JP5314060B2

JP5314060B2 - Light modulator

Info

Publication number: JP5314060B2
Application number: JP2011012555A
Authority: JP
Inventors: 健治河野; 雅也名波; 勇治佐藤; 英司川面; 松本　　聡; 信弘五十嵐; 靖二内田; 中平　　徹
Original assignee: Anritsu Corp
Current assignee: Anritsu Corp
Priority date: 2011-01-25
Filing date: 2011-01-25
Publication date: 2013-10-16
Anticipated expiration: 2031-01-25
Also published as: JP2012155046A

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator operated with a small RF drive voltage as well as a small DC bias voltage. <P>SOLUTION: An optical modulator comprises: a substrate; an optical waveguide formed on the substrate; a progressive wave electrode including a center electrode and a ground electrode for a high frequency electric signal where a high frequency electric signal is transmitted; and a bias electrode for applying a bias voltage. The optical waveguide includes a high frequency electric signal interaction part and a bias interaction part, and a ridge portion is formed by recesses obtained by digging a part of the substrate along the optical waveguide in both the high frequency electric signal interaction part and the DC bias interaction part. The ridge portion has a different height in the high frequency electric signal interaction part from that in the DC bias interaction part, and the height of the ridge portion in the DC bias interaction part is smaller than the height of the ridge portion in the high frequency electric signal interaction part. <P>COPYRIGHT: (C)2012,JPO&INPIT

Description

本発明は、電気光学効果を利用して、光導波路に入射した光を高周波電気信号で変調して光信号パルスとして出射し、ＲＦ駆動電圧とともにＤＣバイアス電圧が小さい光変調器に関する。 The present invention relates to an optical modulator that uses electro-optic effects to modulate light incident on an optical waveguide with a high-frequency electrical signal and emit it as an optical signal pulse, and has a small DC bias voltage together with an RF drive voltage.

近年、高速、大容量の光通信システムが実用化されている。このような高速、大容量の光通信システムに組込むための高速、小型、低価格、かつ高安定な光変調器の開発が求められている。 In recent years, high-speed and large-capacity optical communication systems have been put into practical use. There is a demand for the development of a high-speed, small, low-cost, and highly stable optical modulator for incorporation into such a high-speed, large-capacity optical communication system.

このような要望に応える光変調器として、リチウムナイオベート（ＬｉＮｂＯ_３）のように電界を印加することにより屈折率が変化する、いわゆる電気光学効果を有する基板（以下、ＬＮ基板と略す）に光導波路と進行波電極を形成した進行波電極型リチウムナイオベート光変調器（以下、ＬＮ光変調器と略す）がある。このＬＮ光変調器は、その優れたチャーピング特性から２．５Ｇｂｉｔ／ｓ、１０Ｇｂｉｔ／ｓの大容量光通信システムに適用されている。最近はさらに４０Ｇｂｉｔ／ｓの超大容量光通信システムにも適用が検討されている。 As an optical modulator that meets such demands, a light modulator such as lithium niobate (LiNbO ₃ ) is used for a substrate having a so-called electro-optical effect (hereinafter abbreviated as an LN substrate) whose refractive index changes by applying an electric field. There is a traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as an LN optical modulator) in which a waveguide and a traveling wave electrode are formed. This LN optical modulator is applied to a large capacity optical communication system of 2.5 Gbit / s and 10 Gbit / s because of its excellent chirping characteristics. Recently, application to a 40 Gbit / s ultra-high capacity optical communication system is also being studied.

以下、従来、実用化され、又は提唱されてきたリチウムナイオベートの電気光学効果を利用したＬＮ光変調器について説明する。 Hereinafter, an LN optical modulator using the electro-optic effect of lithium niobate that has been put to practical use or has been proposed will be described.

（第１の従来技術）
特許文献１に開示された、ｚ−カットＬＮ基板を用いて構成した、いわゆるリッジ型ＬＮ光変調器を第１の従来技術の光変調器として図６にその概略斜視図を示す。ここで、図７は図６の概略上面図であり、図８は図６と図７のＡ−Ａ´線における概略断面図である。 (First prior art)
FIG. 6 shows a schematic perspective view of a so-called ridge type LN optical modulator disclosed in Patent Document 1, which is configured using a z-cut LN substrate, as a first conventional optical modulator. 7 is a schematic top view of FIG. 6, and FIG. 8 is a schematic cross-sectional view taken along the line AA ′ of FIGS.

