JP2004078027A - Absolute single mode optical fiber and its designing method - Google Patents

Absolute single mode optical fiber and its designing method Download PDF

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Publication number
JP2004078027A
JP2004078027A JP2002241070A JP2002241070A JP2004078027A JP 2004078027 A JP2004078027 A JP 2004078027A JP 2002241070 A JP2002241070 A JP 2002241070A JP 2002241070 A JP2002241070 A JP 2002241070A JP 2004078027 A JP2004078027 A JP 2004078027A
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refractive index
optical fiber
effective refractive
mode
core portion
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JP3954929B2 (en
Inventor
Hirokazu Kubota
久保田　寛和
Satoki Kawanishi
川西　悟基
Kazunobu Suzuki
鈴木　和宣
Shigeki Koyanagi
小柳　繁樹
Masatoshi Tanaka
田中　正俊
Moriyuki Fujita
藤田　盛行
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Mitsubishi Cable Industries Ltd
Nippon Telegraph and Telephone Corp
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Mitsubishi Cable Industries Ltd
Nippon Telegraph and Telephone Corp
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02357—Property of longitudinal structures or background material varies radially and/or azimuthally in the cladding, e.g. size, spacing, periodicity, shape, refractive index, graded index, quasiperiodic, quasicrystals
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/01205—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments
    • C03B37/01211—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments by inserting one or more rods or tubes into a tube
    • C03B37/01217—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments by inserting one or more rods or tubes into a tube for making preforms of polarisation-maintaining optical fibres
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/01205—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments
    • C03B37/01211—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments by inserting one or more rods or tubes into a tube
    • C03B37/0122—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments by inserting one or more rods or tubes into a tube for making preforms of photonic crystal, microstructured or holey optical fibres
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C03B2203/14—Non-solid, i.e. hollow products, e.g. hollow clad or with core-clad interface
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/30—Polarisation maintaining [PM], i.e. birefringent products, e.g. with elliptical core, by use of stress rods, "PANDA" type fibres
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/42—Photonic crystal fibres, e.g. fibres using the photonic bandgap PBG effect, microstructured or holey optical fibres

