JP2010060640A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator having high-performance optical modulation properties and improved stability. <P>SOLUTION: The optical modulator includes a substrate 1, a traveling wave electrode 4, and a central conductor ridge portion 8a and a ground conductor ridge portion 8b formed by concave portions, wherein an optical waveguide is formed on each of the central conductor ridge portion and the ground conductor ridge portion. The concave portions comprise concave portions 9a to 9c; the second and third concave portions are formed symmetric with respect to the first concave; the central conductor ridge portion is formed, between the first and second concave portions; the ground conductor ridge portion is formed between the first and third concave portions, and a ground conductor is not formed over the first and second concave portions; a portion having the ground conductor formed therein and a portion not having the ground conductor formed therein are alternately formed in one of a position over the third concave and a position of the ground conductor formed on the side opposite to the central conductor ridge portion adjacent to the second concave, which is symmetrical to the third concave with respect to a central conductor; and a portion formed with a thickness thinner than the central conductor and a portion having the same thickness as the central conductor are formed in the other. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical modulator that uses an electro-optic effect to modulate light incident on an optical waveguide with a high-frequency electrical signal and emit it as an optical signal pulse.

  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.

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.

  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.

(First prior art)
FIG. 19 shows a 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. 20 is a cross-sectional view taken along the line AA ′ of FIG.

  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). Therefore, two interaction optical waveguides 3a and 3b (or simply optical waveguides 3a and 3b), that is, Mach-Zehnder optical waveguides, are provided at a portion (referred to as an interaction portion) where the electrical signal and light of the optical waveguide 3 interact. These two arms are formed.

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 made of Au, which is an expensive noble metal material. Reference numeral 5 denotes a conductive layer for suppressing temperature drift caused by a pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, and usually a Si conductive layer is used. The width S of the center conductor 4a is about 7 μm, and the gap W between the center conductor 4a and the ground conductors 4b and 4c is about 15 μm. For simplification of explanation, the Si conductive layer 5 for suppressing temperature drift shown in FIG. 19 is omitted in FIG. In the following, the Si conductive layer 5 is omitted and discussed.

  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, 10a and 10b are regions or portions where the electromagnetic field of the high-frequency electric signal is reduced in the ground conductor, and are referred to as outer peripheral portions. The ridge portions 8a and 8b are also referred to as a central conductor ridge portion and a ground conductor ridge portion, respectively. The ridge portion is also simply called a ridge.

  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, and the like of a high-frequency electric signal. In FIG. 20, the depth of the recesses 9a, 9b, and 9c (or the height of the ridges 8a, 8b) is emphasized, but it is actually about 2 to 5 μm, and the center conductor 4a or ground The value is small compared to the thickness of the conductors 4b and 4c of about 20 μm.

  Now, it has been found that although the first prior art has high modulation characteristics as an LN optical modulator, there is a problem with stability. That is, it has been found that the temperature drift characteristic is poor despite the use of the Si conductive layer 5. The cause is thought to be due to the ridge structure that produces high modulation performance.

  The cause will be described in detail below. As can be seen from FIG. 20, the ridge portion 8a directly below the center conductor 4a is independent of the ground conductors 4b and 4c, and therefore the ridge portion 8a is pulled in a direction parallel to the surface of the z-cut LN substrate 1. There is no power.

However, in the ridge portion 8b, as described above, the thick ground conductor 4b having a thickness of about 20 μm is formed together with the concave portion 9c and the outer peripheral portion 10b. The Au of the ground conductor 4b on the SiO ₂ buffer layer 2 and the thermal expansion coefficient of the z-cut LN substrate 1 are greatly different from each other. Furthermore, the width of the z-cut LN substrate 1 is as wide as several millimeters (for example, 1 mm to 5 mm). On the other hand, since the gap between the interaction optical waveguides 3a and 3b is as narrow as about 15 μm, the width of the ground conductors 4b and 4c is so wide that it can be said to be about half the width of the z-cut LN substrate 1 (in other words, the outer peripheral portion). 10a and 10b are wide). That is, since the width of the ground conductor 4b in FIG. 20 is wide, stresses such as thermal expansion and thermal contraction due to environmental changes accumulate, and a considerably large stress (or moment due to a large thickness) is applied to the ridge portion 8b. Furthermore, since the ridge portion 8b protrudes, it is easily affected by stress (especially stress due to moment). Actually, the width of the ground conductor 4c is wide, and this is also affected.

  However, when a stress is applied to the z-cut LN substrate 1, its refractive index changes (stress birefringence). As a result, the refractive index of the interactive optical waveguide 3a changes, and the LN optical modulator operates. The DC bias point when changing is changed. This is a temperature drift phenomenon peculiar to the ridge structure, and results in impairing the stability as the LN optical modulator. Incidentally, when the environmental temperature of the LN optical modulator was changed from room temperature to 80 ° C., the change of the DC bias point in the first prior art was as large as 6V.

(Second prior art)
In order to solve the problem of the first prior art, a cross-sectional view of the interaction portion of the second prior art disclosed in Patent Document 2 is shown in FIG. As can be seen from FIG. 21, the ground conductor 4b ′ formed on the ridge portion 8b and the ground conductor 4b ″ formed on the outer peripheral portion 10b are thick, but the ground conductor formed in the recess 9c. The thickness of 4b "" is reduced to, for example, about 300 nm or less. Thus, by reducing the thickness of the ground conductor 4b ″ in the concave portion 9c, the stress applied to the ridge portion 8b by the ground conductor 4b ″ having a large area can be reduced, so that the temperature stability can be improved. This is the idea.

  In the following, discussion will be made from the viewpoint of temperature drift and propagation of high-frequency electrical signals. In this second prior art, the recesses 9a, 9b, 9c forming the ridges 8a and 8b and the interaction optical waveguides 3a and 3b have a symmetrical structure with respect to the center line I provided between them. Now, the side surfaces of the ridge portions 8a and 8b are inclined (hence, also referred to as inclined surfaces or inclined portions). Since such an inclined surface is not a −z plane, the generation of charges due to the pyroelectric effect is different from the upper surface of the z-cut LN substrate 1 and the recesses 9a, 9b, and 9c, which are the −z plane.

  Therefore, in order to realize excellent temperature drift characteristics, the recesses 9a, 9b, and 9c are substantially symmetrical with respect to the center line I that can be provided between the interaction optical waveguides 3a and 3b (or the interaction). A structure in which the structure of the z-cut LN substrate 1 including the optical waveguides 3a and 3b is substantially symmetrical with respect to the center line I provided in the middle thereof is desirable. That is, the second prior art can be said to be a structure suitable for temperature drift suppression. In the case of two interaction optical waveguides (interaction optical waveguides 3a and 3b), such as a Mach-Zehnder optical waveguide, the structure in which the interaction optical waveguides 3a and 3b are symmetrical with respect to the center line between them. Then, the number of substantial recesses (9a, 9b, 9c) is an odd number.

