JP2005038481A - Optical system for optical pickup device, optical pickup device, optical information recording and reproducing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element for an optical pickup device which can secure sufficient working distance, is rectified in chromatic aberration and spherical aberration over wavelength change of incoming luminous flux and furthermore, and is high in optical use efficiency; an optical pickup device carrying this optical element as an objective lens; and an optical information recording and reproducing apparatus carrying this optical pickup device. <P>SOLUTION: This optical pickup device 10 in an optical system for the optical pickup device is constituted of; an aberration compensation element 40 made of plastic; and a glass condenser lens 50 for carrying out imaging of the luminous flux ejected from the aberration compensation element. The aberration compensation element has at least one for each of optical surfaces on which a diffraction structure 70 is formed and an optical surfaces on which an optical path difference grant structure 60 is formed, and the condenser lens is a refraction single lens of one group of one piece configuration having at least one non-spherical face. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

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２に表２中の係数を代入した数式で表される。但し、κを円錐係数、Ａ_２ｉを非球面係数とする。
【００８８】
【数２】

【表２】

【００８９】
また、収差補正素子の回折構造は、この回折構造により透過波面に付加される光路差で表される。かかる光路差は、光軸に垂直な方向の高さをｈ（ｍｍ）、ｂ_２ｊを光路差関数係数とするとき次の数３で定義される光路差関数Φ_ｂ（ｍｍ）に表３中の係数を代入した数式で表される。
【００９０】
【数３】

