JP4232523B2

JP4232523B2 - Optical modulator and optical attenuator

Info

Publication number: JP4232523B2
Application number: JP2003120232A
Authority: JP
Inventors: 好晴大井; 篤史小柳
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2003-04-24
Filing date: 2003-04-24
Publication date: 2009-03-04
Anticipated expiration: 2023-04-24
Also published as: JP2004325790A

Description

【０００１】
【発明の属する技術分野】
本発明は光変調素子および光減衰器に関し、特に回折格子と液晶とを組み合わせて、液晶に電圧を印加することにより、液晶の実質的な屈折率を制御して、入射光を回折させて０次回折光（透過光）量を制御する光変調素子とこの光変調素子を用いる光減衰器に関する。
【０００２】
【従来の技術】
回折格子と液晶とを組み合わせて、液晶の実質的屈折率を制御することにより入射光に所望の回折現象を生じさせる光変調素子として、均一屈折率材料からなる断面が凹凸状の位相型回折格子と、この回折格子の凹部に存る液晶と、この液晶の配向状態を変化させる制御手段としての電極からなる素子が報告されている。ここで、電極に電圧を印加しない静的状態において位相型回折格子を形成する材料の屈折率ｎを液晶の常光屈折率ｎ_ｏ（ＬＣ）と等しく設定しておくことにより、入射光は回折作用を受けることなく本素子を直進透過する０次回折光（透過光）が最大となる。
【０００３】
しかしながら、電極に電圧を印加することにより液晶分子の配向方向が変化し、液晶の異常光屈折率ｎ_ｅ（ＬＣ）を感じる入射光の偏光成分は、この異常光屈折率ｎ_ｅ（ＬＣ）と位相型回折格子の屈折率ｎとの屈折率差に起因して回折光が発生し０次回折光の光量が減少するが、液晶の常光屈折率ｎ_ｏ（ＬＣ）を感じる入射光の偏光成分は位相型回折格子との屈折率差が生じないため０次回折光の光量は変化しない。その結果、入射光の偏光方向によっては充分な消光比が得られない問題があった。また、入射光の偏光方向に関わらず入射光を変調するために、それぞれ直交する入射光の偏光成分に位相型回折格子として機能する２個の素子を積層させた構成が必要となり、構造が複雑になる問題があった。
【０００４】
また、回折格子と液晶とを組み合わせて液晶の屈折率を制御することにより入射光に所望の回折現象を生じせさせる他の光変調素子の従来例として次のものも報告されている。すなわち、一対の透明基板の片面に電極層および配向層が形成され、この一対の基板間に配向層と接するように配置した液晶層を有する液晶素子の配向層として複屈折材料を用いた、断面が凹凸状の格子を有する位相型回折格子からなる素子である。ここで、一対の電極間に電圧印加しない状態で、位相型回折格子を構成する複屈折材料の常光屈折率ｎ_ｏおよび異常光屈折率ｎ_ｅと、この回折格子の凹部にある液晶の常光屈折率ｎ_ｏ（ＬＣ）および異常光屈折率ｎ_ｅ（ＬＣ）とを実質的に等しくなるすなわち、ｎ_ｏ＝ｎ_ｏ（ＬＣ）およびｎ_ｅ＝ｎ_ｅ（ＬＣ）となるようにすることにより、入射光はその偏光方向に関わらず回折作用を受けることなく本素子を直進透過する０次回折光が最大となる。
【０００５】
しかしながら、電極間に電圧を印加することにより液晶分子の配向方向が変化し、異常光偏光の入射光に対する液晶の異常光屈折率ｎ_ｅ（ＬＣ）が見かけ上変化するため、液晶の異常光屈折率ｎ_ｅ（ＬＣ）と位相型回折格子の異常光屈折率ｎ_ｅとの屈折率差に起因して回折光が発生し０次回折光の光量が減少するが、常光偏光の入射光に対する液晶の常光屈折率ｎ_ｏ（ＬＣ）は変化しないため、液晶の常光屈折率ｎ_ｏ（ＬＣ）は位相型回折格子の常光屈折率ｎ_ｏと等しいままであり０次回折光の光量は変化しない。
【０００６】
その結果、入射光の偏光方向によっては充分な消光比が得られない問題があった。また、入射光の偏光方向に関わらず入射光を変調するために、一対の透明基板上の配向層を両方とも複屈折材料を用いた位相回折格子とし、各回折格子の格子の配列方向および光軸方向（異常光屈折率の方向）を直交させた構成が必要となり、構造が複雑になるとともに電極間隔の増大に伴い駆動電圧が上昇する問題があった。
【０００７】
また、回折格子の回折効率は波長に依存するため、広い波長帯域で高い消光比を有する光変調素子を得るためには、同一入射偏光に対して回折格子として作用する素子を積層することが有効である。しかし、高消光比と入射偏光依存性の無い光変調素子を実現しようとすると、４素子の積層に相当する極めて複雑な構成が必要となり問題であった。
【０００８】
【特許文献１】
特公平６−５２３５３号公報
【特許文献２】
特表平９−５１２３５６号公報
【０００９】
【発明が解決しようとする課題】
本発明は上述の実情に鑑み、量産性に優れた構成により入射光の偏光方向に関わらず、高い光の利用効率と低い駆動電圧でも高い消光比が実現できる光変調素子および光減衰器を提供することを目的とする。
【００１０】
【課題を解決するための手段】
本発明は、断面が凹凸状の回折格子のそれぞれの凸部が複屈折性固体材料からなり、回折格子のそれぞれの凹部には液晶が充填されており、前記複屈折性固体材料と前記液晶のそれぞれの常光屈折率同士およびそれぞれの異常光屈折率同士が実質的に等しく、
それぞれの前記複屈折性固体材料には前記液晶への電圧印加用の電極が形成されていて、隣接する前記複屈折性固体材料の前記電極間に電圧が印加でき、電圧非印加時において前記複屈折性固体材料の配向方向と前記液晶の配向方向とが揃うように構成されていることを特徴とする光変調素子を提供する。
【００１１】
また、前記複屈折性固体材料が高分子液晶からなる上記の光変調素子を提供する。
【００１２】
また、前記複屈折性固体材料に形成されたそれぞれの前記電極が一つ置きに、異なる２つの共通電極に分配されて接続されている上記の光変調素子を提供する。
【００１３】
また、前記電極は前記複屈折性固体材料の少なくとも側面に形成されている上記の光変調素子を提供する。
【００１４】
また、回折格子の凸部または凹部のいずれか一方に均一屈折率の透光性材料層を重ねて形成し、前記電極間に電圧を印加しない状態において回折格子の凸部の透過光と回折格子の凹部の透過光との間に位相差を発生させない上記の光変調素子を提供する。
【００１５】
また、上記の光変調素子を、それぞれの回折格子の格子長手方向を異ならせて重ねて構成された２層型の光変調素子を提供する。
【００１６】
さらに、前記電極に電圧が印加されて入射光の回折効率が変化する上記の光変調素子と、入射光のうち光変調素子を直進透過する０次回折光と直進透過しない非０次回折光とを分離し、０次回折光のみまたは非０次回折光のみを選別して受光する分別手段とを備え、印加電圧の大きさに応じて受光する光量が調整されることを特徴とする光減衰器を提供する。
【００１７】
【発明の実施の形態】
本発明の光変調素子では、液晶の配向状態を変化させる制御手段である電極に電圧を印加しない静的状態において、断面形状が凹凸状の格子を有する位相型回折格子の凸部を構成する複屈折性固体材料の常光屈折率ｎ_ｏおよび異常光屈折率ｎ_ｅと、回折格子の凹部に存する液晶の常光屈折率ｎ_ｏ（ＬＣ）および異常光屈折率ｎ_ｅ（ＬＣ）とを実質的に等しくするすなわち、ｎ_ｏ＝ｎ_ｏ（ＬＣ）およびｎ_ｅ＝ｎ_ｅ（ＬＣ）となるようにすることにより、入射光はその偏光方向に関わらず回折作用を受けることなく本素子を直進透過する０次回折光が最大となる。
【００１８】
一方、前記電極間に印加する電圧値Ｖの増加に応じて液晶の配向方向が電位勾配の発生する面内すなわち回折格子面内で変化し、位相型回折格子の凸部を構成する複屈折性固体材料の常光屈折率ｎ_ｏに対応する入射光の偏光（常光偏光と定める）に対して該回折格子の凹部に存する液晶の常光屈折率ｎ_ｏ（ＬＣ：Ｖ）はｎ_ｏ（ＬＣ）から見かけ上増加する。一方、位相型回折格子を構成する複屈折材料の異常光屈折率ｎ_ｅに対応する入射光の偏光（異常光偏光と定める）に対して該回折格子の凹部に存する液晶の異常光屈折率ｎ_ｅ（ＬＣ：Ｖ）はｎ_ｅ（ＬＣ）から見かけ上減少する。
【００１９】
ここで、位相型回折格子の凸部をなす複屈折性固体材料と凹部に存する液晶との屈折率が常光偏光および異常光偏光のいずれの入射光に対しても異なるため、回折格子の凹凸段差、すなわち凸部の高さｄに応じて回折光が発生し、０次回折光が減少する。波長λの常光偏光と異常光偏光の入射光に対する凹凸段差に起因した位相差△Φ_ｏ（Ｖ）および△Φ_ｅ（Ｖ）はそれぞれ（式１）および（式２）のように記述される。
【００２０】
【数１】

