JP3909845B2 - Manufacturing method of optical functional element - Google Patents

Manufacturing method of optical functional element Download PDF

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JP3909845B2
JP3909845B2 JP2003166548A JP2003166548A JP3909845B2 JP 3909845 B2 JP3909845 B2 JP 3909845B2 JP 2003166548 A JP2003166548 A JP 2003166548A JP 2003166548 A JP2003166548 A JP 2003166548A JP 3909845 B2 JP3909845 B2 JP 3909845B2
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crystal
single crystal
optical
polarization
linbo
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JP2004163882A (en
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保典 古川
健二 北村
俊二 竹川
優 中村
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National Institute for Materials Science
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National Institute for Materials Science
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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Description

【0001】
【発明の属する技術分野】
本発明は、レーザ光を利用した光情報処理、光加工技術、光通信技術、光計測制御等々の分野で利用する、LiNbO3単結晶基板の分極反転構造を利用して光を制御する光機能素子の製造方法に関する。
【0002】
【従来の技術】
代表的な強誘電体単結晶として知られているニオブ酸リチウム(LiNbO3)単結晶(以下適宜LNと略記する)は、主に表面弾性波素子の基板として使用されている。この結晶は、大口径で組成均質性の高い単結晶が比較的安価で供給可能である。さらに、可視から赤外の広い波長域で透明であり、数十kV/mm程度の高電界を加えることで室温でも強誘電体分極を反転することが可能なことから、近年、分極反転構造を利用した非線形光学素子や電気光学素子など各種光機能素子の基板としても注目されている。
【0003】
特に、近年では、近赤外波長の半導体レーザを非線形光学効果により半波長の青色光に変換する導波路型の光第二高調波発生(SHG)素子の開発が期待されており、なかでも、光ディスクの高密度記録・再生用光源として、LNなどの無機強誘電体単結晶の分極を周期的に反転した構造の素子を用いた波長変換素子は最も良く研究されている。この波長変換素子は疑似位相整合(QuasiPhase Matching; QPM)方式によるもので、基本波と高調波の伝搬定数の差を周期構造で補償して位相整合をとる方式である。
【0004】
この方式では、高い波長変換効率が得られること、出力光の平行ビーム化・回折限界集光が容易であること、適用できる材料や波長に制限がないことなど、多くの優れた特徴を持っている。QPMのための周期構造としては、SHG係数(d33係数)の符号を周期的に反転した構造が高い波長変換効率を得る上で最も有効であり、強誘電体結晶ではd係数の正負は強誘電体分極の極性に対応するので、強誘電分極ドメインを周期的に反転させる構造の形成技術が重要である。
【0005】
この方式を用いて、公知文献(L.E.Myers et al., Optics Letters, 21, p591,1996)にあるように、LN単結晶に約21kV/mmの電界を加え、周期反転構造を作成した、QPM方式によるパラメトリック発振の波長変換素子が報告されている。さらに、公知文献(A.Harada et al., Optics Letters, 22,p805,1997)にあるように、コロナ放電法を用いMgO を添加したLN単結晶に4.75ミクロンの周期で分極反転構造を形成して、波長946nmのレーザ光から473nmの青色SHG光を高効率で変換したSHGレーザについても報告されている。
【0006】
また、電気光学効果を利用した光学素子においては、例えば、公知文献(M. Yamada et al., Appl.Phys.Lett., 69,p3659,1996)によると、強誘電体結晶であるLN単結晶に高電圧を印加することで、結晶中にレンズやプリズム状の分極反転構造を形成し、これを通過したレーザ光を電気光学効果を利用して偏向する光素子やシリンドリカルレンズ、ビームスキャナー、スイッチなどが新しい光素子として注目され、LN単結晶も基板材料として有望とされている。
【0007】
これまでに報告された、強誘電体LN単結晶の分極反転構造を利用した波長変換素子や電気光学素子は、いずれの場合にも基板結晶としては、市販されている無添加またはMgO添加のコングルエント組成のLN単結晶が用いられてきた。
【0008】
この理由は、これまで、入手可能なLN単結晶は、工業的な面から安価で大口径の育成が可能なチョクラルスキー法で育成されたコングルエント組成の結晶に限られているためである。LN結晶では、ストイキオメトリ組成(化学量論組成または以下定比組成とよぶ)とコングルエント組成(一致溶融組成)は一致しないことは、温度-組成比の相関図(相図)から良く知られている。
【0009】
コングルエント組成のみが融液組成と結晶組成とが一致し、結晶全体にわたって均一組成の結晶を育成することが出来る組成であるため、現在、各種用途に製造、使用されているLN単結晶の組成は、Li2O/(Nb2O5+Li2O)のモル分率が約0.485(Li/Nbのモル比は約0.94)のコングルエント組成である。
【0010】
このため、従来のコングルエント組成LN単結晶は、Nb成分が過剰であるため、数%に達するNbイオンがLiイオンを置き換えている(アンチサイト欠陥)し、Liイオンサイトにやはり数%の空位欠陥をもたらしている。この影響は表面弾性波素子応用としては深刻でないとしても、光学素子応用には無視することはできない。このため、光機能素子応用への基板として、不定比の欠陥を減らした定比に近い組成を持つ結晶の開発が望まれていた。
【0011】
相図からわかるように、例えば、LN単結晶の場合、Li濃度が定比よりも高い組成の融液から定比に近い組成の結晶が析出できる。しかし、従来から、大口径のLN結晶を工業的に大量生産する手段として使用されているチョクラルスキー法を用いて定比組成結晶を育成しようとした場合には、結晶の析出に伴ってLi成分の過剰分が坩堝内に残されることになり、融液のLiとNbの組成比が徐々に変化するため、育成開始後すぐに融液組成比は共晶点に至ってしまう。このため、結晶の固化率はわずか10%程度に制限され、析出した結晶の品質も光機能素子応用に使用できるものではなかった。
【0012】
本発明者等は、従来の市販されているコングルエント組成のLN結晶と異なる新規物質として、コングルエント組成の不定比欠陥濃度を大幅に低減したLi2O /(Nb2O5+Li2O)のモル分率が0.495〜0.50(Li/Nbのモル比は約0.98〜1.00)の定比組成に近いニオブ酸リチウム単結晶の発明をなし、特許出願した(特開平10-45497号公報)。また、この新規結晶に関して下記のように文献報告した。
【0013】
この不定比欠陥を低減して高品質結晶を開発する手段として、本発明者等は、例えば、公知文献(K. Kitamura et al., Journal of Crystal Growth 第116巻、1992年発行、第327〜332頁、または北村健二他、応用物理、第65巻、第9号1996年発行第931〜935頁)において、原料を連続的に供給しながら育成する方法(以後原料連続供給二重るつぼ法と略記する)を提案している。
【0014】
例えば、定比組成に近いLN単結晶の育成においては、具体的には、育成融液のLi2O/(Nb2O5+Li2O)のモル分率をLi成分の過剰の0.56〜0.60とし、るつぼを二重構造にして内側のるつぼから定比組成に近いLi2O/(Nb2O5+Li2O)のモル分率が0.498〜0.502(Li/Nbのモル比は約0.99〜1.01)のLN結晶も引き上げることができた。
【0015】
引き上げている結晶の重量を随時測定することで成長レートを求め、そのレートで結晶と同じ定比組成の成分の原料粉末を外るつぼと内るつぼの間に連続的に供給するという方法を用いることで、長尺の結晶育成が可能となり、原料供給量に対して100%の結晶固化率が実現されている。
【0016】
また、本発明者等による最近の公知文献(北村健二他、日本結晶成長学会誌、第25巻、第3号、1998年発行、第A4頁)によれば、上記の定比組成に近い無添加のLN単結晶(Li/Nbモル比で0.98〜1.0)では、分極反転に要する印加電界が従来の1/5程度で済むことを報告した。すなわち、従来のコングルエント組成結晶における数%の不定比欠陥(アンチサイト欠陥や空位欠陥)の存在が、LN結晶が本来有する光学特性や、周期的な分極構造を作成するのに必要な印加電圧を高くしている可能性があることを報告している。
【0017】
さらに、本発明者等による最近の公知文献(Y. Furukawa et al., Journal of Crystal Growth 第211巻、2000年発行、第230〜236頁)によれば、定比組成に近い組成の結晶では、従来のコングルエント組成結晶の耐光損傷性を向上させるために5mol%以上必要とされていたMg等の添加量は1mol%程度の少量の添加でも、十分に耐光損傷性が向上できることを報告している。
【0018】
この場合、MgがLiサイトも置換するのでMgの添加量が増えるに従いLi/Nbモル比は無添加の結晶に較べて小さくなり、得られた結晶のLi/Nbモル比は0.95〜1.0となっている。このように、ストイキオメトリック組成LNはコングルエント組成LNに対し、わずかなモル分率の変化であるが、化学量論比に近づくに従いその結晶特性は大幅に異なる。特に、結晶のLi/Nbのモル比が0.95〜1.01の範囲で従来のコングルエント組成の結晶とは大きく異なる光学特性を有する。
【0019】
【発明が解決しようとする課題】
強誘電体単結晶の基板上に分極反転構造を形成し、分極反転部を通過する光の非線形光学効果や電気光学効果との相互作用を利用した光機能素子を実現する上で最も重要な技術は、数個〜数百個にも及ぶ数ミクロンから数十ミクロンサイズの分極反転構造を精度良くかつ均一に作成することである。
分極反転構造の形成方法として、電子ビーム照射法や電圧印加法がよく知られており一般的によく使用されている。これら光機能素子では分極反転部を光を通過させて使用するために、特に、それぞれの分極反転境界部に光学的歪みがあると素子全体としては非常に大きな光学的な不均一性を引き起こしてしまうため、高効率の素子が実現できなくなる。
【0020】
分極反転境界部には光学的歪みが発生し、10-3〜10-4以上の非常に大きな屈折率変化が生じる。これが通過レーザ光の散乱をもたらし、これによって素子動作も理想条件からずれるため素子効率が低下するという大きな問題があることが、公知例(V.Gopalan et al., J.Appl. Phys.第80巻, 1996年, 6104頁)において指摘されている。
【0021】
このため、前記の公知文献(L.E.Myers et al., Optics Letters, 21, p591,1996)にあるように、LN単結晶に約21kV/mmの電界を加え、周期反転構造を作成した後に、結晶を120℃で1時間加熱し光学的歪みを緩和させなければならないことが報告されている。
【0022】
また、前記公知文献(M. Yamada et al., Appl.Phys.Lett., 69,p3659,1996)によると、強誘電体結晶であるLN単結晶に高電圧を印加することで、結晶中にレンズやプリズム状の分極反転構造を形成した光学素子においても、電圧印加による分極反転構造の形成後に熱処理が必要で、この場合には、結晶基板を500℃に大気中で加熱し5時間も熱処理することが、分極反転部の光学的歪みを除去するために不可欠であることが報告されている。
【0023】
従来の電圧印加法では、通常、zカットのコングルエント組成のLN単結晶を用い、結晶の片面に周期電極を、反対面に一様電極を設けて、試料を室温または200℃程度までに加熱し、電極を通じてパルス電圧を印加することで周期電極直下の部分をz軸方位に向けて分極反転させている。従来のコングルエント組成のLN単結晶の場合には、分極反転に必要な印加電界は21kV/mm以上と高電圧が必要とされている。
【0024】
このような分極反転技術は、キュリー温度以下の温度で強制的に分極の方向、すなわち結晶中のNbやLiイオンの位置を変えるわけである。LN単結晶において分極反転に必要とされる高電界が、光学的歪みを引き起こす直接の原因であるとは必ずしも言えないことがわかっている。
【0025】
すなわち、前記公知文献(A. Harada et al., Optics Letters, 22,p805,1997)において、MgO を5モル%添加したコングルエント組成のLN単結晶では分極反転に必要とされる電界が通常のコングルエント組成より約1/5程度に小さくなるが、この材料を用いた場合でも、コロナ放電法を用いてMgOを添加したLN単結晶に4.75ミクロン周期で分極反転構造を形成したSHGレーザを作成する場合には、光学的歪みを除去するために約500℃で3時間加熱することが必要とされることが報告されている。
