JP2004138895A

JP2004138895A - Optical head device

Info

Publication number: JP2004138895A
Application number: JP2002304640A
Authority: JP
Inventors: Yoshiharu Oi; 大井　好晴
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2002-10-18
Filing date: 2002-10-18
Publication date: 2004-05-13
Anticipated expiration: 2022-10-18
Also published as: JP4300784B2

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an optical head device capable of stably recording and reproducing the information of a variety of optical disks whose using wavelength is different by using one and the same objective. <P>SOLUTION: Annular and stepwise irregularities rotationally symmetric with respect to the optical axis of incident light are formed in the area of an numerical aperture NA<SB>2</SB>regulated by the incident luminous flux of wavelength λ<SB>2</SB>on the plane of a phase correction element 10. The element 10 is equipped with a first phase correction surface 1 where the irregularities are formed so that the phase difference of transmitted light of wavelength λ<SB>1</SB>(λ<SB>1</SB><λ<SB>2</SB>) at the respective steps of the irregularities may be 4π, and is mounted in an optical path between the light source and the objective of the optical head device. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は光ヘッド装置に関し、特に２種以上の光記録媒体の情報の記録または再生に使用する光ヘッド装置に関する。
【０００２】
【従来の技術】
ＣＤ用の光記録媒体（以後、光記録媒体を「光ディスク」という）の情報の記録および再生のために、光源として波長が７９０ｎｍ帯の半導体レーザとＮＡ（開口数）が０．４５から０．５までの対物レンズ、および情報記録面保護用のカバー厚が１．２ｍｍの光ディスクが使用される。一方、ＤＶＤ光ディスクの情報の記録および再生には、光源として波長が６５５ｎｍ帯の半導体レーザとＮＡが０．６から０．６５までの対物レンズおよびカバー厚が０．６ｍｍの光ディスクが使用される。
【０００３】
さらに、記録情報量を増大させるため、光源として波長が４０５ｎｍ帯の半導体レーザとＮＡが０．６５の対物レンズおよびカバー厚が０．６ｍｍの光ディスクまたはＮＡが０．８５の対物レンズおよびカバー厚が０．１ｍｍの光ディスクが提案されている。以下、波長が４０５ｎｍ帯の半導体レーザで使用する光ディスクを特にＨＤ光ディスクという。
【０００４】
ＣＤ光ディスクとＤＶＤ光ディスクとＨＤ用の光ディスクとではカバー厚または使用波長が異なるため、それぞれを互換的に使用する場合、いずれか一種の光ディスクに対して設計された対物レンズを別の光ディスクに用いると大きな球面収差が発生し、情報の記録および再生ができない問題があった。
【０００５】
ＨＤ光ディスクに対して波面収差が最小になるよう設計された対物レンズを用いて、ＤＶＤ光ディスクの情報の記録および再生を行う場合に生じる球面収差を低減するため、ＨＤ用の波長λ_１の入射光の偏光方向とＤＶＤ用の波長λ_２の入射光の偏光方向を直交させ使用する偏光性位相補正素子が提案されている。
【０００６】
従来の偏光性位相補正素子の構成例の断面図を図７に示す。偏光性位相補正素子２０は常光屈折率ｎ_ｏおよび異常光屈折率ｎ_ｅ（ｎ_ｏ≠ｎ_ｅ）の複屈折性材料層２を備え、複屈折材料層が光学結晶のときは主光学軸が、高分子材料のときは分子配向軸が、一方向に揃っており、複屈折材料層には断面形状が鋸歯状であり、かつ入射光の光軸に関して回転対称性を有する鋸歯状の凹凸部が偏光性位相補正素子のＮＡ＝０．６０の領域に形成され、凹凸部の少なくとも凹部に常光屈折率ｎ_ｏとほぼ等しい屈折率ｎ_ｓの均質屈折率透明材料３が充填されている。
【０００７】
ＨＤ用の光ディスクへ波長λ_１の常光偏光が入射した場合、図７（ｂ）に示すように偏光性位相補正素子の透過波面は変化することなく、対物レンズの収差性能を維持する。一方、ＤＶＤ光ディスクへ波長λ_２の異常光偏光が入射した場合、図７（ａ）に示すように光ディスクのカバー厚の相違に起因して発生する球面収差を補正する透過波面となり、ＤＶＤ光ディスクの情報の記録および再生ができる。
【０００８】
また、偏光ビームスプリッタと１／４波長板を用い、半導体レーザから出射する直線偏光が偏光ビームスプリッタを往路で透過し、光ディスクで反射されて復路で１／４波長板を往復することにより、半導体レーザの出射直線偏光と直交する直線偏光に変換して、偏光ビームスプリッタを反射させて光検出器へと集光させることにより、光利用効率の高い信号光検出ができる。
【０００９】
【特許文献１】
特開２００２−５６５６０号公報
【００１０】
【発明が解決しようとする課題】
しかし、偏光ビームスプリッタと１／４波長板を従来の偏光性位相補正素子に用いた場合、往路と復路で偏光性位相補正素子への入射光の偏光状態が異なり、復路で波長λ_１および波長λ_２で大きな球面収差が発生するため問題であった。
【００１１】
また、ＨＤ、ＤＶＤおよびＣＤの３種の光ディスクに対応した、３波長用の位相補正素子が存在しないため、単一の対物レンズを用いてこれら３種の光ディスクの情報の記録および再生を行うことは困難であった。
【００１２】
本発明は、上記の従来技術の欠点を解決し、光学特性に優れ、小型軽量化に適した光ヘッド装置を提供することを目的とする。
【００１３】
【課題を解決するための手段】
本発明は、少なくとも波長λ_１とλ_２（λ_１＜λ_２）の２つの光を出射する光源と、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、光記録媒体の情報記録面からの反射光を検出する光検出器と、光源から対物レンズに至る光路中に波長λ_２の入射光の透過波面を変化させる位相補正素子とを備える、光記録媒体に情報の記録または再生を行う光ヘッド装置において、前記位相補正素子はその平面内における波長λ_２の入射光束が規定する開口数ＮＡ_２の領域に入射光の光軸に関して回転対称性を有する輪帯状でかつ階段状の凹凸部が形成され、さらに凹凸部の各段における波長λ_１の透過光の位相差が２πｍ（ｍは自然数）となるように凹凸部が形成された第１の位相補正面を備えていることを特徴とする光ヘッド装置を提供する。
【００１４】
また、前記位相補正素子は、第１の位相補正面に加えて、前記位相補正素子の平面内における波長λ_２の入射光束の規定する開口数ＮＡ_２の領域に、入射光の光軸に関して回転対称性を有する輪帯状の凹凸部が第１の位相補正面とは異なる面に形成された第２の位相補正面を備え、第２の位相補正面は常光屈折率ｎ_ｏおよび異常光屈折率ｎ_ｅ（ｎ_ｏ≠ｎ_ｅ）で分子配向軸が一方向に揃った有機物の複屈折性材料層からなり、凹凸部の少なくとも凹部に常光屈折率ｎ_ｏと等しい屈折率ｎ_ｓの均質屈折率透明材料が充填されている上記の光ヘッド装置を提供する。
【００１５】
また、前記位相補正素子は、第１の位相補正面に加えて、前記位相補正素子の平面内における波長λ_２の入射光束の規定する開口数ＮＡ_２の領域に、入射光の光軸に関して回転対称性を有する輪帯状の凹凸部が第１の位相補正面とは異なる面に形成された第３の位相補正面を備え、第３の位相補正面は波長λ_１において屈折率が等しくかつ波長λ_２において屈折率がたがいに異なる２種の均質屈折率透明材料からなり、一方の均質屈折率透明材料の凹凸部の少なくとも凹部に他方の均質屈折率透明材料が充填されている上記の光ヘッド装置を提供する。
【００１６】
さらに、前記位相補正素子は、波長λ_１に対する位相差がπ／２の奇数倍となる位相板がさらに追加され一体化されている上記の光ヘッド装置を提供する。
