JP3966978B2 - Optical filter and optical communication system - Google Patents

Description

【００２７】
前記式（１）において、Ａは電界の振幅、ωは光の周波数、ｔは時間、βiは第１の区間１ａの基本モードの伝搬定数である。
ここで、第１の区間１ａの基本モードと第２の区間１ｂの基本モードとの結合効率をａ1、結合時の位相のずれをφ1、第２の区間１ｂの基本モードの伝搬定数をβ01としたとき、第２の区間１ｂの基本モードの電界は、以下の式（２）で表される。
【００２８】
【数２】

【００２９】
また、第１の区間１ａの基本モードと第２の区間１ｂのクラッドモードとの結合効率をａ2とし、結合時の位相のずれをφ2、第２の区間１ｂのクラッドモードの伝搬定数をβ02としたとき、第２の区間１ｂのクラッドモードの電界は、以下の式（３）で表される。
【００３０】
【数３】

【００３１】
ついで、第２の区間１ｂの基本モードから第３の区間１ｃの基本モードに結合する光の電界は、以下の式（４）で表される。
【００３２】
【数４】

【００３３】
前記式（４）において、ｂ1は第１の区間１ａの基本モードと第２の区間１ｂの基本モードとの結合効率、φ3は結合時の位相のずれ、β3は第３の区間１ｃの基本モードの伝搬定数、Ｌは第２の区間１ｂの長さを表している。
また第２の区間１ｂのクラッドモードから第３の区間１ｃのクラッドモードに結合する光の電界は、以下の式（５）で表される。
【００３４】
【数５】

【００３５】
前記式（５）において、ｂ2は第２の区間１ｂのクラッドモードと第３の区間１ｃの基本モードとの結合効率、φ4は結合時の位相のずれを示している。
またここで、ΔΦを以下の式（６）のように定義する。
【００３６】
【数６】

【００３７】
すると第３の区間１ｃを伝搬する基本モードの光の振幅の自乗は、以下の式（７）で表される。
【００３８】
【数７】

【００３９】
したがって、第１の区間１ａを伝搬する基本モードの光の強度と、第３の区間１ｃを伝搬する基本モードの光の強度との比率は、第１の区間１ａから第３の区間１ｃへの光の透過効率を表しており、これは以下の式（８）で表される。
【００４０】
【数８】

