JP2005024666A - Objective lens, optical pickup and optical information processor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens and an optical pickup capable of satisfactorily suppressing spherical aberration in three generations of blue color type/DVD/CD (or two generations of blue color type/DVD) without necessitating an aberration correction element. <P>SOLUTION: The objective lens which focuses light from a light source of wavelength λ1, λ2, λ3 (λ1<λ2<λ3) on an optical recording medium is constituted as a single lens, has aspheric and convex in both surfaces and satisfies the following conditions for performing the focusing on the optical recording medium in the state that wavefront aberration is optimized with respect to the optical recording medium of three wavelengths (or two wavelengths): (1) vd>35, (2) 1.58>nd, (3) 0.58nd-0.29≤R1/f≤0.62nd-0.31 (cf. Fig.). Thereby, the objective lens can satisfactorily suppress the spherical aberration without necessitating the aberration correction element. In the conditions (1)-(3), vd is Abbe number, nd is refractive index of material for d-ray, R1 is paraxial radius of curvature of the light source side surface and f is focal distance. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５２】
図５は、本実施の形態の対物レンズ２Ａを用いて、青色系光記録媒体１ａ、ＤＶＤ系光記録媒体１ｂ、ＣＤ系光記録媒体１ｃの各々について所定の波長を用いて使用したときに発生する波面収差特性を示す特性図である。横軸は各光記録媒体１ａ，１ｂ，１ｃ、縦軸は最良像点での軸上波面収差を表す。各光記録媒体１ａ，１ｂ，１ｃについて、波面収差０．０２λｒｍｓ以下と良好な収差特性を有している。
【００５３】
＜対物レンズの第２の実施の形態＞
本実施の形態を図６及び図７に基づいて説明する。本実施の形態は、使用波長λ１：４０５ｎｍ、基板厚：０．６ｍｍ、開口数ＮＡ：０．７０の青色系光記録媒体１ａと、使用波長λ２：６６０ｎｍ、基板厚：０．６ｍｍ、開口数ＮＡ：０．６５のＤＶＤ系光記録媒体１ｂと、使用波長λ３：７８５ｎｍ、基板厚：１．２ｍｍ、開口数ＮＡ：０．５０のＣＤ系光記録媒体１ｃの３種類の光記録媒体について記録、再生又は消去を行う光ピックアップに用いられる対物レンズ２Ｂに関する。
【００５４】
本実施の形態の対物レンズ２Ｂは、焦点距離ｆ：３．０５ｍｍ、ｄ線の屈折率ｎｄ：１．５５、アッベ数νｄ：６０の硝種（５５００００．６０００００）を用いている。また、本実施の形態の対物レンズ２Ｂは、第１の実施の形態の場合と同じく、青色系光記録媒体１ａに対しては無限系で使用し、ＤＶＤ系光記録媒体１ｂ、ＣＤ系光記録媒体１ｃについては有限系で使用されるレンズである。
【００５５】
青色系光記録媒体１ａでの使用構成を図６（ａ）、表２（ａ）に、ＤＶＤ系光記録媒体１ｂでの使用構成を図６（ｂ）、表２（ｂ）に、ＣＤ系光記録媒体１ｃでの使用構成を図６（ｃ）、表２（ｃ）に示す。対物レンズ２Ｂの非球面形状、表２の記載項目は第１の実施の形態の場合と同様である。
【００５６】
また、本実施の形態の対物レンズ２Ｂも、以下の条件
（１） νｄ＞３５
（２） １．５８＞ｎｄ
（３） ０．５８ｎｄ−０．２９≦Ｒ１／ｆ≦０．６２ｎｄ−０．３１
を満足している。
【００５７】
【表２】

【００５８】
図７は、本実施の形態の対物レンズ２Ｂを用いて、青色系光記録媒体１ａ、ＤＶＤ系光記録媒体１ｂ、ＣＤ系光記録媒体１ｃの各々について所定の波長を用いて使用したときに発生する波面収差特性を示す特性図である。横軸は各光記録媒体１ａ，１ｂ，１ｃ、縦軸は最良像点での軸上波面収差を表す。各光記録媒体１ａ，１ｂ，１ｃについて、波面収差０．０２λｒｍｓ以下と良好な収差特性を有している。
【００５９】
＜対物レンズの第３の実施の形態＞
本実施の形態を図８及び図９に基づいて説明する。本実施の形態は、使用波長λ１：４０５ｎｍ、基板厚：０．６ｍｍ、開口数ＮＡ：０．６７の青色系光記録媒体１ａと、使用波長λ２：６６０ｎｍ、基板厚：０．６ｍｍ、開口数ＮＡ：０．６５のＤＶＤ系光記録媒体１ｂと、使用波長λ３：７８５ｎｍ、基板厚：１．２ｍｍ、開口数ＮＡ：０．５０のＣＤ系光記録媒体１ｃの３種類の光記録媒体について記録、再生又は消去を行う光ピックアップに用いられる対物レンズ２Ｃに関する。
【００６０】
本実施の形態の対物レンズ２Ｃは、焦点距離ｆ：３．０５ｍｍ、ｄ線の屈折率ｎｄ：１．５５、アッベ数νｄ：５５の硝種（５５００００．６０００００）を用いている。また、本実施の形態の対物レンズ２Ｃは、第１の実施の形態の場合と同じく、青色系光記録媒体１ａに対しては無限系で使用し、ＤＶＤ系光記録媒体１ｂ、ＣＤ系光記録媒体１ｃについては有限系で使用されるレンズである。
【００６１】
青色系光記録媒体１ａでの使用構成を図８（ａ）、表３（ａ）に、ＤＶＤ系光記録媒体１ｂでの使用構成を図８（ｂ）、表３（ｂ）に、ＣＤ系光記録媒体１ｃでの使用構成を図８（ｃ）、表３（ｃ）に示す。対物レンズ２Ｃの非球面形状、表３の記載項目は第１の実施の形態の場合と同様である。
【００６２】
また、本実施の形態の対物レンズ２Ｃも、以下の条件
（１） νｄ＞３５
（２） １．５８＞ｎｄ
（３） ０．５８ｎｄ−０．２９≦Ｒ１／ｆ≦０．６２ｎｄ−０．３１
を満足している。
【００６３】
【表３】

【００６４】
図９は、本実施の形態の対物レンズ２Ｃを用いて、青色系光記録媒体１ａ、ＤＶＤ系光記録媒体１ｂ、ＣＤ系光記録媒体１ｃの各々について所定の波長を用いて使用したときに発生する波面収差特性を示す特性図である。横軸は各光記録媒体１ａ，１ｂ，１ｃ、縦軸は最良像点での軸上波面収差を表す。各光記録媒体１ａ，１ｂ，１ｃについて、波面収差０．０２λｒｍｓ以下と良好な収差特性を有している。
【００６５】
［光ピックアップの実施の形態］
＜光ピックアップの第１の実施の形態＞
本実施の形態の光ピックアップは、ＮＡ０．６５青色系／ＤＶＤ／ＣＤへの適用例を示す。
【００６６】
まず、本実施の形態の光ピックアップ１１の光学系全体の構成例を図１０を参照して説明する。本実施の形態の光ピックアップ１１は、青色波長帯域の使用波長λ１＝４０５ｎｍの光源１２、赤色波長帯域の使用波長λ２＝６６０ｎｍの光源１３、及び、赤外波長帯域の使用波長λ３＝７８５ｎｍの光源１４を備え、ＮＡ０．６５、光照射側基板厚０．６ｍｍの青色系光記録媒体１ａと、ＮＡ０．６５、光照射側基板厚０．６ｍｍのＤＶＤ系光記録媒体１ｂと、ＮＡ０．５０、光照射側基板厚１．２ｍｍのＣＤ系光記録媒体１ｃとの各々に対して、記録、再生又は消去が可能な光ピックアップである。
【００６７】
本実施の形態の光ピックアップ１１は、青色波長帯域の半導体レーザ（光源）１２、コリメートレンズ１５、偏光ビームスプリッタ１６、ダイクロイックプリズム１７，１８、偏向プリズム１９、１／４波長板２０、開口部２１、開口切換手段２２、対物レンズ２（本実施の形態では、２Ａ）、検出レンズ２３、光束分割手段２４、受光素子２５より構成される青色波長帯域の光が通過する青色光学系２６と、ホログラムユニット２７、カップリングレンズ２８、ダイクロイックプリズム１７，１８、偏向プリズム１９、１／４波長板２０、開口部２１、開口切換手段２２、対物レンズ２Ａから構成される赤色波長帯域の光が通過するＤＶＤ系の赤色光学系２９と、ホログラムユニット３０、カップリングレンズ３１、ダイクロイックプリズム１８、偏向プリズム１９、１／４波長板２０、開口部２１、開口切換手段２２、対物レンズ２Ａから構成される赤外波長帯域の光が通過するＣＤ系の赤外光学系３２から構成されている。即ち、ダイクロイックプリズム１７，１８、偏向プリズム１９、１／４波長板２０、開口部２１、開口切換手段２２、対物レンズ２Ａは２乃至３つの光学系の共通部品である。
【００６８】
また、ホログラムユニット２７は、半導体レーザ（光源）１３のチップ、ホログラム３３及び受光素子３４を一体化して構成されたものである。同様に、ホログラムユニット３０は、半導体レーザ（光源）１４のチップ、ホログラム３５及び受光素子３６を一体化して構成されたものである。
【００６９】
また、光記録媒体１ａ，１ｂ，１ｃは前述したように各々使用波長が異なる光記録媒体で、光記録媒体１ａは基板厚さが０．６ｍｍの青色系光記録媒体、光記録媒体１ｂは基板厚さが０．６ｍｍのＤＶＤ系光記録媒体、光記録媒体１ｃは基板厚さが１．２ｍｍのＣＤ系光記録媒体である。記録、再生又は消去時には何れかの光記録媒体１ａ，１ｂ又は１ｃのみが図示しない回転機構にセットされて高速回転される。
【００７０】
また、開口部２１は、対物レンズ２をフォーカス方向、トラック方向に可動させるアクチュエータ上の当該対物レンズ２を保持するボビン上で規制することが可能であり、具体的な光学部品を用いる必要はない。
【００７１】
このような構成において、各波長帯域毎の光学系動作例について説明する。まず、青色波長帯域の使用波長λ１＝４０５ｎｍの光源１２、ＮＡ０．６５、光照射側基板厚０．６ｍｍの青色系光記録媒体１ａに記録、再生又は消去を行う場合について説明する。波長４０５ｎｍの半導体レーザ１２から出射した直線偏光の発散光は、コリメートレンズ１５で略平行光とされ、偏光ビームスプリッタ１６、ダイクロイックプリズム１７，１８を透過し、偏向プリズム１９で光路を９０度偏向され、１／４波長板２０を通過し円偏光とされ、開口部２１を透過し、開口切換手段２２においてＮＡ０．６５に制限され、対物レンズ２Ａに入射し、光記録媒体１ａ上に微小スポットとして集光される。このスポットにより、情報の再生、記録又は消去が行われる。
【００７２】
光記録媒体１ａで反射した光は、往路とは反対回りの円偏光となり、再び略平行光とされ、１／４波長板２０を通過して往路と直交した直線偏光になり、偏向プリズム１９、ダイクロイックプリズム１８，１７を通過し偏光ビームスプリッタ１６で反射されて、検出レンズ２３で収束光とされ、光束分割手段２４により複数の光路に偏向分割され受光素子２５に至る。受光素子２５からは、情報信号、サーボ信号が検出される。
【００７３】
次に、赤色波長帯域の使用波長６６０ｎｍの光源１３、ＮＡ０．６５、光照射側基板厚０．６ｍｍのＤＶＤ系光記録媒体１ｂに記録、再生又は消去を行う場合について説明する。ホログラムユニット２７の半導体レーザ１３のチップから出射された６６０ｎｍの光は、ホログラム３３を透過し、カップリングレンズ２８で所定の発散光とされ、青色波長帯域の光は透過し赤色波長帯域の光は反射させるダイクロイックプリズム１７によって偏向プリズム１９の方向に反射され、ダイクロイックプリズム１８通過後、偏向プリズム１９によって光路が９０度偏向され、１／４波長板２０を通過し円偏光とされ、開口部２１においてＮＡ０．６５に制限され、対物レンズ１Ａに入射し、光記録媒体１Ａ上に微小スポットとして集光される。このスポットにより、情報の再生、記録又は消去が行われる。
【００７４】
光記録媒体１Ａで反射した光は、偏向プリズム１９で偏向され、ダイクロイックプリズム１８通過後、ダイクロイックプリズム１７で反射され、カップリングレンズレンズ２８で収束光とされ、ホログラム３３により半導体レーザ１３と同一キャン内にある受光素子３４方向に回折されて受光素子３４に受光される。受光素子３４からは、情報信号、サーボ信号が検出される。
