JP3836730B2

JP3836730B2 - Polarization-maintaining photonic crystal fiber and manufacturing method thereof

Info

Publication number: JP3836730B2
Application number: JP2002019315A
Authority: JP
Inventors: 哲也山本; 正俊田中; 盛行藤田; 悟基川西; 和宣鈴木; 寛和久保田
Original assignee: Mitsubishi Cable Industries Ltd; Nippon Telegraph and Telephone Corp
Current assignee: Mitsubishi Cable Industries Ltd; Nippon Telegraph and Telephone Corp
Priority date: 2002-01-29
Filing date: 2002-01-29
Publication date: 2006-10-25
Anticipated expiration: 2022-01-29
Also published as: JP2003222752A

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarized wave preserving photonic crystal fiber in which a mode of a Ti-diffused waveguide of an LN modulator can be matched to that of a quartz glass waveguide or the like of an AWG. <P>SOLUTION: The polarized wave preserving photonic crystal fiber 20 comprises a solid core 21a, and a clad 21b having a plurality of pores 21c arranged to form a predetermined lattice in a cross section of the fiber. The pore adjacent to the core 21a of the clad 21b has large pore size pores 21c' arranged as a pair via the core 21a, and small pore size pores 21c arranged as a pair via the core 21a so that a mode field of an optical propagation region perpendicularly cross with each other in the cross section of the fiber and has two polarized wave modes having different propagation constants. The large pore size pores 21c' are arranged to be deviated from a position for forming the predetermined lattice in a direction in which an aspect ratio of the pores 21c' is reduced. <P>COPYRIGHT: (C)2003,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、偏波保存フォトニッククリスタルファイバ（以下「偏波保存ＰＦＣ」という）に関する。
【０００２】
【従来の技術】
フォトニッククリスタルファイバは、ファイバ中心をなす中実のコアと、そのコアを被覆するように設けられファイバ横断面に三角格子を形成してファイバ半径方向にフォトニッククリスタル構造を構成するように配設された複数の細孔を有するクラッドとを備えた光ファイバであり、種々の分野での適用が提案されている。
【０００３】
その中の一つとして、図１８（ａ）に示すように、コアを楕円形状とすることにより、その長軸方向と短軸方向とで伝搬定数を異ならしめた偏波保存ＰＣＦがある。
【０００４】
また、Optic letters 2000.9 Vol.25 NO.18には、図１８（ｂ）の示すように、クラッドに三角格子を形成するように複数の細孔が配設されており、コアに隣接した６つの細孔のうちコアを挟んで対向した一対を含む直線上に配設された細孔のみを小孔径のものとすることにより、小孔径の細孔が並ぶ方向とそれに直交する方向とで偏波モードの伝搬定数を異ならしめた偏波保存ＰＣＦが開示されている。このような偏波保存ＰＣＦでは、１０^-3オーダーのモード複屈折率を実現することが可能であり、これは偏波保存ファイバの代表であるＰＡＮＤＡファイバと比較して一桁近く大きく、偏波保存性能が極めて優れる。
【０００５】
さらに、OFC2001.TuM2 2001.3には、図１８（ｃ）に示すように、クラッドに三角格子を形成するように複数の細孔が配設されており、コアに隣接した６つの細孔のうちコアを挟んで対向した一対を他の細孔よりも大孔径のものとすることにより両大孔径の細孔を結ぶ方向とそれに直交する方向とで偏波モードの伝搬定数を異ならしめ、また、その大孔径の細孔のみを三角格子を形成する位置よりも細孔ピッチの８割の長さだけファイバ半径方向内向きにずらして配設することにより、モード複屈折率を高めた偏波保存ＰＣＦが開示されている（図中のコアに示されているのは光強度の分布である）。この偏波保存ＰＣＦでは、モード複屈折率が１．５×１０^-3と計算されており、大孔径の細孔が対向する方向のモードフィールド径に対するそれに直交する方向のモードフィールド径の比であるアスペクト比が図よりおおよそ２．０以上であると推測できる。
【０００６】
【発明が解決しようとする課題】
今日、ＷＤＭ通信において、偏波保存ファイバは、ＬＤ光源（レーザダイオード光源）とＬＮ変調器（リチウムナイオベイト変調器）とを繋ぐピグテールとして使用されており、また、将来、ＳＣ光源（スーパーコンチニウム光源）とフィルタとを備えたＡＷＧ（アレイ導波路）で分波するような場合、ＳＣ光源とＡＷＧとを繋ぐピグテールとして、さらには、ＡＷＧとＬＮ変調器とを繋ぐピグテールとして使用されると考えられる。
【０００７】
ところで、ＬＮ変調器には、導波路としてアスペクト比が１．５であるＴｉ拡散導波路が用いられており、また、ＡＷＧには、導波路としてアスペクト比が１．０に近い石英ガラス系導波路が用いられている。
【０００８】
従って、上記後者の文献に開示された偏波保存ＰＣＦでは、高いモード複屈折率を有するものの、アスペクト比が高いためにＬＮ変調器のＴｉ拡散導波路やＡＷＧの石英ガラス系導波路とモードマッチしないという問題がある。
【０００９】
本発明は、かかる点に鑑みてなされたものであり、その目的とするところは、ＬＮ変調器のＴｉ拡散導波路やＡＷＧの石英ガラス系導波路等とのモードマッチが可能な偏波保存ＰＣＦ及びその製造方法を提供することにある。
【００１０】
【課題を解決するための手段】
本発明は、２つの偏波モードの方向のうち一方のモードフィールド径に対する他方のモードフィールド径の比であるアスペクト比を１．０に近づくように、細孔を三角格子を形成する位置からオフセットして配設したものである。
【００１１】
具体的には、ファイバ中心をなす中実のコアと、該コアを被覆するように設けられファイバ横断面に三角格子を形成してファイバ半径方向にフォトニッククリスタル構造を構成するように配設された複数の細孔を有するクラッドと、を備え、該クラッドにおける該コアに隣接した細孔が、該コアを挟んで対をなして配設された大孔径の細孔と、該コアを挟んで対をなして配設された小孔径の細孔と、からなることにより、光伝搬領域であるモードフィールドがファイバ横断面において相互に直交し且つ伝搬定数が異なる２つの偏波モードを有するように構成された偏波保存ＰＣＦを前提とする。そして、上記コアに隣接した大孔径の細孔及び小孔径の細孔のうち少なくともいずれか一方は、上記モードフィールドにおける上記２つの偏波モードの方向のうち一方のモードフィールド径に対する他方のモードフィールド径の比であるアスペクト比が１．０に近づく向きに、上記三角格子を形成する位置からずれて配設されていることを特徴とする。
【００１２】
上記の構成によれば、高いモード複屈折率を得ることができるのに加え、大孔径の細孔及び小孔径の細孔のうち少なくともいずれか一方が三角格子を形成する位置からずれて配設され、アスペクト比が１．０に近づくようにされているので、ＬＮ変調器のＴｉ拡散導波路やＡＷＧの石英ガラス系導波路等のアスペクト比と近いものとなり、それらとのモードマッチが可能となる。
【００１３】
ここで、本出願において、「アスペクト比」とは、２つの偏波モードの方向のうち一方のモードフィールド径と他方のモードフィールド径とが等しい場合を除いて、小さい方を一方のモードフィールド径、大きい方を他方のモードフィールド径とする。従って、アスペクト比は常に１．０以上となる。
【００１４】
細孔のずれの大きさは、通常、格子ピッチをΛとすると０．１Λ〜０．９Λの範囲である。
【００１５】
偏波保存ＰＣＦの場合、通常、１０^-3以上のモード複屈折率を得ることができ、また、そのため１００ｍでのクロストークが−３０ｄＢ以下と非常に小さい。
【００１６】
本発明の上記アスペクト比は、一般的に存在する導波路とのモードマッチを考慮すれば、１．０以上で且つ２．０よりも小さいことが好ましく、また、アスペクト比は、アスペクト比が１．５であるＴｉ拡散導波路及びアスペクト比が１に近い石英ガラス系導波路を考慮すれば、１．０以上で且つ１．７以下であることがより好ましい。
【００１７】
本発明は、上記コアに隣接した大孔径の細孔又は小孔径の細孔のみが上記三角格子を形成する位置からずれて配設されている構成であってもよい。かかる構成によれば、クラッドのフォトニッククリスタル構造が大きく崩れることがない。
【００１８】
本発明の具体的な構成としては、上記コアに隣接した大孔径の細孔が上記三角格子を形成する位置からファイバ半径方向外向きにずれて配設されているものや、上記コアに隣接した小孔径の細孔が上記三角格子を形成する位置からファイバ半径方向内向きにずれて配設されているものを挙げることができる。
【００１９】
本発明は、ファイバ横断面に、上記コアを挟んで対向するように配設され上記コアに隣接した大孔径の細孔を含む大孔径の細孔群を有する一対の大孔径細孔領域と、上記コアを挟んで対向するように配設され上記コアに隣接した小孔径の細孔を含む小孔径の細孔群を有する一対の小孔径細孔領域と、の４つの領域が形成されており、上記大孔径細孔領域の各細孔及び小孔径細孔領域の各細孔のうち少なくともいずれか一方が上記三角格子を形成する位置からずれて配設されている構成であってもよい。
【００２０】
その具体的な構成としては、上記大孔径細孔領域の各細孔が上記三角格子を形成する位置からファイバ半径方向外向きにずれて配設されているものや、上記小孔径領域の各細孔が上記三角格子を形成する位置からファイバ半径方向内向きにずれて配設されているものを挙げることができる。
【００２１】
以上のような偏波保存ＰＣＦは、
石英製のロッド材にファイバ横断面の孔パタンに対応するようにロッド軸方向に延びる複数の孔を形成して母材を形成する母材形成工程と、
上記母材を線引き加工により細径化する線引き工程と、
を経て製造することができる。
【００２２】
