JP2002196381A - Optical wavelength converting element and method for manufacturing the same - Google Patents
Optical wavelength converting element and method for manufacturing the sameInfo
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- JP2002196381A JP2002196381A JP2000391214A JP2000391214A JP2002196381A JP 2002196381 A JP2002196381 A JP 2002196381A JP 2000391214 A JP2000391214 A JP 2000391214A JP 2000391214 A JP2000391214 A JP 2000391214A JP 2002196381 A JP2002196381 A JP 2002196381A
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- crystal
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- wavelength conversion
- conversion element
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Abstract
Description
【0001】[0001]
【発明の属する技術分野】本発明は、非線形光学効果を
利用した光波長変換素子およびその製造方法に関する。[0001] 1. Field of the Invention [0002] The present invention relates to an optical wavelength conversion element utilizing a nonlinear optical effect and a method of manufacturing the same.
【0002】[0002]
【従来の技術】ニオブ酸リチウム(LiNbO3)単結
晶(以下、LNとする)の温度−組成比の相関図(相
図)は古くから知られており、従来、組成の均質性の高
いLNを製造するためには、結晶と融液が同じ組成で平
衡共存する一致溶融(コングルエント)組成であるモル
分率が48.5%(Li/Nbのモル分率は94%)の
融液から回転引き上げ法で成長されていた。成長された
アズグロウンLN単結晶は多分域状態となっているた
め、成長後の結晶をキュリー温度である1150℃以上
に加熱した状態で結晶のZ軸方向に電圧を印加し、単一
分域化した後、結晶を冷却するポーリング処理を施され
ていた。単一分域化処理された結晶は所定の大きさに加
工された後、各種用途に使用されていた。LNは高い光
学定数を有し、かつ大型結晶の成長が容易なため各種の
光学素子に応用されている。2. Description of the Related Art A correlation diagram (phase diagram) of a temperature-composition ratio of a lithium niobate (LiNbO 3 ) single crystal (hereinafter referred to as LN) has been known for a long time. Is produced from a melt having a mole fraction of 48.5% (a mole fraction of Li / Nb of 94%), which is a congruent melting composition in which the crystal and the melt have the same composition and equilibrium coexist. It was grown by the rotation pulling method. Since the grown as-grown LN single crystal is in a multi-domain state, a voltage is applied in the Z-axis direction of the crystal while the grown crystal is heated to a temperature higher than the Curie temperature of 1150 ° C. to form a single domain. After that, a poling treatment for cooling the crystal was performed. Crystals that have been subjected to single domain processing have been used in various applications after being processed to a predetermined size. LN has a high optical constant and is easy to grow a large crystal, so that it is applied to various optical elements.
【0003】LN結晶の用途の一つとして、LN結晶の
有する高い非線形光学効果を利用した光波長変換素子が
ある。結晶内に周期状の分極反転構造を形成することで
位相整合条件の成立が可能となり任意の波長を高効率で
波長変換することが可能となる。コングルエントのLN
結晶においては周期状の分極反転構造を形成する方法が
種々提案されている。例えば、Ti拡散を利用した方法
がある。コングルエント組成のZ板LN基板表面にスト
ライプ状のTi金属を周期状に形成し、これを基板のキ
ュリー温度近くで熱処理することで周期状の分極反転構
造を形成する。形成された分極反転構造は結晶表面を底
面とする逆三角形形状であり、この分極反転構造を利用
して光波長変換素子が製造されている。[0003] As one of uses of the LN crystal, there is an optical wavelength conversion element utilizing a high nonlinear optical effect of the LN crystal. By forming a periodic domain-inverted structure in the crystal, it is possible to satisfy the phase matching condition, and it is possible to perform wavelength conversion of an arbitrary wavelength with high efficiency. Congruent LN
Various methods for forming a periodically poled structure in a crystal have been proposed. For example, there is a method using Ti diffusion. A stripe-shaped Ti metal is periodically formed on the surface of a Z-plate LN substrate having a congruent composition, and this is heat-treated near the Curie temperature of the substrate to form a periodically poled structure. The formed domain-inverted structure has an inverted triangular shape with the crystal surface as a bottom surface, and an optical wavelength conversion element is manufactured using this domain-inverted structure.
【0004】また、SiO2の堆積と熱処理による方法
もある。この方法はコングルエント組成のZ板LN基板
の表面にストライプ状のSiO2膜を周期状に形成し、
これを熱処理する方法である。基板のキュリー温度近く
まで熱処理することで、Ti拡散による方法と同様の逆
三角形の分極反転構造が形成され、これを利用して光波
長変換素子が製造されている。There is also a method of depositing SiO 2 and heat treatment. In this method, a stripe-shaped SiO 2 film is periodically formed on the surface of a Z-plate LN substrate having a congruent composition,
This is a method of heat treatment. By performing heat treatment near the Curie temperature of the substrate, an inverted triangular domain-inverted structure similar to the method using Ti diffusion is formed, and an optical wavelength conversion element is manufactured using this structure.
【0005】また、従来の他の光波長変換素子の製造方
法として電界印加による分極反転形成を利用した方法が
ある。コングルエント組成のオフカット板LN結晶の表
面に櫛形電極を形成し、電界を印加することで基板のZ
軸方向に向かって分極反転構造を形成する方法で、結晶
内を斜め方向に針状の分極反転構造が周期的に形成でき
る。分極反転は半円に近い逆三角形で基板表面を底面と
して形成される。この方法を利用して光波長変換素子が
製造されている。周期的な分極反転構造による非線形グ
レーティングを利用し、導波路内で基本波と高調波との
位相整合をとることで、高効率の波長変換が行われてい
る。As another conventional method for manufacturing an optical wavelength conversion element, there is a method utilizing polarization inversion by applying an electric field. A comb-shaped electrode is formed on the surface of an off-cut plate LN crystal having a congruent composition, and an electric field is applied to the substrate so that the substrate has a Z-shape.
By forming the domain-inverted structure in the axial direction, a needle-shaped domain-inverted structure can be periodically formed in the crystal in an oblique direction. The polarization inversion is an inverted triangle close to a semicircle and is formed with the substrate surface as the bottom surface. An optical wavelength conversion element is manufactured using this method. A highly efficient wavelength conversion is performed by utilizing a nonlinear grating having a periodically poled structure and performing phase matching between a fundamental wave and a harmonic in a waveguide.
【0006】従来LN単結晶は、従来組成の均質性の高
いLNを製造するためには、結晶と融液が同じ素子で平
衡共存する一致溶融(コングルエント)組成であるLi
2O/(Nb2O5+Li2O)のモル分率が48.5%の
融液から回転引き上げ法で成長されていた。形成される
結晶のモル分率は溶液の組成と等しく48.5%(Li
/Nbのモル分率は94%)であり、キュリー温度は約
1150℃であった。これに対し、Li2O/(Nb2O
5+Li2O)のモル分率が49.5〜50.2%と化学
量論比に近いストイキオメトリックLN結晶の成長が最
近可能になった(特開平10−45498号公報)。作
製方法は、2重坩堝法によるもので、結晶引き上げの際
に、ニオブ酸リチウム溶液の組成をリチウム成分の過剰
なLi 2O/(Nb2O5+Li2O)のモル分率を56〜
60%の特定範囲に保った溶液組成とし、自動的に原料
を供給する手段を備えた2重坩堝法を用いる。ストイキ
オメトリックLNはアズグロウンで単分域化されている
ため成長後のポーリング処理が不要で結晶成長と光学的
均質性の良いことを特徴とする。さらに、コングルエン
ト組成の結晶に対し、キュリー温度が1185〜121
5℃高い特徴を有する。The conventional LN single crystal has a high homogeneity of the conventional composition.
In order to produce a large LN, the crystal and the melt are
Li is a congruent composition that coexists
TwoO / (NbTwoOFive+ LiTwoO) having a molar fraction of 48.5%
It was grown from the melt by the spin-up method. It is formed
The molar fraction of crystals is equal to the composition of the solution, 48.5% (Li
/ Nb mole fraction is 94%) and the Curie temperature is about
1150 ° C. On the other hand, LiTwoO / (NbTwoO
Five+ LiTwoO) with a molar fraction of 49.5 to 50.2%
Growth of stoichiometric LN crystals close to the stoichiometric ratio
(Japanese Patent Application Laid-Open No. 10-45498). Work
The production method is based on the double crucible method, when pulling crystals.