【００７５】
さらに、赤外波長帯域の使用波長７８５ｎｍの光源１４、ＮＡ０．５０、光照射側基板厚１．２ｍｍのＣＤ系光記録媒体１ｃに記録、再生又は消去を行う場合について説明する。ホログラムユニット３０の半導体レーザ１４から出射された７８５ｎｍの光は、ホログラム３５を透過し、カップリングレンズ３１で所定の発散状ビームに変換され、青色と赤色波長域の光は透過し赤外波長域の光は反射させるダイクロイックプリズム１８によって偏向プリズム１９の方向に反射され、偏向プリズム１９によって光路が９０度偏向され、波長板２０を通過し楕円偏光或いは円偏光とされ、開口部２１を通過し、開口切換手段２２でＮＡ０．５０に制限され、対物レンズ２Ａに入射し、光記録媒体１ｃ上に微小スポットとして集光される。このスポットにより、情報の再生、記録或いは消去が行われる。
【００７６】
光記録媒体１ｃで反射した光は、偏向プリズム１９で偏向され、ダイクロイックプリズム１８で反射され、カップリングレンズ３１で収束光とされ、ホログラム３５により半導体レーザ１４と同一キャン内にある受光素子３６方向に回折されて受光素子３６に受光される。受光素子３６からは、情報信号、サーボ信号が検出される。
【００７７】
ここで、対物レンズ２Ａ付近における開口切換部分について説明する。
【００７８】
まず、ＤＶＤ入射光束径について説明する。青色波長帯域で最良の波面を有する対物レンズに、同一光束径の赤色帯域の光を入射させた場合、屈折力が低下し、開口数ＮＡが低くなる。そのため、本実施の形態では赤色波長帯域については、青色波長帯域での入射光束径φ１に比べ、より大きな光束径φ２で入射させる。
【００７９】
図１１は、青色波長帯域において以下の特性
入射光束径φ：３．９６５ｍｍ
開口数（ＮＡ）：０．６５
焦点距離（ｆ）：３．０５ｍｍ
硝種 ：５０００００．６０００００
を有する対物レンズ２Ａにおいて、異なる波長帯域使用時のＮＡ０．６５となる有効径と波長の関係を示すものである。この図１１から、波長６６０ｎｍを使用するＤＶＤ系光記録媒体１ｂの記録、再生又は消去時には光束径φ２を４．０９５ｍｍ程度にする必要があることが分る。なお、波長４０５ｎｍ以外では、適宜物体距離を変更して波面収差最良の状態としている。
【００８０】
また、ＣＤ入射光束径について説明する。ＣＤ系光記録媒体１ｃに対する記録、再生又は消去時の最適な開口数ＮＡは０．５程度であるが、図１１の場合と同様の方法により、その最適な有効径φ３を見積もると、φ３＝３．２８ｍｍ程度とすればよいことが分る。
【００８１】
以上のように対物レンズ２Ａに対する入射光束径φ１、φ２、φ３に関して３段階の開口切換えが必要である。本実施の形態では、入射光束径φ２をアクチュエータ部に設けられた開口部２１で制限し、入射光束径φ１、φ３を切換える開口切換手段２２としては、光源から出射される光束の波長に応じて、例えば、図１２（ａ）〜（ｃ）に示すように回折によって光束径を切換える波長選択性回折パターンが形成された波長選択性回折素子４１を用いる。
【００８２】
図１２（ａ）の青色波長帯域の光束、図１２（ｂ）の赤色波長帯域の光束、図１２（ｃ）の赤外波長帯域の光束における各光束の透過特性は、波長選択性回折素子４１が施されていない中心部領域の入射光束径φ３以内では青色波長帯域、赤色波長帯域、赤外波長帯域の何れの光に対しても透過し、周辺部領域の入射光束径φ３の外周から入射光束径φ１までの領域では、青色波長帯域と赤色波長帯域の光に対しては作用せず、赤外波長帯域の光についてのみ回折させ、周辺部領域の入射光束径φ１の外周から入射光束径φ２までの領域では、赤色波長帯域の光に対しては作用せず、青色波長帯域と赤外波長帯域の光は回折させる。
【００８３】
なお、本実施の形態のような波長選択性回折素子４１に限定されるものでなく、波長選択性の透過／反射特性を持つコーティングを施した素子であってもよい。
【００８４】
＜光ピックアップの第２の実施の形態＞
本実施の形態の光ピックアップは、ＮＡ０．７０青色系／ＤＶＤ／ＣＤへの適用例を示す。
【００８５】
本実施の形態の光ピックアップは、青色波長帯域の使用波長λ１＝４０５ｎｍの光源１２、赤色波長帯域の使用波長λ２＝６６０ｎｍの光源１３、及び、赤外波長帯域の使用波長λ３＝７８５ｎｍの光源１４を備え、ＮＡ０．７０、光照射側基板厚０．６ｍｍの青色系光記録媒体１ａと、ＮＡ０．６５、光照射側基板厚０．６ｍｍのＤＶＤ系光記録媒体１ｂと、ＮＡ０．５０、光照射側基板厚１．２ｍｍのＣＤ系光記録媒体１ｃとの各々に対して、記録、再生又は消去が可能な光ピックアップである。
【００８６】
前述の第１の実施の形態と異なる点は、青色系の開口数がＮＡ０．６５でなくＮＡ０．７０となり、必要入射ビーム径がＤＶＤに比べ青色の方が大きくなった点である。従って、全体構成は、図１０に示したものと同様であるが、本実施の形態では対物レンズとして１Ａに代えて１Ｂが用いられている。ＮＡ０．７０程度であれば、前述の各種変動に伴うマージンの劣化は小さく、また、容量も増やせる。ＮＡ０．７０を用いればＮＡ０．６５に対して、約１５％の容量増加が可能である。
【００８７】
この場合、入射光束径φ１をアクチュエータ部に設けられた開口部２１で制限し、入射光束径φ２、φ３の開口切換手段２２としては、光源から出射される光束の波長に応じて、図１３（ａ）〜（ｃ）に示すように回折によって光束径を切換える波長選択性回折パターンが形成された波長選択性回折素子４２を用いる。
【００８８】
図１３（ａ）の青色波長帯域の光束、図１３（ｂ）の赤色波長帯域の光束、図１３（ｃ）の赤外波長帯域の光束における各光束の透過特性は、波長選択性回折素子４２が施されていない中心部領域の入射光束径φ３では青色波長帯域、赤色波長帯域、赤外波長帯域の何れの光に対しても透過し、周辺部領域の入射光束径φ３の外周から入射光束径φ２までの領域では、青色波長帯域と赤色波長帯域の光に対しては作用せず、赤外波長帯域の光についてのみ回折させ、周辺部領域の入射光束径φ２の外周から入射光束径φ１までの領域では、青色波長帯域の光に対しては作用せず、赤色波長帯域と赤外波長帯域の光は回折させる。
【００８９】
＜光ピックアップの第３の実施の形態＞
本実施の形態の光ピックアップは、ＮＡ０．６７青色系／ＤＶＤ／ＣＤへの適用例を示す。
【００９０】
本実施の形態の光ピックアップは、青色波長帯域の使用波長λ１＝４０５ｎｍの光源１２、赤色波長帯域の使用波長λ２＝６６０ｎｍの光源１３、及び、赤外波長帯域の使用波長λ３＝７８５ｎｍの光源１４を備え、ＮＡ０．６７、光照射側基板厚０．６ｍｍの青色系光記録媒体１ａと、ＮＡ０．６５、光照射側基板厚０．６ｍｍのＤＶＤ系光記録媒体１ｂと、ＮＡ０．５０、光照射側基板厚１．２ｍｍのＣＤ系光記録媒体１ｃとの各々に対して、記録、再生又は消去が可能な光ピックアップである。
【００９１】
前述の第１の実施の形態と異なる点は、青色系の開口数がＮＡ０．６５でなくＮＡ０．６７となり、必要入射ビーム径がＤＶＤに比べ青色の方が大きくなった点である。従って、全体構成は、図１０に示したものと同様であるが、本実施の形態では対物レンズとして１Ａに代えて１Ｃが用いられている。
【００９２】
一般に、青色波長帯域で最良の波面を有する対物レンズに、同一光束径の赤色帯域の光を入射させた場合、屈折力が低下し、開口数が低くなる。逆に言うと、赤色波長帯域の光を通過させた場合に比べ、青色波長帯域の光を通過させたときの方が、ＮＡが高くなり、短波長化と加えて大容量の光ピックアップが実現できる。一方、ＤＶＤ世代は、ＮＡが０．６５近傍と定められている。図１４は、以下の特性
青色での焦点距離：３．０５ｍｍ
硝種 ：５５００００．５５００００
光記録媒体基板厚：０．６ｍｍ
を有する対物レンズを例として、青色波長４０５ｎｍと赤色波長６６０ｎｍの光を透過させたときの開口数ＮＡの関係を示すものである。図１４から、例えば、ＤＶＤ系の赤色波長の光でＮＡ０．６５のとき、同一入射光束径の青色波長の光に対しては、ＮＡ０．６７程度になることを意味する。このような関係を用いれば、第１，２の実施の形態のような３段階の開口切換えでなく、ＤＶＤ、ＣＤ世代同様の２段階の開口切換えとなり、素子の簡素化が図れる。
【００９３】
そして、本実施の形態の開口切換手段２２部分としては、光源から出射される光束の波長に応じて、図１５（ａ）〜（ｃ）に示すように回折によって光束径を切り換える波長選択性回折パターンが形成された波長選択性回折素子４３を用いる。
【００９４】
図１５（ａ）の青色波長帯域の光束、図１５（ｂ）の赤色波長帯域の光束、図１５（ｃ）の赤外波長帯域の光束における各光束の透過特性は、波長選択性回折素子４３が施されていない中心部領域の入射光束径φ３では青色波長帯域、赤色波長帯域、赤外波長帯域の何れの光に対しても透過し、周辺部領域の入射光束径φ３の外周から入射光束径φ１（＝φ２）までの領域では、青色波長帯域と赤色波長帯域の光に対しては作用せず、赤外波長帯域の光についてのみ回折させる。
【００９５】
ところで、図１６は、本実施の形態の対物レンズ２Ｃについて、赤色波長帯域６６０ｎｍの光を通過させたときの入射光束径（Ｘ軸）と、その入射光束径でＤＶＤ系光路がＮＡ０．６５を得るための開口切換手段２２−対物レンズ２Ｃの前側主点の間隔条件（Ｙ軸）の関係を示したものである。また、図１６の第２Ｙ軸には青色波長帯域４０５ｎｍの光を通過させたときの入射光束径（Ｘ軸）に対する開口数：ＮＡを示している。特に、本実施の形態における青色系光記録媒体１ａへの記録再生時の使用開口数はＮＡ０．６７であり、図１６より最適な入射光束径：φと、開口切換手段２２−対物レンズ２Ｃの前側主点間距離：ｔは、
最適な入射光束径：φ＝４．１ｍｍ
最適な開口−対物レンズ前側主点間距離：ｔ＝０ｍｍ
となる。
【００９６】
ここで、図１６の関係について説明する。本実施の形態では、波長λ１＝４０５ｎｍ、波長λ２＝６６０ｎｍの光源と、光源光を光記録媒体１に集光照射する単一の対物レンズ２Ｃとを備え、波長４０５ｎｍの光源１２点灯時には無限系の入射光束で使用し、波長６６０ｎｍの光源１３点灯時には有限系の入射光束で使用するものであって、一定の開口を対物レンズ前側主点から光源側へ所定距離：ｔ、
ｔ≒Ｌ−ＮＡ１・ｆ／ｔａｎ（ａｓｉｎ（ＮＡ２ｏｂｊ））
ここで、ｆ：対物レンズの焦点距離
ＮＡ１：波長λ１＝４０５ｎｍの光源点灯時の像面側開口数
ＮＡ２ｏｂｊ：波長λ２＝６６０ｎｍの光源点灯時の光源側開口数
Ｌ：波長λ２＝６６０ｎｍの光源点灯時の物体距離
の位置に配置しているため、波長４０５ｎｍと波長６６０ｎｍの開口切換えは不要となっている。
【００９７】
ここで、図１７を参照して、上式ｔ≒Ｌ−ＮＡ１・ｆ／ｔａｎ（ａｓｉｎ（ＮＡ２ｏｂｊ））について説明する。まず、波長λ１、開口数ＮＡ１、無限系の光路は図中一点鎖線の如く描かれる。また、対物レンズ１Ｃの焦点距離：ｆと入射光束径：φ、開口数：ＮＡ１は一般に、次式
ＮＡ１＝Φ／２／ｆ
の関係が知られている。一方、波長λ２、開口数ＮＡ２、物体距離Ｌ、物体側開口数ＮＡ２ｏｂｊの有限系光路は図中実線の如く描かれる。ここで、開口の位置は対物レンズ前側主点から光源側に距離：ｔだけ離れた位置で、波長λ２の光束径がφとなるように設置すると、次式
φ／２＝（Ｌ−ｔ）×ｔａｎ（ａｓｉｎ（ＮＡ２ｏｂｊ））
の関係が得られる。
【００９８】
この式を、ＮＡ１に関する式に代入することにより、ｔに関する上式が得られる。ちなみに、本実施の形態では、
青色波長帯域の光路条件：λ１＝４０５ｎｍ、ＮＡ１＝０．６７
赤色波長帯域の光路条件：λ２＝６６０ｎｍ、ＮＡ２＝０．６５、Ｌ＝１４２ｍｍ、ＮＡ２ｏｂｊ＝０．０１５
で、φ＝４．１ｍｍ、ｔ＝０．０ｍｍという構成例を示している。
【００９９】
また、これらの実施の形態では、特に説明しなかったが、ＤＶＤ系やＣＤ系で有限系の構成にする際、その物体距離として波面収差最小となる位置の事例について説明する。例えば、対物レンズの第３の実施の形態の場合は、ＤＶＤ系の波長６６０ｎｍの光を通過させたときの物体距離と波面収差の関係が図１８の如く表されるため、波面収差が最小となる物体距離１４２ｍｍの位置を選択している。
【０１００】
なお、開口切換手段２２としては、回折特性を利用したものに限らず、例えば、反射特性や吸収特性、さらには、偏光を利用したものであってもよい。例えば、反射特性を利用する場合、波長選択性を有する誘電体光学多層膜を波長選択性回折素子４３の場合と同様に設ければよい。より具体的には、当該誘電体光学多層膜が施されていない中心部領域では青色波長帯域、赤色波長帯域、赤外波長帯域の何れの光に対しても高透過率で、周辺部領域φ２より外側領域では青色波長帯域、赤色波長帯域の光に対してのみ高透過率で、赤外波長帯域の光に対しては低透過率となるようにすればよい。吸収特性を利用する場合も、これらの場合と同様に光源側から出射される光の波長に応じてその吸収の如何によって光束径を切換えるように構成すればよい。さらに、偏光を利用する場合、赤色波長帯域と赤外波長帯域の光の偏光方向が直交するように光源を配置し、直交する偏光方向に応じて開口を切換えるようにすればよい。