ｚ−カットＬＮ基板１上に光導波路３が形成されている。この光導波路３は、金属Ｔｉを１０５０℃で約１０時間熱拡散して形成した光導波路であり、マッハツェンダ干渉系（あるいは、マッハツェンダ光導波路）を構成している。したがって、光導波路３の電気信号と光が相互作用する部位（相互作用部と言う）には２本の相互作用光導波路３ａ、３ｂ、つまりマッハツェンダ光導波路の２本のアームが形成されている。 An optical waveguide 3 is formed on the z-cut LN substrate 1. The optical waveguide 3 is an optical waveguide formed by thermally diffusing metal Ti at 1050 ° C. for about 10 hours, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Accordingly, two interaction optical waveguides 3a and 3b, that is, two arms of the Mach-Zehnder optical waveguide are formed at a portion where the electrical signal and light of the optical waveguide 3 interact (referred to as an interaction portion).

この光導波路３の上面にＳｉＯ_２バッファ層２が形成され、このＳｉＯ_２バッファ層２の上面に進行波電極４が形成されている。進行波電極４としては、１つの中心導体４ａと２つの接地導体４ｂ、４ｃを有するコプレーナウェーブガイド（ＣＰＷ）を用いている。なお、通常、進行波電極４は貴金属材料であるＡｕにより形成されている。５はｚ−カットＬＮ基板１を用いて製作したＬＮ光変調器に特有の焦電効果に起因する温度ドリフトを抑圧するための導電層であり、通常はＳｉ導電層を用いるが、ここでは図の簡単化のために省略する。中心導体４ａの幅は各種の値をとるが、多くの場合７μｍ程度であり、中心導体４ａと接地導体４ｂ、４ｃの間のギャップも各種の値をとるが、１５μｍ程度であることが多い。なお、説明を簡単にするために、図６では図示した温度ドリフト抑圧のためのＳｉ導電層５を図７や図８においては省略している。また、以下においてもＳｉ導電層５は省略して議論する。６は高周波電気信号（あるいは、マイクロ波）の給電線であり、高周波コネクタやマイクロ波線路である。７は高周波電気信号の出力線路であり、通常電気的終端が使われるが、高周波コネクタやマイクロ波線路でも良い。 An SiO ₂ buffer layer 2 is formed on the upper surface of the optical waveguide 3, and a traveling wave electrode 4 is formed on the upper surface of the SiO ₂ buffer layer 2. As the traveling wave electrode 4, a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used. Normally, the traveling wave electrode 4 is formed of Au, which is a noble metal material. Reference numeral 5 denotes a conductive layer for suppressing temperature drift caused by the pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, and normally a Si conductive layer is used. Omitted for simplicity. The width of the center conductor 4a takes various values, but in many cases is about 7 μm, and the gap between the center conductor 4a and the ground conductors 4b and 4c also takes various values, but is often about 15 μm. For simplification of description, the Si conductive layer 5 for suppressing temperature drift shown in FIG. 6 is omitted in FIGS. In the following, the Si conductive layer 5 is omitted and discussed. Reference numeral 6 denotes a high-frequency electric signal (or microwave) feeding line, which is a high-frequency connector or a microwave line. Reference numeral 7 denotes an output line for a high-frequency electric signal, which normally uses an electrical termination, but may be a high-frequency connector or a microwave line.

また、図７のＩとして示された領域では中心導体４ａと接地導体４ｂ、４ｃとを伝搬する高周波電気信号と２本の相互作用光導波路３ａ、３ｂを伝搬する光とが相互作用するので高周波電気信号用相互作用部（あるいは簡単に相互作用部）と呼ばれる。 Further, in the region shown as I in FIG. 7, the high-frequency electric signal propagating through the central conductor 4a and the ground conductors 4b and 4c interacts with the light propagating through the two interaction optical waveguides 3a and 3b. It is called an electrical signal interaction part (or simply an interaction part).

