Electronic device

Abstract

An electronic device in which a signal electrode has a bent portion. Earth electrodes are formed with the signal electrode between. A width of a gap at the bent portion between the signal electrode and each earth electrode is narrower than a width of a gap at both ends of the bent portion between the signal electrode and each earth electrode.

Claims

1 . An electronic device comprising: a signal electrode having a bent portion; and earth electrodes formed with the signal electrode therebetween, wherein a gap at the bent portion between the signal electrode and each earth electrode is narrower than a gap at both ends of the bent portion between the signal electrode and each earth electrode. 2 . The electronic device according to claim 1 , wherein the signal electrode and the earth electrodes are formed over or in a ferroelectric substrate. 3 . The electronic device according to claim 1 , wherein a plurality of signal electrodes are formed. 4 . The electronic device according to claim 1 , further comprising an optical waveguide formed over a substrate, wherein the gap at the bent portion between the signal electrode and each earth electrode is narrower than the gap at both ends of the bent portion between the signal electrode and each earth electrode in a front portion of an interaction area in which the signal electrode interferes with the optical waveguide. 5 . The electronic device according to claim 1 , wherein the gap at the bent portion between the signal electrode and each earth electrode is narrower than distance between the signal electrode and a ground layer formed on a reverse of a substrate or in the substrate. 6 . The electronic device according to claim 1 , wherein the gap at the bent portion between the signal electrode and each earth electrode is narrower than thickness of a substrate. 7 . The electronic device according to claim 1 , wherein the gap at the bent portion between the signal electrode and each earth electrode gradually narrows from both ends of the bent portion. 8 . The electronic device according to claim 1 , wherein the bent portion bends at right angles. 9 . The electronic device according to claim 1 , wherein of inner and outer gaps at the bent portion between the signal electrode and the earth electrodes, only the outer gap at the bent portion is narrower than the outer gap at both ends of the bent portion. 10 . The electronic device according to claim 1 , wherein width of the signal electrode at the bent portion is narrower than the width of the signal electrode at both ends of the bent portion. 11 . The electronic device according to claim 1 , wherein thickness at the bent portion of the signal electrode and the earth electrodes is thinner than the thickness at both ends of the bent portion of the signal electrode and the earth electrodes. 12 . The electronic device according to claim 1 , wherein thickness at the bent portion of the earth electrodes is thinner than the thickness at both ends of the bent portion of the earth electrodes. 13 . The electronic device according to claim 1 , wherein a buffer layer is formed at the bent portion between the signal electrode and the earth electrodes and a substrate. 14 . The electronic device according to claim 1 , wherein a buffer layer is formed between the signal electrode and the earth electrodes and a substrate except at the bent portion. 15 . The electronic device according to claim 4 , wherein a buffer layer is formed over the optical waveguide. 16 . The electronic device according to claim 1 , wherein: a buffer layer is formed between the signal electrode and the earth electrodes and a substrate; and thickness at the bent portion of the buffer layer is thicker than the thickness in another area of the buffer layer. 17 . The electronic device according to claim 1 , wherein grooves are formed in a substrate on both sides of the bent portion of the signal electrode. 18 . The electronic device according to claim 1 , wherein: grooves are formed in a substrate on both sides of the signal electrode; and the grooves at the bent portion are deeper than the grooves in another area. 19 . The electronic device according to claim 1 , wherein: grooves are formed in a substrate on both sides of the signal electrode; and width at the bent portion of the grooves is wider than the width in another area of the grooves. 20 . The electronic device according to claim 1 , wherein: grooves are formed in a substrate on both sides of the signal electrode; and distance at the bent portion between the signal electrode and each groove is shorter than the distance in another area between the signal electrode and each groove. 21 . The electronic device according to claim 1 , wherein the earth electrodes formed with the signal electrode therebetween are connected at the bent portion by wires or vias. 22 . The electronic device according to claim 1 , wherein a portion of the gap extending from one end of the bent portion has a different width from a portion of the gap extending from an other end of the bent portion. 23 . The electronic device according to claim 22 , wherein the gap between the signal electrode and each earth electrode gradually narrows from the one end of the bent portion to the other end. 24 . The electronic device according to claim 1 , wherein a portion of the signal electrode extending from one end of the bent portion has a different width from a portion of the signal electrode extending from an other end of the bent portion. 25 . The electronic device according to claim 24 , wherein width of the signal electrode gradually narrows from the one end of the bent portion to the other end. 26 . An electronic device comprising: a first electrode having a bent portion; and second electrodes formed with the first electrode between, wherein a gap at the bent portion between the first electrode and each second electrode is narrower than the gap at both ends of the bent portion between the first electrode and each second electrode.
