JP3550480B2 - Objective lens and optical head using the same - Google Patents

Description

【００１９】
を満たすように整数ｍ、ｎを選択すればよい。またこれで適当なｍ、ｎがない場合には位相シフトのさせかたを図４のようにしてもよい。この場合は輪帯領域を除いたそれ以外の領域にーφの位相シフトを加えることにより、図３と同じ波面形状をで実現できる。したがってこの場合は、
【００２０】
【数２】

【００２１】
を満たしていればよい。これにより例えばλ１を７８０ｎｍ、λ２を６５０ｎｍとすれば、それぞれにおける位相差φは図５のようになる。このように位相差を選べば、ＤＶＤの波面にまったく影響を与えずにＣＤ再生時の球面収差を低減することができる。ここでの逆輪帯位相シフタは空気よりも屈折率の大きい膜を付加する場合など位相ずれを位相遅れによって実現する場合を念頭においた命名である。レンズをけずるなど位相ずれを位相進みによって実現できる場合には輪帯領域を直接けずればよい。これはどちらでも等価であるが、以後はこの場合も含めて逆輪帯位相シフタと呼ぶことにする。
【００２２】
以下、輪帯位相シフタの形状、及び位相差の最適化について説明する。光スポットの評価指標としては無収差スポットの中心強度で規格化した収差のある光スポットの中心強度であるストレール強度があるが、これだと制限開口がある場合のＮＡの違いが現れない。そこで制限開口がある場合も含めて、対物レンズの瞳に入射する全光量に対するスポット中心強度の比を新たな評価指標とする。これを用いると例えば同じ口径でもＮＡが大きく、スポット径が小さくて中心強度が大きい方がこの評価指標が大きいことになる。この評価指標は
【００２３】
【数３】

【００２４】
のようにストレール強度とレンズ全開口半径で規格化した制限開口半径Ｒの２乗の積に比例することがわかる。以下、このストレール強度に規格化制限開口半径の２乗をかけた値をηとする。通常のＣＤピックアップでは波長７８０ｎｍ、対物レンズＮＡ０．４５であるのでＤＶＤの対物レンズＮＡ０．６に対しては無収差であれば、η＝１×（０．４５／０．６）＾２＝０．５６、マレシャルの基準によるストレール強度下限値０．８ではη＝０．４５となる。基板厚誤差による球面収差は４次の球面収差が
【００２５】
【数４】

【００２６】
６次の球面収差が
【００２７】
【数５】

【００２８】
で与えられる。ただしこれらの式におけるｎは屈折率を表している。これらを用いて半径Ｒ１からＲ２までの位相をφ遅らせる輪帯位相シフタを加えた収差は
【００２９】
【数６】

【００３０】
のように表せる。またストレール強度は
【００３１】
【数７】

【００３２】
のように近似できるから、これよりηを最大とするＲ１、Ｒ２、φ、制限開口のＮＡ、Ｗ２０、Ｗ００を求める。実際には数式処理ソフトを用いて、Ｗ２０、Ｗ００は解析的に求め、Ｒ１、Ｒ２、φ、制限開口のＮＡを数値的に求めた。その結果、位相シフタの内径はＮＡ０．２０、外径はＮＡ０．４２、制限開口のＮＡを０．４６とし、位相差を０．２６５λ（λ＝７８０ｎｍ）のとき、η＝０．４８が最大となり、マレシャルの基準によるη＝０．４５を上回っていることがわかった。一方、位相シフタを用いず、制限開口のみで最適化するとＮＡ０．３９でη＝０．３４が最大であった。つまりＮＡ０．４５に換算すれば、ストレール強度で０．６１から０．８６まで改善したことに相当する。この位相差に対してＤＶＤ再生時に生じる収差はＲＭＳ波面収差で０．０３３λ（λ＝６５０ｎｍ）であった。これはほぼレンズの加工精度と同等であり、実際上問題は生じないと考えられる。
【００３３】
この最適な位相差０．２６５λを先に述べたＤＶＤに影響を与えない位相差と比較すると、最も近いのはｍ＝２、ｎ＝２のときの逆輪帯位相マスク、またはｍ＝４、ｎ＝３のときの輪帯位相マスクの０．３３３λであることがわかる。しかしｍが大きくなると位相差を生じさせる膜、あるいはレンズの段差が厚くなり、半導体レーザに波長ずれが生じた場合の位相差のずれが大きくなるので、ここでは逆輪帯位相マスクの方が望ましい。このＤＶＤに影響を与えない位相差に固定した場合の位相シフタの形状を求めると、内径がＮＡ０．２０、外径がＮＡ０．４４、制限開口ＮＡ０．４８のとき、η＝０．４７が最大となった。これは上記の最適な位相差と比べてほとんど遜色ない。
【００３４】
この輪帯位相シフタをＤＶＤ専用レンズに適用した場合の収差低減の効果を、光線追跡で確認した。図６に光線追跡に用いたレンズの仕様、及び面形状を示す。ここで面形状のＲは中心曲率半径、ｋは円錐定数、Ａ４、Ａ６、Ａ８、Ａ１０はそれぞれ、４次、６次、８次、１０次の非球面係数である。これらを用いて面形状は
【００３５】
【数８】

