13.

In FIG. 12, members having the same actions as those of the first embodiment are designated by the same reference numerals.

In the second embodiment, the spherical aberration caused by the difference in the thickness of the substrate is adjusted so that the DVD, the CD and the CD-R having the transparent substrates having the different thicknesses can be reproduced by one optical system. The objective lens is canceled by spherical aberration generated by a change in magnification of the objective lens. Specifically, as shown in FIG. 12, the coupling lens 13 is used during CD and CD-R reproduction than during DVD reproduction. Is also moved to the side of the laser light source (not shown) to cancel the spherical aberration caused by the difference in the thickness of the substrate.

This principle will be described below.

The variation ΔSAt of the spherical aberration with respect to the variation Δt of the substrate thickness has a proportional relationship when the NA of the objective lens on the optical disk side is the same, and can be expressed as follows.

[0097] ^{ΔSAt = Δt · (nt 2 -1} ) / nt 3 · α ··· (a) where, alpha is a proportional constant, nt is the refractive index of the transparent substrate.

It can be considered that the variation ΔSAm of the spherical aberration due to the variation Δm of the lateral magnification of the objective lens has a substantially proportional relationship, and can be expressed as follows.

ΔSAm = F · Δm · β (b) where β is a proportional constant and F is a focal length of the objective lens.

Therefore, in order to correct the spherical aberration as a whole, ΔSAt + ΔSAm = 0 (c) should be satisfied.

That is, Δt · (nt ² −1) / (nt ³ · F · Δm) = − β / α (constant) ... (d).

In expression a, spherical aberration moves in the over direction when nt is a constant value and Δt is positive (> 0), so ΔSAt> 0. Since nt> 1, the proportional constant α is positive.

In the formula b, if the lateral magnification change is positive (> 0) in the lateral magnification change Δm (in the case of a real image system which is not a reflection system, if the absolute value of the lateral magnification is small), the spherical aberration is Since it moves over, ΔSAm> 0. Further, since F> 0, the proportional constant β is also positive.

As a result, Δt (= t2-t1) is positive (>).
In the case of 0), Δm is negative (<0) from the expression d.

Therefore, the lateral magnification at which the spherical aberration on the transparent substrate t ₁ is best corrected is m1, and the transparent substrate t2 (t
1 <t2, Δt = t2-t1), the lateral magnification with which the spherical aberration is best corrected can be represented by Δm = m2-m1 (e), and the optical disc of the transparent substrate t1 can be expressed as follows. (DV
To change from the state of reproducing D) to the state of reproducing the optical disk (CD, CD-R) on the transparent substrate t2, m
It is understood that the coupling lens may be moved so that 1> m2, that is, the coupling lens may be moved to the light source side.

Next, a specific example of the second embodiment (hereinafter referred to as Example 2) will be described.

Example 2 Focal Length of Coupling Lens f _C = 18.70 Focal Length of Objective Lens f _O = 3.62 Optical Information Recording Medium Side Numerical Aperture NA1 = 0.63, NA2 =
0.39 Light source wavelength λ = 680 (nm) The surface numbers of the optical system are as follows: the surface of the coupling lens 13 on the laser light source side is 1, the surface on the objective lens 16 side is 2, and the objective lens 1
6, the surface of the coupling lens 13 side is 3, the transparent substrate 27
The surface on the (28) side is 4, the surface on the objective lens 16 side of the transparent substrate 27 (28) is 5, and the surface on the recording surface 278 (288) side is 6.
And Further, the radius of curvature of each optical surface is r, the lens surface interval when reproducing a DVD having a transparent substrate thickness t1 = 0.6 mm is d1, and the transparent substrate thickness t2 is C = 1.2 mm.
The distance between the lens surfaces when reproducing D and CD-R is d2, and the refractive index of each optical system medium is n.

Table 5 shows lens data of the second embodiment.

[0109]

[Table 5]

Table 6 shows the conical coefficient, the aspherical surface coefficient, and the aspherical power of the s-th surface of the second embodiment.

[0111]

[Table 6]

In FIG. 13A, the substrate thickness t1 = 0.6 m
The spherical aberration diagram at m is shown in (b) of the substrate thickness t2 = 1.2.
FIG. 4 shows a spherical aberration diagram at mm.

