JP3679037B2 - Waveguide type optical circuit - Google Patents

Description

【数４】

ただし、Ｂ _ｓ は多層構造により生じる構造複屈折の値、Ｎは多層構造の層数、ｎ _ｉ 、ｔ _ｉ はそれぞれ各層の屈折率と厚さ、ｃ _１ 、ｃ _２ は、実測または計算によって導波路構造ごとに定まる定数である。
【００４２】
【数２】

【００４３】
ただし、Ｎは多層構造の層数、ｎ_ｉ、ｔ_ｉはそれぞれ各層の屈折率と厚さ、ｃ_１、ｃ_２は、実測または計算によって導波路構造ごとに定まる定数である。
【００４４】
式（１）の右辺において、
【００４５】
【数３】

【００４６】
は層に平行な方向の実効的な屈折率を、
【００４７】
【数４】

【００４８】
は層に垂直な方向の実効的な屈折率を表す。したがって、これらの差が多層構造に由来する構造複屈折となる。そして、これらの計算値は、式（１）のように補正係数ｃ_１、ｃ_２によって、実測または計算によって求められる構造複屈折値Ｂｓと結び付けられる。
【００４９】
補正係数ｃ_１、ｃ_２は、主に、導波路構造(比屈折率差、寸法)による光の閉じ込めに依存し、光の閉じ込めが一定の光導波路では共通の値となる。したがって、適当な多層構造コアを試作して、または適当な多層構造コアについてモード解析を実行して、あらかじめ補正係数を求めておけば、所望の構造複屈折を満たすような多層構造の設計が容易に可能となる。
【００５０】
本発明によれば、スラブ導波路のコアを屈折率の異なる層の多層構造とすることで、層と平行な方向に実効屈折率が高くなる構造複屈折を生じさせることができる。残留熱応力等により生じた複屈折（導波路複屈折）を補償する方向に多層構造となるコアを形成することにより、本発明の目的である、導波路複屈折を低減したスラブ導波路を実現でき、スラブ導波路の導波路複屈折により生じる光回路の偏波依存性を低減できる。特にアレイ導波路格子の各出力ポートの波長シフト量を一様とすることができる。また、従来の偏波依存性低減技術（複屈折補償器）と組み合わせることで、偏波依存性の低減限界を下げた導波型光回路を提供できる。
【００５１】
また、光導波路材料が石英系ガラスである光導波路において、基板をシリコンとした場合、光導波路の実効屈折率が基板に垂直方向の電界（ＴＭモード）に対して大きくなるため、基板に水平方向に多層構造とすることで、応力による導波路複屈折と多層構造による構造複屈折とを打ち消し合わせ、導波路複屈折を低減したスラブ導波路を実現できる。
【００５２】
また、基板が石英の場合には、光導波路の実効屈折率が基板に水平方向の電界（ＴＥモード）に対して大きくなるため、基板に垂直方向に多層構造とすることで、スラブ導波路の導波路複屈折を低減できる。これにより、スラブ導波路の導波路複屈折によって生じる光回路の偏波依存性を低減できる。特にアレイ導波路格子の各出力ポートの波長シフト量を一様とすることができる。
【００５３】
また、石英系光導波路において、従来構造のスラブ導波路は、従来構造の単一モード導波路に比較して導波路複屈折が大きくなっており、この導波路複屈折を打ち消す構造複屈折となる多層構造とするためには、スラブ導波路のコアの多層構造による構造複屈折を、単一モード導波路のコアの多層構造による構造複屈折より大きくする。スラブ導波路及び単一モード導波路に導波路複屈折を解消できる個別の多層構造を適用することで、偏波依存性の小さい回路を提供できる。特にアレイ導波路格子では、各出力ポートの波長シフト量を一様にできるとともに、波長シフトそのものを低減あるいは解消できる。
【００５４】
また、従来の偏波依存性低減技術と組み合わせることで、偏波依存性の低減限界を下げた導波型光回路を提供できる。
【００５５】
また、屈折率が高い層と低い層とを交互に配置した多層構造とすることで、従来と同じ実効屈折率を有し、層に垂直方向での光が感じる屈折率をおおよそ均一となるスラブ導波路を実現できることから、従来と同形状のスラブ導波路を用いて光回路を構成することができる。
【００５６】
また、上述したように、石英系光導波路において、スラブ導波路の導波路複屈折は単一モード導波路の導波路複屈折より大きい。光導波路の寸法等形状により導波路複屈折が異なることがあり、石英系光導波路では、光導波路のコア幅が広がるにつれて導波路複屈折が大きくなる。それゆえ、単一モード導波路、マルチモード導波路、スラブ導波路から形成されるアレイ導波路格子とマルチモード干渉型光結合器（以下、ＭＭＩという）を光結合器としたマッハ・ツェンダー干渉計と組み合わせたような光合分波回路の場合、単一モード導波路、マルチモード導波路、スラブ導波路の順で多層構造による構造複屈折が大きくなるように個別の多層構造を適用することで、それぞれの導波路複屈折を解消し、偏波依存性の小さい回路を提供できる。
【００５７】
また、コアの多層構造の各層の屈折率及びその厚さを、層の中央層に対しておおよそ対称にすることで、伝搬する光の電磁界分布の中心を、従来と同様に、コアのほぼ中心に位置させることができ、また、電磁界分布の形状を基板に垂直、水平方向でコア中心よりほぼ対称にできることから、従来とほぼ同様な回路設計により、所望の回路特性を実現できる。
【００５８】
また、多層構造の各層の屈折率をクラッド側の両端の層からコア内部側の層に向かって高くしたグレーデッドインデックスとすることで、従来の光導波路と異なるスポットサイズを実現しながら、光導波路の複屈折の低減、あるいは、解消できる。
【００５９】
また、多層構造の層数を５〜１０層とすることで、ＦＨＤ法のような厚膜作製プロセスにても、コアとして多層構造を容易に実現できる。また、実施例で述べるように光ファィバとの接続損失を低減できる。
【００６０】
また、多層構造を式（１）〜（３）をおおよそ満たすようにすることで、複雑な解析をすることなく、多層構造を決めることができる。屈折率の異なる少なくとも２層を交互に配置した多層構造では、補正係数ｃ_１＝１はコア内に光が十分に閉じこめられた場合にほぼ相当し、閉じこめが弱い場合は、主に１以下にする。
【００６１】
また、従来の偏波依存性低減技術（複屈折補償器）と組み合わせることでも、導波型光回路の偏波依存性の解消を図れる。例えば、アレイ導波路格子とＭＭＩを光結合器としたマッハ・ツェンダー光干渉計において、スラブ導波路、マルチモード導波路を多層構造のコアとする。単一モード導波路は、その複屈折により光回路の偏波依存性を生じさせアレイ導波路格子の導波路アレイ、及びマッハ・ツェンダー干渉計の光結合器間の光導波路にλ／２波長板を挿入する。これにより、偏波依存性を低減できる。
【００６２】
また、導波型光回路の全ての光導波路のコアを多層構造とする必要はなく、導波型光回路の偏波依存性に影響のある光導波路部分を多層化すれば良い。例えば、上述のようにアレイ導波路格子では、導波路アレイの各導波路とスラブ導波路を多層構造にすれば良い。また、その一部分に適用するだけでも従来に比較して偏波依存性を低減でき、例えば、導波路アレイの各導波路を同程度の割合の長さを多層コアにする、すなわち、偏波依存性に影響のある光導波路部分の内の適当な一部分を多層コアとするなどである。
【００６３】
また、導波型光回路の中で偏波依存性が顕著に現れるのは、一方の光結合器と他方の光結合器を導波路長の異なる複数の導波路で結ぶ光干渉型回路である。光干渉型回路の典型的な構成は、上述のマッハ・ツェンダー干渉計やアレイ導波路格子である。光干渉型回路では、導波路長差分のみ偏波依存性を低減、あるいは解消することで、回路の偏波依存性を低減、あるいは解消できる。すなわち、導波路長が最短の導波路に対する導波路長差分を多層構造コアとすれば良い。あるいは、導波路長差分と一定の長さを加えた長さを多層構造としても良い。また、多層構造コアを連続で形成する必要はなく、合計の導波路長が所定の長さとなれば良い。ラティス型フィルタ等複数の光結合器を複数の導波路で接続した光干渉型回路も本請求項に含まれる。
【００６４】
【発明の実施の形態】
以下、図面を参照して本発明の実施例について説明する。
【００６５】
［実施形態１］
図４は、本発明における導波型光回路の第１の実施形態を示す図で、図４（ａ）は、スラブ導波路の断面図、図４（ｂ）は、単一モード導波路の断面図である。基板４０１にシリコン基板を用い、クラッド４０２及びコア４０３，４０４は石英系ガラスにより形成している。ここで、コア（スラブ導波路）４０３は第１コア４０３ａ及び第２コア４０３ｂを基板に水平方向にそれぞれ複数積層した多層構造を備え、また、コア４０４（単一モード導波路）は第１コア４０４ａ及び第２コア４０４ｂを基板に水平方向にそれぞれ複数積層した多層構造を備えている。
【００６６】
第１コア４０３ａ，４０４ａ及び第２コア４０３ｂ，４０４ｂの比屈折率差、層厚、層数は、コア４０３，４０４の平均比屈折率差が０．７５％、コア厚が６μｍ（但し、コア４０４の導波路幅は６μｍ）、多層構造による構造複屈折値Ｂが−９×１０^−４（単一モード導波路換算−７×１０^−４）となるように、導波路のモード解析により求めた。ここで、平均比屈折率差は、各コア層の比屈折率差の面積平均とした。
【００６７】
本実施の形態で用いた各層の比屈折率差Δｉ、層厚ｔｉ、層数Ｎｉ（但し、ｉ＝１，２）は、
第１コア４０３ａ，４０４ａ：Δ１＝４．４％、ｔ１＝０．１５μｍ、Ｎ１＝７、
第２コア４０３ｂ，４０４ｂ：Δ２＝０％、ｔ２＝０．８３μｍ、Ｎ２＝６ である。
【００６８】
本実施の形態のスラブ導波路は次のように製造した。基板４０１としてシリコン基板を用い、基板４０１上に火炎堆積法により石英系下部クラッド層（コア下部のクラッド）及びコア層を形成する。コア層は、第１コア及び第２コアを交互に堆積している。第１コアには、屈折率を高めるためにＧｅＯ_２が４４ｍｏｌ％添加されており、透明化温度を下げるために微量のＢ_２Ｏ_３とＰ_２Ｏ_５を添加している。次に、コア層の不要部分を反応性イオンエッチング法により除去し、リッジ状のコア４０３，４０４を形成した後、コア４０３，４０４を覆うように下部クラッドと同等の屈折率を有する上部クラッドを火炎堆積法により形成する。クラッド４０２は、この下部クラッドと上部クラッドにより構成される。
【００６９】
この多層構造を有するスラブ導波路及び単一モード導波路により、図１に示すアレイ導波路格子１０１を作製した。即ち、前述した多層構造を有するスラブ導波路により、スラブ導波路１０２を作製し、また、前述した多層構造を有する単一モード導波路により、導波路アレイ１０３、入力導波路１０４、出力導波路１０５を作製した。
【００７０】
作製したアレイ導波路格子１０１の各出力ポートのＴＥモードに対するＴＭモードの波長シフトを図５に示す。導波路アレイ１０３を多層構造にしたことにより、波長シフトが約−０．５１ｎｍとなっているが、出力ポート間のばらつきが約０．００４ｎｍと従来の０．０２ｎｍに比較して１／５に低減している。これにより、スラブ導波路の導波路複屈折が小さくなったことがわかる。
【００７１】
このアレイ導波路格子１０１に、導波路アレイ１０３を垂直に横切るように１／２波長板を挿入した。各出力ポートで波長シフトは、±０．００３ｎｍ以内（出力ポート間のばらつきは約０．００４ｎｍ）となり、従来の±０．０１ｎｍに比較して、偏波依存性低減の限界を下げることができた。
【００７２】
［実施形態２］
本発明の第２の実施形態は、第１の実施形態におけるコアの比屈折率差Δを０．７５％から１．５％としたものである。比屈折率差を大きくすることは、単一モード導波路の曲げ半径を小さくでき、それにより光回路の小型化を図れる利点がある。但し、通常の光ファイバとの接続損失が大きくなるため、入出力導波路部でスポットサイズ変換技術等を適用する必要がある。
【００７３】
第１コア４０３ａ，４０４ａ及び第２コア４０３ｂ，４０４ｂの比屈折率差、層厚、層数は、コア４０３，４０４の平均比屈折率差が１．５％、コア厚が４μｍ（但し、コア４０４の導波路幅は４μｍ）、多層構造による構造複屈折値Ｂが−５×１０^−４となるように、導波路のモード解析により求めた。構造複屈折Ｂを第１の実施の形態に比較して小さくしたのは、従来構造のスラブ導波路の複屈折値が、比屈折率差Δが高くなると小さくなったことによる。
【００７４】
本実施の形態で用いた各層の比屈折率差Δｉ、層厚ｔｉ、層数Ｎｉ（但し、ｉ＝１，２）は、
第１コア４０３ａ，４０４ａ：Δ１＝２．７％、ｔ１＝０．５８μｍ、Ｎ１＝４
第２コア４０３ｂ，４０４ｂ：Δ２＝０％、ｔ２＝０．５７μｍ、Ｎ２＝３
である。
【００７５】
本実施の形態のスラブ導波路は、第１の実施の形態と同様に作製し、第１コアには、屈折率を高めるためにＧｅＯ_２が２７ｍｏｌ％添加されている。
【００７６】
この多層構造を有するスラブ導波路及び単一モード導波路により、第１の実施の形態と同様に、図１に示す回路構成のアレイ導波路格子１０１を作製した。その後、アレイ導波路格子１０１に、導波路アレイ１０３を垂直に横切るように１／２波長板を挿入した。各出力ポートでのＴＥモードに比較したＴＭモードの波長シフト量は、±０．００５ｎｍ以内（出力ポート間のばらつきは０．００５ｎｍ）となり、第１の実施の形態と同様に、従来の波長シフト量に比較して小さな値となっている。
【００７７】
［実施形態３］
図６は、本発明におけるスラブ導波路の第３の実施形態を示す図で、図６（ａ）は、スラブ導波路の断面図、図６（ｂ）は、単一モード導波路の断面図である。基板６０１に石英基板を用い、クラッド６０２及びコア６０３，６０４は石英ガラスにより形成している。ここで、コア（スラブ導波路）６０３は第１コア６０３ａ及び第２コア６０３ｂを基板に垂直方向に複数層（通常、数１００層〜数１０００層）形成した多層構造を備え、また、コア６０４（単一モード導波路）は第１コア６０４ａ及び第２コア６０４ｂを基板に垂直方向に複数層（通常、数層〜数１０層）形成した多層構造を備えている。
【００７８】
第１コア６０３ａ，６０４ａ及び第２コア６０３ｂ，６０４ｂの屈折率、層厚は、コア６０３，６０４の平均比屈折率が０．７５％、コア厚が６μｍ（但し、コア６０４の導波路幅は６μｍ）、多層構造による構造複屈折値Ｂが−９×１０^−４（単一モード導波路換算−７×１０^−４）となるように、導波路のモード解析により求めた。
【００７９】
本実施の形態で用いた各層の比屈折率差Δｉ、層厚ｔｉ（但し、ｉ＝１，２）は、
第１コア６０３ａ，６０４ａ：Δ１＝４．９％、ｔ１＝０．２３μｍ
第２コア６０３ｂ，６０４ｂ：Δ２＝０％、ｔ２＝１．６９μｍ
である。
【００８０】
本発明の実施形態のスラブ導波路は、次のように製造した。基板６０１として石英基板を用い、基板６０１上に火炎堆積法により第１コア６０３ａ，６０４ａに相当するコア層を形成する。この第１コア６０３ａ，６０４ａには、屈折率を高めるためにＧｅＯ_２がそれぞれ４９ｍｏｌ％添加されている。次に、第２コアに相当する部分を反応性イオンエッチング法により除去した後、短冊状になった第１コア６０３ａ，６０４ａを覆うようにクラッドを火炎堆積法により形成する。クラッド６０２と同時に第２コア６０３ｂ，６０４ｂが形成される。
【００８１】
なお、この際、コアを構成する第１コア及び第２コアの各層の界面は、スラブ導波路の場合は入力端の中央と出力端の中央とを結ぶ線分にほぼ平行に設けられ、単一モード導波路の場合は光の進行方向とおおよそ平行、言い換えれば導波路に沿った方向に設けられる。
【００８２】
この多層構造を有するスラブ導波路及び単一モード導波路により、図１に示すようなアレイ導波路格子１０１を作製した。すなわち、前述した多層構造を有するスラブ導波路により、スラブ導波路１０２を作製し、また、前述した多層構造を有する単一モード導波路により、導波路アレイ１０３、入力導波路１０４、出力導波路１０５を作製した。その後、導波路アレイ１０３を垂直に横切るように１／２波長板を挿入した。
【００８３】
各出力ポートでのＴＥモードに比較したＴＭモードの波長シフトは±０．００６ｎｍ以内（出力ポート間のばらつきが約０．００５ｎｍ）となり、従来、±０．０１ｎｍであった偏波依存性低減の限界を下げることができた。
【００８４】
なお、本実施の形態ではコア６０３，６０４を基板６０１の上に直接形成したが、第１、第２の実施の形態と同様、コアの下部にクラッドを備えたものでも実現できる。
【００８５】
［実施形態４］
本発明の第４の実施形態は、第１の実施形態で説明した図１に示すアレイ導波路格子１０１の導波路アレイ１０３のコアの多層構造を、スラブ導波路１０２より小さな構造複屈折となる多層構造としたものである。
【００８６】
導波路アレイ１０３のコアの多層構造は、構造複屈折値Ｂが−２．３×１０^−４となるように、第１コア及び第２コアの比屈折率差Δｉ、層厚ｔｉ、層数Ｎｉ（但し、ｉ＝１，２）を、
