JP3789334B2

JP3789334B2 - Waveguide type optical isolator and manufacturing method thereof

Info

Publication number: JP3789334B2
Application number: JP2001254439A
Authority: JP
Inventors: 義徳太田; 行夫小椋
Original assignee: レーザーフロントテクノロジーズ株式会社
Priority date: 2001-08-24
Filing date: 2001-08-24
Publication date: 2006-06-21
Anticipated expiration: 2021-08-24
Also published as: JP2003066251A

Description

【０００１】
【発明の属する技術分野】
本発明は、磁気光学材料のような特殊な材料を使用することなく、光ファイバを含む光導波路のみによって構成可能な導波路型光アイソレータに関する。
【０００２】
【従来の技術】
近年におけるインターネットの普及やマルチメディア通信等、社会の通信需要の旺盛な拡大に伴って、光ファイバ増幅器を使った伝送距離一万キロ、伝送容量１０テラビット／毎秒を超えなんとする、超長距離でかつ大容量の高密度波長多重（ＤＷＤＭ）光基幹通信技術の開発や、光ファイバ伝送技術を加入者端まで適用して、広帯域（ブロードバンド）のデジタル情報通信サービスを一般家庭やオフィスビルにまで拡大する光アクセスシステムの検討など、これまで以上に光通信システムの大容量化へ向けた技術の開発が勢力的に行われている。
このようなシステムを実現する上で必須の光部品が、光アイソレータ等の光透過特性に強い方向性のある光素子である。光アイソレータは、例えば、送信光源や光ファイバ増幅器から出射される信号光を光ファイバを通して伝送する場合に、光ファイバ接続点等からの反射戻り光が光源である半導体レーザや光ファイバ増幅器に再入射することによって起こる信号光のＳ／Ｎの低下や不安定動作を防止する部品として用いられる。
近年、光伝送方式は、単一モードファイバ（ＳＭＦ）を使った時間分割多重（ＴＤＭ）伝送方式からＤＷＤＭのように多数の波長の信号光を光直接増幅中継して一括して伝送する波長多重伝送方式へ開発の主体が移ってきている。この波長多重伝送方式では、光アイソレータには波長幅が数ｎｍから十数ｎｍの広い波長幅に亘って高いアイソレーション特性を示すことが要求されている。
さらに、近年多モードプラスチック光ファイバ（ＰＯＦ）が石英単一モードファイバに劣らず広帯域光伝送特性を有し、かつサブｍｍオーダのコア径であるがために、光軸合わせのアライメント精度が緩くコネクタによる接続が容易であることから、安価であることが要求される光アクセスシステム用の用途への利用が注目されている。このような多モードＰＯＦに用いる光アイソレータは、波数が広く分布している多モード光に対しても高いアイソレーション特性で動作することが求められる。
【０００３】
光アイソレータは、順方向透過特性と逆方向透過特性とが非可逆な動作をする素子であって、従来から知られている素子の構成例としては、非相反な偏光回転効果であるファラデー効果を示す磁気光学結晶を４５度偏光回転子とし、この偏光回転子を、透過する偏光面が４５度ずれた偏光子と検光子とで挟む構成があり、多く実用に供されている。そして、上に示したような、広い波長範囲を有する波長多重信号光や、波数が広く分布している多モード光に対しても動作する光アイソレータは、現在のところ、従来から知られている磁気的な部品を用いた素子でしか実現できていない。
【０００４】
しかしながら、このような磁気光学結晶や偏光子で構成される光非可逆素子は、前述のファラデー偏光回転子の他に磁石を含めて多数の部品が必要であり、したがって材料の加工や組立に多数の工数を要し、また、アライメントの経時的な信頼性や安定性が乏しく、さらには高価であるという問題がある。
また、光中継器等に用いる場合のように、必ずしも透過する光の偏光状態が定まっていない光ファイバ途中に挿入して使用する（インライン型）ことを可能とするためには、素子特性が偏光に依存しない構成を築かなければならない。このため、更に多数の部品を必要とし、複雑な光路構成を採らなければならず、安定さを欠く。
【０００５】
構成光部品の数が少なく、インライン型で使用できる光アイソレータを構成する方法として、単一モード光ファイバ素線を用いたファイバ型光アイソレータの発明が特許第３０１１１４０号公報に開示されている。この光アイソレータは、コアの周囲をクラッドで包囲した光ファイバの、光軸に垂直な断面内において、コアに接近して水平に引いた直線を境にして、上部と下部とでは異なる屈折率の誘電体クラッドを有し、前記コアには波数ベクトルがコアの光導波方向と前記屈折率の異なる誘電体の境界面に立てた法線とが作る面内にあって、かつ波数ベクトルの光軸となす角度θが、０°＜θ＜９０°となる屈折率の周期的変化による回折格子を有する構成である。
クラッドの屈折率が非軸対称であり回折格子が光軸に対して斜めに設定されていることにより、光ファイバのコア内で伝送される光が、順方向では格子によって回折されることがなく、逆方向では回折されてクラッドへ放射されることになる。この結果、光ファイバのコアを導波されて伝わる光の順方向伝搬と逆方向伝搬とでは透過率が大幅に異なる高アイソレーション特性を有し、さらに偏光依存性の小さい光アイソレータ特性が得られている。
【０００６】
【発明が解決しようとする課題】
しかしながら、上記の従来のファイバ型光部品は、波長が揺らぐことなく一定でかつ単一モード光を対象としたものであって、光源波長や波数の変動及びそれらのばらつきや周囲温度の変化等に対して、アイソレーション特性が極めて敏感に変動し不安定である。また、波長多重光や多モード光に適用しにくいという難点がある。この理由は、導波光をクラッドへ放射させるメカニズムが、単一周期格子によるブラッグ回折効果であって、導波光の波数と回折格子の波数とクラッド放射光の波数との３者の間で、位相整合が常に誤差１０^-5以下程度に正しくとれていなければならないという厳格な条件を満たす必要があるからである。
本発明は、上記の課題に鑑みて成されたものであって、その目的とするところは、広波長幅や多モードのファイバ光に対しても高い非相反透過特性を有し、かつ安価に生産可能な光アイソレータを提供することにある。
【０００７】
【課題を解決するための手段】
本発明の請求項１に係わる発明記載の導波路型光アイソレータは、光導波するコアの周囲がクラッドで包囲された構造をなし、前記クラッドが、光軸に垂直な断面内において前記コアに接近して引いた直線を境にして異なる屈折率の第１のクラッド部及び第２のクラッド部を有し、前記コアが、位相回折格子を有する光導波路であって、前記位相回折格子の波数ベクトルは、前記２つのクラッド部の境界面に直交し且つ前記光軸に沿った面内に在り、前記光導波路は、前記面内において湾曲した湾曲部を有することを特徴とする。
また、本発明の請求項２に係わる発明記載の導波路型光アイソレータは、前記請求項１に係わる発明記載の前記光導波路の湾曲していない部位における前記位相回折格子の波数の大きさは、前記コアに導波される導波光が前記位相回折格子によって回折されて前記第１または第２のクラッド部に放射されて生じる放射光の前記光軸に対する放射角度が、ほぼ９０度となるように設定されていることを特徴とする。
また、本発明の請求項３に係わる発明記載の導波路型光アイソレータは、前記請求項２に係わる発明記載の前記光導波路が、単一モード光導波路であり、前記コアに導波される導波光は、波長がλ1 からλ2 （λ1 ＜λ2 ）まで分布しており、前記第１または第２のクラッド部に放射される放射光の前記光軸に対する放射角度がほぼ９０度となるように設定される導波光の波長は、λ1 であることを特徴とする。
また、本発明の請求項４に係わる発明記載の導波路型光アイソレータは、前記請求項２に係わる発明記載の前記光導波路が、多モード光導波路であり、前記コアに導波される導波光は、波数がｋ1 からｋ2 （ｋ1 ＞ｋ2 ）まで分布しており、前記第１または第２のクラッド部に放射される放射光の前記光軸に対する放射角度がほぼ９０度となるように設定される導波光の波数は、ｋ1 であることを特徴とする。
また、本発明の請求項５に係わる発明記載の導波路型光アイソレータは、前記前記請求項２乃至４に係わる発明記載の前記光導波路が、光ファイバであり、前記湾曲は、前記光ファイバへの応力付与による弾性的な変形によって形成されていることを特徴とする。
また、本発明の請求項６に係わる発明記載の導波路型光アイソレータは、前記前記請求項２乃至４に係わる発明記載の前記光導波路が、平面基板上に形成された光導波路であり、前記湾曲は、前記光導波路形成時にパターン形成によって無応力状態で付与されていることを特徴とする。
また、本発明の請求項７に係わる発明記載の導波路型光アイソレータの製造方法は、光導波するコアの周囲がクラッドで包囲された構造をなし、前記クラッドが、光軸に垂直な断面内において前記コアに接近して引いた直線を境にして異なる屈折率の第１のクラッド部及び第２のクラッド部を有し、前記コアが、位相回折格子を有する光導波路であり、前記位相回折格子の波数ベクトルは、前記２つのクラッド部の境界面に直交し且つ前記光軸に沿った面内に在り、前記光導波路は、前記面内において湾曲した湾曲部を有する導波路型光アイソレータの製造方法であって、直線の前記光導波路に前記位相回折格子を形成する工程と、前記位相回折格子を備えた前記光導波路に前記湾曲を付与する工程を含むことを特徴とする。
また、本発明の請求項８に係わる発明記載の導波路型光アイソレータの製造方法は、光導波するコアの周囲がクラッドで包囲された構造をなし、前記クラッドが、光軸に垂直な断面内において前記コアに接近して引いた直線を境にして異なる屈折率の第１のクラッド部及び第２のクラッド部を有し、前記コアが、位相回折格子を有する光導波路であり、前記位相回折格子の波数ベクトルは、前記２つのクラッド部の境界面に直交し且つ前記光軸に沿った面内に在り、前記光導波路は、前記面内において湾曲した湾曲部を有する導波路型光アイソレータの製造方法であって、前記湾曲を付与した前記光導波路を形成する工程と、前記湾曲した前記光導波路に前記位相回折格子を形成する工程を含むことを特徴とする。
また、本発明の請求項９に係わる発明記載の導波路型光アイソレータの製造方法は、前記請求項７及び８に係わる発明記載の前記位相回折格子を形成する工程が、単一周期で空間変調された照射光を前記光導波路に照射する工程を含むことを特徴とする。
また、本発明の請求項１０に係わる発明記載の導波路型光アイソレータの製造方法は、前記請求項７及び８に係わる発明記載の前記光導波路の湾曲していない部位における前記位相回折格子の波数の大きさは、前記コアに導波される導波光が前記位相回折格子によって回折されて前記第１または第２のクラッド部に放射されて生じる放射光の前記光軸に対する放射角度が、ほぼ９０度となるように設定されていることを特徴とする。
また、本発明の請求項１１に係わる発明記載の導波路型光アイソレータの製造方法は、前記請求項１０に係わる発明記載の前記光導波路が、単一モード光導波路であり、前記コアに導波される導波光は、波長がλ1 からλ2 （λ1 ＜λ2 ）まで分布しており、前記第１または第２のクラッド部に放射される放射光の前記光軸に対する放射角度がほぼ９０度となるように設定される導波光の波長は、λ1 であることを特徴とする。
また、本発明の請求項１２に係わる発明記載の導波路型光アイソレータの製造方法は、前記請求項１１に係わる発明記載の前記光導波路が、多モード光導波路であり、前記コアに導波される導波光は、波数がｋ1 からｋ2 （ｋ1 ＞ｋ2 ）まで分布しており、前記第１または第２のクラッド部に放射される放射光の前記光軸に対する放射角度がほぼ９０度となるように設定される導波光の波数は、ｋ1 であることを特徴とする。
また、本発明の請求項１３に係わる発明記載の導波路型光アイソレータの製造方法は、前記請求項７に係わる発明記載の前記光導波路が、光ファイバであり、前記湾曲は、前記光ファイバへの応力付与による弾性的な変形によって形成されていることを特徴とする。
また、本発明の請求項１４に係わる発明記載の導波路型光アイソレータの製造方法は、前記請求項８に係わる発明記載の前記光導波路が、平面基板上に形成された光導波路であり、前記湾曲は、前記光導波路形成時にパターン形成によって無応力状態で付与されていることを特徴とする。
また、本発明の請求項１５に係わる発明記載の導波路型光アイソレータ並びに導波路型光アイソレータの製造方法は、前記請求項１乃至６及び請求項７乃至１４に係わる発明記載の前記湾曲の曲線が、２次曲線であることを特徴とする。
【０００８】
【発明の実施の形態】
次に、本発明の実施の形態について図面を参照して説明する。
図１（ａ）は、本発明の光アイソレータの第１の実施形態の側面断面図、図１（ｂ）は、端面断面図を示している。この光アイソレータの構造をその製造工程順に説明する。例えば、通常の光通信用石英系単一モードの光ファイバ１を用意し、この光ファイバの側面にＫｒＦエキシマレーザ光の紫外光（〜２４０ｎｍ）を２光束干渉させて照射すると、前記光ファイバ１のコア１ａに、当該コア１１に含まれるＧｅに起因する屈折率変化が生じ、干渉縞に応じた位相格子、すなわち回折格子１２が形成される。回折格子１２の形成方法としては、通常のファイバブラッグ回折格子の形成方法を利用する。このとき、回折格子１２の格子周期Λと、回折格子１２の波数ベクトルの方向と光ファイバの光軸との間の角度θは、後に示すような方法によって予め設定する。図１（ａ）の配置では紙面に向かって垂直に紫外光が照射され、それが作る干渉縞によって、紙面内で波数ベクトルが光軸と角度θだけ傾いた回折格子１２が形成される。
回折格子１２を形成した後、図１（ｂ）のように、光ファイバ１のクラッド１ｂの下側の領域を前記回折格子１２の波数ベクトルと光軸とが作る面に直交した面を研磨によって削除し、コア１ａの円筒面近くまで光学研磨する。前記コア１ａ、石英クラッド１ｂ、空気のそれぞれの屈折率をｎ_core、ｎ_clad、ｎ_air とすると、これらの大小関係は、ｎ_core＞ｎ_clad＞ｎ_air である。
このように構成された光ファイバ１を、研磨面に立てた法線と光ファイバ１の光軸とが作る面内で湾曲させる。ここでは、Ｓ字形状を描くように曲げてある。曲げの曲率は、光ファイバを透過する導波光が曲げによって放射損失を生じない範囲の曲率である。
【０００９】
次に、この第１の実施形態の光アイソレータの動作を説明する。
光ファイバ１の左端からレーザ光を右進行光１０として入射する。光ファイバのコア中を進行した右進行光１０は、コア中に形成されているチルト回折格子１２に進入する。チルト格子１２は湾曲して変形を受けているため、格子の周期間隔は、光軸に沿って微小ながら変化している。このため右進行光１０が強くブラッグ回折を受ける光軸上の位置は、入射光の波長や使用している光ファイバ１のコア径や屈折率によって微妙に異なり、右進行光１０は、光進行方向においてブラッグ整合条件が満足される適切な曲率の湾曲箇所で、空気中へ放射光３０となってほぼ全エネルギーが回折され、光ファイバ１の右端までは到達しない。
一方、光ファイバ１の右端から右進行光１０と同一波長のレーザ光が左進行光２０として入射すると、光ファイバのコア中を進行した左進行光２０は、コア中に形成されているチルト回折格子１２に進入するが、左進行光２０は、ブラッグ条件が満たされないためチルト回折格子１２によって回折は受けず、ほぼ全エネルギーを光ファイバ１の左端に到達させる。
このようにして、光ファイバ１は、進行方向によって光ファイバを透過する光強度が大幅に異なる光アイソレータ動作を行うが、本発明のファイバ型光アイソレータの特徴は、回折格子１２の格子周期Λと、回折格子１２の波数ベクトルの方向と光ファイバの光軸との間のなす角度θが予め適切に設定されており、加えて光ファイバ１を湾曲させてあるために、入射レーザ光の波長が変動またはばらついても、光進行方向において波長に応じた曲率の湾曲箇所から空気中へ回折光３０となってほぼ全エネルギーが回折させる点にある。
【００１０】
次に、上記のような広い動作波長幅を可能にしている理由を説明する。
まず、光ファイバに湾曲を施さず直線形状であって、入射レーザ波長が一定である場合のファイバ型光アイソレータの非可逆動作の原理について図２を参照して説明する。
図２（Ａ）の構造における波数のダイアグラムは、図２（Ｂ）及び（Ｂ‘）に示すように、ｙ＞０には、ｋ_clad＝（２π／λ）・ｎ_cladのみが存在し、ｙ＜０には、ｋ_air ＝（２π／λ）・ｎ_air のみが存在する。右進行光１１０が回折格子１３０の形成されたファイバ型光アイソレータ１０１に入射した時（図２（Ａ））、入射光の波数ベクトルｋと、回折格子の波数ベクトルＫと、放射光の波数ベクトルｋ_air は閉じた三角形を形成してブラッグ条件が成り立ち、入射光は、空気中の放射光１４０へ効率よく回折され、コアを透過する光は消失する。
