JP3832743B2 - Waveguide type multichannel chromatic dispersion compensator and optical module using the same - Google Patents

Description

但し、ｎ_i、ｍ_i、ｋ_iは整数、λ_cは分散補償回路の動作波長域の中心近傍のチャネルの光波長である。又、上記光周波数間隔をΔＦ、光の速度をＣと表記したとき基本光路長差ｌ₁、ｌ₂は次式で表される。
【数６】

【０００９】
そのため、チャネル毎に波長分散補償量を異にすることができなかった。つまり、上記導波路ラティス分散補償回路で報告されている分散補償動作は１回路あたり１チャネルであり、ＷＤＭ方式で用いられる多チャネル信号を補償するには、チャネルの数だけ分散補償量の異なる補償器を設計、作製しなければならない。さらには各チャネルの信号を割り振るためにＡＷＧなどのフィルタを用いる必要がある。必要なＡＷＧの台数は端局で１台、中継局で２台必要になる。以上の理由により分散スロープを補償可能な多チャネルのラティス型の分散補償器は、チップサイズが大型になるかマルチチップ構成にする必要があった。
【００１０】
つまり、上述した従来構成の導波路ラティス型波長分散補償器は、空間光学系部分がなく、部品点数が少なく、挿入損失を小さくできるという特徴を有するが、多チャネル信号を補償するには、チャネルの数だけ分散補償量の異なる補償器を設計、作製しなければならず、さらにはチップサイズが大型になるかマルチチップ構成にしなければならないという問題があった。
【００１１】
本発明は上記課題に鑑みなされたもので、１台あるいは少数台の導波路ラティス型構成の補償器で、多チャネルの光信号の波長分散を補償し、かつ各チャネルで光ファイバの波長分散スロープに応じて異なる波長分散量を補償することができる導波路型多チャネル波長分散補償器を提供することを目的とする。
【００１４】
【課題を解決するための手段】
上記課題を解決する本発明の請求項１に係る導波路型多チャネル波長分散補償器は、第１の光導波路と、第２の光導波路と、前記第１の光導波路及び前記第２の光導波路をそれぞれ異なる箇所で結合するＮ＋１（Ｎは自然数）個の光結合器とを有し、Ｎ＋１個の前記光結合器の内、隣接する２個の前記光結合器で挟まれたＮ個の光路長差付与部を備えた光導波路回路、又は前記光導波路回路を直列に複数台繋いだ光導波路回路であって、入力端から出力端までのＭ個（Ｍは自然数）の光路長差付与部のうち、入力端からｉ番目の光路長差付与部が与える光路長差Ｌ_iが、すべてのｉにおいて、次式で表されるように設定して、光信号の周波数に対して、所定の周期で繰り返す単調減少の波長分散特性と、前記所定の周期より長めの周期で繰り返す単調増加の波長分散特性とを形成し、上記異なる波長分散特性を重畳させることにより、一定の光周波数間隔を有し、光波長が相異なる複数の光チャネルの周辺帯域で、波長分散を一括して補償し、かつ、波長分散補償量を光チャネル毎に異にして、一括して波長分散スロープを補償するようにすることを特徴とする。
【数７】

但し、ｎ_i、ｍ_i、ｋ_iは整数、λ_cはこの多チャネル波長分散補償器の動作波長域の中心近傍のチャネルの光波長であり、上記光周波数間隔をΔＦ、光の速度をＣと表記し、δを光周波数変調度としたとき基本光路長差ｌ₁、ｌ₂とδは次式で表される。
【数８】

【００１５】
上記課題を解決する本発明の請求項２に係る導波路型多チャネル波長分散補償器は、入力側の第１の光導波路回路に、それぞれ直列に並列に接続された第２の光導波路回路及び第３の光導波路回路の３つの前記光導波路回路から構成され、前記第２の光導波路回路と前記第３の光導波路回路との前記光路長差付与部の前記光路長差及び前記光結合器の前記結合強度が同じ構成であり、かつ、前記第２の光導波路回路及び前記第３の光導波路回路の各々の前記光結合器の前記結合強度が、前記第１の光導波路回路との接続部を中心として、前記第１の光導波路回路の前記光結合器の前記結合強度と対称的に等しいことを特徴とする。
【００１６】
上記課題を解決する本発明の請求項３に係る導波路型多チャネル波長分散補償器は、入力端から出力端までのＳ個（Ｓは整数）の光結合器のうち、入力端からｊ番目の光結合器の結合強度κ_jが、すべてのｊにおいて次式で表されることを特徴とする。
【数９】

【００１７】
上記課題を解決する本発明の請求項４に係る導波路型多チャネル波長分散補償器は、１つ又は複数の前記光導波路回路から構成され、各々の前記光導波路回路の真中の前記光路差長付与部を中心として、各々の前記光導波路回路の各々の前記光結合器の前記結合強度が対称的に等しいことを特徴とする。
【００１８】
上記課題を解決する本発明の請求項５に係る導波路型多チャネル波長分散補償器は、基本光路長差ｌ₁、ｌ₂の正負の符号が逆であり、かつ、入力端から出力端までの光路長差付与部の個数Ｍが奇数であり、かつ、入力端からｉ番目もしくは出力端からｉ番目の光路長差付与部が与える光路長差Ｌ_iが、ｉの大小に応じて次式で表されることを特徴とする。
【数１０】