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Optics & Photonics (AREA)
  • Geochemistry & Mineralogy (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
  • Manufacture, Treatment Of Glass Fibers (AREA)

Abstract

<P>PROBLEM TO BE SOLVED: To provide an absolute single mode optical fiber which is easily manufactured and has a high degree of freedom of design and its designing method. <P>SOLUTION: A cladding part 12 constituting the optical fiber is configured in a photonic crystal structure provided with voids which extend in the extension direction of the cladding part 12 and arranged cyclically in a lattice spacing Λ, and the voids are arranged so as to make an axisymmetric property around the central axis of a core part 11 to be less than three times. The lattice spacing Λ of the voids is set so that an effective refractive index of a first mode can be larger than an effective refractive index of the cladding part and also that an effective refractive index of a second mode can be smaller than the effective refractive index of the cladding part in the first mode where the effective refractive index to the wavelength of light propagating through the core part 11 becomes maximum and in the second mode where the effective refractive index becomes the second maximum among a plurality of modes that can exist in an area where the core part 11 and voids adjacently surrounding the core part 11 exist. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、絶対単一モード光ファイバおよびその設計方法に関し、より詳細には、製造が容易で、かつ、設計自由度の高い絶対単一モード光ファイバおよびその設計方法に関する。
【０００２】
【従来の技術】
絶対単一モード光ファイバは、一般に、光ファイバ中に金属などの光吸収媒質を埋め込むことにより偏光に依存した光吸収を起こさせている。従って、絶対単一モード光ファイバを作製するに際しては、ガラス、プラスチック、各種結晶などの母材中にこれらの母材とは異質の材料を埋め込み、これをファイバ化する必要がある。
【０００３】
【発明が解決しようとする課題】
しかしながら、ファイバ化のためには、埋め込まれるべき材料は光ファイバの母材と同程度の融点をもつ必要があり、さらに、母材との親和性も高いことが求められる。このような条件を全て満足する埋め込み材の選択は容易ではなく、これまでは、実用的な特性を示す絶対単一モード光ファイバは作製されていなかった。
【０００４】
本発明は、このような問題に鑑みてなされたものであって、その目的とするところは、製造が容易で、かつ、設計自由度の高い絶対単一モード光ファイバおよびその設計方法を提供することにある。
【０００５】
【課題を解決するための手段】
本発明は、このような目的を達成するために、請求項１に記載の発明は、絶対単一モード光ファイバであって、光ファイバ母材の中心領域に設けられたコア部がクラッド部により取り囲まれており、前記クラッド部は、当該クラッド部の伸長方向に延在し、かつ、格子間隔Λで周期的に配置された空孔を備えることにより屈折率変調するフォトニック結晶構造を有し、前記空孔は、前記コア部の中心軸の周りの軸対称性が３回未満となるように配置され、前記コア部と当該コア部を近接して取り囲む空孔とが存在する領域に存在可能な複数のモードのうち、当該コア部を伝搬する光の波長に対する有効屈折率が最大となる第１モードと２番目に大きな第２モードにおいて、当該第１モードの有効屈折率が前記クラッド部の有効屈折率よりも大きく、かつ、当該第２モードの有効屈折率が前記クラッド部の有効屈折率よりも小さくなるように設定されていることを特徴とする。
【０００６】
また、請求項２に記載の発明は、請求項１に記載の絶対単一モード光ファイバにおいて、前記コア部を近接して取り囲む空孔のうち少なくとも１つの空孔の径が、前記クラッド部に配置された空孔の径よりも大きいことを特徴とする。
【０００７】
また、請求項３に記載の発明は、請求項２に記載の絶対単一モード光ファイバにおいて、前記コア部を近接して取り囲む空孔の径と前記クラッド部に配置された空孔の径とが異なることを特徴とする。
【０００８】
また、請求項４に記載の発明は、請求項１乃至３の何れかに記載の絶対単一モード光ファイバにおいて、前記光ファイバ母材は、ガラスまたはプラスチックであることを特徴とする。
【０００９】
さらに、請求項５に記載の発明は、請求項４に記載の絶対単一モード光ファイバにおいて、前記空孔の内部は、真空、もしくは、前記コア部を伝搬する光に対して透明なガスまたは液体もしくは固体が充填されていることを特徴とする。
