JP4106981B2

JP4106981B2 - Optical attenuator

Info

Publication number: JP4106981B2
Application number: JP2002190086A
Authority: JP
Inventors: 倫央富田; 好晴大井
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2002-06-28
Filing date: 2002-06-28
Publication date: 2008-06-25
Anticipated expiration: 2022-06-28
Also published as: JP2004037480A

Description

【０００１】
【発明の属する技術分野】
本発明は、入射光の偏光方向に関わらず印加電圧の大きさに応じて直進透過光の効率が変化する液晶素子およびそれを用いた電圧可変の光減衰器に関する。
【０００２】
【従来の技術】
液晶を用いた従来の光減衰器である液晶素子の一例を図８に示す。透明電極２、３が形成された透光性基板４と５との間に液晶分子の配向方向が透光性基板面に平行で、Ｘ軸方向と４５°の角度をなす方向に揃ったネマティック液晶の液晶層１が透光性基板の周縁に設けられたシール材６の内部に狭持された液晶セル１１０と、その光出射面側にＸ軸方向に偏光方向を有する直線偏光のみを透過する偏光子１０とを配置した構成となっている。
【０００３】
ここで、透明電極２と３とに矩形波出力の交流電源７を接続し、この電源による電圧非印加時に、波長λでＹ軸方向に偏光方向を有する直線偏光に対する液晶セル１１０のリタデーション値がほぼλ／２となるよう液晶層１の厚さが設定されている。ここで、液晶層１のリタデーション値をほぼλ／２としているのは、電圧非印加時に光減衰器の挿入損失を最小とするため、およびλ／２板として機能させるためである。
【０００４】
この光減衰器において、透明電極間への電圧非印加時に液晶層に入射したＹ軸方向に偏光した直線偏光は、Ｘ軸方向に偏光方向を有する直線偏光となって偏光子を透過する。電圧を印加するとき、印加電圧の増加に伴い液晶層の厚さ方向に、すなわち基板に垂直になるように液晶分子の配向方向が傾く。それに伴い液晶層のリタデーション値がゼロに近づき、液晶セル１１０の透過光はＹ軸方向の直線偏光成分が増大する。その結果、偏光子の透過光量が印加電圧の増加に伴い単調に減少し、電圧可変型の光減衰器となる。
【０００５】
【発明が解決しようとする課題】
従来の、電圧印加量に応じた液晶層のリタデーション値の変化を利用した電圧可変型の光減衰器では、特定の直線偏光を入射光としている。したがって、入射光の偏光方向が任意の場合、偏光子を透過する偏光成分が常に存在するため消光比が劣化して光減衰器の機能が得られなかった。
【０００６】
また、図８において液晶層１の光入射側にＹ軸方向に偏光方向を有する直線偏光のみを透過する偏光子を配置した場合、Ｘ軸方向に偏光方向を有する直線偏光成分は吸収されるため、光挿入損失が大きな光減衰器しか得られなかった。
【０００７】
本発明は、上述の実情に鑑み、入射光の偏光方向に関わらず光挿入損失が低くかつ消光比の高い安定した光減衰器が実現できる液晶素子およびそれを用いた光減衰器を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明は、直交した偏光状態を有する２つの直線偏光で入射する入射光を前記入射光の進行方向または異なる方向へ偏向させて出射する機能を有する液晶素子と、前記進行方向と同じ方向へ前記液晶素子から出射する光を透過するとともに前記進行方向と異なる方向へ前記液晶素子から出射する光を遮断する機構、を備える光減衰器であって、前記液晶素子は、電極および配向膜付き基板間に液晶分子が同一方向に配向処理されて挟持された液晶層からなる液晶単体セルと前記液晶単体セルに積層された位相板とを備える液晶セルと、前記液晶セルの光入射側に配置された第１の偏光性ビームスプリッタと、前記液晶セルの光出射側に配置された第２の偏光性ビームスプリッタと、を備え、前記液晶単体セルは、前記電極間に印加する電圧の大きさに応じて前記液晶セルのリタデーション値が変化し、前記位相板は、前記電極間に特定の電圧Ｖ_２を印加したとき前記液晶セルのリタデーション値をゼロに相殺し、前記第１の偏光ビームスプリッタおよび前記第２の偏光性ビームスプリッタは、高分子液晶からなる複屈折性材料層を備えるとともに、前記複屈折材料層の断面形状が鋸歯状で平面形状が一定のピッチの直線状である偏光性回折格子からなり、前記第１の偏光性ビームスプリッタの高分子液晶の配向方向と前記第２の偏光性ビームスプリッタの高分子液晶の配向方向とが直交し、前記液晶素子へ入射する前記２つの直線偏光は、前記第１の偏光性ビームスプリッタにより偏光方向に応じて進行方向が互いに異なって前記液晶セルを透過し、前記液晶セルが入射する光の波長λに対して前記電圧Ｖ _２と異なる電圧Ｖ _１を印加してλ／２のリタデーション値を有するとき前記第２の偏光性ビームスプリッタを透過する前記２つの直線偏光は、前記液晶素子の前記入射光と同じ進行方向に互いに揃って前記液晶素子から出射し、前記電圧Ｖ _２を印加したときに前記２つの直線偏光は、前記液晶素子の前記入射光と異なる進行方向となって前記液晶素子から出射し、前記電極間に印加される電圧の大きさに応じて前記液晶素子から出射する光を伝搬する光量が変化することを特徴とする光減衰器を提供する。
【０００９】
また、前記位相板の進相軸は、前記液晶層の遅相軸と一致する上記の光減衰器を提供する。
【００１０】
また、前記偏光性回折格子は、特定方向に偏光した直線偏光に対して１次回折光の回折効率が他の次数の回折光の回折効率に比べて高い回折格子であって、かつ第１の偏光性回折格子により回折された直線偏光が第２の偏光性回折格子により回折されて出射する進行方向が、第１の偏光性回折格子に直線偏光が入射する方向と一致する上記の光減衰器を提供する。
【００１２】
【発明の実施の形態】
本発明の液晶素子の構成要素である液晶セルは、電極付き透光性基板間に液晶層が狭持され、その液晶セルの電極間にＶ_１からＶ_２（Ｖ_１≠Ｖ_２）までの電圧が印加されたとき、液晶セルへ入射されて透過される波長λの直線偏光に対するリタデーション値が（ｍ＋１／２）×λからｍ×λ（ｍは整数）まで変化するものとする。
【００１３】
また、本発明の液晶素子の構成要素である偏光性ビームスプリッタは、特定の直線偏光入射光とその偏光方向と直交する直線偏光入射光の進行方向を効率よく異ならせる機能を有するものである。
【００１４】
図１は、本発明の液晶素子の第１の実施態様の構成例を示す側面図である。液晶素子１００は、偏光性ビームスプリッタとして複屈折性材料を備える偏光性回折格子を用いる。透光性基板４と５の片面にそれぞれ透明電極２と３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）が形成されて、シール材６を用いてセル化される。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１とされ、基板と平行に液晶分子の配向方向の揃った液晶セル１１０が得られる。
【００１５】
液晶セル１１０の光入射側に第１の偏光性回折格子１２０が、光出射側に第２の偏光性回折格子１３０が配置されている。何れにも共通した構成について以下に説明する。ガラスなどの透光性基板１１、１２の片面に常光屈折率ｎ_ｏおよび異常光屈折率ｎ_ｅの複屈折材料である高分子液晶層１３、１４を形成する。ここで、液晶モノマーの溶液を透光性基板の配向処理の施された配向膜上に塗布し、液晶分子の配向ベクトル（分子配向軸）を基板と平行面内の特定方向に揃うように配向させた後、紫外線などの光を照射して重合硬化させ高分子液晶層とする。
【００１６】
