JP3661694B2 - Optical head and optical disk apparatus - Google Patents

Description

【００４７】
である。ここでＲｍは光ディスクに振幅１の平行光を垂直に入射した場合のｍ次の回折光の複素振幅に相当しており、
【００４８】
【数２】

【００４９】
のように表せ、特に幅ｗ、波長で規格化した溝深さｄの矩形溝の場合には
【００５０】
【数３】

【００５１】
のようになる。ただしここで
【００５２】
【数４】

【００５３】
である。これらを用いると、入射光に収差がなく、振幅が対物レンズ瞳面内で一様だとすれば光ディスクの周期的な案内溝による０次光と±１次光の干渉強度は
【００５４】
【数５】

【００５５】
のように表せる。ここでただし
【００５６】
【数６】

【００５７】
である。これによりレンズ移動のない場合のプッシュプル方式によるトラッキング信号は
【００５８】
【数７】

【００５９】
のように表せる。
【００６０】
ここで図５に示すように対物レンズ移動により２分割光検出器５０１上の光スポット５０２が動いたとすると、それぞれの２分割光検出器５０１の各分割領域の受光光量の増減から、通常のプッシュプル方式によるトラッキング信号はレンズ移動に起因するパラメタα（０＜α＜１）、β（０＜β＜１）を用いて
【００６１】
【数８】

【００６２】
のように表せる。この右辺第２項がオフセットに相当する。ここで数５において例えば回折格子によるサブスポットをトラックピッチの１／２だけずらすと考えると、右辺第３項のｃｏｓの中の位相がπずれるので、メインスポットのスポット位置ｕ０に対して、サブスポットの干渉強度は、
【００６３】
【数９】

【００６４】
のようになる。また数７においても１／２トラックだけずれれば、トラッキング信号もやはり反転するからレンズ移動のあるときのサブスポットのトラッキング信号は
【００６５】
【数１０】

【００６６】
となる。したがってメインスポットのトラッキング信号とサブスポットのトラッキング信号をさしひくことにより
【００６７】
【数１１】

【００６８】
の信号が得られる。したがってメインスポットがオフトラック０、すなわちｕ０＝０のとき、オフセットは
【００６９】
【数１２】

【００７０】
となる。したがって溝幅がトラックピッチに対して１／２でないときには、オフセットが残留する。これは数１１の上段右辺第２項からわかるように、メインスポットがオントラックにあるときに、その干渉強度がサブスポットの干渉強度と異なることに起因しているため、この強度変化を見込んで図４におけるゲインＧ２を設定しておくことによってキャンセルすることが可能である。またＤＶＤ−ＲＡＭのようにランドグルーブ方式の光ディスクの場合には、このようなゲイン設定をしなくとも、そのままでオフセットがキャンセルされる。
【００７１】
以上においては、トラッキング信号のオフセットキャンセルの効果を説明したが、このように干渉位相の反転した光を同時に検出することで、非点収差焦点ずれ検出方式などで問題となる、スポットが案内溝を横断するのに伴う焦点ずれ信号への外乱もキャンセルしている。以下、その原理について説明する。非点収差焦点ずれ検出のトラック横断に伴う外乱の原因は大きく２つある、１つはディスク上の光スポットに対して加わる非点収差であり、もう１つは４分割検出器のずれである。ここでは非点収差による外乱の混入を例として説明する。非点収差の波面は非点収差係数Ｗ２２、非点収差方位φを用いて
【００７２】
【数１３】

【００７３】
のように表せる。これは有効口径のｘ、ｙ座標で
【００７４】
【数１４】

【００７５】
のように表せるので、非点収差を持った波面が光ディスクによって回折されて、その０次光と±１次回折光が対物レンズ瞳面で±δずれて重なるとすると、非点収差によって加わる干渉の位相差は
【００７６】
【数１５】

【００７７】
のように近似できる。すると０次光と±１次回折光の干渉強度はこれを用いて
【００７８】
【数１６】

【００７９】
のように表せる。ここで図６に示すように非点収差のある光ディスク反射光束６０２において代表点Ａ，Ｂ，Ｃ，Ｄをとるとこれらの位置における干渉強度は数１６より
【００８０】
【数１７】

【００８１】
【数１８】

【００８２】
【数１９】

【００８３】
【数２０】

【００８４】
のように表せる。焦点ずれ検出のための検出器上で、基本的にこれらの強度がそのまま現れるとすると、焦点ずれ信号には
【００８５】
【数２１】

【００８６】
のようにオフトラックｕ０に対してｃｏｓ波形的な外乱が混入する。ここで集光スポットの案内溝横断に伴う強度分布変化が反転したスポットを同時に生成して加え合わせるすると、ψの位相がπずれて、数２１の最初のｓｉｎの符号が反転したものを足し合わせることになるので、外乱は相殺する。このとき焦点ずれ信号の外乱のキャンセルには、ディファレンシャルプッシュプルのオフセットキャンセルのために必要となったような溝幅などによる溝部と溝間部での反射率差のためのゲインの調整は不要である。
【００８７】
図７は、光ディスク上集光スポットが案内溝を横切る場合の強度変化の極性が反転した光束を同時に検出するための別の実施例である。ここでは光ディスクの半径方向に対して平行に配置された直線回折格子７０１を用いる。したがって回折格子による光ディスク上の±１次回折光は、０次光と同じトラック上に配置される。またしたがって反射光束を検出するための光検出器７０２の３つの４分割光検出領域１１２、１１３、１１４は光ディスク接線方向に平行に配置されている。
【００８８】
本実施例で用いている回折格子７０１の詳細構造について図８を用いて説明する。この回折格子は案内溝周期Ｐ、対物レンズ開口数ＮＡ、回折格子挿入位置有効光束径Ｄ、に対して図のようにＤλ／（２ＮＡ・Ｐ）の周期で格子の位相が反転した回折格子になっている。これは光ディスクの案内溝による回折光の０次光８０１に対して±１次回折光８０２、８０３の反射光束のずれによって決まる間隔となっている。このような回折格子による回折光は各周期ごとに回折光の波面の位相がπずれる。これは回折格子がもともとホログラムであることを思い起こせば、容易に理解できる。つまりホログラムはレーザ光のような干渉性の良い２つの光の干渉縞を写真乾板などに露光現像処理したものであり、これに露光したときの２つの光の一方を照射すると、露光したときのもう一方の光がホログラムの回折光として再生されるというものである。そこで上記のように周期的に波面が半波長ずれている光と平坦な波面の光を干渉させて干渉縞をつくったとすると、当然のことながら干渉縞はその位相ずれを反映して不連続に１／２縞の段差をつくる。したがって逆に、そのような回折格子に平坦な波面の光を入射すると、回折光の波面は周期的に１／２波長ずれることになるのである。
【００８９】
図９は位相反転回折格子による回折光のさらに光ディスク案内溝による回折光の位相シフト領域の重なり方を説明する図である。位相反転回折格子による回折光が光ディスクによる案内溝によってさらに回折され、０次光と±１次光がオーバーラップする。しかしながら０次光と±１次光など互いに隣り合った回折光どおしでは、位相反転領域が重なることなく接するようになっている。このときの図の中のａ，ｂ，ｃ，．．．で示した各領域に含まれる回折次数の任意の２つの間に、位相反転回折格子によって加わる位相差を図１０にまとめる。これによれば０次光と±１次回折光など、トラッキング信号に寄与する隣接する回折次数の光の間の位相差は必ずπになっていることがわかる。そして＋１次回折光と−１次回折光、０次光と±２次回折光など回折次数の差が２になるような回折光どうしの位相差は０となっている。したがって数５における干渉の位相差は、サブスポットを１／２トラックオフトラックさせることなく、それと等価な干渉強度反転が実現できる。このようにすると記録マークが中心スポットに対して両側トラックに非対称に存在しても、サブスポットの反射光量にはなんら非対称性が生じない。したがってトラッキング信号のオフセットキャンセルや、焦点ずれ誤差信号の外乱のキャンセルの効果がより安定となる。
【００９０】
ここで位相反転回折格子は、対物レンズと一体とはなっておらず、光ディスクの偏芯に追随して対物レンズが移動した場合に、位相反転回折格子と対物レンズの光軸は相対的にずれることになる。図１１はこの場合の位相シフト領域を示す図である。対物レンズ移動があっても位相シフト領域の継ぎ目が移動するだけで、干渉強度の反転にはなんら支障を生じないことがわかる。
【００９１】
図１２は光ディスク上集光スポットが案内溝を横切る場合の強度変化の極性が反転した光束を同時に検出するためのさらに別の実施例である。ここでは図７の位相反転回折格子にかわりに偏光性位相シフタ１２０１を用いている。これにより偏光性位相シフタ１２０１に入射する特定の方向の直線偏光成分の位相のみをＤλ／（２ＮＡ・Ｐ）周期の領域において反転させ、検出器７０２の直前で、３ビームウォラストンプリズム１２０２により、偏光分離して検出する。このとき位相反転回折格子の場合のような、サブスポットは生じず、光ディスク１０７上の光スポットは１つしか存在しない。これによりサブスポットによる光量損失を軽減でき、記録可能な光ディスクに適した光ヘッドを構成できる。
【００９２】
図１３は偏光性位相シフタの原理を説明するための図である。ここではニオブ酸リチウム（ＬｉＮｂＯ３）を用いた例を示す。ニオブ酸リチウム基板１３０１は紙面内方向に屈折率異方性の主軸１３０２を持ち、そこにグレーティングパターンに合わせてプロトン交換領域１３０３を形成する。さらにそのグレーティングパターンに合わせて、誘電体膜１３０４を形成する。このときグレーティングパターンとその間に入射する常光線１３０５、１３０６の位相差φｏ、異常光線１３０７、１３０８の位相差φｅはそれぞれ以下のように表せる。
【００９３】
【数２２】

【００９４】
ここでそれぞれの位相差を回折効率を考慮して適当な設計値に設定し、誘電体膜の厚さＴｄとプロトン交換領域の深さＴｐを未知数とする連立１次方程式として解けば、
【００９５】
【数２３】

