JP2004063073A - Optical head and optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate disturbance occurring to a defocus signal when an optical spot crosses a track on a recording film surface in accompanying the eccentricity of an optical disk center and to cancel offset of a tracking error signal accompanying lens movement. <P>SOLUTION: A wave front phase of ±one-dimensional diffracted light is shifted by λ/2 in a cycle Dλ/2 (2P*NA) by a phase reverser diffraction grating 701 in which a phase reverses in the same cycle as above in the radial direction of an optical disk and a sub-spot is arranged on the same track as the main spot on the optical disk. By adding a main- and sub-spot defocus error signals in an appropriate gain ratio, the disturbance occurring to the defocus error signal can be canceled. By subtracting the main- and sub-spots tracking error signals by a push/pull system in an appropriate gain ratio, the offset can be canceled. <P>COPYRIGHT: (C)2004,JPO

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]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical head used in an optical disk device and an optical disk device, and more particularly to a technique for improving the performance of detecting an optical spot position control signal.
[0002]
[Prior art]
The prior art of the focus position control method in the optical disk apparatus is described in, for example, Yoshito Tsunoda, edited by the Institute of Electronics, Information and Communication Engineers, "Basics and Application of Optical Disk Storage", first edition (1995, Corona), pp. 79-83. . According to this, there are Foucault method (knife edge method), astigmatism method, beam size detection method, image rotation method, etc., because of simplicity of optical system, easy adjustment, easy combination with tracking detection, etc. At present, the most common method is the astigmatism method. However, the astigmatism method has a problem that disturbances are easily generated in the defocus signal 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 is shifted on the photodetector. As a method of reducing this, for example, a method of shielding the central part of a detection light beam (Japanese Patent Application No. 4-314500, Japanese Patent Application No. 5-102929), a method of reducing by adjusting the rotation of the objective lens (Japanese Patent Application No. 5-68774), detection There is a method of reducing disturbance by calculating light having astigmatism and light having no astigmatism in the system (Japanese Patent Laid-Open No. 5-197980). However, none of these methods is a fundamental solution, and a sufficient reduction effect is not necessarily obtained. There is no present.
[0003]
In addition, this disturbance is particularly large in a land-groove type optical disk used in a DVD-RAM which is scheduled to be commercialized soon. This is because in a land-groove type optical disk, the width of the guide groove (groove) is almost equal to the width of the land between the guide grooves (land), and information is recorded on both sides. This is because the tracking signal by the push-pull method, which will be described later, becomes very large since the optical disk is larger than the conventional optical disk. Therefore, in the optical head for DVD-RAM, it is inevitable to use a beam size method or a Foucault method, which has a complicated configuration and adjustment.
[0004]
The conventional technology of the tracking control method in the optical disk device is similarly described in, for example, supervision of Yoshito Tsunoda, edited by the Institute of Electronics, Information and Communication Engineers, "Basics and Application of Optical Disk Storage", first edition (1995, Corona), pp. 83-92. ing. According to this, there are a three spot type, a diffracted light differential type (push-pull type) and the like. For a read-only type such as a CD, a three-spot type due to the simplicity of the optical system, ease of adjustment, and the strength against disturbances. Push-pull when a large laser emission power is required for recording, such as a magneto-optical disk or DVD-RAM. The method is mainly adopted. At this time, there are circumstances in which the push-pull method cannot be used for the CD and the three spot type cannot be used for the recordable optical disk.
[0005]
In the case of a CD pickup, the need for low cost has been met by mounting and moving an objective lens on an actuator in order to make the focused spot follow the eccentricity of the optical disk. If the push-pull method is used, the detection light beam moves on the photodetector, and appears as an offset. Also, at the pit depth of λ / (4n) (λ: light wavelength, n: substrate refractive index) where the signal amplitude is largest in a read-only optical disk, 0% of the diffracted light due to the periodic structure of the pit row in the radial direction is used. There is a problem that the next light becomes small, and even if the focused spot is off-track, no imbalance occurs in the interference intensity between the 0th-order light and the ± 1st-order diffracted light, and a tracking signal cannot be obtained.
[0006]
On the other hand, in a recordable optical disk, especially a magneto-optical disk, the eccentricity of the optical disk is normally compensated by a coarse actuator for mounting an optical head or only an objective lens and an actuator portion so as to bring an optical spot close to a target track. Performed by actuator. That is, the low frequency component of the tracking error is compensated for by the coarse actuator, and the high frequency component is compensated for by the objective lens actuator. In this way, the reliability required for the recording operation is increased. Accordingly, since the amount of movement of the objective lens actuator is smaller than that of a CD or the like, a push-pull method having higher light use efficiency than the three spot type can be adopted.
[0007]
In addition, when a three-spot type is used as a recordable optical disc, there are the following problems as described in the Optical Memory Symposium '86 Transactions (1986), page 127. First, in an optical disk such as a DVD-RAM which records by changing the reflectance of a recording mark, the light amount of the preceding sub spot and the succeeding 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 return light to the semiconductor laser, the interference state of stray light becomes unbalanced between the sub-spots on both sides due to the inclination of the disk, and an offset also occurs. In addition, as mentioned above, the DVD-RAM uses a land-groove type optical disk, which is another reason why the three-spot type cannot be used. That is, in the land-groove type, the width is originally made equal for the purpose of equalizing the reflected light amount of the land portion and the groove portion. Therefore, even if the light spot is inevitably off-track, the light amount hardly changes and the three-spot type The tracking signal cannot be obtained. Therefore, the push-pull method must be used in the DVD-RAM, but unlike the magneto-optical disk, the DVD-RAM needs to be less expensive than the CD, so that the movement of the objective lens in the push-pull method is required. Accordingly, it is necessary to reduce the tracking signal.
[0008]
A conventional technique for solving this problem in a DVD-RAM is described in, for example, National Technical Report Vol. 40, No. 6, 1994 (pp. 771-778). Here, an objective lens, a λ / 4 plate, and a polarizing diffraction grating are integrally mounted on a two-dimensional actuator, and the interference regions between the + 1st order diffracted light of the disk and the 0th order light of the −1st order diffracted light are different from each other. A polarizing diffraction grating is configured to diffract at a diffraction angle. In this way, the interference region between the + 1st-order diffracted light and the 0th-order diffracted light of the -1st order can be separated on the detector, so that even if the objective lens moves, these lights do not deviate from the detector. It has been shown that the offset caused by the movement of the light spot on the detector can be eliminated by configuring a split photodetector.
[0009]
In addition, since the diffraction grating is made to have a polarization property, the diffraction efficiency becomes substantially zero when the light traveling toward the disk passes through the diffraction grating, and an appropriate diffraction is performed when the disk reflected light again transmits through the diffraction grating. It can be made efficient. If the diffraction grating is a non-polarizing ordinary diffraction grating, the light going to the disk will also be diffracted, so that a loss of light quantity is inevitable. And light loss can be prevented.
[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 movable part of the actuator becomes heavy, and the response speed of the actuator becomes low. There is a problem of being restricted. The optical disk has a higher transfer rate every year as well as the recording density, and the above-mentioned conventional example cannot cope with a further increase in the speed in the future.
[0011]
Another method for eliminating the tracking signal offset accompanying the movement of the objective lens in the push-pull method without mounting any optical components other than the objective lens on the objective lens actuator is disclosed in the Optical Memory Symposium '86, 1986 (1986). It is disclosed on pages 127 to 132. This is to eliminate the tracking signal offset accompanying the movement of the objective lens by subtracting the tracking signal by each push-pull method with the main spot and the sub-spot using three spots. The differential push-pull method (differential push-pull method) )is called. In other words, by disposing the sub-spots on both sides of the main spot by a half of the period of the guide groove, light in which the interference intensity distribution change of the disk reflected light due to off-track is inverted is simultaneously detected, and the offset is included in phase. This is a method in which a tracking signal of the opposite phase is generated and only the offset is canceled by subtracting them. At this time, the gain ratio at the time of subtracting the main spot and the sub spot is selected so as to compensate for the light amount difference generated between the main spot and the sub spot in the diffraction grating that generates the sub spot. By using this conventional example, basically, the tracking signal offset accompanying the movement of the objective lens in the push-pull system can be eliminated without mounting any optical components other than the objective lens on the objective lens actuator. However, in this conventional example, no countermeasures have been taken with respect to the incorporation of disturbance into the defocus signal when the focused spot crosses the guide groove in the astigmatic defocus detection method. Further, as described in the related art, when only one side of the sub spot is on a recorded track and the other is on an unrecorded track, the offset reduction is not sufficient. Further, although not described in the conventional example, the offset remains in the conventional example even when the total amount of reflected light is different between the guide groove and the guide groove. 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 recorded on the track and the two sub spots are recorded on the unrecorded track. In some cases, or vice versa, this situation occurs. Further, since there are a plurality of light spots, there is a disadvantage in light use efficiency during recording.
