JP3547607B2

JP3547607B2 - Wavelength division multiplexing type optical transmission system

Info

Publication number: JP3547607B2
Application number: JP05394298A
Authority: JP
Inventors: 淳一可児; 正彦神野; 匡阪本
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 1998-03-05
Filing date: 1998-03-05
Publication date: 2004-07-28
Anticipated expiration: 2018-03-05
Also published as: JPH11252050A

Description

【０００１】
【発明の属する技術分野】
本発明は、光送受信装置により零分散波長が１５５０ｎｍ付近にないノンゼロ分散シフトファイバを伝送路として波長分割多重光信号を伝送する波長分割多重型光伝送システムに関するものである。
【０００２】
【従来の技術】
光波長分割多重技術とは、互いに波長の異なる複数の光を複数の電気信号により変調し、これらを光波長フィルタ等の光受動部品により波長多重化して、光ファイバ伝送路に送出し、受信側では波長多重化された信号光を波長毎に分離し、それらを光電気変換してもとの複数の電気信号に復調する技術である。この技術は、複数の信号の多重分離を光受動部品のみによって容易に行うことができ、伝送システムの大容量化に有効である。
光波長分割多重に使用する複数の信号光がエルビウム添加ファイバ増幅器（ＥＤＦＡ）の利得帯域である波長１５３０ｎｍ〜１５６５ｎｍに配置された場合、波長分散による波形劣化を最小化するために同じ１５５０ｎｍ付近に零分散波長が配置された分散シフトファイバにおいては、四光波混合の発生によって信号品質が劣化するという問題があった。
【０００３】
ここで「四光波混合」とは、光周波数ｆ_１、ｆ_２、ｆ_３を持つ３つの光が伝搬媒体との非線形相互作用により光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋの四光波混合光を発生する現象である。ここで、ｉ，ｊ，ｋは１から３のいずれかの値をとり、かつ、ｊ≠ｋであるものとする。四光波混合光は、ｆ_ｉとｆ_ｊが一致した場合、つまり２つの光でも発生する。
分散の小さい波長領域を用いた波長分割多重型光通信においては、四光波混合光の発生効率は、位相不整合量Δβが小さいほど大きい。ここで、位相整合量Δβは、
Δβ＝（−λ^４π／ｃ^２）・（ｄＤ／ｄλ）・｛（ｆ_ｉ−ｆ_０）＋（ｆ_ｊ−ｆ_０）｝・（ｆ_ｉ−ｆ_ｋ）・（ｆ_ｊ−ｆ_ｋ）
で表されることが、Ｋ．Ｉｎｏｕｅの論文 ”Ｆｉｂｅｒｆｏｕｒ−ｗａｖｅｍｉｘｉｎｇｉｎｔｈｅｚｅｒｏ−ｄｉｓｐｅｒｓｉｏｎｗａｖｅｌｅｎｇｔｈｒｅｇｉｏｎ”，Ｊ．ＬｉｇｈｔｗａｖｅＴｅｃｈｎｏｌ．，Ｖｏｌ．１０，ｐｐ．１５５３−１５６１，１９９２．に述べられている。ここで、ｆ_０は零分散波長を周波数に換算したものである。また、λは光の波長を、ｃは光の速度を、Ｄは波長分散を表している。この式から、波長多重化された複数の信号光のうち、１つの信号光の光周波数がｆ_０に一致した場合（ｆ_ｉ＝ｆ_ｊ＝ｆ_０）、または、２つの信号光の光周波数が周波数空間でｆ_０を挟む場合（ｆ_ｉ−ｆ_０＝ｆ_０−ｆ_ｊ）にはΔβがゼロとなり、四光波混合の発生効率が最大となることが分かる。発生した四光波混合光の周波数といずれかの信号光の光周波数の差が受信機の受信帯域以内の大きさとなる場合には、四光波混合光は信号光に対して干渉雑音となる。等間隔に配置された光周波数グリッド上に信号光の光周波数が配置されている場合、すなわち等間隔光周波数配置の場合、発生する四光波混合光の光周波数は、必ずこのグリッド上に位置することになる。このため、等間隔光周波数配置の場合には、四光波混合光による干渉雑音の影響が深刻になる。
【０００４】
次に、伝送路となる光ファイバの零分散波長と四光波混合発生効率の関係を示す。
図１は、２００ＧＨｚ間隔１６波の信号光のうち最も波長分散の小さいチャネルの信号光が持つ波長分散を横軸にとり、これらの信号光が光ファイバを伝搬する際に、ある信号光の波長に一致して発生する四光波混合光のパワの信号光パワに対する比率（ｄＢ）を縦軸に示したシミュレーション結果である。ここで、四光波混合光のパワは、Ｋ．Ｉｎｏｕｅ，Ｈ．Ｔｏｂａによる論文”Ｆｉｂｅｒｆｏｕｒ−ｗａｖｅｍｉｘｉｎｇｉｎｍｕｌｔｉ−ｒｅｐｅａｔｅｒｓｙｓｙｔｅｍｓｗｉｔｈｎｏｎｕｎｉｆｏｒｍｃｈｒｏｍａｔｉｃｄｉｓｐｅｒｓｉｏｎ”，Ｊ．ＬｉｇｈｔｗａｖｅＴｅｃｈｎｏｌ．，１３，ｐｐ．８８−９３，１９９５．に示された方法によって見積もった。
ここで、四光波混合光のパワの信号光パワに対する比率が−３０ｄＢ以上になると信号光に劣化が生じることが一般に知られている。よって、図１から、最も波長分散の小さい信号光の波長分散が０．３５ｐｓ／ｋｍ／ｎｍ以下にあると劣化が大きくなることが読み取れる。光ファイバの分散スロープは一般に０．０７ｐｓ／ｎｍ^２／ｋｍ程度である。よって、最も零分散波長に近い信号光の零分散波長からの波長距離が５ｎｍ（＝０．３５／０．０７）より小さいと劣化が生じる、ということが分かる。ただし、零分散波長の両側に信号光が配置される場合にはこの限りではない。
【０００５】
３つの光による四光波混合光の発生は、このうちいずれか一つの光を逆方向に伝搬させることで抑圧される。この理由を以下に述べる。
前述した位相不整合量Δβは、上記３つの光の伝搬定数をそれぞれβｉ， βｊ， βｋとし、四光波混合光の伝搬定数をβ_ＦＷＭとすると、
Δβ＝βｉ＋βｊ−βｋ−β_ＦＷＭ
で表される。伝搬定数は光の波長およびその伝搬方向と関連づけられる物理量で、伝搬方向が変わると符号が反転する。四光波混合光が発生している状態（波長配置）では、Δβはほぼ零であるが、このとき、上記３つの光のいずれかの伝搬方向を逆にすると、Δβは零でなくなることがわかる。つまり、四光波混合光の発生が抑圧されることになる。
【０００６】
また、四光波混合による信号劣化は、Ｆ．Ｆｏｒｇｈｉｅｒｉらの論文”ＲｅｄｕｃｔｉｏｎｏｆＦｏｕｒ−ＷａｖｅＭｉｘｉｎｇＣｒｏｓｓｔａｌｋｉｎＷＤＭＳｙｓｔｅｍｓＵｓｉｎｇＵｎｅｑｕａｌｌｙＳｐａｃｅｄＣｈａｎｎｅｌｓ”，ＩＥＥＥＰｈｏｔｏｎｉｃｓＴｅｃｈｎｏｌｏｇｙＬｅｔｔｅｒｓ，６，ｐｐ．７５４−７５６，１９９４．に詳述されているように、不等間隔波長配置によっても抑圧できる。
ここで「不等間隔波長配置」とは、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋが、多重化されたどの信号光の光周波数とも、受信機の受信帯域以上の差を持つように、各信号光の光周波数差が不等間隔となるような配置とするものである。例えば、波長１５３６．２ｎｍ〜１５５８．２ｎｍの範囲に、周波数間隔が順に１２５，３００，２００，３７５，１５０，１７５，３５０，２５０，１５０，３２５，２２５ＧＨｚとなるように配置された１２波の波長多重信号光の中の任意の３波が発生した四光波混合光は、どの信号光からも最低２５ＧＨｚ離れた周波数位置に生じることになり、干渉雑音とならない。
一方、四光波混合による信号劣化を抑圧する他の従来技術に、伝送路ファイバとしてノンゼロ分散シフトファイバを用いる技術がある。
ここで「ノンゼロ分散シフトファイバ」とは、利用する波長範囲が１５３０ｎｍ〜１５６５ｎｍである場合に四光波混合の発生を回避でき、かつ分散による信号劣化をなるべく小さくする特徴をもった伝送路用光ファイバであり、波長１５３０ｎｍ〜１５６５ｎｍにわたって波長分散の絶対値が０．１ｐｓ／ｎｍ／ｋｍ以上６ｐｓ／ｎｍ／ｋｍ以下である光ファイバ、と規定されている。このファイバは一般に０．０７ｐｓ／ｎｍ^２／ｋｍの分散スロープを持つため、実際には、零分散波長が１５６６〜１６１６ｎｍにあるもの、および１４７９〜１５２９ｎｍにあるものの、２種類のノンゼロ分散シフトファイバがある。
これを伝送路ファイバとすることにより、波長１５３０ｎｍ〜１５６５ｎｍに配置された信号光は、先に述べた四光波混合発生の条件を満たすことが無くなる。
【０００７】
ところで、近年、１５３０ｎｍ〜１５６５ｎｍに増幅利得を持つＥＤＦＡに加え、１５７５ｎｍ〜１６０５ｎｍに利得を持つ利得シフトＥＤＦＡ、１４５０〜１４８５ｎｍに利得を持つツリウム添加ファイバ増幅器（ＴＤＦＡ）が報告された。また、それぞれ、励起光の波長、半導体の組成をかえることで任意の波長の信号光を増幅できるファイバラマン増幅器および半導体光増幅器の研究も進展している。これらの光増幅器による並列増幅を行うことで、より広い利用帯域で信号光を送ることが可能となり、システムのさらなる大容量化が図れるようになる。利用帯域は最終的にはファイバ損失に制限されるが、光ファイバの損失が０．３ｄＢ／ｋｍ以下である領域は、１４５０〜１６５０ｎｍに及ぶことからも、より広い利用帯域で光波長分割多重伝送を行うことが望まれている。
【０００８】
【発明が解決しようとする課題】
しかしながら、先に述べたノンゼロ分散シフトファイバは、利用帯域が１５３０ｎｍ〜１５６５ｎｍであることを前提としている。よって、前述のファイバ増幅器を利用することで利用帯域を広げようとする場合、例えば、零分散波長が１５６６〜１６１６ｎｍにあるノンゼロ分散シフトファイバ上で、１４５０〜１６５０ｎｍの帯域を利用するような場合には、四光波混合の発生が避けられない。
これに加えて、広波長域を利用した場合には、短波長側の信号光が、誘導ラマン散乱と呼ばれる非線形光学効果によって長波長側の信号光の符号に依存した減衰を受け伝送特性が著しく劣化するという問題が指摘されている。この劣化をラマンクロストークによる劣化と呼ぶ。誘導ラマン散乱の発生効率は、使用する光信号の波長間隔が数１０ｎｍとなると増大してゆき、１００ｎｍ程度で最大となることが知られている。
ラマンクロストークによる劣化は、短波長信号光と長波長信号光が光ファイバ伝送路を互いに逆方向に伝搬するような方式、つまり双方向伝送方式によって低減できることが、Ｈ．Ｆ．Ｍａｈｌｅｉｎによる論文 ”ＣｒｏｓｓｔａｌｋｄｕｅｔｏｓｔｉｍｕｌａｔｅｄＲａｍａｎＳｃａｔｔｅｒｉｎｇｉｎｓｉｎｇｌｅ−ｍｏｄｅｆｉｂｒｅｓｆｏｒｏｐｔｉｃａｌｃｏｍｍｕｎｉｃａｔｉｏｎｉｎｗａｖｅｌｅｎｇｔｈｄｉｖｉｓｉｏｎｍｕｌｔｉｐｌｅｘｓｙｓｔｅｍｓ”，ＯｐｔｉｃａｌａｎｄＱｕａｎｔｕｍＥｌｅｃｔｒｏｎｉｃｓ，１６，ｐｐ．４０９−４２５，１９８４に詳述されている。この方式は、ラマンクロストークによる劣化が短波長側の信号光パルスと長波長側の信号光パルスが光ファイバ伝送路を長距離にわたって時間的に重なりながら伝搬することに起因することに着目し、両者を光ファイバ伝送路に対して互いに逆向きに伝搬させることにより、両者の時間的な重なりを平均化させ、長波長側の信号光による短波側信号光の減衰を時間的に平均化しようとするものである。
ただし、Ｈ．Ｆ．Ｍａｈｌｅｉｎによる論文では、短波長側に１波、長波長側に１波を配置する場合についてのみ記載されており、伝送システムの大容量化を目的とする場合には、短波長側の波長帯に複数波、長波長側の波長帯に複数波を、それぞれ配置することになる。しかし、このように各波長帯に複数の信号光を配置すると、近接する波長間の四光波混合による劣化が問題となる。
【０００９】
本発明はこのような事情に鑑みてなされたもので、ノンゼロ分散シフトファイバ上で零分散波長を含む広帯域（５０〜２００ｎｍ）を利用しする場合であっても、ラマンクロストークによる劣化を低減させるとともに、四光波混合による劣化を低減させる波長分割多重型光伝送システム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】
上記目的を達成するために、本願発明のうち請求項１に記載の発明は、零分散波長がおよそ１５６６ｎｍから１６１６ｎｍの範囲に設定されるノンゼロ分散シフトファイバを伝送路として光送受信装置により波長分割多重信号光の送受信を行う波長分割多重型光伝送システムにおいて、前記光送受信装置は、波長分割多重される信号光の波長を１５６１ｎｍ以下、および、１６２１ｎｍ以上にそれぞれ等波長間隔または等周波数間隔で配置し、前記１５６１ｎｍ以下に配置される信号光と前記１６２１ｎｍ以上に配置される信号光とが前記ノンゼロ分散シフトファイバを逆向きに伝搬するよう信号光の伝送を行うことを特徴とする。
また、請求項２に記載の発明は、零分散波長がおよそ１５６６ｎｍから１６１６ｎｍの範囲に設定されるノンゼロ分散シフトファイバを伝送路として光送受信装置により波長分割多重信号光の送受信を行う波長分割多重型光伝送システムおいて、前記光送受信装置は、信号光の波長を、波長１５１１ｎｍから１６２１ｎｍに不等周波数間隔で配置し、波長１５１１ｎｍ以下および１６２１ｎｍ以上の双方に等波長間隔または等周波数間隔で配置し、これらのうち、１６２１ｎｍ以下に配置される信号光と前記１６２１ｎｍ以上に配置される信号光とが前記ノンゼロ分散シフトファイバを逆向きに伝搬するよう信号光の伝送を行うことを特徴とする。
また、請求項３に記載の発明は、零分散波長がおよそ１５６６ｎｍから１６１６ｎｍの範囲に設定されるノンゼロ分散シフトファイバを伝送路として光送受信装置により波長分割多重信号光の送受信を行う波長分割多重型光伝送システムにおいて、前記光送受信装置は、信号光の波長を、波長１５６１ｎｍから１６７１ｎｍに不等周波数間隔で配置し、波長１５６１ｎｍ以下および１６７１ｎｍ以上の双方に等波長間隔または等周波数間隔で配置し、これらのうち、１５６１ｎｍ以下に配置される信号光と１５６１ｎｍ以上に配置される信号光とが前記ノンゼロ分散シフトファイバを逆向きに伝搬するよう信号光の伝送を行うことを特徴とする。
【００１１】
また、請求項４に記載の発明は、零分散波長がおよそ１４７９ｎｍから１５２９ｎｍの範囲に設定されるノンゼロ分散シフトファイバを伝送路として光送受信装置により波長分割多重信号光の送受信を行う波長分割多重型光伝送システムにおいて、前記光送受信装置は、波長分割多重される信号光の波長を１４７４ｎｍ以下、および、１５３４ｎｍ以上にそれぞれ等波長間隔または等周波数間隔で配置し、前記１４７４ｎｍ以下に配置される信号光と前記１５３４ｎｍ以上に配置される信号光とが前記ノンゼロ分散シフトファイバを逆向きに伝搬するよう信号光の伝送を行うことを特徴とする。
また、請求項５に記載の発明は、零分散波長がおよそ１４７９ｎｍから１５２９ｎｍの範囲に設定されるノンゼロ分散シフトファイバを伝送路として光送受信装置により波長分割多重信号光の送受信を行う波長分割多重型光伝送システムにおいて、前記光送受信装置は、信号光の波長を、波長１４２４ｎｍから１５３４ｎｍに不等周波数間隔で配置し、波長１４２４ｎｍ以下および１５３４ｎｍ以上の双方に等波長間隔または等周波数間隔で配置し、これらのうち、１５３４ｎｍ以下に配置される信号光と前記１５３４ｎｍ以上に配置される信号光とが前記ノンゼロ分散シフトファイバを逆向きに伝搬するよう信号光の伝送を行うことを特徴とする。
また、請求項６に記載の発明は、零分散波長がおよそ１４７９ｎｍから１５２９ｎｍの範囲に設定されるノンゼロ分散シフトファイバを伝送路として光送受信装置により波長分割多重信号光の送受信を行う波長分割多重型光伝送システムおいて、前記光送受信装置は、信号光の波長を、波長１４７４ｎｍから１５８４ｎｍに不等周波数間隔で配置し、波長１４７４ｎｍ以下および１５８４ｎｍ以上の双方に等波長間隔または等周波数間隔で配置し、これらのうち、１４７４ｎｍ以下に配置される信号光と前記１４７４ｎｍ以上に配置される信号光とが前記ノンゼロ分散シフトファイバを逆向きに伝搬するよう信号光の伝送を行うことを特徴とする。
【００１２】
また、請求項７に記載の発明は、請求項１乃至請求項６のいずれかに記載の波長分割型光伝送システムにおいて、前記波長分割多重型光伝送システムは、伝送路中に該伝送路の損失を補償するための線形中継光増幅器を１つ以上含むことを特徴としている。
また、請求項８に記載の発明は、請求項１乃至請求項６のいずれかに記載の波長分割多重型光伝送システムにおいて、前記波長分割多重型光伝送システムは、伝走路中に伝送路となるノンゼロ分散シフトファイバと逆符号の分散スロープを持ち零分散波長がほぼ等しい分散補償ファイバを備えた中継器を備え、該中継器により全ての信号光の分散補償を行うことを特徴としている。
また、請求項９に記載の発明は、請求項１乃至請求項７のいずれかに記載の波長分割多重型光伝送システムにおいて、前記波長分割多重型光伝送システムは、伝送路中に伝搬方向の別によって信号光を分離する分波器と、前記分波器により伝搬方向毎に分けられた信号光に対し、光ファイバ伝送路から受ける平均分散とほぼ等しく逆符号の分散を持つ分散補償ファイバとを備えた中継器を備え、伝搬方向毎に信号光の分散補償を行うことを特徴としている。
【００１３】
【発明の実施の形態】
以下、本発明の一実施形態による波長分割多重型光伝送システムを図面を参照して説明する。
本実施の形態においては、波長分割多重される信号光の波長配置について始めに説明し、その後その信号光の波長配置による波長分割多重型光伝送システムの構成を説明する。
【００１４】
まず、本発明の波長分割多重される信号光の波長配置の基本概念について説明する。
光ファイバの零分散波長は、製造上のばらつきからファイバ毎に異なる。ここでは、製造上のばらつきにより零分散波長がλ１〜λ２［ｎｍ］の範囲となる光ファイバを伝送路とする場合について考える。ここで、λ１［ｎｍ］は零分散波長の分布における下限を、λ２［ｎｍ］は零分散波長の分布における上限を表すものとする。
広波長域を利用した波長分割多重型光伝送システムでは、波長距離の離れた信号光間でのラマンクロストークを低減するために、短波長側、長波長側の２つの領域に分けた複数の信号光を伝送路に対して互い逆方向に伝搬させることが有効である。
ここで、互いに逆方向に伝搬すべき複数の信号光の波長を、使用する光ファイバ伝送路の零分散波長に基づいて規定する。これにより、四光波混合による劣化を効率的に抑圧することができる。
【００１５】
具体的には、図２〜図４に示すような信号光の波長配置とすると良い。即ち、
ａ）波長（λ２＋５）［ｎｍ］以上の複数の信号光と、波長（λ１−５）［ｎｍ］以下の複数の信号光を、互いに逆向きに伝搬させる（図２参照）
ｂ）波長（λ２＋５）［ｎｍ］以上の複数の信号光と、波長（λ２＋５）［ｎｍ］以下の複数の信号光を、互いに逆向きに伝搬させ、かつ、波長（２λ１−λ２−５）［ｎｍ］以上（λ２＋５）［ｎｍ］以下の複数の信号光の光周波数差は、不等間隔に配置する（図３参照）
ｃ）波長（λ１−５）［ｎｍ］以上の複数の信号光と、波長（λ１−５）［ｎｍ］以下の複数の信号光を、互いに逆向きに伝搬させ、かつ、波長（λ１−５）［ｎｍ］以上（２λ２−λ１＋５）［ｎｍ］以下の複数の信号光の光周波数差は、不等間隔に配置する（図４参照）
のいずれかにすることにより、四光波混合による劣化が抑圧できる。
【００１６】
次に上述のような信号光の波長配置とする理由を詳細に説明する。
図２に示した波長配置とした理由は次の通りである。
１）図１に示す結果から零分散波長からの信号光の波長距離が５ｎｍ（＝０．３５／０．０７）以上とすることで零分散波長より長波長側の信号光同志、または短波長側の信号光同志による四光波混合の発生を抑えることができる。よって、零分散波長がλ１〜λ２［ｎｍ］の範囲となる光ファイバを伝送路とする場合において、零分散波長の上限λ２を考慮して、波長分割多重される複数の信号光の一部を波長（λ２＋５）［ｎｍ］以上に配置することで、波長（λ２＋５）［ｎｍ］以上に配置された信号光同志での四光波混合の発生を抑えることができる。
２）零分散波長の下限λ１を考慮して、波長分割多重されるその他の複数の信号光を波長（λ１−５）［ｎｍ］以下に配置することで、波長（λ１−５）［ｎｍ］以下に配置された信号光同志での四光波混合の発生を抑えることができる。
３）さらに、波長（λ２＋５）［ｎｍ］以上に配置された複数の信号光と、波長（λ１−５）［ｎｍ］以下に配置された複数の信号光を逆向きに伝搬させることによってこれらの間での四光波混合の発生を抑圧できる。
以上の３点より図２のような信号光の波長配置とするとよい。
【００１７】
次に図３に示した波長配置とした理由は以下の通りである。
１）光ファイバの零分散波長の上限λ２を考慮して、波長（λ２＋５）［ｎｍ］以上に信号光を配置することで、波長（λ２＋５）［ｎｍ］以上に配置された信号光同士での四光波混合を押さえることができる。
２）また、波長（λ２＋５）［ｎｍ］以上に配置された複数の信号光と、波長（λ２＋５）［ｎｍ］以下に配置された複数の信号光とが逆向きに伝搬させることによってこれらの間で四光波混合の発生を抑圧できる。
３）光ファイバの零分散波長λを波長空間で真ん中に挟む複数の信号光が光ファイバを同方向に伝播する場合、上述のように零分散波長λを中心として四光波混合の影響が問題となる同方向に伝搬する範囲の周波数帯において、その信号光の光周波数差を不等間隔に配置することにより干渉雑音が抑圧される。ここで、光ファイバの零分散波長λ［ｎｍ］が下限λ１となる最悪の場合を考慮すると、（２λ１−λ２−５）（＝λ１−｛［λ２＋５］−λ１｝）［ｎｍ］以上（λ２＋５）［ｎｍ］以下の複数の信号光については、その光周波数差を不等間隔に配置することで、確実に干渉雑音が抑圧される。
以上の３点より図３のような光信号の波長配置とするとよい。
【００１８】
次に図４に示した波長配置とした理由は図３で説明した内容と同じであるが、繰り返すと以下の通りとなる。
１）光ファイバの零分散波長の上限λ１を考慮して、波長（λ１−５）［ｎｍ］以上に信号光を配置することで、これらの信号光同士での四光波混合を押さえることができる。
２）また、波長（λ１−５）［ｎｍ］以上に配置された複数の信号光と、波長（λ１−５）［ｎｍ］以下に配置された複数の信号光とが逆向きに伝搬させることによってもこれらの間で四光波混合の発生を抑圧できる。
３）光ファイバの零分散波長λを波長空間で真ん中に挟む複数の信号光が光ファイバを同方向に伝播する場合、上述のように零分散波長λを中心として四光波混合による影響が問題となる同方向に伝搬する範囲の周波数帯において、その信号光の光周波数差を不等間隔に配置することにより干渉雑音が抑圧される。ここで、光ファイバの零分散波長λが上限λ２となる最悪の場合を考慮すると、（λ１−５）［ｎｍ］以上（２λ２−λ１＋５）（＝λ２＋｛λ２−［λ１−５］｝）［ｎｍ］以下の複数の信号光については、その光周波数差を不等間隔に配置することで、確実に干渉雑音が抑圧される。
以上の３点より図４のような光信号の波長配置とするとよい。
【００１９】
加えて、上記に示した３通りの波長配置のすべてにおいては、波長（λ２＋５）［ｎｍ］以上である信号光と（λ１−５）［ｎｍ］以下である信号光との間で発生する相互位相変調が分散と作用することによって信号光に与える波形歪みが低減される。これは、前者と後者の間の非常に大きなウォークオフが相互位相変調を相殺するためである。なお、「相互位相変調」とは、光パルス波により伝送ファイバの局所的な屈折率が変化し、他の光パルス波の瞬時周波数が変えられ光信号の位相が変化することをいう。
【００２０】
以上述べた信号光の波長配置に対する規定は、零分散波長がおよそ１５６６ｎｍから１６１６ｎｍの範囲に配置されるノンゼロ分散シフトファイバを用いる場合、すなわちλ１＝１５６６ｎｍ、λ２＝１６１６ｎｍの場合には、
ａ）波長１５６１（＝１５６６−５）ｎｍ以下の複数の信号光と、波長１６２１（＝１６１６＋５）ｎｍ以上の複数の信号光を、互いに逆向きに伝搬させる
ｂ）波長１６２１（＝１６１６＋５）ｎｍ以上の複数の信号光と、波長１６２１ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させ、波長１５１１（２×１５６６−１６１６−５）ｎｍ以上１６２１ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
ｃ）波長１５６１（＝１５６６−５）ｎｍ以下の複数の信号光と、波長１５６１ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させ、波長１５６１ｎｍ以上１６７１（＝２×１６１６−１５６１＋５）ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
のいずれかの信号光の波長配置となる。
【００２１】
一方、零分散波長がおよそ１４７９ｎｍから１５２９ｎｍの範囲に配置されるノンゼロ分散シフトファイバを用いる場合、すなわちλ１＝１４７９ｎｍ、λ２＝１５２９ｎｍの場合には、
ａ）波長１４７４（＝１４７９−５）ｎｍ以下の複数の信号光と、波長１５３４（＝１５２９＋５）ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させる
ｂ）波長１５３４（＝１５２９＋５）ｎｍ以上の複数の信号光と、波長１５３４ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させ、波長１４２４（＝２×１４７９−１５２９−５）ｎｍ以上１５３４ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