【００３６】
のように表せる。ここでは光軸に対して対称な形状が仮定されており、ｒは半径座標である。このレンズを用いて波長７８０ｎｍの平行光をＣＤの基板厚１．２ｍｍを通して集光したとき、開口数ＮＡ＝０．４５の範囲でのＲＭＳ（ＲｏｏｔＭｅａｎ Ｓｑｕａｒｅ）波面収差は最良像点において、０．１２７９λであった。これに対して上記の輪帯位相板を付加したとき、ＲＭＳ波面収差は０．０７３６６λに低減できた。ただしこのとき用いた光線追跡ソフトでは１λ以上の位相差を扱えなかったため、位相板の厚さはｍ＝１、ｎ＝１の０．３３３３λ相当の厚さを用いた。したがってＤＶＤでの位相差は必然的に０となるため、波長６５０ｎｍでのＤＶＤの基板厚０．６ｍｍを通したスポット収差は０．００１λ以下となった。
【００３７】
以上では制限開口を用いるという前提で説明をしたが、これは必ずしも実際の開口を必要とすることを意味しない。実際にはＲＭＳ波面収差を評価関数として最適な焦点位置を求めるときの、瞳の評価範囲を指定するのとほぼ等価であると考えられる。制限開口の範囲内でなるべくＲＭＳ波面収差が小さくなるように焦点ずれを調整したとすると、制限開口の範囲外の光は当然収差が大きくなり、波面の傾斜も大きくなる。このためそのような領域の光線は焦点からは大きくはずれた位置で焦点面と交差する。したがって集光スポットに対して、このような光線は存在しないのとほぼ等価となる。
【００３８】
このように輪帯位相シフタのみを用いた場合に、スポット性能は改善されるものの、ＮＡ０．４５でのストレール強度換算で０．８６相当では、光学部品のずれや、ディスクの傾き、焦点ずれなどによるスポットの劣化を見込むと必ずしも十分でない可能性がある。そこでさらにこれに組み合わせてレンズの内側と外側で最適化する基板厚を変える。以下これを分割レンズと呼ぶ。これは発明者らによって波長６５０ｎｍでＤＶＤとＣＤの両方の互換をとる方法として発明された（特願平７ー３４２２０３）が、ＣＤを波長７８０ｎｍで再生する場合にはこの分割のＮＡを少なくともＮＡ０．４５以上にする必要があり、この場合にはＤＶＤを再生するときの収差が非常に大きくなってしまうという欠点があった。そこで位相シフタと分割レンズを組み合わせて、位相シフタ形状、位相差、内外分割半径、内側基板厚を同時に最適化したところ、分割レンズで発生する波長７８０ｎｍでのＣＤ再生時の収差と、波長６５０ｎｍでのＤＶＤ再生時の収差を両方とも低減し、ＣＤのスポット性能がさらに改善される解があることがわかった。以下これについて説明する。
【００３９】
分割レンズと位相シフタを組み合わせた場合の波面収差は
【００４０】
【数９】