【００１０】
請求項６に記載の発明は、光ファイバのコア部を取り囲むクラッド部が、当該クラッド部の伸長方向に延在し、かつ、格子間隔Λで周期的に配置された空孔を備えることにより屈折率変調するフォトニック結晶構造を有する絶対単一モード光ファイバの設計方法であって、前記空孔を、前記コア部の中心軸の周りの軸対称性が３回未満となるように配置し、前記コア部と当該コア部を近接して取り囲む空孔とが存在する領域に存在可能な複数のモードのうち、当該コア部を伝搬する光の波長に対する有効屈折率が最大となる第１モードと２番目に大きな第２モードにおいて、当該第１モードの有効屈折率が前記クラッド部の有効屈折率よりも大きく、かつ、当該第２モードの有効屈折率が前記クラッド部の有効屈折率よりも小さくなるように前記格子間隔Λを設定することを特徴とする。
【００１１】
【発明の実施の形態】
以下に、図面を参照して本発明の実施の形態について説明する。
【００１２】
（実施例１）
図１は、本発明の絶対単一モード光ファイバの第１の構造例を説明するための断面図で、この絶対単一モード光ファイバは、光ファイバ材料１３の中心領域に設けられたコア部１１が、多数の空孔を有するクラッド部１２で取り囲まれた構造を有している。
【００１３】
図２は、図１に示した断面図のコア部１１近傍を詳細に説明するための図で、クラッド部に存在する多数の空孔２３は格子間隔Λで周期的に配置されてコア部２１を取り囲み、コア部２１とこのコア部２１を最近接で取り囲む空孔とで「コアとコアを取り囲む空孔を合わせた領域」２２が形成され、いわゆる「フォトニック結晶構造」を利用して光ファイバが形成されている。
【００１４】
フォトニック結晶は、誘電体周期構造を有する結晶であり、図１および図２に示した本発明の絶対単一モード光ファイバでは、ファイバ材料中に空孔が蜂の巣状（六方最密構造）に周期的に配置されて屈折率変調構造を構成している。このようなフォトニック結晶構造を有する光ファイバは、その空孔の配置状態により光伝搬特性を自由に設計することが可能である。
【００１５】
ここで、上述した「コアとコアを取り囲む空孔を合わせた領域」に存在可能な複数のモードのうち、光ファイバ中を伝搬する光の波長に対する伝搬定数（もしくは有効屈折率）の最も大きなモードを「第１モード」、その次に大きなモードを「第２モード」と定義する。
【００１６】
なお、図２においては、後述する計算の便宜のために、コア部２１とこのコア部２１を最近接で取り囲む空孔２３とで形成される六角形の内側の領域を「コアとコアを取り囲む空孔を合わせた領域」２２として定義したが、この領域の定義の仕方はこれに限定されるものではない。
【００１７】
また、空孔の配置の仕方も図２に示したものに限定されるものではなく、所望する光ファイバの特性に応じて自由に設計が可能であるが、図２に示すような六方最密構造のフォトニック結晶とする場合には、同一外径の円柱を隙間無く並べるだけで自然にその横断面が六方最密構造をとることからその製造は極めて容易となるという利点がある。
【００１８】
さらに、クラッド部に存在する多数の空孔のうちの一部の空孔の位置や大きさを他の空孔とは異なるように設計することとすれば、光ファイバ中心軸の周りの対称性を低くして偏波の縮退を解くことも可能となり、偏波保持光ファイバが実現できる。
【００１９】
図３は、図１および図２に示した構造の本発明の絶対単一モード光ファイバの設計方法を説明するための図で、横軸は伝搬光の波長λを空孔格子間隔Λで除した値、縦軸は有効屈折率であり、各波長に対して、第１モードおよび第２モードならびにクラッド部のモードの有効屈折率を計算して求めた結果を概念的に示したものである。なお、この図は、図２に示すように、コア部を取り囲む６つの空孔のうちの２つの空孔の径（ｄ２）を他の空孔の径（ｄ１）よりも大きくし、２回の軸対称性を有する場合について計算を実行した場合を仮定している。
【００２０】
この構造のファイバは３回未満の軸対称性を有するため、第１モードには偏波による縮退はなく、このモードのみが伝搬する光ファイバは真の単一モード光ファイバ（絶対単一モード光ファイバ）となる。
【００２１】
第１モードおよび第２モードの有効屈折率がクラッドのモードの有効屈折率より低くなると、これらのモードはコア部からクラッド部へと漏れ出してゆくようになり、その結果、光ファイバ中を伝搬することができなくなる。
【００２２】
そこで、第１モードは伝搬可能とする一方、第２モードの伝搬が不可となるように設計すれば、上述した「第１モード」および「第２モード」の定義から明らかのように、これらのモード以外のモードは第２モードよりも有効屈折率が低いために光ファイバ中を伝搬することができず、第１モードのみを伝搬させるようにすることが可能となる。
【００２３】
図３において、光ファイバが絶対単一モードで動作する波長領域は、図中に白抜きの矢印で示した領域であり、この領域は、第１モードの有効屈折率がクラッド部の有効屈折率よりも大きく、かつ、第２モードの有効屈折率がクラッド部の有効屈折率よりも小さい波長範囲に対応する。
【００２４】
このように、特定の波長の光を絶対単一モード動作させる光ファイバを設計するためには、その波長λを図３で得られる「波長／空孔格子間隔（λ／Λ）」の値で除して得られた格子間隔Λとすればよい。
【００２５】
図４は、図１および図２に示した構造の本発明の絶対単一モード光ファイバの設計方法を実際の計算により実行した結果を説明するための図で、横軸は伝搬光の波長λを空孔格子間隔Λで除した値、縦軸は有効屈折率（ｎ／ｎ０）であり、空孔格子間隔Λ、クラッド部の空孔径ｄ１、他の空孔よりも径の大きな空孔の径をｄ２とし、ｄ１／Λ＝０．３８、ｄ２／Λ＝１．０４の条件下で求めている。なお、ここでは、ファイバの母材は石英ガラス、空孔の中は真空（もしくは空気）としているが、ファイバの母材は他のガラスやプラスチックであっても良く、また、空孔の内部は、コア中を伝搬する光に対して透明なガスまたは液体もしくは固体が充填されていてもよい。
【００２６】
この図において、実線はクラッド部の有効屈折率、点線が第１モードの有効屈折率、そして、１点鎖線が第２モードの有効屈折率である。この図に示した結果から、この光ファイバの絶対単一動作する波長領域は、λ／Λ＝０．４〜０．４９であることが読み取れる。
【００２７】
