JP4239617B2 - Objective optical element and optical pickup device - Google Patents

Description

【表２】

【０１１２】
表１に示すように、本実施例の対物レンズは、第１の光源から出射される波長λ１＝６７０ｎｍのときの焦点距離ｆ₁＝２．２２ｍｍ、像側開口数ＮＡ１＝０．６０、結像倍率ｍ１＝−１／１０に設定されており、第２の光源から出射される波長λ２＝７８９ｎｍのときの焦点距離ｆ₂＝２．２３ｍｍ、像側開口数ＮＡ２＝０．４６、結像倍率ｍ２＝−１／１０に設定されている。
表１中の面番号２、２´はそれぞれ対物レンズの入射面のうち光軸Ｌからの高さｈが１．１６ｍｍ以下の中央領域Ａ１、ｈが１．１６ｍｍ以上の周辺領域Ａ２、面番号３は出射面を示している。また、ｒｉは曲率半径、ｄｉは第ｉ面から第ｉ＋１面までの光軸Ｌ方向の位置、ｎｉは各面の屈折率を表している。
【０１１３】
対物レンズの第２面、第２´面、第３面は、それぞれ次式（数１）に表１及び表２に示す係数を代入した数式で規定される、光軸Ｌの周りに軸対称な非球面に形成されている。
また、第２面の回折構造に関するブレーズ化波長は１ｍｍ、第２´面の回折構造に関するブレーズ化波長も１ｍｍである。
【０１１４】
【数１】

【０１１５】
ここで、Ｘ（ｈ）は光軸Ｌ方向の軸（光の進行方向を正とする）、κは円錐係数、Ａ_2iは非球面係数である。
【０１１６】
また、第２面と第２´面に形成される回折輪帯のピッチは数２の光路差関数に、表２に示す係数を代入した数式で規定される。
【０１１７】
【数２】

ここで、Ｂ_2iは光路差関数の係数である。
【０１１８】
図９は、波長λ１＝６７０ｎｍの光束を用いた、ＤＶＤにおける球面収差量を示すグラフであり、図１０は、波長λ２＝７８９ｎｍの光束を用いた、ＣＤにおける球面収差量と開口数を示すグラフである。
図９及び図１０から、ＤＶＤとＣＤ共に必要開口数内において球面収差が良好に補正されていることが分かる。
【０１１９】
[実施例２]
次に、対物レンズ及び光ピックアップ装置の第２の実施例について説明する。本実施例においても、図６に示したものと同様に、波長λ１の光束と波長λ２の光束をそれぞれＤＶＤとＣＤに対して用いることが可能な、互換性を有する構成となっている。
【０１２０】
また、図示は省略するが、両面非球面の単レンズである対物レンズの一方の光学面（入射面）上であって、光軸Ｌからの高さｈが０．７８ｍｍ以下の中央領域と０．７８ｍｍ〜１．１６ｍｍの中間領域と、１．１６ｍｍ以上の周辺領域に区分され、中間領域に位相差付与構造としての回折輪帯を備えている。
【０１２１】
また、対物レンズは、光学材料として、温度変化に対する屈折率変化ｄｎ／ｄｔ（温度依存性）が、−１０．０×１０^-6／℃≦ｄｎ／ｄｔ≦−１．０×１０^-6／℃を満たすガラス材料により設計されている。
また、そのガラスの分散ν、屈折率ｎ、融点Ｔは、６５≦ν≦７５、１．４８≦ｎ≦１．５２、２５０℃≦Ｔ≦３００℃を満たす。
表３、表４に対物レンズのレンズデータを示す。
【０１２２】
【表３】

【表４】

【０１２３】
表３に示すように、本実施例の対物レンズは、第１の光源から出射される波長λ１＝６７０ｎｍのときの焦点距離ｆ₁＝２．２２ｍｍ、像側開口数ＮＡ１＝０．６０、結像倍率ｍ１＝−１／１０に設定されており、第２の光源から出射される波長λ２＝７８９ｎｍのときの焦点距離ｆ₂＝２．２３ｍｍ、像側開口数ＮＡ２＝０．４６、結像倍率ｍ２＝−１／１０に設定されている。
表１中の面番号２、２´、２´´はそれぞれ対物レンズの入射面の中央領域、中間領域、周辺領域、面番号３は出射面を示している。また、ｒｉは曲率半径、ｄｉは第ｉ面から第ｉ＋１面までの光軸Ｌ方向の位置、ｎｉは各面の屈折率を表している。
【０１２４】
対物レンズの第２面、第２´、第２´´面、第３面は、それぞれ上記数１に表３及び表４に示す係数を代入した数式で規定される、光軸Ｌの周りに軸対称な非球面に形成されている。
また、第２´面の回折構造に関するブレーズ化波長は１ｍｍである。
【０１２５】
また、回折輪帯のピッチは上記数２の光路差関数に、表４に示す係数を代入した数式で規定される。
【０１２６】
図１１は、波長λ１＝６７０ｎｍの光束を用いた、ＤＶＤにおける球面収差量を示すグラフであり、図１２は、波長λ２＝７８９ｎｍの光束を用いた、ＣＤにおける球面収差量を示すグラフである。
図１１及び図１２から、ＤＶＤとＣＤ共に必要開口数内において球面収差が良好に補正されていることが分かる。
【０１２７】
[実施例３]
次に、対物レンズ及び光ピックアップ装置の第３の実施例について説明する。
本実施例においては、図１に示したものと同様に、青色レーザー光を用いて高密度ＤＶＤに対して情報の再生及び／又は記録を行なうものである。
また、図２に示すように、対物レンズの入射面のほぼ全域に、位相差付与構造としての回折輪帯を設けることにより、例えばモードホップにより、光源からの出射光束の波長が変化した際に生じる色収差を、この位相差付与構造により補正する機能を有している。
【０１２８】
また、対物レンズは、光学材料として、温度変化に対する屈折率変化ｄｎ／ｄｔ（温度依存性）が、−１０．０×１０^-6／℃≦ｄｎ／ｄｔ≦−１．０×１０^-6／℃を満たすガラス材料により設計されている。
また、そのガラスの分散ν、屈折率ｎ、融点Ｔは、６５≦ν≦７５、１．４８≦ｎ≦１．５２、２５０℃≦Ｔ≦３００℃を満たす。
表５、表６に対物レンズのレンズデータを示す。
【０１２９】
【表５】

