JP2004177639A

JP2004177639A - Reflective variable magneto-optical device

Info

Publication number: JP2004177639A
Application number: JP2002343401A
Authority: JP
Inventors: Shohei Abe; 昇平阿部; Makoto Kawaguchi; 誠川口; Nobutake Ezuka; 信武江塚
Original assignee: FDK Corp
Current assignee: FDK Corp
Priority date: 2002-11-27
Filing date: 2002-11-27
Publication date: 2004-06-24
Anticipated expiration: 2022-11-27
Also published as: JP4382344B2

Abstract

<P>PROBLEM TO BE SOLVED: To drastically downsize a device as a whole by reducing not only the dimension in an optical axis direction, but also the dimension in a direction perpendicular to the optical axis. <P>SOLUTION: A reflective variable magneto-optical device is equipped with: a group of optical elements with at least a birefringent element 12 and a variable Faraday rotator 16; and a mirror 18. The device controls the quantity or path of light while light incident from the one end side proceeds through the group of the optical elements and reaches the mirror on the other end side and the reflected light returns through the group of the optical elements and is outputted from the one end side and thus the light is made to go and return through the group of the optical elements. A Faraday element 24 being a variable Faraday rotator is placed on the light path directly before the mirroring means. An electromagnet 28 comprising a rod shaped core 30 with a coil 32 wound thereon is aligned in the vicinity of, and placed opposite to, the mirroring means so as to make one end face thereof placed just behind the mirroring means. A variable magnetic field is applied by the electromagnet to the Faraday element beyond the mirroring means. The device is applicable not only to a variable optical attenuator to control the quantity of light, but also to an optical switch to control the path of light. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、可変ファラデー回転子を用いて光量あるいは光路を制御する反射型の磁気光学光デバイスに関するものである。更に詳しく述べると本発明は、ミラー手段の直前の光路中に可変ファラデー回転子のファラデー素子が位置し、ロッド状磁心にコイルを巻装した電磁石を、その一方の端面がミラー手段の直後に位置するように配列し、電磁石によってミラー手段を越えてファラデー素子に可変磁界を印加するようにした反射型可変磁気光学デバイスに関するものである。
【０００２】
【従来の技術】
【特許文献１】
特開平１０−１６１０７６号公報
【０００３】
光通信システムあるいは光計測システムなどにおいては、透過光量を制御するための光アッテネータ、あるいは光路切り換えや光のオン／オフ制御を行うための光スイッチなど各種の能動型光デバイスが必要となる。光アッテネータは、例えばファラデー回転角可変装置の入力側と出力側に偏光子と検光子を設置し、ファラデー素子に印加する磁界を可変することにより、検光子を透過する光量を制御する光デバイスである。また光スイッチは、入力ポートからの入力光を、異なる出力ポートの選択された任意の一つに出力するというような光路切り換え機能を有する光デバイスであり、１×２型（１入力・２出力）が最も基本的な形態である。
【０００４】
これら光通信用の光デバイスとしては、偏波無依存性であること、信頼性が高いこと、光ファイバとの整合性が良好なことなどが肝要である。そのような要求を満たしうるものとして、例えば偏波面に応じて光路を制御する複屈折素子と偏波面の回転角を制御する可変ファラデー回転子などの各種光素子を組み合わせ配列した光素子群によって必要な光機能部とする構成がある。
【０００５】
かつては、光素子群の一端側から光が入力し、光素子群を進行して、他端側から出力する透過型が一般的であった。しかし近年、光デバイスの小型化などの観点から、配列されている光素子群とミラー手段とを組み合わせ、一端側から入力する光が光素子群を進行して他端側のミラーに達し、該ミラーで反射した光が光素子群を逆行して一端側から出力し、光が光素子群を往復する間、光量の制御あるいは光路の切り換えを行う反射型の光デバイスが開発されている。例えば、特許文献１には、入力ファイバと出力ファイバを備えた２芯フェルールと、レンズと、楔形複屈折板と、磁気光学結晶（ファラデー素子）と、リフレクタとを配列し、ファラデー素子に磁界を印加することでファラデー回転角を変化させる構成が開示されている。
【０００６】
これらの光デバイスにおいて、光量の制御あるいは光路の切り換えは、可変ファラデー回転子の電磁石への通電電流の制御によって行われる。そのために用いられている電磁石は、通常、Ｃ型ヨークにコイルを巻装した構造であり、光路中に挿入されているファラデー素子をそのギャップ部分で挟むように配置し、漏れ磁界により該ファラデー素子に必要な方向への可変磁界が印加できるように構成されている。
【０００７】
【発明が解決しようとする課題】
このような反射型の光デバイスは、反射光が光素子群を通過することで必要な機能をもたせることができるため、必要な部品点数を削減でき光軸方向の長さを大幅に短縮できる利点を有する。しかし、コイルを巻装したＣ型ヨークは、光路の障害とならないようにするために、配列されている光素子群の側方から取り付ける格好となる。そのために、光軸に対して直交する方向の寸法は、透過型も反射型も殆ど変わらず、小さくできないために全体としての小型化（小径化）は不十分であった。
【０００８】
本発明の目的は、光軸方向の寸法のみならず、光軸に直交する方向の寸法も短縮でき、全体として大幅な小型化が可能な反射型可変磁気光学デバイスを提供することである。
【０００９】
【課題を解決するための手段】
本発明は、少なくとも複屈折素子と可変ファラデー回転子を有する光素子群とミラー手段を具備し、一端側から入力する光が光素子群を進行して他端側のミラー手段に達し、該ミラー手段で反射した光が光素子群を逆行して一端側から出力し、光が光素子群を往復する間、光量又は光路が制御される磁気光学デバイスにおいて、前記ミラー手段の直前の光路中に可変ファラデー回転子のファラデー素子が位置しミラー手段を介してファラデー素子に対向するように配列し、ロッド状磁心にコイルを巻装した電磁石を、その一方の端面がミラー手段の直後に位置するように配列し、該電磁石によってミラー手段を越えてファラデー素子に可変磁界を印加する反射型可変磁気光学デバイスである。
