JP4253448B2 - Optical pickup device - Google Patents

Description

【００１０】
（（３）中、Ｉ（ｒcosθ,ｒsinθ）は射出瞳上の強度分布を、Ｓ（ｙ）は焦点誤差信号関数を、ｙ（ｒ）は縦収差量をそれぞれ示す）を満たす内側及び外側半径ｒｍｉｎ及びｒｍａｘを有することを特徴とする。
本発明の光ピックアップ装置においては、前記回折光学素子は、前記輪帯に設けられた回折格子によって透過光を０次回折光と１次回折光に分離するグレーティング又はブレーズ型の透過ホログラムレンズであることを特徴とする。
【００１１】
本発明の光ピックアップ装置においては、前記光検出光学系の前記戻り光の光軸における前記ホログラムレンズの前又は後のいずれかに配置されかつ前記戻り光に非点収差を付与する非点収差発生光学素子を有することを特徴とする。
本発明の光ピックアップ装置においては、前記ホログラムレンズが前記戻り光に非点収差を付与する機能を有することを特徴とする。
【００１２】
本発明の光ピックアップ装置においては、前記スポット受光部は直交する２本の分割線を境界線として各々近接配置されかつ前記分割線の交点を中心に前記回折光学素子の前記輪帯から照射される±１次回折光のいずれかを受光する互いに独立した４個の受光素子から構成され、前記焦点誤差検出回路は対角位置にある前記４個の受光素子の１対の出力和の間の差分を前記光ビームの焦点誤差信号として生成することを特徴とする。
【００１３】
本発明の光ピックアップ装置においては、前記ホログラムレンズが前記戻り光の±１次回折光に対し元の光軸から偏向させ集光せしめる偏芯したレンズ効果を有しかつ該±１次回折光のいずれかに凸レンズ又は凹レンズの作用をする機能を有することを特徴とする。
本発明の光ピックアップ装置においては、前記スポット受光部は各々が前記回折光学素子の前記輪帯から照射される±１次回折光を受光しかつ該±１次回折光のスポットを分割する分割線を境界線として各々近接配置された少なくとも２個の受光素子以上からなりかつ正極性側となる少なくとも１以上の受光素子の面積と負極性側となる少なくとも１以上の受光素子の面積との合計が略等しくなるように構成され、前記焦点誤差検出回路は前記受光素子の正極性側及び負極性側の出力和の間の差分を前記光ビームの焦点誤差信号として生成することを特徴とする。
【００１４】
本発明の光ピックアップ装置においては、前記ホログラムレンズは前記戻り光の±１次回折光のいずれかに凸レンズ又は凹レンズの作用をする機能を有し、前記スポット受光部は直交する２本の分割線を境界線として各々近接配置されかつ前記分割線の交点を中心に前記回折光学素子の前記輪帯から照射される１次回折光を受光する互いに独立した４個の受光素子から構成され、前記焦点誤差検出回路は対角位置にある前記４個の受光素子の１対の出力和の間の差分を前記光ビームの焦点誤差信号として生成することを特徴とする。
【００１５】
【発明の実施の形態】
以下に、本発明による光ピックアップ装置を含む記録再生装置の実施形態について説明する。
図１は、本発明の一実施形態である記録再生装置の構成を示す図である。
光ピックアップ装置３を備えた記録再生装置は、フォーカスアクチュエータ３０１を駆動制御するための駆動制御部５９を備えている。駆動制御部５９は、ピックアップの光検出器４０に接続され、検出された信号に基づき種々の誤差信号を生成し、これらを接続されたフォーカス駆動回路１８などへ供給する。記録再生装置は検出された信号に基づき再生信号を生成する復調回路２０を有し、図示しないがスピンドルモータ、スライダ、トラッキングのためのサーボ駆動回路も備えている。
（第１の実施形態）
図２は、本発明の光ピックアップ装置の構成を示す図である。
【００１６】
光ピックアップ装置３は、光源である半導体レーザ３１と、グレーティング３２と、偏光ビームスプリッタ３３と、コリメータレンズ３４と、ミラー３５と、１／４波長板３６と、対物レンズ３７と、透光性材料からなるシリンドリカルレンズ、マルチレンズなどの非点収差発生光学素子３８と、ホログラムレンズなどの回折光学素子３９と、光検出器４０を備えている。光検出器４０は０次回折光用受光部４００及び±１次回折光用受光部４０１ａ、４０１ｂを備えている。光ディスク１は、記録再生装置のスピンドルモータのターンテーブル（図示せず）上に対物レンズ３７から離間するように載置される。
【００１７】
光ピックアップ装置３にはフォーカスアクチュエータ３０１が内蔵され、これは対物レンズ３７を支持しかつ駆動する。また、光ピックアップ装置３は偏光ビームスプリッタ３３と対物レンズ３７と光路中に透過する光ビームに位相差を付与して波面収差（球面収差）を補償する球面収差補正用レンズ群を挿入できる。
フォーカスアクチュエータ３０１は、フォーカス駆動回路１８から供給された焦点誤差信号のレベルに応じて対物レンズ３７を光ディスク１の表面に垂直な方向（光軸方向）に移動せしめ、光源から発射された光ビームを所定の記録層へ集光するフォーカスサーボを実行する。
【００１８】
図３に示すように、光検出器４０において、光軸上の０次回折光用受光部４００は、直交する２本の分割線を境界線として各々近接配置されかつ互いに独立した４個の等しい面積の受光素子（Ｂ１，Ｂ２，Ｂ３，Ｂ４）から構成され、一方の分割線が光ディスク１のトラック伸長方向に平行になるように構成されている。また、０次回折光用受光部４００から両側に分かれて配置された±１次回折光用受光部４０１ａ、４０１ｂの各々も、各々が直交する２本の分割線を境界線として各々近接配置されかつ互いに独立した４個の等しい面積の受光素子（Ａ１，Ａ２，Ａ３，Ａ４）（Ｃ１，Ｃ２，Ｃ３，Ｃ４）から構成されている。光検出器４０は、光ディスクの記録層上で０次回折光のスポットが合焦となる場合にこれが後述の最小散乱円となり０次回折光用受光部４００の分割線の交点に位置するように、光軸に垂直な平面上に配置されている。これら受光部は０次回折光用受光部４００の中心（分割線の交点）に関して点対称に形成、配置され、すなわち該中心からトラック方向の及び垂直な方向に伸長する直線に関してそれぞれ対称である。
【００１９】
図２に示すように、半導体レーザ３１から射出された光ビームは、グレーティング３２を経て偏光ビームスプリッタ３３に入射する。偏光ビームスプリッタ３３は偏光鏡を有しており、入射した光ビームは偏光ビームスプリッタ３３を通過し、コリメータレンズ３４を経て、ミラー３５により光路を直角に変えられ、１／４波長板３６を通過し、対物レンズ３７から光ディスク１の所定の情報記録面へ照射される。このように、照射光学系は、対物レンズ３７は光ディスク１上に螺旋又は同心円状に形成されたピット列又はトラックへ光ビームを集光して記録面上にスポットを形成する。この照射光ビームスポットにより、光ディスク１の情報記録面に記録情報を書き込む、又は読み出すことができる。
【００２０】
光ディスク１の記録面上の光ビームスポットにて反射された戻り光は光検出光学系により、光検出器４０へ導かれる。すなわち、戻り光は対物レンズ３７、１／４波長板３６、ミラー３５及びコリーメータレンズ３４を経て、再び偏光ビームスプリッタ３３に入射する。この場合、戻り光は偏光ビームスプリッタ３３により半導体レーザ３１への方向とは異なる方向へ光路を変えられ、回折光学素子３９及び非点収差発生光学素子３８へ導かれる。回折光学素子３９及び非点収差発生光学素子３８を通過した戻り光は、非点収差を付与されると共に回折され、光検出器４０における０次回折光用受光部４００及び±１次回折光用受光部４０１ａ、４０１ｂへそれぞれの回折光として入射する。なお、非点収差発生光学素子３８と回折光学素子３９とを逆に配置して戻り光が回折された後に非点収差を付与するようにしてもよい。さらに、シリンドリカルレンズを省いて、ホログラムレンズが戻り光に非点収差を付与する機能を有するようにすることもできる。
【００２１】
光検出器４０の各受光部は、受光した光を光電変換して、光検出電気信号を、図１に示す駆動制御部５９へ供給する。光検出器４０に接続された駆動制御部５９は、所定の演算を行って焦点誤差信号ＦＥ及び再生信号ＲＦ（Radio Frequency）を生成する。すなわち、駆動制御部５９で出力される信号ＦＥ及びＲＦは、図３に示す光検出器４０の各受光部の符号をその出力として示すと、以下の式によって示される。
【００２２】
【数３】
ＦＥ＝（Ａ１＋Ａ４＋Ｃ１＋Ｃ４）−（Ａ２＋Ａ３＋Ｃ２＋Ｃ３）
ＲＦ＝Ｂ１＋Ｂ２＋Ｂ３＋Ｂ４
駆動制御部５９はこれら焦点誤差信号ＦＥ及び再生信号ＲＦを、図１に示すフォーカス駆動回路１８及び復調回路２０にそれぞれ供給する。なお、駆動制御部５９は所定の演算を行って波面収差誤差信号も生成できるように構成され得る。接線方向の分割線で分けた受光素子の光電変換信号はトラッキングエラー信号生成に用いられ得る。
【００２３】
図２に示す光検出光学系の戻り光の光軸に配置された回折光学素子３９のホログラムレンズは光学ガラスからなる平行平板からなる回折格子を形成されたグレーティング又はブレーズ型の透過ホログラムである。
図４に示すように、回折光学素子３９のホログラムレンズは、戻り光から後述する特定の光線成分を環状に抽出する回折格子の輪帯３９Ａを有している。回折光学素子３９の輪帯３９Ａにより抽出される特定の光線成分は、対物レンズ３７などの照射光学系の射出瞳面における光ディスク１の情報記録面上の光透過層によって生じた波面収差の分布の極大値に対応した瞳上の所定半径の近傍の光線成分である。そのため、輪帯３９Ａは、かかる半径として、戻り光の光軸を中心に瞳半径をＲ₀とした場合に０．７１Ｒ₀〜０．７４Ｒ₀を含んでいる部分である。
【００２４】
図５に示すように、回折光学素子３９の輪帯３９Ａは戻り光を回折し、０次回折光及び±１次回折光を光検出器４０の０次回折光用受光部４００並びに±１次回折光用受光部４０１ａ、４０１ｂ上へ非点収差発生光学素子３８を介して導き、円形の０次回折光スポット並びに円形及び環状の±１次回折光スポットを形成して、透過光を０次回折光と１次回折光に分離する。すなわち、回折光学素子３９及び非点収差発生光学素子３８を透過した回折光学素子３９のホログラムレンズの作用を受けない０次回折光は、元の光軸からずれること無く進むが、±１次回折光は該光軸に対称に偏向される。０次回折光用受光部４００は復調回路２０へ、各±１次回折光用受光部は駆動制御部５９へ接続され、それらの出力をそれぞれの回路へ供給する。
【００２５】
図６によって、輪帯３９Ａから得られる±１次回折光を用いて非点収差法のフォーカスサーボを実行する第１の実施形態を詳細に説明する。回折光学素子３９の輪帯３９Ａにより抽出される環状スポットを受光する±１次回折光用受光部４０１ａ、４０１ｂの出力を焦点誤差信号ＦＥの検出に用いる。非点収差法は、戻り光の光学系中にシリンドリカルレンズや平行平板など非点収差発生光学素子を配置し、戻り光を４分割受光部の中心近傍で受光し１つの光スポット形状の変化を検出して焦点誤差信号を生成する方法である。なお、図６において、輪帯３９Ａからの＋１次回折光を代表して動作を説明するために、対物レンズ３７、シリンドリカルレンズの非点収差発生光学素子３８、ホログラムレンズの回折光学素子３９及び＋１次回折光用受光部４０１ａ以外の要素は省略してある。
【００２６】
図６に示すように、対物レンズ３７から回折光学素子３９の輪帯３９Ａ及びシリンドリカルレンズ３８を透過し非点収差を持った戻り光の＋１次回折光は、トラック（接線）伸長方向とディスク半径方向とで直交する２線分によって４分割された受光面を有する＋１次回折光用受光部４０１ａの中心付近に環状の光スポットＳ（後述の最小散乱円）を形成する。
【００２７】
シリンドリカルレンズ３８は、図６に示すように、その中心軸（レンズ面をなす円柱曲面の回転対称軸）が光ディスクのトラック伸長方向に対して４５度の角度で伸長するように、戻り光の光路に配置されている。この構成において、対物レンズ３７により収束する戻り光に非点収差を与え、光線は互いに９０度方向の異なる非点収差となって、光ディスク及び対物レンズ３７間距離に応じて前の線像Ｍ、最小散乱円Ｓ及び後ろの線像Ｍを形成する。検出光学系は、光ビームの合焦時に図６（ａ）の最小散乱円Ｓを＋１次回折光用受光部４０１ａに照射し、デフォーカス時に図６（ｂ）又は（ｃ）のように受光面の対角線方向に延びた線像及び楕円環状の光スポットを＋１次回折光用受光部４０１ａに照射する。＋１次回折光の集光した線像間すなわち図６に示す（ｂ）及び（ｃ）間の距離が焦点誤差信号のキャプチャーレンジＣｐに対応する。
【００２８】
図７は＋１次回折光用受光部４０１ａの出力に基づき生成された焦点誤差信号ＦＥの関数Ｓ(ｙ）、いわゆるＳ字特性であり、縦軸は信号強度Ｓ（ｙ）を、横軸は距離（ｙ）を示す。このＳ字特性において、合焦時に光スポット強度分布が４分割の受光部中心Ｏに関して対称すわなち、接線方向及び半径方向において対称となる図６（ａ）の真円の光スポットが受光素子（Ａ１，Ａ２，Ａ３，Ａ４）に形成されるので、対角線上にある受光素子（Ａ１，Ａ４）（Ａ２，Ａ３）の光電変換出力をそれぞれ加算して得られる値は互いに等しくなり、焦点誤差成分は「０」となる。また、非合焦時には図６（ｂ）又は（ｃ）の如く受光部の対角線方向に楕円又は線状の光スポットが受光部に形成されるので、対角線上にある受光部の光電変換出力をそれぞれ加算して得られる値は極性が互いに反対となる。よって、焦点誤差信号関数のＳ字特性の極大（ｂ）及び極小（ｃ）間がキャプチャーレンジＣｐに対応する。
【００２９】
図８は、ガウス強度分布を有する光ビームの合焦時及び光ディスクに近い又は遠い時における０及び±１次回折光用受光部上の光ビームのスポットの様子を示す。図８（ａ）に示すように、合焦時には、０次回折光は０次回折光用受光部４００上の最小散乱円として集光され、同時に、±１次回折光も±１次回折光用受光部４０１ａ、４０１ｂ上の最小散乱円及び円環として集光される。
【００３０】
対物レンズが光ディスクに遠い場合、図８（ｂ）に示すように、０次及び±１次回折光スポットは対角位置の受光素子に延びた変形した楕円となり集光される。一方、対物レンズが光ディスクに近い場合、図８（ｃ）に示すように、９０度方向の異なる非点収差となって、０次及び±１次回折光スポットはもう一方の対角位置の受光素子に延びた変形した楕円となり集光される。
【００３１】
図８（ａ）に示す光ビームの合焦時であっても、光ディスクのカバー層などの所定膜厚からの厚み誤差がある場合に球面収差が発生するので、光検出器の受光部上の照射光ビームのスポット径が変動する。しかしながら、上記したように、特定輪帯３９Ａに対応する±１次回折光用受光部４０１ａ、４０１ｂ上のスポット形状は安定した形状すなわち受光部中心に点対称の形状を保つのである。ホログラムレンズの回折光学素子３９の回折格子の輪帯３９Ａは、光ディスクのカバー層の厚み変動に強い特定の光線成分を戻り光から環状に抽出している。
【００３２】
発明者は、かかる戻り光の特定の光線成分が球面収差の大きい部分に関係していることを知見して、上記したように、例えば開口数０．８５の対物レンズを用いた光学系において、その射出瞳面における波面収差分布の極大値に対応した光ビーム横断面の規格化半径の近傍の光線成分を、回折光学素子により特定の輪帯環状に抽出して、その光線成分の強度分布を用いて焦点誤差検出を案出したのである。
【００３３】
図９に光ディスクのカバー層の厚み誤差による球面収差がある場合の波面収差（ｂ）及び瞳上光ビーム横断面（ａ）を示す。フォーカスを最良像点に合わせた時、波面収差の分布（ｂ）は対物レンズの瞳半径をＲ₀とすると半径０．７１Ｒ₀の付近でピーク（輪帯収差）を有する。この輪帯収差半径０．７１Ｒ₀の円周上を通過する光線の像点はピックアップに球面収差が発生してもまったく移動がない。この輪帯収差半径は対物レンズのＮＡによって若干異なりＮＡが大きいとわずかではあるが大きくなる。これは発生する球面収差のうち半径の４乗に比例する成分の他に、高次数の成分が大きくなるためである。例えばＮＡが０．８５では輪帯収差半径０．７４Ｒ₀となる。
【００３４】
カバー層などの厚み誤差のある場合すなわち球面収差の発生した場合の±１次回折光用受光部４０１ａ、４０１ｂ上での光強度分布を示した図８から明らかなように、カバー層などの厚みが所定厚さと異なる場合、輪帯収差半径０．７１Ｒ₀を境に内側の光線と外側の光線の分布が異なる。この分布がアンバランスになるため球面収差が発生した場合の焦点誤差検出に誤差（デフォーカス）を生むことになる。しかし、輪帯収差半径０．７１Ｒ₀を透過する光線は、±１次回折光用受光部４０１ａ、４０１ｂ上においてまったく動かないため、この光線のみで焦点誤差検出を行えば球面収差の影響をまったく受けない検出が可能となる。
【００３５】
図１０に焦点誤差検出に用いる光線成分を抽出する大きさの異なる回折光学素子３９の回折格子輪帯を示す。図１０（ａ）は半径０．７１Ｒ₀を含む細い幅の円環状領域の輪帯であり、図１０（ｂ）は半径０．７１Ｒ₀を含む瞳付近の光線も含まれる太い幅の円環状領域であり、図１０（ｃ）は瞳全面領域、すなわちカバー層厚み誤差によって発生するデフォーカス量をすべての反射光を焦点誤差検出に用いる場合である。図１１は厚み誤差とデフォーカスの関係すなわち、輪帯の大きさに対応するデフォーカス量の変化を示す。図１１から明らかなように、図１０（ｂ）（ｃ）に示す領域を用いる場合と比較して、図１０（ａ）の領域すなわち輪帯３９Ａのみ用いれば、カバー層厚み誤差がおおきくてもデフォーカスは全く発生しないことが分かる。図１０（ｂ）の場合は、すべての光線成分を用いる場合（図１０（ｃ））に比べれば効果はあるが厚み誤差が大きいとデフォーカスしてしまう。大きな厚み誤差が予想される場合や多層ディスクなどに用いる場合は図１０（ａ）のホログラムの使用が優れているといえる。図１２には厚み誤差と球面収差エラーの関係を示す。図１２は、図１０（ａ）（ｂ）の円環状領域を用いる場合で同様な特性を示している。
【００３６】
図９で示したように原理上、球面収差の影響が全くない部分は瞳上の半径０．７１Ｒ₀の円周部分であるが、この部分だけの光線成分を用いたのでは光量が足りず、信号のＳ／Ｎが取れない恐れがあるので、実際はこの半径０．７１Ｒ₀部分を含みある幅を持たせる必要がある。瞳上の半径０．７１Ｒ₀より内側の光線の収差と外側の光線の収差が検出器上でバランスさせる必要がある。
【００３７】
そこで、半径を変数ｒとして回折光学素子３９の回折格子輪帯３９Ａの外側半径（ｒｍａｘ）と内側半径（ｒｍｉｍ）の好適な範囲を求める。以下のように、球面収差量に依存せずに最良像点に合焦するための最適輪帯の半径範囲を計算する。
ツェルニケ（Ｚｅｒｎｉｋｅ）収差多項式を用いると、球面収差がある場合の最良像点における波面は次式（１）で表すことができる。
【００３８】
【数４】

