JP3907762B2 - Optical wavelength conversion element, short wavelength light generator and optical pickup - Google Patents

Description

【０１３２】
「第２高調波出力」の欄の記号（◎、△、及び×）は、得られる第２高調波出力の大きさ（波長変換効率の大きさ）を相対的に表すものであって、記号◎が変換効率がよいことを意味し、記号×は変換効率が悪いことを意味し、記号△はその中間を意味している。
【０１３３】
図１６（ａ）〜（ｄ）は、上記表１の（２）に示すクラッド層の厚さが約0.1μｍ〜約0.3μｍの領域における光の伝搬状態を示す図である。図１６（ａ）の屈折率分布に示すようにクラッド層１６２の屈折率が導波層１６１の屈折率より低い場合（クラッド層１６２が設けられていない場合も含む）には、図１６（ｂ）に示すように、導波層１６１の内部に基本モードの基本波１６３及び基本モードの第２高調波１６４が存在している。一方、図１６（ｃ）の屈折率分布に示すようにクラッド層１６２の屈折率が導波層１６１の屈折率より大きい場合には、図１６（ｄ）に示すように、基本モードの第２高調波１６４はクラッド層１６２の内部に閉じ込もるが、ＴＭ01モードの第２高調波１６５は光導波層１６１の内部を伝搬して、基本モードの基本波１６３とのオーバラップが大幅に向上する。特に、ＴＭ01モードの第２高調波の電界分布１６５の大部分は光導波層１６１に存在して、非線形性の小さいクラッド層１６２には電界がほとんど存在せず、しかも基本波１６３の閉じ込めが強くなるために、オーバラップの向上による変換効率の大幅な増大が可能となる。ここで、第２高調波に関しては、その波長選択性により所望の導波モードを効率よく励起できるので、上記に述べた内容に従って、高い変換効率をもたらす所望の導波モードの第２高調波を選択的に励起できる。
【０１３４】
また、高次モードの第２高調波の電界分布について、クラッド層に存在する部分と導波層に存在する部分との比は、クラッド層と導波層との屈折率差に依存する。屈折率差が大きい場合は、電界分布のうちで光導波層に存在する部分が大きくなり、クラッド層での電界のピークが小さくなる。一方、屈折率差が小さい場合は、電界分布のうちで光導波層に存在する部分が小さくなり、クラッド層での電界のピークが大きくなる。
【０１３５】
一方、基本波の電界分布に対しては、クラッド層と導波層との屈折率差が小さい方が有利である。すなわち、屈折率差が小さいと、図１４に示す領域Ｉに相当するクラッド層の厚さの範囲が広がり、厚さの制御に関する制約が緩和される。さらに、第２高調波への変換効率は、ＴＭ01モードの第２高調波とＴＭ00モードの基本波との間の結合によって得られる値よりも、ＴＭ02モードの第２高調波とＴＭ00モードの基本波との間の結合によって得られる値の方が、大きくなる。この条件を満足するには、クラッド層と基板との屈折率差の関係が、ΔＮ_2w（波長４００ｎｍ帯の光に対する基板とクラッド層との屈折率差）＞ΔＮ_w（波長８００ｎｍ帯の光に対する基板とクラッド層との屈折率差）なる関係を満たすことが望ましい。先に図１３を参照して説明したように、Nb₂O₅クラッド層は、LiTaO₃基板に対してΔＮ_2w＞ΔＮ_wの関係を満足するので、非常に有効である。
【０１３６】
また、第２高調波が高次モードの場合、出射された第２高調波の集光特性が劣化する場合がある。例えば、ステップ状の屈折率分布を有する埋め込み型光導波路において、１次の第２高調波の導波モードはＴＭ01モードとなる。従って、この第２高調波を集光すると２つのピークを有する集光ビームとなり、単一のビームスポットに集光するにはビーム成形処理が必要となる。しかし、Nb₂O₅装荷型光導波路を伝搬する第２高調波の高次モードは、基本モードとほぼ同様の集光特性を有し、回折限界までの集光が可能である。これは、第２高調波に対する導波層と基板との間の屈折率差（Δ_n1＝約０．０２）がクラッド層との間の屈折率差（Δ_n2＝約０．３）に比べて非常に小さいために、図１６（ｄ）に示すように、クラッド層における導波モードの電界分布が光導波層における電界分布に比べて非常に小さくなるためである。
【０１３７】
第２高調波の集光特性を劣化させない第２高調波の高次モードを実現するには、３Δ_n1＜Δ_n2が必要である。Δ_n2が小さいとクラッド層における電界分布が大きくなり、変換効率の低下及び集光特性の劣化の原因になる。
【０１３８】
次に、光波長変換素子の効率向上を目的としたクラッド層の最適構造を得るための設計について、以下に述べる。
【０１３９】
まず、最適なクラッド層の厚さについて、Nb₂O₅を例にとって検討した。表２は、クラッド層の厚さに対する、基本波及び第２高調波の導波状態、ならびに第２高調波への変換効率を示す。
【０１４０】
【表２】

【０１４１】
「変換効率」の欄の記号（◎、○、△、及び×）は、波長変換効率の大きさを相対的に表すものであって、記号◎が変換効率がよいことを意味し、記号×は変換効率が悪いことを意味し、記号○及び△はその中間を意味している。
【０１４２】
クラッド層の厚さが約１００ｎｍ以下の場合には、変換効率が最大となるのは基本波及び第２高調波が基本モードである場合であって、この場合の変換効率は、埋め込み型光導波路とあまり変わらない。厚さが約１００ｎｍ以上になると、１次（ＴＭ01）モードの第２高調波との結合が増大しはじめて、変換効率が向上する。さらに約２００ｎｍ以上の厚さでは、１次モードの第２高調波への変換が非常に大きくなり、従来の埋め込み型や装荷型光導波路を用いた光波長変換素子の２倍以上の変換効率が得られる。さらに、クラッド層の厚さが約３００ｎｍ近傍であると、２次（ＴＭ02）の第２高調波への変換が行われて、変換効率はさらに向上する。しかし、クラッド層の厚さが約４００ｎｍ近傍或いはそれ以上になると、クラッド層の内部に基本波が閉じ込もるため、変換効率は大幅に減少する。これより、Nb₂O₅クラッド層の厚さとしては、変換効率の向上のためには約１００ｎｍ〜約３８０ｎｍが必要である。特に、変換効率が向上する厚さとして、約２００ｎｍ〜約３４０ｎｍが望ましい。
【０１４３】
図１７には、LiTaO₃基板上のプロトン交換層の上に形成したクラッド層の屈折率（基本波波長に対する値）とクラッド層の厚さとの関係を示す。
【０１４４】
領域Ｉでは、クラッド層が厚すぎるために基本波はクラッド層内に閉じ込り、変換効率が極端に減少する。領域IIでは、基本波と第２高調波との間のオーバラップが向上して、変換効率が最大になる。領域IIIでも、クラッド層の設置によって変換効率の向上が見られる。このように、領域II及びIIIで、クラッド層の設置による変換効率の向上が可能である。
【０１４５】
図１７から分かるように、クラッド層の屈折率が増大するにつれて、領域II或いはIIIを達成するための最大許容厚さは薄くなり、高精度の厚さの設定が要求される。通常の成膜装置でクラッド層を再現性よく成膜するためには、数１００ｎｍ以上の厚さが適当であり、屈折率としては約２．８以下が良い。
【０１４６】
また、クラッド層と導波層との間の屈折率差が小さすぎると、クラッド層によるオーバラップの向上効果が得られない。クラッド層の屈折率が約２．２以上である場合に、基本波と第２高調波との間のオーバラップが大きくなり、変換効率が大幅に向上する。従って、クラッド層の屈折率は、LiTaO₃基板の場合には、基本波の波長に対して約２．１８以上であることが望ましく、特に高効率化のためには約２．２〜約２．８の範囲に設定することが好ましい。
【０１４７】
本実施形態ではクラッド層の構成材料としてNb₂O₅を用いたが、その他に、LiNbO₃、TiO₂、ZnS、CeO₂、またはこれらを含む混合材料、例えばTa₂O₅、SiO₂、Al₂O₃、SiN等との混合材料も、利用できる。これらによって形成されるクラッド層は、その屈折率を変えることで、より大きなオーバラップを得ることができて、変換効率の大幅な向上が可能となる。
【０１４８】
また、本実施形態では、基板としてLiTaO₃基板を用いているが、他にMgO、Nb或いはNdなどをドープしたLiTaO₃やLiNbO₃、或いはそれらの混合物であるLiTa_1-xNb_xO₃（０≦x≦１）、或いはKTP(KTiOPO₄)等からなる基板を用いても、同様な素子が作製できる。LiTaO₃、LiNbO₃、及びKTPは、いずれも高い非線形性を有し、高効率の光波長変換素子が作製できる。特にKTPは、屈折率が約１．８と低いので、その上に形成されるクラッド層の材料としてTa₂O₅、Al₂O₃、SiN等の利用も可能となり、有効である。また、これらの材料においては、周期的分極反転層の形成方法が確立されており、高効率の光波長変換素子が形成できるため、有効である。
【０１４９】
次に、素子表面に形成されるカバー層について説明する。
【０１５０】
先に説明した図９の構成では、光導波路（詳しくは、プロトン交換層１０５及びクラッド層１１０）の表面を、約４００ｎｍの厚さのSiO₂カバー層１１１で覆っている。これは以下の理由による。
【０１５１】
装荷型光導波路では、クラッド層をエッチングにより形成するため、クラッド層の側面に僅かな凹凸が形成されて、導波損失の原因となる。カバー層１１１でクラッド層を被えば、クラッド層とその他の部分との間の屈折率差が小さくなり、クラッド層の側面の凹凸による導波損失の低減を図ることができる。具体的には、導波損失は、カバー層１１１を設けることで約１／２以下になる。
【０１５２】
なお、本実施形態ではカバー層１１１としてSiO₂を用いたが、カバー層１１１としては基板より屈折率の低い材料が望ましく、例えばTa₂O₅等も有効である。Ta₂O₅膜は屈折率が約２と大きいために、より低損失の光導波路が形成できる。或いは、SiN、Al₂O₃等、高屈折率層よりも小さい屈折率を有し、基本波及び高調波に対して吸収や散乱損失の少ない材料であれば、カバー層として同様の効果が得られる。
【０１５３】
（第２の実施形態）
ここでは、本発明の他の構造による光波長変換素子の特性向上について述べる。具体的には、装荷リッジ型光導波路を適用した光波長変換素子について述べる。
【０１５４】
第１の実施形態で、屈折率の高いクラッド層を用いた装荷型光導波路による光波長変換素子の特性向上が確認できた。ここでは、光波長変換素子の構造にさらに工夫を加えることにより、一層の特性向上を試みた結果について述べる。
【０１５５】
装荷型光導波路により基本波のモードプロファイルの制御が可能になることを前述したが、装荷型光導波路において、第２高調波のモードプロファイルへの影響を抑えながら基本波のモードプロファイルを制御し、オーバラップの向上を図ると、第２高調波の導波モードの幅方向の閉じ込めが弱くなる。その結果、光導波路から出力する第２高調波のビームプロファイルのアスペクト比（出射光をコリメートビームにした場合の縦と横の比）が１：３以上になり、厚み方向の広がり角が幅方向に比べて大きくなる。このため、回折限界の集光特性を得るための光の利用効率が大幅に減少するといった問題が生じる。これを解決するため、本実施形態では、図１８に示す光波長変換素子の構造をとる。
【０１５６】
具体的には、図１８の（ａ）は、光波長変換素子の斜視図であり、（ｂ）は断面図である。本実施形態の光波長変換素子では、Ｃ板のLiTaO₃結晶（すなわち、その上面及び底面が、結晶のＣ軸に垂直な面である）からなる基板２０１の内部に、周期Λ＝約３．５μｍ（周期Λは、図１８では参照番号２０８として示されている）の分極反転層２０４が形成されている。分極反転層２０４の幅Ｗは、約１．７μｍである（幅Ｗは、図１８では参照番号２０９として示されている）。さらに、LiTaO₃結晶基板２０１の表面には、ストライプ状のリッジ２１３を有するプロトン交換層２０５が形成され、リッジ２１３の上面には、Nb₂O₅クラッド層（装荷層）２１０が形成されている。さらに、Nb₂O₅クラッド層２１０を含む結晶基板２０１の表面は、SiO₂からなるカバー層２１１で被われている。リッジ２１３の厚みは約２００ｎｍであり、クラッド層２１０の厚さは約３００ｎｍである。
【０１５７】
導波路を伝搬する波長８５０ｎｍの基本波２０６は基本モード（ＴＭ00モード）で伝搬しており、第２高調波２０７は、深さ方向には１次モード、幅方向には０次の高次モード（ＴＭ01モード）で伝搬していると考えられる。第１の実施形態で説明した装荷型光導波路では、第２高調波１０７がＴＭ01モードで伝搬しているため、横方向の閉じ込めが弱く、導波モードのアスペクト比が大きくなっている。これに対して、本実施形態の構成では、リッジ２１３を付けることで横方向の閉じ込めが改善され、第２高調波の導波モードのアスペクト比が改善される。
【０１５８】
次に、装荷リッジ型光導波路の特性について述べる。
【０１５９】
装荷リッジ型光導波路では、プロトン交換層２０５をリッジ状に加工することにより、第２高調波の幅方向の閉じ込めが強化される。その結果、横方向の第２高調波の導波モードの広がりを、Nb₂O₅装荷型光導波路の２／３〜１／２に抑えることが可能となり、横方向の閉じ込めが強化される。そのため、横方向の基本波と第２高調波とのオーバラップが増大し、装荷型光導波路に比べて変換効率が１．５倍以上に向上する。このように、装荷リッジ構造が光波長変換素子の高効率化に有効であることが明らかになった。
【０１６０】
幅方向の閉じ込めを強くするリッジ２１３の厚さとしては、約１００ｎｍ以上が必要である。また、幅方向のオーバラップの増大による光波長変換素子の効率向上が見られたのは、リッジ２１３の厚さが約２００ｎｍ〜約６００ｎｍの場合である。リッジ２１３の厚さが約１０００ｎｍ以上になると、光導波路の伝搬損失が増大して光波長変換素子の特性が劣化する。
【０１６１】
従って、リッジ２１３の厚みとしては、オーバラップ向上を得るために約１００ｎｍ〜約１０００ｎｍが必要となる。さらに、変換効率の大幅な向上を得るには、約２００ｎｍ〜約６００ｎｍが望ましい。
【０１６２】
次に、リッジ２１３の厚さが約３００ｎｍの光導波路における、光導波路の幅に対する第２高調波のアスペクト比の関係を説明する。
【０１６３】
まず、基本波がマルチモード化することなく導波するには、導波路の幅を約１０μｍ以下にする必要がある。さらに、アスペクト比の改善が可能となるのは、導波路幅が約８μｍ以下の場合である。また、アスペクト比が１：１に近づくのは、導波路幅が約２μｍ〜約４μｍの場合であり、導波路幅が約１μｍ以下の場合は、変換効率が低下する。
【０１６４】
光導波路構造を装荷リッジ構造にすることにより、装荷型光導波路を用いた光波長変換素子の特性に加えて出力ビームの成形が可能となり、特性の優れた光波長変換素子の作製が可能となる。単一モード光導波路として成り立つためには、光導波路幅は約１μｍ〜約１０μｍであるのが望ましい。さらにアスペクト比を改善するには、光導波路幅約１μｍ〜約８μｍであるのが望ましく、アスペクト比を１：１に近づけるためには、光導波路幅を約２μｍ〜約４μｍに制御する必要がある。
【０１６５】
本実施形態ではクラッド層の構成材料としてNb₂O₅を用いたが、その他に、LiNbO₃、TiO₂、ZnS、CeO₂、またはこれらを含む混合材料、例えばTa₂O₅、SiO₂、Al₂O₃、SiN等との混合材料も、利用できる。これらによって形成されるクラッド層は、その屈折率を変えることで、より大きなオーバラップを得ることができて、変換効率の大幅な向上が可能となる。
【０１６６】
また、本実施形態では、基板としてLiTaO₃基板を用いているが、他にMgO、Nb或いはNdなどをドープしたLiTaO₃やLiNbO₃、或いはそれらの混合物であるLiTa_1-xNb_xO₃（０≦x≦１）、或いはKTP等からなる基板を用いても、同様な素子が作製できる。LiTaO₃、LiNbO₃、及びKTPは、いずれも高い非線形性を有し、高効率の光波長変換素子が作製できる。特にKTPは、屈折率が約１．８と低いので、その上に形成されるクラッド層の材料としてTa₂O₅、Al₂O₃、SiN等の利用も可能となり、有効である。また、これらの材料においては、周期的分極反転層の形成方法が確立されており、高効率の光波長変換素子が形成できるため、有効である。
【０１６７】
（第３の実施形態）
Nb₂O₅クラッド層によるオーバラップの向上効果を埋め込み型光導波路に適用することにより、簡便な構造による変換効率の向上が図れる。図１９は、埋め込み型光導波路を用いた本実施形態の光波長変換素子の構成を示す。
【０１６８】
具体的には、図１９は、光波長変換素子の斜視図である。本実施形態の光波長変換素子では、Ｃ板のLiTaO₃結晶（すなわち、その上面及び底面が、結晶のＣ軸に垂直な面である）からなる基板３０１の内部に、周期Λ＝約３．５μｍ（周期Λは、図１９では参照番号３０８として示されている）の分極反転層３０４が形成されている。分極反転層３０４の幅Ｗは、約１．７μｍである（幅Ｗは、図１９では参照番号３０９として示されている）。さらに、LiTaO₃結晶基板３０１の表面には、ストライプ状のプロトン交換層３０５が形成されている。さらに、プロトン交換層３０５を含む結晶基板３０１の表面には、基板３０１よりも高い屈折率を有するNb₂O₅クラッド層３１０が形成されている。さらに、クラッド層３１０を含む結晶基板３０１の表面は、SiO₂からなるカバー層（不図示）で被われている。クラッド層３１０の厚さは、約３００ｎｍである。
【０１６９】
導波路を伝搬する波長８５０ｎｍの基本波３０６は基本モード（ＴＭ00モード）で伝搬しており、第２高調波３０７は、深さ方向には１次モード、幅方向には０次の高次モード（ＴＭ01モード）で伝搬している。分極反転層３０４の周期３０８に対する基本波３０６の波長を選択することによって、第２高調波３０７の導波モード（ＴＭ01モード）を選択的に励振している。また、従来の構成とは異なって、基本波３０６の導波モードを基本モードとし、第２高調波３０７の導波モードを高次モードとすることで、両者の間のオーバラップを増大させて、変換効率の向上を図っている。
【０１７０】
埋め込み型光導波路の上にNb₂O₅クラッド層３１０を約３００ｎｍ堆積すると、基本波３０６の導波モードの閉じ込めが向上し、第２高調波３０７と基本波３０６のオーバラップが増大する。その結果、光波長変換素子の変換効率が約３倍に向上し、高効率化が達成される。具体的には、クラッド層３１０の厚さが約１００ｎｍ程度から、基本波３０６と第２高調波３０７とのオーバラップが増大し始め、厚さ約３００ｎｍで最大となる。ところが、厚さが約４００ｎｍで、急激に変換効率が低下する。これは、装荷型光導波路で得られたクラッド層の厚さと第２高調波への変換効率との関係と、ほぼ同じである。
【０１７１】
埋め込み型光導波路は、装荷型光導波路に比べて横方向の光の閉じ込めが強く、第２高調波の出射ビームのアスペクト比は１：２．５程度であり、比較的良好なビーム形状が得られる。
【０１７２】
埋め込み型光導波路の利点は、伝搬損失の小さな光導波路形成が可能なことである。リッジ形状の光導波路は、導波路側面の凹凸により、伝搬損失が増大する傾向にある。本願発明者らにより作製したリッジ型光導波路でも、約1.5dB/cmの伝搬損失が観測されたのに対し、埋め込み型の光導波路では伝搬損失が1dB/cm以下であって、非常に低損失の光導波路の形成が可能であることが確認された。
【０１７３】
さらに、横方向の閉じ込めを向上させる構造として、クラッド層を光導波路上に選択的に堆積させる方法を試みた。具体的には、幅約５μｍの光導波路に対してクラッド層の幅を５μｍ以下にすることにより、幅方向の閉じ込めが強化され、変換効率が１．２倍に向上した。基本波波長に対するクラッド層の屈折率を約２．１８以上にすれば、オーバラップの向上による変換効率が可能である。特に、屈折率が約２．２〜約２．５の範囲では、オーバラップの向上が高まり、大幅な変換効率の向上が可能である。
【０１７４】
本実施形態ではクラッド層の構成材料としてNb₂O₅を用いたが、その他に、LiNbO₃、TiO₂、ZnS、CeO₂、またはこれらを含む混合材料、例えばTa₂O₅、SiO₂、Al₂O₃、SiN等との混合材料も、利用できる。これらによって形成されるクラッド層は、その屈折率を変えることで、より大きなオーバラップを得ることができて、変換効率の大幅な向上が可能となる。
【０１７５】
また、本実施形態では、基板としてLiTaO₃基板を用いているが、他にMgO、Nb或いはNdなどをドープしたLiTaO₃やLiNbO₃、或いはそれらの混合物であるLiTa_1-xNb_xO₃（０≦x≦１）、或いはKTP等からなる基板を用いても、同様な素子が作製できる。LiTaO₃、LiNbO₃、及びKTPは、いずれも高い非線形性を有し、高効率の光波長変換素子が作製できる。特にKTPは、屈折率が約１．８と低いので、その上に形成されるクラッド層の材料としてTa₂O₅、Al₂O₃、SiN等の利用も可能となり、有効である。また、これらの材料においては、周期的分極反転層の形成方法が確立されており、高効率の光波長変換素子が形成できるため、有効である。
【０１７６】
上記の第１〜第３の実施形態で示した装荷型、装荷リッジ型、及び埋め込み型の構成は、低損失で閉じ込めが強く、光波長変換素子としてだけではなく光導波構造としても有望であり、光通信、光計測デバイス等の光導波路素子への応用が可能である。
【０１７７】
次に、埋め込み型光導波路の耐光損傷性の向上を目的とした導波路構造について述べる。
【０１７８】
埋め込み型光導波路は、高屈折率のクラッド層を設けることにより、導波モード間のオーバラップが増大すると同時に、導波光の閉じ込めが深さ方向及び幅方向ともに非常に強まり、高効率の波長変換が可能になる。また、導波路内での基本波と第２高調波とのオーバラップを高めることで、基本波の電界分布が単独で存在する領域が減少し、耐光損傷強度の向上が図れる。しかし、光損傷の発生を完全に防止することはできず、例えば約１０ｍＷ以上の第２高調波出力に対して、光損傷の発生が観測される。この特性は、装荷型及び装荷リッジ型の光導波路に比べて、わずかに劣っている。
【０１７９】
そこで、光損傷の発生原因について詳細に検討したところ、光導波路の側部近傍で光損傷による屈折率変化が最も大きいことが見いだされた。一方、装荷型及び装荷リッジ型の光導波路では、導波路の側面近傍にプロトン交換層が存在するために、横方向の閉じ込めが緩和されるとともに光導波路側部の耐光損傷強度が向上していると考えられる。
【０１８０】
そこで、埋め込み型の光導波路構造においても、耐光損傷強度の向上を目的として、導波路の側面近傍に導波路より屈折率の低い層（基板より屈折率は高い）を設ける方法を試みた。具体的には、図１９の構成において、濃度の薄いプロトン交換層を光導波路の側部に形成した。その結果、光導波路の横方向の閉じ込めがわずかに（数％）緩和され、変換効率は約１０％低下する。しかし、耐光損傷強度は大幅に増大し、約２０ｍＷ以上の第２高調波に対しても光損傷が発生しない。プロトン交換層を設けたことによるこの光損傷強度の増大は、光導波路の横方向閉じ込めの緩和によって生じるだけでなく、プロトン交換層（導波路側部）の耐光損傷強度の増大によっても生じる。即ち、プロトン交換層は、基板に比べて電気伝導度が高いため、光損傷の原因となる不純物準位での光励起電荷の寿命が短く、光損傷が発生しにくい。そこで、光損傷の発生しやすい導波路側部に電気伝導度の高いプロトン交換部を設けることで、その部分の耐光損傷強度を増大することができる。
【０１８１】
（第４の実施形態）
ここでは、光波長変換素子を用いた短波長光源について述べる。
【０１８２】
前述した実施形態による光波長変換素子の構成により、高効率で安定な光波長変換素子の実現が可能となる。本発明による光波長変換素子を用いた短波長光源の作製の構成を、図２０に示す。
【０１８３】
具体的には、上記短波長光源は、Ｓｉなどから形成されるサブマウント４２０の上に搭載された波長８００ｎｍ帯の基本波光Ｐ１を発する半導体レーザ４２１、及び光波長変換素子４２２より構成され、半導体レーザ４２１の発光領域４２３から出た基本波光Ｐ１を光波長変換素子４２２の導波路４０２の端面に直接に結合させて集光し、導波モードを励起する。光波長変換素子４２２の導波路４０２の他の端面からは、波長変換された第２高調波光Ｐ２が出射する。基板４２０と光波長変換素子４２２との間には、クラッド層４０３が形成されている。
【０１８４】
本発明によって変換効率が高い光波長変換素子４２２が実現するので、出力約１００ｍＷの半導体レーザ４２１を用いて、約１０ｍＷの青色の第２高調波光Ｐ２が得られる。また、使用される光波長変換素子４２２は耐光損傷性に優れ且つ安定な出力が得られるので、出力変動を約２％以下に抑えることができ、安定な出力が得られる。
【０１８５】
４００ｎｍ帯の波長は、印刷製版、バイオ、蛍光分光特性等の特殊計測分野や、光ディスク分野など、広い応用分野において望まれている。本発明による光波長変換素子を用いた短波長光源は、出力特性及び安定性の両方の観点から、これらの応用分野での実用化が可能である。
【０１８６】
なお、半導体レーザ４２１の光を集光光学系を用いて光波長変換素子４２２の導波路４０２に結合させる構成であってもよい。但し、半導体レーザ４２１と光波長変換素子４２２の光導波路４０２を直接結合させる構成のほうが、小型且つ低価格の光源が実現できる。
【０１８７】
（第５の実施形態）
ここでは、短波長光源を光ディスクのピックアップ用光源として用いた光ピックアップについて説明する。
【０１８８】
光ディスクでは高密度記録が望まれており、小型の短波長光源の実現が必要不可欠である。光ディスクを読み取るためのピックアップは、光源、集光光学系、及び受光部分を含む。具体的には、図２１に示すように、Ｓｉサブマウント５２０の上に半導体レーザ５２１及び光波長変換素子５２２が搭載された光源から発せられた光Ｐ１は、レンズ５４０で平行光にされ、ビームスプリッタ５４１を通過してからレンズ５４２で集光されて、光ディスク５４３を照射する。光ディスク５４３からの反射光は、レンズ５４２で平行光にされた後に、ビームスプリッタ５４１及びレンズ５４４を介して、光検出器５４５に集光される。
【０１８９】
光源として本発明による光波長変換素子を用いると、波長４００ｎｍ帯の青色光を光ディスクの読み取り光源として利用できるため、記録密度を２倍に向上させることが可能となる。さらに、高出力の青色光の発生が可能となるため、読み取りだけでなく、光ディスクへ情報を書き込むことも可能となる。半導体レーザを基本波光源として用いることで、非常に小型になり、民生用の小型光ディスク読み取り及び記録装置にも利用できる。
【０１９０】
さらに、光波長変換素子は、光導波路幅を最適化することで、出力ビームのアスペクト比の最適化が行える。例えば、装荷リッジ型導波路構造を有する光波長変換素子において、光導波路の幅を約３μｍにすることにより、アスペクト比を１：１に近づけることが可能となる。この結果、光ピックアップの集光特性を向上させるためのビーム成形プリズムなどが不要になり、高い伝達効率、優れた集光特性、及び低価格化が実現できる。さらに、ビーム成形時に発生する散乱光のノイズが低減でき、ピックアップの簡素化が実現できる。
【０１９１】
（第６の実施形態）
本実施形態では、閉じ込めの強い光導波路の構成について述べる。
【０１９２】
第１の実施形態で説明した光波長変換素子の構造は、光導波路構造としても有効な構造である。そこで、図９に示した光波長変換素子における光導波路の特性について説明する。
【０１９３】
具体的には、図２２（ｈ）に示すように、LiTaO₃基板の上にプロトン交換層からなる光導波路が形成され、さらにその上にNb₂O₅からなるクラッド層が形成されている光波長変換素子に、波長約８６０ｎｍの光を導波させて、導波光の電界分布の観測を行った。比較のために、Ta₂O₅からなるクラッド層が形成されている装荷型光導波路（図２２（ｇ）参照）の導波光の電界分布も測定した。波長約８６０ｎｍの光に対する屈折率は、プロトン交換層（光導波層）が約２．１６、Ta₂O₅層が約２．０、Nb₂O₅層が約２．２５である。
【０１９４】
厚さ約２μｍのプロトン交換層の上に厚さ約３００ｎｍ且つ幅約５μｍのストライプ状のクラッド層を形成した装荷型光導波路の電界分布を、図２２（ａ）〜（ｆ）を参照して説明する。
【０１９５】
まず最初に、深さ方向の分布から説明する。図２２（ａ）及び（ｂ）は、Ta₂O₅からなるクラッド層が形成されている装荷型光導波路とNb₂O₅からなるクラッド層が形成されている装荷型光導波路についての、深さ方向の導波光の電界強度分布であり、図２２（ｃ）及び（ｄ）は、それぞれの構造における深さ方向の屈折率分布である。Ta₂O₅からなるクラッド層が形成されている場合の深さ方向の電界強度分布（図２２（ｃ）参照）は、半値全幅で約３μｍに広がっているのに対し、Nb₂O₅からなるクラッド層が形成されている場合の深さ方向の電界強度分布（図２２（ｄ）参照）の半値全幅は約２．５μｍにまで減少し、光導波路の閉じ込めが大幅に向上している。他の材料（例えば、SiO₂やAl₂O₃）からなるクラッド層についても検討した結果、深さ方向の導波光の電界強度分布は、光導波層の屈折率（約２．１６）以下の屈折率を持つクラッド層ではほとんど変わらない。従って、深さ方向の光の閉じ込めを向上させるには、クラッド層として、導波層より屈折率の高い層を形成する必要がある。
【０１９６】
次に、幅方向の導波光の電界強度分布を、図２２（ｅ）及び（ｆ）にそれぞれ示す。幅方向の閉じ込めに関しても、Ta₂O₅からなるクラッド層が形成されている場合（図２２（ｅ）参照）には、半値全幅で約６μｍ以上に広がっているのに対して、Nb₂O₅クラッド層（図２２（ｆ）参照）では、半値全幅が約５μｍまで低減されて、幅方向に閉じ込めの強い光導波路が形成されていることがわかる。
【０１９７】
以上の結果、導波層より屈折率の高いクラッド層を設けることにより、閉じ込めの強い光導波路が形成できることが明らかになった。閉じ込めの強い光導波路は、導波する光のパワー密度の増大が図れて光の制御効率が向上するため、光導波路を用いた電気光学素子や非線形光学素子に有効である。
【０１９８】
（第７の実施形態）
ここでは、光波長変換素子の特性をさらに向上する方法を検討する。
【０１９９】
クラッド層に屈折率の高い層を設けることにより基本波の導波モードの形状を制御することが可能となるので、導波モードの形状制御を容易にするために図２３に示す構造を形成する。具体的には、クラッド層を多層膜構造にすることにより、導波モードの制御を容易にする。
【０２００】
図２３の（ａ）は、光波長変換素子の斜視図であり、（ｂ）は断面図である。図２３の構成は、クラッド層が、SiO₂層７１２とNb₂O₅層７１０との多層構造になっている以外は、図９を参照して説明した構成と同じである。対応する構成要素には、対応する類似した参照番号を付しており、その説明を省略する。
【０２０１】
導波モードのプロファイルを基板表面側に引き寄せ、基本波と第２高調波とのオーバラップを向上させるには、多層膜内に基板より屈折率の高い層が必要である。ここでは、Ta₂O₅とNb₂O₅との多層膜によるクラッド層を形成する。多層膜の厚さを制御することにより、オーバラップの向上が図れて変換効率が向上することを確認した。
【０２０２】
次に、光導波路の屈折率分布を対称化することによる変換効率の向上について述べる。
【０２０３】
屈折率分布が深さ方向に対して対称な構造になれば、基本波と第２高調波とのオーバラップを大幅に向上させることができる。そのためには、クラッド層として、基板と等しい屈折率を持つ材料が必要となる。基板であるLiTaO₃は、８６０ｎｍの光に対する屈折率が約２．１５である。そこで、この値に近い屈折率をもつクラッド層を形成するため、Ta₂O₅とNb₂O₅との混合膜をクラッド層に用いた。Ta₂O₅及びNb₂O₅の屈折率は、それぞれ約２．０及び約２．２５であり、混合比を制御することにより、全体の屈折率を上記の値（２．１５）に近づけた。
【０２０４】
成膜は、Ta₂O₅及びNb₂O₅の混合ターゲットを用いるスパッタ蒸着により行う。クラッド層を厚さ約５００ｎｍに堆積し、図２３の構成による光波長変換素子を作製したところ、オーバラップの増大による変換効率の大幅な向上を確認できた。