図５は、図２に示した構造を有する実際の光ファイバの損失の波長依存性を説明するための図で、この光ファイバの各パラメータは、Λ＝２．６μｍ、ｄ１＝１μｍ、ｄ２＝２．７μｍである。この図において、各モードの損失が増加し始める波長が各モードの有効屈折率がクラッド部のモードの有効屈折率と等しくなる波長に対応する。
【００２８】
図５の結果より、この光ファイバの絶対単一動作する波長領域は１．１〜１．３μｍであることが読み取れ、図４に示した計算結果で得られる絶対単一動作領域（１．０４〜１．２７μｍ）と良い一致を示し、ほぼ設計どうりの動作を行っていることが理解される。
【００２９】
（実施例２）
本発明の絶対単一モード光ファイバでは、クラッド部の有効屈折率を変化させて絶対単一動作領域を所望の波長に設定するようにすることも可能である。
【００３０】
図６は、本発明の絶対単一モード光ファイバの第２の構造例を説明するための断面図で、この絶対単一モード光ファイバも図２に示したのと同様に、クラッド部に存在する多数の空孔６３は格子間隔Λで周期的に配置されてコア部６１を取り囲み、コア部６１とこのコア部６１を最近接で取り囲む空孔とで「コアとコアを取り囲む空孔を合わせた領域」６２が形成され、いわゆる「フォトニック結晶構造」を利用して光ファイバが形成されているが、「コアとコアを取り囲む空孔を合わせた領域」６２内の空孔の径（ｄ１´、ｄ２´）を実施例１のように取る一方、クラッド部の空孔の径（ｄ３´）をこれらの空孔の径とは異なる大きさとしている点において図２に示す構造と異なっている。
【００３１】
図７は、ｄ１´／Λ＝０．５、ｄ２´／Λ＝１．２５とし、クラッド部の空孔の径ｄ３´をｄ３´／Λ＝０．４７５、０．５、０．５２５、０．５５と変化させた場合の絶対単一動作領域の計算結果を説明するための図である。なお、この計算を実行するに際しても、ファイバの母材は石英ガラス、空孔の中は真空（もしくは空気）としている。
【００３２】
この図において、実線はクラッド部の有効屈折率、点線が第１モードの有効屈折率、そして、１点鎖線が第２モードの有効屈折率である。この図に示した結果から、この光ファイバの絶対単一動作する波長領域は、ｄ３´／Λ＝０．４７５の場合にλ／Λ＝０．３７〜０．５３、ｄ３´／Λ＝０．５の場合にλ／Λ＝０．５８〜０．７２、ｄ３´／Λ＝０．５２５の場合にλ／Λ＝０．６７〜０．８６、そして、ｄ３´／Λ＝０．５５の場合にλ／Λ＝０．８２〜１．０３であることが読み取れる。
【００３３】
ここで示した例では、クラッド部の空孔径ｄ３´を僅か１割程度変化させただけであるが、絶対単一動作領域を倍の波長まで変化させることが可能であり、ｄ３´／Λの値をパラメータとして変化させることで所望の絶対単一動作領域を容易に実現できる。
【００３４】
【発明の効果】
以上説明したように、本発明によれば、光ファイバを構成するクラッド部を、このクラッド部の伸長方向に延在し、かつ、格子間隔Λで周期的に配置された空孔を備えるフォトニック結晶構造で構成し、この空孔を、コア部の中心軸の周りの軸対称性が３回未満となるように配置するとともに、コア部とコア部を近接して取り囲む空孔とが存在する領域に存在可能な複数のモードのうち、コア部を伝搬する光の波長に対する有効屈折率が最大となる第１モードと２番目に大きな第２モードにおいて、第１モードの有効屈折率がクラッド部の有効屈折率よりも大きく、かつ、第２モードの有効屈折率がクラッド部の有効屈折率よりも小さくなるように空孔の格子間隔Λを設定することとしたので、製造が容易で、かつ、設計自由度の高い絶対単一モード光ファイバおよびその設計方法を提供することが可能となる。
【図面の簡単な説明】
【図１】本発明の絶対単一モード光ファイバの第１の構造例を説明するための断面図である。
【図２】図１に示した本発明の絶対単一モード光ファイバのコア部近傍を詳細に説明するための図である。
【図３】図１および図２に示した構造の本発明の絶対単一モード光ファイバの設計方法を説明するための概念図である。
【図４】図１および図２に示した構造の本発明の絶対単一モード光ファイバの設計方法を実際の計算により実行した結果を説明するための図である。
【図５】実際の光ファイバの損失の波長依存性を説明するための図である。
【図６】本発明の絶対単一モード光ファイバの第２の構造例を説明するための断面図である。
【図７】図６に示す構造の本発明の絶対単一モード光ファイバにおいて、クラッド部の空孔の径を変化させた場合の絶対単一動作領域の計算結果を説明するための図である。
【符号の説明】
１１　コア部
１２　クラッド部
１３　光ファイバ材料
２１、６１　コア部
２２、６２　コアとコアを取り囲む空孔を合わせた領域
２３、６３　空孔[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an absolute single mode optical fiber and a design method thereof, and more particularly, to an absolute single mode optical fiber which is easy to manufacture and has a high degree of design freedom, and a design method thereof.
[0002]
[Prior art]
In general, an absolute single mode optical fiber causes light absorption depending on polarization by embedding a light absorbing medium such as a metal in the optical fiber. Therefore, when producing an absolute single mode optical fiber, it is necessary to embed a material different from these base materials into a base material such as glass, plastic, or various crystals, and to convert the material into a fiber.