When the composition of the lithium niobate solution is
Li TwoO / (NbTwoOFive+ LiTwoO) molar fraction of 56 to
The solution composition is maintained within a specific range of 60%.
Is used. Steak
Ometric LN is as-grown single domain
No need for poling after growth, crystal growth and optical
It is characterized by good homogeneity. In addition, congruen
Curie temperature of 1185 to 121
Features 5 ° C higher.
【0007】ストイキオメトリック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。特に結晶のモル分率が49.5〜50.2%(Li
/Nbのモル分率は95〜101%)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。[0007] Stoichiometric LN is a slight change in mole fraction relative to the congruent composition LN.
As they approach the stoichiometric ratio, their crystal properties differ significantly. In particular, when the molar fraction of the crystal is 49.5 to 50.2% (Li
/ Nb molar ratio is in the range of 95 to 101%), and has optical characteristics greatly different from those of the conventional congruent composition crystal.
【0008】[0008]
【発明が解決しようとする課題】コングルエント組成に
おける周期状の分極反転構造の形成およびそれを利用し
た光波長変換素子については、種々の分極反転製造方法
および光波長変換素子の構成が報告されている。しかし
ながら、結晶中のLi,Nb比を制御したストイキオメ
トリック組成の結晶においては、周期状の分極反転構造
の形成およびその特性が明らかにされていないという問
題がある。With respect to the formation of a periodic domain-inverted structure in a congruent composition and an optical wavelength conversion device using the same, various methods of manufacturing a domain-inverted device and the configuration of the optical wavelength conversion device have been reported. . However, in a crystal having a stoichiometric composition in which the Li / Nb ratio in the crystal is controlled, there is a problem that the formation of a periodic domain-inverted structure and its characteristics have not been clarified.
【0009】また、ストイキオメトリック組成のLN結
晶は、未だ分極反転特性が明らかにされておらず、分極
反転特性が従来のコングルエント組成とは大きく異なる
ため従来の分極反転形成方法では、分極反転構造が形成
し難いという問題がある。In addition, the polarization inversion characteristics of the LN crystal having a stoichiometric composition have not been clarified yet, and the polarization inversion characteristics are significantly different from those of the conventional congruent composition. Is difficult to form.
【0010】コングルエント組成のLNを利用した波長
変換素子は高い変換効率が達成されており、高効率で青
色、緑色の波長領域の変換光が確認されている。しかし
ながら、コングルエント組成のLN結晶を用いた光波長
変換素子は、光損傷の発生による出力不安定化が大きな
問題となっている。光波長変換素子の出力安定化には、
耐光損傷強度の向上が望まれている。LNの耐光損傷強
度(光損傷の発生しない最大光強度)の値は、導波路形
状に依存するが波長400nm帯の光に対して1mW以
下、MgドープのLN基板に形成したプロトン交換導波
路でも数10mW程度であり、高出力の光波長変換素子
を実現するには、耐光損傷強度の向上が求められてい
る。A wavelength conversion element using LN having a congruent composition has achieved high conversion efficiency, and it has been confirmed that converted light in the blue and green wavelength regions with high efficiency is obtained. However, in a light wavelength conversion element using an LN crystal having a congruent composition, output instability due to occurrence of optical damage is a serious problem. To stabilize the output of the optical wavelength conversion element,
It is desired to improve the light damage resistance. The value of the light damage resistance (maximum light intensity at which no light damage occurs) of the LN depends on the waveguide shape, but is 1 mW or less for light in the wavelength band of 400 nm. In order to realize a high-output optical wavelength conversion element with a power of about several tens of mW, improvement in light damage resistance is required.
【0011】本発明は、前記従来の問題を解決するた
め、ストイキオメトリックLN結晶に形成した周期状分
極反転構造において、安定な分極反転構造を得、この分
極反転構造を用いることで光導波路プロセス等による分
極反転構造の不均一性の発生を押さえ、高効率の光波長
変換素子構造を実現し、さらに、分極反転形成位置を精
密に制御できるようにし、これによって光波長変換素子
を形成する際の歩留まり向上および第2次高調波(SH
G)素子特性を向上させた光波長変換素子およびその製
造方法を提供することを目的とする。In order to solve the above-mentioned conventional problem, the present invention provides a stable domain-inverted structure in a periodic domain-inverted structure formed in a stoichiometric LN crystal, and uses this domain-inverted structure to produce an optical waveguide process. In order to suppress the occurrence of the non-uniformity of the domain-inverted structure due to the above-mentioned factors, a highly efficient optical wavelength conversion element structure is realized, and the position of domain-inverted formation can be precisely controlled, thereby forming the optical wavelength conversion element. Yield and the second harmonic (SH
G) It is an object of the present invention to provide an optical wavelength conversion element having improved element characteristics and a method for manufacturing the same.
【0012】[0012]
【課題を解決するための手段】上記目的を達成するた
め、本発明の光波長変換素子は、Li2O/(Nb2O5
+Li2O)のモル分率が49.5〜50.2%のスト
イキオメトリックLiNbO3結晶と、前記結晶表面に
形成された分極反転部分とを有し、前記分極反転部分が
周期状に配置されており、前記分極反転部分の少なくと
も一部が連続していることを特徴とする。In order to achieve the above object, an optical wavelength conversion device according to the present invention comprises Li 2 O / (Nb 2 O 5
+ Li 2 O) having a stoichiometric LiNbO 3 crystal having a molar fraction of 49.5 to 50.2%, and domain-inverted portions formed on the crystal surface, wherein the domain-inverted portions are periodically arranged. And at least a part of the domain-inverted portion is continuous.
【0013】次に本発明の光波長変換素子の製造方法
は、Li2O/(Nb2O5+Li2O)のモル分率が4
9.5〜50.2%のストイキオメトリックLiNbO
3結晶の一方の面に櫛形電極を形成し、前記結晶の他方
の面に平面電極を形成し、前記櫛形電極の一部を前記結
晶表面に絶縁膜を介して接触させて連続させ、前記電極
間に電界を印加して周期状の分極反転構造を形成するこ
とを特徴とする。Next, the method for manufacturing an optical wavelength conversion device of the present invention is characterized in that the molar fraction of Li 2 O / (Nb 2 O 5 + Li 2 O) is 4
9.5 to 50.2% stoichiometric LiNbO
(3) forming a comb-shaped electrode on one surface of the crystal, forming a planar electrode on the other surface of the crystal, and bringing a part of the comb-shaped electrode into contact with the crystal surface via an insulating film so as to be continuous; An electric field is applied between them to form a periodically poled structure.
【0014】[0014]
【発明の実施の形態】本発明は、ストイキオメトリック
LN結晶における安定な周期状分極反転構造を見いだし
た点にある。ストイキオメトリックLN結晶における高
効率な光波長変換素子を構成するには均一な周期状分極
反転構造が不可欠であり、結晶的に安定な周期状分極反
転構造が必要とされる。DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the finding of a stable periodically poled structure in a stoichiometric LN crystal. A uniform periodic domain-inverted structure is indispensable for configuring a highly efficient optical wavelength conversion element in a stoichiometric LN crystal, and a crystallographically stable periodic domain-inverted structure is required.
【0015】また、ストイキオメトリックLN結晶にお
いて周期状分極反転構造を形成するための新たな形成方
法を提案する。ストイキオメトリックLN結晶において
は、分極反転構造の形成場所を精度よく決めるのが難し
いという問題が見いだされた。そこで、この問題を解決
する分極反転形成方法を提案することで、分極反転構造
の形成場所を高精度で制御することを可能とし、青色か
ら紫外にかけて高効率な光波長変換素子の実現を可能に
した。Further, a new formation method for forming a periodically poled structure in a stoichiometric LN crystal is proposed. In the stoichiometric LN crystal, it has been found that it is difficult to accurately determine the position where the domain-inverted structure is formed. Therefore, by proposing a polarization inversion formation method that solves this problem, it is possible to control the formation location of the polarization inversion structure with high accuracy, and to realize a highly efficient light wavelength conversion element from blue to ultraviolet. did.
【0016】ストイキオメトリック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。特に結晶のモル分率が49.5〜50.2%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
学特性を有し、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ストイキオメトリックLN特
有の効果である。Stoichiometric LN is a slight change in mole fraction relative to the congruent composition LN.
