JP2004062158A - Optical modulator and communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator of high efficiency which can be mounted on an optical communication system and the like. <P>SOLUTION: The optical modulator is provided with an optical waveguide 12 at least a part of which is formed by using a material having an electrooptic effect, an modulation electrode 13 having first and second conductor lines 13a and 13b electromagnetically bonded to each other and applying a modulation electric field to the optical waveguide 12, a conductive layer 14 forming a first micro strip line together with the first conductor line 13a and a second micro strip line together with the second conductor line 13b, an electric signal inputting section 15 supplying a high frequency signal for optical modulation to the modulation electrode and connection members 16a and 16b interconnecting the first and the second conductor lines 13a and 13b at both ends. The first and the second conductor lines 13a and 13b function as a resonator of an odd mode of the high frequency signal. <P>COPYRIGHT: (C)2004,JPO

Description

【０１０６】
図１６（ｂ）は、表１に示すモード２で共振を生じさせた場合の分岐光導波路３２ｂ、３２ｂ付近における電界の方向を示している。図１６（ｂ）からわかるように、２本の分岐光導波路３２ａ、３２ｂには上下逆方向の電界が印加されるので、光波に位相差が生じ出口側光導波路３２ｙで干渉を生じるので、本実施形態の光変調素子は、光強度変調器として機能する。
【０１０７】
一方、図１６（ｃ）は、表１に示すモード３で共振を生じさせた場合の分岐光導波路３２ｂ、３２ｂ付近における電界の方向を示している。モード３で共振させる場合は、分岐光導波路３２ｂ、３２ｂと、３つの線路３３ａ、３３ｂ、３３ｃとの配置関係が、図１６（ａ）および（ｂ）に示す配置関係とは少し異なっている。より具体的には、分岐光導波路３２ｂ、３２ｂに形成される電界の向きが相互に反対になるように、分岐光導波路３２ｂの位置がシフトしている。
【０１０８】
このように、本実施形態の構成によれば、線路３３ａ、３３ｂ、３３ｃの全てが常に同電位になるモード１では、光強度変調器としては機能しない。このため、本実施形態の光変調素子は、モード１以外のモード２または３で共振するように設計される。
【０１０９】
ここで、２つの分岐光導波路３２ａ、３２ｂの間隔は、光の相互干渉を回避するためにそれほど狭くできないが、間隙部３７ａ、３７ｂの幅は、線路３３ｃが設けられているために、第１、第２の実施形態に比べ、遙かに狭くなる。したがって、間隙部３７ａ、３７ｂには非常に大きな強度の電界が生じる。よって、本実施形態の光変調素子により、高い変調効率が得られる。
【０１１０】
なお、本実施形態においては、両端が接続された半波長共振器として機能する平行結合線路を有する例について説明したが、本発明はかかる実施形態に限定されるものではない。例えば、第２の実施形態のように、両端が開放された平行結合線路や、従来例の図３に示す一端が接続され他端が開放された１／４波長型共振器として機能する平行結合線路を有する光変調素子によっても、本実施形態の基本的な効果を発揮することは可能である。
【０１１１】
また、本実施形態の光変調素子においても、第２の実施形態と同様に、分極反転領域を光導波路の一部に設けても良い。基板に残留分極の極性が相異なる２つの領域を設けることにより、第２の実施形態の効果と第３の実施形態の効果を併せて発揮ざせることができる。
【０１１２】
（実施形態４）
次に、図１７（ａ）から１７（ｃ）を参照しながら、本発明による光変調素子の第４の実施形態を説明する。図１７（ａ）は、本実施形態の光変調素子の平面構成を示す上面図であり、図１７（ｂ）は、図１７（ａ）のＡ０−Ａ１線断面図である。図１７（ｃ）は、本実施形態の一部を拡大して示す斜視図である。
【０１１３】
本実施形態の光変調素子は、図１７（ａ）および（ｂ）に示されるように、同軸コネクタ２０９が取り付けられた第１の基板固定用冶具２１２ａと、基板１１が固定された第２の基板固定用冶具２１２ｂとを備えており、同軸コネクタの中心線２１０を入力線路１５に対して適切に配置するようにして、第１の冶具２１２ａを第２の冶具２１２ａに固定する。第１の冶具２１２ａは、例えばネジなどによって第２の冶具２１２ｂに固定される。
【０１１４】
同軸コネクタの中心線２１０の先端部分には、中間接続部材２１１が取り付けられており、この中間接続部材２１１を介して、中心線２１０は入力線路１５に接続される。このように本実施形態の光変調素子では、その電気信号入力部が入力線路１５とは別に同軸コネクタ２０９および中間接続部材２１１を有している点で前述の実施形態とは異なっている。
【０１１５】
本実施形態では、変調用高周波信号を生成する外部駆動回路と、光変調素子の同軸コネクタとは、例えば同軸ケーブルによって接続される。同軸ケーブルを伝搬してきた高周波信号は、中間接続部材２１１を介して、入力線路１５に与えられる。
【０１１６】
中間接続部材２１１は、図１７（ｃ）に示されるように、同軸コネクタの中心導体２１０の外周と接するように曲げられた部分を有する第１接続部２１４と、入力線路１５に接触する平面状部分を有する第２接続部２１５とを備えている。
【０１１７】
第１接続部２１４と第２接続部２１５とを連結する部分は、弾性部材から形成されていることが好ましい。このような弾性部材を用いることにより、第２接続部材２１５を第１接続部材２１４に対して下方に付勢することができる。このため、図１７（ｂ）に示すように、第２接続部２１５の底面が入力線路１５の上面を押圧し、第２接続部２１５と入力線路１５との間の電気的接触が確保しやすくなる。このような構成を採用すると、第２接続部２１５と入力線路１５との間に、導電性の接着剤を塗布する必要がなく、接続が容易になる。なお、第１接続部２１４と第２接続部２１５とは、一枚の板状導電体から形成されることが好ましい。
【０１１８】
以上の構成を採用することにより、光コネクタ２０９を介して入力線路１５に与えられた高周波信号は、平行結合線路１３に伝搬し、線路１３ａと線路１３ｂとの間で共振を生じさせる。この結果、間隙部１７に高周波の振動電界が生じるため、電気光学効果を有する材料から形成されている光導波路１２の屈折率は高周波信号に応じて変化する。このとき、線路１３ａ及び線路１３ｂの下を通る光導波路には、図１７（ｂ）に示すように、互いに上下逆方向の電界が形成されるため、各分岐光導波路を伝搬する光に位相差が形成される。
【０１１９】
図１８は、本実施形態の構成を有する光変調素子の入力線路１５の端部Ｐにおける反射特性を示している。図１８からわかるように、変調電極の共振周波数は、２６ＧＨｚであり、共振周波数において入力インピーダンスが整合している。図１９は、入力線路１５の周波数に対する透過損出の関係を示すグラフである。
【０１２０】
（第５実施形態）
次に、本発明による光変調素子の第５の実施形態を説明する。
【０１２１】
本実施形態の光変調素子は、第４の実施形態における光変調素子とほぼ同様の構成を有しているが、基板１１上に入力線路を設けられていない点で、第４の実施形態における光変調素子とは異なっている。
【０１２２】
図２０（ａ）は、本実施形態の光変調素子を示す平面面図であり、図２０（ｂ）は、そのＢ０−Ｂ１線断面図である。
【０１２３】
本実施形態の光変調素子の電気信号入力部は、実施形態５と同様に、同軸コネクタ９コネクタ２０９および中間接続部材２１１を有しているが、この中間接続部材２１１が変調電極１３に入力線路を介することなく直接的に接続されている。
【０１２４】
光変調素子を動作させるためには、変調用の高周波信号を平行結合線路１３に供給することによって平行結合線路３に奇対称モードによる共振を効率よく引き起こす必要がある。このことは、中間接続部材２１１を入力インピーダンス整合のとれる位置に接続することにより、実現することができる。
【０１２５】
図２１は、基板１１上の平行結合線路１３と同軸コネクタの中心導体２１０との接続状態を示している。同軸コネクタの中心導体２１０および中間接続部材２１１は、図１７（ｃ）に示す構成を有している。前述のように、第１接続部２１４と第２接続部２１５とを連結する部分は、湾曲した弾性部材から形成されており、第１接続部材２１４に対して第２接続部材２１５を下方に付勢している。このため、第２接続部２１５の底面が平行結合線路１３の上面を押圧し、第２接続部２１５と平行結合線路１３との間の電気的接触が確保される。第２接続部２１５と平行結合線路１３との間に、導電性を有する接着剤を塗布する必要がなく、接続が容易である。
【０１２６】
平行結合線路１３の形成された基板１１は、基板固定用冶具２１２ｂに固定されており、中間接続部材２１１及び同軸コネクタ２０９は、基板固定冶具２１２ａに固定されている。基板固定用冶具２１２ａを基板固定冶具２１２ｂに対してスライドさせることにより、平行結合線路１３と中間接続部材２１１とを適切な位置で接触させることができる。基板固定用冶具２１２ａは、ネジなどによって基板固定用冶具２１２ｂに固定される。
【０１２７】
平行結合線路１３に中間接続部材２１１を接触させるだけで、変調用高周波信号の入力が可能となるため、平行結合線路１３における入力反射特性を測定した後に、平行結合線路１３に対する中間接続部材２１１の接続位置を調節することが簡単に行える。この接続位置の調節は、冶具２１２ａと冶具２１２ｂのねじ止め位置を微調整することによって行うことができる。
【０１２８】
次に、本実施形態の光変調素子について、電磁界シミュレーションによる解析を行った結果を説明する。この具体例では、基板１１として、厚さ０．４００ｍｍのｚカットタンタル酸リチウム結晶（比誘電率４２）を用い、平行結合線路１３の線路幅を０．０５ｍｍ、間隙部の幅を０．０２ｍｍに設定した。
【０１２９】
各線路の材料は、アルミニウムとし、線路を構成するアルミニウム膜の厚さは１μｍに設定した。中間接続部材は、厚さ０．０１ｍｍ、幅０．１５ｍｍの金メッキ処理を施したベリリウム銅とした。
【０１３０】
平行結合線路１３の長さと中間接続部材２１１との接続位置は、入力インピーダンスが整合するように設定した。具体的には、電磁界シミュレーターを用いて、奇対称モードが２６ＧＨｚで共振し、かつその周波数で、同軸コネクタに入力した信号の反射が最も少なくなるように決定した。
【０１３１】
その結果、図２２に示すように、平行結合線路１３の長さは１．２０ｍｍ、中間接続部材の接続位置は平行結合線路１３の中央から０．２５ｍｍとした。また、基板固定用冶具２１２ａから平行結合線路１３までの距離は０．３０ｍｍとした。