[0012]
In addition, as described in the 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 １ ／ of the track pitch, when the spot is on the information track and when the track pitch from the information track ( In a land-groove disk, the reflectivity is different from that at a position shifted by の of the groove pitch). Therefore, in this conventional example, the offset cannot be completely canceled unless compensation is performed including this. It is not taken into account. The allowable value of the offset is stricter as the recording density of the optical disk is higher. In recent years, such a residual offset has become a problem.
[0013]
Further, in this conventional example, an information track is provided between the guide grooves (lands), and an optical disk having no information track is used in the guide grooves (grooves). 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 a DVD-RAM, for example, when the main spot is on the information track, the sub spot is also on the adjacent track, and the fluctuation of the light amount due to the information pits is different from the tracking signal. There is a problem that acts as a disturbance.
[0014]
A method for canceling the incorporation of disturbance into the astigmatism defocus signal is disclosed in JP-A-4-168631. Here, the main spot and the sub-spot by the diffraction grating are arranged on the optical disk with a shift of 案 内 of the guide groove period, and are received by the four-divided photodetectors via the cylindrical lenses. A defocus error signal obtained by taking the difference signal of the sum of two areas in two diagonal directions among the four detection areas is amplified by a gain proportional to the reciprocal of the incident light amount ratio between the main spot and the sub spot. Then, the sum is added to obtain a defocus error signal. Disturbance that occurs when the light spot crosses the guide groove due to astigmatism remaining due to misalignment of the optical head or misalignment of the photodetector naturally 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 the phase shift of 180 ° are effectively added with the same light amount. 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 differential push-pull method described above, and is different from the ratio of the reciprocal of the incident light amount so that the light amount incident on the optical disk is effectively equivalent, as described in the conventional example. What is necessary is just to make it proportional. However, in this conventional example, there is a problem that no tracking method is disclosed. Also, if it is attempted to combine with the above-described differential push-pull method, the amplification gain ratio between the main spot and the sub-spot is equalized between the calculation of the tracking signal and the calculation of the defocus signal in the conventional example. Has 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 a defocus detection method, a disturbance generated in a defocus signal when a light spot crosses a track on a recording film surface due to eccentricity of an optical disk is fundamentally solved. It is to eliminate it.
[0016]
At the same time, the offset accompanying the lens movement occurring in the tracking error signal is basically canceled.
[0017]
In addition, as with the differential push-pull method, a light beam in which the intensity change of the reflected light beam as the light spot on the disk traverses the guide groove is generated at the same time as the normal light beam, and the opposite phase including the in-phase offset component is generated. When a tracking signal is generated to cancel the common-mode offset, the offset caused by the difference in the total reflected light amount of these light beams is canceled.
[0018]
Also, in the differential push-pull method, the offset caused by the lens movement occurring in the tracking error signal is canceled, and in the defocus detection method, the light is focused when the light spot crosses the track on the recording film surface due to the eccentricity of the optical disk. It is to basically eliminate the disturbance generated in the shift signal.
[0019]
Another object of the present invention 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 system on the same track as the main spot.
[0020]
Another advantage is that the same effect as that of the differential push-pull method is achieved so that only one spot is generated on the disk.
[0021]
In addition, these effects are obtained particularly for the astigmatic defocus detection method and the push-pull tracking detection method.
[0022]
Another object of the present invention is to provide a shape of a photodetector that can obtain these effects.
[0023]
Another object of the present invention is to improve the performance of canceling the offset of the tracking signal due to the movement of the objective lens when combining the differential push-pull method and the addition astigmatism method.
[0024]
Another object of the present invention is to remove the influence of disturbance due to information pits when applying the differential push-pull method and the addition astigmatism method to a land groove disk.
[0025]
Another object of the present invention is to solve the above problems in an optical disk device.
[0026]
[Means for Solving the Problems]
In order to solve the above issues,
A semiconductor laser, a condensing optical system that condenses the emitted light as at least one converging spot on an optical disc having a periodic structure in the radial direction, a light detection system that detects reflected light from the optical disc, A light comprising at least one of an electric circuit that obtains at least one of a defocus signal and a tracking error signal of a light spot condensed on the optical disc by photoelectrically converting and calculating, and a data signal recorded on the optical disc. The head has means for generating a plurality of reflected lights in the focusing optical system, in which the polarities of the intensity changes in the reflected light flux as the periodic structure traverses the focused spot on the optical disk are substantially inverted from each other. The light detection system further includes means for separating the plurality of reflected lights and detecting them at the same time, and the electric circuit causes 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 mutually inverted at the same time, is defined as a tracking error signal.
[0028]
Further, at this time, when one of the condensed spots is located on the information track of the optical disc in the electric circuit, the difference between the tracking error signals of the plurality of reflected lights having the inverted polarities is used as the tracking error signal. After amplifying each tracking error signal with a gain proportional to the ratio of the reciprocal of each reflected total light amount, the difference is calculated and used as a tracking error signal.
[0029]
Further, in these, the means for generating a plurality of reflected lights whose polarities of the intensity change are substantially inverted with each other is a diffraction grating arranged between the semiconductor laser and the light splitting element, and the diffraction grating is formed on the optical disk by the diffraction grating. The diffraction grating is positioned in the radial direction of the optical disc such that the converging spot of the ± 1st-order diffracted light is displaced from the converging spot of the 0th-order light in the radial direction in directions opposite to each other by about half of the period of the periodic structure. Install at an angle to
[0030]
Alternatively, the means for generating a plurality of reflected lights whose polarities of the intensity change are substantially inverted from each other is a diffraction grating arranged between the semiconductor laser and the light branching element, and the diffraction grating is substantially in the radial direction of the disk. In the region of the same width every λD / (2NA · P) (λ: light wavelength, NA: objective lens numerical aperture, P: period of the periodic structure on the disk, D: effective light beam diameter on the diffraction grating) The phase of the diffraction grating is tangent to the optical disk so that the converged spot of ± 1st-order diffracted light by this diffraction grating is located on the same track as the converged spot of 0th-order light on the optical disc. They are installed in parallel to the direction, are separated and detected by a light detection system, and a data signal is obtained from a received light amount signal from the zero-order light.
[0031]
Alternatively, the means for generating a plurality of reflected lights in which the polarities of the intensity changes are substantially inverted with each other is a polarizing phase shifter disposed between the semiconductor laser and the light branching element, and the polarizing phase shifter is a disk. A region having the same width every approximately λD / (2NA · P) (λ: light wavelength, NA: numerical aperture of the objective lens, P: period of the periodic structure on the disk, D: effective beam diameter on the diffraction grating) in the radial direction. 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 to this is not changed by the entire phase shifter. A data signal is obtained from a polarization component to which phase inversion is not applied by separating and detecting using an element.
[0032]
In particular, the above configuration is realized by using the astigmatism method for detecting the defocus and the push-pull method for detecting the tracking error.
[0033]
In the photodetection system, a photodetector having at least two sets of photodetection areas for receiving one light spot in four divided photodetection areas is used.
[0034]