【表６】

【０１３０】
表５に示すように、本実施例の対物レンズは、光源から出射される波長λ＝４０５ｎｍのときの焦点距離ｆ＝１．７６ｍｍ、像側開口数ＮＡ＝０．８５、結像倍率ｍ＝０に設定されている。
表５中の面番号２は対物レンズの入射面、面番号３は出射面を示している。また、ｒｉは曲率半径、ｄｉは第ｉ面から第ｉ＋１面までの光軸Ｌ方向の位置、ｎｉは各面の屈折率を表している。
【０１３１】
対物レンズの第２面、第３面は、それぞれ上記数１に表５及び表６に示す係数を代入した数式で規定される、光軸Ｌの周りに軸対称な非球面に形成されている。
また、第２面の回折構造に関するブレーズ化波長は１ｍｍである。
【０１３２】
また、回折輪帯のピッチは上記数２の光路差関数に、表６に示す係数を代入した数式で規定される。
【０１３３】
図１３は、波長λ＝４０５ｎｍの光束を用いた、高密度ＤＶＤにおける球面収差量及びモードホップにより波長λが４０４ｎｍ（−１ｎｍ）と４０６ｎｍ（＋１ｎｍ）に変動した場合の縦球面収差図である。横軸は、波長４０５ｎｍの場合に波面収差が最小となる位置を０にとっている。
図１３から、モードホップにより波長が変動した場合でも、必要開口数内において波面収差が良好に補正されていることが分かる。
【０１３４】
【発明の効果】
本発明によれば、光学素子をガラス材料で成形することが可能となることから、プラスチック性の光学素子を用いる場合と比較して、位相差付与構造に温度特性を補正する機能を持たせる必要が無くなり、収差補正の精度を向上できると共に、レンズ設計の自由度を増やすことができる。
【図面の簡単な説明】
【図１】光ピックアップ装置の構成を示す要部平面図である。
【図２】対物レンズの構造を示す要部横断面図である。
【図３】対物レンズの構造を示す要部横断面図である。
【図４】対物レンズの構造を示す要部横断面図である。
【図５】対物レンズの構造を示す要部横断面図（ａ）及び拡大図（ｂ）である。
【図６】光ピックアップ装置の構成を示す要部平面図である。
【図７】対物レンズの構造を示す要部横断面図である。
【図８】対物レンズの構造を示す要部横断面図である。
【図９】実施例１におけるＤＶＤの縦球面収差図である。
【図１０】実施例１におけるＣＤの縦球面収差図である。
【図１１】実施例２におけるＤＶＤの縦球面収差図である。
【図１２】実施例２におけるＣＤの縦球面収差図である。
【図１３】実施例３におけるモードホップ時の縦球面収差図である。
【符号の説明】
Ｌ 光軸
１０ 光ピックアップ装置
１３ 光情報記録媒体
１４ 情報記録面
２０ 光学素子（対物光学素子）
３０ 位相差付与構造
３２ 輪帯面
３３ 段差
３４ 回折輪帯
３５ 不連続面
４０ 光ピックアップ装置[0001]
BACKGROUND OF THE INVENTION
  The present invention,versusThe present invention relates to an object optical element and an optical pickup device.
[0002]
[Prior art]
An optical pickup device that records and / or reproduces information by condensing a laser beam on an information recording surface of an optical information recording medium (also referred to as an optical disc) such as a CD (compact disc) or a DVD (digital video disc), It is configured by combining various optical elements such as an objective lens, a coupling lens, and a beam expander.
These optical elements are often made of lightweight and inexpensive plastics. However, since plastics have a characteristic that the refractive index changes due to temperature changes, for example, in plastic objective lenses, spherical aberration increases in the over direction due to temperature rise. The problem arises.
[0003]
Therefore, in order to improve the lens characteristics (temperature characteristics) against such temperature changes, a diffractive structure is provided on the optical surface of the objective lens, and spherical aberration is generated in the under direction by the diffractive structure. There is known a technique for canceling the spherical aberration generated in the above.
Also, when the power of the light beam emitted from the light source is increased, if a so-called mode hop occurs in which the wavelength of the light beam fluctuates instantaneously, the position of the focused spot formed on the optical axis is recorded on the information recording on the optical disk. There arises a problem of deviation from the surface.
[0004]
Therefore, a technique using a diffractive structure provided in an optical element is also known as a means for correcting lens characteristics (wavelength characteristics) with respect to such a wavelength change (hereinafter also referred to as “mode hop correction”). ing.
Note that the mode hop correction is to correct the aberration in the focused spot (aberration of axial chromatic aberration and spherical chromatic aberration) before and after the wavelength variation below the diffraction limit.
In addition, for example, the wavelength of the emitted light beam may be different for each laser light source due to individual differences of laser light sources. In this case, the position of the objective lens is made relatively light with respect to the optical information recording medium by an actuator. By moving in the axial direction, axial chromatic aberration among the above aberrations can be corrected, and only spherical aberration is corrected using the diffractive structure.
For environmental changes other than momentary changes such as mode hops, as described above, the objective lens is moved relative to the optical information recording medium using an actuator, as described above. The position of the information recording surface of the medium in the optical axis direction is adjusted to a position where the wavefront aberration of the focused spot is minimized.
[0005]
In addition, in a so-called compatible optical pickup device that uses one objective lens to focus two types of light beams of wavelengths λ1 and λ2 on different types of optical disks, for example, a diffraction structure is formed on a part of the objective lens. The function of limiting the numerical aperture of the objective lens with respect to the light beam with the wavelength λ2 by making the light beam with the wavelength λ2 passing through the diffractive structure flare and not condensing on the optical disk (aperture limiting function) And a sufficient amount of light for recording and / or reproducing information by using the diffracted light with the highest diffraction efficiency among the light beams having the wavelengths λ1 and λ2 that are diffracted by the diffraction structure. Techniques for obtaining are known.
[0006]
In addition, when other optical elements (coupling lenses, beam expanders, etc.) other than the objective lens are also made of plastic, the temperature characteristic of the entire optical system composed of these optical elements is placed at the end of the optical system. There is known a technique for correcting by a diffractive structure provided in an objective lens.
[0007]
As described above, in recent years, the diffractive structure is used for various purposes other than the improvement of temperature characteristics, and research and development have been advanced on an objective lens and an optical pickup device that can satisfy all of these requirements.
For example, an objective lens of a CD / DVD compatible optical pickup device is known in which a concentric diffraction surface (diffraction ring zone) centered on the optical axis is formed on the incident surface of a plastic objective lens. (For example, refer to Patent Document 1).
The pitch of each diffraction zone decreases monotonically in the central area (shared area) occupying a predetermined position in the radial direction of the lens from the optical axis, and decreases in the peripheral area (dedicated area) located around the central area. ing. Of the light flux for CD, the light flux that passes through the peripheral region is used as flare light, so that the light flux that passes through the central region is condensed on the information recording surface of the CD. It is like that.
[0008]
And, as the ratio of flare light (flare amount) of the luminous flux for CD increases, the wavelength characteristic in DVD tends to improve, and the temperature characteristic in DVD tends to improve as the flare amount decreases, By appropriately adjusting the amount of flare, DVD wavelength characteristics and temperature characteristics are compatible while ensuring compatibility between DVD and CD.
[0009]
[Patent Document 1]
JP 2002-109775 A
[0010]
[Problems to be solved by the invention]
Incidentally, since many of the objective lens disclosed in Patent Document 1 and other optical elements constituting a general optical pickup device are made of plastic, it is essential to improve the temperature characteristics. Therefore, there arises a problem that it is difficult to design a lens that has the above-described function for improving temperature characteristics and the function for improving various problems including wavelength characteristics, for example, in a diffractive structure provided in an objective lens. ing.
[0011]
  The object of the present invention is to take the above-mentioned problems into consideration, and there is no need to improve temperature characteristics.VersusAn object optical element and an optical pickup device are provided.
[0012]
[Means for Solving the Problems]
  In order to solve the above-described problems, the invention according to claim 1 is characterized in that at least the light beam having the wavelength λ1 (350 nm ≦ λ1 ≦ 450 nm) emitted from the first light source is used as the information recording surface of the first optical information recording medium. The information is reproduced and / or recorded on the first optical information recording medium by condensing the light onto the first light information recording medium, and the light beam having the wavelength λ2 (630 nm ≦ λ2 ≦ 680 nm) emitted from the second light source is emitted to the second light. Used in an optical pickup device that reproduces and / or records information on the second optical information recording medium by condensing on the information recording surface of the information recording medium.At least when reproducing and / or recording information on the first optical information recording medium and when reproducing and / or recording information on the second optical information recording mediumAn optical element,
  The refractive index change dn / dt with respect to the temperature change is
  -10.0x10^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6/ ℃
And at least one optical surface has a phase difference providing structure that gives a phase difference to the light flux having the wavelength λ1 and / or λ2.Formed with glass, The phase difference providing structure is the light flux of each wavelength,ObjectiveIt has a function of correcting spherical aberration caused by the optical system magnification and / or wavelength difference of the optical element, or the light beam having the wavelength λ2 that has passed through the phase difference providing structure is used as information on the second optical information recording medium. It has an aperture limiting function that prevents light from being condensed on the recording surface.
[0013]
Here, in the present specification, the optical element includes, for example, a member such as an objective optical element (objective lens), a coupling lens, a beam expander, a beam shaper, and a correction plate, which constitutes a condensing optical system of the optical pickup device. Is applicable.
The optical element is not limited to a single lens, and a lens group formed by combining a plurality of lenses in the optical axis direction may be collectively used as the optical element.
The objective optical element (objective lens) is, in a narrow sense, arranged opposite to the optical information recording medium at a position closest to the optical information recording medium in a state where the optical information recording medium is loaded in the optical pickup device. The optical element which has a condensing effect | action is pointed out. In a broad sense, the term “optical element” refers to an optical element that can be moved at least in the optical axis direction by an actuator together with the optical element.
[0014]
In addition to CD and DVD, optical information recording media include optical discs of various standards having different light source wavelengths and protective substrate thicknesses, such as CD-R, RW (recordable compact disc), VD (video disc), MD ( A general optical disk such as a mini disk and an MO (magneto-optical disk), and a high-density optical disk using blue laser light having a wavelength of about 400 nm (hereinafter referred to as “high-density DVD”) are also included.
