JPH09211404A - Optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the resonance of vibration and to obviate the occurrence of the sensitivity change of electric field intensity by a frequency change by forming an optical waveguide forming surface and its opposite surface so as to have an angle above a specific value with the optical axis as the axis of rotation. SOLUTION: An optical waveguide 3 and an electrode 3 are formed on the X surface A of an X-cut lithium niobate substrate 1. The counter surface B of the X surface A is so polished as to have >=35 angle θ of inclination around the optical axis as the axis of rotation with respect to the X surface A. When an AC electric field is impressed on such optical waveguide element, vibration is generated by the displacement of the substrate 1 between the electrodes and propagates as transverse waves in the thickness direction of the substrate. The transverse waves are reflected at 2θ angle with the counter surface B. In the case of ordinarily used 500μm thickness and 10μm electrode width, progressive waves and reflected waves are separated on the A surface A at θ:35' or larger angle and, therefore, both waves do not exert interaction on each other any more. The resonance state of the progressive waves and the reflected waves at an arbitrary frequency do not arise any more.

Description

【発明の詳細な説明】Detailed Description of the Invention

【０００１】[0001]

【発明の属する技術分野】本発明は、光変調器に用いら
れる光導波路素子の構造に関する。TECHNICAL FIELD The present invention relates to a structure of an optical waveguide device used in an optical modulator.

【０００２】[0002]

【従来の技術】光変調器に用いられる光導波路素子は、
図３に示すように、平行平面に加工されたＸカットニオ
ブ酸リチウム基板２１の一方の面（Ｘ面）Ｃ上にチタン
薄膜による光導波路パターンを形成した後、拡散処理し
て光導波路２２を形成し、この光導波路の両側の近傍に
電極薄膜２３を形成することにより製作される。この光
導波路素子は、電極間に印加される電界で基板材料がピ
エゾ効果による変位によって屈折率変化を生ずることに
より、導波路を通過する光を変調する。一般に、この光
導波路素子の製作には、Ｘ面Ｃを鏡面に、Ｘ面Ｃの対向
面Ｄを鏡面または＃２０００程度の平滑研磨面に仕上げ
た基板材料が用いられ、これらが光導波路素子の外形の
うちの上・下２面を形成していた。2. Description of the Related Art An optical waveguide device used in an optical modulator is
As shown in FIG. 3, after forming an optical waveguide pattern of a titanium thin film on one surface (X surface) C of an X-cut lithium niobate substrate 21 processed into a parallel plane, diffusion processing is performed to form an optical waveguide 22. It is manufactured by forming the electrode thin film 23 on both sides of the optical waveguide. This optical waveguide element modulates the light passing through the waveguide by causing the refractive index change due to the displacement of the substrate material due to the piezoelectric effect by the electric field applied between the electrodes. In general, for manufacturing this optical waveguide device, a substrate material in which the X surface C is a mirror surface and the opposite surface D of the X surface C is a mirror surface or a smooth polished surface of about # 2000 is used. The upper and lower two surfaces of the outer shape were formed.

【０００３】[0003]