【００１０】
このように可変ファラデー回転子のファラデー素子をミラー手段の直前に位置させると共に、該ミラー手段の直後に電磁石のロッド状磁心を配置した点に本発明の特徴がある。反射型の光デバイスでは、光はミラー手段で反射されるために該ミラー手段の背後には光は到達しない。つまりミラー手段の背後に非光学部品を配置しても光路の妨げとはならない。本発明は、この点に着目し、ミラー手段の背後に可変ファラデー回転子用の電磁石を設置すると共に、その構造を工夫して、従来のＣ型ヨークではなくロッド状磁心を採用することで、光デバイスの細径化を実現したものなのである。
【００１１】
反射型可変磁気光学デバイスとしては、光素子群は、２芯ファイバフェルールと複屈折素子とレンズをその順序で配列し、２芯ファイバフェルールのファイバ端面とレンズの距離を該レンズの焦点距離にほぼ一致させる構造がある。可変ファラデー回転子によりファラデー回転角をほぼ０〜４５度の範囲で可変することで、出力光量を制御する光アッテネータとして機能する。ここで可変ファラデー回転子は、ファラデー素子の外側に設けた一対の永久磁石と前記電磁石を備え、永久磁石によってファラデー素子の面に平行な方向に飽和磁界を印加し、電磁石によってファラデー素子の面に垂直な方向に可変磁界を印加する構成とするのがよい。
【００１２】
反射型可変磁気光学デバイスとしては、偏波方向が直交関係にある同じ光路の光を分離し異なる光路の光を合成する偏波分離合成用複屈折素子と、異なる光路を通る偏波方向が直交関係にある光を平行関係にし平行関係にある光を直交関係にする偏波選択回転手段と、往路は常光のまま、復路では常光は異常光に異常光は常光に変換する常光−異常光変換手段と、偏波面を０度と４５度のいずれかに切り換える偏波面切換制御手段とを具備し、該偏波面切換手段に２２．５度固定ファラデー回転子と±２２．５度可変ファラデー回転子を用いる構造がある。この反射型可変磁気光学デバイスは、可変ファラデー回転子による偏波面の切り換えによって、光路を制御する光スイッチとして機能する。
【００１３】
ミラー手段は、単体のミラーであってもよいが、可変ファラデー回転子のファラデー素子の背面に形成したミラー膜、あるいは電磁石のロッド状磁心の一方の端面を鏡面加工し形成したミラー膜でもよい。ミラー膜を用いると、ファラデー素子とロッド状磁心の間隔を最小にできるため磁気効率が向上する利点がある。
【００１４】
複屈折素子としては、例えばルチル単結晶あるいはニオブ酸リチウムなどを用いる。またファラデー素子としては、例えば磁性ガーネット単結晶、特にＬＰＥ（液相エピタキシャル）法により育成したビスマス置換希土類鉄ガーネット単結晶膜が好適である。
【００１５】
【発明の実施の形態】
反射型光スイッチの場合は、前記のように、偏波方向が直交関係にある同じ光路の光を分離し異なる光路の光を合成する偏波分離合成用複屈折素子と、異なる光路を通る偏波方向が直交関係にある光を平行関係にし平行関係にある光を直交関係にする偏波選択回転手段と、往路は常光のまま、復路では常光は異常光に異常光は常光に変換する常光−異常光変換手段と、偏波面を０度と４５度のいずれかに切り換える偏波面切換制御手段とを具備している。
【００１６】
ここで偏波分離合成用複屈折素子は、偏波分離合成用の単一の複屈折結晶でもよいが、偏波面を９０度回転する直線位相子を挟んで光学軸が対称となるように２個の複屈折結晶を組み合わせた構成、あるいは偏波分離合成用の複屈折結晶と光路長補正用の複屈折結晶を組み合わせた構成などでもよい。
【００１７】
偏波選択回転手段は、偏波面を９０度回転する２枚の直線位相子からなり、それらは偏波面の回転が必要な光路に１枚の直線位相子が挿入され、偏波面の回転が不要な光路には直線位相子が挿入されないか、あるいは２枚の直線位相子が重なった状態で挿入されるような形状とする。
【００１８】
常光−異常光変換手段は、２個の複屈折結晶の間に、４５度固定ファラデー回転子と、偏波面を４５度回転する直線位相子を挿入した構造とする。
【００１９】
偏波面切換手段は、２２．５度固定ファラデー回転子と±２２．５度可変ファラデー回転子からなり、可変ファラデー回転子がミラー手段寄りに位置するように組み合わせる。
【００２０】
【実施例】
図１は本発明に係る反射型可変磁気光学デバイス（可変光アッテネータ）の一実施例を示す説明図であり、部材の配列構造と各光素子の位置での偏波状態を示している。これは、２芯ファイバフェルール１０と、複屈折素子１２とレンズ１４と、可変ファラデー回転子１６と、ミラー１８を、その順序で配列した磁気光学デバイスである。説明を分かり易くするため、光素子群の配列方向（光軸）をｚ軸とし、該ｚ軸に直交する２方向をそれぞれｘ軸、ｙ軸とする座標軸を設定する。また便宜的に、ｘ方向を右方向（ｘ軸を横軸）、ｙ方向を上方向（ｙ軸を縦軸）とする。
【００２１】
２芯ファイバフェルール１０の一端側には、入力ファイバ２０と出力ファイバ２２が接続されている。入力ファイバ２０は中段右側に設けられ、出力ファイバ２２は中段左側に設けられている。つまり、入力ファイバ２０と出力ファイバ２２は、ｙ方向の位置は一致しているが、ｘ方向ではずれて配置されている。
【００２２】
複屈折素子１２は、ルチルやニオブ酸リチウムなどの複屈折結晶からなる直方体であり、その光学軸はｙ−ｚ面内にあり且つｚ軸から傾いた方向を向くように設定されている。この複屈折素子１２は、偏波方向が直交関係にある同じ光路の光を上下に異なる光路の光に分離し、上下で異なる光路の光を同じ光路の光に合成する機能を果たすものである。
【００２３】
レンズ１４は、例えば均質の透明材料からなる平凸レンズであり、２芯ファイバフェルール１０のファイバ端面とレンズ１４との距離が該レンズ１４の焦点距離にほぼ一致するように設定されている。従って、ファイバ端面からの出射光はレンズで平行光となり、逆に平行光はファイバ端面に集光することになる。
【００２４】
可変ファラデー回転子１６は、ミラー１８の直前の光路中に挿入されているファラデー素子（磁気光学結晶：例えばＢｉ置換希土類鉄ガーネットＬＰＥ膜等）２４と、それに可変外部磁界を印加する磁界印加手段を備えている。磁界印加手段は、ファラデー素子２４の外側にてｙ軸方向で対向するように配置した１対の永久磁石２６ａ，２６ｂと、ミラー１８の直後に配置した電磁石２８からなる。この電磁石２８は、ロッド状磁心３０にコイル３２を巻装した構造であり、該ロッド状磁心３０の一方の端面がミラー１８に対向するように近接配置される。永久磁石２６ａ，２６ｂはファラデー素子１８に対して面内方向（ｙ軸方向）に飽和固定磁界を印加し、電磁石２８はミラー１８を越えてファラデー素子２４に対して面に垂直な方向（ｚ軸方向）に可変磁界を印加する。このようにして、永久磁石２６ａ，２６ｂによる固定磁界と電磁石２８による可変磁界の合成磁界によって、入射光のファラデー回転角を０〜４５度の範囲で自由に可変できるようになっている。
【００２５】
次に、この可変光アッテネータの動作について簡単に説明する。入力ファイバ２０から中段右側光路に入力する光は、複屈折素子１２で常光は中段右側光路を直進し、異常光は＋ｙ方向（上向き）に屈折して上段右側光路を進行する。このように光が分離し、レンズ１４で平行光となって進み、可変ファラデー回転子１６のファラデー素子２４を通過してミラー１８に達する。
【００２６】
（４５度ファラデー回転時：減衰率≒０％）
往路では、分離光は可変ファラデー回転子１６でそれぞれ光の偏波面が４５度回転し、レンズ屈折作用によってミラー１８の同じ位置に入射する。復路では、反射光は光路が上下左右で入れ替わると共に可変ファラデー回転子１６でそれぞれ光の偏波面が更に４５度回転し（従って、往路と復路で合計９０度回転する）、レンズ１４を通って集光され、複屈折素子１２で常光は直進し、異常光は−ｙ方向（下向き）に屈折して、光が合成される。合成した光は全て中段左側光路を通るため出力ファイバ２２に結合する。従って、入力ファイバ２０からの入力光は減衰することなく出力ファイバに出力する。
【００２７】
（０度ファラデー回転時：減衰率≒１００％）
往路では、分離光は可変ファラデー回転子１６では偏波面は回転せず、レンズ屈折作用によってミラー１８の同じ位置に入射する。復路では、反射光は光路が上下左右で入れ替わると共に可変ファラデー回転子１６では偏波面は回転せず、レンズ１４を通り、複屈折素子１２で上段光路の常光は直進し、中段光路の異常光は−ｙ方向（下向き）に屈折するため、光は上段左側光路と下段左側光路を通過するように更に分離され、出力ファイバ２２には全く結合しない。
