JP4432255B2

JP4432255B2 - Optical head device

Info

Publication number: JP4432255B2
Application number: JP2000364978A
Authority: JP
Inventors: 好晴大井; 龍一郎後藤
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2000-11-30
Filing date: 2000-11-30
Publication date: 2010-03-17
Anticipated expiration: 2020-11-30
Also published as: JP2002170272A

Description

【０００１】
【発明の属する技術分野】
本発明は、光ディスクなどの光記録媒体の情報の記録および再生に使用する光ヘッド装置に関する。
【０００２】
【従来の技術】
例えばＣＤやＤＶＤのような光ディスク、または光磁気ディスクなどの光記録媒体（以下、これらをまとめて「光ディスク」という）の情報記録面上に情報を書き込んだり（以下、「記録」という）、または情報記録面上の情報を読み取ったり（以下、「再生」という）する光ヘッド装置が各種用いられている。
【０００３】
図６は従来の光ヘッド装置の一例について概略を示す側面図である。半導体レーザ５からの出射光は、ビームスプリッタ４の半導体レーザ５側の表面に形成された反射膜（図示せず）により反射され、対物レンズ３により光ディスク７の情報記録面に集光される。ここで、反射膜は入射光の一部を透過し残りの光を反射する部分反射膜である。光ディスク７からの反射光は、再び対物レンズ３を透過し、一部の光がビームスプリッタ４を透過した後光検出器６の受光面に集光され、残りの光がビームスプリッタ４の反射膜により反射され戻り光となって半導体レーザ５の発光点に集光される。光検出器６において、受光した光が電気信号に変換され、この受光光量に応じた電気信号はアンプで増幅され、さらに自動ゲイン補正回路で適宜ゲインが補正されて信号レベルが一定範囲に調整されて出力される。
【０００４】
なお、光ヘッド装置の構成として、ビームスプリッタ４と対物レンズ３との間にコリメートレンズを配置する場合や、ビームスプリッタ４と光検出器６との間にシリンドリカルレンズや凹レンズを配置する場合がある。また、ＣＤ用の波長７９０ｎｍ帯域の半導体レーザとＤＶＤ用の波長６５０ｎｍ帯域の半導体レーザとを２個用いた構成の光ヘッド装置などがある。
【０００５】
このような光ヘッド装置において、光源として用いられる半導体レーザの発光点に光ディスクの情報記録面からの反射戻り光が入射すると半導体レーザの出射光強度が不安定になり、その結果正確な情報の記録および再生ができなくなる問題が生じる。この問題を解決するために、半導体レーザ用電源の駆動電流に数百ＭＨｚの高周波電流を重畳する回路を設けて、半導体レーザの発振モードをシングルモードからマルチモードに変えることにより戻り光と発振光との干渉を低減して出射光強度を安定化している。
【０００６】
また、半導体レーザと対物レンズとの間の光路中に半導体レーザの発振波長に対して位相差がπ／２となる１／４波長板２を配置し、１／４波長板を往復した戻り光の偏光状態を発振光の偏光状態から変化させることにより、戻り光と発振光との干渉を低減することで出射光強度を安定化している。しかし、ＣＤ−ＲやＣＤ−ＲＷなどの光ディスクの情報記録面に高速で情報を記録するために半導体レーザの出射光を高出力化すると、半導体レーザの発光点への戻り光強度も増大し、半導体レーザの出射光強度が不安定になり安定した記録および再生ができなくなる問題が生じる。
【０００７】
【発明が解決しようとする課題】
本発明は、上述の実情に鑑み、比較的高出力な半導体レーザを光源として用いる光ヘッド装置において、光ディスクの情報の記録および再生時に安定した信号検出ができる光ヘッド装置を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明は、光源と、前記光源からの出射光を光記録媒体に集光する対物レンズと、前記光記録媒体からの反射光を検出する光検出器と、前記光源からの出射光を前記対物レンズ側へ反射しかつ前記光記録媒体からの反射光の一部を前記光検出器側へ透過するとともに反射光の残りの部分を前記光源側へ反射するビームスプリッタと、を備え、前記光記録媒体に情報の記録および再生を行う光ヘッド装置であって、前記ビームスプリッタと前記対物レンズとの間の光路中に、透過光の位相差がπ／２の奇数倍となる位相板を備え、前記光源と前記ビームスプリッタとの間の光路中に、複屈折性材料からなる下記の偏光性回折素子が設置されていることを特徴とする光ヘッド装置を提供する。
偏光性回折素子：光源からの出射光である第１の直線偏光が前記偏光性回折素子へ入射するときは、回折格子として作用せず第１の直線偏光を透過させ、前記光記録媒体からの前記反射光であって、前記位相板を往復して、前記第１の直線偏光と直交する偏光方向となる第２の直線偏光が前記偏光性回折素子へ入射するときは、回折格子として作用し前記第２の直線偏光を前記光源から遠ざけるように回折させる。
【０００９】
また、前記偏光性回折素子は、透光性平面基板上に、常光屈折率ｎ _ｏおよび異常光屈折率ｎ _ｅ（ｎ _ｏ ≠ｎ _ｅ）の複屈折材料と、前記常光屈折率ｎ _ｏと略等しい屈折率ｎ _ｓの等方性屈折率材料と、によって断面が段差ｄの周期的な凹凸周期構造を有し、かつ、｜ｎ _ｅ −ｎ _ｓ｜×ｄが、前記光源からの出射光の波長λの（ｍ＋１／２）倍（ｍは０を含む正の整数）である上記の光ヘッド装置を提供する。また、前記複屈折材料は、高分子液晶である上記の光ヘッド装置を提供する。また、前記凹凸周期構造のピッチは、５μｍ以下である上記の光ヘッド装置を提供する。さらに、前記偏光性回折素子は、前記光記録媒体へ入射して信号検出するための３ビームの発生素子として用いる、回折効率が偏光方向に依存しない回折格子を有する上記の光ヘッド装置を提供する。
【００１０】
【発明の実施の形態】
図１は本発明の光ヘッド装置の一例について概略を示す側面図である。第１の直線偏光を出射する光源である半導体レーザ５とビームスプリッタ４との光路中に偏光性回折素子１が設置されている以外は図６に示した従来の光ヘッド装置の構成と同じである。したがって、図６と同じ符号は同じ要素を示す。偏光性回折素子１は、光源（半導体レーザ）から出射する第１の直線偏光の入射光に対しては回折格子として作用せず入射光を透過し、かつ光記録媒体からの反射光であって、第１の直線偏光と直交する偏光方向の第２の直線偏光の入射光に対しては回折格子として作用し入射光を光源から遠ざけるように回折する。
【００１１】
このような偏光性回折素子１が設置されているので、光ディスクにより反射され、さらにビームスプリッタ４に反射された第２の直線偏光は、回折されて半導体レーザに届かないので、半導体レーザの出射光強度が安定になり、その結果光ヘッド装置の正確な情報の記録および再生を行うことができる。ここで、入射光を半導体レーザから遠ざけるように回折する場合、半導体レーザに全く届かぬように避けてもよいが、半導体レーザの発光点を避けてもよい。ここでは、この場合も含んでいる。