【００９６】
となり常光線と異常光線で所望の位相差を独立に与える偏光性グレーティングが設計できる。例えば波長λ＝０．６６μm、誘電体層屈折率ｎｄ＝２．２でφｏ＝０゜、φｅ＝＋１８０゜としようとすればＴｐ＝２．０６μm、Ｔｄ＝０．０７μmとすればよい。このようにすればプロトン交換領域について選択的に位相をπずらすことができ、すでに述べた実施例と同様のオフセットキャンセル効果が期待できる。
【００９７】
以下、スカラー回折シミュレーションによる、焦点ずれ検出信号における集光スポットの案内溝横断時の外乱のキャンセル効果、および対物レンズ移動に伴う外乱のキャンセル効果について説明する。図１４は通常の焦点ずれ検出系において、非点収差、球面収差、ディテクタずれが複合して存在するときの、焦点ずれ信号である。中心部分が膨らんでおり、かなり大きな外乱が生じていることがわかる。一方、図１５は位相反転回折格子を用いることを想定した場合の計算結果である。これを見ると外乱がほとんどキャンセルしていることがわかる。図１６はディファレンシャルプッシュプル方式でのメインスポットとサブスポットの焦点ずれ信号を足し合わせたときの焦点ずれ信号である。外乱がほとんどキャンセルできていることがわかる。
【００９８】
図１７は通常の非点収差焦点ずれ検出方式の受光面上でのトラッキング信号を対物レンズ移動を変えて計算したものである。オフセットがかなり大きく発生していることがわかる。図１８は同様の計算を位相反転回折格子を用いたもの行ったものである。オフセットがほとんどキャンセルされていることがわかる。さらに図１９はやはり同様の計算をディファレンテャシャルプッシュプル方式において適用した場合の実施例である。ともにオフセットが非常に小さくなっている。
【００９９】
図２０は特にＣＤ，ＣＤ−Ｒ、ＤＶＤーＲＯＭ、ＤＶＤ−ＲＡＭの再生の可能な本発明による光学系構成の実施例である。半導体レーザはＤＶＤ用の６５０ｎｍ半導体レーザ２００１とＣＤ，ＣＤ−Ｒ用の７８０ｎｍの半導体レーザ２００２の２つを搭載している。ＣＤ−Ｒを再生するにはＣＤーＲの記録膜の反射率の分光特性の関係から、７８０ｎｍの半導体レーザ２００２が必須である。各々の光はそれぞれ回折格子２００３、２００４に入射し、±１次回折光を生じる。ここで６５０ｎｍ用の回折格子２００３は本発明でこれまで説明してきた回折格子であり、７８０ｎｍ用の回折格子はＣＤのトラッキング検出で通常用いられる３ビームトラッキング方式のためのサブスポット形成のための回折格子である。６５０ｎｍの光は次にダイクロイックミラー２００５を反射し、ビームスプリッタ２００６を透過して、立ち上げミラー２００７を反射し、ＤＶＤ／ＣＤ互換対物レンズ２００８により、ＤＶＤ２００９上に集光される。一方７８０ｎｍの光はビームスプリッタ２００６を反射し、立ち上げミラー２００７を反射して、ＤＶＤ／ＣＤ互換対物レンズ２００８により、ＣＤまたはＣＤ−Ｒディスク２００９に集光される。それぞれ反射光は、ＤＶＤ／ＣＤ互換対物レンズ２００８、立ち上げミラー２００７を経て、ビームスプリッタ２００６及びダイクロイックミラー２００５を透過して、光学部品Ｇを透過して光検出器２０１０に集光される。
【０１００】
図２１はこの光学系構成において焦点ずれ検出方式を変えた、複数の実施例での光学部品Ｇ、及び検出器受光パターン構成、及び信号演算方法を説明する図である。焦点ずれ検出方式にビームサイズ検出を用いる場合には光学部品Ｇに曲線回折格子２１０１を用い、回折格子２００３または２００４による０、±１次回折光それぞれについて、光検出器面上で焦点よりもやや前の光スポットと、焦点よりやや後ろの光スポットを曲線回折格子２１０１の±１次回折光として出力させる。このとき曲線回折格子２１０１の回折効率を十分大きくしておき、０次光は生じないようにしておくと検出領域数を減らすことができる。こうして合計６個の光スポットのうち曲線回折格子２１０１の１組の±１次回折光から、ビームサイズ検出方式による焦点ずれ誤差信号が得られる。回折格子２００３または２００４の０次光の一方を４分割光検出器で受光することより、ＤＶＤ−ＲＯＭで用いているＤＰＤ信号（ディファレンシャル・フェーズ・ディテクション）を得ることができる。また回折格子２００３の０次光と回折光の１つから、オフセットキャンセルされたプッシュプル信号をえることができる。ＣＤ用の３ビームトラッキング信号は回折格子２００４の±１次回折光の光量差から検出することができる。また回折格子２００３、２００４の０次光の総光量から再生ＲＦ信号を得ることができる。
【０１０１】
焦点ずれ検出方式としてダブルナイフエッジ方式を用いる場合には光学部品Ｇには、光分割プリズム２１０２を用いる。これにより光検出器上には回折格子２００３、２００４の回折光がそれぞれ４分割されている。回折光の一方の４つの光からダブルナイフエッジ方式による焦点ずれ誤差信号が得られる。プッシュプル方式によるトラッキング信号、ＤＰＤ信号、３ビーム方式によるトラッキング信号、ＲＦ信号などはビームサイズ検出とほぼ同様にして図のように検出することができる。
【０１０２】
これらの焦点ずれ検出方式では検出器の分割線方向の選択によって、ずれや収差にも比較的、案内溝横断に伴う外乱が発生しにくいため、本実施例においては特に分布反転した光と加え合わせる構成をしめさなかったが、光学系の構成によって別の要求から外乱の発生しやすい分割線の構成が必要となることも有り得る。その場合には強度分布変化の反転した光同士の焦点ずれ誤差信号を加え合わせることで、非点収差方式以外の焦点ずれ検出方式においても外乱を低減することが可能となる。したがって本発明は従来、案内溝横断の外乱の観点から採用できなかった光学系構成をも可能にでき、設計の自由度を増大させることができる。
【０１０３】
非点収差焦点ずれ検出方式を採用する場合には、光学部品Ｇは不要である。というのは非点収差焦点ずれ検出のための非点収差は、ダイクロイックミラーを透過するときに生じる非点収差で代用できるからである。これは集束光が平行平板に入射した場合には非点収差が発生する原理を用いている。ここでは焦点ずれ検出においては外乱の観点からこれまで説明したように０次光と１次光の焦点ずれ誤差信号の和をとっている。従来、平行平板を用いて非点収差を導入する場合、平行平板は案内溝横断の外乱が発生しにくいようにトラックに対して４５度をなすように挿入されていたが、本発明により外乱がキャンセルされることにより、このような制限を特に必要でなくなる。したがって場合によっては、光ヘッドの全体的な大きさをコンパクトに抑えるのに有効となり得る。またプッシュプル方式によるトラッキング信号については同様に分布反転したトラッキング信号との差をとっている。その他については他の焦点ずれ検出方式を採用した場合と同等である。
【０１０４】
以下、光磁気ディスク装置を例として、本発明による光ヘッドを光ディスク装置に用いた場合の実施例について説明する。
【０１０５】
光磁気ディスク装置２４００は、光ヘッド２４０１と、制御回路、及び、モータなどの機構系から構成されている。
【０１０６】
まず光ヘッド２４０１において、半導体レーザ２４０２からの直線偏光は本発明による回折格子２４２４、光分岐素子２４０３を透過して、対物レンズ２４０４により光磁気ディスク２４０５上に集光される。このとき信号再生時は光スポットが記録ドメインの上にあるか否かによって反射光の偏光方向がそれぞれ逆向きに約１゜回転することにより記録情報を搬送する。信号記録時は磁界印加コイル２４０６によって光磁気ディスク２４０５があらかじめ初期化されている磁化方向とは逆向きの磁界を印加し、光スポットが照射されて磁気記録膜の転移温度（キュリー点）より高温となった領域だけ磁化を反転させて磁気ドメインを形成する。
【０１０７】
光磁気ディスク２４０６からの反射光は再び対物レンズ２４０４を通り、光分岐素子２４０３を反射して、第２の光分岐素子２４０７に入射する。まず第２の光分岐素子２４０７を反射した光はシリンドリカルレンズ２４２５を透過して、サーボ信号検出用光検出器２４０８に入射し、その中に内蔵された分割光検出器により焦点ずれ信号とトラッキング誤差信号の演算にそれぞれ複数必要となる信号を検出する。ここでシリンドリカルレンズは、同様の非点収差を発生する回折格子でもかまわない。一方、第２の光分岐素子２４０７を透過した光は、偏光分離素子２４０９により、光磁気ディスク２４０６に入射した光の偏光方向に対して、それぞれ＋４５゜、−４５゜をなす直線偏光成分に分離され、２分割光検出器２４１０に入射する。したがって光磁気ディスクによる偏光回転がない場合はそれらは同じ光量となり、偏光回転がある場合はその回転方向に応じて２つの光量が増減する。つまり２分割光検出器の出力の差が光磁気再生信号となる。
【０１０８】
光ヘッド２４０１からの電気的な入出力はフレキシブル基板２４１１、２４１２を介して行われる。本実施例においては半導体レーザ２４０２の駆動とサーボ信号検出用光検出器２４０８の入出力をフレキシブル基板２４１１、２分割光検出器２４１０の入出力をフレキシブル基板２４１２によって行っている。
【０１０９】
半導体レーザ２４０２はユーザデータ２４１３をストアしたバッファメモリ２４１４のディジタル情報に応じて生成された、記録波形発生回路２４１５からの記録波形によりレーザ駆動回路２４１６で、その明滅を制御する。
【０１１０】
サーボ信号検出用光検出器２４０８による電流出力はサーボ信号演算回路２４１７によって電流電圧変換され、増幅され、本発明の演算方法によって演算され、焦点ずれ誤差信号、トラッキング誤差信号、ヘッド位置制御信号を生成する。前記２つの誤差信号は対物レンズアクチュエータ２４１８にフィードバックされ、集光スポットが光磁気ディスク２４０５の記録膜面の情報トラックの上に常にあるように閉ループ制御される。他方、ヘッド位置制御信号は光ヘッド２４０１を再生トラックの近傍にほぼ配置させるよう、ヘッド移動機構２４１９に入力される。こちらの制御はディジタル情報記録用の光磁気ディスクの場合にはやはり通常、閉ループ制御されるが、ＣＤ、ＤＶＤ、ＭＤなどにおいては開ループ制御させることが多い。本発明によりトラッキング信号、焦点ずれ誤差信号とも安定化させ、外乱に強く、記録再生の信頼性が向上する。
【０１１１】
２分割光検出器２４１０からの出力は通常、検出器内部に増幅器を内蔵させ、電圧出力とし、フレキシブル基板２４１２によって信号検出回路２４２０に入力され、差動増幅、イコライザ処理、２値化、復号処理されディジタル情報を再生する。出力情報はバッファメモリ２４１４にストアされる。
【０１１２】
光磁気ディスク２４０５はスピンドルモータ２４２１で回転されており、スピンドルモータ駆動回路２４２２によってその回転が制御されている。また磁界印加コイルは磁界印加コイル制御回路２４２３によってその磁化方向を制御されている。さらにこれらすべての制御回路はコントローラ２４２４で制御されている。
【０１１３】
本実施例においては、光磁気ディスク装置を例としたが、先に述べたＤＶＤ−ＲＡＭにおいても信号検出の光学系が光磁気方式でないだけで、ほぼ同様の構成で実現できる。またＣＤ−ＲＯＭや、ＤＶＤ−ＲＯＭにおいては、記録制御系が不要となるだけでそれ以外はほぼ同様の構成で光ディスク装置を実現できる。いずれについても本発明による光ヘッドを用いることで、焦点ずれ信号とトラッキング信号の安定化が図れ、対物レンズアクチュエータの可動部重量が軽く、高速化に対応でき、安価な光ディスク装置を実現できる。
【０１１４】
【発明の効果】
本発明によれば、回折格子などの安価な部品を対物レンズアクチュエータに搭載することなく、固定光学系に追加するのみで、焦点ずれ検出において光ディスクの偏芯に伴って記録膜面上で光スポットがトラックを横切るときに焦点ずれ信号に生じる外乱を根本的に解消でき、同時にトラッキング誤差信号に生じるレンズ移動に伴うオフセットを根本的にキャンセルできる。
【図面の簡単な説明】
【図１】本発明の基本的な実施例における光学系の構成。
【図２】光ディスク上のスポット配置とそのときの反射光束の強度分布を示す図。
【図３】検出器出力の回路演算方法を示す図。
【図４】検出器出力の演算方法を示す図。
【図５】対物レンズ移動によるトラッキング信号オフセットを説明する図。
【図６】非点収差による焦点ずれ信号への外乱を説明する図。
【図７】位相反転回折格子を用いた実施例における光学系の構成。
【図８】位相反転回折格子の詳細構造を説明する図。
【図９】位相反転光の光ディスク案内溝による回折における位相シフト領域の重なり方を説明する図。
【図１０】位相反転回折格子により光ディスク回折光に加わる干渉位相差。
【図１１】対物レンズ移動がある場合の、位相反転光の光ディスク案内溝による回折における位相シフト領域の重なり方を説明する図。
【図１２】偏光性位相シフタを用いた実施例における光学系の構成。
【図１３】偏光性位相シフタの原理を説明するための図。
【図１４】通常の焦点ずれ信号の案内溝横断による外乱の計算。
【図１５】位相反転回折格子を用いた場合の焦点ずれ信号の案内溝横断による外乱の計算。
【図１６】ディファレンシャルプッシュプル方式における焦点ずれ信号の案内溝横断による外乱の計算。
【図１７】通常の焦点ずれ信号検出光束でのトラッキング信号のレンズシフト特性。
【図１８】位相反転回折格子を用いた場合の焦点ずれ信号検出光束でのトラッキング信号のレンズシフト特性。
【図１９】ディファレンシャルプッシュプル方式における焦点ずれ信号検出光束でのトラッキング信号のレンズシフト特性。
【図２０】ＤＶＤ、ＤＶＤ−ＲＡＭ、ＣＤ，ＣＤ−Ｒの再生の可能な本発明による光学系構成の実施例。
【図２１】図２０の実施例における検出構成の詳細を説明する図。
【図２２】ランドグルーブディスクに対応した検出回路系構成図。
【図２３】図２２における増幅器３０１の周波数特性の説明図。
【図２４】光磁気ディスク装置に用いた場合の実施例。
【符号の説明】
１０１‥‥半導体レーザ、１０２‥‥直線回折格子、１０３‥‥ビームスプリッタ、１０４‥‥立ち上げミラー、１０５‥‥対物レンズアクチュエータ、１０６‥‥対物レンズ、１０７‥‥光ディスク、１０８‥‥０次光、１０９‥‥１次回折光、１１０‥‥−１次回折光、１１１‥‥シリンドリカルレンズ、１１２‥‥０次光用４分割光検出領域、１１３‥‥＋１次回折光用４分割光検出領域、１１４‥‥−１次回折光用４分割光検出領域、１１５‥‥光検出器、２０１‥‥０次光、２０２‥‥＋１次回折光、２０３‥‥−１次回折光、２０４‥‥グルーブ部、２０５‥‥ランド部、２０６‥‥０次光反射光束、２０７‥‥±１次回折光反射光束、３０１、３０２‥‥増幅器、３０３、３０４、３０５‥‥差動増幅器、５０１‥‥２分割光検出器、５０２‥‥反射光束、６０１‥‥非点収差のある光ディスク反射光束、６０２‥‥４分割光検出器、７０１‥‥位相反転回折格子、７０２‥‥光検出器、８０１‥‥光ディスク案内溝による０次回折光、８０２‥‥光ディスク案内溝による＋１次回折光、８０３‥‥光ディスク案内溝による−１次回折光、１２０１‥‥偏光性位相シフタ、１３０１‥‥ニオブ酸リチウム基板、１３０２‥‥結晶主軸方位、１３０３‥‥プロトン交換領域、１３０４‥‥誘電体膜、１３０５、１３０６‥‥常光線、１３０７、１３０８‥‥異常光線、２００１‥‥６５０ｎｍ半導体レーザ、２００２‥‥７８０ｎｍ半導体レーザ、２００３‥‥本発明の回折格子、２００４‥‥３ビームトラッキングサブビーム形成用回折格子、２００５‥‥ダイクロイックミラー、２００６‥‥ビームスプリッタ、２００７‥‥立ち上げミラー、２００８‥‥ＤＶＤ／ＣＤーＲ互換対物レンズ、２００９‥‥ＤＶＤまたはＤＶＤ−ＲＡＭまたはＣＤまたはＣＤ−Ｒ、Ｇ‥‥光学部品、２０１０‥‥光検出器、２１０１‥‥曲線回折格子、２１０２‥‥光分割プリズム、２４００‥‥光磁気ディスク装置、２４０１‥‥光ヘッド、２４０２‥‥半導体レーザ、２４０３‥‥光分岐素子、２４０４‥‥対物レンズ、２４０５‥‥光磁気ディスク、２４０６‥‥磁界印加コイル、２４０７‥‥第２の光分岐素子、２４０８‥‥サーボ信号検出用光検出器、２４０９‥‥偏光分離素子、２４１０‥‥２分割光検出器、２４１１、２４１２‥‥フレキシブル基板、２４１３‥‥ユーザデータ、２４１４‥‥バッファメモリ、２４１５‥‥記録波形発生回路、２４１６‥‥レーザ駆動回路、２４１７‥‥サーボ信号演算回路、２４１８‥‥対物レンズアクチュエータ、２４１９‥‥ヘッド移動機構、２４２０‥‥信号検出回路、２４２１‥‥スピンドルモータ、２４２２‥‥スピンドルモータ駆動回路、２４２３‥‥磁界印加コイル制御回路、２４２４‥‥コントローラ、２４２５‥‥本発明による回折格子、２４２６‥‥シリンドリカルレンズ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head used in an optical disc apparatus and an optical disc apparatus, and more particularly to a technique for improving the performance of detecting a light spot position control signal.
[0002]
[Prior art]
The conventional technique of the focal position control method in the optical disk apparatus is described in, for example, pages 79 to 83, supervised by Yoshito Tsunoda, edited by the Institute of Electronics, Information and Communication Engineers, “Basics and Applications of Optical Disk Storage” (1995, Corona). . According to this, there are Foucault method (knife edge method), astigmatism method, beam size detection method, image rotation method, etc. From the simplicity of the optical system, ease of adjustment, ease of combination with tracking detection, etc. At present, the most common is the astigmatism method. However, the astigmatism method has a problem that disturbance of the defocus signal is likely to occur when the light spot crosses the track on the recording film surface due to the eccentricity of the optical disk. This disturbance is likely to occur particularly when astigmatism occurs in the focused spot or when the light spot shifts on the photodetector. As a method of reducing this, for example, a method of shielding the central portion of the detected light beam (Japanese Patent Application No. 4-314500, Japanese Patent Application No. 5-102928), a method of reducing by rotating the objective lens (Japanese Patent Application No. 5-68774), detection There are methods for reducing disturbances by calculating light with and without astigmatism in the system (Japanese Patent Laid-Open No. 5-197980), but none of them is a fundamental solution, and a sufficient reduction effect is not always obtained. There is no current situation.
[0003]
In addition, this disturbance is particularly large in a land groove type optical disk adopted in a DVD-RAM that is scheduled for commercialization. In the land / groove type optical disc, the width of the guide groove (groove) is substantially equal to the width between the guide grooves (land), and information is recorded on both sides, so the pitch of the guide groove itself is smaller than that of the light spot. This is because the tracking signal based on the push-pull method, which will be described later, is very large because it is larger than the conventional optical disk. Therefore, in an optical head for DVD-RAM, a complicated beam size method and a Foucault method are inevitably used.
[0004]
The conventional technique of the tracking control method in the optical disk apparatus is similarly described in pages 83 to 92 of the first edition (1995, Corona) edited by Yoshito Tsunoda, edited by the Institute of Electronics, Information and Communication Engineers, 1995. ing. According to this, there are a 3-spot type, a diffracted light differential type (push-pull type), and the like. The read-only type such as CD is a three-spot type due to the simplicity of the optical system, ease of adjustment, strength against disturbance, etc. Push-pull when large laser emission power is required for recording such as magneto-optical disk and DVD-RAM The method is mainly adopted. At this time, there is also a situation where the push-pull method cannot be used for the CD and the three-spot type cannot be used for the recordable optical disc.