次に、高分子液晶層を断面形状が鋸歯状（いわゆるブレーズド回折格子型）であり、図１の紙面垂直方向（Ｘ軸）の平面形状が直線状の凹凸部を形成する。すなわち、格子ピッチが一定な直線状ブレーズド回折格子を加工する。
さらに、複屈折性材料層の凹凸部の少なくとも凹部に常光屈折率ｎ_ｏとほぼ等しい屈折率ｎ_ｓの均質屈折率透明材料１５、１６を充填する。
【００１７】
ここで、鋸歯状の凹凸部の断面形状は、入射光の波長λの異常光偏光に対する１次回折効率が最大になるよう、凹凸部の位相差２π×（ｎ_ｅ−ｎ_ｓ）×ｄ／λが２π、すなわち高分子液晶層１３、１４の膜厚ｄをλ／（ｎｅ−ｎｓ）としている。
【００１８】
なお、鋸歯状の断面形状は階段形状で近似した疑似ブレーズ格子としてもよい。その場合、４段から１６段程度の階段状鋸歯とすることが好ましい。また、ブレーズド回折格子に比べて回折効率が低下するが、凹凸部の断面形状が矩形回折格子としてもよい。いずれも、回折光が最大となる凹凸部の位相差条件とすることにより、回折されることなく直進透過する波長λの異常光偏光の０次光成分は１％以下となる。
【００１９】
このようにして得られた偏光性回折格子１２０、１３０に、高分子液晶層に対する常光偏光である直線偏光が入射した場合、高分子液晶層１３、１４と均質屈折率透明材料１５、１６との屈折率に差がないようにするため、入射光は回折されることなく直進透過する。
【００２０】
一方、高分子液晶層に対する異常光偏光である直線偏光が入射した場合、回折格子ピッチに応じて常光偏光とは異なる進行方向に回折光として分離される。この偏光性回折格子を、液晶セル１１０の光入射側に第１の偏光性回折格子１２０および光出射側に第２の偏光性回折格子１３０として液晶セル１１０に接着して一体化し、液晶素子１００としている。
このとき、偏光性回折格子１２０と１３０への入射光は、直進透過する常光偏光成分と特定の方向に回折される異常光偏光成分とに進行経路が分離される。
【００２１】
ここでは、第１の偏光性回折格子１２０を構成する高分子液晶層の配向方向をＸ軸方向とし、第２の偏光性回折格子１３０を構成する高分子液晶層の配向方向をＹ軸方向とし、各偏光性回折格子の配向方向が直交するように配向処理されている。また、液晶セル１１０のネマティック液晶分子の配向方向が第１および第２の偏光性回折格子における高分子液晶層の配向方向と４５°の角度を成すように配置されている。
【００２２】
液晶セル１１０の透過光のリタデーション値がｍ×λ（ｍは整数）のとき、第１の偏光性回折格子１２０を直進透過した偏光成分は第２の偏光性回折格子１３０で回折され、第１の偏光性回折格子１２０で回折された偏光成分は第２の偏光性回折格子１３０を直進透過する。
【００２３】
また、液晶セル１１０の透過光のリタデーション値が（ｍ＋１／２）×λ（ｍは整数）のとき、第１の偏光性回折格子１２０により最大回折された１次回折光が、第２の偏光性回折格子１３０により１次回折光として最大回折されて、非回折光と同じ進行方向となるように、第１の偏光性回折格子１２０と第２の偏光性回折格子１３０の格子形状（ピッチ、直線格子の方向、断面凹凸形状）が調整されている。
【００２４】
図２および図３を用いて、不特定な偏光方向を有する入射光が液晶素子１００に入射した場合の作用を説明する。波長λの入射光のうち、第１の偏光性回折格子１２０に対して常光偏光成分（Ｙ軸）は偏光性回折格子１２０を直線透過し、異常光偏光成分（Ｘ軸）は偏光性回折格子１２０により回折され、液晶セル１１０に入射する。
【００２５】
液晶セル１１０の電極間にＶ_１の電圧を印加した場合（図２）、常光偏光および異常光偏光の各成分に対していずれも（ｍ＋１／２）×λ（ｍは整数）のリタデーション値を有する位相板として機能する。
【００２６】
その結果、第１の偏光性回折格子１２０を直線透過した光は偏光方向が９０°回転して第２の偏光性回折格子１３０に対して常光偏光となって入射するため、第２の偏光性回折格子１３０により回折されることなく直進透過する。また、第１の偏光性回折格子１２０により回折された光は偏光方向が９０°回転して第２の偏光性回折格子１３０に対して異常光偏光となって入射するため、第２の偏光性回折格子１３０により回折され、直進透過光と同じ進行方向となって出射する。したがって、液晶セル１１０の印加電圧がＶ_１のとき、入射光はその偏光方向に関わらず直進透過する。すなわち、液晶セルが特定のリタデーション値、ここでは（ｍ＋１／２）×λ（ｍは整数）、を有するとき第２の偏光性ビームスプリッタを透過する２つの直線偏光は進行方向が液晶素子の入射光と同じ進行方向に互いに揃って液晶素子を出射する。
【００２７】
一方、液晶セル１１０の電極間にＶ_２の電圧を印加した場合（図３）、常光偏光および異常光偏光の各成分に対していずれもｍ×λ（ｍは整数）のリタデーション値を有する位相差板として機能する。
【００２８】
その結果、第１の偏光性回折格子１２０を直線透過した光は偏光方向を保ったまま第２の偏光性回折格子１３０に対して異常光偏光となって入射するため、第２の偏光性回折格子１３０により回折され、液晶素子１００の入射光と異なる進行方向に分離されて出射する。また、第１の偏光性回折格子１２０により回折された光は偏光方向を保ったまま第２の偏光性回折格子１３０に対して常光偏光となって入射するため、第２の偏光性回折格子１３０を直進透過し、液晶素子１００の入射光と異なる進行方向に分離されて出射する。
【００２９】
したがって、液晶セル１１０の印加電圧がＶ_２のとき、入射光はその偏光状態に関わらず回折され、入射光と異なる進行方向に分離されて出射する。
なお、液晶セル１１０の電極間にＶ_１とＶ_２の中間電圧を印加した場合、液晶素子１００の透過光は、入射光と同じ進行方向の成分と入射光と異なる進行方向の成分が印加電圧に応じた比率で混在した出射光となる。
【００３０】
図４と図５は本発明の液晶素子１００を用いた電圧可変の光減衰器の光学系構成およびその作用の一例を示す側面図である。液晶素子１００に不特定な偏光方向を有する平行光が入射し、出射側に集光レンズ８を配置した場合、液晶素子１００を直進透過した光は集光レンズ８の光軸上の焦点面に集光される（図４）。一方、液晶素子１００で回折された光は集光レンズ８の光軸外の焦点面に集光される（図５）。
【００３１】
したがって、集光レンズ８の光軸上の焦点面に開口部を有する開口絞り９を配置することにより、直進透過光のみを透過し進行方向が異なる回折光を遮断するアイソレータとなる。ここで、開口絞り９の代わりに開口部に相当する受光部を有する光検出器を配置することにより直進透過光のみを検出できる。また、光伝送用の光ファイバのコア部を開口部の代わりに配置すれば直進透過光のみを伝送できる。
【００３２】
このようにして、不特定な偏光方向を有する入射光に対して、液晶セル１１０の電極間に印加する電圧を変えることにより、透過光量を調整できる光減衰器が実現する。
【００３３】
本実施態様では、広い入射光波長帯域において消光比の高い光減衰器を得るためには、偏光性回折格子の例えば常光偏光に対する直進透過率が高く、例えば異常光偏光に対する直進透過率が低いことが好ましい。広い入射光波長帯域において異常光偏光に対する高い回折効率を維持して直進透過率を低く保つためには、偏光性回折格子を積層することが有効である。
【００３４】
また、液晶素子１００の印加電圧Ｖ_１およびＶ_２を低電圧化し、波長依存性を低減するためには、液晶層を薄くすなわち液晶層のリタデーション値の電圧印加可変範囲を０〜１／２×λ（ｍ＝０に相当）とすることが好ましい。また、特定印加電圧でリタデーション値ゼロを実現するため、液晶セル１１０に液晶層のリタデーション値を相殺する位相板を積層することが有効である。
【００３５】
また、上記の実施態様では、第１の偏光性回折格子１２０を構成する高分子液晶層の配向方向と第２の偏光性回折格子１３０を構成する高分子液晶層の配向方向が直交する場合について説明したが、両方の高分子液晶層の配向方向を平行としてもよい。その場合は、第１の偏光性回折格子１２０により進行経路が分離された２つの直線偏光は、液晶セルがリタデーション値ｍ×λ（ｍは整数）、すなわち特定のリタデーション値、を有するとき、液晶素子の入射光と同じ進行方向に互いに揃って第２の偏光性ビームスプリッタおよび液晶素子を出射し、液晶セルがリタデーション値（ｍ＋１／２）×λ（ｍは整数）を有するとき、液晶素子の入射光と異なる進行方向に分離されて第２の偏光性ビームスプリッタおよび液晶素子を出射する。
