JP2005025925A - Optical system, optical pickup apparatus, optical information recording and reproducing apparatus, and aberration-correcting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical system as a converging application used for an optical pickup apparatus which is lightweight, can secure a sufficient working distance, and further realizes the correction of a temperature characteristic and chromatic aberration, to provide an optical system for the optical pickup apparatus which is particularly good in the correction of the temperature characteristics, an optical pickup apparatus using the optical system, an optical information recording and reproducing apparatus, and an aberration-correcting element used in the optical system. <P>SOLUTION: The objective optical system 30 comprises an aberration-correcting element 30 made of plastic and a converging lens 50 made of plastic, arranged successively from an object side. The aberration-correcting element has at least one each of the optical surface formed with an optical path difference generating structure 60 constituted of a central region including the optical axis and a plurality of zones 63 divided so as to have fine level differences 62 in the optical axis direction on the outer side of the central region and an optical surface formed with a diffractive structure 70 to diffract an incident luminous flux. The converging lens is a single refractive lens constituted of one lens in one group having at least one aspherical surface. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical system for an optical pickup device, an optical pickup device, an optical information recording / reproducing device, and an aberration correction element for the optical pickup device.

Along with the recent increase in the density of optical discs, an objective lens of an optical pickup device used for recording / reproducing on an optical disc has a high numerical aperture (NA) because of a demand for a smaller focused spot. It is like that.
For example, in an optical pickup device for a high-density optical disk using a blue-violet semiconductor laser light source having a wavelength λ of 405 nm, it has been proposed to use an objective lens having a numerical aperture NA of 0.85 in order to achieve high density. Yes.

As an objective lens having a numerical aperture of 0.85, a glass two-group lens that achieves NA 0.85 by relaxing the manufacturing tolerance of each lens by sharing the refractive power with respect to the incident light flux between the two lenses. It is described in the following Patent Document 1.
However, when molding an aspherical lens made of glass, it is necessary to set the temperature of the mold to a high temperature of about 550 ° C. Therefore, a mold for an aspherical lens made of glass generally has a short life, There is a problem that it is not suitable for mass production.
In addition, a high-density optical disk using a laser beam having a wavelength of 405 nm and an objective lens having an NA of 0.85 employs a protective layer having a thickness of 0.1 mm. The effect on characteristics is large. Therefore, securing the working distance in designing the objective lens is very important in preventing the objective lens from damaging the surface of the protective layer due to interference with the optical disk.

However, in the objective lens having the two-group configuration, the incident light beam is refracted by each of the two lenses and condensed on the information recording surface of the optical disk, so that the light of the marginal light beam when passing through the optical surface facing the optical disk. Since the height from the shaft is reduced and the distance from the objective lens to the optical disk (working distance) is reduced, the objective lens and the optical disk are likely to interfere with each other. In particular, in the two-group objective lens made of glass as described in the above-mentioned Patent Document 1, the weight of the objective lens increases, and therefore the possibility that the objective lens may damage the surface of the protective layer due to interference with the optical disk is very high. Get higher. In order to address the above problems, one of the inventors of the present invention has proposed a plastic single lens with NA of 0.85 as described in Patent Document 2 below.
Plastic lenses can be molded at low temperatures (about 120 ° C) compared to glass lenses, so the mold life is long and material costs are low, so the cost is low. Therefore, mass production is possible while maintaining stable quality. Further, the single lens configuration can ensure a large working distance and is lightweight, so that the problem of damage to the surface of the protective layer due to interference between the objective lens and the optical disk can be solved.

A plastic single lens with NA of 0.85 has the advantages as described above compared with a glass-made two-group lens. On the other hand, the spherical aberration generated by a change in refractive index due to a temperature change is increased. Occurs. This is because a change in spherical aberration that occurs in a plastic single lens with a change in refractive index increases in proportion to NA ⁴ .
In the following description, the characteristic of the optical element at the time of temperature change is referred to as `` temperature characteristic '', and correcting the wavefront aberration change of the optical element due to a predetermined temperature change to be below the diffraction limit is `` This is called “correction of temperature characteristics”.

Also, in an optical pickup device, since the laser power at the time of recording is generally larger than the laser power at the time of reproducing information, the center wavelength of the laser light source instantaneously varies depending on the output change when switching from reproduction to recording. In some cases, mode hopping may occur. The focus position shift caused by such mode hopping can be removed by focusing the objective lens. However, a defect such as a recording failure due to the focus position shift occurs for several nsec until the objective lens is focused. Since this focus position shift becomes larger as the light source wavelength becomes shorter, the wavefront aberration deterioration due to mode hopping becomes larger as the light source wavelength becomes shorter. For the above reasons, an optical pickup device for a high-density optical disk using a blue-violet semiconductor laser as a light source is required to correct a focus position shift of a focused spot with respect to a wavelength change.
In the following description, the axial chromatic aberration and / or spherical aberration change that occurs in the optical element with respect to the wavelength change of the incident light beam is referred to as “chromatic aberration”. The wavefront aberration is deteriorated by shifting the focus position of the light spot. Correcting the “chromatic aberration” of the optical element with respect to a predetermined change in wavelength of the incident light beam to be equal to or less than the diffraction limit is called “correcting chromatic aberration”.

Since the plastic materials that can be used in the short wavelength region are limited, it is impossible to suppress such chromatic aberration by selecting a material having a large Abbe number. Furthermore, the single lens tends to generate a larger amount of chromatic aberration with respect to the same wavelength change of the incident light beam than the two-group lens.
That is, in order to use an NA 0.85 plastic single lens as an objective lens for an optical pickup device using a blue-violet semiconductor laser as a light source, “correction of chromatic aberration” is performed in addition to “correction of temperature characteristics”. It is desirable.

As a technology for correcting the temperature characteristics of a single lens made of plastic, a diffraction structure that has a wavelength dependency of spherical aberration, in which spherical aberration changes in the direction of insufficient correction when the incident light beam changes in the longer direction, A technique of forming on the optical surface of a single lens made of the product is described in Patent Document 3 below.
A technique for correcting chromatic aberration and correcting temperature characteristics by providing a diffraction structure and a plurality of step structures (NPS: non-periodic phase structure) extending in the optical axis direction on the optical surface of a plastic single lens. Is described in Patent Document 4 below.
JP-A-10-123410 (US Patent No. 6,058,095) JP 2001-324673 A JP 11-337818 A (US Pat. No. 6,191,889) International Publication No. 02/41307 Pamphlet

However, in the plastic single lens in which the diffraction structure described in Patent Document 3 is formed, when the temperature rises due to the change of the spherical aberration in the direction of insufficient correction of the diffraction structure due to the change of the incident light beam when the temperature rises. The spherical aberration change in the overcorrection direction due to a decrease in the refractive index of the optical element is canceled and corrected.
In other words, since the technology uses the characteristic that the oscillation wavelength of the semiconductor laser shifts to the longer wavelength side as the temperature rises, the temperature of the objective optical system placed near the actuator that becomes the heat source during the operation of the optical pickup device In a situation where only the temperature rises and the temperature of the semiconductor laser disposed at a position away from the actuator hardly changes, there is a problem that the effect of correcting the temperature characteristic cannot be obtained.

On the other hand, since the technique disclosed in Patent Document 4 uses a diffractive structure for correcting chromatic aberration and NPS for correcting temperature characteristics, only the temperature of the objective optical system increases. Even if the temperature of the semiconductor laser hardly changes, there is an advantage that a temperature characteristic correction effect can be obtained. However, when a diffractive structure or a structure having a fine step in the optical axis direction such as NPS is formed on the optical surface of an objective optical system that requires a large refractive power, the light beam incident on the side surface of the step is In some cases, the path is blocked (light flux is lost) and does not contribute to the formation of a focused spot, leading to a decrease in the amount of light. In particular, in a plastic single lens having an NA of 0.85, the radius of curvature of the optical surface is very small. It is rare.
In addition, since the change in spherical aberration due to temperature characteristics increases in proportion to NA ⁴ , when using a lens with a very high NA, such as a plastic single lens with NA 0.85, the NPS alone has the temperature characteristics. In some cases, it is difficult to sufficiently perform spherical aberration correction, and further improvement in the effect of correcting temperature characteristics has been demanded.

  An object of the present invention is to take the above-mentioned problems into consideration, and is an optical system as a condensing application that is used in an optical pickup device that is lightweight and can secure a sufficient working distance and that can correct temperature characteristics and chromatic aberration. Furthermore, by providing an optical system for an optical pickup device with particularly good correction of temperature characteristics, an optical pickup device using the optical system, an optical information recording / reproducing device, and an aberration correction element used in the optical system. is there.

Said subject is achieved by the following aspects.
According to a first aspect of the present invention, there is provided an optical system for an optical pickup device including an aberration correction element made of plastic and a condensing lens for imaging a light beam emitted from the aberration correction element.
The aberration correction element has at least one first optical surface on which an optical path difference providing structure is formed and one second optical surface on which a diffractive structure is formed,
The condensing lens is a plastic refracting single lens having at least one aspherical surface and having one lens element per group.

According to the first aspect, by using the diffractive structure for correcting chromatic aberration and using the optical path difference providing structure for correcting temperature characteristics, only the refractive index changes due to a change in environmental temperature, and the wavelength of the emitted light beam changes. Even if not, the temperature characteristic can be corrected by the optical path difference providing structure.
In addition, since the aberration correction element and the condenser lens are molded from plastic, it is possible to reduce the weight of the high-NA objective optical system used for high-density optical discs and to produce it at low cost by injection molding using a mold.
Since a structure having a fine step such as a diffractive structure or an optical path difference providing structure is formed on the optical surface of the aberration correction element, the path of the light beam incident on the diffractive structure or the optical path difference providing structure is The proportion of the light beam that is blocked and does not contribute to the formation of the condensed spot can be suppressed, and a reduction in the amount of light can be prevented.

