JP4586309B2

JP4586309B2 - Diffraction element and optical head device

Info

Publication number: JP4586309B2
Application number: JP2001202616A
Authority: JP
Inventors: 好晴大井; 弘昌佐藤; 真弘村川; 龍一郎後藤
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2001-07-03
Filing date: 2001-07-03
Publication date: 2010-11-24
Anticipated expiration: 2021-07-03
Also published as: JP2003014916A

Description

【０００１】
【発明の属する技術分野】
本発明は、回折素子および光ヘッド装置に関し、特に複数のビームを発生する偏光性の回折素子および光記録媒体の情報の記録および再生を行う光ヘッド装置に関する。
【０００２】
【従来の技術】
ＣＤ、ＤＶＤまたは光磁気ディスクなどの光記録媒体（以下、「光ディスク」という）の情報記録面上に情報の記録および再生を行う光ヘッド装置が用いられている。この光ヘッド装置において、光ディスクの回転に追随して情報記録面のトラック上に入射光を集光するために、種々のトラッキング法が用いられる。
【０００３】
そのトラッキング法の中で、再生専用光ディスクの情報記録面の信号（情報）検出法として３ビーム法が、また記録用光ディスクのトラッキング誤差信号検出法として差動プッシュプル法が一般的に用いられている。いずれの方法においても回折格子を用いて０次回折光のメインビームと±１次回折光のサブビームを生成する点が共通している。
【０００４】
また、複数ビーム発生用の回折素子を用いて０次回折光、±１次回折光および±２次回折光、場合によってさらに±３次回折光を発生し、情報記録面の互いに隣り合う複数のトラック上に集光し、情報記録面で反射された複数ビームを光検出器の複数の受光面上に集光して信号検出する。これにより、複数のトラック上に記録されたピット情報を並列（同時）に読み出すことができるため、高速再生を実現できる。ここでは、複数ビームとは５ビーム以上を意味する。
【０００５】
【発明が解決しようとする課題】
複数ビーム発生用の回折素子を記録用光ディスクに用いると、光源である半導体レーザの出射光が複数のトラックに分散されるため、０次回折光を用いて単一トラックに記録するためには高出力半導体レーザが必要であった。また、０次回折光以外のビーム強度が強すぎると複数のトラックに同一の情報を記録することとなり、複数ビーム発生用の回折素子を使用できなかった。
【０００６】
本発明は、上述の実情に鑑み、光ディスクの再生時には複数ビーム発生用の回折素子となり、また光ディスクへの記録時には３ビーム発生用の回折素子となる回折素子を得て、この素子を搭載した高速再生と記録が可能な光ヘッド装置を提供することを目的とする。
【０００７】
【課題を解決するための手段】
本発明は、レーザ光を出射する光源と、出射されたレーザ光を光記録媒体上に集光する対物レンズと、集光され光記録媒体により反射されたレーザ光を検出するための光検出器と、を備える光ヘッド装置において、光源と対物レンズとの間の光路中に、０次回折光および±１次回折光の少なくとも３つの回折光を発生させる非偏光性回折格子と、０次回折光、±１次回折光および±２次回折光の少なくとも５つの回折光を発生させる偏光性回折格子と、前記偏光性回折格子への入射光の偏光面を直交する２つの偏光面間で変化させて、前記偏光性回折格子を機能させるか否かを選択するスイッチング素子とからなる回折素子と、が配置され、光記録媒体上の情報記録面の情報を再生する場合は、前記回折素子における前記偏光性回折格子の回折光を発生させてトラッキング誤差信号検出および複数のトラックに記録された情報を並列に再生する信号検出に用い、光記録媒体上の情報記録面に情報を記録する場合は、前記回折素子における前記偏光性回折格子の回折光を発生させず、前記非偏光性回折格子の±１次回折光をトラッキング誤差信号検出に用いるとともに、０次回折光を情報記録面に情報を記録する信号に用いることを特徴とする光ヘッド装置を提供する。
【０００９】
【発明の実施の形態】
図１は本発明の回折素子の構成要素を模式的に示す断面図であり、回折素子１００は非偏光性回折格子１０と、偏光性回折格子２０と、直線偏光入射光の偏光方向を切り替えるスイッチング素子３０とからなる。各構成要素が個別の部品からなり、それら各構成要素部品を一体化した構成としてもよいし、一体化せずそれらを組み合わせて回折素子１００としてもよい。ここで、偏光性回折格子とは回折特性が格子への入射光の偏光方向に依存するものをいい、非偏光性回折格子とは回折特性が偏光方向に依存しないものをいう。
【００１０】
個別の構成要素部品を一体化した回折素子２００の一例を図２に示す。３つの回折光（３ビーム）発生用の非偏光性回折格子１０は、透光性基板１２の表面を凹凸形状に加工して格子１１としている。回折格子１０により入射光は回折され、発生した０次回折光と±１次回折光を３ビームとして用いる。ここで、０次回折光と±１次回折光との光量総和を最大とするため、格子１１は凹部の幅と凸部の幅の比を１：１とする。０次回折光と±１次回折光のそれぞれの光量の比率は、凹部と凸部における透過光の位相差をその段差ｄにより調整することで定まる。トラッキング信号検出の場合、＋１次回折光または−１次回折光と、０次回折光との回折効率比を１：５〜２０の比率とする。
