JP5162196B2 - 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. 8 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. 9 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 interacting optical waveguides 3a and 3b, that is, two arms of a Mach-Zehnder optical waveguide are formed in a portion (referred to as an interacting portion) where the electrical signal and light of the optical waveguide 3 interact.

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. 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. 8 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 outer peripheral parts. 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. 9, 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 and grounding are shown. The value is small compared to the thickness of the conductors 4b and 4c of about 20 μm.

  Now, it has been found that the first prior art has a wide optical modulation band as an LN optical modulator and has excellent modulation characteristics, but has 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. 9, 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 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, about 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. 9 is wide, stresses such as thermal expansion and thermal shrinkage caused by environmental changes accumulate, and a considerably large stress is applied to the ridge portion 8b. Actually, the width of the ground conductor 4c is wide and the influence is great.

  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. 10, the thickness of the ground conductor 4b ′ formed on the ridge portion 8b and the ground conductor 4b ″ formed on the outer peripheral portion 10b (or called the connection ground conductor 4b ″). Is thick, but the thickness of the ground conductor 4b "" formed in the recess 9c is as thin as about 300 nm, for example. 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.

  However, the second prior art has a serious problem to be solved as follows. In this second prior art, although there are 4b ′, 4b ″ and 4b ′ ″ as the ground conductors, as described above, the thickness of the ground conductor 4b ″ is small, and a high-frequency electric signal of 10 Gbit / s or more is generated. It is difficult to propagate. As a result, the propagation loss of high-frequency electrical signals increases due to the skin effect.

  The ground conductor 4b ′ and the ground conductor 4b ″ are almost completely independent in terms of high frequency. That is, the portion where the most current actually flows corresponding to the center conductor 4a is the ground conductor 4b 'which is opposite to the center conductor 4a and is as narrow as the center conductor 4a. The current flows only through the ground conductor 4b 'formed on the ground conductor ridge 8b.

  Therefore, the high-frequency electric signal is easily lost due to Joule heat, and the modulation band is significantly deteriorated as compared with the first prior art shown in FIG. When the authors actually confirmed through experiments, modulation of 2.5 Gbit / s was finally achieved as a transmission rate in optical communication, and modulation of 10 Gbit / s, which is currently mainstream, was difficult. Also, 40 Gbit / s modulation, which is promising in the near future, could not be performed at all.

(Third prior art)
FIG. 11 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. 12 and FIG. 13 show cross-sectional views along BB ′ and CC ′. Here, 11a, 11b, 11c, and 11d are voids due to the presence of the recesses 9a, 9b, 9c, and 9d. 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⁾ . 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⁾ .

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 embodiment shown in FIG. Further, the ground conductors 4b ⁽⁶⁾ and 4c ⁽⁶⁾ are as wide as the ground conductor 4b '' of the second embodiment 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 embodiment 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 sufficiently suppressed. The problem will be described below.

