JP3558529B2

JP3558529B2 - Optical modulator and optical modulator

Info

Publication number: JP3558529B2
Application number: JP22709098A
Authority: JP
Inventors: 臣一下津; 裕俊永田; 純一郎箕輪
Original assignee: Sumitomo Osaka Cement Co Ltd
Current assignee: Sumitomo Osaka Cement Co Ltd
Priority date: 1998-08-11
Filing date: 1998-08-11
Publication date: 2004-08-25
Anticipated expiration: 2018-08-11
Also published as: JP2000056282A

Description

【０００１】
【発明の技術分野】
本発明は、ニオブ酸リチウム等の電気光学効果を有する材質からなる基板上に光導波路、接地電極および信号電極が形成されている光変調器の形態に関するものであり、特に、高速長距離用に適した光変調器および光変調装置に関するものである。
【０００２】
【従来の技術】
電気光学効果を有する材質からなる基板上に、接地電極、信号電極および光導波路を形成し、信号電極に高周波信号を印加し、光導波路を伝搬する光波を変調する変調器が知られている。図８は、この中でも特に進行波型の信号電極を２つ備えた形態の一例を示す（特開平２−１９６２１２号公報）。
【０００３】
この光変調器４１は、いわゆるマッハツェンダー型の変調器である。この変調器においては、動作速度を一層向上させる目的で、いわゆるコプレナーウエーブガイド型（ＣＰＷ）の形態を有する接地電極および信号電極を使用している。
【０００４】
基板１２は電気光学単結晶からなる。基板１２は、主面１２ｅ、一対の端面１２ａ、１２ｂ、一対の側面１２ｃ、１２ｄを備えている。主面１２ｅには、例えばチタン拡散光導波路９が形成されている。この光導波路９は、入力側の端面１２ａと出力側の端面１２ｂとの間に延びており、入力部分９ａ、入力側分岐部９ｉ、第一の分岐導波路９ｂ、９ｄ、９ｆ、第二の分岐導波路９ｃ、９ｅ、９ｇ、出力部分９ｈを備えている。９ｄと９ｅとは、互いに略平行な分岐導波路であり、この部分に信号電極を設け、高周波信号を作用させる。基板１２の主面１２ｅに、特定形状の接地電極１３Ｇ、１３Ｈ、１３Ｉと、一対の信号電極４６、４７とが設けられている。各接地電極と信号電極との間は、絶縁領域である。
【０００５】
第一の信号電極４６、第二の信号電極４７の各入力部１４Ａ、１４Ｂからそれぞれ高周波信号を印加すると、高周波信号は各接続部４６ａ、４７ａを伝搬し、各電極の各作用部４６ｂ、４７ｂに入り、各作用部中を伝搬し、各接続部４６ｃ、４７ｃから出力部１５Ａ、１５Ｂへと伝搬される。
【０００６】
【発明が解決しようとする課題】
こうしたいわゆる二電極タイプの光変調器においては、マッハツェンダー型の光導波路９を使用していることから、これに合わせるために、基板の中心線（図８において縦横方向の各中心線）に対してそれぞれ線対称をなすように、各信号電極４６、４７を形成している。しかし、本発明者が、このタイプの光変調器をパッケージ内に収容し、外部の高周波電源との結線を行うとき、高周波伝送ケーブルの結線作業が煩雑であった。しかも、高周波電源から同じ振幅の電圧を各信号電極４６、４７へと印加し、各分岐導波路に光を伝搬させ、信号光のオン−オフ制御を行ったところ、実際には所定の信号を印加したときに各分岐導波路９ｄ、９ｅに対して作用する実効電圧が等しくならず、動作電圧に差やバラツキが生じていた。
【０００７】
本発明の課題は、電気光学基板と、光導波路と、光導波路中を伝搬する光波を制御するための複数の進行波型の信号電極と、接地電極と、信号電極に対して高周波信号を入力するための複数の入力部とを備えている光変調器において、各信号電極における各動作電圧の差をなくし、あるいはバラツキを少なくすることである。
【０００８】
【課題を解決するための手段】
本発明に係る光変調器は、電気光学効果を有する材質からなる、主面と一対の端面と一対の側面とを有する基板、基板の主面上において一対の端面の間に延びており、光波を伝搬するための光導波路、光導波路中を伝搬する光波を制御するための複数の進行波型の信号電極、基板の主面上に設けられている接地電極、および各々の信号電極に対して高周波信号を入力するための入力部を少なくとも備えており、各々の入力部が基板の一対の側面の同一面側またはその近傍に設けられていることを特徴とする。
【０００９】
以下、本発明の原理について述べる。図１（ａ）は、光変調器４が容器２中に収容されている装置１を模式的に示す図であり、図１（ｂ）は、容器２の側面図である。容器２は薄い平板形状をしており、容器２中に平板状の光変調器４が収容されている。光変調器４の両方の端面にはそれぞれ光ファイバー５Ａ、５Ｂが接続されており、各光ファイバーは容器２から外部へと引き出されている。光変調器４上には接地電極１３Ａ、１３Ｂ、１３Ｃと信号電極１４Ａ、１４Ｂが載置されている。容器２の側面には一対の高周波信号入力端子６Ａ、６Ｂが設けられており、これらの各端子に対して、外部の高周波信号電源が接続される。
【００１０】
本発明者は、端子６Ａ、６Ｂと光変調器の各入力部１４Ａ、１４Ｂとの各結線に着目し、検討を加えた。各端子６Ａ、６Ｂと各入力部１４Ａ、１４Ｂとは、それそれ高周波伝送ケーブルによって結線されており、各ケーブルは容器中に収容されているのであるが、容器２内の容積は極めて小さいために、入力部１４Ａと端子６Ａとは極めて短いケーブルで結線できるが、入力部１４Ｂと端子６Ｂとは容器２の中で反対側に位置しているために、かなり長いケーブルを必要とする上、このケーブルを容器２の中で光変調器４を避けるように曲折させる必要がある。このため、容器中への光変調器４の収容と結線作業が煩雑となるだけでなく、端子６Ａと６Ｂとに対して同振幅の高周波信号を印加したときに、この電圧が各入力部１４Ａ、１４Ｂに到達するまでの減衰量が異なり、このために各分岐導波路９ｄ、９ｅに印加される実効電圧が異なってくることを見いだした。
【００１１】