【００１７】
【発明の実施の形態】
本発明の光ヘッド装置における位相補正素子１０の一例の断面図を図１に、平面図を図２示す。図１においては、透光性基板５上の第１の位相補正面１と第２の位相補正面２と位相板４と透光性基板６上の開口制限フィルタ７からなる。
【００１８】
使用波長λ_１＝４０５ｎｍでカバー厚０．６ｍｍのＨＤ用の光ディスクに対して、良好な収差となるよう設計されたＮＡ_１＝０．６５のＨＤ用対物レンズを、使用波長λ_２＝６５５ｎｍでカバー厚０．６ｍｍのＤＶＤ用の光ディスクにＮＡ_２＝０．６５で用いたときに発生する波面収差の一例を図３（ａ）に示す。ここでは使用波長の相違に伴い対物レンズおよび光ディスクの屈折率波長分散に起因した球面収差を示し、横軸は開口径に対応した開口数ＮＡであり、縦軸は光軸上の光線（ＮＡ＝０）に対する各ＮＡ値での光線の位相差の断面を表す。実際にはほぼ軸対称の３次元形状の分布をなす。このように大きな球面収差が残留したままでは対物レンズにより光は充分集光されず、光ディスクの記録および再生ができない。
【００１９】
なお、このような球面収差を補正する方法として、対物レンズへの入射光を発散光とするいわゆる有限光学系構成があるが、対物レンズの光軸に対して直交する面内の対物レンズ移動に対して発生する収差が大きいため安定したトラッキング制御を行う上で実用上問題であった。
【００２０】
対物レンズへの入射光を平行光とするいわゆる無限光学系構成において、図３（ａ）に示す波面収差を補正する本発明の第１の位相補正面１について以下に説明する。
【００２１】
ガラスなどの透光性基板５の波長λ_２＝６５５ｎｍに対する開口数ＮＡ_２＝０．６５の領域に、光軸に関して回転対称性を有する輪帯形状でかつ階段状の凹凸部を形成する。ここで、各隣接する輪帯間の凹凸段差ｄは一定で、各段における屈折率ｎの均質材料の透過光と空気の透過光との波長λ_１に対する位相差２π×（ｎ−１）×ｄ／λ_１を２πｍ（ｍは自然数）でｍ＝２、すなわち４πとしている。λ_１＝４０５ｎｍおよびλ_３＝７９０ｎｍの場合、均質材料の屈折率ｎの波長分散を考慮すると、波長λ_１に対して位相差を４πとすれば波長λ_３に対してほぼ２πとなる。すなわち、このような凹凸部段差ｄからなる輪帯形状でかつ階段状の凹凸部に波長λ_１または波長λ_３の光が透過しても透過波面は変化しない。
【００２２】
一方、凹凸段差ｄを透過した波長λ_２の光に対して、位相差２π×（ｎ−１）×ｄ／λ_２位相遅れが生じる。図３を用いて具体例を説明する。段差ｄを透過した波長λ_２の光は空気に対して凸部材料の屈折率波長分散を考慮すると位相差１．２０×２π相当の透過波面の位相遅れが発生する。すなわち、輪帯形状の凹凸部段差ｄ当たり１波長分の位相差２πとの差異が位相差０．２０×２πの透過波面位相遅れとなる。
【００２３】
ここで、ＨＤ波長λ_１の透過光の透過波面が変化しない凹凸段差ｄは位相差２π×（ｎ−１）×ｄ／λ_１＝２π×ｍ（ｍは自然数）の場合であるが、波長λ_２の透過光に対する位相補正作用は位相差２π×（ｎ−１）×ｄ／λ_２と１波長分の位相差２πとの差異が小さな値ほど図３（ａ）に示す球面収差を低減できるので好ましい。また、ｍが小さな値ほど波長λ_１または波長λ_２の±１５ｎｍ程度の波長ばらつきに対する透過波面変動は少なく、かつ精度よく段差加工しやすいため好ましい。その結果、ＤＶＤ波長の位相差０．６０×ｍ×２πでは、ｍ＝２すなわち波長λ_１に対して位相差が４πとなる段差ｄとすることにより、ＤＶＤ波長λ_２に対して最適な位相補正面となる。ｍが４より大きくなると段差加工がやりにくく好ましくない。
【００２４】
したがって、図３（ａ）を示す波面収差を補正するように凹凸部の輪帯半径と凸部の高さを設定することにより、透過波面が図３の（ｂ）に示す断面形状となり、（ａ）と（ｂ）を合成した結果、残留する波面収差は（ｃ）まで低減される。その結果、ＮＡ_１＝０．６５のＨＤ用（４０５ｎｍ波長帯）に収差最小となるよう設計された対物レンズと第１の位相補正面１が形成された位相補正素子を一体で用いることにより、ＮＡ_２＝０．６５のＤＶＤ用（λ_２＝６５５ｎｍ波長帯）においても入射光は充分集光されるため、光ディスクの記録および再生が良好にできる。
【００２５】
次に、第１の位相補正面１により補正された図３（ｃ）に示す残留波面収差をさらに低減する本発明における第２の位相補正面２について以下に説明する。ガラスなどの透光性基板の開口数ＮＡ_２＝０．６５の領域に、常光屈折率ｎ_ｏおよび異常光屈折率ｎ_ｅの複屈折材料層である高分子液晶層を形成する。図１では第１の位相補正面が形成された透光性基板５の反対側面に形成した場合を示す。ここで、液晶モノマーの溶液を透光性基板上の配向処理の施された配向膜上に塗布し、液晶分子の配向ベクトル（分子配向軸）を基板と平行面内の特定方向に揃うように配向させた後、紫外線などの光を照射して重合硬化させ高分子液晶層とする。
【００２６】
次に、高分子液晶層を断面形状が図３の（ｃ）に相当した輪帯形状になるようフォトリソグラフィ法および反応性イオンエッチング法により図１の第２の位相補正面２に示す形状に加工する。このとき、図４（本発明における位相補正素子の第２の位相補正面または第３の位相補正面の波面収差の補正作用を示す波面収差の部分拡大図）の斜線部で示すように高分子液晶層の各凸部を多段の階段形状または１段の凸部形状としてもよい。
【００２７】
さらに、高分子液晶層の凹凸部の少なくとも凹部に常光屈折率ｎ_ｏとほぼ等しい屈折率ｎ_ｓの均質屈折率透明材料３が充填されている。ここで、高分子液晶層の厚さＤは波長λ_２の異常光偏光が高分子液晶層と均質屈折率透明材料を透過するときに発生する光路長差（ｎ_ｅ−ｎ_ｓ）×Ｄが図３の（ｃ）に示す波面収差を補正するように設定される。
【００２８】
この、第２の位相補正面２に、ＨＤ用の波長λ_１およびＣＤ用の波長λ_３の常光偏光が入射し、ＤＶＤ用の波長λ_２の異常光偏光が入射することにより、常光偏光のＨＤおよびＤＶＤでは透過波面収差は変化せず、異常光偏光のＤＶＤでは図３の（ｃ）に示す残留収差を補正する。その結果、第１の位相補正面１にさらに第２の位相補正面２が形成された位相補正素子を対物レンズと一体で用いることにより、ＮＡ_２＝０．６５のＤＶＤ用（λ_２＝６５５ｎｍ波長帯）において入射光の集光性がさらに向上し、安定した光ディスクのさらに良好な記録および再生ができる。
【００２９】
したがって、第２の位相補正面がさらに形成された位相補正素子を用い、ＨＤ用の波長λ_１とＤＶＤ用の波長λ_２との偏光方向の相違により、波長λ_２の入射光に対してのみ残留波面収差をさらに低減できるため、ＤＶＤの記録および再生において安定性がさらに向上する。
【００３０】
第１の位相補正面により補正された図３（ｃ）に示す残留波面収差をさらに低減する第２の位相補正面と同様の収差補正機能を有する本発明の第３の位相補正面について以下に説明する。
【００３１】
図１において、断面形状が図３の（ｃ）に相当した輪帯形状になるように加工された複屈折材料である高分子液晶層（２の斜線部）の代わりに、均質屈折率透明材料３と屈折率波長分散の異なる均質屈折率波長分散透明材料（２の斜線部）を用いている点が異なる。外見上、第３の位相補正面は第２の位相補正面２と同じである。ただし、均質屈折率波長分散透明材料はその凹凸部の少なくとも凹部に充填された均質屈折率透明材料３に対して、ＨＤ用の波長λ_１においては屈折率がほぼ一致しＤＶＤ用の波長においては屈折差Δｎを有する。
【００３２】
このような屈折率波長分散を有する材料として、例えば、３９０ｎｍ近傍以下の波長で光吸収端を有するＴｉＯ_２やＳｉＮなどを成分として含むＳｉＯ_２との混成膜を均質屈折率波長分散透明材料とし、ＴｉＯ_２やＳｉＮなどに比べ光吸収端が短波長域でかつ４０５ｎｍ近傍で上記混成膜と屈折率が等しい均質屈折率透明材料３を用いればよい。ここで、均質屈折率波長分散透明材料（２の斜線部）の厚さＤは、波長λ_２の光が均質屈折率波長分散透明材料と均質屈折率透明材料３を透過するときに発生する光路長差Δｎ×Ｄが図３の（ｃ）に示す波面収差を補正するように設定される。
【００３３】
この、第３の位相補正面にＨＤ用の波長λ_１の光が入射した場合は透過波面収差の変化はないが、ＤＶＤ用の波長λ_２の光が入射した場合は図３の（ｃ）に示す残留収差が補正される。その結果、第１の位相補正面１にさらに第３の位相補正面２が形成された位相補正素子を対物レンズと一体に用いることにより、入射光の偏光状態に関わらず、ＮＡ_２＝０．６５のＤＶＤ用（λ_２＝６５５ｎｍ波長帯）において入射光の集光性がさらに向上し、安定した光ディスクの記録および再生ができる。
【００３４】
さらに、図１において、波長λ_１の光に対して位相差がπ／２の奇数倍となる位相板４が透光性基板６の片面に形成され、充填剤３を用いて位相補正面が形成された透光性基板５に一体化されている。位相板４としては、複屈折性を有する材料であればいずれでもよい。例えば、高分子液晶、水晶などの光学結晶や、一軸延伸により複屈折性が発現するポリカーボネートなどでもよい。また、位相板４として互いの光軸角度と位相差が異なる位相板（後述の実施例にて説明）を積層することにより、波長λ_１および波長λ_２の光に対して１／４波長板として機能する位相板が得られる。