第１コア：Δ１＝２％、ｔ１＝０．４５μｍ、Ｎ１＝５
第２コア：Δ２＝０％、ｔ２＝０．９４μｍ、Ｎ２＝４
とした。
【００８７】
本実施の形態のアレイ導波路格子１０１は、次のように作製した。シリコン基板上に火炎堆積法により下部クラッドを形成し、その上に導波路アレイ用コア層を上記のパラメータで火炎堆積法により作製した。第１コアには、ＧｅＯ_２が２０ｍｏｌ％添加され、微量のＢ_２Ｏ_３とＰ_２Ｏ_５を含有している。スラブ導波路１０２を形成する部分となる導波路アレイ用コア層を反応性イオンエッチング法により除去し、その後、スラブ導波路１０２用コア層を第１の実施の形態と同じパラメータで火炎堆積法により形成した。導波路アレイ用コア層上に形成されたスラブ導波路１０２用コア層を反応性イオンエッチング法により除去した。これにより、下部クラッド上にスラブ導波路形成部分と単一モード導波路形成部分に適した多層構造となるコア層が形成できる。
【００８８】
次に、コア層の不要部分を反応性イオンエッチング法により除去し、リッジ状のコアを形成し、コアを覆うように下部クラッドと同等の屈折率を有する上部クラッドを火炎堆積法により形成した。
【００８９】
作製したアレイ導波路格子１０１の各出力ポートのＴＥモードに対するＴＭモードの波長シフト量は０．０２ｎｍ以下（出力ポート間のばらつき０．００４ｎｍ）であり、偏波依存性を低減する方法として１／２波長板を用いる方法と比較して偏波依存性が若干大きめであるが、偏波依存性を低減する方法適用前の波長シフト０．２〜０．３ｎｍ（出力ポート間のばらつき０．０２ｎｍ）に比較して十分小さな波長シフト及び出力ポート間ばらつきを実現できる。
【００９０】
［実施形態５］
本発明の第５の実施形態では、第１及び第４の実施形態のスラブ導波路の多層構造を、（１）〜（３）式によりさらに厳密に設定し、アレイ導波路格子の波長シフトのばらつきを低減したものである。
【００９１】
図４（ａ）に示すスラブ導波路の第１コア４０３ａ及び第２コア４０３ｂの比屈折率差Δ _ｉ、層厚ｔ_ｉ（ｉ＝１，２）は、次の手順にて設定した。
【００９２】
手順１） 適当な多層構造により光回路を作製し、その特性から多層構造による構造複屈折値を見積もる。その結果と式（１）〜（３）から補正係数ｃ_１、ｃ_２を決める。
【００９３】
手順２） 多層構造による構造複屈折が所定の値となるように、式（１）〜（３）より比屈折率差Δ _ｉ、層厚ｔ_ｉ（ｉ＝１，２）を決める。手順１）は、多層構造の作製精度が良く、屈折率の異なる層を積層した多層構造による構造複屈折以外に新たに応力複屈折等が付加されない場合は、光回路を作製する代わりにモード解析を用いても良い。
【００９４】
コアの平均比屈折率差Δ _ａｖｅが０．７５％、コア厚が６μｍでの多層構造を決めるために、手順１）、図１に示すアレイ導波路格子を、次の３種類の第１コア４０３ａ及び第２コア４０３ｂの比屈折率差Δ _ｉ、層厚ｔ_ｉ（ｉ＝１，２）により作製した。層数は、総層数を１３層とし、第１コアの層数Ｎ１を７層、第２コアの層数Ｎ２を６層で形成した。
【００９５】
１） 第１コア４０３ａ、Δ _１＝１．０％、ｔ_１＝０．６４μｍ
第２コア４０３ｂ、Δ _２＝０％、ｔ_２＝０．２５μｍ
２） 第１コア４０３ａ、Δ _１＝３．０％、ｔ_１＝０．２μｍ
第２コア４０３ｂ、Δ _２＝０％、ｔ_２＝０．７６μｍ
３） 第１コア４０３ａ、Δ _１＝５．１％、ｔ_１＝０．１２μｍ
第２コア４０３ｂ、Δ _２＝０％、ｔ_２＝０．８６μｍ
スラブ導波路及び単一モード導波路は、第１の実施形態と同様に作製し、アレイ導波路格子を作製した。屈折率を高くするためにＧｅＯ_２を比屈折率差Δ１％当たり１０ｍｏｌ％ドープした。
【００９６】
多層構造による導波路複屈折の値Ｂｓを見積もるために、作製したアレイ導波路格子の各出力ポートでのＴＥモードに対するＴＭモードの波長シフトのばらつきを測定した。このばらつきからスラブ導波路での複屈折値Ｂｅを見積もり、このＢｅから従来のコア構造で生じる導波路複屈折値Ｂｏ＝１．１ｘ１０ ^{− 3} を引くことで、多層構造による導波路複屈折の値Ｂｓを見積もった。ここで、Ｂｏ＝１．１ｘ１０ ^{− 3} は、第１の実施形態の波長シフトから再度見積もった値である。第１コアの比屈折率差に対して多層構造による複屈折の大きさ│Ｂｓ│をプロットしたのが図７に示した黒丸である。この結果と（１）〜（３）式より補正係数ｃ_１を１、ｃ_２を０とした。実線は、この補正係数と（１）〜（３）式により計算したものである。
【００９７】
次に、手順２）として、補正係数ｃ_１を１、ｃ_２を０とした（１）〜（３）式より、│Ｂｓ│＝│Ｂｏ│＝１．１ｘ１０ ^{− 3} となる第１コア４０３ａ及び第２コア４０３ｂの比屈折率差Δ _ｉ、層厚ｔ_ｉ（ｉ＝１，２）を求める。図７の計算結果である実線より、│Ｂｓ│＝│Ｂｏ│＝１．１ｘ１０ ^{− 3} となる第１コア４０３ａの比屈折率差Δ _１は５．５％であり、（２），（３）式より、ｔ_１、ｔ_２は下記の値となる。
【００９８】
第１コア４０３ａ、Δ _１＝５．５％、ｔ_１＝０．１１μｍ、Ｎ_１＝７
第２コア４０３ｂ、Δ _２＝０％、ｔ_２＝０．８７μｍ、Ｎ_２＝６
図７より│Ｂｓ│はおおよそΔ _１の１次関数として変化していることから、数点のΔ _１に対する│Ｂｓ│を求め、Δ _１の１次関数として近似曲線を見積もり、それより所定の│Ｂｓ│に対するΔ _１を求めても良い。
【００９９】
この多層構造となるスラブ導波路により、図１に示すアレイ導波路格子を作製した。その後、アレイ導波路格子１０１に、導波路アレイ１０３を垂直に横切るように１／２波長板を挿入した。各出力ポートでのＴＥモードに対するＴＭモードの波長シフト量は、±０．００２ｎｍ以内（出力ポート間のばらつきは０．００２ｎｍ）であった。
【０１００】
第４の実施形態と同様に、導波路アレイ１０３の構造複屈折値が−２．３ｘ１０^{− 4}となる多層構造を、第１コアと第２コアの比屈折率差Δ _ｉ、層厚ｔ_ｉ、層数Ｎ_ｉ（ｉ＝１，２）が
第１コア Δ _１＝２％、ｔ_１＝０．４５μｍ、Ｎ_１＝５
第２コア Δ _２＝０％、ｔ_２＝０．９４μｍ、Ｎ_２＝４
となるように作製した。各出力ポートでのＴＥモードに比較したＴＭモードの波長シフト量は、±０．００８ｎｍ以内（出力ポート間のばらつきは０．００２ｎｍ）であった。
【０１０１】
第１及び第４の実施形態のスラブ導波路の多層構造を、（１）〜（３）式によりさらに厳密に設定することで、アレイ導波路格子の偏波依存性を低減できた。
【０１０２】
［実施形態６］
本発明の第６の実施形態は、第５の実施形態におけるコアの層数を約１／３〜約２倍の範囲で変えた。
【０１０３】
図４（ａ）に示すスラブ導波路のコア４０３の多層構造は、第５の実施形態と同様に、平均比屈折率差Δ _ａｖｅが０．７５％、コア厚が６μｍ、第１コア４０３ａの比屈折率差Δ _１を５．５％、第２コア４０３ｂの比屈折率差Δ _２を０％（多層構造による複屈折の大きさ│Ｂｓ│が１．１ｘ１０^{− 3}）とした。第１コアの層数Ｎ_１を２〜１１層の範囲で変えた１０種類の構造にて、図１に示すアレイ導波路格子を作製した。第２コアの層数Ｎ_２は、（Ｎ_１−１）層である。第１コア４０３ａの層厚ｔ_１と、第２コア４０３ｂの層厚ｔ_２は、次のように設定した。
【０１０４】
１） Ｎ_１＝２、ｔ_１＝０．３９μｍ、ｔ_２＝５．２２μｍ
２） Ｎ_１＝３、ｔ_１＝０．２６μｍ、ｔ_２＝２．６１μｍ
３） Ｎ_１＝４、ｔ_１＝０．２０μｍ、ｔ_２＝１．７４μｍ
４） Ｎ_１＝５、ｔ_１＝０．１６μｍ、ｔ_２＝１．３１μｍ
５） Ｎ_１＝６、ｔ_１＝０．１３μｍ、ｔ_２＝１．０４μｍ
６） Ｎ_１＝７、ｔ_１＝０．１１μｍ、ｔ_２＝０．８７μｍ
７） Ｎ_１＝８、ｔ_１＝０．１０μｍ、ｔ_２＝０．７５μｍ
８） Ｎ_１＝９、ｔ_１＝０．０９μｍ、ｔ_２＝０．６５μｍ
９） Ｎ_１＝１０、ｔ_１＝０．０８μｍ、ｔ_２＝０．５８μｍ
１０） Ｎ_１＝１１、ｔ_１＝０．０７μｍ、ｔ_２＝０．５２μｍ
導波路の製造工程は、第１の実施形態と同じである。
【０１０５】
各出力ポートでのＴＥモードに比較したＴＭモードの波長シフトのポート間ばらつきは、Ｎ_１が２で、０．００５ｎｍであり、│Ｂｓ│が所定値より２．５ｘ１０ ^{− 4} 大きくなっている。これは、従来構造の電磁界分布に比較して波形のひずみが大きいためである。総層数５以上では、ポート間ばらつきは、０．００３ｎｍであり、所定の│Ｂｓ│に対して１．５ｘ１０ ^{− 4} 以内で設定できていることがわかる。これより、総層数は、５以上にすることが望ましい。
【０１０６】
［実施形態７］
本発明の第７の実施形態は、単一モード導波路、マルチモード導波路、及びスラブ導波路の３種類の異なる幅の光導波路により構成される導波型光回路であり、単一モード導波路、マルチモード導波路、スラブ導波路の順に構造複屈折が大きくなる多層構造を適用したものである。導波型光回路は、図１に示すアレイ導波路格子１０１、及び図８に示すＭＭＩを光結合器に用いたマッハ・ツェンダー干渉計８０１にて構成している。
【０１０７】
以下、参考例として、導波型光回路をマッハ・ツェンダー干渉計で構成したものについて説明する。
図８（ａ）は、マッハ・ツェンダー干渉計８０１の構成を示す図で、光結合器であるＭＭＩ８０２と、２つのＭＭＩを繋ぐ光導波路８０３と、一方のＭＭＩと繋がる入力導波路８０４と、他方のＭＭＩと繋がる出力導波路８０５とで構成されている。図８（ｂ）は、ＭＭＩ８１１の構成を示す図で、マルチモード導波路８１２とその一方の端に接続される入力導波路８１３と他方の端に接続される出力導波路８１４より構成されている。
【０１０８】
コアの平均比屈折率差Δ _ａｖｅを０．７５％、コア高さを６μｍとし、単一モード導波路のコア幅は６μｍ、マルチモード導波路のコア幅は２４μｍ、スラブ導波路の幅を２ｍｍとした。多層構造による複屈折の大きさ│Ｂｓ│は、単一モード導波路で、２．３ｘ１０^{− 4}、マルチモード導波路で５ｘ１０^{− 4}、スラブ導波路で１．１ｘ１０^{− 3}となるように、多層構造を（１）〜（３）式を用いて各層の比屈折率差Δ _ｉ、層厚ｔ_ｉ、層数Ｎ_ｉを以下の値に設定した。補正係数は、単一モード導波路でｃ_１＝０．８、ｃ_２＝０、マルチモード導波路でｃ_１＝０．９７、ｃ_２＝０、スラブ導波路でｃ_１＝１、ｃ_２＝０とした。
【０１０９】
単一モード導波路：
第１コア Δ _１＝２％、ｔ_１＝０．４５μｍ、Ｎ_１＝５
第２コア Δ _２＝０％、ｔ_２＝０．９４μｍ、Ｎ_２＝４
マルチモード導波路：
第１コア Δ _１＝３．１％、ｔ_１＝０．２８μｍ、Ｎ_１＝５
第２コア Δ _２＝０％、ｔ_２＝１．１５μｍ、Ｎ_２＝４
スラブ導波路：
第１コア Δ _１＝５．５％、ｔ_１＝０．１６μｍ、Ｎ_１＝５
第２コア Δ _２＝０％、ｔ_２＝１．３１μｍ、Ｎ_２＝４
導波型光回路は、第４の実施形態と同様の手順で作製し、スラブ導波路、マルチモード導波路、単一モード導波路の順にコアを形成した。
【０１１０】
作製した導波型光回路のマッハ・ツェンダー干渉計の特性は、波長シフトが従来の０．２５ｎｍから０．０１ｎｍ、ＴＥモードとＴＭモードのピーク波長の損失差が従来の０．２ｄＢから０．０３ｄＢ以下となり、アレイ導波路格子の特性は各出力ポートでのＴＥモードに対するＴＭモードの波長シフト量は、従来の±０．１３ｎｍ以内（出力ポート間ばらつき０．０２ｎｍ）から±０．０１ｎｍ以内（出力ポート間のばらつきは０．００２ｎｍ）と低減した。これにより、単一モード導波路、マルチモード導波路及びスラブ導波路より構成される導波型光回路に、それぞれ適切な多層構造を適用することで、偏波依存性を低減できることが確認できた。
【０１１１】
［実施形態８］
本発明の第８の実施形態は、３種類の異なる比屈折率差を有する層をほぼ交互に多層化したコア構造である。
【０１１２】
図９は、本発明における第８の実施形態のスラブ導波路の断面図を示す図で、基板９０１にシリコン基板を用い、クラッド９０２及びコア９０３は石英系ガラスにより形成している。コア９０３は、第１コア９０３ａ、第２コア９０３ｂ、及び第３コア９０３ｃを、基板側より第１コア９０３ａ、第２コア９０３ｂ、第３コア９０３ｃ、膜厚が１／２の第２コア９０３ｂ、第１コア９０３ａ、膜厚が１／２の第２コア９０３ｂ、第３コア９０３ｃ、第２コア９０３ｂ、第１コア９０３ａの順で積層した構造であり、コア中央層に対して各層を対称に配置した。
【０１１３】
コアの平均比屈折率差Δ _ａｖｅを０．７５％、コア厚を６μｍ、第１コア９０３ａの比屈折率差Δ _１を６％、第２コア９０３ｂの比屈折率差Δ _２を５％、第３コア９０３ｂの比屈折率差Δ _２を０％とし、多層構造による構造複屈折│Ｂｓ│が１．１ｘ１０^{− 3}となる各層の層厚を、実施の形態５で求めた補正係数ｃ_１＝１、ｃ_２＝０を用いた（１）〜（３）式により次のように設定し、図１に示すアレイ導波路格子を作製した。製造工程は、第１の実施形態と同様である。
【０１１４】
第１コア層９０３ａ、Δ _１＝６％、ｔ_１＝０．１２μｍ、Ｎ_１＝３
第２コア層９０３ｂ、Δ _２＝５％、ｔ_２＝０．１４μｍ、Ｎ_２＝４
（膜厚が１／２の第２コア層９０３ｂの膜厚は０．０７μｍ、
Ｎ_２＝６は、膜厚が１／２の２層を含む）
第３コア層９０３ｂ、Δ _２＝０％、ｔ_２＝２．６１μｍ、Ｎ_２＝２
作製したアレイ導波路格子の各出力ポートでのＴＥモードに比較したＴＭモードの波長シフト量は、±０．０１ｎｍ以内（出力ポート間のばらつきは０．００３ｎｍ）であった。３種類の異なる屈折率層をほぼ交互に配置した構造でも偏波依存性を解消した導波型光回路を実現できることを確認した。
【０１１５】
３種類の異なる比屈折率差の層を交互に配置して多層構造を構成する利点は、２種類の異なる比屈折率差の層を交互に配置して多層構造を構成するのに比べて、比屈折率差、膜厚の選択の自由度が上がることである。コアの平均比屈折率差と多層構造による構造複屈折│Ｂｓ│は、主に、多層構造を構成する層の比屈折率差と膜厚により決まる。２種類の層の交互層で構成する場合、多層構造を規定するパラメータは、２種類の比屈折率差と２種類の膜厚であり、その内の１つを設定すると、平均比屈折率差と構造複屈折│Ｂｓ│が決まっているため、他の３つの値が自動的に設定される。３種類の層の交互配置で構成する場合、多層構造を規定するパラメータは、６種類であり、その内の３つを定めると、平均比屈折率差と│Ｂｓ│が決まっているため、他の３つの値が決まる。それゆえ、例えば、２種類の層の交互配置では、第１〜第７の実施形態のように、第２コアの比屈折率差を０％に設定したため、第１コアの比屈折率差、第１コア及び第２コアの膜厚が自動的に決まった。３種類の層の交互配置では、各層の比屈折率差を設定すれば、各層の膜厚が自動的に決まる。
【０１１６】
すなわち、比屈折率差を適当に選択することができる。これは、作製精度を出しにくいパラメータがある場合、例えば、所定の３種類の比屈折率差にしか設定できない場合、膜厚を高い精度に設定できれば、適切な構造複屈折を与えることができ、複屈折を低減できる利点がある。別の利点として、石英系光導波路においては、光誘起屈折率変化を効率よく生じさせることができる。ＧｅＯ_２を高濃度に添加した石英系ガラスは、高い光誘起屈折率を得ることができる。それゆえ、３種類の層の交互配置において、ＧｅＯ_２を高濃度に添加した層、すなわち、比屈折率の高い層を設定し、構造複屈折の値は膜厚により調整すれば良い。
【０１１７】
本発明は、コアの平均比屈折率差、寸法、多層構造による構造複屈折値を、コアの各層の比屈折率差、厚さ、層数により調整すれば良い。それゆえ、上述した実施形態のコアの平均比屈折率差、寸法、多層構造による複屈折値、コアの各層の比屈折率差、厚さ、層数に限定されるものでない。
【０１１８】
本発明における導波路アレイ等に用いている単一モード導波路は、２モード程度の導波路で、回路内ではおおよそ単一モード導波路として機能する疑似単一モード導波路を含むこととする。
【０１１９】
上述した実施形態では、スラブ導波路を用いた導波型光回路として、アレイ導波路格子を用いたが、これに限定されるものでなく、スターカップラ等に適用できる。
【０１２０】
また、上述した実施形態では、多層構造による構造複屈折値を、従来のコア構造で生じる導波路複屈折値にほぼ等しくなるようにしたが、従来のコア構造で生じる導波路複屈折値の２倍未満の値であれば、少なくとも従来のものよりも光導波路の複屈折を低減でき、光回路の偏波依存性を低減できる。
【０１２１】