  However, it can be seen that the second prior art has the following problems from the viewpoint of low-loss and stable propagation of high-frequency electrical signals. As shown in FIG. 19, a high-frequency electrical signal that is a microwave propagates through the high-frequency electrical signal feed line 6 and a connector (not shown) and is then applied to the traveling wave electrode 4. The electromagnetic field distribution of this connector is axisymmetric with respect to each central conductor. In addition, the electromagnetic field distribution of the high-frequency electromagnetic field is the center of the traveling-wave electrode 4 even at a connection portion (called an input feed-through portion) between a connector (not shown) connected to the high-frequency electric signal feeder 6 and the traveling-wave electrode 4. It is bilaterally symmetric (that is, symmetric with respect to the substrate surface direction) with respect to the conductor (the portion where the central conductor 4a of the interaction portion extends to the connection portion with the core wire of the connector not shown).

  However, in the second prior art, the traveling wave electrode is structurally asymmetric with respect to the line II drawn in the center of the center conductor 4a in FIG. When the structure is asymmetrical, the electromagnetic field distribution is also asymmetrical, so that the matching with the symmetrical electromagnetic field distribution in the input feedthrough portion is not good. As a result, when a high-frequency electric signal propagates through the traveling wave electrode 4 composed of the central conductor 4a, the ground conductors 4b ′, 4b ″, 4b ″, 4c, an asymmetric electromagnetic field from a symmetric electromagnetic field distribution. It must be converted into a distribution, which causes inconveniences such as unstable electromagnetic field distribution or radiation loss (or propagation loss).

(Third prior art)
FIG. 22 shows a top view of the third prior art disclosed in Patent Document 3. As shown in FIG. The width of the z-cut LN substrate 1 is several millimeters, and the gap between the interaction optical waveguides 3a and 3b is about 15 μm. The length of the z-cut LN substrate 1 is about 5 cm to 7 cm.

Here, FIG. 23 and FIG. 24 show cross-sectional views along BB ′ and CC ′. In addition, 11a, 11b, 11c, and 11d are void portions due to the presence of the recesses 9a, 9b, 9c, and 9d, and 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ^{( 5)} and 4c ⁽⁶⁾ are ground conductors. Here, the ground conductor 4b ⁽⁵⁾ connects the ground conductors 4b ⁽⁴⁾ and 4b ⁽⁶⁾ . Reference numeral 10c denotes an outer peripheral portion. Reference numerals 8a, 8b, and 8c denote ridge portions. It can be said that the gaps 11a and 11d are portions where the conductor is missing in the ground conductor (or windows opened in the ground conductor). Reference numerals 13a and 13d denote embedded portions in which the gap portions 11a and 11d are filled with the ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ . 23 and 24, IV is the center line of the center conductor 4a, and the traveling wave electrode is symmetrical with respect to the center line IV.

As can be seen, the widths of the ground conductors 4b ⁽⁴⁾ and 4c ⁽⁴⁾ are as narrow as the ground conductor 4b 'and the center conductor 4a of the second prior art shown in FIG. Further, the ground conductors 4b ⁽⁶⁾ and 4c ⁽⁶⁾ are as wide as the ground conductor 4b '' of the second prior art shown in FIG. Then, this third in the prior art to connect the ground conductor ^4b and ^{(4) 4b (6)} the ground conductor ^{4b (5),} the ground conductor ^4c for connecting with ^{4c (6)} the ground conductor ^{4c (4) (5 )} Is set to be thicker than the ground conductor 4b "" of the second prior art shown in FIG.

  However, when the third prior art was actually manufactured, it was found that this structure has an important problem that the temperature drift due to the ridge structure cannot be suppressed. The problem will be described below from the viewpoint of the pyroelectric effect.

Ridge portions 8a, 8b, 8c (or recesses 9a, 9b, 9c, 9d) are arranged asymmetrically with respect to center line III in the middle of interaction optical waveguides 3a, 3b. Since the inclined surfaces which are the side surfaces of the ridge portions 8a, 8b and 8c are not the -z plane, the distribution of charges due to the pyroelectric effect accompanying the change in the environmental temperature is different from those of the concave portions and the upper surface of the z-cut LN substrate 1. Yes. Therefore, a non-uniform charge distribution (that is, a non-uniform electric field distribution) that changes every moment with a change in the environmental temperature is generated, so that a non-uniform voltage is applied to the interaction optical waveguides 3a and 3b. Since it is necessary to apply a DC bias from an external circuit so as to cancel these non-uniform electric field distributions, it can be concluded that a temperature drift results.
JP-A-4-288518 JP 2004-157500 A JP 2006-84537 A

  As described above, in the conventional first technique proposed as the ridge type LN optical modulator, the stress from the ground conductor (or alternatively, due to the difference in thermal expansion coefficient between Au and the z-cut LN substrate constituting the electrode) The stress due to moment) caused a temperature drift that changed the optimum DC bias point with temperature. In the second prior art proposed to improve this temperature characteristic, although the temperature drift can be improved, the traveling wave electrode is not symmetric with respect to the central conductor, so that the high frequency electric signal is changed from the symmetric mode to the asymmetric mode. It is disadvantageous from the viewpoint of stable propagation and low loss propagation in mode conversion. In the third prior art, the traveling wave electrode is symmetric with respect to the central conductor, which is advantageous from the viewpoint of stable and low-loss propagation of a high-frequency electrical signal. However, the configuration of the ridge (or recess) is advantageous. Since it was asymmetric with respect to the two interacting optical waveguides, the influence of the pyroelectric effect particularly on the inclined surface of the ridge was strong, and there was a problem from the viewpoint of temperature drift. That is, there is an urgent need to develop an optical modulator that can achieve temperature stabilization without sacrificing high speed and low driving voltage as an optical modulator.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optical modulator having high performance in light modulation characteristics and improved stability.

  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, two optical waveguides formed on the substrate, and a buffer formed on the substrate. A layer, a traveling wave electrode comprising a central conductor and a ground conductor disposed above the buffer layer, and at least part of the substrate in a region where the electric field strength of a high-frequency electric signal propagating through the traveling wave electrode is strong A ridge portion constituted by a concave portion formed by a center conductor ridge portion having the center conductor formed upward and a ground conductor ridge portion having the ground conductor formed upward. In the optical modulator in which one of the two optical waveguides is formed in the ridge portion for the central conductor and another optical waveguide is formed in the ridge portion for the ground conductor, the concave portion is First The second and third recesses, and the second and third recesses are formed symmetrically with respect to the center line of the first recess, and the first recess and the second recess The center conductor ridge is formed between the first recess and the third recess, and the ground conductor ridge is formed between the first recess and the third recess. The ground conductor is not formed above the second recess, and the ground conductor is formed above the third recess and on the substrate adjacent to the second recess and not on the ridge portion for the central conductor. A portion where the ground conductor is formed and a portion where the ground conductor is not formed on one side of the ground conductor at a position symmetrical to the third recess with respect to the center line of the center conductor. As a part, it is formed alternately in the direction of the optical waveguide, and on the other side, the conductor is thicker Characterized in that the thin portion and the center conductor or ground conductor opposing to the central conductor and a portion having substantially the same thickness is formed.