【００３６】
図６は本発明の液晶素子の第２の実施形態の構成例と作用例を示す側面図である。液晶素子２００は偏光性ビームスプリッタとして、屈折率の相対的に低いＳｉＯ_２やＭｇＦ_２などの透光性誘電体膜と、屈折率の相対的に高いＴｉＯ_２やＴａ_２Ｏ_５などの透光性誘電体膜とを、交互に光学的膜厚（屈折率×実際の膜厚）が波長オーダとなるように、透光性基板面に積層する。この透光性基板面は光入射面に対して４５°傾いた基板面であり、透光性誘電体膜が形成されて構成された多層膜偏光性ビームスプリッタ２２０、２３０を液晶セル１１０の光入射側と光出射側にそれぞれ用いてある。
【００３７】
図６において、紙面内の偏光成分Ａは多層膜面１７を透過して液晶セルに入射し、紙面に垂直な偏光成分Ｂは多層膜面１７および全反射面１９で反射されて液晶セル１１０に入射する。
【００３８】
液晶セル１１０の電極間印加電圧がＶ_１すなわち液晶層のリタデーション値が（ｍ＋１／２）×λ（ｍは整数）となるとき、すなわち特定のリタデーション値のとき、図６に示すように、偏光成分Ａは９０°偏光方向が回転して液晶セル１１０を透過し、全反射面２０および多層膜面１８で反射され多層膜偏光性ビームスプリッタ２３０から出射する。また、偏光成分Ｂは９０°偏光方向が回転して液晶セル１１０を透過し、多層膜面１８を透過し、偏光成分Ａと同じ光軸に合波されて多層膜偏光性ビームスプリッタ２３０から出射する。その結果、入射光の偏光方向に関わらず、液晶素子２００の入射光の進行方向と同じ方向に直進透過する。
【００３９】
一方、液晶セル１１０の電極間印加電圧がＶ_２すなわち液晶層のリタデーション値がｍ×λ（ｍは整数）となるとき、図７に示すように、偏光成分Ａは偏光状態を保ったまま液晶セル１１０を透過し、全反射面２０で反射され多層膜面１８を透過し、多層膜偏光性ビームスプリッタ２３０から出射する。また、偏光成分Ｂは偏光状態を保ったまま液晶セル１１０を透過し、多層膜面１８で反射され、偏光成分Ａと同じ光軸に合波されて多層膜偏光性ビームスプリッタ２３０を出射する。その結果、入射偏光方向に関わらず、液晶素子２００の入射光と直交する方向に進行方向が分離されて液晶素子２００から出射する。
【００４０】
なお、液晶セル１１０の電極間にＶ_１とＶ_２の中間電圧を印加した場合、液晶素子２００の直進透過光には、入射光と同じ進行方向の成分と入射光と直交する進行方向の成分が印加電圧に応じた比率で混在した出射光となり、一方の進行方向のみ取り出すことにより、印加電圧に応じて出射光量が可変となる光減衰器が実現する。なお、図７中の１７、１９および２２０は、図６における同符号のものと同じ要素である。
【００４１】
本発明の液晶素子および光減衰器は上記実施態様で説明した構成以外の液晶セルおよび偏光性ビームスプリッタの組み合わせにより種々の形態を実現できる。
【００４２】
【実施例】
本例の液晶素子１００について、図１を用いて説明する。常光屈折率ｎｏ（ＬＣ）＝１．５０および異常光屈折率ｎｅ（ＬＣ）＝１．６６のネマティック液晶を、透明電極２、３が片面に形成された透光性基板４、５に挟持し、液晶層１の厚さｄ（ＬＣ）を５．６μｍとした液晶単体セルを作製した。液晶層１の遅相軸方向は、図１に示すＸ軸方向に対して４５°方向で基板に対して平行とした。
【００４３】
透明電極２、３に電圧を印加しない状態では、波長１．５５μｍの光に対する液晶単体セルのリタデーション値は約０．９μｍであり、Ｙ軸方向に偏光方向を有する直線偏光が、液晶単体セルに入射し出射したとき直線偏光の偏光方向はほぼＸ軸方向となった。また、電圧振幅５Ｖの矩形波の交流電圧を印加した状態では、液晶単体セルのリタデーション値は０．１２μｍであった。
【００４４】
さらに、ガラスの透光性基板の片面に、常光屈折率ｎ_ｏ＝１．５５および異常光屈折率ｎ_ｅ＝１．５９で厚さｄ＝３．０μｍの高分子液晶層からなる位相板（図示せず）を液晶単体セルに接着一体化して液晶セル１１０を作製した。高分子液晶層からなる位相板のリタデーション値は−０．１２μｍであり、この位相板の進相軸方向を液晶層１の遅相軸方向と一致させることにより、印加電圧５Ｖでの液晶層１に残留するリタデーション値０．１２μｍを相殺し、液晶セル１１０のリタデーション値はゼロとなる。
【００４５】
さらに、図１において、ガラスの透光性基板１１、１２の片面に常光屈折率ｎ_ｏ（ＰＬＣ）＝１．５５および異常光屈折率ｎ_ｅ（ＰＬＣ）＝１．７０で厚さ９．１μｍの高分子液晶層１３、１４を形成し、フォトリソグラフィとエッチングの技術により格子ピッチ３０μｍで鋸歯状の断面形状を７段で８レベルの階段により近似した階段状の疑似ブレーズ回折格子に加工した。
【００４６】
次に、屈折率１．５５の均質屈折率透明材料１５、１６を用いて、階段状の疑似ブレーズ回折格子の凹部を埋めるとともに、液晶セル１１０の光入射側と光出射側に２種の偏光性回折格子１２０と１３０として接着固定し、液晶素子１００とした。
【００４７】
ここで、液晶セル１１０の光入射側に配置する偏光性回折格子１２０の高分子液晶の配向方向はＸ軸方向とし、液晶セル１１０の光出射側に配置する偏光性回折格子１３０の高分子液晶の配向方向はＹ軸方向としている。
このようにして作製した液晶素子１００に、偏光方向がランダムな波長１．５５μｍの平行光を入射し、その出射光を集光レンズで光ファイバに集光した。
【００４８】
液晶素子１００の液晶層１への矩形交流印加電圧の振幅Ｖを０から５ｖまで変化させたとき、光ファイバ伝送後の光強度Ｉ（Ｖ）において光強度比Ｉ（５ｖ）／Ｉ（０）で規定される消光比は−２２ｄＢと高い値が得られた。また、液晶素子１００の使用に伴う光挿入損失は−０．５ｄＢと低い値であった。
【００４９】
比較例として、液晶セル１１０の光入射面側にＹ軸方向に偏光方向を有する直線偏光のみを透過する偏光子が、液晶セル１１０の光出射面側にＸ軸方向に偏光方向を有する直線偏光のみを透過する偏光子が配置された従来構成の液晶素子の場合、光挿入損失が−３．２ｄＢと大きな値であった。
【００５０】
また、液晶セル１１０の光入射面側には偏光子を配置せず、液晶セル１１０の光出射面側のみにＸ軸方向に偏光方向を有する直線偏光のみを透過する偏光子が配置された従来構成の液晶素子の場合、消光比は−３ｄＢで光減衰器としては不十分であった。
【００５１】
【発明の効果】
以上説明したように、本発明の液晶素子を用いることにより入射光の偏光方向に関わらず光挿入損失が低くかつ消光比の高い安定した液晶素子を用いた光減衰器が実現できる。また、本発明の液晶素子の構成要素である偏光性ビームスプリッタとして多層膜偏光性ビームスプリッタの代わりに複屈折性材料を使用した偏光性回折格子を用いることにより、薄型で小型軽量な液晶素子および光減衰器が実現できる。
【００５２】
また、複屈折性材料を使用した偏光性回折格子として、複屈折材料層の断面形状が鋸歯状の直線格子であり、特定方向の直線偏光に対して１次の回折効率が高い偏光性ブレーズ回折格子とすることにより、光挿入損失が低い光減衰器を実現できる。さらに、光吸収のほとんどない偏光性ビームスプリッタを液晶素子と一体化した本発明の光減衰器では高強度の光が入射しても液晶層の温度上昇が少ないため、安定した消光比が得られる。
【図面の簡単な説明】
【図１】本発明の液晶素子の第１の実施態様の構成例を示す側面図。
【図２】図１の液晶素子の印加電圧がＶ_１時の作用例を示す側面図。
【図３】図１の液晶素子の印加電圧がＶ_２時の作用例を示す側面図。
【図４】図１の液晶素子を光減衰器として用い、印加電圧がＶ_１時の作用例を示す側面図。
【図５】図１の液晶素子を光減衰器として用い、印加電圧がＶ_２時の作用例を示す側面図。