【表３】

【００９１】
また、光路差付与構造の各輪帯の形状は、表４で表される。
【表４】

【００９２】
表４中のｉは光路差付与構造の中心領域及び各輪帯の番号を表し、光軸を含む中心領域をｉ＝１、中心領域の外側（光軸から離れる方向）に隣接する輪帯をｉ＝２、さらに外側に隣接する輪帯をｉ＝３、…とする。即ち、本実施例における対物レンズの収差補正素子には、中心領域の外側に４本の輪帯が形成されている。そして、ｈ_ｉ−１と、ｈ_ｉはそれぞれ中心領域及び各輪帯の始点高さ、終点高さを示す。ｍ_ｉｄは中心領域に対する各輪帯の光軸方向の変移量（光源から光ディスク方向に向かって変移する場合を正とする）を表す。例えば、中心領域の外側に隣接する輪帯（ｉ＝２）は、中心領域に対して光ディスク側に３．８６μｍ変移している。また、ｍ_ｉは、設計基準波長４０５ｎｍにおいて、各輪帯を透過する光束に付与される中心領域を透過する拘束に対する光路差を表し、たとえば中心領域の外側に隣接する輪帯（ｉ＝２、ｍ＝５）を透過する光束は、中心領域を透過する光束に対して５λ（但し、λ＝４０５ｎｍ）の光路差が付与されるので、位相差に換算して２π×５（ｒａｄ）だけ位相が遅れることになる。
【００９３】
入射光束の波長が設計基準波長４０５ｎｍから４１０ｎｍに変化した場合における波面収差は０．０１２λｒｍｓとなった。また、青紫色半導体レーザーのモードホッピングにより、本実施例における対物レンズに対する入射光束の波長が、設計波長の４０５ｎｍから４０６ｎｍになった場合の波面収差のデフォーカス成分は０．０２４λｒｍｓとなった。
また、本実施例の対物レンズ系において、収差補正素子除いた対物レンズのみに対して、入射光束の波長が、設計波長の４０５ｎｍから４０６ｎｍになった場合の波面収差のデフォーカス成分は０．１１１λｒｍｓとなった。
これにより、上記実施例ではガラス製対物レンズのみのときに比べ、収差補正素子の回折構造の作用により、軸上色収差が補正されていることが分る。
【００９４】
【比較例】
次に、上記実施例における光路差付与構造の球面収差補正効果を説明する。比較例として、上記実施例の収差補正素子の第１面に形成した光路差付与構造が無い場合を考える。
【００９５】
この比較例において、入射光束の波長が設計基準波長４０５ｎｍから４１０ｎｍに変化した場合における波面収差は０．０５８λｒｍｓとなった。
これにより上記実施例の対物レンズ系では、光路差付与構造の作用により、入射光の波長が変化したときに発生する球面収差が良好に補正されていることが分る。
【００９６】
【発明の効果】
本発明によれば、十分な作動距離を確保出来、更に色収差及び入射光束の波長が変化した際の球面収差を補正出来、光利用効率の高いピックアップ装置用の光学素子、及びこの光学素子を対物レンズとして搭載した光ピックアップ装置、この光ピックアップ装置を搭載した光情報記録再生装置が得られる。
【図面の簡単な説明】
【図１】本発明に係る光ピックアップ装置の構成を示す平面図である。
【図２】本発明に係る光学系の構成を示す横断面図である。
【図３】回折構造及び光路差付与構造を示す横断面図である。
【図４】本発明に係る光ピックアップ装置の構成を示す平面図である。
【符号の説明】
１０ 光ピックアップ装置
４０ 収差補正素子
５０ 集光レンズ
６０ 光路差付与構造
６１ 中心領域
６２ 輪帯
７０ 回折構造
７１ 中心領域
７２ 輪帯
８０ 接合部材[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical system for an optical pickup device, an optical pickup device, and an optical information recording / reproducing device.
[0002]
[Prior art]
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 is proposed to use an objective lens having a numerical aperture NA of 0.85 in order to achieve high density. Yes.
[0003]
As an objective lens with 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 beam to the two lenses. It is described in the following Patent Document 1.
However, since a high-density disc using a laser beam with a wavelength of 405 nm and an objective lens with NA of 0.85 employs a protective layer with a thickness of 0.1 mm, the scratch on the protective layer surface causes information recording / reproduction. 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.
[0004]
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 disc. Since the height is reduced and the distance (working distance) from the objective lens to the optical disk is shortened, 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 Patent Document 1, the mass of the objective lens becomes large, so that the possibility that the objective lens scratches the surface of the protective layer due to interference with the optical disk becomes very high. .
[0005]
In order to cope with this problem, a single lens made of glass having an NA of 0.85 has been proposed as described in Patent Document 2 below.
With a single lens configuration, a large working distance can be secured and the weight can be reduced, so that it is possible to prevent damage to the surface of the protective layer due to interference between the objective lens and the optical disk. Then, since the curvature of the optical surface on the light source side tends to be large, coma aberration easily occurs due to the optical axis shift between the optical surfaces. Therefore, in order to increase the production efficiency of a single lens with NA of 0.85, it is necessary to secure a sufficient margin for the optical axis deviation between optical surfaces by using a high refractive index glass material.
[0006]
Also, in an optical pickup device, since the laser power during recording is generally larger than the power during information reproduction, a mode in which the center wavelength of the laser light source instantaneously jumps several nm due to output changes when switching from reproduction to recording is performed. May cause hopping. The focus position shift caused by such mode hopping can be removed by focusing the objective lens. However, in the several nsec until the objective lens is focused, problems such as recording failure due to the focus position shift occur. 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.
[0007]
A single lens tends to generate a larger amount of chromatic aberration with respect to the same wavelength change of incident light beam than a two-group lens. In particular, in the above-mentioned single lens made of glass, a margin for optical axis deviation between optical surfaces is increased. When a high refractive index glass material is used in order to ensure sufficiently, the focus position shift becomes larger. This is because a high refractive index glass material generally has a large wavelength dispersion, and a chromatic aberration of a single lens made of such a high refractive index glass material increases. For these reasons, when a blue-violet semiconductor laser is used as a light source and a single lens made of high refractive index glass is used as an objective lens, correction of chromatic aberration of the objective lens is essential.
[0008]
By the way, conventionally, a single lens made of plastic is mainly used as an objective lens used in an optical pickup device for recording / reproducing of an optical information recording medium such as CD, MO, and DVD.
Since the specific gravity of the plastic lens is smaller than that of the glass lens, the burden on the actuator that drives the objective lens can be reduced, and the objective lens can be tracked at high speed.
In addition, plastic lenses manufactured by injection molding of plastic materials with molds can be mass-produced with high precision by producing the desired molds with good production, and the high performance and low cost of the lenses. Can be achieved.
[0009]
Therefore, it is possible to mass-produce an objective lens with a single lens structure for a high-density optical disk, which is advantageous for a two-group lens in terms of securing a large working distance, at a low cost with a light weight and stable performance. It is preferable to use a plastic lens.
However, when a single lens having a high NA is used as a plastic lens, spherical aberration (hereinafter referred to as “temperature aberration” in the present specification) caused by a change in refractive index accompanying a change in temperature becomes a problem. This is because the plastic lens is about two orders of magnitude larger than the glass lens in the refractive index change accompanying the temperature change. Since this temperature aberration is proportional to the fourth power of NA, when the objective lens of NA 0.85 used for the high density optical disk is a plastic lens, there is a problem that the usable temperature range becomes very narrow. .
[0010]
In response to such a problem, a new optical material (hereinafter referred to as “refractive index”) having a reduced refractive index with a change in temperature while maintaining the moldability of the plastic material by mixing an inorganic material having a diameter of 10 nm to 15 nm in the plastic material. In this specification, such an optical material is referred to as “athermal resin”) (Nikkan Kogyo Shimbun, July 4, 2003). According to this announcement, a plastic material whose refractive index decreases as temperature rises (in this example, acrylic resin), and an inorganic material whose refractive index increases as temperature rises (in this example, niobium oxide (Nb ₂ O ₅ )) Can be mixed uniformly at a volume ratio of 4: 1 to cancel the temperature dependence of the refractive index of both.
By using such an athermal resin, it becomes possible to mass-produce a single lens of NA 0.85 that does not deteriorate the light collecting performance even if the environmental temperature changes by injection molding, but there still remains a problem regarding chromatic aberration.
[0011]
As an element for correcting the chromatic aberration of the objective lens with a simple configuration, a diffraction element using a diffraction action is known. In an optical pickup apparatus using a blue-violet semiconductor laser as a light source, an optical pickup apparatus including such a diffraction element for correcting chromatic aberration of an objective lens is described in Patent Document 3 below.
The optical pickup device described in Patent Document 3 corrects chromatic aberration of an objective lens by a diffractive element arranged in a parallel light beam between a blue-violet semiconductor laser light source and an objective lens. When the laser beam length changes, the light beam emitted from the diffractive element toward the objective lens becomes a convergent light beam due to diffraction, and when the semiconductor laser wavelength changes in the shorter direction, it becomes a divergent light beam. Thus, the chromatic aberration of the objective lens is corrected.
[0012]
In the present specification, the condensing position where the diameter of the spot collected by the optical element is the minimum is referred to as “focus position”, and the movement of the focus position due to the wavelength change of the incident light beam is referred to as “focus position deviation”. . In this specification, “focus position shift” and “chromatic aberration” are synonymous, and a change in wavefront aberration caused by chromatic aberration of an optical element with respect to a predetermined amount of wavelength change of an incident light beam is less than the diffraction limit. This correction is called “correction of chromatic aberration”.
[0013]
[Patent Document 1]
JP-A-10-123410
[Patent Document 2]
JP 2003-114383 A
[Patent Document 3]
JP 2001-256672 A
[0014]
[Problems to be solved by the invention]
However, when the divergence of the light beam that passes through the diffraction element and travels toward the objective lens changes due to the wavelength change of the semiconductor laser, spherical aberration occurs because the magnification of the objective lens changes. In particular, in order to correct the chromatic aberration of a single lens made of high refractive index glass, the change in the divergence of the light beam that passes through the diffraction element and travels toward the objective lens as the wavelength of the semiconductor laser changes (that is, the diffractive power in the paraxial direction ) Must be set large, and as a result, when the wavelength of the semiconductor laser changes, the change in spherical aberration accompanying the change in magnification of the objective lens becomes larger.
[0015]
A semiconductor laser has a characteristic that the oscillation wavelength becomes longer when the temperature rises due to a change in environmental temperature or heat radiation from the actuator, but a diffractive element is used to correct chromatic aberration of a single lens made of high refractive index glass. In the optical pickup device, since the change in spherical aberration with respect to the change in wavelength is large, the change in spherical aberration caused by the change in wavelength of the semiconductor laser accompanying the change in temperature is a non-negligible amount.
Furthermore, semiconductor lasers used as light sources in optical pickup devices have wavelength variations of about ± 10 nm in individual oscillation wavelengths due to manufacturing errors. Therefore, if the spherical aberration change with respect to the wavelength change of the objective lens is large, it is not possible to use a semiconductor laser that deviates from the reference wavelength, and it is necessary to select the semiconductor laser, and the manufacturing cost of the optical pickup device is reduced. To rise.