As they approach the stoichiometric ratio, their crystal properties differ significantly. In particular, when the mole fraction of the crystal is in the range of 49.5 to 50.2%, the crystal has optical characteristics that are significantly different from those of the conventional congruent composition crystal, and the polarization inversion characteristics found this time also have this mole fraction. This is an effect unique to the stoichiometric LN.
【0017】ストイキオメトリックLNは、最近作製が
可能になった結晶であり、その光学特性および分極反転
特性については、未だ総てが明らかにされていない。特
に分極反転特性については、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学素子
特性の向上については、さらに未開拓な分野であった。Stoichiometric LN is a crystal that can be manufactured recently, and all of its optical characteristics and polarization reversal characteristics have not been clarified yet. In particular, the polarization inversion characteristics have been clarified for the first time by the present inventors. Further, improvement of optical element characteristics utilizing this characteristic has been an unexplored field.
【0018】本発明においては、前記接触部分は前記分
極反転部分の底部であることが、分極反転構造の安定性
から好ましい。In the present invention, it is preferable that the contact portion is the bottom of the domain-inverted portion from the viewpoint of the stability of the domain-inverted structure.
【0019】また本発明においては、前記結晶のC軸が
前記結晶表面に対し、0.3〜10°の範囲で傾いてい
ることが、深い分極反転構造の形成が可能になることか
らから好ましい。In the present invention, it is preferable that the C axis of the crystal is inclined at 0.3 to 10 ° with respect to the crystal surface, since a deep domain-inverted structure can be formed. .
【0020】また本発明においては、前記ストイキオメ
トリックLiNbO3結晶のキュリー温度が、1185
〜1205℃の範囲であることが、非線型光学案定数の
向上が図れ、高効率の波長変換素子が形成できることか
ら好ましい。In the present invention, the stoichiometric LiNbO 3 crystal has a Curie temperature of 1185.
It is preferable that the temperature is in the range of -1205 ° C., since the nonlinear optical design constant can be improved and a highly efficient wavelength conversion element can be formed.
【0021】また本発明においては、前記結晶がMg,
Zn,Sc,Inのいずれかの添加物を0.03重量%
以上1重量%以下の範囲含有させることが、結晶の耐光
損傷強度を増大させ、高出力特性を向上させることから
好ましい。0.03重量%未満では、前記の作用効果は
発現しにくく、1重量%を超えると結晶構造が不均一に
なり、SHG特性が劣化する傾向となる。Further, in the present invention, the crystal is Mg,
0.03% by weight of any additive of Zn, Sc and In
It is preferable that the content is in the range of 1% by weight or less, since the light damage resistance of the crystal is increased and the high output characteristics are improved. If the content is less than 0.03% by weight, the above-mentioned effects are unlikely to be exhibited, and if it exceeds 1% by weight, the crystal structure becomes non-uniform, and the SHG characteristics tend to deteriorate.
【0022】また本発明においては、前記分極反転部の
周期が3.5μm以下であると、波長400nm帯のS
HG光発生可能になることから好ましい。In the present invention, if the period of the domain-inverted portion is 3.5 μm or less, the S
It is preferable because HG light can be generated.
【0023】次に本発明方法においては、前記櫛形電極
が、周期状のグレーティング構造からなる歯の部分と、
前記歯の部分を互いにつなぐストライプ部分から構成さ
れ、前記ストライプ部分が前記結晶表面と絶縁膜を介し
て接触していることが、形成された分極反転構造が均一
になることから好ましい。Next, in the method of the present invention, the comb-shaped electrode includes a tooth portion having a periodic grating structure,
It is preferable that a stripe portion connecting the tooth portions to each other is formed, and the stripe portion is in contact with the crystal surface via an insulating film, since a formed domain-inverted structure becomes uniform.
【0024】また、前記結晶のC軸が前記結晶表面に対
し、0.3〜10°の範囲で傾いていることが、導波路
の伝播損失が小さくなることから好ましい。In addition, it is preferable that the C axis of the crystal is inclined within a range of 0.3 to 10 ° with respect to the crystal surface, since the propagation loss of the waveguide is reduced.
【0025】また、前記周期状分極反転部分の連続部分
が、断面から見て前記周期状分極反転部分の厚さの5〜
200%の範囲であることが、分極反転構造の安定性が
向上することから好ましい。The continuous portion of the periodically poled portion has a thickness of 5 to 5 times the thickness of the periodically poled portion as viewed from the cross section.
The range of 200% is preferable because the stability of the domain-inverted structure is improved.
【0026】[0026]
【実施例】(実施例1)LN結晶の光学定数の向上を目
的としたストイキオメトリック組成のLN結晶が提案さ
れている(特開平10−45498号公報)。Li2O
/(Nb2O5+Li2O)のモル分率を49.5〜5
0.2%のLNを用いることで、結晶内の欠陥密度の低
減が図れ、電気光学定数、非線型光学定数等の各種定数
の増大が確認されている。さらに光散乱が少なく波長4
00〜600nmの可視光領域での光透過特性にも優れ
ている。発明者らは、このストイキオメトリックLN結
晶における周期的分極反転構造の形成を試みた。コング
ルエント組成のLNはパターン電極による周期状分極反
転構造が可能である。印加電圧は約20kV/mmと非
常に高い印加電圧が必要である。形成された分極反転構
造は安定で500℃程度の熱処理および導波路形成に必
要なプロトン交換によっても反転構造は変化せず安定で
ある。これに対し、ストイキオメトリックLNは分極反
転に必要な印加電圧が極端に低下する。分極反転に必要
な電圧は5kV/mm以下であった。これはLi、Nb
の組成が完全結晶に近いストイキオメトリックLNは結
晶内における結晶欠陥が少ないため、分極反転に対向す
る内部抵抗が少なくなるためである。低電圧で分極反転
が可能なため絶縁破壊が生じにくく厚い結晶にも分極反
転構造の形成が容易である。(Example 1) An LN crystal having a stoichiometric composition for the purpose of improving the optical constant of the LN crystal has been proposed (Japanese Patent Application Laid-Open No. Hei 10-45498). Li 2 O
/ (Nb 2 O 5 + Li 2 O) mole fraction of 49.5 to 5
By using LN of 0.2%, the defect density in the crystal can be reduced, and it has been confirmed that various constants such as an electro-optical constant and a nonlinear optical constant are increased. Further light scattering, wavelength 4
It also has excellent light transmission characteristics in the visible light range of 00 to 600 nm. The inventors have tried to form a periodically poled structure in this stoichiometric LN crystal. LN having a congruent composition can have a periodically poled structure with a pattern electrode. The applied voltage needs a very high applied voltage of about 20 kV / mm. The formed domain-inverted structure is stable and stable without being changed by heat treatment at about 500 ° C. and proton exchange necessary for forming a waveguide. On the other hand, in the stoichiometric LN, the applied voltage required for polarization reversal is extremely reduced. The voltage required for the polarization reversal was 5 kV / mm or less. This is Li, Nb
This is because the stoichiometric LN whose composition is close to a perfect crystal has few crystal defects in the crystal, and therefore has a small internal resistance against polarization inversion. Since polarization inversion can be performed at a low voltage, dielectric breakdown does not easily occur and a polarization inversion structure can be easily formed even in a thick crystal.
【0027】本発明者らは、ストイキオメトリックLN
の分極反転特性について詳細に検討したところ、外乱に
より形成された分極反転が再反転し、分極反転構造に不
均一性を増大させることを見いだした。分極反転電圧が
極端に低下することで分極反転構造の不安定性が増大し
たと考えられる。波長450nm以下の青色〜紫外にか
けての第2次高調波(SHG)の光発生を行うには、周
期3.5μm以下の短周期の分極反転構造が必要であ
る。ところが分極反転構造を3.5μm以下の短周期構
造にした場合、その不安定性の現象が顕著に現れた。原
因は、熱処理時に発生する焦電電界により分極反転が再
反転したためと考えられる。The present inventors have determined that stoichiometric LN
A detailed study of the domain reversal characteristics revealed that the domain reversal formed by the disturbance was re-inverted, increasing nonuniformity in the domain reversal structure. It is considered that the instability of the domain-inverted structure increased due to the extremely low domain-inverted voltage. In order to generate light of the second harmonic (SHG) from blue to ultraviolet with a wavelength of 450 nm or less, a short-period domain-inverted structure with a period of 3.5 μm or less is required. However, when the domain-inverted structure is a short period structure of 3.5 μm or less, the phenomenon of the instability appears remarkably. It is considered that the cause was that the polarization inversion was re-inverted by the pyroelectric electric field generated during the heat treatment.