【００２１】
また、断面が凹凸状（矩形）で凹部と凸部との幅が等しい位相型回折格子において格子ピッチが波長より大きな場合、回折格子の凹凸段差による位相差△Φの０次回折光の効率η_０は（式３）式で近似できる。
【００２２】
【数２】

【００２３】
したがって、電圧Ｖ_０で０次回折光を最小とするためには、△Φ_ｏ（Ｖ_０）および△Φ_ｅ（Ｖ_０）が略πとなるようｎ_ｏ（ＬＣ：Ｖ_０）、ｎ_ｅ（ＬＣ：Ｖ_０）、ｄを規定すればよい。
【００２４】
その結果、単一の回折格子および該回折格子の凹部に存する液晶からなる素子構成により、入射光の偏光方向に関わらず電圧非印加時に直進透過する０次回折光の高い利用効率が得られるとともに印加電圧に応じて０次回折光の効率を変化できて高い消光比を有する光変調素子が実現する。
【００２５】
複屈折性固体材料は液晶と同程度の複屈折性（常光屈折率および異常光屈折率）を有することが好ましいため、液晶分子を高分子化した固体材料である高分子液晶を用いることが好ましい。また、液晶は電界に応じて配向方向が変化する液体材料であればネマティック液晶、強誘電性液晶、反強誘電性液晶、コレステリック液晶などいずれでもよい。
【００２６】
印加電圧に応じて格子の配列方向（長手方向）と角度をなすように液晶分子の配向方向が傾斜する正の誘電異方性を有するネマチチック液晶を用いることにより、回折格子の格子（凸部）毎に形成された電極間に印加された電圧の大きさに応じて液晶分子の配向方向が連続的に変化し、電圧非印加時に液晶分子の配向方向が格子の配列方向に安定しやすいため好ましい。
【００２７】
なお、光変調素子を直進透過する０次回折光と直進透過しない非０次回折光（通常の回折光）とを分離し０次回折光または非０次回折光のみを受光する分別手段としては、例えばレンズや集光鏡などの集光素子がある。光源から放射された光をレンズや集光鏡などの集光素子を用いて光検出器の受光部に集光する光学系において、本発明の光変調素子を光源から光検出器に至る光路中に配置することにより、光変調素子内の電極間に印加する電圧値に応じて光検出器に集光される光量を調整できる光減衰器となる。
【００２８】
すなわち、光変調素子内の電極間印加電圧の大きさに応じて生成された、回折格子により発生する直進透過成分である０次回折光以外の回折光は集光手段により光検出器の受光面に集光されないが、回折格子により回折されない０次回折光は光検出器の受光面に集光される。電極間印加電圧の大きさに応じて０次回折光量が変化するため、光検出器の信号光量が可変な光減衰器となる。
【００２９】
ここで、高い消光比を得るためには直進透過光（０次回折光）と回折光を分離することが必要で、指向性の揃った光束を生成する光源と光変調素子を直進透過した光を光検出器の微少な受光面に集光する集光素子とした構成が好ましい。光源と光変調素子の間、または光検出器と光変調素子との間に光ファイバーや光導波路などの光伝送路が介在してもよい。
【００３０】
本発明の光変調素子および光減衰器のさらなる特徴については、以下に示す実施例により具体的に説明する。
【００３１】
【実施例】
「例１」
本例の光変調素子１０の断面図を図１に、平面図を図２に示す。ガラス基板５の片面にＩＴＯ透明電極を成膜し、フォトリソグラフィによりＸ軸方向に直線状となるよう電極幅５μｍで電極間隔５μｍの櫛形透明電極形状にパターニングし、図２に示すように一本置きに電気的に結合した共通電極である電極３Ａと電極３Ｂを形成する。
【００３２】
次に、ガラス基板５の同じ面上にポリイミド膜を塗布してＸ軸方向に配向処理を施した配向膜４とする。さらに、配向膜上に液晶モノマ−液を塗布して重合固化することによりガラス基板面に対して平行でＸ軸方向に分子配向した膜厚ｄが３μｍの高分子液晶層を形成する。フォトリソグラフィと反応性イオンエッチングにより、櫛形電極形状の電極３Ａおよび電極３Ｂのパターン上を高分子液晶が覆うように加工し、凸部の幅５μｍ格子ピッチ１０μｍで断面が矩形状（凹凸状）の回折格子１とする。すなわち、凸部の複屈折性固体材料が高分子液晶層からなる回折格子とする。
【００３３】
ここで、電極３Ａおよび電極３Ｂは高分子液晶層からなる回折格子１の全面に形成する必要はなく、入射光を光変調するために必要な格子の中心領域のみでよい。すなわち、電極３Ａおよび電極３Ｂの電極幅は格子ピッチの半分以下でもよい。波長λ＝４１０ｎｍにおける回折格子１の複屈折は、Ｘ軸方向に異常光屈折率ｎ_ｅ＝１．７５、Ｙ軸方向に常光屈折率ｎ_ｏ＝１．５５である。
【００３４】
次に、従来の液晶素子と同様にギャップ制御用のスペーサが混入したシール材を用いて、Ｘ軸方向に配向処理を施した配向膜６が形成されたガラス基板７の片面にシール８を印刷塗布し、ガラス基板５に圧着固化することによりセル化する。シールの一部に設けられた注入口（図示せず）から液晶を注入して回折格子１の凹部に液晶２が充填され、注入口を封止して光変調素子１０が完成する。
【００３５】
ここで用いる液晶は、その異常光屈折率ｎ_ｅ（ＬＣ）が高分子液晶の異常光屈折率ｎ_ｅと略等しく、その常光屈折率ｎ_ｏ（ＬＣ）が高分子液晶の常光屈折率ｎ_ｏと略等しいとともに、電界方向に液晶分子が揃う正の誘電異方性のネマティック液晶とし、電圧非印加時にＸ軸方向に分子配向が揃った水平ホモジニアス配向となっている。
【００３６】
また、ガラス基板５および７の光入出射表面には反射防止膜（図示せず）が形成されている。なお、電極３Ａと電極３Ｂに電圧を印加したとき、回折格子１の凹部の液晶が同一方向に揃うように、高分子液晶および液晶の配向方向が電極３Ａと電極３Ｂの直線状方向であるＸ軸に対して少し角度をなすように配向膜４と６を配向処理することが好ましい。また、電極３Ａと電極３Ｂは入射光を光変調するのに必要な領域以外はＣｒやＡｕなどの、透明電極以外の金属電極としてもよい。
【００３７】
このようにして得られた光変調素子１０の光変調機能について以下に説明する。電極３Ａと電極３Ｂに矩形電圧波形の交流電源９を接続し、電極間に電圧Ｖを印加する。電圧Ｖの増加に伴い回折格子１の凹部の液晶にはＹ軸方向に電界が発生し、電界強度に応じて電圧非印加時にＸ軸方向に配向していた液晶のダイレクタ（異常光屈折率方向）は電界方向すなわちＹ軸方向に傾斜する。その結果、液晶の常光屈折率ｎ_ｏ（ＬＣ：Ｖ）は見かけ上ｎ_ｏ（ＬＣ）からｎ_ｅ（ＬＣ）まで増加し、異常光屈折率ｎ_ｅ（ＬＣ：Ｖ）は見かけ上ｎ_ｅ（ＬＣ）からｎ_ｏ（ＬＣ）まで減少する。
【００３８】
Ｚ軸方向に進行する波長λ＝４１０ｎｍの光が光変調素子１０に入射すると、電極間印加電圧Ｖ＝０のときは（式１）および（式２）で記述される常光偏光と異常光偏光の入射光に対する回折格子の凹凸段差に起因した位相差△Φ_ｏ（Ｖ）および△Φ_ｅ（Ｖ）は生じないため、図３（ａ）に示すように入射光は直進透過し９０％以上の高い０次回折光効率が得られる。
【００３９】
一方、電極間の印加電圧Ｖを増加するとＶ_０＝５Ｖから１０Ｖで△Φ_ｏ（Ｖ_０）および△Φ_ｅ（Ｖ_０）が略π、すなわちｎ_ｏ（ＬＣ：Ｖ_０）＝１．６２、ｎ_ｅ（ＬＣ：Ｖ_０）＝１．６８となる。このとき、図３（ｂ）に示すように入射光は回折され０次回折光は略ゼロとなる。したがって、入射光の偏光方向に依存しないで印加電圧Ｖの増加とともに０次回折光効率が減少する。その結果、電圧非印加時に高い０次回折光効率を示すとともに低電圧で消光比の高い光変調素子が得られる。
【００４０】
このようにして得られた光変調素子１０は、例えば、４１０ｎｍ波長帯域で発光する青紫色半導体レーザからの出射光が、光記録媒体の情報記録面に集光されて情報の記録または再生を行う光ヘッド装置において、半導体レーザから光記録媒体に至る光路中に配置され、光記録媒体への光量を調整する光変調器として用いられる。このとき、本素子を用いることにより半導体レーザの出射偏光方向に関わらず情報記録面に集光する０次回折光の光量調整ができるため、レーザ出力が安定する高出力レーザ発振状態で情報記録面の集光光量を減衰させて既に記録された情報を消去することなく安定した再生が可能となる。
【００４１】
「例２」（比較例）
比較例として、回折格子１を形成せずにセル内を液晶２で満たした素子構成とした場合、電極間の印加電圧の増加に伴い電極３Ａおよび電極３Ｂに面する液晶の配向が電界の大きさに応じて変化するため０次回折光が残留し、低電圧で高い消光比が得らない。
【００４２】
「例３」
本例の光変調素子２０の断面図を図４に示す。図１および図２に示す構成要素と同一のものは同じ番号を用いてある。平面図は図２と同様である。例１との素子構成上の相違点は、ＩＴＯ透明電極が高分子液晶からなる回折格子１の壁面（側面）に形成され、電極３Ａと電極３Ｂに電圧印加する構造としている点である。
【００４３】
図５に示す回折格子の部分拡大図を用いて電極３Ａの接続例を説明する。回折格子１の格子側壁面に形成された透明電極３Ａ_１は、格子一本置きに、格子端のガラス基板５の表面に形成された電極３Ａ_２と電気的に接続され、電極３Ａ_２に印加された電圧が格子側壁面の透明電極に伝わる。図４および図５に示す例では透明電極３Ａ_１が格子の側壁面のみに形成されているが、格子のガラス基板５に面する平面、またはその反対側平面にも形成されてもよい。
【００４４】
格子壁面の透明電極３Ａ_１は、高分子液晶からなる回折格子１を加工した後スパッタ成膜などのステップカバレッジにすぐれた成膜法にて透明電極膜を回折格子１の壁面に形成する。そして、回折格子１の凸部間である凹部の平面に成膜された透明電極をフォトリソグラフィとリフトオフまたはエッチングにより除去し、電極３Ａと電極３Ｂの導電接続を断つことにより作製できる。回折格子１の凸部平面（側面ではない）の透明電極は残存したままでもよいが、凸部間の凹部平面の透明電極とともに除去することにより透過率が向上するため好ましい。
【００４５】
本例では光通信波長帯域である波長λ＝１５５０ｎｍの入射光を光変調するために、回折格子１の高分子液晶の膜厚ｄを５μｍとしている。波長１５５０ｎｍでは高分子液晶の複屈折は異常光屈折率ｎ_ｅ＝１．６７および常光屈折率ｎ_ｏ＝１．５０となり、液晶の異常光屈折率ｎ_ｅ（ＬＣ）および常光屈折率ｎ_ｏ（ＬＣ）と略等しい。
【００４６】
このようにして得られた光変調素子２０の光変調機能について、例１である光変調素子１０と比較して以下に説明する。光変調素子１０の電極３Ａと電極３Ｂはガラス基板５の表面に形成されているため、電極間隔が５μｍ程度で印加電圧が１０Ｖ以下の場合、回折格子１の凹部に充填された液晶２の厚さ方向に実効的に生成される電界は３μｍ以下に限られる。
【００４７】
入射光の波長が比較的短い場合は最大回折を得るために必要な位相差△Φ_ｏおよび△Φ_ｅが略πとなる回折格子１の厚さ（膜厚）ｄは薄くてもよい。しかし、入射光の波長が比較的長い場合は回折格子１の厚さｄを厚くする必要があり、回折格子１の凹部に充填された液晶全体の配向方向に揃えるために、電極間に高い電圧を印加することが必要となる。駆動電圧の増加は電極間の短絡を招きやすく素子の信頼性を低下させる。
【００４８】
本例の光変調素子２０では回折格子１の側壁面に電極を形成しているため、回折格子１の厚さｄの大小にかかわらず、その凹部に充填された液晶２の全体に均一な電界を生成できる。その結果、入射光の波長が比較的長い場合でも位相差△Φ_ｏおよび△Φ_ｅが略πとなる構造が可能で、低い駆動電圧で高い消光比が実現できる。
【００４９】
このようにして得られた光変調素子２０は、例えば、波長多重光通信用の１５５０ｎｍ波長帯域で発光する半導体レーザからの出射光をレンズなどの集光素子を用いて光ファイバや光導波路に集光する光学システムの光路中、または光ファイバや光導波路から出射した１５５０ｎｍ波長帯の光を別の光ファイバや光導波路や光検出器などに伝送結合する光路中に配置することにより、印加電圧の大きさに応じて伝送光量を調整できる光減衰器となる。
【００５０】
すなわち、光通信に用いられる光ファイバや光導波路の光結合面および光検出器の受光面は極めて小面積であるため、光変調素子２０を直進透過する０次回折光は伝送または受光できるが、回折光は拡散して伝送されないまたは受光されないため、光強度変調素子である光減衰器として機能する。
【００５１】
例１および例２の光変調素子１０および２０は入射光の偏光方向に関わらず電圧可変光減衰器として機能するため汎用な用途に適用できる。また、光変調機能が発現する部分は回折格子１と液晶２からなる厚さ１０μｍ以下の領域であるため、これらを狭持するガラス基板５および７を薄片化することにより、例えば素子厚５０μｍ程度の光変調素子も実現できる。
【００５２】
その結果、レンズなどの集光素子を用いないで、光ファイバと光ファイバとを直接結合する中間位置や光導波路に幅５０μｍ程度の溝入れ加工してその溝に光変調素子を挿入することにより、容積の増大を抑えて光学システム中の光ファイバ間または光導波路間の光結合効率を電圧により変化させる光減衰器が導入できる。この場合、消光比を維持するためには格子ピッチを１０μｍ以下とすることが好ましい。
【００５３】
上記例１および例３では回折格子の凸部である高分子液晶からなる複屈折性固体材料と格子の凹部に充填された液晶が同程度の複屈折（常光屈折率および異常光屈折率）を有する場合について説明したが、電圧非印加時の複屈折が異なる場合は回折格子の凸部または凹部のいずれか一方に均一屈折率の透光性材料層を形成し、回折格子の凸部の透過光と凹部の透過光との位相差が発生しないように調整すれば高い０次回折光の効率が得られる。
【００５４】
高分子液晶の方が液晶に比べて複屈折が大きい場合は、回折格子の凸部を液晶の常光屈折率ｎ_ｏ（ＬＣ）より小さな均一屈折率材料層と高分子液晶層とを積層した格子とし、液晶の方が高分子液晶に比べて複屈折が大きい場合は回折格子の凹部に液晶の異常光屈折率ｎ_ｅ（ＬＣ）より大きな均一屈折率材料層を所望の厚さ形成した後液晶を充填すればよい。
【００５５】
本発明の光変調素子は光回折現象を利用しているため、０次回折光の効率は入射光の波長に依存する。したがって、広い波長帯域の入射光に対して高い消光比を得るためには、本発明の光変調素子を積層した構成とすることが好ましい。入射光が単一波長や狭帯域波長の場合でも積層することにより消光比は大幅に向上する。ただし、光変調素子を積層する場合は各素子の回折格子の配列方向が異なるようにすることが好ましい。これにより、一つの回折格子で回折された光が別の回折格子で回折されて０次回折光に重畳する多重回折光成分を排除できるため高い消光比が得られる。
【００５６】
上記例１および例３では回折格子の断面形状が矩形の場合について説明したが、鋸波形状のブレーズ格子や三角形状や正弦波形状などでもよい。また、格子ピッチが一定で格子の凹凸部のそれぞれの幅が等しい回折格子でその平面形状が直線状の場合について説明したが、円形などの曲線、格子ピッチが面内で分布しているまたは凹凸部の幅が異なっていてもよい。格子ピッチを空間的に分布させることにより電極間隔も分布するため、回折格子の凹部に充填された液晶に作用する電界も空間的に異なり、その結果回折格子パターンの設計により電圧と０次光回折効率の依存性を空間的に分布させることができる。
【００５７】
ただし、回折格子の凹部に充填された液晶に作用する電界の強さは電極３Ａと電極３Ｂの間隔に反比例して変化するため、駆動電圧を２０Ｖ以下に抑えるためには電極間隔を２０μｍ以下で格子ピッチを４０μｍ以下とすることが好ましい。上記例１および例３では０次回折光成分を用いているが特定の回折次数の回折光を信号光としてもよい。
【００５８】
また、上記例１および例３では電極３Ａと電極３Ｂとを一対として単一電圧を印加しているが、電極を分割してそれぞれ独立に駆動してもよい。例えば、電極３Ａを共通電極とし、電極３Ｂを分割してそれぞれ独立に交流電源に接続する。光ファイバアレイや光導波路アレイなどと組み合わせて各チャネル毎に独立に光変調駆動できる。
【００５９】