【００１９】
上記課題を解決する本発明の請求項６に係る導波路型多チャネル波長分散補償器は、第１の光導波路と、第２の光導波路と、前記第１の光導波路及び前記第２の光導波路をそれぞれ異なる箇所で結合する複数の光結合器とを有し、隣接する前記光結合器で挟まれた光路長差付与部を複数備えた光導波路回路を直列に複数台繋いだ光導波路回路であって、いずれかの前記光導波路回路に、上記多チャネル波長分散補償器を用いたことを特徴とする。
【００２０】
上記課題を解決する本発明の請求項７に係る光モジュールは、上記いずれかの多チャネル波長分散補償器を内部に有する筐体と、前記筐体に支持され、前記多チャネル波長分散補償器に光信号の入出力を行なう光ファイバとを有することを特徴とする。
【００２１】
【発明の実施の形態】
本発明に係る多チャネル波長分散補償器は、１つあるいは少数の導波路型ラティス回路を用いて、複数の光信号の波長分散と波長分散スロープを一括して補償することに特徴がある。ラティス回路の基本的動作を以下に示す。
【００２２】
ラティス回路は、複数の非対称アーム導波路対が光結合器を用いて縦続接続されている構造、つまり、非対称マッハツェンダ型干渉計が多段に縦続接続されている構造である。光結合器は、２本の導波路を近接させ、近接させる導波路長を変えることで、強度結合率を変更するができる。又、非対称アーム導波路対、即ち、光路長差付与部は、２本のアーム導波路の光路長差を変えることで、アーム導波路の位相制御を行うことができる。このことにより、非対称マッハツェンダ型干渉計上で周波数の増加に伴い光波がより多くの光路長を通過するような設定、あるいはより少ない光路長を通過するような設定ができ、周波数の増加に伴い群遅延時間が増加する構成の分散補償器、あるいは減少する構成の分散補償器とすることができる。又、非対称マッハツェンダ型干渉計の段数を増減させた場合も、それぞれ分散値の絶対値を増減させることができる。これは、光路長差の増減によって遅延時間が増減することによる。このような構成により、ラティス回路をそれぞれの波長に対応した分散補償値を有するように設計することができる。本発明の具体的な実施形態を以下に示す。
【００２３】
（第１の実施形態）
図１に、本発明の第１の実施形態における多チャネル波長分散補償器の回路構成を示す。
【００２４】
本回路は、４個の光結合器１０１〜１０４と、３個の光路長差付与部１０９〜１１１から構成される光回路１１５と、４個の光結合器１０５〜１０８と、光路長差付与部１１２〜１１４から構成される光回路１１６を直列接続した回路である。
【００２５】
８個の光結合器１０１〜１０８の結合率をそれぞれκ_j（ｊ：１〜８）とし、６個の光路長差付与部１０９〜１１４が与える光路長差をそれぞれＬ_i（ｉ：１〜６）として、κ_jとＬ_iを以下のように与えた。
【００２６】
まず、波長分散補償器の動作波長域の中心近傍のチャネルの光波長λ_cを１．５５μｍ、光周波数間隔ΔＦを１００ＧＨｚ、光変調度δを５００ＭＨｚとし、式２と式３より基本光路長差ｌ₁とｌ₂を導出した。このときｌ₁＝１５０６．７μｍ、ｌ₂＝１４９１．２μｍとなった。また、６個の各Ｌ_iは、式１に｛ｎ_i｝＝｛１、２、２、０、０、０｝、｛ｍ_i｝＝｛０、０、０、１、２、２｝、｛ｋ_i｝＝｛０、１、１、０、１、１｝を代入して決定した。さらには８個の各κ_jは｛κ_j｝＝｛０．５、０．９３、０．４４、０．０６、０．５、０．９３、０．４４、０．０６｝とした。
【００２７】
上記設計パラメータに従ってフォトマスク設計を行った後、火炎堆積法とフォトリソグラフイーとドライエッチングを用いて石英系導波路回路を作製した。また導波路の比屈折率差が１．２％、導波路コアの断面形状が４μｍ×４μｍになるように作製した。
【００２８】
できあがった光回路のうち１サンプルを、光回路１１５と光回路１１６とが分離できるように切断し、それぞれの光回路の透過特性を評価した結果を図２と図３に示す。
【００２９】
図２には光回路１１５の４つの入出カポート１１７〜１２０のうち、ポート１１８から入射し、ポート１１９とポート１２０に出射される透過光の透過率と波長分散(CD : Chromatic Dispersion)とを示す。ポート１１８からの入射光はその波長に応じてポート１１９とポート１２０に交互に出射され、その繰り返し周期は約１９９ＧＨｚであった。また波長分散は、ポート１１９に出射される場合は右上がりの大きな傾きをもち、ポート１２０に出射される場合は右下がりの大きな傾きを持っていた。
【００３０】
図３には光回路１１６の４つの入出力ポート１２１〜１２４のうち、ポート１２２から入射しポート１２３とポート１２４に出射される透過光の透過率と波長分散とを示す。ポート１２２からの入射光はその波長に応じてポート１２３とポート１２４に交互に出射され、その繰り返し周期は約２０１ＧＨｚであった。また波長分散は、ポート１２３に出射される場合は右上がりの大きな傾きをもち、ポート１２４に出射される場合は右下がりの大きな傾きを持っていた。
【００３１】
本発明の多チャネル波長分散補償器では、波長分散補償量を光チャネル毎に異にすることにより一括して波長分散スロープも補償する。これを図４において、図２と図３とに示した特性を持つ光回路を用いて、本発明の多チャネル波長分散補償器を実現する方法を模式的に示す。
【００３２】
図４（ａ）は、右下がりの波長分散特性をもち、短めの繰り返し周期をもつフィルタの波長分散特性を模式的に表している。これは例えば、図２のポート１１８に入射し、ポート１２０に出射する透過光の波長分散特性に対応する。図４（ｂ）は、右上がりの波長分散特性をもち、長めの繰り返し周期をもつフィルタの波長分散特性を模式的に表している。これは例えば、図３のポート１２１から入射しポート１２３に出射する透過光の波長分散特性に対応する。これらの波長特性をもつフィルタを直列に接続したときの波長分散特性を模式的に表したのが図４（ｃ）である。つまり、光信号の周波数に対して、所定の周期で繰り返す単調減少の波長分散特性と、前記所定の周期より長めの周期で繰り返す単調増加の波長分散特性とを有し、これらの２つのフィルタの波長分散特性を重畳した結果、波長分散特性がほぼ平坦な領域が一定の周期で現れ、かつ、それぞれの領域毎に波長分散値が異なることがわかる。
【００３３】
図５（ｂ）は、光回路１１５と光回路１１６に分離切断しなかった光回路、すなわち光回路１１５と光回路１１６を直列に接続した光回路の透過特性である。光チャネルの間隔を２００ＧＨｚとし、各光チャネルの帯域幅を４０ＧＨｚとしたとき、多チャネル波長分散補償器が動作すべき領域の透過率と波長分散を太線で示してある。
【００３４】
約２００ＧＨｚ間隔で透過率が高い領域が繰り返し現れ、かつ、波長分散がほぼ平坦な領域が繰り返し現れている。また、それぞれの領域の幅は約４０ＧＨｚあり、なにより各領域で波長分散値が異なっている。さらに、この波長分散のグラフに従来から使われている波長分散スロープ値が０．０７ｐｓ／ｎｍ²／ｋｍのＤＳＦファイバ１１ｋｍが必要とする波長分散補償量のグラフ（図５（ａ））を上書きすると、第１の実施形態の光回路が２００ＧＨｚ間隔で４０ＧＨｚの幅で発生する波長分散のグラフとほぼ重なることが確認できる（図５（ｃ））。すなわち、この第１の実施形態の光回路１台により、少なくとも１２チャネルの光信号がＤＳＦファイバを１１ｋｍ伝送した時に光ファイバの波長分散によりうける歪みを一括して補償することが可能であることがわかる。
【００３５】
なお、光回路の透過率の評価には、ＤＳＦファイバを入出カポートに接続した光モジュールで行った。光回路の挿入損失の最良値と最悪値は、波長１．５４μｍから１．５６μｍまでのうち波長分散補償を行う２００ＧＨｚ間隔で４０ＧＨｚ幅の１２個の波長領域では最良１．４ｄＢ、最悪１．６ｄＢであった。このような小さな挿入損失が実現できたのは、導波路ラティス回路が、空間光学系を用いることなく、また、部品点数が少なく、ファイバ結合損失の小さな導波路を用いて作成することができるという特徴に起因するものである。
【００３６】
（第２の実施形態）
図６に、本発明の第２の実施形態における多チャネル波長分散補償器の回路構成を示す。
【００３７】
本回路は、４個の光結合器６０１〜６０４と、３個の光路長差付与部６１３〜６１５から構成される光回路６２２と、４個の光結合器６０５〜６０８と、光路長差付与部６１６〜６１８から構成される光回路６２３と、４個の光結合器６０９〜６１２と、光路長差付与部６１９〜６２１から構成される光回路６２４とから成り立ち、光回路６２３と光回路６２４は、それぞれ光回路６２２と直列に接続した回路である。また光回路６２３と光回路６２４の各回路内でのパラメータは同一のものとした。すなわち、光結合器６０５と光結合器６０９の光結合率は等しく、光結合器６０６と光結合器６１０の光結合率は等しく、光結合器６０７と光結合器６１１の光結合率は等しく、光結合器６０８と光結合器６１２の光結合率は等しく、光路長差付与部６１６と光路長差付与部６１９が与える光路長差は等しく、光路長差付与部６１７と光路長差付与部６２０が与える光路長差は等しく、光路長差付与部６１８と光路長差付与部６２１が与える光路長差は等しいとした。
【００３８】
このとき、８個の光結合器６０１〜６０８の結合率をそれぞれκ_j（ｊ：１〜８）とし、６個の光路長差付与部６１３〜６１８が与える光路長差をそれぞれＬ_i（ｉ：１〜６）とした。さらには式３よりκ_j＝κ_8-j+1とした。このときκ_jとＬ_iを以下のように与えた。
【００３９】
まず、波長分散補償器の動作波長域の中心近傍のチャネルの光波長λ_cを１．５５μｍ、光周波数間隔ΔＦを５０ＧＨｚ、光変調度δを−２２５ＭＨｚとし、式２と式３より基本光路長差ｌ₁とｌ₂を導出し、ｌ₁＝−２９８４．０μｍ、ｌ₂＝３０１１．１μｍを選択した。また、６個の各Ｌ_iは、式１に｛ｎ_i｝＝｛２、２、１、０、０、０｝、｛ｍ_i｝＝｛０、０、０、１、２、２｝、｛ｋ_i｝＝｛−１、−１、０、−１、０、０｝を代入して決定した。さらには８個のκ_jのうち４個の独立なκ_jは｛κ_j｝＝｛０．０６、０．４４、０．９３、０．５｝とした。
【００４０】
上記設計パラメータに従ってフォトマスク設計を行った後、火炎堆積法とフォトリソグラフイーとドライエッチングを用いてシリコン基板上に石英系導波路回路を作製した。また導波路の比屈折率差が０．７５％、導波路コアの断面形状が８μｍ×８μｍになるように作製した。
【００４１】
できあがった光回路のうち１サンプルを、光回路６２２と光回路６２３と光回路６２４とが分離できるように切断し、それぞれの光回路の透過特性を評価した結果を図７と図８と図９に示す。
【００４２】
図７には光回路６２２のポート６２５から入射しポート６２６とポート６２７に出射される透過光の透過率と波長分散とを示す。ポート６２５からの入射光はその波長に応じてポート６２６とポート６２７に交互に出射され、その繰り返し周期は約５０．２３ＧＨｚであった。また波長分散は、ポート６２６に出射される場合もポート６２７に出射される場合も右上がりの大きな傾きを持っていた。
【００４３】
図８には光回路６２３のポート６２６から入射しポート６３０とポート６３１に出射される透過光の透過率と波長分散とを示す。ポート６２６からの入射光はその波長に応じてポート６３０とポート６３１に交互に出射され、その繰り返し周期は約４９．７７ＧＨｚであった。また波長分散は、ポート６３０に出射される場合は右上がりの大きな傾きをもち、ポート６３１に出射される場合は右下がりの大きな傾きを持っていた。
【００４４】
図９には光回路６２４のポート６２７から入射しポート６３２とポート６３３に出射される透過光の透過率と波長分散とを示す。ポート６２７からの入射光はその波長に応じてポート６３２とポート６３３に交互に出射され、その繰り返し周期は約４９．７７ＧＨｚであった。また波長分散は、ポート６３２に出射される場合は右上がりの大きな傾きをもち、ポート６３３に出射される場合は右下がりの大きな傾きを持っていた。
【００４５】