  An optical modulator according to a second aspect of the present invention includes a substrate having an electro-optic effect, two optical waveguides formed on the substrate, a buffer layer formed on the substrate, and an upper portion of the buffer layer. A traveling wave electrode composed of a central conductor and a ground conductor disposed on the substrate, and a recess formed by digging up at least a part of the substrate in a region where the electric field strength of a high-frequency electric signal propagating through the traveling wave electrode is strong A ridge portion, and the ridge portion includes a ridge portion for a central conductor in which the central conductor is formed upward, and a ridge portion for a ground conductor in which the ground conductor is formed upward. In the optical modulator in which one of the two optical waveguides is formed and another optical waveguide is formed in the ridge portion for the ground conductor, the concave portion has first, second and third portions. In the recess of The second conductor recess and the third recess are formed symmetrically with respect to the center line of the first recess, and the central conductor ridge is formed between the first recess and the second recess. And the ground conductor ridge is formed between the first recess and the third recess, and the ground conductor is disposed above the first recess and the second recess. Are formed on the center line of the center conductor of the ground conductor formed above the third recess and on the substrate adjacent to the second recess and not on the ridge portion for the center conductor. On the other hand, at a position symmetrical to the third recess, a portion where the ground conductor is formed and a portion where the ground conductor is not formed are alternately formed in the optical waveguide direction as a connection ground conductor and a gap portion. On the other hand, the ground conductor is thinner than the center conductor on the entire surface. Made is characterized in that is.

  The optical modulator according to claim 3 of the present invention is characterized in that the connection ground conductor has substantially the same thickness as at least a part of the center conductor or the ground conductor.

  The optical modulator according to claim 4 of the present invention is characterized in that the connection ground conductor is thinner than the thickness of the center conductor.

  In the optical modulator according to claim 5 of the present invention, the electromagnetic field of the high-frequency electric signal of the ground conductor formed on the non-ground conductor ridge portion adjacent to the third recess is reduced. The thickness in a region that is a predetermined distance away from the third recess, which is a region, is configured to be thinner than the thickness of the ground conductor in a region other than the region.

  In the optical modulator according to claim 6 of the present invention, the electromagnetic field of the high-frequency electrical signal of the ground conductor formed on the substrate on the side not adjacent to the ridge portion for the central conductor adjacent to the second recess is reduced. The thickness in a region that is a predetermined distance away from the second concave portion that is a region is configured to be thinner than the thickness of the ground conductor in a region other than the region.

  The optical modulator according to claim 7 of the present invention is characterized in that the substrate is made of lithium niobate.

  An optical modulator according to an eighth aspect of the present invention is characterized in that the substrate is made of a semiconductor.

In the optical modulator according to the present invention, the ridge and the concave portion constituting the ridge are symmetrical with respect to the center line provided in the middle of the two interactive optical waveguides. As a result, the charge distribution due to the pyroelectric effect that occurs in response to changes in the environmental temperature (ie, the electric field distribution) is substantially symmetric with respect to the center line provided between the two interacting optical waveguides. Temperature drift can be solved. In addition, by making the basic structure of the traveling wave electrode symmetrical with respect to the center line of the central conductor, it is possible to propagate a high-frequency electric signal stably and with low loss. As a result, there is an excellent effect that it is possible to provide an LN optical modulator with a small thermal drift without impairing the high performance from the viewpoint of modulation of the ridge type optical modulator. Furthermore, the present invention includes a structure in which the thickness of the conductor in the outer peripheral portion, which is a region (or part) where the electromagnetic field distribution of the high-frequency electric signal is reduced in the ground conductor. Due to the structure in which the thickness of the conductor at the outer peripheral portion is reduced, when the environmental temperature changes, a wide ground conductor that is generated due to the difference in thermal expansion coefficient between the ground conductor on the SiO ₂ buffer layer 2 and the z-cut LN substrate. Since the stress of the conductor from the metal can be relieved, it not only contributes to further improvement of the temperature drift characteristic, but also can reduce the amount of use of Au, which is an expensive noble metal material, thereby reducing the cost as an optical modulator. Is also possible.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the related art shown in FIGS. 19 to 24 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

(First embodiment)
FIG. 1 shows a top view of the first embodiment of the present invention. In addition, cross-sectional views taken along DD ′ and EE ′ are shown in FIGS. 2 and 3, respectively. Here, 11a, 11b, and 11c are gaps. 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁵⁾ ′, 4c ⁽⁶⁾ are ground conductors.

The ground conductor 4b ⁽⁵⁾ and the ground conductor 4c ⁽⁵⁾ are thick, so that they are thick. On the other hand, the ground conductor 4c ⁽⁵⁾ 'is thin, so that they are also called thin ground conductors. Reference numerals 10b and 10d are portions where the strength of the high-frequency electrical signal is reduced in the ground conductor, and are referred to as outer peripheral portions. 8a and 8b are ridge portions.

Thick ground conductor 4b ⁽⁵⁾ is ground conductors 4b ⁽⁴⁾ and 4b ^(6), and thick ground conductor 4c ⁽⁵⁾ and thin ground conductor 4c ⁽⁵⁾ 'is ground conductors 4c ⁽⁴⁾ and 4c ^{(6 ). )} connect are (the thickness is thick ground conductor 4b ^{(5) and,} and the ground conductor 4c ⁽⁵⁾ is thicker ground connection conductor, also is thin ground conductor 4c ^{(5) 'is} ground thin connection Also called a conductor).

In this embodiment, the ground conductors 4b ⁽⁵⁾ and 4c ^(5), which are connection ground conductors, are made thick so that they are not easily affected by the skin effect as a high-frequency electrical signal. Further, the thickness of the thin ground conductor 4c ⁽⁵⁾ ′ is set to be as thin as, for example, 50 nm to 500 nm so that the temperature drift is sufficiently small, but the material is made of Au having a small electric resistance. Accordingly, the propagation loss of the high-frequency electric signal is smaller than the loss of Au in these portions. Needless to say, the above numerical values for the thickness of the thin ground conductor 4c ⁽⁵⁾ ′ are examples and not limited thereto.

Reference numerals 10b and 10d are portions where the strength of the high-frequency electrical signal is reduced in the ground conductor, and are referred to as outer peripheral portions. 8a and 8b are ridge portions. It can be said that the gap 11a is a portion where the conductor is missing in the ground conductor (or a window opened in the ground conductor). Reference numeral 13a denotes a buried portion in which the gap portion 11a is filled with the ground conductor 4b ⁽⁵⁾ .

