JP3660043B2

JP3660043B2 - Optical line monitoring method and monitoring system

Info

Publication number: JP3660043B2
Application number: JP04500496A
Authority: JP
Inventors: 正人栗井
Original assignee: Fujikura Ltd
Current assignee: Fujikura Ltd
Priority date: 1996-03-01
Filing date: 1996-03-01
Publication date: 2005-06-15
Anticipated expiration: 2016-03-01
Also published as: JPH09247102A

Description

【０００１】
【発明の属する技術分野】
本発明はＯＴＤＲを用いた光線路の監視方法および監視システムに関し、特に分岐線路を有する光線路全体を一括的に監視できるようにした方法およびシステムに関する。
【０００２】
【従来の技術】
従来より、光ファイバ通信網においては、１つの交換局と複数の基地局（あるいは加入者宅）とを光分岐デバイスを介して光ファイバで結んで、複数系列の光伝送線路（被監視線路）を構築することが行われている。例えば交換局とスターカプラとを１本の基幹ファイバ（基幹線路）で結び、スターカプラの分岐端と複数の基地局とを複数の分岐ファイバ（分岐線路）で結ぶ構成とすることができる。このように光分岐デバイスを用いて光線路を構築すれば、局内配線の合理化や光ケーブルの細径化を図ることができる。
【０００３】
ところで、光ファイバ通信網の信頼性を確保するために、光線路に断線が生じた場合には直ちにそれを検知できるようにする必要がある。そのための装置として、従来よりバックスキャタリング法を用いたＯＴＤＲ（Optical Time Domain Reflectometer）が知られている。
ＯＴＤＲは、被測定光ファイバに高出力、狭パルス幅のレーザ光を入射させると、被測定光ファイバ中で生じる後方散乱光や、入射端面、出射端面、接続面、あるいは破断面等で生じる反射光等が入射端にもどってくるので、もどってきた光信号の強度を測定して得られる波形を観察することによって、被測定光ファイバに断線が生じた場合には、その位置を検出できるようになっている。
【０００４】
【発明が解決しようとする課題】
しかしながら、上述したような分岐デバイスを用いて構築された光線路においては、１本の基幹線路に複数の分岐線路が接続されることによって複数系列の被監視線路が構築されている。したがって、単に交換局内にＯＴＤＲを設置してここから基幹線路に試験光を入射しても、複数の分岐線路からの後方散乱光および反射光が光分岐デバイスで合波されてもどってくるので、どの被監視線路で断線が生じたかを特定するのが難しい。特に、分岐線路の数が増大すると、ＯＴＤＲで得られる後方散乱光および反射光の光信号波形が複雑になり断線箇所を確実に検出することが非常に困難となる。
【０００５】
本発明は前記事情に鑑みてなされたもので、光分岐手段を用いて複数の被監視線路が構築された光線路を、ＯＴＤＲで一括的に監視できるとともに、破断箇所を確実に、かつ容易に検出できるようにした光線路の監視方法を提供することを目的とする。
【０００６】
【課題を解決するための手段】
前記課題を解決するために本発明の光線路の監視方法は、１つの入射端と複数の出射端とを光分岐手段を介して光ファイバで結んでなる複数の被監視線路を、ＯＴＤＲを用いて監視する方法であって、前記複数の被監視線路の線路長はそれぞれ異なる長さであり、かつその線路長の差はＯＴＤＲから出射される試験光のパルス幅に相当する長さの半分以上であり、前記被監視線路に異常がない状態で、前記入射端に前記試験光を入射させたときに該入射端にもどってくる光の強度を経時的に測定して、前記複数の被監視線路の各出射端をそれぞれ認識できる正常状態の波形を得る工程と、前記入射端に前記試験光を入射させたときに該入射端にもどってくる光の強度を経時的に測定して得られる波形を、前記正常状態の波形と比較して、観察されなくなった波形の位置から断線が発生した被監視線路を特定する工程と、新たに観察されたピークの位置から断線位置を測定する工程を有することを特徴とするものである。
また本発明の光線路の監視システムは、１つの入射端と複数の出射端とを光分岐手段を介して光ファイバで結んでなる複数の被監視線路と、前記複数の被監視線路の入射端に接続されたＯＴＤＲとを備えてなり、前記複数の被監視線路の線路長はそれぞれ異なる長さであり、かつその線路長の差はＯＴＤＲから出射される試験光のパルス幅に相当する長さの半分以上であり、前記被監視線路に異常がない状態で、前記入射端に前記試験光を入射させたときに該入射端にもどってくる光の強度を経時的に測定して得られる、前記複数の被監視線路の各出射端をそれぞれ認識できる波形を、正常状態の波形としてメモリーする手段を有することを特徴とするものである。
【０００７】
【発明の実施の形態】
以下、本発明を詳しく説明する。図１は本発明の光線路の監視方法の一実施例を示す光伝送線路の概略構成図である。図中符号１は交換局、２はＯＴＤＲ、３は波長分割多重（ＷＤＭ）カプラ、４は交換局内送受信装置、５，６，７は基地局、８は基地局内送受信装置、１１は基幹線路，１２は光分岐デバイス、１３は第１の分岐線路、１４はダミーファイバ、１５は第２の分岐線路、１６は第３の分岐線路、２０は光線路、２１は被監視線路をそれぞれ示す。
基幹線路１１および第１〜３の分岐線路１３，１５，１６はそれぞれ光ファイバケーブルで構成されている。
【０００８】
本実施例の光伝送線路は、交換局１から送信された光信号（伝送光）が基幹線路１１で伝送され、光分岐デバイス１２で複数に分岐され、分岐された伝送光がそれぞれ第１〜３の分岐線路１３，１５，１６で各基地局５，６，７へ送られるように構成されている。また基地局５，６，７から送信される光信号は、交換局１から送信された光信号とは逆の方向に伝送されて交換局１へ送られるようになっている。
尚、ここでは１本の基幹線路１１と光分岐デバイス１２と１本の分岐線路１３（または１５，１６）とで構成され、交換局（入射端）１と基地局（出射端）５，６，７とを結ぶ１系列の光伝送線路を被監視線路２１という。図１には３系列の被監視線路２１が示されているが、被監視線路２１の数はこれに限らず任意とすることができる。また、入射端を共有する複数の被監視線路２１で構成される光伝送路全体、すなわち１本の基幹線路１１と光分岐デバイス１２と複数の分岐線路１３，１５，１６とで構成される光伝送線路全体を光線路２０という。
【０００９】
交換局１は送受信装置４およびＯＴＤＲ２を備えており、送受信装置４から送信される伝送光、およびＯＴＤＲ２から出射される試験光がそれぞれＷＤＭカプラ３を介して基幹線路１１に入射されるように構成されている。伝送光および試験光としては波長が異なる光がそれぞれ用いられる。例えば、光線路２０を石英系光ファイバを用いて構成した場合には、伝送光として波長１３１０ｎｍの光を好適に用い、試験光として波長１５５０ｎｍの光を好適に用いることができる。光分岐デバイス１２としては波長無依存型カプラ（ＷＩＣ：Wavelength Insensitive Coupler）が用いられ、各種構成の光分岐手段を用いることができる。例えば１：Ｎの双方向用スターカプラを好適に用いることができる。この光分岐デバイス１２は通常クロージャ内に収納されている。
