JP3671768B2 - Optical head device - Google Patents

Description

【００２１】
ここで、複屈折材料のリタデーション値は一般に波長依存性を有し、Ａ、Ｂ、Ｃを材料に依存する分散係数とすると、近似的にＲ＝Ａ＋Ｂ／（λ−Ｃ）と表わせる。また、分散係数であるｋ_Ｌおよびｋ_Ｈは、各々式（２ａ）、（２ｂ）のように定義される。
ここで、数値５８９はリタデーション値の測定基準波長として用いた、ナトリウムのＤ線の波長である。
【００２２】
【数２】

【００２３】
また、既に述べたとおり、上記の広帯域位相差板に入射する２つのレーザ光は同じ直線偏光の方向を有しており、この直線偏光の方向に対して一方の位相差板の光軸のうち進相軸の方向が１０〜２０度の角度をなし他方の位相差板の光軸のうち進相軸の方向が７０〜８０度の角度をなしている。これらの角度のとき、楕円率角の波長依存性が小さく、すなわち楕円率角が波長によって大きく変化せず好ましい。この変化しない効果は、入射する最初（第１）の位相差板における角度が１０〜２０度で後（第２）の位相差板における角度が７０〜８０度であっても、または最初の角度が７０〜８０度で後の角度が１０〜２０度であっても効果に変りはない。
【００２４】
以下、これらの角度について詳しく説明する。入射するレーザ光の直線偏光の方向と２枚の位相差板のそれぞれの進相軸とのなす角度を、入射する位相差板の順にそれぞれθ_１およびθ_２とする。波長λがＲ_１の約２倍およびＲ_２の約４倍、すなわちＲ_１＝λ／２およびＲ_２＝λ／４の関係がほぼ成立するとき、θ_１およびθ_２の関数として楕円率角を計算できる。
【００２５】
図２はこの楕円率角を表わしており、図中の２つの傾いた平面が形成する稜は楕円率角がほぼ９０度の条件を満たしている。図２（ａ）の稜は式（３ａ）で表わされ、θ_２がθ_１より大きい場合のグラフであり、図２（ｂ）の稜は式（３ｂ）で表わされ、θ_１がθ_２よりより大きい場合のグラフである。式（３ａ）、（３ｂ）の条件下で広帯域位相差板は、ほぼ１／４波長板として機能する。
θ_２＝２×θ_１＋π／４・・・（３ａ）
θ_１＝２×θ_２＋π／４・・・（３ｂ）
なお、透過光の楕円偏光度は楕円率角として、透過楕円偏光の長軸強度Ｉ_ａと短軸強度Ｉ_ｂとを用いてｃｏｓ^−１｛（Ｉ_ａ−Ｉ_ｂ）／（Ｉ_ａ＋Ｉ_ｂ）｝と定義される。
【００２６】
使用するレーザ光の波長が、完全に設計波長と一致する場合には、θ_１とθ_２とが式（３ａ）または式（３ｂ）を満足する角度関係のとき、透過光として完全な円偏光が得られる。しかし、波長が設計波長からわずかずれた場合には、θ_１とθ_２とが式（３ａ）または式（３ｂ）の関係を満足する角度の組み合わせの中でも、式（４ａ）、（４ｂ）の関係が成立するとき、もっとも円偏光に近くなる。
（θ_１、θ_２）＝（１５、７５）・・・（４ａ）
（θ_１、θ_２）＝（７５、１５）・・・（４ｂ）
このことから、θ_１とθ_２とが式（４ａ）または式（４ｂ）で与えられる角度のとき、またはこの角度の近傍で±５度の範囲内の角度のときが特に好ましい。例えば、（４ａ）に対しては、上記のとおり（θ_１、θ_２）＝（１０〜２０、７０〜８０）となる。
【００２７】
式（３ａ）を満たすθ_１とθ_２との角度の種々の組み合わせの中で（θ_１、θ_２）＝（１５、７５）および（１０、６５）に対し、透過光の楕円率角の波長依存性を図３に示す。図３より、設計波長（７１８ｎｍ）において楕円率角は９０度であるので、完全な円偏光が得られていること、および（θ_１、θ_２）＝（１５、７５）（実線）に対する波長依存性の方が（θ_１、θ_２）＝（１０、６５）（破線）における波長依存性よりも変化が小さいことがわかる。
【００２８】
また図３から、使用するレーザ光の波長が設計波長から離れるにつれて、楕円率角が９０度からずれ透過光は楕円化している。この楕円化から、例えば高密度記録ディスクに用いられる４００ｎｍおよびＣＤに用いられる７９０ｎｍの波長の組み合わせについては、いずれの波長に対しても大幅に円偏光から乖離することが予想される。
【００２９】
この円偏光からの乖離は、結果として復路の回折特性を劣化させディスクからの反射光の読みとり信号強度を低下させるため、光ヘッド装置においてはこの乖離は問題となる。光ヘッド装置で実際に使用する波長は、搭載した光源の２つの波長を含む全ての波長帯域ではなく、２つの波長のみであり、この波長における偏光性能のみが問題となる。したがって、本発明においては、使用する２つの波長について楕円率角が最も大きくなる２枚の位相差板の進相軸方向の関係を決定した。
【００３０】
参考として、図４（ａ）の斜めの実線は式（３ａ）を、図４（ｂ）のそれは式（３ｂ）を示している。図４（ａ）、（ｂ）中の黒丸が示すように、２つの使用波長で完全な１／４波長板として機能するθ_１とθ_２は、式（５ａ）（図４（ａ））または式（５ｂ）（図４（ｂ））の関係を満たす。
（θ_１、θ_２）＝（１５＋ａ、７５−ａ）・・（５ａ）
（θ_１、θ_２）＝（７５−ａ、１５＋ａ）・・（５ｂ）
ここでａは、２波長の間隔により決まる正の係数である。
【００３１】
【表１】

【００３２】
表１に、条件１（ａ＝０）と条件２（ａ＝３．２）に対して、式（５ｂ）が成立するθ_１とθ_２およびこれらの角度におけるＲ_１とＲ_２の値を示す。
図５に、ａ＝０（破線）とａ＝３．２（実線）について、広帯域位相差板の透過光の楕円率角の波長依存性を示す。設定中心値である（θ_１、θ_２）＝（７５、１５）から、適切な回転角度３．２度を設定することで、２つの使用波長４２５ｎｍと７９０ｎｍに対する楕円率角が大きく改善され、この２つの使用波長に対しては楕円率角が９０度となり完全な円偏光となる。設定中心値が（θ_１、θ_２）＝（１５、７５）の場合にも適切な回転角度を設定すれば同様に完全な円偏光とできる。
【００３３】
上記のように、本発明において任意の２つの波長に対して、式（１ａ）、（１ｂ）を満たすリタデーション値を有する位相差板を用い、最適なａを設定して２枚の位相差板の進相軸方向を決定することで、２つの使用波長に対して完全に広帯域位相差板を１／４波長板として機能させることができる。ここでａの値は、例えば２つの波長が６６０ｎｍと７９０ｎｍの場合は、０．２で、また４２５ｎｍと７９０ｎｍの場合は、３．２である。
【００３４】
したがって、特定の２つの波長の光に対し、式（５ａ）または式（５ｂ）が成立する範囲内で広帯域位相差板が完全に１／４波長板として機能するように、すなわち楕円率角が９０度となるように、ａの値を設定することが極めて好ましい。
【００３５】
本発明における２つの位相差板の接着には、粘着フィルム、ＵＶ硬化型や熱硬化型の接着剤を使用できる。広帯域位相差板の波面収差の低減、温度特性や信頼性の向上のためには、できるだけ薄い接着層として張り合わせることが好ましく、接着層の厚さを１０μｍ以下にすることが特に好ましい。
【００３６】
本発明における広帯域位相差板を使用する際には、透過光の波面収差の劣化を回避するために、表面の平滑化処理や基板による接着保持が好ましい。具体的には、少なくとも１枚の透明基板に広帯域位相差板を接着して使用することが好ましい。他の光学素子と積層一体化せずに広帯域位相差板を単独で用いる場合には、２枚の透明基板により挟み込む構成が波面収差低減・強度確保の点から特に好ましい。
【００３７】
本発明における広帯域位相差板は、単独で使用することもできるが、光ヘッド装置に用いられるその他の光学素子と積層一体化することで、部品点数の削減、光ヘッド装置組み立ての簡略化並びに装置の小型化が実現できる。したがって、広帯域位相差板がレーザ光の光学的性質を変化させる少なくとも１つの光学素子と一体化されていることは好ましい。