【0028】ストイキオメトリックLN結晶は反転電圧
が低いため、焦電効果により発生した電界が分極反転構
造に影響を与える。周期3〜3.5μmの周期状分極反
転構造を形成したストイキオメトリックLN基板の温度
を急激に変化させたところ、周期状の分極反転構造が一
部再反転し、形成された分極反転構造の均一性が劣化し
た。これは従来のコングルエント組成のLNでは観測さ
れなかった現象である。また分極反転の均一性が劣化す
る現象は高温で熱処理する場合にも生じた。Since the stoichiometric LN crystal has a low inversion voltage, the electric field generated by the pyroelectric effect affects the domain-inverted structure. When the temperature of the stoichiometric LN substrate having the periodic domain-inverted structure having a period of 3 to 3.5 μm was rapidly changed, the periodic domain-inverted structure was partially re-inverted. Uniformity deteriorated. This is a phenomenon that was not observed in the conventional LN having a congruent composition. Further, the phenomenon that the uniformity of the domain inversion is deteriorated also occurs when the heat treatment is performed at a high temperature.
【0029】本発明者らは、短周期の分極反転構造の安
定化について種々検討を行ったところ、分極反転構造に
より再反転の発生が異なることを見いだした。即ち、幾
つかの分極反転構造においては急激な温度変化や高温の
熱処理に対しても分極反転構造を安定に保てることを見
いだした。通常分極反転構造は非線形グレーティングを
構成するため、分極反転部と非反転部分が交互に存在す
る。短周期になると形成される分極反転部の断面積は小
さくなる。これによって分極反転部の不安定性が増大し
た。The present inventors conducted various studies on stabilization of a short-period domain-inverted structure, and found that the occurrence of reinversion differs depending on the domain-inverted structure. That is, it has been found that in some domain-inverted structures, the domain-inverted structure can be stably maintained even with rapid temperature changes and high-temperature heat treatment. Usually, since the domain-inverted structure forms a nonlinear grating, domain-inverted portions and non-inverted portions are alternately present. When the period becomes short, the cross-sectional area of the domain-inverted portion formed becomes small. This increased the instability of the domain-inverted portion.
【0030】周期3.5μm以下の短周期構造では、熱
処理等により形成された分極反転構造が変化した。これ
に対し、隣接する分極反転構造の一部が互いに接触して
連続化することで、分極反転構造の安定性が大幅に向上
した。ストイキオメトリックLNにおいても周期が10
μm以上の反転構造においては構造の安定性は問題がな
い。ところが、周期3.5μm以下の短周期構造におい
ては構造の不安定性が顕著になった。これに対し、隣接
する分極反転構造の一部を違いに接触させて分極反転の
断面積を大きくしたところ、分極反転構造の安定性が大
幅に向上することがわかった。In a short-period structure having a period of 3.5 μm or less, a domain-inverted structure formed by heat treatment or the like changed. On the other hand, the stability of the domain-inverted structure was greatly improved by making the adjacent domain-inverted structures contact each other to be continuous. In the stoichiometric LN, the period is 10
There is no problem in the stability of the structure in the inverted structure of μm or more. However, in the short-period structure having a period of 3.5 μm or less, the instability of the structure became remarkable. On the other hand, it was found that the stability of the domain-inverted structure was greatly improved when the cross-sectional area of domain-inverted structure was increased by bringing a part of the domain-inverted structure into contact with the difference.
【0031】次に、分極反転を形成する基板結晶の方位
について検討した。Z板の基板において、形成される分
極反転構造は比較的安定であった。これに対し、オフカ
ット基板に形成した分極反転構造は外乱による再反転現
象の発生が顕著であり、不安定性が増大した。これはオ
フカット基板に形成される個々の分極反転部の断面積が
制約されるためである。オフカット基板で形成される分
極反転は電極先端で発生し、結晶軸に沿って基板内部に
形成される。隣接する分極反転部は互いに非接触でそれ
ぞれの分極反転部が独立しているため、各反転部の断面
積は10μm2程度と小さくなる。そこで、オフカット
のストイキオメトリックLNにおける安定な分極反転構
造について検討した。結晶はオフカット基板のストイキ
オメトリックLN結晶で、結晶のX軸が基板表面の法線
に対し3°傾いている。Next, the orientation of the substrate crystal forming the domain inversion was examined. On the Z-plate substrate, the formed domain-inverted structure was relatively stable. On the other hand, in the domain-inverted structure formed on the off-cut substrate, a re-inversion phenomenon caused by disturbance was remarkable, and the instability increased. This is because the cross-sectional area of each domain-inverted portion formed on the off-cut substrate is restricted. Polarization reversal formed in the off-cut substrate occurs at the tip of the electrode and is formed inside the substrate along the crystal axis. Since the adjacent domain-inverted portions are not in contact with each other and each domain-inverted portion is independent, the cross-sectional area of each domain-inverted portion is as small as about 10 μm 2 . Therefore, a stable domain-inverted structure in the off-cut stoichiometric LN was examined. The crystal is a stoichiometric LN crystal of an off-cut substrate, and the X-axis of the crystal is inclined by 3 ° with respect to the normal to the substrate surface.
【0032】図1(a)は本発明の一実施例の光波長変
換素子の斜視図、図1(b)は同平面図である。FIG. 1A is a perspective view of an optical wavelength conversion device according to one embodiment of the present invention, and FIG. 1B is a plan view of the same.
【0033】図1(a)に示すように、基板1の表面に
櫛形電極4を、基板の裏面に平面電極5を形成し、電極
間に電圧を印加することで櫛形電極から分極反転が成長
し、結晶内部に向かって分極反転部6(図2(a)〜
(c))が形成された。形成した分極反転構造を断面
(図1(b)のX−X線断面)から観測した。この断面
を図2(a)(b)(c)に示す。As shown in FIG. 1A, a comb-shaped electrode 4 is formed on the front surface of the substrate 1 and a plane electrode 5 is formed on the back surface of the substrate. By applying a voltage between the electrodes, domain inversion grows from the comb-shaped electrode. Then, the domain-inverted portions 6 (FIGS.
(C)) was formed. The formed domain-inverted structure was observed from a cross section (cross section taken along line XX in FIG. 1B). This cross section is shown in FIGS. 2 (a), 2 (b) and 2 (c).
【0034】電極構造と電界印加条件を変えて、図2
(a)(b)(c)に示す分極反転構造断面を形成し
た。図2(a)は隣接する分極反転が接触していない構
造、図2(b)は隣接する分極反転場合が上面近傍で接
触している構造、図2(c)は隣接する分極反転が底面
近傍で接触している構造である。図2(a)は従来の分
極反転構造であり、分極反転構造の安定性に問題があっ
た。これに対して本発明の一実施例である図2(b)〜
(c)の構造にすることで温度変化や高温処理に対して
も安定性が大幅に向上した。図2(a)は理想的な形状
である。ただし、ストイキオメトリック結晶において
は、図2(a)の構造を形成するには分極反転部分をか
なり小さく制限する必要があった。すなわち、印加する
電流量を極端に制限することで図2(a)の形状の形成
が可能となった。ただし、この場合の分極反転厚みは
0.5μm以下となり、高効率の光波長変換素子の形成
は難しかった。さらに加える電流量を増やしていくと、
図2(b)の形状の分極反転構造が形成できた。しかし
ながら、さらに電流量を増加させると、隣接する分極反
転部が互いに接触し、部分的に連続化し、周期構造が観
測されなくなった。図2(c)の形の分極反転構造を形
成するには、印加する電圧を短パルス化する必要があ
る。図2(a)〜(b)の方法では印加する電圧は0.
1〜1s以上の印加時間を有する。これに対し、数ms
以下の短パルスの電圧を印加することで、図2(c)の
分極反転構造の形成が可能となった。とくに印加パルス
電圧は5ms以下の短パルスが好ましい。この傾向は、
ストイキオメトリックLNに特有の現象として観測され
た。分極反転形成は、ストイキオメトリックLNとコン
グルエントLNでは大きく異なることが明らかになっ
た。By changing the electrode structure and electric field application conditions, FIG.