他方、左進行光１２０が入射した時（図２（Ａ’））、ここで形成されている回折格子の周期Λによる波数Ｋの大きさでは小さすぎるため、入射光の波数ベクトルｋと、回折格子の波数ベクトルＫと、クラッド放射光の波数ベクトルｋ_cladとは閉じた三角形を形成せずブラッグ条件が成り立たない。このため、入射光はクラッド放射光へ回折されことなく、そのままコア中を透過して出射する。このことによってコア伝搬光に対する強い非可逆透過特性が実現する。
このブラッグベクトル整合条件は、上記３つの波数ベクトルのいずれかが変化する擾乱があるとたちまち崩れ、ブラッグ回折は起こらず、光アイソレータ１０１は、単なる可逆な光ファイバに変ずる。
【００１１】
次に、図１に示した本発明の湾曲した光アイソレータの原理を説明する。
上記の擾乱は、前述のように設計値からの入射波長のずれや変動。温度による屈折率の変化などがある。これらの変動に対して、常にブラッグ回折が起こるようにするためには、光軸に沿った回折格子の何処かで常にブラッグ条件を満たされているように素子の構造に冗長性を持たせるようにすればよい。
図３（Ａ）に示すように、回折格子の波数ベクトルと光軸との成す角をθ、空気中への放射光の光軸との成す角をφとしたとき、図３（Ｂ）に示した、入射光の波数ベクトルｋと、回折格子の波数ベクトルＫと、空気中への放射光の波数ベクトルｋair とが閉じた三角形を形成するブラッグ整合条件は、
｜ｋ｜−｜ｋair ｜ｃｏｓ（φ）＝｜Ｋ｜ｃｏｓ（θ）
｜ｋair ｜ｓｉｎ（φ）＝｜Ｋ｜ｓｉｎ（θ）
の両式を同時に満足することである。
図４は、上式を満たす回折格子波数ベクトルＫ（ベクトルの大きさ｜Ｋ｜と角度θ）の条件を放射光の放射角度φを横軸にとって示したものである。導波モードが存在できる波数ｋの大きさの範囲が、｜ｋclad｜＜｜ｋ｜＜｜ｋcore｜であるため、｜ｋ｜＝｜ｋcore｜の場合を実線で、｜ｋ｜＝｜ｋclad｜の場合を破線でそれぞれ表してある。擾乱によって、入射光の波数ｋは実線と破線の間を単調に変化する。
【００１２】
本発明の光アイソレータにおける、回折格子（｜Ｋ｜、∠θ）の設定方法について説明する。
設定方法は、擾乱によるｋの変動に対して、
１） Δθ＝０の下に、｜ｋ_clad｜＜｜ｋ｜＜｜ｋ_core｜の範囲で常に整合条件を満たすために、回折格子が分布して保持すべき波数の範囲Δ｜Ｋ｜を求める、
２） Δ｜Ｋ｜＝０の下に、｜ｋ_clad｜＜｜ｋ｜＜｜ｋ_core｜の範囲で常に整合条件を満たすために、回折格子が分布して保持すべき光軸に対する角度の範囲Δθを求める、
の２つのアプローチに分類できる。図１の第１の実施形態は、上記の１）のアプローチに基づく。第１の実施形態では、光ファイバを弾性変形させれば回折格子の周期は、微小ながら変化させることができるが、格子の波数ベクトルと光軸との角度θは変化させ得ない場合に相当する。
図４において、φ＝π／２では、θが停留している。すなわち、放射光の放射角度φが光軸に垂直となるような条件付近（ステップａ）では、整合条件を満たすθの変化が小さい。回折格子と光軸とのなす角度θを、波数が最大となる波長の入射光（又は多モードファイバであれば最低次のモード、すなわちｋ＝ｋ_core）に対して定め（ステップｂ：θ＝θ_core）、これに対応するＫ_core（すなわち格子の周期Λ_core）を決定する（ステップｃ）。θ＝θ_core一定下で、入射光の波数ｋが、ｋ＝ｋ_coreからｋ＝ｋ_cladに遷移したとき（ステップｄ）のＫ_cladを求める（ステップｅ）。
光ファイバが湾曲していない直線状態では、Ｋ＝Ｋ_core、θ＝θ_coreとなるように格子の周期と光軸に対する角度を定める。光ファイバを部分的に湾曲させて、湾曲させた部位ではＫ＝Ｋ_cladとなるよう、湾曲していない部位ではＫ＝Ｋ_coreにする。このように、波数が最大となる波長の入射光（又は多モードファイバであれば最低次のモード）に対して、回折光の放射方向が光軸に対して垂直になるように回折格子の周期Λと角度θを選び、光ファイバの伸び縮みによる弾性変形等によって回折格子の周期に或る程度光軸に沿って微小な分布（ΔＫ）を持たせることによって、入射光の波長の変動や波数の分散を吸収して広波長幅ないしは広波数幅（多モード）で光アイソレータを動作させることが可能である。
このことは、光導波路の境界条件が軸対称でないため、コア部を伝わるＴＥ波とＴＭ波とで伝搬波数が多少異なっても、回折格子の波数が分散して設けられているため、整合条件の合ったコア部の曲率位置で双方とも回折されるので、偏光無依存の特性が得られることも意味する。
【００１３】
図１の本発明の第１の実施形態では、光ファイバが直線の状態でコア中に単一周期の回折格子を形成し、その後光ファイバを湾曲させている。これは、上で述べた入射光の波長の変動や波数の分散を吸収して広波長幅ないしは広波数幅（多モード）で光アイソレータを動作させる１）のアプローチによる構成方法に基づいている。光ファイバ１を湾曲させることは、平面に研磨した外側面の表面に接しているコアに伸び縮みを起こし、曲げの変曲点を中心にして光軸に沿って回折格子周期に僅かな伸び縮みの変化を与えている。
【００１４】
図５は本発明の導波路型光アイソレータの第２の実施の形態の構成を示す図である。図５（Ａ）は導波路型光アイソレータ１０１の平面図、（Ｂ）、（Ｃ）は側面図である。正方形断面のコア１０５が一側面を空気１０７に露出させ、３側面をクラッド１０６に覆われて、平面内でＳ字形状に湾曲して形成されている。前記コア部、クラッド部、空気のそれぞれの屈折率をｎ_core、ｎ_clad、ｎ_air とすると、これらの大小関係は、ｎ_core＞ｎ_clad＞ｎ_air である。この導波路は、通常の誘電体成膜技術、リソグラフィー技術及びエッチング技術を用いて製作することができる。導波路製作後第１の実施形態と同様、２光束干渉させた紫外光を照射することによって、単一周期の回折格子１１２をコア部のみに作り込んでいる。ここでは、Ｓ字形状を描くように曲げてあるコア部の曲げの曲率は、コア部を透過する導波光が曲げによって放射損失を生じない範囲の曲率である。
【００１５】
次に、この第２の実施形態の光アイソレータの動作を説明する。
光導波路コア部１０５の左端からレーザ光を右進行光１１０として入射する。コア中を進行した右進行光１１０は、コア中に形成されている回折格子１１２に進入する。回折格子１１２は単一周期で、コア部の変曲点における光軸１２１とθ傾いて形成されている。光透過方向に沿って光軸と回折格子１１２とは異なった角度をなしている。このため右進行光１１０が強くブラッグ回折を受ける光軸上の位置は、入射光の波長や使用しているコア部１０５の屈折率や断面サイズによって微妙に異なり、右進行光１１０は、光進行方向においてブラッグ整合条件が満足される適切な曲率の湾曲箇所で、空気中へ回折光１３０となってほぼ全エネルギーが回折され、コア部１０５の右端までは到達しない。
一方、コア部１０５の右端から右進行光１１０と同一波長のレーザ光が左進行光１２０として入射すると、コア部１０５を進行した左進行光１２０は、コア中に形成されている回折格子１１２に進入するが、左進行光１２０は、ブラッグ条件が満たされないため回折格子１１２によって回折は受けず、ほぼ全エネルギーをコア部１０５の左端に到達させる。
このようにして、光ファイバ１は、進行方向によって光導波路を透過する光強度が大幅に異なる光アイソレータ動作を行うが、本発明の導波路型光アイソレータの特徴は、回折格子１１２の格子周期Λと、回折格子１１２の波数ベクトルの方向とコア部の変曲点における光軸との間のなす角度θが予め適切に設定されており、加えてコア部１０５を湾曲させてあるために、入射レーザ光の波長が変動またはばらついても、光進行方向において波長に応じた曲率の湾曲箇所から空気中へ回折光１３０となってほぼ全エネルギーが回折させる点にある。
【００１６】
本発明の光アイソレータの第２の実施形態における、回折格子（｜Ｋ｜、∠θ）の設定方法について説明する。
ここにおける設定方法は、前述の２）の設定方法、すなわちΔ｜Ｋ｜＝０の下で、Δθが最小となる条件を求め方法である。設定の手順を図６に示す。図６は、図４のグラフの見方を変えたものである。
図６において、図４と同様に導波路コア部の変曲点において、放射光の放射角度φが光軸に垂直となるように回折格子の波数Ｋを、波数が最大となる波長の入射光（又は多モード導波路であれば最低次のモード、すなわちｋ＝ｋ_core）に対して定め（ステップｇ：Ｋ＝Ｋ_core）、これに対応するθ_core（すなわち格子と光軸との成す角度）を決定する（ステップｈ）。Ｋ＝Ｋ_core一定下で、入射光の波数ｋを、ｋ＝ｋ_coreからｋ＝ｋ_cladに遷移させたとき（ステップｉ）のθ_cladを求める（ステップｊ）。
コア部が湾曲していない変曲点では、Ｋ＝Ｋ_core、θ＝θ_coreとなるように格子の周期と光軸に対する角度を定める。コア部が湾曲した部位ではθ＝θ_cladとなるようにコア部の湾曲を決定する。このように、波数が最大となる波長の入射光（又は多モード導波路あれば最低次のモード）に対して、回折光の放射方向がコア部の変曲点における光軸に対して垂直になるように回折格子の周期Λと角度θを選び、回折格子とコア部光軸との間の角度を或る程度光軸に沿って微小な分布（Δθ）を持たせるように、コア部を湾曲させて製作することによって、入射光の波長の変動や波数の分散を吸収して広波長幅ないしは広波数幅（多モード）で光アイソレータを動作させることが可能である。
このことは、光導波路の境界条件が軸対称でないため、コア部を伝わるＴＥ波とＴＭ波とで伝搬波数が多少異なっても、光軸と回折格子との成す角が分散して設けられているため、整合条件のあったコア部の曲率位置で双方とも回折されるので、偏光無依存の特性が得られることを意味する。
【００１７】
本第２の実施形態は、用いる材料を光半導体とすることもでき、この場合には、結晶成長技術に公知の選択成長法を用いることによって、半導体レーザとモノリシック集積された光アイソレータを実現することができる。
【００１８】
尚、上記の２つの実施形態では、回折の整合条件は、実施例では屈折率が低いクラッド部へ放射するような条件で説明しているが、屈折率が高い方のクラッドへ回折光を放射する条件に選んでもよい。
【００１９】
また、上記の説明では、屈折率の異なる２つのクラッドの一方を空気としたが、
他方のクラッドより屈折率が小さければよい。例えば他方のクラッドが石英であれば、それより屈折率の小さいフッ素化樹脂等を用いて覆ってもよい。
【００２０】
また、コア部の光伝搬方向への形状をＳ字形状に設定してあるが、入射光の進行方向と出射光の進行方向を同一にし、素子の組立実装を容易化するためであって。設計原理で示したとおり、単なる円弧や楕円弧等の２次曲線の一部があればよく、必ずしもＳ字である必要はない。
【００２１】
また、用いる導波路材は、石英系やガラス系の材料だけではなく、樹脂等のプラスティック系の材料でも良く、この場合、回折格子の形成には紫外線照射によるカラーセンターの発生のみならず、さらに短波長のシンクロトロン放射光の照射による高分子の主鎖の切断を利用して形成する方法も用いることができる。
や高分子
【００２２】
【発明の効果】
以上説明したように本発明の導波路型光アイソレータは、波数が最大となる波長の入射光（又は多モード導波路であれば最低次のモード）に対して、回折光の放射方向が光軸に対して垂直になるように回折格子の周期Λと角度θを選び、光導波路の湾曲による弾性変形によって回折格子の周期に或る程度光軸に沿って微小な分布（ΔＫ）を持たせることによって、入射光の波長の変動や波数の分散を吸収して広波長幅ないしは広波数幅（多モード）で光アイソレータを動作させることができる。
また、波数が最大となる波長の入射光（又は多モード導波路あれば最低次のモード）に対して、導波路のコア部の光伝搬方向の或る特定の点において、回折光の放射方向が光軸に対して垂直になるように回折格子の周期Λと角度θを選び、回折格子とコア部光軸との間の角度を或る程度光軸に沿って微小な分布（Δθ）を持たせるように、コア部を湾曲させて製作することによって、入射光の波長の変動や波数の分散を吸収して広波長幅ないしは広波数幅（多モード）で光アイソレータを動作させることができる。
これにより、従来の磁気光学効果等の非相反光効果を有する材料や磁石等を特に用いることなく構成簡便で生産性に優れ、伝送通信用単一モード光ファイバのみならず高出力レーザ光伝送用光ファイバや各種の多モード光ファイバに適用できる優れた光アイソレータが実現できる。
また、磁気光学結晶を用いていないため、紫外光から赤外光まで広いスペクトル範囲の光アイソレータを実現できる。
【図面の簡単な説明】
【図１】本発明の導波路型光アイソレータの第１の実施形態の構成を説明する図である。
【図２】本発明の導波路型光アイソレータの非可逆光透過特性を説明する図である。
【図３】本発明の導波路型光アイソレータの第１の実施形態の動作を説明するためのパラメータの構成を示す図である。
【図４】本発明の導波路型光アイソレータの第１の実施形態の動作を説明するための図である。
【図５】本発明の導波路型光アイソレータの第２の実施形態の構成を説明する図である。
【図６】本発明の導波路型光アイソレータの第２の実施形態の動作を説明するための図である。
【符号の説明】
１光ファイバ
１ａコア
１ｂクラッド
１０右進行光
１２回折格子
２０左進行光
３０放射光
１０１導波路型光アイソレータ
１０５コア
１０６クラッド
１０７空気
１１０右進行光
１１２回折格子
１２０左進行光
１３０回折格子
１４０放射光[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a waveguide type optical isolator that can be configured only by an optical waveguide including an optical fiber without using a special material such as a magneto-optical material.
[0002]
[Prior art]
With the spread of the Internet in recent years and the rapid expansion of social communication demands such as multimedia communication, the transmission distance using optical fiber amplifier is 10,000 km, the transmission capacity exceeds 10 terabit / second, and it is very long distance In addition, the development of high-capacity high-density wavelength division multiplexing (DWDM) optical backbone communication technology and the application of optical fiber transmission technology to the subscriber end expand broadband digital information communication services to ordinary homes and office buildings. The development of technologies for increasing the capacity of optical communication systems has been energetically performed, such as the study of optical access systems.
An optical component indispensable for realizing such a system is an optical element having a strong directivity in light transmission characteristics such as an optical isolator. For example, when transmitting signal light emitted from a transmission light source or optical fiber amplifier through an optical fiber, the optical isolator re-enters the semiconductor laser or optical fiber amplifier that is the light source as reflected return light from an optical fiber connection point or the like. This is used as a component for preventing the signal light S / N drop and unstable operation caused by the operation.
