JP3856303B2

JP3856303B2 - Optical fiber characteristic evaluation method and apparatus

Info

Publication number: JP3856303B2
Application number: JP2002082790A
Authority: JP
Inventors: 邦弘戸毛; 和男保苅; 常雄堀口
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2002-03-25
Filing date: 2002-03-25
Publication date: 2006-12-13
Anticipated expiration: 2022-03-25
Also published as: JP2003279447A

Description

【０００１】
【発明の属する技術分野】
本発明は、ラマン光増幅という光ファイバが持つ非線形効果により生じた信号光の波形変化を解析して、光ファイバの長手方向にわたる光増幅特性等の分布を評価する方法、および、そのための装置に関するものである。
【０００２】
【従来の技術】
ラマン光増幅による光増幅効果を用いて信号品質の劣化を抑制し、光信号を長距離伝送するシステムが検討されている。特に、ＷＤＭ（波長多重）システムにこのラマン光増幅効果を適用することが検討されている。そのためには、光線路の波長に対するラマン利得特性を評価することが重要となる。
【０００３】
ラマン光増幅による利得特性を測定する技術として、特開平２−２３８３３９号公報で開示された「光ファイバの特性評価方法および装置」が知られている。特開平２−２３８３３９号公報が開示した技術を従来技術と呼ぶ。
【０００４】
一方、本願出願人は、特願２００１−３５７４８９号として、ラマン光増幅による利得特性を測定する技術「光ファイバの特性評価方法および装置」を提案している。特願２００１−３５７４８９号が提案した技術を関連技術と呼ぶ。
【０００５】
図１に、関連技術（特開２００１−３５７４８９号）の原理を示し、図２に測定波形の模式図を示す。
【０００６】
関連技術では、図１を参照するに、第１光源１からの光パルスＳ１を長さＬの被測定光ファイバ３にその一端（第１端末）３−１から入射させ、他端（第２端末）３−２より第２光源２からのＣＷ光（連続光）Ｓ２を被測定光ファイバ３に入射させ、被測定光ファイバ３中を光パルスＳ１と対向させて伝搬させて、第１光源１からの光パルスＳ１と第２光源２からのＣＷ光Ｓ２との間の相互作用によりラマン光増幅を生じさせる。詳細は後にも述べるが、ラマン光増幅されたＣＷ光は図２のように、ラマン光増幅により光パワーが増加した部分である増幅成分と、増幅前の光パワーである直流成分とに分けられる。ここで、第１端末３−１から第２端末３−２に向けて被測定光ファイバ３上にｚ軸座標（０≦ｚ≦Ｌ）をとると、位置ｚでラマン光増幅を受けたＣＷ光が、光カプラ５、光波長フィルタ８および光カプラまたは光スイッチ１０を介して検出されるときの光パワーＰ_cw ^d(z）は、次の式（１）で表される
【０００７】