【００３９】
この波面関数Ｗ（ｒ）を縦収差量ｙ（ｒ）に変換すると、次式（２）のようになる。
【００４０】
【数５】

【００４１】
上式は、最良像点に合焦した場合に、瞳上の半径ｒ（瞳半径で規格化した値）の円周上を通った光線が、どれだけデフォーカスした位置に焦点を結ぶかを示している（球面収差を半径毎のデフォーカスに換算した）。ここでＡｍｎ（但しｍ，ｎは整数）は球面収差係数を表し、カバー層厚み誤差△Ｔによって生じる球面収差であれば次式（２ａ）で解析的に求められる。
【００４２】
【数６】

（ただし、k0=40,60,80,100,又は120）
【００４３】
例えば、（ＮＡ，λ）＝（０．８５，４０５μｍ）、ｎ＝１．６２、△Ｔ＝１０μｍの場合には、Ａ₄₂＝−０．２６、Ａ₆₃＝−０．０４９、Ａ₈₄＝−０．００７６となり、縦収差ｙ（ｒ）を図示すると図１３に示すようにほぼ放物線になる（ここでは往路のみの球面収差を考えた）。つまり、カバー層厚み誤差によって生じる球面収差をデフォーカスで表現すると、瞳中心を通る光線のデフォーカス量（＝ｙ（０））と瞳最外周を通る光線のデフォーカス量（＝ｙ（１））の間に全ての光線が入ることになる。また、ｙ（ｒ）＝０を解くことで、カバー層厚み誤差△Ｔに依存することなく、半径ｒ＝０．７４近傍の光線が集光する位置が常に最良像点になることが分かる。なお、ここで高次まで考慮しているので半径ｒ＝０．７４が得られるが、半径ｒ＝０．７１はＷ₆₀＝Ｗ₈₀＝Ｗ₁₀₀＝Ｗ₁₂₀＝０とした場合の値である。言い方を変えれば、ｒ＝０．７４近傍の光線のみで焦点誤差信号を生成すれば、△Ｔに関わらず常に最良像点にフォーカスをかけることができるということである。
【００４４】
ここで、半径ｒ＝０．７４近傍の光のみを用いたのでは光量が足りず、検出信号のＳ／Ｎが著しく劣化することが予想されるので、実際にはｒ＝０．７４を内包してある幅の輪帯内の光を用いることになる。図１４に示すように瞳上の半径ｒ＝０．７４を含む輪帯３９Ａの内径ｒｍｉｎと外径ｒｍａｘの範囲を規定する。
この時、輪帯３９Ａの内径ｒｍｉｎと外径ｒｍａｘは光線追跡により数値的に求めることが確実である。さらに、発明者は、球面収差がない時に全面の光を用いて生成した焦点誤差信号関数Ｓ（ｙ）が既知であるならば、解析的に次式（３）を満足するように選んでも良いことを提案する。すなわち、輪帯収差の極大値（ピーク）の内外で光強度がバランスしゼロとなるように、縦収差量をパラメータとした焦点誤差信号関数Ｓ（ｙ）と光強度分布の積が瞳全体においてゼロとして、以下の式（３ａ）を満たす内径ｒｍｉｎと外径ｒｍａｘを算出する。
【００４５】
【数７】