According to a second aspect of the present invention, there is provided an optical system for an optical pickup device including an aberration correction element made of plastic and a condenser lens for imaging a light beam emitted from the aberration correction element.
The aberration correction element has at least one optical surface on which an optical path difference providing structure is formed,
The condensing lens is a single lens having one lens group and having at least one aspherical surface, and the paraxial power P ₁ (mm ⁻¹ ) of the aberration correction element satisfies the following expression (1). It is characterized by.
P ₁ > 0 (1)

According to the second aspect, since the power P ₁ (mm ⁻¹ ) in the paraxial axis of the aberration correction element is greater than 0, the light beam emitted from the aberration correction element is a convergent light beam. This is equivalent to the magnification m of the optical lens being m> 0. In general, a numerical aperture NA∞ (hereinafter referred to as converted NA) converted to infinite luminous flux incidence for a condensing lens with a numerical aperture NA of a finite conjugate type (m ≠ 0) is NA∞ = NA · (1−m ). Therefore, in a condensing lens with a magnification m of m> 0, the converted NA can be reduced, so that the change in spherical aberration due to the temperature characteristic of the condensing lens that increases in proportion to NA ⁴ can be reduced.

  According to a third aspect of the present invention, an optical path between a laser light source and a condensing lens that is disposed at a position facing the optical disk and collects a light beam emitted from the laser light source on an information recording surface of the optical disk. The aberration correction element for an optical pickup device made of plastic is characterized in that an optical path difference providing structure is formed on one optical surface and a diffractive structure is formed on the other optical surface.

According to the third aspect, by using the diffractive structure for correcting chromatic aberration and using the optical path difference providing structure for correcting temperature characteristics, only the refractive index changes due to a change in environmental temperature, and the wavelength of the emitted light beam changes. Even if not, the temperature characteristic can be corrected by the optical path difference providing structure.
In addition, since the aberration correction element and the condenser lens are molded from plastic, it is possible to reduce the weight of the high-NA objective optical system used for high-density optical discs and to produce it at low cost by injection molding using a mold.
In addition, by providing the condensing lens disposed on the optical disc side exclusively with the refractive power with respect to the incident light beam, a large working distance can be secured and interference between the objective lens and the optical disc can be prevented. Since a structure having a fine step such as a diffractive structure or an optical path difference providing structure is formed on the optical surface of the aberration correction element, the path of the light beam incident on the diffractive structure or the optical path difference providing structure is The proportion of the light beam that is blocked and does not contribute to the formation of the condensed spot can be suppressed, and a reduction in the amount of light can be prevented.

According to a fourth aspect of the present invention, there is provided a laser light source, the optical system according to the first or second aspect for condensing a light beam emitted from the laser light source on an information recording surface of the optical disc, and information recording of the optical disc. An optical pickup device comprising: a light detection device that detects a light beam reflected by a surface.
According to a fifth aspect of the present invention, there is provided the optical pickup device according to the fourth aspect and a support member that supports the optical disc, and recording information on the optical disc and reproducing information recorded on the optical disc. Of these, an optical information recording / reproducing apparatus is characterized in that at least one of them can be executed.

  In this specification, an optical disk using a blue-violet semiconductor laser as a light source for recording / reproducing information is generally referred to as a “high-density optical disk”, and information is recorded by an objective optical system with NA of 0.85. In addition to a standard optical disc having a protective layer thickness of about 0.1 mm, information recording / reproduction is performed with an objective optical system with NA of 0.65, and the protective layer thickness is 0. It also includes an optical disc with a standard of about 6 mm. In addition to an optical disk having such a protective layer on its information recording surface, an optical disk having a protective film with a thickness of several to several tens of nanometers on the information recording surface, and the thickness of the protective layer or protective film is An optical disk that is 0 is also included. In this specification, the high-density optical disk includes a magneto-optical disk that uses a blue-violet semiconductor laser as a light source for recording / reproducing information. The present invention is preferably used for an optical pickup device used for recording and / or reproducing of these high-density optical discs, but is not limited thereto.

  In this specification, the term “aberration correction element” is used to suppress changes in wavefront aberration caused by temperature characteristics and / or chromatic aberration of other optical elements due to changes in environmental temperature and changes in wavelength of incident light flux. The “condensing lens” refers to an optical element having a function of condensing and imaging the light beam emitted from the “aberration correction element”. That is, the “aberration correction element” suppresses a change in wavefront aberration caused by the temperature characteristic and / or chromatic aberration of the “condensing lens” accompanying a change in environmental temperature or a change in wavelength of the incident light beam.

When an optical element composed of an “aberration correction element” and a “condensing lens” is used as an objective lens in an optical pickup device, the “aberration correction element” is disposed at a position close to the laser light source. The “condensing lens” is arranged at a position close to the optical disk.
Further, in the present specification, the “optical path difference providing structure” is a structure composed of a central region including the optical axis and a plurality of annular zones divided with fine steps on the outside of the central region, At a predetermined temperature, an optical path difference that is an integral multiple of the wavelength of the incident light beam is generated between wavefronts that pass through adjacent annular zones, and when the temperature changes from the predetermined temperature, along with a change in refractive index, This refers to a structure having such a characteristic that an optical path difference generated between wavefronts passing through adjacent annular zones deviates from an integral multiple of the wavelength of an incident light beam.

According to such a structure, by using the optical path difference providing structure for temperature characteristic correction, for example, even when only the refractive index changes due to a change in the environmental temperature and the wavelength of the incident light beam does not change, the temperature It is possible to effectively correct the characteristics.
A preferable structure of such an “optical path difference providing structure” is that the annular zone adjacent to the outside of the central region is formed by shifting in the optical axis direction so that the optical path length is shortened with respect to the central region, and is most effective. The annular zone at the radial position is formed by shifting in the optical axis direction so that the optical path length is longer with respect to the annular zone adjacent to the inner zone, and the annular zone at a position of 75% of the maximum effective diameter position is It is a structure formed by shifting in the optical axis direction so that the optical path length is shorter with respect to the annular zone adjacent to the inner side and the annular zone adjacent to the outer side.

Here, the “center region” refers to an optical region that includes the optical axis and is surrounded by a step at a position closest to the optical axis.
Further, in this specification, the “diffractive structure” is a structure composed of a central region including the optical axis and a plurality of annular zones divided by minute steps outside the central region, and the incident light flux Is a predetermined wavelength, an optical path difference that is an integral multiple of the wavelength of the incident light flux between wavefronts passing through the adjacent annular zones is generated by diffraction, and the wavelength of the incident light flux changes from the predetermined wavelength. In such a case, it indicates a structure having such a characteristic that an optical path difference generated between wavefronts passing through adjacent annular zones deviates from an integral multiple of the wavelength of the incident light beam.
A preferable structure of such a “diffractive structure” includes a structure in which the cross-sectional shape including the optical axis is a staircase shape in which the optical path length is increased so that the optical path length increases as the distance from the optical axis increases, and the optical axis is included. The cross-sectional shape is a sawtooth shape.

  In the optical system for the optical pickup device according to the first aspect, the optical path difference providing structure is formed on one optical surface of the aberration correcting element, and the diffractive structure is formed on the aberration correcting element. It is one of the preferable aspects that it is formed on the other optical surface.

  In the optical system for the optical pickup device according to the first aspect, it is also preferable that the optical path difference providing structure and the diffractive structure are formed on the same optical surface of the aberration correction element. One.

  In the optical system for the optical pickup device according to the first aspect, it is preferable that the diffractive structure is formed on an optical surface on the condenser lens side of the aberration correction element.

  According to such an aspect, the diffractive structure is formed on the condenser lens side of the aberration correction element, that is, on the exit surface side of the aberration correction element. Here, the incident light beam to the aberration correction element is diffracted by the diffractive structure, so that its path is changed in a direction closer to the optical axis, so that the light beam travels on the incident surface of the aberration correction element. Comparing the exit position on the exit surface of the light beam that receives the diffraction effect on the exit surface and the exit position on the exit surface, the light beam that undergoes the diffractive action on the exit surface is emitted from a position farther away from the optical axis. Even when the light is emitted from the exit surface of the condenser lens, the light beam subjected to the diffracting action on the exit surface of the aberration correction element is emitted from a position further away from the optical axis. This makes it possible to increase the distance (working distance) from the exit surface of the condensing lens to the optical disc surface, and prevents the optical disc surface from interfering with the exit surface of the objective lens when the optical pickup device is driven. it can.

  In the optical system for the optical pickup device according to the first aspect, it is preferable that the optical path difference providing structure is formed on an optical surface whose refractive power in the paraxial axis of the aberration correction element is zero.

  According to such an aspect, since the optical path difference providing structure is formed on a plane, the path of the light flux incident on the optical path difference providing structure is compared with the case where the optical path difference providing structure is formed on a spherical surface. The proportion of the light beam that is blocked and does not contribute to the formation of the condensed spot can be suppressed, and a reduction in the amount of light can be prevented.

  In the optical system for the optical pickup device according to the first aspect, it is preferable that the diffractive structure is formed on an optical surface whose refractive power in the paraxial axis of the aberration correction element is zero.

  According to such an aspect, since the diffractive structure is formed on a plane, the path of the light beam incident on the diffractive structure is blocked and the focused spot compared to the case where the diffractive structure is formed on a spherical surface. The ratio of the luminous flux that does not contribute to the formation of the light can be suppressed, and the decrease in the amount of light can be prevented.