【０００８】
式（１）中、Ｐ_p(0）は第１端末３−１（ｚ＝０）での光パルスの光パワー、Ｐ_cw(L）は第２端末３−２（ｚ＝Ｌ）でのＣＷ光の光パワー、α（λ_p）は光パルスの波長λ_pに対する被測定光ファイバ３の損失係数、α（λ_cw）はＣＷ光の波長λ_cwに対する被測定光ファイバ３の損失係数、Ｃ（λ_cw）は光カプラ５、光波長フィルタ８および光カプラまたは光スイッチ１０の総合した透過率、Ｅ_R(z)は被測定光ファイバ３のラマン利得特性、ｖは被測定光ファイバ３中での光の速度、Ｔは光パルスのパルス幅である。
【０００９】
式（１）中の右辺第１項Ｐ_cw(L)exp［−α（λ_cw）Ｌ］・Ｃ（λ_cw）は、第２光源２からＣＷ光が入射したときにラマン光増幅なしに、α（λ_cw）およびＣ（λ_cw）による減衰を受けた光パワーを示しており、すなわちこれが直流成分である。
【００１０】
また、式（１）中の右辺第２項Ｐ_cw(L)exp［−α（λ_cw）Ｌ］・Ｃ（λ_cw）・Ｅ_R(z）Ｐ_p(0)exp［−α（λ_p）ｚ］(vＴ/2）が、ラマン光増幅で生じた光パワーの増加分であり、すなわち増幅成分を示している。
【００１１】
したがって、式（１）から、ラマン利得特性Ｅ_R(z）は、▲１▼増幅成分、▲２▼直流成分、▲３▼被測定光ファイバ３に入射したＣＷ光Ｓ２のパワー、さらに▲４▼被測定光ファイバ３の損失分布を測定し、時間的波形変化を解析することで、求めることができる。
【００１２】
しかし、式（１）から分かるように、増幅成分は直流成分に依存するため、ラマン利得特性Ｅ_R(z）を測定するには直流成分と増幅成分のそれぞれの値を知る必要があるが、増幅成分は直流成分に比較して非常に小さく、これら２つの成分を同時に測定するには高ダイナミックレンジかつ高分解能な信号処理装置が必要となるという問題がある。
【００１３】
この問題を解決するため、関連技術（特開２００１−３５７４８９号）では、これら増幅成分と直流成分を、光カプラ５、光波長フィルタ８および光カプラまたは光スイッチ１０を介して、第１光検出器６と第２光検出器９で分けて検出し、それぞれを信号処理装置７で信号処理することで、ラマン利得特性Ｅ_R(z）の測定精度を向上させている。
【００１４】
また、図１に示すような装置構成によって、複数の光ファイバを接続したために測定光ファイバ３の区間毎にファイバ損失が異なり途中に接続点が存在する場合においても、各光ファイバや接続点の損失は波長に対して変動するが、２つの光検出器６、９にてその変動を含めて増幅成分と直流成分を測定することから、第２光源２の波長に対する各光ファイバおよび接続点毎での光損失を測定する必要がない。
【００１５】
【発明が解決しようとする課題】
上述したように、ラマン利得特性Ｅ_R(z）は、▲１▼増幅成分、▲２▼直流成分、▲３▼被測定光ファイバ３に入射した光パルスの光パワー、さらに▲４▼被測定光ファイバ３の損失分布を測定し、時間的波形変化を解析することにより求められる。
【００１６】
しかし、上記▲１▼〜▲４▼という多くの測定項目があるため、多くの装置を使用する必要がある。このため、装置構成が複雑になり、かつ，測定に手間がかかるという問題がある。さらに、測定の結果からラマン利得特性Ｅ_R(z）を算出する上での計算が煩雑になるという問題もある。
【００１７】
そこで本発明の目的は、ラマン利得特性が既に分かっている光ファイバを参照光ファイバとして被測定光ファイバと接続させることで、上記の測定項目を削減し、簡易化された光ファイバの特性評価方法および装置構成を提供することにある。
【００１８】
【課題を解決するための手段】
第１発明は上記課題を解決する光ファイバの特性評価方法であり、第１光源からの光パルスを被測定光ファイバの一端から入射させ、第２光源からのＣＷ光を前記被測定光ファイバのもう一方の端から入射させて、前記第１光源からの光パルスと対向させて伝搬させて、前記第１光源からの光パルスと第２光源からのＣＷ光との間の相互作用によりラマン光増幅を生じさせ、光カプラ、光波長フィルタを介して前記ＣＷ光を光検出器に導く光ファイバの特性評価方法において、ラマン利得特性について既知の光ファイバを参照光ファイバとして１本または複数本の被測定光ファイバと位置を特定して接続させて同時に測定を行い、参照光ファイバのラマン利得特性と被測定光ファイバのラマン利得特性の比率を求めることにより、被測定光ファイバのラマン利得特性を算出することを特徴とする。
第２発明は、第１発明において、前記被測定光ファイバの長手方向に前記参照光ファイバを接続することを特徴とする光ファイバの特性評価方法である。
第３発明は、第１発明又は第２発明において、前記比率を、被測定光ファイバの各位置で測定した増幅成分および光パルスの波長に対する損失分布と、参照光ファイバのすくなくとも１つの位置で測定した増幅成分および光パルスの波長に対する損失分布とから求めることを特徴とする光ファイバの特性評価方法である。
第４発明は、第１発明又は第２発明において、前記比率を、一端から前記光パルスを入射し、もう一方の端から前記ＣＷ光を入射して測定した被測定光ファイバの各位置での増幅成分および参照光ファイバのすくなくとも１つの位置での増幅成分と、一端から前記ＣＷ光を入射し、もう一方の端から前記光パルスを入射することにより、前記光パルスと前記ＣＷ光の入射端を入れ替えた後に測定した被測定光ファイバの各位置での増幅成分および参照光ファイバの前記すくなくとも１つの位置での増幅成分とから求めることを特徴とする光ファイバの特性評価方法である。
【００１９】
第５発明は上記課題を解決する光ファイバの特性評価装置であり、第１光源、既にラマン利得特性が分かっている参照光ファイバ、出力する光の波長を変えることができる第２光源、光カプラ、光波長フィルタ、光検出器および信号処理装置から構成され、１本または複数本の被測定光ファイバに、位置を特定して前記参照光ファイバを接続して１本の光ファイバ列とし、第１光源からの光パルスを前記光ファイバ列の一端から入射させ、第２光源からのＣＷ光を前記光ファイバ列のもう一方の端から入射させ、前記第１光源からの光パルスと対向させて伝搬させて、前記第１光源からの光パルスと前記第２光源からのＣＷ光との間の相互作用によりラマン光増幅を生じさせ、光カプラ、光波長フィルタを介して、ＣＷ光の光パワーのうち、ラマン光増幅による成分を検出する光検出器および信号処理装置を配置して、前記光ファイバ列の測定を行い、参照光ファイバのラマン利得特性と被測定光ファイバのラマン利得特性の比率を求めることにより、被測定光ファイバのラマン利得特性を算出することを特徴とする。
第６発明は、第５発明において、前記被測定光ファイバの長手方向に前記参照光ファイバを接続することを特徴とする光ファイバの特性評価装置である。
第７発明は、第５発明又は第６発明において、前記光検出器にて、前記被測定光ファイバの各位置でのＣＷ光の増幅成分および前記参照光ファイバのすくなくとも１つの位置でのＣＷ光の増幅成分を測定し、前記信号処理装置にて、前記被測定光ファイバおよび前記参照光ファイバの前記測定した増幅成分と、前記被測定光ファイバの各位置での光パルスの波長に対する損失分布と、前記参照光ファイバのすくなくとも１つの位置での光パルスの波長に対する損失分布とから、前記参照光ファイバのラマン利得特性と前記被測定光ファイバのラマン利得特性の比率を求め、求めた比率と前記参照光ファイバのラマン利得特性から前記被測定光ファイバのラマン利得特性を算出することを特徴とする光ファイバの特性評価装置である。
第８発明は、第５発明又は第６発明において、前記光検出器にて、前記光パルスを入射する一端と前記ＣＷ光を入射するもう一方の端とを入れ替える前における前記被測定光ファイバの各位置での増幅成分および前記参照光ファイバのすくなくとも１つの位置での増幅成分と、前記一端と前記もう一方の端とを入れ替えた後における前記被測定光ファイバの各位置での増幅成分および前記参照光ファイバの前記すくなくとも１つの位置での増幅成分とを測定し、前記信号処理装置にて、前記一端と前記もう一方の端とを入れ替える前に測定した増幅成分および前記一端と前記もう一方の端とを入れ替えた後に測定した増幅成分とから、前記参照光ファイバのラマン利得特性と前記被測定光ファイバのラマン利得特性の比率を求め、求めた比率と前記参照光ファイバのラマン利得特性から前記被測定光ファイバのラマン利得特性を算出することを特徴とする光ファイバの特性評価装置である。
【００２０】
【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を説明する。
【００２１】
［装置構成例］
図３は、本発明の実施の形態に係る光ファイバの特性評価装置の構成例を示す。図３において、１は光パルスＳ１を出力する第１光源、２はＣＷ光（連続光）Ｓ２を出力する第２光源、３は被測定光ファイバ、４は参照光ファイバ、５は光カプラ、６は光検出器、７は信号処理装置、８は光波長フィルタである。
【００２２】
本発明では、詳細は後述するが基本的な光ファイバの特性評価方法として、第１光源１からの光パルスＳ１を被測定光ファイバ３の一端から入射させ、第２光源２からのＣＷ光Ｓ２を被測定光ファイバ３のもう一方の端から入射させて、第１光源１からの光パルスＳ１と対向させて伝搬させて、第１光源１からの光パルスＳ１と第２光源２からのＣＷ光Ｓ２との間の相互作用によりラマン光増幅を生じさせ、光カプラ５、光波長フィルタ８を介してＣＷ光を光検出器６に導く光ファイバの特性評価方法において、ラマン利得特性について既知の光ファイバ４を参照光ファイバとして１本または複数本の被測定光ファイバ３と接続させて同時に測定し、参照光ファイバ４のラマン利得特性と被測定光ファイバ３のラマン利得特性の比率を求めることにより、被測定光ファイバ３のラマン利得特性を算出する。
【００２３】
また、本発明では、詳細は後述するが基本的な光ファイバの特性評価装置として、第１光源１、既にラマン利得特性が分かっている参照光ファイバ４、出力する光の波長を変えることができる第２光源２、光カプラ５、光波長フィルタ８、光検出器６および信号処理装置７で構成し、被測定光ファイバ３と参照光ファイバ４を接続し、第１光源１からの光パルスＳ１を参照光ファイバ４または被測定光ファイバ３の一端から入射させ、第２光源２からのＣＷ光Ｓ２を被測定光ファイバ３または参照光ファイバ４のもう一方の端から入射させ、第１光源１からの光パルスＳ１と対向させて伝搬させて、第１光源１からの光パルスＳ１と第２光源２からのＣＷ光Ｓ２との間の相互作用によりラマン光増幅を生じさせ、光カプラ５、光波長フィルタ８を介して、ＣＷ光の光パワーのうち、ラマン光増幅による成分を検出する光検出器６および信号処理装置７を配置する。
【００２４】
［第１実施例］
本発明の第１実施例として、図３に示す構成により被測定光ファイバ３のラマン利得特性を評価する方法および装置を説明する。
【００２５】
ここで、参照光ファイバ４の長さをｌ^*（ただし、ｌ^*はアルファベット「Ｌ」の小文字を表す）、被測定光ファイバ３の長さをＬ−ｌ^*とし、互いに接続したものを用いて測定を行う。本実施例では、被測定光ファイバ３と参照光ファイバ４とが接続されてなる全体の光ファイバうち、第１光源１から出射された光パルスＳ１が参照光ファイバ４に入射する位置を第１端末３−１と定義し、第２光源２から出射されたＣＷ光（連続光）Ｓ２が被測定光ファイバ３に入射する位置を第２端末３−２と定義する。また、第１端末３−１から第２端末３−２に向けてｚ軸座標と定義する。このような定義の下で、参照光ファイバ４と被測定光ファイバ３上の任意の位置ｚ（０≦ｚ≦Ｌ）での光パルスの光パワーＰ_p(z）は、次の式（２）となる。また、任意の位置ｚ（０≦ｚ≦Ｌ）でのＣＷ光のパワーＰ_cw(z）は、次の式（３）となる。
Ｐ_p(z）＝Ｐ_p(0)exp［−α（λ_p）ｚ］式（２）
Ｐ_cw(z）＝Ｐ_cw(L)exp［−α（λ_cw）（Ｌ−ｚ）］式（３）
【００２６】
式（２）中、Ｐ_p(0）は第１端末３−１（ｚ＝０）での光パルスの光パワー、α（λ_p）は光パルスの波長λ_pに対する被測定光ファイバ３の損失係数でありる。また、式（３）中、Ｐ_cw(L）は第２端末３−２（ｚ＝Ｌ）でのＣＷ光の光パワー、α（λ_cw）はＣＷ光の波長λ_cwに対する被測定光ファイバ３の損失係数である。
【００２７】
また、参照光ファイバ４と被測定光ファイバ３を合わせた全体の光ファイバでのラマン利得特性をＥ_R(z）＝ｇ（λ_cw,z）／Ａ(z）と定義する。ここで、ｇ（λ_cw,z）は位置ｚにおけるＣＷ光の波長λ_cwでのラマン利得であり、Ａ(z）は位置ｚにおける光ファイバの実効断面積である。さらに、光ファイバ中での光の速度をｖ＝ｃ／ｎ（ｃは光速、ｎは参照光ファイバ４と被測定光ファイバ３のコア屈折率で同一の値とする。）、Ｔを光パルスのパルス幅と定義する。このような定義の下で、位置ｚではラマン増幅によって、ＣＷ光は次の式（４）で与えられるＧ(z）で増幅される。
Ｇ(z）＝exp[Ｅ_R(z）Ｐ_p(z）(vＴ/2)] 式（４）
【００２８】
Ｅ_R(z）Ｐ_p(z）(vＴ/2）が小さな領域では、式（４）は次の式（５）と近似することができる。この条件は通常成立する。
Ｇ(z）≒１＋Ｅ_R(z）Ｐ_p(z）(vＴ/2）式（５）
【００２９】
したがって、位置ｚでは、ラマン光増幅によってＣＷ光の光パワーはＧ(z）Ｐ_cw(z）となる。さらに、ラマン光増幅によって増幅したＣＷ光は光検出器６に到達するまでに、光ファイバの光損失exp[−α（λ_cw）z]で減衰し、また、参照光ファイバ４と被測定光ファイバ３との接続点、光カプラ５および光波長フィルタ８を合わせた透過率Ｃ（λ_cw）によって減衰する。光検出器６の前段に配置された光波長フィルタ８は、第１光源１からの光パルスにより発生する後方レーリー散乱光を除去し、ラマン光増幅による効果のみを取り出す役目を果たす。
【００３０】
これらの減衰（exp[−α（λ_cw）z]およびＣ（λ_cw））を考慮して、式（２）、式（３）および式（５）より、光検出器６にて受光される光パワーＰ_cw ^d(z）は、次の式（６）となる。

