JP3789784B2 - Optical orthogonal frequency division multiplexing transmission system and transmission method - Google Patents

Description

で定まる位相関係をもって前記合波手段が合波して出力するように前記遅延手段から出力される信号Ｅ _k の位相を調整して前記合波手段へ出力する位相調整手段と、前記合波手段のＮ個の出力端子の後に接続され、該合波されたＮ個のそれぞれの信号中から、最も遅延の少ない信号を基準として（Ｎ−１）／Ｎ／Δｆ［ｓ］からＴ［ｓ］（但し、Ｔは１ビットの時間）の時間の信号を取り出すＮ個の時間ゲート手段と、を含む光離散フーリエ変換回路及び、前記時間ゲート手段から出力される信号を受信するＮ個の光受信器を有する光受信部と、前記光送信部、あるいは前記光送信部と前記光受信部との間の光伝送路に配置され、前記光離散フーリエ変換回路の入力における全ての各波長信号のビット位相を一致させるためのビット位相調整手段とを備えたことを特徴とする光直交周波数分割多重伝送方式である。
【０００８】
本発明は、上記に記載の発明において、前記分岐手段は１×Ｎ分岐素子であり、前記遅延手段は、光路長がΔＬ＝ｃ／（Ｎｎ _c Δｆ）（ここで、ｃは光速、ｎ _c は導波路の等価屈折率）ずつ異なるＮ本の遅延導波路であり、前記合波手段はＮ×Ｎ合波素子であり、前記位相調整手段は前記Ｎ本の遅延導波路及び前記Ｎ×Ｎ合波素子に含まれており、前記１×Ｎ分岐素子、前記光路長がΔＬ＝ｃ／（Ｎｎ _c Δｆ）ずつ異なるＮ本の遅延導波路、及び前記Ｎ×Ｎ合波素子がこの順に光学的に結合されて構成されることを特徴とする。
【０００９】
本発明は、上記に記載の発明において、前記１×Ｎ分岐素子が、多モード干渉型１×Ｎ分岐素子であり、前記Ｎ×Ｎ合波素子が、多モード干渉型Ｎ×Ｎ合波素子であることを特徴とする。
【００１０】
本発明は、上記に記載の発明において、前記光離散フーリエ変換回路は、２ ^m ≧Ｎとなる整数ｍに対して、ΔＬ＝ｃ／（２ ^m ｎ _c Δｆ）（ここで、ｃは光速、ｎ _c は導波路の等価屈折率）とおくと、光路長差が２ ^m-1 ΔＬ、２ ^m-2 ΔＬ、…、ΔＬである非対称マッハツェンダカップラを前段の非対称マッハツェンダカップラの２つの出力端子のそれぞれと後段の非対称マッハツェンダカップラの入力端子とが接続するように光学的に多段に結合して構成した手段であって、２ ^ｍ 個の信号を出力し、各非対称マッハツェンダカップラは位相シフタを備えており、位相シフタを調整することにより、光路長がそれぞれΔＬだけ異なる２ ^ｍ 個の信号を以下の式
【数５】

で定まる位相関係をもって合波した２ ^ｍ 個の信号を出力する手段と、該出力された２ ^ｍ 個のそれぞれの信号中から、最も遅延の少ない信号を基準として（２ ^ｍ −１）／２ ^ｍ ／Δｆ［ｓ］からＴ［ｓ］（但し、Ｔは１ビットの時間）の時間の信号を取り出す時間ゲート手段と、を含むことを特徴とする。
【００１１】
本発明は、上記に記載の発明において、前記ビット位相調整手段は、前記光送信器に電気的に接続され、前記光送信器における変調素子への電気的変調信号のビット位相を制御することにより前記光離散フーリエ変換回路の入力における全ての各波長信号のビット位相を一致させることを特徴とする。また、本発明は、上記に記載の発明において、前記ビット位相調整手段は、前記光送信器から前記光受信器の間に配置された光伝送経路に設けられ、光信号の光路長を制御して光学的なビット位相を調整することにより前記光離散フーリエ変換回路の入力における全ての各波長信号のビット位相を一致させることを特徴とする。
【００１２】
本発明は、上記に記載の発明において、前記時間ゲート手段は、光ゲートスイッチであることを特徴とする。また、本発明は、上記に記載の発明において、前記時間ゲート手段は、光受信部における電気ゲート回路であることを特徴とする。
【００１３】
本発明は、光信号を発生する光送信器、及び前記光信号を合波する手段を有する光送信部と、光周波数間隔に等しい標本化周波数Δｆ［Ｈｚ］で、ある一つのビットに注目して離散フーリエ変換の信号処理をし、Ｎ個の出力端子から離散フーリエ変換の結果として、光送信器の光周波数の基準となる光周波数ｆ _０ に対して０、Δｆ、２Δｆ、…、（Ｎ−１）Δｆの各光周波数における係数を出力する光離散フーリエ変換回路、及び前記光離散フーリエ変換回路から出力される信号を受信する光受信器とを有する光受信部と、信号のビット位相を調整するビット位相調整手段とを備えた光直交周波数分割多重伝送方式における伝送方法であって、前記光送信器が、変調素子により光周波数間隔Δｆ［Ｈｚ］、変調速度Ｂ［ｂｉｔ／ｓ］（但し、Ｂ／Δｆ≦１［ｂｉｔ／ｓ／Ｈｚ］）で変調したＮ波（Ｎは２以上の整数）の光信号を発生し、前記合波する手段が、発生される該光信号を合波して送信し、前記ビット位相調整手段が、前記光送信器が発生する信号に対してビット位相を制御することにより前記光離散フーリエ変換回路の入力における全ての各波長信号のビット位相を一致させ、前記光離散フーリエ変換回路が、ビット位相を一致させた多波長信号をＮ分岐し、該分岐した信号に対してそれぞれｋ／Ｎ／Δｆ［ｓ］（ここで、ｋは０からＮ−１までの整数）で定まる時間のビット位相を遅延させたＮ個の信号Ｅ _ｋ を出力し、前記光離散フーリエ変換回路が、出力された信号Ｅ _ｋ を合波して出力する際に、ｌ番目（ここで、ｌは０からＮ−１までの整数）に合波して出力する信号が以下の式
【数６】