[0015]
Further, the phase difference providing structure refers to a structure that gives a specific action to the light flux by giving a predetermined phase difference to the incident light flux, and is not necessarily limited to the diffraction action. Absent.
As the phase difference providing structure, for example, a structure having a sawtooth shape or a step shape along the optical axis direction when the cross section is viewed on a plane including the optical axis (meridian cross section) is indicated.
[0028]
  Claim1According to the invention described in the above, the optical pickup device has compatibility,ObjectiveThe refractive index change dn / dt with respect to the temperature change of the optical element is −10.0 × 10^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6It has a phase difference providing structure that provides a phase difference with respect to the light flux of wavelength λ1 or λ2 on at least one optical surface.
  Therefore, it is common for plastics used in the pastObjectiveCompared with the optical element, it is possible to reduce the aberration change with respect to the temperature change. Therefore, the objective lens etc.ObjectiveIt is possible to use glass having low temperature dependency with respect to the optical element.
[0029]
  Therefore, in the conventional plastic objective lens, it is necessary to design the phase difference providing structure so that a plurality of characteristics such as temperature characteristics and wavelength characteristics can be improved.ObjectiveAccording to the optical element, since it is molded from a glass material, it is not necessary to consider the temperature characteristics even when they are compatible, and the phase difference providing structure does not need to improve (correct) the temperature characteristics. The accuracy of various aberration corrections can be improved, and the degree of freedom in lens design can be increased.
  In addition, an optical pickup device having compatibility between a high-density DVD and a DVD can be obtained.
  ObjectiveThe optical element has a compatible function if a phase difference providing structure is provided in a region through which light of wavelengths λ1 and λ2 used for recording and / or reproduction of the first and second optical information recording media passes in common. And providing a phase difference providing structure in a region where light that is used for recording and / or reproduction of the first optical information recording medium and not used for recording and / or reproduction of the second optical information recording medium passes. It can have an opening limiting function.
  Also,ObjectiveSince the optical element has an aperture limiting function, it is not necessary to arrange a diaphragm on the incident surface side of the objective lens, or even when a diaphragm is disposed, by combining the aperture limiting function and the diaphragm, an optical information recording medium An appropriate numerical aperture according to the type can be obtained.
[0030]
  Claim2The invention described in claim1Described inObjectiveAn optical element,
in frontThe focal length f1 of the light beam having the wavelength λ1 and the numerical aperture NA1 of the focused spot formed on the information recording surface by the light beam of the wavelength λ1 are:
  0.2mm ≦ f1 ≦ 3.5mm
  0.63 ≦ NA1 ≦ 0.95
It is characterized by satisfying.
[0031]
  Claim2According to the invention described in claim1In addition to obtaining the same effect as above, the blue laser beam as the light beam with the wavelength λ1 is appropriately condensed on the information recording surface to record and / or reproduce information on the high-density DVD, and the light beam with the wavelength λ2 is appropriately Thus, it is possible to obtain an optical pickup device that collects light on the information recording surface and records and / or reproduces information on the DVD.
[0033]
  Claim3The invention described in claim1 or 2DescribedObjectiveIn the optical element, the phase difference providing structure includes a plurality of annular surfaces with the optical axis as a center and a plurality of annular surfaces that are continuous through a step substantially parallel to the optical axis. At least one of the light fluxes is characterized in that the phase is substantially uniform on the information recording surface.
[0034]
  Claim3According to the invention described in claim1 or 2The same effect can be obtained, and at least one of the light beams having the wavelength λ1 and the wavelength λ2 that have passed through each ring zone of the phase difference providing structure has almost the same phase on the information recording surface, so that the wavefront aberration can be corrected. . Further, since diffracted light is not used, it is possible to condense on the information recording surface while suppressing a decrease in the light amount of the light beam incident on the phase difference providing structure.
[0055]
  Claim4The invention described in claim 13As described in any one ofObjectiveAn optical element,
  The phase difference providing structure is a diffractive structure.
[0056]
  Claim5The invention described in claim 14As described in any one ofObjectiveAn optical element,
  The phase difference providing structure extends along a direction of the optical axis formed on a plurality of diffraction ring zones centered on the optical axis and at least one of the diffraction ring zones.pluralIt consists of a step-like discontinuous surface.
[0057]
  Claim5According to the invention described in claim 1,4If the same effect as any one of the above can be obtained and the step-like discontinuous surface is a diffractive structure,ObjectiveSince the diffraction order of the light beam having the wavelength λ can be substantially changed in two steps of the diffraction zone formed on the optical element and the stepped discontinuous surface, the optical information recording medium can be changed by appropriately changing the diffraction order. Diffracted light having a sufficient amount of light according to information recording and / or reproduction can be obtained. In addition, the degree of freedom in design with respect to diffraction efficiency and diffraction order can be increased.
  In addition, if the step-like discontinuous surface is a plurality of annular surfaces centering on the optical axis and is continuous through a step substantially parallel to the optical axis, the light flux that has passed through each annular zone Since the phases are almost uniform on the information recording surface, the wavefront aberration can be corrected. Further, since diffracted light is not used, it is possible to condense on the information recording surface while suppressing a decrease in the light amount of the light beam incident on the phase difference providing structure.
[0058]
Claim6The invention described in claim 15As described in any one ofObjectiveAn optical element,
  The dispersion ν and refractive index n of the optical material are
  65 ≦ ν ≦ 75
  1.48 ≦ n ≦ 1.52
It is characterized by satisfying.
[0059]
  Claim7The invention described in claim 16As described in any one ofObjectiveAn optical element,
  The melting point T of the optical material is
  250 ℃ ≦ T ≦ 300 ℃
It is characterized by satisfying.
[0060]
  Claim7According to the invention described in claim 1,6In addition to obtaining the same effect as any one of the above, an optical material having a lower melting point than a general glass material is used. The phase difference imparting structure can be molded relatively easily.
[0061]
  Claim8The optical pickup device according to claim 1, wherein7As described in any one ofObjectiveAn optical element is provided.
[0075]
DETAILED DESCRIPTION OF THE INVENTION
[First embodiment]
A first embodiment of an optical element, objective optical element, and optical pickup device of the present invention will be described with reference to the drawings.
In the present embodiment, the optical element according to the present invention is applied to an objective lens, and the optical pickup apparatus 10 is configured by combining this objective lens and another optical element such as a beam splitter.
[0076]
As shown in FIG. 1, an optical pickup device 10 includes a semiconductor laser as a light source 11, a light beam having a wavelength λ (350 nm ≦ λ ≦ 450 nm) emitted from the semiconductor laser, and an optical information recording medium 13 (this embodiment) In the embodiment, a beam splitter 12 for branching a light beam reflected by a high-density DVD), an objective lens 20 (objective optical element) for condensing the light beam on the information recording surface 14 of the optical information recording medium 13, and a predetermined objective lens 20 A two-dimensional actuator (not shown) that moves in the direction, a concave lens 15, a photodetector 16 that detects reflected light from the optical information recording medium 13, and the like are schematically configured.
In the present embodiment, the light source 11 has a so-called finite configuration in which a light beam enters the objective lens 20 as diverging light. An infinite system configuration in which parallel light is incident on the objective lens 20 may be provided by arranging a collimator or the like.
Reference numeral 17 denotes a protective substrate formed on the information recording surface 14 in order to protect the information recording surface 14 of the optical information recording medium 13.
[0077]
Since the operation of the optical pickup device 10 configured in this manner is well known and will not be described in detail, the light beam having the wavelength λ emitted from the light source 11 passes through the beam splitter 12 and enters the incident surface 21 of the objective lens 20. Thus, the light is emitted by receiving a diffraction action by a phase difference providing structure 30 described later, converges on the information recording surface 14 of the optical information recording medium 13, and forms a spot P on the optical axis L. The light beam condensed on the spot is modulated by the information pits on the information recording surface 14 and reflected. The reflected light beam again passes through the objective lens 20 and is reflected by the beam splitter 12 and branched.
[0078]
Then, the branched light beam enters the photodetector 16 through the concave lens 15, and the photodetector 16 detects a spot of the incident light and outputs a signal, and the optical information recording medium 13 is output using the output signal. The reading signal of the information recorded in is obtained.
In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape or position of the spot on the photodetector 16. Based on the detection result, the two-dimensional actuator moves the objective lens 20 in the focus direction and the tracking direction so that the light beam forms an image on the information recording surface 14 as a spot.
[0079]
As shown in FIG. 2, the objective lens 20 is a single lens in which both the entrance surface 21 and the exit surface 22 are aspheric.
A phase difference imparting structure 30 that imparts a predetermined phase difference to the incident light flux is formed over the entire incident surface 21.