【００３１】
式（６）中の右辺第１項は、第２光源２からＣＷ光が入射したときにラマン光増幅なしに、α（λ_cw）およびＣ（λ_cw）による減衰を受けた光パワーを示しており、すなわちこれが直流成分であり、式の簡略化のために直流成分をＰ_cont ^dと定義すると、次の式（７）となる。また、式（６）中の右辺第２項がラマン光増幅で生じた光パワーの増加分であり、すなわち増幅成分を示しており、これも同様にＰ_amp ^d(z）と定義すると、次の式（８）となる。これより、増幅成分Ｐ_amp ^d(z）は直流成分Ｐ_cont ^dの大きさに依存することが分かる。

【００３２】
ここで、全体のラマン利得特性Ｅ_R(z）（０≦ｚ≦Ｌ）のうち、参照光ファイバ４の既知のラマン利得特性、すなわち０≦ｚ≦１^*の範囲におけるラマン利得特性をＥ_R(z_ref）と定義し、被測定光ファイバ３の未知のラマン利得特性、すなわち１^*≦ｚ≦Ｌの範囲におけるラマン利得特性をＥ_R(z_m）と定義すると、式（８）は次の２つの式（９）、式（10）に分けることができる。ｚ_refは０から１^*の任意の位置、ｚ_mは１^*からＬの任意の位置である。

【００３３】
式（９）を式（10）で除算すると、次の式（11）を得る。

【００３４】
式（11）の右辺で、分子は参照光ファイバ４について検出されたＣＷ光の増幅成分Ｐ_amp ^d(z_ref）と光パルスの波長λ_pに対する損失分布exp[−α（λ_p）_ref］の逆数との積を示しており、分母は被測定光ファイバ３について検出されたＣＷ光の増幅成分Ｐ_amp ^d(z_m）と光パルスの波長λ_pに対する損失分布exp[−α（λ_p）ｚ_m］の逆数との積を示している。
【００３５】
すなわち、参照光ファイバ４と被測定光ファイバ３のラマン利得特性の比率Ｅ_R(z_ref）／Ｅ_R(z_m）は、参照光ファイバ４と被測定光ファイバ３のそれぞれについてＣＷ光の増幅成分および光パルスの波長に対する損失分布の測定結果（Ｐ_amp ^d(z_ref）、Ｐ_amp ^d(z_m）、exp[−α（λ_p）ｚ_ref］、exp[−α（λ_p）ｚ_m］）より求でることができる。
【００３６】
比率Ｅ_R(z_ref）／Ｅ_R(z_m）をＲとすれば、例えば、Ｒ＝｛Ｐ_amp ^d(z_ref）exp[α（λ_p）ｚ_ref］｝／｛Ｐ_amp ^d(z_m）exp[α（λ_p）ｚ_m］｝である。そして、参照光ファイバ４のラマン利得特性Ｅ_R(z_ref）は既知であるので、したがって、得られたラマン利得特性の比率Ｒより、被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）は、一般にＥ_R(z_ref）／Ｒの演算で求めることができる。次の式（12ａ）〜式（12ｄ）に、Ｅ_R(z_m）＝Ｅ_R(z_ref）／Ｒと等価な演算式の例を示す。

【００３７】
式（１１）は、０から１^*の任意の位置ｚ_ref、および、１^*からＬの任意の位置ｚ_mにおいて成立する。したがって、Ｅ_R(z_m）＝Ｅ_R(z_ref）／Ｒや式（12ａ）〜式（12ｄ）で被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）を求めるには、被測定光ファイバ３については、１^*からＬの範囲の各位置ｚ_mでのＰ_amp ^d(z_m）と、光パルスの波長に対する損失分布exp[−α（λ_p）ｚ_m］を知る必要があるが、参照光ファイバ４については、任意の１つの位置におけるラマン利得特性Ｅ_R(z_ref）と、ＣＷ光の増幅成分Ｐ_amp ^d(z_ref）と、光パルスの波長に対する損失分布exp[−α（λ_p）ｚ_ref］が判れば良い。
【００３８】
参照光ファイバ４の増幅成分Ｐ_amp ^d(z_ref）および被測定光ファイバ３の増幅成分Ｐ_amp ^d(z_m）は、光検出器６にて検出する。つまり、参照光ファイバ４および被測定光ファイバ３の直流成分は測定せず、増幅成分Ｐ_amp ^d(z_ref）とＰ_amp ^d(z_m）のみ測定すれば良い。
【００３９】
そのため、高ダイナミックレンジを持つ光検出器は必要なく、関連技術（特願平２００１−３５７４８９号）に述べられているような第２検出器９を用いることもなく、ラマン利得特性Ｅ_R(z_m）が測定可能である。
【００４０】
さらに、第１光源１からの光パルスＳ１の光パワーＰ_p(0) を測定する必要がなく、光パルスの波長λ_pに対する損失分布exp[−α（λ_p）ｚ］を測定すれば良い。
【００４１】
参照光ファイバ４と被測定光ファイバ３のラマン利得特性の比率Ｒの算出および比率Ｒから被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）＝Ｅ_R(z_ref）／Ｒの算出、あるいは式（12ａ）〜式（12ｄ）による被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）の算出は、信号処理装置７にて行われる。その際、被測定光ファイバ３については、１^*からＬの範囲の各位置ｚ_mでのＰ_amp ^d(z_m）と、光パルスの波長に対する損失分布exp[−α（λ_p）ｚ_m］を測定する必要がある。参照光ファイバ４については、少なくとも任意の１つの位置におけるラマン利得特性Ｅ_R(z_ref）と、ＣＷ光の増幅成分Ｐ_amp ^d(z_ref）と、光パルスの波長に対する損失分布exp[−α（λ_p）ｚ_ref］が判れば良い。なお、信号処理装置７は算出結果や測定結果等の表示も行うようにしている。
【００４２】
損失分布exp[−α（λ_p）ｚ］の測定方法としては、公知の技術を用いることができる。例えば、従来技術（特開平２−２３８３３９号公報）のように、第１端末３−１と第２端末３−２を入れ替えてそれぞれラマン光増幅波形を得る両方向測定によって、損失分布exp[−α（λ_p）ｚ］を簡単に求めることができる。あるいは、第１光源１と同じ波長のパルス光源を持つＯＴＤＲ（Optical time Domain Reflectmeter）と呼ばれる光パルス試験器を用いて、損失分布exp[−α（λ_p）ｚ］を測定しても良い。
【００４３】
なお、参照光ファイバ４に複数の被測定光ファイバ３を接続し、長手方向にわたって光ファイバの損失が分布するような場合においても、同様に、上記原理で被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）を簡単に測定することができる。
【００４４】
また、本実施例では参照光ファイバ４は光パルスＳ１が入射する第１端末３−１に位置させているが、被測定光ファイバ３との接続における参照光ファイバ４の位置に関係なく式（11）は成立し、同様に測定可能である。
【００４５】
さらに、波長に対するラマン利得特性を測定する場合においても、式（11）で示す関係が成立し、第２光源２のＣＷ光Ｓ２の波長λ_cwを変化させるだけで、同様に被測定光ファイバ３の波長に対するラマン利得特性Ｅ_R(z_m）を簡単に測定可能である。
【００４６】
［第２実施例」
次に、本発明の第２実施例として、図３に示す構成により被測定光ファイバ３のラマン利得特性を評価する別の方法および装置を説明する。本実施例では、後述するように、被測定光ファイバ３および参照光ファイバ４ともに、光パルスの波長に対する損失分布exp[−α（λ_p）ｚ_m］、損失分布exp[−α（λ_p）ｚ_ref］の測定は不要である。
【００４７】
本実施例の測定方法は、先ず第１端末３−１において第１光源１からの光パルスＳ１を入射させ、第２端末３−２において第２光源２からのＣＷ（連続）光Ｓ２を入射させて光検出器６にて測定した波形と、次に第１端末３−１と第２端末３−２を入れ替えて、第２端末３−２において第１光源１からの光パルスＳ１を入射させ、第１端末３−１において第２光源２からのＣＷ（連続）光Ｓ２を入射させて光検出器６にて測定した波形との両者を比較することにより、被測定光ファイバ３のラマン利得特性を求めるという測定方法である。
【００４８】
第１実施例と同様、第１端末３−１から第１光源１の光パルスＳ１を入射させ、第２端末３−２から第２光源２のＣＷ（連続）光Ｓ２を入射させ、ラマン光増幅によって増幅されたＣＷ光の光パワーを、光カプラ５および光波長フィルタ８を介して、光検出器６で検出して得られる増幅成分Ｐ_amp ^d(z）は、前述した式（８）となる。
Ｐ_amp ^d(z）＝Ｐ_cont ^dＥ_R(z）Ｐ_p(0)exp［−α（λ_p）ｚ］(vＴ/2）式（８）
【００４９】
次に、第１端末３−１と第２端末３−２を入れ替えて同様の測定を行うと、得られる増幅成分Ｐ_amp ^d(z)'、つまり、第２端末３−２から第１光源１の光パルスＳ１を入射させ、第１端末３−１から第２光源２のＣＷ（連続）光Ｓ２を入射させ、ラマン光増幅によって増幅されたＣＷ光の光パワーを同様に光検出器６で検出して得られる増幅成分Ｐ_amp ^d(z)'は、次の式（13ａ）となる。式（13ａ）は、ｚをＬ−ｚと入れ替えることで、次の式（13ｂ）となる
Ｐ_amp ^d(z)'＝Ｐ_cont ^dＥ_R(L-z）Ｐ_p(0)exp［−α（λ_p）ｚ］(vＴ/2）式（13ａ）
Ｐ_amp ^d(L-z)'＝Ｐ_cont ^dＥ_R(z）Ｐ_p(0)exp［−α（λ_p)(L-z)］(vＴ/2）式（13ｂ）
【００５０】
そして、式（８）と式（13ｂ）より、左辺どうし、右辺どうしを掛け合わせてることで、次の式(14)を得る。