[0003]
[Problems to be solved by the invention]
However, in order to form a fiber, the material to be embedded must have a melting point similar to that of the base material of the optical fiber, and further, it is required that the material has high affinity with the base material. It is not easy to select an embedding material that satisfies all of these conditions, and an absolute single-mode optical fiber exhibiting practical characteristics has not been manufactured so far.
[0004]
The present invention has been made in view of such a problem, and an object of the present invention is to provide an absolute single mode optical fiber which is easy to manufacture and has a high degree of design freedom, and a design method thereof. It is in.
[0005]
[Means for Solving the Problems]
In order to achieve such an object, the present invention is directed to an absolute single mode optical fiber, wherein a core provided in a central region of an optical fiber preform is formed by a clad. Being surrounded, the cladding has a photonic crystal structure that extends in the direction in which the cladding extends, and that has a refractive index modulation by providing holes periodically arranged at a lattice interval Λ. The holes are arranged such that the axial symmetry around the central axis of the core portion is less than three times, and are present in a region where the core portion and the holes surrounding the core portion in close proximity exist; Among the plurality of possible modes, in the first mode in which the effective refractive index with respect to the wavelength of light propagating through the core part is the maximum and in the second mode in which the effective refractive index is the second largest, the effective refractive index of the first mode is higher than that of the clad part. Greater than the effective refractive index of Ku, and wherein the effective refractive index of the second mode is set to be smaller than the effective refractive index of the cladding portion.
[0006]
According to a second aspect of the present invention, in the absolute single mode optical fiber according to the first aspect, a diameter of at least one hole among the holes surrounding the core portion in close proximity to the cladding portion. It is characterized in that it is larger than the diameter of the arranged holes.
[0007]
According to a third aspect of the present invention, in the absolute single mode optical fiber according to the second aspect, a diameter of a hole surrounding the core portion and a diameter of a hole arranged in the cladding portion are set to be close to each other. Are different from each other.
[0008]
According to a fourth aspect of the present invention, in the absolute single mode optical fiber according to any one of the first to third aspects, the optical fiber preform is glass or plastic.
[0009]
Further, the invention according to claim 5 is the absolute single mode optical fiber according to claim 4, wherein the inside of the hole is a vacuum or a gas transparent to light propagating through the core. It is characterized by being filled with liquid or solid.
[0010]
The invention according to claim 6 is characterized in that the clad surrounding the core of the optical fiber is extended by extending in the direction of extension of the clad, and is provided with holes periodically arranged at the lattice interval Λ. A method for designing an absolute single mode optical fiber having a photonic crystal structure that modulates the rate, wherein the holes are arranged such that the axial symmetry around a central axis of the core portion is less than three times, Among a plurality of modes that can be present in a region where the core portion and a hole surrounding the core portion in close proximity exist, a first mode in which an effective refractive index with respect to a wavelength of light propagating through the core portion is maximum. In a second largest second mode, the effective refractive index of the first mode is larger than the effective refractive index of the cladding portion, and the effective refractive index of the second mode is smaller than the effective refractive index of the cladding portion. To be And sets the serial lattice spacing lambda.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below with reference to the drawings.
[0012]
(Example 1)
FIG. 1 is a cross-sectional view for explaining a first structural example of an absolute single mode optical fiber according to the present invention. This absolute single mode optical fiber has a core portion provided in a central region of an optical fiber material 13. 11 has a structure surrounded by a cladding part 12 having a large number of holes.
[0013]
FIG. 2 is a view for explaining in detail the vicinity of the core portion 11 in the cross-sectional view shown in FIG. 1. A large number of holes 23 present in the cladding portion are periodically arranged at a lattice interval Λ, and And the core 21 and the hole surrounding the core 21 closest to each other form a “region combining the core and the hole surrounding the core” 22. A fiber is formed.
[0014]
The photonic crystal is a crystal having a dielectric periodic structure. In the absolute single mode optical fiber of the present invention shown in FIG. 1 and FIG. 2, holes are formed in a fiber material in a honeycomb shape (a hexagonal close-packed structure). They are periodically arranged to form a refractive index modulation structure. An optical fiber having such a photonic crystal structure can freely design light propagation characteristics depending on the arrangement of holes.
[0015]
Here, among the plurality of modes that can exist in the above-described “region in which the core and the hole surrounding the core are combined”, the mode having the largest propagation constant (or the effective refractive index) with respect to the wavelength of light propagating in the optical fiber. Is defined as a “first mode”, and the next largest mode is defined as a “second mode”.
[0016]
In FIG. 2, for convenience of calculation described later, a region inside the hexagon formed by the core portion 21 and the hole 23 surrounding the core portion 21 closest to the core portion is referred to as “a core and a core. Although the area is defined as the "area including holes" 22, the manner of defining this area is not limited to this.