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-237314, filed on Sep. 17, 2008, the entire contents of which are incorporated herein by reference. FIELD [0002] The embodiments discussed herein are related to an electronic device including an electrode having a bent portion. BACKGROUND [0003] Each optical communication apparatus includes an optical modulator which modulates light by making use of an electro-optic effect. Such an optical modulator is, for example, an optical waveguide device which modulates continuous wave (CW) light output from a laser by an electrical signal and which outputs the CW light (see, for example, Japanese Laid-open Patent Publication No. 2008-58436). [0004] FIG. 22 is a plan view of an optical waveguide device. With an optical waveguide device depicted in FIG. 22 , an optical waveguide 102 is formed over a dielectric substrate 101 which has an electro-optic effect, and an electrode 103 is formed over the optical waveguide 102 . This optical waveguide device is a dual drive type optical modulator. Electric fields are formed on two waveguides 102 a and 102 b parallel to each other by two signal electrodes 103 a and 103 b respectively. By doing so, input light IN output from a laser is modulated and output light OUT is obtained. [0005] With the optical waveguide device depicted in FIG. 22 , positive data Data and negative data Data (indicated by Data with a bar in FIG. 22 ) which are complementary electrical signals are input to the two signal electrodes 103 a and 103 b , respectively, so that the optical waveguide device will perform push-pull operation. At this time it is necessary to modulate light propagating through the two waveguides 102 a and 102 b parallel to each other at the same timing. To be concrete, it is necessary that the data Data input to the signal electrodes 103 a and 103 b reach the parallel waveguides 102 a and 102 b at the same timing at a line 104 indicated in FIG. 22 . [0006] Therefore, the length of feed portions of the signal electrodes 103 a and 103 b from portions to which the data Data is input to the parallel waveguides 102 a and 102 b is adjusted. By doing so, the timing at which the data Data that are complementary electrical signals reaches the parallel waveguides 102 a and 102 b is adjusted. For example, as indicated in a frame 105 of the FIG. 22 , the signal electrode 103 b is bent to adjust the timing at which the data Data reaches the parallel waveguides 102 a and 102 b. [0007] Plural intrinsic modes of a microwave can propagate through the substrate 101 . The distribution of an electric field and propagation speed for each intrinsic mode depend on the shape of a section of a chip. Coupling between a coplanar mode propagating through an electrode and an intrinsic mode (undesired mode) occurs at a frequency and loss occurs. This coupling tends to occur at a bend where the direction in which the coplanar mode propagates changes. As a result, loss tends to occur at a bend in the electrode. The more significantly an electric field of the coplanar mode overlaps with an electric field of the undesired mode, the stronger the coupling becomes. As an electric field of the coplanar mode spreads, usually the coupling becomes stronger. Therefore, as a gap between a signal electrode and an earth electrode becomes wider, loss becomes heavier. With an electronic device for which light is not used, a dielectric substrate through which light does not pass may be used. [0008] However, a bent portion of an electrode differs from a straight portion in electric field distribution, so signal loss occurs. SUMMARY [0009] According to one aspect of the embodiment, an electronic device includes a signal electrode having a bent portion and earth electrodes formed with the signal electrode between, a gap at the bent portion between the signal electrode and each earth electrode being narrower than a gap at both ends of the bent portion between the signal electrode and each earth electrode. [0010] The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. [0011] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. BRIEF DESCRIPTION OF DRAWING(S) [0012] FIG. 1 is a plan view of an optical waveguide device according to a first embodiment; [0013] FIG. 2 is an enlarged view of a delay region; [0014] FIG. 3 illustrates signal loss which occurs in the case of a gap at a bend being made narrow; [0015] FIG. 4 illustrates signal loss which occurs in the case of the gap at the bend not being made narrow; [0016] FIG. 5 illustrates a delay region of an optical waveguide device according to a second embodiment; [0017] FIG. 6 illustrates a delay region of an optical waveguide device according to a third embodiment; [0018] FIG. 7 illustrates a delay region of an optical waveguide device according to a fourth embodiment; [0019] FIG. 8 illustrates a delay region of an optical waveguide device according to a fifth embodiment; [0020] FIG. 9 illustrates a delay region of an optical waveguide device according to a sixth embodiment; [0021] FIG. 10 is a sectional view taken along the line A-A of FIG. 9 ; [0022] FIG. 11 is a sectional view taken along the line B-B of FIG. 9 ; [0023] FIG. 12 illustrates a delay region of an optical waveguide device according to a seventh embodiment; [0024] FIG. 13 is a sectional