本発明者は、この問題点の発見に立脚して、例えば図２に示すように、基板１２の一方の側面１２ｃ側に両方の入力部１４Ａ、１４Ｂを設けることを想到し、本発明を着想した。これによって、高周波伝送ケーブル７Ａ、７Ｂを容器１の中で光変調器４を避けるように曲折させる必要がなくなり、かつ各端子６Ａ、６Ｂと、対応する各入力部１４Ａ、１４Ｂとの結線を、同じ長さのケーブルを使用して容易に行えるので、ケーブル内の伝送損失の相違に起因する、各分岐導波路９ｄ、９ｅへの実効電圧のずれを防止できる。
【００１２】
また、本発明に係る光変調器は、前記の光変調器と、光変調器の光導波路に対して光学的に接続されている光伝送手段と、高周波信号の複数の入力部に対して連結されている高周波信号伝送手段と、光変調器、光伝送手段および高周波信号伝送手段を収容する容器と、容器中の各高周波信号伝送手段をそれぞれ容器の外部へと連結するために容器に設けられている複数の高周波信号端子とを少なくとも備えていることを特徴とする。
【００１３】
本発明の好適な態様においては、上述した光変調器における、各々の入力部が基板の一対の側面の同一面側又はその近傍に設けられる構成に加え、複数の信号電極が少なくとも第一の信号電極と第二の信号電極とを含んでおり、光導波路が、少なくとも入力側の分岐部、この分岐部よりも下流の少なくとも第一の分岐導波路および第二の分岐導波路を備えており、第一の信号電極が第一の分岐導波路に対して作用するものであり、第二の信号電極が第二の分岐導波路に対して作用するものにおいて、前記第一の信号電極および前記第二の信号電極における、前記各入力部から、高周波信号と各分岐導波路を伝搬する光波との作用開始点までの到達時間の差と、入力側分岐部から各作用開始点への光波の到達時間の差とを、実質的に一致あるいは、２つの差の較差を各信号電極における高周波信号の周期の実質的整数倍とする。
これにより、高周波信号の入力部が基板の一対の側面の同一面側又はその近傍に設けられた場合においても、各作用開始点における各高周波信号の位相を適切に調整することが可能となる。
【００１４】
一実施の対応としては、第一の信号電極の入力部と作用開始点との距離が、第二の信号電極の入力部と作用開始点との距離よりも長く、第一の分岐導波路における入力側分岐部と作用開始点との光路長が、第二の分岐光導波路における入力側分岐部と作用開始点との光路長よりも短い。
【００１５】
この場合において好ましくは、第一の信号電極と第二の電極における高周波信号の各作用開始点への各到達時間の差と、入力側分岐部から各作用開始点への光の各到達時間の差とを、実質的に一致あるいは、２つの差の較差を各信号電極における高周波信号の周期の実質的整数倍とする。ここで、一致とは、前記の到達時間差が次の信号と干渉しないことを表す。例えば、Ａ（Ｇｂ／ｓ）の伝送を行う場合、２／Ａ×１０⁹（ｓ）となることを示す。
【００１６】
また、一実施対応としては、第一の分岐導波路における入力側分岐部と作用開始点との光路長と、第二の分岐導波路における入力側分岐部と作用開始点との光路長とが、実質的に等しい場合、第一の信号電極の入力部と作用開始点との距離と、第二の信号電極の入力部と作用開始点との距離との差が、各信号電極における高周波信号の波長の整数倍である。これによって、各作用開始点における高周波信号の位相が一致する。
【００１７】
また、第一の各信号電極と第二の信号電極とを、基板の主面の中点に対して、基板の主面上で略点対称の形態とすることによって、光変調器に対して温度差が加わったときに、各信号電極における歪みをある程度消去し、温度変化に起因するドリフトを緩和できる。
【００１８】
他の態様においては、複数の信号電極について各々の出力部を備えており、第一の信号電極の入力部と出力部との距離が、第二の信号電極の入力部と出力部との距離と実質的に等しく、各出力部が一方の側面側に設けられている。これによって、高周波信号の出力側においても、入力側とまったく同時に、高周波伝送ケーブルの結線、接続を行うことができるので、実装プロセスが著しく簡略化される。
【００１９】
前記光変調器における信号電極の数は、最低でも２個であるが、場合によっては３個以上設けることができる。例えば、いわゆるカスケード構造の光導波路を基板上に設けた場合においては、分岐導波路が例えば４本、あるいは８本設けられることになるが、本発明に従って、各分岐導波路に対してそれぞれ対応する信号電極を設け、各信号電極について、本発明の前記条件を満足するように、各信号電極および各分岐導波路の位置関係を設計することにより、各動作電圧差等をなくすことができる。
【００２０】
基板の材質としては、ニオブ酸リチウム、タンタル酸リチウム、ニオブ酸リチウム−タンタル酸リチウム固溶体等の電気光学結晶が特に好ましい。また、基板を構成する電気光学結晶の結晶方位は、ｚカットまたはｘカットが好ましい。光導波路の形成方法としては、チタン拡散法等の内拡散法やプロトン交換法を利用できる。本発明の光変調器は、光変調器、光位相変調器、偏波スクランブラ、光スイッチング素子、光コンピューター用の光論理素子等として好適に使用できる。
【００２１】
図２は、本発明の一実施形態に係る光変調器１１を示す平面図であり、図３は、図２のうち基板、各信号電極、光導波路の部分の寸法および位置関係を示す平面図である。光変調器の各構成部分のうち、図８に既に示した構成部分については、同じ符号を付け、その説明は省略する。
【００２２】
本実施形態では、一方の側面１２ｃ側に入力部１４Ａ、１４Ｂが設けられており、他方の側面１２ｄに出力部１５Ａ、１５Ｂが設けられている。第一の信号電極１６は、入力部１４Ａから端面１２ａ、１２ｂに平行に延びる接続部１６ａ、分岐導波路９ｄと重なるように、あるいは分岐導波路９ｄと平行に延びる作用部１６ｂ、および出力部１５Ａと作用部１６ｂとを接続している接続部１６ｃを備えている。
【００２３】
第二の信号電極１７は、入力部１４Ｂから端面に平行に延びる接続部１７ａ、分岐導波路９ｅと重なるように、あるいは分岐導波路９ｅと平行に延びる作用部１７ｂ、および出力部１５Ｂと作用部１７ｂとを接続している接続部１７ｃを備えている。各接続部１６ａ、１７ｃは、それぞれ、各分岐導波路９ｅ、９ｄと交叉している。信号電極１６と１７とは、全体として、基板１２の主面１２ｅの中心点Ｏに対して主面上で点対称の形態をなしている。
【００２４】
信号電極１６と端面１２ａ、他方の側面１２ｄとの間には、接地電極１３Ａが設けられている。信号電極１７と端面１２ｂ、一方の側面１２ｃとの間には、接地電極１３Ｂが設けられている。信号電極１６と１７との間には、接地電極１３Ｃが設けられている。
【００２５】