【図６】本発明の液晶素子の第２の実施形態の構成例と印加電圧がＶ_１時の作用例を示す側面図。
【図７】本発明の液晶素子の第２の実施形態の構成例と印加電圧がＶ_２時の作用例を示す側面図。
【図８】従来の液晶素子の光減衰器の構成例を示す側面図。
【符号の説明】
１：液晶層
２、３：透明電極
４、５、１１、１２：透光性基板
６：シール材
７：矩形波交流電源
８：集光レンズ
９：開口絞り
１０：偏光子
１３、１４：高分子液晶層
１５、１６：均質屈折率透明材料
１７、１８：多層膜面
１９、２０：全反射面
１００、２００：液晶素子
１１０：液晶セル
１２０、１３０：偏光性回折格子
２２０、２３０：多層膜偏光性ビームスプリッタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal element in which the efficiency of linearly transmitted light changes depending on the magnitude of an applied voltage regardless of the polarization direction of incident light, and a voltage variable optical attenuator using the liquid crystal element.
[0002]
[Prior art]
An example of a liquid crystal element which is a conventional optical attenuator using liquid crystal is shown in FIG. A nematic in which the alignment direction of liquid crystal molecules is parallel to the surface of the translucent substrate between the translucent substrates 4 and 5 on which the transparent electrodes 2 and 3 are formed, and is aligned at a 45 ° angle with the X-axis direction. A liquid crystal cell 110 in which a liquid crystal layer 1 of liquid crystal is sandwiched inside a sealing material 6 provided at the periphery of a translucent substrate, and only linearly polarized light having a polarization direction in the X-axis direction on the light exit surface side is transmitted. The polarizer 10 is arranged.
[0003]
Here, a rectangular wave AC power supply 7 is connected to the transparent electrodes 2 and 3, and when no voltage is applied by this power supply, the retardation value of the liquid crystal cell 110 for linearly polarized light having a polarization direction in the Y-axis direction at a wavelength λ is The thickness of the liquid crystal layer 1 is set so as to be approximately λ / 2. Here, the reason why the retardation value of the liquid crystal layer 1 is approximately λ / 2 is to minimize the insertion loss of the optical attenuator when no voltage is applied and to function as a λ / 2 plate.
[0004]
In this optical attenuator, linearly polarized light that is polarized in the Y-axis direction and is incident on the liquid crystal layer when no voltage is applied between the transparent electrodes becomes linearly polarized light having a polarization direction in the X-axis direction, and passes through the polarizer. When a voltage is applied, as the applied voltage increases, the alignment direction of the liquid crystal molecules tilts in the thickness direction of the liquid crystal layer, that is, perpendicular to the substrate. As a result, the retardation value of the liquid crystal layer approaches zero, and the linearly polarized light component in the Y-axis direction increases in the light transmitted through the liquid crystal cell 110. As a result, the amount of light transmitted through the polarizer monotonously decreases as the applied voltage increases, and a voltage variable optical attenuator is obtained.
[0005]
[Problems to be solved by the invention]
In the conventional voltage variable type optical attenuator using the change in the retardation value of the liquid crystal layer according to the voltage application amount, specific linearly polarized light is used as incident light. Therefore, when the polarization direction of incident light is arbitrary, there is always a polarization component that passes through the polarizer, so the extinction ratio is deteriorated and the function of the optical attenuator cannot be obtained.