[0005]
In the CD pickup, the objective lens is mounted on the actuator and moved in order to follow the focused spot on the eccentricity of the optical disk because of the low cost. However, if the push-pull method is used, the detected light flux moves on the photodetector, and this appears as an offset. In addition, at a pit depth of λ / (4n) (λ: optical wavelength, n: substrate refractive index) at which the signal amplitude is the largest in a read-only optical disc, 0 of the diffracted light due to the periodic structure of the pit rows in the radial direction. There is a problem that the secondary light becomes small, and even if the focused spot is off-tracked, the interference intensity between the zero-order light and the ± first-order diffracted light does not generate an imbalance and a tracking signal cannot be obtained.
[0006]
On the other hand, in a recordable optical disc, particularly a magneto-optical disc, compensation for the eccentricity of the optical disc is usually called a coarse actuator, which is equipped with only an optical head or an objective lens and an actuator to bring the light spot close to the target track. It is done with an actuator. That is, the low frequency component of the tracking error is compensated by the coarse actuator, and the high frequency component is compensated by the objective lens actuator. In this way, the reliability required for the recording operation is increased. Therefore, since the amount of movement of the objective lens actuator is small compared to a CD or the like, a push-pull method with higher light utilization efficiency than the three-spot type can be adopted.
[0007]
Further, when a three-spot type is used for a recordable optical disk, there are the following problems as described in Optical Memory Symposium '86 Proceedings (1986), page 127. First, in an optical disk of a type that records by changing the reflectance of a recording mark such as a DVD-RAM, the light quantity of the preceding sub spot and the subsequent sub spot changes during the recording operation, and an offset occurs in the tracking signal. In the case of a magneto-optical disk, since there is light returning to the semiconductor laser, the interference state of stray light becomes unbalanced between the sub-spots on both sides due to the disk tilt, and an offset is also generated. Furthermore, in the DVD-RAM, as described above, the fact that the land-groove type optical disk is used can be cited as the reason why the 3-spot type cannot be used. That is, the land / groove type originally has the same width for the purpose of equalizing the reflected light quantity of the land part and the groove part, so that the light quantity inevitably hardly changes even if the light spot is off-tracked. A tracking signal cannot be taken. Therefore, the push-pull method must be used in the DVD-RAM, but unlike the magneto-optical disk, the DVD-RAM needs to be as low as a CD, and can move the objective lens in the push-pull method. The accompanying tracking signal reduction is essential.
[0008]
A conventional technique for solving this problem in the DVD-RAM is described, for example, in National Technical Report Vol. 40, No. 6 (1994), pages 771 to 778. Here, the objective lens, the λ / 4 plate, and the polarizing diffraction grating are integrally mounted on a two-dimensional actuator, and the interference areas of the + 1st order diffracted light of the disc and the 0th order light of the −1st order diffracted light are different from each other. A polarizing diffraction grating is configured to diffract to a diffraction angle. In this way, the interference region between the + 1st order diffracted light and the 0th order light of the −1st order diffracted light can be separated on the detector, so that even if the objective lens moves, these lights do not leave the detector. It is shown that if a split light detector is configured, an offset caused by the movement of the light spot on the detector can be eliminated.
[0009]
In addition, since the diffraction grating is made polarizing, the diffraction efficiency becomes almost zero when the light traveling to the disk passes through the diffraction grating, and appropriate diffraction is achieved when the disk reflected light again passes through the diffraction grating. To be efficient. If it is a normal diffraction grating that is not polarizable, the light going to the disk will also be diffracted, so light loss is inevitable, but by using it as a polarizing diffraction grating in this way, diffraction with the necessary reflected light is possible. It is possible to prevent light loss.
[0010]
However, in this conventional example, since the objective lens, the λ / 4 plate, and the polarizing diffraction grating are integrally mounted on the two-dimensional actuator, the moving part of the actuator becomes heavy and the response speed of the actuator is low. There is a problem that it is limited. The optical disc has a higher recording speed and a higher transfer speed year by year, and the conventional example cannot cope with further higher speed in the future.
[0011]
Another method of eliminating the tracking signal offset caused by the objective lens movement in the push-pull method without mounting optical components other than the objective lens on the objective lens actuator is described in the Optical Memory Symposium '86 Proceedings (1986). 127 to 132. This method uses 3 spots and subtracts the tracking signal by each push-pull method from the main spot and sub-spot to eliminate the tracking signal offset accompanying the movement of the objective lens. The differential push-pull method (differential push-pull method) )is called. In other words, by shifting the sub-spots to both sides of the main spot by a half of the guide groove period, it is possible to simultaneously detect the light in which the change in the interference intensity distribution of the disc reflected light accompanying off-track is reversed and the offset is included in the same phase. This is a method of canceling only the offset by generating a tracking signal having a reverse phase and subtracting these signals. At this time, the gain ratio when subtracting the main spot and the sub-spot is selected so as to compensate for the light amount difference generated in the main spot and the sub-spot in the diffraction grating that generates the sub-spot. If this conventional example is used, the tracking signal offset due to the movement of the objective lens in the push-pull method can be eliminated basically without mounting optical components other than the objective lens on the objective lens actuator. However, in this conventional example, no countermeasure is taken regarding the mixing of disturbance into the defocus signal when the focused spot crosses the guide groove in the astigmatism defocus detection method. Further, as described in the prior art, offset reduction is not sufficient when only one side of the sub-spot is on a recorded track and the other is on an unrecorded track. Further, although not described in this conventional example, an offset remains in this conventional example even when the total reflected light amount differs between the guide grooves and between the guide grooves. This occurs when the width between the guide groove and the guide groove is not equal. In the case of a DVD-RAM using a land groove disk having the same width, the main spot is a recorded track and the two sub-spots are unrecorded tracks. This situation occurs in some cases and vice versa. Further, since there are a plurality of light spots, it is disadvantageous in terms of light utilization efficiency during recording.
[0012]
In addition, as described in this conventional example, the gain ratio between the main spot and the sub spot is proportional to the reciprocal of the light amount ratio between the main spot and the two sub spots. Can not. This is because, as will be described later, when the guide groove for tracking is not effectively ½ of the track pitch, when the spot is on the information track, the track pitch ( In the land / groove disc, the reflectivity differs depending on the position shifted by ½ of the groove pitch). In this conventional example, the offset cannot be completely canceled unless compensation is included. It is because it is not considered. The allowable offset value is more strict as the recording density of the optical disk is higher. In recent years, such a residual offset has also become a problem.
[0013]
In this conventional example, since there is an information track between the guide grooves (land portion) and an optical disk without the information track is used in the guide groove (groove portion), the sub-spot is used when reproducing information at the main spot. Was not on the information track and was not affected by the information pits. However, in the case of a land / groove disk used for DVD-RAM, for example, when the main spot is on the information track, the sub-spot is also on the adjacent track, and the light quantity fluctuation due to the information pits is caused by the tracking signal. There is a problem that acts as a disturbance.
[0014]
