JP3664305B2

JP3664305B2 - Polarization mode dispersion suppression device

Info

Publication number: JP3664305B2
Application number: JP2002102091A
Authority: JP
Inventors: 敏夫伊藤; 里江子佐藤; 安弘鈴木
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2001-07-27
Filing date: 2002-04-04
Publication date: 2005-06-22
Anticipated expiration: 2022-04-04
Also published as: JP2003156721A

Description

【０００１】
【発明の属する技術分野】
本発明は偏波モード分散抑制装置に関し、光ファイバを伝搬する高速光通信、光交換、光情報処理等の光伝送システムに適用され、特に偏波モード分散を抑制するのに有効な技術である。また本発明は、ＴＥ偏波成分とＴＭ偏波成分との光強度差が大きくても、偏波モード分散を効果的に抑制することができるように工夫している。
【０００２】
【従来の技術】
ＩＴ技術の発展に伴う伝送容量の増加に伴い、光信号のビットレートは２．５Ｇｂ／ｓから１０Ｇｂ／ｓ、さらには４０Ｇｂ／ｓへと増加する傾向にある。ここで問題となるのが偏波モード分散である。図１２は偏波モード分散を説明する図であって、１００１は光ファイバ、１００２は入力光パルス、１００３は入力光パルス１００２のＴＥもしくはＴＭ偏波成分、１００４は入力光パルス１００２のＴＭもしくはＴＥ偏波成分、１００７は出力光パルス、１００５は出力光パルス１００７のＴＥもしくはＴＭ偏波成分、１００６は出力光パルス１００７のＴＭもしくはＴＥ偏波成分である。
【０００３】
一般に光ファイバには偏波モード分散が存在し、光ファイバ中を伝搬する光信号は、偏波面によって、速く進む成分と遅く進む成分に別れて伝搬する。図１２において偏波成分１００３が速く進む成分であり、出力端において、偏波成分１００５となる。一方で偏波成分１００４は遅く進む成分であり、出力端では偏波成分１００６となり偏波成分１００５より遅く到着する。出力光パルス１００７は偏波成分１００５と偏波成分１００６の和となるので、結果として出力光パルス１００７の波形がなまる。
【０００４】
この偏波モード分散の値としては、例えばファイバ長Ｌ（Ｋｍ）に対し、
０．２×Ｌ^1/2（ｐｓ）〜２×Ｌ^1/2（ｐｓ）
程度である。すなわち、最悪の場合１００Ｋｍの光ファイバを仮定すると２０ｐｓの偏波モード分散が生じる。この値は２．５Ｇｂ／ｓ（パルス幅４００ｐｓ）や１０Ｇｂ／ｓ（パルス幅１００ｐｓ）ではさほど大きな問題ではないが、４０Ｇｂ／ｓ（パルス幅２５ｐｓ）では致命的な波形劣化となり、符号誤り率を大きく劣化させる要因となる。
【０００５】
この問題を解決するために、従来では、図１３のような構成で偏波モード分散を抑圧してきた。図１３において、１１０１は入力用光ファイバ、１１０２は入力光パルス、１１０３は入力光パルス１１０２のＴＥもしくはＴＭ偏波成分、１１０４は入力光パルス１１０２のＴＭもしくはＴＥ偏波成分、１１０５は偏波コントローラ、１１０６は偏波モード分散のとくに大きい光ファイバで、例えば偏波面保持光ファイバ、１１０７はＴＥもしくはＴＭ偏波成分、１１０８はＴＭもしくはＴＥ偏波成分、１１０９は光カプラ、１１１０は受光器、１１１１は電気バンドパスフィルタ、１１１２は偏波コントローラ１１０５の制御系、１１１３は出力用光ファイバ、１１１６は波形整形された光パルス、１１１４は光パルス１１１６のＴＥもしくはＴＭ偏波成分、１１１５は光パルス１１１６のＴＭもしくはＴＥ偏波成分である（参考文献 Geogre Ishikawa, Hiroki Ooi, and Yuichi Akiyama, APCC/OECC '99, pp.424-428 ）。
【０００６】
図１３の構成では、遅延した偏波成分１１０３が光ファイバ１１０６の速く進む方向に、先行している偏波成分１１０４を光ファイバ１１０６の遅く進む方向に入力するように、偏波コントローラ１１０５により入力光パルス１１０２の偏波状態を調整することで、偏波モード分散を抑制する方式である。光ファイバ１１０６としては偏波モード分散の特に大きい、例えば偏波面保持光ファイバを用いる。偏波面保持光ファイバの偏波モード分散は、例えば１ｍあたり１ｐｓ程度である。
【０００７】
上記の参考文献に基づく図１３の構成によれば、光信号の一部を光カプラ１１０９にて分岐した後に受光器１１１０にて受光し、受光した光を受光器１１１０にて光／電気変換した電気信号を、伝送速度の半分の帯域の電気バンドパスフィルタ１１１１を介して制御系１１１２に送る。制御系１１１２は、電気信号の強度（つまり受光した光の強度）を最高にするように偏波コントローラ１１０５を制御することで、偏波モード分散を最小にするように、すなわち偏波成分１１１４と偏波成分１１１５の伝搬遅延の差を最小にし、波形整形された光パルス１１６を得る。
【０００８】
【発明が解決しようとする課題】
しかしながら、図１３に示す従来技術には以下のような欠点があった。第１の欠点は光信号のビットレートの制限である。図１３の構成はビットレートのちょうど半分の帯域を有する電気バンドパスフィルタ１１１１を必要とするため、ビットレートを変更することができない。また第２の欠点として、例えば１０１０１０１０…といった１０符号が連続すると、ビットレートの半分の高周波成分が増え、電気のフィードバックに影響を与える問題がある。さらに第３の欠点としてビットレートの半分の速度を有する受光器１１１０を必要とするため、受光器１１１０が高価になる問題があった。
【０００９】
本発明は、上記の欠点に鑑み、ビットレートを変更することができ、１０符号が連続してもフィードバック系に影響を与えず、さらに低速な受光器でも構成が可能な偏波モード分散抑制装置を提供することを目的とする。また、ＴＥ偏波成分とＴＭ偏波成分との光強度差が大きくても、偏波モード分散を効果的に抑制することができ偏波モード分散抑制装置を提供することを目的とする。
【００１０】
【課題を解決するための手段】
上記の問題を解決する本発明の構成は、光ファイバを伝搬してきた光信号が入力されると、この光信号のＴＥ偏波成分とＴＭ偏波成分の伝送遅延の差が最小になるように補償することによって偏波モード分散が抑制された光信号を出力する偏波モード分散補償器と、
前記偏波モード分散補償器から出力された偏波モード分散が抑制された光信号が入力され、この光信号の一部を出力すると共に残りの一部を分岐する１入力２出力の光カプラと、
前記１入力２出力の光カプラにて分岐された光信号をＴＥ偏波成分とＴＭ偏波成分に分ける偏波スプリッタと、
前記光信号のＴＥ偏波成分とＴＭ偏波成分が同一の光路長を経て個別に２つの入力ポートに入力され、光信号のＴＥ偏波成分とＴＭ偏波成分による光ｅｘＯＲ演算をして演算結果を出力する光ｅｘＯＲ回路と、
前記光ｅｘＯＲ回路の論理結果出力が「０」となるように前記偏波モード分散補償器による補償を制御する制御系と、を有することを特徴とする。
【００１１】
また本発明の構成は、光ファイバを伝搬してきた光信号が入力されると、この光信号のＴＥ偏波成分とＴＭ偏波成分の伝送遅延の差が最小になるように補償することによって偏波モード分散が抑制された光信号を出力する偏波モード分散補償器と、
前記偏波モード分散補償器内から偏波モード分散が抑制されたＴＥ偏波成分の一部を分岐する第１の１入力２出力の光カプラと、
前記偏波モード分散補償器内から偏波モード分散が抑制されたＴＭ偏波成分の一部を分岐する第２の１入力２出力の光カプラと、
第１の１入力２出力の光カプラにて分岐された一方の偏波成分と第２の１入力２出力の光カプラにて分岐された他方の偏波成分とが同一の光路長を経て個別に２つの入力ポートに入力され、光信号の一方の偏波成分と他方の偏波成分による光ｅｘＯＲ演算をして演算結果を出力する光ｅｘＯＲ回路と、
前記光ｅｘＯＲ回路の論理結果出力が「０」となるように前記偏波モード分散補償器による補償を制御する制御系と、を有することを特徴とする。
【００１２】
また本発明の構成は、光ファイバを伝搬してきた光信号が入力されこの光信号の偏波状態を制御する偏波コントローラと、前記偏波コントローラから偏波状態が制御された光信号が入力されこの光信号を伝搬させることにより光信号の偏波モード分散を抑制する偏波モード分散制御用光ファイバとで構成された偏波モード分散補償器と、
前記偏波モード分散制御用光ファイバから出力された偏波モード分散が抑制された光信号が入力され、この光信号の一部を出力すると共に残りの一部を分岐する１入力２出力の光カプラと、
前記１入力２出力の光カプラにて分岐された光信号をＴＥ偏波成分とＴＭ偏波成分に分ける偏波スプリッタと、
前記光信号のＴＥ偏波成分とＴＭ偏波成分が同一の光路長を経て個別に２つの入力ポートに入力され、光信号のＴＥ偏波成分とＴＭ偏波成分による光ｅｘＯＲ演算をして演算結果を出力する光ｅｘＯＲ回路と、
前記光ｅｘＯＲ回路の論理結果出力が「０」となるように前記偏波コントローラによる偏波状態を制御する制御系と、を有することを特徴とする。
【００１３】
また本発明の構成は、光ファイバを伝搬してきた光信号が入力されこの光信号の偏波状態を制御する偏波コントローラと、前記偏波コントローラから偏波状態が制御された光信号が入力されこの光信号を伝搬させることにより光信号の偏波モード分散を抑制する偏波モード分散制御用光ファイバとで構成された偏波モード分散補償器と、
前記偏波モード分散制御用光ファイバから出力された偏波モード分散が抑制された光信号が入力され、この光信号の一部を出力すると共に残りの一部を分岐する第１の１入力２出力の光カプラと、
前記第１の１入力２出力の光カプラにて分岐された光信号をＴＥ偏波成分とＴＭ偏波成分に分ける第１の偏波スプリッタと、
第１の偏波スプリッタにて分けられた前記光信号のＴＥ偏波成分とＴＭ偏波成分が同一の光路長を経て個別に２つの入力ポートに入力され、光信号のＴＥ偏波成分とＴＭ偏波成分による光ｅｘＯＲ演算をして演算結果を出力する第１の光ｅｘＯＲ回路と、
前記偏波コントローラと前記偏波モード分散制御用光ファイバとの間に配置され、偏波コントローラから出力された光信号の一部を前記偏波モード分散制御用光ファイバに向けて出力すると共に残りの一部を分岐する第２の１入力２出力の光カプラと、
前記第２の１入力２出力の光カプラにて分岐された光信号をＴＥ偏波成分とＴＭ偏波成分に分ける第２の偏波スプリッタと、
第２の偏波スプリッタにて分けられた前記光信号のＴＥ偏波成分とＴＭ偏波成分が同一の光路長を経て個別に２つの入力ポートに入力され、光信号のＴＥ偏波成分とＴＭ偏波成分による光ｅｘＯＲ演算をして演算結果を出力する第２の光ｅｘＯＲ回路と、
前記第１の光ｅｘＯＲ回路の論理結果出力が「０」となると共に、前記第２の光ｅｘＯＲ回路の論理結果出力が「０」とならないように、前記偏波コントローラによる偏波状態を制御する制御系と、を有することを特徴とする。
【００１４】
また本発明の構成は、光ファイバを伝搬してきた光信号が入力されこの光信号の偏波状態を制御する偏波コントローラと、前記偏波コントローラから偏波状態が制御された光信号が入力されこの光信号を伝搬させることにより光信号の偏波モード分散を抑制する偏波モード分散制御用光ファイバとで構成された偏波モード分散補償器と、
前記偏波モード分散制御用光ファイバから出力された光信号が入力され、この光信号の一部を出力すると共に残りの一部を分岐する１入力２出力の光カプラと、
前記１入力２出力の光カプラにて分岐された光信号をＴＥ偏波成分とＴＭ偏波成分に分ける偏波スプリッタと、
前記偏波スプリッタから出力されたＴＥ偏波成分とＴＭ偏波成分の内の一方の偏波成分を伝搬する第１の偏波用光導波路と、
第１の偏波用光導波路と同一の光路長を有しており、前記偏波スプリッタから出力されたＴＥ偏波成分とＴＭ偏波成分の内の他方の偏波成分を伝搬する第２の偏波用光導波路と、
第１の偏波用光導波路及び第２の偏波用光導波路を経てＴＥ偏波成分とＴＭ偏波成分が個別に入力され、光信号のＴＥ偏波成分とＴＭ偏波成分による光ｅｘＯＲ演算をして演算結果を出力する光ｅｘＯＲ回路と、
前記光ｅｘＯＲ回路の論理出力が「０」となるように前記偏波コントローラによる偏波状態を制御する制御系とを有する偏波モード分散抑制装置において、
第１の偏波用光導波路及び第２の偏波用光導波路に、入力される光の強度が異なっていても出力する光の強度を一定範囲内の強度にして出力する光レベル調整器をそれぞれ備えたことを特徴とする。
【００１５】
また本発明の構成は、前記光レベル調整器は光増幅器を１段もしくは多段に設けて構成されていたり、
第１の偏波用光導波路を経て前記光ｅｘＯＲ回路に入力される偏波成分の強度を検出する第１の受光器と、
第２の偏波用光導波路を経て前記光ｅｘＯＲ回路に入力される偏波成分の強度を検出する第２の受光器と、
第１の受光器で受光した偏波成分の強度と、第２の受光器で受光した偏波成分の強度とが等しくなるように、前記光レベル調整器のそれぞれの増幅度を制御する制御回路とを更に備えていることを特徴とする。
【００１６】
また本発明の構成は、光ファイバを伝搬してきた光信号が入力されこの光信号をＴＥ偏波成分とＴＭ偏波成分に分ける偏波スプリッタと、前記偏波スプリッタにより分岐されたＴＥ偏波成分とＴＭ偏波成分のうちの一方の偏波成分が入力されると共に可変位相遅延線が介装された一方の光導波路と、前記偏波スプリッタにより分岐されたＴＥ偏波成分とＴＭ偏波成分のうちの他方の偏波成分が入力される他方の光導波路と、前記一方の光導波路を伝搬してきた偏波成分と前記他方の光導波路を伝搬してきた偏波成分を合波して出力する光合波器とで構成された偏波モード分散補償器と、
前記一方の光導波路のうち前記可変位相遅延線が介装された位置よりも後段に介装され、一方の偏波成分の一部を前記光合波器に向けて出力すると共に残りの一部を分岐する第１の１入力２出力の光カプラと、
前記他方の光導波路に介装され、他方の偏波成分の一部を前記光合波器に向けて出力すると共に残りの一部を分岐する第２の１入力２出力の光カプラと、
第１の１入力２出力の光カプラにて分岐された一方の偏波成分と第２の１入力２出力の光カプラにて分岐された他方の偏波成分とが同一の光路長を経て個別に２つの入力ポートに入力され、光信号の一方の偏波成分と他方の偏波成分による光ｅｘＯＲ演算をして演算結果を出力する光ｅｘＯＲ回路と、
前記光ｅｘＯＲ回路の論理結果出力が「０」となるように前記可変位相遅延線による位相遅延量を制御する制御系と、を有することを特徴とする。
【００１７】
また本発明の構成は、光ファイバを伝搬してきた光信号が入力されこの光信号をＴＥ偏波成分とＴＭ偏波成分に分ける偏波スプリッタと、前記偏波スプリッタにより分岐されたＴＥ偏波成分とＴＭ偏波成分のうちの一方の偏波成分が入力されると共に可変位相遅延線が介装された一方の光導波路と、前記偏波スプリッタにより分岐されたＴＥ偏波成分とＴＭ偏波成分のうちの他方の偏波成分が入力される他方の光導波路と、前記一方の光導波路を伝搬してきた偏波成分と前記他方の光導波路を伝搬してきた偏波成分を合波して出力する光合波器とにより構成された偏波モード分散補償器と、
前記一方の光導波路のうち前記可変位相遅延線が介装された位置よりも後段に介装され、一方の偏波成分の一部を前記光合波器に向けて出力すると共に残りの一部を分岐する第１の１入力２出力の光カプラと、
前記他方の光導波路に介装され、他方の偏波成分の一部を前記光合波器に向けて出力すると共に残りの一部を分岐する第２の１入力２出力の光カプラと、
第１の１入力２出力の光カプラにて分岐された一方の偏波成分と第２の１入力２出力の光カプラにて分岐された他方の偏波成分とが同一の光路長を経て個別に２つの入力ポートに入力され、光信号の一方の偏波成分と他方の偏波成分による光ｅｘＯＲ演算をして演算結果を出力する光ｅｘＯＲ回路と、
前記光ｅｘＯＲ回路の論理結果出力が「０」となるように前記可変位相遅延線による位相遅延量を制御する第１の制御系と、
前記一方の光導波路のうち前記可変位相遅延線が介装された位置よりも前段に介装され、一方の偏波成分の一部を前記光合波器に向けて出力すると共に残りの一部を分岐する第３の１入力２出力の光カプラと、
前記他方の光導波路に介装され、他方の偏波成分の一部を前記光合波器に向けて出力すると共に残りの一部を分岐する第４の１入力２出力の光カプラと、
前記光ファイバと前記偏波スプリッタとの間に介装され、前記光ファイバから出力された光信号の偏波状態を制御してから前記偏波スプリッタに向けて出力する偏波コントローラと、
第３の光カプラにて分岐された偏波成分と第４の光カプラにて分岐された偏波成分の強度の差が最小になるように、前記偏波コントローラによる偏波状態を制御する第２の制御系と、を有することを特徴とする。
【００１８】
また本発明の構成は、前記偏波モード分散補償器は、プレーナ光回路型の偏波モード分散補償器であり、
このプレーナ光回路型の偏波モード分散補償器は、
第１の偏波スプリッタと、第１の位相調整領域と、第１の可変カプラと、第２の位相調整領域と、第２の可変カプラと、偏波遅延領域と、第２の偏波スプリッタが順に接続されて形成されており、
第１及び第２の偏波スプリッタは、対称型のマッハ・ツェンダ干渉計の一方の導波路にアモルファスシリコンを備えると共に、他方の導波路にヒータを備えて構成され、
第１及び第２の位相調整領域ならびに第１及び第２の可変カプラは、対称型のマッハ・ツェンダ干渉計の両方の導波路にヒータを備えて構成され、
前記偏波遅延領域は、マッハ・ツェンダ干渉計であって一方の導波路と他方の導波路の長さが異なって構成され、
更に、第１の位相調整領域の一方の導波路のうち、ヒータが備えられている部分よりも入力側の位置に第１の半波長板が介装され、
前記偏波遅延領域の２つの導波路の何れか１つに第２の半波長板が介装されており、
このプレーナ光回路型の偏波モード分散補償器のヒータの発熱量を変化させることにより偏波状態を変化させることができるようになっていることを特徴とする。
【００１９】
また本発明の構成は、前記偏波モード分散補償器は、リチウムナイオブレート型の偏波モード分散補償器であり、このリチウムナイオブレート型の偏波モード分散補償器は、
リチウムナイオブレートにより形成された導波路の上に、第１の電圧値の電圧が印加される複数の第１群の櫛型電極と、第２の電圧値の電圧が印加される複数の第２群の櫛形電極とが、交互に配置されると共に、各櫛型電極の間に更にアース電極を配置しており、
光信号のビート長をΛとしたときに、各櫛型電極の相互間の間隔が、入力側から出力側に向かい交互にΛ／４と３Λ／４とになっており、前記櫛形電極に印加する電圧を変化させることにより偏波状態を変化させることができるようになっていることを特徴とする。
【００２０】
また本発明の構成は、前記光ｅｘＯＲ回路の動作速度が、入力する光信号の伝送速度よりも遅く、「０」または「１」の判定を光信号の平均値で行うことを特徴とする。
【００２１】
【発明の実施の形態】
以下に本発明の実施の形態を図面に基づき詳細に説明する。
【００２２】
［第１の実施の形態］
図１は、本発明の第１の実施の形態に係る偏波モード分散抑制装置を示す。同図において、１０１は入力用光ファイバ、１０２は入力光パルス、１０３は入力光パルス１０２のＴＥもしくはＴＭ偏波成分、１０４は入力光パルス１０２のＴＭもしくはＴＥ偏波成分、１０５は偏波コントローラ、１０６は偏波モード分散の特に大きい光ファイバで、例えば偏波面保持光ファイバ、１０７はＴＥもしくはＴＭ偏波成分、１０８はＴＭもしくはＴＥ偏波成分、１０９は光カプラ、１１０は光導波路、１１１は出力用光ファイバ、１１２は偏波スプリッタ、１１３，１１４は光導波路、１１５は相互位相変調型の波長変換素子、１１６は波長変換素子１１５の光源、１１７，１１８は光導波路、１１９は受光器、１２０は偏波コントローラ１０５の制御系、１２３は波形整形された光パルス、１２１は光パルス１２３のＴＥもしくはＴＭ偏波成分、１２２は光パルス１２３のＴＭもしくはＴＥ偏波成分である。
【００２３】
図１に示す第１の実施の形態では、遅延した偏波成分１０３が光ファイバ１０６の速く進む方向に、先行している偏波成分１０４を光ファイバ１０６の遅く進む方向に入力するように、偏波コントローラ１０５により入力光パルス１０２の偏波状態を調整することで、偏波モード分散を抑制する方式である。
【００２４】
本実施の形態では、偏波コントローラ１０５と、偏波モード分散の特に大きい光ファイバ（偏波面保持光ファイバ）１０６とにより、偏波モード分散補償器を構成している。
【００２５】
ここで光信号の一部を光カプラ１０９で分岐して、その偏波成分を偏波スプリッタ１１２でさらに分岐する。この結果、光導波路１１３にはＴＥもしくはＴＭ偏波成分が、光導波路１１４にはＴＭもしくはＴＥ偏波成分が出力される。ここで、光導波路１１３と光導波路１１４の光路長を等しくしたうえで波長変換素子１１５の２つの信号入力ポートに個別に入力する。
【００２６】
波長変換素子１１５は相互位相変調型の波長変換素子であり、入力側の光導波路１１３，１１４の双方の光信号レベルが「０」レベルのときには、光源１１６からの連続光を出力側の光導波路１１７に出力する。すなわち、このとき出力側の光導波路１１８の光信号レベルは「０」レベルである。
一方、入力側の光導波路１１３または光導波路１１４の光信号レベルのうちのどちらか一方だけが「１」レベルになると、波長変換素子１１５の原理により、出力側の光導波路１１８の光信号レベルは「１」レベルとなる。
さらに入力側の光導波路１１３，１１４の双方の光信号レベルが「１」レベルのとき、双方が打ち消し合って、出力側の光導波路１１８の光信号レベルは「０」レベルとなる。
【００２７】
これはすなわち光導波路１１３と光導波路１１４の光信号レベルが等しいときは、光導波路１１８の光信号レベルは「０」レベルとなり、光導波路１１３と光導波路１１４の光信号レベルが等しくない場合には、光導波路１１８の光信号レベルは「１」レベルとなるいわゆるｅｘＯＲ（排他的論理和）動作を意味する。
なお光ｅｘＯＲ回路として動作する波長変換素子１１５の動作速度が、入力する光信号の伝送速度よりも遅く、「０」または「１」の判定を光信号の平均値で行うようにしている。もちろん、動作速度の速い波長変換素子を用いてもよい。
【００２８】
従って光導波路１１８の光信号レベルを常に「０」レベルとすることは、光導波路１１３と光導波路１１４を伝搬する光信号がまったく等しいことになり、出力用光ファイバ１１１を伝搬する偏波成分１２１と偏波成分１２２との間に位相遅延がないことを意味する。
【００２９】
受光器１１９は受光した光強度に応じた電気信号を出力するので、受光器１１９から出力される電気信号の強度を最小にするように（つまり受光した光強度を最小にするように）、制御系１２０にて偏波コントローラ１０５による入力光パルス１０２に対する偏波状態を制御することで、偏波モード分散を最小にするように、すなわち偏波成分１２１と偏波成分１２２の伝搬遅延の差を最小にするように調整する。このような制御動作をすることにより、波形整形された光パルス１２３を得ることができる。
【００３０】
なお、図２に示すように、光源１１６を光導波路１１７に接続して、波長変換素子１１５において、光パルス（ＴＥ偏波成分，ＴＭ偏波成分）と、光源１１６からの光との進行方向が逆方向になるようにしても、図１と同様な動作が行われる。
【００３１】
なお、図１及び図２の実施の形態では、以下のような望ましくない状態になる可能性がある。すなわち偏波コントローラ１０５を制御する際に、偏波成分１０３を光導波路１１３に、偏波成分１０４を光導波路１１４に分離する望ましい状態Ａだけでなく、偏波成分１０３の半分の光出力パワーと偏波成分１０４の半分の出力パワーを光導波路１１３に出力し、残りの偏波成分１０３，１０４の半分の光出力パワーを光導波路１１４に出力した上に、偏波成分１０３，１０４の偏波方向を光ファイバ１０６の偏波モード分散が０になる方向（最も速く進む方向と最も遅く進む方向のちょうど真ん中４５度の角度）に合わせてしまう、望ましくない状態Ｂになる可能性がある。このような不具合を解消するには、次に示す第２の実施の形態を採用すればよい。
【００３２】
［第２の実施の形態］
図３は本発明の第２の実施の形態に係る偏波モード分散抑制装置を示す。同図において、２０１は入力用光ファイバ、２０２は入力光パルス、２０３は入力光パルス２０２のＴＥもしくはＴＭ偏波成分、２０４は入力光パルス２０２のＴＭもしくはＴＥ偏波成分、２０５は偏波コントローラ、２０６は偏波モード分散のとくに大きい光ファイバで、例えば偏波面保持光ファイバ、２０７はＴＥもしくはＴＭ偏波成分、２０８はＴＭもしくはＴＥ偏波成分、２０９は光カプラ、２１０は光導波路、２１１は出力用光ファイバ、２１２は偏波スプリッタ、２１３，２１４は光導波路、２１５は相互位相変調型の波長変換素子、２１６は波長変換素子２１５の光源、２１７，２１８は光導波路、２１９は受光器、２２０は偏波コントローラ２０５の制御系、２２３は波形整形された光パルス、２２１は光パルス２２３のＴＥもしくはＴＭ偏波成分、２２２は光パルス２２３のＴＭもしくはＴＥ偏波成分である。これらの２０１〜２２３は、図１に示すものと同様であり、動作原理も同様である。
【００３３】
そして、偏波コントローラ２０５と、偏波モード分散の特に大きい光ファイバ（偏波面保持光ファイバ）２０６とにより、偏波モード分散補償器を構成している。
【００３４】
第２の実施の形態では、更に、光カプラ２２４、偏波スプリッタ２２５、光導波路２２６，２２７、相互位相変調型の波長変換素子２２８、光源２２９、光導波路２３０，２３１、受光器２３２を具備することで、第１の実施の形態における不具合を解消する。
【００３５】
ここで光信号の一部を、偏波コントローラ２０５と光ファイバ２０６との間に配置した光カプラ２２４によって分岐し、偏波スプリッタ２２５によって、光路長が等しい光導波路２２６，２２７に分岐したうえで波長変換素子２２８に入力する。上述したのと同様に波長変換素子２２８はｅｘＯＲ回路として動作する。
【００３６】
ここで望ましい状態Ａのときには、受光器２３２においては偏波成分２０３，２０４の光信号レベルは等しくなく、受光器２１９においては偏波成分２０３，２０４の光信号レベルは等しい。すなわち、受光器２１９を最小に調整したときに、受光器２３２が最小でなければ、状態Ａであることがわかる。一方、望ましくない状態Ｂでは、受光器２１９を最小にしたとき、受光器２３２も同様に最小になってしまう。この状態は望ましくない状態であるので、制御器２２０により偏波コントローラ２０５による入力光パルス２０２に対する偏波状態を更に調整し、状態Ａになるように調整する。
【００３７】
なお、図３において、光源２１６を光導波路２１７に接続したり、光源２２９を光導波路２３０に接続するようにしてもよい。
【００３８】
ところで、図１〜図３に示す第１及び第２の実施の形態では、以下のような望ましくない状態になる可能性がある。すなわち、入力用光ファイバ１０１（２０１）に入力されるＴＥ偏波成分とＴＭ偏波成分の光強度が等しい、もしくは光強度差が小さい（例えば５ｄＢ程度）場合にはｅｘＯＲ動作が正常動作し、偏波モード分散の抑制を行うことができるが、光強度差が５ｄＢ以上になると光強度が強い方の偏波成分のみ引きずられ、ｅｘＯＲ回路（波長変換素子１１５（２１５））が正常動作しない可能性がある。
【００３９】
実際の光伝送路においては、両偏波成分の強度差は大きく、時として２０ｄＢにも達することがある。このときには、図１〜図３に示す第１及び第２の実施の形態にかかる偏波モード分散抑制装置では、偏波モード分散を抑制することはできない。このようにＴＥ／ＴＭ偏波成分の強度差が大きい場合には、次に示す第３の実施の形態を採用すればよい。
【００４０】
［第３の実施の形態］
図４は本発明の第３の実施の形態にかかる偏波モード分散抑制装置を示す。同図において、３０１は入力用光ファイバ、３０２は入力光パルス、３０３は入力光パルス３０２のＴＥもしくはＴＭ偏波成分、３０４は入力光パルス３０２のＴＭもしくはＴＥ偏波成分、３０５は偏波コントローラ、３０６は偏波モード分散の特に大きい光ファイバで、例えば偏波面保持光ファイバ、３０７はＴＥもしくはＴＭ偏波成分、３０８はＴＭもしくはＴＥ偏波成分、３０９は１入力２出力の光カプラ、３１０は光導波路、３１１は出力用光ファイバ、３１２は偏波スプリッタ、３１３，３１４は偏波用光導波路、３１５は相互位相変調型の波長変換素子、３１６は波長変換素子３１５の光源、３１７，３１８は光導波路、３１９は平均出力のみを検知する受光器、３２０は偏波コントローラ３０５の制御系、３２３は波形整形された光パルス、３２１は光パルス３２３のＴＥもしくはＴＭ偏波成分、３２２は光パルス３２３のＴＭもしくはＴＥ偏波成分である。
【００４１】
更に、光導波路３１３に備えた（介装した）光レベル調整器３２４と、光導波路３１４に備えた（介装した）光レベル調整器３２５とを有する。光レベル調整器３２４，３２５は、入力される光の強度が異なっていても、出力する光の強度を一定範囲内の強度にして出力する機能を有している。その具体的特性は、図５を参照して後述する。