【００４１】
のように表せる。ここではＲ１が輪帯位相シフタ内径、Ｒ２が分割半径、Ｒ３が輪帯位相シフタ外径、Ｒ４が制限開口半径である。分割半径を境として無収差となるためのディスク基板厚が異なり、外側ではＤＶＤに合わせて０．６ｍｍ、内側では最適化によってこれが０．６ｍｍと１．２ｍｍの間となる。したがってそれにともなって球面収差の収差係数Ｗ６０、Ｗ４０が添字１、２をつけて異なるように表示されている。また焦点ずれＷ２０１、Ｗ２０２は分割の内外でＲＭＳ波面収差を最小にするように球面収差量から決まり、定数項Ｗ００１、Ｗ００２は分割の内外で波面収差の平均値が同じなるようにして決まり、全体のＲＭＳ波面収差を最適化する。Ｗ２０１とＷ２０２、Ｗ００１とＷ００２の差はレンズ内外の対応基板厚差で決まり、Ｗ２０２、Ｗ００２を与えられた位相シフタの条件下でＲＭＳを最小とする条件から、数式処理ソフトで解析的に求めることでＷ２０１とＷ００１も求めた。さらに与えられた内側対応基板厚、分割半径Ｒ２について、Ｒ１、Ｒ３、Ｒ４、位相差を数値的に変えてηを最大とする条件を求めた。その結果を図７に示す。ここで横軸は分割レンズの分割半径、縦軸はηであり、中心部基板厚を変えて最適条件での計算結果をプロットしている。またグラフ中にＣＤの無収差と、ストレール強度０．８相当の下限レベル、上記の最適位相シフタ、固定位相シフタを波線で示している。これらは分割レンズを用いていないのでこのグラフ上にはポイントではプロットできない。一方、このときＤＶＤ再生で生じるＲＭＳ波面収差を図８に示した。図７と図８を見比べるとわかるように中心部基板厚を１．２ｍｍに近づければ近づけるほどＣＤの性能は上がり、ＤＶＤの収差は増大する。したがってこれらのポイントの内、どこを最適点として採用するかは、システムのいろいろなマージンの配分によって判断が分かれる。しかし例えば中心基板厚０．７６ｍｍ、分割の境界のＮＡ０．４５のときの、ＣＤ性能η＝０．５２６（ＣＤストレール強度換算０．９４）、ＤＶＤのＲＭＳ波面収差０．０２９９λ程度であればほぼ許容できるのではないかと考えられる。このとき位相シフタがない場合には、ＤＶＤのＲＭＳ波面収差は０．０３１λであり、ＣＤについてはη＝０．４１４であった。したがってＣＤ、ＤＶＤともに位相シフタによって収差が低減していることがわかる。またこのポイントではＣＤ性能の最大値と、ＤＶＤ収差の最小値が一致している。このとき輪帯位相シフタの位相差は０．２９８５λ（λ＝７８０ｎｍ）、内径はＮＡ０．２１４５であり、外径はＮＡ０．４５で分割の境界のＮＡと一致していた。
【００４２】
図９に逆輪帯位相マスク作りつけの分割レンズ模式図を示す。逆輪帯マスクがレンズに作りつけであるため、輪帯位相マスクの領域が凹んでいる。このときディスク側の比較的曲率がゆるやかな面にも分割レンズによる段差を示しているが、これは設計上、像側のみにすることも可能である。
【００４３】
図１０にＣＤ再生波長のずれによるＣＤ再生スポット性能ηの値を示す。横軸の範囲は±２０ｎｍあるが、実際上、温度変化などでずれる波長範囲は±１０ｎｍ程度と考えられる。この範囲だとη＝０．５３から波長ずれー１０ｎｍでη＝０．５２程度の劣化であり、ＮＡ０．４５でのストレール強度換算で０．９３から０．９２程度の変化で、ほとんど影響はない。図中には先に述べた最適輪帯位相シフタ、固定輪帯位相シフタについても合わせて表示している。
【００４４】
図１１は波長６５０ｎｍでのＤＶＤ再生時の波長ずれに対するＲＭＳ波面収差であり、分割レンズと最適位相シフタを組み合わせた場合、収差は０．０３０λから、波長ずれー１０ｎｍで０．０３６λまで増加している。これも十分、許容範囲内と考えられる。また図中には先に述べた最適輪帯位相シフタ、固定輪帯位相シフタについても合わせて表示している。固定位相シフタについてはＤＶＤでは収差が発生しないような位相差が選ばれているので、波長ずれ０で収差は０となっている。最適位相シフタのみについては位相差がＤＶＤで収差を生じない位相差からずれているため、その位相差になる波長ずれ量に向けて線形に波面収差が変化している。
【００４５】
図１２は波長７８０ｎｍによるＣＤ再生時の波面収差形状を示している。それぞれ制限開口のＮＡ範囲で焦点ずれを最適化し、横軸はＮＡ０．６の全開口にわたる瞳の半径座標で示しているため、周辺部は収差が非常に大きくなっている。またそのとき縦軸は±０．５λの範囲内に折り畳んで表示しているため、周辺部は急激に振動しているように見えている。これらは制限開口のみで最適化した場合に比べてより広いＮＡで収差が抑えられている。また制限開口ＮＡの範囲の外側の波面の立ち上がりも急峻となっており、収差の大きいことによる制限開口の効果もより顕著となることが期待される。
【００４６】
図１３は波長６５０ｎｍによるＤＶＤ再生時の波面収差である。図１２での制限開口のみの場合と、固定位相シフタのみの場合には波面収差は完全に０となるので、ここでは分割レンズと最適位相シフタを組み合わせた場合と、最適位相シフタのみの場合を表示している。収差のまったく発生しない最外周部分でも収差が０となっていないことから、全体に若干焦点ずれを生じさせていることがわかる。これは位相シフタで発生した位相差を収差と考えた場合に、若干焦点ずれさせた方が全体のＲＭＳ波面収差が小さくなるためである。いずれにせよグラフ縦軸の値はかなり小さく、波面形状の特異さは実際上影響を及ぼさない程度のＲＭＳ波面収差に抑えられている。
【００４７】
図１４にスポット形状の計算結果を示す。グラフの横軸はスポットのピーク強度に対してｅｘｐ（ー２）倍の強度のスポットの全幅、縦軸はサイドローブの強度を中心強度で規格化した値である。したがってスポット、サイドローブ共に小さい方が望ましいので、プロット点がグラフの左下に近いほど分解能が高いスポットであるということができる。ここで対物レンズの瞳の強度分布としては対称なガウス分布を仮定し、瞳におけるガウス分布の中心の強度に対してｅｘｐ（ー２）倍の強度の範囲の幅に対するレンズ口径の比が０．１、レンズの中心部分の強度に対する周辺部分の強度が０．９８となる場合の計算結果である。図中白抜きの丸印が無収差のＣＤであり、これに近いほどＣＤと同レベルの再生性能が期待できる。黒い四角は通常のＤＶＤレンズに制限開口のみを用いた場合であり、実際に制限開口を挿入した場合、その焦点位置でそのまま制限開口をとりはらった場合、制限開口をとりはらってスポット中心強度が最大となるように焦点位置をずらした場合の３つのプロット点がある。この場合はいずれも無収差ＣＤよりもスポット分解能が劣っている。黒い三角印は最適輪帯位相シフタのみを挿入した場合であり、同様にして３つのプロット点がある。制限開口のみに比べてスポット径はかなり改善しているが、制限開口がないとサイドローブがかなり大きくなっている。白抜きの四角は分割レンズと最適輪帯位相シフタを組み合わせた場合である。同様にして３つのプロット点があるが、この３つがかなり接近していることがわかる。つまりこの場合には制限開口はあってもなくてもほとんど変わらず、仮想的な制限開口の範囲外の光の収差が急峻に増大しているためスポット形成には実質的に影響を与えていないことがわかる。この場合光スポットはＣＤ無収差に比べてスポット径がやや小さく、サイドローブが若干大きめとなっている。これでスポット性能の評価指標であったηの値がＣＤとほぼ同等から若干劣る程度であったのは、おそらく、サイドローブを低減しきれていない影響をスポット径を小さくして相殺している状況となっているのではないかと推測される。一方ＤＶＤを再生する場合のスポットの計算結果を白抜きの三角とひし形でグラフの左下にプロットしている。ひし形がＤＶＤを無収差で再生するスポット、三角が最適分割レンズと最適位相シフタを組み合わせた場合である。ＤＶＤについてはほとんど同じスポット形状となっている。
【００４８】
図１５に光ヘッドの実施例を示す。半導体レーザ４からの光をコリメートレンズ５により平行光としてビーム成形プリズム６１、６２により楕円ビームを円形ビームとする。ビーム成形プリズムは光学系の効率が十分高いか、ディスクのトラックピッチがディスク上の光スポットの主ローブと第１暗線の間隔より広い場合に、取り除いた方が部品点数、隣接トラッククロストーク低減のために有利となる場合もある。さらにこの光はビームスプリッタ７１を透過し、さらに立ち上げミラー８により反射され、２次元アクチュエータ９に搭載された本発明による対物レンズ３により光ディスク上に集光される。光ディスクはＣＤでもＤＶＤでもよい。２次元アクチュエータ９はトラッキング誤差信号により、ディスク半径方向に可動し、光スポットをトラック上に位置決めし、焦点誤差信号により光軸方向に可動し、焦点位置をディスク上に位置決めする。反射光は再び、対物レンズ３、立ち上げミラー８を経由して、ビームスプリッタ７１を反射し、検出光学系に導かれる。ビームスプリッタ７２を透過した光は集光レンズ１１１により集光光束とされ、ビームスプリッタ７３に入射する。ここでは透過光はシリンドリカルレンズ１２を透過し、４分割光検出器１３に入射する。この分割検出器の対角成分の和信号どうしの差動信号を差動増幅器１４１により出力し、焦点ずれ信号とする。一方ビームスプリッタ７３で反射した光は２分割光検出器１５に入射し、それぞれの出力の差動信号を差動増幅器１４２により出力することにより、トラッキング誤差信号を得る。またビームスプリッタ７２を反射した光は集光レンズ１１２により光検出器１６に集光され光電変換された信号は、アンプ１７で増幅され再生信号を得る。再生信号はサーボ信号検出用のディテクタの出力の和信号から検出しても差し支えない。この場合、信号帯域まで検出した信号をローパスフィルタなどで帯域制限してサーボ信号を検出すればよい。サーボ検出光学系は一例であり、他の方式を用いることも可能である。
【００４９】
以上では輪帯位相シフタは対物レンズに作りつけられている実施例を説明してきたが、図１６はＤＶＤ専用の対物レンズ１８と、独立した輪帯位相シフタ１９をハイブリッドに一体化して２次元アクチュエータに搭載した実施例である。ここでは図１５の立ち上げミラーからディスクまでの光学系に相当する部分だけを置き換えることを想定し、その部分だけを示した。
【００５０】
【発明の効果】
輪帯位相シフタ、またはそれとレンズ内外で無収差となる基板厚が異なる対物レンズを最適に組み合わせることにより、波長６５０ｎｍのレーザ光で基板厚０．６ｍｍのＤＶＤを、波長７８０ｎｍのレーザ光で基板厚１．２ｍｍのＣＤを、制限開口を必要とすることなく１つのレンズで再生することが可能となり、小型で安価な光ヘッドを提供できる。
【図面の簡単な説明】
【図１】本発明による対物レンズの基本的なイメージ図。
【図２】球面収差波面形状。
【図３】輪帯位相シフタによる波面収差形状。
【図４】逆輪帯位相シフタによる波面収差形状。
【図５】ＤＶＤに影響のない条件でのＣＤの位相シフト量。
【図６】光線追跡に用いたＤＶＤレンズ仕様。
【図７】分割レンズと位相シフタを組み合わせた場合のＣＤ再生スポット性能。
【図８】ＤＶＤ再生で生じるＲＭＳ波面収差。
【図９】最適逆輪帯位相シフタ作りつけの分割レンズ形状模式図。
【図１０】ＣＤ再生波長のずれによるＣＤ再生スポット性能の変化。
【図１１】ＤＶＤ再生時の波長ずれに対するＲＭＳ波面収差。
【図１２】ＣＤ再生時の波面収差形状。
【図１３】ＤＶＤ再生時の波面収差形状。
【図１４】スポット形状の計算結果。
【図１５】光ヘッドの実施例。
【図１６】対物レンズと輪帯位相シフタがハイブリッドに一体化された実施例。
【符号の説明】