【０１３２】
図２３は、上記構成を有する光変調素子における中間接続部材２１１と平行結合線路１３との接続部での反射特性を示す。図２３からわかるように、共振点で入力信号の反射はなくなり、ほとんど全ての信号電力が平行結合線路１３に入力されている。また、入力線路を設けない場合においても共振特性に影響がないことがわかる。
【０１３３】
本実施形態の構成によれば、入力線路における透過損失をなくすことができる。
【０１３４】
実施形態５の光変調素子では、入力線路１５の存在により、２６ＧＨｚの周波数で０．５ｄＢの透過損出が生じたが、本実施形態の構成によれば、このような入力線路による伝送損失を発生させることなく、高周波信号の入力が可能となり、変調効率をさらに向上させることができる。このため、入力線路を設ける場合に比べて、光出力を０から最大にまで変化させるのに必要な電力を、０．５ｄＢ減少させることが可能となる。
【０１３５】
本実施形態の光変調素子を、図２２に示す設計値に従って作製した場合における作製直後における入力反射特性が、図２４の破線で示すような特性を示したとする。この場合、中間接続部材２１１と平行結合線路１３との接続位置を０．０５ｍｍだけ変調電極の中心に近づけるように移動させるだけで、図２４の実線で示す入力反射特性を得ることができる。すなわち、光変調素子の作製後に、製造プロセスのばらつきなどに起因して入力インピーダンスが設計値からシフトした場合でも、中間接続部材２１１と平行結合線路１３の接続位置を調整することにより、入力インピーダンスを整合させることが容易に実現できる。
【０１３６】
本実施形態によれば、入力線路を設けることなく、共振型電極に信号を入力し、変調効率を改善することができ、また、素子作成後であっても共振型電極の入力インピーダンスの整合を行うことができる。
【０１３７】
なお、本実施形態では、両端を短絡した半波長共振器構造を有する電極構成を採用しているが、本発明における変調電極は、このような構成を有するものに限定されず、共振器構造を有する変調電極であればよい。
【０１３８】
以上に説明してきた各実施形態では、何れも、電気光学効果を有する基板中に光導波路を形成しているが、本発明は、このような構成にも限定されない。基板の表面領域に周囲よりも屈性率の高いコア部を形成し、コア部の上にクラッド部として電気光学効果を有する材料からなる膜を形成する構成を採用してもよい。この場合、コア部を伝搬する光の一部がクラッド部に染み出すため、クラッド部の屈性率を変化させることにより、コア部を伝搬する光の位相を変調することができる。コア部は電気光学効果を有する材料から形成されている必要は無い。
【０１３９】
また、上記実施形態における光導波路は、分岐された少なくとも２つの分枝光導波路と、２つの分枝光導波路を結合する光入力部と、２つの分枝光導波路を結合する光出力部とを有するマッハツェンダ干渉計型の構成を有しているが、本発明の光変調素子は、このような構成を有する光強度変調素子に限定されない。本発明による光変調素子の光導波路が単一の光導波路を有する場合であっても、伝搬する光の位相を効率的に変調することができる。この意味では、本発明の光変調素子は、光の位相を変調する点に本質的な機能を有しており、位相の変調された光を干渉させることによって光強度をも変調させることが可能である。
【０１４０】
なお、第１、第２の実施形態の光変調素子は、光導波路を途中で２つに分岐させて分岐光導波路を有しているが、本発明は、このような分岐光導波路を有する光変調素子に限定されない。例えば、単一の光導波路のみを有する光変調素子に本発明を適用すると、位相変調器として機能する光変調素子が得られる。その場合でも、本発明によれば、線路間における電圧の符号（極性）が均一となり、光の位相変化が相殺されることがないので、光変調素子の光変調効率の向上を図ることができる。
【０１４１】
第１から第３の実施形態に係る光変調素子では、入力線路が平行結合線路の１つの線路に直接接続されているが、本発明は、そのような構成を有するものに限定されない。例えば、入力線路の先端を平行結合線路の１つの線路に間隙を介して対向させることによって、入力結合させることも可能である。特に、線路を構成する材料として超伝導材料を用いた場合など、線路の損失が小さく、共振の無負荷Ｑ値が高い場合などはこのような構成が、特に有効である。
【０１４２】
また、平行結合線路の各線路を接続している接続線路は、添付図面において部分円形状を有しているように描かれているが、接続線路は各線路同士を短い距離で接続できればよいので、多角形の一部を構成するなど、直線部分を有する形状であっても、光変調素子の特性に悪影響を及ぼすことはない。
【０１４３】
素子の基板は、タンタル酸リチウム結晶やニオブ酸リチウムなどの電気光学効果を有する材料以外の電気光学結晶から形成されたものであってもよい。光導波路は、電気光学結晶基板の表面に対して安息香酸中でプロトン交換処理を施す方法によって好適に形成されるが、光導波路は、他の方法で作製されてもよい。例えば、他の機能素子との集積化などのために、タンタル酸リチウム単結晶など以外の基板を利用する必要がある場合には、基板上に、基板よりも屈折率が高く、かつ、電気光学効果を有する材料からなる膜を形成し、その膜を光導波路として用いることもできる。また、基板の表面領域に周囲よりも屈折率の高いコア部を形成し、コア部の上に、クラッド部として電気光学効果を有する材料からなる膜を形成することにより、コア部からしみ出した電界を利用してクラッド部の屈折率変化によって光変調を行うことも同様に有効である。また、平行結合線路は、基板内に埋設されていてもよい。
【０１４４】
（実施形態６）
図２５は、本発明によるファイバ無線システムの構成を示すブロック構成図である。
【０１４５】
本実施形態のファイバ無線システム５０は、第１〜第３の実施形態の光変調素子を内蔵した光変復調器５１を備えている。そして、アンテナ５３により、通常のインターネット等のデータ通信網や、携帯端末との通信、あるいは、ＣＡＴＶからの信号の受信等を、例えばミリ波の搬送波を用いて直接行うことができる。なお、光変復調器５１には、光変調素子とともに光復調素子（例えばフォトダイオード）が内蔵されている。
【０１４６】
一方、ミリ波等の周波数の高い無線信号は長距離の伝送は困難であり、かつ、物体による信号の遮断を受けやすい。そこで、データ通信網６１や、ＣＡＴＶ６２や、携帯電話システム６３との通信を、無線装置６０及び無線装置に付設されたアンテナ６４を用いて行うこともできる。その場合、ファイバ無線通信システム５０と光ファイバ７０を介して接続される光変復調器５５と、これに付設されるアンテナ５４とをさらに備えておく。そして、アンテナ５４、６４及び光変復調器５５を介して、無線装置６０との間で、信号の授受を行うことができる。光変復調器５５には、光変調素子とともに光復調素子（例えばフォトダイオード）が内蔵されている。
【０１４７】
例えば長距離伝送を行いたい場合や、壁等で仕切られた屋内での伝送の際には、ミリ波等の無線信号で変調された光信号を光ファイバ７０を通して伝送することが効果的である。
【０１４８】
【発明の効果】
本発明の光変調素子の構成によれば、位相変調器又は光強度変調器として機能する光変調素子の光変調効率の向上を図ることができ、この光変調素子を通信システムに配置することにより、ミリ波レベルの高周波信号を利用した通信が可能になる。
【図面の簡単な説明】
【図１】（ａ）は、光の直接変調を説明するための図であり、（ｂ）は光の外部変調を説明するための図である。
【図２】電気光学効果を利用した光の外部変調の動作原理を示す断面図である。
【図３】光変調素子の従来例を示す平面図である。
【図４】（ａ）は、本実施形態に係る光変調素子の平面構成を示す平面図、（ｂ）は、その導波路を垂直に横切る断面図、（ｃ）は、この光変調素子の変調電極が形成する電界の強度分布を模式的に示す図である。
【図５】（ａ）および（ｂ）は、それぞれ、図４（ｂ）に示す断面における偶モード、および奇モードの電界（実線）及び磁界（破線）の状態を示す図である。
【図６】第１の実施形態において、電磁界シミュレーションに用いた平行結合線路及び入力線路の平面寸法及び接続位置を示す平面図である。
【図７】第１の実施形態において、電磁界シミュレーションによって得られた光変調素子の共振状態における反射損特性を示す図である。
【図８】（ａ）は、第１の実施形態における平行結合線路１３に入力する高周波信号の波形を示すグラフであり、（ｂ）は、本実施形態の光変調素子の出力光強度／入力光強度の比を示すグラフであり、（ｃ）は、比較例の出力光強度／入力光強度の比を示すグラフである。
【図９】（ａ）および（ｂ）は、それぞれ、１０ＧＨｚで共振する電極構造を有する光変調素子および２６ＧＨｚで共振する電極構造を有する光変調素子の実施例を示す平面図である。
【図１０】図９（ａ）に示す実施例の反射損特性を示すグラフである。
【図１１】図９（ａ）に示す実施例の光出力を時間変化を示すグラフである。
【図１２】図９（ａ）に示す実施例の変調光スペクトルを示すグラフである。
【図１３】図９（ｂ）に示す実施例における変調電極の反射特性（実測値）を示すグラフである。
【図１４】図９（ｂ）に示す実施例の変調光のスペクトルを示すグラフである。
【図１５】（ａ）は、本発明による光変調素子の第２の実施形態の平面構成を示す平面図、（ｂ）は、基板の残留分極の極性が逆の領域を示す平面図、（ｃ）は、平行結合線路における電界強度の分布を示す図である。
【図１６】（ａ）は、本発明による光変調素子の第３の実施形態の平面図、（ｂ）は、モード２で共振する場合の縦断面図であり、（ｃ）は、モード３で共振する場合の縦断面図である。
【図１７】（ａ）は、本発明による光変調素子の第４の実施形態を示す上面図であり、（ｂ）は、（ａ）のＡ０−Ａ１断面図であり、（ｃ）は同軸コネクタ中心導体２１０と中間接続部材２１１の接続を示す斜視図である。
【図１８】第４の実施形態における入力線路部の反射特性を示すグラフである。
【図１９】第４の実施形態における入力線路部の透過損失を示すグラフである。
【図２０】（ａ）は、本発明による光変調素子の第５の実施形態を示す上面図であり、（ｂ）は、（ａ）のＢ０−Ｂ１断面図である。
【図２１】第５の実施形態の一部を示す斜視図である。
【図２２】第５の実施形態における光変調素子の設計パラメータ値を示すレイアウト図である。
【図２３】図２２の光変調素子について行ったシミュレーションの結果を示すグラフである。
【図２４】図２２の光変調素子について行ったシミュレーションの結果を示すグラフである。
【図２５】本発明によるファイバ無線システムの実施形態の構成を示すブロック構成図である。
【符号の説明】
１１　　基板
１２　　光導波路
１２ａ分岐光導波路
１２ｂ　分岐光導波路
１２ｘ　入口側光導波路
１２ｙ　出口側光導波路
１３　　平行結合線路
１３ａ　線路
１３ｂ　線路
１４　　グランドプレーン
１５　　入力線路
１６　　接続線路
１７　　間隙部
１８ａ　分岐点
１８ｂ　分岐点
２０９　同軸コネクタ
２１０　同軸コネクタ中心導体
２１１　中間接続部材
２１２ａ、２１２ｂ　基板固定用冶具
２１３ａ、１３ｂ　光導波路の分岐部分
２１４　第１接続部
２１５　第２接続部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical modulation element and a communication system, and more particularly, to an optical modulation element and a communication system for transmitting a high-frequency signal of several GHz or more used for radio by light.