偏光性回折素子１は複屈折性材料からなり、複屈折性材料には光学結晶、高分子液晶などがある。生産性の容易さなどの観点から、高分子液晶を用いることが好ましい。
【００１２】
図２は、本発明における偏光性回折素子の構成の一つの例を示す側面図である。偏光性回折素子である回折素子１０は、透光性平面基板１１の一方の面に、常光屈折率ｎ_oおよび異常光屈折率ｎ_e（ｎ_o≠ｎ_e）の複屈折性材料１３を断面形状が段差ｄの周期的凹凸状となるよう加工し、その凹部に屈折率ｎ_sの等方性屈折率透明材料１４を充填する。そして、透光性平面基板１１と、透光性平面基板１２とで等方性屈折率透明材料１４を挟み込んだ構造としている。透光性平面基板１１および透光性平面基板１２の空気との界面には反射防止膜が形成されていてもよい。
【００１３】
ここで、例えば屈折率ｎ_sが常光屈折率ｎ_oと略等しい等方性屈折率透明材料１４を用い、｜ｎ_e−ｎ_s｜×ｄが半導体レーザ５の発振波長λの（ｍ＋１／２）倍（ｍは０を含む正の整数）となる段差ｄとしている。ここでｍは０、１または２であることが好ましい。３以上であると段差ｄの大きさが大きくなって、生産効率上好ましくない。ｍが０のときが最も生産効率がよく特に好ましい。
【００１４】
このような構造の回折素子１０を、図１の偏光性回折素子１として光ヘッド装置に搭載し、半導体レーザ５の出射光の第１の直線偏光が回折素子１０に常光偏光（図２（ａ）中の◎）として入射するように配置する。このとき、複屈折性材料１３の凹凸周期構造は，常光屈折率ｎ_oと屈折率ｎ_sとを略等しくしてあるため等方性屈折率層として作用し、第１の直線偏光の入射光は回折されることなく直進透過する。
【００１５】
偏光性回折素子１（回折素子１０）を透過した直線偏光は、光ディスク７で反射されるので、１／４波長板２を往復して透過し、第２の直線偏光の一部がビームスプリッタ４を透過して光検出器６に信号光として入射し、残りはビームスプリッタ４の反射膜（図示せず）で反射され偏光性回折素子１（回折素子１０）に戻り光となって再入射する。このとき、戻り（復路）光の偏光方向が光ディスクへ向かう往路の直線偏光の偏光方向と直交する、第２の直線偏光である異常光直線偏光（図２（ｂ）中の←→）となっているため、複屈折性材料１３の凹凸周期構造は、位相差２π×（ｎ_e−ｎ_s）×ｄ／λすなわち２π×λ×（ｍ＋１／２）／λで位相差π（ｍ＝０としてよい）の位相回折格子として作用する。そして、戻り光のほとんどは±１次以上の高次光として回折されるため、直進透過して半導体レーザ５の発光点に集光される戻り光はわずかとなる。
【００１６】
また、複屈折性材料１３の凹凸周期構造の格子ピッチＰに応じて発生するＱ次回折光の回折角度θは入射光の波長λに対して、ｓｉｎθ＝Ｑλ／Ｐの関係にある。ここで、Ｑは±１、±２、・・・の値をとる。
半導体レーザ５の発光点に最も近い回折光は±１次回折光であるため、±１次回折光が半導体レーザ５の発振に影響しない領域、すなわち発光点と重ならない領域に回折されるように格子ピッチＰを設定する。具体的には５０μｍ以下の格子ピッチＰとすることが好ましい。
【００１７】
凹凸周期構造の格子パターンは格子を平面的に上から見ると通常直線状であるが、直線格子の格子方向が異なる領域に分割した分割パターンや、直線格子の代わりに曲線格子のホログラムパターンとしてもよい。その結果、高出力半導体レーザを光源として用いる光ヘッド装置において、半導体レーザの発光点に集光する戻り光はほとんどなくなり、上記のように半導体レーザの発振光強度が安定するため、光ディスクの情報の記録および再生時に安定した信号検出ができる。
【００１８】
図３は、本発明における偏光性回折素子の構成の他の例を示す側面図である（（ａ）第１の偏光が往路として入射した様子を示し、（ｂ）第２の偏光が復路として入射した様子を示す。）。光ディスクにおける情報の再生のための信号検出法として、一般的に用いられている３ビームを使用する３ビーム法を適用するために、回折素子２０において、透光性平面基板１２の一方の面に光ディスクのトラッキングに用いるためのサブビームを発生する回折格子１５が形成されている。回折格子１５の回折効率は偏光方向に依存しない。他の構成は図２の回折素子１０の構成と同じであり、その他の符号で図２と同じものは図２と同じ要素を示す。
【００１９】
このような構造の回折素子２０を、図１の偏光性回折素子１として光ヘッド装置に用いることにより、光ヘッド装置の部品点数および重量を増やすことなく光ディスクの情報の記録および再生時に安定した信号検出ができるので好ましい。すなわち、反射光の信号検出用の３ビームの発生素子として用いる、回折効率が偏光方向に依存しない回折格子が、偏光性回折素子にさらに形成されていることが好ましい。
【００２０】
図４は本発明における偏光性回折素子の構成の別の例を示す側面図である（（ａ）第１の偏光が往路として入射した様子を示し、（ｂ）第２の偏光が復路として入射した様子を示す。）。図１の光ヘッド装置のビームスプリッタ４と対物レンズ３との間の光路中に配置された１／４波長板２の代わりに配置する。偏光性回折素子である回折素子３０は、等方性屈折率透明材料１４と透光性接着材１７を用いて透光性平面基板１１と透光性平面基板１２との間に位相差がπ／２の奇数倍となる位相板１６を挟み込んだ構成としている。他の構成は図３の回折素子２０の構成と同じであり、その他の符号で図３と同じものは図３と同じ要素を示す。位相板１６はポリカーボネート複屈折膜や高分子液晶などの位相差発生機能を有する有機薄膜位相板や水晶位相板が用いられる。
【００２１】
このような構造の回折素子３０を、図１の偏光性回折素子１として複屈折性材料１３が半導体レーザ５側になるように配置することにより、光ヘッド装置の部品点数および重量が減少するとともに、光ディスクの信号の記録再生時に安定した信号検出ができるので好ましい。
また、位相板を偏光性回折素子と対物レンズとの間の光路中に設置してもよい。
【００２２】
図５は、本発明における偏光性回折素子の構成のさらに別の例を示す側面図である（（ａ）第１の偏光が往路として入射した様子を示し、（ｂ）第２の偏光が復路として入射した様子を示す。）。図４に示す偏光性の回折素子３０では、複屈折性材料１３を構成要素とする偏光性の回折格子と透光性平面基板１２の一方の面に形成された回折格子１５とが別々に形成されている。
【００２３】
しかし、図５に示された回折素子４０では複屈折性材料１３を構成要素とする偏光性の回折格子のみであり、この回折格子４０が図４の回折格子１５の機能も果たす構成となっている。すなわち、図５（ａ）に示すように、第１の直線偏光は、０次回折光とともに、±１次回折光が発生し合計３ビームとなる。第１の直線偏光は、図５（ｂ）に示すように、±１次回折光が発生し、この回折光は光源を避けるように（すなわち光源から遠ざけるように）偏向される。図５の符号で図４の符号と同じものは、図４と同じ要素を表わす。
【００２４】