【００１１】
少なくとも５つの回折光（複数ビーム）発生用の偏光性回折格子２０は、透光性基板１２の格子１１が形成された面とは反対側の面に、常光屈折率ｎ_ｏ、異常光屈折率ｎ_ｅ（ｎ_ｅ≠ｎ_ｏ）で光学軸が透光性基板１２の面に平行で一定方向（後述する）に揃った複屈折性材料層を凹凸格子２１に加工し、常光屈折率ｎ_ｏとほぼ等しい等方的屈折率ｎ_ｓの透光性充填材２２を用いて複屈折性材料層の凹凸部を埋めた構成となっている。
【００１２】
スイッチング素子は、電極付きの透明基板間に挟持された液晶層を有することが、１０Ｖ以下の小さな電圧の印加でスイッチング機能を持たせることができ好ましい。
スイッチング素子３０は、透明電極膜３４、３５が成膜されさらにその表面に配向処理された配向膜（図示せず）が形成された透光性基板３２、３３をシ−ル材３６を用いてシールし、液晶層３１を基板間に狭持した構成としている。配向膜の配向処理方向と、液晶層の材料の種類と、対向する透明電極膜３４、３５への印加電圧Ｖの大きさとに応じて種々のスイッチング素子が実現できる。本発明では、スイッチング素子への入射光の直線偏光に対して、印加電圧がある値Ｖ_１のとき透過光の偏光方向が不変で、印加電圧が他の値Ｖ_２（Ｖ_２≠Ｖ_１）のとき透過光の偏光方向が入射光の偏光方向（以下、「入射偏光方向」という）に対して、直交するように切り替えできる素子を準備する。
次に、回折素子２００の作用について、図３および図４を用いて説明する。スイッチング素子３０への印加電圧がＶ_１のとき、入射偏光方向は９０゜回転した直交偏光に切り替わって回折格子２０に入射する。このとき、入射偏光方向が複屈折性材料層からなる格子２１の異常光偏光方向となるようにすれば、ｎ_ｅとｎ_ｓとは異なるため回折光が発生する。
【００１３】
回折素子２０により発生する各次数の回折光の効率比は、格子２１のピッチ内の凹凸構造を変化させることにより調整できる。具体的には、凹部と凸部が交互に並んでいる格子の、２つの凹部と２つの凸部からなる隣接する４領域をピッチのくり返し単位として、ピッチ内の４領域のそれぞれの領域幅の比を非等分とする。これにより、±１次回折光と±２次回折光、さらに±１次回折光と±３次回折光の効率比率を調整できる。また、０次回折光と±１次回折光との効率比率は、凹部と凸部における透過光の位相差を、その段差ｄの高さを調整することにより、変更できる。このようにして、５ビーム以上の複数ビームが発生する偏光性回折格子２０となる。
【００１４】
さらに、非偏光性回折格子１０を透過するとき、各複数ビームに対して±１次回折光が発生する。ここで、偏光性回折格子２０による複数ビームの発生方向と非偏光性回折格子１０による３ビームの発生方向が重ならないよう、偏光性回折格子２０の格子２１のパタ−ンと非偏光性回折格子１０のパタ−ンが異なるようにする。具体的には、例えばそれぞれの格子の格子方向（長手方向）が角度をなすようにする。
【００１５】
一方、スイッチング素子３０の印加電圧がＶ_２のとき、入射偏光方向は不変のままスイッチング素子３０を透過して回折格子２０に常光偏光として入射し、ｎ_ｏとｎ_ｓはほぼ等しいため回折光は発生せず直進透過する。このとき、偏光性回折格子２０を透過した光は単一ビームであるため、透過光は非偏光性回折格子１０の格子によって規定される回折方向に±１次回折光が発生する。
したがって、スイッチング素子３０の印加電圧をＶ_１とＶ_２との間で切り替えることにより、５ビーム以上の複数ビームと３ビームの回折光の発生を切り替えできる回折素子２００が得られる。なお、図３および図４において、図２と同じ符号の要素は図２と同じ要素を示す。
【００１６】
このようにして得られた本発明の回折素子を光ヘッド装置に搭載した一例を図５に示す。レーザ光源２から出射した光は、本発明の回折素子１を透過し、ビームスプリッタ３により反射され、コリメータレンズ４により平行光化されて、対物レンズ５により光ディスク６の情報記録面に集光される。集光され情報記録面で反射された光は再び対物レンズ５およびコリメータレンズ４、さらにビームスプリッタ３を透過して光検出器７の受光面に集光される。５ビームを用いて５本のトラックの情報を並列読みだしする場合、光検出器７の受光面は例えば図６に示すように分割されている。
【００１７】
再生専用光ディスクを再生する場合、スイッチング素子３０の印加電圧をＶ_１として、本発明の回折素子２００の偏光性回折格子２０により発生した５ビームのうち非偏光性回折格子１０によって回折されなかった光は図６に示すように情報記録面の５本のトラック上に集光する。集光し情報記録面で反射された光を図７に示すように光検出器７の受光面７１、７２、７３、７４および７５で０次、＋１次、−１次、＋２次、−２次の偏光性回折格子２０による回折光として検知する。受光面７１は４分割され、各受光面の信号強度を演算して位相差検出法によるフォーカス位置誤差検出およびヘテロダイン法によるトラッキング位置誤差検出などの公知の制御方式により、複数のトラック上に記録されたピット情報を並列読み出しできる。なお、図６に示すように非偏光性回折格子１０による±１次回折光は偏光性回折格子２０により発生した５ビーム毎にトラック間に集光する。