(A), (b) and (c), the cross section of the domain-inverted structure was formed. 2A is a structure in which adjacent domain inversions are not in contact, FIG. 2B is a structure in which adjacent domain inversions are in contact near the top surface, and FIG. This is a structure that is in close contact. FIG. 2A shows a conventional domain-inverted structure, which has a problem in the stability of the domain-inverted structure. On the other hand, FIG. 2B to FIG.
By adopting the structure of (c), the stability was greatly improved even with temperature changes and high-temperature treatment. FIG. 2A shows an ideal shape. However, in the stoichiometric crystal, it was necessary to limit the domain-inverted portion to a relatively small size in order to form the structure shown in FIG. That is, the shape of FIG. 2A can be formed by extremely limiting the amount of applied current. However, the polarization inversion thickness in this case was 0.5 μm or less, and it was difficult to form a highly efficient light wavelength conversion element. As you add more current,
A domain-inverted structure having the shape shown in FIG. 2B was formed. However, when the current amount was further increased, the adjacent domain-inverted portions contacted each other, became partially continuous, and the periodic structure was not observed. In order to form a domain-inverted structure as shown in FIG. 2C, it is necessary to shorten the voltage to be applied. In the method shown in FIGS.
It has an application time of 1 to 1 s or more. On the other hand, several ms
By applying the following short pulse voltage, the domain-inverted structure shown in FIG. 2C can be formed. In particular, the applied pulse voltage is preferably a short pulse of 5 ms or less. This trend is
It was observed as a phenomenon peculiar to stoichiometric LN. It was revealed that the domain inversion formation was significantly different between the stoichiometric LN and the congruent LN.
【0035】次に形成した分極反転構造を内に光導波路
を形成して図3に示す光波長変換素子を形成した。図3
は本発明の光波長変換素子、すなわち図2(c)の分極
反転構造を有する光波長変換素子の構成を示している。
図3において、1はストイキオメトリック組成のLN基
板、2は周期状分極反転構造、3はプロトン交換光導波
路である。分極反転構造2を形成した後、光導波路3は
プロトン交換とアニール処理により形成した。プロトン
交換層をストライプ状に形成した後アニール処理するこ
とで、高非線形性を有する低損失光導波路が形成でき
た。導波路内に波長820nmの光を入射し、分極反転
構造により波長変換することで波長410nmの紫色光
を発生する光波長変換素子を形成した。Next, an optical waveguide was formed inside the formed domain-inverted structure to form an optical wavelength conversion element shown in FIG. FIG.
2 shows the configuration of the optical wavelength conversion element of the present invention, that is, the optical wavelength conversion element having the domain-inverted structure shown in FIG.
In FIG. 3, 1 is an LN substrate having a stoichiometric composition, 2 is a periodically poled structure, and 3 is a proton exchange optical waveguide. After forming the domain-inverted structure 2, the optical waveguide 3 was formed by proton exchange and annealing. By forming the proton exchange layer in a stripe shape and performing annealing, a low-loss optical waveguide having high nonlinearity was formed. Light having a wavelength of 820 nm was incident on the waveguide, and the wavelength was converted by a domain-inverted structure to form a light wavelength conversion element that generates violet light having a wavelength of 410 nm.
【0036】図2(a)の分極反転構造を用いて光波長
変換素子を形成すると、第2次高調波(SHG)の変換
効率は5%以下であり、高効率の波長変換素子の製造は
難しかった。これは光導波路を形成するプロセスの影響
で分極反転構造に不均一性が生じたためと考えられる。
図2(c)の分極反転構造を有する光波長変換素子は1
00mWの半導体レーザに対し20mWの紫色光の発生
が可能であり効率20%の効率波長変換を達成した。こ
れに対し図2(b)の分極反転構造において変換効率は
1/10の2%に低下した。これは分極反転が光導波路
の表面近傍で接触しているため非線形グレーティングと
して機能が効率よく働かなかったためである。When an optical wavelength conversion element is formed using the domain-inverted structure shown in FIG. 2A, the conversion efficiency of the second harmonic (SHG) is 5% or less, and the production of a highly efficient wavelength conversion element is not possible. was difficult. This is considered to be due to the non-uniformity of the domain-inverted structure caused by the process of forming the optical waveguide.
The optical wavelength conversion element having the domain-inverted structure shown in FIG.
A violet light of 20 mW can be generated with respect to a semiconductor laser of 00 mW, and an efficient wavelength conversion of 20% was achieved. On the other hand, in the domain-inverted structure of FIG. 2B, the conversion efficiency was reduced to 1/10 of 2%. This is because the function as a nonlinear grating did not work efficiently because the polarization inversion was in contact near the surface of the optical waveguide.
【0037】光導波路は2〜3μm程度の厚みであり、
光導波路を伝搬する光のパワー密度が最も高いのは導波
路表面から1μm程度のところである。基本波と高調波
のオーバラップもこの部分で最大となる。分極反転構造
が表面近傍で互いに接触すると光導波路の表面近傍にお
いて波長変換が生じなくなるので変換効率が大幅に減少
する結果となった。これに対し、図2(c)の構造で
は、分極反転が導波路底面部で接触し連続しているた
め、導波路表面近傍における波長変換が阻害されること
なく高効率の波長変換を達成することができた。前記図
2(c)の構造において、周期状分極反転部分6の連続
部分8は、周期状分極反転部分6の厚さの約10%であ
った。分極反転構造の安定性と変換効率特性をまとめる
と、表1の結果となる。電極部分の厚さは、分極反転部
分の5%以上が安定化のために必要である。ただし、そ
の厚みが周期状の部分厚みに対し、2倍を超えると周期
状の反転部分の厚みは減少するため、2倍以下に抑える
のが好ましい。The optical waveguide has a thickness of about 2 to 3 μm,
The highest power density of light propagating through the optical waveguide is about 1 μm from the waveguide surface. The overlap between the fundamental wave and the harmonic is also maximum in this part. When the domain-inverted structures contact each other near the surface, wavelength conversion does not occur near the surface of the optical waveguide, so that the conversion efficiency is greatly reduced. On the other hand, in the structure of FIG. 2C, the polarization reversal is in contact and continuous at the bottom surface of the waveguide, so that high-efficiency wavelength conversion is achieved without hindering the wavelength conversion near the waveguide surface. I was able to. In the structure of FIG. 2C, the continuous portion 8 of the periodically poled portion 6 is about 10% of the thickness of the periodically poled portion 6. Table 1 summarizes the stability and conversion efficiency characteristics of the domain-inverted structure. As for the thickness of the electrode portion, 5% or more of the domain-inverted portion is necessary for stabilization. However, if the thickness exceeds twice the thickness of the periodic portion, the thickness of the periodic inversion portion decreases, so that the thickness is preferably suppressed to twice or less.
【0038】[0038]
【表1】 [Table 1]
【0039】表1において、分極反転構造の安定性の○
は、安定性に問題ないことを示す。具体的には、400
℃程度の熱処理プロセスが分極反転形状に影響を与えな
いことを確認した。同×は400℃程度の熱処理プロセ
スにより分極反転形状に変化が観測された。変換効率の
○は、100mWの基本波に対して20%以上の変換効
率が得られた。同×は100mWの基本波に対して、数
%の変換効率しか得られなかった。In Table 1, the stability of the domain-inverted structure
Indicates that there is no problem in stability. Specifically, 400
It was confirmed that the heat treatment process at about ℃ did not affect the domain-inverted shape. In the case of ×, a change in the domain-inverted shape was observed by a heat treatment process at about 400 ° C. Regarding the conversion efficiency ○, a conversion efficiency of 20% or more was obtained for a fundamental wave of 100 mW. In the case of ×, a conversion efficiency of only several% was obtained for a fundamental wave of 100 mW.
【0040】分極反転構造の安定性と高効率の波長変換
を実現できるのは、図2(c)の構造においてである。It is in the structure of FIG. 2C that the stability of the domain-inverted structure and the highly efficient wavelength conversion can be realized.
【0041】AlGaAs系の波長可変DBR半導体レ
ーザ(波長820nm)が用いて短波長光源を実現でき
た。波長可変DBR半導体レーザは、活性領域とDBR
領域の2電極から構成され、DBR領域への注入電流を
調整することにより、発振波長を調整することができ
る。半導体レーザと導波路型の光波長変換素子を直接結
合することで小型の短波長光源を実現できた。半導体レ
ーザ出力100mWに対して光導波路に60mWの半導
体レーザ光が結合した。A short-wavelength light source was realized using an AlGaAs-based tunable DBR semiconductor laser (wavelength 820 nm). The tunable DBR semiconductor laser has an active region and a DBR.