2. Description of the Related Art Recently, optical transmission systems are wavelength division multiplexing, in which signal light of a large number of wavelengths is transmitted in a lump by directly amplifying and repeating optical signals such as DWDM from time division multiplexing (TDM) transmission systems using a single mode fiber (SMF). The subject of development has shifted to the transmission system. In this wavelength division multiplexing transmission system, the optical isolator is required to exhibit high isolation characteristics over a wide wavelength range of several nm to several tens of nm.
Furthermore, in recent years, multi-mode plastic optical fiber (POF) has a broadband optical transmission characteristic as good as quartz single-mode fiber, and has a core diameter on the order of sub-mm. Therefore, the use for optical access systems that are required to be inexpensive has attracted attention. An optical isolator used for such a multimode POF is required to operate with high isolation characteristics even with respect to multimode light having a wide distribution of wave numbers.
[0003]
An optical isolator is an element in which the forward transmission characteristic and the reverse transmission characteristic operate irreversibly. As a configuration example of a conventionally known element, a Faraday effect that is a non-reciprocal polarization rotation effect is used. The magneto-optical crystal shown is a 45-degree polarization rotator, and this polarization rotator is sandwiched between a polarizer and an analyzer whose transmission plane of polarization is deviated by 45 degrees. In addition, optical isolators that operate for wavelength-division multiplexed signal light having a wide wavelength range as described above and multimode light having a wide distribution of wave numbers are currently known. This can only be realized with elements using magnetic parts.
[0004]
However, such an optical irreversible element composed of a magneto-optical crystal and a polarizer requires a large number of parts including a magnet in addition to the above-mentioned Faraday polarization rotator, and therefore a large number of materials are processed and assembled. In addition, there is a problem that the number of man-hours is long, the reliability and stability of the alignment over time are poor, and the cost is high.
In addition, as in the case of use in an optical repeater, etc., in order to enable insertion (in-line type) in the middle of an optical fiber where the polarization state of transmitted light is not fixed, the element characteristics are polarized. You must build a structure that does not depend on. For this reason, a large number of parts are required, a complicated optical path configuration must be adopted, and stability is lacking.
[0005]
Japanese Patent No. 3011140 discloses a fiber-type optical isolator using a single-mode optical fiber as a method of constructing an optical isolator that has a small number of constituent optical components and can be used in an in-line type. This optical isolator has different refractive indices at the top and bottom, with a straight line approaching the core and drawn horizontally in the cross section perpendicular to the optical axis of the optical fiber surrounded by the cladding around the core. A dielectric clad, wherein the core has a wave vector in a plane formed by the optical waveguide direction of the core and a normal formed on a boundary surface of the dielectric having a different refractive index, and the optical axis of the wave vector And a diffraction grating by a periodic change of the refractive index where the angle θ is 0 ° <θ <90 °.
Since the refractive index of the cladding is non-axisymmetric and the diffraction grating is set obliquely to the optical axis, the light transmitted in the core of the optical fiber is not diffracted by the grating in the forward direction. In the opposite direction, it is diffracted and emitted to the cladding. As a result, it has high isolation characteristics with significantly different transmittances between forward propagation and backward propagation of light guided through the core of the optical fiber, and optical isolator characteristics with little polarization dependence are obtained. ing.
[0006]
[Problems to be solved by the invention]
However, the above-mentioned conventional fiber-type optical components are intended for single-mode light that is constant and does not fluctuate in wavelength, and are subject to fluctuations in the light source wavelength, wave number, variations thereof, changes in ambient temperature, etc. On the other hand, the isolation characteristic varies extremely sensitively and is unstable. In addition, it is difficult to apply to wavelength multiplexed light and multimode light. The reason for this is that the mechanism that radiates guided light to the clad is the Bragg diffraction effect by a single periodic grating, and the phase between the wave number of the guided light, the wave number of the diffraction grating, and the wave number of the clad radiated light. Match is always error 10 ^-Five This is because it is necessary to satisfy the strict condition that it must be taken correctly to the following extent.
The present invention has been made in view of the above-mentioned problems, and the object thereof is to have high nonreciprocal transmission characteristics with respect to a wide wavelength width and multimode fiber light and to be inexpensive. The object is to provide an optical isolator that can be produced.
[0007]
[Means for Solving the Problems]
The waveguide type optical isolator according to the first aspect of the present invention has a structure in which an optical waveguide core is surrounded by a clad, and the clad approaches the core in a cross section perpendicular to the optical axis. A first clad part and a second clad part having different refractive indexes with a straight line drawn as a boundary, and the core is an optical waveguide having a phase diffraction grating, and the wave number vector of the phase diffraction grating Is orthogonal to the boundary surface between the two cladding portions and along the optical axis Face The optical waveguide is in front Writing It has the curved part curved inside.
Further, in the waveguide type optical isolator according to the second aspect of the present invention, the magnitude of the wave number of the phase diffraction grating in the uncurved portion of the optical waveguide according to the first aspect of the invention is as follows: The radiation angle of the radiated light generated when the guided light guided to the core is diffracted by the phase diffraction grating and radiated to the first or second cladding is about 90 degrees. It is characterized by being set.
Further, in the waveguide type optical isolator according to the third aspect of the present invention, the optical waveguide according to the second aspect of the present invention is a single mode optical waveguide, and is guided to the core. The wave light has a wavelength distributed from λ1 to λ2 (λ1 <λ2), and is set so that the radiation angle of the radiated light radiated to the first or second clad portion is about 90 degrees with respect to the optical axis. The wavelength of the guided light is λ1.