In the present embodiment, the phase difference providing structure 30 is formed of a plurality of diffraction ring zones 31 that are formed substantially concentrically around the optical axis L and have an action of diffracting an incident light beam.
[0080]
Each diffraction zone 31 is formed in a sawtooth shape when viewed in a plane (meridian cross section) along the optical axis L, and gives a predetermined phase difference to a light beam having a specific wavelength incident on each diffraction zone 31. By doing so, a diffraction effect is given to the light flux.
The start point 31a and the end point 31b (shown only in one place in FIG. 2) of each diffraction zone 31 are located on a predetermined aspheric surface S (hereinafter referred to as “mother aspheric surface S”) shown in FIG. The shape of the diffraction zone 31 can be defined by the amount of displacement in the direction of the optical axis L with respect to the mother aspheric surface S.
[0081]
The mother aspheric surface S can be defined by a function relating to the distance from the optical axis L with the optical axis L as the center of rotation. In addition, since the design method of the diffraction ring zone 31 is known, description is abbreviate | omitted. Further, such a phase difference providing structure 30 may be provided only on the exit surface 22, or may be provided on both the entrance surface 21 and the exit surface 22.
The objective lens 20 in FIG. 2 is the objective lens 20 configured by one optical element. However, even when the objective lens 20 is configured by combining two or more optical elements, the phase difference providing structure is provided. The optical surface (incident surface and exit surface) on which 30 is provided can be selected as appropriate.
[0082]
As an optical material, the objective lens 20 has a refractive index change dn / dt (temperature dependence) with respect to a temperature change of −10.0 × 10.^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6Designed with a glass material satisfying / ° C.
Further, the dispersion ν, refractive index n, and melting point T of the glass satisfy 65 ≦ ν ≦ 75, 1.48 ≦ n ≦ 1.52, and 250 ° C. ≦ T ≦ 300 ° C.
[0083]
When the optical material satisfies the above conditions, the processability of the lens is high and the refractive index change with respect to the temperature change can be reduced as compared with a general optical material conventionally used. Therefore, since the objective lens 20 can be formed of glass having low temperature dependency and has high workability, it is possible to provide a diffractive structure even for a glass lens that has been difficult in the past.
[0084]
Therefore, in the conventional plastic objective lens 20, it is necessary to design the phase difference providing structure 30 so that a plurality of characteristics such as temperature characteristics and wavelength characteristics can be improved. Therefore, it is not necessary to consider the temperature characteristics because the glass material is molded, the phase difference providing structure 30 is not required to improve (correct) the temperature characteristics, and the accuracy of various aberration corrections can be improved. The degree of freedom in lens design can be increased.
[0085]
Further, the phase difference providing structure 30 reduces the aberration fluctuation amount at the same position in the optical axis L direction before and after the wavelength fluctuation when the wavelength of the light beam emitted from the light source 11 fluctuates by 1 nm from λ due to, for example, mode hopping. .05λrms or less correction function, or correction of aberrations in the focused spot on the information recording surface 14 caused by the total of optical elements arranged on the optical path from the light source 11 to the optical information recording medium 13 when the temperature changes Has the function of
[0086]
In the present embodiment, as described above, the so-called finite optical pickup device 10 in which divergent light is incident on the objective lens 20 is used, and the optical system magnification m with respect to the light flux of wavelength λ of the objective lens 20 is −0. It is preferable to design so that it is in the range of .25 ≦ m ≦ −0.1.
In addition, the focal length f of the light beam having the wavelength λ is set to 0.2 mm ≦ f ≦ 3.5 mm, and the numerical aperture NA (the light beam of the light beam having the wavelength λ) is formed on the information recording surface 14 by the light beam having the wavelength λ. It is preferable to design so that the numerical aperture on the image side is in the range of 0.63 ≦ NA ≦ 0.95.
By satisfying these conditions, it is possible to obtain an optical pickup device 10 that records and / or reproduces information on a high-density DVD that uses blue laser light.
[0087]
Further, the wavelength λ is within the range of 350 nm ≦ λ ≦ 450 nm, the optical system magnification m is negative (preferably within the range of −0.25 ≦ m ≦ −0.1), and the focal length f of the light beam with the wavelength λ is 0. The numerical aperture NA (numerical aperture on the image side of the light beam with wavelength λ) of 0.55 of the condensing spot formed on the information recording surface 14 by the light beam with wavelength λ within the range of 2 mm ≦ f ≦ 1.0 mm is 0.55. By designing within the range of ≦ NA ≦ 0.70, the diameter of the objective lens 20 can be reduced, and the blue laser light can be used in a device smaller than the conventional optical pickup device 10.
[0088]
Further, the wavelength λ is preferably in the range of 630 nm ≦ λ ≦ 680 nm, the optical system magnification m is negative (preferably within the range of −0.30 ≦ m ≦ −0.1), and the focal length f of the light beam having the wavelength λ is preferably set. The numerical aperture NA (numerical aperture on the image side of the light beam with wavelength λ) of the condensing spot formed on the information recording surface 14 by the light beam with wavelength λ within the range of 0.2 mm ≦ f ≦ 3.5 mm is preferably 0. By designing within the range of .55 ≦ NA ≦ 0.75, the optical element (objective lens 20) and the optical pickup device 10 according to the present invention can be suitably used for DVD and MO.
[0089]
In the above embodiment, the structure in which a plurality of diffraction ring zones 31 are formed as the phase difference providing structure 30 has been described. However, the phase difference providing structure 30 according to the present invention is not limited to this, and for example, FIG. 3 may be used.
[0090]
In the objective lens 20 shown in FIG. 3, the phase difference providing structure 30 is configured by a plurality of annular zone surfaces 32 centering on the optical axis L being continuous through a step 33 substantially parallel to the optical axis L. .
Each annular surface 32 is formed so as to protrude toward the light source 11 as it is away from the optical axis L, and by giving a predetermined optical path difference to the light flux of wavelength λ incident on each annular surface 32, A phase difference is generated in each light beam, and as a result, the phase of the light beam that has passed through each annular zone is substantially aligned on the information recording surface 14. The shape of each step 33 can be defined by the amount of displacement in the direction of the optical axis L with respect to the mother aspheric surface S.
[0091]
In the objective lens 20 shown in FIG. 4, the phase difference imparting structure 30 extends along the direction of the optical axis L formed on the optical surfaces of the plurality of diffraction ring zones 34 around the optical axis L and these diffraction ring zones 34. It consists of a step-like discontinuous surface 35.
[0092]
Specifically, the objective lens 20 is a serrated discontinuous surface having a substantial inclination with respect to a predetermined aspherical optical surface (the mother aspheric surface S) with the optical axis L as the center. A plurality of diffraction ring zones 34 are formed. Further, on the optical surface of each diffraction ring zone 34, a predetermined optical path difference is given to the light beam passing through these diffraction ring zones 34. A stepwise discontinuous surface 35 is formed.
[0093]
5 (a) and 5 (b), the line indicated by the alternate long and short dash line represents an optical surface (mother aspheric surface S) having a virtual aspheric shape formed by connecting the starting points of the diffraction ring zones 34. A line indicated by a two-dot chain line represents an outer shape of a conventionally known concentric sawtooth diffractive annular zone 34 formed so as to increase in thickness as the distance from the optical axis L increases with the optical axis L as the center. It is.
[0094]
Lines indicated by solid lines in FIGS. 5A and 5B give a predetermined optical path difference to a light beam passing through each diffraction zone 34 formed on the optical surface of each diffraction zone 34. This represents an actual lens shape including the outer shape of the step-like discontinuous surface 35.
The depth d1 (length in the optical axis L direction) of each discontinuous surface 35 is a value represented by λ / (n−1), where n is the refractive index of the objective lens 20 with respect to the light flux having the wavelength λ. Between the light beam having the wavelength λ that passes through one discontinuous surface 35 and the light beam having the wavelength λ that passes through the adjacent discontinuous surface 35, which corresponds to approximately one wavelength (λ). The length is set such that an optical path difference occurs and a wavefront shift does not occur.
Further, the shape of each discontinuous surface 35 is obtained by dividing the surface shape of the sawtooth diffraction ring zone 34 indicated by a two-dot chain line in FIG. The shape approximates the shape translated in the L direction.
[0095]
As described above, the phase difference providing structure 30 shown in FIGS. 4 and 5 has a function of giving a predetermined optical path difference to a light beam having a wavelength λ passing through the objective optical element (objective lens 20), and each discontinuity. By dividing the surface shape of the surface 35 into a shape in which the diffraction ring zone 34 is divided into sections corresponding to the respective steps 33 and translated in the direction of the optical axis L, the maximum diffraction efficiency of the light flux having the wavelength λ is obtained. It has a function of extracting diffracted light.
Since the diffraction order of the light flux having the wavelength λ can be substantially changed in two steps, that is, the diffraction ring zone 34 formed on the objective optical element and the stepped discontinuous surface 35, for example, the diffraction order is appropriately changed. Thus, it is possible to obtain diffracted light having a sufficient amount of light according to recording and / or reproduction of information with respect to the optical information recording medium 13. In addition, the degree of freedom in design with respect to diffraction efficiency and diffraction order can be increased.
[0096]