【０２０５】
このように、クラッド層に基板と同じ屈折率の膜を用いることで、光波長変換素子の変換効率向上が達成できる。変換効率の向上は、クラッド層の屈折率がLiTaO₃基板の屈折率以上で且つ導波層の屈折率より小さい場合である。
【０２０６】
（第８の実施形態）
ここでは、耐光損傷強度に優れた光波長変換素子構造について述べる。
【０２０７】
光損傷とは、光導波路を導波する光により導波路の屈折率が変化し、出力が不安定になる現象で、特に短波長光に対して顕著に表れる。光波長変換素子は基本波を波長変換して短波長の第２高調波（波長:緑〜青〜紫外）を発生させるが、光損傷により導波路の屈折率が変化すると、第２高調波出力が変動して波長変換特性が不安定になる。
【０２０８】
以下では、光損傷を低減するため構造について行った検討結果を述べる。
【０２０９】
従来の埋め込み型光導波路を用いた光波長変換素子の耐光損傷性を測定した。基板にLiTaO₃を用い、波長約８６０ｎｍの基本波を波長変換して約４３０ｎｍの第２高調波の発生を行ったところ、数ｍＷの第２高調波の出力で光損傷が生じ、出力が不安定になることが明らかになった。そこで、光損傷の原因について検討を行った結果、次のことを発見した。
【０２１０】
第１に、プロトン交換を行うことによりプロトン交換部分の電気伝導度が向上し、これによって耐光損傷強度が向上する。
【０２１１】
第２に、光損傷は、導波路内部の耐光損傷強度だけでなく、導波路周辺部の耐光損傷強度の影響を受ける。埋め込み型光導波路は、光導波路周辺部の電気伝導度が低く光損傷が生じ易くなっているため、プロトン交換光導波路周辺部において光損傷が発生している。
【０２１２】
第３に、分極反転周期のデューティ比（分極反転部の幅Ｗ／分極反転周期Λ）の不均一性が光損傷特性に影響を与える。光損傷は、光により励起される電界により、電気光学効果を介して屈折率変化が生じる現象である。このため、周期的な分極反転構造により光励起電界を相殺し、屈折率変化を低減することが可能となる。しかし、埋め込み型光導波路では、周期状の分極反転構造のデューティ比が不均一になり易く、光損傷が低出力の第２高調波光に対して生じる。
【０２１３】
耐光損傷性に関する検討により、光導波路の周辺部の耐光損傷性が、光導波路の特性に影響を与えることが明らかになった。装荷型光導波路では、導波路側面にプロトン交換層を有するため、埋め込み型光導波路の約１０倍の耐光損傷強度を持っている。しかし、装荷型光導波路の底面は、通常の埋め込み型光導波路と同様の構造となっている。
【０２１４】
そこで、図２４に示す構造を形成する。図２４の（ａ）は、光波長変換素子の斜視図であり、（ｂ）は断面図である。具体的には、プロトン交換層を多層構造にし、導波層である第１のプロトン交換層８０５の底面にプロトン濃度の薄い第２のプロトン交換層８１５を形成する。図２４の構成は、プロトン交換層が上記のように多層構造になっている以外は、図９を参照して説明した構成と同じである。対応する構成要素には対応する類似した参照番号を付しており、その説明を省略する。
【０２１５】
導波層である第１のプロトン交換層８０５の底面に第２のプロトン交換層８１５を設けて電気伝導度を高めることで、光導波路の底部を電気伝導度の高い層で覆って、耐光損傷性をより高めることができる。これにより、耐光損傷性は、装荷型光導波路の２倍以上になる。
【０２１６】
また、本願発明者らの検討により、光損傷強度が分極反転周期にも依存することが明らかになった。周期状の分極反転構造により光励起による電界を相殺する効果があることは既に述べたが、分極反転周期が粗くなると、この効果が薄れてきて光損傷が生じ易くなる。具体的には、分極反転周期が約４μｍ以下で、耐光損傷性が顕著に増大する。周期約８μｍ以下でも、基板に対して２倍以上の耐光損傷性の向上が確認できた。しかし、反転周期が約１０μｍ以上になると、分極反転による耐光損傷性の向上は確認できなかった。
【０２１７】
（第９の実施形態）
前述した実施形態では、高屈折率の導波層を形成する方法として、ピロ燐酸中で基板を熱処理することによりLiTaO₃基板中のLi⁺と酸中のH⁺とを交換し、基板表面に高屈折率のプロトン交換層を形成していた。以下では、プロトン交換法にさらなる改良を加え、低損失で且つ光損傷に強い光導波路を形成する方法を提供する。
【０２１８】
ピロ燐酸は、高温での解離定数が大きく、液中での濃度むらが生じ難いために、ピロ燐酸を用いるプロトン交換処理では、屈折率及び厚みの均一な低損失のプロトン交換層が形成できる。しかし、基板中のLiイオンとプロトンの交換量が大きいので、基板表面に化学損傷を与え、基板の非線形定数を極端に劣化させる。そこで、ピロ燐酸を用いたプロトン交換によるプロトン交換量の制御性について検討した。
【０２１９】
ピロ燐酸のプロトン交換量を制御する方法として、酸を中和して解離定数を低減する方法がある。そこで、燐酸Li塩をピロ燐酸中に加えて、プロトン交換特性の変化を観測した。しかし、燐酸Liをピロ燐酸の10倍以上加えても、プロトン交換特性（交換速度、屈折率）の変化は見られなかった。また、燐酸Liを加えると、溶液の粘度が増して作業性が極端に悪くなる結果となった。
【０２２０】
そこで、基板表面にプロトン透過膜を設けてプロトン交換を行う方法を提案する。
【０２２１】
図２５（ａ）及び（ｂ）に、本実施形態の光導波路の製造方法の工程図を示す。 まず、図２５（ａ）に示すように、スパッタ蒸着によりLiTaO₃基板９０１の＋Ｃ面にTa₂O₅膜（透過膜）９０２を堆積する。次に、基板９０１をピロ燐酸中で熱処理する。Ta₂O₅膜９０２を透過してプロトン交換を行うことにより、図２５（ｂ）に示すように、Ta₂O₅膜９０２とLiTaO₃基板９０１との間に、プロトン交換層９０３を形成する。このようなTa₂O₅膜９０２を介したプロトン交換では、プロトン交換速度や濃度の制御が可能となる。
【０２２２】
図２６は、Ta₂O₅膜９０２の厚さとプロトン交換の拡散定数の関係を示した特性要因図である。約２６０℃でプロトン交換処理を行う場合、拡散定数は、Ta₂O₅膜の厚さがそれぞれ０ｎｍ、１００ｎｍ及び４００ｎｍであるときに、それぞれ０．２２７μｍ²／ｈ、０．０５μｍ²／ｈ、及び０．０１３２μｍ²／ｈとなる。これは、プロトン交換速度がTa₂O₅膜の厚さにより制御されていることを示している。
【０２２３】
形成されたプロトン交換層を評価したところ、化学損傷が非常に小さく、光導波路の伝搬損失が１／２以下に低減できることが見いだされた。さらに、耐光損傷強度が２倍以上に向上し、特性の優れた光導波路が形成されていることが確認された。
【０２２４】
また、プロトン交換濃度を下げることで、プロトン交換層の非線形性を向上させることができる。プロトン交換を行うと、LiTaO₃やLiNbO₃等の材料にはプロトン交換層が形成され、ステップ状の高屈折率層が形成される。プロトン交換層は、交換濃度が高くなると、非線形性が基板の半分以下に劣化することが知られている。非線形性を回復するには、プロトン交換層をアニール処理してプロトン交換濃度を下げる必要があるが、アニール処理を行うとプロトン交換層が熱拡散により広がり、屈折率分布がステップ状からグレーディッド状に変化してしまう。このため、光波長変換素子等へ応用する場合には、導波モードの電界分布形状のコントロールが難しくなり、変換効率低下の原因になっていた。しかし、Ta₂O₅膜を透過膜として用いることでプロトン濃度分布の制御が可能となるので、ステップ形状の屈折率分布を有する高非線形性を保った光導波路の形成が可能となる。
【０２２５】
非線形性の劣化を防ぐTa₂O₅膜の厚さとしては、最低でも約５００ｎｍ以上必要である。この方法を用いて光波長変換素子を形成することにより、変換効率の大幅な向上が可能となる。
【０２２６】
また、図２６に示すように、Ta₂O₅膜の厚さが約１００ｎｍ以下になると、膜厚に対する拡散定数の変化が大きくなり、プロトン交換の深さを制御するためにTa₂O₅膜の厚さの精密なコントロールが必要となって、実質的に制御が難しくなる。さらに、Ta₂O₅膜の厚さが約５０ｎｍ以下では、化学損傷の低減効果が少ない。一方、Ta₂O₅膜の厚さが増大すると、拡散定数が低下し、プロトン交換時間が長くなって生産性が悪くなる。
【０２２７】
このため、Ta₂O₅膜の厚さは、約1μｍ以下が適当である。すなわち、高非線形導波路を形成するには約５００ｎｍ〜約１０００ｎｍの厚さの透過膜（Ta₂O₅膜）が必要であり、プロトン交換速度の制御を行うには、約５０ｎｍ〜約１００ｎｍ程度が適当である。
【０２２８】
さらに、Ta₂O₅透過膜とプロトン交換選択マスクを組み合わせることにより、プロトン交換形状を大幅に変化させることが可能となり、導波モードの電界分布の制御が可能となる。
【０２２９】
具体的には、まず図２７（ａ）に示すように、LiTaO₃基板９１０の＋Ｃ面に、厚さ約６０ｎｍのTa膜９１１で、ストライプ状のマスクパターン９１１を形成する。次に、図２７（ｂ）に示すように、マスクパターン９１１の上に厚さ約４００ｎｍのTa₂O₅層９１２を堆積する。さらに、ピロ燐酸中で熱処理して、Ta₂O₅層９１２を介したプロトン交換処理を行う。このとき、図２７（ｃ）に示すように、Taマスクパターン９１１によって覆われていない部分のみに、ストライプ状のプロトン交換層９１３が形成される。プロトン交換処理は、約２６０℃で約４時間行う。
【０２３０】
図２８（ａ）及び（ｂ）には、プロトン交換プロファイルの断面図を示す。図２８（ａ）は、Ta₂O₅透過膜が無い場合であり、（ｂ）は、厚さ約４００ｎｍのTa₂O₅透過膜９１２を介してプロトン交換を行った場合である。図２８（ｂ）に示すようにTa₂O₅透過膜９１２を設けることにより、Taマスク９１１のエッジ部分のみでプロトン交換が進み、中央部分では拡散速度が遅くなる。そのため、プロトン交換層９１３の深さが、両サイドで深くなる。これは、Taマスク９１１のエッジ部分ではTa₂O₅透過膜９１２が薄くなるため、この部分のプロトン交換速度が速くなるためと考えられる。
【０２３１】
このようなプロトン交換処理によって形成された光導波路では、２ピークを有する２次伝搬モードを選択的に励起することが可能となり、２ピークの光出力を取り出せる。これより、光コンピュータ、作動検出用の計測装置、光ディスクの検出における位置修正用の検出光として、２ビームの出力を利用できる。また、光分波器に応用することにより、導波光を２つの導波路に効率よく分波できる。
【０２３２】
また、プロトン交換速度のTa₂O₅透過膜の厚さへの依存性を利用して、プロトン交換プロファイルの制御が可能となる。通常は、厚さの異なるプロトン交換層を１ウェハ上に形成するには、適切なパターンのプロトン交換マスクを形成して部分的にプロトン交換を行う工程を、繰り返し行う必要がある。しかし、Ta₂O₅透過膜を用いることで、厚さの異なるプロトン交換層を１回のプロトン交換工程で同時に形成することが可能となる。例えば、図２９に示すように、Taマスクパターン９１１とともに使用するTa₂O₅透過膜９１２の厚さを部分的に変えることにより、１回のプロトン交換処理で厚さの異なる複数のプロトン交換層９１３を基板９１０に形成できる。さらに、Ta₂O₅透過膜９１２の厚さをテーパ状に変化させることにより、深さがテーパ状に変化したプロトン交換層の形成も可能となる。
【０２３３】
以上の方法を用いれば、プロトン交換層の分布を自由に変えることができて、プロセスの簡素化が行える。
【０２３４】
次に、長時間にわたるプロトン交換が可能な、耐酸性に優れたマスクについて説明する。
【０２３５】
LiTaO₃、LiNbO₃などに深いプロトン交換層を形成するには、長時間のプロトン交換が必要である。例えば、LiTaO₃に約２μｍ以上の深さのプロトン交換層を形成するには、一般に約２６０℃で約４時間以上のプロトン交換処理が必要となる。ところが、上記のようにして選択的なプロトン交換を行うには、高温で長時間のプロトン処理に耐えるマスクが必要となる。しかし、従来技術によれば、耐酸性に優れたTaマスクを用いても、１時間以上のプロトン交換は難しい。
【０２３６】
これに対して本願発明者らは、金属膜上に酸化膜を堆積することで、長時間のプロトン交換が可能な耐酸性に優れたマスクが得られることを見い出した。具体的には、Ta膜を約６０ｎｍ堆積した後に、その表面にTa₂O₅透過膜を厚さ約４００ｎｍ堆積する。これによって、プロトン交換を４時間以上行ってもマスクは十分な耐プロトン性を示し、マスクの下の基板は全くプロトン交換されていないことが確認された。このように、金属マスク上に酸化膜を堆積することにより、耐酸性に優れたプロトン交換マスクが実現できた。
【０２３７】
なお、本実施形態では透過膜としてTa₂O₅を用いているが、他にNb₂O₅、SiO₂、TiO_2、Al₂O₃、LiNbO₃、LiTaO₃などの耐酸性に優れた酸化膜を用いれば、同様の効果が得られる。
【０２３８】
（第１０の実施形態）
次に、本発明の第１０〜１５の実施形態による光導波路及び光波長変換素子の製造方法における、エッチング速度について説明する。
【０２３９】
加工が困難で溶剤に腐食され難い構造的及び化学的に安定な基板、例えば石英等は、ウェットエッチング、ドライエッチング、反応性イオンエッチング等のエッチング手段を用いても、高精度にエッチングすることは困難である。このような基板は、エッチングにおいてレジストバターンとの選択比が小さく、微細な形状をエッチングする場合には、深いエッチング形状を形成することが困難である。
【０２４０】
これに対して、本発明の以下の光導波路及び光波長変換素子の製造方法においては、エッチングする部分のみを選択的にイオン交換することにより基板の特性を変えて、エッチング速度を向上させている。例えば、LiNbO₃、LiTaO₃等の基板を酸中で熱処理すると、結晶中のLi原子とプロトン（＋Ｈ）が交換されて、プロトン交換層が形成される。プロトン交換層では原子間の結合が元の結晶に比べて弱くなっており、エッチングされ易い状態となっている。この結果、プロトン交換層は、ウエットエッチング、ドライエッチング、反応性イオンエッチング等により、容易にエッチングされる。したがって、本発明の光導波路及び光波長変換素子の製造方法においては、エッチングする部分のみに選択的にイオン交換を施し、この部分をエッチングすることにより、選択比の高い高精度のエッチング処理が可能となる。このため、本発明の製造方法によれば、均一な品質を有する光導波路及び光波長変換素子の形成が可能となる。
【０２４１】
次に、導波方向に直交するように一定周期毎に分極反転構造を有する光波長変換素子の製造方法における、導波路の表面に発生する凹凸について説明する。
【０２４２】
従来の製造方法においては、光導波路の表面はエッチングにより凹凸が形成されて、光の伝搬損失が大きい。この理由は、光導波路の表面における分極反転部分と非反転部分で結晶面が異なるため、エッチング速度に違いが生じるためである。しかし、このような周期的な分極反転構造を有する面に対してイオン交換処理を施すと、そのイオン交換部分においては、周期的に分極反転構造を有する面であっても凹凸が発生しないことが発見された。そこで、エッチングする部分のみに選択的にイオン交換処理を行った後にエッチングすることにより、光導波路表面の凹凸をなくし、光の伝搬損失の少ない高効率の光導波路を有する光波長変換素子の製造が可能となる。
【０２４３】
次に、本発明の製造方法により作製された光導波路及び光波長変換素子における光損傷について説明する。
【０２４４】
従来の埋め込み型光導波路では、光導波路を伝搬する光の電界分布が、光導波路より外部へ染みだしている。このため、強い光が導波路を伝搬した場合、導波路の外周部分において、光損傷（光があたることにより物質の屈折率が変化する現象）が生じる。しかし、本発明の製造方法により作製されたリッジ型の光導波路においては、光導波路の表面にリッジ型の導波層が形成されているため、光の電界分布の染みだしが小さくなる。この結果、本発明による光導波路は、光損傷に対し強い構造となる。
【０２４５】
また、光損傷による屈折率変化は、光の照射により結晶中の不純物から電荷が発生し、結晶内部に電界を生じることにより誘起される。しかし、導波路近傍にイオン交換部分を形成すると、このイオン交換部分において電気伝導度が高くなり、内部電界の発生を抑圧することが可能となる。そこで、本発明の製造方法により形成された構造では、光導波路の表面にリッジ型のイオン交換層を形成しているため、光損傷の発生が抑圧される。
【０２４６】
次に、本発明の光波長変換素子における電界分布の重なりについて説明する。
【０２４７】
光波長変換素子において、基本波と第２高調波の電界分布の重なりが変換効率に大きく影響する。本発明による光波長変換素子においては、光導波路の表面にリッジが形成されているために屈折率分布がステップ状となり、従来の埋め込み型光導波路に比べて、基本波と第２高調波の重なりを大きくすることができる。さらに、光導波路の表面近傍にイオン濃度の高い層が形成されているため、深さ方向の電界の重なりも大きくなり、本発明による光波長変換素子は、高い変換効率を有する。
【０２４８】
図３０（ａ）〜（ｆ）は、本発明の第１０の実施形態における光導波路の製造方法を示す図であり、それぞれ、光の導波方向と直交する面にそった断面図である。
【０２４９】
まず、図３０（ａ）に示すように、非線形光学物質であるＣ板のLiTa0₃単結晶基板１００１（結晶のＣ軸に垂直な面で切り出した基板、以下では、「基板１００１」と略称する）を、約２００℃〜約３００℃のピロ燐酸液中に数分間浸してプロトン交換を行い、基板１００１の表面に帯板状の１次プロトン交換層１００２を形成する。
【０２５０】
次に、基板１００１に対して約４００℃〜約４５０℃でアニール処理を行い、１次プロトン交換層１００２の全部を、図３０（ｂ）に示すように帯板状のアニール処理プロトン交換層１００６にする。
【０２５１】
次に、基板１００１の表面である＋Ｃ面上に、厚さ約６０ｎｍのTa層をスパッタリング又は蒸着により堆積させる。その後、フォトリソグラフィ法とCF₄雰囲気中のドライエッチングによって、所定幅の直線状のTa層１００３を形成する。
【０２５２】
さらに、Ta層１００３を保護マスクとして非マスク部分を再度プロトン交換し、図３０（ｄ）に示すように、アニール処理プロトン交換層１００６の表面から所定の深さでの２次プロトン交換層１００５を形成する。
【０２５３】
次に、基板１００１の表面にネガレジスト（不図示）を塗布した後に、裏面より紫外線を照射してネガレジストを露光して現像し、図３０（ｅ）に示すように直線状のTaマスク１００３の上にレジストパターン１００４を形成する。このとき、直線状のTa層１００３が保護マスクとなって非マスク部分のみ露光されるため、Ta層１００３の上のみに、所定幅を有した直線状のレジストパターン１００４が、選択的に形成される。
【０２５４】
次に、形成されたレジストパターン１００４とTa層１００３を保護マスクとしてCHF₃雰囲気中でドライエッチングを行い、非マスク部分の２次プロトン交換層１００５をエッチングで除去する。さらに、基板１００１をフッ酸：硝酸＝１：２の液中に数秒間浸して、残留するTa層１００３及びレジストパターン１００４を除去する。その後に、光導波路の入出射面となる両端面を光学研磨して、図３０（ｆ）に示すようなリッジ１００７を有する光導波路が製造される。
【０２５５】
なお、リッジ１００７の側面には、プロトン交換部分１００８が形成されている。プロトン交換部分１００８は、Ta層１００３の保護マスクを用いて２次プロトン交換層１００５を形成する際に、プロトンの横方向への拡散により形成される。このプロトン交換部分１００８はアニールされていないため、アニール処理されたプロトン交換層１００６に比べてプロトン濃度か高く、屈折率変化も大きくなっている。
【０２５６】
上記の製造方法におけるエッチングについて説明する。
【０２５７】
上記の製造方法において、エッチング部分のみがTa層１００３を保護マスクとして選択的にプロトン交換されるため、エッチング速度は従来に比べて飛躍的に向上しており、また、プロトン交換部分のみが確実にエッチングされるため、導波部分にプロトン交換による影響を全く与えることがない。
【０２５８】
【表３】

【０２５９】
表３は、LiTaO₃におけるCHF₃雰囲気中でのエッチング速度を比較したデータであり、各種エッチング対象基板のエッチング速度、及びレジストパターンに対する選択比を示している。ここで選択比は、プロトン交換したLiTaO₃基板の値によって規格化した値を示している。表３におけるエッチング対象基板は、LiTaO₃基板、プロトン交換した後にアニール処理したLiTaO₃基板、及びプロトン交換したLiTaO₃基板の３種類である。
【０２６０】
表３に示すように、LiTaO₃基板をプロトン交換処理することにより、他の基板に比べてレジストパターンに対して高い選択比（５倍）がとれる。このため、プロトン交換したLiTaO₃基板では、他の基板に比べて微細かつ深い形状を形成するエッチングが可能となる。
【０２６１】
次に、上記の製造方法において、プロトン交換を用いることによりエッチングの深さを高精度に制御できることを説明する。
【０２６２】
一般的なドライエッチング装置においては、エッチングの厚み制御は±数％の誤差を有する。このため、このようなドライエッチング装置を用いて光導波路を製造する場合には、光導波路の特性がばらつくという問題がある。しかし、本発明に係る上記の光導波路の製造方法を用いることにより、エッチング深さは高精度に制御可能である。その理由を以下に記載する。
【０２６３】
上記の表３に示したように、エッチング速度は、プロトン交換部分とアニールしたプロトン交換部分とで大きく異なっている。このため、本発明の製造方法において、プロトン交換層のエッチングか終了してアニール処理部分に達すると、エッチングの速度は急速に低下し、実質的にエッチングは停止状態となる。従って、エッチング深さは、プロトン交換層の深さとほぼ一致する。また、プロトン交換層の深さは、±１％以下の精度で制御することか可能である。このため、エッチング深さの制御は、プロトン交換層の深さの制御と同等の±１％以下の制御が可能である。
【０２６４】
さらに、プロトン交換したLiTaO₃基板はレジストパターンに対して高い選択比を有しているため、エッチング時に、レジストパターンの形状は確実に保持されている。このため、本発明の製造方法において、プロトン交換していないLiTaO₃基板をエッチングする場合に比べ、プロトン交換したLiTaO₃基板では、より垂直に近いエッチング形状が得られる。
【０２６５】
次に、上記の製造方法におけるセルフアライメントについて説明する。
【０２６６】
上記の製造方法において、直線状のTa層１００３が、２次プロトン交換層１００５を形成するための保護マスク、及びネガレジストを露光するためのマスクの両方を兼用している。このため、Ta層１００３は、エッチング部分のみを選択的にプロトン交換するための保護マスクとして機能するとともに、レジストパターン１００４を裏面から露光してTa層１００３の上のみに確実に形成させるためのマスクとしても、機能している。この結果、レジストパターン１００４はセルフアライメントでTa層１００３の上に正確に形成できるため、上記の製造方法によれば、プロトン交換部分のみを正確にエッチングすることが可能である。この結果、光導波路の均一化及び低損失化を達成することができるので、上記の光導波路の製造方法は、再現性及び量産性に優れた特徴を有している。また、セルフアライメントにより金属マスク上のみに確実にレジストバターンを形成できるため、製造が容易であり、且つ大量生産が可能である。
【０２６７】
次に、上記の製造方法により製造された光導波路の特性評価実験について説明する。
【０２６８】
図３１は、上記の製造方法により作製された光導波路を示す断面図である。図３０（ａ）〜（ｆ）と同じ構成要素には、同じ参照番号を付している、
この光導波路に、波長約８６０ｎｍの光を伝搬させて特性評価実験を行った。具体的には、光導波路に波長約８６０ｎｍの半導体レ−ザの光をレンズを用いて結合させ、導波する光の近視野像を光導波路の出射端面より計測した。この計測結果によれば、上記の光導波路は、リッジ１００７の側面のプロトン交換部分１００８により強い閉じ込めが可能となり、入射したレ−ザ光に対する光導波路内に閉じ込められる光の分布は、従来技術の構造に比べて数％向上していた。さらに、この光導波路には、幅方向にステップ状の屈折率分布が形成されていることが明らかになった。
【０２６９】
次に、上記特性評価実験における各特性の測定結果について説明する。
【０２７０】
最初に、形成された光導波路の伝搬特性について説明する。
【０２７１】
導波する光の伝搬損失の測定値は約２ｄＢ／ｃｍであって、比較的低損失の光導波路が形成された。この値は、LiTaO₃基板自身を直接にエッチングした場合の約１／２の値であり、上記の製造方法によって低損失の光導波路が形成されることが理解できる。
【０２７２】
次に、形成された光導波路の非線形性に関する測定結果について説明する。
【０２７３】
LiTaO₃基板がプロトン交換されると非線形光学定数や電気光学定数などが大きく悪化し、プロトン交換したLiTaO₃基板を有する光導波路を非線形光学素子及び電気光学素子等に適用する際の大きな問題となっていた。しかし、プロトン交換したLiTaO₃基板の光導波路をさらに高温でアニール処理することにより、プロトン濃度が低下し、各定数も通常のLiTaO₃基板と同等の値まで回復することが確認された。
【０２７４】
本実施形態の製造方法により製造した光導波路の非線形光学定数の測定結果より、この光導波路は、アニールしたLiTaO₃基板の光導波路とほぼ等しい値の非線形光学定数を有していることを確認できた。この理由は、２次プロトン交換層１００５が、エッチングにより除去されて光導波路に影響を与えない部分に選択的に形成されるためである。実際にはリッジ１００７の側面に僅かにプロトン交換部分１００８が存在するが、光導波路の内部にはプロトン濃度の高い部分が存在しないために、高い非線形特性を維持することができる。
【０２７５】
次に、形成される光導波路の導波損失特性における測定結果について説明する。
【０２７６】
リッジ型光導波路の導波損失の原因は、リッジ形状をエッチングした際にその表面に生じる僅かな凸凹である。特に、導波路部分（屈折率：約２．２）と空気層（屈折率：１．０）との間の屈折率差が大きいため、光導波路の表面に凹凸が存在すると、導波損失が大きくなる。この影響を低減するためには、光導波路全体を、空気層より屈折率の高い物質で被う必要がある。
【０２７７】
そこで、上記で形成されたリッジ型光導波路の全体を空気層の屈折率より高い物質の膜で被覆した際の、被覆膜の屈折率と導波損失との関係を測定した。図３２に、その測定結果を示す。図３２に示すように、屈折率が１．０より増加すると共に導波損失が低下する。しかし、被覆膜の屈折率が導波路の屈折率を超えると、導波路に光が閉じ込められなくなり、基本波はカットオフされる。このため、被覆膜は、空気層の屈折率（１．０）より大きく且つ光導波路の屈折率（約２．２）より小さい屈折率を有する光透過性材料から形成されることが好ましい。
【０２７８】
なお、上記の説明では、LiTaO₃単結晶基板を用いているが、LiNbO₃の単結晶基板、或いは、LiTaO₃とLiNbO₃との多結晶基板（LiNb_1-xTa_x０₃（０≦Ｘ≦１））でも、上記と同様の効果を得られる。LiNbO₃は電気光学定数及び非線形光学定数などが大きいので、多くの分野における光導波路素子（光スイッチ、非線形光学素子、音響光学素子等）に応用されている。したがって、本実施形態の光導波路及び光波長変換素子の製造方法は、これらの応用分野において非常に有効である。
【０２７９】
さらに、LiTaO₃基板の代わりに、MgOを添加したMgO:LiNbO₃基板やMgO:LiTaO₃基板にも適用できる。これらの材料により形成された基板は光損傷に強いため、高出力の素子が作製できて有効である。
【０２８０】
さらに、基板としては、KTP基板を用いることもできる。KTPを基板材料とした場合には、イオン交換としてＲｂイオン交換が用いられて、上記と同様の効果を有する光導波路が形成できる。KTPは耐光損傷性に優れた特性を有しているため、KTP基板の光導波路は、高出力の光導波路として有効である。
【０２８１】
（第１１の実施形態）
図３３（ａ）〜（ｆ）は、本発明の第１１の実施形態における光導波路の製造方法を示す図であり、それぞれ、光の導波方向と直交する面にそった断面図である。
【０２８２】
まず、図３３（ａ）に示すように、非線形光学物質であるＣ板のLiTa0₃単結晶基板１１０１（結晶のＣ軸に垂直な面で切り出した基板、以下では、「基板１１０１」と略称する）を、約２００℃〜約３００℃のピロ燐酸液中に数分間浸してプロトン交換を行い、基板１１０１の表面に帯板状の１次プロトン交換層１１０２を形成する。
【０２８３】
次に、基板１１０１に対して約４００℃〜約４５０℃でアニール処理を行い、１次プロトン交換層１１０２の全部を、図３３（ｂ）に示すように帯板状のアニール処理プロトン交換層１１０６にする。
【０２８４】
次に、基板を再度ピロ燐酸中で熱処理して、図３３（ｃ）に示すように、アニール処理プロトン交換層１１０６の表面から所定の深さまでの２次プロトン交換層１１０５を形成する。
【０２８５】
さらに、図３３（ｄ）に示すように、２次プロトン交換層１１０５の上にフォトリソグラフィ法により所定幅の直線状レジストパターン１１０４を形成する。
【０２８６】
そして、形成されたレジストパターン１１０４を保護マスクとして用いて、CHF₃雰囲気中でドライエッチングを行い、図３３（ｅ）に示すように、非マスク部分の２次プロトン交換層１１０５をエッチングで除去する。
【０２８７】
その後に、図３３(f)に示すように、残留する直線状の２次プロトン交換層１１０５の上のレジストパターン１１０４を除去する。
【０２８８】
さらに光導波路の入出射面となる両端面を光学研磨して、図面と直交する方向に光が導波する光導波路が製造される。なお、上記の工程で形成されたアニール処理プロトン交換層１１０６は、典型的にはその深さが約２．５μｍであり、最終工程で残存した２次プロトン交換層１１０５によって形成されるリッジの高さは、典型的には約０．４μｍである。
【０２８９】
上記の製造方法によれば、製造工程が簡便なためにリッジ型光導波路を容易に製造することができる。また、上記の製造方法では、エッチングする部分に２次プロトン交換層１１０５を形成することによって、前述のようにエッチング部のみの特性を変えて、エッチング速度を飛躍的に向上させている。さらに、エッチングの深さは２次プロトン交換層１１０５の深さと実質的に一致するため、２次プロトン交換層１１０５の深さを精度高く形成することにより、エッチング深さを所望の深さに制御できる。このため、本実施形態の製造方法によれば、高い精度でリッジを形成できて品質のそろった光導波路が製造できるとともに、プロトン交換部分が光導波路に悪影響を与えることがほとんど無く、リッジにのみプロトン交換部分を有する光導波路を形成することができる。さらに、本実施形態の製造方法によれば、リッジにはプロトン交換濃度の高い高屈折率層が形成されているため、閉じ込めの強い光導波路が形成できる。
【０２９０】
さらに、本実施形態の光導波路は、その表面にプロトン濃度の高い部分が形成されているため、優れた耐光損傷性を有する。これは、プロトン濃度の高い部分が基板に比べて高い電気伝導度を有しているので、光損傷の原因となる光励起による自由電荷の偏りに伴う電界の発生が抑圧されるためである。
【０２９１】
また、光導波路の表面にプロトン濃度の高い部分が形成されているため、光導波路の特性の経時変化が抑圧されている。すなわち、LiNbO₃やLiTaO₃のアニール処理プロトン交換層においては、屈折率の経時変化が発生するが、本実施形態によれば、さらにプロトン交換を行うことにより、そのような屈折率変化は抑圧される。これは、アニール処理プロトン交換層ではその表面近傍の結晶構造がアニール処理後に徐々に変化するのに対して、その表面部分に再度プロトン交換を施すことにより、その部分の結晶構造が変化するためである。
