JP3971660B2 - Wavelength conversion element and wavelength conversion device - Google Patents

Description

【０００６】
このような素子に用いられる非線形光学媒質としては種々の材料が研究開発されており、素子構造としては、例えば、Ｍ．Ｈ．ＣｈｏｕらがＯｐｔｉｃｓ Ｌｅｔｔｅｒｓ ｖｏｌ．２３，１００４（１９９８）で報告しているように、ＬｉＮｂＯ_３などの２次非線形光学材料をその非線形定数が周期的に変化するように周期変調した、いわゆる「擬似位相整合型構造」が有望視されている。
【０００７】
図９は、２次非線形光学効果を利用した従来の差周波発生素子を説明するための図で、図９（ａ）はこの差周波発生素子の構成を概念的に説明するための図であり、図９（ｂ）は変換効率の位相不整合量依存性を説明するための図である。２次非線形光学材料中に周期変調構造を形成するためには、この材料の非線形定数の符号を空間的に交互に反転させるか、あるいは、非線形定数の大きな部分と小さな部分とを交互に配置させる方法が考えられる。
【０００８】
ＬｉＮｂＯ_３のような強誘電体結晶では、非線形定数（ｄ定数）の正負は自発分極の極性に対応する。従って、図９（ａ）に示した差周波発生素子では、非線形光学媒質であるＬｉＮｂＯ_３基板９１上にプロトン交換法で光導波路９２を形成し、ＬｉＮｂＯ_３の自発分極を変調周期Λ_０１４．７５μｍで周期的に反転させることで非線形定数に変調を加えている。この差周波発生素子では、０．７８μｍ帯の励起光９５によって１．５５μｍ帯の信号光９３を波長変換することができる。
【０００９】
この素子に用いられる非線形光学媒質のＬｉＮｂＯ_３においては、波長λ_１の信号光９３に対する屈折率がｎ_１、波長λ_２の変換光（差周波光）９４に対する屈折率がｎ_２、波長λ_３の励起光９５に対する屈折率がｎ_３、非線形定数の変調周期をΛ_０として、位相不整合量Δβは、
【００１０】
【数６】

【００１１】
で与えられ、変換効率ηはこの位相不整合量Δβを用いて、
【００１２】
【数７】

【００１３】
で与えられる。ここで、Ｌは非線形光学媒質の導波方向の長さである。すなわち、この差周波発生素子の変換効率ηは、位相不整合量Δβが２π／Λ_０のとき（擬似位相整合条件）に最大値をとる。上式（２）で与えられる位相不整合量Δβが２π／Λ_０となる擬似位相整合条件を満足する励起光９５の波長は非線形光学媒質の屈折率の波長分散に依存し、信号光９３の波長λ_１を固定し変調周期Λ_０が与えられれば実質的に一意に定まることとなる。
【００１４】
励起光９５の波長を、擬似位相整合条件を満足する擬似位相整合波長から変化させると、上式（２）および（３）に従って変換効率ηは減少する。図９（ｂ）は、変換効率ηの位相不整合量Δβ依存性を示す図で、この図では、変換効率ηの最大値が１となるように規格化して示している。この差周波発生素子のＬｉＮｂＯ_３の光導波路９２の長さを４２ｍｍとすると、変換効率ηがその最大値の半分の値となる位相不整合量Δβの帯域は０．７８μｍ帯の励起波長換算で約０．１ｎｍ程度と非常に狭い。
【００１５】
上式（１）から明らかなように、信号光９３の波長λ_１を任意の波長（λ_２´）の差周波光に変換するためには複数の異なる波長の励起光を用いる必要があるが、図９（ａ）に示すような非線形定数が一定周期で変化する従来の変調構造では、励起光の波長に対する許容範囲が狭いために実質的には励起光の波長を変化させることができず、その結果、任意の波長の差周波光へと変換することはできない。
【００１６】
また、異なる励起光波長に対応させるためには、例えば、複数種類の変調周期を有する変調構造を順次長手方向に配列する方法も考えられるが、この方法で非線形光学媒質の全長を一定にすると、１つの変調周期の配列に用いることが可能な非線形光学媒質の長さが短くなってしまう。２次非線形効果を用いる波長変換素子の変換効率ηは、一般に、非線形光学媒質の長さの２乗に比例するため、例えば４種類の変調周期を配列することとすると、同じ長さの非線形光学媒質を用いた場合に比較して、変換効率ηは６．２５％にまで低下してしまう。
【００１７】
複数の励起光波長に対応可能な波長変換素子を構成するために、Ｍ．Ｈ．ＣｈｏｕらがＯｐｔｉｃｓ Ｌｅｔｔｅｒｓ，ｖｏｌ．２４，１１５７（１９９９）で報告するように、周期変調構造に位相反転構造を付与する方法が提案されている。
【００１８】
図１０および図１１は、周期変調構造に位相反転構造を付与することで複数の励起光波長に対応可能とした従来の差周波発生素子を説明するための図で、図１０（ａ）はこの差周波発生素子の構成を概念的に示す平面図、図１０（ｂ）はその一部の拡大図であり、図１１はこの差周波発生素子における位相変調の様子および変換効率の位相不整合量依存性を説明するための図である。
【００１９】
この差周波発生素子は、図９で示した差周波発生素子と同様に、非線形媒質としてＬｉＮｂＯ_３基板１０１にプロトン交換法で光導波路１０２を形成し、ＬｉＮｂＯ_３の自発分極を基本変調周期Λ_０１４．７５μｍで周期的に反転させて非線形定数に変調を加えている。すなわち、この差周波発生素子では、一定の長周期Λ_ｐｈで一定周期Λ_０（基本変調周期１４．７５μｍ）の分極反転構造の位相を１８０°反転させて分極反転構造を形成しており、これにより、複数の位相不整合量Δβにおいて変換効率ηがピークをもつようにすることができる。なお、この差周波発生素子も、０．７８μｍ帯の波長λ_３の励起光１０５によって１．５５μｍ帯の波長λ_１の信号光１０３を波長変換して波長λ_２の差周波光１０４とすることができる。
【００２０】
図１１に示した変換効率の位相不整合量依存性において、図１１（ａ）は、位相反転周期Λ_ｐｈを１４ｍｍ、デューティ比を５０％として位相反転させた分極反転構造の非線形光学媒質中での長手方向への位相変化を示す図であり、図１１（ｂ）は、この差周波発生素子の変換効率を、図９に示した非線形光学媒質と同じ長さではあるが位相反転構造をもたない非線形光学媒質を用いて構成した差周波発生素子の変換効率を1として規格化した場合の位相不整合量依存性を示す図である。なお、この差周波発生素子において分極反転が施された光導波路の長さは４２ｍｍである。
【００２１】
図１１（ｂ）から分るように、変換効率は、位相不整合量が{（２π／Λ_０）−（２π／Λ_ｐｈ）}と{（２π／Λ_０）＋（２π／Λ_ｐｈ）}のときに最大となり、２つの励起波長を用いて波長変換を行うことが可能となる。
【００２２】
また、図１１（ｃ）および図１１（ｄ）に示すように、長周期Λ_ｐｈを７ｍｍとし位相反転のデューティ比を２６．５％とすることで、位相不整合量が、それぞれ、{（２π／Λ_０）−（２π／Λ_ｐｈ）}、（２π／Λ_０）、および、{（２π／Λ_０）＋（２π／Λ_ｐｈ）}のときに変換効率が最大となり、３つの励起波長を用いて波長変換を行うことが可能となる。
【００２３】
さらに、図１１（ｅ）および図１１（ｆ）に示すように、長周期Λ_ｐｈを１４ｍｍと倍にしてさらに位相反転を重畳させることで、位相不整合量が、それぞれ{（２π／Λ_０）−（６π／Λ_ｐｈ）}、{（２π／Λ_０）−（２π／Λ_ｐｈ）}、{（２π／Λ_０）＋（２π／Λ_ｐｈ）}、および、{（２π／Λ_０）＋（６π／Λ_ｐｈ）}のときに変換効率が最大となり、４π／Λ_ｐｈごとに４つのピークが得られるため、４つの励起波長を用いて波長変換を行うことが可能となる。
【００２４】
【発明が解決しようとする課題】
しかしながら、上述した従来の構成の波長変換素子では、以下のような問題があった。
【００２５】
第１に、例えば図１１（ｆ）に示すような４つのピークが得られるようにした構造の規格化変換効率は、必要な波長以外にも変換効率の副次的なピークが生じることとなる結果、素子の変換効率が１７％まで低下してしまうという問題である。
【００２６】
第２に、狭い励起光波長間隔で変換効率のピークを得ようとすると必然的に長い周期反転構造が必要となり、通常用いられる３〜４インチ程度のサイズの基板内に位相反転周期を配置する際に制限が生じてしまうという問題がある。
【００２７】
図１１（ａ）、（ｃ）、（ｅ）の位相整合曲線のそれぞれのピーク間隔は０．７８μｍ帯の励起波長に換算すると０．８ｎｍとなり、これは、励起波長を４００ＧＨｚ間隔で変化できることを意味している。すなわち、式（１）の関係から、励起光波長を変化させると励起光波長の変化分だけ変換光波長も変化するため、変換光波長を４００ＧＨｚ間隔で変化できることとなる。
【００２８】
ＷＤＭ通信への応用を考えると、２００ＧＨｚ、１００ＧＨｚ等の更に短い間隔でピークをもつ素子も必要と考えられる。例えば図１１（ｆ）に示す４つのピークをもたせるようにした構造の規格化変換効率は、Λ_ｐｈの位相反転周期に対して位相不整合が４π／Λ_ｐｈごとにピークをもつため、位相反転周期を長くすることでピーク間隔を短くすることができる。ＬｉＮｂＯ_３光導波路で２００ＧＨｚ間隔や１００ＧＨｚ間隔の励起波長に対応させることを想定すると、４つの励起波長に対応させるために必要な位相反転周期は、各々２８ｍｍおよび５６ｍｍと極めて長いものとなってしまうのである。
【００２９】
第３に、上述したように、位相反転パターンの重畳により１〜４までの励起光波長数に対応する方法は開示されていたものの、それ以外の励起光波長数に対応させる方法は現在まで知られておらず、任意の励起光波長数に柔軟に対応させることが困難であった。
【００３０】
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、任意の励起光波長数に対応した設計が可能で、変換効率の低下がなく、かつ、実用的な大きさの非線形光学媒質を用いて構成可能な波長変換素子および光増幅素子並びにこれらを用いた波長変換装置および光増幅装置を提供することにある。
【００３１】
【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、非線形光学媒質を備え、当該非線形光学媒質に式（Ａ）を満足する３つの波長（λ_１、λ_２、λ_３：但しλ_１＝λ_２の場合を含む）のうちの１つまたは２つの波長の光を入射させ、当該非線形光学媒質内で生じる２次非線形光学効果によって、前記３つの波長のうちの何れかであり、かつ、少なくとも入射光の１つの波長とは異なる波長の出射光に変換する波長変換素子であって、前記非線形光学媒質は、光の進行方向の周期Λ_０ごとに略連続的に位相が変化する非線形定数の周期変調構造と当該周期変調構造の位相変化が周期Λ_ｐｈごとに繰り返される位相変調構造とで構成された位相変化周期変調構造を有し、前記非線形光学媒質内での波長変換に関わる前記３つの波長（λ _１ 、λ _２ 、λ _３ ）の光に対する前記非線形光学媒質の屈折率を各々ｎ _１ 、ｎ _２ およびｎ _３ とする場合に、式（Ｂ）で与えられる位相不整合量Δβが２π／Λ _０ ±２πｉ／Λ _ｐｈ （ｉ＝０，１，…，ｎ：ｎは正の整数）で与えられる複数の変換効率のピークにおける効率の合計が最大となるように前記周期変調構造の周期Λ _０ および前記位相変調構造の周期Λ _ｐｈ ならびに前記位相変化周期変調構造の位相変調曲線を設定したことを特徴とする。
【００３２】
【数１０】

【数１１】

【００３３】
また、請求項２に記載の発明は、非線形光学媒質を備え、当該非線形光学媒質に式（Ａ）を満足する３つの波長（λ _１ 、λ _２ 、λ _３ ：但しλ _１ ＝λ _２ の場合を含む）のうちの１つまたは２つの波長の光を入射させ、当該非線形光学媒質内で生じる２次非線形光学効果によって、前記３つの波長のうちの何れかであり、かつ、少なくとも入射光の１つの波長とは異なる波長の出射光に変換する波長変換素子であって、前記非線形光学媒質は、光の進行方向の周期Λ _０ ごとに略連続的に位相が変化する非線形定数の周期変調構造と当該周期変調構造の位相変化が周期Λ _ｐｈ ごとに繰り返される位相変調構造とで構成された位相変化周期変調構造を有し、前記非線形光学媒質内での波長変換に関わる前記３つの波長（λ_１、λ_２、λ_３）の光に対する前記非線形光学媒質の屈折率を各々ｎ_１、ｎ_２およびｎ_３とする場合に、式（Ｂ）で与えられる位相不整合量Δβが２π／Λ _０ ±２π（２ｉ＋１）／Λ _ｐｈ （ｉ＝０，１，…，ｎ：ｎは正の整数）で与えられる複数の変換効率のピークにおける効率の合計が最大となるように前記周期変調構造の周期Λ_０および前記位相変調構造の周期Λ_ｐｈならびに前記位相変化周期変調構造の位相変調曲線を設定したことを特徴とする。
【００３４】
【数１２】