【００３８】
具体的に光学素子とは、ディスク上での集光特性を改善する例えば液晶を用いた位相補正素子や、回折により信号光を検出器に導く回折格子、特に偏光による回折特性の違いを用いた偏光型回折素子などが挙げられる。
本発明における広帯域位相差板は、偏光による特性の違いを利用した光学素子を有する光ヘッド装置に用いると特に効果が大きく、さらに小型化・軽量化が要求される光情報の記録再生に用いる光ヘッド装置用の部品に適している。
【００３９】
図６において、偏光ホログラム４０１および１／４波長板４０２は一体化され、アクチュエータ４０３に搭載された対物レンズ４０４に取り付けられている。ここで使用している１／４波長板４０２は本発明における広帯域位相差位板である。波長６６０ｎｍの半導体レーザ４０５および波長７９０ｎｍの半導体レーザ４０６からのそれぞれのレーザ光は、コリメートレンズ４０７、４０８で平行化され、プリズム４０９を介して、偏光ホログラム４０１および１／４波長板４０２を透過し、対物レンズ４０４でディスク４１０に集光される。
【００４０】
ディスク４１０の表面上に形成されたピットの情報を含んだ反射光は、それぞれの経路を逆方向に進行する。プリズム４０９を透過または反射した戻り光は、それぞれコリメートレンズ４０７、４０８を透過後６６０ｎｍ用のフォトダイオード４１１および７９０ｎｍ用のフォトダイオード４１２により検出される。
【００４１】
この図６の構成で、２つの波長のうちいずれか一方の波長のレーザ光に対して最適化した、または２つの波長の中間値の波長のレーザ光に対して最適化した偏光ホログラムを使用すると、いずれの波長の光に対しても、往路は高透過の特性を示し、復路は問題となる効率の低下が発生しない。
【００４２】
【実施例】
［例１］
本例を図１（ａ）、図１（ｂ）に基づいて説明する。厚さ０．５ｍｍのガラス基板１０１にＵＶ硬化型の接着剤１０２を滴下し、ポリカーボネートを延伸して得られたリタデーション値が２６０ｎｍ、厚さ３０μｍの複屈折フィルム１０３を接着剤１０２上に重ねて、積層体を形成した。
【００４３】
その後、この積層体を複屈折フィルム１０３側を上にし、この複屈折フィルム１０３上に加重のための厚さ１ｍｍのガラス基板（図示せず）を乗せた後、１０００ｒｐｍの回転速度で２０秒間、５０００ｒｐｍで１００秒間回転させ、基板１０１と複屈折フィルム１０３の間の接着層の厚さを５μｍとした。さらに、積層体に波長３６５ｎｍのＵＶ光を５０００ｍＪ照射し、接着層を硬化させ位相差板１０４とした。同様にして、新たなガラス基板１０５に同じくポリカーボネートを延伸して得られたリタデーション値が１３０ｎｍ、厚さ３０μｍの複屈折フィルム１０６を張り合わせ、位相差板１０７を作製した。また、この位相差板１０７にも同様にＵＶ光の照射を行った。
【００４４】
各々の位相差板１０４、１０７について、波長６６０ｎｍの半導体レーザ光を用いてリタデーション値および光学軸方位を測定した。測定した光学軸を用いて、位相差板１０４の光軸のうちの進相軸方向１０８に対して、位相差板１０７の光軸のうちの進相軸方向１０９が約５４度の角度となるように配置した。角度の測定方向は、位相差板１０４が上のときに、位相差板１０４側から見て反時計回り方向を正（＋）とした。
【００４５】
２枚の位相差板１０４、１０７の間にＵＶ接着剤１０２を滴下し、１０００ｒｐｍの回転速度で２０秒間、５０００ｒｐｍで１００秒間回転させ、接着層の厚さを５μｍとした。回転時の軸方向のずれを修正した後に、５０００ｍＪのＵＶ光を照射し接着層を硬化させ広帯域位相差板とした。
【００４６】
位相差板１０４の進相軸方向１０８に対して、−１８度（角度方向の符号は上に定義）の方向を基準として、広帯域位相差板を外形５ｍｍ×５ｍｍにダイシング・ソーにより切断し、広帯域位相差板素子を得た。ここで、広帯域位相差板の進相軸方向１１３は、積層された２枚の位相差板のそれぞれの進相軸方向の中間の方向と定義した。
【００４７】
波長８６０ｎｍの半導体レーザからの出射光を基本波とし、非線形光学結晶ＫＮｂＯ_３を用いて発生させた第２高調波の波長４３０ｎｍのレーザ光と波長７８９ｎｍの半導体レーザからの出射光とを用いて広帯域位相差板素子の楕円率角を測定した。上記２種類の波長のレーザ光の直線偏光方向１１４は、広帯域位相差板素子の進相軸方向１１３に対して−４５度となっている広帯域位相差板素子外辺と平行な方向であり、入射はガラス基板１０１側から行った。測定した楕円率角は、波長４３０ｎｍのレーザ光に対し約８６度、７８９ｎｍに対し約８８度となって、実用上充分な特性であった。
【００４８】
また、広帯域位相差素子の透過波面収差は、波長６３３ｎｍのＨｅ−Ｎｅレーザを用いて測定した結果、２５ｍλ_ｒｍｓ（２乗平均）以下であり、光学素子として充分使用できるレベルであった。
【００４９】
この広帯域位相差素子を図６の光ヘッド装置の１／４波長板４０２として組み込んだ。１つの光源は波長７８９ｎｍの半導体レーザ４０６とした。他方の光源は波長６６０ｎｍの半導体レーザ４０５の代わりに８６０ｎｍの半導体レーザを設置し、半導体レーザとコリメートレンズ４０７との間に非線形光学結晶ＫＮｂＯ₃（図示せず）を配置して波長を４３０ｎｍとした。その結果、２つの波長４３０ｎｍおよび７８９ｎｍに対して満足できる円偏光が得られ、光利用効率の高い信号光を得ることができた。
【００５０】
［例２］
本例を図７（ａ）、図７（ｂ）に基づいて説明する。図７（ａ）に示すように、レーザ光の入射側面（図中、下側の面）に低反射コート膜２０１が施された直径１２．５ｃｍ、厚さ０．５ｍｍのガラス基板２０２を用意し、ガラス基板２０２の光ディスク側の面（図中、上側の面）にポリイミドの膜を形成し、ラビングによる水平配向処理を施してポリイミド配向膜２０３とした。
【００５１】
配向処理を施したガラス基板上に、液晶セルとなるガラス基板面間のギャップを保持するために図示しない直径３μｍのＳｉＯ_２ビーズを５０００個／ｃｍ^２の密度で散布した。その後、離型化処理を施された図示しない水平配向ガラス基板と上記配向処理を施したガラス基板と対向させ、基板外周部に印刷された図示しない熱硬化型のエポキシシール剤を用いて、２枚のガラス基板間のギャップを３μｍとした。そのギャップに４−（３−アクリロイルオキシプロピル）オキシ−４’−シアノビフェニルと、アクリル酸４−（４−ｎ−ブチルベンゾイルオキシ）フェニルを主成分とする液状の液晶材料（液晶性モノマー）を注入し、２枚のガラス基板間に挟持させた。
【００５２】
このとき、液晶性モノマーには光重合開始材としてベンゾインイソプロピルエーテルを１％添加してＵＶ硬化性の液晶性モノマー組成物とした。その後、波長３６５ｎｍのＵＶ光を液晶材料全体に照射し、水平配向状態のまま液晶性モノマー組成物全体を重合・固化することによって、ガラス基板による構成物全体を固定した。１４０℃、３０分間の熱処理の後に、上記の図示しない水平配向対向ガラス基板を離型除去して、厚さ３μｍの水平配向した高分子液晶２０４の有機薄膜を形成した。
【００５３】
この高分子液晶２０４の有機薄膜上にスパッタ法によりＳｉＯ_２の無機薄膜を約５０ｎｍ成膜し、フォトリソグラフィー法によりレジストパターン（図示せず）を作製した。このレジストパターンを使用し、ドライエッチング法により流量１００ＳＣＣＭのＣＦ_４ガスを用いて圧力０．２Ｔｏｒｒ、出力３００Ｗの条件下で３分間のエッチングを行い、ピッチ６μｍの格子状に加工しＳｉＯ_２マスク２０５を作製した。
【００５４】