【００１８】
また、ＣＤ−ＲやＣＤ−ＲＷなどの記録用光ディスクを再生する場合、偏光性回折格子２０の０次回折光が非偏光性回折格子１０により±１次回折光に分割されたビームを、２分割された光検出器７の受光面７６と７７とで検知し、差動プッシュプル法によるトラッキング位置誤差検出を行う。ここで、偏光性回折格子２０の回折光方向と非偏光性回折格子１０の回折光方向は平行とならないようにする必要がある。
【００１９】
一方、記録用光ディスクに記録する場合、スイッチング素子３０の印加電圧をＶ_２として、偏光性回折格子２０では回折光が発生しないで非偏光性回折格子１０で３ビームを発生させ、差動プッシュプル法によるトラッキング位置誤差検出および記録を行う。
【００２０】
以下の実施例において、本発明の回折素子について具体的に説明する。
【００２１】
【実施例】
本実施例の回折素子について、図２を用いて説明する。ガラス基板１２上に屈折率ｎ＝１．４５のＳｉＯ_２膜を膜厚２２１ｎｍとなるよう成膜した後、フォトエッチングにより周期的にＳｉＯ_２膜を除去するよう凹凸形状に加工し、空気と界面をなす非偏光性回折格子１０とした。格子１１の格子ピッチＰは３０μｍとし、１つの凹部と１つの凸部から構成され、凹部と凸部の幅の比は１：１であった。この非偏光性回折格子１０に波長が７８５ｎｍの光が入射したとき、約８５％が０次回折光として透過し、約６％の±１次回折光が発生した。
【００２２】
次に、ＩＴＯからなる透明電極膜３４、３５、さらに配向処理された配向膜が形成（図示せず）されたガラス基板３２と３３を用いて、基板周辺部でシ−ル材３６によりセルギャップ５μｍとなるようにし、基板間にネマティック液晶を注入して液晶層３１を形成し、スイッチング素子３０を作製した。基板３２と３３の配向膜の配向方向は平行とすることにより液晶分子は基板面と平行で一定方向に配向した。
【００２３】
この液晶分子の配向方向に対して入射偏光方向が４５゜の角度をなす、波長が７８５ｎｍの直線偏光レーザ光が入射した場合、透明電極膜３４、３５に電圧を印加しないとき（Ｖ_１＝０）スイッチング素子３０は位相差πの位相板となり、入射偏光方向が９０゜回転（直交）した出射光となった。また、透明電極膜３４、３５に周波数１ｋＨｚで振幅５Ｖの矩形電圧を印加したとき（Ｖ_２＝５Ｖ）は液晶分子が基板面に対して垂直方向に配向が揃い位相差がほとんど発生しないため、出射光の偏光方向は変わらなかった。
【００２４】
次に、ガラス基板１２の片面に形成された配向処理後の配向膜上に、液晶モノマ−溶液を厚さが一定となるよう塗布した後、紫外線を照射して重合固化することにより、常光屈折率ｎ_ｏが１．５５、異常光屈折率ｎ_ｅが１．６０で層厚が３．８６μｍの高分子液晶からなる複屈折性材料層（高分子液晶層）を形成した。さらに、フォトエッチング法により高分子液晶層を断面形状が凹凸状の格子２１に加工し、等方的屈折率ｎ_ｓが１．５５である透光性の充填材２２を用いて高分子液晶層の凹凸部を埋めるとともにスイッチング素子３０のガラス基板３２に接着して偏光性回折格子２０とした。
【００２５】
ここで、格子２１の格子ピッチＰを３０μｍとし、格子ピッチ内を幅がそれぞれ１．５μｍ、９．６μｍ、８．７μｍおよび１０．２μｍ、凸部の高さｄが３．８６μｍの４つの領域に分割した。この偏光性回折格子２０に波長が７８５ｎｍの異常光偏光方向のレーザ光を入射すると、約４２％が０次回折光として透過し、約１０．２％の±１次および±２次回折光が発生する５ビーム用の回折素子が得られた。また、常光偏光方向のレーザ光を入射すると、回折光は発生しなかった。
【００２６】
このようにして得られた本発明の回折素子２００において、格子１１の格子方向と偏光性回折格子２０の格子方向とが約４５゜の角度をなし、偏光性回折格子２０の常光屈折率方向とスイッチング素子３０の配向膜の配向方向が約４５゜の角度をなしている。
【００２７】
図５に示すように、光ヘッド装置のレーザ光源２とビームスプリッタ３との間の回折素子１の位置に、本実施例の回折素子２００を配置し、レーザ光源２からの出射偏光方向が偏光性回折格子２０の常光偏光方向と一致するようにした。ここで、使用したレーザ光源は、出射波長が７８５ｎｍの高出力半導体レーザであった。
【００２８】
スイッチング素子３０に電圧を印加しないとき、入射偏光方向がスイッチング素子３０により９０゜回転した直交偏光に切り替わって透過し、偏光性回折格子２０に異常光偏光として入射する。このとき、偏光性回折格子２０により５ビームの回折光が発生し、さらに非偏光性回折格子１０により各ビームに対して３ビームづつ、合計１５本のビームが回折光として発生した。そして図６に示すように、１５本のビーム中５本のビームが光ディスクの情報記録面の５本のトラック上に光スポットとなって集光され、また１０本のビーム（非偏光性回折格子１０による±１次回折光）が各トラックから０．５ピッチ離れた位置（トラックとトラックとの間）に光スポットとなって集光された。
【００２９】
