JP3854590B2 - Objective lens - Google Patents

Description

【００１５】
このことは、基板の厚さが異なる光ディスクのいずれに対しても、その基板の厚さに対応する波長のレーザビームを用いた場合、このレーザビームの対物レンズと基板を通った全ての光線がこの基板の情報記録面上で良好に集光するような光路長を経るようにするものである。
なお、この時本発明の一実施形態のレンズは後の実施例で具体的に説明するように、そのレンズ面が複数の非球面に分割されてなり、分割されたいずれの非球面も各単色光の固有の波長に対応した単一の焦点を有するとともに、前記各単色光の固有の波長に対応した焦点はそれぞれ異なる位置に配置されるよう設計されている。
【００１６】
いま、図３において、対物レンズ１を用いて基板２の情報記録面２ａにレーザビームを集光させる場合について説明する。ここで、対物レンズ１の面Ａは光入射側面、面Ｂは光出射側面であり、基板２の情報記録面２ａは対物レンズ１側とは反対側にある。
【００１７】
図３は、対物レンズ１に入射するレーザビームは平行光とし（従って、図３に示す光学系は、いわゆる無限光学系である）、対物レンズ１の光軸ＯＡからこれに垂直な方向の距離（光線高さ）ｈの位置Ｐ₁を通る光線が光軸ＯＡを横切る点（集光点）Ｐ₅に達するまでの光路を摸式的に示すものである。ここで、かかる光路での対物レンズ１への入射点をＰ₂、対物レンズ１からの出射点をＰ₃、透明基板２への入射点をＰ₄とし、
点Ｐ₁〜入射点Ｐ₂：空間距離＝Ｓ_1h 屈折率＝ｎ₁
入射点Ｐ₂〜出射点Ｐ₃：空間距離＝Ｓ_2h 屈折率＝ｎ₂
出射点Ｐ₃〜入射点Ｐ₄：空間距離＝Ｓ_3h 屈折率＝ｎ₃
入射点Ｐ₄〜集光点Ｐ₅：空間距離＝Ｓ_4h 屈折率＝ｎ₄
とすると、点Ｐ₁から集光点Ｐ₅までの光路長Ｌ_hは、
【数４】

で表わされる。なお、光軸ＯＡ上での光路長Ｌ_hは、この数４において、ｈ＝０の場合である。
【００１８】
この数４は任意の光線高さｈについて該当するものであり、収差補正されている場合には、夫々の光線高さｈに対する集光点Ｐ₅が夫々の許容範囲内で情報記録面２ａ上にある。すなわち、本発明は、例えば厚さが異なる複数の基板夫々毎に異なる波長のレーザビームを用いることにより、色収差と球面収差とが相殺し合って夫々の光線高さｈに対する集光点Ｐ₅が夫々の許容範囲内で情報記録面２ａ上にあるようにするものである。
例えば、ＣＤにおける７９０ｎｍの単色光（λ_１）とＤＶＤにおける６５５ｎｍの単色光（λ_２）が用いられる場合で、これらの両波長が共通して使用される領域を複数の非球面部に分割したレンズ面とする手法では、任意の前記非球面部の光路長が他の非球面部の光路長と該各単色光の波長λｉのほぼ整数倍異ならせ、なおかつ前記各非球面部における前記各単色光の波面収差の最大値と最小値の差を△Ｖｄ（λ_１）と△Ｖｄ（λ_２）（ｄは１，２・・・・の整数で各非球面部を意味する）とした時に、いずれの非球面部においても各単色光の前記差の比を０．４以上２．５以下、好ましくは０．５以上２．０以下とすることにより両波長においてレンズ全体として許容範囲のＲＭＳ波面収差を確保することができる。なお、ここでいう波面収差は光線高さ（ｈ）をｈ＝０の場合の光路長をＬ_０とし、各光線高さにおける光路長をＬｈとすると、波面収差Ｖｈは、数５で表される。
【数５】

図１０は、ＣＤとＤＶＤの波長におけるレンズの波面収差を対比して模式図的に示したものであり、横軸は光線高さ、縦軸は波面収差で、上側がＣＤの各非球面部の波面収差を、下側がＤＶＤの各非球面部の上記式で求められる波面収差を表している。例えば非球面部の第１領域におけるその非球面部内の波面収差の最大値と最小値の差は、△Ｖ₁（λ_１）、△Ｖ₁（λ_２）で定義される。本発明では、後の実施例で明らかにされているように、いずれの非球面部においても各波長の波面収差の最大値と最小値の差の比は０．４以上２．５以下である。すなわち、本発明はいずれの波長においても各非球面部で波面収差に一定の分布を有する点でも、従来の一方の波長を基準にレンズ面を構成し、他方の波長においてのみ位相ずれを利用して波面収差を補正する方式と異なる。なお、前記整数倍としては分割する非球面の数にもよるが０倍〜±１０倍とすることが好ましく、０倍〜±５倍とすることが好ましい。
また、本発明の多波長用レンズはいずれの非球面部の各領域においても各波長の前記波面収差の最大値と最小値の差が０．１４λｉ以下（例えば波長が７９０ｎｍである場合には±１１０．６ｎｍ以下、波長が６５５ｎｍである場合には±９１．７ｎｍ以下）、好ましくは、０．１２λｉ以下、さらに好ましくは０．１０λｉ以下とすることにより各波長においてさらに良好な光学特性を確保することができる。
さらに、本発明では、二波長用光学系の場合、各波長の波面収差をそれらがほぼ対称形となる多波長用レンズを用いることにより、二波長のバランスが取れ、さらにＲＭＳ波面収差を低減することができる。
なお、ＲＭＳ波面収差を低減することを考えると、ＣＤの場合には図１０の光線高さ１．５８ｍｍまでのＤＶＤとＣＤの共通使用領域のみの波面収差からＲＭＳ波面収差が決まってしまうが、ＤＶＤの場合には、前記共通使用領域の外側にＤＶＤ専用領域（図１０では光線高さ１．５８〜２．０２ｍｍの範囲）があり、前記共通使用領域と前記専用領域の両方の波面収差からＲＭＳ（Root Mean Square）波面収差値が求まる。よってＤＶＤの場合には、前記共通使用領域の波面収差が多少悪くても、ＤＶＤ専用領域の波面収差については、ＣＤを全く無視してＤＶＤのみを良くするようにすれば、ＤＶＤのＲＭＳ波面収差を許容値内に十分低減することができる。例えば、図１０の模式図では、ＤＶＤとＣＤの共通使用領域においてＤＶＤの波面収差は０〜−０．１０６λであり、ＣＤの波面収差は０〜＋０．０８８であって、ＣＤの波面収差の方がＤＶＤの波面収差よりも小さい。またＤＶＤ専用領域の波面収差は−０．０５２λである。この結果、ＲＭＳ波面収差としては、ＤＶＤは０．０２１２λＲＭＳ、ＣＤは０．０２２２λＲＭＳと、ＲＭＳ波面収差はＤＶＤもＣＤもほぼ等しい値となっている。このように、ＲＭＳ波面収差として、ＤＶＤもＣＤも同一の値としたい場合には、ＤＶＤとＣＤの共通使用領域においてはＣＤの波面収差をＤＶＤの波面収差よりも良くしておき、ＤＶＤのＲＭＳ波面収差については共通使用領域で劣化している分をＤＶＤ専用領域で補ってやるようにすることが有効である。ＤＶＤとＣＤのＲＭＳ波面収差の比率を変えたい場合にも同様に、ＤＶＤについては、共通使用領域の波面収差が多少悪くても専用領域で補えることを考慮すると良い。
本発明の実施形態により、例えば基板の厚さが異なるいずれの光ディスクに対しても、情報記録面に良好な光スポットを形成することが可能となる。なおこのことは、ディスク基板の厚みが異なっていなくても、つまり、厚みが同じで波長が異なるような場合でも前記集光点Ｐ₅を夫々の許容範囲内にすることにより適用可能である。また、光記録媒体に限らず、光通信などで異なる波長のレーザビームを同一のレンズもしくは光学系を通過させるような場合にも適用可能である。
【００１９】
以下、本発明の実施形態を、透明基板の厚さが異なる２種類の光ディスク、即ち、ＤＶＤとＣＤとを例に、図面を用いて説明する。なお、本発明の第１実施形態のレンズは製造上の容易さから非晶質ポリオレフィンからなる樹脂を射出成形により作成したものである。第２実施形態のレンズはガラスの屈折率のものであるが、レンズ材質をプラスチック樹脂としたい場合にはプラスチック樹脂の屈折率で設計すれば良い。
【００２０】
図１は本発明による対物レンズの第１の実施形態の作用を示す図であって、同図（ａ）はＤＶＤに対するもの、同図（ｂ）はＣＤに対するものである。図において、１はこの実施形態の対物レンズ、２はＤＶＤの透明基板（以下、ＤＶＤ基板という）、３はＣＤの透明基板（以下、ＣＤ基板という）、４，５はレーザビームである。
【００２１】
まず、図１（ａ）において、対物レンズ１が図示しない光ディスク装置の光ヘッドに設けられている。そして、ＤＶＤがこの光ディスク装置に装着されて、対物レンズ１によって平行光として入射されるレーザビーム４が集光されることにより、記録再生が行なわれる。ここで、ＤＶＤ基板２の厚さｔ₁は0．6ｍｍであり、このときのレーザビーム４としては、波長λ₁＝655ｎｍのレーザビームが開口数ＮＡ＝0.63の光束として用いられる。かかる条件のもとに、かかるレーザビームは、ＤＶＤ基板２の対物レンズ１側とは反対側の面の情報記録面２ａに集光される。
【００２２】
図１（ｂ）は上記と同じ光ディスク装置にＣＤが装着され、同じ対物レンズ１を用いて記録再生が行なわれる場合を示す。ここで、ＣＤ基板３の厚さｔ₂は1.2ｍｍであり、このときのレーザビーム５としては、波長λ₂＝790ｎｍのレーザビームがほぼ開口数ＮＡ＝0.63の光束として用いられるが、実質的には、開口数ＮＡ＝0.47の光束がＣＤ基板３の情報記録面３ａに集光し、ハッチングして示すほぼＮＡ＝0.47〜0.63の対物レンズ１の光軸ＯＡから離れた部分を通る光束はこの情報記録面３ａで集光しない。このように、この開口数ＮＡがほぼ0.47までの上記のレンズ領域は、ＤＶＤ，ＣＤの共通使用領域となる。
【００２３】
このように、この第１の実施形態は、ＤＶＤ，ＣＤともに収差が良好に低減されて、情報記録面２ａ，３ａで良好な光スポットが得られるようにするものであるが、このために、ＤＶＤ，ＣＤの両方共に、任意の光線高さｈに対して上記数４で示す光路長Ｌ_hが収差を低減して許容値内とするような値とするように、対物レンズ１のレンズ面形状を設定するものである。以下、かかるレンズ面形状の一具体例を図２により説明する。
【００２４】
図２において、対物レンズ１の光出射側面Ｂについて、光線高さｈの点をｃ、この点ｃから光軸ＯＡに平行な方向での光出射側面Ｂ上の点をｄとすると、この光出射側面Ｂの面形状は、任意の光線高さｈに対する点ｃ，ｄ間の距離Ｚ_Bにより、
【数６】