view taken along the line A-A of FIG. 12 ; [0025] FIG. 14 is a sectional view of a delay region of an optical waveguide device according to an eighth embodiment; [0026] FIG. 15 is a sectional view of a delay region of an optical waveguide device according to a ninth embodiment; [0027] FIG. 16 is a sectional view of a bend of an optical waveguide device according to a tenth embodiment; [0028] FIG. 17 is a sectional view of a portion other than the bend of the optical waveguide device according to the tenth embodiment; [0029] FIG. 18 illustrates a delay region of an optical waveguide device according to an eleventh embodiment; [0030] FIG. 19 illustrates a delay region of an optical waveguide device in which earth electrodes are connected by vias; [0031] FIG. 20 illustrates a delay region of an optical waveguide device according to a twelfth embodiment; [0032] FIG. 21 illustrates a delay region of an optical waveguide device in which the width of a signal electrode is changed; and [0033] FIG. 22 is a plan view of an optical waveguide device. DESCRIPTION OF EMBODIMENT(S) [0034] A first embodiment will now be described in detail with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout. [0035] FIG. 1 is a plan view of an optical waveguide device according to a first embodiment. With an optical waveguide device depicted in FIG. 1 , an optical waveguide 2 is formed over a dielectric substrate 1 (in an area of the dielectric substrate 1 near the surface). The dielectric substrate 1 is made of an electro-optic crystal such as LiNbO 3 or LiTaO 2 . [0036] The optical waveguide 2 includes an input waveguide 2 a , parallel waveguides 2 b and 2 c , and an output waveguide 2 d . The optical waveguide 2 is formed by forming a metal film of titanium (Ti) or the like over part of the dielectric substrate 1 and performing thermal diffusion of titanium. The optical waveguides 2 may be formed by making a proton exchange in benzoic acid after patterning. [0037] An electrode 3 indicated by oblique lines includes signal electrodes 3 a and 3 b through which electrical signals (data) propagate and earth electrodes 3 c . The electrode 3 is a coplanar electrode. If a z-cut substrate is used as the dielectric substrate 1 , a change in refractive index caused by a z-direction electric field is used. Therefore, the electrode 3 is formed over the optical waveguide 2 . A pattern of the electrode 3 is formed over the optical waveguide 2 . However, in order to prevent light propagating through the parallel waveguides 2 b and 2 c from being absorbed by the electrode 3 , a buffer layer is formed between the dielectric substrate 1 and the electrode 3 . For example, a SiO 2 film with a thickness of about 0.2 to 2 μm is used as the buffer layer. [0038] The optical waveguide device depicted in FIG. 1 is a dual drive type optical modulator. Electric fields are formed on the two parallel waveguides 2 b and 2 c by the two signal electrodes 3 a and 3 b respectively. By doing so, input light IN output from a laser is modulated and output light OUT is obtained. [0039] When the optical waveguide device is driven at a high speed, traveling-wave type electrodes are formed by connecting ends of the signal electrodes 3 a and 3 b and the earth electrodes 3 c via resistors and a microwave signal is applied from an input side. At this time refractive indexes in the parallel waveguides 2 b and 2 c change to +Δna and −Δnb, respectively, because of the electric fields. As a result, while the input light IN is propagating through the parallel waveguides 2 b and 2 c , a phase difference changes. Accordingly, the intensity-modulated output light OUT is output from the output waveguide 2 d because of Mach-Zehnder interference. The effective refractive index of a microwave is controlled by changing the shape of a section of each of the signal electrodes 3 a and 3 b . The speed of light is made to match the speed of the microwave. By doing so, high-speed optical response characteristics can be obtained. [0040] As indicated in a frame 4 of FIG. 1 , the signal electrode 3 b of the optical waveguide device is bent. That is to say, the optical waveguide device includes a delay region by which timing at which the data Data reaches the parallel waveguides 2 b and 2 c is adjusted. A gap at a bent portion between the signal electrode 3 b and the earth electrode 3 c is narrower than the gap in another area between the signal electrode 3 b and the earth electrode 3 c so that microwave signal loss will be controlled. [0041] FIG. 2 is an enlarged view of the delay region. A delay region depicted in FIG. 2 is obtained by enlarging the delay region depicted in the frame 4 of FIG. 1 . Members in FIG. 2 that are the same as those depicted in FIG. 1 are marked with the same symbols and descriptions of them will be omitted. [0042] In order to adjust the timing of the data Data, the signal electrode 3 b has a bent portion (bend) 4 a . As depicted in FIG. 2 , for example, the direction of the signal electrode 3 b is changed at the bend 4 a by 90°. [0043] Width S 1 of a gap at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c is narrower than the width S 2 of the gap in another area (straight area of a feeder) between the signal electrode 3 b and the earth electrode 3 c . That is to say, the signal electrode 