【００１０】
基地局５は送受信装置８を備えており、第１の分岐線路１３がこの送受信装置８に接続されている。また基地局５内において第１の分岐線路１３には必要に応じた長さのダミーファイバ１４が挿入されている。ダミーファイバ１４は、第１の分岐線路１３を含む被監視線路２１の線路長を調節するために必要に応じて用いられるもので、被監視線路２１を構成する光ファイバケーブルと同じものが使用される。ここで被監視線路２１の線路長とは、ＯＴＤＲ２における入射端から基地局５内における出射端までの長さをいう。ダミーファイバ１４は分岐線路１３の任意の位置に挿入することができるが、基地局５内で挿入する構成とすれば挿入時の作業性や収納性の点で好ましい。
基地局５内における被監視線路２１の出射端、すなわち被監視線路２１と送受信装置８との接続点は無反射であることが望ましく、分岐線路１３またはダミーファイバ１４と送受信装置８との接続は低反射コネクタ等の反射を抑える光部品（図示略）を用いて行うことが好ましい。
また他の基地局６，７も同様に送受信装置（図示略）を備えた構成となっており、各基地局６，７内において、第２の分岐線路１５および第３の分岐線路１６には必要に応じてダミーファイバ（図示略）が挿入されている。
【００１１】
本発明においては、光線路２０を構成する複数の被監視線路２１の線路長が、それぞれ異なる長さとなるように構成される。また複数の被監視線路２１の線路長の差はＯＴＤＲ２から出射される試験光のパルス幅に相当する長さの半分以上とすることが必要である。したがって、例えば各被監視線路２１の線路長は、試験光のパルス幅に相当する長さの半分の値を公差とする等差数列的に増加するように設定することができる。尚、試験光のパルス幅に相当する長さは、
（光速／コアの屈折率）×（ＯＴＤＲから出射される試験光のパルス幅）で求められる。
よって、例えば（光速／コアの屈折率）＝０．２ｍ／ｎｓ、試験光のパルス幅＝５０ｎｓとすると、このパルス幅に相当する線路長は約１０ｍとなる。よって、第１〜３の分岐線路１３，１５，１６を構成している光ファイバケーブルの長さがいずれも等しいとすると、第３の分岐線路１６にはダミーファイバを挿入せず、第２の分岐線路１５には長さ５ｍのダミーファイバを挿入し、第１の分岐線路１３には長さ１０ｍのダミーファイバ１４を挿入し…というように、ダミーファイバの長さを５ｍずつ長くすることによって、各被監視線路２１の長さを好ましく違えることができる。
【００１２】
尚、複数の被監視線路２１の線路長の差は、理論的には上述の通りＯＴＤＲ２から出射される試験光のパルス幅に相当する長さの半分以上とすれば測定可能であるが、実際には、使用するＯＴＤＲ装置の分解能によってはこれよりも長くすることが好ましい。すなわち、ＯＴＤＲ２で得られる波形において、断線等による後方散乱光への影響が後方散乱光の定常値から±０．１ｄＢ以内に回復するのに必要な距離（アッテネーション）を考慮して、複数の被監視線路２１の線路長の差が、ＯＴＤＲ２から出射される試験光のパルス幅に相当する長さの値以上となるようにするのが望ましい。このようにすれば現在市販されているＯＴＤＲによっても好ましい分解能が得られ、断線位置等を容易かつ正確に検出することができる。
【００１３】
このように構成された光線路２０の各被監視線路２１の監視は次のようにして行うことができる。まず、光線路２０に異常がない状態で、交換局１内においてＯＴＤＲ２からの試験光を基幹線路１１に入射させる。そして光線路２０からＯＴＤＲ２へもどってくる光の強度を経時的に測定すれば、例えば図２に示すような波形が得られるので、これを正常状態の波形としてメモリーする。
図２の例は、第１の分岐線路１３を含む被監視線路２１の線路長がＬ₁、第２の分岐線路１５を含む被監視線路の線路長がＬ₂、第３の分岐線路１６を含む被監視線路の線路長がＬ₃（Ｌ₁＞Ｌ₂＞Ｌ₃）で、Ｌ₁とＬ₂との差およびＬ₂とＬ₃との差が、いずれも試験光のパルス幅に相当する長さの半分以上とした場合に得られる波形である。この図において縦軸はＯＴＤＲ２へもどってきた後方散乱光または反射光の強度を示す。また横軸は時間軸であるが、光線路の各点のＯＴＤＲ２からの距離に換算して示している。この図の波形は、入射端、出射端、あるいは接続点等における反射や接続損がないと仮定した場合に理論的に得られる波形を示している。
この図に示されるように、複数の分岐線路１３，１５，１６からの後方散乱光が光分岐デバイス１２で合波されてＯＴＤＲ２へもどってきても、各被監視線路の線路長Ｌ₁，Ｌ₂，Ｌ₃が異なり、かつその差が試験光のパルス幅に相当する長さの半分以上となっているので、各分岐線路１３，１５，１６の出射端での後方散乱ａ，ｂ，ｃをそれぞれ認識することができる。
尚、線路長Ｓ₁の地点で得られた立上り部は、光分岐デバイス１２による後方散乱を示している。
【００１４】
このようなＯＴＤＲ２による波形の測定を常時行い、正常状態の波形と比較して、正常状態の波形との差を検出することによって、光線路における断線の発生と断線位置を知ることができる。例えば光線路２０に断線が生じた場合には、図３（ａ）または（ｂ）に示すように、図２に示した正常状態の波形とは異なる波形が得られる。図３（ａ）は、第２の分岐線路を含む被監視線路（線路長Ｌ₂）において、線路長Ｌ₀（Ｓ₁＜Ｌ₀＜Ｌ₃）の位置で断線が発生した場合の波形を示し、図３（ｂ）は、線路長Ｌ₀（Ｌ₃＜Ｌ₀＜Ｌ₁）の位置で断線が発生した場合の波形を示す。
そして図３（ａ）（ｂ）の異常状態の波形と図２の正常状態の波形とを比較すると、正常状態では距離Ｌ₂で観察されていた後方散乱ｂが異常状態の波形では観察されない。このことから線路長Ｌ₂の被監視線路、すなわち第２の分岐線路１５を含む被監視線路に断線が生じたことがわかる。そして断線箇所では光ファイバの破断面によって試験光の反射が生じるので、異常状態の波形にはこの破断面からの反射光によるピークＰが新たに観察されることになる。したがって、このピークＰの位置（Ｌ₀）を算出することによって断線位置をほぼ正確に測定することができる。
このように本実施例によれば、複数の分岐線路１３，１５，１６を有する光線路２の基幹線路１１に試験光を入射させることにより、この光線路２０全体を一括的に監視することができる。また断線等の異常が発生した場合には、ＯＴＤＲ２で測定される波形とメモリーされた正常状態の波形との差を読み取ることにより、断線が生じた被監視線路の特定し、かつ断線箇所を断定することができる。
【００１５】
本実施例では、３系列の被監視線路２１を備えた光線路２０について常時監視を行う方法を例に挙げて説明したが、被監視線路２１の数が増えても同様にして常時監視を行うことができる。
また本実施例において、ＯＴＤＲ２で測定される波形は、負の傾きを有する傾斜部分と、段差の立上り部ａ，ｂ，ｃとから概略なっており、各被監視線路の線路長の差は２つの立上り部の間の距離に相当する。したがって各被監視線路の線路長の差が大きい方が、立上り部ａ，ｂ，ｃの間隔が大きくなるので波形を観察する際に各被監視線路の出射端を認識し易い。ただし被監視線路の数が多い場合には、各被監視線路の線路長の差を大きくすると最長の被監視線路の長さが長くなるため、伝送損失の増加等の問題が生じる恐れがある。よって、各被監視線路の線路長が、試験光のパルス幅に相当する長さの半分の値、好ましくは試験光のパルス幅に相当する長さの値を公差とする等差数列的に増加するように設定すれば、被監視線路の数の増加に好適に対応できる。また各被監視線路の線路長がいずれも等しくなるように設定すれば、立上り部が等間隔に観察される波形が得られるので、視覚的に波形の変化を観察し易い。