１‥‥輪帯位相シフタつき対物レンズ、１０１‥‥輪帯位相シフト領域、２‥‥球面収差波面、１０２‥‥逆輪帯位相シフト領域、３‥‥逆輪帯位相シフタ一体型分割レンズ、４‥‥半導体レーザ、５‥‥コリメートレンズ、６１、６２‥‥ビーム成形プリズム、７１、７２、７３‥‥ビームスプリッタ、８‥‥立ち上げミラー、９‥‥２次元アクチュエータ、１０‥‥光ディスク、１１１、１１２‥‥集光レンズ、１２‥‥シリンドリカルレンズ、１３‥‥４分割ディテクタ、１４１、１４２‥‥差動アンプ、１５‥‥２分割ディテクタ、１６‥‥ディテクタ、１７‥‥アンプ、１８‥‥ＤＶＤ用対物レンズ、１９‥‥輪帯位相シフタ。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical disk device for optically reproducing information from an optical recording medium, and more particularly to an optical head for reproducing signals from optical disks having different substrate thicknesses using light sources having different optical wavelengths, and an objective lens used for the optical head.
[0002]
[Prior art]
Optical discs have been remarkably advanced in recent years as large-capacity interchangeable information recording media. For this reason, recording / reproducing methods, recording densities, and disk sizes are diversified, and it is becoming difficult to ensure their compatibility. In particular, CDs (Compact Discs) are the most widely used so far, and CD-Rs (Compact Disk-Recordables), which are recordable CDs compatible with the CDs, have also been widely used. In developing new optical disks, there is a great demand for compatibility with these CDs and CD-Rs. A DVD (Digital Video Disk) has recently been released as a next-generation high-density ROM following these. Here, in order to improve the recording density, the numerical aperture (NA) of the objective lens is increased from 0.45 of a conventional CD to 0.6. When the wavelength of the laser light source used is λ, the size of the condensed spot on the optical disk is proportional to λ / NA. Therefore, the shorter the wavelength and the larger the NA, the smaller the light spot can be. If the light spot is small, high-density information pits can be reproduced with good quality, so that the recording density of the optical disk can be improved. Therefore, the wavelength of the semiconductor laser used for DVDs is set to be 780 nm to 650 nm for CDs. On the other hand, an increase in NA sharply increases coma aberration generated when the disk is tilted, and rather deteriorates the light spot. Therefore, the thickness of the substrate of the DVD is reduced from 1.2 mm of the CD to 0.6 mm of the CD, and the coma of the disc tilt due to the increase in NA is suppressed. However, if the thickness of the substrate is changed from that of a CD, a spherical aberration is generated at the time of reproducing a CD with an objective lens dedicated to a DVD, and the light spot is blurred. This is because an objective lens for an optical disk is designed in advance so as to have a spherical aberration that compensates for a specific substrate thickness.
[0003]
A conventional means for solving this problem is described in, for example, Optical Review, Vol. 1, No. 1, 1994, pp. 27-29 (Optical Review, Vol. 1, No. 1 (1994) pp. 27-29.). I have. Here, a hologram is formed on the surface of the 0.6 mm objective lens, a CD is reproduced by the diffracted light, and a DVD is reproduced by the transmitted light. Here, the hologram pattern is designed in advance so as to compensate for the spherical aberration that occurs when reproducing a CD. However, since a hologram is used in this case, a light spot for DVD is generated even when reproducing a CD, and a light spot for CD is generated when reproducing a DVD. Also, the light reflected by the disk is diffracted again. Due to these, there is a disadvantage that a loss of light quantity cannot be avoided.
[0004]
The second conventional example is Mitsubishi Electric News Release, Development No. 9507 (June 21, 1995). In this method, both an objective lens for 0.6 mm and an objective lens for 1.2 mm are mounted on an optical head, and the two lenses are switched and used by a movable actuator. However, in this case, since two lenses are switched, there are problems such as an increase in cost due to the use of two lenses, reproducibility of lens positions, and deterioration of response characteristics due to a large and heavy actuator.
[0005]
A third conventional example is described in Nikkei Electronics, January 29, 1996, No. 654, pp. 15-16. Here, a limiting aperture made of liquid crystal is provided, and in reproducing a CD, the NA is reduced to 0.35 to reduce aberration. However, in this case, since the semiconductor laser having a wavelength of 635 nm is used for both the CD and DVD, the NA of the CD can be reduced to this level. However, when reproducing a CD-R in which the reflectance is significantly reduced with light having a wavelength shorter than 780 nm, this method is used. Has the disadvantage that it cannot be used.
[0006]
On the other hand, the inventors of the present invention have previously filed Japanese Patent Application No. 7-342203 in which the substrate thickness to be optimized inside and outside the objective lens is changed in order to achieve compatibility between DVD and CD at a wavelength of 650 nm. Has been proposed. However, when reproducing a CD at a wavelength of 780 nm, it is necessary to set the NA of this division to at least NA 0.45 or more. In this case, there is a disadvantage that aberrations when reproducing a DVD become extremely large. .
[0007]