[0006]
In FIG. 8, when a polarizer that transmits only linearly polarized light having a polarization direction in the Y-axis direction is disposed on the light incident side of the liquid crystal layer 1, linearly polarized light components having a polarization direction in the X-axis direction are absorbed. Only an optical attenuator with a large optical insertion loss was obtained.
[0007]
In view of the above circumstances, the present invention provides a liquid crystal element capable of realizing a stable optical attenuator with low optical insertion loss and high extinction ratio regardless of the polarization direction of incident light, and an optical attenuator using the liquid crystal element. With the goal.
[0008]
[Means for Solving the Problems]
The present invention provides a liquid crystal element having a function of deflecting and emitting incident light incident as two linearly polarized lights having orthogonal polarization states in the traveling direction of the incident light or in different directions, and in the same direction as the traveling direction. A light attenuator comprising a mechanism for transmitting light emitted from the liquid crystal element and blocking light emitted from the liquid crystal element in a direction different from the traveling direction, wherein the liquid crystal element is disposed between the electrode and the substrate with the alignment film. A liquid crystal cell comprising a liquid crystal layer composed of a liquid crystal layer sandwiched by liquid crystal molecules aligned in the same direction, and a phase plate laminated on the liquid crystal single cell, and disposed on the light incident side of the liquid crystal cell. A first polarizing beam splitter; and a second polarizing beam splitter disposed on the light exit side of the liquid crystal cell, wherein the single liquid crystal cell has a voltage applied between the electrodes. The retardation value of the liquid crystal cell is changed in accordance with of can, the phase plate, the re Tadeshon value of the liquid crystal cell upon application of a specific voltage V ₂ between the electrodes offset to zero, the first The polarizing beam splitter and the second polarizing beam splitter each include a birefringent material layer made of a polymer liquid crystal , and the birefringent material layer has a sawtooth cross-sectional shape and a linear shape with a constant planar shape. polarized made of a light diffraction grating, and the first orientation direction of polymer liquid crystal of the polarizing beam splitter polarizing beam splitter direction and the second orientation of the polymer liquid crystal of perpendicular is incident to the liquid crystal element the two linearly polarized, the direction of travel according to the polarization directions different from each other and transmitted through the liquid crystal cell by the first polarizing beam splitter, a wave of light the liquid crystal cell is incident to the two linearly polarized transmitted through the second polarizing beam splitter when having retardation values of the applied voltage V ₂ different voltages V ₁ lambda / 2 with respect to lambda is entered prior to the liquid crystal element aligned in the same direction of travel as Shako each other emitted from the liquid crystal element, the two linearly polarized upon application of the voltage V ₂ from the liquid crystal element becomes the incident light different from the traveling direction of the liquid crystal element the emitted, to provide an optical attenuator, wherein the amount of light propagating light emitted from the liquid crystal element is changed according to the magnitude of the voltage applied between the electrodes.
[0009]
Further, the optical attenuator is provided in which the fast axis of the phase plate coincides with the slow axis of the liquid crystal layer.
[0010]
Further, the polarizing diffraction grating is a high diffraction grating than the diffraction efficiency of first order diffracted light to linearly polarized light polarized in a specific direction in the diffraction efficiency of other order diffracted light and first The above-mentioned optical attenuator in which the traveling direction in which the linearly polarized light diffracted by the polarizing diffraction grating is diffracted and emitted by the second polarizing diffraction grating coincides with the direction in which the linearly polarized light enters the first polarizing diffraction grating. I will provide a.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the liquid crystal cell which is a constituent element of the liquid crystal element of the present invention, a liquid crystal layer is sandwiched between translucent substrates with electrodes, and V ₁ to V ₂ (V ₁ ≠ V ₂ ) between the electrodes of the liquid crystal cell. It is assumed that when a voltage is applied, the retardation value for linearly polarized light having a wavelength λ that is transmitted through the liquid crystal cell changes from (m + 1/2) × λ to m × λ (m is an integer).
[0013]
The polarizing beam splitter, which is a constituent element of the liquid crystal element of the present invention, has a function of efficiently changing the traveling direction of specific linearly polarized incident light and linearly polarized incident light orthogonal to the polarization direction.
[0014]
FIG. 1 is a side view showing a configuration example of the first embodiment of the liquid crystal element of the present invention. The liquid crystal element 100 uses a polarizing diffraction grating including a birefringent material as a polarizing beam splitter. Transparent electrodes 2 and 3 are formed on one side of translucent substrates 4 and 5, respectively, and an alignment film (not shown) that is aligned in the same direction is formed thereon, and a cell is formed using sealant 6. It becomes. Moreover, nematic liquid crystal in the cell ordinary refractive index _n o (LC) and extraordinary refractive index _{_{n e (LC) (n o}} (LC) <n e (LC)) is injected in the liquid crystal layer 1, the substrate As a result, a liquid crystal cell 110 in which the alignment directions of the liquid crystal molecules are aligned is obtained.
[0015]
The first polarizing diffraction grating 120 is disposed on the light incident side of the liquid crystal cell 110, and the second polarizing diffraction grating 130 is disposed on the light emitting side. A configuration common to both will be described below. Forming a polymer liquid crystal layer 13, 14 is a birefringent material of the ordinary refractive index n _o and extraordinary index n _e on one side of the transparent substrate 11 such as glass. Here, the liquid crystal monomer solution is applied on the alignment film of the translucent substrate, and the alignment vector (molecular alignment axis) of the liquid crystal molecules is aligned in a specific direction in a plane parallel to the substrate. Then, the polymer liquid crystal layer is formed by polymerizing and curing by irradiating light such as ultraviolet rays.
[0016]
Next, the polymer liquid crystal layer has a sawtooth shape (so-called blazed diffraction grating type) in cross-sectional shape, and a concavo-convex portion in which the planar shape in the vertical direction (X axis) in FIG. 1 is linear. That is, a linear blazed diffraction grating having a constant grating pitch is processed.
Moreover, filling the homogeneous refractive index transparent material 15 and 16 of substantially equal refractive index n _s and at least concave portions of the concavo-convex portion of the birefringent material layer ordinary refractive index n _o.
[0017]
Here, the cross-sectional shape of the serrated concavo-convex portion, so that the first-order diffraction efficiency for extraordinarily polarized light of the wavelength of the incident light λ becomes maximum, the phase difference 2 [pi × uneven portion _{_{(n e -n s) × d}} / λ is 2π, that is, the film thickness d of the polymer liquid crystal layers 13 and 14 is λ / (ne-ns).
[0018]
The sawtooth cross-sectional shape may be a pseudo blazed grating approximated by a staircase shape. In that case, it is preferable to use a stepped sawtooth of about 4 to 16 steps. Moreover, although diffraction efficiency falls compared with a blazed diffraction grating, the cross-sectional shape of an uneven | corrugated | grooved part is good also as a rectangular diffraction grating. In any case, by setting the phase difference condition of the concavo-convex portion where the diffracted light is maximum, the 0th-order light component of the extraordinary light polarized light having the wavelength λ that is transmitted straight without being diffracted is 1% or less.
[0019]
When linearly polarized light, which is ordinary polarized light with respect to the polymer liquid crystal layer, is incident on the polarizing diffraction gratings 120 and 130 thus obtained, the polymer liquid crystal layers 13 and 14 and the homogeneous refractive index transparent materials 15 and 16 In order to prevent a difference in refractive index, incident light is transmitted straight without being diffracted.