ｃ）波長１４７４（＝１４７９−５）ｎｍ以下の複数の信号光と、波長１４７４ｎｍ以上の複数の信号光を、互いに逆向きに伝搬させ、波長１４７４ｎｍ以上１５８４（＝２×１５２９−１４７９＋５）ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
のいずれかの信号光の波長配置となる。
【００２２】
次に、上記の信号光の波長配置について図５〜図１０を用いてその具体例を示す。なお、以下の例では、ファイバ伝送路の各端に光送受信装置があり、送信および受信のために所定数の信号光を波長分割多重するものとして説明する。
【００２３】
図５は、ファイバ伝送路として零分散波長が１５６６ｎｍ以上１６１６ｎｍ以下であるノンゼロ分散シフトファイバを使用する場合の信号光の波長配置を示した図である。図５のように一方の光送受信装置から送出する１６波の信号光６１は波長１５６１ｎｍ以下の領域に１００ＧＨｚ間隔で配置する。また、他方の光送受信装置から送出する１６の信号光６２は波長１６２１ｎｍ以上の領域に１００ＧＨｚ間隔で配置する。
これにより、互いに波長間隔の離れた複数の信号光６１と複数の信号光６２が互いに逆方向に伝搬するため、これらの間で生じるラマンクロストークを回避することができる。また、ノンゼロ分散シフトファイバの零分散波長からの波長距離が５ｎｍ以内である複数の信号光、または、零分散波長を波長空間で真ん中に挟む複数の信号光が同一方向に伝搬することがないため、四光波混合の発生による伝送品質の劣化を回避できるようになる。ただし、波長間隔および波長数は、図５の例に限定されるものではない。
【００２４】
図６も図５と同様にファイバ伝送路として零分散波長が１５６６ｎｍ以上１６１６ｎｍ以下であるノンゼロ分散シフトファイバを使用する場合の信号光の波長配置を示した図であるが、その波長配置が異なる。図６の例では、一方の光送受信装置から送出する１６波の信号光６３は、波長１６２１ｎｍ以上の領域に１００ＧＨｚ間隔に配置される。また、他方の光送受信装置から送出する１２波の信号光６４は波長１６２１ｎｍ以下の領域に、波長１５７５．８ｎｍを最短波長として、順に、１２５，３００，２００，３７５，１５０，１７５，３５０，２５０，１５０，３２５，２２５ＧＨｚ間隔で配置される。このような不等間隔配置とすることで、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋは、いずれの信号光からも最低２５ＧＨｚ離れた周波数位置にしか発生せず、四光波混合光と信号光との干渉雑音が低減できる。
これにより、互いに波長間隔の離れた複数の信号光６３と複数の信号光６４が互いに逆方向に伝搬するため、これらの間で生じるラマンクロストークを回避することができる。さらに、同一方向に伝搬した場合に零分散波長を間に挟んで四光波混合を発生し得る複数の信号光６３と複数の信号光６４が逆方向に伝搬することでその間での四光波混合の発生を回避できる。さらに、同一方向に伝搬するため四光波混合を発生する波長１５１１ｎｍ以上１６２１ｎｍ以下の複数の信号光は、不等間隔配置とすることで、発生した四光波混合光が信号光に与える干渉雑音を低減できる。
ただし、波長間隔および波長数は図６の例に限定されるものではなく、不等間隔波長配置については、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋが、多重化されたどの信号光の光周波数とも、受信機の受信帯域以上の差を持つような配置であればよい。
【００２５】
図７も図５、図６と同様にファイバ伝送路として零分散波長が１５６６ｎｍ以上１６１６ｎｍ以下であるノンゼロ分散シフトファイバを使用する場合の信号光の波長配置を示した図であるが、その波長配置が異なる。図７の例では、一方の光送受信装置から送出する１６波の信号光９１は、波長１５６１ｎｍ以下の領域に１００ＧＨｚ間隔に配置される。また、他方の光送受信装置から送出する１２波の信号光９２は波長１５６１ｎｍ以上の領域に、波長１５７５．８ｎｍを最短波長として、順に、１２５，３００，２００，３７５，１５０，１７５，３５０，２５０，１５０，３２５，２２５ＧＨｚ間隔で配置される。このような不等間隔配置とすることで、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋは、いずれの信号光からも最低２５ＧＨｚ離れた周波数位置にしか発生せず、四光波混合光と信号光との干渉雑音が低減できる。
これにより、互いに波長間隔の離れた複数の信号光９１と複数の信号光９２が互いに逆方向に伝搬するため、これらの間で生じるラマンクロストークを回避することができる。さらに、同一方向に伝搬した場合に零分散波長を間に挟んで四光波混合を発生し得る複数の信号光９１と複数の信号光９２とが逆方向に伝搬することでその間での四光波混合の発生を回避できる。さらに、同一方向に伝搬するため四光波混合を発生する波長１５６１ｎｍ以上１６７１ｎｍ以下の複数の信号光は、不等間隔配置とすることで、発生した四光波混合光が信号光に与える干渉雑音を低減できる。
ただし、波長間隔および波長数は図７の例に限定されるものではなく、不等間隔波長配置については、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋが、多重化されたどの信号光の光周波数とも、受信機の受信帯域以上の差を持つような配置であればよい。
【００２６】
図８はファイバ伝送路として零分散波長が１４７９ｎｍ以上１５２９ｎｍ以下であるノンゼロ分散シフトファイバを使用する場合の信号光の波長配置を示した図である。図８において、一方の光送受信装置から送出する１６波の信号光１０１は波長１４７４ｎｍ以下の領域に１００ＧＨｚ間隔で配置される。また、他方の光送受信装置から送出する１６の信号光１０２は波長１５３４ｎｍ以上の領域に１００ＧＨｚ間隔で配置される。
これにより、互いに波長間隔の離れた複数の信号光１０１と複数の信号光１０２が互いに逆方向に伝搬するため、これらの間で生じるラマンクロストークを回避することができる。また、零分散波長からの波長距離が５ｎｍ以内である複数の信号光、または、零分散波長を波長空間で真ん中に挟む複数の信号光が同一方向に伝搬することがないため、四光波混合の発生による伝送品質の劣化を回避できる。ただし、波長間隔および波長数は、図８の例に限定されるものではない。
【００２７】
図９も図８と同様にファイバ伝送路として零分散波長が１４７９ｎｍ以上１５２９ｎｍ以下であるノンゼロ分散シフトファイバを使用する場合の信号光の波長配置を示した図であるが、その波長配置が異なる。図９の例では、一方の光送受信装置から送出する１６波の信号光１１１は、波長１５３４ｎｍ以上の領域に１００ＧＨｚ間隔に配置される。また、他方の光送受信装置から送出する１２波の信号光１１２は波長１５３４ｎｍ以下の領域に、波長１５００．８ｎｍを最短波長として、順に、１２５，３００，２００，３７５，１５０，１７５，３５０，２５０，１５０，３２５，２２５ＧＨｚ間隔で配置される。このような不等間隔配置とすることで、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋは、いずれの信号光からも最低２５ＧＨｚ離れた周波数位置にしか発生せず、四光波混合光と信号光との干渉雑音が低減できる。
これにより、互いに波長間隔の離れた複数の信号光１１１と複数の信号光１１２が互いに逆方向に伝搬するため、これらの間で生じるラマンクロストークを回避することができる。また、同一方向に伝搬した場合に零分散波長を間に挟んで四光波混合を発生し得る複数の信号光１１１と複数の信号光１１２とが逆方向に伝搬することでその間での四光波混合の発生を回避できる。さらに、同一方向に伝搬するため四光波混合を発生する波長１４２４ｎｍ以上１５３４ｎｍ以下の複数の信号光は、不等間隔配置とすることで、発生した四光波混合光が信号光に与える干渉雑音を低減できる。
ただし、波長間隔および波長数は図９の例に限定されるものではなく、不等間隔波長配置については、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋが、多重化されたどの信号光の光周波数とも、受信機の受信帯域以上の差を持つような配置であればよい。
【００２８】
図１０も図８、図９と同様にファイバ伝送路として零分散波長が１４７９ｎｍ以上１５２９ｎｍ以下であるノンゼロ分散シフトファイバを使用する場合の信号光の波長配置を示した図であるが、その波長配置が異なる。図１０の例では、一方の光送受信装置から送出する１６波の信号光１０３は、波長１４７４ｎｍ以下の領域に１００ＧＨｚ間隔に配置される。また、他方の光送受信装置から送出する１２波の信号光１０４は波長１４７４ｎｍ以上の領域に、波長１５００．８ｎｍを最短波長として、順に、１２５，３００，２００，３７５，１５０，１７５，３５０，２５０，１５０，３２５，２２５ＧＨｚ間隔で配置される。このような不等間隔配置とすることで、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋは、いずれの信号光からも最低２５ＧＨｚ離れた周波数位置にしか発生せず、四光波混合光と信号光との干渉雑音が低減できる。
これにより、互いに波長間隔の離れた複数の信号光１０３と複数の信号光１０４が互いに逆方向に伝搬するため、これらの間で生じるラマンクロストークを回避することができる。さらに、同一方向に伝搬した場合に零分散波長を間に挟んで四光波混合を発生し得る複数の信号光１０３と複数の信号光１０４が逆方向に伝搬することでその間での四光波混合の発生を回避できる。さらに、同一方向に伝搬するため四光波混合を発生する波長１４７４ｎｍ以上１５８４ｎｍ以下の複数の信号光は、不等間隔配置とすることで、発生した四光波混合光が信号光に与える干渉雑音を低減できる。
ただし、波長間隔および波長数は図１０の例に限定されるものではなく、不等間隔波長配置については、光周波数ｆ_ｉ，ｆ_ｊ，ｆ_ｋの任意の３波から発生した四光波混合光の光周波数ｆ_ＦＷＭ＝ｆ_ｉ＋ｆ_ｊ−ｆ_ｋが、多重化されたどの信号光の光周波数とも、受信機の受信帯域以上の差を持つような配置であればよい。
【００２９】
次に、ノンゼロ分散シフトファイバを伝送路とし、上記で説明した信号光の波長配置を用いる波長分割多重型光伝送システムの構成例を示す。
図１１は、光波長分割多重双方向伝送方式を適用されるノンゼロ分散シフトファイバ上でのポイントツーポイント波長分割多重型光伝送システム構成を示すブロック構成図である。
図１１より、波長分割多重型光伝送システムは、一方の光送受信装置５４と他方の光送受信装置５６、そして光送受信装置５４、５６を結ぶ一本のノンゼロ分散シフトファイバ伝送路５５により構成されている。ここで、光送受信装置５４から送出される複数の信号光と光送受信装置５６から送出される複数の信号光はノンゼロ分散シフトファイバ伝送路５５を互いに逆向きに伝搬するものである。
なお、光送受信装置５４、５６は、互いに波長の異なる複数の光を複数の電気信号により変調し、これらを波長多重化して送信する送信回路５１と、波長多重化された互いに波長の異なる複数の信号光を波長毎に分離し、それらを光電気変換して複数の電気信号に復調する受信回路５２、および、送信回路からは伝送路に、伝送路からは受信回路に信号光を導くフィルタまたはサーキュレータ５３から構成される。さらに、送信回路５１は互いに異なる波長に設定された複数の光源（レーザ）、光源から出力される光搬送波をデータ信号で変調する複数の変調器、複数の信号光を波長多重化する合波器等により構成さる。また、受信回路５２は複数の信号光を分離する分波器および分波された信号光から電気信号を復調する複数の受信機等により構成される。なお、送信回路５１は光源のバイアス等を直接変調する方式も適応可能である。
【００３０】
図１１に示す構成において、複数の信号光が光送受信装置５４、５６で受ける損失を補償するために、送信回路５１の後またはノンゼロ分散シフトファイバ伝送路５５の前に光増幅器をさらに配置してもよい。また、信号の受信感度を増加させるために、受信回路５２の前または受信機の前に光増幅器をさらに配置してもよい。利用できる光増幅器としては、エルビウム添加ファイバ増幅器、利得シフトエルビウム添加ファイバ増幅器、ツリウム添加ファイバ増幅器、ファイバラマン増幅器、半導体光増幅器がある。なお、エルビウム添加ファイバ増幅器は波長１５３０〜１５６５ｎｍの信号光を、利得シフトエルビウム添加ファイバ増幅器は波長１５７０〜１６０５ｎｍの信号光を、ツリウム添加ファイバ増幅器は波長１４４０〜１４８０ｎｍの信号光を、それぞれ一括に増幅できる。また、ファイバラマン増幅器および半導体光増幅器は、それぞれ、励起光の波長、半導体の組成により、任意の波長の信号光を増幅できる。
【００３１】
図１２は、ポイントツーポイント多中継波長分割多重型光伝送システム構成を示すブロック構成図である。図１２に示したように、その構成は、図１１に示した構成において、ノンゼロ分散シフトファイバ伝送路５５の途中に、その損失を補償するために、線形中継光増幅器７１を挿入した構成となっている。ここで、図１２において図１１の各部に対応する部分には同一の符号を付け、その説明を省略する。なお、図１２は、線形中継光増幅器７１が１個の例を示しているが、この線形中継光増幅器７１は伝送路中に所定間隔で複数設けてもよい。
このように、伝送路中に線形中継光増幅器を設けることにより、信号光の伝送距離を長くすることができる。
ここで、線形中継光増幅器７１は、互いに逆方向に伝搬する複数の信号光を一括に増幅する双方向光増幅器であってもよいし、図１３に示したように、フィルタまたはサーキュレータ８１によって伝搬方向の別に分離された複数の信号光が光増幅器６２で増幅され、フィルタまたはサーキュレータ８１によって再び伝搬方向の異なる複数の信号光と多重されるような構成であってもよい。
また、双方向に伝送するすべての信号光は線形中継時に分散補償されてもよい。例えば、光ファイバ伝送路と逆符号の分散スロープを持ち零分散波長がほぼ等しい分散補償ファイバを用いて、一括に分散補償されてもよい。これにより、少ない部品点数で分散補償を行うことができる。また、図１３に示したように、伝搬方向の別によって分離され、この際、それぞれの伝搬方向の信号光が光ファイバ伝送路から受ける平均分散とほぼ等しく逆符号の分散を持つ分散補償ファイバによって、伝搬方向毎に分散補償されてもよい。これにより、より高い精度で分散補償を行うことができる。
【００３２】
なお、上述の光信号の波長配置における利用形態は、図１１から図１３に示すポイント・ツー・ポイント伝送システムに限らず、あらゆるネットワーク形態の波長分割多重型光伝送システムに適用可能である。
【００３３】
本実施の形態では、ノンゼロ分散シフトファイバを例にして、説明を行ったが、零分散波長が１５３０〜１５７０ｎｍに配置される分散シフトファイバを用いた場合には、
ａ）波長１５２５（１５３０−５）ｎｍ以下の複数の信号光と、波長１５７５（＝１５７０＋５）ｎｍ以上の複数の信号光を、互いに逆向きに伝搬させる
ｂ）波長１５７５ｎｍ以上の複数の信号光と、波長１５７５ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させ、波長１４８５（＝２×１５３０−１５７０−５）ｎｍ以上１５７５ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
ｃ）波長１５２５ｎｍ以下の複数の信号光と、波長１５２５ｎｍ以上の複数の信号光を、互いに逆向きに伝搬させ、波長１５２５ｎｍ以上１６１５（＝２×１５７０−１５３０＋５）ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
のいずれかの光信号の波長配置となる。
【００３４】
また、零分散波長が１３００〜１３３０ｎｍに配置される通常シングルモードファイバを用いた場合には、
ａ）波長１３３５（＝１３３０＋５）ｎｍ以上の複数の信号光と、波長１２９５（＝１３００−５）ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させる
ｂ）波長１３３５ｎｍ以上の複数の信号光と、波長１３３５ｎｍ以下の複数の信号光を、互いに逆向きに伝搬させ、波長１２６５（＝２×１３００−１３３０−５）ｎｍ以上１３３５ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
ｃ）波長１２９５ｎｍ以下の複数の信号光と、波長１２９５ｎｍ以上の複数の信号光を、互いに逆向きに伝搬させ、波長１２９５ｎｍ以上１３６５（＝２×１３３０−１３００＋５）ｎｍ以下の複数の信号光の光周波数差は、不等間隔に配置する
のいずれかの光信号の波長配置となる。
【００３５】
また、零分散波長が１５６６ｎｍから１６１６ｎｍの範囲に配置されるノンゼロ分散シフトファイバを伝送路とする場合において、
１）１５６１（１５６６−５）ｎｍ以下に信号光を配置する
２）１６２１（１６１６＋５）ｎｍ以上に信号光を配置する
３）１５６１ｎｍ以下、および、１６２１ｎｍ以上に信号光を配置し、信号光の伝送路に対する伝搬方向を特定しない
という信号光配置であってもよい。なぜならば、ノンゼロ分散シフトファイバの零分散波長に対する信号光の波長距離を５ｎｍ以上とすることで、零分散波長より長波長側の信号光同志、または、短波長側の信号光同志による四光波混合の発生は抑圧されるからである。零分散波長が１４７９ｎｍから１５２９ｎｍの範囲に配置されるノンゼロ分散シフトファイバを伝送路とした場合にも同様のことがいえる。さらに、上述した零分散波長が１５３０ｎｍから１５７０ｎｍに配置される分散シフトファイバ、零分散波長が１３００ｎｍから１３３０ｎｍに配置される通常シングルモードファイバについても同様のことがいえる。
【００３６】
また、信号光の光周波数差を不等間隔に配置する場合の波長範囲は、伝送路の零分散波長の最悪値を考慮して決定している。これを、光ファイバの零分散波長の製造上のばらつきの分布統計値を利用してより狭い波長範囲において信号光の光周波数差を不等間隔に配置するようにしてもよい。
例えば、零分散波長が１５６６ｎｍから１６１６ｎｍの範囲に配置されるノンゼロ分散シフトファイバを伝送路として用いた場合、図７に示した第３の具体例では、不等間隔に配置すべき信号光の範囲を１５６１ｎｍから１６７１ｎｍと定めた。しかしながら、分布統計を考えたとき実際にはほとんどの伝送路の零分散波長が１５７１ｎｍから１５８６ｎｍにあったとすると、不等間隔に配置すべき信号光の範囲は、１５６１ｎｍ（信号光を光ファイバ伝送路に対して逆向きに伝搬させる境界波長）から１６０６（２×１５８６−１５７１＋５）ｎｍまで狭めてもよい。同様のことは、図６、図９、図１０のそれぞれに示す第２、第５、第６のそれぞれの具体例についてもいえる。
【００３７】
【発明の効果】
以上説明したように、本発明は、零分散波長を含む広帯域（５０〜２００ｎｍ）を利用した光波長分割多重双方向伝送システムにおいて、所定波長を境に信号光を光ファイバ伝送路に対して互いに逆向きに伝搬させることにより、両者の時間的な重なりを平均化させ、長波長側信号光が誘導ラマン散乱を通して短波長側信号光に与える減衰を時間的に平均化できる。よって、ラマンクロストークによる劣化を低減できる。この時間的重なりの平均化は、両者の間で生じる相互位相変調も平均化させるため、相互位相変調とファイバ分散による波形歪みによる劣化も回避できる。また、同じく所定波長を境に信号光を光ファイバ伝送路に対して互いに逆向きに伝搬させることにより、両者の間の位相不整合量を大きくすることができる。よって長波長側信号光と短波長側信号光の間での四光波混合の発生を抑圧できる。
また、光ファイバ伝送路の零分散波長に対する波長距離を５ｎｍ以上とすることで、零分散波長に対して長波長側の信号光同志、または、短波長側の信号光同志の四光波混合の発生を抑えることができる。
さらに、光ファイバ伝送路の零分散波長を含む波長帯域に信号光を配置する場合、信号光の光周波数差が不等間隔となるように配置することで、四光波混合による信号劣化を押さえて、使用光周波数帯域をさらに広げることができる。
【図面の簡単な説明】
【図１】信号光の波長分散と四光波混合光強度の関係のシミュレーション結果を示す図である。
【図２】信号光の波長配置を説明する第１の図である。
【図３】信号光の波長配置を説明する第２の図である。
【図４】信号光の波長配置を説明する第３の図である。
【図５】ノンゼロ分散シフトファイバを伝送路とする場合の信号光の長配置の第１の具体例を説明するための図である。
【図６】ノンゼロ分散シフトファイバを伝送路とする場合の信号光の長配置の第２の具体例を説明するための図である。
【図７】ノンゼロ分散シフトファイバを伝送路とする場合の信号光の長配置の第３の具体例を説明するための図である。
【図８】ノンゼロ分散シフトファイバを伝送路とする場合の信号光の長配置の第４の具体例を説明するための図である。
【図９】ノンゼロ分散シフトファイバを伝送路とする場合の信号光の長配置の第５の具体例を説明するための図である。
【図１０】ノンゼロ分散シフトファイバを伝送路とする場合の信号光の長配置の第６の具体例を説明するための図である。
【図１１】波長分割多重型光伝送システムのブロック構成図である。
【図１２】波長分割多重型光伝送システムの他のブロック構成図である。
【図１３】線形中継光増幅器の構成例を示した図である。
【符号の説明】
５１送信回路
５２受信回路
５３フィルタまたはサーキュレータ
５４、５６光送受信装置
５５ノンゼロ分散シフトファイバ伝送路[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength division multiplexing type optical transmission system for transmitting a wavelength division multiplexed optical signal by using a non-zero dispersion shifted fiber whose zero dispersion wavelength is not near 1550 nm by an optical transceiver.
[0002]
[Prior art]
Optical wavelength division multiplexing technology is a technique in which a plurality of lights having different wavelengths are modulated by a plurality of electric signals, wavelength-multiplexed by optical passive components such as an optical wavelength filter, transmitted to an optical fiber transmission line, and received on a receiving side. In this technology, the wavelength-multiplexed signal light is separated for each wavelength, and the signals are photoelectrically converted and demodulated into a plurality of original electric signals. This technique can easily demultiplex a plurality of signals using only optical passive components, and is effective for increasing the capacity of a transmission system.
When a plurality of signal lights used for optical wavelength division multiplexing are arranged in a wavelength of 1530 nm to 1565 nm which is a gain band of an erbium-doped fiber amplifier (EDFA), the same 1550 nm is used to minimize waveform deterioration due to chromatic dispersion. In a dispersion-shifted fiber in which a zero-dispersion wavelength is disposed in the vicinity, there is a problem that the signal quality is deteriorated due to the occurrence of four-wave mixing.
[0003]