Ridges 8a, 8b, and 8c (or recesses 9a, 9b, 9c, and 9d) are arranged asymmetrically with respect to the center line between the optical waveguides 3a and 3b. Since the inclined surfaces which 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 the environmental temperature is different from the concave portion and the upper surface of the z-cut LN substrate 1. Further, the arrangement of the traveling wave electrodes is also asymmetric. For this reason, a non-uniform charge distribution (that is, a non-uniform electric field distribution) that changes every moment with the change of 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 due to the difference in thermal expansion coefficient between Au and the z-cut LN substrate constituting the electrode increases with temperature. A temperature drift occurred that changed the optimum DC bias point. In the second prior art proposed to improve this temperature characteristic, the thickness of the connecting ground conductor in the concave portion connecting the thick ground conductor adjacent to the central conductor and the thick ground conductor other than the concave portion is small. Due to the skin effect, Joule heat is generated in the connecting ground conductor, the propagation loss of the high-frequency electric signal is increased, and the light modulation band is narrowed. Further, most of the current flows through the ground conductor at a portion formed on the narrow ridge portion for the ground conductor, and therefore, the high-frequency electric signal is easily lost due to Joule heat. As described above, the second prior art has a problem in basic characteristics as an optical modulator from the viewpoint of high-speed modulation. In the third prior art, the traveling wave electrode is symmetrical with respect to the central conductor, which is advantageous from the viewpoint of stable and low-loss propagation of a high-frequency electric signal, but has a problem in temperature drift characteristics. This is mainly because the structure of the ridge portion (or recess) is asymmetric with respect to the two optical waveguides, so that the charge induced on the bottom surface of the recess or the top surface of the ridge and the charge induced on the inclined surface of the ridge. This is due to the large difference between the two. As a result, there is an urgent need to develop an optical modulator that can achieve temperature stabilization without sacrificing high speed and low drive 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. comprising a layer, the upwardly disposed central conductor of the buffer layer and the traveling wave electrode formed of the ground conductor, and a front Symbol ridge composed of a plurality of recesses formed by digging at least a portion of the substrate The ridge portion is adjacent to the central conductor ridge portion with the central conductor formed above , through a common recess, and the ground conductor ridge portion with the ground conductor formed upward. In the optical modulator, one of the two optical waveguides is formed in the central conductor ridge portion, and the ground conductor is formed in the concave portions on both sides of the central conductor ridge portion. Not before The ground conductors in said common recess in the recess on the side not the ridge ground conductor, with its missing part sites are formed, other than the region that the missing (hereinafter, referred to as the ground connection conductor) Is formed to be substantially the same as at least a part of the center conductor or the ground conductor, and the plurality of recesses and the traveling wave electrode are paired with respect to a center line provided in the middle of the two optical waveguides . universal arrangement der is, a plurality in the recess of the non-common recess side of the ground conductor ridge portion, the ground connection conductor is a length of 5μm~500μm in the optical waveguide direction in the optical waveguide A plurality of the missing portions are formed with a length of 30 μm to 3 mm in the optical waveguide direction of the optical waveguide, and the length of each of the missing portions is the connection ground conductor. It characterized that you have been formed alternately in longer combinations than the length of each.

  The optical modulator according to claim 2 of the present invention is characterized in that a ground conductor adjacent to the recess has substantially the same thickness as at least a part of the center conductor or the ground conductor.

The optical modulator according to claim 3 of the present invention, except for the plurality of recesses, at least a portion of the thickness of the ground conductor in the optical waveguide and the region in which a direction a predetermined distance away from the optical waveguide crossing, the It is characterized in that it is made at least partially thinner than the thickness of the ground conductor in a region other than the region.

  According to a fourth aspect of the present invention, there is provided an optical modulator having a structure in which the concave portion is not provided below the ground conductor which is not provided with the optical waveguide below.

  The optical modulator according to claim 5 of the present invention is characterized in that the substrate is made of lithium niobate.

  The optical modulator according to claim 6 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 arrangement of the ridge and the concave portion constituting the ridge and the electrode is substantially symmetrical with respect to the center line provided in the middle of the two optical waveguides, and the connection ground in the concave portion is provided. While increasing the thickness of the conductor, a part of the conductor is omitted to relieve stress. As a result, the distribution of electric charges (that is, the distribution of the electric field) that occurs in response to changes in the environmental temperature is substantially symmetric with respect to the center line provided between the two optical waveguides. Can achieve extremely improved temperature drift characteristics. The traveling wave electrode is also substantially symmetric with respect to the center line provided in the middle of the two optical waveguides. As a result, although the symmetry of the electromagnetic field propagating through the traveling wave electrode with respect to the center line of the central conductor is broken, unlike the second conventional technique, the thickness of the connection ground conductor is thick, so that propagation due to the skin effect in the connection ground conductor is caused. Loss is small. In addition, since the ground conductor on the side where the optical waveguide is not formed does not have a portion where the conductor is missing, it is somewhat advantageous from the viewpoint of propagation loss of high-frequency electrical signals. Furthermore, the present invention includes a structure in which the thickness of the conductor on the outer peripheral portion is reduced. By reducing the thickness of the conductor at the outer peripheral portion, when the environmental temperature changes, the stress of the conductor from the wide ground conductor caused by the difference in thermal expansion coefficient between the conductor and the z-cut LN substrate is relieved. Therefore, not only can the temperature drift characteristics be further improved, but also the amount of Au, which is an expensive noble metal material, can be reduced, so that the cost as an optical modulator can be reduced.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the prior art shown in FIG. 8 to FIG. 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, 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁶⁾ and 4c are ground conductors. As can be seen from the figure, a ground conductor 4b ⁽⁵⁾ thickened so as not to be affected by the skin effect as a high-frequency electrical signal connects the ground conductors 4b ⁽⁴⁾ and 4b ⁽⁶⁾ (ground). Conductor 4b ⁽⁵⁾ is called a connecting ground conductor).