【００４１】
ここで、モードの実効屈折率をｎe、真空中での光の波長をλとすると、モードの伝搬定数は、以下の式（９）で表される。
【００４２】
【数９】

【００４３】
この式（９）を、前記式（８）に代入すると、光の透過効率は以下の式（１０）のようになる。
【００４４】
【数１０】

【００４５】
この式（１０）において、ｎ01は第２の区間１ｂの基本モードの実効屈折率、ｎ02は第２の区間１ｂのクラッドモードの実効屈折率を示している。
この式（１０）の右辺の第３項は、光の透過効率を表しており、これにより、この光ファイバの構成は、波長λに対して周期的な透過損失変化を有するものであって、光フィルタとして用いることができることがわかる。
【００４６】
そしてこの式（１０）より、損失ピークの波長間隔は、第２の区間１ｂの長さＬによって決定されることがわかる。
すなわち第２の区間１ｂの長さＬが大きい場合には、損失ピークの波長間隔は短くなり、第２の区間１ｂの長さＬが小さい場合には、損失ピークの波長間隔は長くなる。
したがって、第２の区間１ｂの長さＬを変化させることによって、損失ピークの波長間隔を調整し、光フィルタの特性を変化させることができる。
さらに前記式（１０）より、摂動付与部２ｃ，２ｄにおける結合効率ａ1，ａ2，ｂ1，ｂ2を調整することによって、損失ピークの大きさを調整できることがわかる。
【００４７】
ところで、本発明の光フィルタは、例えば以下のようにして製造することができる。
図５（ａ）は、本発明の光フィルタの製造装置と製造方法の一例を示す概略構成図である。
図中符号５ａは炭酸ガスレーザで、この炭酸ガスレーザ５ａから発生したレーザ光は、ミラー５ｂを介して進行方向が調整され、さらにレンズ５ｃによって集光され、照射されるようになっている。
【００４８】
一方光ファイバ２は、その中央部の被覆層２ｂが除去され、裸光ファイバ２ａが露出された状態になっている。
またクランプ５ｄ，５ｄは支持ネジ５ｅによって支持されており、これらのクランプ５ｄ，５ｄは、前記光ファイバ２の裸光ファイバ２ａが露出された部分の両側の被覆層２ｂの終端部分を把持している。
これらのクランプ５ｄ，５ｄは支持ネジ５ｅを回転させることによって、この支持ネジ５ｅの長さ方向にそって移動させることができるようになっている。
【００４９】
摂動付与部２ｃ，２ｄを形成するにおいては、炭酸ガスレーザ５ａから発生したレーザ光を、ミラー５ｂとレンズ５ｃを介して裸光ファイバ２ａに照射して加熱し、この裸光ファイバ２ａを局所的に軟化させる。ついで一方のクランプ５ｄを、他方のクランプ５ｄから遠ざかる方向に移動させる。
すると図５（ｂ）に示すように、裸光ファイバ２ａにおいて、前記レーザ光の照射によって軟化した部分が延伸されて縮径部２ｅが形成され、これが摂動付与部２ｃとなる。
ついで、レーザ光の照射位置を裸光ファイバ２ａの長さ方向にそって移動させ、同様の操作を繰り返すと、図５（ｂ）に示すように縮径部２ｆが形成され、これが摂動付与部２ｄとなる。
裸光ファイバ２ａを軟化させて延伸する際の加熱温度は、通常１５００〜２２００℃とされる。また加熱時間は０．１〜１０秒とされる。
【００５０】
また、延伸を行わずに摂動付与部２ｃ，２ｄを形成することもできる。
例えばレーザ光の照射によって、裸光ファイバ２ａを局所的に８００〜２０００℃に加熱すると、裸光ファイバ２ａを構成している石英ガラスに添加されているドーパントが拡散する。この結果、この加熱部分のモードフィールドが変化し、摂動付与部２ｃ，２ｄを形成することができる。
また、紫外光を照射して、露光することによって摂動付与部２ｃ，２ｄを形成することもできる。
すなわち、ゲルマニウム添加石英ガラスに、紫外光を照射すると、屈折率が上昇することが知られている。したがって、ゲルマニウム添加石英ガラスからなるコアを有する裸光ファイバ２ａに、その長さ方向において局所的に紫外光を照射すると、この部分のコアの屈折率が上昇する。
この結果、モードフィールドが変化し、摂動付与部２ｃ，２ｄを形成することができる。
【００５１】
また、この光フィルタの特性に係るパラメータである前記第２の区間１ｂの長さＬ、あるいは結合効率ａ1，ａ2，ｂ1，ｂ2は、以下のようにして調整することができる。
第２の区間１ｂの長さＬは、設計時に設定することもできるし、摂動付与部２ｃ，２ｄを形成した後に、第２の区間１ｂの一部あるいは全体を加熱し、長さ方向に延伸することによって変化させることができる。
このときの加熱温度は、一般に約１０００℃以上とされる。また、第２の区間１ｂのモードフィールドに大きな変化を生じないように、第２の区間１ｂ全体を加熱し、緩やかに延伸すると好ましい。
【００５２】
また、摂動付与部２ｃ，２ｄを形成する際の条件を変更することによって、結合効率ａ1，ａ2，ｂ1，ｂ2を調整することができる。
例えば加熱、延伸によって縮径部２ｅ，２ｆを形成する場合には、延伸長などを調整することによって、結合効率ａ1，ａ2，ｂ1，ｂ2を調整することができる。
加熱によってドーパンドを拡散させる場合には、加熱温度、加熱時間を変更することによって、結合効率ａ1，ａ2，ｂ1，ｂ2を調整することができる。
また紫外光の照射によって屈折率の上昇を図る場合には、紫外光の照射時間、照射回数などを変更することによって、結合効率ａ1，ａ2，ｂ1，ｂ2を調整することができる。
【００５３】
実際の摂動付与部２ｃ，２ｄの形成作業は、光フィルタの波長−透過損失特性を測定しながら、条件を調整しつつ行う。
あるいは、摂動付与部２ｃ，２ｄを形成した後に、光フィルタの特性をモニターしながら、さらに上述の操作を繰り返して、その特性を調整することもできる。
また、上述の摂動付与部２ｃ，２ｄの形成方法から２つ以上の方法を組み合わせて用いてもよい。
さらに摂動付与部２ｃ，２ｄは、２箇所以上であればよく、少なくとも基本モードとクラッドモードとのモード結合関係を制御することができれば、その数を限定するものではない。しかし前記モード結合関係の制御が容易なので、２箇所とするのが通常である。
【００５４】
図６は、本発明の光フィルタの第１の実施例において得られた波長−透過損失特性の関係を示すグラフである。
この第１の実施例において、光ファイバ２の裸光ファイバ２ａは、デプレスドクラッド型の屈折率分布を有するものである。そのコア３ａと第１のクラッド３ｂとの比屈折率差は０．４％、第１のクラッド３ｂと中間部３ｃとの比屈折率差は０．１５％、中間部３ｃと第２のクラッド３ｄとの比屈折率差は０．１５％であった。またコア３ａの外径は９μｍ、第１のクラッド３ｂの外径は３５μｍ、中間部３ｃの外径は５０μｍ、第２のクラッド３ｄの外径は約１２５μｍであった。 摂動付与部２ｃ，２ｄは、加熱、延伸によって形成した縮径部２ｅ，２ｆであった。第２の区間１ｂの長さＬは２０ｍｍであった。
縮径部２ｅ，２ｆは、光ファイバ２の長さ方向においていずれも０．５ｍｍの長さの範囲に形成し、いずれも最小外径は１０５μｍであった。
【００５５】
この第１の実施例の光フィルタにおいては、図６に示すように、波長λに対して周期的な損失ピークが生じた。このグラフにおいて、隣接する損失ピークの中心波長間の間隔は２５〜３０ｎｍであった。また、損失ピークの中心波長の損失値は０．８〜１．２ｄＢであった。
さらに、この光フィルタの第２の区間１ｂを加熱して延伸し、第２の区間１ｂの長さＬを２２ｍｍとして第２の実施例の光フィルタを得た。この光フィルタの波長−透過損失特性のグラフにおいて、隣接する損失ピークの中心波長間の間隔は２２〜２７ｎｍであった。
したがって、第２の区間１ｂの長さＬを調整することによって光フィルタの特性を調節することができることが確認できた。
【００５６】
また第１の実施例と同様の光フィルタを形成した後、摂動付与部２ｃ，２ｄをそれぞれ、１１００℃，１０時間加熱してドーパントを拡散させて、第３の実施例の光フィルタを得た。
この結果、この光フィルタの波長−透過損失特性のグラフにおいて、損失ピークの中心波長の損失値は０．６〜１．０ｄＢであった。
したがって、結合効率ａ1，ａ2，ｂ1，ｂ2を調整することによって、損失ピークの大きさを変化させることができることが確認できた。
【００５７】
このように本発明の光フィルタは、光ファイバ２に摂動付与部２ｃ，２ｄを形成するだけで製造することができるので、光軸の煩雑な調整や、精密な加工を行わずに作製することができる。
また、第２の区間１ｂの長さＬによって損失ピークの波長間隔を調整することができる。また、摂動付与部２ｃ，２ｄにおける結合効率ａ1，ａ2，ｂ1，ｂ2を調整することによって、損失ピークの大きさを調整することができる。
したがって、前記第２の区間１ｂの長さＬや摂動付与部２ｃ，２ｄにおける結合効率ａ1，ａ2，ｂ1，ｂ2を変更することによって、異なる特性を有する様々な光フィルタを提供することができる。
また、光フィルタを製造した後にも、前記第２の区間１ｂの長さＬや摂動付与部２ｃ，４ｂにおける結合効率ａ1，ａ2，ｂ1，ｂ2を調整することができるので、特性の制御が容易である。
【００５８】
ところで、裸光ファイバ２ａが露出した部分は脆弱なので、保護された状態で使用するのが好ましい。
図７（ａ）、図７（ｂ）は、図５（ｂ）に示す光フィルタを保護ケースに収めた状態の一例を示すものであって、図７（ａ）は平面図、図７（ｂ）は、図７（ａ）におけるＡ−Ａで切断した断面図である。
【００５９】
符号５ａは台座であって、この台座６ａは断面半円状で、平面部６ａ’を有している。この平面部６ａ’には、光フィルタの裸光ファイバ２ａが露出した部分と、被覆層２ｂの終端が配置され、裸光ファイバ２ａが露出した部分の両側付近と、被覆層２ｂが、それぞれ接着剤６ｃによって固定されている。
そして、前記台座６ａの平面部６ａ’の上から、断面半円状の中空部６ｂ’を有する蓋６ｂがかぶせられて、外形円筒状の保護ケース６が形成され、裸光ファイバ２ａが露出した部分が保護されている。
前記保護ケース６の材料としては、好ましくは石英ガラスなどが用いられるが、金属、セラミックスなどを用いることもできる。
【００６０】
また、保護ケースにかわって熱収縮チューブを用いることもできる。
図８（ａ）、図８（ｂ）、図８（ｃ）は、熱収縮チューブを用いて図５（ｂ）に示す光フィルタを保護した場合の構造の一例を示すものである。
図８（ａ）、図８（ｂ）は、熱収縮チューブを加熱する前の状態を示したもので、図８（ｃ）は熱収縮チューブを加熱し、収縮させた後の状態を示している。
以下、この構造の作製手順を追って説明する。
【００６１】
すなわち、最初に図８（ａ）、図８（ｂ）に示すように、裸光ファイバ２ａが露出した部分と、被覆層２ｂの終端を、細径チューブ（熱収縮チューブ）６ａ内に配置する。
さらにこの細径チューブ７ａを、テンションメンバ７ｂと平行に、太径チューブ（熱収縮チューブ）６ｃ内に配置する。
テンションメンバ７ｂの長さは、前記細径チューブ７ａの長さと同程度とされる。
太径チューブ７ｃの長さは、収縮後に細径チューブ７ａと太径チューブ７ｃの両端を覆うことができるように、細径チューブ７ａの長さよりも長くなっている。
【００６２】
この状態で所定の温度にまで加熱すると、図８（ｃ）に示すように細径チューブ７ａと太径チューブ７ｃが収縮する。
そして細径チューブ７ａの内面は、裸光ファイバ２ａの外面と、被覆層２ｂの終端付近の外面に密着する。
一方、太径チューブ７ｃの内面は、細径チューブ７ａの外面と、テンションメンバ７ｂの外面に密着する。そして、太径チューブ７ｃの両端は、前記細径チューブ７ａとテンションメンバ７ｂの両端を覆うようにして、被覆層２ｂの外面に密着した状態となる。
【００６３】
前記テンションメンバ７ｂは、裸光ファイバ２ａが露出した部分に曲げが付与されて劣化するのを防ぐために設けられているもので、金属、セラミックス、ガラスなどからなるものである。
熱収縮チューブ（細径チューブ７ａ、太径チューブ７ｃ）はポリエチレンなどのプラスチックからなるものであって、通常１００〜２５０℃に加熱することによって、収縮させることができるものである。
【００６４】
また本発明の光フィルタの用途の一例として、エルビウムドープ光ファイバ増幅器があげられる。
図９は従来のエルビウムドープ光ファイバ増幅器の一例を示す概略構成図である。
図中符号１１は光ファイバ（光伝送路）１１であって、この光ファイバ１１には、入射側から順に、エルビウムドープ光ファイバを収容したエルビウムドープ光ファイバ収容部１２、ＷＤＭ（波長合分波）カプラ１３、光アイソレータ１５が設けられている。
前記ＷＤＭカプラ１３は入射側にひとつのポートを有し、出射側にふたつのポートを有している。
そして入射側のポートと、出射側のひとつポートとが光ファイバ１１の途中に挿入、接続されている。一方、出射側の他方のポートは、ポンプ光源１４に接続されている。
【００６５】
すなわち、光ファイバ１１に入射した光はエルビウムドープ光ファイバ収容部１２を経て伝搬し、途中でポンプ光源１４から光ファイバ１１にポンプ光が入射され、このポンプ光はＷＤＭカプラ１３において光ファイバ１１を進行する伝搬光と合波し、これにより増幅された光が光ファイバ１１から出射される。
光アイソレータ１５は、光ファイバ１１において反射をおさえ、安定させるために設けられているものである。
【００６６】
しかし、従来のエルビウムドープ光ファイバ増幅器においては、利得に波長依存性があるという問題がある。波長多重伝送を行うにおいて、波長依存性が大きいと、波長によって利得がばらつくため不都合である。
このため本発明の光フィルタを、このエルビウムドープ光ファイバ増幅器に挿入し、利得が大きい波長範囲の光を損失させることによって、波長−利得特性の平坦化を図ることができる。
このとき、光フィルタの挿入位置は光ファイバ１１の入射端から出射端までの間であれば特に限定するものではなく、図９に符号１１ａ，１１ｂ，１１ｃ，１１ｄで示した挿入位置のいずれでもよい。
ここで、本発明の光フィルタの光ファイバの基本モード（ＬＰ01モード）のモードフィールドと、光ファイバ（光伝送路）１１に使用されるシングルモード光ファイバの基本モード（ＬＰ01モード）のモードフィールドとがほぼ一致するように設定されることが望ましい。
前記モードフィールドがほぼ一致していないと、光フィルタの入射側に接続されている光ファイバ１１の基本モードから、光フィルタの光ファイバの第１の区間に、基本モード以外にクラッドモードも励起されてしまい、このクラッドモードが、上述のモード結合に加わって、光フィルタの波長に対する損失の周期的変化の特性が乱れてしまうからである。
モードフィールドをほぼ一致させるには、光ファイバ１１のシングルモード光ファイバのモードフィールド径と、光フィルタの光ファイバの、コアと第１のクラッドとからなるシングルモード光ファイバ構造の部分のモードフィールド径とを、ほぼ等しくしておいてもよい。
さらに望ましくは、光フィルタの光ファイバのコアと第１のクラッドとからなるシングルモード光ファイバ構造の部分と、光伝送路１１に使用するシングルモード光ファイバとにおいて、コア径、屈折率分布などをほぼ等しくするか、さらには、光フィルタの前記シングルモード光ファイバ構造の部分に、光伝送路１１と同じ光ファイバを使用してもよい。
なお本発明の光フィルタは、上述の組み合わせに限定されず、光伝送装置や光線路設備などの光通信システムに広く適用されるものである。この場合、前述のように本発明の光フィルタの光ファイバの基本モードのモードフィールドと、これらの装置の光伝送路に使用されるシングルモード光ファイバのモードフィールドとがほぼ等しくなるように設定されることが望ましい。
【００６７】
【発明の効果】
以上説明したように本発明の光フィルタは、クラッドに導波構造を有する光ファイバの２箇所以上に摂動付与部を形成するだけで構成することができるので、光軸の煩雑な調整や、精密な加工を行わずに作製することができる。
