JP2003015032A - Objective lens, condensing optical system, optical pickup device and recording and reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an objective lens for recording and reproducing the information of an optical information recording medium whose diameter is small and whose working distance is large even though it is an objective lens consisting of two positive lenses and having high NA, and to provide a condensing optical system for recording and reproducing the information of an optical information recording medium using the objective lens, an optical pickup device using the same and a recording and reproducing device using the same. SOLUTION: This objective lens 6 is used for recording and reproducing the information of the optical information recording medium, and is constituted of a first lens group 6a having positive refractive power and a second lens group 6b having positive refractive power arranged in order from a light source side. It is a finite conjugate type one for condensing divergent luminous flux from a light source 1 on the information recording surface 7' of the optical information recording medium, and satisfies NA>=0.65. Then, it satisfies 0.10<=NA×WD/f<=0.35 or 0.01<=|m|<=0.30.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens, a focusing optical system, an optical pickup device, and a recording / reproducing apparatus for recording and / or reproducing information on an optical information recording medium.

[0002]

2. Description of the Related Art In recent years, with the practical use of red semiconductor lasers, conventional optical disks (also called "optical information recording media")
The DVD (Digital Versatile Disk), which is a high-density optical disk with a capacity similar to that of a CD (Compact Disk), has been developed and commercialized. It is expected that next-generation optical discs will appear. In the optical system of the optical information recording / reproducing apparatus using such a next-generation optical disc as a medium,
In order to increase the density of the recording signal or reproduce the high density recording signal, it is required to reduce the diameter of the spot condensed on the information recording surface via the objective lens. For that purpose, it is necessary to further shorten the wavelength of the laser as the light source and further increase the numerical aperture of the objective lens. A blue-violet semiconductor laser having an oscillation wavelength of about 400 nm is expected to be put to practical use as a short-wavelength laser light source.

Further, when a high NA objective lens having an NA of 0.65 or more is constructed by using two positive lenses, the refracting power is distributed to four surfaces to increase the curvature of each surface, thereby making it possible to perform die processing. A lens with reduced error sensitivity during lens molding has been proposed. However, such a large NA,
In addition, if the small-diameter objective lens is composed of two positive lenses, the working distance tends to be small, and there is a problem that the objective lens may come into contact with the optical information recording medium due to the warp of the optical information recording medium.

By the way, when the wavelength of the laser light source is shortened and the numerical aperture of the objective lens is increased, a relatively long time such as recording or reproducing information on a conventional optical disk such as a CD or a DVD is required. It is expected that problems that can be almost ignored will be actualized in the optical pickup device including a combination of a laser light source of wavelength and an objective lens of low numerical aperture.

One of the problems is the problem of axial chromatic aberration that occurs in the objective lens due to minute fluctuations in the oscillation wavelength of the laser light source. Since the wavelength of a light beam emitted from a semiconductor laser used as a light source in an optical pickup device is generally monochromatic, it is considered that axial chromatic aberration does not occur in the objective lens, but the wavelength changes instantaneously due to a change in output. nm
A mode hop phenomenon that changes to some extent may occur. If the axial chromatic aberration of the objective lens is not corrected, the condensing position may change due to the mode hop phenomenon, and an error may occur in recording and / or reproducing information. As the wavelength of the light source becomes shorter, the amount of change in the condensing position becomes larger.
Short-wavelength semiconductor lasers of m or less, especially 400 nm oscillation wavelength
If a blue-violet semiconductor laser of a certain degree is used, it is necessary to correct the axial chromatic aberration generated in the objective lens.

Further, since the plastic lens has a larger change in the refractive index and the shape due to the change in temperature and humidity than the glass lens, the performance deterioration due to the change is likely to become a problem. Since this deterioration in performance, that is, the increase in spherical aberration, increases as the NA increases (generally increases in proportion to the fourth power of NA), N
In the case of an objective lens made of a plastic material with A of 0.65 or more, if the temperature changes by about ± 30 ° C., there is a possibility that information recording and / or reproduction may be hindered.

Further, another problem which becomes apparent when the wavelength of the laser light source is shortened and the numerical aperture of the objective lens is increased is the fluctuation of the spherical aberration generated in the objective lens due to the minute fluctuation of the oscillation wavelength of the light source. A semiconductor laser used as a light source in an optical pickup device has an oscillation wavelength of ± 10n.
There are about m variations among individuals. When a semiconductor laser with an oscillation wavelength that deviates from the reference wavelength is used as the light source, the spherical aberration that occurs in the objective lens increases as the numerical aperture increases, so a semiconductor laser with an oscillation wavelength that deviates from the reference wavelength cannot be used. It is necessary to select the semiconductor laser used as the light source.

Further, another problem which becomes apparent in shortening the wavelength of the laser light source and increasing the numerical aperture of the objective lens is the spherical surface of the optical system caused by the thickness error of the protective layer (also called "transparent substrate") of the optical disc. This is the variation of aberration. The spherical aberration caused by the thickness error of the protective layer is generated in proportion to the fourth power of the numerical aperture of the objective lens. Therefore, as the numerical aperture of the objective lens increases, the influence of the thickness error of the protective layer increases and stable information Recording or playback may not be possible.

[0009]

SUMMARY OF THE INVENTION The present invention is for recording and / or reproducing information on an optical information recording medium having a small diameter and a large working distance even if it is a high NA objective lens composed of two positive lenses. The objective is to provide an objective lens of.

Even with an objective lens having a high NA composed of two positive lenses, light having a small diameter, a large working distance, and axial chromatic aberration, which is a problem when a short wavelength light source is used, is corrected. It is an object to provide an objective lens for recording and / or reproducing information on an information recording medium.

Further, in the condensing optical system for recording and / or reproducing information on the optical information recording medium, the oscillation wavelength variation of the laser light source, the temperature / humidity variation, and the thickness error of the transparent substrate of the optical information recording medium. An object of the present invention is to provide a condensing optical system that can effectively correct fluctuations of spherical aberration generated on each optical surface of the condensing optical system with a simple configuration.

Another object of the present invention is to provide a converging optical system in which axial chromatic aberration, which is a problem when a short wavelength light source is used, is corrected.

Another object of the present invention is to provide an optical pickup device equipped with this objective lens and / or a condensing optical system, and a recording / reproducing device equipped with this optical pickup device.

[0014]

In order to achieve the above object, a first objective lens according to the present invention is an objective lens for recording and / or reproducing information on an optical information recording medium, which is from a light source side. It is a finite conjugate type that is composed of a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are arranged in order, and focuses a divergent light beam from a light source on the information recording surface of the optical information recording medium. And satisfy the following equation.

NA ≧ 0.65 (1) where NA is a predetermined image-side numerical aperture required for recording and / or reproducing on an optical information recording medium.

According to this objective lens, the image-side numerical aperture (NA) of the predetermined objective lens required for recording and / or reproducing on the optical information recording medium is increased to 0.65 or more (as in the conventional case). 0.4 for optical information recording media, eg CD
5 and 0.60 for DVD), the size of the spot condensed on the information recording surface can be made smaller, so that the recording and / or the reproduction of the information recorded at a higher density than the conventional optical information recording medium can be performed. This is possible for optical information recording media.

Further, by constructing this high NA lens with two positive lens groups, the refracting power for light rays can be distributed to four surfaces, so that the amount of aberration generated on each surface is small and the high N is high.
It is possible to satisfactorily correct various aberrations including spherical aberration in the light flux of A, and it is possible to obtain an objective lens that is less likely to be deteriorated due to errors such as decentering of each surface and is easy to manufacture.

Furthermore, since the objective lens is of the finite conjugate type for converging the divergent light beam from the light source on the information recording surface of the optical information recording medium, a large working distance can be secured, and thus a large NA is obtained. 2 objective lenses
It is possible to prevent the contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium, which is a problem in the case of being constituted by one positive lens group. In general, if the objective lens with a single lens structure is a finite conjugate type, the incident angle of the light beam on the surface on the light source side tends to be large, so the aberration deterioration due to the eccentricity error on the surface on the light source side increases, but Since the objective lens according to the present invention has a two-group configuration that has a margin for aberration correction, even if the objective lens is of the finite conjugate type, aberration deterioration due to decentering error does not become too large.

The finite conjugate type objective lens is generally an objective lens at a finite position in which both the object point and the image point which are conjugate with each other are not infinite, and is in a finite position. In addition to focusing the divergent light flux from the real object point on the information recording surface of the optical information recording medium, it also focuses the convergent light flux toward the virtual object point at a finite position on the information recording surface of the optical information recording medium. The objective lens of the finite conjugate type according to the present invention includes a divergent light beam from an actual point at a finite position
It is an objective lens that focuses light on the information recording surface of the optical information recording medium.

The above-mentioned first objective lens preferably satisfies the following expression (2).

[0021] NA ≧ 0.75 (2)

Further, it is preferable that the following expression (3) is satisfied.

0.10 ≦ NA · WD / f ≦ 0.35 (3) where NA: a predetermined image-side numerical aperture WD required for recording and / or reproduction on an optical information recording medium: the objective lens Working distance (m
m) f: focal length of the whole system of the objective lens (mm)

In order to secure a large working distance, it is effective to increase the focal length of the objective lens, but in this case, the optical pickup device becomes large, which is not preferable in practice. It is preferable to satisfy the formula (3) in order to downsize the pickup device, secure the working distance, and balance the performance of the objective lens. If the upper limit of Expression (3) is not exceeded, the angle between the contact surface of the light source side surface of the second lens and the plane perpendicular to the optical axis will not become too large, so die machining with a diamond bite is more effective. Can be done accurately. Furthermore, it is possible to obtain a lens in which the sine condition is corrected well. If the lower limit of expression (3) is not exceeded, a large working distance can be secured even with a small diameter, so contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium can be prevented. You can

The first objective lens preferably satisfies the following expression (4), and more preferably satisfies the following expression (4 ') in order to obtain the effects described later.

0.01 ≦ | m | ≦ 0.30 (4) 0.03 ≦ | m | ≦ 0.20 (4 ′) where m: the object side numerical aperture of the objective lens is N
When A _OBJ and the image-side numerical aperture are NA _{IM G} , NA
Lateral magnification of the objective lens defined by _OBJ / NA _IMG

The lateral magnification of the first objective lens preferably satisfies the expression (4). If the upper limit of Expression (4) is not exceeded, the angle of incidence of light rays on the surface of the first lens group on the light source side will not become too large. Therefore, the first lens group such as surface decentering or group decentering Aberration deterioration due to the eccentricity error can be suppressed to be small. Further, since the distance between the objective lens and the light source does not become too small, it becomes easy to dispose the optical elements such as the polarization beam splitter and the wavelength plate. If the lower limit of Expression (4) is not exceeded, the object-image distance of the objective lens will not become too small, and therefore the optical pickup device equipped with the first objective lens can be made compact.

Further, the above-mentioned first objective lens has a use wavelength of 600 nm or less, and has a thickness of 3 mm in the use wavelength region.
Is preferably formed of an optical material having an internal transmittance of 85% or more. By setting the wavelength used to be 600 nm or less, the size of the spot condensed on the information recording surface can be made smaller, so that recording and / or reproduction of information recorded at high density can be performed more densely than the conventional optical information recording medium. This is possible for optical information recording media. Further, in order to suppress the absorption of light by the optical material caused by the use wavelength becoming a short wavelength, the first objective lens has an internal transmittance of 85% or more in the use wavelength region at a thickness of 3 mm. By being formed from an optical material that is
Since the output of the light source when recording information on the optical information recording medium can be small, the life of the light source can be extended, and
The S / N ratio of the read signal at the time of reproducing information can be improved.

The second objective lens according to the present invention is
An objective lens for recording and / or reproducing information on an optical information recording medium, comprising a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are sequentially arranged from the light source side,
It is a finite conjugate type that converges a divergent light beam from a light source on the information recording surface of the optical information recording medium, and is characterized by satisfying the following expression (5).

0.10 ≦ NA · WD / f ≦ 0.35 (5) where NA: a predetermined image-side numerical aperture WD required for recording and / or reproducing on an optical information recording medium: the objective lens Working distance (m
m) f: focal length of the whole system of the objective lens (mm)

According to this objective lens, the objective lens is
By constructing one positive lens group, the refracting power for light rays can be distributed to four surfaces, so the amount of aberration generated on each surface is small, and various aberrations including spherical aberration are favorable even in a high NA light flux. It is possible to obtain an objective lens which can be easily corrected and can be manufactured easily with little deterioration of various aberrations due to errors such as decentering of each surface. In addition, since the objective lens is a finite conjugate type in which the divergent light flux from the light source is condensed on the information recording surface of the optical information recording medium, a large working distance can be secured, so that an objective lens with a large NA can be obtained. It is possible to prevent the contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium, which is a problem when the two positive lens groups are used. In general, when the objective lens having a single-lens structure is a finite conjugate type, the incident angle of the light ray on the surface on the light source side tends to be large, and thus the aberration deterioration due to the decentering error on the surface on the light source side becomes large. Since the second objective lens has a two-group configuration with a margin for aberration correction, aberration deterioration due to an eccentricity error does not become too large even when it is of a finite conjugate type.