図３において、Ａ、Ｃは、各分岐導波路９ｄ、９ｅと各信号電極１６、１７との相互作用が始まる作用開始点であり、Ｂ、Ｄは作用終結点である。Ｅ、Ｆは、それぞれ、各信号電極と各分岐導波路とが直交する交叉点である。各入力部１４Ａ、１４Ｂから高周波信号を入力すると、信号電極１６と１７との間では、各入力部から作用開始点Ａ、Ｃに到達するまでの距離は相違しているので、作用開始点において高周波信号に遅延が生ずる。遅延時間の大きさは、高周波信号の実効屈折率をｎｍとし、距離の差をＬ１とすると、ｎｍ×Ｌ１／ｃとなる。
【００２６】
しかし、これと同時に、９ａ側から入射してきた光には、分岐導波路９ｄ側と９ｅ側との間で、Ｌ２の距離の差があり、光の屈折率をｎ０とすると、ｎ０×Ｌ２／ｃの時間差が生ずる。従って、ｎｍ×Ｌ１／ｃ−ｎ０×Ｌ２／ｃを、各信号電極における高周波信号の周期の整数倍近くにすることにより、各作用開始点における各高周波信号の位相を合わせることができる。また、ｎｍ×Ｌ１／ｃ−ｎ０×Ｌ２／ｃを０近くにすることによって、分岐導波路を伝搬する光に対する高周波信号の作用が開始する時間の遅延を少なくし、あるいは実質的に０とすることができる。また、各信号電極における作用長Ｌ３を等しくすることによって、各分岐導波路における光の位相シフト量を等しくできる。なお、この設計では、作用長Ｌ３を一定にしたときに必要な信号電極の全長を短くできるので、広帯域特性が得られる。
【００２７】
図４は、本発明の他の実施形態に係る光変調器２１を示す平面図であり、図５は、図４のうち基板、各信号電極、光導波路の部分の寸法および位置関係を示す平面図である。
【００２８】
本実施形態でも、一方の側面１２ｃ側に入力部１４Ａ、１４Ｂが設けられており、他方の側面１２ｄに出力部１５Ａ、１５Ｂが設けられている。第一の信号電極２６は、入力部１４Ａから端面１２ａ、１２ｂに平行に延びる接続部２６ａ、分岐導波路９ｄと重なっている作用部２６ｃ、作用部２６ｃと接続部２６ａとを接続し、かつ分岐導波路９ｄとは平行に延びる接続部２６ｂ、および出力部１５Ａと作用部２６ｃとを接続している接続部２６ｄを備えている。
【００２９】
第二の信号電極２７は、入力部１４Ｂから端面に平行に延びる接続部２７ａ、分岐導波路９ｅと重なっている作用部２７ｂ、作用部２７ｂから更に端面１２ｂ側へと延びる接続部２７ｃ、および出力部１５Ｂと接続部２７ｃとを接続している接続部２７ｄを備えている。各信号電極２６、２７は、光導波路９の入力部分９ａ、出力部分９ｈとＧ、Ｈで交叉している。信号電極２６と２７とは、全体として、基板１２の主面１２ｅの中心点Ｏに対して点対称の形態をなしている。
【００３０】
図５において、Ａ、Ｃは、各分岐導波路９ｄ、９ｅと各信号電極２６、２７との相互作用が始まる作用開始点であり、Ｂ、Ｄは作用終結点である。各入力部１４Ａ、１４Ｂから高周波信号を入力すると、信号電極２６と２７との間では、各入力部から作用開始点Ａ、Ｃに到達するまでの距離は相違しているので、作用開始点において高周波信号に遅延が生ずる。遅延時間の大きさは、高周波信号の実効屈折率をｎｍとし、距離の差をＬ４＋Ｌ５とすると、ｎｍ×（Ｌ４＋Ｌ５）に比例する。
【００３１】
本例においては、図２、３の例とは異なり、光導波路の分岐部分９ｉから各作用開始点Ａ、Ｃへの光路長は等しいので、これによる上記遅延の緩和作用はない。このため、Ｌ４＋Ｌ５を、この信号電極中における高周波信号の波長の整数倍とすることによって、作用開始点ＡとＣとにおける各高周波信号の位相を合わせることができる。Ｌ６は作用長である。
【００３２】
しかも、図２、３の場合とは異なり、本例では各信号電極が分岐前の入力部分９ａおよび結合後の出力部分９ｈにおいて光導波路と交叉しており（Ｇ、Ｈ）、各分岐導波路９ｄ、９ｅに対して交叉していない。従って、交叉点Ｅ、Ｆの周辺における不要な変調を防止できる。
【００３３】
また、図２、または図４の光変調器においては、各信号電極の全長を等しくできるので、各信号電極を伝搬する間の高周波信号の振幅の減衰量を等しくすることができる。
【００３４】
図６は、本発明の他の実施形態に係る光変調器３１を示す平面図であり、図７は、図６のうち基板、各信号電極、光導波路の部分の寸法および位置関係を示す平面図である。
【００３５】
本実施形態では、一方の側面１２ｃ側に、入力部１４Ａ、１４Ｂ、出力部１５Ａ、１５Ｂが設けられている。信号電極３６は、入力部１４Ａから端面１２ａ、１２ｂに平行に延びる接続部３６ａ、側面１２ｃと平行に延びる接続部３６ｂ、分岐導波路９ｄと重なっている作用部３６ｃ、側面１２ｃと平行に延びる接続部３６ｄ、および接続部３６ｄと出力部１５Ａとを接続している、端面と平行に延びる接続部３６ｅを備えている。信号電極３７は、入力部１４Ｂから端面に平行に延びる接続部３７ａ、分岐導波路９ｅと重なっている作用部３７ｂ、作用部３７ｂから端面と平行に延びる接続部３７ｃ、接続分岐導波路３７ｃから蛇行して入力部１４Ｂ側へと戻っている接続部３７ｄを備えており、接続部３７ｄの末端が出力部１５Ｂにつながっている。信号電極３６は、光導波路９の入力部分９ａ、出力部分９ｈとＧ、Ｈで交叉している。１３Ｄ、１３Ｅ、１３Ｆは接地電極である。
【００３６】
図７において、Ａ、Ｃは、各分岐導波路９ｄ、９ｅと各信号電極３６、３７との相互作用が始まる作用開始点であり、Ｂ、Ｄは作用終結点である。各入力部１４Ａ、１４Ｂから高周波信号を入力すると、信号電極３６と３７との間では、各入力部から作用開始点Ａ、Ｃに到達するまでの距離は相違しているので、作用開始点において高周波信号に遅延が生ずる。遅延時間の大きさは、高周波信号の実効屈折率をｎｍとし、距離の差をＬ４＋Ｌ５とすると、ｎｍ×（Ｌ４＋Ｌ５）に比例する。
【００３７】
本例においても、光導波路の分岐部分９ｉから各作用開始点Ａ、Ｃへの光路長は等しいので、これによる上記遅延の緩和作用はない。このため、Ｌ４＋Ｌ５を、この信号電極中における高周波信号の波長の整数倍とすることによって、作用開始点ＡとＣとにおける各高周波信号の位相を合わせることができる。
【００３８】
本例では、更に、蛇行する接続部３７ｃ、３７ｄを設けることによって、各信号電極の長さの差を小さくすることができ、あるいは設計によっては両方の信号電極の長さを等しくすることができる。
【００３９】
図２−図７に示すような各光変調器は、通常法によって製造できる。例えばＺカットニオブ酸リチウムウエハー上に、フォトリソグラフィー法によってチタンをパターニングし、熱拡散法で光導波路を形成できる。この際には、例えば、チタン厚さを８００オングストロームとし、幅を７μｍとし、拡散温度を１０００℃とし、拡散時間を２０時間とし、基板の主面に酸化珪素をパターニングして（厚さ０．５−２．０μｍ）バッファー層を形成する。更に、バッファー層の上に、レジスト等をパターニングし、その上に各電極のパターンを、厚さ１５−３０μｍのメッキによって形成できる。
