JP3883118B2 - Optical directional coupler and manufacturing method of optical directional coupler - Google Patents

Description

【０００８】
ここで、Ｌは直線導波路２２５，２２６の長さ、ｄＬは第１及び第２曲げ導波路２２３，２２４，２２７，２２８において生じる結合成分を直線部の長さに変換した結合長、Ｌ０は定数である。
ビーム伝搬法を用いた数値計算によれば、導波路パラメータとして、幅Ｗ＝７μｍ、厚さＨ＝６μｍ、Ｒ＝１０ｍｍ、比屈折率差Δ＝０．７５％、直線導波路間隔Ｇ＝３μｍにて、図９の構造の光方向性結合器ではＬ０＝３３４９μｍ，ｄＬ＝−６４６３μｍとなり、入力光が出力導波路２２９及び２３０に等分配される５０％結合長Ｌ３ｄＢ＝１４３９μｍが得られる。
また、その損失は０．０２ｄＢである。
【０００９】
前述の利得等化器フィルタでは、熱光学効果位相シフタの熱干渉を避けるため、入力導波路間隔Ｓ及び出力導波路間隔Ｓは２５０μｍ程度とする必要がある。本従来例における導波路パラメータを用いれば、Ｓ＝２５０μｍでは単位光方向性結合器の回路長は、その第１曲げ導波路の始端から第２曲げ導波路の終端までで５９００μｍとなる。
従って、利得等化器フィルタでは全回路長２７．９ｃｍのうち１１．８ｃｍを光方向性結合器が占める。
【００１０】
一方、光回路の導波路幅を狭くすることで光結合を強め、光方向性結合器を短長化することが可能である。
以下にビーム伝搬法による数値例を挙げて、従来手法を説明する。
導波路幅Ｗに対する５０％結合長Ｌ３ｄＢの関係、及び、その導波路幅Ｗにおける単独導波路の半径５ｍｍの９０度曲げ導波路の伝搬損失を図１１に示す。
導波路幅Ｗ以外の導波路パラメータは前出の従来例と同様である。
【００１１】
導波路幅を狭めることにより、直線導波路２２５，２２６での光電界の導波路コアからの染み出しが大きくなるため、光結合を生じる領域１での光結合を強めることができる。
図１０より、例えば、Ｗ＝３μｍではＬ３ｄＢ＝１０５μｍであるので、上述の条件（Ｓ＝２５０μｍ）では、光方向性結合器の第１曲げ導波路の始端から第２曲げ導波路の終端までの回路長は４５７０μｍへの短長化が可能である。
しかしながら、Ｗ＝３μｍの光導波路では同時に光方向性結合器以外の導波路部分での曲げ半径５ｍｍでの９０度曲げによる伝搬損失は２．５ｄＢまで増加する傾向が見て取れる。
【００１２】
【非特許文献１】
T． Matsuda et al ， ″Ultra−ｂroadband Raman−amplified transoceanic system with adaptive gain equalization″， the ４th International Convention on Undersea Communication （SubOptic 2001）， PDP4， Kyoto， May， 2001．
【００１３】
【発明が解決しようとする課題】
前述のように、従来の光方向性結合器は光導波路上において機能回路を構成するために必要不可欠な構成要素であるが、例えば前述の例においては、光方向性結合器の全長はおよそ６ｍｍにも及び、その短長化が切実な問題であった。
更に、光方向性結合器の短長化の方策のひとつである導波路幅の狭幅化を適用したとしても、光方向性結合器以外の領域での伝搬損失が極端に大きくなり、実用的な解ではなかった。
本発明は、光方向性結合器以外の領域での伝搬損失を増大させることなく、光方向性結合器の短長化を図ることを目的とする。
【００１４】
【課題を解決するための手段】
本発明において開示される発明の概要を簡単に説明すれば次の通りである。
本発明における光方向性結合器は、基板上に形成された光導波路により構成され、光方向性結合器の結合領域以外の領域においては、孤立導波路としての伝搬損失が十分無視できるほど小さい。
【００１５】
加えて光方向性結合器は、２本の入力導波路が、光結合が生じない程度十分離れた間隔から第１曲げ導波路を介して徐々に接近し、直線導波路において一定間隔を保ちつつ平行に延在し、再び第２曲げ導波路を介して光結合が生じない位置まで離遠する構造を有し、前記第１曲げ導波路及び前記第２曲げ導波路において導波路パラメータが断熱的に変化することで、その光結合を生じる領域においては導波路Ｖ値が、光結合を生じない領域における導波路Ｖ値よりも小さく、かつ光結合を生じる領域において導波路は単一モードであり、前記第１曲げ導波路を構成する２本の導波路におけるＶ値の伝搬方向変化率は等しく、また、前記第２曲げ導波路を構成する２本の導波路におけるＶ値の伝搬方向変化率は等しく、前記光結合を生じる領域におけるＶ値は１以上かつ零次ベッセル関数の第１の零点未満である光学的特性を有する。
光結合を生じる領域における最小Ｖ値を１以上とすることで、光方向性結合器自体の放射損失増加も防ぐことができる。
【００１６】
このような光方向性結合器は、光結合を生じない領域から光結合を生じる領域にかけて、導波路の屈折率を断熱的に小さくすることにより実現される。
この場合、光結合を生じる領域におけるＶ値が光結合を生じない領域におけるＶ値に比べて小さく設定される。
ここで用いた「断熱的」とは、光伝搬の解析において用いられる″adiabatic″を意味し、熱力学で用いられる「断熱変化」を意味するものではない。
また、前記入力導波路と前記第１曲げ導波路の接点、前記出力導波路と前記第２曲げ導波路の接点、前記直線導波路と前記第１曲げ導波路の接点、前記直線導波路と前記第２曲げ導波路の接点、及び前記第１、第２曲げ導波路において導波路屈折率の連続性を保ちつつ、導波路屈折率を十分滑らかに変化させることで、当該光方向性結合器の伝搬損失の増加を回避できる。
さらに、入力導波路における屈折率変化ｎ ( ｘ ) が、直線導波路における屈折率ｎ１、光結合を生じない領域における屈折率ｎ２、入力導波路と第１曲げ導波路の接点の伝搬方向位置ｘ１、直線導波路と第１曲げ導波路の接点との境界の伝搬方向位置ｘ２について、
【００１７】
【数４】

【００１８】
を満足する構成としてもよい。
【００１９】
このような光方向性結合器の構造は、基板上に形成された光導波路において、前記第１曲げ導波路、前記第２曲げ導波路に対して導波路屈折率を調整可能なレーザを照射して、前記入力導波路から前記直線導波路にかけて、及び前記出力導波路から前記直線導波路にかけて、導波路屈折率を徐々に断熱的に変化させることにより作製される。
また、前記第１曲げ導波路、前記第２曲げ導波路の屈折率の変化率は、光方向性結合器に対する導波路屈折率を調整可能なレーザ光の照射位置を適切な速度で移動させることにより実現される。
【００２０】
上記方法に加え、光結合を生じない領域から光結合を生じる領域にかけて、前記第１曲げ導波路及び前記第２曲げ導波路の幅を断熱的に狭くすることによっても、実現可能である。
この場合、光結合を生じる領域における横方向Ｖ値が光結合を生じない領域における横方向Ｖ値に比べて小さく設定される。
また、前記入力導波路と前記第１曲げ導波路の接点、前記出力導波路と前記第２曲げ導波路の接点、前記直線導波路と前記第１曲げ導波路の接点、前記直線導波路と前記第２曲げ導波路の接点、及び前記第１、第２曲げ導波路において導波路屈折率の連続性を保ちつつ、導波路幅を十分滑らかに変化させることで、当該光方向性結合器の伝搬損失の増加を回避できる。
また、第１曲げ導波路における導波路幅変化Ｗ ( ｘ ) が、光結合を生じない領域における導波路幅Ｗ１、光結合を生じる領域における導波路幅Ｗ２、第１曲げ導波路の光結合を生じない領域との接点の伝搬方向位置ｘ１、第１曲げ導波路の光結合を生じる領域との接点 の伝搬方向位置ｘ２について、
【００２１】
【数５】