Furthermore, in order to secure a large working distance, it is effective to increase the focal length of the objective lens, but in this case, the optical pickup device becomes large in size, which is not preferable in practice. In order to reduce the size of the pickup device, secure the working distance, and balance the performance of the objective lens, it is preferable to satisfy the formula (5). If the upper limit of Expression (5) is not exceeded, the angle formed between the contact surface of the light source side surface of the second lens and the plane perpendicular to the optical axis will not become too large, so die machining with a diamond tool is more effective. Can be done accurately. Furthermore, it is possible to obtain a lens in which the sine condition is corrected well. If the lower limit of expression (5) is not exceeded, a large working distance can be secured even with a small diameter, so contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium can be prevented. You can

In this case, the second objective lens is expressed by the following equation (6).
It is preferable to satisfy the following condition, and it is more preferable to satisfy the following formula (6 ′) in order to obtain the effects described later.

0.01 ≦ | m | ≦ 0.30 (6) 0.03 ≦ | m | ≦ 0.20 (6 ′) where m: the object-side numerical aperture of the objective lens is N
When A _OBJ and the image-side numerical aperture are NA _{IM G} , NA
Lateral magnification of the objective lens defined by _OBJ / NA _IMG

If the upper limit of equation (6) is not exceeded,
Since the incident angle of the light ray on the surface of the first lens group on the light source side does not become too large, it is possible to suppress aberration deterioration due to decentering errors of the first lens group such as surface decentering and group decentering. . Further, since the distance between the objective lens and the light source does not become too small, it becomes easy to dispose the optical elements such as the polarization beam splitter and the wavelength plate. If the lower limit of expression (6) is not exceeded, the object-to-image distance of the objective lens will not become too small, and the optical pickup device equipped with the second objective lens can be made compact.

The second objective lens has a usable wavelength of 60.
It is preferably 0 nm or less and is formed of an optical material having an internal transmittance of 85% or more at a thickness of 3 mm in the used wavelength region. By setting the wavelength used to be 600 nm or less, the size of the spot condensed on the information recording surface can be made smaller, so that recording and / or reproduction of information recorded at high density can be performed more densely than the conventional optical information recording medium. This is possible for optical information recording media. Further, in order to suppress the absorption of light by the optical material caused by the use wavelength becoming a short wavelength, the second objective lens has an internal transmittance of 85% in the use wavelength region with a thickness of 3 mm.
By forming the optical material as described above, the output of the light source at the time of recording information on the optical information recording medium can be small, so that the life of the light source can be extended and the reading at the time of reproducing the information can be performed. The S / N ratio of the signal can be improved.

The third objective lens according to the present invention is
An objective lens for recording and / or reproducing information on an optical information recording medium, comprising a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are sequentially arranged from the light source side,
It is a finite conjugate type that converges a divergent light beam from a light source on the information recording surface of the optical information recording medium, and satisfies the following expression (7).

0.01 ≦ | m | ≦ 0.30 (7) where m: the object side numerical aperture of the objective lens is N
When A _OBJ and the image-side numerical aperture are NA _{IM G} , NA
Lateral magnification of the objective lens defined by _OBJ / NA _IMG

According to this objective lens, the objective lens is
By constructing one positive lens group, the refracting power for light rays can be distributed to four surfaces, so the amount of aberration generated on each surface is small, and various aberrations including spherical aberration are favorable even in a high NA light flux. It is possible to obtain an objective lens which can be easily corrected and can be manufactured easily with little deterioration of various aberrations due to errors such as decentering of each surface. In addition, since the objective lens is a finite conjugate type in which the divergent light flux from the light source is condensed on the information recording surface of the optical information recording medium, a large working distance can be secured, so that an objective lens with a large NA can be obtained. It is possible to prevent the contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium, which is a problem when the two positive lens groups are used. In general, when the objective lens having a single-lens structure is a finite conjugate type, the incident angle of the light ray on the surface on the light source side tends to be large, and thus the aberration deterioration due to the decentering error on the surface on the light source side becomes large. Since the second objective lens has a two-group configuration with a margin for aberration correction, aberration deterioration due to an eccentricity error does not become too large even when it is of a finite conjugate type.

Further, if the upper limit of the expression (7) is not exceeded, the incident angle of the light ray on the surface of the first lens group on the light source side will not become too large, so that surface decentering, group decentering, etc. It is possible to suppress the aberration deterioration due to the eccentricity error of the first lens group. Further, since the distance between the objective lens and the light source does not become too small, it becomes easy to dispose the optical elements such as the polarization beam splitter and the wavelength plate. If the lower limit of expression (7) is not exceeded, the object-image distance of the objective lens will not become too small, and the optical pickup device equipped with the third objective lens can be made compact.
In order to obtain the above effect, the formula (7) is expressed by the following formula (7 ′).
Is more preferable.

0.03≤ | m | ≤0.20 (7 ')

The third objective lens has a usable wavelength of 60.
It is preferably 0 nm or less and is formed of an optical material having an internal transmittance of 85% or more at a thickness of 3 mm in the used wavelength region. By setting the wavelength used to be 600 nm or less, the size of the spot condensed on the information recording surface can be made smaller, so that recording and / or reproduction of information recorded at high density can be performed more densely than the conventional optical information recording medium. This is possible for optical information recording media. Further, in order to suppress the absorption of light by the optical material caused by the use wavelength becoming a short wavelength, the third objective lens has an internal transmittance of 85% in the use wavelength region at a thickness of 3 mm.
By forming the optical material as described above, the output of the light source at the time of recording information on the optical information recording medium can be small, so that the life of the light source can be extended and the reading at the time of reproducing the information can be performed. The S / N ratio of the signal can be improved.

The fourth objective lens according to the present invention is
An objective lens for recording and / or reproducing information on an optical information recording medium, comprising a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are sequentially arranged from the light source side,
It is a finite conjugate type that converges a divergent light beam from a light source on the information recording surface of the optical information recording medium, and has a usable wavelength of 600n.
It is less than or equal to m, and has a ring-shaped diffractive structure on at least one surface.

According to this objective lens, the usable wavelength is 60
By setting the thickness to 0 nm or less, the size of the spot condensed on the information recording surface can be reduced, and therefore, the optical information recording medium can be recorded at a higher density than that of the conventional optical information recording medium and / or the information recorded at a high density can be reproduced. Will be possible. Also,
By constructing this objective lens with two positive lens groups,
Since the refracting power for a light beam can be distributed to four surfaces, the amount of aberration generated on each surface is small, and various aberrations including spherical aberration can be satisfactorily corrected even in a high NA light flux, and each surface It is possible to obtain an objective lens that is easy to manufacture with little deterioration of various aberrations due to errors such as eccentricity.

Further, since the fourth objective lens is of the finite conjugate type in which the divergent light beam from the light source is condensed on the information recording surface of the optical information recording medium, a large working distance can be secured. It is possible to prevent contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium, which is a problem when the objective lens having a large NA is composed of two positive lens groups. In general, if the objective lens with a single lens structure is a finite conjugate type, the angle of incidence of the light beam on the surface on the light source side tends to be large, so the aberration deterioration due to the eccentricity error on the surface on the light source side increases, but Since the objective lens according to the present invention has a two-group configuration that has a margin for aberration correction, even if the objective lens is of the finite conjugate type, aberration deterioration due to decentering error does not become too large.

On the other hand, the axial chromatic aberration generated in the objective lens, which is a problem due to the shortened wavelength used, is diffracted by the ring-shaped diffractive structure provided on at least one surface and refracted by the refractive lens. It is possible to obtain an objective lens in which deterioration of aberration is suppressed to a small level even when a momentary wavelength variation occurs due to a mode hopping phenomenon of a laser light source, because the objective lens is appropriately corrected.

In the above-mentioned fourth objective lens, the light source generates a wavelength variation of ± 10 nm or less, and the diffractive structure is
It is preferable to have a function of suppressing the axial chromatic aberration caused by the refractive index dispersion of the optical material of the objective lens due to the wavelength variation of the light source. This diffraction structure is ± 10 of the light source.
It has an axial chromatic aberration characteristic of causing an axial chromatic aberration having a polarity opposite to that of the axial chromatic aberration caused by the dispersion of the refractive index of the optical material forming the objective lens due to the wavelength variation of about nm. That is, when the wavelength of the light source changes to the long wavelength side,
The axial chromatic aberration caused by the refractive index dispersion of the optical material is in the direction in which the back focus of the objective lens becomes longer than before the wavelength change, but when the wavelength of the light source changes to the long wavelength side, it occurs due to the diffractive structure. On-axis chromatic aberration is a direction in which the back focus of the objective lens becomes shorter than before the wavelength change. In order to obtain a state in which the axial chromatic aberration of the wavefront that has passed through the objective lens and is condensed on the information recording surface is well corrected, the diffractive structure and the refractive power are appropriately combined so that the diffractive structure causes the axial chromatic aberration. The magnitude of the chromatic aberration may be made substantially equal to the axial chromatic aberration caused by the dispersion of the refractive index of the optical material.

Further, in the fourth objective lens, it is preferable that the following expressions (8), (9) and (10) are satisfied.

[0049] NA ≧ 0.65 (8)

[0050] 0.10 ≦ NA ・ WD / f ≦ 0.35 (9)

[0051]             0.01 ≦ | m | ≦ 0.30 (10)

0.03 ≦ | m | ≦ 0.20 (10 ′) where NA: a predetermined image-side numerical aperture WD required for recording and / or reproducing on an optical information recording medium WD: of the objective lens Working distance (m
m) f: focal length of the entire system of the objective lens (mm) m: When the objective lens object side numerical aperture _{NA OBJ,} the image side numerical aperture was _{NA IMG,} defined by _NA OBJ _{/ NA IMG} Lateral magnification of the objective lens

By increasing the image-side numerical aperture (NA) of the predetermined objective lens required for recording and / or reproducing on the optical information recording medium as in the formula (8) to 0.65 or more (conventional Optical information recording medium, eg 0.45 for CD, DVD
However, since the size of the spot condensed on the information recording surface can be reduced, the recording and / or reproduction of the information recorded at a higher density than that of the conventional optical information recording medium is an optical information recording. It is possible for the medium.

Further, in order to secure a large working distance, it is effective to increase the focal length of the objective lens, but in this case, the optical pickup device becomes large, which is not practically preferable. It is preferable to satisfy the expression (9) in order to downsize the pickup device, secure the working distance, and balance the performance of the objective lens. If the upper limit of Expression (9) is not exceeded, the angle between the contact surface of the light source side surface of the second lens and the plane perpendicular to the optical axis does not become too large, so die machining with a diamond tool is more effective. Can be done accurately. Furthermore, it is possible to obtain a lens in which the sine condition is corrected well. If the lower limit of Expression (9) is not exceeded, a large working distance can be secured even with a small diameter, so contact between the objective lens and the optical information recording medium due to the warp of the optical information recording medium can be prevented. You can

Regarding the lateral magnification of the fourth objective lens,
If the upper limit of Expression (10) is not exceeded, the angle of incidence of the light ray on the surface of the first lens group on the light source side will not become too large, so the first lens group such as surface decentering or group decentering Aberration deterioration due to the eccentricity error can be suppressed to be small. Further, since the distance between the objective lens and the light source does not become too small, it becomes easy to dispose the optical elements such as the polarization beam splitter and the wavelength plate. If the lower limit of expression (10) is not exceeded, the object-to-image distance of the objective lens will not become too small, and therefore the optical pickup device equipped with the fourth objective lens can be made compact. Note that the formula (1
0) is more preferably within the range of formula (10 ′) in order to obtain the above-mentioned effects.

Further, it is preferable that the fourth objective lens satisfies the following expression (11).

0.01 ≦ PD / PT ≦ 0.15 (11) Here, PD: Φb = b _2i h ² + b _4i h ⁴ + b _6i h ⁶ + ... with the diffractive structure formed on the i-th surface. When expressed by the optical path difference function defined by (A) (here, h
Is the height (mm) from the optical axis, and is b _2i , b _4i , b
_6i , ... _Are optical path difference function coefficients of the 2nd, 4th, 6th, ..., respectively, and the power of only the diffractive structure defined by PD = Σ (−2 · b _2i ) (mm ^{− 1} ) PT: Power of the entire objective lens system that combines the power of the refracting lens and the power of the diffractive structure (mm ⁻¹ ).

By determining the diffractive structure of the objective lens so as to satisfy the expression (11), it is possible to excellently correct the axial chromatic aberration generated in the objective lens for the wavelength of 600 nm or less. The axial chromatic aberration of the wavefront when a spot is formed on the information recording surface of the optical information recording medium above the lower limit of the expression (11) does not become overcorrected, and below the upper limit, on the information recording surface of the optical information recording medium. The axial chromatic aberration of the wavefront when connecting the spots is not overcorrected.

In the fourth objective lens, the first lens group has a meniscus shape with a convex surface facing the light source side, and the first lens group has a ring-shaped zone on the surface closest to the optical information recording medium. It is preferable to have the diffractive structure of and satisfy the following expression (11 ′).

0.05 ≦ PD / PT ≦ 0.25 (11 ′) However, PD: Φb = b _2i h ² + b _4i h ⁴ + b _6i h ⁶ + ... When expressed by the optical path difference function defined by
Is the height (mm) from the optical axis, and is b _2i , b _4i , b
_6i , ... _Are optical path difference function coefficients of the 2nd, 4th, 6th, ..., respectively, and the power of only the diffractive structure defined by PD = Σ (−2 · b _2i ) (mm ^{− 1} ) PT: Power of the entire objective lens system that combines the power of the refracting lens and the power of the diffractive structure (mm ⁻¹ ).

