JP2003322794A - Aberration compensating optical element, optical system, optical pickup device, and recorder and reproducer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aberration compensating optical element which can compensate change in spherical aberration of an objective due to temperature change even when a high-density recording optical pickup system uses a high-NA (numerical aperture) plastic objective. <P>SOLUTION: The aberration compensating optical element 14 is a plastic lens of 1-group and 1-element constitution which is disposed on an optical path between a light source 2 and the objective 13 of ≥0.75 in image-side numerical aperture having at least one plastic lens and has a diffractive structure having a plurality of ring-shaped zone steps on at least one surface; and the change in tertiary spherical aberration of the objective due to change in refractive index of at least one plastic lens of the objective accompanying the temperature change of the objective is made small by changing the tilt angle of a marginal light beam of projection luminous flux from the aberration compensating optical element accompanying the temperature change of the aberration compensating optical element. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element for aberration correction arranged in an optical path between a light source and a high NA objective lens having at least one plastic lens, which is a spherical surface of the objective lens due to temperature change. An optical correction optical element capable of suppressing a change in aberration to a small extent, and a high NA objective lens having this aberration correction optical element and at least one plastic lens are provided, and An optical system for an optical pickup for performing at least one of recording and reproducing information, and an optical system including the optical system for performing at least one of recording and reproducing information on an optical information recording medium. A pickup device and an optical pickup device including the optical pickup device for recording audio and / or images and recording audio on an optical information recording medium. Preliminary / or a recording and reproducing apparatus for performing at least one of the image reproduction.

[0002]

2. Description of the Related Art In recent years, research on a new high-density recording optical pickup system using a blue-violet semiconductor laser light source with an oscillation wavelength λ of about 400 nm and an objective lens whose numerical aperture (NA) is increased to about 0.85.・ Development is progressing.
DVD (NA = 0.6, λ = 650 nm, recording capacity 4.
For example, for an optical disc having a diameter of 12 cm and the same size as 7 GB), 25 GB of information can be recorded on an optical disc having NA = 0.85 and λ = 400 nm.

In such a high-density optical pickup system, in order to reduce the cost and the weight, it is desirable to use a plastic lens as the objective lens like the conventional CD system and DVD system. The inventors of the present invention have proposed a two-group, two-element objective lens as described in Japanese Patent Application No. 2001-256422 as a plastic objective lens suitable for a high-density recording optical pickup system.

However, in a high NA plastic objective lens having an NA of about 0.85, the change of the spherical aberration caused by the change of the refractive index of the plastic is large when the temperature changes (the change of the spherical aberration is the fourth power of NA). However, it may be a problem in actual use. FIG. 17 shows, as an example, how spherical aberration changes with temperature change in a plastic objective lens having a two-group two-lens structure having an NA of 0.85, a design reference wavelength of 405 nm, a focal length of 1.76, and a design reference temperature of 25 ° C. Show. In addition, Table 1 shows lens data of this objective lens.

[0005]

[Table 1]

In order to record / reproduce information on / from an optical disc, the performance of the entire optical system for an optical pickup must satisfy the Marechal limit (wavefront aberration is 0.07λrms or less when λ is a wavelength). is there. Since an optical system for an actual optical pickup includes optical elements other than the objective lens such as a collimator and a prism, the upper limit of the wavefront aberration allowed in the objective lens is 0.03λrms.
It will be about. The operation guarantee temperature of the optical disk player is 8
Although it is about 5 ° C., the wavefront aberration of this objective lens at 85 ° C. exceeds 0.03 λrms from FIG. 17, and there is a possibility that stable information recording / reproduction cannot be performed on the optical disc.

Further, when using a high NA objective lens or a short wavelength light source of about 400 nm, axial chromatic aberration generated in the objective lens becomes a problem. Laser light emitted from a semiconductor laser is generally considered to have a single wavelength (single mode), and it is considered that there is no axial chromatic aberration, but in reality the center wavelength is momentarily several nm due to temperature changes, output changes, etc. Jumping or mode hopping may occur. Mode hopping is a wavelength change that occurs instantaneously so that focusing of the objective lens cannot follow, so if the chromatic aberration of the objective lens is not corrected, a defocus component is added and the wavefront aberration deteriorates. The deterioration of the wavefront aberration at the time of mode hopping becomes particularly large when an objective lens having a high NA or a short wavelength light source is used, as described below. Wavelength fluctuation Δλ
Spherical aberration does not vary by the objective lens for, and the back focus fb varies by .DELTA.FB, root mean square value W _rms of the wave front aberration when focusing the objective lens in the optical axis direction with respect to the back focus variation Is 0
However, if focusing is not performed, W _rms is expressed by the following equation (1).

W _rms = 0.145 · {(NA) ² / λ} / | Δfb | (1)

For example, DVD (NA = 0.6, λ = 6
(50 nm) and an optical disc with NA = 0.85 and λ = 400 nm, the wavefront aberration is degraded by 3.26 times in the latter even if Δfb is the same. That is, assuming that the allowable values of the wavefront aberration are the same, the allowable value of | Δfb | is 1
It becomes as small as /3.26, and it is necessary to reduce the axial chromatic aberration of the wavefront that is transmitted through the objective lens and focused on the recording surface of the optical disc.

Further, the upper limit of the axial chromatic aberration allowed in the objective lens used in the high density optical pickup system will be calculated from the equation (1). From the equation (1), the defocus amount (axial chromatic aberration) ΔfB _MAX that satisfies the wavefront aberration of 0.03λrms or less is ΔfB _MAX = 0.117 (μm) (2) when the used wavelength is 405 nm. FIG. 18 shows the wavelength 405 ± 1 of the objective lens in Table 1.
It is a figure showing the spherical aberration and axial chromatic aberration in nm. The blue-violet semiconductor laser is
It is said that the wavelength changes by about 1 nm, but from FIG. 18, when this objective lens changes from 405 nm to 406 nm, an axial chromatic aberration of about 0.25 μm occurs, and the axis allowed by the objective lens is increased. The upper limit of the upper chromatic aberration exceeds 0.117 μm.

As an objective lens for the cemented doublet type optical disc in which the chromatic aberration is corrected, Japanese Patent Laid-Open No. 61-3110 has been proposed.
Japanese Patent Laid-Open No. 62-28609 and Japanese Patent Laid-Open No. 62-28609 are known, in which a lens made of a low dispersion material having a positive refractive power and a lens made of a high dispersion material having a negative refractive power are combined. The lens is not suitable for an objective lens for an optical disc, which is essential to be lightweight. Because there is a limit to the dispersion of materials, when trying to correct chromatic aberration for short wavelengths, the refractive power of each lens increases, but the positive lens is thick to secure the edge thickness of the positive lens. As a result, the objective lens itself becomes heavy.

Further, Japanese Laid-Open Patent Publication No. 11-174318 discloses an objective lens having a two-element structure having a numerical aperture of 0.85 on the optical disk side and a hologram provided on an optical surface for correcting axial chromatic aberration. Have been described. However, when the hologram has a ring-shaped structure having concentric minute steps, in a high NA objective lens in which the curvature of the optical surface tends to be small, the influence of the shadow of the ring-shaped structure is large and the light transmittance is high. Therefore, it is not suitable for a high-density recording optical pickup system as an information writing system that requires high light utilization efficiency.

A chromatic aberration correcting optical element for correcting the axial chromatic aberration of the objective lens is disclosed in Japanese Patent Laid-Open No. 6-82.
An optical element for correcting chromatic aberration described in Japanese Patent No. 725 is known, but an optical element for correcting chromatic aberration in which a plane perpendicular to the optical axis is formed stepwise as a concentric ring zone with respect to the optical axis is disclosed. When arranged in a parallel light flux, the reflected light from the diffractive structure returns in the same direction as the incident light, and a ghost signal is generated in the detection system of the optical pickup, so it is not suitable as a chromatic aberration correction optical element for an optical pickup system. .