図５において、複屈折性材料１３の常光屈折率ｎ_oおよび異常光屈折率ｎ_e（ｎ_o≠ｎ_e）が等方性屈折率透明材料１４の屈折率ｎ_sと異なり、｜ｎ_e−ｎ_s｜×ｄが半導体レーザの発振波長λの（ｍ＋１／２）倍（ｍは０を含む正の整数）となる段差ｄであるとともに、｜ｎ_o−ｎ_s｜×ｄが発振波長λの（ｋ＋１／４）倍から（ｋ＋１／１２）倍のまでの範囲（ｋは０を含む整数）とすることが好ましい。ここでｍとｋは０、１または２であることが好ましい。３以上であると段差ｄの大きさが大きくなって、生産効率上好ましくない。ｍとｋが０のときが最も生産効率がよく特に好ましい。
【００２５】
このような構造の回折素子４０を、図１の偏光性回折素子１として複屈折性材料１３が半導体レーザ５側になるとともに、光ヘッド装置において半導体レーザ５の出射光の第１の直線偏光が複屈折性材料１３に常光偏光として入射するように配置することが好ましい。この配置により、光ヘッド装置の部品点数および重量が減少するとともに、光ディスクの情報の記録および再生時に安定した信号検出ができる。
【００２６】
このとき、往路において複屈折性材料１３の凹凸周期構造は位相差が２π×（ｎ_o−ｎ_s）×ｄ／λである、すなわち２π×λ×（ｋ＋１／４）／λから２π×λ×（ｋ＋１／１２）／λまでであるから、位相差がπ／２からπ／６までの位相回折格子として作用するため（ｋ＝０としてよい）、回折効率の式から、５０％から９３％程度の０次回折光と２０％から３％程度の±１次回折光が算出され、凹凸周期構造の格子ピッチを適宜設定することにより光ディスクの情報を含む信号検出用の３ビーム発生素子とすることができる。
【００２７】
複屈折性材料１３を透過した０次回折光と±１次回折光の直線偏光は位相板１６を透過した後円偏光となって回折素子４０を出射し、光ディスクで反射して戻り光となって再び回折素子４０に入射する。位相板１６を透過した後、往路の入射常光偏光と直交する偏光方向を有する異常光偏光となって複屈折性材料１３に入射する。このとき、複屈折性材料１３の凹凸周期構造は位相差が２π×（ｎ_e−ｎ_s）×ｄ／λ（すなわち２π×λ×（ｍ＋１／２）／λ）であり、位相差がπの位相回折格子として作用し、戻り光の大部分は±１次以上の高次回折光となるため、直進透過して半導体レーザの発光点に集光される戻り光はわずかとなる。ここで、ｍ＝０としてよい。
【００２８】
このような構造の回折素子４０を用いることにより、一つの偏光性回折格子のみで、光ディスクの情報を含む信号検出用の３ビームの発生素子としての機能と半導体レーザの発光点への戻り光を偏向する機能を同時に実現できる。その結果、上記のように光ヘッド装置の部品点数および重量が減少するとともに、光ディスクの情報の記録および再生時に安定した信号検出ができる。
【００２９】
【実施例】
「例１」
まず、本例の光ヘッド装置に用いられた回折素子について、図４を用いて説明する。一方の面に反射防止膜が形成された透光性平面基板１１であるガラス基板の他方の面上に、複屈折性材料１３として常光屈折率ｎ_oが１．５５および異常光屈折率ｎ_eが１．７０の高分子液晶層を形成し、フォトリソグラフィとエッチングの技術により直線状の格子ピッチが５μｍ、凹部の深さが２．６μｍの凹凸周期構造を作製した。また、透光性平面基板１２の外側の面にＳｉＯ₂の膜を厚さ０．２４μｍに成膜した後、フォトリソグラフィとエッチングの技術により格子ピッチが２５μｍ、凹凸段差が０．２４μｍの回折格子１５を作製し、その表面に反射防止膜を形成した。
【００３０】
さらに、屈折率ｎ_s＝１．５５の等方性屈折率透明材料１４である均質屈折率透明樹脂を用いて、高分子液晶層の凹凸状に加工された凹部を充填した。また、ポリカーボネート複屈折膜製の１／４波長板１６を、透光性平面基板１１、１２である２枚のガラス基板を用いて挟み込んで、均質屈折透明樹脂と同じ屈折率の透光性接着材１７により接着し、回折素子３０を作製した。ここで、ポリカーボネート複屈折膜製の１／４波長板１６は、光ヘッド装置に用いられる、発振波長λが７９０ｎｍである半導体レーザ５の入射直線偏光に対して、透過光の位相差がπ／２となるものを使用した。
【００３１】
このような構造の回折素子３０を、図１の偏光性回折素子１として光ヘッド装置に搭載し、半導体レーザ５から出射する直線偏光が高分子液晶層に常光偏光（第１の直線偏光）として入射するように配置した。このとき、回折素子３０を透過する光は、高分子液晶層の凹凸周期構造による回折はしないが、回折格子１５により０次回折光と±１次回折光が発生し、これらの光を光ディスクの情報を含む信号検出用の３ビームとして用いた。発生した回折光は、入射光に対しての約８３％の０次回折光と約７％の±１次回折光であり、出射光は円偏光であった。
【００３２】
さらに、ビームスプリッタ４の表面に形成された反射膜（図示せず）により、入射光の８５％が反射され、対物レンズ３により光ディスクの情報記録面に集光し、情報記録面からの反射光となって発散した。この発散した反射光は、そのうちの１５％の光が再び対物レンズ３によりビームスプリッタ４を透過し、光検出器６の受光面に集光され、残りの８５％がビームスプリッタ４により反射され戻り光となって偏光性回折素子１(回折素子３０)に入射した。
【００３３】
偏光性回折素子１(回折素子３０)に入射した光は１／４波長板１６を透過して異常光偏光（第２の直線偏光）となって高分子液晶からなる回折格子に入射し、入射光の９５％以上が±１次以上の高次回折光となって拡散され、半導体レーザ５の発光点に集光される光はほぼゼロとなった。その結果、高出力の半導体レーザを光源として用いる光ヘッド装置において、半導体レーザの発振光強度が安定し、光ディスクの情報の記録および再生時に安定した信号検出ができた。
【００３４】
「例２」
まず、本例の光ヘッド装置に用いられた回折素子について、図５を用いて説明する。一方の面に反射防止膜が形成された透光性平面基板１１であるガラス基板の他方の面上に、複屈折性材料１３として常光屈折率ｎ_oが１．５５および異常光屈折率ｎ_eが１．７０の高分子液晶層を形成し、フォトリソグラフィとエッチングの技術により直線状の格子ピッチが２５μｍ、凹部の深さが１．９μｍの凹凸周期構造を作製した。透光性平面基板１２の片面に反射防止膜を形成した。
【００３５】
さらに、屈折率ｎ_s＝１．４９の等方性屈折率透明材料１４である均質屈折率透明樹脂を用いて、高分子液晶層の凹凸状に加工された凹部を充填した。また、ポリカーボネート複屈折膜製の１／４波長板１６を、透光性平面基板１１、１２である２枚のガラス基板を用いて挟み込んで、均質屈折透明樹脂と同じ屈折率の透光性接着材１７により接着し、回折素子４０を作製した。ここで、ポリカーボネート複屈折膜製の１／４波長板１６は、光ヘッド装置に用いられる、発振波長λが７９０ｎｍである半導体レーザ５の入射直線偏光に対して、透過光の位相差がπ／２となるものを使用した。
【００３６】
このような構造の回折素子４０を、図１の偏光性回折素子１として光ヘッド装置に搭載し、半導体レーザ５の出射光の直線偏光が高分子液晶層からなる偏光性の回折格子に常光偏光（第１の直線偏光）として入射するように配置した。このとき、高分子液晶層の凹凸周期構造は位相差２π×（ｎ_o−ｎ_s）×ｄ／λ、すなわち位相差０．２９πの位相回折格子として作用し、入射光に対しての約８１％の０次回折光と約８％の±１次回折光が発生した。出射光は円偏光であった。この０次回折光と±１次回折光を光ディスクの情報を含む信号検出用の３ビームとして用いた。
【００３７】