【０１０１】
なお、開口切換手段２２としては、図１２等に示したように別素子として設けずに、対物レンズ２（２Ａ，２Ｂ，２Ｃ）面にコーティング或いは回折格子を設けた構成であってもよい。例えば、図１９では波長選択性の透過／反射のコーティング４４を対物レンズ２の光源側面に形成したものであって、図１９（ａ）は波長４０５ｎｍの光で青色系光記録媒体１ａに収束照射している場合、図１９（ｂ）は波長６６０ｎｍの光でＤＶＤ系光記録媒体１ｂに収束照射している場合、図１９（ｃ）は波長７８５ｎｍの光でＣＤ系光記録媒体１ｃに収束照射している場合を示す。コーティング４４は波長７８５ｎｍの光に対してのみ反射するものであって、波長４０５ｎｍ、６６０ｎｍの光については透過する特性を有する。
【０１０２】
［光情報処理装置の実施の形態］
本実施の形態の光情報処理装置の構成例を図２０の概略斜視図を参照して説明する。本実施の形態の光情報処理装置５１は、適用波長、開口数ＮＡの異なる複数種類の光記録媒体１ａ，１ｂ又は１ｃに対して、図１０に示したような構成の光ピックアップ５２を用いて情報の記録、再生又は消去を互換性を持って行う装置である。本実施の形態において、光記録媒体１（１ａ，１ｂ又は１ｃ）はディスク状であって、保護ケース５３内に格納されている。光記録媒体１（１ａ，１ｂ又は１ｃ）は保護ケース５３ごと、挿入口５４から当該光情報処理装置５１内に対して矢印「入」方向へ挿入セットされ、スピンドルモータ５５により回転され、光ピックアップ５２により情報の記録や再生、或いは消去が行われる。なお、光記録媒体１（１ａ，１ｂ又は１ｃ）は保護ケース５３に入れられている必要はなく裸の状態であってもよい。
【０１０３】
対物レンズや光ピックアップ５２として、前述の実施の形態で説明したものを用いることにより、収差補正素子を必要とすることなく、青色系／ＤＶＤ／ＣＤの３世代（或いは、青色系／ＤＶＤの２世代）において、球面収差が十分に抑制された対物レンズ及び光ピックアップを備える光情報処理装置５１が提供される。
【０１０４】
【発明の効果】
請求項１ないし６記載の発明の対物レンズによれば、使用波長との関係から、当該対物レンズの硝種条件、形状条件、使用波長と基板厚条件等を適正に規定したので、従来技術の貼り合せレンズや、位相補正素子付加などを行うことなく、青色系、ＤＶＤ、ＣＤのうちの２波長或いは３波長について互換性を持たせることができる。
【０１０５】
請求項７，９記載の発明の光ピックアップによれば、条件（４）（５）を満足することにより、近年規格化が検討されているＮＡ０．６５の青色系光記録媒体ＨＤ−ＤＶＤ、ＮＡ０．６５のＤＶＤ系光記録媒体の互換が可能となる。
【０１０６】
請求項８，９記載の発明の光ピックアップによれば、例えばＮＡ０．６７などのように青色系の開口数ＮＡを若干大きくすれば、入射光束径φ１、φ２を共通化することが可能となり、開口切換えを不要にすることができる。
【０１０７】
請求項１０ないし１３記載の発明の光ピックアップによれば、いわゆる青色系、ＤＶＤ、ＣＤの３世代各々について最適ＮＡを得ることができる。
【０１０８】
請求項１４，１５記載の発明の光ピックアップによれば、これらの請求項７ないし１３記載の発明の光束径を、対物レンズと一体に可動する開口部或いは波長選択性開口切換手段によって規定することができる。
【０１０９】
請求項１６記載の発明の光ピックアップによれば、開口部のみ（開口切換手段なし）で２世代、開口部と１段階の切換のみで（２段階切換手段でなく）３世代各々について最適ＮＡを得ることができる。
【０１１０】
請求項１７記載の発明の光ピックアップによれば、開口切換用の専用素子を設けずに済み、アクチュエータの軽量化、組付工数の削減が可能となる。
【０１１１】
請求項１８記載の発明の光情報処理装置によれば、請求項１ないし１７の発明と同様の作用・効果を奏することができる。
【図面の簡単な説明】
【図１】青色系、ＤＶＤ系、ＣＤ系各々の光記録媒体での屈折率とアッベ数とに応じた波面収差の分布を示す特性図である。
【図２】アッベ数と屈折率との関係で硝種条件を示す特性図である。
【図３】屈折率と第１面曲率半径との関係を示す特性図である。
【図４】対物レンズの第１の実施の形態を示す説明図である。
【図５】その波面収差特性図である。
【図６】対物レンズの第２の実施の形態を示す説明図である。
【図７】その波面収差特性図である。
【図８】対物レンズの第３の実施の形態を示す説明図である。
【図９】その波面収差特性図である。
【図１０】光ピックアップの実施の形態を示す光学系構成図である。
【図１１】波長とＮＡ０．６５にするための有効径との関係を示す特性図である。
【図１２】光ピックアップの第１の実施の形態における開口切換えを示す構成図である。
【図１３】光ピックアップの第２の実施の形態における開口切換えを示す構成図である。
【図１４】青色波長と赤色波長とのＮＡ相関を示す特性図である。
【図１５】光ピックアップの第３の実施の形態における開口切換えを示す概略構成図である。
【図１６】対物レンズの入射光束径−対物レンズ間隔の特性図である。
【図１７】開口位置に関する算術式を説明するための原理図である。
【図１８】対物レンズの波長６６０ｎｍでの物体距離と波面収差との関係を示す特性図である。
【図１９】対物レンズに対するコーティング例を示す概略構成図である。
【図２０】光情報処理装置の実施の形態を示す概略斜視図である。
【図２１】一般に想定される青色系／ＤＶＤ／ＣＤ互換光ピックアップのブロック構成図である。
【図２２】対物レンズの従来例を示す構成図である。
【図２３】従来の光源波長−波面収差特性図である。
【符号の説明】
１ 光記録媒体
２ 対物レンズ
１１ 光ピックアップ
１２〜１４ 光源
２１ 開口部
２２ 開口切換手段
４１〜４３ 波長選択性回折素子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens, an optical pickup, and an optical information processing apparatus.
[0002]
[Prior art]
As means for storing video information, audio information or data on a computer, optical recording media such as a CD having a recording capacity of 0.65 GB and a DVD having a recording capacity of 4.7 GB are becoming widespread. In recent years, there has been an increasing demand for further improvement in recording density and increase in capacity.
[0003]
As means for increasing the recording density of such an optical recording medium, in an optical pickup that writes or calls information on the optical recording medium, the numerical aperture (NA) of the objective lens is increased, or the wavelength of the light source is shortened. By doing so, it is effective to reduce the diameter of the beam spot condensed by the objective lens and formed on the optical recording medium.
[0004]
Therefore, for example, in the “CD optical recording medium”, the numerical aperture of the objective lens is 0.45 to 0.50 and the wavelength of the light source is approximately 785 nm, whereas from the “CD optical recording medium”. In the “DVD optical recording medium” in which the recording density has been increased, the numerical aperture of the objective lens is 0.60 to 0.65, and the wavelength of the light source is approximately 660 nm. As described above, the optical recording medium is desired to further improve the recording density and increase the capacity. For this purpose, the numerical aperture of the objective lens is further larger than 0.65, or the wavelength of the light source. Is desired to be shorter than 660 nm.
[0005]
Meanwhile, while new standards with such high NA or short wavelength have been proposed in recent years, CDs and DVDs, which are conventional optical recording media, exist at the user's hand. It is desirable that both these optical recording media and recently proposed new standard optical recording media can be handled by the same optical information processing apparatus. As the simplest method for realizing this, there is a method in which both the conventional optical pickup and the new standard optical pickup are mounted. However, with this method, it is difficult to achieve downsizing and cost reduction.
[0006]
Therefore, in order to achieve miniaturization and cost reduction, as an optical pickup that can be compatible with a blue (large capacity) optical recording medium using a light source in the blue wavelength band and an existing DVD or CD, FIG. As shown in FIG. 5, the light sources having different wavelengths of the blue light source 100, the DVD light source 101, and the CD light source 102, and the light emitted from these light sources 100, 101, and 102 converge on a predetermined optical recording medium 103. It can be said that a configuration including a single objective lens 104 for irradiation is desirable.