In the optical system for the optical pickup device according to the second aspect, it is preferable that the refractive power P _R (mm ⁻¹ ) on the paraxial axis of the aberration correction element satisfies the following expression (2).
P _R > 0 (2)

According to such an aspect, since the refractive power P _R (mm ⁻¹ ) on the paraxial axis of the aberration correction element is greater than 0, the light beam that has received the refractive power is emitted from the aberration correction element as convergent light.
When the refractive index of the aberration correction element decreases as the environmental temperature increases, the refractive power of the aberration correction element decreases, so the degree of convergence of the light beam emitted from the aberration correction element decreases the refractive index. Smaller than before. This is equivalent to changing the magnification of the condensing lens in a direction in which the magnification of the condensing lens decreases as the environmental temperature rises. Therefore, the spherical aberration changes in the direction of insufficient correction in the condensing lens due to this magnification change. Accordingly, it is possible to cancel the spherical aberration that changes in the overcorrection direction due to a decrease in the refractive index of the condenser lens as the environmental temperature increases.
As described above, the correction amount of the temperature characteristic of the condensing lens by the optical path difference providing structure can be reduced by such an aspect, so the number of annular zones can be reduced and the width in the direction perpendicular to the optical axis of the annular zones can be reduced. The effect of shortening the time required for mold processing and improving the transferability of the optical path difference providing structure to the plastic material during molding can be obtained.

  In the optical system for an optical pickup device according to the second aspect, it is preferable that the aberration correction optical element has at least one optical surface on which a diffractive structure is formed.

  In the optical system for an optical pickup device according to the present invention, it is preferable that the diffractive structure corrects at least axial chromatic aberration generated in the condenser lens with a change in wavelength of an incident light beam.

Furthermore, the optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +. Obtained by combining the paraxial diffraction power P _D (mm ⁻¹ ) defined by P _D = −2 · b ₂ , the aberration correction element, and the condenser lens. The resultant paraxial combined power P _T (mm ⁻¹ ) preferably satisfies the following expression (3).
0.03 ≦ P _D / P _T ≦ 0.15 (3)
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient

  In the optical system for an optical pickup device according to the present invention, it is preferable that the diffractive structure corrects at least a spherical aberration change generated in the condenser lens with a change in wavelength of an incident light beam.

Furthermore, the optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +. When the optical path difference is expressed, it is preferable that at least one optical path difference function coefficient other than the second order has a non-zero value.
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient

Further, the diffractive structure is composed of a central region including the optical axis and a plurality of annular zones divided with a fine step outside the central region, and the diffraction structure has a direction perpendicular to the optical axis of the annular zone at the maximum effective diameter position. It is preferable that the width P _f (mm) and the width P _h (mm) in the direction perpendicular to the optical axis of the annular zone at the position of 50% of the maximum effective diameter satisfy the following expression (4).
0 <| P _h / P _f −2 | <5 (4)

  In the optical system for an optical pickup device of the present invention, the diffractive structure is composed of a central region including the optical axis and a plurality of annular zones divided with fine steps outside the central region, and a cross section including the optical axis. It is one of the preferred embodiments that the shape is a stepped shape.

  In the optical system for an optical pickup device of the present invention, the diffractive structure is composed of a central region including the optical axis and a plurality of annular zones divided with fine steps outside the central region, and a cross section including the optical axis. It is also a preferable aspect that the shape is a sawtooth shape.

In the optical system for an optical pickup device of the present invention, the wavelength of the laser light source of the optical pickup device on which the objective optical system is mounted is λ (nm), and the aperture is obtained by combining the aberration correction element and the condenser lens. When the number is NA, it is preferable to satisfy the following expressions (5) and (6).
λ ≦ 450nm (5)
0.60 ≦ NA ≦ 0.95 (6)
Furthermore, in the optical system for an optical pickup device of the present invention, when the numerical aperture obtained by combining the aberration correcting element and the condenser lens is NA and the focal length is f (mm), the following (9 ) To (10) are preferably satisfied.
0.75 ≦ NA ≦ 0.90 (9)
0.7 <f <2.5 (10)

  In the optical system for an optical pickup device of the present invention, it is preferable that the aberration correction element and the condenser lens are designed so that aberrations can be evaluated independently.

  In the optical system for an optical pickup device of the present invention, each of the aberration correction element and the condenser lens has a flange portion formed around the optical function portion, and the flange portion of the condenser lens It is preferable that a part and a part of the flange portion of the aberration correction element are joined.

  In the optical system for an optical pickup device of the present invention, the aberration correction element and the condenser lens are bonded via a bonding member.

  In the aberration correction element for an optical pickup device according to the third aspect, the condensing lens is a single lens made of plastic and includes one group, and the optical path difference providing structure includes an optical axis. It consists of a central region and a plurality of annular zones divided with fine steps outside the central region, and when the environmental temperature rises, the optical path difference is such that the spherical aberration of the transmitted wavefront changes in the direction of insufficient correction. It is preferable to correct the spherical aberration change in the overcorrection direction generated in the condenser lens by being generated between the adjacent annular zones.

  According to such an aspect, by using the optical path difference providing structure for temperature characteristic correction, for example, even when only the refractive index changes due to a change in the environmental temperature and the wavelength of the incident light beam does not change, the temperature It is possible to effectively correct the characteristics.

  In the aberration correction element according to the third aspect, in the optical path difference providing structure, the annular zone adjacent to the outside of the central region is arranged in the optical axis direction so that the optical path length is shorter than the central region. The annular zone formed at the maximum effective diameter position is formed by shifting in the optical axis direction so that the optical path length is longer than the annular zone adjacent to the inside at the maximum effective diameter position. The annular zone at the position of% is formed so as to be shifted in the optical axis direction so that the optical path length becomes shorter with respect to the annular zone adjacent to the inside and the annular zone adjacent to the outside. preferable.

  In the aberration correction element for an optical pickup device according to the third aspect, it is preferable that the diffractive structure corrects at least axial chromatic aberration generated in the condenser lens in accordance with a change in wavelength of an incident light beam.

Furthermore, the power P _M (mm ⁻¹ ) of the aberration correction element for a predetermined wavelength of 450 nm or less and the power P _L (mm) of the aberration correction element for a wavelength longer than the predetermined wavelength by 10 nm. ⁻¹ ) and the power P _S (mm ⁻¹ ) on the paraxial axis of the aberration correction element for a wavelength shorter by 10 nm than the predetermined wavelength preferably satisfy the following expression (7).
P _S <P _M <P _L (7)

  In the aberration correction element for an optical pickup device according to the third aspect, it is preferable that the diffractive structure corrects a spherical aberration change generated in the condenser lens in accordance with a wavelength change of an incident light beam.

Furthermore, the optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +. When the optical path difference is expressed, it is preferable that at least one optical path difference function coefficient other than the second order has a non-zero value.
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient

The diffractive structure is composed of a central region including the optical axis and a plurality of annular zones divided with fine steps outside the central region, and is perpendicular to the optical axis of the annular zone at the maximum effective diameter position. More preferably, the width Pf (mm) in the direction and the width Ph (mm) in the direction perpendicular to the optical axis of the annular zone at the position of 50% of the maximum effective diameter satisfy the following expression (8).
0 <| P _h / P _f −2 | <5 (8)

  In the aberration correction element for an optical pickup device according to the third aspect, the optical surface on which the optical path difference providing structure is formed is an optical surface on the laser light source side, and the optical surface on which the diffraction structure is formed is The optical surface on the condenser lens side is preferable.

  In the aberration correction element for an optical pickup device according to the third aspect, it is preferable that the optical path difference providing structure is formed on an optical surface whose refractive power in the paraxial axis of the aberration correction element is zero. .

  According to such an aspect, since the optical path difference providing structure is formed on a plane, the path of the light flux incident on the optical path difference providing structure is compared with the case where the optical path difference providing structure is formed on a spherical surface. The proportion of the light beam that is blocked and does not contribute to the formation of the condensed spot can be suppressed, and a reduction in the amount of light can be prevented.

  In the aberration correction element for an optical pickup device according to the third aspect, the diffractive structure is formed on an optical surface whose refractive power in the paraxial axis of the aberration correction element is zero. .

  According to such an aspect, since the diffractive structure is formed on a plane, the path of the light beam incident on the diffractive structure is blocked and the focused spot compared to the case where the diffractive structure is formed on a spherical surface. The ratio of the luminous flux that does not contribute to the formation of the light can be suppressed, and the decrease in the amount of light can be prevented.

  In the aberration correction element for an optical pickup device according to the third aspect, the diffractive structure includes a central region including the optical axis and a plurality of annular zones that are divided with a minute step outside the central region. In one preferred embodiment, the cross-sectional shape including the optical axis is a stepped shape.

  In the aberration correction element for an optical pickup device according to the third aspect, the diffractive structure includes a central region including the optical axis and a plurality of annular zones that are divided with a minute step outside the central region. In another preferred embodiment, the cross-sectional shape including the optical axis is a sawtooth shape.

  According to another aspect of the present invention, in the optical element for an optical pickup device including a plastic aberration correcting element and a condenser lens for forming an image of a light beam emitted from the aberration correcting element, the aberration is The correction element has at least one optical surface on which a diffractive structure is formed, and the condensing lens has one optical element on each group having at least one optical surface on which an optical path difference providing structure is formed and one aspherical surface. It is the single lens of a structure, It is characterized by the above-mentioned.

In this aspect, the diffraction structure for diffracting the incident light beam is formed in the aberration correction element, and the optical path difference providing structure is formed in the condenser lens.
Here, spherical aberration occurs in the light beam passing through the optical path difference providing structure due to a change in the environmental temperature, but when the aberration correcting element and the condensing lens are assembled in a state where the axial deviation occurs. Is incident on the condensing lens in a state where spherical aberration occurs, and the light beam emitted from the condensing lens causes coma aberration at the condensing spot. Therefore, by forming the optical path difference providing structure on the condensing lens, even if the aberration correction element and the condensing lens are assembled in a state where the axial deviation occurs, the coma aberration is generated. Can be prevented.