【0026】
このような従来のコングルエント組成LN結晶を基板に用い、基板上に分極反転構造を形成した素子の分極反転境界を偏光顕微鏡で観察すると、図1の(a) に様子を示したように大きな光学的歪みがすべての分極反転境界部において観察された。さらに分極反転部を横切るように使用するレーザ光を通過させると数%から十数%もの非常に大きな光の伝搬ロスが観察された。このような分極反転境界における光学的歪みの発生は、大きな光の伝搬ロスの問題だけではなく、この光学的歪みを緩和するための光機能素子の製作における余分な熱処理工程を必要とさせることにもなる。
【0027】
さらに大きな問題は、歪み除去のための熱処理中に、単一分極基板の一部に電圧印加法などで一旦形成された数ミクロンサイズの分極反転部で、焦電効果が発生し結晶が破壊したり、反転分極のサイズや位置がほんのわずかであるが変化させることである。この変化は高効率の素子を再現性良く作成するうえで大きな問題となった。
【0028】
【課題を解決するための手段】
本発明者は、前記従来の問題を解決するため、強誘電体単結晶としてLN単結晶の特性究明を鋭意継続していたところ、定比組成に近い組成のLN単結晶は、分極反転構造を形成しても分極反転境界部での光学的歪や光の伝搬ロスが非常に小さく、これを基板に用いることで分極反転構造を持つ光機能素子として優れた特性を有することを見いだした。
【0029】
すなわち、本発明は、(1)強誘電体単結晶基板の一部に、電子ビーム走査照射法または電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、この分極反転部を通過した光を制御する光機能素子の製造方法であって、該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることにより、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、前記分極反転構造を形成直後の該分極反転部を通過させた該光の伝搬ロスを2%以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記2%以下の値をさらに低減させることを特徴とする光機能素子の製造方法、である。
【0030】
また、本発明は、(2)強誘電体単結晶基板の一部に、電子ビーム走査照射法または電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、この分極反転部を通過した光を制御する光機能素子の製造方法であって、該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることにより、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、分極反転境界部の屈折率変化が1×10 −4 以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記1×10 −4 以下の値をさらに低減させることを特徴とする光機能素子の製造方法、である。
【0031】
また、本発明は、(3)両面光学研磨された厚み0.30mm〜3.0mmの強誘電体単結晶基板の一部に、3〜4kV/mmの電界を印加する電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、非線形光学効果を利用して周期的反転分極構造を持つ単結晶内に入射したレーザの波長変換を行う光波長変換素子の製造方法であって、該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることにより、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、該分極反転構造を形成直後の分極反転部を通過させた光の伝搬ロスが2%以下、かつ、分極反転境界部の屈折率変化が1×10 −4 以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記2%以下の値、かつ、前記1×10 −4 以下の値をさらに低減させることを特徴とするレーザの波長変換素子の製造方法、である。
【0032】
また、本発明は、(4)両面光学研磨された厚み0.20mm〜2.0mmの強誘電体単結晶基板の一部に、2.5〜5kV/mmのパルス状の電圧を印加する電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、電気光学効果を利用してプリズムまたはレンズ形状に反転した分極構造を持つ単結晶内に入射されたレーザ光の偏向または集光を制御する光機能素子の製造方法であって、該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることによって、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施していない、または、100℃以下の温度で熱処理を施さないで、該分極反転構造を形成直後の分極反転部を通過させた光の伝搬ロスが2%以下、かつ、分極反転境界部の屈折率変化が1×10 −4 以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記2%以下の値、かつ、前記1×10 −4 以下の値をさらに低減させることを特徴とするレーザ光の偏向または集光を制御する光機能素子の製造方法、である。
【0033】
また、本発明は、(5)前記強誘電体単結晶基板は、原料連続供給二重るつぼで育成されることを特徴とする上記(1)〜(4)のいずれかに記載の方法、である。
【0034】
また、本発明は、(6)前記強誘電体単結晶基板は、原料連続供給二重るつぼで育成したMg、Zn、Sc、Inから選ばれる少なくとも一つの元素を0.1〜4.8モル%ドーピングして含有するLi/Nbのモル比が0.95〜1.00の範囲のLiNbO結晶であることを特徴とする上記(5)に記載の方法、である。
【0035】
本発明者らは、強誘電体単結晶の分極反転構造を利用した光機能素子における素子性能や分極反転制御性の問題点は単結晶基板にあることを突き止めた。本発明は、強誘電体単結晶の分極反転構造を利用した光機能素子用途として、ある組成範囲にあるLN結晶単結晶基板に着目した点にある。Li/Nbのモル比が0.95〜1.01の範囲であるニオブ酸リチウム単結晶が従来の材料の特性と異なり、分極反転構造を利用した光機能素子材料の品質を大幅に向上させることが可能になった。これを利用することで、光機能素子の特性も飛躍的に向上することが明らかになった。
【0036】
今回見いだされた分極反転特性についても、このモル分率を有するLN単結晶特有の効果である。定比組成に近いLN単結晶は、原料連続供給二重坩堝法によって、最近ようやく光学的に均質な基板作製が可能になった結晶であり、その光学特性については、未だ総てが明らかにされていない。
【0037】
特にこれらの結晶の分極反転境界の光学特性については、本発明者らが初めて明らかにしたものである。また、この特性を利用した光機能素子特性の大幅な向上については、さらに未開拓な分野であった。
【0038】
【発明の実施の形態】
次に本発明の光機能素子として用いられるLN単結晶の製造方法と物性を示す。市販の高純度Li2O、Nb2O5の原料粉末を準備し、Li2O:Nb2O5の比が0.54:0.46〜0.60:0.40のLi成分過剰原料を混合した。また、Li2O:Nb2O5=0.50:0.50の定比組成原料を混合した。次に、1ton/cm2の静水圧でラバープレス成形し、それぞれを約1050℃の大気中で焼結し原料棒を作成した。また、混合済みの定比組成原料を連続供給用原料として、約1150℃の大気中で焼結し、粉砕し、大きさが50ミクロン以上500ミクロンのサイズの範囲で分級した。
【0039】
次に、二重るつぼ法による単結晶育成に際して、作成したLi成分過剰原料からなる原料棒を内側および外側るつぼに予め充填し、次にるつぼを加熱してLi成分過剰な融液を作成した。Mg添加の効果を確認する実験では、この充填の際に、市販の高純度MgCO3を内側および外側るつぼに予め0.1〜4.8mol%の範囲で充填した。
【0040】
次に、原料連続供給型二重坩堝法を用いて定比組成に近いLN単結晶の育成を行った。二重るつぼ内のLi成分過剰組成の融液に種結晶を漬け、定比組成に近い、すなわち、不定比欠陥濃度を極力抑えた単結晶を得た。不定比欠陥の密度や構造を精密に制御するために、結晶化した成長量に見合った量のLi2O/(Nb2O5+Li2O)のモル分率が0.50の化学量論組成比の原料を外側坩堝に自動的に供給しながら結晶を育成した。
【0041】
ここで、育成に用いた坩堝は白金でできており、外側るつぼは直径125mm高さ70mm、内側るつぼは直径85mm高さ90mmとした。この場合にも融液組成を均一化させるために育成に際して坩堝を3rpmの速度で種結晶と反対方向に回転させた。育成条件は結晶回転速度を15rpm、引き上げ速度は0.5mm/hで一定とし、育成雰囲気を大気中とした。約1週間の育成により直径約49〜52mm、長さ約65〜75mmの大きさで、クラックのない無色透明のLN結晶体を得た。
【0042】
得られた全ての結晶に関して、結晶の上部、中心、下部の3ヶ所から試料を切り出しLi/Nbモル比を化学分析より求めた。化学分析では組成比の絶対値を精度良く求めるために、非常に慎重に組成を分析した。分析は同一試料について数カ所の異なる分析装置を用いて評価した結果の平均値として求めた。その結果、LN単結晶の場合、定比に最も近い組成ではLi/Nbモル比が0.99〜1.01であった。
【0043】
一方、Mgを添加した結晶ではMgがLiやNbサイトを置換していくので、Mgの添加量が増えるに従いLi/Nbモル比は変化し、得られた結晶のLi/Nbモル比は0.95より大きく1.0より小さい範囲にあった。Mg以外にZn、Sc、Inを添加した場合には元素の種類によって結晶内での偏析係数は異なるため、添加量に対する結晶内含有量は異なるものの、いずれの添加元素においても、添加元素がLiやNbサイトを置換していくので、添加元素の添加量が増えるに従いLi/Nbモル比は変化し、得られた結晶のLi/Nbモル比は0.95より大きく1.0より小さい範囲にあった。
【0044】
一方、キュリー温度測定による組成評価においては、予め定比組成に調合し1150℃で焼結した定比組成の標準焼結試料のキュリー温度は1200℃であることを確認し、この値と上記原料連続供給二重るつぼで育成したLN単結晶のキュリー温度を比較した。キュリー温度測定によるLi/Nbモル比の評価結果も化学分析の結果とほぼ一致し、また、1本の結晶内での結晶組成の均質性も極めて良いことを確認した。
【0045】
次に、単一分域化状態にあるLN単結晶から大きさが35mm×35mm×40mmのブロック状試料を切り出し、メカノケミカル研磨により表面研磨を行った。試料の光学的均質性をマッハツエンダー干渉法により評価したところ、マクロな欠陥や光学的に不均一な部分は見られず、試料内の屈折率変化は1×10-5以下が得られ光学的均質性に優れていることが確認された。
【0046】
従来から市販されてきたコングルエント組成のLN単結晶基板は単結晶育成技術の制約から多量のNb成分が過剰のものである。Li/Nbモル比が0.94であるため数%にもおよぶ多量の不定比欠陥を含んでいる。一方、本発明者等は、原料連続供給二重坩堝法によってLi成分過剰の融液から結晶を育成し、より定比組成に近いLi/Nbモル比が0.95〜1.01のLN単結晶が育成でき、Nb成分過剰による不定比欠陥濃度を低減した単結晶が光機能素子基板として優れた特性を示すことを初めて明らかにしたものである。
【0047】
すなわち、従来の結晶における過剰なNbにより形成される多量の不定比欠陥が、分極反転構造を利用する光機能素子応用にとって大きな問題を引き起こすことを見い出した。この欠陥の存在によって、分極反転に必要な印加電圧と自発分極の関係を示すヒステリシス曲線は非対称的になり、分極反転には数十kV/mmの高電界が必要とされ、しかも分極反転を行うと分極反転境界部には大きな光学的歪みと光の伝搬ロスが導入されることが分かった。さらに、不定比欠陥が多く結晶内部で欠陥が不均一に分布しており欠陥濃度が高いような箇所では分極反転がピンニングされやすいために、より大きな光学的歪みが蓄積され結晶の破壊の原因になることが明らかになった。
【0048】
図1は、LN単結晶を基板に用いて、両面光学研磨された厚み0.5mmの基板1、4上に電圧印加法により周期的分極反転部2、5を形成した後に、分極反転部2、5を偏光顕微鏡で詳細に観察した様子の一例を示している。分極反転の周期は約3〜4μmとし、波長850nm帯の基本波に対して擬似位相整合するように設計した。
【0049】
図1(a)は、従来のコングルエント組成のLN単結晶を基板に用いた場合の様子である。図1 (b)は、基板に上述した定比組成に近い組成のLN単結晶を用いた場合の様子である。両者の違いは明らかで、図1(a)では、分極反転部に非常に大きな光学的歪みがあるのが観察されたのに対し、図1(b)では、偏光顕微鏡下で光は均一に透過し光学的歪みは観察されなかった。
【0050】
さらに、結晶基板にさまざまな形状と大きさの分極反転構造を形成し、基板の違いによる分極反転境界部での光学的歪みを観察し、レーザ干渉法により分極反転境界での屈折率の大きさを評価すると従来結晶では8×10-3〜3×10-4と非常に大きな屈折率変化が観察された。一方、本発明での結晶基板を用いた光機能素子では、熱処理を行わなくても光学的歪みは屈折率変化で1×10-4以下が得られることが確認された。
【0051】
さらに図2は、室温〜200℃付近の温度で電子ビーム照射法および電圧印加法で周期的分極反転構造を形成した、長さ5mm、厚さ0.5mmの各種組成のLN単結晶の両端面を鏡面研磨し、分極反転部を伝搬していく光が結晶内部で散乱や光学的歪みなどで引き起こされる光の伝搬ロスの大きさを評価した結果を示したグラフである。