この第１の従来技術では、ｚ−カットＬＮ基板１をエッチングなどで掘り込むことにより、凹部９ａ、９ｂ、及び９ｃ（あるいは、リッジ部８ａ、８ｂとも言える）を形成している。ここで、１２はリッジ部（ここでは８ａ）の側壁である。 In the first prior art, the recesses 9a, 9b and 9c (or ridges 8a and 8b) are formed by digging the z-cut LN substrate 1 by etching or the like. Here, 12 is a side wall of the ridge portion (here, 8a).

このリッジ構造を採ることにより、高周波電気信号（あるいは、マイクロ波）の実効屈折率（あるいは、マイクロ波実効屈折率）、特性インピーダンス、変調帯域、駆動電圧などにおいて優れた特性を実現することができる。なお、図８では凹部９ａ、９ｂ、及び９ｃの深さ（あるいはリッジ部８ａ、８ｂの高さ）を強調して描いているが、実際には数μｍ程度の深さであり、中心導体４ａや接地導体４ｂ、４ｃの厚みである数十μｍに比較すると、その値は小さい。 By adopting this ridge structure, it is possible to realize excellent characteristics in the effective refractive index (or microwave effective refractive index), characteristic impedance, modulation band, driving voltage, etc. of a high-frequency electric signal (or microwave). . In FIG. 8, the depths of the recesses 9a, 9b, and 9c (or the heights of the ridges 8a and 8b) are emphasized, but the actual depth is about several μm, and the central conductor 4a Compared with the thickness of the ground conductors 4b and 4c, which is several tens of μm, the value is small.

次に、この第１の従来技術であるＬＮ光変調器の動作について説明する。このＬＮ光変調器を動作させるには、中心導体４ａと接地導体４ｂ、４ｃ間にＤＣバイアス電圧と高周波電気信号とを印加する必要がある。 Next, the operation of the first conventional LN optical modulator will be described. In order to operate this LN optical modulator, it is necessary to apply a DC bias voltage and a high-frequency electric signal between the center conductor 4a and the ground conductors 4b and 4c.

図９に示す電圧−光出力特性はＬＮ光変調器の電圧−光出力特性であり、Ｖｂはその際のＤＣバイアス電圧である。この図９に示すように、通常、ＤＣバイアス電圧Ｖｂは光出力特性の山と底の中点に設定される。この第１の従来技術では、高周波電気信号と光とが相互作用する相互作用部ＩにＤＣバイアス電圧も印加するので、高周波電気信号の出力部に設ける不図示の電気的終端にコンデンサーを具備させることによりバイアスＴの機能を持たせる必要があり、構造が複雑となってしまう。 The voltage-light output characteristic shown in FIG. 9 is the voltage-light output characteristic of the LN optical modulator, and Vb is the DC bias voltage at that time. As shown in FIG. 9, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic. In the first prior art, since a DC bias voltage is also applied to the interaction part I where the high-frequency electrical signal and light interact, a capacitor is provided at the electrical terminal (not shown) provided at the output part of the high-frequency electrical signal. Therefore, it is necessary to provide the function of the bias T, and the structure becomes complicated.

（第２の従来技術）
図１０は第２の従来技術の上面図であって、第１の従来技術において必要であったバイアスＴを無くすために、不図示の電気的終端を抵抗のみとし、ＤＣバイアスを新たに設けたＤＣバイアス部ＩＩに印加する構造とした、いわゆるバイアス分離構造の光変調器である。この構造では、高周波電気信号が相互作用光導波路３ａ、３ｂと相互作用する高周波電気信号用相互作用部Ｉと、ＤＣバイアス電圧が相互作用光導波路３ａ、３ｂに印加されるＤＣバイアス用相互作用部ＩＩを具備しており、バイアス分離型構造と呼ばれる。その一例が特許文献２に開示されている。 (Second prior art)
FIG. 10 is a top view of the second prior art, and in order to eliminate the bias T required in the first prior art, the electrical termination (not shown) is only a resistor and a DC bias is newly provided. This is an optical modulator having a so-called bias separation structure that is applied to the DC bias unit II. In this structure, a high frequency electrical signal interaction unit I in which a high frequency electrical signal interacts with the interaction optical waveguides 3a and 3b, and a DC bias interaction unit in which a DC bias voltage is applied to the interaction optical waveguides 3a and 3b. II, which is called a bias-separated structure. An example thereof is disclosed in Patent Document 2.