Japanese Patent Laid-Open No. 4-168631 discloses a method for canceling the mixing of disturbance into an astigmatism defocus signal. Here, the main spot and the sub spot due to the diffraction grating are also arranged on the optical disc so as to be shifted by ½ of the guide groove period, and are received by the quadrant photodetector through the cylindrical lens. In each of the four detection areas, the defocus error signal obtained by taking the difference signal of the sum of the two areas in the diagonal direction is amplified with a gain proportional to the reciprocal of the incident light quantity ratio between the main spot and the sub spot. And adding them to obtain a defocus error signal. Naturally, the disturbance that occurs when the light spot crosses the guide groove due to astigmatism remaining due to misalignment of the optical head, misalignment of the photodetector, etc., has the same period as the guide groove. Since the function is a function, the disturbance is canceled if the main spot and the sub spot corresponding to a phase shift of 180 ° are effectively added with the same amount of light. Hereinafter, this conventional example will be referred to as an additive astigmatism method for convenience. In this case, the gain to be added is different from the above-described differential push-pull method, as described in the conventional example, in the ratio of the reciprocal of the incident light quantity so that the light quantity incident on the optical disk is effectively equivalent. What is necessary is just to make it proportional. However, this conventional example has a problem that no tracking method is disclosed. Also, if it is attempted to combine with the differential push-pull method described above, the amplification gain ratio of the main spot and the sub-spot will be made equal in the tracking signal calculation and the defocus signal calculation in the conventional example. However, there is a problem that the offset of the tracking signal cannot be canceled when there is an objective lens shift.
[0015]
In view of the above prior art, the problem to be solved by the present invention is that in the defocus detection method, the disturbance that occurs in the defocus signal when the light spot crosses the track on the recording film surface due to the eccentricity of the optical disk is fundamental. It is to eliminate it.
[0016]
At the same time, the offset accompanying the lens movement that occurs in the tracking error signal is fundamentally canceled.
[0017]
Also, like the differential push-pull method, a light beam in which the intensity change of the reflected light beam is reversed as the light spot on the disk crosses the guide groove is generated at the same time as the normal light beam. When canceling the in-phase offset by creating a tracking signal, the offset caused by the difference in the total reflected light quantity of these light beams is canceled.
[0018]
Also, in the differential push-pull method, the offset caused by the lens movement that occurs in the tracking error signal is canceled, and at the same time, in the defocus detection method, the focal point when the light spot crosses the track on the recording film surface due to the eccentricity of the optical disk. This is to fundamentally eliminate the disturbance generated in the deviation signal.
[0019]
Another object is to obtain the same tracking signal offset canceling effect accompanying the movement of the objective lens while keeping the sub spot in the differential push-pull method on the same track as the main spot.
[0020]
Further, the same effect as that of the differential push-pull method is configured such that only one spot is generated on the disk.
[0021]
Further, these effects are obtained particularly for the astigmatism defocus detection method and the push-pull tracking detection method.
[0022]
Moreover, it is to show a photodetector shape that obtains these effects.
[0023]
Further, in combining the differential push-pull method and the additive astigmatism method, it is to improve the performance of canceling the tracking signal offset by moving the objective lens.
[0024]
In addition, when the differential push-pull method and the additive astigmatism method are combined and applied to a land groove disk, the influence of disturbance due to information pits is removed.
[0025]
Another object of the present invention is to solve the above problems in an optical disc apparatus.
[0026]
[Means for Solving the Problems]
In order to solve the above problems,
A semiconductor laser, a condensing optical system that condenses the emitted light as at least one condensing spot on an optical disk having a periodic structure in a radial direction, a light detection system that detects reflected light from the optical disk, and a received light amount Light comprising at least an electric circuit that obtains at least one of a defocus signal and a tracking error signal of a light spot focused on the optical disc by photoelectric conversion and calculation, and a data signal recorded on the optical disc In the head, the condensing optical system has means for generating a plurality of reflected lights in which the polarities of the intensity changes in the reflected light flux as the periodic structure traverses the condensing spot on the optical disc are substantially reversed. And a means for separating and simultaneously detecting the plurality of reflected lights in the light detection system, which is caused by the intensity change in the electric circuit. Obtaining a focus error signal by variation of the focus error signal is the sum of focus error signals of the reflected light of each such offset each other.
[0027]
At this time, the difference between the tracking error signals of the plurality of reflected lights whose polarities are reversed at the same time is defined as a tracking error signal.
[0028]
Further, at this time, when the difference between the tracking error signals of the plurality of reflected lights whose polarities are reversed from each other is used as the tracking error signal, when one of the focused spots is on the information track of the optical disc in the electric circuit Each of the tracking error signals is amplified by a gain proportional to the ratio of the reciprocal of the total reflected light quantity of each of the signals, and the difference between them is calculated to obtain a tracking error signal.
[0029]
Further, in these, the means for generating a plurality of reflected lights whose polarities of intensity change are substantially inverted from each other is a diffraction grating disposed between the semiconductor laser and the optical branching element, and the diffraction grating is formed on the optical disk by the diffraction grating. The diffraction grating is arranged in the radial direction of the optical disc so that the condensing spot of ± 1st order diffracted light is radially displaced in the opposite direction by about half the period of the periodic structure with respect to the condensing spot of 0th order light. Installed at an angle with respect to.
[0030]
Alternatively, the means for generating a plurality of reflected lights whose intensities of the polarities are substantially reversed from each other is a diffraction grating disposed between the semiconductor laser and the optical branching element, and the diffraction grating is substantially in the disk radial direction. λD / (2NA · P) (λ: optical wavelength, NA: numerical aperture of objective lens, P: period of radial periodic structure on disk, D: effective beam diameter on diffraction grating) every other region of the same width The phase of the diffraction grating is tangent to the optical disc so that the converging spot of ± 1st order diffracted light by this diffraction grating is placed on the same track as the condensing spot of 0th order light on the optical disc. It is installed in parallel to the direction, and these are separated and detected by the light detection system, and a data signal is obtained from the received light amount signal from the 0th order light.
[0031]
Alternatively, the means for generating a plurality of reflected lights whose intensity change polarities are substantially reversed from each other is a polarizing phase shifter disposed between the semiconductor laser and the optical branching element, and the polarizing phase shifter is a disk. Areas of the same width in the radial direction every approximately λD / (2NA · P) (λ: optical wavelength, NA: objective lens numerical aperture, P: period of the periodic structure on the disk, D: effective beam diameter on the diffraction grating) The phase of the linearly polarized light component polarized in a specific direction is relatively inverted, and the phase of the linearly polarized light component orthogonal thereto is not changed by the entire phase shifter. A data signal is obtained from a polarization component not subjected to phase inversion by detecting it separately using an element.
[0032]
In particular, the above configuration is realized using the astigmatism method for defocus detection and the push-pull method for tracking error detection.
[0033]
In the light detection system, a light detector having at least two pairs of light detection regions for receiving one light spot in the light detection region divided into four is used.
[0034]
Further, a semiconductor laser, a condensing optical system for condensing the emitted light on an optical disk having a periodic structure such as a guide groove in the radial direction, a light detection system for detecting reflected light from the optical disk, and light from the reflected light In an optical head having an electric circuit that obtains both a spot defocus signal and a tracking error signal, the sub-spot is shifted from the main spot by a half of the guide groove period by a diffraction grating or the like. At least one kind of reflected light is generated in each of two types in which the polarities of the intensity changes in the reflected light flux accompanying the periodic structure traversing the spot are substantially reversed from each other. The light detection system separately detects the plurality of reflected lights, and amplifies a defocus error signal obtained by adding a defocus error signal obtained by at least one of the two kinds of reflected lights in the electric circuit, and further A defocus error signal is obtained by addition, and a tracking error signal is obtained by amplifying and subtracting the tracking error signal obtained by adding the tracking error signals of at least one of the two types of reflected light. At this time, the optical disk is a land / groove disk or the like, in which the periodic structure is constituted by a guide groove, and the error of the reflectance when the light spot is between the guide grooves is ± 30% or less as compared to when the light spot is in the guide groove. By using it, the ratio of the amplification gains of the tracking error signals of the two types of reflected light is matched with the ratio of the amplification gains of the defocus error signals of the two types of reflected light.