[0002]
[Prior art]
In a system that performs communication and information processing using an optical signal, it is necessary to modulate the phase and intensity of light with an electric signal (for example, a high-frequency signal such as a microwave or a millimeter wave). Such light modulation methods include direct modulation and external modulation.
[0003]
In the direct modulation, as shown in FIG. 1A, the intensity of the light output from the light source itself is modulated by directly modulating a current for driving a light source such as a semiconductor laser. Direct modulation does not require a modulator outside the light source, so it is suitable for miniaturization of the system. However, it is difficult to modulate light at a high frequency of several GHz or more. Ping limits long distance fiber transmission.
[0004]
On the other hand, in the external modulation, as shown in FIG. 1B, light output from a light source such as a semiconductor laser (light with stable output) is input to the light modulation element, and the light And modulate the intensity. The light can be modulated using an electro-optic effect, an acousto-optic effect, a magneto-optic effect, a non-linear optical effect, or the like.
[0005]
As described above, it is difficult to achieve ultra-high-speed optical modulation by a method of directly modulating a semiconductor laser, and therefore, development of an external modulation type element capable of high-speed operation is urgently required. Among the external modulation type devices, the electro-optic light modulation device using a dielectric crystal having the Pockels effect has the advantages that it can operate at an ultra-high speed and that the phase disturbance due to modulation is small. ing. For this reason, the electro-optic light modulator is very effective for high-speed information transmission, long-distance optical fiber communication, and the like. In addition, if an optical waveguide structure is manufactured using an electro-optic light modulation element, there is a possibility that downsizing and efficiency improvement of the element can be realized at once.
[0006]
In general, an electro-optic light modulation element includes a transmission line that propagates a modulation signal as a modulation electrode (signal electrode) on an electro-optic crystal, and an optical waveguide formed near the transmission line. Then, the refractive index of the optical waveguide portion is changed by the electric field induced around the modulation electrode, thereby changing the phase of the light wave propagating in the optical waveguide.
[0007]
An ordinary crystal used for an electro-optic light modulation element has a relatively small electro-optic coefficient. The electro-optic coefficient is a basic parameter of light modulation. Therefore, it is important for the electro-optic light modulation element to efficiently apply an electric field to the optical waveguide.
[0008]
FIG. 2 is a sectional view showing a basic structure of the electro-optic light modulation element. An optical waveguide is formed in a substrate surface region of a crystal having an electro-optical effect (electro-optical crystal), and a modulation electrode is formed on the optical waveguide.
[0009]
The electro-optic crystal has optical anisotropy, and the refractive index changes substantially in proportion to the magnitude of the applied electric field (Pockels effect). Therefore, by adjusting the potential V applied to the modulation electrode, the refractive index n of the optical waveguide can be changed. The amount of change Δn in the refractive index of the optical waveguide is proportional to the electric field E applied to the optical waveguide. When the refractive index of the optical waveguide changes by Δn, the phase of the output light changes by Δφ as shown in FIG. In general, the phase change amount Δφ is proportional to the product of the electric field strength E and the length L of the optical waveguide.
[0010]
Since a modulation signal for forming an electric field in the optical waveguide is given from the outside of the light modulation element to an electrode of the light modulation element via an input line, it is important to efficiently input the modulation signal.
[0011]
Next, a conventional light modulation element will be described in more detail with reference to FIG. FIG. 3 is a plan view of a conventional light modulation element. This light modulation element is disclosed in Patent Document 1.
[0012]
3 includes a substrate 101 made of a material having an electro-optic effect, and an optical waveguide 112 formed on the surface of the substrate 101. The optical waveguide 112 is formed by, for example, thermally diffusing a metal into a part of the substrate 101.
[0013]
On the surface of the substrate 101, parallel coupling lines 13 made of a metal film such as aluminum or gold are provided on both left and right sides of the optical waveguide 112, and a ground plane 114 made of a metal film is provided on the back surface of the substrate 101. Is provided. The parallel coupling line 113 includes two lines 113a and 113b that are parallel to each other.
[0014]
In this conventional example, the lines 113a and 113b of the parallel coupled line 113 are coupled to each other by a single line 124. However, the publication also discloses a structure in which the two lines 113a and 113b are not coupled. .
[0015]
An input terminal 129 connected via a tap 128 is provided on a part of the line 113b, and a high-frequency signal source 119 is connected between the input terminal 129 and the ground plane 114.
[0016]
The input light is introduced from one end of the optical waveguide 112, passes through a portion of the optical waveguide 112 that is located in the gap 116 between the lines 113 a and 113 b of the parallel coupling line 113, and then enters the other end of the optical waveguide 112. Output from the end as output light. At this time, since the input terminal 129 and the parallel coupling line 113 are magnetically coupled, when a high frequency signal is supplied from the high frequency signal source 119, the high frequency signal propagates to each of the lines 113a and 113b of the parallel coupling line 113. As a result, an electric field is generated in the gap 116. In accordance with the electric field intensity, the refractive index of the optical waveguide 112 changes due to the electro-optic effect. As a result, the phase of the output light changes, and the present light modulation element operates as a phase modulator.
[0017]
Here, two types of modes, an even mode and an odd mode, usually exist in the parallel coupling line. In the odd mode, the voltages of the two lines constituting the parallel coupling line are inverted with each other, so that a very large electric field is induced in the gap. In the optical modulation element shown in FIG. 3, high-efficiency optical modulation can be achieved by exciting an odd mode in each of the lines 113 a and 113 b of the parallel coupling line 113 with a modulation signal.
[0018]
[Patent Document 1]
US Patent No. 5,400,416
[0019]
[Problems to be solved by the invention]
In the future, in order for the light modulation device to be used for general purposes in an optical communication system or the like, there is room for improvement in the characteristics of the conventional light modulation device. That is, there is a demand for the development of a more efficient light modulation element.
[0020]
A main object of the present invention is to provide a highly efficient light modulation element that can be incorporated in an optical communication system or the like.
[0021]
[Means for Solving the Problems]
The light modulation element of the present invention has an optical waveguide at least partially formed of a material having an electro-optic effect, and first and second conductor lines electromagnetically coupled to each other, and applies a modulation electric field to the optical waveguide. A modulating electrode to be applied, a conductive layer forming a first microstrip line together with the first conductor line, and forming a second microstrip line together with the second conductor line; An electric signal input unit for supplying a high-frequency signal; and a connecting member for connecting the first and second conductor lines to each other at both ends, wherein the first and second conductor lines are provided for the high-frequency signal. Functions as an odd mode resonator.
[0022]
In a preferred embodiment, the optical waveguide has at least two branched optical waveguides, an optical input unit for coupling the two optical waveguides, and an optical output unit for coupling the two optical waveguides. The portion of the optical waveguide to which the modulation electric field is applied is divided into the two branch optical waveguides, and the modulation electrode has a different polarity with respect to each of the two branch optical waveguides. It is arranged so as to exert an electric field, and functions as an intensity modulator for modulating the intensity of light input to the optical waveguide.
[0023]
In a preferred embodiment, the modulation electrode is arranged to modulate a refractive index of a portion of the optical waveguide to which the modulation electric field is applied, and modulates a phase of light input to the optical waveguide. Functions as a modulator.
[0024]
In a preferred embodiment, the optical waveguide has at least two portions having different remanent polarizations.
[0025]
In a preferred embodiment, the optical waveguide is formed on a substrate having an electro-optic effect.
[0026]
In a preferred embodiment, the electric signal input unit has an input line forming a microstrip line together with the conductive layer, and the input line is connected to one of the first and second conductor lines. I have.
[0027]
In a preferred embodiment, the electric signal input section has a coaxial connector connected to a line for transmitting the high frequency signal for optical modulation, and an intermediate connection member for electrically connecting the coaxial connector and the modulation electrode. are doing.
[0028]
Another light modulation element of the present invention has an optical waveguide at least partially formed of a material having an electro-optic effect, and first and second conductor lines electromagnetically coupled to each other. A modulating electrode for applying a modulating electric field, a conductive layer forming a first microstrip line with the first conductor line, and forming a second microstrip line with the second conductor line; An electrical signal input unit for supplying a high-frequency signal for optical modulation, wherein the optical waveguide has at least two portions having different remanent polarizations, and the first and second conductor lines are provided with the high-frequency signal. Function as an odd-mode resonator.
[0029]
In a preferred embodiment, the optical waveguide has at least two branched optical waveguides, an optical input unit for coupling the two optical waveguides, and an optical output unit for coupling the two optical waveguides. The portion of the optical waveguide to which the modulation electric field is applied is divided into the two branch optical waveguides, and the first and second conductor lines are connected to each of the two branch optical waveguides. On the other hand, they are arranged so as to exert electric fields having different polarities, and function as an intensity modulator for modulating the intensity of light input to the optical waveguide.
[0030]
In a preferred embodiment, the modulation electrode is arranged to modulate a refractive index of a portion of the optical waveguide to which the modulation electric field is applied, and modulates a phase of light input to the optical waveguide. Functions as a modulator.
[0031]
In a preferred embodiment, the semiconductor device further includes a connecting member that connects the first and second conductor lines to each other at at least one end.
[0032]
In a preferred embodiment, the optical waveguide is formed on a substrate having an electro-optic effect.
[0033]
In a preferred embodiment, the electric signal input unit has an input line forming a microstrip line together with the conductive layer, and the input line is connected to one of the first and second conductor lines. I have.
[0034]
In a preferred embodiment, the electric signal input section has a coaxial connector connected to a line for transmitting the high frequency signal for optical modulation, and an intermediate connection member for electrically connecting the coaxial connector and the modulation electrode. are doing.
[0035]
Still another light modulation element of the present invention has an optical waveguide at least partially formed of a material having an electro-optical effect, and first, second, and third conductor lines electromagnetically coupled to each other. Forming a first microstrip line with the modulation electrode for applying a modulation electric field to the optical waveguide, the first conductor line, and forming a second microstrip line with the second conductor line; A conductive layer forming a third microstrip line together with the third conductor line; and an electric signal input unit for supplying a high-frequency signal for light modulation to the modulation electrode.