また、上述した実施形態では、火炎堆積法によりコア形成したが、作製法に限定されるものでなく、他の石英ガラスによる多層構造を形成できる手段、例えば、スパッタ、プラズマＣＶＤ、ＥＣＲ−ＣＶＤなどにより形成しても導波路複屈折を低減・解消できる。また、屈折率を調整するのにＧｅＯ_２を用いたが、屈折率を所望の値に設定できれば良いので、ＴｉＯ_２等、他のドーパントを適用しても良い。さらに、導波路材料として石英系ガラスを用いたが、屈折率の異なる層を多層化できれば良く、他のガラス系材料、高分子材料なども適用できる。
【０１２２】
本実施形態の層数では、単一モード導波路と光ファィバとの接続損失を０．１〜０．３ｄＢ程度低減でき、層数が少ない方が小さな値となる。作製した光回路の回路としての過剰損失は、従来と同程度である。それゆえ、層数を適切に選択することで、回路特性を損なわずに、入出力導波路を伝搬する光のスポットサイズを光ファイバに近づけることができ、光ファィバとの接続損失を低減することができる。
【０１２３】
また、上述した実施形態では、屈折率の異なる数種類の層をほぼ交互に配置した多層構造としたが、これに限定されるものでなく、クラッド側の層からコア中心に向かって屈折率が高くなるグレーデッドインデックス構造等にも適用できる。交互に配置の場合、光は、応力による複屈折分布の平均的な値を感じ、その大きさは従来の光導波路構造での導波路複屈折とほぼ同程度になっていたが、グレーデッドインデックスの場合、コア中央付近の層で電界分布が強くなるためコア内に顕著な応力分布がある場合に、上述した実施形態の補正係数ｃ_１をそのまま用いることができなくなる。それゆえ、数種類の適当な構造を作製し、それにフィットするような補正係数ｃ_１、ｃ_２を見積もる必要がある。
【０１２４】
【発明の効果】
以上説明したように本発明によれば、スラブ導波路の導波路複屈折を低減でき、スラブ導波路の導波路複屈折により生じていた偏波依存性を低減でき、特にアレイ導波路格子の各出力ポートの波長シフトを一様とすることができる。また、従来の偏波依存性低減技術と組み合わせることで、偏波依存性の低減限界を下げることができ、高性能な導波型光回路を提供することができる。これは、光合分波器では、光合分波器を通過する信号の損失やクロストークの揺らぎを低減することであり、光ＷＤＭ技術を用いた光通信システムにおける信号の信頼性を高めることになる。
【図面の簡単な説明】
【図１】従来のアレイ導波路格子の構成図である。
【図２】従来のスラブ導波路及び導波路アレイの断面図である。
【図３】従来のアレイ導波路格子における各出力ポートの波長シフトを示すグラフである。
【図４】本発明の導波型光回路の第１、第２、第４の実施形態を示す断面図である。
【図５】第１の実施形態によるアレイ導波路格子における各出力ポートの波長シフトを示すグラフである。
【図６】本発明の導波型光回路の第３の実施形態を示す断面図である。
【図７】本発明の第５の実施形態を説明するための多層構造のコアにおける第１コアの比屈折率差Δ１に対する多層構造による複屈折の大きさ│Ｂｓ│のグラフである。
【図８】（ａ）はマッハ・ツェンダー干渉計、（ｂ）はマルチモード干渉型光結合器を示す図である。
【図９】本発明の第８の実施形態を示す断面図である。
【符号の説明】
１０１ アレイ導波路格子
１０２ スラブ導波路
１０３ 導波路アレイ
１０４ 入力導波路
１０５ 出力導波路
２０１ 基板
２０２ クラッド
２０３ コア（スラブ導波路）
２０４ コア（単一モード導波路）
４０１ 基板
４０２ クラッド
４０３ コア（スラブ導波路）
４０４ コア（単一モード導波路）
４０３ａ，４０４ａ 第１コア
４０３ｂ，４０４ｂ 第２コア
６０１ 基板
６０２ クラッド
６０３ コア（スラブ導波路）
６０４ コア（単一モード導波路）
６０３ａ，６０４ａ 第１コア
６０３ｂ，６０４ｂ 第２コア
８０１ マッハ・ツェンダー干渉計
８０２ マルチモード干渉型光結合器（ＭＭＩ）
８０３ 光導波路
８０４ 入力導波路
８０５ 出力導波路
８１１ マルチモード干渉型光結合器（ＭＭＩ）
８１２ マルチモード導波路
８１３ 入力導波路
８１４ 出力導波路
９０１ 基板
９０２ クラッド
９０３ コア
９０３ａ 第１コア
９０３ｂ 第２コア
９０３ｃ 第３コア[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a waveguide optical circuit, and more particularly to a waveguide optical circuit including a slab waveguide.
[0002]
[Prior art]
Due to the worldwide spread of Internet use, there is an urgent need to construct a communication system that can simultaneously transmit a large amount of large-capacity data at high speed. An optical communication system using an optical wavelength division multiplexing (WDM) technique has been attracting attention as a system that satisfies this demand, and has been introduced globally mainly in the United States.
[0003]
In the optical WDM technology, an optical multiplexer / demultiplexer that can multiplex / demultiplex a plurality of different wavelengths is indispensable. As one of the realization modes, there is a waveguide type optical circuit in which an optical circuit is constituted by an optical waveguide on a substrate.
[0004]
A waveguide type optical circuit is an IC in light, and applies an LSI microfabrication technique or the like to collectively form an optical waveguide on a flat substrate. Therefore, it is possible to realize a high-performance circuit having excellent integration and mass productivity and a complicated circuit configuration. In recent years, research and development have been actively promoted along with the growing interest in optical communication systems.
[0005]
Waveguide-type optical circuits have been realized with various materials such as semiconductors, LN, plastics, and silica-based glass. Silica-based optical waveguides that are formed of silica-based glass on a silicon substrate are used for optical communication transmission. Other waveguides have good compatibility with silica-based optical fiber, which is a path, and have high stability and long-term reliability that are characteristics of forming materials, and can realize an optical circuit with stable operation characteristics. Practical use is progressing compared to materials.
[0006]
One basic configuration of an optical multiplexer / demultiplexer that has been put to practical use by a quartz optical waveguide is an arrayed waveguide grating. An arrayed waveguide grating is a high-performance optical circuit that can combine and demultiplex a plurality of light beams with different wavelengths in a simple configuration that connects a waveguide array and a slab waveguide, compared to other configurations. This is a configuration that can be reduced in size.
[0007]
FIG. 1 is a diagram showing a basic configuration of a conventional arrayed waveguide grating. In an arrayed waveguide grating 101, a waveguide array 103 is connected between two slab waveguides 102, and an input waveguide 104 and an output waveguide 105 are connected. Are connected by different slab waveguides 102. In the waveguide array 103, the waveguide lengths of adjacent waveguides are different, and the waveguide length difference from the adjacent waveguide determines the wavelength interval at which optical multiplexing / demultiplexing is performed.
[0008]
2 (a) and 2 (b) are diagrams showing a cross-sectional structure of a conventional slab waveguide and a waveguide array. FIG. 2 (a) is a cross-sectional view of the slab waveguide, and FIG. It is sectional drawing of a waveguide array. In the cross-sectional view of the slab waveguide 102 in FIG. 2A, the core 203 is covered on the substrate 201 with the clad 202, and the width of the core 203 is wide in the horizontal direction of the substrate. In addition, in the cross-sectional view of the waveguide array 103 in FIG. 2B, the optical waveguide of the core 204 having a structure in which the core width of the core 203 of the slab waveguide 102 is made substantially the same as the core thickness. It is a waveguide. In such a silica-based optical waveguide, an optical multiplexer / demultiplexer up to 128 waves is realized.
[0009]
However, the optical characteristics of the arrayed waveguide grating formed by a silica-based optical waveguide using silicon as the substrate are the modes in which the spectrum of the mode having the electric field perpendicular to the substrate (TM mode) has the electric field in the horizontal direction (TE mode). ) And polarization dependency shifted to the longer wavelength side. This wavelength shift amount is 0.2 to 0.3 nm in an arrayed waveguide grating having a demultiplexing interval of 0.8 nm. Such polarization dependence in an optical multiplexer / demultiplexer is due to the fact that the polarization direction of the signal light transmitted through the optical fiber is uncertain and fluctuates over time, so transmission loss and crosstalk vary over time. As a result, the reliability of the signal is deteriorated.
[0010]
The cause of the polarization dependence of the arrayed waveguide grating is that each optical waveguide of the waveguide array 103 has a waveguide birefringence in which the effective refractive index felt by the TM mode is larger than the effective refractive index felt by the TE mode. Because. For an optical waveguide with a relative refractive index difference Δ = 0.75%, (2-3) × 10^-4There is a waveguide birefringence. The cause of this waveguide birefringence is due to the residual thermal stress caused by the difference in thermal expansion coefficient between silicon as the substrate and quartz glass as the material of the optical waveguide.
[0011]