[0007]
By the way, in order to condense on the optical recording medium 103 of different standards such as blue, DVD, or CD with one objective lens 104 as described above, the difference in wavelength used / substrate thickness of the optical recording medium 103 is as follows. There are challenges associated with. That is, Patent Document 1 shows the amount of aberration generated when the objective lens 110 shown in FIG. 22 designed with a light source wavelength of 405 nm is used in the wavelength range of 400 to 800 nm (FIG. 23). Reference numeral 111 denotes an optical recording medium. As will be described later, the wavefront aberration generally needs to be about 0.02 λrms or less, but according to FIG. 23, it becomes 0.10 λrms or more near the wavelengths 660 nm and 785 nm used in the DVD and CD systems. Yes. Therefore, Patent Document 1 proposes to use a bonded lens as the objective lens. However, the bonded lens requires a bonding process and has a problem such as an increase in weight due to the overlap of two lenses.
[0008]
Other conventional examples include Non-Patent Document 1 and Patent Document 2. In these proposed examples, spherical aberration occurs when a beam is focused on a DVD-type optical recording medium or a CD-type optical recording medium with an objective lens exclusively for blue. Therefore, the optical paths of the DVD system and the CD system are configured as a finite system, and the phase distribution is not changed for the light of the blue wavelength band, and the phase distribution is changed for the light of the other DVD and CD. A correction element is provided. Setting the incident light beam to the finite system, that is, the divergent state, has an effect of reducing spherical aberration. Nevertheless, since the spherical aberration remains, the spherical aberration is corrected by adding a phase opposite to the spherical aberration by the phase correction element.
[0009]
Further, as another problem, the numerical aperture NA used is different in a plurality of types of optical recording media having different standards. That is, in the DVD optical recording medium, the reproduction system is 0.60, the recording system is 0.65, the CD reproduction system is 0.45, and the recording system is 0.50. . Currently, optical information processing apparatuses that are widely used are capable of recording both DVDs and CDs, and it is desirable to use NA: 0.65 for DVD systems and NA: 0.50 for CD systems. . On the other hand, for blue optical recording media, an HD-DVD standard of NA 0.65 has been proposed. Thus, it is necessary to switch the NA according to the optical recording medium on which recording / reproduction is performed. Therefore, the aperture switching element described in Patent Documents 3 to 6 when the DVD and CD are compatible with the second generation, and the aperture described in Patent Document 7 when the large-capacity optical recording medium and the DVD are compatible with the second generation. In order to achieve the switching element, and further, three-generation compatibility, a three-stage aperture switching element as described in Patent Documents 8 and 9 is required.
[0010]
[Patent Document 1]
JP 2002-107617 A
[Patent Document 2]
JP 2003-67972 A
[Patent Document 3]
Japanese Patent No. 3240846
[Patent Document 4]
Japanese Patent No. 2713257
[Patent Document 5]
Japanese Patent No. 2725653
[Patent Document 6]
Japanese Patent No. 3036314
[Patent Document 7]
JP 2001-216676 A
[Patent Document 8]
JP 2000-187870 A
[Patent Document 9]
JP 2003-67972 A
[Non-Patent Document 1]
ISO 2001 Proceedings "BLUE / DVD / CD COMPATIBLE OPTIMAL HEAD WITH THREE WAVELENGTHS AND A WAVELENGTH SELECTIVE FILTER" Ryuichi Katayama and Yuichi Koi 31
[0011]
[Problems to be solved by the invention]
With regard to such a conventional technique, first, the phase correction element according to Non-Patent Document 1 and Patent Document 2 is moved integrally with the objective lens because coma aberration occurs when the optical axis is shifted from the objective lens. On the other hand, as an actuator that moves the objective lens in the focus direction and the track direction, it is desired to reduce the number of parts in order to reduce the weight and reduce the number of assembling steps. Further, in order to suppress the coma aberration, it is necessary to adjust the alignment between the objective lens and the phase correction element.
[0012]
The same applies to the aperture switching element. If the optical axis of the aperture switching element and the objective lens are deviated, a desired NA cannot be obtained.
[0013]
An object of the present invention is to provide an objective lens and an optical pickup in which spherical aberration is sufficiently suppressed in the second generation of blue / DVD or the third generation of blue / DVD / CD without the need for an aberration correction element. Is to provide.
[0014]
Another object of the present invention is to provide an objective lens and an optical pickup that can reduce cost and reduce assembly man-hours by reducing the number of components without requiring an aberration correction element and an aperture switching element.
[0015]
[Means for Solving the Problems]
The invention described in claim 1 is an objective lens that converges and irradiates light from a light source with wavelengths λ1 and λ2 (λ1 <λ2) onto an optical recording medium, and is configured as a single lens, and both surfaces are aspherical and convex. Yes, with respect to the d-line material: refractive index: nd and Abbe number: νd, paraxial radius of curvature of light source side surface: R1, focal length: f, conditions:
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Satisfied.
[0016]
According to a fourth aspect of the present invention, there is provided an objective lens that converges and irradiates light from a light source having wavelengths λ1, λ2, and λ3 (λ1 <λ2 <λ3) onto an optical recording medium, and is configured as a single lens. Is an aspherical surface and a convex surface, the refractive index for the d-line material: nd and Abbe number: νd, the paraxial radius of curvature of the light source side surface: R1, and the focal length: f, the conditions are:
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Satisfied.
[0017]
That is, the objective lens is a positive lens that condenses the parallel light flux incident from the light source side, and the objective lens for the optical pickup of the present invention has a single lens configuration and both surfaces are aspherical. Can be a biconvex lens or a meniscus lens. However, since the biconvex lens can relax the curvature on the light source side, a biconvex lens is preferable from the viewpoint of manufacturing feasibility. Conditions (1), (2), and (3) are conditions for convergent irradiation on the optical recording medium in the best wavefront aberration state for the optical recording medium with two or three wavelengths. For example, when the condition (2) is not satisfied, the refractive index of the objective lens is too small, and when realizing a desired NA, the curvature of the lens surface on the light source side must be increased, and the objective lens surface is highly accurate. Therefore, it is difficult to form the lens, and the cost as an objective lens is increased. Further, if the conditions (1) and (2) for the refractive index: nd and the Abbe number: νd are not satisfied, the chromatic aberration when passing light having different wavelengths becomes too large, and it is difficult to make two or three wavelengths compatible. It becomes. If the surface on the light source side of the objective lens is “convex to the light source side”, increasing the paraxial radius of curvature R1 means decreasing the positive refractive power on this surface. Since the objective lens for optical pickup has a large numerical aperture NA of about 0.65, the positive refractive power of the lens must be increased. Therefore, if the paraxial radius of curvature: R1 is increased and the numerical aperture is increased as described above, the refractive index of the lens material must be increased, and “the refractive index of the lens material: nd is the paraxial The radius of curvature is increased as R1 increases.
[0018]
According to a second aspect of the present invention, in the objective lens according to the first aspect, the conditions (1), (2), and (3) are based on an infinite system incident light beam with respect to light from the light source having the wavelength λ1. With respect to light from the light source having the wavelength λ2, it is defined for incident light beams of a finite system. According to a third aspect of the present invention, in the objective lens according to the first or second aspect, the conditions (1), (2), and (3) are such that an incident light flux in a blue wavelength band as the wavelength λ1 and a red wavelength band as the wavelength λ2 It is prescribed for. According to a fifth aspect of the present invention, in the objective lens according to the third aspect, the conditions (1), (2), and (3) are based on an infinite system incident light beam with respect to light from the light source having the wavelength λ1. With respect to the light from the light sources having the wavelengths λ 2 and λ 3, it is defined with respect to a finite incident light beam. According to a sixth aspect of the present invention, in the objective lens according to the fourth or fifth aspect, the conditions (1), (2), and (3) include a blue wavelength band as the wavelength λ1, a red wavelength band as the wavelength λ2, and the wavelength. λ3 is defined for the incident light flux in the infrared wavelength band.
[0019]
That is, the objective lens of these inventions is used with a finite system incident light beam for long-wavelength light. As an infinite objective lens in the blue wavelength band, when used under the wavelength and substrate thickness conditions of DVD and CD, differences in substrate thickness (0.6 mm, 0.6 mm, 1.2 mm), differences in wavelength (405 nm, (Spherical aberration associated with 660 nm and 785 nm) occurs. In order to suppress this spherical aberration, it is possible to correct the incident light beam to the objective lens as divergent light when recording, reproducing or erasing data on a DVD or CD. That is, when recording, reproducing or erasing on a DVD or CD, it is used as a finite objective lens.
[0020]
Consider these inventions. First, when forming a light spot with a "desired spot diameter" on the recording surface via a light source of a predetermined wavelength and a substrate of a predetermined thickness, "the upper limit value of the wavefront aberration allowed to form a good light spot" ”Must be suppressed to 0.04λrms (λ: wavelength) or less. The 0.04λrms includes a wavefront deterioration due to a manufacturing error such as a curvature radius shift, a thickness shift, an aspherical shape shift, a shift of each surface, and a tilt of each surface of the first and second surfaces of the lens. . Therefore, the standard value of the median value of the wavefront deterioration amount is about 0.02λrms. Hereinafter, explanation will be made using a median standard of 0.02λrms or less. In the present invention, recording, reproduction, or erasing is performed in the three generations of blue, DVD, and CD as described above. Therefore, it is necessary to realize a wavefront aberration of 0.02λrms or less at each wavelength. The lens material is represented by a refractive index: nd and an Abbe number: νd, “an infinite system with respect to an optical recording medium having a substrate thickness of 0.6 mm, a numerical aperture NA of 0.65 to 0.70, and a working wavelength of 405 nm. Incident blue optical system ”,“ Substrate thickness: 0.6 mm, numerical aperture NA: 0.65, DVD optical system of finite incidence with respect to an optical recording medium having a working wavelength of 660 nm ”,“ Substrate thickness: 1.2 mm, When the wavefront aberration of the CD optical system having a finite incidence on an optical recording medium having a numerical aperture NA of 0.50 and a working wavelength of 785 nm is obtained, the wavefront aberration is as shown in FIGS. Both DVD, CD and 0.02λ rms are within the range of conditions (1) and (2) indicated by hatching in FIG. In FIG. 2, the range indicated by the broken line generally corresponds to a region where glass is mass-produced. If these conditions (1) and (2) are not satisfied, it can be seen from FIG. 1 (b) that the chromatic aberration due to the difference in wavelength band of the DVD system becomes too large and the aberration of 0.02λrms or less cannot be satisfied.
[0021]
At the same time, under the condition that “wavefront aberration: 0.02λrms or less” can be achieved, the relationship in which the paraxial radius of curvature: R1 and the refractive index: nd are satisfied, “the surface with a strong curvature is directed toward the light source side. The focal length of the objective lens for an optical pickup formed as a “biconvex lens”: f = 3.05 mm, and the numerical aperture: NA 0.65. +). A mark (● ■ Δ × * ○ +) indicates an Abbe number: νd, which is 65, 60, 55, 50, 45, 40, 35 in order, and falls within the range of the straight lines a and b. Since the refractive index of the material depends not only on the refractive index nd of the d-line but also on the Abbe number νd, the relationship between the paraxial radius of curvature: R1 and the refractive index: nd is not uniquely determined, but the range of the straight lines a and b By satisfying the condition (3) of the paraxial radius of curvature: R1 and the refractive index: nd determined by: and satisfying the refractive index: nd determined by the condition (1), “wavefront aberration: 0.02λrms or less” Can be achieved.
[0022]
According to the objective lens of the invention described in claims 1 to 6 as described above, two wavelengths or three of blue, DVD, and CD can be used without adding a conventional pasted lens or adding a phase correction element. Wavelength compatibility is possible.
[0023]
Next, in configuring the optical pickup, the optimum NA is different for each of the blue light recording medium, the DVD optical recording medium, and the CD optical recording medium. Therefore, it is necessary to switch the incident light beam diameter as appropriate.
[0024]
First, an optical pickup according to a seventh aspect of the present invention includes a plurality of light sources that emit light of wavelengths λ1 and λ2 and a single light source that converges and irradiates light from these light sources onto an optical recording medium. 3 and an effective numerical aperture NA (λ1), NA (λ2) of each wavelength λ1, λ2, and each wavelength incident on the objective lens. The incident beam diameters φ1 and φ2 of λ1 and λ2 are as follows:
(4) NA (λ1) = NA (λ2)
(5) φ2> φ1
Satisfied.
[0025]
In general, the effective numerical aperture NA is such that the diameter of the light beam incident on the objective lens is φ and the focal length is f.