[0036]
In a preferred embodiment, the optical waveguide has at least two branched optical waveguides, an optical input unit for coupling the two optical waveguides, and an optical output unit for coupling the two optical waveguides. The portion of the optical waveguide to which the modulation electric field is applied is divided into the two branch optical waveguides, and the first and second conductor lines are connected to one of the two branch optical waveguides. On the other hand, the third and second conductor lines are arranged so as to exert electric fields having different polarities, and the third and second conductor lines are arranged so as to exert electric fields having different polarities to the other of the two branch optical waveguides. It functions as an intensity modulator that modulates the intensity of light input to the optical waveguide.
[0037]
In a preferred embodiment, the modulation electrode is arranged to modulate a refractive index of a portion of the optical waveguide to which the modulation electric field is applied, and modulates a phase of light input to the optical waveguide. Functions as a modulator.
[0038]
In a preferred embodiment, the semiconductor device further includes a connecting member that connects the first, second, and third conductor lines to each other at at least one end.
[0039]
In a preferred embodiment, the optical waveguide has at least two portions having different remanent polarizations.
[0040]
In a preferred embodiment, the optical waveguide is formed on a substrate having an electro-optic effect.
[0041]
In a preferred embodiment, the electric signal input section has an input line forming a microstrip line together with the conductive layer, and the input line is connected to one of the first and third conductor lines. I have.
[0042]
In a preferred embodiment, the electric signal input section has a coaxial connector connected to a line for transmitting the high frequency signal for optical modulation, and an intermediate connection member for electrically connecting the coaxial connector and the modulation electrode. are doing.
[0043]
The communication system of the present invention includes any one of the above-described light modulation elements, an input unit that inputs light to the light modulation element, and a control unit that supplies the modulation high-frequency signal to the light modulation element. .
[0044]
BEST MODE FOR CARRYING OUT THE INVENTION
(Embodiment 1)
Hereinafter, a first embodiment of the light modulation device according to the present invention will be described with reference to FIGS. 4 (a) to 4 (c). FIG. 4A shows a plan configuration of the light modulation element according to the present embodiment, and FIG. 4B shows a cross section perpendicular to the waveguide. FIG. 4C schematically shows the intensity distribution of the electric field formed by the modulation electrode of the light modulation element.
[0045]
As shown in FIG. 4A, the light modulation element according to the present embodiment includes an optical waveguide 12 formed on a surface portion of a substrate 11 having an electro-optic effect by using a proton exchange method using benzoic acid or the like. Have. The substrate 11 is made of, for example, lithium tantalate (LiTaO ₃ ) Single crystal or lithium niobate (LiNbO ₃ ) It is formed from a material such as a single crystal.
[0046]
The optical waveguide 12 branches into two branch optical waveguides 12a and 12b at two branch points 18a and 18b. The input light input from the entrance-side optical waveguide 12x branches at one branch point 18a, passes through the two branch optical waveguides 12a and 12b, and is coupled at the other branch point 18b to form a common exit-side optical waveguide. Continue on 12y. The optical waveguide 12 having such a configuration is called “Mach-Zehnder interferometer type optical waveguide”.
[0047]
On the substrate 11, a parallel coupling line 13 formed of two lines 13a and 13b extending along each of the branch optical waveguides 12a and 12b of the optical waveguide 12 is provided. Each inner end of each of the lines 13a and 13b is formed so as to be located immediately above the center of each of the branch optical waveguides 12a and 12b (FIG. 4B). Both ends of each line 13a, 13b are connected to each other via connection lines 16a, 16b.
[0048]
Further, an input line (feeding line) 15 connected to one line 13b of the parallel coupling line 13 is provided on the substrate 11, and an electric signal for modulation (high-frequency signal) is transmitted through the input line. Is entered.
[0049]
The lines 13a and 13b, the connection lines 16a and 16b, and the input line 15 of the parallel coupling line 13 are formed by patterning a film made of a metal such as aluminum or gold deposited by a vacuum evaporation method using photolithography and etching techniques. Obtained by On the back surface of the substrate 11, a ground plane (grounded conductive layer) 14 manufactured by the same method is provided.
[0050]
Although not shown in FIG. 4B, a SiO 2 line is provided between the lines 13a and 13b and the substrate 11. ₂ It is preferable that an insulating buffer layer made of, for example, be formed.
[0051]
The modulation electrode in the light modulation element of the present embodiment is constituted by electromagnetically coupled lines 13a and 13b and connection lines 16a and 16b. Then, a first microstrip line is formed between the line 13a and the ground plane 14, and a second microstrip line is formed between the line 13b and the ground plane 4. An electric signal input for modulation propagates through these microstrip lines.
[0052]
The light to be modulated (laser light) is input from the entrance-side optical waveguide 12x, and undergoes a modulation action as described below when passing through the branch optical waveguides 12a and 12b.
[0053]
In the present embodiment, an electric signal (high-frequency signal) for modulating light is supplied to each of the lines 13 a and 13 b of the parallel coupling line 13 via the input line 15. At this time, the wavelength of the high-frequency signal propagating through the parallel coupling line 13 via the input line 15 is determined by the frequency, the dielectric constant of the substrate, and the like. In the present embodiment, since the design parameters such as the length and width of each of the lines 13a and 13b are set to appropriate values according to the wavelength of the input high-frequency signal, when a predetermined high-frequency signal is given to the input line 15, , Resonance occurs in the parallel coupling line 13.
[0054]
When such resonance occurs, an electric field is formed in the gap 17 of the parallel coupling line 13 as shown by a dotted line in FIG. At this time, since the signal power is stored in the resonator, the intensity of the electric field becomes extremely large. This electric field oscillates at the resonance frequency, and its direction and intensity periodically change. When such an oscillating electric field is formed in or near the optical waveguide, the refractive index of the material forming the branch optical waveguides 12a and 12b periodically changes in accordance with the electric field strength due to the electro-optic effect.
[0055]
In the present embodiment, as shown in FIG. 4B, electric fields in the up and down directions are applied to the branch optical waveguide 12a and the branch optical waveguide 12b. For this reason, when the substrate 11 is formed of, for example, a z-cut lithium tantalate crystal, opposite phase changes are given to light passing through the two branch optical waveguides 12a and 12b. As a result, in the exit side optical waveguide 12y, interference of the two lights passing through the branch optical waveguides 12a and 12b occurs, and the intensity of the output light changes due to the interference. Thus, the light modulation element of the present embodiment operates as a light intensity modulator.
[0056]
Here, the resonance mode in the parallel coupling line 13 will be described.
[0057]
The lines 13a and 13b of the parallel coupling line 13 in the present embodiment are two parallel transmission lines independent of each other, and are arranged so as to be electromagnetically coupled. One transmission line (microstrip line) is constituted by one line 13a and the ground plane 14, and another transmission line (microstrip line) is constituted by the other line 13b and the ground plane 14. Since these two transmission lines are arranged parallel and close to each other, they are electromagnetically coupled to form the parallel coupling line 13.
[0058]
The transmission line is not limited to the one having the above-described configuration, and may be any as long as it has two lines, an outgoing path and a return path, and can transmit electromagnetic waves. In this case, one of the outgoing path and the return path (the line 13a or 13b in the present embodiment) is a normal line electrode (called a strip electrode, a hot electrode, or the like), and the other line is a ground electrode (the present embodiment). Is the ground plane 14).
[0059]
Since the parallel coupling line 13 is formed by coupling two transmission lines that may exist alone, there are two types of propagation modes independent of each other as even modes and odd modes as resonance modes.
[0060]
FIGS. 5A and 5B schematically show the electric field (solid line) and the magnetic field (dashed line) of the even mode and the odd mode in the cross section shown in FIG. 4B, respectively. The even mode is a mode in which the voltages of the two line electrodes are equal (in phase), as shown in FIG. In the even mode, an electric field is formed between each line electrode and the ground electrode, but almost no electric field is formed in the gap between the lines (the gap 17 shown in FIG. 4B).
[0061]
On the other hand, the odd mode is a mode in which the voltages of the two coupled parallel lines are opposite (negative phase) as shown in FIG. In the odd mode, an electric field is formed not only between each line and the ground electrode, but also in the gap between the lines (the gap 17 shown in FIG. 4B). If the two coupled lines are close together, a particularly large electric field is formed in the gap between the lines.
[0062]
In the present embodiment, the lengths and widths of the lines 13a and 13b, and the connection positions between the input lines and the lines 13b are adjusted so that odd-mode resonance occurs in the lines 13a and 13b of the parallel coupling lines. . Specifically, the length of each of the lines 13a and 13b is set to half the wavelength of the high frequency modulation signal. Further, as described later, the connection between the input line 15 and the line 13a is set to an appropriate position so as to suppress reflection of signal propagation in an odd mode. As a result, an odd-mode resonance of 波長 wavelength occurs in the parallel coupling line 3, and as a result, a large electric field is induced in the gap 17 between the two lines 13 a and 13 b, so that light modulation with extremely high efficiency is performed. Becomes possible.
[0063]
In this embodiment, since both ends of the parallel coupling line 13 are connected by the connection lines 16a and 16b, the distribution state of the voltage generated between the line 13a and the line 13b is as shown in FIG. In addition, the triangular function shape is 0 at both ends of the lines 13a and 13b and becomes maximum at the center. Since the direction of the electric field between the lines 13a and 13b is the same in any part, the light is transmitted from the branch optical waveguide 12a (or 13b) below one line 13a (or 13b) over the entire length of the half-wavelength line. Or 12b), the phase changes received while passing through them are not canceled out and continue to be added, so that a high modulation efficiency is obtained.
[0064]
In order to properly operate the light modulation device of the present embodiment, it is necessary to efficiently cause the parallel coupling line 13 to resonate in an odd mode by a modulation signal. In the present embodiment, by connecting the input line 15 to a position that matches the input impedance, resonance in an odd mode can be easily realized.
[0065]
Hereinafter, results of analyzing the element characteristics of the light modulation element of the present embodiment and the light modulation element having a conventional structure (comparative example) by electromagnetic field simulation will be described.
[0066]
FIG. 6 is a plan view showing plane dimensions and connection positions of the parallel coupling line and the input line used in the electromagnetic field simulation. FIG. 7 is a graph showing the reflection loss characteristics in the resonance state of the light modulation element obtained by the electromagnetic field simulation.
[0067]
In this analysis, the substrate 11 is a z-cut lithium tantalate crystal having a thickness of 0.4 mm (dielectric constant 41), the width of each of the lines 13a and 13b of the parallel coupling line 13 is 0.05 mm, and the gap 17 Is set to 0.02 mm, and the width of the input line 15 is set to 0.05 mm so that the characteristic impedance becomes 50Ω. The material forming each of the lines 13a, 13b, 16a, 16b, and 15 is gold, and the film thickness is 2 μm. The length of each of the lines 13a and 13b of the parallel coupling line 13 and the connection position between the input line 17 and the line 13b are determined by using an electromagnetic field simulator so that the odd mode resonates at 10 GHz and the input line 17 was determined so that the reflection of the signal input to 17 was minimized, in other words, the input impedance was matched.