In each case, the spherical aberration is satisfactorily corrected, and the DVD, CD and CD-R can be satisfactorily reproduced.

In the second embodiment, the coupling lens 13 is moved to change the magnification of the objective lens 16 alone. However, a hologram is arranged on the light source side of the objective lens and the primary light and the secondary light are used. The light can be used to change the magnification of the objective lens alone, or the correction lens can be detachably installed on the light source side of the objective lens to change the magnification of the objective lens alone. The magnification of the objective lens alone can be changed by moving the light source in the optical axis direction or by giving different movements to the coupling lens and the laser light source.

The method of moving the coupling lens and the laser light source has an effect that the light quantity loss can be reduced and the number of optical components need not be increased.

[0116]

According to the present invention, it is possible to record and / or reproduce a DVD by using a semiconductor laser light source having a relatively long wavelength, and to record and record an optical information recording medium having different substrate thicknesses in one optical pickup device. / Or regeneration and C
It becomes possible to reproduce D-R, and to provide an optical system for recording and / or reproducing an optical information recording medium having a particularly simple structure and compact.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an optical pickup optical system of a conventional optical information recording medium.

FIG. 2 is a diagram showing a relationship between a disc substrate thickness and wavefront aberration.

FIG. 3 is a configuration diagram showing an example of an optical system for reproducing both DVD and CD.

FIG. 4 is a characteristic diagram showing the reflectance and wavelength dependence of a CD-R disc.

FIG. 5 is an optical path diagram showing a basic configuration of an example of a recording and / or reproducing condensing optical system of the optical information recording medium of the present invention.

FIG. 6 is an optical path diagram illustrating a state in which a light beam emitted from an objective lens is condensed on a recording surface via a transparent substrate.

FIG. 7 is a spherical aberration diagram when the substrate thickness t1 = 0.6 mm and a spherical aberration diagram when the substrate thickness t2 = 1.2 mm in the conventional example.

FIG. 8 is a spherical aberration diagram when the substrate thickness t1 = 0.6 mm in Example 1, and a spherical aberration diagram and a light spot interference fringe when the substrate thickness t2 = 1.2 mm.

FIG. 9 is a diagram showing a spot shape at the time of best defocus when the substrate thickness t1 is used in the conventional example.

FIG. 10 is a substrate thickness t1 in Example 1 according to the present invention.
The figure which shows the spot shape at the time of the best defocus when passing through.

FIG. 11 is a substrate thickness t2 in Example 1 according to the present invention.
FIG. 6 is a diagram showing a spot shape when a defocus amount is about 9 μm when the light beam is passed through.

FIG. 12 is an optical path diagram showing a second embodiment of a recording and / or reproducing condensing optical system of the optical information recording medium of the present invention.

FIG. 13 is a spherical aberration diagram of the second embodiment of the condensing optical system.

[Explanation of symbols]

 1 Laser Light Source 7 Substrate (Transparent Substrate) 13 Coupling Lens 16 Objective Lens 17, 27, 28 Transparent Substrate of Optical Disc 18, 278, 288 Recording Surface of Optical Disc

Claims (10)