【００２２】
を満足する構成としてもよい。
【００２３】
上記方法に加え、光結合を生じない領域から光結合を生じる領域にかけて、前記第１曲げ導波路及び前記第２曲げ導波路の厚さを断熱的に薄くすることにより、光結合を生じる領域における縦方向Ｖ値が光結合を生じない領域における縦方向Ｖ値に比べて小さくすることができる。
【００２４】
更に、その光方向性結合器の構造は、下部クラッド層を成膜する工程とコア膜を成膜する工程と導波路コアパターンを形成する工程と、上部クラッドを形成する工程を及び、前記コアパターンを形成する工程と上部クラッド層を成膜する工程の間に、前記第１曲げ導波路の前記直線導波路側の領域、前記第２曲げ導波路の前記直線導波路側の領域、及び前記直線導波路全体を除く基板の上部に、適切な高さにシャドウマスクを配置することで、前記第１曲げ導波路の前記直線導波路側の領域、前記第２曲げ導波路の前記直線導波路側の領域、及び前記直線導波路全体を、異方性エッチングすることで、前記入力導波路から前記直線導波路にかけて、及び前記出力導波路から前記直線導波路にかけて、導波路厚を徐々に断熱的に変化させる工程を有する作製方法により作製される。
【００２５】
本発明の光方向性結合器では、光結合を生じない領域に比べて光結合を生じる領域でのＶ値は小さい、即ち導波路コアからの光電界の染み出し成分は大きくなる。
従って、光結合を生じる領域での２本の導波路のモードの重なりは従来型の光方向性結合器に比べて大きくなる。
【００２６】
即ち、光結合を生じる領域における２本の導波路間の光結合も強められ、光方向性結合器の短長化が図られる。
加えて、本光方向性結合器では、光方向性結合器以外の回路要素において損失の増大を引き起こすことがない。
これは、光方向性結合器の曲げ導波路部にモード変換器を内包しているので、光結合を生じない領域及び当該光方向性結合器以外の回路要素においては大きなＶ値の確保が可能、即ち、導波路コアに対して強い光電界の閉じ込めを維持できるからである。
【００２７】
また、光方向性結合器自体においても、その曲げ導波路部のモード変換器は光結合を生じない領域と光結合を生じる領域の間で断熱的に、即ち導波路の閉じ込めを徐々に変化させることでモード変換を行うので、光方向性結合器においても過剰な損失を生じない。
以上のように、本発明によれば、その他の回路要素での伝搬損失増大という光学特性の劣化を引き起こすことなく、光方向性結合器の短長化が可能となる。
【００２８】
【発明の実施の形態】
以下に図面を参照して本発明の実施の形態を詳細に説明する。
なお、発明の実施の形態を説明するための全図において、同一機能を有するものは同一符号をつけ、その繰り返しの説明は省略する。
以下の例では石英系光導波路における光方向性結合器について説明する。
【００２９】
〔実施例１〕
本発明の第１の実施例に係る光方向性結合器の概略図及びその導波路コアの屈折率の分布を図１に示す。
本実施例の光方向性結合器は、入力導波路３２１及び３２２、第１曲げ導波路３２３及び３２４、直線導波路３２５及び３２６、第２曲げ導波路３２７及び３２８、出力導波路３２９及び３３０からなる。
【００３０】
距離Ｓだけ離れた位置にある導波路幅Ｗの入力導波路３２１及び３２２は、第１曲げ導波路３２３及び３２４を介して直線導波路３２５及び３２６に接続される。
更に、直線導波路３２５及び３２６は第２曲げ導波路３２７及び３２８を介して、距離Ｓだけ離れた位置における出力導波路３２９及び３３０に接続される。直線導波路３２５及び３３６は間隔Ｇを隔てて、長さＬに渡って平行に延在する。
【００３１】
また、第１曲げ導波路３２３及び３２４は、それぞれ入力導波路３２１及び３２２から出力導波路３２５及び３２６にかけて、その屈折率がｎ２からｎ１へと変化する。
即ち、入力導波路３２１及び３２２と第１曲げ導波路３２３及び３２４の境界Ａから第１曲げ導波路３２３及び３２４と直線導波路３２５及び３２６の境界Ｂにかけて、導波路の屈折率はｎ２からｎ１までに小さくなる。
【００３２】
一方、直線導波路３２５及び３２６と第２曲げ導波路３２７及び３２８の境界Ｃから第２曲げ導波路３２７及び３２８と出力導波路３２９と３３０の境界Ｄにかけて、導波路の屈折率はｎ１からｎ２まで大きくなる。
また、入力導波路３２１及び３２２と出力導波路３２９及び３３０は一様なｎ２の屈折率を有し、直線導波路３２５及び３２６は一様な屈折率ｎ１を有する。
【００３３】
上述の構造を有する光方向性結合器をＷ＝５μｍ，コア厚みＨ＝５μｍ，Ｇ＝２μｍ，Ｒ＝１０ｍｍ，Ｓ＝２５０μｍ，ｎ２＝１．４８０４，ｎ１＝１．４６９２，クラッド屈折率ｎ０＝１．４５８２として作製した。
作製した光方向性結合器の結合率と直線導波路３２５及び３２６の長さＬの関係を図２に示す。
前記第１曲げ導波路３２３，３２４における屈折率変化は、境界Ａ及び境界Ｂでの連続性を保つため、下式（２）とした。
【００３４】
【数６】