で定まる位相関係を有するように信号Ｅ _k の位相を調整し、前記光離散フーリエ変換回路が、位相が調整された信号を合波して出力し、該合波されたＮ個のそれぞれの信号中から、最も遅延の少ない信号を基準として（Ｎ−１）／Ｎ／Δｆ［ｓ］からＴ［ｓ］（但し、Ｔは１ビットの時間）の時間の信号を取り出して出力し、前記光受信器が、前記光離散フーリエ変換回路から出力される信号を受信することを特徴とする伝送方法である。
【００１４】
【発明の実施の形態】
以下、図面を参照して本発明の実施の形態について説明する。図１に本発明の実施の第１の形態を示す。送信部において、光周波数間隔Δｆ［Ｈｚ］で並ぶＮ個の光送信器１−１₀〜１−１_N-1は、変調速度Ｂ［ｂｉｔ／ｓ］で変調しており、帯域利用効率Ｂ／Δｆ［ｂｉｔ／ｓ／Ｈｚ］が１以下且つ１に近い値になるように変調速度が設定されている。すなわち、各送信器１−１₀〜１−１_N-1は、光周波数ｆ₀、ｆ₁＝ｆ₀＋Δｆ、ｆ₂＝ｆ₀＋２Δｆ、…、ｆ_N-1＝ｆ₀＋（Ｎ−１）Δｆの周波数の光信号を発生するとともに、これを変調速度Ｂでそれぞれ変調している。
【００１５】
光送信器１−１₀〜１−１_N-1は電気的なビット位相調整手段１−２₀〜２−１_N-1によって光送信器における変調素子への電気的変調信号のビット位相を制御できるようになっており、ある一つのビットに注目して信号処理ができるように、光離散フーリエ変換回路１−５の入力端において全光周波数（＝波長）の信号のビット位相が一致するように位相シフタなどを用いて電気信号のビット位相を調整する。送信部の最後ではＮ個の光送信器からの光信号が合波器１−３により合波され、一括して一つの光伝送媒体１−４を伝送される。
【００１６】
ここで、光信号が等しい光周波数間隔Δｆ［Ｈｚ］で並ぶとき、標本化周波数Δｆの離散フーリエ変換回路により各波長に分離できることを説明する。
【００１７】
波長多重信号は光伝送媒体１−４を伝送された後、光受信部において、光離散フーリエ変換回路１−５によって標本化周波数Δｆで離散フーリエ変換され、Ｎ個の出力端子からフーリエ変換の結果が出力される。Ｎ個の出力端子から出力される離散フーリエ変換の結果はそれぞれ、周波数０、Δｆ、２Δｆ、…、（Ｎ−１）Δｆの係数であり、送信器１−１₀〜１−１_N-1の光周波数も基準となる光周波数ｆ₀に対して０、△ｆ、２△ｆ、…、（Ｎ−１）Δｆの差を有しているので、離散フーリエ変換回路により各光周波数（＝波長）の信号成分が得られることがわかる。すなわち、離散フーリエ変換回路は光周波数分離回路の働きをする。
【００１８】
各波長に分離された光信号は光受信器１−６₀〜１−６_N-1により受信され、高密度な波長多重光伝送が実現される。
【００１９】
光離散フーリエ変換回路１−５の出力端子からみると、フーリエ変換により所望以外の他の周波数成分はすべて打ち消しあう。すなわち、入力側の光周波数成分が互いに直交しているということである。したがって、類似の無線通信方式の呼び名を取って、本発明の波長多重光伝送方式を光直交周波数分割多重伝送方式（光ＯＦＤＭ伝送方式）と呼ぶことにする。
【００２０】
図２に本発明の実施の第２の形態を示す。ここで、図１に示すものに対応する構成には同一の参照符号を付けている（以下、同じ）。第１の実施の形態との差はビット位相調整手段の違いである。ビット位相調整手段として、光送信器１−１₀〜１−１_N-1の各出力と合波器１−３との間に設けたものである可変遅延線などの光学的なビット位相調整手段２−１₀〜２−１_N-1を用い、光路長を調整して、光離散フーリエ変換回路１−５の入力端における全波長の信号のビット位相を一致させる。
【００２１】
また、図２では送信部で光学的なビット位相調整手段を用いているが、光送信部から受信部の間の光伝送経路１−４において、任意の地点で波長分散媒質など波長によって光路長が変化する素子による光学的なビット位相調整手段を用いて、各波長のビット位相を一致させても良いもよい。
【００２２】
図３に本発明の実施の第３の形態を示す。図３は、図１および図２を参照して説明した光離散フーリエ変換回路１−５（１−５ａとする。）の具体例を示したものである。図は波長数Ｎ＝４の場合を示している。この発明の構成および動作を図を用いて説明する。送信部は本発明の実施の第１の形態と同じであるので構成および動作の説明を省略する。
【００２３】
受信部の光離散フーリエ変換回路１−５ａは、１×Ｎ分岐素子３−１、Ｎ本の遅延導波路３−２₀〜３−２₃、およびＮ×Ｎ合波素子３−４、ならびに光時間ゲートスイッチ３−５₀〜３−５₃から構成される。この構成がなぜ離散フーリエ変換回路になるかという説明は後で説明することにして、まず、各素子の動作を説明する。
【００２４】
光伝送媒体１−４を伝送された後、波長多重信号は、まず、１×Ｎ分岐素子３−１によりＮ分岐される。遅延導波路３−２₀〜３−２₃はそれぞれ光路長がΔＬ＝ｃ／（Ｎｎ_cΔｆ）（ここで、ｃは光速、ｎ_c導波路の等価屈折率）だけ異なっており、分岐された信号は最も短い導波路を基準にして、それぞれ０、１／Ｎ／△ｆ、２／Ｎ／Δｆ、…、（Ｎ−１）／Ｎ／Δｆの時間だけ遅延される。それぞれの信号をＥｉｎ_k（ｋ＝０...Ｎ−１）とすると、Ｎ×Ｎ合波素子３−４により、それぞれ以下の式
【００２５】
【数７】

【００２６】
で定まる位相関係をもって合波され、Ｎ個の端子に出力される。一般に波長の長さ以下の光路長差は位相差に相当する。位相シフタ３−３₁〜３−３₃は製造の誤差を調整するためのものである。製造の誤差が充分小さい場合は、式（１）にしたがって、遅延導波路の光路長差を波長の長さ以下の精度まで制御することにより、位相シフタは省略できる。このことは、他の実施例の位相シフタについても同じである。式（１）では位相の符号は後の数学的説明に合わせるためにマイナス符号を取ったが、プラスでもマイナスでも良い。
【００２７】
１×Ｎ分岐素子３−１は、より具体的には、多モード干渉型１×Ｎ分岐素子、１×Ｎスターカップラなど光を直接Ｎ分岐する様々な手段、あるいは、１×２カップラの多段接続など多段接続によりＮ分岐する様々な手段によって実現できる。
【００２８】
Ｎ×Ｎ合波素子３−４は、より具体的には、図４に示すようにＮ本（この場合Ｎ＝４）の遅延導波路の出力をそれぞれ１×Ｎ分岐素子３−１₀〜３−１₃で分岐した後に位相調整用の位相シフタ３−３、３−３、…を介してＮ個のＮ×１合波素子４−１₀〜４−１₃（１×Ｎ分岐素子の入出力を逆にしたものによって実現できる）で合波する方法や単一素子としては多モード干渉型Ｎ×Ｎ合波素子など、Ｎ本の遅延導波路の出力を式（１）の位相関係で合波する様々な手段によって実現できる。多モード干渉型Ｎ×Ｎ合波素子を用いる場合は、多モード干渉型合波素子内の位相変化を考慮して式（１）の位相項を補正する必要があるが、その補正値ついては学術論文「L.O.Lierstuen et al. IEEE Photon. Tech. Lett., Vol.7, No.9, pp. 1037-1036 (1995)」に示された方法によって解くことができる。
【００２９】
さらに、Ｎ×Ｎ合波素子３−４のＮ個の出力端子から、時間ゲート３−５₀〜３−５₃によって、最も遅延の少ない信号を基準として（Ｎ−１）／Ｎ／△ｆ〜Ｔ（但し、Ｔは１ビットの時間）の時間に於ける信号が取り出され、各波長（＝光周波数）に分離される。
【００３０】
前述したように、離散フーリエ変換回路により各光周波数（＝波長）の信号成分が得られることから、各波長に分離された信号は光受信器１−６₀〜１−６₃により受信され、高密度な波長多重光伝送が実現される。
【００３１】
以下数式をもって、１×Ｎ分岐素子３−１、Ｎ本の遅延導波路３−２₀〜３−２₃、およびＮ×Ｎ合波素子３−４および光時間ゲートスイッチ３−５から構成される光回路が光離散フーリエ変換回路の動作をすることを説明する。
【００３２】
ある１つのビットに注目し、時間間隔Δｔ＝１／Ｎ／Δｆ毎に標本化したと仮定し、ｋ番目の標本化値をｘ_k＝ｘ（ｋΔｔ）とすると、離散的フーリエスペクトルＸｌ＝Ｘｌ（ｌΔω）（但し、Δω＝２πΔｆ）は離散フーリエ変換の公式により
【００３３】
【数８】