図１および図４に示す光変調素子において、例えば表面に反射層が形成されたガラス基板５を用い、その上に配向膜４および回折格子１などを形成することにより、反射型の光変調素子を得ることができる。すなわち、ガラス基板７から入射した光が反射層により反射され回折格子を往復してガラス基板７から出射する。透過型の光変調素子に比べ、回折格子１の複屈折性固体材料からなる凸部と凹部に充填された液晶とをそれぞれ往復するため、回折格子の凹凸段差により発生する同じ位相差を得るのに、格子の厚さｄは半分でよい。したがって、回折格子１が加工しやすくなる。なお、反射層として金属などの導電体を使用できないため、ＴｉＯ_２やＴａ_２Ｏ_５などの高屈折率誘電体とＳｉＯ_２などの低屈折率誘電体を交互に波長程度の光学膜厚で積層した誘電体多層膜ミラーを用いることが好ましい。
【００６０】
【発明の効果】
以上説明したように、本発明の光変調素子では複屈折性固体材料からなる断面が凹凸状の回折格子の凹部に充填された液晶の配向方向が、格子毎に分割して形成された電極間に電圧を印加することにより変化するため、入射光の偏光方向に関わらず印加電圧に応じて直進透過する０次回折光の効率を変化できる光変調素子が実現する。
【００６１】
また、回折格子を形成する複屈折性固体材料として高分子液晶を用いることにより回折格子の凹部に充填された液晶と同程度の複屈折が得やすいため、電圧非印加時に直進透過する０次回折光の高い利用効率が得られる。
【００６２】
また、正の誘電異方性を有するネマチツク液晶を用いることにより、回折格子の格子毎に形成された電極間に印加された電圧の大きさに応じて液晶の配向方向が連続的に変化し、電圧非印加時には回折格子の格子長手方向に液晶分子の配向方向が安定して揃いやすいため、連続的かつ安定に０次回折光の効率を変化できる光変調素子が実現する。
【００６３】
また、回折格子の側壁面に電極を形成することにより格子の厚さの大小にかかわらずその凹部に充填された液晶全体に均一な電界を印加できるため、入射光の波長が比較的長い場合でも、低い駆動電圧で高い消光比が実現できる。
【００６４】
また、回折格子を形成する複屈折性固体材料と回折格子の凹部に充填された液晶との複屈折が異なる場合、回折格子の凸部または凹部のいずれか一方に均一屈折率の透光性材料層を形成することにより回折格子の凸部の透過光と凹部の透過光とに位相差が発生しないよう調整できるため、高い０次回折光の透過効率が得られる。
【００６５】
また、本発明の光変調素子を少なくとも２個、各光変調素子に形成された回折格子の格子長手方向が互いに異なるように積層することにより、消光比が大幅に向上する。
【００６６】
本発明の光変調素子を、例えば光ヘッド装置において半導体レーザから光記録媒体に至る光路中に配置して、光記録媒体への光量を調整する光変調器として用いることにより、レーザ出力が安定する高出力レーザ発振状態で情報記録面の集光光量を減衰させて既に記録された情報を消去することなく安定した再生ができる。
【００６７】
本発明の光変調素子を、例えば波長多重光通信用の１５５０ｎｍ波長帯域で発光する半導体レーザからの出射光をレンズなどの集光素子を用いて光ファイバや光導波路に集光する光学システムの光路中、または光ファイバや光導波路から出射した１５５０ｎｍ波長帯の光を別の光ファイバや光導波路や光検出器などに伝送結合する光路中に配置することにより、印加電圧に応じて伝送光量を調整できる光減衰器となる。
【図面の簡単な説明】
【図１】本発明の光変調素子の構成例を示す側面図。
【図２】本発明の光変調素子の構成例を示す平面図。
【図３】図１に示す光変調素子に光が入射した場合の作用を示す側面図。
（ａ）は電圧非印加時の透過状態、（ｂ）は電圧印加時で最大回折状態。
【図４】本発明の光変調素子の他の構成例を示す側面図。
【図５】本発明の光変調素子の他の構成例における回折格子と電極との関係を説明するための部分的に拡大した模式図。
【符号の説明】
１：回折格子
２：液晶
３Ａ、３Ｂ、３Ａ２：電極
３Ａ１：透明電極
４、６：配向膜
５、７：ガラス基板
８：シール
９：交流電源
１０、２０：光変調素子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light modulation element and an optical attenuator, and in particular, by combining a diffraction grating and a liquid crystal and applying a voltage to the liquid crystal, the substantial refractive index of the liquid crystal is controlled to diffract incident light. The present invention relates to an optical modulation element that controls the amount of next diffracted light (transmitted light) and an optical attenuator using the optical modulation element.
[0002]
[Prior art]
A phase-type diffraction grating with a concavo-convex cross section made of a uniform refractive index material as a light modulation element that produces a desired diffraction phenomenon in incident light by controlling the substantial refractive index of the liquid crystal by combining a diffraction grating and liquid crystal An element comprising a liquid crystal in a concave portion of the diffraction grating and an electrode as a control means for changing the alignment state of the liquid crystal has been reported. Here, the refractive index n of the material forming the phase-type diffraction grating in the static state where no voltage is applied to the electrode is the normal refractive index n of the liquid crystal. _o By setting it equal to (LC), the 0th-order diffracted light (transmitted light) that goes straight through the element without diffracting the incident light is maximized.
[0003]
However, by applying a voltage to the electrode, the orientation direction of the liquid crystal molecules changes, and the extraordinary refractive index n of the liquid crystal _e The polarization component of incident light that feels (LC) is the extraordinary refractive index n. _e Diffracted light is generated due to the refractive index difference between (LC) and the refractive index n of the phase type diffraction grating, and the amount of 0th-order diffracted light is reduced. _o Since the polarization component of the incident light that feels (LC) does not cause a difference in refractive index from the phase type diffraction grating, the light amount of the 0th-order diffracted light does not change. As a result, there is a problem that a sufficient extinction ratio cannot be obtained depending on the polarization direction of incident light. In addition, in order to modulate the incident light regardless of the polarization direction of the incident light, a configuration in which two elements that function as a phase diffraction grating are stacked on the polarization components of the incident light that are orthogonal to each other is required, and the structure is complicated. There was a problem to become.
[0004]
The following is also reported as a conventional example of another light modulation element that causes a desired diffraction phenomenon in incident light by controlling the refractive index of the liquid crystal by combining a diffraction grating and liquid crystal. That is, a cross section using a birefringent material as an alignment layer of a liquid crystal element in which an electrode layer and an alignment layer are formed on one surface of a pair of transparent substrates and a liquid crystal layer is disposed between the pair of substrates so as to be in contact with the alignment layer Is an element composed of a phase type diffraction grating having an uneven grating. Here, the ordinary refractive index n of the birefringent material constituting the phase diffraction grating in a state where no voltage is applied between the pair of electrodes. _o And extraordinary refractive index n _e And the ordinary refractive index n of the liquid crystal in the concave portion of the diffraction grating. _o (LC) and extraordinary refractive index n _e (LC) is substantially equal, i.e., n _o = N _o (LC) and n _e = N _e By setting (LC), the 0th-order diffracted light that travels straight through the element without diffracting the incident light regardless of its polarization direction is maximized.
[0005]
However, by applying a voltage between the electrodes, the orientation direction of the liquid crystal molecules changes, and the extraordinary refractive index n of the liquid crystal with respect to the incident light of extraordinary light polarization _e (LC) apparently changes, so that the extraordinary refractive index n of the liquid crystal _e (LC) and extraordinary refractive index n of phase type diffraction grating _e Diffracted light is generated due to the difference in refractive index between the first and second diffracted light, and the amount of zero-order diffracted light is reduced. _o Since (LC) does not change, the ordinary refractive index n of the liquid crystal _o (LC) is the ordinary refractive index n of the phase type diffraction grating _o And the amount of 0th-order diffracted light does not change.
[0006]