【００４２】
そして、偏波コントローラ３０５と、偏波モード分散の特に大きい光ファイバ（偏波面保持光ファイバ）３０６とにより、偏波モード分散補償器を構成している。
【００４３】
図４に示す偏波モード分散抑制装置では、遅延した偏波成分３０３が偏波面保持光ファイバ３０６の速く進む方向に、先行している偏波成分３０４が偏波面保持光ファイバ３０６の遅く進む方向に入力するように、制御系３２０にて偏波コントローラ３０５を制御することにより、入力光パルス３０２の偏波状態を調整して、偏波モード分散を抑制している。
【００４４】
ここで光信号の一部を光カプラ３０９で分岐して、その偏波成分を偏波スプリッタ３１２でさらに分岐する。この結果、偏波用光導波路３１３にはＴＥもしくはＴＭ偏波成分が、偏波用光導波路３１４にはＴＭもしくはＴＥ偏波成分が出力される。ここで、偏波用光導波路３１３と偏波用光導波路３１４の光路長を等しくしたうえで、波長変換素子３１５の信号入力ポートに、ＴＥ偏波成分とＴＭ偏波成分が個別に入力する。
【００４５】
波長変換素子３１５は相互位相変調型の波長変換素子であり、偏波用光導波路３１３，３１４の双方の光強度が「０」レベルのときには光源３１６からの連続光を光導波路３１７に出力する。すなわち、このとき出力側の光導波路３１８は「０」レベルである。
一方、偏波用光導波路３１３または偏波用光導波路３１４の光強度のうちのどちらか一方だけが「１」レベルになると、波長変換素子３１５の原理により出力側の光導波路３１８の光強度は「１」レベルとなる。
さらに偏波用光導波路３１３，３１４の双方の光強度が「１」レベルのとき、双方が打ち消し合って、出力側の光導波路３１８の光強度は「０」レベルとなる。
【００４６】
光レベル調整器３２４を通過して波長変換素子３１５に入力される偏波成分の光強度と、光レベル調整器３２５を通過して波長変換素子３１５に入力される偏波成分の光強度は、一定範囲内の強度になっており、ほぼ一定である。
【００４７】
したがって、偏波スプリッタ３１２にて分岐して、偏波用光導波路３１３，３１４に入力したＴＥ偏波成分とＴＭ偏波成分の光強度差が大きい場合であっても、光レベル調整器３２４を通過して波長変換素子３１５に入力される偏波成分の光強度と、光レベル調整器３２５を通過して波長変換素子３１５に入力される偏波成分の光強度は、ほぼ等しくなるため、
▲１▼偏波用光導波路３１３に入力された偏波成分と、偏波用光導波路３１４に入力された偏波成分との入力タイミングが等しいときには、光導波路３１８の光強度は「０」レベルとなり、
▲２▼偏波用光導波路３１３に入力された偏波成分と、偏波用光導波路３１４に入力された偏波成分との入力タイミングが等しくないときには、光導波路３１８の光強度は「１」レベルとなる。
つまり、偏波スプリッタ３１２にて分岐して偏波用光導波路３１３，３１４に入力したＴＥ偏波成分とＴＭ偏波成分の光強度に大きな差があった場合でも、波長変換素子３１５における、ｅｘＯＲ動作を確保することができる。
【００４８】
受光器３１９は受光した光強度に応じた電気信号を出力するので、受光器３１９から出力される電気信号の強度を最小にするように（つまり受光した光強度を最小にするように）、制御系３２０にて偏波コントローラ３０５による入力光パルス３０２に対する偏波状態を制御することで、偏波モード分散を最小にするように、すなわち偏波成分３２１と偏波成分３２２の伝搬遅延の差を最小にするように調節する。このような制御動作をすることにより、波形成形された光パルス３２３を得ることができる。しかも、偏波成分３０３と偏波成分３０４の光強度差が大きくても、ｅｘＯＲ動作を確保することができるので、波形成形された光パルス３２３を得ることができる。
【００４９】
なお、図４において、光源３１６を光導波路３１７に接続するようにしてもよい。
【００５０】
［光レベル調整器の特性］
図５は、第３の実施の形態における光レベル調整器３２４，３２５の特性を示す特性図であって、具体的には増幅度２０ｄＢ、飽和出力３ｄＢｍの半導体光増幅器の光入力に対する光出力を図示したものである。同図により、−２０ｄＢｍを越えるいかなる光強度に対しても、出力強度は０〜＋３ｄＢｍの範囲に収まる。すなわち例えば、入力される光強度が−２０ｄＢｍと０ｄＢｍのように２０ｄＢを越える光強度差が存在しても、光レベル調整の結果、出力する光強度を０〜＋３ｄＢｍと３ｄＢの範囲に収めることができる。
【００５１】
なお通常、半導体光増幅器をこのように光レベル調整器として用いると信号のパターンによって増幅度が変化するいわゆるパターン効果による波形歪みが問題となるが、本発明においては受光器３１９における平均出力のみを検知するため、波形が歪んでも問題にならない。
【００５２】
なお、半導体光増幅器１段で増幅度２０ｄＢの達成が困難な場合には、半導体光増幅器を２段、３段とカスケード接続しても構わない。
また、入力される光の強度が低い光伝送システムに用いる場合には、増幅度が高い光増幅器を採用し、入力される光の強度が高い光伝送システムに用いる場合には、増幅度が低い光増幅器を採用する。
また、半導体光増幅器の代わりに、光ファイバアンプ等を用いて光レベル調整器を構成しても良い。
【００５３】
［第４の実施の形態］
図６は本発明の第４の実施の形態にかかる偏波モード分散抑制装置を示す。この実施の形態では、図４に示す第３の実施の形態にかかる偏波モード分散抑制装置に、更に、受光器３２６，３２７と、制御回路３２８と、フィードバック回路３２９，３３０を付加した構成となっている。
【００５４】
受光器３２６には、光レベル調整器３２４を通過して波長変換素子３１５に入力される偏波成分が入力され、受光器３２６は、この偏波成分の強度に応じた電気信号を出力する。また、受光器３２７には、光レベル調整器３２５を通過して波長変換素子３１５に入力される偏波成分が入力され、受光器３２７は、この偏波成分の強度に応じた電気信号を出力する。制御回路３２８は、受光器３２６及び受光器３２７からの電気信号を監視することにより、偏波用光導波路３１３を介して波長変換素子３１５に入力される偏波光の強度、及び、偏波用光導波路３１４を介して波長変換素子３１５に入力される偏波光の強度をモニタすることができる。
【００５５】
そして、制御回路３２８は、両偏波光の強度の差が最小になるように、フィードバック回路３２９，３３０を介して、光レベル調整器３２４，３２５の増幅度を個別に調整している。つまり、光強度が弱いときには光レベル調整器３２４，３２５の電流を増加することで増幅度を上げ、光強度が強いときには光レベル調整器３２４，３２５の電流を減少することで増幅度を下げる。このような光強度調整制御によって、偏波用光導波路３１３を介して波長変換素子３１５に入力される偏波光の強度と、偏波用光導波路３１４を介して波長変換素子３１５に入力される偏波光の強度を、より厳密に一致させることができ、更に確実に偏波モード分散の制御が可能になる。
【００５６】
［第５の実施の形態］
図７は本発明の第５の実施の形態に係る偏波モード分散抑制装置を示す。図７において、４０１は入力用光ファイバ、４０２は入力光パルス、４０３は入力光パルス４０２のＴＥもしくはＴＭ偏波成分、４０４は入力光パルス４０２のＴＭもしくはＴＥ偏波成分、４０５は偏波スプリッタ、４０６，４０７は光導波路、４０８は光導波路４０６に介装された可変位相遅延線、４０９は光合波器、４１１，４１２は光カプラ、４１３は相互位相変調型の波長変換素子、４１４は波長変換素子４１３の光源、４１５，４１６は光導波路、４１７は受光器、４１８は可変位相遅延線４０８の制御系、４１９は出力用光ファイバ、４２２は波形整形された光パルス、４２０は光パルス４２２のＴＥもしくはＴＭ偏波成分、４２１は光パルス４２２のＴＭもしくはＴＥ偏波成分である。
【００５７】
本実施の形態では、偏波スプリッタ４０５と、光導波路４０６，４０７と、可変位相遅延線４０８と光合波器４０９とにより、偏波モード分散補償器を構成している。
【００５８】
図７に示す第５の実施の形態では、入力光パルス４０２を偏波スプリッタ４０５にて分岐することにより、遅延した（もしくは先行した）偏波成分４０３を光導波路４０６に、偏波成分４０４を光導波路４０７に分岐すると共に、遅延した（もしくは先行した）偏波成分４０３の光路長を短く（もしくは長く）するよう可変位相遅延線４０８を調整することで、偏波モード分散を抑制する方式である。
【００５９】
ここで光カプラ４１１，４１２によって一部の光信号を分岐し、同一光路長の光導波路を経て波長変換素子４１３に入力する。前述のように波長変換素子４１３はｅｘＯＲ回路として動作するので、光導波路４１６の出力値を最小にするように制御系４１８にて可変位相遅延線４０８による位相遅延量を制御することで、偏波モード分散を最小にするように、すなわち偏波成分４２０と偏波成分４２１の伝搬遅延の差を最小にし、波形整形された光パルス４２２を得る。
【００６０】
なお、図８に示すように、光源４１４を光導波路４１５に接続して、波長変換素子４１３において、光パルス（ＴＥ偏波成分，ＴＭ偏波成分）と、光源４１４からの光との進行方向が逆方向になるようにしても、図７と同様な動作が行われる。
【００６１】
しかしながら、図７及び図８に示す第５の実施の形態では、偏波成分４０３が光導波路４０６に、偏波成分４０４が光導波路４０７にうまく分岐するような偏波状態にあるとは限らないという不具合がある。このような不具合を解消するには、次に示す第６の実施の形態を採用すればよい。
【００６２】
［第６の実施の形態］
図９は本発明の第６の実施の形態に係る偏波モード分散抑制装置を示す。図６において、４０１は入力用光ファイバ、４０２は入力光パルス、４０３は入力光パルス４０２のＴＥもしくはＴＭ偏波成分、４０４は入力光パルス４０２のＴＭもしくはＴＥ偏波成分、４０５は偏波スプリッタ、４０６，４０７は光導波路、４０８は光導波路４０６に介装された可変位相遅延線、４０９は光合波器、４１１，４１２は光カプラ、４１３は相互位相変調型の波長変換素子、４１４は波長変換素子４１３の光源、４１５，４１６は光導波路、４１７は受光器、４１８は可変位相遅延線４０８の制御系、４１９は出力用光ファイバ、４２２は波形整形された光パルス、４２０は光パルス４２２のＴＥもしくはＴＭ偏波成分、４２１は光パルス４２２のＴＭもしくはＴＥ偏波成分である。これらの４０１〜４２２は、図８に示すものと同様であり、動作原理も同様である。
【００６３】
そして、本実施の形態では、偏波スプリッタ４０５と、光導波路４０６，４０７と、可変位相遅延線４０８と光合波器４０９とにより、偏波モード分散補償器を構成している。
【００６４】
第６の実施の形態では、更に、光カプラ４２３，４２４、受光器４２５，４２６、偏波コントローラ４２７、制御系４２８を具備することで、第５の実施の形態における不具合を解消する。すなわち、光導波路４０６，４０７を伝搬する光信号の一部を光カプラ４２３，４２４にて分岐し、分岐した光信号の強度を受光器４２５，４２６で測定し、その強度の差が最小になるように、制御系４２８にて偏波コントローラ４２７による入力光パルス４０２に対する偏波状態を制御することで、偏波成分４０３が光導波路４０６に、偏波成分４０４が光導波路４０７にうまく分岐するような偏波状態にすることができる。
【００６５】
なお、図９において、光源４１６を光導波路４１５に接続するようにしてもよい。
【００６６】
本発明で特筆すべきことは、図１〜図９における位相変調型波長変換素子１１５，２１５，２２８，３１５，４１３がいずれも入力信号に追従しなくてもいい点である。すなわち、ｅｘＯＲ動作が１ビット１ビットに追従する必要はなく、平均値として検出できればよい。同様のことは受光器１１９，２１９，２３２，３１９，３２６，３２７，４１７，４２５，４２６にも言える。すなわち本発明の構成はいずれも光信号の平均出力パワーで動作するので、ビットレートに依存しない。また１０１０信号が連続しても、フィードバック系に影響を与えない。さらには安価な低速な受光器を使うことができる。
【００６７】
〔偏波モード分散補償器の他の例〕
上述した実施の形態では、偏波モード分散補償器として、
（イ）偏波コントローラ（偏波ローテータ）と、偏波モード分散の特に大きい光ファイバ（例えば偏波面保持光ファイバ）との組合せ（図１〜図４，図６参照）や、
（ロ）偏波スプリッタと、２本の光導波路と、一方の光導波路に介装した可変位相遅延線と、光合波器との組合せ（図７〜図９）を採用しているが、
偏波モード分散補償器としては他の種類の物も採用することができる。以下に他の偏波モード分散補償器を説明する。
【００６８】
〔ＰＬＣ（プレーナ光波回路）を使った偏波モード分散補償器〕
ＰＬＣ（シリコン基板上の石英系導波路：プレーナ光波回路）を使った偏波モード分散補償器を、図１０を参照して説明する。このタイプの偏波モード分散補償器は、T.Saida et al.,"planar Lightwave Circuit Polarization Mode Compensator",ECOC’2001,Amsterdamに開示されており、２つの偏波スプリッタ５０１，５０２と、２つの半波長板５０３，５０４と、２つの位相調整領域５０５，５０６と、２つの可変カプラ５０７，５０８と、１つの偏波遅延領域５０９とにより構成されている。つまり、第１の偏波スプリッタ５０１と、第１の位相調整領域５０５と、第１の可変カプラ５０７と、第２の位相調整領域５０６と、第２の可変カプラ５０８と、偏波遅延領域５０９と、第２の偏波スプリッタ５０２がこの順に直列的に接続されて構成されている。
【００６９】
偏波スプリッタ５０１，５０２は対称型のマッハ・ツェンダ干渉計の一方の導波路にアモルファスシリコンＡＳ１，ＡＳ２を、他方の導波路にヒータＨ１，Ｈ２を備えて構成されたものであり、ＴＥ・ＴＭ偏波を分離または結合する。
【００７０】
可変カプラ５０７，５０８も同じく対称型のマッハ・ツェンダ干渉計の構造をとり、両方の導波路に備えたヒータＨ７−１，Ｈ７−２，Ｈ８−１，Ｈ８−２に加える電力を調整することで、光信号の分岐比を変化させることができる。
【００７１】
位相調整領域５０５，５０６も同じく対称型のマッハ・ツェンダ干渉計の構造をとり、各導波路にヒータＨ５−１，Ｈ５−２，Ｈ６−１，Ｈ６−２を備えており、光信号の位相を変化させる。
【００７２】
半波長板５０３は、位相調整領域５０５の下側の導波路のうち、ヒータＨ５−１よりも入力側の位置に介装されており、半波長板５０４は、偏波遅延領域５０９の短い方の導波路に介装されている。半波長板５０３，５０４はＴＥ偏波をＴＭ偏波に、ＴＭ偏波をＴＥ偏波へと逆転する機能を持つ。
【００７３】
偏波遅延領域５０９は２つの長さの異なる導波路からなり、両者を伝搬する光信号の伝搬遅延の差は２０ｐｓに設定されている。
【００７４】
図１０に示す偏波モード分散補償器は、以下のように動作する。入力された光信号は第１の偏波スプリッタ５０１によってＴＥ偏波とＴＭ偏波に分離される。さらにＴＭ偏波は半波長板５０３によってＴＥ偏波に変換される。すなわち、入力信号はＴＥ偏波成分と、ＴＭ偏波成分（但し偏光はＴＥ偏波）に分離される。
【００７５】
このＴＥ偏波成分とＴＭ偏波成分を、２つの位相調整領域５０５，５０６と２つの可変カプラ５０７，５０８に導波させる。ここで、各ヒータＨ５−１，Ｈ５−２，Ｈ６−１，Ｈ６−２，Ｈ７−１，Ｈ７−２，Ｈ８−１，Ｈ８−２に加える電力を調整し（この結果、発熱量を調整し）、位相調整領域５０５，５０６の位相と可変カプラ５０７，５０８の分岐比を変化させることで、ＴＥ偏波成分とＴＭ偏波成分（ただし偏光はＴＥ成分）の比を自由に変化させることができる（これは、上記（イ）のタイプの偏波モード分散補償器において、偏波コントローラを調整することで、ＴＥ偏波成分とＴＭ偏波成分の比を変化させることに対応する）。
【００７６】
その後、ＴＥ偏波成分は偏波遅延領域５０９の長い方の導波路に、ＴＭ偏波成分（偏光はＴＥ偏波）は短い方の導波路に入力される。導波路の伝搬時間の差は２０ｐｓであるので、ＴＥ偏波成分が２０ｐｓ遅れることになる（これは上記（イ）のタイプの偏波モード分散補償器において、偏波面保持光ファイバを用いて一方の偏波成分を他方の偏波成分に比べて遅延させることに対応する）。
【００７７】
なお、ＴＭ偏波成分（ＴＥ偏光）は半波長板５０４によって再びＴＭ偏波に戻され、ＴＥ偏波成分とＴＭ偏波成分を偏波スプリッタ５０２によって結合して出力する。
【００７８】
本構成は基本的に、偏波コントローラ（偏波ローテータ）と偏波面保持光ファイバによって偏波モード分散を補償する、上記（イ）のタイプの偏波モード分散補償器と等しい。即ち、位相調整領域５０５と可変カプラ５０７が偏波コントローラに、偏波遅延領域５０９が偏波面保持光ファイバに相当する。
【００７９】
〔ＬＮを使った偏波モード分散補償器〕
ＬＮ（リチウムナイオブレート、LiNbO₃）を使った偏波モード分散補償器を、図１１を参照して説明する。このタイプの偏波モード分散補償器は、R.Noe et al .,"Integrated optical LiNbO₃ distributed polarization mode dispersion compensator in 20 Gb/s transmission system"Electronics Letters,vol.35,no.8,pp.652-654,1999に開示されており、光信号が伝搬する光導波路６００の上に、電圧Ｖ１を印加する櫛型電極６０１ａ，６０１ｂと、０Ｖのアース電極６０２と、電圧Ｖ２を印加する櫛型電極６０３ａ，６０３ｂを順に配置したものである。つまり、櫛型電極６０３ａ，６０１ａ，６０３ｂ，６０１ｂがこの順に配列され、これら櫛型電極６０３ａ，６０１ａ，６０３ｂ，６０１ｂの間（櫛型電極の相互間および各櫛の間）にアース電極６０２が配置されている。
【００８０】
光信号のビート長をΛとしたとき、櫛型電極間の間隔を、図１１のようにΛ／４と３Λ／４となるようにする。ＬＮ基板としてはＸ軸方向にカットし、Ｙ軸方向に光信号が伝搬するものを利用する。Ｖ１，Ｖ２に±６９Ｖの電圧を印加することで、光信号の偏波を±４５度回転することができる。ＬＮ導波路には０．２６ｐｓ／ｍｍの偏波モード分散があるので、光信号を回転することで光信号の偏波モード分散をわずかに変えることができる。Noe らは、Λ／４と３Λ／４の間隔を持つ櫛型電極２つを１組として、７３組を縦列接続することによって４３ｐｓの偏波モード分散補償に成功している。挿入損は約７ｄＢである。
【００８１】
なお、上述した他にも、偏波モード分散補償器は各種存在するが、これらの偏波モード分散補償器を組み込んで、本発明の偏波モード分散抑制装置を構成することができる。
【００８２】
例えば図１における偏波コントローラ１０５と光ファイバ１０６による偏波モード分散補償器の代わりに、上述した各種の偏波モード分散補償器を採用することにより、本発明の偏波モード分散抑制装置を構成することができる。
また例えば図７における偏波スプリッタ４０５と、光導波路４０６，４０７と、可変位相遅延線４０８と、光合波器４０９とによる偏波モード分散補償器の代わりに、上述した各種の偏波モード分散補償器を採用することにより、本発明の偏波モード分散抑制装置を構成することができる。
【００８３】
【発明の効果】
以上説明したように本発明によれば、ビットレートを変更することができ、１０符号が連続してもフィードバック系に影響を与えず、さらに低速な受光器でも構成が可能な偏波モード分散抑制装置を提供することができる。
【００８４】
また、本発明によれば、ＴＥ偏波成分とＴＭ偏波成分との強度差が大きい場合にもｅｘＯＲ回路が正常に動作し、偏波モード分散を抑制することが可能な偏波モード分散抑制装置を提供することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態に係る偏波モード分散抑制装置を示す構成図。
【図２】本発明の第１の実施の形態に係る偏波モード分散抑制装置の変形例を示す構成図。
【図３】本発明の第２の実施の形態に係る偏波モード分散抑制装置を示す構成図。
【図４】本発明の第３の実施の形態に係る偏波モード分散抑制装置を示す構成図。
【図５】第３の実施の形態における光レベル調整器の特性を示す特性図
【図６】本発明の第４の実施の形態に係る偏波モード分散抑制装置を示す構成図。
【図７】本発明の第５の実施の形態に係る偏波モード分散抑制装置を示す構成図。
【図８】本発明の第５の実施の形態に係る偏波モード分散抑制装置の変形例を示す構成図。
【図９】本発明の第６の実施の形態に係る偏波モード分散抑制装置を示す構成図。
【図１０】ＰＬＣを使った偏波モード分散補償器を示す構成図。
【図１１】ＬＮを使った偏波モード分散補償器を示す構成図。
【図１２】偏波モード分散を説明する説明図。
【図１３】従来の偏波モード分散抑制装置を示す構成図。
【符号の説明】
１０１は入力用光ファイバ、
１０２は入力光パルス、
１０３，１０４は偏波成分、
１０５は偏波コントローラ、
１０６は偏波モード分散のとくに大きい光ファイバで、例えば偏波面保持光ファイバ、
１０７，１０８は偏波成分、
１０９は光カプラ、
１１０は光導波路、
１１１は出力用光ファイバ、
１１２は偏波スプリッタ、
１１３，１１４は光導波路、
１１５は相互位相変調型の波長変換素子、
１１６は光源、
１１７，１１８は光導波路、
１１９は受光器、
１２０は制御系、
１２１，１２２は偏波成分、
１２３は波形整形された光パルス、
２０１は入力用光ファイバ、
２０２は入力光パルス、
２０３，２０４は偏波成分、
２０５は偏波コントローラ、
２０６は偏波モード分散のとくに大きい光ファイバで、例えば偏波面保持光ファイバ、
２０７，２０８は偏波成分、
２０９は光カプラ、
２１０は光導波路、
２１１は出力用光ファイバ、
２１２は偏波スプリッタ、
２１３，２１４は光導波路、
２１５は相互位相変調型の波長変換素子、
２１６は光源、
２１７，２１８は光導波路、
２１９は受光器、
２２０は制御系、
２２１，２２２は偏波成分、
２２３は波形整形された光パルス、
２２４は光カプラ、
２２５は偏波スプリッタ、
２２６，２２７は光導波路、
２２８は相互位相変調型の波長変換素子、
２２９は光源、
２３０，２３１は光導波路、
２３２は受光器、
３０１は入力用光ファイバ、
３０２は入力光パルス、
３０３，３０４は偏波成分、
３０５は偏波スプリッタ、
３０６は偏波面保持光ファイバ、
３０７，３０８は偏波成分、
３０９は光カプラ、
３１０は光導波路、
３１１は出力用光ファイバ、
３１２は偏波スプリッタ、
３１３，３１４は偏波用光導波路、
３１５は波長変換素子、
３１６は光源、
３１７，３０８は光導波路、
３１９は受光器、
３２０は制御系、
３２１，３２２は偏波成分、
３２３は波形整形された光パルス、
３２４，３２５は光レベル調整器、
３２６，３２７は受光器、
３２８は制御回路、
３２９，３３０はフィードバック回路、
４０１は入力用光ファイバ、
４０２は入力光パルス、
４０３，４０４は偏波成分、
４０５は偏波スプリッタ、
４０６，４０７は光導波路、
４０８は可変位相遅延線、
４０９は光合波器、
４１１，４１２は光カプラ、
４１３は相互位相変調型の波長変換素子、
４１４は光源、
４１５，４１６は光導波路、
４１７は受光器、
４１８は制御系、
４１９は出力用光ファイバ、
４２０，４２１は偏波成分、
４２２は波形整形された光パルス、
４２３，４２４は光カプラ、
４２５，４２６は受光器、
４２７は偏波コントローラ、
４２８は制御系、
５０１，５０２は偏波スプリッタ、
５０３，５０４は半波長板、
５０５，５０６は位相調整領域、
５０７，５０８は可変カプラ、
５０９は偏波遅延領域、
６０１ａ，６０１ｂ，，６０３ａ，６０３ｂは櫛型電極、
６０２ａ〜６０２ｄはアース電極、
１００１は光ファイバ、
１００２は入力光パルス、
１００３，１００４，１００５，１００６は偏波成分、
１００７は光パルスである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polarization mode dispersion suppressing device, and is applied to an optical transmission system such as high-speed optical communication, optical switching, and optical information processing propagating through an optical fiber, and is particularly effective for suppressing polarization mode dispersion. . Further, the present invention is devised so that the polarization mode dispersion can be effectively suppressed even if the difference in light intensity between the TE polarization component and the TM polarization component is large.
[0002]
[Prior art]
As the transmission capacity increases with the development of IT technology, the bit rate of optical signals tends to increase from 2.5 Gb / s to 10 Gb / s, and further to 40 Gb / s. The problem here is polarization mode dispersion. FIG. 12 is a diagram illustrating polarization mode dispersion, where 1001 is an optical fiber, 1002 is an input optical pulse, 1003 is a TE or TM polarization component of the input optical pulse 1002, and 1004 is TM or TE of the input optical pulse 1002. The polarization component, 1007 is the output light pulse, 1005 is the TE or TM polarization component of the output light pulse 1007, and 1006 is the TM or TE polarization component of the output light pulse 1007.
[0003]
In general, polarization mode dispersion exists in an optical fiber, and an optical signal propagating through the optical fiber propagates into a component that advances fast and a component that advances slowly depending on the plane of polarization. In FIG. 12, the polarization component 1003 is a component that progresses fast, and becomes a polarization component 1005 at the output end. On the other hand, the polarization component 1004 is a component that progresses slowly, and becomes a polarization component 1006 at the output end and arrives later than the polarization component 1005. Since the output light pulse 1007 is the sum of the polarization component 1005 and the polarization component 1006, the waveform of the output light pulse 1007 is rounded as a result.
[0004]
As the polarization mode dispersion value, for example, for the fiber length L (Km),
0.2 × L ^1/2 (Ps) ~ 2 × L ^1/2 (Ps)