【００５１】
この式（14）について、第１実施例と同様に参照光ファイバ４、すなわち０≦ｚ≦１^*の範囲におけるラマン利得特性をＥ_R(z_ref）と定義し、被測定光ファイバ３の未知のラマン利得特性、すなわち１^*≦ｚ≦Ｌの範囲におけるラマン利得特性をＥ_R(z_m）と定義すると、式（14）は次の２つの式（15）、式（16）に分けることができる。ｚ_refは０から１^*の任意の位置、ｚ_mは１^*からＬの任意の位置である。

【００５２】
式（15）を式（16）で除算すると、次の式（17）の関係が得られる。

【００５３】
第１端末３−１と第２端末３−２を入れ替える前の位置ｚと、入れ替えた後の位置Ｌ−ｚは、各光ファイバの同じ部分を示している。そこで、式（17）の右辺は、参照光ファイバ４について両方向から測定した任意の同一位置での増幅成分どうしを掛け合わせた時の値と、被測定光ファイバ３について両方向から測定した任意の同一位置での増幅成分どうしを掛け合わせた時の値との比であり、この比が参照光ファイバ４のラマン利得特性をＥ_R(z_ref）と、被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）との比の自乗となることを示している。式（17）の増幅成分Ｐ_amp ^d(z_ref）、Ｐ_amp ^d(L-z_ref)'、Ｐ_amp ^d(z_m）およびＰ_amp ^d(L-z_m)'は光検出器６にて測定される。
【００５４】
したがって、参照光ファイバ４と被測定光ファイバ３のラマン利得特性の比率Ｅ_R(z_ref）／Ｅ_R(z_m）は、参照光ファイバ４と被測定光ファイバ３のそれぞれについてＣＷ光の増幅成分の測定結果（Ｐ_amp ^d(z_ref）、Ｐ_amp ^d(L-z_ref)'、Ｐ_amp ^d(z_m）、Ｐ_amp ^d(L-z_m)'）より求めることができる。
【００５５】
比率Ｅ_R(z_ref）／Ｅ_R(z_m）をＲとすれば、例えば、Ｒ＝｛Ｐ_amp ^d(z_ref）Ｐ_amp ^d(L-z_ref)'｝^1/2／｛Ｐ_amp ^d(z_m）Ｐ_amp ^d(L-z_m)'｝^1/2である。そして、参照光ファイバ４のラマン利得特性Ｅ_R(z_ref）は既知であるので、第１実施例と同様、得られたラマン利得特性の比率Ｒより、被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）は、一般にＥ_R(z_ref）／Ｒの演算で求めることができる。次の式（18）に、Ｅ_R(z_m）＝Ｅ_R(z_ref）／Ｒと等価な演算式の例を示す。

【００５６】
式（１７）は、０から１^*の任意の位置ｚ_ref、および、１^*からＬの任意の位置ｚ_mにおいて成立する。したがって、Ｅ_R(z_m）＝Ｅ_R(z_ref）／Ｒや式（18）で被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）を求めるには、被測定光ファイバ３については、１^*からＬの範囲の各位置ｚ_mでのＣＷ光の増幅成分Ｐ_amp ^d(z_m）およびＰ_amp ^d(L-z_m)'を知る必要があるが、参照光ファイバ４については、任意の１つの位置におけるラマン利得特性Ｅ_R(z_ref）と、ＣＷ光の増幅成分Ｐ_amp ^d(z_ref）およびＰ_amp ^d(L-z_ref)'が判れば良い。
【００５７】
本実施例では、式（17）から見ても分かるように、光パルスはもとより、直流成分の光パワーを測定する必要がなく、簡単に被測定光ファイバ３のラマン利得特性を測定することが可能である。さらに、本実施例では、測定結果からラマン利得特性を算出する過程において、第１実施例で求めていた光パルスの波長λ_pにおける損失分布exp[−α（λ_p）ｚ］を測定する必要がなく、利得の算出が容易にできるという利点がある。
【００５８】
参照光ファイバ４と被測定光ファイバ３のラマン利得特性の比率Ｒの算出および比率Ｒから被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）＝Ｅ_R(z_ref）／Ｒの算出、あるいは式（18）による被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）の算出は、信号処理装置７にて行われる。その際、被測定光ファイバ３については、１^*からＬの範囲の各位置ｚ_mでのＣＷ光の増幅成分Ｐ_amp ^d(z_m）およびＰ_amp ^d(L-z_m)'を測定する必要がある。参照光ファイバ４については、少なくとも任意の１つの位置におけるラマン利得特性Ｅ_R(z_ref）と、ＣＷ光の増幅成分Ｐ_amp ^d(z_ref）およびＰ_amp ^d(L-z_ref)'とが判れば良い。
【００５９】
なお本実施例でも、第１実施例と同様、参照光ファイバ４に複数の被測定光ファイバ３を接続する場合、被測定光ファイバ３に対する参照光ファイバ４の位置を変えた場合、さらに、第２光源２の波長λ_cwを変化させて波長に対するラマン利得特性を測定する場合、いずれについても式（17）の関係が成立し、同様に上記原理で被測定光ファイバ３のラマン利得特性Ｅ_R(z_m）を簡単に測定することができる。
【００６０】
【発明の効果】
以上の説明したように、本発明では、ラマン利得特性（ラマン光増幅の特性分布）の評価において、ラマン利得特性が既知の参照光ファイバを被測定光ファイバと接続し、同時に測定することによって、参照光ファイバの既知のラマン利得特性と被測定光ファイバのラマン利得特性との比率を求めることにより、被測定光ファイバのラマン利得特性を算出する。このような本発明の光ファイバの特性評価方法および装置を用いることで、ラマン利得特性を算出する上で従来技術では必要な測定項目であった第１光源からの光パルスのパワーの測定および検出されるＣＷ光の直流成分の測定が不要となった。したがって、ラマン利得特性を簡単に求めることが可能であり、測定方法および測定装置を大幅に簡素化でき、短時間で光ファイバのラマン利得特性の分布を測定することができる。
【図面の簡単な説明】
【図１】関連技術（特開２００１−３５７４８９号）の測定装置の構成を示す図。
【図２】同関連技術における測定波形例を模式的に示す図。
【図３】本発明の実施の形態における装置構成を示す図。
【符号の説明】
１第１光源
２第２光源
３被測定光ファイバ
３−１第１端末
３−２第２端末
４参照光ファイバ
５光カプラ
６光検出器
７信号処理装置
８光波長フィルタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for evaluating the distribution of optical amplification characteristics and the like in the longitudinal direction of an optical fiber by analyzing a change in waveform of signal light caused by a nonlinear effect of an optical fiber called Raman optical amplification, and an apparatus therefor Is.
[0002]
[Prior art]
A system that transmits optical signals over long distances by suppressing degradation of signal quality using an optical amplification effect by Raman optical amplification has been studied. In particular, it has been studied to apply this Raman light amplification effect to a WDM (wavelength multiplexing) system. For that purpose, it is important to evaluate the Raman gain characteristic with respect to the wavelength of the optical line.
[0003]
As a technique for measuring gain characteristics by Raman light amplification, an “optical fiber characteristic evaluation method and apparatus” disclosed in Japanese Patent Laid-Open No. 2-238339 is known. The technique disclosed in Japanese Patent Laid-Open No. 2-238339 is called a conventional technique.
[0004]
On the other hand, the applicant of the present application has proposed, as Japanese Patent Application No. 2001-357489, a technique “optical fiber characteristic evaluation method and apparatus” for measuring gain characteristics by Raman amplification. The technique proposed by Japanese Patent Application No. 2001-357489 is referred to as a related technique.
[0005]
FIG. 1 shows the principle of the related art (Japanese Patent Laid-Open No. 2001-357489), and FIG. 2 shows a schematic diagram of a measured waveform.
[0006]
In the related art, referring to FIG. 1, the light pulse S1 from the first light source 1 is incident on the measured optical fiber 3 having a length L from one end (first terminal) 3-1 and the other end (second Terminal) 3-2 causes CW light (continuous light) S2 from the second light source 2 to enter the optical fiber 3 to be measured, and propagates in the optical fiber 3 to be measured so as to oppose the optical pulse S1. The Raman light amplification is caused by the interaction between the light pulse S1 from 1 and the CW light S2 from the second light source 2. As will be described in detail later, as shown in FIG. 2, the CW light amplified by Raman light is divided into an amplification component that is a portion where the optical power is increased by Raman light amplification and a direct current component that is the optical power before amplification. . Here, when taking a z-axis coordinate (0 ≦ z ≦ L) on the optical fiber 3 to be measured from the first terminal 3-1 to the second terminal 3-2, the CW subjected to Raman light amplification at the position z. Optical power P when light is detected through the optical coupler 5, the optical wavelength filter 8 and the optical coupler or optical switch 10_cw ^d(z) is expressed by the following equation (1).
[0007]