According to a fourth aspect of the present invention, there is provided a waveguide type optical isolator according to the fourth aspect of the present invention, wherein the optical waveguide according to the second aspect of the present invention is a multimode optical waveguide and is guided to the core. Has a wave number distributed from k1 to k2 (k1> k2), and is set so that the radiation angle of the radiated light radiated to the first or second cladding portion is about 90 degrees with respect to the optical axis. The wave number of the guided light is k1.
Further, in the waveguide type optical isolator according to the fifth aspect of the present invention, the optical waveguide according to the second to fourth aspects of the present invention is an optical fiber, and the bending is applied to the optical fiber. It is formed by elastic deformation by applying stress.
In the waveguide type optical isolator according to the sixth aspect of the present invention, the optical waveguide according to the second to fourth aspects of the present invention is an optical waveguide formed on a flat substrate, The curve is provided in a stress-free state by pattern formation when the optical waveguide is formed.
According to a seventh aspect of the present invention, there is provided a method for manufacturing a waveguide type optical isolator having a structure in which an optical waveguide core is surrounded by a clad, and the clad is in a cross section perpendicular to the optical axis. A first clad portion and a second clad portion having different refractive indexes with a straight line approaching the core as a boundary, the core being an optical waveguide having a phase diffraction grating, and the phase diffraction The wave vector of the grating is perpendicular to the boundary surface between the two cladding parts and along the optical axis. Face The optical waveguide is in front Writing A method of manufacturing a waveguide-type optical isolator having a curved portion inside thereof, the step of forming the phase diffraction grating in the straight optical waveguide, and the bending of the optical waveguide including the phase diffraction grating. Including a step of imparting.
Further, the waveguide type optical isolator manufacturing method according to claim 8 of the present invention has a structure in which the periphery of the optical waveguide core is surrounded by a clad, and the clad is in a cross section perpendicular to the optical axis. A first clad portion and a second clad portion having different refractive indexes with a straight line approaching the core as a boundary, the core being an optical waveguide having a phase diffraction grating, and the phase diffraction The wave vector of the grating is perpendicular to the boundary surface between the two cladding parts and along the optical axis. Face The optical waveguide is in front Writing A method of manufacturing a waveguide type optical isolator having a curved portion inside, the step of forming the curved optical waveguide, and the step of forming the phase diffraction grating in the curved optical waveguide Include thing It is characterized by.
According to a ninth aspect of the present invention, there is provided a method for manufacturing a waveguide type optical isolator according to the ninth aspect of the present invention, wherein the step of forming the phase diffraction grating according to the seventh and eighth aspects of the present invention includes spatial modulation in a single period. A step of irradiating the optical waveguide with the irradiated light.
According to a tenth aspect of the present invention, there is provided a method of manufacturing a waveguide type optical isolator according to the tenth aspect of the present invention, wherein the wavenumber of the phase diffraction grating at the uncurved portion of the optical waveguide according to the seventh and eighth aspects of the present invention. The radiation angle of the radiated light with respect to the optical axis of the radiated light generated when the guided light guided to the core is diffracted by the phase diffraction grating and radiated to the first or second cladding is approximately 90. It is set so as to be a degree.
Further, in a method for manufacturing a waveguide type optical isolator according to an eleventh aspect of the present invention, the optical waveguide according to the tenth aspect of the present invention is a single mode optical waveguide, and is guided to the core. The guided light has a wavelength distributed from λ1 to λ2 (λ1 <λ2), and the radiation angle of the radiated light radiated to the first or second cladding portion is approximately 90 degrees with respect to the optical axis. The wavelength of the guided light set in this way is λ1.
According to a twelfth aspect of the present invention, there is provided a waveguide type optical isolator manufacturing method, wherein the optical waveguide according to the eleventh aspect of the invention is a multimode optical waveguide and is guided to the core. The guided light has a wave number distributed from k1 to k2 (k1> k2), and the radiation angle of the radiated light radiated to the first or second clad portion with respect to the optical axis is approximately 90 degrees. The wave number of the guided light set to is k1.
According to a thirteenth aspect of the present invention, there is provided a waveguide type optical isolator manufacturing method according to the thirteenth aspect of the present invention, wherein the optical waveguide according to the seventh aspect of the present invention is an optical fiber, and the bending is applied to the optical fiber. It is formed by elastic deformation by applying stress.
According to a fourteenth aspect of the present invention, there is provided a waveguide type optical isolator manufacturing method, wherein the optical waveguide according to the eighth aspect of the present invention is an optical waveguide formed on a flat substrate, The curve is provided in a stress-free state by pattern formation when the optical waveguide is formed.
Further, the waveguide type optical isolator according to the fifteenth aspect of the present invention and the manufacturing method of the waveguide type optical isolator are the curved curves according to the first to sixth aspects and the seventh to seventh aspects of the present invention. Is a quadratic curve.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1A is a side sectional view of a first embodiment of an optical isolator according to the present invention, and FIG. 1B is an end sectional view. The structure of this optical isolator will be described in the order of its manufacturing process. For example, when an optical fiber 1 of a normal silica-based single mode for optical communication is prepared and the side surface of the optical fiber is irradiated with ultraviolet light (up to 240 nm) of KrF excimer laser light with two light beam interference, the optical fiber 1 In the core 1a, a refractive index change caused by Ge contained in the core 11 occurs, and a phase grating corresponding to the interference fringes, that is, a diffraction grating 12 is formed. As a method for forming the diffraction grating 12, a normal fiber Bragg diffraction grating forming method is used. At this time, the grating period Λ of the diffraction grating 12 and the angle θ between the direction of the wave vector of the diffraction grating 12 and the optical axis of the optical fiber are set in advance by a method as described later. In the arrangement of FIG. 1A, ultraviolet light is irradiated vertically toward the paper surface, and the diffraction fringes 12 whose wave vector is inclined with respect to the optical axis by an angle θ are formed in the paper surface by the interference fringes created by the ultraviolet light.