[0014]

SUMMARY OF THE INVENTION Even when a high NA plastic objective lens is used in a high density recording optical pickup system or the like, the present invention has a relatively simple structure and spherical aberration of the objective lens due to temperature change. It is an object of the present invention to provide an optical system for an optical pickup capable of suppressing a change in the above, an optical pickup device including the optical system, and a recording / reproducing device including the optical system.

It is also an object of the present invention to provide an aberration correcting optical element capable of correcting a change in spherical aberration of an objective lens due to temperature change when used with a plastic objective lens having a large change in spherical aberration due to temperature change. To aim.

Further, even when a light source having poor monochromaticity or a light source whose wavelength changes abruptly is used in a high density recording optical pickup system or the like, axial chromatic aberration can be corrected with a relatively simple structure, and It is also an object of the present invention to provide an optical pickup optical system that can be manufactured at low cost, an optical pickup device including the same, and a recording / reproducing device including the same.

Further, since the numerical aperture on the side of the optical information recording medium is large, correction of spherical aberration, correction of sine conditions and miniaturization,
It is also an object of the present invention to provide an aberration correction optical element capable of correcting axial chromatic aberration when used with an objective lens in which axial chromatic aberration remains for the purpose of thinning, weight reduction, and cost reduction. is there.

[0018]

In order to achieve the above object, an aberration correction optical element according to the present invention comprises a light source, an image side numerical aperture of 0.75 or more, and at least one plastic lens. An aberration correction optical element disposed in an optical path between the objective lens and the aberration correction optical element, wherein the aberration correction optical element has a diffractive structure including a plurality of annular zone steps on at least one surface. The aberration correction optical element is a single-lens plastic lens, and the aberration correction optical element is caused by a change in refractive index ΔN _OBJ of at least one plastic lens included in the objective lens due to a temperature change of the objective lens. the change Deruta3SA _OBJ of the third-order spherical aberration of the objective lens, the refractive index change of the aberration correcting optical element due to the temperature change of the aberration correcting optical element Δ Change in the inclination angle of the marginal ray of the light flux from the aberration correcting optical element due to _AC, by characterized by small.

As described above, the aberration-correcting optical element according to the present invention is a plastic lens comprising a light source and an objective lens having an image-side numerical aperture of 0.75 or more and having at least one plastic lens. It has a diffractive structure that is arranged in the optical path between and has a plurality of annular zone steps on at least one surface. A general plastic material has the characteristic that when the temperature rises, the refractive index decreases, and when the temperature decreases, the refractive index increases. Is an order of magnitude larger than glass materials. Therefore, the diffractive structure as a diffractive lens is formed by setting the power of the entire system of the aberration correction optical element, which is a plastic lens, to be constant, and forming a diffractive structure composed of a plurality of annular zone steps on at least one surface of the aberration correction optical element. By appropriately setting the distribution of the power and the refracting power of the refraction lens, when the refractive index of the aberration correction optical element changes due to temperature change, the marginal ray of the exit light beam from the aberration correction optical element is changed. It is possible to freely select the amount of change in the tilt angle.

When such an aberration correction optical element is arranged in the optical path between the light source and the objective lens having at least one plastic lens, the refractive index of the plastic lens changes due to temperature change. It is possible to cause a change in the third-order spherical aberration of the objective lens to cancel the change due to a change in the tilt angle of the marginal ray of the light beam emitted from the aberration correction optical element. Therefore, even a high NA objective lens having a narrow usable temperature range and having at least one plastic lens can be used in combination with the aberration correction optical element according to the present invention to widen the usable temperature range. be able to. As a result, in a high-density recording optical pickup system that requires an objective lens having an image-side numerical aperture of 0.75 or more, the objective lens can be a plastic lens.
The cost of the optical pickup device can be reduced. In addition,
In this specification, the refracting power (as a refraction lens) refers to the power determined by the radius of curvature, the axial thickness, and the paraxial amount of the refractive index, and the diffractive power (as a diffractive lens) is the diffractive structure. When the optical path difference to be added to the wave front that passes through is expressed by an optical path difference function of Equation 2 described later, it means the power of the diffractive structure defined by PD = Σ (−2 · b _2i · n _i ).

It is preferable that the above-mentioned aberration correcting optical element has a temperature characteristic in which the paraxial power increases as the temperature rises and that the following expression (A) is satisfied.

P _T1 <P _T0 <P _T2 (A)

[0023] However, _{P T0:} paraxial power of the aberration correcting optical element at a given temperature ^{_{T 0 (mm - 1) P}} T1: at temperatures _{T 1} lower by a predetermined temperature difference than the predetermined temperature _{T 0} the aberration paraxial power of the correcting optical element ^(mm _{-1) P T2:} paraxial power of the aberration correcting optical element in temperature _{T 2} higher by a predetermined temperature difference than the predetermined temperature _{T 0} ^{(mm -1} )

Since this aberration correction optical element satisfies the above expression (A), when the temperature rises, the inclination angle of the upper marginal ray of the light beam emitted from the aberration correction optical element is changed before the temperature changes. In comparison, the direction that becomes smaller, that is,
It changes clockwise with respect to the optical axis. This corresponds to the direction in which the imaging magnification of the objective lens increases. Therefore, in the optical pickup optical system, when this aberration correction optical element is used in combination with an objective lens having a temperature characteristic such that the third-order spherical aberration component changes in the direction of insufficient correction when the temperature rises,
It is possible to act in a direction to cancel the change of the third-order spherical aberration when the temperature of the objective lens changes. Here, the “predetermined temperature T ₀ ” is preferably “25 ° C.” and the “predetermined temperature difference” is preferably “30 ° C. temperature difference”. In this specification, the paraxial power (of the entire system) refers to the power obtained by combining the refracting power and the diffracting power. That is, if the refracting power is zero,
The paraxial power is equal to the diffractive power, and if the diffractive power is zero, the paraxial power is equal to the refractive power.

The objective lens is an objective lens having a two-group, two-element configuration in which an l-th lens having a positive refracting power and a second lens having a positive refracting power are arranged in this order from the light source side. It is preferable that the first lens is a plastic lens.

As an objective lens having a temperature characteristic in which the third-order spherical aberration component changes in the direction of undercorrection when the temperature rises, the objective lens having the two-group, two-element construction as described above, at least the first lens. There is an objective lens that is a plastic lens. From the viewpoint of cost reduction and weight reduction, it is more preferable that both the first lens and the second lens are plastic lenses.

When used in combination with the objective lens having the two-group two-lens structure as described above, the aberration correcting optical element according to the present invention preferably satisfies the following expressions (B) and (C). .

P _R <0 (B) 0 <ΔP _AC / ΔT _AC <1 × 10 ⁻⁴ (C)

Where P _R : refraction power (mm ⁻¹ ) ΔP _{AC of the} aberration correction optical element as a refraction lens: the aberration correction optical element due to temperature change ΔT _AC (° C.) of the aberration correction optical element Amount of paraxial power change (mm ^-1 )

When the refracting power P _{R of the} aberration correcting optical element as a refraction lens satisfies the expression (B), the amount of change in the paraxial power of the aberration correcting optical element with respect to temperature change (ΔP _AC
Since the sign of / ΔT _AC ) can be positive, it can be made to act in the direction of canceling the change of the third-order spherical aberration of the objective lens having the two-group two-lens structure when the temperature changes. On the other hand, Δ
When the value of P _AC / ΔT _AC is smaller than the upper limit of the expression (B), the correction of the third-order spherical aberration of the objective lens having the two-group two-lens structure at the time of temperature change does not become excessive. From the above, ΔP _AC
When the value of / ΔT _AC satisfies the expression (B), the correction amount of the change in spherical aberration at the time of temperature change can be made appropriate for the objective lens having the two-group two-lens structure.