[Claims] 1. The divergent light from the laser light source is converted into convergent light so that the light flux from the laser light source is condensed as a light spot on the information recording surface of the high density optical information recording medium through the transparent substrate. 670 nm, where a coupling lens and an objective lens for converging the convergent light on the information recording surface are used, where λ is the wavelength of the light source and NA1 is the numerical aperture of the objective lens on the information recording surface side. ≤ λ ≤ 700 nm 0.61 ≤ NA1 ≤ 0.65 A converging optical system for recording and / or reproducing of an optical information recording medium. 2. The thickness of the transparent substrate of the high density optical information recording medium is t1, and the thickness of the transparent substrate of the low density optical information recording medium whose information recording density is lower than that of the high density optical information recording medium is t1.
2. When the required numerical aperture on the optical information recording medium side of the objective lens for obtaining a light spot capable of recording and / or reproducing on the low density optical information recording medium is NA2, a transparent substrate having a thickness t1 is interposed. When the wavefront aberration is 0.05
λrms or less, and (１ ／) NA from the luminous flux of NA2
2. The condensing optical system for recording and / or reproducing of the optical information recording medium according to claim 1, wherein the second luminous flux is overcorrected. However, t1 <t2, NA1> NA2
It is. 3. The thickness of the transparent substrate of the high density optical information recording medium is t1, and the thickness of the transparent substrate of the low density optical information recording medium having an information recording density lower than that of the high density optical information recording medium is t1.
2. When the required numerical aperture on the optical information recording medium side of the objective lens for obtaining a light spot capable of recording and / or reproducing on the low density optical information recording medium is NA2, a transparent substrate having a thickness t1 is used. The wavefront aberration at that time is 0.05 λrms or less, and the numerical aperture is observed when the interference fringes of the light spot in the focused state are erected so as to be almost linear through the transparent substrate with the thickness t1 by the interferometer. Recording and / or recording on the optical information recording medium according to claim 1, wherein the optical information recording medium has a wavefront aberration that causes interference fringes having a V-shaped bent portion near NA3.
Or a condensing optical system for reproduction. However, t1 <t2, (1/2) · NA2 ≦ NA3 <NA1. 4. The thickness of the transparent substrate of the high density optical information recording medium is t1, and the thickness of the transparent substrate of the low density optical information recording medium having an information recording density lower than that of the high density optical information recording medium is t1.
2. When the required numerical aperture on the optical information recording medium side of the objective lens for obtaining a light spot enabling recording and / or reproduction of the low density optical information recording medium is NA2, the optical information recording of the objective lens The numerical aperture on the medium side is NA1,
Recording and / or reproduction is performed on the high-density optical information recording medium with a magnification m1 of the objective lens alone, and the numerical aperture of the objective lens on the optical information recording medium side is NA2,
Magnification m2 of the objective lens alone (however, m1> m2)
2. The condensing optical system for recording and / or reproducing the optical information recording medium according to claim 1, wherein recording and / or reproducing is performed on the high density optical information recording medium. 5. The thickness of the transparent substrate of the high density optical information recording medium is t1, and the thickness of the transparent substrate of the low density optical information recording medium having an information recording density lower than that of the high density optical information recording medium is t1.
2. When the required numerical aperture on the optical information recording medium side of the objective lens for obtaining a light spot enabling recording and / or reproduction of the low density optical information recording medium is NA2, the optical information recording of the objective lens The numerical aperture on the medium side is NA1,
Recording and / or reproduction is performed on the high-density optical information recording medium with the position on the optical axis of the coupling lens as a first position, and the numerical aperture of the objective lens on the optical information recording medium side is NA2,
The recording and / or reproduction of the high-density optical information recording medium is performed by setting a position on the optical axis of the coupling lens as a second position farther from the objective lens than the first position. 2. A condensing optical system for recording and / or reproducing of the optical information recording medium described in 1. 6. A light beam from a laser light source is focused as a light spot on a data recording surface of a high density optical data recording medium via a transparent substrate to record information on the data recording surface and / or the data recording. A condensing optical system for recording and / or reproducing an optical information recording medium for reproducing information on a surface, wherein the wavelength of the light source is λ, and the numerical aperture of the condensing optical system on the information recording surface side is When NA1 is set, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, the thickness of the transparent substrate of the high density optical information recording medium is t1, and the information recording density is higher than that of the high density optical information recording medium. The thickness of the transparent substrate of the low-density optical information recording medium is t2, and the optical information recording medium side of the condensing optical system for obtaining a light spot enabling recording and / or reproduction of the low-density optical information recording medium is required. When the numerical aperture is NA2, the thickness t 1 has a wavefront aberration of 0.05 λrms or less when passing through the transparent substrate,