作製したＳｉＯ₂マスク２０５を使用し、ドライエッチング法により、流量１００ＳＣＣＭのＯ_２ガスを用いて、圧力０．２Ｔｏｒｒ、出力３００Ｗの条件下で２０分間エッチングを行い、図示しないフォトレジストを除去すると同時に、ピッチが６μｍで、厚さが３μｍの高分子液晶２０４の有機格子を作製した。
【００５５】
図７（ａ）のように、上側の片面に低反射コート膜２０１を施された厚さ０．５ｍｍのカバーガラス２０６に、ＵＶ接着剤を滴下し、ポリカーボネートを延伸して得られたリタデーション値が１８０ｎｍ、厚さ３０μｍの複屈折フィルム２０７を張り合わせた。１０００ｒｐｍの回転速度で２０秒間、５０００ｒｐｍで１００秒間回転させ、接着層２０８を厚さ５μｍとした。その後、５０００ｍＪのＵＶ光を照射し接着層２０８を硬化させた。
【００５６】
同様にして、ポリカーボネートを延伸して得られたリタデーション値が３６０ｎｍ、厚さ３０μｍの複屈折フィルム２０９の進相軸方向２１０が、複屈折フィルム２０７の進相軸方向２１１に対して−６０度となるよう張り合わせた。その後、１０００ｒｐｍで２０秒間、５０００ｒｐｍで１００秒間回転させ、接着層２０８を厚さ５μｍとしたのち、波長３６５ｎｍのＵＶ光を５０００ｍＪ照射し、広帯域位相差板付きカバーガラス２１２とした。ここで、カバーガラスの進相軸方向２１３は、積層された２枚の位相差板のそれぞれの進相軸方向の中間の方向と定義した。角度は、広帯域位相差板を下からみて、反時計回りを正とした。
【００５７】
その後、広帯域位相差板付きカバーガラス２１２とガラス基板２０２の有機格子側との間にＵＶ接着剤２１４を滴下し、カバーガラスの進相軸方向２１３と往路入射のレーザ光の直線偏光方向２１５が４５度の角度をなすように配置した。その後、広帯域位相差板付きカバーガラス２１２と基板２０２を同時に１０００ｒｐｍの回転速度で２０秒間、５０００ｒｐｍで１００秒間回転させて、接着層２１４を厚さ５μｍとするとともに高分子液晶２０４の有機格子の格子間に充填接着した。回転時の軸方位のずれを修正した後に、５０００ｍＪのＵＶ光を照射し接着層２１４硬化させた。
【００５８】
ここで、使用した接着剤は、有機格子に用いた高分子液晶２０４（常光屈折率ｎ_ｏ＝１．５、異常光屈折率ｎ_ｅ＝１．６）の、常光屈折率ｎ_ｏと等しい屈折率（ｎ＝１．５）を、硬化後に有する紫外線硬化型の接着剤であった。最後にダイシング・ソーにより切断して、外径４ｍｍ×４ｍｍ、厚さ約１．１ｍｍの広帯域位相差板付き偏光型回折素子２１６を作製した。
【００５９】
このように作製された広帯域位相差板付き偏光型回折素子の波長６５８ｎｍおよび７８７ｎｍにおける光学特性を表２に示す。６５８ｎｍと７８７ｎｍのいずれの波長に対しても、８５度以上の楕円率角が得られており、広帯域位相差板付き偏光型回折素子は実用上充分使用できるレベルの１／４波長板として機能していることが確認された。
【００６０】
【表２】

【００６１】
この広帯域位相差板付き偏光型回折素子は、波長６６０ｎｍ付近での回折特性が最適となるように設計されているため、７９０ｎｍ付近での回折特性は６６０ｎｍと比べると多少劣るが、実用上充分に高い透過率が得られた。
さらに、透過光の波面収差は、偏光型回折素子の光の入出射面の中心部（直径２．５ｍｍの円内）において、２５ｍλ_ｒｍｓ（２乗平均）以下であり良好であった。
【００６２】
この広帯域位相差板付き偏光型回折素子２１６を図６の光ヘッド装置の偏光ホログラム４０１と１／４波長板４０２の代わりに組み込んだ。その結果、使用した２つの波長６５８ｎｍと７８７ｎｍに対し充分な円偏光が得られ、また偏光型回折素子も充分機能して波面収差が抑えられて、光利用効率が極めて高い信号光を得ることができた。
【００６３】
【発明の効果】
本発明は、光ヘッド装置中の光源と対物レンズとの間に広帯域位相差板が設置されており、この広帯域位相差板は２枚の位相差板がそれぞれの光軸を交差するように重ねられていて、レーザ光が第１に入射する位相差板のリタデーション値が第２に入射する位相差板のリタデーション値より大きく、それらの比が１．８〜２．２であり、前記２つのレーザ光が前記広帯域位相差板を透過するときの楕円偏光度がいずれのレーザ光に対しても８５度以上となるように、前記広帯域位相差板に入射する前記２つのレーザ光は同じ直線偏光の方向を有しており、前記方向と一方の位相差板の進相軸との方向が１０〜２０度の角度をなし、前記方向と他方の位相差板の進相軸との方向が７０〜８０度の角度をなすように２枚の位相差板の光軸の方向がそれぞれ調整されている。
【００６４】
この構成をとることによって、この広帯域位相差板は透過する異なる波長の直線偏光のレーザ光に対してほぼ１／４波長板として機能し、直線偏光をほぼ円偏光に変換できる。したがって、この広帯域位相差板を光ヘッド装置中組み込むことによって、ディスクからの反射戻り光として検出された異なる波長の信号光はそれぞれ光利用効率の高い信号光となる。
【図面の簡単な説明】
【図１】本発明における広帯域位相差板の構成を示し、（ａ）２枚の位相差板を重ねた断面図、（ｂ）重ねられた２枚の位相差板の各光軸などの角度関係を示す平面図。
【図２】本発明における広帯域位相差板の透過光に対する楕円率角の設置角度（θ_１、θ_２）依存性を示し、（ａ）θ_２がθ_１より大きい場合のグラフ、（ｂ）θ_１がθ_２よりより大きい場合のグラフ。
【図３】本発明における広帯域位相差板の透過光に対する楕円率角の波長依存性を示すグラフ。
【図４】本発明における参考例として広帯域位相差板の、２波長光に対して１／４波長板となる設置角度（θ_１、θ_２）を示し、（ａ）θ_１＝１５＋ａおよびθ_２＝７５−ａで、ａが１０度以内のグラフ、（ｂ）θ_１＝７５−ａおよびθ_２＝１５＋ａで、ａが１０度以内のグラフ。
【図５】本発明における参考例として広帯域位相差板に対する透過光の楕円率角の波長依存性を示すグラフ（図４（ｂ）のａ＝０とａ＝３．２の場合）。
【図６】本発明の光ヘッド装置の構成例を示す図。
【図７】本発明における広帯域位相差板付き偏光型回折素子の構成を示し、（ａ）広帯域位相差板と偏光型回折素子とを重ねた断面図、（ｂ）重ねられた２枚の位相差板の各光軸などの角度関係を示す平面図。
【図８】従来の位相差板の透過光に対する楕円率角の波長依存性を示すグラフ。
【図９】従来の光ヘッド装置の構成例を示す図。
【符号の説明】
２０１：低反射コート膜
１０１、１０５、２０２：ガラス基板
２０３：ポリイミド配向膜
２０４：高分子液晶
２０５：ＳｉＯ_２マスク
２０６：カバーガラス
１０３、１０６、２０７、２０９：複屈折フィルム
１０４、１０７：位相差板
１０２、２０８：接着層
１０８、１０９、２１０、２１１：進相軸方向
２１２：広帯域位相差板付きカバーガラス
１１３：広帯域位相差板（素子）の進相軸方向
２１３：カバーガラスの進相軸方向
１０２、２１４：接着剤
１１４、２１５：直線偏光方向
２１６：広帯域位相差板付き偏光型回折素子
３０１、３０２、４０５、４０６：半導体レーザ
３０３、３０４、４０１：偏光ホログラム
３０５、３０６、４０２：１／４波長板
３０７、３０８、４０７、４０８：コリメートレンズ
３０９、４０９：プリズム
３１０、４０３：アクチュエータ
３１１、４０４：対物レンズ
３１２、４１０：ディスク
３１３、３１４、４１１、４１２：フォトダイオード[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an optical head device equipped with a broadband retardation plate for controlling the phase state of laser light.