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 is 15 μm. Here, the width of the center conductor 4a and the width of the ground conductor 4b ⁽⁴⁾ are approximately the same (or even if they are different from each other, the difference is about several μm, so that they do not differ greatly). Reference numerals 10a and 10b denote outer peripheral portions of the ground conductor. In FIG. 3, reference numeral 13a denotes a buried portion in which the gap portion 11a is filled with the ground conductor 4b ⁽⁵⁾ .

  There are two important things in the present invention. First, as can be seen from FIG. 2, the structure of the traveling wave electrode including the center conductor 4a and the ground conductor 4b and the ridges 8a and 8b including the recesses 9a, 9b and 9c for the optical waveguides 3a and 3b is the same as that of the optical waveguides 3a and 3b. This is a point that is substantially symmetric with respect to the center line V provided in the middle. As a result, there is a recess or a ridge inclined surface that is asymmetric with respect to the optical waveguide, and an electric field caused by charges induced in the recess, the upper surface of the ridge, and the inclined surface of the ridge is applied nonuniformly to the two optical waveguides. Unlike the third prior art shown in FIG. 12, extremely stable temperature drift characteristics can be realized.

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 widths Ww of the gap portions 11a and 11b were 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 provided in the middle of the optical waveguides 3a and 3b. It has become possible to greatly suppress the temperature drift compared to the prior art. Thus, it was proved from the experimental results that the concept of the present invention is correct. Here, even when the lengths Lw of the gap portions 11a and 11b 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.

The second important point is the light modulation characteristic. Although the second prior art shown in FIG. 10 is also excellent in terms of temperature drift, as described above, the thickness of the connecting ground conductor in the second prior art is thin. There is a problem that the propagation loss of the signal becomes large. As can be seen from FIG. 3 of the present invention, the ground conductor 4b ⁽⁵⁾ connecting the ground conductors 4b ^{( 4)} and 4b ⁽⁶⁾ is thick. Accordingly, the propagation loss of the high-frequency electrical signal is small compared to the second prior art, which is advantageous as an optical modulation band.

If the traveling wave electrode is not strictly symmetrical with respect to the center line provided between the optical waveguides 3a and 3b, it is not correct if the effect of the present invention can be exhibited. At least one of the widths of the ground conductors 4b ⁽⁴⁾ and 4c may be different from the width of the center conductor 4a by several μm, and the center of the structure related to the traveling wave electrode including the above is provided between the optical waveguides 3a and 3b. Symmetric (or substantially nearly symmetrical) with respect to the line. Moreover, you may newly provide a recessed part in the location which does not exert influence of the grounding conductors 4b ⁽⁶⁾ and 4c.

  The concept of making the arrangement of the two optical waveguides and the traveling wave electrode conductors 4a in these first embodiments substantially symmetric can be said for all the embodiments of the present invention.

(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, 4b ⁽¹⁴⁾ , 4b ⁽¹⁵⁾ , 4b ⁽¹⁶⁾ , 4b ⁽¹⁷⁾ and 4c ⁽¹⁴⁾ , 4c ⁽¹⁵⁾ are ground conductors. As can be seen from the figure, a ground conductor 4b ⁽¹⁵⁾ thickened so as not to be affected by the skin effect as a high-frequency electrical signal connects the ground conductors 4b ⁽¹⁴⁾ and 4b ⁽¹⁶⁾ .