In view of this, an object of the present invention is to reproduce a CD having a substrate thickness of 1.2 mm with light of a wavelength of 780 nm accurately and at a low cost without loss of light amount, and to reproduce a DVD having a substrate thickness of 0.6 mm with light of a wavelength of 650 nm. It is to play.
[0008]
[Means for Solving the Problems]
In order to solve the above-described problems, in the present invention, when condensing two wavelengths of laser light onto optical discs having different substrate thicknesses,
An annular phase shifter for reducing the aberration of the condensed spot of each wavelength is added integrally with the objective lens.
[0009]
Alternatively, an annular phase shifter for reducing the aberration of the condensed spot of one wavelength at least without changing the aberration of the condensed spot of one wavelength is added integrally with the objective lens.
[0010]
Alternatively, an annular phase shifter that reduces both aberrations of the condensed spots of laser light of two wavelengths is integrated with a lens having different substrate thicknesses for condensing without aberrations inside and outside the objective lens. Add it.
[0011]
Alternatively, as an optical head, a semiconductor laser of two wavelengths, branching means for branching reflected light from the optical disk from the optical path from the semiconductor laser to the optical disk, and a focused spot position control signal and reproduction from the branched light The above-described objective lens is used for condensing light of each wavelength on an optical disk having a different substrate thickness by an optical head comprising at least a detecting means for detecting a signal.
[0012]
Alternatively, two semiconductor lasers having different wavelengths, an objective lens for condensing light of each wavelength on an optical disk having a different substrate thickness, and a light reflected from the optical disk being branched from an optical path from the semiconductor laser to the optical disk. In an optical head including at least a branching unit and a detecting unit for detecting a focused spot position control signal and a reproduction signal from the reflected light branched by the branching unit, the aberration of the focused spot of each wavelength is reduced. An annular phase shifter to be reduced is added.
[0013]
Alternatively, two semiconductor lasers having different wavelengths, branching means for branching the reflected light from the optical disk from the optical path from the semiconductor laser to the optical disk, and a condensed spot position control signal from the reflected light branched by the branching means. In an optical head comprising at least a detecting means for detecting a reproduction signal, a substrate thickness for condensing light of each wavelength on an optical disc having a different substrate thickness is different between an inner side and an outer side without aberration. Using an objective lens, a ring-shaped phase shifter that reduces both aberrations of the condensed spot of each wavelength is added.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0015]
FIG. 1 is a basic image diagram of an objective lens according to the present invention. In a DVD objective lens 1 according to the present invention, a donut-shaped annular phase shift region 101 is added to a normal DVD objective lens. The annular phase shift region 101 may be loaded with a thin film, or the lens may be directly processed into such a shape from the beginning. Since a normal DVD lens is designed to have no aberration when the substrate thickness is 0.6 mm, when reproducing a DVD with a laser beam having a wavelength of 650 nm, the aberration applied by the phase shifter is minimized. I do. On the other hand, when reproducing a CD having a substrate thickness of 1.2 mm with a laser beam having a wavelength of 780 nm, the spherical aberration generated due to a substrate thickness error of 0.6 mm is reduced.
[0016]
Hereinafter, the fact that the aberration is qualitatively reduced will be described. FIG. 2 shows a schematic diagram of the wavefront shape of the spherical aberration when the focal position is optimized. Here, the horizontal axis is the radius coordinate of the pupil of the objective lens, and the vertical axis is the amount of wavefront aberration. Due to the difference in the substrate thickness between the CD and the DVD, the light spot when reproducing the CD with the DVD-only lens has a wavefront shape roughly represented by such a quartic function. On the other hand, FIG. 3 shows a schematic diagram of the wavefront shape when the phase shift is performed in an annular shape. It can be seen that the maximum value of the aberration is reduced by the phase shift.
[0017]
On the other hand, when reproducing a DVD using this lens, the aberration of the DVD must not increase. One method for achieving this is to use the difference between the wavelength for reproducing a CD and the wavelength for reproducing a DVD so that the phase is shifted only for the CD and not for the DVD. For this purpose, the CD reproduction wavelength is λ1, the DVD reproduction wavelength is λ2, and the phase difference generated during CD reproduction is φ.
[0018]
(Equation 1)