図１０のＢ−Ｂ´線とＣ−Ｃ´線における断面図を、各々図１１（ａ）と（ｂ）に示す。ここで、４ａ´は中心導体、４ｂ´と４ｃ´は接地導体である。９ａ´、９ｂ´、及び９ｃ´はＤＣバイアス用相互作用部の凹部であり、リッジ部８ａ´、８ｂ´を形成している。１１ａと１１ｂはＤＣバイアス電極である。 Sectional views taken along lines BB ′ and CC ′ in FIG. 10 are shown in FIGS. 11 (a) and 11 (b), respectively. Here, 4a ′ is a central conductor, and 4b ′ and 4c ′ are ground conductors. Reference numerals 9a ′, 9b ′, and 9c ′ are concave portions of the DC bias interaction portion, and form ridge portions 8a ′ and 8b ′. 11a and 11b are DC bias electrodes.

ここで、図１０と図１１に示した第２の従来技術の問題点について考察する。これまで、高周波電気信号用相互作用部Ｉのリッジ部８ａ、８ｂとＤＣバイアス用相互作用部ＩＩのリッジ部８ａ´、８ｂ´はドライエッチングにより同時に形成されて来た。そのため、高周波電気信号用相互作用部Ｉのリッジ部８ａ、８ｂの高さＨ_ｉとＤＣバイアス用相互作用部ＩＩのリッジ部８ａ´、８ｂ´の高さＨ_Ｂ´は同じ高さ、

Ｈ_ｉ＝Ｈ_Ｂ´ （１）

である。つまり、一般的には、高周波電気信号用相互作用部のリッジ部の高さＨ_ｉとＤＣバイアス用相互作用部のリッジ部の高さＨ_Ｂ´とを同じ高さに設定することが、高周波電気信号の電圧（つまり、ＲＦ駆動電圧、あるいはＲＦＶπ）とＤＣバイアス電圧（あるいはＤＣＶπ）の面から最も好適であると考えられていた。 Here, the problem of the second prior art shown in FIGS. 10 and 11 will be considered. Until now, the ridges 8a and 8b of the high frequency electrical signal interaction part I and the ridges 8a 'and 8b' of the DC bias interaction part II have been simultaneously formed by dry etching. Therefore, the height H _i of the ridge portions 8a and 8b of the high frequency electrical signal interaction portion I and the height H _B ′ of the ridge portions 8a ′ and 8b ′ of the DC bias interaction portion II are the same,

H _i = H _B ′ (1)

It is. That is, generally, the height H _i of the ridge portion of the high frequency electrical signal interaction portion and the height H _B ′ of the ridge portion of the DC bias interaction portion are set to the same height. In view of the voltage of the electric signal (that is, the RF drive voltage or RFVπ) and the DC bias voltage (or DCVπ), it was considered most preferable.

ところが、本出願人は詳細なシミュレーションの結果、一般に高周波電気信号用相互作用部Ｉに要求される最適リッジ部の高さＨ_{ｉ，ｏｐｔ}とＤＣバイアス用相互作用部ＩＩに要求される最適リッジ部の高さＨ_{Ｂ，ｏｐｔ}とは異なっていることを見出した。従って、高周波電気信号用相互作用部Ｉのリッジ部８ａ、８ｂの高さＨ_ｉとＤＣバイアス用相互作用部ＩＩのリッジ部８ａ´、８ｂ´の高さＨ_Ｂとを同じ高さに設定する第２の従来技術では、ＲＦＶπとＤＣＶπとを同時に低減することはできないことがわかった。 However, as a result of detailed simulations, the present applicant has found that the optimum ridge portion H _{i, opt} and the optimum bias ridge portion required for the DC bias interaction portion II are generally required for the high frequency electrical signal interaction portion I. Was found to be different from the height H _{B, opt} . Therefore, to set the ridge portion 8a of the high frequency electric signal interaction portion I, the height H _i and DC bias interaction portion II of 8b ridge 8a ', and a height H _B of 8b' the same height In the second prior art, it has been found that RFVπ and DCVπ cannot be reduced simultaneously.

特開平４−２８８５１８号公報JP-A-4-288518 特開２００８−１２２７８６号公報JP 2008-122786 A

以上のように、従来技術では高周波電気信号用相互作用部のリッジ部の高さとＤＣバイアス用相互作用部のリッジ部の高さとを同じに設定していたので、高周波電気信号の電圧とＤＣバイアス電圧とを同時に充分に低減することはできなかった。 As described above, in the prior art, the height of the ridge portion of the high frequency electrical signal interaction portion and the height of the ridge portion of the DC bias interaction portion are set to be the same. The voltage could not be reduced sufficiently at the same time.