[0002]
[Prior art]
  An optical head device is used for writing optical information on an optical recording medium such as an optical disk and a magneto-optical disk and reading optical information from the optical recording medium. The optical head device writes and reads information by condensing the emitted light of a semiconductor laser as a light source on a recording surface of a disk-shaped optical recording medium (hereinafter referred to as a disk).
[0003]
  The shorter the wavelength of the laser light used, the more the recording density can be improved. On the other hand, it is necessary to be able to reproduce with long-wavelength (near-infrared) laser light for many disks that have been widely used so far. Conventional near-infrared laser light and short-wavelength laser light Various schemes that are compatible with are proposed. In order to ensure this compatibility with conventional disks, there is a method in which a light source in the near infrared region is installed in addition to a short wavelength light source for high recording density.
[0004]
  On the other hand, in an optical head device corresponding to a high-density disk and a write-type disk such as a CD-R / RW, high light use efficiency is required for any wavelength for each disk. A polarizing optical head device using a polarizing diffraction element having transmittance / return path high diffraction efficiency is conceivable.
[0005]
  FIG. 9 shows an example of the configuration of a conventional polarizing optical head device using two laser beams. In FIG. 9, linearly polarized laser beams from the semiconductor laser 301 having an emission wavelength of 660 nm and the semiconductor laser 302 having an emission wavelength of 790 nm are highly transmitted with respect to the incident polarization direction. Each pass through 304. The linearly polarized laser light is converted into circularly polarized light by the ¼ nm plate 305 for 660 nm and the ¼ wavelength plate 306 for 790 nm integrated with the respective polarization holograms. Thereafter, the collimating lenses 307 and 308 arranged individually make the laser light parallel light, and are transmitted and reflected through the prism 309 having the characteristics of 660 nm transmission / 790 nm reflection, and are held by the actuator 310 and are common to the two wavelengths. The light is condensed on the surface of the disk 312 by the objective lens 311.
[0006]
  Further, the reflected light from the disk 312 including information on the pits formed on the surface of the disk 312 travels in the opposite direction along each path. Circularly polarized light whose rotational direction is reversed by the reflection on the surface of the disk 312 is transmitted again through the quarter-wave plate 305 and the quarter-wave plate 306, respectively, and is converted into linearly polarized light having a polarization direction orthogonal to the incident polarization direction. Diffracted by the polarization hologram 303 and the polarization hologram 304. Information recorded on the disk 312 is read by detecting the pit information of the disk possessed by the diffracted light by the photodiode 313 for 660 nm and the photodiode 314 for 790 nm.
[0007]
[Problems to be solved by the invention]
  In the conventional optical head device, for example, when two wavelengths are used, two sets of optical elements such as a polarization diffraction element and a quarter wavelength plate are arranged for each wavelength, the number of parts is large, and the volume of the device is increased. Furthermore, there is a problem that assembly adjustment takes time.
[0008]
  On the other hand, in order to reduce the size of the optical head device, it has been proposed to arrange two semiconductor lasers close to each other or to use a semiconductor laser capable of oscillating a plurality of wavelengths. Even if different prisms are used, it is difficult to switch the optical path for each wavelength.
[0009]
  However, as shown in FIG. 8, the ellipticity angle is one wavelength of light, for example, a phase difference plate that is circularly polarized light (1/4 wavelength) with respect to 660 nm light, with respect to the other light with a wavelength of 790 nm. Becomes elliptically polarized light and the desired characteristics cannot be obtained. Similarly, a retardation plate that is circularly polarized light (1/4 wavelength) with respect to 790 nm light is elliptically polarized light with respect to light having a wavelength of 660 nm, and a desired characteristic cannot be obtained. Here, in FIG. 8, to be a quarter wavelength plate for light of a certain wavelength means that the ellipticity angle is 90 degrees at that wavelength, and thus it is desired to obtain circularly polarized light. It is that.
[0010]
  Further, for example, even when a quarter wavelength plate for 728 nm, which is an intermediate value between wavelengths 660 nm and 790 nm, is used, the output polarization becomes elliptical for light of any wavelength of 660 nm and 790 nm, and the quarter wavelength. It has insufficient properties as a plate. Due to the characteristic failure of the quarter-wave plate, the polarization characteristic of the return path deviates from the designed return-path linearly polarized light, and the diffraction efficiency is reduced. As a result, the performance of the optical head device is deteriorated due to the decrease of the signal level.
[0011]
  As described above, the problem of this deterioration is that a prism or the like is used when a semiconductor laser chip is placed close to each other to reduce the size of the optical head device or when a laser chip capable of oscillating two wavelengths of light is used. Thus, there arises a problem that the optical paths of the two wavelengths of light cannot be separated.
[0012]
  Further, in order to further improve the recording density, the wavelength used in the disk is shortened, but this short wavelength has a large wavelength difference compared to the wavelength used in the conventional disk, 790 nm, and after passing through the quarter wavelength plate. The ellipticity angle is greatly deviated from 90 degrees.
  An object of the present invention is to solve the above-described problems in an optical head device using two different wavelength laser beams as a light source.
[0013]
[Means for Solving the Problems]
  The present invention provides an optical head device in which linearly polarized laser light emitted from a light source is condensed by an objective lens and guided to an optical recording medium, and reflected light from the optical recording medium is received by a photodetector. Used in optical head devices with different wavelengths430 nm and 789 nm, 658 nm and 787 nmOf the two laser beams, and the shorter wavelength of the two laser beams is λ_L, The longer wavelength λ_HAnd wavelength λ_L, Λ_HThe wideband phase difference plate for controlling the phase state of the two laser beams is installed between the light source and the objective lens.
  The broadband retardation plate is overlapped so that the two retardation plates intersect each other, and the retardation value of the retardation plate on which the two laser beams are incident first is expressed as R.₁The retardation value of the second incident phase difference plate is R₂Then R₁= Λ / 2 and R₂= Λ / 4 is substantially established, and the ratio R of the two retardation values₁/ R₂The elliptical angle when the two laser beams are transmitted through the broadband retardation plate is 85 degrees or more with respect to any of the laser beams. The two laser beams incident on the phase difference plate have the same direction of linear polarization, and the direction between the direction and the fast axis of one phase difference plate forms an angle of 10 to 20 degrees, An optical head device characterized in that the directions of the optical axes of the two phase difference plates are respectively adjusted so that the direction of the phase difference axis of the other phase difference plate forms an angle of 70 to 80 degrees.