[0019]
The integers m and n may be selected so as to satisfy. If there is no appropriate m or n, the phase shift may be made as shown in FIG. In this case, the same wavefront shape as that of FIG. 3 can be realized by adding a phase shift of −φ to the region other than the annular region. So in this case,
[0020]
(Equation 2)

[0021]
What is necessary is just to satisfy. Thus, for example, if λ1 is 780 nm and λ2 is 650 nm, the phase difference φ in each case is as shown in FIG. If the phase difference is selected in this manner, the spherical aberration during CD reproduction can be reduced without affecting the wavefront of the DVD at all. The name of the inverted orbicular zone phase shifter is a name in consideration of a case where a phase shift is realized by a phase delay, such as when a film having a refractive index larger than that of air is added. When the phase shift can be realized by the phase advance, such as by scratching the lens, the annular zone may be directly scratched. These are equivalent in both cases, but hereinafter, including this case, will be referred to as an inverted orbicular zone phase shifter.
[0022]
Hereinafter, the shape of the annular phase shifter and the optimization of the phase difference will be described. As an evaluation index of the light spot, there is a Strehl intensity, which is the center intensity of the light spot having an aberration, normalized by the center intensity of the non-aberration spot, but this does not cause a difference in NA when there is a limiting aperture. Therefore, the ratio of the spot center intensity to the total amount of light incident on the pupil of the objective lens is used as a new evaluation index even when there is a limiting aperture. If this is used, for example, the larger the NA, the smaller the spot diameter and the larger the central intensity even for the same aperture, the larger the evaluation index. This metric is
[0023]
(Equation 3)

[0024]
It can be seen that it is proportional to the product of the square of the limiting aperture radius R standardized by the Strehl strength and the total aperture radius of the lens as shown in FIG. Hereinafter, a value obtained by multiplying the streak strength by the square of the standardized limiting opening radius is defined as η. In a normal CD pickup, the wavelength is 780 nm and the objective lens NA is 0.45. Therefore, if there is no aberration with respect to the DVD objective lens NA of 0.6, η = 1 × (0.45 / 0.6) ＾ 2 = 0 .56, η = 0.45 at the Strehl strength lower limit of 0.8 according to Marechal's criterion. The fourth order spherical aberration is caused by the substrate thickness error.
[0025]
(Equation 4)

[0026]
6th order spherical aberration
[0027]
(Equation 5)

