JP2005003758A - Reflective light modulator and variable optical attenuator - Google Patents

Abstract

Translated fromJapanese

【課題】小型で消光比の高い反射型光変調素子を得る。
【解決手段】反射ミラー３と、反射型偏光回折格子２０と、液晶素子３０と、液晶素子３０を構成する液晶分子の制御手段とを備える反射型光変調素子２０であって、入射光が液晶素子を透過して反射型偏光回折格子に入射し、入射光のうち第１の偏光方向を有する第１直線偏光は回折されることなく反射型偏光回折格子２０を透過して反射ミラー３で反射されて液晶素子を出射し、第１直線偏光と直交する偏光方向を有する第２直線偏光は反射型偏光回折格子により回折されて第１直線偏光の出射光とは異なる方向に液晶素子を出射する反射型光変調素子とする。
【選択図】図１A reflection type light modulation device having a small size and a high extinction ratio is obtained.
A reflection type light modulation element comprising a reflection mirror, a reflection type polarization diffraction grating, a liquid crystal element, and a liquid crystal molecule control means constituting the liquid crystal element, the incident light being a liquid crystal. The first linearly polarized light having the first polarization direction out of the incident light is transmitted through the reflective polarization diffraction grating 20 and reflected by the reflection mirror 3 without being diffracted. The second linearly polarized light having a polarization direction orthogonal to the first linearly polarized light is diffracted by the reflective polarization diffraction grating and emitted from the liquid crystal element in a direction different from the light emitted from the first linearly polarized light. A reflective light modulation element is used.
[Selection] Figure 1

Description

Translated fromJapanese

【０００１】
【発明の属する技術分野】
本発明は、反射型光変調素子および可変光減衰器に関し、さらに詳しくは反射型光変調素子は反射型偏光回折格子と液晶素子とを備え、液晶分子の配向状態を電圧印加により制御する制御手段を用いて液晶素子の透過光の偏光状態を変化させ、反射型偏光回折格子による回折状態を変化させる反射型光変調素子に関する。また、可変光減衰器は反射型光変調素子による回折光または非回折光にいずれか一方を分別手段により受光し、印加電圧の大きさに応じて受光光量が変化するものである。
【０００２】
【従来の技術】
液晶を利用した光変調素子は液晶ディスプレイとして商品化されているとともに、液晶層に印加する電圧を変化させて透過光を制御する可変光減衰器や光スイッチなどの各種光デバイスに使用されている。例えば、特開２００３−６６４５０には位相板を液晶セルに一体化し、印加電圧の大きさに応じて透過光量が変化する可変光減衰器が開示されている。また、同じ特開２００３−６６４５０において、偏光回折型の偏光子をさらに一体化することにより、高強度の光が入射しても液晶層の温度上昇が少ないとともに安定した消光比が得られる構成例が開示されている。この従来の透過型光変調素子の構成例を図８に示す。位相板付き液晶セル５０に偏光回折型の偏光子６０が一体化され可変光減衰器３００としている。Ｙ軸方向の直線偏光は液晶層へ印加する電圧に応じて液晶セル５０を透過後の偏光状態が変化し、偏光回折型偏光子６０によりＹ軸方向の直線偏光成分は回折され、Ｘ軸方向の直線偏光成分は直進透過する。回折光と直進透過光とを分離し直進透過光のみを取りだすことにより電圧による可変減衰器となる。
【０００３】
また、同じ特開２００３−６６４５０には、液晶セル内の一方の電極を反射電極とすることにより、液晶層の厚さを半分にできるため低駆動電圧化および高速応答化につながることが開示されている。
【０００４】
波長多重光通信ネットワークにおいて、外部制御信号により波長多重信号に対し所定の波長の光を通過または通過を阻止する光スイッチが必要であり、阻止時と通過時との光量比Ｉ_ｏｆｆ／Ｉ_ｏｎとして−４０ｄＢ以上の高い消光比が求められる。
【０００５】
図８に示す従来の可変減衰器は、印加電圧の大きさに応じて消光比を調整する用途には適しているが、−４０ｄＢ以上の高い消光比が得られる印加電圧範囲は狭いため、安定して高い消光比を維持する光スイッチとして使用するためにはＩ_ｏｆｆレベルの低い電圧領域を拡大する必要があった。
【０００６】
消光比向上策として、可変光減衰器３００を直列に２個配置することが有効であるが、電圧印加配線が複雑化するとともに大型化を招く問題があった。特に、波長多重信号における所定の波長の光を通過または阻止する光スイッチや可変光減衰器とするために、液晶層に電圧を印加する電極を波長ごとに分割した画素電極構造として各画素を独立に外部制御信号によりスイッチングする必要があり、可変光減衰器の大型化を招くことなく２個の可変光減衰器間の画素電極を位置調整することは困難であった。
【０００７】
また、透過型の可変光減衰器３００の光出射側に反射ミラーを設けることにより可変光減衰器３００を直列に２個配置したのと同様の構成が単一の可変光減衰器３００を用いて実現する。しかし、偏光回折型の偏光子を用いる場合、往路で回折された光が反射ミラーで反射された後復路でも回折され、回折されない直進透過光に重畳するため高い阻止量が得られない問題があった。
【０００８】
【特許文献１】
特開２００３−６６４５０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、量産性に優れた小型で安定して高い阻止量が得られる反射型光変調素子および反射型光変調素子を用いた可変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、入射光を反射する反射ミラーと、複屈折材料が鋸歯状の断面形状に加工された回折格子からなる反射型偏光回折格子と、入射光が透過することによりその偏光状態を変化させる液晶素子と、液晶素子内の液晶分子の配向状態を変化させる制御手段とを備える反射型光変調素子であって、外部からの入射光が液晶素子を透過して反射型偏光回折格子に入射し、反射型偏光回折格子への入射光のうち第１の偏光方向を有する第１直線偏光は回折されることなく反射型偏光回折格子を透過して反射ミラーで反射されて液晶素子を出射し、第１の偏光方向と直交する偏光方向を有する第２直線偏光は反射型偏光回折格子により回折されて第１直線偏光の出射光とは異なる方向に液晶素子を出射することを特徴とする反射型光変調素子を提供する。
【００１１】
また、前記反射型偏光回折格子を構成する複屈折材料が高分子液晶からなる上記の反射型光変調素子を提供する。
【００１２】
また、前記液晶素子と前記反射型偏光回折格子との間に、第１直線偏光を回折しかつ第２直線偏光を回折することなく透過する透過型偏光回折格子が配置されている上記の反射型光変調素子を提供する。
【００１３】
また、前記透過型偏光回折格子は、高分子液晶層が鋸歯状の断面形状に加工された回折格子からなる上記の反射型光変調素子を提供する。
【００１４】
さらに、上記の反射型光変調素子を用いる可変光減衰器であって、前記制御手段は前記液晶素子に電圧を印加する手段であり、印加電圧により前記液晶素子を透過する光の偏光状態が変化して、反射型偏光回折格子により回折された光と回折されない光とが分離して２つの出射光とされ、いずれか一方の出射光のみを受光する分別手段が設けられていて、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可変光減衰器を提供する。
【００１５】
【発明の実施の形態】
［第１の実施態様］
図１は本発明の第１の実施態様の反射型光変調素子の構成例を示す側面図である。
透光性基板５と６の片面にそれぞれ透明電極８と９が形成され、さらにその上に同一方向に配向処理された配向膜（図示せず）が形成され、シール材１０を用いてセル化されている。ここで、配向膜の配向処理はＸＹ面内でＸ軸に対して４５度の角度をなす。さらに、セル内に常光屈折率ｎ_ｏ（ＬＣ）および異常光屈折率ｎ_ｅ（ＬＣ）（ｎ_ｏ（ＬＣ）＜ｎ_ｅ（ＬＣ））のネマティック液晶が注入されて液晶層７とし、透光性基板と平行でかつＸ軸に対して４５度の角度をなす方向に液晶分子の配向方向すなわち異常光屈折率ｎ_ｅ（ＬＣ）が揃った液晶素子３０が得られる。
【００１６】
また、片面に反射ミラー３が形成された透光性基板４の反射ミラー表面にＸ軸方向に配向処理された配向膜（図示せず）を形成し、その上に液晶モノマー溶液を塗布した後、紫外線を照射してＸ軸方向に液晶分子の配向方向の揃った常光屈折率ｎ_ｏ（ＰＬＣ）および異常光屈折率ｎ_ｅ（ＰＬＣ）（ｎ_ｏ（ＰＬＣ）＜ｎ_ｅ（ＰＬＣ））の高分子液晶層を形成する。さらに、フォトリソグラフィ法と反応性イオンエッチング法により、高分子液晶層を断面が鋸波形状でＸ軸方向に直線形状となるブレーズ型回折格子１に加工する。
【００１７】
次に、常光屈折率ｎ_ｏ（ＰＬＣ）と略等しい等方性屈折率ｎ_ｓの透明接着材２をブレーズ型回折格子１の凹部に充填するとともに透光性基板５に接着固定することにより反射型偏光回折格子２０が得られる。このようにして、高分子液晶を複屈折材料とした回折格子からなる反射型偏光回折格子２０と液晶素子３０とが一体化された反射型光変調素子１００となる。
【００１８】
ここで、液晶層の複屈折率（常光屈折率と異常光屈折率との差）を△ｎ（ＬＣ）＝ｎ_ｅ（ＬＣ）−ｎ_ｏ（ＬＣ）とすると、波長λの入射光に対して液晶層７のリタデーション値△ｎ（ＬＣ）×ｄ（ＬＣ）がλ／２となるように液晶層の厚さｄ（ＬＣ）を調整する。
【００１９】
また、高分子液晶からなるブレーズ型回折格子１の格子高さｄ（ＰＬＣ）は、入射光に対して＋１次回折効率が最大となるようにする。すなわち、高分子液晶の複屈折率を△ｎ（ＰＬＣ）＝ｎ_ｅ（ＰＬＣ）−ｎ_ｏ（ＰＬＣ）とすると、△ｎ（ＰＬＣ）×ｄ（ＰＬＣ）が略λ／２としている。図１では断面が鋸波形状のブレーズ型回折格子１を示すが、鋸波形状の斜辺部分を多段の階段で近似した疑似のブレーズ型回折格子としてもよい。
【００２０】
反射ミラー３に対して入射角θで反射型光変調素子１００に入射した光が反射型偏光回折格子２０で＋１次回折光として反射して入射光と同じ方向に出射する場合、ブレーズ型回折格子１の格子ピッチＰをＰ＝λ／（２×ｓｉｎθ）とし、光入出射面をブレーズ型回折格子１の鋸波形状の断面であるＹＺ面内とする。