NA = φ / 2 / f
It is represented by Focal length: f becomes longer as the wavelength is longer. Therefore, if the conditions (4) and (5) are satisfied, the compatibility of the NA 0.65 blue optical recording medium HD-DVD and the NA 0.65 DVD optical recording medium, which are being standardized in recent years, becomes possible. .
[0026]
An optical pickup according to a ninth aspect of the invention is a single optical pickup according to any one of the first, second, and third aspects, wherein a plurality of light sources that emit light having wavelengths λ1 and λ2 and light from these light sources converges on the optical recording medium. Objective lenses, and the effective numerical apertures NA (λ1) and NA (λ2) of the wavelengths λ1 and λ2 and the wavelengths λ1 and λ1 incident on the objective lens. The incident light beam diameters φ1 and φ2 of λ2 are:
(6) NA (λ1)> NA (λ2)
(7) φ1 = φ2
Satisfied.
[0027]
For example, if the blue numerical aperture NA is slightly increased, such as NA 0.67, the incident light beam diameters φ1 and φ2 can be made common, and aperture switching is not required. In general, wavefront deterioration due to the thickness error of the transparent substrate of the optical recording medium is proportional to the fourth power of NA, and wavefront deterioration due to the tilt error of the transparent substrate is proportional to the third power of NA. Can not. However, if the HD-DVD has a level where NA 0.65 is used at about NA 0.67, the deterioration of these margins is very small and can be used without any problem.
[0028]
An optical pickup according to an eleventh aspect of the invention includes a plurality of light sources that emit light of wavelengths λ1, λ2, and λ3 and a single light source that converges and irradiates light from these light sources onto an optical recording medium. 6 and an effective numerical aperture NA (λ1), NA (λ2), NA (λ3) of each of the wavelengths λ1, λ2, and λ3 for the light of the wavelengths λ1, λ2, and λ3, And incident beam diameters φ1, φ2, and φ3 of the wavelengths λ1, λ2, and λ3 incident on the objective lens are as follows:
(8) NA (λ1) = NA (λ2)> NA (λ3)
(9) φ2> φ1> φ3
Satisfied.
[0029]
This makes it possible to obtain an optimum NA for each of the three generations of blue, DVD, and CD.
[0030]
An optical pickup according to a twelfth aspect of the present invention includes a plurality of light sources that emit light of wavelengths λ1, λ2, and λ3 and a single light source that converges and irradiates light from these light sources onto an optical recording medium. 6 and an effective numerical aperture NA (λ1), NA (λ2), NA (λ3) of each of the wavelengths λ1, λ2, and λ3 for the light of the wavelengths λ1, λ2, and λ3, And incident beam diameters φ1, φ2, and φ3 of the wavelengths λ1, λ2, and λ3 incident on the objective lens are as follows:
(10) NA (λ1)> NA (λ2)> NA (λ3)
(11) φ1> φ2> φ3
Satisfied.
[0031]
This makes it possible to obtain an optimum NA for each of the three generations of blue, DVD, and CD.
[0032]
An optical pickup according to a thirteenth aspect of the present invention includes a plurality of light sources that emit light having wavelengths λ1, λ2, and λ3, and a single light source that converges and irradiates light from these light sources onto an optical recording medium. 6 and an effective numerical aperture NA (λ1), NA (λ2), NA (λ3) of each of the wavelengths λ1, λ2, and λ3 for the light of the wavelengths λ1, λ2, and λ3, And incident beam diameters φ1, φ2, and φ3 of the wavelengths λ1, λ2, and λ3 incident on the objective lens are as follows:
(12) NA (λ1)> NA (λ2)> NA (λ3)
(13) φ1 = φ2> φ3
Satisfied.
[0033]
This makes it possible to obtain an optimum NA for each of the three generations of blue, DVD, and CD.
[0034]
The beam diameters of the inventions according to the seventh to fifteenth aspects of the present invention are defined by an aperture or wavelength-selective aperture switching means that can move integrally with the objective lens. That is, the invention according to claim 14 is the optical pickup according to any one of claims 7 to 13, further comprising an opening having a diameter substantially equal to the maximum incident light beam diameter, and the opening is integrated with the objective lens. Move. The invention according to claim 15 is the optical pickup according to claim 7, 10, 11 or 12, further comprising an aperture having substantially the same diameter as the maximum incident light beam diameter and wavelength-selective aperture switching means. The opening and the opening switching means move integrally with the objective lens.
[0035]
According to a sixteenth aspect of the present invention, in the optical pickup according to the eighth or twelfth aspect, an infinite system incident light beam is used when the light source having the wavelength λ1 is turned on, and a finite system incident light beam is used when the light source having the wavelength λ2 is turned on. An optical pickup having an opening having substantially the same diameter as the incident light beam diameter φ1 (= φ2) from the front principal point of the objective lens to the light source side at a predetermined distance: t,
t≈L-NA1 · f / tan (asin (NA2obj))
Where f: focal length of the objective lens
NA1: Numerical aperture on the image plane side when the light source with wavelength λ1 is turned on
NA2obj: the numerical aperture on the light source side when the light source of wavelength λ2 is turned on
L: Object distance when the light source of wavelength λ2 is turned on
It is arranged at the position.
[0036]
Therefore, it is possible to obtain the optimum NA for each of the three generations only with the opening (no opening switching means) for two generations and only with the opening and the one-stage switching (not with the two-stage switching means).
[0037]
According to a seventeenth aspect of the present invention, in the optical pickup according to the fifteenth aspect, the aperture switching means is a wavelength-selective coating or a wavelength-selective diffraction grating, and is provided on either the light source side or the medium side of the objective lens. Is formed.
[0038]
Therefore, it is not necessary to provide a dedicated element for switching the opening, and the actuator can be reduced in weight and the number of assembling steps can be reduced.
[0039]
An optical information processing apparatus according to an eighteenth aspect of the present invention is an optical recording apparatus using the optical pickup including the objective lens according to any one of the first to sixth aspects or the optical pickup according to any one of the seventh to seventeenth aspects. Records, reproduces or erases information on the medium.
[0040]
Accordingly, the same effect as in the first to seventeenth aspects of the invention can be achieved.
[0041]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an objective lens of the present invention, an optical pickup using the objective lens, and an optical information processing apparatus including the optical pickup will be described below. In the following description, the case of blue generation / DVD / CD three generation compatibility is described, but two generations of three generations such as blue / DVD or blue / CD may be selected.
[0042]
[Embodiment of objective lens]
<First Embodiment of Objective Lens>
The present embodiment will be described with reference to FIGS. In this embodiment, a blue-based optical recording medium having a used wavelength λ1: 405 nm, a substrate thickness: 0.6 mm, and a numerical aperture NA: 0.65, a used wavelength λ2: 660 nm, a substrate thickness: 0.6 mm, and a numerical aperture NA Recording, reproduction, or recording on three types of optical recording media: a 0.65 DVD optical recording medium and a CD optical recording medium having a use wavelength λ3: 785 nm, a substrate thickness: 1.2 mm, and a numerical aperture NA: 0.50 The present invention relates to an objective lens used in an optical pickup that performs erasing.
[0043]
First, with reference to FIG. 4A and Table 1A, the optical performance of the objective lens 2A when used for a blue-based optical recording medium 1a having a working wavelength λ1: 405 nm is shown. The objective lens 2A of the present embodiment uses a glass type (500000.600,000) having a numerical aperture NA: 0.65, a focal length f: 3.05 mm, a d-line refractive index nd: 1.50, and an Abbe number νd: 60. Used.
[0044]
Further, the objective lens 2A of the present embodiment has an aspherical shape of the lens surface thereof: optical axis direction coordinate: X, optical axis orthogonal direction coordinate: Y, paraxial radius of curvature: R, conic constant: K, Using the higher order coefficients: A, B, C, D, E, F,...
X = (Y²/ R) / [1 + √ {1- (1 + K) Y / R²} + AY⁴+ BY⁶+ CY⁸+ DY¹⁰+ EY¹²+ FY¹⁴+ GY¹⁶+ HY¹⁸+ JY²⁰+ ...
The shape is specified by giving R, K, A, B, C, D,.
[0045]
Table 1 (a) shows specific data. The symbols in the table are as follows. “OBJ” means an object point (semiconductor laser as a light source), but the objective lens 2A is an “infinite system”, and “INFINITY (infinity)” with a radius of curvature: RDY and a thickness: THI has an infinite light source. It means being in the distance. “STO” is the entrance pupil plane, the radius of curvature is “INFINITY”, and the thickness is “0” by design. Here, unless otherwise specified, the unit of the quantity having the dimension of length is “mm”.
[0046]
“S2” means “light source side surface” of the objective lens 2A, and “S3” means “optical recording medium side surface”. The thickness of the objective lens 2A is 1.85 mm, and the thickness described in “right side of curvature radius” in the column of S3: 1.651193 mm indicates “working distance”. “S4” is the light source side surface of the light irradiation side substrate of the optical recording medium 1, and “IMG” is the surface matching the recording surface. The distance between these surfaces S4 and IMG, ie, the light irradiation side substrate thickness is 0. 6 mm, n: 1.62.
[0047]
“EPD: entrance pupil diameter” represents an incident light beam diameter (3.965 mm), and “WL: wavelength” represents a use wavelength (405 nm). In the display of the aspheric coefficient, for example, “D: 0.306790E-05” means that “D = 0.306790 × 10^{− 5}"Means.
[0048]
Moreover, the objective lens 2A of the present embodiment has the following conditions:
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Is satisfied.
[0049]
Next, a case where the present invention is applied to a DVD optical recording medium 1b having a used wavelength of 660 nm will be described with reference to FIG. 4B and Table 1B. The shape of the objective lens 2A is the same as that in FIG. 1A and Table 1A, but the refractive index and the working distance are different. Further, the DVD optical recording medium 1b is used with finite incidence, and the distance from the object point “OBJ” (semiconductor laser as a light source) to a single opening on the first surface “STO”. It is set to 137 mm. This is a value selected to minimize wavefront aberration.
[0050]
Further, the case of applying to the CD optical recording medium 1c having a working wavelength of 785 nm will be described with reference to FIG. 4C and Table 1C. The shape of the objective lens 2A is the same as in FIGS. 1A and 1A, but the refractive index, working distance, and thickness of the light irradiation side substrate are different. Further, the CD-type optical recording medium 1c is used with finite incidence, and the distance from the object point “OBJ” (semiconductor laser as a light source) to a single opening on the first surface “STO”. 45.0 mm. Similar to the DVD system, the value is selected so that the wavefront aberration is minimized.
[0051]
[Table 1]

[0052]
FIG. 5 occurs when each of the blue optical recording medium 1a, the DVD optical recording medium 1b, and the CD optical recording medium 1c is used with a predetermined wavelength using the objective lens 2A of the present embodiment. It is a characteristic view which shows the wavefront aberration characteristic to do. The horizontal axis represents each optical recording medium 1a, 1b, 1c, and the vertical axis represents the on-axis wavefront aberration at the best image point. Each of the optical recording media 1a, 1b, and 1c has a favorable aberration characteristic with a wavefront aberration of 0.02λrms or less.
[0053]
<Second Embodiment of Objective Lens>
The present embodiment will be described with reference to FIGS. In this embodiment, a blue-based optical recording medium 1a having a used wavelength λ1: 405 nm, a substrate thickness: 0.6 mm, and a numerical aperture NA: 0.70, a used wavelength λ2: 660 nm, a substrate thickness: 0.6 mm, and a numerical aperture. Recording is performed on three types of optical recording media: a DVD optical recording medium 1b with NA: 0.65, and a CD optical recording medium 1c with a used wavelength λ3: 785 nm, a substrate thickness: 1.2 mm, and a numerical aperture NA: 0.50. The present invention relates to an objective lens 2B used for an optical pickup for reproducing or erasing.
[0054]
The objective lens 2B of the present embodiment uses a glass type (550000.600,000) having a focal length f: 3.05 mm, a d-line refractive index nd: 1.55, and an Abbe number νd: 60. The objective lens 2B of the present embodiment is used in an infinite system for the blue optical recording medium 1a, as in the first embodiment, and is used for the DVD optical recording medium 1b and the CD optical recording. The medium 1c is a lens used in a finite system.