[0016]
In addition, in the above-described single lens made of athermal resin, when the chromatic aberration is corrected by the diffraction element, when the incident wavelength to the diffraction element is changed due to a change in the environmental temperature or individual oscillation wavelength variation, The change in spherical aberration accompanying the change in magnification of the objective lens is an amount that cannot be ignored.
This is because the amount of change in spherical aberration accompanying the change in magnification increases in proportion to the fourth power of NA and 1 / λ, and it is assumed that the wavelength dispersion of the athermal resin is currently a material for optical elements for optical disks. Even if the glass material that can be used as a material is improved to the same level as that of the material having the smallest wavelength dispersion (the Abbe number in d-line is about 70), when the incident wavelength to the diffraction element changes, the magnification changes. Correction of the accompanying spherical aberration is necessary.
[0017]
Here, it is possible to correct the spherical aberration change with respect to the wavelength change by giving the above-mentioned diffraction element the wavelength dependence of the spherical aberration, but in addition to the focus position shift with respect to the wavelength change of the incident light flux, When correcting the change in spherical aberration with respect to the change in the wavelength of the light beam, the width in the direction perpendicular to the annular zone of the diffractive structure is reduced, resulting in a decrease in diffraction efficiency due to improper transfer during molding, and in the optical axis direction of the annular zone. There is a concern that the light use efficiency may be lowered due to the light beam incident on the step being blocked in its path. Furthermore, since the number of annular zones increases, the time required for mold processing becomes longer and the manufacturing cost of the diffraction element increases.
[0018]
An object of the present invention is to take into account the above-mentioned problems, can secure a sufficient working distance, correct chromatic aberration and spherical aberration with respect to a wavelength change of an incident light beam, and further, for an optical pickup device having high light utilization efficiency. An optical element, an optical pickup device in which the optical element is mounted as an objective lens, and an optical information recording / reproducing device in which the optical pickup device is mounted are provided.
[0019]
[Means for Solving the Problems]
In order to solve the above-described problems, the invention described in claim 1 includes a plastic aberration correction element and a glass condensing lens for forming an image of a light beam emitted from the aberration correction element. In the optical system for an optical pickup device, the aberration correction element has at least one optical surface on which a diffractive structure is formed and one optical surface on which an optical path difference providing structure is formed. The lens is a refracting single lens having at least one aspherical surface and having one lens element per group.
[0020]
According to the first aspect of the present invention, it is possible to secure a large working distance by providing the condensing lens disposed on the optical disc side exclusively with the refractive power with respect to the incident light beam. Interference can be prevented. Further, since the chromatic aberration of the condenser lens is corrected by the diffractive structure, good recording / reproducing characteristics can be maintained even when the wavelength of the incident light beam changes instantaneously by laser mode hopping.
[0021]
In addition, since the chromatic spherical aberration is corrected by the optical path difference providing structure, the spherical aberration does not become too large even when the wavelength of the semiconductor laser changes due to temperature change, and the oscillation wavelength is the reference due to manufacturing errors. Even if a semiconductor laser deviated from the wavelength is used, the spherical aberration does not become too large.
Then, by using the diffractive structure for correcting axial chromatic aberration and using the optical path difference providing structure for correcting chromatic spherical aberration, the length of the zonal zone of the diffractive structure in the direction perpendicular to the optical axis is increased, and the aberration correction element is formed. Transferability is improved. Furthermore, since a structure having fine steps such as a diffractive structure and 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 is blocked and condensed. The ratio of the light flux that does not contribute to the formation of the spot can be suppressed, and a decrease in the amount of light can be prevented.
[0022]
Note that the condenser lens that bears most of the refractive power for incident light flux is used in the optical pickup device in which the optical system according to the present invention is mounted in order to ensure a sufficient margin for optical axis misalignment between optical surfaces in manufacturing. It is desirable that the laser light source is made of a glass material having a refractive index of 1.55 or more at the oscillation wavelength.
In this specification, an optical disk that uses a blue-violet laser such as a blue-violet semiconductor laser or a blue-violet SHG laser as a light source for recording / reproducing information is collectively referred to as a “high-density optical disk”, and NA0. Recording / reproducing information with an objective optical system of .85, recording / reproducing information with an objective optical system of NA 0.65, in addition to a standard optical disc with a protective layer thickness of about 0.1 mm, It also includes a standard optical disc having a protective layer thickness of about 0.6 mm. In addition to an optical disc having such a protective layer on its information recording surface, an optical disc having a protective film with a thickness of several to several tens of nm on the information recording surface, or a protective film having a thickness of 0 Some optical disks are also included. In this specification, the high-density optical disk includes a magneto-optical disk that uses a blue-violet laser as a light source for recording / reproducing information.
[0023]
In this specification, “aberration correction element” refers to an optical element having a function of suppressing a change in wavefront aberration that occurs in another optical element due to a change in wavelength of an incident light beam. , An optical element having a function of condensing and focusing the light beam emitted from the “aberration correction element”. That is, the “aberration correction element” suppresses a change in wavefront aberration that occurs in the “condensing lens” with 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 closer to the laser light source than the condensing lens. The “condensing lens” is arranged closer to the optical disc than the aberration correction element.
[0024]
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 a fine step outside the central region. When the wavelength of the incident light beam is a predetermined wavelength, an optical path difference that is an integral multiple of the wavelength of the incident light beam is generated between wavefronts that pass through the adjacent annular zones, and when the wavelength changes from the predetermined wavelength, It 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.
A specific 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 becomes shorter with respect to the central region, and the maximum The annular zone at the position of 75% of the effective diameter position is formed by shifting 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. Structure.
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.
[0025]
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 on the outside of the central region, and the incident light flux When the wavelength is a predetermined wavelength, an optical path difference that is an integral multiple of the wavelength of the incident light between wavefronts passing through the adjacent annular zones is generated by diffraction, and the wavelength of the incident light beam is changed from the predetermined wavelength. In some cases, it 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 the incident light beam.
The specific structure of such a “diffractive structure” is a structure in which the cross-sectional shape including the optical axis is a staircase shape in which the optical path length increases as the distance from the optical axis increases, or the optical axis A cross-sectional shape including the sawtooth shape.
[0026]
The invention according to claim 2 is an optical system for an optical pickup device comprising 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 optical surface on which a diffractive structure is formed and one optical surface on which an optical path difference providing structure is formed, and the condenser lens has a diameter of 30 nm in a plastic material. It is a refractive single lens of one group and one lens having at least one aspherical surface formed by using a material in which the following particles are dispersed.
[0027]
According to a third aspect of the present invention, in the optical system for an optical pickup device according to the second aspect, the condenser lens satisfies the following expression (1).
| A | <10 × 10 ^-5 (1)
However, A is the rate of change of the refractive index of the condenser lens with respect to temperature change, the refractive index n of the condenser lens with respect to the wavelength of the laser light source of the optical pickup device on which the optical system is mounted, The linear expansion coefficient α and the molecular refractive power [R] of the condenser lens are expressed by the above equation (1).
[0028]
According to a fourth aspect of the present invention, in the optical system for an optical pickup device according to the third aspect, the condenser lens satisfies the following expression (2).
| A | <8 × 10 ^-5 (2)
[0029]
According to a fifth aspect of the present invention, in the optical system for an optical pickup device according to the fourth aspect, the condenser lens satisfies the following expression (3).
| A | <6 × 10 ^-5 (3)
[0030]
According to a sixth aspect of the present invention, in the optical system for an optical pickup device according to any one of the second to fifth aspects, the particles are an inorganic material.
[0031]
According to a seventh aspect of the present invention, in the optical system for an optical pickup device according to the sixth aspect, the inorganic material is an oxide.
[0032]
According to an eighth aspect of the present invention, in the optical system for an optical pickup device according to the seventh aspect, the oxide is in a saturated oxidation state.
[0033]
The invention according to claim 9 is the optical system for an optical pickup device according to any one of claims 2 to 8, wherein the volume ratio of the plastic material and the particles in the condenser lens is as follows: It is in the range of 9: 1 to 9: 6.
[0034]
According to the second aspect of the present invention, similarly to the optical system according to the first aspect of the present invention, the converging lens disposed exclusively on the optical disc side has a refractive power with respect to the incident light beam, thereby reducing the working distance. A large amount can be secured, and interference between the objective lens and the optical disk can be prevented.