【００２８】
（２２．５度ファラデー回転時：減衰率≒５０％）
ファラデー回転角が４５度と０度の間の場合は次のようになる。代表的な例として中間の２２．５度の場合を説明する。往路では、分離光は可変ファラデー回転子１６でそれぞれ光の偏波面が２２．５度回転し、レンズ屈折作用によってミラー１８の同じ位置に入射する。復路では、反射光は光路が上下左右で入れ替わると共に可変ファラデー回転子１６でそれぞれ光の偏波面が更に２２．５度回転し（従って、往路と復路で合計４５度回転する）、レンズ１４を通って集光される。上段光路の光は、複屈折素子１２で常光は直進し、異常光は−ｙ方向（下向き）に屈折する。中段光路の光も、複屈折素子１２で常光は直進し、異常光は−ｙ方向（下向き）に屈折する。つまり、上段光路の異常光と中段光路の常光は合成され、残りの光は分離されたままとなる。従って、合成した中段左側光路の光だけが出力ファイバ２２に結合し、残りの光（上段光路の常光と下段光路の異常光）は出力ファイバ２２には結合しない。このようにして、入力ファイバ２０からの入力光は、ファラデー回転角に応じて出力ファイバ２２に結合することになり、結果的に出力ファイバへの出力が減衰することになる。
【００２９】
このようにして、可変ファラデー回転子１６のファラデー回転角を０度から４５度の範囲で自由に可変することで、制御したファラデー回転角に応じて出力ファイバ２２への光の結合割合が得られ、出力光量を制御できるのである。
【００３０】
このように反射型の光デバイスでは、光はミラーで反射されるために、該ミラーの背後には光は到達せず、ミラーの背後に非光学部品を配置しても光路の妨げとはならない。本発明は、このことを利用して、ミラーの背後にロッド状磁心の電磁石を設置することで、光デバイスの細径化を実現しているのである。その構造の例を図２に示す。
【００３１】
図２のＡは、図１の実施例で示したものと同様であり、別個の部材であるミラー１８をファラデー素子２４とロッド状磁心３０の間に設置した構造である。この構造は、最も単純であるが、ミラー１８を挿入するためにファラデー素子２４とロッド状磁心３０との間隔を広げる必要があり、磁気効率が若干悪くなる。図２のＢは、ファラデー素子２４の背面にミラー膜３４を形成した例である。ファラデー素子２４を両面研磨した後、一方の面にミラー膜３４を形成する加工を施す。この構成は、作業も比較的容易で最適である。図２のＣは、ロッド状磁心３０の一方の端面にミラー膜３６を形成した例である。ロッド状磁心３０の一方の端面を研磨した後、ミラー膜３６を形成する加工を施す。ロッド状磁心の材質によっては、このような構成も可能である。図２のＢ及びＣの例は、いずれもロッド状磁心３０の一端面とファラデー素子２４との間隔を最小にできるため、磁気効率的には極めて良好である。
【００３２】
図２からも分かるように、本発明では、ロッド状磁心３０にコイル３２を巻き付ける構成のため、光素子群の外側に電磁石ヨークが大きく張り出すことが無くなり、十分に小径化できる。なお、ロッド状磁心３０を用いることで、漏洩磁界が大きく広がり、他のデバイスへの影響が懸念されるが、磁気シールド材で取り囲むか、デバイスのケースを磁気シールド材で構成することで、そのような懸念は払拭できる。
【００３３】
図３は、本発明に係る反射型可変磁気光学デバイス（光スイッチ）の他の実施例を示す説明図であり、部材の配列構造と各光素子の位置での偏波状態を示している。偏波方向が直交関係にある同じ光路の光を分離し異なる光路の光を合成する偏波分離合成用複屈折素子４０と、異なる光路を通る偏波方向が直交関係にある光を平行関係にし平行関係にある光を直交関係にする偏波選択回転手段４２と、往路は常光のまま、復路では常光は異常光に異常光は常光に変換する常光−異常光変換手段４４と、偏波面を０度と４５度のいずれかに切り換える偏波面切換制御手段４６と、ミラー４８を、その順序で配列した磁気光学デバイスである。説明を分かり易くするため、光素子群の配列方向（入力光の進行方向）をｚ軸とし、該ｚ軸に直交する２方向をそれぞれｘ軸、ｙ軸とする座標軸を設定する。また便宜的に、ｘ方向を右方向（ｘ軸を横軸）、ｙ方向を上方向（ｙ軸を縦軸）とする。
【００３４】
偏波分離合成用複屈折素子４０は、ここではルチルやニオブ酸リチウムなどの複屈折結晶からなる単一の直方体であり、その光学軸はｘ−ｚ面内にあり且つｚ軸から傾いた方向を向くように設定されている。この複屈折素子４０は、偏波方向が直交関係にある同じ光路の光を異なる光路の光に分離し、異なる光路の光を同じ光路の光に合成する機能を果たすものである。従って、原理的には、このように単一の複屈折結晶で構成できる。
【００３５】
偏波選択回転手段４２は、偏波面を９０度回転する２枚の１／２波長板５０，５２からなり、偏波面の回転が必要な光路に１枚の１／２波長板が挿入され、偏波面の回転が不要な光路には２枚の１／２波長板ともに挿入されないか、あるいは２枚の１／２波長板が重なった状態で挿入されるような構造とする（図４のＡ参照）。両１／２波長板５０，５２は、光学軸はｘ−ｙ面内で且つｘ軸に対して４５度同じ方向に傾いて設定される。これによって、偏波選択回転手段４２は、異なる光路を通る偏波方向が直交関係にある光を平行関係にし平行関係にある光を直交関係にする機能を果たす。
【００３６】
常光−異常光変換手段４４は、２個の複屈折結晶５４，５６の間に、４５度固定ファラデー回転子５８と、偏波面を４５度回転する１／２波長板６０を挿入した構造とする。第１の複屈折結晶５４及び第２の複屈折結晶５６は、共に光学軸がｙ−ｚ面内で且つｙ軸に傾いている同一構造であり、光学軸がｙ軸に対称となるように配置されている。１／２波長板６０は、その光学軸がｘ−ｙ面内で且つｘ軸に対して２２．５度傾いた状態となっている。
【００３７】
偏波面切換手段４６は、２２．５度固定ファラデー回転子６２と±２２．５度可変ファラデー回転子６４からなり、可変ファラデー回転子６４がミラー４８寄りに位置するように組み合わせる。±２２．５度可変ファラデー回転子６４は、光路内に位置するファラデー素子６６と、該ファラデー素子６６に可変外部磁界を印加する電磁石６８の組み合わせからなる。電磁石６８は、ロッド状磁心７０にコイル７２を巻装した構造であってミラー４８の背後に位置し、ミラー４８を越えてファラデー素子６６に可変外部磁界を印加するすようになっている。
【００３８】
次に、この光スイッチの動作について説明する。偏波面の回転方向は、時計回りを正方向とする。入力→出力１とするには、電磁石６８のコイル７２に通電することで±２２．５度可変ファラデー回転子６４によって偏波面を−２２．５度回転するように外部磁界を印加し、入力→出力２とするには、電磁石６８への通電電流を切り換えて±２２．５度可変ファラデー回転子６４によって偏波面を＋２２．５度回転するように外部磁界を印加する。
【００３９】
（入力→出力１）
中段左側の入力ポートからの入力光は、偏波分離合成用複屈折素子４０で常光は直進し異常光はｘ方向（右方向）に屈折するため、左右の光路に分離する。偏波選択回転手段４２では、左側光路の光が１／２波長板５０を通過して偏波面が９０度回転し、右側光路の光は両１／２波長板５０，５２をバイパスするため、左右両光路の光の偏波面はｘ軸に平行になって互いに平行な関係となる。これらの光は、常光−異常光変換手段４４の第１の複屈折結晶５４に対しては常光となるためそのまま直進し、４５度固定ファラデー回転子５８で＋４５度回転し、１／２波長板６０で−４５度回転して元に戻り、第２の複屈折結晶５６に対しては常光となるためそのまま直進する。偏波面切換制御手段４６の２２．５度固定ファラデー回転子６２で偏波面が＋２２．５度回転し、±２２．５度可変ファラデー回転子６４で−２２．５度回転するため元に戻り、ミラー４８で反射する。
【００４０】
ミラー４８での反射光は、偏波面切換制御手段４６の±２２．５度可変ファラデー回転子６４で偏波面が−２２．５度回転し、２２．５度固定ファラデー回転子６２で＋２２．５度回転するため元に戻る。これらの光は、常光−異常光変換手段４４の第２の複屈折結晶５６に対しては常光となるためそのまま直進し、１／２波長板６０で＋４５度回転し、４５度固定ファラデー回転子５８で更に＋４５度回転するため、第１の複屈折結晶５４に対しては異常光となるためｙ方向に（上向きに）屈折し上段光路を進行する。そして、偏波選択回転手段４２では、右上光路の光は１／２波長板５２を通過することで偏波面が９０度回転し、左上の光路の光は、両１／２波長板５２，５０を通過するために偏波面は変化せず、左右両光路の光の偏波面は互いに直交の関係となる。これらの光は偏波分離合成用複屈折素子４０で常光は直進し異常光は−ｙ方向に屈折するため、上段左側光路に合成され、出力ポート１から出力する。