本発明はこのような事情に鑑みてなされたものであり、高周波電気信号の電圧とＤＣバイアス電圧とを同時に充分に低減した光変調器を提供することを目的とする。 The present invention has been made in view of such circumstances, and an object thereof is to provide an optical modulator in which the voltage of a high-frequency electrical signal and the DC bias voltage are sufficiently reduced simultaneously.

上記課題を解決するために、本発明の請求項１に記載の光変調器は、電気光学効果を有する基板と、該基板に形成された光を導波するための少なくとも２本の光導波路と、前記基板の一方の面側に形成され、前記光を変調する高周波電気信号が伝搬する高周波電気信号用の中心電極及び接地電極を有する進行波電極と、前記光にバイアス電圧を印加するバイアス電極とを有し、前記光導波路には前記進行波電極に前記高周波電気信号が印加されることにより前記光の位相を変調するための高周波電気信号用相互作用部と、前記バイアス電極にバイアス電圧を印加することにより前記光の位相を調整するためのバイアス用相互作用部とを具備し、前記高周波電気信号用相互作用部と前記ＤＣバイアス用相互作用部の両方において前記光導波路に沿って前記基板の一部を掘り下げて形成された凹部によりリッジ部をなす光変調器において、前記高周波電気信号用相互作用部の前記リッジの高さが、前記高周波電気信号の電圧値が略最小となるような所定高さで形成され、前記ＤＣバイアス用相互作用部の前記リッジの高さが、前記バイアス電圧値が略最小となるような所定高さで形成され、前記ＤＣバイアス用相互作用部における前記リッジ部の高さが、前記高周波電気信号用相互作用部における前記リッジ部の高さよりも低いことを特徴としている。 In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, and at least two optical waveguides for guiding light formed on the substrate. A traveling wave electrode formed on one side of the substrate and having a center electrode and a ground electrode for a high frequency electrical signal through which the high frequency electrical signal for modulating the light propagates; and a bias electrode for applying a bias voltage to the light A high-frequency electrical signal interaction unit for modulating the phase of the light by applying the high-frequency electrical signal to the traveling-wave electrode, and a bias voltage applied to the bias electrode. And a bias interaction unit for adjusting the phase of the light by applying, to the optical waveguide in both the high-frequency electrical signal interaction unit and the DC bias interaction unit. In the optical modulator constituting the ridge part by recesses formed digging a portion of the substrate it, wherein the height of the ridge of the interaction portion for high frequency electric signal is substantially the minimum voltage value of the high frequency electric signal The DC bias interaction portion is formed at a predetermined height such that the bias voltage value is substantially minimized. the height of the ridge portion in section is, it is characterized in that not lower than the height of the ridge portion in the high-frequency electric signal interaction portion.

上記課題を解決するために、本発明の請求項２に記載の光変調器は、請求項１に記載の光変調器において、前記ＤＣバイアス用相互作用部における前記２本の光導波路の間隔が、前記高周波電気信号用相互作用部における前記２本の光導波路の間隔よりも狭く形成されていることを特徴としている。 In order to solve the above-described problem, an optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect, wherein an interval between the two optical waveguides in the DC bias interaction unit is set. The high-frequency electrical signal interaction portion is formed narrower than the interval between the two optical waveguides.

上記課題を解決するために、本発明の請求項３に記載の光変調器は、請求項１または２に記載の光変調器において、前記基板がｚ−カットＬＮ基板であることを特徴としている。 In order to solve the above-mentioned problem, an optical modulator according to claim 3 of the present invention is characterized in that in the optical modulator according to claim 1 or 2 , the substrate is a z-cut LN substrate. .

本発明に係る光変調器では、高周波電気信号用相互作用部のリッジ部の高さとＤＣバイアス用相互作用部のリッジ部の高さとを異ならしめることにより、高周波電気信号の電圧（ＲＦＶπ）を充分低くした場合においても、同時にＤＣバイアス電圧（ＤＣＶπ）を充分に低減することが可能となるという優れた利点がある。 In the optical modulator according to the present invention, the height of the ridge portion of the high frequency electrical signal interaction portion and the height of the ridge portion of the DC bias interaction portion are made different from each other, thereby sufficiently increasing the voltage (RFVπ) of the high frequency electrical signal. Even when it is lowered, there is an excellent advantage that the DC bias voltage (DCVπ) can be sufficiently reduced at the same time.