It is preferable that the light source is a light source that emits light having a wavelength of 550 nm or less, and the aberration correction optical element satisfies the following expression (D).

P _λ1 <P _λ0 <P _λ2 (D)

[0033] However, P _.lambda.0: the light source paraxial power of the aberration correcting optical element at the wavelength lambda ₀ of the light generated _^(mm -l) P ^λ1: the at 10nm wavelength shorter lambda ₁ than the wavelength lambda ₀ aberrations Paraxial power (mm ⁻¹ ) P _{λ2 of the} correction optical element: Paraxial power (mm ⁻¹ ) of the aberration correction optical element at the wavelength λ _{2 which} is 10 nm longer than the wavelength λ ⁰

As described above, the aberration correction optical element is (D).
By having the wavelength characteristics that satisfy the formula, when the light having the wavelength different from the light having the predetermined wavelength by the predetermined wavelength difference is incident, the axial chromatic aberration generated by the aberration correction optical element and the objective lens are Since the axial chromatic aberration that occurs is corrected by canceling it, the focal spot of the light passing through the aberration correction optical element and the objective lens on the information recording surface of the optical information recording medium has a small axial chromatic aberration. It will be suppressed. By combining with the aberration correction optical element according to the present invention, even an objective lens that does not strictly correct axial chromatic aberration,
It can be used as an objective lens for a high density optical information recording medium.

Further, in order to suppress the residual aberration of the entire optical system when it acts in a direction to cancel the change of the third-order spherical aberration component of the objective lens due to the temperature change, it is combined with the above-mentioned aberration correcting optical element. The objective lens used preferably satisfies the following expression (E). As a result, it is possible to suppress the residual aberration when acting in a direction to cancel the change of the third-order spherical aberration component of the objective lens due to the temperature change.

Further, the following expressions (F) to (H) work in a direction that favorably cancels the change of the spherical aberration of the objective lens when the temperature changes, and the aberration correcting optical element and the objective lens are This is a condition for suppressing the axial chromatic aberration of the focused spot on the information recording surface of the optical information recording medium of the light passing through the optical axis. When the change amount of the third-order spherical aberration of the objective lens at the time of temperature change satisfies the expression (F) and the axial chromatic aberration of the objective lens satisfies the expression (G), (H)
By forming the diffractive structure having the diffractive power satisfying the formula in the aberration correction optical element, it is possible to achieve both the temperature characteristic compensation of the objective lens and the axial chromatic aberration compensation.

| Δ3SA _OBJ | / | Δ5SA _OBJ |> 1 (E) −30.0 × 10 ⁻⁴ <Δ3SA _OBJ / (ΔT _OBJ · NA ⁴ · f _OBJ ) <0 (F) 3 × 10 ⁻² < ΔfB _OBJ · νd _OBJ / f _OBJ <14 × 10 ⁻² (G) 1.0 × 10 ⁻² <P _D <10.0 × 10 ⁻² (H)

Where Δ3SA _OBJ : temperature change of the objective lens ΔT _OBJ
(3) A change amount of the third-order spherical aberration component when the aberration of the objective lens is expanded into a Zernike polynomial when the refractive index of the plastic lens of the objective lens changes by ΔN _OBJ due to (° C.). Is expressed as an RMS (Root Mean Square) value in which the wavelength λ ₀ (nm) of the light generated by is generated. The sign is “+” when the third-order spherical aberration component changes in the direction of overcorrection, and the third-order spherical surface. The case where the aberration component changes in the direction of insufficient correction is defined as "-". Δ5SA _OBJ : Temperature change of the objective lens ΔT _OBJ
A change amount of the fifth-order spherical aberration component when the aberration of the objective lens is expanded into a Zernike polynomial when the refractive index of the plastic lens of the objective lens changes by ΔN _OBJ due to (° C.), RMS (Root Mean Square) in units of wavelength λ ₀ (nm) of light generated by the light source
Expressed as a value. NA: A predetermined image-side numerical aperture f _{OBJ of the} objective lens required for recording and / or reproducing of the optical information recording medium: Focal length (mm) of the objective lens ΔfB _OBJ : The light source is generated in the objective lens Axial chromatic aberration (mm) νd _{OBJ of the} objective lens when light having a wavelength longer than the wavelength λ ₀ of the light by a wavelength difference of +10 nm is incident νd _OBJ : Abbe number of d line of the first lens of the objective lens, Average value P _{D of} the second lens with the Abbe number of the d-line: a wavefront that is transmitted through the diffractive structure formed on the i-th surface of the aberration correction optical element The additional optical path difference Φ _bi is the height h from the optical axis.
_As a function of _i (mm), Φ _bi = ni · (b _2i · h _i ² + b _4i · h _i ⁴ + b
_6i · h _i ⁶ + ...) In the case of being represented by the optical path difference function (here, b
_2i , b _4i , b _6i ... _Are secondary, quaternary, and 6 respectively.
Next, an optical path difference function coefficient, ni is a diffraction order having the maximum diffracted light amount among the diffracted light generated by the diffractive structure formed on the i-th surface), P _D = Σ (−2. b _2i · ni) diffractive power (mm ⁻¹ ) as a diffractive lens

Further, in the aberration correction optical element, when viewed macroscopically, one optical surface on which a diffractive structure composed of a plurality of flat annular zones is formed, and a concave refracting surface on the opposite surface. And the other optical surface formed.

When the plane is a diffractive surface having a diffractive structure, the diffracted light reflected by the diffractive structure travels in a direction different from that of the incident light, so that detection of a ghost signal in the detection system of the optical pickup device can be prevented. Furthermore, the diffractive structure can be manufactured with high precision by electron beam drawing. A method for producing a diffractive structure composed of minute steps of an annular zone by electron beam drawing is described in "OPTICS DESIGN Optical Design Research Group Journal No. 20 2000.2.25", pages 26 to 31. Further, the optical surface on the opposite side of the plane on which the diffractive structure is formed is a concave refracting surface, but at this time, the refractive power of the concave surface and the diffractive power of the diffractive structure are substantially equal in absolute value, If the signs are reversed, the paraxial power of the aberration-correcting optical element can be set to 0, which facilitates placement in the optical system.

Here, "a plane when viewed macroscopically" means that an annular step is formed on a plane optical surface,
In FIG. 2A, which will be described later, it is synonymous with that a line L1 (envelope) connecting the vertices of each ring zone step becomes a straight line.

In each of the above-mentioned aberration correction optical elements, it is preferable that the paraxial power P ₀ of the aberration correction optical element at the wavelength λ ₀ of the light generated by the light source is substantially zero, and the paraxial light flux It is easy to place it in. Specifically, it is to satisfy the following expressions (I) to (K).

P _D > 0 (I) P _R <0 (J) −0.9> P _D / P _R > −1.1 (K)

It is preferable that the above-mentioned aberration correcting optical element has a temperature characteristic that the paraxial power decreases in the direction of decreasing temperature, and satisfies the following expression (L).

P _T2 <P _T0 <P _T1 (L)

[0046] However, _{P T0:} paraxial power of the aberration correcting optical element at a given temperature ^{_{T 0 (mm - 1) P}} T1: at temperatures _{T 1} lower by a predetermined temperature difference than the predetermined temperature _{T 0} the aberration paraxial power of the correcting optical element ^(mm _{-1) P T2:} paraxial power of the aberration correcting optical element in temperature _{T 2} higher by a predetermined temperature difference than the predetermined temperature _{T 0} ^{(mm -1} )

Since this aberration correction optical element satisfies the above equation (L), when the temperature rises, the inclination angle of the upper marginal ray of the light beam emitted from the aberration correction optical element is changed before the temperature changes. In comparison, the direction of increasing, that is,
It changes counterclockwise with respect to the optical axis. This corresponds to the direction in which the imaging magnification of the objective lens decreases. Therefore, in the optical pickup optical system, when this aberration correction optical element is used in combination with an objective lens having a temperature characteristic such that the third-order spherical aberration component changes in the overcorrection direction when the temperature rises,
It is possible to act in a direction to cancel the change of the third-order spherical aberration when the temperature of the objective lens changes. Here, the “predetermined temperature T ₀ ” is preferably “25 ° C.” and the “predetermined temperature difference” is preferably “30 ° C. temperature difference”.