【発明が解決しようとする課題】このような光導波路素
子を用いて交流電界で光を変調する場合、基板材料の変
位は、基板の面内振動を生ずる。この面内振動は、Ｘ軸
方向、即ち、基板の厚み方向に横波となって伝播する。
この現象では、光導波路素子の導波路形成面であるＸ面
Ｃおよびその対向面Ｄが平滑である場合、基板厚みが振
動の二分の一波長の整数倍となる周波数において、振動
の進行波と対向面からの反射波（図中↓↑で示してい
る）が作用しあい、共振状態となる。このため、従来の
平行かつ平坦な面で構成された光導波路素子では、使用
する周波数帯域に上記の共振周波数が含まれていた場
合、共振により電界強度に対する感度変化を生じてい
た。一例として、厚さ５００μｍのＸカットニオブ酸リ
チウムを基板として、電極幅１０μｍで製作した光導波
路素子について、１７０ＭＨｚから２３０ＭＨｚまで周
波数を変化させた時の一定強度の電界に対する感度特性
を測定したところ、約５ＭＨｚ間隔で急激な感度変化が
観測された。この周波数特性は、ニオブ酸リチウムを振
動子として設計する時に用いられる、厚み−共振周波数
定数２.４０ＧＨｚ・μｍから、厚さ５００μｍに対し
求められる共振周波数４.８ＭＨｚと約一致し、共振現
象によることが確認された。また、従来の光導波路素子
のＸ面Ｃの対向面Ｄの仕上げ粗さは、＃２０００の研磨
面で、算術平均粗さ（Ｒａ）で０.７μｍであり、この
粗さでは、進行波を散乱するには有効でないことも確認
された。When light is modulated by an alternating electric field using such an optical waveguide device, displacement of the substrate material causes in-plane vibration of the substrate. The in-plane vibration propagates as a transverse wave in the X-axis direction, that is, the thickness direction of the substrate.
In this phenomenon, when the X-plane C, which is the waveguide forming surface of the optical waveguide element, and the opposing surface D are smooth, a traveling wave of vibration is generated at a frequency at which the substrate thickness is an integral multiple of a half wavelength of vibration. Reflected waves from the opposing surfaces (shown by ↓ ↑ in the figure) act on each other, resulting in a resonance state. Therefore, in the conventional optical waveguide element constituted by parallel and flat surfaces, when the above-mentioned resonance frequency is included in the frequency band to be used, the resonance causes a change in sensitivity to the electric field strength. As an example, when an optical waveguide element manufactured by using an X-cut lithium niobate having a thickness of 500 μm as a substrate and having an electrode width of 10 μm, the sensitivity characteristic to an electric field of constant strength when the frequency was changed from 170 MHz to 230 MHz was measured, A rapid change in sensitivity was observed at intervals of about 5 MHz. This frequency characteristic is approximately equal to the resonance frequency of 4.8 MHz required for the thickness of 500 μm from the thickness-resonance frequency constant of 2.40 GHz · μm, which is used when designing lithium niobate as a vibrator, and it depends on the resonance phenomenon. It was confirmed. Further, the finish roughness of the facing surface D of the X surface C of the conventional optical waveguide device is 0.7 μm in terms of arithmetic average roughness (Ra) on the # 2000 polished surface, and with this roughness, traveling waves It was also confirmed that it was not effective for scattering.

【０００４】この周波数による感度変化は、例えば、光
変調器を電界強度測定に利用しようとする場合に有害な
誤差の発生源となる。The change in sensitivity due to the frequency becomes a source of harmful errors when the optical modulator is used for measuring the electric field strength.

【０００５】それゆえに、本発明の目的は、振動の共振
を無くし、周波数変化による電界強度の感度変化を生じ
ないようにした光導波路素子を提供することにある。Therefore, it is an object of the present invention to provide an optical waveguide device which eliminates resonance of vibration and prevents the sensitivity change of the electric field strength due to the frequency change.

【０００６】[0006]

【課題を解決するための手段】本発明によれば、Ｘカッ
トニオブ酸リチウム基板上にチタン薄膜による光導波路
パターンを形成した後、拡散処理して光導波路を形成
し、光導波路の両側の近傍に電極薄膜を形成することに
より構成されるＹ伝播またはＺ伝播光導波路素子におい
て、光導波路形成面とその対向面が光軸を回転軸として
３５分以上の傾斜角を有することを特徴とする光導波路
素子が得られる。According to the present invention, after forming an optical waveguide pattern made of a titanium thin film on an X-cut lithium niobate substrate, diffusion processing is performed to form the optical waveguide, and the vicinity of both sides of the optical waveguide is formed. In a Y-propagation or Z-propagation optical waveguide device formed by forming an electrode thin film on the optical waveguide, the optical waveguide formation surface and the opposing surface have an inclination angle of 35 minutes or more with the optical axis as a rotation axis. A waveguide element is obtained.

【０００７】又、本発明によれば、光導波路形成面の対
向面の面粗さＲａが４μｍ以上であることを特徴とする
光導波路素子が得られる。Further, according to the present invention, there can be obtained an optical waveguide element characterized in that the surface roughness Ra of the surface opposite to the optical waveguide forming surface is 4 μm or more.

【０００８】図１に、本発明による光導波路素子を光軸
方向（光の入射方向）から見た形状を示す。Ｘカットニ
オブ酸リチウム基板１のＸ面Ａ上に光導波路２および電
極３が形成され、Ｘ面Ａの対向面Ｂは、Ｘ面Ａに対し光
軸を回転軸とした傾斜角θを持つように研磨加工されて
いる。この光導波路素子に交流電界を印加した時、電極
間の基板の変位により振動が発生し、基板厚み方向に横
波となって伝播する。この横波は、対向面Ｂにおいて、
２θの角度で反射する。通常、用いられる厚み５００μ
ｍ、電極幅１０μｍの場合、θ＝３５分、又はそれ以上
の角度において、進行波と反射波は、Ｘ面Ａ上で分離さ
れるため、相互作用を及ぼすことがなくなる。このた
め、任意の周波数で進行波と反射波の共振状態を生じる
ことがない。即ち、周波数変化による電界強度の感度変
化のない光導波路素子を得ることができる。FIG. 1 shows a shape of an optical waveguide device according to the present invention viewed from the optical axis direction (light incident direction). The optical waveguide 2 and the electrode 3 are formed on the X plane A of the X-cut lithium niobate substrate 1, and the facing surface B of the X plane A has an inclination angle θ with respect to the X plane A with the optical axis as the rotation axis. It has been polished. When an AC electric field is applied to this optical waveguide element, vibration occurs due to the displacement of the substrate between the electrodes and propagates as a transverse wave in the substrate thickness direction. This transverse wave is
It reflects at an angle of 2θ. Normally used thickness 500μ
In the case of m and the electrode width of 10 μm, the traveling wave and the reflected wave are separated on the X plane A at an angle of θ = 35 minutes or more, so that they do not interact with each other. Therefore, the resonant state of the traveling wave and the reflected wave does not occur at an arbitrary frequency. That is, it is possible to obtain an optical waveguide device in which the sensitivity of the electric field strength does not change due to the frequency change.