Further, a semiconductor laser, a condensing optical system for condensing the emitted light on an optical disc having a periodic structure such as a guide groove in a radial direction, a light detection system for detecting reflected light from the optical disc, and a light In an optical head having an electric circuit for obtaining both a defocus signal and a tracking error signal of a spot, light is condensed by disposing a sub-spot by a diffraction grating or the like and displacing it from a main spot by a half of a guide groove cycle. At least one of two types of reflected light, in which the polarities of the intensity changes in the reflected light flux as the periodic structure traverses the spot are substantially reversed, is generated. The light detection system separates and detects the plurality of reflected lights, and amplifies a defocus error signal obtained by adding a defocus error signal due to at least one of each of the two types of reflected lights in the electric circuit. By adding, a tracking error signal is obtained by amplifying and subtracting a tracking error signal obtained by adding a tracking error signal by at least one of each of the two types of reflected light. At this time, the optical disk may be a land-groove disk or the like in which the periodic structure is formed of guide grooves, and the error of the reflectance when the light spot is between the guide grooves is ± 30% or less, when the light spot is in the guide grooves. By using this, the ratio of the amplification gain of the tracking error signal of the two types of reflected light is made to match the ratio of the amplification gain of the defocus error signal of the two types of reflected light.
[0035]
In a similar optical head, when the optical disk is not a land-groove disk, that is, when one of the optical spots is located on an information track of the optical disk, the optical track is shifted from the information track by の of the period of the periodic structure. In the case of an optical disc having different reflectivities at the position, the ratio of the amplification gain of the tracking error signal of the plurality of reflected lights is made different from the ratio of the amplification gain of the defocus error signal of the plurality of reflected lights. .
[0036]
Further, in the optical head, the electric circuit for detecting the sub spot has a frequency characteristic of cutting off a frequency band of a reproduction signal of information recorded on the optical disk.
[0037]
Also, an optical disk device having the above optical head and at least a disk rotating means and a reproduction control circuit from the optical disk is provided.
[0038]
BEST MODE FOR CARRYING OUT 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 according to a basic embodiment of the present invention. The light from the semiconductor laser 101 generates diffracted light by the diffraction grating 102, passes through the beam splitter 103, the rising mirror 104, and the objective lens 106, and forms a main spot 108 of the zero-order light and ± first-order diffracted light on the optical disc 107. Are formed. The reflected light again passes through the objective lens 106 and the rising mirror 104, reflects off the beam splitter 103, is given astigmatism for defocus detection by the cylindrical lens 111, and is received by the photodetector 115. The photodetector 115 is divided into a quadrant photodetection region 112 for zero-order light and quadrant photodetection regions 113 and 114 for ± 1st-order diffracted light, which are independently detected. Here, the diffraction grating 102 is arranged at a slight angle such that the ± 1st-order diffracted light on the disk is shifted from the 0th-order light by half of the pitch of the guide grooves on both sides.
[0040]
FIG. 2 is a diagram showing the spot arrangement on the optical disk at this time and the intensity distribution of the reflected light beam at that time. The figure shows a case where the 0th-order light 201 and the ± 1st-order diffracted lights 202 and 203 are slightly shifted to the left with respect to the groove portion 204 and the land portion 205, just on track, and slightly shifted to the right. At this time, the intensity change of the zero-order light detection light beam 206 and the ± first-order diffraction light detection light beam 207 shifts in the opposite direction with respect to the track shift as shown in the figure. This is because the ± first-order diffracted lights 202 and 203 on the disk are arranged with a 1/2 track shift from the zero-order light. The fact that such a detected light flux intensity distribution occurs at the position of the condensed spot on the optical disk is known as described in the above-mentioned documents and the like.
[0041]
FIG. 3 is a diagram showing a circuit operation method of the detector output at this time. However, on the detector 115, the intensity distribution is rotated by 90 degrees due to astigmatism for defocus detection. Here, the defocus signal (AF signal) is obtained by adding the same diagonal components of the corresponding split outputs of the split detectors 112, 113, 114 for the main spot and the sub spot, and then calculating the differential signal by the differential amplifier 303. It is obtained by doing. The change in the intensity distribution of the sub-spot when the focused spot on the disk crosses the guide groove is inverted from that of the main spot, so that only the disturbance is canceled out in this way. At this time, usually, in order to keep the light intensity of the sub-spot smaller than that of the main spot, the signal output of the sub-spot is amplified by the amplifier 301 by the light intensity ratio before the calculation. However, since there are two sub-spots here, the gain on the sub-spot side is actually the signal intensity of the two sub-spots with respect to the main spot, where A is the intensity of the main spot and B is the intensity of the sub spot. The amplification gains may be multiplied by A / (2B). On the other hand, a tracking signal (TR signal) is obtained by alternately adding outputs of two regions divided into left and right in a main spot and a sub spot in the drawing, and then obtaining a differential output by a differential amplifier 304. . When the focused spot on the disk crosses the guide groove, the change in intensity distribution of the sub spot is inverted from that of the main spot, and the offset due to the lens shift is not inverted. Thus, tracking in which only the offset component is canceled A signal is obtained. Here, when the main spot is on the land, the sub-spot is on the groove, and when the land and the groove have different widths, a difference occurs in the amount of reflected light. As a result, the offset cancellation becomes insufficient. In such a case, the signal of the sub-spot is further amplified by the amplifier 302 so as to compensate for the light amount difference between the main spot and the sub-spot. For example, when the information track is on the land, the reflectance of the land is a and the reflectance of the groove is b, and the gain of the sub-spot amplifier 302 may be a / b. 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, a tracking error signal having no offset due to the lens shift and a defocus error signal having no disturbance when the guide groove is traversed are simultaneously obtained. On the other hand, for the reproduction signal, the sum of the light amounts of the main spots may be output by the differential amplifier 305. However, here, an optical disk that reproduces a signal with a reflected light amount, such as a read-only optical disk or a phase-change optical disk, is assumed. However, in the case of a magneto-optical disk, the only difference is that the data signal is a differential signal of two signal outputs obtained by polarization separation of the data signal. The defocus error signal and the tracking error signal can be detected by this embodiment. is there.
[0042]
FIG. 4 is a diagram summarizing the calculation method at this time. Four outputs a, b, c, and d of the 0th-order light four-split photodetection area 112, and four outputs e, f, g, h, i, and four outputs of the ± first-order light four-split photodetection areas 113 and 114, respectively. After all, the calculation shown in the figure may be performed on j, k, and l. Note that 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 generally assumed that the reflectance is 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 a land-groove disk used for a DVD-RAM, the width of the guide groove is almost の of the track pitch, and the reflectance is substantially equal when the light spot is on the land and when the light spot is on the groove. For this reason, the amplifier 302 in FIG. 3 can be omitted, and the circuit configuration can be simplified as shown in FIG. However, even in the case of a land-groove disk, the difference in the reflectance of the land portion with respect to the groove portion may be up to about ± 10% due to a manufacturing error. However, according to a computer simulation, if the difference is about this degree, when the effective aperture of the objective lens is 4 mm, the track offset is 0.01 μm in a DVD-RAM disk having a groove pitch of 1.48 μm even if the lens shift is 0.4 mm. Therefore, the track offset can be tolerated even in the configuration of FIG. Conversely, if the allowable value of the track offset is assumed to be 0.05 μm, the reflectance difference between the land and the groove is about 1.6 times under the same conditions. Since this is twice or more in an optical disk other than a normal land groove disk, the configuration of FIG. 22 can be applied only to a land groove disk.
[0044]
In the land groove disk, since the information track is present in both the groove portion and the groove portion, when the main spot is located in the information track, the sub spot is naturally located on the adjacent information track. In this case, recording information is mixed into sub-spots which is not expected except for the land / groove disk. To avoid this, the amplifier 301 in FIG. 22 may have a frequency characteristic as shown in FIG. In FIG. 23, the horizontal axis represents the frequency, the left vertical axis represents the frequency characteristic of the gain of the amplifier, and the right vertical axis represents the intensity of the reproduced signal of the information track in the detector. The reproduction signal is in a higher frequency band than control signals such as a defocus error signal and a tracking error signal, and the signals detected by the detector are combined. Here, a control signal without disturbance can be obtained by giving the amplifier such a characteristic that the gain becomes low in the reproduction signal band.
[0045]
Next, an explanation will be made analytically that the offset of the tracking signal due to the movement of the objective lens is canceled by such a method. According to the literature of Journal of Optical Society of America, Vol. 69, No. 1, 1979, pp. 4 to 24 (J. Opt. Soc. Am, 69, 1 (1979) 4-24), the period of an optical disc is described. In the scalar diffraction approximation, the distribution of the reflected light due to the structure is such that the m-th order Fourier coefficient Rm of the reflectance distribution of the optical disk and the incident light amplitude distribution a (x, y) are displaced by the m-th order diffraction mNA / Pλ (NA : Numerical aperture, P: guide groove period, λ: wavelength, and added by multiplying by a phase component exp (i2πmu0 / P) based on the spot position u0. Ie
[0046]
(Equation 1)