これらの光スポットは情報記録面で反射され、図７に示すように光検出器７の受光面に集光された。再生用光ディスクにおいて、受光面７１、７２、７３、７４および７５により情報記録面の信号を検出し、４分割された受光面７１でフォ−カス位置誤差検出およびトラッキング位置誤差検出を行い、５本のトラック上に記録されたピット情報を並列に読み出すことができた。
【００３０】
また、ＣＤ−ＲやＣＤ−ＲＷなどの記録用光ディスクを再生する場合、光検出器７の２分割受光面７６と７７で検知した信号、および４分割受光面７１で検知した信号を用い、差動プッシュプル法による信号検出を行った。
【００３１】
一方、記録用光ディスクに記録する場合、スイッチング素子３０の周波数１ｋＨｚで振幅５Ｖの矩形電圧を印加して、偏光性回折格子２０への入射偏光が常光偏光のまま入射することにより偏光性回折格子２０では回折光を発生させず、非偏光性回折格子１０で３ビームを発生させて、光検出器７の受光面７１、７６および７７を用いて信号検出および０次回折光による記録を行うことができた。
【００３２】
【発明の効果】
以上説明したように、本発明の回折素子を用いれば５ビーム以上の複数ビームの発生と３ビームの発生を切り替えできる。この回折素子を光ディスクの再生において、複数のトラックの並列（同時）信号読み出し用の複数ビーム発生回折素子として用いることにより、多くの記録信号を同時に高精度で読み出すことができるため、高速再生が実現できる。また、光ディスクの記録においては、３ビーム発生回折素子として用いることによりレーザ光の光量損失を抑制できるため、正確な高速記録が実現できる。
【図面の簡単な説明】
【図１】本発明の回折素子の構成要素を模式的に示す構成図。
【図２】本発明の回折素子の１例を示す断面図。
【図３】図２の回折素子に、低い電圧Ｖ_１を印加したときの回折素子の作用を示す断面図。
【図４】図２の回折素子に、電圧Ｖ_２（Ｖ_２≠Ｖ_１）を印加したときの回折素子の作用を示す断面図。の側面図。
【図５】本発明の光ヘッド装置の構成の１例を示す側面図。
【図６】本発明の回折素子を使用した場合の、光ディスクの情報記録面におけるスポット集光形態の例を示す平面図。
【図７】本発明の回折素子を使用した場合の、光検出器の受光面におけるスポット集光形態の例を示す平面図。
【符号の説明】
１、１００、２００：回折素子
１０：非偏光性回折格子
２０：偏光性回折格子
３０：スイッチング素子
１１、２１、３１：格子
１２、３２、３３：透光性基板
２２：充填材
３１：液晶層
３４、３５：透明電極膜
３６：シール材
２：レーザ光源
３：ビームスプリッタ
７：光検出器
７１、７２、７３、７４、７５、７６、７７：受光面[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diffraction element and an optical head device, and more particularly to a polarizing diffraction element that generates a plurality of beams and an optical head device that records and reproduces information on an optical recording medium.
[0002]
[Prior art]
2. Description of the Related Art An optical head device that records and reproduces information on an information recording surface of an optical recording medium (hereinafter referred to as “optical disk”) such as a CD, DVD, or magneto-optical disk is used. In this optical head device, various tracking methods are used to collect incident light on a track on the information recording surface following the rotation of the optical disk.
[0003]
Among the tracking methods, a three-beam method is generally used as a signal (information) detection method for an information recording surface of a read-only optical disc, and a differential push-pull method is used as a tracking error signal detection method for a recording optical disc. Yes. Both methods share the point that a diffraction grating is used to generate a main beam of 0th-order diffracted light and sub-beams of ± 1st-order diffracted light.