【００３４】
と表される。式（１）と式（２）を比較すると、０、１／Ｎ／Δｆ、２／Ｎ／Δｆ、…、（Ｎ−１）／Ｎ／Δｆの時間だけ遅延した信号ｘ（ｋ／（ＮΔｆ））を式（１）のような位相関係で合波したＮ×Ｎ合波素子の出力が離散フーリエ変換の結果を示すことがわかる。以上により、本発明の実施の第３の形態により離散フーリエ変換が実現されることがわかる。
【００３５】
注目しているある１つのビットの合波部分のみを取り出すためには、（Ｎ−１）／Ｎ／Δｆ〜Ｔの時間の信号を時間ゲートスイッチにより取り出すことが必要である。以下図５をもって、この光離散フーリエ変換回路１−５ａの動作のうち、時間ゲートスイッチ３−５₀〜３−５₃の動作を説明する。ある一つのビットに注目すると遅延導波路の出力Ｅｉｎ_k（ｋ＝０...３）はそれぞれ図の様に遅延される。これらを合波すると、３／（４Δｆ）〜Ｔの時間部分のみが全てのＥｉｎ_kの合波になっていることがわかる。この時間部分のみが式（２）の結果を示すので、時間ゲートスイッチにより３／（４Δｆ）〜Ｔの時間の時間の信号を取り出すとＥｉｎ_kの合波成分が取り出せて、光離散フーリエ変換が実現される。
【００３６】
図３の形態では、時間ゲートとして光時間ゲートスイッチ３−５〜３−５₃を用いている。光時間ゲートスイッチとして例えば、電界吸収型変調器やマッハツェンダ型の強度変調器をスイッチとして用いたものや非線形光学効果を用いた全光スイッチなどで実現できる。
【００３７】
あるいは、図６の本発明の実施の第４の形態のように、光受信器５−２₀〜５−２₃で電気信号に変換した後、識別回路の識別タイミングを調整して、時間ゲートとして特定の時間の電圧を検出する方法など、電気的な時間ゲート回路５−１₀〜５−１₃によっても実現できる。すなわち、図６に示す光離散フーリエ変換回路１−５ｂでは、図３に示す光離散フーリエ変換回路１−５ａにおける光時間ゲートスイッチ３−５₀〜３−５₃を省略するとともに、光受信器１−６₀〜１−６₃に対応する光受信器５−２₀〜５−２₃内に電気的時間ゲート回路５−１₀〜５−１₃を設けている。
【００３８】
図７に本発明の実施の第５の形態を示す。光離散フーリエ変換回路１−５（１−５ｃとする。）の別の具体例を示したものである。図は波長数Ｎ＝４の場合を示している。
【００３９】
この発明の構成および動作を図を用いて説明する。光離散フーリエ変換回路以外の部分は実施の第１および第３の形態と同でであるので、光離散フーリエ変換回路１−５ｃのみを説明する。
【００４０】
整数ｍを２^m≧Ｎとなるように選び、△Ｌ＝ｃ／（２^mｎ_cΔｆ）（ここで、ｃは光速、ｎ_c導波路の等価屈折率）とおく。光離散フーリエ変換回路１−５ｃでは周波数多重信号は、まず、非対称マッハツェンダカップラ６−１₁により２^m-1ΔＬの光路長差、即ち１／２／Δｆの遅延時間差をもって合波される。このとき、信号波長の一つと非対称マッハツェンダカップラの透過スペクトルのピークが一致するように位相シフタ３−６₁を調整する。出力のそれぞれは次の非対称マッハツェンダカップラ６−１₂〜６−１₃に結合され、さらに２^m-2ΔＬの光路長差、即ち１／４／Δｆの遅延時間差をもって合波される。このとき、信号波長のピークと非対称マッハツェンダカップラの透過スペクトルのピークが一致するように位相シフタ３−６₂〜３−６₃を調整する。
【００４１】
これらをｍ回繰り返し、最後は２^m-1個の非対称マッハツェンダカップラにより、ΔＬの光路長差、即ち１／Ｎ／Δｆの遅延時間差をもって合波され、２^m個の出力を得る。このとき、信号波長のピークと非対称マッハツェンダカップラの透過スペクトルのピークが一致するように位相シフタを調整する。このように構成すると、２^m個の出力信号は光路長がそれぞれΔＬだけ異なる２^m個の信号を式（１）の位相関係でもって合波したことになる。
【００４２】
ここで、非対称マッハツェンダカップラの結合順序を遅延時間差の大きい順に並べて説明したが、結合する順序は任意であり、どのような順番で結合してもよい。
【００４３】
出力された２^m個の各端子から、時間ゲート３−５₀〜３−５₃によって、最も遅延の少ない信号を基準として（２^m−１）／２^m／Δｆ〜Ｔの時間の信号を取り出す。以上のようにして光離散フーリエ変換が実現される。
【００４４】
図８〜図１０に本発明の実施の第３の形態の構成を用いたシミュレーション結果を示す。波長間隔５［ＧＨｚ］、ビットレート５［Ｇｂ／ｓ］、Ｎ＝４とした。帯域利用効率は１［ｂｉｔ／ｓ／Ｈｚ］となる。光離散フーリエ変換回路の前、および後、そして光ゲートの後のアイダイアグラムを示す。十分なアイダイアグラムが開いていることがわかる。したがって、本発明により帯域利用効率の良い波長多重光伝送方式が実現できる。
【００４５】
【発明の効果】
以上、説明したように、本発明により、帯域利用効率の良い波長多重光伝送方式及び伝送方法が実現できる。これにより、従来と同じ波長帯域幅で、より多くの波長数を有する波長多重伝送方式を実現でき、光通信の伝送容量の増大に貢献することができる。
【００４６】
また、帯域の狭い光伝送路は帯域の広い光伝送路に比べて安価に構築できるため、伝送路の構築コストが光伝送方式のコストの多くを占めるような光伝送方式においては、本発明によりコストの削減が可能となる。
【図面の簡単な説明】
【図１】本発明の第１の実施形態の構成を示すブロック図
【図２】本発明の第２の実施形態の構成を示すブロック図
【図３】本発明の第３の実施形態の構成を示すブロック図
【図４】図３のＮ×Ｎ合波素子３−４の構成例を示すブロック図
【図５】図３の光時間ゲートスイッチ３−５₀〜３−５₃の動作を説明するためのタイミングチャート
【図６】本発明の第４の実施形態の構成を示すブロック図
【図７】本発明の第５の実施形態の構成を示すブロック図
【図８】本発明の第３の実施形態によるシミュレーション結果を示すアイダイアグラム（光離散フーリエ変換回路１−５ａ前の波形）
【図９】本発明の第３の実施形態によるシミュレーション結果を示すアイダイアグラム（光離散フーリエ変換回路１−５ａ後の波形）
【図１０】本発明の第３の実施形態によるシミュレーション結果を示すアイダイアグラム（光時間ゲート回路３−５₀〜３−５₃の後の波形）
【図１１】従来の波長多重光伝送方式の構成を示すブロック図
【符号の説明】
１−１₀〜１−１_N-1 光送信器
１−２₀〜１−２_N-1 電気的ビット位相調整手段
１−４ 光伝送媒体
１−５，１−５ａ〜１−５ｃ 光離散フーリエ変換回路
１−６₀〜１−６_N-1，５−２₀〜５−２₃ 光受信器
２−１₀〜２−１_N-1 光学的ビット位相調整手段
３−１，３−１₀〜３−１₃ １×Ｎ分岐素子
３−２₀〜３−２₃ 遅延導波路
３−３₁〜３−３₃，３−６₁〜３−６₃ 位相シフタ
３−４ Ｎ×Ｎ合波素子
３−５₀〜３−５₃ 光時間ゲートスイッチ
４−１₀〜４−１₃ Ｎ×１合波素子
５−１₀〜５−１₃ 電気的時間ゲートスイッチ
６−１₁〜６−１₃ 非対称マッハツェンダカップラ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength division multiplexing optical transmission system and transmission method in the field of optical communication.
[0002]
[Prior art]
In order to increase the transmission capacity of optical communication, a wavelength division multiplexing optical transmission system that transmits signals using a large number of wavelengths is used.
[0003]
FIG. 11 is a block diagram of a conventional wavelength division multiplexing optical transmission system. In the transmission unit, output signals of a plurality of optical transmitters 1-1 having different wavelengths (= optical frequencies) are multiplexed by the wavelength multiplexer 8-1. A number of combined optical signals are transmitted using a single optical transmission medium 1-4. In the reception unit, the received optical signal is demultiplexed for each wavelength by the wavelength demultiplexer 8-2, and the optical signals of each wavelength are received by the plurality of optical receivers 1-6. The wavelength division multiplexing optical transmission system has been realized with the above configuration.
[0004]
From the viewpoint of effective use of the wavelength range of light, high-density wavelength division multiplexing transmission in which the wavelength intervals of light are close to each other is desired. When the optical frequency interval is Δf [Hz] and the transmission speed is B [bit / s], B / Δf [bit / s / Hz] is referred to as band efficiency (Spectral Efficiency). The theoretical limit of the band use efficiency of the ON / OFF modulation method of the double sideband is 1 [bit / s / Hz].
[0005]
In the conventional wavelength division multiplex transmission system, a desired signal light is extracted from a receiving unit by a wavelength selection filter using a wavelength demultiplexer such as an arrayed waveguide type diffraction grating. However, when a large number of signals are transmitted at a high density, signals of adjacent wavelengths are overlapped with each other, so that there is a problem that a desired signal cannot be separated by the wavelength selection filter. If the wavelength interval is widened to prevent interference with adjacent frequencies, the band utilization efficiency decreases. In the conventional wavelength division multiplexing transmission system, the band utilization efficiency is usually about 0.4 [bit / s / Hz] or less.
[0006]
[Problems to be solved by the invention]
When a large number of signals are transmitted at a high density, signals of adjacent wavelengths are overlapped with each other, so that there is a problem that the wavelength selection filter used in the conventional wavelength multiplexing transmission system cannot separate the signals. On the other hand, when the wavelength interval is widened, the conventional wavelength multiplexing transmission system has a problem that the band use efficiency is poor. An object of the present invention is to provide a wavelength division multiplexing optical transmission system and transmission method with good band utilization efficiency.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the present invention provides N modulated by a modulation element at an optical frequency interval Δf [Hz] and a modulation speed B [bit / s] (B / Δf ≦ 1 [bit / s / Hz]). An optical transmitter that generates an optical signal of a wave (N is an integer of 2 or more), an optical transmitter that is connected after the optical transmitter and multiplexes the optical signal, and sampling that is equal to the optical frequency interval At a frequency Δf [Hz], signal processing of discrete Fourier transform is performed by paying attention to a certain bit, and the optical frequency f serving as a reference of the optical frequency of the optical transmitter is obtained as a result of the discrete Fourier transform from N output terminals. ₀ for 0, Δf, 2Δf, ..., (N-1) an optical discrete Fourier transform circuit for outputting a coefficient in each light frequency Delta] f, are connected after the means for multiplexing said optical signal, input N-branch multi-wavelength signal with matched bit phase And a bit phase of a time determined by k / N / Δf [s] (where k is an integer from 0 to N−1) for each of the branched signals connected after the branching means. delay means for outputting N signals E _k obtained by delaying, and multiplexing means for outputting a signal E _k delayed bit phase k / N / Δf [s] only multiplexes by said delay means, said Provided between the delay means and the multiplexing means, to the l-th output terminal of the N output terminals of the multiplexing means (where l is an integer from 0 to N-1), Number 4]