【００４６】
ここでＩ（ｒ）は対物レンズ射出瞳上の強度分布であるが、回転対称で表せない場合は、射出瞳上の強度分布をＩ（ｒcosθ,ｒsinθ）として、以下の式（３）を満たすように、内径ｒｍｉｎと外径ｒｍａｘを算出する。
【００４７】
【数８】

【００４８】
例えば、球面収差がない時の瞳全面の光を用いたＳ字特性の焦点誤差信号関数Ｓ（ｙ）が、上記図７のようである場合に、球面収差があまり大きくなければ、言いかえるとｙ（ｒ）がＳ（ｙ）の線形領域内に分布するならば、以下の式（４）のように簡単に書ける。すなわち図７に示すＳ（ｙ）のｙ＝０の傾きと縦収差量ｙ（ｒ）との積で表せる。
【００４９】
【数９】

【００５０】
モデルを簡単にするために、Ｉ（ｒ）＝１．０（一様光入射かつ鏡面反射）とし、（１）、（４）式を（３）式に代入して整理すると、以下の式（５）となる。
【００５１】
【数１０】

【００５２】
但し、ＮＡ＝０．８５、λ＝４０５ｎｍ、ｎ＝１．６２とした。
（５）式を満たすような輪帯の内径ｒｍｉｎと外径ｒｍａｘ（内側及び外側半径）は図１５に示すグラフのようになる。図１５から明らかなように、回折光学素子の輪帯幅を瞳半径のｒｍｉｎ＝ｒｍａｘ＝０．７１と極めて狭い範囲からｒｍｉｎ＝０．２５〜ｒｍａｘ＝０．９５の幅の広い範囲までの範囲で設定することにより、光透過層によって生じた波面収差の射出瞳面における波面収差分布の極大値に対応した好適な光線成分を、戻り光から、環状に抽出することができる。
【００５３】
実際には、Ｉ（ｒ）はガウス分布であったり、光ディスクでの回折を考えると更に複雑な分布になるが、上述と同様に式（３）を満たすように回折光学素子の輪帯幅の内径ｒｍｉｎと外径ｒｍａｘの関係が導出可能であり、射出瞳上の強度分布、焦点誤差信号関数から内径及び外径の最適値を求めることができる。
このように第１の実施形態では、焦点誤差検出に非点収差法を用いつつ、射出瞳上の強度分布から輪帯収差を含む円環状部分の光線は、その円環状部分に対応した部分（輪帯３９Ａ）に回折格子を設けた回折光学素子を用いて、本来の光軸より分離する。そして、円環回折格子によって偏向分離された円環状光線を焦点誤差検出専用の光検出器受光部（±１次回折光用受光部４０１ａ、４０１ｂ）に入射させ、焦点誤差を検出するので、球面収差が発生しても受光部上で全く影響を受けない。
【００５４】
さらに第１の実施形態では、輪帯収差の領域（瞳半径において０．７１Ｒ₀を含む領域）を境に内側を透過する光線と外側の光線の挙動が球面収差によって異なることを利用して光学系に発生する球面収差を検出する。
これらの構成によって、フォーカスサーボが回折光学素子の輪帯３９Ａを透過する光線のみでかけられているので、球面収差の影響を全く受けない高精度のデフォーカス検出が可能となる。
【００５５】
具体的に、上記装置の図８に示すような対物レンズの光ビームの合焦時及び光ディスクに近い又は遠い時における０次及び±１次回折光用受光部にて得られる焦点誤差信号強度（フォーカスエラー）とデフォーカス量との関係を第１の実施形態につい調べた。資料となる光ディスクのカバー層の厚み誤差を０．１ｍｍ±０．０２ｍｍの範囲で調べた。図１６に比較例として０次回折光用受光部４００からの出力に基づいて非点収差法により得た信号(B1+B4)−(B2+B3)を示す。図１７は第１の実施形態の±１次回折光用受光部４０１ａ、４０１ｂからの出力に基づいた信号(A1+A4+C1+C4)−(A2+A3+C2+C3)を示す。図１６から明らかなように、図１７を比較して、射出瞳面における波面収差分布の極大値に対応した光ビーム横断面の規格化半径の近傍の抽出された光線成分を用いた場合、良好なＳ字特性が得られることが分かる。
【００５６】
このように、焦点誤差検出に非点収差法を用いた第１の実施形態は、円環状部分の分離はその領域に対応した輪帯部分に回折格子を設け本来の光軸より分離するとともに、当該回折格子によって偏向された円環状光線を焦点誤差検出専用受光部に入射させる。焦点誤差検出用受光部の形状は非点収差法で用いるいわゆる4分割ディテクタである。回折素子は０次光の強度が大きくなるように設定されている。ＲＦ信号やトラッキングエラー信号などは０次光が入射する０次光専用受光部で検出するようにしているので、ＲＦ信号を得るための加算アンプの数を少なくでき不要なノイズの増加を抑えることができる。また、第１の実施形態では回折素子による±1次回折光を両方用いたがとちらか一方のみでもかまわない。さらに、回折素子をフレーズ形状にしてどちらか一方の回折光のみを用いてもよい。
（第２の実施形態）
第２の実施形態は、第１の実施形態における非点収差発生光学素子３８を省略し、光検出器４０、回折光学素子３９及び駆動制御部５９を差動スポットサイズ法に対応して変更した以外、第１の実施形態と基本的に同一である。上記第１の実施形態ではフォーカスアクチュエータ３０１のフォーカシングサーボ制御の方式として非点収差法を用いたが、第２の実施形態では、差動スポットサイズ法を用いる。スポットサイズ法は、光ディスクからの戻り光を２つの光路に分割し、それぞれ焦点距離の異なる前方及び後方の焦点を生じるように構成して、合焦点の前後に受光部設け、その上の光スポットの大きさを比較して焦点誤差信号を生成する方法である。
【００５７】
図１８に示すように、第２の実施形態の光検出器４０において、光軸上の０次回折光用受光部４００は、単一の受光素子（Ｂ）から構成されている。また、０次回折光用受光部４００からディスク半径方向の両側に分かれて配置された±１次回折光用受光部４０１ａ、４０１ｂのそれぞれは、中心に配置された受光素子（Ａ２）（Ｃ２）に半径方向に伸長する直線に対称に配置された等しい面積の受光素子対（Ａ１，Ａ３）（Ｃ１，Ｃ３）から構成されている。光検出器４０は、光ディスクの記録層上で０次回折光のスポットが合焦となる場合にこれが後述の最小散乱円となり０次回折光用受光部４００の中心に位置するように、光軸に垂直な平面上に配置されている。これら受光部は該中心からトラック方向の及び垂直な方向に伸長する直線に関してそれぞれ対称である。
【００５８】
図１９に示すように、第２の実施形態の回折光学素子３９のホログラムレンズは、光ディスク１の情報記録面上の光透過層によって生じた波面収差の対物レンズ３７などの照射光学系の射出瞳面における波面収差分布の極大値に対応した光ビーム横断面の規格化半径の近傍の特定の光線成分を、戻り光から、環状に抽出する回折格子の輪帯３９Ａを備えている。輪帯３９Ａは±１次回折光に対し元の光軸から略対称に偏向させ集光せしめるように、偏芯したレンズ効果を有している。また、輪帯３９Ａは±１次回折光のいずれかに凸レンズ又は凹レンズの作用をなすように設定されている。さらに、回折光学素子３９の輪帯３９Ａは、戻り光の光軸を中心に瞳半径をＲ₀とした場合に瞳上の半径０．７１Ｒ₀〜０．７４Ｒ₀を含んでいる。
【００５９】
図２０に示すように、回折光学素子３９の輪帯３９Ａは戻り光を回折し、それぞれ０次回折光及び±１次回折光を光検出器４０の０次回折光用受光部４００並びに±１次回折光用受光部４０１ａ、４０１ｂ上へ導き、円形の０次回折光スポット並びに円形及び環状の±１次回折光スポットを形成して、透過光を０次回折光と１次回折光に分離する。すなわち、回折光学素子３９を透過した回折光学素子３９のホログラムレンズの作用を受けない０次回折光は、元の光軸からずれること無く進むが、±１次回折光は該光軸に対称に偏向される。０次回折光用受光部４００は復調回路２０へ、各±１次回折光用受光部は駆動制御部５９へ接続され、それらからの出力それぞれの回路へ供給される。回折光学素子３９の輪帯３９Ａにより抽出される環状スポットを受光する±１次回折光用受光部４０１ａ、４０１ｂの出力を焦点誤差信号ＦＥの検出に用いる。
【００６０】
図２１によって、輪帯３９Ａから得られる±１次回折光を用いて差動スポットサイズ法のフォーカスサーボを実行する第２の実施形態を詳細に説明する。なお、図２１において、輪帯３９Ａからの＋１次回折光を代表して動作を説明するために、対物レンズ３７、回折光学素子３９、０次回折光用受光部４００及び＋１次回折光用受光部４０１ａ，４０１ｂ以外の要素は省略してある。
【００６１】
図２１に示すように、回折光学素子３９は、光ディスクのトラック上で光ビームが合焦したとき、０次回折光が光軸の０次回折光用受光部４００上に集光点を結び、同時に、＋１次回折光が光軸から離れ光検出器４０の手前に焦点ｆ１を結び、かつ、−１次回折光が光軸から離れ光検出器４０の遠方に焦点ｆ２を結び、±１次回折光用受光部４０１ａ，４０１ｂの中央の受光素子（Ａ２）（Ｃ２）上に環状スポットが照射されるように、構成されている。よって、デフォーカス時の対物レンズが近づく場合と遠ざかる場合で±１次回折光用受光部４０１ａ，４０１ｂにおける環状スポットの大きさ異なることになるので、それぞれの中央の受光素子の幅を適宜設定すると、例えば、図２１に示す（ｄ）及び（ｅ）間の距離が焦点誤差信号のキャプチャーレンジとして、規定される。上記実施形態と同様にＳ字特性を示す信号が得られる。
【００６２】
駆動制御部５９は、出力する焦点誤差信号ＦＥ及び再生信号ＲＦが、図２２に示す光検出器４０の各受光部の符号をその出力として示すと、以下の式によって示されるように、構成されている。
【００６３】
【数１１】
ＦＥ＝（Ａ１＋Ａ３＋Ｃ２）−（Ａ２＋Ｃ１＋Ｃ３）
ＲＦ＝Ｂ
図２２（ａ）に示すように、光ディスクのカバー層が所定膜厚である場合の合焦時には、０次回折光は０次回折光用受光部４００上の最小散乱円として集光され、同時に、±１次回折光も±１次回折光用受光部４０１ａ、４０１ｂ上の最小散乱円及び円環として集光される。対物レンズが光ディスクに遠い場合、図２２（ｂ）に示すように、０次回折光スポットは若干拡大し、±１次回折光用受光部上に拡大及び縮小した円として集光される。一方、対物レンズが光ディスクに近い場合、図２２（ｃ）に示すように、０次回折光スポットは若干拡大し、±１次回折光用受光部上に縮小及び拡大した円として集光される。
（第３の実施形態）
第３の実施形態は、第１の実施形態における光検出器４０、回折光学素子３９及び駆動制御部５９を非点収差法及び差動スポットサイズ法に対応して変更した以外、第１の実施形態と基本的に同一である。上記第１の実施形態ではフォーカスアクチュエータ３０１のフォーカシングサーボ制御の方式として非点収差法のみを用いたが、第３の実施形態では、焦点誤差検出に非点収差法及び差動スポットサイズ法を用いたハイブリッド法を用いる。
【００６４】
図２３に示すように、第３の実施形態の光検出器４０において、光軸上の０次回折光用受光部４００は、直交する２本の分割線を境界線として各々近接配置されかつ互いに独立した４個の等しい面積の受光素子（Ｂ１，Ｂ２，Ｂ３，Ｂ４）から構成され、一方の分割線が光ディスク１のトラック伸長方向に平行になるように構成されている。また、０次回折光用受光部４００からさらに離れてディスク半径方向の両側に分かれて配置された±１次回折光用受光部４０１ａ、４０１ｂのそれぞれは、中心に配置された受光素子（Ａ２）（Ｃ２）に半径方向に伸長する直線に対称に配置された等しい面積の受光素子対（Ａ１，Ａ３）（Ｃ１，Ｃ３）から構成されている。光検出器４０は、光ディスクの記録層上で０次回折光のスポットが合焦となる場合にこれが後述の最小散乱円となり０次回折光用受光部４００の分割線の交点に位置するように、光軸に垂直な平面上に配置されている。これら受光部は該中心からトラック方向の及び垂直な方向に伸長する直線に関してそれぞれ対称である。
【００６５】
第３の実施形態の回折光学素子３９のホログラムレンズは、図１９に示す実施形態のものと同様、±１次回折光のいずれかに凸レンズ又は凹レンズの作用をするように設定された輪帯３９Ａを備える。輪帯３９Ａの内外側には回折格子が設けられていない透過平行板部分がある。回折光学素子３９の輪帯３９Ａは、戻り光の光軸を中心に瞳半径をＲ₀とした場合に瞳上の半径０．７１Ｒ₀〜０．７４Ｒ₀を含んでいる。
【００６６】
図２４に示すように、回折光学素子３９の輪帯３９Ａは戻り光を回折し、０次回折光及び±１次回折光を光検出器４０の０次回折光用受光部４００並びに±１次回折光用受光部４０１ａ、４０１ｂ上へ非点収差発生光学素子３８を介して導き、円形の０次回折光スポット並びに円形及び環状の±１次回折光スポットを形成して、透過光を０次回折光と１次回折光に分離する。０次回折光用受光部４００は復調回路２０へ、各±１次回折光用受光部は駆動制御部５９へ接続され、それらからの出力それぞれの回路へ供給される。回折光学素子３９の輪帯３９Ａにより抽出される楕円環状スポットを受光する±１次回折光用受光部４０１ａ、４０１ｂの出力を焦点誤差信号ＦＥの検出に用いる。
【００６７】
駆動制御部５９は、出力する焦点誤差信号ＦＥ及び再生信号ＲＦが、図２５に示す光検出器４０の各受光部の符号をその出力として示すと、以下の式によって示されるように、構成されている。
【００６８】
【数１２】
ＦＥ＝（Ａ１＋Ａ３＋Ｃ２）−（Ａ２＋Ｃ１＋Ｃ３）
ＲＦ＝Ｂ１＋Ｂ２＋Ｂ３＋Ｂ４
上記のいずれの実施形態においても、ＤＰＰやＣＴＣなど３ビーム仕様のピックアップに本発明を適用する場合、サイドビームの戻り光にも±１次回折光が発生するがサイドビームの±１次回折光はかなり少ない光量となるためこの光を受光する光検出器はあえて設ける必要はなくなる。３ビーム用光検出器は前記回折光学素子による０次回折光のみを受光するようにできる。
【００６９】
【発明の効果】
本発明によれば、球面収差の影響が全くない円環状領域を透過する光線のみをもちいて焦点誤差検出を行うように構成したので、ディスクのカバー層厚み誤差によって球面収差が発生した場合でも焦点誤差検出に誤差（デフォーカス）が発生することがなく良好な焦点誤差検出が行える。
【図面の簡単な説明】
【図１】本発明による光ピックアップ装置を備えた記録再生装置の構成を示す概略ブロック図。
【図２】本発明による光ピックアップ装置の構成を示す概略斜視図。
【図３】本発明による光ピックアップ装置の光検出器の概略平面図。
【図４】本発明による光ピックアップ装置の回折光学素子のホログラムレンズの構成を示す概略平面図。