【００１６】
なお本実施例においては、ＯＴＤＲ２で波形を測定したときに、各被監視線路の出射端での後方散乱が観察され、これによって各被監視線路を認識できるように構成されているが、被監視線路の出射端からの反射光が観察されるように構成すれば、これによっても各被監視線路を認識することが可能である。
また光伝送線路の構成は図１のものに限らず、各種の変形が可能である。例えば基地局５，６，７は加入者宅であってもよい。あるいは１系列の被監視線路に対して２つ以上の光分岐デバイスを用いて２段階に分岐させた構成の光線路においても、各被監視線路の入射端から出射端までの線路長を上記のように設定することによって、ＯＴＤＲを用いて同様に監視することが可能である。
【００１７】
【実施例】
以下、具体的な実施例を示して、本発明の効果を明らかにする。以下の実施例において、測定には、市販のＯＴＤＲ（光ファイバアナライザＡＱ−７１４０Ｃ、ＡＮＤＯ、安藤電気株式会社製）を使用した。
（実施例１）
図４に示すような分岐線路を有する光伝送線路を構成して、ＯＴＤＲを用いた光線路の監視を行った。図４において図１と同じ構成要素には同一符号を付してその説明を簡略化する。
本実施例の光伝送線路において、光分岐デバイス１２としては波長無依存型の１×２双方向用スターカプラを用いた。そして交換局１のＯＴＤＲ２から出射された試験光がＷＤＭカプラ３を介して基幹線路１１に入射され、スターカプラ１２で２つに分岐された後、第１の分岐線路３１および第２の分岐線路３２にそれぞれ入射されて第１の送受信装置ＣＳ₁および第２の送受信装置ＣＳ₂へそれぞれ出射されるようにした。第１の分岐線路３１と第１の送受信装置ＣＳ₁との接続，および第２の分岐線路３２と第２の送受信装置ＣＳ₂との接続には低反射コネクタ３３，３４をそれぞれ用い、これによって第１の分岐線路３１および第２の分岐線路３２の出射端における反射が抑えられるようにした。また基幹線路１１および各分岐線路３１，３２とスターカプラ１２との接続は融着により行った。
基幹線路１１および分岐線路３１，３２は石英系光ファイバで構成し、交換局１から出射される伝送光の波長は１３１０ｎｍとし、ＯＴＤＲから出射される試験光の波長は１５５０ｎｍとした。試験光のパルス幅は１μｍとした。またＯＴＤＲ２から第１の送受信装置ＣＳ₁までの線路長の実測値は２７１２ｍであり、ＯＴＤＲ２から第２の送受信装置ＣＳ₂までの線路長の実測値は８５６１ｍであった。
【００１８】
まず、光線路に異常がない状態でＯＴＤＲ２による測定を行ったところ、図５に示すような波形が得られた。これを正常状態の波形としてメモリーした。この波形には、第１の送受信装置ＣＳ₁での出射端からの後方散乱ａ、および第２の送受信装置ＣＳ₂での出射端からの後方散乱ｂがそれぞれ観察される。図５の波形を用いてＯＴＤＲ２から第１の送受信装置ＣＳ₁までの線路長を算出したところ２７１０ｍであり、実測値との誤差は２ｍであった。また図５の波形を用いてＯＴＤＲ２から第２の送受信装置ＣＳ₂までの線路長を算出したところ８５４０ｍであり、実測値との誤差は２１ｍであった。
【００１９】
次に、第１の分岐線路３１が途中で断線しており（破断点Ａ）、第２の分岐線路３２には異常がない状態でＯＴＤＲ２による測定を行ったところ、図６に示すような波形が得られた。この波形には第２の送受信装置ＣＳ₂での出射端からの後方散乱ｂは観察されるが、第１の送受信装置ＣＳ₁での出射端からの後方散乱ａは観察されない。その代わりに、正常状態では観察されなかったピークＰ₁が観察される。このピークＰ₁のＯＴＤＲ２からの距離を算出したところ１０６２ｍであった。また破断点ＡのＯＴＤＲ２からの距離を実測したところ１０４９ｍであった。測定により得られたピークＰ₁の位置と破断点Ａの実測値との誤差は１３ｍであり、ＯＴＤＲ２を用いて破断点Ａをほぼ正確に検出できることが認められた。
【００２０】
次に、第１の分岐線路３１が途中で断線しており（破断点Ａ）、また第２の分岐線路３２も途中で断線している（破断点Ｂ）状態でＯＴＤＲ２による測定を行ったところ、図７に示すような波形が得られた。この波形においては、図６と同様に第１の送受信装置ＣＳ₁での出射端からの後方散乱ａは観察されず、その代わりに、正常状態では観察されなかったピークＰ₁が観察される。それに加えて第２の送受信装置ＣＳ₂での出射端からの後方散乱ｂも観察されず、その代わりに、正常状態では観察されなかったピークＰ₂が観察される。このピークＰ₁およびピークＰ₂のＯＴＤＲ２からの距離を算出したところ、それぞれ１０６２ｍおよび８１９３ｍであった。また破断点Ａおよび破断点ＢのＯＴＤＲ２からの距離を実測したところ、それぞれ１０４９ｍおよび８１９６ｍであった。測定により得られたピークＰ₁の位置と破断点Ａの実測値との誤差は１３ｍであり、測定により得られたピークＰ₂の位置と破断点Ｂの実測値との誤差は３ｍであった。このことからＯＴＤＲ２を用いて破断点Ａおよび破断点Ｂをほぼ正確に検出できることが認められた。
【００２１】
（実施例２）
図８に示すように、８系列の分岐線路を有する光線路を構成した。すなわち、基幹線路１１の一端をＯＴＤＲ２に接続し、他端を１×８スプリッタ４０に融着接続した。１×８スプリッタ４０の８つの分岐端に第１〜８の分岐線路４１，４２，４３，４４，４５，４６，４７，４８をそれぞれ融着接続した。ＯＴＤＲ２の入射端から基幹線路１１と１×８スプリッタ４０との接続点までの実測距離は５３５ｍ、基幹線路１１と１×８スプリッタ４０との接続点から１×８スプリッタ４０の分岐端までの実測距離は７．５ｍとし、第１〜８の分岐線路には、約２００ｍずつ長さが異なる光ファイバケーブルをそれぞれ用いた。すなわち、
第１の分岐線路４１の実測長は２０９ｍ、
第２の分岐線路４２の実測長は４１４ｍ、
第３の分岐線路４３の実測長は６１２ｍ、
第４の分岐線路４４の実測長は８１４ｍ、
第５の分岐線路４５の実測長は１０１４ｍ、
第６の分岐線路４６の実測長は１２１３ｍ、
第７の分岐線路４７の実測長は１４１４ｍ、
第８の分岐線路４８の実測長は１６１６ｍとした。また各分岐線路４１，４２，４３，４４，４５，４６，４７，４８の出射端は直角クリープとした。基幹線路１１および第１〜８の分岐線路４１，４２，４３，４４，４５，４６，４７，４８は石英系光ファイバケーブルで構成した。
【００２２】
ＯＴＤＲ２から波長１３１０ｍ、パルス幅５０ｎｓの試験光を基幹線路２へ入射させて、各線路の長手方向の伝搬光の強度変化を測定したところ、図９に示すような波形が得られた。この波形では、第１〜８の分岐線路の各出射端面での後方散乱光による８つの立上り部▲１▼〜▲８▼がそれぞれ観察される。これらの立上り部▲１▼〜▲８▼のＯＴＤＲ２からの位置（測定長）をそれぞれ算出したところ下記表１に示すような値が得られた。また表１には第１〜８の分岐線路をそれぞれ含む被監視線路の線路長（ＯＴＤＲ２から分岐線路の出射端までの長さ）の実測値、および測定値と実測値との誤差を合わせて示す。
【００２３】
【表１】