【数１３】

【００３５】
また、請求項３に記載の発明は、非線形光学媒質を備え、当該非線形光学媒質に式（Ａ）を満足する３つの波長（λ _１ 、λ _２ 、λ _３ ：但しλ _１ ＝λ _２ の場合を含む）のうちの１つまたは２つの波長の光を入射させ、当該非線形光学媒質内で生じる２次非線形光学効果によって、前記３つの波長のうちの何れかであり、かつ、少なくとも入射光の１つの波長とは異なる波長の出射光に変換する波長変換素子であって、前記非線形光学媒質は、光の進行方向の周期Λ _０ ごとに略連続的に位相が変化する非線形定数の周期変調構造と当該周期変調構造の位相変化が周期Λ _ｐｈ ごとに繰り返される位相変調構造とで構成された位相変化周期変調構造を有し、前記非線形光学媒質内での波長変換に関わる前記３つの波長（λ_１、λ_２、λ_３）の光に対する前記非線形光学媒質の屈折率を各々ｎ_１、ｎ_２およびｎ_３とする場合に、式（Ｂ）で与えられる位相不整合量Δβが２π／Λ _０ ＋２πｉ／Λ _ｐｈ （ｉ＝ｍ，ｍ＋１，ｎ：ｍ，ｎは正または負の整数であり、｜ｍ｜≠｜ｎ｜）で与えられる複数の変換効率のピークにおける効率の合計が最大となるように前記周期変調構造の周期Λ_０および前記位相変調構造の周期Λ_ｐｈならびに前記位相変化周期変調構造の位相変調曲線を設定したことを特徴とする。
【００３６】
【数１４】

【数１５】

【００３７】
請求項４に記載の発明は、請求項１乃至３の何れかに記載の波長変換素子であって、式（Ｃ）で与えられる評価関数Ｔが最小になるとき、複数の前記変換効率のピークの効率の合計が最大になることを特徴とする。
【００３８】
【数１６】

（η（ｊ）（ｊ＝１、２、…、Ｌ：Ｌは前記変換効率のピークの数で、正の整数）は各変換効率のピークにおける効率、η _ｎｏｒｍ は光導波路の長さが同じで位相変調をもたない場合の波長変換素子の効率）
【００３９】
さらに、請求項５に記載の発明は、波長変換装置であって、発振波長が可変あるいは複数の発振波長を切替可能な光源と、請求項１乃至４の何れかに記載の波長変換素子とを備え、前記光源から射出される光を前記非線形光学媒質に入射させ、当該光源から射出される光の波長に応じて変換光の波長を切り替えることを特徴とする。
また、請求項６に記載の発明は、非線形光学媒質を備え、当該非線形光学媒質に式（Ａ）を満足する３つの波長（λ _１ 、λ _２ 、λ _３ ：但しλ _１ ＝λ _２ の場合を含む）のうちの１つまたは２つの波長の光を入射させ、当該非線形光学媒質内で生じる２次非線形光学効果によって、前記３つの波長のうちの何れかであり、かつ、少なくとも入射光の１つの波長とは異なる波長の出射光に変換する波長変換素子の製造方法であって、光の進行方向の周期Λ _０ ごとに略連続的に位相が変化する非線形定数の周期変調構造と当該周期変調構造の位相変化が周期Λ _ｐｈ ごとに繰り返される位相変調構造とで構成された位相変化周期変調構造を前記非線形光学媒質に形成し、前記非線形光学媒質内での波長変換に関わる前記３つの波長（λ _１ 、λ _２ 、λ _３ ）の光に対する前記非線形光学媒質の屈折率を各々ｎ _１ 、ｎ _２ およびｎ _３ とする場合に、式（Ｂ）で与えられる位相不整合量Δβが２π／Λ _０ ±２πｉ／Λ _ｐｈ （ｉ＝０，１，…，ｎ：ｎは正の整数）で与えられる複数の変換効率のピークにおける効率の合計が最大となるように前記周期変調構造の周期Λ _０ および前記位相変調構造の周期Λ _ｐｈ ならびに前記位相変化周期変調構造の位相変調曲線を設定することを特徴とする。
【数１７】

【数１８】

【００４０】
【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が、本明細書において用いられている「波長変換素子（および波長変換装置）」という用語は、波長変換素子（および波長変換装置）が光増幅機能をも併せ持つ場合には、単に波長変換素子（および波長変換装置）のみを意味するものではなく、光増幅素子（および光増幅装置）をも意味する。
【００４１】
図１は、本発明の波長変換素子の構成を概念的に説明するための図で、ここでは、分極を反転することにより非線形定数の符号を反転させることのできるＬｉＮｂＯ_３のような強誘電体結晶材料を非線形光学媒質として用いた差周波発生素子を例に説明する。
【００４２】
図１（ａ）に示すように、この差周波発生素子では、非線形光学媒質である非線形材料基板１１上に光導波路１２を形成し、非線形光学媒質の自発分極を周期的に反転させることで非線形定数に変調を加え、波長λ_３の励起光１５によって波長λ_１の信号光１３を波長λ_２の変換光（差周波光）１４に変換している。
【００４３】
この差周波発生素子は、従来の波長変換素子と同様に、非線形媒質の長手方向に非線形定数が周期的に変調されているが、図１（ｂ）に示すように、光導波路１２方向の周期Λ_０ごとに略連続的に位相が変化する非線形定数の周期変調構造とこの周期変調構造の位相変化が周期Λ_ｐｈごとに繰り返される位相変調構造とで構成された位相変化周期変調構造を有し、波長λ_１の信号光１３と波長λ_３の励起光１５とを非線形材料基板１１上に形成した非線形光学媒質の光導波路１２に入射させ、励起光１５により非線形光学媒質内に生じる２次非線形光学効果によって信号光１３とは異なる波長λ_２の変換光１４を生成する。
【００４４】
なお、この図では高い波長変換効率が得られるように、非線形媒質として光の閉じ込めが強く、長い相互作用が得られる光導波路型の構造の素子を示しているが、高パワーのレーザ波長を変換する素子の場合にはバルク型の構造としてもよい。
【００４５】
図２は、図１に示した差周波発生素子の非線形媒質の２次非線形定数の周期変調構造を詳細に説明するための図で、図２（ａ）には、この周期変調構造の一部における非線形定数の長手方向での変化の様子が示されている。この図に示すように、非線形定数の変化を周期Λ_０ごとに区切ると、非線形定数は一定の周期Λ_０で反転（＋１から−１、もしくは、−１から＋１）しているが、周期変調構造がどの位相から始まっているかが、各周期ごと、あるいは、数周期ごとに徐々に変化している。図２（ｂ）は、図２（ａ）に示した周期変調構造の各周期ごとの位相変化を示したものである。
【００４６】
このような位相変調を加えると、例えば図２（ａ）のように長手方向に向かって位相が進んでゆく構造とした場合には、等価的に周期が短くなったのと同様な効果を与える。逆に、長手方向に向かって位相が遅れてゆく構造とした部分は、等価的に周期が長くなったのと同様な効果を与える。このような位相変調を加えた非線形定数の周期変調構造は、図２（ｃ）に示すように、Λ_０より長い周期Λ_ｐｈで繰り返す構造となっている。
【００４７】
本発明の波長変換素子では、従来の素子のように位相を１８０°反転させる方法とは異なり、図２（ｃ）のように、ほぼ連続的に位相が変化する非線形定数の周期変調構造を採用し、この周期変調構造の位相変化が周期Λ_ｐｈで繰り返される位相変調構造で構成された位相変化周期変調構造を導入している。更に、発明者らはこのような長周期繰り返し構造を有する周期変調型の非線形材料に位相変調を施す際の位相変調の方法について鋭意検討を行った結果、位相変調波形の形状（位相変調曲線）を変化させることで変換効率を大幅に損なうことなく任意の励起波長数に対応できることを見出した。
【００４８】
以下に、位相変調曲線の設定方法について説明する。光の伝搬方向軸上の位置ｚにおける非線形定数をｄ（ｚ）とすると、光導波路がｚ＝０からｚ＝Ｌまで存在すると仮定して、光導波路を励起光と信号光が伝搬した後（ｚ＝Ｌ）の変換効率は、位相不整合量Δβに対して次式で与えられる。
【００４９】
【数１２】

【００５０】
この式から非線形定数の空間的な変化ｄ（ｚ）を与え、フーリエ変換を行うことで位相不整合量Δβに対する変換効率の変化を計算することができる。本発明では、非線形定数が周期Λ_０ごとに変調され、かつ、周期Λ_ｐｈで位相が変調されているために、２π／Λ_０を中心に２π／Λ_ｐｈごとに離れた位相不整合量Δβ（Δβ＝２π／Λ_０、２π／Λ_０±２π／Λ_ｐｈ、２π／Λ_０±４π／Λ_ｐｈ、…）において、変換効率のピークをもつようになる。
【００５１】
所望の励起光波長数において最大の効率を得るためには、所望のピークのみが大きくなり、それ以外のピークが小さくなるようにすればよい。例えば、３つの励起波長に対応させる場合は、Δβが２π／Λ_０および２π／Λ_０±２π／Λ_ｐｈの３つのピークが最大となるようにすればよい。また、Ｌ個のピークをもつようにする場合、分極反転構造単位ごとの位相変調曲線を変化させては非線形定数の空間変化ｄ（ｚ）を計算し、フーリエ変換を行って所望の各ピークにおける変換効率を求めて以下に与える評価関数Ｔを計算し、その値が最小になるように逐次計算を行って最適化すればよい。
【００５２】
【数１９】