After forming the diffraction grating 12, as shown in FIG. 1B, the region under the cladding 1 b of the optical fiber 1 is polished by polishing a surface orthogonal to the surface formed by the wave vector of the diffraction grating 12 and the optical axis. Delete and optically polish to near the cylindrical surface of the core 1a. Refractive indexes of the core 1a, the quartz cladding 1b, and air are n _core , N _clad , N _air Then, these magnitude relationships are n _core > N _clad > N _air It is.
The optical fiber 1 configured as described above is curved in a plane formed by the normal line standing on the polished surface and the optical axis of the optical fiber 1. Here, it is bent so as to draw an S shape. The bending curvature is a curvature in a range in which the guided light transmitted through the optical fiber does not cause a radiation loss due to bending.
[0009]
Next, the operation of the optical isolator according to the first embodiment will be described.
Laser light is incident as right traveling light 10 from the left end of the optical fiber 1. The right traveling light 10 that has traveled through the core of the optical fiber enters the tilt diffraction grating 12 formed in the core. Since the tilt grating 12 is curved and deformed, the periodic interval of the grating changes slightly along the optical axis. Therefore, the position on the optical axis where the right traveling light 10 is strongly subjected to Bragg diffraction differs slightly depending on the wavelength of the incident light, the core diameter of the optical fiber 1 being used, and the refractive index. At a curved portion having an appropriate curvature satisfying the Bragg alignment condition in the direction, almost all energy is diffracted into the air as radiated light 30 and does not reach the right end of the optical fiber 1.
On the other hand, when laser light having the same wavelength as the right traveling light 10 is incident as the left traveling light 20 from the right end of the optical fiber 1, the left traveling light 20 traveling in the core of the optical fiber is tilted diffraction formed in the core. Although entering the grating 12, the left traveling light 20 is not diffracted by the tilt diffraction grating 12 because the Bragg condition is not satisfied, and almost all energy reaches the left end of the optical fiber 1.
In this way, the optical fiber 1 performs an optical isolator operation in which the light intensity transmitted through the optical fiber varies greatly depending on the traveling direction. The fiber type optical isolator of the present invention is characterized by the grating period Λ of the diffraction grating 12 and Since the angle θ formed between the direction of the wave vector of the diffraction grating 12 and the optical axis of the optical fiber is set appropriately in advance, and the optical fiber 1 is curved, the wavelength of the incident laser light is Even if it fluctuates or varies, there is a point where almost all energy is diffracted from the curved portion having the curvature according to the wavelength in the light traveling direction into the air as diffracted light 30.
[0010]
Next, the reason why such a wide operating wavelength width is possible will be described.
First, the principle of the irreversible operation of the fiber type optical isolator in the case where the optical fiber is linear without bending and the incident laser wavelength is constant will be described with reference to FIG.
As shown in FIGS. 2B and 2B, the wavenumber diagram in the structure of FIG. _clad = (2π / λ) · n _clad Only exists, and for y <0, k _air = (2π / λ) · n _air There exists only. When the right traveling light 110 is incident on the fiber-type optical isolator 101 on which the diffraction grating 130 is formed (FIG. 2A), the wave number vector k of the incident light, the wave number vector K of the diffraction grating, and the wave number vector of the emitted light k _air Forms a closed triangle and the Bragg condition is satisfied, and the incident light is efficiently diffracted into the radiation light 140 in the air, and the light transmitted through the core disappears.
On the other hand, when the left traveling light 120 is incident (FIG. 2 (A ′)), the wave number vector k of the incident light and the diffraction are small because the wave number K by the period Λ of the diffraction grating formed here is too small. Lattice wave vector K and clad radiation light wave vector k _clad Does not form a closed triangle and the Bragg condition does not hold. Therefore, the incident light is transmitted through the core as it is without being diffracted into the clad radiation. This realizes a strong irreversible transmission characteristic for the core propagation light.
This Bragg vector matching condition collapses immediately when there is a disturbance in which any of the above three wave number vectors changes, Bragg diffraction does not occur, and the optical isolator 101 changes to a simple reversible optical fiber.
[0011]
Next, the principle of the curved optical isolator of the present invention shown in FIG. 1 will be described.
The above disturbance is a shift or fluctuation of the incident wavelength from the design value as described above. There is a change in refractive index due to temperature. In order to ensure that Bragg diffraction always takes place against these fluctuations, it is necessary to make the element structure redundant so that the Bragg condition is always satisfied at some point of the diffraction grating along the optical axis. You can do it.
As shown in FIG. 3A, when the angle formed by the wave vector of the diffraction grating and the optical axis is θ, and the angle formed by the optical axis of the emitted light into the air is φ, FIG. The Bragg matching condition that forms the closed wave-number vector k of the incident light, the wave-number vector K of the diffraction grating, and the wave-number vector kair of the emitted light into the air is as follows:
| K | − | kair | cos (φ) = | K | cos (θ)
| Kair | sin (φ) = | K | sin (θ)
Both of these formulas are satisfied at the same time.
FIG. 4 shows the conditions of the diffraction grating wave vector K (vector magnitude | K | and angle θ) satisfying the above expression with the radiation angle φ of the emitted light as the horizontal axis. Since the range of the magnitude of the wave number k in which the waveguide mode can exist is | kclad | <| k | <| kcore |, the case of | k | = | kcore | is indicated by a solid line, and | k | = | kclad | These cases are represented by broken lines. Due to the disturbance, the wave number k of the incident light changes monotonically between the solid line and the broken line.
[0012]
A method for setting the diffraction grating (| K |, ∠θ) in the optical isolator of the present invention will be described.
The setting method is for the fluctuation of k due to disturbance.
1) Below Δθ = 0, _clad | <| K | <| k _core In order to always satisfy the matching condition in the range of |, the wave number range Δ | K |