Here, “four-wave mixing” means the optical frequency f₁, F₂, F₃Three lights having the optical frequency f due to nonlinear interaction with the propagation medium_FWM= F_i+ F_j−f_kIs a phenomenon that generates four-wave mixing light. Here, it is assumed that i, j, and k take any value from 1 to 3 and j ≠ k. Four-wave mixing light is f_iAnd f_jAre generated, that is, two light beams are generated.
In wavelength division multiplexing optical communication using a wavelength region with small dispersion, the generation efficiency of four-wave mixing light increases as the amount of phase mismatch Δβ decreases. Here, the phase matching amount Δβ is
Δβ = (− λ⁴π / c²) · (DD / dλ) · ｛(f_i−f₀) + (F_j−f₀)｝・ (F_i−f_k) ・ (F_j−f_k)
Is represented by K. Inoue, "Fiber four-wave mixing in the zero-dispersion wavelength region," J. Org. Lightwave Technology. , Vol. 10, pp. 1553-1561, 1992. It is stated in. Where f₀Is a value obtained by converting the zero dispersion wavelength into a frequency. Further, λ represents the wavelength of light, c represents the speed of light, and D represents chromatic dispersion. From this equation, the optical frequency of one signal light among a plurality of wavelength-multiplexed signal lights is f₀If (f_i= F_j= F₀) Or the optical frequency of the two signal lights is f₀When sandwiching (f_i−f₀= F₀−f_j) Indicates that Δβ is zero, and the generation efficiency of four-wave mixing is maximized. If the difference between the frequency of the generated four-wave mixed light and the optical frequency of any signal light is within the receiving band of the receiver, the four-wave mixed light becomes interference noise with the signal light. When the optical frequency of the signal light is arranged on the optical frequency grid arranged at equal intervals, that is, in the case of the equidistant optical frequency arrangement, the optical frequency of the generated four-wave mixing light is always located on this grid. Will be. For this reason, in the case of the equally spaced optical frequency arrangement, the influence of interference noise due to the four-wave mixing light becomes serious.
[0004]
Next, the relationship between the zero-dispersion wavelength of an optical fiber serving as a transmission line and the efficiency of four-wave mixing generation will be described.
FIG. 1 shows, on the horizontal axis, the chromatic dispersion of the signal light of the channel with the smallest chromatic dispersion among the 16 signal lights at 200 GHz intervals, and when these signal lights propagate through the optical fiber, the wavelength dispersion of the signal light is It is the simulation result which showed the ratio (dB) of the power of the four-wave mixing light generated coincident with the power of the signal light on the vertical axis. Here, the power of the four-wave mixing light is described in K.K. Inoue, H .; Toba, "Fiberfour-wave mixing in multi-repeator systems with the nonuniform chromatographic dispersion", J. Amer. Lightwave Technology. , 13, pp. 88-93, 1995. Was estimated by the method shown in (1).
Here, it is generally known that the signal light is deteriorated when the ratio of the power of the four-wave mixing light to the power of the signal light is -30 dB or more. Therefore, it can be seen from FIG. 1 that the degradation increases when the chromatic dispersion of the signal light having the smallest chromatic dispersion is 0.35 ps / km / nm or less. The dispersion slope of an optical fiber is generally 0.07 ps / nm.²/ Km. Therefore, it can be seen that deterioration occurs when the wavelength distance of the signal light closest to the zero-dispersion wavelength from the zero-dispersion wavelength is smaller than 5 nm (= 0.35 / 0.07). However, this does not apply when the signal light is arranged on both sides of the zero dispersion wavelength.
[0005]
Generation of four-wave mixing light by three lights is suppressed by propagating any one of the lights in the opposite direction. The reason will be described below.
The above-mentioned phase mismatch amount Δβ is obtained by setting the propagation constants of the above three lights to βi, βj, βk, and the propagation constant of the four-wave mixing light to β_FWMThen
Δβ = βi + βj-βk-β_FWM
Is represented by The propagation constant is a physical quantity associated with the wavelength of light and its propagation direction, and the sign is inverted when the propagation direction changes. In the state where the four-wave mixing light is generated (wavelength arrangement), Δβ is almost zero. At this time, if the propagation direction of any of the three lights is reversed, Δβ is not zero. . That is, the generation of four-wave mixing light is suppressed.
[0006]
The signal degradation due to four-wave mixing is described in F.A. Forghieri et al., "Reduction of Four-Wave Mixing Crosstalk in WDM Systems Using Uniquely Spaced Channels," IEEE Photonics Technology, Technology. 754-756, 1994. As described in detail above, it is possible to suppress even by the unequally-spaced wavelength arrangement.
Here, the “unequally-spaced wavelength arrangement” means the optical frequency f_i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_{FWM =}f_i+ F_j−f_kHowever, the arrangement is such that the optical frequency difference of each signal light is unequally spaced so that the optical frequency of any multiplexed signal light has a difference equal to or greater than the reception band of the receiver. For example, 12 wavelengths are arranged in the range of 1536.2 nm to 1558.2 nm so that the frequency intervals are 125, 300, 200, 375, 150, 175, 350, 250, 150, 325, and 225 GHz. Four-wave mixing light in which three arbitrary waves in the wavelength-multiplexed signal light are generated is generated at a frequency position at least 25 GHz away from any signal light, and does not become interference noise.
On the other hand, another conventional technique for suppressing signal deterioration due to four-wave mixing includes a technique using a non-zero dispersion shift fiber as a transmission line fiber.
Here, the "non-zero dispersion shifted fiber" is a transmission line having a feature that, when the wavelength range to be used is 1530 nm to 1565 nm, generation of four-wave mixing can be avoided and signal deterioration due to dispersion is minimized. The optical fiber is defined as an optical fiber having an absolute value of chromatic dispersion of 0.1 ps / nm / km or more and 6 ps / nm / km over a wavelength of 1530 nm to 1565 nm. This fiber is typically 0.07 ps / nm²In fact, there are two types of non-zero dispersion-shifted fibers, one having a dispersion slope of 1566-1616 nm and one having a dispersion slope of 1479-1529 nm.
By using this as a transmission line fiber, the signal light arranged at a wavelength of 1530 nm to 1565 nm does not satisfy the above-described condition of four-wave mixing generation.
[0007]
Incidentally, in recent years, in addition to the EDFA having an amplification gain of 1530 nm to 1565 nm, a gain shift EDFA having a gain of 1575 nm to 1605 nm and a thulium-doped fiber amplifier (TDFA) having a gain of 1450 to 1485 nm have been reported. . In addition, researches on fiber Raman amplifiers and semiconductor optical amplifiers that can amplify signal light of an arbitrary wavelength by changing the wavelength of the pump light and the composition of the semiconductor, respectively, are also progressing. By performing parallel amplification by these optical amplifiers, it becomes possible to transmit signal light in a wider use band, and the capacity of the system can be further increased. The bandwidth used is ultimately limited by the fiber loss. However, the area where the loss of the optical fiber is 0.3 dB / km or less extends from 1450 to 1650 nm. It is desired to perform multiplex transmission.
[0008]
[Problems to be solved by the invention]
However, the above-described non-zero dispersion shifted fiber is based on the premise that the usable band is 1530 nm to 1565 nm. Therefore, when trying to widen the utilization band by using the above-described fiber amplifier, for example, using a band of 1450 to 1650 nm on a non-zero dispersion shift fiber having a zero dispersion wavelength of 1566 to 1616 nm. In such cases, the occurrence of four-wave mixing is inevitable.
In addition, when a wide wavelength region is used, the signal light on the short wavelength side is attenuated depending on the sign of the signal light on the long wavelength side due to a nonlinear optical effect called stimulated Raman scattering, and the transmission characteristics are remarkably increased. The problem of deterioration has been pointed out. This deterioration is called Raman crosstalk deterioration. It is known that the generation efficiency of stimulated Raman scattering increases as the wavelength interval of the used optical signal becomes several tens of nm, and becomes maximum at about 100 nm.
It has been reported that degradation due to Raman crosstalk can be reduced by a system in which short-wavelength signal light and long-wavelength signal light propagate in opposite directions through an optical fiber transmission line, that is, a bidirectional transmission system. F. Mahlein's paper, "Crostalk talk to simulated Raman Scattering insole-mode fibres for optical communication in wavelength distribution, and a description of the proposal. 409-425, 1984. This method focuses on the fact that the degradation due to Raman crosstalk is caused by the signal light pulse on the short wavelength side and the signal light pulse on the long wavelength side propagating over the optical fiber transmission path over time over time and overlapping. By making the two propagate in opposite directions to the optical fiber transmission line, the temporal overlap between them is averaged, and the attenuation of the short-wave signal light due to the long-wave signal light is averaged over time. Is what you do.
However, H. F. In the paper by Mahlein, only the case where one wave is arranged on the short wavelength side and one wave is arranged on the long wavelength side is described. In order to increase the capacity of the transmission system, the wavelength band on the short wavelength side is used. A plurality of waves and a plurality of waves are arranged in the wavelength band on the long wavelength side. However, when a plurality of signal lights are arranged in each wavelength band in this way, deterioration due to four-wave mixing between adjacent wavelengths becomes a problem.
[0009]