As an objective lens having a temperature characteristic in which the third-order spherical aberration component changes in the direction of overcorrection when the temperature rises, there is a plastic lens having one lens in one group.
If the numerical aperture of a plastic lens with a one-group, one-lens structure is increased, the usable temperature range will become extremely narrow. (The plastic lens with a one-group, one-lens structure has a focal length, an image-side numerical aperture, and a usable wavelength. , The amount of change in spherical aberration due to temperature change is about 5 to 10 times larger than that of a plastic lens having a two-group, two-element configuration with the same imaging magnification), and a temperature control mechanism for controlling the temperature of the plastic lens is specially required. Next to
This increases the manufacturing cost of the optical pickup device and complicates the optical pickup device. Therefore, a high-NA plastic lens having a simple structure and a low cost can be used by combining a high-NA plastic lens having a one-group one-lens configuration and an aberration correction optical element according to the present invention. The usable temperature range can be expanded.

M represented by the following equations (M) and (M ') is 0,
Step difference Δ (m
It is preferable that at least one of the annular zone steps having m) is formed within the effective diameter.

M = INT (Y) (M) Y = Δ × (n−1) / (λ ₀ × 10 ⁻³ ) (M ′) where INT (Y): an integer n obtained by rounding Y : Refractive index λ ₀ of the chromatic aberration correcting optical element at the wavelength λ ₀ (nm) of the light generated by the light source: Wavelength (nm) of the light generated by the light source

In the above equation (M), when the incident light beam is diffracted by the diffractive structure formed on the optical surface of the aberration correction optical element, the diffracted light amount of the diffracted light having the second or higher diffraction order is So that the amount of diffracted light of any of the diffracted orders becomes larger than the amount of step difference Δ (m
m) to determine. As a result, the minimum value of the interval between the adjacent ring zone steps in the optical axis direction becomes m times as compared with the case where the step amount Δ is determined so that the first-order diffracted light has the maximum diffracted light amount. It is possible to reduce the influence of the light amount loss due to the phase mismatching portion that occurs due to the transfer failure of the ring zone step at the time. Further, since the number of steps of the ring zone is 1 / m times, the processing time of the molding die can be shortened.

Further, it is preferable that the aberration correcting optical element has a diffractive structure having a plurality of annular zone steps on both surfaces. By distributing the diffractive power of the diffractive structure to two optical surfaces, the minimum value of the interval between the adjacent ring zone steps in the optical axis direction can be increased as compared with the case where the diffractive structure is formed only on one surface. it can. As a result, it is possible to reduce the influence of the light amount loss due to the phase mismatch portion caused by the transfer failure of the ring-shaped step at the time of molding.

Further, when the wavelength of the incident light beam changes to the long wavelength side, the spherical aberration of the outgoing light beam has a spherical aberration characteristic such that the spherical aberration changes in the direction of undercorrection or overcorrection. It is preferable to have at least one surface having a diffractive structure that satisfies the formula (N).

0.2 ≦ | (P _hf / P _hm ) −2 | ≦ 6.0 (N) where P _hf is a direction perpendicular to the optical axis of the diffractive structure at a position hf that is half the maximum effective diameter hm. Ring spacing P _hm : It is preferable that the ring spacing is in the direction perpendicular to the optical axis of the diffractive structure at the maximum effective diameter hm.

When a short wavelength light source having a wavelength of 550 nm or less, particularly about 400 nm, is used, the refractive index changes greatly with respect to a minute wavelength change of the lens material. Therefore, when a minute wavelength change occurs, axial chromatic aberration occurs in the objective lens. Besides that,
The spherical aberration also changes. For example, in an objective lens having a single-lens configuration, when the wavelength changes to the long wavelength side by 10 nm from the design wavelength, the spherical aberration changes in the overcorrection direction. Further, in the two-group objective lens, the spherical aberration when the wavelength changes to the long wavelength side of 10 nm from the design wavelength changes to the overcorrection direction or the undercorrection direction depending on the paraxial power arrangement of the lens groups. To do

The above formula (N) is a condition for correcting the spherical aberration change of the objective lens by the diffraction action of the aberration correction optical element when a slight wavelength change occurs. If the optical path difference function has only a second-order optical path difference function coefficient (also referred to as a diffractive surface coefficient), then (P _hf / P _hm ) −2 = 0, but the aberration correction optical element according to the present invention is designed. In order to satisfactorily correct the change in spherical aberration that occurs in the objective lens due to a slight change in wavelength from the wavelength due to the diffractive action of the diffractive structure of the aberration correction optical element, the higher order optical path difference function coefficient is used. . At this time, (P _hf /
It is preferable that P _hm ) −2 has a value that is apart from 0 to some extent, and if the expression (N) is satisfied, the above-mentioned change in spherical aberration can be favorably canceled by the diffraction effect.

Further, it is preferable that the above-mentioned aberration-correcting optical element has a wavelength at which the diffraction efficiency is maximized at 550 nm or less. More preferably, the design wavelength of the objective lens and the wavelength that maximizes the diffraction efficiency are substantially the same.

In the optical system according to the present invention, a light source, an objective lens having an image-side numerical aperture of 0.75 or more, and having at least one plastic lens, and between the light source and the objective lens are provided. An optical system for reproducing information from an information recording surface of an optical information recording medium and / or recording information on the information recording surface, which comprises an aberration correcting optical element arranged in an optical path, The above-mentioned aberration-correcting optical element is included as the aberration-correcting optical element.

The optical pickup device according to the present invention has a light source, an image-side numerical aperture of 0.75 or more, and
An optical system having an objective lens having at least one plastic lens, and an aberration correction optical element arranged in an optical path between the light source and the objective lens,
An optical pickup device for reproducing information from an information recording surface of an optical information recording medium and / or recording information on the information recording surface, characterized by including the above optical system as the optical system.

According to the above optical system and optical pickup device, even when a high NA plastic objective lens is used in a high density recording optical pickup system or the like, the above-mentioned aberration correcting optical element is provided, so that it is relatively simple. The structure can suppress the change in spherical aberration of the objective lens due to the temperature change to be small.

Further, the recording / reproducing apparatus according to the present invention is equipped with the above-mentioned optical pickup device and is capable of recording voice and / or images and / or reproducing voice and / or images.

In the present specification, the diffractive surface refers to a surface provided with a relief on the surface of an optical element, for example, the surface of a lens, to have a function of diffracting an incident light beam. When there is a region where diffraction occurs and a region where diffraction does not occur, it means a region where diffraction occurs. Further, the diffractive structure or the diffractive pattern means a region where this diffraction occurs. As the relief shape, for example, on the surface of the optical element, it is formed as a ring of substantially concentric circles centered on the optical axis, and when viewed in a plane including the optical axis, each ring zone is sawtooth-shaped or stairs. A shape like a shape is known, but includes such a shape.

In the present specification, recording and reproducing of information means recording information on the information recording surface of the optical information recording medium as described above and reproducing the information recorded on the information recording surface. Say that. The condensing optical system of the present invention may be used to perform only recording or reproduction, or may be used to perform both recording and reproduction. Further, it may be used for recording on one optical information recording medium and reproducing on another optical information recording medium, or for one optical information recording medium. Alternatively, it may be used for performing reproduction and recording and reproduction for another optical information recording medium. Note that the reproduction here includes simply reading information.