[0008]
In formula (1), P_p(0) is the optical power of the optical pulse at the first terminal 3-1 (z = 0), P_cw(L) is the optical power of the CW light at the second terminal 3-2 (z = L), α (λ_p) Is the wavelength λ of the light pulse_pLoss factor of the optical fiber 3 to be measured, α (λ_cw) Is the wavelength λ of the CW light_cwLoss factor of the optical fiber 3 to be measured, C (λ_cw) Is the total transmittance of the optical coupler 5, optical wavelength filter 8 and optical coupler or optical switch 10, E_R(z) is the Raman gain characteristic of the optical fiber 3 to be measured, v is the speed of light in the optical fiber 3 to be measured, and T is the pulse width of the optical pulse.
[0009]
The first term P on the right-hand side in equation (1)_cw(L) exp [−α (λ_cw) L] · C (λ_cw) Is expressed by α (λ) without Raman light amplification when CW light is incident from the second light source 2._cw) And C (λ_cw) Shows the optical power that has been attenuated by (i.e., the DC component).
[0010]
Further, the second term P on the right side in the formula (1)_cw(L) exp [−α (λ_cw) L] · C (λ_cw) ・ E_R(z) P_p(0) exp [−α (λ_p) Z] (vT / 2) is an increase in optical power generated by Raman amplification, that is, an amplification component.
[0011]
Therefore, from the equation (1), the Raman gain characteristic E_R(z) measures (1) amplification component, (2) DC component, (3) power of the CW light S2 incident on the measured optical fiber 3, and (4) loss distribution of the measured optical fiber 3, It can be obtained by analyzing the temporal waveform change.
[0012]
However, as can be seen from the equation (1), since the amplification component depends on the DC component, the Raman gain characteristic E_RTo measure (z), it is necessary to know the values of the DC component and the amplified component, but the amplified component is very small compared to the DC component, and a high dynamic range is required to measure these two components simultaneously. In addition, there is a problem that a high-resolution signal processing device is required.
[0013]
In order to solve this problem, in the related technique (Japanese Patent Laid-Open No. 2001-357589), the amplification component and the direct current component are subjected to the first optical detection via the optical coupler 5, the optical wavelength filter 8, and the optical coupler or the optical switch 10. The detector 6 and the second photodetector 9 detect the signals separately, and signal processing is performed by the signal processing device 7 so that the Raman gain characteristic E_RThe measurement accuracy of (z) is improved.
[0014]
In addition, even when a plurality of optical fibers are connected by the apparatus configuration as shown in FIG. 1 and the fiber loss is different for each section of the measurement optical fiber 3 and connection points exist in the middle, each optical fiber or connection point Although the loss fluctuates with respect to the wavelength, since the amplification component and the direct current component are measured by the two