この製造方法において、上記母材の複数の孔を封止した状態で線引き加工するようにしてもよい。このようにすれば、線引き加工時の線引き張力によって母材の孔を押し潰す方向に作用する力が孔内の圧力と均衡し、孔が押し潰されることなく線引き加工を行うことができる。ここで、封止とは、母材の複数の孔への外気の進入を阻止する手段全てを意味する。
【００２３】
その場合、複数の孔の封止前に、その複数の孔の内壁の不純物を除去する不純物除去処理を施すことが好ましい。このようにすれば、母材の孔内の不純物を除去することができるので、製造される偏波保存ＰＣＦは低損失なものとなる。不純物除去処理としては、フッ化水素酸によるエッチング処理や塩素ガスに晒すことにより水酸基を除去する塩素ガス処理等を挙げることができる。
【００２４】
また、上記偏波保存ＰＣＦであって、クラッドを被覆するように設けられた被覆部を備えたものは、
石英製のロッド材に上記ファイバ横断面の孔パタンに対応するようにロッド軸方向に延びる複数の孔を形成する穿孔工程と、
上記複数の孔を形成したロッド材をロッド軸方向に加熱延伸することにより細径化して母材本体を形成する母材本体形成工程と、
上記母材本体と上記被覆部になる石英製の被覆部形成材とで母材を構成し、該母材を線引き加工により細径化する線引き工程と、
を経て製造することもできる。
【００２５】
この場合、上記母材本体を石英製の筒状の被覆部形成材内に配置してそれらで母材を構成し、該母材を線引き加工により該母材本体と該被覆部形成材とを一体化させつつ細径化するようにしても、上記母材本体の外周に石英系材料を堆積一体化させることにより上記被覆部になる被覆部形成材を形成してそれらで母材を構成し、該母材を線引き加工により細径化するようにしてもよい。
【００２６】
この製造方法において、上記母材本体の複数の孔を封止した状態で線引き加工するようにしてもよい。このようにすれば、線引き加工時の線引き張力によって母材本体の孔を押し潰す方向に作用する力が孔内の圧力と均衡し、孔が押し潰されることなく線引き加工を行うことができる。ここで、封止とは、母材本体の複数の孔への外気の進入を阻止する手段全てを意味する。
【００２７】
その場合、複数の孔の封止前に、その複数の孔の内壁の不純物を除去する不純物除去処理を施すことが好ましい。このようにすれば、母材本体の孔内の不純物を除去することができるので、製造される偏波保存ＰＣＦは低損失なものとなる。不純物除去処理としては、フッ化水素酸によるエッチング処理や塩素ガスに晒すことにより水酸基を除去する塩素ガス処理等を挙げることができる。
【００２８】
また、いずれの製造方法においても、上記石英製のロッド材を水酸基含有量が１ｐｐｍ以下の無水合成ガラスで形成されたものとすることが好ましい。このようにすれば、製造される偏波保存ＰＣＦに含まれる水酸基の数が少なく低損失なものとなる。
【００２９】
【発明の効果】
以上説明したように、本発明によれば、ＬＮ変調器のＴｉ拡散導波路やＡＷＧの石英ガラス系導波路等のアスペクト比と近いものとなり、それらとのモードマッチが可能となる。
【００３０】
【発明の実施の形態】
以下、本発明の実施形態を図面に基づいて詳細に説明する。
【００３１】
（実施形態１）
＜偏波保存ＰＣＦの構成＞
図１は、本発明の実施形態１に係る偏波保存ＰＣＦ２０を示す。図２は、その横断面における中心部分を示す。
【００３２】
この偏波保存ＰＣＦ２０は、石英（ＳｉＯ₂）で形成されたものであって、ファイバ中心をなす中実のコア２１ａ及びそのコア２１ａを被覆するように設けられたクラッド２１ｂからなるファイバ本体２１と、ファイバ本体２１のクラッド２１ｂをさらに被覆するように設けられた被覆部２２とで構成されている。
【００３３】
クラッド２１ｂには、複数の細孔が配設されており、それらがファイバ横断面において三角格子を形成してファイバ半径方向にフォトニッククリスタル構造を構成している。
【００３４】
ファイバ横断面において、コア２１ａに隣接した６つの細孔のうち、コア２１ａを挟んで対向するように配設された一対の細孔２１ｃ’は他の細孔２１ｃよりも大孔径のものである一方、コア２１ａを挟んで対向するように配設された他の二対の細孔２１ｃはコア２１ａに隣接したもの以外の細孔２１ｃと同径の小孔径のものである。これによって、偏波保存ＰＣＦ２０が光伝搬領域であるモードフィールドがファイバ横断面において相互に直交し且つ伝搬定数が異なる２つの偏波モードを有するものとなっている。
【００３５】
そして、大孔径の細孔２１ｃ’は、図２に仮想線で示す三角格子を形成する位置からファイバ半径方向外向き、すなわち、コア２１ａから遠ざかる向きにずれて配設されている。この配設位置のずれの向きは、モードフィールドにおける２つの偏波モードの方向のうち一方のモードフィールド径に対する他方のモードフィールド径の比であるアスペクト比が１．０に近づく向きである。このように、三角格子を形成する位置からずれて配設されているのは大孔径の細孔２１ｃ’のみである。
【００３６】
この偏波保存ＰＣＦ２０は、アスペクト比が１．０以上で且つ２．０より小さい（好ましくは１．０以上で且つ１．７以下）。
【００３７】
以上の構成の偏波保存ＰＣＦ２０によれば、高いモード複屈折率を得ることができるのに加え、大孔径の細孔２１ｃ’が三角格子を形成する位置からファイバ半径方向外向きにずれて配設されているので、それによってアスペクト比が下げられるので、ファイバＬＮ変調器のＴｉ拡散導波路やＡＷＧの石英ガラス系導波路等のアスペクト比と近いものとなり、それらとのモードマッチが可能となる。
【００３８】
また、大孔径の細孔２１ｃ’のみが三角格子を形成する位置からずれて配設されているので、クラッド２１ｂのフォトニッククリスタル構造が大きく崩れることがない。
【００３９】
＜偏波保存ＰＣＦの製造方法＞
次に、本発明の実施形態１に係る偏波保存ＰＣＦ２０の製造方法を工程の順を追って説明する。
【００４０】
−準備工程−
図３に示すように、石英製の円柱のロッド材１を準備する。このロッド材１として、水酸基含有量が１ｐｐｍ以下の無水合成ガラスで形成されたものを用いる。
【００４１】
−穿孔工程−
図３に示すように、準備したロッド材１に対し、中心軸部分をコア形成部１ａとして中実のまま残し、そのコア形成部１ａを囲うようにロッド軸方向に延びる貫通した孔１ｃを複数配設することによりクラッド形成部１ｂを形成する。これらの孔１ｃは、上記偏波保存ＰＣＦ２０のクラッド２１ｂに構成された細孔パターンが形成されるように配設する。孔の形成は、例えば、ドリルによる穿孔加工、棒状の研磨具による粗仕上げ、中仕上げ、最終仕上げ加工及びブラシと酸化セリウム研磨材とによる最終研磨加工により行う。
【００４２】
−母材本体形成工程−
コア形成部１ａ及びクラッド形成部１ｂを形成したロッド材１を電気炉延伸器にセットし、図４に示すように、ロッド材１をロッド延伸用ヒータ２で加熱すると共に延伸して細径化した母材本体３を形成する。このとき、ロッド材１が細径化された母材本体３では、ロッド材１のものより孔径及び孔間隔が縮小された複数の孔３ｃが保持され、これに伴いコア形成部３ａ及びクラッド形成部３ｂがロッド材１のものより縮小されたものとなる。
【００４３】
−不純物除去処理工程−
母材本体３をフッ化水素酸に浸漬して表面をエッチングし、母材本体３表面に付着した金属等の不純物を除去する。
【００４４】
次いで、図５に示すように、母材本体３の両端にそれぞれ補助パイプ４を溶着した後、１０００〜１２００℃の温度雰囲気下で、一方の補助パイプ４に塩素ガスを送り、それを母材本体３の孔３ｃに流通させ、他方の補助パイプ４から排出する塩素ガス処理を行うことにより、孔３ｃの内壁に形成された水酸基を除去する。
【００４５】
−封止工程−
塩素ガス処理を行った後、図６に示すように、直ちに両方の補助パイプ４の端を加熱して閉じ、母材本体３の孔３ｃの両端を封止した状態にする。この際、孔内に塩素ガスが封入されるようにしてもよい。
【００４６】
−線引き工程−
図７に示すように、母材本体３を石英製の筒状の被覆部形成材１２内に配置してそれらで母材１３を構成し、その母材１３を線引き用ヒータ１１で加熱すると共に延伸する線引き加工により母材本体３と被覆部形成材１２とを一体化させつつ細径化して偏波保存ＰＣＦ２０を製造する。この際、母材本体３と被覆部形成材１２との間の空隙内を負圧に減圧する。
【００４７】
母材本体を被覆部形成材と加熱一体化させて母材を構成した後に線引き加工する場合、その加熱一体化の過程で孔の孔径の変動を生じる虞があるが、以上のような偏波保存ＰＣＦ２０の製造方法によれば、母材本体３と被覆部形成材１２とを線引き時に一体化するようにしているので、かかる孔径の変動を防止することができる。
【００４８】
また、線引き加工時に、母材本体３と被覆部形成材１２との間の空隙内を負圧に減圧し、ファイバ本体２１と被覆部２２との間への空気の閉じ込めが防がれるので、製造される偏波保存ＰＣＦ２０の伝送損失がかかる空気の閉じ込めにより高くなることを防止することができる。
【００４９】
以上のような偏波保存ＰＣＦ２０の製造方法によれば、複数の孔１ｃを形成したロッド材１を加熱延伸して母材本体３を形成するので、ロッド材１の段階で有する孔径及び孔間隔の誤差が母材本体３の段階で縮小され、高寸法精度の偏波保存ＰＣＦ２０を製造することができる。
【００５０】
また、母材本体３の複数の孔３ｃを封止した状態で線引き加工するようにしているので、線引き加工時の線引き張力によって母材本体３の孔３ｃを押し潰す方向に作用する力が孔内の圧力と均衡し、孔３ｃが押し潰されることなく線引き加工を行うことができる。
【００５１】
さらに、石英製の円柱のロッド材１として水酸基含有量が１ｐｐｍ以下の無水合成ガラスで形成されたものを用いており、しかも、母材本体３の複数の孔３ｃの封止前に、母材本体３表面の金属等を除去するフッ化水素酸によるエッチング処理及び孔３ｃの内壁の水酸基を除去するための塩素ガス処理を施しているので、低損失の偏波保存ＰＣＦ２０を製造することができる。
【００５２】
＜実験１＞
以下の実施例１及び比較例１の偏波保存ＰＣＦを作製し、それぞれのアスペクト比を求めた。
【００５３】
−実施例１−
ＶＡＤにより作成した直径４５ｍｍの無水合成石英のロッド材に、図８（ａ）に示す孔パターンの孔をあけた後、それを直径６ｍｍとなるまで延伸して母材本体を形成した。このとき、小孔径の細孔２１ｃとなるものの孔径Ｄ₁を１．４ｍｍ、大孔径の細孔２１ｃ’となるものの孔径Ｄ₂を３．９ｍｍ、小孔径の細孔２１ｃとなるものの孔間のピッチＰ₁を３．９ｍｍ、及びロッド中心から大孔径の細孔２１ｃ’となるものまでのピッチＰ₂を４．７ｍｍとした。母材本体の孔内部を塩素ガス処理して両端を封止した後、それを外径２５ｍｍで内孔径８ｍｍの石英ガラス管の中心に配置し、これを母材としてファイバ径１２５μｍに線引き加工して作製された偏波保存ＰＣＦ２０を実施例１とした。
【００５４】
作製された実施例１の偏波保存ＰＣＦ２０の構成は、小孔径の細孔２１ｃの孔径ｄ₁が１．０μｍ、大孔径の細孔２１ｃ’の孔径ｄ₂が２．７μｍ、小孔径の細孔２１ｃ間の孔ピッチΛ₁が２．７μｍ及びファイバ中心から大孔径の細孔２１ｃ’までのピッチΛ₂が３．２μｍ（すなわち、大孔径の細孔２１ｃ’が三角格子を形成する位置からファイバ半径方向外向きに０．１９Λ₁ずらして配設されている。）であった。この偏波保存ＰＣＦ２０について、波長１．５５μｍで光強度分布を測定してシングルモード伝送を確認し、光強度が中心の最大値の１／ｅ²となる径を測定した（光強度分布がガウシアン分布である場合、光強度が中心の最大値の１／ｅ²となる径がモードフィールド径となる）。この結果、長径が４．４μｍ、短径が３．５μｍであり、従って、アスペクト比が１．２６であった。また、モード複屈折率は１．６×１０^-3であり、高い偏波保存性を示した。以上の特性を表１に示す。
【００５５】
【表１】