【図５】本発明による光ピックアップ装置の回折光学素子のホログラムレンズから光検出器までの戻り光の光路を示す概略図。
【図６】本発明による光ピックアップ装置の光検出光学系の構成を示す概略斜視図。
【図７】本発明による光ピックアップ装置のキャプチャーレンジを有する焦点誤差信号の変化を示すグラフ。
【図８】本発明による光ピックアップ装置における戻り光の０及び±１次回折光用受光部上の光スポットの様子を示す光検出器の概略平面図。
【図９】光ディスクのカバー層の厚み誤差による球面収差がある場合の波面収差及び瞳上光ビーム横断面の関係を示す概略図。
【図１０】本発明による光ピックアップ装置の焦点誤差検出に用いる光線成分を抽出する大きさの異なる３つの回折光学素子の回折格子輪帯を示す概略平面図。
【図１１】本発明による光ピックアップ装置における光ディスクのカバー層の厚み誤差に対するデフォーカス量の変化を示すグラフ。
【図１２】本発明による光ピックアップ装置における光ディスクのカバー層の厚み誤差に対する球面収差誤差の変化を示すグラフ。
【図１３】本発明による光ピックアップ装置における光ディスクのカバー層の厚み誤差により生じる球面収差を示すグラフ。
【図１４】本発明による光ピックアップ装置の焦点誤差検出に用いる光線成分を抽出する回折光学素子の回折格子輪帯の内径及び外径を説明する概略平面図。
【図１５】本発明による光ピックアップ装置の焦点誤差検出に用いる光線成分を抽出する回折光学素子の回折格子輪帯の内径及び外径の関係の一例を示すグラフ。
【図１６】比較として０次回折光を用いた、光ディスクのカバー層の厚み誤差ごとのデフォーカスに関する焦点誤差検出信号の変化を示すグラフ。
【図１７】本発明による光ピックアップ装置における±１次回折光を用いた、光ディスクのカバー層の厚み誤差ごとのデフォーカスに関する焦点誤差検出信号の変化を示すグラフ。
【図１８】本発明による他の実施形態の光ピックアップ装置の光検出器の概略平面図。
【図１９】本発明による他の実施形態の光ピックアップ装置の回折光学素子のホログラムレンズの構成を示す概略平面図。
【図２０】本発明による他の実施形態の光ピックアップ装置の回折光学素子のホログラムレンズから光検出器までの戻り光の光路を示す概略図。
【図２１】本発明による他の実施形態の光ピックアップ装置の光検出光学系の構成を示す概略斜視図。
【図２２】本発明による他の実施形態の光ピックアップ装置における戻り光の０及び±１次回折光用受光部上の光スポットの様子を示す光検出器の概略平面図。
【図２３】本発明による他の実施形態の光ピックアップ装置の光検出器の概略平面図。
【図２４】本発明による他の実施形態の光ピックアップ装置の回折光学素子のホログラムレンズから光検出器までの戻り光の光路を示す概略図。
【図２５】本発明による他の実施形態の光ピックアップ装置における戻り光の０及び±１次回折光用受光部上の光スポットの様子を示す光検出器の概略平面図。
【符号の説明】
１ 光ディスク
３ ピックアップ
１８ フォーカス駆動回路
２０ 復調回路
３１ 半導体レーザ
３２ グレーティング
３３ 偏光ビームスプリッタ
３４ コリーメータレンズ
３５ ミラー
３６ １／４波長板
３７ 対物レンズ
３８ 非点収差発生光学素子
３９ 回折光学素子
３９Ａ 輪帯
４０ 光検出器
５９ 制御部
３０１ フォーカスアクチュエータ
４００ ０次回折光用受光部
４０１ａ、４０１ｂ ±１次回折光用受光部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device in a recording / reproducing apparatus for an optical information recording medium such as an optical disk.
[0002]
[Prior art]
In recent years, there is a high recording density and large capacity optical disk called DVD (Digital Versatile Disc) and a system using the same. A single-layer DVD with a single recording layer is 4.7 Gbytes, but by increasing the number of recording layers, the recording capacity can be doubled by using two recording layers in the DVD standard. Yes.
[0003]
In order to further increase the capacity of optical discs, there is also a next-generation multilayer optical disc system that uses a high NA and short wavelength optical system and a light source and has two, three, and four recording layers laminated under the cover layer. It is considered. In a multilayer optical disc in which a plurality of recording surfaces are alternately stacked with a spacer layer interposed therebetween, in order to read information from the surface side of one of the optical discs, the focal point of the light beam with respect to the recording surface in one of the desired layers Must be precisely aligned with the in-focus position or the optimum condensing position, that is, it is necessary to irradiate the desired recording layer with the condensed spot.
[0004]
In a condensing optical system with a large effective numerical aperture (NA), the thickness of the cover layer of the optical disc is increased, and the total thickness (depth) of the predetermined light transmission layer including the cover layer up to the desired recording layer in the case of a multilayer structure. ), A large wavefront aberration (mainly spherical aberration) proportional to the fourth power of the numerical aperture occurs.
[0005]
[Problems to be solved by the invention]
Since the spot diameter of the irradiation light beam on the recording layer is greatly expanded due to the spherical aberration caused by the thickness error, even an optical pickup equipped with an optical system in which a focus servo system is optimally designed for a predetermined light transmission layer, In the high NA condensing optical system, the focus error signal during the focus servo operation is greatly affected by the spherical aberration. That is, when the thickness error of the light transmission layer on the recording layer is large, the optimum focus position of the spot may be shifted and the focus error signal may be deteriorated.
[0006]
The present invention has been made in view of the above points. Even in an optical system using an objective lens having a high numerical aperture, the stability of the focus servo is improved and the optimum alignment of the light beam with respect to the target recording surface is achieved. An object of the present invention is to provide an optical pickup device capable of following the focal position satisfactorily.
[0007]
[Means for Solving the Problems]
The optical pickup device of the present invention includes an irradiation optical system that focuses a light beam on a recording surface through a light transmission layer of an optical recording medium to form a spot, and return light that is reflected from the spot and returns. An optical pickup device that has a light detection optical system that collects the light beam on a light detector and detects a focus error of the light beam,
In the vicinity of a predetermined radius on the pupil corresponding to the maximum value of the wavefront aberration distribution on the exit pupil plane of the irradiation optical system of the wavefront aberration generated in the optical system, which is disposed on the optical axis of the return light of the light detection optical system Comprising a diffractive optical element having an annular zone for extracting the light beam component in a ring shape from the return light,
The photodetector includes a spot light receiving unit that receives the extracted light component passing through the annular zone, and is connected to the spot light receiving unit and detects a focus error of the light beam based on a photoelectric conversion output from the spot light receiving unit. And a focus error detecting circuit.
[0008]
In the optical pickup device of the present invention, the predetermined radius on the pupil is a pupil radius R around the optical axis of the return light of the light detection optical system. _o 0.71R ₀ ~ 0.74R ₀ It is characterized by being.
In the optical pickup device of the present invention, the annular zone has the following formula (3):
[0009]
[Expression 2]