【００３５】
さらに、図１において、透光性基板６の片面の開口数ＮＡ_３＝０．５０の領域の外に、波長λ_１および波長λ_２の光を透過しかつ波長λ_３の光を反射する開口制限フィルタ７が成膜されている。なお、ＮＡ_３＝０．５０の領域には波長λ_１、波長λ_２および波長λ_３の光を透過するとともにＮＡ_１≒ＮＡ_２＝０．６５の領域で位相差が発生しない反射防止膜（図示せず）が形成されている。開口制限フィルタ７はＴｉＯ_２やＴａ_２Ｏ_５などの高屈折率透明膜とＳｉＯ_２やＭｇＦ_２などの低屈折率透明膜を波長程度の膜厚で交互に１０から２０層程度積層されている。
【００３６】
次に、このようにして得られた位相補正素子１０を搭載した本発明の光ヘッド装置の例を、図６を用いて説明する。半導体レーザ１４Ａから放射された波長λ_１＝４０５ｎｍの直線偏光が偏光ビームスプリッタ１９で反射され、合波プリズム１７を透過し、コリメートレンズ１３により平行光となり位相補正素子１０に入射する。さらに、位相補正素子１０内の位相板は波長λ_１に対して１／４波長板として作用するため、円偏光に変換されて図５（ａ）に示すように位相補正素子１０を直進透過し、開口数ＮＡ_１＝０．６５に相当する光束がＨＤ用に設計された対物レンズ１２によりＨＤ用の光ディスク１１の情報記録面へ集光される。
【００３７】
情報記録面で反射した信号光は元の経路を逆方向に進行して、位相補正素子１０内の位相板により偏光面が９０°回転した直線偏光に変換されて位相補正素子１０を直進透過し、合波プリズム１７および偏光ビームスプリッタ１９を透過して光検出器１５Ａの受光面へ集光され、電気信号に変換される。
【００３８】
また、半導体レーザ１４Ｂから放射された波長λ_２＝６５５ｎｍの直線偏光の光は、その一部がホログラムビームスプリッタ１６Ｂを透過し、合波プリズム１８を透過し、合波プリズム１７で反射された後、コリメートレンズ１３により集光され平行光となり位相補正素子１０に入射する。開口数ＮＡ_２＝０．６５に相当する光束は、位相補正素子１０により波長の相違に起因して発生する球面収差を補正するよう図５（ｂ）に示す透過波面に変換されるため、対物レンズ１２によりＤＶＤ用の光ディスク１１の情報記録面で充分小さなスポットに集光される。情報記録面で反射した信号光は元の経路を逆方向に進行して、一部がホログラムビームスプリッタ１６Ｂにより回折されて光検出器１５Ｂの受光面へ集光され、電気信号に変換される。
【００３９】
ここで、位相補正素子１０内の位相板を波長λ_２に対しても１／４波長板とし、ホログラムビームスプリッタ１６Ｂとして常光偏光を透過し異常光偏光を回折する偏光ホログラムビームスプリッタを用いることにより、光利用効率が向上するため特に光ディスクへの情報の記録に有利となる。
【００４０】
また、半導体レーザ１４Ｃから放射された波長λ_３＝７９０ｎｍの直線偏光の光は、その一部がホログラムビームスプリッタ１６Ｃを透過し、合波プリズム１８および合波プリズム１７で反射され、コリメートレンズ１３により集光されてわずかに発散した光として位相補正素子１０に入射する。発散した光の入射により、球面収差を補正する。位相補正素子内の開口制限フィルタにより開口数ＮＡ_３＝０．５０内の光束のみが直進透過し、図５（ｃ）に示すように透過波面は不変のまま、対物レンズ１２によりＣＤ用の光ディスク１１の情報記録面へ集光される。
【００４１】
ここで、光ディスクのカバー層および波長の相違に起因して発生する球面収差は、対物レンズ１２への入射光を発散光とすることにより補正されている。情報記録面で反射した信号光は元の経路を逆方向に進行して、一部がホログラムビームスプリッタ１６Ｃにより回折されて光検出器１５Ｃの受光面へ集光され、電気信号に変換される。
【００４２】
ここで、位相補正素子１０内の位相板を波長λ_３に対しても１／４波長板とすることにより、半導体レーザ１４Ｃに帰還する戻り光の偏光面は出射光と直交するため、レーザ発信を乱すことなく安定した記録および再生が実現する。
【００４３】
したがって、本発明における位相補正素子１０をＨＤ用の光ディスクに対して設計された対物レンズ１２と一体で光ヘッド装置に搭載することにより、ＤＶＤ用の光ディスクおよびＣＤ用の光ディスクの記録および再生に用いた場合に発生する波面収差を補正できる。このため、半導体レーザから出射した光を安定して光ディスクの情報記録面へ集光し、ＨＤ、ＤＶＤとＣＤの記録および再生が実現できる。
【００４４】
なお、上記の例ではＨＤ用の対物レンズとしてカバー厚０．６ｍｍのＨＤ用の光ディスクに対してＮＡ＝０．６５で用いたとき良好な収差となるよう設計された場合について説明したが、カバー厚０．１ｍｍのＨＤ用の光ディスクに対してＮＡ＝０．８５で最適設計された対物レンズに対しても同様に、本発明における位相補正素子を用いることによりＨＤ、ＤＶＤとＣＤの記録および再生が実現できる。
【００４５】
また、本発明におけるの位相補正素子１０を用いてＨＤおよびＤＶＤのみの記録または再生を実現する場合、図１に示す開口制限フィルタ７は不要である。さらに、図６に示す光ヘッド装置において、ＣＤ用の半導体レーザ１４Ｃ、光検出器１５Ｃ、ホログラムビームスプリッタ１６Ｃおよび合波プリズム１８を用いない構成とすればよい。
【００４６】
【実施例】
「例１」
本例の光ヘッド装置における位相補正素子１０の断面図を図１に、平面図を図２に示す。３波長λ_１＝４０５ｎｍ、λ_２＝６５５ｎｍ、λ_３＝７９０ｎｍに対するそれぞれの屈折率が、１．４７０、１．４５６、１．４５４である透光性基板５であるガラス基板を用意した。焦点距離３ｍｍの対物レンズを用いた場合に、このガラス基板の表面で波長λ_２の入射光束を規定する開口数ＮＡ_２＝０．６５の領域（直径３．９ｍｍ）を直接エッチングにより、断面形状が５レベル（４段）で等段差の階段の凹凸形状に加工され、かつ光軸に関して回転対称性を有する輪帯状の階段の凹凸形状を有する第１の位相補正面１を形成した。さらに、第１の位相補正面１が加工されたガラス基板表面に、３波長での反射率が１％以下の反射防止膜を形成した。
【００４７】
ここで、凹凸形状の１段の高さｄ_１を１．７２３μｍとし、空気との光路差が波長λ_１に対して２×λ_１となり、位相差４πに相当する。このとき、空気との光路差が波長λ_２に対しては１．２×λ_２すなわち０．２×λ_２相当となり、波長λ_３に対しては約λ_３となっている。したがって、第１の位相補正面１に入射する波長λ_１および波長λ_３の透過波面は変化しないが、波長λ_２の透過波面は階段の凹凸形状格子の輪帯分布に応じて変化する。
【００４８】
凹凸形状の各段の輪帯半径は次のように決定された。すなわち、使用波長λ_１＝４０５ｎｍでカバー厚０．６ｍｍのＨＤ用の光ディスクに対して良好な収差となるよう設計されたＮＡ_１＝０．６５のＨＤ用対物レンズを、使用波長λ_２＝６５５ｎｍでカバー厚０．６ｍｍのＤＶＤ用の光ディスクにＮＡ_２＝０．６５で用いたときに発生する図３の（ａ）に示す透過波面収差を補正するように決定された。具体的には、半径０．６１４ｍｍ領域を凹部基準面とすると、半径０．６１４ｍｍから０．８７８ｍｍ領域を高さｄ_１（１段）、半径０．８７８ｍｍから１．０９８ｍｍ領域を高さ２×ｄ_１（２段）、半径１．０９８ｍｍから１．３３４ｍｍ領域を高さ３×ｄ_１（３段）、半径１．３３４ｍｍから１．６７３ｍｍ領域を高さ４×ｄ_１（４段）、半径１．６７３ｍｍから１．７８９ｍｍ領域を高さ３×ｄ_１（３段）、半径１．７８９ｍｍから１．８５７ｍｍ領域を高さ２×ｄ_１（２段）、半径１．８５７ｍｍから１．９０８ｍｍ領域を高さｄ_１（１段）、半径１．９０８ｍｍから１．９５０ｍｍ領域を基準面と同じ高さに加工した。
【００４９】
このとき、第１の位相補正面を透過した波長λ_２の透過波面を図３の（ｂ）に、図３の（ａ）と（ｂ）を合成した結果残留する透過波面収差を（ｃ）に示す。
【００５０】
図３の（ａ）のＲＭＳ（Ｒｏｏｔ　Ｍｅａｎ　Ｓｑｕａｒｅ）波面収差は２４０ｍλと極めて大きな３次の球面収差であったが、（ｃ）では３次のＲＭＳ球面収差成分は３ｍλで高次成分も含めた全体のＲＭＳ波面収差も５４ｍλに減少し、波長λ_２でＤＶＤ用の光ディスクに適用した場合も回折限界の集光性能が得られた。
【００５１】
次に、第１の位相補正面１が形成されたガラス基板の反対側の面に、波長λ_２で常光屈折率ｎ_ｏ＝１．５４および異常光屈折率ｎ_ｅ＝１．６０の複屈折材料である高分子液晶層を形成した。そして、図３の（ｃ）に示す残留透過波面収差を低減するように高分子液晶層をエッチング加工し、凹部に常光屈折率ｎ_ｏとほぼ等しい屈折率ｎ_ｓ＝１．５４の均質屈折率透明材料３を充填して第２の位相補正面２とした。
【００５２】