3 b and the earth electrode 3 c are formed so that the condition S 1 <S 2 will be met. By making the gap at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c narrower than the gap in another area between the signal electrode 3 b and the earth electrode 3 c , the distribution (spread) of an electric field at the bend 4 a can be controlled and signal loss can be reduced. [0044] FIG. 3 illustrates signal loss which occurs in the case of the gap at the bend being made narrow. FIG. 4 illustrates signal loss which occurs in the case of the gap at the bend not being made narrow. In each of FIGS. 3 and 4 , a horizontal axis indicates a frequency and a vertical axis indicates S 21 of S parameters (which indicates electrical signal loss and which is equal to (output power)/(input power)). [0045] In FIG. 2 , the gap at the bend 4 a is narrower than the gap in another area. As indicated by a waveform W 1 of FIG. 3 , partial loss in the power of an electrical signal input to the signal electrode 3 b is controlled, compared with a waveform W 2 of FIG. 4 which is obtained in the case of the gap at the bend 4 a not being made narrow. In addition, power loss in a high-frequency band is reduced. [0046] As stated above, the gap at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c is made narrower than the gap in another area between the signal electrode 3 b and the earth electrode 3 c . By doing so, the distribution of an electric field at the bend 4 a can be controlled and signal loss at the bend 4 a can be reduced. [0047] In addition, by controlling the distribution of an electric field, crosstalk with another signal or interference with light can be controlled. Controlling the distribution of an electric field is effective, for example, in the case where a plurality of optical waveguides 2 are formed over the dielectric substrate 1 and where a plurality of signal electrodes are formed. [0048] If the dielectric substrate 1 is made of a ferroelectric such as LiNbO 3 or LiTaO 2 , the characteristic impedance of the signal electrodes 3 a and 3 b is low. In order to make the characteristic impedance of the signal electrodes 3 a and 3 b high, a gap between the signal electrode 3 a and the earth electrode 3 c and a gap between the signal electrode 3 b and the earth electrode 3 c are made wide. As a result, if the dielectric substrate 1 is made of a ferroelectric, signal loss at the bend 4 a in the signal electrode 3 b increases. By making the gap at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c narrow, however, signal loss at the bend 4 a can be reduced effectively. [0049] Furthermore, in order to enhance the effect of reducing signal loss which is obtained by making the gap at the bend 4 a narrow, the gap at the bend 4 a is made narrower than the thickness of the dielectric substrate 1 . The reason for this is as follows. If the gap at the bend 4 a is wider than the thickness of the dielectric substrate 1 , then the distribution of an electric field reaches ground formed on a reverse of the dielectric substrate 1 . As a result, the effect obtained by making the gap at the bend 4 a narrow lessens. If a ground layer is formed in the dielectric substrate 1 , the gap at the bend 4 a is made narrower than the distance between the signal electrode 3 a and the ground layer formed in the dielectric substrate 1 and the distance between the signal electrode 3 b and the ground layer formed in the dielectric substrate 1 . [0050] If the gap at the bend 4 a is made narrow, the impedance of the signal electrode 3 b at the bend 4 a becomes low and the problem of the reflection of a signal may arise. In this case, the width of the signal electrode 3 b at the bend 4 a is made narrower than the width of the signal electrode 3 b in another area (portion of the signal electrode 3 b except at the bend 4 a ). By doing so, a drop in impedance can be controlled and therefore the reflection of a signal can be controlled. [0051] In the above description the gap at the bend 4 a in the signal electrode 3 b depicted in the frame 4 of FIG. 1 is made narrow. However, the same applies to a bend in the signal electrode 3 a or another bend in the signal electrode 3 b . There is no need to consider electrical signal loss at an end (rear portion where the interference of an electrical signal and light has ended) of an interaction area (corresponding to the parallel waveguides 2 b and 2 c ) where the interference of the electrical signal and light occurs. Therefore, the above description is applicable only to a bend in a front portion of the interaction area. [0052] In addition, the above description is related to the electrode of the optical waveguide device. However, the above description is also applicable to an electrode formed in another electronic device. That is to say, if an electrode formed over or in a substrate of a electronic device has a bent portion, a gap at the bent portion between a signal electrode and an earth electrode may be made narrower than the gap in another area between the signal electrode and the earth electrode. By doing so, signal loss caused by the distribution of an electric field can be reduced. [0053] A second embodiment will now be described. With the optical waveguide device according to the first embodiment, the signal electrode and