JP5033084B2 - Light modulator - Google Patents

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. 15 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. 16 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). Accordingly, two interaction optical waveguides 3a and 3b, that is, two arms of the Mach-Zehnder optical waveguide are formed at a portion where the electrical signal and light of the optical waveguide 3 interact (referred to as an interaction portion).

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. In order to simplify the description, the Si conductive layer 5 for suppressing temperature drift shown in FIG. 15 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.

  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. 16, the depth of the recesses 9a, 9b, and 9c (or the height of the ridges 8a and 8b) is emphasized, but is actually about 2 to 5 μm, and the center conductor 4a and the 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. 16, the ridge portion 8a immediately below the central 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. 16 is wide, stresses such as thermal expansion and thermal contraction due to environmental changes accumulate, and a considerably large stress (or a 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. 17, 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 the second prior art, the recesses 9a, 9b, 9c and the optical waveguides 3a, 3b forming the ridges 8a, 8b are symmetrical 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 optical waveguides 3a and 3b (or with the optical waveguide 3a). The structure of the z-cut LN substrate 1 including 3b is substantially symmetric with respect to the center line I provided in the middle thereof. 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 (optical waveguides 3a and 3b) such as a Mach-Zehnder optical waveguide, the structure in which the optical waveguides 3a and 3b are symmetric with respect to the center line between them is substantially The number of concave portions (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. 15, the high-frequency electric signal that is a microwave propagates through the high-frequency electric 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 this 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 ″, and 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. 18 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. 19 and FIG. 20 show sectional views taken along the lines 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⁾ . 19 and 20, IV is a 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. 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.

Ridges 8a, 8b, and 8c (or recesses 9a, 9b, 9c, and 9d) are arranged asymmetrically with respect to the center line III between the optical waveguides 3a and 3b. Since the inclined surfaces that are the side surfaces of the ridges 8a, 8b, and 8c are not the -z plane, the distribution of charges due to the pyroelectric effect accompanying the change in environmental temperature is different from those of the concave portions and the upper surface of the z-cut LN substrate 1. . 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 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 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 portion where the ground conductor is formed and the portion where the ground conductor is not formed are above the third recess. The ground conductor formed on the substrate on the side that is not the ridge portion for the central conductor adjacent to the second recess and is formed in the optical waveguide direction alternately as the conductor and the first gap portion includes the central conductor. The ground conductor is formed at a position symmetrical to the third recess with respect to the center line of The portion and the portion not formed are alternately formed in the direction of the optical waveguide as a second connecting ground conductor and a second gap, and the first gap and the first gap of the ground conductor are formed. At least one of the position, length, or width in the direction of the optical waveguide is formed differently for at least a part of the two gaps.

  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. The portion where the ground conductor is formed and the portion where the ground conductor is not formed are formed above the third recess as an optical waveguide as the first connection ground conductor and the first gap portion. The ground conductor formed on the substrate on the side that is not the ridge portion for the central conductor adjacent to the second recess and is alternately formed in the direction, the third conductor with respect to the center line of the central conductor The portion where the ground conductor is formed and the portion where it is not formed at a position symmetrical to the recess Are alternately formed in the direction of the optical waveguide as a second connecting ground conductor and a second gap, and at least one of the first connecting ground conductor and the second connecting ground conductor. It is characterized in that at least one of the position, the length, the width, or the thickness in the direction of the optical waveguide is different for each part.

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

  The optical modulator according to claim 4 of the present invention is characterized in that the first connection ground conductor or the second 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 optical waveguides. As a result, the charge distribution (that is, the electric field distribution) due to the pyroelectric effect that occurs in response to a change in the environmental temperature is substantially symmetric with respect to the center line provided between the two optical waveguides. 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. 15 to 20 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, 11c, and 11e are gaps. 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁶⁾ are ground conductors. The ground conductor 4b ⁽⁵⁾ connects the ground conductors 4b ⁽⁴⁾ and 4b ⁽⁶⁾ , and the ground conductor 4c ⁽⁵⁾ connects the ground conductors 4c ⁽⁴⁾ and 4c ⁽⁶⁾ (ground conductor 4b ^{(5). )} And the ground conductor 4c ⁽⁵⁾ are also called connection ground conductors).