具体的には、図４において各凸部が２レベル（１段）の矩形凹凸形状によって近似された形状で、かつ光軸に関して回転対称性を有する輪帯状の凹凸形状とし、第２の位相補正面２を透過する波長λ_２の異常光偏光の透過波面が図４の（ｃ）を近似的に補正するようにした。すなわち、波長λ_２の異常光偏光に対する凹凸形状の階段の屈折率差（ｎ_ｅ−ｎ_ｓ）は０．０６であるため、図３の（ｃ）に示す波面収差０．２λ_２を２レベル（１段）の高分子液晶層で補正する場合、高分子液晶層の高さを約１．１μｍとすればよい。また、各矩形凸部に相当する高分子液晶層の同心輪帯領域をｍｍ単位で最小半径と最大半径の組（　，）で表記すると、（０．４２９，０．６１４）、（０．７５６，０．８７８）、（０．９８９，１．０９８）、（１．２０７，１．３３４）、（１．６７３，１．７４０）、（１．７８９，１．８２６）、（１．８５７，１．８８４）、（１．９０８、１．９３１）となった。
【００５３】
したがって、波長λ_２の入射光を異常光偏光とすることにより透過波面は高分子液晶層の凹凸形状の輪帯分布に応じて変化し、所望の残留収差を補正できる。具体的には、図３の（ｃ）に示すＲＭＳ波面収差の５４ｍλが２７ｍλに半減し、情報の記録および再生の安定性が向上した。
【００５４】
一方、第２の位相補正面２に入射する波長λ_１および波長λ_３の光を常光偏光とすることにより透過波面は変化しないため、良好な透過波面収差レベルに保たれている。
【００５５】
また、波長λ_１および波長λ_２の透過率が９８％以上で波長λ_３の透過率が１０％以下の開口制限フィルタ７を、透光性基板６であるガラス基板の開口数ＮＡ＝０．５０の領域を除いた図２に示す領域に形成した。また、開口数ＮＡ_３＝０．５０の領域には、波長λ_１、λ_２およびλ_３の３波長で反射率が１％以下の反射防止膜を形成した。
【００５６】
さらに、透光性基板６であるガラス基板の片面に、第２の位相補正面２の高分子液晶層と同じ材料を用い、液晶分子の配向方向のそろった膜厚１．９μｍの第１層と膜厚３．８μｍの第２層とを積層した位相板４を形成した。ここで、第１層と第２層の高分子液晶層の分子の配向方向を、透光性基板５であるガラス基板の１側から見て第２の位相補正面２の高分子液晶の分子の配向方向に対して反時計回りに７２．５°および１６．５°とした。その結果、波長λ_１および波長λ_２に対して１／４波長板、波長λ_３に対しても１／４波長板に近い位相板の機能が得られた。
【００５７】
位相補正素子１０は、開口制限フィルタ７と位相板４とが形成された透光性基板６であるガラス基板と、第１の位相補正面１と第２の位相補正面２の形成された透光性基板５であるガラス基板を、均質屈折率透明充填材３を用いて接着一体化されている。
【００５８】
このようにして作製された位相補正素子１０と対物レンズ１２をアクチュエータに一体化固定し、図６に示す光ヘッド装置に搭載した。この光ヘッド装置をＨＤ用およびＤＶＤ用の光ディスクの記録および再生に用いると、対物レンズのみのときに発生したＤＶＤでの波面収差が補正された。さらに、ＣＤ用の光ディスクの記録および再生においても従来の有限系構成による収差補正法が適用できた。
【００５９】
また、ＨＤ用には偏光ビームスプリッタ１９を、ＤＶＤ用には偏光ホログラムビームスプリッタ１６Ｂを用いることにより光利用効率が大幅に向上した。その結果、安定してＨＤ用とＤＶＤ用とＣＤ用の光ディスクの記録および再生が実現した。
【００６０】
「例２」
光ヘッド装置における位相補正素子の他の例として、第２の位相補正面の代わりに本発明の第３の位相補正面を用いた構成について以下に記す。例１において、第２の位相補正面として用いられた高分子液晶層（図１において２の斜線部）の代わりに、ＳｉＮとＳｉＯ_２の体積比が４５：５５の混成膜としたＳｉＮＯの均質屈折率膜を用い、均質屈折率透明材料３である充填材として高屈折率透明樹脂を用いた。
【００６１】
波長λ_１、λ_２、λ_３におけるＳｉＮＯのそれぞれの屈折率は１．７４７、１．７２５、１．７２２、高屈折率透明樹脂の屈折率はそれぞれ１．７４７、１．７０４、１．７００で、ＳｉＮＯと高屈折率透明樹脂の屈折率差Δｎは波長λ_１では生じないが波長λ_２では０．０２１だけ生じる。
【００６２】
透光性基板５であるガラス基板の第１の位相補正面１が形成された面とは反対の面に塗布したフォトレジストをパターニングした後厚さ３．１μｍのＳｉＮＯ膜を成膜し、フォトレジストを剥離するリフトオフ加工により、第２の位相補正面と同様に各凸部が２レベル（１段）の矩形状で、かつ光軸に関して回転対称性を有する輪帯状の凹凸部を形成し、その凹部に高屈折率透明樹脂を充填して第３の位相補正面２とした。各矩形凸部に相当するＳｉＯＮ膜の同心輪帯領域の最小半径と最大半径および他の位相補正素子の構成は例１と同じである。
【００６３】
ＳｉＮＯ膜と高屈折率透明樹脂の光路差は０．１λ_２に相当し、ＤＶＤの図３の（ｃ）に示すＲＭＳ波面収差が５４ｍλから２７ｍλにまで半減し、光ヘッド装置に搭載した場合記録および再生の安定性が向上した。
【００６４】
第１の位相補正面と第３の位相補正面の収差補正機能は入射光の偏光状態に依存しないため透過波面は位相板４の性能にかかわらず一定であり、往路および復路においてＤＶＤで発生する波面収差を補正できる特徴がある。
【００６５】
一方、ＣＤ用の光ディスクにおいては第３の位相補正面を波長λ_３の光が透過するとき波面収差が発生するが、３次のＲＭＳ球面収差成分は１ｍλと充分小さな値で、高次成分も含めた全体のＲＭＳ波面収差も５０ｍλ程度であり、使用上問題とならなかった。
【００６６】
【発明の効果】
本発明における第１の位相補正面が形成された位相補正素子を、ＨＤ用の光ディスクにおいて収差が最小となるよう設計された対物レンズと一体で光ヘッド装置に搭載することにより、ＤＶＤ用の光ディスクにおいて発生する球面収差が低減されるため、ＨＤおよびＤＶＤに対して安定した情報の記録および再生ができる。
【００６７】
また、第２の位相補正面または第３の位相補正面がさらに形成された位相補正素子を用いることにより、残留波面収差をさらに低減できるため、ＤＶＤの記録および再生において安定性がさらに向上する。
【００６８】
また、第２の位相補正面は入射偏光方向を調整することにより、ＨＤ用およびＣＤ用の波長に対して収差劣化がない位相補正素子とできる。
【００６９】
また、第３の位相補正面は入射光の波長の相違にのみ依存した透過波面変化を及ぼし偏光状態には依存しないため、位相板を併用して往路と復路で位相補正素子入射偏光が異なる場合でも同じ透過波面性能とできる。
【００７０】
また、１／４波長板相当の位相板が一体化された位相補正素子を用い、偏光ビームスプリッタと併用することにより光学系の光利用効率が向上するため、半導体レーザ光源の消費電力の低減またはより高速な記録および再生ができる。さらに、半導体レーザ光源への戻り光に起因したレーザ発信強度変動が抑制され、光ディスクの記録および再生の安定性が向上する。
【００７１】
また、本発明における位相補正素子にさらに開口制限フィルタを一体に形成することにより、ＨＤおよびＤＶＤに加えてＣＤの光ディスクの記録および再生ができる光ヘッド装置となる。
【図面の簡単な説明】
【図１】本発明における位相補正素子の構造を示す断面図。
【図２】本発明における位相補正素子の構造を示す平面図で、（ａ）は一方の面から見た平面図、（ｂ）は他方の面から見た平面図。
【図３】ＤＶＤ用の光ディスクにおける透過光の波面収差を示すグラフで、（ａ）は本発明における位相補正素子を用いないときに発生する波面収差、（ｂ）は本発明における位相補正素子を用いたとき、本素子の第１の収差補正面が発生する補正用の波面収差、（ｃ）は本発明における位相補正素子を用いたとき、本素子の第１の収差補正面により補正された結果残留する波面収差。
【図４】本発明における位相補正素子の第２の位相補正面または第３の位相補正面の波面収差の補正作用を示す波面収差の部分拡大図。
【図５】本発明における位相補正素子へ３種の波長の光が入射したときの光束と波面を示す図で、（ａ）は波長λ_１の光が入射したときの断面図、（ｂ）は波長λ_２の光が入射したときの断面図、（ｃ）は波長λ_３の光が入射したときの断面図。
【図６】本発明における位相補正素子を搭載した光ヘッド装置を示す構成図。
【図７】従来の位相補正素子の構造および光束と波面を示す図で、（ａ）は波長λ_２の異常光偏光が入射したときの断面図、（ｂ）波長λ_１の常光偏光が入射したときの断面図。
【符号の説明】
１：第１の位相補正面
２：第２の位相補正面または第３の位相補正面
３：均質屈折率透明材料
４：位相板
５、６：透光性基板
７：開口制限フィルタ
１０：位相補正素子
１１：光ディスク
１２：対物レンズ
１３：コリメートレンズ
１４Ａ、１４Ｂ、１４Ｃ：半導体レーザ
１５Ａ、１５Ｂ、１５Ｃ：光検出器
１６Ｂ、１６Ｃ：ホログラムビームスプリッタ
１７、１８：合波プリズム
１９：偏光ビームスプリッタ
２０：偏光性位相補正素子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device, and more particularly to an optical head device used for recording or reproducing information on two or more types of optical recording media.