また第２の区間の長さによって損失ピークの波長間隔を調整することができる。また、接続部における結合効率を調整することによって、損失ピークの大きさを調整することができる。
したがって、第２の区間の長さや摂動付与部における結合効率を変更することによって、異なる特性を有する様々な光フィルタを提供することができる。
また、例えば光フィルタの製造後においても前記第２の区間の長さや摂動付与部における結合効率を調整することができるので、特性の制御が容易である。
【図面の簡単な説明】
【図１】 本発明の光フィルタの構成の一例を示す一部側断面図である。
【図２】 図２（ａ）、図２（ｂ）は、本発明の光ファイバの屈折率分布の例を示すものであって、図２（ａ）はデプレスドクラッド型の屈折率分布を示すグラフ、図２（ｂ）は階段型クラッド型の屈折率分布を示すグラフである。
【図３】 階段型クラッド型の屈折率分布を有する光ファイバにおける、ＬＰ01モード（基本モード）とＬＰ02モードとＬＰ03モードの電界分布を示すグラフである。
【図４】 本発明の光フィルタの動作の一例を示す説明図である。
【図５】 図５（ａ）は、加熱、延伸によって摂動付与部を形成する本発明の光フィルタの製造装置と製造方法の一例を示す概略構成図である。
図５（ｂ）は図５（ａ）に示す製造装置によって製造された本発明の光フィルタの一例を示す平面図である。
【図６】 本発明の光フィルタの第１の実施例において得られる波長−透過損失特性の関係を示すグラフである。
【図７】 図７（ａ）、図７（ｂ）は、図５（ｂ）に示す光フィルタを、保護ケースに収めた状態の一例を示すものであって、図７（ａ）は平面図、図７（ｂ）は、図７（ａ）におけるＡ−Ａで切断した断面図である。
【図８】 図８（ａ）、図８（ｂ）、図８（ｃ）は、熱収縮チューブを用いて本発明の光フィルタを保護する場合の構造の一例を示すものである。
図８（ａ）、図８（ｂ）は、それぞれ熱収縮チューブを加熱する前の状態を示した斜視図と断面図で、図８（ｃ）は熱収縮チューブを加熱し、収縮させた状態を示す一部側断面図である。
【図９】 従来のエルビウムドープ光ファイバ増幅器の一例を示す概略構成図である。
【図１０】 従来の光フィルタの一例を示す概略構成図である。
【図１１】 従来の光フィルタの他の例を示す斜視図である。
【符号の説明】
１ａ…第１の区間、１ｂ…第２の区間、１ｃ…第３の区間、
２…光ファイバ、２ａ…裸光ファイバ、２ｂ…被覆層、２ｃ，２ｄ…摂動付与部、２ｅ，２ｆ…縮径部、３ａ…コア、３ｂ…第１のクラッド（第１の層）、３ｃ…中間部（第２の層）、３ｄ…第２のクラッド、３ｅ…クラッド、
４ａ…コア、４ｂ…第１のクラッド（第１の層）、４ｃ…第２のクラッド（第２の層）、４ｄ…クラッド。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical filter capable of selectively attenuating a specific wavelength, and more particularly to an optical filter that can be manufactured without complicated adjustment of an optical axis and precise processing.
[0002]
[Prior art]
FIG. 10 is a schematic configuration diagram showing an example of a conventional optical filter.
Reference numerals 21 and 21 denote optical fibers, and these optical fibers 21 and 21 are held by optical fiber holding portions 22 and 22, respectively.
On the other hand, reference numerals 23 and 23 denote collimating lenses, and these collimating lenses 23 and 23 are held by lens holding portions 24 and 24, respectively, with a dielectric multilayer filter 25 interposed therebetween.
In this optical filter, the light output from one optical fiber 21 enters the dielectric multilayer filter 25 through one collimator lens 23, where light of a specific wavelength is lost. Then, the light having the specific wavelength lost enters the other optical fiber 21 through the other collimator lens 23 and is output.
[0003]
FIG. 11 is a perspective view showing another example of a conventional optical filter. Reference numeral 31 is an input / output optical fiber, 32 is a waveguide substrate, 33 is a slit, 34 is a waveguide, and 35 is a dielectric multilayer filter. is there.
Input / output optical fibers 31, 31 are connected to both ends of the waveguide 34 of the waveguide substrate 32, respectively.
On the other hand, in the vicinity of the center of the waveguide substrate 32, a dielectric multilayer filter 35 is inserted into a slit 33 formed obliquely with respect to the waveguide 34 and across the waveguide 34, and bonded and fixed. Has been.
In this optical filter, light output from one input / output optical fiber 31 is incident on the dielectric multilayer filter 35 via the waveguide 34, where light of a specific wavelength is lost. The light having a specific wavelength lost through the dielectric multilayer filter 35 enters the other input / output optical fiber 31 from the other waveguide 34 and is output.
[0004]
However, in the manufacture of the optical filter having the configuration shown in FIG. 10, it is necessary to accurately arrange the optical fibers 21 and 21, the collimating lenses 23 and 23, and the dielectric multilayer filter 25 to adjust the optical axis. Efficiency and product yield were low.
Further, in the manufacture of the optical filter having the configuration shown in FIG. 11, there is a process in which a slit 33 is formed in the waveguide substrate 32, and then a dielectric multilayer filter 35 is inserted into the slit 33 and bonded and fixed. Processing was necessary. In addition, in the connection between the waveguide 34 and the input / output optical fiber 31, there is a problem that adjustment of the optical axis is troublesome.
[0005]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical filter that can be manufactured without complicated adjustment of the optical axis and precise processing.
[0006]
[Means for Solving the Problems]
  In order to solve the above-described problem, in the present invention, in addition to the LP01 mode of the fundamental mode, a perturbation is locally generated at one location in the length direction of an optical fiber having a waveguide structure in which at least one mode is guided. GrantedByThe perturbation imparting part that changes the mode field,Two or more places are formed,The perturbation is a change in the refractive index distribution or a change in the refractive index. In each perturbation imparting section, a mode field change causes a coupling between modes between the LP01 mode of the fundamental mode and at least one other mode. In addition, a specific wavelength is selectively attenuated by utilizing the fact that the phase difference of the coupling light between the modes changes periodically with respect to the change of the wavelength of the propagating light.An optical filter characterized by the above is proposed.
  The optical fiber has a core and a clad, and the clad is provided on the circumference of the core, and is provided on the circumference of the first layer having a lower refractive index than the core and the circumference of the first layer. It is preferable that a second layer having a lower refractive index than that of the first layer is provided. Further, it is preferable that the optical fiber guides only the LP02 mode in addition to the LP01 mode of the fundamental mode. Further, the present invention proposes an optical communication system characterized by using the optical filter of the present invention. In the present invention, the optical fiber includes a bare optical fiber, an optical fiber, an optical fiber core, and the like.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example of the configuration of the optical filter of the present invention.
Reference numeral 2 denotes an optical fiber, and this optical fiber 2 is formed by providing a coating layer 2b on a bare optical fiber 2a.
In this optical fiber 2, the coating layer 2b in the middle is partially removed, and the bare optical fiber 2a is exposed.
Two perturbation imparting portions 2c and 2d are formed in the exposed portion of the bare optical fiber 2a.
These perturbation imparting portions 2c and 2d divide the optical fiber 2 into a first section 1a, a second section 1b, and a third section 1c, thereby constituting this optical filter.
[0008]
The bare optical fiber 2a is made of pure silica glass or quartz glass whose refractive index is adjusted by adding a dopant such as fluorine or germanium.