このように、光回路６２２〜６２４に示す光回路ではκ＝０．０６側から入射するかあるいはκ＝０．５側から入射するかを選択することによって、波長分散の傾きの正負を出射ポート間でそろえる、あるいは、逆にすることができる。
【００４６】
一般に光ファイバのもつ波長分散の傾きは、波長に対し緩やかな右上がりであるため、光ファイバで構成される伝送路の波長分散を補償する波長分散補償器には、波長に対し緩やかな右下がりの波長分散が要求される。
【００４７】
第１の実施形態でも説明したとおり、直列接続した光回路全体で波長分散の傾きをゆるやかな右下がりにするためには、繰り返し周期が長めで右上がりの波長分散特性をもつフィルタと、繰り返し周期が短めで右下がりの波長分散特性をもつフィルタを用いる必要がある。第２の実施形態の光回路では１段目の光回路６２６の入力ポートをκ＝０．０６側に選択したが、仮にκ＝０．５側のポートを１段目の入力ポートに選択すると、２つの出力ポートでの波長分散の傾きが正負逆向きになるので、２段目のフィルタの片方は繰り返し周期を長くする必要が生じ、残りの片方の繰り返し周期は短くする必要が生じる。このため光回路全体では３種類の繰り返し周期をもつ。その結果、波長分散が平坦になる領域の中心を光チャネルのグリッドに合わせるのが難しくなる。ちなみに、κ＝０．０６側を１段目の光回路の入力ポートに選択した場合は、出力ポートでの波長分散の傾きは１種類であるため、２段目のフィルタの繰り返し周期は１種類だけで十分であり、光回路全体では２種顆の繰り返し周期をもつこととなる。このように２種類の繰り返し周期のみ持つ場合には、それぞれの周期を（ΔＦ−δ）と（ΔＦ＋δ）とに選択すれば、全体の回路の周期はΔＦとなり、これを光チャネルの周期に合わせるように設計すればよい。
【００４８】
図１０には、これらの光回路６２２と光回路６２３と光回路６２４とを用いて、本発明の目的である波長分散補償量を光チャネル毎に異にすることにより一括して波長分散スロープも補償することができる多チャネル波長分散補償器を実現する方法を模式的に示す。
【００４９】
図１０（ａ）は、右上がりの波長分散特性をもち長めの繰り返し周期をもつフィルタの波長分散特性を模式的に表している。さらには図１０では２つある出力ポートの両方の波長分散特性を示している。これは例えば図７のポート６２５に入射しポート６２６に出射する透過光の波長分散特性が図１０（ａ）のｐｏｒｔＡの特性に対応し、ポート６２５に入射しポート６２７に出射する透過光の波長分散特性が図１０（ａ）のｐｏｒｔＢの特性に対応する。
【００５０】
図１０（ｂ）は、右下がりの波長分散特性をもち短めの繰り返し周期をもつフィルタの波長分散特性を模式的に表している。これは例えば図８のポート６２６から入射しポート６３１に出射する透過光の波長分散特性に対応する。図１０（ａ）のｐｏｒｔＡと図１０（ｂ）の波長特性をもつフィルタを直列に接続したときの波長分散特性を模式的に表したのが図１０（ｃ）である。２つのフィルタの波長分散特性を重畳した結果、波長分散特性がほぼ平坦な領域が一定の周期で現れ、かつ、それぞれの領域毎に波長分散値が異なる。
【００５１】
図１０（ｄ）は、図１０（ｂ）とは異なる波長位置で、右下がりの波長分散特性をもち短めの繰り返し周期をもつフィルタの波長分散特性を模式的に表している。これは例えば図９のポート６２７から入射しポート６３３に出射する透過光の波長分散特性に対応する。図１０（ａ）のｐｏｒｔＢと図１０（ｄ）の波長特性をもつフィルタを直列に接続したときの波長分散特性を模式的に表したのが図１０（ｅ）である。２つのフィルタの波長分散特性を重畳した結果、この場合も、波長分散特性がほぼ平坦な領域が一定の周期で現れ、かつ、それぞれの領域毎に波長分散値が異なる。
【００５２】
図１０（ｆ）には、図１０（ｃ）と図１０（ｅ）で得られた波長分散特性を重ね合わせたものを示す。
【００５３】
１段目の光回路６２２の入力ポートをκ＝０．０６側に選択したので、図１０（ｃ）に示した繰り返し周期と、図１０（ｅ）に示した繰り返し周期とが等しくなり、さらには図１０（ｆ）において、波長分散がほぼ平坦な領域の中心の繰り返し周期が一定にすることが可能になる。
【００５４】
図１１（ｂ）、（ｃ）は、光回路６２２と光回路６２３と光回路６２４に分離切断しなかった光回路、すなわち光回路６２２の出力ポートそれぞれに光回路６２３と光回路６２４を直列に接続した光回路の各々の出力ポートでの透過特性である。光チャネルの間隔を５０ＧＨｚとし、各光チャネルの帯域幅を２０ＧＨｚとしたとき、多チャネル波長分散補償器が動作すべき領域の透過率と波長分散を太線で示してある。
【００５５】
出力ポートがポート６３１（図１１（ｂ）参照）とポート６３３（図１１（ｃ）参照）とに交互に入れ替わるものの約５０ＧＨｚ間隔で透過率が高い領域が繰り返し現れ、かつ、波長分散がほぼ平坦な領域が繰り返し現れている。また、それぞれの領域の幅は約２０ＧＨｚあり、各領域間での波長分散値がゆるやかな右下がりになっている。さらに、これらの波長分散のグラフに従来から使われている波長分散スロープ値が０．０７ｐｓ／ｎｍ²／ｋｍのＤＳＦファイバ８０ｋｍが必要とする波長分散補償量のグラフ（図１１（ａ））を上書きすると、第２の実施形態の光回路が５０ＧＨｚ間隔で２０ＧＨｚの幅で発生する波長分散のグラフとほぼ重なることが確認できる（図１１（ｄ））。すなわち、この第２の実施形態の光回路１台により、少なくとも５０チャネルの光信号がＤＳＦファイバを８０ｋｍ伝送した時に光ファイバの波長分散によりうける歪みを一括して補償することが可能であることがわかる。
【００５６】
なお、光回路の透過率の評価には、ＤＳＦファイバを入出力ポートに接続した光モジュールで行った。光回路の挿入損失の最良値と最悪値は、波長１．５４μｍから１．５６μｍまでのうち波長分散補償を行う５０ＧＨｚ間隔で２０ＧＨｚ幅の５０個の波長領域では最良１．３ｄＢ、最悪１．６ｄＢであった。このような小さな挿入損失が実現できたのは、導波路ラティス型は、空間光学系を用いることなく、また、部品点数が少なく、ファイバ結合損失の小さな導波路を用いて作成することができるという特徴に起因するものである。
【００５７】
（第３の実施形態）
図１２に、本発明の第３の実施形態における多チャネル波長分散補償器の回路構成を示す。
【００５８】
本回路は、８個の光結合器１２０１〜１２０８と、７個の光路長差付与部１２０９〜１２１５から構成された光回路である。光回路中央の光路長差付与部１２１２が与える光路長差はゼロとし、基本光路長差をのぞくと左右対称の構造をしている。すなわち、８個の光結合器１２０１〜１２０８の結合率をそれぞれκ_j（ｊ：１〜８）とし、７個の光路長差付与部１２０９〜１２１５が与える光路長差をそれぞれＬ_i（ｉ：１〜７）としたとき、κ_jとＬ_iを以下のように与えた。各結合率は式３よりκ_j＝κ_8-j+1とし、８個のκ_jのうち４個の独立であるκ_jを｛κ_j｝＝｛０．０１５、０．６、０．７、０．５｝とした。次に、波長分散補償器の動作波長域の中心近傍のチャネルの光波長λ_cを１．５５μｍ、光周波数間隔ΔＦを２００ＧＨｚ、光変調度δを−４ＧＨｚとし、式２と式３より基本光路長差ｌ₁とｌ₂を導出し、ｌ₁＝７３４．４μｍ、ｌ₂＝７６４．６μｍを選択した。また、６個の各Ｌ_iは、式１に｛ｎ_i｝＝｛−２、−２、−１、０、０、０、０｝、｛ｍ_i｝＝｛０、０、０、０、−１、−２、−２｝、｛ｋ_i｝＝｛−１、−１、０、０、−１、０、０｝を代入して決定した。
【００５９】
この図１２に記載の第３の実施形態の光回路の基本構成は、図６に記載の第２の実施形態の光回路の基本構成と以下のように関連づけることができる。光回路６２３と光回路６２４は全く同一のものであるので、ポート６２７からポート６３３への透過特性はポート６２８からポート６３１への透過特性と同一のものである。また、光チャネル周辺の波長領域では光信号は光回路６２２によってポート６２６またはポート６２７の片方に集中し、他方のポートには殆ど光信号が来ないので、第１次近似として相互に独立なこと象と考えられる。そこで、ポート６２７の場所で光回路６２４を切り離し、光回路６２３のポート６２８を光回路６２２のポート６２７に接続する。このポート６２２とポート６２７との接続に際し、交差導波路が生じないように、光回路６２３を上下反転する。以上の変形作業により、第２の実施形態の光回路の基本構成と第３の実施形態の光回路の基本構成とは、光チャネル周辺の波長領域では、ほぼ等価に近い光回路であることがわかる。
【００６０】
この図１２に記載の第３の実施形態の光回路では、第２の実施形態の光回路にくらべ、光結合器の数を１２個から８個に削減し、光路長差付与部の数を９個から７個に削減できるというメリットがある。さらには１入力１出力構成になるので中継局での使用に向いたものになっている。
【００６１】
上記設計パラメータに従ってフォトマスク設計を行った後、火炎堆積法とフォトリソグラフイーとドライエッチングを用いてシリコン基板上に石英系導波路回路を作製した。また導波路の比屈折率差が０．７５％、導波路コアの断面形状が８μｍ×８μｍになるように作製した。
【００６２】
できあがった光回路の透過特性を評価した結果を図１３に示す。光チャネルの間隔を２００ＧＨｚとし、各光チャネルの帯域幅を８０ＧＨｚとしたとき、多チャネル波長分散補償器が動作すべき領域の透過率と波長分散を太線で示してある。
【００６３】
図１３（ｂ）では、透過率はほぼ全域で高く、かつ、波長分散がほぼ平坦な領域が繰り返し現れている。また、波長分散がほぼ平坦な領域の幅は約８０ＧＨｚあり、各領域間での波長分散値がゆるやかな右下がりになっている。さらに、この波長分散のグラフに従来から使われている波長分散スロープ値が０．０７ｐｓ／ｎｍ²／ｋｍのＤＳＦファイバ１０ｋｍが必要とする波長分散補償量のグラフ（図１３（ａ））を上書きすると、第３の実施形態の光回路が２００ＧＨｚ間隔で８０ＧＨｚの幅で発生する波長分散のグラフとほぼ重なることが確認できる（図１３（ｃ））。すなわち、この第３の実施形態の光回路１台により、少なくとも１２チャネルの光信号がＤＳＦファイバを１０ｋｍ伝送した時に光ファイバの波長分散によりうける歪みを一括して補償することが可能であることがわかる。
【００６４】
なお、光回路の透過率の評価には、ＤＳＦファイバを入出力ポートに接続した光モジュールで行った。光回路の挿入損失の最良値と最悪値は、波長１．５４μｍから１．５６μｍまでのうち波長分散補償を行う２００ＧＨｚ間隔で８０ＧＨｚ幅の１２個の波長領域では最良０．６ｄＢ、最悪２．２ｄＢであった。このような小さな挿入損失が実現できたのは、導波路ラティス型は、空間光学系を用いることなく、また、部品点数が少なく、ファイバ結合損失の小さな導波路を用いて作成することができるという特徴に起因するものである。
【００６５】
（第４の実施形態）
図１４に、本発明の第４の実施形態における多チャネル波長分散補償器の回路構成を示す。
【００６６】
本回路は、６個の光結合器１４０１〜１４０６と、５個の光路長差付与部１４１３〜１４１７から構成された光回路１４２３と、６個の光結合器１４０７〜１４１２と、５個の光路長差付与部１４１８〜１４２２から構成された光回路１４２４を直列に接続した光回路である。
【００６７】
光回路１４２３は公知の波長分散補償器（非特許文献２参照）をベースに設計したもので、一定の間隔のチャネル近傍で波長分散を補償できるものの、チャネル毎に波長分散補償量を異にすることによって波長分散スロープまで補償することはできない回路である。
【００６８】
光回路１４２３の回路パラメータの詳細は、光結合器１４０１〜１４０６の結合強度をκ'_jとし、５個の光路長差付与部１４１３〜１４１７が与える光路長差をそれぞれＬ'_i（ｉ：１〜５）としたとき、｛κ'_j：ｊ＝１〜６｝＝｛０．５、０．３３、０．３３、０．３３、０．３３、０．５｝とした。また基本光路長差をｌ'₁とｌ'₂をそれぞれ１４９８．６μｍと１４９９．４μｍとし、さらには
【数１１】