[0002]
[Prior art]
For recording and reproducing information on an optical recording medium for CD (hereinafter, optical recording medium is referred to as “optical disk”), a semiconductor laser having a wavelength of 790 nm band and NA (numerical aperture) of 0.45 to 0.00 are used as a light source. An objective lens up to 5 and an optical disk with a cover thickness of 1.2 mm for protecting the information recording surface are used. On the other hand, for recording and reproducing information on a DVD optical disk, a semiconductor laser having a wavelength of 655 nm, an objective lens having an NA of 0.6 to 0.65, and an optical disk having a cover thickness of 0.6 mm are used as a light source.
[0003]
Furthermore, in order to increase the amount of recorded information, a semiconductor laser having a wavelength of 405 nm as a light source and an objective lens having a NA of 0.65 and an optical disk having a cover thickness of 0.6 mm or an objective lens having a NA of 0.85 and a cover thickness are used. A 0.1 mm optical disk has been proposed. Hereinafter, an optical disk used in a semiconductor laser having a wavelength of 405 nm band is particularly referred to as an HD optical disk.
[0004]
Since the CD optical disc, DVD optical disc, and HD optical disc have different cover thicknesses or operating wavelengths, when using each interchangeably, an objective lens designed for one type of optical disc is used for another optical disc. There is a problem that large spherical aberration occurs and information cannot be recorded and reproduced.
[0005]
In order to reduce spherical aberration that occurs when recording and reproducing information on a DVD optical disk using an objective lens designed to minimize wavefront aberration with respect to the HD optical disk, the wavelength λ for HD is reduced. ₁ Direction of incident light and wavelength λ for DVD ₂ There has been proposed a polarizing phase correction element that uses the incident light with the polarization directions orthogonal to each other.
[0006]
A cross-sectional view of a configuration example of a conventional polarizing phase correction element is shown in FIG. Polarizing phase correction element 20 has ordinary refractive index n _o And extraordinary refractive index n _e (N _o ≠ n _e ), And when the birefringent material layer is an optical crystal, the main optical axis is aligned in one direction. When the birefringent material layer is a polymer material, the molecular orientation axis is aligned in one direction. A sawtooth uneven portion having a sawtooth shape in cross section and having rotational symmetry with respect to the optical axis of incident light is formed in a region of NA = 0.60 of the polarizing phase correction element, and at least the concave portion of the uneven portion is ordinary light. Refractive index n _o Refractive index n approximately equal to _s The homogeneous refractive index transparent material 3 is filled.
[0007]
Wavelength λ to optical disc for HD ₁ When the ordinary light polarized light is incident, the transmitted wavefront of the polarizing phase correction element does not change as shown in FIG. 7B, and the aberration performance of the objective lens is maintained. On the other hand, the wavelength λ ₂ When the extraordinary polarized light is incident, as shown in FIG. 7A, the transmitted wavefront corrects the spherical aberration caused by the difference in the cover thickness of the optical disc, and information can be recorded and reproduced on the DVD optical disc.
[0008]
In addition, a polarization beam splitter and a quarter wavelength plate are used, and linearly polarized light emitted from the semiconductor laser is transmitted through the polarization beam splitter in the forward path, reflected by the optical disk, and reciprocated through the quarter wavelength plate in the return path. Signal light detection with high light utilization efficiency can be performed by converting the light into linearly polarized light orthogonal to the outgoing linearly polarized light of the laser, reflecting the polarized beam splitter, and condensing it onto the photodetector.
[0009]
[Patent Document 1]
JP 2002-56560 A
[0010]
[Problems to be solved by the invention]
However, when a polarizing beam splitter and a quarter wave plate are used for a conventional polarizing phase correction element, the polarization state of incident light to the polarizing phase correction element differs between the forward path and the return path, and the wavelength λ ₁ And wavelength λ ₂ This is a problem because large spherical aberration occurs.
[0011]
In addition, since there is no three-wavelength phase correction element corresponding to three types of optical disks of HD, DVD and CD, information recording and reproduction of these three types of optical disks can be performed using a single objective lens. Was difficult.
[0012]
An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide an optical head device which is excellent in optical characteristics and suitable for reduction in size and weight.
[0013]
[Means for Solving the Problems]
The present invention has at least a wavelength λ ₁ And λ ₂ (Λ ₁ <Λ ₂ ), The objective lens for condensing the light emitted from the light source onto the optical recording medium, the photodetector for detecting the reflected light from the information recording surface of the optical recording medium, and the objective from the light source. Wavelength λ in the optical path to the lens ₂ And an optical head device for recording or reproducing information on an optical recording medium, the phase correction element having a wavelength λ in the plane ₂ Numerical aperture NA defined by the incident light flux ₂ In this region, a ring-shaped and stepped uneven portion having rotational symmetry with respect to the optical axis of the incident light is formed, and the wavelength λ at each step of the uneven portion is formed. ₁ There is provided an optical head device comprising a first phase correction surface having a concavo-convex portion so that a phase difference of transmitted light of 2πm (m is a natural number).
[0014]
The phase correction element has a wavelength λ in the plane of the phase correction element in addition to the first phase correction surface. ₂ Numerical aperture NA defined by the incident light flux ₂ The second phase correction surface is provided with a second phase correction surface in which the annular concavo-convex portion having rotational symmetry with respect to the optical axis of the incident light is formed on a surface different from the first phase correction surface. Is the ordinary refractive index n _o And extraordinary refractive index n _e (N _o ≠ n _e ) With an organic birefringent material layer having molecular orientation axes aligned in one direction. _o A refractive index n equal to _s There is provided the above optical head device filled with a transparent material having a uniform refractive index.
[0015]
The phase correction element has a wavelength λ in the plane of the phase correction element in addition to the first phase correction surface. ₂ Numerical aperture NA defined by the incident light flux ₂ The third phase correction surface is provided with a third phase correction surface in which a ring-shaped uneven portion having rotational symmetry with respect to the optical axis of incident light is formed on a surface different from the first phase correction surface. Is the wavelength λ ₁ At the same refractive index and wavelength λ ₂ The above optical head device is composed of two types of transparent materials having different refractive indexes, each of which has a refractive index different from each other, and at least a concave portion of the concave and convex portions of the transparent material with uniform refractive index is filled with the transparent material with the other uniform refractive index. provide.
[0016]
Further, the phase correction element has a wavelength λ ₁ The above optical head device is provided in which a phase plate whose phase difference with respect to is an odd multiple of π / 2 is further added and integrated.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a cross-sectional view of an example of the phase correction element 10 in the optical head device of the present invention, and FIG. 2 shows a plan view thereof. In FIG. 1, the first phase correction surface 1, the second phase correction surface 2, the phase plate 4, and the aperture limiting filter 7 on the translucent substrate 6 are formed on the translucent substrate 5.
[0018]
Working wavelength λ ₁ = NA designed to provide good aberration for HD optical disc with 405 nm and cover thickness of 0.6 mm ₁ = 0.65 HD objective lens, use wavelength λ ₂ = NA for DVD optical disk with 655 nm and cover thickness of 0.6 mm ₂ FIG. 3A shows an example of wavefront aberration that occurs when used at = 0.65. Here, spherical aberration due to refractive index wavelength dispersion of the objective lens and the optical disk is shown with the difference in the used wavelength, the horizontal axis is the numerical aperture NA corresponding to the aperture diameter, and the vertical axis is a ray on the optical axis (NA = 0) represents a cross section of the phase difference of the light beam at each NA value. Actually, it has a substantially axisymmetric three-dimensional shape distribution. If such large spherical aberration remains, light is not sufficiently collected by the objective lens, and recording and reproduction of the optical disk cannot be performed.
[0019]
In addition, as a method for correcting such spherical aberration, there is a so-called finite optical system configuration in which the incident light to the objective lens is divergent light, but the objective lens is moved in a plane orthogonal to the optical axis of the objective lens. On the other hand, since the generated aberration is large, it is a practical problem in performing stable tracking control.
[0020]
The first phase correction surface 1 of the present invention for correcting the wavefront aberration shown in FIG. 3A in a so-called infinite optical system configuration in which the incident light to the objective lens is parallel light will be described below.
[0021]
Wavelength λ of translucent substrate 5 such as glass ₂ = NA for 655nm ₂ A stepped concavo-convex portion having an annular shape having rotational symmetry with respect to the optical axis is formed in the region of 0.65. Here, the uneven step d between the adjacent annular zones is constant, and the wavelength λ between the transmitted light of the homogeneous material having the refractive index n and the transmitted light of air at each step. ₁ Phase difference 2π × (n−1) × d / λ ₁ Is 2πm (m is a natural number) and m = 2, that is, 4π. λ ₁ = 405 nm and λ ₃ = 790 nm, considering the wavelength dispersion of the refractive index n of the homogeneous material, the wavelength λ ₁ If the phase difference is 4π with respect to the wavelength λ ₃ Is approximately 2π. That is, the wavelength λ is formed in the ring-shaped and step-like uneven portion including such uneven portion step d. ₁ Or wavelength λ ₃ The transmitted wavefront does not change even if the light of the same is transmitted.
[0022]
On the other hand, the wavelength λ transmitted through the uneven step d ₂ Phase difference 2π × (n−1) × d / λ ₂ A phase lag occurs. A specific example will be described with reference to FIG. Wavelength λ transmitted through the step d ₂ When the refractive index wavelength dispersion of the convex material is taken into consideration with respect to air, a phase delay of the transmitted wave front corresponding to a phase difference of 1.20 × 2π occurs. That is, the difference from the phase difference 2π for one wavelength per ring-shaped uneven portion step d is a transmitted wavefront phase delay of phase difference 0.20 × 2π.