【００２４】
（実施例３）
上記実施例２において、ＯＴＤＲ２から出射される試験光のパルス幅を２００ｎｓとした他は同様にして、各線路の長手方向の伝搬光の強度変化を測定したところ、図１０に示すような波形が得られた。この波形で観察される立上り部▲１▼〜▲８▼のＯＴＤＲ２からの位置（測定長）をそれぞれ算出したところ下記表２に示すような値が得られた。また表２には第１〜８の分岐線路をそれぞれ含む被監視線路の線路長（ＯＴＤＲ２から分岐線路の出射端までの長さ）の実測値、および測定値と実測値との誤差を合わせて示す。
【００２５】
【表２】

【００２６】
（比較例１）
上記実施例２において、ＯＴＤＲ２から出射される試験光のパルス幅を１μｓとした他は同様にして、各線路の長手方向の伝搬光の強度変化を測定したところ、図１１に示すような波形が得られた。この波形では立上り部▲１▼〜▲８▼はなだらかになっていた。各立上り部▲１▼〜▲８▼のＯＴＤＲ２からの位置（測定長）をそれぞれ算出したところ下記表３に示すような値が得られた。また表３には第１〜８の分岐線路をそれぞれ含む被監視線路の線路長（ＯＴＤＲ２から分岐線路の出射端までの長さ）の実測値、および測定値と実測値との誤差を合わせて示す。
【００２７】
【表３】