[0028]
Given by However, n in these equations represents a refractive index. The aberration obtained by adding an annular phase shifter that delays the phase from the radius R1 to the radius R2 by φ using them is
[0029]
(Equation 6)

[0030]
Can be expressed as The streak strength is
[0031]
(Equation 7)

[0032]
From this, R1, R2, φ that maximizes η, the NA of the limiting aperture, W20, and W00 are obtained. Actually, using mathematical formula processing software, W20 and W00 were analytically obtained, and R1, R2, φ, and the NA of the limiting aperture were numerically obtained. As a result, when the inner diameter of the phase shifter is NA 0.20, the outer diameter is NA 0.42, the NA of the limiting aperture is 0.46, and the phase difference is 0.265λ (λ = 780 nm), η = 0.48 is the maximum. It turned out that it exceeded η = 0.45 according to Marechal's criterion. On the other hand, when optimization was performed only with the limiting aperture without using the phase shifter, η = 0.34 was the maximum at NA 0.39. That is, when converted to NA of 0.45, this corresponds to an improvement in the Strehl strength from 0.61 to 0.86. With respect to this phase difference, the aberration generated during DVD reproduction was 0.033λ (λ = 650 nm) in RMS wavefront aberration. This is almost equal to the processing accuracy of the lens, and it is considered that no problem actually occurs.
[0033]
Comparing this optimum phase difference of 0.265λ with the above-mentioned phase difference which does not affect the DVD, the closest one is the reverse orbicular zone phase mask when m = 2, n = 2, or m = 4, It can be seen that the annular phase mask is 0.333λ when n = 3. However, when m increases, the film or phase difference of the lens that causes a phase difference increases, and the phase difference shift increases when a wavelength shift occurs in the semiconductor laser. Therefore, the reverse annular phase mask is more preferable here. . When the shape of the phase shifter is fixed to a phase difference that does not affect the DVD, when the inner diameter is NA 0.20, the outer diameter is NA 0.44, and the limiting aperture NA is 0.48, η = 0.47 is maximum. It became. This is almost equal to the above-mentioned optimal phase difference.
[0034]
The effect of reducing aberration when this annular phase shifter was applied to a DVD-only lens was confirmed by ray tracing. FIG. 6 shows the specifications and surface shape of the lens used for ray tracing. Here, R of the surface shape is a central radius of curvature, k is a conic constant, and A4, A6, A8, and A10 are fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients, respectively. The surface shape using these
[0035]
(Equation 8)

[0036]
Can be expressed as Here, a shape symmetric with respect to the optical axis is assumed, and r is a radial coordinate. When collimated light having a wavelength of 780 nm is condensed through a CD substrate thickness of 1.2 mm using this lens, the RMS (Root Mean Square) wavefront aberration in the range of the numerical aperture NA = 0.45 is 0.1 mm at the best image point. 1279λ. On the other hand, when the above-mentioned annular phase plate was added, the RMS wavefront aberration could be reduced to 0.07366λ. However, since the ray tracing software used at this time could not handle a phase difference of 1λ or more, the thickness of the phase plate was 0.3333λ corresponding to m = 1 and n = 1. Therefore, the phase difference in the DVD is inevitably zero, and the spot aberration at a wavelength of 650 nm through the DVD substrate thickness of 0.6 mm is 0.001λ or less.
[0037]
Although the above description has been made on the assumption that the limiting aperture is used, this does not necessarily mean that an actual aperture is required. Actually, it is considered that this is almost equivalent to specifying an evaluation range of a pupil when obtaining an optimum focus position using the RMS wavefront aberration as an evaluation function. Assuming that the defocus is adjusted so that the RMS wavefront aberration is reduced as much as possible within the range of the limiting aperture, the light outside the range of the limiting aperture naturally has a large aberration and a large wavefront inclination. Thus, the light rays in such an area intersect the focal plane at a position far from the focal point. Therefore, it is almost equivalent to the absence of such a light beam with respect to the focused spot.
[0038]
When only the annular phase shifter is used, the spot performance is improved. However, when the Strehl intensity is equivalent to 0.86 in terms of NA of 0.45, displacement of optical components, disc tilt, defocus, etc. There is a possibility that it is not always sufficient if the deterioration of the spot due to is considered. Therefore, in addition to this, the substrate thickness to be optimized inside and outside the lens is changed. Hereinafter, this is referred to as a split lens. This was invented by the inventors as a method of obtaining compatibility between both DVD and CD at a wavelength of 650 nm (Japanese Patent Application No. 7-342203). However, when reproducing a CD at a wavelength of 780 nm, the NA of this division must be at least NA0. In this case, there is a disadvantage that aberrations when reproducing a DVD become extremely large. Therefore, when the phase shifter and the split lens were combined to optimize the phase shifter shape, phase difference, inner and outer split radii, and inner substrate thickness at the same time, the aberration at the time of CD reproduction at the wavelength of 780 nm generated by the split lens and the wavelength of 650 nm were generated. It has been found that there is a solution that reduces both aberrations during DVD reproduction and further improves the spot performance of CD. This will be described below.
[0039]
The wavefront aberration when combining the split lens and the phase shifter is
[0040]
(Equation 9)