[0035]
Further, in the same optical head, when the optical disk is not a land-groove disk, that is, when one of the light spots is positioned on the information track of the optical disk, it is shifted from the information track by 1/2 of the period of the periodic structure. In the case of an optical disc having different reflectivity when positioned at a position, the amplification gain ratio of the tracking error signal of the plurality of reflected lights is made different from the amplification gain ratio of the defocus error signal of the plurality of reflected lights. .
[0036]
In the optical head, the electric circuit for detecting the sub spot has a frequency characteristic for cutting off a frequency band of a reproduction signal of recorded information on the optical disk.
[0037]
Further, an optical disk apparatus is provided in which the above optical head is provided with at least a disk rotating means and a reproduction control circuit for the optical disk.
[0038]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0039]
FIG. 1 shows the configuration of an optical system in a basic embodiment of the present invention. The light from the semiconductor laser 101 is diffracted by the diffraction grating 102, passes through the beam splitter 103, the rising mirror 104, and the objective lens 106, and then on the optical disc 107, the zero-order main spot 108 and the ± first-order diffracted light. The two sub-spots 109 and 110 are formed. The reflected light passes through the objective lens 106 and the rising mirror 104 again, reflects off the beam splitter 103, is given astigmatism for detecting a defocus by the cylindrical lens 111, and is received by the photodetector 115. The photodetector 115 is divided into a 0-order light quadrant light detection region 112 and a ± 1st-order diffracted light quadrant light detection regions 113, 114, which are independently detected. Here, the diffraction grating 102 is arranged slightly tilted so that the ± 1st order diffracted light on the disk is shifted from the 0th order light by half of the guide groove pitch on both sides.
[0040]
FIG. 2 is a diagram showing the spot arrangement on the optical disc at this time and the intensity distribution of the reflected light beam at that time. In the figure, the 0th-order light 201 and the ± 1st-order diffracted lights 202 and 203 are slightly shifted to the left side with respect to the groove part 204 and the land part 205, just on track, and slightly shifted to the right side. At this time, the intensity change of the 0th-order light detection light beam 206 and the ± 1st-order diffracted light detection light beam 207 shifts in the opposite direction with respect to the track deviation as shown in the figure. This is because the ± first-order diffracted lights 202 and 203 on the disk are arranged with a shift of ½ track with respect to the zero-order light. The fact that such a detected light beam intensity distribution is generated at the position of the focused spot on the optical disk itself is known as described in the above-mentioned literature.
[0041]
FIG. 3 is a diagram showing a circuit calculation method of the detector output at this time. However, on the detector 115, the intensity distribution is rotated 90 degrees due to astigmatism for defocus detection. Here, the defocus signal (AF signal) is calculated by the differential amplifier 303 after adding the same diagonal components of the corresponding divided outputs of the main spot and sub spot split detectors 112, 113 and 114. Can be obtained. Since the change in the intensity distribution of the sub-spot when the focused spot on the disk crosses the guide groove is reversed from that of the main spot, only the disturbance is canceled out in this way. At this time, since the light amount of the sub spot is usually smaller than that of the main spot, the signal output of the sub spot is amplified by the amplifier 301 by the light amount ratio, and the calculation is performed. However, since there are two sub-spots here, the gain on the sub-spot side is actually that the main spot intensity is A and the sub-spot intensity is B. The amplification gain may be increased by A / (2B) times. On the other hand, the tracking signal (TR signal) is obtained by taking the differential output by the differential amplifier 304 after adding the outputs of the two regions divided into right and left alternately in the main spot and the sub spot in the figure. . The change in intensity distribution of the sub-spot when the focused spot on the disk crosses the guide groove is reversed from the main spot, and the offset due to lens shift is not reversed. A signal is obtained. Here, when the main spot is on the land, the sub-spot is on the groove. Therefore, when the width of the land portion is different from that of the groove portion, a difference occurs in the amount of reflected light. As a result, offset cancellation becomes insufficient, and in such a case, the sub-spot signal is further amplified by the amplifier 302 so as to compensate for the difference in light quantity between the main spot and the sub-spot. For example, when the information track is on a land, the gain of the sub-spot amplifier 302 may be a / b, where a is the reflectance of the land and b is the reflectance of the groove. In some cases, the output of the main spot may be smaller. In such a case, the main spot side may be amplified. Alternatively, the gain of the amplifier 302 may be set to 1 or less. As described above, the tracking error signal without offset due to the lens shift and the defocus error signal without disturbance when crossing the guide groove can be obtained simultaneously. On the other hand, the reproduction signal may be output by the differential amplifier 305 with the total light amount of the main spot. However, here, it is assumed that the optical disk reproduces a signal with the amount of reflected light, such as a read-only optical disk or a phase change optical disk. However, in the case of the magneto-optical disk, the difference is only due to the difference between the data signal and the differential signal of the two signal outputs obtained by polarization separation, and the defocus error signal and tracking error signal can be detected by this embodiment. is there.
[0042]
FIG. 4 summarizes the calculation method at this time. Four outputs a, b, c, and d of the zero-order light quadrant light detection region 112 and four outputs e, f, g, h, i, and ± 4 primary light quadrant light detection regions 113 and 114, respectively. For j, k, and l, an operation as shown in the figure may be performed. However, RF is a data signal, AF is a defocus signal, and TR is a tracking error signal.
[0043]
In the above-described embodiment, it is assumed that the reflectance is generally different between the case where the light spot is in the guide groove and the case where the light spot is between the guide grooves. However, in the land / groove disk used in the DVD-RAM, the width of the guide groove is approximately ½ of the track pitch, and the reflectance is almost the same when the light spot is in the land and in the groove. For this reason, the amplifier 302 in FIG. 3 can be omitted, and the circuit configuration as shown in FIG. 22 can be simplified. However, even in the case of a land / groove disc, the difference in reflectance of the land portion relative to the groove portion can be about ± 10% at maximum due to manufacturing errors. However, if the difference is about this, according to computer simulation, when the effective aperture of the objective lens is 4 mm, the track offset is 0.01 μm in a DVD-RAM disk with a groove pitch of 1.48 μm even if the lens shift is 0.4 mm. Therefore, the track offset is allowable even in the configuration of FIG. On the contrary, if the allowable value of the track offset is 0.05 μm, the difference in reflectance of the land portion with respect to the groove portion is about 1.6 times under the same conditions. In an optical disc that is not a normal land / groove disc, this is more than twice, so the configuration of FIG. 22 can be applied only to a land / groove disc.
[0044]
In the land / groove disc, since there are information tracks in both the groove and the groove, when the main spot is in the information track, the sub spot is naturally on the adjacent information track. In this case, recording information is mixed into sub-spots that are not assumed except for land-groove discs. In order to avoid this, the amplifier 301 in FIG. 22 may have frequency characteristics as shown in FIG. In FIG. 23, the horizontal axis represents the frequency, the left vertical axis represents the frequency characteristics of the gain of the amplifier, and the right vertical axis represents the intensity of the reproduction signal of the information track in the detector. The reproduction signal is in a higher frequency band than the control signal such as the defocus error signal and the tracking error signal, but these signals are synthesized by the detector. Here, by giving the amplifier a characteristic that provides a low gain in the reproduction signal band, a control signal free from disturbance can be obtained.
[0045]
Next, it will be analytically described that the offset of the tracking signal due to the movement of the objective lens is canceled by such a method. According to the Journal of Optical Society of America, 1979, Vol. 69, No. 1, pages 4 to 24 (J. Opt. Soc. Am, 69, 1 (1979) 4-24), the period of the optical disc In the scalar diffraction approximation, the distribution of the reflected light by the structure is obtained by converting the m-th order Fourier coefficient Rm of the reflectance distribution of the optical disk and the incident light amplitude distribution a (x, y) to a distribution shift mNA / Pλ (NA : Shifted by the numerical aperture, P: guide groove period, λ: wavelength) and added by applying the phase component exp (i2πmu0 / P) due to the spot position u0. Ie
[0046]
[Expression 1]