A phase adjusting means for adjusting the phase of the signal E _k output from the delay means so that the combining means outputs a combined signal having a phase relationship determined by: Are connected after N output terminals, and (N−1) / N / Δf [s] to T [s] with reference to the signal with the least delay among the combined N signals. An optical discrete Fourier transform circuit including N time gate means for extracting a signal having a time of T (where T is a time of 1 bit), and N optical receivers for receiving a signal output from the time gate means. A bit of each wavelength signal at the input of the optical discrete Fourier transform circuit, arranged in the optical transmission path between the optical receiver having the optical device and the optical transmitter or between the optical transmitter and the optical receiver. Bit phase adjuster for phase matching An optical orthogonal frequency division multiplex transmission system characterized by comprising a stage.
[0008]
According to the present invention, in the invention described above, the branching unit is a 1 × N branching element, and the delay unit has an optical path length of ΔL = c / (Nn _c Δf) (where c is the speed of light, n _c Are N delay waveguides, each of which is equivalent to the equivalent refractive index of the waveguide), the multiplexing means is an N × N multiplexing element, and the phase adjusting means is the N delay waveguides and the N × N included in the multiplexing element, said 1 × N splitter, delay waveguide of the optical path length _{ΔL = c / (Nn c Δf} ) by different N present, and the N × N multiplexer element optical in this order It is characterized by being connected to each other.
[0009]
The present invention is the above-described invention, wherein the 1 × N branching element is a multimode interference type 1 × N branching element, and the N × N multiplexing element is a multimode interference type N × N multiplexing element. It is characterized by being.
[0010]
The present invention, in the invention described above, the optical discrete Fourier transform circuit, for integer m to be ^{2 m ≧ N, ΔL = c} / (2 m n c Δf) ( where, c is the speed of light, n _c is the effective refractive index of the waveguide) and putting, the optical path length difference ^{2 m-1 ΔL, 2 m} -2 ΔL, ..., asymmetric Mach-da coupler of the preceding asymmetric Mach-Zehnder coupler two output terminals is [Delta] L Each of the asymmetrical Mach-Zehnder couplers is a means that is optically coupled in multiple stages so that the input terminals of the subsequent asymmetrical Mach-Zehnder couplers are connected to each other and outputs 2 ^m signals. By adjusting the phase shifter, 2 ^m signals whose optical path lengths are different from each other by ΔL are expressed as follows :