A condensing optical system for recording and / or reproducing of an optical information recording medium, wherein a light flux of (1/2) NA2 is overcorrected rather than a light flux of 2. However, t1 <t2,
NA1> NA2. 7. A light beam from a laser light source is condensed as a light spot on a data recording surface of a high density optical data recording medium through a transparent substrate to record information on the data recording surface and / or the data recording. A condensing optical system for recording and / or reproducing an optical information recording medium for reproducing information on a surface, wherein the wavelength of the light source is λ, and the numerical aperture of the condensing optical system on the information recording surface side is When NA1 is set, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, the thickness of the transparent substrate of the high-density optical information recording medium is t1, and the information recording density is higher than that of the high-density optical information recording medium. The thickness of the transparent substrate of the low-density optical information recording medium is t2, and the optical information recording medium side of the condensing optical system for obtaining a light spot enabling recording and / or reproduction of the low-density optical information recording medium is required. When the numerical aperture is NA2, the thickness The wavefront aberration when passing through the transparent substrate of No. 1 is 0.05 λrms or less, and the interference fringes of the light spot in the focused state is set up by the interferometer so as to be almost linear through the transparent substrate of thickness t1. The recording and / or recording of the optical information recording medium according to claim 1, wherein the optical information recording medium has a wavefront aberration that causes interference fringes having a V-shaped bent portion near a numerical aperture NA3 when observed.
Or a condensing optical system for reproduction. However, t1 <t2, (1/2)
-NA2 <NA3 <NA1. 8. A light beam from a laser light source is condensed as a light spot through a transparent substrate by a coupling lens and an objective lens on an information recording surface of a high density optical information recording medium,
Recording and / or recording of an optical information recording medium for recording information on the information recording surface and / or reproducing information on the information recording surface
Alternatively, in the condensing optical system for reproduction, the wavelength of the light source is λ,
When the numerical aperture of the objective lens on the information recording surface side is NA1, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, and the thickness of the transparent substrate of the high density optical information recording medium is t1, The thickness of the transparent substrate of the low density optical information recording medium having an information recording density lower than that of the high density optical information recording medium is t2, and an optical spot for recording and / or reproducing the low density optical information recording medium is obtained. When the numerical aperture of the objective lens on the optical information recording medium side is NA2, the numerical aperture of the objective lens on the optical information recording medium side is NA1,
Recording and / or reproduction of the high-density optical information recording medium is performed at a magnification m1 of the objective lens alone, a numerical aperture of the objective lens on the optical information recording medium side is NA2, and a magnification m2 of the objective lens alone (however, m1 > M2), the recording and / or reproducing of the high density optical information recording medium is performed, and the condensing optical system for recording and / or reproducing of the optical information recording medium. 9. A light beam from a laser light source is condensed as a light spot through a transparent substrate by a coupling lens and an objective lens on an information recording surface of a high density optical information recording medium,
Recording and / or recording of an optical information recording medium for recording information on the information recording surface and / or reproducing information on the information recording surface
Or a reproducing condensing optical system, where λ is the wavelength of the light source and NA1 is the numerical aperture of the objective lens on the information recording surface side: 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65 The thickness of the transparent substrate of the high density optical information recording medium is t1, the thickness of the transparent substrate of the low density optical information recording medium having an information recording density lower than that of the high density optical information recording medium is t2, and the low density optical information is When the required numerical aperture on the optical information recording medium side of the objective lens for obtaining a light spot enabling recording and / or reproduction of the recording medium is NA2, the aperture on the optical information recording medium side of the condensing optical system is Number is NA1,
Recording and / or reproduction is performed on the high-density optical information recording medium with the position on the optical axis of the coupling lens as the first position, and the numerical aperture of the objective lens on the optical information recording medium side is NA.
2. The position on the optical axis of the coupling lens is set to the first
A recording and / or reproducing condensing optical system for the optical information recording medium, wherein recording and / or reproducing of the high-density optical information recording medium is performed at a second position farther from the objective lens than a position. 10. An objective lens for recording and / or reproducing of an optical information recording medium, which converges a light beam from a laser light source as a light spot on a information recording surface of a high density optical information recording medium via a transparent substrate. When the wavelength of the light source is λ, the numerical aperture on the information recording surface side during use is NA1, and the lateral magnification is m, 670 nm ≦ λ ≦ 700 nm 0.61 ≦ NA1 ≦ 0.65, and 0.03 An objective lens for recording and / or reproducing of an optical information recording medium, which has a minimum wavefront aberration when entering convergent light in the range of ≦ m 2 through the transparent substrate and is within a Marechal limit.