本発明の第１の実施形態に係わる光変調器の概略構成を示す上面図1 is a top view showing a schematic configuration of an optical modulator according to a first embodiment of the present invention. （ａ）図１のＢ−Ｂ´線における断面図、（ｂ）図１のＤ−Ｄ´線における断面図(A) Sectional view taken along line BB 'in FIG. 1, (b) Sectional view taken along line DD' in FIG. 本発明の原理を説明する図The figure explaining the principle of this invention 本発明の第２の実施形態に係わる光変調器の概略構成を示す上面図FIG. 5 is a top view showing a schematic configuration of an optical modulator according to a second embodiment of the present invention. （ａ）図４のＢ−Ｂ´における断面図、（ｂ）図４のＥ−Ｅ´における断面図(A) Cross-sectional view taken along line BB 'in FIG. 4, (b) Cross-sectional view taken along line EE' in FIG. 第１の従来技術の光変調器についての概略構成を示す斜視図The perspective view which shows schematic structure about the optical modulator of 1st prior art 第１の従来技術の光変調器についての概略構成を示す上面図The top view which shows schematic structure about the optical modulator of 1st prior art 図７のＡ−Ａ´線における断面図Sectional drawing in the AA 'line of FIG. 光変調器の動作原理を説明する図The figure explaining the principle of operation of an optical modulator 第２の従来技術の光変調器についての概略構成を示す上面図The top view which shows schematic structure about the optical modulator of 2nd prior art （ａ）図１０のＢ−Ｂ´線における断面図、（ｂ）図１０のＣ−Ｃ´線における断面図(A) Sectional view taken along line BB 'in FIG. 10, (b) Sectional view taken along line CC' in FIG.

以下、本発明の実施形態について説明するが、図６から図１１に示した従来技術と同一の符号は同一機能部に対応しているため、ここでは同一の符号を持つ機能部の説明を省略する。 Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the related art shown in FIGS. 6 to 11 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

（第１の実施形態）
図１は本発明における第１の実施形態の上面図である。図１のＢ−Ｂ´線とＤ−Ｄ´線における断面図を各々図２（ａ）と（ｂ）に示す。第２の従来技術と同様に、高周波電気信号用相互作用部ＩとＤＣバイアス用相互作用部ＩＩＩとを備えるバイアス分離型の光変調器である。 (First embodiment)
FIG. 1 is a top view of a first embodiment of the present invention. 2A and 2B are cross-sectional views taken along lines BB ′ and DD ′ in FIG. 1, respectively. Similar to the second prior art, this is a bias-separated type optical modulator comprising a high-frequency electrical signal interaction section I and a DC bias interaction section III.

高周波電気信号用相互作用部Ｉに対応する図２（ａ）からわかるように、第１の従来技術や第２の従来技術と同様に、中心導体４ａ´と接地導体４ｂ´、４ｃ´とからなる進行波電極を伝搬する高周波電気信号の実効屈折率が高周波電気信号用相互作用光導波路３ｂを伝搬する光の等価屈折率に近くなるように、リッジ部８ａ、８ｂの高さＨ_ｉは従来技術と同じく高く設定されている。 As can be seen from FIG. 2 (a) corresponding to the high-frequency electrical signal interaction section I, the center conductor 4a ′ and the ground conductors 4b ′ and 4c ′ are used in the same manner as in the first conventional technique and the second conventional technique. The height H _i of the ridge portions 8a and 8b is conventionally set so that the effective refractive index of the high-frequency electrical signal propagating through the traveling-wave electrode becomes close to the equivalent refractive index of the light propagating through the high-frequency electrical signal interaction optical waveguide 3b. It is set as high as the technology.