【００４１】
（入力→出力２）
中段左側の入力ポートからの入力光は、偏波分離合成用複屈折素子４０で常光は直進し異常光はｘ方向に屈折するため、左右の光路に分離する。偏波選択回転手段４２では、左側光路の光の偏波面が９０度回転し、左右両光路の光の偏波面はｘ軸に平行になって互いに平行な関係となる。これらの光は、常光−異常光変換手段４４の第１の複屈折結晶５４に対しては常光となるためそのまま直進し、４５度固定ファラデー回転子５８で＋４５度回転し、１／２波長板６０で−４５度回転して元に戻り、第２の複屈折結晶５６に対しては常光となるためそのまま直進する。偏波面切換制御手段４６の２２．５度固定ファラデー回転子６２で偏波面が＋２２．５度回転し、±２２．５度可変ファラデー回転子６４で更に＋２２．５度回転するため合計＋４５度回転してミラー４８に達し反射する。
【００４２】
ミラー４８での反射光は、偏波面切換制御手段４６の±２２．５度可変ファラデー回転子６４で偏波面が＋２２．５度回転し、２２．５度固定ファラデー回転子６２で更に＋２２．５度回転するため合計で＋４５度更に回転する。これらの光は、常光−異常光変換手段４４の第２の複屈折結晶５６に対しては異常光となるため−ｙ方向に（下向きに）屈折して下段光路を進行する。１／２波長板６０で＋４５度回転し、４５度固定ファラデー回転子５８で更に＋４５度回転するため、第１の複屈折結晶５４に対しては常光となるためそのまま下段光路を進行する。そして偏波選択回転手段４２では、左下光路の光の偏波面が９０度回転し、左右両光路の光の偏波面は互いに直交の関係となる。これらの光は偏波分離合成用複屈折素子４０で常光は直進し異常光は−ｙ方向（下向き）に屈折するため、下段左側光路に合成され、出力ポート２から出力する。
【００４３】
この実施例でも、可変ファラデー回転子６４は、ミラー４８の直前にファラデー素子６６を配置できるように全体の光素子の構成を工夫し、ミラー４８の背後にロッド状磁心７０にコイル７２を巻装した構造の電磁石６８を配置している。この光スイッチにおいても、図２に示すような各種のミラー手段（例えばミラー膜構造）をそのまま用いることができる。
【００４４】
偏波選択回転手段は、図４のＡに示すように、偏波面を９０度回転する２枚の１／２波長板５０，５２からなり、それらは偏波面の回転が必要な光路に１枚の１／２波長板が挿入され、偏波面の回転が不要な光路には２枚の１／２波長板ともに挿入されないか、あるいは２枚の１／２波長板が重なった状態で挿入されるような構造としている。それに代えて、図４のＢに示すように、偏波面を９０度回転する２枚の１／２波長板８０，８２からなり、それらは偏波面の回転が必要な光路に１枚の１／２波長板が挿入され、偏波面の回転が不要な光路には１／２波長板が挿入されない構造としてもよい。
【００４５】
偏波分離合成用複屈折素子は、前記のように原理的には偏波分離合成用の単一の複屈折結晶４０で構成できる。しかし、図５のＡに示すように、偏波面を９０度回転する直線位相子（光学軸がｘ軸に対して４５度傾いている１／２波長板８４）を挟んで光学軸が対称となるように２個の複屈折結晶８６，８８を組み合わせた（ｙ−ｚ面内でｚ軸に対して傾いている同一の２個の複屈折結晶を対向配置した）構成、あるいは図５のＢに示すように、偏波分離合成用の複屈折結晶４０と光路長補正用の複屈折結晶９０（光学軸がｙ軸に平行）を組み合わせた構成などでもよい。このような構成とすると、部品点数は増えるが、分離した光の光路長を揃えて偏波モード分散を補償することができる。
【００４６】
【発明の効果】
本発明は上記のように、可変ファラデー回転子のファラデー素子をミラー手段の直前に配置し、その電磁石はロッド状磁心にコイルを巻装した構造として前記ミラー手段の直後に配置した反射型可変磁気光学デバイスであるから、光軸方向の寸法を短縮できることは無論のこと、反射型の特性を活かしてファラデー素子に外部可変磁界を印加する電磁石を光路を妨げずに光軸に沿って配置でき、大幅な小型化、特に小径化が可能となる。
【図面の簡単な説明】
【図１】本発明に係る反射型可変磁気光学デバイス（可変光アッテネータ）の一実施例を示す説明図。
【図２】ミラー手段の例を示す説明図。
【図３】本発明に係る反射型可変磁気光学デバイス（光スイッチ）の他の実施例を示す説明図。
【図４】その偏波選択回転手段の例を示す説明図。
【図５】その偏波分離合成用複屈折素子の例を示す説明図。
【符号の説明】
１０２芯ファイバフェルール
１２複屈折素子
１４レンズ
１６可変ファラデー回転子
１８ミラー
２０入力ファイバ
２２出力ファイバ
２４ファラデー素子
２６ａ，２６ｂ永久磁石
２８電磁石
３０ロッド状磁心
３２コイル[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reflection-type magneto-optical device that controls a light amount or an optical path using a variable Faraday rotator. More specifically, in the present invention, the Faraday element of the variable Faraday rotator is located in the optical path immediately before the mirror means, and an electromagnet in which a coil is wound around a rod-shaped magnetic core has one end face located immediately after the mirror means. The present invention relates to a reflection-type variable magneto-optical device in which a variable magnetic field is applied to a Faraday element beyond an mirror by an electromagnet.
[0002]
[Prior art]
[Patent Document 1]
JP-A-10-161076
[0003]
In an optical communication system or an optical measurement system, various active optical devices such as an optical attenuator for controlling the amount of transmitted light or an optical switch for switching an optical path and controlling light on / off are required. An optical attenuator is, for example, an optical device that installs a polarizer and an analyzer on an input side and an output side of a Faraday rotation angle varying device, and controls a light amount transmitted through the analyzer by changing a magnetic field applied to the Faraday element. is there. The optical switch is an optical device having an optical path switching function of outputting input light from an input port to an arbitrary selected one of different output ports, and is a 1 × 2 type (1 input / 2 output). ) Is the most basic form.