【００５３】
但し、ここでη（ｊ）はｊ番目のピークにおける効率、η_ｎｏｒｍは長さが同じで位相変調をもたない場合の波長変換素子の効率である。
【００５４】
発明者らは、この方法により、様々な励起波長数の場合において位相変調の設定を検討した結果、本発明によれば、変換効率を損なうことなく任意の励起波長数に対応可能であることを見出した。
【００５５】
図３は、種々の励起波長数に対応可能な本発明の波長変換素子の、位相変調曲線および変換効率の位相不整合量依存性の例を説明するための図である。これらの図において、変換効率は、非線形媒質の長さが同じで位相変調をもたない場合の波長変換素子の変換効率を１として規格化して示している。
【００５６】
例えば、図３（ａ）および図３（ｂ）の各々は、３つの励起光波長に対応させるように構成した波長変換素子の位相変調曲線と変換効率の位相不整合量依存性を示す図であるが、これらの図を図１１（ｃ）および図１１（ｄ）に示した従来の構成の波長変換素子の特性と比較すると、変換効率の位相不整合量依存性の余分な副次的ピークが抑制され、かつ、３０％という従来よりも高い変換効率が得られている。
【００５７】
本発明の波長変換素子では、これらの周期変調構造の基本反転周期Λ_０およびこの繰り返しのための位相変調構造の周期Λ_ｐｈ並びにこれら周期変調構造と位相変調構造とで構成される位相変化周期変調構造の位相変調曲線を上述した方法によって適当に設定して位相整合曲線の形状を変化させることで、任意の励起波長数に容易に対応させることができる。これらの周期Λ_０、Λ_ｐｈおよび位相変調曲線の設定の仕方はその目的に応じて異なるが、何れの方法においても、位相不整合量Δβが周期Λ_０、Λ_ｐｈで決定される特定の値を取るときに変換効率が極大値を取るように位相変調曲線が設定される。
【００５８】
具体的には、第１の方法は、位相不整合量Δβが２π／Λ_０±２πｉ／Λ_ｐｈ（ｉ＝０，１，…，ｎ：ｎは正の整数）において変換効率が極大となるように設定する方法であり、第２の方法は、位相不整合量Δβが２π／Λ_０±２π（２ｉ＋１）／Λ_ｐｈ（ｉ＝０，１，…，ｎ：ｎは正の整数）において変換効率が極大となるように設定する方法であり、そして、第３の方法は、位相不整合量Δβが２π／Λ_０＋２πｉ／Λ_ｐｈ（ｉ＝ｍ，ｍ＋１，…，ｎ：ｍ，ｎは正または負の整数であり、｜ｍ｜≠｜ｎ｜）において変換効率が極大となるように設定する方法である。
【００５９】
例えば、４つの励起光波長に対応可能な波長変換素子を構成するためには、その位相変調曲線を図３（ｃ）に示すようにすることで、図３（ｄ）に示すように、位相不整合量{（２π／Λ_０）−（２π／Λ_ｐｈ）}、（２π／Λ_０）、{（２π／Λ_０）＋（２π／Λ_ｐｈ）}、および、{（２π／Λ_０）＋（４π／Λ_ｐｈ）}のときに変換効率を最大とすることができる。なお、図３（ｂ）は上述した第１の方法においてｎ＝１とした場合に相当し、図３（ｄ）は第３の方法において、ｍ＝−１、ｎ＝２とした場合に相当する。
【００６０】
このように、本発明の波長変換素子によれば、位相不整合量が２π／Λ_ｐｈごとに４つのピークが得られるため、図１１に示した従来の構成の波長変換素子に比較して、同じ位相変調周期で半分の励起光波長間隔が得られる。従って、同じ励起波長間隔を得るためには半分の周期で済むことになる。
【００６１】
非線形媒質としてＬｉＮｂＯ_３にプロトン交換法で形成した光導波路を用いることとし、ＬｉＮｂＯ_３の自発分極を周期的に反転させて非線形定数に変調を加え、図３中の周期Λ_ｐｈを約１４ｍｍあるいは２８ｍｍとした場合を想定すると、位相整合カーブのピークに相当する使用可能な励起波長の間隔はそれぞれ２００ＧＨｚおよび１００ＧＨｚとなり、従来の波長変換素子に比較して半分の繰り返し周期で同じ励起波長間隔を実現でき、その結果、一般的に使用される３〜４インチ径の基板上に容易に配置することが可能となる。
【００６２】
以上説明したように、本発明の波長変換素子の構成では、非線形光学媒質内での光の進行方向に略連続的に位相が変化する周期変調構造が周期Λ_ｐｈで繰り返す位相変調構造を非線形定数の周期反転構造に導入することにより、任意の励起波長数に対応可能で、かつ、変換効率の低下が抑制され、さらに、実用的な大きさの非線形材料を用いて容易に構成可能な励起波長可変型の波長変換素子（およびそれを用いて構成される波長変換装置）を実現することが可能となる。
【００６３】
（実施例１）
図４は、本発明の波長変換素子の第１の実施例の諸特性を説明するための図で、この波長変換素子は、波長０．７８μｍ帯の励起光を入射して波長１．５５μｍ帯の信号光を差周波光へ変換するように構成されている。ここで、図４（ａ）は本実施例に用いた非線形定数の変調構造における位相変調曲線、図４（ｂ）は１．５５μｍ帯の波長可変光源を用いてＳＨＧ特性を評価した規格化変換効率、図４（ｃ）は１．５５μｍ帯における変換光のスペクトルである。
【００６４】
この波長変換装置では、ＬｉＮｂＯ_３のＺ板（Ｚ軸に垂直な面となるように切出された基板）を用い、分極反転部を電界印加法により基本周期１５．５μｍで分極反転させている。このようにして分極反転させた基板にフォトリソグラフィー技術によりＳｉＯ_２をパターンニングし、約１８０度の温度で安息香酸中に浸漬させた後に酸素雰囲気中で熱処理して光導波路を形成した。なお、この波長変換素子は５つの励起波長に対応可能なように構成されている。
【００６５】
この波長変換素子の分極反転部は、位相変調周期Λ_ｐｈを１４．２６ｍｍとし、その全体の長さが５７．０４ｍｍであり、位相変調パターンが４周期分（５７．０４ｍｍ／１４．２６ｍｍ）繰り返されるように構成されており、位相変調周期Λ_ｐｈ当たりに配置される分極反転構造は９２０周期（１４．２６ｍｍ／１５．５μｍ）となる。本実施例においては、この周期１５．５μｍの分極反転構造を２周期ごとを単位として位相変調周期を４６０分割し、それぞれの分極反転構造単位ごとの位相を最適化して５つの励起波長において最大の変換効率が得られるように構成している。
【００６６】
この波長変換素子に備えられた非線形光学媒質には、図４（ａ）に示した非線形定数の変調構造における位相変調曲線のように、１周期の間に位相が０から約１．６πまで滑らかに変化するように位相変調が施されている。
【００６７】
本実施例で用いた分極反転構造を有するＬｉＮｂＯ_３基板は、この基板の＋Ｚ面にレジストを塗布した後にフォトリソグラフィ技術によりパターン化して電極を蒸着し、基板の両側に電解液を接触させて電界を印加することでレジストのない電極が基板に直接触れている部分の分極を反転させるようにして作成される。なお、分極反転するドメインの幅は電極の幅よりも若干広くなるため、その広がりを考慮してフォトリソグラフィに用いるマスクを設計しておく必要がある。本実施例では、計算上理想的な周期位相変調分極反転構造を計算した後に反転ドメイン幅の広がり分だけレジスト幅が広くなるようにマスクを設計している。
【００６８】
図４（ｂ）の横軸は本実施例の波長変換装置に備える波長変換素子から発生する０．７８μｍ帯の第２高調波の波長であり、縦軸の変換効率は５７．０４ｍｍの長さに一定周期１５．５μｍの分極反転構造で同様に作成した素子の変換効率を１として規格化して示しており、上述した第１の方法において、ｎ＝２とした場合に相当する。この図に示した結果から、この波長変換素子に０．７８μｍ帯の励起光を入射して差周波発生させた際の変換効率の波長依存性が評価できる。
【００６９】
この図に示すように、波長７７８．７ｎｍを中心として約０．４ｎｍ間隔で５つのピークが得られており、このことは、励起光波長を２００ＧＨｚ間隔で変化させることができることに相当する。また、一定周期の素子に比較した場合の変換効率は約１８％であり、図１１（ｆ）に示した従来の構成の４波長対応の波長変換素子と比較すると、励起波長数が多いのにも関わらず同等の変換効率が実現されていることがわかる。
【００７０】
図４（ｃ）は、信号光の波長を１５４８．９ｎｍとし、励起光の波長を７７７．９、７７８．３、７７８．７、７７９．１、および、７７９．５ｎｍと約０．４ｎｍ間隔で変化させた場合の１．５５μｍ帯のスペクトルで、この図に示すように、励起光の波長が変化することに伴って変換光の波長を１．６ｎｍ間隔で変化させることが可能である。
【００７１】
（実施例２）
図５は、本発明の波長変換素子の第２の実施例の諸特性を説明するための図で、図５（ａ）はこの波長変換素子の位相変調曲線、図５（ｂ）は１．５５μｍ帯の波長可変光源を用いてＳＨＧ特性を評価した規格化効率、図５（ｃ）は１．５５μｍ帯における変換光のスペクトルである。
【００７２】
この波長変換素子は、第１の実施例が奇数の励起波長に対応するように構成されているのに対して、偶数の励起波長を用いて波長変換を行えるように構成されている。図４からわかるように、周期変調構造の基本周期Λ_０に周期Λ_ｐｈで位相変調を施した第１の実施例の構成では位相不整合量が２π／Λ_０を中心に２π／Λ_ｐｈごとに変換効率のピークが現れる。従って、偶数のピークをもたせるためには、中心ピークを０次ピークとすると、この中心から数えて奇数番目のピークのみを大きくし、０次ピークを含めた偶数番目のピークが小さくなるように位相変調曲線を設定すればよい。
【００７３】
本実施例では、図中に示すように、＋１次、＋３次、−１次、および、−３次の４つのピークが最大となるように構成されている。なお、本実施例では、分極反転の基本周期を１５．５μｍとし、分極反転部の全長を５７．０４ｍｍ、位相変調周期を１４．２６ｍｍとして、位相変調パターンが４周期分繰り返すようにしている。従って、位相変調パターン１周期当たりに配置される分極反転構造は９２０周期となる。本実施例では、この周期１５．５μｍの分極反転構造を２周期を単位として位相変調周期を４６０分割してそれぞれの分極反転構造単位ごとの位相を最適化し、４つの励起波長において最大の変換効率が得られる構造とされ、図５（ａ）に示す位相変調曲線ように、１周期の間に位相が０から約１．６πまで滑らかに変化するように位相変調が施されている。
【００７４】
この波長変換素子からは、図５（ｂ）に示すように、波長７７８．７ｎｍを中心として約０．８ｎｍ間隔で４つのピークが得られ、これは、励起光波長を４００ＧＨｚ間隔で変化させ得ることに相当する。本実施例では、偶数次のピークを間引いた間隔でピークが得られるように位相変調曲線を構成したことに対応して、第１の実施例と同様の位相変調周期を用いたにも関わらず各ピーク間隔が倍となっている。このように、本発明では、位相変調曲線を様々に変更することでピーク数やピーク間隔を自在に変更することができる。
【００７５】
図５（ｂ）の縦軸の変換効率は、５７．０４ｍｍの長さに一定周期１５．５μｍの分極反転を有する構造とした素子の変換効率を１として規格化して示しており、上述した第２の方法において、ｎ＝１とした場合に相当する。この一定周期の分極反転を有する構造の素子に比較した場合の変換効率は約２３％であり、図１１（ｂ）の４波長対応の従来技術と比較して、励起波長数が同じでありながら１．２５倍程度の効率が実現できていることがわかる。
【００７６】
第１の実施例では、励起光として０．７８μｍ帯の光を外部から入射して１．５５μｍ帯の波長変換動作させる例を示したが、この他にも、１．５５μｍ帯の光源を外部励起光として用い、非線形媒質中のＳＨＧにより媒質内部で０．７８μｍ帯の光を発生して励起光として用いるいわゆるカスケード励起を行うことも可能である。
【００７７】
図５（ｃ）は、本発明の波長変換素子をカスケード励起方式で動作確認を行い、信号光の波長を１５４２．７ｎｍとし、励起光の波長を１５５９．８、１５５８．２、１５５６．６、および、１５５５．０ｎｍと約１．６ｎｍ間隔で変化させた場合の１．５５μｍ帯のスペクトルで、この図に示すように、励起光の波長が変化することに伴って変換光の波長を３．２ｎｍ間隔で変化させることが可能である。
【００７８】
（実施例３）
図６は、本発明の波長変換素子の第３の実施例の諸特性を説明するための図で、図６（ａ）はこの波長変換素子の位相変調曲線、図６（ｂ）は１．５５μｍ帯の波長可変光源を用いてＳＨＧ特性を評価した規格化効率、図６（ｃ）は１．５５μｍ帯における変換光のスペクトルである。
【００７９】
この波長変換素子は、第２の実施例が偶数の励起波長に対応するように構成されているのに対して、偶数の励起波長を用いて波長変換を行えるように構成されていることに加え、励起光波長間隔を短くすることができるように構成している。第２の実施例では、偶数次のピークを間引いてピークが得られるように位相変調曲線を構成したが、偶数のピークをもたせるためには０次ピークも含めて０次ピークの周りに非対称にピークが得られるように位相変調曲線を設定する方法もある。本実施例では、図中に示すように、０次、−１次、−２次、および、＋１次の４つのピークが最大となるように構成している。なお、本実施例では、分極反転の基本周期を１５．５μｍとし、分極反転部の全長を５７．０４ｍｍ、位相変調周期を１４．２６ｍｍとして、位相変調パターンが４周期分繰り返すようにしている。従って、位相変調パターン１周期当たりに配置される分極反転構造は９２０周期となる。本実施例では、この周期１５．５μｍの分極反転構造を２周期を単位として位相変調周期を４６０分割してそれぞれの分極反転構造単位ごとの位相を最適化し、４つの励起波長において最大の変換効率が得られる構造とされ、図６（ａ）に示す位相変調曲線ように、１周期の間に位相が−０．１πから約１．１πまでほぼ滑らかに変化するように位相変調が施されている。
【００８０】
この波長変換素子からは、図６（ｂ）に示すように、波長７７８．７ｎｍを中心として約０．４ｎｍ間隔で４つのピークが得られ、これは、励起光波長を２００ＧＨｚ間隔で変化させ得ることに相当する。本実施例では、０次のピークの周りに非対称に４つのピークが得られるように位相変調曲線を構成した結果として、第２の実施例と同様の位相変調周期を用いたにも関わらず各ピーク間隔が倍となっている。このように、本発明では、位相変調曲線を様々に変更することでピーク数やピーク間隔を自在に変更することができる。
【００８１】
図６（ｂ）の縦軸の変換効率は、５７．０４ｍｍの長さに一定周期１５．５μｍの分極反転を有する構造の素子の変換効率を１として規格化して示しており、上述した第３の方法において、ｍ＝−１、ｎ＝２とした場合に相当する。これから分るように、一定周期の素子に比較した場合の本実施例の素子の変換効率は約２３％であり、図１１（ｂ）の４波長対応の従来技術と比較して、励起波長数が同じでありながら１．２５倍程度の効率が実現できていることがわかる。
【００８２】
本実施例では、１．５５μｍ帯の波長の光を信号光とし、励起光の波長をそれぞれ７７８．３、７７８．７、７７９．１、および、７７９．５ｎｍと約０．４ｎｍ間隔で変化させることにより、変換光の波長を１．６ｎｍ間隔で変化させることが可能である。なお、本実施例では２００ＧＨｚ間隔で励起光波長を変化させるために位相変調周期を１４．２６ｎｍとしたが、例えば励起光波長の間隔を半分の１００ＧＨｚにしたい場合には、位相変調周期を倍の２８．５２ｍｍとして同様の手順で装置を構成すればよく、この周期とすれば一般的に使用される３〜４インチ径の基板上に容易に配置することが可能となる。このように、本発明によれば、位相変調関数を工夫することで従来よりも短い周期構造を用いても１００ＧＨｚ等の狭い励起波長間隔に対応可能となる。
【００８３】
本発明の波長変換素子に入射する励起光のパワーが充分に大きな場合には、差周波光を発生するだけではなく、パラメトリック効果により入力光を増幅することも可能である。このことを確認するため、本実施例では、１．５５μｍ帯の光源を外部励起光として用い、非線形媒質中のＳＨＧにより媒質内部で０．７８μｍ帯の光を発生して励起光とするカスケード方式で増幅動作の確認を行った。
【００８４】
励起光としては、繰り返し周波数１００ＭＨｚ、時間幅１００ｐｓの励起光パルスを用い、信号光には繰り返し周波数１００ＭＨｚ、時間幅１０ｐｓのパルスを励起光パルスと時間的に同期させて本実施例の素子に入射させて増幅動作を確認した。
【００８５】
図６（ｃ）は、本実施例の素子をカスケード励起方式で動作確認を行い、信号光の波長を１５４０．０ｎｍとし、励起光の波長を１５５７．４と１５５９．０ｎｍとに変化させた場合の１．５５μｍ帯のスペクトルで、この図に示すように、励起光の波長が変化することに伴って変換光の波長を変化させることが可能であるとともに、励起光を入射させない場合に比較して１２ｄＢ程入射信号が増幅されており、変換光のパワーも信号光と同等のパワーが得られることがわかる。
【００８６】
（実施例４）
図７は、本発明の波長変換素子の第４の実施例の諸特性を説明するための図で、図７（ａ）はこの波長変換素子の位相変調曲線であり、図７（ｂ）は１．５５μｍ帯の波長可変光源を用いてＳＨＧ特性を評価した規格化効率である。
【００８７】
この波長変換素子は、第１〜３の実施例が４または５の励起波長に対応するように構成されているのに対して、更に多くの励起波長を用いて波長変換が可能であり、図中に示すように、奇数次の８つのピークが最大になるように構成されている。
【００８８】
本実施例では、分極反転の基本周期を１５．５μｍとし、分極反転部の全長を５７．０４ｍｍ、位相変調周期を１４．２６ｍｍとして、位相変調パターンが４周期分繰り返すようにしている。従って、位相変調パターン１周期当たりに配置される分極反転構造は９２０周期となる。本実施例では、この周期１５．５μｍの分極反転構造を２周期を単位として位相変調周期を４６０分割してそれぞれの分極反転構造単位ごとの位相を最適化し、４つの励起波長において最大の変換効率が得られる構造とされ、本実施例の波長変換素子には、図７（ａ）に示す位相変調曲線ように、１周期の間に位相が０から約２．７πまでほぼ滑らかに変化するように位相変調が施されている。
【００８９】
この波長変換素子における周期設定は、上述した第２の方法においてｎ＝３とした場合に相当し、図７（ｂ）に示すように、波長７７８．７ｎｍを中心として約０．８ｎｍ間隔で８つのピークが得られ、これは、励起光波長を４００ＧＨｚ間隔で変化させ得ることに相当する。
【００９０】
本実施例では、１．５５μｍ帯の波長の光を信号光とし、励起光の波長をそれぞれ７８０ｎｍ近傍で約０．４ｎｍ間隔で変化させることにより、変換光の波長を３．２ｎｍ間隔で変化させることが可能である。このように、本発明によれば、位相変調関数を工夫することで、励起波長数が非常に多い場合においても、容易に設計・作成を行うことが可能となる。
【００９１】
（実施例５）
図８は、本発明の波長変換素子を備える波長変換装置の構成を説明するための図で、この波長変換装置８０の励起発生部８１には１．５５μｍ帯の異なる波長でそれぞれ発振する５つの半導体レーザが励起光源８２として用いられている。これらの励起光源８２からのレーザ光をアレイ導波路格子からなる合波器８３で合波し、Ｅｒドープの光ファイバ増幅器８４で増幅して励起光を発生させる。信号光８８と励起光は、誘電体多層膜からなる合波器８６により合波され、本発明の波長変換素子８７に入射して変換光８９を射出する。なお、この実施例では、波長変換素子８７として、第１の実施例と同様に５つの励起波長に対応可能な素子を用いている。
【００９２】
この実施例では、１．５５μｍ帯の外部励起光を用いてカスケード励起方式を採用している。この他にも、０．７８μｍ帯の異なる波長でそれぞれ発振する５つの半導体レーザを用意すれば同様の装置が構成できる。その場合、Ｅｒドープの光ファイバ増幅器８４を省略するか半導体レーザ増幅器を用いることとすればよい。本実施例では、励起光源８２として用いられるそれぞれの半導体レーザの波長を１５５５．８、１５５６．６、１５５７．４、１５５８．２、および、１５５９．０ｎｍとして約０．８ｎｍ間隔で用意した。
【００９３】
励起波長制御部８５によりこれらの半導体レーザのどれか１つを選択して発振させることにより、図４（ｃ）に示したのと同様に、変換光の波長を１．６ｎｍ間隔で変化させることができる。また、複数の半導体レーザを同時に発振させることにより１．６ｎｍ間隔ごとに離れた複数の変換光８９を同時に発生させることもできる。
【００９４】
なお、本実施例では、複数の励起光源を用いて励起光発生部８１を構成することとしたが、発振波長が可変な単体の光源、あるいは、複数の波長の切替可能な光源を用いる構成としても同様の装置が構成可能である。
【００９５】
また、本実施例では、非線形材料としてＬｉＮｂＯ_３を用いたが、これに限定されるものではなく、非線形定数の反転もしくは変調が可能な２次非線形材料（例えば、ＬｉＴａＯ_３、ＫＮｂＯ_３、ＫＴａＯ_３、Ｌｉ_ｘＫ_１−ｘＴａ_ｙＮｂ_１−ｙＯ_３、ＫＴＰ等の酸化物結晶、ＡｌＧａＡｓ等の半導体、有機材料など）を用いることとしてもよい。
【００９６】
さらに、本実施例では、本発明の波長変換素子を、光の閉じ込めを強くし、長い相互作用長が得られ、高効率化に有利な非線形媒質として光導波路型の素子構成で説明したが、これに限定されるものではなく、例えば高パワーのレーザ波長を変換するような場合においては、バルク型の素子構成を採用するようにしてもよい。
【００９７】
以上、本発明の波長変換素子を、この波長変換素子に備える非線形光学媒質に信号光と励起光という波長の異なる２種の光を入射させ、これらの光と波長の異なる変換光を出射する差周波発生素子を例として説明したが、入射光または出射光の種類はこれに限定されるものではなく、次式を満足する３つの波長（λ_１、λ_２、λ_３：但しλ_１＝λ_２の場合を含む）のうちの１つまたは２つの波長の光を入射させ、これら３つの波長のうちの何れかであり、かつ、少なくとも入射光の１つの波長とは異なる波長の光に変換するように構成することも可能である。
【００９８】
【数１４】