[0017]
Also, the way of arranging the holes is not limited to that shown in FIG. 2, and it is possible to design freely according to the characteristics of the desired optical fiber. In the case of a photonic crystal having a structure, there is an advantage that the manufacture is extremely easy since the cross-section naturally takes a hexagonal close-packed structure simply by arranging columns having the same outer diameter without gaps.
[0018]
Furthermore, if the position and size of some of the many holes present in the cladding are designed to be different from those of other holes, the symmetry around the central axis of the optical fiber can be achieved. Can be reduced to solve the polarization degeneration, and a polarization maintaining optical fiber can be realized.
[0019]
FIG. 3 is a diagram for explaining a method of designing the absolute single mode optical fiber of the present invention having the structure shown in FIGS. 1 and 2. The horizontal axis represents the wavelength λ of the propagating light divided by the hole lattice spacing Λ. The vertical axis represents the effective refractive index, and the conceptual diagram shows a result obtained by calculating the effective refractive index of the first mode, the second mode, and the mode of the cladding portion for each wavelength. . In this figure, as shown in FIG. 2, the diameter (d2) of two of the six holes surrounding the core portion is made larger than the diameter (d1) of the other holes, and two times. It is assumed that the calculation has been performed for the case having the axial symmetry.
[0020]
Since the fiber of this structure has less than three axial symmetries, the first mode is not degenerated by polarization, and the optical fiber that propagates only this mode is a true single mode optical fiber (absolute single mode optical fiber). Fiber).
[0021]
When the effective refractive index of the first mode and the second mode becomes lower than the effective refractive index of the cladding mode, these modes leak from the core portion to the cladding portion, and as a result, propagate in the optical fiber. You can't do that.
[0022]
Therefore, if the first mode is designed to be able to propagate while the second mode is not allowed to propagate, as is clear from the definitions of the “first mode” and the “second mode”, these Since the modes other than the mode have a lower effective refractive index than the second mode, they cannot propagate in the optical fiber, and can propagate only the first mode.
[0023]
In FIG. 3, the wavelength region in which the optical fiber operates in the absolute single mode is a region indicated by a white arrow in the figure, and the effective refractive index of the first mode is the effective refractive index of the cladding. This corresponds to a wavelength range that is larger than the effective refractive index of the second mode and smaller than the effective refractive index of the cladding.
[0024]
As described above, in order to design an optical fiber for operating light of a specific wavelength in an absolute single mode, the wavelength λ is determined by the value of “wavelength / hole lattice spacing (λ / Λ)” obtained in FIG. What is necessary is just to obtain the lattice spacing Λ obtained by the division.
[0025]
FIG. 4 is a diagram for explaining the result of executing the design method of the absolute single mode optical fiber of the present invention having the structure shown in FIGS. 1 and 2 by actual calculation, and the horizontal axis indicates the wavelength λ of the propagating light. Is divided by the pore lattice spacing Λ, the vertical axis is the effective refractive index (n / n0), and the pore lattice spacing Λ, the pore diameter d1 of the cladding part, and the pore diameter larger than the other pores. The diameter is d2, and it is determined under the conditions of d1 / Λ = 0.38 and d2 / Λ = 1.04. Here, the base material of the fiber is quartz glass and the inside of the hole is vacuum (or air). However, the base material of the fiber may be other glass or plastic, and the inside of the hole is It may be filled with a gas or liquid or solid that is transparent to light propagating in the core.
[0026]
In this figure, the solid line is the effective refractive index of the cladding, the dotted line is the effective refractive index of the first mode, and the dashed line is the effective refractive index of the second mode. From the results shown in this figure, it can be seen that the wavelength region of the optical fiber in which the absolute single operation is performed is λ / Λ = 0.4 to 0.49.
[0027]
FIG. 5 is a diagram for explaining the wavelength dependence of the loss of the actual optical fiber having the structure shown in FIG. 2. The parameters of this optical fiber are as follows: Λ = 2.6 μm, d1 = 1 μm, d2 = 2.7 μm. In this figure, the wavelength at which the loss of each mode begins to increase corresponds to the wavelength at which the effective refractive index of each mode is equal to the effective refractive index of the mode of the cladding.
[0028]
From the results of FIG. 5, it can be seen that the wavelength region of the optical fiber in which the absolute single operation is performed is 1.1 to 1.3 μm, and the absolute single operation region (1.04 μm) obtained from the calculation result shown in FIG. １．２1.27 μm), indicating that the operation is almost as designed.
[0029]
(Example 2)
In the absolute single mode optical fiber of the present invention, it is possible to change the effective refractive index of the cladding to set the absolute single operation region to a desired wavelength.