[0004]
It is important for these optical devices for optical communication to be polarization-independent, have high reliability, and have good matching with an optical fiber. Such a requirement can be satisfied, for example, by an optical element group in which various optical elements such as a birefringent element that controls the optical path according to the polarization plane and a variable Faraday rotator that controls the rotation angle of the polarization plane are arranged. There is a configuration in which the optical function unit is used.
[0005]
In the past, a transmission type in which light is input from one end of an optical element group, travels through the optical element group, and is output from the other end has been common. However, in recent years, from the viewpoint of miniaturization of the optical device and the like, the arrayed optical element group and the mirror unit are combined, and light input from one end travels through the optical element group and reaches the mirror on the other end. Reflection type optical devices have been developed in which light reflected by a mirror travels backward through the optical element group and is output from one end, and while the light reciprocates through the optical element group, controls the amount of light or switches the optical path. For example, in Patent Document 1, a two-core ferrule having an input fiber and an output fiber, a lens, a wedge-shaped birefringent plate, a magneto-optical crystal (Faraday element), and a reflector are arranged, and a magnetic field is applied to the Faraday element. A configuration for changing the Faraday rotation angle by applying the voltage is disclosed.
[0006]
In these optical devices, the control of the light amount or the switching of the optical path is performed by controlling the current supplied to the electromagnet of the variable Faraday rotator. The electromagnet used for this purpose usually has a structure in which a coil is wound around a C-type yoke, and the Faraday element inserted in the optical path is arranged so as to be sandwiched between the gaps. In such a manner that a variable magnetic field can be applied in a necessary direction.
[0007]
[Problems to be solved by the invention]
Such a reflection type optical device has the advantage that the required function can be provided by passing the reflected light through the optical element group, so that the required number of components can be reduced and the length in the optical axis direction can be greatly reduced. Having. However, the C-type yoke around which the coil is wound is suitable for mounting from the side of the arranged optical element group in order not to hinder the optical path. Therefore, the size in the direction orthogonal to the optical axis is almost the same for the transmission type and the reflection type, and cannot be reduced. Therefore, the overall size (small diameter) is insufficient.
[0008]
An object of the present invention is to provide a reflective variable magneto-optical device that can reduce not only the dimension in the optical axis direction but also the dimension in the direction perpendicular to the optical axis, and can greatly reduce the size as a whole.
[0009]
[Means for Solving the Problems]
The present invention comprises an optical element group having at least a birefringent element and a variable Faraday rotator and a mirror means, and light input from one end travels through the optical element group to reach the mirror means on the other end side, and the mirror means In the magneto-optical device in which the amount of light or the optical path is controlled while the light reciprocatingly travels through the optical element group and is output from one end side while the light reciprocates through the optical element group, the light reflected by the means is located in the optical path immediately before the mirror means. An electromagnet in which the Faraday element of the variable Faraday rotator is arranged so as to face the Faraday element via the mirror means, and a coil is wound around a rod-shaped core, one end face of which is located immediately after the mirror means. And a reflective variable magneto-optical device that applies a variable magnetic field to the Faraday element beyond the mirror means by the electromagnet.
[0010]
The feature of the present invention lies in that the Faraday element of the variable Faraday rotator is located immediately before the mirror means and the rod-shaped core of the electromagnet is arranged immediately after the mirror means. In a reflection type optical device, light does not reach behind the mirror means because the light is reflected by the mirror means. In other words, disposing the non-optical component behind the mirror means does not hinder the optical path. Focusing on this point, the present invention installs an electromagnet for the variable Faraday rotator behind the mirror means, devises its structure, and adopts a rod-shaped core instead of the conventional C-type yoke. This is because the diameter of the optical device has been reduced.
[0011]
As a reflective variable magneto-optical device, the optical element group includes a two-core fiber ferrule, a birefringent element, and a lens arranged in that order, and the distance between the fiber end face of the two-core fiber ferrule and the lens is substantially equal to the focal length of the lens. There is a structure to match. The variable Faraday rotator functions as an optical attenuator for controlling the amount of output light by varying the Faraday rotation angle in a range of approximately 0 to 45 degrees. Here, the variable Faraday rotator includes a pair of permanent magnets and the electromagnet provided outside the Faraday element, applies a saturation magnetic field by the permanent magnet in a direction parallel to the plane of the Faraday element, and applies a magnetic field to the surface of the Faraday element by the electromagnet. It is preferable to apply a variable magnetic field in a vertical direction.
[0012]
As a reflection-type variable magneto-optical device, a polarization splitting / combining birefringent element that separates light beams in the same optical path having orthogonal polarization directions and combines light beams in different optical paths, and a polarization direction passing through different optical paths is orthogonal. Polarization selective rotation means to make the related light parallel and make the parallel light orthogonal, and ordinary light to extraordinary light conversion to convert ordinary light to extraordinary light in the return path and ordinary light to extraordinary light in the return path Means, and a polarization plane switching control means for switching the polarization plane between 0 degree and 45 degrees, wherein the polarization plane switching means comprises a 22.5 degree fixed Faraday rotator and a ± 22.5 degree variable Faraday rotator. There is a structure using. This reflective variable magneto-optical device functions as an optical switch that controls the optical path by switching the polarization plane by a variable Faraday rotator.
[0013]
The mirror means may be a single mirror, or may be a mirror film formed on the back surface of the Faraday element of the variable Faraday rotator, or a mirror film formed by mirror-finishing one end face of a rod-shaped magnetic core of an electromagnet. The use of the mirror film has the advantage of improving the magnetic efficiency because the distance between the Faraday element and the rod-shaped magnetic core can be minimized.
[0014]
As the birefringent element, for example, rutile single crystal or lithium niobate is used. As the Faraday element, for example, a magnetic garnet single crystal, particularly a bismuth-substituted rare earth iron garnet single crystal film grown by the LPE (liquid phase epitaxial) method is suitable.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
In the case of a reflection type optical switch, as described above, a polarization splitting / combining birefringent element that separates lights in the same optical path having orthogonal polarization directions and combines lights in different optical paths, and a polarization passing through different optical paths. Polarization selective rotation means for converting light whose wave direction is orthogonal to parallel and making light parallel to the orthogonal relationship orthogonal, and ordinary light on the forward path while converting ordinary light to extraordinary light on the return path. An extraordinary light conversion means, and a polarization plane switching control means for switching the plane of polarization between 0 degree and 45 degrees.
[0016]
Here, the birefringent element for polarization separation / combination may be a single birefringent crystal for polarization separation / combination, but the birefringent element may be symmetrical with respect to the optical axis with a linear phase shifter rotating the polarization plane by 90 degrees. A configuration in which two birefringent crystals are combined, or a configuration in which a birefringent crystal for polarization separation / synthesis and a birefringent crystal for optical path length correction are combined may be used.
[0017]
The polarization selecting / rotating means is composed of two linear phase shifters for rotating the plane of polarization by 90 degrees, and one linear phase shifter is inserted in an optical path requiring rotation of the plane of polarization, so that rotation of the plane of polarization is not required. The shape is such that no linear phase shifter is inserted in the optical path, or two linear phase shifters are inserted in an overlapping state.
[0018]
The ordinary light-extraordinary light conversion means has a structure in which a 45-degree fixed Faraday rotator and a linear phase shifter that rotates the polarization plane by 45 degrees are inserted between two birefringent crystals.
[0019]
The polarization plane switching means includes a 22.5-degree fixed Faraday rotator and a ± 22.5-degree variable Faraday rotator, and is combined so that the variable Faraday rotator is located closer to the mirror means.