但し、 C=-0.12301、K=3.312138、A₄=0.01628151、A₆=-0.004311717、A₈=0.000682316、A₁₀=-0.00004157469で表わされるようにする。
【００２５】
なお、数６において、上記係数Ｃ，Ｋ，Ａ₄，Ａ₆，Ａ₈，Ａ₁₀の値を代入して任意の光線高さｈ（≠０）に対する距離Ｚ_Bを求めると、その値は負の値となるが、これは光出射側面Ｂ上の点ｄが点ｃ、従って、この光出射側面Ｂの光軸ＯＡが通る面頂点ｅよりも出射面側（図２での左側）に位置することを示している。距離Ｚ_Bが正の値である場合には、逆の右側に位置することを示している。
【００２６】
次に、対物レンズ１の光入射側面Ａについて、光線高さｈの点をａ、この点ａから光軸ＯＡに平行な方向での光入射側面Ａ面上の点をｂとすると、光入射側面Ａの面形状は、光線高さｈ（ｍｍ）とこの光線高さｈに対する点ａ，ｂ間の距離Ｚ_A（ｍｍ）とが次の表１に示す関係となるレンズ面形状に設定される。
【００２７】
【表１】

対物レンズ１の上記数６で表わされる光出射側面Ｂも、また、上記表１の点列データで表わされる光入射側面Ａも、連続した非球面をなすものである。また、対物レンズ１の光軸上の面頂点ｆ，ｅ間の距離、即ち、中心厚ｔ₀は２.２ｍｍであって、波長λ₁＝655ｎｍ(ＤＶＤ）での屈折率ｎは1.54014であり、波長λ₂＝790ｎｍ（ＣＤ）での屈折率ｎは1.5365である。
【００２８】
（ｉ）ここで、収差を評価するための上記の収差の許容値としては、対物レンズ１への入射レーザビームが入射角０゜である場合（即ち、光軸ＯＡに平行な平行光）について、ＤＶＤ（波長λ₁＝655ｎｍ），ＣＤ（波長λ₂＝790ｎｍ）ともに、ＲＭＳ波面収差で0.035λ、好ましくは、0.033λ、さらに好ましくは、0.030λとする。この第１の実施形態では、ＤＶＤ，ＣＤの波面収差がかかる許容値以下となるように、光出射面Ｂと光入射面Ａを上記の面形状に設定しているものである。
【００２９】
この第１の実施形態では、２種類の異なる波長λ₁，λ₂を用いた場合を示しているが、一般に、ｎ種類（但し、ｎは２以上の整数）の異なる波長λ_i（但し、ｉ＝１，２，......，ｎ）を用いる場合も、同様である。
【００３０】
(ii)また、このようにｎ種類の波長λ_iを用いた場合について、これら波長λ_iの入射レーザビームが入射角０゜である場合の夫々のＲＭＳ波面収差をＷ_i・λ_iとすると、これら収差は、
【数７】

（但し、ｉ番目の該光ビームの波長をλ_i（ｉ＝１，２，......）、全ての波長にわたる個々のＲＭＳ波面収差の二乗の総和をΣＷ_i ²、波長λ_iの光ビームのＲＭＳ波面収差をＷ_i・λ_iとする）
を満足するようにする。このときの許容値Ｗ₀としては、0.028、好ましくは0.026，さらに好ましくは0.025、さらに好ましくは0.023とする。上記第１の実施形態では、ＤＶＤのＲＭＳ波面収差をＷ₁、ＣＤのＲＭＳ波面収差をＷ₂とし、かつｉ＝１，２であるから、上記数７は、
【数８】

となる。
【００３１】
(iii）あるいはまた、異なるｎ種類の波長λ_iのレーザビームを用いる場合、夫々の波長λ_iのうちで最大のＲＭＳ波面収差をＷmax、最小のＲＭＳ波面収差をＷminとすると、
１≦Ｗmax／Ｗmin＜Ｗ_th
とする。この場合の許容値Ｗ_thとしては、1.8、好ましくは1.6、さらに好ましくは1.4とする。上記第１の実施形態の場合には、ＤＶＤのＲＭＳ波面収差Ｗ₁とＣＤのＲＭＳ波面収差Ｗ₂とのいずれか一方が最大のＲＭＳ波面収差Ｗmaxとなり、他方が最小のＲＭＳ波面収差Ｗminとする。
【００３２】
図４はこの第１の実施形態でのＲＭＳ波面収差の計算結果を示すものであって、横軸に像高（ｍｍ）を取り、縦軸にＲＭＳ波面収差を取っている。
【００３３】
図４（ａ）はＤＶＤ（波長λ₁＝655ｎｍ）に対するＲＭＳ波面収差を示しており、像高＝０ｍｍのときには、ＲＭＳ波面収差＝0.02130λ₁である。また、図４（ｂ）はＣＤ（波長λ₂＝790ｎｍ）に対するＲＭＳ波面収差を示しており、像高＝０ｍｍのときには、ＲＭＳ波面収差＝0.02410λ₂である。
【００３４】
かかる数値を評価するために、上記の各条件式に挿入すると、
（ｉ）まず、ＤＶＤ，ＣＤについて、ＲＭＳ波面収差が0.02130λ，0.02410λと上記の許容値0.035λ、好ましくは、0.033λ、さらに好ましくは、0.030λよりも小さい。
【００３５】
（ii）ＤＶＤ，ＣＤについて、上記数８により、
【数９】

であるから、上記の許容値0.028、好ましくは0.026、さらに好ましくは0.025、さらに好ましくは0.023以下となっている。
【００３６】
(iii)ＤＶＤ，ＣＤについて、Ｗmax／Ｗminをみると、
Ｗmax／Ｗmin＝0.02410／0.02130＝1.1315
となるから、上記の許容値1.8、好ましくは1.6、さらに好ましくは1.4以下となっている。
【００３７】
図５は上記数６で示す面形状の光出射側面Ｂと上記表１で示す面形状の入射側面Ａとを有する対物レンズ１を用いたことによるＤＶＤ，ＣＤの情報記録面上での光スポットの計算結果を示す図であって、横軸は情報記録面での光軸を基準点とした光軸に垂直方向の位置を距離（ｍｍ）で表わしたものであり、縦軸はこの基準点（＝0ｍｍ）での光強度を１としたときの各位置の相対的光強度を表わしている。
【００３８】
図５（ａ）はＤＶＤに対する光スポットを示すものであって、相対的光強度が1／e²（＝13.5％）となる光スポット直径φ_Dは0.85μｍである。また、図５（ｂ）はＣＤに対する光スポットを示すものであって、相対的光強度が1／e²となる光スポット直径φ_Cは1.37μｍである。このように、ＤＶＤ，ＣＤともに、情報記録面に良好な光スポットが得られる。
【００３９】
次に、本発明による対物レンズの第２の実施形態について説明する。
【００４０】
この第２の実施形態は、その基本的構成は上記の第１の実施形態と同様であるが、光入射面Ａを光軸から半径方向に複数の区間に区分し、夫々の区間の面形状を、ＤＶＤ，ＣＤともに収差が許容値内に良好に低減されるように、設定するものである。
【００４１】
この第２の実施形態の光入射面Ａの面形状を図２を用いて説明する。いま、この光入射面Ａの光線高さｈ方向（半径方向）の光軸ＯＡ側からｊ番目の区間での点ａ，ｂ間の距離を次の関数Ｚ_Ajで、即ち、
【数１０】

で表わされる。なお、数１０での光源高さｈは、ｊ番目の区間でのものである。
【００４２】
そして、ＤＶＤ，ＣＤともに収差を許容値内に良好に低減するための数１０での区間毎に、その範囲(ｈの範囲）とその各定数B，C，K，A₄，A₆，A₈，A₁₀，A₁₂，A₁₄，A₁₆を示すと、次の表２に示すようになる。
【００４３】
【表２】

また、この第２の実施形態での光出射面Ｂの面形状Ｚ_Bは、次の数１１で表わされる。
【００４４】
【数１１】

但し、 C=-0.0747792、K=15.7398、A₄=0.012308、A₆=-0.0037652、A₈=0.00068571、A₁₀=-0.000048284
また、対物レンズ１の光軸上の面頂点ｆ，ｅ間の距離、即ち、中心厚さｔ₀は2.2ｍｍであって、波長λ₁＝655ｎｍ（ＤＶＤ）での屈折率ｎは1.604194であり、波長λ₂＝790ｎｍ（ＣＤ）での屈折率ｎは1.599906である。
【００４５】
ここで、収差を評価するための上記収差の許容値としては、上記第１の実施形態と同様である。
【００４６】
図６はこの第２の実施形態でのＲＭＳ波面収差の計算結果を示すものであって、横軸、縦軸は図４と同様である。
【００４７】
図６（ａ）はＤＶＤ（波長λ₁＝655ｎｍ）に対するＲＭＳ波面収差を示しており、像高＝０ｍｍのときには、ＲＭＳ波面収差＝0.01945λ₁である。また、図６（ｂ）はＣＤ（波長λ₂＝790ｎｍ）に対するＲＭＳ波面収差を示しており、像高＝０ｍｍのときには、ＲＭＳ波面収差＝0.02525λ₂である。
図１１に上記のレンズの共通使用領域の波面収差を計算した結果を、各非球面部における波面収差の差及びその比を表３に示す。
【表３】

表３に示すように、７９０ｎｍと６５５ｎｍの共通使用領域において各波面収差の差の比ΔＶｄ(λ790)／ΔＶｄ(λ655)は、１．００〜１．０４の間に入っている。また、比ΔＶｄ(λ655)／ΔＶｄ(λ790)は、０．９６〜１．００の間に入っている。そして、その各領域の波面収差自体も両波長において０．１４λ以下となっている。また、このレンズでは波面収差が７９０ｎｍの波長において＋側に、６５５ｎｍの波長において−側に現れるようにしていて、両波面収差がほぼ対称形となる。
なお、光軸を中心に分割された隣接する各非球面部で光路長の差が生じているが、その差は各波長に対応してほぼ整数倍になるように設計されていて、またこの実施例では偶数の分割された非球面部からなっている。
【００４８】
かかる数値を評価するために、第１の実施形態と同様、上記の各条件式に挿入すると、
（ｉ）まず、ＤＶＤ，ＣＤについて、ＲＭＳ波面収差が0.01945λ₁，0.02525λ₂と上記の許容値0.035λ、好ましくは、0.033λ、さらに好ましくは、0.030λよりも小さい。
【００４９】
（ii）ＤＶＤ，ＣＤについて、上記数８により、
【数１２】