[0047]
It is. Here, Rm corresponds to the complex amplitude of the mth-order diffracted light when the parallel light having an amplitude of 1 is vertically incident on the optical disk,
[0048]
[Expression 2]

[0049]
In the case of a rectangular groove having a groove depth d normalized by a width w and a wavelength,
[0050]
[Equation 3]

[0051]
become that way. Where
[0052]
[Expression 4]

[0053]
It is. If these are used, if the incident light has no aberration and the amplitude is uniform in the objective lens pupil plane, the interference intensity of the zero-order light and the ± first-order light by the periodic guide groove of the optical disk is
[0054]
[Equation 5]

[0055]
It can be expressed as Where
[0056]
[Formula 6]

[0057]
It is. As a result, the tracking signal by push-pull method when there is no lens movement is
[0058]
[Expression 7]

[0059]
It can be expressed as
[0060]
Here, as shown in FIG. 5, if the light spot 502 on the two-divided photodetector 501 is moved by the objective lens movement, the normal push is caused by the increase / decrease in the received light quantity of each divided area of each two-divided photodetector 501. The tracking signal by the pull method uses parameters α (0 <α <1) and β (0 <β <1) due to lens movement.
[0061]
[Equation 8]

[0062]
It can be expressed as The second term on the right side corresponds to the offset. Here, assuming that the sub-spot by the diffraction grating is shifted by ½ of the track pitch in Formula 5, for example, the phase in the cos of the third term on the right side is shifted by π. The interference intensity of the spot is
[0063]
[Equation 9]

[0064]
become that way. Also, in Formula 7, if the track is shifted by ½ track, the tracking signal is also inverted, so the sub-spot tracking signal when the lens moves is
[0065]
[Expression 10]

[0066]
It becomes. Therefore, by subtracting the main spot tracking signal and the sub spot tracking signal,
[0067]
[Expression 11]

[0068]
Is obtained. Therefore, when the main spot is off-track 0, that is, u0 = 0, the offset is
[0069]
[Expression 12]

[0070]
It becomes. Therefore, an offset remains when the groove width is not ½ of the track pitch. As can be seen from the second term on the upper right side of Equation 11, when the main spot is on-track, the interference intensity is different from the interference intensity of the sub-spot, so this intensity change is expected. Cancellation is possible by setting the gain G2 in FIG. Further, in the case of a land groove type optical disk such as a DVD-RAM, the offset is canceled as it is without setting such a gain.
[0071]
In the above, the effect of the offset cancellation of the tracking signal has been described. However, by simultaneously detecting the light with the reversed interference phase in this way, the spot becomes a problem in the astigmatism defocus detection method, etc. Disturbances to the defocus signal accompanying crossing are also cancelled. Hereinafter, the principle will be described. There are two major causes of disturbance associated with track crossing in astigmatism defocus detection. One is astigmatism applied to the light spot on the disk, and the other is a four-divided detector shift. . Here, description will be given by taking as an example the mixing of disturbance due to astigmatism. The wave front of astigmatism uses astigmatism coefficient W22 and astigmatism azimuth φ.
[0072]
[Formula 13]

[0073]
It can be expressed as This is the x and y coordinates of the effective aperture
[0074]
[Expression 14]

[0075]
Therefore, if the wavefront having astigmatism is diffracted by the optical disk, and the zero-order light and the ± first-order diffracted light overlap with each other by ± δ on the objective lens pupil plane, interference caused by astigmatism is caused. The phase difference is
[0076]
[Expression 15]

[0077]
It can be approximated as follows. Then, the interference intensity between 0th order light and ± 1st order diffracted light is used
[0078]
[Expression 16]

[0079]
It can be expressed as Here, when representative points A, B, C, and D are taken in an optical disc reflected light beam 602 having astigmatism as shown in FIG.
[0080]
[Expression 17]

[0081]
[Expression 18]

[0082]
[Equation 19]

[0083]
[Expression 20]

[0084]
It can be expressed as Assuming that these intensities appear as they are on the detector for defocus detection, the defocus signal is
[0085]
[Expression 21]

[0086]
As shown, a disturbance having a cosine waveform is mixed into the off-track u0. Here, when a spot in which the intensity distribution change accompanying the crossing of the guide groove of the condensing spot is reversed is generated and added at the same time, the phase of ψ is shifted by π, and the one in which the sign of the first sin in Equation 21 is reversed is added. Therefore, the disturbance will cancel out. At this time, it is not necessary to adjust the gain for the difference in reflectivity between the groove and the groove due to the groove width, etc., which is necessary for canceling the differential push-pull offset, to cancel the disturbance of the defocus signal. is there.
[0087]
FIG. 7 shows another embodiment for simultaneously detecting a light beam in which the polarity of the intensity change when the focused spot on the optical disk crosses the guide groove is reversed. Here, a linear diffraction grating 701 disposed in parallel to the radial direction of the optical disk is used. Therefore, the ± first-order diffracted light on the optical disk by the diffraction grating is arranged on the same track as the zero-order light. Accordingly, the three quadrant light detection areas 112, 113, and 114 of the photodetector 702 for detecting the reflected light beam are arranged in parallel to the optical disc tangential direction.
[0088]
A detailed structure of the diffraction grating 701 used in this embodiment will be described with reference to FIG. This diffraction grating is a diffraction grating in which the phase of the grating is inverted with a period of Dλ / (2NA · P) as shown in the figure with respect to the guide groove period P, the objective lens numerical aperture NA, and the diffraction grating insertion position effective beam diameter D. It has become. This is an interval determined by the deviation of the reflected light beams of the ± 1st order diffracted beams 802 and 803 with respect to the 0th order beam 801 of the diffracted beam by the guide groove of the optical disk. The diffracted light by such a diffraction grating has a phase shift of π of the wavefront of the diffracted light every period. This can be easily understood by recalling that the diffraction grating is originally a hologram. In other words, the hologram is obtained by exposing and developing an interference fringe of two light beams having good coherence such as a laser beam to a photographic dry plate and the like. The other light is reproduced as diffracted light of the hologram. Therefore, if interference fringes are created by causing interference between light with a wavefront whose wavefront is periodically shifted by half a wavelength and light with a flat wavefront as described above, the interference fringes naturally reflect the phase shift and become discontinuous. Create a 1 / 2-striped step. Therefore, conversely, when light having a flat wavefront is incident on such a diffraction grating, the wavefront of the diffracted light is periodically shifted by ½ wavelength.
[0089]
FIG. 9 is a diagram for explaining how the phase shift areas of the diffracted light by the phase inversion diffraction grating and the diffracted light by the optical disc guide groove overlap. The diffracted light by the phase inversion diffraction grating is further diffracted by the guide groove by the optical disc, and the zero order light and the ± first order light overlap. However, the diffracted lights adjacent to each other such as 0th order light and ± 1st order light are in contact with each other without overlapping the phase inversion regions. A, b, c,. . . The phase differences applied by the phase inversion diffraction grating between any two diffraction orders included in each region shown in FIG. 10 are summarized in FIG. This shows that the phase difference between adjacent diffraction orders that contribute to the tracking signal, such as 0th order light and ± 1st order diffracted light, is always π. The phase difference between the diffracted lights, such as + 1st order diffracted light and −1st order diffracted light, 0th order light and ± 2nd order diffracted light, and the like so that the difference in diffraction order is 2, is zero. Therefore, the interference phase difference in Equation 5 can realize the equivalent inversion of the interference intensity without causing the sub-spot to be ½ track off-track. In this way, even if the recording mark is present asymmetrically on the tracks on both sides with respect to the center spot, no asymmetry occurs in the reflected light quantity of the sub spot. Accordingly, the effect of canceling the offset of the tracking signal and canceling the disturbance of the defocus error signal becomes more stable.
[0090]
Here, the phase inversion diffraction grating is not integrated with the objective lens, and when the objective lens moves following the eccentricity of the optical disk, the optical axes of the phase inversion diffraction grating and the objective lens are relatively shifted. It will be. FIG. 11 is a diagram showing a phase shift region in this case. It can be seen that even if the objective lens is moved, only the seam of the phase shift region is moved, and there is no problem in reversing the interference intensity.
[0091]
FIG. 12 shows still another embodiment for simultaneously detecting a light beam in which the polarity of the intensity change when the focused spot on the optical disk crosses the guide groove is reversed. Here, a polarizing phase shifter 1201 is used instead of the phase inversion diffraction grating of FIG. As a result, only the phase of the linearly polarized light component in a specific direction incident on the polarizing phase shifter 1201 is inverted in the region of Dλ / (2NA · P) period, and immediately before the detector 702, the three-beam Wollaston prism 1202 Detect with polarization separation. At this time, no sub-spot is generated as in the case of the phase-inversion diffraction grating, and there is only one light spot on the optical disc 107. As a result, the light loss due to the sub-spot can be reduced, and an optical head suitable for a recordable optical disk can be configured.
[0092]
FIG. 13 is a diagram for explaining the principle of the polarizing phase shifter. Here, an example using lithium niobate (LiNbO3) is shown. The lithium niobate substrate 1301 has a principal axis 1302 with refractive index anisotropy in the in-plane direction, and forms a proton exchange region 1303 in accordance with the grating pattern. Further, a dielectric film 1304 is formed in accordance with the grating pattern. At this time, the grating pattern and the phase difference φo between the ordinary rays 1305 and 1306 incident therebetween and the phase difference φe between the extraordinary rays 1307 and 1308 can be expressed as follows.
[0093]
[Expression 22]

[0094]
Here, if each phase difference is set to an appropriate design value in consideration of diffraction efficiency, and solved as a simultaneous linear equation with the thickness Td of the dielectric film and the depth Tp of the proton exchange region as unknowns,
[0095]
[Expression 23]