[0023]
Where HD wavelength λ ₁ The uneven step d where the transmitted wavefront of the transmitted light does not change has a phase difference of 2π × (n−1) × d / λ. ₁ = 2π × m (m is a natural number), but the wavelength λ ₂ The phase correction effect on the transmitted light is 2π × (n−1) × d / λ. ₂ And the smaller the phase difference 2π for one wavelength is, the smaller the spherical aberration shown in FIG. Also, the smaller the value of m, the wavelength λ ₁ Or wavelength λ ₂ The transmission wavefront variation with respect to the wavelength variation of about ± 15 nm is small, and it is preferable because it is easy to process a step with high accuracy. As a result, when the phase difference of the DVD wavelength is 0.60 × m × 2π, m = 2, that is, the wavelength λ ₁ By setting the step d so that the phase difference is 4π, the DVD wavelength λ ₂ It becomes an optimal phase correction surface. If m is larger than 4, it is difficult to perform step processing, which is not preferable.
[0024]
Accordingly, by setting the ring zone radius of the concavo-convex portion and the height of the convex portion so as to correct the wavefront aberration shown in FIG. 3A, the transmitted wavefront becomes the cross-sectional shape shown in FIG. As a result of combining a) and (b), the remaining wavefront aberration is reduced to (c). As a result, NA ₁ = 0.65 for HD (405 nm wavelength band), the objective lens designed to minimize the aberration and the phase correction element on which the first phase correction surface 1 is formed are used as a unit. ₂ = 0.65 for DVD (λ ₂ (= 655 nm wavelength band), since the incident light is sufficiently condensed, recording and reproduction of the optical disk can be performed satisfactorily.
[0025]
Next, the second phase correction surface 2 in the present invention for further reducing the residual wavefront aberration shown in FIG. 3C corrected by the first phase correction surface 1 will be described below. Numerical aperture NA of translucent substrates such as glass ₂ = Normal light refractive index n in the region of 0.65 _o And extraordinary refractive index n _e A polymer liquid crystal layer which is a birefringent material layer is formed. In FIG. 1, the case where it forms in the opposite side surface of the translucent board | substrate 5 in which the 1st phase correction surface was formed is shown. Here, the solution of the liquid crystal monomer is applied onto the alignment film subjected to the alignment treatment on the translucent substrate, and the alignment vector (molecular alignment axis) of the liquid crystal molecules is aligned in a specific direction in a plane parallel to the substrate. After the alignment, light such as ultraviolet rays is irradiated and polymerized to form a polymer liquid crystal layer.
[0026]
Next, the polymer liquid crystal layer is formed into the shape shown in the second phase correction surface 2 of FIG. 1 by photolithography and reactive ion etching so that the cross-sectional shape of the polymer liquid crystal layer becomes an annular shape corresponding to FIG. Process. At this time, as shown by the hatched portion in FIG. 4 (partially enlarged view of the wavefront aberration showing the correction effect of the wavefront aberration of the second phase correction surface or the third phase correction surface of the phase correction element in the present invention). Each convex portion of the liquid crystal layer may have a multi-step staircase shape or a single-step convex portion shape.
[0027]
Further, at least the concave portion of the concavo-convex portion of the polymer liquid crystal layer has an ordinary refractive index n. _o Refractive index n approximately equal to _s The homogeneous refractive index transparent material 3 is filled. Here, the thickness D of the polymer liquid crystal layer has a wavelength λ ₂ Difference in optical path length (n) generated when the extraordinary light polarization passes through the polymer liquid crystal layer and the homogeneous refractive index transparent material. _e -N _s ) × D is set so as to correct the wavefront aberration shown in FIG.
[0028]
The second phase correction surface 2 has a wavelength λ for HD. ₁ And λ for CD ₃ Of ordinary light polarized and the wavelength λ for DVD ₂ When the extraordinary light polarized light is incident, the transmitted wavefront aberration does not change in the ordinary light polarized HD and DVD, and the residual aberration shown in FIG. 3C is corrected in the extraordinary light polarized DVD. As a result, by using the phase correction element in which the second phase correction surface 2 is further formed on the first phase correction surface 1 and the objective lens, the NA is obtained. ₂ = 0.65 for DVD (λ ₂ = 655 nm wavelength band), the condensing property of incident light is further improved, and stable recording and reproduction of an optical disc can be performed.
[0029]
Therefore, using a phase correction element further formed with a second phase correction surface, the wavelength λ for HD ₁ And wavelength for DVD ₂ The wavelength λ ₂ Since the residual wavefront aberration can be further reduced only with respect to the incident light, stability in DVD recording and reproduction is further improved.
[0030]
The third phase correction surface of the present invention having the same aberration correction function as the second phase correction surface that further reduces the residual wavefront aberration shown in FIG. 3C corrected by the first phase correction surface will be described below. explain.
[0031]
In FIG. 1, a transparent material having a uniform refractive index is used instead of the polymer liquid crystal layer (the hatched portion in FIG. 2) which is a birefringent material processed so that the cross-sectional shape becomes a ring-shaped shape corresponding to FIG. 3 is different from that in FIG. 3 in that a homogeneous refractive index wavelength-dispersed transparent material (hatched portion 2) having a different refractive index wavelength dispersion is used. In appearance, the third phase correction surface is the same as the second phase correction surface 2. However, the homogeneous refractive index wavelength-dispersed transparent material is different from the homogeneous refractive index transparent material 3 filled in at least the concave portions of the concave and convex portions with respect to the wavelength λ for HD. ₁ In FIG. 4, the refractive indexes are almost the same, and the wavelength for DVD has a refractive difference Δn.
[0032]
As a material having such refractive index wavelength dispersion, for example, TiO having a light absorption edge at a wavelength of about 390 nm or less. ₂ SiO containing SiN and SiN as components ₂ TiO2 with a homogeneous refractive index wavelength dispersion transparent material ₂ A homogeneous refractive index transparent material 3 having a light absorption edge shorter than that of SiN or the like and having a refractive index equal to that of the above-mentioned mixed film in the vicinity of 405 nm may be used. Here, the thickness D of the homogeneous refractive index wavelength-dispersed transparent material (shaded portion of 2) has a wavelength λ ₂ Is set so as to correct the wavefront aberration shown in FIG. 3C. The optical path length difference Δn × D generated when the light passes through the homogeneous refractive index wavelength dispersion transparent material and the homogeneous refractive index transparent material 3.
[0033]
This third phase correction surface has a wavelength λ for HD ₁ Is not changed, but the wavelength λ for DVD is not changed. ₂ 3 is corrected, the residual aberration shown in FIG. 3C is corrected. As a result, by using the phase correction element in which the third phase correction surface 2 is further formed on the first phase correction surface 1 and the objective lens, the NA can be obtained regardless of the polarization state of the incident light. ₂ = 0.65 for DVD (λ ₂ = 655 nm wavelength band), the condensing property of incident light is further improved, and stable recording and reproduction of an optical disk can be performed.
[0034]
Further, in FIG. 1, the wavelength λ ₁ A phase plate 4 whose phase difference is an odd multiple of π / 2 is formed on one side of the translucent substrate 6, and the translucent substrate 5 on which the phase correction surface is formed using the filler 3. It is integrated. As the phase plate 4, any material having birefringence may be used. For example, it may be an optical crystal such as polymer liquid crystal or quartz, or polycarbonate that exhibits birefringence by uniaxial stretching. Further, by laminating phase plates (explained in the examples described later) having different optical axis angles and phase differences as the phase plate 4, the wavelength λ ₁ And wavelength λ ₂ A phase plate that functions as a quarter-wave plate can be obtained with respect to the light.
[0035]
Further, in FIG. 1, the numerical aperture NA on one side of the translucent substrate 6. ₃ = Λ outside the region of 0.50, wavelength λ ₁ And wavelength λ ₂ Of light and wavelength λ ₃ An aperture limiting filter 7 that reflects the light is formed. NA ₃ = 0.50 is the wavelength λ ₁ , Wavelength λ ₂ And wavelength λ ₃ Of light and NA ₁ ≒ NA ₂ An antireflection film (not shown) that does not generate a phase difference in the region of 0.65 is formed. The aperture limiting filter 7 is TiO ₂ And Ta ₂ O ₅ High refractive index transparent film such as SiO and SiO ₂ And MgF ₂ About 10 to 20 layers of low refractive index transparent films are alternately laminated with a film thickness of about a wavelength.
[0036]
Next, an example of the optical head device of the present invention on which the phase correction element 10 obtained in this way is mounted will be described with reference to FIG. Wavelength λ emitted from the semiconductor laser 14A ₁ = 405 nm linearly polarized light is reflected by the polarization beam splitter 19, passes through the combining prism 17, becomes parallel light by the collimating lens 13, and enters the phase correction element 10. Further, the phase plate in the phase correction element 10 has a wavelength λ. ₁ 5 is converted into circularly polarized light and transmitted straight through the phase correction element 10 as shown in FIG. ₁ = 0.65 is converged on the information recording surface of the HD optical disk 11 by the objective lens 12 designed for HD.
[0037]
The signal light reflected from the information recording surface travels in the reverse direction along the original path, is converted into linearly polarized light whose polarization plane is rotated by 90 ° by the phase plate in the phase correction element 10, and travels straight through the phase correction element 10. Then, the light passes through the multiplexing prism 17 and the polarization beam splitter 19 and is condensed on the light receiving surface of the photodetector 15A and converted into an electric signal.
[0038]
Further, the wavelength λ emitted from the semiconductor laser 14B ₂ = 655 nm linearly polarized light is partially transmitted through the hologram beam splitter 16B, transmitted through the combining prism 18, reflected by the combining prism 17, and then condensed by the collimating lens 13 to become parallel light. The light enters the correction element 10. NA ₂ = 0.65 is converted into the transmitted wavefront shown in FIG. 5B so that the phase correction element 10 corrects the spherical aberration caused by the difference in wavelength, and the objective lens 12 causes the DVD to The light is focused on a sufficiently small spot on the information recording surface of the optical disk 11 for use. The signal light reflected on the information recording surface travels in the reverse direction along the original path, and a part thereof is diffracted by the hologram beam splitter 16B, collected on the light receiving surface of the photodetector 15B, and converted into an electric signal.