[0041]
Can be expressed as Here, R1 is the inner diameter of the annular phase shifter, R2 is the division radius, R3 is the outer diameter of the annular phase shifter, and R4 is the limiting aperture radius. The thickness of the disk substrate for achieving no aberration differs at the boundary of the division radius, and is 0.6 mm on the outside according to the DVD and between 0.6 mm and 1.2 mm by optimization on the inside. Accordingly, the aberration coefficients W60 and W40 of the spherical aberration are displayed differently with the suffixes 1 and 2 accordingly. The defocus W201 and W202 are determined from the amount of spherical aberration so as to minimize the RMS wavefront aberration inside and outside the division, and the constant terms W001 and W002 are determined so that the average value of the wavefront aberration inside and outside the division becomes the same. RMS wavefront aberration is optimized. The difference between W201 and W202 and between W001 and W002 is determined by the corresponding substrate thickness difference between the inside and outside of the lens. W202 and W002 are analytically obtained by the mathematical processing software from the condition of minimizing the RMS under the condition of the given phase shifter. Then, W201 and W001 were also obtained. Further, for the given inner corresponding substrate thickness and division radius R2, conditions for maximizing η were obtained by numerically changing R1, R3, R4, and the phase difference. FIG. 7 shows the result. Here, the horizontal axis is the division radius of the division lens, and the vertical axis is η, and the calculation results under the optimal conditions are plotted while changing the central substrate thickness. Further, in the graph, the stigmatic aberration of the CD, the lower limit level corresponding to the Strehl intensity of 0.8, the optimum phase shifter and the fixed phase shifter are indicated by broken lines. Since these do not use a split lens, they cannot be plotted as points on this graph. On the other hand, FIG. 8 shows the RMS wavefront aberration generated in DVD reproduction at this time. As can be seen by comparing FIGS. 7 and 8, the closer the central substrate thickness is to 1.2 mm, the higher the performance of the CD and the greater the aberration of the DVD. Therefore, which of these points is to be adopted as the optimum point depends on the distribution of various margins of the system. However, for example, when the CD performance η = 0.526 (CD Strehl intensity conversion 0.94) and the DVD RMS wavefront aberration are about 0.0299λ when the center substrate thickness is 0.76 mm and the NA at the boundary of division is 0.45, almost It may be acceptable. At this time, when there was no phase shifter, the RMS wavefront aberration of DVD was 0.031λ, and that of CD was η = 0.414. Therefore, it can be seen that the aberration is reduced by the phase shifter for both CD and DVD. At this point, the maximum value of the CD performance and the minimum value of the DVD aberration coincide. At this time, the phase difference of the annular phase shifter was 0.2985λ (λ = 780 nm), the inner diameter was NA 0.2145, and the outer diameter was NA 0.45, which coincided with the NA at the boundary of division.
[0042]
FIG. 9 shows a schematic diagram of a split lens provided with an inverted annular phase mask. Since the inverted annular mask is built into the lens, the area of the annular phase mask is concave. At this time, a step due to the split lens is also shown on the surface of the disk having a relatively gentle curvature, but it is also possible to design this only on the image side.
[0043]
FIG. 10 shows the value of the CD reproduction spot performance η due to the shift of the CD reproduction wavelength. The range of the abscissa is ± 20 nm, but in practice, the wavelength range shifted by a temperature change or the like is considered to be about ± 10 nm. In this range, the degradation is about η = 0.52 at a wavelength shift of −10 nm from η = 0.53, and the change is about 0.93 to 0.92 in terms of the Strehl intensity conversion at NA 0.45. Absent. In the figure, the optimum annular phase shifter and the fixed annular phase shifter described above are also shown.
[0044]
FIG. 11 shows the RMS wavefront aberration with respect to the wavelength shift at the time of DVD reproduction at the wavelength of 650 nm. When the split lens and the optimal phase shifter are combined, the aberration increases from 0.030λ to 0.036λ at the wavelength shift of −10 nm. I have. This is also considered sufficiently within the allowable range. In the figure, the optimum annular phase shifter and the fixed annular phase shifter described above are also shown. For the fixed phase shifter, a phase difference is selected so that no aberration occurs in the DVD. Therefore, the aberration is 0 at a wavelength shift of 0. The phase difference of only the optimal phase shifter is shifted from the phase difference that does not cause aberration in the DVD, so that the wavefront aberration linearly changes toward the wavelength shift amount that becomes the phase difference.
[0045]
FIG. 12 shows a wavefront aberration shape at the time of CD reproduction at a wavelength of 780 nm. Since the defocus is optimized in the NA range of the limiting aperture, and the horizontal axis is indicated by the radial coordinates of the pupil over the entire aperture of NA 0.6, the peripheral portion has a very large aberration. At this time, since the vertical axis is folded and displayed within the range of ± 0.5λ, the peripheral portion seems to vibrate rapidly. In these, the aberration is suppressed in a wider NA as compared with the case where optimization is performed only with the limiting aperture. Further, the rise of the wavefront outside the range of the limiting aperture NA is also steep, and it is expected that the effect of the limiting aperture due to the large aberration will be more remarkable.
[0046]
FIG. 13 shows wavefront aberrations at the time of DVD reproduction at a wavelength of 650 nm. Since the wavefront aberration is completely zero in the case of only the limiting aperture in FIG. 12 and the case of only the fixed phase shifter, here, the case where the split lens and the optimum phase shifter are combined and the case where only the optimum phase shifter are used are described. it's shown. Since the aberration is not 0 even at the outermost peripheral portion where no aberration occurs, it is understood that a slight defocus occurs on the whole. This is because, when the phase difference generated by the phase shifter is considered as an aberration, slightly defocusing reduces the overall RMS wavefront aberration. In any case, the value on the vertical axis of the graph is considerably small, and the peculiarity of the wavefront shape is suppressed to the RMS wavefront aberration that does not substantially affect the wavefront shape.
[0047]
FIG. 14 shows the calculation results of the spot shape. The horizontal axis of the graph is the full width of the spot having an intensity of exp (-2) times the peak intensity of the spot, and the vertical axis is the value obtained by standardizing the intensity of the side lobe with the central intensity. Therefore, since it is desirable that both the spot and the side lobe are small, it can be said that the closer the plot point is to the lower left of the graph, the higher the resolution is. Here, a symmetrical Gaussian distribution is assumed as the intensity distribution of the pupil of the objective lens, and the ratio of the lens aperture to the width of the intensity range of exp (−2) times the intensity of the center of the Gaussian distribution in the pupil is 0. 1. Calculation results when the intensity of the peripheral portion is 0.98 with respect to the intensity of the central portion of the lens. The white circles in the figure indicate CDs having no aberration. The closer to this, the higher the level of reproduction performance can be expected from CDs. The black square indicates the case where only the limiting aperture is used in a normal DVD lens. When the limiting aperture is actually inserted, when the limiting aperture is taken as it is at the focal position, when the limiting aperture is taken, the spot center intensity is reduced. There are three plot points when the focal position is shifted so as to be the maximum. In each case, the spot resolution is inferior to that of the aberration-free CD. The black triangles indicate the case where only the optimum annular phase shifter is inserted, and there are three plot points in the same manner. Although the spot diameter is considerably improved as compared with the limiting aperture alone, the side lobe is considerably larger without the limiting aperture. The white squares indicate the case where the split lens and the optimum annular zone phase shifter are combined. Similarly, there are three plot points, and it can be seen that these three points are quite close. In other words, in this case, there is almost no change whether or not there is a limiting aperture, and since the aberration of light outside the range of the virtual limiting aperture sharply increases, it does not substantially affect spot formation. You can see that. In this case, the light spot has a slightly smaller spot diameter and slightly larger side lobes than the CD no aberration. The reason why the value of η, which is an evaluation index of the spot performance, was almost equal to or slightly inferior to that of the CD probably offset the effect of not reducing the side lobe by reducing the spot diameter. It is presumed that the situation has been reached. On the other hand, the calculation result of the spot when the DVD is reproduced is plotted at the lower left of the graph by a white triangle and a diamond. The diamonds represent spots for reproducing DVDs with no aberration, and the triangles represent the case where the optimum division lens and the optimum phase shifter are combined. DVDs have almost the same spot shape.
[0048]
FIG. 15 shows an embodiment of the optical head. The light from the semiconductor laser 4 is converted into parallel light by the collimating lens 5 and the elliptical beam is converted into a circular beam by the beam forming prisms 61 and 62. If the efficiency of the optical system is sufficiently high or the track pitch of the disk is wider than the distance between the main lobe of the light spot on the disk and the first dark line, the beam shaping prism should be removed to reduce the number of parts and to reduce adjacent track crosstalk. In some cases, it may be advantageous. Further, this light passes through the beam splitter 71, is further reflected by the rising mirror 8, and is focused on the optical disk by the objective lens 3 according to the present invention mounted on the two-dimensional actuator 9. The optical disk may be a CD or a DVD. The two-dimensional actuator 9 moves in the disk radial direction by the tracking error signal, positions the light spot on the track, moves in the optical axis direction by the focus error signal, and positions the focus position on the disk. The reflected light is reflected by the beam splitter 71 again via the objective lens 3 and the rising mirror 8, and is guided to the detection optical system. The light transmitted through the beam splitter 72 is condensed by the condensing lens 111 and is incident on the beam splitter 73. Here, the transmitted light passes through the cylindrical lens 12 and enters the four-divided photodetector 13. The differential signal between the sum signals of the diagonal components of the split detector is output by the differential amplifier 141 to be a defocus signal. On the other hand, the light reflected by the beam splitter 73 is incident on the two-split photodetector 15, and the differential signal of each output is output by the differential amplifier 142 to obtain a tracking error signal. The light reflected by the beam splitter 72 is condensed on the photodetector 16 by the condensing lens 112, and the photoelectrically converted signal is amplified by the amplifier 17 to obtain a reproduced signal. The reproduction signal may be detected from the sum signal of the outputs of the servo signal detection detector. In this case, the servo signal may be detected by band-limiting the signal detected up to the signal band with a low-pass filter or the like. The servo detection optical system is an example, and other methods can be used.
[0049]
The embodiment in which the annular phase shifter is built in the objective lens has been described above. FIG. 16 shows a two-dimensional actuator in which the objective lens 18 dedicated to DVD and the independent annular phase shifter 19 are integrated into a hybrid. This is an embodiment mounted on a computer. Here, it is assumed that only a portion corresponding to the optical system from the rising mirror to the disk in FIG. 15 is replaced, and only that portion is shown.
[0050]
【The invention's effect】
By optimally combining an annular phase shifter or an objective lens having a different substrate thickness with no aberration inside and outside of the lens, a DVD having a substrate thickness of 0.6 mm with a laser beam of 650 nm and a substrate thickness of 780 nm with a laser beam of 780 nm. A 1.2 mm CD can be reproduced by one lens without requiring a limiting aperture, and a compact and inexpensive optical head can be provided.
[Brief description of the drawings]
FIG. 1 is a basic image diagram of an objective lens according to the present invention.
FIG. 2 shows a spherical aberration wavefront shape.
FIG. 3 is a wavefront aberration shape obtained by an annular phase shifter.
FIG. 4 is a wavefront aberration shape by an inverted orbicular zone phase shifter.
FIG. 5 shows the phase shift amount of a CD under conditions that do not affect the DVD.
FIG. 6 shows a DVD lens specification used for ray tracing.
FIG. 7 is a CD reproduction spot performance when a split lens and a phase shifter are combined.
FIG. 8 shows RMS wavefront aberration generated in DVD reproduction.
FIG. 9 is a schematic view of the shape of a split lens in which an optimum inverted annular phase shifter is built.
FIG. 10 shows a change in CD reproduction spot performance due to a shift in CD reproduction wavelength.
FIG. 11 is an RMS wavefront aberration with respect to a wavelength shift during DVD reproduction.
FIG. 12 is a wavefront aberration shape during CD reproduction.
FIG. 13 is a wavefront aberration profile during DVD reproduction.
FIG. 14 is a calculation result of a spot shape.
FIG. 15 is an embodiment of an optical head.
FIG. 16 shows an embodiment in which the objective lens and the annular phase shifter are integrated in a hybrid.
[Explanation of symbols]
1} objective lens with annular phase shifter, 101 ° annular phase shift area, 2 ° spherical aberration wavefront, 102 ° inverted annular phase shift area, 3 ‥‥ inverted annular phase shifter integrated split lens, 4 ‥‥ semiconductor laser, 5 ‥‥ collimating lens, 61, 62 ‥‥ beam shaping prism, 71, 72, 73 ‥‥ beam splitter, 8 ‥‥ rising mirror, 9 ‥‥ two-dimensional actuator, 10 ‥‥ optical disk, 111, 112 ‥‥ condenser lens, 12 ‥‥ cylindrical lens, 13 ‥‥ quadrant detector, 141, 142 ‥‥ differential amplifier, 15 ‥‥ two-part detector, 16 ‥‥ detector, 17 ‥‥ amplifier, 18 ‥ {Object lens for DVD, 19} annular phase shifter.

Claims (4)

A laser beam having a first wavelength is focused on a substrate having a first thickness, and a laser beam having a second wavelength different from the first wavelength has a second thickness different from the first thickness. An objective lens for focusing light on a substrate having a thickness of 2;
The substrate thickness for focusing without aberration inside and outside the objective lens is different,
The objective lens has an annular phase shifter in a concave shape,
An objective lens, wherein the depth of the concave shape is adjusted so as to reduce both the aberration that occurs when the first wavelength is used and the aberration that occurs when the second wavelength is used. The objective lens according to claim 1, wherein the first wavelength is 780 nm, and the second wavelength is 650 nm. The objective lens according to claim 1, wherein the first thickness is 1.2 mm, and the second thickness is 0.6 mm. A semiconductor laser light source that emits laser light of a first wavelength and a second wavelength different from the first wavelength,
An objective lens that focuses the laser light on a substrate,
The substrate thickness for focusing without aberration inside and outside the objective lens is different,
The objective lens has a concave annular phase shifter,
An optical head , wherein the depth of the concave shape is adjusted so as to reduce both the aberration generated when the first wavelength is used and the aberration generated when the second wavelength is used.

Images