[0004]
In addition, 0th-order diffracted light, ± 1st-order diffracted light, ± 2nd-order diffracted light, and in some cases ± 3rd-order diffracted light are generated using a diffraction element for generating multiple beams, and are collected on a plurality of adjacent tracks on the information recording surface. The light is reflected and reflected by the information recording surface to collect light on a plurality of light receiving surfaces of the photodetector to detect signals. Thereby, pit information recorded on a plurality of tracks can be read out in parallel (simultaneously), so that high-speed reproduction can be realized. Here, a plurality of beams means 5 beams or more.
[0005]
[Problems to be solved by the invention]
When a diffractive element for generating multiple beams is used in a recording optical disk, the light emitted from the semiconductor laser, which is the light source, is dispersed in a plurality of tracks. Therefore, a high output is required for recording on a single track using zero-order diffracted light. A semiconductor laser was required. Further, if the beam intensity other than the 0th-order diffracted light is too strong, the same information is recorded on a plurality of tracks, and a diffraction element for generating a plurality of beams cannot be used.
[0006]
In view of the above circumstances, the present invention obtains a diffractive element for generating a plurality of beams during reproduction of an optical disc, and a diffractive element for generating three beams during recording on an optical disc. An object of the present invention is to provide an optical head device capable of reproduction and recording.
[0007]
[Means for Solving the Problems]
The present invention relates to a light source that emits laser light, an objective lens that condenses the emitted laser light on an optical recording medium, and a photodetector for detecting the laser light that is collected and reflected by the optical recording medium And a non-polarization diffraction grating that generates at least three diffracted lights of zero-order diffracted light and ± first-order diffracted light in the optical path between the light source and the objective lens, and zero-order diffracted light, ± 1 and polarizing diffraction grating for generating at least five diffracted light diffracted light and ± 2-order diffracted light, is varied between two polarization planes orthogonal to the polarization plane of the incident light to the polarizing diffraction grating, the polarization A diffraction element comprising a switching element for selecting whether or not to function the diffractive diffraction grating, and when reproducing information on the information recording surface on the optical recording medium, the polarizing diffraction grating in the diffractive element Diffraction In the case where information is recorded on an information recording surface on an optical recording medium and used for detecting a tracking error signal by generating light and detecting signals recorded in a plurality of tracks in parallel, the polarization in the diffraction element is used. The first-order diffracted light of the non-polarized diffraction grating is used for tracking error signal detection, and the zero-order diffracted light is used as a signal for recording information on the information recording surface. An optical head device is provided.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a cross-sectional view schematically showing components of a diffraction element according to the present invention. A diffraction element 100 is a non-polarization diffraction grating 10, a polarization diffraction grating 20, and switching for switching the polarization direction of linearly polarized incident light. An element 30 is included. Each component may be composed of individual parts, and each component part may be integrated, or may be combined to form the diffraction element 100 without being integrated. Here, a polarizing diffraction grating refers to a diffraction characteristic that depends on the polarization direction of light incident on the grating, and a non-polarizing diffraction grating refers to a diffraction characteristic that does not depend on the polarization direction.
[0010]
An example of a diffraction element 200 in which individual component parts are integrated is shown in FIG. The non-polarizing diffraction grating 10 for generating three diffracted lights (three beams) is formed into a grating 11 by processing the surface of the translucent substrate 12 into an uneven shape. The incident light is diffracted by the diffraction grating 10, and the generated 0th order diffracted light and ± 1st order diffracted light are used as 3 beams. Here, in order to maximize the total light amount of the 0th-order diffracted light and the ± 1st-order diffracted light, the grating 11 has a ratio of the width of the concave portion to the width of the convex portion of 1: 1. The ratio of the light amounts of the 0th-order diffracted light and the ± 1st-order diffracted light is determined by adjusting the phase difference of transmitted light between the concave portion and the convex portion by the step d. In the case of tracking signal detection, the diffraction efficiency ratio between the + 1st order diffracted light or the −1st order diffracted light and the 0th order diffracted light is set to a ratio of 1: 5 to 20.
[0011]
The polarizing diffraction grating 20 for generating at least five diffracted lights (multiple beams) has an ordinary light refractive index n _o and an extraordinary light refractive index on the surface of the translucent substrate 12 opposite to the surface on which the grating 11 is formed. A birefringent material layer whose optical axis is parallel to the surface of the light-transmitting substrate 12 and aligned in a certain direction (described later) at n _e ( _ne ≠ n _o ) is processed into a concavo-convex grating 21, and the ordinary refractive index n _o The concavo-convex portion of the birefringent material layer is filled with a translucent filler 22 having an isotropic refractive index n _s that is substantially equal to the above.
[0012]
It is preferable that the switching element has a liquid crystal layer sandwiched between transparent substrates with electrodes because a switching function can be provided by applying a small voltage of 10 V or less.