The condensing lens used in the present invention is made of plastic for condensing the convergent light beam emitted from the convergent light beam generating means for converting the light beam emitted from the laser light source into a converged light beam on the information recording surface of the optical disk. When the wavefront aberration within the diffraction limit is minimum, the magnification is m, the lens thickness on the optical axis is d (mm), the focal length is f (mm), and the numerical aperture is NA. In this case, it is preferable that the lens is a single lens having one lens element per group that satisfies the following expressions (9) to (11) and has at least one aspheric surface.
0 <m <1 (9)
0.75 <d / f <1.5 (10)
0.75 ≦ NA ≦ 0.95 (11)

In this condensing lens, it is preferable that the convergent light beam generation means is the aberration correction element according to the third aspect.
As a plastic material used for the condensing lens and aberration correction element of the present invention, performance such as transparency, heat resistance, low water absorption, birefringence, light resistance, etc. required as an optical element for an optical pickup device It does not specifically limit as long as it satisfies. As a plastic material preferably used in the present invention, it is particularly preferable that the plastic material is excellent in light resistance to a low wavelength laser such as a blue-violet laser.
The structure of the plastic used for the condenser lens and aberration correction element of the present invention is not particularly limited, but from the viewpoint of achieving the above performance, a polymer having an alicyclic hydrocarbon structure is preferable, and such a polymer is more preferable. Preferred examples include hydrides of norbornene-based ring-opening polymers, hydrides of norbornene-α olefin copolymers, hydrides of styrene-butadiene block copolymers, and the like. In particular, a hydride of a styrene-butadiene block copolymer is preferable from the viewpoint of light resistance to a low wavelength laser such as a blue-violet laser. As such a polymer, a hydride of a styrene-butadiene block copolymer described in JP-A No. 2002-148401 is particularly preferably used.

  According to the present invention, an optical system as a light collecting application used for an optical pickup device that is lightweight and can secure a sufficient working distance, and that can correct temperature characteristics and chromatic aberration, and further, correction of temperature characteristics is particularly good. An optical system for an optical pickup device, an optical pickup device using the optical system, an optical information recording / reproducing device, and an aberration correction element used in the optical system can be obtained.

Embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram of an optical pickup device 10 according to the present embodiment. The optical pickup device 10 includes a blue-violet semiconductor laser 11 as a light source, a polarizing beam splitter 12, a quarter wavelength plate 16, a collimator 13, and an aperture. 18, an objective lens (also referred to as an objective optical system) 30, a focusing / tracking biaxial actuator 19, a cylindrical lens 17, a concave lens 14, and a photodetector 15.
The divergent light beam emitted from the blue-violet semiconductor laser 11 passes through the polarization beam splitter 12, passes through the quarter-wave plate 16 and the collimating lens 13, becomes a circularly polarized parallel light beam, and then the diameter of the light beam is reduced by the diaphragm 18. It is regulated and becomes a spot formed on the information recording surface 21 by the objective lens 30 via the protective layer 22 of the high-density optical disc 20.

The reflected light beam modulated by the information pits on the information recording surface 21 becomes a converged light beam again through the objective lens 30, the diaphragm 18 and the collimating lens 13, and then linearly polarized light transmitted through the quarter-wave plate 16. Astigmatism is given by being reflected by the polarization beam splitter 12, passing through the cylindrical lens 17 and the concave lens 14, and converges on the photodetector 15. Then, information recorded on the high-density optical disk 20 can be read using the output signal of the photodetector 15.
In the present embodiment, the objective lens 30 is an infinite conjugate type that condenses the parallel light flux from the collimating lens 13 on the information recording surface 21 of the high-density optical disc 20. A finite conjugate type in which the divergent light beam is condensed on the information recording surface 21 of the high-density optical disk 20 may be used.

The objective lens 30 has a function of condensing the laser light emitted from the blue-violet semiconductor laser 11 on the information recording surface 21 via the protective layer 22 of the high-density optical disc 20 (for example, a Blu-ray disc). A plastic aberration correction element 40 disposed on the semiconductor laser 11 side and a plastic condenser lens 50 disposed on the high-density optical disk 20 side are included. The NA obtained by combining the aberration correction element 40 and the condenser lens 50 is 0.85.
Further, the aberration correction element 40 and the condensing lens 50 are flange portions formed integrally with the optical function portion at the peripheral side of the optical function portion (region through which the light beam from the blue-violet semiconductor laser 11 passes). 43 and the flange portion 53, and the flange portions 43 and 53 are joined to each other, so that the aberration correction element 40 and the condenser lens 50 perform focusing and tracking together.

As shown in FIG. 2, the objective optical system 30 includes a plastic aberration correction element 40 and a plastic condensing lens 50 arranged in order from the object side.
The condenser lens 50 is a refracting single lens of one group and one lens having at least one aspherical surface (two surfaces of the incident surface 51 and the light exiting surface 52 in the present embodiment).
As shown in FIG. 3, the aberration correction element 40 is a so-called parallel plate optical element whose optical surfaces (incident surface and outgoing surface) have a planar shape near the optical axis. A difference providing structure 60 is formed, and a diffractive structure 70 is formed on the exit surface 42.

As shown in FIG. 3, the optical path difference providing structure 60 includes a central region 61 including a plane that includes the optical axis L and is orthogonal to the optical axis L, and a plurality of annular zones 62 formed outside the central region 61. Consists of
Each annular zone 62 is formed concentrically around the optical axis L, and an inner peripheral edge portion and an outer peripheral edge portion are divided by a fine step 63 extending in the optical axis L direction.
Among the plurality of annular zones 62, the annular zone (annular zone 62 a) adjacent to the outside of the central region 61 has an optical path length shorter than that of the central region 61, that is, along the optical axis L direction. The annular zone (annular zone 62b) at a position corresponding to the maximum effective diameter D of the optical surface is formed so that the optical path length is longer than the annular zone 62 adjacent to the inner zone, that is, The annular zone (annular zone 62c) at a position corresponding to 75% of the maximum effective diameter D is formed by shifting to the light source 11 side along the optical axis L direction. The optical path length with respect to the annular zone 62 adjacent to the optical disc 20 is changed to the optical disc 20 side along the optical axis L direction.

Then, when the light beam emitted from the light source 11 passes through each annular zone 62 on the incident surface 41 of the aberration correction element 40, it corresponds to the amount of displacement of the annular zone 62 (the length of the step 63 in the optical axis L direction). Thus, a phase difference is generated between the light beams.
When the ambient temperature is not changed, an optical path difference is given to each light flux so that the phases of the light fluxes that have passed through the respective annular zones 62 are substantially aligned on the information recording surface 21, and the environmental temperature rises. In this case, an optical path difference that causes the spherical aberration of the transmitted wavefront to change in the direction of insufficient correction is generated between the adjacent annular zones, thereby correcting the change in spherical aberration in the overcorrection direction that occurs in the condenser lens 50. It is supposed to be.

The diffractive structure 70 includes a central region 71 including the optical axis L, and a plurality of annular zones (diffractive annular zones 72) divided by fine steps 72a in the optical axis L direction outside the central region 71. The cross-sectional shape including the optical axis L of the band 72 is formed in a sawtooth shape.
Then, it is added to the wavefront transmitted through the diffraction structure 70 by the optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +. Is obtained by combining the paraxial diffraction power P _D (mm ⁻¹ ) defined by P _D = −2 · b ₂ , the aberration correction element 40 and the condenser lens 50. When the resultant paraxial combined power P _T (mm ⁻¹ ) satisfies the expression (3), the diffractive structure 70 corrects the longitudinal chromatic aberration generated in the condenser lens 50 in accordance with the wavelength change of the incident light beam. It has become.
0.03 ≦ P _D / P _T ≦ 0.15 (3)
Where h is the height (mm) from the optical axis L, b ₂ , b ₄ , b ₆ ,... Are secondary, fourth, sixth,. Difference function coefficient,

Further, by having a non-zero value for at least one optical path difference function coefficient other than the second order, the diffractive structure 70 corrects a change in spherical aberration generated in the condenser lens 50 with a change in wavelength of the incident light beam. It has become.
Further, the width P _f (mm) in the direction perpendicular to the optical axis L of the annular zone 62 at the position of the maximum effective diameter D and the direction perpendicular to the optical axis L of the annular zone 62 at a position of 50% of the maximum effective diameter D. The diffractive structure 70 is designed so that the width P _h (mm) of the lens satisfies the equation (4).
0 <| P _h / P _f −2 | <5 (4)
Further, the paraxial power P ₁ (mm ⁻¹ ) of the aberration correction element 40 satisfies Expression (1), and the refractive power P _R (mm ⁻¹ ) of the aberration correction element 40 along the paraxial satisfies Expression (2). Thus, the aberration correction element 40 is designed.
P ₁ > 0 (1)
P _R > 0 (2)

Since the power P ₁ (mm ⁻¹ ) on the paraxial axis of the aberration correction element is greater than 0, the light beam emitted from the aberration correction element is a convergent light beam. This is because the magnification m of the condenser lens is m> 0. Is equivalent to In general, a numerical aperture NA∞ (hereinafter referred to as converted NA) converted to infinite luminous flux incidence for a condensing lens with a numerical aperture NA of a finite conjugate type (m ≠ 0) is NA∞ = NA · (1−m ). Therefore, in a condensing lens with a magnification m of m> 0, the converted NA can be reduced, so that the change in spherical aberration due to the temperature characteristic of the condensing lens that increases in proportion to NA ⁴ can be reduced.