[0010]
The inner and outer radii satisfying (in (3), I (r cos θ, r sin θ) represents the intensity distribution on the exit pupil, S (y) represents the focus error signal function, and y (r) represents the amount of longitudinal aberration). It has rmin and rmax.
In the optical pickup device of the present invention, the diffractive optical element is a grating or a blazed transmission hologram lens that separates transmitted light into zero-order diffracted light and first-order diffracted light by a diffraction grating provided in the annular zone. Features.
[0011]
In the optical pickup device of the present invention, astigmatism generation is provided either before or after the hologram lens on the optical axis of the return light of the light detection optical system and imparts astigmatism to the return light. It has an optical element.
In the optical pickup device of the present invention, the hologram lens has a function of imparting astigmatism to the return light.
[0012]
In the optical pickup device of the present invention, the spot light receiving unit is disposed close to each other with two orthogonal dividing lines as boundaries, and is irradiated from the annular zone of the diffractive optical element around the intersection of the dividing lines. It is composed of four independent light receiving elements that receive either ± 1st order diffracted light, and the focus error detection circuit calculates a difference between a pair of output sums of the four light receiving elements at diagonal positions. It is generated as a focus error signal of the light beam.
[0013]
In the optical pickup device of the present invention, the hologram lens has an eccentric lens effect in which the ± 1st order diffracted light of the return light is deflected from the original optical axis and condensed, and any one of the ± 1st order diffracted light It has a function of acting as a convex lens or a concave lens.
In the optical pickup device of the present invention, each of the spot light receiving portions receives a ± first-order diffracted light emitted from the annular zone of the diffractive optical element and borders a dividing line that divides the spot of the ± first-order diffracted light. The total of the area of at least one or more light receiving elements on the positive polarity side and the area of at least one or more light receiving elements on the negative polarity side is composed of at least two light receiving elements or more arranged close to each other as lines. The focus error detection circuit is configured to generate a difference between the positive and negative output sums of the light receiving element as a focus error signal of the light beam.
[0014]
In the optical pickup device of the present invention, the hologram lens has a function of acting as a convex lens or a concave lens on any of the ± first-order diffracted lights of the return light, and the spot light receiving unit has two orthogonal dividing lines. The focus error detection is composed of four light receiving elements that are arranged close to each other as boundaries and receive the first-order diffracted light irradiated from the annular zone of the diffractive optical element around the intersection of the dividing lines. The circuit generates a difference between a pair of output sums of the four light receiving elements at diagonal positions as a focus error signal of the light beam.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a recording / reproducing apparatus including an optical pickup device according to the present invention will be described below.
FIG. 1 is a diagram showing a configuration of a recording / reproducing apparatus according to an embodiment of the present invention.
The recording / reproducing apparatus including the optical pickup device 3 includes a drive control unit 59 for driving and controlling the focus actuator 301. The drive control unit 59 is connected to the optical detector 40 of the pickup, generates various error signals based on the detected signals, and supplies them to the connected focus drive circuit 18 and the like. The recording / reproducing apparatus includes a demodulation circuit 20 that generates a reproduction signal based on the detected signal, and also includes a spindle motor, a slider, and a servo drive circuit for tracking (not shown).
(First embodiment)
FIG. 2 is a diagram showing the configuration of the optical pickup device of the present invention.
[0016]
The optical pickup device 3 includes a semiconductor laser 31, which is a light source, a grating 32, a polarization beam splitter 33, a collimator lens 34, a mirror 35, a quarter wavelength plate 36, an objective lens 37, and a translucent material. An astigmatism generation optical element 38 such as a cylindrical lens or a multi lens, a diffractive optical element 39 such as a hologram lens, and a photodetector 40. The photodetector 40 includes a 0th-order diffracted light receiving unit 400 and ± 1st-order diffracted light receiving units 401a and 401b. The optical disk 1 is placed on a turntable (not shown) of a spindle motor of the recording / reproducing apparatus so as to be separated from the objective lens 37.
[0017]
The optical pickup device 3 includes a focus actuator 301 that supports and drives the objective lens 37. Further, the optical pickup device 3 can insert a spherical aberration correcting lens group that gives a phase difference to the polarizing beam splitter 33, the objective lens 37, and the light beam transmitted in the optical path to compensate for wavefront aberration (spherical aberration).
The focus actuator 301 moves the objective lens 37 in a direction (optical axis direction) perpendicular to the surface of the optical disc 1 in accordance with the level of the focus error signal supplied from the focus drive circuit 18, and the light beam emitted from the light source is emitted. A focus servo for condensing light onto a predetermined recording layer is executed.
[0018]
As shown in FIG. 3, in the photodetector 40, the 0th-order diffracted light receiving unit 400 on the optical axis is arranged in proximity to each other using two orthogonal dividing lines as boundary lines and is independent of each other. Light receiving elements (B 1, B 2, B 3, B 4), and one dividing line is configured to be parallel to the track extending direction of the optical disc 1. Also, each of the ± 1st order diffracted light receiving portions 401a and 401b arranged separately on both sides from the 0th order diffracted light receiving portion 400 is disposed close to each other with two orthogonal dividing lines as boundaries. It is composed of four independent light receiving elements (A1, A2, A3, A4) (C1, C2, C3, C4) having the same area. When the spot of the 0th order diffracted light is focused on the recording layer of the optical disc, the light detector 40 becomes a minimum scattering circle to be described later and is positioned at the intersection of the dividing lines of the light receiving unit 400 for 0th order diffracted light. It is arranged on a plane perpendicular to the axis. These light receiving portions are formed and arranged symmetrically with respect to the center (intersection of dividing lines) of the light receiving portion 400 for 0th-order diffracted light, that is, are symmetrical with respect to straight lines extending from the center in the track direction and in the perpendicular direction.
[0019]
As shown in FIG. 2, the light beam emitted from the semiconductor laser 31 enters the polarization beam splitter 33 through the grating 32. The polarizing beam splitter 33 has a polarizing mirror, and the incident light beam passes through the polarizing beam splitter 33, passes through the collimator lens 34, the optical path is changed to a right angle by the mirror 35, and passes through the quarter wavelength plate 36. Then, the predetermined information recording surface of the optical disk 1 is irradiated from the objective lens 37. Thus, in the irradiation optical system, the objective lens 37 condenses the light beam onto the pit row or track formed on the optical disc 1 in a spiral or concentric manner to form a spot on the recording surface. With this irradiated light beam spot, recorded information can be written to or read from the information recording surface of the optical disc 1.
[0020]
The return light reflected by the light beam spot on the recording surface of the optical disc 1 is guided to the light detector 40 by the light detection optical system. That is, the return light passes through the objective lens 37, the quarter-wave plate 36, the mirror 35, and the collimator lens 34 and then enters the polarization beam splitter 33 again. In this case, the return light has its optical path changed in a direction different from the direction toward the semiconductor laser 31 by the polarization beam splitter 33 and is guided to the diffractive optical element 39 and the astigmatism generating optical element 38. The return light that has passed through the diffractive optical element 39 and the astigmatism generating optical element 38 is given astigmatism and is diffracted, and the 0th-order diffracted light receiving unit 400 and the ± 1st-order diffracted light receiving unit in the photodetector 40. The light beams enter the respective 401a and 401b as diffracted light. The astigmatism generation optical element 38 and the diffractive optical element 39 may be reversely arranged to give astigmatism after the return light is diffracted. Further, the cylindrical lens can be omitted, and the hologram lens can have a function of giving astigmatism to the return light.
[0021]
Each light receiving unit of the photodetector 40 photoelectrically converts the received light and supplies a light detection electric signal to the drive control unit 59 shown in FIG. The drive control unit 59 connected to the photodetector 40 performs a predetermined calculation to generate a focus error signal FE and a reproduction signal RF (Radio Frequency). That is, the signals FE and RF output from the drive control unit 59 are expressed by the following equations when the signs of the respective light receiving units of the photodetector 40 shown in FIG.
[0022]
[Equation 3]
FE = (A1 + A4 + C1 + C4) − (A2 + A3 + C2 + C3)
RF = B1 + B2 + B3 + B4
The drive control unit 59 supplies the focus error signal FE and the reproduction signal RF to the focus drive circuit 18 and the demodulation circuit 20 shown in FIG. The drive control unit 59 can be configured to perform a predetermined calculation and generate a wavefront aberration error signal. The photoelectric conversion signal of the light receiving element divided by the dividing line in the tangential direction can be used for generating a tracking error signal.
[0023]
The hologram lens of the diffractive optical element 39 disposed on the optical axis of the return light of the light detection optical system shown in FIG. 2 is a grating or blazed transmission hologram formed with a diffraction grating made of a parallel plate made of optical glass.
As shown in FIG. 4, the hologram lens of the diffractive optical element 39 has a diffraction grating ring zone 39 </ b> A for extracting a specific light ray component to be described later from the return light in a ring shape. The specific light component extracted by the annular zone 39A of the diffractive optical element 39 is a distribution of wavefront aberration generated by the light transmission layer on the information recording surface of the optical disc 1 on the exit pupil plane of the irradiation optical system such as the objective lens 37. It is a light ray component in the vicinity of a predetermined radius on the pupil corresponding to the maximum value. Therefore, the annular zone 39A has a pupil radius R around the optical axis of the return light as the radius. ₀ 0.71R ₀ ~ 0.74R ₀ It is a part that contains.
[0024]
As shown in FIG. 5, the annular zone 39A of the diffractive optical element 39 diffracts the return light, and the 0th-order diffracted light and the ± 1st-order diffracted light are received as the 0th-order diffracted light receiving unit 400 and the ± 1st-order diffracted light received by the photodetector 40. Are guided through the astigmatism generating optical element 38 onto the parts 401a and 401b to form a circular 0th-order diffracted light spot and circular and annular ± 1st-order diffracted light spots to convert the transmitted light into 0th-order diffracted light and first-order diffracted light. To separate. That is, the 0th-order diffracted light that does not receive the action of the hologram lens of the diffractive optical element 39 that has passed through the diffractive optical element 39 and the astigmatism generating optical element 38 travels without deviating from the original optical axis, but the ± 1st-order diffracted light is It is deflected symmetrically with respect to the optical axis. The 0th-order diffracted light receiving unit 400 is connected to the demodulation circuit 20, and each ± 1st-order diffracted light receiving unit is connected to the drive control unit 59, and supplies their outputs to the respective circuits.
[0025]
With reference to FIG. 6, a detailed description will be given of a first embodiment in which astigmatism focus servo is performed using ± first-order diffracted light obtained from the annular zone 39A. The outputs of the ± 1st-order diffracted light receiving portions 401a and 401b that receive the annular spot extracted by the annular zone 39A of the diffractive optical element 39 are used to detect the focus error signal FE. In the astigmatism method, an astigmatism generating optical element such as a cylindrical lens or a parallel plate is arranged in a return light optical system, and the return light is received in the vicinity of the center of the four-divided light receiving portion to change the shape of one light spot. This is a method of detecting and generating a focus error signal. In FIG. 6, in order to explain the operation on behalf of the + 1st order diffracted light from the annular zone 39A, the objective lens 37, the cylindrical lens astigmatism generation optical element 38, the hologram lens diffractive optical element 39, and the +1 next time. Elements other than the folded light receiving portion 401a are omitted.
[0026]
As shown in FIG. 6, the + 1st order diffracted light of astigmatism that passes through the annular zone 39A of the diffractive optical element 39 and the cylindrical lens 38 from the objective lens 37 and has astigmatism is in the track (tangential) extension direction and the disk radial direction. An annular light spot S (minimum scattering circle described later) is formed in the vicinity of the center of the light receiving part 401a for + 1st order diffracted light having a light receiving surface divided into four by two line segments orthogonal to each other.
[0027]
As shown in FIG. 6, the cylindrical lens 38 has an optical path for return light so that its central axis (rotational symmetry axis of the cylindrical curved surface forming the lens surface) extends at an angle of 45 degrees with respect to the track extending direction of the optical disk. Is arranged. In this configuration, astigmatism is given to the return light converged by the objective lens 37, and the light rays become astigmatism different from each other by 90 degrees, and the previous line image M, depending on the distance between the optical disk and the objective lens 37, A minimum scattering circle S and a back line image M are formed. The detection optical system irradiates the light receiving section 401a for the + 1st order diffracted light with the minimum scattering circle S of FIG. 6A when the light beam is focused, and the light receiving surface as shown in FIG. 6B or 6C at the time of defocusing. + 1-order diffracted light receiving portion 401a is irradiated with a line image extending in the diagonal direction and an elliptical annular light spot. The distance between the line images collected by the + 1st order diffracted light, that is, the distance between (b) and (c) shown in FIG. 6, corresponds to the capture range Cp of the focus error signal.
[0028]
FIG. 7 shows a function S (y) of the focus error signal FE generated based on the output of the light receiving unit 401a for + 1st order diffracted light, so-called S-characteristics, where the vertical axis represents the signal intensity S (y) and the horizontal axis represents the distance. (Y) is shown. In this S-characteristic, the light spot intensity distribution is symmetric with respect to the four-part light receiving part center O at the time of focusing, and the perfect light spot in FIG. 6A symmetric in the tangential direction and the radial direction is the light receiving element. (A1, A2, A3, A4), the values obtained by adding the photoelectric conversion outputs of the light receiving elements (A1, A4) (A2, A3) on the diagonal line are equal to each other, resulting in a focus error. The component is “0”. Further, at the time of out of focus, an elliptical or linear light spot is formed on the light receiving portion in the diagonal direction of the light receiving portion as shown in FIG. 6B or FIG. 6C, so that the photoelectric conversion output of the light receiving portion on the diagonal line is obtained. The values obtained by adding each have opposite polarities. Therefore, the range between the maximum (b) and the minimum (c) of the S-characteristic of the focus error signal function corresponds to the capture range Cp.
[0029]
FIG. 8 shows the state of the spot of the light beam on the light receiving unit for 0 and ± 1st order diffracted light when the light beam having a Gaussian intensity distribution is in focus and close to or far from the optical disk. As shown in FIG. 8A, at the time of focusing, the 0th-order diffracted light is collected as the minimum scattering circle on the 0th-order diffracted light receiving unit 400, and at the same time, the ± 1st-order diffracted light is also received by the ± 1st-order diffracted light receiving unit 401a. , 401b are collected as a minimum scattering circle and an annulus.
[0030]
When the objective lens is far from the optical disk, as shown in FIG. 8 (b), the 0th-order and ± 1st-order diffracted light spots are condensed into a deformed ellipse extending to the light receiving element at the diagonal position. On the other hand, when the objective lens is close to the optical disk, astigmatism different in the direction of 90 degrees occurs as shown in FIG. 8C, and the zeroth and ± first order diffracted light spots are the light receiving elements at the other diagonal position. It becomes a deformed ellipse extending in the direction of focusing.
[0031]
Even when the light beam shown in FIG. 8A is in focus, spherical aberration occurs when there is a thickness error from a predetermined film thickness such as the cover layer of the optical disc. The spot diameter of the irradiation light beam varies. However, as described above, the spot shape on the ± 1st-order diffracted light receiving portions 401a and 401b corresponding to the specific annular zone 39A maintains a stable shape, that is, a point-symmetric shape at the center of the light receiving portion. The annular zone 39A of the diffraction grating of the diffractive optical element 39 of the hologram lens extracts a specific light component that is resistant to fluctuations in the thickness of the cover layer of the optical disc in an annular shape from the return light.
[0032]
The inventor found out that a specific light ray component of such return light is related to a portion having a large spherical aberration, and as described above, for example, in an optical system using an objective lens having a numerical aperture of 0.85, The light component in the vicinity of the normalized radius of the light beam cross section corresponding to the maximum value of the wavefront aberration distribution on the exit pupil plane is extracted into a specific annular zone by the diffractive optical element, and the intensity distribution of the light component is calculated. It was used to devise focus error detection.
[0033]
FIG. 9 shows a wavefront aberration (b) and a pupil light beam cross section (a) when there is spherical aberration due to the thickness error of the cover layer of the optical disk. When focusing on the best image point, the wavefront aberration distribution (b) indicates the pupil radius of the objective lens as R ₀ Then radius 0.71R ₀ Near the peak (annular aberration). This zone aberration radius 0.71R ₀ The image point of the light beam passing on the circumference of the lens does not move at all even if spherical aberration occurs in the pickup. The radius of the zonal aberration varies slightly depending on the NA of the objective lens, and increases slightly if the NA is large. This is because, in the generated spherical aberration, in addition to the component proportional to the fourth power of the radius, the higher order component becomes larger. For example, when NA is 0.85, the annular aberration radius is 0.74R. ₀ It becomes.
[0034]
As is apparent from FIG. 8 showing the light intensity distribution on the light receiving portions 401a and 401b for ± 1st order diffracted light when there is a thickness error of the cover layer or the like, that is, when spherical aberration occurs, the thickness of the cover layer or the like is clear. If it is different from the predetermined thickness, the zonal aberration radius is 0.71R ₀ The distribution of the inner ray and the outer ray is different at the boundary. Since this distribution is unbalanced, an error (defocus) is generated in focus error detection when spherical aberration occurs. However, annulus aberration radius 0.71R ₀ Since the light beam that passes through does not move at all on the light receiving sections 401a and 401b for ± 1st order diffracted light, detection with no influence of spherical aberration becomes possible if focus error detection is performed with only this light beam.
[0035]
FIG. 10 shows a diffraction grating ring zone of the diffractive optical element 39 having a different size for extracting a light beam component used for focus error detection. FIG. 10A shows a radius of 0.71R. ₀ And FIG. 10 (b) shows a radius 0.71R. ₀ FIG. 10C shows the entire pupil area, that is, the defocus amount caused by the cover layer thickness error, and uses all reflected light for focus error detection. Is the case. FIG. 11 shows the relationship between the thickness error and the defocus, that is, the change of the defocus amount corresponding to the size of the annular zone. As is clear from FIG. 11, compared to the case of using the regions shown in FIGS. 10B and 10C, if only the region of FIG. 10A, that is, the annular zone 39A is used, even if the cover layer thickness error is large. It can be seen that no defocusing occurs. In the case of FIG. 10B, there is an effect as compared with the case where all light components are used (FIG. 10C), but defocusing occurs when the thickness error is large. When a large thickness error is expected or when it is used for a multilayer disk, it can be said that the use of the hologram of FIG. FIG. 12 shows the relationship between thickness error and spherical aberration error. FIG. 12 shows similar characteristics in the case of using the annular region of FIGS. 10 (a) and 10 (b).
[0036]
As shown in FIG. 9, in principle, a portion that is not affected by spherical aberration has a radius 0.71R on the pupil. ₀ However, if the light component of only this part is used, the amount of light is insufficient and the signal S / N may not be obtained. ₀ It needs to have a certain width including the part. Radius 0.71R on the pupil ₀ The aberrations of the inner and outer rays need to be balanced on the detector.
[0037]
Therefore, a suitable range of the outer radius (rmax) and the inner radius (rmim) of the diffraction grating ring zone 39A of the diffractive optical element 39 is obtained with the radius as a variable r. As described below, the radius range of the optimum annular zone for focusing on the best image point is calculated without depending on the amount of spherical aberration.
When a Zernike aberration polynomial is used, the wavefront at the best image point when there is spherical aberration can be expressed by the following equation (1).
[0038]
[Expression 4]