であるから、上記の許容値0.028、好ましくは0.026，さらに好ましくは0.025、さらに好ましくは0.023以下となっている。
【００５０】
(iii)ＤＶＤ，ＣＤについて、Ｗmax／Ｗminをみると、
Ｗmax／Ｗmin＝0.02525／0.01945＝1.298
となるから、上記の許容値1.8、好ましくは1.6、さらに好ましくは1.4以下となっている。
【００５１】
図７は上記数１１で示す面形状の光出射側面Ｂと上記数１０及び表２で示す面形状の入射側面Ａとを有する対物レンズ１を用いたことによるＤＶＤ，ＣＤの情報記録面上での光スポットの計算結果に示す図であって、横軸，縦軸は図５と同様である。
【００５２】
図７（ａ）はＤＶＤに対する光スポットを示すものであって、相対的光強度が1／e²（＝13.5％）となる光スポット直径φ_Dは0.89μｍである。また、図７（ｂ）はＣＤに対する光スポットを示すものであって、相対的光強度が1／e²となる光スポット直径φ_Cは1.30μｍである。このように、ＤＶＤ，ＣＤともに、情報記録面に良好な光スポットが得られる。
なお、本実施形態２では、前記の表３において前記比が０．９６〜１．０４であって、ＲＭＳ波面収差としてはＤＶＤが0.01945λ₁でＣＤが0.02525λ₂であるが、前記図１０の説明で述べたように、共通使用領域におけるＤＶＤの波面収差をもう少し劣化させて、ＣＤの波面収差を良くすれば、ＤＶＤ、ＣＤ共にＲＭＳ波面収差として０．０２２〜０．０２３λ程度の同等のＲＭＳ波面収差とすることも可能である。
【００５３】
なお、一例として、先の特開２００１−５１１９２号公報に記載のＤＶＤとＣＤとのＲＭＳ波面収差をみると、
例１） ＤＶＤ：0.001λ₁ ＣＤ：0.047λ₂
例２） ＤＶＤ：0.019λ₁ ＣＤ：0.037λ₂
但し、λ₁＝640ｎｍ λ₂＝780ｎｍ
の２つの例が挙げられているが、いずれにおいても、ＣＤについては、上記の許容値0.035λを越えるものである。
また、この例２）のレンズの各波長における波面収差を公報記載のレンズデ−タを用いて計算による求めると下記表４及び図１２に示すように、その比が０．０３〜３３．４４と本発明の範囲外であり、そのため両者のバランスがずれている。さらにＤＶＤ側の波面収差は０．１４λ以下となるが、ＣＤ側の波面収差が大きくなり、レンズ全体のＲＭＳ波面収差も大きくなってしまう。
【表４】

【数１３】

は、上記夫々について、0.0332，0.0294となり、いずれも上記の許容値0.028、好ましくは0.026，さらに好ましくは0.025、さらに好ましくは0.023を越えているし、さらに、これらのＷ_max／Ｗ_minも夫々、47，1.847となり、いずれも上記の許容値1.8、好ましくは1.6、さらに好ましくは1.4を越えている。
【００５４】
このように、上記第１，第２の実施例とも、収差を上記の許容値内に抑えることができるものであるが、これは、収差がかかる許容値内に収まるように、基板厚の差による球面収差と色収差とがキャンセルし合うレンズ面形状としていることによるものである。これに対し、先の特開２００１−５１１９２号公報では、単に入射レーザビームをＤＶＤレーザビームの波長の整数倍分位相シフトすることにより、ＣＤの収差低減を図るようにしたものであるから、いずれか１つの波長に対しては、収差を充分小さく抑えることができるとしても、全ての波長に対して、上記のような小さい値の許容値内に収差を同時に納めることができないのである。
【００５５】
以上の実施形態では、ＤＶＤとＣＤとで基板厚さが夫々0.6ｍｍと1.2ｍｍと異なることによる球面収差が655ｎｍと790ｎｍとの波長の差による色収差により打ち消して総合的な収差が低減されていることが、図５及び図７に示す光スポット及び図４，図６に示す波面収差のグラフから明らかである。また、以上の実施形態では、対物レンズ１の光入射側面Ａの面形状は上記表１に示す点列データ，数１０及び表２で与えられ、光出射側面Ｂの面形状は上記数６，数１１に示す非球面の式により与えられるので、先の従来例のような回折レンズ構造を用いておらず、また、記録または再生に必要な開口（ＮＡ）に対してほぼ全ての光束を集光することができるので、高い光利用効率が得られることになる。
【００５６】
なお、以上の実施形態では、図１に示すように、ほぼ開口数ＮＡ＝0.47から開口数ＮＡ＝0.63までの対物レンズ１の外側領域はＤＶＤのみに使用され、ＣＤでは使用しないので、かかる外側領域での光入射側面Ａ，光出射側面Ｂのいずれか一方または双方にＤＶＤのときの波長655ｎｍの光を透過し、ＣＤのときの波長790ｎｍの光を透過しない薄膜処理を施したり、あるいは、かかる外側領域での光入射側面Ａ，光出射側面Ｂのいずれか一方または双方二波長655ｎｍの光には作用しないが、波長790ｎｍの光に作用するような回折格子を形成して、波長655ｎｍの光利用効率を落とさず二波長790ｎｍの光利用効率を落とすようにしてもよい。
【００５７】
即ち、以上の実施形態のごとく、異なる開口数の系に共用する際に、開口数に応じた絞りを設定できない場合には、開口数の小さな光学系においては、余分の光束をも受容することになるので、開口数の大きな光学系に合致して設計されたレンズの外側領域部分を通過する光が、開口数が小さな光学系に悪影響を及ぼさないような配慮をすることが望ましい。例えば、レンズの外側領域を通過した光がディスク面には集光しないように、横収差量が0.015ｍｍ以上となるようにするのが望ましい。
【００５８】
また、以上の実施形態では、ＤＶＤとＣＤとの２種類の光ディスクを例としたが、本発明は、これに限らず、これら以外の種類が異なる光ディスクであってもよいし、また、基板の厚みが異なる３種類以上の光ディスクに対しても、適用可能であり、夫々毎に使用するレーザビームの波長を異ならせ、これらに応じて、色収差が波面収差を打ち消すように、レンズ面形状を設定すればよい。
またさらに、基板厚みが同じでも使用する波長が異なるために従来の通常のレンズでは大きな収差が発生してしまうような場合にも本発明を適用することにより収差低減をすることができる。
【００５９】
図８は本発明による対物レンズを用いた光ヘッドの一実施形態を示す構成図であって、１１はＤＶＤレーザ、１２はＣＤレーザ、１３，１４はハーフプリズム、１５はコリメータレンズ、１６は検出レンズ、１７は光検出器、１８は回析格子、１９はアクチュエータであり、図１に対応する部分には同一符号をつけている。
【００６０】
同図において、ＤＶＤディスク２を記録または再生する場合には、ＤＶＤレーザ１１を駆動する。ＤＶＤレーザ１１から発生される波長655ｎｍのレーザビームが、ハーフプリズム１３で反射し、ハーフプリズム１４を透過してコリメータレンズ１５に入射する。コリメータレンズ１５を通過して平行光となってレーザビームは、対物レンズ１に入射して集光され、ＤＶＤディスク２の情報記録面に光スポットを形成する。そして、ＤＶＤディスク２で反射した反射光が対物レンズ１により平行光となり、コリメータレンズ１５に入射する。コリメータレンズ１５はこの平行光を収束光にし、この収束光はハーフプリズム１４，１３を透過し、検出レンズ１６を通って光検出器１７に到達する。光検出器１７の検出出力信号は信号処理回路（図示せず）に供給され、情報記録再生信号やフォーカス誤差信号，トラッキング誤差信号が得られる。図示しないシステム制御回路は、得られたフォーカス誤差信号とトラッキング誤差信号をもとに、適正なフォーカス位置とトラッキング位置に対物レンズ１が位置するように、アクチュエータ駆動回路（図示せず）を制御してアクチュエータ１９を駆動する。
【００６１】
ＣＤディスク３を記録または再生する場合には、ＣＤレーザ１２を駆動する。ＣＤレーザ１２から発生される波長790ｎｍのレーザビームが回折格子１８を通り、ハーフプリズム１４で反射されてコリメータレンズ１５に入射する。コリメータレンズ１５を通過して平行光となったレーザビームは、対物レンズ１に入射して集光され、ＣＤディスク３の情報記録面に光スポットを形成する。そして、ＣＤディスク３で反射した反射光が対物レンズ１により平行光となり、コリメータレンズ１５に入射する。コリメータレンズ１５はこの平行光を収束光にし、この収束光はハーフプリズム１４，１３を透過し、検出レンズ１６を通って光検出器１７に到達する。光検出器１７の検出出力信号は図示しない信号処理回路に供給され、情報記録再生信号やフォーカス誤差信号，トラッキング誤差信号が得られる。
【００６２】
なお、ＣＤディスク３の場合のトラッキング誤差信号は、ＣＤレーザ１２からのレーザビームを、回折格子１８により、０次光と土１次光の３ビームに分岐し、これら±１次光によりトラッキング誤差信号を得るようにしている。
【００６３】
このようにして得られたトラッキング誤差信号とフォーカス誤差信号とにより、ＤＶＤディスク２と同様にして、適正なフォーカス位置とトラッキング位置に対物レンズ１が位置するように、アクチュエータ１９を駆動する。
【００６４】
なお、本発明において、対物レンズ１の代わりに、コリメータレンズ１５あるいはハーフプリズム１４など両ディスクに共通する光学系において、本発明における対物レンズと同様の機能を持つように光学設計することもできる。また、図示しないが、本発明の対物レンズと同等の機能を有する他の光学要素をハーフプリズム１４からディスク２またはディスク３に至る光路に配置することによってもよい。
【００６５】
なお、コリメータレンズ１５は必ずしも必要ではなく、いわゆる有限系の光学系でも、本発明は適用可能である。
【００６６】
図９は本発明による対物レンズを用いた光ディスク装置の一実施形態を示す構成図であって、２０はアクチュエータ駆動回路、２１は信号処理回路、２２はレーザ駆動回路、２３はシステム制御回路、２４はディスク判別手段であり、図８に対応する部分には同一符号をつけている。
【００６７】
同図において、光ピックアップ装置部分については、図８に示す構成と同様である。
【００６８】
まず、装着されたディスクの種類をディスク判別手段２４により判別する。そのディスク判別方法としては、ディスクの基板の厚さを光学的もしくは機械的な方法で検出する方法、ディスクまたはディスクのカートリッジに予め記録された識別マークを検出する方法などが考えられる。もしくは、ディスクの厚さ，種類を仮定してディスクの信号を再生し、正常な信号が得られなければ、別の厚さ，種類のディスクであると判断する方法でもよい。ディスク判別結果は、ディスク判別手段２４からシステム制御回路２３に伝達される。
【００６９】
ＤＶＤディスクであると判別された場合には、システム制御回路２３よりレーザ駆動回路２２に対してＤＶＤレーザを点灯させるような信号が伝達され、レーザ駆動回路２２によりＤＶＤレーザ１１が点灯される。これにより、光ヘッドでは、図８に示した実施形態と同様二波長655ｎｍのレーザビームが光検出器１７に到達する。この光検出器１７からの検出信号が信号処理回路２１に送られて情報記録再生信号とフォーカス誤差信号とトラッキング誤差信号とが生成され、システム制御回路２３に送られる。システム制御回路２３では、これらフォーカス誤差信号とトラッキング誤差信号とに基づいて、アクチュエータ駆動回路２０を制御し、この制御に基づいてアクチュエータ駆動回路２０がアクチュエータ１９を駆動して対物レンズ１をフォーカス方向及びトラッキング方向に移動させる、いわゆるサーボ回路の動作により、フォーカス制御及びトラッキング制御が正規に行なわれて、対物レンズ１がＤＶＤディスク２に対して正しい位置に位置するように、上記の各回路及びアクチュエータ１９が動作するものとし、その結果、情報記録再生信号が良好に得られる。
【００７０】
装着されたディスクがＣＤディスク３であると判別された場合には、システム制御回路２３より、レーザ駆動回路２２に対してＣＤレーザ１２を点灯させるような信号が伝達される。これにより、ＣＤレーザ１２から波長790ｎｍのレーザビームが発生する。これ以降の動作は図８に光ヘッドの場合と同様であり、このレーザビームが光検出器１７に到達し、上記のＤＶＤディスク２の場合と同様に、各回路やアクチュエータ１９が作動してサーボ動作が行なわれ、情報記録再生信号が良好に得られる。
【００７１】
第２の実施の形態について、次のような点を見出すことができた。即ち、前述した波面収差を得るためのレンズ面形状としては、ＤＶＤ、ＣＤ複数種類の単色光をそれぞれの屈折作用により集光させる多波長用のレンズにおける少なくとも一方のレンズ面のうちの全ての単色光での共通使用領域において屈折率の異なる複数の非球面部同士の隣接段差量をレンズ光軸に近い順にＤ１２、Ｄ２３、Ｄ３４、Ｄ４５、・・・・、隣接部のＤＶＤ時のＮＡをレンズ光軸の近い順にＮＡ１２、ＮＡ２３、ＮＡ３４、ＮＡ４５、・・・・、隣接部のＣＤ時のＮＡをレンズ光軸に近い順にＣＮＡ１２、ＣＮＡ２３、ＣＮＡ３４、ＣＮＡ４５、・・・・、としたときに、下記数１４の式を満足するようなレンズ面形状とするのが良い。
【００７２】
【数１４】