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. 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 gaps 11a and 11e are portions where the conductor is missing in the ground conductor (or windows 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 optical waveguide is not formed. In FIG. 2, V is a center line provided between the optical waveguides 3a and 3b, and the optical waveguides 3a and 3b (or the ridges 8a and 8b or the recesses 9a, 9b, and 9c) are located with respect to the center line V. It has a symmetrical structure. Therefore, V can be said to be an axis of symmetry about the optical waveguide. 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). 8a and 8b, also called inclined surfaces).

  What is important is that in the first embodiment, the concave portions 9a, 9b, 9c including the inclined surfaces of the optical waveguides 3a, 3b are symmetrical with respect to the center line V. 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 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, it is a change of the position that does not affect the present invention, and therefore the present invention. Belongs. 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 optical waveguides, a larger number of recesses are provided. However, 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 figure, by adopting this embodiment, the traveling wave electrode is symmetrical with respect to the center line of the center conductor 4a, which is advantageous from the viewpoint of high-frequency light modulation, but the recesses 11a, 11b, 11c. , 11d can be greatly suppressed in temperature drift as compared with the third prior art in which the optical waveguides 3a and 3b are not symmetric, and the idea of the present invention was proved to be 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. Needless to say, 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 equal to the width Ww ′ and the length Lw ′ of the gap portion 11e, respectively. Although it is assumed that the length Le of 4b ^{(5) and} the length Le ′ of the ground conductor 4c ⁽⁵⁾ are equal, the present invention is not limited to this. Further, the number of the gap portions 11a and the ground conductors 4b ⁽⁵⁾ may be different from the number of the gap portions 11e 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. That the 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. Accordingly, the conversion from the symmetric mode of the symmetric high-frequency electrical signal of the connector and the input feedthrough 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 symmetric with respect to the center line V provided between the optical waveguides 3a and 3b. The width of the ground conductor 4b ⁽⁴⁾ may be different from the width of the center conductor 4a by several μm. Including this, the center of the optical waveguides 3a and 3b with respect to the center line V provided in the middle of the optical waveguides 3a and 3b. The center conductor and the ground conductor on the top are symmetrical (or substantially symmetrical). 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 optical waveguide is symmetric with respect to the center line V provided in the middle of the two optical waveguides, and the basic structure related to the traveling wave electrode is symmetric with respect to the center line VI of the central conductor. Thus, compared with the case where they do not have symmetry, 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 propagated.

(Second Embodiment)
FIG. 5 shows a top view of the second embodiment of the present invention. In addition, cross-sectional views taken along lines FF ′ and GG ′ are shown in FIGS. 6 and 7, respectively. Here, 11a, 11b, 11c, and 11f are gaps. 4b ⁽⁷⁾ , 4b ⁽⁸⁾ , 4b ⁽⁹⁾ , 4c ⁽⁷⁾ , 4c ⁽⁸⁾ , 4c ⁽⁹⁾ are ground conductors. The ground conductor 4b ⁽⁸⁾ connects the ground conductors 4b ⁽⁷⁾ and 4b ⁽⁹⁾ , and the ground conductor 4c ⁽⁸⁾ connects the ground conductors 4c ⁽⁷⁾ and 4c ⁽⁹⁾ (the ground conductor 4b ^{(8). )} And the ground conductor 4c ⁽⁸⁾ 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 air gap portions 11a and 11f are portions where the conductor is missing in the ground conductor (or windows opened in the ground conductor).

  Here, in FIG. 6, V is a center line provided in the middle of the optical waveguides 3a and 3b, and the optical waveguides 3a and 3b (or the ridges 8a and 8b or the recesses 9a, 9b, and 9c) are the center lines V. It has a symmetrical structure. Therefore, as described in FIG. 2 shown as the first embodiment, V can be said to be an axis of symmetry with respect to the optical waveguides 3a and 3b. As described above, the ridge portions 8a and 8b are inclined portions (side surfaces of the ridges 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. Or have an inclined surface). Also in this second embodiment of the present invention, the optical waveguides 3a and 3b have a structure in which the recesses 9a, 9b, and 9c are symmetrical with respect to the center line V including the inclined portion. The charge distribution, that is, the electric field distribution is also symmetric with respect to the center line V, and is extremely stable with respect to temperature drift accompanying environmental changes.