【０２９２】
なお、上記の説明では、LiTaO₃単結晶基板を用いているが、LiNbO₃の単結晶基板、或いは、LiTaO₃とLiNbO₃との多結晶基板（LiNb_1-xTa_x０₃（０≦Ｘ≦１））でも、上記と同様の効果を得られる。LiNbO₃は電気光学定数及び非線形光学定数などが大きいので、多くの分野における光導波路素子（光スイッチ、非線形光学素子、音響光学素子等）に応用されている。したがって、本実施形態の光導波路及び光波長変換素子の製造方法は、これらの応用分野において非常に有効である。
【０２９３】
さらに、LiTaO₃基板の代わりに、MgOを添加したMgO:LiNbO₃基板やMgO:LiTaO₃基板にも適用できる。これらの材料により形成された基板は光損傷に強いため、高出力の素子が作製できて有効である。
【０２９４】
さらに、基板としては、KTP基板を用いることもできる。KTPを基板材料とした場合には、イオン交換としてＲｂイオン交換が用いられて、上記と同様の効果を有する光導波路が形成できる。KTPは耐光損傷性に優れた特性を有しているため、KTP基板の光導波路は、高出力の光導波路として有効である。
【０２９５】
さらに、上記では光導波路にプロトン交換導波路を用いているが、他に、Ｔｉ、Ｃｕ、Ｎｄ、Ｃｄ、Ｚなどの金属拡散導波路も適用できる。これらの金属拡散導波路は、非線形定数や電気光学定数の劣化が少ないために、優れた特性を有する光学素子が製造できる。
【０２９６】
（第１２の実施形態）
図３４（ａ）〜（ｇ）は、本発明の第１０の実施形態で説明した光導波路の製造方法を利用した光波長変換素子の製造方法を示す図であり、それぞれ、光の導波方向と直交する面にそった断面図である。
【０２９７】
まず、図３４（ａ）に示すように、非線形光学物質であるＣ板のLiTa0₃単結晶基板１２０１（結晶のＣ軸に垂直な面で切り出した基板、以下では、「基板１２０１」と略称する）を、約２００℃〜約３００℃のピロ燐酸液中に数分間浸してプロトン交換を行い、基板１２０１の表面に帯板状の１次プロトン交換層１２０２を形成する。
【０２９８】
次に、基板１２０１に対して約４００℃〜約４５０℃でアニール処理を行い、１次プロトン交換層１２０２の全部を、図３４（ｂ）に示すように帯板状のアニール処理プロトン交換層１２０６にする。
【０２９９】
次に、基板１２０１の表面である＋Ｃ面上に櫛形電極１２０９ａを設け、基板１２０１の裏面である−Ｃ面には平面電極１２０９ｂを形成する。このように取付けられた両電極間に電圧を印加して、図３４（ｃ）に示すように、光導波方向に直交する分極反転層１２１０が形成される。この分極反転層１２１０は、櫛形電極１２０９ａの形状に応じた一定周期毎に形成される。
【０３００】
さらに、両電極を取り除いた後に、基板１２０１の＋Ｃ面上に、厚さ約６０ｎｍのTa層をスパッタリング又は蒸着により堆積させる。その後、フォトリソグラフィ法とCF₄雰囲気中のドライエッチングによって、図３４（ｄ）に示すような所定幅の直線状のTa層１２０３を形成する。
【０３０１】
さらに、Ta層１２０３を保護マスクとして非マスク部分を再度プロトン交換し、図３４（ｅ）に示すように、アニール処理プロトン交換層１２０６の表面から所定の深さでの２次プロトン交換層１２０５を形成する。
【０３０２】
次に、基板１２０１の表面にネガレジスト（不図示）を塗布した後に、裏面より紫外線を照射してネガレジストを露光して現像し、図３４（ｆ）に示すように直線状のTaマスク１２０３の上にレジストパターン１２０４を形成する。このとき、直線状のTa層１２０３が保護マスクとなって非マスク部分のみ露光されるため、Ta層１２０３の上のみに、所定幅を有した直線状のレジストパターン１２０４が、選択的に形成される。
【０３０３】
次に、形成されたレジストパターン１２０４とTa層１２０３を保護マスクとしてCHF₃雰囲気中でドライエッチングを行い、図３４（ｇ）に示すように、非マスク部分の２次プロトン交換層１２０５をエッチングで除去する。さらに、基板１２０１をフッ酸：硝酸＝１：２の液中に数秒間浸して、残留するTa層１２０３及びレジストパターン１２０４を除去する。その後に、光導波路の入出射面となる両端面を光学研磨して、図３４（ｇ）に示すようなリッジ１２０７を有する光導波路が製造される。
【０３０４】
なお、リッジ１２０７の側面には、プロトン交換部分１２０８が形成されている。プロトン交換部分１２０８は、Ta層１２０３の保護マスクを用いて２次プロトン交換層１２０５を形成する際に、プロトンの横方向への拡散により形成される。このプロトン交換部分１２０８はアニールされていないため、アニール処理されたプロトン交換層１２０６に比べてプロトン濃度が高く、屈折率変化も大きくなっている。
【０３０５】
本実施形態の製造方法においては、帯板状の光導波路を形成した後に分極反転層１２１０が形成されるため、分極反転層１２１０は光導波路形成時の製造工程により影響されることがない。従って、均一な分極反転層１２１０を確実に形成ができる。
【０３０６】
通常は、分極反転層をエッチングすると、非分極反転層と分極反転層間でエッチング速度に違いがあるために光導波路の表面に凹凸が形成される。しかし、本実施形態の製造方法では、エッチングすべき分極反転層１２１０を有する表面には２次プロトン交換層１２０５が形成されているため、分極反転層１２１０の表面にエッチングによる凹凸が形成されない。この結果、本実施形態の製造方法は、低損失の光波長変換素子を形成するのに適した製造方法である。
【０３０７】
さらに、上記の製造方法によれば、２次プロトン交換層１２０５の深さの制御により、エッチングの深さを精度高く制御できるため、特性のそろった光波長変換素子の製造が可能である。
【０３０８】
次に、本実施形態の製造方法により製造された光波長変換素子の原理について説明する．
図３５は、本実施形態の製造方法により製造された光波長変換素子の構造を示す斜視図である。図３４（ａ）〜（ｇ）と同じ構成要素には同じ参照番号を付しており、その説明は省略する。
【０３０９】
図３５の光波長変換素子は、擬似位相整合型の第２高調波発生素子であり、光導波路内の光の伝搬の方向に周期的に形成された分極反転層１２１０により、光導波路を伝搬する基本波が半分の波長の第２高調波に変換される。例えば、波長８６０ｎｍの赤外光をこの光導波路に入射すれば、半分の波長の４３０ｎｍの青色光が出射される。光波長変換素子の特性は、光導波路の非線形光学定数、及び光導波路を伝搬する基本波と第２高調波との電界の重なりに大きく依存している。
【０３１０】
次に、上記の製造方法により製造された光波長変換素子の特性評価結果について説明する。
【０３１１】
最初に、リッジ型の光導波路を有する光波長変換素子の導波モード（基本波と第２高調波）の重なりについて説明する。
【０３１２】
本実施形態の光波長変換素子の評価実験において、光導波路を伝搬する導波光の近視野像を観測して、導波光の電界分布の横方向（図３５におけるＸ方向）の重なりを求めた。図３６（ａ）及び（ｂ）は、基本波光及び第２高調波光の光導波路における強度分布を示すグラフであり、図３６（ａ）は従来の埋め込み型導波路の場合を示し、図３６（ｂ）は本実施形態のリッジ型光導波路の場合を示している。これらのグラフより、横方向の重なりは、従来の埋め込み型光導波路の強度分布に比べて、本実施形態のリッジ型光導波路で非常に大きくなっていることが分かる。この理由は、リッジ型光導波路の屈折率分布がリッジ１２０７によってステップ状になるとともに、リッジ１２０７の側面に形成されたプロトン交換部分１２０８が高屈折率部となって、光導波路の閉じ込めに大きく貢献しているためである。この結果、リッジ型光導波路を用いた本実施形態の光波長変換素子では、従来の埋め込み型光導波路を用いた光波長変換素子に比べて、変換効率が１．５倍に向上した。
【０３１３】
次に、本実施形態で製造された光波長変換素子における耐光損傷性について説明する。
【０３１４】
光導波路における光損傷は、光導波路等の光の閉じ込めの強い場所で発生しやすい。また、このような光損傷の発生は、光の波長が短い程、顕著になる。従来の光波長変換素子においては、第２高調波の発生により光損傷が生じて、出力持性か不安定になる。そこで、従来の埋め込み型光導波路においては、耐光損傷性に優れるとともに基板に比較して大きな耐光損傷強度を有するプロトン交換したLiTaO₃が用いられていた。しかし、従来の埋め込み型光導波路では、導波する光がプロトン交換領域以外の部分へも染み出すため、導波路の外周部で光損傷が生じて、結果として高出力の第２高調波を発生させることは困難であった。
【０３１５】
これに対して、本実施形態の製造方法により製造された光波長変換素子について耐光損傷性の評価実験を行ったところ、従来の埋め込み型光導波路を用いた光波長変換素子の１０倍以上の耐光損傷性を有していた。その理由は、リッジ型光導波路を用いた光波長変換素子は、光導波路の表面にリッジ形状のプロトン交換層を形成して、光導波路からの光の染みだしを抑制しているためである。また、プロトン交換層は電気伝導度が高いため、光損傷の原因となる光励起自由電子の偏りによる電界の発生を抑圧している。このため、本実施形態の波長変換素子では、導波路の表面に形成されたリッジ形状のプロトン交換層により、耐光損傷性に優れた導波路構造が実現されている。さらに、本実施形態の光波長変換素子では、リッジ１２０７の側面に形成されたプロトン交換部分１２０８が高いプロトン濃度を有して電気伝導度を高めているため、この点でも耐光損傷性が向上している。
【０３１６】
次に、本実施形態の光波長変換素子の変換効率について説明する。
【０３１７】
リッジ型光導波路において、リッジ１２０７の側面に微細な凹凸が形成されると導波損失が大きくなる。このような導波損失を低減するするために、前述の第１０の実施形態では、空気層の屈折率より高く且つ光導波路の屈折率より低い膜で、光導波路を覆う。このような被膜は、本実施形態の波長変換素子についても有効である。
【０３１８】
また、本実施形態の光波長変換素子では、リッジ型光導波路を用いることにより、光導波路における横方向の伝搬光の重なりを大きくすることが可能になっている。しかし、光導波路における深さ方向（図３５におけるＺ方向）の光の強度分布の重なりはあまり改善されておらず、高効率の光導波路を有する光波長変換素子を形成するためには、その深さ方向の重なりを大きくする必要が有る。そこで、深さ方向の導波光の強度分布を制御する方法として、光導波路のリッジ１２０７の表面に高屈折率層を形成する（すなわち、リッジ１２０７の表面のみを選択的に屈折率の高い層で覆う）。このように高屈折率の膜をリッジ１２０７の表面に形成することにより、光導波路の深さ方向の閉じこめを強くして、導波モード間の重なりを大きくすることができる。
【０３１９】
図３７（ａ）及び（ｂ）は、光波長変換素子における基本波と第２高周波の深さ方向の重なり状態を示した状態図であり、（ａ）は前述の本実施形態の製造方法により製造された光波長変換素子における導波モード間の重なり状態を示し、（ｂ）は、リッジ１２０７の表面にさらに高屈折率層１２２０を形成した光波長変換素子における導波モード間の重なり状態を示す。
【０３２０】
図３７（ｂ）に示した光波長変換素子で、リッジ１２０７の表面上のみに選択的に形成した高屈折率層１２２０は、アモルファス状のLiNbO₃である。光導波路の閉じ込めを強くするには、光導波路の屈折率より高い屈折率を持つ膜を、リッジ１２０７の表面上に形成することが有効である。このため、LiTaO₃の光導波路の屈折率（約２．１５）よりも大きい屈折率（約２．２５）を有するアモルファス状のLiNbO₃を、高屈折率層１２２０の構成材料として用いている。高屈折率層１２２０の厚さは、光導波路を伝搬する第２高調波がこの高屈折率層１２２０を導波しない程度の厚さ、例えば約１００ｎｍ〜約５００ｎｍとする。この理由は、第２高調波がLiNbO₃の表面層を導波すると、この表面層に第２高調波が閉じ込もり、変換効率か大幅に低下してしまうためである。図３７（ｂ）に示すように、LiNbO₃の高屈折率層１２２０を有する光波長変換素子では、深さ方向の導波光における基本波と第２高調波間の強度分布の重なりが大きいために、図３７（ａ）に示した光波長変換素子に比べて変換効率が１．３倍向上した。
【０３２１】
なお、上記の説明では、LiTaO₃単結晶基板を用いているが、LiNbO₃の単結晶基板、或いは、LiTaO₃とLiNbO₃との多結晶基板（LiNb_1-xTa_x０₃（０≦Ｘ≦１））でも、上記と同様の効果を得られる。LiNbO₃は電気光学定数及び非線形光学定数などが大きいので、多くの分野における光導波路素子（光スイッチ、非線形光学素子、音響光学素子等）に応用されている。したがって、本実施形態の光導波路及び光波長変換素子の製造方法は、これらの応用分野において非常に有効である。
【０３２２】
さらに、LiTaO₃基板の代わりに、MgOを添加したMgO:LiNbO₃基板やMgO:LiTaO₃基板にも適用できる。これらの材料により形成された基板は光損傷に強いため、高出力の素子が作製できて有効である。
【０３２３】
さらに、基板としては、KTP基板を用いることもできる。KTPを基板材料とした場合には、イオン交換としてＲｂイオン交換が用いられて、上記と同様の効果を有する光導波路が形成できる。KTPは耐光損傷性に優れた特性を有しているため、KTP基板の光導波路は、高出力の光導波路として有効である。
【０３２４】
さらに、基板としては、他にZnS、GaAsなどの半導体基板を用いることもできる。半導体材料は大きな非線形光学定数を有し、製造技術も発達しているため、高出力の光波長変換素子が作製できる。
【０３２５】
さらに、上記では光導波路にプロトン交換導波路を用いているが、他に、Ｔｉ、Ｃｕ、Ｎｄ、Ｃｄ、Ｚなどの金属拡散導波路も適用できる。これらの金属拡散導波路は、非線形定数や電気光学定数の劣化が少ないために、優れた特性を有する光学素子が製造できる。
【０３２６】
（第１３の実施形態）
図３８（ａ）〜（ｆ）は、本発明の第１１の実施形態で説明した光導波路の製造方法を利用した光波長変換素子の製造方法を示す図であり、それぞれ、光の導波方向と直交する面にそった断面図である。
【０３２７】
まず、図３８（ａ）に示すように、非線形光学物質であるＣ板のLiTa0₃単結晶基板１３０１（結晶のＣ軸に垂直な面で切り出した基板、以下では、「基板１３０１」と略称する）を、約２００℃〜約３００℃のピロ燐酸液中に数分間浸してプロトン交換を行い、基板１３０１の表面に帯板状の１次プロトン交換層１３０２を形成する。
【０３２８】
次に、基板１３０１に対して約４００℃〜約４５０℃でアニール処理を行い、１次プロトン交換層１３０２の全部を、図３８（ｂ）に示すように帯板状のアニール処理プロトン交換層１３０６にする。
【０３２９】
次に、基板を再度ピロ燐酸中で熱処理して、図３８（ｃ）に示すように、アニール処理プロトン交換層１３０６の表面から所定の深さまでの２次プロトン交換層１３０５を形成する。
【０３３０】
次に、基板１３０１の表面である＋Ｃ面上に櫛形電極１３０９ａを設け、基板１３０１の裏面である−Ｃ面には平面電極１３０９ｂを形成する。このように取付けられた両電極間に電圧を印加して、図３８（ｄ）に示すように、光導波方向に直交する分極反転層１３１０が形成される。この分極反転層１３１０は、櫛形電極１３０９ａの形状に応じた一定周期毎に形成される。
【０３３１】
さらに、両電極を取り除いた後に、さらに、図３８（ｅ）に示すように、基板１３０１の上にフォトリソグラフィ法により所定幅の直線状レジストパターン１３０４を形成する。
【０３３２】
そして、形成されたレジストパターン１３０４を保護マスクとして用いて、CHF₃雰囲気中でドライエッチングを行い、非マスク部分の２次プロトン交換層１３０５をエッチングで除去する。その後に、図３８（ｆ）に示すように、残留する直線状の２次プロトン交換層１３０５の上のレジストパターン１３０４を除去する。さらに光導波路の入出射面となる両端面を光学研磨して、光波長変換素子が製造される。なお、上記の工程で形成されたアニール処理プロトン交換層１１０６は、典型的にはその深さが約２．５μｍであり、最終工程で残存した２次プロトン交換層１３０５によって形成されるリッジの高さは、典型的には約０．４μｍである。
【０３３３】
上記の本実施形態の光波長変換素子の製造方法では、簡便な製造工程で、リッジ型光導波路を有する光波長変換素子を容易に製造できる。さらに、形成される光波長変換素子は、そのリッジがプロトン濃度が高く屈折率変化の大きな２次プロトン交換層１３０５に設けられているため、光の閉じ込めの強い導波路構造が実現される。
【０３３４】
さらに、上記の製造方法によれば、２次プロトン交換層１３０５の深さの制御により、エッチングの深さを精度高く制御できるため、特性のそろった光波長変換素子の製造が可能である。
【０３３５】
次に、上記の製造方法により製造された光波長変換素子の特性評価結果について説明する。
【０３３６】
図３９は、本実施形態の製造方法により製造された光波長変換素子の構造を示す斜視図である。図３８（ａ）〜（ｆ）と同じ構成要素には同じ参照番号を付している。具体的には、基板１３０１の上に、アニール処理プロトン交換層１３０６が形成されている。このアニール処理プロトン交換層１３０６の上には、リッジ１３０７であるプロトン交換部分１３８０が形成されている。また、光の導波方向と直交するように、一定周期毎に分極反転層１３１０が形成されている。プロトン交換部分１３８０は、プロトン交換時に形成されたプロトン濃度の高い２次プロトン交換層１３０５の一部分である。
【０３３７】
上記の構成を有する光波長変換素子は、リッジ１３０７として屈折率の高いプロトン交換層１３０５を有するため、深さ方向の導波モード間（基本波と第２高調波）の電界分布の重なりが大きく、高効率の光波長変換素子となっている。以下に、その理由を述べる。
【０３３８】
図４０は、リッジの厚さと光波長変換素子の変換効率との関係を示したグラフである。図４０の（ａ）、（ｂ）及び（ｃ）は、リッジの高さの異なる３種類の光波長変換素子における各導波モード間（基本波と第２高調波）の厚さ方向の重なりを示している。
【０３３９】
図４０の（ａ）に示す光波長変換素子では、リッジ１３０７が非常に薄く、実質的には表面に高屈折率層が存在しない。この場合には、導波路の屈折率分布がグレーデイッド状（中心部の屈折率か高く、外周部分の屈折率か低い状態）になり、基本波と第２高調波の重なりが小さくて、変換効率が低い。
【０３４０】
図４０の（ｂ）に示す光波長変換素子では、やや厚いリッジ１３０７が設けられている。このような表面における高屈折率層により、基本波の強度分布が表面近傍に引き寄せられ、基本波と第２高調波の重なりか増大して、高い変換効率か得られる。具体的には、図４０の（ｂ）に示す光波長変換素子の変換効率は、リッジに高屈折率層を形成しない場合に比べて約１．５倍に増大しており、変換効率の高効率化を達成するためにはリッジに高屈折率層を形成することが有効な手段であることが、明らかである。
【０３４１】
図４０の（ｃ）に示す光波長変換素子では、さらに厚い高屈折率層が設けられて、基本波と第２高調波の重なりが増大している。このとき、基本波及び第２高調波は、いずれも、表面の高屈折率層ではカットオフされている。しかし、図４０の（ｃ）の光波長変換素子では、第２高調波が高屈折率層を導波するので、変換効率は逆に低下している。この理由は、高屈折率層におけるプロトン濃度が高く非線形光学定数か劣化しているために、この高屈折率層では高効率の波長変換が生じないからである。
【０３４２】
従って、高効率の光波長変換素子を構成するためには、リッジの厚さを第２高調波のみがカットオフとなる程度にするのが望ましい。
【０３４３】
さらに、本実施形態の光波長変換素子では、光導波路の表面にプロトン濃度の高い部分が形成されているため、優れた耐光損傷性を有する。これは、プロトン濃度の高い部分が基板に比べて高い電気伝導度を有しているので、光損傷の原因となる光励起による自由電荷の偏りに伴う電界の発生が抑圧されるためである。
【０３４４】
また、光導波路の表面にプロトン濃度の高い部分が形成されているため、光導波路の特性の経時変化が抑圧されている。すなわち、LiNbO₃やLiTaO₃のアニール処理プロトン交換層においては、屈折率の経時変化が発生するが、本実施形態によれば、さらにプロトン交換を行うことにより、そのような屈折率変化は抑圧される。これは、アニール処理プロトン交換層ではその表面近傍の結晶構造がアニール処理後に徐々に変化するのに対して、その表面部分に再度プロトン交換を施すことにより、その部分の結晶構造が変化するためである。
【０３４５】
なお、上記の説明では、LiTaO₃単結晶基板を用いているが、LiNbO₃の単結晶基板、或いは、LiTaO₃とLiNbO₃との多結晶基板（LiNb_1-xTa_x０₃（０≦Ｘ≦１））でも、上記と同様の効果を得られる。LiNbO₃は電気光学定数及び非線形光学定数などが大きいので、多くの分野における光導波路素子（光スイッチ、非線形光学素子、音響光学素子等）に応用されている。したがって、本実施形態の光導波路及び光波長変換素子の製造方法は、これらの応用分野において非常に有効である。
【０３４６】
さらに、LiTaO₃基板の代わりに、MgOを添加したMgO:LiNbO₃基板やMgO:LiTaO₃基板にも適用できる。これらの材料により形成された基板は光損傷に強いため、高出力の素子が作製できて有効である。
【０３４７】
さらに、基板としては、KTP基板を用いることもできる。KTPを基板材料とした場合には、イオン交換としてＲｂイオン交換が用いられて、上記と同様の効果を有する光導波路が形成できる。KTPは耐光損傷性に優れた特性を有しているため、KTP基板の光導波路は、高出力の光導波路として有効である。
【０３４８】
さらに、基板としては、他にZnS、GaAsなどの半導体基板を用いることもできる。半導体材料は大きな非線形光学定数を有し、製造技術も発達しているため、高出力の光波長変換素子が作製できる。
【０３４９】
さらに、上記では光導波路にプロトン交換導波路を用いているが、他に、Ｔｉ、Ｃｕ、Ｎｄ、Ｃｄ、Ｚなどの金属拡散導波路も適用できる。これらの金属拡散導波路は、非線形定数や電気光学定数の劣化が少ないために、優れた特性を有する光学素子が製造できる。
【０３５０】
なお、本発明における光導波路の構成は、例えばファイバ状の導波路に適用することもできる。その場合には、光学材料によって形成される円筒状コアが上記の説明における光導波層に相当し、そのコアの表面を覆うように、上述の特徴を有するクラッド層が形成されている。
【０３５１】
【発明の効果】
以上に説明したように、光導波路上にクラッド層を設け、光導波路を伝搬する基本モードの基本波と一部がクラッド層を導波する高次モードの第２高調波との間で位相整合をとることにより、モード間のオーバラップを高めて変換効率を大幅に向上することができるので、実用上、大きな効果が得られる。
【０３５２】
また、屈折率の高い材料でクラッド層を形成することにより、導波路の幅方向の閉じ込めも、強くすることができる。これによって、導波路から出射される光の出射角を幅方向と深さ方向でほぼ等しくすることができて、出射ビームのアスペクト比を改善し、光の利用効率を大幅に向上させることができる。これによって、実用上、大きな効果が得られる。
【０３５３】
また、光波長変換素子において、高屈折率層と基板より屈折率を有するクラッド層とを設けることにより、光導波路を伝搬する光の閉じ込めを強くして、光のパワー密度を向上することができる。さらに、これによって、光導波路内を伝搬する基本波と高調波との電界分布のオーバラップが増大するため、変換効率を大幅に向上することが可能となり、実用上、大きな効果が得られる。
【０３５４】
また、本発明の光波長変換素子の構造により、光損傷による第２高調波出力の変動を大幅に低減することが可能となる。これにより、高出力で安定な第２高調波出力が得られるようになり、実用上、大きな効果が得られる。
【０３５５】
さらに、本発明によれば、高屈折率のクラッド層により電界分布の制御が可能となるので、光導波路を伝搬するモードプロファイルの制御性が向上する。これによって、第２高調波出力の放射パターンのアスペクト比を１に近づけることが可能となり、光の利用効率が大幅に向上し、実用上、大きな効果が得られる。
【０３５６】
さらに、光導波路の製造方法として、プロトン交換時に透過膜を通してプロトン交換を行うことにより、プロトン交換層表面の化学損傷を低減できる。これによって、導波損失の小さな光導波路が形成できて、実用上、大きな効果が得られる。
【０３５７】
また、透過膜の厚さの制御によって形成されるプロトン交換層の厚さをコントロールすることができるため、１回のプロトン交換処理により異なった深さを有するプロトン交換層が形成できる。さらに、形成されるプロトン交換層の形状を透過膜の形状により制御できるため、作業の単純化、プロセス工程の削減による量産性の向上が可能となり、実用上、大きな効果が得られる。
【０３５８】
さらに、本発明の光導波路及び光波長変換素子の製造方法では、金属マスクを用いてエッチング部分のみを選択的にプロトン交換し、さらに金属マスク上にレジストバターンを裏面露光により形成してエッチングすることにより、容易に光導波路を形成できる。また、上記製造工程において、エッチング部分をイオン交換にすることにより、エッチング速度及びエッチング精度の向上を達成することができ、品質のそろった光導波路の形成か可能となる。
【０３５９】
さらに、製造された光導波路は、低損失で且つ耐光損傷性に優れ、導波路を伝搬する導波モードの電界分布の最適化が図られており、高効率且つ高出力の光波長変換素子へ適用が可能である。
【０３６０】
例えば、第１０の実施形態の製造方法によれば、エッチングの深さを高精度に制御できるため、光導波路の均一化及び導波損失の低減化を達成することができる。第１１の実施形態における製造方法では、リッジにプロトン交換濃度の高い高屈折率層を形成して、光の閉じ込めの強い素子を実現するとともに、耐光損傷性に優れた光導波路を容易に製造することができる。さらに、第１２の実施形態によれば、光導波路表面にリッジを形成し、このリッジに高屈折率部分を形成することによって、高い変換効率を有し、且つ耐光損傷性に優れた光波長変換素子を得ることができる。また、第１３の実施形態に示した製造方法によれば、エッチングの深さを高精度に制御できるため、均一な品質を有する光波長変換素子を容易に製造できるとともに、変換効率の高い光波長変換素子を得ることができる。
【図面の簡単な説明】
【図１】（ａ）〜（ｄ）は、従来の埋め込み型光導波路の製造方法を説明するための断面図である。
【図２】（ａ）は、従来の光波長変換素子の構成の一例を示す斜視図であり、（ｂ）は、（ａ）の線２Ｂ−２Ｂにおける断面図であり、（ｃ）は、（ａ）の線２Ｃ−２Ｃにおける断面図である。
【図３】従来の光波長変換素子の構成の他の一例を示す斜視図である。
【図４】従来の光波長変換素子の構成のさらに他の一例を示す斜視図である。
【図５】（ａ）及び（ｂ）は、光導波層の導波モードと分極反転層とのオーバラップを模式的に示す図である。
【図６】（ａ）は、光導波路の模式的な断面図であり、（ｂ）は、（ａ）の構成における導波モードの電界分布を表す図である。
【図７】（ａ）〜（ｄ）は、様々な構成の光導波路の模式的な断面図とそれぞれの構成における導波モードの電界分布を表す図である。
【図８】クラッド層及び光導波層における屈折率比と第２高調波強度比との関係を示す図である。
【図９】本発明の第１の実施形態における光波長変換素子の構成を示す図である。
【図１０】基本波と第２高調波とのオーバラップを表す図である。
【図１１】（ａ）及び（ｂ）は、それぞれクラッド層を有する光波長変換素子の構成を示す断面図であり、（ｃ）及び（ｄ）は、それぞれ（ａ）及び（ｂ）の構成における導波光の電界強度分布を示す図である。
【図１２】（ａ）〜（ｄ）は、本発明の第１の実施形態における光波長変換素子の製造工程を説明する断面図である。
【図１３】 LiTaO₃及びNb₂O₅の屈折率分散特性を表す図である。
【図１４】導波光の波長及びクラッド層の厚さに対する、導波モードの特性を表す図である。
【図１５】（ａ）〜（ｃ）は、異なる厚さのクラッド層に対する導波モードの電界分布を表す図である。
【図１６】（ａ）〜（ｄ）は、光の伝搬状態（基本波及び第２高調波の導波モード）を説明するための図である。
【図１７】クラッド層の屈折率と厚さとの関係を表す図である。
【図１８】本発明の第２の実施形態における光波長変換素子の構成を示す図である。
【図１９】本発明の第３の実施形態における光波長変換素子の構成を示す図である。
【図２０】短波長光源の構成を示す図である。
【図２１】光ピックアップの構成を示す図である。
【図２２】（ａ）〜（ｈ）は、異なる材料から構成されたクラッド層が形成されている場合の導波モードの電界分布を説明するための図である。
【図２３】本発明の第７の実施形態における光波長変換素子の構成を示す図である。
【図２４】本発明の第８の実施形態における光波長変換素子の構成を示す図である。
【図２５】（ａ）及び（ｂ）は、本発明の第９の実施形態における光導波路の製造工程を説明する断面図である。
【図２６】 Ta₂O₅透過膜の厚さと拡散定数との関係を表す図である。
【図２７】（ａ）〜（ｃ）は、本発明の第９の実施形態における光導波路の他の製造工程を説明する断面図である。
【図２８】（ａ）及び（ｂ）は、形成されるプロトン交換層の断面図である。
【図２９】透過膜を用いるプロトン交換処理で形成されるプロトン交換層の他の断面図である。
【図３０】（ａ）〜（ｆ）は、本発明の第１０の実施形態における光導波路の製造工程を説明する断面図である。
【図３１】光導波路の構成を示す断面図である。
【図３２】図３１の光導波路の表面に形成される層の屈折率と導波損失との関係を示す図である。
【図３３】（ａ）〜（ｆ）は、本発明の第１１の実施形態における光導波路の製造工程を説明する断面図である。
【図３４】（ａ）〜（ｇ）は、本発明の第１２の実施形態における光波長変換素子の製造工程を説明する断面図である。
【図３５】光波長変換素子の構成を示す斜視図である。
【図３６】（ａ）及び（ｂ）は、光導波路を伝搬する光の横方向強度分布を示す図である。
【図３７】（ａ）及び（ｂ）は、光導波路を伝搬する光の深さ方向強度分布を示す図である。
【図３８】（ａ）〜（ｆ）は、本発明の第１３の実施形態における光波長変換素子の製造工程を説明する断面図である。
【図３９】光波長変換素子の構成を示す斜視図である。
【図４０】光波長変換素子におけるリッジの厚さと変換効率との関係を示す図である。
【符号の説明】
１０１ LiTaO₃基板
１０４ 分極反転層
１０５ プロトン交換層
１０６ 基本波光
１０７ 第２高調波光
１０８ 分極反転層の周期Λ
１０９ 分極反転層の幅Ｗ
１１０ クラッド層
１１１ カバー層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical waveguide used for optical information processing or optical applied measurement using a coherent light source, an optical wavelength conversion element using the optical waveguide, and a manufacturing method thereof. Furthermore, the present invention relates to a short-wavelength light generator configured using the above-described optical waveguide or optical wavelength conversion element, and an optical pickup using the same.