[0047]
It is. Here, Rm corresponds to the complex amplitude of the m-th order diffracted light when parallel light having an amplitude of 1 is perpendicularly incident on the optical disk.
[0048]
(Equation 2)

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

[0051]
become that way. But here
[0052]
(Equation 4)

[0053]
It is. When these are used, if the incident light has no aberration and the amplitude is uniform in the pupil plane of the objective lens, the interference intensity of the 0th-order light and ± 1st-order light due to the periodic guide groove of the optical disk becomes
[0054]
(Equation 5)

[0055]
Can be expressed as But here
[0056]
(Equation 6)

[0057]
It is. With this, the tracking signal by the push-pull method when there is no lens movement is
[0058]
(Equation 7)

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

[0062]
Can be expressed as The second term on the right side corresponds to the offset. Here, assuming that the sub-spot due to the diffraction grating is shifted by の of the track pitch in Equation 5, for example, the phase in 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 equation (7), if the track is shifted by 1/2 track, the tracking signal is also inverted, so that the tracking signal of the sub spot when the lens is moved is
[0065]
(Equation 10)

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

[0068]
Is obtained. Therefore, when the main spot is off-track 0, ie, u0 = 0, the offset is
[0069]
(Equation 12)

[0070]
It becomes. Therefore, when the groove width is not 1/2 of the track pitch, the offset remains. As can be seen from the second term in the upper right-hand side of Equation 11, when the main spot is on track, the interference intensity is different from the interference intensity of the sub-spot. It is possible to cancel 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 without setting such a gain.
[0071]
In the above, the effect of offset cancellation of the tracking signal has been described. By simultaneously detecting the light whose interference phase is inverted in this way, the spot becomes a problem in the astigmatism defocus detection method and the like. The disturbance to the defocus signal accompanying the traversal is also canceled. Hereinafter, the principle will be described. There are two main causes of disturbance due to track traversal in astigmatism defocus detection. One is astigmatism added to a light spot on a disk, and the other is a displacement of a quadrant detector. . Here, an example will be described in which disturbance due to astigmatism is mixed. The astigmatism wavefront is calculated using the astigmatism coefficient W22 and the astigmatism azimuth φ.
[0072]
(Equation 13)

[0073]
Can be expressed as This is the x and y coordinates of the effective aperture
[0074]
[Equation 14]

[0075]
Therefore, if the wavefront having astigmatism is diffracted by the optical disc and the 0th-order light and ± 1st-order diffracted light overlap by ± δ on the objective lens pupil plane, the interference added by the astigmatism The phase difference is
[0076]
[Equation 15]