As described above, when a ring-shaped diffractive structure is formed on the most optical information recording medium side surface (its shape is concave) of the first lens group having the meniscus shape with the convex surface facing the light source side. , The angle of the tip of each diffractive ring zone having a sawtooth shape becomes looser compared to the case where a diffractive structure is formed on a convex surface. Will be good. As a result, it is possible to prevent a decrease in diffraction efficiency due to a shape error of the diffractive ring zone, and it is possible to obtain high light utilization efficiency. At this time, the diffractive structure formed on the surface of the first lens group that is closest to the optical information recording medium is determined so as to satisfy the following expression, whereby axial chromatic aberration generated in the objective lens with respect to a wavelength of 600 nm or less. Can be satisfactorily corrected.

The axial chromatic aberration of the wavefront when a spot is formed on the information recording surface of the optical information recording medium above the lower limit of the above formula (11 ') does not become overcorrected, and below the upper limit of the optical information recording medium. It is possible to prevent the axial chromatic aberration of the wavefront when a spot is formed on the information recording surface from being overcorrected.

In the fourth objective lens, the reference wavelength is λ (mm), and the focal length of the whole objective lens system is f (m).
m), ni is the order of the diffracted light having the largest amount of diffracted light among the diffracted light generated by the diffractive structure formed on the i-th surface, Mi is the number of ring zones of the diffractive structure within the effective diameter of the i-th surface, The minimum value of the interval between adjacent ring zones of the diffractive structure within the effective diameter of the i-plane is Pi (m
When m), it is preferable that the following expression (12) is satisfied.

0.04 ≦ f · λ · Σ (ni / (Mi · Pi ² )) ≦ 0.50 (12)

By determining the diffraction structure of the objective lens so as to satisfy the expression (12), it is possible to excellently correct the axial chromatic aberration generated in the objective lens with respect to the wavelength of 600 nm or less. The axial chromatic aberration of the wavefront when a spot is formed on the information recording surface of the optical information recording medium above the lower limit of the expression (12) does not become undercorrected too much, and below the upper limit, on the information recording surface of the optical information recording medium. The axial chromatic aberration of the wavefront when connecting the spots is not overcorrected.

In the fourth objective lens, the following equation (1
It is preferable to satisfy 3).

0.2 ≦ | (Ph / Pf) −2 | ≦ 3.0 (13) where Pf: at a predetermined image-side numerical aperture required for recording and / or reproducing on the optical information recording medium Diffraction ring zone spacing (mm) Ph: Diffraction ring zone spacing (mm) at a numerical aperture of 1/2 of a predetermined image-side numerical aperture required for recording and / or reproducing on an optical information recording medium

As described above, if the ring-shaped zone spacing of the diffractive structure, that is, the spacing between the ring-shaped zones in the direction perpendicular to the optical axis satisfies Expression (13), spherical aberration during wavelength fluctuation can be corrected well. Therefore, it is not necessary to adjust the position of the coupling lens, the objective lens, or the light source in the optical axis direction when the laser light source having the oscillation wavelength deviated from the reference wavelength is incorporated into the optical pickup device, and the assembly time of the optical pickup device is significantly increased. Can be shortened. Further, if the optical path difference function has only a second-order optical path difference function coefficient (also referred to as a diffractive surface coefficient), (Ph / Pf) −2 = 0 and the fourth objective lens changes a minute wavelength from the reference wavelength. In order to satisfactorily correct the change in spherical aberration caused by the action of diffraction, a high-order optical path difference function coefficient of the optical path difference function is used. At this time, it is preferable that (Ph / Pf) −2 be a value that is apart from 0 to some extent, and if Expression (13) is satisfied, the change in spherical aberration due to the wavelength change can be canceled well by the action of diffraction. it can. Spherical aberration when the wavelength changes from the reference wavelength above the lower limit of Expression (13) does not become overcorrected, and spherical aberration when the wavelength changes from the reference wavelength below the upper limit does not become overcorrected.

In the case where the fourth objective lens has a diffractive action as a diffractive lens and a refractive action as a refraction lens, the back focus becomes shorter when the wavelength of the light source changes to the long wavelength side. It is preferable to have an axial chromatic aberration characteristic that changes to and satisfy the following expression (14).

−1 <ΔCA / ΔSA <0 (14) where ΔCA: amount of change in axial chromatic aberration with respect to change in wavelength (mm) ΔSA: amount of change in spherical aberration of marginal ray with respect to change in wavelength (mm)

As described above, when the diffractive action as the diffractive lens and the refraction action as the refraction lens are combined,
The back focus when the wavelength of the light source fluctuates to the long wavelength side has the axial chromatic aberration characteristic such that it changes in the direction of becoming shorter than the back focus before the fluctuation, and the formula (14)
It is preferable to satisfy. The wavelength of the light source fluctuated by crossing the spherical aberration curve of the reference wavelength and the spherical aberration curve of the long and short wavelength sides (also called chromatic spherical aberration) by overcorrecting the axial chromatic aberration of the objective lens due to the diffraction effect. In this case, the movement of the optimum writing position can be suppressed to be small, and the objective lens with a mode hop phenomenon of the light source and the deterioration of the wavefront aberration at the time of high frequency superposition can be obtained. Also,
When correcting chromatic aberration as described above by the diffraction effect,
Since the diffraction ring zone spacing can be made larger than when correcting both axial chromatic aberration and chromatic spherical aberration, the processing time of the mold can be shortened, and the reduction of diffraction efficiency due to the manufacturing error of the ring zone shape can be prevented. Can be achieved.

In the fourth objective lens, the ni-th order diffracted light quantity generated by the diffractive structure formed on the i-th surface is larger than the diffracted light quantity of any other order, and the information on the optical information recording medium is It is preferable that the ni-order diffracted light generated in the diffractive structure for recording and / or reproducing can be condensed on the information recording surface of the optical information recording medium. Where n
Is an integer other than 0 and ± 1.

This structure relates to an objective lens which records and / or reproduces information on an optical information recording medium by using diffracted light of second or higher order. If the ring-shaped diffractive structure is formed so that the diffraction efficiency of the diffracted light of the second or higher order is maximized, the step difference between the ring-shaped zones and the interval between the ring-shaped zones are increased, and the required accuracy of the shape of the diffractive structure is increased. Don't get too strict. In general, compared with the case of using the 1st-order diffracted light, the decrease of the diffraction efficiency due to the wavelength change is large when the 2nd-order or more is used, but when the light source close to a single wavelength is used, the above-mentioned diffraction efficiency Since the amount of decrease due to wavelength change is so small that it can be ignored, it is possible to obtain an objective lens having a diffractive structure that is easy to manufacture and has a sufficient diffraction efficiency.

In the above-described first to fourth objective lenses, at least one of the surfaces between the surface of the first lens group closest to the light source and the surface of the second lens group closest to the light source. It is preferable that the two surfaces are aspherical surfaces. In addition,
"The surface between the most light source side surface of the first lens group and the most light source side surface of the second lens group" means the most light source side surface of the first lens group and the most light source side surface of the second lens group. The surface on the light source side is also included.

Thus, if at least two of the surfaces between the surface of the first lens group closest to the light source and the surface of the second lens group closest to the light source are aspherical surfaces, spherical aberration In addition, it is possible to satisfactorily correct coma and astigmatism. At this time, it is preferable that at least two surfaces, that is, the surface closest to the light source of the first lens group and the surface closest to the light source of the second lens group are aspherical surfaces, because the aberration can be more precisely corrected. Further, by making the surface of the first lens group closest to the optical information recording medium aspherical surface, it is possible to suppress aberrations caused by the deviation of the optical axes of the first lens group and the second lens group, which is more preferable. .

In the above-mentioned first to fourth objective lenses, both the first lens group and the second lens group are plastic lenses, and the following formulas (1A) and (1B) are used.
It is preferable to satisfy.

NA ≧ 0.75 (1A) 0.06>ΔSAG> −0.08 (1B) ΔSAG = (X1′−X2 ′) / (NA ⁴ · f · (1+ | m |)) X1 ′ = X1 · (N1-1) ³ / f1 X2 ′ = X2 · (N2-1) ³ / f2 where NA: of the predetermined objective lens necessary for recording and / or reproducing on the optical information recording medium. Image-side numerical aperture X1: a plane that is perpendicular to the optical axis and is in contact with the apex of the optical surface of the i-th lens unit that is closest to the light source, and the effective diameter outermost periphery (the NA
At the position on the optical surface closest to the light source of the i-th lens group) on which the marginal ray of light is incident, the difference (mm) in the optical axis direction from the surface closest to the light source of the i-th lens group As a reference, the case of measuring in the direction of the optical information recording medium is positive, and the case of measuring in the direction of the light source is negative (i = 1 or 2) f: focal length (mm) of the entire system of the objective lens f1: the first Focal length of lens group (mm) f2: Focal length of the second lens group (mm) m: _NAOBJ / NA, where _NAOBG is the object-side numerical aperture of the objective lens and _{NAIMG is} the image-side numerical aperture. Lateral magnification N1 of the objective lens defined by _IMG : Refractive index of the first lens group at the used wavelength N2: Refractive index of the second lens group at the used wavelength

As described above, in a high NA plastic objective lens, since the amount of change in spherical aberration due to the change in refractive index due to temperature change is large, it may pose a problem in practical use. In a high NA objective lens in which both the first lens group and the second lens group are plastic lenses, by satisfying the above formulas (1A) and (1B), a change in spherical aberration caused by a temperature change The amount can be reduced.

The sag amount X of the surface of the first lens unit closest to the light source
When the value of ΔSAG, which is the difference between 1 and the sag amount X2 of the surface closest to the light source of the second lens group, is larger than −0.08, the change in spherical aberration due to temperature change can be reduced. On the other hand, when the value of ΔSAG is 0.06 or more, it is advantageous to reduce the change in spherical aberration due to temperature change, but the working distance becomes too short, so the objective lens and the optical information recording are reduced. The probability of collision with the medium increases dramatically. Further, since the manufacturing tolerance of each lens group becomes too small, the production efficiency of each lens group deteriorates. Furthermore, the amount of astigmatism generated by the optical axis shift between the first lens group and the second lens group becomes too large, so that the efficiency of assembling the first lens group and the second lens group deteriorates. Therefore, the condition relating to ΔSAG for suppressing the change of spherical aberration due to temperature change to be small, the manufacturing error sensitivity being small, the manufacturing being facilitated, and the working distance being sufficiently secured is expressed by the above formula (1B). ) Conditions are provided.

If the value of ΔSAG is larger than the lower limit of the expression (1B), the distance on the optical axis between the surface of the second lens group closest to the optical information recording medium and the light incident surface of the optical information recording medium becomes small. Therefore, the possibility of collision between the objective lens and the optical information recording medium can be reduced. Further, since the amount of astigmatism generated due to the optical axis shift between the first lens group and the second lens group does not become too large, the efficiency of assembling the first lens group and the second lens group can be improved. Further, since the degree of meniscus of the second lens group does not become too large, it is possible to reduce the amount of coma aberration caused by the optical axis shift of the 1-plane of the second lens group, and improve the production efficiency of the second lens group. further,
Since the curvature of the surface of the first lens group closest to the light source does not become too small, it is possible to reduce the amount of coma aberration that occurs due to the optical axis shift of the surface of the first lens group and improve the production efficiency of the first lens group.

On the other hand, when the value of ΔSAG is smaller than the upper limit of the equation (1B), it is possible to obtain a lens in which the change in spherical aberration due to temperature change is small. Further, since the lens thickness on the optical axis of the first lens group does not become too large, a compact objective lens can be obtained, which is advantageous from the viewpoint of downsizing of the optical pickup device. Furthermore, since the curvature of the surface of the second lens group closest to the light source does not become too small,
The expected angle, which is the angle formed by the normal line of the aspherical surface and the optical axis, does not become too large, and the die machining can be performed accurately.

In the first to fourth objective lenses,
It is preferable that the following expression (15) is satisfied.

3 ≦ √ (f1 / f2) / | m | ≦ 50 (15) where fi: focal length (mm) of the i-th lens group (i =
1 or 2) (however, when the i-th lens group has a diffractive structure, the focal length of the entire system of the i-th lens group (m
m)) m: When the objective lens object side numerical aperture _{NA OBJ,} the image side numerical aperture was _{NA IMG, NA} OBJ _{/ NA IMG}
Lateral magnification of the objective lens defined by

It is preferable that the above-mentioned objective lens satisfies the expression (15). However, in the objective lens according to the present invention, as the lateral magnification increases, the ratio of the focal lengths of the first lens group and the second lens group (f1 / If f1 / f2 becomes too small, that is, if the power of the second lens group becomes too large, the radius of curvature of the light source side surface of the second lens group becomes small, and f2) becomes small. Aberration deterioration due to the optical axis shift of the two lens groups increases. Further, the error sensitivity with respect to the thickness of the central lens of the second lens group becomes large. On the other hand, f1 /
If f2 becomes too large, that is, if the power of the first lens group becomes too large, the image height characteristics such as coma and astigmatism cannot be satisfactorily corrected. From the above,
The value of (f1 / f2) / | m | needs to be within a certain range in order to obtain an objective lens that is easy to make and has good performance. When the value is less than or equal to the upper limit of Expression (15), a lens that is easy to manufacture can be obtained, and since the lateral magnification does not become too small, a compact optical pickup device can be obtained. Below the lower limit, it is possible to make a lens with good image height characteristics, and since the lateral magnification does not become too large,
It becomes easy to arrange optical elements such as a beam shaping element, a deflecting beam splitter, and a wave plate.