Degree. That is, assuming a 100 km optical fiber in the worst case, polarization mode dispersion of 20 ps occurs. This value is not a big problem at 2.5 Gb / s (pulse width 400 ps) or 10 Gb / s (pulse width 100 ps), but at 40 Gb / s (pulse width 25 ps), the waveform becomes fatal and the code error rate is reduced. It becomes a factor that greatly deteriorates.
[0005]
In order to solve this problem, conventionally, polarization mode dispersion has been suppressed with the configuration shown in FIG. In FIG. 13, 1101 is an input optical fiber, 1102 is an input optical pulse, 1103 is a TE or TM polarization component of the input optical pulse 1102, 1104 is a TM or TE polarization component of the input optical pulse 1102, and 1105 is a polarization controller. 1106 is an optical fiber having a particularly large polarization mode dispersion. For example, a polarization maintaining optical fiber, 1107 is a TE or TM polarization component, 1108 is a TM or TE polarization component, 1109 is an optical coupler, 1110 is a light receiver, and 1111. Is an electric bandpass filter, 1112 is a control system of the polarization controller 1105, 1113 is an optical fiber for output, 1116 is an optical pulse whose waveform is shaped, 1114 is a TE or TM polarization component of the optical pulse 1116, 1115 is an optical pulse 1116 TM or TE polarization component of gre Ishikawa, Hiroki Ooi, and Yuichi Akiyama, APCC / OECC '99, pp.424-428).
[0006]
In the configuration of FIG. 13, the polarization controller 1105 inputs the delayed polarization component 1103 so that the preceding polarization component 1104 is input in the direction in which the optical fiber 1106 progresses slowly in the direction in which the optical fiber 1106 advances. This is a method of suppressing polarization mode dispersion by adjusting the polarization state of the optical pulse 1102. As the optical fiber 1106, for example, a polarization maintaining optical fiber having a particularly large polarization mode dispersion is used. The polarization mode dispersion of the polarization-maintaining optical fiber is, for example, about 1 ps per meter.
[0007]
According to the configuration of FIG. 13 based on the above-mentioned reference, a part of the optical signal is branched by the optical coupler 1109 and then received by the light receiver 1110, and the received light is subjected to optical / electrical conversion by the light receiver 1110. The electric signal is sent to the control system 1112 through the electric bandpass filter 1111 having a band half the transmission rate. The control system 1112 controls the polarization controller 1105 so as to maximize the intensity of the electric signal (that is, the intensity of the received light) so as to minimize the polarization mode dispersion, that is, the polarization component 1114 and A difference in propagation delay of the polarization component 1115 is minimized, and an optical pulse 116 having a waveform shaped is obtained.
[0008]
[Problems to be solved by the invention]
However, the prior art shown in FIG. 13 has the following drawbacks. The first drawback is a limitation on the bit rate of the optical signal. Since the configuration of FIG. 13 requires an electric bandpass filter 1111 having a band that is exactly half the bit rate, the bit rate cannot be changed. Further, as a second drawback, when 10 codes such as 10101010... Continue, a high-frequency component that is half the bit rate increases, which affects electric feedback. Further, as a third drawback, since the light receiver 1110 having a half speed of the bit rate is required, the light receiver 1110 becomes expensive.
[0009]
In view of the above-described drawbacks, the present invention can change the bit rate, does not affect the feedback system even if 10 codes are continuous, and can also be configured with a low-speed light receiver. The purpose is to provide. It is another object of the present invention to provide a polarization mode dispersion suppressing device that can effectively suppress polarization mode dispersion even if the difference in light intensity between the TE polarization component and the TM polarization component is large.
[0010]
[Means for Solving the Problems]
The configuration of the present invention that solves the above problem is such that when an optical signal propagating through an optical fiber is input, the difference in transmission delay between the TE polarization component and the TM polarization component of the optical signal is minimized. A polarization mode dispersion compensator that outputs an optical signal in which polarization mode dispersion is suppressed by compensation;
A 1-input 2-output optical coupler that receives an optical signal with suppressed polarization mode dispersion output from the polarization mode dispersion compensator, outputs a part of the optical signal and branches the other part; ,
A polarization splitter that divides an optical signal branched by the 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and TM polarization component of the optical signal are individually input to two input ports via the same optical path length, and are calculated by performing an optical exOR operation using the TE polarization component and TM polarization component of the optical signal. An optical exOR circuit that outputs the result;
And a control system for controlling compensation by the polarization mode dispersion compensator so that a logical result output of the optical exOR circuit becomes “0”.
[0011]
In addition, the configuration of the present invention compensates for an optical signal propagating through an optical fiber by compensating so that a difference in transmission delay between the TE polarization component and the TM polarization component of the optical signal is minimized. A polarization mode dispersion compensator that outputs an optical signal with suppressed wave mode dispersion;
A first 1-input 2-output optical coupler for branching a part of the TE polarization component in which polarization mode dispersion is suppressed from within the polarization mode dispersion compensator;
A second 1-input 2-output optical coupler for branching a part of the TM polarization component whose polarization mode dispersion is suppressed from within the polarization mode dispersion compensator;
One polarization component branched by the first 1-input 2-output optical coupler and the other polarization component branched by the second 1-input 2-output optical coupler individually through the same optical path length And an optical exOR circuit that performs an optical exOR operation on one polarization component and the other polarization component of the optical signal and outputs a calculation result,
And a control system for controlling compensation by the polarization mode dispersion compensator so that a logical result output of the optical exOR circuit becomes “0”.
[0012]
In the configuration of the present invention, an optical signal propagating through an optical fiber is input and a polarization controller that controls the polarization state of the optical signal, and an optical signal whose polarization state is controlled by the polarization controller are input. A polarization mode dispersion compensator configured with a polarization mode dispersion control optical fiber that suppresses polarization mode dispersion of the optical signal by propagating the optical signal;
1-input 2-output light that is input from the polarization mode dispersion controlling optical fiber and whose polarization mode dispersion is suppressed, outputs a part of the optical signal and branches the remaining part. A coupler,
A polarization splitter that divides an optical signal branched by the 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and TM polarization component of the optical signal are individually input to two input ports via the same optical path length, and are calculated by performing an optical exOR operation using the TE polarization component and TM polarization component of the optical signal. An optical exOR circuit that outputs the result;
And a control system for controlling a polarization state by the polarization controller so that a logical result output of the optical exOR circuit becomes “0”.
[0013]
In the configuration of the present invention, an optical signal propagating through an optical fiber is input and a polarization controller that controls the polarization state of the optical signal, and an optical signal whose polarization state is controlled by the polarization controller are input. A polarization mode dispersion compensator configured with a polarization mode dispersion control optical fiber that suppresses polarization mode dispersion of the optical signal by propagating the optical signal;
A first input 1 that receives an optical signal with suppressed polarization mode dispersion output from the polarization mode dispersion controlling optical fiber, outputs a part of the optical signal, and branches the remaining part. An output optical coupler;
A first polarization splitter that splits an optical signal branched by the first 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and TM polarization component of the optical signal divided by the first polarization splitter are individually input to two input ports through the same optical path length, and the TE polarization component and TM of the optical signal are input. A first optical exOR circuit that performs an optical exOR operation on the polarization component and outputs a calculation result;
A portion of the optical signal that is arranged between the polarization controller and the polarization mode dispersion control optical fiber and that is output from the polarization controller is output toward the polarization mode dispersion control optical fiber and the rest A second 1-input 2-output optical coupler that branches a portion of
A second polarization splitter for dividing the optical signal branched by the second 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and the TM polarization component of the optical signal divided by the second polarization splitter are individually input to the two input ports through the same optical path length, and the TE polarization component of the optical signal and TM A second optical exOR circuit that performs an optical exOR operation on the polarization component and outputs a calculation result;
The polarization state is controlled by the polarization controller so that the logical result output of the first optical exOR circuit becomes “0” and the logical result output of the second optical exOR circuit does not become “0”. And a control system.
[0014]
In the configuration of the present invention, an optical signal propagating through an optical fiber is input and a polarization controller that controls the polarization state of the optical signal, and an optical signal whose polarization state is controlled by the polarization controller are input. A polarization mode dispersion compensator configured with a polarization mode dispersion control optical fiber that suppresses polarization mode dispersion of the optical signal by propagating the optical signal;
A 1-input 2-output optical coupler that receives an optical signal output from the polarization mode dispersion controlling optical fiber, outputs a part of the optical signal, and branches the remaining part;
A polarization splitter that divides an optical signal branched by the 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
A first polarization optical waveguide that propagates one of the TE polarization component and the TM polarization component output from the polarization splitter;
A second polarization component having the same optical path length as the first polarization optical waveguide and propagating the other polarization component of the TE polarization component and the TM polarization component output from the polarization splitter; An optical waveguide for polarization;
The TE polarization component and the TM polarization component are individually input via the first polarization optical waveguide and the second polarization optical waveguide, and the optical exOR operation is performed using the TE polarization component and the TM polarization component of the optical signal. An optical exOR circuit that outputs a calculation result and
In a polarization mode dispersion suppressing apparatus having a control system for controlling a polarization state by the polarization controller so that a logical output of the optical exOR circuit is “0”,
An optical level adjuster that outputs the intensity of output light within a certain range even if the intensity of input light is different between the first polarization optical waveguide and the second polarization optical waveguide. It is characterized by having each.
[0015]