(2 ^m −1) / 2 ^m on the basis of a signal with the least delay among the 2 ^m signals output from the 2 ^m signals combined with a phase relationship determined by Time gate means for extracting a signal having a time of T [s] (where T is a time of 1 bit) from / Δf [s].
[0011]
According to the present invention, in the above-described invention, the bit phase adjusting means is electrically connected to the optical transmitter, and controls the bit phase of the electric modulation signal to the modulation element in the optical transmitter. The bit phases of all the wavelength signals at the input of the optical discrete Fourier transform circuit are matched. According to the present invention, in the above-described invention, the bit phase adjusting means is provided in an optical transmission path disposed between the optical transmitter and the optical receiver, and controls an optical path length of the optical signal. By adjusting the optical bit phase, the bit phases of all the wavelength signals at the input of the optical discrete Fourier transform circuit are matched.
[0012]
The present invention is characterized in that, in the invention described above, the time gate means is an optical gate switch. In the invention described above, the time gate means is an electric gate circuit in an optical receiver.
[0013]
The present invention focuses on an optical transmitter that generates an optical signal, an optical transmitter having means for multiplexing the optical signal, and a single bit at a sampling frequency Δf [Hz] equal to the optical frequency interval. The signal processing of the discrete Fourier transform is performed, and as a result of the discrete Fourier transform from the N output terminals , 0, Δf, 2Δf,..., (N) with respect to the optical frequency f _{0 serving as} a reference of the optical frequency of the optical transmitter. -1) An optical receiver having an optical discrete Fourier transform circuit that outputs a coefficient at each optical frequency of Δf, and an optical receiver that receives a signal output from the optical discrete Fourier transform circuit, and a bit phase of the signal A transmission method in an optical orthogonal frequency division multiplex transmission system including a bit phase adjustment means for adjusting, wherein the optical transmitter uses a modulation element to modulate an optical frequency interval Δf [Hz] and a modulation speed B [bit / s] ( However, B [Delta] f≤1 [bit / s / Hz]) modulated N-wave (N is an integer of 2 or more) optical signal is generated, and the multiplexing means multiplexes the generated optical signal and transmits it The bit phase adjustment means controls the bit phase of the signal generated by the optical transmitter to match the bit phases of all the wavelength signals at the input of the optical discrete Fourier transform circuit; A discrete Fourier transform circuit N-branches a multi-wavelength signal having the same bit phase and k / N / Δf [s] (where k is an integer from 0 to N−1). N signals E _k delayed by the bit phase of the time determined by (1)), and when the optical discrete Fourier transform circuit combines and outputs the output signal E _k , , L is an integer from 0 to N-1) Is the following formula :