さらに、ビームスプリッタ４の表面に形成された反射膜（図示せず）により、入射光の８５％が反射され、対物レンズ３により光ディスクの情報記録面に集光し、情報記録面からの反射光となって発散した。この発散した反射光は、そのうちの１５％の光が再び対物レンズ３によりビームスプリッタ４を透過し、光検出器６の受光面に集光され、残りの８５％がビームスプリッタ４により反射され戻り光となって偏光性回折素子１(回折素子4０)に入射した。
【００３８】
偏光性回折素子１(回折素子4０)に入射した光は１／４波長板１６を透過して異常光偏光（第２の直線偏光）となって高分子液晶からなる回折格子に入射し、入射光の９５％以上が±１次以上の高次回折光となって拡散され、半導体レーザ５の発光点に集光される光はほぼゼロとなった。その結果、高出力の半導体レーザを光源として用いる光ヘッド装置において、半導体レーザの発振光強度が安定し、光ディスクの情報の記録および再生時に安定した信号検出ができた。
【００３９】
本例では、一つの偏光性回折素子のみを用いて、往路で光ディスクの情報を含む信号検出用の３ビームを発生し、復路で半導体レーザの発光点への戻り光を回折するため、例１に比べて単純な構成で同じ作用効果が得られた。
【００４０】
【発明の効果】
以上説明したように、本発明における偏光性回折素子を用いた光ヘッド装置において、半導体レーザの発光点への戻り光がなくなるため、半導体レーザの発振光強度が安定し、光ディスクの情報の記録および再生時に安定した信号検出ができる。また、部品点数の増加を招くことなく、光ディスクの情報を含む信号検出用の３ビームを発生する回折格子機能と、半導体レーザの発光点への戻り光を遮断する機能とを併せ持った偏光性回折素子からなる光ヘッド装置が実現する。
【図面の簡単な説明】
【図１】本発明の光ヘッド装置の構成の一例を示す側面図。
【図２】本発明における偏光性回折素子の一例を示す図で、（ａ）第１の直線偏光が往路として入射した様子を示す側面図、（ｂ）第２の直線偏光が復路として入射した様子を示す側面図。
【図３】本発明における偏光性回折素子の他の例を示す図で、（ａ）第１の直線偏光が往路として入射した様子を示す側面図、（ｂ）第２の直線偏光が復路として入射した様子を示す側面図。
【図４】本発明における偏光性回折素子の別の例を示す図で、（ａ）第１の直線偏光が往路として入射した様子を示す側面図、（ｂ）第２の直線偏光が復路として入射した様子を示す側面図。
【図５】本発明における偏光性回折素子のさらに別の例を示す図で、（ａ）第１の直線偏光が往路として入射した様子を示す側面図、（ｂ）第２の直線偏光が復路として入射した様子を示す側面図。
【図６】従来の光ヘッド装置の構成の概略を示す側面図。
【符号の説明】
１：偏光性回折素子
１０、２０，３０，４０：回折素子
２、１６：１／４波長板（位相板）
３：対物レンズ
４：ビ−ムスプリッタ
５：半導体レーザ
６：光検出器
７：光ディスク（光記録媒体）
１１、１２：透光性平面基板
１３：複屈折性材料
１４、１７：等方性屈折率透明材料（透光性接着材）
１５：回折格子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device used for recording and reproducing information on an optical recording medium such as an optical disk.
[0002]
[Prior art]
For example, information is written on an information recording surface of an optical recording medium such as an optical disc such as a CD or a DVD or a magneto-optical disc (hereinafter collectively referred to as “optical disc”) (hereinafter referred to as “recording”), or Various optical head devices that read (hereinafter referred to as “reproduction”) information on an information recording surface are used.
[0003]
FIG. 6 is a side view schematically showing an example of a conventional optical head device. The emitted light from the semiconductor laser 5 is reflected by a reflection film (not shown) formed on the surface of the beam splitter 4 on the semiconductor laser 5 side, and is focused on the information recording surface of the optical disc 7 by the objective lens 3. Here, the reflection film is a partial reflection film that transmits a part of incident light and reflects the remaining light. The reflected light from the optical disk 7 is transmitted again through the objective lens 3, and part of the light is transmitted through the beam splitter 4, then condensed on the light receiving surface of the photodetector 6, and the remaining light is reflected on the reflecting film of the beam splitter 4. The light is reflected by the laser beam and returned to the light emitting point of the semiconductor laser 5. In the photodetector 6, the received light is converted into an electrical signal, the electrical signal corresponding to the received light amount is amplified by an amplifier, and the gain is appropriately corrected by an automatic gain correction circuit to adjust the signal level within a certain range. Is output.