【００２８】
（比較例２）
上記実施例２において、ＯＴＤＲ２から出射される試験光のパルス幅を１０μｓとした他は同様にして、各線路の長手方向の伝搬光の強度変化を測定したところ、図１２に示すような波形が得られた。この波形では第１〜８の分岐線路の各出射端面での後方散乱光による８つの立上り部を認識することができず、線路長を測定できなかった。
【００２９】
実施例２，３の結果より、分岐線路の数が８系統と多くなっても各分岐線路の出射端をそれぞれ認識し、かつ各分岐線路を含む被監視線路の線路長をほぼ正確に測定することができた。したがって、上記実施例１と同様にして光線路の常時監視を行えば、断線が生じた場合にはそれを検知し、かつ破断している分岐線路を特定し破断位置を断定することができる。
また比較例１，２では、ＯＴＤＲから出射される試験光のパルス幅（１μｓ，１０μｓ）に相当する線路長が比較例１では約２０５ｍ、比較例２では約２０５０ｍであるのに対して、各分岐線路の線路長の差がこれよりも小さいために、各分岐線路の線路長を正確に測定することができなかった。
これらの結果より、スプリッタ４０で分岐した後の各分岐線路を容易かつ正確に見分けるためには、線路長（ファイバ長）で試験光のパルス幅の１パルス分に相当する長さ以上さがなければならないことが確認できた。
【００３０】
【発明の効果】
以上説明したように本発明の光線路の監視方法は、１つの入射端と複数の出射端とを光分岐手段を介して光ファイバで結んでなる複数の被監視線路を、ＯＴＤＲを用いて監視する方法であって、前記複数の被監視線路の線路長をそれぞれ異なる長さとし、かつその線路長の差をＯＴＤＲから出射される試験光のパルス幅に相当する長さの半分以上とすることを特徴とするものである。
したがって、複数の被監視線路から後方散乱光や反射光が光分岐手段で合波されて１つの入射端へもどってきても、各被監視線路の出射端での後方散乱、あるいは反射による光信号が重なり合うことがないので、これら複数の被監視線路から入射端へもどってくる光をＯＴＤＲで測定したときに、各被監視線路の出射端をそれぞれ認識することができる。よって光分岐手段を用いて複数系列の被監視線路が構築された光線路全体を、ＯＴＤＲを用いて一括的に監視することができる。
そして前記入射端に前記試験光を入射させたときに該入射端にもどってくる光の強度を経時的に測定して得られる波形を、正常状態で得られる波形と比較して、正常状態の波形との差により断線の発生および断線位置を検知すれば、断線が生じた被監視線路の特定および断線位置の断定をほぼ正確に行うことができる。
【図面の簡単な説明】
【図１】本発明の光線路の監視方法の一実施例を示す光伝送線路の概略構成図である。
【図２】本発明の光線路の監視方法により得られる正常状態の波形の例を示すグラフである。
【図３】本発明の光線路の監視方法により得られる異常状態の波形の例を示すグラフである。
【図４】実施例１の光伝送線路の概略構成図である。
【図５】実施例１で得られた正常状態の波形である。
【図６】実施例１で得られた異常状態の波形である。
【図７】実施例１で得られた異常状態の波形である。
【図８】実施例２の光伝送線路の概略構成図である。
【図９】実施例２で得られた正常状態の波形である。
【図１０】実施例３で得られた正常状態の波形である。
【図１１】比較例１で得られた正常状態の波形である。
【図１２】比較例２で得られた正常状態の波形である。
【符号の説明】
１…交換局（入射端）、２…ＯＴＤＲ、５，６，７…基地局（出射端）、
１１…基幹線路、１２…光分岐デバイス（光分岐手段）、
１３，１５，１６…分岐線路、２１…被監視線路、２０…光線路、
３１…第１の分岐線路、３２…第２の分岐線路、
４０…１×８スプリッタ（光分岐手段）、
４１…第１の分岐線路、４２…第２の分岐線路、４３…第３の分岐線路、
４４…第４の分岐線路、４５…第５の分岐線路、４６…第６の分岐線路、
４７…第７の分岐線路、４８…第８の分岐線路。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an optical line monitoring method using OTDR.And monitoring systemIn particular, a method for monitoring an entire optical line having a branch line in a lumpAnd systemAbout.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an optical fiber communication network, a single switching center and a plurality of base stations (or subscriber premises) are connected by an optical fiber via an optical branching device, and a plurality of optical transmission lines (monitored lines) are connected. Has been done to build. For example, the switching station and the star coupler can be connected by a single basic fiber (main line), and the branch end of the star coupler and a plurality of base stations can be connected by a plurality of branch fibers (branch lines). If an optical line is constructed using an optical branching device in this way, it is possible to rationalize the intra-office wiring and reduce the diameter of the optical cable.
[0003]
By the way, in order to ensure the reliability of the optical fiber communication network, it is necessary to be able to immediately detect when a disconnection occurs in the optical line. As an apparatus for this, an OTDR (Optical Time Domain Reflectometer) using a backscattering method has been conventionally known.
In OTDR, when high-power, narrow-pulse-width laser light is incident on the optical fiber to be measured, the backscattered light generated in the optical fiber to be measured, and the reflection generated on the incident end face, the outgoing end face, the connection face, or the fracture surface, etc. Since light and the like return to the incident end, it is possible to detect the position of the optical fiber under measurement when the optical fiber to be measured is broken by observing the waveform obtained by measuring the intensity of the returned optical signal. It has become.
[0004]
[Problems to be solved by the invention]
However, in the optical line constructed by using the branch device as described above, a plurality of series of monitored lines are constructed by connecting a plurality of branch lines to one main line. Therefore, even if the OTDR is simply installed in the exchange and the test light is incident on the trunk line from here, the backscattered light and the reflected light from the plurality of branch lines are returned by the optical branching device. It is difficult to specify which monitored line has caused the disconnection. In particular, when the number of branch lines increases, the optical signal waveforms of backscattered light and reflected light obtained by OTDR become complicated, and it is very difficult to reliably detect a disconnection point.
[0005]
The present invention has been made in view of the above circumstances, and an optical line in which a plurality of monitored lines are constructed using an optical branching unit can be collectively monitored by OTDR, and a broken part can be surely and easily obtained. An object of the present invention is to provide a method of monitoring an optical line that can be detected.
[0006]
[Means for Solving the Problems]
  To solve the above problemsOf the present inventionThe optical line monitoring method is a method of monitoring, using OTDR, a plurality of monitored lines in which one incident end and a plurality of outgoing ends are connected by an optical fiber via an optical branching means. Line length of the monitored lineIsDifferent lengthsAndAnd the difference of the line lengthIsMore than half of the length corresponding to the pulse width of the test light emitted from OTDRAnd measuring the intensity of light that returns to the incident end over time when the test light is incident on the incident end with no abnormality in the monitored line, Obtaining a waveform in a normal state that can recognize each outgoing end of the line, and measuring the intensity of light that returns to the incident end over time when the test light is incident on the incident end Comparing the waveform with the waveform in the normal state, identifying the monitored line where the disconnection has occurred from the position of the waveform that is no longer observed, and measuring the disconnection position from the position of the newly observed peak HaveIt is characterized by this.
  AlsoOf the present inventionOptical line monitoringThe system includes a plurality of monitored lines in which one incident end and a plurality of emitting ends are connected by an optical fiber via an optical branching unit, and an OTDR connected to the incident ends of the plurality of monitored lines. The lengths of the plurality of monitored lines are different from each other, and the difference between the line lengths is more than half of the length corresponding to the pulse width of the test light emitted from the OTDR. Each of the plurality of monitored lines obtained by measuring with time the intensity of light returning to the incident end when the test light is incident on the incident end with no abnormality in the monitored line. It has a means for memorizing the waveform which can recognize each outgoing end as a waveform of a normal state.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below. FIG. 1 is a schematic configuration diagram of an optical transmission line showing an embodiment of the optical line monitoring method of the present invention. In the figure, reference numeral 1 is an exchange station, 2 is an OTDR, 3 is a wavelength division multiplexing (WDM) coupler, 4 is an intra-exchange station transmitter / receiver, 5, 6 and 7 are base stations, 8 is an intra-base station transmitter / receiver, 11 is a trunk line, Reference numeral 12 denotes an optical branch device, 13 denotes a first branch line, 14 denotes a dummy fiber, 15 denotes a second branch line, 16 denotes a third branch line, 20 denotes an optical line, and 21 denotes a monitored line.
The trunk line 11 and the first to

third branch lines

13, 15, and 16 are each configured by an optical fiber cable.
[0008]
In the optical transmission line of the present embodiment, the optical signal (transmission light) transmitted from the switching center 1 is transmitted through the trunk line 11, branched into a plurality by the optical branching device 12, and the branched transmission lights are first to first, respectively. 3

branch lines

13, 15, 16 are sent to the base stations 5, 6, 7. The optical signals transmitted from the base stations 5, 6, and 7 are transmitted in the opposite direction to the optical signal transmitted from the switching station 1 and sent to the switching station 1.
Here, it is composed of one trunk line 11, an optical branching device 12, and one branch line 13 (or 15, 16), and an exchange station (incoming end) 1 and base stations (outgoing ends) 5, 6 , 7 is referred to as a monitored line 21. Although three series of monitored lines 21 are shown in FIG. 1, the number of monitored lines 21 is not limited to this and may be arbitrary. Further, the entire optical transmission line composed of a plurality of monitored lines 21 sharing an incident end, that is, light composed of one trunk line 11, an optical branching device 12, and a plurality of