として、｛ｎ'_i｝＝｛１、−１、１、−１、０｝、｛ｍ'_i｝＝｛０、−１、１、−１、１｝、｛ｋ'_i｝＝｛０、０、０、０、０、０｝とした。
【００６９】
一方、光回路１４２４の基本構成は、図１２に記載の第３の実施形態の光回路の基本構成と以下のように関連づけることができる。
【００７０】
（Ｉ）光路長差付与部１２１７は等長であり、光結合器１２０４と光結合器１２０５の結合率は共に０．５である。すなわち光結合器１２０４と光路長差付与部１２１７と光結合器１２０５は、全体で交差導波路と光学的に等価であり置き換えることが可能である。
【００７１】
（ＩＩ）光路長差付与部１２１３〜１２１５が与える光路長差の正負を逆にすることにより、前述の交差導波路を平行導波路と置き換えることができる。
【００７２】
（ＩＩＩ）光路長差付与部１２１１と光路長差付与部１２１３との間に光結合部分がなくなったので、２つの光路長差付与部をまとめて１つの光路長差付与部として扱うことができ、必要な光路長差は極めて小さくなり、作製誤差の影響が受けにくくなる。
【００７３】
以上の変形操作により第４の実施形態の光回路１４２４の基本構成は、第３の実施形態の光回路の基本構成と光学的に等価であることがわかる。従って、第４の実施形態の光回路は、従来の光回路１４２３と本発明に係る光回路１４２４とを、直列に繋いだ構成と言える。
【００７４】
この図１４に記載の第４の実施形態の光回路では、第３の実施形態の光回路にくらべ、光結合器の数を８個から６個に削減し、光路長差付与部の数を７個から５個に削減できるというメリットがある。
【００７５】
光回路１４２４の回路パラメータの詳細は、６個の光結合器１４０７〜１４１２の結合強度をκ_jとし、５個の光路長差付与部１４１８〜１４２２が与える光路長差をそれぞれＬ_iとしたとき、κ_jとＬ_iを以下のように与えた。
【００７６】
まず、波長分散補償器の動作波長域の中心近傍のチャネルの光波長λ_cを１．５７５μｍ、光周波数間隔ΔＦを１００ＧＨｚ、光変調度δを−７５０ＭＨｚとし、式２と式３より基本光路長差ｌ₁とｌ₂を導出し、ｌ₁＝−１４８７．９μｍ、ｌ₂＝１５０９．２μｍを選択した。つまり、この光回路１４２４において、入力端から出力端までの光路長差付与部１４１８〜１４２２の個数が奇数であり、基本光路長差ｌ₁、ｌ₂の正負の符号が逆であると言える。
【００７７】
また、５個の各Ｌ_iは、式４に｛ｎ_i｝＝｛２、２｝、｛ｋ_i｝＝｛−１、−１、−１、０、０｝を代入して決定した。より具体的にはＬ１＝Ｌ２＝−２９７５．１μｍ、Ｌ３＝２１．２６μｍ、Ｌ４＝Ｌ５＝３０１７．６μｍである。さらには６個のκ_jのうち３個の独立なκ_jは｛κ_j｝＝｛０．０１５、０．６、０．７｝とした。
【００７８】
上記設計パラメータに従ってフォトマスク設計を行った後、火炎堆積法とフォトリソグラフイーとドライエッチングを用いてシリコン基板上に石英系導波路回路を作製した。また導波路の比屈折率差が１．２％、導波路コアの断面形状が４μｍ×４μｍになるように作製した。
【００７９】
できあがった光回路の透過特性を評価した結果を図１５に示す。光チャネルの間隔を２００ＧＨｚとし、各光チャネルの帯域幅を４０ＧＨｚとしたとき、多チャネル波長分散補償器が動作すべき領域の透過率と波長分散を太線で示してある。
【００８０】
図１５に示すように、透過率はほぼ全域で高く、かつ、波長分散がほぼ平坦な領域が繰り返し現れている。また、波長分散がほぼ平坦な領域の幅は約４０ＧＨｚあり、各領域間での波長分散値がゆるやかな右下がりになっている。さらに、この波長分散のグラフに従来から使われている波長分散スロープ値が０．０７ｐｓ／ｎｍ²／ｋｍのＤＳＦファイバ２５ｋｍが必要とする波長分散補償量のグラフを上書きすると、第４の実施形態の光回路が２００ＧＨｚ間隔で４０ＧＨｚの幅で発生する波長分散のグラフとほぼ重なることが確認できる。すなわち、この第４の実施形態の光回路により、少なくとも１８チャネルの光信号がＤＳＦファイバ２５ｋｍを伝送した時に光ファイバの波長分散によりうける歪みを一括して補償することが可能であることがわかる。
【００８１】
なお、光回路の透過率の評価には、ＤＳＦファイバを入出力ポートに接続した光モジュールで行った。光回路の挿入損失の最良値と最悪値は、波長１．５４μｍから１．５６μｍまでのうち波長分散補償を行う２００ＧＨｚ間隔で４０ＧＨｚ幅の１８個の波長領域では最良１．７ｄＢ、最悪３．６ｄＢであった。このような小さな挿入損失が実現できたのは、導波路ラティス型は、空間光学系を用いることなく、また、部品点数が少なく、ファイバ結合損失の小さな導波路を用いて作成することができるという特徴に起因するものである。
【００８２】
なお、以上述べた第１、第２、第３、第４の各実施形態では、火炎堆積法とフォトリソグラフイーとドライエッチングを用いてシリコン基板上に石英系導波路回路を作製したが、その導波路材料がポリイミド、シリコーン、半導体、ＬｉＮｂＯ₃などであっても、本発明を適用可能である。また、製造方法も、スピンコート法、ゾルゲル法、スパック法、ＣＶＤ法、イオン拡散法、イオンビーム直接描画法などであっても本発明を適用可能である。さらには基板材質もシリコンに限定されるものではなく、石英基板でも、同様な効果が得られる。
【００８３】
図１６に、実施形態１で用いた本発明の導波路型多チャネル波長分散補償器（ＰＬＣチップ）を用いた光モジュールを示す。
【００８４】
光モジュールは、熱伝導性の良い筐体１６０１の内部にペルチェ保持板１６０２を固定ネジ１６０３で固定し、図示はしないがペルチェ保持板１６０２を掘削して作製した凹部にペルチェ素子と温度センサ（熱伝対）をその近傍に配置している。ペルチェ素子、及び温度センサの直上に実施形態１の光回路１６０４が来るように配している（図示せず）。
【００８５】
光回路１６０４の端部にはガラス板１６０５を接着剤で接着し、ファイバ１６０６を保持しているファイバブロック１６０７と光接合するように接着している。ファイバ１６０６は筐体１６０１の縁に設けた凹部に断熱性弾性接着剤１６０８で接着してあり、さらにファイバコード１６０９を有するファイバブーツ１６１０を筐体１６０１に埋め込むように保持している。
【００８６】
実施形態１で用いた本発明のＰＬＣチップはペルチェ保持板に断熱性弾性接着剤１６０８で接着されている。
【００８７】
最後に、これらを被うように蓋をかぶせてネジ止めして本発明の光モジュールを作製した。なお、蓋とネジ止め部は図示していない。
【００８８】
このような光モジュールを作製し、実施形態１と同様な測定を行なったところ、図５と同じ結果が得られ実施形態１と同様な結果が得られたことが分かった。
【００８９】
同様に、実施形態２、３、４で用いた本発明の導波路型多チャネル波長分散補償器を用いた光モジュールを作製し、それぞれ実施形態２、３、４と同様な効果が得られた。なお、本発明の導波路型多チャネル波長分散補償器が２つ以上の部分に別れる場合もあるが、それぞれの光モジュールを作製し、光モジュール間をファイバで結合し同様な効果を得られることが分かった。また、図示はしないが、１つの筐体内部に上記の２つ以上のチップをそれぞれペルチェ保持板の上に保持させた光モジュールも同様な効果が得られた。
【００９０】
【発明の効果】
以上説明したように、本発明によれば、光ファイバのもつ波長分散と波長分散スロープの両方に対応する多チャネル波長分散補償器を、空間光学系がなく部品点数が少なく挿入損失を小さくできる導波路ラティス型で供給することが可能になる。
【図面の簡単な説明】
【図１】本発明の第１の実施形態の回路構成を示す図である。
【図２】光回路１１５の透過スペクトル特性を示す図である。
【図３】光回路１１６の透過スペクトル特性を示す図である。
【図４】光回路１１５と光回路１１６の透過スペクトル特性の合成を模式的に示す図である。
【図５】図１の多チャネル波長分散補償器の透過スペクトル特性を示す図である。
【図６】本発明の第２の実施形態の回路構成を示す図である。
【図７】光回路６２２の透過スペクトル特性を示す図である。
【図８】光回路６２３の透過スペクトル特性を示す図である。
【図９】光回路６２４の透過スペクトル特性を示す図である。
【図１０】光回路６２２と光回路６２３と光回路６２４の透過スペクトル特性の合成を模式的に示す図である。
【図１１】図６の多チャネル波長分散補償器の透過スペクトル特性を示す図である。
【図１２】本発明の第３の実施形態の回路構成を示す図である。
【図１３】図１２の多チャネル波長分散補償器の透過スペクトル特性を示す図である。
【図１４】本発明の第４の実施形態の回路構成を示す図である。
【図１５】図１５の多チャネル波長分散補償器の透過スペクトル特性を示す図である。
【図１６】本発明の導波路型多チャネル波長分散補償器（ＰＬＣチップ）を用いた光モジュールを示す図である。
【符号の説明】
１０１〜１０８ 光結合器
１０９〜１１４ 光路長差付与部
１１５、１１６ 光回路
１１７〜１２４ ポート
６０１〜６１２ 光結合器
６１３〜６２１ 光路長差付与部
６２２〜６２４ 光回路
６２５〜６３３ ポート
１２０１〜１２０８ 光結合器
１２０９〜１２１５ 光路長差付与部
１２１６〜１２１９ ポート
１４０１〜１４１２ 光結合器
１４１３〜１４２２ 光路長差付与部
１４２３、１４２４ 光回路
１４２５、１４２６ ポート
１６０１ 筐体
１６０２ ペルチェ保持板
１６０３ 固定ねじ
１６０４ ＰＬＣチップ
１６０５ ガラス板
１６０６ ファイバ
１６０７ ファイバブロック
１６０８ 断熱性弾性接着剤
１６０９ ファイバコード
１６１０ ファイバブーツ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a chromatic dispersion compensator used in the field of optical communication or optical information processing, and is suitable, for example, as a multi-channel chromatic dispersion compensator used in WDM (Wavelength Division Multiplexing) optical communication.
[0002]
[Prior art]
Long-distance transmission is possible by using low-loss optical fiber. However, in optical fibers with chromatic dispersion, the propagation speed of light differs depending on the wavelength, so when trying to send a signal modulated at high speed over a long distance, the distortion of the optical signal pulse increases, resulting in transmission capacity or transmission. The distance will be limited.
[0003]
In order to solve this problem, various methods for compensating the chromatic dispersion of an optical fiber have been proposed so far.
[0004]
Broadly divided, there are a compensation method using fibers having different chromatic dispersion and a compensation method using a dispersion compensator composed of small devices or optical modules. Therefore, there is an advantage that management such as fiber switching becomes easy. As such a small compensator, for example, there are the following reports.
(1) Fiber grating type (see Non-Patent Document 1)
(2) Waveguide lattice type (see Non-Patent Document 2)
(3) Waveguide ring type (see Non-Patent Document 3)
(4) VIPA (Virtually Imaged Phased Array) type (see Non-Patent Document 4)
(5) Waveguide AWG (Arrayed Waveguide Grating) type (see Non-Patent Document 5)
(6) Etalon type (see Non-Patent Document 6)
(7) Multi-layer film type (see Non-Patent Document 7)
[0005]
Among them, the (2) waveguide lattice type and (3) waveguide ring type, which do not require a circulator or a collimator, have a feature that they are easy to manufacture because they do not have a space optical system part and have a small number of parts. Furthermore, (2) the waveguide lattice type can be produced using a waveguide with a small fiber coupling loss, and (3) has an advantage that a device with a smaller insertion loss can be produced than the waveguide ring type. .
[0006]
On the other hand, a WDM system using an optical channel with a plurality of wavelengths has been studied and put into practical use so that a larger capacity can be transmitted using an optical fiber. However, since the chromatic dispersion of the optical fiber varies depending on the wavelength, the chromatic dispersion compensator is also required to compensate the chromatic dispersion amount corresponding to the chromatic dispersion slope with a plurality of channels.
[0007]
[Non-Patent Document 1]
(1) Fiber grating type
(For example, "Dynamic dispersion Compensation devices for high speed transmission systems" by B. J. Eggleton et al., Optical Fiber Communication Conference 2001, Paper WH1)
[Non-Patent Document 2]
(2) Waveguide lattice type
(For example, "Integrated-0ptic dispersion slope equalizer for Nx40Gb / s WDM transmission" by K. Takigushi et al., European Conference on Optical Communication 2000, paper 7.1.3)
[Non-Patent Document 3]
(3) Waveguide ring type
(For example, "Integrated Waveguide Allpass Filter Tunable Dispersion Compensators" by C. K. Madsen et al., Optical Fiber Communications Conference 2002, paper TuT1)
[Non-Patent Document 4]
▲ 4 ▼ VIPA type
(For example, "Compensation of chromatic dispersion and dispersion Slope using a virtually imaged phased array" by M. Shirasaki et al., Optical Fiber Communications Conference 2001, paper TuS1)
[Non-Patent Document 5]
(5) Waveguide AWG type
(For example, "2X40-Channel dispersion-Slope compensator for 40-Gbit / s WDM transmission systems covering entire C- and L-bands" by K. Takenouchi et al., Optical Fiber Communications Conference 2001, paper TuS2)
[Non-Patent Document 6]
▲ 6 ▼ Etalon type
(For example, “Multichannel tunable dispersion Compensation using all-pass multicavity etalons” by D. J. Moss et al., Optical Fiber Communications Conference 2002, paper TuT2)
[Non-Patent Document 7]
(7) Multilayer type
(For example, "Entirely Thin-Film Allpass Coupled-Cavity Filters in Parallel Configuration for Adjustable Dispersion-slope Compensation" by M. Jablonski et al., IEEE Photonics Technology Letters, (2001) p.1188)
[0008]
[Problems to be solved by the invention]
However, in the conventionally reported waveguide lattice dispersion compensation circuit (see Non-Patent Document 2), the design is such that the same transmission characteristic appears at a constant frequency interval, that is, the optical path provided by the optical path length difference providing unit at each stage. Long difference L_iProvided an optical path length difference represented by the following mathematical formula.
[Equation 5]