【0052】
同一組成の試料であっても、分極反転構造の作成時の印加電圧、電極の形状、電極材質、温度などによって光の伝搬ロスにはばらつきが見られた。Li/Nbモル比が0.94のコングルエント組成結晶では、光の伝搬ロスは4〜8%と非常に大きいことが分かった。これに対して、Li/Nbモル比が0.98〜1.01と定比に近い無添加のLN単結晶や、3モル%程度のMgOを含むLi/Nbモル比0.95のLN単結晶など多くの結晶で光の伝搬ロスが2%以下が得られ、中には0.1%以下の光の伝搬ロスの小さい良質な分極反転構造を形成できる結晶も得られた。
【0053】
さらに図3は、室温付近の温度で電子ビーム照射法および電圧印加法で周期的分極反転構造を形成した、長さ5mm、厚さ0.5mmの結晶の分極反転部を通過していく光の伝搬ロスが熱処理によってどれだけ低減できるかを示したグラフである。
【0054】
従来のコングルエント組成結晶を用いると分極反転構造の形成後は、散乱や光学歪みなどの影響により光の伝搬ロスは非常に大きく、コングルエントLN結晶では、かなり高温度に基板を加熱してやらないと光の伝搬ロスが下がらない結果が得られた。これに対して、本発明の定比組成に近いLN単結晶を基板に用いた場合には高温での熱処理をしなくても光の伝搬ロスは小さく光機能素子の性能向上が期待できることが明らかであるが、熱処理をする場合、図3に示すように、光の伝搬ロスをさらに小さくする効果が認められる 100 ℃以下の温度で熱処理を施すだけで十分である。
【0055】
LN単結晶では、キュリー温度より高温の常誘電相において、LiとNbイオンは電気的中性位置に配置しているが、キュリー温度以下の強誘電相ではLiおよびNbイオンが+zもしくは-z方向に少しずれる。このイオンのずれの方向によってドメインの正負の分極方向が決定されている。分極反転構造を持つ光機能素子では、高電界を加えることでこのイオンを低温で強制的に移動させることが必要になる。
【0056】
一致溶融組成の不定比欠陥が多い場合にはLiサイトに入った過剰のNbを移動させることは容易ではないため、分極反転には大きな印加電圧が必要となる。さらに、高電圧を印加して強制的に分極を反転させるわけであるから、分極反転境界部には大きな光学的歪みが導入されると考えられる。現状では、本発明で見られた光学的歪みや光の伝搬ロスの低減の原因について、結晶の反転電圧や内部電界の大きさだけでは十分な説明ができているわけではない。
【0057】
しかしながら、不定比欠陥を多量に含む従来のコングルエント組成結晶よりも、不定比欠陥を1桁以上低減した定比組成に近いLN単結晶が分極反転素子の基板として優れることは明らかである。このことから、強誘電体単結晶基板として定比組成に近い組成のLN単結晶を用いることで、分極反転構造を形成しても分極反転境界部での光学的歪みを示さず、分極反転境界部での光学的歪みを除去するための加熱工程なしに、分極反転境界部の屈折率変化が1×10-4以下が得られるため、分極反転制御性に優れ、レーザ光の散乱がなく光の伝搬ロスが小さく光機能素子として優れた特性を有する。
【0058】
【実施例】
以下実施例を用いて、本発明をさらに具体的に説明する。
実施例1
LN単結晶を光波長変換素子に適用した場合の特性について説明する。図4は定比組成に近い単結晶(Li/Nbモル比が0.98〜1.01の無添加LN単結晶)を基板に用いて、基板上に周期的分極反転構造を形成したQPMデバイスの概略構成図である。両面光学研磨された厚み0.30mm〜3.0mmの基板6の+z面に櫛形電極と平行電極をパターニングした。周期は約3.2μmで、波長約850nmの基本波に対して擬似位相整合するように設計された。上記組成の結晶基板の−z面は、電極を全面に蒸着した。櫛形電極と平行電極の間、および櫛形電極と−z面の裏面電極に、それぞれ3〜4kV/mm程度の従来のコングルエント結晶より1/5程度の低い電界を印加して、絶縁破壊なしに周期的分極反転幅8で分極反転領域を形成した。
【0059】
本実施例においては周期状分極反転構造の分極反転部を偏光顕微鏡で詳細に観察したが光学的歪みは見られなかった。また、分極反転部に波長可変レーザ9からレンズ10を介して照射したレーザ光を通過させたが、レーザ光の散乱は全く観察されず、このため、熱処理は全く不要で、しかも高効率の波長変換が得られた。用いたLN結晶は予め分極状態は非常に均一化されている。結晶に周期状の分極反転構造を形成する際にも、定比組成に近いLN単結晶においては、結晶の均一性に優れているため、均一な分極反転構造の形成が可能になる。
【0060】
このように、従来のコングルエント組成のLN結晶を基板として用いたときに見られた問題は解決されていた。さらに、分極反転構造を形成した後、結晶を取り外し、断面となる結晶のy面を研摩、フッ酸・硝酸の混合液でエッチングして、分極の反転の様子を調べた。周期分極反転幅比その分極の形は印加電圧のパルス幅や電流を最適化することで、試料全体にわたり周期分極の分極反転幅比を理想的な比に精度よく作成することができていることが確認された。
【0061】
周期分極反転構造の形成は厚みが1mm以上の試料についても同様に高精度に形成が可能であった。しかも光学的歪みは見られず、伝搬損失も0.2%以下と非常に少なかった。これらの厚い試料では、特に、分極反転構造の形成後の熱処理が不要なことは大きなメリットとなった。これは、1mm以上の厚さを持つ試料では、部分的な結晶のマクロな欠陥や、電極の不均一、熱的な不均一があると、光学的歪みを除去する熱処理中に分極反転境界部が容易に移動したり、焦電効果で結晶が破壊してしまう問題があったからである。このため、本実施例で作成された光学的均一性と分極反転制御性に優れた光機能素子は、特に光の伝搬ロスの小さなことが要求される内部共振器型の波長変換素子として最適であると考えられる。
【0062】
QPM-SHGデバイスの特性の評価は基本波として、波長可変高出力Tiサファイヤレーザ(波長850nm)を用いて行い、高効率の光波長変換が確認できた。その様子を図5に示す。従来のコングルエント組成LN結晶を基板に用いた場合、熱処理前ではほとんど効率良い波長変換は得られない。熱処理により波長変換効率が改善される様子が見られたが、本発明の光素子ではより高い波長変換効率が得られている。
【0063】
この理由は、光の伝搬ロスが小さいことが大きな理由として考えられる。さらに、基板の非線形光学定数が大きいことに加え、光学的歪みがなく、かつ熱処理不要のため分極反転構造の完全性がより高いことも高性能な光機能素子が得られるのに寄与していると考えられる。
【0064】
また、ここでは、850nm付近の近赤外光の基本波に対して青色光を発生するQPM-SHG素子を作成した実施例に付いて詳しく述べたが、本発明によれば、基本波がこの二つの波長に限ることはなく、LN単結晶が透明でかつ位相整合が可能である波長域に関して適用することが可能である。
【0065】
さらに、LN単結晶の分極構造を周期的に反転させ、可視から近赤外域の波長を持った入射レーザの波長を短波長化あるいは長波長化させる本発明の光機能素子は第二高調波発生素子に限らず、光パラメトリック発振素子や差周波、和周波発生素子をはじめ、光スイッチや光変調器など分極反転構造を利用する高性能光素子を実現することが可能である。その応用も、さらにはリモートセンシング、ガス検知をはじめとする応用分野や、波長ミキサーやパルス成形素子などの光通信分野への適用も可能である。
【0066】
ここでは、強誘電体単結晶基板の一部にキュリー温度以下の温度において分極反転構造を形成する実施例として、電圧印加法を用いたLN単結晶の光機能素子について説明したが、キュリー温度以下の温度における分極反転構造を形成する方法として、電子ビーム走査照射法であっても同様の効果が得られる。
【0067】
実施例2
定比組成に近い (Li/Nbモル比が0.98〜1.01の無添加LN単結晶)を基板に用いて、レンズやプリズム状の分極反転構造を作製し電気光学効果を利用した偏向素子や、シリンドリカルレンズ、ビームスキャナー、スイッチなどの光素子を製作した。
【0068】
図6および図7は、それぞれレンズ14およびプリズム19状の分極反転構造を作成し、作成した電気光学効果を利用して単結晶内に入射されたレーザ光を制御するフォーカシングおよびスキャンニングを行う光機能素子の概略構成図である。直径1.5インチ、厚み0.2〜2.0mm、両面研摩されたz-カットの上述したLN単結晶11、16を準備し、両z面に厚さ約200nmのAl電極をスパッタリングにより形成し、リソグラフを用いて、レンズ14やプリズム19状パターンを形成した。その後、+z面にパルス状の印加電圧15、20を約2.5〜5KV/mmで印加し分極を反転させた。
【0069】
本実施例においては分極反転部を偏光顕微鏡で詳細に観察したが光学的歪みは見られなかった。また、半導体レーザ12、17により分極反転領域13、18にレーザ光を通過させたが、レーザ光の散乱は全く観察されず、このため、熱処理は全く不要で、しかも光機能素子が得られた。用いたLN結晶は予め分極状態は非常に均一化されている。さらに結晶の端面を鏡面研磨仕上げを行い、レーザ光の入出射面とした。
【0070】
試作した分極反転構造による屈折率の反転構造を形成したLN単結晶の電気光学効果を利用した光素子の性能は、レンズやプリズム状の分極反転構造の設計や分極反転構造の作製プロセスの精度、および材料の持つ電気光学定数の大きさで決定された。本実施例のレンズやプリズム状パターンの分極反転構造で、特筆すべきことは分極反転境での光の伝搬ロスと光学的歪みがなく、かつ分極反転性の制御が非常に容易であることから良好な素子特性が得られ、光機能素子の駆動効率が向上したことである。
【0071】
従来の一致溶融組成のLN結晶では反転周期が短くなり反転構造が複雑になると、精度の良いレンズやプリズム状の分極反転構造の作製は困難で、かつ熱処理が必要であった。これに対し、定比組成に近いLN単結晶を、分極反転構造を利用した光機能素子用途として用いることにより、光機能素子の高精度な分極反転構造の形成が可能であった。
【0072】
さらに、本結晶は一致溶融組成の結晶よりも大きな電気光学定数r33を有しているので、より小さな動作電圧でより優れたデバイス性能が得られた。例えば偏向素子の場合には約600V/mmの電界で約6℃と大きな偏向角が得られた。また、約100V/mm近傍で動作するレンズや、約500V/mmでのスイッチング動作も得られた。
【0073】
ここでは、強誘電体単結晶基板の一部にキュリー温度以下の温度において分極反転構造を形成する実施例として、電圧印加法を用いたLN単結晶の光機能素子について説明したが、キュリー温度以下の温度における分極反転構造を形成する方法として、電子ビーム走査照射法であっても同様の効果が得られる。
【0074】
【発明の効果】
以上詳しく述べたように、本発明によれば、強誘電体単結晶基板の一部に、キュリー温度以下の温度において、電子ビーム走査照射法、または電圧印加法を用いて分極反転構造を形成し、この分極反転部を通過した光を制御する光機能素子において、強誘電体単結晶としてLi/Nbのモル比が0.95〜1.01の範囲の定比組成に近い組成のLN単結晶を用いることによって、2%以下の所望の値の光の伝搬ロスが得られ、自発分極の方向反転に伴う分極反転境界部での光学的歪みを除去するための加熱工程なしに、分極反転境界部の屈折率変化が1×10-4以下を得ることができるため、分域境界で歪みがなく、かつ光学的均質性と分極反転制御性とに優れた素子が実現できるため、光機能素子特性の大幅な向上が期待できる。
【0075】
これにより、本発明は、レーザ光を利用した光情報処理、光加工技術、光通信技術、光計測制御等々の分野での光機能素子の実用化を促進させる大きな効果をもたらす。
【図面の簡単な説明】
【図1】LN単結晶基板に周期分極反転構造を形成後、+z面を透過偏光観察した外観図であり、(a)は、従来のコングルエント組成LN結晶基板、(b)は、定比組成に近いLN結晶基板を示す。
【図2】結晶組成と分極反転部を伝搬した結晶内部の光の伝搬ロスの関係を示したグラフ。
【図3】熱処理温度と分極反転部を通過した結晶内部の光の伝搬ロスの関係を示したグラフ。
【図4】本発明の一実施例の光波長変換素子を示す概念図。
【図5】基本入力光とSHG光出力の関係を示したグラフ。
【図6】本発明の一実施例の集光素子を示す概念図。
【図7】本発明の一実施例の偏向素子を示す概念図。
【符号の説明】
1.コングルエント組成LN単結晶基板+z面
2.周期的分極反転部
3.光学歪み
4.定比組成に近いLN単結晶基板+z面
5.周期的分極反転部
6.定比組成に近いLN単結晶基板
7.分極反転領域
8.周期的分極反転幅
9.波長可変レーザ
10. レンズ
11. 定比組成に近いLN単結晶基板
12. 半導体レーザ
13. 分極反転領域
14. レンズ
15. 印加電圧
16. 定比組成に近いLN単結晶基板
17. 半導体レーザ
18. 分極反転領域
19. プリズム
20. 印加電圧
[0001]
BACKGROUND OF THE INVENTION
The present invention relates to LiNbO used in fields such as optical information processing using laser light, optical processing technology, optical communication technology, and optical measurement control.ThreeOptical functional device that controls light using the polarization inversion structure of a single crystal substrateManufacturing ofRegarding the method.
[0002]
[Prior art]