[0039]
When this wavefront function W (r) is converted into a longitudinal aberration amount y (r), the following equation (2) is obtained.
[0040]
[Equation 5]

[0041]
The above equation shows how much the defocused position of the light beam that passes through the circumference of the radius r (value normalized by the pupil radius) on the pupil when focusing on the best image point. (Spherical aberration is converted into defocus for each radius). Here, Amn (where m and n are integers) represents a spherical aberration coefficient. If the spherical aberration is caused by the cover layer thickness error ΔT, it is analytically obtained by the following equation (2a).
[0042]
[Formula 6]

(However, k0 = 40, 60, 80, 100, or 120)
[0043]
For example, when (NA, λ) = (0.85, 405 μm), n = 1.62, ΔT = 10 μm, A ₄₂ = -0.26, A ₆₃ = -0.049, A ₈₄ = −0.0076, and the longitudinal aberration y (r) becomes almost parabolic as shown in FIG. 13 (here, spherical aberration only in the forward path is considered). That is, when spherical aberration caused by the cover layer thickness error is expressed by defocus, the defocus amount of light rays passing through the center of the pupil (= y (0)) and the defocus amount of light rays passing through the outermost periphery of the pupil (= y (1)) ) All the light rays enter. Also, by solving for y (r) = 0, it can be seen that the best image point is always the position at which the light rays in the vicinity of the radius r = 0.74 are focused without depending on the cover layer thickness error ΔT. Here, since the higher order is taken into consideration, the radius r = 0.74 is obtained, but the radius r = 0.71 is W ₆₀ = W ₈₀ = W ₁₀₀ = W ₁₂₀ This is the value when = 0. In other words, if the focus error signal is generated only with light rays in the vicinity of r = 0.74, the best image point can always be focused regardless of ΔT.
[0044]
Here, if only light in the vicinity of the radius r = 0.74 is used, the amount of light is insufficient, and it is expected that the S / N of the detection signal is significantly deteriorated, so that actually r = 0.74 is included. The light in the ring with a certain width is used. As shown in FIG. 14, the range of the inner diameter rmin and the outer diameter rmax of the annular zone 39A including the radius r = 0.74 on the pupil is defined.
At this time, it is certain that the inner diameter rmin and the outer diameter rmax of the annular zone 39A are obtained numerically by ray tracing. Further, if the focus error signal function S (y) generated by using the entire surface light is known when there is no spherical aberration, the inventor may choose to satisfy the following expression (3) analytically. Propose that. That is, the product of the focus error signal function S (y) using the longitudinal aberration amount as a parameter and the light intensity distribution in the entire pupil so that the light intensity is balanced and becomes zero within the maximum value (peak) of the zonal aberration. As zero, an inner diameter rmin and an outer diameter rmax that satisfy the following expression (3a) are calculated.
[0045]
[Expression 7]