However, n_i, M_i, K_iIs an integer, λ_cIs the optical wavelength of the channel near the center of the operating wavelength region of the dispersion compensation circuit. When the optical frequency interval is expressed as ΔF and the speed of light as C, the basic optical path length difference l₁, L₂Is expressed by the following equation.
[Formula 6]

[0009]
Therefore, the chromatic dispersion compensation amount cannot be made different for each channel. That is, the dispersion compensation operation reported in the waveguide lattice dispersion compensation circuit is one channel per circuit, and in order to compensate a multi-channel signal used in the WDM system, compensations having different dispersion compensation amounts by the number of channels. The vessel must be designed and manufactured. Furthermore, it is necessary to use a filter such as AWG in order to allocate the signal of each channel. The number of AWGs required is one at the terminal station and two at the relay station. For the above reasons, the multi-channel lattice type dispersion compensator capable of compensating the dispersion slope has to have a large chip size or a multi-chip configuration.
[0010]
In other words, the waveguide lattice chromatic dispersion compensator having the conventional configuration described above has a feature that there is no spatial optical system part, the number of parts is small, and insertion loss can be reduced. There are problems that the compensators having different dispersion compensation amounts must be designed and manufactured, and that the chip size must be increased or a multi-chip configuration.
[0011]
The present invention has been made in view of the above problems, and compensates the chromatic dispersion of a multi-channel optical signal with one or a few waveguide lattice type compensators, and the chromatic dispersion slope of the optical fiber in each channel. An object of the present invention is to provide a waveguide type multi-channel chromatic dispersion compensator capable of compensating for different chromatic dispersion amounts according to the above.
[0014]
[Means for Solving the Problems]
  Claims of the present invention for solving the above-mentioned problems1The waveguide type multi-channel chromatic dispersion compensator according toA first optical waveguide, a second optical waveguide, and N + 1 (N is a natural number) optical couplers for coupling the first optical waveguide and the second optical waveguide at different locations, Among N + 1 optical couplers, an optical waveguide circuit provided with N optical path length difference providing units sandwiched between two adjacent optical couplers, or a plurality of optical waveguide circuits connected in series An optical waveguide circuit,Of the M optical path length difference providing units from the input end to the output end (M is a natural number), the optical path length difference L given by the i-th optical path length difference providing unit from the input end_iIs expressed by the following equation for all iIn this way, a monotonically decreasing chromatic dispersion characteristic that repeats with a predetermined period and a monotonically increasing chromatic dispersion characteristic that repeats with a period longer than the predetermined period are formed with respect to the frequency of the optical signal, and the above differences By superimposing chromatic dispersion characteristics, chromatic dispersion is compensated in a lump in the peripheral band of a plurality of optical channels having different optical wavelengths and having a constant optical frequency interval, and the amount of chromatic dispersion compensation is set to the optical channel. Compensate for chromatic dispersion slope at once, different for eachIt is characterized by that.
[Expression 7]

  However, n_i, M_i, K_iIs an integer, λ_cIs the optical wavelength of the channel near the center of the operating wavelength region of this multi-channel chromatic dispersion compensator, where the optical frequency interval is expressed as ΔF, the speed of light as C, and δ as the optical frequency modulation degree. Long difference l₁, L₂And δ are expressed by the following equations.
[Equation 8]

[0015]
  Claims of the present invention for solving the above-mentioned problems2The waveguide type multi-channel chromatic dispersion compensator according to the third optical waveguide circuit of the second optical waveguide circuit and the third optical waveguide circuit respectively connected in parallel to the first optical waveguide circuit on the input side. The optical path length difference of the optical path length difference providing unit between the second optical waveguide circuit and the third optical waveguide circuit and the coupling strength of the optical coupler are the same, and The coupling strength of the optical coupler of each of the second optical waveguide circuit and the third optical waveguide circuit is such that the first optical waveguide is centered on a connection portion with the first optical waveguide circuit. It is symmetrically equal to the coupling strength of the optical coupler of the circuit.
[0016]
  Claims of the present invention for solving the above-mentioned problems3The waveguide type multi-channel chromatic dispersion compensator according to the present invention includes a coupling strength κ of the j-th optical coupler from the input end among S optical couplers (S is an integer) from the input end to the output end._jIs expressed by the following equation for all j.
[Equation 9]

[0017]
  Claims of the present invention for solving the above-mentioned problems4The waveguide type multi-channel chromatic dispersion compensator according to the present invention is composed of one or a plurality of the optical waveguide circuits, and each of the optical waveguides is centered on the optical path difference length providing portion in the middle of each of the optical waveguide circuits. The coupling strength of each of the optical couplers of each circuit is symmetrically equal.
[0018]
  Claims of the present invention for solving the above-mentioned problems5The waveguide type multi-channel chromatic dispersion compensator according to FIG.₁, L₂And the number M of optical path length difference giving units from the input end to the output end is an odd number, and the i th optical path length giving unit from the input end or the i th optical path length giving unit from the output end Optical path length difference L_iIs expressed by the following equation according to the magnitude of i.
[Expression 10]