The coating layer 2b includes a primary coating provided on the bare optical fiber 2a to form an optical fiber, and further, the primary coating of the primary coating to form the optical fiber in an optical fiber core. It consists of a secondary coating provided on top.
The primary coating constituting the coating layer 2b is usually made of an ultraviolet curable resin, a silicone resin or the like, and the secondary coating is made of nylon or the like.
[0009]
As will be described later, the bare optical fiber 2a has a structure in which the clad has two or more layers. In the structure up to the core at the center and the first layer of the clad provided on the core, the fundamental mode ( This is a single mode optical fiber structure in which only the LP01 mode) is guided.
Furthermore, in the present invention, the bare optical fiber 2a is required to have a waveguide structure in the clad so that at least one of the modes other than the fundamental mode propagates through the clad almost without loss.
Hereinafter, modes other than the fundamental mode propagating through the cladding of the bare optical fiber 2a with almost no loss are referred to as cladding modes.
On the other hand, the mode that guides the central portion, that is, the LP01 mode is called a fundamental mode.
Further, modes other than these modes are referred to as radiation modes.
The present invention guides the fundamental mode and the clad mode in the optical fiber 2, and further controls the mode coupling relationship between the fundamental mode and the clad mode in the perturbation imparting portions 2c and 2d, thereby providing the characteristics as an optical filter. Is what you get.
[0010]
The bare optical fiber 2a has a core and a clad, and the clad is provided on the circumference of the core, and has a first layer having a lower refractive index than the core and on the circumference of the first layer. It is preferable to include a second layer having a lower refractive index than the first layer.
[0011]
2A and 2B show examples of the refractive index distribution of the bare optical fiber 2a.
The depressed clad type shown in FIG. 2 (a) has a high refractive index core 3a, and a first clad (first layer) 3b having a lower refractive index than the core 3a formed on the circumference, An intermediate part (second layer) 3c having a lower refractive index than the first cladding 3b formed on the circumference and a higher refractive index than the intermediate part 3c formed on the circumference, the core 3a And a second cladding 3d having a lower refractive index than that of the second cladding 3d.
That is, in this depressed clad type, the clad 3e is formed from three layers of the first clad 3b, the intermediate portion 3c, and the second clad 3d.
The core 3a, the first clad 3b, the intermediate portion 3c, and the second clad 3d are arranged concentrically.
[0012]
For example, the relative refractive index difference between the core 3a and the first cladding 3b is 0.2-4%, and the relative refractive index difference between the first cladding 3b and the intermediate portion 3c is 0.1-0. The relative refractive index difference between the intermediate portion 3c and the second cladding 3d is 4% or more.
The refractive index of the first cladding 3b and the refractive index of the second cladding 3d may be equal or different.
The outer diameter of the core 3a is 3 to 10 μm, the outer diameter of the first cladding 3b is 20 to 50 μm, the outer diameter of the intermediate portion 3c is equal to or more than the value obtained by adding 5 μm to the outer diameter of the first cladding 3b. The outer diameter of the cladding 3d is about 125 μm.
For example, the core 3a is made of germanium-added quartz glass, the first cladding 3b and the second cladding 3d are made of pure silica glass, and the intermediate portion 3c is made of fluorine-added quartz glass.
[0013]
The step-type clad type shown in FIG. 2B has a high refractive index core 4a, a first clad (first layer) 4b having a lower refractive index than the core 4a formed on the periphery thereof, A second clad (second layer) 4c having a lower refractive index than the first clad 4b formed on the periphery is formed.
That is, in this step-type clad type, the clad 4d is formed from two layers of the first clad 4b and the second clad 4c.
The core 4a, the first cladding 4b, and the second cladding 4c are arranged concentrically.
[0014]
The relative refractive index difference between the core 4a and the first cladding 4b is 0.2-4%, and the relative refractive index difference between the first cladding 4b and the second cladding 4c is 0.1-0. 4%.
The outer diameter of the core 4a is 3 to 10 μm, the outer diameter of the first cladding 4b is 20 to 50 μm, and the outer diameter of the second cladding 4c is about 125 μm.
For example, the core 4a is made of germanium-added quartz glass, the first clad 4b is made of pure quartz glass, and the second clad 4c is made of fluorine-added quartz glass.
[0015]
FIG. 3 shows an example of the LP01 mode (fundamental mode), LP02 mode, and LP03 mode electric field distributions in the wavelength band of 1500-1600 nm of the bare optical fiber 2a having the step-type cladding type refractive index distribution shown in FIG. It is a graph which shows.
Here, these electric field distributions are obtained in consideration of the refractive index distribution structure up to the core 4a, the first cladding 4b, and the second cladding 4c.
In this example, the radii of the core 4a, the first cladding 4b, and the second cladding 4c are 4.5 μm, 20 μm, and 62.5 μm, respectively (in terms of layer thickness, 4.5 μm, 15.5 μm, 42.5 μm).
[0016]
As can be seen from this graph, the LP01 mode (fundamental mode) electric field of this optical fiber is distributed in the core 4a at the center of the bare optical fiber 2a and in the vicinity thereof.
For example, when a single mode optical fiber composed of a core 4a and a first cladding 4b is assumed, the distribution is almost the same as the LP01 mode electric field of the single mode optical fiber.
The LP02 mode electric field is distributed in and around the first cladding 4b provided on the circumference of the core 4a. That is, it can be seen that this bare optical fiber 2a has a waveguide structure in which the LP02 mode guides the cladding 4d.
The LP03 mode electric field is distributed over the core 4a, the first cladding 4b, and the second cladding 4c.
[0017]
Also in the bare optical fiber 2a having a depressed clad type refractive index profile shown in FIG. 2A, the LP01 mode electric field is distributed in and around the core 3a, and the LP02 mode electric field is generated around the core 3a. The first clad 3b provided above is distributed in the vicinity thereof.
[0018]