The configuration of the present invention is such that the optical level adjuster is configured by providing an optical amplifier in one stage or multiple stages,
A first light receiver for detecting the intensity of the polarization component input to the optical exOR circuit via the first polarization optical waveguide;
A second light receiver for detecting the intensity of the polarization component input to the optical exOR circuit via the second polarization optical waveguide;
A control circuit for controlling each amplification degree of the optical level adjuster so that the intensity of the polarization component received by the first light receiver is equal to the intensity of the polarization component received by the second light receiver. And further comprising.
[0016]
The configuration of the present invention also includes a polarization splitter that receives an optical signal propagated through an optical fiber and divides the optical signal into a TE polarization component and a TM polarization component, and a TE polarization component branched by the polarization splitter. And one of the TM polarization components and one optical waveguide having a variable phase delay line interposed therebetween, and the TE polarization component and the TM polarization component branched by the polarization splitter. The other optical waveguide to which the other polarization component is input, the polarization component propagating through the one optical waveguide, and the polarization component propagating through the other optical waveguide are combined and output. A polarization mode dispersion compensator configured with an optical multiplexer;
Among the one optical waveguide, it is interposed downstream from the position where the variable phase delay line is interposed, and outputs a part of one polarization component to the optical multiplexer and the remaining part. A first 1-input 2-output optical coupler that branches;
A second one-input two-output optical coupler that is interposed in the other optical waveguide, outputs a part of the other polarization component to the optical multiplexer, and branches the remaining part;
One polarization component branched by the first 1-input 2-output optical coupler and the other polarization component branched by the second 1-input 2-output optical coupler individually through the same optical path length And an optical exOR circuit that performs an optical exOR operation on one polarization component and the other polarization component of the optical signal and outputs a calculation result,
And a control system for controlling a phase delay amount by the variable phase delay line so that a logical result output of the optical exOR circuit becomes “0”.
[0017]
The configuration of the present invention also includes a polarization splitter that receives an optical signal propagated through an optical fiber and divides the optical signal into a TE polarization component and a TM polarization component, and a TE polarization component branched by the polarization splitter. And one of the TM polarization components and one optical waveguide having a variable phase delay line interposed therebetween, and the TE polarization component and the TM polarization component branched by the polarization splitter. The other optical waveguide to which the other polarization component is input, the polarization component propagating through the one optical waveguide, and the polarization component propagating through the other optical waveguide are combined and output. A polarization mode dispersion compensator configured with an optical multiplexer;
Among the one optical waveguide, it is interposed downstream from the position where the variable phase delay line is interposed, and outputs a part of one polarization component to the optical multiplexer and the remaining part. A first 1-input 2-output optical coupler that branches;
A second one-input two-output optical coupler that is interposed in the other optical waveguide, outputs a part of the other polarization component to the optical multiplexer, and branches the remaining part;
One polarization component branched by the first 1-input 2-output optical coupler and the other polarization component branched by the second 1-input 2-output optical coupler individually through the same optical path length And an optical exOR circuit that performs an optical exOR operation on one polarization component and the other polarization component of the optical signal and outputs a calculation result,
A first control system for controlling a phase delay amount by the variable phase delay line so that a logical result output of the optical exOR circuit is “0”;
Among the one optical waveguide, it is interposed before the position where the variable phase delay line is interposed, and outputs a part of one polarization component to the optical multiplexer and the remaining part. A third 1-input 2-output optical coupler that branches;
A fourth one-input two-output optical coupler that is interposed in the other optical waveguide and outputs a part of the other polarization component to the optical multiplexer and branches the remaining part;
A polarization controller that is interposed between the optical fiber and the polarization splitter, and controls the polarization state of the optical signal output from the optical fiber and then outputs the polarization signal to the polarization splitter;
The polarization state is controlled by the polarization controller so that the difference in intensity between the polarization component branched by the third optical coupler and the polarization component branched by the fourth optical coupler is minimized. And 2 control systems.
[0018]
In the configuration of the present invention, the polarization mode dispersion compensator is a planar optical circuit type polarization mode dispersion compensator,
This planar optical circuit type polarization mode dispersion compensator
First polarization splitter, first phase adjustment region, first variable coupler, second phase adjustment region, second variable coupler, polarization delay region, and second polarization splitter Are connected in order,
The first and second polarization splitters include amorphous silicon in one waveguide of a symmetric Mach-Zehnder interferometer and a heater in the other waveguide.
The first and second phase adjustment regions and the first and second variable couplers are configured with heaters in both waveguides of the symmetric Mach-Zehnder interferometer,
The polarization delay region is a Mach-Zehnder interferometer, and the length of one waveguide is different from the other waveguide,
Furthermore, among the one waveguide of the first phase adjustment region, the first half-wave plate is interposed at a position closer to the input side than the portion provided with the heater,
A second half-wave plate is interposed in any one of the two waveguides in the polarization delay region;
This planar optical circuit type polarization mode dispersion compensator is characterized in that the polarization state can be changed by changing the amount of heat generated by the heater.
[0019]
Further, in the configuration of the present invention, the polarization mode dispersion compensator is a lithium niobate type polarization mode dispersion compensator, and the lithium niobate type polarization mode dispersion compensator is:
A plurality of first group of comb electrodes to which a voltage of a first voltage value is applied and a plurality of second electrodes to which a voltage of a second voltage value is applied on a waveguide formed of lithium niobate. The comb electrodes of the group are alternately arranged, and a ground electrode is further arranged between the comb electrodes,
When the beat length of the optical signal is Λ, the interval between the comb electrodes is alternately Λ / 4 and 3Λ / 4 from the input side to the output side, and is applied to the comb electrodes. The polarization state can be changed by changing the voltage to be applied.
[0020]
The configuration of the present invention is characterized in that the operation speed of the optical exOR circuit is slower than the transmission speed of the input optical signal, and “0” or “1” is determined based on the average value of the optical signal.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the drawings.
[0022]
[First Embodiment]
FIG. 1 shows a polarization mode dispersion suppressing apparatus according to a first embodiment of the present invention. In the figure, 101 is an input optical fiber, 102 is an input optical pulse, 103 is a TE or TM polarization component of the input optical pulse 102, 104 is a TM or TE polarization component of the input optical pulse 102, and 105 is a polarization controller. , 106 is an optical fiber having a particularly large polarization mode dispersion, for example, a polarization maintaining optical fiber, 107 is a TE or TM polarization component, 108 is a TM or TE polarization component, 109 is an optical coupler, 110 is an optical waveguide, 111 Is an output optical fiber, 112 is a polarization splitter, 113 and 114 are optical waveguides, 115 is a phase conversion type wavelength conversion element, 116 is a light source of the wavelength conversion element 115, 117 and 118 are optical waveguides, and 119 is a light receiver. , 120 is a control system of the polarization controller 105, 123 is an optical pulse whose waveform is shaped, and 121 is a TE of the optical pulse 123. Is properly TM polarization component, 122 is a TM or TE polarization component of the optical pulse 123.
[0023]
In the first embodiment shown in FIG. 1, the delayed polarization component 103 is input in the direction in which the optical fiber 106 travels fast, and the preceding polarization component 104 is input in the direction in which the optical fiber 106 travels slowly. This is a method of suppressing polarization mode dispersion by adjusting the polarization state of the input optical pulse 102 by the polarization controller 105.
[0024]
In this embodiment, a polarization mode dispersion compensator is configured by the polarization controller 105 and the optical fiber (polarization plane maintaining optical fiber) 106 having particularly large polarization mode dispersion.
[0025]
Here, a part of the optical signal is branched by the optical coupler 109 and the polarization component is further branched by the polarization splitter 112. As a result, a TE or TM polarization component is output to the optical waveguide 113, and a TM or TE polarization component is output to the optical waveguide 114. Here, after the optical path lengths of the optical waveguide 113 and the optical waveguide 114 are made equal, they are individually input to the two signal input ports of the wavelength conversion element 115.
[0026]
The wavelength conversion element 115 is a mutual phase modulation type wavelength conversion element, and when the optical signal levels of both of the optical waveguides 113 and 114 on the input side are “0” level, the continuous light from the light source 116 is output to the optical waveguide on the output side. It outputs to 117. That is, at this time, the optical signal level of the output-side optical waveguide 118 is “0” level.
On the other hand, when only one of the optical signal levels of the input-side optical waveguide 113 or the optical waveguide 114 becomes “1” level, the optical signal level of the output-side optical waveguide 118 is determined by the principle of the wavelength conversion element 115. “1” level.
Further, when the optical signal levels of both of the input-side optical waveguides 113 and 114 are “1” level, both cancel each other, and the optical signal level of the output-side optical waveguide 118 becomes “0” level.
[0027]
That is, when the optical signal levels of the optical waveguide 113 and the optical waveguide 114 are equal, the optical signal level of the optical waveguide 118 is “0” level, and when the optical signal levels of the optical waveguide 113 and the optical waveguide 114 are not equal. The optical signal level of the optical waveguide 118 means a so-called exOR (exclusive OR) operation where the level is “1”.
The operating speed of the wavelength conversion element 115 operating as an optical exOR circuit is slower than the transmission speed of the input optical signal, and “0” or “1” is determined based on the average value of the optical signal. Of course, a wavelength conversion element having a high operating speed may be used.
[0028]