The oscillation wavelength can be adjusted by adjusting the injection current into the DBR region, which is composed of two electrodes in the region. By directly coupling the semiconductor laser and the waveguide-type optical wavelength conversion device, a compact short-wavelength light source was realized. A semiconductor laser beam of 60 mW was coupled to the optical waveguide for a semiconductor laser output of 100 mW.
【0042】導波損失は−0.5dB/cmと従来の導
波損失の1/2となり、低損失の光導波路が実現でき
た。ストイキオメトリックLNを用いることで結晶の透
過率を上げることが可能となり、同時に、低電圧の電界
印加による分極反転が形成されるため、分極反転時に結
晶に与えるダメージが少なくなり、分極反転構造内に低
損失の光導波路の形成が可能となった。The waveguide loss was -0.5 dB / cm, which is 1/2 of the conventional waveguide loss, and a low-loss optical waveguide was realized. By using the stoichiometric LN, it is possible to increase the transmittance of the crystal, and at the same time, polarization inversion is caused by application of a low-voltage electric field. Thus, a low-loss optical waveguide can be formed.
【0043】さらに、隣接する分極反転部を違いに接触
することで分極反転部の境界で発生していたわずかな屈
折率変化による光導波路の伝搬損失の増大が低減したた
めである。このため、非常に低損失の光導波路形成が可
能になった。さらに、光波長変換素子の変換効率は約5
0%になり、従来の2倍以上に向上することができた。
これは、導波路損失の低減に加え、基板の非線形光学定
数が向上したためで、ストイキオメトリックLN結晶は
コングルエント組成LN結晶に比べ1.2倍以上の非線
形光学定数を有するためである。さらに高い屈折率変化
を利用して、光導波路のプロトン濃度を低減することが
可能であるため、導波路内の非線形光学定数の増大が実
現し、高効率の光波長変換素子が形成できた。Furthermore, this is because the increase in the propagation loss of the optical waveguide due to a slight change in the refractive index caused at the boundary between the domain-inverted portions due to the contact between the adjacent domain-inverted portions is reduced. Therefore, it is possible to form an optical waveguide with very low loss. Further, the conversion efficiency of the optical wavelength conversion element is about 5
0%, which is more than double the conventional value.
This is because the nonlinear optical constant of the substrate is improved in addition to the reduction of the waveguide loss, and the stoichiometric LN crystal has a nonlinear optical constant that is 1.2 times or more as large as the congruent composition LN crystal. Since the proton concentration in the optical waveguide can be reduced by utilizing a higher refractive index change, the nonlinear optical constant in the waveguide is increased, and a highly efficient optical wavelength conversion element can be formed.
【0044】なお、本実施例では、基板としては3°オ
フカットのX板ストイキオメトリックLNについて検討
したが、基板のオフカット角(基板表面の法線と結晶の
X軸のなす角度)は0.3〜10°が望ましい。オフカ
ット角はわずかに存在することで分極反転が結晶のZ軸
に沿って成長するため深い分極反転構造が形成される。
しかしながら、オフカット角が浅くなるに従い分極反転
の厚みは減少し、0.3°以下になると通常のX板に形
成される1μm程度の分極反転構造しか形成されなくな
る。In this embodiment, an X-plate stoichiometric LN with a 3 ° off-cut was examined as the substrate. However, the off-cut angle of the substrate (the angle between the normal to the substrate surface and the X-axis of the crystal) is 0.3-10 degrees is desirable. When the off-cut angle is slightly present, the domain inversion grows along the Z axis of the crystal, so that a deep domain inversion structure is formed.
However, as the off-cut angle becomes shallower, the thickness of the domain inversion decreases, and when it becomes 0.3 ° or less, only a domain-inverted structure of about 1 μm formed on a normal X plate is formed.
【0045】一方、オフカット角が厚くなると深い分極
反転構造が形成され高効率の光波長変換素子の製造が可
能となる。しかし、オフカット角の増大とともに形成さ
れるプロトン交換光導波路の伝搬損失が増大する。10
°以上の角度では伝搬損失が3dB/cm以上になり、
光波長変換素子の変換効率が大幅に低下するため好まし
くない。On the other hand, when the off-cut angle is large, a deep domain-inverted structure is formed, and a highly efficient optical wavelength conversion element can be manufactured. However, as the off-cut angle increases, the propagation loss of the proton exchange optical waveguide formed increases. 10
At an angle of more than °, the propagation loss becomes more than 3 dB / cm,
It is not preferable because the conversion efficiency of the light wavelength conversion element is significantly reduced.
【0046】なお、本実施例では、ストイキオメトリッ
クLNにおける分極反転構造について検討したが、スト
イキオメトリックLNにMg,Zn,Sc,Inのいず
れかの添加物を0.03重量%以上添加することで、光
波長変換素子の特性を大幅に向上させることが可能とな
った。耐光損傷強度を向上させることで出力特性が向上
する。添加物を含まない結晶では1mW程度のSHG出
力に対しても、光損傷による出力の不安定現象が観測さ
れたが、添加物を入れることで50mW以上のSHG出
力も安定に出力することが可能となった。In this embodiment, the polarization inversion structure in the stoichiometric LN was examined. However, an additive of Mg, Zn, Sc, or In is added to the stoichiometric LN in an amount of 0.03% by weight or more. This has made it possible to greatly improve the characteristics of the optical wavelength conversion element. Output characteristics are improved by increasing the light damage resistance. In the case of crystals containing no additive, an unstable phenomenon due to optical damage was observed even for an SHG output of about 1 mW, but the SHG output of 50 mW or more can be output stably by adding an additive. It became.
【0047】なお、本実施の形態では、光波長変換素子
への応用について説明したが、その他、分極反転構造を
利用した光スイッチ、光偏光器、変調器等への応用につ
いても同様の素子特性の向上が図れた。例えば、3次元
導波路に周期状の分極反転構造を形成し、これに電界を
印加することで導波路内にグレーティング構造を形成/
消去することが可能となる。グレーティングを利用した
方向性結合器やTE/TMモード変換器、DBRグレー
ティング等の機能を電界印加により制御することが可能
となる。このようなグレーティング構造を形成する場合
にも、本発明の構成をとることで安定な形状を保つこと
が可能となる。In this embodiment, the application to the optical wavelength conversion element has been described. However, the same applies to the application to the optical switch, the optical deflector, the modulator and the like using the domain-inverted structure. Was improved. For example, a periodic domain-inverted structure is formed in a three-dimensional waveguide, and an electric field is applied thereto to form a grating structure in the waveguide.
It can be erased. Functions such as a directional coupler using a grating, a TE / TM mode converter, and a DBR grating can be controlled by applying an electric field. Even when such a grating structure is formed, a stable shape can be maintained by adopting the configuration of the present invention.
【0048】また、光偏光器においてもグレーティング
によるDBRを利用するもの3角の反転形状を利用しプ
リズム効果による偏光を実現するものがあるが、微細な
分極反転形成を利用する場合には、いずれも、分極反転
の一部を接触させることで安定な形状が実現できる。電
界印加による屈折率変化を利用するこれらの素子におい
ては、ストイキオメトリックLNを用いることで、高屈
折率の光導波路の利用が可能になることと、電気光学定
数の増大が図れることで、光学素子の特性が大幅に向上
できた。Also, there are optical deflectors that use a DBR by a grating and those that use a triangular inversion shape to realize polarization by the prism effect. However, when a fine polarization inversion is used, Also, a stable shape can be realized by contacting a part of the domain inversion. In these devices utilizing the change in the refractive index due to the application of an electric field, the use of a stoichiometric LN enables the use of an optical waveguide having a high refractive index, and the increase in the electro-optical constant allows the optical device to be used. The characteristics of the device were greatly improved.