2) Below Δ | K | = 0, _clad | <| K | <| k _core In order to always satisfy the matching condition in the range of |, the range of angles Δθ with respect to the optical axis to be distributed and held by the diffraction grating is obtained.
These two approaches can be classified. The first embodiment of FIG. 1 is based on the above approach 1). In the first embodiment, if the optical fiber is elastically deformed, the period of the diffraction grating can be changed while being minute, but this corresponds to the case where the angle θ between the wave number vector of the grating and the optical axis cannot be changed. .
In FIG. 4, when φ = π / 2, θ is stationary. That is, near the condition (step a) where the radiation angle φ of the emitted light is perpendicular to the optical axis, the change in θ satisfying the matching condition is small. The angle θ formed by the diffraction grating and the optical axis is defined as incident light having a wavelength with the maximum wave number (or the lowest mode in the case of a multimode fiber, that is, k = k _core ) (Step b: θ = θ _core ), K corresponding to this _core (Ie lattice period Λ _core ) Is determined (step c). θ = θ _core Under certain conditions, the wave number k of the incident light is k = k _core To k = k _clad At the time of transition to (step d) _clad (Step e).
In the straight state where the optical fiber is not bent, K = K _core , Θ = θ _core The period of the grating and the angle with respect to the optical axis are determined so that The optical fiber is partially bent, and K = K at the bent portion. _clad So that K = K at the uncurved part _core To. As described above, the period of the diffraction grating is set so that the radiation direction of the diffracted light is perpendicular to the optical axis with respect to the incident light having the maximum wave number (or the lowest order mode in the case of a multimode fiber). By selecting Λ and angle θ and giving the diffraction grating period a small distribution (ΔK) along the optical axis to some extent by elastic deformation due to the expansion and contraction of the optical fiber, the wavelength variation and wave number of the incident light It is possible to operate the optical isolator with a wide wavelength width or a wide wave number width (multimode) by absorbing the dispersion.
This is because the boundary condition of the optical waveguide is not axially symmetric, and even if the propagation wave number is slightly different between the TE wave and the TM wave transmitted through the core portion, the wave number of the diffraction grating is provided in a distributed manner. Since both are diffracted at the curvature position of the matching core part, it also means that polarization independent characteristics can be obtained.
[0013]
In the first embodiment of the present invention shown in FIG. 1, a single-period diffraction grating is formed in a core in a state where the optical fiber is straight, and then the optical fiber is bent. This is based on the above-described configuration method based on the approach 1) in which the optical isolator is operated with a wide wavelength width or a wide wave number width (multimode) by absorbing the wavelength variation of the incident light and the wave number dispersion described above. Bending the optical fiber 1 causes expansion and contraction of the core in contact with the surface of the outer surface polished to a flat surface, and slightly expands and contracts along the optical axis around the bending inflection point. Is giving changes.
[0014]
FIG. 5 is a diagram showing the configuration of the second embodiment of the waveguide type optical isolator of the present invention. 5A is a plan view of the waveguide type optical isolator 101, and FIGS. 5B and 5C are side views. A core 105 having a square cross section is formed such that one side surface is exposed to air 107 and three side surfaces are covered with a clad 106 and curved in an S shape in a plane. Refractive indexes of the core part, the clad part, and air are expressed as n. _core , N _clad , N _air Then, these magnitude relationships are n _core > N _clad > N _air It is. The waveguide can be manufactured using a normal dielectric film forming technique, a lithography technique, and an etching technique. Similar to the first embodiment, after the waveguide is manufactured, the single-period diffraction grating 112 is formed only in the core portion by irradiating with ultraviolet light that has been subjected to two-beam interference. Here, the curvature of bending of the core portion bent so as to draw an S shape is a curvature in a range in which the waveguide light transmitted through the core portion does not cause radiation loss due to bending.
[0015]
Next, the operation of the optical isolator of the second embodiment will be described.
Laser light is incident as right traveling light 110 from the left end of the optical waveguide core portion 105. The right traveling light 110 traveling in the core enters a diffraction grating 112 formed in the core. The diffraction grating 112 is formed in a single period and inclined by θ with respect to the optical axis 121 at the inflection point of the core portion. The optical axis and the diffraction grating 112 have different angles along the light transmission direction. For this reason, the position on the optical axis where the right traveling light 110 is strongly subjected to Bragg diffraction differs slightly depending on the wavelength of the incident light, the refractive index and the cross-sectional size of the core 105 used, and the right traveling light 110 At a curved portion having an appropriate curvature that satisfies the Bragg alignment condition in the direction, almost all energy is diffracted into the air as diffracted light 130 and does not reach the right end of the core portion 105.
On the other hand, when a laser beam having the same wavelength as the right traveling light 110 is incident as the left traveling light 120 from the right end of the core unit 105, the left traveling light 120 traveling through the core unit 105 is incident on the diffraction grating 112 formed in the core. Although it enters, the left traveling light 120 is not diffracted by the diffraction grating 112 because the Bragg condition is not satisfied, and almost all energy reaches the left end of the core portion 105.
In this way, the optical fiber 1 performs an optical isolator operation in which the light intensity transmitted through the optical waveguide varies greatly depending on the traveling direction. The waveguide type optical isolator of the present invention is characterized by the grating period Λ of the diffraction grating 112. And the angle θ between the direction of the wave vector of the diffraction grating 112 and the optical axis at the inflection point of the core portion is set in advance, and in addition, the core portion 105 is curved. Even if the wavelength of the laser beam fluctuates or varies, it is at a point where almost all energy is diffracted from the curved portion having the curvature according to the wavelength in the light traveling direction into the air as the diffracted light 130.
[0016]
A method for setting the diffraction grating (| K |, ∠θ) in the second embodiment of the optical isolator of the present invention will be described.