[0055]
FIG. 6 (a) and Table 2 (a) show the usage configuration in the blue optical recording medium 1a, and FIG. 6 (b) and Table 2 (b) show the usage configuration in the DVD optical recording medium 1b. The configuration used in the optical recording medium 1c is shown in FIG. 6C and Table 2C. The aspherical shape of the objective lens 2B and the items described in Table 2 are the same as those in the first embodiment.
[0056]
The objective lens 2B of the present embodiment also has the following conditions
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Is satisfied.
[0057]
[Table 2]

[0058]
FIG. 7 occurs when each of the blue optical recording medium 1a, the DVD optical recording medium 1b, and the CD optical recording medium 1c is used with a predetermined wavelength using the objective lens 2B of the present embodiment. It is a characteristic view which shows the wavefront aberration characteristic to do. The horizontal axis represents each optical recording medium 1a, 1b, 1c, and the vertical axis represents the on-axis wavefront aberration at the best image point. Each of the optical recording media 1a, 1b, and 1c has a favorable aberration characteristic with a wavefront aberration of 0.02λrms or less.
[0059]
<Third Embodiment of Objective Lens>
The present embodiment will be described with reference to FIGS. In this embodiment, a blue-based optical recording medium 1a having a used wavelength λ1: 405 nm, a substrate thickness: 0.6 mm, and a numerical aperture NA: 0.67, a used wavelength λ2: 660 nm, a substrate thickness: 0.6 mm, and a numerical aperture Recording is performed on three types of optical recording media: a DVD optical recording medium 1b with NA: 0.65, and a CD optical recording medium 1c with a used wavelength λ3: 785 nm, a substrate thickness: 1.2 mm, and a numerical aperture NA: 0.50. The present invention relates to an objective lens 2C used for an optical pickup for reproducing or erasing.
[0060]
The objective lens 2C of the present embodiment uses a glass type (550000.600,000) having a focal length f: 3.05 mm, a d-line refractive index nd: 1.55, and an Abbe number νd: 55. The objective lens 2C of the present embodiment is used in an infinite system for the blue optical recording medium 1a, as in the case of the first embodiment, and the DVD optical recording medium 1b and CD optical recording. The medium 1c is a lens used in a finite system.
[0061]
FIG. 8 (a) and FIG. 3 (a) show the usage configuration in the blue optical recording medium 1a, and FIG. 8 (b) and Table 3 (b) show the usage configuration in the DVD optical recording medium 1b. The configuration used in the optical recording medium 1c is shown in FIG. 8C and Table 3C. The aspherical shape of the objective lens 2C and the items described in Table 3 are the same as in the case of the first embodiment.
[0062]
The objective lens 2C of the present embodiment also has the following conditions:
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Is satisfied.
[0063]
[Table 3]

[0064]
FIG. 9 occurs when each of the blue optical recording medium 1a, the DVD optical recording medium 1b, and the CD optical recording medium 1c is used with a predetermined wavelength using the objective lens 2C of the present embodiment. It is a characteristic view which shows the wavefront aberration characteristic to do. The horizontal axis represents each optical recording medium 1a, 1b, 1c, and the vertical axis represents the on-axis wavefront aberration at the best image point. Each of the optical recording media 1a, 1b, and 1c has a favorable aberration characteristic with a wavefront aberration of 0.02λrms or less.
[0065]
[Embodiment of optical pickup]
<First Embodiment of Optical Pickup>
The optical pickup of this embodiment shows an application example to NA 0.65 blue system / DVD / CD.
[0066]
First, a configuration example of the entire optical system of the optical pickup 11 of the present embodiment will be described with reference to FIG. The optical pickup 11 of the present embodiment includes a light source 12 having a used wavelength λ1 = 405 nm in a blue wavelength band, a light source 13 having a used wavelength λ2 = 660 nm in a red wavelength band, and a light source having a used wavelength λ3 = 785 nm in an infrared wavelength band. 14, a blue optical recording medium 1a with NA 0.65 and light irradiation side substrate thickness 0.6 mm, a DVD optical recording medium 1b with NA 0.65 and light irradiation side substrate thickness 0.6 mm, NA 0.50, This is an optical pickup capable of recording, reproducing or erasing with respect to each of the CD-type optical recording medium 1c having a light irradiation side substrate thickness of 1.2 mm.
[0067]
The optical pickup 11 of this embodiment includes a semiconductor laser (light source) 12 in a blue wavelength band, a collimator lens 15, a polarization beam splitter 16, dichroic prisms 17 and 18, a deflection prism 19, a quarter wavelength plate 20, and an opening 21. , Aperture switching means 22, objective lens 2 (2A in the present embodiment), detection lens 23, light beam splitting means 24, blue optical system 26 through which light in the blue wavelength band passes, and hologram A DVD through which light in the red wavelength band, which is composed of the unit 27, the coupling lens 28, the dichroic prisms 17 and 18, the deflection prism 19, the quarter wavelength plate 20, the aperture 21, the aperture switching means 22, and the objective lens 2A passes. Red optical system 29, hologram unit 30, coupling lens 31, dichroic prism 18, a deflection prism 19, a quarter-wave plate 20, an aperture 21, aperture switching means 22, and an infrared optical system 32 of a CD system through which light in the infrared wavelength band, which is composed of the objective lens 2 </ b> A, passes. Yes. That is, the dichroic prisms 17 and 18, the deflecting prism 19, the quarter wavelength plate 20, the aperture 21, the aperture switching means 22, and the objective lens 2A are common components of two to three optical systems.
[0068]
The hologram unit 27 is configured by integrating a chip of a semiconductor laser (light source) 13, a hologram 33 and a light receiving element 34. Similarly, the hologram unit 30 is configured by integrating the chip of the semiconductor laser (light source) 14, the hologram 35 and the light receiving element 36.
[0069]
Further, as described above, the optical recording media 1a, 1b, and 1c are optical recording media having different use wavelengths, the optical recording medium 1a is a blue optical recording medium having a substrate thickness of 0.6 mm, and the optical recording medium 1b is a substrate. The DVD optical recording medium having a thickness of 0.6 mm and the optical recording medium 1c are CD optical recording media having a substrate thickness of 1.2 mm. At the time of recording, reproducing or erasing, only one of the optical recording media 1a, 1b or 1c is set on a rotating mechanism (not shown) and rotated at high speed.
[0070]
The opening 21 can be regulated on a bobbin that holds the objective lens 2 on an actuator that moves the objective lens 2 in the focus direction and the track direction, and it is not necessary to use a specific optical component. .
[0071]
In such a configuration, an operation example of the optical system for each wavelength band will be described. First, a description will be given of a case where recording, reproduction, or erasing is performed on the blue-type optical recording medium 1a having the use wavelength λ1 = 405 nm of the blue wavelength band, NA 0.65, and light irradiation side substrate thickness 0.6 mm. The linearly polarized divergent light emitted from the semiconductor laser 12 having a wavelength of 405 nm is made substantially parallel light by the collimator lens 15, passes through the polarization beam splitter 16 and the dichroic prisms 17 and 18, and is deflected by 90 degrees by the deflecting prism 19. The circularly polarized light passes through the quarter-wave plate 20, passes through the aperture 21, is limited to NA 0.65 by the aperture switching means 22, enters the objective lens 2A, and forms a minute spot on the optical recording medium 1a. Focused. Information is reproduced, recorded, or erased by this spot.
[0072]
The light reflected by the optical recording medium 1a becomes circularly polarized light in the opposite direction to the outward path, becomes again substantially parallel light, passes through the quarter-wave plate 20 and becomes linearly polarized light orthogonal to the forward path, and the deflection prism 19, The light passes through the dichroic prisms 18 and 17, is reflected by the polarization beam splitter 16, is converged by the detection lens 23, is deflected and divided into a plurality of optical paths by the light beam dividing means 24, and reaches the light receiving element 25. An information signal and a servo signal are detected from the light receiving element 25.
[0073]
Next, a case where recording, reproduction, or erasing is performed on the light source 13 having a use wavelength of 660 nm in the red wavelength band, NA 0.65, and DVD-type optical recording medium 1b having a light irradiation side substrate thickness of 0.6 mm will be described. The light of 660 nm emitted from the chip of the semiconductor laser 13 of the hologram unit 27 is transmitted through the hologram 33 and is made a predetermined divergent light by the coupling lens 28, the light in the blue wavelength band is transmitted, and the light in the red wavelength band is transmitted. The light is reflected in the direction of the deflecting prism 19 by the dichroic prism 17 to be reflected, and after passing through the dichroic prism 18, the optical path is deflected by 90 degrees by the deflecting prism 19, passes through the quarter wavelength plate 20, and becomes circularly polarized light. It is limited to NA 0.65, enters the objective lens 1A, and is condensed as a minute spot on the optical recording medium 1A. Information is reproduced, recorded, or erased by this spot.
[0074]
The light reflected by the optical recording medium 1 A is deflected by the deflecting prism 19, passes through the dichroic prism 18, is reflected by the dichroic prism 17, is converged by the coupling lens lens 28, and is converged with the semiconductor laser 13 by the hologram 33. The light is diffracted in the direction of the light receiving element 34 inside and received by the light receiving element 34. An information signal and a servo signal are detected from the light receiving element 34.
[0075]
Further, a case will be described in which recording, reproduction, or erasure is performed on the light source 14 having an operating wavelength of 785 nm in the infrared wavelength band, NA 0.50, and CD-type optical recording medium 1c having a light irradiation side substrate thickness of 1.2 mm. The 785 nm light emitted from the semiconductor laser 14 of the hologram unit 30 is transmitted through the hologram 35 and converted into a predetermined divergent beam by the coupling lens 31, and the light in the blue and red wavelength regions is transmitted and the infrared wavelength region is transmitted. Is reflected in the direction of the deflecting prism 19 by the reflecting dichroic prism 18, the optical path is deflected by 90 degrees by the deflecting prism 19, passes through the wave plate 20, becomes elliptically polarized light or circularly polarized light, passes through the opening 21, The aperture switching means 22 limits the NA to 0.50, enters the objective lens 2A, and is condensed as a minute spot on the optical recording medium 1c. Information is reproduced, recorded or erased by this spot.
[0076]
The light reflected by the optical recording medium 1 c is deflected by the deflecting prism 19, reflected by the dichroic prism 18, converged by the coupling lens 31, and directed to the light receiving element 36 within the same can as the semiconductor laser 14 by the hologram 35. And is received by the light receiving element 36. An information signal and a servo signal are detected from the light receiving element 36.
[0077]
Here, the aperture switching portion in the vicinity of the objective lens 2A will be described.
[0078]
First, the DVD incident light beam diameter will be described. When light in the red band having the same luminous flux diameter is incident on an objective lens having the best wavefront in the blue wavelength band, the refractive power decreases and the numerical aperture NA decreases. Therefore, in the present embodiment, the red wavelength band is made incident with a larger light beam diameter φ2 than the incident light beam diameter φ1 in the blue wavelength band.
[0079]
FIG. 11 shows the following characteristics in the blue wavelength band.
Incident beam diameter φ: 3.965 mm
Numerical aperture (NA): 0.65
Focal length (f): 3.05 mm
Glass type: 500000.600,000
2 shows the relationship between the effective diameter and wavelength, which is NA 0.65 when using different wavelength bands. From FIG. 11, it can be seen that the beam diameter φ2 needs to be about 4.095 mm when recording, reproducing or erasing the DVD optical recording medium 1b using the wavelength of 660 nm. For wavelengths other than 405 nm, the object distance is changed as appropriate to obtain the best wavefront aberration.
[0080]
Also, the CD incident light beam diameter will be described. The optimum numerical aperture NA at the time of recording, reproducing or erasing with respect to the CD optical recording medium 1c is about 0.5. However, when the optimum effective diameter φ3 is estimated by the same method as in FIG. 11, φ3 = It can be seen that it should be about 3.28 mm.