【００７３】
数１４において、λ１は短い方の波長（ｍｍ）、λ２は長い方の波長（ｍｍ）、ｎ１は波長λ１におけるレンズの屈折率、ｎ２は波長λ２におけるレンズの屈折率である。数式におけるｍａは、ａｉｊに最も近い整数である。また、ｍｂは、ｂｉｊに最も近い整数である。換言すると、ｍａは、ｍａ−ａｉｊの絶対値が最小となる整数であり、また、ｍｂは、ｍｂ−ｂｉｊの絶対値が最小となる整数である。例えば、１．１０４の場合には「１」であり、０．４９の場合には「０」である。数１４において用いられている記号を図１３に示す。図１３に示されるように、この例では、レンズは、ＤＶＤ／ＣＤ共通使用領域と、ＤＶＤ専用領域を有している。そして、ＤＶＤ／ＣＤ共通使用領域は、▲１▼乃至▲６▼の６面の非球面部を有している。そして、これらの非球面部の段差をＤ１２、Ｄ２３・・・・として表している。ＤＶＤの場合における段差Ｄ１２間のＮＡをＮＡ１２と、段差Ｄ２３間のＮＡをＮＡ２３としている。ＣＤの場合における段差Ｄ１２間のＮＡをＣＮＡ１２と、段差Ｄ２３間のＮＡをＣＮＡ２３としている。数１４では、Ａｉｊ／Ｂｉｊは、０．３３３から３の範囲内にあるが、さらに好ましい範囲は、０．５から２の範囲である。
【００７４】
尚、この例における具体的な数値を表５に纏める。
【００７５】
【表５】