[0068]
As a result, as shown in FIG. 6, the length of each of the lines 13a and 13b of the parallel coupling line 13 is 3 mm, and the position of the input line 15 is 0.69 mm from the center of the parallel coupling line 13. In this case, as can be seen from FIG. 6, the reflection of the input signal is eliminated at the resonance point, and almost all the signal power is input to the resonator. According to the calculation by the conformal mapping method, the light modulation efficiency in this case gives a phase difference of π to the light waves in the two branched optical waveguides. In other words, it was found that the power required to change the light output from 0 to the maximum was about 0.43 W. This power is smaller than that of a conventional light modulation element, and according to the present embodiment, high modulation efficiency can be realized.
[0069]
FIG. 8A shows a waveform of a high-frequency signal input to the parallel coupling line 13, and FIG. 8B shows a ratio of output light intensity / input light intensity of the optical modulation element according to the present embodiment. (C) shows the ratio of output light intensity / input light intensity of the comparative example. The vertical axis of the graph in FIG. 8A is the voltage of the high-frequency signal, and the horizontal axis is time. The vertical axes of the graphs in FIGS. 8B and 8C are the ratio of the output light intensity to the input light intensity, and the horizontal axis is time. The ratio of the output light intensity to the input light intensity was calculated ignoring the loss of the optical waveguide.
[0070]
The simulation was performed under the condition that a π / 2 phase bias was applied between the two branch optical waveguides. In the optical modulation element of the comparative example used for the analysis, the resonance frequency (10 GHz), the width and the thickness of the lines 113a and 113b are the same as those of the lines 13a and 13b of the present embodiment, and two optical waveguides 112 are used. It has branched to. The length (1.5 mm) of each line and the connection position between the input line 129 and the line 113 are set using an electromagnetic field simulator so that the odd mode resonates at 10 GHz and the input impedance matches. .
[0071]
Comparing the graph of FIG. 8B with the graph of FIG. 8C, it can be seen that the light modulation efficiency of the light modulation element of the present embodiment is much higher than the light modulation efficiency of the comparative example. The reason why the light modulation efficiency is improved in the present embodiment is considered to be as follows.
[0072]
First, by connecting both ends of the two lines 13a and 13b constituting the parallel coupling line 13 by the connection lines 16a and 16b, they function as a half-wavelength resonator as shown in FIG. 4C. The parallel coupling line 13 can be realized. On the other hand, the parallel coupling line 113 of the conventional light modulation device shown in FIG. 3 can realize only a quarter-wave resonance state.
[0073]
As described above, according to the present embodiment, when the odd-mode resonance is generated, the two connection lines 16a and 16b can serve as nodes of the resonance. However, the parallel coupling line 113a in the conventional light modulation element shown in FIG. , 113b, when the odd mode impedance matching is performed, the open end becomes an antinode instead of a resonance node. In the present embodiment, since the half-mode odd mode resonance can be generated, light that passes through a portion of the branch optical waveguides 12a and 12b having a length corresponding to a half wavelength of the high-frequency modulation signal is used. The light modulation efficiency is improved as compared with the light modulation device of FIG.
[0074]
Next, with reference to FIGS. 9A and 9B, an embodiment of a light modulation element having an electrode structure resonating at 10 GHz and a light modulation element having an electrode structure resonating at 26 GHz will be described.
[0075]
Each of the two light modulation elements has a modulation electrode structure having the size and layout shown in FIGS. 9A and 9B. Substrate is z-cut LiTaO ₃ (Thickness: 0.4 mm), and a Mach-Zehnder optical waveguide having a width of 5 μm was formed on the substrate surface by proton exchange using benzoic acid.
[0076]
The surface of the substrate on which the optical waveguide is formed is made of SiO3 having a thickness of 0.13 μm. ₂ After being covered with a buffer layer composed of a layer, an aluminum film (thickness: 0.9 μm) was deposited on the buffer layer by a vacuum evaporation method. This aluminum film was patterned using photolithography and etching techniques to simultaneously form parallel coupling lines and input lines made of aluminum. The width of the parallel coupling line was 50 μm, the width of the gap was 20 μm, and the width of the input line was 110 μm. In FIGS. 9A and 9B, the magnitude relationship between the width of the parallel coupling line and the width of the input line does not reflect the actual scale.
[0077]
The modulation experiment was performed using light having a wavelength of 1.3 μm. FIG. 10 is a graph showing the modulation characteristics obtained by the measurement. In the graph shown in FIG. 10, the vertical axis indicates the reflection loss, and the horizontal axis indicates the frequency. In the graph, measured data is indicated by a solid line, and data obtained by electromagnetic field simulation is indicated by a broken line.
[0078]
From the results shown in FIG. 10, it was found that the no-load Q value, which is an index indicating the degree of accumulation of the signal power of the resonator, was about 30. This means that approximately 30 times the power of the signal input to the resonator is stored in the resonator. From these results, it was confirmed that extremely high modulation efficiency can be realized by using the modulation electrode of the present embodiment.
In FIG. 11, the vertical axis indicates the output of the light modulation element, and the horizontal axis indicates time.
[0079]
As can be seen from these figures, an optical modulator having a modulation electrode that resonates at a high frequency of 10 GHz was obtained. The modulation index when a high-frequency signal of 100 mW was applied was 0.2 rad. FIG. 12 shows the spectrum of the modulated light.
[0080]
FIG. 13 is a graph showing the reflection characteristics (measured values) of the modulation electrode in the light modulation element having the electrode that resonates at 26 GHz.
[0081]
From the results shown in FIG. 13, it was found that the no-load Q value was about 60. From these results, it was confirmed that extremely high modulation efficiency can be realized by using the modulation electrode of the present embodiment. FIG. 14 shows the spectrum of the modulated light.
[0082]
In any of the above light modulation elements, resonance in an odd mode occurs. In the parallel coupling line, even mode resonance may occur, but even in the same electrode structure, there is a difference in the resonance frequency and impedance between the odd mode and the even mode. In this embodiment, the length of the parallel coupling line, the width of the gap, the connection position with the input line, and the like are adjusted so that only the odd mode is excited for the high-frequency signal of the predetermined frequency.
[0083]
(Embodiment 2)
Next, a second embodiment of the light modulation device of the present invention will be described with reference to FIGS. FIG. 15A shows a plan configuration of the light modulation element according to the present embodiment, and FIG. 15B shows a region where the polarity of the remanent polarization of the substrate is opposite. FIG. 15C shows the distribution of the electric field intensity in the parallel coupling line.
[0084]
As shown in FIG. 15A, an optical waveguide 22 formed on the surface of a substrate 21 having an electro-optic effect by a proton exchange method using benzoic acid or the like is used in the light modulation element according to the present embodiment. have. The substrate 21 is made of, for example, lithium tantalate (LiTaO ₃ ) Single crystal, lithium niobate (LiNbO) ₃ ) It is formed from a material such as a single crystal.
[0085]
The optical waveguide 22 branches into two branch optical waveguides 22a and 22b at two branch points 28a and 28b. The input light input from the entrance-side optical waveguide 22x branches at one branch point 28a and After passing through one of the branch optical waveguides 22a and 22b, the other branch point 28b is configured to travel along a common exit-side optical waveguide 22y.
[0086]
On the substrate 21, a parallel coupling line 23 including two lines 23a and 23b extending along the branch optical waveguides 22a and 22b of the optical waveguide 22 is provided. Each inner end of each of the lines 23a and 23b is formed so as to be located immediately above a substantially central portion of each of the branch optical waveguides 22a and 22b. Further, on the substrate 21, there is provided an input line 25 which is connected to one of the lines 23b of the parallel coupling line 23 and applies an input signal which causes the parallel coupling line 23 to resonate. Each of the lines 23a and 23b, the connection lines 26a and 26b, and the input line 25 of the parallel coupling line 23 are respectively formed of a metal film such as aluminum or gold formed using a process such as a vacuum deposition method, photolithography, and etching. ing. On the back surface of the substrate 21, a ground plane 24 formed by using a metal film deposition method or the like is provided. The above configuration is basically the same as the light modulation device of the first embodiment.
[0087]
In the present embodiment, unlike the first embodiment, both ends of each of the lines 23a and 23b are open ends without being connected to each other. The substrate 21 is divided into two regions 21a and 21b (domains) having different remanent polarization directions (positive and negative electro-optic coefficients). That is, in the present embodiment, a region located below each first half of the two lines 23a and 23b constituting the parallel coupling line 23, and a region located below each second half of the two lines 23a and 23b. Have remanent polarizations with different polarities.
[0088]
In the present embodiment, the region 21a located below the latter half of each of the two lines 23a and 23b has the remanent polarization of the first polarity (positive or negative), and the other region 21b, that is, the two lines A region 21b including a region located below each first half of 23a and 23b has remanent polarization of the second polarity (negative or positive). In other words, the remanent polarization of the region 21a has a reversal polarization that is opposite to that of the other region 21b.
[0089]
The input light is introduced from the entrance-side optical waveguide 22x, and undergoes an optical modulation action as described below when passing through the branch optical waveguides 22a and 22b.
[0090]
When a high-frequency signal is input from the input line 25 and resonance occurs in each of the lines 23a and 23b of the parallel coupling line 23, an electric field similar to the electric field indicated by a dotted line in FIG. . Then, due to the electro-optic effect, the refractive index of the material forming the branch optical waveguides 22a and 22b changes according to the electric field intensity. In the present embodiment, similarly to the first embodiment, electric fields in the directions opposite to each other are applied to the branch optical waveguide 22a and the branch optical waveguide 22b. Therefore, when the substrate 21 is made of, for example, a z-cut lithium tantalate crystal or the like, the light passing through the two branch optical waveguides 22a and 22b is given opposite phase changes. Therefore, in the exit side optical waveguide 22y, interference between the two lights passing through the branch optical waveguides 22a and 22b occurs, and the intensity of the output light changes due to the interference. Acts as a vessel.
[0091]
In this embodiment, since both ends of the parallel coupling line 23 are open ends, the distribution state of the voltage generated between the line 23a and the line 23b is, as shown in FIG. Is a trigonometric function in which the sign is 0 at the center and the opposite sign at both ends is the maximum. Therefore, the direction of the electric field between the lines 23a and 23b is opposite between the first half and the second half. If the polarity of the remanent polarization of the entire substrate 21 is uniform, while the light passes through the branch optical waveguides 22a and 22b below the lines 23a and 23b of the parallel coupling line 23, the first half is Since the opposite phase change is given to the latter half, the phase change is canceled (canceled), and high modulation efficiency cannot be obtained. However, in the light modulation device of the present embodiment, the region 21b of the substrate 21 located below the first half of the parallel coupling line 23 and the region 21a of the substrate 21 located below the first half of the parallel coupling line 23 are different. , The positive and negative of the potential optical coefficient are opposite. As a result, as shown by the dotted line in FIG. 15C, the phase modulation substantially the same as when the electric field having the same sign as that in the first half is applied to each of the lines 23a and 23b in each of the second half and the second half. , 22b. Therefore, the phase change due to the modulation received while the light passes through the branch optical waveguide 22a (or 22b) below the line 23a (or 23b) of the parallel coupling line 23 continues to be added without being canceled out. Modulation efficiency is obtained.