【００９９】
例えば、波長λ_１、λ_２の光を入射させて和周波である波長λ_３の光を出射させたり、あるいは、λ_１＝λ_２の関係を満足する２つの光を入射させて第２高調波である波長λ_３＝２λ_１の光を出射させるように波長変換素子を構成するようにしてもよい。本発明によれば、その様な構成とした場合にも、入射光の波長を変化させて出射光の波長を変換させることができる。
【０１００】
【発明の効果】
以上説明したように、本発明によれば、波長変換素子に備える非線形光学媒質を、光の進行方向の周期Λ_０ごとに略連続的に変化する非線形定数の周期変調構造とこの周期変調構造の位相変化が周期Λ_ｐｈごとに繰り返される位相変調構造を有するように構成し、位相不整合量Δβが、２π／Λ_０±２πｉ／Λ_ｐｈ（ｉ＝０，１，…，ｎ：ｎは正の整数）、または、２π／Λ_０±２π（２ｉ＋１）／Λ_ｐｈ（ｉ＝０，１，…，ｎ：ｎは正の整数）、もしくは、２π／Λ_０＋２πｉ／Λ_ｐｈ（ｉ＝ｍ，ｍ＋１，…，ｎ：ｍ，ｎは正または負の整数であり、｜ｍ｜≠｜ｎ｜）において変換効率を極大ならしめるようにしたので、任意の励起波長数に対応した設計が可能で、変換効率の低下がなく、かつ、実用的な大きさの非線形材料を用いて構成可能な波長変換素子および光増幅素子並びにこれらを用いた波長変換装置および光増幅装置を提供することが可能となる。
【図面の簡単な説明】
【図１】本発明の波長変換素子の構成を概念的に説明するための図である。
【図２】図１に示した本発明の差周波発生素子が備える非線形媒質の２次非線形定数の周期変調構造を詳細に説明するための図である。
【図３】種々の励起波長数に対応させるために考案した本発明の波長変換素子の、位相変調曲線と変換効率の位相不整合量依存性とを説明するための図である。
【図４】本発明の波長変換素子の第１の実施例の諸特性を説明するための図である。
【図５】本発明の波長変換素子の第２の実施例の諸特性を説明するための図である。
【図６】本発明の波長変換素子の第３の実施例の諸特性を説明するための図である。
【図７】本発明の波長変換素子の第４の実施例の諸特性を説明するための図である。
【図８】本発明の波長変換装置の構成を説明するための図である。
【図９】２次非線形光学効果を利用した従来の差周波発生素子を説明するための図で、（ａ）はこの差周波発生素子の構成を説明するための図であり、（ｂ）は位相不整合量に対する変換効率依存性を説明するための図である。
【図１０】周期変調構造に位相反転構造を付与することで複数の励起光波長に対応可能とした、従来の差周波発生素子の構成を示す図である。
【図１１】周期変調構造に位相反転構造を付与することで複数の励起光波長に対応可能とした、従来の差周波発生素子における変換効率の位相不整合量依存性を説明するための図である。
【符号の説明】
１１、９１、１０１ 非線形材料基板
１２、９２、１０２ 光導波路
１３、９３、１０３ 信号光
１４、９４、１０４ 変換光
１５、９５、１０５ 励起光
８０ 波長変換装置
８１ 励起発生部
８２ 励起光源
８３ アレイ導波路格子からなる合波器
８４ 光ファイバ増幅器
８５ 励起波長制御部
８６ 誘電体多層膜からなる合波器
８７ 波長変換素子
８８ 信号光
８９ 変換光[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength conversion element and a wavelength conversion device. More specifically, the present invention relates to a wavelength conversion element and a wavelength conversion device, and can be designed for any number of excitation wavelengths. The present invention relates to a wavelength conversion element and a wavelength conversion device that can be configured.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, wavelength conversion elements using various second-order nonlinear optical effects and wavelength conversion devices configured using the same have been known. For example, a second harmonic generation device converts incident light into half of its wavelength. The sum frequency generator can convert light of two different wavelengths into light of a frequency corresponding to the sum of the frequencies of these lights. Can do.
[0003]
Further, the difference frequency generator can convert light of two different wavelengths into light having a frequency corresponding to the difference between the frequencies of these lights, and the intensity of one of the incident lights is different from that of the other. If it is sufficiently large compared to the light intensity, it can be configured as an optical amplifying device that amplifies the intensity of incident light by the parametric effect, or a parametric resonator using such a parametric effect can be configured as a wavelength variable light source. Is also possible.
[0004]
Hereinafter, the operation principle of a conventional wavelength conversion element will be briefly described by taking a difference frequency generation element using a second-order nonlinear optical effect as an example. For these elements, the wavelength λ₁Signal light of wavelength λ₃The wavelength λ is incident on a nonlinear optical medium excited by the excitation light of₂Convert to converted light. A relationship given by the following equation holds between these three wavelengths.
[0005]
[Equation 5]

[0006]
Various materials have been researched and developed as nonlinear optical media used in such elements. H. Chou et al., Optics Letters vol. 23, 1004 (1998), as reported in LiNbO.₃A so-called “pseudo phase matching structure” in which a second-order nonlinear optical material such as the above is periodically modulated so that its nonlinear constant periodically changes is promising.
[0007]
FIG. 9 is a diagram for explaining a conventional difference frequency generating element using a second-order nonlinear optical effect, and FIG. 9A is a diagram for conceptually explaining the configuration of the difference frequency generating element. FIG. 9B is a diagram for explaining the dependency of the conversion efficiency on the phase mismatch amount. In order to form a periodic modulation structure in the second-order nonlinear optical material, the sign of the nonlinear constant of this material is alternately inverted spatially, or large portions and small portions of the nonlinear constant are alternately arranged. A method is conceivable.
[0008]
LiNbO₃In such a ferroelectric crystal, the sign of the nonlinear constant (d constant) corresponds to the polarity of the spontaneous polarization. Therefore, in the difference frequency generating element shown in FIG. 9A, LiNbO which is a nonlinear optical medium is used.₃An optical waveguide 92 is formed on the substrate 91 by a proton exchange method, and LiNbO₃The spontaneous polarization of the modulation period Λ₀The nonlinear constant is modulated by periodically inverting at 14.75 μm. In this difference frequency generation element, the wavelength of the signal light 93 in the 1.55 μm band can be converted by the excitation light 95 in the 0.78 μm band.
[0009]
LiNbO as a nonlinear optical medium used in this element₃At wavelength λ₁The refractive index for the signal light 93 is n₁, Wavelength λ₂The refractive index for the converted light (difference frequency light) 94 is n₂, Wavelength λ₃The refractive index for the excitation light 95 is n₃, The modulation period of the nonlinear constant is Λ₀The phase mismatch amount Δβ is
[0010]
[Formula 6]