As a method of reducing the polarization dependence, (1) a groove is formed on both sides of the waveguide, stress applied to the waveguide is reduced, and waveguide birefringence is reduced. (2) on the waveguide. (3) A method for reducing the polarization dependence of the entire optical circuit by controlling the birefringence by trimming the stress applying film while monitoring the optical circuit characteristics after forming a stress applying film such as a-Si. A method of controlling the birefringence by irradiating the waveguide with ultraviolet rays while monitoring the optical circuit characteristics, and reducing the polarization dependence in the entire optical circuit. (4) Inserting a half-wave plate in the optical circuit. , A method for reducing the polarization dependence of the entire optical circuit by switching the polarization mode, and (5) GeO on the cladding covering the core₂, B₂O₃, P₂O₅A method to reduce birefringence by doping a material that increases the coefficient of thermal expansion, such as, and bringing it closer to the coefficient of thermal expansion of the substrate has been developed, and the wavelength shift described above can be reduced to the order of 0.01 nm. It is.
[0012]
[Problems to be solved by the invention]
However, the wavelength shift amount of the arrayed waveguide grating differs depending on the output port of the output waveguide (105 in FIG. 1), and the wavelength shift amount varies between the output ports even by the technique for reducing the polarization dependence described in the prior art. Was hardly reduced, limiting the reduction of polarization dependence.
[0013]
FIG. 3 is a graph plotting the wavelength shift of the TM mode with respect to the TE mode of each output port in an arrayed waveguide grating that splits 32 waves with a demultiplexing interval of 0.8 nm. No. 104). The variation of the wavelength shift amount is about 0.02 nm, and has a constant inclination between the output ports 1 to 32. This is because the waveguide birefringence of the slab waveguide 102 is approximately 1.1 × 10 6.^-4Because there is.
[0014]
The present invention has been made in view of such problems. The object of the present invention is to reduce the waveguide birefringence of the slab waveguide and to reduce the polarization dependence caused thereby. It is to provide an optical circuit.
[0015]
It is another object of the present invention to provide a waveguide type optical circuit having a uniform polarization dependency by reducing variation in the amount of wavelength shift between output ports of an arrayed waveguide grating.
[0016]
Furthermore, an object of the present invention is to provide a waveguide type optical circuit in which the polarization dependence reduction limit is lowered by combining with the polarization dependence reduction technique described in the prior art.
[0017]
[Means for Solving the Problems]
  In order to achieve such an object, the present invention is formed on a substrate according to claim 1,Made of glass material or polymer materialClad and coreAnd the core is covered with the cladA waveguide-type optical circuit constituting an arrayed waveguide grating having a slab waveguide, wherein at least a part of the core of the slab waveguide is parallel to the substrate surface and includes at least three layers, and adjacent layers The multi-layer structure has a different refractive index, the layer direction of the multilayer structure is formed such that the structural birefringence due to the multilayer structure compensates for other waveguide birefringence, and the structural birefringence due to the multilayer structure is The size is less than twice the size of the waveguide birefringence value due to other waveguide birefringence.
[0018]
According to a second aspect of the present invention, in the first aspect of the present invention, the substrate is a silicon substrate, and the optical waveguide is made of quartz glass.
[0020]
  The invention according to claim 3 is formed on a substrate,Made of glass material or polymer materialClad and coreAnd the core is covered with the cladA waveguide-type optical circuit constituting an arrayed waveguide grating having a slab waveguide, wherein at least a part of the core of the slab waveguide is perpendicular to the substrate surface and includes at least three layers, and adjacent layers The multi-layer structure has a different refractive index, the interface of the layers is parallel to the line connecting the input center and output center of the slab waveguide, and the structure birefringence due to the multilayer structure compensates for other waveguide birefringence. Thus, the layer direction of the multilayer structure is formed, and the magnitude of the structural birefringence due to the multilayer structure is less than twice the magnitude of the waveguide birefringence value due to the other waveguide birefringence. Features.
[0021]
  Further, the invention according to claim 4 is the claim.3In the invention described in item 1, the substrate is a quartz substrate, and the optical waveguide is made of quartz glass.
[0026]
  Claims5The invention described in claim 1Any one of 1 to 4The birefringence compensator is provided in the waveguide array of the arrayed waveguide grating.
[0027]
  Claims6The invention described in claim 15The birefringence compensator is a birefringence compensator using a λ / 2 wavelength plate.
[0032]
  Claims7The invention described in claim 1 to claim 16In any one of the inventions, at least one of the cores of the multilayer structure is characterized in that layers having different refractive indexes are alternately arranged so that the refractive index distribution is uniform in the core.
[0033]
  Claims8The invention described in claim 1 to claim 17In any one of the inventions, at least one of the cores of the multilayer structure is arranged such that the refractive index of each layer and the thickness of each layer are symmetrical with respect to the central layer.
[0034]
  Claims9The invention described in claim 1 to claim 16, 8In the invention according to any one of the above, at least one of the multilayer structure cores is arranged such that the refractive index of each layer increases from the both end layers in contact with the clad toward the core inner layer. And
[0035]
  Claims10The invention described in claim 1 to claim 19In any one of the inventions, the total number of layers of at least one of the cores of the multilayer structure is 5 to 10 layers.
[0036]
  Claims11The invention described in claim 17Thru10In any one of the inventions, a birefringence compensator is provided.
[0037]
  According to a twelfth aspect of the invention, in the invention of the eleventh aspect, the birefringence compensator is a birefringence compensator using a λ / 2 wavelength plate.
  The invention according to claim 13 is the invention according to any one of claims 1 to 11, wherein the birefringence value of the waveguide is 1.5 × 10. ^-4 It is characterized by the following.
The invention according to claim 14 is the structure according to any one of claims 1 to 11, wherein the value of structural birefringence generated by the multilayer structure satisfies the following expressions (1) and (2): It is characterized by being.
[Equation 3]