Here, no recess is formed under the ground conductors 4c ⁽⁵⁾ and 4c ⁽⁶⁾ on the side where the interactive optical waveguide is not formed. In FIG. 2, V is a center line provided in the middle of the interaction optical waveguides 3a and 3b, and the interaction optical waveguides 3a and 3b (or the ridge portions 8a and 8b or the recesses 9a, 9b, and 9c) are the center lines. The structure is symmetric with respect to V. Therefore, V can be said to be an axis of symmetry about the optical waveguide (or ridge). As described above, the ridge portions 8a and 8b are inclined portions (as described above, the distribution of charges generated by the pyroelectric effect is different from the bottom surfaces of the recesses 9a, 9b, and 9c and the top surface of the z-cut LN substrate 1). It is a side surface of the parts 8a and 8b and is also called an inclined surface).

  What is important is that in the first embodiment, the concave portions 9a, 9b, and 9c are symmetrical with respect to the center line V including the inclined surfaces of the interactive optical waveguides 3a and 3b. As a result, the charge distribution due to the pyroelectric effect, that is, the electric field distribution is also symmetric with respect to the center line V, so that extremely stable characteristics can be realized with respect to the temperature drift accompanying the environmental change.

  It should be noted that even if the symmetry about the interaction optical waveguides 3a and 3b is broken by forming a recess at a position that does not affect the temperature drift below each ground conductor, this is a change of the position that does not affect the present. Belongs to the invention. 2 and 3, the number of recesses is three. However, as long as the structure is symmetrical with respect to the center line provided in the middle of the two interactive optical waveguides, a larger number of recesses can be obtained. Even if provided, it can be said to belong to the present invention. Needless to say, these are true for all embodiments of the present invention.

FIG. 4 shows the experimental results for the first embodiment of the present invention when the environmental temperature T is changed from 20 ° C. to 80 ° C. For comparison, the figure also shows the measurement results for the first prior art, the second prior art, and the third prior art. Here, the width S of the center conductor 4a is 7 μm, and the gap W between the center conductor 4a and the ground conductor 4b ⁽⁴⁾ or the ground conductor 4c ⁽⁴⁾ is 15 μm. Further, the width Ww of the gap portion 11a was 15 μm, and the length Lw thereof and the length Le of the ground conductor 4b ⁽⁵⁾ were 1 mm and 100 μm, respectively. As can be seen from the drawing, since the traveling wave electrode is symmetrical with respect to the center line of the center conductor 4a, the arrangement of the recesses 9a, 9b, 9c, and 9d is an interactive optical waveguide, which is advantageous from the viewpoint of high-frequency light modulation. For 3a and 3b, the temperature drift can be greatly suppressed as compared with the third prior art which is not symmetrical, and it was proved that the idea of the present invention is correct. Note that the second conventional technique is excellent in terms of temperature drift, but as described above, the second conventional technique has a problem from the viewpoint of mode conversion of a high-frequency electric signal. Here, even when the length Lw of the gap 11a and the length Le of the ground conductor 4b ⁽⁵⁾ were changed to about 30 μm to 3 mm and about ⁵ μm to 500 μm, respectively, the temperature drift could be efficiently suppressed. However, it goes without saying that the present invention is not limited to these numerical values.

In the above description, in order to simplify the explanation of the principle of the present invention, the width Ww and the length Lw of the gap portion 11a are the width Ww ′ and the length Lw ′ of the thin ground conductor 4c ⁽⁵⁾ ′, respectively. It is assumed that the length Le of the ground conductor 4b ⁽⁵⁾ is equal to the length Le ′ of the ground conductor 4c ⁽⁵⁾ , but the present invention is not limited to this. Further, the number of the gaps 11a and the ground conductors 4b ⁽⁵⁾ may be different from the number of the thin ground conductors 4c ⁽⁵⁾ ′ and the ground conductors 4c ⁽⁵⁾ . This is true for all embodiments of the present invention. The length and width of the ground conductor (connection ground conductor) and the gap described above are assumed to be in the length direction of the interaction optical waveguides 3a and 3b.

Next, it considers from a viewpoint of electromagnetic field distribution of a high frequency electric signal. As in all other embodiments of the present invention, the basic components as traveling wave electrodes in FIGS. 1 to 3 are the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁶⁾ . As can be seen from FIGS. 2 and 3, the center line VI drawn to the center of the center conductor 4a is obtained from the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ^(6). This is the axis of symmetry of the traveling wave electrode. Thus, the basic structure of the traveling wave electrode in the present embodiment has structural symmetry with the center line of the center conductor 4a as the axis of symmetry. The fact that the basic structure of the traveling wave electrode is symmetric means that the electromagnetic field distribution of the high-frequency electrical signal propagating through the traveling wave electrode is also symmetric. Therefore, the conversion from the symmetric mode of the symmetric high-frequency electrical signal of the connector and the input feed-through portion to the asymmetric mode of the traveling-wave electrode, which is necessary in the second prior art shown in FIG. It is possible to propagate the electric signal stably and with low loss.

Now, it is not correct if the concave portion is not strictly symmetrical with respect to the center line V provided in the middle of the interaction optical waveguides 3a and 3b. Further, the width of the ground conductors 4b ⁽⁴⁾ and 4c ⁽⁴⁾ may differ from the width of the center conductor 4a by several μm, and the center line V provided in the middle of the interaction optical waveguides 3a and 3b including this may be included. The recesses are symmetric (or substantially symmetric). Furthermore, similarly, the effect of the present invention can be exhibited even if the structure of the traveling wave electrode is not strictly symmetrical with respect to the center line VI. These are true for all embodiments of the invention.

  As described above, in the present embodiment, the structure related to the interactive optical waveguide is symmetric with respect to the center line V provided in the middle of the two interactive optical waveguides, and the basic structure related to the traveling wave electrode is the center of the central conductor. By making symmetry about the line VI, compared with the case where they are not symmetrical, the temperature drift associated with the environmental temperature change is suppressed, and the mode of the high frequency electric signal is stabilized and propagated to low loss. Yes.

As described above, the center line VI drawn to the center of the center conductor 4a is the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽ the basic structure of the traveling wave electrode ⁾ . ⁶⁾ The axis of symmetry. From the viewpoint that the symmetry of the basic structure of the traveling wave electrode is important, for example, in FIG. 2, instead of the gap 11a, a thin connection for connecting the ground conductor 4b ⁽⁴⁾ and the ground conductor 4b ⁽⁶⁾ An air gap may be provided in place of the ground conductor and in place of the thin ground conductor 4c ⁽⁵⁾ ′. That is, the center line of the center electrode 4a as a symmetrical axis in FIG. 2, the void portion 11a and the thin ground conductor 4c ^{(5) 'and} it may be replaced with structures course. The thin connection ground conductor and the gap may be interchanged (in other words, the structures on the left and right of the traveling wave electrode may be interchanged with respect to the center line VI drawn at the center of the center conductor 4a). The idea is true for all embodiments of the invention.