【００３５】
ここで、ｘ２は境界Ａにおける伝搬方向位置，ｘ１は境界Ｂにおける伝搬方向位置である。
更に、ここでは、第１曲げ導波路３２３，３２４にともに式（２）を適用した。即ち、第１曲げ導波路３２３及び第１曲げ導波路３２４におけるＶ値の伝搬方向変化率は等しく設定されることになる。
前記第２曲げ導波路３２７，３２８についても、境界Ｃ及び境界Ｄでの連続性を保つように設定した。
【００３６】
上記パラメータにおけるＶ値は、同面積のコアを有する光ファイバのＶ値に換算して、光結合を生じない領域２ではＶ２＝２．７４、光結合を生じる領域１ではＶ１＝１．９３である。
５０％結合長はＬ３ｄＢ＝１２８μｍ、光方向性結合器単体の損失は０．０５ｄＢであった。
また、ビーム伝搬法による計算によれば、光方向性結合器以外の領域における、半径２ｍｍの９０度曲げの伝搬損失は、わずか０．１ｄＢであった。
更に、本パラメータでは、第１曲げ導波路の始端から第２曲げ導波路の終端までの全長は４．６ｍｍであった。
【００３７】
本実施例の光方向性結合器は以下の方法により作製した。
本実施例における光方向性結合器の作製方法の概略を図３に示す。
まず、火炎堆積法及び反応性イオンエッチングによる一般的な石英系光導波路の作製法に基づいて、光方向性結合器３３９を作製した。
このとき、導波路パラメータは導波路屈折率をｎ２＝ｎ１＝１．４６０６とした以外は、前出のパラメータと同様となるように作製した。
【００３８】
次に、波長１９３ｎｍのＡｒＦエキシマレーザ３３５からのレーザ光３３８を１００％ミラー３３６及びレンズ３３７を介して光方向性結合器３３９へ垂直に照射した。
ここで、レンズ３３７を調整しレーザ光３３８を導波路コア３３３にて結像させ、導波路コア３３３での光強度を２６０ｍＪ／ｃｍ２／ｐｕ１ｓｅとした。
レーザパルスの繰り返しは８０ｐｐｓであった。
レーザ光照射にあたっては、レーザ光の結像位置が、入力導波路３２１，３２２から出力導波路３２９，３３０へ向かって移動するように、光方向性結合器３３９を図４に示す速度で矢印Ａの方向へ移動させた。
【００３９】
即ち、レーザ光３３８が入力導波路３２１，３２２に照射されているときは、一定速度ｖ２で、第１曲げ導波路３２３，３２４に照射されているときは式（２）と逆の特性の速度で、直線導波路３２５，３２６に照射されているときは一定速度ｖ１で、第２曲げ導波路に照射されているときには式（２）と逆の特性の速度で、更に出力導波路３２９，３３０に照射されているときには一定速度ｖ２にて、光方向性結合器３３９を移動させた。
ここで、移動速度はｖ２＝０．２ｍｍ／ｈ，ｖ１＝２ｍｍ／ｈとした。
本設定により、導波路の初期屈折率１．４６０６を、光結合を生じない領域２での最大屈折率ｎ２＝１．４８０４へと、光結合を生じる領域１での最小屈折率ｎ１＝１．４６９２へと変化させた。
【００４０】
本実施例では、導波路屈折率を調整可能なレーザとしてＡｒＦエキシマレーザを用いたが、例えばＫｒＦエキシマレーザやＣＯ２レーザなどの屈折率変化を誘起可能な光源であれば、光源の種類によらず本発明が適用可能であることは明らかである。
また、石英系光導波路の作製法として火炎堆積法及び反応性イオンエッチングによる方法を用いたが、ＣＶＤ法などの気相堆積法やスピンオングラス（ＳＯＧ）のようなゾルゲル法による堆積方法及びイオンミリングなどのエッチング方法を用いても本発明は実現可能であり、光導波路の作製方法によらないことも明らかである。
【００４１】
〔実施例２〕
本発明の第２の実施例に係る光方向性結合器の概略図を図５に示す。
本実施例の光方向性結合器は、２つの入力導波路１０１及び１０２、第１曲げ導波路１０３及び１０４、直線導波路１０５及び１０６、第２曲げ導波路１０７及び１０８、出力導波路１０９及び１１０からなる。
距離Ｓだけ離れた位置にある導波路幅Ｗ１の入力導波路１０１及び１０２は、第１曲げ導波路１０３及び１０４を介して直線導波路１０５及び１０６へ接続される。
【００４２】
第１曲げ導波路１０３は、入力導波路１０１及び直線導波路１０５と連続的に接続されつつ、その導波路幅がＷ１からＷ２へと断熱的に狭くなる構造を有する。
同様に第１曲げ導波路１０４は、入力導波路１０２及び直線導波路１０６と連続的に接続されつつ、その導波路幅がＷ１からＷ２へと断熱的に狭くなる構造を有する。
更に、直線導波路１０５及び１０６は長さＬｌに渡って平行に延在した後、第２曲げ導波路１０７及び１０８を経由して出力導波路１０９及び１１０へと接続される。
【００４３】
ここで、第２曲げ導波路１０７及び１０８は第１曲げ導波路１０３及び１０４と線対称の構造を有する。
即ち、第２曲げ導波路１０７は、直線導波路１０５から出力導波路１０９に向かってその幅を断熱的にＷ２からＷ１まで広げる。
また、第２曲げ導波路１０８は直線導波路１０６から出力導波路１１０に向かって、第１曲げ導波路１０３（１０４）と対称に、その幅を断熱的にＷ２からＷ１まで広げる。
【００４４】
第１、第２曲げ導波路では入力導波路１０１，１０２側、及び出力導波路１０９，１１０側における横方向Ｖ値に比べて、直線導波路１０５，１０６側での横方向Ｖ値は小さくなる。
即ち、後者の領域では光電界の染み出し成分を大きく設定することができ、同領域で大きな光結合を実現することができる。
本実施例では、Ｗ１＝７μｍ，Ｗ２＝３μｍ，Ｓ＝２５０μｍ，第１曲げ導波路１０３，１０４及び第２曲げ導波路１０５及び１０６の曲げ半径Ｒ＝１０ｍｍ，導波路厚さＨ＝６μｍ，導波路の比屈折率差Δ＝０．７５％として石英系光導波路による光方向性結合器を作製した。
【００４５】
作製にあたっては、火炎堆積法、及び反応性イオンエッチングによる一般的な作製工程を採用した。
上記パラメータでは光結合を生じない領域２における横方向Ｖ値はＶ２＝２．５２であり、光結合を生じる領域１における横方向Ｖ値はＶ１＝１．０８である。
また、本実施例においては第１曲げ導波路１０３，１０４における導波路幅の変化は下式（３）に従った。
【００４６】
【数７】