[0092]
In the above description, the traveling time of light is ignored. In an actual light modulation element, the speed of light is finite, so that the electric field intensity felt by the light is different from the solid line in FIG. Therefore, the optimal pattern of the region 21a is strictly different from the pattern shown in FIG. Specifically, it is preferable that the region 21a in the substrate 21 is shifted slightly downstream of the latter half of each of the lines 23a and 23b by the phase delay of the high-frequency signal.
[0093]
As shown in FIG. 15B, the pattern of the region 21a does not necessarily need to invert the remnant polarization of a large area from that of the other region 21b, and each of the branch optical waveguides 22a and 22b and the necessary minimum. What is necessary is just to invert the remanent polarization only in the peripheral portion.
[0094]
According to the light modulation device of the present embodiment, the substrate 21 is provided with the regions 21a, 21b having different electro-optic coefficients by utilizing the difference in the direction of the remanent polarization, so that the light passes through the branch optical waveguides 22a, 22b. In this case, the phase change due to the modulation received during the operation can be continued to be added without being cancelled. That is, in the present embodiment, the effect obtained by connecting both ends of each of the lines 13a and 13b in the first embodiment to form a half-wavelength resonator is obtained in the substrate 21 with the residual polarization. Providing regions having different positive / negative values can be exerted, and high modulation efficiency can be obtained.
[0095]
Note that the light modulation element of the present embodiment has the parallel coupling line 23 that functions as a half-wavelength resonator with both ends open, but the present invention is not limited to the embodiment having such a configuration. For example, a parallel coupling line having both ends connected by a connection line and functioning as a one-wavelength resonator is provided, and the polarity of the remanent polarization of the branch optical waveguide of the substrate corresponding to the latter half of each line of the parallel coupling line is set to the other region. May be reversed. According to such a configuration, it is possible to add the phase changes of the optical modulation received by one wavelength without canceling each other. According to the light modulation element of the present embodiment, when the frequency is the same, the resonator length is twice as long as the resonator length of the light modulation element shown in FIG. 4, so that the modulation efficiency is improved accordingly. Further, by using higher-order resonance, it is possible to further increase the modulation efficiency.
[0096]
As described above, by using the higher-order resonance due to the odd mode, it is possible to dramatically increase the modulation efficiency. Even in the case of an optical modulation element having a structure in which one end of a parallel coupling line is connected and the other end is open, the same effect can be exerted by utilizing the difference in remanent polarization.
[0097]
Note that the number of regions in which the polarity of remanent polarization is inverted is not limited to one. By arranging an appropriate number of domain-inverted regions, the length of the modulation electrode can be increased.
[0098]
(Embodiment 3)
Next, a third embodiment of the light modulation device of the present invention will be described with reference to FIGS. 16 (a) and 16 (b). FIG. 16A shows a plan configuration of the light modulation element according to the present embodiment, and FIG. 16B is a longitudinal sectional view thereof.
[0099]
As shown in FIG. 16A, the light modulation element according to this embodiment includes lithium tantalate (LiTaO ₃ ) Single crystal, lithium niobate (LiNbO) ₃ An optical waveguide 32 formed by a proton exchange method using benzoic acid or the like is provided on the surface of a substrate 31 having an electro-optical effect such as a single crystal. The optical waveguide 32 is branched into two branched optical waveguides 32a and 32b at two branch points 38a and 38b, and the input light input from the entrance-side optical waveguide 32x is branched at one branch point 38a to form a second branch. After passing through the two branch optical waveguides 32a and 32b, the other branch point 38b is configured to travel along the common exit side optical waveguide 32y.
[0100]
Further, on the substrate 31, a parallel coupling line 33 including three lines 33a, 33b, and 33c extending along the branch optical waveguides 32a and 32b of the optical waveguide 32 is provided. Each inner end of each of the lines 33a, 33b is formed so as to be located almost immediately above the center of each of the branch optical waveguides 32a, 32b. The line 33c is located between the two lines 33a and 33b. Both ends of each line 33a, 33b, 33c are connected to each other via connection lines 36a, 36b. Further, on the substrate 31, there is provided an input line 35 connected to one line 33b of the parallel coupling line 33 and for applying an input signal causing resonance in the parallel coupling line 33. Each of the lines 33a to 33c, the connection lines 36a and 36b, and the input line 35 of the parallel coupling line 33 are respectively formed of a metal film such as aluminum or gold formed by a process such as a vacuum deposition method, photolithography, and etching. ing. On the back surface of the substrate 31, a ground plane 34 formed by using a metal film deposition method or the like is provided.
[0101]
The input light is introduced from the entrance-side optical waveguide 32x, and undergoes a light modulation action as described below when passing through the branch optical waveguides 32a and 32b.
[0102]
When a high-frequency signal is input from the input line 35 and resonance occurs in each of the lines 33a, 33b, and 33c of the parallel coupling line 33, an electric field as shown by a dotted line in FIG. Occurs. Then, due to the electro-optical effect, the refractive index of the material forming the branch optical waveguides 32a and 32b changes according to the electric field intensity. Therefore, in the exit side optical waveguide 32y, interference of the two lights passing through the branch optical waveguides 32a and 32b occurs, and the intensity of the output light changes due to the interference. Acts as a vessel.
[0103]
Here, in a parallel coupling line 33 having three lines 33a to 33c as shown in FIGS. 16A and 16B, three types of propagation modes 1 to 3 usually exist.
[0104]
Table 1 below shows the codes of the potentials of the lines 33a to 33c in modes 1 to 3 in a table.
[0105]
[Table 1]

[0106]
FIG. 16B shows the direction of the electric field in the vicinity of the branch optical waveguides 32b when the resonance occurs in the mode 2 shown in Table 1. As can be seen from FIG. 16B, since an electric field in the upside down direction is applied to the two branch optical waveguides 32a and 32b, a phase difference occurs in the light waves and interference occurs in the exit side optical waveguide 32y. The light modulation element of the embodiment functions as a light intensity modulator.
[0107]
On the other hand, FIG. 16C shows the direction of the electric field in the vicinity of the branch optical waveguides 32b when the resonance is generated in the mode 3 shown in Table 1. When resonating in mode 3, the positional relationship between the branch optical waveguides 32b, 32b and the three lines 33a, 33b, 33c is slightly different from the positional relationship shown in FIGS. 16A and 16B. More specifically, the position of the branch optical waveguide 32b is shifted so that the directions of the electric fields formed in the branch optical waveguides 32b and 32b are opposite to each other.
[0108]
Thus, according to the configuration of the present embodiment, in the mode 1 in which all of the lines 33a, 33b, and 33c are always at the same potential, the line does not function as a light intensity modulator. For this reason, the light modulation element of the present embodiment is designed to resonate in mode 2 or mode 3 other than mode 1.
[0109]
Here, the interval between the two branch optical waveguides 32a and 32b cannot be so narrow to avoid mutual interference of light, but the width of the gaps 37a and 37b is the first because the line 33c is provided. , Is much smaller than in the second embodiment. Therefore, an electric field having a very large intensity is generated in the gaps 37a and 37b. Therefore, high modulation efficiency can be obtained by the light modulation device of the present embodiment.
[0110]
Note that, in the present embodiment, an example has been described in which a parallel coupling line functioning as a half-wavelength resonator having both ends connected is provided, but the present invention is not limited to such an embodiment. For example, as in the second embodiment, a parallel coupling line having both ends open or a parallel coupling functioning as a quarter-wave resonator having one end connected and the other end open as shown in FIG. The basic effects of the present embodiment can also be exerted by an optical modulation element having a line.
[0111]
Also, in the light modulation device of the present embodiment, similarly to the second embodiment, the domain-inverted region may be provided in a part of the optical waveguide. By providing two regions having different remanent polarizations on the substrate, the effects of the second embodiment and the effects of the third embodiment can be exhibited together.
[0112]
(Embodiment 4)
Next, a fourth embodiment of the light modulation device according to the present invention will be described with reference to FIGS. 17 (a) to 17 (c). FIG. 17A is a top view illustrating a planar configuration of the light modulation element of the present embodiment, and FIG. 17B is a cross-sectional view taken along line A0-A1 of FIG. FIG. 17C is an enlarged perspective view showing a part of the present embodiment.
[0113]
As shown in FIGS. 17A and 17B, the light modulation element of the present embodiment includes a first substrate fixing jig 212a to which a coaxial connector 209 is attached, and a second substrate fixing jig 212a to which a substrate 11 is fixed. A substrate fixing jig 212b is provided, and the first jig 212a is fixed to the second jig 212a such that the center line 210 of the coaxial connector is appropriately arranged with respect to the input line 15. The first jig 212a is fixed to the second jig 212b with, for example, screws.
[0114]
An intermediate connection member 211 is attached to a distal end portion of the center line 210 of the coaxial connector, and the center line 210 is connected to the input line 15 via the intermediate connection member 211. As described above, the optical modulation element according to the present embodiment is different from the above-described embodiment in that the electric signal input unit has the coaxial connector 209 and the intermediate connecting member 211 separately from the input line 15.
[0115]
In the present embodiment, the external drive circuit that generates the high-frequency signal for modulation and the coaxial connector of the light modulation element are connected by, for example, a coaxial cable. The high-frequency signal transmitted through the coaxial cable is provided to the input line 15 via the intermediate connecting member 211.
[0116]
As shown in FIG. 17C, the intermediate connection member 211 has a first connection portion 214 having a portion bent so as to be in contact with the outer periphery of the center conductor 210 of the coaxial connector, and a planar shape that is in contact with the input line 15. And a second connection portion 215 having a portion.
[0117]
It is preferable that a portion connecting the first connection portion 214 and the second connection portion 215 is formed of an elastic member. By using such an elastic member, the second connection member 215 can be urged downward with respect to the first connection member 214. For this reason, as shown in FIG. 17B, the bottom surface of the second connection portion 215 presses the upper surface of the input line 15, and it is easy to secure electrical contact between the second connection portion 215 and the input line 15. Become. When such a configuration is employed, there is no need to apply a conductive adhesive between the second connection portion 215 and the input line 15, and the connection is facilitated. Note that the first connection portion 214 and the second connection portion 215 are preferably formed from a single plate-shaped conductor.