Lithium niobate (LiNbO), known as a typical ferroelectric single crystalThree) Single crystals (hereinafter abbreviated as LN where appropriate) are mainly used as substrates for surface acoustic wave devices. As this crystal, a single crystal having a large diameter and high composition homogeneity can be supplied at a relatively low cost. Furthermore, it is transparent in a wide wavelength range from visible to infrared, and it is possible to invert ferroelectric polarization even at room temperature by applying a high electric field of several tens of kV / mm. It attracts attention as a substrate for various optical functional elements such as nonlinear optical elements and electro-optical elements.
[0003]
In particular, in recent years, the development of a waveguide-type optical second harmonic generation (SHG) element that converts a semiconductor laser with a near-infrared wavelength into blue light with a half-wavelength by a nonlinear optical effect is expected. As a high-density recording / reproducing light source for an optical disk, a wavelength conversion element using an element having a structure in which the polarization of an inorganic ferroelectric single crystal such as LN is periodically reversed is most studied. This wavelength conversion element is based on a quasi-phase matching (QPM) method, and a phase matching is performed by compensating for the difference in propagation constant between the fundamental wave and the harmonic wave by a periodic structure.
[0004]
This method is expensivewavelengthIt has many excellent features such as high conversion efficiency, easy parallelization of output light and diffraction-limited focusing, and no restrictions on applicable materials and wavelengths. The periodic structure for QPM includes the SHG coefficient (d33(Structure of the coefficient) is reversed periodicallyWavelength conversionIt is most effective in obtaining efficiency. In ferroelectric crystals, the sign of the d coefficient corresponds to the polarity of the ferroelectric polarization, so the ferroelectric domain is periodically inverted.StructuralForming technology is important.
[0005]
Using this method, a periodic inversion structure was created by applying an electric field of about 21 kV / mm to an LN single crystal as described in known literature (LEMyers et al., Optics Letters, 21, p591,1996). A parametric oscillation wavelength conversion element using a method has been reported. Furthermore, as shown in the publicly known literature (A. Harada et al., Optics Letters, 22, p805, 1997), a domain-inverted structure was formed at a period of 4.75 microns on an LN single crystal to which MgO was added using the corona discharge method. In addition, an SHG laser in which blue light of 473 nm is converted with high efficiency from laser light having a wavelength of 946 nm has also been reported.
[0006]
In addition, in an optical element using the electro-optic effect, for example, according to a known document (M. Yamada et al., Appl. Phys. Lett., 69, p3659, 1996), an LN single crystal that is a ferroelectric crystal. By applying a high voltage to the lens, a lens or prism-like domain-inverted structure is formed in the crystal, and the optical element, cylindrical lens, beam scanner, or switch that deflects the laser light that has passed through it using the electro-optic effect Are attracting attention as new optical elements, and LN single crystals are also promising as substrate materials.
[0007]
Wavelength conversion elements and electro-optic elements using the polarization reversal structure of ferroelectric LN single crystals reported so far are in any case commercially available additive-free or MgO-added congruent. LN single crystals of composition have been used.
[0008]
This is because the LN single crystals that have been available so far are limited to crystals of congruent composition grown by the Czochralski method, which is inexpensive and capable of growing a large diameter from an industrial viewpoint. It is well known from the temperature-composition ratio correlation diagram (phase diagram) that the stoichiometric composition (stoichiometric composition or “stoichiometric composition”) and congruent composition (coincident melt composition) do not match in LN crystals. ing.
[0009]
Since only the congruent composition has the same melt composition and crystal composition and can grow crystals with a uniform composition over the entire crystal, the composition of LN single crystals currently manufactured and used for various applications is , Li2O / (Nb2OFive+ Li2The congruent composition has a molar fraction of O) of about 0.485 (the molar ratio of Li / Nb is about 0.94).
[0010]
For this reason, in the conventional congruent composition LN single crystal, the Nb component is excessive, so Nb ions reaching several percent replace Li ions (antisite defects), and several percent of vacancies are also present at the Li ion sites. Has brought. Although this effect is not serious for surface acoustic wave device applications, it cannot be ignored for optical device applications. Therefore, it has been desired to develop a crystal having a composition close to a constant ratio with reduced non-stoichiometric defects as a substrate for optical functional element applications.
[0011]
As can be seen from the phase diagram, for example, in the case of an LN single crystal, a crystal having a composition close to the constant ratio can be precipitated from a melt having a composition in which the Li concentration is higher than the constant ratio. However, when trying to grow a stoichiometric crystal using the Czochralski method, which has been conventionally used as a means for industrially mass-producing large-diameter LN crystals, Li The excess component is left in the crucible, and the composition ratio of Li and Nb in the melt gradually changes. Therefore, the melt composition ratio reaches the eutectic point immediately after the start of growth. For this reason, the solidification rate of the crystals is limited to only about 10%, and the quality of the precipitated crystals cannot be used for optical functional device applications.