[0081]
As described above, three-stage aperture switching is required for the incident light beam diameters φ1, φ2, and φ3 with respect to the objective lens 2A. In the present embodiment, the incident light beam diameter φ2 is limited by the opening 21 provided in the actuator section, and the aperture switching means 22 for switching the incident light beam diameters φ1 and φ3 is in accordance with the wavelength of the light beam emitted from the light source. For example, as shown in FIGS. 12A to 12C, a wavelength-selective diffraction element 41 on which a wavelength-selective diffraction pattern for switching the beam diameter by diffraction is formed is used.
[0082]
The transmission characteristics of each light beam in the light beam in the blue wavelength band in FIG. 12A, the light beam in the red wavelength band in FIG. 12B, and the light beam in the infrared wavelength band in FIG. Within the incident light beam diameter φ3 in the central region where no light is applied, light in any of the blue wavelength band, red wavelength band, and infrared wavelength band is transmitted and incident from the outer periphery of the incident light beam diameter φ3 in the peripheral region In the region up to the light beam diameter φ1, it does not act on the light in the blue wavelength band and the red wavelength band, only the light in the infrared wavelength band is diffracted, and the incident light beam diameter from the outer periphery of the incident light beam diameter φ1 in the peripheral region. In the region up to φ2, it does not act on the light in the red wavelength band, and diffracts the light in the blue wavelength band and the infrared wavelength band.
[0083]
Note that the present invention is not limited to the wavelength selective diffraction element 41 as in the present embodiment, and may be an element provided with a coating having wavelength selective transmission / reflection characteristics.
[0084]
<Second Embodiment of Optical Pickup>
The optical pickup of this embodiment shows an application example to NA 0.70 blue / DVD / CD.
[0085]
The optical pickup of the present embodiment includes a light source 12 having a used wavelength λ1 = 405 nm in a blue wavelength band, a light source 13 having a used wavelength λ2 = 660 nm in a red wavelength band, and a light source 14 having a used wavelength λ3 = 785 nm in an infrared wavelength band. A blue optical recording medium 1a having an NA of 0.70 and a light irradiation side substrate thickness of 0.6 mm, a DVD optical recording medium 1b having an NA of 0.65 and a light irradiation side substrate thickness of 0.6 mm, an NA of 0.50, and light. This is an optical pickup capable of recording, reproducing or erasing with respect to each of the CD-type optical recording medium 1c having a substrate thickness of 1.2 mm on the irradiation side.
[0086]
The difference from the first embodiment described above is that the blue numerical aperture is not NA 0.65 but NA 0.70, and the required incident beam diameter is larger in blue than in DVD. Therefore, the overall configuration is the same as that shown in FIG. 10, but 1B is used instead of 1A as the objective lens in the present embodiment. If the NA is about 0.70, the deterioration of the margin due to the above-described various changes is small, and the capacity can be increased. When NA 0.70 is used, the capacity can be increased by about 15% with respect to NA 0.65.
[0087]
In this case, the incident light beam diameter φ1 is limited by the opening 21 provided in the actuator portion, and the aperture switching means 22 for the incident light beam diameters φ2 and φ3 is shown in FIG. As shown in a) to (c), a wavelength-selective diffraction element 42 on which a wavelength-selective diffraction pattern for switching the beam diameter by diffraction is formed is used.
[0088]
The transmission characteristics of each light beam in the light beam in the blue wavelength band in FIG. 13A, the light beam in the red wavelength band in FIG. 13B, and the light beam in the infrared wavelength band in FIG. Is transmitted through any of the blue wavelength band, the red wavelength band, and the infrared wavelength band at the incident light beam diameter φ3 in the central region where no light is applied, and the incident light beam from the outer periphery of the incident light beam diameter φ3 in the peripheral region. In the region up to the diameter φ2, it does not act on the light in the blue wavelength band and the red wavelength band, only the light in the infrared wavelength band is diffracted, and the incident light beam diameter φ1 from the outer periphery of the incident light beam diameter φ2 in the peripheral region. In the region up to this point, it does not act on the light in the blue wavelength band, and diffracts the light in the red wavelength band and the infrared wavelength band.
[0089]
<Third Embodiment of Optical Pickup>
The optical pickup of the present embodiment shows an application example to NA 0.67 blue system / DVD / CD.
[0090]
The optical pickup of the present embodiment includes a light source 12 having a used wavelength λ1 = 405 nm in a blue wavelength band, a light source 13 having a used wavelength λ2 = 660 nm in a red wavelength band, and a light source 14 having a used wavelength λ3 = 785 nm in an infrared wavelength band. A blue optical recording medium 1a with an NA of 0.67 and a light irradiation side substrate thickness of 0.6 mm, a DVD optical recording medium 1b with an NA of 0.65 and a light irradiation side substrate thickness of 0.6 mm, an NA of 0.50 and an optical This is an optical pickup capable of recording, reproducing or erasing with respect to each of the CD-type optical recording medium 1c having a substrate thickness of 1.2 mm on the irradiation side.
[0091]
The difference from the first embodiment described above is that the blue numerical aperture is NA 0.67 instead of NA 0.65, and the required incident beam diameter is larger in blue than in DVD. Therefore, the overall configuration is the same as that shown in FIG. 10, but in this embodiment, 1C is used as the objective lens instead of 1A.
[0092]
In general, when light in the red band having the same luminous flux diameter is incident on an objective lens having the best wavefront in the blue wavelength band, the refractive power decreases and the numerical aperture decreases. In other words, the NA is higher when light in the blue wavelength band is passed than when light in the red wavelength band is allowed to pass, and a large-capacity optical pickup is realized in addition to shortening the wavelength. it can. On the other hand, in the DVD generation, NA is determined to be around 0.65. FIG. 14 shows the following characteristics:
Focal length in blue: 3.05mm
Glass type: 550000.550,000
Optical recording medium substrate thickness: 0.6 mm
The relationship between the numerical aperture NA when transmitting light with a blue wavelength of 405 nm and a red wavelength of 660 nm is shown as an example. From FIG. 14, for example, when the red wavelength light of the DVD system is NA 0.65, it means that the NA is about 0.67 for the blue wavelength light with the same incident light beam diameter. If such a relationship is used, not the three-stage aperture switching as in the first and second embodiments, but the two-stage aperture switching similar to the DVD and CD generations, and the device can be simplified.
[0093]
As the aperture switching means 22 portion of the present embodiment, wavelength selective diffraction that switches the beam diameter by diffraction as shown in FIGS. 15A to 15C according to the wavelength of the beam emitted from the light source. A wavelength selective diffraction element 43 on which a pattern is formed is used.
[0094]
The transmission characteristics of each light beam in the light beam in the blue wavelength band in FIG. 15A, the light beam in the red wavelength band in FIG. 15B, and the light beam in the infrared wavelength band in FIG. Is transmitted through any of the blue wavelength band, the red wavelength band, and the infrared wavelength band at the incident light beam diameter φ3 in the central region where no light is applied, and the incident light beam from the outer periphery of the incident light beam diameter φ3 in the peripheral region. In the region up to the diameter φ1 (= φ2), it does not act on the light in the blue wavelength band and the red wavelength band, and diffracts only the light in the infrared wavelength band.
[0095]
FIG. 16 shows the incident light beam diameter (X axis) when the light of red wavelength band 660 nm is passed through the objective lens 2C of the present embodiment, and the DVD optical path has an NA of 0.65 with the incident light beam diameter. This shows the relationship between the aperture switching means 22 to obtain and the space condition (Y axis) of the front principal point of the objective lens 2C. The second Y axis in FIG. 16 shows the numerical aperture: NA with respect to the incident light beam diameter (X axis) when light in the blue wavelength band of 405 nm is passed. In particular, the numerical aperture used at the time of recording / reproducing on the blue optical recording medium 1a in the present embodiment is NA 0.67. From FIG. 16, the optimum incident beam diameter: φ and the aperture switching means 22-objective lens 2C Distance between front principal points: t is
Optimum incident beam diameter: φ = 4.1mm
Optimum aperture-front lens principal point distance: t = 0 mm
It becomes.
[0096]
Here, the relationship of FIG. 16 will be described. In this embodiment, a light source having a wavelength λ1 = 405 nm and a wavelength λ2 = 660 nm and a single objective lens 2C for condensing and irradiating the light source light onto the optical recording medium 1 are provided. Is used when the light source 13 with a wavelength of 660 nm is turned on, and is used with a finite system incident light beam, and a predetermined opening from the front principal point of the objective lens to the light source side is a predetermined distance: t,
t≈L-NA1 · f / tan (asin (NA2obj))
Where f: focal length of the objective lens
NA1: Image plane side numerical aperture when a light source with wavelength λ1 = 405 nm is turned on
NA2obj: the numerical aperture on the light source side when the light source with wavelength λ2 = 660 nm is turned on
L: Object distance when the light source with wavelength λ2 = 660 nm is turned on
Therefore, it is not necessary to switch the aperture between the wavelength of 405 nm and the wavelength of 660 nm.
[0097]
Here, with reference to FIG. 17, the above expression t≈L−NA1 · f / tan (asin (NA2obj)) will be described. First, the wavelength λ1, the numerical aperture NA1, and the infinite optical path are drawn as shown by a one-dot chain line in the figure. Further, the focal length: f, the incident beam diameter: φ, and the numerical aperture: NA1 of the objective lens 1C are generally given by
NA1 = Φ / 2 / f
The relationship is known. On the other hand, a finite optical path having a wavelength λ2, a numerical aperture NA2, an object distance L, and an object-side numerical aperture NA2obj is drawn as a solid line in the figure. Here, when the aperture is located at a position away from the front principal point of the objective lens by a distance: t and the diameter of the light beam having the wavelength λ2 is φ,
φ / 2 = (L−t) × tan (asin (NA2obj))
The relationship is obtained.
[0098]
By substituting this equation into the equation for NA1, the above equation for t is obtained. By the way, in this embodiment,
Optical path condition of blue wavelength band: λ1 = 405 nm, NA1 = 0.67
Optical path condition in the red wavelength band: λ2 = 660 nm, NA2 = 0.65, L = 142 mm, NA2obj = 0.015
In this example, φ = 4.1 mm and t = 0.0 mm.
[0099]
Although not specifically described in these embodiments, an example of a position where the wavefront aberration is minimized as the object distance when the DVD system or the CD system is configured as a finite system will be described. For example, in the case of the third embodiment of the objective lens, the relationship between the object distance and the wavefront aberration when the light of wavelength 660 nm of the DVD system is passed is expressed as shown in FIG. A position with an object distance of 142 mm is selected.
[0100]
The aperture switching means 22 is not limited to using the diffraction characteristics, and may be, for example, a reflection characteristic, an absorption characteristic, and further using polarized light. For example, when the reflection characteristic is used, a dielectric optical multilayer film having wavelength selectivity may be provided similarly to the case of the wavelength selective diffraction element 43. More specifically, in the central region where the dielectric optical multilayer film is not applied, the peripheral region φ2 has high transmittance with respect to any light in the blue wavelength band, red wavelength band, and infrared wavelength band. In the outer region, high transmittance is required only for light in the blue wavelength band and red wavelength band, and low transmittance is required for light in the infrared wavelength band. Also in the case of using the absorption characteristic, the light beam diameter may be switched depending on the absorption according to the wavelength of the light emitted from the light source side as in these cases. Further, when using polarized light, the light source may be arranged so that the polarization directions of the light in the red wavelength band and the infrared wavelength band are orthogonal, and the aperture may be switched according to the orthogonal polarization direction.
[0101]
The aperture switching means 22 may have a configuration in which a coating or a diffraction grating is provided on the surface of the objective lens 2 (2A, 2B, 2C) without being provided as a separate element as shown in FIG. For example, in FIG. 19, a wavelength-selective transmission / reflection coating 44 is formed on the light source side surface of the objective lens 2, and FIG. 19A shows convergent irradiation of the blue optical recording medium 1a with light having a wavelength of 405 nm. 19 (b) shows a case where the DVD-type optical recording medium 1b is converged and irradiated with light having a wavelength of 660 nm, and FIG. 19 (c) shows a case where the CD-type optical recording medium 1c is converged and irradiated with light having a wavelength of 785nm. This is the case. The coating 44 reflects only light with a wavelength of 785 nm, and has a characteristic of transmitting light with wavelengths of 405 nm and 660 nm.