photodetectors

6 and 9 including the fluctuation, each optical fiber and each connection point with respect to the wavelength of the second light source 2 are measured. There is no need to measure the optical loss at.
[0015]
[Problems to be solved by the invention]
As described above, the Raman gain characteristic E_R(z) measures (1) amplification component, (2) DC component, (3) optical power of optical pulse incident on the optical fiber 3 to be measured, and (4) loss distribution of the optical fiber 3 to be measured. It is obtained by analyzing the temporal waveform change.
[0016]
However, since there are many measurement items (1) to (4), it is necessary to use many devices. For this reason, there exists a problem that an apparatus structure becomes complicated and a measurement takes time. Furthermore, the Raman gain characteristic E_RThere is also a problem that the calculation for calculating (z) becomes complicated.
[0017]
Therefore, an object of the present invention is to connect an optical fiber whose Raman gain characteristic is already known as a reference optical fiber to the optical fiber to be measured, thereby reducing the above measurement items and simplifying the optical fiber characteristic evaluation method. And providing an apparatus configuration.
[0018]
[Means for Solving the Problems]
  The first invention is an optical fiber characteristic evaluation method that solves the above-mentioned problems. A light pulse from a first light source is incident from one end of the optical fiber to be measured, and CW light from a second light source is incident on the optical fiber to be measured. The light is incident from the other end, propagates in opposition to the light pulse from the first light source, and Raman light is generated by the interaction between the light pulse from the first light source and the CW light from the second light source. In an optical fiber characteristic evaluation method that causes amplification and guides the CW light to a photodetector through an optical coupler and an optical wavelength filter, one or a plurality of optical fibers whose Raman gain characteristics are known are used as reference optical fibers. The position of the optical fiber to be measured is specified and connected to perform measurement simultaneously, and the ratio of the Raman gain characteristic of the reference optical fiber to the Raman gain characteristic of the optical fiber to be measured is obtained. And calculates the Raman gain characteristics of the driver.
  A second invention is the optical fiber characteristic evaluation method according to the first invention, wherein the reference optical fiber is connected in the longitudinal direction of the optical fiber to be measured.
  According to a third invention, in the first invention or the second invention, the ratio is measured at at least one position of the amplification component and the loss distribution with respect to the wavelength of the optical pulse measured at each position of the measured optical fiber and the reference optical fiber. It is an optical fiber characteristic evaluation method characterized in that it is obtained from the amplified component and the loss distribution with respect to the wavelength of the optical pulse.
  A fourth invention is the first invention or the second invention, wherein the ratio isThe light pulse is incident from one end and the CW light is incident from the other end.The measured amplifying component at each position of the measured optical fiber and the amplifying component at at least one position of the reference optical fiber;By entering the CW light from one end and the light pulse from the other end, the incident ends of the light pulse and the CW light are changed.An optical fiber characteristic evaluation method characterized in that it is obtained from an amplification component at each position of an optical fiber to be measured measured after replacement and an amplification component at at least one position of a reference optical fiber.
[0019]
  First5The present invention is an optical fiber characteristic evaluation apparatus that solves the above-mentioned problems, and includes a first light source, a reference optical fiber whose Raman gain characteristics are already known, a second light source capable of changing the wavelength of output light, an optical coupler, and an optical device. It consists of a wavelength filter, a photodetector and a signal processing device,One or moreOptical fiber to be measuredAnd locateConnect the reference optical fiberOne optical fiber arrayThe light pulse from the first light sourceOptical fiber arrayOf CW light from the second light source.Optical fiber arrayThe light is incident from the other end of the light source, propagated in opposition to the light pulse from the first light source, and Raman is generated by the interaction between the light pulse from the first light source and the CW light from the second light source. A photodetector and a signal processing device are provided that detect optical components of the CW light through the optical coupler and the optical wavelength filter, and detect the component due to the Raman optical amplification through the optical coupler.Then, the optical fiber array is measured, and the Raman gain characteristic of the measured optical fiber is calculated by obtaining the ratio of the Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the measured optical fiber.It is characterized by that.
  A sixth invention is an optical fiber characteristic evaluation apparatus according to the fifth invention, wherein the reference optical fiber is connected in the longitudinal direction of the optical fiber to be measured.
  First7Invention5inventionOr the sixth inventionThe optical detector measures the CW light amplification component at each position of the optical fiber to be measured and the CW light amplification component at at least one position of the reference optical fiber. The measured amplification component of the measured optical fiber and the reference optical fiber, the loss distribution with respect to the wavelength of the optical pulse at each position of the measured optical fiber, and at least one position of the reference optical fiber A ratio between the Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the measured optical fiber is obtained from the loss distribution with respect to the wavelength of the optical pulse, and the measured light is calculated from the obtained ratio and the Raman gain characteristic of the reference optical fiber. An optical fiber characteristic evaluation apparatus for calculating a Raman gain characteristic of a fiber.
  First8Invention5inventionOr the sixth inventionIn the optical detector, the amplification component at each position of the optical fiber to be measured and the reference optical fiber before the one end where the optical pulse is incident and the other end where the CW light is incident are replaced with each other. At least the amplification component at one position and the amplification component at each position of the optical fiber to be measured after the one end and the other end are exchanged, and the reference optical fiberSaidAt least the amplified component at one position is measured, and the signal processing device replaces the measured amplified component and the one end with the other end before exchanging the one end with the other end. A ratio between the Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the optical fiber to be measured is obtained from the amplification component measured later, and the ratio of the Raman gain characteristic of the optical fiber to be measured is determined from the obtained ratio and the Raman gain characteristic of the reference optical fiber. An optical fiber characteristic evaluation apparatus that calculates Raman gain characteristics.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0021]
[Device configuration example]
FIG. 3 shows a configuration example of the optical fiber characteristic evaluation apparatus according to the embodiment of the present invention. In FIG. 3, 1 is a first light source that outputs an optical pulse S1, 2 is a second light source that outputs CW light (continuous light) S2, 3 is an optical fiber to be measured, 4 is a reference optical fiber, 5 is an optical coupler, 6 is a photodetector, 7 is a signal processing device, and 8 is an optical wavelength filter.
[0022]
In the present invention, as will be described later in detail, as a basic optical fiber characteristic evaluation method, the optical pulse S1 from the first light source 1 is incident from one end of the optical fiber 3 to be measured, and the CW light S2 from the second light source 2 is entered. Is incident from the other end of the optical fiber 3 to be measured and propagates in opposition to the light pulse S1 from the first light source 1, and the light pulse S1 from the first light source 1 and the CW from the second light source 2 are transmitted. In a method for evaluating characteristics of an optical fiber that causes Raman light amplification by interaction with the light S2 and guides CW light to the photodetector 6 through the optical coupler 5 and the optical wavelength filter 8, the Raman gain characteristics are known. The optical fiber 4 is connected to one or a plurality of optical fibers to be measured 3 as a reference optical fiber and simultaneously measured, and the ratio between the Raman gain characteristic of the reference optical fiber 4 and the Raman gain characteristic of the optical fiber 3 to be measured is obtained. And the calculated Raman gain characteristics of the measured optical fiber 3.
[0023]
In the present invention, as will be described in detail later, as a basic optical fiber characteristic evaluation apparatus, the first light source 1, the reference optical fiber 4 whose Raman gain characteristic is already known, and the wavelength of the output light can be changed. A second light source 2, an optical coupler 5, an optical wavelength filter 8, a photodetector 6, and a signal processing device 7 are connected to connect the measured optical fiber 3 and the reference optical fiber 4, and an optical pulse S 1 from the first light source 1. Is incident from one end of the reference optical fiber 4 or the measured optical fiber 3, and the CW light S2 from the second light source 2 is incident from the other end of the measured optical fiber 3 or the reference optical fiber 4, and the first light source 1 The optical pulse S1 from the first light source 1 is propagated to cause Raman amplification by the interaction between the light pulse S1 from the first light source 1 and the CW light S2 from the second light source 2, and the optical coupler 5, Optical wavelength filter Through, of the light power of the CW light, placing a light detector 6 and the signal processing unit 7 for detecting a component due to Raman amplification.
[0024]
[First embodiment]
As a first embodiment of the present invention, a method and apparatus for evaluating the Raman gain characteristic of the optical fiber 3 to be measured with the configuration shown in FIG. 3 will be described.
[0025]
Here, the length of the reference optical fiber 4 is l^*(However, l^*Represents the lowercase letter of the alphabet “L”), and the length of the optical fiber 3 to be measured is L−l.^*And measure using those connected to each other. In the present embodiment, the position at which the light pulse S1 emitted from the first light source 1 is incident on the reference optical fiber 4 among the entire optical fibers formed by connecting the optical fiber 3 to be measured and the reference optical fiber 4 to the first optical fiber 1 is determined. The terminal 3-1 is defined, and the position where the CW light (continuous light) S <b> 2 emitted from the second light source 2 enters the optical fiber 3 to be measured is defined as a second terminal 3-2. Moreover, it defines as a z-axis coordinate toward the 2nd terminal 3-2 from the 1st terminal 3-1. Under such a definition, the optical power P of the optical pulse at an arbitrary position z (0 ≦ z ≦ L) on the reference optical fiber 4 and the optical fiber 3 to be measured._p(z) becomes the following equation (2). Further, the power P of the CW light at an arbitrary position z (0 ≦ z ≦ L)_cw(z) becomes the following equation (3).
P_p(z) = P_p(0) exp [−α (λ_pZ] Formula (2)
P_cw(z) = P_cw(L) exp [−α (λ_cw(Lz)] Formula (3)
[0026]
In formula (2), P_p(0) is the optical power of the optical pulse at the first terminal 3-1 (z = 0), α (λ_p) Is the wavelength λ of the light pulse_pIs the loss coefficient of the optical fiber 3 to be measured. In formula (3), P_cw(L) is the optical power of the CW light at the second terminal 3-2 (z = L), α (λ_cw) Is the wavelength λ of the CW light_cwIs the loss coefficient of the optical fiber 3 to be measured.
[0027]
Further, the Raman gain characteristic of the entire optical fiber including the reference optical fiber 4 and the measured optical fiber 3 is expressed as E_R(z) = g (λ_cw, z) / A (z). Where g (λ_cw, z) is the wavelength λ of the CW light at position z_cwA (z) is the effective area of the optical fiber at the position z. Further, the speed of light in the optical fiber is v = c / n (where c is the speed of light, n is the same value of the core refractive index of the reference optical fiber 4 and the optical fiber 3 to be measured), and T is an optical pulse. The pulse width is defined as Under such a definition, CW light is amplified by G (z) given by the following equation (4) by Raman amplification at the position z.
G (z) = exp [E_R(z) P_p(z) (vT / 2)] Equation (4)
[0028]
E_R(z) P_pIn the region where (z) (vT / 2) is small, Equation (4) can be approximated by the following Equation (5). This condition usually holds.
G (z) ≒ 1 + E_R(z) P_p(z) (vT / 2) Equation (5)
[0029]
Therefore, at position z, the optical power of the CW light is G (z) P by Raman amplification._cw(z). Further, the CW light amplified by the Raman light amplification reaches the optical detector 6 until the optical loss exp [−α (λ_cw) Z], and the transmittance C (λ) including the connection point of the reference optical fiber 4 and the optical fiber 3 to be measured, the optical coupler 5 and the optical wavelength filter 8 is combined._cw). The optical wavelength filter 8 arranged in the front stage of the photodetector 6 serves to remove the backward Rayleigh scattered light generated by the light pulse from the first light source 1 and to extract only the effect of Raman light amplification.
[0030]
These attenuations (exp [−α (λ_cw) Z] and C (λ_cw)), The optical power P received by the photodetector 6 from the equations (2), (3), and (5)._cw ^d(z) becomes the following equation (6).

[0031]
The first term on the right side in equation (6) is expressed as α (λ without Raman light amplification when CW light is incident from the second light source 2._cw) And C (λ_cw), That is, this is a direct current component, and the direct current component is changed to P for simplification of the equation._cont ^dThen, the following equation (7) is obtained. In addition, the second term on the right side of the equation (6) is an increase in optical power generated by Raman light amplification, that is, an amplification component, which is also represented by P_amp ^dWhen defined as (z), the following equation (8) is obtained. From this, the amplification component P_amp ^d(z) DC component P_cont ^dIt turns out that it depends on the size of.

[0032]
Here, the overall Raman gain characteristic E_R(z) (0 ≦ z ≦ L), the known Raman gain characteristic of the reference optical fiber 4, that is, 0 ≦ z ≦ 1^*The Raman gain characteristics in the range_R(z_ref) And the unknown Raman gain characteristic of the optical fiber 3 to be measured, that is, 1^*The Raman gain characteristic in the range of ≦ z ≦ L is expressed as E_R(z_m), Equation (8) can be divided into the following two equations (9) and (10). z_refIs 0 to 1^*Any position of z_mIs 1^*To any position of L.

[0033]
When equation (9) is divided by equation (10), the following equation (11) is obtained.

[0034]
On the right side of equation (11), the numerator is the amplified component P of the CW light detected for the reference optical fiber 4._amp ^d(z_ref) And optical pulse wavelength λ_pLoss distribution exp [−α (λ_p)_ref], And the denominator is the amplified component P of the CW light detected for the optical fiber 3 to be measured._amp ^d(z_m) And optical pulse wavelength λ_pLoss distribution exp [−α (λ_pZ_m] And the reciprocal of the product.
[0035]
That is, the ratio E of the Raman gain characteristics of the reference optical fiber 4 and the measured optical fiber 3_R(z_ref) / E_R(z_m) Is a measurement result (P of the loss distribution with respect to the wavelength of the amplified component of the CW light and the wavelength of the optical pulse for each of the reference optical fiber 4 and the measured optical fiber 3._amp ^d(z_ref), P_amp ^d(z_m), Exp [−α (λ_pZ_ref], Exp [−α (λ_pZ_m]).
[0036]
Ratio E_R(z_ref) / E_R(z_m) Is R, for example, R = {P_amp ^d(z_ref) Exp [α (λ_pZ_ref]} / {P_amp ^d(z_m) Exp [α (λ_pZ_m]}. And the Raman gain characteristic E of the reference optical fiber 4_R(z_ref) Is known, therefore, the Raman gain characteristic E of the optical fiber 3 to be measured is determined from the ratio R of the obtained Raman gain characteristics._R(z_m) Is generally E_R(z_ref) / R. In the following formulas (12a) to (12d), E_R(z_m) = E_R(z_refAn example of an arithmetic expression equivalent to /) R is shown.