(Second Embodiment)
FIG. 5 shows a top view of the second embodiment of the present invention. As can be seen when compared to the first embodiment shown in FIG. 1, the void portion 11a and the thin ground connection conductor 4c ^{(5) 'in} the present embodiment, but also a thick connecting ground conductors 4b and ⁽⁵⁾ 4c ⁽⁵⁾ are displaced from each other in the direction of the interaction optical waveguide, and as a result, the cross-sectional views at FF ′ and GG ′ are the same as those in FIGS.

As in the second embodiment, electrode components such as a gap portion of a traveling wave electrode and a grounding conductor for connection are arranged in the longitudinal direction of the interaction optical waveguides 3a and 3b with respect to the center line VI of the center conductor 4a. The idea that they may be offset can be said for all embodiments of the present invention. Here, the components of the traveling wave electrode that may be displaced in the longitudinal direction of the interaction optical waveguides 3a and 3b are, for example, the gap portion 11a and the thin connection ground conductor 4c ⁽⁵⁾ ′ in the second embodiment. Or the thick connection ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ . However, the present invention is not limited to this embodiment, and the present invention refers to a gap, a thick connection ground conductor, or a thin connection ground conductor. .

As in the other embodiments of the present invention, also in the second embodiment, the gap portion 11a and the thin ground conductor 4c ⁽⁵⁾ 'in FIG. 5 are replaced, that is, the first embodiment is described. As described above, it goes without saying that the structure may be such that the thin connecting ground conductor 4c ⁽⁵⁾ ′ and the gap 11a are interchanged on the left and right of the center line VI of the center conductor 4a.

(Third embodiment)
FIG. 6 shows a top view of the third embodiment of the present invention. In addition, cross-sectional views taken along lines H-H 'and I-I' are shown in FIGS. 7 and 8, respectively. Here, as in the first embodiment, 11a is a gap. In addition, 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁶⁾ , 4c ⁽⁷⁾ are ground conductors. Ground conductor ^{4b (5)} is the ground conductor ^{4b (4)} and ^{4b (6),} also the ground conductor ^{4c (7)} is connected to the ^{4c (6)} the ground conductor ^{4c (4)} (the ground conductor ^{4b (5 )} And the ground conductor 4c ⁽⁷⁾ are also called connection ground conductors). Moreover, 10b and 10d are outer peripheral parts. 8a and 8b are ridge portions. It can be said that the gap 11a is a portion where the conductor is missing in the ground conductor (or a window opened in the ground conductor).

  Here, in FIGS. 7 and 8, V is a center line provided between the interaction optical waveguides 3a and 3b, and the interaction optical waveguides 3a and 3b (or the ridge portions 8a and 8b, or the recesses 9a and 9b, 9c) has a symmetrical structure with respect to the center line V. Therefore, V can be said to be an axis of symmetry with respect to the interactive optical waveguides 3a and 3b. As described above, the ridge portions 8a and 8b are inclined portions (distributions of the ridge portions 8a and 8b) in which the distribution of charges generated by the pyroelectric effect is different from the bottom surfaces of the recesses 9a, 9b, and 9c and the top surface of the z-cut LN substrate 1. Side or inclined surface). Also in this second embodiment of the present invention, the interaction optical waveguides 3a and 3b are symmetrical with respect to the center line V, including the inclined portion, so that the pyroelectric The electric charge distribution due to the effect, that is, the electric field distribution is also symmetric with respect to the center line V, and the temperature drift accompanying the environmental change is extremely stable.

Next, as in the first embodiment, consideration is given from the viewpoint of the electromagnetic field distribution of the high-frequency electric signal. As in all other embodiments of the present invention, the basic components as traveling wave electrodes in FIGS. 6 to 8 are the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁶⁾ . 7 and 8, the center line VI drawn to the center of the center conductor 4a is obtained from the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ^(6). This is the axis of symmetry of the basic components of the traveling wave electrode. Thus, in the present embodiment, the basic structure of the traveling wave electrode has a structural symmetry with the center of the center conductor 4a as the axis of symmetry, and therefore the second structure shown in FIG. As compared with the second prior art, the symmetrical electromagnetic fields of the connector and the input feedthrough portion can be propagated stably and with low loss.

  As described above, also in the present embodiment, the structure related to the interactive optical waveguide and the ridge is symmetric with respect to the center line V provided in the middle of the two interactive optical waveguides, and the basic structure related to the traveling wave electrode is centered. By making the conductor center line VI symmetric, the temperature drift associated with the environmental temperature change is suppressed, the mode of the high-frequency electric signal is stabilized, and the loss is reduced as compared with the case of not having the symmetry. Propagating.

However, as can be seen by comparing FIG. 3 and FIG. 8, the connecting ground conductor 4c ⁽⁷⁾ in FIG. 8 is thinner than the connecting ground conductor 4c ⁽⁵⁾ in FIG. Somewhat disadvantageous.

As described in the first embodiment, the center line VI drawn to the center of the center conductor 4a is the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ which are the basic structures of traveling wave electrodes. 4c ⁽⁴⁾ and 4c ⁽⁶⁾ . From the viewpoint that the symmetry of the basic structure of the traveling wave electrode is important, for example, in FIG. 7, instead of the gap 11a, a thin connection for connecting the ground conductor 4b ⁽⁴⁾ and the ground conductor 4b ⁽⁶⁾ A gap is provided in place of the ground conductor and the thin connection ground conductor 4c ⁽⁷⁾ . In FIG. 8, the thick connection ground conductor 4b ⁽⁵⁾ and the thin connection ground conductor 4c ⁽⁷⁾ may be interchanged. good. That is, in FIG. 7 and FIG. 8, the left and right components of the traveling wave electrode may be interchanged with each other with respect to the center line VI. , Note that the structure is the same as the original).

(Fourth embodiment)
FIG. 9 shows a top view of the fourth embodiment of the present invention. Sectional views taken along lines JJ ′ and KK ′ are shown in FIGS. 10 and 11, respectively. Here, as in the first embodiment, 11a is a gap. 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4b ⁽⁷⁾ , 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁵⁾ ′, 4c ⁽⁶⁾ are ground conductors. The thin ground conductor 4b ⁽⁷⁾ is the ground conductor 4b ⁽⁴⁾ and 4b ⁽⁶⁾ , and the thick ground conductor 4c ⁽⁵⁾ and the thin ground conductor 4c ⁽⁵⁾ 'is the ground conductor 4c ⁽⁴⁾ and 4c ^{(6 ). )} connects the (each thin ground conductor ^{4b (7),} and a thick ground conductor ^{4c (5)} and the thin ground conductor ^{4c (5)} 'is also referred to as the ground connection conductor). Moreover, 10b and 10d are outer peripheral parts. 8a and 8b are ridge portions. It can be said that the gap 11a is a portion where the conductor is missing in the ground conductor (or a window opened in the ground conductor).