【００２１】
なお、反射型光変調素子１００に対して入射光の入射角θ_１と出射光の出射角θ_２が異なっていてもよい。その場合、ブレーズ型回折格子１の格子ピッチＰをＰ＝λ／（ｓｉｎθ_１＋ｓｉｎθ_２）とする。
【００２２】
図２は、本発明の第１の実施態様の反射型光変調素子の他の構成例を示す側面図である。なお、図２に示す反射型光変調素子１１０の構成例のように、高分子液晶層からなるブレーズ型回折格子１を透光性基板５に形成し、常光屈折率ｎ_ｏ（ＰＬＣ）と略等しい等方性屈折率ｎ_ｓの透明接着材２をブレーズ型回折格子１の凹部に充填するとともに反射ミラー３が形成された透光性基板５に接着固定する構成としてもよい。なお、図２において図１と同符号のものは、同じ要素を示す。
【００２３】
次に、図３を用いて図１に示す反射型光変調素子１００の動作について説明する。以下、図１も参照しながら説明する。透明電極８と９とに接続された交流電源１１を通して液晶層７に電圧が印加され、液晶分子の配向状態が電圧により変化する。
【００２４】
印加電圧Ｖ_０（＝０Ｖ）のとき、液晶分子の配向は透明基板と平行でかつＸ軸に対して４５度の角度をなす方向に揃い、波長λで偏光方向がＹＺ平面内で入射角θの入射光に対して、液晶層７のリタデーション値は略λ／２となる。その結果、図３（ａ）に示すように、液晶素子３０を透過した波長λの光の偏光方向は９０度回転してＸ軸方向の第２直線偏光に変化し、反射型偏光回折格子２０に入射する。Ｘ軸方向の直線偏光は高分子液晶からなるブレーズ型回折格子１の異常光屈折率ｎ_ｅ（ＰＬＣ）に対応するため、ブレーズ型回折格子１と透明接着材２とで高分子液晶の複屈折率に相当する屈折率差△ｎ（ＰＬＣ）が生じ、＋１次回折光が最大である反射型偏光回折格子２０となり、入射光と同じ経路を戻り反射型光変調素子１００から出射する。ここで、反射型偏光回折格子２０により回折されたＸ軸方向の第２直線偏光の反射光は、液晶素子３０を再び透過することにより偏光方向がＹＺ平面内の出射光となる。
【００２５】
一方、大半の液晶分子が電界方向に揃うのに充分な印加電圧Ｖ（＞Ｖ_０）としたとき、液晶層７のリタデーション値は略ゼロとなり、液晶素子３０を透過後の入射光の偏光方向は変化せず、ＹＺ平面内の偏光方向を保ったまま第１直線偏光で反射型偏光回折格子２０に入射する。このときの偏光方向は高分子液晶層の常光屈折率ｎ_ｏ（ＰＬＣ）に対応し、透明接着材２の等方性屈折率ｎ_ｓと一致するためブレーズ型回折格子１で回折されず直進透過する。その結果、図３（ｂ）に示すように、反射ミラー３により鏡面反射されて高分子液晶層を往復し、反射型光変調素子１００から入射光と異なる方向に出射する。
【００２６】
反射型光変調素子１００において、電圧印加時に残留する液晶層７のリタデーション値を位相板を用いて相殺することにより低電圧で高い消光比が実現できる。例えば、図１において液晶素子３０の透光性基板６の片面に液晶層７の液晶分子の配向方向と直交する配向を有する高分子液晶からなる位相板を形成し、そのリタデーション値が電圧印加時に残留する液晶層７のリタデーション値と一致する高分子液晶層の厚さとすればよい。高分子液晶からなる位相板の代わりに水晶や１軸延伸後のポリカーボネートからなる位相板を用いてもよい。
【００２７】
ここで、反射型光変調素子１００の光出射側に入射光と同じ方向に出射する戻り光を信号光として受光し、入射光と異なる方向に出射する戻り光を受光しない光学系を用いることにより、透明電極８と９に印加する電圧をＶ_０からＶまで変化させることにより信号光量を変化できる電圧可変光減衰器や、電圧をＶ_０とＶとで切り替えることによりオン・オフ動作する光スイッチとなる。
【００２８】
図４に本発明の反射型光変調素子１００を、電圧による可変光減衰器や光スイッチとして用いる場合の光学系構成例を示す。光ファイバ１４から出射した光をレンズ１３により集光し偏光子１２に入射する。
偏光子１２はルチル（Ｔ_ｉＯ_２）結晶などの複屈折結晶を光入射面と光出射面とが角度をなすよう加工したもので、ＹＺ平面内の偏光とそれに直交するＸ軸方向の偏光の光路が分離して結晶中を伝搬するように結晶方位が決められ、反射型光変調素子１００に入射する。ここで、例えば反射型光変調素子１００への入射光の内、Ｘ軸方向の偏光は反射ミラー面に垂直に入射し、ＹＺ平面内の偏光は反射ミラー面に入射角θで入射するように、偏光子１２の光入射面と光出射面のなす角度を設定している。
【００２９】
図１の透明電極８と９に印加する電圧がＶ_０のとき、伝搬する光線を図４（ａ）に示す。反射型光変調素子１００への入射光の内、Ｘ軸方向の偏光は液晶素子を透過してＹＺ平面内の第１直線偏光となって反射型偏光回折格子で回折されることなく反射ミラーに垂直に入射および反射され、入射光と同じ光路で反射型光変調素子１００と偏光子１２とレンズ１３とを経て光ファイバ１４に集光される。また、ＹＺ平面内の偏光は液晶素子を透過してＸ軸方向の第２直線偏光となって反射型偏光回折格子で主に＋１次回折にされ、入射光と同じ光路で反射型光変調素子１００と偏光子１２とレンズ１３とを経て光ファイバ１４に集光される。すなわち、光ファイバ１４の出射光がその偏光に関わらず同じ光ファイバ１４に帰還して伝搬する。
【００３０】
また、透明電極８と９とに印加する電圧がＶのとき、伝搬する光線を図３（ｂ）に示す。反射型光変調素子１００の入射光の内、異常光偏光は異常光偏光のまま液晶素子を透過して反射型偏光回折格子で回折されるため、入射光と異なる光路で反射型光変調素子１００と偏光子１２とレンズ１３とを経ても光ファイバ１４に集光されない。また、常光偏光は常光偏光のまま液晶素子を透過して反射型偏光回折格子で回折されることなく反射ミラー面で鏡面反射されて入射光と異なる光路で反射型光変調素子１００と偏光子１２とレンズ１３とを経ても光ファイバ１４には集光されない。すなわち、光ファイバ１４の出射光がその偏光方向に関わらず同じ光ファイバ１４に帰還しない。
【００３１】
液晶素子３０の透明電極８と９の少なくとも一方が分割された複数の画素電極からなり、電極ごとに独立に電圧を印加できる反射型光変調素子としてもよい。このとき、図１において画素電極はＸ軸方向に分割され、Ｙ軸方向には分割されていないパターンとすることが好ましい。
【００３２】
また、図４において複屈折結晶を加工した偏光子１２を反射型光変調素子１００に組み合わせて用いた例を示したが、偏光回折型偏光子を偏光子１２として用いてもよい。図５は、反射型光変調素子１００の光入射側に偏光回折型偏光子２２を用いた場合の光減衰器の作用を示す側面図である。（ａ）は印加電圧がＶ_０の状態を、（ｂ）は印加電圧がＶの状態を示す。ここでは透過型のブレーズ型回折格子からなる偏光回折型偏光子２２を偏光子１２として用いている。偏光回折型偏光子２２は反射型光変調素子１００のブレーズ型回折格子１と同じ高分子液晶からなる回折格子だが、その格子ピッチは２倍の２Ｐとし、高分子液晶の常光屈折率と異常光屈折率とが逆になるように高分子液晶分子の配向方向がブレーズ型回折格子１と９０度異なる。また、鋸波断面形状の斜辺の傾斜方向が逆で、偏光回折型偏光子２２では透過型のブレーズ型回折格子の＋１次回折が最大となるよう、反射型光変調素子１００のブレーズ型回折格子１に比べ高分子液晶層からなるブレーズ型回折格子の格子高さが２倍となっている。
【００３３】
印加電圧Ｖ_０のとき、図５（ａ）に示すように偏光回折型偏光子２２を直進透過するＸ軸方向の偏光は反射型光変調素子１００の反射ミラーに垂直に入射および反射されて元の経路を帰還する。一方、偏光回折型偏光子２２で＋１次回折されたＹＺ平面内の偏光は反射型光変調素子１００の反射型偏光回折格子で＋１次回折されてレンズ１３を経て光ファイバ１４へと、元の経路を帰還する。
【００３４】
また、印加電圧Ｖのとき、図５（ｂ）に示すように偏光回折型偏光子２２を直進透過するＸ軸方向の偏光は反射型光変調素子１００の反射型偏光回折格子で回折されて光ファイバ１４に集光されない。一方、偏光回折型偏光子２２で＋１次回折されたＹＺ平面内の偏光は反射型光変調素子１００の反射ミラーに斜入射して鏡面反射されるため光ファイバ１４に集光されない。すなわち、図４に示した場合と同様に電圧可変光減衰器や光スイッチとして機能する。
【００３５】
図４および図５の減衰器の作用を示す図では、ＹＺ平面内の偏光とそれに直交するＸ軸方向の偏光が入射する場合について説明したが、図１から図３に示したように、ＹＺ平面内の偏光のみが入射してもよい。
【００３６】
波長多重光通信ネットワークに用いられる光スイッチや可変光減衰器では−４０ｄＢ以上の高い消光比を安定して実現すること、すなわちＩ_ｏｆｆレベルを充分小さな値にすることが重要である。
【００３７】
Ｉ_ｏｆｆレベルは反射型光変調素子１００の入射光と同じ光路を帰還する光量であり、図１において液晶層７のリタデーション値がゼロのときに反射型偏光回折格子に入射するＹＺ平面内の第１直線偏光が＋１次回折されて入射光と同じ光路を帰還する光量である。ここで、反射型偏光回折格子の消光比は、Ｉ_ｏｆｆレベルに関係する第１直線偏光（高分子液晶の常光偏光）の入射光に対して発生する＋１次回折光と、Ｉ_ｏｎレベルに関係する第２直線偏光（高分子液晶の異常光偏光）の入射光に対して発生する＋１次回折光との光量比に相当する。すなわち、ブレーズ型回折格子１の高分子液晶の常光屈折率ｎ_ｏ（ＰＬＣ）と透明接着材２の等方性屈折率ｎ_ｓとが完全に一致すれば、常光偏光の入射光に対して＋１次回折光は発生せず、原理的にＩ_ｏｆｆレベルはゼロとなり、消光比は無限大となる。しかし、ｎ_ｏ（ＰＬＣ）とｎ_ｓとのわずかな相違に起因して第１直線偏光に対し＋１次回折光が発生し、反射型偏光回折格子の消光比を劣化させる原因となる。
【００３８】
［第２の実施態様］
図６は本発明の第２の実施態様の反射型光変調素子２００の構成例を示す側面図である。反射型偏光回折格子の消光比を向上することにより反射型光変調素子１００の安定して高い消光比を実現する。
【００３９】
図１に示した反射型光変調素子１００と比較し、反射型偏光回折格子２０と液晶素子３０との間に、高分子液晶層をブレーズ型回折格子１５に加工して透明接着材２でその凹部が充填された透過型偏光回折格子４０が形成されている点が異なる。図６において、図２と同符号のものは同じ要素を表す。
【００４０】
ブレーズ型回折格子１５は透光性基板５の表面にＹ軸方向に配向処理された配向膜（図示せず）を形成し、その上に液晶モノマー溶液を塗布した後、紫外線などを照射してブレーズ型回折格子１と同じ常光屈折率ｎ_ｏ（ＰＬＣ）および異常光屈折率ｎ_ｅだが、Ｙ軸方向に液晶分子の配向方向の揃った高分子液晶層を形成する。すなわち、高分子液晶の配向方向がブレーズ型回折格子１とブレーズ型回折格子１５で直交している。さらに、フォトリソグラフィ法と反応性イオンエッチング法により高分子液晶層を断面が鋸波形状でＹ軸方向に直線形状となるブレーズ型回折格子１５に加工する。図６では断面形状が視認できるようにＹＺ平面内に鋸波形状の断面を示すが、後述するようにブレーズ型回折格子１とブレーズ型回折格子１５の多重回折光に起因する迷光を抑制するため、実際には鋸波形状の断面は紙面と垂直なＸＺ平面となるよう加工する。
【００４１】
高分子液晶からなるブレーズ型回折格子１５の格子高さＤ（ＰＬＣ）は、入射光に対して＋１次回折効率が最大となるようにする。すなわち、高分子液晶の複屈折率を△ｎ（ＰＬＣ）＝ｎ_ｅ（ＰＬＣ）−ｎ_ｏ（ＰＬＣ）とすると、△ｎ（ＰＬＣ）×Ｄ（ＰＬＣ）＝λとしている。図６では断面が鋸波形状のブレーズ型回折格子１５を示すが、鋸波形状の斜辺部分を多段の階段で近似した疑似のブレーズ型回折格子としてもよい。