Further, since the refractive power P _R (mm ⁻¹ ) on the paraxial axis of the aberration correction element is greater than 0, the light beam that has received the refractive power is emitted from the aberration correction element as convergent light.
When the refractive index of the aberration correction element decreases as the environmental temperature increases, the refractive power of the aberration correction element decreases, so the degree of convergence of the light beam emitted from the aberration correction element decreases the refractive index. Smaller than before. This is equivalent to changing the magnification of the condensing lens in a direction in which the magnification of the condensing lens decreases as the environmental temperature rises. Therefore, the spherical aberration changes in the direction of insufficient correction in the condensing lens due to this magnification change. Accordingly, it is possible to cancel the spherical aberration that changes in the overcorrection direction due to a decrease in the refractive index of the condenser lens as the environmental temperature increases.
As described above, the correction amount of the temperature characteristic of the condenser lens by the optical path difference providing structure can be reduced, so the number of annular zones can be reduced, the width in the direction perpendicular to the optical axis of the annular zones can be increased, and mold processing can be performed. The effects of shortening the time required and improving the transferability of the optical path difference providing structure to the plastic material during molding can be obtained.

As described above, according to the objective optical system 30 shown in the present embodiment, the diffraction structure 70 is used for correcting chromatic aberration, and the optical path difference providing structure 60 is used for correcting temperature characteristics. Even when only the refractive index changes and the wavelength of the emitted light beam does not change, the optical path difference providing structure 60 can correct the temperature characteristics.
In addition, since the aberration correction element 40 and the condensing lens 50 constituting the objective optical system 30 are molded from plastic, it is possible to reduce the weight of the high NA objective optical system 30 used for the high-density optical disc 20 and to form a mold. It can be produced at low cost by the injection molding used. Further, by providing the condensing lens 50 disposed on the optical disc side exclusively with the refractive power with respect to the incident light beam, a large working distance can be secured, and interference between the objective lens 30 and the optical disc 20 can be prevented. . Further, since a structure having fine steps such as the diffraction structure 70 and the optical path difference providing structure 60 is formed on the optical surface of the aberration correction element 40, the light flux incident on the diffraction structure 70 and the optical path difference providing structure 60 is reduced. Among them, the ratio of the light flux that does not contribute to the formation of the condensed spot can be suppressed because the path is blocked, and the decrease in the light amount can be prevented.

In addition, since the optical path difference providing structure 60 and the diffractive structure 70 are formed on a plane (the incident surface 41 and the exit surface 42 of the aberration correction element 40), the optical path difference providing structure 60 and the diffractive structure 70 are formed on a spherical surface. In comparison, among the light beams incident on the optical path difference providing structure 60 and the diffractive structure 70, the ratio of the light beams whose path is blocked and does not contribute to the formation of the condensed spot can be suppressed, and the decrease in the light amount can be prevented.
In addition, although illustration is abbreviate | omitted, it is good also as using the member (joining member) which can connect the aberration correction element 40 and the condensing lens 50 integrally, without providing the flange parts 43 and 53. FIG.
Although not shown in the drawings, information recording on the optical disc 20 is performed by combining the optical pickup device 10 described above with a rotation drive device that rotatably holds the optical disc 20 and a control device that controls driving of these various devices. In addition, an optical information recording / reproducing apparatus capable of executing at least one of reproduction of information recorded on the optical disc 20 can be obtained.

Further, in the present embodiment, the optical pickup device 10 includes one laser light source 11, and for one type of standard (or recording density) optical disc (in the present embodiment, the high-density optical disc 20). However, the present invention is not limited to this, and a configuration of an optical pickup device having compatibility between two or more types of optical discs using two or more light sources may be used.
In the following, when two or more light sources are used to form an optical pickup device having compatibility between two or more types of optical discs, 2 is a preferred form of the diffractive structure 70 of the aberration correction element 40. The types of diffraction structures will be described. First, the depth of the step in the optical axis direction of the diffractive structure 70 is set so that the diffraction orders of the generated diffracted light are different from each other with respect to the incident light beam having the wavelength λ1 and the incident light beam having the wavelength λ2 (λ1> λ2). Is to design. As a result, it is possible to correct chromatic aberration optimal for each wavelength region and to obtain sufficient diffraction efficiency for each wavelength region.

The other is that the diffractive structure 70 is configured so that one of the incident light flux of wavelength λ1 and the incident light flux of wavelength λ2 (λ1> λ2) is given an optical path difference and the other is not substantially given an optical path difference. In other words, it is a structure having wavelength selectivity in the diffraction action.
Specifically, as shown in FIGS. 4A and 4B, the diffractive structure 80 is formed on a plurality of annular zones 81 centered on the optical axis L and the optical surfaces of these annular zones 81. It comprises a step-like discontinuous surface 82 along the optical axis L direction. FIG. 4 shows a case where the diffractive structure 80 is formed on the incident surface 41 of the aberration correction element 40. The annular zone 81 is a sawtooth-shaped discontinuous surface having a substantial inclination with respect to the optical surface S having a predetermined shape that is rotationally symmetric with respect to the optical axis L. Further, on the optical surface of each annular zone 81, A step-like discontinuous surface 82 is formed along the optical axis L that gives a predetermined optical path difference to the light flux passing through the annular zone 81. Here, the optical surface S having a predetermined shape may be a spherical surface or an aspherical surface. Further, it may be an optical surface whose refractive power in the paraxial is 0, or an optical surface whose refractive power in the paraxial is not 0.

The depth d1 (length in the optical axis L direction) of each discontinuous surface 82 is a value represented by j × λ / (n−1), where n is a refractive index for a light beam having a predetermined wavelength λ. (Where j is a positive integer), and a light beam having a wavelength λ that passes through one discontinuous surface 82 and a light beam having a wavelength λ that passes through a discontinuous surface 82 adjacent thereto. In the meantime, the optical path difference corresponding to approximately one wavelength (λ) is generated, and the length is designed so that the wavefront does not shift.
Further, the shape of each discontinuous surface 82 is obtained by dividing the surface shape of the serrated annular zone 81 indicated by a two-dot chain line in FIG. It approximates the shape translated in the direction.
When the diffractive structure has such a configuration, an optical path difference is given to only one of the incident light flux of wavelength λ1 and the incident light flux of wavelength λ2 (λ1> λ2). It is possible to correct a change in spherical aberration caused by a difference in thickness, a chromatic aberration in a blue-violet region, a temperature characteristic in a blue-violet region, and the like.

Further, the optical path difference providing structure 60 may be formed on the exit surface 42 side of the aberration correction element 40, and the diffractive structure 70 may be formed on the incident surface 41 side of the aberration correction element 40. In this case, the light beam subjected to the diffracting action on the exit surface 42 is emitted from a position far from (distant from) the optical axis L on the exit surface 42, and further away from the optical axis L on the exit surface 52 of the condenser lens 50. Since the light is emitted from the position, the distance (working distance) from the light emission surface 52 of the condenser lens 50 to the surface of the optical disk 20 can be increased, and the surface of the optical disk 20 is collected when the optical pickup device 10 is driven. The situation of interfering with the exit surface 52 of the optical lens 50 can be prevented beforehand.
Further, the optical path difference providing structure 60 and the diffractive structure 70 may be formed on the same optical surface of the aberration correction element 40.

  Alternatively, the diffraction structure 70 may be formed in the aberration correction element 40, and the optical path difference providing structure 60 may be formed in the condenser lens 50. Here, when the light beam passing through the optical path difference providing structure 60 normally has a spherical aberration due to a change in the environmental temperature, the aberration correction element 40 and the condenser lens 50 are assembled in a state in which an axial deviation occurs. In this case, the light beam incident on the condensing lens 50 in a state where spherical aberration is generated and emitted from the condensing lens 50 causes coma aberration at the condensing spot, but the optical path difference providing structure 60 is added to the condensing lens 50. The formation of the coma aberration can be prevented even if the aberration correction element 40 and the condensing lens 50 are assembled in a state in which an axial deviation occurs.

Next, examples of the above-described objective optical system will be described.
[Example 1]
The objective optical system in this example has a focal length f = 1.76 mm, a paraxial power PT = 0.568 mm ⁻¹ , a numerical aperture NA = 0.85, a design wavelength λ = 405 nm, and a design reference temperature T = 25 ° C. Is set.
The aberration correction element is a plastic lens having both surfaces flat, and an optical path difference providing structure is formed on the first surface (incident surface), and a diffractive structure (diffraction ring zone is sawtooth shape) is formed on the second surface (exit surface). ing. Further, paraxial power (as the sum of the power P _D of the diffractive and power P _R of refraction) P ₁ = 0.05mm ^-1, paraxial refractive power P _R = 0 mm ^-1, paraxial diffractive power P _D = 0.05 mm ⁻¹ , ring width P _f = 5.4 μm at the maximum effective diameter position, ring width P _h = 10.8 μm at the position of 50% of the maximum effective diameter, and wavefront aberration of the aberration correction element alone ( 405 nm, 25 ° C.) = 0.014λrms, λ = 405 nm paraxial power P _M : 0.05 mm ⁻¹ , λ = 410 nm luminous flux paraxial power P _L : 0.0507 mm ⁻¹ , λ = 400 nm The paraxial power P _{S with} respect to the luminous flux is set to 0.0494 mm ⁻¹ , and the refractive index change rate with temperature change = −9.0 × E−5.

The condensing lens is a plastic lens whose both surfaces (third surface and fourth surface) are aspherical, focal length = 1.89 mm, magnification m = 0.088, numerical aperture NA = 0.85, condensing. The wavefront aberration (405 nm, 25 ° C.) of the lens alone is set to 0.000 λrms, and the refractive index change rate with temperature change is set to −10.8 × E-5.
Table 1 shows the curvature radius r of the aberration correction element and the condenser lens, the position d in the optical axis L direction from the i-th surface to the (i + 1) -th surface, and the refractive index N for the light flux having the wavelength indicated by the subscript.