【００５６】
−比較例１−
小孔径の細孔２１０ｃとなるものの孔間のピッチＰ₁及びロッド中心から大孔径の細孔２１０ｃ’となるものまでのピッチＰ₂のいずれをもを３．９ｍｍとし、実施例１の場合と同様にして作製した偏波保存ＰＣＦ２００を比較例１とした。
【００５７】
作製された比較例１の偏波保存ＰＣＦ２００の構成は、図８（ｂ）に示すように、小孔径２１０ｃの細孔間の孔ピッチΛ₁及びファイバ中心から大孔径の細孔２１０ｃ’までのピッチΛ₂のいずれもが２．７μｍであることを除いては実施例１と同一であった。波長１．５５μｍで光強度分布を測定し、シングルモード伝送を確認した。この偏波保存ＰＣＦ２００について、波長１．５５μｍで光強度分布を測定してシングルモード伝送を確認し、光強度が中心の最大値の１／ｅ²となる径を測定した。この結果、長径が４．４μｍ、短径が２．９μｍであり、従って、アスペクト比が１．５２であった。また、モード複屈折率は１．７×１０^-3であり、実施例１と同等の高い偏波保存性を示した。以上の特性を表２に示す。
【００５８】
【表２】

【００５９】
−実験結果−
上記のように、アスペクト比は、大孔径の細孔２１ｃ’を三角格子を形成する位置からずらして配設した実施例１の方が、大孔径の細孔２１０ｃ’を三角格子を形成する位置に配設した比較例１よりも小さい。
【００６０】
（実施形態２）
＜偏波保存ＰＣＦの構成＞
図９は、本発明の実施形態２に係る偏波保存ＰＣＦ２０を示す。なお、実施形態１と同一部分は同一符号で示す。
【００６１】
この偏波保存ＰＣＦ２０では、ファイバ横断面において、コア２１ａに隣接した６つの細孔のうち、コア２１ａを挟んで対向するように配設された一対の細孔２１ｃ’は他の細孔２１ｃよりも大孔径のものである一方、コア２１ａを挟んで対向するように配設された他の二対の細孔２１ｃはコア２１ａに隣接したもの以外の他の細孔２１ｃと同径の小孔径のものである。これによって、偏波保存ＰＣＦ２０が光伝搬領域であるモードフィールドがファイバ横断面において相互に直交し且つ伝搬定数が異なる２つの偏波モードを有するものとなっている。
【００６２】
そして、コア２１ａに隣接した大孔径の細孔２１ｃ’は、図９に仮想線で示す三角格子を形成する位置からファイバ半径方向外向き、すなわち、コア２１ａから遠ざかる向きにずれて配設されている。この配設位置のずれの向きは、モードフィールドにおける２つの偏波モードの方向のうち一方のモードフィールド径に対する他方のモードフィールド径の比であるアスペクト比が１．０に近づく向きである。また、２つの大孔径の細孔２１ｃ’を結ぶ線上にある、それらのそれぞれの外側の小孔径の細孔２１ｃもまた、図９に仮想線で示す三角格子を形成する位置からファイバ半径方向外向き、すなわち、コア２１ａから遠ざかる向きにずれて配設されている。
【００６３】
以上の構成の偏波保存ＰＣＦ２０では、大孔径の細孔２１ｃ’の孔径が大きい場合等、これを三角格子を形成する位置からずらして配設した際に、大孔径の細孔２１ｃ’がその外側にある小孔径の細孔２１ｃと干渉するのが防がれる。
【００６４】
その他の構成、作用及び効果、並びに製造方法は実施形態１と同一である。
【００６５】
＜実験２＞
以下の実施例２及び比較例２の偏波保存ＰＣＦを作製し、それぞれのアスペクト比を求めた。
【００６６】
−実施例２−
ＶＡＤにより作成した直径４５ｍｍの無水合成石英のロッド材に、図１０（ａ）に示す孔パターンの孔をあけた後、それを直径６ｍｍとなるまで延伸して母材本体を形成した。このとき、小孔径の細孔２１ｃとなるものの孔径Ｄ₁を２．７ｍｍ、大孔径の細孔２１ｃ’となるものの孔径Ｄ₂を６．７ｍｍ、小孔径の細孔２１ｃとなるものの孔間のピッチＰ₁を５．８ｍｍ、及びロッド中心から大孔径の細孔２１ｃ’となるものまでのピッチＰ₂を７．０ｍｍとした。母材本体の孔内部を塩素ガス処理して両端を封止した後、それを外径２５ｍｍで内孔径８ｍｍの石英ガラス管の中心に配置し、これを母材としてファイバ径１２５μｍに線引き加工して作製された偏波保存ＰＣＦ２０を実施例２とした。
【００６７】
作製された実施例２の偏波保存ＰＣＦ２０の構成は、小孔径の細孔２１ｃの孔径ｄ₁が１．９μｍ、大孔径の細孔２１ｃ’の孔径ｄ₂が４．７μｍ、小孔径の細孔２１ｃ間の孔ピッチΛ₁が４．０μｍ及びファイバ中心から大孔径の細孔２１ｃ’までのピッチΛ₂が４．８μｍ（すなわち、大孔径の細孔２１ｃ’及びその外側の小孔径の細孔２１ｃが、それぞれ三角格子を形成する位置からファイバ半径方向外向きに０．２０Λ₁ずらして配設されている。）であった。この偏波保存ＰＣＦ２０について、波長１．５５μｍで光強度分布を測定してシングルモード伝送を確認し、光強度が中心の最大値の１／ｅ²となる径を測定した。この結果、長径が６．１μｍ、短径が４．４μｍであり、従って、アスペクト比が１．３９であった。また、モード複屈折率は１．４×１０^-3であり、実施例１と比較すると若干低いが、モードフィールド径が大きいので他の光ファイバとの接続性に優れるといえる。以上の特性を表３に示す。
【００６８】
【表３】

【００６９】
−比較例２−
小孔径の細孔２１０ｃとなるものの孔間のピッチＰ₁及びロッド中心から大孔径の細孔２１０ｃ’となるものまでのピッチＰ₁のいずれをもを５．８ｍｍとし、実施例１の場合と同様にして作製した偏波保存ＰＣＦ２００を比較例２とした。
【００７０】
作製された比較例２の偏波保存ＰＣＦ２００の構成は、図１０（ｂ）に示すように、小孔径の細孔２１０ｃ間の孔ピッチΛ₁及びファイバ中心から大孔径の細孔２１０ｃ’までのピッチΛ₂のいずれもが４．０μｍであることを除いては実施例１と同一であった。この偏波保存ＰＣＦ２００について、波長１．５５μｍで光強度分布を測定してシングルモード伝送を確認し、光強度が中心の最大値の１／ｅ²となる径を測定した。この結果、長径が６．１μｍ、短径が３．５μｍであり、従って、アスペクト比が１．７１であった。また、モード複屈折率は１．５×１０^-3であり、実施例２と同等の性能を示した。以上の特性を表４に示す。
【００７１】
【表４】

【００７２】
−実験結果−
上記のように、アスペクト比は、大孔径の細孔２１ｃ’及びその外側の小孔径の細孔２１ｃを三角格子を形成する位置からずらして配設した実施例２の方が、大孔径の細孔２１０ｃ’を三角格子を形成する位置に配設した比較例２よりも小さい。
【００７３】
（実施形態３）
＜偏波保存ＰＣＦの構成＞
図１１は、本発明の実施形態３に係る偏波保存ＰＣＦ２０を示す。なお、実施形態１と同一部分は同一符号で示す。
【００７４】
この偏波保存ＰＣＦ２０では、ファイバ横断面において、コア２１ａに隣接した６つの細孔のうち、コア２１ａを挟んで対向するように配設された一対の細孔２１ｃを結ぶ直線上にある細孔は他の細孔よりも小孔径のものである一方、コア２１ａを挟んで対向するように配設された他の二対の細孔２１ｃ’を含むその他の細孔は大孔径のものである。また、ファイバ横断面において、コア２１ａを挟んで対向するように配設されコア２１ａに隣接した大孔径の細孔２１ｃ’を含む大孔径の細孔群を有する一対の領域が大孔径細孔領域３１を構成している一方、コア２１ａを挟んで対向するように配設されコア２１ａに隣接した小孔径の細孔２１ｃを含む小孔径の細孔群を有する一対の領域が小孔径細孔領域３２を構成している。以上の構成によって、偏波保存ＰＣＦ２０が光伝搬領域であるモードフィールドがファイバ横断面において相互に直交し且つ伝搬定数が異なる２つの偏波モードを有するものとなっている。
【００７５】
そして、コア２１ａに隣接した小孔径の細孔２１ｃは、図１１に仮想線で示す三角格子を形成する位置からファイバ半径方向内向き、すなわち、コア２１ａに近づく向きにずれて配設されている。この配設位置のずれの向きは、モードフィールドにおける２つの偏波モードの方向のうち一方のモードフィールド径に対する他方のモードフィールド径の比であるアスペクト比が１．０に近づく向きである。また、コア２１ａに隣接した小孔径の細孔２１ｃを結ぶ線上にある、それらの外側の小孔径の細孔２１ｃもまた、図９に仮想線で示す三角格子を形成する位置からファイバ半径方向内向き、すなわち、コア２１ａに近づく向きにずれて配設されている。つまり、大孔径細孔領域３１の各細孔２１ｃ’が三角格子を形成する位置に配設されている一方、小孔径細孔領域３２の各細孔２１ｃが三角格子を形成する位置からファイバ半径方向外向きにずれて配設されている。
【００７６】
以上の構成の偏波保存ＰＣＦ２０は、クラッド２１ｂに構成されたフォトニッククリスタル構造の孔間隔に対する細孔の孔径の比が全体的に大きいためにゼロ分散波長が短波長側にシフトしたものである。
【００７７】
その他の構成、作用及び効果、並びに製造方法は実施形態１と同一である。
【００７８】
＜実験３＞
以下の実施例３及び比較例３の偏波保存ＰＣＦを作製し、それぞれのアスペクト比を求めた。
【００７９】
−実施例３−
ＶＡＤにより作成した直径４５ｍｍの無水合成石英のロッド材に、図１２（ａ）に示す孔パターンの孔をあけた後、それを直径６ｍｍとなるまで延伸して母材本体を形成した。このとき、小孔径の細孔２１ｃとなるものの孔径Ｄ₁を１．０ｍｍ、大孔径の細孔２１ｃ’となるものの孔径Ｄ₂を２．３ｍｍ、小孔径の細孔２１ｃとなるものの孔間のピッチＰ₁を３．３ｍｍ、及びロッド中心から大孔径の細孔２１ｃ’となるものまでのピッチＰ₂を２．８ｍｍとした。母材本体の孔内部を塩素ガス処理して両端を封止した後、それを外径２５ｍｍで内孔径８ｍｍの石英ガラス管の中心に配置し、これを母材としてファイバ径１２５μｍに線引き加工して作製された偏波保存ＰＣＦ２０を実施例３とした。
【００８０】
作製された実施例３の偏波保存ＰＣＦ２０の構成は、小孔径の細孔２１ｃの孔径ｄ₁が０．７μｍ、大孔径の細孔２１ｃ’の孔径ｄ₂が１．６μｍ、小孔径の細孔２１ｃ間の孔ピッチΛ₁が２．３μｍ及びファイバ中心から大孔径の細孔２１ｃ’までのピッチΛ₂が２．０μｍ（すなわち、小孔径の細孔２１ｃが三角格子を形成する位置からファイバ半径方向内向きに０．１３Λ₁ずれて配設されている。）であった。そして、アスペクト比は１．１０であった。また、モード複屈折率は１．６×１０^-3であり、高い偏波保存性を示した。以上の特性を表５に示す。
【００８１】
【表５】