【００４２】
また、ブレーズ型回折格子１５からなる透過型偏光回折格子４０の回折光は信号光として用いられないため、回折光が信号光に重畳しない回折角度となる格子ピッチであればよい。
【００４３】
次に、透過型偏光回折格子４０を付加することによる反射型光変調素子２００の消光比改善作用について説明する。
透過型偏光回折格子４０に入射する光のうち、液晶素子３０を透過してＸ軸方向の偏光となった第２直線偏光は、透過型偏光回折格子４０に対しては常光偏光であるため回折されることなく直進透過し、反射型偏光回折格子２０に対しては異常光偏光であるため＋１次回折光が発生し、反射ミラー３で反射され、入射光と同じ経路を戻り透過型偏光回折格子４０を直進透過して反射型光変調素子２００から信号光として出射する。このとき、透過型偏光回折格子４０の回折光、反射型偏光回折格子２０の＋１次回折光以外の回折光と回折されない光は少ないほどＩ_ｏｎレベルの低下を招かないので、少ないことが好ましい。
【００４４】
また、液晶素子３０を透過してＸＺ平面内の偏光となった第１直線偏光偏光は、透過型偏光回折格子４０に対しては異常光偏光であるため回折され、反射型偏光回折格子２０に対しては常光偏光であるため大半の光は回折されることなく反射ミラー３で反射され、透過型偏光回折格子４０により再び回折されて入射光と異なる経路を経て反射型光変調素子２００から出射し、信号光には重畳しない。このとき、ｎ_ｏ（ＰＬＣ）とｎ_ｓとのわずかな相違に起因して反射型偏光回折格子２０で発生する＋１次回折光は透過型偏光回折格子４０で回折され（図６の点線で示す）、ＹＺ平面外に出射して信号光には重畳しないため高い消光比が実現できる。また、透過型偏光回折格子４０の格子形状をブレーズ型回折格子１５としているため、反射ミラー３の反射前後である透過型偏光回折格子４０の往路（前）と復路（後）で回折された多重回折光の内、信号光に重畳する光量は極めて低い値になっている。
【００４５】
その結果、反射型偏光回折格子２０に透過型偏光回折格子４０を積層した本発明の反射型光変調素子２００の構成とすることにより、Ｉ_ｏｆｆレベルを安定して充分小さな値となるため、−４０ｄＢ以上の高い消光比を実現する電圧による光スイッチや可変光減衰器が得られる。特に、波長多重光通信ネットワークに用いられる入射光の波長帯域幅に対し、広い印加電圧マージンにおいて高い消光比を確保できる。
【００４６】
また、板厚の薄い透光性基板５を用いることにより反射ミラー３と液晶層７との間隔を小さくできるため、液晶素子３０の透明電極８と９の少なくとも一方が分割された複数の画素電極からなる場合でも、液晶素子の往路と復路とで画素電極の透過光の隣接電極への漏れが低減され、クロストークが抑制できる。
【００４７】
また、透過型偏光回折格子４０と反射型偏光回折格子２０との間隔を広くし、透過型偏光回折格子４０の空間格子パターンを分布させることによりブレーズ型回折格子１５を矩形状の格子とした場合でも、透過型偏光回折格子４０の往路と復路で回折された多重回折光が信号光に重畳することを回避できる。
【００４８】
また、反射型光変調素子１００（図５）および２００（図６）において、透光性基板５および６の屈折率の異なる媒質との界面で発生する反射光は、透光性基板の表面に微細な凹凸を形成することにより拡散光となり、信号光に重畳する迷光を軽減できる。また、液晶素子３０と反射型偏光回折格子２０または透過型偏光回折格子４０が一体化された反射型偏光回折格子２０を別々に作製し、接着剤を用いて一体化してもよい。
【００４９】
なお、上記第１および第２の実施態様では印加電圧Ｖ_０とＶの間の動作について説明したが、他の印加電圧またはこの印加電圧に応じた液晶の配向状態を変化させてもよい。
また、液晶配向としてツイスト配向や垂直配向やハイブリッド配向でもよい。また、液晶の配向状態の変化に応じて透過光の偏光状態が変化する液晶であればネマティック液晶以外にスメクティック液晶やコレスティック液晶またはそれ以外の液晶材料を用いてもよい。
【００５０】
また、液晶の配向状態を変化させる制御手段として透明電極間に印加する交流電圧の場合について示したが、電圧の代わりに磁場、温度、光などの制御手段を用いて液晶の配向状態を変化させてもよい。
また、複屈折材料として高分子液晶を用い鋸歯状の断面形状に加工された回折格子からなる反射型偏光回折格子とした例について説明したが、誘電体結晶などの複屈折材料を用いてもよい。また、格子ピッチ内で厚さ方向に液晶分子をチルト配向の空間分布をつけることにより、鋸歯状の断面形状に加工しない同じ高分子液晶層の厚さで異常光偏光に対してブレーズ型回折格子と同等の機能とできる。
【００５１】
反射ミラーとして、ガラス基板などの表面にアルミニウム、金、銀などの金属膜や、Ｔａ_２Ｏ_５やＴｉＯ_２などの高屈折率誘電体膜とＳｉＯ_２などの低屈折率誘電体膜を交互に積層した誘電体多層膜ミラーを蒸着法やスパッタ法で形成して用いることができる。
【００５２】
透明電極を形成する材料としては、ＩＴＯなどの酸化物膜や、アルミニウム、クロムなどの金属箔膜が使用できるが、伝導性、光透過性、耐久性などの点からＩＴＯ膜を用いることが好ましい。
【００５３】
偏光状態を変えるとは、液晶素子へ入射する直線偏光を円偏光と直線偏光を含む楕円偏光に変換することを意味し、上記実施態様においては、電圧Ｖ_０で直線偏光の偏光方向を９０度回転する場合を示す。
【００５４】
本発明の反射型光変調素子および可変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。
【００５５】
【実施例】
本実施例を図６に示す反射型光変調素子２００の断面図を用いて説明する。透光性基板５であるガラス基板の片面に成膜されたＩＴＯ膜からなる透明電極８をフォトリソグラフィ法とエッチング法によりＹ軸方向に直線状（断面形状が視認できるようＸ軸方向に直線形状として表示）となるよう電極幅１００μｍで電極間隔５μｍの画素電極形状にパターニングし、透光性基板６であるガラス基板の片面にＩＴＯ膜からなる透明電極９を成膜する。透明電極８および９上にポリイミド膜を塗布してＸ軸に対して４５度の角度をなす方向に配向処理を施した配向膜（図示せず）とする。
【００５６】
次に、セル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配向膜が形成された透光性基板６（ガラス基板）の片面にシール１０を印刷塗布し、透光性基板５（ガラス基板）に圧着固化することにより空セルを作製する。シールの一部に設けられた注入口（図示せず）から液晶を注入した後、注入口を封止して液晶素子３０ができあがる。ここで用いる液晶は波長λ＝１．５６μｍに対して常光屈折率ｎ_ｏ（ＬＣ）＝１．４９および異常光屈折率ｎ_ｅ（ＬＣ）＝１．６５で、セルギャップをｄ（ＬＣ）＝５．５μｍとする。このとき液晶層７のリタデーション値は０．８８μｍである。
【００５７】
また、図６には示されていないが、常光屈折率方向が液晶層の異常光屈折率方向と一致するとともにリタデーション値０．１μｍの高分子液晶からなる位相板を透光性基板６と別の透明基板（図示せず）で狭持して形成する。透明電極８と９に電圧を印加しないとき、この位相板と液晶層７とを一体化した液晶素子３０全体のリタデーション値が０．７８μｍとなり、波長λ＝１．５６μｍに対してλ／２波長板に相当するようにしている。このときＹＺ平面内の偏光を有する入射光は液晶素子３０を透過してＸ軸方向の偏光を有する出射光に偏光方向が変化する。
【００５８】
透明電極８と９に電圧を印加し、液晶層７の液晶分子を透光性基板に対して平行配向から垂直配向に変化させることにより、液晶層のリタデーション値が減少し印加電圧６Ｖで０．１μｍ残留するが、位相板と液晶層７とを一体化することで位相板がこの残留分を打ち消して全体のリタデーション値はゼロとなり、入射光は液晶素子３０を透過して偏光状態が不変のまま出射する。
【００５９】
次に、位相板付き液晶素子３０の透光性基板５の片面にポリイミド膜を塗布してＹ軸方向に配向処理を施した配向膜（図示せず）とする。その上に液晶モノマー溶液を塗布した後紫外線を照射して常光屈折率ｎ_ｏ（ＰＬＣ）＝１．５５および異常光屈折率ｎ_ｅ（ＰＬＣ）＝１．７５で、Ｙ軸方向に配向方向の揃った膜厚６．８μｍの高分子液晶層を形成する。さらに、フォトリソグラフィ法と反応性イオンエッチング法により高分子液晶層に対し、断面が鋸波形状を８段の階段形状で近似した深さ６．８μｍでＹ軸方向に直線形状（断面形状が視認できるようＸ軸方向に直線形状として表示）となる疑似のブレーズ型回折格子１５に加工する。
【００６０】
次に、片面に反射ミラー３が形成された透光性基板４であるガラス基板の反射ミラー３の表面にＸ軸方向に配向処理された配向膜（図示せず）を形成し、その上にブレーズ型回折格子１５と同じ液晶モノマーを用い同様のプロセスにて、Ｘ軸方向に液晶分子の配向方向の揃った高分子液晶からなり、断面が鋸波形状を８段の階段形状で近似したＸ軸方向に直線形状となる疑似のブレーズ型回折格子１に加工する。ここで、斜辺部を階段状で近似した疑似のブレーズ型回折格子１の格子高さは３．４μｍとしている。また、反射ミラー３は高屈折率誘電体膜Ｔａ_２Ｏ_５と低屈折率誘電体膜ＳｉＯ_２を交互に光学膜厚が略λ／４となるよう３０層以上積層し、１．４８μｍから１．６２μｍまでの波長帯域で９９．９％以上の反射率を有する多層膜反射ミラーを用いる。
【００６１】
さらに、常光屈折率ｎ_ｏ（ＰＬＣ）と略等しい等方性屈折率ｎ_ｓ＝１．５６の透明接着材２を用いて、疑似のブレーズ型回折格子１および疑似のブレーズ型回折格子１５の凹部に充填するとともに透光性基板４と５に接着固定することにより反射型偏光回折格子２０と透過型偏光回折格子４０が得られる。このようにして、反射型光変調素子２００を作製する。このとき、Ｘ軸方向の第２直線偏光は透過型偏光回折格子４０を回折されることなく直進透過し、反射型偏光回折格子２０で８０％以上が＋１次回折される。また、ＹＺ平面内の第１直線偏光は透過型偏光回折格子４０で８０％以上が＋１次回折され、反射型偏光回折格子２０を直進透過する。
【００６２】
この反射型光変調素子２００にＹＺ面内で反射ミラー面の法線方向であるＺ軸に対して入射角θ＝５°で光が入射する。入射光は液晶素子３０の画素電極ごとに１．４８μｍから１．６２μｍまでの波長帯の異なる波長の光が入射し、各画素電極に対応した入射光波長に対して＋１次回折光の回折角度が入射角θと同じになるように、反射型偏光回折格子２０の疑似のブレーズ型回折格子１５の格子ピッチが８．５μｍから９．３μｍまで分布している。
【００６３】
透明電極８と９とに接続された交流電源を通して画素電極ごとに液晶層７に印加する電圧Ｖを調整する。ＹＺ平面内の偏光（第１直線偏光）は位相板付き液晶素子３０を透過すると、Ｖ＝０ではほぼ直交するＸ軸方向の偏光（第２直線偏光）に変化し、Ｖ＝６Ｖ前後で偏光状態は変化しない。
【００６４】
したがって、反射型光変調素子２００の各画素電極に入射する１．４８μｍから１．６２μｍまでの波長帯の光に対して、反射型偏光回折格子２０で回折されて入射光と同じ角度方向に反射型光変調素子２００を出射する光は、印加電圧Ｖ＝０では大半であるが、印加電圧Ｖ＝６Ｖ前後ではほぼゼロとなる。また、印加電圧Ｖが０と６Ｖの中間領域ではその中間の光量となる。
【００６５】
すなわち、入射光と同じ角度の出射光と、異なる角度の出射光とを分離する光学系と組み合わせることにより高い消光比の電圧による光スイッチまたは可変減衰器が得られる。例えば、光ファイバから出射する１．４８μｍから１．６２μｍまでの波長帯の波長多重光を回折格子により分光した後、レンズを用いて反射型光変調素子２００に入射角θで入射させるとともに各画素電極に集光する。