[0118]
By adopting the above configuration, the high-frequency signal given to the input line 15 via the optical connector 209 propagates to the parallel coupling line 13 and causes resonance between the line 13a and the line 13b. As a result, a high-frequency oscillating electric field is generated in the gap 17, so that the refractive index of the optical waveguide 12 formed of a material having an electro-optic effect changes according to the high-frequency signal. At this time, as shown in FIG. 17 (b), electric fields in the directions opposite to each other are formed in the optical waveguide passing under the line 13a and the line 13b, so that the light propagating through each branch optical waveguide has a phase difference. Is formed.
[0119]
FIG. 18 shows the reflection characteristics at the end P of the input line 15 of the optical modulation device having the configuration of the present embodiment. As can be seen from FIG. 18, the resonance frequency of the modulation electrode is 26 GHz, and the input impedance matches at the resonance frequency. FIG. 19 is a graph showing the relationship between the transmission loss and the frequency of the input line 15.
[0120]
(Fifth embodiment)
Next, a fifth embodiment of the light modulation device according to the present invention will be described.
[0121]
The light modulation element of the present embodiment has substantially the same configuration as the light modulation element of the fourth embodiment, but differs from the fourth embodiment in that an input line is not provided on the substrate 11. It is different from a light modulation element.
[0122]
FIG. 20A is a plan view showing the light modulation element of the present embodiment, and FIG. 20B is a cross-sectional view taken along the line B0-B1.
[0123]
The electric signal input section of the light modulation element of the present embodiment has a coaxial connector 9 connector 209 and an intermediate connection member 211 as in the fifth embodiment. Are directly connected without going through.
[0124]
In order to operate the light modulation element, it is necessary to efficiently generate resonance in the parallel coupling line 3 by an odd symmetric mode by supplying a high frequency signal for modulation to the parallel coupling line 13. This can be realized by connecting the intermediate connecting member 211 to a position where the input impedance can be matched.
[0125]
FIG. 21 shows a connection state between the parallel coupling line 13 on the substrate 11 and the center conductor 210 of the coaxial connector. The central conductor 210 and the intermediate connecting member 211 of the coaxial connector have the configuration shown in FIG. As described above, the portion connecting the first connection portion 214 and the second connection portion 215 is formed of a curved elastic member, and the second connection member 215 is attached to the first connection member 214 downward. I'm going. For this reason, the bottom surface of the second connection portion 215 presses the upper surface of the parallel coupling line 13, and electrical contact between the second connection portion 215 and the parallel coupling line 13 is secured. There is no need to apply a conductive adhesive between the second connection part 215 and the parallel coupling line 13, and the connection is easy.
[0126]
The substrate 11 on which the parallel coupling lines 13 are formed is fixed to a substrate fixing jig 212b, and the intermediate connecting member 211 and the coaxial connector 209 are fixed to the substrate fixing jig 212a. By sliding the substrate fixing jig 212a with respect to the substrate fixing jig 212b, the parallel coupling line 13 and the intermediate connecting member 211 can be brought into contact at an appropriate position. The substrate fixing jig 212a is fixed to the substrate fixing jig 212b with screws or the like.
[0127]
Since the high-frequency signal for modulation can be input only by bringing the intermediate connection member 211 into contact with the parallel coupling line 13, the input reflection characteristic of the parallel coupling line 13 is measured, and then the intermediate connection member 211 with respect to the parallel coupling line 13 is measured. The connection position can be easily adjusted. The adjustment of the connection position can be performed by finely adjusting the screwing positions of the jigs 212a and 212b.
[0128]
Next, the results of analysis of the light modulation element of the present embodiment by electromagnetic field simulation will be described. In this specific example, a z-cut lithium tantalate crystal having a thickness of 0.400 mm (dielectric constant 42) is used as the substrate 11, the line width of the parallel coupling line 13 is 0.05 mm, and the width of the gap is 0.02 mm. Set to.
[0129]
The material of each line was aluminum, and the thickness of the aluminum film forming the line was set to 1 μm. The intermediate connecting member was beryllium copper having a thickness of 0.01 mm and a width of 0.15 mm and subjected to gold plating.
[0130]
The length of the parallel coupling line 13 and the connection position between the intermediate connection member 211 were set so that the input impedances matched. Specifically, using an electromagnetic field simulator, it was determined that the odd symmetric mode resonated at 26 GHz and that the signal input to the coaxial connector had the least reflection at that frequency.
[0131]
As a result, as shown in FIG. 22, the length of the parallel connection line 13 was 1.20 mm, and the connection position of the intermediate connection member was 0.25 mm from the center of the parallel connection line 13. The distance from the substrate fixing jig 212a to the parallel coupling line 13 was 0.30 mm.
[0132]
FIG. 23 shows the reflection characteristics at the connection between the intermediate connection member 211 and the parallel coupling line 13 in the optical modulation element having the above configuration. As can be seen from FIG. 23, the reflection of the input signal is eliminated at the resonance point, and almost all of the signal power is input to the parallel coupling line 13. Further, it can be seen that the resonance characteristics are not affected even when the input line is not provided.
[0133]
According to the configuration of the present embodiment, transmission loss in the input line can be eliminated.
[0134]
In the optical modulation element according to the fifth embodiment, transmission loss of 0.5 dB occurs at a frequency of 26 GHz due to the presence of the input line 15. According to the configuration of the present embodiment, transmission loss due to such an input line is reduced. A high-frequency signal can be input without generating the signal, and the modulation efficiency can be further improved. For this reason, the power required to change the optical output from 0 to the maximum can be reduced by 0.5 dB as compared with the case where the input line is provided.
[0135]
It is assumed that, when the light modulation element of the present embodiment is manufactured according to the design values shown in FIG. 22, the input reflection characteristics immediately after the manufacturing show the characteristics shown by the broken line in FIG. In this case, the input reflection characteristic shown by the solid line in FIG. 24 can be obtained only by moving the connection position between the intermediate connection member 211 and the parallel coupling line 13 so as to approach the center of the modulation electrode by 0.05 mm. That is, even if the input impedance is shifted from the design value due to a variation in the manufacturing process after the fabrication of the light modulation element, the input impedance is adjusted by adjusting the connection position between the intermediate connection member 211 and the parallel coupling line 13. Matching can be easily realized.
[0136]
According to the present embodiment, it is possible to improve the modulation efficiency by inputting a signal to the resonant electrode without providing an input line, and to adjust the input impedance of the resonant electrode even after the element is created. It can be carried out.
[0137]
In the present embodiment, an electrode configuration having a half-wavelength resonator structure in which both ends are short-circuited is adopted. However, the modulation electrode in the present invention is not limited to one having such a configuration. Any modulation electrode may be used.
[0138]
In each of the embodiments described above, the optical waveguide is formed in the substrate having the electro-optic effect. However, the present invention is not limited to such a configuration. A configuration may be adopted in which a core portion having a higher refractive index than the periphery is formed in the surface region of the substrate, and a film made of a material having an electro-optical effect is formed as a clad portion on the core portion. In this case, since a part of the light propagating in the core oozes into the clad, the phase of the light propagating in the core can be modulated by changing the refractive index of the clad. The core does not need to be formed from a material having an electro-optic effect.
[0139]
Further, the optical waveguide in the above embodiment includes at least two branched optical waveguides, an optical input unit coupling the two branched optical waveguides, and an optical output unit coupling the two branched optical waveguides. Although it has a Mach-Zehnder interferometer type configuration, the light modulation element of the present invention is not limited to a light intensity modulation element having such a configuration. Even when the optical waveguide of the optical modulation element according to the present invention has a single optical waveguide, the phase of the propagating light can be efficiently modulated. In this sense, the light modulation element of the present invention has an essential function in modulating the phase of light, and can modulate the light intensity by interfering the phase-modulated light. It is.
[0140]
Note that the optical modulators of the first and second embodiments have a branch optical waveguide by branching the optical waveguide into two in the middle, and the present invention relates to a light modulator having such a branch optical waveguide. It is not limited to a modulation element. For example, when the present invention is applied to an optical modulator having only a single optical waveguide, an optical modulator that functions as a phase modulator can be obtained. Even in such a case, according to the present invention, the sign (polarity) of the voltage between the lines becomes uniform, and the phase change of light is not canceled out, so that the light modulation efficiency of the light modulation element can be improved. .
[0141]
In the optical modulation elements according to the first to third embodiments, the input line is directly connected to one of the parallel coupling lines, but the present invention is not limited to such a configuration. For example, it is also possible to input-couple by making the tip of the input line face one of the parallel coupling lines via a gap. In particular, such a configuration is particularly effective when the loss of the line is small and the unloaded Q value of resonance is high, such as when a superconducting material is used as the material forming the line.
[0142]
Also, the connection lines connecting the respective lines of the parallel coupling lines are drawn as having a partial circular shape in the attached drawings, but the connection lines only need to be able to connect the respective lines at a short distance. Even if the shape has a linear portion, such as forming a part of a polygon, the characteristics of the light modulation element are not adversely affected.
[0143]
The substrate of the element may be formed of an electro-optic crystal other than a material having an electro-optic effect, such as lithium tantalate crystal or lithium niobate. The optical waveguide is preferably formed by a method in which the surface of the electro-optic crystal substrate is subjected to a proton exchange treatment in benzoic acid, but the optical waveguide may be manufactured by another method. For example, when it is necessary to use a substrate other than lithium tantalate single crystal or the like for integration with other functional elements, the refractive index is higher on the substrate than on the substrate, A film made of a material having an effect can be formed, and the film can be used as an optical waveguide. In addition, a core portion having a higher refractive index than the surroundings is formed in the surface region of the substrate, and a film made of a material having an electro-optical effect is formed as a clad portion on the core portion, so that the core portion exudes from the core portion. It is similarly effective to perform optical modulation by changing the refractive index of the cladding using an electric field. Further, the parallel coupling line may be buried in the substrate.
[0144]
(Embodiment 6)
FIG. 25 is a block diagram showing the configuration of the fiber radio system according to the present invention.
[0145]
The fiber radio system 50 of the present embodiment includes an optical modulator / demodulator 51 incorporating the optical modulation device of the first to third embodiments. The antenna 53 can directly perform communication with a normal data communication network such as the Internet or the like, communication with a portable terminal, or reception of a signal from a CATV, for example, using a millimeter wave carrier. The optical modulator / demodulator 51 incorporates an optical demodulator (for example, a photodiode) together with the optical modulator.
[0146]
On the other hand, high-frequency radio signals such as millimeter waves are difficult to transmit over long distances and are susceptible to signal interruption by objects. Thus, communication with the data communication network 61, the CATV 62, and the mobile phone system 63 can be performed using the wireless device 60 and the antenna 64 attached to the wireless device. In that case, an optical modulator / demodulator 55 connected to the fiber wireless communication system 50 via the optical fiber 70 and an antenna 54 attached thereto are further provided. Then, signals can be exchanged with the wireless device 60 via the antennas 54 and 64 and the optical modulator / demodulator 55. The optical modulator / demodulator 55 incorporates an optical demodulator (for example, a photodiode) together with the optical modulator.