[0020]
On the other hand, when linearly polarized light, which is extraordinary light polarization, enters the polymer liquid crystal layer, it is separated as diffracted light in a traveling direction different from that of ordinary light polarization according to the diffraction grating pitch. The polarizing diffraction grating is bonded to and integrated with the liquid crystal cell 110 as the first polarizing diffraction grating 120 on the light incident side of the liquid crystal cell 110 and the second polarizing diffraction grating 130 on the light emitting side. It is said.
At this time, the traveling path of the incident light to the polarizing diffraction gratings 120 and 130 is separated into an ordinary light polarization component that travels straight and an extraordinary light polarization component that is diffracted in a specific direction.
[0021]
Here, the alignment direction of the polymer liquid crystal layer constituting the first polarizing diffraction grating 120 is defined as the X-axis direction, and the alignment direction of the polymer liquid crystal layer constituting the second polarizing diffraction grating 130 is defined as the Y-axis direction. The orientation treatment is performed so that the orientation directions of the polarizing diffraction gratings are orthogonal to each other. Further, the alignment direction of the nematic liquid crystal molecules of the liquid crystal cell 110 is arranged to form an angle of 45 ° with the alignment direction of the polymer liquid crystal layer in the first and second polarizing diffraction gratings.
[0022]
When the retardation value of the transmitted light of the liquid crystal cell 110 is m × λ (m is an integer), the polarization component that has been transmitted straight through the first polarizing diffraction grating 120 is diffracted by the second polarizing diffraction grating 130, and The polarized light component diffracted by the polarizing diffraction grating 120 passes through the second polarizing diffraction grating 130 in a straight line.
[0023]
Further, when the retardation value of the transmitted light of the liquid crystal cell 110 is (m + 1/2) × λ (m is an integer), the first-order diffracted light diffracted by the first polarizing diffraction grating 120 is the second polarizing property. The grating shapes (pitch, linear grating) of the first polarizing diffraction grating 120 and the second polarizing diffraction grating 130 are diffracted as the first-order diffracted light by the diffraction grating 130 and have the same traveling direction as the non-diffracted light. , The cross-sectional uneven shape) is adjusted.
[0024]
The operation when incident light having an unspecified polarization direction enters the liquid crystal element 100 will be described with reference to FIGS. Of the incident light having the wavelength λ, the ordinary light polarization component (Y axis) is linearly transmitted through the polarization diffraction grating 120 and the extraordinary light polarization component (X axis) is the polarization diffraction grating. The light is diffracted by 120 and enters the liquid crystal cell 110.
[0025]
When a voltage is applied to the V ₁ between the electrodes of the liquid crystal cell 110 retardation value (FIG. 2), both with respect to each component of the ordinarily polarized light and extraordinarily polarized light (m + 1/2) × λ (m is an integer) It functions as a phase plate.
[0026]
As a result, the light that has been linearly transmitted through the first polarizing diffraction grating 120 is rotated by 90 ° and enters the second polarizing diffraction grating 130 as ordinary light polarized light. The light passes straight without being diffracted by the diffraction grating 130. In addition, since the light diffracted by the first polarizing diffraction grating 120 is rotated by 90 ° and enters the second polarizing diffraction grating 130 as an extraordinary light polarization, the second polarizing property The light is diffracted by the diffraction grating 130 and is emitted in the same traveling direction as the linearly transmitted light. Therefore, the voltage applied to the liquid crystal cells 110 when the V _1, the incident light is straightly transmitted regardless of the polarization direction. That is, when the liquid crystal cell has a specific retardation value, here (m + 1/2) × λ (m is an integer), the two linearly polarized light beams transmitted through the second polarizing beam splitter are traveling in the direction of incidence of the liquid crystal element. The liquid crystal elements are emitted in alignment with each other in the same traveling direction as the light.
[0027]
On the other hand, when applying a voltage of V ₂ between the electrodes of the liquid crystal cell 110 (FIG. 3), ordinary position having retardation values of both for each component of the polarized light and the abnormal light polarization m × lambda (m is an integer) Functions as a phase difference plate.
[0028]
As a result, the light linearly transmitted through the first polarizing diffraction grating 120 enters the second polarizing diffraction grating 130 as extraordinary light polarization while maintaining the polarization direction, so that the second polarizing diffraction The light is diffracted by the grating 130 and separated and emitted in a traveling direction different from the incident light of the liquid crystal element 100. Further, since the light diffracted by the first polarizing diffraction grating 120 enters the second polarizing diffraction grating 130 as ordinary light polarization while maintaining the polarization direction, the second polarizing diffraction grating 130 is incident. Are transmitted in a straight line, separated in a traveling direction different from the incident light of the liquid crystal element 100, and emitted.
[0029]
Therefore, when the voltage applied to the liquid crystal cell 110 is V ₂ , the incident light is diffracted regardless of its polarization state, separated and emitted in a traveling direction different from the incident light.
When an intermediate voltage of V ₁ and V ₂ is applied between the electrodes of the liquid crystal cell 110, the transmitted light of the liquid crystal element 100 has a component in the same traveling direction as the incident light and a component in the traveling direction different from the incident light. It becomes the emitted light mixed in the ratio according to.
[0030]
4 and 5 are side views showing an example of an optical system configuration and an operation of a voltage variable optical attenuator using the liquid crystal element 100 of the present invention. When parallel light having an unspecified polarization direction is incident on the liquid crystal element 100 and the condenser lens 8 is arranged on the exit side, the light transmitted through the liquid crystal element 100 in a straight line passes through the focal plane on the optical axis of the condenser lens 8. It is condensed (FIG. 4). On the other hand, the light diffracted by the liquid crystal element 100 is condensed on the focal plane outside the optical axis of the condenser lens 8 (FIG. 5).
[0031]
Therefore, by arranging the aperture stop 9 having an opening on the focal plane on the optical axis of the condenser lens 8, an isolator that transmits only the straight transmitted light and blocks the diffracted light having different traveling directions is obtained. Here, instead of the aperture stop 9, only a straight-forward transmitted light can be detected by arranging a photodetector having a light receiving portion corresponding to the opening. Further, if the core part of the optical fiber for optical transmission is arranged instead of the opening part, only the straight transmitted light can be transmitted.
[0032]
In this way, an optical attenuator that can adjust the amount of transmitted light is realized by changing the voltage applied between the electrodes of the liquid crystal cell 110 with respect to incident light having an unspecified polarization direction.
[0033]
In this embodiment, in order to obtain an optical attenuator having a high extinction ratio in a wide incident light wavelength band, the linear diffraction transmittance of the polarizing diffraction grating, for example, for ordinary light polarization is high, for example, the straight transmittance for abnormal light polarization is low. Is preferred. In order to maintain high diffraction efficiency for extraordinary light polarization in a wide incident light wavelength band and keep the straight transmittance low, it is effective to stack a polarizing diffraction grating.
[0034]
Further, the applied voltage V ₁ and V ₂ of the liquid crystal element 100 and lower voltage, in order to reduce the wavelength dependence, voltage application pressure variable range 0-1 / 2 of the retardation value of the liquid crystal layer thinner i.e. liquid crystal layer It is preferable to set to xλ (corresponding to m = 0). In order to achieve a retardation value of zero at a specific applied voltage, it is effective to stack a phase plate on the liquid crystal cell 110 that cancels the retardation value of the liquid crystal layer.