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. 5 to 7 are the center conductor 4a and the ground conductors 4b ⁽⁷⁾ , 4b ⁽⁹⁾ , 4c ⁽⁷⁾ , 4c ⁽⁹⁾ . As can be seen from FIGS. 6 and 7, the center line VII drawn to the center of the center conductor 4a is derived from the center conductor 4a and the ground conductors 4b ⁽⁷⁾ , 4b ⁽⁹⁾ , 4c ⁽⁷⁾ , 4c ^(9). This is the axis of symmetry of the traveling wave electrode. As described above, in this embodiment, the basic structure of the traveling wave electrode has structural symmetry with the center of the center conductor 4a as the axis of symmetry, and therefore the first 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 this embodiment, the structure related to the optical waveguide is symmetric with respect to the center line V provided in the middle of the two optical waveguides, and the basic structure related to the traveling wave electrode is the center line VII of the central conductor. By making it symmetrical, the temperature drift accompanying the environmental temperature change is suppressed and the mode of the high-frequency electric signal is stabilized and propagated with low loss compared to the case where they are not symmetrical.

However, when FIG. 3 is compared with FIG. 7, the connection ground conductors 4b ⁽⁸⁾ and 4c ⁽⁸⁾ in FIG. 7 are thinner than the connection ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ in FIG. From the viewpoint of effect, it is somewhat disadvantageous for high-speed light modulation.

(Third embodiment)
FIG. 8 shows a top view of the third embodiment of the present invention. 9 and 10 are sectional views taken along lines H-H 'and I-I', respectively. Here, 11a, 11b, 11c, and 11e are gaps. Note that 4b ⁽¹⁰⁾ , 4b ⁽¹²⁾ , 4b ⁽¹³⁾ , 4c ⁽¹⁰⁾ , 4c ⁽¹²⁾ , and 4c ⁽¹³⁾ are ground conductors. The ground conductor 4b ⁽¹¹⁾ connects the ground conductors 4b ⁽¹⁰⁾ and 4b ⁽¹²⁾ , and the ground conductor 4c ⁽¹¹⁾ connects the ground conductors 4c ⁽¹⁰⁾ and 4c ⁽¹²⁾ (the ground conductor 4b ^{(11). )} And the ground conductor 4c ⁽¹¹⁾ are also called connection ground conductors). Reference numerals 10b and 10d denote portions where the high-frequency electrical signal is reduced, that is, outer peripheral portions. 8a and 8b are ridge portions. It can be said that the air gap portions 11a and 11e are portions where the conductor is missing in the ground conductor (or windows opened in the ground conductor).

Also in the present embodiment shown in FIG. 9, the structure relating to the optical waveguide is such that the recesses 9a, 9b, 9c are arranged symmetrically with respect to the center line V in the middle between the two optical waveguides 3a and 3b. It is an important factor in having excellent temperature drift characteristics. 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 ⁽¹⁰⁾ , 4b ⁽¹²⁾ , 4b ⁽¹³⁾ , 4c ⁽¹⁰⁾ , 4c ⁽¹²⁾ , 4c ^(13). This is the axis of symmetry of the traveling wave electrode. Thus, the present embodiment also has a structural symmetry with respect to the basic structure of the traveling wave electrode, with the center of the center conductor 4a as the axis of symmetry, so that the high-frequency electrical signal propagating through the traveling wave electrode is in a symmetric mode. It is. Therefore, it is possible to propagate the high-frequency electric signal in a stable mode and with low loss, with good matching with the symmetrical electromagnetic field distribution of the connector and the input feed-through portion.

What should be noted in the third embodiment of the present invention is that the thickness of the ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ is 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 ⁽¹³⁾ of the outer peripheral portion 10b. Furthermore, in order to make the effect of the present invention more remarkable, the thickness of the ground conductor 4c ⁽¹³⁾ formed on the outer peripheral portion 10d is also reduced. By adopting this device, as shown in FIG. 11, 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.

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 ⁽¹³⁾ and 4c ⁽¹³⁾ , the amount of expensive Au used can be significantly reduced, which can contribute to cost reduction.

The ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ , 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 that is the electrical ground by wires or ribbons. It is useful from the point of view. This is true for all embodiments of the invention.

In this way, the idea of improving the temperature drift without reducing the modulation characteristics by reducing the thickness of the ground conductor in the outer peripheral portion where the high-frequency electrical signal is small is the first aspect of the present invention shown in FIGS. Needless to say, the present invention can also be applied to the second embodiment. 9 and 10, there is a certain effect even if only one of the thicknesses of the ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ is reduced. 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.