[0004]
As a configuration of the optical head device, a collimator lens may be disposed between the beam splitter 4 and the objective lens 3, or a cylindrical lens or a concave lens may be disposed between the beam splitter 4 and the photodetector 6. . In addition, there is an optical head device configured using two semiconductor lasers for a wavelength of 790 nm for CD and two semiconductor lasers for a wavelength of 650 nm for DVD.
[0005]
In such an optical head device, when the reflected return light from the information recording surface of the optical disk is incident on the light emitting point of the semiconductor laser used as the light source, the intensity of the emitted light from the semiconductor laser becomes unstable, resulting in accurate information recording. Further, there arises a problem that reproduction cannot be performed. In order to solve this problem, a circuit that superimposes a high frequency current of several hundred MHz on the drive current of the power supply for the semiconductor laser is provided, and the return light and the oscillation light are changed by changing the oscillation mode of the semiconductor laser from the single mode to the multimode. And the output light intensity is stabilized.
[0006]
In addition, a quarter-wave plate 2 having a phase difference of π / 2 with respect to the oscillation wavelength of the semiconductor laser is disposed in the optical path between the semiconductor laser and the objective lens, and the return light travels back and forth through the quarter-wave plate. The output light intensity is stabilized by reducing the interference between the return light and the oscillation light by changing the polarization state of the oscillation light from the polarization state of the oscillation light. However, when the output power of the semiconductor laser is increased in order to record information on the information recording surface of an optical disc such as a CD-R or CD-RW at high speed, the intensity of the return light to the light emitting point of the semiconductor laser also increases. There is a problem that the intensity of the emitted light of the semiconductor laser becomes unstable and stable recording and reproduction cannot be performed.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide an optical head device capable of stable signal detection at the time of recording and reproducing information on an optical disk in an optical head device using a relatively high-power semiconductor laser as a light source in view of the above circumstances. To do.
[0008]
[Means for Solving the Problems]
The present invention relates to a light source and an objective lens for focusing the optical recording medium the light emitted from the light source, a photodetector for detecting light reflected from the optical recording medium, the light emitted from the light source objective the part of the reflected light from the reflection vital the optical recording medium to the lens side and a beam splitter for reflecting the remaining portion of the reflected light to the light source side as well as transmitted to the photodetector side, the optical recording An optical head device that records and reproduces information on a medium, and includes a phase plate in the optical path between the beam splitter and the objective lens, the phase difference of transmitted light being an odd multiple of π / 2, in an optical path between the light source and the beam splitter, to provide an optical head device, wherein a polarizing diffraction element below consisting of birefringent material is disposed.
Polarizing diffraction element: when the first linearly polarized light being emitted from the light source is incident on the polarizing diffraction element is transmitted through the first linear polarization does not act as a diffraction grating, from said optical recording medium a the reflected light, back and forth the phase plate, when the first second linearly polarized light as the polarized direction orthogonal to the linearly polarized light is incident on the polarizing diffraction element acts as a diffraction grating diffracting said second linearly polarized light away from the light source.
[0009]
Further, the polarizing diffraction element is a translucent planar substrate, a birefringent material of the ordinary refractive index n _o and extraordinary refractive index _{_{_{n e (n o ≠ n e}}} ), the ordinary refractive index n _o and substantially An isotropic refractive index material having an equal refractive index n _s and a periodic concavo-convex periodic structure having a step d in cross section, and | n _e −n _s | × d is the output light from the light source The above optical head device is provided which is (m + 1/2) times the wavelength λ (m is a positive integer including 0). Further, the above-described optical head device is provided in which the birefringent material is a polymer liquid crystal. In addition, the above-described optical head device is provided in which the pitch of the uneven periodic structure is 5 μm or less. Furthermore, the polarizing diffraction element is used as a three-beam generating element for detecting a signal incident on the optical recording medium, and provides the above optical head device having a diffraction grating whose diffraction efficiency does not depend on the polarization direction. .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a side view schematically showing an example of the optical head device of the present invention. The configuration of the conventional optical head device shown in FIG. 6 is the same as that of the conventional optical head device shown in FIG. 6 except that the polarizing diffractive element 1 is installed in the optical path between the semiconductor laser 5 which is a light source emitting the first linearly polarized light and the beam splitter 4. is there. Therefore, the same reference numerals as those in FIG. 6 denote the same elements. The polarizing diffractive element 1 does not act as a diffraction grating for incident light of the first linearly polarized light emitted from a light source (semiconductor laser), transmits incident light, and is reflected light from an optical recording medium. The incident light of the second linearly polarized light having the polarization direction orthogonal to the first linearly polarized light acts as a diffraction grating and diffracts the incident light away from the light source.
[0011]
Since such a polarizing diffraction element 1 is installed, the second linearly polarized light reflected by the optical disk and further reflected by the beam splitter 4 is diffracted and does not reach the semiconductor laser. The intensity becomes stable, and as a result, accurate information recording and reproduction of the optical head device can be performed. Here, when the incident light is diffracted away from the semiconductor laser, it may be avoided not to reach the semiconductor laser at all, but the light emitting point of the semiconductor laser may be avoided. Here, this case is also included.
The polarizing diffraction element 1 is made of a birefringent material, and examples of the birefringent material include an optical crystal and a polymer liquid crystal. From the viewpoint of ease of productivity and the like, it is preferable to use a polymer liquid crystal.
[0012]
FIG. 2 is a side view showing one example of the configuration of the polarizing diffraction element in the present invention. Diffractive element 10 is a polarizing diffraction element on one surface of the translucent planar substrate 11, cross the birefringent material 13 in the ordinary refractive index n _o and extraordinary refractive index _{_{_{n e (n o ≠ n e}}} ) The shape is processed so as to have a periodic unevenness with a level difference d, and the concave portion is filled with an isotropic refractive index transparent material 14 having a refractive index n _s . In addition, the isotropic refractive index transparent material 14 is sandwiched between the translucent flat substrate 11 and the translucent flat substrate 12. An antireflection film may be formed at the interface between the light-transmitting flat substrate 11 and the light-transmitting flat substrate 12 with air.
[0013]
Here, substantially with equal isotropic refractive index transparent material 14 for example the refractive index n _s is the ordinary refractive index _{_{_{n o, | n e -n s}}} | × d is the oscillation wavelength λ of the semiconductor laser 5 (m + 1/2 ) Is a step d which is a multiple (m is a positive integer including 0). Here, m is preferably 0, 1 or 2. If it is 3 or more, the level difference d becomes large, which is not preferable in terms of production efficiency. When m is 0, the production efficiency is highest and particularly preferable.