[0002]
[Prior art]
Optical waveguides are applied in a wide range of fields such as communication, optical information processing, and measurement as a light wave control technique. In particular, if an optical waveguide is applied to an optical wavelength conversion element, the wavelength of laser light (fundamental wave) emitted from a semiconductor laser is converted by such an optical wavelength conversion element, and light having a shorter wavelength (second harmonic wave) is converted. ) Has been actively researched.
[0003]
As a conventional optical waveguide, a buried optical waveguide is generally used. FIGS. 1A to 1D are cross-sectional views illustrating a conventional method for manufacturing a buried optical waveguide by proton exchange treatment for exchanging proton ions (+ H) and heat treatment.
[0004]
Specifically, first, as shown in FIG. _Three A Ta layer 12 having a linear opening is formed on the surface (+ C surface) of the substrate 11. Ta layer 12 is made of LiTaO. _Three The surface of the substrate 11 is masked except for a predetermined area.
[0005]
Next, LiTaO masked with Ta layer 12 _Three Substrate 11 is treated with pyrophosphoric acid at about 220 ° C. to about 300 ° C. to form unmasked LiTaO as shown in FIG. _Three A region 13 (hereinafter also referred to as “proton exchange region”) subjected to proton exchange treatment is formed near the surface of the substrate 11.
[0006]
Subsequently, the Ta layer 12 is immersed in a solution in which hydrofluoric acid and nitric acid are mixed at a ratio of 1: 2 for several seconds to remove the Ta layer 12 as shown in FIG.
[0007]
After that, LiTaO _Three The substrate 11 is annealed to form the annealed proton exchange layer 14 as shown in FIG. The annealed proton exchange layer 14 formed in this way functions as a buried optical waveguide.
[0008]
On the other hand, an optical wavelength conversion element employing a ridge-type optical waveguide structure for enhancing light confinement is disclosed in, for example, Japanese Patent Application Laid-Open No. 1-238631.
[0009]
FIG. 2A shows an example of the configuration of a conventional optical wavelength conversion element including such a ridge-type optical waveguide 22, and LiNbO. _Three The surface of the optical waveguide 22 formed on the substrate 21 is processed into a ridge shape. That is, as shown in FIG. 2B, which is a cross-sectional view taken along line 2B-2B in FIG. 2A, the thickness d of the portion 22a through which light is guided (that is, the ridge portion) is the side portion. It is larger than the thickness h of 22b. By providing a ridge in the optical waveguide 22 and causing it to function as a light waveguide portion 22a, the lateral confinement of light is strengthened, and the power density of the fundamental wave inside the waveguide 22 is improved. The conversion efficiency to the second harmonic is improved.
[0010]
Further, in the configuration of the optical wavelength conversion element in FIG. 2A, the thickness of the optical waveguide 22 is thicker than other portions in the vicinity of the end face 24 on which the fundamental wave P1 is incident. _Three An incident portion 23 to the substrate 21 is configured. As shown in FIG. 2C, which is a cross-sectional view taken along line 2C-2C in FIG. _Three The fundamental wave P1 incident on the substrate 21 is converted into a second harmonic wave P2 after the wavelength is converted, and then the LiNbO is emitted from the emission part 25. _Three Go out of the substrate 21.
[0011]
Furthermore, for example, Japanese Patent Application Laid-Open No. 61-94031 discloses an optical wavelength conversion element using a loaded optical waveguide. An example of the configuration of such an optical wavelength conversion element is shown in FIG.
[0012]
Specifically, LiNbO _Three An optical waveguide 32 is formed on the substrate 31 by proton exchange, and an SiO 2 having a lower refractive index than that of the optical waveguide 32 is further formed thereon. ₂ Thus, a striped clad layer (loading layer) 33 is formed. With the loaded optical waveguide having such a configuration, a low-loss waveguide can be formed, and an optical wavelength conversion element that converts the fundamental wave P1 into the second harmonic P2 with high efficiency is realized.
[0013]
LiNbO _Three The following method for forming a ridge waveguide on a substrate has already been reported.
[0014]
LiNbO _Three Is a mechanically and chemically stable material and is difficult to be etched, so that the etching selectivity with respect to the resist is small. Therefore, LiNbO _Three It is generally difficult to produce a deep etched shape on the surface of the film. However, proton exchanged LiNbO _Three Then, the etching rate is several times faster than that of an untreated substrate. By utilizing this phenomenon, the following ridge type optical waveguide manufacturing method has been proposed. That is, LiNbO of C plate _Three The substrate is heat treated in a suitable acid to form a proton exchange layer on the surface. Further, a striped Ti protective mask layer is formed on the formed proton exchange layer by photolithography, and the non-mask portion is etched by ECR etching. Thereafter, the Ti protective mask layer is removed, and both end faces of the optical waveguide are optically polished to form an incident / exit portion.
[0015]
On the other hand, FIG. 4 shows another configuration of an optical wavelength conversion element using an embedded optical waveguide. That is, LiNbO _Three A proton exchange optical waveguide 42 is formed on the substrate 41, and further, TiO having a higher refractive index than that of the optical waveguide 42 is formed thereon. ₂ Thus, a clad layer (loading layer) 43 is formed. A plurality of polarization inversion layers 44 are formed so as to be orthogonal to the optical waveguide 42 periodically. The fundamental wave P1 incident from the incident part 45 having such a structure is overlapped with the polarization inversion layer 44 while propagating through the optical waveguide 42 and converted into the second harmonic wave P2, and comes out from the emission part 46. .
[0016]
FIGS. 5A and 5B are diagrams schematically showing the overlap between the waveguide mode of the optical waveguide 42 and the polarization inversion layer 44 in a configuration without the clad layer (loading layer) 43 and a certain configuration. is there. 5A and 5B, the left side of the figure shows the cross-sectional structure of the optical wavelength conversion element, and the right side of the figure shows the electric field distribution of the propagation light in the depth direction of the waveguide cross section.
[0017]
In the configuration having no cladding layer shown in FIG. 5A, it is about half of the propagation light (fundamental wave P1) that overlaps with the domain-inverted layer 44 and is converted into a harmonic (of FIG. 5A). (The hatched area in the electric field distribution shown on the right side) and the resulting harmonic output is not very large. On the other hand, as shown in FIG. 5B, in the configuration having the cladding layer 43 having a large refractive index on the optical waveguide 42, most of the propagating light (fundamental wave P1) (shown on the right side of FIG. 5B). Since the hatched region in the electric field distribution overlaps with the polarization inversion layer 44, an optical wavelength conversion element that converts the fundamental wave P1 into the second harmonic wave P2 with high efficiency is realized.
[0018]
[Problems to be solved by the invention]
When an optical waveguide is applied to an optical wavelength conversion element, an overlap between waveguide modes (fundamental wave and second harmonic) is an important factor for realizing a highly efficient optical wavelength conversion element. That is, the greater the overlap between the electric field distribution of the fundamental wave, the second harmonic, and the electric field distribution, the greater the conversion efficiency of the optical wavelength conversion element. In order to increase the overlap between the electric field distribution of the fundamental wave, the second harmonic, and the electric field distribution in order to realize a highly efficient optical wavelength conversion element, for example, the refractive index distribution of the optical waveguide may be stepped. Conceivable.
[0019]
However, in the prior art method of manufacturing an embedded optical waveguide by proton exchange treatment and heat treatment as described above, the refractive index distribution of the optical waveguide to be formed depends on the thermal diffusion state of the proton and is near the surface. It becomes a graded shape in which the refractive index is high and gradually decreases in the depth direction. Under such a refractive index distribution state, the electric field distribution differs greatly between the fundamental wave to be guided and the second harmonic wave, and it is difficult to realize a highly efficient optical wavelength conversion element. Also, with this conventional method, the refractive index distribution of the optical waveguide cannot be freely controlled.
[0020]
Further, in the conventional embedded optical waveguide, optical damage is likely to occur due to leakage light to the periphery of the waveguide, and light having a high power density is guided to generate a high-output second harmonic. difficult.
[0021]
On the other hand, in the configuration in which the clad layer is formed on the optical waveguide, confinement of the optical waveguide is enhanced to achieve high efficiency. That is, the mode distribution (fundamental mode) of the guided light propagating through the optical waveguide is moved to the vicinity of the surface by the cladding layer, and the overlap between the polarization inversion portion and the fundamental wave of the fundamental mode or the second harmonic of the fundamental mode is caused. Trying to improve.
[0022]
However, the conventional configuration can increase the overlap between the polarization inversion unit and the waveguide mode, but increases the overlap between the fundamental wave and the second harmonic that have the greatest influence on the conversion efficiency. Since it cannot be achieved, there is a limit to improving the conversion efficiency. This is because the fundamental wave and the second harmonic wave are greatly different from each other in the distribution of the waveguide modes in the optical waveguide due to the difference in wavelength, and thus the increase in overlap between the two modes is limited.
[0023]
Furthermore, since the portion where the fundamental wave and the second harmonic do not overlap is large, instability of the second harmonic output due to the occurrence of optical damage may occur.
[0024]
On the other hand, in the conventional optical wavelength conversion element using the ridge type optical waveguide, the conversion efficiency is improved by increasing the power density due to the confinement effect of the optical waveguide. However, the increase of the confinement effect by the ridge waveguide with respect to the fundamental wave is limited to the lateral direction, and the confinement in the depth direction is not improved. As a result, an increase in overlap (especially in the depth direction) between the fundamental wave that has the greatest effect on conversion efficiency and the second harmonic cannot be achieved with the ridge structure, and there is a limit to improving conversion efficiency. is there.
[0025]
The present invention has been made in order to solve the above-mentioned problems, and the object thereof is (1) the refractive index distribution can be controlled, and the overlap between the fundamental wave and the second harmonic is increased. And providing an optical waveguide having a waveguide structure with excellent light damage resistance, and (2) using the optical waveguide as described above, stably providing a high output second harmonic output. Providing an optical waveguide that can be supplied efficiently, (3) Providing a method for manufacturing such an optical waveguide and an optical wavelength conversion element, and (4) Forming using the optical waveguide and optical wavelength conversion element described above It is to provide a short wavelength light generator and an optical pickup.
[0026]
[Means for Solving the Problems]
The optical waveguide of the present invention includes an optical material, an optical waveguide layer formed on the optical material, and a clad layer formed on the surface of the optical waveguide layer, and the optical waveguide layer includes light of wavelength λ1 and Light of wavelength λ2 (λ1> λ2) can be guided, and the refractive index and thickness of the cladding layer satisfy the waveguide conditions for the light of wavelength λ2 and are cut for the light of wavelength λ1. It is set so as to satisfy the off-state, whereby the above object is achieved.
[0027]
In one embodiment, the optical waveguide layer has a stripe shape.
[0028]
In one embodiment, the optical material further comprises a high refractive index layer having a refractive index n1 formed in the vicinity of the surface of the optical material, the optical material having a refractive index ns, and the optical waveguide layer in the vicinity of the surface of the optical material. It is formed in a stripe shape and has a refractive index nf, and the refractive index satisfies the relationship of nf>n1> ns.
[0029]
In one embodiment, the clad layer is striped. Furthermore, the optical waveguide layer may be striped.
[0030]
In one embodiment, a ridge is formed in a stripe shape on the surface of the optical waveguide layer, the cladding layer is formed in a stripe shape on the ridge, and the light in the optical waveguide layer is Waveguide the ridge.
[0031]
In one embodiment, the optical material forms a cylindrical core, and the cladding layer covers the periphery of the core.
[0032]
Preferably, in the optical waveguide, the light of wavelength λ1 propagates in the fundamental mode, and the light of wavelength λ2 propagates in the higher order mode.
[0033]
Preferably, the effective refractive index Nc of the cladding layer and the effective refractive index Nf of the optical waveguide layer satisfy a relationship of Nc> 1.02 · Nf.
[0034]
The clad layer may be a multilayer film.
[0035]
An optical wavelength conversion element of the present invention comprises a substrate made of a material having a nonlinear optical effect, an optical waveguide layer formed on the substrate, and a cladding layer formed on the surface of the optical waveguide layer, The optical waveguide layer can guide a fundamental wave having a wavelength λ and a second harmonic wave having a wavelength λ / 2, and the cladding layer has a fundamental wave of a fundamental mode and a higher-order mode guided in the optical waveguide layer. The overlap of the electric field distribution with the second harmonic is increased, and the fundamental wave is converted into the second harmonic, thereby achieving the above object.
[0036]
Preferably, the refractive index and the thickness of the cladding layer are set so as to satisfy a waveguide condition for the second harmonic and a cutoff condition for the fundamental wave.
[0037]
In one embodiment, the optical waveguide layer has a stripe shape.
[0038]
In one embodiment, the substrate further comprises a high refractive index layer having a refractive index n1 formed near the surface of the substrate, the substrate has a refractive index ns, and the optical waveguide layer is striped near the surface of the substrate. Formed and has a refractive index nf, the refractive index satisfies a relationship of nf>n1> ns, and the refractive index and thickness of the cladding layer satisfy the waveguide conditions for the second harmonic. Satisfactory and the cut-off condition is set for the fundamental wave.
[0039]
In one embodiment, the clad layer has a stripe shape, and the refractive index and thickness of the clad layer satisfy a waveguide condition for the second harmonic wave and satisfy a cutoff condition for the fundamental wave. Is set to Preferably, the optical waveguide layer has a stripe shape.
[0040]
In one embodiment, a ridge is formed in a stripe shape on the surface of the optical waveguide layer, the cladding layer is formed in a stripe shape on the ridge, and the light in the optical waveguide layer is The refractive index and the thickness of the clad layer are set so as to satisfy the waveguide condition for the second harmonic and the cutoff condition for the fundamental wave.
[0041]
Preferably, in the optical waveguide layer, the fundamental wave of the fundamental mode and the second harmonic of the higher order mode are phase-matched with each other.
[0042]
Preferably, the effective refractive index Nc of the cladding layer and the effective refractive index Nf of the optical waveguide layer satisfy a relationship of Nc> 1.02 · Nf.
[0043]
The clad layer may be a multilayer film.
[0044]
The order of the second harmonic mode propagating through the optical waveguide layer may be one greater than the order of modes capable of propagating through the cladding layer.
[0045]
The cladding layer is Nb ₂ O _Five May be included. Alternatively, the cladding layer may be made of a linear material.
[0046]
Preferably, the substrate is LiNb _1-x Ta _x O _Three (0 ≦ x ≦ 1), a periodic domain-inverted structure is formed in the substrate.
[0047]
The short wavelength light generation device of the present invention includes a semiconductor laser and a light wavelength conversion element, the wavelength of light emitted from the semiconductor laser is converted by the light wavelength conversion element, and the light wavelength conversion element is the above-mentioned Thus, the above object can be achieved.
[0048]
The optical pickup of the present invention includes a short wavelength light generator and a condensing optical system, and the short wavelength light emitted from the short wavelength light generator is condensed by the condensing optical system, and the short wavelength light is collected. The generator has the above characteristics, whereby the above object is achieved.
[0049]
An optical waveguide manufacturing method of the present invention includes a step of forming a first ion exchange layer in the vicinity of a surface of a substrate made of a nonlinear optical material, and annealing the first ion exchange layer to form an annealed ion exchange layer. Forming, a step of forming a second ion exchange layer at a predetermined position of the annealing ion exchange layer, and a step of forming a resist mask having a predetermined pattern above the second ion exchange layer; And removing the non-masked portion of the second ion exchange layer by etching using the resist mask to form a ridge, thereby achieving the above object.
[0050]
In one embodiment, the step of forming the second ion exchange layer includes the step of forming a linear metal mask on the surface of the annealed ion exchange layer, and the step of forming the metal mask in the annealed ion exchange layer. Forming the second ion exchange layer on an uncovered portion, and the resist mask is selectively formed only on the metal mask. Preferably, the resist pattern is selectively formed only on the metal mask using backside exposure.
[0051]
In another embodiment, the step of forming the second ion exchange layer includes a step of forming the second ion exchange layer in the vicinity of the surface of the annealed ion exchange layer, and the resist mask includes the second ion exchange layer. It is formed linearly on the surface of the ion exchange layer.
[0052]
The method may further include forming a dielectric film on the surface of the ridge.
[0053]
The substrate is a C-plate LiNb _1-X Ta _x O _Three (0 ≦ x ≦ 1) substrate.
[0054]
Both the first ion exchange layer and the second ion exchange layer may be proton exchange layers.
[0055]
The method for manufacturing an optical wavelength conversion element according to the present invention includes a step of forming a first ion exchange layer in the vicinity of a surface of a substrate made of a nonlinear optical material, and an annealing treatment ion exchange by annealing the first ion exchange layer. Forming a layer, forming a domain-inverted layer in the substrate, forming a second ion exchange layer at a predetermined position of the annealed ion exchange layer, and the second ion exchange layer Forming a resist mask having a predetermined pattern above the substrate, and etching the non-masked portion of the second ion exchange layer by using the resist mask to form a ridge. This achieves the above objective.
[0056]
In one embodiment, the step of forming the second ion exchange layer includes the step of forming a linear metal mask on the surface of the annealed ion exchange layer, and the metal mask of the annealed ion exchange layer. Forming the second ion exchange layer in a portion not covered with the resist mask, and the resist mask is selectively formed only on the metal mask. Preferably, the resist pattern is selectively formed only on the metal mask using backside exposure.
[0057]
In another embodiment, the step of forming the second ion exchange layer includes a step of forming the second ion exchange layer in the vicinity of the surface of the annealed ion exchange layer, and the resist mask includes the second ion exchange layer. It is formed linearly on the surface of the ion exchange layer.
[0058]
The method may further include forming a dielectric film on the surface of the ridge.
[0059]
The substrate is a C-plate LiNb _1-X Ta _x O _Three (0 ≦ x ≦ 1) substrate.
[0060]
Both the first ion exchange layer and the second ion exchange layer may be proton exchange layers.
[0061]
According to another aspect of the present invention, a substrate made of a nonlinear optical material, a first ion exchange layer including an optical waveguide region formed in the vicinity of the surface of the substrate, and formed in the vicinity of the optical waveguide region. In addition, an optical waveguide provided with a second ion exchange layer having an ion exchange concentration higher than that of the first ion exchange layer is provided, thereby achieving the above object.
[0062]
In one embodiment, the first ion exchange layer has a linear ridge substantially parallel to the waveguide direction, the ridge includes the optical waveguide region, and the second ion exchange layer includes the ridge. It is formed on the side.
[0063]
In another embodiment, the second ion exchange layer forms a linear ridge substantially parallel to the waveguide direction on the surface of the first ion exchange layer.
[0064]
The substrate is a C-plate LiNb _1-X Ta _x O _Three (0 ≦ x ≦ 1) substrate.
[0065]
Both the first ion exchange layer and the second ion exchange layer may be proton exchange layers.
[0066]
According to still another aspect of the present invention, a substrate including a substrate made of a nonlinear optical material, a polarization inversion layer formed in the substrate at a constant period, and an optical waveguide region formed near the surface of the substrate. An optical wavelength conversion element comprising: an ion exchange layer of 1; and a second ion exchange layer formed in the vicinity of the optical waveguide region and having an ion exchange concentration higher than that of the first ion exchange layer Which achieves the above objective.
[0067]
In one embodiment, the first ion exchange layer has a linear ridge substantially parallel to the waveguide direction, the ridge includes the optical waveguide region, and the second ion exchange layer includes the ridge. It is formed on the side.
[0068]
In another embodiment, the second ion exchange layer forms a linear ridge substantially parallel to the waveguide direction on the surface of the first ion exchange layer.
[0069]
The substrate is a C-plate LiNb _1-X Ta _x O _Three (0 ≦ x ≦ 1) substrate.
[0070]
Both the first ion exchange layer and the second ion exchange layer may be proton exchange layers.
[0071]
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an optical wavelength conversion element using the generation of the second harmonic due to the non-linear optical effect, and using the phase matching between the fundamental wave of the fundamental mode and the second harmonic wave of the higher order mode, the overshoot of both. The wrap is enlarged to increase the conversion efficiency from the fundamental wave to the second harmonic. First, the principle will be described below.
[0072]
First, optical wavelength conversion in a general optical waveguide will be described.
[0073]
Normally, as shown in the sectional view of FIG. 6A, the optical waveguide is composed of a substrate 61 (refractive index: ns) and an optical waveguide 62 (refractive index: nf) formed thereon. The refractive index satisfies the relationship of nf> ns, and the electric field distribution of the guided modes (fundamental fundamental wave 64, fundamental mode second harmonic 65, and primary mode second harmonic 66) is shown in FIG. As shown in (b). As can be seen from FIG. 6B, the overlap of the electric field between the fundamental wave and the second harmonic wave is the largest between the fundamental wave 64 of the fundamental mode and the second harmonic wave 65 of the fundamental mode. As the wave guide mode order increases, the overlap decreases. Since the conversion efficiency of the optical wavelength conversion element is proportional to the overlap amount of the waveguide mode, the phase matching between the fundamental modes having the largest overlap amount as described above is the conversion with the highest conversion efficiency.
[0074]
Next, a case where a clad layer having a refractive index nc higher than the refractive index nf is formed on the optical waveguide will be described below.
[0075]
When a cladding layer having a higher refractive index is provided on the optical waveguide, the mode distribution of the fundamental wave is biased toward the cladding layer, and the waveguide mode is strongly confined near the surface of the device. Therefore, strong confinement of the fundamental wave is realized. In a conventional optical wavelength conversion element having a cladding layer, conversion efficiency is improved by utilizing the strong confinement of the fundamental wave. However, since the fundamental wave and the second harmonic have different wavelengths and their refractive index dispersion exists, the dispersion of the waveguide mode is different (the second harmonic is more strongly attracted to the cladding layer than the fundamental wave). ). Therefore, the increase in overlap between modes is still limited.
[0076]
In order to solve the above-mentioned problems, the present inventors have found a new method for controlling the waveguide mode of the second harmonic using a clad layer having a high refractive index. The contents will be described below.
[0077]
As described above, the confinement of the fundamental wave can be enhanced by using a clad layer having a high refractive index. However, as the cladding layer is made thicker, the confinement of the fundamental wave becomes stronger, while the second harmonic with a short wavelength is confined inside the cladding layer, and the conversion efficiency of the device is extremely reduced.
[0078]
However, through the analysis by the inventors of the present application, it has been found that the higher-order waveguide mode of the second harmonic has an electric field distribution in the optical waveguide portion and can achieve sufficient overlap with the fundamental wave. This will be described with reference to FIGS. In each of FIGS. 7A to 7D, the cross-sectional structure of the optical wavelength conversion element is shown on the left side of the figure, and the electric field distribution in the depth direction of the cross section of the waveguide is shown on the right side of the figure. These figures show the mode of the waveguide mode when the thickness of the cladding layer is changed in the optical waveguide having the cladding layer with a high refractive index.
[0079]
FIG. 7A shows a configuration without the cladding layer described with reference to FIGS. 6A and 6B. Here, the description is omitted.
[0080]
FIG. 7B shows a case where a thinner cladding layer 63 is provided on the optical waveguide 62 provided on the substrate 61. The cladding layer 63 has a cutoff condition (that is, the waveguide mode of the cladding layer 63 is different from that of the fundamental wave 64 of the 0th order (fundamental mode) and the second harmonic wave 65 of the 0th order (fundamental mode). Satisfies the conditions that do not stay inside. Accordingly, both the 0th-order (fundamental mode) fundamental wave 64 and the 0th-order (fundamental mode) second harmonic wave 65 cannot propagate only inside the cladding layer 63, but propagate inside the optical waveguide 62. To do. However, the clad layer 63 draws the electric field distribution of the fundamental wave 64 of the 0th order (fundamental mode) and the second harmonic wave 65 of the 0th order (fundamental mode) near the surface of the element.
[0081]
FIG. 7C shows a case where a thicker cladding layer 63 is provided, and the cladding layer 63 satisfies the cutoff condition with respect to the fundamental wave 64, but the second harmonic wave Can propagate the low-order mode 65 (in this case, the fundamental mode). At this time, the confinement of the fundamental wave becomes stronger than that in the configuration of FIG. However, since the second harmonic 65 of the fundamental mode is confined inside the cladding layer 63, the conversion efficiency from the fundamental to the second harmonic of the fundamental mode is extremely reduced. On the other hand, among the second harmonics, a mode (in this case, the first order mode 66) which is one order higher than the order of the mode in which the clad layer 63 can be guided (in this case, the 0th order mode 65) is the majority. Propagates through the optical waveguide 62.