[0014]
  Also, the shorter wavelength is λ_L= 658nm, the longer wavelength λ_H= 787nm, and the angle between the direction of the same linearly polarized light of the two laser beams and the direction of the fast axis of one phase difference plate is θ₁, The angle formed by the direction between the direction and the fast axis of the other retardation plate is θ₂Where θ₁And θ₂The optical head device according to claim 1, satisfying the relationship of (3a) or (3b).
  θ₂= 2 × θ₁+ Π / 4 (3a)
  θ₁= 2 × θ₂+ Π / 4 (3b)
  MaIn addition, the optical head device described above is such that the ratio of the retardation values is 2, and the degree of elliptical polarization when the two laser beams pass through a broadband retardation plate is substantially equal.
  Furthermore, the present invention provides the above optical head device in which the broadband retardation plate is integrated with at least one optical element that changes the optical properties of the two laser beams.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
  In the optical head device of the present invention, the laser beams emitted from the light source are two laser beams used in the optical head device having different wavelengths, and the phase state of two or more laser beams between the light source and the objective lens A broadband retardation plate is installed to control this. This broadband phase difference plate is a phase difference in which two retardation plates are overlapped so that their optical axes intersect each other, and the retardation value of the phase difference plate on which the laser light is incident first is incident on the second. It is configured to be larger than the retardation value of the plate, and the ratio of the retardation values is 1.8 to 2.2. Then, the two laser beams incident on the broadband retardation plate are such that the ellipticity angle when the two laser beams pass through the broadband retardation plate is 85 degrees or more with respect to any laser beam. The direction of the linearly polarized light is the same, and the direction between the direction and the fast axis of one phase difference plate forms an angle of 10 to 20 degrees, and the direction and the fast axis of the other phase difference plate The directions of the optical axes of the two retardation plates are adjusted so that the directions form an angle of 70 to 80 degrees.
  With this configuration, the broadband retardation plate functions as a substantially quarter-wave plate with respect to linearly polarized laser light of any wavelength that is transmitted, and the linearly polarized light can be substantially circularly polarized.
[0016]
  As the birefringent material used as the retardation plate, for example, a polycarbonate film having birefringence by stretching is preferable. Furthermore, an organic polymer film having birefringence by stretching can also be used. Further, as the birefringent material, an organic material having birefringence may be aligned on the substrate. In addition, TiO formed on the substrate with birefringence by a film formation method such as anisotropic vapor deposition.₂Inorganic materials such as quartz substrate, LiNbO₃The substrate itself may have birefringence, such as.
[0017]
  The number of laser beams having different wavelengths to be emitted is two.
  For example, typical phase difference combinations of the two phase difference plates constituting the broadband phase difference plate in the present invention and typical arrangement of the fast axis are as follows.
  In addition, the retardation values generated by the first and second retardation plates in the order in which the laser beams are incident on the stacked retardation plates are respectively expressed as R.₁And R₂And Furthermore, the shorter wavelength of the two different wavelengths of the two laser beams is λ_L, The longer wavelength λ_HAnd λ_L≦ λ ≦ λ_HIs the wavelength λ of the two laser beams to be used._L, Λ_HIs defined as an intermediate wavelength.
[0018]
  At this time, R₁Is approximately half the wavelength λ and R₂Is about 1/4 times the ratio R of these retardation values₁/ R₂Is 1.8 to 2.2, and the optical axes of the two retardation plates intersect each other. The crossing angle is preferably 45 to 75 degrees. At this time, the above-described effect, that is, the effect of converting linearly polarized light into substantially circularly polarized light is provided.
[0019]
  On the other hand, the retardation value R of the retardation plate that is incident first on the two laser beams.₁Retardation value R of the phase difference plate incident on the second side₂Larger, the ratio of the retardation values is 2, and the ellipticity angles when the two laser beams are transmitted through the broadband retardation plate are substantially equal, that is, in the equations (1a) and (1b) When the relationship is established, it is preferable that the light use efficiency is equal for any wavelength used. Here, the retardation dispersion coefficient for a short-wavelength laser beam is expressed as k._LK to that for longer wavelengths_HIt was.
[0020]
[Expression 1]

[0021]
  Here, the retardation value of the birefringent material generally has wavelength dependency, and can be approximately expressed as R = A + B / (λ−C), where A, B, and C are dispersion coefficients depending on the material. Also, the dispersion coefficient k_LAnd k_HAre defined as in equations (2a) and (2b), respectively.
  Here, the numerical value 589 is the wavelength of the sodium D line used as the measurement reference wavelength of the retardation value.
[0022]
[Expression 2]

[0023]
  Further, as already described, the two laser beams incident on the broadband retardation plate have the same linearly polarized light direction, and one of the optical axes of one of the retardation plates with respect to the linearly polarized light direction. The direction of the fast axis forms an angle of 10 to 20 degrees, and the direction of the fast axis of the optical axis of the other phase difference plate forms an angle of 70 to 80 degrees. These angles are preferable because the wavelength dependence of the ellipticity angle is small, that is, the ellipticity angle does not vary greatly with wavelength. This unchanging effect can be achieved even if the angle at the first (first) retardation plate incident is 10 to 20 degrees and the angle at the later (second) retardation plate is 70 to 80 degrees, or the first angle. Even if the angle is 70 to 80 degrees and the later angle is 10 to 20 degrees, the effect is not changed.
[0024]
  Hereinafter, these angles will be described in detail. The angle formed by the direction of the linearly polarized light of the incident laser beam and the fast axis of each of the two retardation plates is expressed as θ in the order of the incident retardation plates.₁And θ₂And Wavelength λ is R₁About twice and R₂About 4 times, that is, R₁= Λ / 2 and R₂When the relationship of = λ / 4 is almost established, θ₁And θ₂The ellipticity angle can be calculated as a function of
[0025]
  FIG. 2 shows the ellipticity angle, and the ridge formed by the two inclined planes in the drawing satisfies the condition that the ellipticity angle is approximately 90 degrees. The ridge in FIG. 2A is expressed by equation (3a), and θ₂Is θ₁FIG. 2B is a graph in the case of being larger, the edge of FIG. 2B is expressed by the equation (3b), and θ₁Is θ₂It is a graph when larger than. Under the conditions of the expressions (3a) and (3b), the broadband retardation plate functions almost as a quarter wavelength plate.
θ₂= 2 × θ₁+ Π / 4 (3a)
θ₁= 2 × θ₂+ Π / 4 (3b)
  The elliptical polarization degree of the transmitted light is the ellipticity angle, and the major axis intensity I of the transmitted elliptically polarized light is I._aAnd minor axis strength I_bAnd cos^-1{(I_a-I_b) / (I_a+ I_b)}.
[0026]
  If the wavelength of the laser beam to be used completely matches the design wavelength, θ₁And θ₂Is an angular relationship satisfying the expression (3a) or the expression (3b), complete circularly polarized light is obtained as transmitted light. However, if the wavelength deviates slightly from the design wavelength,₁And θ₂Among the combinations of angles satisfying the relationship of the formula (3a) or the formula (3b), when the relationship of the formulas (4a) and (4b) is established, the combination is closest to circularly polarized light.
(Θ₁, Θ₂) = (15, 75) (4a)
(Θ₁, Θ₂) = (75, 15) (4b)
  From this, θ₁And θ₂And is an angle given by the formula (4a) or (4b), or an angle within a range of ± 5 degrees in the vicinity of this angle. For example, for (4a), (θ₁, Θ₂) = (10-20, 70-80).