【００８２】
−比較例３−
小孔径の細孔２１０ｃとなるものの孔間のピッチＰ₁及びロッド中心から大孔径の細孔２１０ｃ’となるものまでのピッチＰ₂のいずれをもを３．３ｍｍとし、実施例１の場合と同様にして作製した偏波保存ＰＣＦ２００を比較例３とした。
【００８３】
作製された比較例３の偏波保存ＰＣＦ２００の構成は、図１２（ｂ）に示すように、小孔径の細孔２１０ｃ間の孔ピッチΛ₁及びファイバ中心から大孔径の細孔２１０ｃ’までのピッチΛ₂のいずれもが２．３μｍであることを除いては実施例１と同一であった。そして、アスペクト比は１．３０であった。また、モード複屈折率は１．６×１０^-3であり、実施例３と同等の性能を示した。以上の特性を表６に示す。
【００８４】
【表６】

【００８５】
−実験結果−
上記のように、アスペクト比は、小孔径の細孔２１ｃを三角格子を形成する位置からずらして配設した実施例３の方が、小孔径の細孔２１０ｃを三角格子を形成する位置に配設した比較例３よりも小さい。
【００８６】
（実施形態４）
＜偏波保存ＰＣＦの構成＞
図１３は、本発明の実施形態４に係る偏波保存ＰＣＦ２０を示す。なお、実施形態１と同一部分は同一符号で示す。
【００８７】
この偏波保存ＰＣＦ２０では、ファイバ横断面において、コア２１ａに隣接した６つの細孔のうち、コア２１ａを挟んで対向するように配設された一対の細孔２１ｃ’を結ぶ直線上にある細孔は他の細孔よりも大孔径のものである一方、コア２１ａを挟んで対向するように配設された他の二対の細孔２１ｃを含むその他の細孔は小孔径のものである。また、ファイバ横断面において、コア２１ａを挟んで対向するように配設されコア２１ａに隣接した大孔径の細孔２１ｃ’を含む大孔径の細孔群を有する一対の領域が大孔径細孔領域３１を構成している一方、コア２１ａを挟んで対向するように配設されコア２１ａに隣接した小孔径の細孔２１ｃを含む小孔径の細孔群を有する一対の領域が小孔径細孔領域３２を構成している。以上の構成によって、偏波保存ＰＣＦ２０が光伝搬領域であるモードフィールドがファイバ横断面において相互に直交し且つ伝搬定数が異なる２つの偏波モードを有するものとなっている。
【００８８】
そして、コア２１ａに隣接した大孔径の細孔２１ｃ’は、図１３に仮想線で示す三角格子を形成する位置からファイバ半径方向内向き、すなわち、コア２１ａから遠ざかる向きにずれて配設されている。この配設位置のずれの向きは、モードフィールドにおける２つの偏波モードの方向のうち一方のモードフィールド径に対する他方のモードフィールド径の比であるアスペクト比が１．０に近づく向きである。また、コア２１ａに隣接した大孔径の細孔２１ｃ’を結ぶ線上にある、それらの外側の大孔径の細孔２１ｃ’もまた、図１３に仮想線で示すように三角格子を形成する位置からファイバ半径方向外向き、すなわち、コア２１ａから遠ざかる向きにずれて配設されている。つまり、小孔径細孔領域３２の各細孔２１ｃが三角格子を形成する位置に配設されている一方、大孔径細孔領域３１の各細孔２１ｃ’が三角格子を形成する位置からファイバ半径方向外向きにずれて配設されている。
【００８９】
実施形態１及び２のものでは、コア２１ａに隣接した大孔径の細孔２１ｃ’の外側に小孔径の細孔２１ｃがあるので、長波長の光がその方向に放射してしまい長波長の伝送が遮断される虞があるが、以上の構成の偏波保存ＰＣＦ２０では、コア２１ａに隣接した大孔径の細孔２１ｃ’の外側にさらに大孔径の細孔２１ｃ’があるので、かかる不都合が解消される。
【００９０】
その他の構成、作用及び効果、並びに製造方法は実施形態１と同一である。
【００９１】
（その他の実施形態）
上記実施形態１〜４では、母材本体３を石英製の筒状の被覆部形成材１２内に配置してそれらで母材１３を構成し、その母材１３を線引き用ヒータ１１で加熱すると共に延伸する線引き加工により母材本体３と被覆部形成材１２とを一体化させつつ細径化して偏波保存ＰＣＦ２０を製造したが、特にこれに限定されるものではなく、図１４に示すように、孔３ｃが封止された母材本体３の外周に、酸水素バーナ５の火炎にＳｉＣｌ₄を供給して生成した石英微粒子を堆積一体化させ石英多孔質層６を形成し、次いで、図１５に示すように、石英多孔質層６が外周に一体に堆積した母材本体３をヘリウムと塩素ガスとを流通させた加熱容器７に配置し、外部より焼成用ヒータ８で加熱することにより石英多孔質層６を焼成して透明な被覆部形成材９に変化させて母材１０とした後、その母材１０を線引き装置にセットし、図１６に示すように、線引き用ヒータ１１で線引き加工により細径化するようにしてもよい。
【００９２】
また、石英製のロッド材にファイバ横断面の孔パターンに対応するようにロッド軸方向に延びる複数の孔を形成して母材を形成する母材形成工程と、母材を線引き加工により細径化する線引き工程と、を経て偏波保存ＰＣＦを製造するようにしてもよい。この場合、母材の複数の孔を封止した状態で線引き加工するようにすれば、線引き加工時の線引き張力によって母材の孔を押し潰す方向に作用する力が孔内の圧力と均衡し、孔が押し潰されることなく線引き加工を行うことができる。また、複数の孔の封止前に、その複数の孔の内壁の不純物を除去する不純物除去処理を施すようにすれば、母材の孔内の不純物を除去することができるので、製造される偏波保存ＰＣＦは低損失なものとなる。さらに、石英製のロッド材を水酸基含有量が１ｐｐｍ以下の無水合成ガラスで形成されたものとすれば、製造される偏波保存ＰＣＦに含まれる水酸基の数が少なく低損失なものとなる。
【００９３】
また、実施形態３では、ファイバ横断面に、コア２１ａを挟んだ一対の帯状領域でよる小孔径細孔領域３２を構成する一方、その他の領域で大孔径細孔領域３１を構成し、実施形態４では、ファイバ横断面に、コア２１ａを挟んだ一対の帯状領域でよる大孔径細孔領域３１を構成する一方、その他の領域で小孔径細孔領域３２を構成したが、特にこれに限定されるものではなく、図１７に示すように、コア２１ａを中心とした例えば中心角６０°や１２０°の扇形領域で大孔径細孔領域３１及び小孔径細孔領域３２をそれぞれ構成するようにしてもよい。
【００９４】
また、上記実施形態１〜３では、純粋な石英製の偏波保存ＰＣＦ２０としたが、特にこれに限定されるものではなく、主成分が石英のものであれば、他の元素がドープされたものであってもよい。
【図面の簡単な説明】
【図１】実施形態１に係る偏波保存ＰＣＦの斜視図である。
【図２】実施形態１に係る偏波保存ＰＣＦの横断面中心部分の正面図である。
【図３】実施形態１に係る準備工程及び穿孔工程の説明図である。
【図４】実施形態１に係る母材本体形成工程の説明図である。
【図５】実施形態１に係る不純物除去処理工程の塩素ガス処理の説明図である。
【図６】実施形態１に係る封止工程の説明図である。
【図７】実施形態１に係る線引き工程の説明図である。
【図８】実験１の実施例１及び比較例１のそれぞれの偏波保存ＰＣＦの横断面中心部分の正面図である。
【図９】実施形態２に係る偏波保存ＰＣＦの横断面中心部分の正面図である。
【図１０】実験２の実施例２及び比較例２のそれぞれの偏波保存ＰＣＦの横断面中心部分の正面図である。
【図１１】実施形態３に係る偏波保存ＰＣＦの横断面中心部分の正面図である。
【図１２】実験３の実施例３及び比較例３のそれぞれの偏波保存ＰＣＦの横断面中心部分の正面図である。
【図１３】実施形態４に係る偏波保存ＰＣＦの横断面中心部分の正面図である。
【図１４】その他の実施形態の偏波保存ＰＣＦの製造方法に係る被覆部形成材形成工程の石英多孔質層の形成過程を示す説明図である。
【図１５】その他の実施形態の偏波保存ＰＣＦの製造方法に係る被覆部形成材形成工程の石英多孔質層の焼成による被覆部形成材の形成過程を示す説明図である。
【図１６】その他の実施形態の偏波保存ＰＣＦの製造方法に係る線引き工程を示す説明図である。
【図１７】その他の実施形態に係る偏波保存ＰＣＦの横断面中心部分の模式的な正面図である。
【図１８】従来技術に係る偏波保存ＰＣＦの横断面中心部分の正面図である。
【符号の説明】
１ロッド材
１ａ，３ａコア形成部
１ｂ，３ｂクラッド形成部
１ｃ，３ｃ孔
２ロッド延伸用ヒータ
３母材本体
４補助パイプ
５酸水素バーナ
６石英多孔質層
７加熱容器
８焼成用ヒータ
９，１２被覆部形成材
１０，１３母材
１１線引き用ヒータ
２０，２００偏波保存ＰＣＦ
２１ファイバ本体
２１ａ，２１０ａコア
２１ｂ，２１０ｂクラッド
２１ｃ，２１０ｃ小孔径の細孔
２１ｃ’，２１０ｃ’ 大孔径の細孔
２２被覆部
３１大孔径細孔領域
３２小孔径細孔領域[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polarization maintaining photonic crystal fiber (hereinafter referred to as “polarization maintaining PFC”).
[0002]
[Prior art]
A photonic crystal fiber has a solid core that forms the center of the fiber and a cross section of the fiber that covers the core. triangle An optical fiber including a clad having a plurality of pores arranged so as to form a photonic crystal structure in the fiber radial direction by forming a lattice, and has been proposed for application in various fields.
[0003]
As one of them, as shown in FIG. 18 (a), there is a polarization preserving PCF in which the propagation constant is different between the major axis direction and the minor axis direction by making the core elliptical.
[0004]
In addition, in Optic letters 2000.9 Vol.25 NO.18, as shown in FIG. 18 (b), a plurality of pores are arranged so as to form a triangular lattice in the cladding, and there are six adjacent to the core. By making only the pores arranged on a straight line including a pair of pores facing each other across the core among the pores having a small pore diameter, polarization is performed in the direction in which the small pore diameters are aligned and in the direction perpendicular thereto. A polarization-maintaining PCF with different mode propagation constants is disclosed. In such a polarization preserving PCF, 10 ^-3 It is possible to realize an order mode birefringence, which is almost an order of magnitude higher than that of a PANDA fiber, which is a typical polarization-maintaining fiber, and has extremely excellent polarization-maintaining performance.
[0005]
Furthermore, in OFC2001.TuM2 2001.3, as shown in FIG. 18 (c), a plurality of pores are disposed so as to form a triangular lattice in the clad, and the core among the six pores adjacent to the core is disposed. By making the pair facing each other with a larger pore diameter than the other pores, the propagation constant of the polarization mode is made different between the direction connecting the pores of both large pore diameters and the direction orthogonal thereto, Polarization-preserving PCF with an increased mode birefringence by disposing only large pores with a length of 80% of the pore pitch inward in the fiber radial direction from the position where the triangular lattice is formed. Is disclosed (the light intensity distribution is shown in the core in the figure). In this polarization-maintaining PCF, the mode birefringence is 1.5 × 10 ^-3 From the figure, it can be estimated that the aspect ratio, which is the ratio of the mode field diameter in the direction perpendicular to the mode field diameter in the direction in which the large pores face each other, is approximately 2.0 or more.
[0006]
[Problems to be solved by the invention]
Today, in WDM communication, a polarization-maintaining fiber is used as a pigtail connecting an LD light source (laser diode light source) and an LN modulator (lithium niobate modulator), and in the future, an SC light source (super continuum). In the case of demultiplexing by an AWG (arrayed waveguide) provided with a light source) and a filter, it is considered to be used as a pigtail connecting the SC light source and the AWG and further as a pigtail connecting the AWG and the LN modulator. It is done.
[0007]
By the way, a Ti diffusion waveguide having an aspect ratio of 1.5 is used as a waveguide in the LN modulator, and a quartz glass-based waveguide having an aspect ratio close to 1.0 as a waveguide is used in the AWG. A waveguide is used.
[0008]
Therefore, although the polarization maintaining PCF disclosed in the latter document has a high mode birefringence, the aspect ratio is high, so that it matches the Ti diffusion waveguide of the LN modulator and the silica glass waveguide of the AWG. There is a problem of not.
[0009]
The present invention has been made in view of this point, and the object of the present invention is to maintain a polarization-maintaining PCF capable of mode matching with a Ti diffusion waveguide of an LN modulator, a quartz glass-based waveguide of AWG, or the like. And a manufacturing method thereof.
[0010]
[Means for Solving the Problems]
In the present invention, an aspect ratio which is a ratio of one mode field diameter to the other mode field diameter in two polarization mode directions is set. Approaching 1.0 So that the pores triangle It is arranged offset from the position where the lattice is formed.
[0011]
Specifically, a solid core forming the center of the fiber and a cross section of the fiber provided to cover the core. triangle A clad having a plurality of pores arranged to form a photonic crystal structure in a fiber radial direction by forming a lattice, and the pores adjacent to the core in the clad sandwich the core The mode field, which is the light propagation region, is made of a fiber having a large pore diameter arranged in pairs and a small pore diameter arranged in pairs across the core. Assume a polarization-maintaining PCF configured to have two polarization modes that are orthogonal to each other in cross section and have different propagation constants. At least one of the large pore and the small pore adjacent to the core is the other mode field with respect to one mode field diameter of the two polarization modes in the mode field. The aspect ratio that is the ratio of the diameter is Approaching 1.0 Orientation above triangle It is characterized by being disposed offset from the position where the lattice is formed.
[0012]
According to the above configuration, in addition to being able to obtain a high mode birefringence, at least one of a large pore diameter pore and a small pore diameter pore is present. triangle Disposed from the position where the lattice is formed, the aspect ratio Is 1 . Since it is close to 0, it becomes close to the aspect ratio of the Ti diffusion waveguide of the LN modulator, the silica glass-based waveguide of the AWG, etc., and mode matching with them becomes possible.
[0013]
Here, in the present application, the “aspect ratio” means that one of the two polarization mode directions has the smaller mode field diameter, except for one mode field diameter being equal to the other mode field diameter. The larger one is the other mode field diameter. Therefore, the aspect ratio is always 1.0 or more.
[0014]
The magnitude of the pore displacement is usually in the range of 0.1 Λ to 0.9 Λ when the lattice pitch is Λ.
[0015]
For polarization-maintaining PCF, usually 10 ^-3 The above mode birefringence can be obtained, and for that reason, the crosstalk at 100 m is as very low as −30 dB or less.
[0016]
The aspect ratio of the present invention is preferably 1.0 or more and smaller than 2.0 in consideration of a mode match with a generally existing waveguide, and the aspect ratio is 1 Considering a Ti diffusion waveguide having a thickness of 0.5 and a silica glass-based waveguide having an aspect ratio close to 1, it is more preferably 1.0 or more and 1.7 or less.
[0017]
In the present invention, only a large pore or a small pore adjacent to the core is the above. triangle It may be configured to be shifted from the position where the lattice is formed. According to such a configuration, the photonic crystal structure of the cladding does not collapse greatly.
[0018]
As a specific configuration of the present invention, the large pores adjacent to the core are triangle Those that are displaced from the position where the lattice is formed outward in the radial direction of the fiber, and small pores adjacent to the core triangle There may be mentioned those arranged so as to be shifted inward in the radial direction of the fiber from the position where the grating is formed.
[0019]
The present invention is a pair of large pore diameter regions having a large pore group including a large pore diameter adjacent to the core, which is disposed so as to face the fiber cross section across the core; Four regions are formed: a pair of small pore regions having a small pore group including small pores adjacent to the core and arranged to face each other across the core. , At least one of the pores in the large pore region and the pores in the small pore region is the above triangle It may be configured to be shifted from the position where the lattice is formed.
[0020]
As its specific configuration, each pore in the large pore area is triangle Each of the pores in the small-diameter region is arranged so as to be displaced from the position where the lattice is formed outward in the radial direction of the fiber. triangle There may be mentioned those arranged so as to be shifted inward in the radial direction of the fiber from the position where the grating is formed.
[0021]
The polarization preserving PCF as described above is
A base material forming step of forming a base material by forming a plurality of holes extending in the rod axis direction so as to correspond to a hole pattern of a fiber cross section in a quartz rod material;
A drawing step of reducing the diameter of the base material by drawing,
Can be manufactured.
[0022]
In this manufacturing method, the drawing may be performed in a state where the plurality of holes of the base material are sealed. By doing so, the force acting in the direction of crushing the hole of the base material by the drawing tension at the time of drawing is balanced with the pressure in the hole, and the drawing can be performed without crushing the hole. Here, the sealing means all means for preventing the outside air from entering the plurality of holes of the base material.
[0023]
In that case, it is preferable to perform an impurity removal treatment for removing impurities on the inner walls of the plurality of holes before sealing the plurality of holes. In this way, impurities in the hole of the base material can be removed, so that the manufactured polarization maintaining PCF has a low loss. Examples of the impurity removal treatment include etching treatment with hydrofluoric acid, chlorine gas treatment for removing hydroxyl groups by exposure to chlorine gas, and the like.