  Here, in FIGS. 10 and 11, V is a center line provided between the interaction optical waveguides 3a and 3b, and the interaction optical waveguides 3a and 3b (or the ridge portions 8a and 8b, or the recesses 9a and 9b, 9c) has a symmetrical structure with respect to the center line V. Therefore, V can be said to be an axis of symmetry with respect to the interactive optical waveguides 3a and 3b. As described above, the ridge portions 8a and 8b are inclined portions (distributions of the ridge portions 8a and 8b) in which the distribution of charges generated by the pyroelectric effect is different from the bottom surfaces of the recesses 9a, 9b, and 9c and the top surface of the z-cut LN substrate 1. Side or inclined surface). Also in this second embodiment of the present invention, the interaction optical waveguides 3a and 3b are symmetrical with respect to the center line V, including the inclined portion, so that the pyroelectric The electric charge distribution due to the effect, that is, the electric field distribution is also symmetric with respect to the center line V, and the temperature drift accompanying the environmental change is extremely stable.

Next, as in the first embodiment, consideration is given from the viewpoint of the electromagnetic field distribution of the high-frequency electric signal. As in all other embodiments of the present invention, the basic components as traveling wave electrodes in FIGS. 9 to 11 are the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁶⁾ . As can be seen from FIGS. 10 and 11, the center line VI drawn to the center of the center conductor 4a is obtained from the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ^(6). This is the axis of symmetry of the basic components of the traveling wave electrode. Thus, in the present embodiment, the basic structure of the traveling wave electrode has a structural symmetry with the center of the center conductor 4a as the axis of symmetry, and therefore the second structure shown in FIG. As compared with the second prior art, the symmetrical electromagnetic fields of the connector and the input feedthrough portion can be propagated stably and with low loss.

  As described above, also in the present embodiment, the structure related to the interactive optical waveguide and the ridge is symmetric with respect to the center line V provided in the middle of the two interactive optical waveguides, and the basic structure related to the traveling wave electrode is centered. By making the conductor center line VI symmetric, the temperature drift associated with the environmental temperature change is suppressed, the mode of the high-frequency electric signal is stabilized, and the loss is reduced as compared with the case of not having the symmetry. Propagating.

However, as can be seen when comparing FIGS. 3 and 11, the ground connection conductor 4b ⁽⁷⁾ in FIG. 11 because thinner than the ground connection conductor 4b ⁽⁵⁾ in FIG. 3, high-speed optical modulation in terms of skin effect Somewhat disadvantageous.

As described in the first embodiment, the center line VI drawn to the center of the center conductor 4a is the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ which are the basic structures of traveling wave electrodes. , ^4c (4), has a symmetry axis of ^{4c (6).} From the viewpoint that the symmetry of the basic structure of the traveling wave electrode is important, for example, in FIG. 10, instead of the gap portion 11a, a thin connection for connecting the ground conductor 4b ⁽⁴⁾ and the ground conductor 4b ⁽⁶⁾ A gap is provided in place of the ground conductor and the thin connection ground conductor 4c ⁽⁷⁾ . In FIG. 11, the thick connection ground conductor 4b ⁽⁵⁾ and the thin connection ground conductor 4c ⁽⁷⁾ may be interchanged. good. That is, in FIG. 10 and FIG. 11, the left and right components of the traveling wave electrode may be interchanged with respect to the center line VI.

Further, the fourth embodiment is also the gap in the form portion 11a and a thin connecting ground conductor ^{4c (5)} ', also has a thick ground connection conductor ^{4b (7)} and ^{4c (5),} the second as shown in FIG. 5 It goes without saying that, similarly to the embodiment, they may be displaced from each other in the direction of the interaction optical waveguides 3a and 3b.

(Fifth embodiment)
FIG. 12 shows a top view of the fifth embodiment of the present invention. Further, cross-sectional views taken along lines LL ′ and MM ′ are shown in FIGS. 13 and 14, respectively. Here, as in the first embodiment, 11a is a gap. 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4b ⁽⁷⁾ , 4c ⁽⁴⁾ , 4c ⁽⁶⁾ , 4c ⁽⁷⁾ are ground conductors. The ground conductor 4b ⁽⁷⁾ connects the ground conductors 4b ⁽⁴⁾ and 4b ⁽⁶⁾ , and the ground conductor 4c ⁽⁷⁾ connects the ground conductors 4c ⁽⁴⁾ and 4c ⁽⁶⁾ (the ground conductor 4b ^{(7). )} And the ground conductor 4c ⁽⁷⁾ are also called connection ground conductors). Moreover, 10b and 10d are outer peripheral parts. 8a and 8b are ridge portions. It can be said that the gap 11a is a portion where the conductor is missing in the ground conductor (or a window opened in the ground conductor).

  Here, in FIGS. 13 and 14, V is a center line provided in the middle of the interaction optical waveguides 3a and 3b, and the interaction optical waveguides 3a and 3b (or the ridge portions 8a and 8b or the recesses 9a and 9b, 9c) has a symmetrical structure with respect to the center line V. Therefore, V can be said to be an axis of symmetry with respect to the interactive optical waveguides 3a and 3b. As described above, the ridge portions 8a and 8b are inclined portions (distributions of the ridge portions 8a and 8b) in which the distribution of charges generated by the pyroelectric effect is different from the bottom surfaces of the recesses 9a, 9b, and 9c and the top surface of the z-cut LN substrate 1. Side or inclined surface). Also in this second embodiment of the present invention, the interaction optical waveguides 3a and 3b are symmetrical with respect to the center line V, including the inclined portion, so that the pyroelectric The electric charge distribution due to the effect, that is, the electric field distribution is also symmetric with respect to the center line V, and the temperature drift accompanying the environmental change is extremely stable.

Next, as in the first embodiment, consideration is given from the viewpoint of the electromagnetic field distribution of the high-frequency electric signal. As with all other embodiments of the present invention, the basic components as traveling wave electrodes in FIGS. 12-14 are the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , is a 4c ^(6). As can be seen from FIGS. 13 and 14, the center line VI drawn to the center of the center conductor 4a is obtained from the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ^(6). This is the axis of symmetry of the basic components of the traveling wave electrode. Thus, in the present embodiment, the basic structure of the traveling wave electrode has a structural symmetry with the center of the center conductor 4a as the axis of symmetry, and therefore the second structure shown in FIG. As compared with the second prior art, the symmetrical electromagnetic fields of the connector and the input feedthrough portion can be propagated stably and with low loss.

  As described above, also in the present embodiment, the structure related to the interactive optical waveguide and the ridge is symmetric with respect to the center line V provided in the middle of the two interactive optical waveguides, and the basic structure related to the traveling wave electrode is centered. By making the conductor center line VI symmetric, the temperature drift associated with the environmental temperature change is suppressed, the mode of the high-frequency electric signal is stabilized, and the loss is reduced as compared with the case of not having the symmetry. Propagating.

However, as can be seen by comparing FIG. 13 and FIG. 14, the connecting ground conductor 4c ⁽⁷⁾ in FIG. 13 and the connecting ground conductor 4b ⁽⁷⁾ in FIG. Slightly disadvantageous to modulation.