【００４０】
【発明の効果】
本発明によれば、電気光学基板と、光導波路と、複数の進行波型の信号電極と、接地電極と、各々の信号電極に対して高周波信号を入力するための入力部とを備えている光変調器において、各信号電極における各動作電圧の差をなくし、あるいはバラツキを少なくでき、また、容器中への光変調器の収容と高周波信号伝送手段の各信号電極への結線作業を容易にできる。
【図面の簡単な説明】
【図１】（ａ）は、容器１内に光変調器４を収容した状態を模式的に示す図であり、（ｂ）は、容器側面の各高周波信号端子を示す図である。
【図２】本発明の一実施形態に係る光変調器１１を示す平面図である。
【図３】図２の光変調器１１から、信号電極および光導波路の位置と寸法との関係を抽出して示す平面図である。
【図４】本発明の他の実施形態に係る光変調器２１を示す平面図である。
【図５】図４の光変調器２１から、信号電極および光導波路の位置と寸法との関係を抽出して示す平面図である。
【図６】本発明の他の実施形態に係る光変調器３１を示す平面図である。
【図７】図６の光変調器３１から、信号電極および光導波路の位置と寸法との関係を抽出して示す平面図である。
【図８】従来の光変調器の平面図である。
【符号の説明】
１光変調装置
２容器（パッケージ）
４、１１、２１、３１光変調器
５Ａ、５Ｂ光ファイバー（光伝送手段）
１２基板
１２ａ、１２ｂ端面
１２ｃ一方の側面
１２ｄ他方の側面
１２ｅ主面
１３Ａ、１３Ｂ、１３Ｃ、１３Ｄ、１３Ｅ、１３Ｆ接地電極
１４Ａ、１４Ｂ入力部
１５Ａ、１５Ｂ出力部
１６、１７、２６、２７、３６、３７信号電極
Ａ、Ｃ作用開始点
Ｂ、Ｄ作用終結点
Ｅ、Ｆ、Ｇ、Ｈ信号電極と光導波路との交叉点[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a form of an optical modulator in which an optical waveguide, a ground electrode, and a signal electrode are formed on a substrate made of a material having an electro-optical effect such as lithium niobate. The present invention relates to a suitable light modulator and light modulation device.
[0002]
[Prior art]
2. Description of the Related Art A modulator that forms a ground electrode, a signal electrode, and an optical waveguide on a substrate made of a material having an electro-optic effect, applies a high-frequency signal to the signal electrode, and modulates a light wave propagating through the optical waveguide is known. FIG. 8 shows an example of a mode in which two traveling wave signal electrodes are provided (Japanese Patent Laid-Open No. 2-196212).
[0003]
The optical modulator 41 is a so-called Mach-Zehnder type modulator. In this modulator, a ground electrode and a signal electrode having a so-called coplanar wave guide (CPW) form are used for the purpose of further improving the operation speed.
[0004]
The substrate 12 is made of an electro-optic single crystal. The substrate 12 has a main surface 12e, a pair of end surfaces 12a and 12b, and a pair of side surfaces 12c and 12d. On the main surface 12e, for example, a titanium diffused optical waveguide 9 is formed. The optical waveguide 9 extends between the input-side end face 12a and the output-side end face 12b, and includes an input portion 9a, an input-side branch portion 9i, first branch waveguides 9b, 9d, 9f, and a second branch waveguide 9b. Branch waveguides 9c, 9e, 9g and an output portion 9h are provided. Reference numerals 9d and 9e denote branch waveguides substantially parallel to each other. A signal electrode is provided in this portion to allow a high-frequency signal to act. On a main surface 12e of the substrate 12, ground electrodes 13G, 13H, and 13I having a specific shape and a pair of signal electrodes 46 and 47 are provided. An insulating region is provided between each ground electrode and the signal electrode.