【００４７】
導波路幅変化の概略を図５の下部に示す。
ここで、ｘ２は入力導波路１０１，１０２と第１曲げ導波路１０３，１０４の接点Ａの伝搬方向の位置であり、ｘ１は第１曲げ導波路１０３，１０４と直線導波路１０５，１０６の接点Ｂの伝搬方向の位置である。
また、第２曲げ導波路１０７，１０８に対しても、式（３）と同様の特性を有する導波路幅変化を与えた。
【００４８】
本実施例における光方向性結合器の結合率の直線部結合長Ｌ１依存性を図６に示す。
５０％結合長は１１５μｍであった。
これは従来の光方向性結合器の５０％結合長である１４３９μｍに比較して１／１０以下の長さである。
また、第１曲げ導波路の始端から第２曲げ導波路の終端までの全長は４．６ｍｍてあり、従来の光歩行性結合器に比較しておよそ３／４に短長化された。
また、本光方向性結合器単体の損失は、０．０４ｄＢであった。
これは、従来の光方向性結合器の損失０．０２ｄＢに比べて０．０２ｄＢの増加が見られるのみである。
【００４９】
〔実施例３〕
本発明の第３の実施例に係る光方向性結合器の概略について上面図及び側面図を図７に示す。
本実施例の光方向性結合器は、入力導波路１５１及び１５２、第１曲げ導波路１５３及び１５４、直線導波路１５５及び１５６、第２曲げ導波路１５７及び１５８、出力導波路１５９及び１６０からなる。
距離Ｓ１だけ離れた位置にある導波路幅Ｗの入力導波路１５１及び１５２は、第１曲げ導波路１５３及び１５４を介して直線導波路１５５及び１５６に接続される。
【００５０】
更に、直線導波路１５５及び１５６は第２曲げ導波路１５７及び１５８を介して距離Ｓ２だけ離れた位置にある出力導波路１５９及び１６０に接続される。
本実施例における光方向性結合器では、図７側面図に示すように、第１曲げ導波路１５３及び１５４、第２曲げ導波路１５７及び１５８に、それぞれ、入力導波路１５１及び１５２から、出力導波路１５９及び１６０から直線導波路１５５及び１５６に向かって、断熱的にその厚みがＨ１からＨ２となる構造を有する。
【００５１】
この構造においては、第１、第２曲げ導波路では入力導波路１５１，１５２側、及び出力導波路１５９，１６０側における縦方向Ｖ値に比べて、直線導波路１５５，１５６側での縦方向Ｖ値は小さく設定できる。
即ち、後者の領域では光電界の染み出し成分を大きく設定することができ、同領域で大きな光結合を実現することができる。
【００５２】
上記構造を有する光方向性結合器をＷ＝６μｍ，Ｈ１＝６μｍ，Ｈ２＝３μｍ，Ｓ１＝４００μｍ，Ｓ２＝２５０μｍ、導波路の比屈折率差Δ＝０．７５％の石英系光導波路により作製した。
本実施例の光方向性結合器の結合比と直線導波路長Ｌの関係を図８に示す。
５０％結合長はＬ３ｄＢ＝１４０μｍであり、第１曲げ導波路の始端から第２曲げ導波路の終端までの全長は５．５ｍｍであった。
また、９０度曲げ導波路の損失も０．０２ｄＢと非常に低い値が得られた。
【００５３】
本実施例の光方向性結合器は、図９に示す方法により作製した。
まず、図９（ａ）に示すように、工程１として、基板１６１上に、石英系ガラス微粒子を火炎堆積法により堆積した後、１０００℃以上の高温にて溶融固化することで、厚さ３０μｍの下部クラッド層１６２を成膜した。
更に、図９（ｂ）に示すように、工程２として、同様にゲルマニウムを添加した石英系ガラス微粒子を火炎堆積法により堆積した後、１０００℃以上の高温にて溶融固化し、厚さ６μｍのコア層１６３を成膜した。
本工程２においては、ゲルマニウムの添加量を適切に調整することで、導波路の比屈折率差Δを０．７５％に設定した。
【００５４】
引き続き、図９（ｃ）に示すように、工程３として、反応性イオンエッチングにより導波路パターン１６４を形成した。
本工程３により、入力導波路１５１，１５２、第１曲げ導波路１５３，１５４、直線導波路１５５，１５６、第２曲げ導波路１５７，１５８及び出力導波路１５９，１６０を形成した。
更に、図９（ｄ）に示すように、工程４として、長さＬｓの窓を有するシャドウマスク１６５を導波路パターン１６４の上面から距離Ｄｓ＝１．２ｍｍの位置に配置した後、工程５において再び反応性イオンエッチングにより、導波路パターン表面を異方性エッチングした。
ここで、シャドウマスク１６５はパイレックスガラスによるものを用いた。
【００５５】
その後、図９（ｅ）に示すように、工程５として、シャドウマスクの窓の直下のコア厚が３μｍとなるようにエッチング時間を調整した。
最後に、図９（ｆ）に示すように、工程６として、工程１，２と同様に火炎堆積法で石英系ガラス微粒子を堆積し、１０００℃以上の高温にて溶融固化することで、厚さ３０μｍの上部クラッド層１６６を成膜した。
本実施例では、石英系光導波路による実現方法に基づいて説明したが、導波路材料及びその成腹方法には拠らず、例えばインジウムリン化合物などの半導体導波路やポリマー導波路を用いても実現可能であり、更にその作製方法はＣＶＤ法などの気相堆積法によっても可能である。
【００５６】
このように説明したように、本発明は、光方向性結合器を構成している光導波路のＶ値を断熱的に（連続的に滑らかに）変化させ、光方向性結合器の長さを短く出来るものである。
Ｖ値の変化方法としては、屈折率を変化させる方法と、コア幅を変化させる方法、コアの高さを変化させる方法、及びその組み合わせがある。
本発明を用いることにより、結合比５０％の通常の光方向性結合器の場合でも、任意の結合比のものでも方向性結合器全体の長さを短くする事が可能である。
【００５７】
なお、実施例１では導波路の屈折率変化、実施例２では導波路のコア幅変化、実施例３では導波路のコア厚さ変化の例を示したが、これらＶ値を変化させる方法は単独に用いても良いし、適宜組み合わせても用いることもできる。
例えば、「コア幅と高さ」の両方とも変化させる、「コア幅と屈折率」の両方とも変化させる、「コア高さと屈折率」の両方とも変化させる、「コア幅と高さと屈折率」の全てを適宜変化させることも本発明の範囲である。
これらの変化方法を適宜組み合わせて用いれば、Ｖ値の変化を大きくすることができ、光回路全体の規模を小型化することができる。
また、小型化することにより基板１枚あたりの光回路数を多く取ることができ、価格的にも歩留まり的にも効果がある。
【００５８】
【発明の効果】
以上、実施例に基づいて具体的に説明したように、本発明によれば、光方向性結合器以外の導波路領域での損失の増加及び光方向性結合器自体の損失の増加を招くことなく、光方向性結合器の短長化が可能となる。
【図面の簡単な説明】
【図１】 本発明の第１の実施例における光方向性結合器を示す概略図及びその屈折率分布を示すグラフである。
【図２】 本発明の第１の実施例における光方向性結合器の結合比と直線導波路長の関係を示すグラフである。
【図３】 本発明の第１の実施例の光方向性結合器の作製方法を示す説明図である。
【図４】 本発明の第１の実施例の光方向性結合器の作製方法におけるレーザ光の光方向性結合器への照射位置と照射位置の移動速度の関係を示すグラフである。
【図５】 本発明の第２の実施例における光方向性結合器を示す概略図及びその屈折率分布を示すグラフである。
【図６】 本発明の第２の実施例における光方向性結合器の結合比と直線導波路長の関係を示すグラフである。
【図７】 本発明の第３の実施例における光方向性結合器を示す概略図及び側面図である。
【図８】 本発明の第３の実施例の光方向性結合器の結合比と直線導波路長Ｌの関係を示す図である。
【図９】 本発明の第３の実施例の光方向性結合器の作製方法を示す工程図である。
【図１０】 従来の光方向性結合器を示す概略図である。
【図１１】 従来の光方向性結合器の５０％結合長と導波路幅の関係、及び９０度曲げ導波路の伝搬損失と導波路幅の関係を示すグラフである。
【符号の説明】
１０１，１０２，１５１，１５２，２２１，２２２，３２１，３２２ 入力導波路
１０３，１０４，１５３，１５４，２２３，２２４，３２３，３２４ 第１曲げ導波路
１０５，１０６，１５５，１５６，２２５，２２６，３２５，３２６ 直線導波路
１０７，１０８，１５７，１５８，２２７，２２８，３２７，３２８ 第２曲げ導波路
１０９，１１０，１５９，１６０，２２９，２３０，３２９，３３０ 出力導波路
１６１ 基板
１６２ 下部クラッド層
１６３ コア層
１６４ 導波路パターン
１６５ シャドウマスク
１６６ 上部クラッド層[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an optical directional coupler and a method for manufacturing the optical directional coupler. Specifically, the present invention relates to an optical directional coupler that multiplexes and demultiplexes optical power in a waveguide optical metrology product used for optical fiber communication.
[0002]
[Prior art]
  Due to the rapid development of optical communication technology, various optical components have been researched and developed. Among them, the waveguide type optical component based on the optical waveguide on the flat substrate occupies the most important position.
  This is because the waveguide-type optical component has a feature that it can be mass-produced with good reproducibility with accuracy below the light wavelength by photolithography technology and microfabrication technology.
  In particular, in recent years, waveguide-type optical components have been increased in scale and complexity.
As the waveguide scale becomes larger and more complicated, space saving per unit chip is eagerly desired in order to improve mass productivity.
[0003]
  By the way, in such a waveguide type optical component, the optical directional coupler is the most basic circuit element.
  For example, a large-scale gain equalizer using an optical waveguide used in a long-distance transmission experiment has 20 50% coupled optical directional couplers and 5 sets of delay lines in order to realize desired optical characteristics. It is configured by cascade connection, and its total circuit length reaches 27.9 cm (see Non-Patent Document 1).
[0004]
  Hereinafter, an outline of a conventional optical directional coupler used in the above-described document will be described with reference to the drawings.
  A schematic diagram of a conventional optical directional coupler is shown in FIG.
  The conventional optical directional coupler includes two input waveguides 221 and 222, first bending waveguides 223 and 224, straight waveguides 225 and 226, second bending waveguides 227 and 228, and two output light beams. It consists of waveguides 229 and 230.
[0005]
  Two input waveguides 221 and 222 having a distance S at which optical coupling does not occur are connected to two straight waveguides 225 and 226 via first bending waveguides 223 and 224, respectively.
  Similarly, the two straight waveguides 225 and 226 are connected to the two input waveguides 229 and 230 that are spaced apart from each other through the second bending waveguides 227 and 228, respectively, so that no optical coupling occurs. The
[0006]
  In the optical directional coupler, for example, the light wave incident from the input waveguide 221 is optically coupled to the other optical waveguide in the region 1 where the optical coupling occurs via the first bending waveguide 223. And output to the output waveguides 229 and 230 via the second bending waveguides 227 and 228.
  In general, the directional coupler coupling ratio C.I. R. Is given by the following equation (1).
[0007]
【number3]