【００７６】
【発明の効果】
以上説明したように、本発明によれば、透明基板の厚さが異なる２種類以上の光ディスクに対して、回折レンズ構造を用いずに、屈折作用によって記録または再生に必要な開口（ＮＡ）で全ての光束を所望とする位置に可及的に少ない収差で集光させることができ、光利用効率をより高めることができる。また、上記説明から分かるように、本発明は複数の単色光を用いる多波長用光学系において、分割されたいずれの非球面も各単色光の固有の波長に対応した単一の焦点を有するとともに、前記各単色光の固有の波長に対応した焦点はそれぞれ異なる位置に配置することができ、光通信等における光学系においてもその利用が可能である。
【図面の簡単な説明】
【図１】本発明による対物レンズの第１の実施形態を示す図である。
【図２】図１に示す第１の実施形態のレンズ面形状の一具体例を示す図である。
【図３】対物レンズと光ディスクの透明基板とからなる光学系での光路長を説明するための図である。
【図４】図１に示す第１の実施形態の第１の実施形態の波面収差の測定結果の一具体例を示すグラフ図である。
【図５】図１に示す第１の実施形態を用いた光ディスク装置での種類が異なる光ディスクに対する光スポットの計算結果を示す図である。
【図６】本発明による対物レンズの第２の実施形態の波面収差の測定結果の一具体例を示すグラフ図である。
【図７】本発明による対物レンズの第２の実施形態を用いた光ディスク装置での種類が異なる光ディスクに対する光スポットの計算結果を示す図である。
【図８】本発明による光ヘッドの一実施形態を示す図である。
【図９】本発明による光ディスク装置の一実施形態を示す図である。
【図１０】光線高さに対する各波長の波面収差を示す模式図である。
【図１１】実施例２における光線高さに対する各波長の波面収差を示す図である。
【図１２】特開２００１−５１１９２号に記載のレンズを用いた場合の光線高さに対する各波長の波面収差を示す図である。
【図１３】実施例２における数式を説明するための図である。
【符号の説明】
１ 実施形態の対物レンズ
２ ＤＶＤの透明基板
２ａ 情報記録面
３ ＣＤの透明基板
３ａ 情報記録面
４，５ レーザビーム
１１ ＤＶＤレーザ
１２ ＣＤレーザ
１３，１４ ハーフプリズム
１５ コリメータレンズ
１６ 検出レンズ
１７ 光検出器
１８ 回析格子
１９ アクチュエータ
２０ アクチュエータ駆動回路
２１ 信号処理回路
２２ レーザ駆動回路
２３ システム制御回路
２４ ディスク判別手[0001]
BACKGROUND OF THE INVENTION
    The present invention is a multi-wavelength optical system that uses a plurality of types of monochromatic light. For example, the present invention can be applied to different types of optical recording media such as CDs (compact discs including CDs such as CD-R) and DVDs (digital versatile discs). Can be used in compatible compatible recording and playback devicesObjective lensAbout.
[0002]
[Prior art]
Conventionally, compatible optical disk devices have been proposed that can play back together optical disks of different types, such as CDs and DVDs. A CD or DVD (hereinafter collectively referred to as an optical disk) uses a transparent substrate, and an information recording surface is provided on one surface of the transparent substrate. The optical disk has a structure in which two transparent substrates are bonded together with their information recording surfaces facing each other, or such a transparent substrate is a transparent protective substrate, and the information recording surface of the transparent substrate is a protective substrate. It is configured to be pasted so as to face each other. When reproducing the information signal stored in the optical disk having such a configuration, it is necessary to focus the laser beam from the light source on the information recording surface of the optical disk via a transparent substrate by the optical disk device. The wavelength of the laser beam differs depending on whether it is used on a CD or a DVD as described later. In order to focus the laser beam, an objective lens is used in the optical disc apparatus. Here, the thickness of the transparent substrate used in the CD is 1.2 mm, whereas the thickness of the transparent substrate used in the DVD is 0.6 mm, which depends on the type of optical disk (difference in laser beam wavelength). The thickness of the transparent substrate provided with the information recording surface is different. In an optical disc apparatus that reproduces optical discs of different types, it is necessary to focus the laser beam on the information recording surface even if the thickness of the transparent substrate varies depending on the type of optical disc. In addition, it has been proposed that a new optical disk apparatus that has been recently proposed uses a blue laser having a wavelength of about 400 nm for information reproduction. Therefore, it is expected that such a new optical disc can be used simultaneously with the optical disc apparatus in addition to the CD and the current DVD for backward compatibility.
[0003]
As such a compatible optical disc device, an objective lens is provided for each type of optical disc in the pickup, and the objective lens is exchanged according to the type of the optical disc to be used, or a pickup is provided for each type of optical disc. It is conceivable to change the pickup depending on the type. However, in order to realize cost reduction and downsizing of the apparatus, it is desirable that the same lens can be used for any kind of optical disk as the objective lens.
[0004]
As a representative example of such an objective lens, there is one described in Patent Document 1. The objective lens described in this document is divided into three or more annular lens surfaces in the radial direction, and every other annular lens surface and the other annular lens surfaces have different refractive powers. ing. Then, for each laser beam having the same wavelength, every other annular lens surface condenses the laser beam onto the information recording surface of an optical disk (DVD) on a thin transparent substrate (0.6 mm), for example. The laser beam is condensed on the information recording surface of the optical disk (CD) of the thick transparent substrate (1.2 mm), for example, by the annular ring-shaped lens surface.
[0005]
Another typical example is described in Patent Document 2 or 3. These documents use a short wavelength (635 nm or 650 nm) laser beam for a thin transparent substrate DVD and a long wavelength (780 nm) laser beam for a thick transparent substrate CD. An optical disc device is disclosed. This optical disc apparatus has an objective lens commonly used for these laser beams. In this objective lens, a diffractive lens structure is formed in which minute steps of an annular zone are densely provided on one surface of a refractive lens having a positive power. Such a diffractive lens structure condenses the diffracted light of the short wavelength laser beam on the DVD of the thin transparent substrate and the diffracted light of the long wavelength laser beam on the information recording surface of the CD of the thick transparent substrate. Designed. Each diffracted light is designed to collect the same order of diffracted light on the information recording surface. The reason for using a laser beam with a short wavelength for DVD is that the recording density of DVD is higher than that of CD, and it is therefore necessary to narrow down the beam spot. As is well known, the size of the light spot is proportional to the wavelength and inversely proportional to the numerical aperture NA.
An objective lens of an annular phase correction lens system in which an annular phase shifter is provided on the lens surface has also been proposed (see, for example, Patent Document 4). In this objective lens, first, the wavelength λ used for the DVD is used.₁Is based on a lens surface that eliminates wavefront aberration caused by a 640 nm laser beam. Further, this objective lens is divided into a plurality of annular refracting surfaces in the radial direction, and these refracting surfaces are respectively separated from the reference lens surface by a predetermined step (i-th step from the lens center is d)._iAnd). Such step d_iDue to the respective refracting surfaces, the DVD laser beam has this wavelength λ with respect to the reference lens surface.₁An integer m_iBy shifting the phase by twice, the wavefront aberration of the CD system is reduced.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 9-145995
[Patent Document 2]
JP 2000-81666 A
[Patent Document 3]
US Pat. No. 6,118,594
[Patent Document 4]
JP 2001-51192 A
[0007]
[Problems to be solved by the invention]
In any of the above conventional examples, since a common objective lens can be used for both DVD and CD, means for exchanging members used for each DVD and CD including the objective lens becomes unnecessary, and the cost and configuration are reduced. This is advantageous in terms of simplification.
However, in Patent Document 1 (Japanese Patent Application Laid-Open No. 9-145959), since the annular lens surface used for the objective lens is different for each DVD and CD, there are many ineffective portions with respect to the incident laser beam, and light utilization is performed. There is a problem that the efficiency is remarkably low.
[0008]
Further, in Patent Document 2 or 3 (Japanese Patent Laid-Open No. 2000-81566, US Pat. No. 6,118,594), since the diffracted light by the diffractive lens structure is used, the diffraction efficiency for each of different wavelengths is simultaneously set to 100%. There is a problem that you can't. In this diffractive lens, the diffraction efficiency is 100% at a wavelength almost intermediate between the short wavelength (635 nm or 650 nm) laser beam used for DVD and the long wavelength (780 nm) laser beam used for CD. Thus, the diffraction efficiency is balanced with respect to the used laser beam. In addition, since a diffractive lens structure is provided on the lens surface, a minute step is required, but it is easily affected by manufacturing errors, and if the diffractive structure deviates from the design, the diffraction efficiency is deteriorated. As described above, the deterioration of the diffraction efficiency or the fact that the diffraction efficiency does not reach 100% means that all of the incident light cannot be condensed on the information recording surface provided on the transparent substrate of the optical disk. This is a light loss.
[0009]
Furthermore, in the annular phase correction lens system disclosed in Patent Document 4 (Japanese Patent Laid-Open No. 2001-51192), the lens surface is designed to eliminate wavefront aberration with respect to the DVD laser beam, although the light utilization efficiency is high. From this reference plane, so that the wavefront aberration for the CD laser beam is reduced.₁An integer m_iDouble step d_iIt is only dented to make a refracting surface. Further, at this time, since the condensing point position in each annular zone is shifted by the formation of the step due to the formation of the step, the curved surface shape of each annular zone is designed so as not to shift the condensing point position. However, in the conventional example of Document 4, the wavefront aberration of the DVD can be sufficiently reduced, but the wavefront aberration cannot be sufficiently reduced with respect to the CD laser beam. It is described in many prior arts that the wavefront aberration value may be 0.07λ RMS or less of the Marechal evaluation standard as the RMS wavefront aberration value. However, in the case of an optical disc apparatus, 0.07λ RMS or less is a value that should be a target value for the entire optical disc apparatus, and is still insufficient as a target value for the objective lens alone. The optical disk device as a whole has many factors that degrade the RMS wavefront aberration, such as the laser astigmatism, the aberration of the collimator lens, the aberration of the reflection mirror and the transmission mirror, and the tilt shift between the optical pickup and the optical disk. Therefore, it is required that the RMS wavefront aberration value be as small as possible, not 0.07λ RMS or less. Specifically, it is desirable that the objective lens alone has an RMS wavefront aberration value of about 0.02λ RMS. However, in Patent Document 4, in Example 1, DVD is 0.001λ RMS and CD is 0.047λ RMS. 2, DVD is 0.019λRMS and CD is 0.037λRMS, and the wavefront aberration of DVD is a good value, but it is only 0.037λRMS or more for CD, and both DVD and CD are good. The wavefront aberration value has not been obtained.
[0010]
  An object of the present invention is to solve such a problem and to reduce the wavefront aberration as much as possible for each of a plurality of types of optical recording media having different transparent substrate thicknesses and to achieve high light utilization efficiency. The beam can be focused on the information recording surface.Objective lensIs to provide.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, a light beam having a different wavelength is incident on each of a plurality of types of optical recording media having different transparent substrate thicknesses, and the light beam is condensed on the optical recording medium side by refraction. In the design method of an objective lens having a positive power, the lens surface is designed so as to almost cancel out the spherical aberration caused by the difference in the thickness of the transparent substrate of the optical recording medium due to the chromatic aberration caused by the difference in the wavelength of the light beam. Invented a method.
[0012]
  Further, according to this, the present invention provides a monochromatic light beam having a different wavelength via a transparent substrate for each of a plurality of types of optical recording media having different transparent substrate thicknesses.CondensationOn the information recording surface provided on the transparent substrate of the optical recording medium.Form a light spotAn objective lens having positive power,The exit surface consists of a single aspheric surface, and the entrance surface isA lens surface region through which light beams of different wavelengths focused on information recording surfaces of different types of optical recording media pass is divided into a plurality of aspherical portions in the radial direction from the optical axis. The difference between the maximum value and the minimum value of the wavefront aberration of each monochromatic light condensed corresponding to the optical recording medium is expressed as ΔVd (λi) (d is an integer of 1, 2,... And i is an integer of 1, 2,..., In any aspherical portion, the light is condensed under the condition that the ratio of the differences of each monochromatic light satisfies 0.4 or more and 2.5 or less. The aspherical lens surface shape that cancels out the chromatic aberration caused by the difference in the wavelength λ of the light beam and the spherical aberration caused by the difference in the thickness of the transparent substrate of the optical recording medium is formed on the aspherical part. An objective lens characterized by being set is provided.
  Here, each of the aspherical portionsInsideSo that the difference (ΔVd (λi)) between the maximum value and the minimum value of the wavefront aberration of each monochromatic light at 0.14λi (i is an integer of 1, 2,...) Or less is collected.SaidIt is preferable to set the lens surface shape of the aspheric part.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Now thickness t₁In the first optical disk using the transparent substrate, the objective lens in the optical disk apparatus using the first optical disk is corrected for aberrations, and the laser beam is focused on the information recording surface provided on the substrate. . Such an optical disc apparatus has a thickness t different from that of the transparent substrate.₂When the second optical disk using the transparent substrate is used, the thickness t of the transparent substrate is₂Is thickness t₁This objective lens and thickness t₂A spherical aberration is caused by the transparent substrate, and this thickness t₂The laser beam does not converge well on the information recording surface provided on the transparent substrate.
[0014]
On the other hand, when laser beams having different wavelengths are used in an optical system composed of such an objective lens and a transparent substrate, chromatic aberration occurs. Here, chromatic aberration refers to a difference in spherical aberration that occurs corresponding to each laser beam when the objective lens is irradiated with laser beams having different wavelengths. For example, when the objective lens is irradiated with a laser beam with a wavelength of 655 nm and a laser beam with a wavelength of 790 nm, the chromatic aberration is the spherical aberration that occurs when the objective lens is irradiated with the laser beam with a wavelength of 655 nm, and the laser beam with a wavelength of 790 nm is the objective. It is the difference in spherical aberration that occurs when the lens is irradiated. The present invention reduces spherical aberration caused by the above-described difference in substrate thickness by utilizing such chromatic aberration. That is, a laser beam having a different wavelength is used for each optical disc having a different substrate thickness, and the spherical aberration caused by the difference in the substrate thickness is canceled by the chromatic aberration caused by the difference in the wavelength of the laser beam. In contrast, the overall aberration is within the allowable range.
That is, the thickness is t₁Spherical aberration in the case of S_A(T₁), Thickness is t₂Spherical aberration in the case of S_A(T₂), And the spherical aberration generated with respect to the laser beam of wavelength λ1 is S_A(Λ₁), Wavelength λ₂Spherical aberration that occurs for a laser beam of S_A(Λ₂), The chromatic aberration due to the difference in wavelength is the difference between the spherical aberrations (S_A(Λ₂-S_A(Λ₁)). At this time, in the present invention, the lens surface is designed so that the following mathematical formula is established as much as possible.
[Equation 3]

[0015]
This means that for any optical disc having a different substrate thickness, when a laser beam having a wavelength corresponding to the thickness of the substrate is used, all the light beams that have passed through the objective lens and the substrate of the laser beam are transmitted. The optical path length is such that the light is well condensed on the information recording surface of the substrate.
At this time, the lens according to the embodiment of the present invention has a lens surface divided into a plurality of aspheric surfaces, as will be described in detail in the following examples. It has a single focal point corresponding to the specific wavelength of light, and the focal points corresponding to the specific wavelength of each monochromatic light are designed to be arranged at different positions.
[0016]
Now, with reference to FIG. 3, a case where a laser beam is focused on the information recording surface 2a of the substrate 2 using the objective lens 1 will be described. Here, the surface A of the objective lens 1 is a light incident side surface, the surface B is a light emission side surface, and the information recording surface 2a of the substrate 2 is on the side opposite to the objective lens 1 side.
[0017]
In FIG. 3, the laser beam incident on the objective lens 1 is parallel light (the optical system shown in FIG. 3 is a so-called infinite optical system), and the distance from the optical axis OA of the objective lens 1 in the direction perpendicular thereto. (Light height) h position P₁Point (condensing point) P where the light beam passing through the optical axis OA crosses the optical axis OA_FiveThis schematically shows the optical path to reach. Here, the incident point to the objective lens 1 in this optical path is P₂, P is the exit point from the objective lens 1_Three, P is the incident point on the transparent substrate 2_Fourage,
Point P₁~ Incident point P₂: Spatial distance = S_1h    Refractive index = n₁
Incident point P₂~ Exit point P_Three: Spatial distance = S_2h    Refractive index = n₂
Output point P_Three~ Incident point P_Four: Spatial distance = S_3h    Refractive index = n_Three
Incident point P_Four~ Focusing point P_Five: Spatial distance = S_4h    Refractive index = n_Four
Then point P₁To condensing point P_FiveOptical path length L to_hIs
[Expression 4]

It is represented by The optical path length L on the optical axis OA_hIs the case where h = 0 in this equation (4).
[0018]
This equation 4 corresponds to an arbitrary light beam height h, and when aberration correction is performed, the condensing point P for each light beam height h._FiveAre on the information recording surface 2a within the respective allowable ranges. That is, according to the present invention, for example, by using laser beams having different wavelengths for each of a plurality of substrates having different thicknesses, the chromatic aberration and the spherical aberration cancel each other, and the condensing point P with respect to each ray height h._FiveAre on the information recording surface 2a within the respective allowable ranges.
For example, 790 nm monochromatic light (λ₁) And 655 nm monochromatic light (λ₂) Is used, and a method of using a lens surface in which a region in which both wavelengths are used in common is divided into a plurality of aspherical portions, the optical path length of any of the aspherical portions is other aspherical portions. And the difference between the maximum value and the minimum value of the wavefront aberration of each monochromatic light at each aspherical surface portion is made ΔVd (λ₁) And ΔVd (λ₂) (D is an integer of 1, 2,..., Meaning each aspherical portion), and the ratio of the difference of each monochromatic light is 0.4 to 2.5 in any aspherical portion. Preferably, by setting it to 0.5 or more and 2.0 or less, the RMS wavefront aberration within the allowable range can be secured as a whole lens at both wavelengths. The wavefront aberration referred to here is the optical path length L when the ray height (h) is h = 0.₀Assuming that the optical path length at each light beam height is Lh, the wavefront aberration Vh is expressed by Equation 5.
[Equation 5]