(Fourth embodiment)
FIG. 12 shows a top view of the fourth embodiment of the present invention. Sectional views taken along lines JJ ′ and KK ′ are shown in FIGS. 13 and 14, respectively. Here, 11a, 11b, 11c, and 11e are gaps. Note that 4b ⁽¹⁴⁾ , 4b ⁽¹⁵⁾ , 4b ⁽¹⁶⁾ , 4c ⁽¹⁴⁾ , 4c ⁽¹⁵⁾ , and 4c ⁽¹⁶⁾ are ground conductors. The ground conductor 4b ⁽¹⁵⁾ is a ground conductor for connection that connects the ground conductors 4b ⁽¹⁴⁾ and 4b ⁽¹⁶⁾ , and the ground conductor 4c ⁽¹⁵⁾ is a ground conductor for connection that connects the ground conductors 4c ⁽¹⁴⁾ and 4c ⁽¹⁶⁾ . 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 air gap portions 11a and 11e are portions where the conductor is missing in the ground conductor (or windows 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 ⁽¹⁵⁾ .

What is characteristic in this embodiment is that the thicknesses of the ground conductor 4b ⁽¹⁵⁾ and the ground conductor 4c ⁽¹⁵⁾ , which are connection ground conductors, are different. That is, the thickness of the ground conductor 4b ⁽¹⁵⁾ is made larger than the thickness of the ground conductor 4c ⁽¹⁵⁾ . Thereby, there is an advantage that it is less susceptible to the skin effect of the high-frequency electrical signal than the thickness of both the ground conductor 4b ⁽¹⁵⁾ and the ground conductor 4c ⁽¹⁵⁾ is reduced. Although this embodiment was larger than the thickness of the ground conductor ^4c and the thickness of the ground conductor ^{4b (15) (15),} than the thickness of the reverse to ground thickness conductor ^4b of the ground conductor ^{4c (15) (15)} Needless to say, it may be thicker.

The width Ww and the length Lw of the gap 11a are equal to the width Ww ′ and the length Lw ′ of the gap 11e, respectively, and the length Le of the ground conductor 4b ^{(15) and} the length of the ground conductor 4c ⁽¹⁵⁾ . Although the figures are drawn assuming that the length Le ′ is also equal, the present invention is not limited to this. Further, the number of the gap portions 11a and the ground conductors 4b ⁽¹⁵⁾ may be different from the number of the gap portions 11e and the ground conductors 4c ⁽¹⁵⁾ . 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 in the length direction of the interaction optical waveguides 3a and 3b. The width Ww of the gap 11a and the width Ww ′ of the gap 11e can be said to be the widths of the connection ground conductor 4b ⁽¹⁵⁾ and the connection ground conductor 4c ⁽¹⁵⁾ , respectively.

Similarly to the first and second embodiments shown in FIGS. 1 and 5, the ground conductor 4b ⁽¹⁵⁾ and the ground conductor 4c ⁽¹⁵⁾ , which are connection ground conductors, are connected to the optical waveguides 3a and 3b. It may be shifted in the longitudinal direction. Furthermore, it goes without saying that the thickness of the ground conductor on the outer peripheral portion may be reduced as in the third embodiment shown in FIG.

  As described above, also in this embodiment, the structure related to the optical waveguide is symmetric about the center line V provided in the middle of the two optical waveguides, and the basic structure related to the traveling wave electrode is the center line VI of the center conductor. By making it symmetrical, compared with the case where there is no such symmetry, temperature drift due to environmental temperature changes is suppressed, and the mode of the high-frequency electric signal is stabilized and propagated with low loss.

(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. 1, FIG. 5, and FIG. 8, the connecting ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ , 4b ⁽⁸⁾ and 4c ⁽⁸⁾ , 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ are the length of the optical waveguide. The center conductor 4a, which is the basic structure of the traveling wave electrode, and the ground conductor (4b) that is continuous in the longitudinal direction of the optical waveguide, are shifted from each other in the direction (up and down in the paper planes of FIGS. 1, 5, and 8). ⁽⁴⁾ and 4c ⁽⁴⁾ , 4b ⁽⁶⁾ and 4c ⁽⁶⁾ , 4b ⁽⁷⁾ and 4c ⁽⁷⁾ , 4b ⁽⁹⁾ and 4c ⁽⁹⁾ , and 4b ⁽¹⁰⁾ and 4c ⁽¹⁰⁾ 4b ⁽¹²⁾ and 4c ⁽¹²⁾ ) are symmetrical with respect to the central axis of the central conductor 4, so that the high-frequency electric signal feels an almost symmetrical electrode structure. 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.