また、集光レンズの非球面は、その面の頂点に接する平面からの変形量をＸ（ｍｍ）、光軸に垂直な方向の高さをｈ（ｍｍ）、曲率半径をｒ（ｍｍ）とするとき、次の数１に表２中の係数を代入した数式で表される。ただし、κを円錐係数、Ａ_2iを非球面係数とする。

In addition, the aspherical surface of the condenser lens has a deformation amount from a plane in contact with the apex of the surface as X (mm), a height perpendicular to the optical axis as h (mm), and a radius of curvature as r (mm). Is expressed by a mathematical formula in which the coefficient in Table 2 is substituted into the following formula 1. Here, κ is a conic coefficient, and A _2i is an aspheric coefficient.

Further, the diffractive structure of the aberration correction element is represented by an optical path difference added to the transmitted wavefront by this diffractive structure. This optical path difference is shown in Table 3 in the optical path difference function Φ _b (mm) defined by the following equation 2 where the height in the direction perpendicular to the optical axis is h (mm) and b _2j is the optical path difference function coefficient. It is expressed by a mathematical formula in which the coefficient is substituted.

Further, the shape of each ring zone of the optical path difference providing structure is shown in Table 4.

  In Table 4, i represents the center region of the optical path difference providing structure and the number of each annular zone. The central region including the optical axis is i = 1, and the annular zone adjacent to the outside of the central region (the direction away from the optical axis). I = 2, and the annular zone adjacent to the outside is i = 3,. That is, in the aberration correction element of the objective optical system in the present embodiment, nine annular zones are formed outside the central region. Hi-1 and hi represent the start point height and end point height of the center region and each ring zone, respectively. mid represents the distance in the optical axis direction of each annular zone with respect to the center region (positive when displaced from the light source toward the optical disc). For example, the annular zone (i = 2) adjacent to the outside of the central region is shifted 3.86 μm toward the optical disc side with respect to the central region, and the outermost annular zone (i = 10) is relative to the central region. And 3.86 μm is shifted to the light source side. Further, mi represents an optical path difference given to a light beam transmitted through each annular zone with respect to a light beam transmitted through the central region at a design reference temperature T = 25 ° C., for example, a ring adjacent to the outside of the central region. The light flux that passes through the band (i = 2, m = 5) is given an optical path difference of 5λ (where λ = 405 nm) with respect to the light flux that passes through the central region. The phase is delayed by 5 (rad), and the light beam transmitted through the outermost annular zone (i = 10, m = −5) is −5λ (provided that λ = 405 nm) with respect to the light beam transmitted through the central region. ), The phase advances by 2π × 5 (rad) in terms of a phase difference.

FIG. 5 shows the wavefront aberration when the temperature of the objective lens in this embodiment increases from the design reference temperature of 25 ° C. to 55 ° C. along with the environmental temperature change, the horizontal axis shows the pupil coordinate h, and the vertical axis. The axis represents wavefront aberration (unit: λ). FIG. 6 shows the wavefront aberration when evaluated with only the condenser lens.
At 55 ° C., the rms value of the wavefront aberration is 0.075 λrms with only the condensing lens, whereas it is 0.010 λrms when the aberration correcting element is combined with the condensing lens. 5 and 6, the wavelength change of the blue-violet semiconductor laser accompanying the temperature change is also considered, and the change rate is assumed to be +0.05 nm / ° C.

  Further, when the wavelength change of the blue-violet semiconductor laser due to the temperature change is not taken into consideration, that is, when only the refractive index change of the condenser lens and the aberration correcting element is taken into consideration, at 55 ° C., the rms of the wavefront aberration with the condenser lens alone. While the value was 0.070 λrms, when an aberration correction element was combined with the condenser lens, the value was 0.022 λrms. As a result, the objective lens of the present invention is an optical element having excellent temperature characteristics despite having a plastic single lens with NA of 0.85, and in particular, providing an optical path difference for correcting the temperature characteristics. Since the structure is used, it can be seen that a good temperature characteristic correction effect is obtained regardless of whether or not the wavelength of the blue-violet semiconductor laser changes as the temperature rises.

FIG. 7 shows the wavefront aberration when the wavelength of the incident light beam with respect to the objective lens in this embodiment is increased by 1 nm from the design wavelength of 405 nm by the mode hopping of the blue-violet semiconductor laser, and the horizontal axis shows the pupil coordinate h. The vertical axis represents wavefront aberration (unit: λ). FIG. 8 shows the wavefront aberration when evaluated with only the condenser lens.
At a wavelength of 406 nm, the rms value of the wavefront aberration is 0.112 λrms when only the condensing lens is used, whereas it is 0.022 λrms when the aberration correcting element is combined with the condensing lens. In FIGS. 7 and 8, the focus positions of the objective optical system and the condenser lens are fixed to the focus position at 405 nm.
As a result, it can be seen that the objective lens of the present invention is well corrected for chromatic aberration by the action of the diffractive structure.

[Example 2]
The objective optical system in this example has a focal length f = 1.00 mm, paraxial power PT = 1.002 mm ⁻¹ , numerical aperture NA = 0.85, design wavelength λ = 408 nm, and design reference temperature T = 35 ° C. Is set.
The aberration correction element is a plastic lens whose first surface (incident surface) is flat and second surface (exit surface) is an aspheric surface. The first surface has an optical path difference providing structure, and the second surface has a diffractive structure (diffraction). An annular zone is a sawtooth shape). In addition, paraxial power P ₁ = 0.116 mm ⁻¹ , paraxial refraction power P _R = 0.051 mm ⁻¹ , paraxial diffraction power P _D = 0.064 mm ⁻¹ , and ring width P at the maximum effective diameter position. _f = 12.2 μm, annular width P _h at 50% position of maximum effective diameter = 53.8 μm, wavefront aberration (408 nm, 35 ° C.) of aberration correction element alone = 0.003 λrms, λ = 408 nm paraxial power ^{_{P M: 0.1156mm -1, λ =}} 413nm paraxial power for the light flux ^{_{P L: 0.1163mm -1, λ =}} 403nm paraxial power for the light flux P _S: 0.1149mm ^-1, temperature change The change rate of the refractive index associated with is set to -1.0 × E-4.

The condensing lens is a plastic lens whose both surfaces (third surface and fourth surface) are aspherical, focal length = 1.09 mm, magnification m = 0.115, numerical aperture NA = 0.85, condensing. The wavefront aberration (408 nm, 35 ° C.) of the lens alone is set to 0.001 λrms, and the refractive index change rate with temperature change is set to −1.0 × E−4.
Table 5 shows paraxial data of the aberration correction element and the condenser lens.

Further, the aspherical surfaces of the aberration correction element and the condenser lens are expressed by mathematical formulas obtained by substituting the coefficients in Table 6 into Equation 1 above. The aspherical shape of the second surface is perpendicular to the optical axis L, and as the distance from the optical axis increases, the amount of shift in the optical axis L direction with respect to the spherical surface represented by the radius of curvature r increases. It has become.

The diffractive structure of the aberration correction element is represented by an optical path difference added to the transmitted wavefront by this diffractive structure. The optical path difference is defined by substituting the coefficient in Table 7 into the above equation (2). _b Expressed in mm.

Table 8 shows the shape of each ring zone of the optical path difference providing structure.

In the aberration correction element of the objective optical system in the present embodiment, six annular zones are formed outside the central region. In addition, mi represents an optical path difference given to a light beam transmitted through each annular zone with respect to a light beam transmitted through the central region at a design reference temperature T = 35 ° C., for example, a ring adjacent to the outside of the central region. The light flux that passes through the band (i = 2, m = 3) is given an optical path difference of 3λ (where λ = 408 nm) with respect to the light flux that passes through the central region. The phase is delayed by 3 (rad), and the light beam transmitted through the outermost annular zone (i = 6, m = -3) is −3λ (where λ = 408 nm) with respect to the light beam transmitted through the central region. ), The phase advances by 2π × 3 (rad) in terms of phase difference.
When the incident angle of the light beam is 0.5 ° and the rms value of the coma component of the wavefront aberration is 0.009λ rms with only the condenser lens, the aberration correction element is combined with the condenser lens. 0.003λrms. That is, an optical system that satisfies the sine condition is obtained by combining a condenser lens and an aberration correction element.

At 65 ° C., the rms value of the wavefront aberration is 0.047 λrms with the condenser lens alone, whereas it is 0.006 λrms when the aberration correction element is combined with the condenser lens. In addition, the wavelength change of the blue-violet semiconductor laser accompanying the temperature change was also considered, and the change rate was assumed to be +0.05 nm / ° C.
Further, when the wavelength change of the blue-violet semiconductor laser due to the temperature change is not taken into consideration, that is, only the change in the refractive index of the condenser lens and the aberration correcting element is taken into consideration, at 65 ° C., the rms of the wavefront aberration is obtained only with the condenser lens. While the value was 0.045 λrms, when the aberration correction element was combined with the condenser lens, the value was 0.014 λrms. As a result, the objective lens of the present invention is an optical element having excellent temperature characteristics despite having a plastic single lens with NA of 0.85, and in particular, providing an optical path difference for correcting the temperature characteristics. Since the structure is used, it can be seen that a good temperature characteristic correction effect is obtained regardless of whether or not the wavelength of the blue-violet semiconductor laser changes as the temperature rises.

At a wavelength of 409 nm, the rms value of the wavefront aberration is 0.058 λrms with only the condensing lens, whereas when the aberration correction element is combined with the condensing lens, it is 0.006 λrms. The focus positions of the objective optical system and the condenser lens are fixed at the focus position at 408 nm.
As a result, it can be seen that the objective lens of the present invention is well corrected for chromatic aberration by the action of the diffractive structure.