[0037]
Equation (11) is from 0 to 1^*Any position z_refAnd 1^*To any position z from L_mThis holds true. Therefore, E_R(z_m) = E_R(z_ref) / R and the Raman gain characteristic E of the optical fiber 3 to be measured by the equations (12a) to (12d)_R(z_m) For the optical fiber 3 to be measured^*Each position z in the range from to L_mP in_amp ^d(z_m) And loss distribution exp [−α (λ_pZ_mIt is necessary to know the Raman gain characteristic E at any one position for the reference optical fiber 4._R(z_ref) And amplification component P of CW light_amp ^d(z_ref) And loss distribution exp [−α (λ_pZ_ref] Should be understood.
[0038]
Amplification component P of reference optical fiber 4_amp ^d(z_ref) And the amplification component P of the optical fiber 3 to be measured_amp ^d(z_m) Is detected by the photodetector 6. That is, the direct current components of the reference optical fiber 4 and the optical fiber 3 to be measured are not measured, and the amplification component P_amp ^d(z_ref) And P_amp ^d(z_m) Only need to be measured.
[0039]
Therefore, a photodetector having a high dynamic range is not necessary, and the Raman gain characteristic E is not used without using the second detector 9 as described in the related art (Japanese Patent Application No. 2001-357589)._R(z_m) Is measurable.
[0040]
Furthermore, the optical power P of the optical pulse S1 from the first light source 1_pIt is not necessary to measure (0) and the wavelength λ of the optical pulse_pLoss distribution exp [−α (λ_p) Z] may be measured.
[0041]
Calculation of the ratio R of the Raman gain characteristic between the reference optical fiber 4 and the measured optical fiber 3 and the Raman gain characteristic E of the measured optical fiber 3 from the ratio R_R(z_m) = E_R(z_ref) / R or the Raman gain characteristic E of the optical fiber 3 to be measured according to the equations (12a) to (12d)._R(z_m) Is calculated by the signal processing device 7. At that time, for the optical fiber 3 to be measured, 1^*Each position z in the range from to L_mP in_amp ^d(z_m) And loss distribution exp [−α (λ_pZ_m] Need to be measured. For the reference optical fiber 4, the Raman gain characteristic E at least in any one position_R(z_ref) And amplification component P of CW light_amp ^d(z_ref) And loss distribution exp [−α (λ_pZ_ref] Should be understood. The signal processing device 7 also displays calculation results, measurement results, and the like.
[0042]
Loss distribution exp [−α (λ_p) As a measuring method of z], a known technique can be used. For example, as in the prior art (Japanese Patent Laid-Open No. 2-238339), the loss distribution exp [−α is obtained by bidirectional measurement in which the first terminal 3-1 and the second terminal 3-2 are interchanged to obtain a Raman optical amplification waveform. (Λ_p) Z] can be easily determined. Alternatively, using an optical pulse tester called OTDR (Optical time Domain Reflectmeter) having a pulse light source having the same wavelength as that of the first light source 1, the loss distribution exp [−α (λ_p) Z] may be measured.
[0043]
In the case where a plurality of optical fibers to be measured 3 are connected to the reference optical fiber 4 and the loss of the optical fiber is distributed over the longitudinal direction, the Raman gain characteristic E of the optical fiber to be measured 3 is similarly determined based on the above principle._R(z_m) Can be measured easily.
[0044]
In this embodiment, the reference optical fiber 4 is positioned at the first terminal 3-1 on which the optical pulse S 1 is incident. However, regardless of the position of the reference optical fiber 4 in connection with the optical fiber 3 to be measured, 11) holds and can be measured in the same way.
[0045]
Further, in the case of measuring the Raman gain characteristic with respect to the wavelength, the relationship expressed by the equation (11) is established, and the wavelength λ of the CW light S2 of the second light source 2 is satisfied._cwIn the same manner, the Raman gain characteristic E with respect to the wavelength of the optical fiber 3 to be measured is changed._R(z_m) Can be measured easily.
[0046]
[Second Example]
Next, as a second embodiment of the present invention, another method and apparatus for evaluating the Raman gain characteristic of the optical fiber 3 to be measured with the configuration shown in FIG. 3 will be described. In this embodiment, as will be described later, in both the measured optical fiber 3 and the reference optical fiber 4, the loss distribution exp [−α (λ_pZ_m], Loss distribution exp [−α (λ_pZ_ref] Is not required.
[0047]
In the measurement method of the present embodiment, first, the light pulse S1 from the first light source 1 is incident on the first terminal 3-1, and the CW (continuous) light S2 from the second light source 2 is incident on the second terminal 3-2. Then, the waveform measured by the photodetector 6 and the first terminal 3-1 and the second terminal 3-2 are exchanged, and the light pulse S1 from the first light source 1 is incident on the second terminal 3-2. Then, the CW (continuous) light S2 from the second light source 2 is incident on the first terminal 3-1, and the waveform measured by the photodetector 6 is compared with the Raman of the optical fiber 3 to be measured. This is a measurement method for obtaining gain characteristics.
[0048]
As in the first embodiment, the light pulse S1 of the first light source 1 is incident from the first terminal 3-1, the CW (continuous) light S2 of the second light source 2 is incident from the second terminal 3-2, and Raman light is incident. An amplification component P obtained by detecting the optical power of the CW light amplified by the amplification by the photodetector 6 through the optical coupler 5 and the optical wavelength filter 8._amp ^d(z) becomes the above-described formula (8).
P_amp ^d(z) = P_cont ^dE_R(z) P_p(0) exp [−α (λ_pZ] (vT / 2) Equation (8)
[0049]
Next, when the same measurement is performed by exchanging the first terminal 3-1 and the second terminal 3-2, the obtained amplification component P is obtained._amp ^d(z) ′, that is, the light pulse S1 of the first light source 1 is incident from the second terminal 3-2, the CW (continuous) light S2 of the second light source 2 is incident from the first terminal 3-1, and the Raman light is incident. Similarly, an amplified component P obtained by detecting the optical power of the CW light amplified by the amplification with the photodetector 6._amp ^d(z) ′ becomes the following expression (13a). Expression (13a) becomes the following expression (13b) by replacing z with Lz.
P_amp ^d(z) '= P_cont ^dE_R(L-z) P_p(0) exp [−α (λ_pZ] (vT / 2) Equation (13a)
P_amp ^d(L-z) '= P_cont ^dE_R(z) P_p(0) exp [−α (λ_p) (L-z)] (vT / 2) Equation (13b)
[0050]
Then, the following equation (14) is obtained by multiplying the left sides and the right sides from the equations (8) and (13b).

[0051]
For this equation (14), the reference optical fiber 4, that is, 0 ≦ z ≦ 1, as in the first embodiment.^*The Raman gain characteristics in the range_R(z_ref) And the unknown Raman gain characteristic of the optical fiber 3 to be measured, that is, 1^*The Raman gain characteristic in the range of ≦ z ≦ L is expressed as E_R(z_m), The equation (14) can be divided into the following two equations (15) and (16). z_refIs 0 to 1^*Any position of z_mIs 1^*To any position of L.

[0052]
When equation (15) is divided by equation (16), the following equation (17) is obtained.

[0053]
The position z before replacing the first terminal 3-1 and the second terminal 3-2 and the position Lz after the replacement indicate the same part of each optical fiber. Therefore, the right side of the equation (17) represents the value obtained by multiplying the amplified components at arbitrary identical positions measured from both directions with respect to the reference optical fiber 4, and the arbitrary identical measured from both directions with respect to the measured optical fiber 3. This ratio is a ratio to the value obtained by multiplying amplification components at positions, and this ratio represents the Raman gain characteristic of the reference optical fiber 4 as E_R(z_ref) And Raman gain characteristic E of the optical fiber 3 to be measured_R(z_m) To the square of the ratio. Amplification component P in equation (17)_amp ^d(z_ref), P_amp ^d(L-z_ref) ', P_amp ^d(z_m) And P_amp ^d(L-z_m) ′ Is measured by the photodetector 6.
[0054]
Therefore, the ratio E of the Raman gain characteristics of the reference optical fiber 4 and the measured optical fiber 3_R(z_ref) / E_R(z_m) Is a measurement result (P of amplification component of CW light for each of the reference optical fiber 4 and the optical fiber 3 to be measured._amp ^d(z_ref), P_amp ^d(L-z_ref) ', P_amp ^d(z_m), P_amp ^d(L-z_m) ').
[0055]
Ratio E_R(z_ref) / E_R(z_m) Is R, for example, R = {P_amp ^d(z_ref) P_amp ^d(L-z_ref) '}^1/2/ {P_amp ^d(z_m) P_amp ^d(L-z_m) '}^1/2It is. And the Raman gain characteristic E of the reference optical fiber 4_R(z_ref) Is already known, the Raman gain characteristic E of the measured optical fiber 3 is determined from the ratio R of the obtained Raman gain characteristics as in the first embodiment._R(z_m) Is generally E_R(z_ref) / R. In the following equation (18), E_R(z_m) = E_R(z_refAn example of an arithmetic expression equivalent to /) R is shown.