[0011]
The conversion efficiency η is given by this phase mismatch amount Δβ,
[0012]
[Expression 7]

[0013]
Given in. Here, L is the length of the nonlinear optical medium in the waveguide direction. That is, the conversion efficiency η of this difference frequency generating element is such that the phase mismatch amount Δβ is 2π / Λ.₀(Pseudo phase matching condition) takes the maximum value. The phase mismatch amount Δβ given by the above equation (2) is 2π / Λ.₀The wavelength of the excitation light 95 that satisfies the quasi phase matching condition depends on the wavelength dispersion of the refractive index of the nonlinear optical medium, and the wavelength λ of the signal light 93₁Is fixed and the modulation period Λ₀Is given, it will be determined uniquely.
[0014]
When the wavelength of the excitation light 95 is changed from the quasi phase matching wavelength that satisfies the quasi phase matching condition, the conversion efficiency η decreases according to the above equations (2) and (3). FIG. 9B shows the dependence of the conversion efficiency η on the phase mismatch amount Δβ. In this figure, the conversion efficiency η is normalized so that the maximum value is 1. LiNbO of this difference frequency generator₃If the length of the optical waveguide 92 is 42 mm, the band of the phase mismatch amount Δβ where the conversion efficiency η is half of the maximum value is about 0.1 nm in terms of the excitation wavelength in the 0.78 μm band. narrow.
[0015]
As is apparent from the above equation (1), the wavelength λ of the signal light 93 is₁For any wavelength (λ₂It is necessary to use a plurality of different wavelengths of excitation light in order to convert to the difference frequency light of ′). However, in the conventional modulation structure in which the nonlinear constant as shown in FIG. Since the allowable range with respect to the wavelength of light is narrow, the wavelength of the excitation light cannot be changed substantially, and as a result, it cannot be converted into a difference frequency light of an arbitrary wavelength.
[0016]
Further, in order to correspond to different excitation light wavelengths, for example, a method of sequentially arranging modulation structures having a plurality of types of modulation periods in the longitudinal direction is conceivable, but if the total length of the nonlinear optical medium is made constant by this method, The length of the nonlinear optical medium that can be used for an array of one modulation period is shortened. Since the conversion efficiency η of the wavelength conversion element using the second-order nonlinear effect is generally proportional to the square of the length of the nonlinear optical medium, for example, if four types of modulation periods are arranged, nonlinear optics having the same length Compared to the case where a medium is used, the conversion efficiency η is reduced to 6.25%.
[0017]
In order to construct a wavelength conversion element that can handle a plurality of excitation light wavelengths, H. Chou et al., Optics Letters, vol. 24, 1157 (1999), a method for providing a phase inversion structure to a periodic modulation structure has been proposed.
[0018]
FIG. 10 and FIG. 11 are diagrams for explaining a conventional difference frequency generating element that can cope with a plurality of pumping light wavelengths by adding a phase inversion structure to a periodic modulation structure. FIG. FIG. 10B is an enlarged view of a part of the plan view conceptually showing the configuration of the difference frequency generating element, and FIG. 11 shows the phase modulation state and the amount of phase mismatch of the conversion efficiency in the difference frequency generating element. It is a figure for demonstrating dependence.
[0019]
This difference frequency generating element is similar to the difference frequency generating element shown in FIG. 9 as LiNbO as a nonlinear medium.₃An optical waveguide 102 is formed on the substrate 101 by a proton exchange method, and LiNbO₃Spontaneous modulation of the fundamental modulation period Λ₀The modulation is applied to the nonlinear constant by periodically inverting at 14.75 μm. That is, in this difference frequency generating element, a constant long period Λ_phAnd constant period Λ₀The domain-inverted structure is formed by inverting the phase of the domain-inverted structure (basic modulation period: 14.75 μm) by 180 °, so that the conversion efficiency η has a peak in a plurality of phase mismatch amounts Δβ. be able to. This difference frequency generating element also has a wavelength λ of 0.78 μm band.₃The wavelength λ of 1.55 μm band by the excitation light 105 of₁Wavelength of the signal light 103 is converted to wavelength λ₂The difference frequency light 104 can be obtained.
[0020]
In the dependence of the conversion efficiency shown in FIG. 11 on the amount of phase mismatch, FIG._phIs a diagram showing a phase change in the longitudinal direction in a nonlinear optical medium having a domain-inverted structure in which the phase is inverted with a duty ratio of 50%, and FIG. 11B shows the conversion efficiency of the difference frequency generating element. Is a phase mismatch when the conversion efficiency of a difference frequency generating element configured using a nonlinear optical medium having the same length as that of the nonlinear optical medium shown in FIG. It is a figure which shows quantity dependency. The length of the optical waveguide subjected to polarization inversion in this difference frequency generating element is 42 mm.
[0021]
As can be seen from FIG. 11 (b), the conversion efficiency is such that the phase mismatch amount is {(2π / Λ₀)-(2π / Λ_ph)} And {(2π / Λ₀) + (2π / Λ_ph)}, The wavelength conversion can be performed using two excitation wavelengths.
[0022]
Further, as shown in FIGS. 11 (c) and 11 (d), the long period Λ_phIs 7 mm and the phase inversion duty ratio is 26.5%, so that the amount of phase mismatch becomes {(2π / Λ₀)-(2π / Λ_ph)}, (2π / Λ₀) And {(2π / Λ₀) + (2π / Λ_ph)}, The conversion efficiency is maximized, and wavelength conversion can be performed using three excitation wavelengths.
[0023]
Further, as shown in FIGS. 11 (e) and 11 (f), the long period Λ_phIs multiplied by 14 mm and phase inversion is further superimposed, so that the amount of phase mismatch becomes {(2π / Λ₀)-(6π / Λ_ph)}, {(2π / Λ₀)-(2π / Λ_ph)}, {(2π / Λ₀) + (2π / Λ_ph)} And {(2π / Λ₀) + (6π / Λ_ph)}, The conversion efficiency is maximized and 4π / Λ_phSince four peaks are obtained for each, wavelength conversion can be performed using four excitation wavelengths.
[0024]
[Problems to be solved by the invention]
However, the conventional wavelength conversion element described above has the following problems.
[0025]
First, the normalized conversion efficiency of the structure in which, for example, four peaks as shown in FIG. 11 (f) are obtained, a secondary peak of conversion efficiency occurs in addition to the necessary wavelength. As a result, the conversion efficiency of the element is reduced to 17%.
[0026]
Secondly, in order to obtain the peak of conversion efficiency with a narrow excitation light wavelength interval, a long period inversion structure is inevitably required, and the phase inversion period is arranged in a normally used substrate of about 3 to 4 inches in size. There is a problem that a restriction occurs.
[0027]
Each peak interval of the phase matching curves in FIGS. 11A, 11C, and 11E is 0.8 nm when converted to an excitation wavelength in the 0.78 μm band, which means that the excitation wavelength can be changed at 400 GHz intervals. I mean. That is, from the relationship of the expression (1), when the pumping light wavelength is changed, the converted light wavelength is also changed by the change of the pumping light wavelength, so that the converted light wavelength can be changed at intervals of 400 GHz.
[0028]
Considering application to WDM communication, an element having peaks at shorter intervals such as 200 GHz and 100 GHz is considered necessary. For example, the normalized conversion efficiency of the structure having four peaks shown in FIG._phThe phase mismatch is 4π / Λ with respect to the phase inversion period of_phSince each has a peak, the peak interval can be shortened by lengthening the phase inversion period. LiNbO₃Assuming that the optical waveguide is adapted to the excitation wavelengths at intervals of 200 GHz and 100 GHz, the phase inversion periods necessary to correspond to the four excitation wavelengths are as extremely long as 28 mm and 56 mm, respectively.
[0029]
Thirdly, as described above, although a method corresponding to the number of pumping light wavelengths from 1 to 4 by superposition of the phase inversion pattern has been disclosed, methods for responding to other pumping light wavelength numbers are known to date. Therefore, it is difficult to flexibly cope with an arbitrary number of excitation light wavelengths.
[0030]
The present invention has been made in view of such problems, and the object of the present invention is to design a design corresponding to an arbitrary number of excitation light wavelengths, no reduction in conversion efficiency, and practical use. An object of the present invention is to provide a wavelength conversion element and an optical amplification element that can be configured using a nonlinear optical medium of a large size, and a wavelength conversion apparatus and an optical amplification apparatus that use these.
[0031]
[Means for Solving the Problems]
  In order to achieve such an object, the present invention includes a nonlinear optical medium, and the nonlinear optical medium includes the nonlinear optical medium.Formula (A)3 wavelengths satisfying (λ₁, Λ₂, Λ₃: However, λ₁= Λ₂Of one of the three wavelengths), and is incident on at least one of the three wavelengths by the second-order nonlinear optical effect generated in the nonlinear optical medium. A wavelength conversion element that converts the light into outgoing light having a wavelength different from one wavelength of the light, wherein the nonlinear optical medium has a period Λ in the traveling direction of the light;₀A periodic modulation structure with a nonlinear constant whose phase changes substantially continuously every time and the phase change of the periodic modulation structure is a period Λ_phHaving a phase change period modulation structure composed of a phase modulation structure repeated every time,The three wavelengths (λ) related to wavelength conversion in the nonlinear optical medium ₁ , Λ ₂ , Λ ₃ ) For the refractive index of the nonlinear optical medium ₁ , N ₂ And n ₃ When the phase mismatch amount Δβ given by the equation (B) is 2π / Λ ₀ ± 2πi / Λ _ph (I = 0, 1,..., N: n is a positive integer) The period Λ of the periodic modulation structure is such that the sum of the efficiency at a plurality of conversion efficiency peaks is maximized. ₀ And the period Λ of the phase modulation structure _ph And a phase modulation curve of the phase change period modulation structure is set..
[0032]
[Expression 10]

## EQU11 ##

[0033]
  The invention according to claim 2A nonlinear optical medium is provided, and the nonlinear optical medium has three wavelengths (λ ₁ , Λ ₂ , Λ ₃ : However, λ ₁ = Λ ₂ Of one of the three wavelengths), and is incident on at least one of the three wavelengths by the second-order nonlinear optical effect generated in the nonlinear optical medium. A wavelength conversion element that converts the light into outgoing light having a wavelength different from one wavelength of the light, wherein the nonlinear optical medium has a period Λ in the traveling direction of the light; ₀ A periodic modulation structure with a nonlinear constant whose phase changes substantially continuously every time and the phase change of the periodic modulation structure is a period Λ _ph Having a phase change period modulation structure composed of a phase modulation structure repeated every time,The three wavelengths (λ) related to wavelength conversion in the nonlinear optical medium₁, Λ₂, Λ₃) For the refractive index of the nonlinear optical medium₁, N₂And n₃IfFormula (B)The phase mismatch amount Δβ given by2π / Λ ₀ ± 2π (2i + 1) / Λ _ph (I = 0, 1,..., N: n is a positive integer)The period Λ of the periodic modulation structure to be₀And the period Λ of the phase modulation structure_phIn addition, a phase modulation curve of the phase change period modulation structure is set.
[0034]
[Expression 12]

[Formula 13]

[0035]
  The invention according to claim 3A nonlinear optical medium is provided, and the nonlinear optical medium has three wavelengths (λ ₁ , Λ ₂ , Λ ₃ : However, λ ₁ = Λ ₂ Of one of the three wavelengths), and is incident on at least one of the three wavelengths by the second-order nonlinear optical effect generated in the nonlinear optical medium. A wavelength conversion element that converts the light into outgoing light having a wavelength different from one wavelength of the light, wherein the nonlinear optical medium has a period Λ in the traveling direction of the light; ₀ A periodic modulation structure with a nonlinear constant whose phase changes substantially continuously every time and the phase change of the periodic modulation structure is a period Λ _ph Having a phase change period modulation structure composed of a phase modulation structure repeated every time,The three wavelengths (λ) related to wavelength conversion in the nonlinear optical medium₁, Λ₂, Λ₃) For the refractive index of the nonlinear optical medium₁, N₂And n₃IfFormula (B)The phase mismatch amount Δβ given by2π / Λ ₀ + 2πi / Λ _ph (I = m, m + 1, n: m, n is a positive or negative integer and | m | ≠ | n |) is the maximum sum of the efficiency at the peak of the plurality of conversion efficiencies given byThe period Λ of the periodic modulation structure to be₀And the period Λ of the phase modulation structure_phIn addition, a phase modulation curve of the phase change period modulation structure is set.
[0036]
[Expression 14]

[Expression 15]

[0037]
  The invention according to claim 4 is the wavelength conversion element according to any one of claims 1 to 3, wherein when the evaluation function T given by the formula (C) is minimized, a plurality of peaks of the conversion efficiency are obtained. The total efficiency is maximized.
[0038]
[Expression 16]