[0024]
Further, the polarization-maintaining PCF having a covering portion provided so as to cover the clad is
A drilling step of forming a plurality of holes extending in the rod axis direction so as to correspond to the hole pattern of the fiber cross section in the quartz rod material;
A base material body forming step of forming a base material body by reducing the diameter by heating and stretching the rod material in which the plurality of holes are formed in the rod axis direction;
A base material is constituted by the base material main body and the quartz covering portion forming material to be the covering portion, and a drawing step for reducing the diameter of the base material by drawing processing;
Can also be manufactured.
[0025]
In this case, the base material body is arranged in a quartz cylindrical covering portion forming material to form the base material, and the base material body and the covering portion forming material are drawn by drawing the base material. Even if the diameter is reduced while integrating, a base material is formed by forming a covering portion forming material that becomes the covering portion by depositing and integrating a quartz-based material on the outer periphery of the base material body. The base material may be reduced in diameter by drawing.
[0026]
In this manufacturing method, the drawing may be performed in a state where the plurality of holes of the base material body are sealed. By doing so, the force acting in the direction of crushing the hole of the base material body by the drawing tension at the time of the drawing process is balanced with the pressure in the hole, and the drawing process can be performed without crushing the hole. Here, the sealing means all means for preventing the outside air from entering the plurality of holes of the base material body.
[0027]
In that case, it is preferable to perform an impurity removal treatment for removing impurities on the inner walls of the plurality of holes before sealing the plurality of holes. In this way, since impurities in the hole of the base material body can be removed, the manufactured polarization-maintaining PCF has low loss. Examples of the impurity removal treatment include etching treatment with hydrofluoric acid, chlorine gas treatment for removing hydroxyl groups by exposure to chlorine gas, and the like.
[0028]
In any of the manufacturing methods, the quartz rod material is preferably formed of anhydrous synthetic glass having a hydroxyl group content of 1 ppm or less. By doing so, the number of hydroxyl groups contained in the produced polarization-maintaining PCF is small and the loss is low.
[0029]
【The invention's effect】
As described above, according to the present invention, the aspect ratio is close to that of the Ti diffusion waveguide of the LN modulator, the silica glass-based waveguide of the AWG, and the like, and mode matching with them is possible.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0031]
(Embodiment 1)
<Configuration of polarization-maintaining PCF>
FIG. 1 shows a polarization preserving PCF 20 according to Embodiment 1 of the present invention. FIG. 2 shows the central part in the cross section.
[0032]
This polarization maintaining PCF 20 is made of quartz (SiO 2 ₂ ), A fiber main body 21 comprising a solid core 21a forming the center of the fiber and a clad 21b provided so as to cover the core 21a, and a clad 21b of the fiber main body 21 are further covered It is comprised with the coating | coated part 22 provided in this way.
[0033]
The clad 21b is provided with a plurality of pores, which form a triangular lattice in the cross section of the fiber to form a photonic crystal structure in the fiber radial direction.
[0034]
Of the six pores adjacent to the core 21a in the fiber cross section, the pair of pores 21c ′ disposed so as to face each other with the core 21a therebetween is larger in diameter than the other pores 21c. On the other hand, the other two pairs of pores 21c disposed so as to face each other with the core 21a interposed therebetween have the same small diameter as the pores 21c other than those adjacent to the core 21a. As a result, the mode field in which the polarization-maintaining PCF 20 is an optical propagation region has two polarization modes that are orthogonal to each other in the cross section of the fiber and have different propagation constants.
[0035]
The large-diameter pores 21c ′ are arranged so as to be shifted outward from the position where the triangular lattice shown in phantom lines in FIG. 2 is formed in the fiber radial direction, that is, away from the core 21a. The orientation of the disposition position is determined by the aspect ratio which is the ratio of the mode field diameter of one mode field to the other of the two polarization modes in the mode field. Approaching 1.0 The direction. As described above, only the large pores 21c ′ are arranged so as to be displaced from the positions where the triangular lattice is formed.
[0036]
The polarization preserving PCF 20 has an aspect ratio of 1.0 or more and smaller than 2.0 (preferably 1.0 or more and 1.7 or less).
[0037]
According to the polarization-maintaining PCF 20 having the above configuration, in addition to obtaining a high mode birefringence, the large pore 21c ′ is shifted outward from the position where the triangular lattice is formed in the fiber radial direction. Since the aspect ratio is lowered by this, it becomes close to the aspect ratio of the Ti diffusion waveguide of the fiber LN modulator, the silica glass-based waveguide of AWG, and the like, and mode matching with them is possible. .
[0038]
In addition, since only the large pores 21c 'are arranged so as to be shifted from the positions where the triangular lattice is formed, the photonic crystal structure of the clad 21b is not greatly broken.
[0039]
<Method of manufacturing polarization-maintaining PCF>
Next, the manufacturing method of the polarization preserving PCF 20 according to the first embodiment of the present invention will be described in the order of steps.
[0040]
-Preparation process-
As shown in FIG. 3, a cylindrical rod rod 1 made of quartz is prepared. The rod material 1 is made of anhydrous synthetic glass having a hydroxyl group content of 1 ppm or less.
[0041]
-Drilling process-
As shown in FIG. 3, with respect to the prepared rod material 1, a plurality of through-holes 1c extending in the rod axis direction so as to surround the core forming portion 1a are left with the central shaft portion remaining solid as the core forming portion 1a. By disposing, the clad forming portion 1b is formed. These holes 1c are arranged so that a pore pattern formed in the clad 21b of the polarization maintaining PCF 20 is formed. The hole is formed by, for example, drilling with a drill, rough finishing with a rod-shaped polishing tool, intermediate finishing, final finishing, and final polishing with a brush and a cerium oxide abrasive.
[0042]
-Base material body formation process-
The rod material 1 on which the core forming portion 1a and the clad forming portion 1b are formed is set in an electric furnace stretching device, and the rod material 1 is heated by the rod stretching heater 2 and stretched to reduce the diameter as shown in FIG. The base material main body 3 is formed. At this time, in the base material main body 3 in which the rod material 1 is reduced in diameter, a plurality of holes 3c whose hole diameter and hole interval are reduced as compared with those of the rod material 1 are held, and accordingly, the core forming portion 3a and the clad formation The portion 3b is reduced from that of the rod material 1.
[0043]
-Impurity removal process-
The base material body 3 is immersed in hydrofluoric acid to etch the surface to remove impurities such as metal adhering to the surface of the base material body 3.
[0044]
Next, as shown in FIG. 5, after the auxiliary pipes 4 are welded to both ends of the base material body 3, chlorine gas is sent to one of the auxiliary pipes 4 in a temperature atmosphere of 1000 to 1200 ° C. Hydrochloric acid formed on the inner wall of the hole 3c is removed by performing a chlorine gas treatment that is circulated through the hole 3c of the main body 3 and discharged from the other auxiliary pipe 4.
[0045]
-Sealing process-
After performing the chlorine gas treatment, as shown in FIG. 6, both ends of both auxiliary pipes 4 are immediately heated and closed, and both ends of the holes 3 c of the base material body 3 are sealed. At this time, chlorine gas may be sealed in the hole.
[0046]
-Drawing process-
As shown in FIG. 7, the base material body 3 is arranged in a quartz cylindrical covering portion forming material 12 to form a base material 13, and the base material 13 is heated by a drawing heater 11. A polarization-maintaining PCF 20 is manufactured by reducing the diameter while integrating the base material body 3 and the covering portion forming material 12 by drawing process. At this time, the gap between the base material main body 3 and the covering portion forming material 12 is reduced to a negative pressure.
[0047]
When wire drawing is performed after the base material body is heated and integrated with the covering portion forming material to form the base material, there is a risk that the hole diameter may fluctuate during the heating integration process. According to the method for manufacturing the storage PCF 20, the base material main body 3 and the covering portion forming material 12 are integrated at the time of drawing, so that fluctuations in the hole diameter can be prevented.
[0048]
Further, during the drawing process, the gap between the base body 3 and the covering portion forming material 12 is reduced to a negative pressure, and air confinement between the fiber body 21 and the covering portion 22 is prevented. It is possible to prevent the transmission loss of the polarization maintaining PCF 20 to be manufactured from becoming high due to such air confinement.
[0049]
According to the method for manufacturing the polarization maintaining PCF 20 as described above, the base material body 3 is formed by heating and stretching the rod material 1 in which the plurality of holes 1c are formed. Is reduced at the stage of the base material main body 3, and the polarization maintaining PCF 20 with high dimensional accuracy can be manufactured.
[0050]
Further, since the drawing process is performed in a state where the plurality of holes 3c of the base material body 3 are sealed, the force acting in the direction of crushing the holes 3c of the base material body 3 by the drawing tension at the time of the drawing process is applied to the hole. It is possible to perform the drawing process in balance with the internal pressure without the hole 3c being crushed.
[0051]
Further, a quartz cylindrical rod material 1 is made of anhydrous synthetic glass having a hydroxyl group content of 1 ppm or less, and before the plurality of holes 3c of the base material body 3 are sealed, the base material is used. Since the etching treatment with hydrofluoric acid for removing the metal and the like on the surface of the main body 3 and the chlorine gas treatment for removing the hydroxyl group on the inner wall of the hole 3c are performed, a low-loss polarization-maintaining PCF 20 can be manufactured. .
[0052]
<Experiment 1>
The polarization preserving PCFs of Example 1 and Comparative Example 1 below were produced, and the aspect ratios were obtained.
[0053]
Example 1
A hole of the hole pattern shown in FIG. 8 (a) was drilled in an anhydrous synthetic quartz rod material having a diameter of 45 mm prepared by VAD, and then stretched to a diameter of 6 mm to form a base material main body. At this time, the pore diameter D of the small pore 21c ₁ Is the diameter D of the pore 21c ′ having a large diameter of 1.4 mm. ₂ 3.9 mm, the pitch P between the holes of the small pores 21c ₁ 3.9 mm, and the pitch P from the center of the rod to the one having the large pore diameter 21c ′ ₂ Was 4.7 mm. After the inside of the hole of the base metal body is treated with chlorine gas to seal both ends, it is placed at the center of a quartz glass tube having an outer diameter of 25 mm and an inner hole diameter of 8 mm, and this is drawn into a fiber diameter of 125 μm as the base material. The polarization-maintaining PCF 20 produced in this way was taken as Example 1.
[0054]
The configuration of the polarization-maintaining PCF 20 of Example 1 thus manufactured is that the pore diameter d of the small pore diameter 21c. ₁ Is 1.0 μm, the pore diameter d of the large pore 21c ′ ₂ Is 2.7 μm and the hole pitch Λ between the small pores 21c ₁ Is 2.7 μm, and the pitch Λ from the fiber center to the large pore 21c ′ ₂ Is 3.2 μm (that is, 0.19 Λ from the position where the large pore 21c ′ forms a triangular lattice outward in the fiber radial direction) ₁ They are offset. )Met. For this polarization-maintaining PCF 20, the light intensity distribution is measured at a wavelength of 1.55 μm to confirm single mode transmission, and the light intensity is 1 / e of the maximum value at the center. ² (When the light intensity distribution is a Gaussian distribution, the light intensity is 1 / e of the center maximum value. ² Is the mode field diameter). As a result, the major axis was 4.4 μm and the minor axis was 3.5 μm. Therefore, the aspect ratio was 1.26. The mode birefringence is 1.6 × 10. ^-3 And showed high polarization preservability. The above characteristics are shown in Table 1.
[0055]
[Table 1]