[0005]
When a high-frequency signal is applied from each of the input portions 14A and 14B of the first signal electrode 46 and the second signal electrode 47, the high-frequency signal propagates through each of the connection portions 46a and 47a, and each of the action portions 46b and 47b of each electrode. , And propagates through each action section, and is propagated from each connection section 46c, 47c to the output sections 15A, 15B.
[0006]
[Problems to be solved by the invention]
In such a so-called two-electrode type optical modulator, since the Mach-Zehnder type optical waveguide 9 is used, in order to match this, with respect to the center line of the substrate (the center lines in the vertical and horizontal directions in FIG. 8). The signal electrodes 46 and 47 are formed so as to be line-symmetric with each other. However, when the inventor accommodates this type of optical modulator in a package and performs connection with an external high-frequency power supply, connection work of a high-frequency transmission cable is complicated. In addition, when a voltage having the same amplitude is applied to each of the signal electrodes 46 and 47 from the high-frequency power supply, light is propagated through each branch waveguide, and on / off control of the signal light is performed. When applied, the effective voltages acting on the respective branch waveguides 9d and 9e are not equal, and a difference or variation occurs in operating voltages.
[0007]
An object of the present invention is to input a high-frequency signal to an electro-optic substrate, an optical waveguide, a plurality of traveling wave signal electrodes for controlling light waves propagating in the optical waveguide, a ground electrode, and the signal electrode. Another object of the present invention is to eliminate a difference between operating voltages of signal electrodes in an optical modulator including a plurality of input sections for performing the operation, or to reduce variations.
[0008]
[Means for Solving the Problems]
An optical modulator according to the present invention includes a substrate made of a material having an electro-optical effect, a substrate having a main surface, a pair of end surfaces, and a pair of side surfaces, extending between the pair of end surfaces on the main surface of the substrate, An optical waveguide for propagating light, a plurality of traveling wave type signal electrodes for controlling light waves propagating in the optical waveguide, a ground electrode provided on the main surface of the substrate, and each signal electrode. At least an input unit for inputting a high-frequency signal is provided, and each input unit is provided on or near the same surface of a pair of side surfaces of the substrate.
[0009]
Hereinafter, the principle of the present invention will be described. FIG. 1A is a diagram schematically illustrating a device 1 in which an optical modulator 4 is housed in a container 2, and FIG. 1B is a side view of the container 2. The container 2 has a thin flat plate shape, and the flat optical modulator 4 is accommodated in the container 2. Optical fibers 5A and 5B are connected to both end faces of the optical modulator 4, respectively, and each optical fiber is drawn out of the container 2 to the outside. On the optical modulator 4, ground electrodes 13A, 13B, 13C and signal electrodes 14A, 14B are mounted. A pair of high-frequency signal input terminals 6A and 6B are provided on the side surface of the container 2, and an external high-frequency signal power supply is connected to each of these terminals.
[0010]
The present inventor paid attention to each connection between the terminals 6A and 6B and each of the input sections 14A and 14B of the optical modulator, and made a study. Each of the terminals 6A, 6B and each of the input sections 14A, 14B are connected by a high-frequency transmission cable, and each cable is housed in a container. However, since the volume in the container 2 is extremely small, Although the input section 14A and the terminal 6A can be connected by an extremely short cable, the input section 14B and the terminal 6B are located on the opposite side in the container 2, so that a considerably long cable is required. The cable must be bent in the container 2 so as to avoid the light modulator 4. For this reason, not only is the accommodation and connection work of the optical modulator 4 in the container complicated, but also when a high-frequency signal having the same amplitude is applied to the terminals 6A and 6B, this voltage is applied to each input section 14A. , And 14B, so that the effective voltage applied to each of the branch waveguides 9d and 9e is different.
[0011]
Based on the discovery of this problem, the inventor of the present invention has conceived of providing both input sections 14A and 14B on one side surface 12c side of the substrate 12, for example, as shown in FIG. did. Thereby, it is not necessary to bend the high-frequency transmission cables 7A and 7B in the container 1 so as to avoid the optical modulator 4, and the connection between the terminals 6A and 6B and the corresponding input sections 14A and 14B is Since it is easy to use cables of the same length, it is possible to prevent the effective voltage from being shifted to the branch waveguides 9d and 9e due to the difference in transmission loss in the cables.
[0012]
In addition, the optical modulator according to the present invention includes the optical modulator, an optical transmission unit optically connected to an optical waveguide of the optical modulator, and a plurality of input units for a high-frequency signal. A high-frequency signal transmission means, a light modulator, a container for housing the light transmission means and the high-frequency signal transmission means, and a high-frequency signal transmission means provided in the container for connecting each high-frequency signal transmission means in the container to the outside of the container. And at least a plurality of high-frequency signal terminals.
[0013]
In a preferred aspect of the present invention, in the optical modulator described above, in addition to the configuration in which each input unit is provided on or near the same surface of a pair of side surfaces of the substrate, a plurality of signal electrodes are provided at least for the first signal. Comprising an electrode and a second signal electrode, the optical waveguide comprises at least a branch portion on the input side, at least a first branch waveguide and a second branch waveguide downstream of the branch portion, The first signal electrode acts on the first branch waveguide, and the second signal electrode acts on the second branch waveguide, wherein the first signal electrode and the second In the two signal electrodes, the difference in arrival time from the input portion to the operation start point between the high-frequency signal and the light wave propagating through each branch waveguide, and the arrival of the light wave from the input side branch portion to each operation start point. Substantially match the time difference The hidden two difference be substantially integral multiple of the period of the high frequency signals in the signal electrodes.
This makes it possible to appropriately adjust the phase of each high-frequency signal at each operation start point, even when the input section for the high-frequency signal is provided on the same side of the pair of side surfaces of the substrate or in the vicinity thereof.
[0014]
In one embodiment, the distance between the input portion of the first signal electrode and the operation start point is longer than the distance between the input portion of the second signal electrode and the operation start point, and The optical path length between the input side branch and the operation start point is shorter than the optical path length between the input side branch and the operation start point in the second branch optical waveguide.