(Η (j) (j = 1, 2,..., L: L is the number of peaks of the conversion efficiency, a positive integer) is the efficiency at the peak of each conversion efficiency, η _norm Is the efficiency of the wavelength conversion element when the optical waveguide length is the same and there is no phase modulation)
[0039]
  Furthermore, the invention described in claim 5 is a wavelength conversion device, comprising: a light source whose oscillation wavelength is variable or capable of switching a plurality of oscillation wavelengths; and the wavelength conversion element according to any one of claims 1 to 4. The light emitted from the light source is incident on the nonlinear optical medium, and the wavelength of the converted light is switched according to the wavelength of the light emitted from the light source.
  The invention according to claim 6 includes a nonlinear optical medium, and the nonlinear optical medium has three wavelengths (λ ₁ , Λ ₂ , Λ ₃ : However, λ ₁ = Λ ₂ Of one of the three wavelengths), and is incident on at least one of the three wavelengths by the second-order nonlinear optical effect generated in the nonlinear optical medium. A method of manufacturing a wavelength conversion element for converting into outgoing light having a wavelength different from one wavelength of light, wherein the period Λ in the traveling direction of the light ₀ A periodic modulation structure with a nonlinear constant whose phase changes substantially continuously every time and the phase change of the periodic modulation structure is a period Λ _ph A phase change periodic modulation structure configured with a phase modulation structure repeated every time is formed in the nonlinear optical medium, and the three wavelengths (λ) related to wavelength conversion in the nonlinear optical medium are formed. ₁ , Λ ₂ , Λ ₃ ) For the refractive index of the nonlinear optical medium ₁ , N ₂ And n ₃ When the phase mismatch amount Δβ given by the equation (B) is 2π / Λ ₀ ± 2πi / Λ _ph (I = 0, 1,..., N: n is a positive integer) The period Λ of the periodic modulation structure is such that the sum of the efficiency at a plurality of conversion efficiency peaks is maximized. ₀ And the period Λ of the phase modulation structure _ph And a phase modulation curve of the phase change period modulation structure is set.
[Expression 17]

[Expression 18]

[0040]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. The term “wavelength conversion element (and wavelength conversion device)” used in this specification is a wavelength conversion element (and wavelength conversion device). ) Also has an optical amplification function, it means not only a wavelength conversion element (and a wavelength conversion device) but also an optical amplification element (and an optical amplification device).
[0041]
FIG. 1 is a diagram for conceptually explaining the configuration of the wavelength conversion element of the present invention. Here, LiNbO that can invert the sign of a nonlinear constant by inverting polarization.₃A difference frequency generating element using a ferroelectric crystal material as described above as a nonlinear optical medium will be described as an example.
[0042]
As shown in FIG. 1 (a), in this difference frequency generating element, an optical waveguide 12 is formed on a nonlinear material substrate 11 which is a nonlinear optical medium, and the spontaneous polarization of the nonlinear optical medium is periodically inverted to nonlinearly generate the difference frequency generating element. Modulate the constant, wavelength λ₃Of the wavelength λ by the excitation light 15 of₁Signal light 13 of wavelength λ₂Converted light (difference frequency light) 14.
[0043]
In this difference frequency generation element, the nonlinear constant is periodically modulated in the longitudinal direction of the nonlinear medium as in the conventional wavelength conversion element. However, as shown in FIG. Λ₀A periodic modulation structure with a nonlinear constant whose phase changes substantially every time and the phase change of this periodic modulation structure is the period Λ_phHaving a phase change period modulation structure composed of a phase modulation structure repeated every time, and having a wavelength λ₁Signal light 13 and wavelength λ₃The excitation light 15 is made incident on the optical waveguide 12 of the nonlinear optical medium formed on the nonlinear material substrate 11, and has a wavelength λ different from that of the signal light 13 due to the second-order nonlinear optical effect generated in the nonlinear optical medium by the excitation light 15.₂The converted light 14 is generated.
[0044]
Note that this figure shows an optical waveguide-type element that has strong light confinement and a long interaction as a nonlinear medium so that high wavelength conversion efficiency can be obtained, but it converts high-power laser wavelengths. In the case of an element to be used, a bulk type structure may be used.
[0045]
FIG. 2 is a diagram for explaining in detail the periodic modulation structure of the second-order nonlinear constant of the nonlinear medium of the difference frequency generating element shown in FIG. 1. FIG. 2 (a) shows a part of this periodic modulation structure. The state of the change in the longitudinal direction of the nonlinear constant at is shown. As shown in this figure, the change of the nonlinear constant is represented by the period Λ₀Each non-linear constant has a constant period Λ₀Is inverted (+1 to -1 or -1 to +1), but the phase from which the periodic modulation structure starts is gradually changed every period or every several periods. FIG. 2B shows a phase change for each period of the periodic modulation structure shown in FIG.
[0046]
When such phase modulation is applied, for example, in the case of a structure in which the phase advances in the longitudinal direction as shown in FIG. 2A, an effect similar to that in which the period is equivalently shortened is provided. . On the contrary, the part where the phase is delayed in the longitudinal direction gives the same effect as the period is equivalently longer. As shown in FIG. 2 (c), a periodic modulation structure with a nonlinear constant added with such phase modulation is₀Longer period Λ_phIt has a structure that repeats.
[0047]
The wavelength conversion element of the present invention employs a periodic modulation structure with a nonlinear constant whose phase changes almost continuously as shown in FIG. 2C, unlike the conventional method of inverting the phase by 180 °. The phase change of this periodic modulation structure is the period Λ_phA phase change period modulation structure composed of a phase modulation structure repeated in step (1) is introduced. Furthermore, as a result of intensive studies on a phase modulation method when performing phase modulation on a periodic modulation type nonlinear material having such a long-period repetitive structure, the inventors have determined the shape of a phase modulation waveform (phase modulation curve). It was found that by changing the value, it is possible to cope with an arbitrary number of excitation wavelengths without significantly impairing the conversion efficiency.
[0048]
Hereinafter, a method for setting a phase modulation curve will be described. Assuming that the nonlinear constant at the position z on the light propagation direction axis is d (z), it is assumed that the optical waveguide exists from z = 0 to z = L, and after the excitation light and the signal light propagate through the optical waveguide ( The conversion efficiency of z = L) is given by the following equation with respect to the phase mismatch amount Δβ.
[0049]
[Expression 12]

[0050]
By giving a spatial change d (z) of the nonlinear constant from this equation and performing Fourier transform, the change in conversion efficiency with respect to the phase mismatch amount Δβ can be calculated. In the present invention, the nonlinear constant is the period Λ₀Every period and the period Λ_ph2π / Λ because the phase is modulated by₀Around 2π / Λ_phPhase mismatch amount Δβ (Δβ = 2π / Λ)₀2π / Λ₀± 2π / Λ_ph2π / Λ₀± 4π / Λ_ph,..., Have a peak conversion efficiency.
[0051]
In order to obtain the maximum efficiency at the desired number of excitation light wavelengths, only the desired peak may be increased and the other peaks may be decreased. For example, when it corresponds to three excitation wavelengths, Δβ is 2π / Λ.₀And 2π / Λ₀± 2π / Λ_phThe three peaks may be maximized. In addition, when having L peaks, the phase modulation curve for each domain-inverted structural unit is changed to calculate the non-linear constant spatial change d (z), and Fourier transform is performed to obtain a desired peak at each peak. What is necessary is to calculate the evaluation function T given below by calculating the conversion efficiency, and to perform optimization so as to minimize the value.
[0052]
[Equation 19]