[0077]
Can be approximated as follows. Then, the interference intensity between the 0th-order light and ± 1st-order diffracted light is
[0078]
(Equation 16)

[0079]
Can be expressed as Here, as shown in FIG. 6, when representative points A, B, C, and D are taken in the reflected light beam 602 of the optical disk having astigmatism, the interference intensity at these positions is expressed by the following equation (16).
[0080]
[Equation 17]

[0081]
(Equation 18)

[0082]
[Equation 19]

[0083]
(Equation 20)

[0084]
Can be expressed as If these intensities basically appear as they are on the detector for defocus detection, the defocus signal
[0085]
(Equation 21)

[0086]
, A disturbance like a cos waveform is mixed into the off-track u0. Here, when spots in which the intensity distribution change due to the convergence spot traversing the guide groove are generated and added at the same time, the phase of ψ is shifted by π and the sign of the first sin in Equation 21 is added. Therefore, the disturbance cancels out. At this time, in order to cancel the disturbance of the defocus signal, it is not necessary to adjust the gain due to the reflectance difference between the groove and the groove due to the groove width or the like, which is necessary for canceling the offset of the differential push-pull. is there.
[0087]
FIG. 7 shows another embodiment for simultaneously detecting a light beam having an inverted polarity of a change in intensity when a condensed spot on an optical disk crosses a guide groove. Here, a linear diffraction grating 701 arranged 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. Therefore, the three quadrant light detection areas 112, 113, and 114 of the photodetector 702 for detecting the reflected light flux are arranged in parallel with the tangential direction of the optical disk.
[0088]
The 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 at a period of Dλ / (2NA · P) with respect to the guide groove period P, the objective lens numerical aperture NA, and the effective beam diameter D of the diffraction grating insertion position as shown in the figure. Has become. This is an interval determined by the shift of the reflected light flux of the ± first-order diffracted lights 802 and 803 with respect to the zero-order light 801 of the diffracted light by the guide groove of the optical disc. The phase of the wavefront of the diffracted light of the diffracted light by such a diffraction grating is shifted by π 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 laser light on a photographic dry plate or the like. The other light is reproduced as hologram diffracted light. Therefore, as described above, if the interference fringes are created by interfering the light whose wavefront is shifted by a half wavelength and the light having a flat wavefront periodically, as a matter of course, the interference fringes are discontinuous reflecting the phase shift. Create a 1/2 stripe step. Therefore, conversely, when light with a flat wavefront is incident on such a diffraction grating, the wavefront of the diffracted light is periodically shifted by 波長 wavelength.
[0089]
FIG. 9 is a view for explaining how the phase shift regions of the diffracted light by the phase inversion diffraction grating and the diffracted light by the optical disk guide groove overlap. The diffracted light by the phase inversion diffraction grating is further diffracted by the guide groove of the optical disc, and the 0th order light and the ± 1st order light overlap. However, the diffracted lights adjacent to each other, such as the 0th-order light and the ± 1st-order lights, are in contact with each other without overlapping the phase inversion regions. At this time, a, b, c,. . . FIG. 10 summarizes the phase difference applied by the phase inversion diffraction grating between any two diffraction orders included in each region indicated by. According to this, it can be seen that the phase difference between adjacent diffraction order lights that contribute to the tracking signal, such as the 0th order light and the ± 1st order diffracted lights, is always π. The phase difference between the diffracted lights, such as the + 1st-order diffracted light and the -1st-order diffracted light, and the 0th-order light and the ± 2nd-order diffracted lights, whose diffractive order is 2, is zero. Therefore, the phase difference of the interference in Equation 5 can realize the equivalent inversion of the interference intensity without making the sub spot off-track by 1/2 track. In this way, even if the recording mark exists asymmetrically on both tracks with respect to the center spot, no asymmetry occurs in the reflected light amount of the sub spot. Therefore, the effects of offset cancellation of the tracking signal and cancellation of disturbance of the defocus error signal become more stable.
[0090]
Here, the phase inversion diffraction grating is not integrated with the objective lens, and the optical axis of the phase inversion diffraction grating and the optical axis of the objective lens relatively shift when the objective lens moves following the eccentricity of the optical disk. 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 moves, only the seam of the phase shift region moves, and there is no problem in reversing the interference intensity.
[0091]
FIG. 12 shows still another embodiment for simultaneously detecting light fluxes of which the polarity of the intensity change is reversed when the condensed spot on the optical disk crosses the guide groove. 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 Polarized light is detected. At this time, there is no sub-spot as in the case of the phase inversion diffraction grating, and there is only one light spot on the optical disk 107. As a result, the light amount 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 polarization phase shifter. Here, an example using lithium niobate (LiNbO3) is shown. The lithium niobate substrate 1301 has a main axis 1302 having a refractive index anisotropy in an in-plane direction of the paper, and a proton exchange region 1303 is formed there along with the grating pattern. Further, a dielectric film 1304 is formed according to the grating pattern. At this time, the phase difference φo between the grating patterns and 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]
(Equation 22)

[0094]
Here, each phase difference is set to an appropriate design value in consideration of diffraction efficiency, and solved as a simultaneous linear equation in which the thickness Td of the dielectric film and the depth Tp of the proton exchange region are unknown.
[0095]
(Equation 23)