[0014]
The diffractive element 10 having such a structure is mounted on the optical head device as the polarizing diffractive element 1 in FIG. 1, and the first linearly polarized light of the emitted light from the semiconductor laser 5 is applied to the diffractive element 10 by ordinary light polarization (FIG. ) Is placed so as to be incident as ◎). At this time, uneven periodic structure of the birefringent material 13 acts as isotropic refractive index layer because that is substantially equal to the refractive index n _s and ordinary refractive index n _o, the incident light of first linearly polarized light Transmits straight without being diffracted.
[0015]
The linearly polarized light that has passed through the polarizing diffraction element 1 (diffraction element 10) is reflected by the optical disc 7, so that it travels back and forth through the quarter-wave plate 2, and part of the second linearly polarized light is the beam splitter 4. And is incident on the light detector 6 as signal light, and the rest is reflected by a reflective film (not shown) of the beam splitter 4 and returns to the polarizing diffractive element 1 (diffractive element 10) as incident light. . At this time, the polarization direction of the return (return path) light becomes the extraordinary light linear polarization (← → in FIG. 2B), which is the second linear polarization, which is orthogonal to the polarization direction of the linear polarization of the forward path toward the optical disk. and for which, uneven periodic structure of the birefringent material 13, the phase difference _{_{2π × (n e -n s)}} × d / λ That is 2π × λ × (m + 1 /2) / phase difference λ π (m = 0 It may act as a phase diffraction grating. Since most of the return light is diffracted as higher-order light of ± 1st order or higher, there is little return light that passes through straight and is collected at the light emitting point of the semiconductor laser 5.
[0016]
Further, the diffraction angle θ of the Q-order diffracted light generated according to the grating pitch P of the concavo-convex periodic structure of the birefringent material 13 has a relationship of sin θ = Qλ / P with respect to the wavelength λ of the incident light. Here, Q takes values of ± 1, ± 2,.
Since the diffracted light closest to the light emitting point of the semiconductor laser 5 is ± first order diffracted light, the grating pitch is such that the ± first order diffracted light is diffracted into a region that does not affect the oscillation of the semiconductor laser 5, that is, a region that does not overlap the light emitting point. Set P. Specifically, the lattice pitch P is preferably 50 μm or less.
[0017]
The grating pattern of the concavo-convex periodic structure is usually linear when the grating is viewed from above, but it can also be divided into areas with different grating directions of the linear grating, or a hologram pattern with a curved grating instead of a linear grating. Good. As a result, in an optical head device using a high-power semiconductor laser as a light source, almost no return light is collected at the light emitting point of the semiconductor laser, and the oscillation light intensity of the semiconductor laser is stabilized as described above. Stable signal detection can be performed during recording and reproduction.
[0018]
FIG. 3 is a side view showing another example of the configuration of the polarizing diffraction element according to the present invention ((a) shows a state where the first polarized light is incident as an outward path, and (b) shows a second polarized light as a return path. Shows the incident state.) As a signal detection method for reproducing information on an optical disc, a three-beam method using three beams, which is generally used, is applied to one surface of the translucent flat substrate 12 in the diffraction element 20. A diffraction grating 15 for generating a sub beam to be used for tracking of the optical disk is formed. The diffraction efficiency of the diffraction grating 15 does not depend on the polarization direction. The other configuration is the same as the configuration of the diffraction element 10 in FIG. 2, and the same reference numerals as those in FIG. 2 denote the same elements as those in FIG.
[0019]
By using the diffraction element 20 having such a structure in the optical head device as the polarizing diffraction element 1 of FIG. 1, a stable signal at the time of recording and reproducing information on the optical disk without increasing the number of parts and the weight of the optical head device. This is preferable because it can be detected. That is, it is preferable that a diffraction grating whose diffraction efficiency does not depend on the polarization direction, which is used as a three-beam generating element for detecting a signal of reflected light, is further formed on the polarizing diffraction element.
[0020]
FIG. 4 is a side view showing another example of the configuration of the polarizing diffraction element according to the present invention ((a) shows a state where the first polarized light is incident as the forward path, and (b) the second polarized light is incident as the backward path. Shows how it was done.) 1 is disposed instead of the quarter-wave plate 2 disposed in the optical path between the beam splitter 4 and the objective lens 3 of the optical head device of FIG. The diffractive element 30 which is a polarizing diffractive element has a phase difference of π between the translucent flat substrate 11 and the translucent flat substrate 12 using the isotropic refractive index transparent material 14 and the translucent adhesive 17. The phase plate 16 that is an odd multiple of / 2 is sandwiched. The other configuration is the same as the configuration of the diffraction element 20 in FIG. 3, and the same reference numerals as those in FIG. 3 denote the same elements as those in FIG. As the phase plate 16, an organic thin film phase plate or a crystal phase plate having a phase difference generating function such as a polycarbonate birefringent film or a polymer liquid crystal is used.
[0021]
By disposing the diffraction element 30 having such a structure as the polarizing diffraction element 1 of FIG. 1 so that the birefringent material 13 is on the semiconductor laser 5 side, the number of parts and the weight of the optical head device are reduced. It is preferable because stable signal detection can be performed at the time of recording / reproducing the signal of the optical disk.
In addition, a phase plate may be installed in the optical path between the polarizing diffraction element and the objective lens.
[0022]
FIG. 5 is a side view showing still another example of the configuration of the polarizing diffraction element in the present invention ((a) shows a state in which the first polarized light is incident as the forward path, and (b) the second polarized light is in the return path. Shows the incident state.) In the polarizing diffraction element 30 shown in FIG. 4, the polarizing diffraction grating having the birefringent material 13 as a constituent element and the diffraction grating 15 formed on one surface of the translucent flat substrate 12 are separately formed. Has been.
[0023]
However, the diffraction element 40 shown in FIG. 5 has only a polarizing diffraction grating having the birefringent material 13 as a constituent element, and this diffraction grating 40 also functions as the diffraction grating 15 of FIG. Yes. That is, as shown in FIG. 5A, in the first linearly polarized light, ± first-order diffracted light is generated together with zero-order diffracted light, resulting in a total of three beams. As shown in FIG. 5B, ± first-order diffracted light is generated from the first linearly polarized light, and this diffracted light is deflected so as to avoid the light source (that is, away from the light source). 5 that are the same as those in FIG. 4 represent the same elements as those in FIG.
[0024]
5, unlike the ordinary refractive index n _o and extraordinary refractive index _{_{_{n e (n o ≠ n e}}} ) the refractive index n _s of the isotropic refractive index transparent material 14 of the birefringent material 13, | n _e - n _s | × d is a step d that is (m + 1/2) times the oscillation wavelength λ of the semiconductor laser (m is a positive integer including 0), and | n _o −n _s | × d is the oscillation wavelength λ. It is preferable that the range is (k + 1/4) times (k + 1/12) times (k is an integer including 0). Here, m and k are preferably 0, 1 or 2. If it is 3 or more, the level difference d becomes large, which is not preferable in terms of production efficiency. When m and k are 0, the production efficiency is highest and particularly preferable.