[0056]
-Comparative Example 1-
Pitch P between holes of the small pores 210c ₁ And the pitch P from the center of the rod to the one having a large pore diameter 210c ′ ₂ Both of these were set to 3.9 mm, and a polarization-maintaining PCF 200 manufactured in the same manner as in Example 1 was used as Comparative Example 1.
[0057]
As shown in FIG. 8B, the configuration of the polarization-maintaining PCF 200 of Comparative Example 1 thus produced has a hole pitch Λ between pores having a small pore diameter 210c. ₁ And the pitch Λ from the fiber center to the large pore 210c ′ ₂ These were the same as Example 1 except that each of them was 2.7 μm. The light intensity distribution was measured at a wavelength of 1.55 μm, and single mode transmission was confirmed. For this polarization-maintaining PCF 200, the light intensity distribution is measured at a wavelength of 1.55 μm to confirm single mode transmission, and the light intensity is 1 / e of the maximum value at the center. ² The diameter was measured. As a result, the major axis was 4.4 μm and the minor axis was 2.9 μm, so the aspect ratio was 1.52. The mode birefringence is 1.7 × 10. ^-3 And high polarization preservability equivalent to that of Example 1 was exhibited. The above characteristics are shown in Table 2.
[0058]
[Table 2]

[0059]
-Experimental results-
As described above, the aspect ratio of the first embodiment in which the large pore diameter 21c ′ is shifted from the position where the triangular lattice is formed is the position where the large pore diameter 210c ′ is the triangular lattice position. It is smaller than the comparative example 1 arrange | positioned.
[0060]
(Embodiment 2)
<Configuration of polarization-maintaining PCF>
FIG. 9 shows a polarization preserving PCF 20 according to the second embodiment of the present invention. In addition, the same part as Embodiment 1 is shown with the same code | symbol.
[0061]
In this polarization-maintaining PCF 20, a pair of pores 21c ′ disposed so as to face each other across the core 21a among the six pores adjacent to the core 21a in the fiber cross section is more than the other pores 21c. The other two pairs of pores 21c arranged so as to face each other with the core 21a interposed therebetween are small pore diameters having the same diameter as other pores 21c other than those adjacent to the core 21a. belongs to. As a result, the mode field in which the polarization-maintaining PCF 20 is an optical propagation region has two polarization modes that are orthogonal to each other in the cross section of the fiber and have different propagation constants.
[0062]
The large pore 21c ′ adjacent to the core 21a is disposed so as to be shifted outward in the fiber radial direction from the position where the triangular lattice shown by phantom lines in FIG. 9 is formed, that is, away from the core 21a. Yes. The orientation of the disposition position is determined by the aspect ratio which is the ratio of the mode field diameter of one mode field to the other of the two polarization modes in the mode field. Approaching 1.0 The direction. In addition, the small pores 21c on the outer sides of the two large pores 21c ′ on the line connecting the two large pores 21c ′ are also outside the radial direction of the fiber from the position where the triangular lattice shown in FIG. 9 is formed. It is displaced in the direction, that is, the direction away from the core 21a.
[0063]
In the polarization-maintaining PCF 20 having the above configuration, when the pore diameter of the large pore diameter 21c ′ is large, the large pore diameter 21c ′ is changed from the position where the triangular lattice is formed. Interference with the small pores 21c on the outside is prevented.
[0064]
Other configurations, operations and effects, and the manufacturing method are the same as those in the first embodiment.
[0065]
<Experiment 2>
Polarization-preserving PCFs of Example 2 and Comparative Example 2 below were produced, and the respective aspect ratios were obtained.
[0066]
-Example 2-
A hole of the hole pattern shown in FIG. 10 (a) was drilled in an anhydrous synthetic quartz rod material having a diameter of 45 mm prepared by VAD, and then stretched to a diameter of 6 mm to form a base material main body. At this time, the pore diameter D of the small pore 21c ₁ Is 2.7 mm, the pore diameter D of the large pore diameter 21c ′ ₂ 6.7 mm, the pitch P between the holes of the small pores 21c ₁ 5.8 mm, and the pitch P from the center of the rod to the one having the large pore diameter 21c ′ ₂ Was set to 7.0 mm. After the inside of the hole of the base metal body is treated with chlorine gas to seal both ends, it is placed at the center of a quartz glass tube having an outer diameter of 25 mm and an inner hole diameter of 8 mm, and this is drawn into a fiber diameter of 125 μm as the base material. The polarization-maintaining PCF 20 produced in this way was taken as Example 2.
[0067]
The configuration of the polarization-maintaining PCF 20 of Example 2 thus manufactured is such that the pore diameter d of the small pore diameter 21c. ₁ Is 1.9 μm, the pore diameter d of the large pore 21c ′ ₂ Is 4.7 μm and the hole pitch Λ between the small pores 21c ₁ Is 4.0 μm and the pitch Λ from the center of the fiber to the large pore 21c ′ ₂ 4.8 μm (that is, the large pore 21c ′ and the small pore 21c on the outside thereof are 0.20Λ outward from the position where the triangular lattice is formed, respectively, outward in the fiber radial direction. ₁ They are offset. )Met. For this polarization-maintaining PCF 20, the light intensity distribution is measured at a wavelength of 1.55 μm to confirm single mode transmission, and the light intensity is 1 / e of the maximum value at the center. ² The diameter was measured. As a result, the major axis was 6.1 μm and the minor axis was 4.4 μm. Therefore, the aspect ratio was 1.39. The mode birefringence is 1.4 × 10. ^-3 Although it is slightly lower than that of Example 1, it can be said that the mode field diameter is large and the connectivity with other optical fibers is excellent. The above characteristics are shown in Table 3.
[0068]
[Table 3]