[0012]
As a new substance different from conventional commercially available congruent composition LN crystals, the present inventors have significantly reduced the non-stoichiometric defect concentration of congruent composition.2O / (Nb2OFive+ Li2The invention of a lithium niobate single crystal close to a stoichiometric composition having a molar fraction of O) of 0.495 to 0.50 (Li / Nb molar ratio of about 0.98 to 1.00) was filed (JP-A-10-45497). ). Moreover, the literature was reported as follows about this novel crystal.
[0013]
As means for developing high-quality crystals by reducing this non-stoichiometric defect, the present inventors have disclosed, for example, a known document (K. Kitamura et al., Journal of Crystal Growth vol. 116, published in 1992, 327- 332 or Kenji Kitamura et al., Applied Physics, Vol. 65, No. 9, Issue 1996, pages 931 to 935), a method of growing while continuously supplying raw materials (hereinafter referred to as raw material continuous supply double crucible method) Abbreviated).
[0014]
For example, in the growth of LN single crystals close to the stoichiometric composition, specifically, the Li2O / (Nb2OFive+ Li2The molar fraction of O) is 0.56 to 0.60, which is an excess of the Li component, and the crucible is made into a double structure so that the Li is close to the stoichiometric composition from the inner crucible.2O / (Nb2OFive+ Li2LN crystals having a molar fraction of O) of 0.498 to 0.502 (Li / Nb molar ratio of about 0.99 to 1.01) could also be pulled.
[0015]
Use a method in which the growth rate is obtained by measuring the weight of the crystal being pulled up as needed, and the raw material powder having the same ratio composition as the crystal is continuously supplied between the outer crucible and the crystal at that rate. Thus, it is possible to grow a long crystal, and a crystal solidification rate of 100% is realized with respect to the raw material supply amount.
[0016]
Further, according to the recent publicly known literature by the present inventors (Kenji Kitamura et al., Journal of Japanese Society for Crystal Growth, Vol. 25, No. 3, 1998, page A4), there is no It was reported that the applied electric field required for polarization reversal was about 1/5 of that of the conventional LN single crystal (Li / Nb molar ratio 0.98 to 1.0). In other words, the presence of several percent of non-stoichiometric defects (anti-site defects and vacancy defects) in conventional congruent composition crystals can reduce the optical characteristics inherent in LN crystals and the applied voltage required to create a periodically polarized structure. It is reported that it may be high.
[0017]
Furthermore, according to the recent known literature by the present inventors (Y. Furukawa et al., Journal of Crystal Growth Vol. 211, 2000, pp. 230 to 236), crystals having a composition close to the stoichiometric composition In addition, it has been reported that the light damage resistance can be sufficiently improved even with the addition of a small amount of about 1 mol% of Mg, etc., which was required to be 5 mol% or more in order to improve the light damage resistance of conventional congruent composition crystals. Yes.
[0018]
In this case, since the Mg also replaces the Li site, the Li / Nb molar ratio becomes smaller than that of the additive-free crystal as the amount of Mg added increases, and the Li / Nb molar ratio of the obtained crystal becomes 0.95-1.0. ing. Thus, the stoichiometric composition LN is a slight change in molar fraction with respect to the congruent composition LN, but its crystal characteristics are significantly different as it approaches the stoichiometric ratio. In particular, the crystal has Li / Nb molar ratio in the range of 0.95 to 1.01 and has optical characteristics that are significantly different from those of conventional congruent composition crystals.
[0019]
[Problems to be solved by the invention]
The most important technology for realizing an optical functional device that uses a nonlinear optical effect and electro-optic effect of light passing through the domain-inverted part by forming a domain-inverted structure on a ferroelectric single crystal substrate Is to produce a domain-inverted structure with a size of several to several tens of microns ranging from several to several hundreds with high accuracy and uniformity.
Polarization inversionStructuralAs a forming method, an electron beam irradiation method and a voltage application method are well known and generally used. In these optical functional elements, in order to use the domain-inverted part through the light, optical distortion is particularly applied to each domain-inverted boundary part.MigaIn such a case, the entire element causes very large optical non-uniformity, so that a highly efficient element cannot be realized.
[0020]
AntipolarizationRolling boundaryHas optical distortion, 10-3~Ten-FourThe above extremely large refractive index change occurs. This causes scattering of the passing laser beam, and this causes a major problem that the device operation is also deviated from the ideal condition and the device efficiency is lowered, which is known in the known example (V. Gopalan et al., J. Appl. Phys. No. 80 Volume, 1996, p. 6104).
[0021]
Therefore, as described in the above-mentioned known literature (LEMyers et al., Optics Letters, 21, p591, 1996), an electric field of about 21 kV / mm is applied to the LN single crystal to create a periodic inversion structure. Has been reported to have to relieve optical distortion by heating at 120 ° C. for 1 hour.
[0022]
In addition, according to the known literature (M. Yamada et al., Appl. Phys. Lett., 69, p3659, 1996), by applying a high voltage to the LN single crystal which is a ferroelectric crystal, Even in optical elements with a lens or prism-shaped polarization reversal structure, polarization reversal by voltage applicationStructuralHeat treatment is required after formation, and in this case, it is reported that heating the crystal substrate to 500 ° C in the air and heat-treating for 5 hours is indispensable for removing the optical distortion of the domain-inverted part. ing.
[0023]
In the conventional voltage application method, an LN single crystal with a z-cut congruent composition is usually used, a periodic electrode is provided on one side of the crystal, and a uniform electrode is provided on the opposite side, and the sample is heated to room temperature or about 200 ° C. By applying a pulse voltage through the electrode, the portion directly under the periodic electrode is inverted in polarization toward the z-axis direction. In the case of an LN single crystal having a conventional congruent composition, an applied electric field necessary for polarization reversal is required to be 21 kV / mm or higher.
[0024]
Such a polarization inversion technique forcibly changes the direction of polarization, that is, the position of Nb or Li ions in the crystal at a temperature lower than the Curie temperature. It has been found that the high electric field required for polarization reversal in LN single crystals is not necessarily the direct cause of optical distortion.
[0025]
That is, in the known literature (A. Harada et al., Optics Letters, 22, p805, 1997), the electric field required for polarization reversal is normal congruent in an LN single crystal having a congruent composition to which 5 mol% of MgO is added. Although it is about 1/5 smaller than the composition, even when this material is used, an SHG laser in which a domain-inverted structure is formed with a 4.75 micron period on an LN single crystal doped with MgO using the corona discharge method is used. Reported that it is necessary to heat at about 500 ° C. for 3 hours to remove optical distortion.
[0026]
Such a conventional congruent composition LN crystal is used as a substrate, and the domain inversion boundary of a device in which a domain inversion structure is formed on the substratePartWas observed with a polarization microscope, a large optical distortion was observed at all the domain-inverted boundaries as shown in FIG. 1 (a). Furthermore, when the laser beam used across the domain-inverted portion is passed, it is very large, from several to tens of percent.light'sPropagation loss was observed. Such a polarization inversion boundaryPartThe occurrence of optical distortion inlight'sIn addition to the problem of propagation loss, an extra heat treatment step is required in the production of an optical functional element for alleviating this optical distortion.
[0027]
A more serious problem is that during the heat treatment for strain removal, a pyroelectric effect occurs in a domain-inverted part of several microns that is once formed on a part of a single polarization substrate by the voltage application method, etc., and the crystal is destroyed. Or change the size and position of the reversal polarization, though only slightly. This change has become a major problem in producing highly efficient devices with good reproducibility.
[0028]
[Means for Solving the Problems]
In order to solve the above-mentioned conventional problems, the present inventor has been continually investigating the characteristics of LN single crystals as ferroelectric single crystals.ConstructionEven when formingpolarizationIt was found that optical distortion at the inversion boundary and light propagation loss are very small, and that it has excellent characteristics as an optical functional element having a domain-inverted structure by using this for the substrate.
[0029]
That is, the present invention (1) forms a domain-inverted structure on a part of a ferroelectric single crystal substrate at a temperature equal to or lower than the Curie temperature using an electron beam scanning irradiation method or a voltage application method. A method of manufacturing an optical functional device for controlling light that has passed through, wherein the single crystal is LiNbO.3Using crystals, the LiNbO3By making the molar ratio of Li / Nb of the crystal in the range of 0.95 to 1.01, the LiNbO after forming the domain-inverted structure3To relieve optical distortion of crystalsHeat ofProcessingWithout reducing the propagation loss of the light that has passed through the domain-inverted part immediately after forming the domain-inverted structure to a desired value of 2% or less.OrAfter forming the domain-inverted structureAt temperatures below 100 ° CSaidHeat treatmentJust above the value of 2% or lessA method for manufacturing an optical functional element, wherein the optical functional element is reduced.
[0030]
In the present invention, (2) a domain-inverted structure is formed on a part of a ferroelectric single crystal substrate at a temperature equal to or lower than the Curie temperature by using an electron beam scanning irradiation method or a voltage application method. A method of manufacturing an optical functional device for controlling light that has passed through, wherein the single crystal is LiNbO.3Using crystals, the LiNbO3By making the molar ratio of Li / Nb of the crystal in the range of 0.95 to 1.01, the LiNbO after forming the domain-inverted structure3To relieve optical distortion of crystalsHeat ofProcessingThe refractive index change at the domain inversion boundary is 1 × 10 -4 Reduce to the desired value below,OrAfter forming the domain-inverted structureAt temperatures below 100 ° CSaidHeat treatmentJust 1 × 10 -4 The following valuesA method for manufacturing an optical functional element, wherein the optical functional element is reduced.