branch lines

13, 15, and 16. The entire transmission line is referred to as an optical line 20.
[0009]
The exchange 1 includes a transmission / reception device 4 and an OTDR 2, and is configured such that transmission light transmitted from the transmission / reception device 4 and test light emitted from the OTDR 2 are incident on the trunk line 11 via the WDM coupler 3. Has been. Light having different wavelengths is used as the transmission light and the test light, respectively. For example, when the optical line 20 is configured using a silica-based optical fiber, light having a wavelength of 1310 nm can be suitably used as transmission light, and light having a wavelength of 1550 nm can be suitably used as test light. As the optical branching device 12, a wavelength-independent coupler (WIC: Wavelength Insensitive Coupler) is used, and optical branching means having various configurations can be used. For example, a 1: N bidirectional star coupler can be suitably used. This optical branching device 12 is usually housed in a closure.
[0010]
The base station 5 includes a transmission / reception device 8, and the first branch line 13 is connected to the transmission / reception device 8. In addition, a dummy fiber 14 having a length as required is inserted into the first branch line 13 in the base station 5. The dummy fiber 14 is used as needed to adjust the line length of the monitored line 21 including the first branch line 13, and the same optical fiber cable as the monitored line 21 is used. The Here, the line length of the monitored line 21 refers to the length from the incident end in the OTDR 2 to the outgoing end in the base station 5. The dummy fiber 14 can be inserted at any position on the branch line 13. However, the configuration of inserting the dummy fiber 14 in the base station 5 is preferable in terms of workability and storage property at the time of insertion.
The exit end of the monitored line 21 in the base station 5, that is, the connection point between the monitored line 21 and the transmission / reception device 8 is preferably non-reflective, and the connection between the branch line 13 or the dummy fiber 14 and the transmission / reception device 8 is It is preferable to use an optical component (not shown) that suppresses reflection, such as a low-reflection connector.
Similarly, the other base stations 6 and 7 are also provided with a transmission / reception device (not shown). In each base station 6 and 7, the second branch line 15 and the third branch line 16 are A dummy fiber (not shown) is inserted as necessary.
[0011]
In the present invention, the lengths of the plurality of monitored lines 21 constituting the optical line 20 are configured to be different from each other. Further, the difference in line length between the plurality of monitored lines 21 needs to be at least half of the length corresponding to the pulse width of the test light emitted from the OTDR 2. Therefore, for example, the line length of each monitored line 21 can be set so as to increase in an arithmetic progression having a tolerance that is half the length corresponding to the pulse width of the test light. The length corresponding to the pulse width of the test light is
(Speed of light / refractive index of core) × (pulse width of test light emitted from OTDR)
Therefore, for example, if (light speed / core refractive index) = 0.2 m / ns and the pulse width of the test light = 50 ns, the line length corresponding to this pulse width is about 10 m. Therefore, if the lengths of the optical fiber cables constituting the first to

third branch lines

13, 15, and 16 are all equal, no dummy fiber is inserted into the third branch line 16, and the second By inserting a dummy fiber having a length of 5 m into the branch line 15, inserting a dummy fiber 14 having a length of 10 m into the first branch line 13, etc. The lengths of the monitored lines 21 can be preferably changed.
[0012]
The difference in the line lengths of the plurality of monitored lines 21 can theoretically be measured as long as it is at least half the length corresponding to the pulse width of the test light emitted from the OTDR 2 as described above. Depending on the resolution of the OTDR device to be used, it is preferable to make it longer than this. In other words, in the waveform obtained by OTDR2, a plurality of coverages are taken into consideration in consideration of the distance (attenuation) necessary for the influence on the backscattered light due to the disconnection or the like to recover within ± 0.1 dB from the steady value of the backscattered light. It is desirable that the difference in the line length of the monitoring line 21 be equal to or greater than the length value corresponding to the pulse width of the test light emitted from the OTDR2. In this way, a preferable resolution can be obtained even with OTDR currently on the market, and the disconnection position and the like can be detected easily and accurately.
[0013]
Monitoring of each monitored line 21 of the optical line 20 configured as described above can be performed as follows. First, test light from the OTDR 2 is made incident on the trunk line 11 in the exchange 1 with no abnormality in the optical line 20. If the intensity of light returning from the optical line 20 to the OTDR 2 is measured over time, a waveform as shown in FIG. 2 is obtained, for example, and stored as a waveform in a normal state.
In the example of FIG. 2, the length of the monitored line 21 including the first branch line 13 is L.₁The line length of the monitored line including the second branch line 15 is L₂The line length of the monitored line including the third branch line 16 is L_Three(L₁> L₂> L_Three) And L₁And L₂And L and₂And L_ThreeIs a waveform obtained when the difference between the two is more than half the length corresponding to the pulse width of the test light. In this figure, the vertical axis indicates the intensity of backscattered light or reflected light that has returned to OTDR2. Further, although the horizontal axis is a time axis, it is shown in terms of the distance from the OTDR 2 at each point of the optical line. The waveform in this figure shows a waveform theoretically obtained when it is assumed that there is no reflection or connection loss at the entrance end, the exit end, or the connection point.
As shown in this figure, even if the backscattered light from the plurality of

branch lines

13, 15, 16 is combined by the optical branch device 12 and returned to the OTDR 2, the line length L of each monitored line₁, L₂, L_ThreeAnd the difference is more than half of the length corresponding to the pulse width of the test light, so that backscattering a, b, and c at the exit ends of the

branch lines

13, 15, and 16 are recognized. be able to.
Line length S₁The rising portion obtained at the point of (2) shows backscattering by the optical branching device 12.
[0014]
Such a measurement of the waveform by OTDR2 is always performed, and compared with the waveform in the normal state, by detecting a difference from the waveform in the normal state, it is possible to know the occurrence of the disconnection and the position of the disconnection in the optical line. For example, when disconnection occurs in the optical line 20, a waveform different from the waveform in the normal state shown in FIG. 2 is obtained as shown in FIG. 3 (a) or (b). FIG. 3A shows a monitored line (line length L) including the second branch line.₂) In line length L₀(S₁<L₀<L_Three) Shows a waveform when a disconnection occurs at the position of FIG. 3, and FIG.₀(L_Three<L₀<L₁The waveform when a disconnection occurs at the position) is shown.
3A and 3B is compared with the waveform in the normal state in FIG. 2, the distance L in the normal state is compared.₂The backscattering b observed in (1) is not observed in the abnormal waveform. From this, the line length L₂It can be seen that a disconnection has occurred in the monitored line, that is, the monitored line including the second branch line 15. Then, since the test light is reflected by the broken surface of the optical fiber at the broken portion, the peak P due to the reflected light from the broken surface is newly observed in the abnormal waveform. Therefore, the position of this peak P (L₀) Can be measured almost accurately.
As described above, according to the present embodiment, the entire optical line 20 can be collectively monitored by making the test light incident on the main line 11 of the optical line 2 having the plurality of