As a result, there is a problem that a sufficient extinction ratio cannot be obtained depending on the polarization direction of incident light. In addition, in order to modulate the incident light regardless of the polarization direction of the incident light, both of the alignment layers on the pair of transparent substrates are phase diffraction gratings using a birefringent material, and the arrangement direction of each diffraction grating and the light A configuration in which the axial direction (the direction of the extraordinary light refractive index) is orthogonal is required, and there is a problem in that the structure becomes complicated and the drive voltage increases as the electrode spacing increases.
[0007]
In addition, since the diffraction efficiency of a diffraction grating depends on the wavelength, it is effective to stack elements that act as diffraction gratings for the same incident polarized light in order to obtain a light modulation element having a high extinction ratio in a wide wavelength band. It is. However, when trying to realize a light modulation element having a high extinction ratio and no dependence on incident polarization, a very complicated configuration corresponding to a stack of four elements is required.
[0008]
[Patent Document 1]
Japanese Patent Publication No. 6-52353
[Patent Document 2]
JP 9-512356 JP
[0009]
[Problems to be solved by the invention]
In view of the above circumstances, the present invention provides an optical modulator and an optical attenuator that can realize a high extinction ratio even with a high light utilization efficiency and a low driving voltage, regardless of the polarization direction of incident light, by a configuration excellent in mass productivity. The purpose is to do.
[0010]
[Means for Solving the Problems]
In the present invention, each convex part of the diffraction grating having an uneven cross section is made of a birefringent solid material, and each concave part of the diffraction grating is filled with liquid crystal, Said Birefringent solid materials and Said Each ordinary light refractive index and each extraordinary refractive index of the liquid crystal are substantially equal,
each Said For birefringent solid materials Said An electrode for applying voltage to the liquid crystal is formed and adjacent Said Of birefringent solid material Said Voltage can be applied between the electrodes When no voltage is applied, the alignment direction of the birefringent solid material is aligned with the alignment direction of the liquid crystal. An optical modulation element configured as described above is provided.
[0011]
Further, the present invention provides the above light modulation element, wherein the birefringent solid material is a polymer liquid crystal.
[0012]
Further, each of the birefringent solid materials formed in the birefringent solid material Said The light modulation element is provided in which every other electrode is distributed and connected to two different common electrodes.
[0013]
Further, the above-mentioned light modulation element is provided in which the electrode is formed on at least a side surface of the birefringent solid material.
[0014]
Further, a transparent material layer having a uniform refractive index is formed on either one of the convex portion or concave portion of the diffraction grating, and the transmitted light and the diffraction grating of the convex portion of the diffraction grating are not applied between the electrodes. There is provided the above-described light modulation element that does not generate a phase difference with the transmitted light of the concave portion.
[0015]
In addition, the present invention provides a two-layer type light modulation element configured by overlapping the above light modulation elements with different grating longitudinal directions of the respective diffraction gratings.
[0016]
Further, the above-mentioned light modulation element that changes the diffraction efficiency of incident light when voltage is applied to the electrode is separated from the 0th-order diffracted light that travels straight through the light modulation element and the non-zero-order diffracted light that does not travel straight through the incident light. And an optical attenuator characterized in that the light attenuating means is adjusted according to the magnitude of the applied voltage, and includes a sorting means for selecting and receiving only the 0th-order diffracted light or only the non-0th-order diffracted light. .
[0017]
DETAILED DESCRIPTION OF THE INVENTION
In the light modulation element of the present invention, in the static state in which no voltage is applied to the electrode, which is a control means for changing the alignment state of the liquid crystal, the convex portion of the phase-type diffraction grating having a grating with a concavo-convex cross section is formed. Ordinary refractive index n of refractive solid material _o And extraordinary refractive index n _e And the ordinary refractive index n of the liquid crystal present in the concave portion of the diffraction grating. _o (LC) and extraordinary refractive index n _e (LC) is substantially equal, i.e. n _o = N _o (LC) and n _e = N _e By setting (LC), the 0th-order diffracted light that travels straight through the element without diffracting the incident light regardless of its polarization direction is maximized.
[0018]
On the other hand, as the voltage value V applied between the electrodes increases, the orientation direction of the liquid crystal changes in the plane where the potential gradient occurs, that is, in the diffraction grating plane, and forms birefringence constituting the convex portion of the phase type diffraction grating. Ordinary refractive index n of solid material _o The ordinary refractive index n of the liquid crystal existing in the concave portion of the diffraction grating with respect to the polarization of the incident light (determined as ordinary light polarization) corresponding to _o (LC: V) is n _o Apparently increases from (LC). On the other hand, the extraordinary refractive index n of the birefringent material constituting the phase type diffraction grating _e Anomalies in the liquid crystal present in the recesses of the diffraction grating with respect to the polarization of incident light (determined as extraordinary light polarization) corresponding to light Refractive index n _e (LC: V) is n _e Apparently decreases from (LC).
[0019]
Here, since the refractive index of the birefringent solid material forming the convex portion of the phase type diffraction grating and the liquid crystal existing in the concave portion is different for both incident light of ordinary light polarization and extraordinary light polarization, the uneven step of the diffraction grating That is, diffracted light is generated according to the height d of the convex portion, and 0th-order diffracted light is reduced. Phase difference △ Φ due to uneven step for incident light of ordinary and polarized light of wavelength λ _o (V) and ΔΦ _e (V) is described as (Equation 1) and (Equation 2), respectively.
[0020]
[Expression 1]