In addition, by using a material in which particles with a diameter of 30 nm or less are dispersed in a plastic material as the material for the condensing lens, the refractive index accompanying a change in temperature is reduced while maintaining the moldability of the plastic material. Therefore, it is possible to suppress a change in spherical aberration due to a change in refractive index even though it is a single lens having a high NA.
Since the chromatic aberration of the condensing lens is corrected by the diffractive structure, good recording / reproducing characteristics can be maintained even when the wavelength of the incident light beam is instantaneously changed by laser mode hopping.
[0035]
In addition, since the chromatic spherical aberration is corrected by the optical path difference providing structure, the spherical aberration does not become too large even when the wavelength of the semiconductor laser changes due to temperature change, and the oscillation wavelength is the reference due to manufacturing errors. Even if a semiconductor laser deviated from the wavelength is used, the spherical aberration does not become too large.
Then, by using the diffractive structure for correcting axial chromatic aberration and using the optical path difference providing structure for correcting chromatic spherical aberration, the length of the zonal zone of the diffractive structure in the direction perpendicular to the optical axis is increased, and the aberration correction element is formed. Transferability is improved. Furthermore, since a structure having fine steps such as a diffractive structure and 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 is blocked and condensed. The ratio of the light flux that does not contribute to the formation of the spot can be suppressed, and a decrease in the amount of light can be prevented.
[0036]
Here, the temperature change of the refractive index of the condensing lens in the optical system of the present invention will be described. The rate of change of the refractive index with respect to the temperature change is expressed by A in the above formula 1 by differentiating the refractive index n with respect to the temperature t based on the Lorentz-Lorenz formula.
In the case of a general plastic material, since the contribution of the second term is smaller than the first term of Equation 1, the second term can be almost ignored. For example, in the case of acrylic resin (PMMA), the linear expansion coefficient α is 7 × 10. ^-5 Substituting into the above equation, A = -12 × 10 ^-5 Which is almost consistent with the measured value.
[0037]
Here, in the condensing lens in the optical system of the present invention, the contribution of the second term of the above formula is substantially increased by dispersing it in a fine particle plastic material having a diameter of 30 nm or less, and the linear expansion of the first term. To counteract changes caused by
Specifically, as described in the invention of claim 3, the conventional technique is −12 × 10 6. ^-5 The refractive index change rate with respect to the temperature change, which is about 10 × 10 6 in absolute value ^-5 It is preferable to suppress to less than. More preferably, as described in claim 4, 8 × 10 ^-5 Less, more preferably 6 × 10 6 as described in the invention of claim 5. ^-5 In order to reduce the change in spherical aberration associated with the change in temperature of the condenser lens, it is preferable to keep it below.
For example, acrylic resin (PMMA) and niobium oxide (Nb) ₂ O ₅ ) Is dispersed, the dependence of the refractive index change on the temperature change can be eliminated.
[0038]
The plastic material used as the base material has a volume ratio of 80 and niobium oxide in a ratio of about 20, and these are uniformly mixed. There is a problem that the fine particles are likely to aggregate, but a technique of applying a charge to the particle surface to disperse the particles is also known, and a necessary dispersion state can be generated.
The volume ratio can be appropriately increased or decreased in order to control the rate of change of the refractive index with respect to the temperature change, and a plurality of types of nano-sized inorganic particles can be blended and dispersed.
In the above example, the volume ratio is 80:20. However, as described in the invention of claim 9, the volume ratio can be appropriately adjusted between 90:10 and 60:40. If the volume ratio is smaller than 90:10, the effect of suppressing the change in refractive index is reduced.
[0039]
Further, as described in the sixth aspect of the invention, the fine particles are preferably inorganic, and further, as described in the seventh aspect of the invention, are preferably oxides. And as described in the invention of claim 8, it is preferable that the oxide is saturated and the oxide is not oxidized any more.
An inorganic substance is preferable in order to keep the reaction with a plastic material, which is a high molecular organic compound, low, and the oxide causes deterioration in transmittance and wavefront aberration due to long-term irradiation of a blue-violet laser. Can be prevented. In particular, oxidation is likely to be accelerated under the severe condition of being irradiated with a blue-violet laser at a high temperature. However, with such an inorganic oxide, it is possible to prevent transmittance deterioration and wave aberration deterioration due to oxidation.
[0040]
If the diameter of the fine particles dispersed in the plastic material is large, the incident light beam is likely to be scattered and the transmittance of the condensing lens is lowered. In a blue-violet semiconductor laser used for recording / reproducing information in a high-density optical disk, the laser power with which stable laser oscillation can be obtained over a long time is about 30 mW, so that the transmittance of the optical element to the blue-violet laser is low. This is disadvantageous in terms of speeding up of information recording and compatibility with multi-layer discs. Therefore, the diameter of the fine particles dispersed in the plastic material is preferably 20 nm or less, and more preferably 10 to 15 nm or less in order to prevent a decrease in the transmittance of the condenser lens.
[0041]
According to a tenth aspect of the present invention, in the optical system for an optical pickup device according to any one of the first to ninth aspects, the aberration correction element is a single lens having one lens group. And
[0042]
The invention according to claim 11 is the optical system for the optical pickup device according to any one of claims 1 to 10, wherein _b = B ₂ ・ H ² + B ₄ ・ H ⁴ + B ₆ ・ H ⁶ Optical path difference function Φ defined by + ... _b (Mm) represents the optical path difference added to the wavefront transmitted through the diffractive structure (where h is the height from the optical axis (mm), b ₂ , B ₄ , B ₆ ,... Are second-order, fourth-order, sixth-order,... Optical path difference function coefficients), P _D = -2 · b ₂ Diffraction power P in the paraxial axis defined by _D (Mm ^-1 ) And the paraxial combined power P obtained by combining the aberration correction element and the condenser lens. _T (Mm ^-1 ) Satisfies the following expression (4).
0.03 ≦ P _D / P _T ≦ 0.15 (4)
[0043]
According to the invention of claim 11, P _D / P _T Indicates the ratio of the diffraction power in the entire system, that is, the degree of correction of axial chromatic aberration. If this value is determined so as to satisfy the expression (4), the longitudinal chromatic aberration can be corrected satisfactorily.
[0044]
The invention according to claim 12 is the optical system for the optical pickup device according to any one of claims 1 to 11, wherein _b = B ₂ ・ H ² + B ₄ ・ H ⁴ + B ₆ ・ H ⁶ Optical path difference function Φ defined by + ... _b (Mm) represents the optical path difference added to the wavefront transmitted through the diffractive structure (where h is the height from the optical axis (mm), b ₂ , B ₄ , B ₆ ,... Are second-order, fourth-order, sixth-order,... Optical path difference function coefficients). ₂ The optical path difference function coefficients of orders other than are all zero.
[0045]
According to the twelfth aspect of the present invention, the optical path difference function coefficients other than the second order are all set to 0, so that the optical axis perpendicular to the zonal zone of the diffractive structure is corrected compared to the case where the chromatic spherical aberration is corrected by the diffractive structure. Since the length of the direction becomes longer, the transferability at the time of forming the aberration correction element is improved, the diffraction efficiency is improved, the ratio of the luminous flux that does not contribute to the formation of the condensed spot can be suppressed, and the light quantity can be prevented from being reduced.
[0046]
The invention according to claim 13 is the optical system for an optical pickup device according to any one of claims 1 to 12, wherein the diffractive structure includes a central region including an optical axis and an outer side of the central region. A width P in the direction perpendicular to the optical axis of the annular zone, which is composed of a plurality of annular zones divided with fine steps, at the maximum effective diameter position. _f (Mm) and the width P in the direction perpendicular to the optical axis of the annular zone at a position of 50% of the maximum effective diameter. _h (Mm) satisfies the following expression (5).
0 ≦ | P _h / P _f -2 | <0.5 (5)
[0047]
According to the invention of claim 13, P _h And P _f Satisfies the formula (5), the length of the zonal zone of the diffractive structure in the direction perpendicular to the optical axis is longer than that in the case of correcting chromatic spherical aberration with the diffractive structure. The diffraction efficiency is improved, the ratio of the luminous flux that does not contribute to the formation of the condensing spot can be suppressed, and the reduction of the light amount can be prevented.
[0048]
The invention according to claim 14 is the optical system for an optical pickup device according to any one of claims 1 to 13, wherein _b = B ₂ ・ H ² + B ₄ ・ H ⁴ + B ₆ ・ H ⁶ Optical path difference function Φ defined by + ... _b (Mm) represents the optical path difference added to the wavefront transmitted through the diffractive structure (where h is the height from the optical axis (mm), b ₂ , B ₄ , B ₆ ,... Are second-order, fourth-order, sixth-order,... Optical path difference function coefficients), P _D = -2 · b ₂ Diffraction power P in the paraxial axis defined by _D (Mm ^-1 ) And the refractive power P at the paraxial axis of the aberration correcting element. _R (Mm ^-1 ) Satisfies the following expression (6).
-1.2 ≦ P _D / P _R ≦ −0.8 (6)
[0049]
According to the invention of claim 14, P _D / P _R Indicates the degree of convergence of the light beam emitted from the aberration correction element, and if the condition of the expression (6) is satisfied, the emitted light becomes substantially parallel light, which facilitates the design and manufacture of the condenser lens.
[0050]