branch lines

13, 15, and 16. it can. Also, when an abnormality such as disconnection occurs, the difference between the waveform measured by OTDR2 and the normal waveform stored in memory is read to identify the monitored line where the disconnection has occurred and determine the disconnection location. can do.
[0015]
In this embodiment, the method of constantly monitoring the optical line 20 including the three lines of monitored lines 21 has been described as an example. However, even if the number of monitored lines 21 increases, the constant monitoring is similarly performed. be able to.
In the present embodiment, the waveform measured by OTDR2 is roughly composed of an inclined portion having a negative inclination and rising portions a, b, and c of the step, and the difference in line length of each monitored line is 2 This corresponds to the distance between the two rising parts. Therefore, the larger the difference in the line lengths of the monitored lines, the larger the intervals between the rising portions a, b, and c. Therefore, when observing the waveform, it is easier to recognize the emission end of each monitored line. However, when the number of monitored lines is large, the length of the longest monitored line is increased when the difference in the line length of each monitored line is increased, which may cause problems such as an increase in transmission loss. Therefore, the line length of each monitored line increases in an arithmetic progression with a tolerance of a value corresponding to half the length corresponding to the pulse width of the test light, preferably a length corresponding to the pulse width of the test light. If it sets so, it can respond suitably to the increase in the number of monitored lines. Further, if the lengths of the monitored lines are set to be equal to each other, a waveform in which the rising portions are observed at equal intervals can be obtained, so that it is easy to visually observe the change in the waveform.
[0016]
In this embodiment, when the waveform is measured with OTDR2, backscattering at the output end of each monitored line is observed, and thereby, each monitored line can be recognized. If the configuration is such that the reflected light from the outgoing end of the line is observed, it is possible to recognize each monitored line also by this.
The configuration of the optical transmission line is not limited to that shown in FIG. 1, and various modifications can be made. For example, the base stations 5, 6, and 7 may be subscriber homes. Alternatively, even in an optical line having a configuration in which two or more optical branching devices are used to branch a series of monitored lines in two stages, the line length from the incident end to the outgoing end of each monitored line is set as described above. By setting in this way, it is possible to similarly monitor using OTDR.
[0017]
【Example】
Hereinafter, specific examples will be shown to clarify the effects of the present invention. In the following examples, a commercially available OTDR (optical fiber analyzer AQ-7140C, ANDO, manufactured by Ando Electric Co., Ltd.) was used for the measurement.
Example 1
An optical transmission line having a branch line as shown in FIG. 4 was constructed, and the optical line was monitored using OTDR. In FIG. 4, the same components as those in FIG.
In the optical transmission line of the present embodiment, a wavelength-independent type 1 × 2 bidirectional star coupler is used as the optical branching device 12. Then, after the test light emitted from the OTDR 2 of the exchange 1 is incident on the trunk line 11 via the WDM coupler 3 and branched into two by the star coupler 12, the first branch line 31 and the second branch line 32 are respectively incident on the first transmitter / receiver CS₁And the second transceiver CS₂It was made to emit to each. First branch line 31 and first transmission / reception device CS₁And the second branch line 32 and the second transceiver CS₂The

low reflection connectors

33 and 34 are used for the connection to each other so that reflection at the emission ends of the first branch line 31 and the second branch line 32 is suppressed. The main line 11 and the

branch lines

31 and 32 and the star coupler 12 were connected by fusion.
The trunk line 11 and the

branch lines

31 and 32 are made of silica optical fibers, the wavelength of the transmission light emitted from the exchange 1 is 1310 nm, and the wavelength of the test light emitted from the OTDR is 1550 nm. The pulse width of the test light was 1 μm. Also, from OTDR2 to the first transmission / reception device CS₁The measured value of the line length up to 2712 m is from OTDR2 to the second transmission / reception device CS.₂The actual measured line length was 8561 m.
[0018]
First, when measurement was performed with OTDR2 in a state where there was no abnormality in the optical line, a waveform as shown in FIG. 5 was obtained. This was stored as a normal waveform. In this waveform, the first transceiver CS₁Backscattering a from the output end of the second transmission / reception device CS₂Backscattering b from the emission end at is observed. The first transmitter / receiver CS from OTDR2 using the waveform of FIG.₁The calculated line length was 2710 m, and the error from the actually measured value was 2 m. Further, from the OTDR 2 to the second transmitter / receiver CS using the waveform of FIG.₂The length of the line up to 8540 m was calculated, and the error from the actually measured value was 21 m.
[0019]
Next, when the first branch line 31 is broken in the middle (breaking point A) and the second branch line 32 is measured with the OTDR 2 without any abnormality, a waveform as shown in FIG. 6 is obtained. was gotten. In this waveform, the second transceiver CS₂Although the backscattering b from the output end is observed, the first transmitter / receiver CS₁Backscattering a from the emission end at is not observed. Instead, the peak P that was not observed in the normal state₁Is observed. This peak P₁The calculated distance from OTDR2 was 1062 m. Further, when the distance from the OTDR2 at the break point A was measured, it was 1049 m. Peak P obtained by measurement₁The error between the position of and the measured value of the breaking point A was 13 m, and it was confirmed that the breaking point A could be detected almost accurately using OTDR2.
[0020]
Next, when the first branch line 31 is disconnected in the middle (break point A) and the second branch line 32 is also disconnected in the middle (break point B), measurement by OTDR2 was performed. A waveform as shown in FIG. 7 was obtained. In this waveform, as in FIG.₁No backscatter a from the exit end at the peak P, which was not observed in the normal state instead.₁Is observed. In addition, the second transceiver CS₂In addition, backscattering b from the emission end is not observed, and instead, the peak P not observed in the normal state₂Is observed. This peak P₁And peak P₂The distance from OTDR2 was calculated to be 1062 m and 8193 m, respectively. Moreover, when the distance from OTDR2 of the breaking point A and the breaking point B was measured, they were 1049m and 8196m, respectively. Peak P obtained by measurement₁The error between the position of and the measured value of the breaking point A is 13 m, and the peak P obtained by the measurement is₂The error between this position and the measured value of the breaking point B was 3 m. From this, it was confirmed that the breaking point A and the breaking point B can be detected almost accurately using OTDR2.
[0021]
(Example 2)
As shown in FIG. 8, an optical line having 8 branch lines was configured. That is, one end of the trunk line 11 was connected to the OTDR 2 and the other end was fusion-connected to the 1 × 8 splitter 40. The first to

eighth branch lines

41, 42, 43, 44, 45, 46, 47, 48 were fused and connected to the eight branch ends of the 1 × 8 splitter 40. The measured distance from the incident end of the OTDR 2 to the connection point between the trunk line 11 and the 1 × 8 splitter 40 is 535 m, and the measured distance from the connection point between the trunk line 11 and the 1 × 8 splitter 40 to the branch end of the 1 × 8 splitter 40. The distance was 7.5 m, and optical fiber cables having different lengths of about 200 m were used for the first to eighth branch lines. That is,
The measured length of the first branch line 41 is 209 m,
The measured length of the second branch line 42 is 414 m,
The measured length of the third branch line 43 is 612 m,
The measured length of the fourth branch line 44 is 814 m,
The measured length of the fifth branch line 45 is 1014 m,
The measured length of the sixth branch line 46 is 1213 m,
The measured length of the seventh branch line 47 is 1414 m,
The actually measured length of the eighth branch line 48 was 1616 m. Further, the exit ends of the

branch lines

41, 42, 43, 44, 45, 46, 47, and 48 were formed as a right-angle creep. The main line 11 and the first to

eighth branch lines

41, 42, 43, 44, 45, 46, 47, and 48 were made of silica-based optical fiber cables.
[0022]
When test light having a wavelength of 1310 m and a pulse width of 50 ns was incident on the trunk line 2 from the OTDR 2 and the change in the intensity of the propagation light in the longitudinal direction of each line was measured, a waveform as shown in FIG. 9 was obtained. In this waveform, eight rising portions {circle around (1)} to {circle around (8)} are observed by the backscattered light at the respective exit end faces of the first to eighth branch lines. When the positions (measurement lengths) of these rising portions (1) to (8) from the OTDR 2 were calculated, the values shown in Table 1 below were obtained. Table 1 also shows the measured length of the monitored line including each of the first to eighth branch lines (the length from OTDR2 to the exit end of the branch line) and the error between the measured value and the measured value. Show.
[0023]
[Table 1]