[0096]
Therefore, it is possible to design a polarizing grating that gives a desired phase difference independently between ordinary rays and extraordinary rays. For example, if the wavelength λ = 0.66 μm, the dielectric layer refractive index nd = 2.2, and φo = 0 ° and φe = + 180 °, then Tp = 2.06 μm and Td = 0.07 μm. In this way, the phase can be selectively shifted by π with respect to the proton exchange region, and the same offset canceling effect as in the above-described embodiment can be expected.
[0097]
In the following, a description will be given of the effect of canceling disturbance when the focused spot crosses the guide groove in the defocus detection signal and the effect of canceling disturbance due to the movement of the objective lens by the scalar diffraction simulation. FIG. 14 shows a defocus signal when an astigmatism, a spherical aberration, and a detector shift are present in combination in an ordinary defocus detection system. It can be seen that the central part is swollen and a considerably large disturbance has occurred. On the other hand, FIG. 15 shows a calculation result when it is assumed that a phase inversion diffraction grating is used. As you can see, the disturbance is almost cancelled. FIG. 16 shows a defocus signal when the defocus signals of the main spot and the sub spot in the differential push-pull method are added. It can be seen that the disturbance is almost cancelled.
[0098]
FIG. 17 shows the tracking signal on the light receiving surface of the normal astigmatism defocus detection method calculated by changing the objective lens movement. It can be seen that the offset is considerably large. FIG. 18 shows a similar calculation using a phase-inversion diffraction grating. It can be seen that the offset is almost cancelled. Further, FIG. 19 shows an embodiment when the same calculation is applied to the differential push-pull method. Both offsets are very small.
[0099]
FIG. 20 shows an embodiment of an optical system configuration according to the present invention that can reproduce CDs, CD-Rs, DVD-ROMs, and DVD-RAMs. Two semiconductor lasers, a 650 nm semiconductor laser 2001 for DVD and a 780 nm semiconductor laser 2002 for CD and CD-R, are mounted. In order to reproduce the CD-R, a semiconductor laser 2002 of 780 nm is indispensable from the relationship of the spectral characteristics of the reflectance of the recording film of the CD-R. Each light is incident on diffraction gratings 2003 and 2004, respectively, to generate ± first-order diffracted light. Here, the diffraction grating 2003 for 650 nm is the diffraction grating described so far in the present invention, and the diffraction grating for 780 nm is diffraction for forming a sub-spot for the three-beam tracking method that is usually used for CD tracking detection. It is a lattice. Next, the 650 nm light is reflected by the dichroic mirror 2005, transmitted through the beam splitter 2006, reflected by the rising mirror 2007, and condensed on the DVD 2009 by the DVD / CD compatible objective lens 2008. On the other hand, the 780 nm light is reflected by the beam splitter 2006, reflected by the rising mirror 2007, and condensed on the CD or CD-R disc 2009 by the DVD / CD compatible objective lens 2008. The reflected light passes through the DVD / CD compatible objective lens 2008 and the rising mirror 2007, passes through the beam splitter 2006 and the dichroic mirror 2005, passes through the optical component G, and is collected on the photodetector 2010.
[0100]
FIG. 21 is a diagram for explaining the optical component G, detector light-receiving pattern configuration, and signal calculation method in a plurality of embodiments in which the defocus detection method is changed in this optical system configuration. When beam size detection is used for the defocus detection method, a curved diffraction grating 2101 is used for the optical component G, and the 0 and ± 1st order diffracted lights by the diffraction grating 2003 or 2004 are slightly ahead of the focus on the photodetector surface. And the light spot slightly behind the focal point are output as ± first-order diffracted light of the curved diffraction grating 2101. At this time, if the diffraction efficiency of the curved diffraction grating 2101 is sufficiently increased so that zero-order light is not generated, the number of detection regions can be reduced. Thus, a defocus error signal by the beam size detection method is obtained from one set of ± first-order diffracted lights of the curved diffraction grating 2101 out of a total of six light spots. A DPD signal (differential phase detection) used in a DVD-ROM can be obtained by receiving one of the zero-order lights of the diffraction grating 2003 or 2004 with a 4-split photodetector. In addition, a push-pull signal whose offset has been canceled can be obtained from one of the 0th-order light and the diffracted light of the diffraction grating 2003. The three-beam tracking signal for CD can be detected from the light amount difference of ± first-order diffracted light from the diffraction grating 2004. In addition, a reproduction RF signal can be obtained from the total amount of zero-order light of the diffraction gratings 2003 and 2004.
[0101]
When the double knife edge method is used as the defocus detection method, the light splitting prism 2102 is used as the optical component G. As a result, the diffracted light of the diffraction gratings 2003 and 2004 is divided into four on the photodetector. A defocus error signal by the double knife edge method is obtained from one of the four lights of the diffracted light. Push-pull tracking signals, DPD signals, 3-beam tracking signals, RF signals, and the like can be detected as shown in the figure in substantially the same manner as beam size detection.
[0102]
In these defocus detection methods, disturbances due to crossing of the guide grooves are less likely to occur due to the selection of the dividing line direction of the detector. Therefore, in this embodiment, in addition to the light whose distribution has been reversed. Although the configuration has not been shown, it is possible that a configuration of a dividing line that is likely to generate a disturbance due to another requirement is required depending on the configuration of the optical system. In that case, by adding the defocus error signals of the lights whose intensity distributions are inverted, disturbance can be reduced even in defocus detection methods other than the astigmatism method. Therefore, the present invention can also enable an optical system configuration that has not been conventionally adopted from the viewpoint of disturbance across the guide groove, and can increase the degree of freedom in design.
[0103]
When the astigmatism defocus detection method is employed, the optical component G is not necessary. This is because astigmatism for detecting astigmatism defocus can be substituted by astigmatism generated when passing through the dichroic mirror. This is based on the principle that astigmatism occurs when focused light is incident on a parallel plate. Here, in the defocus detection, the sum of defocus error signals of the 0th order light and the 1st order light is taken from the viewpoint of disturbance as described above. Conventionally, when introducing astigmatism using a parallel plate, the parallel plate has been inserted at 45 degrees with respect to the track so that the disturbance across the guide groove does not occur easily. By being canceled, such a restriction is not particularly necessary. Therefore, in some cases, it can be effective to keep the overall size of the optical head compact. Similarly, the tracking signal using the push-pull method is different from the tracking signal whose distribution is inverted. Others are the same as when other defocus detection methods are employed.
[0104]
Hereinafter, an example in which the optical head according to the present invention is used in an optical disk apparatus will be described by taking a magneto-optical disk apparatus as an example.
[0105]
The magneto-optical disk device 2400 includes an optical head 2401, a control circuit, and a mechanism system such as a motor.
[0106]
First, in the optical head 2401, the linearly polarized light from the semiconductor laser 2402 passes through the diffraction grating 2424 and the optical branching element 2403 according to the present invention, and is condensed on the magneto-optical disk 2405 by the objective lens 2404. At this time, at the time of signal reproduction, the recording information is conveyed by rotating the polarization direction of the reflected light in the opposite direction by about 1 ° depending on whether or not the light spot is on the recording domain. At the time of signal recording, the magnetic field application coil 2406 applies a magnetic field opposite to the magnetization direction in which the magneto-optical disk 2405 is initialized in advance, and is irradiated with a light spot, which is higher than the transition temperature (Curie point) of the magnetic recording film. The magnetic domain is formed by reversing the magnetization only in the region.
[0107]
The reflected light from the magneto-optical disk 2406 passes through the objective lens 2404 again, reflects off the optical branch element 2403, and enters the second optical branch element 2407. First, the light reflected from the second optical branching element 2407 passes through the cylindrical lens 2425 and enters the servo signal detection photodetector 2408, and the defocusing signal and tracking error are incorporated by the split photodetector incorporated therein. A plurality of signals necessary for signal calculation are detected. Here, the cylindrical lens may be a diffraction grating that generates the same astigmatism. On the other hand, the light transmitted through the second optical branching element 2407 is separated into linearly polarized light components that form + 45 ° and −45 ° with respect to the polarization direction of the light incident on the magneto-optical disk 2406 by the polarization separating element 2409, respectively. And enters the two-divided photodetector 2410. Accordingly, when there is no polarization rotation by the magneto-optical disk, they have the same light amount, and when there is polarization rotation, the two light amounts increase or decrease according to the rotation direction. That is, the difference in output from the two-divided photodetector becomes the magneto-optical reproduction signal.
[0108]
Electrical input / output from the optical head 2401 is performed via flexible substrates 2411 and 2412. In this embodiment, driving of the semiconductor laser 2402 and input / output of the servo signal detection photodetector 2408 are performed by the flexible substrate 2411 and input / output of the split photodetector 2410 by the flexible substrate 2412.
[0109]
The laser drive circuit 2416 controls the blinking of the semiconductor laser 2402 by the recording waveform from the recording waveform generation circuit 2415 generated according to the digital information in the buffer memory 2414 storing the user data 2413.
[0110]
The current output from the servo signal detection photodetector 2408 is converted into a current voltage by a servo signal calculation circuit 2417, amplified, and calculated by the calculation method of the present invention to generate a defocus error signal, tracking error signal, and head position control signal. To do. The two error signals are fed back to the objective lens actuator 2418 and controlled in a closed loop so that the focused spot is always on the information track on the recording film surface of the magneto-optical disk 2405. On the other hand, the head position control signal is input to the head moving mechanism 2419 so that the optical head 2401 is disposed almost in the vicinity of the reproduction track. This control is usually closed loop control in the case of a magneto-optical disk for recording digital information, but in many cases, open loop control is performed in a CD, DVD, MD or the like. The present invention stabilizes both the tracking signal and the defocus error signal, is resistant to disturbance, and improves the reliability of recording and reproduction.
[0111]
The output from the two-divided photodetector 2410 is usually an amplifier built in the detector, is used as a voltage output, is input to the signal detection circuit 2420 by the flexible board 2412, and is differentially amplified, equalized, binarized, and decoded. The digital information is reproduced. The output information is stored in the buffer memory 2414.
[0112]
The magneto-optical disk 2405 is rotated by a spindle motor 2421, and its rotation is controlled by a spindle motor drive circuit 2422. The magnetization direction of the magnetic field application coil is controlled by a magnetic field application coil control circuit 2423. Furthermore, all these control circuits are controlled by a controller 2424.
[0113]
In the present embodiment, the magneto-optical disk device is taken as an example, but the DVD-RAM described above can be realized with substantially the same configuration only if the signal detection optical system is not a magneto-optical system. In addition, in the CD-ROM and DVD-ROM, an optical disc apparatus can be realized with substantially the same configuration except that a recording control system is not required. In any case, by using the optical head according to the present invention, the defocus signal and the tracking signal can be stabilized, the moving part weight of the objective lens actuator is light, the speed can be increased, and an inexpensive optical disc apparatus can be realized.
[0114]
【The invention's effect】
According to the present invention, an inexpensive spot such as a diffraction grating is not added to the objective lens actuator, but only added to the fixed optical system. Disturbances that occur in the defocus signal when the lens crosses the track can be fundamentally eliminated, and at the same time, the offset due to lens movement that occurs in the tracking error signal can be fundamentally canceled.
[Brief description of the drawings]
FIG. 1 shows the configuration of an optical system in a basic embodiment of the present invention.
FIG. 2 is a diagram showing a spot arrangement on an optical disc and an intensity distribution of a reflected light beam at that time.
FIG. 3 is a diagram showing a circuit calculation method of detector output.
FIG. 4 is a diagram illustrating a calculation method of a detector output.
FIG. 5 is a diagram illustrating a tracking signal offset due to objective lens movement.
FIG. 6 is a diagram for explaining a disturbance to a defocus signal due to astigmatism.
FIG. 7 shows a configuration of an optical system in an embodiment using a phase inversion diffraction grating.
FIG. 8 illustrates a detailed structure of a phase inversion diffraction grating.
FIG. 9 is a diagram for explaining how phase shift regions overlap in diffraction of phase-inverted light by an optical disc guide groove.
FIG. 10 shows an interference phase difference applied to the optical disc diffracted light by the phase inversion diffraction grating.
FIG. 11 is a diagram for explaining how phase shift regions overlap in diffraction by an optical disc guide groove of phase-inverted light when there is an objective lens movement.
FIG. 12 shows a configuration of an optical system in an example using a polarizing phase shifter.
FIG. 13 is a diagram for explaining the principle of a polarizing phase shifter.
FIG. 14 shows a calculation of a disturbance due to traversing a guide groove of a normal defocus signal.
FIG. 15 is a diagram illustrating a calculation of a disturbance due to crossing of a guide groove of a defocus signal when a phase inversion diffraction grating is used.
FIG. 16 shows the calculation of disturbance caused by crossing the guide groove of the defocus signal in the differential push-pull method.
FIG. 17 shows a lens shift characteristic of a tracking signal with a normal defocus signal detection light beam.
FIG. 18 shows a lens shift characteristic of a tracking signal with a defocus signal detection light beam when a phase inversion diffraction grating is used.
FIG. 19 is a lens shift characteristic of a tracking signal with a defocus signal detection light beam in a differential push-pull method.
FIG. 20 shows an embodiment of an optical system configuration according to the present invention capable of reproducing DVD, DVD-RAM, CD, and CD-R.
FIG. 21 is a diagram for explaining the details of the detection configuration in the embodiment of FIG. 20;
FIG. 22 is a configuration diagram of a detection circuit system corresponding to a land groove disk.
23 is an explanatory diagram of frequency characteristics of the amplifier 301 in FIG.
FIG. 24 shows an embodiment when used in a magneto-optical disk apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 ... Semiconductor laser, 102 ... Linear diffraction grating, 103 ... Beam splitter, 104 ... Rising mirror, 105 ... Objective lens actuator, 106 ... Objective lens, 107 ... Optical disc, 108 ... Zero order light 109 ... 1st order diffracted light, 110 ... 1st order diffracted light, 111 ... Cylindrical lens, 112 ... 4th order light detection area for 0th order light, 113 ... 4th order light detection area for 1st order diffracted light, 114 ... -1st order diffracted light quadrant light detection region, 115 ... photodetector, 201 ... 0th order light, 202 ... + 1st order diffracted light, 203 ... -1st order diffracted light, 204 ... groove part, 205 ... Land portion, 206... 0th order light reflected light beam, 207... ± 1st order diffracted light reflected light beam, 301, 302... Amplifier, 303, 304, 305 .. Differential amplifier, 501. Ejector 502... Reflected light beam 601. Astigmatic optical disk reflected light beam 602... 4 split photodetector 701... Phase inversion diffraction grating 702. 0th order diffracted light by groove, 802... + 1st order diffracted light by optical disk guide groove, 803... 1st order diffracted light by optical disk guide groove, 1201... Polarizing phase shifter, 1301. Direction 1303 ... Proton exchange region, 1304 ... Dielectric film, 1305, 1306 ... Ordinary ray, 1307, 1308 ... Abnormal ray, 2001 ... 650 nm semiconductor laser, 2002 ... 780 nm semiconductor laser, 2003 ... Inventive diffraction grating, 2004... Three beam tracking sub beam forming diffraction grating, 2005... Dichroic ························ Beam splitter, 2007 ································· DVD / CD-R compatible objective lens, 2009 ··· DVD or DVD-RAM or CD or CD-R, G ··· Optical components, 2010 ··· Optical detector, 2101 ... Curved diffraction grating, 2102 ... Optical splitting prism, 2400 ... Magneto-optical disk device, 2401 ... Optical head, 2402 ... Semiconductor laser, 2403 ... Optical branching element, 2404 ... Objective Lens, 2405, magneto-optical disk, 2406, magnetic field application coil, 2407, second optical branching element, 2408, photodetector for detecting servo signal, 2409, polarization separating element, 2410, split light Detector, 2411, 2412 ... Flexible substrate, 2413 ... User data, 2414 ... Buffer memo 2415 ... Recording waveform generation circuit, 2416 ... Laser drive circuit, 2417 ... Servo signal calculation circuit, 2418 ... Objective lens actuator, 2419 ... Head moving mechanism, 2420 ... Signal detection circuit, 2421 ... Spindle motor , 2422 ... spindle motor drive circuit, 2423 ... magnetic field application coil control circuit, 2424 ... controller, 2425 ... diffraction grating according to the present invention, 2426 ... cylindrical lens.