[Expression 4]

  However, B _s Is the value of structural birefringence caused by the multilayer structure, N is the number of layers in the multilayer structure, n _i , T _i Is the refractive index and thickness of each layer, c ₁ , C ₂ Is a constant determined for each waveguide structure by actual measurement or calculation.
[0042]
[Expression 2]

[0043]
Where N is the number of layers in the multilayer structure and n_i, T_iIs the refractive index and thickness of each layer, c₁, C₂Is a constant determined for each waveguide structure by actual measurement or calculation.
[0044]
On the right side of equation (1):
[0045]
[Equation 3]

[0046]
Is the effective refractive index in the direction parallel to the layer,
[0047]
[Expression 4]

[0048]
Represents the effective refractive index in the direction perpendicular to the layer. Therefore, these differences are structural birefringence derived from the multilayer structure. These calculated values are calculated by the correction coefficient c as shown in the equation (1).₁, C₂Is linked to the structural birefringence value Bs obtained by actual measurement or calculation.
[0049]
Correction coefficient c₁, C₂Is mainly dependent on light confinement due to the waveguide structure (relative refractive index difference, size), and is a common value for optical waveguides with constant light confinement. Therefore, it is easy to design a multilayer structure that satisfies the desired structure birefringence if an appropriate multilayer structure core is prototyped or mode analysis is performed on the appropriate multilayer structure core to obtain a correction coefficient in advance. It becomes possible.
[0050]
According to the present invention, by forming the core of the slab waveguide with a multilayer structure of layers having different refractive indexes, structural birefringence in which the effective refractive index increases in a direction parallel to the layers can be generated. Realizing the slab waveguide with reduced waveguide birefringence, which is the object of the present invention, by forming a core with a multilayer structure in the direction to compensate for birefringence (waveguide birefringence) caused by residual thermal stress, etc. The polarization dependence of the optical circuit caused by the waveguide birefringence of the slab waveguide can be reduced. In particular, the wavelength shift amount of each output port of the arrayed waveguide grating can be made uniform. Further, by combining with a conventional polarization dependence reduction technique (birefringence compensator), a waveguide type optical circuit with a lower limit of polarization dependence can be provided.
[0051]
In addition, in an optical waveguide whose optical waveguide material is quartz glass, when the substrate is made of silicon, the effective refractive index of the optical waveguide increases with respect to the electric field (TM mode) in the direction perpendicular to the substrate. By adopting a multilayer structure, a waveguide slab waveguide with reduced waveguide birefringence can be realized by canceling out waveguide birefringence due to stress and structural birefringence due to the multilayer structure.
[0052]
In addition, when the substrate is quartz, the effective refractive index of the optical waveguide is increased with respect to the electric field (TE mode) in the horizontal direction to the substrate. Waveguide birefringence can be reduced. Thereby, the polarization dependence of the optical circuit caused by the waveguide birefringence of the slab waveguide can be reduced. In particular, the wavelength shift amount of each output port of the arrayed waveguide grating can be made uniform.
[0053]
In addition, in the silica-based optical waveguide, the slab waveguide with the conventional structure has a larger waveguide birefringence than the single-mode waveguide with the conventional structure, and the structure birefringence cancels this waveguide birefringence. In order to obtain a multilayer structure, the structural birefringence due to the multilayer structure of the core of the slab waveguide is made larger than the structural birefringence due to the multilayer structure of the core of the single mode waveguide. By applying an individual multilayer structure capable of eliminating the waveguide birefringence to the slab waveguide and the single mode waveguide, a circuit with small polarization dependence can be provided. Particularly in an arrayed waveguide grating, the wavelength shift amount of each output port can be made uniform, and the wavelength shift itself can be reduced or eliminated.
[0054]
Further, by combining with a conventional polarization dependence reduction technique, a waveguide type optical circuit with a lower limit of polarization dependence reduction can be provided.
[0055]
In addition, a slab that has the same effective refractive index as before, and the refractive index perceived by light in the direction perpendicular to the layer is approximately uniform by adopting a multilayer structure in which high refractive index layers and low refractive index layers are alternately arranged. Since a waveguide can be realized, an optical circuit can be configured using a slab waveguide having the same shape as a conventional one.
[0056]
As described above, in the silica-based optical waveguide, the waveguide birefringence of the slab waveguide is larger than the waveguide birefringence of the single mode waveguide. The waveguide birefringence may vary depending on the dimensions and the shape of the optical waveguide. In the silica-based optical waveguide, the waveguide birefringence increases as the core width of the optical waveguide increases. Therefore, a Mach-Zehnder interferometer using an optical waveguide of an arrayed waveguide grating formed of a single mode waveguide, a multimode waveguide, and a slab waveguide and a multimode interference optical coupler (hereinafter referred to as MMI). In the case of an optical multiplexing / demultiplexing circuit such as combined with a single-mode waveguide, a multi-mode waveguide, and a slab waveguide, by applying an individual multilayer structure so that the structural birefringence due to the multilayer structure is increased, Each waveguide birefringence is eliminated, and a circuit with small polarization dependence can be provided.
[0057]
Also, by making the refractive index and thickness of each layer of the multilayer structure of the core approximately symmetric with respect to the central layer of the layer, the center of the electromagnetic field distribution of the propagating light can be set substantially the same as in the conventional case. Since it can be positioned at the center and the shape of the electromagnetic field distribution can be made almost symmetrical with respect to the core center in the vertical and horizontal directions with respect to the substrate, desired circuit characteristics can be realized by circuit design almost the same as in the prior art.
[0058]
In addition, by using a graded index in which the refractive index of each layer of the multilayer structure is increased from the layers on both ends on the clad side toward the layer on the core inner side, while realizing a spot size different from that of the conventional optical waveguide, the optical waveguide Birefringence can be reduced or eliminated.
[0059]
In addition, by setting the number of layers in the multilayer structure to 5 to 10, a multilayer structure can be easily realized as a core even in a thick film manufacturing process such as the FHD method. Further, as described in the embodiment, the connection loss with the optical fiber can be reduced.
[0060]
Further, by making the multilayer structure approximately satisfy the expressions (1) to (3), the multilayer structure can be determined without complicated analysis. In a multilayer structure in which at least two layers having different refractive indexes are alternately arranged, the correction coefficient c₁= 1 substantially corresponds to the case where light is sufficiently confined in the core, and is mainly 1 or less when the confinement is weak.
[0061]
Also, the polarization dependence of the waveguide optical circuit can be eliminated by combining with a conventional polarization dependence reduction technique (birefringence compensator). For example, in a Mach-Zehnder optical interferometer using an arrayed waveguide grating and an MMI as an optical coupler, a slab waveguide and a multimode waveguide are used as the core of a multilayer structure. The single-mode waveguide causes the polarization dependence of the optical circuit due to its birefringence, and the λ / 2 wavelength plate in the optical waveguide between the waveguide array of the arrayed waveguide grating and the optical coupler of the Mach-Zehnder interferometer Insert. Thereby, polarization dependence can be reduced.
[0062]
In addition, it is not necessary that the cores of all the optical waveguides of the waveguide optical circuit have a multilayer structure, and the optical waveguide portion that affects the polarization dependence of the waveguide optical circuit may be multilayered. For example, as described above, in the arrayed waveguide grating, each waveguide of the waveguide array and the slab waveguide may have a multilayer structure. In addition, even if it is applied to a part of it, the polarization dependence can be reduced as compared with the conventional case. For example, each waveguide of the waveguide array has a multi-layer core with the same proportion of length, that is, polarization dependence. For example, an appropriate portion of the optical waveguide portion that affects the property may be a multi-layer core.
[0063]
Also, the polarization dependence among the waveguide optical circuits is noticeable in the optical interference circuit in which one optical coupler and the other optical coupler are connected by a plurality of waveguides having different waveguide lengths. . A typical configuration of the optical interference circuit is the above-described Mach-Zehnder interferometer or arrayed waveguide grating. In the optical interference type circuit, the polarization dependence of the circuit can be reduced or eliminated by reducing or eliminating the polarization dependence only for the waveguide length difference. That is, the waveguide length difference with respect to the waveguide having the shortest waveguide length may be used as the multilayer structure core. Or it is good also considering the length which added waveguide length difference and fixed length as a multilayer structure. Further, it is not necessary to continuously form the multilayer structure core, and the total waveguide length may be a predetermined length. An optical interference type circuit in which a plurality of optical couplers such as a lattice type filter are connected by a plurality of waveguides is also included in the claims.
[0064]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0065]
[Embodiment 1]
4A and 4B are diagrams showing a first embodiment of a waveguide optical circuit according to the present invention. FIG. 4A is a sectional view of a slab waveguide, and FIG. 4B is a single mode waveguide. It is sectional drawing. A silicon substrate is used as the substrate 401, and the clad 402 and the cores 403 and 404 are made of quartz glass. Here, the core (slab waveguide) 403 has a multilayer structure in which a plurality of first cores 403a and second cores 403b are stacked in the horizontal direction on the substrate, and the core 404 (single mode waveguide) is the first core. It has a multilayer structure in which a plurality of 404a and second cores 404b are stacked in the horizontal direction on the substrate.
[0066]
The relative refractive index difference, the layer thickness, and the number of layers of the first core 403a, 404a and the second core 403b, 404b are the average relative refractive index difference of the cores 403, 404 is 0.75%, and the core thickness is 6 μm (however, the core The waveguide width of 404 is 6 μm), and the structural birefringence value B due to the multilayer structure is −9 × 10^-4(Single mode waveguide conversion -7 × 10^-4) Was obtained by mode analysis of the waveguide. Here, the average relative refractive index difference was an area average of the relative refractive index differences of the core layers.
[0067]
The relative refractive index difference Δi, the layer thickness ti, and the number of layers Ni (where i = 1, 2) of each layer used in the present embodiment are:
First core 403a, 404a: Δ1 = 4.4%, t1 = 0.15 μm, N1 = 7,
Second core 403b, 404b: Δ2 = 0%, t2 = 0.83 μm, N2 = 6.
[0068]
The slab waveguide of the present embodiment was manufactured as follows. A silicon substrate is used as the substrate 401, and a quartz-based lower clad layer (clad under the core) and a core layer are formed on the substrate 401 by a flame deposition method. In the core layer, the first core and the second core are alternately deposited. The first core has GeO to increase the refractive index.₂Is added in an amount of 44 mol%, and a small amount of B is added to lower the clearing temperature.₂O₃And P₂O₅Is added. Next, unnecessary portions of the core layer are removed by reactive ion etching to form ridge-like cores 403 and 404, and then an upper clad having a refractive index equivalent to that of the lower clad so as to cover the cores 403 and 404 is formed. Formed by flame deposition. The clad 402 is composed of the lower clad and the upper clad.
[0069]
An arrayed waveguide grating 101 shown in FIG. 1 is manufactured by using the slab waveguide and the single mode waveguide having the multilayer structure. That is, the slab waveguide 102 is manufactured by the slab waveguide having the multilayer structure described above, and the waveguide array 103, the input waveguide 104, and the output waveguide 105 are formed by the single mode waveguide having the multilayer structure described above. Was made.
[0070]
FIG. 5 shows the wavelength shift of the TM mode with respect to the TE mode of each output port of the fabricated arrayed waveguide grating 101. Since the waveguide array 103 has a multilayer structure, the wavelength shift is about −0.51 nm, but the variation between output ports is about 0.004 nm, which is 1/5 of the conventional 0.02 nm. Reduced. This shows that the waveguide birefringence of the slab waveguide is reduced.
[0071]
A half-wave plate was inserted into the arrayed waveguide grating 101 so as to cross the waveguide array 103 vertically. The wavelength shift at each output port is within ± 0.003 nm (the variation between output ports is about 0.004 nm), and the limit of polarization dependence reduction can be lowered compared to the conventional ± 0.01 nm. It was.
[0072]
[Embodiment 2]
In the second embodiment of the present invention, the relative refractive index difference Δ of the core in the first embodiment is changed from 0.75% to 1.5%. Increasing the relative refractive index difference has the advantage that the bending radius of the single mode waveguide can be reduced, thereby reducing the size of the optical circuit. However, since a connection loss with a normal optical fiber increases, it is necessary to apply a spot size conversion technique or the like in the input / output waveguide section.