[Example 3]
The objective optical system in this example has a focal length f = 1.76 mm, paraxial power PT = 0.569 mm ⁻¹ , numerical aperture NA = 0.85, design wavelength λ = 407.5 nm, design reference temperature T = 35. It is set to ℃.
The aberration correction element is a plastic lens whose first surface (incident surface) is flat and second surface (exit surface) is an aspheric surface. The first surface has an optical path difference providing structure, and the second surface has a diffractive structure (diffraction). An annular zone is a sawtooth shape). Further, paraxial power P ₁ = 0.065 mm ^-1, paraxial refractive power P _R = 0.026 mm ^-1, paraxial diffractive power P _D = 0.039 mm ^-1, annular width P at the maximum effective diameter position _f = 11.2 μm, the zone width P _h at 50% of the maximum effective diameter = 46.6 μm, the wavefront aberration (407.5 nm, 35 ° C.) of the aberration correction element alone = 0.002λrms, λ = 407. paraxial power P _M for the light flux with 5nm: 0.0649mm ^-1, paraxial power for the light flux ^{_{λ = 412.5nm P L: 0.0654mm -1}} , paraxial power for the light flux λ = 402.5nm P _S: It is set to 0.0645 mm ⁻¹ and the rate of change in refractive index with temperature change = −1.0 × E−4.

  The condensing lens is a plastic lens whose both surfaces (third surface and fourth surface) are aspheric surfaces, focal length = 1.94 mm, magnification m = 0.114, numerical aperture NA = 0.85, condensing. The wavefront aberration (407.5 nm, 35 ° C.) of the lens alone is set to 0.001 λrms, and the refractive index change rate with temperature change is set to −1.0 × E-4.

Table 9 shows paraxial data of the aberration correction element and the condenser lens.

Further, the aspherical surfaces of the aberration correction element and the condenser lens are expressed by mathematical formulas obtained by substituting the coefficients in Table 10 into Equation 1 above. The aspherical shape of the second surface is perpendicular to the optical axis L, and as the distance from the optical axis increases, the amount of shift in the optical axis L direction with respect to the spherical surface represented by the radius of curvature r increases. It has become.

The diffractive structure of the aberration correcting element is represented by an optical path difference added to the transmitted wavefront by the diffractive structure. The optical path difference is defined by substituting the coefficient in Table 11 into the above formula 2. _b Expressed in mm.

Table 12 shows the shape of each annular zone of the optical path difference providing structure.

In the aberration correction element of the objective optical system in the present embodiment, eight annular zones are formed outside the central region. In addition, mi represents an optical path difference given to a light beam transmitted through each annular zone with respect to a light beam transmitted through the central region at a design reference temperature T = 35 ° C., for example, a ring adjacent to the outside of the central region. The light flux that passes through the band (i = 2, m = 4) is given an optical path difference of 4λ (where λ = 407.5 nm) with respect to the light flux that passes through the central region. The phase is delayed by 2π × 4 (rad), and the light beam transmitted through the outermost annular zone (i = 8, m = −4) is −4λ (where λ = 407.5 nm), the phase advances by 2π × 4 (rad) in terms of phase difference.
When the incident angle of the light beam is 0.5 ° and the rms value of the coma component of the wavefront aberration is 0.016λ rms with only the condensing lens, an aberration correction element is combined with the condensing lens. 0.005λrms. That is, an optical system that satisfies the sine condition is obtained by combining a condenser lens and an aberration correction element.

At 65 ° C., the rms value of the wavefront aberration is 0.090 λrms with only the condensing lens, whereas it is 0.009 λrms when the aberration correcting element is combined with the condensing lens. In addition, the wavelength change of the blue-violet semiconductor laser accompanying the temperature change was also considered, and the change rate was assumed to be +0.05 nm / ° C.
Further, when the wavelength change of the blue-violet semiconductor laser due to the temperature change is not taken into consideration, that is, only the change in the refractive index of the condenser lens and the aberration correcting element is taken into consideration, at 65 ° C., the rms of the wavefront aberration is obtained only with the condenser lens. While the value was 0.085 λrms, when the aberration correction element was combined with the condenser lens, the value was 0.027 λrms. As a result, the objective lens of the present invention is an optical element having excellent temperature characteristics despite having a plastic single lens with NA of 0.85, and in particular, providing an optical path difference for correcting the temperature characteristics. Since the structure is used, it can be seen that a good temperature characteristic correction effect is obtained regardless of whether or not the wavelength of the blue-violet semiconductor laser changes as the temperature rises.

At the wavelength of 408.5 nm, the rms value of the wavefront aberration is 0.109 λrms with only the condensing lens, whereas when the aberration correction element is combined with the condensing lens, it is 0.011 λrms. Note that the focus positions of the objective optical system and the condenser lens are fixed at a focus position of 407.5 nm.
As a result, it can be seen that the objective lens of the present invention is well corrected for chromatic aberration by the action of the diffractive structure.

It is a principal part top view which shows the structure of an optical pick-up apparatus. It is a principal part top view which shows the structure of an objective optical system. It is a principal part top view of an aberration correction element. It is the principal part top view (a) and enlarged view (b) which show a diffraction structure. It is a graph which shows the relationship between the wavefront aberration in an Example, and the height from an optical axis. It is a graph of a comparative example. It is a graph which shows the relationship between the wavefront aberration in an Example, and the height from an optical axis. It is a graph of a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical pick-up apparatus 30 Objective optical system 40 Aberration correction element 43 Flange part 50 Condensing lens 53 Flange part 60 Optical path difference providing structure 61 Center area 62 Ring zone 63 Step 70 Diffraction structure 72 Ring zone (Diffraction structure)
72a step

Claims (52)