The invention according to claim 15 is the optical system for an optical pickup device according to any one of claims 1 to 14, wherein the optical path difference providing structure includes a central region including an optical axis, and Consists of a plurality of annular zones divided with fine steps on the outside, and adjacent optical path differences that cause the spherical aberration of the transmitted wavefront to change in the direction of insufficient correction when the wavelength of the incident light beam changes to the longer wavelength side The spherical surface of the transmitted wavefront is corrected when the spherical aberration change in the overcorrection direction generated in the condenser lens is corrected and the wavelength of the incident light beam is changed to the short wavelength side. It is characterized in that a spherical aberration change in the undercorrection direction generated in the condenser lens is corrected by generating an optical path difference between adjacent annular zones so that the aberration changes in the overcorrection direction.
[0051]
According to the fifteenth aspect of the present invention, chromatic spherical aberration can be corrected by the optical path difference providing structure when the wavelength of the incident light beam changes.
[0052]
According to a sixteenth aspect of the present invention, in the optical system for an optical pickup device according to the fifteenth aspect, a change in the wavelength of the incident light beam to a longer wavelength side and / or a short wavelength side of the wavelength of the incident light beam. The change to is an oscillation wavelength change accompanying an environmental temperature change of a laser light source of an optical pickup device on which the optical system is mounted.
[0053]
According to the sixteenth aspect of the present invention, since the chromatic spherical aberration is corrected by the optical path difference providing structure, the spherical aberration does not become too large even when the oscillation wavelength of the semiconductor laser changes with the temperature change. .
[0054]
According to a seventeenth aspect of the present invention, in the optical system for an optical pickup device according to the fifteenth or sixteenth aspect, the wavelength of the incident light beam is changed to the longer wavelength side and / or the wavelength of the incident light beam is short. The change to the wavelength side is a variation in oscillation wavelength due to a manufacturing error of a laser light source of an optical pickup device on which the optical system is mounted.
[0055]
According to the seventeenth aspect of the present invention, since the chromatic spherical aberration is corrected by the optical path difference providing structure, the spherical aberration does not occur even when a semiconductor laser whose laser oscillation wavelength is deviated from the reference wavelength due to a manufacturing error is used. It never gets too big.
[0056]
The invention according to claim 18 is the optical system for an optical pickup device according to any one of claims 15 to 17, wherein, in the optical path difference providing structure, the annular zone adjacent to the outside of the central region is The annular zone is formed by shifting in the optical axis direction so that the optical path length is shorter than the central region, and the annular zone at the maximum effective diameter position is longer than the annular zone adjacent to the inner zone. And the annular zone at a position of 75% of the maximum effective diameter position is an optical path with respect to the annular zone adjacent to the inner side and the annular zone adjacent to the outer side thereof. It is formed by shifting in the direction of the optical axis so that the length becomes shorter.
[0057]
According to the invention described in claim 18, in the optical path difference providing structure, the annular zone adjacent to the outside of the central region is formed so as to be shifted with respect to the central region so that the optical path length becomes shorter. Therefore, it is possible to cancel the spherical aberration in the overcorrected direction that occurs in the condenser lens when the wavelength of the incident light beam changes to the long wavelength side, and when the wavelength of the incident light beam changes to the short wavelength side Spherical aberration in the direction of insufficient correction that occurs in the condenser lens can be canceled out.
[0058]
According to a nineteenth aspect of the present invention, in the optical system for an optical pickup device according to any one of the first to eighteenth aspects, the depth in the optical axis direction of the fine step of the optical path difference providing structure is d. (Μm), where the wavelength of the laser light source of the optical pickup device on which the optical system is mounted is λ (nm), and the refractive index with respect to the wavelength λ of the aberration correction element is n, the following expression (7) is satisfied. It is characterized by that.
3 ≦ d · (n−1) / (λ × 10 ^-3 ) ≦ 10 (7)
[0059]
According to the nineteenth aspect of the present invention, d · (n−1) / (λ × 10 ^-3 ) ≦ 10, the spherical aberration due to the change of the oscillation wavelength of the semiconductor laser can be corrected well, and 3 ≦ d · (n−1) / (λ × 10 ^-3 ), The length in the direction perpendicular to the optical axis of the ring zone of the optical path difference providing structure can be ensured so that the optical path difference providing structure can be easily molded, and the transferability at the time of forming the aberration correction element is good. The transmittance is improved, the ratio of the luminous flux that does not contribute to the formation of the condensed spot can be suppressed, and the decrease in the light amount can be prevented.
[0060]
The invention according to claim 20 is the optical system for an optical pickup device according to any one of claims 1 to 19, wherein the wavelength of the laser light source of the optical pickup device on which the optical system is mounted is λ (nm ), Where NA is a numerical aperture obtained by combining the aberration correcting element and the condenser lens, the following equations (8) and (9) are satisfied.
λ ≦ 450nm (8)
0.60 ≦ NA ≦ 0.95 (9)
[0061]
According to a twenty-first aspect of the present invention, in the optical system for an optical pickup device according to any one of the first to twentieth aspects, both the aberration correction element and the condenser lens have a wavefront aberration within the Marshall limit. Aberration correction is performed so that
[0062]
According to the twenty-first aspect of the present invention, the aberration correction element and the condenser lens can be independently evaluated, and thus the manufacture is facilitated.
[0063]
According to a twenty-second aspect of the present invention, in the optical system for an optical pickup device according to any one of the first to twenty-first aspects, the aberration correction element and the condenser lens are bonded via a bonding member, respectively. It is characterized by being.
[0064]
According to the twenty-second aspect of the present invention, since the condenser lens can be tracked integrally with the aberration correcting element, no coma is generated even when the incident light beam deviates from the design wavelength, and good tracking characteristics are obtained. Can be obtained.
[0065]
The invention according to claim 23 is an optical pickup device comprising: a laser light source; and an objective optical system for condensing a light beam emitted from the laser light source onto an information recording surface of an optical disc. The optical system according to any one of claims 1 to 22 is provided.
[0066]
According to a twenty-fourth aspect of the present invention, the optical pickup device according to the twenty-third aspect is mounted, and at least one of recording of information on the optical disk and reproduction of information recorded on the optical disk can be executed. To do.
[0067]
DETAILED DESCRIPTION OF THE INVENTION
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 a diaphragm. 18, an objective lens 30, a focusing / tracking biaxial actuator 19, a cylindrical lens 17, a concave lens 14, and a photodetector 15.
Although described in detail later, the optical system for the optical pickup device according to the present invention includes an aberration correction element 40 and a condenser lens 50 that constitute the objective lens 30.
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, and becomes a circularly polarized parallel light beam. 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.
[0068]
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 collimator lens 13, and then passes through the quarter-wave plate 16 to become linearly polarized light. Reflected by the polarization beam splitter 12, passes through the cylindrical lens 17 and the concave lens 14, gives a critical aberration, and converges to the photodetector 15. The information recorded on the high density optical disk 20 can be read using the output signal of the photodetector 15.
As a light source for emitting laser light having a wavelength of about 400 nm, an SHG blue-violet laser using a second harmonic generation method may be used instead of the blue-violet semiconductor laser.
[0069]
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). 11 includes a plastic aberration correction element 40 disposed on the 11 side and a glass condensing lens 50 disposed on the high density optical disk 20 side. The NA obtained by combining the aberration correction element 40 and the condenser lens 50 is 0.85.
Further, since the aberration correction element 40 and the condenser lens 50 are joined using the joining member 80, the aberration correction element 40 and the condenser lens 50 are integrated to perform focusing and tracking.
[0070]
As shown in FIG. 2, the objective lens 30 includes a plastic aberration correction element 40, a glass condensing lens 50, and a joining member 80 that are sequentially arranged 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 (in this embodiment, two surfaces of an incident surface 51 and an output surface 52).
As shown in FIG. 3, the aberration correction element 40 is an optical element in which the shape of the incident surface 41 in the vicinity of the optical axis is planar and the exit surface 42 is concave, and the incident surface 41 is provided with an optical path difference. A structure 60 is formed, and a diffractive structure 70 is formed on the emission surface 42.
[0071]
In the aberration correction element 40 according to the embodiment of the present invention, the optical path difference providing structure 60 is formed on the incident surface 41, and the diffractive structure 70 is formed on the exit surface 42. The diffraction structure 70 may be formed on the surface 41, and the optical path difference providing structure 60 may be formed on the emission surface 42. Further, the optical path difference providing structure 60 may be formed on one of the optical surfaces of the incident surface 41 and the exit surface 42, and the diffractive structure 70 may be formed on each annular zone of the optical path difference providing structure 60. Further, the optical path difference providing structure 60 may be formed on the concave optical surface, and the diffractive structure 70 may be formed on the flat optical surface.
[0072]