If the thickness of the ground conductor is large, the stress (moment) applied to the ridge portion 8b due to the lever principle increases. Therefore, in the second embodiment of the present invention, the thickness of the ground conductor 4b ⁽¹⁷⁾ of the outer peripheral portion 10b is reduced. The stress is reduced. In order to make the effect of the present invention more remarkable, the thickness of the ground conductor 4c ⁽¹⁵⁾ formed on the outer peripheral portion 10a is also reduced.

As can be seen from FIG. 6, also in this embodiment, the recesses 9a, 9b, 9c and the ridges 8a, 8b, the center conductor 4a, the ground conductor 4b ⁽¹⁴⁾ , 4b ⁽¹⁵⁾ , 4b ⁽¹⁶⁾ , 4b ^{(17 ) And} 4c ⁽¹⁴⁾ and 4c ⁽¹⁵⁾ are arranged symmetrically with respect to a center line V provided in the middle between the two optical waveguides 3a and 3b.

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 ⁽¹⁷⁾ and 4c ⁽¹⁵⁾ , the amount of expensive Au used can be remarkably reduced, and the cost of the LN optical modulator can be reduced.

The ground conductors 4b ⁽¹⁷⁾ and 4c ⁽¹⁵⁾ , 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 housing 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.

(Each embodiment)
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. 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.

  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 has excellent temperature drift characteristics and reduced cost by reducing the thickness of a large-area ground conductor in a high-performance ridge-type optical modulator. It is useful as an optical modulator.

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 temperature drift 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. 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 drawing in BB 'of FIG. Sectional drawing in CC 'of FIG.

Explanation of symbols

1: z-cut LN substrate (LN substrate)
2, 14, 15: SiO2 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 (14), 4b (15), 4b (16), 4b (17), 4c, 4c (4), 4c (5), 4c (6), 4c (14), 4c (15): 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 portion for central conductor)
8b, 8c: Ridge portion (ridge portion for grounding conductor)
9a, 9b, 9c, 9d: concave portions 10a, 10b, 10c: outer peripheral portions 11a, 11b, 11c, 11d: void portions (portions where conductors are missing )
1 3a, 13d: embedded portion

Claims (6)

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 and wave electrode, and a pre-SL; and a formed ridge portion of a plurality of recesses formed by digging at least a portion of the substrate, the ridge center conductor ridge portion in which the center conductor is formed above A ground conductor ridge portion adjacent to the central conductor ridge portion through a common recess, and the ground conductor is formed on the upper side. The central conductor ridge portion is formed of the two optical waveguides. In the optical modulator in which one is formed,
The ground conductor is not formed in the recesses on both sides of the central conductor ridge,
The ground conductor in the concave portion on the side that is not the common concave portion of the ground conductor ridge portion has a missing portion formed in a part thereof, and other than the missing portion (hereinafter referred to as a connecting ground conductor). ) Is formed to be substantially the same as at least a part of the center conductor or the ground conductor,
The are two pieces of the traveling wave electrode is symmetric arrangement der and the plurality of recesses against the middle provided with the center line of the optical waveguide,
In the concave portion on the side that is not the common concave portion of the ridge portion for the ground conductor,
A plurality of the connection ground conductors are formed with a length of 5 μm to 500 μm in the optical waveguide direction in the optical waveguide,
A plurality of the missing portions are formed with a length of 30 μm to 3 mm in the optical waveguide direction in the optical waveguide, and each of the missing portions is longer than each of the connection ground conductors. optical modulator, wherein that you have been formed alternately in combination.   2. The optical modulator according to claim 1, wherein a ground conductor adjacent to the recess has substantially the same thickness as at least a part of the center conductor or the ground conductor. Except for the plurality of recesses, the thickness of at least a part of the ground conductor in a region intersecting the optical waveguide and separated from the optical waveguide by a predetermined distance is at least greater than the thickness of the ground conductor in a region other than the region. The optical modulator according to claim 1, wherein the optical modulator is partially thinned.   4. The optical modulator according to claim 1, wherein the optical modulator has a structure in which the concave portion is not provided below the ground conductor that does not include the optical waveguide below. 5.   The optical modulator according to any one of claims 1 to 4, wherein the substrate is made of lithium niobate.   The optical modulator according to any one of claims 1 to 4, wherein the substrate is made of a semiconductor.

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