the earth electrode are formed so that the gap at the bend will change like a step. With an optical waveguide device according to a second embodiment, a signal electrode and an earth electrode are formed so that a gap at a bend will gradually narrow. [0054] FIG. 5 illustrates a delay region of an optical waveguide device according to a second embodiment. Members in FIG. 5 that are the same as those depicted in FIG. 2 are marked with the same symbols and descriptions of them will be omitted. [0055] As depicted in FIG. 5 , an earth electrode 3 c is formed so that a gap will have tapers 11 a and 11 b at both ends of a bend 4 a . The gap at the bend 4 a between a signal electrode 3 b and the earth electrode 3 c has the tapers 11 a and 11 b and gradually narrows. After the width of the gap changes from S 2 to S 1 , the width of the gap is S 1 and is constant. [0056] By gradually narrowing the gap at the bend 4 a in this way, signal loss caused by a sudden change in the distribution of an electric field can be reduced. [0057] A third embodiment will now be described. With the optical waveguide device according to the first embodiment, the signal electrode and the earth electrode are formed so that each of the signal electrode and the earth electrode will describe a curve at the bend. With an optical waveguide device according to a third embodiment, a signal electrode and an earth electrode are formed so that the signal electrode and the earth electrode will bend at right angles at a bend. [0058] FIG. 6 illustrates a delay region of an optical waveguide device according to a third embodiment. Members in FIG. 6 that are the same as those depicted in FIG. 5 are marked with the same symbols and descriptions of them will be omitted. [0059] As depicted in FIG. 6 , a signal electrode 3 b and an earth electrode 3 c are formed so that the signal electrode 3 b and the earth electrode 3 c will bend at right angles at a bend 4 a. [0060] The signal electrode 3 b and the earth electrode 3 c are formed so that the signal electrode 3 b and the earth electrode 3 c will bend at right angles at the bend 4 a . By doing so, a pattern of an electrode 3 can be formed easily. In addition, the area of the bend 4 a can be made small. [0061] A fourth embodiment will now be described. With the optical waveguide device according to the first embodiment, both of an outer gap and an inner gap formed at the bend are made narrow. With an optical waveguide device according to a fourth embodiment, only one of two gaps formed at a bend is made narrow. [0062] FIG. 7 illustrates a delay region of an optical waveguide device according to a fourth embodiment. Members in FIG. 7 that are the same as those depicted in FIG. 5 are marked with the same symbols and descriptions of them will be omitted. [0063] As depicted in FIG. 7 , an outer gap 21 a at a bend 4 a is narrower than a gap in another area. [0064] An inner gap 21 b formed at the bend 4 a is equal in width to a gap formed in another area. [0065] That is to say, if the width at the bend 4 a of the outer gap 21 a is S 1 and the width of a gap formed in another area is S 2 , then S 1 <S 2 . The width of the inner gap 21 b is S 2 and the width of a gap formed in another area is S 2 . That is to say, S 2 =S 2 . [0066] If a portion of a gap between a signal electrode 3 b and an earth electrode 3 c is made narrow, the impedance of the signal electrode 3 b at the narrow gap portion becomes low and the problem of the reflection of a signal may arise. In this case, only the outer gap 21 a at the bend 4 a is made narrow. By doing so, signal loss can be reduced. In addition, a drop in impedance can be controlled and therefore the reflection of a signal can be controlled. [0067] As stated above, only the outer gap 21 a at the bend 4 a is made narrow. By doing so, signal loss can be reduced and the reflection of a signal can be controlled. [0068] As described in the first embodiment, a drop in impedance can be controlled by making the width of the signal electrode 3 b narrow. However, if the width of the signal electrode 3 b is made too narrow, the signal electrode 3 b may break. If the above method of making only one of the gaps 21 a and 21 b at the bend 4 a narrow is adopted, there is no need to make the width of the signal electrode 3 b narrow. Therefore, the possibility that the signal electrode 3 b will break becomes small. [0069] In the above description the outer gap 21 a is made narrow. However, only the inner gap 21 b may be made narrow. From the viewpoint of reducing signal loss, however, a great effect is obtained by making the outer gap 21 a narrow, compared with the case where the inner gap 21 b is made narrow. [0070] A fifth embodiment will now be described. With the optical waveguide device according to the fourth embodiment, the outer gap 21 a at the bend 4 a is made narrow by forming tapers at both ends of the bend 4 a . With an optical waveguide device according to a fifth embodiment, a gap is gradually narrowed from both ends of a bend. [0071] FIG. 8 illustrates a delay region of an optical waveguide device according to a fifth embodiment. Members in FIG. 8 that are the same as those depicted in FIG. 7 are marked with the same symbols and descriptions of them will be omitted. [0072] As depicted in FIG. 8 , an