[0015]
In this case, preferably, the difference between the respective arrival times of the high-frequency signals at the first signal electrode and the second electrode to the respective operation start points, and the respective arrival times of the light from the input side branch to the respective operation start points. The difference is substantially equal to the difference, or the difference between the two differences is a substantially integral multiple of the period of the high-frequency signal at each signal electrode. Here, the agreement means that the arrival time difference does not interfere with the next signal. For example, when transmission of A (Gb / s) is performed, 2 / A × 10 ⁹ (s) is indicated.
[0016]
Further, as one implementation correspondence, the optical path length between the input side branch portion and the operation start point in the first branch waveguide and the optical path length between the input side branch portion and the operation start point in the second branch waveguide are different. , When substantially equal, the difference between the distance between the input portion of the first signal electrode and the operation start point and the distance between the input portion of the second signal electrode and the operation start point is a high-frequency signal at each signal electrode. Is an integral multiple of the wavelength of. As a result, the phases of the high-frequency signals at the respective operation start points match.
[0017]
Further, the first signal electrode and the second signal electrode are substantially point-symmetric on the main surface of the substrate with respect to the center of the main surface of the substrate, so that the When a temperature difference is applied, distortion in each signal electrode can be eliminated to some extent, and drift caused by a temperature change can be reduced.
[0018]
In another aspect, each of the plurality of signal electrodes has an output portion, and the distance between the input portion and the output portion of the first signal electrode is equal to the distance between the input portion and the output portion of the second signal electrode. And each output section is provided on one side. As a result, the high-frequency transmission cable can be connected and connected on the output side of the high-frequency signal at the same time as the input side, so that the mounting process is significantly simplified.
[0019]
The number of signal electrodes in the optical modulator is at least two, but may be three or more in some cases. For example, when an optical waveguide having a so-called cascade structure is provided on a substrate, for example, four or eight branch waveguides are provided. According to the present invention, each branch waveguide is provided with a corresponding one. By providing a signal electrode and designing the positional relationship between each signal electrode and each branch waveguide so as to satisfy the above condition of the present invention, each operating voltage difference and the like can be eliminated.
[0020]
As the material of the substrate, an electro-optic crystal such as lithium niobate, lithium tantalate, or a lithium niobate-lithium tantalate solid solution is particularly preferable. Further, the crystal orientation of the electro-optic crystal constituting the substrate is preferably z-cut or x-cut. As a method for forming the optical waveguide, an internal diffusion method such as a titanium diffusion method or a proton exchange method can be used. The optical modulator of the present invention can be suitably used as an optical modulator, an optical phase modulator, a polarization scrambler, an optical switching element, an optical logic element for an optical computer, and the like.
[0021]
FIG. 2 is a plan view showing an optical modulator 11 according to an embodiment of the present invention, and FIG. 3 is a plan view showing dimensions and a positional relationship of portions of a substrate, each signal electrode, and an optical waveguide in FIG. It is. Of the components of the optical modulator, the components already shown in FIG. 8 are denoted by the same reference numerals, and description thereof will be omitted.
[0022]
In the present embodiment, the input units 14A and 14B are provided on one side surface 12c, and the output units 15A and 15B are provided on the other side surface 12d. The first signal electrode 16 includes a connecting portion 16a extending parallel to the end faces 12a and 12b from the input portion 14A, an operating portion 16b extending so as to overlap with or parallel to the branch waveguide 9d, and an output portion 15A. And a connecting portion 16c for connecting the operating portion 16b.
[0023]
The second signal electrode 17 includes a connecting portion 17a extending in parallel with the end face from the input portion 14B, an operating portion 17b extending so as to overlap with or parallel to the branching waveguide 9e, and an output portion 15B and an operating portion. 17b is provided with a connection portion 17c for connecting the connection portion 17b to the connection portion 17b. Each connection part 16a, 17c crosses each branch waveguide 9e, 9d, respectively. The signal electrodes 16 and 17 have a point-symmetrical shape on the main surface with respect to the center point O of the main surface 12e of the substrate 12 as a whole.
[0024]
A ground electrode 13A is provided between the signal electrode 16, the end face 12a, and the other side face 12d. A ground electrode 13B is provided between the signal electrode 17 and the end face 12b, and one side face 12c. A ground electrode 13C is provided between the signal electrodes 16 and 17.
[0025]
In FIG. 3, A and C are operation start points at which the interaction between the branch waveguides 9d and 9e and the signal electrodes 16 and 17 starts, and B and D are operation end points. E and F are cross points where each signal electrode and each branch waveguide are orthogonal to each other. When a high-frequency signal is input from each of the input sections 14A and 14B, the distance from each of the input sections to the operation start points A and C is different between the signal electrodes 16 and 17. The high-frequency signal is delayed. The magnitude of the delay time is nm × L1 / c, where the effective refractive index of the high-frequency signal is nm and the distance difference is L1.
[0026]
However, at the same time, the light incident from the 9a side has a difference in the distance L2 between the branch waveguides 9d and 9e. If the refractive index of the light is n0, then n0 × L2 / A time difference of c occurs. Therefore, by making nm × L1 / c−n0 × L2 / c close to an integral multiple of the period of the high-frequency signal at each signal electrode, the phase of each high-frequency signal at each operation start point can be matched. Also, by setting nm × L1 / c−n0 × L2 / c close to 0, the delay of the time when the action of the high-frequency signal on the light propagating through the branch waveguide starts is reduced, or substantially set to 0. be able to. Further, by making the working lengths L3 of the respective signal electrodes equal, the phase shift amounts of light in the respective branch waveguides can be made equal. In this design, since the required total length of the signal electrode can be shortened when the operation length L3 is constant, a wide band characteristic can be obtained.
[0027]
FIG. 4 is a plan view showing an optical modulator 21 according to another embodiment of the present invention, and FIG. 5 is a plan view showing dimensions and a positional relationship of a substrate, signal electrodes, and an optical waveguide portion in FIG. FIG.