[0008]
  Here, L is the length of the straight waveguides 225 and 226, dL is the coupling length obtained by converting the coupling component generated in the first and second bending waveguides 223, 224, 227, and 228 into the length of the straight portion, and L0 is It is a constant.
  According to the numerical calculation using the beam propagation method, the waveguide parameters are as follows: width W = 7 μm, thickness H = 6 μm, R = 10 mm, relative refractive index difference Δ = 0.75%, linear waveguide spacing G = 3 μm. In the optical directional coupler having the structure shown in FIG. 9, L0 = 3349 μm and dL = −6463 μm, and 50% coupling length L3dB = 1439 μm is obtained in which the input light is equally distributed to the output waveguides 229 and 230.
  The loss is 0.02 dB.
[0009]
  In the aforementioned gain equalizer filter, the input waveguide interval S and the output waveguide interval S need to be about 250 μm in order to avoid thermal interference of the thermo-optic effect phase shifter. Using the waveguide parameter in this conventional example, when S = 250 μm, the circuit length of the unit optical directional coupler is 5900 μm from the start end of the first bending waveguide to the end of the second bending waveguide.
  Therefore, in the gain equalizer filter, the optical directional coupler occupies 11.8 cm out of the total circuit length of 27.9 cm.
[0010]
  On the other hand, by reducing the waveguide width of the optical circuit, the optical coupling can be strengthened and the optical directional coupler can be shortened.
  The conventional method will be described below by giving numerical examples based on the beam propagation method.
  FIG. 11 shows the relationship of the 50% coupling length L3 dB to the waveguide width W and the propagation loss of a 90-degree bent waveguide having a radius of 5 mm of the single waveguide in the waveguide width W.
  Waveguide parameters other than the waveguide width W are the same as in the conventional example described above.
[0011]
  By narrowing the waveguide width, the optical field in the straight waveguides 225 and 226 increases from the waveguide core, so that the optical coupling in the region 1 where optical coupling occurs can be strengthened.
  From FIG. 10, for example, when W = 3 μm, L3 dB = 105 μm. Therefore, under the above-described condition (S = 250 μm), from the start end of the first bending waveguide of the optical directional coupler to the end of the second bending waveguide. The circuit length can be shortened to 4570 μm.
  However, in the optical waveguide of W = 3 μm, it can be seen that the propagation loss due to the 90 ° bending at the bending radius of 5 mm in the waveguide portion other than the optical directional coupler tends to increase to 2.5 dB.
[0012]
[Non-Patent Document 1]
T. Matsuda et al, “Ultra-broadband Raman-amplified transoceanic system with adaptive gain equalization”, the 4th International Convention on Undersea Communication (SubOptic 2001), PDP4, Kyoto, May, 2001.
[0013]
[Problems to be solved by the invention]
  As described above, the conventional optical directional coupler is an indispensable component for configuring a functional circuit on the optical waveguide. For example, in the above-described example, the total length of the optical directional coupler is approximately 6 mm. However, shortening the length was a serious problem.
  Furthermore, even if narrowing of the waveguide width, which is one of the measures for shortening the optical directional coupler, is applied, the propagation loss in the region other than the optical directional coupler becomes extremely large, which is practical. It was not a good solution.
  An object of the present invention is to shorten the length of an optical directional coupler without increasing propagation loss in a region other than the optical directional coupler.
[0014]
[Means for Solving the Problems]
  The outline of the invention disclosed in the present invention will be briefly described as follows.
  The optical directional coupler according to the present invention is constituted by an optical waveguide formed on a substrate, and in a region other than the coupling region of the optical directional coupler, the propagation loss as an isolated waveguide is sufficiently small to be negligible.
[0015]
  In addition, the optical directional coupler allows the two input waveguides to gradually approach through the first bending waveguide from an interval sufficiently separated so that optical coupling does not occur, while maintaining a constant interval in the linear waveguide. It has a structure that extends in parallel and separates to a position where optical coupling does not occur again through the second bending waveguide, and the waveguide parameters are adiabatic in the first bending waveguide and the second bending waveguide. The waveguide V value in the region where the optical coupling occurs is smaller than the waveguide V value in the region where the optical coupling does not occur, and the waveguide is single mode in the region where the optical coupling occurs. The rate of change in the propagation direction of the V value in the two waveguides constituting the first bent waveguide is equal, and the rate of change in the propagation direction of the V value in the two waveguides constituting the second bent waveguide. Are equalIn the region where the optical coupling occurs, the V value is 1 or more and less than the first zero of the zero-order Bessel functionHas optical properties.
  By setting the minimum V value in the region where optical coupling occurs to 1 or more, an increase in radiation loss of the optical directional coupler itself can be prevented.
[0016]
  Such an optical directional coupler is realized by adiabaticly reducing the refractive index of the waveguide from a region where no optical coupling occurs to a region where optical coupling occurs.
  In this case, the V value in the region where optical coupling occurs is set smaller than the V value in the region where optical coupling does not occur.
  As used herein, “adiabatic” means “adiabatic” used in the analysis of light propagation, and does not mean “adiabatic change” used in thermodynamics.
  A contact point between the input waveguide and the first bending waveguide; a contact point between the output waveguide and the second bending waveguide; a contact point between the linear waveguide and the first bending waveguide; By changing the waveguide refractive index sufficiently smoothly while maintaining the continuity of the waveguide refractive index in the contact point of the second bending waveguide and the first and second bending waveguides, the optical directional coupler An increase in propagation loss can be avoided.
  Furthermore, the refractive index change n in the input waveguide ( x ) Are the refractive index n1 in the linear waveguide, the refractive index n2 in the region where no optical coupling occurs, the propagation direction position x1 of the contact between the input waveguide and the first bending waveguide, and the contact between the linear waveguide and the first bending waveguide. About the propagation direction position x2 of the boundary of
[0017]
[Expression 4]

0018]
It is good also as composition which satisfies.
0019]
  In such an optical directional coupler structure, in the optical waveguide formed on the substrate, the first bending waveguide and the second bending waveguide are irradiated with a laser whose waveguide refractive index can be adjusted. Thus, the refractive index of the waveguide is gradually adiabatically changed from the input waveguide to the linear waveguide and from the output waveguide to the linear waveguide.
  Also, the rate of change in the refractive index of the first bending waveguide and the second bending waveguide is to move the irradiation position of the laser beam capable of adjusting the waveguide refractive index with respect to the optical directional coupler at an appropriate speed. It is realized by.
0020]
  In addition to the above method, it can also be realized by adiabaticly reducing the width of the first bending waveguide and the second bending waveguide from a region where optical coupling does not occur to a region where optical coupling occurs.
  In this case, the lateral V value in the region where optical coupling occurs is set smaller than the lateral V value in the region where optical coupling does not occur.
  A contact point between the input waveguide and the first bending waveguide; a contact point between the output waveguide and the second bending waveguide; a contact point between the linear waveguide and the first bending waveguide; Propagation of the optical directional coupler by changing the waveguide width sufficiently smoothly while maintaining the continuity of the waveguide refractive index in the contact points of the second bending waveguide and the first and second bending waveguides. Increase in loss can be avoided.
  Further, the waveguide width change W in the first bending waveguide ( x ) Is the waveguide width W1 in the region where no optical coupling occurs, the waveguide width W2 in the region where optical coupling occurs, the propagation direction position x1 of the contact with the region where no optical coupling occurs in the first bending waveguide, the first bending guidance Contact point with the optical coupling region of the waveguide About the propagation direction position x2 of
0021]
[Equation 5]