【0049】(実施例2)ここでは、ストイキオメトリ
ックLNにおける周期状分極反転構造の製造方法につい
て述べる。オフカット基板における周期状分極反転構造
の形成方法は図1に示す電極構造により分極反転を形成
した。分極反転は電極指の先端近傍で発生し、結晶のZ
軸に沿って基板内部に形成された。これは、櫛形電極と
平面電極間に電圧を印加した場合、電極指の先端で電界
強度が最大に成るためである。Embodiment 2 Here, a method of manufacturing a periodically poled structure in a stoichiometric LN will be described. In the method of forming the periodic domain-inverted structure on the off-cut substrate, domain-inverted was formed by the electrode structure shown in FIG. Polarization reversal occurs near the tip of the electrode finger,
Formed inside the substrate along the axis. This is because when a voltage is applied between the comb-shaped electrode and the plane electrode, the electric field strength is maximized at the tip of the electrode finger.
【0050】これに対し、ストイキオメトリックLNに
おいては分極反転の形成される位置が電極指先端に限ら
れず、電極指の付け根やストライプ部分等でランダムに
発生することが見いだされた。これは、ストイキオメト
リックLNの反転電圧が低いため、電極内の電界分布に
おいて電界強度差が小さく成り、分極反転の発生場所が
比較的ランダム生じたためと考えられる。分極反転の形
成される位置が再現性よく決定されないと、光導波路位
置が決まらず光波長変換素子の高効率設計が再現性よく
できない。そこで、分極反転の形成される部分のみ電極
を基板に接触させる方法を見つけた。On the other hand, it has been found that in the stoichiometric LN, the position where the polarization inversion is formed is not limited to the tip of the electrode finger, but occurs randomly at the base of the electrode finger or at the stripe portion. This is presumably because the inversion voltage of the stoichiometric LN is low, the electric field intensity difference in the electric field distribution in the electrode is small, and the location of the domain inversion is relatively random. If the position where the polarization inversion is formed is not determined with good reproducibility, the position of the optical waveguide is not determined, and high-efficiency design of the optical wavelength conversion element cannot be performed with good reproducibility. Therefore, a method was found in which the electrode was brought into contact with the substrate only at the portion where the domain inversion was formed.
【0051】図4に本発明の光波長変換素子の製造方法
の構成を示す。ストイキオメトリック組成LNオフカッ
ト基板11の表面にストライプ状の電気絶縁膜12を形
成した。電気絶縁膜12としては、市販の感光性樹脂を
用いた。次に電気絶縁膜12を介して櫛形電極13を形
成した。基板の裏面には平面電極14を形成した。櫛形
電極は周期状に電極部分が並んだ歯の部分15と、歯の
部分をつなぐストライプ部分16からなり、分極反転構
造の形成に必要な電極15のみを基板に接触させて連続
させ、他の部分は電気絶縁膜により基板から絶縁した。
電極間に電圧を印加すると基板に接触した電極部分15
から分極反転が発生し、周期状の分極反転構造が形成さ
れた。従来の方法では分極反転の形成位置は数10μm
以上ばらついた。これに対し、本実施例の方法により製
造した分極反転構造は1μm以下の精度で分極反転構造
の位置を再現性よく決定することが可能となった。周期
状分極反転部分の連続部分は、実施例1と同様に周期状
分極反転部分の厚さの約10%であった。FIG. 4 shows a configuration of a method for manufacturing an optical wavelength conversion element according to the present invention. A striped electrical insulating film 12 was formed on the surface of a stoichiometric composition LN off-cut substrate 11. As the electric insulating film 12, a commercially available photosensitive resin was used. Next, a comb-shaped electrode 13 was formed via the electric insulating film 12. The plane electrode 14 was formed on the back surface of the substrate. The comb-shaped electrode is composed of a tooth portion 15 in which electrode portions are arranged periodically and a stripe portion 16 connecting the tooth portions. Only the electrode 15 necessary for forming the domain-inverted structure is brought into contact with the substrate to be continuous. The part was insulated from the substrate by an electric insulating film.
When a voltage is applied between the electrodes, the electrode portion 15 in contact with the substrate
From this, domain inversion occurred, and a periodic domain inversion structure was formed. In the conventional method, the position where the domain inversion is formed is several tens μm.
That was all. On the other hand, in the domain-inverted structure manufactured by the method of the present embodiment, the position of the domain-inverted structure can be determined with a high reproducibility with an accuracy of 1 μm or less. The continuous portion of the periodically poled portion was about 10% of the thickness of the periodically poled portion as in Example 1.
【0052】従来の方法と本発明の方法により製造した
分極反転構造を用いて光波長変換素子を作製した。作製
方法としては、周期状の分極反転構造を横切るように光
導波路を形成した。光導波路は、選択マスクを用いてス
トライプ状にプロトン交換を行い作製した。作製した光
波長変換素子の変換効率を測定したところ、従来の方法
で作製した光波長変換素子の歩留まりは5%以下と低か
った。これは、分極反転部分の形成位置が決定できない
ため、光導波路と分極反転部分が効率よく重なる確率が
低いためである。An optical wavelength conversion element was manufactured using the domain-inverted structure manufactured by the conventional method and the method of the present invention. As a manufacturing method, an optical waveguide was formed so as to cross the periodically poled structure. The optical waveguide was manufactured by performing proton exchange in a stripe shape using a selection mask. When the conversion efficiency of the manufactured light wavelength conversion device was measured, the yield of the light wavelength conversion device manufactured by the conventional method was as low as 5% or less. This is because the position where the domain-inverted portion is formed cannot be determined, and the probability that the optical waveguide and the domain-inverted portion overlap efficiently is low.
【0053】これに対して、本発明の製造方法により作
製した光波長変換素子は80%以上の高い歩留まりで作
製することができた。また、作製した光波長変換素子の
変換効率も従来の方法に対し2倍以上の変換効率が得ら
れた。本発明の製造方法により光導波路内に均一な周期
状分極反転構造の形成が可能になったからである。On the other hand, the optical wavelength conversion device manufactured by the manufacturing method of the present invention could be manufactured with a high yield of 80% or more. Further, the conversion efficiency of the manufactured light wavelength conversion element was more than twice as high as that of the conventional method. This is because a uniform periodic polarization inversion structure can be formed in the optical waveguide by the manufacturing method of the present invention.
【0054】Mgの高濃度添加に関しては、0.2mo
l%以上の添加で耐光損傷強度は大幅に改善されるが、
2mol%以上添加することで、耐損傷強度は青色光に
対して70mW以上に増大し、さらに3mol%以上添
加することで100mWの青色光の発生も可能になっ
た。これらのMgドープ量の増大によっても、結晶欠
陥、散乱損失の増大は観測されず、良好な光学特性が得
られた。As for the high concentration addition of Mg, 0.2 mol
The light damage resistance is significantly improved by adding 1% or more,
By adding 2 mol% or more, the damage resistance increased to 70 mW or more with respect to blue light, and by adding 3 mol% or more, it became possible to generate 100 mW of blue light. No crystal defects and no increase in scattering loss were observed even with an increase in the Mg doping amount, and good optical characteristics were obtained.
【0055】[0055]
【発明の効果】以上のように本発明によれば、ストイキ
オメトリックLN結晶に形成した周期状分極反転構造に
おいて、隣接する分極反転部を一部接触させることで安
定な分極反転構造を得ることが可能となった。この分極
反転構造を用いることで光導波路プロセス等による分極
反転構造の不均一性の発生が押さえられ、高効率の光波
長変換素子構造を実現できるため、その実用効果は大き
い。As described above, according to the present invention, in the periodically poled structure formed in the stoichiometric LN crystal, a stable poled structure can be obtained by partially contacting adjacent poled parts. Became possible. By using this domain-inverted structure, the occurrence of non-uniformity in the domain-inverted structure due to an optical waveguide process or the like is suppressed, and a highly efficient optical wavelength conversion element structure can be realized.
【0056】また、分極反転構造を形成する際に、電極
の一部を基板と絶縁することで、分極反転形成位置を精
密に制御することが可能となった。これによって光波長
変換素子を形成する際の歩留まり向上およびSHG素子
特性の向上が可能となりその実用効果は大きい。In forming the domain-inverted structure, it is possible to precisely control the domain-inverted formation position by insulating a part of the electrode from the substrate. This makes it possible to improve the yield and the SHG element characteristics when forming the optical wavelength conversion element, and the practical effect is large.
【図1】(a)は本発明の一実施例の光波長変換素子の
斜視図、(b)は同平面図である。FIG. 1A is a perspective view of an optical wavelength conversion device according to an embodiment of the present invention, and FIG. 1B is a plan view of the same.