[0019]
  Claims of the present invention for solving the above-mentioned problems6The waveguide type multi-channel chromatic dispersion compensator according to the present invention includes a first optical waveguide, a second optical waveguide, and a plurality of lights that couple the first optical waveguide and the second optical waveguide at different locations. An optical waveguide circuit having a plurality of optical waveguide circuits each having a plurality of optical path length difference providing portions sandwiched between adjacent optical couplersBecauseThe multi-channel chromatic dispersion compensator is used for any one of the optical waveguide circuits.
[0020]
  Claims of the present invention for solving the above-mentioned problems7An optical module according to the present invention includes a housing having any one of the multi-channel chromatic dispersion compensators inside, and an optical fiber that is supported by the housing and that inputs and outputs optical signals to the multi-channel chromatic dispersion compensator. It is characterized by having.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The multi-channel chromatic dispersion compensator according to the present invention is characterized in that chromatic dispersion and chromatic dispersion slope of a plurality of optical signals are collectively compensated using one or a small number of waveguide type lattice circuits. The basic operation of the lattice circuit is shown below.
[0022]
The lattice circuit has a structure in which a plurality of pairs of asymmetric arm waveguides are cascaded using an optical coupler, that is, a structure in which asymmetric Mach-Zehnder interferometers are cascaded in multiple stages. The optical coupler can change the strength coupling ratio by bringing two waveguides close to each other and changing the length of the approaching waveguide. Further, the asymmetric arm waveguide pair, that is, the optical path length difference providing unit can control the phase of the arm waveguide by changing the optical path length difference between the two arm waveguides. As a result, the asymmetric Mach-Zehnder type interferometer can be set so that the light wave passes through more optical path lengths as the frequency increases or passes through a smaller optical path length, and the group delay increases as the frequency increases. The dispersion compensator can be configured to increase in time, or can be a dispersion compensator configured to decrease. Also, when the number of stages of the asymmetric Mach-Zehnder interferometer is increased or decreased, the absolute value of the dispersion value can be increased or decreased. This is because the delay time increases or decreases as the optical path length difference increases or decreases. With such a configuration, the lattice circuit can be designed to have a dispersion compensation value corresponding to each wavelength. Specific embodiments of the present invention are shown below.
[0023]
(First embodiment)
FIG. 1 shows a circuit configuration of a multi-channel chromatic dispersion compensator according to the first embodiment of the present invention.
[0024]
This circuit includes four optical couplers 101 to 104, three optical path length difference providing units 109 to 111, four optical couplers 105 to 108, and optical path length difference application. This is a circuit in which optical circuits 116 including units 112 to 114 are connected in series.
[0025]
The coupling rates of the eight optical couplers 101 to 108 are each κ_j(J: 1 to 8), and the optical path length differences given by the six optical path length difference providing units 109 to 114 are respectively L_i(I: 1 to 6)_jAnd L_iWas given as follows.
[0026]
First, the optical wavelength λ of the channel near the center of the operating wavelength region of the chromatic dispersion compensator_cIs 1.55 μm, the optical frequency interval ΔF is 100 GHz, and the optical modulation degree δ is 500 MHz.₁And l₂Was derived. At this time₁= 1506.7 μm, l₂= 1491.2 μm. Each of the six L_iIs represented by {n_i} = {1, 2, 2, 0, 0, 0}, {m_i} = {0, 0, 0, 1, 2, 2}, {k_i} = {0, 1, 1, 0, 1, 1} and determined. In addition, each of the 8 κ_jIs {κ_j} = {0.5, 0.93, 0.44, 0.06, 0.5, 0.93, 0.44, 0.06}.
[0027]
After designing a photomask according to the above design parameters, a quartz-based waveguide circuit was fabricated by using a flame deposition method, photolithography and dry etching. The waveguide was fabricated so that the relative refractive index difference of the waveguide was 1.2% and the waveguide core had a cross-sectional shape of 4 μm × 4 μm.
[0028]
One sample of the completed optical circuit is cut so that the optical circuit 115 and the optical circuit 116 can be separated, and the results of evaluating the transmission characteristics of each optical circuit are shown in FIGS.
[0029]
FIG. 2 shows the transmittance and wavelength dispersion (CD: Chromatic Dispersion) of transmitted light that enters from the port 118 and exits to the port 119 and the port 120 among the four input / output ports 117 to 120 of the optical circuit 115. . Incident light from the port 118 was alternately emitted to the port 119 and the port 120 according to the wavelength, and the repetition period was about 199 GHz. Further, the chromatic dispersion has a large slope that rises to the right when emitted to the port 119, and has a large slope that falls to the right when emitted to the port 120.
[0030]
FIG. 3 shows the transmittance and chromatic dispersion of transmitted light that enters from the port 122 and exits to the port 123 and the port 124 among the four input / output ports 121 to 124 of the optical circuit 116. Incident light from the port 122 is alternately emitted to the port 123 and the port 124 according to the wavelength, and the repetition period is about 201 GHz. Further, the chromatic dispersion has a large slope that rises to the right when emitted to the port 123, and has a large slope that falls to the right when emitted to the port 124.
[0031]
In the multi-channel chromatic dispersion compensator of the present invention, the chromatic dispersion slope is also compensated collectively by making the chromatic dispersion compensation amount different for each optical channel. FIG. 4 schematically shows a method for realizing the multi-channel chromatic dispersion compensator of the present invention using the optical circuit having the characteristics shown in FIGS.
[0032]
FIG. 4A schematically shows the chromatic dispersion characteristics of a filter having a chromatic dispersion characteristic that descends to the right and having a shorter repetition period. This corresponds to, for example, the wavelength dispersion characteristic of transmitted light that enters the port 118 in FIG. FIG. 4B schematically shows the wavelength dispersion characteristics of a filter having a wavelength dispersion characteristic that rises to the right and has a longer repetition period. This corresponds to, for example, the wavelength dispersion characteristic of transmitted light that enters from the port 121 of FIG. FIG. 4C schematically shows chromatic dispersion characteristics when filters having these wavelength characteristics are connected in series. That is, the optical signal frequency has a monotonically decreasing chromatic dispersion characteristic that repeats at a predetermined period and a monotonically increasing chromatic dispersion characteristic that repeats at a period longer than the predetermined period. As a result of superimposing the chromatic dispersion characteristics, it can be seen that areas where the chromatic dispersion characteristics are almost flat appear at a constant period, and the chromatic dispersion values differ for each area.
[0033]
FIG. 5B shows transmission characteristics of an optical circuit in which the optical circuit 115 and the optical circuit 116 are not separated and cut, that is, an optical circuit in which the optical circuit 115 and the optical circuit 116 are connected in series. When the interval between the optical channels is 200 GHz and the bandwidth of each optical channel is 40 GHz, the transmittance and chromatic dispersion in the region where the multichannel chromatic dispersion compensator should operate are indicated by bold lines.
[0034]
Regions with high transmittance appear repeatedly at intervals of about 200 GHz, and regions with almost flat chromatic dispersion appear repeatedly. The width of each region is about 40 GHz, and the chromatic dispersion value is different among the regions. Furthermore, the chromatic dispersion slope value conventionally used in this chromatic dispersion graph is 0.07 ps / nm.²Overwriting the chromatic dispersion compensation amount graph (FIG. 5A) required by the 11 km / km DSF fiber 11km, a graph of chromatic dispersion generated by the optical circuit of the first embodiment with a width of 40 GHz at intervals of 200 GHz; It can be confirmed that they almost overlap (FIG. 5C). In other words, the single optical circuit of the first embodiment can collectively compensate for distortions caused by chromatic dispersion of an optical fiber when an optical signal of at least 12 channels is transmitted through a DSF fiber for 11 km. Recognize.
[0035]
The transmittance of the optical circuit was evaluated using an optical module in which a DSF fiber was connected to the input / output port. The best and worst values of the insertion loss of the optical circuit are 1.4 dB at the best and 1.6 dB at the worst in 12 wavelength regions of 40 GHz width at 200 GHz intervals among the wavelengths from 1.54 μm to 1.56 μm. Met. The reason why such a small insertion loss can be realized is that the waveguide lattice circuit can be created without using a spatial optical system, using a waveguide with a small number of components, and a small fiber coupling loss. This is due to the characteristics.
[0036]
(Second Embodiment)
FIG. 6 shows a circuit configuration of a multi-channel chromatic dispersion compensator according to the second embodiment of the present invention.
[0037]
This circuit includes four optical couplers 601 to 604, three optical path length difference providing units 613 to 615, four optical couplers 605 to 608, and optical path length difference application. The optical circuit 623 includes optical units 616 to 618, four optical couplers 609 to 612, and the optical circuit 624 includes optical path length difference providing units 619 to 621. Are circuits connected in series with the optical circuit 622, respectively. The parameters in the optical circuit 623 and the optical circuit 624 are the same. That is, the optical coupling rates of the optical coupler 605 and the optical coupler 609 are equal, the optical coupling rates of the optical coupler 606 and the optical coupler 610 are equal, and the optical coupling rates of the optical coupler 607 and the optical coupler 611 are equal, The optical coupling ratios of the optical coupler 608 and the optical coupler 612 are equal, the optical path length differences given by the optical path length difference providing unit 616 and the optical path length difference providing unit 619 are equal, and the optical path length difference providing unit 617 and the optical path length difference providing unit 620. Are equal, and the optical path length difference provided by the optical path length difference providing unit 618 and the optical path length difference providing unit 621 are equal.
[0038]
At this time, the coupling rates of the eight optical couplers 601 to 608 are respectively expressed as κ._j(J: 1 to 8), and the optical path length differences given by the six optical path length difference providing units 613 to 618 are respectively L_i(I: 1 to 6). Furthermore, from equation 3, κ_j= Κ_{8-j + 1}It was. At this time κ_jAnd L_iWas given as follows.
[0039]
First, the optical wavelength λ of the channel near the center of the operating wavelength region of the chromatic dispersion compensator_cIs 1.55 μm, the optical frequency interval ΔF is 50 GHz, the optical modulation degree δ is −225 MHz, and the basic optical path length difference l from Equation 2 and Equation 3₁And l₂To derive l₁= −2984.0 μm, l₂= 3011.1 μm was selected. Each of the six L_iIs represented by {n_i} = {2, 2, 1, 0, 0, 0}, {m_i} = {0, 0, 0, 1, 2, 2}, {k_i} = {− 1, −1, 0, −1, 0, 0} was assigned. 8 more kappa_j4 independent kappa_jIs {κ_j} = {0.06, 0.44, 0.93, 0.5}.
[0040]
After designing a photomask according to the above design parameters, a quartz-based waveguide circuit was fabricated on a silicon substrate by using a flame deposition method, photolithography and dry etching. The waveguide was fabricated so that the relative refractive index difference was 0.75% and the waveguide core had a cross-sectional shape of 8 μm × 8 μm.
[0041]
One sample of the completed optical circuit was cut so that the optical circuit 622, the optical circuit 623, and the optical circuit 624 can be separated, and the results of evaluating the transmission characteristics of the respective optical circuits are shown in FIGS. Shown in
[0042]
FIG. 7 shows the transmittance and chromatic dispersion of transmitted light that enters from the port 625 of the optical circuit 622 and exits to the port 626 and the port 627. Incident light from the port 625 was alternately emitted to the port 626 and the port 627 according to the wavelength, and the repetition period was about 50.23 GHz. Further, the chromatic dispersion has a large upward slope when it is emitted to the port 626 and the port 627.
[0043]
FIG. 8 shows the transmittance and chromatic dispersion of transmitted light that enters from the port 626 of the optical circuit 623 and exits to the port 630 and the port 631. Incident light from the port 626 was alternately emitted to the port 630 and the port 631 according to the wavelength, and the repetition period was about 49.77 GHz. In addition, the chromatic dispersion has a large upward slope when emitted to the port 630 and a large downward slope when emitted to the port 631.
[0044]
FIG. 9 shows the transmittance and chromatic dispersion of transmitted light that enters from the port 627 of the optical circuit 624 and exits to the ports 632 and 633. Incident light from the port 627 was alternately emitted to the port 632 and the port 633 according to the wavelength, and the repetition period was about 49.77 GHz. In addition, the chromatic dispersion has a large upward slope when emitted to the port 632 and a large downward slope when emitted to the port 633.
[0045]
As described above, in the optical circuits shown in the optical circuits 622 to 624, by selecting whether the light is incident from the κ = 0.06 side or from the κ = 0.5 side, the positive / negative of the slope of the chromatic dispersion can be set. It can be arranged in the middle or vice versa.
[0046]
In general, the slope of chromatic dispersion of an optical fiber rises slowly to the right with respect to the wavelength. Therefore, a chromatic dispersion compensator that compensates for the chromatic dispersion of the transmission line composed of the optical fiber has a gentle downward slope with respect to the wavelength. Chromatic dispersion is required.
[0047]
As described in the first embodiment, in order to make the slope of chromatic dispersion gently lower to the right in the entire optical circuit connected in series, a filter having a longer repetition period and an upward chromatic dispersion characteristic, and a repetition period However, it is necessary to use a filter having a shorter wavelength drop characteristic. In the optical circuit of the second embodiment, the input port of the first-stage optical circuit 626 is selected to be κ = 0.06, but if the κ = 0.5-side port is selected as the first-stage input port, Since the slopes of chromatic dispersion at the two output ports are opposite to each other, one of the second-stage filters needs to have a longer repetition period, and the remaining one has to have a shorter repetition period. For this reason, the entire optical circuit has three types of repetition periods. As a result, it becomes difficult to align the center of the region where the chromatic dispersion is flat with the grid of the optical channel. By the way, when the κ = 0.06 side is selected as the input port of the first stage optical circuit, there is only one type of chromatic dispersion slope at the output port, so there is one type of repetition period for the second stage filter. Is sufficient, and the entire optical circuit has a repetition cycle of two kinds of condyles. In this way, when only two types of repetition periods are provided, if each period is selected as (ΔF−δ) and (ΔF + δ), the entire circuit period becomes ΔF, and this is matched with the period of the optical channel. Should be designed as follows.
[0048]
In FIG. 10, the chromatic dispersion slope is collectively obtained by using these optical circuits 622, 623, and 624 to change the chromatic dispersion compensation amount, which is the object of the present invention, for each optical channel. A method for realizing a multi-channel chromatic dispersion compensator capable of compensation is schematically shown.
[0049]
FIG. 10A schematically shows the chromatic dispersion characteristics of a filter having a chromatic dispersion characteristic that rises to the right and has a long repetition period. Furthermore, FIG. 10 shows the chromatic dispersion characteristics of both of the two output ports. This is because, for example, the wavelength dispersion characteristic of the transmitted light that enters the port 625 and exits from the port 626 in FIG. 7 corresponds to the characteristic of port A in FIG. The dispersion characteristic corresponds to the characteristic of port B in FIG.
[0050]
FIG. 10B schematically shows the chromatic dispersion characteristic of a filter having a chromatic dispersion characteristic with a downward slope and a short repetition period. This corresponds to, for example, the wavelength dispersion characteristic of transmitted light that enters from the port 626 of FIG. 8 and exits to the port 631. FIG. 10C schematically shows the chromatic dispersion characteristics when the port A of FIG. 10A and the filter having the wavelength characteristics of FIG. 10B are connected in series. As a result of superimposing the chromatic dispersion characteristics of the two filters, a region where the chromatic dispersion property is substantially flat appears at a constant period, and the chromatic dispersion value differs for each region.
[0051]
FIG. 10D schematically shows the wavelength dispersion characteristics of a filter having a wavelength dispersion characteristic that is lower right and having a shorter repetition period at a wavelength position different from that in FIG. This corresponds to, for example, the wavelength dispersion characteristic of transmitted light that enters from the port 627 in FIG. 9 and exits to the port 633. FIG. 10E schematically shows the chromatic dispersion characteristics when the port B of FIG. 10A and the filter having the wavelength characteristics of FIG. 10D are connected in series. As a result of superimposing the chromatic dispersion characteristics of the two filters, in this case as well, a region where the chromatic dispersion property is substantially flat appears at a constant period, and the chromatic dispersion value differs for each region.
[0052]
FIG. 10 (f) shows a superposition of the chromatic dispersion characteristics obtained in FIG. 10 (c) and FIG. 10 (e).
[0053]
Since the input port of the first-stage optical circuit 622 is selected to be κ = 0.06, the repetition period shown in FIG. 10C is equal to the repetition period shown in FIG. In FIG. 10 (f), the repetition period at the center of the region where the chromatic dispersion is almost flat can be made constant.
[0054]
11B and 11C illustrate an optical circuit that is not separated into the optical circuit 622, the optical circuit 623, and the optical circuit 624, that is, the optical circuit 623 and the optical circuit 624 in series at the output ports of the optical circuit 622, respectively. It is a transmission characteristic in each output port of the connected optical circuit. When the interval between the optical channels is 50 GHz and the bandwidth of each optical channel is 20 GHz, the transmittance and chromatic dispersion of the region where the multichannel chromatic dispersion compensator should operate are shown by bold lines.
[0055]
Although the output port alternates between port 631 (see FIG. 11 (b)) and port 633 (see FIG. 11 (c)), regions with high transmittance appear repeatedly at intervals of about 50 GHz, and chromatic dispersion is almost flat. Area appears repeatedly. In addition, the width of each region is about 20 GHz, and the chromatic dispersion value between the regions is gradually lowered to the right. Furthermore, the chromatic dispersion slope value conventionally used in these chromatic dispersion graphs is 0.07 ps / nm.²Overwriting the chromatic dispersion compensation graph (FIG. 11A) required by the 80 km / km DSF fiber 80 km, the graph of chromatic dispersion generated by the optical circuit of the second embodiment with a width of 20 GHz at 50 GHz intervals It can be confirmed that they almost overlap (FIG. 11 (d)). In other words, the single optical circuit of the second embodiment can collectively compensate for the distortion caused by the chromatic dispersion of the optical fiber when optical signals of at least 50 channels are transmitted through the DSF fiber by 80 km. Recognize.
[0056]
The transmittance of the optical circuit was evaluated using an optical module in which a DSF fiber was connected to the input / output port. The best and worst values of the insertion loss of the optical circuit are 1.3 dB at the best and 50 dB at the wavelength of 1.54 μm to 1.56 μm in the 50 wavelength region of 20 GHz width at 50 GHz intervals, and the worst 1.6 dB. Met. The reason why such a small insertion loss can be realized is that the waveguide lattice type can be produced without using a spatial optical system, using a waveguide with a small number of parts, and a small fiber coupling loss. This is due to the characteristics.
[0057]
(Third embodiment)
FIG. 12 shows a circuit configuration of a multi-channel chromatic dispersion compensator according to the third embodiment of the present invention.
[0058]
This circuit is an optical circuit composed of eight optical couplers 1201 to 1208 and seven optical path length difference providing units 1209 to 1215. The optical path length difference provided by the optical path length difference providing unit 1212 at the center of the optical circuit is set to zero, and the structure is symmetrical with respect to the basic optical path length difference. That is, the coupling rates of the eight optical couplers 1201 to 1208 are set to κ, respectively._j(J: 1 to 8), and the optical path length differences given by the seven optical path length difference providing units 1209 to 1215 are respectively L_i(I: 1 to 7), κ_jAnd L_iWas given as follows. Each coupling rate is κ from Equation 3._j= Κ_{8-j + 1}And 8 kappa_jOf which 4 are independent κ_j{Κ_j} = {0.015, 0.6, 0.7, 0.5}. Next, the optical wavelength λ of the channel near the center of the operating wavelength range of the chromatic dispersion compensator_cIs 1.55 μm, the optical frequency interval ΔF is 200 GHz, the optical modulation degree δ is −4 GHz, and the basic optical path length difference l from Equation 2 and Equation 3₁And l₂To derive l₁= 734.4 μm, l₂= 764.6 μm was selected. Each of the six L_iIs represented by {n_i} = {− 2, −2, −1, 0, 0, 0, 0}, {m_i} = {0, 0, 0, 0, -1, -2, -2}, {k_i} = {− 1, −1, 0, 0, −1, 0, 0} was determined.
[0059]
The basic configuration of the optical circuit of the third embodiment described in FIG. 12 can be associated with the basic configuration of the optical circuit of the second embodiment described in FIG. 6 as follows. Since the optical circuit 623 and the optical circuit 624 are exactly the same, the transmission characteristic from the port 627 to the port 633 is the same as the transmission characteristic from the port 628 to the port 631. Also, in the wavelength region around the optical channel, the optical signals are concentrated on one of the ports 626 and 627 by the optical circuit 622, and almost no optical signal comes to the other port. Considered an elephant. Therefore, the optical circuit 624 is disconnected at the port 627 and the port 628 of the optical circuit 623 is connected to the port 627 of the optical circuit 622. When the port 622 and the port 627 are connected, the optical circuit 623 is turned upside down so that a crossed waveguide does not occur. By the above modification work, the basic configuration of the optical circuit of the second embodiment and the basic configuration of the optical circuit of the third embodiment are almost equivalent optical circuits in the wavelength region around the optical channel. Recognize.
[0060]
In the optical circuit of the third embodiment shown in FIG. 12, the number of optical couplers is reduced from 12 to 8 compared to the optical circuit of the second embodiment, and the number of optical path length difference providing units is increased. There is an advantage that the number can be reduced from 9 to 7. Furthermore, since it has a 1-input 1-output configuration, it is suitable for use in a relay station.
[0061]
After designing a photomask according to the above design parameters, a quartz-based waveguide circuit was fabricated on a silicon substrate by using a flame deposition method, photolithography and dry etching. The waveguide was fabricated so that the relative refractive index difference was 0.75% and the waveguide core had a cross-sectional shape of 8 μm × 8 μm.
[0062]
The results of evaluating the transmission characteristics of the completed optical circuit are shown in FIG. When the interval between the optical channels is 200 GHz and the bandwidth of each optical channel is 80 GHz, the transmittance and chromatic dispersion of the region in which the multichannel chromatic dispersion compensator should operate are indicated by bold lines.
[0063]
In FIG. 13B, a region where the transmittance is high in almost the entire region and the wavelength dispersion is almost flat appears repeatedly. In addition, the width of the region where the chromatic dispersion is almost flat is about 80 GHz, and the chromatic dispersion value between the regions is gradually lowered to the right. Furthermore, the chromatic dispersion slope value conventionally used in this chromatic dispersion graph is 0.07 ps / nm.²Overwriting the chromatic dispersion compensation graph (FIG. 13A) required by the 10 km / km DSF fiber 10km, the graph of chromatic dispersion generated by the optical circuit of the third embodiment with a width of 80 GHz at intervals of 200 GHz; It can be confirmed that they almost overlap (FIG. 13C). In other words, with one optical circuit of the third embodiment, it is possible to collectively compensate for distortion caused by chromatic dispersion of an optical fiber when an optical signal of at least 12 channels is transmitted through a DSF fiber by 10 km. Recognize.
[0064]
The transmittance of the optical circuit was evaluated using an optical module in which a DSF fiber was connected to the input / output port. The best and worst values of the insertion loss of the optical circuit are the best 0.6 dB and the worst 2.2 dB in 12 wavelength regions of 80 GHz width at 200 GHz intervals of wavelength dispersion compensation from wavelengths 1.54 μm to 1.56 μm. Met. The reason why such a small insertion loss can be realized is that the waveguide lattice type can be produced without using a spatial optical system, using a waveguide with a small number of parts, and a small fiber coupling loss. This is due to the characteristics.
[0065]
(Fourth embodiment)
FIG. 14 shows a circuit configuration of a multi-channel chromatic dispersion compensator according to the fourth embodiment of the present invention.
[0066]
This circuit includes six optical couplers 1401 to 1406, five optical path length difference giving units 1413 to 1417, six optical couplers 1407 to 1412, and five optical paths. This is an optical circuit in which optical circuits 1424 composed of length difference providing units 1418 to 1422 are connected in series.
[0067]
The optical circuit 1423 is designed based on a known chromatic dispersion compensator (see Non-Patent Document 2), and can compensate for chromatic dispersion in the vicinity of a channel at a constant interval, but the amount of chromatic dispersion compensation is different for each channel. Therefore, the circuit cannot compensate even the chromatic dispersion slope.
[0068]
For details of the circuit parameters of the optical circuit 1423, the coupling strength of the optical couplers 1401 to 1406 is set to κ ′._jAnd the optical path length differences given by the five optical path length difference providing units 1413 to 1417 are represented by L ′._i(I: 1 to 5), {κ '_j: J = 1-6} = {0.5, 0.33, 0.33, 0.33, 0.33, 0.5}. The basic optical path length difference is l '₁And l '₂Are 1498.6 μm and 1499.4 μm, respectively,
## EQU11 ##