[0027]
  Θ satisfying equation (3a)₁And θ₂Among various combinations of angles with (θ₁, Θ₂) = (15, 75) and (10, 65), the wavelength dependence of the ellipticity angle of the transmitted light is shown in FIG. As shown in FIG. 3, since the ellipticity angle is 90 degrees at the design wavelength (718 nm), complete circular polarization is obtained, and (θ₁, Θ₂) = (15,75) (solid line) has a wavelength dependence of (θ₁, Θ₂) = (10, 65) (dashed line) shows that the change is smaller than the wavelength dependence.
[0028]
  Also, from FIG. 3, as the wavelength of the laser beam used deviates from the design wavelength, the ellipticity angle deviates from 90 degrees and the transmitted light becomes elliptical. From this ovalization, for example, the combination of wavelengths of 400 nm used for high-density recording disks and 790 nm used for CDs is expected to deviate significantly from circularly polarized light for any wavelength.
[0029]
  This divergence from circularly polarized light results in degradation of the diffraction characteristics of the return path and decreases the read signal intensity of the reflected light from the disk, and this divergence becomes a problem in the optical head device. The wavelengths actually used in the optical head device are not all the wavelength bands including the two wavelengths of the mounted light source, but only two wavelengths, and only the polarization performance at these wavelengths becomes a problem. Therefore, in the present invention, the relationship in the fast axis direction of the two retardation plates that have the largest ellipticity angle for the two wavelengths used is determined.
[0030]
  As reference,The oblique solid line in FIG. 4 (a) shows the equation (3a), and that in FIG. 4 (b) shows the equation (3b). As indicated by the black circles in FIGS. 4 (a) and 4 (b), θ functions as a perfect quarter-wave plate at two wavelengths used.₁And θ₂Satisfies the relationship of equation (5a) (FIG. 4 (a)) or equation (5b) (FIG. 4 (b)).
(Θ₁, Θ₂) = (15 + a, 75−a) (5a)
(Θ₁, Θ₂) = (75−a, 15 + a) (5b)
  Here, a is a positive coefficient determined by the interval between two wavelengths.
[0031]
[Table 1]

[0032]
  Table 1 shows that θ for which Equation (5b) is satisfied for Condition 1 (a = 0) and Condition 2 (a = 3.2).₁And θ₂And R at these angles₁And R₂Indicates the value of.
  FIG. 5 shows the wavelength dependence of the ellipticity angle of the transmitted light of the broadband retardation plate for a = 0 (broken line) and a = 3.2 (solid line). The set center value (θ₁, Θ₂) = (75, 15), by setting an appropriate rotation angle of 3.2 degrees, the ellipticity angles for the two used wavelengths of 425 nm and 790 nm are greatly improved. For these two used wavelengths, the ellipticity is The angle is 90 degrees and complete circular polarization is achieved. Set center value is (θ₁, Θ₂) = (15, 75), a perfect circularly polarized light can be obtained by setting an appropriate rotation angle.
[0033]
  As described above, for the two arbitrary wavelengths in the present invention, the retardation plate having the retardation value satisfying the expressions (1a) and (1b) is used, and the optimum a is set to set the two retardation plates. By determining the fast axis direction, it is possible to make the broadband retardation plate function as a quarter wave plate completely for the two used wavelengths. Here, the value of a is, for example, 0.2 when the two wavelengths are 660 nm and 790 nm, and is 3.2 when the two wavelengths are 425 nm and 790 nm.
[0034]
  Therefore, with respect to light of two specific wavelengths, the broadband retardation plate functions completely as a quarter-wave plate within the range where the formula (5a) or the formula (5b) is satisfied, that is, the ellipticity angle is It is very preferable to set the value of a so as to be 90 degrees.
[0035]
  For adhesion of the two retardation plates in the present invention, an adhesive film, a UV curable adhesive, or a thermosetting adhesive can be used. In order to reduce wavefront aberration and improve temperature characteristics and reliability of the broadband retardation plate, it is preferable that the adhesive layers are laminated as thin as possible, and the thickness of the adhesive layer is particularly preferably 10 μm or less.
[0036]
  When using the broadband retardation plate in the present invention, it is preferable to smooth the surface or to maintain the adhesion with the substrate in order to avoid the deterioration of the wavefront aberration of the transmitted light. Specifically, it is preferable to use a broadband retardation plate adhered to at least one transparent substrate. In the case where a broadband retardation plate is used alone without being laminated and integrated with other optical elements, a configuration of sandwiching between two transparent substrates is particularly preferable from the viewpoint of reducing wavefront aberration and ensuring strength.
[0037]
  The broadband retardation plate in the present invention can be used alone, but by laminating and integrating with other optical elements used in the optical head device, the number of components is reduced, the optical head device assembly is simplified, and the device Can be miniaturized. Therefore, it is preferable that the broadband retardation plate is integrated with at least one optical element that changes the optical properties of the laser light.
[0038]
  Specifically, the optical element is a phase correction element using, for example, liquid crystal that improves the light condensing characteristic on the disk, or a diffraction grating that guides signal light to the detector by diffraction, particularly a difference in diffraction characteristic due to polarization. Examples thereof include a polarizing diffraction element.
  The broadband retardation plate of the present invention is particularly effective when used in an optical head device having an optical element that utilizes the difference in characteristics due to polarization, and is used for recording and reproducing optical information that is required to be smaller and lighter. Suitable for parts for head devices.
[0039]
  In FIG. 6, a polarization hologram 401 and a quarter wavelength plate 402 are integrated and attached to an objective lens 404 mounted on an actuator 403. The quarter wave plate 402 used here is a broadband retardation plate in the present invention. The respective laser beams from the semiconductor laser 405 having a wavelength of 660 nm and the semiconductor laser 406 having a wavelength of 790 nm are collimated by collimating lenses 407 and 408, and pass through the polarization hologram 401 and the quarter-wave plate 402 through the prism 409. The light is condensed on the disk 410 by the objective lens 404.
[0040]
  Reflected light including information on pits formed on the surface of the disk 410 travels in the opposite direction along each path. The return light transmitted or reflected by the prism 409 is detected by the photodiode 411 for 660 nm and the photodiode 412 for 790 nm after passing through the collimating lenses 407 and 408, respectively.
[0041]
  In the configuration of FIG. 6, when a polarization hologram optimized for laser light having one of the two wavelengths or optimized for laser light having an intermediate value between the two wavelengths is used. For any wavelength of light, the forward path exhibits high transmission characteristics, and the return path does not cause a reduction in efficiency.
[0042]
【Example】
  [Example 1]
  This example will be described with reference to FIGS. 1 (a) and 1 (b). A UV curable adhesive 102 is dropped on a glass substrate 101 having a thickness of 0.5 mm, and a retardation value obtained by stretching the polycarbonate is 260 nm, and a birefringent film 103 having a thickness of 30 μm is stacked on the adhesive 102. A laminate was formed.
[0043]
  Then, after placing this laminate on the birefringent film 103 side and placing a 1 mm thick glass substrate (not shown) on the birefringent film 103 for 20 seconds at a rotational speed of 1000 rpm, The substrate was rotated at 5000 rpm for 100 seconds, and the thickness of the adhesive layer between the substrate 101 and the birefringent film 103 was set to 5 μm. Further, the laminate was irradiated with 5000 mJ of UV light having a wavelength of 365 nm to cure the adhesive layer, and the phase difference plate 104 was obtained. Similarly, a retardation plate 107 was produced by laminating a birefringent film 106 having a retardation value of 130 nm and a thickness of 30 μm obtained by stretching a polycarbonate on a new glass substrate 105. Similarly, the retardation film 107 was irradiated with UV light.