[0024]
(Example 3)
In Example 2 described above, the intensity change of the propagation light in the longitudinal direction of each line was measured in the same manner except that the pulse width of the test light emitted from the OTDR 2 was set to 200 ns. Obtained. When the positions (measurement lengths) of the rising portions (1) to (8) observed from this waveform from the OTDR2 were calculated, the values shown in Table 2 below were obtained. Table 2 also shows the measured length of the monitored line including each of the first to eighth branch lines (the length from OTDR2 to the exit end of the branch line) and the error between the measured value and the measured value. Show.
[0025]
[Table 2]

[0026]
(Comparative Example 1)
In Example 2, the change in the intensity of the propagation light in the longitudinal direction of each line was measured in the same manner except that the pulse width of the test light emitted from the OTDR 2 was 1 μs. As a result, a waveform as shown in FIG. 11 was obtained. Obtained. In this waveform, the rising portions (1) to (8) were gentle. When the position (measurement length) from each OTDR 2 of each rising portion (1) to (8) was calculated, the values shown in Table 3 below were obtained. Table 3 also shows the measured length of the monitored line including the first to eighth branch lines (the length from OTDR2 to the exit end of the branch line) and the error between the measured value and the measured value. Show.
[0027]
[Table 3]

[0028]
(Comparative Example 2)
In Example 2 above, except that the pulse width of the test light emitted from the OTDR 2 was set to 10 μs, the intensity change of the propagation light in the longitudinal direction of each line was measured. As a result, a waveform as shown in FIG. 12 was obtained. Obtained. In this waveform, it was impossible to recognize the eight rising parts due to the backscattered light at the exit end faces of the first to eighth branch lines, and the line length could not be measured.
[0029]
From the results of Examples 2 and 3, even when the number of branch lines increases to eight, the output end of each branch line is recognized, and the line length of the monitored line including each branch line is measured almost accurately. I was able to. Therefore, if the optical line is constantly monitored in the same manner as in the first embodiment, it is possible to detect a broken line, identify a broken branch line, and determine the broken position.
In Comparative Examples 1 and 2, the line length corresponding to the pulse width (1 μs, 10 μs) of the test light emitted from the OTDR is about 205 m in Comparative Example 1 and about 2050 m in Comparative Example 2, whereas Since the difference in the line length of the branch lines is smaller than this, the line length of each branch line cannot be measured accurately.
From these results, in order to distinguish each branch line after branching by the splitter 40 easily and accurately, the line length (fiber length) must be longer than the length corresponding to one pulse of the pulse width of the test light. It was confirmed that it was necessary.
[0030]
【The invention's effect】
As described above, the optical line monitoring method of the present invention uses OTDR to monitor a plurality of monitored lines in which one incident end and a plurality of outgoing ends are connected by an optical fiber via an optical branching unit. The lengths of the plurality of monitored lines are different from each other, and the difference between the line lengths is set to be not less than half the length corresponding to the pulse width of the test light emitted from the OTDR. It is a feature.
Therefore, even if backscattered light or reflected light is multiplexed from a plurality of monitored lines and returned to one incident end, an optical signal due to backscattering or reflection at the outgoing end of each monitored line Therefore, when the light returning from the plurality of monitored lines to the incident end is measured by OTDR, the emission end of each monitored line can be recognized. Therefore, the entire optical line in which a plurality of monitored lines are constructed using the optical branching unit can be collectively monitored using OTDR.
Then, when the test light is incident on the incident end, the waveform obtained by measuring the intensity of the light that returns to the incident end over time is compared with the waveform obtained in the normal state, If the occurrence of a disconnection and the disconnection position are detected based on the difference from the waveform, the monitored line where the disconnection has occurred can be identified and the disconnection position can be determined almost accurately.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an optical transmission line showing an embodiment of an optical line monitoring method of the present invention.
FIG. 2 is a graph showing an example of a waveform in a normal state obtained by the optical line monitoring method of the present invention.
FIG. 3 is a graph showing an example of a waveform in an abnormal state obtained by the optical line monitoring method of the present invention.
4 is a schematic configuration diagram of an optical transmission line according to Embodiment 1. FIG.
5 is a waveform in a normal state obtained in Example 1. FIG.
6 is a waveform of an abnormal state obtained in Example 1. FIG.
7 is a waveform of an abnormal state obtained in Example 1. FIG.
FIG. 8 is a schematic configuration diagram of an optical transmission line according to a second embodiment.
9 is a waveform in a normal state obtained in Example 2. FIG.
10 is a waveform in a normal state obtained in Example 3. FIG.
11 is a waveform in a normal state obtained in Comparative Example 1. FIG.
12 is a waveform in a normal state obtained in Comparative Example 2. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Switching station (incident end), 2 ... OTDR, 5, 6, 7 ... Base station (exit end),
11 ... trunk line, 12 ... optical branching device (optical branching means),
13, 15, 16 ... branch line, 21 ... monitored line, 20 ... optical line,
31 ... 1st branch line, 32 ... 2nd branch line,
40 ... 1 × 8 splitter (optical branching means),
41 ... 1st branch line, 42 ... 2nd branch line, 43 ... 3rd branch line,
44 ... 4th branch line, 45 ... 5th branch line, 46 ... 6th branch line,
47 ... seventh branch line, 48 ... eighth branch line.

Claims

A plurality of monitored lines formed by connecting one incident end (1) and a plurality of emission ends (5, 6, 7) via optical branching means (12) with optical fibers (11, 13, 15, 16) (21) is a method of monitoring using OTDR (2),
The line length of the plurality of monitored line are different lengths, respectively, and the difference between the line length is at least half the length corresponding to the pulse width of the test light emitted from the OTDR,
In a state where there is no abnormality in the monitored line, the intensity of light returning to the incident end when the test light is incident on the incident end is measured over time, and each of the plurality of monitored lines is measured. A step of obtaining a waveform in a normal state in which each of the emission ends can be recognized, and a waveform obtained by measuring with time the intensity of light that returns to the incidence end when the test light is incident on the incidence end, Compared with the waveform in the normal state, the step of identifying the monitored line where the disconnection has occurred from the position of the waveform that is no longer observed, and the step of measuring the disconnection position from the position of the newly observed peak An optical line monitoring method as a feature.

2. The optical line monitoring method according to claim 1, wherein the output end of the monitored line is a non-reflective end, and the waveform that is no longer observed is a waveform due to backscattering.

A plurality of monitored lines formed by connecting one incident end and a plurality of emission ends with an optical fiber via an optical branching unit;
  OTDR connected to the incident ends of the plurality of monitored lines,
  The line lengths of the plurality of monitored lines are different from each other, and the difference between the line lengths is more than half of the length corresponding to the pulse width of the test light emitted from the OTDR,
  The plurality of monitored lines obtained by measuring, over time, the intensity of light that returns to the incident end when the test light is incident on the incident end in a state where the monitored line is normal. An optical line monitoring system comprising means for storing, as a waveform in a normal state, a waveform capable of recognizing each emission end of the optical line.

4. The optical line monitoring system according to claim 3, wherein a dummy fiber is inserted into the monitored line, and the dummy fiber is accommodated in a base station.