The present invention has been made in view of such circumstances, and reduces degradation due to Raman crosstalk even when a wide band (50 to 200 nm) including a zero dispersion wavelength is used on a non-zero dispersion shift fiber. It is another object of the present invention to provide a wavelength division multiplexing type optical transmission system in which deterioration due to four-wave mixing is reduced.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 of the present invention provides a wavelength division multiplexing method using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of about 1566 nm to 1616 nm as a transmission line by an optical transceiver. In a wavelength division multiplexing type optical transmission system for transmitting and receiving signal light, the optical transceiver is wavelength division multiplexed.RushinIncrease the wavelength of the signal light to 1561 nm or less and 1621 nm or moreSoEqual wavelength interval or equal frequency interval, respectivelyDistributed inAnd transmitting the signal light so that the signal light arranged at 1561 nm or less and the signal light arranged at 1621 nm or more propagate in the non-zero dispersion shifted fiber in opposite directions.
According to a second aspect of the present invention, there is provided a wavelength division multiplexing type in which a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of about 1566 nm to 1616 nm is used as a transmission line to transmit and receive wavelength division multiplexed signal light by an optical transceiver. In the optical transmission system, the optical transmitting and receiving device includes:FaithWavelength of signal light is unequal frequency interval from wavelength 1511nm to 1621nmDistributed inAnd wavelength of 1511 nm or less and 1621 nm or moreTo bothEqual wavelength interval or equal frequency intervalDistributed inAnd transmitting the signal light such that the signal light arranged at 1621 nm or less and the signal light arranged at 1621 nm or more propagate in the opposite direction through the non-zero dispersion shifted fiber. .
Claims3The invention described in the above is a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexing signal light by an optical transceiver using a non-zero dispersion shift fiber whose zero dispersion wavelength is set in a range of approximately 1566 nm to 1616 nm, The optical transceiver is, FaithThe wavelength of the signal light is varied from 1561 nm to 1671 nm at unequal frequency intervals.Distributed inWith a wavelength of 1561 nm or less and 1671 nm or moreBoth sidesThe signal light is arranged at equal wavelength intervals or equal frequency intervals, and of these, the signal light arranged at 1561 nm or less and the signal light arranged at 1561 nm or more propagate in the non-zero dispersion shifted fiber in the opposite direction. Is performed.
[0011]
Claims4The invention described in is a wavelength division multiplexing type optical transmission system that transmits and receives wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of about 1479 nm to 1529 nm, The optical transceiver is wavelength division multiplexed.RushinMake the wavelength of the signal light equal to or less than 1474 nm and equal to or more than 1534 nm at equal wavelength intervals or equal frequency intervals, respectively.Distributed inAnd transmitting the signal light so that the signal light arranged at 1474 nm or less and the signal light arranged at 1534 nm or more propagate through the non-zero dispersion-shifted fiber in opposite directions.
Claims5The invention described in (1) has a zero dispersion wavelength in the range of about 1479 nm to 1529 nm.ConfigurationIn a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transmission and reception device using a non-zero dispersion shifted fiber as a transmission path, the optical transmission and reception device, FaithThe wavelength of the signal light is unequal frequency interval from the wavelength of 1424 nm to 1534 nm.Distributed inWavelength of 1424 nm or less and 1534 nm or moreBoth sidesArranged at equal wavelength intervals or equal frequency intervals. Of these, the signal light arranged at 1534 nm or less and the signal light arranged at 1534 nm or more propagate the non-zero dispersion shifted fiber in the opposite direction. Transmission is performed.
Claims6The invention described in (1) relates to a wavelength division multiplexing type optical transmission system that transmits and receives wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of approximately 1479 nm to 1529 nm. , The optical transceiver,FaithThe wavelength of the signal light is varied from 1474 nm to 1584 nm at unequal frequency intervals.Distributed inWavelengths of 1474 nm or less and 1584 nm or moreBoth sidesEqual wavelength interval or equal frequency intervalDistributed inAnd transmitting the signal light such that the signal light arranged at 1474 nm or less and the signal light arranged at 1474 nm or more propagate in the opposite direction through the non-zero dispersion shifted fiber. .
[0012]
Claims7The invention described in claim 1 is a claim 1 to claim6The wavelength division multiplexing type optical transmission system according to any one of the above, wherein the wavelength division multiplexing type optical transmission system includes one or more linear repeater optical amplifiers in a transmission line for compensating for a loss in the transmission line. And
Claims8The invention described in claim 1 is a claim 1 to claim6In the wavelength division multiplexing type optical transmission system according to any one of the above, the wavelength division multiplexing type optical transmission system has a non-zero dispersion-shifted fiber serving as a transmission line in a transmission path and a dispersion slope of an opposite sign, and has a zero dispersion wavelength substantially. A repeater having equal dispersion compensating fibers is provided, and the repeater performs dispersion compensation of all signal lights.
Claims9The invention described in claim 1 is a claim 1 to claim7In the wavelength division multiplexing type optical transmission system according to any one of the above, the wavelength division multiplexing type optical transmission system may include a demultiplexer that separates signal light in a transmission path according to a propagation direction, For the signal light divided in each direction, a repeater having a dispersion compensation fiber having a dispersion of the same sign as the average dispersion received from the optical fiber transmission line is provided, and dispersion compensation of the signal light is performed in each propagation direction. It is characterized by doing.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a wavelength division multiplexing type optical transmission system according to an embodiment of the present invention will be described with reference to the drawings.
In this embodiment, the wavelength allocation of signal light to be wavelength division multiplexed will be described first, and then the configuration of the wavelength division multiplexing type optical transmission system based on the wavelength allocation of the signal light will be described.
[0014]
First, the basic concept of the wavelength allocation of signal light to be wavelength division multiplexed according to the present invention will be described.
The zero-dispersion wavelength of an optical fiber differs from fiber to fiber due to manufacturing variations. Here, a case is considered where an optical fiber whose zero dispersion wavelength is in the range of λ1 to λ2 [nm] due to manufacturing variations is used as the transmission line. Here, λ1 [nm] represents the lower limit in the distribution of the zero dispersion wavelength, and λ2 [nm] represents the upper limit in the distribution of the zero dispersion wavelength.
In a wavelength division multiplexing type optical transmission system using a wide wavelength range, in order to reduce Raman crosstalk between signal lights at different wavelength distances, a plurality of regions divided into two regions on a short wavelength side and a long wavelength side are used. It is effective to propagate the signal light in the opposite directions to the transmission path.
Here, the wavelengths of a plurality of signal lights to be propagated in mutually opposite directions are defined based on the zero dispersion wavelength of an optical fiber transmission line to be used. This makes it possible to efficiently suppress deterioration due to four-wave mixing.
[0015]
More specifically, the wavelength arrangement of the signal light is preferably as shown in FIGS. That is,
a) A plurality of signal lights having a wavelength (λ2 + 5) [nm] or more and a plurality of signal lights having a wavelength (λ1-5) [nm] or less are propagated in opposite directions (see FIG. 2).
b) A plurality of signal lights having a wavelength of (λ2 + 5) [nm] or more and a plurality of signal lights having a wavelength of (λ2 + 5) [nm] or less propagate in opposite directions to each other, and have a wavelength of (2λ1-λ2-5) [ [nm] or more and (λ2 + 5) [nm] or less are arranged at unequal intervals (see FIG. 3).
c) A plurality of signal lights having a wavelength of (λ1-5) [nm] or more and a plurality of signal lights having a wavelength of (λ1-5) [nm] or less are propagated in opposite directions to each other, and have a wavelength of (λ1-5). ) Differences in optical frequency between a plurality of signal lights of [nm] or more and (2λ2−λ1 + 5) [nm] or less are arranged at unequal intervals (see FIG. 4).
By adopting any one of the above, deterioration due to four-wave mixing can be suppressed.
[0016]
Next, the reason for setting the wavelength arrangement of the signal light as described above will be described in detail.
The reason for using the wavelength arrangement shown in FIG. 2 is as follows.
1) From the results shown in FIG. 1, by setting the wavelength distance of the signal light from the zero-dispersion wavelength to 5 nm (= 0.35 / 0.07) or more, the signal light on the longer wavelength side than the zero-dispersion wavelength or the shorter wavelength It is possible to suppress the generation of four-wave mixing caused by the signal lights on the sides. Therefore, when an optical fiber having a zero-dispersion wavelength in the range of λ1 to λ2 [nm] is used as a transmission line, a part of a plurality of wavelength-division-multiplexed signal lights is divided in consideration of the upper limit λ2 of the zero-dispersion wavelength. By arranging at a wavelength of (λ2 + 5) [nm] or more, occurrence of four-wave mixing in signal lights arranged at a wavelength of (λ2 + 5) [nm] or more can be suppressed.
2) Considering the lower limit λ1 of the zero-dispersion wavelength, the wavelength (λ1-5) [nm] is obtained by arranging a plurality of other wavelength division multiplexed signal lights at a wavelength (λ1-5) [nm] or less. The occurrence of four-wave mixing in the signal lights arranged below can be suppressed.
3) Further, a plurality of signal lights arranged at a wavelength of (λ2 + 5) [nm] or more and a plurality of signal lights arranged at a wavelength of (λ1-5) [nm] or less are propagated in opposite directions to each other. It is possible to suppress the generation of four-wave mixing between them.
From the above three points, it is preferable to set the wavelength arrangement of the signal light as shown in FIG.
[0017]
Next, the reason for using the wavelength arrangement shown in FIG. 3 is as follows.