【００６６】
その結果、入射光と同じ角度方向に反射型光変調素子２００を出射した光は光ファイバに帰還して伝搬されるがそれ以外の光は遮断されるため−４０ｄＢ以上の高い消光比を安定して実現する電圧による光スイッチまたは可変減衰器となる。また、反射ミラーと電極との間隔を小さくできるため、画素電極構造であっても画素間の光信号クロストークが抑制できる。
【００６７】
図７中の○線は本発明の反射型光変調素子２００液晶層へ電圧を０Ｖから１０Ｖまで印加したとき、光ファイバから出射した光の内で反射型光変調素子を反射して同じ光ファイバに戻る信号光効率を示すグラフである。４．６Ｖから１０Ｖ以上の広い電圧印加範囲で−４０ｄＢ以上の高い消光比が得られる。
【００６８】
比較のために、従来の可変光減衰器３００を用いた場合の印加電圧に対する信号光効率を図７中に実線で示す。−４０ｄＢ以上の消光比が得られるのは６．０Ｖから６．４Ｖまでの狭い印加電圧範囲に留まっている。
【００６９】
【発明の効果】
以上説明したように、本発明の反射型光変調素子では液晶分子の配向状態の変化に応じて入射光の偏光状態を変化させる液晶素子と反射基板面上に形成された複屈折材料が鋸歯状の断面形状に加工された回折格子からなり、第１直線偏光を鏡面反射し、第２直線偏光を回折した反射する反射型偏光回折格子とが一体化されているため、小型で消光比の高い反射型光変調素子が得られる。また、液晶の配向状態を電圧により変化させる場合、反射ミラーと電圧印加用電極の間隔を小さくできるため、画素電極構造であっても画素間の光信号クロストークが抑制できる。
【００７０】
また、反射型偏光回折格子として複屈折性を有する高分子液晶を用いることにより、微細な格子ピッチでも正確に鋸歯状の断面形状に加工できるため、高い信号光回折効率が得られる。
【００７１】
また、液晶素子と反射型偏光回折格子との間に、第１直線偏光を回折しかつこれと直交する偏光方向の第２直線偏光を回折することなく透過する透過型偏光回折格子を配置することによりさらに高い消光比が得られる。
【００７２】
特に、透過型偏光回折格として複屈折性を有する高分子液晶を鋸歯状の断面形状に加工した回折格子とすることにより、往路および復路で透過型偏光回折格子により回折された多重回折光がＩ_ｏｆｆレベルに重畳しないため、広い印加電圧範囲において安定してＩ_ｏｆｆレベルを低い値に維持できる。その結果、波長多重光通信ネットワークなどの使用に適する光スイッチや光減衰器となる。
【００７３】
また、反射型光変調素子を出射する反射型偏光回折格子により回折された光と回折されない光とを分離して一方の出射光のみを受光する分別手段を付加することにより、印加電圧の大きさに応じて出射光の光量を調整する可変光減衰器となる。
【図面の簡単な説明】
【図１】本発明の第１の実施態様の反射型光変調素子の構成例を示す断面図。
【図２】本発明の第１の実施態様の反射型光変調素子の他の構成例を示す断面図。
【図３】図１に示す反射型光変調素子に光が入射した場合の作用を示す断面図、（ａ）電圧非印加時の最大＋１次回折反射状態、（ｂ）電圧印加時でミラー反射状態。
【図４】図１に示す反射型光変調素子の光入射側に複屈折結晶からなる偏光子を用いた場合の光減衰器の作用を説明する断面図、（ａ）電圧非印加時の状態、（ｂ）は電圧印加時の状態。
【図５】図１に示す反射型光変調素子の光入射側に偏光回折型偏光子を用いた場合の光減衰器の作用を示す断面図、（ａ）電圧非印加時の状態、（ｂ）電圧印加時の状態。
【図６】本発明の第２の実施態様の反射型光変調素子の構成例を示す断面図。
【図７】本発明の実施例の反射型光変調素子を用いた場合の、印加電圧と信号光効率を示すグラフ（○線は実施例を、実線は従来構成の比較例）。
【図８】従来例の透過型光変調素子の構成例を示す断面図。
【符号の説明】
１、１５：ブレーズ型回折格子
２：透明接着材
３：反射ミラー
４、５、６：透光性基板
７：液晶層
８、９：透明電極
１０：シール
１１：交流電源
１２：偏光子
１３：レンズ
１４：光ファイバ
２２、６０：偏光回折型偏光子
２０：反射型偏光回折格子
３０：液晶素子
４０：透過型偏光回折格子
５０：位相板付き液晶セル
１００、１１０、２００：反射型光変調素子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a reflection type light modulation element and a variable optical attenuator, and more specifically, the reflection type light modulation element includes a reflection type polarization diffraction grating and a liquid crystal element, and controls the alignment state of liquid crystal molecules by applying a voltage. The present invention relates to a reflection-type light modulation element that changes the polarization state of transmitted light of a liquid crystal element using a light source and changes the diffraction state of a reflection-type polarization diffraction grating. Further, the variable optical attenuator receives either diffracted light or non-diffracted light by the reflective light modulation element by the separating means, and the amount of received light changes according to the magnitude of the applied voltage.
[0002]
[Prior art]
Light modulation elements using liquid crystals are commercialized as liquid crystal displays, and are used in various optical devices such as variable optical attenuators and optical switches that control transmitted light by changing the voltage applied to the liquid crystal layer. . For example, Japanese Patent Application Laid-Open No. 2003-66450 discloses a variable optical attenuator in which a phase plate is integrated with a liquid crystal cell and the amount of transmitted light changes according to the magnitude of an applied voltage. Further, in the same Japanese Patent Application Laid-Open No. 2003-66450, a configuration example in which a polarization diffraction type polarizer is further integrated so that a temperature rise of the liquid crystal layer is small and a stable extinction ratio can be obtained even when high-intensity light is incident. Is disclosed. An example of the configuration of this conventional transmissive light modulation element is shown in FIG. A polarizationdiffraction type polarizer 60 is integrated with theliquid crystal cell 50 with a phase plate to form a variableoptical attenuator 300. The linearly polarized light in the Y-axis direction changes its polarization state after being transmitted through theliquid crystal cell 50 in accordance with the voltage applied to the liquid crystal layer, and the linearly polarized light component in the Y-axis direction is diffracted by the polarizationdiffractive polarizer 60, and the X-axis direction The linearly polarized light component is transmitted in a straight line. By separating the diffracted light and the straight transmitted light and extracting only the straight transmitted light, a variable attenuator by voltage is obtained.
[0003]
Also, the same Japanese Patent Application Laid-Open No. 2003-66450 discloses that by using one electrode in the liquid crystal cell as a reflective electrode, the thickness of the liquid crystal layer can be halved, leading to lower drive voltage and faster response. ing.
[0004]
In a wavelength division multiplexing optical communication network, an optical switch is required to pass or block light of a predetermined wavelength with respect to the wavelength division multiplexed signal by an external control signal._off / I_on Therefore, a high extinction ratio of -40 dB or more is required.