outer gap 31 a formed at a bend 4 a is narrower than a gap formed in another area. The outer gap 31 a is formed so that the outer gap 31 a will gradually narrow from both ends of the bend 4 a . The width of the narrowest portion of the outer gap 31 a is S 1 . [0073] An inner gap 31 b formed at the bend 4 a is equal in width to a gap formed in another area. That is to say, if the width of the narrowest portion of the outer gap 31 a is S 1 and the width of a gap formed in another area is S 2 , then S 1 <S 2 . The width of the inner gap 31 b is S 2 and the width of a gap formed in another area is S 2 . That is to say, S 2 =S 2 . [0074] As stated above, the gap can be formed so that the gap will gradually narrow from both ends of the bend 4 a. [0075] A sixth embodiment will now be described. With an optical waveguide device according to a sixth embodiment, the reflection of a signal caused by a drop in impedance is controlled by making the thickness of a signal electrode and an earth electrode at a bend thin. [0076] FIG. 9 illustrates a delay region of an optical waveguide device according to a sixth embodiment. FIG. 10 is a sectional view taken along the line A-A of FIG. 9 . FIG. 11 is a sectional view taken along the line B-B of FIG. 9 . Members in FIGS. 9 through 11 that are the same as those depicted in FIG. 5 are marked with the same symbols and descriptions of them will be omitted. [0077] As depicted in FIG. 10 , the thickness of a signal electrode 3 b and an earth electrode 3 c formed is set to t 1 at a bend 4 a . For example, the signal electrode 3 b and the earth electrode 3 c are formed so that the thickness of a bent portion of the signal electrode 3 b and the thickness of a portion of the earth electrode 3 c of predetermined width which extends along the bent portion of the signal electrode 3 b will be t 1 . [0078] As depicted in FIG. 11 , the signal electrode 3 b and the earth electrode 3 c are formed so that the thickness of a portion except at the bend 4 a of each of the signal electrode 3 b and the earth electrode 3 c will be t 2 . The relationship between t 1 and t 2 is t 1 <t 2 . [0079] The signal electrode 3 b and the earth electrode 3 c are formed in this way so that the thickness of the signal electrode 3 b and the earth electrode 3 c at the bend 4 a will be thinner than the thickness of the signal electrode 3 b and the earth electrode 3 c in another area. By doing so, a drop in impedance at the bend 4 a is controlled and therefore the reflection of a signal can be controlled. [0080] In addition, there is no need to narrow the width of the signal electrode 3 b . As a result, the possibility that the signal electrode 3 b will break becomes small. [0081] In the above description the thickness at the bend 4 a of both of the signal electrode 3 b and the earth electrode 3 c is made thinner than the thickness of the signal electrode 3 b and the earth electrode 3 c in another area. However, the thickness of only the earth electrode 3 c may be made thin. Even in this case, a drop in impedance at the bend 4 a can be controlled. Furthermore, the thickness of only the earth electrode 3 c is made thin, so the possibility that the signal electrode 3 b will break can be decreased. [0082] A seventh embodiment will now be described. With an optical waveguide device according to a seventh embodiment, a buffer layer is formed at a bend between electrodes and a substrate. [0083] FIG. 12 illustrates a delay region of an optical waveguide device according to a seventh embodiment. FIG. 13 is a sectional view taken along the line A-A of FIG. 12 . Members in FIGS. 12 and 13 that are the same as those depicted in FIG. 5 are marked with the same symbols and descriptions of them will be omitted. [0084] As depicted in FIG. 13 , a buffer layer 41 is formed at a bend 4 a between a substrate 1 and a signal electrode 3 b and an earth electrode 3 c . The buffer layer 41 is made of, for example, SiO 2 . [0085] The buffer layer 41 is not formed in an area other than the bend 4 a . Accordingly, a sectional view taken along the line B-B of FIG. 12 is the same as that depicted in FIG. 11 . [0086] By forming the buffer layer 41 in this way at the bend 4 a between the substrate 1 and the signal electrode 3 b and the earth electrode 3 c , loss in the power of light propagating through an optical waveguide 2 can be reduced. Such loss is caused by the distribution of an electric field at the bend 4 a. [0087] A buffer layer may be formed in an entire area except at the bend 4 a . In addition, a buffer layer may be formed over an area where loss in the power of light occurs. For example, a buffer layer may be formed over the optical waveguide 2 . [0088] Furthermore, a buffer layer is formed in the entire area between the substrate 1 and an electrode 3 and only a portion of the buffer layer under the bend 4 a may be made thicker than the rest of the buffer layer. [0089] An eighth embodiment will now be described. With an optical waveguide device according to an eighth embodiment, grooves are formed at a bend. By doing so, a drop in impedance is controlled and the reflection of a signal is controlled. [0090] FIG. 14 is a sectional view of a delay region of an optical waveguide device according to an eighth embodiment. A sectional view depicted in FIG. 14 