[0073]
The relative refractive index difference, the layer thickness, and the number of layers of the first core 403a, 404a and the second core 403b, 404b are 1.5% of the average relative refractive index difference of the cores 403, 404, and the core thickness is 4 μm. The waveguide width of 404 is 4 μm), and the structural birefringence value B due to the multilayer structure is −5 × 10^-4It was determined by mode analysis of the waveguide. The reason why the structural birefringence B is made smaller than that of the first embodiment is that the birefringence value of the slab waveguide having the conventional structure becomes smaller as the relative refractive index difference Δ becomes higher.
[0074]
The relative refractive index difference Δi, the layer thickness ti, and the number of layers Ni (where i = 1, 2) of each layer used in the present embodiment are:
First core 403a, 404a: Δ1 = 2.7%, t1 = 0.58 μm, N1 = 4
Second cores 403b and 404b: Δ2 = 0%, t2 = 0.57 μm, N2 = 3
It is.
[0075]
The slab waveguide of the present embodiment is manufactured in the same manner as in the first embodiment, and the first core has GeO in order to increase the refractive index.₂27 mol% is added.
[0076]
The arrayed waveguide grating 101 having the circuit configuration shown in FIG. 1 was produced by using the slab waveguide and the single mode waveguide having the multilayer structure as in the first embodiment. Thereafter, a half-wave plate was inserted into the arrayed waveguide grating 101 so as to cross the waveguide array 103 vertically. The wavelength shift amount of the TM mode compared to the TE mode at each output port is within ± 0.005 nm (the variation between the output ports is 0.005 nm), and the conventional wavelength shift is the same as in the first embodiment. The value is small compared to the amount.
[0077]
[Embodiment 3]
FIGS. 6A and 6B are diagrams showing a third embodiment of the slab waveguide according to the present invention. FIG. 6A is a sectional view of the slab waveguide, and FIG. 6B is a sectional view of the single mode waveguide. It is. A quartz substrate is used for the substrate 601, and the clad 602 and the cores 603 and 604 are made of quartz glass. Here, the core (slab waveguide) 603 has a multilayer structure in which a first core 603a and a second core 603b are formed in a plurality of layers (usually several hundred to several thousand layers) in a direction perpendicular to the substrate. The (single-mode waveguide) has a multilayer structure in which a first core 604a and a second core 604b are formed in a plurality of layers (usually several layers to several tens layers) in a direction perpendicular to the substrate.
[0078]
The refractive index and layer thickness of the first cores 603a and 604a and the second cores 603b and 604b are such that the average relative refractive index of the cores 603 and 604 is 0.75% and the core thickness is 6 μm (however, the waveguide width of the core 604 is 6 μm), and the structure birefringence value B by the multilayer structure is −9 × 10^-4(Single mode waveguide conversion -7 × 10^-4) Was obtained by mode analysis of the waveguide.
[0079]
The relative refractive index difference Δi and the layer thickness ti (where i = 1, 2) of each layer used in the present embodiment are:
First core 603a, 604a: Δ1 = 4.9%, t1 = 0.23 μm
Second core 603b, 604b: Δ2 = 0%, t2 = 1.69 μm
It is.
[0080]
The slab waveguide of the embodiment of the present invention was manufactured as follows. A quartz substrate is used as the substrate 601, and core layers corresponding to the first cores 603a and 604a are formed on the substrate 601 by a flame deposition method. The first cores 603a and 604a have GeO in order to increase the refractive index.₂49 mol% of each is added. Next, after removing a portion corresponding to the second core by the reactive ion etching method, a clad is formed by a flame deposition method so as to cover the first cores 603a and 604a having a strip shape. The second cores 603b and 604b are formed simultaneously with the cladding 602.
[0081]
At this time, in the case of a slab waveguide, the interface between the layers of the first core and the second core constituting the core is provided substantially parallel to the line segment connecting the center of the input end and the center of the output end. In the case of a single-mode waveguide, it is provided approximately parallel to the light traveling direction, in other words, in the direction along the waveguide.
[0082]
An arrayed waveguide grating 101 as shown in FIG. 1 was manufactured by using the slab waveguide and the single mode waveguide having the multilayer structure. That is, the slab waveguide 102 is manufactured by the slab waveguide having the multilayer structure described above, and the waveguide array 103, the input waveguide 104, and the output waveguide 105 are formed by the single mode waveguide having the multilayer structure described above. Was made. Thereafter, a half-wave plate was inserted so as to cross the waveguide array 103 vertically.
[0083]
The TM mode wavelength shift compared to the TE mode at each output port is within ± 0.006 nm (variation between output ports is approximately 0.005 nm), which reduces the polarization dependence that was previously ± 0.01 nm. We were able to lower the limit.
[0084]
In this embodiment, the cores 603 and 604 are formed directly on the substrate 601. However, similar to the first and second embodiments, the core 603 and 604 may be realized by providing a clad below the core.
[0085]
[Embodiment 4]
In the fourth embodiment of the present invention, the multilayer structure of the core of the waveguide array 103 of the arrayed waveguide grating 101 shown in FIG. 1 described in the first embodiment has a structural birefringence smaller than that of the slab waveguide 102. It has a multilayer structure.
[0086]
The multilayer structure of the core of the waveguide array 103 has a structural birefringence value B of −2.3 × 10.^-4The relative refractive index difference Δi, the layer thickness ti, and the number of layers Ni (where i = 1, 2) of the first core and the second core are
First core: Δ1 = 2%, t1 = 0.45 μm, N1 = 5
Second core: Δ2 = 0%, t2 = 0.94 μm, N2 = 4
It was.
[0087]
The arrayed waveguide grating 101 of the present embodiment was manufactured as follows. A lower clad was formed on a silicon substrate by a flame deposition method, and a waveguide array core layer was fabricated thereon by the flame deposition method with the above parameters. The first core has GeO₂Is added in an amount of 20 mol%, and a small amount of B is added.₂O₃And P₂O₅Contains. The core layer for the waveguide array, which is a part for forming the slab waveguide 102, is removed by the reactive ion etching method, and then the core layer for the slab waveguide 102 is formed by the flame deposition method with the same parameters as in the first embodiment. Formed. The core layer for the slab waveguide 102 formed on the core layer for the waveguide array was removed by a reactive ion etching method. Thereby, a core layer having a multilayer structure suitable for the slab waveguide forming portion and the single mode waveguide forming portion can be formed on the lower clad.
[0088]
Next, unnecessary portions of the core layer were removed by a reactive ion etching method to form a ridge-shaped core, and an upper clad having a refractive index equivalent to that of the lower clad was formed by a flame deposition method so as to cover the core.
[0089]
The wavelength shift amount of the TM mode with respect to the TE mode of each output port of the manufactured arrayed waveguide grating 101 is 0.02 nm or less (the variation between output ports is 0.004 nm). As a method for reducing the polarization dependence, 1 / Although the polarization dependency is slightly larger than the method using the two-wavelength plate, the wavelength shift before application of the method for reducing the polarization dependency is 0.2 to 0.3 nm (the variation between the output ports is 0.02 nm). Compared to (2), sufficiently small wavelength shift and output port variation can be realized.
[0090]
[Embodiment 5]
In the fifth embodiment of the present invention, the multilayer structure of the slab waveguide of the first and fourth embodiments is set more strictly by the equations (1) to (3), and the wavelength shift of the arrayed waveguide grating is set. The variation is reduced.
[0091]
  The relative refractive index difference between the first core 403a and the second core 403b of the slab waveguide shown in FIG.Δ _i, Layer thickness t_i(I = 1, 2) was set by the following procedure.
[0092]
Procedure 1) An optical circuit is fabricated with an appropriate multilayer structure, and the structural birefringence value due to the multilayer structure is estimated from the characteristics. From the result and equations (1) to (3), the correction coefficient c₁, C₂Decide.
[0093]
  Procedure 2) Relative refractive index difference from equations (1) to (3) so that the structural birefringence due to the multilayer structure becomes a predetermined value.Δ _i, Layer thickness t_i(I = 1, 2) is determined. In step 1), if the multi-layer structure is manufactured with good accuracy and stress birefringence is not newly added in addition to the structure birefringence of the multi-layer structure in which layers having different refractive indexes are laminated, mode analysis is performed instead of manufacturing an optical circuit. May be used.
[0094]
  Average relative refractive index difference of the coreΔ _aveIn order to determine a multilayer structure with a core thickness of 6 μm, the procedure 1), the arrayed waveguide grating shown in FIG. 1 is used for the following three types of relative refraction of the first core 403a and the second core 403b. Rate differenceΔ _i, Layer thickness t_i(I = 1, 2). The total number of layers was 13, the number of first core layers N1 was 7 and the number of second core layers N2 was 6 layers.
[0095]
  1) First core 403a,Δ ₁= 1.0%, t₁= 0.64 μm
        Second core 403b,Δ ₂= 0%, t₂= 0.25μm
  2) First core 403a,Δ ₁= 3.0%, t₁= 0.2 μm
        Second core 403b,Δ ₂= 0%, t₂= 0.76 μm
  3) First core 403a,Δ ₁= 5.1%, t₁= 0.12 μm
        Second core 403b,Δ ₂= 0%, t₂= 0.86 μm
  The slab waveguide and the single mode waveguide were produced in the same manner as in the first embodiment, and an arrayed waveguide grating was produced. GeO to increase the refractive index₂Was doped at 10 mol% per 1% relative refractive index difference.
[0096]
  In order to estimate the value Bs of the waveguide birefringence due to the multilayer structure, the dispersion of the TM mode wavelength shift with respect to the TE mode at each output port of the fabricated arrayed waveguide grating was measured. The birefringence value Be in the slab waveguide is estimated from this variation, and the waveguide birefringence value Bo generated in the conventional core structure from this Be = 1.1x.10 ^{− 3} The value Bs of the waveguide birefringence due to the multilayer structure was estimated. Where Bo = 1.1x10 ^{− 3} Is a value estimated again from the wavelength shift of the first embodiment. The black circles shown in FIG. 7 plot the magnitude of the birefringence | Bs | by the multilayer structure against the relative refractive index difference of the first core. From this result and the equations (1) to (3), the correction coefficient c₁1 and c₂Was set to 0. The solid line is calculated by this correction coefficient and the equations (1) to (3).
[0097]
  Next, as procedure 2), the correction coefficient c₁1 and c₂From the expressions (1) to (3) where 0 is 0, | Bs | = | Bo | = 1.1x10 ^{− 3} The relative refractive index difference between the first core 403a and the second core 403bΔ _i, Layer thickness t_i(I = 1, 2) is obtained. From the solid line as the calculation result of FIG. 7, | Bs | = | Bo | = 1.1x10 ^{− 3} The relative refractive index difference of the first core 403aΔ ₁Is 5.5%, and from equations (2) and (3), t₁, T₂Takes the following values:
[0098]
  First core 403a,Δ ₁= 5.5%, t₁= 0.11 μm, N₁= 7
  Second core 403b,Δ ₂= 0%, t₂= 0.87 μm, N₂= 6
  From Fig. 7, | Bs | is roughlyΔ ₁Since it changes as a linear function ofΔ ₁For | Bs |Δ ₁Estimate the approximate curve as a linear function ofΔ ₁You may ask for.
[0099]
The arrayed waveguide grating shown in FIG. 1 was produced from the slab waveguide having a multilayer structure. Thereafter, a half-wave plate was inserted into the arrayed waveguide grating 101 so as to cross the waveguide array 103 vertically. The amount of wavelength shift of the TM mode with respect to the TE mode at each output port was within ± 0.002 nm (the variation between the output ports was 0.002 nm).
[0100]
  Similar to the fourth embodiment, the structural birefringence value of the waveguide array 103 is −2.3 × 10 6.^{− Four}The multi-layer structure becomes the difference in relative refractive index between the first core and the second core.Δ _i, Layer thickness t_i, Number of layers N_i(I = 1, 2) is
  1st coreΔ ₁= 2%, t₁= 0.45 μm, N₁= 5
  2nd coreΔ ₂= 0%, t₂= 0.94 μm, N₂= 4
  It produced so that it might become. The amount of wavelength shift in the TM mode compared to the TE mode at each output port was within ± 0.008 nm (the variation between the output ports was 0.002 nm).
[0101]
The polarization dependency of the arrayed waveguide grating can be reduced by setting the multilayer structure of the slab waveguides of the first and fourth embodiments more strictly by the equations (1) to (3).
[0102]
[Embodiment 6]
In the sixth embodiment of the present invention, the number of core layers in the fifth embodiment is changed in a range of about 1/3 to about 2 times.
[0103]
  The multilayer structure of the core 403 of the slab waveguide shown in FIG. 4A is similar to the fifth embodiment in the average relative refractive index difference.Δ _aveIs 0.75%, the core thickness is 6 μm, the relative refractive index difference of the first core 403aΔ ₁5.5%, relative refractive index difference of the second core 403bΔ ₂0% (birefringence due to multilayer structure | Bs | is 1.1 × 10^{− Three}). Number of first core layers N₁The arrayed waveguide grating shown in FIG. Number of layers of second core N₂(N₁-1) layer. Layer thickness t of the first core 403a₁And the layer thickness t of the second core 403b₂Was set as follows.
[0104]
1) N₁= 2, t₁= 0.39 μm, t₂= 5.22 μm
2) N₁= 3, t₁= 0.26 μm, t₂= 2.61 μm
3) N₁= 4, t₁= 0.20 μm, t₂= 1.74 μm
4) N₁= 5, t₁= 0.16 μm, t₂= 1.31 μm
5) N₁= 6, t₁= 0.13 μm, t₂= 1.04 μm
6) N₁= 7, t₁= 0.11 μm, t₂= 0.87μm
7) N₁= 8, t₁= 0.10 μm, t₂= 0.75μm
8) N₁= 9, t₁= 0.09 μm, t₂= 0.65 μm
9) N₁= 10, t₁= 0.08 μm, t₂= 0.58 μm
10) N₁= 11, t₁= 0.07 μm, t₂= 0.52 μm
The manufacturing process of the waveguide is the same as that of the first embodiment.
[0105]
  The dispersion between ports in the wavelength shift of the TM mode compared to the TE mode at each output port is N₁Is 2 and 0.005 nm, and | Bs | is 2.5x from the predetermined value.10 ^{− 4} It is getting bigger. This is because the waveform distortion is larger than the electromagnetic field distribution of the conventional structure. When the total number of layers is 5 or more, the port-to-port variation is 0.003 nm, which is 1.5 × for a predetermined | Bs |.10 ^{− 4} It can be seen that it can be set within. Accordingly, the total number of layers is desirably 5 or more.