0022]
It is good also as composition which satisfies.
0023]
  In addition to the above method, the thickness of the first bending waveguide and the second bending waveguide from the region where no optical coupling occurs to the region where optical coupling occursRefuseBy making it thermally thin, the vertical direction V value in the region where optical coupling occurs can be made smaller than the vertical direction V value in the region where optical coupling does not occur.
0024]
  Further, the structure of the optical directional coupler includes a step of forming a lower clad layer, a step of forming a core film, a step of forming a waveguide core pattern, and a step of forming an upper clad. Between the step of forming a pattern and the step of forming the upper cladding layer, the region of the first bent waveguide on the straight waveguide side, the region of the second bent waveguide on the straight waveguide side, and the An area on the linear waveguide side of the first bending waveguide and the linear waveguide of the second bending waveguide are arranged by arranging a shadow mask at an appropriate height above the substrate excluding the entire linear waveguide. The thickness of the waveguide is gradually insulated from the input waveguide to the linear waveguide and from the output waveguide to the linear waveguide by anisotropic etching of the entire region and the linear waveguide. Changing process It is manufactured according to the manufacturing method having.
0025]
  In the optical directional coupler of the present invention, the V value in the region where the optical coupling occurs is smaller than that in the region where the optical coupling does not occur, that is, the oozing component of the optical electric field from the waveguide core becomes large.
  Therefore, the overlap of the modes of the two waveguides in the region where the optical coupling occurs is larger than that of the conventional optical directional coupler.
0026]
  That is, the optical coupling between the two waveguides in the region where the optical coupling occurs is strengthened, and the optical directional coupler can be shortened.
  In addition, this optical directional coupler does not cause an increase in loss in circuit elements other than the optical directional coupler.
  This is because the mode converter is included in the bending waveguide portion of the optical directional coupler, so that a large V value can be secured in a region where no optical coupling occurs and in circuit elements other than the optical directional coupler. That is, confinement of a strong optical electric field with respect to the waveguide core can be maintained.
0027]
  Also in the optical directional coupler itself, the mode converter of the bent waveguide section adiabatically changes the region where no optical coupling occurs and the region where optical coupling occurs, that is, gradually changes the waveguide confinement. Mode conversion is performed to prevent excessive loss even in the optical directional coupler..
  Less thanAs described above, according to the present invention, it is possible to shorten the length of the optical directional coupler without causing deterioration of optical characteristics such as propagation loss increase in other circuit elements.
0028]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
  Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof is omitted.
  In the following example, an optical directional coupler in a silica-based optical waveguide will be described.
0029]
[Example 1]
  FIG. 1 shows a schematic diagram of the optical directional coupler according to the first embodiment of the present invention and the refractive index distribution of the waveguide core.
  The optical directional coupler of this embodiment includes input waveguides 321 and 322, first bending waveguides 323 and 324, straight waveguides 325 and 326, second bending waveguides 327 and 328, and output waveguides 329 and 330. Become.
0030]
  The input waveguides 321 and 322 having the waveguide width W, which are separated by a distance S, are connected to the straight waveguides 325 and 326 via the first bending waveguides 323 and 324, respectively.
  Further, the straight waveguides 325 and 326 are connected to the output waveguides 329 and 330 at positions separated by the distance S via the second bending waveguides 327 and 328. The straight waveguides 325 and 336 extend in parallel over the length L with a gap G therebetween.
0031]
  Further, the refractive indexes of the first bending waveguides 323 and 324 change from n2 to n1 from the input waveguides 321 and 322 to the output waveguides 325 and 326, respectively.
  That is, from the boundary A of the input waveguides 321 and 322 and the first bending waveguides 323 and 324 to the boundary B of the first bending waveguides 323 and 324 and the straight waveguides 325 and 326, the refractive index of the waveguide is n2 to n1. It gets smaller by
0032]
  On the other hand, from the boundary C of the straight waveguides 325 and 326 and the second bending waveguides 327 and 328 to the boundary D of the second bending waveguides 327 and 328 and the output waveguides 329 and 330, the refractive index of the waveguide is n1 to n2. Grows up.
  The input waveguides 321 and 322 and the output waveguides 329 and 330 have a uniform refractive index of n2, and the straight waveguides 325 and 326 have a uniform refractive index n1.
0033]
  The optical directional coupler having the above-described structure is W = 5 μm, core thickness H = 5 μm, G = 2 μm, R = 10 mm, S = 250 μm, n2 = 1.4804, n1 = 1.4692, cladding refractive index n0 = It was produced as 1.4582.
  FIG. 2 shows the relationship between the coupling rate of the produced optical directional coupler and the length L of the straight waveguides 325 and 326.
  The refractive index change in the first bending waveguides 323 and 324 is expressed by the following equation (2) in order to maintain continuity at the boundary A and the boundary B.
0034]
【number6]

0035]
  Here, x2 is a propagation direction position at the boundary A, and x1 is a propagation direction position at the boundary B.
  Further, here, the expression (2) is applied to both the first bending waveguides 323 and 324. That is, the propagation rate change rate of the V value in the first bending waveguide 323 and the first bending waveguide 324 is set equal.
  The second bending waveguides 327 and 328 were also set so as to maintain continuity at the boundary C and the boundary D.
0036]
  The V value in the above parameters is converted to the V value of an optical fiber having a core of the same area, V2 = 2.74 in the region 2 where no optical coupling occurs, and V1 = 1.93 in the region 1 where optical coupling occurs. is there.
  The 50% coupling length was L3 dB = 128 μm, and the loss of the optical directional coupler alone was 0.05 dB.
  Further, according to the calculation by the beam propagation method, the propagation loss of 90 ° bending with a radius of 2 mm in the region other than the optical directional coupler was only 0.1 dB.
  Furthermore, with this parameter, the total length from the start end of the first bending waveguide to the end of the second bending waveguide was 4.6 mm.
0037]
  The optical directional coupler of this example was manufactured by the following method.
  An outline of a method for manufacturing the optical directional coupler in this embodiment is shown in FIG.
  First, an optical directional coupler 339 was fabricated based on a general quartz-based optical waveguide fabrication method using flame deposition and reactive ion etching.
  At this time, the waveguide parameters were made to be the same as those described above except that the waveguide refractive index was set to n2 = n1 = 1.4606.
0038]
  Next, a laser beam 338 from an ArF excimer laser 335 having a wavelength of 193 nm was vertically irradiated onto the optical directional coupler 339 via a 100% mirror 336 and a lens 337.
  Here, the lens 337 was adjusted so that the laser beam 338 was imaged by the waveguide core 333, and the light intensity at the waveguide core 333 was 260 mJ / cm 2 / pu1se.
  The repetition of the laser pulse was 80 pps.
  In the laser light irradiation, the optical directional coupler 339 is moved at the speed shown in FIG. 4 by the arrow A so that the imaging position of the laser light moves from the input waveguides 321 and 322 toward the output waveguides 329 and 330. Moved in the direction of.
0039]
  That is, when the laser beam 338 is irradiated to the input waveguides 321 and 322, the speed is constant velocity v2, and when the laser beam 338 is irradiated to the first bending waveguides 323 and 324, the speed is opposite to the formula (2). Thus, when the linear waveguides 325 and 326 are irradiated, the output waveguides 329 and 330 are further driven at a constant velocity v1 and when the second bending waveguide is irradiated, the velocity is opposite to that of the equation (2). When the light is irradiated, the optical directional coupler 339 is moved at a constant speed v2.
  Here, the moving speed was set to v2 = 0.2 mm / h and v1 = 2 mm / h.
  With this setting, the initial refractive index 1.4606 of the waveguide is changed to the maximum refractive index n2 = 1.4804 in the region 2 where no optical coupling occurs, and the minimum refractive index n1 = 1. It was changed to 4692.
0040]
  In this embodiment, an ArF excimer laser is used as a laser capable of adjusting the refractive index of the waveguide. For example, any light source capable of inducing a refractive index change such as a KrF excimer laser or a CO2 laser can be used. It is clear that the present invention is applicable.
  In addition, although a flame deposition method and a reactive ion etching method were used as a method for producing a quartz optical waveguide, a vapor deposition method such as a CVD method, a deposition method using a sol-gel method such as spin-on-glass (SOG), and ion milling. It is obvious that the present invention can be realized even by using an etching method such as the above, and is not based on a method for manufacturing an optical waveguide.
0041]
[Example 2]
  FIG. 5 shows a schematic diagram of an optical directional coupler according to the second embodiment of the present invention.
  The optical directional coupler of this embodiment includes two input waveguides 101 and 102, first bending waveguides 103 and 104, straight waveguides 105 and 106, second bending waveguides 107 and 108, output waveguide 109 and 110.
  The input waveguides 101 and 102 having the waveguide width W1 that are separated by a distance S are connected to the straight waveguides 105 and 106 via the first bending waveguides 103 and 104, respectively.
0042]
  The first bending waveguide 103 has a structure in which the waveguide width is adiabatically narrowed from W1 to W2 while being continuously connected to the input waveguide 101 and the straight waveguide 105.
  Similarly, the first bending waveguide 104 has a structure in which the waveguide width is adiabatically narrowed from W1 to W2 while being continuously connected to the input waveguide 102 and the straight waveguide 106.
  Further, the linear waveguides 105 and 106 extend in parallel over the length Ll, and then are connected to the output waveguides 109 and 110 via the second bending waveguides 107 and 108.
0043]
  Here, the second bending waveguides 107 and 108 have a line-symmetric structure with the first bending waveguides 103 and 104.
  That is, the second bending waveguide 107 expands its width from the linear waveguide 105 to the output waveguide 109 in adiabatic manner from W2 to W1.
  In addition, the second bending waveguide 108 expands its width from W2 to W1 in an adiabatic manner, symmetrically with the first bending waveguide 103 (104), from the straight waveguide 106 to the output waveguide 110.
0044]
  In the first and second bending waveguides, the lateral V value on the straight waveguides 105 and 106 side is smaller than the lateral V value on the input waveguides 101 and 102 side and the output waveguides 109 and 110 side. .
  That is, in the latter region, the leakage component of the optical electric field can be set large, and a large optical coupling can be realized in the same region.
  In this embodiment, W1 = 7 μm, W2 = 3 μm, S = 250 μm, the bending radius R of the first bending waveguides 103 and 104 and the second bending waveguides 105 and 106, R = 10 μm, the waveguide thickness H = 6 μm, An optical directional coupler using a silica-based optical waveguide was manufactured with a relative refractive index difference Δ = 0.75% of the waveguide.
0045]
  In the production, a general production process using a flame deposition method and reactive ion etching was adopted.
  With the above parameters, the lateral V value in region 2 where no optical coupling occurs is V2 = 2.52, and the lateral V value in region 1 where optical coupling occurs is V1 = 1.08.
  In this embodiment, the change in the waveguide width in the first bending waveguides 103 and 104 follows the following equation (3).
0046]
【number7]