corresponds to, for example, a sectional view taken along the line A-A of FIG. 12 . Members in FIG. 14 that are the same as those depicted in FIG. 10 are marked with the same symbols and descriptions of them will be omitted. [0091] As depicted in FIG. 14 , grooves 51 a and 51 b are formed in a substrate 1 at a bend 4 a between a signal electrode 3 b and an earth electrode 3 c . By forming the grooves 51 a and 51 b , a space occupied by air becomes wider and impedance increases. [0092] Grooves are not formed in gaps except at the bend 4 a . Therefore, a sectional view (corresponding to, for example, a sectional view taken along the line B-B of FIG. 12 ) taken along a line not passing through the bend 4 a is the same as that depicted in FIG. 11 . [0093] The grooves 51 a and 51 b are formed in this way in the gaps at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c . By doing so, a drop in impedance at the bend 4 a can be controlled and the reflection of a signal can be controlled. [0094] Grooves may be formed in the entire substrate 1 between the signal electrode 3 b and the earth electrode 3 c . In this case, the grooves 51 a and 51 b formed in the gaps at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c are made deeper than the grooves formed in another area. [0095] A ninth embodiment will now be described. With an optical waveguide device according to a ninth embodiment, the width of grooves at a bend is made wider than the width of the grooves in another area. [0096] FIG. 15 is a sectional view of a delay region of an optical waveguide device according to a ninth embodiment. A sectional view depicted in FIG. 15 corresponds to, for example, a sectional view taken along the line A-A of FIG. 12 . Members in FIG. 15 that are the same as those depicted in FIG. 10 are marked with the same symbols and descriptions of them will be omitted. [0097] Grooves 61 a and 61 b are formed on both sides of a signal electrode 3 b . As depicted in FIG. 15 , however, the width of the grooves 61 a and 61 b at a bend 4 a is made wider than the width of the grooves 61 a and 61 b in another area. As depicted in FIG. 15 , an earth electrode 3 c is formed in part of the grooves 61 a and 61 b at the bend 4 a the width of which is wider than that of the grooves 61 a and 61 b in another area. [0098] The width of the grooves 61 a and 61 b at the bend 4 a is made wider than the width of the grooves 61 a and 61 b in another area in this way. By doing so, a drop in impedance at the bend 4 a can be controlled and the reflection of a signal can be controlled. [0099] A tenth embodiment will now be described. With an optical waveguide device according to a tenth embodiment, a signal electrode is formed so that the distance at a bend between the signal electrode and a groove will be shorter than the distance in another area between the signal electrode and the groove. [0100] FIG. 16 is a sectional view of a bend of an optical waveguide device according to a tenth embodiment. FIG. 17 is a sectional view of a portion other than the bend of the optical waveguide device according to the tenth embodiment. The sectional view depicted in FIG. 16 corresponds to, for example, a sectional view taken along the line A-A of FIG. 12 . The sectional view depicted in FIG. 17 corresponds to, for example, a sectional view taken along the line B-B of FIG. 12 . [0101] As depicted in FIGS. 16 and 17 , grooves 71 a and 71 b are formed on both sides of a signal electrode 3 b . The distance at a bend 4 a between the signal electrode 3 b and each of the grooves 71 a and 71 b is made shorter than the distance in another area between the signal electrode 3 b and each of the grooves 71 a and 71 b. [0102] As depicted in FIG. 16 , for example, the distance at the bend 4 a between the signal electrode 3 b and each of the grooves 71 a and 71 b is 0. As depicted in FIG. 17 , on the other hand, the distance in another area between the signal electrode 3 b and each of the grooves 71 a and 71 b is d. [0103] As the distance d becomes shorter, impedance increases. Accordingly, by making the distance at the bend 4 a between the signal electrode 3 b and each of the grooves 71 a and 71 b shorter than the distance in another area between the signal electrode 3 b and each of the grooves 71 a and 71 b , a drop in impedance at the bend 4 a can be controlled. [0104] The distance at the bend 4 a between the signal electrode 3 b and each of the grooves 71 a and 71 b is made shorter in this way than the distance in another area between the signal electrode 3 b and each of the grooves 71 a and 71 b . By doing so, the reflection of a signal can be controlled. [0105] An eleventh embodiment will now be described. With an optical waveguide device according to an eleventh embodiment, earth electrodes having a signal electrode therebetween are connected at a bend in order to reduce signal loss. [0106] FIG. 18 illustrates a delay region of an optical waveguide device according to an eleventh embodiment. Members in FIG. 18 that are the same as those depicted in FIG. 5 are marked with the same symbols and descriptions of them will be omitted. [0107] As depicted in FIG. 18 , earth electrodes 3 c having a signal electrode 3 b therebetween are connected at a bend 4 a by wires 81 . The wires 81 extend across the signal