[0025]
The diffractive element 40 having such a structure is used as the polarizing diffractive element 1 in FIG. 1, and the birefringent material 13 is on the semiconductor laser 5 side. In the optical head device, the first linearly polarized light emitted from the semiconductor laser 5 is The birefringent material 13 is preferably disposed so as to be incident as ordinary light polarized light. With this arrangement, the number of parts and the weight of the optical head device are reduced, and stable signal detection can be performed during recording and reproduction of information on the optical disk.
[0026]
At this time, in the forward path, the uneven structure of the birefringent material 13 has a phase difference of 2π × (n _o −n _s ) × d / λ, that is, 2π × λ × (k + 1/4) / λ to 2π × λ. Since it is up to x (k + 1/12) / λ, it acts as a phase diffraction grating with a phase difference of π / 2 to π / 6 (it may be k = 0). About 0% 0th order diffracted light and 20% to about 3% ± 1st order diffracted light are calculated, and by appropriately setting the grating pitch of the concavo-convex periodic structure, a 3-beam generating element for signal detection including information on the optical disk is obtained. Can do.
[0027]
The linearly polarized light of the 0th-order diffracted light and the ± 1st-order diffracted light transmitted through the birefringent material 13 is transmitted through the phase plate 16 and then becomes circularly polarized light, exits the diffraction element 40, is reflected by the optical disk, and becomes return light again. The light enters the diffraction element 40. After passing through the phase plate 16, the light enters the birefringent material 13 as an extraordinary light polarized light having a polarization direction orthogonal to the outgoing ordinary light polarized light. At this time, uneven periodic structure of birefringent material 13 is the phase difference is _{_{2π × (n e -n s)}} × d / λ ( i.e. 2π × λ × (m + 1 /2) / λ), the phase difference is π Most of the return light is high-order diffracted light of ± 1st order or higher, so that the amount of return light that is transmitted straight and collected at the light emitting point of the semiconductor laser is small. Here, m = 0 may be set.
[0028]
By using the diffraction element 40 having such a structure, only one polarizing diffraction grating can function as a three-beam generating element for signal detection including information on the optical disk and return light to the light emitting point of the semiconductor laser. The deflecting function can be realized at the same time. As a result, the number of parts and the weight of the optical head device are reduced as described above, and stable signal detection can be performed during recording and reproduction of information on the optical disk.
[0029]
【Example】
"Example 1"
First, the diffraction element used in the optical head device of this example will be described with reference to FIG. On the other surface of the glass substrate is a light-transmissive flat substrate 11 on which the antireflection film is formed on one surface, the ordinary refractive index n _o as a birefringent material 13 is 1.55 and the extraordinary refractive index n _e Was formed, and a concavo-convex periodic structure having a linear lattice pitch of 5 μm and a recess depth of 2.6 μm was produced by photolithography and etching techniques. Further, after a SiO ₂ film having a thickness of 0.24 μm is formed on the outer surface of the translucent flat substrate 12, a diffraction grating having a grating pitch of 25 μm and an uneven step of 0.24 μm by photolithography and etching techniques. 15 was prepared, and an antireflection film was formed on the surface.
[0030]
Furthermore, using the homogeneous refractive index transparent resin which is the isotropic refractive index transparent material 14 having a refractive index n _s = 1.55, the concave portions processed into the concave and convex shape of the polymer liquid crystal layer were filled. Further, a quarter-wave plate 16 made of a polycarbonate birefringent film is sandwiched between two glass substrates which are translucent flat substrates 11 and 12, and a translucent adhesive having the same refractive index as that of a homogeneous refractive transparent resin. The diffraction element 30 was produced by bonding with the material 17. Here, the quarter-wave plate 16 made of a polycarbonate birefringent film has a phase difference of transmitted light of π / with respect to the incident linearly polarized light of the semiconductor laser 5 having an oscillation wavelength λ of 790 nm used in the optical head device. 2 was used.
[0031]
The diffractive element 30 having such a structure is mounted on the optical head device as the polarizing diffractive element 1 of FIG. 1, and the linearly polarized light emitted from the semiconductor laser 5 is converted into ordinary light polarized light (first linearly polarized light) in the polymer liquid crystal layer. It arrange | positioned so that it might inject. At this time, the light transmitted through the diffractive element 30 is not diffracted by the rugged periodic structure of the polymer liquid crystal layer, but the diffraction grating 15 generates 0th-order diffracted light and ± 1st-order diffracted light. Used as 3 beams for signal detection. The generated diffracted light was about 83% 0th order diffracted light and about 7% ± 1st order diffracted light with respect to the incident light, and the emitted light was circularly polarized light.
[0032]
Further, 85% of the incident light is reflected by a reflection film (not shown) formed on the surface of the beam splitter 4, and is condensed on the information recording surface of the optical disk by the objective lens 3, and reflected light from the information recording surface. And diverged. Of this diverged reflected light, 15% of the light is again transmitted through the beam splitter 4 by the objective lens 3 and condensed on the light receiving surface of the photodetector 6, and the remaining 85% is reflected by the beam splitter 4 and returned. The light was incident on the polarizing diffraction element 1 (diffraction element 30).
[0033]
Light incident on the polarizing diffraction element 1 (diffraction element 30) is transmitted through the quarter-wave plate 16 and becomes an extraordinary light polarization (second linearly polarized light) and is incident on a diffraction grating made of a polymer liquid crystal. More than 95% of the light is diffused as high-order diffracted light of ± 1st order or higher, and the light collected at the light emitting point of the semiconductor laser 5 becomes almost zero. As a result, in an optical head device using a high-power semiconductor laser as a light source, the oscillation light intensity of the semiconductor laser was stabilized, and stable signal detection was possible during recording and reproduction of information on the optical disk.
[0034]
"Example 2"
First, the diffraction element used in the optical head device of this example will be described with reference to FIG. On the other surface of the glass substrate is a light-transmissive flat substrate 11 on which the antireflection film is formed on one surface, the ordinary refractive index n _o as a birefringent material 13 is 1.55 and the extraordinary refractive index n _e A polymer liquid crystal layer of 1.70 was formed, and a concavo-convex periodic structure with a linear lattice pitch of 25 μm and a recess depth of 1.9 μm was produced by photolithography and etching techniques. An antireflection film was formed on one side of the translucent flat substrate 12.
[0035]
Furthermore, using a homogeneous refractive index transparent resin, which is an isotropic refractive index transparent material 14 having a refractive index n _s = 1.49, the concave portions processed into the concavo-convex shape of the polymer liquid crystal layer were filled. Further, a quarter-wave plate 16 made of a polycarbonate birefringent film is sandwiched between two glass substrates which are translucent flat substrates 11 and 12, and a translucent adhesive having the same refractive index as that of a homogeneous refractive transparent resin. The diffraction element 40 was produced by bonding with the material 17. Here, the quarter-wave plate 16 made of a polycarbonate birefringent film has a phase difference of transmitted light of π / with respect to the incident linearly polarized light of the semiconductor laser 5 having an oscillation wavelength λ of 790 nm used in the optical head device. 2 was used.