As in the fourth embodiment of the present invention shown in FIG. 12, the connection ground conductors 4b ⁽¹⁵⁾ and 4c are also provided in the structure in which the connection ground conductor is disposed at a position symmetrical to the center line VI. ⁽¹⁵⁾ may be shifted in the longitudinal direction of the interaction optical waveguides 3a and 3b.

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, in the optical modulator according to the present invention, in the high-performance ridge type optical modulator, the structure related to the recess and the optical waveguide provided in the substrate is symmetrical with respect to the center line provided in the middle of the two optical waveguides. To improve the temperature drift characteristics, improve the propagation characteristics of the high-frequency electrical signal by making the basic structure of the traveling wave electrode symmetrical with respect to the center line of the central conductor, and It is useful as an optical modulator that realizes further improvement in temperature drift characteristics and reduction in cost by reducing the thickness.

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. Sectional drawing in FF 'of FIG. Sectional drawing in GG 'of FIG. 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 view taken along line II ′ of FIG. The figure explaining the characteristic of the 3rd Embodiment of this invention 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. The perspective view which shows schematic structure about the optical modulator of 1st prior art Sectional view in AA 'of 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 view taken along the line BB 'in FIG. Sectional view along CC 'in 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⁾ , 4b ⁽¹²⁾ , 4b ⁽¹³⁾ , 4b ⁽¹⁴⁾ , 4b ⁽¹⁵⁾ , 4b ^{(16 ) )} , 4c, 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁶⁾ , 4c ⁽⁷⁾ , 4c ⁽⁸⁾ , 4c ⁽⁹⁾ , 4c ⁽¹⁰⁾ , 4c ⁽¹¹⁾ , 4c ⁽¹²⁾ , 4c ⁽¹³⁾ , 4c ⁽¹⁴⁾ , 4c ⁽¹⁵⁾ , 4c ⁽¹⁶⁾ : Ground conductor 5: Si conductive layer 6: High frequency (RF) electric signal feeder 7: High frequency (RF) electric signal output line 8a: Ridge portion (Ridge for central conductor)
8b, 8c: Ridge portion (ridge portion for grounding conductor)
9a, 9b, 9c, 9d: recessed portions 10a, 10b, 10c, 10d: outer peripheral portions 11a, 11b, 11c, 11d, 11e, 11f: void portions (portions where conductors are missing)
13a: 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,
Above 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 direction of the optical waveguide as the first connection ground conductor and the first gap portion. And
The grounding conductor formed on the substrate on the side that is not the ridge portion for the central conductor adjacent to the second concave portion has a position that is symmetrical to the third concave portion with respect to the center line of the central conductor. The portion where the ground conductor is formed and the portion where the ground conductor is not formed are alternately formed in the direction of the optical waveguide as the second connection ground conductor and the second gap portion,
At least one of the position, the length, or the width in the direction of the optical waveguide is formed differently for at least a part of the first gap and the second gap of the ground conductor. Light modulator. 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,
Above 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 direction of the optical waveguide as the first connection ground conductor and the first gap portion. And
The grounding conductor formed on the substrate on the side that is not the ridge portion for the central conductor adjacent to the second concave portion has a position that is symmetrical to the third concave portion with respect to the center line of the central conductor. The portion where the ground conductor is formed and the portion where the ground conductor is not formed are alternately formed in the direction of the optical waveguide as the second connection ground conductor and the second gap portion,
At least one of the first connecting ground conductor and the second connecting ground conductor is formed so that at least one of the position, length, width, or thickness in the direction of the optical waveguide is different. An optical modulator.   3. The first connection ground conductor or the second connection ground conductor has substantially the same thickness as at least a part of the center conductor or the ground conductor. Light modulator.   3. The optical modulator according to claim 1, wherein the first connection ground conductor or the second connection ground conductor is thinner than a thickness of the center conductor. 4.   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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