[0005]
Although the conventional variable attenuator shown in FIG. 8 is suitable for an application in which the extinction ratio is adjusted according to the magnitude of the applied voltage, the applied voltage range in which a high extinction ratio of −40 dB or more can be obtained is stable. For use as an optical switch that maintains a high extinction ratio, I_off It was necessary to expand the low voltage range.
[0006]
As a measure for improving the extinction ratio, it is effective to arrange two variableoptical attenuators 300 in series. However, there is a problem that the voltage application wiring becomes complicated and increases in size. In particular, in order to make an optical switch or variable optical attenuator that passes or blocks light of a predetermined wavelength in a wavelength multiplexed signal, each pixel is made independent as a pixel electrode structure in which an electrode for applying a voltage to a liquid crystal layer is divided for each wavelength. Therefore, it is difficult to adjust the position of the pixel electrode between the two variable optical attenuators without increasing the size of the variable optical attenuator.
[0007]
In addition, a single variableoptical attenuator 300 has a configuration similar to that in which two variableoptical attenuators 300 are arranged in series by providing a reflection mirror on the light output side of the transmission type variableoptical attenuator 300. Realize. However, when a polarization diffraction type polarizer is used, the light diffracted in the forward path is reflected by the reflection mirror and then diffracted in the return path, and is superimposed on the straight transmitted light that is not diffracted. It was.
[0008]
[Patent Document 1]
JP 2003-66450 A
[0009]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and aims to provide a reflection type light modulation element that is excellent in mass productivity and that can stably obtain a high blocking amount and a variable optical attenuator using the reflection type light modulation element. To do.
[0010]
[Means for Solving the Problems]
The present invention is a reflection mirror that reflects incident light, a reflective polarization diffraction grating that is formed of a diffraction grating in which a birefringent material is processed into a sawtooth cross-sectional shape, and changes its polarization state by transmitting incident light. A reflection type light modulation element comprising a liquid crystal element and a control means for changing the alignment state of liquid crystal molecules in the liquid crystal element, and incident light from the outside passes through the liquid crystal element and enters the reflection type polarization diffraction grating. The first linearly polarized light having the first polarization direction out of the incident light to the reflective polarization diffraction grating passes through the reflective polarization diffraction grating without being diffracted and is reflected by the reflection mirror to exit the liquid crystal element. The second linearly polarized light having a polarization direction orthogonal to the first polarization direction is diffracted by the reflective polarization diffraction grating and is emitted from the liquid crystal element in a direction different from the outgoing light of the first linearly polarized light. Provides light modulation elements That.
[0011]
Further, the present invention provides the above-described reflection type light modulation element in which the birefringent material constituting the reflection type polarization diffraction grating is made of a polymer liquid crystal.
[0012]
Further, the reflection type diffraction grating described above, wherein a transmission type polarization diffraction grating that diffracts the first linearly polarized light and transmits the second linearly polarized light without diffracting is disposed between the liquid crystal element and the reflective polarization diffraction grating. A light modulation element is provided.
[0013]
Further, the transmission type polarization diffraction grating provides the above-mentioned reflection type light modulation element comprising a diffraction grating in which a polymer liquid crystal layer is processed into a sawtooth cross-sectional shape.
[0014]
Further, in the variable optical attenuator using the reflection type light modulation element, the control means is means for applying a voltage to the liquid crystal element, and a polarization state of light transmitted through the liquid crystal element is changed by the applied voltage. Then, the light diffracted by the reflective polarization diffraction grating and the light not diffracted are separated into two outgoing lights, and a separating means for receiving only one of the outgoing lights is provided. Provided is a variable optical attenuator characterized in that the amount of light received is adjusted according to the size.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
[First Embodiment]
FIG. 1 is a side view showing a configuration example of a reflective light modulation device according to a first embodiment of the present invention.
Transparent electrodes 8 and 9 are respectively formed on one side of thetranslucent substrates 5 and 6, and an alignment film (not shown) that is aligned in the same direction is formed thereon, and is formed into a cell by using thesealing material 10. Has been. Here, the alignment treatment of the alignment film makes an angle of 45 degrees with respect to the X axis in the XY plane. In addition, the ordinary refractive index n in the cell_o (LC) and extraordinary refractive index n_e (LC) (n_o (LC) <n_e (LC)) nematic liquid crystal is injected to form theliquid crystal layer 7, and the alignment direction of the liquid crystal molecules, that is, the extraordinary light refractive index n is parallel to the translucent substrate and forms an angle of 45 degrees with respect to the X axis._e Aliquid crystal element 30 with uniform (LC) is obtained.
[0016]
Further, after forming an alignment film (not shown) aligned in the X-axis direction on the reflection mirror surface of thetranslucent substrate 4 having thereflection mirror 3 formed on one side, and applying a liquid crystal monomer solution thereon. Ordinary refractive index n in which the alignment direction of liquid crystal molecules is aligned in the X-axis direction by irradiating ultraviolet rays_o (PLC) and extraordinary refractive index n_e (PLC) (n_o (PLC) <n_e (PLC)) polymer liquid crystal layer is formed. Further, the polymer liquid crystal layer is processed into a blazeddiffraction grating 1 having a sawtooth cross section and a linear shape in the X-axis direction by photolithography and reactive ion etching.
[0017]
Next, ordinary light refractive index n_o Isotropic refractive index n substantially equal to (PLC)_s The reflectivepolarizing diffraction grating 20 is obtained by filling thetransparent adhesive 2 in the concave portion of the blazeddiffraction grating 1 and adhering and fixing it to thetranslucent substrate 5. In this way, the reflection typelight modulation element 100 in which the reflection typepolarization diffraction grating 20 composed of a diffraction grating using a polymer liquid crystal as a birefringent material and theliquid crystal element 30 are integrated is obtained.
[0018]
Here, the birefringence (difference between the ordinary light refractive index and the extraordinary light refractive index) of the liquid crystal layer is expressed by Δn (LC) = n_e (LC) -n_o Assuming (LC), the thickness d (LC) of the liquid crystal layer is adjusted so that the retardation value Δn (LC) × d (LC) of theliquid crystal layer 7 becomes λ / 2 with respect to incident light of wavelength λ. .
[0019]
Further, the grating height d (PLC) of the blazeddiffraction grating 1 made of polymer liquid crystal is set so that the + 1st order diffraction efficiency is maximized with respect to the incident light. That is, the birefringence of the polymer liquid crystal is expressed by Δn (PLC) = n_e (PLC) -n_o If (PLC), Δn (PLC) × d (PLC) is approximately λ / 2. Although FIG. 1 shows a blazeddiffraction grating 1 having a sawtooth cross section, a pseudo blazed diffraction grating having a sawtooth-shaped hypotenuse approximated by a multi-step staircase may be used.
[0020]
When light incident on the reflectivelight modulation element 100 at an incident angle θ with respect to thereflective mirror 3 is reflected as + 1st order diffracted light by the reflectivepolarizing diffraction grating 20 and emitted in the same direction as the incident light, the blazeddiffraction grating 1 Is set to P = λ / (2 × sin θ), and the light incident / exit surface is in the YZ plane, which is a sawtooth cross section of the blazeddiffraction grating 1.
[0021]
Note that the incident angle θ of the incident light with respect to the reflectivelight modulation element 100₁ And the outgoing angle of outgoing light θ₂ May be different. In that case, the grating pitch P of the blazeddiffraction grating 1 is set to P = λ / (sin θ₁ + Sinθ₂ ).
[0022]
FIG. 2 is a side view showing another configuration example of the reflective light modulation element according to the first embodiment of the present invention. As in the configuration example of the reflectivelight modulation element 110 shown in FIG. 2, a blazeddiffraction grating 1 made of a polymer liquid crystal layer is formed on atranslucent substrate 5, and the ordinary light refractive index n_o Isotropic refractive index n substantially equal to (PLC)_s Thetransparent adhesive 2 may be filled in the concave portion of the blazeddiffraction grating 1 and adhered and fixed to thetranslucent substrate 5 on which thereflection mirror 3 is formed. 2, the same reference numerals as those in FIG. 1 denote the same elements.
[0023]
Next, the operation of the reflectivelight modulation device 100 shown in FIG. 1 will be described with reference to FIG. Hereinafter, description will be given with reference to FIG. A voltage is applied to theliquid crystal layer 7 through theAC power supply 11 connected to thetransparent electrodes 8 and 9, and the alignment state of the liquid crystal molecules changes according to the voltage.
[0024]
Applied voltage V₀ When (= 0V), the alignment of the liquid crystal molecules is aligned in a direction parallel to the transparent substrate and at an angle of 45 degrees with respect to the X axis, and is incident light having an incident angle θ in the YZ plane with a wavelength λ. On the other hand, the retardation value of theliquid crystal layer 7 is approximately λ / 2. As a result, as shown in FIG. 3A, the polarization direction of the light having the wavelength λ transmitted through theliquid crystal element 30 is rotated by 90 degrees to be changed to the second linearly polarized light in the X-axis direction. Is incident on. The linearly polarized light in the X-axis direction is the extraordinary refractive index n of the blazeddiffraction grating 1 made of polymer liquid crystal._e In order to cope with (PLC), a refractive index difference Δn (PLC) corresponding to the birefringence of the polymer liquid crystal is generated between the blazeddiffraction grating 1 and thetransparent adhesive 2, and the + 1st order diffracted light is maximum. It becomes thepolarization diffraction grating 20, and returns from the reflection typelight modulation element 100 through the same path as the incident light. Here, the reflected light of the second linearly polarized light in the X-axis direction diffracted by the reflectivepolarization diffraction grating 20 is transmitted through theliquid crystal element 30 again, so that the polarization direction becomes outgoing light in the YZ plane.
[0025]
On the other hand, an applied voltage V (> V) sufficient to align most liquid crystal molecules in the electric field direction.₀ ), The retardation value of theliquid crystal layer 7 becomes substantially zero, the polarization direction of the incident light after passing through theliquid crystal element 30 does not change, and the first linearly polarized light is reflected while maintaining the polarization direction in the YZ plane. The light enters thepolarization diffraction grating 20. The polarization direction at this time is the ordinary refractive index n of the polymer liquid crystal layer._o (PLC) and isotropic refractive index n oftransparent adhesive 2_s Therefore, the light is transmitted through the blazeddiffraction grating 1 without being diffracted. As a result, as shown in FIG. 3B, the light is reflected by the reflectingmirror 3 and reciprocates in the polymer liquid crystal layer, and is emitted from the reflectivelight modulation element 100 in a direction different from the incident light.
[0026]
In the reflectivelight modulation element 100, a high extinction ratio can be realized at a low voltage by canceling out the retardation value of theliquid crystal layer 7 remaining when a voltage is applied, using a phase plate. For example, in FIG. 1, a phase plate made of a polymer liquid crystal having an orientation orthogonal to the orientation direction of the liquid crystal molecules of theliquid crystal layer 7 is formed on one surface of thetranslucent substrate 6 of theliquid crystal element 30, and the retardation value thereof is The thickness of the polymer liquid crystal layer may be the same as the retardation value of the remainingliquid crystal layer 7. Instead of a phase plate made of a polymer liquid crystal, a phase plate made of quartz or uniaxially stretched polycarbonate may be used.