The phase of the signal E _k is adjusted so as to have a phase relationship determined by: the optical discrete Fourier transform circuit multiplexes and outputs the signals whose phases are adjusted, and each of the combined N signals A signal having a time of (N−1) / N / Δf [s] to T [s] (where T is a time of 1 bit) is extracted from the signal with the least delay as a reference, and is output. In the transmission method, the receiver receives a signal output from the optical discrete Fourier transform circuit.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention. In the transmission unit, N optical transmitters 1-1 _{0 to} 1-1 _N-1 arranged at the optical frequency interval Δf [Hz] are modulated at the modulation speed B [bit / s], and the band use efficiency B The modulation speed is set so that / Δf [bit / s / Hz] is 1 or less and close to 1. That is, the transmitters 1-1 _{0 to} 1-1 _N-1 have optical frequencies f ₀ , f ₁ = f ₀ + Δf, f ₂ = f ₀ + 2Δf,..., F _N-1 = f ₀ + (N− 1) An optical signal having a frequency of Δf is generated and modulated at a modulation speed B.
[0015]
The optical transmitters 1-1 _{0 to} 1-1 _N-1 use the electric bit phase adjustment means 1-2 _{0 to} 2-1 _N-1 to change the bit phase of the electrical modulation signal to the modulation element in the optical transmitter. The bit phase of the signal of all optical frequencies (= wavelengths) matches at the input end of the optical discrete Fourier transform circuit 1-5 so that signal processing can be performed while paying attention to one bit. As described above, the bit phase of the electric signal is adjusted using a phase shifter or the like. At the end of the transmission unit, the optical signals from the N optical transmitters are multiplexed by the multiplexer 1-3, and are transmitted collectively through one optical transmission medium 1-4.
[0016]
Here, it will be described that when optical signals are arranged at equal optical frequency intervals Δf [Hz], they can be separated into wavelengths by a discrete Fourier transform circuit having a sampling frequency Δf.
[0017]
The wavelength multiplexed signal is transmitted through the optical transmission medium 1-4, and then subjected to discrete Fourier transform at the sampling frequency Δf by the optical discrete Fourier transform circuit 1-5 in the optical receiver, and the result of Fourier transform from the N output terminals. Is output. The results of the discrete Fourier transform output from the N output terminals are coefficients of frequencies 0, Δf, 2Δf,..., (N−1) Δf, respectively, and the transmitters 1-1 _{0 to} 1-1 _N−1. .., (N−1) Δf with respect to the reference optical frequency f ₀ , each optical frequency (= It can be seen that a signal component of (wavelength) is obtained. That is, the discrete Fourier transform circuit functions as an optical frequency separation circuit.
[0018]
The optical signals separated into the respective wavelengths are received by the optical receivers 1-6 _{0 to} 1-6 _N-1, thereby realizing high-density wavelength division multiplexing optical transmission.
[0019]
When viewed from the output terminal of the optical discrete Fourier transform circuit 1-5, all other frequency components than desired are canceled by the Fourier transform. That is, the optical frequency components on the input side are orthogonal to each other. Therefore, taking the name of a similar wireless communication system, the wavelength division multiplexing optical transmission system of the present invention is called an optical orthogonal frequency division multiplexing transmission system (optical OFDM transmission system).
[0020]
FIG. 2 shows a second embodiment of the present invention. Here, the same reference numerals are assigned to the components corresponding to those shown in FIG. 1 (hereinafter the same). The difference from the first embodiment is the difference in the bit phase adjusting means. As a bit phase adjusting means, optical bit phase adjustment such as the variable delay line in which is provided between the optical transmitter 1-1 ₀ ~1-1 _N-1 of each output and multiplexer 1-3 Means 2-1 _{0 to} 2-1 _N-1 are used to adjust the optical path length so that the bit phases of the signals of all wavelengths at the input end of the optical discrete Fourier transform circuit 1-5 match.
[0021]
In FIG. 2, the optical bit phase adjusting means is used in the transmission unit. However, in the optical transmission path 1-4 between the optical transmission unit and the reception unit, the optical path length depends on the wavelength such as the wavelength dispersion medium at an arbitrary point. The bit phase of each wavelength may be matched by using an optical bit phase adjusting means using an element that changes.
[0022]
FIG. 3 shows a third embodiment of the present invention. FIG. 3 shows a specific example of the optical discrete Fourier transform circuit 1-5 (referred to as 1-5a) described with reference to FIGS. The figure shows the case where the number of wavelengths N = 4. The configuration and operation of the present invention will be described with reference to the drawings. Since the transmission unit is the same as that of the first embodiment of the present invention, description of the configuration and operation is omitted.
[0023]
Light discrete Fourier transform circuit 1-5a of the receiving unit, 1 × N splitter 3-1, N present delay waveguides 3-2 ₀ ～3-2 _3, and N × N multiplexer element 3-4, and It comprises optical time gate switches 3-5 _{0 to} 3-5 ₃ . The reason why this configuration becomes a discrete Fourier transform circuit will be described later. First, the operation of each element will be described.
[0024]
After being transmitted through the optical transmission medium 1-4, the wavelength multiplexed signal is first N-branched by the 1 × N branch element 3-1. (Where, c is the speed of light, the equivalent refractive index of n _c waveguide) delay waveguides 3-2 ₀ ～3-2 ₃ each optical path length _{ΔL = c / (Nn c Δf} ) differing by, branched The signals are delayed by 0, 1 / N / Δf, 2 / N / Δf,..., (N−1) / N / Δf, respectively, with the shortest waveguide as a reference. Assuming that each signal is Ein _k (k = 0... N−1), each of the following equations is obtained by the N × N multiplexing element 3-4.
[Expression 7]

[0026]
Are combined and output to N terminals. In general, an optical path length difference equal to or less than the wavelength length corresponds to a phase difference. The phase shifters 3-3 _{1 to} 3-3 ₃ are for adjusting manufacturing errors. When the manufacturing error is sufficiently small, the phase shifter can be omitted by controlling the optical path length difference of the delay waveguide to an accuracy equal to or less than the wavelength length according to the equation (1). The same applies to the phase shifters of other embodiments. In equation (1), the sign of the phase is a minus sign to match later mathematical explanations, but it may be plus or minus.
[0027]
More specifically, the 1 × N branch element 3-1 is a multimode interference type 1 × N branch element, 1 × N star coupler, or other various means for directly N-branching light, or 1 × 2 coupler multistage. It can be realized by various means for N-branching by multistage connection such as connection.
[0028]
N × N multiplexer element 3-4, and more specifically, N the respective 1 × N splitter 3-1 ₀ output of the delay waveguide (in this case N = 4) as shown in FIGS. 4 to 3-1 ₃ and then N N × 1 multiplexers 4-1 _{0 to} 4-1 ₃ (1 × N branch elements) through phase shifters 3-3, 3-3,. The output of N delay waveguides such as a multimode interference type N × N multiplexing element such as a multi-mode interference type N × N multiplexing element or the like as a single element can be realized by the phase of equation (1). It can be realized by various means of combining in relation. When a multimode interference type N × N multiplexing element is used, it is necessary to correct the phase term of Equation (1) in consideration of the phase change in the multimode interference type multiplexing element. It can be solved by the method shown in the paper “LOLierstuen et al. IEEE Photon. Tech. Lett., Vol.7, No.9, pp. 1037-1036 (1995)”.
[0029]
Further, from the N output terminals of the N × N multiplexing element 3-4, the time gates 3-5 _{0 to} 3-5 ₃ are used to set (N−1) / N / Δf based on the signal with the least delay. A signal at time T to T (where T is a time of 1 bit) is taken out and separated into each wavelength (= optical frequency).
[0030]
As described above, since the signal component of each optical frequency (= wavelength) is obtained by the discrete Fourier transform circuit, the signal separated into each wavelength is received by the optical receivers 1-6 _{0 to} 1-6 ₃ , High-density wavelength division multiplexing optical transmission is realized.
[0031]
It is composed of 1 × N branching element 3-1, N delay waveguides 3-2 _{0 to} 3-2 ₃ , N × N multiplexing element 3-4, and optical time gate switch 3-5 with the following formula. It will be described that the optical circuit operates as an optical discrete Fourier transform circuit.
[0032]
Assuming that one bit is focused and sampled every time interval Δt = 1 / N / Δf, and the k-th sampled value is x _k = x (kΔt), the discrete Fourier spectrum Xl = Xl (LΔω) (where Δω = 2πΔf) is expressed by the formula of the discrete Fourier transform.
[Equation 8]