[0147]
For example, when performing long-distance transmission or when transmitting indoors partitioned by a wall or the like, it is effective to transmit an optical signal modulated by a radio signal such as a millimeter wave through the optical fiber 70. .
[0148]
【The invention's effect】
According to the configuration of the light modulation element of the present invention, it is possible to improve the light modulation efficiency of the light modulation element that functions as a phase modulator or a light intensity modulator. By disposing this light modulation element in a communication system, Thus, communication using a high-frequency signal at the millimeter wave level becomes possible.
[Brief description of the drawings]
1A is a diagram for explaining direct modulation of light, and FIG. 1B is a diagram for explaining external modulation of light.
FIG. 2 is a cross-sectional view illustrating an operation principle of external modulation of light using an electro-optic effect.
FIG. 3 is a plan view showing a conventional example of a light modulation element.
4A is a plan view showing a planar configuration of the light modulation device according to the present embodiment, FIG. 4B is a cross-sectional view perpendicular to the waveguide, and FIG. It is a figure which shows typically the intensity distribution of the electric field which a modulation electrode forms.
5 (a) and 5 (b) are diagrams respectively showing states of an electric field (solid line) and a magnetic field (dashed line) in an even mode and an odd mode in the cross section shown in FIG. 4 (b).
FIG. 6 is a plan view showing plane dimensions and connection positions of a parallel coupling line and an input line used in an electromagnetic field simulation in the first embodiment.
FIG. 7 is a diagram showing reflection loss characteristics in a resonance state of the light modulation element obtained by an electromagnetic field simulation in the first embodiment.
FIG. 8A is a graph illustrating a waveform of a high-frequency signal input to the parallel coupling line 13 according to the first embodiment, and FIG. 8B is a graph illustrating an output light intensity / input of the optical modulation element according to the present embodiment. It is a graph which shows the ratio of light intensity, and (c) is a graph which shows the ratio of the output light intensity / input light intensity of a comparative example.
FIGS. 9A and 9B are plan views showing examples of a light modulation element having an electrode structure resonating at 10 GHz and a light modulation element having an electrode structure resonating at 26 GHz, respectively.
FIG. 10 is a graph showing the reflection loss characteristics of the embodiment shown in FIG.
FIG. 11 is a graph showing a time change of the optical output of the embodiment shown in FIG. 9 (a).
FIG. 12 is a graph showing a modulated light spectrum of the example shown in FIG.
FIG. 13 is a graph showing the reflection characteristics (measured values) of the modulation electrode in the example shown in FIG. 9 (b).
FIG. 14 is a graph showing the spectrum of the modulated light of the example shown in FIG. 9 (b).
FIG. 15A is a plan view showing a planar configuration of a light modulation element according to a second embodiment of the present invention, FIG. 15B is a plan view showing a region where the polarity of the remanent polarization of the substrate is opposite, (c) is a diagram showing the distribution of the electric field intensity in the parallel coupling line.
16A is a plan view of a light modulation element according to a third embodiment of the present invention, FIG. 16B is a longitudinal sectional view in the case of resonance in mode 2, and FIG. FIG. 6 is a vertical cross-sectional view in a case where resonance occurs in FIG.
17A is a top view showing a fourth embodiment of the light modulation device according to the present invention, FIG. 17B is a sectional view taken along line A0-A1 of FIG. 17A, and FIG. It is a perspective view showing connection of connector central conductor 210 and intermediate connection member 211.
FIG. 18 is a graph illustrating reflection characteristics of an input line unit according to the fourth embodiment.
FIG. 19 is a graph illustrating transmission loss of an input line unit according to the fourth embodiment.
20A is a top view showing a fifth embodiment of the light modulation device according to the present invention, and FIG. 20B is a sectional view taken along line B0-B1 of FIG.
FIG. 21 is a perspective view showing a part of the fifth embodiment.
FIG. 22 is a layout diagram illustrating design parameter values of a light modulation element according to a fifth embodiment.
FIG. 23 is a graph showing the results of a simulation performed on the light modulation device of FIG. 22;
24 is a graph showing the results of a simulation performed on the light modulation device of FIG.
FIG. 25 is a block diagram showing a configuration of an embodiment of a fiber radio system according to the present invention.
[Explanation of symbols]
11 Substrate
12 Optical waveguide
12a branch optical waveguide
12b Branch optical waveguide
12x entrance side optical waveguide
12y exit side optical waveguide
13 Parallel coupled line
13a railway
13b railway
14 Ground plane
15 Input line
16 connecting lines
17 gap
18a junction
18b junction
209 Coaxial connector
210 Coaxial connector center conductor
211 Intermediate connecting member
212a, 212b Jig for fixing substrate
213a, 13b Branch of optical waveguide
214 first connection part
215 Second connection unit

Claims (23)

An optical waveguide formed at least in part from a material having an electro-optic effect,
A modulation electrode having first and second conductor lines electromagnetically coupled, and applying a modulation electric field to the optical waveguide;
A conductive layer that forms a first microstrip line with the first conductor line, and forms a second microstrip line with the second conductor line;
An electric signal input unit for supplying a high-frequency signal for light modulation to the modulation electrode,
A connecting member for connecting the first and second conductor lines to each other at both ends;
With
The light modulation element, wherein the first and second conductor lines function as an odd-mode resonator of the high-frequency signal. The optical waveguide has at least two branched optical waveguides, an optical input unit that couples the two optical waveguides, and an optical output unit that couples the two optical waveguides,
A portion of the optical waveguide to which the modulation electric field is applied is divided into the two branch optical waveguides,
The said modulation | alteration electrode is arrange | positioned so that an electric field of a different polarity may be applied to each of the said two branch optical waveguides, and functions as an intensity modulator which modulates the intensity | strength of the light input into the said optical waveguide. 3. The light modulation element according to claim 1. The modulation electrode is arranged to modulate a refractive index of a portion of the optical waveguide to which the modulation electric field is applied, and functions as a phase modulator that modulates a phase of light input to the optical waveguide. The light modulation device according to claim 1. The light modulation element according to claim 1, wherein the optical waveguide has at least two portions having different remanent polarizations. The light modulation device according to claim 1, wherein the optical waveguide is formed on a substrate having an electro-optic effect. The electric signal input unit has an input line that forms a microstrip line with the conductive layer,
The light modulation element according to claim 1, wherein the input line is connected to one of the first and second conductor lines. The electric signal input unit has a coaxial connector connected to a line that propagates the optical modulation high-frequency signal, and an intermediate connection member that electrically connects the coaxial connector and the modulation electrode. Item 2. The light modulation element according to item 1. An optical waveguide formed at least in part from a material having an electro-optic effect,
A modulation electrode having first and second conductor lines electromagnetically coupled to each other and applying a modulation electric field to the optical waveguide;
A conductive layer that forms a first microstrip line with the first conductor line, and forms a second microstrip line with the second conductor line;
An electric signal input unit for supplying a high-frequency signal for light modulation to the modulation electrode,
With
The optical waveguide has at least two portions having different remanent polarizations,
The light modulation element, wherein the first and second conductor lines function as an odd-mode resonator of the high-frequency signal. The optical waveguide has at least two branched optical waveguides, an optical input unit that couples the two optical waveguides, and an optical output unit that couples the two optical waveguides,
A portion of the optical waveguide to which the modulation electric field is applied is divided into the two branch optical waveguides,
The first and second conductor lines are arranged so as to exert electric fields having different polarities on each of the two branch optical waveguides, and an intensity modulator for modulating the intensity of light input to the optical waveguides The light modulation element according to claim 8, which functions as: The modulation electrode is arranged to modulate a refractive index of a portion of the optical waveguide to which the modulation electric field is applied, and functions as a phase modulator that modulates a phase of light input to the optical waveguide. An optical modulator according to claim 8. The light modulation element according to claim 8, further comprising a connecting member that connects the first and second conductor lines to each other at at least one end. The light modulation device according to claim 8, wherein the optical waveguide is formed on a substrate having an electro-optic effect. The electric signal input unit has an input line that forms a microstrip line with the conductive layer,
The light modulation device according to claim 8, wherein the input line is connected to one of the first and second conductor lines. The electric signal input unit,
A coaxial connector connected to a line for transmitting the high-frequency signal for optical modulation,
The light modulation device according to claim 8, further comprising an intermediate connection member that electrically connects the coaxial connector and the modulation electrode. An optical waveguide formed at least in part from a material having an electro-optic effect,
A modulation electrode having first, second, and third conductor lines electromagnetically coupled to each other and applying a modulation electric field to the optical waveguide;
A first microstrip line is formed with the first conductor line, a second microstrip line is formed with the second conductor line, and a third microstrip line is formed with the third conductor line. A conductive layer;
An electric signal input unit for supplying a high-frequency signal for light modulation to the modulation electrode,
A light modulation element comprising: The optical waveguide has at least two branched optical waveguides, an optical input unit that couples the two optical waveguides, and an optical output unit that couples the two optical waveguides,
A portion of the optical waveguide to which the modulation electric field is applied is divided into the two branch optical waveguides,
The first and second conductor lines are arranged so as to exert electric fields having different polarities on one of the two branch optical waveguides, and the third and second conductor lines are arranged on the two branch optical waveguides. The light modulation element according to claim 15, wherein the light modulation element is arranged so as to apply electric fields having different polarities to the other of the branch optical waveguides, and functions as an intensity modulator that modulates the intensity of light input to the optical waveguide. The modulation electrode is arranged to modulate a refractive index of a portion of the optical waveguide to which the modulation electric field is applied, and functions as a phase modulator that modulates a phase of light input to the optical waveguide. The light modulation device according to claim 15. The light modulation device according to claim 15, further comprising a connecting member that connects the first, second, and third conductor lines to each other at at least one end. The light modulation device according to claim 15, wherein the optical waveguide has at least two portions having different remanent polarizations. The light modulation device according to claim 15, wherein the optical waveguide is formed on a substrate having an electro-optic effect. The electric signal input unit has an input line that forms a microstrip line with the conductive layer,
The light modulation device according to claim 15, wherein the input line is connected to one of the first and third conductor lines. The electric signal input unit,
A coaxial connector connected to a line for transmitting the high-frequency signal for optical modulation,
The light modulation element according to claim 15, further comprising an intermediate connection member that electrically connects the coaxial connector and the modulation electrode. An optical modulation device according to any one of claims 1 to 22,
An input unit for inputting light to the light modulation element,
A control unit that supplies the modulation high-frequency signal to the light modulation element,
A communication system comprising:

Images