[0020]
【Example】
FIG. 1 is an explanatory view showing an embodiment of a reflection type variable magneto-optical device (variable optical attenuator) according to the present invention, showing an arrangement structure of members and a polarization state at the position of each optical element. This is a magneto-optical device in which a two-core fiber ferrule 10, a birefringent element 12, a lens 14, a variable Faraday rotator 16, and a mirror 18 are arranged in that order. In order to make the description easy to understand, a coordinate axis is set with the arrangement direction (optical axis) of the optical element group as the z-axis and two directions orthogonal to the z-axis as the x-axis and the y-axis, respectively. For convenience, the x direction is defined as a right direction (x axis is a horizontal axis), and the y direction is defined as an upward direction (a y axis is a vertical axis).
[0021]
An input fiber 20 and an output fiber 22 are connected to one end of the two-core fiber ferrule 10. The input fiber 20 is provided on the middle right side, and the output fiber 22 is provided on the middle left side. In other words, the input fiber 20 and the output fiber 22 have the same position in the y direction, but are shifted in the x direction.
[0022]
The birefringent element 12 is a rectangular parallelepiped made of a birefringent crystal such as rutile or lithium niobate, and its optical axis is set in the yz plane and in a direction inclined from the z axis. The birefringent element 12 has a function of separating light of the same optical path whose polarization direction is orthogonal to light of a vertically different optical path, and combining light of a vertically different optical path with light of the same optical path. .
[0023]
The lens 14 is a plano-convex lens made of, for example, a homogeneous transparent material, and is set so that the distance between the fiber end face of the two-core fiber ferrule 10 and the lens 14 substantially matches the focal length of the lens 14. Therefore, the light emitted from the fiber end face becomes parallel light by the lens, and the parallel light is condensed on the fiber end face.
[0024]
The variable Faraday rotator 16 includes a Faraday element (magneto-optical crystal: for example, Bi-substituted rare earth iron garnet LPE film) 24 inserted in the optical path immediately before the mirror 18 and magnetic field applying means for applying a variable external magnetic field thereto. Have. The magnetic field applying means includes a pair of permanent magnets 26a and 26b arranged so as to face each other outside the Faraday element 24 in the y-axis direction, and an electromagnet 28 arranged immediately after the mirror 18. The electromagnet 28 has a structure in which a coil 32 is wound around a rod-shaped magnetic core 30, and is arranged close to one end face of the rod-shaped magnetic core 30 so as to face the mirror 18. The permanent magnets 26a and 26b apply a saturation fixed magnetic field to the Faraday element 18 in the in-plane direction (y-axis direction), and the electromagnet 28 passes over the mirror 18 and extends in a direction perpendicular to the plane with respect to the Faraday element 24 (z axis). Direction). In this way, the Faraday rotation angle of the incident light can be freely varied in the range of 0 to 45 degrees by the combined magnetic field of the fixed magnetic field by the permanent magnets 26a and 26b and the variable magnetic field by the electromagnet 28.
[0025]
Next, the operation of the variable optical attenuator will be briefly described. The light input from the input fiber 20 to the middle right optical path is birefringent element 12 where ordinary light goes straight through the middle right optical path, and extraordinary light is refracted in the + y direction (upward) and travels along the upper right optical path. In this way, the light is separated, travels as parallel light by the lens 14, passes through the Faraday element 24 of the variable Faraday rotator 16, and reaches the mirror 18.
[0026]
(During 45 degree Faraday rotation: attenuation rate ≒ 0%)
On the outward path, the polarization of the separated light is rotated by 45 degrees by the variable Faraday rotator 16 and is incident on the same position of the mirror 18 by the lens refraction. In the return path, the reflected light is switched up and down and left and right, and the polarization plane of the light is further rotated by 45 degrees by the variable Faraday rotator 16 (accordingly, a total of 90 degrees is rotated in the forward path and the return path). The birefringent element 12 emits light, the ordinary light goes straight on, and the extraordinary light is refracted in the −y direction (downward), whereby light is synthesized. All the combined lights pass through the middle-stage left optical path and are coupled to the output fiber 22. Therefore, the input light from the input fiber 20 is output to the output fiber without being attenuated.
[0027]
(At 0 degree Faraday rotation: attenuation rate ≒ 100%)
On the outward path, the polarization of the separated light does not rotate in the variable Faraday rotator 16 but enters the mirror 18 at the same position by the lens refraction. On the return path, the reflected light changes its optical path up, down, left, and right, and the polarization plane does not rotate in the variable Faraday rotator 16, passes through the lens 14, and the ordinary light in the upper optical path goes straight through the birefringent element 12, and the extraordinary light in the middle optical path Since the light is refracted in the −y direction (downward), the light is further separated so as to pass through the upper left optical path and the lower left optical path, and is not coupled to the output fiber 22 at all.
[0028]
(During 22.5 degree Faraday rotation: attenuation rate ≒ 50%)
If the Faraday rotation angle is between 45 degrees and 0 degrees, then: A typical example of an intermediate case of 22.5 degrees will be described. On the outward path, the separated light is rotated by the variable Faraday rotator 16 with its polarization plane rotated by 22.5 degrees, and is incident on the same position of the mirror 18 by the lens refraction. On the return path, the reflected light is switched up and down and right and left, and the polarization plane of the light is further rotated by 22.5 degrees by the variable Faraday rotator 16 (accordingly, a total of 45 degrees is rotated on the outward path and the return path), and passes through the lens 14. Is collected. As for light in the upper optical path, ordinary light goes straight through the birefringent element 12, and extraordinary light is refracted in the -y direction (downward). The ordinary light also travels straight through the birefringent element 12, and the extraordinary light is refracted in the −y direction (downward) in the middle optical path. That is, the extraordinary light in the upper optical path and the ordinary light in the middle optical path are combined, and the remaining light remains separated. Therefore, only the combined light in the middle left optical path is coupled to the output fiber 22, and the remaining light (the ordinary light in the upper optical path and the extraordinary light in the lower optical path) is not coupled to the output fiber 22. In this way, the input light from the input fiber 20 is coupled to the output fiber 22 according to the Faraday rotation angle, and as a result, the output to the output fiber is attenuated.
[0029]
In this way, by freely changing the Faraday rotation angle of the variable Faraday rotator 16 within the range of 0 to 45 degrees, the coupling ratio of light to the output fiber 22 can be obtained according to the controlled Faraday rotation angle. , The amount of output light can be controlled.
[0030]
As described above, in the reflection type optical device, since light is reflected by the mirror, the light does not reach behind the mirror, and even if a non-optical component is arranged behind the mirror, it does not hinder the optical path. . According to the present invention, by utilizing this, an electromagnet having a rod-shaped magnetic core is installed behind the mirror, thereby realizing a smaller optical device. FIG. 2 shows an example of the structure.
[0031]
FIG. 2A is similar to that shown in the embodiment of FIG. 1, and has a structure in which the mirror 18 as a separate member is installed between the Faraday element 24 and the rod-shaped magnetic core 30. Although this structure is the simplest, it is necessary to increase the distance between the Faraday element 24 and the rod-shaped core 30 in order to insert the mirror 18, and the magnetic efficiency is slightly deteriorated. FIG. 2B is an example in which a mirror film 34 is formed on the back surface of the Faraday element 24. After the two sides of the Faraday element 24 are polished, a process of forming a mirror film 34 on one surface is performed. This configuration is relatively easy to work and optimal. FIG. 2C shows an example in which a mirror film 36 is formed on one end surface of the rod-shaped magnetic core 30. After polishing one end surface of the rod-shaped magnetic core 30, a process of forming the mirror film 36 is performed. Such a configuration is also possible depending on the material of the rod-shaped magnetic core. In each of the examples B and C in FIG. 2, since the distance between the one end surface of the rod-shaped magnetic core 30 and the Faraday element 24 can be minimized, the magnetic efficiency is extremely good.