[0053]
Where η (j) is the efficiency at the j-th peak, η_normIs the efficiency of the wavelength conversion element when the length is the same and there is no phase modulation.
[0054]
The inventors have studied the setting of phase modulation in the case of various excitation wavelength numbers by this method, and as a result, according to the present invention, it is possible to cope with any number of excitation wavelength without impairing the conversion efficiency. I found it.
[0055]
FIG. 3 is a diagram for explaining an example of the phase mismatch curve dependency of the phase modulation curve and the conversion efficiency on the phase mismatch amount of the wavelength conversion element of the present invention that can cope with various numbers of excitation wavelengths. In these figures, the conversion efficiencies are normalized by assuming that the conversion efficiencies of the wavelength conversion elements are 1 when the length of the nonlinear medium is the same and no phase modulation is performed.
[0056]
For example, each of FIGS. 3A and 3B is a diagram showing the phase mismatch curve of the phase modulation curve and the conversion efficiency of a wavelength conversion element configured to correspond to three excitation light wavelengths. However, when these figures are compared with the characteristics of the wavelength conversion element having the conventional configuration shown in FIGS. 11 (c) and 11 (d), an extra secondary peak of the phase mismatch amount dependency of the conversion efficiency is obtained. And a higher conversion efficiency of 30% than before is obtained.
[0057]
In the wavelength conversion element of the present invention, the basic inversion period Λ of these periodic modulation structures₀And the period Λ of the phase modulation structure for this iteration_phIn addition, by appropriately setting the phase modulation curve of the phase change periodic modulation structure composed of the period modulation structure and the phase modulation structure by the above-described method and changing the shape of the phase matching curve, the number of excitation wavelengths can be set arbitrarily. It can be easily handled. These periods Λ₀, Λ_phThe method of setting the phase modulation curve differs depending on the purpose, but in any method, the phase mismatch amount Δβ is equal to the period Λ.₀, Λ_phThe phase modulation curve is set so that the conversion efficiency takes a maximum value when taking a specific value determined in (1).
[0058]
Specifically, in the first method, the phase mismatch amount Δβ is 2π / Λ.₀± 2πi / Λ_ph(I = 0, 1,..., N: n is a positive integer) is a method for setting the conversion efficiency to a maximum, and the second method has a phase mismatch amount Δβ of 2π / Λ.₀± 2π (2i + 1) / Λ_ph(I = 0, 1,..., N: n is a positive integer) is a method for setting the conversion efficiency to a maximum, and the third method has a phase mismatch amount Δβ of 2π / Λ.₀+ 2πi / Λ_ph(I = m, m + 1,..., N: m, n is a positive or negative integer and | m | ≠ | n |) is set so that the conversion efficiency becomes maximum.
[0059]
For example, in order to construct a wavelength conversion element that can handle four excitation light wavelengths, the phase modulation curve is as shown in FIG. 3 (c), and as shown in FIG. Mismatch amount {(2π / Λ₀)-(2π / Λ_ph)}, (2π / Λ₀), {(2π / Λ₀) + (2π / Λ_ph)} And {(2π / Λ₀) + (4π / Λ_ph)}, The conversion efficiency can be maximized. FIG. 3B corresponds to the case where n = 1 in the first method described above, and FIG. 3D corresponds to the case where m = −1 and n = 2 in the third method. To do.
[0060]
Thus, according to the wavelength conversion element of the present invention, the phase mismatch amount is 2π / Λ._phSince four peaks are obtained for each, a half excitation light wavelength interval is obtained with the same phase modulation period as compared with the wavelength conversion element having the conventional configuration shown in FIG. Therefore, in order to obtain the same excitation wavelength interval, half the period is sufficient.
[0061]
LiNbO as nonlinear medium₃An optical waveguide formed by a proton exchange method is used for LiNbO.₃3 is periodically inverted to modulate the nonlinear constant, and the period Λ in FIG._phIs assumed to be about 14 mm or 28 mm, the usable excitation wavelength intervals corresponding to the peak of the phase matching curve are 200 GHz and 100 GHz, respectively, and the same excitation with half the repetition period compared to the conventional wavelength conversion element. The wavelength interval can be realized, and as a result, it can be easily arranged on a 3 to 4 inch diameter substrate that is generally used.
[0062]
As described above, in the configuration of the wavelength conversion element of the present invention, the periodic modulation structure whose phase changes substantially continuously in the traveling direction of light in the nonlinear optical medium has the period Λ._phBy introducing a phase modulation structure that repeats in step 1 into a periodic inversion structure with a nonlinear constant, it is possible to cope with an arbitrary number of excitation wavelengths, a reduction in conversion efficiency is suppressed, and a practically sized nonlinear material is used. Therefore, it is possible to realize an excitation wavelength variable type wavelength conversion element (and a wavelength conversion device configured using the same) that can be easily configured.
[0063]
Example 1
FIG. 4 is a diagram for explaining various characteristics of the first embodiment of the wavelength conversion element according to the present invention. This wavelength conversion element receives excitation light having a wavelength of 0.78 μm and receives a wavelength of 1.55 μm. The signal light is converted into difference frequency light. 4A is a phase modulation curve in the nonlinear constant modulation structure used in this embodiment, and FIG. 4B is a normalized conversion in which SHG characteristics are evaluated using a wavelength variable light source in the 1.55 μm band. Efficiency, FIG. 4 (c) is a spectrum of converted light in the 1.55 μm band.
[0064]
In this wavelength converter, LiNbO₃The Z-plate (substrate cut out so as to be a surface perpendicular to the Z-axis) is used, and the domain-inverted portion is domain-inverted at a basic period of 15.5 μm by the electric field application method. The substrate with the polarization inverted in this way is subjected to SiO 2 by photolithography.₂The film was patterned, immersed in benzoic acid at a temperature of about 180 ° C., and then heat-treated in an oxygen atmosphere to form an optical waveguide. In addition, this wavelength conversion element is comprised so that it can respond to five excitation wavelengths.
[0065]
The polarization inversion part of this wavelength conversion element has a phase modulation period Λ_phIs 14.26 mm, the entire length is 57.04 mm, and the phase modulation pattern is configured to repeat four periods (57.04 mm / 14.26 mm), and the phase modulation period Λ_phThe domain-inverted structure disposed at the center has 920 periods (14.26 mm / 15.5 μm). In this example, the phase inversion structure of this 15.5 μm period inversion structure is divided into 460 units every two periods, and the phase of each domain inversion structure unit is optimized to obtain the maximum at five excitation wavelengths. The conversion efficiency is obtained.
[0066]
The nonlinear optical medium provided in this wavelength conversion element has a smooth phase from 0 to about 1.6π during one period as shown in the phase modulation curve in the nonlinear constant modulation structure shown in FIG. Phase modulation is performed so as to change to.
[0067]
LiNbO having a domain-inverted structure used in this example₃The substrate is coated with a resist on the + Z surface of the substrate, patterned by photolithography technique, electrodes are deposited, and an electrode without resist is directly applied to the substrate by applying an electric field by contacting an electrolyte on both sides of the substrate. It is created by reversing the polarization of the touched part. Note that the width of the domain that undergoes polarization inversion is slightly wider than the width of the electrode, and therefore it is necessary to design a mask used for photolithography in consideration of the spread. In this embodiment, the mask is designed so that the resist width is increased by the extent of the inversion domain width after calculating the ideal periodic phase modulation polarization inversion structure.
[0068]
The horizontal axis of FIG. 4B is the wavelength of the second harmonic wave in the 0.78 μm band generated from the wavelength conversion element provided in the wavelength conversion device of the present embodiment, and the vertical conversion efficiency is 57.04 mm long. The conversion efficiency of an element similarly produced with a domain-inverted structure with a constant period of 15.5 μm is standardized as 1, which corresponds to the case where n = 2 in the first method described above. From the results shown in this figure, it is possible to evaluate the wavelength dependence of the conversion efficiency when a difference frequency is generated by making 0.78 μm band excitation light incident on the wavelength conversion element.
[0069]
As shown in this figure, five peaks are obtained at intervals of about 0.4 nm centering on the wavelength of 778.7 nm, which corresponds to the fact that the excitation light wavelength can be changed at intervals of 200 GHz. In addition, the conversion efficiency when compared with an element having a constant period is about 18%, and the number of excitation wavelengths is larger than that of the conventional wavelength conversion element corresponding to four wavelengths shown in FIG. Nevertheless, it can be seen that equivalent conversion efficiency is achieved.
[0070]
In FIG. 4C, the wavelength of the signal light is 1548.9 nm, and the wavelengths of the excitation light are 777.9, 778.3, 778.7, 779.1, and 779.5 nm at intervals of about 0.4 nm. In the 1.55 μm band spectrum when changed, the wavelength of the converted light can be changed at intervals of 1.6 nm as the wavelength of the excitation light changes as shown in this figure.
[0071]
(Example 2)
FIG. 5 is a diagram for explaining various characteristics of the second embodiment of the wavelength conversion element of the present invention. FIG. 5 (a) is a phase modulation curve of this wavelength conversion element, and FIG. Normalization efficiency obtained by evaluating SHG characteristics using a wavelength variable light source in the 55 μm band, FIG. 5C shows a spectrum of converted light in the 1.55 μm band.
[0072]
This wavelength conversion element is configured to perform wavelength conversion using even-numbered excitation wavelengths, whereas the first embodiment is configured to correspond to odd-numbered excitation wavelengths. As can be seen from FIG. 4, the fundamental period Λ of the periodic modulation structure₀Period Λ_phIn the configuration of the first embodiment in which the phase modulation is performed with the phase mismatch amount is 2π / Λ.₀Around 2π / Λ_phEach time a peak in conversion efficiency appears. Therefore, in order to have even-numbered peaks, if the center peak is the 0th order peak, only the odd-numbered peaks counted from this center are increased, and the even-numbered peaks including the 0th-order peak are decreased. What is necessary is just to set a modulation curve.
[0073]
In the present embodiment, as shown in the figure, the four peaks of + 1st order, + 3rd order, −1st order, and −3rd order are maximized. In this embodiment, the basic period of polarization inversion is 15.5 μm, the total length of the polarization inversion part is 57.04 mm, the phase modulation period is 14.26 mm, and the phase modulation pattern is repeated for four periods. Therefore, the domain-inverted structure arranged per phase modulation pattern period is 920 periods. In the present embodiment, the polarization inversion structure with a period of 15.5 μm is divided into 460 phase modulation periods in units of two periods, and the phase for each polarization inversion structure unit is optimized, and the maximum conversion efficiency is obtained at four excitation wavelengths. As shown in FIG. 5A, the phase modulation is performed so that the phase smoothly changes from 0 to about 1.6π during one period.
[0074]
From this wavelength conversion element, as shown in FIG. 5B, four peaks are obtained at intervals of about 0.8 nm centered on a wavelength of 778.7 nm, which can change the excitation light wavelength at intervals of 400 GHz. It corresponds to that. In this embodiment, the phase modulation curve is configured so that peaks are obtained at intervals obtained by thinning out even-order peaks, although the same phase modulation period as that in the first embodiment is used. Each peak interval is doubled. Thus, in the present invention, the number of peaks and the peak interval can be freely changed by changing the phase modulation curve in various ways.
[0075]
The conversion efficiency on the vertical axis in FIG. 5B is normalized by assuming that the conversion efficiency of an element having a structure having a polarization inversion of 15.5 μm with a fixed period of 15.5 μm in length of 57.04 mm. This corresponds to the case of n = 1 in the method of 2. The conversion efficiency when compared with an element having a structure having polarization inversion at a constant period is about 23%, and the number of excitation wavelengths is the same as that of the conventional technology corresponding to the four wavelengths shown in FIG. It can be seen that an efficiency of about 1.25 times can be realized.
[0076]
In the first embodiment, an example in which 0.78 μm band light is incident from the outside as excitation light and the wavelength conversion operation in the 1.55 μm band is performed. It is also possible to perform so-called cascade excitation that is used as pumping light and generates 0.78 μm band light inside the medium by SHG in the nonlinear medium and used as pumping light.
[0077]
FIG. 5 (c) shows the operation of the wavelength conversion element of the present invention by the cascade excitation method, the signal light wavelength is set to 1542.7 nm, and the excitation light wavelengths are 1559.8, 1558.2, 1556.6, In the 1.55 μm band spectrum when it is changed at intervals of about 1.6 nm and 1555.0 nm, as shown in this figure, the wavelength of the converted light is changed according to the change of the wavelength of the excitation light. It is possible to change at intervals of 2 nm.
[0078]
(Example 3)
6A and 6B are diagrams for explaining various characteristics of the third embodiment of the wavelength conversion element of the present invention. FIG. 6A is a phase modulation curve of the wavelength conversion element, and FIG. Normalization efficiency obtained by evaluating SHG characteristics using a wavelength variable light source in the 55 μm band, FIG. 6C shows a spectrum of converted light in the 1.55 μm band.
[0079]
This wavelength conversion element is configured to be able to perform wavelength conversion using an even number of excitation wavelengths, whereas the second embodiment is configured to correspond to an even number of excitation wavelengths. The excitation light wavelength interval can be shortened. In the second embodiment, the phase modulation curve is configured so as to obtain the peak by thinning out the even-order peak. However, in order to have the even-numbered peak, it is asymmetric around the zero-order peak including the zero-order peak. There is also a method of setting a phase modulation curve so that a peak is obtained. In this embodiment, as shown in the figure, the four peaks of 0th order, −1st order, −2nd order, and + 1st order are maximized. In this embodiment, the basic period of polarization inversion is 15.5 μm, the total length of the polarization inversion part is 57.04 mm, the phase modulation period is 14.26 mm, and the phase modulation pattern is repeated for four periods. Therefore, the domain-inverted structure arranged per phase modulation pattern period is 920 periods. In the present embodiment, the polarization inversion structure with a period of 15.5 μm is divided into 460 phase modulation periods in units of two periods, and the phase for each polarization inversion structure unit is optimized, and the maximum conversion efficiency is obtained at four excitation wavelengths. As shown in FIG. 6A, the phase modulation is performed so that the phase changes almost smoothly from −0.1π to about 1.1π during one period. Yes.
[0080]
From this wavelength conversion element, as shown in FIG. 6B, four peaks are obtained at an interval of about 0.4 nm centering on a wavelength of 778.7 nm, which can change the excitation light wavelength at an interval of 200 GHz. It corresponds to that. In this embodiment, as a result of configuring the phase modulation curve so that four peaks are obtained asymmetrically around the zeroth order peak, each phase modulation period similar to that of the second embodiment is used. The peak interval is doubled. Thus, in the present invention, the number of peaks and the peak interval can be freely changed by changing the phase modulation curve in various ways.
[0081]
The conversion efficiency on the vertical axis in FIG. 6B is normalized by assuming that the conversion efficiency of an element having a polarization inversion with a length of 57.04 mm and a constant period of 15.5 μm is 1. This method corresponds to the case where m = −1 and n = 2. As can be seen, the conversion efficiency of the device of this example when compared to a device with a constant period is about 23%, which is the number of excitation wavelengths compared to the conventional technology for four wavelengths shown in FIG. It can be seen that an efficiency of about 1.25 times can be realized with the same.
[0082]
In the present embodiment, light having a wavelength of 1.55 μm is used as signal light, and the wavelengths of the excitation light are changed at intervals of about 0.4 nm to 778.3, 778.7, 779.1, and 779.5 nm, respectively. Thus, the wavelength of the converted light can be changed at intervals of 1.6 nm. In this embodiment, the phase modulation period is 14.26 nm in order to change the excitation light wavelength at 200 GHz intervals. For example, when the excitation light wavelength interval is to be reduced to 100 GHz, the phase modulation period is doubled. The apparatus may be configured in the same procedure as 28.52 mm, and with this cycle, it can be easily arranged on a 3 to 4 inch diameter substrate that is generally used. Thus, according to the present invention, by devising the phase modulation function, it is possible to cope with a narrow excitation wavelength interval such as 100 GHz even if a periodic structure shorter than the conventional one is used.
[0083]
When the power of the excitation light incident on the wavelength conversion element of the present invention is sufficiently large, it is possible not only to generate the difference frequency light but also to amplify the input light by the parametric effect. In order to confirm this, in this embodiment, a 1.55 μm band light source is used as the external pumping light, and 0.78 μm band light is generated inside the medium by SHG in the nonlinear medium and used as the pumping light. The amplification operation was confirmed with
[0084]
As the pumping light, a pumping light pulse having a repetition frequency of 100 MHz and a time width of 100 ps is used. As the signal light, a pulse having a repetition frequency of 100 MHz and a time width of 10 ps is temporally synchronized with the pumping light pulse and is incident on the element of this embodiment. The amplification operation was confirmed.
[0085]
FIG. 6C shows the case where the operation of the device of this embodiment is confirmed by the cascade excitation method, the wavelength of the signal light is 1540.0 nm, and the wavelength of the excitation light is changed to 1557.4 and 1559.0 nm. As shown in this figure, it is possible to change the wavelength of the converted light with the change of the wavelength of the excitation light as compared to the case where the excitation light is not incident. It can be seen that the incident signal is amplified by about 12 dB, and the power of the converted light can be equivalent to that of the signal light.
[0086]
Example 4
FIG. 7 is a diagram for explaining various characteristics of the wavelength conversion element according to the fourth embodiment of the present invention. FIG. 7A is a phase modulation curve of the wavelength conversion element, and FIG. It is the standardization efficiency which evaluated the SHG characteristic using the wavelength variable light source of a 1.55 micrometer band.
[0087]
This wavelength conversion element is configured so that the first to third embodiments correspond to the excitation wavelengths of 4 or 5, whereas wavelength conversion is possible using more excitation wavelengths. As shown in the figure, the odd-numbered eight peaks are configured to be maximum.
[0088]
In this embodiment, the basic period of polarization inversion is 15.5 μm, the total length of the polarization inversion part is 57.04 mm, the phase modulation period is 14.26 mm, and the phase modulation pattern is repeated for four periods. Therefore, the domain-inverted structure arranged per phase modulation pattern period is 920 periods. In the present embodiment, the polarization inversion structure with a period of 15.5 μm is divided into 460 phase modulation periods in units of two periods, and the phase for each polarization inversion structure unit is optimized, and the maximum conversion efficiency is obtained at four excitation wavelengths. In the wavelength conversion element of this example, the phase changes almost smoothly from 0 to about 2.7π during one period as shown in the phase modulation curve shown in FIG. Is phase-modulated.
[0089]
The period setting in this wavelength conversion element corresponds to the case where n = 3 in the second method described above. As shown in FIG. 7B, the period is set to 8 at intervals of about 0.8 nm with a wavelength of 778.7 nm as the center. Two peaks are obtained, which corresponds to the fact that the excitation light wavelength can be changed at 400 GHz intervals.
[0090]
In this embodiment, the wavelength of the converted light is changed at intervals of 3.2 nm by changing the wavelength of the excitation light at intervals of about 0.4 nm in the vicinity of 780 nm by using light having a wavelength of 1.55 μm band as signal light. It is possible. As described above, according to the present invention, it is possible to easily design and create even when the number of excitation wavelengths is very large by devising the phase modulation function.
[0091]
(Example 5)
FIG. 8 is a diagram for explaining the configuration of a wavelength conversion device including the wavelength conversion element of the present invention. The excitation generator 81 of this wavelength conversion device 80 has five oscillations at different wavelengths in the 1.55 μm band. A semiconductor laser is used as the excitation light source 82. The laser light from these pumping light sources 82 is multiplexed by a multiplexer 83 comprising an arrayed waveguide grating, and amplified by an Er-doped optical fiber amplifier 84 to generate pumping light. The signal light 88 and the excitation light are combined by a multiplexer 86 made of a dielectric multilayer film, enter the wavelength conversion element 87 of the present invention, and emit converted light 89. In this embodiment, as the wavelength conversion element 87, an element that can cope with five excitation wavelengths is used as in the first embodiment.
[0092]
In this embodiment, a cascade excitation method is employed using external excitation light in the 1.55 μm band. In addition, a similar device can be configured by preparing five semiconductor lasers that respectively oscillate at different wavelengths in the 0.78 μm band. In that case, the Er-doped optical fiber amplifier 84 may be omitted or a semiconductor laser amplifier may be used. In this example, the wavelengths of the respective semiconductor lasers used as the excitation light source 82 were prepared at intervals of about 0.8 nm with 1555.8, 1556.6, 1557.4, 1558.2, and 1559.0 nm.
[0093]
By selecting one of these semiconductor lasers to oscillate by the excitation wavelength controller 85, the wavelength of the converted light is changed at intervals of 1.6 nm, as shown in FIG. 4C. Can do. It is also possible to simultaneously generate a plurality of converted lights 89 separated at intervals of 1.6 nm by simultaneously oscillating a plurality of semiconductor lasers.
[0094]
In this embodiment, the excitation light generator 81 is configured using a plurality of excitation light sources. However, as a configuration using a single light source whose oscillation wavelength is variable or a light source capable of switching a plurality of wavelengths. A similar apparatus can be configured.
[0095]
In this example, LiNbO is used as the nonlinear material.₃However, the present invention is not limited to this, and a second-order nonlinear material that can invert or modulate a nonlinear constant (for example, LiTaO) is used.₃, KNbO₃, KTaO₃, Li_xK_1-xTa_yNb_1-yO₃, Oxide crystals such as KTP, semiconductors such as AlGaAs, and organic materials).
[0096]
Furthermore, in the present embodiment, the wavelength conversion element of the present invention has been described with an optical waveguide type element configuration as a nonlinear medium that is strong in light confinement, has a long interaction length, and is advantageous for high efficiency. For example, in the case of converting a high-power laser wavelength, a bulk type element configuration may be adopted.
[0097]
As described above, in the wavelength conversion element according to the present invention, the difference in which two types of light having different wavelengths, ie, signal light and excitation light, are incident on the nonlinear optical medium provided in the wavelength conversion element and the converted light having different wavelengths from these lights is emitted Although the frequency generating element has been described as an example, the type of incident light or outgoing light is not limited to this, and three wavelengths (λ₁, Λ₂, Λ₃: However, λ₁= Λ₂1 or two of these wavelengths are incident, and the light is converted into light having a wavelength different from at least one of the three wavelengths and different from at least one wavelength of the incident light. It is also possible to configure as described above.
[0098]
[Expression 14]