As described above, the one having the depressed clad type or stepped clad type refractive index distribution is provided on the circumference of the first clad 3b, 4b on the intermediate portion 3c or the second clad having a lower refractive index than these. Since 4c is provided, higher-order modes such as the LP02 mode are selectively distributed in or around the first clads 3b and 4b provided on the circumference of the cores 3a and 4a. The clad mode propagates almost without loss.
[0019]
The perturbation imparting portions 2c and 2d are portions where perturbation is locally imparted to the bare optical fiber 2a in the length direction. Perturbation means “change in refractive index distribution” or “change in refractive index”.
That is, in the perturbation imparting units 2c and 2d, the mode field is changed from that of the other portions due to “change in refractive index distribution” or “change in refractive index”.
Due to the change of the mode field, mode coupling occurs between the plurality of modes in the perturbation imparting units 2c and 2d.
Specifically, “change in refractive index distribution” or “change in refractive index” means, for example, reduction or expansion of the core diameter of the bare optical fiber 2a, or diffusion of dopant added to the bare optical fiber 2a, Or it is formation of the refractive index change of the core of the bare optical fiber 2a.
[0020]
Hereinafter, the basic operation of the optical filter will be described with reference to FIG.
When light enters the first section 1a, the fundamental mode propagates through the first section 1a.
The fundamental mode propagated through the first section 1a is coupled to the fundamental mode and the cladding mode of the second section 1b in the perturbation imparting unit 2c, and propagates through the second section 1b in a divided state.
Next, in the perturbation imparting unit 2d, most of the fundamental modes propagated through the second section 1b are coupled to the fundamental mode of the third section 1c. On the other hand, the cladding mode propagated through the second section 1b is coupled to the fundamental mode and the cladding mode of the third section 1c.
[0021]
At this time, if most of the cladding modes propagated through the second section 1b are coupled to the fundamental mode of the third section 1c, the light of the fundamental mode propagated through the first section 1a is transmitted from the third section 1c. Output with little loss. That is, the difference between the fundamental mode light intensity in the first section 1a and the fundamental mode light intensity output from the third section 1c is reduced.
[0022]
On the other hand, if most of the cladding mode light propagated through the second section 1b is not coupled to the fundamental mode of the third section 1c, the fundamental mode light propagated through the first section 1a is converted into the third mode. It is possible to output with a large loss from the section 1c.
That is, the difference between the intensity of the fundamental mode light in the first interval 1a and the intensity of the fundamental mode light output from the third interval 1c is increased.
[0023]
Furthermore, the difference between the fundamental mode light intensity of the first section 1a and the fundamental mode light intensity of the third section 1c is wavelength dependent.
That is, in the perturbation imparting unit 2d, the phase of light coupled from the fundamental mode of the second section 1b to the fundamental mode of the third section 1c, and the cladding mode of the second section 1b to the fundamental mode of the third section 1c. If the phase of the coupled light matches and there is no difference between them (hereinafter referred to as phase difference), most of the clad mode light in the second section 1b becomes the fundamental mode in the third section 1c. Join.
On the other hand, when the phase difference is a half wavelength, most of the light in the cladding mode in the second section 1b is not coupled to the fundamental mode in the third section 1c.
The phase difference is wavelength-dependent and changes periodically in response to changes in the wavelength of propagating light.
As a result, in the wavelength-transmission loss characteristic of this optical filter, the increase and decrease of the periodic transmission loss are repeated corresponding to the change of the wavelength.
[0024]
Strictly speaking, the fundamental mode of the first section 1a is coupled to the partial radiation mode in addition to the fundamental mode and the cladding mode of the second section 1b.
Further, the fundamental mode of the second section 1b is coupled to a partial cladding mode and a radiation mode in addition to the fundamental mode of the third section 1c.
The cladding mode in the second section 1b is coupled to the partial radiation mode in addition to the fundamental mode and the cladding mode in the third section 1c.
Here, considering the coupling to a plurality of cladding modes and radiation modes, the mode coupling relationship becomes complicated, and periodic loss variation with respect to a clear wavelength cannot be obtained. For this reason, it is desirable that the fundamental mode be coupled to only one cladding mode.
However, in the case of a depressed cladding type or stepped cladding type refractive index profile, the fundamental mode is distributed in the cores 3a and 4a, and the cladding mode is selective to the first claddings 3b and 4b adjacent thereto. In effect, the coupling to the radiation mode can be neglected.
Therefore, it is desirable that only the LP02 mode is guided in the cladding mode. This can be achieved, for example, as follows.
By setting the normalized frequency ν in the first cladding 3b, 4b in the used wavelength range to a value within the range of 4 to 7, the higher order mode higher than the LP03 mode is cut off, and the LP02 mode Only is guided. Where ν is
ν = 2π / λ · na · (2Δ)^1/2
It is represented by In the equation, λ is the wavelength of light, n is the refractive index of the first cladding 3b, 4b, a is the radius of the first cladding 3b, 4b, Δ is the first cladding 3b, 4b and the intermediate portion 3c, second The relative refractive index difference with the clad 4c is shown.
Here, in the numerical range of the normalized frequency ν, a low-order mode in the LPkm mode (k ≠ 0) such as the LP11 mode may be guided, but due to stretching of the optical fiber, ultraviolet light exposure, or the like. By giving the perturbation so that there is no anisotropy, there is almost no coupling from the LP01 mode to the LPkm mode.
[0025]
Hereinafter, it demonstrates still in detail using numerical formula.
The electric field of the fundamental mode propagating through the first section 1a is expressed by the following formula (1).
[0026]
[Expression 1]