[0069]
-Comparative Example 2-
Pitch P between holes of the small pores 210c ₁ And the pitch P from the center of the rod to the one having a large pore diameter 210c ′ ₁ Each of these was 5.8 mm, and a polarization-maintaining PCF 200 produced in the same manner as in Example 1 was used as Comparative Example 2.
[0070]
As shown in FIG. 10B, the configuration of the polarization-maintaining PCF 200 of Comparative Example 2 thus manufactured has a hole pitch Λ between small pores 210c. ₁ And the pitch Λ from the fiber center to the large pore 210c ′ ₂ These were the same as Example 1 except that all of them were 4.0 μm. For this polarization-maintaining PCF 200, the light intensity distribution is measured at a wavelength of 1.55 μm to confirm single mode transmission, and the light intensity is 1 / e of the maximum value at the center. ² The diameter was measured. As a result, the major axis was 6.1 μm and the minor axis was 3.5 μm. Therefore, the aspect ratio was 1.71. The mode birefringence is 1.5 × 10 ^-3 The same performance as in Example 2 was exhibited. Table 4 shows the above characteristics.
[0071]
[Table 4]

[0072]
-Experimental results-
As described above, the aspect ratio of Example 2 in which the large pore diameter 21c ′ and the small pore diameter 21c outside thereof are shifted from the positions where the triangular lattice is formed is smaller. The hole 210c ′ is smaller than the comparative example 2 arranged at the position where the triangular lattice is formed.
[0073]
(Embodiment 3)
<Configuration of polarization-maintaining PCF>
FIG. 11 shows a polarization preserving PCF 20 according to Embodiment 3 of the present invention. In addition, the same part as Embodiment 1 is shown with the same code | symbol.
[0074]
In this polarization-maintaining PCF 20, in the fiber cross section, among the six pores adjacent to the core 21a, the pores are on a straight line connecting a pair of pores 21c arranged to face each other with the core 21a interposed therebetween. Is smaller in diameter than the other pores, while the other pores including the other two pairs of pores 21c 'disposed so as to face each other with the core 21a interposed therebetween have a large pore diameter. . In the cross section of the fiber, a pair of regions having a large pore group including a large pore 21c 'adjacent to the core 21a and arranged to face each other with the core 21a therebetween is a large pore region. 31, a pair of regions having a small pore group including small pores 21c disposed adjacent to the core 21a so as to face each other across the core 21a is a small pore region. 32. With the above configuration, the mode field in which the polarization-maintaining PCF 20 is an optical propagation region has two polarization modes that are orthogonal to each other in the fiber cross section and have different propagation constants.
[0075]
The small pores 21c adjacent to the core 21a are arranged so as to be shifted inward in the fiber radial direction from the position where the triangular lattice shown by phantom lines in FIG. 11 is formed, that is, toward the core 21a. . The orientation of the disposition position is determined by the aspect ratio which is the ratio of the mode field diameter of one mode field to the other of the two polarization modes in the mode field. Approaching 1.0 The direction. Further, the small pores 21c outside the pores on the line connecting the small pores 21c adjacent to the core 21a are also located in the fiber radial direction from the position where the triangular lattice shown by the phantom lines in FIG. 9 is formed. It is arranged so as to be displaced in the direction, that is, the direction approaching the core 21a. That is, each pore 21c 'of the large pore diameter region 31 is disposed at a position where a triangular lattice is formed, while the fiber radius from the position where each pore 21c of the small pore diameter region 32 forms a triangular lattice. It is arranged so as to be displaced outward in the direction.
[0076]
The polarization-maintaining PCF 20 having the above configuration is one in which the zero dispersion wavelength is shifted to the short wavelength side because the ratio of the pore diameter to the pore spacing of the photonic crystal structure formed in the clad 21b is large as a whole. .
[0077]
Other configurations, operations and effects, and the manufacturing method are the same as those in the first embodiment.
[0078]
<Experiment 3>
Polarization-preserving PCFs of the following Example 3 and Comparative Example 3 were produced, and the respective aspect ratios were obtained.
[0079]
-Example 3-
A hole of the hole pattern shown in FIG. 12 (a) was made in an anhydrous synthetic quartz rod material having a diameter of 45 mm prepared by VAD, and then stretched to a diameter of 6 mm to form a base material main body. At this time, the pore diameter D of the small pore 21c ₁ Pore diameter D of the pore 21c ′ having a large pore diameter of 1.0 mm ₂ Is 2.3 mm and the pitch P between the pores of the small pore 21c. ₁ 3.3 mm, and the pitch P from the center of the rod to the pore 21c ′ having a large pore diameter ₂ Was 2.8 mm. After the inside of the hole of the base metal body is treated with chlorine gas to seal both ends, it is placed at the center of a quartz glass tube having an outer diameter of 25 mm and an inner hole diameter of 8 mm, and this is drawn into a fiber diameter of 125 μm as the base material. The polarization-maintaining PCF 20 produced in this way was taken as Example 3.
[0080]
The configuration of the polarization-maintaining PCF 20 of Example 3 thus manufactured is that the pore diameter d of the small pore diameter 21c. ₁ Is 0.7 μm and the pore diameter d of the large pore 21c ′ ₂ Is 1.6 μm and the hole pitch Λ between the small pores 21c ₁ Is 2.3 μm, and the pitch Λ from the fiber center to the large pore 21c ′ ₂ Is 2.0 μm (that is, 0.13Λ inward in the radial direction of the fiber from the position where the small pores 21c form a triangular lattice. ₁ They are offset. )Met. The aspect ratio was 1.10. The mode birefringence is 1.6 × 10. ^-3 And showed high polarization preservability. The above characteristics are shown in Table 5.
[0081]
[Table 5]

[0082]
-Comparative Example 3-
Pitch P between holes of the small pores 210c ₁ And the pitch P from the center of the rod to the one having a large pore diameter 210c ′ ₂ Each of these was 3.3 mm, and a polarization-maintaining PCF 200 produced in the same manner as in Example 1 was used as Comparative Example 3.
[0083]
As shown in FIG. 12B, the configuration of the polarization-maintaining PCF 200 of Comparative Example 3 thus manufactured has a hole pitch Λ between the small pores 210c. ₁ And the pitch Λ from the fiber center to the large pore 210c ′ ₂ These were the same as Example 1 except that each of them was 2.3 μm. The aspect ratio was 1.30. The mode birefringence is 1.6 × 10. ^-3 The same performance as in Example 3 was exhibited. The above characteristics are shown in Table 6.
[0084]
[Table 6]