[0106]
[Embodiment 7]
The seventh embodiment of the present invention is a waveguide type optical circuit composed of three types of optical waveguides having different widths, a single mode waveguide, a multimode waveguide, and a slab waveguide. A multilayer structure in which structural birefringence increases in the order of a waveguide, a multimode waveguide, and a slab waveguide is applied. The waveguide type optical circuit includes an arrayed waveguide grating 101 shown in FIG. 1 and a Mach-Zehnder interferometer 801 using the MMI shown in FIG. 8 as an optical coupler.
[0107]
  Hereinafter, as a reference example, a waveguide optical circuit configured with a Mach-Zehnder interferometer will be described.
  FIG. 8A is a diagram showing the configuration of a Mach-Zehnder interferometer 801, which is an MMI 802 that is an optical coupler, an optical waveguide 803 that connects two MMIs, an input waveguide 804 that is connected to one MMI, and the other And an output waveguide 805 connected to the MMI. FIG. 8B is a diagram showing the configuration of the MMI 811, which includes a multimode waveguide 812, an input waveguide 813 connected to one end thereof, and an output waveguide 814 connected to the other end. .
[0108]
  Average relative refractive index difference of the coreΔ _aveWas 0.75%, the core height was 6 μm, the core width of the single mode waveguide was 6 μm, the core width of the multimode waveguide was 24 μm, and the width of the slab waveguide was 2 mm. The birefringence magnitude | Bs | due to the multilayer structure is 2.3 × 10 in a single mode waveguide.^{− Four}5x10 with multimode waveguide^{− Four}1.1x10 with slab waveguide^{− Three}The specific refractive index difference of each layer using the equations (1) to (3)Δ _i, Layer thickness t_i, Number of layers N_iWas set to the following values: The correction factor is c in a single mode waveguide.₁= 0.8, c₂= 0, c in multimode waveguide₁= 0.97, c₂= 0, c in slab waveguide₁= 1, c₂= 0.
[0109]
  Single mode waveguide:
    1st coreΔ ₁= 2%, t₁= 0.45 μm, N₁= 5
    2nd coreΔ ₂= 0%, t₂= 0.94 μm, N₂= 4
  Multimode waveguide:
    1st coreΔ ₁= 3.1%, t₁= 0.28 μm, N₁= 5
    2nd coreΔ ₂= 0%, t₂= 1.15 μm, N₂= 4
  Slab waveguide:
    1st coreΔ ₁= 5.5%, t₁= 0.16 μm, N₁= 5
    2nd coreΔ ₂= 0%, t₂= 1.31 μm, N₂= 4
  The waveguide type optical circuit was manufactured in the same procedure as in the fourth embodiment, and a core was formed in the order of a slab waveguide, a multimode waveguide, and a single mode waveguide.
[0110]
The characteristics of the Mach-Zehnder interferometer of the fabricated waveguide type optical circuit are as follows. The wavelength shift is from 0.25 nm to 0.01 nm, and the peak wavelength loss difference between the TE mode and the TM mode is from 0.2 dB to 0.2 mm. The characteristic of the arrayed waveguide grating is that the wavelength shift amount of the TM mode with respect to the TE mode at each output port is within ± 0.01 nm from the conventional ± 0.13 nm (variation between output ports 0.02 nm) ( The variation between output ports was reduced to 0.002 nm). As a result, it was confirmed that the polarization dependency can be reduced by applying an appropriate multilayer structure to the waveguide type optical circuit composed of a single mode waveguide, a multimode waveguide, and a slab waveguide. .
[0111]
[Embodiment 8]
The eighth embodiment of the present invention has a core structure in which three types of layers having different relative refractive index differences are multilayered almost alternately.
[0112]
FIG. 9 is a cross-sectional view of the slab waveguide according to the eighth embodiment of the present invention. A silicon substrate is used as the substrate 901, and the clad 902 and the core 903 are formed of quartz glass. The core 903 includes a first core 903a, a second core 903b, and a third core 903c. The first core 903a, the second core 903b, the third core 903c, and a second core 903b having a film thickness of ½ from the substrate side. The first core 903a, the second core 903b having a thickness of 1/2, the third core 903c, the second core 903b, and the first core 903a are stacked in this order, and each layer is symmetrical with respect to the core central layer. Arranged.
[0113]
  Average relative refractive index difference of the coreΔ _aveIs 0.75%, the core thickness is 6 μm, and the relative refractive index difference of the first core 903aΔ ₁6%, relative refractive index difference of the second core 903bΔ ₂5%, relative refractive index difference of the third core 903bΔ ₂Is 0%, and the structural birefringence | Bs | by the multilayer structure is 1.1 × 10^{− Three}Is the correction coefficient c obtained in the fifth embodiment.₁= 1, c₂The following settings were made according to the equations (1) to (3) using = 0 to produce the arrayed waveguide grating shown in FIG. The manufacturing process is the same as in the first embodiment.
[0114]
  First core layer 903a,Δ ₁= 6%, t₁= 0.12 μm, N₁= 3
  A second core layer 903b,Δ ₂= 5%, t₂= 0.14 μm, N₂= 4
                (The thickness of the second core layer 903b having a thickness of 1/2 is 0.07 μm,
        N₂= 6 includes two layers with a film thickness of 1/2)
  A third core layer 903b,Δ ₂= 0%, t₂= 2.61 μm, N₂= 2
  The wavelength shift amount of the TM mode compared to the TE mode at each output port of the produced arrayed waveguide grating was within ± 0.01 nm (variation between output ports was 0.003 nm). It was confirmed that a waveguide-type optical circuit in which the polarization dependence was eliminated could be realized even with a structure in which three different refractive index layers were arranged almost alternately.
[0115]
The advantage of configuring a multilayer structure by alternately arranging three types of layers having different relative refractive index differences compared to configuring a multilayer structure by alternately arranging two types of layers having different relative refractive index differences, This is to increase the degree of freedom in selecting the relative refractive index difference and the film thickness. The average relative refractive index difference of the core and the structural birefringence | Bs | due to the multilayer structure are mainly determined by the relative refractive index difference and the film thickness of the layers constituting the multilayer structure. In the case of an alternating layer of two types of layers, the parameters that define the multilayer structure are two types of relative refractive index differences and two types of film thickness. If one of them is set, the average relative refractive index difference Since the structure birefringence | Bs | is determined, the other three values are automatically set. In the case of three layers of alternating arrangement, there are six parameters that define the multilayer structure, and if three of them are defined, the average relative refractive index difference and | Bs | The three values are determined. Therefore, for example, in the alternate arrangement of two types of layers, the relative refractive index difference of the second core is set to 0% as in the first to seventh embodiments. The film thicknesses of the first core and the second core were automatically determined. In the alternate arrangement of the three types of layers, the film thickness of each layer is automatically determined by setting the relative refractive index difference of each layer.
[0116]
That is, the relative refractive index difference can be appropriately selected. This is because if there are parameters that are difficult to produce, for example, it can only be set to three different types of relative refractive index differences, if the film thickness can be set with high accuracy, an appropriate structural birefringence can be given, There is an advantage that birefringence can be reduced. As another advantage, in a silica-based optical waveguide, a light-induced refractive index change can be efficiently generated. GeO₂Quartz glass with a high concentration of can obtain a high light-induced refractive index. Therefore, in the alternating arrangement of the three layers, GeO₂May be set, that is, a layer having a high relative refractive index, and the value of the structural birefringence may be adjusted by the film thickness.
[0117]
In the present invention, the average relative refractive index difference of the core, the size, and the structural birefringence value due to the multilayer structure may be adjusted by the relative refractive index difference of each layer of the core, the thickness, and the number of layers. Therefore, the average relative refractive index difference, the size, the birefringence value due to the multilayer structure, the relative refractive index difference of each layer of the core, the thickness, and the number of layers of the above-described embodiment are not limited.
[0118]
The single mode waveguide used in the waveguide array or the like in the present invention is a waveguide of about two modes and includes a pseudo single mode waveguide that functions as a single mode waveguide in the circuit.
[0119]
In the above-described embodiment, the arrayed waveguide grating is used as the waveguide type optical circuit using the slab waveguide. However, the present invention is not limited to this and can be applied to a star coupler or the like.
[0120]
In the above-described embodiment, the structural birefringence value due to the multilayer structure is made substantially equal to the waveguide birefringence value generated in the conventional core structure, but the waveguide birefringence value generated in the conventional core structure is 2 If the value is less than double, at least the birefringence of the optical waveguide can be reduced as compared with the conventional one, and the polarization dependence of the optical circuit can be reduced.
[0121]
In the above-described embodiment, the core is formed by the flame deposition method. However, the core is not limited to the manufacturing method, and other means capable of forming a multilayer structure using quartz glass, such as sputtering, plasma CVD, ECR-CVD, etc. Even if formed by this, waveguide birefringence can be reduced or eliminated. GeO is also used to adjust the refractive index.₂However, as long as the refractive index can be set to a desired value, TiO₂Other dopants may be applied. Further, although quartz-based glass is used as the waveguide material, it is only necessary that layers having different refractive indexes can be multilayered, and other glass-based materials and polymer materials can be applied.
[0122]
With the number of layers in this embodiment, the connection loss between the single mode waveguide and the optical fiber can be reduced by about 0.1 to 0.3 dB, and the smaller the number of layers, the smaller the value. The excess loss as a circuit of the manufactured optical circuit is about the same as the conventional one. Therefore, by appropriately selecting the number of layers, the spot size of light propagating through the input / output waveguide can be made closer to that of the optical fiber without damaging the circuit characteristics, and the connection loss with the optical fiber can be reduced. Can do.
[0123]
In the above-described embodiment, a multilayer structure in which several types of layers having different refractive indexes are arranged alternately is used. However, the present invention is not limited to this, and the refractive index increases from the clad side layer toward the core center. It can be applied to a graded index structure. In the case of alternating arrangement, the light feels the average value of the birefringence distribution due to stress, and its magnitude is almost the same as the waveguide birefringence in the conventional optical waveguide structure, but the graded index In this case, since the electric field distribution is strong in the layer near the center of the core, the correction coefficient c of the above-described embodiment is obtained when there is a significant stress distribution in the core.₁Cannot be used as it is. Therefore, a correction factor c that makes several suitable structures and fits them.₁, C₂Need to be estimated.
[0124]
【The invention's effect】
As described above, according to the present invention, the waveguide birefringence of the slab waveguide can be reduced, and the polarization dependence caused by the waveguide birefringence of the slab waveguide can be reduced. The wavelength shift of the output port can be made uniform. Further, by combining with the conventional polarization dependence reduction technology, the reduction limit of polarization dependence can be lowered, and a high performance waveguide optical circuit can be provided. This is to reduce loss of signals passing through the optical multiplexer / demultiplexer and fluctuation of crosstalk in the optical multiplexer / demultiplexer, and to improve signal reliability in an optical communication system using the optical WDM technology. .
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a conventional arrayed waveguide grating.
FIG. 2 is a cross-sectional view of a conventional slab waveguide and waveguide array.
FIG. 3 is a graph showing the wavelength shift of each output port in a conventional arrayed waveguide grating.
FIG. 4 is a cross-sectional view showing first, second, and fourth embodiments of the waveguide type optical circuit of the present invention.
FIG. 5 is a graph showing the wavelength shift of each output port in the arrayed waveguide grating according to the first embodiment.
FIG. 6 is a cross-sectional view showing a third embodiment of the waveguide optical circuit of the present invention.
FIG. 7 is a graph of the birefringence magnitude | Bs | due to the multilayer structure with respect to the relative refractive index difference Δ1 of the first core in the multilayer structure core for explaining the fifth embodiment of the present invention;
8A is a diagram illustrating a Mach-Zehnder interferometer, and FIG. 8B is a diagram illustrating a multimode interference optical coupler.
FIG. 9 is a cross-sectional view showing an eighth embodiment of the present invention.
[Explanation of symbols]
101 arrayed waveguide grating
102 Slab waveguide
103 Waveguide Array
104 Input waveguide
105 Output waveguide
201 substrate
202 clad
203 core (slab waveguide)
204 core (single mode waveguide)
401 substrate
402 clad
403 core (slab waveguide)
404 core (single mode waveguide)
403a, 404a 1st core
403b, 404b second core
601 substrate
602 clad
603 core (slab waveguide)
604 core (single mode waveguide)
603a, 604a First core
603b, 604b Second core
801 Mach-Zehnder interferometer
802 Multimode interference optical coupler (MMI)
803 Optical waveguide
804 Input waveguide
805 output waveguide
811 Multimode interference optical coupler (MMI)
812 Multimode waveguide
813 Input waveguide
814 Output waveguide
901 substrate
902 clad
903 core
903a 1st core
903b Second core
903c 3rd core