1) Considering the upper limit λ2 of the zero-dispersion wavelength of the optical fiber, by arranging the signal light at the wavelength (λ2 + 5) [nm] or more, the signal light arranged at the wavelength (λ2 + 5) [nm] or more can be used. Four-wave mixing can be suppressed.
2) Further, a plurality of signal lights arranged at a wavelength of (λ2 + 5) [nm] or more and a plurality of signal lights arranged at a wavelength of (λ2 + 5) [nm] or less are propagated in opposite directions, thereby forming Can suppress the generation of four-wave mixing.
3) When a plurality of signal lights sandwiching the zero-dispersion wavelength λ of the optical fiber in the middle in the wavelength space propagate in the same direction through the optical fiber, the effect of four-wave mixing with the zero-dispersion wavelength λ as a center as described above is a problem. In a frequency band in a range of propagating in the same direction, interference noise is suppressed by arranging the optical frequency differences of the signal light at unequal intervals. Here, considering the worst case where the zero dispersion wavelength λ [nm] of the optical fiber becomes the lower limit λ1, (2λ1−λ2-5) (= λ1 − {[λ2 + 5] −λ1}) [nm] or more (λ2 + 5) ) With respect to a plurality of signal lights of [nm] or less, interference noise is reliably suppressed by arranging the optical frequency differences at unequal intervals.
From the above three points, it is preferable to set the wavelength of the optical signal as shown in FIG.
[0018]
Next, the reason for setting the wavelength arrangement shown in FIG. 4 is the same as that described in FIG. 3, but the following is repeated.
1) Considering the upper limit λ1 of the zero dispersion wavelength of the optical fiber, by arranging the signal light at the wavelength (λ1-5) [nm 2] or more, four-wave mixing between these signal lights can be suppressed. .
2) In addition, a plurality of signal lights arranged at a wavelength (λ1-5) [nm] or more and a plurality of signal lights arranged at a wavelength (λ1-5) [nm] or less propagate in opposite directions. The generation of four-wave mixing can be suppressed between them.
3) When a plurality of signal lights sandwiching the zero-dispersion wavelength λ of the optical fiber in the middle in the wavelength space propagate in the same direction in the optical fiber, the influence of four-wave mixing around the zero-dispersion wavelength λ as described above is a problem. In a frequency band in a range of propagating in the same direction, interference noise is suppressed by arranging the optical frequency differences of the signal light at unequal intervals. Here, considering the worst case where the zero dispersion wavelength λ of the optical fiber becomes the upper limit λ2, (λ1-5) [nm] or more and (2λ2−λ1 + 5) (= λ2 + {λ2- [λ1-5]}) [ nm] or less, interference noise is reliably suppressed by arranging the optical frequency differences at unequal intervals.
From the above three points, it is preferable to set the wavelength arrangement of the optical signal as shown in FIG.
[0019]
In addition, in all three wavelength arrangements described above, the mutual light generated between the signal light having a wavelength of (λ2 + 5) [nm] or more and the signal light having a wavelength of (λ1-5) [nm] or less is obtained. The waveform distortion given to the signal light by the phase modulation acting on the dispersion is reduced. This is because the very large walk-off between the former and the latter cancels the cross-phase modulation. The “cross-phase modulation” means that the local refractive index of the transmission fiber changes due to an optical pulse wave, the instantaneous frequency of another optical pulse wave changes, and the phase of an optical signal changes.
[0020]
The definition for the wavelength arrangement of the signal light described above is based on the case where a non-zero dispersion shift fiber whose zero dispersion wavelength is arranged in a range of about 1566 nm to 1616 nm, that is, when λ1 = 1566 nm and λ2 = 1616 nm,
a) A plurality of signal lights having a wavelength of 1561 (= 1566-5) nm or less and a plurality of signal lights having a wavelength of 1621 (= 1616 + 5) nm or more propagate in opposite directions.
b) A plurality of signal lights having a wavelength of 1621 (= 1616 + 5) nm or more and a plurality of signal lights having a wavelength of 1621 nm or less propagate in opposite directions to each other, and have a wavelength of 1511 (2 × 1566-1616-5) nm or more and 1621 nm or less. Optical frequency differences between multiple signal lights are arranged at unequal intervals
c) A plurality of signal lights having a wavelength of 1561 (= 1566-5) nm or less and a plurality of signal lights having a wavelength of 1561 nm or less propagate in opposite directions to each other, and have a wavelength of 1561 nm to 1671 (= 2 × 1616-1561 + 5) nm or less. The optical frequency differences of the multiple signal lights are arranged at unequal intervals
The wavelength arrangement of any one of the above signal light.
[0021]
On the other hand, when a non-zero dispersion shifted fiber having a zero dispersion wavelength arranged in a range of about 1479 nm to 1529 nm is used, that is, when λ1 = 1479 nm and λ2 = 1529 nm,
a) A plurality of signal lights having a wavelength of 1474 (= 1479-5) nm or less and a plurality of signal lights having a wavelength of 1534 (= 1529 + 5) nm or less propagate in opposite directions.
b) A plurality of signal lights having a wavelength of 1534 (= 1529 + 5) nm or more and a plurality of signal lights having a wavelength of 1534 nm or less propagate in opposite directions to each other, and have a wavelength of 1424 (= 2 × 1479-1529-5) nm or more and 1534 nm or less. The optical frequency differences of the multiple signal lights are arranged at unequal intervals
c) A plurality of signal lights having a wavelength of 1474 (= 1479-5) nm or less and a plurality of signal lights having a wavelength of 1474 nm or more propagate in opposite directions to each other, and have a wavelength of 1474 nm or more and 1584 (= 2 × 1529-1479 + 5) nm or less. The optical frequency differences of the multiple signal lights are arranged at unequal intervals
The wavelength arrangement of any one of the above signal light.
[0022]
Next, a specific example of the wavelength arrangement of the signal light will be described with reference to FIGS. In the following example, an optical transmission / reception device is provided at each end of a fiber transmission line, and a description will be given on the assumption that a predetermined number of signal lights are wavelength division multiplexed for transmission and reception.
[0023]
FIG. 5 is a diagram showing the wavelength arrangement of signal light when a non-zero dispersion shift fiber having a zero dispersion wavelength of 1566 nm or more and 1616 nm or less is used as a fiber transmission line. As shown in FIG. 5, signal light 61 of 16 waves transmitted from one of the optical transmitting and receiving apparatuses is arranged at an interval of 100 GHz in a region having a wavelength of 1561 nm or less. The 16 signal lights 62 transmitted from the other optical transmitting / receiving apparatus are arranged at an interval of 100 GHz in a region having a wavelength of 1621 nm or more.
Accordingly, the plurality of signal lights 61 and the plurality of signal lights 62 separated from each other by the wavelength interval propagate in opposite directions, and thus Raman crosstalk generated therebetween can be avoided. In addition, a plurality of signal lights whose wavelength distance from the zero dispersion wavelength of the non-zero dispersion shifted fiber is within 5 nm or a plurality of signal lights sandwiching the zero dispersion wavelength in the center of the wavelength space do not propagate in the same direction. In addition, it is possible to avoid deterioration of transmission quality due to generation of four-wave mixing. However, the wavelength interval and the number of wavelengths are not limited to the example of FIG.
[0024]
FIG. 6 is also a diagram showing the wavelength arrangement of signal light when a non-zero dispersion shift fiber having a zero dispersion wavelength of 1566 nm or more and 1616 nm or less is used as a fiber transmission line as in FIG. 5, but the wavelength arrangement is different. In the example of FIG. 6, the signal light 63 of 16 waves transmitted from one of the optical transmitting and receiving apparatuses is arranged at an interval of 100 GHz in a region having a wavelength of 1621 nm or more. The twelve signal lights 64 transmitted from the other optical transmitting and receiving device are arranged in a region of a wavelength of 1621 nm or less, with a wavelength of 1575.8 nm being the shortest wavelength, in order of 125, 300, 200, 375, 150, 175, 350, 250. , 150, 325 and 225 GHz. With such an uneven spacing, the optical frequency f_i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kIs generated only at a frequency position at least 25 GHz away from any signal light, and interference noise between the four-wave mixing light and the signal light can be reduced.
Accordingly, the plurality of signal lights 63 and the plurality of signal lights 64 separated from each other by the wavelength interval propagate in opposite directions, so that Raman crosstalk generated between them can be avoided. Further, when a plurality of signal lights 63 and a plurality of signal lights 64 capable of generating four-wave mixing with the zero-dispersion wavelength interposed therebetween when propagating in the same direction propagate in the opposite direction, four-wave mixing between them is performed. Occurrence can be avoided. Furthermore, a plurality of signal lights having a wavelength of 1511 nm or more and 1621 nm or less that generate four-wave mixing to propagate in the same direction are arranged at unequal intervals, thereby reducing interference noise given to the signal light by the generated four-wave mixing light. it can.
However, the wavelength interval and the number of wavelengths are not limited to the example of FIG._i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kHowever, the arrangement may be such that the optical frequency of any multiplexed signal light has a difference equal to or greater than the reception band of the receiver.
[0025]
FIG. 7 also shows the wavelength arrangement of signal light when a non-zero dispersion shift fiber having a zero dispersion wavelength of 1566 nm or more and 1616 nm or less is used as a fiber transmission line, similarly to FIGS. 5 and 6. Are different. In the example of FIG. 7, the signal light 91 of 16 waves transmitted from one optical transmission / reception device is arranged at an interval of 100 GHz in a region having a wavelength of 1561 nm or less. The twelve signal lights 92 transmitted from the other optical transmitting and receiving device are in the region of a wavelength of 1561 nm or more, and the wavelength of 1575.8 nm is the shortest wavelength, and are 125, 300, 200, 375, 150, 175, 350, and 250 in that order. , 150, 325, 225 GHz. With such an uneven spacing, the optical frequency f_i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kIs generated only at a frequency position at least 25 GHz away from any signal light, and interference noise between the four-wave mixing light and the signal light can be reduced.