The aberration correcting optical element of the present invention is defined as follows. That is, an objective lens including at least one plastic lens and having an image-side numerical aperture of 0.75 or more is left at an ambient temperature of 25 ° C., and after a sufficient time has passed (that is, the refractive index of the plastic lens is When the wavefront aberration of this objective lens is measured (after the change reaches a steady state), the 3rd-order spherical aberration component is set to 3SA.
(25 ° C), this objective lens is left at an ambient temperature of 55 ° C, and after a sufficient time, the wavefront aberration of this objective lens is measured in the same manner, and the third-order spherical aberration component is 3SA.
(55 ℃). Furthermore, an optical system in which a certain optical element is arranged in the optical path between the light source and this objective lens is
When the wavefront aberration of this optical system is measured after leaving it at an ambient temperature of ℃ for 3 hours, the third-order spherical aberration component is set to 3SA (25 ° C) ', and this optical system is at an ambient temperature of 55 ℃. After leaving it for a long time, the wavefront aberration of this optical system is measured in the same manner after a sufficient time, and the third-order spherical aberration component is measured by 3SA.
(55 ° C) '. Between these four third-order spherical aberration component measurements, | 3SA (55 ° C) -3SA (25 ° C) |> | 3SA (55 ° C) '-
When the relation 3SA (25 ° C.) ′ | Holds, this “certain optical element” is defined as the aberration correcting optical element according to the present invention.

[0065]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram schematically showing the structure of an optical pickup device including an optical system for an optical pickup according to the present embodiment, and FIG. 2A is a side view of the aberration correcting optical element of FIG. 2 (b) is a plan view seen from the arrow direction A in FIG. 2 (a).

The optical pickup device 1 of FIG. 1 includes a semiconductor laser 2 as a light source, a diffraction-integrated type aberration correction optical element 14, and an objective lens 13. The semiconductor laser 2 is a GaN-based blue-violet laser that emits a light flux with a wavelength of about 400 nm. In addition to the above-mentioned GaN-based blue-violet laser, as a light source that emits a light flux with a wavelength of about 400 nm,
It may be an SHG blue-violet laser.

Further, as shown in FIGS. 2 (a) and 2 (b),
On the surface S1 of the aberration correction optical element 14 on the semiconductor laser 2 side, a substantially concentric circular diffraction pattern p is provided on a flat optical surface. Objective lens 1 of optical element 14 for aberration correction
The surface S2 on the third side is a concave surface having a negative refractive power, and the paraxial power of the aberration correcting optical element 14 is set to 0 by setting the absolute values of the diffraction power of the diffraction pattern p and the refractive power of the concave surface to be equal. There is.

The substantially concentric circular diffraction pattern may be provided on the surface of the aberration correcting optical element 14 on the side of the objective lens 13 or may be provided on both the surface on the light source side and the surface on the objective lens 13 side. May be. Although the paraxial power of the aberration correction optical element 14 is set to 0, the paraxial power of the aberration correction optical element 14 may be positive or negative. Although the diffraction pattern of the aberration correction optical element 14 is substantially concentric with the optical axis, other diffraction patterns may be provided.

Further, in the optical pickup device 1 of FIG.
Although the aberration correction optical element 14 is arranged as a separate element from the objective lens 13, the aberration correction optical element 14 and the objective lens 13 may be integrated by a lens frame or an adhesive agent. Since the correction optical element 14 and the objective lens 13 are integrally tracking-controlled by the actuator 10, good tracking characteristics can be obtained.

The objective lens 13 is the aberration correction optical element 1.
4 is a lens for condensing the light flux from the optical disc 4 on the information recording surface 5 ′ of the optical disc 5 within the diffraction limit, and is composed of two plastic lenses integrated with the holding member 13a, and at least one non-lens It has a spherical surface, and the numerical aperture on the optical disk 5 side is 0.85. Also, the objective lens 1
3 is a flange portion 13 having a surface extending perpendicular to the optical axis.
b, and the objective lens 1
3 can be accurately attached to the optical pickup device 1.

The divergent light beam emitted from the semiconductor laser 2 passes through the polarization beam splitter 6, passes through the collimating lens 7 and the quarter wavelength plate 8, becomes a circularly polarized parallel light beam, and passes through the aberration correction optical element 14. After that, the light passes through the diaphragm 9 and becomes a spot formed by the objective lens 13 on the information recording surface 5 ′ through the transparent substrate 5 ″ of the high density recording optical disc 5. The objective lens 13 is focus-controlled and tracking-controlled by the actuator 10 arranged around the objective lens 13.

The reflected light flux modulated by the information pits on the information recording surface 5'is again the objective lens 13, the aberration correcting optical element 14, the quarter wavelength plate 8 and the collimating lens 7.
After passing through, the light beam becomes a convergent light beam, is reflected by the polarization beam splitter 6, passes through the cylindrical lens 11, is given astigmatism, and is converged on the photodetector 12. Then, the information recorded on the optical disk 5 can be read using the output signal of the photodetector 12.

In this embodiment, the aberration correcting optical element 14 is a plastic lens, and the substantially concentric diffraction pattern as described above is provided on the optical surface, so that the oscillation of the semiconductor laser 2 is oscillated. Axial chromatic aberration is generated that has a sign opposite to that of the objective lens 13 with respect to the wavelength, and the absolute values thereof are substantially the same. Therefore, the semiconductor laser 2
The light beam emitted from the optical system passes through the aberration correction optical element 14 and the objective lens 13 and is condensed on the information recording surface 5 ′ of the optical disc 5 with almost no axial chromatic aberration.

In the present embodiment, heat is dissipated from the focusing coil or tracking coil mounted near the objective lens 13 or the temperature outside the optical pickup device 1 rises, so that the objective lens 13 and the aberration correcting optical element 14 are removed. When the temperature rises, the refractive index of the plastic lens changes so as to decrease, so that in the objective lens 13, the third-order spherical aberration component changes in the undercorrection direction. At this time, since the refracting power of the entire system of the aberration correction optical element 14 as the refraction lens satisfies the above expression (B), the light beam emitted from the aberration correction optical element 14 becomes a convergent light beam. When the convergent light beam is incident on the objective lens 13, the third-order spherical aberration changed in the undercorrection direction is canceled, so that the light beam emitted from the semiconductor laser 2 passes through the aberration correction optical element 14 and the objective lens 13. By doing so, even if the temperature rises, the light is focused on the information recording surface 5 ′ with the spherical aberration suppressed to a small level.

Further, the optical pickup device as shown in FIG. 1 is, for example, a next-generation high-density recording medium such as a high-density DVD, CD, CD-R, CD-RW, CD-Video, CD-ROM. , DVD, DVD-RO
A player or drive compatible with optical information recording media such as M, DVD-RAM, DVD-R, DVD-RW, DVD + RW, and MD,
Alternatively, it can be mounted on an audio and / or image recording device and / or reproducing device such as an AV device, a personal computer, or another information terminal incorporating them.

[0077]

EXAMPLES FIGS. 3 and 4 show examples of objective lenses in which changes in spherical aberration and axial chromatic aberration due to temperature changes are corrected by the aberration correcting optical elements of Examples 1 to 3 according to the present invention. FIG. 3 shows an objective lens (focal length 1.76 m) in which axial chromatic aberration is corrected by the aberration correcting optical element of the present invention.
FIG. 3 is an optical path diagram with m and an image-side numerical aperture of 0.85). This objective lens is formed of an olefin resin having an Abbe number of 56.5 for d line. FIG. 4 is a graph showing spherical aberration and axial chromatic aberration at a wavelength of 405 ± 10 nm of this objective lens, showing that when the wavelength changes to the 10 nm long wavelength side, the focus changes to the over side by about 3 μm. There is.