[0102]
[Embodiment of optical information processing apparatus]
A configuration example of the optical information processing apparatus according to the present embodiment will be described with reference to a schematic perspective view of FIG. The optical information processing apparatus 51 of the present embodiment uses an optical pickup 52 configured as shown in FIG. 10 for a plurality of types of optical recording media 1a, 1b, or 1c having different applied wavelengths and numerical apertures NA. This is an apparatus for recording, reproducing or erasing information with compatibility. In the present embodiment, the optical recording medium 1 (1 a, 1 b or 1 c) has a disk shape and is stored in the protective case 53. The optical recording medium 1 (1 a, 1 b, or 1 c) is inserted and set in the arrow “on” direction from the insertion port 54 into the optical information processing apparatus 51 together with the protective case 53, rotated by the spindle motor 55, and optical pickup Information is recorded, reproduced, or erased by 52. The optical recording medium 1 (1a, 1b or 1c) does not need to be placed in the protective case 53 and may be in a bare state.
[0103]
By using the objective lens and the optical pickup 52 described in the above-described embodiment, three generations of blue / DVD / CD (or 2 of blue / DVD) can be used without requiring an aberration correction element. Generation), an optical information processing apparatus 51 including an objective lens and an optical pickup in which spherical aberration is sufficiently suppressed is provided.
[0104]
【The invention's effect】
According to the objective lens of the first to sixth aspects of the present invention, since the glass type condition, shape condition, use wavelength and substrate thickness condition, etc. of the objective lens are appropriately defined from the relationship with the use wavelength, the pasting of the prior art is performed. It is possible to provide compatibility for two or three wavelengths of blue, DVD, and CD without adding a matching lens or a phase correction element.
[0105]
According to the optical pickups of the seventh and ninth aspects of the invention, the NA 0.65 blue-based optical recording medium HD-DVD, NA0, which has recently been studied for standardization, by satisfying the conditions (4) and (5). .65 DVD optical recording media can be interchanged.
[0106]
According to the optical pickups of the eighth and ninth aspects, the incident light beam diameters φ1 and φ2 can be made common by slightly increasing the blue numerical aperture NA such as NA 0.67. Opening switching can be eliminated.
[0107]
According to the optical pickup of the invention described in claims 10 to 13, the optimum NA can be obtained for each of the three generations of so-called blue, DVD and CD.
[0108]
According to the optical pickups of the fourteenth and fifteenth aspects of the present invention, the light beam diameters of the seventh to thirteenth inventions are defined by the aperture or wavelength selective aperture switching means that is movable integrally with the objective lens. Can do.
[0109]
According to the optical pickup of the sixteenth aspect of the present invention, the optimum NA is obtained for each of the three generations with only the opening (no opening switching means) for two generations and with only one stage switching with the opening (not the two-stage switching means). Obtainable.
[0110]
According to the optical pickup of the seventeenth aspect of the invention, it is not necessary to provide a dedicated element for switching the opening, and the actuator can be reduced in weight and the number of assembling steps can be reduced.
[0111]
According to the optical information processing apparatus of the eighteenth aspect of the invention, the same operation and effect as the first aspect of the invention can be achieved.
[Brief description of the drawings]
FIG. 1 is a characteristic diagram showing the distribution of wavefront aberration according to the refractive index and Abbe number in each of blue, DVD, and CD optical recording media.
FIG. 2 is a characteristic diagram showing glass type conditions in relation to the Abbe number and the refractive index.
FIG. 3 is a characteristic diagram showing a relationship between a refractive index and a first surface radius of curvature.
FIG. 4 is an explanatory diagram showing a first embodiment of an objective lens.
FIG. 5 is a wavefront aberration characteristic diagram thereof.
FIG. 6 is an explanatory diagram showing a second embodiment of the objective lens.
FIG. 7 is a characteristic diagram of the wavefront aberration.
FIG. 8 is an explanatory diagram showing a third embodiment of the objective lens.
FIG. 9 is a characteristic diagram of the wavefront aberration.
FIG. 10 is a configuration diagram of an optical system showing an embodiment of an optical pickup.
FIG. 11 is a characteristic diagram showing a relationship between a wavelength and an effective diameter for setting NA to 0.65.
FIG. 12 is a configuration diagram showing aperture switching in the first embodiment of the optical pickup.
FIG. 13 is a configuration diagram showing aperture switching in the second embodiment of the optical pickup;
FIG. 14 is a characteristic diagram showing an NA correlation between a blue wavelength and a red wavelength.
FIG. 15 is a schematic configuration diagram showing aperture switching in the third embodiment of the optical pickup;
FIG. 16 is a characteristic diagram of an incident light beam diameter of an objective lens and an objective lens interval;
FIG. 17 is a principle diagram for explaining an arithmetic expression related to the opening position;
FIG. 18 is a characteristic diagram showing the relationship between the object distance and wavefront aberration of the objective lens at a wavelength of 660 nm.
FIG. 19 is a schematic configuration diagram illustrating a coating example for an objective lens.
FIG. 20 is a schematic perspective view showing an embodiment of an optical information processing apparatus.
FIG. 21 is a block diagram of a blue / DVD / CD compatible optical pickup that is generally assumed.
FIG. 22 is a configuration diagram illustrating a conventional example of an objective lens.
FIG. 23 is a conventional light source wavelength-wavefront aberration characteristic diagram.
[Explanation of symbols]
1 Optical recording media
2 Objective lens
11 Optical pickup
12-14 Light source
21 opening
22 Opening switching means
41-43 Wavelength selective diffraction element

Claims (18)

An objective lens that converges and irradiates light from a light source with wavelengths λ1 and λ2 (λ1 <λ2) onto an optical recording medium,
Constructed as a single lens, both surfaces are aspherical and convex, refractive index for the d-line material: nd and Abbe number: νd, paraxial radius of curvature of light source side surface: R1, focal length: f, conditions:
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Objective lens characterized by satisfying The conditions (1), (2), and (3) are defined for an infinite incident light beam with respect to the light from the light source having the wavelength λ1, and a finite system for the light from the light source with the wavelength λ2. The objective lens according to claim 1, wherein the objective lens is defined with respect to an incident light beam. The conditions (1), (2), and (3) are defined for an incident light flux in a blue wavelength band as the wavelength λ1 and in a red wavelength band as the wavelength λ2. Objective lens. An objective lens that converges and irradiates light from a light source with wavelengths λ1, λ2, and λ3 (λ1 <λ2 <λ3) onto an optical recording medium,
Constructed as a single lens, both surfaces are aspherical and convex, refractive index for the d-line material: nd and Abbe number: νd, paraxial radius of curvature of light source side surface: R1, focal length: f, conditions:
(1) νd> 35
(2) 1.58> nd
(3) 0.58nd−0.29 ≦ R1 / f ≦ 0.62nd−0.31
Objective lens characterized by satisfying The conditions (1), (2), and (3) are defined for an infinite incident light beam with respect to light from the light source having the wavelength λ1 and for light from light sources with the wavelengths λ2 and λ3. The objective lens according to claim 4, wherein the objective lens is defined for an incident light beam of a finite system. The conditions (1), (2), and (3) are defined for the incident light flux in the blue wavelength band as the wavelength λ1, the red wavelength band as the wavelength λ2, and the infrared wavelength band as the wavelength λ3. The objective lens according to claim 4 or 5. A plurality of light sources emitting light of wavelengths λ1 and λ2,
A single objective lens according to claim 1, 2 or 3, which converges and irradiates light from these light sources onto an optical recording medium;
With
For the light beams having the wavelengths λ1 and λ2, the effective numerical apertures NA (λ1) and NA (λ2) of the wavelengths λ1 and λ2, and the incident light beam diameters φ1 and φ2 of the wavelengths λ1 and λ2 that enter the objective lens. But the condition:
(4) NA (λ1) = NA (λ2)
(5) φ2> φ1
An optical pickup characterized by satisfying A plurality of light sources emitting light of wavelengths λ1 and λ2,
A single objective lens according to claim 1, 2 or 3, which converges and irradiates light from these light sources onto an optical recording medium;
With
For the light beams having the wavelengths λ1 and λ2, the effective numerical apertures NA (λ1) and NA (λ2) of the wavelengths λ1 and λ2, and the incident light beam diameters φ1 and φ2 of the wavelengths λ1 and λ2 that enter the objective lens. But the condition:
(6) NA (λ1)> NA (λ2)
(7) φ1 = φ2
An optical pickup characterized by satisfying 9. The optical pickup according to claim 7, wherein the optical recording media used for the wavelengths [lambda] 1 and [lambda] 2 have substantially the same substrate thickness. A plurality of light sources that emit light of wavelengths λ1, λ2, and λ3;
A single objective lens according to claim 4, 5 or 6, which converges and irradiates light from these light sources onto an optical recording medium;
With
With respect to the light of the wavelengths λ1, λ2, and λ3, the effective numerical apertures NA (λ1), NA (λ2), NA (λ3) of the wavelengths λ1, λ2, and λ3, and the wavelengths λ1 that enter the objective lens , Λ2, λ3 incident beam diameters φ1, φ2, and φ3 satisfy the following conditions:
(8) NA (λ1) = NA (λ2)> NA (λ3)
(9) φ2>φ1> φ3
An optical pickup characterized by satisfying A plurality of light sources that emit light of wavelengths λ1, λ2, and λ3;
A single objective lens according to claim 4, 5 or 6, which converges and irradiates light from these light sources onto an optical recording medium;
With
With respect to the light of the wavelengths λ1, λ2, and λ3, the effective numerical apertures NA (λ1), NA (λ2), NA (λ3) of the wavelengths λ1, λ2, and λ3, and the wavelengths λ1 that enter the objective lens , Λ2, λ3 incident beam diameters φ1, φ2, and φ3 satisfy the following conditions:
(10) NA (λ1)> NA (λ2)> NA (λ3)
(11) φ1>φ2> φ3
An optical pickup characterized by satisfying A plurality of light sources that emit light of wavelengths λ1, λ2, and λ3;
A single objective lens according to claim 4, 5 or 6, which converges and irradiates light from these light sources onto an optical recording medium;
With
With respect to the light of the wavelengths λ1, λ2, and λ3, the effective numerical apertures NA (λ1), NA (λ2), NA (λ3) of the wavelengths λ1, λ2, and λ3, and the wavelengths λ1 that enter the objective lens , Λ2, λ3 incident beam diameters φ1, φ2, and φ3 satisfy the following conditions:
(12) NA (λ1)> NA (λ2)> NA (λ3)
(13) φ1 = φ2> φ3
An optical pickup characterized by satisfying The substrate thickness of the optical recording medium used for the light of the wavelengths λ1 and λ2 is substantially the same, and the substrate thickness of the optical recording medium used for the light of the wavelength λ3 is used at the wavelengths λ1 and λ2. The optical pickup according to claim 10, wherein the optical pickup is approximately twice the thickness of the substrate of the optical recording medium. 14. The optical pickup according to claim 7, further comprising an opening having a diameter substantially the same as a maximum incident light beam diameter, and the opening is movable integrally with the objective lens. 11. An aperture having substantially the same diameter as the maximum incident beam diameter and an aperture switching means having wavelength selectivity are provided, and these aperture and aperture switching means are movable integrally with the objective lens. 11 or 12. An optical pickup that is used with an infinite system incident light beam when the light source with wavelength λ1 is turned on, and is used with a finite system incident light beam when the light source with wavelength λ2 is turned on,
An opening having substantially the same diameter as the incident beam diameter φ1 (= φ2) is passed through the objective lens front principal point to the light source side by a predetermined distance: t,
t≈L-NA1 · f / tan (asin (NA2obj))
Where f: focal length NA1 of objective lens: image plane side numerical aperture NA2obj when light source with wavelength λ1 is lit: light source side numerical aperture when light source with wavelength λ2 is lit L: position of object distance when light source with wavelength λ2 is lit The optical pickup according to claim 8, wherein the optical pickup is arranged in the above. 16. The optical pickup according to claim 15, wherein the aperture switching means is a wavelength selective coating or a wavelength selective diffraction grating, and is formed on either the light source side surface or the medium side surface of the objective lens. Recording, reproducing or erasing information on an optical recording medium using the optical pickup provided with the objective lens according to any one of claims 1 to 6 or the optical pickup according to any one of claims 7 to 17. A characteristic optical information processing apparatus.

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