[0082]
At this time, the electric field distribution of the second harmonic 66 of the first mode in the optical waveguide 62 is almost the same as the electric field distribution of the second harmonic 65 of the fundamental mode in the configuration without the cladding layer shown in FIG. does not change. On the other hand, the waveguide mode 64 of the 0th-order fundamental wave is strongly attracted to the cladding layer 63. That is, the fundamental wave 64 in the fundamental mode (0th order) becomes a waveguide mode with strong confinement, whereas the second harmonic in the higher order mode (for example, the first second harmonic 66) is generated by the cladding layer 63. Almost no change from non-existing configuration. As a result, the overlap between the two modes can be dramatically increased, thereby greatly improving the conversion efficiency.
[0083]
FIG. 7D shows a case where a thicker cladding layer 63 is provided, and a zero-order fundamental wave 64 can be guided through the cladding layer 63. However, when the fundamental wave can be guided in the cladding layer 63 in this way, the overlap between the fundamental wave and the second harmonic wave inside the optical waveguide 62 is extremely reduced, so that the conversion efficiency is greatly increased. To reduce.
[0084]
As described above, as a result of the study by the present inventor, in the optical waveguide 62 having the clad layer 63 with a high refractive index, the fundamental wave 64 of the fundamental mode (0th order) and the high wave in the configuration of FIG. It has been found that the overlap with the second mode 66 of the next mode (first order) is increased, enabling highly efficient wavelength conversion.
[0085]
However, in order to realize the configuration of FIG. 7C, it is necessary to satisfy several conditions.
[0086]
First, the clad layer 63 needs to have a thickness and a refractive index that satisfy the cutoff condition for the fundamental wave that “the fundamental wave cannot be guided therethrough”. On the other hand, the state in which the cladding layer 63 satisfies the waveguide condition for the fundamental wave that “the fundamental wave can be guided in the inside” actually corresponds to the configuration of FIG. Wavelength conversion is impossible.
[0087]
Secondly, as shown in FIG. 7C, it is necessary to guide a higher-order mode having a peak of the electric field distribution in the portion of the optical waveguide 62 as the second harmonic waveguide mode. The reason is that it is the overlap between the electric field of the fundamental wave and the electric field of the second harmonic wave in the optical waveguide 62 that contributes to the wavelength conversion from the fundamental wave to the second harmonic wave. In order to satisfy this condition, the cladding layer 63 satisfies the waveguide condition of the second harmonic, and the higher-order waveguide having a higher order than the waveguide mode of the order capable of guiding the cladding layer 63 is achieved. The second harmonic of the wave mode needs to be selectively guided inside the optical waveguide 62. For example, in the case of FIG. 7C, the second harmonic wave guide mode capable of guiding the clad layer 63 is the fundamental mode (0th mode) 65, while the optical waveguide 62 has a peak. The second harmonic wave guide mode is the primary mode 66.
[0088]
Therefore, in order to realize a highly efficient wavelength conversion element having a configuration in which the clad layer 63 having a high refractive index is provided on the optical waveguide 62, the thickness and refractive index of the clad layer 63 are set to the fundamental wave. It is sufficient to set so that the cutoff condition is satisfied and the waveguide condition is satisfied for the second harmonic.
[0089]
Third, in order to realize highly efficient wavelength conversion, in the state of FIG. 7C, the electric field distribution of the second harmonic 66 of the higher order mode inside the optical waveguide 62 is changed to the inside of the cladding layer 63. It is necessary to make the electric field distribution sufficiently large (that is, make the electric field distribution inside the cladding layer 63 sufficiently small).
[0090]
Here, the ratio (nc / nf) between the refractive index nc of the cladding layer 63 and the refractive index nf of the optical waveguide 62, the peak value Ic of the second harmonic intensity of the cladding layer 63, and the second harmonic of the optical waveguide 62. FIG. 8 shows the relationship between the intensity peak value Iw and the ratio (Ic / Iw). Accordingly, the ratio of the intensity distribution of the second harmonic in the cladding layer 63 and the optical waveguide 62 is inversely proportional to the magnitude of the ratio of the respective refractive indexes, and the refractive index of the cladding layer 63 with respect to the refractive index of the optical waveguide 62. As the rate increases, the electric field distribution in the cladding layer 63 decreases.
[0091]
From the relationship shown in FIG. 8, in order to increase the use efficiency of the second harmonic light by setting the electric field intensity in the cladding layer 63 to 1/10 or less of the electric field intensity in the optical waveguide 62, the refractive index nc of the cladding layer 63 is changed. It is sufficient if the refractive index nf is set so as to satisfy the condition of nc> 1.02 · nf. Although the second harmonic in the sub-peak portion is a loss, if the second harmonic emitted under the above conditions is condensed, the output characteristics are not deteriorated due to the sub-peak, and the condensing characteristic up to the diffraction limit can be realized.
[0092]
In the above description, the configuration in which the clad layer is a single layer is described, but the same effect can be obtained when the clad layer is a multilayer film. In particular, when a clad layer composed of a multilayer film is used, the refractive index distribution inside the clad layer can be controlled, so that the degree of freedom in element design is increased, and an element with high tolerance can be manufactured. Become.
[0093]
The clad layer is preferably formed of a linear material having no nonlinear effect. If the cladding layer is made of a non-linear material, wavelength conversion occurs inside the cladding layer, but such wavelength conversion inside the cladding layer is a loss for the generation efficiency of the second harmonic light inside the optical waveguide. is there.
[0094]
In addition, when the configuration of the present invention is applied to a three-dimensional optical waveguide, the electric field distribution in the lateral direction of the optical waveguide can be greatly affected. That is, by forming a high refractive index cladding layer on the optical waveguide, the effective refractive index for the second harmonic and the fundamental wave increases. For this reason, confinement of the electric field distribution in the lateral direction is greatly increased for each of the fundamental wave and the second harmonic. As a result, an increase in light power density and an increase in overlap in the lateral direction can be obtained, and as a result, conversion efficiency can be improved.
[0095]
Further, according to the present invention, by selecting the wavelength of the fundamental wave, it is possible to uniquely select the second harmonic waveguide mode propagating through the optical waveguide. For this reason, it is possible to selectively perform phase matching between the fundamental wave and the second harmonic of the higher-order mode to increase the overlap between the two, thereby realizing highly efficient wavelength conversion. .
[0096]
Further, according to the study by the inventors of the present application, it has been found that the structure of the optical waveguide according to the present invention is very effective in improving the light damage resistance.
[0097]
Optical damage occurs when impurities inside the crystal are excited by short-wavelength light (second harmonic) to generate an internal electric field. Further, when the fundamental wave is present, the level for trapping impurities is excited and the internal electric field due to optical damage tends to be fixed. This phenomenon is prominent in the portion where the fundamental wave exists alone in the vicinity of the electric field distribution of the second harmonic. On the other hand, if the configuration of the present invention is adopted, the overlap between the fundamental wave and the second harmonic wave in the optical waveguide increases, and the portion of the electric field distribution of the second harmonic wave and the fundamental wave that does not overlap. Decrease. Therefore, the occurrence of optical damage that occurs as described above can be greatly reduced.
[0098]
The structure of the optical waveguide of the present invention described above is effective in increasing the overlap of light of different wavelengths propagating through the optical waveguide and increasing the interaction between the two. Specifically, it is effective not only for an optical wavelength conversion element but also for an optical waveguide that simultaneously guides two or more lights having different wavelengths. Furthermore, since the light distribution can be drawn to the vicinity of the device surface, it is possible to increase the influence on guided light by components (for example, electrodes) formed on the surface, and optical integration with high operating efficiency. A circuit element can be configured.
[0099]
Next, an embodiment of the optical wavelength conversion element of the present invention will be described with reference to a specific element structure.
[0100]
As a specific element structure,
(1) An element structure using a loaded optical waveguide having a high refractive index layer on the surface of a crystal having a periodic domain-inverted layer, and further having a striped clad layer on the high refractive index layer,
(2) A loaded ridge type optical waveguide having a high refractive index layer processed into a striped ridge shape on the surface of a crystal having a periodic domain-inverted layer, and further having a cladding layer on the surface of the ridge. Element structure used, and
(3) An element structure using a buried optical waveguide having a stripe-shaped high refractive index layer on the surface of a crystal having a periodic domain-inverted layer and further having a cladding layer on the crystal surface;
Is mentioned. Hereinafter, each element structure will be described in the embodiment.
[0101]
(First embodiment)
In this embodiment, an optical wavelength conversion element using a loaded optical waveguide will be described.
[0102]
FIG. 9 is a diagram showing the structure of the optical wavelength conversion element in the present embodiment. Specifically, FIG. 9A is a perspective view of an optical wavelength conversion element, and FIG. 9B is a cross-sectional view.
[0103]
In the optical wavelength conversion element of this embodiment, the LiTaO of C plate _Three In a substrate 101 made of a crystal (that is, its top and bottom surfaces are planes perpendicular to the C-axis of the crystal), a period Λ = about 3.5 μm (the period Λ is shown as reference numeral 108 in FIG. The domain-inverted layer 104 is formed. The width W of the domain inversion layer 104 is about 1.7 μm (the width W is shown as reference numeral 109 in FIG. 9). In addition, LiTaO _Three The surface of the crystal substrate 101 has a proton exchange layer 105 functioning as an optical waveguide having a depth of about 2 μm, and a striped Nb. ₂ O _Five A clad layer (loading layer) 110 is formed. In addition, Nb ₂ O _Five The surface of the crystal substrate 101 including the cladding layer 110 is made of SiO. ₂ It is covered with a cover layer 111 made of
[0104]
The fundamental wave 106 having a wavelength of about 850 nm incident from the end face of the element is converted into the second harmonic 107 having a wavelength of about 425 nm while propagating through the proton exchange layer 105 and is emitted from the other end face.
[0105]
In the configuration of FIG. 9, unlike the configuration according to the prior art, the cladding layer 110 has Nb having a higher refractive index than the substrate 101. ₂ O _Five Is used. Specifically, for light having a wavelength of about 800 nm, Nb ₂ O _Five The refractive index of LiTaO is about 2.25. _Three Has a refractive index of about 2.15. However, the thickness of the cladding layer 110 is set to about 300 nm so that the fundamental wave 106 propagating inside the device does not remain inside the cladding layer 110.
[0106]
The fundamental wave 106 having a wavelength of about 850 nm that propagates inside the device propagates in the fundamental mode (TM00 mode). On the other hand, the second harmonic 107 propagates in the first-order mode in the depth direction of the element, and propagates in the 0th-order higher-order mode (TM01 mode) in the width direction of the element. By selecting the wavelength of the fundamental wave 106 with respect to the period 108 of the domain-inverted layer 104, the waveguide mode (TM01) of the second harmonic 107 is selectively excited. In the configuration of the present embodiment, unlike the configuration according to the prior art, the fundamental wave is propagated in the fundamental mode, and the second harmonic is propagated in the higher order mode, thereby increasing the overlap between the two and wavelength conversion. Improves efficiency.
[0107]
Next, the principle of improving the wavelength conversion efficiency by increasing the overlap between the fundamental wave and the second harmonic in the structure of the optical wavelength conversion element of the present embodiment will be described below.
[0108]
As shown in the present embodiment, in the optical wavelength conversion element having a structure in which the cladding layer 110 is further formed on the optical waveguide (proton exchange layer) 105 provided on the substrate 101, the second fundamental wave is transmitted. The efficiency of converting to a harmonic depends largely on the overlap between the fundamental and the second harmonic. Referring to FIG. 10, the overlap 122 between the fundamental wave and the second harmonic is a non-linearity between the distribution 120 of the fundamental electric field (Ew) and the distribution 121 of the second harmonic electric field (E2w). It is represented by the overlap in the substance (the hatched area in FIG. 10).
[0109]
A cladding layer having a refractive index lower than that of the substrate 101 (for example, Ta having a refractive index n = 2.0) is formed on the optical waveguide layer 105. ₂ O _Five In an optical waveguide having a film) 131 or a buried optical waveguide (see FIG. 11A), as shown in FIG. 11C, the electric field distribution 123 and the second harmonic of the fundamental wave in the fundamental mode (TM00 mode). The maximum overlap 125 is obtained with the electric field distribution 124 of the wave (TM00 mode). However, the size of the overlap 125 in the depth direction of the element is limited due to the wavelength dispersion characteristics of the fundamental wave and the second harmonic.
[0110]
On the other hand, a clad layer having a refractive index higher than that of the substrate 101 (for example, Nb having a refractive index n = 2.25) is formed on the optical waveguide layer 105. ₂ O _Five FIG. 11 (d) shows the electric field distribution of the light guided through the optical wavelength conversion element (see FIG. 11 (b)) of the present invention having the film (132). In this case, the fundamental wave (TM00 mode) is strongly confined by the clad layer 132 having a high refractive index, and the electric field distribution 126 is biased near the surface of the element. On the other hand, the electric field distribution 127 of the second harmonic in the TM01 mode in this configuration is substantially the same as the electric field distribution 124 of the second harmonic in the TM00 mode in FIG. 11C, and the overlap 128 is the same as that in FIG. ) Significantly increased compared to the overlap 125 in FIG. As a result, the conversion efficiency of the optical wavelength conversion element of the present invention is about twice or more higher than that of the conventional optical wavelength conversion element having a loading structure. Further, by forming the clad layer 132 having a high refractive index, as shown in FIG. 11 (d), the ratio existing in the clad layer 132 in the electric field distribution 127 of the second harmonic in the TM01 mode is very high. Since it can be made smaller, it has been found that the shape can be made almost the same as the electric field distribution 124 of the second harmonic in the TM00 mode shown in FIG.
[0111]
The beam pattern of the second harmonic output obtained by the wavelength conversion element of the present invention is not different from the beam pattern of the second harmonic output of the fundamental wave mode propagating through the conventional buried optical waveguide, and is close to the diffraction limit. Wavefront characteristics that can be focused up to
[0112]
The inventors of the present application have also studied light generation of the second harmonic of the TM00 mode in the structure of FIG. 11B, but the second harmonic of the TM00 mode is confined inside the cladding layer 132. Therefore, it was confirmed that the overlap with the fundamental wave is extremely reduced. That is, in the configuration of the present invention, it is possible to greatly increase the overlap of the electric field distribution between the fundamental wave of the TM00 mode and the second harmonic of the higher order mode (TM01 mode).
[0113]
Next, with reference to FIGS. 12A to 12D, a method for manufacturing the optical wavelength conversion element of this embodiment will be described.
[0114]
First, as shown in FIG. 12A, the LiTaO of the + C plate _Three A proton exchange layer 105 that functions as an optical waveguide is formed on the surface of the substrate 101. Specifically, after performing a heat treatment for about 14 minutes in pyrophosphoric acid at about 260 ° C., an annealing process is performed at about 420 ° C. for about 8 minutes to form the optical waveguide (proton exchange layer) 105.
[0115]
Next, the comb-shaped electrode 112 is formed on the + C surface of the substrate 101 (actually the surface of the proton exchange layer 105), and the planar electrode (not shown) is formed on the −C surface (back surface) of the substrate 101. Then, a high voltage pulse voltage (about 21 kV / mm) is applied between both electrodes to form a periodic domain-inverted layer 104 as shown in FIG.
[0116]
Subsequently, after removing both electrodes, as shown in FIG. 12C, Nb is formed on the + C surface of the substrate 101 (the surface of the proton exchange layer 105) by sputtering. ₂ O _Five Film 110 is deposited to a thickness of about 300 nm. In addition, the deposited Nb ₂ O _Five A stripe mask pattern (not shown) is formed on the film 110 by photolithography, and CHF _Three Dry etching is performed in a gas atmosphere, and as shown in FIG. ₂ O _Five The film 110 is processed into a stripe shape.
[0117]
What is important in the process of forming a domain-inverted layer by applying an electric field as described above is an applied voltage waveform. Specifically, LiTaO _Three In order to form a domain inversion layer 104 having a short period of time, an inversion voltage level (a voltage level at which the polarization is inverted, as a pulse voltage waveform, LiTaO _Three In this case, it is necessary to apply a voltage waveform in which a pulse voltage is superimposed on a bias voltage having a magnitude of about 21 kV / mm or less. At this time, the sum of the bias voltage level and the pulse voltage level needs to be equal to or higher than the level of the inversion voltage.
[0118]
Further, in the above process, only the bias voltage is applied after applying the voltage on which the pulse voltage is superimposed. According to the study by the inventors of the present application, the bias voltage application time (Tb) is the time of the polarization inversion layer. It was found that it affects the uniformity of shape. That is, when Tb is about 2 seconds or less, the non-uniformity of the periodic structure of the domain-inverted layer 104 to be formed increases. In order to obtain the uniformity of the periodic structure of the domain inversion layer 104, it is necessary to set Tb to about 3 seconds or more. According to further studies by the inventors of the present application, it is clear that the periodical polarization inversion layer 104 having a very high uniformity can be formed and the optical wavelength conversion element with high efficiency can be manufactured preferably by setting the time to about 5 seconds or longer. Became.
[0119]
Next, the relationship between the waveguide mode propagating through the optical waveguide and the characteristics of the optical wavelength conversion element will be described in detail below.
[0120]
Nb ₂ O _Five FIG. 13 shows the measurement result of the refractive index dispersion characteristic performed for the purpose of analyzing the waveguide characteristic of the optical waveguide having the cladding layer. In FIG. 13, LiTaO _Three Substrate (line 132) and Nb ₂ O _Five The measurement result of the refractive index dispersion of the cladding layer (line 131) is shown.
[0121]
From this, depending on the wavelength, LiTaO _Three Substrate and Nb ₂ O _Five The refractive index difference between the cladding layer and the refractive index difference between the substrate and the cladding layer in the 800 nm band corresponding to the fundamental wave wavelength is different from the refractive index difference in the 400 nm band corresponding to the second harmonic wavelength. The rate difference is increasing.
[0122]
FIG. 14 shows the analysis result of the waveguide characteristic of the loaded optical waveguide performed based on this result. FIG. 14 is a characteristic factor diagram showing the relationship between the wavelength of light guided through the optical waveguide and the thickness of the cladding layer (loading layer) and the waveguide mode capable of propagating inside the cladding layer.
[0123]
In the region I, light cannot propagate through the cladding layer, and only the waveguide layer functions as a region where light can propagate. In region II, only fundamental mode light can propagate through the cladding layer, while in region III, fundamental mode and TM01 mode light can propagate through the cladding layer.
[0124]
The waveguide mode in each region will be further described below.
[0125]
First, as a first case, when the thickness of the cladding layer 152 is smaller than about 0.1 μm, the second harmonic having a wavelength of about 430 nm and the fundamental wave having a wavelength of about 860 nm are both classified into the region I. Is done. Accordingly, there is no waveguide mode capable of propagating inside the cladding layer 152. Therefore, the shape of the electric field distribution of the propagation light is as shown in FIG. 15A, and both the fundamental wave 153 in the fundamental wave mode and the second harmonic wave 154 in the fundamental mode propagate through the inside of the optical waveguide layer 151. Thus, the electric field distribution of the propagating light inside the cladding layer 152 becomes small.
[0126]
Next, as a second case, when the thickness of the clad layer 152 is between about 0.1 μm and about 0.36 μm, the second harmonic having a wavelength of about 430 nm is classified into the region II, and the clad layer The second harmonic wave 154 of the fundamental mode can propagate through the inside of 152. On the other hand, the fundamental wave 153 having a wavelength of about 860 nm is classified into the region I and cannot propagate inside the cladding layer 152. As a result, the second harmonic wave 154 of the fundamental mode is confined inside the cladding layer 152 as shown in FIG. 15B, but the fundamental wave 153 propagates inside the waveguide layer 151.
[0127]
Further, as a third case, when the thickness of the cladding layer 152 is larger than about 0.36 μm, the second harmonic having a wavelength of about 430 nm is classified into the region III, while the fundamental wave having a wavelength of about 860 nm. Is classified in Region II. Accordingly, the fundamental mode and the second harmonic can be propagated in the cladding mode 152, and both the fundamental mode fundamental wave 153 and the fundamental mode second harmonic 154 are contained in the cladding layer 152. Close up.
[0128]
Considering the overlap between the electric field distributions of the fundamental wave and the second harmonic in each of the above cases, in the first case, the electric field distribution in the optical waveguide layer having nonlinearity is large. Exists inside. However, in the second case, since the second harmonic is confined in the cladding layer, the overlap in the optical waveguide layer is very small. Further, in the third case, since both the fundamental wave and the second harmonic are confined in the cladding layer, the overlap between the two in the optical waveguide is expected to be close to zero.
[0129]
However, according to the experiments by the inventors of the present application, for the cladding layer having a thickness corresponding to the third case, the fundamental wave is confined in the cladding layer, the overlap is reduced, and the conversion efficiency is greatly increased. Although a decrease is observed, for the cladding layer having a thickness corresponding to the second case, a significant improvement in the conversion efficiency to the second harmonic is achieved, and a value larger than the value in the first case was gotten. Therefore, when the waveguide characteristics are further examined, in the second case where the conversion efficiency is greatly improved, the second harmonic waveguide mode is not a fundamental mode but a higher-order waveguide mode. It was discovered. That is, in the second case, it is found that the conversion efficiency is improved by increasing the overlap of the electric field distribution between the fundamental wave of the fundamental mode and the second harmonic of the higher order mode. It was.
[0130]
The relationship between the thickness of the cladding layer and the waveguide mode of the second harmonic that maximizes the conversion efficiency is shown in Table 1 below.
[0131]
[Table 1]

[0132]
Symbols (◎, Δ, and ×) in the “second harmonic output” column relatively represent the magnitude of the obtained second harmonic output (the magnitude of wavelength conversion efficiency). “” Means that the conversion efficiency is good, “x” means that the conversion efficiency is bad, and “Δ” means the middle.
[0133]
FIGS. 16A to 16D are diagrams showing light propagation states in the region where the thickness of the cladding layer shown in (1) of Table 1 is about 0.1 μm to about 0.3 μm. When the refractive index of the cladding layer 162 is lower than the refractive index of the waveguide layer 161 as shown in the refractive index profile of FIG. 16A (including the case where the cladding layer 162 is not provided), FIG. ), A fundamental mode fundamental wave 163 and a fundamental mode second harmonic 164 exist inside the waveguide layer 161. On the other hand, when the refractive index of the cladding layer 162 is larger than the refractive index of the waveguide layer 161 as shown in the refractive index profile of FIG. 16C, the second of the fundamental mode is shown in FIG. Although the harmonic wave 164 is confined inside the cladding layer 162, the second harmonic wave 165 in the TM01 mode propagates inside the optical waveguide layer 161, and the overlap with the fundamental wave 163 in the fundamental mode is greatly improved. To do. In particular, most of the electric field distribution 165 of the second harmonic in the TM01 mode is present in the optical waveguide layer 161, and there is almost no electric field in the cladding layer 162 having a small nonlinearity, and the fundamental wave 163 is strongly confined. Therefore, the conversion efficiency can be greatly increased by improving the overlap. Here, with respect to the second harmonic, since the desired waveguide mode can be efficiently excited by the wavelength selectivity, the second harmonic of the desired waveguide mode that provides high conversion efficiency can be obtained in accordance with the contents described above. Can be selectively excited.
[0134]
In addition, regarding the electric field distribution of the second harmonic of the higher order mode, the ratio between the portion existing in the cladding layer and the portion existing in the waveguide layer depends on the refractive index difference between the cladding layer and the waveguide layer. When the difference in refractive index is large, the portion of the electric field distribution existing in the optical waveguide layer becomes large, and the electric field peak in the cladding layer becomes small. On the other hand, when the difference in refractive index is small, the portion of the electric field distribution existing in the optical waveguide layer becomes small, and the electric field peak in the cladding layer becomes large.
[0135]
On the other hand, with respect to the electric field distribution of the fundamental wave, it is advantageous that the refractive index difference between the cladding layer and the waveguide layer is small. That is, when the refractive index difference is small, the range of the thickness of the cladding layer corresponding to the region I shown in FIG. 14 is widened, and restrictions on the thickness control are relaxed. Further, the conversion efficiency to the second harmonic is higher than the value obtained by the coupling between the second harmonic of the TM01 mode and the fundamental wave of the TM00 mode, and the fundamental wave of the TM02 mode and the TM00 mode. The value obtained by the connection between and becomes larger. In order to satisfy this condition, the relationship of the refractive index difference between the cladding layer and the substrate is ΔN _2w (Refractive index difference between substrate and clad layer for light of wavelength 400 nm)> ΔN _w It is desirable to satisfy the relationship of (refractive index difference between the substrate and the clad layer with respect to light having a wavelength of 800 nm). As described above with reference to FIG. 13, Nb ₂ O _Five The cladding layer is LiTaO _Three ΔN against the substrate _2w > ΔN _w This is very effective.
[0136]
In addition, when the second harmonic is in the higher order mode, the light collection characteristic of the emitted second harmonic may be deteriorated. For example, in a buried optical waveguide having a stepped refractive index profile, the first-order second-harmonic waveguide mode is a TM01 mode. Therefore, when this second harmonic is condensed, it becomes a condensed beam having two peaks, and a beam shaping process is required to condense it into a single beam spot. But Nb ₂ O _Five The higher-order mode of the second harmonic propagating through the loaded optical waveguide has light collection characteristics almost the same as those of the fundamental mode, and light collection up to the diffraction limit is possible. This is because the refractive index difference between the waveguide layer and the substrate for the second harmonic (Δ _n1 = Approximately 0.02) is the difference in refractive index from the cladding layer (Δ _n2 == about 0.3), because the electric field distribution of the waveguide mode in the cladding layer is much smaller than the electric field distribution in the optical waveguide layer, as shown in FIG. is there.
[0137]
In order to realize a higher-order mode of the second harmonic that does not deteriorate the condensing characteristic of the second harmonic, 3Δ _n1 <Δ _n2 is required. Δ _n2 If it is small, the electric field distribution in the clad layer becomes large, which causes a decrease in conversion efficiency and a deterioration in light collecting characteristics.
[0138]
Next, the design for obtaining the optimum structure of the cladding layer for the purpose of improving the efficiency of the optical wavelength conversion element will be described below.
[0139]
First, for the optimum cladding layer thickness, Nb ₂ O _Five Was considered as an example. Table 2 shows the waveguide state of the fundamental wave and the second harmonic, and the conversion efficiency to the second harmonic with respect to the thickness of the cladding layer.
[0140]
[Table 2]

[0141]
Symbols (◎, ○, Δ, and ×) in the column of “conversion efficiency” represent the relative magnitude of wavelength conversion efficiency, and symbol ◎ means that conversion efficiency is good, and symbol × Means that the conversion efficiency is poor, and the symbols ◯ and Δ mean the middle.
[0142]
When the thickness of the cladding layer is about 100 nm or less, the conversion efficiency is maximized when the fundamental wave and the second harmonic are in the fundamental mode, and the conversion efficiency in this case is the embedded optical waveguide. And not much different. When the thickness is about 100 nm or more, coupling with the second harmonic of the first order (TM01) mode begins to increase, and the conversion efficiency is improved. Furthermore, when the thickness is about 200 nm or more, the conversion of the first-order mode to the second harmonic becomes very large, and the conversion efficiency is more than twice that of a conventional optical wavelength conversion element using a buried type or loaded type optical waveguide. can get. Further, when the thickness of the cladding layer is about 300 nm, conversion to the second-order (TM02) second harmonic is performed, and the conversion efficiency is further improved. However, when the thickness of the cladding layer is about 400 nm or more, the fundamental wave is confined inside the cladding layer, and the conversion efficiency is greatly reduced. From this, Nb ₂ O _Five The thickness of the clad layer needs to be about 100 nm to about 380 nm in order to improve the conversion efficiency. In particular, the thickness that improves the conversion efficiency is preferably about 200 nm to about 340 nm.
[0143]
FIG. 17 shows LiTaO _Three The relationship between the refractive index (value with respect to a fundamental wave wavelength) of the clad layer formed on the proton exchange layer on the substrate and the thickness of the clad layer is shown.
[0144]
In region I, since the cladding layer is too thick, the fundamental wave is confined in the cladding layer, and the conversion efficiency is extremely reduced. In the region II, the overlap between the fundamental wave and the second harmonic is improved, and the conversion efficiency is maximized. Even in region III, the conversion efficiency is improved by the installation of the cladding layer. Thus, in the regions II and III, the conversion efficiency can be improved by installing the cladding layer.
[0145]
As can be seen from FIG. 17, as the refractive index of the cladding layer increases, the maximum allowable thickness for achieving region II or III decreases, and a highly accurate thickness setting is required. In order to form a clad layer with high reproducibility by a normal film forming apparatus, a thickness of several hundred nm or more is appropriate, and a refractive index is preferably about 2.8 or less.
[0146]
Further, if the refractive index difference between the cladding layer and the waveguide layer is too small, the effect of improving the overlap by the cladding layer cannot be obtained. When the refractive index of the cladding layer is about 2.2 or more, the overlap between the fundamental wave and the second harmonic becomes large, and the conversion efficiency is greatly improved. Therefore, the refractive index of the cladding layer is LiTaO _Three In the case of a substrate, it is desirable that it is about 2.18 or more with respect to the wavelength of the fundamental wave.
[0147]
In this embodiment, the constituent material of the cladding layer is Nb ₂ O _Five In addition, LiNbO _Three , TiO ₂ , ZnS, CeO ₂ Or mixed materials containing these, for example Ta ₂ O _Five , SiO ₂ , Al ₂ O _Three A mixed material with SiN or the like can also be used. By changing the refractive index of the clad layer formed by these, a larger overlap can be obtained, and the conversion efficiency can be greatly improved.
[0148]
In this embodiment, the substrate is LiTaO. _Three LiTaO doped with MgO, Nb, Nd, etc. _Three Or LiNbO _Three LiTa, or a mixture of them _1-x Nb _x O _Three (0 ≦ x ≦ 1) or KTP (KTiOPO _Four A similar element can be manufactured even if a substrate made of) is used. LiTaO _Three , LiNbO _Three , And KTP have high nonlinearity, and a highly efficient optical wavelength conversion element can be produced. In particular, since KTP has a low refractive index of about 1.8, Ta clad is used as a material for the clad layer formed thereon. ₂ O _Five , Al ₂ O _Three SiN and other materials can be used and are effective. In addition, these materials are effective because a method for forming a periodic polarization inversion layer has been established and a highly efficient optical wavelength conversion element can be formed.
[0149]
Next, the cover layer formed on the element surface will be described.
[0150]
In the configuration of FIG. 9 described above, the surface of the optical waveguide (specifically, the proton exchange layer 105 and the cladding layer 110) is made to have a thickness of about 400 nm of SiO. ₂ Covered with a cover layer 111. This is due to the following reason.
[0151]
In the loaded optical waveguide, since the cladding layer is formed by etching, slight irregularities are formed on the side surface of the cladding layer, which causes waveguide loss. If the cover layer 111 covers the clad layer, the difference in refractive index between the clad layer and other portions is reduced, and the waveguide loss due to the irregularities on the side surfaces of the clad layer can be reduced. Specifically, the waveguide loss becomes about ½ or less by providing the cover layer 111.