FIG. 10 schematically shows the wavefront aberration of the lens at the wavelengths of CD and DVD, with the horizontal axis indicating the ray height, the vertical axis indicating the wavefront aberration, and the upper side of each aspherical portion of the CD. The lower side represents the wavefront aberration obtained by the above formula of each aspherical portion of the DVD. For example, the difference between the maximum value and the minimum value of the wavefront aberration in the aspheric surface in the first region of the aspheric surface is ΔV₁(Λ₁) 、 △ V₁(Λ₂). In the present invention, as will be clarified in the following examples, the ratio of the difference between the maximum value and the minimum value of the wavefront aberration at each wavelength is 0.4 or more and 2.5 or less in any aspherical portion. . That is, in the present invention, the lens surface is configured based on one of the conventional wavelengths and the phase shift is used only at the other wavelength even in the point that each aspheric surface has a constant distribution of wavefront aberration at any wavelength. This is different from the method of correcting wavefront aberration. The integer multiple is preferably 0 times to ± 10 times, more preferably 0 times to ± 5 times, although it depends on the number of aspheric surfaces to be divided.
Further, in the multi-wavelength lens of the present invention, the difference between the maximum value and the minimum value of the wavefront aberration at each wavelength is 0.14λi or less (for example, when the wavelength is 790 nm in each region of any aspherical surface, ± 110.6 nm or less, and when the wavelength is 655 nm, ± 91.7 nm or less), preferably 0.12 λi or less, more preferably 0.10 λi or less to ensure better optical characteristics at each wavelength. be able to.
Furthermore, in the present invention, in the case of a two-wavelength optical system, by using a multi-wavelength lens in which the wavefront aberrations of the respective wavelengths are almost symmetrical, the two wavelengths are balanced, and the RMS wavefront aberration is further reduced. be able to.
In consideration of reducing the RMS wavefront aberration, in the case of a CD, the RMS wavefront aberration is determined from the wavefront aberration of only the common use region of DVD and CD up to a ray height of 1.58 mm in FIG. In the case of a DVD, there is a DVD dedicated area (in the range of 1.58 to 2.02 mm in ray height in FIG. 10) outside the common use area, and from the wavefront aberration of both the common use area and the dedicated area. RMS (Root Mean Square) wavefront aberration value is obtained. Therefore, in the case of a DVD, even if the wavefront aberration in the common use area is somewhat worse, the wavefront aberration in the DVD dedicated area can be improved by ignoring the CD at all and improving only the DVD. Can be sufficiently reduced within an allowable value. For example, in the schematic diagram of FIG. 10, the wavefront aberration of DVD is 0 to −0.106λ and the wavefront aberration of CD is 0 to +0.088 in the common use region of DVD and CD. Is smaller than the wavefront aberration of DVD. The wavefront aberration in the DVD-dedicated region is -0.052λ. As a result, the RMS wavefront aberration is 0.0212λRMS for DVD and 0.0222λRMS for CD, and the RMS wavefront aberration is almost equal for both DVD and CD. As described above, when it is desired to set the same value for both the DVD and the CD as the RMS wavefront aberration, the CD wavefront aberration is made better than the DVD wavefront aberration in the common use region of the DVD and CD. For the wavefront aberration, it is effective to compensate for the deterioration in the common use area in the DVD dedicated area. Similarly, in the case where it is desired to change the ratio of the RMS wavefront aberration of DVD and CD, it may be considered that the DVD can be compensated for in the dedicated area even if the wavefront aberration in the common use area is somewhat worse.
According to the embodiment of the present invention, it is possible to form a good light spot on the information recording surface, for example, for any optical disc having a different substrate thickness. Note that this is not necessary even if the thickness of the disk substrate is different, that is, even when the thickness is the same and the wavelength is different._FiveCan be applied by setting each within the allowable range. Further, the present invention is not limited to an optical recording medium, and can also be applied to cases where laser beams having different wavelengths are passed through the same lens or optical system in optical communication or the like.
[0019]
Hereinafter, an embodiment of the present invention will be described with reference to the drawings, taking two types of optical disks having different transparent substrate thicknesses, that is, a DVD and a CD as an example. In addition, the lens of 1st Embodiment of this invention produced the resin which consists of amorphous polyolefin by injection molding from the ease of manufacture. The lens according to the second embodiment has a refractive index of glass. However, when the lens material is a plastic resin, the lens may be designed with the refractive index of the plastic resin.
[0020]
FIGS. 1A and 1B are views showing the operation of the first embodiment of the objective lens according to the present invention. FIG. 1A is for a DVD and FIG. 1B is for a CD. In the figure, 1 is the objective lens of this embodiment, 2 is a DVD transparent substrate (hereinafter referred to as a DVD substrate), 3 is a CD transparent substrate (hereinafter referred to as a CD substrate), and 4 and 5 are laser beams.
[0021]
First, in FIG. 1A, an objective lens 1 is provided in an optical head of an optical disk device (not shown). Then, the DVD is mounted on the optical disk apparatus, and the laser beam 4 incident as parallel light is condensed by the objective lens 1 to perform recording / reproduction. Here, the thickness t of the DVD substrate 2₁Is 0.6 mm, and the laser beam 4 at this time has a wavelength λ₁= 655 nm laser beam is used as the luminous flux with numerical aperture NA = 0.63. Under such conditions, the laser beam is focused on the information recording surface 2a on the opposite side of the DVD substrate 2 from the objective lens 1 side.
[0022]
FIG. 1B shows a case where a CD is mounted on the same optical disk apparatus as described above, and recording / reproduction is performed using the same objective lens 1. Here, the thickness t of the CD substrate 3₂Is 1.2 mm, and the laser beam 5 at this time has a wavelength λ₂= 790 nm laser beam is used as a light beam with a numerical aperture NA = 0.63, but substantially a light beam with a numerical aperture NA = 0.47 is condensed on the information recording surface 3a of the CD substrate 3 and is shown by hatching. A light beam passing through a portion of the objective lens 1 with NA = 0.47 to 0.63 away from the optical axis OA is not condensed on the information recording surface 3a. As described above, the lens area having a numerical aperture NA up to about 0.47 is a common use area for DVD and CD.
[0023]
As described above, in the first embodiment, aberrations are reduced well for both DVD and CD, and a good light spot is obtained on the information recording surfaces 2a and 3a. For both DVD and CD, the optical path length L shown in the above equation 4 with respect to an arbitrary beam height h._hIs to set the lens surface shape of the objective lens 1 so that the aberration is reduced within the allowable value. Hereinafter, a specific example of the lens surface shape will be described with reference to FIG.
[0024]
In FIG. 2, regarding the light exit side B of the objective lens 1, if the point of the light beam height h is c, and the point on the light exit side B in a direction parallel to the optical axis OA from this point c is d, The surface shape of the exit side surface B is the distance Z between the points c and d with respect to an arbitrary ray height h._BBy
[Formula 6]

However, C = -0.12301, K = 3.312138, A_Four= 0.01628151, A₆= -0.004311717, A₈= 0.000682316, A_Ten= -0.00004157469.
[0025]
In Equation 6, the coefficients C, K, A_Four, A₆, A₈, A_TenThe distance Z for any ray height h (≠ 0) by substituting the value of_BIs a negative value. This is because the point d on the light emission side surface B is the point c, and therefore the light emission side surface B is more than the surface vertex e through which the optical axis OA of the light emission side surface B passes ( It is located on the left side in FIG. Distance Z_BIs a positive value, it indicates that it is located on the opposite right side.
[0026]
Next, with respect to the light incident side A of the objective lens 1, the point of the light beam height h is a, and the point on the light incident side A surface in a direction parallel to the optical axis OA from this point a is b. The surface shape of the side surface A is a light ray height h (mm) and a distance Z between points a and b with respect to the light ray height h._A(Mm) is set to the lens surface shape having the relationship shown in Table 1 below.
[0027]
[Table 1]

The light emission side surface B represented by the above equation 6 of the objective lens 1 and the light incident side surface A represented by the point sequence data of Table 1 also form a continuous aspherical surface. Further, the distance between the surface vertices f and e on the optical axis of the objective lens 1, that is, the center thickness t₀Is 2.2 mm and wavelength λ₁= Refractive index n at 655 nm (DVD) is 1.54014, wavelength λ₂= Refractive index n at 790 nm (CD) is 1.5365.
[0028]
(I) Here, as an allowable value of the aberration for evaluating the aberration, the incident laser beam to the objective lens 1 has an incident angle of 0 ° (that is, parallel light parallel to the optical axis OA). DVD (wavelength λ₁= 655nm), CD (wavelength λ₂In both cases, the RMS wavefront aberration is 0.035λ, preferably 0.033λ, and more preferably 0.030λ. In the first embodiment, the light exit surface B and the light entrance surface A are set to the above-described surface shapes so that the wavefront aberration of DVD and CD is less than the allowable value.
[0029]
In this first embodiment, two different wavelengths λ₁, Λ₂In general, there are n types of different wavelengths λ (where n is an integer of 2 or more)._iThe same applies when (where i = 1, 2,..., N) is used.
[0030]
(ii) In addition, n types of wavelengths λ_iFor the wavelength λ_iRMS wavefront aberration when the incident laser beam is incident angle 0 °, W_i・ Λ_iThen, these aberrations are
[Expression 7]

(However, the wavelength of the i-th light beam is λ_i(I = 1, 2,...), ΣW is the sum of squares of individual RMS wavefront aberrations over all wavelengths._i ², Wavelength λ_iRMS wavefront aberration of the light beam of W_i・ Λ_iAnd)
To be satisfied. Allowable value W at this time₀Is 0.028, preferably 0.026, more preferably 0.025, and still more preferably 0.023. In the first embodiment, the RMS wavefront aberration of DVD is set to W₁, W RMS wavefront aberration of CD₂And i = 1, 2, the above formula 7 is
[Equation 8]

It becomes.
[0031]
(iii) or n different wavelengths λ_iEach of the wavelengths λ_iThe maximum RMS wavefront aberration is Wmax and the minimum RMS wavefront aberration is Wmin.
1 ≦ Wmax / Wmin <W_th
And Allowable value W in this case_thIs 1.8, preferably 1.6, and more preferably 1.4. In the case of the first embodiment, the RMS wavefront aberration W of the DVD₁And CD RMS wavefront aberration W₂Is the maximum RMS wavefront aberration Wmax, and the other is the minimum RMS wavefront aberration Wmin.
[0032]
FIG. 4 shows the calculation result of the RMS wavefront aberration in the first embodiment. The horizontal axis represents the image height (mm), and the vertical axis represents the RMS wavefront aberration.
[0033]
4A shows a DVD (wavelength λ₁RMS wavefront aberration with respect to 655 nm), and when the image height = 0 mm, the RMS wavefront aberration = 0.02130λ.₁It is. FIG. 4B shows the CD (wavelength λ).₂RMS wavefront aberration with respect to 790 nm), and when the image height = 0 mm, the RMS wavefront aberration = 0.02410λ.₂It is.
[0034]
In order to evaluate such a numerical value, if inserted into the above conditional expressions,
(I) First, for DVD and CD, the RMS wavefront aberration is 0.02130λ, 0.02410λ, and the allowable value of 0.035λ, preferably 0.033λ, and more preferably smaller than 0.030λ.
[0035]
(Ii) For DVD and CD,
[Equation 9]