[0027]
Here, the return light emitted in the same direction as the incident light is received as signal light on the light emitting side of the reflectivelight modulation element 100, and an optical system that does not receive the return light emitted in the direction different from the incident light is used. The voltage applied to thetransparent electrodes 8 and 9 is V₀ Voltage variable optical attenuator that can change the amount of signal by changing from V to V₀ By switching between V and V, the optical switch is turned on / off.
[0028]
FIG. 4 shows a configuration example of an optical system when the reflection typelight modulation element 100 of the present invention is used as a variable optical attenuator or an optical switch by voltage. The light emitted from theoptical fiber 14 is collected by thelens 13 and enters thepolarizer 12.
Polarizer 12 is rutile (T_i O₂ ) A birefringent crystal, such as a crystal, processed so that the light incident surface and the light output surface form an angle. The light path of polarized light in the YZ plane and polarized light in the X-axis direction perpendicular to it is separated and propagated in the crystal. Thus, the crystal orientation is determined, and the light enters the reflectivelight modulation element 100. Here, for example, in the incident light to the reflection typelight modulation element 100, the polarization in the X-axis direction is perpendicularly incident on the reflection mirror surface, and the polarization in the YZ plane is incident on the reflection mirror surface at an incident angle θ. The angle between the light incident surface and the light exit surface of thepolarizer 12 is set.
[0029]
The voltage applied to thetransparent electrodes 8 and 9 in FIG.₀ FIG. 4 (a) shows the propagating light rays. Of the incident light to the reflectivelight modulation element 100, the polarized light in the X-axis direction is transmitted through the liquid crystal element to become the first linearly polarized light in the YZ plane, and is not diffracted by the reflective polarization diffraction grating. The light is incident and reflected vertically, and is collected on theoptical fiber 14 through the reflectivelight modulator 100, thepolarizer 12, and thelens 13 along the same optical path as the incident light. Also, the polarized light in the YZ plane is transmitted through the liquid crystal element to become the second linearly polarized light in the X-axis direction, and is mainly + 1st-order diffracted by the reflective polarization diffraction grating, and the reflective light modulation element in the same optical path as the incident light The light is condensed on theoptical fiber 14 through 100, thepolarizer 12, and thelens 13. That is, the light emitted from theoptical fiber 14 propagates back to the sameoptical fiber 14 regardless of its polarization.
[0030]
Further, when the voltage applied to thetransparent electrodes 8 and 9 is V, the propagating light beam is shown in FIG. Of the incident light of the reflection typelight modulation element 100, the extraordinary light polarization is transmitted through the liquid crystal element while being extraordinary light polarization and is diffracted by the reflection type polarization diffraction grating. Even after passing through thepolarizer 12 and thelens 13, the light is not condensed on theoptical fiber 14. Further, the ordinary light polarized light is transmitted through the liquid crystal element as the ordinary light polarized light and is not diffracted by the reflective polarization diffraction grating, but is specularly reflected by the reflecting mirror surface and reflected by a light path different from the incident light. And thelens 13, the light is not condensed on theoptical fiber 14. That is, the light emitted from theoptical fiber 14 does not return to the sameoptical fiber 14 regardless of the polarization direction.
[0031]
Theliquid crystal element 30 may be a reflective light modulation element in which at least one of thetransparent electrodes 8 and 9 is composed of a plurality of divided pixel electrodes and a voltage can be applied independently for each electrode. At this time, in FIG. 1, it is preferable that the pixel electrode has a pattern that is divided in the X-axis direction and not divided in the Y-axis direction.
[0032]
4 shows an example in which thepolarizer 12 processed with a birefringent crystal is used in combination with the reflectivelight modulation element 100, a polarization diffraction polarizer may be used as thepolarizer 12. FIG. 5 is a side view showing the operation of the optical attenuator when the polarization diffraction type polarizer 22 is used on the light incident side of the reflection typelight modulation element 100. (A) is applied voltage V₀ (B) shows a state where the applied voltage is V. Here, a polarization diffraction type polarizer 22 composed of a transmission type blazed diffraction grating is used as thepolarizer 12. The polarization diffractive polarizer 22 is a diffraction grating made of the same polymer liquid crystal as the blazeddiffraction grating 1 of the reflectivelight modulator 100, but the grating pitch is doubled to 2P, and the normal refractive index and extraordinary light of the polymer liquid crystal are used. The orientation direction of the polymer liquid crystal molecules is 90 degrees different from that of the blazeddiffraction grating 1 so that the refractive index is reversed. In addition, the blazed diffraction grating of the reflectivelight modulation device 100 is arranged so that the inclined direction of the hypotenuse of the sawtooth cross-sectional shape is opposite and the + 1st order diffraction of the transmission blazed diffraction grating is maximized in the polarization diffraction polarizer 22. Compared to 1, the blazed diffraction grating composed of a polymer liquid crystal layer has a grating height twice as high.
[0033]
Applied voltage V₀ In this case, as shown in FIG. 5A, the polarized light in the X-axis direction that travels straight through the polarization diffraction type polarizer 22 is incident and reflected perpendicularly to the reflection mirror of the reflection typelight modulation element 100 and returns to the original path. To do. On the other hand, the polarized light in the YZ plane + 1st order diffracted by the polarization diffraction type polarizer 22 is + 1st order diffracted by the reflective polarization diffraction grating of the reflectivelight modulation element 100 and passes through thelens 13 to theoptical fiber 14. Return the route.
[0034]
In addition, when the applied voltage is V, as shown in FIG. 5B, the X-axis direction polarized light that travels straight through the polarization diffraction type polarizer 22 is diffracted by the reflection type polarization diffraction grating of the reflection typelight modulation element 100 and light. It is not focused on thefiber 14. On the other hand, the polarized light in the YZ plane + 1st order diffracted by the polarization diffraction type polarizer 22 is obliquely incident on the reflection mirror of the reflection typelight modulation element 100 and is specularly reflected, so that it is not condensed on theoptical fiber 14. That is, it functions as a voltage variable optical attenuator and an optical switch as in the case shown in FIG.
[0035]
FIGS. 4 and 5 illustrate the operation of the attenuator. In the above description, the polarized light in the YZ plane and the polarized light in the X-axis direction orthogonal thereto are incident. However, as shown in FIGS. Only polarized light in the plane may be incident.
[0036]
In an optical switch and a variable optical attenuator used in a wavelength division multiplexing optical communication network, it is possible to stably realize a high extinction ratio of −40 dB or more, that is, I_off It is important to make the level sufficiently small.
[0037]
I_off The level is the amount of light returning on the same optical path as the incident light of the reflectivelight modulation element 100. In FIG. 1, the first straight line in the YZ plane that enters the reflective polarizing diffraction grating when the retardation value of theliquid crystal layer 7 is zero. This is the amount of light whose polarization is + 1st order diffracted and returned on the same optical path as the incident light. Here, the extinction ratio of the reflective polarization grating is I_off + 1st order diffracted light generated with respect to incident light of first linearly polarized light (ordinary light polarization of polymer liquid crystal) related to the level, and I_on This corresponds to the light amount ratio with the + 1st order diffracted light generated with respect to the incident light of the second linearly polarized light (abnormal light polarization of the polymer liquid crystal) related to the level. That is, the ordinary refractive index n of the polymer liquid crystal of the blazeddiffraction grating 1_o Isotropic refractive index n of (PLC) andtransparent adhesive 2_s And + 1st order diffracted light is not generated with respect to the incident light of ordinary light polarization, and in principle I_off The level is zero and the extinction ratio is infinite. However, n_o (PLC) and n_s + 1st order diffracted light is generated with respect to the first linearly polarized light due to a slight difference from the above, causing the extinction ratio of the reflective polarization diffraction grating to deteriorate.
[0038]
[Second Embodiment]
FIG. 6 is a side view showing a configuration example of the reflectivelight modulation element 200 according to the second embodiment of the present invention. By improving the extinction ratio of the reflection type polarization diffraction grating, the reflection typelight modulation element 100 can stably achieve a high extinction ratio.
[0039]
Compared to the reflectivelight modulation element 100 shown in FIG. 1, a polymer liquid crystal layer is processed into a blazed diffraction grating 15 between the reflectivepolarization diffraction grating 20 and theliquid crystal element 30, and thetransparent adhesive 2 The difference is that a transmissivepolarization diffraction grating 40 filled with a concave portion is formed. 6, the same reference numerals as those in FIG. 2 represent the same elements.
[0040]
The blazed diffraction grating 15 is formed by forming an alignment film (not shown) aligned in the Y-axis direction on the surface of thetranslucent substrate 5, applying a liquid crystal monomer solution thereon, and then irradiating it with ultraviolet rays or the like. Same ordinary refractive index n as blazed diffraction grating 1_o (PLC) and extraordinary refractive index n_e However, a polymer liquid crystal layer in which the alignment direction of liquid crystal molecules is aligned in the Y-axis direction is formed. That is, the alignment direction of the polymer liquid crystal is perpendicular to the blazeddiffraction grating 1 and the blazed diffraction grating 15. Further, the polymer liquid crystal layer is processed into a blazed diffraction grating 15 having a sawtooth cross section and a linear shape in the Y-axis direction by photolithography and reactive ion etching. In FIG. 6, a sawtooth cross section is shown in the YZ plane so that the cross section can be visually recognized. However, in order to suppress stray light caused by multiple diffracted light of the blazeddiffraction grating 1 and the blazed diffraction grating 15 as will be described later. Actually, the sawtooth cross section is processed to be an XZ plane perpendicular to the paper surface.
[0041]
The grating height D (PLC) of the blazed diffraction grating 15 made of polymer liquid crystal is set so that the + 1st-order diffraction efficiency is maximized with respect to incident light. That is, the birefringence of the polymer liquid crystal is expressed by Δn (PLC) = n_e (PLC) -n_o If (PLC), Δn (PLC) × D (PLC) = λ. Although FIG. 6 shows a blazed diffraction grating 15 having a sawtooth shape in cross section, a pseudo blazed diffraction grating in which the oblique side of the sawtooth shape is approximated by multiple steps may be used.
[0042]
Further, since the diffracted light of the transmissivepolarization diffraction grating 40 composed of the blazed diffraction grating 15 is not used as signal light, it is sufficient that the diffraction pitch is a diffraction angle at which the diffracted light does not overlap the signal light.
[0043]
Next, the effect of improving the extinction ratio of the reflectivelight modulation element 200 by adding the transmissivepolarization diffraction grating 40 will be described.