[0096]
It is possible to design a polarizing grating that gives a desired phase difference independently between the ordinary ray and the extraordinary ray. For example, if it is assumed that φo = 0 ° and φe = + 180 ° with the wavelength λ = 0.66 μm and the refractive index of the dielectric layer nd = 2.2, then Tp = 2.06 μm and Td = 0.07 μm. In this manner, the phase can be selectively shifted by π in the proton exchange region, and the same offset canceling effect as in the above-described embodiment can be expected.
[0097]
Hereinafter, a description will be given of the effect of canceling the disturbance when the focused spot crosses the guide groove and the effect of canceling the disturbance due to the movement of the objective lens in the defocus detection signal by the scalar diffraction simulation. FIG. 14 shows a defocus signal when astigmatism, spherical aberration, and detector deviation are present in a complex manner in a normal defocus detection system. It can be seen that the central portion is swollen, and a considerably large disturbance has occurred. On the other hand, FIG. 15 shows a calculation result on the assumption that a phase inversion diffraction grating is used. This shows that the disturbance is almost canceled. 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 was almost canceled.
[0098]
FIG. 17 shows a calculation result of the tracking signal on the light receiving surface of the ordinary astigmatism defocus detection method by changing the movement of the objective lens. It can be seen that the offset is quite large. FIG. 18 shows a similar calculation performed using a phase inversion diffraction grating. It can be seen that the offset is almost canceled. Further, FIG. 19 shows an embodiment in which the same calculation is applied in the differential push-pull method. In both cases, the offset is very small.
[0099]
FIG. 20 shows an embodiment of the optical system according to the present invention which can reproduce CDs, CD-Rs, DVD-ROMs and DVD-RAMs. As the semiconductor laser, 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 a CD-R, a semiconductor laser 2002 of 780 nm is indispensable from the relation of the spectral characteristics of the reflectance of the recording film of the CD-R. Each light is incident on the diffraction gratings 2003 and 2004, respectively, and generates ± 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 a diffraction grating for forming a sub-spot for a three-beam tracking method usually used for tracking detection of CD. It is a lattice. The light of 650 nm is then 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 light of 780 nm reflects the beam splitter 2006, reflects the rising mirror 2007, and is focused on the CD or CD-R disc 2009 by the DVD / CD compatible objective lens 2008. The respective reflected light passes through a DVD / CD compatible objective lens 2008 and a rising mirror 2007, passes through a beam splitter 2006 and a dichroic mirror 2005, passes through an optical component G, and is focused on a photodetector 2010.
[0100]
FIG. 21 is a diagram for explaining an optical component G, a detector light receiving pattern configuration, and a 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 as the optical component G, and each of the 0 and ± 1st-order diffracted lights by the diffraction grating 2003 or 2004 is slightly before the focus on the photodetector surface. And a light spot slightly behind the focal point are output as ± 1st-order diffracted light of the curved diffraction grating 2101. At this time, if the diffraction efficiency of the curved diffraction grating 2101 is set to be sufficiently large so that zero-order light is not generated, the number of detection regions can be reduced. In this way, out of a total of six light spots, a defocus error signal by the beam size detection method is obtained from a set of ± 1st-order diffracted lights of the curved diffraction grating 2101. The DPD signal (differential phase detection) used in the DVD-ROM can be obtained by receiving one of the zero-order lights of the diffraction grating 2003 or 2004 with the four-divided photodetector. Further, an offset-cancelled push-pull signal can be obtained from one of the zero-order light and the diffracted light of the diffraction grating 2003. The three-beam tracking signal for CD can be detected from the difference in the amount of ± 1st-order diffracted light of the diffraction grating 2004. Further, a reproduction RF signal can be obtained from the total light amount of the zero-order light of the diffraction gratings 2003 and 2004.
[0101]
When the double knife edge method is used as the defocus detection method, a light splitting prism 2102 is used for the optical component G. Thereby, the diffracted lights of the diffraction gratings 2003 and 2004 are each divided into four on the photodetector. A defocus error signal by the double knife edge method is obtained from one of the four diffracted lights. A tracking signal, a DPD signal, a tracking signal, an RF signal, and the like using a push-pull method can be detected as shown in the drawing in substantially the same manner as the beam size detection.
[0102]
In these defocus detection methods, since disturbance due to traversing the guide groove is relatively unlikely to occur due to the selection of the direction of the dividing line of the detector, the present embodiment is particularly combined with the light whose distribution has been inverted. Although the configuration is not shown, the configuration of the optical system may necessitate a configuration of a dividing line in which disturbance is likely to occur due to another requirement. In this case, by adding the defocus error signals of the lights whose intensity distribution changes have been inverted, disturbance can be reduced even in a defocus detection method other than the astigmatism method. Therefore, the present invention can also enable an optical system configuration that cannot be 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 adopted, the optical component G is unnecessary. This is because astigmatism for detecting astigmatism defocus can be substituted by astigmatism generated when the light passes through the dichroic mirror. This uses the principle that astigmatism occurs when the focused light is incident on a parallel plate. Here, in the defocus detection, the sum of the defocus error signals of the zero-order light and the first-order light is calculated as described above from the viewpoint of disturbance. Conventionally, when astigmatism is introduced using a parallel plate, the parallel plate has been inserted at 45 degrees to the track so that disturbances across the guide groove hardly occur. 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. The difference between the tracking signal by the push-pull method and the tracking signal whose distribution is inverted is similarly calculated. Others are the same as those in the case where another defocus detection method is adopted.
[0104]
Hereinafter, an embodiment in which an optical head according to the present invention is used in an optical disk device will be described using a magneto-optical disk device as an example.
[0105]
The magneto-optical disk device 2400 includes an optical head 2401, a control circuit, and a mechanical system such as a motor.
[0106]
First, in the optical head 2401, linearly polarized light from the semiconductor laser 2402 passes through the diffraction grating 2424 and the light splitting 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 polarization direction of the reflected light is rotated by about 1 ° in the opposite direction depending on whether or not the light spot is on the recording domain, thereby conveying the recorded information. At the time of signal recording, the magneto-optical disk 2405 applies a magnetic field in a direction opposite to the previously initialized magnetization direction by the magnetic field applying coil 2406 to irradiate a light spot and to be 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 where.
[0107]