Therefore, the allowable value is 0.028, preferably 0.026, more preferably 0.025, and further preferably 0.023 or less.
[0036]
(iii) For DVD and CD, looking at Wmax / Wmin,
Wmax / Wmin = 0.02410 / 0.02130 = 1.1315
Therefore, the allowable value is 1.8, preferably 1.6, and more preferably 1.4 or less.
[0037]
FIG. 5 shows a light spot on the information recording surface of a DVD or CD by using the objective lens 1 having the light emitting side surface B having the surface shape shown in Equation 6 and the incident side surface A having the surface shape shown in Table 1 above. The horizontal axis represents the position in the direction perpendicular to the optical axis with respect to the optical axis on the information recording surface as a reference point, and the vertical axis represents this reference point. The relative light intensity at each position when the light intensity at (= 0 mm) is 1 is shown.
[0038]
FIG. 5A shows a light spot with respect to a DVD, and the relative light intensity is 1 / e.²Light spot diameter φ (= 13.5%)_DIs 0.85 μm. FIG. 5B shows a light spot with respect to the CD, and the relative light intensity is 1 / e.²Light spot diameter φ_CIs 1.37 μm. Thus, a good light spot can be obtained on the information recording surface for both DVD and CD.
[0039]
Next, a second embodiment of the objective lens according to the present invention will be described.
[0040]
The basic configuration of the second embodiment is the same as that of the first embodiment, but the light incident surface A is divided into a plurality of sections in the radial direction from the optical axis, and the surface shape of each section is as follows. Are set so that the aberration is reduced well within the allowable range for both DVD and CD.
[0041]
The surface shape of the light incident surface A of the second embodiment will be described with reference to FIG. Now, the distance between the points a and b in the j-th section from the optical axis OA side in the light beam height h direction (radial direction) of the light incident surface A is expressed by the following function Z._AjIn other words,
[Expression 10]

It is represented by Note that the light source height h in Equation 10 is for the j-th section.
[0042]
For both sections DVD and CD, the range (range h) and its constants B, C, K, A for each section of Equation 10 for reducing aberrations well within an allowable value._Four, A₆, A₈, A_Ten, A₁₂, A₁₄, A₁₆Is shown in Table 2 below.
[0043]
[Table 2]

In addition, the surface shape Z of the light exit surface B in the second embodiment_BIs represented by the following equation (11).
[0044]
## EQU11 ##

However, C = -0.0747792, K = 15.7398, A_Four= 0.012308, A₆= -0.0037652, A₈= 0.00068571, A_Ten= -0.000048284
Further, the distance between the surface vertices f and e on the optical axis of the objective lens 1, that is, the center thickness t.₀Is 2.2 mm and wavelength λ₁= Refractive index n at 655 nm (DVD) is 1.604194, wavelength λ₂= Refractive index n at 790 nm (CD) is 1.599906.
[0045]
Here, the allowable value of the aberration for evaluating the aberration is the same as that in the first embodiment.
[0046]
FIG. 6 shows the calculation result of the RMS wavefront aberration in the second embodiment, and the horizontal axis and the vertical axis are the same as those in FIG.
[0047]
FIG. 6A shows a DVD (wavelength λ₁RMS wavefront aberration with respect to (= 655 nm), and when the image height = 0 mm, the RMS wavefront aberration = 0.01945λ.₁It is. FIG. 6B shows a CD (wavelength λ).₂RMS wavefront aberration with respect to 790 nm), and when the image height is 0 mm, the RMS wavefront aberration is 0.02525λ.₂It is.
FIG. 11 shows the result of calculating the wavefront aberration in the common use region of the above-mentioned lens, and Table 3 shows the difference in wavefront aberration and the ratio thereof in each aspheric surface.
[Table 3]

As shown in Table 3, the ratio ΔVd (λ790) / ΔVd (λ655) of the difference between the wavefront aberrations in the common use region of 790 nm and 655 nm is between 1.00 and 1.04. Further, the ratio ΔVd (λ655) / ΔVd (λ790) is between 0.96 and 1.00. The wavefront aberration of each region is also 0.14λ or less at both wavelengths. In this lens, the wavefront aberration appears on the + side at a wavelength of 790 nm and on the − side at a wavelength of 655 nm, and both wavefront aberrations are substantially symmetrical.
Note that there is a difference in optical path length between adjacent aspherical parts divided around the optical axis, and the difference is designed to be an integer multiple corresponding to each wavelength. In the embodiment, it consists of an even number of divided aspheric surfaces.
[0048]
In order to evaluate such numerical values, as in the first embodiment, when inserted into the above conditional expressions,
(I) First, for DVD and CD, the RMS wavefront aberration is 0.01945λ.₁, 0.02525λ₂And the allowable value of 0.035λ, preferably 0.033λ, and more preferably smaller than 0.030λ.
[0049]
(Ii) For DVD and CD,
[Expression 12]

Therefore, the allowable value is 0.028, preferably 0.026, more preferably 0.025, and further preferably 0.023 or less.
[0050]
(iii) For DVD and CD, looking at Wmax / Wmin,
Wmax / Wmin = 0.02525 / 0.01945 = 1.298
Therefore, the allowable value is 1.8, preferably 1.6, and more preferably 1.4 or less.
[0051]
FIG. 7 shows an information recording surface of a DVD or CD obtained by using the objective lens 1 having the light emitting side surface B having the surface shape shown in the above equation 11 and the light incident side surface A having the surface shape shown in the above equation 10 and Table 2. The horizontal axis and the vertical axis are the same as those in FIG. 5.
[0052]
FIG. 7A shows a light spot with respect to a DVD, and the relative light intensity is 1 / e.²Light spot diameter φ (= 13.5%)_DIs 0.89 μm. FIG. 7B shows a light spot with respect to the CD, and the relative light intensity is 1 / e.²Light spot diameter φ_CIs 1.30 μm. Thus, a good light spot can be obtained on the information recording surface for both DVD and CD.
In the second embodiment, the ratio in Table 3 is 0.96 to 1.04, and the RMS wavefront aberration is 0.01945λ for DVD.₁And CD is 0.02525λ₂However, as described in the explanation of FIG. 10, if the wavefront aberration of the DVD in the common use region is further deteriorated to improve the wavefront aberration of the CD, the RMS wavefront aberration of both DVD and CD is 0.022 to An equivalent RMS wavefront aberration of about 0.023λ may be used.
[0053]
As an example, looking at the RMS wavefront aberration of DVD and CD described in JP-A-2001-51192,
Example 1) DVD: 0.001λ₁        CD: 0.047λ₂
Example 2) DVD: 0.019λ₁        CD: 0.037λ₂
Where λ₁= 640nm λ₂= 780nm
In any case, the CD exceeds the above allowable value of 0.035λ.
Further, when the wavefront aberration at each wavelength of the lens of Example 2) is obtained by calculation using the lens data described in the publication, the ratio is 0.03 to 33.44 as shown in Table 4 and FIG. It is outside the scope of the present invention, and therefore the balance between the two is shifted. Further, the wavefront aberration on the DVD side is 0.14λ or less, but the wavefront aberration on the CD side becomes large, and the RMS wavefront aberration of the entire lens also becomes large.
[Table 4]

[Formula 13]