As {n '_i} = {1, -1, 1, -1, 0}, {m '_i} = {0, -1, 1, -1, 1}, {k '_i} = {0, 0, 0, 0, 0, 0}.
[0069]
On the other hand, the basic configuration of the optical circuit 1424 can be associated with the basic configuration of the optical circuit of the third embodiment shown in FIG. 12 as follows.
[0070]
(I) The optical path length difference providing unit 1217 has the same length, and the coupling rates of the optical coupler 1204 and the optical coupler 1205 are both 0.5. In other words, the optical coupler 1204, the optical path length difference providing unit 1217, and the optical coupler 1205 are optically equivalent to the crossed waveguide as a whole and can be replaced.
[0071]
(II) By reversing the positive / negative of the optical path length difference provided by the optical path length difference providing units 1213 to 1215, the above-described crossed waveguide can be replaced with a parallel waveguide.
[0072]
(III) Since there is no optical coupling portion between the optical path length difference providing unit 1211 and the optical path length difference providing unit 1213, the two optical path length difference providing units can be collectively handled as one optical path length difference providing unit. The required optical path length difference is extremely small and is not easily affected by manufacturing errors.
[0073]
It can be understood that the basic configuration of the optical circuit 1424 of the fourth embodiment is optically equivalent to the basic configuration of the optical circuit of the third embodiment by the above modification operation. Therefore, it can be said that the optical circuit of the fourth embodiment has a configuration in which the conventional optical circuit 1423 and the optical circuit 1424 according to the present invention are connected in series.
[0074]
In the optical circuit of the fourth embodiment shown in FIG. 14, the number of optical couplers is reduced from eight to six compared to the optical circuit of the third embodiment, and the number of optical path length difference providing units is reduced. There is an advantage that the number can be reduced from 7 to 5.
[0075]
For details of the circuit parameters of the optical circuit 1424, the coupling strength of the six optical couplers 1407 to 1412 is represented by κ._jAnd the optical path length differences given by the five optical path length difference providing units 1418 to 1422 are respectively L_iWhen κ_jAnd L_iWas given as follows.
[0076]
First, the optical wavelength λ of the channel near the center of the operating wavelength region of the chromatic dispersion compensator_cIs 1.575 μm, the optical frequency interval ΔF is 100 GHz, the optical modulation degree δ is −750 MHz, and the basic optical path length difference l from Equation 2 and Equation 3₁And l₂To derive l₁= −1487.9 μm, l₂= 1509.2 μm was selected. That is, in this optical circuit 1424, the number of optical path length difference providing units 1418 to 1422 from the input end to the output end is an odd number, and the basic optical path length difference l₁, L₂It can be said that the sign of is opposite.
[0077]
Each of the 5 L_iIs expressed as {n_i} = {2, 2}, {k_i} = {-1, -1, -1, 0, 0} and determined. More specifically, L1 = L2 = −2975.1 μm, L3 = 21.26 μm, and L4 = L5 = 3017.6 μm. 6 kappa_j3 independent kappa_jIs {κ_j} = {0.015, 0.6, 0.7}.
[0078]
After designing a photomask according to the above design parameters, a quartz-based waveguide circuit was fabricated on a silicon substrate by using a flame deposition method, photolithography and dry etching. The waveguide was fabricated so that the relative refractive index difference of the waveguide was 1.2% and the waveguide core had a cross-sectional shape of 4 μm × 4 μm.
[0079]
FIG. 15 shows the result of evaluating the transmission characteristics of the completed optical circuit. When the interval between the optical channels is 200 GHz and the bandwidth of each optical channel is 40 GHz, the transmittance and chromatic dispersion in the region where the multichannel chromatic dispersion compensator should operate are indicated by bold lines.
[0080]
As shown in FIG. 15, a region where the transmittance is high in almost the entire region and the wavelength dispersion is almost flat appears repeatedly. Further, the width of the region where the chromatic dispersion is almost flat is about 40 GHz, and the chromatic dispersion value between the regions is gradually lowered to the right. Furthermore, the chromatic dispersion slope value conventionally used in this chromatic dispersion graph is 0.07 ps / nm.²Overwriting the graph of the chromatic dispersion compensation amount required by the 25 km / km DSF fiber, it can be confirmed that the optical circuit of the fourth embodiment substantially overlaps the graph of chromatic dispersion generated at a width of 40 GHz at intervals of 200 GHz. That is, it can be seen that the optical circuit according to the fourth embodiment can collectively compensate for distortion caused by wavelength dispersion of the optical fiber when optical signals of at least 18 channels are transmitted through the DSF fiber 25 km.
[0081]
The transmittance of the optical circuit was evaluated using an optical module in which a DSF fiber was connected to the input / output port. The best and worst values of the insertion loss of the optical circuit are 1.7 dB best and worst 3.6 dB in 18 wavelength regions of 40 GHz width at 200 GHz intervals among wavelengths from 1.54 μm to 1.56 μm. Met. The reason why such a small insertion loss can be realized is that the waveguide lattice type can be produced without using a spatial optical system, using a waveguide with a small number of parts, and a small fiber coupling loss. This is due to the characteristics.
[0082]
In each of the first, second, third, and fourth embodiments described above, a quartz-based waveguide circuit is fabricated on a silicon substrate by using a flame deposition method, photolithography, and dry etching. Waveguide material is polyimide, silicone, semiconductor, LiNbO_ThreeHowever, the present invention can be applied. Further, the present invention can be applied even if the manufacturing method is a spin coating method, a sol-gel method, a spack method, a CVD method, an ion diffusion method, an ion beam direct drawing method, or the like. Furthermore, the substrate material is not limited to silicon, and the same effect can be obtained with a quartz substrate.
[0083]
FIG. 16 shows an optical module using the waveguide type multi-channel chromatic dispersion compensator (PLC chip) of the present invention used in the first embodiment.
[0084]
In the optical module, a Peltier holding plate 1602 is fixed inside a casing 1601 with good thermal conductivity with a fixing screw 1603, and a Peltier element and a temperature sensor (thermal Is placed in the vicinity. The optical circuit 1604 according to the first embodiment is arranged directly above the Peltier element and the temperature sensor (not shown).
[0085]
A glass plate 1605 is bonded to an end of the optical circuit 1604 with an adhesive, and is bonded so as to be optically bonded to the fiber block 1607 holding the fiber 1606. The fiber 1606 is bonded to a recess provided at the edge of the housing 1601 with a heat insulating elastic adhesive 1608, and a fiber boot 1610 having a fiber cord 1609 is held so as to be embedded in the housing 1601.
[0086]
The PLC chip of the present invention used in Embodiment 1 is bonded to a Peltier holding plate with a heat insulating elastic adhesive 1608.
[0087]
Finally, the optical module of the present invention was manufactured by covering with a cover and screwing. The lid and the screwing portion are not shown.
[0088]
When such an optical module was manufactured and the same measurement as in the first embodiment was performed, it was found that the same result as in FIG. 5 was obtained and the same result as in the first embodiment was obtained.
[0089]
Similarly, an optical module using the waveguide type multi-channel chromatic dispersion compensator of the present invention used in the second, third, and fourth embodiments was manufactured, and the same effects as those of the second, third, and fourth embodiments were obtained. . Although the waveguide type multi-channel chromatic dispersion compensator of the present invention may be divided into two or more parts, the same effect can be obtained by fabricating each optical module and coupling the optical modules with a fiber. I understood. Although not shown, the same effect was obtained with an optical module in which the above two or more chips were held on a Peltier holding plate in one casing.
[0090]
【The invention's effect】
As described above, according to the present invention, a multi-channel chromatic dispersion compensator that supports both the chromatic dispersion and the chromatic dispersion slope of an optical fiber can be realized without a spatial optical system and with a small number of components and a small insertion loss. It becomes possible to supply a waveguide lattice type.
[Brief description of the drawings]
FIG. 1 is a diagram showing a circuit configuration of a first embodiment of the present invention.
FIG. 2 is a diagram illustrating transmission spectrum characteristics of an optical circuit 115. FIG.
FIG. 3 is a diagram showing transmission spectrum characteristics of an optical circuit.
FIG. 4 is a diagram schematically illustrating the synthesis of transmission spectrum characteristics of an optical circuit 115 and an optical circuit 116. FIG.
5 is a diagram showing transmission spectrum characteristics of the multi-channel chromatic dispersion compensator of FIG. 1. FIG.
FIG. 6 is a diagram showing a circuit configuration of a second embodiment of the present invention.
FIG. 7 is a diagram showing transmission spectrum characteristics of an optical circuit 622;
FIG. 8 is a diagram showing a transmission spectrum characteristic of an optical circuit 623;
FIG. 9 is a diagram showing transmission spectrum characteristics of an optical circuit 624;
10 is a diagram schematically showing the composition of transmission spectrum characteristics of the optical circuit 622, the optical circuit 623, and the optical circuit 624. FIG.
11 is a diagram showing transmission spectrum characteristics of the multichannel chromatic dispersion compensator of FIG. 6. FIG.
FIG. 12 is a diagram showing a circuit configuration of a third embodiment of the present invention.
13 is a diagram showing transmission spectrum characteristics of the multi-channel chromatic dispersion compensator of FIG. 12. FIG.
FIG. 14 is a diagram showing a circuit configuration of a fourth embodiment of the present invention.
15 is a diagram showing transmission spectrum characteristics of the multi-channel chromatic dispersion compensator of FIG. 15. FIG.
FIG. 16 is a diagram showing an optical module using a waveguide type multi-channel chromatic dispersion compensator (PLC chip) according to the present invention.
[Explanation of symbols]
101-108 optical coupler
109 to 114 Optical path length difference giving unit
115, 116 Optical circuit
117 to 124 ports
601-612 optical coupler
613 to 621 Optical path length difference giving unit
622-624 optical circuit
625-633 ports
1201-1208 Optical coupler
1209 to 1215 optical path length difference giving unit
1216-1219 ports
1401-1414 Optical coupler
1413-1422 Optical path length difference giving unit
1423, 1424 Optical circuit
1425, 1426 ports
1601 housing
1602 Peltier holding plate
1603 Fixing screw
1604 PLC chip
1605 glass plate
1606 fiber
1607 Fiber block
1608 Insulating elastic adhesive
1609 Fiber cord
1610 Fiber Boot