[0035]
In the above embodiment, the orientation direction of the polymer liquid crystal layer constituting the first polarizing diffraction grating 120 and the orientation direction of the polymer liquid crystal layer constituting the second polarizing diffraction grating 130 are orthogonal to each other. As described above, the alignment directions of both polymer liquid crystal layers may be parallel. In that case, when the liquid crystal cell has a retardation value m × λ (m is an integer), that is, a specific retardation value, the two linearly polarized light whose traveling paths are separated by the first polarizing diffraction grating 120 is a liquid crystal. When the second polarizing beam splitter and the liquid crystal element are emitted in alignment with each other in the same traveling direction as the incident light of the element, and the liquid crystal cell has a retardation value (m + 1/2) × λ (m is an integer), The light is separated in the traveling direction different from that of the incident light, and is emitted from the second polarizing beam splitter and the liquid crystal element.
[0036]
FIG. 6 is a side view showing a configuration example and an operation example of the second embodiment of the liquid crystal element of the present invention. The liquid crystal element 200 is a polarizing beam splitter, such as a light-transmitting dielectric film such as SiO ₂ or MgF ₂ having a relatively low refractive index, and a light transmitting material such as TiO ₂ or Ta ₂ O ₅ having a relatively high refractive index. The conductive dielectric film is laminated on the surface of the translucent substrate so that the optical film thickness (refractive index × actual film thickness) is in the wavelength order alternately. The translucent substrate surface is a substrate surface inclined by 45 ° with respect to the light incident surface, and the multilayer polarizing beam splitters 220 and 230 formed by forming the translucent dielectric film are used as the light of the liquid crystal cell 110. They are used on the incident side and the light exit side, respectively.
[0037]
In FIG. 6, the polarization component A in the paper surface is transmitted through the multilayer film surface 17 and enters the liquid crystal cell, and the polarization component B perpendicular to the paper surface is reflected by the multilayer film surface 17 and the total reflection surface 19 to be reflected on the liquid crystal cell 110. Incident.
[0038]
When the inter-electrode voltage applied to the liquid crystal cell 110 is a retardation value of V ₁ i.e. the liquid crystal layer becomes (m + 1/2) × λ (m is an integer), that is, when a particular retardation value, as shown in FIG. 6, the polarization The component A rotates 90 ° polarization direction and transmits through the liquid crystal cell 110, is reflected by the total reflection surface 20 and the multilayer film surface 18, and is emitted from the multilayer film polarizing beam splitter 230. Further, the polarization component B rotates through the 90 ° polarization direction and passes through the liquid crystal cell 110, passes through the multilayer film surface 18, is combined with the same optical axis as the polarization component A, and is emitted from the multilayer film polarizing beam splitter 230. To do. As a result, regardless of the polarization direction of the incident light, the liquid crystal element 200 transmits straight in the same direction as the traveling direction of the incident light.
[0039]
On the other hand, when the inter-electrode voltage applied to the liquid crystal cell 110 is a retardation value of V ₂ i.e. the liquid crystal layer is made of a m × λ (m is an integer), as shown in FIG. 7, while the polarization component A keeping the polarization state liquid The light passes through the cell 110, is reflected by the total reflection surface 20, passes through the multilayer film surface 18, and exits from the multilayer film polarizing beam splitter 230. Further, the polarization component B passes through the liquid crystal cell 110 while maintaining the polarization state, is reflected by the multilayer film surface 18, is combined with the same optical axis as the polarization component A, and exits from the multilayer polarization beam splitter 230. As a result, regardless of the incident polarization direction, the traveling direction is separated in the direction orthogonal to the incident light of the liquid crystal element 200 and is emitted from the liquid crystal element 200.
[0040]
When an intermediate voltage of V ₁ and V ₂ is applied between the electrodes of the liquid crystal cell 110, the component of the traveling direction that is the same as the incident light and the component of the traveling direction that is orthogonal to the incident light are included in the linearly transmitted light of the liquid crystal element 200. Becomes a mixed emission light at a ratio corresponding to the applied voltage, and an optical attenuator in which the amount of emitted light is variable according to the applied voltage is realized by taking out only one traveling direction. Note that reference numerals 17, 19 and 220 in FIG. 7 are the same elements as those in FIG.
[0041]
The liquid crystal element and the optical attenuator of the present invention can be realized in various forms by a combination of a liquid crystal cell and a polarizing beam splitter other than those described in the above embodiments.
[0042]
【Example】
A liquid crystal element 100 of this example will be described with reference to FIG. A nematic liquid crystal having ordinary refractive index no (LC) = 1.50 and extraordinary refractive index ne (LC) = 1.66 is sandwiched between translucent substrates 4 and 5 having transparent electrodes 2 and 3 formed on one side. A liquid crystal unit cell in which the thickness d (LC) of the liquid crystal layer 1 was 5.6 μm was produced. The slow axis direction of the liquid crystal layer 1 was 45 ° with respect to the X axis direction shown in FIG.
[0043]
When no voltage is applied to the transparent electrodes 2 and 3, the retardation value of the liquid crystal single cell with respect to light having a wavelength of 1.55 μm is about 0.9 μm, and linearly polarized light having a polarization direction in the Y-axis direction is applied to the liquid crystal single cell. When entering and exiting, the polarization direction of the linearly polarized light was substantially the X-axis direction. In addition, in a state where a rectangular wave AC voltage having a voltage amplitude of 5 V was applied, the retardation value of the single liquid crystal cell was 0.12 μm.
[0044]
Further, on one side of a glass translucent substrate, a phase plate made of a polymer liquid crystal layer having an ordinary light refractive index n _o = 1.55 and an extraordinary light refractive index n _e = 1.59 and a thickness d = 3.0 μm ( A liquid crystal cell 110 was manufactured by bonding and integrating a liquid crystal cell (not shown) to a single liquid crystal cell. The retardation value of the phase plate made of the polymer liquid crystal layer is −0.12 μm. By making the fast axis direction of the phase plate coincide with the slow axis direction of the liquid crystal layer 1, the liquid crystal layer 1 at an applied voltage of 5V is used. The retardation value 0.12 μm remaining in the liquid crystal is canceled out, and the retardation value of the liquid crystal cell 110 becomes zero.
[0045]
Further, in FIG. 1, the thickness in the ordinary refractive index _n o (PLC) = 1.55 and an extraordinary refractive index _n e (PLC) = 1.70 on one surface of the glass of the transparent substrate 11, 12 9.1 microns The polymer liquid crystal layers 13 and 14 were formed, and processed into a staircase-like pseudo-blazed diffraction grating having a sawtooth-like cross-sectional shape approximated by seven steps and eight steps with a grating pitch of 30 μm by photolithography and etching techniques.
[0046]
Next, the homogeneous refractive index transparent materials 15 and 16 having a refractive index of 1.55 are used to fill the concave portion of the stepped pseudo-blazed diffraction grating, and two types of polarized light are provided on the light incident side and the light output side of the liquid crystal cell 110. The liquid crystal element 100 was obtained by bonding and fixing as the diffractive diffraction gratings 120 and 130.