Further, in order to further achieve the above-mentioned action, the range of the following formula (15 ') is particularly desirable.

[0086]               6 ≦ √ (f1 / f2) / | m | ≦ 40 (15 ')

Further, it is preferable that the first to fourth objective lenses satisfy the following expression (16).

0.5 ≦ (r2 + r1) / (r2-r1) ≦ 4.0 (16) where, r1: paraxial radius of curvature (mm) of the surface of the first lens group closest to the light source, r2: first lens Paraxial radius of curvature (mm) of the surface of the group closest to the optical information recording medium

The above equation (16) relates to an appropriate shape of the first lens group. If the upper limit of expression (16) is not exceeded, the meniscus degree of the first lens group will not become too large, and the surface of the first lens group that is closest to the light source and the surface that is closest to the optical information recording medium side will not be formed. The aberration deterioration due to the axis deviation between the two does not become too large. If the lower limit is not exceeded, correction of spherical aberration will not be insufficient.

Further, in order to further achieve the above-mentioned action, the range of the following formula (16 ') is particularly desirable.

[0091]         0.8 ≦ (r2 + r1) / (r2-r1) ≦ 3.0 (16 ′)

It is preferable that the first to fourth objective lenses satisfy the following expression (17).

−0.02 ≦ NA · (X2−X1) /f≦0.30 (17) where NA is a predetermined image-side numerical aperture necessary for recording and / or reproducing on the optical information recording medium. Xi: a plane that is perpendicular to the optical axis and is in contact with the apex of the surface of the i-th lens group that is closest to the light source, and the periphery of the effective diameter (the position on the surface of the i-th lens group that is closest to the light source and on which the marginal ray of NA is incident) The difference in the optical axis direction from the surface closest to the light source of the i-th lens group in) is positive when measured in the direction of the optical information recording medium with the tangential plane as a reference, and negative when measured in the direction of the light source ( i = 1 or 2) f: focal length of the whole system of the objective lens (mm)

The above expression (17) is a conditional expression regarding the sag amount of the surface on the light source side of the first lens group and the surface on the light source side of the second lens group for favorably correcting the spherical aberration. In the objective lens according to the present invention, as the lateral magnification increases, the ratio (f1 / f2) of the focal lengths of the first lens group and the second lens group decreases, and the curvature of the light source side surface of the first lens group decreases. The curvature of the surface of the second lens unit on the light source side is loose and becomes tight. Therefore, as shown in FIG. 29, the sag amount (X1) on the light source side surface of the first lens group necessary for correcting the spherical aberration is small, and the sag amount (X2) on the light source side surface of the second lens group is small. growing. By the way, as the value of X1 is positive and its absolute value is smaller, the effect of overcorrecting the spherical aberration of the marginal ray becomes greater, and as the value of X2 is positive and its absolute value is larger, the spherical aberration of the marginal ray is corrected. Since the effect of making it insufficient becomes large, in order to satisfactorily correct spherical aberration, X2-
X1 needs to be within a certain range. From the above, it is preferable that the objective lens of the present invention satisfies the expression (17). If the lower limit of the expression (17) is exceeded, the spherical aberration of the marginal ray is not overcorrected. Don't run short

Further, in order to further achieve the above-mentioned action, the range of the following formula (17 ') is particularly desirable.

[0096]     −0.01 ≦ (X2−X1) /f·NA≦0.20 (17 ′)

Further, it is preferable that the first to fourth objective lenses include a lens group made of an optical plastic material. As described above, when at least one lens group is made of an optical plastic material, even a high NA objective lens composed of two lens groups having a large lens volume is lightweight, so that it can be used as a focusing actuator. It is possible to reduce the load on the product, to follow it at high speed, to drive with a small actuator, and to mass-produce at a lower cost than by injection molding. From the above, it is more preferable that both of the two lens groups are made of an optical plastic material.

Further, since the plastic lens has a larger change in the refractive index and the shape due to the change in temperature and humidity than the glass lens, the performance deterioration due to the change is likely to become a problem. This deterioration in performance, that is, the increase in spherical aberration, increases in proportion to the fourth power of NA, and therefore becomes more problematic as the objective lens has a higher NA. Generally, the change in the refractive index of a plastic lens with respect to the temperature change is about -10 × 10 ^-5 / ° C. When the high NA objective lens according to the present invention is composed of two lenses made of a plastic material, if the working distance is small with respect to the focal length of the objective lens, an uncorrected third-order spherical aberration occurs when the temperature rises. When the temperature drops, overcorrected third-order spherical aberration occurs. On the other hand, when the working distance is increased with respect to the focal length of the objective lens, it is possible to generate a higher-order spherical aberration of the fifth order or more, which has a polarity opposite to that of the above-described third-order spherical aberration when the temperature changes. At this time, the objective lens is expressed by the above formula (3).
By and / or satisfying the expression (4), it becomes possible to satisfactorily balance the amount of generation of the third-order spherical aberration and the amount of generation of the high-order spherical aberration of opposite polarity, and it is possible to achieve a high balance between high-order spherical aberrations made of plastic materials. Even with an NA objective lens, it is possible to obtain an objective lens having a wide usable temperature range with little deterioration of the wavefront aberration when the temperature changes. If the lower limit of expression (3) and / or expression (4) is exceeded, the spherical aberration will not be overcorrected when the temperature rises, and if it is below the upper limit, the spherical aberration will not be overcorrected when the temperature rises. Further, if the lower limit of the formula (3) and / or the formula (4) is exceeded, the spherical aberration will not be overcorrected when the temperature drops, and if it is below the upper limit, the spherical aberration will not be overcorrected when the temperature drops.

If at least one lens group in the first to fourth objective lenses is made of an optical glass material, the second lens group of the two lens groups which tends to have a small radius of curvature is d. Refractive index at the line is 1.5
If the optical glass material of 5 or more is used, the angle between the contact surface of the second lens group on the light source side and the plane perpendicular to the optical axis does not become too large, so that the die machining with the diamond tool can be performed more accurately. It can be carried out. Furthermore, it is more preferable to form the first lens group from an optical glass material having an Abbe number of 65 or more at the d-line, because it is possible to suppress the occurrence of axial chromatic aberration when using a short wavelength light source.

In the first to fourth objective lenses, a plane which is perpendicular to the optical axis and is in contact with the apex of the most light source side surface of the i-th lens group and the most light source side surface of the i-th lens group. It is preferable to provide a diaphragm for regulating the light flux between them (i =
1 or 2).

As described above, when the diaphragm for restricting the luminous flux is arranged immediately before the most light source side surface of the i-th lens group, it is located at the apex of the most light source side surface of the first lens group perpendicular to the optical axis. It is preferably arranged between the contacting plane and the surface of the first lens closest to the light source. As a result, the distance between the diaphragm and the surface of the i-th lens unit closest to the light source is reduced, and it is possible to prevent the light rays from passing through a portion higher than the area where the aberration correction is guaranteed. The aberration does not increase even when light is incident.

When a diaphragm is provided for the first to fourth objective lenses, it corresponds to the image-side numerical aperture necessary for recording and / or reproducing information on the optical information recording medium on at least one surface. It is more preferable to regulate the diameter of the condensed light flux by providing a portion at which the normal direction of the surface changes discontinuously. As a result, a light beam that passes through the surface outside the part where the normal direction of the surface changes discontinuously and a light beam that passes through the surface on the optical axis side of the part where the normal direction of the refractive surface changes discontinuously It can be focused at different points. By integrally molding a lens that provides a part where the normal direction of the surface changes discontinuously and a part where the normal direction of the surface changes discontinuously, it is not necessary to attach a separate diaphragm to the objective lens. The time and cost can be reduced. Further, since the diaphragm is not required when the bobbin is molded, the total weight of the drive unit including the objective lens and the bobbin can be reduced.

The first condensing optical system according to the present invention is
A finite conjugate that includes a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are sequentially arranged from the light source side, and focuses a divergent light beam from the light source on the information recording surface of the optical information recording medium. Is a condensing optical system for recording and / or reproducing information on an optical information recording medium including an objective lens that is a mold, and is a spherical surface generated by the condensing optical system between the light source and the information recording surface. The following formula (18) is provided with means for correcting fluctuations in aberration.
It is characterized by satisfying.

NA ≧ 0.65 (18) where NA is a predetermined image-side numerical aperture of the objective lens required for recording and / or reproducing on the optical information recording medium.

As described above, by the first to fourth objective lenses, even if the objective lens has a high NA consisting of two positive lens groups, the objective lens has a small diameter and a large working distance, and the wavelength used. Even if the wavelength is short, an objective lens with a high NA composed of two positive lens groups in which axial chromatic aberration caused by mode hopping of a laser light source is effectively corrected can be obtained. When an attempt is made to increase the recording density by using an objective lens having a large NA and a light source having a shorter wavelength, variations in spherical aberration due to various errors cannot be ignored.

Therefore, if a means for correcting the fluctuation of the spherical aberration is provided between the light source and the information recording surface of the optical information recording medium like the above-mentioned first condensing optical system, even if there are various errors. A condensing optical system that can maintain good condensing characteristics can be obtained.

The causes of the variation of the spherical aberration are, specifically, the variation of the oscillation wavelength among the individual laser light sources, the change of the refractive index of the optical plastic material caused by the change of the temperature and humidity, and the change of the optical information recording medium. Variations in the thickness of the transparent substrate, manufacturing errors of the objective lens (surface shape error, thickness error on the optical axis, air gap error between lenses, etc.) are included. By providing a means for correcting the spherical aberration (particularly the third-order spherical aberration) that occurs due to these various causes, there are the following advantages (1) to (3), and a condensing optical system that can always maintain good condensing characteristics. Obtainable.

Since it is not necessary to select the laser light source,
Since the requirement for manufacturing accuracy of the laser light source does not become too strict, mass productivity of the laser light source can be improved. Also,
The manufacturing time of the optical pickup device can be shortened.

Since the constituent elements included in the condensing optical system can be formed of a plastic material, the cost can be significantly reduced.

Since the required accuracy for manufacturing error of the optical information recording medium does not become too strict, mass productivity of the optical information recording medium can be improved.

Since the requirement for the manufacturing accuracy of the objective lens does not become too strict, the mass productivity of the objective lens can be improved.

The second condensing optical system according to the present invention is
The first lens group having a positive refracting power and the second lens group having a positive refracting power, which are arranged in order from the light source side, are emitted from the light source.
A condensing optical system for recording and / or reproducing information of an optical information recording medium including a finite conjugate type objective lens for converging a divergent light beam having a wavelength of 00 nm or less on an information recording surface of the optical information recording medium. It is characterized in that a ring-shaped diffractive structure is provided on at least one surface of the optical element constituting the condensing optical system.

Like this condensing optical system, the back focus of the objective lens is shortened when the wavelength of the light source slightly fluctuates to the long wavelength side on at least one surface of the optical element constituting the condensing optical system. By providing the diffractive structure having such wavelength characteristics, it is possible to effectively correct the axial chromatic aberration that occurs in the objective lens, which is a problem when using a short wavelength light source such as a blue-violet semiconductor laser. By providing the optical element having the above-mentioned diffractive structure between the light source and the objective lens separately from the objective lens, even the objective lens in which axial chromatic aberration is not strictly corrected is applied to the condensing optical system according to the present invention. It will be possible.

In this case, the light source causes a wavelength variation of ± 10 nm or less, and the diffractive structure is caused by the refractive index dispersion of the optical material of the optical element constituting the condensing optical system accompanying the wavelength variation of the light source. It is preferable to have a function of suppressing the axial chromatic aberration that occurs.

The above-mentioned diffractive structure has an axial chromatic aberration having a polarity opposite to that of the axial chromatic aberration caused by the refractive index dispersion of the optical material of the optical element forming the condensing optical system, which is caused by the wavelength variation of about ± 10 nm of the light source. It has an axial chromatic aberration characteristic as it occurs. That is, when the wavelength of the light source changes to the long wavelength side,
The axial chromatic aberration caused by the refractive index dispersion of the optical material of the optical element that constitutes the condensing optical system is in the direction in which the back focus of the condensing optical system becomes longer than before the wavelength fluctuation, but the light source has a long wavelength. When the wavelength fluctuates to the side, the axial chromatic aberration caused by the diffractive structure is in a direction in which the back focus of the condensing optical system becomes shorter than before the wavelength fluctuates. In order to obtain a state in which the axial chromatic aberration of the wavefront that has been transmitted through the condensing optical system and condensed on the information recording surface is well corrected, the axis generated by the diffractive structure should be obtained by appropriately combining the diffractive power and the refractive power. The magnitude of the upper chromatic aberration may be made substantially equal to the axial chromatic aberration generated by the refractive index dispersion of the optical material. In the above description, the condensing optical system is assumed to be one positive lens having a condensing effect.

In the above-mentioned second condensing optical system,
In the case where the objective lens has a diffractive action as a diffractive lens and a refractive action as a refraction lens, on-axis such that the back focus changes in the direction of shortening when the wavelength of the light source changes to the long wavelength side. It is preferable to have chromatic aberration characteristics and to satisfy the following expression (19).