The switching element 30 uses a sealing material 36 for transparent substrates 32 and 33 having transparent electrode films 34 and 35 formed thereon and an alignment film (not shown) formed on the surface thereof. The liquid crystal layer 31 is sandwiched between the substrates by sealing. Various switching elements can be realized according to the alignment treatment direction of the alignment film, the type of material of the liquid crystal layer, and the magnitude of the voltage V applied to the opposing transparent electrode films 34 and 35. In the present invention, when the applied voltage is a certain value V ₁ with respect to the linearly polarized light incident on the switching element, the polarization direction of the transmitted light is unchanged, and the applied voltage is another value V ₂ (V ₂ ≠ V ₁ ). In this case, an element capable of switching so that the polarization direction of transmitted light is orthogonal to the polarization direction of incident light (hereinafter referred to as “incident polarization direction”) is prepared.
Next, the effect | action of the diffraction element 200 is demonstrated using FIG. 3 and FIG. When the voltage applied to the switching element 30 is V ₁ , the incident polarization direction is switched to orthogonal polarization rotated by 90 ° and incident on the diffraction grating 20. In this case, the incident polarization direction if such a extraordinarily polarized light direction of the grating 21 consisting of birefringent material layer, different because diffracted light is generated from the n _e and n _s.
[0013]
The efficiency ratio of each order of diffracted light generated by the diffraction element 20 can be adjusted by changing the concavo-convex structure within the pitch of the grating 21. Specifically, in the lattice in which the concave portions and the convex portions are alternately arranged, the adjacent four regions composed of the two concave portions and the two convex portions are set as the repeating unit of the pitch, and the width of each of the four regions in the pitch is set. The ratio is unequal. Thereby, the efficiency ratio of ± 1st order diffracted light and ± 2nd order diffracted light, and further ± 1st order diffracted light and ± 3rd order diffracted light can be adjusted. Further, the efficiency ratio between the 0th-order diffracted light and the ± 1st-order diffracted light can be changed by adjusting the height of the level difference d of the phase difference of the transmitted light in the concave and convex portions. In this way, the polarizing diffraction grating 20 in which a plurality of beams of 5 beams or more are generated.
[0014]
Further, when passing through the non-polarizing diffraction grating 10, ± first-order diffracted light is generated for each of the plurality of beams. Here, the pattern of the grating 21 of the polarizing diffraction grating 20 and the non-polarizing diffraction grating so that the generation direction of the plurality of beams by the polarizing diffraction grating 20 and the generation direction of the three beams by the non-polarizing diffraction grating 10 do not overlap. Make 10 patterns different. Specifically, for example, the lattice direction (longitudinal direction) of each lattice forms an angle.
[0015]
On the other hand, when the applied voltage of the switching element 30 is V _2, the incident polarization direction is incident as ordinary light polarized in the diffraction grating 20 passes through the left switching device 30 unchanged, n _o and n _s is substantially equal because the diffracted light It does not occur and passes straight. At this time, since the light transmitted through the polarizing diffraction grating 20 is a single beam, ± 1st-order diffracted light is generated in the diffraction direction defined by the grating of the non-polarizing diffraction grating 10.
Therefore, by switching the voltage applied to the switching element 30 between V ₁ and V ₂ , the diffractive element 200 that can switch the generation of diffracted light of five or more beams and three beams is obtained. 3 and FIG. 4, the same reference numerals as those in FIG. 2 indicate the same elements as those in FIG.
[0016]
An example in which the diffraction element of the present invention thus obtained is mounted on an optical head device is shown in FIG. The light emitted from the laser light source 2 passes through the diffraction element 1 of the present invention, is reflected by the beam splitter 3, is collimated by the collimator lens 4, and is condensed on the information recording surface of the optical disk 6 by the objective lens 5. The The light that has been collected and reflected by the information recording surface passes through the objective lens 5 and the collimator lens 4 and the beam splitter 3 again, and is collected on the light receiving surface of the photodetector 7. When information on five tracks is read in parallel using five beams, the light receiving surface of the photodetector 7 is divided, for example, as shown in FIG.
[0017]
When reproducing a read-only optical disk, light applied to the switching element 30 is set to V _{1 and} light that has not been diffracted by the non-polarizing diffraction grating 10 out of the five beams generated by the polarizing diffraction grating 20 of the diffraction element 200 of the present invention. As shown in FIG. 6, the light is condensed on five tracks on the information recording surface. As shown in FIG. 7, the light collected and reflected by the information recording surface is received on the light receiving surfaces 71, 72, 73, 74 and 75 of the photodetector 7 by the 0th order, the + 1st order, the −1st order, the + 2nd order, and −2. It is detected as diffracted light by the next polarizing diffraction grating 20. The light receiving surface 71 is divided into four parts and is recorded on a plurality of tracks by a known control method such as focus position error detection by the phase difference detection method and tracking position error detection by the heterodyne method by calculating the signal intensity of each light receiving surface. Pit information can be read in parallel. As shown in FIG. 6, the ± first-order diffracted light by the non-polarizing diffraction grating 10 is condensed between tracks for every five beams generated by the polarizing diffraction grating 20.