Is 0.0332, 0.0294 for each of the above values, both of which exceed the above allowable value of 0.028, preferably 0.026, more preferably 0.025, and even more preferably 0.023._max/ W_minRespectively, 47 and 1.847, both of which exceed the allowable value 1.8, preferably 1.6, and more preferably 1.4.
[0054]
As described above, in both the first and second embodiments, the aberration can be suppressed within the allowable value. However, this is because the difference in the substrate thickness is such that the aberration is within the allowable value. This is due to the lens surface shape in which spherical aberration and chromatic aberration cancel each other. On the other hand, in Japanese Patent Laid-Open No. 2001-51192, the CD aberration is reduced by simply shifting the phase of the incident laser beam by an integral multiple of the wavelength of the DVD laser beam. Even if the aberration can be suppressed to a sufficiently small value for one wavelength, the aberration cannot be simultaneously accommodated within the allowable value of the small value as described above for all wavelengths.
[0055]
In the above embodiment, the spherical aberration due to the difference in substrate thickness between 0.6 mm and 1.2 mm for DVD and CD is canceled by the chromatic aberration due to the difference in wavelength between 655 nm and 790 nm, and the total aberration is reduced. This is apparent from the light spot shown in FIGS. 5 and 7 and the wavefront aberration graphs shown in FIGS. In the above embodiment, the surface shape of the light incident side surface A of the objective lens 1 is given by the point sequence data shown in Table 1 above, Equation 10 and Table 2, and the surface shape of the light exit side surface B is expressed by Equation 6 above. Since it is given by the aspherical formula shown in Equation 11, the diffractive lens structure as in the prior art is not used, and almost all the light flux is collected with respect to the aperture (NA) necessary for recording or reproduction. Since light can be emitted, high light utilization efficiency can be obtained.
[0056]
In the above embodiment, as shown in FIG. 1, the outer region of the objective lens 1 having a numerical aperture NA = 0.47 to a numerical aperture NA = 0.63 is used only for DVD and not used for CD. Either one or both of the light incident side surface A and the light emitting side surface B in the region is subjected to a thin film treatment that transmits light with a wavelength of 655 nm for DVD and does not transmit light with a wavelength of 790 nm for CD, or In such an outer region, either one or both of the light incident side surface A and the light exit side surface B does not act on light having a dual wavelength of 655 nm, but forms a diffraction grating that acts on light having a wavelength of 790 nm, and forms a wavelength of 655 nm. The light utilization efficiency of the two wavelengths 790 nm may be decreased without decreasing the light utilization efficiency.
[0057]
In other words, as in the above-described embodiment, when the diaphragm corresponding to the numerical aperture cannot be set when sharing the system with different numerical apertures, an extra light beam can be received in an optical system having a small numerical aperture. Therefore, it is desirable to take care that the light passing through the outer region of the lens designed to match the optical system with a large numerical aperture does not adversely affect the optical system with a small numerical aperture. For example, it is desirable that the amount of lateral aberration be 0.015 mm or more so that light that has passed through the outer region of the lens is not collected on the disk surface.
[0058]
In the above embodiment, two types of optical disks, DVD and CD, are taken as an example. However, the present invention is not limited to this, and other types of optical disks may be used. It is also applicable to three or more types of optical discs with different thicknesses, and the lens surface shape is set so that chromatic aberration cancels wavefront aberration according to the wavelength of the laser beam used for each. do it.
Further, even when the substrate thickness is the same, the wavelength used is different, so that a large aberration occurs in the conventional normal lens, the aberration can be reduced by applying the present invention.
[0059]
FIG. 8 is a block diagram showing an embodiment of an optical head using an objective lens according to the present invention, in which 11 is a DVD laser, 12 is a CD laser, 13 and 14 are half prisms, 15 is a collimator lens, and 16 is detection. A lens, 17 is a photodetector, 18 is a diffraction grating, and 19 is an actuator. The same reference numerals are given to portions corresponding to those in FIG.
[0060]
In the figure, when the DVD disk 2 is recorded or reproduced, the DVD laser 11 is driven. A laser beam having a wavelength of 655 nm generated from the DVD laser 11 is reflected by the half prism 13, passes through the half prism 14, and enters the collimator lens 15. The laser beam that has passed through the collimator lens 15 and becomes parallel light is incident on the objective lens 1 and collected, and forms a light spot on the information recording surface of the DVD disk 2. Then, the reflected light reflected by the DVD disk 2 becomes parallel light by the objective lens 1 and enters the collimator lens 15. The collimator lens 15 converts the parallel light into convergent light, and the convergent light passes through the half prisms 14 and 13 and reaches the photodetector 17 through the detection lens 16. The detection output signal of the photodetector 17 is supplied to a signal processing circuit (not shown), and an information recording / reproducing signal, a focus error signal, and a tracking error signal are obtained. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuator 19 is driven.
[0061]
When recording or reproducing the CD disk 3, the CD laser 12 is driven. A laser beam having a wavelength of 790 nm generated from the CD laser 12 passes through the diffraction grating 18, is reflected by the half prism 14, and enters the collimator lens 15. The laser beam that has passed through the collimator lens 15 and becomes parallel light is incident on the objective lens 1 and condensed to form a light spot on the information recording surface of the CD disk 3. Then, the reflected light reflected by the CD disk 3 becomes parallel light by the objective lens 1 and enters the collimator lens 15. The collimator lens 15 converts the parallel light into convergent light, and the convergent light passes through the half prisms 14 and 13 and reaches the photodetector 17 through the detection lens 16. The detection output signal of the photodetector 17 is supplied to a signal processing circuit (not shown), and an information recording / reproducing signal, a focus error signal, and a tracking error signal are obtained.
[0062]
Note that the tracking error signal in the case of the CD disk 3 is obtained by branching the laser beam from the CD laser 12 into three beams of zero-order light and soil primary light by the diffraction grating 18, and these ± first-order light causes tracking error. I try to get a signal.
[0063]
The actuator 19 is driven so that the objective lens 1 is positioned at an appropriate focus position and tracking position in the same manner as the DVD disk 2 by using the tracking error signal and the focus error signal thus obtained.
[0064]
In the present invention, instead of the objective lens 1, an optical system common to both disks such as the collimator lens 15 or the half prism 14 can be optically designed to have the same function as the objective lens in the present invention. Although not shown, another optical element having a function equivalent to that of the objective lens of the present invention may be disposed in the optical path from the half prism 14 to the disk 2 or the disk 3.
[0065]
The collimator lens 15 is not necessarily required, and the present invention can be applied to a so-called finite optical system.
[0066]
FIG. 9 is a block diagram showing an embodiment of an optical disk apparatus using an objective lens according to the present invention, wherein 20 is an actuator drive circuit, 21 is a signal processing circuit, 22 is a laser drive circuit, 23 is a system control circuit, 24 Is disc discriminating means, and parts corresponding to those in FIG.
[0067]
In the figure, the optical pickup device is the same as the configuration shown in FIG.
[0068]
First, the disc discriminating unit 24 discriminates the type of disc loaded. As the disc discrimination method, a method of detecting the thickness of the substrate of the disc by an optical or mechanical method, a method of detecting an identification mark recorded in advance on the disc or the cartridge of the disc, and the like can be considered. Alternatively, a method may be used in which a disc signal is reproduced assuming a disc thickness and type, and if a normal signal cannot be obtained, it is determined that the disc has a different thickness and type. The disc discrimination result is transmitted from the disc discrimination means 24 to the system control circuit 23.
[0069]
When the disc is determined to be a DVD disc, a signal for turning on the DVD laser is transmitted from the system control circuit 23 to the laser drive circuit 22, and the DVD laser 11 is turned on by the laser drive circuit 22. Thereby, in the optical head, the laser beam having the two wavelengths of 655 nm reaches the photodetector 17 as in the embodiment shown in FIG. The detection signal from the photodetector 17 is sent to the signal processing circuit 21 to generate an information recording / reproducing signal, a focus error signal, and a tracking error signal, and sent to the system control circuit 23. The system control circuit 23 controls the actuator drive circuit 20 based on the focus error signal and the tracking error signal. Based on this control, the actuator drive circuit 20 drives the actuator 19 to move the objective lens 1 in the focus direction and By the operation of a so-called servo circuit that moves in the tracking direction, focus control and tracking control are normally performed so that the objective lens 1 is positioned at the correct position with respect to the DVD disk 2. As a result, an information recording / reproducing signal can be obtained satisfactorily.
[0070]
When it is determined that the loaded disk is the CD disk 3, a signal for turning on the CD laser 12 is transmitted from the system control circuit 23 to the laser driving circuit 22. As a result, a laser beam having a wavelength of 790 nm is generated from the CD laser 12. The subsequent operation is the same as that in the case of the optical head in FIG. 8. This laser beam reaches the photodetector 17, and each circuit and the actuator 19 are operated and the servo is operated as in the case of the DVD disk 2 described above. The operation is performed, and an information recording / reproducing signal can be obtained satisfactorily.
[0071]
The following points were found for the second embodiment. That is, as the lens surface shape for obtaining the above-mentioned wavefront aberration, all the monochromatic surfaces of at least one lens surface in a multi-wavelength lens that condenses a plurality of types of monochromatic light of DVD and CD by their respective refraction effects. D12, D23, D34, D45,..., The NA of the adjacent portion during DVD in order of the adjacent step amount between the plurality of aspherical portions having different refractive indexes in the light common use region in order from the lens optical axis. NA12, NA23, NA34, NA45,... In the order of closeness of the optical axis, and the NA at the time of CD of the adjacent part as CNA12, CNA23, CNA34, CNA45,. The lens surface shape should satisfy the following formula (14).
[0072]
[Expression 14]

[0073]
In Equation 14, λ1 is the shorter wavelength (mm), λ2 is the longer wavelength (mm), n1 is the refractive index of the lens at wavelength λ1, and n2 is the refractive index of the lens at wavelength λ2. In the mathematical formula, ma is an integer closest to aij. Mb is an integer closest to bij. In other words, ma is an integer that minimizes the absolute value of ma-aij, and mb is an integer that minimizes the absolute value of mb-bij. For example, it is “1” in the case of 1.104, and “0” in the case of 0.49. The symbols used in Equation 14 are shown in FIG. As shown in FIG. 13, in this example, the lens has a DVD / CD common use area and a DVD dedicated area. The DVD / CD common use area has six aspherical surfaces (1) to (6). And the level | step difference of these aspherical surfaces is represented as D12, D23 .... In the case of a DVD, the NA between the steps D12 is NA12, and the NA between the steps D23 is NA23. In the case of a CD, the NA between the steps D12 is CNA12, and the NA between the steps D23 is CNA23. In Equation 14, Aij / Bij is in the range of 0.333 to 3, but a more preferable range is in the range of 0.5 to 2.
[0074]
In addition, specific numerical values in this example are summarized in Table 5.
[0075]
[Table 5]

[0076]
【The invention's effect】
As described above, according to the present invention, the aperture (NA) required for recording or reproduction by refraction without using a diffractive lens structure for two or more types of optical disks having different transparent substrate thicknesses. All light beams can be condensed at a desired position with as little aberration as possible, and the light utilization efficiency can be further increased. Further, as can be seen from the above description, the present invention is a multi-wavelength optical system using a plurality of monochromatic lights, and each of the divided aspherical surfaces has a single focal point corresponding to the unique wavelength of each monochromatic light. The focal points corresponding to the unique wavelength of each monochromatic light can be arranged at different positions, and can be used in an optical system in optical communication or the like.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of an objective lens according to the present invention.
FIG. 2 is a diagram showing a specific example of a lens surface shape according to the first embodiment shown in FIG. 1;
FIG. 3 is a diagram for explaining an optical path length in an optical system including an objective lens and a transparent substrate of an optical disc.
4 is a graph showing a specific example of a measurement result of wavefront aberration according to the first embodiment of the first embodiment shown in FIG. 1; FIG.
FIG. 5 is a diagram showing a calculation result of a light spot for optical discs of different types in the optical disc apparatus using the first embodiment shown in FIG. 1;
FIG. 6 is a graph showing a specific example of a measurement result of wavefront aberration of the second embodiment of the objective lens according to the present invention.
FIG. 7 is a diagram showing calculation results of light spots for optical discs of different types in the optical disc apparatus using the second embodiment of the objective lens according to the present invention.
FIG. 8 is a diagram showing an embodiment of an optical head according to the present invention.
FIG. 9 is a diagram showing an embodiment of an optical disc apparatus according to the present invention.
FIG. 10 is a schematic diagram showing wavefront aberration of each wavelength with respect to light beam height.
11 is a diagram showing wavefront aberrations of respective wavelengths with respect to the light beam height in Example 2. FIG.
FIG. 12 is a diagram showing wavefront aberration of each wavelength with respect to the light beam height when the lens described in Japanese Patent Laid-Open No. 2001-51192 is used.
FIG. 13 is a diagram for explaining mathematical expressions in the second embodiment.
[Explanation of symbols]
Objective lens of 1 embodiment
2 Transparent substrate for DVD
2a Information recording surface
3 CD transparent substrate
3a Information recording surface
4,5 laser beam
11 DVD laser
12 CD laser
13,14 half prism
15 Collimator lens
16 detection lens
17 Photodetector
18 diffraction grating
19 Actuator
20 Actuator drive circuit
21 Signal processing circuit
22 Laser drive circuit
23 System control circuit
24 disc discriminator

Claims (2)

Is for each different types of optical recording media thickness of the transparent substrate, a light beam of monochromatic light of different wavelengths, is condensed through the transparent substrate, an information recording provided on the transparent substrate of the optical recording medium An objective lens having a positive power to form a light spot on a surface,
The exit surface consists of a single aspheric surface,
Plane of incidence, the lens surface area through which different kinds of different wavelengths of the light beam to be focused on the information recording surface of the optical recording medium, is divided into a plurality of non-spherical portion in a radial direction from the optical axis,
The difference between the maximum value and the minimum value of the wavefront aberration of each monochromatic light condensed corresponding to the optical recording medium in each aspheric surface portion is expressed as ΔVd (λi) (d is an integer of 1, 2,... Each aspheric surface portion, i is an integer of 1, 2,...), And in any aspheric surface portion, the ratio of the difference of each monochromatic light is 0.4 to 2.5. The non-spherical portion cancels out the chromatic aberration caused by the difference in the wavelength λ of the light beam and the spherical aberration caused by the difference in the thickness of the transparent substrate of the optical recording medium so that the light is condensed under the condition satisfying An objective lens having a spherical lens surface shape. Focusing is performed when the difference (ΔVd (λi)) between the maximum value and the minimum value of the wavefront aberration of each monochromatic light within each aspherical portion is 0.14λi (i is an integer of 1, 2,...) Or less. The objective lens according to claim 1, wherein the lens surface shape of the aspherical portion is set so as to cause

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