[0030]
FIG. 6 is a cross-sectional view for explaining a second example of the structure of the absolute single mode optical fiber of the present invention. This absolute single mode optical fiber also exists in the clad portion as shown in FIG. A large number of holes 63 are periodically arranged at a lattice interval Λ to surround the core portion 61, and the core portion 61 and the hole that surrounds the core portion 61 closest to each other make “the core and the hole surrounding the core match. A region 62 is formed, and an optical fiber is formed using a so-called “photonic crystal structure”. The diameter (d1) of the hole in the “region combining the core and the hole surrounding the core” 62 ', D2') are taken as in the first embodiment, while the diameter (d3 ') of the holes in the cladding is different from the diameters of these holes, which is different from the structure shown in FIG. I have.
[0031]
FIG. 7 shows that d1 ′ / Λ = 0.5 and d2 ′ / Λ = 1.25, and the diameter d3 ′ of the holes in the cladding is d3 ′ / Λ = 0.475, 0.5, 0.525, It is a figure for explaining the calculation result of the absolute single operation area at the time of changing to 0.55. When performing this calculation, the base material of the fiber is quartz glass, and the inside of the holes is vacuum (or air).
[0032]
In this figure, the solid line is the effective refractive index of the cladding, the dotted line is the effective refractive index of the first mode, and the dashed line is the effective refractive index of the second mode. From the results shown in this figure, the wavelength region of this optical fiber in which absolute single operation is performed is λ / Λ = 0.37 to 0.53 and d3 ′ / Λ = 0 when d3 ′ / Λ = 0.475. .Lambda ./. Lambda. = 0.58 to 0.72 for 0.5, .lambda ./. Lambda. = 0.67 to 0.86 for d3 '/. Lambda. = 0.525, and d3' /. Lambda. = 0.55 In this case, it can be read that λ / Λ = 0.82 to 1.03.
[0033]
In the example shown here, the hole diameter d3 'of the cladding portion is changed by only about 10%, but the absolute single operation region can be changed up to twice the wavelength, and d3' / Λ By changing the value as a parameter, a desired absolute single operation area can be easily realized.
[0034]
【The invention's effect】
As described above, according to the present invention, a photonic comprising a cladding part constituting an optical fiber, the void extending in a direction in which the cladding part extends, and having holes periodically arranged at lattice intervals Λ. It is composed of a crystal structure, and the holes are arranged so that the axial symmetry around the central axis of the core portion is less than three times, and there are a core portion and a hole surrounding the core portion in close proximity. Among the plurality of modes that can exist in the region, in the first mode in which the effective refractive index with respect to the wavelength of light propagating through the core portion is the largest and in the second mode in which the effective refractive index is the second largest, the effective refractive index of the first mode is the And the lattice spacing 空 of the holes is set so that the effective refractive index of the second mode is smaller than the effective refractive index of the cladding part. Absolute single with high design freedom A mode optical fiber and a method for designing the same can be provided.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating a first structural example of an absolute single mode optical fiber of the present invention.
FIG. 2 is a diagram for explaining in detail the vicinity of a core portion of the absolute single mode optical fiber of the present invention shown in FIG.
FIG. 3 is a conceptual diagram for explaining a method for designing an absolute single mode optical fiber of the present invention having the structure shown in FIGS. 1 and 2;
FIG. 4 is a diagram for explaining the result of executing the method of designing the absolute single mode optical fiber of the present invention having the structure shown in FIGS. 1 and 2 by actual calculation.
FIG. 5 is a diagram for explaining the wavelength dependence of the loss of an actual optical fiber.
FIG. 6 is a cross-sectional view for explaining a second structural example of the absolute single mode optical fiber of the present invention.
FIG. 7 is a diagram for explaining a calculation result of an absolute single operation region when the diameter of a hole in a cladding portion is changed in the absolute single mode optical fiber of the present invention having the structure shown in FIG. 6; .
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 Core part 12 Cladding part 13 Optical fiber material 21, 61 Core part 22, 62 Region 23, 63 where core and hole surrounding core are combined.