[0028]
Also in the present embodiment, the input portions 14A and 14B are provided on one side surface 12c side, and the output portions 15A and 15B are provided on the other side surface 12d. The first signal electrode 26 is connected to the connecting portion 26a extending from the input portion 14A in parallel to the end faces 12a and 12b, the operating portion 26c overlapping the branch waveguide 9d, connecting the operating portion 26c to the connecting portion 26a, and branching. The waveguide 9d includes a connecting portion 26b extending in parallel with the waveguide 9d, and a connecting portion 26d connecting the output portion 15A and the operating portion 26c.
[0029]
The second signal electrode 27 includes a connection portion 27a extending from the input portion 14B in parallel with the end face, an operation portion 27b overlapping the branch waveguide 9e, a connection portion 27c extending further from the operation portion 27b to the end face 12b side, and an output. A connection portion 27d connecting the portion 15B and the connection portion 27c is provided. Each of the signal electrodes 26 and 27 crosses the input portion 9a and the output portion 9h of the optical waveguide 9 at G and H. The signal electrodes 26 and 27 have a point symmetry with respect to the center point O of the main surface 12e of the substrate 12 as a whole.
[0030]
In FIG. 5, A and C are operation start points at which the interaction between the branch waveguides 9d and 9e and the signal electrodes 26 and 27 starts, and B and D are operation end points. When a high-frequency signal is input from each of the input sections 14A and 14B, the distance from each of the input sections to the operation start points A and C is different between the signal electrodes 26 and 27. The high-frequency signal is delayed. The magnitude of the delay time is proportional to nm × (L4 + L5), where the effective refractive index of the high-frequency signal is nm and the difference in distance is L4 + L5.
[0031]
In the present example, unlike the examples of FIGS. 2 and 3, the optical path lengths from the branching portion 9i of the optical waveguide to the respective operation start points A and C are equal, so that there is no effect of reducing the delay. Therefore, by setting L4 + L5 to be an integral multiple of the wavelength of the high-frequency signal in the signal electrode, the phases of the high-frequency signals at the operation start points A and C can be matched. L6 is the working length.
[0032]
Further, unlike the cases of FIGS. 2 and 3, in this example, each signal electrode crosses the optical waveguide at the input portion 9a before branching and the output portion 9h after coupling (G, H), and each branching waveguide 9d and 9e do not intersect. Therefore, unnecessary modulation around the intersections E and F can be prevented.
[0033]
Further, in the optical modulator of FIG. 2 or FIG. 4, since the total length of each signal electrode can be made equal, the attenuation of the amplitude of the high-frequency signal during propagation through each signal electrode can be made equal.
[0034]
FIG. 6 is a plan view showing an optical modulator 31 according to another embodiment of the present invention, and FIG. 7 is a plan view showing dimensions and a positional relationship of a substrate, signal electrodes, and an optical waveguide in FIG. FIG.
[0035]
In the present embodiment, the input units 14A and 14B and the output units 15A and 15B are provided on one side surface 12c side. The signal electrode 36 includes a connection portion 36a extending parallel to the end surfaces 12a and 12b from the input portion 14A, a connection portion 36b extending parallel to the side surface 12c, an operation portion 36c overlapping the branch waveguide 9d, and a connection extending parallel to the side surface 12c. A connecting portion 36d that connects the connecting portion 36d and the output portion 15A and that extends in parallel with the end face. The signal electrode 37 includes a connecting portion 37a extending in parallel from the input portion 14B to the end face, an operating portion 37b overlapping the branch waveguide 9e, a connecting portion 37c extending in parallel from the operating portion 37b to the end face, and meandering from the connecting branch waveguide 37c. A connection portion 37d that returns to the input portion 14B side is provided, and an end of the connection portion 37d is connected to the output portion 15B. The signal electrode 36 crosses the input portion 9a and the output portion 9h of the optical waveguide 9 at G and H. 13D, 13E and 13F are ground electrodes.
[0036]
In FIG. 7, A and C are operation start points at which the interaction between the branch waveguides 9d and 9e and the signal electrodes 36 and 37 starts, and B and D are operation end points. When a high-frequency signal is input from each of the input portions 14A and 14B, the distance from each of the input portions to the operation start points A and C is different between the signal electrodes 36 and 37. The high-frequency signal is delayed. The magnitude of the delay time is proportional to nm × (L4 + L5), where the effective refractive index of the high-frequency signal is nm and the difference in distance is L4 + L5.
[0037]
Also in this example, since the optical path length from the branch portion 9i of the optical waveguide to each of the operation start points A and C is equal, there is no effect of reducing the delay. Therefore, by setting L4 + L5 to be an integral multiple of the wavelength of the high-frequency signal in the signal electrode, the phases of the high-frequency signals at the operation start points A and C can be matched.
[0038]
In this example, furthermore, by providing the meandering connection portions 37c and 37d, the difference between the lengths of the respective signal electrodes can be reduced, or the lengths of both the signal electrodes can be made equal depending on the design. .
[0039]
Each optical modulator as shown in FIGS. 2 to 7 can be manufactured by a usual method. For example, titanium can be patterned by photolithography on a Z-cut lithium niobate wafer, and an optical waveguide can be formed by thermal diffusion. At this time, for example, the thickness of titanium is set to 800 Å, the width is set to 7 μm, the diffusion temperature is set to 1000 ° C., the diffusion time is set to 20 hours, and silicon oxide is patterned on the main surface of the substrate (thickness: 0.1 μm). 5-2.0 μm) to form a buffer layer. Further, a resist or the like is patterned on the buffer layer, and a pattern of each electrode can be formed thereon by plating with a thickness of 15 to 30 μm.
[0040]
【The invention's effect】
According to the present invention, an electro-optic substrate, an optical waveguide, a plurality of traveling-wave signal electrodes, a ground electrode, and an input unit for inputting a high-frequency signal to each signal electrode are provided. In the optical modulator, the difference between the operating voltages of the signal electrodes can be eliminated or the variation can be reduced, and the housing of the optical modulator in the container and the connection work of the high-frequency signal transmission means to each signal electrode can be easily performed. it can.
[Brief description of the drawings]
FIG. 1A is a diagram schematically illustrating a state in which an optical modulator 4 is accommodated in a container 1, and FIG. 1B is a diagram illustrating each high-frequency signal terminal on a side surface of the container.