Accordingly, the plurality of signal lights 91 and the plurality of signal lights 92 which are separated from each other by a wavelength interval propagate in opposite directions, and thus Raman crosstalk generated between them can be avoided. Furthermore, a plurality of signal lights 91 and a plurality of signal lights 92, which can generate four-wave mixing with a zero-dispersion wavelength interposed therebetween when propagating in the same direction, propagate in opposite directions, thereby causing four-wave mixing between them. Can be avoided. Furthermore, a plurality of signal lights having a wavelength of 1561 nm or more and 1671 nm or less, which generate four-wave mixing to propagate in the same direction, are arranged at unequal intervals, thereby reducing interference noise given to the signal light by the generated four-wave mixed light. it can.
However, the wavelength interval and the number of wavelengths are not limited to the example of FIG._i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kHowever, the arrangement may be such that the optical frequency of any multiplexed signal light has a difference equal to or greater than the reception band of the receiver.
[0026]
FIG. 8 is a diagram showing the wavelength arrangement of signal light when a non-zero dispersion shifted fiber having a zero dispersion wavelength of 1479 nm or more and 1529 nm or less is used as a fiber transmission line. In FIG. 8, signal light 101 of 16 waves transmitted from one optical transmitting / receiving device is arranged at an interval of 100 GHz in a region having a wavelength of 1474 nm or less. The 16 signal lights 102 transmitted from the other optical transmitting / receiving apparatus are arranged at a wavelength of 1534 nm or more at intervals of 100 GHz.
Thus, the plurality of signal lights 101 and the plurality of signal lights 102 separated from each other by the wavelength interval propagate in directions opposite to each other, so that Raman crosstalk generated between them can be avoided. Further, since a plurality of signal lights whose wavelength distance from the zero-dispersion wavelength is within 5 nm or a plurality of signal lights sandwiching the zero-dispersion wavelength in the middle of the wavelength space do not propagate in the same direction, four-wave mixing is not performed. Deterioration of transmission quality due to occurrence can be avoided. However, the wavelength interval and the number of wavelengths are not limited to the example of FIG.
[0027]
9 is a diagram showing the wavelength arrangement of signal light when a non-zero dispersion shift fiber having a zero dispersion wavelength of 1479 nm or more and 1529 nm or less is used as a fiber transmission line as in FIG. 8, but the wavelength arrangement is different. In the example of FIG. 9, the 16 signal light beams 111 transmitted from one of the optical transmitting and receiving apparatuses are arranged at an interval of 100 GHz in a region having a wavelength of 1534 nm or more. Also, the 12 signal light 112 transmitted from the other optical transmitting / receiving apparatus is 125, 300, 200, 375, 150, 175, 350, 250 in the range of 1534 nm or less with the wavelength of 1500.8 nm being the shortest wavelength. , 150, 325, 225 GHz. With such an uneven spacing, the optical frequency f_i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kIs generated only at a frequency position at least 25 GHz away from any signal light, and interference noise between the four-wave mixing light and the signal light can be reduced.
Accordingly, the plurality of signal lights 111 and the plurality of signal lights 112 which are separated from each other by a wavelength interval propagate in opposite directions, and thus Raman crosstalk generated therebetween can be avoided. Also, a plurality of signal lights 111 and a plurality of signal lights 112 that can generate four-wave mixing with a zero-dispersion wavelength interposed therebetween when propagating in the same direction are propagated in opposite directions, so that four-wave mixing between them is performed. Can be avoided. Furthermore, a plurality of signal lights having wavelengths of not less than 1424 nm and not more than 1534 nm, which generate four-wave mixing to propagate in the same direction, are arranged at unequal intervals to reduce interference noise given to the signal light by the generated four-wave mixing light. it can.
However, the wavelength interval and the number of wavelengths are not limited to the example of FIG._i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kHowever, the arrangement may be such that the optical frequency of any multiplexed signal light has a difference equal to or greater than the reception band of the receiver.
[0028]
FIG. 10 is a diagram showing the wavelength arrangement of signal light when a non-zero dispersion shift fiber having a zero dispersion wavelength of 1479 nm or more and 1529 nm or less is used as a fiber transmission line as in FIGS. 8 and 9. Are different. In the example of FIG. 10, the 16 signal lights 103 transmitted from one of the optical transmitting and receiving apparatuses are arranged at an interval of 100 GHz in a region having a wavelength of 1474 nm or less. Also, the 12 signal light 104 transmitted from the other optical transmitting and receiving device is set to a wavelength of 1474 nm or more and a wavelength of 1500.8 nm is set as the shortest wavelength, and 125, 300, 200, 375, 150, 175, 350, 250 in order. , 150, 325, 225 GHz. With such an uneven spacing, the optical frequency f_i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kIs generated only at a frequency position at least 25 GHz away from any signal light, and interference noise between the four-wave mixing light and the signal light can be reduced.
Accordingly, the plurality of signal lights 103 and the plurality of signal lights 104 which are separated from each other by a wavelength interval propagate in opposite directions, and thus Raman crosstalk generated between them can be avoided. Further, when the plurality of signal lights 103 and the plurality of signal lights 104 that can generate four-wave mixing with the zero-dispersion wavelength interposed therebetween when propagating in the same direction propagate in opposite directions, the four-wave mixing between them is performed. Occurrence can be avoided. Furthermore, a plurality of signal lights having a wavelength of 1474 nm or more and 1584 nm or less that generate four-wave mixing to propagate in the same direction are arranged at unequal intervals, thereby reducing interference noise given to the signal light by the generated four-wave mixed light. it can.
However, the wavelength interval and the number of wavelengths are not limited to the example of FIG._i, F_j, F_kOptical frequency f of four-wave mixing light generated from any three waves_FWM= F_i+ F_j−f_kHowever, the arrangement may be such that the optical frequency of any multiplexed signal light has a difference equal to or greater than the reception band of the receiver.
[0029]
Next, a configuration example of a wavelength division multiplexing type optical transmission system using a non-zero dispersion shifted fiber as a transmission line and using the wavelength arrangement of signal light described above will be described.
FIG. 11 is a block diagram showing a configuration of a point-to-point wavelength division multiplexing type optical transmission system on a non-zero dispersion shift fiber to which the optical wavelength division multiplexing bidirectional transmission system is applied.
As shown in FIG. 11, the wavelength division multiplexing type optical transmission system includes one optical transmitting / receiving device 54, the other optical transmitting / receiving device 56, and one non-zero dispersion shift fiber transmission line 55 connecting the optical transmitting / receiving devices 54, 56. I have. Here, the plurality of signal lights transmitted from the optical transmitting / receiving device 54 and the plurality of signal lights transmitted from the optical transmitting / receiving device 56 propagate in the non-zero dispersion shifted fiber transmission line 55 in opposite directions.
The optical transmitting / receiving devices 54 and 56 are configured to modulate a plurality of lights having different wavelengths with a plurality of electric signals, multiplex these signals, and transmit the multiplexed light, and a plurality of wavelength-multiplexed plurality of lights having different wavelengths. A receiving circuit 52 that separates the signal light for each wavelength, performs photoelectric conversion on the signal light, and demodulates the signal light into a plurality of electric signals; and a filter or a filter that guides the signal light from the transmission circuit to the transmission path and from the transmission path to the reception circuit. It is composed of a circulator 53. Further, the transmission circuit 51 includes a plurality of light sources (lasers) set to different wavelengths, a plurality of modulators for modulating an optical carrier output from the light source with a data signal, and a multiplexer for wavelength-multiplexing a plurality of signal lights. And so on. The receiving circuit 52 includes a duplexer for separating a plurality of signal lights, a plurality of receivers for demodulating an electric signal from the split signal lights, and the like. Note that the transmission circuit 51 is also applicable to a system that directly modulates the bias or the like of the light source.
[0030]
In the configuration shown in FIG. 11, an optical amplifier is further arranged after the transmission circuit 51 or before the non-zero dispersion shift fiber transmission line 55 in order to compensate for the loss of a plurality of signal lights received by the optical transmission and reception devices 54 and 56. Is also good. Further, an optical amplifier may be further arranged before the receiving circuit 52 or before the receiver in order to increase the signal receiving sensitivity. Available optical amplifiers include erbium-doped fiber amplifiers, gain-shifted erbium-doped fiber amplifiers, thulium-doped fiber amplifiers, fiber Raman amplifiers, and semiconductor optical amplifiers. The erbium-doped fiber amplifier emits signal light having a wavelength of 1530-1565 nm, the gain-shifted erbium-doped fiber amplifier emits signal light having a wavelength of 1570-1605 nm, and the thulium-doped fiber amplifier emits signal light having a wavelength of 1440-1480 nm. Can be amplified all at once. Further, the fiber Raman amplifier and the semiconductor optical amplifier can amplify signal light of any wavelength depending on the wavelength of the pump light and the composition of the semiconductor, respectively.