0047]
  An outline of the change in the waveguide width is shown in the lower part of FIG.
  Here, x2 is the position in the propagation direction of the contact A between the input waveguides 101 and 102 and the first bending waveguides 103 and 104, and x1 is the contact between the first bending waveguides 103 and 104 and the straight waveguides 105 and 106. B is the position in the propagation direction.
  In addition, a change in the waveguide width having the same characteristics as the expression (3) was given to the second bending waveguides 107 and 108.
0048]
  FIG. 6 shows the dependence of the coupling ratio of the optical directional coupler in this embodiment on the linear portion coupling length L1.
  The 50% bond length was 115 μm.
  This is 1/10 or less of the length of 1439 μm, which is the 50% coupling length of the conventional optical directional coupler.
  Further, the total length from the start end of the first bending waveguide to the end of the second bending waveguide was 4.6 mm, which was shortened to about 3/4 compared with the conventional optical walking coupler.
  The loss of the present optical directional coupler alone was 0.04 dB.
  This is only an increase of 0.02 dB compared to the loss of 0.02 dB of the conventional optical directional coupler.
0049]
Example 3
  FIG. 7 shows a top view and a side view of the outline of the optical directional coupler according to the third embodiment of the present invention.
  The optical directional coupler of this embodiment includes input waveguides 151 and 152, first bending waveguides 153 and 154, straight waveguides 155 and 156, second bending waveguides 157 and 158, and output waveguides 159 and 160. Become.
  The input waveguides 151 and 152 having the waveguide width W at positions separated by the distance S1 are connected to the straight waveguides 155 and 156 via the first bending waveguides 153 and 154.
0050]
  Further, the straight waveguides 155 and 156 are connected to the output waveguides 159 and 160 located at a distance S2 through the second bending waveguides 157 and 158, respectively.
  In the optical directional coupler in the present embodiment, as shown in the side view of FIG. 7, the first bending waveguides 153 and 154 and the second bending waveguides 157 and 158 are output from the input waveguides 151 and 152, respectively. From the waveguides 159 and 160 toward the straight waveguides 155 and 156, the thickness is adiabatically changed from H1 to H2.
0051]
  In this structure, in the first and second bending waveguides, the vertical direction on the straight waveguides 155 and 156 side is longer than the vertical V value on the input waveguides 151 and 152 side and on the output waveguides 159 and 160 side. The V value can be set small.
  That is, in the latter region, the leakage component of the optical electric field can be set large, and a large optical coupling can be realized in the same region.
0052]
  An optical directional coupler having the above structure is manufactured by a quartz optical waveguide having W = 6 μm, H1 = 6 μm, H2 = 3 μm, S1 = 400 μm, S2 = 250 μm, and a relative refractive index difference Δ = 0.75% of the waveguide. did.
  FIG. 8 shows the relationship between the coupling ratio of the optical directional coupler of this embodiment and the straight waveguide length L.
  The 50% coupling length was L3 dB = 140 μm, and the total length from the start end of the first bending waveguide to the end of the second bending waveguide was 5.5 mm.
  Also, the loss of the 90-degree bent waveguide was 0.02 dB, a very low value.
0053]
  The optical directional coupler of this example was manufactured by the method shown in FIG.
  First, as shown in FIG. 9A, as step 1, quartz glass particles are deposited on a substrate 161 by a flame deposition method, and then melted and solidified at a high temperature of 1000 ° C. or higher to obtain a thickness of 30 μm. The lower cladding layer 162 was formed.
  Further, as shown in FIG. 9B, as step 2, quartz-based glass particles added with germanium are similarly deposited by a flame deposition method, and then melted and solidified at a high temperature of 1000 ° C. or more to obtain a thickness of 6 μm. A core layer 163 was formed.
  In Step 2, the relative refractive index difference Δ of the waveguide was set to 0.75% by appropriately adjusting the amount of germanium added.
0054]
  Subsequently, as shown in FIG. 9C, as step 3, a waveguide pattern 164 was formed by reactive ion etching.
  By this step 3, input waveguides 151 and 152, first bending waveguides 153 and 154, straight waveguides 155 and 156, second bending waveguides 157 and 158, and output waveguides 159 and 160 are formed.
  Further, as shown in FIG. 9D, in step 4, after a shadow mask 165 having a window having a length Ls is arranged at a distance Ds = 1.2 mm from the upper surface of the waveguide pattern 164, in step 5, The waveguide pattern surface was anisotropically etched again by reactive ion etching.
  Here, the shadow mask 165 is made of Pyrex glass.
0055]
  Thereafter, as shown in FIG. 9E, in step 5, the etching time was adjusted so that the core thickness directly under the shadow mask window was 3 μm.
  Finally, as shown in FIG. 9 (f), as step 6, quartz glass fine particles are deposited by the flame deposition method as in steps 1 and 2, and melted and solidified at a high temperature of 1000 ° C. An upper cladding layer 166 having a thickness of 30 μm was formed.
  Although the present embodiment has been described based on a realization method using a silica-based optical waveguide, it does not depend on a waveguide material and its abdominal method, and for example, a semiconductor waveguide such as an indium phosphide compound or a polymer waveguide may be used. Further, it can be realized by a vapor deposition method such as a CVD method.
0056]
  As described above, the present invention changes the length of the optical directional coupler by adiabatically (continuously and smoothly) changing the V value of the optical waveguide constituting the optical directional coupler. It can be shortened.
  As a method of changing the V value, there are a method of changing the refractive index, a method of changing the core width, a method of changing the height of the core, and a combination thereof.
  By using the present invention, the length of the entire directional coupler can be shortened even in the case of a normal optical directional coupler having a coupling ratio of 50% or an arbitrary coupling ratio.
0057]
  In the first embodiment, the refractive index of the waveguide is changed, in the second embodiment, the core width of the waveguide is changed. In the third embodiment, the core thickness of the waveguide is changed. They may be used alone or in appropriate combinations.
  For example, change both "core width and height", change both "core width and refractive index", change both "core height and refractive index", "core width and height and refractive index" It is also within the scope of the present invention to change all of these as appropriate.
  If these changing methods are used in appropriate combination, the change of the V value can be increased, and the scale of the entire optical circuit can be reduced.
  Further, by downsizing, it is possible to increase the number of optical circuits per substrate, which is effective in terms of price and yield.
0058]
【The invention's effect】
  As described above based on the embodiments, according to the present invention, the loss in the waveguide region other than the optical directional coupler and the loss of the optical directional coupler itself are increased. Therefore, the optical directional coupler can be shortened.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an optical directional coupler in a first embodiment of the present invention and a graph showing its refractive index distribution.
FIG. 2 is a graph showing the relationship between the coupling ratio of the optical directional coupler and the straight waveguide length in the first embodiment of the present invention.
FIG. 3 is an explanatory diagram showing a method of manufacturing the optical directional coupler according to the first embodiment of the present invention.
FIG. 4 is a graph showing the relationship between the irradiation position of laser light on the optical directional coupler and the moving speed of the irradiation position in the method of manufacturing the optical directional coupler according to the first embodiment of the present invention.
FIG. 5 is a schematic view showing an optical directional coupler according to a second embodiment of the present invention and a graph showing its refractive index distribution.
FIG. 6 is a graph showing the relationship between the coupling ratio of the optical directional coupler and the straight waveguide length in the second embodiment of the present invention.
7A and 7B are a schematic view and a side view showing an optical directional coupler according to a third embodiment of the present invention.
FIG. 8 is a diagram showing the relationship between the coupling ratio of the optical directional coupler according to the third embodiment of the present invention and the straight waveguide length L;
FIG. 9 is a process diagram showing a method of manufacturing an optical directional coupler according to a third embodiment of the present invention.
FIG. 10 is a schematic view showing a conventional optical directional coupler.
FIG. 11 is a graph showing the relationship between 50% coupling length and waveguide width of a conventional optical directional coupler, and the relationship between propagation loss and waveguide width of a 90-degree bent waveguide.
[Explanation of symbols]
  101, 102, 151, 152, 221, 222, 321, 322 Input waveguide
  103,104,153,154,223,224,323,324 1st bending waveguide
  105, 106, 155, 156, 225, 226, 325, 326 linear waveguide
  107, 108, 157, 158, 227, 228, 327, 328 Second bending waveguide
  109, 110, 159, 160, 229, 230, 329, 330 Output waveguide
  161 substrate
  162 Lower cladding layer
  163 Core layer
  164 Waveguide pattern
  165 shadow mask
  166 Upper cladding layer