[0046]
Here, I (r) is the intensity distribution on the exit pupil of the objective lens, but when it cannot be expressed in rotational symmetry, the intensity distribution on the exit pupil is taken as I (r cos θ, r sin θ) and the following expression (3) is satisfied. Thus, the inner diameter rmin and the outer diameter rmax are calculated.
[0047]
[Equation 8]

[0048]
For example, when the S-shaped focus error signal function S (y) using the light of the entire pupil surface when there is no spherical aberration is as shown in FIG. If y (r) is distributed in the linear region of S (y), it can be simply written as the following equation (4). That is, it can be represented by the product of the slope of S = 0 (y = 0) and the longitudinal aberration amount y (r) shown in FIG.
[0049]
[Equation 9]

[0050]
In order to simplify the model, when I (r) = 1.0 (uniform light incidence and specular reflection), and substituting Equations (1) and (4) into Equation (3), the following equation is obtained: (5)
[0051]
[Expression 10]

[0052]
However, NA = 0.85, λ = 405 nm, and n = 1.62.
The inner diameter rmin and outer diameter rmax (inner and outer radii) of the annular zone that satisfies the equation (5) are as shown in the graph of FIG. As is apparent from FIG. 15, the annular zone width of the diffractive optical element ranges from a very narrow range of the pupil radius, rmin = rmax = 0.71, to a wide range of the range of rmin = 0.25-rmax = 0.95. Thus, a suitable light component corresponding to the maximum value of the wavefront aberration distribution on the exit pupil plane of the wavefront aberration generated by the light transmission layer can be extracted from the return light in a ring shape.
[0053]
Actually, I (r) has a Gaussian distribution or a more complicated distribution in consideration of diffraction on the optical disk, but the annular width of the diffractive optical element satisfies the expression (3) as described above. The relationship between the inner diameter rmin and the outer diameter rmax can be derived, and the optimum values of the inner diameter and the outer diameter can be obtained from the intensity distribution on the exit pupil and the focus error signal function.
As described above, in the first embodiment, while using the astigmatism method for focus error detection, the light beam of the annular portion including the annular aberration from the intensity distribution on the exit pupil is a portion corresponding to the annular portion ( Separation from the original optical axis is performed using a diffractive optical element provided with a diffraction grating in the annular zone 39A). Since the annular light beam deflected and separated by the annular diffraction grating is incident on a photodetector light receiving unit (± 1st order diffracted light receiving units 401a and 401b) dedicated to focus error detection, the focus error is detected. Even if this occurs, there is no influence on the light receiving unit.
[0054]
Furthermore, in the first embodiment, the region of the zonal aberration (0.71R in pupil radius) ₀ Spherical aberration generated in the optical system is detected by utilizing the fact that the behavior of the light beam transmitted through the inner side and the outer light beam differs depending on the spherical aberration.
With these configurations, since the focus servo is applied only by the light beam that passes through the annular zone 39A of the diffractive optical element, highly accurate defocus detection that is not affected by spherical aberration becomes possible.
[0055]
Specifically, the focus error signal intensity (focus) obtained by the light receiving unit for the 0th order and ± 1st order diffracted light when the light beam of the objective lens as shown in FIG. The relationship between the error) and the defocus amount was examined in the first embodiment. The thickness error of the cover layer of the optical disk serving as a document was examined in the range of 0.1 mm ± 0.02 mm. FIG. 16 shows a signal (B1 + B4) − (B2 + B3) obtained by the astigmatism method based on the output from the 0th-order diffracted light receiving unit 400 as a comparative example. FIG. 17 shows a signal (A1 + A4 + C1 + C4) − (A2 + A3 + C2 + C3) based on the outputs from the light receiving sections 401a and 401b for ± first-order diffracted light according to the first embodiment. As is clear from FIG. 16, when the extracted light component near the normalized radius of the light beam cross section corresponding to the maximum value of the wavefront aberration distribution on the exit pupil plane is compared with FIG. It can be seen that a good S-characteristic can be obtained.
[0056]
Thus, in the first embodiment using the astigmatism method for focus error detection, the annular portion is separated from the original optical axis by providing a diffraction grating in the annular portion corresponding to the region, The annular light beam deflected by the diffraction grating is made incident on the focus error detection light receiving unit. The shape of the focus error detecting light receiving portion is a so-called quadrant detector used in the astigmatism method. The diffraction element is set so that the intensity of the 0th-order light is increased. Since the RF signal, tracking error signal, and the like are detected by the 0th-order light receiving part on which the 0th-order light is incident, the number of addition amplifiers for obtaining the RF signal can be reduced to suppress an increase in unnecessary noise. Can do. In the first embodiment, both the ± first-order diffracted light beams from the diffraction element are used, but only one of them may be used. Furthermore, the diffraction element may be formed into a phrase shape and only one of the diffracted lights may be used.
(Second Embodiment)
In the second embodiment, the astigmatism generation optical element 38 in the first embodiment is omitted, and the photodetector 40, the diffractive optical element 39, and the drive control unit 59 are changed corresponding to the differential spot size method. Other than this, it is basically the same as the first embodiment. In the first embodiment, the astigmatism method is used as the focusing servo control method of the focus actuator 301. In the second embodiment, the differential spot size method is used. In the spot size method, the return light from the optical disc is divided into two optical paths, and front and rear focal points with different focal lengths are formed, and a light receiving unit is provided before and after the focal point, and the light spot thereon Is a method of generating a focus error signal by comparing the magnitudes of.
[0057]
As shown in FIG. 18, in the photodetector 40 of the second embodiment, the 0th-order diffracted light receiving unit 400 on the optical axis is configured by a single light receiving element (B). Also, each of the ± 1st order diffracted light receiving portions 401a and 401b arranged separately from the 0th order diffracted light receiving portion 400 on both sides in the disk radial direction has a radius to the light receiving elements (A2) and (C2) disposed at the center. It is composed of light receiving element pairs (A1, A3) (C1, C3) of equal area, which are arranged symmetrically with a straight line extending in the direction. The light detector 40 is perpendicular to the optical axis so that when the spot of the 0th-order diffracted light is in focus on the recording layer of the optical disc, this becomes the minimum scattering circle described later and is positioned at the center of the light-receiving unit 400 for 0th-order diffracted light. Are arranged on a flat surface. These light receiving portions are symmetrical with respect to a straight line extending from the center in the track direction and in the perpendicular direction.
[0058]
As shown in FIG. 19, the hologram lens of the diffractive optical element 39 of the second embodiment is an exit pupil of an irradiation optical system such as an objective lens 37 having a wavefront aberration caused by a light transmission layer on the information recording surface of the optical disc 1. A diffraction grating ring zone 39A for extracting a specific ray component in the vicinity of the normalized radius of the light beam cross section corresponding to the maximum value of the wavefront aberration distribution on the surface from the return light is provided. The annular zone 39A has a decentered lens effect so that the first-order diffracted light is deflected and condensed substantially symmetrically from the original optical axis. Further, the annular zone 39A is set so that a convex lens or a concave lens acts on any of the ± first-order diffracted lights. Further, the annular zone 39A of the diffractive optical element 39 has a pupil radius R around the optical axis of the return light. ₀ Radius 0.71R on the pupil ₀ ~ 0.74R ₀ Is included.
[0059]
As shown in FIG. 20, the annular zone 39A of the diffractive optical element 39 diffracts the return light, and converts the 0th-order diffracted light and the ± 1st-order diffracted light into the 0th-order diffracted light receiving unit 400 and the ± 1st-order diffracted light, respectively. The light is guided onto the light receiving portions 401a and 401b, and a circular 0th-order diffracted light spot and circular and annular ± 1st-order diffracted light spots are formed, and the transmitted light is separated into 0th-order diffracted light and first-order diffracted light. That is, 0th-order diffracted light that does not receive the action of the hologram lens of the diffractive optical element 39 that has passed through the diffractive optical element 39 travels without deviating from the original optical axis, but ± 1st-order diffracted light is deflected symmetrically with respect to the optical axis. The The 0th-order diffracted light receiving unit 400 is connected to the demodulation circuit 20, and each ± 1st-order diffracted light receiving unit is connected to the drive control unit 59, and the output from each is supplied to each circuit. The outputs of the ± 1st-order diffracted light receiving portions 401a and 401b that receive the annular spot extracted by the annular zone 39A of the diffractive optical element 39 are used to detect the focus error signal FE.
[0060]
With reference to FIG. 21, a second embodiment in which focus servo of the differential spot size method is executed using ± first-order diffracted light obtained from the annular zone 39A will be described in detail. In FIG. 21, the objective lens 37, the diffractive optical element 39, the 0th-order diffracted light receiving unit 400, and the + 1st-order diffracted light receiving unit 401a, in order to explain the operation representatively of the + 1st-order diffracted light from the annular zone 39A, Elements other than 401b are omitted.
[0061]
As shown in FIG. 21, when the light beam is focused on the track of the optical disc, the diffractive optical element 39 connects the condensing point of the 0th-order diffracted light on the 0th-order diffracted light receiving unit 400 of the optical axis, The + 1st order diffracted light is separated from the optical axis and forms a focal point f1 in front of the light detector 40, and the −1st order diffracted light is separated from the optical axis and connected to a focal point f2 far from the light detector 40. An annular spot is irradiated on the light receiving elements (A2) and (C2) at the center of 401a and 401b. Therefore, the size of the annular spot in the light receiving portions 401a and 401b for ± 1st order diffracted light differs depending on whether the objective lens at the time of defocusing approaches or moves away. For example, the distance between (d) and (e) shown in FIG. 21 is defined as the focus error signal capture range. As in the above embodiment, a signal showing S-characteristic is obtained.
[0062]
The drive control unit 59 is configured such that the output focus error signal FE and the reproduction signal RF indicate the signs of the respective light receiving units of the photodetector 40 shown in FIG. ing.
[0063]
[Expression 11]
FE = (A1 + A3 + C2) − (A2 + C1 + C3)
RF = B
As shown in FIG. 22 (a), at the time of focusing when the cover layer of the optical disc has a predetermined film thickness, the 0th-order diffracted light is condensed as a minimum scattering circle on the light-receiving unit 400 for 0th-order diffracted light, and at the same time ± The first-order diffracted light is also collected as a minimum scattering circle and an annulus on the ± 1st-order diffracted light receiving portions 401a and 401b. When the objective lens is far from the optical disk, as shown in FIG. 22B, the 0th-order diffracted light spot is slightly enlarged and condensed as an enlarged and reduced circle on the ± 1st-order diffracted light receiving section. On the other hand, when the objective lens is close to the optical disk, as shown in FIG. 22C, the 0th-order diffracted light spot is slightly enlarged and condensed as a reduced and enlarged circle on the ± 1st-order diffracted light receiving section.
(Third embodiment)
The third embodiment is the same as the first embodiment except that the photodetector 40, the diffractive optical element 39, and the drive control unit 59 in the first embodiment are changed corresponding to the astigmatism method and the differential spot size method. Basically the form. In the first embodiment, only the astigmatism method is used as the focusing servo control method of the focus actuator 301. In the third embodiment, the astigmatism method and the differential spot size method are used for focus error detection. The hybrid method used was used.
[0064]
As shown in FIG. 23, in the photodetector 40 according to the third embodiment, the 0th-order diffracted light receiving units 400 on the optical axis are arranged close to each other with two orthogonal dividing lines as boundaries, and are independent from each other. The four light receiving elements (B 1, B 2, B 3, B 4) having the same area are configured such that one dividing line is parallel to the track extending direction of the optical disc 1. Further, each of the ± 1st order diffracted light receiving portions 401a and 401b, which are further separated from the 0th order diffracted light receiving portion 400 and separated on both sides in the disk radial direction, has a light receiving element (A2) (C2) disposed at the center. ) Are arranged with a pair of light receiving elements (A1, A3) (C1, C3) of equal area arranged symmetrically with a straight line extending in the radial direction. When the spot of the 0th order diffracted light is focused on the recording layer of the optical disc, the light detector 40 becomes a minimum scattering circle to be described later and is positioned at the intersection of the dividing lines of the light receiving unit 400 for 0th order diffracted light. It is arranged on a plane perpendicular to the axis. These light receiving portions are symmetrical with respect to a straight line extending from the center in the track direction and in the perpendicular direction.
[0065]
The hologram lens of the diffractive optical element 39 of the third embodiment has an annular zone 39A set so as to act as a convex lens or a concave lens on any of ± first-order diffracted light, as in the embodiment shown in FIG. Prepare. There is a transmission parallel plate portion on which the diffraction grating is not provided on the inner and outer sides of the annular zone 39A. The annular zone 39A of the diffractive optical element 39 has a pupil radius R around the optical axis of the return light. ₀ Radius 0.71R on the pupil ₀ ~ 0.74R ₀ Is included.
[0066]
As shown in FIG. 24, the annular zone 39A of the diffractive optical element 39 diffracts the return light, and the 0th-order diffracted light and the ± 1st-order diffracted light are received by the 0th-order diffracted light receiving unit 400 and the ± 1st-order diffracted light received by the photodetector 40. Are guided through the astigmatism generating optical element 38 onto the parts 401a and 401b to form a circular 0th-order diffracted light spot and circular and annular ± 1st-order diffracted light spots to convert the transmitted light into 0th-order diffracted light and first-order diffracted light. To separate. The 0th-order diffracted light receiving unit 400 is connected to the demodulation circuit 20, and each ± 1st-order diffracted light receiving unit is connected to the drive control unit 59, and the output from each is supplied to each circuit. The outputs of the light receiving sections 401a and 401b for ± first-order diffracted light that receive the elliptical annular spot extracted by the annular zone 39A of the diffractive optical element 39 are used for detection of the focus error signal FE.
[0067]
The drive control unit 59 is configured so that the output focus error signal FE and the reproduction signal RF indicate the signs of the respective light receiving units of the photodetector 40 shown in FIG. ing.
[0068]
[Expression 12]
FE = (A1 + A3 + C2) − (A2 + C1 + C3)
RF = B1 + B2 + B3 + B4
In any of the above-described embodiments, when the present invention is applied to a three-beam pickup such as DPP or CTC, ± first-order diffracted light is generated in the return light of the side beam, but the ± 1st-order diffracted light of the side beam is considerable. Since the amount of light is small, it is not necessary to provide a photodetector for receiving this light. The three-beam photodetector can receive only the 0th-order diffracted light from the diffractive optical element.
[0069]
【The invention's effect】
According to the present invention, since the focus error detection is performed using only the light beam that passes through the annular region that is not affected by the spherical aberration at all, even if the spherical aberration occurs due to the disc cover layer thickness error, the focus error is detected. An error (defocus) does not occur in error detection, and good focus error detection can be performed.
[Brief description of the drawings]
FIG. 1 is a schematic block diagram showing a configuration of a recording / reproducing apparatus including an optical pickup device according to the present invention.
FIG. 2 is a schematic perspective view showing a configuration of an optical pickup device according to the present invention.
FIG. 3 is a schematic plan view of a photodetector of the optical pickup device according to the present invention.
FIG. 4 is a schematic plan view showing a configuration of a hologram lens of a diffractive optical element of the optical pickup device according to the present invention.
FIG. 5 is a schematic view showing an optical path of return light from the hologram lens of the diffractive optical element of the optical pickup device according to the present invention to the photodetector.
FIG. 6 is a schematic perspective view showing a configuration of a light detection optical system of the optical pickup device according to the present invention.
FIG. 7 is a graph showing a change in a focus error signal having a capture range of the optical pickup device according to the present invention.
FIG. 8 is a schematic plan view of a photodetector showing a state of a light spot on a light receiving portion for 0th and ± 1st order diffracted light of return light in the optical pickup device according to the present invention.
FIG. 9 is a schematic diagram showing the relationship between wavefront aberration and pupillary light beam cross section when there is spherical aberration due to the thickness error of the cover layer of the optical disc.
FIG. 10 is a schematic plan view showing diffraction grating zones of three diffractive optical elements having different sizes for extracting a light beam component used for focus error detection of the optical pickup device according to the present invention.
FIG. 11 is a graph showing a change in defocus amount with respect to the thickness error of the cover layer of the optical disc in the optical pickup device according to the present invention.
FIG. 12 is a graph showing a change in spherical aberration error with respect to the thickness error of the cover layer of the optical disc in the optical pickup device according to the present invention.
FIG. 13 is a graph showing spherical aberration caused by the thickness error of the cover layer of the optical disc in the optical pickup device according to the present invention.
FIG. 14 is a schematic plan view illustrating an inner diameter and an outer diameter of a diffraction grating ring zone of a diffractive optical element that extracts a light beam component used for focus error detection of the optical pickup device according to the present invention.
FIG. 15 is a graph showing an example of the relationship between the inner and outer diameters of a diffraction grating ring zone of a diffractive optical element that extracts a light beam component used for focus error detection of the optical pickup device according to the present invention;
FIG. 16 is a graph showing a change in a focus error detection signal related to defocus for each thickness error of a cover layer of an optical disk using 0th-order diffracted light as a comparison.
FIG. 17 is a graph showing a change in a focus error detection signal related to defocus for each thickness error of a cover layer of an optical disc using ± first-order diffracted light in the optical pickup device according to the present invention.
FIG. 18 is a schematic plan view of a photodetector of an optical pickup device according to another embodiment of the present invention.
FIG. 19 is a schematic plan view showing the configuration of a hologram lens of a diffractive optical element of an optical pickup device according to another embodiment of the present invention.
FIG. 20 is a schematic diagram showing an optical path of return light from a hologram lens of a diffractive optical element of an optical pickup device according to another embodiment of the present invention to a photodetector.
FIG. 21 is a schematic perspective view showing a configuration of a light detection optical system of an optical pickup device according to another embodiment of the present invention.
FIG. 22 is a schematic plan view of a photodetector showing a state of a light spot on a light receiving unit for 0th and ± first-order diffracted light of return light in an optical pickup device of another embodiment according to the present invention.
FIG. 23 is a schematic plan view of a photodetector of an optical pickup device according to another embodiment of the present invention.
FIG. 24 is a schematic diagram showing an optical path of return light from a hologram lens of a diffractive optical element of an optical pickup device according to another embodiment of the present invention to a photodetector.
FIG. 25 is a schematic plan view of a photodetector showing a state of a light spot on a light receiving unit for 0 and ± first-order diffracted light of return light in an optical pickup device of another embodiment according to the present invention.
[Explanation of symbols]
1 Optical disc
3 Pickup
18 Focus drive circuit
20 Demodulator circuit
31 Semiconductor laser
32 grating
33 Polarizing Beam Splitter
34 Collimator lens
35 mirror
36 1/4 wave plate
37 Objective lens
38 Astigmatism generating optical element
39 Diffractive optical element
39A Ring
40 photodetectors
59 Control unit
301 Focus actuator
400 Light receiving part for 0th order diffracted light
401a, 401b ± 1st order diffracted light detector