In the present embodiment, the case where the optical element of the present invention is applied to the objective lens 20 has been described. However, the present invention is not limited to this, and any optical element that constitutes the optical system of the optical pickup device 10 may be used. Further, the present invention may be applied to a beam expander, a coupling lens, and an optical element configured to be shaped and emitted so as to have a substantially uniform light intensity distribution different from the light intensity distribution of the incident light beam.
[0097]
[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to the drawings.
In the present embodiment, the optical pickup device 40 is a so-called compatible device that uses two types of light beams of wavelengths λ 1 and λ 2 for two types of optical information recording media 13. Since the focal length, the numerical aperture, and the like are the main differences from the first embodiment, the differences will be mainly described below.
[0098]
  As shown in FIG. 6, the optical pickup device 40 uses a light beam having a wavelength λ1 (350 nm ≦ λ1 ≦ 450 nm) emitted from the first light source 11a as the first optical information recording medium 13.High densityBy focusing on the information recording surface 14 of the DVD, information is reproduced and / or recorded on the first optical information recording medium 13, and the wavelength λ2 (630 nm ≦ λ2 ≦ 680 nm) emitted from the second light source 11b is obtained. ) As the second optical information recording medium 13DVDThe information is recorded on the information recording surface 14 to reproduce and / or record information on the second optical information recording medium 13.
  In the semiconductor laser light source 11, a first light source 11a that emits a light beam having a wavelength λ1 and a second light source 11b that emits a light beam having a wavelength λ2 are integrally formed (one package). Of course, these light sources 11a and 11b may be arranged separately.
  The optical paths of the light beams having the wavelengths λ1 and λ2 are substantially the same as those described in the first embodiment, and the description thereof will be omitted because they are well known.
[0099]
As shown in FIG. 7, the objective lens 20 is a single lens in which both the entrance surface 21 and the exit surface 22 are aspherical surfaces, and a phase difference providing structure 30 is formed over almost the entire entrance surface 21.
As the phase difference providing structure 30, the same structure as the diffraction ring zone 31 in the first embodiment shown in FIG. 2 is used.
Then, by applying a diffractive action to only one or both of the light beams having the wavelengths λ1 and λ2 that pass through the diffraction ring zone 31, the optical system magnification and / or the wavelength of the light beams of these wavelengths are set. The phase difference providing structure has a function of correcting the spherical aberration caused by the difference.
[0100]
As an optical material, the objective lens 20 has a refractive index change dn / dt (temperature dependence) with respect to a temperature change of −10.0 × 10.^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6Designed with a glass material satisfying / ° C.
Further, the dispersion ν, refractive index n, and melting point T of the glass satisfy 65 ≦ ν ≦ 75, 1.48 ≦ n ≦ 1.52, and 250 ° C. ≦ T ≦ 300 ° C.
[0101]
When the optical material satisfies the above conditions, the processability of the lens is high and the refractive index change with respect to the temperature change can be reduced as compared with a general optical material conventionally used. Therefore, since the objective lens 20 can be formed of glass having low temperature dependency and has high workability, it is possible to provide a diffractive structure even for a glass lens that has been difficult in the past.
Accordingly, as in the first embodiment, since the objective lens 20 is formed of a glass material, it is not necessary to consider the temperature characteristics, and the phase difference providing structure 30 needs to improve (correct) the temperature characteristics. The accuracy of various aberration corrections can be improved, and the degree of freedom in lens design can be increased.
[0102]
The focal length f1 of the objective lens 20 with respect to the light beam having the wavelength λ1 is set to 0.2 mm ≦ f1 ≦ 3.5 mm, and the numerical aperture NA1 (wavelength λ1) of the condensing spot formed on the information recording surface 14 by the light beam having the wavelength λ1. The numerical aperture on the image side of the luminous flux is preferably designed to be in the range of 0.63 ≦ NA1 ≦ 0.95.
By satisfying these conditions, the blue laser beam as the light beam having the wavelength λ1 is appropriately condensed on the information recording surface 14, information is recorded and / or reproduced on the high-density DVD, and the light beam having the wavelength λ2 is appropriately collected. Thus, it is possible to obtain the optical pickup device 10 that collects light on the information recording surface 14 and records and / or reproduces information on the DVD.
[0103]
Further, the wavelength λ1 is in the range of 630 nm ≦ λ1 ≦ 680 nm, the wavelength λ2 is in the range of 750 nm ≦ λ2 ≦ 800 nm, and the optical system magnification m1 for the light beam having the wavelength λ1 is m1 ≠ 0 (preferably −0.3 ≦ m1 ≦ − 0.1), the optical system magnification m2 for the light beam with wavelength λ2 is set to m2 ≠ 0, and the focal length f1 of the light beam with wavelength λ1 is preferably within the range of 0.5 mm ≦ f1 ≦ 3.5 mm, and the wavelength λ1. The numerical aperture NA1 of the condensing spot formed on the information recording surface 14 by the light beam (the numerical aperture on the image side of the light beam having the wavelength λ1) is preferably designed in the range of 0.55 ≦ NA1 ≦ 0.75. preferable.
By satisfying these conditions, the light beam having the wavelength λ1 is appropriately condensed on the information recording surface 14, information is recorded on and / or reproduced from the DVD, and the light beam having the wavelength λ2 is appropriately formed on the information recording surface 14. It is possible to obtain the optical pickup device 10 that collects information and records and / or reproduces information on the CD.
[0104]
In the above embodiment, the structure in which a plurality of diffraction ring zones 31 are formed as the phase difference providing structure 30 has been described. However, the phase difference providing structure 30 according to the present invention is not limited to this, and the first structure The structure as shown in the embodiment (see FIGS. 2 to 5) may be used.
[0105]
Further, as shown in FIG. 8, the incident surface 21 of the objective lens 20 has a range with a height h or less around the optical axis L (hereinafter referred to as “central region A1”) and the periphery of the central region A1. The phase difference providing structure 30 may be provided only in the peripheral region A2 or in both the central region A1 and the peripheral region A2. The incident surface 21 may be divided into two or more regions.
[0106]
In this way, by dividing the incident surface 21 into a plurality of regions and designing the lens so as to give different diffractive effects to the light beams passing through each region, for example, the light beam having the wavelength λ1 is separated from the central region A1. When passing through the peripheral area A2, it is diffracted and converged on the information recording surface 14 of the DVD. Of the light flux having the wavelength λ2, the light flux passing through the central area A1 is diffracted to receive the information recording face 14 of the CD. The light beam that is converged upward and passes through the peripheral area A2 is given a diffracting action so as to be flare light, so that it does not converge on the information recording surface 14 of the CD.
With such a configuration, the objective lens 20 can be provided with a so-called aperture limiting function that does not condense part of the light flux having the wavelength λ2 on the information recording surface 14.
[0107]
In the present embodiment, when used for high-density DVD / DVD compatibility with a wavelength λ1 of 350 nm to 450 nm and a wavelength λ2 of 630 to 680 nm, the phase difference providing structure has the above-described light fluxes with wavelengths of λ1 and λ2. And the function of correcting spherical aberration caused by the difference in optical system magnification and / or wavelength, and the light beam having the second wavelength λ2 that has passed through the phase difference providing structure is collected on the information recording surface of the second optical information recording medium. It has both an aperture limiting function that does not allow light.
In addition, when used for DVD / CD compatibility with a wavelength λ1 of 630 nm to 680 nm and a wavelength λ2 of 750 to 800 nm, the phase difference providing structure corrects spherical aberration caused mainly by the difference in substrate thickness with respect to the light flux of each wavelength. Or an aperture limiting function that does not collect the light beam having the wavelength λ2 that has passed through the phase difference providing structure on the information recording surface of the second optical information recording medium.
In Examples 1 and 3 to be described later, the phase difference providing structure has both a compatible function and an aperture limiting function, and in Example 2, the phase difference providing structure has only a compatible function.
[0108]
【Example】
[Example 1]
Next, a first embodiment of the objective lens and the optical pickup device will be described. In the present embodiment, in the same manner as shown in FIG. 6, a compatible optical pickup device capable of using a light beam having a wavelength λ1 and a light beam having a wavelength λ2 for a DVD and a CD, respectively. ing.
[0109]
Similarly to the one shown in FIG. 8, the height h from the optical axis L is 1.16 mm or less on one optical surface (incident surface) of the objective lens which is a single lens with double aspheric surfaces. A diffraction zone as a phase difference providing structure is provided in each of the central region A1 and the peripheral region A2 of 1.16 mm or more.
The objective lens is an optical material having a refractive index change dn / dt (temperature dependency) with respect to a temperature change of −10.0 × 10.^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6Designed with a glass material satisfying / ° C.
Further, the dispersion ν, refractive index n, and melting point T of the glass satisfy 65 ≦ ν ≦ 75, 1.48 ≦ n ≦ 1.52, and 250 ° C. ≦ T ≦ 300 ° C.
[0110]
FIG. 8 shows a schematic diagram of the objective lens used in this embodiment. Therefore, the number of diffracting ring zones 31, the ratio of the central region A1 and the peripheral region A2 to the lens diameter in the radial direction, and the like do not necessarily match those of the objective lens of the present embodiment.
Tables 1 and 2 show the lens data of the objective lens.
[0111]
[Table 1]