As described in the first embodiment, the center line VI drawn to the center of the center conductor 4a is the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁶⁾ which are the basic structures of traveling wave electrodes. 4c ⁽⁴⁾ and 4c ⁽⁶⁾ . From the viewpoint that the symmetry of the basic structure of the traveling wave electrode is important, for example, in FIG. 13, instead of the gap 11a, a thin connection for connecting the ground conductor 4b ⁽⁴⁾ and the ground conductor 4b ⁽⁶⁾ A space may be provided in place of the ground conductor and the thin connection ground conductor 4c ⁽⁷⁾ . That is, in FIG. 13 and FIG. 14, the left and right components of the traveling wave electrode may be interchanged with each other with respect to the center line VI. , Note that the structure is the same as the original).

(Sixth embodiment)
FIG. 15 shows a top view of the sixth embodiment of the present invention. Further, cross-sectional views taken along lines NN ′ and OO ′ are shown in FIGS. 16 and 17, respectively. Here, as in the first embodiment, 11a is a gap. 4b ⁽⁸⁾ , 4b ⁽⁹⁾ , 4b ⁽¹⁰⁾ , 4b ⁽¹¹⁾ , 4c ⁽⁸⁾ , 4c ⁽⁹⁾ , 4c ⁽⁹⁾ ', 4c ⁽¹⁰⁾ , 4c ⁽¹¹⁾ are ground conductors It is. The ground conductor 4b ⁽⁹⁾ connects the ground conductors 4b ⁽⁸⁾ and 4b ⁽¹⁰⁾ , and the ground conductors 4c ⁽⁹⁾ and 4c ⁽⁹⁾ 'connect the ground conductors 4c ⁽⁸⁾ and 4c ⁽¹⁰⁾ . (The ground conductor 4b ⁽⁹⁾ and the ground conductors 4c ⁽⁹⁾ , 4c ⁽⁹⁾ 'are also called connection ground conductors). Moreover, 10b and 10d are outer peripheral parts. 8a and 8b are ridge portions. It can be said that the gap 11a is a portion where the conductor is missing in the ground conductor (or a window opened in the ground conductor).

What should be noted in the sixth embodiment of the present invention is that the thicknesses of the ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ are as thin as 300 nm, for example. When the thickness of the grounding conductor is large, the stress (or stress due to moment) applied to the z-cut LN substrate 1 and eventually the ridges 8a and 8b by the lever principle increases. Therefore, in this embodiment, the stress is reduced by reducing the thickness of the ground conductor 4b ⁽¹¹⁾ of the outer peripheral portion 10b. Furthermore, in order to more remarkable the effect of the present invention, are also thinner thickness of the ground conductor 4c formed on an outer peripheral portion 10d ^(11). By incorporating this contrivance, as shown in FIG. 18, the temperature drift of the DC bias with respect to the change in the environmental temperature from 20 ° C. to 80 ° C. can be further suppressed as compared with the first embodiment.

In this embodiment, the first embodiment is taken as an example, and the thickness of the ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ above the outer peripheral portions 10b and 10d is reduced. The present invention can be applied to all the embodiments of the present invention including the second to fifth embodiments.

As described above, considering that the gap between the interaction optical waveguides 3a and 3b is about 15 μm, the width of the interaction portion where the high-frequency electric signal interacts with the light propagating through the interaction optical waveguides 3a and 3b. Is significantly narrower than the width of the z-cut LN substrate 1 (about 1 mm to 5 mm). Therefore, by reducing the thickness of the ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ , the amount of expensive Au used can be significantly reduced, which can contribute to cost reduction.

The ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ , which are thin but have a large area, establish a firm electrical ground from the viewpoint of high-frequency electrical signals and connect them to the casing, which is an electrical ground, using wires or ribbons. It is useful from the point of view. This is true for all embodiments of the invention.

In addition, even if only one of the thicknesses of the ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ is reduced in FIGS. Further, since the electromagnetic field of the high-frequency electric signal is originally small at the outer peripheral portion, even if the asymmetry is broken to this extent, the propagation characteristic of the high-frequency electric signal is hardly affected. This is true for all embodiments of the invention.

(Each embodiment)
In the present specification, it has been stated that the traveling wave electrode is symmetric with respect to the center line of the center conductor, which is most desirable from the viewpoint of propagation of a high-frequency electrical signal. is there. That is, in FIG. 5, the connecting ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵ ) are shifted from each other in the longitudinal direction of the optical waveguide (up and down in the plane of FIG. 1, FIG. 5, and FIG. 8). The center conductor 4a and the ground conductors (4b ⁽⁴⁾ and 4c ⁽⁴⁾ , 4b ⁽⁶⁾ and 4c ⁽⁶⁾ ) continuous in the longitudinal direction of the optical waveguide, which are the basic structures of the electrodes, are the center of the center conductor 4a. Since it is symmetrical with respect to the axis, it feels like a substantially symmetrical electrode structure for high-frequency electrical signals. Needless to say, the gaps 11b and 11c may be formed to the limit of the longitudinal direction of the interaction part. That is, it goes without saying that even if the structure is not strictly symmetrical, if it is almost symmetrical at the main part, it also belongs to the present invention.

  Further, in the first embodiment shown in FIG. 3 and the third embodiment shown in FIG. 8, it has been described that the connecting ground conductor is as thick as the central conductor, but the thickness of the connecting ground conductor is one. Needless to say, you don't have to. In particular, when the connection ground conductor is formed in the recess, the electrode is formed in the recessed portion, so the connection ground conductor may be thicker than the thickness of the center conductor depending on the process method such as resist coating and plating conditions. Although it may be thin, it may be said that these also belong to the present invention.

Although the Mach-Zehnder optical waveguide is used as an example of the branched optical waveguide, it goes without saying that the present invention can be applied to other branched / multiplexed optical waveguides such as directional couplers. The present invention is also applicable to a phase modulator having a single optical waveguide. In the case of the phase modulator, the one optical waveguide and the traveling wave electrode are symmetric with respect to the center line of the central conductor. 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.

  Further, although the z-cut LN substrate has been described, an LN substrate having other plane orientation such as x-cut and y-cut may be used, or a lithium tantalate substrate or a substrate made of a different material such as a semiconductor substrate may be used. Furthermore, although the electrode has been described as a traveling wave electrode, in principle, a lumped constant electrode may be used. Therefore, the traveling wave electrode in this specification includes a lumped constant electrode.

  Usually, each recess is formed with the same width, but when etching is performed so that the recess close to the outer periphery becomes very wide (the outer periphery is almost the same height as the bottom of the recess). It is possible to apply the present invention by considering the widely etched portion as the actual outer peripheral portion.

  As described above, the optical modulator according to the present invention is a high-performance ridge type optical modulator, in which the structure related to the recess, ridge, or interactive optical waveguide provided in the substrate is located between the two interactive optical waveguides. The temperature drift characteristics are improved by making the center line symmetrical, and the propagation characteristics of high-frequency electrical signals are improved by making the basic structure of the traveling wave electrode symmetrical to the center line of the central conductor. In addition, it is useful as an optical modulator that realizes further improvement in temperature drift characteristics and reduction in cost by reducing the thickness of Au in the outer peripheral portion.