−1 <ΔCA / ΔSA <0 (19) where ΔCA: amount of change in axial chromatic aberration with respect to change in wavelength (mm) ΔSA: amount of change in spherical aberration of marginal ray with respect to change in wavelength (mm)

In the converging optical system according to the present invention, when the diffractive action as the diffractive lens and the refractive action as the refraction lens are combined, the back focus when the wavelength of the light source fluctuates to the long wavelength side changes. It is preferable to have an axial chromatic aberration characteristic that changes in a direction in which it becomes shorter than the back focus before the adjustment, and to satisfy the expression (19). By diffracting, the axial chromatic aberration of the entire focusing optical system is overcorrected, and the spherical aberration curve of the reference wavelength and the spherical aberration curve of the long / short wavelength side (also called chromatic spherical aberration) are crossed to each other. It is possible to suppress the movement of the optimum writing position when the wavelength fluctuates, and it is possible to provide a focusing optical system in which the mode hop phenomenon of the light source and the deterioration of the wavefront aberration at the time of superposition of high frequencies are small.

Further, in the condensing optical system in which the chromatic aberration is corrected by the diffractive action as described above, the diffraction ring zone spacing can be made larger than in the condensing optical system in which the axial chromatic aberration and the chromatic spherical aberration are both corrected. Therefore, it is possible to shorten the processing time of the die and prevent the diffraction efficiency from being lowered due to the manufacturing error of the ring shape. In the above description, the condensing optical system is assumed to be one positive lens having a condensing effect.

The amount of n-th order diffracted light generated in the diffractive structure is larger than that of any other orders, and is generated in the diffractive structure to record and / or reproduce information on the optical information recording medium. It is preferable that the n-th order diffracted light can be condensed on the information recording surface of the optical information recording medium. Here, n is an integer other than 0 and ± 1.

This structure relates to a condensing optical system for recording and / or reproducing information on the optical information recording medium by using the diffracted light of the second or higher order. If the ring-shaped diffractive structure is formed so that the diffraction efficiency of the diffracted light of the second or higher order is maximized, the step difference between the ring-shaped zones and the interval between the ring-shaped zones are increased, and the required accuracy of the shape of the diffractive structure is increased. Don't get too strict. In general, when the second or higher order is used, the decrease in the diffraction efficiency due to the wavelength change is larger than when the first order diffracted light is used. Since the amount of decrease due to the change is so small that it can be ignored, it is possible to obtain a condensing optical system having a diffractive structure that is easy to manufacture and has a sufficient diffraction efficiency.

Further, in the above second condensing optical system, means for correcting a variation of spherical aberration generated in the condensing optical system is provided between the light source and the information recording surface, and the following equation (2)
It is preferable to satisfy 0).

NA ≧ 0.65 (20) where NA is a predetermined image-side numerical aperture of the objective lens required for recording and / or reproducing on the optical information recording medium.

As described above, when an objective lens having a large NA satisfying the expression (20) is used in the condensing optical system according to the present invention, the fluctuation of spherical aberration due to various errors cannot be ignored. Therefore, if a means for correcting the fluctuation of the spherical aberration is provided between the light source and the information recording surface of the optical information recording medium, good condensing characteristics can be maintained even if there are various errors, and yet, due to the diffractive action of the diffractive structure. Thus, it is possible to obtain the condensing optical system in which the axial chromatic aberration is well corrected. As the cause of the fluctuation of the spherical aberration, specifically, the variation of the oscillation wavelength between the individual laser light sources, the change of the refractive index of the optical plastic material caused by the change of the temperature and humidity, the transparent substrate of the optical information recording medium Variations in thickness and manufacturing errors of the objective lens (surface shape error, thickness error on optical axis, air gap error between lenses, etc.) are included.

In the above-mentioned first and second focusing optical systems,
It is preferable that the spherical aberration corrector includes at least one optical element capable of changing the divergence of the emitted light flux by changing along the optical axis. With a configuration including at least one such optical element, it is possible to excellently correct fluctuations in spherical aberration with a simple configuration.

More specifically, one of the two objective lens groups may be movable along the optical axis, or an optical element provided between the light source and the objective lens may be used as an optical element. The displacement may be possible along the axial direction. In any case, temperature / humidity changes, or fluctuations in the thickness of the transparent substrate of the optical information recording medium, or slight fluctuations in the oscillation wavelength of the light source,
Alternatively, it is possible to satisfactorily correct the spherical aberration caused by the manufacturing error of the objective lens.

Further, by forming the optical element that can be displaced along the optical axis from a lightweight optical plastic material, it becomes possible to reduce the load on the actuator and to make a high-speed response to the fluctuation of spherical aberration.

Further, the means for correcting the fluctuation of the spherical aberration is an element which is arranged between the light source and the information recording surface and whose refractive index distribution along the direction perpendicular to the optical axis is variable. Is preferred. In such a device that produces a distribution of the refractive index along the direction perpendicular to the optical axis by applying a voltage, etc., if the variation of spherical aberration is corrected, there is no moving part and the mechanically simple structure An optical system can be obtained.

Further, the means for correcting the fluctuation of the spherical aberration may be an element having a variable refractive index arranged between the objective lens and the optical information recording medium. When it is arranged between the objective lens and the optical information recording medium, spherical aberration can be corrected if the refractive index changes instead of the refractive index distribution.

Further, the above-mentioned first to fourth objective lenses can be applied to the first and second focusing optical systems.

Further, in each condensing optical system, the condensing optical system is capable of recording and / or reproducing information on a plurality of information recording layers from the same light beam incident surface side, and focus between the information recording layers. It is preferable that the fluctuation of spherical aberration caused by the difference in the thickness of the transparent substrate from the light beam incident surface to each information recording layer at the time of jump is corrected by a means for correcting the fluctuation of spherical aberration.

With this structure, recording and / or recording of information on a multi-layer recording type optical information recording medium having a structure in which a plurality of transparent substrates and information recording layers are laminated in order from the light beam incident surface side.
Alternatively, the present invention relates to a condensing optical system used in a reproducible optical pickup device. The spherical aberration correction means corrects the variation of spherical aberration caused by the difference in the thickness of the transparent substrate from the light beam incident surface to each information recording layer during the focus jump between the information recording layers.
Even the next-generation optical pickup device using the above-mentioned objective lens with a high numerical aperture can record and / or reproduce information on / from a multi-layer recording type optical information recording medium.

Further, the first optical pickup device according to the present invention comprises a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are arranged in order from the light source side for the light flux emitted from the light source. The divergent light beam from the light source is condensed on the information recording surface of the optical information recording medium by a condensing optical system including a finite conjugate type objective lens for condensing on the information recording surface of the optical information recording medium. An optical pickup device for recording information on a recording surface and / or reproducing information on the information recording surface, wherein the light source is displaced along an optical axis direction so that the divergence of a light beam incident on the objective lens is increased. Detecting means for detecting a variation in spherical aberration generated in the condensing optical system by detecting reflected light from the information recording surface, and detecting by the detecting means. Depending on the result, the sphere And having a driving means for displacing along the optical axis direction the light source in order to reduce variations in aberrations.

According to this optical pickup device, in the optical pickup device having the finite conjugate type objective lens of the two-group structure, by displacing the light source along the optical axis, variations in oscillation wavelength among individual laser light sources, Refractive index change of optical plastic material caused by change of temperature and humidity, thickness variation of transparent substrate of optical information recording medium, manufacturing error of objective lens (surface shape error, thickness error on optical axis, between lenses) It is possible to correct fluctuations in spherical aberration that occur due to (air gap error, etc.). Specifically, it is generated in the condensing optical system while monitoring the signal in the detecting means for detecting the condensed state of the light beam condensed on the information recording surface by detecting the reflected light from the information recording surface. The driving means for displacing the light source along the optical axis is operated so that the spherical aberration is optimally corrected. A piezo actuator or a voice coil type actuator can be used as the driving means for shifting the light source along the optical axis.

The above optical pickup device includes the above first
~ The fourth objective lens can be applied.

Further, the second optical pickup device according to the present invention comprises a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are arranged in order from the light source side for the luminous flux emitted from the light source. A finite conjugate type objective lens for converging the divergent light flux from the light source on the information recording surface of the optical information recording medium, and a condensing optical system including a means for correcting the fluctuation of the spherical aberration of the optical information recording medium. An optical pickup device for condensing light on an information recording surface, recording information on the information recording surface and / or reproducing information on the information recording surface, wherein the condensing optical system is the condensing optical system described above. By detecting reflected light from the information recording surface, detecting means for detecting the fluctuation of spherical aberration generated in the condensing optical system, and according to the detection result of the detecting means, In order to reduce the fluctuation of spherical aberration, And having a driving means for driving the means for correcting the variation of the surface aberration.

According to this optical pickup device, by having the above-mentioned condensing optical system, even in an optical pickup device equipped with an objective lens having a high NA, variations in oscillation wavelength among individual laser light sources and temperature fluctuations can be prevented. Changes in the refractive index of the optical plastic material caused by changes in humidity, variations in the thickness of the transparent substrate of the optical information recording medium, manufacturing errors of the objective lens (surface shape error, thickness error on the optical axis, between lenses) Since the fluctuation of the spherical aberration caused by the air gap error or the like) is corrected, it is possible to obtain an optical pickup device which is always in a good focusing state. Specifically, while monitoring the signal in the detection means for detecting the condensed state of the light flux condensed on the information recording surface by detecting the reflected light from the information recording surface,
The driving means for driving the means for correcting the fluctuation of the spherical aberration is operated so that the spherical aberration generated in the focusing optical system is optimally corrected. As the driving device, a voice coil type actuator, a piezo actuator, or the like can be used.

Since the audio / image recording apparatus / reproducing apparatus according to the present invention is equipped with the above-described optical pickup apparatus, the audio / image recording apparatus / reproducing apparatus for the next-generation optical information recording medium having a higher density and a larger capacity than a DVD can be used. Image recording or reproduction can be performed well.

[0139]

BEST MODE FOR CARRYING OUT THE INVENTION Optical pickup devices according to first to sixth embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment>

FIG. 1 is a diagram schematically showing an optical pickup device according to the first embodiment of the present invention.

In the optical pickup device shown in FIG. 1, a light beam emitted from a light source 1 made of a semiconductor laser is a polarization beam splitter 2, correction means for spherical aberration variation, and drive means 11.
After passing through the refractive index distribution variable element 3, the 1/4 wavelength plate 4 and the diaphragm 5 driven by, the transparent substrate 7 of the optical information recording medium is formed by the second group objective lens 6 including the first lens 6a and the second lens 6b. It is configured to be focused on the information recording surface 7 ′ via the. Further, the reflected light from the information recording surface 7'is again provided with the second group objective lens 6 and the refractive index distribution variable element 3.
And the like, and is then reflected by the polarization beam splitter 2 to pass through a condenser lens 12 for imparting astigmatism to the reflected light from the information recording surface 7 and travel toward a photodetector 13. It

In the second group objective lens 6, the first lens 6a and the second lens 6b are integrally formed by the holding member 8, and the flange portion 9 of the holding member 8 accurately attaches to the optical pickup device. . Further, the optical pickup device of FIG. 1 includes a biaxial actuator 10 that drives the objective lens 6 in two axial directions for tracking / focusing, as a driving means of the objective lens.

In the optical pickup device shown in FIG. 1, an element having a variable refractive index distribution is used as the spherical aberration variation correction means.
An optically transparent electrode layer a electrically connected to each other,
b and c are electrically insulated from the electrode layers a, b and c, and refractive index distribution variable layers d and e whose refractive index distribution changes according to an applied voltage are alternately laminated, and The transparent electrode layers a, b, and c are divided into a plurality of regions.

When the fluctuation of the spherical aberration is detected by the photodetector 13, a voltage is applied to the refractive index distribution variable element 3 by the driving means 11 to the electrode layers a, b and c to change the refractive index distribution variable layer. The refractive indices of d and e are changed depending on the location, and the distribution of the refractive index is generated along the direction perpendicular to the optical axis, so that the phase of the light emitted from the refractive index distribution variable element 3 has zero fluctuation of spherical aberration. To control.

According to the optical pickup device of the present embodiment, the light flux from the light source 1 is focused on the information recording surface 7'of the optical information recording medium by the second group objective lens 6 via the variable refractive index distribution element 3. Then, the reflected light modulated thereby is received by the photodetector 13 through the reverse path, whereby reproduction can be performed. Recording can be performed on the information recording surface of the optical information recording medium in the same manner.

At the time of the above-mentioned recording or reproduction, the driving means 11 applies a voltage to the variable refractive index distribution element 3 based on the detection result of the spherical aberration variation so that the phase of the emitted light becomes zero. Control, the variation of the oscillation wavelength among laser light sources, the change of the refractive index of the optical plastic material caused by the change of temperature and humidity, the change of the thickness of the transparent substrate of the optical information recording medium, the manufacture of the objective lens. Recording or reproduction can be satisfactorily performed while correcting variations in spherical aberration caused by errors or the like.

Further, as described above, when a distribution of the refractive index along the direction perpendicular to the optical axis is generated by applying a voltage and the variation of the spherical aberration is corrected, a mechanically simple structure without moving parts is obtained. An optical optical system can be obtained.

<Second Embodiment>

FIG. 2 is a diagram schematically showing an optical pickup device according to the second embodiment of the present invention.

The optical pickup device shown in FIG. 2 has basically the same configuration as that of FIG. 1 except that a liquid crystal element is used as an element whose refractive index distribution is variable. Therefore, the same parts are designated by the same reference numerals. , The description is omitted.