Claims (7)

A first optical waveguide, a second optical waveguide, and N + 1 (N is a natural number) optical couplers for coupling the first optical waveguide and the second optical waveguide at different locations, Among N + 1 optical couplers, an optical waveguide circuit provided with N optical path length difference providing units sandwiched between two adjacent optical couplers, or a plurality of optical waveguide circuits connected in series An optical waveguide circuit,
Of the M optical path length difference providing units from the input end to the output end (M is a natural number), the optical path length difference L _i given by the i-th optical path length difference providing unit from the input end is Set as shown in the following formula ,
A monotonically decreasing chromatic dispersion characteristic that repeats at a predetermined period and a monotonically increasing chromatic dispersion characteristic that repeats at a period longer than the predetermined period are formed with respect to the frequency of the optical signal, and the different chromatic dispersion characteristics are superimposed. By
Compensate the chromatic dispersion in a lump in the peripheral band of multiple optical channels with different optical wavelengths with a constant optical frequency interval, and collectively change the chromatic dispersion compensation amount for each optical channel. A multichannel chromatic dispersion compensator characterized by compensating for a chromatic dispersion slope .

Where n _i , m _i , and k _i are integers, λ _c is the optical wavelength of the channel near the center of the operating wavelength region of the multichannel chromatic dispersion compensator, the optical frequency interval is ΔF, and the speed of light is C The basic optical path length differences l ₁ , l ₂ and δ are expressed by the following equations, where δ is the optical frequency modulation degree.

The multi-channel chromatic dispersion compensator according to claim 1 ,
The first optical waveguide circuit on the input side includes the three optical waveguide circuits of the second optical waveguide circuit and the third optical waveguide circuit connected in series, respectively, and the second optical waveguide circuit and the The optical path length difference of the optical path length difference providing unit with the third optical waveguide circuit and the coupling strength of the optical coupler are the same, and the second optical waveguide circuit and the third optical waveguide The coupling strength of each of the optical couplers of each circuit is symmetrically equal to the coupling strength of the optical coupler of the first optical waveguide circuit, centering on a connection portion with the first optical waveguide circuit. A multi-channel chromatic dispersion compensator. The multi-channel chromatic dispersion compensator according to claim 1 ,
Among the S optical couplers from the input end to the output end (S is an integer), the coupling strength κ _{j of the} j-th optical coupler from the input end is expressed by the following equation for all j. A multi-channel chromatic dispersion compensator characterized by

The multi-channel chromatic dispersion compensator according to claim 1 ,
The coupling strength of each of the optical couplers of each of the optical waveguide circuits is composed of one or a plurality of the optical waveguide circuits, with the optical path difference length providing portion in the middle of each of the optical waveguide circuits as a center. Multi-channel chromatic dispersion compensator characterized by being symmetrically equal. The multi-channel chromatic dispersion compensator according to claim 3 ,
The signs of the basic optical path length differences l ₁ and l ₂ are opposite to each other, the number M of the optical path length difference providing units from the input end to the output end is an odd number, and the i th or The multi-channel chromatic dispersion compensator characterized in that the optical path length difference L _i given by the i-th optical path length difference providing unit from the output end is expressed by the following equation depending on the magnitude of i.

The first optical waveguide, the second optical waveguide, and a plurality of optical couplers for coupling the first optical waveguide and the second optical waveguide at different locations, respectively, and the adjacent optical couplers a waveguide circuit was connected a plurality of optical waveguide circuit in series with a plurality of optical path length difference imparting portion sandwiched by,
6. A multi-channel chromatic dispersion compensator according to claim 5 , wherein the multi-channel chromatic dispersion compensator according to claim 5 is used for any one of the optical waveguide circuits. A housing having the multichannel chromatic dispersion compensator according to any one of claims 1 to 6 inside, and light that is held in the housing and that inputs and outputs optical signals to the multichannel chromatic dispersion compensator An optical module comprising a fiber.

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