Next, Examples 1 to 5 of the optical system for an optical pickup according to the present invention will be described. In addition,
The aspherical surface in this embodiment is represented by the following formula 1 when the optical axis direction is the X axis, the height in the direction perpendicular to the optical axis is h, and the radius of curvature of the refracting surface is r. However, κ is a cone coefficient and A _2i is an aspherical surface coefficient.

[0079]

[Equation 1]

The ring-shaped diffractive surface provided on the lens of this embodiment can be expressed by the following equation 2 as the optical path difference function Φ _b . Here, n is the diffraction order of the diffracted light having the maximum diffracted light amount among the diffracted light generated on the diffractive surface, and h
Is the height perpendicular to the optical axis, and b _2j is the coefficient of the optical path difference function.

[0081]

[Equation 2]

<Example 1>

Table 2 shows data on the optical system for the optical pickup of the first embodiment, and FIG. 5 shows an optical path diagram of the optical system for the optical pickup of the first embodiment.

[0084]

[Table 2]

In Example 1, the aberration correcting optical element is formed of an olefin resin, and the axial chromatic aberration of the objective lens is corrected by using both the light source side surface and the objective lens side optical surface as diffractive surfaces. . In addition, the refracting power on each optical surface is negative, the absolute value is the same as the absolute value of the diffraction power, and the paraxial power on each optical surface is 0. Was set so as not to change in comparison with the incident light beam diameter. Further, the coefficient of the optical path difference function in Table 2 was determined so that the second-order diffracted light has the maximum diffracted light amount.

FIG. 6 is a graph showing spherical aberration and axial chromatic aberration at a wavelength of 405 ± 10 nm of the optical system for the optical pickup of Example 1, showing that the focal point hardly moves regardless of the wavelength.

Further, the diffraction power of the aberration correcting optical element is determined so that the axial chromatic aberration of the combined system with the objective lens is overcorrected, and further, the spherical aberration on the long wavelength side and the short wavelength side of the combined system are determined. The remaining spherical aberration of over-correction and over-correction of each are left, and the spherical aberration graph of the reference wavelength and the spherical aberration graph of the long wavelength side and the short wavelength side are tolerated to obtain the best image point position when the wavelength changes. , The defocus component of the wavefront aberration when the mode hopping of +1 nm occurs can be reduced to 0.002λrms (calculated value).

When the chromatic aberration of the composite system is corrected as described above, the axial chromatic aberration of the composite system is almost completely corrected, and the spherical aberration on the long wavelength side and the spherical aberration on the short wavelength side are almost completely corrected. Since the gap between the diffraction ring zone structures of the optical element for correcting chromatic aberration can be increased as compared with the case where the movement of the best image point position at the time of wavelength change is suppressed by correction, the shape error of the diffraction ring zone structure at the time of fabrication can be increased. It is possible to reduce the light amount loss due to.

<Example 2>

Table 3 shows data on the optical system for the optical pickup of the second embodiment, and FIG. 7 shows an optical path diagram of the optical system for the optical pickup of the second embodiment.

[0091]

[Table 3]

In Example 2, the aberration correcting optical element was formed of an olefin resin, and the optical surface on the light source side was macroscopically a flat diffraction surface to correct the axial chromatic aberration of the objective lens. Further, the paraxial power of the aberration correction optical element was set to 0 by making the refracting power of the optical surface on the objective lens side negative and making its absolute value the same as the absolute value of the diffraction power of the optical surface on the light source side. Furthermore, the coefficient of the optical path difference function in Table 3 is set to 2
It was determined that the secondary diffracted light had the maximum amount of diffracted light.

FIG. 8 is a graph showing spherical aberration and axial chromatic aberration at a wavelength of 405 ± 10 nm in the optical system for the optical pickup of Example 2, showing that the focus hardly moves regardless of the wavelength.

<Example 3>

Table 4 shows data on the optical system for the optical pickup of the third embodiment, and FIG. 9 shows an optical path diagram of the optical system for the optical pickup of the third embodiment.

[0096]

[Table 4]

In Example 3, the aberration correcting optical element was formed of an olefin resin, and the optical surface on the light source side was a diffractive surface to correct the axial chromatic aberration of the objective lens. Also, by making the refracting power of the optical surface on the light source side negative and making its absolute value the same as the absolute value of the diffraction power, and making the optical surface on the objective lens side a plane, the paraxial axis of the aberration correction optical element is The power was set to 0. Further, the coefficient of the optical path difference function in Table 4 was determined so that the second-order diffracted light had the maximum light amount. In addition, by using a term of the second or higher order of the coefficient of the optical path difference function, and by using a diffractive surface having a spherical aberration characteristic such that the spherical aberration changes in the undercorrection direction when the wavelength changes to the long wavelength side, The change in spherical aberration that occurs in the objective lens when the wavelength changes to the long wavelength side is offset and corrected.

FIG. 10 is a graph showing the spherical aberration and the axial chromatic aberration at the wavelength of 405 ± 10 nm of the optical system for the optical pickup of Example 3, showing that the focus and the spherical aberration hardly change regardless of the wavelength. ing.

<Example 4>

Table 5 shows data relating to the optical system of Example 4, and FIG. 11 shows an optical path diagram. Further, FIG. 12 shows a spherical aberration diagram and an axial chromatic aberration at 405 nm ± 1 nm of the optical system of the present example.

[0101]

[Table 5]

In this embodiment, an optical element for aberration correction, which is a plastic lens, is provided on the light source side of the plastic objective lens shown in Table 1. The aberration correction optical element of this example is the same optical element as the aberration correction optical element of Example 1.

<Example 5>

Table 6 shows data relating to the optical system of Example 5, and FIG. 13 shows an optical path diagram. Further, FIG. 14 shows a spherical aberration diagram and an axial chromatic aberration at 405 nm ± 1 nm of the optical system of the present example.

[0105]

[Table 6]

In this embodiment, an optical element for aberration correction, which is a plastic lens, is provided on the light source side of the plastic objective lens shown in Table 1. The aberration correction optical element of this example is the same optical element as the aberration correction optical element of Example 2.

In the lens data of Tables 2, 3, and 4, r (mm) is the radius of curvature, d (mm) is the surface spacing,
N _λ represents the refractive index at a wavelength of 405 nm. In addition, the design reference wavelength of the aberration correction optical element and the objective lens in each of Examples 1 to 3 is 405 nm, and the design reference temperature is 25 ° C.

In the lens data of Table 1, Table 5 and Table 6, r (mm) is the radius of curvature, d (mm) is the surface spacing,
N ₄₀₄ , N ₄₀₅ , and N ₄₀₆ are wavelengths 404, respectively.
represents the refractive index at nm, 405 nm, 406 nm,
The design reference wavelength of the aberration correction optical element and the objective lens in each of Example 4 and Example 5 is 405 nm, and the design reference temperature is 25 ° C.

The aberration correction optical elements of Examples 1 to 5 are as follows:
The diffractive power of the diffractive lens (its sign is positive) and the refractive power of the diffractive lens (its sign is negative) have different signs, and the absolute values are the same. Has a paraxial power of 0 at the design reference wavelength of 405 nm. Therefore, since the refracting power of the aberration correction optical element is negative, a convergent light beam is emitted from the aberration correction optical element when the temperature rises. As a result, when the temperature rises, the third-order spherical aberration that changes in the direction of undercorrection in the objective lens is canceled out.

FIG. 15 shows the results of correcting the axial chromatic aberration generated in the objective lens shown in Table 1 by the instantaneous wavelength change of the light source by the aberration correcting optical elements of Examples 4 and 5. When a wavelength change of ± 1 nm, which is equivalent to a wavelength change due to mode hopping of a blue-violet semiconductor laser, occurs, the RMS value of the wavefront aberration in the objective lens alone (“without aberration correction optical element” in FIG. 15) is 0.03λrms
As described above, by combining this objective lens with the aberration correction optical element, the RMS value of the wavefront aberration is 0.03 λrms with respect to the wavelength change of ± 1 nm in any of Examples 4 and 5. It is below.