[0031]
In addition, the present invention uses (3) a voltage application method in which an electric field of 3 to 4 kV / mm is applied to a part of a ferroelectric single crystal substrate having a thickness of 0.30 mm to 3.0 mm subjected to double-side optical polishing. A method of manufacturing an optical wavelength conversion element that forms a polarization inversion structure at a temperature equal to or lower than a Curie temperature and performs wavelength conversion of a laser incident in a single crystal having a periodically inverted polarization structure using a nonlinear optical effect, LiNbO as the single crystal3Using crystals, the LiNbO3By making the molar ratio of Li / Nb of the crystal in the range of 0.95 to 1.01, the LiNbO after forming the domain-inverted structure3To relieve optical distortion of crystalsHeat ofProcessingHowever, the propagation loss of light that has passed through the domain-inverted part immediately after forming the domain-inverted structure is 2% or less, and the refractive index change at the domain-inverted boundary part is 1 × 10 -4 Whether to reduce to the following desired valueOrAfter forming the domain-inverted structureAt temperatures below 100 ° CSaidHeat treatmentAlone, the value of 2% or less, and the 1 × 10 -4 The following valuesA method of manufacturing a wavelength conversion element of a laser, wherein
[0032]
In the present invention, (4) a voltage for applying a pulse voltage of 2.5 to 5 kV / mm to a part of a ferroelectric single crystal substrate having a thickness of 0.20 mm to 2.0 mm that has been optically polished on both sides. A polarization inversion structure is formed at a temperature equal to or lower than the Curie temperature using an application method, and deflection or condensing of laser light incident on a single crystal having a polarization structure inverted to a prism or lens shape using the electro-optic effect. Is a method of manufacturing an optical functional device that controls LiNbO as the single crystal3Using crystals, the LiNbO3The LiNbO after forming the domain-inverted structure by adjusting the molar ratio of Li / Nb of the crystal to 0.95 to 1.01.3To relieve optical distortion of crystalsHeat ofNot treatedsoOr heat treatment at a temperature of 100 ° C or lessHowever, the propagation loss of light that has passed through the domain-inverted part immediately after forming the domain-inverted structure is 2% or less, and the refractive index change at the domain-inverted boundary part is 1 × 10 -4 Whether to reduce to the following desired valueOrAfter forming the domain-inverted structureAt temperatures below 100 ° CSaidHeat treatmentAlone, the value of 2% or less, and the 1 × 10 -4 The following valuesA method for manufacturing an optical functional element that controls deflection or condensing of laser light, characterized in that it is reduced.
[0033]
In addition, the present invention provides: (5) The method according to any one of (1) to (4), wherein the ferroelectric single crystal substrate is grown in a raw material continuous supply double crucible. is there.
[0034]
Further, according to the present invention, (6) the ferroelectric single crystal substrate comprises 0.1 to 4.8 mol of at least one element selected from Mg, Zn, Sc, and In grown in a raw material continuous supply double crucible. LiNbO having a molar ratio of Li / Nb contained by doping of 0.95 to 1.003The method according to (5) above, which is a crystal.
[0035]
The present inventors have found that there are problems in device performance and polarization inversion controllability in an optical functional device using a polarization inversion structure of a ferroelectric single crystal in a single crystal substrate. The present invention is based on an LN crystal single crystal substrate in a certain composition range as an optical functional device application using a polarization inversion structure of a ferroelectric single crystal. Unlike the characteristics of conventional materials, lithium niobate single crystals with a Li / Nb molar ratio in the range of 0.95 to 1.01 are polarized.Optical function using structureThe quality of device materials can be greatly improved. By using this, it became clear that the characteristics of the optical functional element are also improved dramatically.
[0036]
The polarization reversal characteristics found this time are also unique to LN single crystals having this molar fraction. The LN single crystal close to the stoichiometric composition is a crystal that has finally made it possible to produce an optically homogeneous substrate by the raw material continuous supply double crucible method, and all of its optical properties have yet to be clarified. Not.
[0037]
Especially the polarization reversal boundaries of these crystalsPartThe present inventors have clarified the optical characteristics for the first time. In addition, a significant improvement in the characteristics of the optical functional element utilizing this characteristic has been an unexplored field.
[0038]
DETAILED DESCRIPTION OF THE INVENTION
Next, a production method and physical properties of an LN single crystal used as the optical functional element of the present invention will be shown. Commercially available high purity Li2O, Nb2OFivePrepare raw material powder of Li2O: Nb2OFiveThe Li component excess raw material with a ratio of 0.54: 0.46 to 0.60: 0.40 was mixed. Li2O: Nb2OFive= 0.50: 0.50 stoichiometric composition raw materials were mixed. Next, 1ton / cm2Rubber press molding was performed at a hydrostatic pressure of 1, and each was sintered in an atmosphere of about 1050 ° C. to prepare a raw material rod. Moreover, the mixed stoichiometric composition raw material was sintered in the atmosphere at about 1150 ° C. as a raw material for continuous supply, pulverized, and classified in a size range of 50 microns to 500 microns.
[0039]
Next, when the single crystal was grown by the double crucible method, the prepared raw material rod made of the excessive Li component material was filled in the inner and outer crucibles in advance, and then the crucible was heated to prepare an excessive Li component melt. In an experiment to confirm the effect of Mg addition, a commercial high-purity MgCOThreeWere filled in the inner and outer crucibles in the range of 0.1 to 4.8 mol% in advance.
[0040]
Next, an LN single crystal close to a stoichiometric composition was grown using a raw material continuous supply type double crucible method. A seed crystal was soaked in the melt of the Li component excess composition in the double crucible to obtain a single crystal that was close to the stoichiometric composition, that is, suppressed the nonstoichiometric defect concentration as much as possible. In order to precisely control the density and structure of non-stoichiometric defects, an amount of Li commensurate with the amount of crystallized growth2O / (Nb2OFive+ Li2Crystals were grown while automatically supplying a raw material having a stoichiometric composition ratio of O) of 0.50 to the outer crucible.
[0041]
Here, the crucible used for the growth was made of platinum, the outer crucible had a diameter of 125 mm and a height of 70 mm, and the inner crucible had a diameter of 85 mm and a height of 90 mm. Also in this case, in order to make the melt composition uniform, the crucible was rotated in the direction opposite to the seed crystal at a speed of 3 rpm during the growth. The growth conditions were such that the crystal rotation speed was 15 rpm, the pulling speed was constant at 0.5 mm / h, and the growth atmosphere was in the air. By growing for about 1 week, a colorless and transparent LN crystal having a diameter of about 49 to 52 mm and a length of about 65 to 75 mm and having no cracks was obtained.
[0042]
With respect to all the crystals obtained, samples were cut out from the upper, center, and lower portions of the crystal, and the Li / Nb molar ratio was determined by chemical analysis. In chemical analysis, the composition was analyzed very carefully in order to obtain the absolute value of the composition ratio with high accuracy. The analysis was performed as an average value of the results of evaluating the same sample using several different analyzers. As a result, in the case of the LN single crystal, the Li / Nb molar ratio was 0.99 to 1.01 at the composition closest to the constant ratio.
[0043]
On the other hand, in the crystals with Mg added, Mg replaces Li and Nb sites, so the Li / Nb molar ratio changes as the amount of Mg added increases, and the Li / Nb molar ratio of the obtained crystal is 0.95 It was in a large range less than 1.0. When Zn, Sc, or In is added in addition to Mg, the segregation coefficient in the crystal differs depending on the type of element, so the content in the crystal differs depending on the added amount. As the Nb sites were replaced, the Li / Nb molar ratio changed as the amount of additive elements increased, and the Li / Nb molar ratio of the obtained crystals was in the range of more than 0.95 and less than 1.0.
[0044]
On the other hand, in the composition evaluation by measuring the Curie temperature, it was confirmed that the Curie temperature of the standard sintered sample of the stoichiometric composition prepared in advance to the stoichiometric composition and sintered at 1150 ° C was 1200 ° C. The Curie temperatures of LN single crystals grown in a continuous supply double crucible were compared. The results of evaluation of the Li / Nb molar ratio by Curie temperature measurement were almost consistent with the results of chemical analysis, and the homogeneity of the crystal composition within one crystal was confirmed to be very good.
[0045]
Next, a block sample having a size of 35 mm × 35 mm × 40 mm was cut out from the LN single crystal in a single domain state, and surface polishing was performed by mechanochemical polishing. When the optical homogeneity of the sample was evaluated by Mach-Zehnder interferometry, no macro defects or optically nonuniform portions were found, and the refractive index change in the sample was 1 × 10-FiveThe following was obtained and it was confirmed that the optical homogeneity was excellent.
[0046]
Conventionally commercially available LN single crystal substrates of congruent composition have excessive amounts of Nb components due to limitations of single crystal growth technology. Since the Li / Nb molar ratio is 0.94, it contains a large amount of non-stoichiometric defects as much as several percent. On the other hand, the present inventors can grow crystals from a Li component excess melt by the raw material continuous supply double crucible method, and grow a LN single crystal having a Li / Nb molar ratio closer to a stoichiometric composition of 0.95 to 1.01. This is the first elucidation that a single crystal with reduced non-stoichiometric defect concentration due to excessive Nb component exhibits excellent characteristics as an optical functional device substrate.
[0047]
That is, it has been found that a large amount of non-stoichiometric defects formed by excess Nb in the conventional crystal causes a large problem for optical functional device applications utilizing the domain-inverted structure. Due to the presence of this defect, the hysteresis curve indicating the relationship between the applied voltage necessary for polarization reversal and spontaneous polarization becomes asymmetric, and a high electric field of several tens of kV / mm is required for polarization reversal, and the polarization reversal is performed. WhenpolarizationThere is a large optical distortion at the inversion boundary.light'sIt was found that propagation loss was introduced. Furthermore, since there are many non-stoichiometric defects and the defects are unevenly distributed inside the crystal and the defect concentration is high, the polarization inversion tends to be pinned.OpticalIt became clear that strain accumulated and caused the destruction of the crystal.
[0048]
Figure 1 shows a 0.5mm thick substrate that is optically polished on both sides using LN single crystal.1, 4Periodic polarization inversion by applying voltage on topPart 2, 5After forming the polarization inversion part2, 5Shows an example of a detailed observation with a polarizing microscope. The period of polarization inversion was about 3 to 4 μm, and it was designed to be quasi-phase matched to the fundamental wave in the wavelength range of 850 nm.
[0049]
FIG. 1 (a) shows a state in which a conventional congruent composition LN single crystal is used as a substrate. FIG. 1 (b) shows a state where an LN single crystal having a composition close to the above-described stoichiometric composition is used for the substrate. The difference between the two is obvious, and in Fig. 1 (a), a very large optical distortion is observed in the domain-inverted part.3On the other hand, in Fig. 1 (b), light is transmitted uniformly under a polarizing microscope.OpticalNo distortion was observed.
[0050]
Furthermore, polarization inversion of various shapes and sizes on the crystal substrateConstructionFormed at the boundary of the domain inversion due to the difference in substrateOpticalObserve strain and use laser interferometry to reverse polarizationPartWhen evaluating the refractive index, the conventional crystal is 8 × 10-3~ 3 × 10-FourA very large refractive index change was observed. On the other hand, in the optical functional element using the crystal substrate according to the present invention, the optical distortion is not caused even without performing the heat treatment.With refractive index change1 × 10-FourIt was confirmed that the following was obtained.
[0051]
Furthermore, Fig. 2 shows the end faces of LN single crystals with various compositions of length 5mm and thickness 0.5mm, in which a periodic domain-inverted structure was formed by electron beam irradiation and voltage application at temperatures from room temperature to around 200 ° C. The light that is mirror-polished and propagates through the domain-inverted part is scattered inside the crystal.OpticalCaused by distortionlight'sIt is the graph which showed the result of having evaluated the size of propagation loss.