[0018]
When reproducing a recording optical disk such as a CD-R or CD-RW, a beam obtained by dividing the zero-order diffracted light of the polarizing diffraction grating 20 into ± first-order diffracted light by the non-polarizing diffraction grating 10 is divided into two. Detection is performed by the light receiving surfaces 76 and 77 of the photodetector 7, and tracking position error detection is performed by a differential push-pull method. Here, it is necessary to prevent the diffracted light direction of the polarizing diffraction grating 20 and the diffracted light direction of the non-polarizing diffraction grating 10 from being parallel.
[0019]
On the other hand, when recording on a recording optical disk, the applied voltage of the switching element 30 is set to V ₂ , the polarizing diffraction grating 20 does not generate diffracted light, but the non-polarizing diffraction grating 10 generates three beams, and differential push-pull. Tracking position error detection and recording by the method.
[0020]
In the following examples, the diffraction element of the present invention will be specifically described.
[0021]
【Example】
The diffraction element of the present embodiment will be described with reference to FIG. After a SiO ₂ film having a refractive index n = 1.45 is formed on the glass substrate 12 so as to have a film thickness of 221 nm, it is processed into a concavo-convex shape so as to periodically remove the SiO ₂ film by photoetching, and the interface with air The non-polarizing diffraction grating 10 is formed. The grating pitch P of the grating 11 was 30 μm, and it was composed of one concave part and one convex part, and the ratio of the width between the concave part and the convex part was 1: 1. When light having a wavelength of 785 nm was incident on the non-polarizing diffraction grating 10, about 85% was transmitted as 0th order diffracted light, and about 6% ± 1st order diffracted light was generated.
[0022]
Next, using transparent electrode films 34 and 35 made of ITO and glass substrates 32 and 33 on which an alignment film subjected to alignment treatment is formed (not shown), a cell gap is formed by a seal material 36 around the substrate. A nematic liquid crystal was injected between the substrates to form a liquid crystal layer 31 so that the switching element 30 was produced. By aligning the alignment directions of the alignment films of the substrates 32 and 33, the liquid crystal molecules were aligned in a certain direction parallel to the substrate surface.
[0023]
When linearly polarized laser light having a wavelength of 785 nm is incident on the liquid crystal molecules with an incident polarization direction of 45 °, no voltage is applied to the transparent electrode films 34 and 35 (V ₁ = 0). ) The switching element 30 became a phase plate having a phase difference π, and became an outgoing light whose incident polarization direction was rotated (orthogonal) by 90 °. In addition, when a rectangular voltage having a frequency of 1 kHz and an amplitude of 5 V is applied to the transparent electrode films 34 and 35 (V ₂ = 5 V), the liquid crystal molecules are aligned in the direction perpendicular to the substrate surface, so that almost no phase difference occurs. The polarization direction of the emitted light was not changed.
[0024]
Next, a liquid crystal monomer solution is applied on the alignment film formed on one side of the glass substrate 12 so as to have a constant thickness, and then irradiated with ultraviolet rays to be polymerized and solidified, thereby allowing ordinary light refraction. rate _{n o} is 1.55, the extraordinary refractive index _{n e} is formed a birefringent material layer thickness at 1.60 of a polymer liquid crystal 3.86Myuemu (polymer liquid crystal layer). Further, the polymer liquid crystal layer cross section was processed in an uneven shape of the grid 21 by a photo-etching method, the polymer liquid crystal layer with a filler 22 of the translucent isotropic refractive index n _s is 1.55 The polarizing diffraction grating 20 was formed by filling the concave and convex portions of the switching element 30 and adhering to the glass substrate 32 of the switching element 30.
[0025]
Here, the lattice pitch P of the lattice 21 is set to 30 μm, the width within the lattice pitch is 1.5 μm, 9.6 μm, 8.7 μm and 10.2 μm, and the height d of the convex portion is 3.86 μm. Divided into When a laser beam with an extraordinary polarization direction having a wavelength of 785 nm is incident on the polarizing diffraction grating 20, about 42% is transmitted as 0th order diffracted light, and about 10.2% ± 1st order and ± 2nd order diffracted light is generated. A diffraction element for 5 beams was obtained. In addition, when laser light in the ordinary light polarization direction was incident, no diffracted light was generated.
[0026]
In the diffraction element 200 of the present invention thus obtained, the grating direction of the grating 11 and the grating direction of the polarizing diffraction grating 20 form an angle of about 45 °, and the normal refractive index direction of the polarizing diffraction grating 20 is The alignment direction of the alignment film of the switching element 30 forms an angle of about 45 °.
[0027]
As shown in FIG. 5, the diffractive element 200 of the present embodiment is disposed at the position of the diffractive element 1 between the laser light source 2 and the beam splitter 3 of the optical head device, and the direction of polarized light emitted from the laser light source 2 is polarized. In accordance with the ordinary light polarization direction of the diffractive diffraction grating 20. Here, the laser light source used was a high-power semiconductor laser having an emission wavelength of 785 nm.