一方、図２（ｂ）からわかるように、ＤＣバイアス用相互作用部ＩＩＩにおけるリッジ部８ａ´、８ｂ´の高さＨ_Ｂは高周波電気信号用相互作用部Ｉのリッジ部８ａ、８ｂの高さＨ_ｉよりも低く設定されている。つまり、

Ｈ_ｉ＞Ｈ_Ｂ（２）

としている。 On the other hand, as can be seen from FIG. 2 (b), the ridge portions 8a ', the height _{H B} ridge portion 8a, the height of 8b of the high frequency electric signal interaction portion I of 8b' in DC bias interaction portion III It is set to be lower than H _i. That means

H _i > H _B (2)

It is said.

次に、本発明の原理について説明する。図３には、図２におけるリッジ部８ａ´、８ｂ´の高さＨ_Ｂを変数とした際のＤＣＶπを左縦軸に、またリッジ部８ａ、８ｂの高さＨ_ｉを変数とした際のＲＦＶπを右縦軸にとったグラフを示す。図に示すように、ＤＣＶπが最小となるリッジ部８ａ´、８ｂ´の高さＨ_{Ｂ，ｏｐｔ}とＲＦＶπが最小となるリッジ部８ａ、８ｂの高さＨ_{ｉ，ｏｐｔ}とは値が異なり、（２）式が成り立っていることがわかる。 Next, the principle of the present invention will be described. In FIG. 3, the ridge portion 8a' in FIG 2, the DCVπ when used as a variable height _{H B} of 8b' the left vertical axis and the ridge portion 8a, the height _{H i} of 8b variables were when The graph which took RFV (pi) on the right vertical axis | shaft is shown. As shown, the ridge portion 8a' which DCVπ is minimum, the height _{H B} of the 8B _', the ridge portion 8a _{which opt} and RFVπ is minimized, 8b height _{H i,} different values and _opt, ( 2) It turns out that Formula is formed.

（第２の実施形態）
図４は本発明における第２の実施形態の概略上面図である。第１の実施形態とは、ＤＣバイアス用相互作用部の構成が異なっている。図４のＢ−Ｂ´線とＥ−Ｅ´線における断面図を各々図５（ａ）と（ｂ）に示す。 (Second Embodiment)
FIG. 4 is a schematic top view of the second embodiment of the present invention. The configuration of the DC bias interaction unit is different from that of the first embodiment. Cross-sectional views taken along lines BB ′ and EE ′ of FIG. 4 are shown in FIGS. 5A and 5B, respectively.

図５に示すように、本実施形態ではＥ−Ｅ´線における断面においてＤＣバイアス用相互作用部ＩＶにおける光導波路間３ａ、３ｂのギャップが、高周波電気信号相互作用部Ｉにおける光導波路間３ａ、３ｂのギャップよりも狭くなっている。このように光導波路間３ａ、３ｂのギャップを狭く構成すると、ＤＣＶπが最小となるリッジ部８ａ´、８ｂ´の高さＨ_{Ｂ，ｏｐｔ}の値はより小さくなる。光導波路間３ａ、３ｂのギャップが狭くなるとＤＣＶπが低くなるとともに、Ｈ_{Ｂ，ｏｐｔ}の値が小さいので製作にかかる時間も短くなり、さらにリッジ部８ａ´や８ｂ´の側壁１２に起因する光の挿入損失を低減できるという利点が生じる。 As shown in FIG. 5, in this embodiment, the gap between the optical waveguides 3a and 3b in the DC bias interaction unit IV in the cross section taken along the line EE ′ is the gap between the optical waveguides 3a and 3b in the high-frequency electrical signal interaction unit I. It is narrower than the gap of 3b. When the gap between the optical waveguides 3a and 3b is thus narrowed, the heights H _{B and opt} of the ridge portions 8a ′ and 8b ′ at which DCVπ is minimized become smaller. When the gap between the optical waveguides 3a and 3b is narrowed, DCVπ is lowered, and the time required for the production is shortened because the values of HB and _opt are small. Further, the light caused by the side walls 12 of the ridges 8a ′ and 8b ′ is reduced. The advantage that the insertion loss can be reduced occurs.