[0031]
FIG. 12 is a block diagram showing a configuration of a point-to-point multi-relay wavelength division multiplexing type optical transmission system. As shown in FIG. 12, the configuration is such that a linear repeater optical amplifier 71 is inserted in the configuration shown in FIG. 11 in the non-zero dispersion shifted fiber transmission line 55 in order to compensate for the loss. ing. Here, in FIG. 12, parts corresponding to the respective parts in FIG. 11 are denoted by the same reference numerals, and description thereof will be omitted. Although FIG. 12 shows an example in which one linear repeater optical amplifier 71 is provided, a plurality of linear repeater optical amplifiers 71 may be provided at predetermined intervals in a transmission path.
Thus, by providing the linear repeater optical amplifier in the transmission path, the transmission distance of the signal light can be lengthened.
Here, the linear repeater optical amplifier 71 may be a bidirectional optical amplifier that collectively amplifies a plurality of signal lights propagating in opposite directions to each other, or as illustrated in FIG. A configuration in which a plurality of signal lights separated in different directions are amplified by the optical amplifier 62 and multiplexed again by the filter or the circulator 81 with a plurality of signal lights having different propagation directions is also possible.
Also, all signal lights transmitted in two directions may be dispersion-compensated at the time of linear relay. For example, dispersion compensation may be performed collectively using a dispersion compensating fiber having a dispersion slope of the opposite sign to the optical fiber transmission line and having substantially the same zero dispersion wavelength. Thereby, dispersion compensation can be performed with a small number of components. Further, as shown in FIG. 13, the light is separated according to the direction of propagation, and at this time, the signal light in each direction of propagation is distributed by a dispersion compensating fiber having a dispersion substantially equal to the average dispersion received from the optical fiber transmission line and having the opposite sign. , May be dispersion-compensated for each propagation direction. Thereby, dispersion compensation can be performed with higher accuracy.
[0032]
The usage of the above-described optical signal in the wavelength allocation is not limited to the point-to-point transmission systems shown in FIGS. 11 to 13, but can be applied to any network type wavelength division multiplexing type optical transmission system.
[0033]
In the present embodiment, a description has been given by taking a non-zero dispersion-shifted fiber as an example, but when a dispersion-shifted fiber having a zero-dispersion wavelength arranged at 1530 to 1570 nm is used,
a) A plurality of signal lights having a wavelength of 1525 (1530-5) nm or less and a plurality of signal lights having a wavelength of 1575 (= 1570 + 5) nm or more propagate in opposite directions.
b) A plurality of signal lights having a wavelength of 1575 nm or more and a plurality of signal lights having a wavelength of 1575 nm or less propagate in opposite directions, and a plurality of signal lights having a wavelength of 1485 (= 2 × 1530-1570-5) nm or more and 1575 nm or less. Are arranged at unequal intervals
c) A plurality of signal lights having a wavelength of 1525 nm or less and a plurality of signal lights having a wavelength of 1525 nm or more are propagated in opposite directions to each other, and a plurality of signal lights having a wavelength of 1525 nm or more and 1615 (= 2 × 1570-1530 + 5) nm or less. Frequency differences are arranged at unequal intervals
Wavelength arrangement of the optical signal.
[0034]
When a normal single mode fiber having a zero dispersion wavelength of 1300 to 1330 nm is used,
a) A plurality of signal lights having a wavelength of 1335 (= 1330 + 5) nm or more and a plurality of signal lights having a wavelength of 1295 (= 1300-5) nm or less propagate in opposite directions.
b) A plurality of signal lights having a wavelength of 1335 nm or more and a plurality of signal lights having a wavelength of 1335 nm or less propagate in opposite directions, and a plurality of signal lights having a wavelength of 1265 (= 2 × 1300-1330-5) nm or more and 1335 nm or less. Are arranged at unequal intervals
c) A plurality of signal lights having a wavelength of 1295 nm or less and a plurality of signal lights having a wavelength of 1295 nm or more are propagated in opposite directions, and a plurality of signal lights having a wavelength of 1295 nm or more and 1365 (= 2 × 1330-1300 + 5) nm or less. Frequency differences are arranged at unequal intervals
Wavelength arrangement of the optical signal.
[0035]
Further, when a non-zero dispersion shifted fiber whose zero dispersion wavelength is arranged in a range of 1566 nm to 1616 nm is used as a transmission line,
1) Signal light is arranged below 1561 (1566-5) nm
2) Signal light is arranged at 1621 (1616 + 5) nm or more
3) Signal light is arranged at 1561 nm or less and 1621 nm or more, and the propagation direction of the signal light to the transmission path is not specified.
Signal light arrangement. This is because by setting the wavelength distance of the signal light to the zero dispersion wavelength of the non-zero dispersion shifted fiber to be 5 nm or more, four-wave mixing by the signal lights on the longer wavelength side or the shorter wavelength side than the zero dispersion wavelength is performed. Is suppressed. The same can be said for the case where a non-zero dispersion shifted fiber having a zero dispersion wavelength in the range of 1479 nm to 1529 nm is used as a transmission line. Further, the same can be said for a dispersion-shifted fiber having a zero dispersion wavelength of 1530 nm to 1570 nm and a normal single mode fiber having a zero dispersion wavelength of 1300 nm to 1330 nm.
[0036]
Further, the wavelength range when the optical frequency differences of the signal light are arranged at unequal intervals is determined in consideration of the worst value of the zero dispersion wavelength of the transmission line. The optical frequency differences of the signal light may be arranged at unequal intervals in a narrower wavelength range using the distribution statistical value of the manufacturing dispersion of the zero-dispersion wavelength of the optical fiber.
For example, when a non-zero dispersion shifted fiber having a zero dispersion wavelength arranged in the range of 1566 nm to 1616 nm is used as the transmission line, the third specific example shown in FIG. From 1561 nm to 1671 nm. However, assuming that the zero dispersion wavelength of most transmission lines is actually 1571 nm to 1586 nm when considering distribution statistics, the range of signal light to be arranged at unequal intervals is 1561 nm (the signal light is May be narrowed to 1606 (2 × 1586-1571 + 5) nm. The same applies to the second, fifth, and sixth specific examples shown in FIGS. 6, 9, and 10, respectively.
[0037]
【The invention's effect】
As described above, according to the present invention, in an optical wavelength division multiplexing bidirectional transmission system using a wide band (50 to 200 nm) including a zero dispersion wavelength, signal light is transmitted to an optical fiber transmission line at a predetermined wavelength boundary. By propagating in the opposite direction, the temporal overlap between the two can be averaged, and the attenuation that the long-wavelength signal light gives to the short-wavelength signal light through stimulated Raman scattering can be averaged over time. Therefore, deterioration due to Raman crosstalk can be reduced. The averaging of the temporal overlap also averages the cross-phase modulation generated between the two, so that deterioration due to waveform distortion due to cross-phase modulation and fiber dispersion can be avoided. Similarly, by propagating the signal light in opposite directions to the optical fiber transmission line at the predetermined wavelength, the amount of phase mismatch between the two can be increased. Therefore, generation of four-wave mixing between the long-wavelength signal light and the short-wavelength signal light can be suppressed.
By setting the wavelength distance of the optical fiber transmission line to the zero-dispersion wavelength to 5 nm or more, four-wave mixing of long-wavelength signal light or short-wavelength signal light with respect to the zero-dispersion wavelength occurs. Can be suppressed.
Furthermore, when arranging signal light in a wavelength band including the zero-dispersion wavelength of an optical fiber transmission line, signal deterioration due to four-wave mixing is suppressed by arranging the signal light so that the optical frequency difference of the signal light is unequally spaced. In addition, the used optical frequency band can be further expanded.
[Brief description of the drawings]
FIG. 1 is a diagram showing a simulation result of a relationship between chromatic dispersion of signal light and four-wave mixing light intensity.
FIG. 2 is a first diagram illustrating a wavelength arrangement of signal light.
FIG. 3 is a second diagram illustrating the wavelength arrangement of signal light.
FIG. 4 is a third diagram illustrating the wavelength arrangement of signal light.
FIG. 5 is a diagram illustrating a first specific example of a long arrangement of signal light when a non-zero dispersion shifted fiber is used as a transmission line.
FIG. 6 is a diagram for explaining a second specific example of a long arrangement of signal light when a non-zero dispersion shifted fiber is used as a transmission line.
FIG. 7 is a diagram for explaining a third specific example of a long arrangement of signal light when a non-zero dispersion shifted fiber is used as a transmission line.
FIG. 8 is a diagram for explaining a fourth specific example of a long arrangement of signal light when a non-zero dispersion shifted fiber is used as a transmission line.
FIG. 9 is a diagram for explaining a fifth specific example of long arrangement of signal light when a non-zero dispersion shifted fiber is used as a transmission line.
FIG. 10 is a diagram for explaining a sixth specific example of a long arrangement of signal light when a non-zero dispersion shifted fiber is used as a transmission line.
FIG. 11 is a block diagram of a wavelength division multiplexing type optical transmission system.
FIG. 12 is another block diagram of the wavelength division multiplexing type optical transmission system.
FIG. 13 is a diagram illustrating a configuration example of a linear repeater optical amplifier.
[Explanation of symbols]
51 Transmission circuit
52 receiving circuit
53 Filter or circulator
54, 56 Optical transceiver
55 Non-zero dispersion shifted fiber transmission line