[0021]
Further, in the case of a phase-type diffraction grating having a concavo-convex cross section (rectangular shape) and the widths of the concave and convex portions being equal, the grating pitch is larger than the wavelength. ₀ Can be approximated by Equation (3).
[0022]
[Expression 2]

[0023]
Therefore, the voltage V ₀ In order to minimize the 0th-order diffracted light, ΔΦ _o (V ₀ ) And △ Φ _e (V ₀ ) To be approximately π _o (LC: V ₀ ), N _e (LC: V ₀ ) And d may be defined.
[0024]
As a result, the element configuration consisting of a single diffraction grating and a liquid crystal in the concave portion of the diffraction grating provides high utilization efficiency of zero-order diffracted light that is transmitted straight when no voltage is applied, regardless of the polarization direction of incident light. An optical modulator having a high extinction ratio that can change the efficiency of the zero-order diffracted light according to the voltage is realized.
[0025]
Since the birefringent solid material preferably has the same birefringence (ordinary refractive index and extraordinary refractive index) as the liquid crystal, it is preferable to use a polymer liquid crystal that is a solid material obtained by polymerizing liquid crystal molecules. . The liquid crystal may be a nematic liquid crystal, a ferroelectric liquid crystal, an antiferroelectric liquid crystal, a cholesteric liquid crystal, or the like as long as it is a liquid material whose orientation direction changes according to an electric field.
[0026]
By using nematic liquid crystal having positive dielectric anisotropy in which the alignment direction of liquid crystal molecules is inclined so as to form an angle with the arrangement direction (longitudinal direction) of the grating according to the applied voltage, the grating (convex part) of the diffraction grating The orientation direction of the liquid crystal molecules changes continuously according to the magnitude of the voltage applied between the electrodes formed every time, and it is preferable because the orientation direction of the liquid crystal molecules easily stabilizes in the lattice alignment direction when no voltage is applied. .
[0027]
As the separating means for separating the 0th-order diffracted light that travels straight through the light modulation element and the non-0th-order diffracted light that does not travel straight (normal diffracted light) and receives only the 0th-order diffracted light or the non-zeroth-order diffracted light, for example, a lens or There are condensing elements such as condensing mirrors. In an optical system that condenses light emitted from a light source onto a light receiving portion of a photodetector using a condensing element such as a lens or a condensing mirror, the light modulation element of the present invention is in the optical path from the light source to the photodetector. By disposing the optical attenuator, the optical attenuator can adjust the amount of light condensed on the photodetector in accordance with the voltage value applied between the electrodes in the light modulation element.
[0028]
That is, diffracted light other than the 0th-order diffracted light, which is a linearly transmitted component generated by the diffraction grating, generated according to the magnitude of the voltage applied between the electrodes in the light modulation element is applied to the light receiving surface of the photodetector by the condensing means. The 0th-order diffracted light that is not collected but not diffracted by the diffraction grating is collected on the light receiving surface of the photodetector. Since the 0th-order diffracted light quantity changes in accordance with the magnitude of the voltage applied between the electrodes, an optical attenuator with variable signal light quantity of the photodetector is obtained.
[0029]
Here, in order to obtain a high extinction ratio, it is necessary to separate the straight transmitted light (0th order diffracted light) from the diffracted light. It is preferable to use a condensing element that condenses light on the minute light receiving surface of the photodetector. An optical transmission line such as an optical fiber or an optical waveguide may be interposed between the light source and the light modulation element or between the photodetector and the light modulation element.
[0030]
Further features of the light modulation element and the light attenuator of the present invention will be described in detail with reference to the following examples.
[0031]
【Example】
"Example 1"
A sectional view of the light modulation element 10 of this example is shown in FIG. 1, and a plan view thereof is shown in FIG. An ITO transparent electrode is formed on one side of the glass substrate 5 and patterned into a comb-like transparent electrode shape with an electrode width of 5 μm and an electrode interval of 5 μm so as to be linear in the X-axis direction by photolithography, as shown in FIG. An electrode 3A and an electrode 3B, which are common electrodes electrically coupled to each other, are formed.
[0032]
Next, a polyimide film is applied on the same surface of the glass substrate 5 to form an alignment film 4 subjected to an alignment process in the X-axis direction. Further, a liquid crystal monomer liquid is applied onto the alignment film and polymerized and solidified to form a polymer liquid crystal layer having a film thickness d of 3 μm that is parallel to the glass substrate surface and molecularly oriented in the X-axis direction. The pattern of the comb-shaped electrode 3A and electrode 3B is processed so as to cover the polymer liquid crystal by photolithography and reactive ion etching, and the convex portions are 5 μm wide and the lattice pitch is 10 μm, and the cross section is rectangular (uneven) The diffraction grating 1 is assumed. In other words, the birefringent solid material of the convex portion is a diffraction grating composed of a polymer liquid crystal layer.
[0033]
Here, the electrode 3A and the electrode 3B do not need to be formed on the entire surface of the diffraction grating 1 made of the polymer liquid crystal layer, but only the central region of the grating necessary for optically modulating incident light. That is, the electrode width of the electrode 3A and the electrode 3B may be half or less of the lattice pitch. The birefringence of the diffraction grating 1 at the wavelength λ = 410 nm is an extraordinary refractive index n in the X-axis direction. _e = 1.75, normal refractive index n in the Y-axis direction _o = 1.55.
[0034]
Next, the seal 8 is printed on one side of the glass substrate 7 on which the alignment film 6 that has been subjected to the alignment process is formed using a seal material mixed with a spacer for controlling the gap as in the conventional liquid crystal element. It is formed into a cell by coating and solidifying by pressure on the glass substrate 5. Liquid crystal is injected from an injection port (not shown) provided in a part of the seal to fill the concave portion of the diffraction grating 1 with the liquid crystal 2, and the injection port is sealed to complete the light modulation element 10.
[0035]
The liquid crystal used here has an extraordinary refractive index n. _e (LC) is the extraordinary refractive index n of the polymer liquid crystal _e And its ordinary refractive index n _o (LC) is the ordinary light refractive index n of the polymer liquid crystal _o And a nematic liquid crystal having positive dielectric anisotropy in which liquid crystal molecules are aligned in the electric field direction, and a horizontal homogeneous alignment in which molecular alignment is aligned in the X-axis direction when no voltage is applied.
[0036]
An antireflection film (not shown) is formed on the light incident / exit surfaces of the