The setting method here is a setting method of the above-mentioned 2), that is, a method for obtaining a condition under which Δθ is minimum under Δ | K | = 0. The setting procedure is shown in FIG. FIG. 6 shows a different view of the graph of FIG.
In FIG. 6, as in FIG. 4, at the inflection point of the waveguide core, the wave number K of the diffraction grating is set so that the radiation angle φ of the radiated light is perpendicular to the optical axis, and the incident light having the wavelength at which the wave number is maximum. (Or the lowest order mode for multimode waveguides, ie k = k _core ) (Step g: K = K _core ), Corresponding θ _core (That is, the angle formed by the grating and the optical axis) is determined (step h). K = K _core Under constant conditions, the wave number k of the incident light is k = k _core To k = k _clad When transitioning to (step i) _clad (Step j).
At the inflection point where the core is not curved, K = K _core , Θ = θ _core The period of the grating and the angle with respect to the optical axis are determined so that Θ = θ at the curved part of the core _clad The curvature of the core part is determined so that As described above, the incident direction of the diffracted light is perpendicular to the optical axis at the inflection point of the core portion with respect to the incident light having the maximum wave number (or the lowest order mode in the case of a multimode waveguide). Select the period Λ and the angle θ of the diffraction grating so that the angle between the diffraction grating and the core optical axis has a very small distribution (Δθ) along the optical axis. By making it bend, it is possible to operate the optical isolator with a wide wavelength width or a wide wavenumber width (multimode) by absorbing fluctuations in the wavelength of incident light and dispersion of wavenumbers.
This is because the boundary condition of the optical waveguide is not axially symmetric, and even if the propagation wave number is slightly different between the TE wave and TM wave transmitted through the core, the angles formed by the optical axis and the diffraction grating are distributed. Therefore, both are diffracted at the curvature position of the core portion where the matching condition is satisfied, which means that polarization-independent characteristics can be obtained.
[0017]
In the second embodiment, an optical semiconductor can be used as a material to be used. In this case, an optical isolator monolithically integrated with a semiconductor laser is realized by using a known selective growth method for crystal growth technology. be able to.
[0018]
In the above-described two embodiments, the diffraction matching condition is explained under the condition that the diffraction is radiated to the clad portion having a low refractive index in the examples. However, the diffracted light is radiated to the clad having the higher refractive index. You may choose the conditions to do.
[0019]
In the above description, one of the two clads having different refractive indexes is air.
It is sufficient that the refractive index is smaller than that of the other cladding. For example, if the other clad is quartz, it may be covered with a fluorinated resin having a smaller refractive index.
[0020]
In addition, although the shape of the core portion in the light propagation direction is set to an S shape, the traveling direction of the incident light and the traveling direction of the emitted light are made the same to facilitate assembly and mounting of the element. As shown in the design principle, it is only necessary to have a part of a quadratic curve such as a simple arc or an elliptical arc, and it is not necessarily S-shaped.
[0021]
Further, the waveguide material used may be not only a quartz or glass material but also a plastic material such as a resin. In this case, the formation of the diffraction grating is not only the generation of a color center due to ultraviolet irradiation, A method can also be used in which the main chain of the polymer is broken by irradiation with synchrotron radiation having a short wavelength.
And polymer
[0022]
【The invention's effect】
As described above, the waveguide type optical isolator according to the present invention is configured such that the radiation direction of the diffracted light is the optical axis with respect to the incident light having the maximum wave number (or the lowest mode in the case of a multimode waveguide). The period Λ and angle θ of the diffraction grating are selected so as to be perpendicular to each other, and the period of the diffraction grating has a slight distribution (ΔK) along the optical axis to some extent by elastic deformation due to the curvature of the optical waveguide. Thus, it is possible to operate the optical isolator with a wide wavelength width or a wide wave number width (multi-mode) by absorbing fluctuations in the wavelength of incident light and wave number dispersion.
In addition, with respect to incident light having a wavelength with the maximum wave number (or the lowest mode if a multimode waveguide), the radiation direction of diffracted light at a specific point in the light propagation direction of the core portion of the waveguide Select the period Λ and angle θ of the diffraction grating so that is perpendicular to the optical axis, and set the angle between the diffraction grating and the core optical axis to a certain extent along the optical axis (Δθ). The optical isolator can be operated with a wide wavelength width or a wide wave number width (multimode) by absorbing the fluctuation of the wavelength of the incident light and the dispersion of the wave number by making the core part curved so as to have .
This makes it easy to construct and have excellent productivity without using materials or magnets that have nonreciprocal light effects such as conventional magneto-optical effects, and is suitable not only for single-mode optical fibers for transmission communication but also for high-power laser light transmission. An excellent optical isolator applicable to optical fibers and various multimode optical fibers can be realized.
Further, since no magneto-optic crystal is used, an optical isolator having a wide spectral range from ultraviolet light to infrared light can be realized.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of a first embodiment of a waveguide type optical isolator according to the present invention.
FIG. 2 is a diagram for explaining irreversible light transmission characteristics of a waveguide type optical isolator according to the present invention.
FIG. 3 is a diagram showing the configuration of parameters for explaining the operation of the first embodiment of the waveguide type optical isolator of the present invention.
FIG. 4 is a diagram for explaining the operation of the first embodiment of the waveguide type optical isolator of the present invention.
FIG. 5 is a diagram illustrating a configuration of a second embodiment of a waveguide type optical isolator according to the present invention.
FIG. 6 is a diagram for explaining the operation of the second embodiment of the waveguide type optical isolator of the present invention.
[Explanation of symbols]
1 Optical fiber
1a core
1b cladding
10 Right traveling light
12 Diffraction grating
20 Left traveling light
30 Synchrotron radiation
101 Waveguide type optical isolator
105 core
106 clad
107 air
110 Right traveling light
112 diffraction grating
120 Left traveling light
130 diffraction grating
140 Synchrotron radiation