[0152]
In the present embodiment, the cover layer 111 is made of SiO. ₂ However, the cover layer 111 is preferably made of a material having a refractive index lower than that of the substrate. ₂ O _Five Etc. are also effective. Ta ₂ O _Five Since the film has a large refractive index of about 2, a lower loss optical waveguide can be formed. Or SiN, Al ₂ O _Three If the material has a refractive index smaller than that of the high refractive index layer and has little absorption or scattering loss with respect to the fundamental wave and harmonics, the same effect as the cover layer can be obtained.
[0153]
(Second Embodiment)
Here, the improvement of the characteristics of the optical wavelength conversion element by another structure of the present invention will be described. Specifically, an optical wavelength conversion element using a loaded ridge type optical waveguide will be described.
[0154]
In the first embodiment, it was confirmed that the characteristics of the optical wavelength conversion element were improved by the loaded optical waveguide using the clad layer having a high refractive index. Here, the results of attempts to further improve the characteristics by further devising the structure of the optical wavelength conversion element will be described.
[0155]
As described above, the mode profile of the fundamental wave can be controlled by the loaded optical waveguide. In the loaded optical waveguide, the mode profile of the fundamental wave is controlled while suppressing the influence on the mode profile of the second harmonic, When the overlap is improved, confinement in the width direction of the waveguide mode of the second harmonic becomes weak. As a result, the aspect ratio of the second harmonic beam profile output from the optical waveguide (the ratio of length to width when the emitted light is a collimated beam) is 1: 3 or more, and the spread angle in the thickness direction is the width direction. Larger than For this reason, the problem that the utilization efficiency of the light for obtaining the condensing characteristic of a diffraction limit significantly reduces arises. In order to solve this, the present embodiment adopts the structure of the optical wavelength conversion element shown in FIG.
[0156]
Specifically, FIG. 18A is a perspective view of an optical wavelength conversion element, and FIG. 18B is a cross-sectional view. In the optical wavelength conversion element of this embodiment, the LiTaO of C plate _Three A period Λ = about 3.5 μm (period Λ is shown as reference numeral 208 in FIG. 18) inside a substrate 201 made of a crystal (that is, its top and bottom surfaces are planes perpendicular to the C axis of the crystal). The domain-inverted layer 204 is formed. The width W of the domain inversion layer 204 is about 1.7 μm (the width W is shown as a reference number 209 in FIG. 18). In addition, LiTaO _Three A proton exchange layer 205 having a striped ridge 213 is formed on the surface of the crystal substrate 201, and Nb is formed on the upper surface of the ridge 213. ₂ O _Five A clad layer (loading layer) 210 is formed. In addition, Nb ₂ O _Five The surface of the crystal substrate 201 including the clad layer 210 is made of SiO. ₂ It is covered with a cover layer 211 made of The thickness of the ridge 213 is about 200 nm, and the thickness of the cladding layer 210 is about 300 nm.
[0157]
The fundamental wave 206 having a wavelength of 850 nm propagating through the waveguide propagates in the fundamental mode (TM00 mode), and the second harmonic 207 is a first-order mode in the depth direction and a 0th-order higher-order mode in the width direction. It is considered that propagation is in (TM01 mode). In the loaded optical waveguide described in the first embodiment, since the second harmonic 107 propagates in the TM01 mode, the lateral confinement is weak and the waveguide mode aspect ratio is large. On the other hand, in the configuration of the present embodiment, the confinement in the lateral direction is improved by adding the ridge 213, and the aspect ratio of the waveguide mode of the second harmonic is improved.
[0158]
Next, the characteristics of the loaded ridge type optical waveguide will be described.
[0159]
In the loaded ridge type optical waveguide, confinement of the second harmonic in the width direction is enhanced by processing the proton exchange layer 205 into a ridge shape. As a result, the spread of the waveguide mode of the second harmonic in the lateral direction is expressed as Nb ₂ O _Five It becomes possible to suppress to 2/3 to 1/2 of the loaded optical waveguide, and lateral confinement is enhanced. Therefore, the overlap between the fundamental wave in the lateral direction and the second harmonic increases, and the conversion efficiency is improved by 1.5 times or more compared with the loaded optical waveguide. Thus, it has become clear that the loaded ridge structure is effective in increasing the efficiency of the optical wavelength conversion element.
[0160]
The thickness of the ridge 213 that strengthens the confinement in the width direction needs to be about 100 nm or more. Further, the efficiency of the optical wavelength conversion element is improved by increasing the overlap in the width direction when the thickness of the ridge 213 is about 200 nm to about 600 nm. When the thickness of the ridge 213 is about 1000 nm or more, the propagation loss of the optical waveguide increases and the characteristics of the optical wavelength conversion element deteriorate.
[0161]
Accordingly, the thickness of the ridge 213 needs to be about 100 nm to about 1000 nm in order to obtain an improvement in overlap. Furthermore, about 200 nm to about 600 nm is desirable to obtain a significant improvement in conversion efficiency.
[0162]
Next, the relationship of the aspect ratio of the second harmonic to the width of the optical waveguide in an optical waveguide having a ridge 213 thickness of about 300 nm will be described.
[0163]
First, in order for the fundamental wave to be guided without becoming a multimode, the width of the waveguide needs to be about 10 μm or less. Furthermore, the aspect ratio can be improved when the waveguide width is about 8 μm or less. Also, the aspect ratio approaches 1: 1 when the waveguide width is about 2 μm to about 4 μm. When the waveguide width is about 1 μm or less, the conversion efficiency decreases.
[0164]
By making the optical waveguide structure into a loaded ridge structure, it becomes possible to form an output beam in addition to the characteristics of the optical wavelength conversion element using the loaded optical waveguide, and it becomes possible to produce an optical wavelength conversion element with excellent characteristics. . In order to be realized as a single mode optical waveguide, the width of the optical waveguide is desirably about 1 μm to about 10 μm. In order to further improve the aspect ratio, the optical waveguide width is desirably about 1 μm to about 8 μm. In order to make the aspect ratio close to 1: 1, it is necessary to control the optical waveguide width to about 2 μm to about 4 μm. .
[0165]
In this embodiment, the constituent material of the cladding layer is Nb ₂ O _Five In addition, LiNbO _Three , TiO ₂ , ZnS, CeO ₂ Or mixed materials containing these, for example Ta ₂ O _Five , SiO ₂ , Al ₂ O _Three A mixed material with SiN or the like can also be used. By changing the refractive index of the clad layer formed by these, a larger overlap can be obtained, and the conversion efficiency can be greatly improved.
[0166]
In this embodiment, the substrate is LiTaO. _Three LiTaO doped with MgO, Nb, Nd, etc. _Three Or LiNbO _Three LiTa, or a mixture of them _1-x Nb _x O _Three A similar element can be manufactured even when a substrate made of (0 ≦ x ≦ 1) or KTP is used. LiTaO _Three , LiNbO _Three , And KTP both have high nonlinearity, and a highly efficient optical wavelength conversion element can be produced. In particular, since KTP has a low refractive index of about 1.8, Ta clad is used as a material for the cladding layer formed thereon. ₂ O _Five , Al ₂ O _Three SiN and other materials can be used and are effective. In addition, these materials are effective because a method for forming a periodic polarization inversion layer has been established and a highly efficient optical wavelength conversion element can be formed.
[0167]
(Third embodiment)
Nb ₂ O _Five By applying the overlap improvement effect of the clad layer to the buried optical waveguide, conversion efficiency can be improved with a simple structure. FIG. 19 shows the configuration of the optical wavelength conversion element of this embodiment using an embedded optical waveguide.
[0168]
Specifically, FIG. 19 is a perspective view of an optical wavelength conversion element. In the optical wavelength conversion element of this embodiment, the LiTaO of C plate _Three In a substrate 301 made of a crystal (ie, the top and bottom surfaces are planes perpendicular to the C-axis of the crystal), the period Λ = about 3.5 μm (the period Λ is shown as reference numeral 308 in FIG. The domain-inverted layer 304 is formed. The width W of the polarization inversion layer 304 is about 1.7 μm (the width W is shown as a reference number 309 in FIG. 19). In addition, LiTaO _Three A striped proton exchange layer 305 is formed on the surface of the crystal substrate 301. Further, the surface of the crystal substrate 301 including the proton exchange layer 305 has Nb having a higher refractive index than that of the substrate 301. ₂ O _Five A clad layer 310 is formed. Further, the surface of the crystal substrate 301 including the cladding layer 310 is made of SiO. ₂ It is covered with a cover layer (not shown). The thickness of the cladding layer 310 is about 300 nm.
[0169]
The fundamental wave 306 having a wavelength of 850 nm propagating through the waveguide propagates in the fundamental mode (TM00 mode), and the second harmonic 307 is a first-order mode in the depth direction and a 0th-order higher-order mode in the width direction. Propagating in (TM01 mode). By selecting the wavelength of the fundamental wave 306 with respect to the period 308 of the domain-inverted layer 304, the waveguide mode (TM01 mode) of the second harmonic 307 is selectively excited. Further, unlike the conventional configuration, the fundamental mode 306 is set to the fundamental mode and the second harmonic 307 is set to the higher order mode, thereby increasing the overlap therebetween. To improve the conversion efficiency.
[0170]
Nb on buried optical waveguide ₂ O _Five When the cladding layer 310 is deposited by about 300 nm, the confinement of the guided mode of the fundamental wave 306 is improved, and the overlap between the second harmonic 307 and the fundamental wave 306 is increased. As a result, the conversion efficiency of the optical wavelength conversion element is improved about three times, and high efficiency is achieved. Specifically, when the thickness of the cladding layer 310 is about 100 nm, the overlap between the fundamental wave 306 and the second harmonic 307 starts to increase, and becomes maximum at a thickness of about 300 nm. However, when the thickness is about 400 nm, the conversion efficiency rapidly decreases. This is almost the same as the relationship between the thickness of the cladding layer obtained in the loaded optical waveguide and the conversion efficiency to the second harmonic.
[0171]
The buried optical waveguide is more confined in the lateral direction than the loaded optical waveguide, and the aspect ratio of the output beam of the second harmonic is about 1: 2.5, and a relatively good beam shape can be obtained. It is done.
[0172]
The advantage of the embedded optical waveguide is that an optical waveguide with a small propagation loss can be formed. Ridge-shaped optical waveguides tend to increase propagation loss due to irregularities on the side surfaces of the waveguide. In the ridge type optical waveguide produced by the inventors of the present application, a propagation loss of about 1.5 dB / cm was observed, whereas in the buried type optical waveguide, the propagation loss was 1 dB / cm or less, which is very low loss. It was confirmed that the optical waveguide can be formed.
[0173]
Furthermore, as a structure for improving the lateral confinement, an attempt was made to selectively deposit a clad layer on an optical waveguide. Specifically, by making the width of the clad layer 5 μm or less for an optical waveguide having a width of about 5 μm, confinement in the width direction is enhanced and the conversion efficiency is improved by 1.2 times. If the refractive index of the clad layer with respect to the fundamental wavelength is set to about 2.18 or more, conversion efficiency due to improved overlap is possible. In particular, when the refractive index is in the range of about 2.2 to about 2.5, the overlap is improved, and the conversion efficiency can be greatly improved.
[0174]
In this embodiment, the constituent material of the cladding layer is Nb ₂ O _Five In addition, LiNbO _Three , TiO ₂ , ZnS, CeO ₂ Or mixed materials containing these, for example Ta ₂ O _Five , SiO ₂ , Al ₂ O _Three A mixed material with SiN or the like can also be used. By changing the refractive index of the clad layer formed by these, a larger overlap can be obtained, and the conversion efficiency can be greatly improved.
[0175]
In this embodiment, the substrate is LiTaO. _Three LiTaO doped with MgO, Nb, Nd, etc. _Three Or LiNbO _Three LiTa, or a mixture of them _1-x Nb _x O _Three A similar element can be manufactured even when a substrate made of (0 ≦ x ≦ 1) or KTP is used. LiTaO _Three , LiNbO _Three , And KTP both have high nonlinearity, and a highly efficient optical wavelength conversion element can be produced. In particular, since KTP has a low refractive index of about 1.8, Ta clad is used as a material for the cladding layer formed thereon. ₂ O _Five , Al ₂ O _Three SiN and other materials can be used and are effective. In addition, these materials are effective because a method for forming a periodic polarization inversion layer has been established and a highly efficient optical wavelength conversion element can be formed.
[0176]
The loaded type, loaded ridge type, and buried type configurations shown in the first to third embodiments are low loss and strong confinement, and are promising not only as an optical wavelength conversion element but also as an optical waveguide structure. Application to optical waveguide elements such as optical communication and optical measurement devices is possible.
[0177]
Next, a waveguide structure for the purpose of improving the optical damage resistance of the buried optical waveguide will be described.
[0178]
By providing a cladding layer with a high refractive index, the buried optical waveguide increases the overlap between guided modes, and at the same time, the confinement of the guided light is very strong in both the depth and width directions, resulting in highly efficient wavelength conversion. Is possible. Further, by increasing the overlap between the fundamental wave and the second harmonic wave in the waveguide, the region where the electric field distribution of the fundamental wave exists alone is reduced, and the light damage resistance can be improved. However, the occurrence of optical damage cannot be completely prevented. For example, the occurrence of optical damage is observed for the second harmonic output of about 10 mW or more. This characteristic is slightly inferior to the loaded and loaded ridge type optical waveguides.
[0179]
Therefore, when the cause of the optical damage was examined in detail, it was found that the refractive index change due to the optical damage was the largest in the vicinity of the side portion of the optical waveguide. On the other hand, in the loading type and loading ridge type optical waveguides, since the proton exchange layer exists in the vicinity of the side surface of the waveguide, lateral confinement is relaxed and the optical damage resistance at the side of the optical waveguide is improved. it is conceivable that.
[0180]
Therefore, in the buried optical waveguide structure, an attempt was made to provide a layer having a refractive index lower than that of the waveguide (having a higher refractive index than that of the substrate) in the vicinity of the side surface of the waveguide in order to improve the light damage resistance. Specifically, in the configuration of FIG. 19, a proton exchange layer with a low concentration was formed on the side portion of the optical waveguide. As a result, the lateral confinement of the optical waveguide is slightly relaxed (several percent), and the conversion efficiency is reduced by about 10%. However, the light damage resistance is greatly increased, and no light damage occurs even for the second harmonic of about 20 mW or more. This increase in the optical damage strength due to the provision of the proton exchange layer is caused not only by the relaxation of the lateral confinement of the optical waveguide, but also by the increase in the optical damage resistance of the proton exchange layer (waveguide side portion). That is, since the proton exchange layer has higher electrical conductivity than the substrate, the lifetime of the photoexcited charge at the impurity level that causes photodamage is short, and photodamage is less likely to occur. Therefore, by providing a proton exchange part with high electrical conductivity at the side of the waveguide where light damage is likely to occur, the light damage resistance at that part can be increased.
[0181]
(Fourth embodiment)
Here, a short wavelength light source using an optical wavelength conversion element will be described.
[0182]
With the configuration of the optical wavelength conversion element according to the above-described embodiment, a highly efficient and stable optical wavelength conversion element can be realized. FIG. 20 shows a configuration for producing a short wavelength light source using the light wavelength conversion element according to the present invention.
[0183]
Specifically, the short wavelength light source includes a semiconductor laser 421 that emits fundamental wave light P1 having a wavelength of 800 nm and is mounted on a submount 420 formed of Si or the like, and an optical wavelength conversion element 422, and a semiconductor. The fundamental light P1 emitted from the light emitting region 423 of the laser 421 is directly coupled to the end face of the waveguide 402 of the optical wavelength conversion element 422 and condensed to excite the waveguide mode. The wavelength-converted second harmonic light P2 is emitted from the other end face of the waveguide 402 of the optical wavelength conversion element 422. A clad layer 403 is formed between the substrate 420 and the light wavelength conversion element 422.
[0184]
Since the optical wavelength conversion element 422 with high conversion efficiency is realized by the present invention, the blue second harmonic light P2 of about 10 mW can be obtained using the semiconductor laser 421 with an output of about 100 mW. Further, since the light wavelength conversion element 422 used has excellent light damage resistance and a stable output can be obtained, the output fluctuation can be suppressed to about 2% or less, and a stable output can be obtained.
[0185]
The wavelength in the 400 nm band is desired in a wide range of application fields such as printing plate making, biotechnology, special measurement fields such as fluorescence spectroscopic characteristics, and optical disk fields. The short wavelength light source using the light wavelength conversion element according to the present invention can be put into practical use in these application fields from the viewpoints of both output characteristics and stability.
[0186]
In addition, the structure which couple | bonds the light of the semiconductor laser 421 with the waveguide 402 of the optical wavelength conversion element 422 using a condensing optical system may be sufficient. However, a configuration in which the semiconductor laser 421 and the optical waveguide 402 of the light wavelength conversion element 422 are directly coupled can realize a light source with a small size and a low price.
[0187]
(Fifth embodiment)
Here, an optical pickup using a short wavelength light source as an optical disk pickup light source will be described.
[0188]
For optical discs, high-density recording is desired, and realization of a small short-wavelength light source is indispensable. A pickup for reading an optical disc includes a light source, a condensing optical system, and a light receiving portion. Specifically, as shown in FIG. 21, light P1 emitted from a light source in which a semiconductor laser 521 and an optical wavelength conversion element 522 are mounted on an Si submount 520 is converted into parallel light by a lens 540, and the beam After passing through the splitter 541, the light is condensed by the lens 542 to irradiate the optical disk 543. The reflected light from the optical disk 543 is collimated by the lens 542 and then condensed on the photodetector 545 via the beam splitter 541 and the lens 544.
[0189]
When the light wavelength conversion element according to the present invention is used as a light source, blue light having a wavelength of 400 nm band can be used as a reading light source of an optical disk, so that the recording density can be doubled. Further, since high-output blue light can be generated, not only reading but also information can be written to the optical disc. By using a semiconductor laser as a fundamental wave light source, it becomes very small and can be used for a consumer-use compact optical disk reading and recording apparatus.
[0190]
Furthermore, the optical wavelength conversion element can optimize the aspect ratio of the output beam by optimizing the width of the optical waveguide. For example, in an optical wavelength conversion element having a loaded ridge-type waveguide structure, the aspect ratio can be made close to 1: 1 by setting the width of the optical waveguide to about 3 μm. As a result, a beam shaping prism or the like for improving the light collecting characteristic of the optical pickup becomes unnecessary, and high transmission efficiency, excellent light collecting characteristic, and cost reduction can be realized. Furthermore, noise of scattered light generated during beam shaping can be reduced, and pickup can be simplified.
[0191]
(Sixth embodiment)
In the present embodiment, a configuration of an optical waveguide having strong confinement will be described.
[0192]
The structure of the optical wavelength conversion element described in the first embodiment is an effective structure as an optical waveguide structure. Therefore, the characteristics of the optical waveguide in the optical wavelength conversion element shown in FIG. 9 will be described.
[0193]
Specifically, as shown in FIG. _Three An optical waveguide consisting of a proton exchange layer is formed on the substrate, and Nb is further formed thereon. ₂ O _Five A light having a wavelength of about 860 nm was guided through an optical wavelength conversion element in which a clad layer made of was formed, and the electric field distribution of the guided light was observed. For comparison, Ta ₂ O _Five The electric field distribution of the guided light of the loaded optical waveguide (see FIG. 22 (g)) in which the clad layer made of is formed was also measured. The refractive index for light with a wavelength of about 860 nm is about 2.16 for the proton exchange layer (optical waveguide layer), Ta ₂ O _Five Layer is about 2.0, Nb ₂ O _Five The layer is about 2.25.
[0194]
The electric field distribution of a loaded optical waveguide in which a striped clad layer having a thickness of about 300 nm and a width of about 5 μm is formed on a proton exchange layer having a thickness of about 2 μm is described with reference to FIGS. explain.
[0195]
First, the distribution in the depth direction will be described. 22 (a) and 22 (b) show Ta ₂ O _Five Nb and a loaded optical waveguide with a cladding layer made of ₂ O _Five FIG. 22 (c) and FIG. 22 (d) show the refractive index in the depth direction in each structure for the loaded optical waveguide in which the clad layer made of is formed. Distribution. Ta ₂ O _Five The electric field intensity distribution in the depth direction when the clad layer made of (see FIG. 22C) spreads to about 3 μm at the full width at half maximum, whereas Nb ₂ O _Five The full width at half maximum of the electric field strength distribution in the depth direction (see FIG. 22 (d)) when the clad layer made of is reduced to about 2.5 μm, and the optical waveguide confinement is greatly improved. . Other materials (eg SiO ₂ Or Al ₂ O _Three As a result of studying the clad layer made of (3), the electric field intensity distribution of the guided light in the depth direction is hardly changed in the clad layer having a refractive index equal to or lower than the refractive index (about 2.16) of the optical waveguide layer. Therefore, in order to improve the light confinement in the depth direction, it is necessary to form a layer having a higher refractive index than the waveguide layer as the cladding layer.
[0196]
Next, the electric field intensity distribution of the guided light in the width direction is shown in FIGS. 22 (e) and 22 (f), respectively. For confinement in the width direction, Ta ₂ O _Five When the clad layer made of (see FIG. 22 (e)) is formed, the full width at half maximum spreads to about 6 μm or more, whereas Nb ₂ O _Five In the clad layer (see FIG. 22F), it can be seen that the full width at half maximum is reduced to about 5 μm, and an optical waveguide having strong confinement in the width direction is formed.
[0197]
As a result, it has been clarified that a highly confined optical waveguide can be formed by providing a cladding layer having a higher refractive index than the waveguide layer. An optical waveguide with strong confinement is effective for an electro-optical element and a nonlinear optical element using the optical waveguide because the power density of the guided light is increased and the light control efficiency is improved.
[0198]
(Seventh embodiment)
Here, a method for further improving the characteristics of the optical wavelength conversion element is examined.
[0199]
By providing a layer having a high refractive index in the cladding layer, the shape of the waveguide mode of the fundamental wave can be controlled. Therefore, the structure shown in FIG. 23 is formed in order to facilitate the shape control of the waveguide mode. . Specifically, the waveguide mode is easily controlled by making the clad layer a multilayer film structure.
[0200]
FIG. 23A is a perspective view of the light wavelength conversion element, and FIG. 23B is a cross-sectional view. In the configuration of FIG. 23, the cladding layer is made of SiO ₂ Layer 712 and Nb ₂ O _Five Except for the multilayer structure with the layer 710, the configuration is the same as that described with reference to FIG. Corresponding components are given corresponding similar reference numerals and their description is omitted.
[0201]
In order to draw the profile of the waveguide mode toward the substrate surface side and improve the overlap between the fundamental wave and the second harmonic, a layer having a refractive index higher than that of the substrate is required in the multilayer film. Here, Ta ₂ O _Five And Nb ₂ O _Five And forming a clad layer of a multilayer film. It was confirmed that by controlling the thickness of the multilayer film, the overlap was improved and the conversion efficiency was improved.
[0202]
Next, improvement in conversion efficiency by symmetrizing the refractive index distribution of the optical waveguide will be described.
[0203]
If the refractive index distribution has a symmetrical structure with respect to the depth direction, the overlap between the fundamental wave and the second harmonic can be greatly improved. For this purpose, a material having a refractive index equal to that of the substrate is required as the cladding layer. LiTaO as a substrate _Three Has a refractive index of about 2.15 for light of 860 nm. Therefore, in order to form a cladding layer having a refractive index close to this value, Ta ₂ O _Five And Nb ₂ O _Five Was used as the cladding layer. Ta ₂ O _Five And Nb ₂ O _Five Were approximately 2.0 and 2.25, respectively, and the overall refractive index was brought close to the above value (2.15) by controlling the mixing ratio.
[0204]
Deposition is Ta ₂ O _Five And Nb ₂ O _Five Sputter deposition using a mixed target of When the clad layer was deposited to a thickness of about 500 nm and an optical wavelength conversion device having the configuration shown in FIG. 23 was produced, it was confirmed that the conversion efficiency was greatly improved due to an increase in overlap.
[0205]
Thus, by using a film having the same refractive index as that of the substrate for the cladding layer, the conversion efficiency of the optical wavelength conversion element can be improved. The conversion efficiency is improved because the refractive index of the cladding layer is LiTaO. _Three This is a case where the refractive index is higher than the refractive index of the substrate and lower than the refractive index of the waveguide layer.
[0206]
(Eighth embodiment)
Here, an optical wavelength conversion element structure having excellent light damage resistance is described.
[0207]
Optical damage is a phenomenon in which the refractive index of a waveguide changes due to light guided through the optical waveguide and the output becomes unstable, and is particularly noticeable for short wavelength light. The optical wavelength conversion element converts the fundamental wave to generate a second short wavelength (wavelength: green to blue to ultraviolet). If the refractive index of the waveguide changes due to optical damage, the second harmonic output Changes and the wavelength conversion characteristics become unstable.
[0208]
In the following, the results of studies conducted on the structure to reduce optical damage will be described.
[0209]
The optical damage resistance of an optical wavelength conversion element using a conventional embedded optical waveguide was measured. LiTaO on the substrate _Three When the wavelength of a fundamental wave of about 860 nm is converted by using a wavelength to generate the second harmonic of about 430 nm, optical damage occurs due to the output of the second harmonic of several mW, and the output becomes unstable. Became clear. As a result of investigating the cause of photodamage, we discovered the following.
[0210]
First, by conducting proton exchange, the electrical conductivity of the proton exchange part is improved, thereby improving the light damage resistance.
[0211]
Secondly, optical damage is affected not only by the optical damage resistance inside the waveguide but also by the optical damage resistance around the waveguide. Since the embedded optical waveguide has low electrical conductivity at the periphery of the optical waveguide and is likely to cause optical damage, optical damage occurs at the periphery of the proton exchange optical waveguide.
[0212]
Third, the non-uniformity of the duty ratio of the polarization inversion period (the width W of the polarization inversion portion / the polarization inversion period Λ) affects the optical damage characteristics. Optical damage is a phenomenon in which a refractive index change occurs through an electro-optic effect due to an electric field excited by light. For this reason, it becomes possible to cancel the optical excitation electric field by the periodic domain-inverted structure and reduce the refractive index change. However, in the buried optical waveguide, the duty ratio of the periodic domain-inverted structure is likely to be non-uniform, and optical damage occurs with respect to the low-power second harmonic light.
[0213]
Examination on the light damage resistance revealed that the light damage resistance around the optical waveguide affects the characteristics of the light guide. Since the loaded optical waveguide has a proton exchange layer on the side surface of the waveguide, it has a light damage resistance about 10 times that of the embedded optical waveguide. However, the bottom surface of the loaded optical waveguide has a structure similar to that of a normal buried optical waveguide.
[0214]
Therefore, the structure shown in FIG. 24 is formed. FIG. 24A is a perspective view of the light wavelength conversion element, and FIG. 24B is a cross-sectional view. Specifically, the proton exchange layer has a multilayer structure, and a second proton exchange layer 815 having a low proton concentration is formed on the bottom surface of the first proton exchange layer 805 that is a waveguide layer. The configuration of FIG. 24 is the same as the configuration described with reference to FIG. 9 except that the proton exchange layer has a multilayer structure as described above. Corresponding components are given corresponding similar reference numerals and their description is omitted.
[0215]
By providing a second proton exchange layer 815 on the bottom surface of the first proton exchange layer 805 that is a waveguide layer to increase the electrical conductivity, the bottom portion of the optical waveguide is covered with a layer having a high electrical conductivity, so that light damage is prevented. The sex can be increased. As a result, the light damage resistance is more than twice that of the loaded optical waveguide.
[0216]
Moreover, it became clear by examination of this inventor that optical damage intensity also depends on a polarization inversion period. As described above, the periodic domain-inverted structure has the effect of canceling out the electric field caused by light excitation. However, when the domain-inverted period becomes coarse, this effect is diminished and light damage is likely to occur. Specifically, when the polarization inversion period is about 4 μm or less, the light damage resistance is remarkably increased. Even at a period of about 8 μm or less, it was confirmed that the light damage resistance was improved by 2 times or more with respect to the substrate. However, when the inversion period was about 10 μm or more, improvement in light damage resistance due to polarization inversion could not be confirmed.
[0217]
(Ninth embodiment)
In the embodiment described above, as a method of forming a high refractive index waveguide layer, LiTaO is obtained by heat-treating the substrate in pyrophosphoric acid. _Three Li in the substrate ⁺ And H in acid ⁺ And a proton exchange layer having a high refractive index was formed on the substrate surface. The following provides a method for further improving the proton exchange method to form an optical waveguide that is low loss and resistant to optical damage.
[0218]
Since pyrophosphoric acid has a large dissociation constant at high temperature and it is difficult for uneven concentration in the liquid to occur, a proton exchange layer using pyrophosphoric acid can form a low-loss proton exchange layer having a uniform refractive index and thickness. However, since the exchange amount of Li ions and protons in the substrate is large, the substrate surface is chemically damaged, and the nonlinear constant of the substrate is extremely deteriorated. Therefore, the controllability of proton exchange amount by proton exchange using pyrophosphoric acid was examined.
[0219]
As a method of controlling the proton exchange amount of pyrophosphoric acid, there is a method of neutralizing the acid to reduce the dissociation constant. Therefore, a change in proton exchange characteristics was observed by adding Li phosphate to pyrophosphoric acid. However, even when Li phosphate was added more than 10 times that of pyrophosphoric acid, no change in proton exchange characteristics (exchange rate, refractive index) was observed. Moreover, when Li phosphate was added, the viscosity of the solution increased, resulting in extremely poor workability.
[0220]
Therefore, a method for exchanging protons by providing a proton permeable membrane on the substrate surface is proposed.
[0221]
FIG. 25A and FIG. 25B are process diagrams of the optical waveguide manufacturing method of the present embodiment. First, as shown in FIG. 25 (a), LiTaO is formed by sputter deposition. _Three Ta on the + C surface of the substrate 901 ₂ O _Five A film (permeable film) 902 is deposited. Next, the substrate 901 is heat-treated in pyrophosphoric acid. Ta ₂ O _Five By performing proton exchange through the membrane 902, as shown in FIG. ₂ O _Five Film 902 and LiTaO _Three A proton exchange layer 903 is formed between the substrate 901 and the substrate 901. Ta like this ₂ O _Five In proton exchange via the membrane 902, the proton exchange rate and concentration can be controlled.