【０００９】図２に、本発明による光導波路素子を光軸
方向から見た形状の他の例を示す。ニオブ酸リチウム基
板１１のＸ面Ａ上に光導波路１２および電極１３が形成
され、Ｘ面Ａの対向面Ｂは、粒度が＃１５０〜＃４００
の砥材で研磨加工されている。この光導波路素子に交流
電界を印加した時、電極間の基板の変位により振動が発
生し、基板厚み方向に横波となって伝播する。この横波
は、対向面Ｂにおいて乱反射することから、任意の周波
数で進行波と反射波の共振状態を生じることがない。な
お、図中、矢印で横波の進行波と反射波を模式的に示し
ている。即ち、周波数変化による電界強度の感度変化の
ない光導波路素子を得ることができる。FIG. 2 shows another example of the shape of the optical waveguide device according to the present invention viewed from the optical axis direction. The optical waveguide 12 and the electrode 13 are formed on the X surface A of the lithium niobate substrate 11, and the facing surface B of the X surface A has a grain size of # 150 to # 400.
It is polished with the abrasive material. When an AC electric field is applied to this optical waveguide element, vibration occurs due to the displacement of the substrate between the electrodes and propagates as a transverse wave in the substrate thickness direction. Since this transverse wave is diffusely reflected on the facing surface B, the traveling wave and the reflected wave do not resonate at an arbitrary frequency. In addition, in the figure, the traveling wave and the reflected wave of the transverse wave are schematically shown by arrows. That is, it is possible to obtain an optical waveguide device in which the sensitivity of the electric field strength does not change due to the frequency change.

【００１０】[0010]

【発明の実施の形態】以下に、実施例により本発明の具
体的な実施の形態を詳細に示す。BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, specific embodiments of the present invention will be described in detail with reference to Examples.

【００１１】（実施例１）図１に示すように、光導波路
素子を厚さ５００μｍのＸカットニオブ酸リチウム基板
を用い、電極間幅１０μｍ，Ｘ面Ａに対する対向面Ｂの
光軸方向（図面に垂直方向）を回転軸とした傾斜角度θ
を１°として製作した。この光導波路素子の電界検出感
度を９０ＭＨｚから８００ＭＨｚまで測定した結果、有
害な感度変化は観測されなかった。(Example 1) As shown in FIG. 1, an optical waveguide element was used with an X-cut lithium niobate substrate having a thickness of 500 μm, an interelectrode width of 10 μm, and an optical axis direction of a facing surface B with respect to an X surface A (drawing: Angle perpendicular to the axis of rotation)
Was manufactured with 1 °. As a result of measuring the electric field detection sensitivity of this optical waveguide device from 90 MHz to 800 MHz, no harmful sensitivity change was observed.

【００１２】なお、傾斜角θについては、実施例の角度
に限らず、Ｘ面Ａ上で進行波と反射波を分離できる３５
分以上の任意の角度でよい。The inclination angle θ is not limited to the angle of the embodiment, and the traveling wave and the reflected wave can be separated on the X plane A 35.
Any angle of minutes or more may be used.

【００１３】（実施例２）図２に示すように、光導波路
素子を、厚さ５００μｍのＸカットニオブ酸リチウム基
板を用い、電極間幅１０μｍ，Ｘ面Ａの対向面Ｂを粒度
が＃１５０および＃４００の砥材を用いて研磨して製作
した。＃４００の粒度の砥材を用いた研磨による仕上げ
粗さの実測値は、Ｒａで４μｍであった。(Example 2) As shown in FIG. 2, an optical waveguide element was used with an X-cut lithium niobate substrate having a thickness of 500 μm, the interelectrode width was 10 μm, and the opposite surface B of the X surface A had a grain size of # 150. And it manufactured by polishing using the abrasive material of # 400. An actual measurement value of finish roughness by polishing using an abrasive having a grain size of # 400 was 4 μm in Ra.