Claims

A first clad portion having a refractive index different from that of a straight line drawn close to the core in a cross section perpendicular to the optical axis in a cross section perpendicular to the optical axis. And a second clad portion, wherein the core is an optical waveguide having a phase diffraction grating,
The wave vector of the phase diffraction grating is in a plane perpendicular to the boundary surface between the two cladding portions and along the optical axis,
The optical waveguide, a waveguide-type optical isolator and having a curved portion that is curved in front Symbol plane.

The magnitude of the wave number of the phase diffraction grating in the uncurved part of the optical waveguide is that the guided light guided to the core is diffracted by the phase diffraction grating and radiated to the first or second cladding part. The waveguide type optical isolator according to claim 1, wherein a radiation angle of the emitted light with respect to the optical axis is set to be approximately 90 degrees.

The optical waveguide is a single mode optical waveguide, and the wavelength of the guided light guided to the core is distributed from λ1 to λ2 (λ1 <λ2), and the first or second clad portion 3. A waveguide type optical isolator according to claim 2, wherein the wavelength of the guided light set so that the radiation angle of the emitted light with respect to the optical axis is approximately 90 degrees is .lambda.1.

The optical waveguide is a multimode optical waveguide, and the guided light guided to the core has a wave number distributed from k1 to k2 (k1> k2), and radiates to the first or second cladding portion. 3. The waveguide type optical isolator according to claim 2, wherein the wave number of the guided light set so that the radiation angle of the emitted light with respect to the optical axis is approximately 90 degrees is k1.

5. The waveguide type optical isolator according to claim 2, wherein the optical waveguide is an optical fiber, and the curve is formed by elastic deformation by applying stress to the optical fiber.

5. The optical waveguide according to claim 2, wherein the optical waveguide is an optical waveguide formed on a flat substrate, and the curvature is applied in a stress-free state by pattern formation when the optical waveguide is formed. Waveguide type optical isolator.

A first clad portion having a refractive index different from that of a straight line drawn close to the core in a cross section perpendicular to the optical axis in a cross section perpendicular to the optical axis. And the second clad part, the core is an optical waveguide having a phase diffraction grating, and a wave number vector of the phase diffraction grating is orthogonal to a boundary surface between the two clad parts and along the optical axis. and lies in a plane, the optical waveguide is a manufacturing method of waveguide-type optical isolator having a curved portion that is curved in front Symbol plane,
A method of manufacturing a waveguide type optical isolator, comprising: a step of forming the phase diffraction grating in the straight optical waveguide; and a step of imparting the curvature to the optical waveguide provided with the phase diffraction grating.

A first clad portion having a refractive index different from that of a straight line drawn close to the core in a cross section perpendicular to the optical axis in a cross section perpendicular to the optical axis. And the second clad part, the core is an optical waveguide having a phase diffraction grating, and a wave number vector of the phase diffraction grating is orthogonal to a boundary surface between the two clad parts and along the optical axis. and lies in a plane, the optical waveguide is a manufacturing method of waveguide-type optical isolator having a curved portion that is curved in front Symbol plane,
A method of manufacturing a waveguide type optical isolator, comprising: forming the curved optical waveguide; and forming the phase diffraction grating on the curved optical waveguide.

9. The waveguide type optical isolator according to claim 7, wherein the step of forming the phase diffraction grating includes a step of irradiating the optical waveguide with irradiation light spatially modulated with a single period. Method.

The magnitude of the wave number of the phase diffraction grating in the uncurved part of the optical waveguide is that the guided light guided to the core is diffracted by the phase diffraction grating and radiated to the first or second cladding part. 10. The method of manufacturing a waveguide type optical isolator according to claim 7, wherein a radiation angle of the emitted light with respect to the optical axis is set to be approximately 90 degrees.

The optical waveguide is a single mode optical waveguide, and the wavelength of the guided light guided to the core is distributed from λ1 to λ2 (λ1 <λ2), and the first or second clad portion 11. The waveguide type optical isolator according to claim 10, wherein the wavelength of the guided light set so that the radiation angle of the emitted light with respect to the optical axis is approximately 90 degrees is λ1. Method.

The optical waveguide is a multimode optical waveguide, and the guided light guided to the core has a wave number distributed from k1 to k2 (k1> k2), and radiates to the first or second cladding portion. 12. The method of manufacturing a waveguide type optical isolator according to claim 11, wherein the wave number of the guided light set so that the radiation angle of the emitted light with respect to the optical axis is approximately 90 degrees is k1. .

8. The method of manufacturing a waveguide type optical isolator according to claim 7, wherein the optical waveguide is an optical fiber, and the curve is formed by elastic deformation by applying stress to the optical fiber. .

9. The waveguide according to claim 8, wherein the optical waveguide is an optical waveguide formed on a flat substrate, and the curvature is applied in a stress-free state by pattern formation when the optical waveguide is formed. Type optical isolator manufacturing method.

The waveguide type optical isolator according to any one of claims 1 to 6 and the method for producing a waveguide type optical isolator according to claims 7 to 14, wherein the curved curve is a quadratic curve.