glass substrates

5 and 7. Note that when a voltage is applied to the

electrodes

3A and 3B, the alignment direction of the polymer liquid crystal and the liquid crystal is the linear direction of the

electrodes

3A and 3B so that the liquid crystals in the concave portions of the diffraction grating 1 are aligned in the same direction. The

alignment films

4 and 6 are preferably subjected to an alignment treatment so as to form a slight angle with respect to the axis. Further, the

electrodes

3A and 3B may be metal electrodes other than transparent electrodes such as Cr and Au except for a region necessary for optically modulating incident light.
[0037]
The light modulation function of the light modulation element 10 thus obtained will be described below. An AC power supply 9 having a rectangular voltage waveform is connected to the

electrodes

3A and 3B, and a voltage V is applied between the electrodes. As the voltage V increases, an electric field is generated in the liquid crystal in the concave portion of the diffraction grating 1 in the Y-axis direction, and the director of the liquid crystal aligned in the X-axis direction when no voltage is applied according to the electric field strength ) Is inclined in the electric field direction, that is, in the Y-axis direction. As a result, the ordinary refractive index n of the liquid crystal _o (LC: V) is apparently n _o (LC) to n _e Increased to (LC), abnormal light Refractive index n _e (LC: V) is apparently n _e (LC) to n _o Decrease to (LC).
[0038]
When light having a wavelength λ = 410 nm traveling in the Z-axis direction is incident on the light modulation element 10, when the interelectrode applied voltage V = 0, ordinary light polarization and extraordinary light polarization described by (Expression 1) and (Expression 2) Phase difference ΔΦ due to uneven step of diffraction grating with respect to incident light of _o (V) and ΔΦ _e Since (V) does not occur, as shown in FIG. 3 (a), incident light is transmitted straight and high zero-order diffracted light efficiency of 90% or more is obtained.
[0039]
On the other hand, when the applied voltage V between the electrodes is increased, V ₀ = △ Φ from 5V to 10V _o (V ₀ ) And △ Φ _e (V ₀ ) Is approximately π, that is, n _o (LC: V ₀ ) = 1.62, n _e (LC: V ₀ ) = 1.68. At this time, as shown in FIG. 3B, the incident light is diffracted and the zero-order diffracted light becomes substantially zero. Therefore, the 0th-order diffracted light efficiency decreases as the applied voltage V increases without depending on the polarization direction of the incident light. As a result, it is possible to obtain a light modulation element that exhibits high zero-order diffracted light efficiency when no voltage is applied, and has a low voltage and a high extinction ratio.
[0040]
In the light modulation element 10 obtained in this way, for example, light emitted from a blue-violet semiconductor laser emitting in the 410 nm wavelength band is condensed on the information recording surface of the optical recording medium to record or reproduce information. In an optical head device, it is disposed in an optical path from a semiconductor laser to an optical recording medium, and is used as an optical modulator that adjusts the amount of light to the optical recording medium. At this time, by using this element, the amount of 0th-order diffracted light condensed on the information recording surface can be adjusted regardless of the outgoing polarization direction of the semiconductor laser, so that the information recording surface is in a high-power laser oscillation state where the laser output is stable. Stable reproduction is possible without reducing the amount of collected light and erasing information already recorded.
[0041]
"Example 2" (comparative example)
As a comparative example, when the element configuration is such that the cell is filled with the liquid crystal 2 without forming the diffraction grating 1, the orientation of the liquid crystal facing the