Therefore, when the optical signal level of the optical waveguide 118 is always set to “0” level, the optical signals propagating through the optical waveguide 113 and the optical waveguide 114 are exactly the same, and the polarization component 121 propagating through the output optical fiber 111. And there is no phase delay between the polarization component 122 and the polarization component 122.
[0029]
Since the light receiver 119 outputs an electrical signal corresponding to the received light intensity, the control is performed so that the intensity of the electrical signal output from the light receiver 119 is minimized (that is, the received light intensity is minimized). By controlling the polarization state of the input optical pulse 102 by the polarization controller 105 in the system 120, the polarization mode dispersion is minimized, that is, the propagation delay difference between the polarization component 121 and the polarization component 122 is reduced. Adjust to minimize. By performing such a control operation, it is possible to obtain a light pulse 123 having a waveform shaped.
[0030]
As shown in FIG. 2, the light source 116 is connected to the optical waveguide 117, and in the wavelength conversion element 115, the traveling direction of the light pulse (TE polarization component, TM polarization component) and the light from the light source 116. Even if is in the opposite direction, the same operation as in FIG. 1 is performed.
[0031]
In the embodiment shown in FIGS. 1 and 2, there is a possibility that the following undesirable state may occur. That is, when controlling the polarization controller 105, not only the desired state A in which the polarization component 103 is separated into the optical waveguide 113 and the polarization component 104 into the optical waveguide 114, but also the optical output power half that of the polarization component 103. Half of the output power of the polarization component 104 is output to the optical waveguide 113, half of the remaining polarization components 103 and 104 are output to the optical waveguide 114, and then the polarization of the polarization components 103 and 104 is output. There is a possibility that the state is adjusted to a direction in which the polarization mode dispersion of the optical fiber 106 becomes 0 (an angle of 45 degrees between the fastest traveling direction and the slowest traveling direction), which is an undesirable state B. To solve such a problem, the second embodiment shown below may be adopted.
[0032]
[Second Embodiment]
FIG. 3 shows a polarization mode dispersion suppressing apparatus according to the second embodiment of the present invention. In the figure, 201 is an input optical fiber, 202 is an input light pulse, 203 is a TE or TM polarization component of the input light pulse 202, 204 is a TM or TE polarization component of the input light pulse 202, and 205 is a polarization controller. , 206 is an optical fiber having a particularly large polarization mode dispersion, for example, a polarization maintaining optical fiber, 207 is a TE or TM polarization component, 208 is a TM or TE polarization component, 209 is an optical coupler, 210 is an optical waveguide, 211 Is an output optical fiber, 212 is a polarization splitter, 213 and 214 are optical waveguides, 215 is a cross-phase modulation type wavelength conversion element, 216 is a light source of the wavelength conversion element 215, 217 and 218 are optical waveguides, and 219 is a light receiver , 220 is a control system of the polarization controller 205, 223 is an optical pulse whose waveform is shaped, and 221 is the T of the optical pulse 223. Or TM polarization component, 222 is a TM or TE polarization component of the optical pulse 223. These 201 to 223 are the same as those shown in FIG. 1, and the operation principle is also the same.
[0033]
A polarization mode dispersion compensator is configured by the polarization controller 205 and an optical fiber (polarization plane maintaining optical fiber) 206 having a particularly large polarization mode dispersion.
[0034]
The second embodiment further includes an optical coupler 224, a polarization splitter 225, optical waveguides 226 and 227, a mutual phase modulation type wavelength conversion element 228, a light source 229, optical waveguides 230 and 231, and a light receiver 232. This solves the problem in the first embodiment.
[0035]
Here, a part of the optical signal is branched by an optical coupler 224 disposed between the polarization controller 205 and the optical fiber 206, and is branched by the polarization splitter 225 into optical waveguides 226 and 227 having the same optical path length. Input to the wavelength conversion element 228. As described above, the wavelength conversion element 228 operates as an exOR circuit.
[0036]
Here, in the desired state A, the optical signal levels of the polarization components 203 and 204 are not equal in the light receiver 232, and the optical signal levels of the polarization components 203 and 204 are equal in the light receiver 219. That is, when the light receiver 219 is adjusted to the minimum, if the light receiver 232 is not the minimum, the state A is known. On the other hand, in the undesirable state B, when the light receiver 219 is minimized, the light receiver 232 is similarly minimized. Since this state is an undesirable state, the controller 220 further adjusts the polarization state with respect to the input optical pulse 202 by the polarization controller 205 to adjust to the state A.
[0037]
In FIG. 3, the light source 216 may be connected to the optical waveguide 217, or the light source 229 may be connected to the optical waveguide 230.
[0038]
By the way, in the 1st and 2nd embodiment shown in FIGS. 1-3, there exists a possibility of the following undesirable states. That is, when the light intensity of the TE polarization component and the TM polarization component input to the input optical fiber 101 (201) is equal or the difference in light intensity is small (for example, about 5 dB), the exOR operation operates normally. Polarization mode dispersion can be suppressed, but when the light intensity difference is 5 dB or more, only the polarization component with the higher light intensity is dragged, and the exOR circuit (wavelength conversion element 115 (215)) may not operate normally. There is sex.
[0039]
In an actual optical transmission line, the difference in intensity between both polarization components is large, and sometimes reaches 20 dB. At this time, the polarization mode dispersion suppression device according to the first and second embodiments shown in FIGS. 1 to 3 cannot suppress polarization mode dispersion. Thus, when the intensity difference of the TE / TM polarization component is large, the following third embodiment may be adopted.
[0040]
[Third Embodiment]
FIG. 4 shows a polarization mode dispersion suppressing apparatus according to a third embodiment of the present invention. In the figure, 301 is an input optical fiber, 302 is an input light pulse, 303 is a TE or TM polarization component of the input light pulse 302, 304 is a TM or TE polarization component of the input light pulse 302, and 305 is a polarization controller. , 306 is an optical fiber having a particularly large polarization mode dispersion, for example, a polarization maintaining optical fiber, 307 is a TE or TM polarization component, 308 is a TM or TE polarization component, 309 is a 1-input 2-output optical coupler, 310 Is an optical waveguide, 311 is an output optical fiber, 312 is a polarization splitter, 313 and 314 are optical waveguides for polarization, 315 is a cross-phase modulation type wavelength conversion element, 316 is a light source of the wavelength conversion element 315, 317 and 318 Is an optical waveguide, 319 is a light receiver that detects only the average output, 320 is a control system of the polarization controller 305, 323 is a waveform shaping Light pulses, 321 TE or TM polarization component of the optical pulse 323, 322 is a TM or TE polarization component of the optical pulse 323.
[0041]
Furthermore, the optical level adjuster 324 provided (interposed) in the optical waveguide 313 and the optical level adjuster 325 provided (interposed) in the optical waveguide 314 are provided. The light level adjusters 324 and 325 have a function of setting the intensity of output light to an intensity within a certain range even when the intensity of input light is different. The specific characteristics will be described later with reference to FIG.
[0042]
A polarization mode dispersion compensator is configured by the polarization controller 305 and an optical fiber (polarization plane maintaining optical fiber) 306 having a particularly large polarization mode dispersion.
[0043]
In the polarization mode dispersion suppressing apparatus shown in FIG. 4, the delayed polarization component 303 is advanced in the direction in which the polarization-maintaining optical fiber 306 travels fast, and the preceding polarization component 304 is traveled in the slow direction in the polarization-maintaining optical fiber 306. The polarization mode dispersion is suppressed by controlling the polarization controller 305 by the control system 320 so that the polarization state of the input optical pulse 302 is adjusted.
[0044]
Here, a part of the optical signal is branched by the optical coupler 309 and the polarization component is further branched by the polarization splitter 312. As a result, a TE or TM polarization component is output to the polarization optical waveguide 313, and a TM or TE polarization component is output to the polarization optical waveguide 314. Here, after making the optical path lengths of the polarization optical waveguide 313 and the polarization optical waveguide 314 equal, the TE polarization component and the TM polarization component are individually input to the signal input port of the wavelength conversion element 315.
[0045]
The wavelength conversion element 315 is a cross-phase modulation type wavelength conversion element, and outputs continuous light from the light source 316 to the optical waveguide 317 when the light intensities of both the polarization optical waveguides 313 and 314 are “0” level. That is, at this time, the output-side optical waveguide 318 is at the “0” level.
On the other hand, when only one of the light intensities of the polarization optical waveguide 313 and the polarization optical waveguide 314 becomes “1” level, the light intensity of the optical waveguide 318 on the output side is determined by the principle of the wavelength conversion element 315. “1” level.
Further, when the light intensities of both of the polarization optical waveguides 313 and 314 are “1” level, both cancel each other, and the light intensity of the output-side optical waveguide 318 becomes “0” level.
[0046]
The light intensity of the polarization component input to the wavelength conversion element 315 through the light level adjuster 324 and the light intensity of the polarization component input to the wavelength conversion element 315 through the light level adjuster 325 are as follows: The intensity is within a certain range and is almost constant.
[0047]
Therefore, even if the light intensity difference between the TE polarization component and the TM polarization component branched by the polarization splitter 312 and input to the polarization optical waveguides 313 and 314 is large, the optical level adjuster 324 is provided. The light intensity of the polarization component that passes through and is input to the wavelength conversion element 315 is substantially equal to the light intensity of the polarization component that passes through the optical level adjuster 325 and is input to the wavelength conversion element 315.
(1) When the input timings of the polarization component input to the polarization optical waveguide 313 and the polarization component input to the polarization optical waveguide 314 are equal, the light intensity of the optical waveguide 318 is at “0” level. And
(2) When the input timings of the polarization component input to the polarization optical waveguide 313 and the polarization component input to the polarization optical waveguide 314 are not equal, the light intensity of the optical waveguide 318 is “1”. Become a level.
In other words, even if there is a large difference in light intensity between the TE polarization component and the TM polarization component branched by the polarization splitter 312 and input to the polarization optical waveguides 313 and 314, the exOR in the wavelength conversion element 315 Operation can be secured.
[0048]