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 with the optical axis L as the center, and an inner peripheral edge portion and an outer peripheral edge portion are divided by a minute 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 maximum effective diameter D of the optical surface ₁ The annular zone (annular zone 62b) at a position corresponding to is shifted to the light source 11 side along the optical axis L direction so that the optical path length becomes longer than the annular zone 62 (c) adjacent to the inner zone. Maximum effective diameter D ₁ The annular zone 62 (d) at a position corresponding to 75% of the optical zone length is shorter than the annular zone 62 (e) adjacent to the inner side and the annular zone 62 (f) adjacent to the outer side. It is formed by shifting to the optical disc 20 side along the optical axis L direction.
[0073]
FIG. 4 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 polarization beam splitter 12, a quarter wavelength plate 16, and a collimator 13. , Aperture 18, objective lens 30 ′, focusing / tracking biaxial actuator 19, cylindrical lens 17, concave lens 14, and photodetector 15.
Although described in detail later, the optical system for an optical pickup device according to the present invention includes an aberration correction element 40 ′ and a condensing lens 50 ′ constituting the objective lens 30 ′.
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, and becomes a circularly polarized parallel light beam. 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.
[0074]
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 passes through the quarter-wave plate 16 to be linearly polarized. As a result, the light is reflected by the polarization beam splitter 12, passes through the cylindrical lens 17 and the concave lens 14, is given astigmatism, and converges on the photodetector 15. The information recorded on the high density optical disk 20 can be read using the output signal of the photodetector 15.
As a light source for emitting laser light having a wavelength of about 400 nm, an SHG blue-violet laser using a second harmonic generation method may be used instead of the blue-violet semiconductor laser.
[0075]
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 laser 11 side, and an athermal resin condensing lens 50 ′ disposed on the high density optical disk 20 side. The NA obtained by combining the aberration correction element 40 ′ and the condenser lens 50 ′ is 0.85.
Further, since the aberration correction element 40 ′ and the condenser lens 50 ′ are joined using the joining member 80, the aberration correction element 40 ′ and the condenser lens 50 ′ are integrated to perform focusing and tracking.
[0076]
The condensing lens 50 ′ is a refractive single lens made of athermal resin having one group and one lens having at least one aspherical surface (in this embodiment, two surfaces of an incident surface 51 and an output surface 52).
The aberration correction element 40 ′ is an optical element in which the shape of the incident surface near the optical axis is planar and the exit surface is concave. A diffraction structure 70 is formed on the entrance surface 41, and the exit surface 42 is formed on the exit surface 42. An optical path difference providing structure 60 is formed.
[0077]
In the aberration correction element 40 ′ according to the embodiment of the present invention, the diffraction structure 70 is formed on the incident surface 41 and the optical path difference providing structure 60 is formed on the output surface 42. The optical path difference providing structure 60 may be formed on the surface 41, and the diffractive structure 70 may be formed on the exit surface 42. Further, the optical path difference providing structure 60 may be formed on one of the optical surfaces of the incident surface 41 and the exit surface 42, and the diffractive structure 70 may be formed on each annular zone of the optical path difference providing structure 60.
The configuration and function of the optical path difference providing structure 60 and the diffractive structure 70 formed in the aberration correction preventing 40 ′ in the present embodiment are the same as those of the optical path difference providing structure 60 and the diffractive structure 70 of the aberration correcting element 40 described above. Therefore, detailed explanation is omitted.
[0078]
Then, when the light beam emitted from the blue-violet semiconductor laser 11 passes through each annular zone 62 on the incident surface 41 of the aberration correction element 40, the amount of displacement from the central region 61 of the annular zone 62 (the optical axis of the step 63). An optical path difference corresponding to the length in the L direction is given, and a phase difference is generated in each light beam.
[0079]
When the oscillation wavelength of the blue-violet semiconductor laser 11 is the same as the design wavelength of the aberration correction element 40, the light fluxes that have passed through the respective annular zones 62 are aligned with each other so that the phases of the light fluxes are substantially aligned on the information recording surface 21. On the other hand, when the optical path difference is given and the oscillation wavelength of the blue-violet semiconductor laser is longer than the design wavelength of the aberration correction element 40, the adjacent optical path difference is set so that the spherical aberration of the transmitted wavefront changes in the direction of insufficient correction. By occurring between the bands 62, the change in spherical aberration in the overcorrection direction that occurs in the condenser lens 50 is corrected. Similarly, when the oscillation wavelength of the blue-violet semiconductor laser is shorter than the design wavelength of the aberration correction element 40, an optical path difference that causes the spherical aberration of the transmitted wavefront to change in the overcorrected direction is generated between adjacent annular zones 62. As a result, the change in spherical aberration in the direction of insufficient correction that occurs in the condenser lens 50 is corrected.
[0080]
The diffractive structure 70 includes a central region 71 including the optical axis L, and a plurality of annular zones 72 (diffractive annular zones) divided with a fine step 74 in the optical axis L direction outside the central region 71. A sectional shape including the optical axis L of the diffraction ring zone 72 is formed on the concave surface 73 in a stepped shape.
And Φ _b = B ₂ ・ H ² + B ₄ ・ H ⁴ + B ₆ ・ H ⁶ Optical path difference function Φ defined by + ... _b (Mm) represents the optical path difference added to the wavefront transmitted through the diffractive structure 70, and P _D = -2 · b ₂ Diffraction power P in the paraxial axis defined by _D (Mm ^-1 ) And the combined power P in the paraxial obtained by combining the aberration correction element 40 and the condenser lens 50. _T (Mm ^-1 ) And (4) satisfy the expression (4), the diffractive structure 70 corrects the axial chromatic aberration generated in the condenser lens 50 in accordance with the wavelength change of the incident light beam.
0.03 ≦ P _D / P _T ≦ 0.15 (4)
Where h is the height from the optical axis L (mm), b ₂ , B ₄ , B ₆ ,... Represent second-order, fourth-order, sixth-order,... Optical path difference function coefficients, respectively.
Also, the refractive power (mm ^-1 ) And the diffraction power P on the paraxial axis of the diffraction structure 70 _D (Mm ^-1 ) And the diffractive structure 70 are designed so that the exit surface 42 of the aberration correcting element and the diffraction structure 70 are designed so as to satisfy the expression (6).
-1.2 ≦ P _D / P _R ≦ −0.8 (6)
[0081]
Maximum effective diameter D ₂ The width P in the direction perpendicular to the optical axis L of the diffraction ring zone 72a at the position _f (Mm) and maximum effective diameter D ₂ Width P in the direction perpendicular to the optical axis L of the annular zone 72b at the position of 50% of _h The diffractive structure 70 is designed so that (mm) satisfies the expression (5).
0 ≦ | P _h / P _f -2 | <0.5 (5)
[0082]
As described above, according to the objective lens system 30 shown in the present embodiment, it is possible to ensure a large working distance by providing the condenser lens 50 disposed exclusively on the optical disc side with the refractive power with respect to the incident light beam. Thus, interference between the objective lens system 30 and the protective layer 22 of the optical disk can be prevented.
Further, since the chromatic aberration of the condenser lens 50 is corrected by the diffractive structure 70, good recording / reproducing characteristics can be maintained even when the wavelength of the incident light beam is instantaneously changed by the mode hopping of the blue-violet laser 11.
[0083]
Further, since the chromatic spherical aberration is corrected by the optical path difference providing structure 60, even when the oscillation wavelength of the blue-violet semiconductor laser 11 is changed due to the temperature change, the spherical aberration is not excessively increased. The maximum effective diameter D is obtained by using the diffraction structure 70 for correcting axial chromatic aberration and using the optical path difference providing structure 60 for correcting chromatic spherical aberration. ₂ The width P in the direction perpendicular to the optical axis L of the diffraction ring zone 72a at the position _f (Mm) becomes longer, and the transferability when the aberration correction element 40 is molded is improved.
Further, since a structure having fine steps such as the diffractive structure 70 and the optical path difference providing structure 60 is formed on the optical surface of the aberration correction element 40, the path of the light beam incident on the diffractive structure 70 is blocked. Thus, the ratio of the luminous flux that does not contribute to the formation of the condensing spot can be suppressed, and a decrease in the amount of light can be prevented.
[0084]
Although not shown, the optical pickup devices 10 and 10 'described above are combined with a rotation driving device that rotatably holds the optical disc 20, a control device that controls the driving of these various devices, and the like. An optical information recording / reproducing apparatus capable of executing at least one of information recording and information recording on the optical disc 20 can be obtained.
[0085]
Further, in the present embodiment, the optical pickup devices 10 and 10 'are provided with one laser light source 11, and an optical disc of one type (or recording density) (in this embodiment, a 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.
[0086]
【Example】
Next, examples of the objective lens system described above will be described. The optical system in this embodiment has a focal length f = 1.765 mm and a paraxial power P. _T = 0.567mm ^-1 The numerical aperture NA = 0.85, the design wavelength λ = 405 nm, and the design reference temperature T = 25 ° C.
The aberration correction element is a plastic lens in which the first surface (incident surface) is flat and the second surface (exit surface) is a concave lens, the optical path difference providing structure on the first surface, and the diffractive structure (diffractive wheel) on the second surface. A band is a staircase shape). Diffraction ring zone width P at the maximum effective diameter position _f = 6.75 μm, diffraction zone width P at 50% of maximum effective diameter _h = 13.5 μm.
The condensing lens is a glass lens having aspheric surfaces on both sides (third surface and fourth surface), and is set to have a focal length f = 1.765 nm, a magnification m = 0, and a numerical aperture NA = 0.85. ing. 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 at the design wavelength 405 nm ₄₀₅ , The refractive index at 406 nm is n ₄₀₆ , The refractive index at 410 nm is n ₄₁₀ , The Abbe number of d line is v _d Represented by
[0087]
[Table 1]