【図2】本発明の一実施例と従来例の光波長変換素子に
おける分極反転構造の断面図で、(a)は従来例の光波
長変換素子の分極反転構造を示す図、(b)は本発明の
一実施例の上面が接触している分極反転構造を示す図、
(c)は本発明の一実施例の下面が接触している分極反
転構造を示す図である。FIGS. 2A and 2B are cross-sectional views of a domain-inverted structure of an embodiment of the present invention and a conventional optical wavelength conversion element, where FIG. 2A shows a domain-inverted structure of a conventional optical wavelength conversion element, and FIG. A diagram showing a domain-inverted structure in which the upper surface of one embodiment of the present invention is in contact,
(C) is a diagram showing a domain-inverted structure in which the lower surface of one embodiment of the present invention is in contact.
【図3】本発明の一実施例の光波長変換素子の斜視図で
ある。FIG. 3 is a perspective view of an optical wavelength conversion element according to one embodiment of the present invention.
【図4】本発明の一実施例の光波長変換素子の製造方法
を示す斜視図である。FIG. 4 is a perspective view illustrating a method for manufacturing an optical wavelength conversion element according to one embodiment of the present invention.
1 ストイキオメトリック組成のLN基板 2 周期状分極反転構造 3 プロトン交換光導波路 4,13 櫛形電極 5,14 平面電極 6 分極反転部 7 分極非反転部 8 分極反転部の連続部 11 ストイキオメトリック組成LNオフカット基板 12 ストライプ状の電気絶縁膜 15 電極の歯の部分 16 電極のストライプ部分 DESCRIPTION OF SYMBOLS 1 LN substrate of stoichiometric composition 2 Periodic polarization inversion structure 3 Proton exchange optical waveguide 4, 13 Comb electrode 5, 14 Planar electrode 6 Polarization inversion part 7 Polarization non-inversion part 8 Continuous part of polarization inversion part 11 Stoichiometric composition LN off-cut substrate 12 Striped electrical insulating film 15 Electrode tooth portion 16 Electrode stripe portion
フロントページの続き (71)出願人 500121621 古川 保典 埼玉県深谷市上柴町西四丁目17番地15号 (71)出願人 500121643 竹川 俊二 茨城県つくば市吾妻二丁目11番地801棟403 号 (71)出願人 500583151 中村 優 茨城県つくば市吾妻1丁目18−1番地406 棟402号 (72)発明者 水内 公典 大阪府門真市大字門真1006番地 松下電器 産業株式会社内 (72)発明者 山本 和久 大阪府門真市大字門真1006番地 松下電器 産業株式会社内 (72)発明者 北村 健二 茨城県つくば市吾妻4丁目13番61号 (72)発明者 古川 保典 茨城県つくば市竹園一丁目801番602号 (72)発明者 竹川 俊二 茨城県つくば市吾妻二丁目11番地801棟403 号 (72)発明者 中村 優 茨城県つくば市吾妻1丁目18−1番地406 棟402号 Fターム(参考) 2K002 AB12 BA01 CA03 DA06 EA04 FA26 FA27 GA05 GA07 HA20Continuing from the front page (71) Applicant 500121621 Yasunori Furukawa 4-17-17 Nishi, Kamishiba-cho, Fukaya-shi, Saitama (71) Applicant 500121643 Shunji Takekawa 2-1-111, Azuma, Tsukuba-shi, Ibaraki 801-403 (71) Application Person 500583151 Yu Nakamura 1-18-1 Azuma, Tsukuba-shi, Ibaraki Pref. 406 Building 402 (72) Inventor Kiminori Mizuuchi 1006 Ojidoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (72) Inventor Kazuhisa Yamamoto Osaka 1006 Kadoma, Kadoma City Matsushita Electric Industrial Co., Ltd. (72) Inventor Kenji Kitamura 4-13-61, Azuma, Tsukuba-shi, Ibaraki Prefecture ) Inventor Shunji Takegawa 2-Chome, 11-11, Azuma, Tsukuba-shi, Ibaraki Pref. EA04 FA26 FA27 GA05 GA07 HA20
Claims (11)
率が49.5〜50.2%のストイキオメトリックLi
NbO3結晶と、前記結晶表面に形成された分極反転部
分とを有し、前記分極反転部分が周期状に配置されてい
る光波長変換素子であって、 前記分極反転部分の少なくとも一部が連続していること
を特徴とする光波長変換素子。1. A stoichiometric Li having a molar fraction of Li 2 O / (Nb 2 O 5 + Li 2 O) of 49.5 to 50.2%.
An optical wavelength conversion element having an NbO 3 crystal and a domain-inverted portion formed on the crystal surface, wherein the domain-inverted portion is periodically arranged, and at least a part of the domain-inverted portion is continuous. An optical wavelength conversion element, comprising:
に存在する請求項1に記載の光波長変換素子。2. The optical wavelength conversion device according to claim 1, wherein the continuous portion exists at a bottom of the domain-inverted portion.
0.3〜10°の範囲で傾いている請求項1または2に
記載の光波長変換素子。3. The crystal according to claim 1, wherein the C axis of the crystal is aligned with the crystal surface.
3. The light wavelength conversion device according to claim 1, wherein the light wavelength conversion device is inclined in a range of 0.3 to 10 degrees.
晶のキュリー温度が、1185〜1205℃の範囲であ
る請求項1〜3のいずれかに記載の光波長変換素子。4. The light wavelength conversion device according to claim 1, wherein the Curie temperature of the stoichiometric LiNbO 3 crystal is in the range of 1185 to 1205 ° C.
れかの添加物を0.03重量%以上含有する請求項1〜
4のいずれかに記載の光波長変換素子。5. The crystal according to claim 1, wherein said crystal contains at least 0.03% by weight of an additive of Mg, Zn, Sc or In.
5. The optical wavelength conversion element according to any one of 4.
ある請求項1〜5のいずれかに記載の光波長変換素子。6. The optical wavelength conversion element according to claim 1, wherein a period of said domain inversion section is 3.5 μm or less.
面から見て前記周期状分極反転部分の厚さの5〜200
%の範囲である請求項1〜6のいずれかに記載の光波長
変換素子。7. A continuous portion of said periodically poled portion has a thickness of 5 to 200 times the thickness of said periodically poled portion as viewed from the cross section.
%. The optical wavelength conversion device according to claim 1, wherein the ratio is in the range of%.
率が49.5〜50.2%のストイキオメトリックLi
NbO3結晶の一方の面に櫛形電極を形成し、 前記結晶の他方の面に平面電極を形成し、 前記櫛形電極の一部を前記結晶表面に絶縁膜を介して接
触させて連続させ、 前記電極間に電界を印加して周期状の分極反転構造を形
成する光波長変換素子の製造方法。8. A stoichiometric Li having a molar fraction of Li 2 O / (Nb 2 O 5 + Li 2 O) of 49.5 to 50.2%.
Forming a comb-shaped electrode on one surface of the NbO 3 crystal, forming a planar electrode on the other surface of the crystal, and bringing a part of the comb-shaped electrode into contact with the crystal surface via an insulating film so as to be continuous; A method for manufacturing an optical wavelength conversion element in which an electric field is applied between electrodes to form a periodically poled structure.
構造からなる歯の部分と、前記歯の部分を互いにつなぐ
ストライプ部分から構成され、前記ストライプ部分が前
記結晶表面と絶縁膜を介して接触させて連続させている
請求項7に記載の光波長変換素子の製造方法。9. The comb-shaped electrode includes teeth having a periodic grating structure and stripes connecting the teeth to each other, and the stripes are brought into contact with the crystal surface via an insulating film. The method for manufacturing an optical wavelength conversion element according to claim 7, wherein the light wavelength conversion element is continuous.
0.3〜10°の範囲で傾いている請求項8または9に
記載の光波長変換素子の製造方法。10. The crystal according to claim 1, wherein a C axis of said crystal is aligned with said crystal surface.
The method for manufacturing a light wavelength conversion element according to claim 8 or 9, wherein the light wavelength conversion element is inclined in a range of 0.3 to 10 °.
断面から見て前記周期状分極反転部分の厚さの5〜20
0%の範囲である請求項8〜10のいずれかに記載の光
波長変換素子の製造方法。11. A continuous portion of said periodic domain-inverted portion,
5-20 of the thickness of the periodically poled portion as viewed from the cross section
The method for producing an optical wavelength conversion element according to any one of claims 8 to 10, wherein the range is 0%.
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