[0052]
Polarization reversal even for samples with the same compositionStructuralDepending on applied voltage, electrode shape, electrode material, temperature, etc.light'sThere was variation in the propagation loss. For congruent composition crystals with a Li / Nb molar ratio of 0.94,light'sPropagation loss was found to be very large at 4-8%. On the other hand, there are many crystals such as an additive-free LN single crystal with a Li / Nb molar ratio of 0.98 to 1.01 and a LN single crystal with a Li / Nb molar ratio of 0.95 containing about 3 mol% of MgO.light'sPropagation loss is 2% or less, including 0.1% or lesslight'sPropagation losssmallHigh quality polarization reversalCan form structureCrystals were also obtained.
[0053]
Furthermore, Figure 3 shows the propagation of light passing through a domain-inverted part of a 5 mm long and 0.5 mm-thick crystal in which a periodic domain-inverted structure is formed by electron beam irradiation and voltage application at temperatures near room temperature It is the graph which showed how much loss can be reduced by heat processing.
[0054]
Polarization reversal with conventional congruent composition crystalsFormation of structureAfter that, scattering and opticsTargetDue to distortion and other effectsLight propagationLoss is very large, and congruent LN crystals must be heated to a fairly high temperature.Light propagationThe result that the loss did not decrease was obtained. In contrast, when an LN single crystal close to the stoichiometric composition of the present invention is used for the substrate,At high temperatureWithout heat treatmentLight propagationIt is clear that the loss is small and the performance improvement of the optical functional element can be expected.When heat treatment is performed, as shown in FIG. 3, the effect of further reducing the light propagation loss is recognized. 100 It is sufficient to perform the heat treatment at a temperature of ℃ or less.
[0055]
In the LN single crystal, Li and Nb ions are placed in the electrically neutral position in the paraelectric phase higher than the Curie temperature, but in the ferroelectric phase below the Curie temperature, Li and Nb ions are + z or -z. Slightly out of direction. The positive and negative polarization directions of the domain are determined by the direction of this ion shift. In an optical functional element having a domain-inverted structure, it is necessary to forcibly move these ions at a low temperature by applying a high electric field.
[0056]
When there are many non-stoichiometric defects having a coincidence melting composition, it is not easy to move excess Nb entering the Li site, so a large applied voltage is required for polarization reversal. In addition, since the high voltage is applied to forcibly invert the polarization,Polarization inversionIt is considered that a large optical distortion is introduced at the boundary portion. At present, the optical distortion andlight'sThe cause of the reduction of the propagation loss cannot be sufficiently explained only by the inversion voltage of the crystal and the magnitude of the internal electric field.
[0057]
However, the non-stoichiometric defects are reduced by an order of magnitude or more compared to the conventional congruent composition crystal containing a large amount of non-stoichiometric defects.ConstantIt is clear that an LN single crystal close to the specific composition is excellent as a substrate for a polarization inversion element. From this, polarization inversion is achieved by using an LN single crystal with a composition close to the stoichiometric composition as a ferroelectric single crystal substrate.ConstructionEven when formingpolarizationThe refractive index change at the polarization inversion boundary is 1 × 10 without showing optical distortion at the inversion boundary and without a heating step to remove the optical distortion at the polarization inversion boundary.-FourBecause the following is obtained,ConversionExcellent control and no laser light scatteringlight'sIt has small propagation loss and excellent characteristics as an optical functional element.
[0058]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples.
Example 1
The characteristics when an LN single crystal is applied to an optical wavelength conversion element will be described. Figure 4 shows the substrate of a single crystal close to the stoichiometric composition (undoped LN single crystal with a Li / Nb molar ratio of 0.98 to 1.01).6Used for the substrate6It is a schematic block diagram of the QPM device which formed the periodic polarization inversion structure on it. A comb-shaped electrode and a parallel electrode were patterned on the + z plane of the substrate 6 having a thickness of 0.30 mm to 3.0 mm which was optically polished on both sides. The period was about 3.2 μm, and it was designed to be quasi-phase matched to the fundamental wave with a wavelength of about 850 nm. Crystal substrate with the above composition6On the -z plane, electrodes were deposited on the entire surface. Apply an electric field about 1/5 lower than the conventional congruent crystal of about 3 to 4 kV / mm between the comb electrode and the parallel electrode, and the back electrode of the comb electrode and the −z plane, respectively, and cycle without dielectric breakdown TargetWith polarization reversal width of 8Polarization inversion region7Formed.
[0059]
In this example, the domain-inverted portion of the periodic domain-inverted structure was observed in detail with a polarization microscope, but no optical distortion was observed. In the polarization inversion partIrradiated from the wavelength tunable laser 9 through the lens 10Although the laser beam was allowed to pass through, no scattering of the laser beam was observed. Therefore, no heat treatment was required, and highly efficient wavelength conversion was obtained. The LN crystal used has a very uniform polarization state in advance. Periodic polarization reversal in crystalsConstructionEven when forming the LN single crystal close to the stoichiometric composition, since the crystal uniformity is excellent, a uniform domain-inverted structure can be formed.
[0060]
Thus, the problems seen when using conventional congruent composition LN crystals as a substrate have been solved. Further, after forming the domain-inverted structure, the crystal was removed, and the y-plane of the crystal in the cross section was polished and etched with a mixed solution of hydrofluoric acid and nitric acid to examine the state of polarization inversion. Periodic polarization reversal width ratio The shape of the polarization is optimized by optimizing the pulse width and current of the applied voltage, and the polarization reversal width ratio of the periodic polarization can be accurately created to the ideal ratio over the entire sample. Was confirmed.
[0061]
The periodic domain-inverted structure can be formed with high accuracy in the same manner even for a sample having a thickness of 1 mm or more. Moreover, no optical distortion was observed, and the propagation loss was very small at 0.2% or less. For these thick samples, in particular, polarization inversionFormation of structureThe fact that no subsequent heat treatment was required was a great merit. This is because a sample having a thickness of 1 mm or more has a partial crystal macro defect, electrode non-uniformity, and thermal non-uniformity. This is because there is a problem that the crystal moves easily or the crystal is broken by the pyroelectric effect. For this reason, the optical functional element with excellent optical uniformity and polarization reversal controllability created in this example is particularlylight'sIt is considered to be optimal as an internal resonator type wavelength conversion element that requires a small propagation loss.
[0062]
The characteristics of the QPM-SHG device were evaluated using a tunable high-power Ti sapphire laser (wavelength 850 nm) as the fundamental wave, and high-efficiency optical wavelength conversion was confirmed. This is shown in FIG. When a conventional congruent composition LN crystal is used for the substrate, almost efficient wavelength conversion cannot be obtained before the heat treatment. By heat treatmentwavelengthAlthough the conversion efficiency was seen to be improved, the optical device of the present invention is higherwavelengthConversion efficiency is obtained.
[0063]
The reason islight'sA major reason is that the propagation loss is small. Furthermore, in addition to the large non-linear optical constant of the substrate, the fact that there is no optical distortion and no heat treatment is required, and the higher integrity of the domain-inverted structure contributes to obtaining a high-performance optical functional device. it is conceivable that.
[0064]
In addition, here, the example in which the QPM-SHG element that generates blue light with respect to the fundamental wave of near-infrared light near 850 nm has been created has been described in detail. The present invention is not limited to two wavelengths, and can be applied to a wavelength region where the LN single crystal is transparent and phase matching is possible.
[0065]
Furthermore, the optical functional element of the present invention that periodically inverts the polarization structure of the LN single crystal and shortens or lengthens the wavelength of an incident laser having a wavelength in the visible to near-infrared region generates second harmonics. It is possible to realize not only an element but also a high performance optical element using a polarization inversion structure such as an optical parametric oscillation element, a difference frequency and a sum frequency generation element, and an optical switch and an optical modulator. The application can also be applied to application fields such as remote sensing and gas detection, and to optical communication fields such as wavelength mixers and pulse shaping elements.
[0066]
Here, as an example of forming a polarization inversion structure on a part of a ferroelectric single crystal substrate at a temperature equal to or lower than the Curie temperature, an LN single crystal optical functional element using a voltage application method has been described. As a method for forming the domain-inverted structure at the temperature, the same effect can be obtained even by the electron beam scanning irradiation method.
[0067]
Example 2
Using a near-stoichiometric composition (non-added LN single crystal with a Li / Nb molar ratio of 0.98 to 1.01) as a substrate, a lens or prism-like domain-inverted structure is fabricated to use a deflecting element that utilizes the electro-optic effect, or a cylindrical Optical elements such as lenses, beam scanners, and switches were manufactured.
[0068]
6 and 7 show the lens.14And prism19FIG. 2 is a schematic configuration diagram of an optical functional element that performs a focusing and scanning for controlling a laser beam incident on a single crystal using a created electro-optic effect by creating a domain-inverted structure. The above-mentioned LN single crystal with a diameter of 1.5 inches, a thickness of 0.2 to 2.0 mm and double-side polished z-cut11, 16Prepare an Al electrode with a thickness of about 200 nm on both z planes by sputtering, and use a lithograph to create a lens.14And prism19A pattern was formed. After that, pulse shape on + z planeAppliedVoltage15, 20Was applied at about 2.5 to 5 KV / mm to reverse the polarization.
[0069]
In this example, the polarization inversion portion was observed in detail with a polarizing microscope, but no optical distortion was observed. Also,With semiconductor lasers 12 and 17Polarization inversionRegions 13 and 18Although no laser light was observed at all, no heat treatment was required, and an optical functional element was obtained. The LN crystal used has a very uniform polarization state in advance. Further, the end face of the crystal was mirror-polished to form a laser light incident / exit surface.
[0070]
Reversal of refractive index by a prototyped domain-inverted structureConstructionThe performance of an optical element using the electro-optic effect of a LN single crystal formed by the design depends on the design of the lens or prism-like domain-inverted structure, the accuracy of the fabrication process of the domain-inverted structure, and the size of the electro-optic constant of the material It has been determined. The polarization reversal structure of the lens or prismatic pattern of this example is notable for the polarization reaction.TransitionWorldPartInlight'sThis is because there is no propagation loss and optical distortion, and control of the polarization inversion property is very easy, so that good device characteristics are obtained and the drive efficiency of the optical functional device is improved.
[0071]
In conventional congruent composition LN crystals, if the inversion period is shortened and the inversion structure is complicated, it is difficult to produce a lens or prism-like domain inversion structure with high accuracy, and heat treatment is required. On the other hand, by using an LN single crystal close to the stoichiometric composition as an optical functional device application using a polarization inversion structure, high-precision polarization inversion of the optical functional deviceConstructionCan be formed.