[0027]
In the equation (1), A is the amplitude of the electric field, ω is the frequency of light, t is time, and β i is the propagation constant of the fundamental mode in the first section 1a.
Here, the coupling efficiency between the fundamental mode of the first section 1a and the fundamental mode of the second section 1b is a1, the phase shift at the time of coupling is φ1, and the propagation constant of the fundamental mode of the second section 1b is β01. Then, the electric field of the fundamental mode in the second section 1b is expressed by the following formula (2).
[0028]
[Expression 2]

[0029]
The coupling efficiency between the fundamental mode of the first section 1a and the cladding mode of the second section 1b is a2, the phase shift at the time of coupling is φ2, and the propagation constant of the cladding mode of the second section 1b is β02. Then, the electric field of the clad mode in the second section 1b is expressed by the following formula (3).
[0030]
[Equation 3]

[0031]
Next, the electric field of light coupled from the fundamental mode of the second section 1b to the fundamental mode of the third section 1c is expressed by the following equation (4).
[0032]
[Expression 4]

[0033]
In the above equation (4), b1 is the coupling efficiency between the fundamental mode of the first section 1a and the fundamental mode of the second section 1b, φ3 is the phase shift during coupling, and β3 is the fundamental mode of the third section 1c. The propagation constant of L represents the length of the second section 1b.
The electric field of light coupled from the cladding mode of the second section 1b to the cladding mode of the third section 1c is expressed by the following equation (5).
[0034]
[Equation 5]

[0035]
In the equation (5), b2 represents the coupling efficiency between the cladding mode of the second section 1b and the fundamental mode of the third section 1c, and φ4 represents the phase shift at the time of coupling.
Here, ΔΦ is defined as in the following formula (6).
[0036]
[Formula 6]

[0037]
Then, the square of the amplitude of the light in the fundamental mode propagating through the third section 1c is expressed by the following equation (7).
[0038]
[Expression 7]

[0039]
Therefore, the ratio between the intensity of the fundamental mode light propagating in the first interval 1a and the intensity of the fundamental mode light propagating in the third interval 1c is from the first interval 1a to the third interval 1c. The light transmission efficiency is represented by the following formula (8).
[0040]
[Equation 8]

[0041]
Here, when the effective refractive index of the mode is ne and the wavelength of light in vacuum is λ, the propagation constant of the mode is expressed by the following equation (9).
[0042]
[Equation 9]

[0043]
When this equation (9) is substituted into the equation (8), the light transmission efficiency is expressed by the following equation (10).
[0044]
[Expression 10]