The reflected light from the magneto-optical disk 2406 passes through the objective lens 2404 again, is reflected by the light splitting element 2403, and is incident on the second light splitting element 2407. First, the light reflected by the second light splitting element 2407 passes through the cylindrical lens 2425 and enters the servo signal detecting photodetector 2408. The defocused signal and tracking error are detected by the split photodetector incorporated therein. A plurality of signals required for each of the signal operations are detected. Here, the cylindrical lens may be a diffraction grating that generates similar astigmatism. On the other hand, the light transmitted through the second optical branching element 2407 is separated by the polarization splitting element 2409 into linearly polarized light components of + 45 ° and −45 ° with respect to the polarization direction of the light incident on the magneto-optical disk 2406, respectively. Then, the light enters the two-segment photodetector 2410. Therefore, 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 between the outputs of the two-segment photodetectors is the magneto-optical reproduction signal.
[0108]
Electrical input and output from the optical head 2401 are performed via flexible substrates 2411 and 2412. In the present embodiment, the flexible substrate 2411 drives the semiconductor laser 2402 and inputs / outputs the photodetector 2408 for detecting a servo signal, and the flexible substrate 2412 inputs / outputs the two-divided photodetector 2410.
[0109]
The laser drive circuit 2416 controls the blinking of the semiconductor laser 2402 by a recording waveform from a recording waveform generating circuit 2415 generated according to digital information in a buffer memory 2414 storing user data 2413.
[0110]
The current output from the servo signal detecting photodetector 2408 is converted from current to 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, a tracking error signal, and a head position control signal. I do. The two error signals are fed back to the objective lens actuator 2418, and closed-loop control is performed 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 as to position the optical head 2401 substantially near the reproduction track. In the case of a magneto-optical disc for recording digital information, this control is normally performed in a closed loop, but in the case of a CD, DVD, MD or the like, open loop control is often performed. 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-segment photodetector 2410 is usually made into a voltage output by incorporating an amplifier inside the detector, input to the signal detection circuit 2420 by the flexible substrate 2412, and subjected to differential amplification, equalizer processing, binarization, and decoding processing. And reproduces the digital information. 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. Further, all these control circuits are controlled by the controller 2424.
[0113]
In the present embodiment, a magneto-optical disk device is taken as an example. However, the DVD-RAM described above can be realized with almost the same configuration except that the signal detection optical system is not a magneto-optical system. In the case of a CD-ROM or a DVD-ROM, an optical disk device can be realized with almost the same configuration except that the 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 weight of the movable portion of the objective lens actuator can be reduced, the speed can be increased, and an inexpensive optical disk device can be realized.
[0114]
【The invention's effect】
According to the present invention, an inexpensive component such as a diffraction grating is not mounted on an objective lens actuator, but is simply added to a fixed optical system. Can fundamentally eliminate the disturbance that occurs in the defocus signal when crossing the track, and at the same time can fundamentally cancel the offset due to lens movement that occurs in the tracking error signal.
[Brief description of the drawings]
FIG. 1 shows the configuration of an optical system according to 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 operation method of a detector output.
FIG. 4 is a diagram showing a calculation method of a detector output.
FIG. 5 is a diagram illustrating a tracking signal offset due to movement of an objective lens.
FIG. 6 is a view for explaining 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 is a diagram illustrating a detailed structure of a phase inversion diffraction grating.
FIG. 9 is a view for explaining how the phase shift regions overlap in the diffraction of the phase-inverted light by the optical disc guide grooves.
FIG. 10 shows an interference phase difference applied to diffracted light from an optical disk by a phase inversion diffraction grating.
FIG. 11 is a view for explaining how the phase shift regions overlap in the diffraction of the phase-reversed light by the optical disc guide groove when the objective lens moves.
FIG. 12 shows a configuration of an optical system in an embodiment using a polarizing phase shifter.
FIG. 13 is a view for explaining the principle of a polarizing phase shifter.
FIG. 14 is a diagram illustrating calculation of disturbance due to crossing a guide groove of a normal defocus signal.
FIG. 15 shows calculation of disturbance due to crossing of a guide groove of a defocus signal when a phase inversion diffraction grating is used.
FIG. 16 is a diagram illustrating calculation of disturbance caused by crossing a guide groove of a defocus signal in the differential push-pull method.
FIG. 17 shows a lens shift characteristic of a tracking signal in a normal defocus signal detection light beam.
FIG. 18 shows a lens shift characteristic of a tracking signal in a light beam for detecting a defocus signal when a phase inversion diffraction grating is used.
FIG. 19 shows a lens shift characteristic of a tracking signal with a light beam for detecting a defocus signal in a differential push-pull method.
FIG. 20 is 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 view for explaining details of a 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 drive.
[Explanation of symbols]
101 ‥‥ semiconductor laser, 102 ‥‥ linear diffraction grating, 103 ‥‥ beam splitter, 104 ‥‥ rising mirror, 105 ‥‥ objective lens actuator, 106 ‥‥ objective lens, 107 ‥‥ optical disk, 108 ‥‥ 0th order light , 109 ° 1st-order diffracted light, 110 ° -1st-order diffracted light, 111 ° cylindrical lens, 112 ° 0-order diffracted light detection area for 1st-order diffracted light, 113 ° + 1-order diffracted light detection area for 1st-order diffracted light, 114 ° {-1st-order diffracted light 4-division light detection area, 115} photodetector, 201 {0th-order light, 202} + 1st-order diffracted light, 203 {-1st-order diffracted light, 204} groove portion, 205} Land, 206 {0-order reflected light flux, 207} ± first-order reflected light reflected light, 301, 302} amplifier, 303, 304, 305 # differential amplifier, 501 # 2 split light Output device, 502 reflected light beam, 601 reflected light beam on optical disc with astigmatism, 602 quadrant photodetector, 701 phase inverted grating, 702 photodetector, 801 optical disc guide 0th-order diffracted light by the groove, 802 ° + 1st-order diffracted light by the optical disk guide groove, 803 ° -1st-order diffracted light by the optical disk guide groove, 1201１２０polarizing phase shifter, 1301 ‥‥ lithium niobate substrate, 1302 ‥‥ crystal principal axis Orientation, 1303 proton exchange region, 1304 dielectric film, 1305, 1306 ordinary light, 1307, 1308 extraordinary light, 2001 650 nm semiconductor laser, 2002 780 nm semiconductor laser, 2003 lines Inventive diffraction grating, 2004 # 3 beam tracking sub-beam forming diffraction grating, 2005 # dichroic Mirror, 2006 Beam splitter, 2007 Mirror, 2008 DVD / CD-R compatible objective lens, 2009 DVD or DVD-RAM or CD or CD-R, G optical component, 2010 {Photodetector, 2101} Curved diffraction grating, 2102} Light splitting prism, 2400} Magneto-optical disk device, 2401 {Optical head, 2402} Semiconductor laser, 2403} Optical splitting element, 2404} Object Lens, 2405 magneto-optical disk, 2406 magnetic field application coil, 2407 second light splitting element, 2408 servo photodetector photodetector, 2409 polarization splitting element, 2410 split light Detector, 2411, 2412 Flexible board, 2413 User data, 2414 Buffer memo , 2415 ‥‥ recording waveform generation circuit, 2416 ‥‥ laser drive circuit, 2417 ‥‥ servo signal operation 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 (15)