electrodes

3A and 3B increases as the applied voltage between the electrodes increases. Since it varies depending on the thickness, 0th-order diffracted light remains, and a high extinction ratio cannot be obtained at low voltage.
[0042]
"Example 3"
A sectional view of the light modulation element 20 of this example is shown in FIG. The same components as those shown in FIGS. 1 and 2 are denoted by the same reference numerals. The plan view is the same as FIG. The difference in element configuration from Example 1 is that the ITO transparent electrode is formed on the wall surface (side surface) of the diffraction grating 1 made of a polymer liquid crystal, and a voltage is applied to the

electrodes

3A and 3B.
[0043]
A connection example of the electrode 3A will be described using a partially enlarged view of the diffraction grating shown in FIG. Transparent electrode 3A formed on the grating side wall surface of diffraction grating 1 ₁ Is an electrode 3A formed on the surface of the glass substrate 5 at the end of the lattice every other lattice. ₂ And the electrode 3A ₂ The voltage applied to is transmitted to the transparent electrode on the grating side wall surface. In the example shown in FIGS. 4 and 5, the transparent electrode 3A ₁ Is formed only on the side wall surface of the lattice, but may also be formed on the plane of the lattice facing the glass substrate 5 or on the opposite plane.
[0044]
Transparent electrode 3A on the lattice wall ₁ After processing the diffraction grating 1 made of polymer liquid crystal, a transparent electrode film is formed on the wall surface of the diffraction grating 1 by a film forming method having excellent step coverage such as sputtering film formation. Then, the transparent electrode formed on the flat surface of the concave portion between the convex portions of the diffraction grating 1 is removed by photolithography and lift-off or etching, and the conductive connection between the electrode 3A and the electrode 3B is cut off. The transparent electrode on the convex part plane (not the side surface) of the diffraction grating 1 may remain, but it is preferable because the transmittance is improved by removing it together with the transparent electrode on the concave part plane between the convex parts.
[0045]
In this example, the film thickness d of the polymer liquid crystal of the diffraction grating 1 is set to 5 μm in order to optically modulate incident light having a wavelength λ = 1550 nm which is an optical communication wavelength band. At a wavelength of 1550 nm, the birefringence of the polymer liquid crystal is an extraordinary refractive index n. _e = 1.67 and ordinary refractive index n _o = 1.50 and the extraordinary refractive index n of the liquid crystal _e (LC) and ordinary light refractive index n _o It is approximately equal to (LC).
[0046]
The light modulation function of the light modulation element 20 thus obtained will be described below in comparison with the light modulation element 10 of Example 1. Since the

electrodes

3A and 3B of the light modulation element 10 are formed on the surface of the glass substrate 5, the thickness of the liquid crystal 2 filled in the recesses of the diffraction grating 1 when the electrode spacing is about 5 μm and the applied voltage is 10 V or less. The electric field that is effectively generated in the vertical direction is limited to 3 μm or less.
[0047]
When the wavelength of incident light is relatively short, the phase difference ΔΦ necessary to obtain maximum diffraction _o And △ Φ _e The thickness (film thickness) d of the diffraction grating 1 where is approximately π may be small. However, when the wavelength of incident light is relatively long, it is necessary to increase the thickness d of the diffraction grating 1, and a high voltage is applied between the electrodes in order to align the entire liquid crystal filled in the concave portion of the diffraction grating 1. Must be applied. An increase in the driving voltage tends to cause a short circuit between the electrodes, and decreases the reliability of the element.
[0048]
In the light modulation element 20 of this example, since the electrode is formed on the side wall surface of the diffraction grating 1, a uniform electric field is applied to the entire liquid crystal 2 filled in the recess regardless of the thickness d of the diffraction grating 1. Can be generated. As a result, even when the wavelength of the incident light is relatively long, the phase difference ΔΦ _o And △ Φ _e Is possible to achieve a high extinction ratio with a low driving voltage.
[0049]
The light modulation element 20 obtained in this way collects, for example, light emitted from a semiconductor laser emitting in the 1550 nm wavelength band for wavelength division multiplexing optical communication into an optical fiber or an optical waveguide using a condensing element such as a lens. By placing in the optical path of the optical system that illuminates, or in the optical path that transmits and couples light in the 1550 nm wavelength band emitted from the optical fiber or optical waveguide to another optical fiber, optical waveguide, or photodetector, the applied voltage The optical attenuator can adjust the amount of transmitted light according to the size.
[0050]
That is, since the optical coupling surface of the optical fiber and optical waveguide used for optical communication and the light receiving surface of the photodetector are extremely small in area, 0th-order diffracted light that passes straight through the light modulation element 20 can be transmitted or received. Since light is not diffused and transmitted or received, it functions as an optical attenuator that is a light intensity modulation element.
[0051]
Since the

light modulation elements

10 and 20 of Examples 1 and 2 function as voltage variable optical attenuators regardless of the polarization direction of incident light, they can be applied to general purposes. In addition, since the portion where the light modulation function is manifested is an area composed of the diffraction grating 1 and the liquid crystal 2 and having a thickness of 10 μm or less, the

glass substrates

5 and 7 sandwiching these are made into thin pieces, for example, an element thickness of about 50 μm. The light modulation element can also be realized.
[0052]
As a result, without using a condensing element such as a lens, the optical fiber and the optical fiber can be directly coupled to each other or by inserting a light modulation element into the groove. In addition, it is possible to introduce an optical attenuator that suppresses an increase in volume and changes the optical coupling efficiency between optical fibers or optical waveguides in an optical system according to voltage. In this case, in order to maintain the extinction ratio, the grating pitch is preferably set to 10 μm or less.
[0053]
In Examples 1 and 3, the birefringent solid material made of polymer liquid crystal, which is the convex part of the diffraction grating, and the liquid crystal filled in the concave part of the grating have the same birefringence (ordinary refractive index and extraordinary refractive index). We explained the case of having no voltage applied Time If the birefringence of the diffraction grating is different, a transparent material layer having a uniform refractive index is formed on either the convex part or concave part of the diffraction grating, and the phase difference between the transmitted light of the convex part of the diffraction grating and the transmitted light of the concave part is If it is adjusted so that it does not occur, high zero-order diffracted light efficiency can be obtained.
[0054]
When the birefringence of the polymer liquid crystal is larger than that of the liquid crystal, the convex part of the diffraction grating is Rate n _o (LC) A grating with a uniform refractive index material layer and a polymer liquid crystal layer laminated. If the birefringence of the liquid crystal is larger than that of the polymer liquid crystal, the abnormal light reflection of the liquid crystal is formed in the concave portion of the diffraction grating Rate n _e (LC) A liquid crystal may be filled after forming a larger uniform refractive index material layer to a desired thickness.
[0055]
Since the light modulation element of the present invention utilizes the light diffraction phenomenon, the efficiency of the 0th-order diffracted light depends on the wavelength of the incident light. Therefore, in order to obtain a high extinction ratio with respect to incident light in a wide wavelength band, a configuration in which the light modulation elements of the present invention are stacked is preferable. Even when the incident light has a single wavelength or a narrow band wavelength, the extinction ratio is greatly improved by stacking. However, when the light modulation elements are stacked, it is preferable that the arrangement directions of the diffraction gratings of the elements are different. As a result, the light diffracted by one diffraction grating is diffracted by another diffraction grating and the multiple diffracted light components superimposed on the 0th-order diffracted light can be eliminated, so that a high extinction ratio can be obtained.
[0056]
In the first and third examples, the case where the cross-sectional shape of the diffraction grating is rectangular has been described. However, a sawtooth blazed grating, a triangular shape, a sine wave shape, or the like may be used. In addition, a case has been described in which the grating pitch is constant and the width of each of the concave and convex portions of the grating is the same, and the planar shape thereof is a straight line. However, a curved line such as a circle, the grating pitch is distributed in the plane, or irregular The widths of the parts may be different. Since the electrode pitch is also distributed by spatially distributing the grating pitch, the electric field acting on the liquid crystal filled in the concave portion of the diffraction grating is also spatially different. As a result, the voltage and zeroth-order light diffraction are determined by the design of the diffraction grating pattern. Efficiency dependencies can be spatially distributed.
[0057]
However, the strength of the electric field acting on the liquid crystal filled in the concave portion of the diffraction grating changes in inverse proportion to the distance between the