In FIG. 2, as the refractive index distribution variable element 3,
A liquid crystal element 15a in which liquid crystal molecules are aligned in an arbitrary X direction in a plane perpendicular to the optical axis, and a liquid crystal molecule is aligned in a Y direction perpendicular to the X direction in a plane perpendicular to the optical axis. The liquid crystal element 15b arranged is used. Liquid crystal element 15
a and the liquid crystal element 15b are respectively the glass substrate 15c-
It has a configuration in which stripe electrodes arranged perpendicularly to the alignment direction of the liquid crystal molecules-liquid crystal molecules-planar electrodes-glass substrate 15c are arranged in this order, and further between the liquid crystal elements 15a and 15b. The half-wave plate 15d is sandwiched. The interval between the striped electrodes is about 10 μm. By applying a voltage to the electrodes of the liquid crystal element 15a by the driving means 11, a refractive index distribution in the X direction is formed, and by applying a voltage to the electrodes of the liquid crystal element 15b by the driving means 11, the Y direction. Since the refractive index distribution is formed, the optical path difference can be added to the wavefront that has passed through the variable refractive index distribution element 3 in the shape of a ring around the optical axis. With the variable refractive index distribution element 3, it is possible to correct the variation in spherical aberration, as in the optical pickup device of FIG.

Further, in the optical pickup device according to the present invention, the refractive index distribution variable element 3 as a means for correcting the fluctuation of the spherical aberration has a ring-shaped optical path difference about the optical axis with respect to the transmitted wavefront. It may be added as long as it can be added, and is not limited to the form of the refractive index distribution variable element 3 in the optical pickup device of FIGS. 1 and 2.

<Third Embodiment>

FIG. 3 is a diagram schematically showing an optical pickup device according to the third embodiment of the present invention.

The optical pickup device shown in FIG. 3 has two components for correcting spherical aberration variation instead of the refractive index distribution variable element.
The configuration is basically the same as that of FIG. 1 except that one of the group objective lenses can be moved in the optical axis direction, and therefore, the same portions are denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 3, the first lens 6a of the second-group objective lens is constructed to be movable in the optical axis direction by the uniaxial actuator 21 in order to correct the fluctuation of the spherical aberration, and the second lens 6b is biaxial. The actuator 10 drives in two axial directions for tracking / focusing. When the fluctuation of the spherical aberration is detected by the photodetector 13, the first lens 6a of the second group objective lens shifts in the optical axis direction so that the spherical aberration becomes zero. According to the optical pickup device of FIG.
The same effect as that of FIG. 1 can be obtained.

<Fourth Embodiment>

FIG. 4 is a diagram schematically showing an optical pickup device according to the fourth embodiment of the present invention.

The optical pickup device shown in FIG. 4 has basically the same configuration as that of FIG. 1 except that the light source can be displaced in the optical axis direction in order to correct the spherical aberration variation instead of the refractive index distribution variable element. Therefore, the same parts are designated by the same reference numerals, and the description thereof will be omitted.

As shown in FIG. 4, the light source 1 is constructed so that it can be displaced in the optical axis direction by a uniaxial actuator 21 in order to correct fluctuations in spherical aberration. When the fluctuation of the spherical aberration is detected by the photodetector 13, the light source 1 shifts in the optical axis direction so that the spherical aberration becomes zero. According to the optical pickup device of FIG. 4, the same effect as that of FIG. 1 can be obtained.

<Fifth Embodiment>

FIG. 5 is a diagram schematically showing an optical pickup device according to the fifth embodiment of the present invention.

The optical pickup device shown in FIG. 5 has basically the same configuration as that of FIG. 1 except that the lens can be displaced in the optical axis direction in order to correct the spherical aberration variation instead of the refractive index distribution variable element. Therefore, the same parts are given the same reference numerals,
The description is omitted.

As shown in FIG. 5, the lens 23 is constructed to be movable in the optical axis direction by the uniaxial actuator 21 in order to correct the fluctuation of the spherical aberration. When the fluctuation of the spherical aberration is detected by the photodetector 13, the lens 23 shifts in the optical axis direction so that the spherical aberration becomes zero. According to the optical pickup device of FIG. 5, the same effect as that of FIG. 1 can be obtained.

<Sixth Embodiment>

FIG. 6 is a diagram schematically showing an optical pickup device according to a sixth embodiment of the present invention.

The optical pickup device shown in FIG.
Between the polarization beam splitter 2 and the polarization beam splitter 2, a coupling lens 14 and a quarter-wave plate 4 are arranged, and between the polarization beam splitter 2 and the condenser lens 12 of the photodetector 13 is a quarter wavelength.
Since the configuration is basically the same as that of FIG. 5 except that the wave plate 4 is arranged, the same portions are denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 6, when the photodetector 13 detects the variation of the spherical aberration, the lens 23 is displaced in the optical axis direction by the uniaxial actuator 21 so that the spherical aberration becomes zero, and the parallel light The divergence angle is changed to correct the variation of spherical aberration. According to the optical pickup device of FIG. 6, the same effect as that of FIG. 1 can be obtained.

<Seventh Embodiment>

FIG. 30 is a diagram schematically showing an optical pickup device according to the seventh embodiment of the present invention.

The optical pickup device shown in FIG. 30 is the same as that shown in FIG. 1 except that a variable refractive index element is arranged between the objective lens and the optical information recording medium instead of the variable refractive index distribution element.
Since the configuration is basically the same as the above, the same portions are denoted by the same reference numerals, and the description thereof will be omitted.

In the optical pickup device of FIG. 30, the element 24 having a variable refractive index is arranged between the objective lens and the optical information recording medium in order to correct the spherical aberration variation. As the refractive index variable element 24, for example, an optical element whose refractive index changes according to an applied voltage can be used. When the photodetector 13 detects the variation of the spherical aberration, the driving means 25 of the variable refractive index element 24 changes the refractive index of the variable refractive index element 24 so that the variation of the spherical aberration becomes zero. Since the image side numerical aperture of the objective lens used in the optical pickup device of the present embodiment is as large as 0.65 or more, when the refractive index variable element 24 is arranged between the objective lens and the optical information recording medium, it is very small. It is possible to correct a larger variation in spherical aberration by changing the refractive index, and it is possible to reduce the driving voltage of the variable refractive index element 24 and downsize the variable refractive index element 24.
According to the optical pickup device of FIG. 30, the same effect as that of FIG. 1 can be obtained.

[0174]

EXAMPLES Examples 1 to 12 according to the present invention will be described below. Table 1 shows a list of data regarding the objective lenses and the like in Examples 1 to 12.

[0175]

[Table 1]

The diffractive surface provided on the objective lens of each example is represented by the optical path difference function Φb according to the above-mentioned formula (A). Also,
The aspherical surface in the objective lens of each example is x in the optical axis direction.
The height in the direction perpendicular to the axis and the optical axis is represented by h and is expressed by the following expression (B).

X = (h ² / r) / {1 + √ (1- (1 + κ) (h ² / r ² ))} + A ₄ h ⁴ + A ₆ h ⁶ + ... (B) where A ₄ , a _6, · · ·: aspheric coefficient, kappa: conical coefficient, r: a paraxial curvature radius, expressed r, d, n, the radius of curvature of the lens, surface interval, refractive index.

<Examples 1 to 7>

The respective lens data of the objective lenses of Examples 1 to 7 are shown in Tables 2 to 8 below. In addition, Examples 1 to 1
7 are optical path diagrams for FIG. 7, FIG. 9, FIG. 11, FIG.
5, FIG. 17, and FIG. 19, respectively, and FIG. 8, FIG. 10, FIG. 12, FIG. 14, and FIG.
6, FIG. 18 and FIG. 20 respectively.

[0180]

[Table 2]

[0181]

[Table 3]

[0182]

[Table 4]

[0183]

[Table 5]

[0184]

[Table 6]

[0185]

[Table 7]

[0186]

[Table 8]

In the above-mentioned Examples 1 to 7, the used wavelength is 405n.
Although the objective lens has a two-group two-lens configuration with an m and an image-side numerical aperture of 0.7 or more, it has a large working distance of 0.30 mm or more by being a finite conjugate type.

Since the objective lenses of Examples 1, 2, 4, 5, 6, and 7 are made of an optical plastic material for both the first group and the second group, they can be mass-produced at low cost. Among them, in the objective lenses of Examples 1, 4, 5 and 6, the surface on the light source side of the first lens is a diffractive surface to correct the axial chromatic aberration generated in the objective lens.

In particular, in the objective lens of the fifth embodiment, the axial chromatic aberration is overcorrected so that the above equation (14) is satisfied, so that the objective lens has a small deterioration of the wavefront aberration at the time of mode hopping. , The objective lenses of Examples 1, 4, and 6 in which the axial chromatic aberration and the chromatic spherical aberration are both corrected,
It was possible to increase the minimum value of the diffraction ring zone spacing. The variation amount ΔCA of the axial chromatic aberration in the equation (14) means the spherical aberration curves of 405 nm and 415 nm in the spherical aberration diagram of the objective lens of Example 5 when the wavelength of the light source is shifted by +10 nm to the long wavelength side. Is indicated by the moving width of the lower end of the light source. In addition, the variation ΔSA of the spherical aberration of the marginal ray is 405n
It is indicated by the width between the upper end of the spherical aberration curve and the upper end of the 415 nm spherical aberration curve when the lower end of the spherical aberration curve of m is moved in parallel to the position where the lower end thereof overlaps the lower end of the spherical aberration curve of 415 nm.

In each of the objective lenses of Examples 1, 2, 5, 6, and 7, the amount of third-order spherical aberration generated when the temperature changes and 3
Formed from an optical plastic material that has a greater change in refractive index when the temperature changes than the optical glass material by balancing the amount of higher-order spherical aberration of the fifth order or higher that occurs in the direction opposite to the secondary spherical aberration. Even with the objective lens having a high NA, it was possible to suppress the deterioration of the wavefront aberration when the temperature changes.

Further, in the objective lens of the seventh embodiment, the light flux is restricted to the position corresponding to the image side numerical aperture of 0.85 on the light source side refracting surface of the first lens (A portion in FIG. 19A). Therefore, a step as shown in FIG. 19B is formed. When h is the height from the optical axis, the function expressing the shape of the surface on the optical axis side of the step is f (h), and the shape of the surface outside the step is g (h), its differential function f '(h) and g' (h) are
By determining the shape of g (h) so as to satisfy f ′ (h) ≠ g ′ (h), the light flux passing through the surface outside the step is the light passing through the surface on the optical axis side of the step. Is focused on a different point.

In the objective lens of Example 3, both the first lens and the second lens were made of optical glass material.
By forming the second lens, which tends to have a small radius of curvature, from an optical glass material (SK12) having a d-line refractive index of 1.55 or more, a contact surface on the light source side and a plane perpendicular to the optical axis are formed. Since the angle formed by does not become too large, the die machining with the diamond tool can be performed more accurately. Furthermore, by forming the first lens from an optical glass material (FK01) having an Abbe number of 65 or more at the d-line, the generation of axial chromatic aberration that occurs in the objective lens is suppressed.

<Examples 8 to 11>

The respective lens data of the objective lenses in Examples 8 to 11 are shown in Tables 9 to 12 below. Also,
The optical path diagrams of Examples 8 to 11 are shown in FIGS.
5 and 27, and spherical aberration diagrams for Examples 8 to 11 are shown in FIGS. 22, 24, 26, and 28, respectively.

[0195]

[Table 9]

[0196]

[Table 10]

[0197]

[Table 11]

[0198]

[Table 12]

Examples 8 to 11 are two-group construction, a finite conjugate type objective lens, and a condensing optical system having means for correcting fluctuations in spherical aberration.
The image side numerical aperture of the objective lens is 05 nm and is 0.85.

In the converging optical system of Example 8, as shown in FIG. 3, the first lens of the objective lens is displaced along the optical axis to correct the variation of spherical aberration. Since the first lens is formed of a lightweight optical plastic material, the burden on the actuator can be reduced and a high-speed response to changes in spherical aberration can be achieved. Further, the surface on the light source side is a diffractive surface to correct axial chromatic aberration. Further, the second lens is made of an optical glass material (LaK1) having a refractive index of 1.55 or more at d-line.
By forming from 3), the angle formed by the contact surface of the surface on the light source side and the plane perpendicular to the optical axis was prevented from becoming too large.

Table 13 shows the results of correcting the wavefront aberration caused by the wavelength fluctuation of the light source, the temperature change, and the thickness error of the transparent substrate in Example 8, and in any case, it is corrected well.

[0202]

[Table 13]

Further, in the converging optical system of Example 9, the variation of the spherical aberration was corrected by shifting the light source along the optical axis as shown in FIG. Further, by making the surface on the light source side a diffraction surface, the axial chromatic aberration generated in the objective lens was corrected.

Table 14 shows the results of correcting the wavefront aberration caused by the wavelength variation of the light source, the temperature change, and the thickness error of the transparent substrate in Example 9, and in any case, it is well corrected.

[0205]

[Table 14]

In the converging optical system of the tenth embodiment, as shown in FIG.
As described above, the variation of the spherical aberration is corrected by shifting the lens disposed between the light source and the objective lens along the optical axis. Furthermore, by using this lens as a diffractive lens, the axial chromatic aberration generated in the entire condensing optical system was corrected. At this time, the axial chromatic aberration was overcorrected so that the expression (19) was satisfied, so that the diffraction ring zone spacing was prevented from becoming too small.