[0085]
-Experimental results-
As described above, the aspect ratio is such that in Example 3 in which the small pore diameter 21c is shifted from the position where the triangular lattice is formed, the small pore 210c is disposed at the position where the triangular lattice is formed. It is smaller than the comparative example 3 provided.
[0086]
(Embodiment 4)
<Configuration of polarization-maintaining PCF>
FIG. 13 shows a polarization preserving PCF 20 according to Embodiment 4 of the present invention. In addition, the same part as Embodiment 1 is shown with the same code | symbol.
[0087]
In this polarization preserving PCF 20, in the fiber cross-section, among the six pores adjacent to the core 21a, the fine pores on a straight line connecting a pair of pores 21c ′ disposed so as to face each other with the core 21a interposed therebetween. The pores have a larger pore size than the other pores, while the other pores including the other two pairs of pores 21c arranged so as to face each other with the core 21a interposed therebetween have a small pore size. . In the cross section of the fiber, a pair of regions having a large pore group including a large pore 21c 'adjacent to the core 21a and arranged to face each other with the core 21a therebetween is a large pore region. 31, a pair of regions having a small pore group including small pores 21c disposed adjacent to the core 21a so as to face each other across the core 21a is a small pore region. 32. With the above configuration, the mode field in which the polarization-maintaining PCF 20 is an optical propagation region has two polarization modes that are orthogonal to each other in the fiber cross section and have different propagation constants.
[0088]
The large pores 21c ′ adjacent to the core 21a are disposed so as to be shifted inward in the radial direction of the fiber from the position where the triangular lattice shown by phantom lines in FIG. 13 is formed, that is, away from the core 21a. Yes. The orientation of the disposition position is determined by the aspect ratio which is the ratio of the mode field diameter of one mode field to the other of the two polarization modes in the mode field. Approaching 1.0 The direction. Further, the outer large pores 21c 'on the line connecting the large pores 21c' adjacent to the core 21a are also located from the position where a triangular lattice is formed as shown by phantom lines in FIG. They are arranged so as to be shifted outward in the fiber radial direction, that is, away from the core 21a. That is, while each pore 21c of the small pore diameter region 32 is disposed at a position where a triangular lattice is formed, the fiber radius from the position where each pore 21c ′ of the large pore diameter region 31 forms a triangular lattice. It is arranged so as to be displaced outward in the direction.
[0089]
In the first and second embodiments, since there is a small pore 21c outside the large pore 21c 'adjacent to the core 21a, long wavelength light is emitted in that direction and long wavelength transmission is performed. However, in the polarization-maintaining PCF 20 having the above-described configuration, the large pore 21c ′ further outside the large pore 21c ′ adjacent to the core 21a eliminates such inconvenience. Is done.
[0090]
Other configurations, operations and effects, and the manufacturing method are the same as those in the first embodiment.
[0091]
(Other embodiments)
In the first to fourth embodiments, the base material body 3 is disposed in the quartz cylindrical covering portion forming material 12 to form the base material 13, and the base material 13 is heated by the wire drawing heater 11. The polarization-maintaining PCF 20 was manufactured by reducing the diameter while integrating the base material main body 3 and the covering portion forming material 12 by drawing together with the drawing, but the invention is not particularly limited thereto, as shown in FIG. Further, the flame of the oxyhydrogen burner 5 is applied to the outer periphery of the base material body 3 in which the hole 3c is sealed. _Four 15 is formed to integrate the quartz fine particles generated by the formation of the quartz porous layer 6, and then, as shown in FIG. 15, the base material body 3 on which the quartz porous layer 6 is integrally deposited on the outer periphery is formed with helium. Placed in a heating vessel 7 in which chlorine gas is circulated, the quartz porous layer 6 is baked by heating with a baking heater 8 from the outside to be changed into a transparent covering portion forming material 9 to obtain a base material 10. Thereafter, the base material 10 may be set in a drawing apparatus, and the diameter may be reduced by drawing with a drawing heater 11 as shown in FIG.
[0092]
In addition, a base material forming step of forming a base material by forming a plurality of holes extending in the rod axis direction so as to correspond to the hole pattern of the fiber cross section in the quartz rod material, and the base material having a small diameter by drawing The polarization preserving PCF may be manufactured through a drawing process. In this case, if the drawing process is performed in a state where a plurality of holes in the base material are sealed, the force acting in the direction of crushing the holes in the base material by the drawing tension at the time of the drawing process is balanced with the pressure in the hole. The drawing process can be performed without crushing the hole. Further, if the impurity removal treatment for removing the impurities on the inner walls of the plurality of holes is performed before the plurality of holes are sealed, the impurities in the holes of the base material can be removed, and thus the manufacturing is performed. The polarization preserving PCF has a low loss. Further, if the quartz rod material is made of anhydrous synthetic glass having a hydroxyl group content of 1 ppm or less, the number of hydroxyl groups contained in the produced polarization-maintaining PCF is small and the loss is low.
[0093]
In the third embodiment, a small pore diameter region 32 constituted by a pair of band-like regions sandwiching the core 21a is formed in the fiber cross section, while a large pore diameter region 31 is constituted in the other regions, In FIG. 4, the large pore diameter region 31 constituted by a pair of band-shaped regions sandwiching the core 21 a is formed in the fiber cross section, while the small pore diameter region 32 is configured in the other regions, but is not particularly limited thereto. Instead, as shown in FIG. 17, the large pore region 31 and the small pore region 32 are configured by a sector region having a central angle of 60 ° or 120 °, for example, with the core 21a as the center. Also good.
[0094]
In the first to third embodiments, the polarization-maintaining PCF 20 made of pure quartz is used. However, the present invention is not particularly limited to this. If the main component is quartz, other elements are doped. It may be a thing.
[Brief description of the drawings]
FIG. 1 is a perspective view of a polarization maintaining PCF according to a first embodiment.
FIG. 2 is a front view of the central portion of the cross section of the polarization maintaining PCF according to the first embodiment.
3 is an explanatory diagram of a preparation process and a drilling process according to Embodiment 1. FIG.
FIG. 4 is an explanatory diagram of a base material main body forming process according to the first embodiment.
FIG. 5 is an explanatory diagram of chlorine gas treatment in the impurity removal treatment step according to the first embodiment.
6 is an explanatory diagram of a sealing process according to Embodiment 1. FIG.
FIG. 7 is an explanatory diagram of a drawing process according to the first embodiment.
8 is a front view of the central portion of the cross section of each polarization-maintaining PCF in Example 1 and Comparative Example 1 of Experiment 1. FIG.
FIG. 9 is a front view of the central portion of the cross section of the polarization maintaining PCF according to the second embodiment.
10 is a front view of the central portion of the cross section of each polarization-maintaining PCF in Example 2 and Comparative Example 2 of Experiment 2. FIG.
FIG. 11 is a front view of a central portion of a cross section of a polarization maintaining PCF according to a third embodiment.
12 is a front view of the central portion of the transverse cross section of each polarization-maintaining PCF of Example 3 and Comparative Example 3 of Experiment 3. FIG.
FIG. 13 is a front view of a central portion of a cross section of a polarization maintaining PCF according to a fourth embodiment.
FIG. 14 is an explanatory diagram showing a formation process of a quartz porous layer in a covering portion forming material forming step according to a method of manufacturing a polarization maintaining PCF of another embodiment.
FIG. 15 is an explanatory diagram showing a formation process of a covering portion forming material by firing a quartz porous layer in a covering portion forming material forming step according to a method of manufacturing a polarization maintaining PCF of another embodiment.
FIG. 16 is an explanatory diagram showing a drawing process according to a method of manufacturing a polarization maintaining PCF of another embodiment.
FIG. 17 is a schematic front view of a central portion of a cross section of a polarization maintaining PCF according to another embodiment.
FIG. 18 is a front view of the central portion of the cross section of the polarization maintaining PCF according to the prior art.
[Explanation of symbols]
1 Rod material
1a, 3a Core forming part
1b, 3b Clad formation part
1c, 3c hole
2 Rod extension heater
3 Base material body
4 Auxiliary pipe
5 Oxyhydrogen burner
6 Quartz porous layer
7 Heating container
8 Heating heater
9,12 Covering part forming material
10, 13 Base material
11 Wire drawing heater
20,200 Polarization-preserving PCF
21 Fiber body
21a, 210a core
21b, 210b cladding
21c, 210c Small pores
21c ', 210c' Large pore diameter
22 Covering part
31 Large pore area
32 Small pore area

Claims

A solid core that forms the center of the fiber, and a plurality of pores that are provided so as to cover the core and that form a triangular lattice in the cross section of the fiber to form a photonic crystal structure in the fiber radial direction A fine pore adjacent to the core in the clad is arranged in a pair with a large-diameter pore arranged in a pair with the core in between. A polarized wave configured such that a mode field that is a light propagation region has two polarization modes that are orthogonal to each other and have different propagation constants in the cross section of the fiber. A storage photonic crystal fiber,
At least one of the large pore and the small pore adjacent to the core is the other mode field diameter with respect to one mode field diameter of the two polarization modes in the mode field. A polarization-maintaining photonic crystal fiber, wherein the aspect ratio, which is the ratio of the above, is arranged so as to deviate from the position where the triangular lattice is formed in a direction approaching 1.0 .

In the polarization maintaining photonic crystal fiber according to claim 1,
A polarization-maintaining photonic crystal fiber having an aspect ratio of 1.0 or more and less than 2.0.

In the polarization maintaining photonic crystal fiber according to claim 2,
A polarization-maintaining photonic crystal fiber having an aspect ratio of 1.0 or more and 1.7 or less.

In the polarization maintaining photonic crystal fiber according to claim 1,
A polarization-maintaining photonic crystal fiber, characterized in that only large pores or small pores adjacent to the core are disposed so as to deviate from the positions where the triangular lattice is formed.

In the polarization maintaining photonic crystal fiber according to claim 1,
A polarization-maintaining photonic crystal fiber characterized in that large pores adjacent to the core are arranged so as to be shifted outward in the fiber radial direction from a position where the triangular lattice is formed.

In the polarization maintaining photonic crystal fiber according to claim 1,
A polarization-maintaining photonic crystal fiber characterized in that small-diameter pores adjacent to the core are displaced inward in the fiber radial direction from a position where the triangular lattice is formed.

In the polarization maintaining photonic crystal fiber according to claim 1,
A pair of large pore diameter regions having a large pore group including a large pore diameter adjacent to the core and disposed across the fiber cross section across the core, and the core sandwiched between the core And a pair of small-pore diameter pore regions having a small-pore diameter pore group including small-pore diameter pores adjacent to the core, the four regions are formed,
Polarization preserving characterized in that at least one of each pore in the large pore diameter region and each pore in the small pore size region is displaced from a position where the triangular lattice is formed Photonic crystal fiber.

The polarization maintaining photonic crystal fiber according to claim 7,
A polarization-maintaining photonic crystal fiber, wherein each of the pores in the large pore diameter pore region is arranged to be shifted outward in a fiber radial direction from a position where the triangular lattice is formed.

The polarization maintaining photonic crystal fiber according to claim 7,
A polarization-maintaining photonic crystal fiber, wherein each of the pores in the small pore region is displaced inward in the fiber radial direction from a position where the triangular lattice is formed.

A solid core that forms the center of the fiber, and a plurality of pores that are provided so as to cover the core and that form a triangular lattice in the cross section of the fiber to form a photonic crystal structure in the fiber radial direction A fine pore adjacent to the core in the clad is arranged in a pair with a large-diameter pore arranged in a pair with the core in between. A mode field that is a light propagation region is configured to have two polarization modes that are orthogonal to each other in the fiber cross section and have different propagation constants. in the direction of the aspect ratio is the ratio of the other of the mode field diameter to the mode field diameter of one of the directions of the two polarization modes in the field approaches 1.0, adjacent to the core At least one of the pores of the pores and small pore size of the large pore size is a method for producing a polarization maintaining photonic crystal fiber displaced from the position disposed to form the triangular lattice,
A base material forming step of forming a base material by forming a plurality of holes extending in the rod axis direction so as to correspond to the hole pattern of the fiber cross section in the quartz rod material;
A drawing step of reducing the diameter of the base material by drawing,
A method of manufacturing a polarization-maintaining photonic crystal fiber, comprising:

The method of manufacturing a polarization-maintaining photonic crystal fiber according to claim 10,
A method of manufacturing a polarization-maintaining photonic crystal fiber, wherein the drawing is performed in a state where a plurality of holes of the base material are sealed.

The method of manufacturing a polarization maintaining photonic crystal fiber according to claim 11,
A method of manufacturing a polarization-maintaining photonic crystal fiber, wherein an impurity removing process for removing impurities on the inner walls of the plurality of holes is performed before sealing the plurality of holes.

A solid core that forms the center of the fiber, and a plurality of pores that are provided so as to cover the core and that form a triangular lattice in the cross section of the fiber to form a photonic crystal structure in the fiber radial direction A large pore diameter in which the pores adjacent to the core in the clad are disposed in pairs with the core sandwiched therebetween. , And small-diameter pores arranged in pairs with the core in between, the mode fields as light propagation regions are orthogonal to each other in the fiber cross section and the propagation constant is It is configured to have two different polarization modes, and is a ratio of one mode field diameter to the other mode field diameter in the direction of the two polarization modes in the mode field. In the direction of the aspect ratio approaches 1.0, at least one of the pores of the pores and small pore size of the large pore size adjacent to the core is disposed offset from the position of forming the triangular lattice polarization A method for producing a preserved photonic crystal fiber comprising:
A drilling step of forming a plurality of holes extending in the rod axis direction so as to correspond to the hole pattern of the fiber cross section in the quartz rod material;
A base material body forming step of forming a base material body by reducing the diameter by heating and stretching the rod material in which the plurality of holes are formed in the rod axis direction;
A base material is constituted by the base material main body and the quartz covering portion forming material to be the covering portion, and a drawing step for reducing the diameter of the base material by drawing processing;
A method of manufacturing a polarization-maintaining photonic crystal fiber, comprising:

The method of manufacturing a polarization maintaining photonic crystal fiber according to claim 13,
A method of manufacturing a polarization-maintaining photonic crystal fiber, wherein the drawing is performed in a state where a plurality of holes of the base material body are sealed.

The method of manufacturing a polarization maintaining photonic crystal fiber according to claim 14,
A method of manufacturing a polarization-maintaining photonic crystal fiber, wherein an impurity removing process for removing impurities on the inner walls of the plurality of holes is performed before sealing the plurality of holes.

In the method for manufacturing a polarization-maintaining photonic crystal fiber according to claim 10 or 13,
A method for producing a polarization-maintaining photonic crystal fiber, wherein the quartz rod material is formed of anhydrous synthetic glass having a hydroxyl group content of 1 ppm or less.