electrodes

3A and 3B. Therefore, in order to suppress the drive voltage to 20 V or less, the electrode distance should be 20 μm or less. The grating pitch is preferably 40 μm or less. In the first and third examples, the 0th-order diffracted light component is used, but diffracted light of a specific diffraction order may be used as signal light.
[0058]
In Examples 1 and 3, a single voltage is applied with the electrode 3A and the electrode 3B as a pair, but the electrodes may be divided and driven independently. For example, the electrode 3A is a common electrode, and the electrode 3B is divided and connected independently to an AC power source. In combination with an optical fiber array, an optical waveguide array, or the like, optical modulation driving can be performed independently for each channel.
[0059]
In the light modulation element shown in FIGS. 1 and 4, for example, a glass substrate 5 having a reflection layer formed on the surface thereof is used, and an alignment film 4, a diffraction grating 1, and the like are formed on the glass substrate 5. Can be obtained. That is, the light incident from the glass substrate 7 is reflected by the reflection layer and travels back and forth through the diffraction grating and exits from the glass substrate 7. Compared with the transmission type light modulation element, the convex portion made of the birefringent solid material of the diffraction grating 1 and the liquid crystal filled in the concave portion are reciprocated, so that the same phase difference generated by the uneven step of the diffraction grating is obtained. In addition, the thickness d of the grating may be half. Therefore, the diffraction grating 1 can be easily processed. In addition, since a conductor such as metal cannot be used as the reflective layer, TiO ₂ And Ta ₂ O ₅ High refractive index dielectric such as SiO and SiO ₂ It is preferable to use a dielectric multilayer mirror in which low-refractive-index dielectric materials such as those are alternately laminated with an optical film thickness of about the wavelength.
[0060]
【The invention's effect】
As described above, in the light modulation element of the present invention, the alignment direction of the liquid crystal in which the cross section made of a birefringent solid material is filled in the concave portion of the concave-convex diffraction grating is divided between the electrodes formed for each grating. Therefore, an optical modulation element that can change the efficiency of the 0th-order diffracted light that travels straight according to the applied voltage regardless of the polarization direction of the incident light is realized.
[0061]
In addition, by using a polymer liquid crystal as the birefringent solid material forming the diffraction grating, it is easy to obtain the same birefringence as that of the liquid crystal filled in the concave portion of the diffraction grating. High utilization efficiency can be obtained.
[0062]
In addition, by using a nematic liquid crystal having positive dielectric anisotropy, the orientation direction of the liquid crystal changes continuously according to the magnitude of the voltage applied between the electrodes formed for each grating of the diffraction grating, When no voltage is applied, the alignment direction of the liquid crystal molecules is easily and stably aligned in the grating longitudinal direction of the diffraction grating, so that an optical modulation element capable of changing the efficiency of the 0th-order diffracted light continuously and stably is realized.
[0063]
In addition, by forming an electrode on the side wall surface of the diffraction grating, a uniform electric field can be applied to the entire liquid crystal filled in the recess regardless of the thickness of the grating, so even if the wavelength of incident light is relatively long A high extinction ratio can be realized with a low driving voltage.
[0064]
Further, when the birefringence of the birefringent solid material forming the diffraction grating and the liquid crystal filled in the concave portion of the diffraction grating is different, the light-transmitting material having a uniform refractive index in either the convex portion or the concave portion of the diffraction grating By forming the layer, it is possible to adjust so that a phase difference does not occur between the transmitted light of the convex portion and the transmitted light of the concave portion of the diffraction grating, so that a high transmission efficiency of 0th-order diffracted light can be obtained.
[0065]
Further, the extinction ratio is greatly improved by stacking at least two light modulation elements of the present invention so that the grating longitudinal directions of the diffraction gratings formed in each light modulation element are different from each other.
[0066]
By using the optical modulation element of the present invention in an optical path from the semiconductor laser to the optical recording medium in an optical head device, for example, and using it as an optical modulator for adjusting the amount of light to the optical recording medium, the laser output is stabilized. Stable reproduction can be performed without erasing information already recorded by attenuating the amount of light collected on the information recording surface in a high-power laser oscillation state.
[0067]
An optical path of an optical system for condensing light emitted from a semiconductor laser that emits light in a wavelength band of 1550 nm for wavelength multiplexing optical communication on an optical fiber or optical waveguide using a condensing element such as a lens. The amount of transmitted light is adjusted according to the applied voltage by placing it in the optical path that transmits or couples light in the 1550 nm wavelength band emitted from the optical fiber or optical waveguide to another optical fiber, optical waveguide, or photodetector. It becomes a possible optical attenuator.
[Brief description of the drawings]
FIG. 1 is a side view showing a configuration example of a light modulation element of the present invention.
FIG. 2 is a plan view showing a configuration example of a light modulation element of the present invention.
FIG. 3 is a side view showing an operation when light is incident on the light modulation element shown in FIG. 1;
(A) is a transmission state when no voltage is applied, and (b) is a maximum diffraction state when a voltage is applied.
FIG. 4 is a side view showing another configuration example of the light modulation element of the present invention.
FIG. 5 is a partially enlarged schematic view for explaining the relationship between a diffraction grating and an electrode in another configuration example of the light modulation element of the present invention.
[Explanation of symbols]
1: Diffraction grating
2: Liquid crystal
3A, 3B, 3A2: Electrode
3A1: Transparent electrode
4, 6: Alignment film
5, 7: Glass substrate
8: Seal
9: AC power supply
10, 20: Light modulation element

Claims

Section becomes each of the convex portion of the concavo-convex grating a birefringent solid material, each of the recesses of the diffraction grating and the liquid crystal is filled, each ordinary refractive of the said birefringent solid material crystal And the extraordinary refractive indices are substantially equal,
Each of the birefringent solid material be formed an electrode for applying a voltage to the liquid crystal, can apply a voltage between the electrodes of the birefringent solid material adjacent the double in the de-energized A light modulation element characterized in that the alignment direction of the refractive solid material and the alignment direction of the liquid crystal are aligned .

The light modulation element according to claim 1, wherein the birefringent solid material is made of a polymer liquid crystal.

The light modulation element according to claim 1, wherein each of the electrodes formed on the birefringent solid material is distributed and connected to two different common electrodes.

The light modulation element according to claim 1, wherein the electrode is formed on at least a side surface of the birefringent solid material.

A transparent material layer having a uniform refractive index is formed on either one of the convex portion or concave portion of the diffraction grating, and the transmitted light of the convex portion of the diffraction grating and the concave portion of the diffraction grating in a state where no voltage is applied between the electrodes. light modulation element according to claims 1 which does not cause a phase difference 4 any one between the transmitted light.

2-layer optical modulation element to the optical modulation element, which is formed by stacking with different grating longitudinal direction of the respective diffraction grating according to claims 1 to 5 or.

7. The light modulation element according to claim 1, wherein a voltage is applied to the electrode to change the diffraction efficiency of incident light, and 0th-order diffracted light that travels straight through the light modulation element of incident light and does not travel straight. A separating means for separating non-zero-order diffracted light and selecting and receiving only zero-order diffracted light or non-zero-order diffracted light; and the amount of light received is adjusted according to the magnitude of applied voltage Optical attenuator.