Table 15 shows the results of correcting the wavefront aberration caused by the wavelength fluctuation of the light source, the temperature change, and the thickness error of the transparent substrate in Example 10, and in any case, it is well corrected.

[0208]

[Table 15]

Further, in the condensing optical system of the eleventh embodiment, as shown in FIG.
As described above, the variation of the spherical aberration is corrected by shifting the lens disposed between the collimator lens and the objective lens along the optical axis. In this case, since the light emitted from the collimator lens is a parallel light beam, optical elements such as a polarization beam splitter, a beam shaping element, and a wave plate can be easily arranged in this parallel light beam. In addition, the surface on the light source side of the first lens of the objective lens is a diffractive surface to correct the axial chromatic aberration generated in the objective lens. Furthermore, by using a diffractive lens as the collimator lens, the light emitted from the collimator lens is made to be a substantially parallel light flux even when the wavelength of the light source changes or the temperature and humidity change.

Table 16 shows the results of correcting the wavefront aberration caused by the wavelength fluctuation of the light source, the temperature change, and the thickness error of the transparent substrate in the eleventh embodiment, and in any case, it is well corrected.

[0211]

[Table 16]

Further, since the lens capable of shifting along the optical axis in the condensing optical system of the tenth and eleventh embodiments is formed of a lightweight optical plastic material, it is possible to reduce the load on the actuator and the fluctuation of spherical aberration. High-speed response is possible.

Further, the condensing optical systems of Examples 8 to 11 are
It is also possible to correct the occurrence of spherical aberration due to variation in the thickness of the transparent substrate of the optical information recording medium exceeding 0.02 mm. Therefore, it is possible to record and / or reproduce information on / from a multilayer optical information recording medium having a structure in which a plurality of transparent substrates and information recording layers are sequentially laminated from the front surface side.

<Example 12>

Lens data of the objective lens in Example 12 are shown in Table 17 below. 31 shows an optical path diagram of Example 12, and FIG. 32 shows a spherical aberration diagram thereof.

[0216]

[Table 17]

The objective lens of Example 12 is an objective lens of NA 0.85 composed of two plastic lenses. By using the first lens as a diffractive lens, axial chromatic aberration, which is a problem when using a short wavelength semiconductor laser light source, is corrected. At this time, by using a concave surface, which is a surface on the optical information recording medium side, as a diffractive surface, the angle of the tip of each diffractive ring zone having a sawtooth shape is more Then relaxed. Therefore, the first
When the lens is molded, the diffractive ring zone can be satisfactorily transferred.

Furthermore, in the objective lens of the twelfth embodiment, since the conditional expressions (1A) and (1B) described above are satisfied, the change amount of spherical aberration at the time of temperature change is high even though the objective lens is a high NA plastic lens. Very small. On the other hand, the axial wavefront aberration at the design reference temperature of 25 degrees is 0.002λrms, whereas the axial wavefront aberration at 55 degrees is 0.003λrms when the temperature rises by 30 °. If it is zero.

When calculating the above wavefront aberration,
Change in refractive index of plastic lens due to temperature change (-1
2 × 10 ⁻⁵ / degree) and the change in wavelength of the light source due to temperature change (+0.05 nm / degree), regarding the shape change of the plastic lens due to temperature change, the change in wavefront aberration compared to the change in refractive index. It is not considered because it has a small effect on.

In each of the tables and figures described above, E (or e) is used to express the power of 10, for example, E-02
It may be expressed as (= 10 ⁻² ).

Further, in the present specification, the optical information recording medium is not limited to the one having a protective layer on the light beam incident surface side,
Those without a protective layer are also included. When the optical information recording medium has a protective layer, the objective lens used in the optical pickup device of the present invention corrects the aberration so that the spherical aberration is minimized under the combination with the protective layer having a specific thickness. Is preferably provided.

Further, in this specification, the minute fluctuation of the oscillation wavelength of the light source means ± 10 n with respect to the oscillation wavelength of the light source.
It refers to the wavelength variation within the range of m. Further, in the present specification, (correctly) correcting various aberrations means that the so-called diffraction limit performance is 0.07λrms or less when the wavefront aberration is obtained (where λ is the oscillation wavelength of the light source used). Is preferable, and more preferably 0.05 λrms or less in consideration of the assembling accuracy of the optical pickup device.

Further, the objective lens of the present invention, the condensing optical system,
As a short-wavelength light source that is preferably used in the optical pickup device, in addition to the above-mentioned blue-violet semiconductor laser, a wavelength conversion element for converting the wavelength of light from the semiconductor laser in half in front of the semiconductor laser, so-called SHG ( Second Harm
onic Generation: second harmonic generation) There is a light source in which an element is formed.

[0224]

According to the present invention, an objective for recording and / or reproducing information on an optical information recording medium having a small diameter and a large working distance, even if the objective lens has a high NA and is composed of two positive lenses. A lens can be provided. Also,
Information of an optical information recording medium in which even a high NA objective lens composed of two positive lenses has a small diameter, a large working distance, and axial chromatic aberration which is a problem when a short wavelength light source is used is corrected. An objective lens for recording and / or reproducing can be provided.

Further, in the condensing optical system for recording and / or reproducing the information of the optical information recording medium, the fluctuation of the oscillation wavelength of the laser light source, the temperature / humidity change, the error of the thickness of the transparent substrate of the optical information recording medium. It is possible to provide a condensing optical system that can effectively correct the fluctuation of spherical aberration generated on each optical surface of the condensing optical system due to the above reasons with a simple configuration. Further, it is possible to provide a condensing optical system in which axial chromatic aberration which is a problem when a short wavelength light source is used is corrected.

Furthermore, it is possible to provide an optical pickup device equipped with this objective lens and / or a condensing optical system, and a recording / reproduction device equipped with this optical pickup device.

[Brief description of drawings]

FIG. 1 is a schematic view of an optical pickup device according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram of an optical pickup device according to a second embodiment of the present invention.

FIG. 3 is a schematic diagram of an optical pickup device according to a third embodiment of the present invention.

FIG. 4 is a schematic diagram of an optical pickup device according to a fourth embodiment of the present invention.

FIG. 5 is a schematic diagram of an optical pickup device according to a fifth embodiment of the present invention.

FIG. 6 is a schematic diagram of an optical pickup device according to a sixth embodiment of the present invention.

FIG. 7 is an optical path diagram of Example 1.

8 is a spherical aberration diagram for Example 1. FIG.

FIG. 9 is an optical path diagram of Example 2.

FIG. 10 is a spherical aberration diagram of Example 2.

FIG. 11 is an optical path diagram of Example 3.

FIG. 12 is a spherical aberration diagram of Example 3.

FIG. 13 is an optical path diagram of Example 4.

FIG. 14 is a spherical aberration diagram of Example 4.

FIG. 15 is an optical path diagram of Example 5.

FIG. 16 is a spherical aberration diagram of Example 5.

FIG. 17 is an optical path diagram of Example 6.

FIG. 18 is a spherical aberration diagram of Example 6;

19 (a) is an optical path diagram of Example 7, and FIG. 19 (b) is an enlarged view of part A of FIG. 19 (a).

FIG. 20 is a spherical aberration diagram of Example 7.

FIG. 21 is an optical path diagram of Example 8.

22 is a spherical aberration diagram for Example 8. FIG.

FIG. 23 is an optical path diagram of Example 9.

24 is a spherical aberration diagram for Example 9. FIG.

FIG. 25 is an optical path diagram of Example 10.

FIG. 26 is a spherical aberration diagram of Example 10.

27 is an optical path diagram of Example 11. FIG.

FIG. 28 is a spherical aberration diagram of Example 11;

FIG. 29 is a diagram for explaining the definitions of X1 and X2 in Expression (17).

FIG. 30 is a schematic diagram of an optical pickup device according to a seventh embodiment of the present invention.

FIG. 31 is an optical path diagram of Example 12;

FIG. 32 is a spherical aberration diagram of Example 12;

[Explanation of symbols]

1 Semiconductor laser (light source) 3 Refractive index distribution variable element 6 2nd group objective lens 6a first lens 6b Second lens 7 Transparent substrate 7'information recording surface 10 2-axis actuator 11 Driving means for variable refractive index distribution element 3 13 Photodetector 21 1-axis actuator 23 lenses

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H087 KA13 LA01 NA01 NA08 NA14                       PA02 PA03 PA17 PB02 PB03                       QA01 QA02 QA05 QA12 QA13                       QA18 QA21 QA22 QA25 QA33                       QA41 QA46 RA05 RA12 RA13                       RA22 RA28 RA32 RA34 RA42                       RA43 RA46 UA01                 5D119 AA41 BA01 DA01 DA05 EC01                       JA09 JA44 JA46 JA54 JB01                       JB02 JB03

Claims (46)