[0045]
In this equation (10), n01 represents the effective refractive index of the fundamental mode in the second section 1b, and n02 represents the effective refractive index of the cladding mode in the second section 1b.
The third term on the right side of the equation (10) represents the light transmission efficiency, whereby the configuration of this optical fiber has a periodic transmission loss change with respect to the wavelength λ. It can be seen that it can be used as an optical filter.
[0046]
From this equation (10), it can be seen that the wavelength interval of the loss peak is determined by the length L of the second section 1b.
That is, when the length L of the second section 1b is large, the wavelength interval of the loss peak is short, and when the length L of the second section 1b is small, the wavelength interval of the loss peak is long.
Therefore, by changing the length L of the second section 1b, the wavelength interval of the loss peak can be adjusted and the characteristics of the optical filter can be changed.
Furthermore, from the above equation (10), it can be seen that the magnitude of the loss peak can be adjusted by adjusting the coupling efficiencies a1, a2, b1, b2 in the perturbation imparting units 2c, 2d.
[0047]
By the way, the optical filter of this invention can be manufactured as follows, for example.
Fig.5 (a) is a schematic block diagram which shows an example of the manufacturing apparatus and manufacturing method of the optical filter of this invention.
In the figure, reference numeral 5a denotes a carbon dioxide laser, and the laser light generated from the carbon dioxide laser 5a is adjusted in the traveling direction via a mirror 5b, and further condensed and irradiated by a lens 5c.
[0048]
On the other hand, the optical fiber 2 is in a state where the coating layer 2b at the center is removed and the bare optical fiber 2a is exposed.
The clamps 5d and 5d are supported by a support screw 5e, and these clamps 5d and 5d hold the end portions of the covering layer 2b on both sides of the portion of the optical fiber 2 where the bare optical fiber 2a is exposed. Yes.
These clamps 5d and 5d can be moved along the length direction of the support screw 5e by rotating the support screw 5e.
[0049]
In forming the perturbation imparting portions 2c and 2d, the bare optical fiber 2a is irradiated with the laser light generated from the carbon dioxide laser 5a through the mirror 5b and the lens 5c and heated, and the bare optical fiber 2a is locally applied. Soften. Then, one clamp 5d is moved away from the other clamp 5d.
Then, as shown in FIG. 5 (b), in the bare optical fiber 2a, the softened portion by the laser light irradiation is stretched to form the reduced diameter portion 2e, which becomes the perturbation imparting portion 2c.
Next, when the irradiation position of the laser beam is moved along the length direction of the bare optical fiber 2a and the same operation is repeated, a reduced diameter portion 2f is formed as shown in FIG. 2d.
The heating temperature when the bare optical fiber 2a is softened and stretched is usually 1500 to 2200 ° C. The heating time is 0.1 to 10 seconds.
[0050]
Further, the perturbation imparting portions 2c and 2d can be formed without stretching.
For example, when the bare optical fiber 2a is locally heated to 800-2000 ° C. by laser light irradiation, the dopant added to the quartz glass constituting the bare optical fiber 2a diffuses. As a result, the mode field of the heated portion is changed, and the perturbation imparting portions 2c and 2d can be formed.
Further, the perturbation imparting portions 2c and 2d can be formed by irradiating with ultraviolet light for exposure.
That is, it is known that when a germanium-doped quartz glass is irradiated with ultraviolet light, the refractive index increases. Accordingly, when the bare optical fiber 2a having a core made of germanium-added quartz glass is locally irradiated with ultraviolet light in the length direction, the refractive index of the core in this portion increases.
As a result, the mode field changes and the perturbation imparting portions 2c and 2d can be formed.
[0051]
The length L of the second section 1b or the coupling efficiency a1, a2, b1, b2 that is a parameter relating to the characteristics of the optical filter can be adjusted as follows.
The length L of the second section 1b can be set at the time of design. After the perturbation imparting portions 2c and 2d are formed, a part or the whole of the second section 1b is heated and stretched in the length direction. It can be changed by doing.
The heating temperature at this time is generally about 1000 ° C. or higher. Further, it is preferable that the entire second section 1b is heated and gently stretched so as not to cause a large change in the mode field of the second section 1b.
[0052]
Further, the coupling efficiencies a1, a2, b1, and b2 can be adjusted by changing the conditions for forming the perturbation imparting units 2c and 2d.
For example, when the reduced diameter portions 2e and 2f are formed by heating and stretching, the coupling efficiencies a1, a2, b1, and b2 can be adjusted by adjusting the stretching length and the like.
When the dopant is diffused by heating, the coupling efficiencies a1, a2, b1, and b2 can be adjusted by changing the heating temperature and the heating time.
When the refractive index is increased by irradiation with ultraviolet light, the coupling efficiencies a1, a2, b1, and b2 can be adjusted by changing the irradiation time and the number of times of irradiation with ultraviolet light.
[0053]
The actual perturbation imparting units 2c and 2d are formed while adjusting the conditions while measuring the wavelength-transmission loss characteristics of the optical filter.
Alternatively, after the perturbation imparting portions 2c and 2d are formed, the above-described operation can be repeated while adjusting the characteristics while monitoring the characteristics of the optical filter.
Moreover, you may use combining two or more methods from the formation method of the above-mentioned perturbation provision parts 2c and 2d.
Further, the number of perturbation imparting units 2c and 2d may be two or more, and the number is not limited as long as at least the mode coupling relationship between the fundamental mode and the clad mode can be controlled. However, since it is easy to control the mode coupling relationship, two locations are usually used.
[0054]
FIG. 6 is a graph showing the relationship between wavelength and transmission loss characteristics obtained in the first embodiment of the optical filter of the present invention.
In the first embodiment, the bare optical fiber 2a of the optical fiber 2 has a depressed clad type refractive index distribution. The relative refractive index difference between the core 3a and the first cladding 3b is 0.4%, the relative refractive index difference between the first cladding 3b and the intermediate portion 3c is 0.15%, and the intermediate portion 3c and the second cladding. The relative refractive index difference from 3d was 0.15%. The outer diameter of the core 3a was 9 μm, the outer diameter of the first cladding 3b was 35 μm, the outer diameter of the intermediate portion 3c was 50 μm, and the outer diameter of the second cladding 3d was about 125 μm. The perturbation imparting portions 2c and 2d were the reduced diameter portions 2e and 2f formed by heating and stretching. The length L of the second section 1b was 20 mm.
The reduced diameter portions 2e and 2f are both formed in a length range of 0.5 mm in the length direction of the optical fiber 2, and the minimum outer diameter of each is 105 μm.
[0055]
In the optical filter of the first embodiment, as shown in FIG. 6, a periodic loss peak occurs with respect to the wavelength λ. In this graph, the interval between the central wavelengths of adjacent loss peaks was 25 to 30 nm. The loss value at the center wavelength of the loss peak was 0.8 to 1.2 dB.
Further, the second section 1b of the optical filter was heated and stretched, and the length L of the second section 1b was set to 22 mm to obtain the optical filter of the second embodiment. In the wavelength-transmission loss characteristic graph of this optical filter, the interval between the center wavelengths of adjacent loss peaks was 22 to 27 nm.
Therefore, it was confirmed that the characteristics of the optical filter can be adjusted by adjusting the length L of the second section 1b.
[0056]
Further, after forming the same optical filter as in the first example, the perturbation imparting portions 2c and 2d were heated at 1100 ° C. for 10 hours, respectively, to diffuse the dopant, thereby obtaining the optical filter of the third example. .
As a result, in the graph of wavelength-transmission loss characteristics of this optical filter, the loss value at the center wavelength of the loss peak was 0.6 to 1.0 dB.
Therefore, it was confirmed that the magnitude of the loss peak can be changed by adjusting the coupling efficiencies a1, a2, b1, and b2.
[0057]
As described above, since the optical filter of the present invention can be manufactured by simply forming the perturbation imparting portions 2c and 2d in the optical fiber 2, it is manufactured without complicated adjustment of the optical axis and precise processing. Can do.
Further, the wavelength interval of the loss peak can be adjusted by the length L of the second section 1b. Further, the magnitude of the loss peak can be adjusted by adjusting the coupling efficiencies a1, a2, b1, and b2 in the perturbation imparting units 2c and 2d.
Therefore, various optical filters having different characteristics can be provided by changing the length L of the second section 1b and the coupling efficiencies a1, a2, b1, and b2 in the perturbation imparting units 2c and 2d.
Even after the optical filter is manufactured, the length L of the second section 1b and the coupling efficiencies a1, a2, b1, b2 in the perturbation imparting units 2c, 4b can be adjusted, so that the characteristics can be easily controlled. It is.
[0058]
By the way, since the part where the bare optical fiber 2a is exposed is fragile, it is preferably used in a protected state.
FIGS. 7A and 7B show an example of a state in which the optical filter shown in FIG. 5B is housed in a protective case. FIG. 7A is a plan view, and FIG. FIG. 7B is a cross-sectional view taken along line AA in FIG.
[0059]
Reference numeral 5a denotes a pedestal, and this pedestal 6a is semicircular in cross section and has a flat portion 6a '. The flat portion 6a ′ is provided with a portion where the bare optical fiber 2a of the optical filter is exposed and a terminal end of the coating layer 2b, and the vicinity of both sides of the portion where the bare optical fiber 2a is exposed and the coating layer 2b are bonded. It is fixed by the agent 6c.
Then, a cover 6b having a hollow part 6b 'having a semicircular cross section is placed on the flat part 6a' of the pedestal 6a to form an outer cylindrical protective case 6, and the bare optical fiber 2a is exposed. The part is protected.
As a material of the protective case 6, quartz glass or the like is preferably used, but metal, ceramics, or the like can also be used.
[0060]
Further, a heat shrinkable tube can be used instead of the protective case.
FIG. 8A, FIG. 8B, and FIG. 8C show an example of the structure when the optical filter shown in FIG. 5B is protected using a heat shrinkable tube.