[Table 2]

[0112]
As shown in Table 1, the objective lens of this example has a focal length f at a wavelength λ1 = 670 nm emitted from the first light source.₁= 2.22 mm, the image-side numerical aperture NA1 = 0.60, the imaging magnification m1 = −1 / 10, and the focal length f at the wavelength λ2 = 789 nm emitted from the second light source.₂= 2.23 mm, the image-side numerical aperture NA2 = 0.46, and the imaging magnification m2 = −1 / 10.
Surface numbers 2 and 2 'in Table 1 are a central region A1 having a height h from the optical axis L of 1.16 mm or less, a peripheral region A2 having a height of 1.16 mm or more, and a surface number of the incident surface of the objective lens. Reference numeral 3 denotes an exit surface. Also, ri represents the radius of curvature, di represents the position in the optical axis L direction from the i-th surface to the (i + 1) -th surface, and ni represents the refractive index of each surface.
[0113]
The second surface, the second 'surface, and the third surface of the objective lens are axially symmetric around the optical axis L, which is defined by a mathematical formula in which the coefficients shown in Tables 1 and 2 are substituted into the following formula (Equation 1). It is formed in a non-spherical surface.
The blazed wavelength for the diffractive structure on the second surface is 1 mm, and the blazed wavelength for the diffractive structure on the second surface is also 1 mm.
[0114]
[Expression 1]