In an optical system for an optical pickup device composed of a plastic aberration correction element and a plastic condensing lens for imaging a light beam emitted from the aberration correction element,
The aberration correction element has at least one first optical surface on which an optical path difference providing structure is formed and one second optical surface on which a diffractive structure is formed,
2. The optical system according to claim 1, wherein the condensing lens is a refracting single lens having one lens element in a group having at least one aspherical surface.   The optical system according to claim 1, wherein the first optical surface and the second optical surface are different optical surfaces.   The optical system according to claim 1, wherein the first optical surface and the second optical surface are the same optical surface.   The optical system according to any one of claims 1 to 3, wherein the second optical surface is positioned on the condenser lens side of the aberration correction element.   5. The optical system according to claim 1, wherein a refractive power in a paraxial axis of the aberration correction element on the first optical surface is zero.   6. The optical system according to claim 1, wherein a refractive power in a paraxial direction of the aberration correction element on the second optical surface is zero.   The optical path difference providing structure is composed of a central region including an optical axis and a plurality of annular zones divided with fine steps outside the central region, and the annular zone adjacent to the outside of the central region is the It is formed by shifting in the optical axis direction so that the optical path length becomes shorter with respect to the central region, and the annular zone at the maximum effective diameter position is longer than the annular zone adjacent to the inner zone. The annular zone formed by shifting in the direction of the optical axis and having a position of 75% of the maximum effective diameter has a shorter optical path length than the annular zone adjacent to the inner side and the annular zone adjacent to the outer side. The optical system according to any one of claims 1 to 6, wherein the optical system is formed by shifting in the optical axis direction.   The optical system according to claim 1, wherein the diffractive structure corrects axial chromatic aberration generated in the condenser lens in accordance with a change in wavelength of an incident light beam. The optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +... Is added to the wavefront transmitted through the diffractive structure. When the optical path difference is expressed, the near power obtained by combining the paraxial diffraction power P _D (mm ⁻¹ ) defined by P _D = −2 · b ₂ , the aberration correction element, and the condenser lens. The optical system according to claim 8, wherein the combined power P _T (mm ⁻¹ ) in the shaft satisfies the following expression (3).
0.03 ≦ P _D / P _T ≦ 0.15 (3)
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient   The optical system according to claim 1, wherein the diffractive structure corrects a spherical aberration change generated in the condenser lens in accordance with a wavelength change of an incident light beam. The optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +... Is added to the wavefront transmitted through the diffractive structure. 11. The optical system according to claim 10, wherein when the optical path difference is expressed, the at least one optical path difference function coefficient other than the second order has a non-zero value. The diffractive structure is composed of a central region including the optical axis and a plurality of annular zones divided with fine steps outside the central region, and a width P in a direction perpendicular to the optical axis of the annular zone at the maximum effective diameter position. _{The f} (mm) and the width P _h (mm) in the direction perpendicular to the optical axis of the annular zone at a position of 50% of the maximum effective diameter satisfy the following expression (4): 11. The optical system according to 11.
0 <| P _h / P _f −2 | <5 (4)   The diffractive structure includes a central region including an optical axis and a plurality of annular zones divided with a fine step outside the central region, and a cross-sectional shape including the optical axis is a staircase shape. Item 13. The optical system according to any one of Items 1 to 12.   The diffractive structure is composed of a central region including an optical axis and a plurality of annular zones divided with a minute step outside the central region, and a cross-sectional shape including the optical axis is a sawtooth shape. Item 13. The optical system according to any one of Items 1 to 12. When the wavelength of the laser light source of the optical pickup device on which the optical system is mounted is λ (nm) and the numerical aperture obtained by combining the aberration correction element and the condenser lens is NA, the following (5) and (6) Formula is satisfy | filled, The optical system as described in any one of Claims 1-14 characterized by the above-mentioned.
λ ≦ 450nm (5)
0.60 ≦ NA ≦ 0.95 (6)   The optical system according to claim 1, wherein the aberration correction element and the condenser lens are each designed to be capable of evaluating aberration independently.   Each of the aberration correction element and the condenser lens has a flange portion formed around the optical function portion, and a part of the flange portion of the condenser lens and a flange portion of the aberration correction element. The optical system according to claim 1, wherein a part thereof is bonded.   The optical system according to claim 1, wherein the aberration correction element and the condenser lens are joined via a joining member.   19. An optical system according to claim 1, wherein a light beam emitted from a laser light source and the laser light source is collected on an information recording surface of a disk, and a light beam reflected by the information recording surface is detected. An optical pickup device comprising:   20. The optical pickup device according to claim 19, and a support member that supports the optical disc, wherein at least one of recording information on the optical disc and reproducing information recorded on the optical disc can be executed. An optical information recording / reproducing apparatus. In an optical element for an optical pickup device comprising a plastic aberration correcting element and a plastic condensing lens for imaging a light beam emitted from the aberration correcting element,
The aberration correction element has at least one optical surface on which an optical path difference providing structure is formed,
The condensing lens is a single lens having one lens group and having at least one aspherical surface, and the paraxial power P ₁ (mm ⁻¹ ) of the aberration correction element satisfies the following expression (1). An optical system characterized by
P ₁ > 0 (1) The optical system according to claim 21, wherein a refractive power P _R (mm ^-1 ) on the paraxial axis of the aberration correcting element satisfies the following expression (2).
P _R > 0 (2)   23. The optical system according to claim 21, wherein the aberration correction optical element has at least one optical surface on which a diffractive structure is formed.   The optical path difference providing structure is composed of a central region including an optical axis and a plurality of annular zones divided with fine steps outside the central region, and the annular zone adjacent to the outside of the central region is the It is formed by shifting in the optical axis direction so that the optical path length becomes shorter with respect to the central region, and the annular zone at the maximum effective diameter position is longer than the annular zone adjacent to the inner zone. The annular zone formed by shifting in the direction of the optical axis and having a position of 75% of the maximum effective diameter has a shorter optical path length than the annular zone adjacent to the inner side and the annular zone adjacent to the outer side. The optical system according to any one of claims 21 to 23, wherein the optical system is formed by shifting in the optical axis direction.   The optical system according to claim 23, wherein the diffractive structure corrects axial chromatic aberration generated in the condenser lens in accordance with a change in wavelength of an incident light beam. The optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +... Is added to the wavefront transmitted through the diffractive structure. When the optical path difference is expressed, the near power obtained by combining the paraxial diffraction power P _D (mm ⁻¹ ) defined by P _D = −2 · b ₂ , the aberration correction element, and the condenser lens. 26. The optical system according to claim 25, wherein the combined power P _T (mm ⁻¹ ) at the shaft satisfies the following expression (3).
0.03 ≦ P _D / P _T ≦ 0.15 (3)
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient   24. The optical system according to claim 23, wherein the diffractive structure corrects a spherical aberration change generated in the condenser lens in accordance with a wavelength change of an incident light beam. The optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +... Is added to the wavefront transmitted through the diffractive structure. 28. The optical system according to claim 27, wherein when the optical path difference is expressed, at least one optical path difference function coefficient other than the second order has a non-zero value.
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient The diffractive structure is composed of a central region including the optical axis and a plurality of annular zones divided with fine steps outside the central region, and a width P in a direction perpendicular to the optical axis of the annular zone at the maximum effective diameter position. 28 or 27, wherein _f (mm) and a width P _h (mm) in a direction perpendicular to the optical axis of the annular zone at a position of 50% of the maximum effective diameter satisfy the following expression (4): 28. The optical system according to 28.
0 <| P _h / P _f −2 | <5 (4)   The diffractive structure includes a central region including an optical axis and a plurality of annular zones divided with a fine step outside the central region, and a cross-sectional shape including the optical axis is a staircase shape. Item 24. The optical system according to Item 23.   The diffractive structure is composed of a central region including an optical axis and a plurality of annular zones divided with a minute step outside the central region, and a cross-sectional shape including the optical axis is a sawtooth shape. Item 24. The optical system according to Item 23. When the wavelength of the laser light source of the optical pickup device on which the optical system is mounted is λ (nm) and the numerical aperture obtained by combining the aberration correction element and the condenser lens is NA, the following (5) and (6) Formula is satisfy | filled, The optical system as described in any one of Claims 21-31 characterized by the above-mentioned.
λ ≦ 450nm (5)
0.60 ≦ NA ≦ 0.95 (6)   The optical system according to any one of claims 21 to 32, wherein the aberration correction element and the condenser lens are each designed to be capable of evaluating aberrations independently.   Each of the aberration correction element and the condenser lens has a flange portion formed around the optical function portion, and a part of the flange portion of the condenser lens and a flange portion of the aberration correction element. The optical system according to claim 21, wherein a part thereof is bonded.   The optical system according to any one of claims 21 to 35, wherein the aberration correction element and the condenser lens are bonded via a bonding member.   The optical pickup device having a laser light source and an objective lens for condensing a light beam emitted from the laser light source on an information recording surface of an optical disc, wherein the objective lens is any one of claims 21 to 35. An optical pickup device, characterized in that the optical pickup device is described in the above.   37. The optical pickup device according to claim 36, and a support member that supports the optical disc, wherein at least one of recording of information on the optical disc and reproduction of information recorded on the optical disc can be executed. An optical information recording / reproducing apparatus. A plastic light source disposed in a light path between a laser light source and a plastic condensing lens that is disposed at a position facing the optical disk and collects a light beam emitted from the laser light source on an information recording surface of the optical disk. In an aberration correction element for an optical pickup device,
An aberration correction element, wherein an optical path difference providing structure is formed on one optical surface, and a diffractive structure is formed on the other optical surface.   The aberration correction element is disposed in an optical path between a laser light source and a plastic condensing lens that is disposed at a position facing the optical disk and collects a light beam emitted from the laser light source on an information recording surface of the optical disk. The aberration correction element according to claim 38, wherein the aberration correction element is arranged.   The optical path difference providing structure is composed of a central region including an optical axis and a plurality of annular zones divided with fine steps outside the central region, and the annular zone adjacent to the outside of the central region is the It is formed by shifting in the optical axis direction so that the optical path length becomes shorter with respect to the central region, and the annular zone at the maximum effective diameter position is longer than the annular zone adjacent to the inner zone. The annular zone formed at the position of 75% of the maximum effective diameter position with a shift in the optical axis direction has a shorter optical path length than the annular zone adjacent to the inner side and the annular zone adjacent to the outer side. 40. The aberration correction element according to claim 38, wherein the aberration correction element is formed so as to be shifted in the optical axis direction.   41. The aberration correcting element according to claim 38, wherein the diffractive structure corrects axial chromatic aberration generated in the condenser lens in accordance with a change in wavelength of an incident light beam. The paraxial power P _N (mm ⁻¹ ) of the aberration correction element for a predetermined wavelength of 450 nm or less and the paraxial power P _L (mm ^{−1) of the} aberration correction element for a wavelength 10 nm longer than the predetermined wavelength. And the paraxial power P _S (mm ⁻¹ ) of the aberration correction element for a wavelength shorter by 10 nm than the predetermined wavelength satisfies the following expression (7): Aberration correction element.
P _S <P _N <P _L (7)   41. The aberration correction element according to claim 38, wherein the diffractive structure corrects a spherical aberration change generated in the condenser lens with a change in wavelength of an incident light beam. The optical path difference function Φ _b (mm) defined by Φ _b = b ₂ · h ² + b ₄ · h ⁴ + b ₆ · h ⁶ +... Is added to the wavefront transmitted through the diffractive structure. 44. The aberration correction element according to claim 43, wherein when the optical path difference is represented, at least one optical path difference function coefficient other than the second order has a non-zero value.
Where h is the height (mm) from the optical axis, b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth,. Function coefficient The diffractive structure is composed of a central region including the optical axis and a plurality of annular zones that are divided with fine steps outside the central region, and is in a direction perpendicular to the optical axis of the annular zone at the maximum effective diameter position. 44. The width Pf (mm) and the width Ph (mm) in the direction perpendicular to the optical axis of the annular zone at a position of 50% of the maximum effective diameter satisfy the following expression (8): Or the aberration correction element of 44.
0 <| P _h / P _f −2 | <5 (8)   The optical surface on which the optical path difference providing structure is formed is an optical surface on a laser light source side, and the optical surface on which the diffractive structure is formed is an optical surface on the condenser lens side. The aberration correction element according to any one of 38 to 45.   The aberration correction element according to any one of claims 38 to 46, wherein the optical path difference providing structure is formed on an optical surface whose refractive power in the paraxial axis of the aberration correction element is zero. .   48. The aberration correction element according to claim 38, wherein the diffractive structure is formed on an optical surface whose refractive power in the paraxial axis of the aberration correction element is zero.   The diffractive structure includes a central region including an optical axis and a plurality of annular zones divided with a fine step outside the central region, and a cross-sectional shape including the optical axis is a staircase shape. Item 49. The aberration correction element according to any one of Items 38 to 48.   The diffractive structure is composed of a central region including an optical axis and a plurality of annular zones divided with a minute step outside the central region, and a cross-sectional shape including the optical axis is a sawtooth shape. Item 49. The aberration correction element according to any one of Items 38 to 48. The following (9) to (10) are satisfied, where NA is a numerical aperture obtained by combining the aberration correcting element and the condenser lens and f is a focal length: Optical system.
0.75 ≦ NA ≦ 0.90 (9)
0.7 <f <2.5 (10) The following (9) to (10) are satisfied, where NA is a numerical aperture obtained by combining the aberration correcting element and the condenser lens and f is a focal length: Optical system.
0.75 ≦ NA ≦ 0.90 (9)
0.7 <f <2.5 (10)

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