Claims (10)

An irradiation optical system for condensing a light beam on a recording surface through a light transmission layer of an optical recording medium to form a spot, and return light reflected from the spot and returning to a photodetector. An optical pickup device having a light detection optical system and detecting a focus error of the light beam,
In the vicinity of a predetermined radius on the pupil corresponding to the maximum value of the wavefront aberration distribution on the exit pupil plane of the irradiation optical system of the wavefront aberration generated in the optical system, which is disposed on the optical axis of the return light of the light detection optical system Comprising a diffractive optical element having an annular zone for extracting the light beam component in a ring shape from the return light,
The photodetector includes a spot light receiving unit that receives the extracted light component passing through the annular zone, and is connected to the spot light receiving unit and detects a focus error of the light beam based on a photoelectric conversion output from the spot light receiving unit. An optical pickup device provided with a focus error detection circuit. The predetermined radius on the pupil is 0.71R _{0 to} 0.74R ₀ when the pupil radius is R _o around the optical axis of the return light of the light detection optical system. 1. The optical pickup device according to 1. The annular zone is expressed by the following formula (3)

The inner and outer radii satisfying (in (3), I (r cos θ, r sin θ) represents the intensity distribution on the exit pupil, S (y) represents the focus error signal function, and y (r) represents the amount of longitudinal aberration). 2. The optical pickup device according to claim 1, wherein the optical pickup device has rmin and rmax. 2. The light according to claim 1, wherein the diffractive optical element is a grating or a blazed transmission hologram lens that separates transmitted light into zero-order diffracted light and first-order diffracted light by a diffraction grating provided in the annular zone. Pickup device. And an astigmatism generating optical element that is disposed either before or after the hologram lens on the optical axis of the return light of the light detection optical system and imparts astigmatism to the return light. The optical pickup device according to claim 4. The optical pickup device according to claim 4, wherein the hologram lens has a function of imparting astigmatism to the return light. The spot light-receiving unit receives either one of ± first-order diffracted lights that are arranged close to each other with two perpendicular dividing lines as a boundary line and irradiated from the annular zone of the diffractive optical element around the intersection of the dividing lines. The focus error detection circuit generates a difference between a pair of output sums of the four light receiving elements at diagonal positions as a focus error signal of the light beam. The optical pickup device according to claim 5 or 6, wherein: The hologram lens has a decentered lens effect that deflects and condenses the ± 1st order diffracted light of the return light from the original optical axis, and functions as a convex lens or a concave lens on any of the ± 1st order diffracted light The optical pickup device according to claim 4, further comprising: Each of the spot light receiving sections receives ± first-order diffracted light emitted from the annular zone of the diffractive optical element, and is arranged in proximity to each other with a dividing line dividing the spot of the ± first-order diffracted light as a boundary line. A total of an area of at least one light receiving element that is more than one light receiving element and that is on the positive polarity side and an area of at least one light receiving element that is on the negative polarity side is substantially equal to the focus error. 9. The optical pickup device according to claim 8, wherein the detection circuit generates a difference between the output sums of the positive polarity side and the negative polarity side of the light receiving element as a focus error signal of the light beam. The hologram lens has a function of acting as a convex lens or a concave lens on any one of the ± first-order diffracted lights of the return light, and the spot light receiving portions are disposed close to each other with two orthogonal dividing lines as boundary lines, and The four diffractive optical elements that receive the first-order diffracted light irradiated from the annular zone of the diffractive optical element around the intersection of the dividing lines are configured to be independent of each other, and the focus error detection circuit is located at the diagonal position. 7. The optical pickup device according to claim 5, wherein a difference between a pair of output sums of the light receiving elements is generated as a focus error signal of the light beam.

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