[0044]
  About each phase difference plate 104,107, the retardation value and the optical axis direction were measured using the semiconductor laser beam of wavelength 660nm. Using the measured optical axis, the fast axis direction 109 of the optical axis of the phase difference plate 107 forms an angle of about 54 degrees with respect to the fast axis direction 108 of the optical axis of the phase difference plate 104. Arranged. The angle measurement direction was positive (+) in the counterclockwise direction when viewed from the phase difference plate 104 side when the phase difference plate 104 was up.
[0045]
  The UV adhesive 102 was dropped between the two retardation plates 104 and 107 and rotated for 20 seconds at a rotation speed of 1000 rpm and 100 seconds at 5000 rpm, so that the thickness of the adhesive layer was 5 μm. After correcting the axial displacement during rotation, the adhesive layer was cured by irradiating 5000 mJ UV light to obtain a broadband retardation plate.
[0046]
  With respect to the fast axis direction 108 of the phase difference plate 104, the broadband phase difference plate is cut into a 5 mm × 5 mm outer shape by a dicing saw with reference to the direction of −18 degrees (the sign of the angle direction is defined above), A broadband retardation plate element was obtained. Here, the fast axis direction 113 of the broadband phase difference plate is defined as an intermediate direction between the respective fast axis directions of the two stacked phase difference plates.
[0047]
  The emission light from a semiconductor laser with a wavelength of 860 nm is used as a fundamental wave, and the nonlinear optical crystal KNbO₃The ellipticity angle of the broadband retardation plate element was measured using laser light having a second harmonic wavelength of 430 nm and light emitted from a semiconductor laser having a wavelength of 789 nm. The linear polarization direction 114 of the laser light of the two types of wavelengths is a direction parallel to the outer periphery of the broadband retardation plate element that is −45 degrees with respect to the fast axis direction 113 of the broadband retardation plate element. Incidence was performed from the glass substrate 101 side. The measured ellipticity angle was about 86 degrees for laser light with a wavelength of 430 nm and about 88 degrees for 789 nm, which was a practically sufficient characteristic.
[0048]
  Further, the transmission wavefront aberration of the broadband phase difference element was measured using a He—Ne laser having a wavelength of 633 nm._rmsIt was below (root-mean-square) and was a level which can be sufficiently used as an optical element.
[0049]
  This broadband phase difference element was incorporated as a quarter wavelength plate 402 of the optical head device of FIG. One light source was a semiconductor laser 406 having a wavelength of 789 nm. The other light source is a 860 nm semiconductor laser installed in place of the 660 nm semiconductor laser 405, and the nonlinear optical crystal KNbO is interposed between the semiconductor laser and the collimating lens 407._Three(Not shown) was arranged to set the wavelength to 430 nm. As a result, satisfactory circularly polarized light was obtained for the two wavelengths of 430 nm and 789 nm, and signal light with high light utilization efficiency could be obtained.
[0050]
  [Example 2]
  This example will be described with reference to FIGS. 7 (a) and 7 (b). As shown in FIG. 7A, a glass substrate 202 having a diameter of 12.5 cm and a thickness of 0.5 mm is prepared, in which a low-reflection coating film 201 is applied to a laser light incident side surface (lower surface in the drawing). Then, a polyimide film was formed on the surface of the glass substrate 202 on the optical disk side (the upper surface in the figure), and a horizontal alignment process by rubbing was performed to obtain a polyimide alignment film 203.
[0051]
  In order to maintain the gap between the glass substrate surfaces to be liquid crystal cells on the glass substrate subjected to the alignment treatment, SiO 3 having a diameter of 3 μm (not shown).₂5000 beads / cm²Sprayed at a density of. Then, using a thermosetting epoxy sealant (not shown) printed on the outer periphery of the substrate, facing a horizontally oriented glass substrate (not shown) subjected to the release treatment and the glass substrate subjected to the orientation process, The gap between the glass substrates was 3 μm. A liquid liquid crystal material (liquid crystalline monomer) mainly composed of 4- (3-acryloyloxypropyl) oxy-4′-cyanobiphenyl and 4- (4-n-butylbenzoyloxy) phenyl acrylate is formed in the gap. Poured and sandwiched between two glass substrates.
[0052]
  At this time, 1% of benzoin isopropyl ether was added to the liquid crystalline monomer as a photopolymerization initiator to obtain a UV curable liquid crystalline monomer composition. Thereafter, the entire liquid crystal material was irradiated with UV light having a wavelength of 365 nm, and the entire liquid crystal monomer composition was polymerized and solidified in the horizontal alignment state, thereby fixing the entire structure of the glass substrate. After heat treatment at 140 ° C. for 30 minutes, the above-mentioned horizontally aligned counter glass substrate (not shown) was removed from the mold to form an organic thin film of polymer liquid crystal 204 having a thickness of 3 μm and horizontally aligned.
[0053]
  SiO 2 is sputtered onto the organic thin film of the polymer liquid crystal 204.₂An inorganic thin film was formed to a thickness of about 50 nm, and a resist pattern (not shown) was produced by photolithography. Using this resist pattern, a dry etching method with a flow rate of 100 SCCM CF₄Etching is performed for 3 minutes using gas with a pressure of 0.2 Torr and an output of 300 W, and processed into a lattice shape with a pitch of 6 μm.₂A mask 205 was produced.
[0054]
  Produced SiO₂Using the mask 205, the dry etching method is used and the flow rate is 100 SCCM.₂Etching is performed for 20 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W using gas to remove the photoresist (not shown), and at the same time, an organic lattice of a polymer liquid crystal 204 having a pitch of 6 μm and a thickness of 3 μm is produced did.
[0055]
  As shown in FIG. 7 (a), a retardation value obtained by dripping a UV adhesive on a cover glass 206 having a thickness of 0.5 mm and having a low-reflective coating film 201 on one upper surface and stretching a polycarbonate. A birefringent film 207 having a thickness of 180 nm and a thickness of 30 μm was laminated. The adhesive layer 208 was rotated at a rotational speed of 1000 rpm for 20 seconds and at 5000 rpm for 100 seconds, so that the adhesive layer 208 had a thickness of 5 μm. Thereafter, 5000 mJ of UV light was irradiated to cure the adhesive layer 208.
[0056]
  Similarly, the retardation value obtained by stretching the polycarbonate is 360 nm, and the fast axis direction 210 of the birefringent film 209 having a thickness of 30 μm is −60 degrees with respect to the fast axis direction 211 of the birefringent film 207. Laminated together. Then, after rotating at 1000 rpm for 20 seconds and 5000 rpm for 100 seconds to make the adhesive layer 208 have a thickness of 5 μm, UV light with a wavelength of 365 nm was irradiated with 5000 mJ to obtain a cover glass 212 with a broadband retardation plate. Here, the fast axis direction 213 of the cover glass was defined as an intermediate direction between the respective fast axis directions of the two stacked phase difference plates. The angle was positive in the counterclockwise direction when the broadband retardation plate was viewed from below.
[0057]
  Thereafter, a UV adhesive 214 is dropped between the cover glass 212 with a broadband retardation plate and the organic lattice side of the glass substrate 202, and the fast axis direction 213 of the cover glass and the linear polarization direction 215 of the laser light incident on the outward path are They were arranged to make an angle of 45 degrees. Thereafter, the cover glass 212 with the broadband retardation plate and the substrate 202 are simultaneously rotated at a rotation speed of 1000 rpm for 20 seconds and at 5000 rpm for 100 seconds to make the adhesive layer 214 have a thickness of 5 μm and an organic lattice of the polymer liquid crystal 204. Filled and adhered in between. After correcting the deviation of the axial direction during rotation, the adhesive layer 214 was cured by irradiation with UV light of 5000 mJ.
[0058]
  Here, the adhesive used was a polymer liquid crystal 204 (ordinary refractive index n) used for the organic lattice._o= 1.5, extraordinary light refractive index n_e= Normal light refractive index n of 1.6)_oWas an ultraviolet curable adhesive having a refractive index (n = 1.5) equal to that after curing. Finally, the substrate was cut by a dicing saw to produce a polarizing diffraction element 216 with a broadband retardation plate having an outer diameter of 4 mm × 4 mm and a thickness of about 1.1 mm.