[0072]
Furthermore, this crystal has a larger electro-optic constant r33Therefore, better device performance was obtained with a smaller operating voltage. For example, in the case of a deflection element, a large deflection angle of about 6 ° C. was obtained with an electric field of about 600 V / mm. In addition, a lens that operates around 100V / mm and a switching operation at about 500V / mm were obtained.
[0073]
Here, as an example of forming a polarization inversion structure on a part of a ferroelectric single crystal substrate at a temperature equal to or lower than the Curie temperature, an LN single crystal optical functional element using a voltage application method has been described. As a method for forming the domain-inverted structure at the temperature, the same effect can be obtained even by the electron beam scanning irradiation method.
[0074]
【The invention's effect】
As described in detail above, according to the present invention, a domain-inverted structure is formed on a part of a ferroelectric single crystal substrate by using an electron beam scanning irradiation method or a voltage application method at a temperature equal to or lower than the Curie temperature. By using an LN single crystal having a composition close to the stoichiometric composition in which the molar ratio of Li / Nb is in the range of 0.95 to 1.01 as the ferroelectric single crystal in the optical functional element that controls the light that has passed through the polarization inversion portion Less than 2%Of the desired value of lightPropagation loss is obtained, and the refractive index change at the domain inversion boundary is 1 × 10 without a heating step to remove the optical distortion at the domain inversion boundary due to the direction reversal of the spontaneous polarization.-FourSince the following can be obtained, an element having no distortion at the domain boundary and having excellent optical homogeneity and polarization reversal controllability can be realized, so that significant improvement in optical functional element characteristics can be expected.
[0075]
Thus, the present invention brings about a great effect of promoting the practical application of optical functional elements in fields such as optical information processing using laser light, optical processing technology, optical communication technology, and optical measurement control.
[Brief description of the drawings]
[Fig.1] Periodic polarization inversion on LN single crystal substrateStructureFIG. 4 is an external view of the + z plane observed through transmission polarization after formation, where (a) shows a conventional congruent composition LN crystal substrate and (b) shows an LN crystal substrate close to a stoichiometric composition.
[Fig. 2] Crystal composition and the inside of the crystal propagated through the domain-inverted partLight propagationA graph showing the relationship of loss.
[Fig. 3] Heat treatment temperature and the inside of the crystal that passed through the domain-inverted partlight'sA graph showing the relationship of propagation loss.
FIG. 4 is a conceptual diagram illustrating an optical wavelength conversion element according to an embodiment of the present invention.
FIG. 5 is a graph showing the relationship between basic input light and SHG light output.
FIG. 6 is a conceptual diagram showing a light collecting element according to an embodiment of the present invention.
FIG. 7 is a conceptual diagram illustrating a deflection element according to an embodiment of the present invention.
[Explanation of symbols]
1. Congruent composition LN single crystal substrate + z plane
2. Periodic polarization inversion
3.OpticsTargetdistortion
4. LN single crystal substrate close to stoichiometric composition + z plane
5. Periodic polarization reversal part
6. LN single crystal substrate close to stoichiometric composition
7. Polarization inversion region
8. Periodic polarization inversion width
9. Tunable laser
10. Lens
11. LN single crystal substrate close to stoichiometric composition
12. Semiconductor laser
13. Polarization inversion region
14. Lens
15. Applied voltage
16. LN single crystal substrate close to stoichiometric composition
17. Semiconductor laser
18. Polarization inversion region
19. Prism
20. Applied voltage

Claims (6)

強誘電体単結晶基板の一部に、電子ビーム走査照射法または電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、この分極反転部を通過した光を制御する光機能素子の製造方法であって、
該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることにより、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、前記分極反転構造を形成直後の該分極反転部を通過させた該光の伝搬ロスを2%以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記2%以下の値をさらに低減させることを特徴とする光機能素子の製造方法。
An optical functional element that forms a domain-inverted structure at a temperature equal to or lower than the Curie temperature by using an electron beam scanning irradiation method or a voltage application method on a part of a ferroelectric single crystal substrate, and controls light passing through the domain-inverted part A manufacturing method of
Using LiNbO 3 crystal as a single crystal, of the LiNbO 3 crystal by a range of Li / Nb molar ratio from 0.95 to 1.01,該分pole reversal structure after the formation of the LiNbO 3 crystal without the facilities to heat treatment to alleviate optical distortion, reduce or to a desired value of the propagation loss of the optical obtained by passing the該分pole reversal unit immediately after forming the polarization inversion structure 2% or less, or, only subjected to the heat treatment at 100 ° C. below the temperature after forming the該分pole reversals, manufacturing method of an optical functional element, characterized by further reducing the 2% value.
強誘電体単結晶基板の一部に、電子ビーム走査照射法または電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、この分極反転部を通過した光を制御する光機能素子の製造方法であって、
該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることにより、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、分極反転境界部の屈折率変化が1×10 −4 以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記1×10 −4 以下の値をさらに低減させることを特徴とする光機能素子の製造方法。
An optical functional element that forms a domain-inverted structure at a temperature equal to or lower than the Curie temperature by using an electron beam scanning irradiation method or a voltage application method on a part of a ferroelectric single crystal substrate, and controls light passing through the domain-inverted part A manufacturing method of
Using LiNbO 3 crystal as a single crystal, of the LiNbO 3 crystal by a range of Li / Nb molar ratio from 0.95 to 1.01,該分pole reversal structure after the formation of the LiNbO 3 crystal without the facilities to heat treatment to alleviate optical distortion, or the refractive index change of the polarization-inverted boundaries is reduced to a desired value of 1 × 10 -4 or less, or, after forming the該分pole reversals only subjected to the heat treatment at 100 ° C. below the temperature method for manufacturing an optical functional element, characterized in that to further reduce the 1 × 10 -4 or less of the value.
両面光学研磨された厚み0.30mm〜3.0mmの強誘電体単結晶基板の一部に、3〜4kV/mmの電界を印加する電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、非線形光学効果を利用して周期的反転分極構造を持つ単結晶内に入射したレーザの波長変換を行う光波長変換素子の製造方法であって、
該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることにより、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、該分極反転構造を形成直後の分極反転部を通過させた光の伝搬ロスが2%以下、かつ、分極反転境界部の屈折率変化が1×10 −4 以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記2%以下の値、かつ、前記1×10 −4 以下の値をさらに低減させることを特徴とするレーザの波長変換素子の製造方法。
A domain-inverted structure at a temperature equal to or lower than the Curie temperature using a voltage application method in which an electric field of 3 to 4 kV / mm is applied to a part of a ferroelectric single crystal substrate having a thickness of 0.30 mm to 3.0 mm that is optically polished on both sides. A method of manufacturing an optical wavelength conversion element that converts the wavelength of a laser incident in a single crystal having a periodically inverted polarization structure using a nonlinear optical effect,
Using LiNbO 3 crystal as a single crystal, of the LiNbO 3 crystal by a range of Li / Nb molar ratio from 0.95 to 1.01,該分pole reversal structure after the formation of the LiNbO 3 crystal without the facilities to heat treatment to alleviate optical distortion, the propagation loss of the light passing through the polarization inversion immediately after forming a該分pole inversion structure 2% or less and the refractive index change of the polarization-inverted boundaries There reduce or to a desired value of 1 × 10 -4 or less, or, only subjected to the heat treatment at 100 ° C. below the temperature after forming the該分pole reversal structure, the 2% value and the 1 method for manufacturing a wavelength conversion element of the laser, characterized in that to reduce × 10 -4 or less of the value further.
両面光学研磨された厚み0.20mm〜2.0mmの強誘電体単結晶基板の一部に、2.5〜5kV/mmのパルス状の電圧を印加する電圧印加法を用いてキュリー温度以下の温度で分極反転構造を形成し、電気光学効果を利用してプリズムまたはレンズ形状に反転した分極構造を持つ単結晶内に入射されたレーザ光の偏向または集光を制御する光機能素子の製造方法であって、
該単結晶としてLiNbO結晶を用い、該LiNbO結晶のLi/Nbのモル比を0.95〜1.01の範囲とすることによって、該分極反転構造を形成した後の該LiNbO結晶の光学的歪みを緩和するための熱処理を施さないで、該分極反転構造を形成直後の分極反転部を通過させた光の伝搬ロスが2%以下、かつ、分極反転境界部の屈折率変化が1×10 −4 以下の所望の値まで低減させるか、または、該分極反転構造を形成した後に100℃以下の温度で前記熱処理を施すだけで、前記2%以下の値、かつ、前記1×10 −4 以下の値をさらに低減させることを特徴とするレーザ光の偏向または集光を制御する光機能素子の製造方法。
Using a voltage application method in which a pulsed voltage of 2.5 to 5 kV / mm is applied to a part of a ferroelectric single crystal substrate having a thickness of 0.20 mm to 2.0 mm that has been subjected to double-sided optical polishing, the Curie temperature or lower. Method of manufacturing an optical functional element that controls polarization or condensing of laser light incident on a single crystal having a polarization structure in which a polarization inversion structure is formed by temperature and is inverted into a prism or lens shape by using an electro-optic effect Because
Using LiNbO 3 crystal as a single crystal, of the LiNbO 3 crystal by a range of Li / Nb molar ratio from 0.95 to 1.01,該分pole reversal structure after the formation of the LiNbO 3 crystal without the facilities to heat treatment to alleviate optical distortion, the propagation loss of the light passing through the polarization inversion immediately after forming a該分pole inversion structure 2% or less and the refractive index change of the polarization-inverted boundaries There reduce or to a desired value of 1 × 10 -4 or less, or, only subjected to the heat treatment at 100 ° C. below the temperature after forming the該分pole reversal structure, the 2% value and the 1 A method for producing an optical functional element for controlling the deflection or condensing of laser light, wherein the value of × 10 −4 or less is further reduced.
前記強誘電体単結晶基板は、原料連続供給二重るつぼで育成されることを特徴とする請求項1〜4のいずれかに記載の方法。The method according to claim 1, wherein the ferroelectric single crystal substrate is grown in a raw material continuous supply double crucible. 前記強誘電体単結晶基板は、原料連続供給二重るつぼで育成したMg、Zn、Sc、Inから選ばれる少なくとも一つの元素を0.1〜4.8モル%ドーピングして含有するLi/Nbのモル比が0.95〜1.00の範囲のLiNbO結晶であることを特徴とする請求項5に記載の方法。The ferroelectric single crystal substrate contains Li / Nb doped with 0.1 to 4.8 mol% of at least one element selected from Mg, Zn, Sc and In grown in a raw material continuous supply double crucible. The method according to claim 5, wherein the molar ratio of LiNbO 3 is in the range of 0.95 to 1.00.
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