[0115]
Here, X (h) is an axis in the direction of the optical axis L (the light traveling direction is positive), κ is a conical coefficient, A_2iIs the aspheric coefficient.
[0116]
Further, the pitch of the diffraction zone formed on the second surface and the second surface is defined by a mathematical formula obtained by substituting the coefficients shown in Table 2 into the optical path difference function of Formula 2.
[0117]
[Expression 2]

Where B_2iIs the coefficient of the optical path difference function.
[0118]
FIG. 9 is a graph showing the spherical aberration amount in a DVD using a light beam having a wavelength λ1 = 670 nm, and FIG. 10 is a graph showing the spherical aberration amount and the numerical aperture in a CD using a light beam having a wavelength λ2 = 789 nm. It is.
From FIG. 9 and FIG. 10, it can be seen that the spherical aberration is well corrected within the necessary numerical aperture for both DVD and CD.
[0119]
[Example 2]
Next, a second embodiment of the objective lens and the optical pickup device will be described. In the present embodiment as well, as shown in FIG. 6, the light beam having the wavelength λ1 and the light beam having the wavelength λ2 can be used for the DVD and the CD, respectively.
[0120]
Although not shown in the figure, a central region having a height h from the optical axis L of 0.78 mm or less on one optical surface (incident surface) of the objective lens which is a single lens having a double-sided aspheric surface and 0 It is divided into an intermediate region of .78 mm to 1.16 mm and a peripheral region of 1.16 mm or more, and a diffraction ring zone as a phase difference providing structure is provided in the intermediate region.
[0121]
The objective lens is an optical material having a refractive index change dn / dt (temperature dependency) with respect to a temperature change of −10.0 × 10.^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6Designed with a glass material satisfying / ° C.
Further, the dispersion ν, refractive index n, and melting point T of the glass satisfy 65 ≦ ν ≦ 75, 1.48 ≦ n ≦ 1.52, and 250 ° C. ≦ T ≦ 300 ° C.
Tables 3 and 4 show the lens data of the objective lens.
[0122]
[Table 3]

[Table 4]

[0123]
As shown in Table 3, the objective lens of this example has a focal length f when the wavelength λ1 emitted from the first light source is 670 nm.₁= 2.22 mm, the image-side numerical aperture NA1 = 0.60, the imaging magnification m1 = −1 / 10, and the focal length f at the wavelength λ2 = 789 nm emitted from the second light source.₂= 2.23 mm, the image-side numerical aperture NA2 = 0.46, and the imaging magnification m2 = −1 / 10.
In Table 1, surface numbers 2, 2 ′, and 2 ″ indicate the central region, intermediate region, peripheral region, and surface number 3 of the incident surface of the objective lens, respectively. Also, ri represents the radius of curvature, di represents the position in the optical axis L direction from the i-th surface to the (i + 1) -th surface, and ni represents the refractive index of each surface.
[0124]
The second surface, the second 'surface, the second' surface, and the third surface of the objective lens are respectively defined around the optical axis L defined by the mathematical formulas obtained by substituting the coefficients shown in Tables 3 and 4 into Equation 1 above. It is formed into an axisymmetric aspherical surface.
The blazed wavelength for the diffractive structure on the second 'surface is 1 mm.
[0125]
Further, the pitch of the diffraction zone is defined by an equation in which the coefficients shown in Table 4 are substituted into the optical path difference function of the above formula 2.
[0126]
FIG. 11 is a graph showing the amount of spherical aberration in a DVD using a light beam having a wavelength λ1 = 670 nm, and FIG. 12 is a graph showing the amount of spherical aberration in a CD using a light beam having a wavelength λ2 = 789 nm.
From FIG. 11 and FIG. 12, it can be seen that the spherical aberration is well corrected within the required numerical aperture for both DVD and CD.
[0127]
[Example 3]
Next, a third embodiment of the objective lens and the optical pickup device will be described.
In the present embodiment, information is reproduced and / or recorded on a high-density DVD using blue laser light in the same manner as shown in FIG.
In addition, as shown in FIG. 2, by providing a diffraction ring zone as a phase difference providing structure over almost the entire entrance surface of the objective lens, for example, when the wavelength of the emitted light beam from the light source changes due to mode hopping. It has a function of correcting generated chromatic aberration by this phase difference providing structure.
[0128]
The objective lens is an optical material having a refractive index change dn / dt (temperature dependency) with respect to a temperature change of −10.0 × 10.^-6/ ° C. ≦ dn / dt ≦ −1.0 × 10^-6Designed with a glass material satisfying / ° C.
Further, the dispersion ν, refractive index n, and melting point T of the glass satisfy 65 ≦ ν ≦ 75, 1.48 ≦ n ≦ 1.52, and 250 ° C. ≦ T ≦ 300 ° C.
Tables 5 and 6 show the lens data of the objective lens.
[0129]
[Table 5]