[0032]
As can be seen from FIG. 2, in the present invention, since the coil 32 is wound around the rod-shaped magnetic core 30, the electromagnet yoke does not protrude greatly outside the optical element group, and the diameter can be sufficiently reduced. The use of the rod-shaped core 30 greatly expands the leakage magnetic field, which may affect other devices. However, by enclosing the device with a magnetic shield material or forming a device case with a magnetic shield material, the Such concerns can be dispelled.
[0033]
FIG. 3 is an explanatory view showing another embodiment of the reflection type variable magneto-optical device (optical switch) according to the present invention, showing the arrangement of members and the polarization state at the position of each optical element. The polarization splitting / combining birefringent element 40, which separates the light beams in the same optical path having orthogonal polarization directions and combines the light beams in different optical paths, and makes the light beams whose polarization directions passing through different optical paths are orthogonal to each other have a parallel relationship. A polarization selecting / rotating means 42 for converting the light in a parallel relation into an orthogonal relation; an ordinary light-extraordinary light converting means 44 for converting the ordinary light to the extraordinary light on the return path and the extraordinary light to the extraordinary light on the return path; This is a magneto-optical device in which a polarization plane switching control means 46 for switching between 0 degree and 45 degrees and a mirror 48 are arranged in that order. In order to make the description easy to understand, a coordinate axis is set with the arrangement direction of the optical element group (the traveling direction of the input light) as the z-axis and two directions orthogonal to the z-axis as the x-axis and the y-axis, respectively. For convenience, the x direction is defined as a right direction (x axis is a horizontal axis), and the y direction is defined as an upward direction (a y axis is a vertical axis).
[0034]
Here, the birefringent element 40 for polarization separation / combination is a single rectangular parallelepiped made of a birefringent crystal such as rutile or lithium niobate, and its optical axis is in the xz plane and in a direction inclined from the z axis. Is set to face. The birefringent element 40 has a function of separating light of the same optical path whose polarization directions are orthogonal to light of different optical paths, and combining light of different optical paths with light of the same optical path. Therefore, in principle, it can be composed of a single birefringent crystal as described above.
[0035]
The polarization selecting / rotating means 42 includes two half-wave plates 50 and 52 for rotating the plane of polarization by 90 degrees, and one half-wave plate is inserted into an optical path that requires rotation of the plane of polarization. In the optical path that does not require the rotation of the polarization plane, the two half-wave plates are not inserted together, or the two half-wave plates are inserted in an overlapping state (A in FIG. 4). reference). The optical axes of both the half-wave plates 50 and 52 are set in the xy plane and inclined in the same direction by 45 degrees with respect to the x-axis. As a result, the polarization selecting / rotating means 42 has a function of converting light beams having polarization directions passing through different optical paths that are orthogonal to each other into a parallel relationship, and changing light beams having parallel polarization directions to an orthogonal relationship.
[0036]
The ordinary light-extraordinary light conversion means 44 has a structure in which a 45-degree fixed Faraday rotator 58 and a half-wave plate 60 that rotates the polarization plane by 45 degrees are inserted between two birefringent crystals 54 and 56. . Each of the first birefringent crystal 54 and the second birefringent crystal 56 has the same structure in which the optical axis is in the yz plane and is inclined to the y axis, and the optical axis is symmetric with respect to the y axis. Are located. The half-wave plate 60 is in a state where its optical axis is inclined in the xy plane and 22.5 degrees with respect to the x-axis.
[0037]
The polarization plane switching means 46 includes a 22.5-degree fixed Faraday rotator 62 and a ± 22.5-degree variable Faraday rotator 64, and is combined so that the variable Faraday rotator 64 is located closer to the mirror 48. The ± 22.5-degree variable Faraday rotator 64 includes a combination of a Faraday element 66 located in the optical path and an electromagnet 68 for applying a variable external magnetic field to the Faraday element 66. The electromagnet 68 has a structure in which a coil 72 is wound around a rod-shaped magnetic core 70, is located behind the mirror 48, and applies a variable external magnetic field to the Faraday element 66 beyond the mirror 48.
[0038]
Next, the operation of the optical switch will be described. The clockwise rotation direction of the polarization plane is the positive direction. To change from input to output 1, an external magnetic field is applied so that the polarization plane is rotated by −22.5 degrees by the ± 22.5-degree variable Faraday rotator 64 by energizing the coil 72 of the electromagnet 68, and input → In order to make the output 2, the current supplied to the electromagnet 68 is switched, and an external magnetic field is applied by the ± 22.5 degrees variable Faraday rotator 64 so that the polarization plane is rotated +22.5 degrees.
[0039]
(Input → output 1)
The input light from the input port on the left side of the middle stage is split into the left and right optical paths because the ordinary light goes straight and the extraordinary light is refracted in the x direction (right direction) by the polarization splitting / combining birefringent element 40. In the polarization selecting / rotating means 42, the light in the left optical path passes through the half-wave plate 50 and the plane of polarization is rotated by 90 degrees, and the light in the right optical path bypasses both the half-wave plates 50 and 52. The planes of polarization of the light on both the left and right optical paths are parallel to the x-axis and have a parallel relationship with each other. Since these lights become ordinary light with respect to the first birefringent crystal 54 of the ordinary light-extraordinary light conversion means 44, they travel straight as they are, are rotated +45 degrees by the 45-degree fixed Faraday rotator 58, and have a half wavelength plate. It rotates -45 degrees at 60 and returns to its original position, and it goes straight to the second birefringent crystal 56 because it becomes ordinary light. The polarization plane is rotated by +22.5 degrees by the 22.5-degree fixed Faraday rotator 62 of the polarization plane switching control means 46, and returned by -22.5 degrees by the ± 22.5-degree variable Faraday rotator 64, The light is reflected by the mirror 48.
[0040]
The reflected light from the mirror 48 has its polarization plane rotated by −22.5 degrees by the ± 22.5 degrees variable Faraday rotator 64 of the polarization plane switching control means 46 and +22.5 degrees by the 22.5 degree fixed Faraday rotator 62. Return to original position to rotate. Since these lights become ordinary light with respect to the second birefringent crystal 56 of the ordinary light-extraordinary light conversion means 44, they travel straight as they are, are rotated by +45 degrees by the half-wave plate 60, and are fixed at 45 degrees by the Faraday rotator. Since it is further rotated by +45 degrees at 58, it becomes extraordinary light with respect to the first birefringent crystal 54, so that it refracts in the y direction (upward) and travels along the upper optical path. Then, in the polarization selecting / rotating means 42, the light on the upper right optical path is rotated by 90 degrees by passing through the half-wave plate 52, and the light on the upper left optical path is rotated by the two half-wave plates 52 and 50. , The plane of polarization does not change, and the planes of polarization of the light on the left and right optical paths are orthogonal to each other. The ordinary light travels straight through the polarization splitting / combining birefringent element 40 and the extraordinary light is refracted in the −y direction, so that these lights are combined in the upper left optical path and output from the output port 1.