[Claims] 1. An objective lens for recording and / or reproducing information on an optical information recording medium, comprising: a first lens group having a positive refractive power and a second lens group having a positive refractive power, which are arranged in order from a light source side. The objective lens is composed of a finite conjugate type that converges a divergent light beam from a light source on the information recording surface of the optical information recording medium, and satisfies the following formula. NA ≧ 0.65 where NA is a predetermined image-side numerical aperture required for recording and / or reproducing on the optical information recording medium. 2. The following formula is satisfied: 1.
The objective lens described in. NA ≧ 0.75 3. The following expression is satisfied: 1.
Alternatively, the objective lens described in 2. 0.10 ≦ NA · WD / f ≦ 0.35 where NA is a predetermined image-side numerical aperture necessary for recording and / or reproducing on an optical information recording medium WD is the working distance (m) of the objective lens.
m) f: focal length of the whole system of the objective lens (mm) 4. The method according to claim 1, wherein the following expression is satisfied.
4. The objective lens according to any one of items 1 to 3. 0.01 ≦ | m | ≦ 0.30 where m: the object side numerical aperture of the objective lens is N
When A _OBJ and the image-side numerical aperture are NA _{IM G} , NA
Lateral magnification of the objective lens defined by _OBJ / NA _IMG 5. The optical material according to any one of claims 1 to 4, wherein the wavelength used is 600 nm or less, and the optical transmittance is 85% or more in the wavelength used region when the thickness is 3 mm. Item 1. The objective lens according to Item 1. 6. An objective lens for recording and / or reproducing information on an optical information recording medium, comprising: a first lens group having a positive refractive power and a second lens group having a positive refractive power, which are arranged in order from a light source side. The objective lens is composed of a finite conjugate type that converges a divergent light beam from a light source on the information recording surface of the optical information recording medium, and satisfies the following formula. 0.10 ≦ NA · WD / f ≦ 0.35 where NA is a predetermined image-side numerical aperture necessary for recording and / or reproducing on an optical information recording medium WD is the working distance (m) of the objective lens.
m) f: focal length of the whole system of the objective lens (mm) 7. The method according to claim 6, wherein the following expression is satisfied.
The objective lens described in. 0.01 ≦ | m | ≦ 0.30 where m: the object side numerical aperture of the objective lens is N
When A _OBJ and the image-side numerical aperture are NA _{IM G} , NA
Lateral magnification of the objective lens defined by _OBJ / NA _IMG 8. The optical material according to claim 6 or 7, wherein the wavelength used is 600 nm or less, and the optical transmittance is 85% or more in the wavelength used region when the thickness is 3 mm. Objective lens. 9. An objective lens for recording and / or reproducing information on an optical information recording medium, comprising: a first lens group having a positive refractive power and a second lens group having a positive refractive power, which are sequentially arranged from a light source side. The objective lens is composed of a finite conjugate type that converges a divergent light beam from a light source on the information recording surface of the optical information recording medium, and satisfies the following formula. 0.01 ≦ | m | ≦ 0.30 where m: the object side numerical aperture of the objective lens is N
When A _OBJ and the image-side numerical aperture are NA _{IM G} , NA
Lateral magnification of the objective lens defined by _OBJ / NA _IMG 10. The internal transmittance is 85% when the wavelength used is 600 nm or less and the thickness is 3 mm in the wavelength range used.
The objective lens according to claim 9, which is formed of the above optical material. 11. Recording and / or recording of information on an optical information recording medium
Alternatively, the reproducing objective lens includes a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are sequentially arranged from the light source side. The divergent light beam from the light source is directed to the optical information recording medium. An objective lens which is of a finite conjugate type for condensing light on an information recording surface, has an operating wavelength of 600 nm or less, and has a ring-shaped diffractive structure on at least one surface. 12. The light source generates a wavelength variation of ± 10 nm or less, and the diffractive structure is accompanied by a wavelength variation of the light source.
The objective lens according to claim 11, which has a function of suppressing axial chromatic aberration caused by refractive index dispersion of the optical material of the objective lens. 13. The objective lens according to claim 11, which satisfies the following expression. NA ≧ 0.65 0.10 ≦ NA · WD / f ≦ 0.35 0.01 ≦ | m | ≦ 0.30 where NA is a predetermined value necessary for recording and / or reproducing on an optical information recording medium. Image-side numerical aperture WD of the objective lens: Working distance (m
m) f: focal length of the entire system of the objective lens (mm) m: When the objective lens object side numerical aperture _{NA OBJ,} the image side numerical aperture was _{NA IMG,} defined by _NA OBJ _{/ NA IMG} Lateral magnification of the objective lens 14. The objective lens according to claim 11, which satisfies the following expression. 0.01 ≦ PD / PT ≦ 0.15 where PD is an optical path difference defined by Φb = b _2i h ² + b _4i h ⁴ + b _6i h ⁶ + ... When expressed as a function (where h
Is the height (mm) from the optical axis, and is b _2i , b _4i , b
_6i , ... _Are optical path difference function coefficients of the 2nd, 4th, 6th, ..., respectively, and the power of only the diffractive structure defined by PD = Σ (−2 · b _2i ) (mm ^{− 1} ) PT: Power of the entire objective lens system that combines the power of the refracting lens and the power of the diffractive structure (mm ⁻¹ ). 15. The first lens group has a meniscus shape with a convex surface facing the light source side, and has a ring-shaped diffractive structure on the surface of the first lens group closest to the optical information recording medium.
The objective lens according to any one of claims 11 to 13, which satisfies the following expression. 0.05 ≦ PD / PT ≦ 0.25 where PD is an optical path difference defined by Φb = b _2i h ² + b _4i h ⁴ + b _6i h ⁶ + ... When expressed as a function (where h
Is the height (mm) from the optical axis, and is b _2i , b _4i , b
_6i , ... _Are optical path difference function coefficients of the 2nd, 4th, 6th, ..., respectively, and the power of only the diffractive structure defined by PD = Σ (−2 · b _2i ) (mm ^{− 1} ) PT: Power of the entire objective lens system that combines the power of the refracting lens and the power of the diffractive structure (mm ⁻¹ ). 16. Diffraction having a maximum amount of diffracted light among diffracted light generated by a diffractive structure formed on the i-th surface, wherein a reference wavelength is λ (mm), a focal length of the entire objective lens system is f (mm). Let ni be the order of light, Mi be the number of ring zones of the diffractive structure within the effective diameter of the i-th surface, and Pi (mm) be the minimum value of the interval between adjacent ring zones of the diffractive structure within the effective diameter of the i-th surface. In this case, the objective lens according to any one of claims 11 to 15, wherein the following expression is satisfied. 0.04 ≦ f · λ · Σ (ni / (Mi · Pi ² )) ≦
0.50 17. The objective lens according to claim 11, wherein the following expression is satisfied. 0.2 ≦ | (Ph / Pf) −2 | ≦ 3.0 where Pf: Diffraction ring zone spacing (mm) at a predetermined image-side numerical aperture required for recording and / or reproducing on an optical information recording medium ) Ph: Diffraction ring zone spacing (mm) at a numerical aperture of 1/2 of a predetermined image-side numerical aperture required for recording and / or reproducing on an optical information recording medium 18. An axis which, when the diffractive action as a diffractive lens and the refractive action as a refraction lens are combined, changes in a direction in which the back focus becomes shorter when the wavelength of the light source fluctuates toward the long wavelength side. The objective lens according to any one of claims 11 to 17, which has an upper chromatic aberration characteristic and satisfies the following expression. -1 <ΔCA / ΔSA <0 where ΔCA: amount of change in axial chromatic aberration with respect to change in wavelength (mm) ΔSA: amount of change in spherical aberration of marginal ray with respect to change in wavelength (mm) 19. The recording and / or reproducing of information on the optical information recording medium, wherein the amount of diffracted light of ni order generated in the diffractive structure formed on the i-th surface is larger than that of any other orders. N generated in the diffraction structure
The objective lens according to any one of claims 11 to 18, wherein the i-th order diffracted light can be condensed on the information recording surface of the optical information recording medium. Here, n is an integer other than 0 and ± 1. 20. Among the surfaces between the surface of the first lens group closest to the light source and the surface of the second lens group closest to the light source, at least two surfaces are aspherical surfaces. The objective lens according to any one of claims 1 to 19. 21. The objective lens according to claim 1, wherein both the first lens group and the second lens group are plastic lenses and satisfy the following formula. NA ≧ 0.75 0.06>ΔSAG> −0.08 ΔSAG = (X1′−X2 ′) / (NA ⁴ · f · (1+
| M |)) X1 ′ = X1 · (N1-1) ³ / f1 X2 ′ = X2 · (N2-1) ³ / f2, where NA is for recording and / or reproducing on the optical information recording medium. A necessary predetermined image-side numerical aperture X1 of the objective lens, which is perpendicular to the optical axis and is in contact with the apex of the optical surface of the i-th lens group that is closest to the light source, and the periphery of the effective diameter (above NA
At the position on the optical surface closest to the light source of the i-th lens group) on which the marginal ray of light is incident, the difference (mm) in the optical axis direction from the surface closest to the light source of the i-th lens group As a reference, the case of measuring in the direction of the optical information recording medium is positive, and the case of measuring in the direction of the light source is negative (i = 1 or 2) f: focal length (mm) of the entire system of the objective lens f1: the first Focal length of lens group (mm) f2: Focal length of the second lens group (mm) m: _NAOBJ / NA, where _NAOBG is the object-side numerical aperture of the objective lens and _{NAIMG is} the image-side numerical aperture. Lateral magnification N1 of the objective lens defined by _IMG : Refractive index of the first lens group at the used wavelength N2: Refractive index of the second lens group at the used wavelength 22. The objective lens according to claim 1, which satisfies the following expression. 3 ≦ √ (f1 / f2) / | m | ≦ 50 where fi: focal length (mm) of the i-th lens group (i =
1 or 2) (however, when the i-th lens group has a diffractive structure, the focal length of the entire system of the i-th lens group (m
m)) m: When the objective lens object side numerical aperture _{NA OBJ,} the image side numerical aperture was _{NA IMG, NA} OBJ _{/ NA IMG}
Lateral magnification of the objective lens defined by 23. The objective lens according to claim 1, wherein the following expression is satisfied. 0.5 ≦ (r2 + r1) / (r2-r1) ≦ 4.0 where r1: paraxial radius of curvature (mm) of the surface of the first lens group closest to the light source, r2: most optical information recording of the first lens group Paraxial radius of curvature of the medium side surface (mm) 24. The objective lens according to claim 1, which satisfies the following expression. −0.02 ≦ NA · (X2−X1) /f≦0.30 where NA is a predetermined image-side numerical aperture necessary for recording and / or reproducing on an optical information recording medium Xi: perpendicular to the optical axis At a plane that is in contact with the vertex of the surface of the i-th lens group that is closest to the light source, and the i-th lens group at the outermost periphery of the effective diameter (the position on the surface that is closest to the light source of the i-th lens group where the marginal ray of NA is incident) The difference in the optical axis direction from the surface closest to the light source is positive when measured in the direction of the optical information recording medium with reference to the tangential plane, and negative when measured in the direction of the light source (i = 1 or 2). f: focal length of the whole system of the objective lens (mm) 25. The objective lens according to claim 1, further comprising a lens group formed of an optical plastic material. 26. The objective lens according to claim 1, further comprising a lens group formed of an optical glass material. 27. A diaphragm for restricting a light beam is provided between a plane which is perpendicular to the optical axis and is in contact with the apex of the most light source side surface of the i-th lens group and the most light source side surface of the i-th lens group. The objective lens according to any one of claims 1 to 26, wherein (I = 1 or 2) 28. The normal direction of the surface is discontinuously changed to a position on at least one surface corresponding to an image-side numerical aperture required for recording and / or reproducing information on an optical information recording medium. The objective lens according to any one of claims 1 to 27, characterized in that the diameter of the condensed light flux is regulated by providing a portion to be formed. 29. A first lens group having a positive refracting power and a second lens group having a positive refracting power which are sequentially arranged from the light source side, and a divergent light beam from the light source is formed on an information recording surface of the optical information recording medium. A condensing optical system for recording and / or reproducing information on an optical information recording medium including a finite conjugate type objective lens for condensing light, wherein the condensing optical system is provided between the light source and the information recording surface. A condensing optical system provided with means for correcting fluctuations in spherical aberration generated in the system and satisfying the following expression. NA ≧ 0.65, where NA is a predetermined image-side numerical aperture of the objective lens required for recording and / or reproducing on the optical information recording medium. 30. The optical information recording medium, comprising a first lens group having a positive refracting power and a second lens group having a positive refracting power, which are sequentially arranged from the light source side, and emits a divergent light flux having a wavelength of 600 nm or less emitted from a light source. Is a condensing optical system for recording and / or reproducing information on an optical information recording medium including a finite conjugate type objective lens that condenses light on the information recording surface, and an optical element constituting the condensing optical system. A condensing optical system having a ring-shaped diffractive structure provided on at least one surface of the. 31. The light source generates a wavelength variation of ± 10 nm or less, and the diffractive structure is accompanied by a wavelength variation of the light source.
31. The condensing optical system according to claim 30, which has a function of suppressing axial chromatic aberration caused by refractive index dispersion of an optical material of an optical element forming the condensing optical system. 32. When the objective lens has a diffractive action as a diffractive lens and a refractive action as a refraction lens, the back focus is shortened when the wavelength of the light source fluctuates to a long wavelength side. 32. The condensing optical system according to claim 30 or 31, wherein the condensing optical system has the following axial chromatic aberration characteristics and satisfies the following expression. -1 <ΔCA / ΔSA <0 where ΔCA: amount of change in axial chromatic aberration with respect to change in wavelength (mm) ΔSA: amount of change in spherical aberration of marginal ray with respect to change in wavelength (mm) 33. The n-th order diffracted light amount generated in the diffractive structure is larger than any other diffracted light amount, and is generated in the diffractive structure for recording and / or reproducing information on the optical information recording medium. The condensing optical system according to any one of claims 30 to 32, characterized in that the n-th order diffracted light can be condensed on the information recording surface of the optical information recording medium. Here, n is an integer other than 0 and ± 1. 34. Between the light source and the information recording surface,
The condensing optical system according to any one of claims 30 to 33, further comprising: a unit that corrects a variation in spherical aberration generated in the condensing optical system, and satisfies the following expression. NA ≧ 0.65, where NA is a predetermined image-side numerical aperture of the objective lens required for recording and / or reproducing on the optical information recording medium. 35. The spherical aberration correction means includes at least one optical element capable of changing the divergence of an emitted light beam by shifting along the optical axis. Or the condensing optical system described in 34. 36. The optical element capable of changing the divergence of the emitted light beam by shifting along the optical axis is one of the lens groups of the objective lens. 35. The condensing optical system described in 35. 37. An optical element capable of changing the divergence of an emitted light beam by shifting along the optical axis is arranged between the light source and the objective lens. The condensing optical system according to claim 36. 38. The optical element capable of changing the divergence of the emitted light flux by shifting along the optical axis is formed of an optical plastic material. The condensing optical system according to any one of 37. 39. The means for correcting the fluctuation of the spherical aberration is an element which is arranged between the light source and the information recording surface and has a variable refractive index distribution along a direction perpendicular to an optical axis. The condensing optical system according to claim 29 or 34. 40. The means for correcting the variation of the spherical aberration is an element having a variable refractive index arranged between the objective lens and the optical information recording medium. 34. The condensing optical system according to 34. 41. The objective lens according to any one of claims 1 to 2.
41. The condensing optical system according to claim 29, comprising the objective lens according to claim 6. 42. A condensing optical system capable of recording and / or reproducing information on a plurality of information recording layers from the same light flux incident surface side, wherein the light flux incident surface is used for focus jump between information recording layers. 42. Any one of claims 29, 34 to 41, characterized in that the variation of spherical aberration caused by the difference in the thickness of the transparent substrate up to each information recording layer is corrected by the means for correcting the variation of spherical aberration. The condensing optical system described in. 43. A divergent light beam from a light source, which comprises a first lens group having a positive refracting power and a second lens group having a positive refracting power, arranged in order from the light source side. Of the information recording surface of the optical information recording medium by a condensing optical system including an objective lens of a finite conjugate type for condensing on the information recording surface, and recording and / or the information on the information recording surface. An optical pickup device for reproducing information on an information recording surface, wherein the light source is configured to change the divergence of a light beam incident on the objective lens by changing along the optical axis direction, By detecting the reflected light from the information recording surface, a detecting means for detecting the variation of the spherical aberration generated in the condensing optical system, and the variation of the spherical aberration according to the detection result of the detecting means. Optical axis to reduce the light source An optical pickup device, comprising: a driving unit for shifting along a direction. 44. The objective lens according to any one of claims 1 to 2.
The optical pickup device according to claim 43, comprising the objective lens according to claim 8. 45. The optical information recording medium, wherein a light beam emitted from a light source is composed of a first lens group having a positive refractive power and a second lens group having a positive refractive power, which are sequentially arranged from the light source side, and a light beam diverging from the light source is recorded on the optical information recording medium. Of the objective lens which is a finite conjugate type that converges on the information recording surface of, and a condensing optical system including a means for correcting the fluctuation of spherical aberration, and condenses on the information recording surface of the optical information recording medium.
43. An optical pickup device for recording information on the information recording surface and / or reproducing information on the information recording surface, wherein the condensing optical system is any one of claims 29, 34 to 42. Having a condensing optical system, by detecting the reflected light from the information recording surface, a detecting unit for detecting the variation of the spherical aberration generated in the condensing optical system, and the detection result of the detecting unit. Accordingly, there is provided an optical pickup device, comprising: a driving unit that drives a unit that corrects the fluctuation of the spherical aberration in order to reduce the fluctuation of the spherical aberration. 46. An audio and / or image recording device comprising the optical pickup device according to any one of claims 43 to 45, and / or
Audio and / or image playback device.