The aspherical surface of the condensing lens has an amount of deformation 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). Then, it is expressed by a mathematical expression in which the coefficient in Table 2 is substituted into the following formula 2. Where κ is the cone coefficient, A _2i Is the aspheric coefficient.
[0088]
[Expression 2]

[Table 2]

[0089]
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. The optical path difference is obtained by setting the height in the direction perpendicular to the optical axis to h (mm), b _2j Is the optical path difference function coefficient defined by the following equation (3) _b It is expressed by a mathematical formula in which the coefficient in Table 3 is substituted for (mm).
[0090]
[Equation 3]

[Table 3]

[0091]
Further, the shape of each ring zone of the optical path difference providing structure is shown in Table 4.
[Table 4]

[0092]
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 (in the direction away from the optical axis). It is assumed that i = 2 and the annular zone adjacent to the outside is i = 3,. That is, in the aberration correction element of the objective lens in the present embodiment, four annular zones are formed outside the central region. And h _i-1 And h _i Indicates the center region and the start point height and end point height of each ring zone. m _i d represents the amount of shift in the optical axis direction of each annular zone with respect to the central region (positive when the light source is shifted toward the optical disc). For example, an annular zone (i = 2) adjacent to the outside of the central region has shifted 3.86 μm toward the optical disc with respect to the central region. M _i Represents the optical path difference with respect to the constraint passing through the central region given to the light beam transmitted through each annular zone at the design reference wavelength of 405 nm. For example, the annular zone adjacent to the outside of the central region (i = 2, m = 5) Since the optical path difference of 5λ (provided that λ = 405 nm) is given to the light beam transmitted through the central region, the phase is delayed by 2π × 5 (rad) in terms of the phase difference. Become.
[0093]
When the wavelength of the incident light beam is changed from the design reference wavelength 405 nm to 410 nm, the wavefront aberration is 0.012 λrms. Further, due to the mode hopping of the blue-violet semiconductor laser, the defocus component of the wavefront aberration when the wavelength of the incident light beam with respect to the objective lens in this embodiment is changed from the design wavelength of 405 nm to 406 nm is 0.024 λrms.
Further, in the objective lens system of the present embodiment, the defocus component of the wavefront aberration when the wavelength of the incident light beam is changed from the design wavelength of 405 nm to 406 nm is 0.111 λrms only for the objective lens excluding the aberration correction element. It became.
Thus, it can be seen that the axial chromatic aberration is corrected by the action of the diffractive structure of the aberration correcting element in the above embodiment, compared to the case of using only the glass objective lens.
[0094]
[Comparative example]
Next, the spherical aberration correction effect of the optical path difference providing structure in the above embodiment will be described. As a comparative example, consider a case where there is no optical path difference providing structure formed on the first surface of the aberration correction element of the above embodiment.
[0095]
In this comparative example, the wavefront aberration when the wavelength of the incident light beam was changed from the design reference wavelength 405 nm to 410 nm was 0.058 λrms.
Accordingly, it can be seen that in the objective lens system of the above embodiment, the spherical aberration generated when the wavelength of the incident light is changed is favorably corrected by the action of the optical path difference providing structure.
[0096]
【The invention's effect】
According to the present invention, it is possible to secure a sufficient working distance, further correct chromatic aberration and spherical aberration when the wavelength of an incident light beam is changed, and an optical element for a pickup device with high light utilization efficiency, and the optical element is an object. An optical pickup device mounted as a lens and an optical information recording / reproducing device mounted with the optical pickup device can be obtained.
[Brief description of the drawings]
FIG. 1 is a plan view showing a configuration of an optical pickup device according to the present invention.
FIG. 2 is a cross-sectional view showing a configuration of an optical system according to the present invention.
FIG. 3 is a cross-sectional view showing a diffractive structure and an optical path difference providing structure.
FIG. 4 is a plan view showing a configuration of an optical pickup device according to the present invention.
[Explanation of symbols]
10 Optical pickup device
40 Aberration correction element
50 condenser lens
60 Optical path difference providing structure
61 Central area
62 Ring
70 Diffraction structure
71 Central area
72
80 Joining members