Since the light receiver 319 outputs an electrical signal corresponding to the received light intensity, the control is performed so that the intensity of the electrical signal output from the light receiver 319 is minimized (that is, the received light intensity is minimized). By controlling the polarization state of the input optical pulse 302 by the polarization controller 305 in the system 320, the polarization mode dispersion is minimized, that is, the difference in propagation delay between the polarization component 321 and the polarization component 322 is reduced. Adjust to minimize. By performing such a control operation, an optical pulse 323 having a waveform shape can be obtained. In addition, since the exOR operation can be ensured even if the light intensity difference between the polarization component 303 and the polarization component 304 is large, the optical pulse 323 having a waveform shape can be obtained.
[0049]
In FIG. 4, the light source 316 may be connected to the optical waveguide 317.
[0050]
[Characteristics of light level adjuster]
FIG. 5 is a characteristic diagram showing the characteristics of the optical level adjusters 324 and 325 in the third embodiment. Specifically, the optical output with respect to the optical input of the semiconductor optical amplifier having an amplification degree of 20 dB and a saturation output of 3 dBm is shown. It is illustrated. According to the figure, the output intensity falls within the range of 0 to +3 dBm for any light intensity exceeding −20 dBm. That is, for example, even if there is a difference in light intensity exceeding 20 dB such as -20 dBm and 0 dBm, the output light intensity can be within the range of 0 to +3 dBm and 3 dB as a result of light level adjustment. it can.
[0051]
Normally, when a semiconductor optical amplifier is used as an optical level adjuster in this way, waveform distortion due to a so-called pattern effect in which the degree of amplification changes depending on the signal pattern becomes a problem. However, in the present invention, only the average output from the light receiver 319 is used. Because it is detected, it does not matter if the waveform is distorted.
[0052]
When it is difficult to achieve an amplification degree of 20 dB with one stage of the semiconductor optical amplifier, the semiconductor optical amplifiers may be cascaded in two stages and three stages.
In addition, when used in an optical transmission system with low intensity of input light, an optical amplifier with high amplification is adopted, and when used in an optical transmission system with high intensity of input light, the amplification is low. Adopt an optical amplifier.
Further, the optical level adjuster may be configured using an optical fiber amplifier or the like instead of the semiconductor optical amplifier.
[0053]
[Fourth Embodiment]
FIG. 6 shows a polarization mode dispersion suppressing apparatus according to a fourth embodiment of the present invention. In this embodiment, a configuration in which light receivers 326 and 327, a control circuit 328, and feedback circuits 329 and 330 are further added to the polarization mode dispersion suppressing apparatus according to the third embodiment shown in FIG. It has become.
[0054]
The light receiving unit 326 receives a polarization component that passes through the light level adjuster 324 and is input to the wavelength conversion element 315, and the light receiving unit 326 outputs an electrical signal corresponding to the intensity of the polarization component. In addition, a polarization component input to the wavelength conversion element 315 through the light level adjuster 325 is input to the light receiver 327, and the light receiver 327 outputs an electrical signal corresponding to the intensity of this polarization component. To do. The control circuit 328 monitors the electrical signals from the light receiver 326 and the light receiver 327 to thereby detect the intensity of the polarized light input to the wavelength conversion element 315 via the polarization optical waveguide 313 and the polarization light. The intensity of the polarized light input to the wavelength conversion element 315 via the waveguide 314 can be monitored.
[0055]
The control circuit 328 individually adjusts the amplification levels of the optical level adjusters 324 and 325 via the feedback circuits 329 and 330 so that the difference in intensity between the two polarized lights is minimized. That is, when the light intensity is weak, the amplification is increased by increasing the current of the light level adjusters 324 and 325, and when the light intensity is strong, the amplification is decreased by decreasing the current of the light level adjusters 324 and 325. By such light intensity adjustment control, the intensity of the polarized light input to the wavelength conversion element 315 via the polarization optical waveguide 313 and the polarization input to the wavelength conversion element 315 via the polarization optical waveguide 314 are detected. The intensity of wave light can be matched more strictly, and polarization mode dispersion can be controlled more reliably.
[0056]
[Fifth Embodiment]
FIG. 7 shows a polarization mode dispersion suppressing apparatus according to the fifth embodiment of the present invention. In FIG. 7, 401 is an input optical fiber, 402 is an input optical pulse, 403 is a TE or TM polarization component of the input optical pulse 402, 404 is a TM or TE polarization component of the input optical pulse 402, and 405 is a polarization splitter. , 406 and 407 are optical waveguides, 408 is a variable phase delay line interposed in the optical waveguide 406, 409 is an optical multiplexer, 411 and 412 are optical couplers, 413 is a mutual phase modulation type wavelength conversion element, and 414 is a wavelength. A light source of the conversion element 413, 415 and 416 are optical waveguides, 417 is a light receiver, 418 is a control system of the variable phase delay line 408, 419 is an optical fiber for output, 422 is an optical pulse whose waveform is shaped, 420 is an optical pulse 422 The TE or TM polarized wave component 421 is the TM or TE polarized wave component of the optical pulse 422.
[0057]
In this embodiment, the polarization mode dispersion compensator is configured by the polarization splitter 405, the optical waveguides 406 and 407, the variable phase delay line 408, and the optical multiplexer 409.
[0058]
In the fifth embodiment shown in FIG. 7, the input optical pulse 402 is branched by the polarization splitter 405 so that the delayed (or preceding) polarization component 403 is converted into the optical waveguide 406 and the polarization component 404 is converted into the polarization component 404. A method of suppressing polarization mode dispersion by branching to the optical waveguide 407 and adjusting the variable phase delay line 408 so as to shorten (or lengthen) the optical path length of the delayed (or preceding) polarization component 403. is there.
[0059]
Here, some optical signals are branched by the optical couplers 411 and 412 and input to the wavelength conversion element 413 through the optical waveguide having the same optical path length. As described above, since the wavelength conversion element 413 operates as an exOR circuit, the phase delay amount by the variable phase delay line 408 is controlled by the control system 418 so that the output value of the optical waveguide 416 is minimized. A waveform-shaped optical pulse 422 is obtained so as to minimize the mode dispersion, that is, minimize the difference in propagation delay between the polarization component 420 and the polarization component 421.
[0060]
As shown in FIG. 8, the light source 414 is connected to the optical waveguide 415, and in the wavelength conversion element 413, the traveling direction of the light pulse (TE polarization component, TM polarization component) and the light from the light source 414 Even in the reverse direction, the same operation as in FIG. 7 is performed.
[0061]
However, in the fifth embodiment shown in FIGS. 7 and 8, the polarization component 403 is not necessarily in a polarization state in which the polarization component 403 is well branched to the optical waveguide 406 and the polarization component 404 is well branched to the optical waveguide 407. There is a problem that. To solve such a problem, the following sixth embodiment may be adopted.
[0062]
[Sixth Embodiment]
FIG. 9 shows a polarization mode dispersion suppressing apparatus according to the sixth embodiment of the present invention. In FIG. 6, 401 is an input optical fiber, 402 is an input optical pulse, 403 is a TE or TM polarization component of the input optical pulse 402, 404 is a TM or TE polarization component of the input optical pulse 402, and 405 is a polarization splitter. , 406 and 407 are optical waveguides, 408 is a variable phase delay line interposed in the optical waveguide 406, 409 is an optical multiplexer, 411 and 412 are optical couplers, 413 is a mutual phase modulation type wavelength conversion element, and 414 is a wavelength. A light source of the conversion element 413, 415 and 416 are optical waveguides, 417 is a light receiver, 418 is a control system of the variable phase delay line 408, 419 is an optical fiber for output, 422 is an optical pulse whose waveform is shaped, 420 is an optical pulse 422 The TE or TM polarized wave component 421 is the TM or TE polarized wave component of the optical pulse 422. These 401 to 422 are the same as those shown in FIG. 8, and the operation principle is also the same.
[0063]
In this embodiment, the polarization mode dispersion compensator is configured by the polarization splitter 405, the optical waveguides 406 and 407, the variable phase delay line 408, and the optical multiplexer 409.
[0064]
In the sixth embodiment, the optical couplers 423 and 424, the light receivers 425 and 426, the polarization controller 427, and the control system 428 are further provided to solve the problems in the fifth embodiment. That is, a part of the optical signal propagating through the optical waveguides 406 and 407 is branched by the optical couplers 423 and 424, the intensity of the branched optical signal is measured by the light receivers 425 and 426, and the difference in intensity is minimized. As described above, the control system 428 controls the polarization state of the input optical pulse 402 by the polarization controller 427 so that the polarization component 403 is successfully branched into the optical waveguide 406 and the polarization component 404 is branched into the optical waveguide 407. It is possible to achieve a simple polarization state.
[0065]
In FIG. 9, the light source 416 may be connected to the optical waveguide 415.
[0066]
What should be noted in the present invention is that none of the phase modulation type wavelength conversion elements 115, 215, 228, 315, and 413 in FIGS. 1 to 9 need to follow the input signal. That is, it is not necessary for the exOR operation to follow 1 bit and 1 bit, as long as it can be detected as an average value. The same applies to the light receivers 119, 219, 232, 319, 326, 327, 417, 425, 426. That is, since all the configurations of the present invention operate with the average output power of the optical signal, they do not depend on the bit rate. Even if the 1010 signal continues, the feedback system is not affected. In addition, an inexpensive low-speed receiver can be used.
[0067]
[Other examples of polarization mode dispersion compensator]
In the embodiment described above, as a polarization mode dispersion compensator,
(A) a combination of a polarization controller (polarization rotator) and an optical fiber having a particularly large polarization mode dispersion (for example, a polarization-maintaining optical fiber) (see FIGS. 1 to 4 and 6),
(B) A combination of a polarization splitter, two optical waveguides, a variable phase delay line interposed in one optical waveguide, and an optical multiplexer (FIGS. 7 to 9) is employed.
Other types of polarization mode dispersion compensators can also be employed. Other polarization mode dispersion compensators will be described below.
[0068]
[Polarization mode dispersion compensator using PLC (planar lightwave circuit)]
A polarization mode dispersion compensator using a PLC (quartz waveguide on a silicon substrate: planar lightwave circuit) will be described with reference to FIG. This type of polarization mode dispersion compensator is disclosed in T. Saida et al., “Planar Lightwave Circuit Polarization Mode Compensator”, ECOC '2001, Amsterdam, two polarization splitters 501, 502, and two Half-wave plates 503 and 504, two phase adjustment regions 505 and 506, two variable couplers 507 and 508, and one polarization delay region 509 are configured. That is, the first polarization splitter 501, the first phase adjustment region 505, the first variable coupler 507, the second phase adjustment region 506, the second variable coupler 508, and the polarization delay region 509 The second polarization splitter 502 is connected in series in this order.