FIG. 2 is a plan view showing an optical modulator 11 according to one embodiment of the present invention.
FIG. 3 is a plan view extracting and showing the relationship between the positions and dimensions of a signal electrode and an optical waveguide from the optical modulator 11 of FIG. 2;
FIG. 4 is a plan view showing an optical modulator 21 according to another embodiment of the present invention.
5 is a plan view extracting and showing the relationship between the positions and dimensions of a signal electrode and an optical waveguide from the optical modulator 21 of FIG.
FIG. 6 is a plan view showing an optical modulator 31 according to another embodiment of the present invention.
FIG. 7 is a plan view extracting and showing the relationship between the positions and dimensions of a signal electrode and an optical waveguide from the optical modulator 31 of FIG. 6;
FIG. 8 is a plan view of a conventional optical modulator.
[Explanation of symbols]
1 light modulator 2 container (package)
4, 11, 21, 31 Optical modulators 5A, 5B Optical fiber (optical transmission means)
12 Board 12a, 12b End face 12c One side face 12d The other side face 12e Principal faces 13A, 13B, 13C, 13D, 13E, 13F Ground electrodes 14A, 14B Input parts 15A, 15B Output parts 16, 17, 26, 27, 36, 37 Signal electrodes A, C Operation start point B, D Operation end points E, F, G, H Intersection points between signal electrode and optical waveguide

Claims

A substrate having a main surface, a pair of end surfaces, and a pair of side surfaces, made of a material having an electro-optic effect, an optical waveguide extending between the pair of end surfaces on the main surface of the substrate and propagating a light wave, A plurality of traveling wave type signal electrodes for controlling light waves propagating in the optical waveguide, a ground electrode provided on the main surface of the substrate, and a high frequency signal for inputting to each signal electrode An optical modulator comprising at least an input unit,
The plurality of signal electrodes include at least a first signal electrode and a second signal electrode, and the optical waveguide has at least a branch portion on an input side, and at least a first branch waveguide downstream of the branch portion. And a second branch waveguide, wherein the first signal electrode acts on the first branch waveguide, and the second signal electrode acts on the second branch waveguide. Thing,
Each of the input units is provided on or near the same surface of a pair of side surfaces of the substrate,
In the first signal electrode and the second signal electrode, from each of the input sections, the difference between the arrival time of the high-frequency signal and the action start point of the light wave propagating through each branch waveguide, and from the input side branch section An optical modulator characterized in that the difference between the arrival times of the light waves at the respective action start points substantially coincides with each other, or the difference between the two differences is substantially an integral multiple of the period of the high-frequency signal at each signal electrode. .

A substrate having a main surface, a pair of end surfaces, and a pair of side surfaces, made of a material having an electro-optic effect, an optical waveguide extending between the pair of end surfaces on the main surface of the substrate and propagating a light wave, A plurality of traveling wave type signal electrodes for controlling light waves propagating in the optical waveguide, a ground electrode provided on the main surface of the substrate, and a high frequency signal for inputting to each signal electrode An optical modulator comprising at least an input unit,
The plurality of signal electrodes include at least a first signal electrode and a second signal electrode, and the optical waveguide has at least a branch portion on an input side, and at least a first branch waveguide downstream of the branch portion. And a second branch waveguide, wherein the first signal electrode acts on the first branch waveguide, and the second signal electrode acts on the second branch waveguide. Thing,
Each of the input units is provided on or near the same surface of a pair of side surfaces of the substrate,
The distance between the input portion of the first signal electrode and the operation start point is longer than the distance between the input portion of the second signal electrode and the operation start point, and the input in the first branch waveguide is An optical modulator wherein the optical path length between the side branch and the operation start point is shorter than the optical path length between the input side branch and the operation start point in the second branch optical waveguide, and wherein the first signal electrode and The difference between the time of arrival of the high-frequency signal at the second signal electrode to each operation start point and the difference between the time of light arrival from the input side branch to each operation start point substantially coincides with each other, or two differences. Wherein the difference is set to a substantially integral multiple of the period of the high-frequency signal at each signal electrode.

A substrate having a main surface, a pair of end surfaces, and a pair of side surfaces, made of a material having an electro-optic effect, an optical waveguide extending between the pair of end surfaces on the main surface of the substrate and propagating a light wave, A plurality of traveling wave type signal electrodes for controlling light waves propagating in the optical waveguide, a ground electrode provided on the main surface of the substrate, and a high frequency signal for inputting to each signal electrode An optical modulator comprising at least an input unit,
The plurality of signal electrodes include at least a first signal electrode and a second signal electrode, and the optical waveguide has at least a branch portion on an input side, and at least a first branch waveguide downstream of the branch portion. And a second branch waveguide, wherein the first signal electrode acts on the first branch waveguide, and the second signal electrode acts on the second branch waveguide. Thing,
Each of the input units is provided on or near the same surface of a pair of side surfaces of the substrate,
The optical path length of the input side branch and the operation start point in the first branch waveguide, and the optical path length of the input side branch and the operation start point in the second branch optical waveguide are substantially In the same optical modulator, the difference between the distance between the input portion of the first signal electrode and the operation start point, and the distance between the input portion of the second signal electrode and the operation start point, An optical modulator characterized by being an integral multiple of the wavelength of a high-frequency signal at a signal electrode.

The said 1st signal electrode and the said 2nd signal electrode are making substantially point symmetry on a main surface with respect to the midpoint of the said main surface of the said board | substrate, The Claims 1-3 characterized by the above-mentioned. An optical modulator according to claim 1.

Each of the signal electrodes has an output unit for a high-frequency signal, and a distance between an input unit and an output unit of the first signal electrode is a distance between an input unit and an output unit of the second signal electrode. The optical modulator according to any one of claims 1 to 4, wherein the output units are substantially equal, and each output unit is provided on or near the same side of the side surface.

An optical modulator according to any one of claims 1 to 5, an optical transmission unit optically connected to the optical waveguide of the optical modulator, and the input unit for the high-frequency signal. High-frequency signal transmission means respectively connected to the optical modulator, a container accommodating the optical transmission means and the high-frequency signal transmission means, and each high-frequency signal transmission means in the container for connecting to the outside of the container. At least a plurality of high-frequency signal terminals provided in a container.