[0034]
It is expressed. Comparing the equations (1) and (2), the signal x (k / (NΔf) delayed by the time of 0, 1 / N / Δf, 2 / N / Δf,..., (N−1) / N / Δf. It can be seen that the output of the N × N multiplexing element obtained by multiplexing ()) with the phase relationship as in Expression (1) indicates the result of the discrete Fourier transform. From the above, it can be seen that the discrete Fourier transform is realized by the third embodiment of the present invention.
[0035]
In order to extract only the combined portion of one bit of interest, it is necessary to extract a signal having a time of (N−1) / N / Δf to T by a time gate switch. The operation of the time gate switches 3-5 _{0 to} 3-5 ₃ will be described below with reference to FIG. 5 among the operations of the optical discrete Fourier transform circuit 1-5a. When attention is paid to one bit, the output Ein _k (k = 0... 3) of the delay waveguide is delayed as shown in the figure. When these are combined, it can be seen that only the time portion of the 3 / (4Δf) ~T is in combining all Ein _k. Since only this time part shows the result of the formula (2), and retrieve the combined components of retrieving the Ein _k time signal time 3 / (4Δf) ~T by the time gate switches, optical discrete Fourier transform Realized.
[0036]
In the form of FIG. 3, optical time gate switches 3-5 to 3-5 ₃ are used as time gates. As an optical time gate switch, for example, an electroabsorption modulator or a Mach-Zehnder intensity modulator can be used as a switch, or an all-optical switch using a nonlinear optical effect.
[0037]
Alternatively, as in the fourth embodiment of the present invention shown in FIG. 6, after the optical receivers 5-2 _{0 to} 5-2 ₃ convert the signals into electric signals, the identification timing of the identification circuit is adjusted to obtain a time gate. It can also be realized by electrical time gate circuits 5-1 _{0 to} 5-1 ₃ such as a method for detecting a voltage at a specific time. That is, in the optical discrete Fourier transform circuit 1-5b 6, while omitting the optical time gate switch 3-5 ₀ ～3-5 ₃ in the optical discrete Fourier transform circuit 1-5a shown in FIG. 3, the optical receiver is provided with electrical time gate circuit 5-1 ₀ ～5-1 ₃ 1-6 ₀ ～1-6 ₃ corresponding optical receiver 5-2 ₀ ～5-2 ₃ in.
[0038]
FIG. 7 shows a fifth embodiment of the present invention. Another specific example of the optical discrete Fourier transform circuit 1-5 (referred to as 1-5c) is shown. The figure shows the case where the number of wavelengths N = 4.
[0039]
The configuration and operation of the present invention will be described with reference to the drawings. Since parts other than the optical discrete Fourier transform circuit are the same as those of the first and third embodiments, only the optical discrete Fourier transform circuit 1-5c will be described.
[0040]
Select an integer m such that ^{2 m ≧ N, △ L =} c / (2 m n c Δf) ( where, c is the speed of light, the equivalent refractive index of n _c waveguide) put the. Frequency division multiplexed signal in the optical discrete Fourier transform circuit 1-5c, first, the optical path length difference of the asymmetric Mach-Zehnder coupler 6-1 ₁ by 2 ^m-1 [Delta] L, i.e. are combined with a delay time difference between 1/2 / Δf. At this time, the peak of the transmission spectrum of one signal wavelength and the asymmetric Mach-Zehnder coupler adjusts the phase shifter 3-6 ₁ so as to coincide. Each of the outputs is coupled to the following asymmetric Mach-Zehnder couplers 6-1 _{2 to} 6-1 ₃ and further combined with an optical path length difference of 2 ^m−2 ΔL, that is, a delay time difference of 1/4 / Δf. At this time, the phase shifters 3-6 _{2 to} 3-6 ₃ are adjusted so that the peak of the signal wavelength matches the peak of the transmission spectrum of the asymmetric Mach-Zehnder coupler.
[0041]
These operations are repeated m times, and finally, 2 ^m−1 asymmetric Mach-Zehnder couplers are combined with an optical path length difference of ΔL, that is, a delay time difference of 1 / N / Δf, to obtain 2 ^m outputs. At this time, the phase shifter is adjusted so that the peak of the signal wavelength matches the peak of the transmission spectrum of the asymmetric Mach-Zehnder coupler. With this configuration, the 2 ^m output signals are combined with 2 ^m signals having optical path lengths different from each other by ΔL according to the phase relationship of Expression (1).
[0042]
Here, the coupling order of the asymmetric Mach-Zehnder couplers has been described in order of increasing delay time difference. However, the coupling order is arbitrary, and the coupling order may be any order.
[0043]
From the output 2 ^m terminals, the time gates 3-5 _{0 to} 3-5 ₃ generate signals having a time of (2 ^m -1) / 2 ^m / Δf to T with reference to the signal with the least delay. Take out. The optical discrete Fourier transform is realized as described above.
[0044]
8 to 10 show simulation results using the configuration of the third embodiment of the present invention. The wavelength interval was 5 [GHz], the bit rate was 5 [Gb / s], and N = 4. The band utilization efficiency is 1 [bit / s / Hz]. Figure 2 shows an eye diagram before and after an optical discrete Fourier transform circuit and after an optical gate. You can see that enough eye diagrams are open. Therefore, according to the present invention, it is possible to realize a wavelength division multiplexing optical transmission system with good band utilization efficiency.
[0045]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a wavelength division multiplexing optical transmission system and transmission method with good band utilization efficiency. As a result, it is possible to realize a wavelength division multiplexing transmission system having a larger number of wavelengths with the same wavelength bandwidth as that of the prior art, thereby contributing to an increase in the transmission capacity of optical communication.
[0046]
In addition, since an optical transmission line with a narrow band can be constructed at a lower cost than an optical transmission line with a wide band, in the optical transmission system in which the construction cost of the transmission path occupies much of the cost of the optical transmission system, the present invention Cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a first embodiment of the present invention. FIG. 2 is a block diagram showing a configuration of a second embodiment of the present invention. FIG. 3 is a configuration of a third embodiment of the present invention. the block diagram FIG. 4 is a block diagram showing a configuration example of a N × N multiplexer element 3-4 of FIG. 3 and FIG. 5 optical time operation of the gate switch 3-5 ₀ ～3-5 ₃ of Figure 3 showing a FIG. 6 is a block diagram showing a configuration of a fourth embodiment of the present invention. FIG. 7 is a block diagram showing a configuration of a fifth embodiment of the present invention. 3 is an eye diagram showing a simulation result according to the third embodiment (waveform before the optical discrete Fourier transform circuit 1-5a).
FIG. 9 is an eye diagram showing a simulation result according to the third embodiment of the present invention (waveform after the optical discrete Fourier transform circuit 1-5a);
FIG. 10 is an eye diagram showing a simulation result according to the third embodiment of the present invention (waveform after optical time gate circuits 3-5 _{0 to} 3-5 ₃ );
FIG. 11 is a block diagram showing a configuration of a conventional wavelength division multiplexing optical transmission system.
1-1 _{0 to} 1-1 _N-1 optical transmitter 1-2 _{0 to} 1-2 _N-1 electric bit phase adjusting means 1-4 optical transmission media 1-5, 1-5a to 1-5c optical discrete Fourier transform circuits 1-6 _{0 to} 1-6 _N-1 , 5-2 _{0 to} 5-2 ₃ optical receivers 2-1 _{0 to} 2-1 _N-1 optical bit phase adjusting means 3-1, 3- 1 _{0 to} 3-1 ₃ 1 × N branch element 3-2 _{0 to} 3-2 ₃ delay waveguide 3-3 _{1 to} 3-3 ₃ , 3-6 _{1 to} 3-6 ₃ phase shifter 3-4 N × N multiplexing element 3-5 _{0 to} 3-5 ₃ optical time gate switch 4-1 _{0 to} 4-1 ₃ N × 1 multiplexing element 5-1 _{0 to} 5-1 ₃ electrical time gate switch 6-1 ₁ ～6-1 ₃ asymmetric Mach-Zehnder coupler

Claims (9)