Claims

In a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of approximately 1566 nm to 1616 nm,
The optical transceiver,
Hereinafter 1561nm wavelength of the signal light that will be wavelength division multiplexed, and then placed at equal wavelength intervals or equal frequency intervals, respectively it in the above 1621Nm, disposed in the 1561nm or less in place the signal light the 1621Nm more A wavelength division multiplexing type optical transmission system, wherein signal light is transmitted such that the signal light propagates in the opposite direction through the non-zero dispersion shift fiber.

In a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of about 1566 nm to 1616 nm,
The optical transceiver,
The wavelength of the signal light, and placed at unequal frequency intervals from the wavelength 1511Nm to 1621Nm, and placed at equal wavelength intervals or equal frequency intervals in both inclusive and 1621Nm wavelength 1511Nm, among these, arranged below 1621Nm A wavelength division multiplexing type optical transmission system, wherein the signal light is transmitted such that the signal light to be transmitted and the signal light arranged at 1621 nm or more propagate in the non-zero dispersion shift fiber in opposite directions.

In a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of approximately 1566 nm to 1616 nm,
The optical transceiver,
The wavelength of the signal light, and placed at unequal frequency intervals 1671nm wavelength 1561 nm, and arranged at equal wavelength intervals or equal frequency intervals in both inclusive and 1671nm wavelengths 1561 nm, among these, it is arranged below 1561 nm A wavelength division multiplexing type optical transmission system, wherein signal light is transmitted such that the signal light and the signal light arranged at 1561 nm or more propagate in the non-zero dispersion shift fiber in opposite directions.

In a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of approximately 1479 nm to 1529 nm,
The optical transceiver,
Hereinafter 1474nm wavelength of the signal light that will be wavelength division multiplexed, and then placed in a wavelength interval or equal frequency intervals or the like, respectively or 1534 nm, the signal disposed in the 1474nm or less in place the signal light the 1534 nm or more A wavelength division multiplexing type optical transmission system, wherein signal light is transmitted so that light propagates in the non-zero dispersion shift fiber in the opposite direction.

In a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of approximately 1479 nm to 1529 nm,
The optical transceiver,
The wavelength of the signal light, and placed at unequal frequency intervals 1534nm wavelength 1424Nm, arranged at equal wavelength intervals or equal frequency intervals in both inclusive and 1534nm wavelength 1424Nm, among these, it is arranged below 1534nm A wavelength division multiplexing type optical transmission system, wherein the signal light is transmitted such that the signal light and the signal light arranged at 1534 nm or more propagate in the non-zero dispersion shift fiber in opposite directions.

In a wavelength division multiplexing type optical transmission system for transmitting and receiving wavelength division multiplexed signal light by an optical transceiver using a non-zero dispersion shifted fiber whose zero dispersion wavelength is set in a range of about 1479 nm to 1529 nm,
The optical transceiver,
The wavelength of the signal light, and placed at unequal frequency intervals from the wavelength 1474Nm to 1584 nm, and placed at equal wavelength intervals or equal frequency intervals in both inclusive and 1584 nm wavelength 1474Nm, among these, arranged below 1474Nm A wavelength division multiplexing type optical transmission system, wherein the signal light is transmitted such that the signal light to be transmitted and the signal light arranged at 1474 nm or more propagate in the non-zero dispersion shifted fiber in opposite directions.

The wavelength division multiplexing optical transmission system, according to any one of claims 1 to 6, characterized in that it comprises a linear repeater optical amplifier for compensating for loss of the transmission path in the transmission line 1 or more Wavelength division multiplexing type optical transmission system.

The wavelength division multiplexing type optical transmission system includes a repeater having a dispersion compensation fiber having a dispersion slope of an opposite sign and a substantially equal zero dispersion wavelength to a non-zero dispersion shift fiber serving as a transmission line in a transmission path, the repeater comprising: The wavelength division multiplexing type optical transmission system according to any one of claims 1 to 6 , wherein dispersion compensation of all signal lights is performed by:

The wavelength division multiplexing type optical transmission system includes a demultiplexer that separates signal light in a transmission path according to a propagation direction, and an optical fiber transmission path for the signal light divided for each propagation direction by the demultiplexer. with an average dispersion and repeater that includes a dispersion compensating fiber having a dispersion of substantially equal opposite sign received from any of claims 1 to 6, characterized in that performing dispersion compensation of the signal light for each propagation direction 2. A wavelength division multiplexing type optical transmission system according to claim 1.