[0099]
For example, wavelength λ₁, Λ₂Wavelength λ which is the sum frequency₃Of light or λ₁= Λ₂The wavelength λ which is the second harmonic by making two lights satisfying the relationship₃= 2λ₁The wavelength conversion element may be configured to emit the light. According to the present invention, even in such a configuration, the wavelength of the emitted light can be converted by changing the wavelength of the incident light.
[0100]
【The invention's effect】
As described above, according to the present invention, the nonlinear optical medium provided in the wavelength conversion element is made to have a period Λ in the light traveling direction.₀A periodic modulation structure with a nonlinear constant that changes substantially continuously every time and the phase change of this periodic modulation structure is the period Λ_phAnd having a phase modulation structure repeated every time, the phase mismatch amount Δβ is 2π / Λ₀± 2πi / Λ_ph(I = 0, 1,..., N: n is a positive integer) or 2π / Λ₀± 2π (2i + 1) / Λ_ph(I = 0, 1,..., N: n is a positive integer), or 2π / Λ₀+ 2πi / Λ_ph(I = m, m + 1,..., N: m, n is a positive or negative integer, and conversion efficiency is maximized in | m | ≠ | n |), so it corresponds to an arbitrary number of excitation wavelengths. A wavelength conversion element and an optical amplifying element that can be designed using a non-linear material of a practical size with no reduction in conversion efficiency, and a wavelength conversion apparatus and an optical amplifying apparatus using them It becomes possible to do.
[Brief description of the drawings]
FIG. 1 is a diagram for conceptually explaining the configuration of a wavelength conversion element of the present invention.
FIG. 2 is a diagram for explaining in detail a periodic modulation structure of a second-order nonlinear constant of a nonlinear medium included in the difference frequency generating element of the present invention shown in FIG. 1;
FIG. 3 is a diagram for explaining the phase modulation curve and the dependence of the conversion efficiency on the phase mismatch amount of the wavelength conversion element of the present invention devised to cope with various numbers of excitation wavelengths.
FIG. 4 is a diagram for explaining various characteristics of the first embodiment of the wavelength conversion element of the present invention.
FIG. 5 is a diagram for explaining various characteristics of the second embodiment of the wavelength conversion element of the present invention;
FIG. 6 is a diagram for explaining various characteristics of the third embodiment of the wavelength conversion element of the present invention.
FIG. 7 is a diagram for explaining various characteristics of a wavelength conversion element according to a fourth embodiment of the present invention.
FIG. 8 is a diagram for explaining a configuration of a wavelength conversion device of the present invention.
9A and 9B are diagrams for explaining a conventional difference frequency generating element using a second-order nonlinear optical effect. FIG. 9A is a diagram for explaining the configuration of the difference frequency generating element. FIG. It is a figure for demonstrating the conversion efficiency dependence with respect to the amount of phase mismatching.
FIG. 10 is a diagram showing a configuration of a conventional difference frequency generating element that can cope with a plurality of excitation light wavelengths by adding a phase inversion structure to a periodic modulation structure.
FIG. 11 is a diagram for explaining the dependence of the conversion efficiency on the phase mismatch amount in a conventional difference frequency generating element that can cope with a plurality of pumping light wavelengths by adding a phase inversion structure to the periodic modulation structure; is there.
[Explanation of symbols]
11, 91, 101 Non-linear material substrate
12, 92, 102 Optical waveguide
13, 93, 103 Signal light
14, 94, 104 Conversion light
15, 95, 105 Excitation light
80 wavelength converter
81 Excitation generator
82 Excitation light source
83 A multiplexer comprising an arrayed waveguide grating
84 Optical fiber amplifier
85 Excitation wavelength controller
86 Multiplexer made of dielectric multilayer
87 Wavelength conversion element
88 Signal light
89 Conversion light

Claims (6)

One or two of three wavelengths (λ ₁ , λ ₂ , λ ₃ : including the case of λ ₁ = λ ₂ ) that includes the nonlinear optical medium and satisfies the formula (A) in the nonlinear optical medium A light having a wavelength is incident, and a second-order nonlinear optical effect generated in the nonlinear optical medium causes any one of the three wavelengths to be emitted to a light having a wavelength different from at least one wavelength of the incident light. A wavelength conversion element for conversion,
The nonlinear optical medium includes a periodic modulation structure having a nonlinear constant whose phase changes substantially continuously for each period Λ _{0 in the} light traveling direction , and a phase modulation structure in which the phase change of the periodic modulation structure is repeated for each period Λ _ph. Having a phase change period modulation structure composed of
When the refractive indexes of the nonlinear optical medium with respect to light of the _three wavelengths (λ ₁ , λ ₂ , λ ₃ ) related to wavelength conversion in the nonlinear optical medium are n ₁ , n _2, and n ₃ , respectively. Efficiency at the peak of a plurality of conversion efficiencies where the phase mismatch amount Δβ given by the equation (B) is given by 2π / Λ ₀ ± 2πi / Λ _ph (i = 0, 1,..., N: n is a positive integer) The wavelength conversion element is characterized in that the period Λ ₀ of the period modulation structure, the period Λ _ph of the phase modulation structure, and the phase modulation curve of the phase change period modulation structure are set such that the sum of the phase modulation structure is maximized .

One or two of three wavelengths (λ ₁ , λ ₂ , λ ₃ : including the case of λ ₁ = λ ₂ ) that includes the nonlinear optical medium and satisfies the formula (A) in the nonlinear optical medium A light having a wavelength is incident, and a second-order nonlinear optical effect generated in the nonlinear optical medium causes any one of the three wavelengths to be emitted to a light having a wavelength different from at least one wavelength of the incident light. A wavelength conversion element for conversion,
The nonlinear optical medium includes a periodic modulation structure having a nonlinear constant whose phase changes substantially continuously for each period Λ _{0 in the} light traveling direction , and a phase modulation structure in which the phase change of the periodic modulation structure is repeated for each period Λ _ph. Having a phase change period modulation structure composed of
When the refractive indexes of the nonlinear optical medium with respect to light of the _three wavelengths (λ ₁ , λ ₂ , λ ₃ ) related to wavelength conversion in the nonlinear optical medium are n ₁ , n _2, and n ₃ , respectively. The phase mismatch amount Δβ given by the equation (B) is 2π / Λ ₀ ± 2π (2i + 1) / Λ _ph (i = 0, 1,..., N: n is a positive integer) . The wavelength conversion element, wherein the period Λ ₀ of the period modulation structure, the period Λ _ph of the phase modulation structure, and the phase modulation curve of the phase change period modulation structure are set so that the total efficiency at the peak is maximized .

One or two of three wavelengths (λ ₁ , λ ₂ , λ ₃ : including the case of λ ₁ = λ ₂ ) that includes the nonlinear optical medium and satisfies the formula (A) in the nonlinear optical medium A light having a wavelength is incident, and a second-order nonlinear optical effect generated in the nonlinear optical medium causes any one of the three wavelengths to be emitted to a light having a wavelength different from at least one wavelength of the incident light. A wavelength conversion element for conversion,
The nonlinear optical medium includes a periodic modulation structure having a nonlinear constant whose phase changes substantially continuously for each period Λ _{0 in the} light traveling direction , and a phase modulation structure in which the phase change of the periodic modulation structure is repeated for each period Λ _ph. Having a phase change period modulation structure composed of
When the refractive indexes of the nonlinear optical medium with respect to light of the _three wavelengths (λ ₁ , λ ₂ , λ ₃ ) related to wavelength conversion in the nonlinear optical medium are n ₁ , n _2, and n ₃ , respectively. The phase mismatch amount Δβ given by equation (B) is 2π / Λ ₀ + 2πi / Λ _ph (i = m, m + 1, n: m, n is a positive or negative integer, | m | ≠ | n |) The period Λ ₀ of the period modulation structure, the period Λ _ph of the phase modulation structure, and the phase modulation curve of the phase change period modulation structure are set so that the sum of the efficiency at the peak of the plurality of conversion efficiencies given in FIG . The wavelength conversion element characterized by the above-mentioned.

4. The wavelength conversion element according to claim 1, wherein when the evaluation function T given by the expression (C) is minimized, the sum of the plurality of peak conversion efficiency is maximized. 5. .

(Η (j) (j = 1, 2,..., L: L is the number of peaks of the conversion efficiency, a positive integer) is the efficiency at the peak of each conversion efficiency, η _norm Is the efficiency of the wavelength conversion element when the length of the optical waveguide is the same and there is no phase modulation) A light source capable of changing an oscillation wavelength or switching a plurality of oscillation wavelengths, and the wavelength conversion element according to any one of claims 1 to 4,
A wavelength conversion device, wherein light emitted from the light source is incident on the nonlinear optical medium, and the wavelength of the converted light is switched according to the wavelength of light emitted from the light source. A nonlinear optical medium is provided, and the nonlinear optical medium has three wavelengths (λ ₁ , Λ ₂ , Λ ₃ : However, λ ₁ = Λ ₂ 1) or two of the three wavelengths, and at least incident by the second-order nonlinear optical effect generated in the nonlinear optical medium. A method for manufacturing a wavelength conversion element for converting into outgoing light having a wavelength different from one wavelength of light,
Period Λ of light traveling direction ₀ A periodic modulation structure with a nonlinear constant whose phase changes substantially continuously every time and the phase change of the periodic modulation structure is a period Λ _ph Forming a phase change periodic modulation structure composed of a phase modulation structure repeated every time in the nonlinear optical medium,
The three wavelengths (λ) related to wavelength conversion in the nonlinear optical medium ₁ , Λ ₂ , Λ ₃ ) For the refractive index of the nonlinear optical medium ₁ , N ₂ And n ₃ When the phase mismatch amount Δβ given by the equation (B) is 2π / Λ ₀ ± 2πi / Λ _ph (I = 0, 1,..., N: n is a positive integer) and the period Λ of the periodic modulation structure is such that the sum of the efficiency at the peak of the plurality of conversion efficiencies ₀ And the period Λ of the phase modulation structure _ph And a method of manufacturing a wavelength conversion element, wherein a phase modulation curve of the phase change period modulation structure is set.

Images