1 is a top view showing a schematic configuration of an optical modulator according to a first embodiment of the present invention. Sectional drawing in DD 'of FIG. Sectional view taken along line EE ′ of FIG. The figure explaining the characteristic of the 1st Embodiment of this invention FIG. 5 is a top view showing a schematic configuration of an optical modulator according to a second embodiment of the present invention. FIG. 6 is a top view showing a schematic configuration of an optical modulator according to a third embodiment of the present invention. Sectional drawing in HH 'of FIG. Sectional drawing in II 'of FIG. The top view which shows schematic structure of the optical modulator concerning the 4th Embodiment of this invention. Sectional drawing in JJ 'of FIG. Sectional drawing in KK 'of FIG. Top view showing a schematic configuration of an optical modulator according to a fifth embodiment of the present invention. Sectional drawing in LL 'of FIG. Sectional drawing in MM 'of FIG. The top view which shows schematic structure of the optical modulator concerning the 6th Embodiment of this invention. Sectional drawing in NN 'of FIG. Sectional view taken along line OO 'in FIG. The figure explaining the characteristic of the 6th Embodiment of this invention The perspective view which shows schematic structure about the optical modulator of 1st prior art Sectional view taken along the line AA 'in FIG. Sectional drawing which shows schematic structure about the optical modulator of 2nd prior art 3 is a top view showing a schematic configuration of a third conventional optical modulator. FIG. Sectional drawing in BB 'of FIG. Sectional drawing in CC 'of FIG.

Explanation of symbols

1: z-cut LN substrate (LN substrate)
_2, 14, 15: SiO ₂ buffer layer (buffer layer)
3: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b: interaction optical waveguide constituting Mach-Zehnder optical waveguide 4: traveling wave electrode 4a: central conductor 4b, 4b ′, 4b ″, 4b ″, 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ^{(6 )} , 4b ⁽⁷⁾ , 4b ⁽⁸⁾ , 4b ⁽⁹⁾ , 4b ⁽¹⁰⁾ , 4b ⁽¹¹⁾ , 4c, 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁵⁾ ', 4c ⁽⁶⁾ , 4c ⁽⁷⁾ , 4c ⁽⁸⁾ , 4c ⁽⁹⁾ , 4c ⁽⁹⁾ ', 4c ⁽¹⁰⁾ , 4c ⁽¹¹⁾ : Ground conductor 5: Si conductive layer 6: High frequency (RF) electric signal feeder 7: High frequency (RF) electrical signal output line 8a: Ridge part (ridge part for central conductor)
8b, 8c: Ridge portion (ridge portion for grounding conductor)
9a, 9b, 9c, 9d: concave portions 10a, 10b, 10c, 10d: outer peripheral portions 11a, 11b, 11c, 11d: void portions (portions where the conductor is missing)
13a, 13d: embedded part

Claims (8)

Progression comprising a substrate having an electro-optic effect, two optical waveguides formed on the substrate, a buffer layer formed on the substrate, a central conductor disposed above the buffer layer, and a ground conductor A wave electrode, and a ridge portion formed by a recess formed by digging at least part of the substrate in a region where the electric field strength of the high-frequency electric signal propagating through the traveling wave electrode is strong, A center conductor ridge portion formed above the center conductor and a ground conductor ridge portion formed above the ground conductor, and one of the two optical waveguides on the center conductor ridge portion. In the optical modulator in which another optical waveguide is formed in the ridge portion for the ground conductor,
The concave portion includes first, second, and third concave portions, and the second concave portion and the third concave portion are formed at symmetrical positions with respect to the center line of the first concave portion, and the first concave portion is formed. The center conductor ridge portion is formed between the first recess and the second recess, and the ground conductor ridge portion is formed between the first recess and the third recess,
The ground conductor is not formed above the first recess and the second recess,
The third recess with respect to the center line of the center conductor of the ground conductor formed above the third recess and on the substrate adjacent to the second recess but not on the ridge portion for the center conductor And a portion where the ground conductor is formed on one side and a portion where the ground conductor is not formed are alternately formed in the direction of the optical waveguide as a connection ground conductor and a gap, and the center conductor is formed on the other side. An optical modulator comprising: a portion where a conductor is formed thinner than a thickness; and a portion having substantially the same thickness as the center conductor or a ground conductor opposite to the center conductor. Progression comprising a substrate having an electro-optic effect, two optical waveguides formed on the substrate, a buffer layer formed on the substrate, a central conductor disposed above the buffer layer, and a ground conductor A wave electrode, and a ridge portion formed by a recess formed by digging at least part of the substrate in a region where the electric field strength of the high-frequency electric signal propagating through the traveling wave electrode is strong, A center conductor ridge portion formed above the center conductor and a ground conductor ridge portion formed above the ground conductor, and one of the two optical waveguides on the center conductor ridge portion. In the optical modulator in which another optical waveguide is formed in the ridge portion for the ground conductor,
The concave portion includes first, second, and third concave portions, and the second concave portion and the third concave portion are formed at symmetrical positions with respect to the center line of the first concave portion, and the first concave portion is formed. The center conductor ridge portion is formed between the first recess and the second recess, and the ground conductor ridge portion is formed between the first recess and the third recess,
The ground conductor is not formed above the first recess and the second recess,
The third recess with respect to the center line of the center conductor of the ground conductor formed above the third recess and on the substrate adjacent to the second recess but not on the ridge portion for the center conductor And a portion where the ground conductor is formed on one side and a portion where the ground conductor is not formed are alternately formed in the direction of the optical waveguide as a connection ground conductor and a gap, and the ground conductor is formed on the other side. An optical modulator characterized by being formed on the entire surface with a thickness smaller than the thickness of the central conductor.   The optical modulator according to claim 1, wherein the connection ground conductor has substantially the same thickness as at least a part of the center conductor or the ground conductor.   The optical modulator according to claim 1, wherein the connection ground conductor is thinner than the thickness of the center conductor.   A predetermined distance away from the third recess, which is a region where the electromagnetic field of the high-frequency electrical signal is reduced, on a ground conductor formed on the substrate that is not adjacent to the ridge portion for the ground conductor adjacent to the third recess. 5. The optical modulator according to claim 1, wherein a thickness in the region is smaller than a thickness of the ground conductor in a region other than the region.   A ground conductor formed on a substrate adjacent to the second recess and not the ridge portion for the central conductor is separated from the second recess, which is a region where the electromagnetic field of the high-frequency electric signal is reduced, by a predetermined distance. 6. The optical modulator according to claim 1, wherein a thickness in the region is smaller than a thickness of the ground conductor in a region other than the region.   The optical modulator according to any one of claims 1 to 6, wherein the substrate is made of lithium niobate.   The optical modulator according to any one of claims 1 to 6, wherein the substrate is made of a semiconductor.

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