Claims (6)

光ファイバ母材の中心領域に設けられたコア部がクラッド部により取り囲まれており、
前記クラッド部は、当該クラッド部の伸長方向に延在し、かつ、格子間隔Λで周期的に配置された空孔を備えることにより屈折率変調するフォトニック結晶構造を有し、
前記空孔は、前記コア部の中心軸の周りの軸対称性が３回未満となるように配置され、
前記コア部と当該コア部を近接して取り囲む空孔とが存在する領域に存在可能な複数のモードのうち、当該コア部を伝搬する光の波長に対する有効屈折率が最大となる第１モードと２番目に大きな第２モードにおいて、当該第１モードの有効屈折率が前記クラッド部の有効屈折率よりも大きく、かつ、当該第２モードの有効屈折率が前記クラッド部の有効屈折率よりも小さくなるように設定されていることを特徴とする絶対単一モード光ファイバ。The core provided in the central region of the optical fiber preform is surrounded by the clad,
The clad portion has a photonic crystal structure that extends in the direction in which the clad portion extends, and that modulates the refractive index by including holes periodically arranged at a lattice interval Λ,
The holes are arranged such that the axial symmetry around the central axis of the core portion is less than three times,
Among a plurality of modes that can be present in a region where the core portion and a hole surrounding the core portion in close proximity exist, a first mode in which an effective refractive index with respect to a wavelength of light propagating through the core portion is maximum. In a second largest second mode, the effective refractive index of the first mode is larger than the effective refractive index of the cladding portion, and the effective refractive index of the second mode is smaller than the effective refractive index of the cladding portion. An absolute single mode optical fiber, wherein the optical fiber is set to be: 前記コア部を近接して取り囲む空孔のうち少なくとも１つの空孔の径が、前記クラッド部に配置された空孔の径よりも大きいことを特徴とする請求項１に記載の絶対単一モード光ファイバ。2. The absolute single mode according to claim 1, wherein a diameter of at least one of the holes surrounding the core portion is larger than a diameter of the hole arranged in the clad portion. Optical fiber. 前記コア部を近接して取り囲む空孔の径と前記クラッド部に配置された空孔の径とが異なることを特徴とする請求項２に記載の絶対単一モード光ファイバ。3. The absolute single mode optical fiber according to claim 2, wherein a diameter of a hole surrounding the core portion and a diameter of a hole arranged in the cladding portion are different from each other. 前記光ファイバ母材は、ガラスまたはプラスチックであることを特徴とする請求項１乃至３の何れかに記載の絶対単一モード光ファイバ。The absolute single mode optical fiber according to any one of claims 1 to 3, wherein the optical fiber preform is glass or plastic. 前記空孔の内部は、真空、もしくは、前記コア部を伝搬する光に対して透明なガスまたは液体もしくは固体が充填されていることを特徴とする請求項４に記載の絶対単一モード光ファイバ。The absolute single mode optical fiber according to claim 4, wherein the inside of the hole is filled with a vacuum or a gas or a liquid or a solid that is transparent to light propagating through the core portion. . 光ファイバのコア部を取り囲むクラッド部が、当該クラッド部の伸長方向に延在し、かつ、格子間隔Λで周期的に配置された空孔を備えることにより屈折率変調するフォトニック結晶構造を有する絶対単一モード光ファイバの設計方法であって、
前記空孔を、前記コア部の中心軸の周りの軸対称性が３回未満となるように配置し、
前記コア部と当該コア部を近接して取り囲む空孔とが存在する領域に存在可能な複数のモードのうち、当該コア部を伝搬する光の波長に対する有効屈折率が最大となる第１モードと２番目に大きな第２モードにおいて、当該第１モードの有効屈折率が前記クラッド部の有効屈折率よりも大きく、かつ、当該第２モードの有効屈折率が前記クラッド部の有効屈折率よりも小さくなるように前記格子間隔Λを設定することを特徴とする絶対単一モード光ファイバの設計方法。The clad part surrounding the core part of the optical fiber has a photonic crystal structure that extends in the direction in which the clad part extends, and that has a refractive index modulation by providing holes periodically arranged with a lattice spacing Λ. An absolute single mode optical fiber design method,
The holes are arranged such that the axial symmetry around the central axis of the core portion is less than three times,
Among a plurality of modes that can be present in a region where the core portion and a hole surrounding the core portion in close proximity exist, a first mode in which an effective refractive index with respect to a wavelength of light propagating through the core portion is maximum. In a second largest second mode, the effective refractive index of the first mode is larger than the effective refractive index of the cladding portion, and the effective refractive index of the second mode is smaller than the effective refractive index of the cladding portion. A method for designing an absolute single mode optical fiber, wherein the lattice spacing Λ is set so as to be as follows.
JP2002241070A 2002-08-21 2002-08-21 Absolute single-mode optical fiber and design method thereof Expired - Fee Related JP3954929B2 (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111812771A (en) * 2020-06-15 2020-10-23 艾菲博(宁波)光电科技有限责任公司 Solid core polarization maintaining high nonlinear photonic crystal fiber and preparation process thereof

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111812771A (en) * 2020-06-15 2020-10-23 艾菲博(宁波)光电科技有限责任公司 Solid core polarization maintaining high nonlinear photonic crystal fiber and preparation process thereof

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