A semiconductor laser, a condensing optical system for converging the emitted light as at least one converging spot on an optical disk having a periodic structure in a radial direction, a light detection system for detecting light reflected from the optical disk, and the reflected light An optical circuit for obtaining both a defocus signal and a tracking error signal of the focused spot from the optical head, wherein the periodic structure crosses the focused spot on the optical disk in the focusing optical system. The electric circuit includes means for generating a plurality of reflected lights in which the polarities of the intensity changes in the reflected light flux are substantially inverted from each other, and means for separating and detecting the plurality of reflected lights in the light detection system, Obtaining the defocus error signal by summing the defocus error signals of the respective reflected lights so that the fluctuations of the defocus error signals caused by the intensity change cancel each other. Light head is characterized. 2. The optical head according to claim 1, wherein a difference between the tracking error signals of the plurality of reflected lights whose polarities are inverted from each other is used as a tracking error signal. 3. The optical head according to claim 2, wherein one of the condensed spots in the electric circuit is the optical disc when a difference between the tracking error signals of the plurality of reflected lights whose polarities are mutually inverted is set as a tracking error signal. Amplifying each tracking error signal with a gain proportional to the ratio of the reciprocal of the total amount of reflected light when it is on the information track, and calculating the difference to obtain a tracking error signal. head. 4. The optical head according to claim 1, further comprising: a light splitting element for splitting the reflected light from the optical disc from an optical path from a semiconductor laser, and generating a plurality of reflected lights whose polarities of the intensity change are substantially inverted with each other. The means is a diffraction grating disposed between the semiconductor laser and the light splitting element, and a focused spot of ± 1st-order diffracted light by the diffraction grating on the optical disk is focused on a focused spot of the 0th-order light. The optical head according to claim 1, wherein the diffraction grating is arranged to be inclined with respect to the radial direction of the optical disc so as to be arranged in a radial direction in a direction opposite to each other by about half of a period of the periodic structure. 4. The optical head according to claim 1, further comprising: a light splitting element for splitting the reflected light from the optical disc from an optical path from a semiconductor laser, and generating a plurality of reflected lights whose polarities of the intensity change are substantially inverted with each other. The means is a diffraction grating disposed between the semiconductor laser and the light splitting element, and the diffraction grating is substantially λD / (2NA · P) (λ: light wavelength, NA: objective lens aperture) in the disk radial direction. Number, P: period of the radial periodic structure on the disk, D: effective beam diameter on the diffraction grating), the phase of the grating is inverted in the region of the same width, and ± 1st-order diffracted light by this diffraction grating on the optical disk Is set in parallel to the tangential direction of the optical disc so that the condensed spot is located on the same track as the condensed spot of the zero-order light. detection An optical head, characterized in that to obtain the data signal from the received light amount signal from the zero-order light. 4. The optical head according to claim 1, further comprising: a light splitting element for splitting the reflected light from the optical disc from an optical path from a semiconductor laser, and generating a plurality of reflected lights whose polarities of the intensity change are substantially inverted with each other. The means is a polarizing phase shifter disposed between the semiconductor laser and the optical branching element, and the polarizing phase shifter is substantially λD / (2NA · P) (λ: light wavelength, NA : Objective lens numerical aperture, P: Period of the periodic structure on the disk, D: Effective luminous flux diameter on the diffraction grating) The phase of the linearly polarized light component polarized in a specific direction in the same width region is relatively inverted. The phase of the linearly polarized light component orthogonal to this is a structure that does not change in the entire phase shifter.The light detection system separates and detects these polarized light components using a polarization separating element, and detects the polarized light components from which no phase inversion is applied. An optical head for obtaining a data signal. 7. The optical head according to claim 1, wherein an astigmatism method is used for detecting the defocus, and a push-pull method is used for detecting a tracking error. 8. The optical head according to claim 7, further comprising: a photodetector having at least two sets of photodetection areas for receiving one light spot in four divided photodetection areas in the photodetection system. . A semiconductor laser, a condensing optical system that condenses the emitted light on an optical disk having a periodic structure in a radial direction, an optical branching element that branches reflected light from the optical disk from an optical path from the semiconductor laser, In an optical head having a light detection system for detecting reflected light and an electric circuit for obtaining at least one of a defocus signal of a light spot and a tracking error signal from the reflected light, the optical head is disposed between the semiconductor laser and the light branching element. The radius of the optical disc is set such that the ± 1st-order diffracted light spot on the optical disc is radially offset from the 0th-order light spot by approximately half of the period of the periodic structure in directions opposite to each other. An optical head further comprising a diffraction grating installed at an angle to a direction. A semiconductor laser, a condensing optical system that condenses the emitted light on an optical disk having a periodic structure in a radial direction, an optical branching element that branches reflected light from the optical disk from an optical path from the semiconductor laser, In an optical head having a light detection system for detecting reflected light and an electric circuit for obtaining at least one of a defocus signal of a light spot and a tracking error signal from the reflected light, the optical head is disposed between the semiconductor laser and the light branching element. And approximately every λD / (2NA · P) (λ: light wavelength, NA: numerical aperture of the objective lens, P: period of the radial periodic structure on the disk, D: effective light beam diameter on the diffraction grating) in the radial direction of the optical disk. The phase of the grating is inverted in the same width region, so that the converging spot of ± 1st-order diffracted light on the optical disk is arranged on the same track as the converging spot of 0th-order light. An optical head, characterized by further having a diffraction grating direction is parallel installed to the optical disc tangential direction. A semiconductor laser, a condensing optical system that condenses the emitted light on an optical disk having a periodic structure in a radial direction, an optical branching element that branches reflected light from the optical disk from an optical path from the semiconductor laser, In an optical head having a light detection system for detecting reflected light and an electric circuit for obtaining at least one of a defocus signal of a light spot and a tracking error signal from the reflected light, the optical head is disposed between the semiconductor laser and the light branching element. And the same in the radial direction of the optical disk at approximately λD / (2NA · P) (λ: light wavelength, NA: numerical aperture of the objective lens, P: period of the periodic structure on the disk, D: effective beam diameter on the diffraction grating) A polarizing phase shifter having a structure in which the phase of a linearly polarized light component polarized in a specific direction in the width region is relatively inverted, and the phase of the linearly polarized light component orthogonal to this is not changed as a whole. An optical head, comprising: A semiconductor laser, a condensing optical system for converging the emitted light as at least one converging spot on an optical disk having a periodic structure in a radial direction, a light detection system for detecting light reflected from the optical disk, and the reflected light An optical circuit for obtaining both a defocus signal and a tracking error signal of the focused spot from the optical head, wherein the periodic structure crosses the focused spot on the optical disk in the focusing optical system. Means for generating at least one or more types of reflected light of two types, in which the polarities of the intensity changes in the reflected light flux are substantially inverted from each other, and means for separating and detecting the plurality of reflected lights in the light detection system Amplifying a defocus error signal obtained by adding a defocus error signal due to at least one of each of the two types of reflected light in the electric circuit. The optical head obtains a tracking error signal by amplifying and subtracting a tracking error signal obtained by adding a tracking error signal by at least one of each of the two types of reflected light. An optical disc, wherein a periodic structure of the optical disc is constituted by a guide groove, and a reflectance difference between the guide grooves when one of the condensed spots is located on the guide groove is ± 10% or less, An optical head, wherein the ratio of the amplification gains of the tracking error signals of the two types of reflected light is equal to the ratio of the amplification gains of the defocus error signals of the two types of reflected light. A semiconductor laser, a condensing optical system for converging the emitted light as at least one converging spot on an optical disk having a periodic structure in a radial direction, a light detection system for detecting light reflected from the optical disk, and the reflected light An optical circuit that obtains both a defocus signal and a tracking error signal of the converging spot from the converging spot. Means for generating at least one or more reflected lights of two types whose polarities of the intensity change are substantially inverted with each other, and means for separating and detecting these plural reflected lights in the light detection system. Amplifying and further adding a defocus error signal obtained by adding a defocus error signal due to at least one of each of the two types of reflected light in the electric circuit. An optical head for obtaining a defocus error signal and amplifying and subtracting a tracking error signal obtained by adding a tracking error signal by at least one of each of the two types of reflected light to obtain a tracking error signal. An optical disc having different reflectivity when one of the converging spots is located on an information track of the optical disc and when it is located at a position shifted from the information track by a half of the period of the periodic structure. An optical head, wherein the ratio of the amplification gains of the tracking error signals of the plurality of reflected lights is different from the ratio of the amplification gains of the defocus error signals of the plurality of reflected lights. 14. The optical head according to claim 12, wherein the electric circuit has a frequency characteristic for cutting off a frequency band of a reproduction signal of information recorded on the optical disk. 15. An optical disc device comprising at least the optical head according to claim 1 and a means for controlling rotation of the optical disc and a reproduction control circuit for the optical disc.

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