[0047]
Here, the alignment direction of the polymer liquid crystal of the polarizing diffraction grating 120 disposed on the light incident side of the liquid crystal cell 110 is the X-axis direction, and the polymer liquid crystal of the polarizing diffraction grating 130 disposed on the light emitting side of the liquid crystal cell 110. The orientation direction is the Y-axis direction.
Parallel light with a wavelength of 1.55 μm having a random polarization direction was incident on the liquid crystal element 100 thus manufactured, and the emitted light was condensed on an optical fiber by a condenser lens.
[0048]
When the amplitude V of the rectangular alternating voltage applied to the liquid crystal layer 1 of the liquid crystal element 100 is changed from 0 to 5 v, the light intensity ratio I (5 v) / I (0) in the light intensity I (V) after the optical fiber transmission. The extinction ratio specified by is as high as -22 dB. Further, the optical insertion loss due to the use of the liquid crystal element 100 was a low value of -0.5 dB.
[0049]
As a comparative example, a polarizer that transmits only linearly polarized light having a polarization direction in the Y-axis direction on the light incident surface side of the liquid crystal cell 110 is linearly polarized light having a polarization direction in the X-axis direction on the light exit surface side of the liquid crystal cell 110. In the case of a liquid crystal element having a conventional configuration in which a polarizer that only transmits light is disposed, the optical insertion loss is a large value of -3.2 dB.
[0050]
In addition, a polarizer that does not arrange a polarizer on the light incident surface side of the liquid crystal cell 110 and a polarizer that transmits only linearly polarized light having a polarization direction in the X-axis direction is disposed only on the light emitting surface side of the liquid crystal cell 110. In the case of the liquid crystal element having the configuration, the extinction ratio was −3 dB, which was insufficient as an optical attenuator.
[0051]
【The invention's effect】
As described above, by using the liquid crystal element of the present invention, an optical attenuator using a stable liquid crystal element having a low optical insertion loss and a high extinction ratio can be realized regardless of the polarization direction of incident light. Further, by using a polarizing diffraction grating using a birefringent material instead of a multilayer polarizing beam splitter as a polarizing beam splitter which is a constituent element of the liquid crystal element of the present invention, a thin, small and light liquid crystal element and An optical attenuator can be realized.
[0052]
In addition, as a polarizing diffraction grating using a birefringent material, a cross-sectional shape of the birefringent material layer is a sawtooth-shaped linear grating, and a polarization blazed diffraction having high first-order diffraction efficiency for linearly polarized light in a specific direction. By using a grating, an optical attenuator with low optical insertion loss can be realized. Furthermore, in the optical attenuator of the present invention in which a polarizing beam splitter having almost no light absorption is integrated with a liquid crystal element, a stable extinction ratio can be obtained because the temperature rise of the liquid crystal layer is small even when high intensity light is incident. .
[Brief description of the drawings]
FIG. 1 is a side view showing a configuration example of a first embodiment of a liquid crystal element of the present invention.
FIG. 2 is a side view showing an example of operation when the applied voltage of the liquid crystal element of FIG. 1 is V ₁ ;
FIG. 3 is a side view showing an example of operation when the applied voltage of the liquid crystal element of FIG. 1 is V ₂ ;
4 is a side view showing an example of operation when the liquid crystal element of FIG. 1 is used as an optical attenuator and the applied voltage is V _1. FIG.
[5] using a liquid crystal element in FIG. 1 as an optical attenuator, a side view of the applied voltage indicates the action example o'clock V _2.
FIG. 6 is a side view showing a configuration example of a liquid crystal element according to a second embodiment of the present invention and an operation example when an applied voltage is V ₁ .
FIG. 7 is a side view showing a configuration example of a liquid crystal element according to a second embodiment of the present invention and an operation example when an applied voltage is V ₂ .
FIG. 8 is a side view showing a configuration example of a conventional optical attenuator of a liquid crystal element.
[Explanation of symbols]
1: Liquid crystal layer 2, 3: Transparent electrodes 4, 5, 11, 12: Translucent substrate 6: Sealing material 7: Rectangular wave AC power supply 8: Condensing lens 9: Aperture stop 10: Polarizers 13, 14: High Molecular liquid crystal layers 15, 16: Homogeneous refractive index transparent material 17, 18: Multilayer film surface 19, 20: Total reflection surface 100, 200: Liquid crystal element 110: Liquid crystal cell 120, 130: Polarizing diffraction grating 220, 230: Multilayer film Polarizing beam splitter

Claims

A liquid crystal element having a function of deflecting and emitting incident light incident as two linearly polarized lights having orthogonal polarization states in a traveling direction or different directions of the incident light, and exiting from the liquid crystal element in the same direction as the traveling direction A light attenuator comprising a mechanism that transmits light to be transmitted and blocks light emitted from the liquid crystal element in a direction different from the traveling direction,
The liquid crystal element includes: a liquid crystal cell comprising a liquid crystal layer in which liquid crystal molecules are sandwiched by being aligned in the same direction between an electrode and a substrate with an alignment film; and a phase plate laminated on the liquid crystal single cell; A first polarizing beam splitter disposed on the light incident side of the liquid crystal cell, and a second polarizing beam splitter disposed on the light emitting side of the liquid crystal cell,
In the liquid crystal single cell, the retardation value of the liquid crystal cell changes according to the magnitude of the voltage applied between the electrodes,
It said phase plate is offset re Tadeshon value of the liquid crystal cell upon application of a specific voltage V ₂ between the electrodes to zero,
The first polarizing beam splitter and the second polarizing beam splitter each include a birefringent material layer made of a polymer liquid crystal , and the birefringent material layer has a sawtooth cross-sectional shape and a constant planar shape. Consisting of a linear polarizing diffraction grating,
The alignment direction of the polymer liquid crystal of the first polarizing beam splitter is orthogonal to the alignment direction of the polymer liquid crystal of the second polarizing beam splitter,
The two linearly polarized light incident on the liquid crystal element are transmitted through the liquid crystal cell with different traveling directions according to the polarization direction by the first polarizing beam splitter , and the wavelength λ of light incident on the liquid crystal cell. When the voltage V ₁ different from the voltage V ₂ is applied and the retardation value is λ / 2 , the two linearly polarized light transmitted through the second polarizing beam splitter is the incident light of the liquid crystal element. Emanating from the liquid crystal element aligned with each other in the same traveling direction,
The two linearly polarized upon application of the voltage V _2, the emitted from the liquid crystal element becomes the traveling direction different from the incident light of the liquid crystal element,
An optical attenuator characterized in that the amount of light propagating light emitted from the liquid crystal element changes according to the magnitude of the voltage applied between the electrodes.

The optical attenuator according to claim 1, wherein a fast axis of the phase plate coincides with a slow axis of the liquid crystal layer.

The polarizing diffraction grating is a high diffraction grating than the diffraction efficiency of first order diffracted light to linearly polarized light polarized in a specific direction in the diffraction efficiency of other order diffracted light and the first polarizing The traveling direction in which the linearly polarized light diffracted by the diffraction grating is diffracted and emitted by the second polarizing diffraction grating coincides with the direction in which the linearly polarized light enters the first polarizing diffraction grating. The optical attenuator according to item 1.