Claims (7)

A semiconductor laser;
In the condensing optical system that condenses the emitted light emitted from the semiconductor laser as at least one condensing spot on an optical disc having a periodic structure in the radial direction, the periodic structure traverses the condensing spot on the optical disc. Means for generating a plurality of reflected lights in which the polarities of the intensity changes in the reflected light flux accompanying the above are substantially reversed;
A light detection system for separately detecting the plurality of reflected lights from the optical disc;
A light branching element for branching the reflected light from the optical disk from the optical path from the semiconductor laser;
An electrical circuit for obtaining a defocus error signal and a tracking error signal of the focused spot from the plurality of reflected lights,
The means for generating a plurality of reflected lights whose polarities of intensity change are substantially reversed from each other is a diffraction grating disposed between the semiconductor laser and the optical branching element,
This diffraction grating is approximately λD / (2NA · P) (λ: optical wavelength, NA objective lens numerical aperture, P: period of the periodic structure on the disk, D: effective beam diameter on the diffraction grating) in the disk radial direction. The diffraction grating is such that the phase of the grating is reversed in the region of the same width, and the condensing spot of ± 1st order diffracted light by this diffraction grating is arranged on the same track as the condensing spot of 0th order light on the optical disk. The direction of the grating is set parallel to the optical disc tangential direction, and these are separated and detected by the light detection system, and a data signal is obtained from the received light amount signal from the zero order light,
In the electric circuit, a defocus error signal is obtained by taking the sum of the defocus error signals of each of the plurality of reflected lights so that fluctuations in the defocus error signal due to the intensity change cancel each other. A tracking error signal of each of a plurality of reflected lights having substantially opposite polarities is obtained with a gain proportional to the ratio of the reciprocal of the total amount of reflected light when one of the focused spots is on the information track of the optical disc. An optical head characterized by obtaining a tracking error signal by calculating a difference between the amplified tracking error signals after amplification. A semiconductor laser;
In the condensing optical system that condenses the emitted light emitted from the semiconductor laser as at least one condensing spot on an optical disc having a periodic structure in the radial direction, the periodic structure traverses the condensing spot on the optical disc. Means for generating a plurality of reflected lights in which the polarities of the intensity changes in the reflected light flux accompanying the above are substantially reversed;
A light detection system for separately detecting the plurality of reflected lights from the optical disc;
A light branching element for branching the reflected light from the optical disk from the optical path from the semiconductor laser;
An electrical circuit for obtaining a defocus error signal and a tracking error signal of the focused spot from the plurality of reflected lights,
A means for generating a plurality of reflected lights whose polarities of intensity change are substantially reversed from each other is a polarization phase shifter disposed between the semiconductor laser and the optical branching element,
The polarizing phase shifter is approximately λD / (2NA · P) in the disc radial direction (λ: optical wavelength, NA objective lens numerical aperture, P: period of the periodic structure on the disc, D: effective beam diameter on the diffraction grating) Relatively invert the phase of the linearly polarized light component polarized in a specific direction in the same width region,
The phase of the linearly polarized light component orthogonal to the linearly polarized light component polarized in the specific direction is a structure that does not change throughout the phase shifter,
In the light detection system, the polarization component is separated and detected using a polarization separation element, and a data signal is obtained from the polarization component not subjected to phase inversion,
In the electric circuit, a defocus error signal is obtained by taking the sum of the defocus error signals of each of the plurality of reflected lights so that fluctuations in the defocus error signal due to the intensity change cancel each other. A tracking error signal of each of a plurality of reflected lights having substantially opposite polarities is obtained with a gain proportional to the ratio of the reciprocal of the total amount of reflected light when one of the focused spots is on the information track of the optical disc. An optical head characterized by obtaining a tracking error signal by calculating a difference between the amplified tracking error signals after amplification. The optical head according to claim 1 or 2,
The optical disk has a periodic structure constituted by guide grooves, and the difference in reflectance when positioned between the guide grooves is ± 10% or less with respect to when one of the focused spots is positioned on the guide grooves, An optical head, wherein a ratio of amplification gains of tracking error signals of the two types of reflected light is equal to a ratio of amplification gains of defocus error signals of the two types of reflected light. The optical head according to claim 1 or 2,
The optical disc has a reflectivity when one of the focused spots is located on an information track of the optical disc and at a location shifted from the information track by a half of the period of the periodic structure. An optical head, which is a different optical disk, wherein an amplification gain ratio of tracking error signals of the plurality of reflected lights is different from an amplification gain ratio of defocus error signals of the plurality of reflected lights. The optical head according to any one of claims 1 to 4,
An optical head using an astigmatism method for detecting the defocus and a push-pull method for detecting the tracking error. The optical head according to claim 5, wherein
An optical head comprising: a photodetector having at least two sets of light detection regions for receiving one light spot in a light detection region divided into four in the light detection system. The optical head according to claim 3 or 4,
Cutting off the frequency band of the reproduction signal of the recording information of the optical disc in the electric circuit;
An optical head having frequency characteristics.

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