[0036]
The diffractive element 40 having such a structure is mounted on the optical head device as the polarizing diffractive element 1 of FIG. 1, and the linearly polarized light of the emitted light from the semiconductor laser 5 is polarized on the polarizing diffraction grating made of a polymer liquid crystal layer. It arrange | positioned so that it may inject as (1st linearly polarized light). At this time, the concave-convex periodic structure of the polymer liquid crystal layer acts as a phase diffraction grating having a phase difference of 2π × (n _o −n _s ) × d / λ, that is, a phase difference of 0.29π, and is approximately 81 with respect to incident light. % 0th order diffracted light and about 8% ± 1st order diffracted light were generated. The outgoing light was circularly polarized. The 0th-order diffracted light and ± 1st-order diffracted light were used as three beams for signal detection including information on the optical disk.
[0037]
Further, 85% of the incident light is reflected by a reflection film (not shown) formed on the surface of the beam splitter 4, and is condensed on the information recording surface of the optical disk by the objective lens 3, and reflected light from the information recording surface. And diverged. Of this diverged reflected light, 15% of the light is again transmitted through the beam splitter 4 by the objective lens 3 and condensed on the light receiving surface of the photodetector 6, and the remaining 85% is reflected by the beam splitter 4 and returned. The light was incident on the polarizing diffraction element 1 (diffraction element 40).
[0038]
The light incident on the polarizing diffractive element 1 (diffractive element 40) passes through the quarter-wave plate 16 and becomes an extraordinary light polarized light (second linearly polarized light) and enters a diffraction grating made of a polymer liquid crystal. More than 95% of the light is diffused as high-order diffracted light of ± 1st order or higher, and the light collected at the light emitting point of the semiconductor laser 5 becomes almost zero. As a result, in an optical head device using a high-power semiconductor laser as a light source, the oscillation light intensity of the semiconductor laser was stabilized, and stable signal detection was possible during recording and reproduction of information on the optical disk.
[0039]
In this example, only one polarizing diffraction element is used to generate three signal detection beams including information on the optical disk in the forward path, and diffract the return light to the light emitting point of the semiconductor laser in the backward path. The same effect was obtained with a simple configuration.
[0040]
【The invention's effect】
As described above, in the optical head device using the polarizing diffraction element according to the present invention, the return light to the light emitting point of the semiconductor laser is eliminated, so that the oscillation light intensity of the semiconductor laser is stabilized, and information recording on the optical disk is performed. Stable signal detection during playback. Polarization diffraction that has both a diffraction grating function for generating three beams for signal detection including information on the optical disk and a function for blocking the return light to the light emitting point of the semiconductor laser without increasing the number of parts. An optical head device composed of elements is realized.
[Brief description of the drawings]
FIG. 1 is a side view showing an example of the configuration of an optical head device of the present invention.
2A and 2B are diagrams showing an example of a polarizing diffraction element according to the present invention, in which FIG. 2A is a side view showing a state in which first linearly polarized light is incident as a forward path, and FIG. 2B is a side view illustrating second linearly polarized light is incident as a backward path. The side view which shows a mode.
FIGS. 3A and 3B are diagrams showing another example of the polarizing diffraction element according to the present invention, in which FIG. 3A is a side view showing a state in which first linearly polarized light is incident as an outward path, and FIG. 3B is a diagram showing second linearly polarized light as a return path. The side view which shows a mode that it injected.
FIGS. 4A and 4B are diagrams showing another example of the polarizing diffraction element according to the present invention, in which FIG. 4A is a side view showing a state in which first linearly polarized light is incident as an outward path, and FIG. 4B is a diagram showing second linearly polarized light as a return path. The side view which shows a mode that it injected.
5A and 5B are diagrams showing still another example of the polarizing diffraction element according to the present invention, in which FIG. 5A is a side view showing a state in which the first linearly polarized light is incident as an outward path, and FIG. 5B is a diagram showing the second linearly polarized light being the return path. The side view which shows a mode that it injected as.
FIG. 6 is a side view schematically showing the configuration of a conventional optical head device.
[Explanation of symbols]
1: Polarizing diffraction element 10, 20, 30, 40: Diffraction element 2, 16: 1/4 wavelength plate (phase plate)
3: Objective lens 4: Beam splitter 5: Semiconductor laser 6: Photo detector 7: Optical disc (optical recording medium)
11, 12: Translucent flat substrate 13: Birefringent material 14, 17: Isotropic refractive index transparent material (translucent adhesive)
15: Diffraction grating

Claims

A light source;
An objective lens for focusing the light emitted from the light source to the optical recording medium,
A photodetector for detecting reflected light from the optical recording medium;
Beam and reflects the remainder of the reflected light to the light source side while transmitting a part of the reflected light from the reflection vital the optical recording medium to the objective lens side light emitted from the light source to the photodetector side comprising a splitter, a, there is provided an optical head device for recording and reproducing information on the optical recording medium,
In the optical path between the beam splitter and the objective lens, provided with a phase plate in which the phase difference of transmitted light is an odd multiple of π / 2,
An optical head and wherein in the optical path, that polarizing diffraction element below consisting of birefringent material is placed between the light source and the beam splitter.
Polarization diffraction element:
When the first linear polarized light which is emitted from the light source is incident on the polarizing diffraction element is transmitted through the first linear polarization does not act as a diffraction grating,
A the light reflected from the optical recording medium, back and forth the phase plate, when the first second linearly polarized light as the polarized direction orthogonal to the linearly polarized light is incident on the polarizing diffraction element diffracts to act as a diffraction grating away the second linearly polarized light from the light source.

The polarizing diffraction element is substantially equal refractive the transparent planar substrate, a birefringent material of the ordinary refractive index n _o and extraordinary refractive index _{_{_{n e (n o ≠ n e}}} ), and the ordinary refractive index n _o An isotropic refractive index material having a refractive index n _s and a periodic concave-convex periodic structure with a step d in cross section, and | n _e −n _s | × d is a wavelength λ of light emitted from the light source The optical head device according to claim 1, wherein (m + ½) times (m is a positive integer including 0) .

The optical head device according to claim 2 , wherein the birefringent material is a polymer liquid crystal .

4. The optical head device according to claim 2, wherein the pitch of the irregular periodic structure is 5 μm or less. 5 .

5. The polarizing diffraction element according to any one of claims 1 to 4, wherein the polarizing diffraction element has a diffraction grating that is used as a three-beam generation element for detecting a signal by being incident on the optical recording medium and whose diffraction efficiency does not depend on a polarization direction. The optical head device described.