[0028]
When no voltage is applied to the switching element 30, the incident polarization direction is switched to orthogonal polarization rotated by 90 ° by the switching element 30, and is transmitted to the polarizing diffraction grating 20 as extraordinary light polarization. At this time, five beams of diffracted light were generated by the polarizing diffraction grating 20, and three beams for each beam were generated by the non-polarizing diffraction grating 10 as a diffracted light. As shown in FIG. 6, 5 out of 15 beams are collected as light spots on 5 tracks on the information recording surface of the optical disk, and 10 beams (non-polarizing diffraction grating) are collected. 10 first-order diffracted light) was collected as a light spot at a position 0.5 pitch away from each track (between the tracks).
[0029]
These light spots were reflected by the information recording surface and focused on the light receiving surface of the photodetector 7 as shown in FIG. In the optical disc for reproduction, signals on the information recording surface are detected by the light receiving surfaces 71, 72, 73, 74 and 75, and the focus position error detection and the tracking position error detection are performed on the light receiving surface 71 divided into four. The pit information recorded on the track of this was able to be read in parallel.
[0030]
When reproducing a recording optical disk such as a CD-R or CD-RW, the signal detected by the two-divided light receiving surfaces 76 and 77 of the photodetector 7 and the signal detected by the four-divided light receiving surface 71 are used. Signal detection by dynamic push-pull method was performed.
[0031]
On the other hand, when recording on a recording optical disk, a rectangular voltage having a frequency of 1 kHz and an amplitude of 5 V of the switching element 30 is applied, and the polarized light incident on the polarizing diffraction grating 20 is incident on the polarizing diffraction grating 20 as ordinary polarized light. In this case, diffracted light is not generated, three beams are generated by the non-polarizing diffraction grating 10, and signal detection and recording with zero-order diffracted light can be performed using the light receiving surfaces 71, 76 and 77 of the photodetector 7. It was.
[0032]
【The invention's effect】
As described above, by using the diffraction element of the present invention, it is possible to switch between the generation of a plurality of five or more beams and the generation of three beams. By using this diffractive element as a multiple beam generating diffractive element for parallel (simultaneous) signal readout of multiple tracks during optical disc playback, many recorded signals can be read simultaneously with high precision, enabling high-speed playback. it can. Further, in the recording of the optical disc, the use of the three-beam generating diffraction element can suppress the light amount loss of the laser beam, so that accurate high-speed recording can be realized.
[Brief description of the drawings]
FIG. 1 is a configuration diagram schematically showing components of a diffraction element of the present invention.
FIG. 2 is a cross-sectional view showing an example of the diffraction element of the present invention.
3 is a cross-sectional view showing the operation of the diffraction element when a low voltage V ₁ is applied to the diffraction element of FIG. 2;
4 is a cross-sectional view showing the operation of the diffraction element when a voltage V ₂ (V ₂ ≠ V ₁ ) is applied to the diffraction element of FIG. 2; Side view.
FIG. 5 is a side view showing an example of the configuration of the optical head device of the present invention.
FIG. 6 is a plan view showing an example of a spot condensing form on an information recording surface of an optical disc when the diffraction element of the present invention is used.
FIG. 7 is a plan view showing an example of a spot condensing form on the light receiving surface of the photodetector when the diffraction element of the present invention is used.
[Explanation of symbols]
1, 100, 200: the diffraction element 10: Non-polarizing diffraction grating 20: polarizing diffraction grating 30: switching elements 11, 21, 31: grid 12,32,33: translucent substrate 22: the filler 31: liquid crystal layer 34, 35: Transparent electrode film 36: Sealing material 2: Laser light source 3: Beam splitter 7: Photo detectors 71, 72, 73, 74, 75, 76, 77: Light receiving surface

Claims

A light source for emitting laser light, an objective lens for condensing the emitted laser light on an optical recording medium, and a photodetector for detecting the laser light condensed and reflected by the optical recording medium. In the optical head device,
A non-polarization diffraction grating that generates at least three diffracted lights of 0th order and ± 1st order diffracted light in the optical path between the light source and the objective lens, 0th order diffracted light, ± 1st order diffracted light, and ± 2nd order diffracted light a polarizing diffraction grating for generating at least five of the diffracted light, wherein by changing between two polarization planes orthogonal to the polarization plane of the incident light to the polarizing diffraction grating, whether to function the polarizing diffraction grating a diffraction element formed of a switching element for selecting, been arranged,
When reproducing information on an information recording surface on an optical recording medium, diffracted light from the polarizing diffraction grating in the diffraction element is generated to detect a tracking error signal and reproduce information recorded on a plurality of tracks in parallel. Used for signal detection
When recording information on the information recording surface on the optical recording medium, the diffraction light of the polarizing diffraction grating in the diffraction element is not generated, and the ± first-order diffraction light of the non-polarizing diffraction grating is used for tracking error signal detection. An optical head device characterized by using zero-order diffracted light as a signal for recording information on an information recording surface .

The optical head device according to claim 1, wherein the switching element is a switching element having a liquid crystal layer sandwiched between transparent substrates with electrodes.