[0222]
FIG. 26 shows Ta ₂ O _Five FIG. 6 is a characteristic factor diagram showing the relationship between the thickness of the membrane 902 and the diffusion constant of proton exchange. When proton exchange treatment is performed at about 260 ° C., the diffusion constant is Ta ₂ O _Five When the film thickness is 0 nm, 100 nm and 400 nm, respectively, 0.227 μm ² / H, 0.05μm ² / H and 0.0132 μm ² / H. This is because the proton exchange rate is Ta ₂ O _Five It shows that it is controlled by the thickness of the film.
[0223]
When the formed proton exchange layer was evaluated, it was found that chemical damage was very small and the propagation loss of the optical waveguide could be reduced to 1/2 or less. Furthermore, it was confirmed that the optical waveguide having excellent characteristics was formed by improving the light damage resistance strength more than twice.
[0224]
Moreover, the nonlinearity of the proton exchange layer can be improved by lowering the proton exchange concentration. When proton exchange is performed, LiTaO _Three Or LiNbO _Three A proton exchange layer is formed on such a material, and a step-like high refractive index layer is formed. It is known that the non-linearity of the proton exchange layer deteriorates to less than half that of the substrate when the exchange concentration increases. To recover the nonlinearity, it is necessary to anneal the proton exchange layer to lower the proton exchange concentration. However, when the annealing treatment is performed, the proton exchange layer expands due to thermal diffusion, and the refractive index distribution changes from stepped to graded. Will change. For this reason, when applied to an optical wavelength conversion element or the like, it is difficult to control the electric field distribution shape of the waveguide mode, which causes a decrease in conversion efficiency. But Ta ₂ O _Five Since the proton concentration distribution can be controlled by using the membrane as the permeable membrane, it is possible to form an optical waveguide having a step-shaped refractive index distribution and maintaining high nonlinearity.
[0225]
Ta to prevent nonlinear deterioration ₂ O _Five The minimum thickness of the film is about 500 nm. By forming the optical wavelength conversion element using this method, the conversion efficiency can be greatly improved.
[0226]
Further, as shown in FIG. ₂ O _Five When the film thickness is about 100 nm or less, the change of the diffusion constant with respect to the film thickness increases, and Ta is controlled to control the proton exchange depth. ₂ O _Five Precise control of the film thickness is required, making it difficult to control. In addition, Ta ₂ O _Five When the thickness of the film is about 50 nm or less, the effect of reducing chemical damage is small. On the other hand, Ta ₂ O _Five As the thickness of the membrane increases, the diffusion constant decreases and the proton exchange time becomes longer, resulting in poor productivity.
[0227]
For this reason, Ta ₂ O _Five The film thickness is suitably about 1 μm or less. That is, in order to form a highly nonlinear waveguide, a permeable membrane (Ta having a thickness of about 500 nm to about 1000 nm is used. ₂ O _Five In order to control the proton exchange rate, about 50 nm to about 100 nm is suitable.
[0228]
In addition, Ta ₂ O _Five By combining the permeable membrane and the proton exchange selection mask, the proton exchange shape can be greatly changed, and the electric field distribution in the waveguide mode can be controlled.
[0229]
Specifically, first, as shown in FIG. _Three A stripe-shaped mask pattern 911 is formed on the + C surface of the substrate 910 with a Ta film 911 having a thickness of about 60 nm. Next, as shown in FIG. 27B, a Ta pattern having a thickness of about 400 nm is formed on the mask pattern 911. ₂ O _Five Layer 912 is deposited. In addition, heat treatment in pyrophosphoric acid, Ta ₂ O _Five A proton exchange treatment through the layer 912 is performed. At this time, as shown in FIG. 27C, a stripe-shaped proton exchange layer 913 is formed only in a portion not covered with the Ta mask pattern 911. The proton exchange treatment is performed at about 260 ° C. for about 4 hours.
[0230]
FIGS. 28A and 28B are cross-sectional views of proton exchange profiles. FIG. 28 (a) shows Ta ₂ O _Five This is a case where there is no permeable membrane, and (b) shows a Ta film having a thickness of about 400 nm. ₂ O _Five This is a case where proton exchange is performed through the permeable membrane 912. As shown in FIG. ₂ O _Five By providing the permeable membrane 912, proton exchange proceeds only at the edge portion of the Ta mask 911, and the diffusion rate decreases at the central portion. Therefore, the depth of the proton exchange layer 913 increases on both sides. This is because the Ta mask 911 edge portion is Ta. ₂ O _Five This is probably because the proton exchange rate in this portion is increased because the permeable membrane 912 is thinned.
[0231]
In the optical waveguide formed by such a proton exchange process, it is possible to selectively excite a secondary propagation mode having two peaks, and it is possible to take out a light output of two peaks. As a result, the output of two beams can be used as an optical computer, a measurement device for detecting operation, and detection light for position correction in detection of an optical disk. In addition, by applying to an optical demultiplexer, the guided light can be efficiently demultiplexed into two waveguides.
[0232]
Also, proton exchange rate Ta ₂ O _Five The proton exchange profile can be controlled using the dependence on the thickness of the permeable membrane. Usually, in order to form proton exchange layers having different thicknesses on one wafer, it is necessary to repeat a process of forming a proton exchange mask having an appropriate pattern and performing partial proton exchange. But Ta ₂ O _Five By using the permeable membrane, it is possible to simultaneously form proton exchange layers having different thicknesses in one proton exchange step. For example, as shown in FIG. 29, Ta used with the Ta mask pattern 911 ₂ O _Five By partially changing the thickness of the permeable membrane 912, a plurality of proton exchange layers 913 having different thicknesses can be formed on the substrate 910 by one proton exchange treatment. In addition, Ta ₂ O _Five By changing the thickness of the permeable membrane 912 in a tapered shape, it is possible to form a proton exchange layer whose depth is changed in a tapered shape.
[0233]
If the above method is used, the distribution of the proton exchange layer can be freely changed, and the process can be simplified.
[0234]
Next, a mask with excellent acid resistance capable of proton exchange over a long period of time will be described.
[0235]
LiTaO _Three , LiNbO _Three In order to form a deep proton exchange layer, etc., proton exchange for a long time is required. For example, LiTaO _Three In general, in order to form a proton exchange layer having a depth of about 2 μm or more, a proton exchange treatment at about 260 ° C. for about 4 hours or more is generally required. However, in order to perform selective proton exchange as described above, a mask that can withstand proton treatment for a long time at a high temperature is required. However, according to the prior art, it is difficult to exchange protons for more than 1 hour even if a Ta mask having excellent acid resistance is used.
[0236]
On the other hand, the present inventors have found that a mask excellent in acid resistance capable of long-time proton exchange can be obtained by depositing an oxide film on a metal film. Specifically, after depositing a Ta film about 60 nm, Ta film is deposited on the surface. ₂ O _Five A permeable membrane is deposited to a thickness of about 400 nm. As a result, it was confirmed that even when proton exchange was performed for 4 hours or more, the mask showed sufficient proton resistance, and the substrate under the mask was not proton-exchanged at all. Thus, the proton exchange mask excellent in acid resistance was realizable by depositing an oxide film on a metal mask.
[0237]
In this embodiment, Ta is used as the permeable membrane. ₂ O _Five But Nb ₂ O _Five , SiO ₂ , TiO _2, Al ₂ O _Three , LiNbO _Three , LiTaO _Three Similar effects can be obtained by using an oxide film having excellent acid resistance such as the above.
[0238]
(Tenth embodiment)
Next, the etching rate in the manufacturing method of the optical waveguide and the optical wavelength conversion element according to the tenth to fifteenth embodiments of the present invention will be described.
[0239]
Structurally and chemically stable substrates that are difficult to process and difficult to be corroded by solvents, such as quartz, can be etched with high precision even using etching means such as wet etching, dry etching, and reactive ion etching. Have difficulty. Such a substrate has a small selection ratio with respect to a resist pattern in etching, and it is difficult to form a deep etching shape when a fine shape is etched.
[0240]
On the other hand, in the manufacturing method of the following optical waveguide and optical wavelength conversion element of the present invention, the etching rate is improved by changing the characteristics of the substrate by selectively ion-exchange only the portion to be etched. . For example, LiNbO _Three , LiTaO _Three When a substrate such as the above is heat-treated in an acid, Li atoms in the crystal and protons (+ H) are exchanged to form a proton exchange layer. In the proton exchange layer, the bond between atoms is weaker than that of the original crystal, and it is easily etched. As a result, the proton exchange layer is easily etched by wet etching, dry etching, reactive ion etching, or the like. Therefore, in the manufacturing method of the optical waveguide and the optical wavelength conversion element of the present invention, the ion exchange is selectively performed only on the portion to be etched, and this portion is etched, so that the etching process with high selectivity can be performed with high accuracy. It becomes. For this reason, according to the manufacturing method of the present invention, it is possible to form an optical waveguide and an optical wavelength conversion element having uniform quality.
[0241]
Next, unevenness generated on the surface of the waveguide in the method for manufacturing an optical wavelength conversion element having a polarization inversion structure at regular intervals so as to be orthogonal to the waveguide direction will be described.
[0242]
In the conventional manufacturing method, the surface of the optical waveguide is uneven by etching, and the propagation loss of light is large. This is because the crystal plane is different between the domain-inverted portion and the non-inverted portion on the surface of the optical waveguide, resulting in a difference in etching rate. However, when ion exchange treatment is performed on a surface having such a periodically poled structure, unevenness does not occur even on the surface having a periodically poled structure in the ion exchange portion. It's been found. Therefore, by selectively performing ion exchange treatment only on the portion to be etched, etching is performed to eliminate the irregularities on the surface of the optical waveguide, and to produce an optical wavelength conversion element having a highly efficient optical waveguide with little light propagation loss. It becomes possible.
[0243]
Next, optical damage in the optical waveguide and the optical wavelength conversion element manufactured by the manufacturing method of the present invention will be described.
[0244]
In a conventional embedded optical waveguide, the electric field distribution of light propagating through the optical waveguide oozes out from the optical waveguide. For this reason, when strong light propagates through the waveguide, optical damage (a phenomenon in which the refractive index of the substance changes due to exposure to light) occurs in the outer periphery of the waveguide. However, in the ridge type optical waveguide manufactured by the manufacturing method of the present invention, since the ridge type waveguide layer is formed on the surface of the optical waveguide, the oozing of the electric field distribution of light is reduced. As a result, the optical waveguide according to the present invention has a structure strong against optical damage.
[0245]
In addition, a change in refractive index due to light damage is induced by generating an electric field from impurities in the crystal due to light irradiation and generating an electric field inside the crystal. However, when an ion exchange part is formed in the vicinity of the waveguide, the electric conductivity is increased in the ion exchange part, and the generation of an internal electric field can be suppressed. Therefore, in the structure formed by the manufacturing method of the present invention, since the ridge-type ion exchange layer is formed on the surface of the optical waveguide, the occurrence of optical damage is suppressed.
[0246]
Next, the overlapping of the electric field distribution in the optical wavelength conversion element of the present invention will be described.
[0247]
In the optical wavelength conversion element, the overlap of the electric field distribution between the fundamental wave and the second harmonic greatly affects the conversion efficiency. In the optical wavelength conversion device according to the present invention, since the ridge is formed on the surface of the optical waveguide, the refractive index distribution is stepped, and the fundamental wave and the second harmonic are overlapped as compared with the conventional embedded optical waveguide. Can be increased. Furthermore, since a layer having a high ion concentration is formed in the vicinity of the surface of the optical waveguide, the overlapping of electric fields in the depth direction also increases, and the optical wavelength conversion device according to the present invention has high conversion efficiency.
[0248]
30A to 30F are views showing a method of manufacturing an optical waveguide according to the tenth embodiment of the present invention, and are cross-sectional views taken along a plane orthogonal to the light guiding direction.
[0249]
First, as shown in FIG. 30A, the LiTa0 of the C plate, which is a nonlinear optical material. _Three A single crystal substrate 1001 (a substrate cut along a plane perpendicular to the C-axis of the crystal, hereinafter abbreviated as “substrate 1001”) is immersed in a pyrophosphoric acid solution at about 200 ° C. to about 300 ° C. for several minutes for proton exchange. Then, a strip-shaped primary proton exchange layer 1002 is formed on the surface of the substrate 1001.
[0250]
Next, the substrate 1001 is annealed at about 400 ° C. to about 450 ° C., and the entire primary proton exchange layer 1002 is annealed as shown in FIG. To.
[0251]
Next, a Ta layer having a thickness of about 60 nm is deposited on the + C plane, which is the surface of the substrate 1001, by sputtering or evaporation. Then photolithography and CF _Four A linear Ta layer 1003 having a predetermined width is formed by dry etching in an atmosphere.
[0252]
Further, the non-mask part is proton exchanged again using the Ta layer 1003 as a protective mask, and as shown in FIG. 30D, the secondary proton exchange layer 1005 at a predetermined depth from the surface of the annealed proton exchange layer 1006 is formed. Form.
[0253]
Next, after applying a negative resist (not shown) to the front surface of the substrate 1001, the negative resist is exposed and developed by irradiating ultraviolet rays from the back surface, and a linear Ta mask 1003 as shown in FIG. A resist pattern 1004 is formed thereon. At this time, since only the non-mask portion is exposed using the linear Ta layer 1003 as a protective mask, a linear resist pattern 1004 having a predetermined width is selectively formed only on the Ta layer 1003. The
[0254]
Next, using the formed resist pattern 1004 and Ta layer 1003 as a protective mask, CHF _Three Dry etching is performed in an atmosphere, and the secondary proton exchange layer 1005 in the non-mask portion is removed by etching. Further, the substrate 1001 is immersed in a solution of hydrofluoric acid: nitric acid = 1: 2 for several seconds to remove the remaining Ta layer 1003 and the resist pattern 1004. Thereafter, both end surfaces which are the entrance and exit surfaces of the optical waveguide are optically polished to manufacture an optical waveguide having a ridge 1007 as shown in FIG.
[0255]
A proton exchange portion 1008 is formed on the side surface of the ridge 1007. The proton exchange portion 1008 is formed by diffusion of protons in the lateral direction when the secondary proton exchange layer 1005 is formed using the protective mask for the Ta layer 1003. Since the proton exchange portion 1008 is not annealed, the proton concentration is higher than that of the annealed proton exchange layer 1006, and the refractive index change is also large.
[0256]
Etching in the above manufacturing method will be described.
[0257]
In the above manufacturing method, only the etched portion is selectively proton-exchanged using the Ta layer 1003 as a protective mask, so that the etching rate is dramatically improved as compared with the conventional method, and only the proton-exchanged portion is reliably ensured. Since the etching is performed, the waveguide portion is not affected at all by proton exchange.
[0258]
[Table 3]

[0259]
Table 3 shows LiTaO _Three CHF in _Three It is data comparing the etching rates in the atmosphere, and shows the etching rates of various substrates to be etched and the selectivity with respect to the resist pattern. The selectivity here is proton exchanged LiTaO _Three The values normalized by the substrate values are shown. The substrate to be etched in Table 3 is LiTaO. _Three Substrate, LiTaO annealed after proton exchange _Three Substrate and proton-exchanged LiTaO _Three There are three types of substrates.
[0260]
As shown in Table 3, LiTaO _Three By subjecting the substrate to proton exchange treatment, a high selectivity (5 times) can be obtained with respect to the resist pattern as compared with other substrates. For this reason, proton-exchanged LiTaO _Three The substrate can be etched to form a finer and deeper shape than other substrates.
[0261]
Next, it will be described that the depth of etching can be controlled with high accuracy by using proton exchange in the above manufacturing method.
[0262]
In a general dry etching apparatus, the etching thickness control has an error of ± several percent. For this reason, when manufacturing an optical waveguide using such a dry etching apparatus, there exists a problem that the characteristic of an optical waveguide varies. However, the etching depth can be controlled with high accuracy by using the method for manufacturing an optical waveguide according to the present invention. The reason is described below.
[0263]
As shown in Table 3 above, the etching rate is greatly different between the proton exchange part and the annealed proton exchange part. For this reason, in the manufacturing method of the present invention, when the etching of the proton exchange layer is completed and the annealing treatment part is reached, the etching rate rapidly decreases and the etching is substantially stopped. Therefore, the etching depth substantially matches the depth of the proton exchange layer. The depth of the proton exchange layer can be controlled with an accuracy of ± 1% or less. For this reason, the control of the etching depth can be controlled to ± 1% or less equivalent to the control of the depth of the proton exchange layer.
[0264]
In addition, proton exchanged LiTaO _Three Since the substrate has a high selectivity with respect to the resist pattern, the shape of the resist pattern is reliably maintained during etching. Therefore, in the production method of the present invention, LiTaO that is not proton exchanged _Three LiTaO with proton exchange compared to etching the substrate _Three In the substrate, an etching shape closer to vertical is obtained.
[0265]
Next, self-alignment in the above manufacturing method will be described.
[0266]
In the above manufacturing method, the linear Ta layer 1003 serves as both a protective mask for forming the secondary proton exchange layer 1005 and a mask for exposing the negative resist. For this reason, the Ta layer 1003 functions as a protective mask for selectively exchanging only the etched portion, and a mask for reliably forming the resist pattern 1004 only on the Ta layer 1003 by exposing the resist pattern 1004 from the back surface. As well as functioning. As a result, since the resist pattern 1004 can be accurately formed on the Ta layer 1003 by self-alignment, only the proton exchange portion can be accurately etched according to the manufacturing method described above. As a result, since the optical waveguide can be made uniform and the loss can be reduced, the above-described optical waveguide manufacturing method has excellent reproducibility and mass productivity. Further, since the resist pattern can be reliably formed only on the metal mask by self-alignment, manufacturing is easy and mass production is possible.
[0267]
Next, a characteristic evaluation experiment of the optical waveguide manufactured by the above manufacturing method will be described.
[0268]
FIG. 31 is a cross-sectional view showing an optical waveguide manufactured by the above manufacturing method. The same components as those in FIGS. 30A to 30F are denoted by the same reference numerals.
A characteristic evaluation experiment was conducted by propagating light having a wavelength of about 860 nm through the optical waveguide. Specifically, light of a semiconductor laser having a wavelength of about 860 nm was coupled to the optical waveguide using a lens, and a near-field image of the guided light was measured from the exit end face of the optical waveguide. According to this measurement result, the optical waveguide can be strongly confined by the proton exchange portion 1008 on the side surface of the ridge 1007, and the distribution of the light confined in the optical waveguide with respect to the incident laser light is the same as that of the prior art. It was improved by several percent compared to the structure. Further, it has been clarified that a step-like refractive index profile is formed in the width direction in this optical waveguide.
[0269]
Next, measurement results of each characteristic in the characteristic evaluation experiment will be described.
[0270]
First, the propagation characteristics of the formed optical waveguide will be described.
[0271]
The measured value of the propagation loss of the guided light was about 2 dB / cm, and a relatively low loss optical waveguide was formed. This value is LiTaO _Three The value is about ½ when the substrate itself is directly etched, and it can be understood that a low-loss optical waveguide is formed by the above manufacturing method.
[0272]
Next, the measurement result regarding the nonlinearity of the formed optical waveguide will be described.
[0273]
LiTaO _Three When the substrate is proton-exchanged, nonlinear optical constants and electro-optic constants greatly deteriorate, and proton-exchanged LiTaO _Three This has been a serious problem when an optical waveguide having a substrate is applied to a nonlinear optical element, an electro-optical element, or the like. However, proton exchanged LiTaO _Three By annealing the optical waveguide of the substrate at a higher temperature, the proton concentration decreases, and each constant is also normal LiTaO _Three It was confirmed that it recovered to the same value as the substrate.
[0274]
From the measurement result of the nonlinear optical constant of the optical waveguide manufactured by the manufacturing method of this embodiment, this optical waveguide is an annealed LiTaO _Three It was confirmed that the nonlinear optical constant was almost equal to the optical waveguide of the substrate. This is because the secondary proton exchange layer 1005 is selectively formed in a portion that is removed by etching and does not affect the optical waveguide. Actually, the proton exchange portion 1008 slightly exists on the side surface of the ridge 1007. However, since there is no portion with a high proton concentration inside the optical waveguide, high nonlinear characteristics can be maintained.
[0275]
Next, the measurement result in the waveguide loss characteristic of the optical waveguide to be formed will be described.
[0276]
The cause of the waveguide loss of the ridge type optical waveguide is a slight unevenness generated on the surface when the ridge shape is etched. In particular, since the refractive index difference between the waveguide portion (refractive index: about 2.2) and the air layer (refractive index: 1.0) is large, if there are irregularities on the surface of the optical waveguide, the waveguide loss will be reduced. growing. In order to reduce this influence, it is necessary to cover the entire optical waveguide with a material having a higher refractive index than the air layer.
[0277]
Therefore, the relationship between the refractive index of the coating film and the waveguide loss when the entire ridge-type optical waveguide formed as described above was coated with a film of a material higher than the refractive index of the air layer was measured. FIG. 32 shows the measurement results. As shown in FIG. 32, the waveguide loss decreases as the refractive index increases from 1.0. However, when the refractive index of the coating film exceeds the refractive index of the waveguide, light is not confined in the waveguide, and the fundamental wave is cut off. For this reason, it is preferable that the coating film is formed of a light transmissive material having a refractive index larger than the refractive index (1.0) of the air layer and smaller than the refractive index of the optical waveguide (about 2.2).
[0278]
In the above description, LiTaO _Three A single crystal substrate is used, but LiNbO _Three Single crystal substrate or LiTaO _Three And LiNbO _Three And polycrystalline substrate (LiNb _1-x Ta _x 0 _Three (0 ≦ X ≦ 1)), the same effect as described above can be obtained. LiNbO _Three Has a large electro-optic constant and nonlinear optical constant, and is applied to optical waveguide elements (optical switches, nonlinear optical elements, acousto-optic elements, etc.) in many fields. Therefore, the manufacturing method of the optical waveguide and the optical wavelength conversion element of the present embodiment is very effective in these application fields.
[0279]
In addition, LiTaO _Three MgO: LiNbO with MgO added instead of substrate _Three Substrate or MgO: LiTaO _Three It can also be applied to substrates. Since a substrate formed of these materials is resistant to optical damage, a high-power element can be manufactured and effective.
[0280]
Further, a KTP substrate can be used as the substrate. When KTP is used as the substrate material, Rb ion exchange is used as ion exchange, and an optical waveguide having the same effect as described above can be formed. Since KTP has excellent light damage resistance, the optical waveguide of the KTP substrate is effective as a high-power optical waveguide.
[0281]
(Eleventh embodiment)
FIGS. 33A to 33F are views showing a method of manufacturing an optical waveguide according to the eleventh embodiment of the present invention, and are cross-sectional views taken along a plane orthogonal to the light guiding direction.
[0282]
First, as shown in FIG. 33A, the LiTa0 of the C plate, which is a nonlinear optical material. _Three Single-crystal substrate 1101 (a substrate cut in a plane perpendicular to the C-axis of the crystal, hereinafter abbreviated as “substrate 1101”) is immersed in a pyrophosphoric acid solution at about 200 ° C. to about 300 ° C. for several minutes for proton exchange. Then, a strip-shaped primary proton exchange layer 1102 is formed on the surface of the substrate 1101.
[0283]
Next, the substrate 1101 is annealed at about 400 ° C. to about 450 ° C., and the entire primary proton exchange layer 1102 is strip-like annealed proton exchange layer 1106 as shown in FIG. To.
[0284]
Next, the substrate is again heat-treated in pyrophosphoric acid to form a secondary proton exchange layer 1105 from the surface of the annealed proton exchange layer 1106 to a predetermined depth, as shown in FIG.
[0285]
Further, as shown in FIG. 33D, a linear resist pattern 1104 having a predetermined width is formed on the secondary proton exchange layer 1105 by photolithography.
[0286]
Then, using the formed resist pattern 1104 as a protective mask, CHF _Three Dry etching is performed in an atmosphere, and the secondary proton exchange layer 1105 in the non-mask portion is removed by etching as shown in FIG.
[0287]
Thereafter, as shown in FIG. 33F, the resist pattern 1104 on the remaining linear secondary proton exchange layer 1105 is removed.
[0288]
Further, both end faces that are the entrance and exit surfaces of the optical waveguide are optically polished to produce an optical waveguide in which light is guided in a direction orthogonal to the drawing. The annealed proton exchange layer 1106 formed in the above step typically has a depth of about 2.5 μm, and the height of the ridge formed by the secondary proton exchange layer 1105 remaining in the final step is high. The thickness is typically about 0.4 μm.
[0289]
According to said manufacturing method, since a manufacturing process is simple, a ridge type | mold optical waveguide can be manufactured easily. Further, in the above manufacturing method, by forming the secondary proton exchange layer 1105 in the portion to be etched, the characteristics of only the etched portion are changed as described above, and the etching rate is dramatically improved. Furthermore, since the etching depth substantially matches the depth of the secondary proton exchange layer 1105, the etching depth is controlled to a desired depth by forming the depth of the secondary proton exchange layer 1105 with high accuracy. it can. For this reason, according to the manufacturing method of the present embodiment, a ridge can be formed with high accuracy and an optical waveguide with uniform quality can be manufactured, and the proton exchange portion hardly affects the optical waveguide, and only the ridge is formed. An optical waveguide having a proton exchange portion can be formed. Furthermore, according to the manufacturing method of the present embodiment, since the high refractive index layer having a high proton exchange concentration is formed on the ridge, an optical waveguide with strong confinement can be formed.
[0290]
Furthermore, the optical waveguide of the present embodiment has excellent light damage resistance because a portion having a high proton concentration is formed on the surface thereof. This is because the portion having a high proton concentration has a higher electric conductivity than the substrate, so that the generation of an electric field accompanying the bias of free charge due to photoexcitation that causes photodamage is suppressed.
[0291]
In addition, since a portion having a high proton concentration is formed on the surface of the optical waveguide, a change with time in the characteristics of the optical waveguide is suppressed. That is, LiNbO _Three Or LiTaO _Three In the annealed proton exchange layer, a change in refractive index with time occurs, but according to the present embodiment, such a change in refractive index is suppressed by further proton exchange. This is because in the annealed proton exchange layer, the crystal structure in the vicinity of the surface gradually changes after the annealing treatment, whereas when the surface part is subjected to proton exchange again, the crystal structure of that part changes. is there.
[0292]
In the above description, LiTaO _Three A single crystal substrate is used, but LiNbO _Three Single crystal substrate or LiTaO _Three And LiNbO _Three And polycrystalline substrate (LiNb _1-x Ta _x 0 _Three (0 ≦ X ≦ 1)), the same effect as described above can be obtained. LiNbO _Three Has a large electro-optic constant and nonlinear optical constant, and is applied to optical waveguide elements (optical switches, nonlinear optical elements, acousto-optic elements, etc.) in many fields. Therefore, the manufacturing method of the optical waveguide and the optical wavelength conversion element of the present embodiment is very effective in these application fields.
[0293]
In addition, LiTaO _Three MgO: LiNbO with MgO added instead of substrate _Three Substrate or MgO: LiTaO _Three It can also be applied to substrates. Since a substrate formed of these materials is resistant to optical damage, a high-power element can be manufactured and effective.
[0294]
Further, a KTP substrate can be used as the substrate. When KTP is used as the substrate material, Rb ion exchange is used as ion exchange, and an optical waveguide having the same effect as described above can be formed. Since KTP has excellent light damage resistance, the optical waveguide of the KTP substrate is effective as a high-power optical waveguide.
[0295]
Furthermore, in the above description, a proton exchange waveguide is used for the optical waveguide, but metal diffusion waveguides such as Ti, Cu, Nd, Cd, and Z can also be applied. Since these metal diffusion waveguides have little deterioration of nonlinear constants or electro-optic constants, optical elements having excellent characteristics can be manufactured.
[0296]
(Twelfth embodiment)
FIGS. 34A to 34G are views showing a method of manufacturing an optical wavelength conversion element using the method of manufacturing an optical waveguide described in the tenth embodiment of the present invention, and each shows a light waveguide direction. It is sectional drawing along the surface orthogonal to.
[0297]
First, as shown in FIG. 34A, the LiTa0 of the C plate, which is a nonlinear optical material. _Three Single-crystal substrate 1201 (a substrate cut in a plane perpendicular to the C-axis of the crystal, hereinafter abbreviated as “substrate 1201”) is immersed in a pyrophosphoric acid solution at about 200 ° C. to about 300 ° C. for several minutes for proton exchange. Then, a strip-like primary proton exchange layer 1202 is formed on the surface of the substrate 1201.
[0298]
Next, the substrate 1201 is annealed at about 400 ° C. to about 450 ° C., and the entire primary proton exchange layer 1202 is annealed in the form of a strip-like proton exchange layer 1206 as shown in FIG. To.
[0299]
Next, a comb-shaped electrode 1209a is provided on the + C surface which is the surface of the substrate 1201, and a planar electrode 1209b is formed on the −C surface which is the back surface of the substrate 1201. As shown in FIG. 34C, a polarization inversion layer 1210 orthogonal to the optical waveguide direction is formed by applying a voltage between the two electrodes thus mounted. The domain-inverted layer 1210 is formed at regular intervals according to the shape of the comb-shaped electrode 1209a.
[0300]
Further, after removing both electrodes, a Ta layer having a thickness of about 60 nm is deposited on the + C surface of the substrate 1201 by sputtering or vapor deposition. Then photolithography and CF _Four A linear Ta layer 1203 having a predetermined width as shown in FIG. 34D is formed by dry etching in the atmosphere.
[0301]
Further, the non-mask portion is proton exchanged again using the Ta layer 1203 as a protective mask, and as shown in FIG. 34 (e), the secondary proton exchange layer 1205 at a predetermined depth from the surface of the annealed proton exchange layer 1206 is formed. Form.
[0302]
Next, after applying a negative resist (not shown) to the front surface of the substrate 1201, the negative resist is exposed and developed by irradiating ultraviolet rays from the back surface, and a linear Ta mask 1203 as shown in FIG. A resist pattern 1204 is formed thereon. At this time, since only the non-mask portion is exposed using the linear Ta layer 1203 as a protective mask, a linear resist pattern 1204 having a predetermined width is selectively formed only on the Ta layer 1203. The
[0303]
Next, using the formed resist pattern 1204 and Ta layer 1203 as a protective mask, CHF _Three Dry etching is performed in an atmosphere, and the secondary proton exchange layer 1205 in the non-masked portion is removed by etching as shown in FIG. Further, the substrate 1201 is immersed in a solution of hydrofluoric acid: nitric acid = 1: 2 for several seconds to remove the remaining Ta layer 1203 and the resist pattern 1204. After that, both end faces which are the entrance / exit surfaces of the optical waveguide are optically polished to manufacture an optical waveguide having a ridge 1207 as shown in FIG.