[0039]
Here, the phase plate in the phase correction element 10 is changed to the wavelength λ. ₂ In contrast, the use of a polarizing hologram beam splitter that transmits ordinary light polarization and diffracts extraordinary light polarization as the hologram beam splitter 16B improves the light utilization efficiency. Is advantageous.
[0040]
Further, the wavelength λ emitted from the semiconductor laser 14C ₃ = Part of the linearly polarized light of 790 nm passes through the hologram beam splitter 16C, is reflected by the combining prism 18 and the combining prism 17, is condensed by the collimating lens 13, and is phase-corrected as slightly diverging light. Incident on the element 10. Spherical aberration is corrected by the incidence of divergent light. A numerical aperture NA by an aperture limiting filter in the phase correction element ₃ = 0.50, only the light beam is transmitted in a straight line, and is condensed on the information recording surface of the optical disk 11 for CD by the objective lens 12 with the transmitted wavefront unchanged as shown in FIG.
[0041]
Here, the spherical aberration caused by the difference between the cover layer and the wavelength of the optical disc is corrected by making the incident light to the objective lens 12 into divergent light. The signal light reflected on the information recording surface travels in the reverse direction along the original path, and a part thereof is diffracted by the hologram beam splitter 16C, collected on the light receiving surface of the photodetector 15C, and converted into an electric signal.
[0042]
Here, the phase plate in the phase correction element 10 is changed to the wavelength λ. ₃ On the other hand, by using the quarter wavelength plate, the polarization plane of the return light returning to the semiconductor laser 14C is orthogonal to the outgoing light, so that stable recording and reproduction can be realized without disturbing the laser transmission.
[0043]
Therefore, by mounting the phase correction element 10 according to the present invention on the optical head device integrally with the objective lens 12 designed for the HD optical disk, it can be used for recording and reproduction of the optical disk for DVD and the optical disk for CD. The wavefront aberration that occurs in the event of an error can be corrected. For this reason, the light emitted from the semiconductor laser can be stably collected on the information recording surface of the optical disc, and recording and reproduction of HD, DVD and CD can be realized.
[0044]
In the above example, the case where the objective lens for HD is designed to have good aberration when used at NA = 0.65 with respect to the optical disc for HD having a cover thickness of 0.6 mm is explained. Similarly, for the objective lens optimally designed with NA = 0.85 for the HD optical disk having a thickness of 0.1 mm, the recording and reproduction of HD, DVD and CD can be performed by using the phase correction element of the present invention Can be realized.
[0045]
In addition, when recording or reproducing only HD and DVD using the phase correction element 10 of the present invention, the aperture limiting filter 7 shown in FIG. 1 is unnecessary. Further, the optical head device shown in FIG. 6 may be configured such that the semiconductor laser 14C for CD, the photodetector 15C, the hologram beam splitter 16C, and the multiplexing prism 18 are not used.
[0046]
【Example】
"Example 1"
A sectional view of the phase correction element 10 in the optical head device of this example is shown in FIG. 1, and a plan view thereof is shown in FIG. 3 wavelengths λ ₁ = 405 nm, λ ₂ = 655 nm, λ ₃ = The glass substrate which is the translucent board | substrate 5 whose each refractive index with respect to 790nm is 1.470, 1.456, 1.454 was prepared. When an objective lens with a focal length of 3 mm is used, the wavelength λ ₂ Numerical aperture NA that defines the incident light flux ₂ = 0.65 area (diameter 3.9 mm) is directly etched to form a stepped concavo-convex shape with 5 steps (4 steps) in cross-section, and an annular shape having rotational symmetry with respect to the optical axis A first phase correction surface 1 having a stepped concavo-convex shape was formed. Further, an antireflection film having a reflectance at 3 wavelengths of 1% or less was formed on the glass substrate surface on which the first phase correction surface 1 was processed.
[0047]
Here, the height d of one step of the concavo-convex shape ₁ Is 1.723 μm, and the optical path difference from the air is the wavelength λ ₁ 2 × λ ₁ This corresponds to a phase difference of 4π. At this time, the optical path difference from the air is the wavelength λ ₂ Is 1.2 × λ ₂ That is, 0.2 × λ ₂ Equivalent to wavelength λ ₃ Is about λ ₃ It has become. Therefore, the wavelength λ incident on the first phase correction surface 1 ₁ And wavelength λ ₃ The transmitted wavefront of the light does not change, but the wavelength λ ₂ The transmitted wavefront changes depending on the annular distribution of the concavo-convex shaped grid of stairs.
[0048]
The annular radius of each step of the uneven shape was determined as follows. That is, use wavelength λ ₁ = NA designed to have good aberration for HD optical disc with 405 nm and cover thickness of 0.6 mm ₁ = 0.65 HD objective lens, use wavelength λ ₂ = NA for DVD optical disk with 655 nm and cover thickness of 0.6 mm ₂ = 0.65 was determined so as to correct the transmitted wavefront aberration shown in FIG. Specifically, if a region having a radius of 0.614 mm is defined as a concave reference surface, a region having a radius of 0.614 mm to 0.878 mm is defined as a height d. ₁ (1 step), radius 0.878mm to 1.098mm area 2 × d ₁ (2 steps), radius 1.098mm to 1.334mm area 3 × d ₁ (3 steps), radius 1.334mm to 1.673mm area 4 × d height ₁ (4 steps), radius 1.673mm to 1.789mm area 3xd ₁ (3 steps), radius 1.789mm to 1.857mm area height 2xd ₁ (2 steps), radius d from 1.857mm to 1.908mm height d ₁ (Step 1) A radius 1.908 mm to 1.950 mm region was processed to the same height as the reference surface.
[0049]
At this time, the wavelength λ transmitted through the first phase correction surface ₂ FIG. 3B shows the transmitted wavefront of FIG. 3, and FIG. 3C shows the transmitted wavefront aberration that remains as a result of combining FIGS. 3A and 3B.
[0050]
The RMS (Root Mean Square) wavefront aberration in FIG. 3A is a very large third-order spherical aberration of 240 mλ, but in FIG. 3C, the third-order RMS spherical aberration component is 3 mλ and includes higher-order components. The overall RMS wavefront aberration is also reduced to 54 mλ, and the wavelength λ ₂ Thus, when applied to an optical disc for DVD, a diffraction limited light condensing performance was obtained.
[0051]
Next, the wavelength λ is applied to the surface on the opposite side of the glass substrate on which the first phase correction surface 1 is formed. ₂ Ordinary refractive index n _o = 1.54 and extraordinary light refractive index n _e = 1.60 A polymer liquid crystal layer which is a birefringent material was formed. Then, the polymer liquid crystal layer is etched so as to reduce the residual transmitted wavefront aberration shown in FIG. _o Refractive index n approximately equal to _s = 1.54 homogeneous refractive index transparent material 3 was filled to form the second phase correction surface 2.
[0052]
Specifically, in FIG. 4, each convex portion has a shape approximated by a two-level (one step) rectangular uneven shape, and has a ring-shaped uneven shape having rotational symmetry with respect to the optical axis, and the second phase correction. Wavelength λ transmitted through surface 2 ₂ The transmitted wavefront of the extraordinary light polarization is approximately corrected as shown in FIG. That is, the wavelength λ ₂ Difference in refractive index (n _e -N _s ) Is 0.06, so the wavefront aberration of 0.2λ shown in FIG. ₂ Is corrected with a two-level (one-stage) polymer liquid crystal layer, the height of the polymer liquid crystal layer may be about 1.1 μm. Further, when the concentric annular zone region of the polymer liquid crystal layer corresponding to each rectangular convex portion is expressed by a pair (,) of the minimum radius and the maximum radius in mm units, (0.429, 0.614), (0.756). , 0.878), (0.989, 1.098), (1.207, 1.334), (1.673, 1.740), (1.789, 1.826), (1.857). , 1.884) and (1.908, 1.931).
[0053]
Therefore, the wavelength λ ₂ By making the incident light into an extraordinary light polarized light, the transmitted wavefront changes in accordance with the uneven distribution of the polymer liquid crystal layer, and the desired residual aberration can be corrected. Specifically, the RMS wavefront aberration of 54 mλ shown in (c) of FIG. 3 is halved to 27 mλ, and the stability of information recording and reproduction is improved.
[0054]
On the other hand, the wavelength λ incident on the second phase correction surface 2 ₁ And wavelength λ ₃ Since the transmitted wavefront does not change by making the light of the ordinary light polarized, the transmitted wavefront aberration level is kept good.
[0055]
Also, wavelength λ ₁ And wavelength λ ₂ The wavelength λ is greater than 98% ₃ The aperture limiting filter 7 having a transmittance of 10% or less was formed in the region shown in FIG. 2 excluding the region of the numerical aperture NA = 0.50 of the glass substrate which is the translucent substrate 6. The numerical aperture NA ₃ = 0.50, the wavelength λ ₁ , Λ ₂ And λ ₃ An antireflection film having a reflectance of 1% or less at these three wavelengths was formed.
[0056]
Further, the same material as that of the polymer liquid crystal layer of the second phase correction surface 2 is used on one side of the glass substrate which is the translucent substrate 6, and the first layer having a thickness of 1.9 μm aligned in the alignment direction of the liquid crystal molecules. And a phase plate 4 in which a second layer having a thickness of 3.8 μm is laminated. Here, the molecules of the polymer liquid crystal layers of the second phase correction surface 2 are viewed from the side of the glass substrate which is the translucent substrate 5 with respect to the orientation direction of the molecules of the first and second polymer liquid crystal layers. The angle was 72.5 ° and 16.5 ° counterclockwise with respect to the orientation direction. As a result, the wavelength λ ₁ And wavelength λ ₂ 1/4 wavelength plate, wavelength λ ₃ In contrast, the function of a phase plate close to a quarter-wave plate was obtained.