Claims (14)

A waveguide-type optical circuit which is formed on a substrate and has a clad and a core made of a glass-based material or a polymer material , and the core comprises an arrayed waveguide grating having a slab waveguide covered with the clad Because
At least a part of the core of the slab waveguide has a multilayer structure that is parallel to the substrate surface and includes at least three layers, and the adjacent layers have different refractive indexes. The layer direction of the multilayer structure is formed so as to compensate for waveguide birefringence, and the magnitude of the structural birefringence due to the multilayer structure is twice the magnitude of the waveguide birefringence value due to other waveguide birefringence. A waveguide-type optical circuit characterized by being less than.   The waveguide optical circuit according to claim 1, wherein the substrate is a silicon substrate, and the optical waveguide is made of silica glass. A waveguide-type optical circuit which is formed on a substrate and has a clad and a core made of a glass-based material or a polymer material , and the core comprises an arrayed waveguide grating having a slab waveguide covered with the clad Because
At least a part of the core of the slab waveguide is a multilayer structure that is perpendicular to the substrate surface and includes at least three layers, and the adjacent layers have different refractive indexes, and the interface between the layers is the input center and the output of the slab waveguide. The layer direction of the multilayer structure is formed so that the structure birefringence by the multilayer structure compensates for other waveguide birefringence, and is parallel to the line connecting the center, and the structure birefringence by the multilayer structure Is less than twice the size of the waveguide birefringence value due to other waveguide birefringence.   4. The waveguide optical circuit according to claim 3, wherein the substrate is a quartz substrate, and the optical waveguide is made of silica glass.   5. The waveguide optical circuit according to claim 1, wherein a birefringence compensator is provided in a waveguide array of the arrayed waveguide grating.   6. The waveguide optical circuit according to claim 5, wherein the birefringence compensator is a birefringence compensator using a λ / 2 wavelength plate.   7. The layer according to claim 1, wherein at least one of the multi-layered cores is configured such that layers having different refractive indexes are alternately arranged so that a refractive index distribution is uniform in the core. Waveguide type optical circuit.   The at least one of the cores of the multilayer structure is disposed so that the refractive index of each layer and the thickness of each layer are symmetric with respect to the central layer. Waveguide type optical circuit.   9. The multilayer core according to claim 1, wherein at least one of the multi-layered cores is arranged such that the refractive index of each layer increases from the both end layers in contact with the clad toward the inner layer of the core. The waveguide type optical circuit according to any one of the above.   The waveguide type optical circuit according to claim 1, wherein the total number of layers of at least one core of the multilayer structure is 5 to 10 layers.   11. The waveguide optical circuit according to claim 7, further comprising a birefringence compensator.   12. The waveguide optical circuit according to claim 11, wherein the birefringence compensator is a birefringence compensator using a λ / 2 wavelength plate. The birefringence value of the waveguide is 1.5 × 10 ^-4 The waveguide optical circuit according to claim 1, wherein: The waveguide optical circuit according to claim 1, wherein a value of structural birefringence generated by the multilayer structure satisfies the following formulas (1) and (2).

  However, B _s Is the value of structural birefringence caused by the multilayer structure, N is the number of layers in the multilayer structure, n _i , T _i Is the refractive index and thickness of each layer, c ₁ , C ₂ Is a constant determined for each waveguide structure by actual measurement or calculation.

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