[0069]
The polarization splitters 501 and 502 include amorphous silicon AS1 and AS2 in one waveguide of a symmetrical Mach-Zehnder interferometer, and heaters H1 and H2 in the other waveguide. Separate or combine polarizations.
[0070]
The variable couplers 507 and 508 also have a symmetrical Mach-Zehnder interferometer structure, and adjust the power applied to the heaters H7-1, H7-2, H8-1, and H8-2 provided in both waveguides. Thus, the branching ratio of the optical signal can be changed.
[0071]
Similarly, the phase adjustment regions 505 and 506 have a symmetrical Mach-Zehnder interferometer structure, and are provided with heaters H5-1, H5-2, H6-1, and H6-2 in each waveguide, and the phase of the optical signal. To change.
[0072]
The half-wave plate 503 is interposed at a position closer to the input side than the heater H5-1 in the waveguide below the phase adjustment region 505. The half-wave plate 504 is shorter in the polarization delay region 509. It is interposed in the waveguide. The half-wave plates 503 and 504 have a function of reversing the TE polarization into the TM polarization and the TM polarization into the TE polarization.
[0073]
The polarization delay region 509 is composed of two waveguides having different lengths, and the difference in propagation delay of the optical signal propagating through both is set to 20 ps.
[0074]
The polarization mode dispersion compensator shown in FIG. 10 operates as follows. The input optical signal is separated into TE polarization and TM polarization by the first polarization splitter 501. Further, the TM polarized wave is converted into the TE polarized wave by the half-wave plate 503. That is, the input signal is separated into a TE polarization component and a TM polarization component (where the polarization is TE polarization).
[0075]
The TE polarization component and the TM polarization component are guided to the two phase adjustment regions 505 and 506 and the two variable couplers 507 and 508. Here, the electric power applied to each heater H5-1, H5-2, H6-1, H6-2, H7-1, H7-2, H8-1, H8-2 is adjusted (as a result, the heat generation amount is adjusted). By changing the phase of the phase adjustment regions 505 and 506 and the branching ratio of the variable couplers 507 and 508, the ratio of the TE polarization component and the TM polarization component (where polarization is the TE component) can be freely changed. (This corresponds to changing the ratio of the TE polarization component and the TM polarization component by adjusting the polarization controller in the polarization mode dispersion compensator of type (A)).
[0076]
Thereafter, the TE polarization component is input to the longer waveguide of the polarization delay region 509, and the TM polarization component (polarization is TE polarization) is input to the shorter waveguide. Since the difference in propagation time of the waveguide is 20 ps, the TE polarization component is delayed by 20 ps (this is because the polarization mode dispersion compensator of the above type (a) uses a polarization maintaining optical fiber). This corresponds to delaying one polarization component of the other compared with the other polarization component).
[0077]
The TM polarization component (TE polarization) is returned to the TM polarization again by the half-wave plate 504, and the TE polarization component and the TM polarization component are combined by the polarization splitter 502 and output.
[0078]
This configuration is basically the same as the polarization mode dispersion compensator of the type (a) in which the polarization mode dispersion is compensated by the polarization controller (polarization rotator) and the polarization maintaining optical fiber. That is, the phase adjustment region 505 and the variable coupler 507 correspond to a polarization controller, and the polarization delay region 509 corresponds to a polarization-maintaining optical fiber.
[0079]
[Polarization mode dispersion compensator using LN]
LN (lithium niobate, LiNbO _Three ) Will be described with reference to FIG. This type of polarization mode dispersion compensator is described by R. Noe et al., “Integrated optical LiNbO _Three distributed polarization mode dispersion compensator in 20 Gb / s transmission system "Electronics Letters, vol. 35, no. 8, pp. 652-654, 1999, and a voltage is applied to the optical waveguide 600 through which the optical signal propagates. Comb electrodes 601a and 601b to which V1 is applied, a ground electrode 602 of 0 V, and comb electrodes 603a and 603b to which voltage V2 is applied are sequentially arranged, that is, comb electrodes 603a, 601a, 603b, and 601b. Are arranged in this order, and a ground electrode 602 is disposed between the comb electrodes 603a, 601a, 603b, and 601b (between the comb electrodes and between the combs).
[0080]
When the beat length of the optical signal is Λ, the interval between the comb electrodes is set to Λ / 4 and 3Λ / 4 as shown in FIG. An LN substrate that cuts in the X-axis direction and propagates an optical signal in the Y-axis direction is used. By applying a voltage of ± 69 V to V1 and V2, the polarization of the optical signal can be rotated by ± 45 degrees. Since the LN waveguide has a polarization mode dispersion of 0.26 ps / mm, the polarization mode dispersion of the optical signal can be slightly changed by rotating the optical signal. Noe et al. Succeeded in polarization mode dispersion compensation of 43 ps by cascading 73 pairs of two comb-shaped electrodes having an interval of Λ / 4 and 3Λ / 4 as one set. The insertion loss is about 7 dB.
[0081]
In addition to the above, there are various types of polarization mode dispersion compensators. However, the polarization mode dispersion suppression apparatus of the present invention can be configured by incorporating these polarization mode dispersion compensators.
[0082]
For example, in place of the polarization mode dispersion compensator using the polarization controller 105 and the optical fiber 106 in FIG. 1, the above-described various polarization mode dispersion compensators are adopted to constitute the polarization mode dispersion suppression device of the present invention. can do.
Further, for example, instead of the polarization mode dispersion compensator using the polarization splitter 405, the optical waveguides 406 and 407, the variable phase delay line 408, and the optical multiplexer 409 in FIG. By adopting the detector, the polarization mode dispersion suppressing device of the present invention can be configured.
[0083]
【The invention's effect】
As described above, according to the present invention, the bit rate can be changed, and even if 10 codes are continuous, the feedback system is not affected, and the polarization mode dispersion suppression can be configured even with a low-speed receiver. An apparatus can be provided.
[0084]
Further, according to the present invention, the polarization mode dispersion suppression that allows the exOR circuit to operate normally and suppress polarization mode dispersion even when the difference in intensity between the TE polarization component and the TM polarization component is large. An apparatus can be provided.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a polarization mode dispersion suppressing apparatus according to a first embodiment of the present invention.
FIG. 2 is a configuration diagram showing a modification of the polarization mode dispersion suppressing apparatus according to the first embodiment of the present invention.
FIG. 3 is a configuration diagram showing a polarization mode dispersion suppressing apparatus according to a second embodiment of the present invention.
FIG. 4 is a configuration diagram showing a polarization mode dispersion suppressing apparatus according to a third embodiment of the present invention.
FIG. 5 is a characteristic diagram showing characteristics of an optical level adjuster according to the third embodiment.
FIG. 6 is a configuration diagram showing a polarization mode dispersion suppressing apparatus according to a fourth embodiment of the present invention.
FIG. 7 is a block diagram showing a polarization mode dispersion suppressing apparatus according to a fifth embodiment of the present invention.
FIG. 8 is a configuration diagram showing a modification of the polarization mode dispersion suppressing apparatus according to the fifth embodiment of the present invention.
FIG. 9 is a block diagram showing a polarization mode dispersion suppressing apparatus according to a sixth embodiment of the present invention.
FIG. 10 is a configuration diagram showing a polarization mode dispersion compensator using a PLC.
FIG. 11 is a configuration diagram showing a polarization mode dispersion compensator using LN.
FIG. 12 is an explanatory diagram for explaining polarization mode dispersion.
FIG. 13 is a configuration diagram showing a conventional polarization mode dispersion suppressing device.
[Explanation of symbols]
101 is an input optical fiber,
102 is an input light pulse,
103 and 104 are polarization components,
105 is a polarization controller,
106 is an optical fiber having particularly large polarization mode dispersion, for example, a polarization maintaining optical fiber,
107 and 108 are polarization components,
109 is an optical coupler,
110 is an optical waveguide,
111 is an optical fiber for output,
112 is a polarization splitter,
113 and 114 are optical waveguides,
115 is a mutual phase modulation type wavelength conversion element,
116 is a light source,
117 and 118 are optical waveguides,
119 is a light receiver,
120 is a control system,
121 and 122 are polarization components,
123 is a light pulse whose waveform is shaped,
201 is an input optical fiber;
202 is an input light pulse;
203 and 204 are polarization components,
205 is a polarization controller,
206 is an optical fiber having particularly large polarization mode dispersion, for example, a polarization maintaining optical fiber,
207 and 208 are polarization components,
209 is an optical coupler,
210 is an optical waveguide,
211 is an output optical fiber,
212 is a polarization splitter,
213 and 214 are optical waveguides,
215 is a mutual phase modulation type wavelength conversion element,
216 is a light source,
217 and 218 are optical waveguides,
219 is a light receiver,
220 is a control system,
221 and 222 are polarization components,
223 is a light pulse whose waveform is shaped,
224 is an optical coupler,
225 is a polarization splitter,
226 and 227 are optical waveguides,
228 is a mutual phase modulation type wavelength conversion element,
229 is a light source,
230 and 231 are optical waveguides,
232 is a light receiver,
301 is an input optical fiber;
302 is an input light pulse;
303 and 304 are polarization components,
305 is a polarization splitter,
306 is a polarization maintaining optical fiber,
307 and 308 are polarization components,
309 is an optical coupler,
310 is an optical waveguide;
311 is an optical fiber for output,
312 is a polarization splitter,
313 and 314 are optical waveguides for polarization,
315 is a wavelength conversion element,
316 is a light source,
317 and 308 are optical waveguides,
319 is a light receiver,
320 is a control system,
321 and 322 are polarization components,
323 is a light pulse whose waveform is shaped,
324 and 325 are light level adjusters,
326 and 327 are light receivers,
328 is a control circuit,
329 and 330 are feedback circuits,
401 is an input optical fiber;
402 is an input light pulse;
403 and 404 are polarization components,
405 is a polarization splitter,
406 and 407 are optical waveguides,
408 is a variable phase delay line,
409 is an optical multiplexer,
411 and 412 are optical couplers,
413 is a mutual phase modulation type wavelength conversion element,
414 is a light source,
415 and 416 are optical waveguides,
417 is a light receiver,
418 is a control system,
419 is an optical fiber for output,
420 and 421 are polarization components,
422 is a light pulse whose waveform is shaped,
423 and 424 are optical couplers,
425 and 426 are light receivers,
427 is a polarization controller,
428 is a control system,
501 and 502 are polarization splitters,
503 and 504 are half-wave plates,
505 and 506 are phase adjustment regions,
507 and 508 are variable couplers,
509 is a polarization delay region,
601a, 601b, 603a, 603b are comb electrodes,
602a to 602d are ground electrodes,
1001 is an optical fiber,
1002 is an input light pulse,
1003, 1004, 1005, 1006 are polarization components,
Reference numeral 1007 denotes an optical pulse.