Claims (11)

The first bending waveguide is composed of an optical waveguide formed on a substrate, and two waveguides having such a small propagation loss as an isolated waveguide are separated from an input waveguide separated by an interval at which no optical coupling occurs. Through the second bending waveguide and again away from the output waveguide separated by an interval where no optical coupling occurs, and The two waveguides are optical directional couplers having a line-symmetric structure, and the optical coupling occurs when the waveguide V value changes adiabatically in the first bending waveguide and the second bending waveguide. In the region where the optical coupling occurs, the waveguide V value is smaller than the waveguide V value in the region where optical coupling does not occur, and the waveguide is a single mode waveguide in the region where optical coupling occurs, and the first bending is performed. 2 constituting the waveguide The propagation direction changing rate of the V values in the waveguide of equal and the propagation direction changing rate of the V values of the two waveguides constituting the second bend waveguide rather equal, in the region causing the optical coupling An optical directional coupler having a V value of 1 or more and less than a first zero of a zero-order Bessel function .   By reducing the refractive index of the first bending waveguide and the second bending waveguide in an adiabatic manner from the region where optical coupling does not occur to the region where optical coupling occurs, the V value in the region where optical coupling occurs is optically coupled. 2. The optical directional coupler according to claim 1, wherein the optical directional coupler is smaller than a V value in a region where no sag occurs.   The adiabatic change in the refractive index of the waveguide includes a contact point between the input waveguide and the first bending waveguide, a contact point between the output waveguide and the second bending waveguide, and the straight waveguide and the first bending waveguide. 3. The optical directional coupler according to claim 2, wherein the contact point is sufficiently smooth at the contact point of the linear waveguide and the second bending waveguide, and at the first and second bending waveguides. Refractive index change n in the first bent waveguide ( x ) The refractive index n1 in the region where the optical coupling occurs, the refractive index n2 in the region where the optical coupling does not occur, the propagation direction position x1 of the contact point between the input waveguide and the first bending waveguide, the linear waveguide and the About the propagation direction position x2 of the boundary with the contact of the first bending waveguide,

The optical directional coupler according to claim 2 or 3, wherein:   By reducing the width of the first bending waveguide and the second bending waveguide in an adiabatic manner from a region where optical coupling does not occur to a region where optical coupling occurs, the lateral V value in the region where optical coupling occurs is light. 2. The optical directional coupler according to claim 1, wherein the optical directional coupler is smaller than a lateral V value in a region where no coupling occurs. The adiabatic change in the waveguide width is caused by the contact between the input waveguide and the first bending waveguide, the contact between the output waveguide and the second bending waveguide, the linear waveguide and the first bending waveguide. 6. The optical directional coupler according to claim 5 , wherein the contact point, the contact point between the straight waveguide and the second bending waveguide, and the first and second bending waveguides are sufficiently smooth. Waveguide width change W in the first bent waveguide ( x ) In areas where optical coupling does not occur A waveguide width W1, a waveguide width W2 in the region where the optical coupling occurs, a propagation direction position x1 of a contact of the first bending waveguide with the region where the optical coupling does not occur, the light of the first bending waveguide For the propagation direction position x2 of the contact point with the region where the coupling occurs,

The optical directional coupler according to claim 5 or 6, wherein:   By reducing the thickness of the first bending waveguide and the second bending waveguide in an adiabatic manner from a region where no optical coupling occurs to a region where optical coupling occurs, the vertical V value in the region where optical coupling occurs is light. 2. The optical directional coupler according to claim 1, wherein the optical directional coupler is smaller than a vertical V value in a region where no coupling occurs. 4. The method of manufacturing an optical directional coupler according to claim 2, wherein the first bent waveguide and the second bent waveguide are manufactured in a general optical waveguide manufacturing process. The waveguide refractive index is gradually adiabatic from the input waveguide to the linear waveguide and from the output waveguide to the linear waveguide. the method of manufacturing an optical directional coupler characterized by having a step of changing the. The optical directionality according to claim 9 , further comprising a step of adiabatically changing the waveguide refractive index by moving an irradiation position of the laser on the optical directional coupler at an appropriate speed. A method for manufacturing a coupler. 9. The method of manufacturing an optical directional coupler according to claim 8 , wherein the optical directional coupler is formed by an optical waveguide formed on a substrate, and the lower clad layer is formed. And a step of forming a core film, a step of forming a waveguide core pattern, and a step of forming an upper clad, and between the step of forming the core pattern and the step of forming an upper clad layer, or Between the step of forming a core film and the step of forming a waveguide core pattern, a region on the straight waveguide side of the first bent waveguide, a region on the straight waveguide side of the second bent waveguide, And a shadow mask is disposed at an appropriate height above the substrate excluding the entire linear waveguide, and the linear waveguide side region of the first bending waveguide, the linear waveguide of the second bending waveguide Side area, and the straight line A step of gradually adiabatically reducing the waveguide thickness from the input waveguide to the linear waveguide and from the output waveguide to the linear waveguide by anisotropic etching of the entire waveguide; A manufacturing method of a characteristic optical directional coupler.

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