【００１４】これらの光導波路素子の電界検出感度を９
０ＭＨｚから８００ＭＨｚまで測定した結果、有害な感
度変化は観測されなかった。The electric field detection sensitivity of these optical waveguide devices is 9
As a result of measuring from 0 MHz to 800 MHz, no harmful sensitivity change was observed.

【００１５】以上の結果から、粒度が＃１５０〜＃４０
０の砥材で研磨加工を行ってＲａを４μｍ以上の面粗さ
としたＸ面Ａの対向面Ｂは、Ｘ面Ａからの進行波を有効
に散乱することから、有害な周波数特性を解消する形状
効果を有する構造の光導波路素子が得られることが確認
された。From the above results, the grain size is # 150 to # 40.
The opposite surface B of the X surface A, which has been polished with an abrasive material of 0 and has a surface roughness Ra of 4 μm or more, effectively scatters the traveling wave from the X surface A, thereby eliminating harmful frequency characteristics. It was confirmed that an optical waveguide device having a structure having a shape effect was obtained.

【００１６】[0016]

【発明の効果】このように、本発明によれば、従来の光
導波路素子に比べ、基板の厚み方向に伝播する波による
共振を防ぎ、周波数変化による電界強度の感度変化がな
い、高品質の光導波路素子を容易に得ることができる。As described above, according to the present invention, as compared with the conventional optical waveguide device, resonance due to the wave propagating in the thickness direction of the substrate is prevented, and the sensitivity of the electric field strength does not change due to the frequency change, and it is of high quality. The optical waveguide device can be easily obtained.

【図面の簡単な説明】[Brief description of drawings]

【図１】本発明の光導波路素子を光軸方向から見た図。FIG. 1 is a view of an optical waveguide device of the present invention viewed from the optical axis direction.

【図２】本発明の光導波路素子を光軸方向から見た任意
の位置の断面図。なお、図中の矢印は基板の変位によっ
て生じた振動の進行波と反射波を示す。FIG. 2 is a cross-sectional view of the optical waveguide device of the present invention at an arbitrary position when viewed from the optical axis direction. The arrows in the figure indicate the traveling wave and the reflected wave of the vibration generated by the displacement of the substrate.

【図３】従来の光導波路素子を光軸方向から見た任意の
位置の断面図。なお、矢印は基板の変位によって生じた
振動の進行波と反射波を示す。FIG. 3 is a cross-sectional view of a conventional optical waveguide device viewed from the optical axis direction at an arbitrary position. The arrows indicate the traveling wave and the reflected wave of the vibration generated by the displacement of the substrate.

【符号の説明】[Explanation of symbols]

１，１１，２１ ニオブ酸リチウム基板 ２，１２，２２ 光導波路 ３，１３，２３ 電極 Ａ，Ｃ Ｘ面 Ｂ，Ｄ Ｘ面の対向面 θ （Ｘ面Ａに対する対向面Ｂの）傾斜角 1,11,21 Lithium niobate substrate 2,12,22 Optical waveguide 3,13,23 Electrodes A, C X plane B, DX Opposed surface θ (angle of opposed surface B to X surface A)

Claims (2)

【特許請求の範囲】[Claims] 【請求項１】 Ｘカットニオブ酸リチウム基板上にチタ
ン薄膜による光導波路パターンを形成した後、拡散処理
して光導波路を形成し、前記光導波路の両側の近傍に電
極薄膜を形成することにより構成される光導波路素子に
おいて、光導波路形成面とその対向面が光軸を回転軸と
して３５分以上の角度を有することを特徴とする光導波
路素子。1. A structure comprising forming an optical waveguide pattern of a titanium thin film on an X-cut lithium niobate substrate, diffusing treatment to form the optical waveguide, and forming electrode thin films near both sides of the optical waveguide. In the optical waveguide element described above, the optical waveguide forming surface and the opposing surface have an angle of 35 minutes or more with the optical axis as a rotation axis. 【請求項２】 Ｘカットニオブ酸リチウム基板上にチタ
ン薄膜による光導波路パターンを形成した後、拡散処理
して光導波路を形成し、前記光導波路の両側の近傍に電
極薄膜を形成することにより構成される光導波路素子に
おいて、前記光導波路を形成した面の対向面の面粗さＲ
ａが４μｍ以上であることを特徴とする光導波路素子。2. An X-cut lithium niobate substrate on which an optical waveguide pattern made of a titanium thin film is formed, diffusion treatment is performed to form the optical waveguide, and electrode thin films are formed near both sides of the optical waveguide. In the optical waveguide element, the surface roughness R of the surface opposite to the surface on which the optical waveguide is formed is
An optical waveguide element, wherein a is 4 μm or more.