In FIG. 15, the wavelength change 0 corresponds to the design reference wavelength of 405 nm, and the RM of the wavefront aberration is
When calculating the S value, the objective lens is fixed at the best focus position at 405 nm.

FIG. 16 shows the result of correction of the third-order spherical aberrations changed by the objective lens shown in Table 1 by the aberration correction optical elements of Examples 4 and 5 due to the temperature rise.
At 85 ° C. which is the operation compensation temperature of the optical disk player, the RMS value of the wavefront aberration is 0.03 λrm in the objective lens alone (“without aberration correction optical element” in FIG. 16).
s (maximum capacity of wavefront aberration) or more, but by combining this objective lens with no aberration correction optical element, the RMS value of wavefront aberration is 0.03 λrms in any of Examples 4 and 5. It is below.

When calculating the RMS value of the wavefront aberration in FIG. 16, only the change in the refractive index of the plastic lens with respect to the change in environmental temperature is considered, and the change amount is about −1.
It is ^0x10 < ^-4 > / ^degreeC .

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

[0115]

According to the present invention, even when a high NA plastic objective lens is used in a high density recording optical pickup system or the like, a change in spherical aberration of the objective lens due to a temperature change can be achieved with a relatively simple structure. It is possible to provide an optical system for an optical pickup capable of suppressing the above, an optical pickup device including the optical system, and a recording / reproducing device including the optical system.

Further, it is possible to provide an aberration correcting optical element capable of correcting the change of the spherical aberration of the objective lens due to the temperature change when used together with the plastic objective lens in which the change of the spherical aberration due to the temperature change is large.

Further, even when a light source having poor monochromaticity or a light source whose wavelength changes abruptly is used in a high-density recording optical pickup system or the like, axial chromatic aberration can be corrected with a relatively simple structure, and An optical pickup optical system that can be manufactured at low cost, an optical pickup apparatus including the same, and a recording / reproducing apparatus including the same can be provided.

Further, since the numerical aperture on the optical information recording medium side is large, correction of spherical aberration, correction of sine conditions and miniaturization,
An optical element for aberration correction capable of correcting axial chromatic aberration when used with an objective lens in which axial chromatic aberration remains for the purpose of reducing thickness, weight, and cost.

[Brief description of drawings]

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

2 (a) is a side view of the aberration correction optical element of FIG. 1, and FIG. 2 (b) is an aberration correction optical element of FIG. 1 viewed from the arrow direction A of FIG. 2 (a). FIG.

FIG. 3 is a diagram showing an example of an objective lens in which axial chromatic aberration is corrected by the aberration correction optical elements of Examples 1 to 3 according to the present invention, and is an optical path diagram of this objective lens.

4 is a graph showing spherical aberration and axial chromatic aberration at a wavelength of 405 ± 10 nm of the objective lens in FIG.

FIG. 5 is an optical path diagram of an optical system for an optical pickup of Example 1.

FIG. 6 is a wavelength 4 of the optical system for the optical pickup according to the first embodiment.
7 is a graph showing spherical aberration and axial chromatic aberration at 05 ± 10 nm.

FIG. 7 is an optical path diagram of an optical system for an optical pickup according to a second embodiment.

FIG. 8 is a wavelength 4 of the optical system for the optical pickup according to the second embodiment.
7 is a graph showing spherical aberration and axial chromatic aberration at 05 ± 10 nm.

FIG. 9 is an optical path diagram of an optical system for an optical pickup according to a third embodiment.

FIG. 10 is a graph showing spherical aberration and axial chromatic aberration at a wavelength of 405 ± 10 nm in the optical system for the optical pickup of Example 3.

FIG. 11 is an optical path diagram regarding the optical system of Example 4;

FIG. 12 is a diagram showing a spherical aberration diagram and an axial chromatic aberration at 405 nm ± 1 nm of the optical system of Example 4;

FIG. 13 is an optical path diagram regarding the optical system of Example 5;

FIG. 14 is a diagram showing a spherical aberration diagram and an axial chromatic aberration at 405 nm ± 1 nm of the optical system of Example 5;

FIG. 15 is a diagram showing a result of correcting axial chromatic aberration generated in the objective lens shown in Table 1 due to an instantaneous wavelength change of a light source by the aberration correction optical elements of Examples 4 and 5.

16 is a diagram showing a result of correcting the third-order spherical aberration changed by the objective lens shown in Table 1 by the aberration correcting optical elements of Examples 4 and 5. FIG.

FIG. 17 shows a conventional NA 0.85 and design reference wavelength 405.
It is a figure which shows the mode of spherical aberration change with respect to temperature change of the plastic objective lens of 2 groups 2 elements which is 25 nm, focal length 1.76, and a design reference temperature is 25 degreeC.

FIG. 18 is a diagram showing spherical aberration and axial chromatic aberration at a wavelength of 405 ± 1 nm of the objective lens in Table 1.

[Explanation of symbols]

1 Optical Pickup Device 2 Semiconductor Laser (Light Source) 5 Optical Disc (Optical Information Recording Medium) 5 ′ Information Recording Surface 13 Objective Lens 14 Aberration Correction Optical Element S1 Chromatic Aberration Correction Optical Element 3 Surface S2 on the Light Source Side Chromatic Aberration Correction Optical Element Objective lens 3
Side surface p diffraction pattern

Continued front page    F-term (reference) 2H087 KA13 LA01 NA04 NA08 NA14                       PA03 PA17 PB03 QA01 QA03                       QA05 QA18 QA19 QA21 QA25                       QA33 QA41 QA46 RA05 RA12                       RA13 RA42 RA46 UA01                 5D119 AA02 AA11 AA22 AA40 BA01                       BB01 BB02 BB03 BB04 BB05                       DA01 DA05 EB02 EC01 EC07                       FA05 JA02 JA09 JA12 JA29                       JA32 JA43 JB03 JB04 JB06

Claims (18)