Of the light incident on the transmissivepolarization diffraction grating 40, the second linearly polarized light that has been transmitted through theliquid crystal element 30 and becomes polarized in the X-axis direction is diffracted because it is ordinary light polarization with respect to the transmissivepolarization diffraction grating 40. + 1st order diffracted light is generated because of the extraordinary light polarization with respect to the reflectivepolarization diffraction grating 20, and is reflected by thereflection mirror 3 and returns along the same path as the incident light. The light is transmitted through thestraight line 40 and emitted from the reflectivelight modulation element 200 as signal light. At this time, as the diffracted light other than the diffracted light of the transmissivepolarizing diffraction grating 40 and the + 1st order diffracted light of the reflectivepolarizing diffraction grating 20 and the light that is not diffracted are smaller, I_on Since it does not cause a decrease in level, it is preferable that the amount is small.
[0044]
Further, the first linearly polarized light that has been transmitted through theliquid crystal element 30 and becomes polarized light in the XZ plane is diffracted to the transmissivepolarization diffraction grating 40 because it is an extraordinary light polarization, and is reflected by the reflectionpolarization diffraction grating 20. On the other hand, since it is ordinary light polarized light, most of the light is reflected by the reflectingmirror 3 without being diffracted, and is diffracted again by the transmissivepolarizing diffraction grating 40 and is emitted from the reflectivelight modulation element 200 through a path different from the incident light. However, it is not superimposed on the signal light. At this time, n_o (PLC) and n_s + 1st order diffracted light generated by the reflectivepolarization diffraction grating 20 due to a slight difference from the diffracted light is diffracted by the transmissive polarization diffraction grating 40 (shown by a dotted line in FIG. 6), and is emitted out of the YZ plane to become signal light. Can not realize superposition, so a high extinction ratio can be realized. In addition, since the grating shape of the transmissivepolarization diffraction grating 40 is the blazed diffraction grating 15, the multiplexed light diffracted in the forward path (front) and the return path (rear) of the transmissionpolarization diffraction grating 40 before and after the reflection by thereflection mirror 3. Of the diffracted light, the amount of light superimposed on the signal light has a very low value.
[0045]
As a result, by adopting the configuration of the reflection typelight modulation element 200 of the present invention in which the transmission typepolarization diffraction grating 40 is laminated on the reflection typepolarization diffraction grating 20, I_off Since the level is stabilized and becomes a sufficiently small value, an optical switch or a variable optical attenuator using a voltage that realizes a high extinction ratio of −40 dB or more can be obtained. In particular, a high extinction ratio can be ensured in a wide applied voltage margin with respect to the wavelength bandwidth of incident light used in the wavelength division multiplexing optical communication network.
[0046]
Moreover, since the space | interval of thereflective mirror 3 and theliquid crystal layer 7 can be made small by using the thin translucent board |substrate 5, the several pixel electrode by which at least one of thetransparent electrodes 8 and 9 of theliquid crystal element 30 was divided | segmented Even in this case, leakage of light transmitted through the pixel electrode to the adjacent electrode is reduced in the forward path and the backward path of the liquid crystal element, and crosstalk can be suppressed.
[0047]
When the blazed diffraction grating 15 is a rectangular grating by increasing the distance between the transmissionpolarization diffraction grating 40 and the reflectionpolarization diffraction grating 20 and distributing the spatial grating pattern of the transmissionpolarization diffraction grating 40. However, it is possible to avoid the multiple diffracted light diffracted in the forward path and the return path of the transmission typepolarization diffraction grating 40 from being superimposed on the signal light.
[0048]
In addition, in the reflective light modulation elements 100 (FIG. 5) and 200 (FIG. 6), the reflected light generated at the interface between thetranslucent substrates 5 and 6 and the medium having a different refractive index is reflected on the surface of the translucent substrate. By forming fine unevenness, it becomes diffused light, and stray light superimposed on signal light can be reduced. Alternatively, the reflectivepolarizing diffraction grating 20 in which theliquid crystal element 30 and the reflectivepolarizing diffraction grating 20 or the transmissivepolarizing diffraction grating 40 are integrated may be separately manufactured and integrated using an adhesive.
[0049]
In the first and second embodiments, the applied voltage V₀ Although the operation between V and V has been described, other applied voltage or the alignment state of the liquid crystal according to the applied voltage may be changed.
The liquid crystal alignment may be twist alignment, vertical alignment, or hybrid alignment. In addition to nematic liquid crystals, smectic liquid crystals, cholestic liquid crystals, or other liquid crystal materials may be used as long as the polarization state of transmitted light changes according to the change in the alignment state of the liquid crystals.
[0050]
In addition, as an example of the control means for changing the alignment state of the liquid crystal, an AC voltage applied between the transparent electrodes has been shown. However, instead of the voltage, the alignment state of the liquid crystal is changed using a control means such as a magnetic field, temperature, or light. May be.
Further, although an example in which a reflective polarizing diffraction grating made of a diffraction grating processed into a sawtooth cross section using a polymer liquid crystal as a birefringent material has been described, a birefringent material such as a dielectric crystal may be used. . In addition, by providing a spatial distribution of tilt alignment of liquid crystal molecules in the thickness direction within the grating pitch, a blaze-type diffraction grating against extraordinary light polarization with the same polymer liquid crystal layer thickness that is not processed into a sawtooth cross-sectional shape It is possible to have the same function.
[0051]
As a reflection mirror, a metal film such as aluminum, gold, silver, etc.₂ O₅ And TiO₂ High refractive index dielectric film such as SiO and SiO₂ A dielectric multilayer film mirror in which low-refractive-index dielectric films such as these are alternately laminated can be formed by vapor deposition or sputtering.
[0052]
As a material for forming the transparent electrode, an oxide film such as ITO or a metal foil film such as aluminum or chromium can be used, but it is preferable to use an ITO film from the viewpoint of conductivity, light transmittance, durability, and the like. .
[0053]
Changing the polarization state means converting linearly polarized light incident on the liquid crystal element into elliptically polarized light including circularly polarized light and linearly polarized light. In the above embodiment, the voltage V₀ Shows a case where the polarization direction of linearly polarized light is rotated by 90 degrees.
[0054]
Further features of the reflective light modulation element and the variable optical attenuator of the present invention will be specifically described with reference to the following embodiments.
[0055]
【Example】
This embodiment will be described with reference to a cross-sectional view of the reflectivelight modulation element 200 shown in FIG. Atransparent electrode 8 made of an ITO film formed on one side of a glass substrate that is a light-transmittingsubstrate 5 is linear in the Y-axis direction by a photolithography method and an etching method (linear shape in the X-axis direction so that the cross-sectional shape can be visually recognized). And atransparent electrode 9 made of an ITO film is formed on one side of a glass substrate which is a light-transmittingsubstrate 6 so as to form a pixel electrode shape having an electrode width of 100 μm and an electrode interval of 5 μm. An alignment film (not shown) is obtained by applying a polyimide film on thetransparent electrodes 8 and 9 and performing an alignment process in a direction that forms an angle of 45 degrees with respect to the X axis.
[0056]
Next, aseal 10 is printed and applied on one side of a translucent substrate 6 (glass substrate) on which an alignment film is formed using a sealing material in which spacers for controlling the cell gap are mixed, and the translucent substrate 5 (glass substrate). ) To produce an empty cell. After injecting liquid crystal from an injection port (not shown) provided in a part of the seal, the injection port is sealed to complete theliquid crystal element 30. The liquid crystal used here has an ordinary refractive index n for a wavelength λ = 1.56 μm._o (LC) = 1.49 and extraordinary light refractive index n_e (LC) = 1.65 and the cell gap is d (LC) = 5.5 μm. At this time, the retardation value of theliquid crystal layer 7 is 0.88 μm.
[0057]
Although not shown in FIG. 6, a phase plate made of a polymer liquid crystal having a normal light refractive index direction coincident with the extraordinary light refractive index direction of the liquid crystal layer and a retardation value of 0.1 μm is separated from thetranslucent substrate 6. And a transparent substrate (not shown). When no voltage is applied to thetransparent electrodes 8 and 9, the retardation value of the entireliquid crystal element 30 in which the phase plate and theliquid crystal layer 7 are integrated is 0.78 μm, and λ / 2 wavelength with respect to the wavelength λ = 1.56 μm. It corresponds to a board. At this time, the incident light having the polarization in the YZ plane is transmitted through theliquid crystal element 30 and the polarization direction is changed to the outgoing light having the polarization in the X-axis direction.
[0058]
By applying a voltage to thetransparent electrodes 8 and 9 and changing the liquid crystal molecules of theliquid crystal layer 7 from the parallel alignment to the vertical alignment with respect to the translucent substrate, the retardation value of the liquid crystal layer is reduced, and the applied voltage is 6V. 1 μm remains, but by integrating the phase plate and theliquid crystal layer 7, the phase plate cancels out this residue and the overall retardation value becomes zero, and the incident light is transmitted through theliquid crystal element 30 and the polarization state remains unchanged. The light is emitted as it is.
[0059]
Next, an alignment film (not shown) is obtained by applying a polyimide film on one surface of the light-transmittingsubstrate 5 of theliquid crystal element 30 with a phase plate and performing an alignment process in the Y-axis direction. A liquid crystal monomer solution is applied thereon, then irradiated with ultraviolet rays, and an ordinary refractive index n_o (PLC) = 1.55 and extraordinary refractive index n_e (PLC) = 1.75, and a polymer liquid crystal layer having a film thickness of 6.8 μm and aligned in the Y-axis direction is formed. Furthermore, the cross-sectional shape of the polymer liquid crystal layer is 6.8 μm, approximating a sawtooth wave shape with an 8-step staircase shape by a photolithography method and a reactive ion etching method, and the cross-sectional shape is visible in the Y-axis direction. It is processed into a pseudo blazed diffraction grating 15 having a linear shape in the X-axis direction.
[0060]
Next, an alignment film (not shown) that is aligned in the X-axis direction is formed on the surface of thereflection mirror 3 of the glass substrate, which is thetranslucent substrate 4 having thereflection mirror 3 formed on one side, and on that. In the same process using the same liquid crystal monomer as the blazed diffraction grating 15, the liquid crystal molecules are aligned in the X-axis direction, and the cross section is approximated by an 8-step staircase shape. Processing into a pseudo blazeddiffraction grating 1 having a linear shape in the axial direction. Here, the height of the pseudo blazeddiffraction grating 1 in which the hypotenuse is approximated by a staircase is set to 3.4 μm. Thereflection mirror 3 is made of a high refractive index dielectric film Ta.₂ O₅ And low refractive indexdielectric film SiO₂ 30 or more layers are alternately laminated so that the optical film thickness becomes approximately λ / 4, and a multilayer film reflection mirror having a reflectance of 99.9% or more in a wavelength band from 1.48 μm to 1.62 μm is used.