Claims (24)

In an optical system for an optical pickup device comprising a plastic aberration correction element and a glass condensing lens for imaging a light beam emitted from the aberration correction element,
The aberration correction element has at least one optical surface on which a diffractive structure is formed and one optical surface on which an optical path difference providing structure is formed,
The optical system for an optical pickup device, wherein the condensing lens is a refractive single lens of one group and one lens having at least one aspherical surface. In an optical system for an optical pickup device composed of 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 optical surface on which a diffractive structure is formed and one optical surface on which an optical path difference providing structure is formed,
The condensing lens is a refractive single lens of one group and one lens having at least one aspherical surface formed by using a material in which particles having a diameter of 30 nm or less are dispersed in a plastic material. An optical system for an optical pickup device. The optical system for an optical pickup device according to claim 2, wherein the condenser lens satisfies the following expression (1).
| A | <10 × 10 ⁻⁵ (1)
However, A is the rate of change of the refractive index of the condenser lens with respect to temperature change, the refractive index n of the condenser lens with respect to the wavelength of the laser light source of the optical pickup device on which the optical system is mounted, The linear expansion coefficient α and the molecular refractive power [R] of the condenser lens are expressed by the following equation.

The optical system for an optical pickup device according to claim 3, wherein the condenser lens satisfies the following expression (2).
| A | <8 × 10 ⁻⁵ (2) The optical system for an optical pickup device according to claim 4, wherein the condenser lens satisfies the following expression (3).
| A | <6 × 10 ⁻⁵ (3) The optical system for an optical pickup device according to claim 2, wherein the particles are an inorganic material. The optical system for an optical pickup device according to claim 6, wherein the inorganic material is an oxide. 8. The optical system for an optical pickup device according to claim 7, wherein the oxide is in a saturated oxidation state. 9. The light according to claim 2, wherein a volume ratio of the plastic material and the particles in the condenser lens is in a range of 9: 1 to 9: 6. Optical system for pickup device. The optical system for an optical pickup device according to claim 1, wherein the aberration correction element is a single lens having one lens element per group. 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 (where h is the height from the optical axis (mm), b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth, ..., Diffraction power P _D (mm ⁻¹ ) on the paraxial axis defined by P _D = −2 · b ₂ , the aberration correction element, and the condenser lens 11. The optical pickup device according to claim 1, wherein the combined power P _T (mm ⁻¹ ) in the paraxial obtained by combining the two satisfies the following expression (4): Optical system.
0.03 ≦ P _D / P _T ≦ 0.15 (4) 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 (where h is the height from the optical axis (mm), b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth, The optical path difference function coefficients of orders other than the second order optical path difference function coefficient b ₂ are all zero. An optical system for an optical pickup device according to one item. 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. The width P _f (mm) and the 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 (5): Item 13. An optical system for an optical pickup device according to any one of Items 1 to 12.
0 ≦ | P _h / P _f −2 | <0.5 (5) 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 (where h is the height from the optical axis (mm), b ₂ , b ₄ , b ₆ ,... Are the second, fourth, sixth, ..., Diffraction power P _D (mm ⁻¹ ) in the paraxial axis defined by P _D = −2 · b ₂ , and refraction in the paraxial axis of the aberration correction element a power P _{R (mm} ^-1), the following (6) an optical system for the optical pickup device written in any one of claims 1 to 13, characterized in that satisfies the equation.
−1.2 ≦ P _D / P _R ≦ −0.8 (6) The optical path difference providing 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 when the wavelength of the incident light beam changes to the long wavelength side The spherical aberration of the transmitted wave front is changed between the adjacent annular zones so that the spherical aberration changes in the direction of undercorrection, thereby correcting the change of spherical aberration in the overcorrection direction generated in the condenser lens, and When the wavelength of the incident light beam is changed to the short wavelength side, an optical path difference that causes the spherical aberration of the transmitted wavefront to change in the overcorrected direction is generated between the adjacent annular zones, thereby generating in the condenser lens. The optical system for an optical pickup device according to claim 1, wherein a change in spherical aberration in a direction of insufficient correction is corrected. The change of the wavelength of the incident light beam to the long wavelength side and / or the change of the wavelength of the incident light beam to the short wavelength side is caused by a change in the environmental temperature of the laser light source of the optical pickup device on which the optical system is mounted. The optical system for an optical pickup device according to claim 15, wherein the oscillation wavelength change is accompanied. The change of the wavelength of the incident light beam to the longer wavelength side and / or the change of the wavelength of the incident light beam to the shorter wavelength side is caused by the manufacturing error of the laser light source of the optical pickup device on which the optical system is mounted. The optical system for an optical pickup device according to claim 15 or 16, wherein the optical system has wavelength variation. In the optical path difference providing structure, the annular zone adjacent to the outside of the central region is formed by shifting in the optical axis direction so as to shorten the optical path length with respect to the central region, and at the maximum effective diameter position. The annular zone is formed by shifting in the optical axis direction so that the optical path length becomes 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 18. An annular zone that is adjacent to the inner side and an annular zone that is adjacent to the outer side thereof are formed by being shifted in the optical axis direction so that the optical path length is shortened. An optical system for the optical pickup device according to the item. The depth of the fine step of the optical path difference providing structure in the optical axis direction is d (μm), the wavelength of the laser light source of the optical pickup device on which the optical system is mounted is λ (nm), and the aberration correction element has the wavelength The optical system for an optical pickup device according to claim 1, wherein the following expression (7) is satisfied, where n is a refractive index with respect to the wavelength λ.
3 ≦ d · (n−1) / (λ × 10 ⁻³ ) ≦ 10 (7) 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 (8) and The optical system for an optical pickup device according to claim 1, wherein the formula (9) is satisfied.
λ ≦ 450nm (8)
0.60 ≦ NA ≦ 0.95 (9) 21. The optical pickup device according to claim 1, wherein both the aberration correction element and the condensing lens are subjected to aberration correction so that a wavefront aberration is within a Marechal limit. Optical system. The optical system for an optical pickup device according to any one of claims 1 to 21, wherein the aberration correction element and the condensing lens are each joined via a joining member. 23. An optical pickup apparatus comprising: a laser light source; and an objective optical system for condensing a light beam emitted from the laser light source on an information recording surface of an optical disc. An optical pickup device comprising the optical system according to claim 1. 24. An optical information recording / reproducing apparatus comprising the optical pickup device according to claim 23, wherein at least one of recording of information on an optical disk and reproduction of information recorded on the optical disk can be executed.

Images