[0057]
The phase correction element 10 includes a glass substrate, which is a translucent substrate 6 on which the aperture limiting filter 7 and the phase plate 4 are formed, and a transmission layer on which the first phase correction surface 1 and the second phase correction surface 2 are formed. The glass substrate which is the optical substrate 5 is bonded and integrated using the homogeneous refractive index transparent filler 3.
[0058]
The phase correction element 10 and the objective lens 12 manufactured in this way were integrally fixed to an actuator and mounted on the optical head device shown in FIG. When this optical head device was used for recording and reproduction of optical disks for HD and DVD, wavefront aberration in DVD that occurred only with the objective lens was corrected. Furthermore, the conventional aberration correction method using a finite system configuration can also be applied to recording and reproduction of an optical disk for CD.
[0059]
Further, the use efficiency of light is greatly improved by using the polarization beam splitter 19 for HD and the polarization hologram beam splitter 16B for DVD. As a result, recording and reproduction of optical disks for HD, DVD, and CD were realized stably.
[0060]
"Example 2"
As another example of the phase correction element in the optical head device, a configuration using the third phase correction surface of the present invention instead of the second phase correction surface will be described below. In Example 1, instead of the polymer liquid crystal layer (the hatched portion in FIG. 1) used as the second phase correction surface, SiN and SiO ₂ A uniform refractive index film of SiNO having a mixed film ratio of 45:55 was used, and a high refractive index transparent resin was used as the filler that is the homogeneous refractive index transparent material 3.
[0061]
Wavelength λ ₁ , Λ ₂ , Λ ₃ Each of SiNO has a refractive index of 1.747, 1.725, 1.722, and a high refractive index transparent resin has a refractive index of 1.747, 1.704, 1.700, respectively, and SiNO and a high refractive index transparent resin. Is the wavelength λ ₁ Does not occur, but wavelength λ ₂ Then, only 0.021 occurs.
[0062]
After patterning the photoresist applied on the surface opposite to the surface on which the first phase correction surface 1 of the glass substrate which is the translucent substrate 5 is formed, a SiNO film having a thickness of 3.1 μm is formed. By lift-off processing to peel off the resist, each convex part is a two-level (one step) rectangular shape like the second phase correction surface, and an annular concavo-convex part having rotational symmetry with respect to the optical axis is formed. The concave portion was filled with a high refractive index transparent resin to form a third phase correction surface 2. The minimum and maximum radii of the concentric annular zone region of the SiON film corresponding to each rectangular protrusion and the configuration of the other phase correction elements are the same as in Example 1.
[0063]
The optical path difference between the SiNO film and the high refractive index transparent resin is 0.1λ. ₂ The RMS wavefront aberration shown in FIG. 3C of the DVD was halved from 54 mλ to 27 mλ, and the stability of recording and reproduction was improved when mounted on an optical head device.
[0064]
Since the aberration correction function of the first phase correction surface and the third phase correction surface does not depend on the polarization state of the incident light, the transmitted wavefront is constant regardless of the performance of the phase plate 4, and is generated in the DVD in the forward path and the return path. It has the feature that wavefront aberration can be corrected.
[0065]
On the other hand, in the optical disk for CD, the third phase correction surface has a wavelength λ. ₃ Wavefront aberration is generated when the light of the above is transmitted, but the third-order RMS spherical aberration component is a sufficiently small value of 1 mλ, and the total RMS wavefront aberration including the higher-order component is also about 50 mλ, which is a problem in use. There wasn't.
[0066]
【The invention's effect】
By mounting the phase correction element having the first phase correction surface according to the present invention on an optical head device integrally with an objective lens designed to minimize aberration in an HD optical disk, an optical disk for DVD Since the spherical aberration generated in is reduced, stable information recording and reproduction can be performed for HD and DVD.
[0067]
Further, since the residual wavefront aberration can be further reduced by using the phase correction element in which the second phase correction surface or the third phase correction surface is further formed, the stability in DVD recording and reproduction is further improved.
[0068]
In addition, the second phase correction surface can be a phase correction element having no aberration deterioration with respect to the wavelengths for HD and CD by adjusting the incident polarization direction.
[0069]
In addition, since the third phase correction surface changes the transmitted wavefront depending only on the difference in the wavelength of the incident light and does not depend on the polarization state, the phase correction element incident polarization is different between the forward path and the return path using a phase plate. However, the same transmission wavefront performance can be achieved.
[0070]
In addition, the use of a phase correction element in which a phase plate equivalent to a quarter-wave plate is integrated and used in combination with a polarization beam splitter improves the light utilization efficiency of the optical system, thereby reducing the power consumption of the semiconductor laser light source or Higher speed recording and playback are possible. Further, fluctuations in the intensity of the laser transmission due to the return light to the semiconductor laser light source are suppressed, and the stability of recording and reproduction of the optical disk is improved.
[0071]
Further, by forming the aperture limiting filter integrally with the phase correction element in the present invention, an optical head device capable of recording and reproducing an optical disk of CD in addition to HD and DVD is obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing the structure of a phase correction element in the present invention.
2A and 2B are plan views showing the structure of a phase correction element in the present invention, wherein FIG. 2A is a plan view seen from one surface, and FIG. 2B is a plan view seen from the other surface;
FIGS. 3A and 3B are graphs showing wavefront aberrations of transmitted light in an optical disk for DVD, where FIG. 3A shows wavefront aberrations that occur when the phase correction element of the present invention is not used, and FIG. 3B shows the phase correction element of the present invention. When used, the wavefront aberration for correction generated by the first aberration correction surface of this element, (c) was corrected by the first aberration correction surface of this element when the phase correction element of the present invention was used. Resulting residual wavefront aberration.
FIG. 4 is a partially enlarged view of wavefront aberration showing a correction effect of wavefront aberration of the second phase correction surface or the third phase correction surface of the phase correction element in the present invention.
5A and 5B are diagrams showing light fluxes and wavefronts when light of three wavelengths enters the phase correction element in the present invention, and FIG. ₁ (B) is a wavelength λ ₂ (C) is a wavelength λ ₃ Sectional view when the light of.
FIG. 6 is a configuration diagram showing an optical head device equipped with a phase correction element according to the present invention.
7A and 7B are diagrams showing a structure of a conventional phase correction element, a light beam, and a wavefront, where FIG. ₂ Sectional view when the extraordinary light polarized light is incident, (b) wavelength λ ₁ Sectional drawing when the ordinary light polarized light enters.
[Explanation of symbols]
1: First phase correction surface
2: Second phase correction surface or third phase correction surface
3: Transparent material with uniform refractive index
4: Phase plate
5, 6: Translucent substrate
7: Aperture limit filter
10: Phase correction element
11: Optical disc
12: Objective lens
13: Collimating lens
14A, 14B, 14C: Semiconductor laser
15A, 15B, 15C: photodetector
16B, 16C: Hologram beam splitter
17, 18: multiplexing prism
19: Polarizing beam splitter
20: Polarization phase correction element

Claims

A light source that emits at least two light beams having wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), an objective lens that condenses the light emitted from the light source on the optical recording medium, and an information recording surface of the optical recording medium. Light that records or reproduces information on an optical recording medium, comprising: a photodetector that detects reflected light; and a phase correction element that changes the transmitted wavefront of incident light having a wavelength λ _{2 in} the optical path from the light source to the objective lens. In the head device,
The phase correction element has a ring-shaped and stepped concavo-convex portion having rotational symmetry with respect to the optical axis of incident light in a numerical aperture NA ₂ region defined by an incident light beam having a wavelength λ ₂ in the plane, An optical head device comprising: a first phase correction surface having a concavo-convex portion formed so that a phase difference of transmitted light having a wavelength λ _{1 at} each step of the concavo-convex portion is 2πm (m is a natural number). .

In addition to the first phase correction surface, the phase correction element is rotationally symmetric with respect to the optical axis of the incident light in the area of the numerical aperture NA ₂ defined by the incident light beam having the wavelength λ ₂ in the plane of the phase correction element. a second phase correction surface formed on a surface different from the uneven portion of the annular first phase correction surface having a second phase correction surface is the ordinary refractive index n _o and extraordinary index n _e A homogeneous refractive index transparent material having a refractive index n _s equal to the ordinary refractive index n _{o in} at least the concave portion of the concavo-convex portion, comprising an organic birefringent material layer having molecular orientation axes aligned in one direction (n _o ≠ n _e ). The optical head device according to claim 1, wherein

In addition to the first phase correction surface, the phase correction element is rotationally symmetric with respect to the optical axis of the incident light in the area of the numerical aperture NA ₂ defined by the incident light beam having the wavelength λ ₂ in the plane of the phase correction element. A ring-shaped uneven portion having a third phase correction surface formed on a surface different from the first phase correction surface, and the third phase correction surface has the same refractive index at the wavelength λ ₁ and the wavelength λ _2. 2. The material according to claim 1, comprising two kinds of transparent materials having different refractive indexes, each of which has a refractive index different from each other, and at least a concave portion of the concave and convex portions of the one homogeneous refractive index transparent material is filled with the other homogeneous refractive index transparent material. Optical head device.

4. The optical head device according to claim 1, wherein the phase correction element is further integrated with a phase plate whose phase difference with respect to the wavelength λ ₁ is an odd multiple of π / 2. 5.