[Table 6]

[0130]
As shown in Table 5, the objective lens of this example has a focal length f = 1.76 mm when the wavelength λ = 405 nm emitted from the light source, an image-side numerical aperture NA = 0.85, and an imaging magnification m =. It is set to 0.
In Table 5, surface number 2 indicates the entrance surface of the objective lens, and surface number 3 indicates the exit surface. Also, ri represents the radius of curvature, di represents the position in the optical axis L direction from the i-th surface to the (i + 1) -th surface, and ni represents the refractive index of each surface.
[0131]
The second surface and the third surface of the objective lens are formed as axisymmetric aspherical surfaces around the optical axis L, which are defined by mathematical expressions obtained by substituting the coefficients shown in Tables 5 and 6 into Equation 1 above. .
The blazed wavelength for the diffractive structure of the second surface is 1 mm.
[0132]
Further, the pitch of the diffraction zone is defined by a mathematical formula in which the coefficients shown in Table 6 are substituted into the optical path difference function of the above formula 2.
[0133]
FIG. 13 is a longitudinal spherical aberration diagram when the wavelength λ varies between 404 nm (−1 nm) and 406 nm (+1 nm) due to the amount of spherical aberration and mode hop in a high-density DVD using a light beam with a wavelength λ = 405 nm. On the horizontal axis, the position at which the wavefront aberration is minimized when the wavelength is 405 nm is set to zero.
From FIG. 13, it can be seen that even when the wavelength varies due to mode hops, the wavefront aberration is well corrected within the required numerical aperture.
[0134]
【The invention's effect】
According to the present invention, since it becomes possible to mold the optical element with a glass material, it is necessary to have a function of correcting the temperature characteristics in the phase difference providing structure as compared with the case of using a plastic optical element. Thus, the accuracy of aberration correction can be improved and the degree of freedom in lens design can be increased.
[Brief description of the drawings]
FIG. 1 is a plan view of a principal part showing a configuration of an optical pickup device.
FIG. 2 is a cross-sectional view of a main part showing the structure of an objective lens.
FIG. 3 is a cross-sectional view of a main part showing the structure of an objective lens.
FIG. 4 is a cross-sectional view of a main part showing the structure of an objective lens.
FIGS. 5A and 5B are a cross-sectional view (a) and an enlarged view (b) of a main part showing the structure of the objective lens. FIGS.
FIG. 6 is a plan view of a principal part showing the configuration of the optical pickup device.
FIG. 7 is a cross-sectional view of the main part showing the structure of the objective lens.
FIG. 8 is a cross-sectional view of a main part showing the structure of an objective lens.
9 is a longitudinal spherical aberration diagram of a DVD in Example 1. FIG.
10 is a longitudinal spherical aberration diagram of CD in Example 1. FIG.
11 is a longitudinal spherical aberration diagram of a DVD in Example 2. FIG.
12 is a longitudinal spherical aberration diagram of CD in Example 2. FIG.
13 is a longitudinal spherical aberration diagram for a mode hop in Example 3. FIG.
[Explanation of symbols]
L Optical axis
10 Optical pickup device
13 Optical information recording media
14 Information recording surface
20 Optical elements (objective optical elements)
30 Phase difference imparting structure
32 Ring surface
33 steps
34 Diffraction ring zone
35 Discontinuous surface
40 Optical pickup device

Claims (8)

Information on the first optical information recording medium is obtained by condensing at least a light beam having a wavelength λ1 (350 nm ≦ λ1 ≦ 450 nm) emitted from the first light source on the information recording surface of the first optical information recording medium. Is reproduced and / or recorded, and the light beam having the wavelength λ2 (630 nm ≦ λ2 ≦ 680 nm) emitted from the second light source is condensed on the information recording surface of the second optical information recording medium. Used in an optical pickup device for reproducing and / or recording information on the optical information recording medium , and at least when reproducing and / or recording information on the first optical information recording medium, and the second An objective optical element shared when reproducing and / or recording information on an optical information recording medium ,
The refractive index change dn / dt with respect to the temperature change is
-10.0 × ^{10 -6} /℃≦dn/dt≦-1.0×10 ^-6 / ℃
The filled, to at least one optical surface, the formed glass to have a phase difference providing structure that imparts a phase difference to the light flux of the wavelength λ1 and / or .lambda.2, the phase difference providing structure, wherein each wavelength It has a function of correcting spherical aberration caused by a difference in optical system magnification and / or wavelength of the objective optical element with respect to the light beam, or the light beam having the wavelength λ2 that has passed through the phase difference providing structure is used as the second light. An objective optical element having an aperture limiting function that prevents light from being condensed on an information recording surface of an information recording medium. The objective optical element according to claim 1 ,
The focal length f1 with respect to the light beam before Symbol wavelength .lambda.1, the information recording condenser is formed on the surface spot numerical aperture NA1 of the light flux with wavelength .lambda.1 is
0.2mm ≦ f1 ≦ 3.5mm
0.63 ≦ NA1 ≦ 0.95
An objective optical element characterized by satisfying: The objective optical element according to claim 1 or 2 ,
In the phase difference providing structure, a plurality of annular surfaces with the optical axis as the center is continuous through a step substantially parallel to the optical axis, and at least one light flux among the wavelengths λ1 and λ2 passing through each annular zone is An objective optical element characterized in that the phase is substantially uniform on the information recording surface. The objective optical element according to any one of claims 1 to 3 ,
The objective optical element, wherein the phase difference providing structure is a diffractive structure. The objective optical element according to any one of claims 1 to 4 ,
The phase difference providing structure includes a plurality of diffraction ring zones centered on the optical axis, and a plurality of stepped shapes along the optical axis direction formed on the optical surface of at least one of the diffraction ring zones. An objective optical element comprising a discontinuous surface. The objective optical element according to any one of claims 1 to 5 ,
The dispersion ν and refractive index n of the optical material are
65 ≦ ν ≦ 75
1.48 ≦ n ≦ 1.52
An objective optical element characterized by satisfying: The objective optical element according to any one of claims 1 to 6 ,
The melting point T of the optical material is
250 ℃ ≦ T ≦ 300 ℃
An objective optical element characterized by satisfying: An optical pickup device comprising the objective optical element according to any one of claims 1 to 7 .

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