（各実施形態）
以上の説明では、分岐光導波路の例としてマッハツェンダ光導波路を用いたが、方向性結合器などその他の分岐合波型の光導波路にも本発明を適用可能であることは言うまでもない。また光導波路の形成法としてはＴｉ熱拡散法の他に、プロトン交換法など光導波路の各種形成法を適用できるし、バッファ層としてＡｌ_２Ｏ_３等のＳｉＯ_２以外の各種材料も適用できる。 (Each embodiment)
In the above description, a Mach-Zehnder optical waveguide is used as an example of the branched optical waveguide. However, it goes without saying that the present invention can be applied to other branched / multiplexed optical waveguides such as directional couplers. As a method for forming the optical waveguide, various methods for forming the optical waveguide such as a proton exchange method can be applied in addition to the Ti thermal diffusion method, and various materials other than SiO ₂ such as Al ₂ O ₃ can be applied as the buffer layer.

また、本発明はＤＣバイアス用相互作用部の電極構造に依存せず、ＣＰＷ構造や非対称コプレーナストリップ（ＡＣＰＳ）構造、あるいは対称コプレーナストリップ（ＣＰＳ）構造など、各種の電極構造について成り立つことはいうまでもない。 In addition, the present invention does not depend on the electrode structure of the DC bias interaction section, and can be applied to various electrode structures such as a CPW structure, an asymmetric coplanar strip (ACPS) structure, or a symmetric coplanar strip (CPS) structure. Nor.

また、図１〜５においては、高周波電気信号用相互作用部におけるリッジ部のギャップとＤＣバイアス用相互作用部におけるリッジ部のギャップとを等しく図示しているが、本発明はこれに限定されるものではない。 1 to 5, the gap of the ridge portion in the high frequency electrical signal interaction portion and the gap of the ridge portion in the DC bias interaction portion are equally shown, but the present invention is limited to this. It is not a thing.

１：ｚ−カットＬＮ基板（ＬＮ基板）
２：ＳｉＯ_２バッファ層（バッファ層）
３：マッハツェンダ光導波路（光導波路）
３ａ、３ｂ：マッハツェンダ光導波路を構成する相互作用光導波路
４：進行波電極
４ａ、４ａ´：中心導体
４ｂ、４ｂ´、４ｃ、４ｃ´：接地導体
６：高周波電気信号給電線
７：高周波電気信号出力線
８ａ、８ａ´、８ｂ、８ｂ´：リッジ部
９ａ、９ａ´、９ｂ、９ｂ´、９ｃ、９ｃ´：凹部
１１ａ、１１ｂ：ＤＣバイアス電極
１２：リッジ部の側壁
Ｉ：高周波電気信号用相互作用部
ＩＩ、ＩＩＩ、ＩＶ：ＤＣバイアス用相互作用部 1: z-cut LN substrate (LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b: Interaction optical waveguide constituting the Mach-Zehnder optical waveguide 4: Traveling wave electrode 4a, 4a ′: Center conductor 4b, 4b ′, 4c, 4c ′: Ground conductor 6: High frequency electric signal feeder 7: High frequency electric signal Output lines 8a, 8a ′, 8b, 8b ′: Ridge portions 9a, 9a ′, 9b, 9b ′, 9c, 9c ′: Recesses 11a, 11b: DC bias electrodes 12: Side walls of the ridge portions I: Mutual use for high-frequency electric signals Action part II, III, IV: Interaction part for DC bias

Claims

A substrate having an electro-optic effect, at least two optical waveguides for guiding light formed on the substrate, and a high-frequency electric signal that is formed on one surface side of the substrate and modulates the light propagates A traveling wave electrode having a center electrode and a ground electrode for high-frequency electrical signals, and a bias electrode for applying a bias voltage to the light,
The optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage applied to the bias electrode. A bias interaction unit for adjusting the phase of the light,
In the optical modulator forming a ridge portion by a recess formed by digging down a part of the substrate along the optical waveguide in both the high-frequency electrical signal interaction portion and the DC bias interaction portion,
The height of the ridge of the high-frequency electrical signal interaction part is formed at a predetermined height such that the voltage value of the high-frequency electrical signal is substantially minimum,
The height of the ridge of the DC bias interaction portion is formed at a predetermined height such that the bias voltage value is substantially minimum,
The DC level of the ridge portion in the bias interaction portion, an optical modulator, wherein not lower than the height of the ridge portion in the high-frequency electric signal interaction portion.

2. The interval between the two optical waveguides in the DC bias interaction unit is formed to be narrower than the interval between the two optical waveguides in the high frequency electrical signal interaction unit. An optical modulator according to 1.

The optical modulator according to claim 1, wherein the substrate is a z-cut LN substrate .