[0304]
A proton exchange portion 1208 is formed on the side surface of the ridge 1207. The proton exchange portion 1208 is formed by diffusion of protons in the lateral direction when the secondary proton exchange layer 1205 is formed using the protective mask for the Ta layer 1203. Since the proton exchange portion 1208 is not annealed, the proton concentration is higher than that of the annealed proton exchange layer 1206, and the refractive index change is also large.
[0305]
In the manufacturing method of this embodiment, since the domain-inverted layer 1210 is formed after the strip-shaped optical waveguide is formed, the domain-inverted layer 1210 is not affected by the manufacturing process when the optical waveguide is formed. Therefore, the uniform domain inversion layer 1210 can be reliably formed.
[0306]
Normally, when the domain-inverted layer is etched, irregularities are formed on the surface of the optical waveguide due to the difference in etching rate between the non-domain-inverted layer and the domain-inverted layer. However, in the manufacturing method of this embodiment, since the secondary proton exchange layer 1205 is formed on the surface having the domain-inverted layer 1210 to be etched, unevenness due to etching is not formed on the surface of the domain-inverted layer 1210. As a result, the manufacturing method of this embodiment is a manufacturing method suitable for forming a low-loss optical wavelength conversion element.
[0307]
Furthermore, according to the manufacturing method described above, the etching depth can be controlled with high accuracy by controlling the depth of the secondary proton exchange layer 1205, and therefore, an optical wavelength conversion element with uniform characteristics can be manufactured.
[0308]
Next, the principle of the optical wavelength conversion element manufactured by the manufacturing method of this embodiment will be described.
FIG. 35 is a perspective view showing the structure of an optical wavelength conversion element manufactured by the manufacturing method of this embodiment. The same components as those in FIGS. 34A to 34G are denoted by the same reference numerals, and the description thereof is omitted.
[0309]
The optical wavelength conversion element in FIG. 35 is a quasi-phase matching type second harmonic generation element, and propagates through the optical waveguide by the polarization inversion layer 1210 formed periodically in the direction of light propagation in the optical waveguide. The fundamental wave is converted to a second harmonic having a half wavelength. For example, if infrared light having a wavelength of 860 nm is incident on this optical waveguide, blue light having a half wavelength of 430 nm is emitted. The characteristics of the optical wavelength conversion element greatly depend on the nonlinear optical constant of the optical waveguide and the overlap of the electric field between the fundamental wave and the second harmonic wave propagating through the optical waveguide.
[0310]
Next, the characteristic evaluation result of the optical wavelength conversion element manufactured by the above manufacturing method will be described.
[0311]
First, the overlapping of waveguide modes (fundamental wave and second harmonic) of an optical wavelength conversion element having a ridge-type optical waveguide will be described.
[0312]
In the evaluation experiment of the optical wavelength conversion element of the present embodiment, a near-field image of the guided light propagating through the optical waveguide was observed to obtain the overlap in the horizontal direction (X direction in FIG. 35) of the electric field distribution of the guided light. 36 (a) and 36 (b) are graphs showing the intensity distributions of the fundamental wave light and the second harmonic light in the optical waveguide. FIG. 36 (a) shows the case of a conventional buried waveguide, and FIG. b) shows the case of the ridge type optical waveguide of the present embodiment. From these graphs, it can be seen that the horizontal overlap is much larger in the ridge type optical waveguide of the present embodiment than the intensity distribution of the conventional buried type optical waveguide. This is because the refractive index distribution of the ridge type optical waveguide is stepped by the ridge 1207, and the proton exchange portion 1208 formed on the side surface of the ridge 1207 becomes a high refractive index portion, which greatly contributes to confinement of the optical waveguide. It is because it is doing. As a result, in the optical wavelength conversion element of this embodiment using the ridge type optical waveguide, the conversion efficiency is improved by 1.5 times compared to the optical wavelength conversion element using the conventional embedded optical waveguide.
[0313]
Next, the light damage resistance in the optical wavelength conversion element manufactured in this embodiment will be described.
[0314]
Optical damage in an optical waveguide is likely to occur in a place where light confinement is strong, such as an optical waveguide. Moreover, the occurrence of such optical damage becomes more remarkable as the wavelength of light is shorter. In the conventional optical wavelength conversion element, optical damage is caused by the generation of the second harmonic, and the output property becomes unstable. Therefore, in a conventional embedded optical waveguide, proton-exchanged LiTaO is excellent in light damage resistance and has a high light damage resistance compared to the substrate. _Three Was used. However, in the conventional buried type optical waveguide, the guided light oozes out to the part other than the proton exchange region, so that optical damage occurs at the outer periphery of the waveguide, resulting in the generation of the second harmonic with high output. It was difficult to do.
[0315]
On the other hand, when an evaluation experiment of optical damage resistance was performed on the optical wavelength conversion element manufactured by the manufacturing method of the present embodiment, the optical resistance conversion was 10 times or more that of an optical wavelength conversion element using a conventional embedded optical waveguide. It was damaging. This is because an optical wavelength conversion element using a ridge-type optical waveguide has a ridge-shaped proton exchange layer formed on the surface of the optical waveguide to suppress the leakage of light from the optical waveguide. In addition, since the proton exchange layer has high electrical conductivity, generation of an electric field due to bias of photoexcited free electrons that causes photodamage is suppressed. For this reason, in the wavelength conversion element of this embodiment, the waveguide structure excellent in light damage resistance is realized by the ridge-shaped proton exchange layer formed on the surface of the waveguide. Furthermore, in the light wavelength conversion element of this embodiment, the proton exchange portion 1208 formed on the side surface of the ridge 1207 has a high proton concentration to increase the electrical conductivity. ing.
[0316]
Next, the conversion efficiency of the optical wavelength conversion element of this embodiment will be described.
[0317]
In a ridge-type optical waveguide, if fine irregularities are formed on the side surface of the ridge 1207, the waveguide loss increases. In order to reduce such waveguide loss, in the tenth embodiment described above, the optical waveguide is covered with a film that is higher than the refractive index of the air layer and lower than the refractive index of the optical waveguide. Such a coating is also effective for the wavelength conversion element of this embodiment.
[0318]
Moreover, in the optical wavelength conversion element of this embodiment, it is possible to increase the overlap of the propagation light in the lateral direction in the optical waveguide by using the ridge type optical waveguide. However, the overlap of the light intensity distribution in the depth direction (Z direction in FIG. 35) in the optical waveguide has not been improved so much, and in order to form an optical wavelength conversion element having a highly efficient optical waveguide, the depth It is necessary to increase the overlap in the vertical direction. Therefore, as a method for controlling the intensity distribution of the guided light in the depth direction, a high refractive index layer is formed on the surface of the ridge 1207 of the optical waveguide (that is, only the surface of the ridge 1207 is selectively made of a layer having a high refractive index. cover). By forming a film having a high refractive index on the surface of the ridge 1207 in this way, the confinement in the depth direction of the optical waveguide can be strengthened, and the overlap between the waveguide modes can be increased.
[0319]
FIGS. 37A and 37B are state diagrams showing the overlapping state of the fundamental wave and the second high frequency in the depth direction in the optical wavelength conversion element. FIG. 37A is a diagram illustrating the manufacturing method of the present embodiment. FIG. 5B shows the overlapping state between the waveguide modes in the manufactured optical wavelength conversion element, and FIG. 6B shows the overlapping state between the waveguide modes in the optical wavelength conversion element in which the high refractive index layer 1220 is further formed on the surface of the ridge 1207. Show.
[0320]
In the optical wavelength conversion element shown in FIG. 37B, the high refractive index layer 1220 selectively formed only on the surface of the ridge 1207 is made of amorphous LiNbO. _Three It is. In order to strengthen the confinement of the optical waveguide, it is effective to form a film having a refractive index higher than that of the optical waveguide on the surface of the ridge 1207. For this reason, LiTaO _Three Amorphous LiNbO having a refractive index (about 2.25) larger than that of the optical waveguide of _Three Is used as a constituent material of the high refractive index layer 1220. The thickness of the high refractive index layer 1220 is set to such a thickness that the second harmonic propagating through the optical waveguide does not guide the high refractive index layer 1220, for example, about 100 nm to about 500 nm. This is because the second harmonic is LiNbO _Three This is because the second harmonic wave is confined in this surface layer and the conversion efficiency is greatly reduced. As shown in FIG. 37 (b), LiNbO _Three In the optical wavelength conversion element having the high refractive index layer 1220, since the overlap of the intensity distribution between the fundamental wave and the second harmonic wave in the guided light in the depth direction is large, the optical wavelength conversion element shown in FIG. The conversion efficiency improved 1.3 times.
[0321]
In the above description, LiTaO _Three A single crystal substrate is used, but LiNbO _Three Single crystal substrate or LiTaO _Three And LiNbO _Three And polycrystalline substrate (LiNb _1-x Ta _x 0 _Three (0 ≦ X ≦ 1)), the same effect as described above can be obtained. LiNbO _Three Has a large electro-optic constant and nonlinear optical constant, and is applied to optical waveguide elements (optical switches, nonlinear optical elements, acousto-optic elements, etc.) in many fields. Therefore, the manufacturing method of the optical waveguide and the optical wavelength conversion element of the present embodiment is very effective in these application fields.
[0322]
In addition, LiTaO _Three MgO: LiNbO with MgO added instead of substrate _Three Substrate or MgO: LiTaO _Three It can also be applied to substrates. Since a substrate formed of these materials is resistant to optical damage, a high-power element can be manufactured and effective.
[0323]
Further, a KTP substrate can be used as the substrate. When KTP is used as the substrate material, Rb ion exchange is used as ion exchange, and an optical waveguide having the same effect as described above can be formed. Since KTP has excellent light damage resistance, the optical waveguide of the KTP substrate is effective as a high-power optical waveguide.
[0324]
In addition, a semiconductor substrate such as ZnS or GaAs can also be used as the substrate. Since semiconductor materials have a large nonlinear optical constant and manufacturing technology has been developed, a high-output optical wavelength conversion element can be produced.
[0325]
Furthermore, in the above description, a proton exchange waveguide is used for the optical waveguide, but metal diffusion waveguides such as Ti, Cu, Nd, Cd, and Z can also be applied. Since these metal diffusion waveguides have little deterioration of nonlinear constants or electro-optic constants, optical elements having excellent characteristics can be manufactured.
[0326]
(13th Embodiment)
38 (a) to 38 (f) are diagrams showing a method of manufacturing an optical wavelength conversion element using the method of manufacturing an optical waveguide described in the eleventh embodiment of the present invention, and each shows a light waveguide direction. It is sectional drawing along the surface orthogonal to.
[0327]
First, as shown in FIG. 38A, the LiTa0 of the C plate, which is a nonlinear optical material. _Three A single crystal substrate 1301 (a substrate cut along a plane perpendicular to the C-axis of the crystal, hereinafter abbreviated as “substrate 1301”) is immersed in a pyrophosphoric acid solution at about 200 ° C. to about 300 ° C. for several minutes to exchange protons. Then, a strip-shaped primary proton exchange layer 1302 is formed on the surface of the substrate 1301.
[0328]
Next, the substrate 1301 is annealed at about 400 ° C. to about 450 ° C., and the entire primary proton exchange layer 1302 is annealed in the form of a strip-like proton exchange layer 1306 as shown in FIG. To.
[0329]
Next, the substrate is again heat-treated in pyrophosphoric acid to form a secondary proton exchange layer 1305 from the surface of the annealed proton exchange layer 1306 to a predetermined depth as shown in FIG.
[0330]
Next, a comb-shaped electrode 1309 a is provided on the + C surface which is the surface of the substrate 1301, and a planar electrode 1309 b is formed on the −C surface which is the back surface of the substrate 1301. As shown in FIG. 38D, a polarization inversion layer 1310 orthogonal to the optical waveguide direction is formed by applying a voltage between the two electrodes thus mounted. The domain-inverted layer 1310 is formed at regular intervals according to the shape of the comb-shaped electrode 1309a.
[0331]
Further, after removing both electrodes, a linear resist pattern 1304 having a predetermined width is formed on the substrate 1301 by photolithography as shown in FIG.
[0332]
Then, using the formed resist pattern 1304 as a protective mask, CHF _Three Dry etching is performed in an atmosphere, and the secondary proton exchange layer 1305 in the non-mask portion is removed by etching. Thereafter, as shown in FIG. 38F, the resist pattern 1304 on the remaining linear secondary proton exchange layer 1305 is removed. Furthermore, the optical wavelength conversion element is manufactured by optically polishing both end faces which are the incident / exit surfaces of the optical waveguide. The annealed proton exchange layer 1106 formed in the above step typically has a depth of about 2.5 μm, and the height of the ridge formed by the secondary proton exchange layer 1305 remaining in the final step is high. The thickness is typically about 0.4 μm.
[0333]
In the manufacturing method of the optical wavelength conversion element of the present embodiment, the optical wavelength conversion element having the ridge type optical waveguide can be easily manufactured by a simple manufacturing process. Furthermore, since the ridge of the optical wavelength conversion element to be formed is provided in the secondary proton exchange layer 1305 having a high proton concentration and a large refractive index change, a waveguide structure with strong light confinement is realized.
[0334]
Furthermore, according to the above manufacturing method, the etching depth can be controlled with high accuracy by controlling the depth of the secondary proton exchange layer 1305, and therefore, an optical wavelength conversion element with uniform characteristics can be manufactured.
[0335]
Next, the characteristic evaluation result of the optical wavelength conversion element manufactured by the above manufacturing method will be described.
[0336]
FIG. 39 is a perspective view showing the structure of an optical wavelength conversion element manufactured by the manufacturing method of the present embodiment. The same components as those in FIGS. 38A to 38F are denoted by the same reference numerals. Specifically, an annealed proton exchange layer 1306 is formed on the substrate 1301. A proton exchange portion 1380 that is a ridge 1307 is formed on the annealed proton exchange layer 1306. In addition, a domain-inverted layer 1310 is formed at regular intervals so as to be orthogonal to the light guiding direction. The proton exchange portion 1380 is a part of the secondary proton exchange layer 1305 having a high proton concentration formed during proton exchange.
[0337]
Since the optical wavelength conversion element having the above configuration includes the proton exchange layer 1305 having a high refractive index as the ridge 1307, the overlapping of the electric field distribution between the waveguide modes in the depth direction (fundamental wave and second harmonic) is large. It is a highly efficient optical wavelength conversion element. The reason is described below.
[0338]
FIG. 40 is a graph showing the relationship between the thickness of the ridge and the conversion efficiency of the optical wavelength conversion element. (A), (b) and (c) of FIG. 40 show the overlap in the thickness direction between the respective waveguide modes (fundamental wave and second harmonic) in three types of optical wavelength conversion elements having different ridge heights. Is shown.
[0339]
In the optical wavelength conversion element shown in FIG. 40A, the ridge 1307 is very thin and there is substantially no high refractive index layer on the surface. In this case, the refractive index distribution of the waveguide is in a gradual shape (the refractive index at the central part is high and the refractive index at the outer peripheral part is low), the overlap between the fundamental wave and the second harmonic is small, and the conversion efficiency Is low.
[0340]
In the optical wavelength conversion element shown in FIG. 40B, a slightly thick ridge 1307 is provided. By such a high refractive index layer on the surface, the intensity distribution of the fundamental wave is attracted to the vicinity of the surface, and the overlap between the fundamental wave and the second harmonic increases, thereby obtaining high conversion efficiency. Specifically, the conversion efficiency of the optical wavelength conversion element shown in FIG. 40B is increased by about 1.5 times compared to the case where the high refractive index layer is not formed on the ridge, and the conversion efficiency is high. It is clear that the formation of a high refractive index layer on the ridge is an effective means for achieving efficiency.
[0341]
In the optical wavelength conversion element shown in FIG. 40C, a thicker high refractive index layer is provided, and the overlap between the fundamental wave and the second harmonic is increased. At this time, both the fundamental wave and the second harmonic are cut off in the high refractive index layer on the surface. However, in the optical wavelength conversion element of FIG. 40 (c), the second harmonic is guided through the high refractive index layer, so that the conversion efficiency is lowered. This is because the proton concentration in the high refractive index layer is high and the nonlinear optical constant is deteriorated, so that highly efficient wavelength conversion does not occur in the high refractive index layer.
[0342]
Therefore, in order to construct a highly efficient optical wavelength conversion element, it is desirable that the thickness of the ridge be set to such an extent that only the second harmonic is cut off.
[0343]
Furthermore, the optical wavelength conversion element of this embodiment has excellent light damage resistance because a portion with a high proton concentration is formed on the surface of the optical waveguide. This is because the portion having a high proton concentration has a higher electric conductivity than the substrate, so that the generation of an electric field accompanying the bias of free charge due to photoexcitation that causes photodamage is suppressed.
[0344]
In addition, since a portion having a high proton concentration is formed on the surface of the optical waveguide, a change with time in the characteristics of the optical waveguide is suppressed. That is, LiNbO _Three Or LiTaO _Three In the annealed proton exchange layer, a change in refractive index with time occurs, but according to the present embodiment, such a change in refractive index is suppressed by further proton exchange. This is because in the annealed proton exchange layer, the crystal structure in the vicinity of the surface gradually changes after the annealing treatment, whereas when the surface part is subjected to proton exchange again, the crystal structure of that part changes. is there.
[0345]
In the above description, LiTaO _Three A single crystal substrate is used, but LiNbO _Three Single crystal substrate or LiTaO _Three And LiNbO _Three And polycrystalline substrate (LiNb _1-x Ta _x 0 _Three (0 ≦ X ≦ 1)), the same effect as described above can be obtained. LiNbO _Three Has a large electro-optic constant and nonlinear optical constant, and is applied to optical waveguide elements (optical switches, nonlinear optical elements, acousto-optic elements, etc.) in many fields. Therefore, the manufacturing method of the optical waveguide and the optical wavelength conversion element of the present embodiment is very effective in these application fields.
[0346]
In addition, LiTaO _Three MgO: LiNbO with MgO added instead of substrate _Three Substrate or MgO: LiTaO _Three It can also be applied to substrates. Since a substrate formed of these materials is resistant to optical damage, a high-power element can be manufactured and effective.
[0347]
Further, a KTP substrate can be used as the substrate. When KTP is used as the substrate material, Rb ion exchange is used as ion exchange, and an optical waveguide having the same effect as described above can be formed. Since KTP has excellent light damage resistance, the optical waveguide of the KTP substrate is effective as a high-power optical waveguide.
[0348]
In addition, a semiconductor substrate such as ZnS or GaAs can also be used as the substrate. Since semiconductor materials have a large nonlinear optical constant and manufacturing technology has been developed, a high-output optical wavelength conversion element can be produced.
[0349]
Furthermore, in the above description, a proton exchange waveguide is used for the optical waveguide, but metal diffusion waveguides such as Ti, Cu, Nd, Cd, and Z can also be applied. Since these metal diffusion waveguides have little deterioration of nonlinear constants or electro-optic constants, optical elements having excellent characteristics can be manufactured.
[0350]
The configuration of the optical waveguide in the present invention can also be applied to, for example, a fiber-shaped waveguide. In that case, the cylindrical core formed of the optical material corresponds to the optical waveguide layer in the above description, and the cladding layer having the above-described characteristics is formed so as to cover the surface of the core.
[0351]
【The invention's effect】
As described above, a clad layer is provided on the optical waveguide, and the phase matching is performed between the fundamental wave of the fundamental mode propagating through the optical waveguide and the second harmonic of the higher order mode in which part of the wave is guided through the clad layer. Since the conversion efficiency can be greatly improved by increasing the overlap between the modes, a great effect can be obtained in practical use.
[0352]
Moreover, confinement in the width direction of the waveguide can be strengthened by forming the clad layer with a material having a high refractive index. As a result, the emission angles of the light emitted from the waveguide can be made substantially equal in the width direction and the depth direction, the aspect ratio of the emitted beam can be improved, and the light utilization efficiency can be greatly improved. . As a result, a great effect is obtained in practical use.
[0353]
Further, in the optical wavelength conversion element, by providing a high refractive index layer and a cladding layer having a refractive index from the substrate, confinement of light propagating through the optical waveguide can be strengthened, and the light power density can be improved. . Further, this increases the overlap of the electric field distribution between the fundamental wave and the harmonic wave propagating in the optical waveguide, so that the conversion efficiency can be greatly improved, and a great effect can be obtained practically.
[0354]
Moreover, the structure of the optical wavelength conversion element of the present invention can greatly reduce the fluctuation of the second harmonic output due to optical damage. As a result, a high-output and stable second harmonic output can be obtained, and a great effect can be obtained in practical use.
[0355]
Furthermore, according to the present invention, since the electric field distribution can be controlled by the high refractive index cladding layer, the controllability of the mode profile propagating through the optical waveguide is improved. As a result, the aspect ratio of the radiation pattern of the second harmonic output can be made close to 1, and the light utilization efficiency is greatly improved, and a great effect is obtained in practical use.
[0356]
Furthermore, as a method for producing an optical waveguide, chemical exchange on the surface of the proton exchange layer can be reduced by performing proton exchange through the permeable membrane during proton exchange. As a result, an optical waveguide with a small waveguide loss can be formed, and a great effect can be obtained in practical use.
[0357]
Further, since the thickness of the proton exchange layer formed by controlling the thickness of the permeable membrane can be controlled, proton exchange layers having different depths can be formed by one proton exchange treatment. Furthermore, since the shape of the proton exchange layer to be formed can be controlled by the shape of the permeable membrane, it is possible to improve the mass productivity by simplifying the operation and reducing the process steps, and a great effect is obtained in practical use.
[0358]
Furthermore, in the manufacturing method of the optical waveguide and the optical wavelength conversion element of the present invention, only the etched portion is selectively proton-exchanged using a metal mask, and a resist pattern is formed on the metal mask by backside exposure and etched. Thus, an optical waveguide can be easily formed. Further, in the above manufacturing process, the etching portion is ion-exchanged to improve the etching rate and the etching accuracy, and it becomes possible to form an optical waveguide with uniform quality.
[0359]
Furthermore, the manufactured optical waveguide has low loss and excellent optical damage resistance, and the electric field distribution of the waveguide mode propagating through the waveguide is optimized, so that the optical wavelength conversion element with high efficiency and high output can be obtained. Applicable.
[0360]
For example, according to the manufacturing method of the tenth embodiment, since the etching depth can be controlled with high accuracy, the optical waveguide can be made uniform and the waveguide loss can be reduced. In the manufacturing method according to the eleventh embodiment, a high refractive index layer having a high proton exchange concentration is formed on the ridge to realize an element with strong light confinement, and an optical waveguide having excellent light damage resistance is easily manufactured. be able to. Furthermore, according to the twelfth embodiment, by forming a ridge on the surface of the optical waveguide and forming a high refractive index portion on the ridge, optical wavelength conversion having high conversion efficiency and excellent light damage resistance is achieved. An element can be obtained. Further, according to the manufacturing method shown in the thirteenth embodiment, since the etching depth can be controlled with high accuracy, an optical wavelength conversion element having uniform quality can be easily manufactured, and an optical wavelength with high conversion efficiency. A conversion element can be obtained.
[Brief description of the drawings]
FIGS. 1A to 1D are cross-sectional views for explaining a conventional method for manufacturing a buried optical waveguide.
2A is a perspective view showing an example of a configuration of a conventional optical wavelength conversion element, FIG. 2B is a cross-sectional view taken along line 2B-2B in FIG. 2A, and FIG. It is sectional drawing in line 2C-2C of (a).
FIG. 3 is a perspective view showing another example of the configuration of a conventional optical wavelength conversion element.
FIG. 4 is a perspective view showing still another example of the configuration of a conventional optical wavelength conversion element.
FIGS. 5A and 5B are diagrams schematically illustrating an overlap between a waveguide mode of an optical waveguide layer and a polarization inversion layer. FIGS.
6A is a schematic cross-sectional view of an optical waveguide, and FIG. 6B is a diagram illustrating an electric field distribution of a waveguide mode in the configuration of FIG.
7A to 7D are schematic cross-sectional views of optical waveguides having various configurations, and diagrams showing electric field distributions of waveguide modes in the respective configurations.
FIG. 8 is a diagram showing a relationship between a refractive index ratio and a second harmonic intensity ratio in a cladding layer and an optical waveguide layer.
FIG. 9 is a diagram illustrating a configuration of an optical wavelength conversion element according to the first embodiment of the present invention.
FIG. 10 is a diagram illustrating an overlap between a fundamental wave and a second harmonic.
FIGS. 11A and 11B are cross-sectional views showing the configuration of an optical wavelength conversion element having a cladding layer, respectively, and FIGS. 11C and 11D show the configurations of FIGS. It is a figure which shows the electric field strength distribution of the guided light in.
FIGS. 12A to 12D are cross-sectional views illustrating a manufacturing process of an optical wavelength conversion device according to the first embodiment of the present invention. FIGS.
[Figure 13] LiTaO _Three And Nb ₂ O _Five It is a figure showing the refractive index dispersion characteristic.
FIG. 14 is a diagram illustrating the characteristics of a waveguide mode with respect to the wavelength of guided light and the thickness of a cladding layer.
FIGS. 15A to 15C are diagrams showing electric field distributions of waveguide modes with respect to clad layers having different thicknesses. FIGS.
FIGS. 16A to 16D are diagrams for explaining a light propagation state (waveguide mode of fundamental wave and second harmonic);
FIG. 17 is a diagram illustrating a relationship between a refractive index and a thickness of a clad layer.
FIG. 18 is a diagram showing a configuration of an optical wavelength conversion element according to a second embodiment of the present invention.
FIG. 19 is a diagram showing a configuration of an optical wavelength conversion element according to a third embodiment of the present invention.
FIG. 20 is a diagram illustrating a configuration of a short wavelength light source.
FIG. 21 is a diagram illustrating a configuration of an optical pickup.
FIGS. 22A to 22H are diagrams for explaining an electric field distribution in a waveguide mode when a clad layer made of a different material is formed. FIGS.
FIG. 23 is a diagram showing a configuration of an optical wavelength conversion element in a seventh embodiment of the present invention.
FIG. 24 is a diagram showing a configuration of an optical wavelength conversion element in an eighth embodiment of the present invention.
FIGS. 25A and 25B are cross-sectional views illustrating a manufacturing process of an optical waveguide in a ninth embodiment of the present invention. FIGS.
FIG. 26 Ta ₂ O _Five It is a figure showing the relationship between the thickness of a permeable film, and a diffusion constant.
FIGS. 27A to 27C are cross-sectional views illustrating another manufacturing process of the optical waveguide in the ninth embodiment of the present invention. FIGS.
28 (a) and 28 (b) are cross-sectional views of the proton exchange layer to be formed.
FIG. 29 is another cross-sectional view of a proton exchange layer formed by proton exchange treatment using a permeable membrane.
FIGS. 30A to 30F are cross-sectional views illustrating a manufacturing process of an optical waveguide in a tenth embodiment of the present invention.
FIG. 31 is a cross-sectional view showing a configuration of an optical waveguide.
32 is a diagram showing the relationship between the refractive index of a layer formed on the surface of the optical waveguide of FIG. 31 and waveguide loss.
33 (a) to 33 (f) are cross-sectional views illustrating an optical waveguide manufacturing process in an eleventh embodiment of the present invention.
34 (a) to (g) are cross-sectional views illustrating a manufacturing process of an optical wavelength conversion device in a twelfth embodiment of the present invention.
FIG. 35 is a perspective view showing a configuration of an optical wavelength conversion element.
36 (a) and 36 (b) are diagrams showing the lateral intensity distribution of light propagating through the optical waveguide.
FIGS. 37A and 37B are diagrams showing the intensity distribution in the depth direction of light propagating through the optical waveguide. FIGS.
FIGS. 38A to 38F are cross-sectional views illustrating a manufacturing process of an optical wavelength conversion device in a thirteenth embodiment of the present invention.
FIG. 39 is a perspective view showing a configuration of an optical wavelength conversion element.
FIG. 40 is a diagram showing the relationship between the ridge thickness and the conversion efficiency in the optical wavelength conversion element.
[Explanation of symbols]
101 LiTaO _Three substrate
104 Polarization inversion layer
105 Proton exchange layer
106 fundamental light
107 Second harmonic light
108 Period Λ of polarization inversion layer
109 Width W of polarization inversion layer
110 Clad layer
111 Cover layer

Claims (9)

A substrate made of a material having a nonlinear optical effect;
An optical waveguide layer formed on the substrate;
A cladding layer formed on the surface of the substrate on the optical waveguide layer,
In the optical wavelength conversion element in which the fundamental wave of wavelength λ is converted into a harmonic of wavelength λ / 2 in the optical waveguide layer,
A periodic domain-inverted structure is formed in the optical waveguide layer,
The thickness of the cladding layer is set so as to satisfy a waveguide condition for the light of the wavelength λ / 2 and to satisfy a cutoff condition for the light of the wavelength λ,
The optical wavelength conversion element, wherein the effective refractive index Nc of the cladding layer and the effective refractive index Nf of the optical waveguide layer satisfy a relationship of Nc> 1.02 · Nf.   The optical wavelength conversion element according to claim 1, wherein the optical waveguide layer has a stripe shape.   The optical wavelength conversion element according to claim 1, wherein the cladding layer has a stripe shape.   The optical wavelength conversion element according to claim 1, wherein a ridge is formed in a stripe shape on a surface of the optical waveguide layer, and the cladding layer is formed in a stripe shape on the ridge. A cylindrical core made of a material having a nonlinear optical effect;
A cladding layer covering the periphery of the core,
In the optical wavelength conversion element in which the fundamental wave of wavelength λ is converted into a harmonic of wavelength λ / 2 in the core,
A periodic domain-inverted structure is formed in the core,
The thickness of the cladding layer is set so as to satisfy a waveguide condition for the light of the wavelength λ / 2 and to satisfy a cutoff condition for the light of the wavelength λ,
The optical wavelength conversion element, wherein the effective refractive index Nc of the cladding layer and the effective refractive index Nf of the core satisfy a relationship of Nc> 1.02 · Nf. In the optical waveguide layer, the light of the wavelength lambda is propagated in the fundamental mode, the light of the wavelength lambda / 2 propagates in higher order modes, the optical wavelength conversion element according to any one of claims 1 to 4.   The optical wavelength conversion element according to claim 1, wherein the cladding layer is formed of a multilayer film. A short-wavelength light generator comprising a semiconductor laser and an optical wavelength conversion element, wherein the wavelength of light emitted from the semiconductor laser is converted by the optical wavelength conversion element, wherein the optical wavelength conversion element is claimed. those described in any of 1 to 7, the short-wavelength light generating apparatus. An optical pickup comprising a short wavelength light generator and a condensing optical system, wherein the short wavelength light emitted from the short wavelength light generator is collected by the condensing optical system, the short wavelength light An optical pickup, wherein the generator is the one according to claim 8 .

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