[0056]
Equation (17) is from 0 to 1^*Any position z_refAnd 1^*To any position z from L_mThis holds true. Therefore, E_R(z_m) = E_R(z_ref) / R and Equation (18), the Raman gain characteristic E of the optical fiber 3 to be measured_R(z_m) For the optical fiber 3 to be measured^*Each position z in the range from to L_mAmplification component P of CW light at_amp ^d(z_m) And P_amp ^d(L-z_m) ′, It is necessary to know the Raman gain characteristic E at any one position for the reference optical fiber 4._R(z_ref) And amplification component P of CW light_amp ^d(z_ref) And P_amp ^d(L-z_ref) 'Should be understood.
[0057]
In this embodiment, as can be seen from the equation (17), it is not necessary to measure the optical power of the direct current component as well as the optical pulse, and the Raman gain characteristic of the optical fiber 3 to be measured can be easily measured. Is possible. Furthermore, in this embodiment, in the process of calculating the Raman gain characteristics from the measurement result, the wavelength λ of the optical pulse obtained in the first embodiment is obtained._pLoss distribution exp [−α (λ_p) Z] does not need to be measured, and the gain can be easily calculated.
[0058]
Calculation of the ratio R of the Raman gain characteristic between the reference optical fiber 4 and the measured optical fiber 3 and the Raman gain characteristic E of the measured optical fiber 3 from the ratio R_R(z_m) = E_R(z_ref) / R or the Raman gain characteristic E of the optical fiber 3 to be measured according to the equation (18)_R(z_m) Is calculated by the signal processing device 7. At that time, for the optical fiber 3 to be measured, 1^*Each position z in the range from to L_mAmplification component P of CW light at_amp ^d(z_m) And P_amp ^d(L-z_m) 'Must be measured. For the reference optical fiber 4, the Raman gain characteristic E at least in any one position_R(z_ref) And amplification component P of CW light_amp ^d(z_ref) And P_amp ^d(L-z_ref) '.
[0059]
Also in this embodiment, as in the first embodiment, when a plurality of optical fibers to be measured 3 are connected to the reference optical fiber 4, when the position of the reference optical fiber 4 with respect to the optical fiber to be measured 3 is changed, Wavelength λ of two light sources 2_cwWhen the Raman gain characteristic with respect to the wavelength is measured while changing the relationship, the relationship of the equation (17) is established in both cases, and similarly, the Raman gain characteristic E of the optical fiber 3 to be measured is based on the above principle._R(z_m) Can be measured easily.
[0060]
【The invention's effect】
As described above, in the present invention, in the evaluation of Raman gain characteristics (characteristic distribution of Raman light amplification), a reference optical fiber having a known Raman gain characteristic is connected to the optical fiber to be measured, and simultaneously measured, The ratio of the known Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the measured optical fiber is obtained to calculate the Raman gain characteristic of the measured optical fiber. By using such an optical fiber characteristic evaluation method and apparatus of the present invention, the measurement and detection of the power of the optical pulse from the first light source, which is a necessary measurement item in the prior art for calculating the Raman gain characteristic, is performed. Measurement of the direct current component of the CW light is no longer necessary. Therefore, the Raman gain characteristic can be easily obtained, the measurement method and the measurement apparatus can be greatly simplified, and the distribution of the Raman gain characteristic of the optical fiber can be measured in a short time.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a measuring apparatus according to related technology (Japanese Patent Laid-Open No. 2001-357589).
FIG. 2 is a diagram schematically showing an example of a measurement waveform in the related technology.
FIG. 3 is a diagram showing an apparatus configuration according to the embodiment of the present invention.
[Explanation of symbols]
1 First light source
2 Second light source
3 Optical fiber to be measured
3-1 First terminal
3-2 Second terminal
4 Reference optical fiber
5 Optical coupler
6 Light detector
7 Signal processor
8 Optical wavelength filter

Claims

A light pulse from the first light source is incident from one end of the optical fiber to be measured, and CW light (continuous light) from the second light source is incident from the other end of the optical fiber to be measured. Propagating in opposition to the light pulse of
An optical fiber that causes Raman light amplification by the interaction between the light pulse from the first light source and the CW light from the second light source, and guides the CW light to a photodetector through an optical coupler and an optical wavelength filter. In the characterization method of
The Raman gain characteristics are measured using a known optical fiber as a reference optical fiber, specifying and connecting the position to one or more optical fibers to be measured,
An optical fiber characteristic evaluation method, comprising: calculating a Raman gain characteristic of a measured optical fiber by calculating a ratio between a Raman gain characteristic of a reference optical fiber and a Raman gain characteristic of the measured optical fiber.

In claim 1,
A method for evaluating characteristics of an optical fiber, wherein the reference optical fiber is connected in a longitudinal direction of the optical fiber to be measured.

In claim 1 or claim 2,
The ratio is
Loss distribution with respect to the wavelength of the amplified component and the optical pulse measured at each position of the measured optical fiber,
A method for evaluating characteristics of an optical fiber, which is obtained from an amplification component measured at at least one position of a reference optical fiber and a loss distribution with respect to a wavelength of an optical pulse.

In claim 1 or claim 2,
The ratio is
An amplification component at each position of the optical fiber to be measured and an amplification component at at least one position of the reference optical fiber measured by entering the optical pulse from one end and entering the CW light from the other end ;
Amplified components at each position of the optical fiber to be measured measured after the CW light is incident from one end and the light pulse is incident from the other end to switch the light pulse and the incident end of the CW light. And an optical fiber characteristic evaluation method characterized in that it is obtained from the amplification component at at least one position of the reference optical fiber.

A first light source, a reference optical fiber whose Raman gain characteristics are already known, a second light source capable of changing the wavelength of the output light, an optical coupler, an optical wavelength filter, a photodetector, and a signal processing device,
The reference optical fiber is connected to one or a plurality of optical fibers to be measured and the reference optical fiber is connected to form one optical fiber array, and an optical pulse from the first light source is incident from one end of the optical fiber array. The CW light (continuous light) from the second light source is incident from the other end of the optical fiber array, propagates in opposition to the light pulse from the first light source, and the light pulse from the first light source. That causes Raman light amplification by the interaction between the second light source and the CW light from the second light source, and detects components of the optical power of the CW light from the optical power of the CW light through an optical coupler and an optical wavelength filter. Arrange the detector and signal processing device,
An optical fiber comprising: measuring the optical fiber array; and calculating a Raman gain characteristic of the optical fiber to be measured by obtaining a ratio between the Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the optical fiber to be measured. Characterization equipment.

In claim 5,
An apparatus for evaluating characteristics of an optical fiber, wherein the reference optical fiber is connected in a longitudinal direction of the optical fiber to be measured.

In claim 5 or claim 6,
The optical detector measures an amplification component of CW light at each position of the optical fiber to be measured and an amplification component of CW light at at least one position of the reference optical fiber,
In the signal processing device, the measured amplification components of the measured optical fiber and the reference optical fiber, a loss distribution with respect to the wavelength of the optical pulse at each position of the measured optical fiber, and at least the reference optical fiber From the loss distribution with respect to the wavelength of the optical pulse at one position, the ratio of the Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the measured optical fiber is obtained,
An optical fiber characteristic evaluation apparatus, wherein the Raman gain characteristic of the optical fiber to be measured is calculated from the obtained ratio and the Raman gain characteristic of the reference optical fiber.

In claim 5 or claim 6,
In the optical detector, at least one of the amplification component at each position of the optical fiber to be measured and at least one of the reference optical fiber before switching one end where the light pulse is incident and the other end where the CW light is incident. An amplification component at one position, an amplification component at each position of the optical fiber to be measured after switching the one end and the other end, and an amplification component at at least one position of the reference optical fiber Measure and
In the signal processing device, from the amplification component measured before exchanging the one end and the other end and the amplification component measured after exchanging the one end and the other end of the reference optical fiber A ratio of a Raman gain characteristic and a Raman gain characteristic of the measured optical fiber is obtained, and a Raman gain characteristic of the measured optical fiber is calculated from the obtained ratio and the Raman gain characteristic of the reference optical fiber. Characterization equipment.