[Claims] 1. A light source and an image-side numerical aperture of 0.75 or more.
And has at least one plastic lens
Aberration correction light placed in the optical path between the objective lens and
An academic element, The aberration-correcting optical element is duplicated on at least one surface.
1-group 1-element structure having a diffractive structure consisting of several ring-shaped steps
Is a plastic lens of The aberration-correcting optical element is configured to change the temperature of the objective lens.
Plastic lens included in the objective lens according to
Index change of at least one plastic lens
Change ΔN_OBJThird-order spherical aberration of the objective lens due to
Change Δ3SA _OBJIs the temperature of the aberration correction optical element.
Change ΔN of the aberration correction optical element due to the change in degree
_ACLight flux emitted from the aberration correction optical element due to
Change of inclination angle of marginal ray of
Aberration correction optical element. 2. The following formula is satisfied: 1.
The aberration-correcting optical element described in. P _T1 <proviso _{P T0 <P T2, P T0} : paraxial power of the aberration correcting optical element at a given temperature ^{_{T 0 (mm - 1) P}} T1: predetermined temperature difference than the predetermined temperature _{T 0} Paraxial power (mm ⁻¹ ) P _{T2 of the} aberration correction optical element at a low temperature T ₁ : Paraxial of the aberration correction optical element at a temperature T _{2 that} is higher than the predetermined temperature T ₀ by a predetermined temperature difference. Power (mm ^-1 ) 3. The objective lens, in order from the light source side,
A first lens having a positive refracting power and a second lens having a positive refracting power
The aberration correction optical element according to claim 1 or 2, wherein the objective lens is a two-group two-element objective lens in which a lens is arranged, and at least the first lens is a plastic lens. 4. The method according to claim 3, wherein the following expression is satisfied.
The aberration-correcting optical element described in. P _R <0 0 <ΔP _AC / ΔT _AC <1 × 10 ⁻⁴ , where P _R : refraction power (mm ⁻¹ ) as a refraction lens of the aberration correction optical element ΔP _AC : of the aberration correction optical element Temperature change ΔT _AC
Change amount of paraxial power of the aberration correction optical element due to (° C.) (mm ⁻¹ ) 5. The aberration correction optical element according to claim 2, wherein the light source is a light source that generates light having a wavelength of 550 nm or less, and the aberration correction optical element satisfies the following expression. . P _λ1 <P _λ0 <P _λ2 where P _λ0 : paraxial power (mm ^−l ) of the aberration correction optical element at the wavelength λ ₀ of the light generated by the light source P _{λ 1} : wavelength 10 nm shorter than the wavelength λ ₀ Paraxial power of the aberration correction optical element at λ ₁ (mm ⁻¹ ) P _λ2 : Paraxial power of the aberration correction optical element at the wavelength λ ₂ longer by 10 nm than the wavelength λ ₀ (mm ⁻¹ ). 6. The aberration-correcting optical element according to claim 3, 4 or 5, wherein the following expression is satisfied. | Δ3SA _OBJ | / | Δ5SA _OBJ |> 1 −30.0 × 10 ⁻⁴ <Δ3SA _OBJ / (ΔT _OBJ
・ NA ⁴ · f _OBJ ) <03 × 10 ⁻² <ΔfB _OBJ · νd _OBJ / f _OBJ <
14 × 10 ⁻² 1.0 × 10 ⁻² <P _D <10.0 × 10 ⁻² However, Δ3SA _OBJ : Temperature change of the objective lens ΔT _OBJ
(3) A change amount of the third-order spherical aberration component when the aberration of the objective lens is expanded into a Zernike polynomial when the refractive index of the plastic lens of the objective lens changes by ΔN _OBJ due to (° C.). Is expressed as an RMS (Root Mean Square) value in which the wavelength λ ₀ (nm) of the light generated by is generated. The sign is “+” when the third-order spherical aberration component changes in the direction of overcorrection, and the third-order spherical surface. The case where the aberration component changes in the direction of insufficient correction is defined as "-". Δ5SA _OBJ : Temperature change of the objective lens ΔT _OBJ
A change amount of the fifth-order spherical aberration component when the aberration of the objective lens is expanded into a Zernike polynomial when the refractive index of the plastic lens of the objective lens changes by ΔN _OBJ due to (° C.), RMS (Root Mean Square) in units of wavelength λ ₀ (nm) of light generated by the light source
Expressed as a value. NA: A predetermined image-side numerical aperture f _{OBJ of the} objective lens required for recording and / or reproducing of the optical information recording medium: Focal length (mm) of the objective lens ΔfB _OBJ : The light source is generated in the objective lens Axial chromatic aberration (mm) νd _{OBJ of the} objective lens when light having a wavelength longer than the wavelength λ ₀ of the light by a wavelength difference of +10 nm is incident νd _OBJ : Abbe number of d line of the first lens of the objective lens, Average value P _{D of} the second lens with the Abbe number of the d-line: a wavefront that is transmitted through the diffractive structure formed on the i-th surface of the aberration correction optical element The additional optical path difference Φ _bi is the height h from the optical axis.
_As a function of _i (mm), Φ _bi = ni · (b _2i · h _i ² + b _4i · h _i ⁴ + b
_6i · h _i ⁶ + ...) In the case of being represented by the optical path difference function (here, b
_2i , b _4i , b _6i ... _Are secondary, quaternary, and 6 respectively.
Next, an optical path difference function coefficient, ni is a diffraction order having the maximum diffracted light amount among the diffracted light generated by the diffractive structure formed on the i-th surface), P _D = Σ (−2. b _2i · ni) diffractive power (mm ⁻¹ ) as a diffractive lens 7. The aberration-correcting optical element macroscopically includes one optical surface on which a diffractive structure formed of a plurality of plane ring-shaped steps is formed, and a concave refracting surface on the opposite surface. The optical element for aberration correction according to any one of claims 1 to 6, further comprising: the other optical surface formed. 8. The aberration correction optical element comprises:
Wavelength of generated light λ ₀The optical element for aberration correction in
Paraxial power P of child₀Is characterized by being substantially zero
Item 9. An aberration correction optical element according to any one of items 1 to 7.
Child. 9. The aberration correcting optical element according to claim 8, wherein the aberration correcting optical element satisfies the following expression. P _D > 0 P _R <0-0.9> P _D / P _R > -1.1 10. The aberration-correcting optical element according to claim 1, wherein the following expression is satisfied. P _T2 <proviso _{P T0 <P T1, P T0} : paraxial power of the aberration correcting optical element at a given temperature ^{_{T 0 (mm - 1) P}} T1: predetermined temperature difference than the predetermined temperature _{T 0} Paraxial power (mm ⁻¹ ) P _{T2 of the} aberration correcting optical element at a low temperature T ₁ : Paraxial of the aberration correcting optical element at a temperature T ₂ higher than the predetermined temperature T ₀ by a predetermined temperature difference. Power (mm ^-1 ) 11. The aberration-correcting optical element according to claim 10, wherein the objective lens is a plastic lens having a structure of one lens in one group. 12. At least one ring zone step having a step difference Δ (mm) in the optical axis direction such that m represented by the following equation is an integer other than 0 and ± 1 is formed within an effective diameter. The aberration-correcting optical element according to any one of claims 1 to 11, characterized in that m = INT (Y) Y = Δ × (n−1) / (λ ₀ × 10 ⁻³ ) where INT (Y): an integer obtained by rounding Y: n: wavelength λ of light generated by the light source Refractive index λ ₀ of the aberration correction optical element at ₀ (nm): wavelength (nm) of light generated by the light source 13. The aberration-correcting optical element according to claim 1, further comprising a diffractive structure having a plurality of annular zone steps on both surfaces. 14. Spherical aberration characteristics such that when the wavelength of an incident light beam changes to the long wavelength side, the spherical aberration of the outgoing light beam changes in the direction of undercorrection or overcorrection. 2. At least one surface having a diffractive structure satisfying the formula is provided.
The aberration-correcting optical element according to any one of 3 above. 0.2 ≦ | (P _hf / P _hm ) −2 | ≦ 6.0, where P _hf : annular zone spacing P _{hm in} a direction perpendicular to the optical axis of the diffractive structure at a position hf that is half the maximum effective diameter _hm. : Ring spacing in the direction perpendicular to the optical axis of the diffractive structure at the maximum effective diameter hm 15. The wavelength at which the diffraction efficiency is maximum is 550 n.
The aberration-correcting optical element according to any one of claims 1 to 14, wherein the optical element is m or less. 16. A light source, an objective lens having an image-side numerical aperture of 0.75 or more and having at least one plastic lens, and an aberration arranged in an optical path between the light source and the objective lens. And a correction optical element,
An optical system for reproducing information from an information recording surface of an optical information recording medium and / or recording information on the information recording surface, wherein the aberration correcting optical element is any one of claims 1 to 15. An optical system having the aberration correction optical element described in the item. 17. A light source, an objective lens having an image-side numerical aperture of 0.75 or more and having at least one plastic lens, and an aberration arranged in an optical path between the light source and the objective lens. An optical pickup device for reproducing information from an information recording surface of an optical information recording medium and / or recording information on the information recording surface, comprising: an optical system having a correcting optical element; An optical pickup device comprising the optical system according to claim 16 as a system. 18. An audio and / or image recording device, and / or an audio and / or image reproducing device, comprising the optical pickup device according to claim 17.