electrode 3 b. [0108] By connecting the earth electrodes 3 c having the signal electrode 3 b therebetween in this way at the bend 4 a by the wires 81 , grounding is performed completely and signal loss can be reduced further. [0109] The earth electrodes 3 c having the signal electrode 3 b therebetween may be connected by vias. [0110] FIG. 19 illustrates a delay region of an optical waveguide device in which earth electrodes are connected by vias. Members in FIG. 19 that are the same as those depicted in FIG. 5 are marked with the same symbols and descriptions of them will be omitted. [0111] As depicted in FIG. 19 , earth electrodes 3 c having a signal electrode 3 b therebetween are connected at a bend 4 a by vias 82 . The vias 82 are connected to, for example, a reverse of a substrate 1 or a ground layer formed in the substrate 1 . [0112] By connecting the earth electrodes 3 c having the signal electrode 3 b therebetween in this way at the bend 4 a by the vias 82 , grounding is performed completely and signal loss can be controlled further. [0113] A twelfth embodiment will now be described. With an optical waveguide device according to a twelfth embodiment, an x-cut substrate of an anisotropic material is used. [0114] FIG. 20 illustrates a delay region of an optical waveguide device according to a twelfth embodiment. Members in FIG. 20 that are the same as those depicted in FIG. 2 are marked with the same symbols and descriptions of them will be omitted. [0115] If an x-cut substrate is used as a substrate 1 , in FIG. 20 a dielectric constant in a vertical direction differs from a dielectric constant in a horizontal direction. If the direction of a signal electrode 3 b is changed at a bend 4 a , impedance changes. Therefore, as depicted in FIG. 20 , a gap between the signal electrode 3 b and an earth electrode 3 c has widths of S 3 and S 4 at a portion extending from one end of the bend 4 a and at a portion extending from the other end of the bend 4 a , respectively, that is to say, different widths. This means that the width of the gap in a straight portion of the signal electrode 3 b included in a feeder is changed according to the direction of the signal electrode 3 b formed. [0116] In order to reduce signal loss at the bend 4 a , the gap at the bend 4 a between the signal electrode 3 b and the earth electrode 3 c is made narrower than the gap in another area between the signal electrode 3 b and the earth electrode 3 c . To be concrete, as depicted in FIG. 20 , the signal electrode 3 b and the earth electrode 3 c are formed so that the widths S 5 and S 6 of the gap at the bend 4 a will be narrower than the widths S 3 and S 4 respectively. [0117] In addition, in order to control a change in impedance caused by a bend of the signal electrode 3 b , the width of the gap at the bend 4 a is not constant and is gradually changed at a bend in the signal electrode 3 b . To be concrete, as depicted in FIG. 20 , the width of the gap is S 6 at one end of the bend 4 a , gradually widens, and is S 5 (S 6 <S 5 ) at the other end of the bend 4 a. [0118] As stated above, even if the substrate 1 is made of an anisotropic material, a change in impedance can be controlled and signal loss can be reduced. [0119] By changing the widths of the signal electrode 3 b at respective portions extending from both ends of the bend 4 a , a change in impedance can be controlled. [0120] FIG. 21 illustrates a delay region of an optical waveguide device in which the width of a signal electrode is changed. Members in FIG. 21 that are the same as those depicted in FIG. 2 are marked with the same symbols and descriptions of them will be omitted. [0121] As depicted in FIG. 21 , a signal electrode 3 b has widths of W 1 and W 2 (W 1 <W 2 ) at its portion extending from one end of the bend 4 a and its portion extending from the other end of the bend 4 a , respectively, that is to say, different widths. In addition, the width of the signal electrode 3 b at the bend 4 a gradually narrows from W 2 to W 1 . As a result, even if an x-cut substrate of an anisotropic material is used as a substrate 1 , a change in impedance can be controlled. [0122] Furthermore, the width of a gap at both ends of the bend 4 a is wider than the width of the gap at the bend 4 a . As a result, signal loss at the bend 4 a can be reduced. [0123] Some of the first through twelfth embodiments described above may be combined. For example, grooves are formed in gaps, earth electrodes are connected by wires, and the width of the gaps at one end of a bend is different from the width of the gaps at the other end of the bend. [0124] The above examples are related to an optical modulator. However, the above examples are also applicable to an electronic circuit for which light is not used. In this case, a dielectric through which light does not pass may be used as a substrate. [0125] With the electronic device disclosed, signal loss at a bent portion of an electrode can be reduced. [0126] All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention has(have) been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

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    US-2010264855-A1October 21, 2010Sumitomo Osaka Cement Co., Ltd.Light control device
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