8 (a) and 8 (b) show the state before heating the heat-shrinkable tube, and FIG. 8 (c) shows the state after heating and shrinking the heat-shrinkable tube. Yes.
Hereinafter, the manufacturing procedure of this structure will be described.
[0061]
That is, first, as shown in FIGS. 8A and 8B, the exposed portion of the bare optical fiber 2a and the end of the coating layer 2b are disposed in the small-diameter tube (heat-shrinkable tube) 6a. .
Further, the small-diameter tube 7a is disposed in a large-diameter tube (heat-shrinkable tube) 6c in parallel with the tension member 7b.
The length of the tension member 7b is approximately the same as the length of the small diameter tube 7a.
The length of the large diameter tube 7c is longer than the length of the small diameter tube 7a so that both ends of the small diameter tube 7a and the large diameter tube 7c can be covered after contraction.
[0062]
When heated to a predetermined temperature in this state, the small-diameter tube 7a and the large-diameter tube 7c contract as shown in FIG.
The inner surface of the small-diameter tube 7a is in close contact with the outer surface of the bare optical fiber 2a and the outer surface near the end of the coating layer 2b.
On the other hand, the inner surface of the large-diameter tube 7c is in close contact with the outer surface of the small-diameter tube 7a and the outer surface of the tension member 7b. The both ends of the large diameter tube 7c are in close contact with the outer surface of the coating layer 2b so as to cover both ends of the small diameter tube 7a and the tension member 7b.
[0063]
The tension member 7b is provided to prevent the exposed portion of the bare optical fiber 2a from being bent and deteriorated, and is made of metal, ceramics, glass, or the like.
The heat-shrinkable tubes (the small-diameter tube 7a and the large-diameter tube 7c) are made of a plastic such as polyethylene and can be shrunk by heating to 100 to 250 ° C.
[0064]
An example of the use of the optical filter of the present invention is an erbium-doped optical fiber amplifier.
FIG. 9 is a schematic diagram showing an example of a conventional erbium-doped optical fiber amplifier.
In the figure, reference numeral 11 denotes an optical fiber (optical transmission line) 11. The optical fiber 11 includes, in order from the incident side, an erbium-doped optical fiber accommodating portion 12 that accommodates an erbium-doped optical fiber, and WDM (wavelength multiplexing / demultiplexing). ) A coupler 13 and an optical isolator 15 are provided.
The WDM coupler 13 has one port on the incident side and two ports on the output side.
The incident-side port and the emission-side port are inserted and connected in the middle of the optical fiber 11. On the other hand, the other port on the emission side is connected to the pump light source 14.
[0065]
That is, the light incident on the optical fiber 11 propagates through the erbium-doped optical fiber housing 12, and the pump light is incident on the optical fiber 11 from the pump light source 14 on the way, and this pump light passes through the optical fiber 11 in the WDM coupler 13. The light propagated by the traveling light is multiplexed and the light amplified thereby is emitted from the optical fiber 11.
The optical isolator 15 is provided to suppress reflection in the optical fiber 11 and stabilize it.
[0066]
However, the conventional erbium-doped optical fiber amplifier has a problem that the gain has wavelength dependency. In wavelength division multiplexing transmission, if the wavelength dependency is large, the gain varies depending on the wavelength, which is inconvenient.
Therefore, the wavelength-gain characteristic can be flattened by inserting the optical filter of the present invention into this erbium-doped optical fiber amplifier and losing light in a wavelength range with a large gain.
At this time, the insertion position of the optical filter is not particularly limited as long as it is between the incident end and the emission end of the optical fiber 11, and any of the insertion positions indicated by reference numerals 11a, 11b, 11c, and 11d in FIG. Good.
Here, the mode field of the fundamental mode (LP01 mode) of the optical fiber of the optical filter of the present invention, and the mode field of the fundamental mode (LP01 mode) of the single mode optical fiber used for the optical fiber (optical transmission line) 11 Are preferably set so as to substantially match.
If the mode fields do not substantially match, the cladding mode is excited in addition to the fundamental mode from the fundamental mode of the optical fiber 11 connected to the incident side of the optical filter to the first section of the optical fiber of the optical filter. This is because this cladding mode is added to the above-described mode coupling, and the characteristic of the periodic change of the loss with respect to the wavelength of the optical filter is disturbed.
In order to make the mode fields substantially coincide with each other, the mode field diameter of the single mode optical fiber of the optical fiber 11 and the mode field diameter of the single mode optical fiber structure composed of the core and the first cladding of the optical fiber of the optical filter are used. May be substantially equal.
More preferably, the core diameter, refractive index distribution, and the like of the single mode optical fiber structure portion formed of the optical fiber core and the first clad of the optical filter and the single mode optical fiber used for the optical transmission line 11 are as follows. It is also possible to use the same optical fiber as that of the optical transmission line 11 for the part of the single mode optical fiber structure of the optical filter.
In addition, the optical filter of this invention is not limited to the above-mentioned combination, but is widely applied to optical communication systems, such as an optical transmission apparatus and optical line equipment. In this case, as described above, the mode field of the fundamental mode of the optical fiber of the optical filter of the present invention and the mode field of the single mode optical fiber used in the optical transmission line of these devices are set to be substantially equal. It is desirable.
[0067]
【The invention's effect】
As described above, the optical filter of the present invention can be configured only by forming the perturbation imparting portions at two or more locations of the optical fiber having a waveguide structure in the clad. It can be produced without performing any processing.
Further, the wavelength interval of the loss peak can be adjusted by the length of the second section. Moreover, the magnitude of the loss peak can be adjusted by adjusting the coupling efficiency in the connection portion.
Therefore, various optical filters having different characteristics can be provided by changing the length of the second section or the coupling efficiency in the perturbation imparting unit.
Further, for example, even after the optical filter is manufactured, the length of the second section and the coupling efficiency in the perturbation imparting unit can be adjusted, so that the characteristics can be easily controlled.
[Brief description of the drawings]
FIG. 1 is a partial side sectional view showing an example of a configuration of an optical filter of the present invention.
FIGS. 2A and 2B show examples of the refractive index distribution of the optical fiber of the present invention, and FIG. 2A shows a depressed cladding type refractive index distribution. FIG. 2B is a graph showing a refractive index distribution of a staircase clad type.
FIG. 3 is a graph showing electric field distributions of an LP01 mode (fundamental mode), an LP02 mode, and an LP03 mode in an optical fiber having a step-type cladding type refractive index distribution.
FIG. 4 is an explanatory diagram showing an example of the operation of the optical filter of the present invention.
FIG. 5 (a) is a schematic configuration diagram showing an example of an optical filter manufacturing apparatus and a manufacturing method of the present invention in which a perturbation imparting portion is formed by heating and stretching.
FIG.5 (b) is a top view which shows an example of the optical filter of this invention manufactured with the manufacturing apparatus shown to Fig.5 (a).
FIG. 6 is a graph showing the relationship between wavelength and transmission loss characteristics obtained in the first example of the optical filter of the present invention.
7 (a) and 7 (b) show an example of a state in which the optical filter shown in FIG. 5 (b) is housed in a protective case, and FIG. 7 (a) is a plan view. FIG. 7B is a cross-sectional view taken along line AA in FIG.
8A, FIG. 8B, and FIG. 8C show an example of a structure in the case of protecting the optical filter of the present invention using a heat-shrinkable tube.
FIGS. 8A and 8B are a perspective view and a cross-sectional view showing a state before heating the heat-shrinkable tube, respectively, and FIG. 8C is a state where the heat-shrinkable tube is heated and contracted. FIG.
FIG. 9 is a schematic configuration diagram showing an example of a conventional erbium-doped optical fiber amplifier.
FIG. 10 is a schematic configuration diagram illustrating an example of a conventional optical filter.
FIG. 11 is a perspective view showing another example of a conventional optical filter.
[Explanation of symbols]
1a ... 1st section, 1b ... 2nd section, 1c ... 3rd section,
2 ... optical fiber, 2a ... bare optical fiber, 2b ... coating layer, 2c, 2d ... perturbation imparting part, 2e, 2f ... reduced diameter part, 3a ... core, 3b ... first clad (first layer), 3c ... intermediate part (second layer), 3d ... second clad, 3e ... clad,
4a ... core, 4b ... first clad (first layer), 4c ... second clad (second layer), 4d ... clad.

Claims (4)

Besides LP01 mode of the fundamental mode, at least one mode of an optical fiber having a waveguide structure for guiding in the longitudinal direction, perturbations locally perturbed in one place changes the mode field by the Rukoto granted imparting portion is formed at two or more places becomes, the perturbation is the change or changes in the refractive index of the refractive index distribution, in each of the perturbation applying unit, by a change in the mode field, LP01 mode and the other of at least the fundamental mode Inter-mode coupling occurs with one mode, and a specific wavelength is selectively selected by utilizing the fact that the phase difference of the light of each mode coupling periodically changes with changes in the wavelength of the propagating light. An optical filter characterized by being attenuated .   The optical fiber has a core and a clad, and the clad is provided on the circumference of the core, and is provided on the circumference of the first layer having a lower refractive index than the core and the circumference of the first layer. The optical filter according to claim 1, further comprising a second layer having a lower refractive index than the first layer.   3. The optical filter according to claim 1, wherein the optical fiber guides only the LP02 mode in addition to the LP01 mode of the fundamental mode.   An optical communication system using the optical filter according to claim 1.

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