[0041]
(Input → Output 2)
The input light from the input port on the left side of the middle stage is split into the left and right optical paths because the ordinary light goes straight and the extraordinary light is refracted in the x direction by the polarization splitting / combining birefringent element 40. In the polarization selecting / rotating means 42, the polarization plane of the light in the left optical path is rotated by 90 degrees, and the polarization planes of the light in the left and right optical paths are parallel to the x-axis and have a parallel relationship with each other. Since these lights become ordinary light with respect to the first birefringent crystal 54 of the ordinary light-extraordinary light conversion means 44, they travel straight as they are, are rotated +45 degrees by the 45-degree fixed Faraday rotator 58, and have a half wavelength plate. The light is rotated by −45 degrees at 60 and returns to the original state. The 22.5-degree fixed Faraday rotator 62 of the polarization-plane switching control means 46 rotates the polarization plane by +22.5 degrees, and the ± 22.5-degree variable Faraday rotator 64 further rotates by +22.5 degrees. Then, the light reaches the mirror 48 and is reflected.
[0042]
The reflected light from the mirror 48 has its polarization plane rotated by +22.5 degrees by the ± 22.5 degree variable Faraday rotator 64 of the polarization plane switching control means 46, and further +22.5 degrees by the 22.5 degree fixed Faraday rotator 62. It rotates by +45 degrees in total because it rotates by 45 degrees. Since these lights become extraordinary light with respect to the second birefringent crystal 56 of the ordinary light-extraordinary light conversion means 44, they refract in the -y direction (downward) and travel in the lower optical path. Since the light is rotated by +45 degrees by the half-wave plate 60 and further rotated by +45 degrees by the 45-degree fixed Faraday rotator 58, the first birefringent crystal 54 becomes ordinary light and travels along the lower optical path as it is. Then, in the polarization selecting / rotating means 42, the polarization plane of the light in the lower left optical path is rotated by 90 degrees, and the polarization planes of the light in the left and right optical paths are orthogonal to each other. The ordinary light goes straight through the polarization splitting / combining birefringent element 40 and the extraordinary light is refracted in the −y direction (downward), so that these lights are combined in the lower left optical path and output from the output port 2.
[0043]
In this embodiment as well, the variable Faraday rotator 64 is devised so that the Faraday element 66 can be disposed immediately before the mirror 48, and the coil 72 is wound around the rod-shaped core 70 behind the mirror 48. The electromagnet 68 having the above structure is arranged. Also in this optical switch, various mirror means (for example, a mirror film structure) as shown in FIG. 2 can be used as it is.
[0044]
The polarization selective rotation means, as shown in FIG. 4A, comprises two half-wave plates 50 and 52 for rotating the polarization plane by 90 degrees, and one of them is provided in an optical path that requires rotation of the polarization plane. The half-wave plate is inserted, and neither of the two half-wave plates is inserted into the optical path that does not require the rotation of the polarization plane, or the two half-wave plates are inserted in an overlapping state. It has such a structure. Instead, as shown in FIG. 4B, it is composed of two half-wave plates 80 and 82 for rotating the plane of polarization by 90 degrees, and these are placed on the optical path that requires rotation of the plane of polarization. A structure may be employed in which a two-wavelength plate is inserted, and a half-wavelength plate is not inserted in an optical path that does not require rotation of the polarization plane.
[0045]
As described above, the polarization splitting / combining birefringent element can be composed of a single birefringent crystal 40 for polarization splitting / combining in principle. However, as shown in FIG. 5A, the optical axis is symmetric with respect to a linear phase shifter (a half-wave plate 84 whose optical axis is inclined at 45 degrees with respect to the x-axis) that rotates the polarization plane by 90 degrees. A configuration in which two birefringent crystals 86 and 88 are combined so that the same two birefringent crystals tilted with respect to the z-axis in the yz plane are arranged facing each other, or B in FIG. As shown in (2), a birefringent crystal 40 for polarization separation / synthesis and a birefringent crystal 90 for optical path length correction (the optical axis is parallel to the y-axis) may be used. With this configuration, although the number of components increases, the polarization mode dispersion can be compensated by aligning the optical path lengths of the separated lights.
[0046]
【The invention's effect】
As described above, according to the present invention, the Faraday element of the variable Faraday rotator is disposed immediately before the mirror means, and the electromagnet is a structure in which a coil is wound around a rod-shaped magnetic core, and the reflection type variable magnet is disposed immediately after the mirror means. Of course, since it is an optical device, it is possible to shorten the dimension in the optical axis direction, and it is possible to arrange an electromagnet that applies an external variable magnetic field to the Faraday element by utilizing the characteristics of the reflection type along the optical axis without obstructing the optical path, Substantial miniaturization, especially miniaturization, becomes possible.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing one embodiment of a reflective variable magneto-optical device (variable optical attenuator) according to the present invention.
FIG. 2 is an explanatory diagram showing an example of a mirror unit.
FIG. 3 is an explanatory view showing another embodiment of the reflective variable magneto-optical device (optical switch) according to the present invention.
FIG. 4 is an explanatory diagram showing an example of the polarization selection rotation means.
FIG. 5 is an explanatory view showing an example of the polarization splitting / combining birefringent element.
[Explanation of symbols]
10 Two-core fiber ferrule
12 Birefringent element
14 lenses
16 Variable Faraday rotator
18 mirror
20 Input fiber
22 Output fiber
24 Faraday element
26a, 26b permanent magnet
28 electromagnet
30 rod-shaped magnetic core
32 coils

Claims

An optical element group having at least a birefringent element and a variable Faraday rotator; and mirror means. Light input from one end travels through the optical element group to reach the mirror means on the other end side and is reflected by the mirror means. In the magneto-optical device in which the amount of light or the optical path is controlled while the light that has traveled backward through the optical element group and is output from one end side and the light reciprocates through the optical element group,
The Faraday element of the variable Faraday rotator is located in the optical path immediately before the mirror means. A reflection-type variable magneto-optical device, which is arranged so as to face an element, and applies a variable magnetic field to the Faraday element beyond the mirror means by the electromagnet.

The optical element group has a structure in which a two-core fiber ferrule, a birefringent element, and a lens are arranged in that order, and the distance between the fiber end face of the two-core fiber ferrule and the lens substantially matches the focal length of the lens. 2. The reflection-type variable magneto-optical device according to claim 1, wherein the reflection type variable magneto-optical device has an optical attenuator function of controlling an output light amount by varying a Faraday rotation angle in a range of approximately 0 to 45 degrees by a child.

The variable Faraday rotator includes a pair of permanent magnets and the electromagnet provided outside the Faraday element, applies a saturation magnetic field in a direction parallel to the plane of the Faraday element by the permanent magnet, and is perpendicular to the plane of the Faraday element by the electromagnet. 3. The reflective variable magneto-optical device according to claim 2, wherein the variable magnetic field is applied in various directions.

A polarization splitting / combining birefringent element that separates light beams in the same optical path with orthogonal polarization directions and combines light beams in different optical paths, and parallel lights that have orthogonal polarization directions passing through different optical paths. Polarization selective rotation means for making the related light orthogonal, an ordinary light on the forward path, an ordinary light to an extraordinary light on the return path, an ordinary light-extraordinary light conversion means for converting the extraordinary light to ordinary light, and a polarization plane of 0 degree. A polarization plane switching control means for switching to any one of 45 degrees, wherein a 22.5 degree fixed Faraday rotator and a ± 22.5 degree variable Faraday rotator are used for the polarization plane switching means, 2. The reflection-type variable magneto-optical device according to claim 1, wherein the reflection type variable magneto-optical device has an optical switch function of controlling an optical path by switching a polarization plane by a rotator.

5. The reflection type variable magneto-optical device according to claim 1, wherein the mirror means comprises a mirror film formed on a back surface of the Faraday element of the variable Faraday rotator.

5. The reflective variable magneto-optical device according to claim 1, wherein the mirror means comprises a mirror film formed by mirror-finishing one end face of a rod-shaped magnetic core of the electromagnet.