Light of N waves (N is an integer of 2 or more) modulated by a modulation element at an optical frequency interval Δf [Hz] and modulation speed B [bit / s] (B / Δf ≦ 1 [bit / s / Hz]) An optical transmitter that generates a signal, and an optical transmitter that is connected after the optical transmitter and has means for combining the optical signals;
Discrete Fourier transform signal processing is performed with a sampling frequency Δf [Hz] equal to the optical frequency interval, paying attention to one bit, and the optical frequency of the optical transmitter is obtained as a result of the discrete Fourier transform from N output terminals. An optical discrete Fourier transform circuit that outputs a coefficient at each optical frequency of 0, Δf, 2Δf,..., (N−1) Δf with respect to the optical frequency f _{0 serving as} a reference of
Branching means connected after the means for multiplexing the optical signals, and for branching the multi-wavelength signal having the same bit phase input into N branches;
N signals which are connected after the branching means and delay the bit phase of the time determined by k / N / Δf [s] (where k is an integer from 0 to N−1) for the branched signals. Delay means for outputting a signal E _k of
Combining means for combining and outputting the signal E _k delayed by k / N / Δf [s] by the delay means by the bit phase;
To the l-th output terminal (where l is an integer from 0 to N−1) of the N output terminals of the multiplexing means provided between the delay means and the multiplexing means,

Phase adjusting means for adjusting the phase of the signal E _k output from the delay means so that the combining means outputs a combined signal having a phase relationship determined by :
From (N−1) / N / Δf [s] , which is connected after the N output terminals of the multiplexing means and is based on the signal with the least delay among the N combined signals. N time gate means for extracting a signal of time T [s] (where T is a time of 1 bit);
An optical discrete Fourier transform circuit including: an optical receiver having N optical receivers for receiving signals output from the time gate means ;
A bit phase that is arranged in the optical transmission path between the optical transmission section or the optical transmission section and the optical reception section, and matches the bit phases of all the wavelength signals at the input of the optical discrete Fourier transform circuit. And an optical orthogonal frequency division multiplex transmission system. The branch unit is 1 × N splitter, the delay means, the optical path length _{ΔL = c / (Nn c Δf} ) ( where, c is the speed of light, n _c is the effective refractive index of the waveguide) different by N A plurality of delay waveguides, the multiplexing means is an N × N multiplexing element, and the phase adjusting means is included in the N delay waveguides and the N × N multiplexing element. × N branching elements, characterized in that the optical path length _{ΔL = c / (Nn c Δf} ) by different N number of delay waveguides, and the N × N multiplexer element is constituted by optically coupled in this order The optical orthogonal frequency division multiplex transmission system according to claim 1 . The 1 × N splitter is a multimode interference-type 1 × N splitter, the N × N multiplexer element, to claim 2, characterized in that the multi-mode interference type N × N multiplexer element The optical orthogonal frequency division multiplexing transmission system described. The optical discrete Fourier transform circuit is:
For an integer ^m satisfying 2 ^m ≧ N, ΔL = c / (2 ^m n _c Δf) (where c is the speed of light and n _c is the equivalent refractive index of the waveguide)
Optical path length difference ^{2 m-1 ΔL, 2 m} -2 ΔL, ..., an input terminal of each and subsequent asymmetric Mahhatsu Endakappura of two output terminals of the asymmetric Mach-Zehnder coupler the preceding asymmetric Mach-Zehnder coupler is [Delta] L is connected In this way, optically coupled in multiple stages, outputs 2 ^m signals, each asymmetric Mach-Zehnder coupler has a phase shifter, and the optical path length is adjusted by adjusting the phase shifter. 2 ^m signals that differ by ΔL

Means for outputting 2 ^m signals combined with a phase relationship determined by
From the 2 ^m output signals, (2 ^m −1) / 2 ^m / Δf [s] to T [s] (where T is a time of 1 bit ) based on the signal with the least delay. ) Time gate means for retrieving the time signal;
The optical orthogonal frequency division multiplex transmission system according to claim 1 , comprising: The bit phase adjusting means includes
The bit phase of all the wavelength signals at the input of the optical discrete Fourier transform circuit is controlled by controlling the bit phase of the electrical modulation signal to the modulation element in the optical transmitter , electrically connected to the optical transmitter . The optical orthogonal frequency division multiplex transmission system according to claim 1, wherein the optical frequency division multiplexing transmission systems are matched . The bit phase adjusting means includes
At the input of the optical discrete Fourier transform circuit by the provided arranged optical transmission path between the optical receiver from the optical transmitter to adjust the optical bit phase by controlling the optical path length of the optical signal 2. The optical orthogonal frequency division multiplexing transmission system according to claim 1, wherein the bit phases of all the wavelength signals are matched . The time gate means is
5. The optical orthogonal frequency division multiplex transmission system according to claim 1 , wherein the optical orthogonal frequency division multiplex transmission system is an optical gate switch. It said time gate means, optical orthogonal frequency division multiplexing transmission method according to any one of claims 1 to 4, characterized in that an electric gate circuit in the optical receiver. An optical transmitter that generates an optical signal, an optical transmitter having means for multiplexing the optical signals, and a discrete Fourier transform focusing on a single bit at a sampling frequency Δf [Hz] equal to the optical frequency interval As a result of the discrete Fourier transform from the N output terminals, the optical frequency f serving as a reference for the optical frequency of the optical transmitter is obtained. ₀ , .DELTA.f, 2.DELTA.f,..., (N-1) .DELTA.f optical frequency discrete Fourier transform circuit that outputs coefficients and optical receivers that receive signals output from the optical discrete Fourier transform circuit. A transmission method in an optical orthogonal frequency division multiplex transmission system, comprising: an optical receiving unit including: a bit phase adjusting unit that adjusts a bit phase of a signal;
N wave (N is 2) modulated by the optical transmitter with an optical frequency interval Δf [Hz] and modulation speed B [bit / s] (where B / Δf ≦ 1 [bit / s / Hz]) by the modulation element. The optical signal of the above integer), and the means for multiplexing combines and transmits the generated optical signal,
The bit phase adjustment means matches the bit phase of all the wavelength signals at the input of the optical discrete Fourier transform circuit by controlling the bit phase with respect to the signal generated by the optical transmitter,
The optical discrete Fourier transform circuit N-branches a multi-wavelength signal having the same bit phase, and k / N / Δf [s] (where k is from 0 to N−1), respectively. N signals E obtained by delaying the bit phase for a time determined by _k Output
The optical discrete Fourier transform circuit outputs an output signal E _k When the signal is combined and output, the signal that is combined and output to the l-th (where l is an integer from 0 to N-1) is

The signal E so as to have a phase relationship determined by _k Adjust the phase of
The optical discrete Fourier transform circuit synthesizes and outputs the signal whose phase is adjusted, and uses the signal with the smallest delay as a reference from among the N multiplexed signals. Extract and output a signal from N / Δf [s] to T [s] (where T is a 1-bit time),
The optical receiver receives a signal output from the optical discrete Fourier transform circuit.
  A transmission method characterized by the above.

Images