[0061]
Further, ordinary light refractive index n_o Isotropic refractive index n substantially equal to (PLC)_s = 1.56 transparentadhesive material 2 is used to fill the concave portions of the pseudo blazeddiffraction grating 1 and the pseudo blazed diffraction grating 15 and to adhere to and fix to thetranslucent substrates 4 and 5, thereby reflecting polarization. Thediffraction grating 20 and the transmissivepolarization diffraction grating 40 are obtained. In this way, the reflectivelight modulation element 200 is manufactured. At this time, the second linearly polarized light in the X-axis direction passes straight through the transmissivepolarization diffraction grating 40 without being diffracted, and 80% or more is + 1st-order diffracted by the reflectivepolarization diffraction grating 20. Further, 80% or more of the first linearly polarized light in the YZ plane is + 1st-order diffracted by the transmission typepolarization diffraction grating 40 and passes straight through the reflection typepolarization diffraction grating 20.
[0062]
Light is incident on the reflection-typelight modulation element 200 at an incident angle θ = 5 ° with respect to the Z-axis which is the normal direction of the reflection mirror surface in the YZ plane. Incident light is incident on each pixel electrode of theliquid crystal element 30 with light having different wavelengths in the wavelength band from 1.48 μm to 1.62 μm, and the diffraction angle of the + 1st order diffracted light with respect to the incident light wavelength corresponding to each pixel electrode. The grating pitch of the pseudo blazed diffraction grating 15 of the reflectivepolarization diffraction grating 20 is distributed from 8.5 μm to 9.3 μm so as to be the same as the incident angle θ.
[0063]
The voltage V applied to theliquid crystal layer 7 is adjusted for each pixel electrode through an AC power source connected to thetransparent electrodes 8 and 9. When the polarized light in the YZ plane (first linearly polarized light) is transmitted through theliquid crystal element 30 with a phase plate, it changes to polarized light in the X-axis direction (second linearly polarized light) substantially orthogonal at V = 0, and polarized at around V = 6V. The state does not change.
[0064]
Therefore, the light in the wavelength band from 1.48 μm to 1.62 μm incident on each pixel electrode of the reflectivelight modulation element 200 is diffracted by the reflectivepolarization diffraction grating 20 and reflected in the same angle direction as the incident light. Most of the light emitted from the typelight modulation element 200 is almost zero at the applied voltage V = 0, but is almost zero at around the applied voltage V = 6V. Further, in the intermediate region where the applied voltage V is 0 and 6V, the light quantity is intermediate.
[0065]
That is, an optical switch or variable attenuator with a high extinction ratio voltage can be obtained by combining with an optical system that separates outgoing light having the same angle as incident light and outgoing light having a different angle. For example, after wavelength-multiplexed light in a wavelength band from 1.48 μm to 1.62 μm emitted from an optical fiber is dispersed by a diffraction grating, it is made incident on the reflectivelight modulation element 200 at an incident angle θ using a lens and each pixel. Focus on the electrode.
[0066]
As a result, the light emitted from the reflectivelight modulation element 200 in the same angle direction as the incident light is propagated back to the optical fiber, but the other light is blocked, so that a high extinction ratio of −40 dB or more is stabilized. It becomes an optical switch or a variable attenuator based on the voltage realized. In addition, since the distance between the reflecting mirror and the electrode can be reduced, optical signal crosstalk between pixels can be suppressed even with a pixel electrode structure.
[0067]
In FIG. 7, when the voltage is applied to the liquid crystal layer of the reflection typelight modulation element 200 of the present invention from 0 V to 10 V, the same optical fiber reflects the reflection type light modulation element in the light emitted from the optical fiber. It is a graph which shows the signal light efficiency which returns to. A high extinction ratio of -40 dB or more can be obtained in a wide voltage application range of 4.6 V to 10 V or more.
[0068]
For comparison, the signal light efficiency with respect to the applied voltage when the conventional variableoptical attenuator 300 is used is shown by a solid line in FIG. An extinction ratio of -40 dB or higher is obtained in a narrow applied voltage range from 6.0 V to 6.4 V.
[0069]
【The invention's effect】
As described above, in the reflection type light modulation element of the present invention, the liquid crystal element that changes the polarization state of incident light according to the change in the alignment state of the liquid crystal molecules and the birefringent material formed on the reflective substrate surface are sawtooth-shaped. And a reflection type polarization diffraction grating that reflects the first linearly polarized light and reflects the second linearly polarized light is integrated, and thus is compact and has a high extinction ratio. A reflective light modulation element is obtained. Further, when the alignment state of the liquid crystal is changed by voltage, the distance between the reflection mirror and the voltage application electrode can be reduced, so that optical signal crosstalk between pixels can be suppressed even in the pixel electrode structure.
[0070]
Further, by using a polymer liquid crystal having birefringence as the reflective polarization diffraction grating, it is possible to accurately process a sawtooth cross-sectional shape even with a fine grating pitch, so that high signal light diffraction efficiency can be obtained.
[0071]
In addition, a transmissive polarization diffraction grating that diffracts the first linearly polarized light and transmits the second linearly polarized light in the polarization direction orthogonal to the first linearly polarized light without diffracting is disposed between the liquid crystal element and the reflective polarization diffraction grating. As a result, a higher extinction ratio can be obtained.
[0072]
In particular, by using a diffraction grating obtained by processing a polymer liquid crystal having birefringence as a transmission polarization diffraction grating into a sawtooth cross-sectional shape, multiple diffracted light diffracted by the transmission polarization diffraction grating in the forward path and the return path is I_off Since it does not overlap with the level, I can be stable over a wide applied voltage range._off The level can be maintained at a low value. As a result, an optical switch or an optical attenuator suitable for use in a wavelength division multiplexing optical communication network or the like is obtained.
[0073]
In addition, by adding a separation means for separating the light diffracted by the reflective polarizing diffraction grating that exits the reflective light modulation element and the light that is not diffracted and receiving only one of the emitted light, the magnitude of the applied voltage is increased. Thus, a variable optical attenuator that adjusts the amount of emitted light according to the above is obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration example of a reflective light modulation element according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view showing another configuration example of the reflective light modulation element according to the first embodiment of the present invention.
3 is a cross-sectional view showing the operation when light is incident on the reflective light modulation element shown in FIG. 1, (a) maximum + 1st order diffraction reflection state when no voltage is applied, and (b) mirror reflection when voltage is applied. Status.
4 is a cross-sectional view for explaining the operation of an optical attenuator when a polarizer made of a birefringent crystal is used on the light incident side of the reflective light modulation element shown in FIG. 1, (a) state when no voltage is applied. , (B) shows the state when a voltage is applied.
5 is a cross-sectional view showing the operation of an optical attenuator when a polarization diffraction type polarizer is used on the light incident side of the reflective light modulation element shown in FIG. 1, (a) state when no voltage is applied, (b) ) State when voltage is applied.
FIG. 6 is a cross-sectional view showing a configuration example of a reflective light modulation element according to a second embodiment of the present invention.
FIG. 7 is a graph showing applied voltage and signal light efficiency in the case of using the reflective light modulation device of the example of the present invention (circle line indicates an example, solid line indicates a comparative example of a conventional configuration).
FIG. 8 is a cross-sectional view showing a configuration example of a conventional transmissive light modulation element.
[Explanation of symbols]
1, 15: Blaze diffraction grating
2: Transparent adhesive
3: Reflection mirror
4, 5, 6: Translucent substrate
7: Liquid crystal layer
8, 9: Transparent electrode
10: Seal
11: AC power supply
12: Polarizer
13: Lens
14: Optical fiber
22, 60: Polarization diffraction type polarizer
20: Reflective polarization diffraction grating
30: Liquid crystal element
40: Transmission polarization grating
50: Liquid crystal cell with phase plate
100, 110, 200: reflection type light modulation element

Claims (5)

Translated fromJapanese

入射光を反射する反射ミラーと、複屈折材料が鋸歯状の断面形状に加工された回折格子からなる反射型偏光回折格子と、入射光が透過することによりその偏光状態を変化させる液晶素子と、液晶素子内の液晶分子の配向状態を変化させる制御手段とを備える反射型光変調素子であって、外部からの入射光が液晶素子を透過して反射型偏光回折格子に入射し、反射型偏光回折格子への入射光のうち第１の偏光方向を有する第１直線偏光は回折されることなく反射型偏光回折格子を透過して反射ミラーで反射されて液晶素子を出射し、第１の偏光方向と直交する偏光方向を有する第２直線偏光は反射型偏光回折格子により回折されて第１直線偏光の出射光とは異なる方向に液晶素子を出射することを特徴とする反射型光変調素子。A reflection mirror that reflects incident light, a reflective polarization diffraction grating that is formed of a diffraction grating in which a birefringent material is processed into a sawtooth cross-sectional shape, a liquid crystal element that changes its polarization state by transmitting incident light, and A reflection type light modulation element comprising a control means for changing the alignment state of liquid crystal molecules in the liquid crystal element, wherein incident light from the outside passes through the liquid crystal element and enters the reflection type polarization diffraction grating, Of the incident light to the diffraction grating, the first linearly polarized light having the first polarization direction passes through the reflective polarization diffraction grating without being diffracted, is reflected by the reflection mirror, and exits the liquid crystal element. A reflection type light modulation element, wherein the second linearly polarized light having a polarization direction orthogonal to the direction is diffracted by a reflection type polarization diffraction grating and emitted from the liquid crystal element in a direction different from the outgoing light of the first linearly polarized light.前記反射型偏光回折格子を構成する複屈折材料が高分子液晶からなる請求項１記載の反射型光変調素子。The reflective light modulation element according to claim 1, wherein the birefringent material constituting the reflective polarization diffraction grating is made of a polymer liquid crystal.前記液晶素子と前記反射型偏光回折格子との間に、第１直線偏光を回折しかつ第２直線偏光を回折することなく透過する透過型偏光回折格子が配置されている請求項１または２記載の反射型光変調素子。The transmission type polarization diffraction grating that diffracts the first linearly polarized light and transmits the second linearly polarized light without diffracting is disposed between the liquid crystal element and the reflective polarization diffraction grating. Reflection type light modulation element.前記透過型偏光回折格子は、高分子液晶層が鋸歯状の断面形状に加工された回折格子からなる請求項３記載の反射型光変調素子。4. The reflective light modulation element according to claim 3, wherein the transmission type polarization diffraction grating is a diffraction grating in which a polymer liquid crystal layer is processed into a sawtooth cross-sectional shape.請求項１から４のいずれかに記載の反射型光変調素子を用いる可変光減衰器であって、前記制御手段は前記液晶素子に電圧を印加する手段であり、印加電圧により前記液晶素子を透過する光の偏光状態が変化して、反射型偏光回折格子により回折された光と回折されない光とが分離して２つの出射光とされ、いずれか一方の出射光のみを受光する分別手段が設けられていて、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可変光減衰器。5. A variable optical attenuator using the reflective light modulation element according to claim 1, wherein the control means is means for applying a voltage to the liquid crystal element, and transmits the liquid crystal element by the applied voltage. The polarization state of the light to be changed changes, the light diffracted by the reflective polarization diffraction grating and the light not diffracted are separated into two outgoing lights, and a separating means for receiving only one of the outgoing lights is provided. The variable optical attenuator is characterized in that the amount of light received is adjusted according to the magnitude of the applied voltage.

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