[0059]
  Table 2 shows the optical characteristics of the polarizing diffraction element with a broadband retardation plate manufactured in this way at wavelengths of 658 nm and 787 nm. An ellipticity angle of 85 degrees or more is obtained for both wavelengths of 658 nm and 787 nm, and the polarizing diffraction element with a broadband retardation plate functions as a quarter-wave plate that can be used practically. It was confirmed that
[0060]
[Table 2]

[0061]
  Since this polarizing diffraction element with a broadband retardation plate is designed so that the diffraction characteristics near the wavelength of 660 nm are optimal, the diffraction characteristics near 790 nm are somewhat inferior to those of 660 nm. A high transmittance was obtained.
  Furthermore, the wavefront aberration of the transmitted light is 25 mλ at the center of the light incident / exit surface of the polarizing diffraction element (inside a circle with a diameter of 2.5 mm)._rmsIt was below (root mean square) and was favorable.
[0062]
  This polarizing diffraction element 216 with a broadband retardation plate is incorporated in place of the polarization hologram 401 and the quarter-wave plate 402 of the optical head device of FIG. As a result, sufficient circularly polarized light can be obtained with respect to the two wavelengths 658 nm and 787 nm used, and the polarization type diffraction element functions sufficiently to suppress wavefront aberration, thereby obtaining signal light with extremely high light utilization efficiency. did it.
[0063]
【The invention's effect】
  In the present invention, a broadband retardation plate is installed between a light source and an objective lens in an optical head device, and the broadband retardation plate is overlapped so that two retardation plates cross each optical axis. The retardation value of the retardation plate on which the laser beam is incident first is larger than the retardation value of the retardation plate on which the laser beam is incident second, and the ratio thereof is 1.8 to 2.2. The two laser beams incident on the broadband retardation plate are the same linearly polarized light so that the degree of elliptical polarization when the laser beam passes through the broadband retardation plate is 85 degrees or more for any laser beam. The direction between the direction and the phase advance axis of one phase difference plate forms an angle of 10 to 20 degrees, and the direction between the direction and the phase advance axis of the other phase difference plate is 70 degrees. The direction of the optical axis of the two retardation plates is at an angle of ~ 80 degrees. Respectively have been adjusted.
[0064]
  By adopting this configuration, the broadband retardation plate functions as a substantially quarter-wave plate with respect to linearly polarized laser beams having different wavelengths to be transmitted, and linearly polarized light can be converted into substantially circularly polarized light. Therefore, by incorporating this wide-band retardation plate in the optical head device, signal lights of different wavelengths detected as reflected return light from the disk become signal lights with high light utilization efficiency.
[Brief description of the drawings]
FIG. 1 shows a configuration of a broadband retardation plate according to the present invention, (a) a sectional view in which two retardation plates are stacked, and (b) an angle of each optical axis of the two stacked retardation plates. The top view which shows a relationship.
FIG. 2 is an installation angle (θ of an ellipticity angle with respect to transmitted light of a broadband retardation plate in the present invention;₁, Θ₂) Dependency, (a) θ₂Is θ₁Graph when larger, (b) θ₁Is θ₂Graph for greater than.
FIG. 3 is a graph showing the wavelength dependence of the ellipticity angle with respect to the transmitted light of the broadband retardation plate in the present invention.
FIG. 4 in the present inventionAs a reference exampleInstallation angle (θ of a broadband retardation plate that becomes a quarter-wave plate for two-wavelength light₁, Θ₂) And (a) θ₁= 15 + a and θ₂= 75−a, a is a graph within 10 degrees, (b) θ₁= 75-a and θ₂= 15 + a and a is within 10 degrees.
FIG. 5 in the present inventionAs a reference exampleThe graph which shows the wavelength dependence of the ellipticity angle of the transmitted light with respect to a broadband phase difference plate (in the case of a = 0 and a = 3.2 of FIG.4 (b)).
FIG. 6 is a diagram showing a configuration example of an optical head device of the present invention.
7 shows a configuration of a polarizing diffraction element with a broadband retardation plate in the present invention, (a) a cross-sectional view in which the broadband retardation plate and the polarizing diffraction element are overlapped, and (b) two stacked layers. FIG. The top view which shows angular relationships, such as each optical axis of a phase difference plate.
FIG. 8 is a graph showing the wavelength dependence of the ellipticity angle with respect to the transmitted light of a conventional phase difference plate.
FIG. 9 is a diagram illustrating a configuration example of a conventional optical head device.
[Explanation of symbols]
201: Low reflection coating film
101, 105, 202: glass substrate
203: Polyimide alignment film
204: Polymer liquid crystal
205: SiO₂mask
206: Cover glass
103, 106, 207, 209: birefringent film
104, 107: Retardation plate
102, 208: adhesive layer
108, 109, 210, 211: fast axis direction
212: Cover glass with broadband retardation plate
113: Fast axis direction of broadband retardation plate (element)
213: Fast axis direction of cover glass
102, 214: Adhesive
114, 215: Linear polarization direction
216: Polarization type diffraction element with broadband retardation plate
301, 302, 405, 406: Semiconductor laser
303, 304, 401: Polarization hologram
305, 306, 402: 1/4 wavelength plate
307, 308, 407, 408: Collimating lens
309, 409: Prism
310, 403: Actuator
311 and 404: Objective lens
312, 410: Disc
313, 314, 411, 412: Photodiode

Claims (4)

In an optical head device in which linearly polarized laser light emitted from a light source is condensed by an objective lens and guided to an optical recording medium, and reflected light from the optical recording medium is received by a photodetector, the laser light has a different wavelength. It is any two of the wavelengths of 430 nm and 789 nm and 658 nm and 787 nm used in the optical head device, and the shorter wavelength of the two laser beams is λ _L and the longer wavelength Is λ _H, and an intermediate wavelength λ of wavelengths λ _L and λ _H is a design wavelength,
In addition, a broadband retardation plate that controls the phase state of the two laser beams is installed between the light source and the objective lens,
The broadband retardation plate is overlapped so that the two retardation plates intersect each other, and the retardation value of the retardation plate on which the two laser beams are incident _first is R ₁ , When the retardation value of the retardation plate incident on ₂ is R ₂ , the relationship of R ₁ = λ / 2 and R ₂ = λ / 4 is substantially established, and the ratio R ₁ / R _{2 of the} two retardation values is The broadband phase difference is 1.8 to 2.2, and the ellipticity angle when the two laser beams pass through the broadband phase difference plate is 85 degrees or more with respect to any laser beam. The two laser beams incident on the plate have the same linearly polarized direction, and the direction and the direction of the fast axis of one phase difference plate form an angle of 10 to 20 degrees, and the direction and the other The direction of the phase difference plate with the fast axis is 70 to 80 degrees An optical head apparatus, wherein the direction of the optical axis of the two retardation plate is adjusted respectively. The shorter wavelength is set to λ _L = 6 58 nm, the longer wavelength is set to λ _H = 7 87 nm, and the direction of the same linearly polarized light of the two laser beams and the phase advance of one phase difference plate When the angle formed by the direction with the axis is θ ₁ , and the angle formed by the direction between the direction and the fast axis of the other phase difference plate is θ ₂ , θ ₁ and θ ₂ are either (3a) or (3b) The optical head device according to claim 1, wherein the relationship is satisfied.
θ ₂ = 2 × θ ₁ + π / 4 (3a)
θ ₁ = 2 × θ ₂ + π / 4 (3b) The optical head device according to claim 1 or 2 ellipticity angle is substantially equal when the ratio of the two retardation value is 2 a and the two laser light passes through the broadband retarder. 4. The optical head device according to claim 1 , wherein the broadband retardation plate is integrated with at least one optical element that changes an optical property of the two laser beams.

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