Claims

When an optical signal propagated through an optical fiber is input, polarization mode dispersion is suppressed by compensating for the difference in transmission delay between the TE polarization component and the TM polarization component of the optical signal. A polarization mode dispersion compensator for outputting an optical signal;
A 1-input 2-output optical coupler that receives an optical signal with suppressed polarization mode dispersion output from the polarization mode dispersion compensator, outputs a part of the optical signal and branches the other part; ,
A polarization splitter that divides an optical signal branched by the 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and TM polarization component of the optical signal are individually input to two input ports via the same optical path length, and are calculated by performing an optical exOR operation using the TE polarization component and TM polarization component of the optical signal. An optical exOR circuit that outputs the result;
And a control system that controls compensation by the polarization mode dispersion compensator so that a logical result output of the optical exOR circuit becomes “0”.

When an optical signal propagated through an optical fiber is input, polarization mode dispersion is suppressed by compensating for the difference in transmission delay between the TE polarization component and the TM polarization component of the optical signal. A polarization mode dispersion compensator for outputting an optical signal;
A first 1-input 2-output optical coupler for branching a part of the TE polarization component in which polarization mode dispersion is suppressed from within the polarization mode dispersion compensator;
A second 1-input 2-output optical coupler for branching a part of the TM polarization component whose polarization mode dispersion is suppressed from within the polarization mode dispersion compensator;
One polarization component branched by the first 1-input 2-output optical coupler and the other polarization component branched by the second 1-input 2-output optical coupler individually through the same optical path length And an optical exOR circuit that performs an optical exOR operation on one polarization component and the other polarization component of the optical signal and outputs a calculation result,
And a control system that controls compensation by the polarization mode dispersion compensator so that a logical result output of the optical exOR circuit becomes “0”.

A polarization controller that controls the polarization state of an optical signal that has been propagated through an optical fiber, and a polarization controller that controls the polarization state of the optical signal is input to propagate the optical signal. A polarization mode dispersion compensator configured with a polarization mode dispersion control optical fiber that suppresses polarization mode dispersion of an optical signal,
1-input 2-output light that is input from the polarization mode dispersion controlling optical fiber and whose polarization mode dispersion is suppressed, outputs a part of the optical signal and branches the remaining part. A coupler,
A polarization splitter that divides an optical signal branched by the 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and TM polarization component of the optical signal are individually input to two input ports via the same optical path length, and are calculated by performing an optical exOR operation using the TE polarization component and TM polarization component of the optical signal. An optical exOR circuit that outputs the result;
A polarization mode dispersion suppressing apparatus, comprising: a control system that controls a polarization state by the polarization controller so that a logical result output of the optical exOR circuit becomes “0”.

A polarization controller that controls the polarization state of an optical signal that has been propagated through an optical fiber, and a polarization controller that controls the polarization state of the optical signal is input to propagate the optical signal. A polarization mode dispersion compensator configured with a polarization mode dispersion control optical fiber that suppresses polarization mode dispersion of an optical signal,
A first input 1 that receives an optical signal with suppressed polarization mode dispersion output from the polarization mode dispersion controlling optical fiber, outputs a part of the optical signal, and branches the remaining part. An output optical coupler;
A first polarization splitter that splits an optical signal branched by the first 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and TM polarization component of the optical signal divided by the first polarization splitter are individually input to two input ports through the same optical path length, and the TE polarization component and TM of the optical signal are input. A first optical exOR circuit that performs an optical exOR operation on the polarization component and outputs a calculation result;
A portion of the optical signal that is arranged between the polarization controller and the polarization mode dispersion control optical fiber and that is output from the polarization controller is output toward the polarization mode dispersion control optical fiber and the rest A second 1-input 2-output optical coupler that branches a portion of
A second polarization splitter for dividing the optical signal branched by the second 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
The TE polarization component and the TM polarization component of the optical signal divided by the second polarization splitter are individually input to the two input ports through the same optical path length, and the TE polarization component of the optical signal and TM A second optical exOR circuit that performs an optical exOR operation on the polarization component and outputs a calculation result;
The polarization state is controlled by the polarization controller so that the logical result output of the first optical exOR circuit becomes “0” and the logical result output of the second optical exOR circuit does not become “0”. And a polarization mode dispersion suppressing device.

A polarization controller that controls the polarization state of an optical signal that has been propagated through an optical fiber, and a polarization controller that controls the polarization state of the optical signal is input to propagate the optical signal. A polarization mode dispersion compensator configured with a polarization mode dispersion control optical fiber that suppresses polarization mode dispersion of an optical signal,
A 1-input 2-output optical coupler that receives an optical signal output from the polarization mode dispersion controlling optical fiber, outputs a part of the optical signal, and branches the remaining part;
A polarization splitter that divides an optical signal branched by the 1-input 2-output optical coupler into a TE polarization component and a TM polarization component;
A first polarization optical waveguide that propagates one of the TE polarization component and the TM polarization component output from the polarization splitter;
A second polarization component having the same optical path length as the first polarization optical waveguide and propagating the other polarization component of the TE polarization component and the TM polarization component output from the polarization splitter; An optical waveguide for polarization;
The TE polarization component and the TM polarization component are individually input via the first polarization optical waveguide and the second polarization optical waveguide, and the optical exOR operation is performed using the TE polarization component and the TM polarization component of the optical signal. An optical exOR circuit that outputs a calculation result and
In a polarization mode dispersion suppressing apparatus having a control system for controlling a polarization state by the polarization controller so that a logical output of the optical exOR circuit is “0”,
An optical level adjuster that outputs the intensity of output light within a certain range even if the intensity of input light is different between the first polarization optical waveguide and the second polarization optical waveguide. A polarization mode dispersion suppressing device characterized by comprising each.

6. The polarization mode dispersion suppressing apparatus according to claim 5, wherein the optical level adjuster is configured by providing optical amplifiers in one or more stages.

In claim 5 or claim 6,
A first light receiver for detecting the intensity of the polarization component input to the optical exOR circuit via the first polarization optical waveguide;
A second light receiver for detecting the intensity of the polarization component input to the optical exOR circuit via the second polarization optical waveguide;
A control circuit for controlling each amplification degree of the optical level adjuster so that the intensity of the polarization component received by the first light receiver is equal to the intensity of the polarization component received by the second light receiver. And a polarization mode dispersion suppressing device.

A polarization splitter that splits the optical signal into a TE polarization component and a TM polarization component, and a TE polarization component and a TM polarization component branched by the polarization splitter. One polarization component is input and a variable phase delay line is interposed, and the other polarization of the TE polarization component and the TM polarization component branched by the polarization splitter. The other optical waveguide to which the component is input, and an optical multiplexer that combines and outputs the polarization component propagating through the one optical waveguide and the polarization component propagating through the other optical waveguide. Polarization mode dispersion compensator
Among the one optical waveguide, it is interposed downstream from the position where the variable phase delay line is interposed, and outputs a part of one polarization component to the optical multiplexer and the remaining part. A first 1-input 2-output optical coupler that branches;
A second one-input two-output optical coupler that is interposed in the other optical waveguide, outputs a part of the other polarization component to the optical multiplexer, and branches the remaining part;
One polarization component branched by the first 1-input 2-output optical coupler and the other polarization component branched by the second 1-input 2-output optical coupler individually through the same optical path length And an optical exOR circuit that performs an optical exOR operation on one polarization component and the other polarization component of the optical signal and outputs a calculation result,
And a control system that controls a phase delay amount by the variable phase delay line so that a logical result output of the optical exOR circuit becomes “0”.

A polarization splitter that splits the optical signal into a TE polarization component and a TM polarization component, and a TE polarization component and a TM polarization component branched by the polarization splitter. One polarization component is input and a variable phase delay line is interposed, and the other polarization of the TE polarization component and the TM polarization component branched by the polarization splitter. The other optical waveguide to which the component is input, and an optical multiplexer that combines and outputs the polarization component propagating through the one optical waveguide and the polarization component propagating through the other optical waveguide. Polarization mode dispersion compensator
Among the one optical waveguide, it is interposed downstream from the position where the variable phase delay line is interposed, and outputs a part of one polarization component to the optical multiplexer and the remaining part. A first 1-input 2-output optical coupler that branches;
A second one-input two-output optical coupler that is interposed in the other optical waveguide, outputs a part of the other polarization component to the optical multiplexer, and branches the remaining part;
One polarization component branched by the first 1-input 2-output optical coupler and the other polarization component branched by the second 1-input 2-output optical coupler individually through the same optical path length And an optical exOR circuit that performs an optical exOR operation on one polarization component and the other polarization component of the optical signal and outputs a calculation result,
A first control system for controlling a phase delay amount by the variable phase delay line so that a logical result output of the optical exOR circuit is “0”;
Among the one optical waveguide, it is interposed before the position where the variable phase delay line is interposed, and outputs a part of one polarization component to the optical multiplexer and the remaining part. A third 1-input 2-output optical coupler that branches;
A fourth one-input two-output optical coupler that is interposed in the other optical waveguide and outputs a part of the other polarization component to the optical multiplexer and branches the remaining part;
A polarization controller that is interposed between the optical fiber and the polarization splitter, and controls the polarization state of the optical signal output from the optical fiber and then outputs the polarization signal to the polarization splitter;
The polarization state is controlled by the polarization controller so that the difference in intensity between the polarization component branched by the third optical coupler and the polarization component branched by the fourth optical coupler is minimized. And a polarization mode dispersion suppressing apparatus.

The polarization mode dispersion compensator according to any one of claims 1 to 9, wherein the polarization mode dispersion compensator is a planar optical circuit type polarization mode dispersion compensator.
This planar optical circuit type polarization mode dispersion compensator
First polarization splitter, first phase adjustment region, first variable coupler, second phase adjustment region, second variable coupler, polarization delay region, and second polarization splitter Are connected in order,
The first and second polarization splitters include amorphous silicon in one waveguide of a symmetric Mach-Zehnder interferometer and a heater in the other waveguide.
The first and second phase adjustment regions and the first and second variable couplers are configured with heaters in both waveguides of the symmetric Mach-Zehnder interferometer,
The polarization delay region is a Mach-Zehnder interferometer, and the length of one waveguide is different from the other waveguide,
Furthermore, among the one waveguide of the first phase adjustment region, the first half-wave plate is interposed at a position closer to the input side than the portion provided with the heater,
A second half-wave plate is interposed in any one of the two waveguides in the polarization delay region;
A polarization mode dispersion suppressing apparatus characterized in that the polarization state can be changed by changing the amount of heat generated by a heater of a polarization mode dispersion compensator of this planar optical circuit type.

11. The polarization mode dispersion compensator according to claim 1, wherein the polarization mode dispersion compensator is a lithium niobate type polarization mode dispersion compensator, and the lithium niobate type polarization mode dispersion compensator is ,
A plurality of first group of comb electrodes to which a voltage of a first voltage value is applied and a plurality of second electrodes to which a voltage of a second voltage value is applied on a waveguide formed of lithium niobate. The comb electrodes of the group are alternately arranged, and a ground electrode is further arranged between the comb electrodes,
When the beat length of the optical signal is Λ, the interval between the comb electrodes is alternately Λ / 4 and 3Λ / 4 from the input side to the output side, and is applied to the comb electrodes. A polarization mode dispersion suppressing device characterized in that the polarization state can be changed by changing the voltage to be applied.

12. The operation speed of the optical exOR circuit is slower than the transmission speed of the input optical signal, and the determination of “0” or “1” is performed based on the average value of the optical signal. The polarization mode dispersion suppressing device according to any one of the above.