Optical modulator

Abstract

To provide an optical modulator capable of improving the ratio of an electric field applied to an optical waveguide.SOLUTION: An optical modulator (100) includes an optical waveguide (2), a first electrode (31), two second electrodes (32), and a third electrode (4) that forms a potential difference with a group of the first electrode (31) and the second electrodes (32). A voltage having the same phase as that of the first electrode (31) is applied to each of the second electrodes (32). In a cross-sectional view perpendicular to an extending direction of the optical waveguide (2), the first electrode (31) is provided on one side in a thickness direction of the optical waveguide (2), and one second electrode (32) of the two second electrodes (32) is provided on one side in a width direction of the optical waveguide (2) with respect to the first electrode (31) at an interval from the first electrode (31), the other second electrode (32) is provided at an interval from the first electrode (31) on the other side in the width direction of the optical waveguide (2) with respect to the first electrode (31). The third electrode (4) is provided on the other side in the thickness direction of the optical waveguide (2).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical modulators. [Background technology]

[0002] The spread of mobile devices and cloud computing has led to a dramatic increase in internet traffic. This has led to an expansion in demand for optical communications. Optical communications require optical transceivers to convert optical signals into electrical signals and vice versa. An optical transceiver has an optical modulator as its main component. The optical modulator converts electrical signals into optical signals.

[0003] A conventional optical modulator is disclosed, for example, in Japanese Patent Laid-Open No. 2008-250081 (Patent Document 1). The optical modulator in Patent Document 1 includes a thin plate having an electro-optic effect, an optical waveguide formed in the thin plate, and a control electrode for controlling light passing through the optical waveguide. The control electrode includes a first electrode and a second electrode, which are arranged to sandwich the thin plate. The first electrode includes a coplanar electrode including at least a first signal electrode and a ground electrode. The second electrode includes at least a second signal electrode. Modulation signals having mutually inverted phases are input to the first signal electrode and the second signal electrode, which cooperate to apply an electric field to the optical waveguide. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-250081 Summary of the Invention [Problem to be solved by the invention]

[0005] In the optical modulator of Patent Document 1, it is undeniable that a portion of the electric field from the first signal electrode leaks to the left and right ground electrodes without passing through the optical waveguide. Furthermore, it is undeniable that a portion of the electric field from the second signal electrode leaks to the left and right ground electrodes without passing through the optical waveguide. For this reason, it is difficult to say that the proportion of the electric field applied to the optical waveguide is high.

[0006] An object of the present disclosure is to provide an optical modulator that can improve the ratio of the electric field applied to the optical waveguide. [Means for solving the problem]

[0007] The optical modulator according to the present disclosure includes an optical waveguide made of a material having an electro-optic effect and a control electrode for controlling light passing through the optical waveguide. The control electrode includes a first electrode, two second electrodes, and a third electrode that forms a potential difference with the group of the first and second electrodes. A voltage of the same phase as that of the first electrode is applied to each of the second electrodes. In a cross-sectional view perpendicular to the extension direction of the optical waveguide, the first electrode is provided on one side in the thickness direction of the optical waveguide. In this cross-sectional view, one of the two second electrodes is provided on one side in the width direction of the optical waveguide with a gap between it and the first electrode, and the other second electrode is provided on the other side in the width direction of the optical waveguide with a gap between it and the first electrode. In this cross-sectional view, the third electrode is provided on the other side in the thickness direction of the optical waveguide. [Effects of the Invention]

[0008] According to the optical modulator according to the present disclosure, the ratio of the electric field applied to the optical waveguide can be improved. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing a cross section of an optical modulator according to the first embodiment. [Figure 2] FIG. 2 is a schematic diagram showing a cross section of the optical modulator of the first modification. [Figure 3] FIG. 3 is a schematic diagram showing a cross section of the optical modulator of the first modification. [Figure 4] FIG. 4 is a schematic diagram showing a cross section of the optical modulator of the first modification. [Figure 5] FIG. 5 is a schematic diagram showing a cross section of the optical modulator according to the second embodiment. [Figure 6] FIG. 6 is a schematic diagram showing a cross section of an optical modulator according to the third embodiment. [Figure 7] FIG. 7 is a schematic diagram showing a cross section of an optical modulator according to the second modification. [Figure 8] FIG. 8 is a schematic diagram showing a cross section of an optical modulator according to the second modification. [Figure 9] FIG. 9 is a schematic diagram showing a cross section of an optical modulator according to the second modification. [Figure 10] FIG. 10 is a schematic diagram showing a cross section of an optical modulator according to the third modification. [Figure 11] FIG. 11 is a schematic diagram showing a cross section of an optical modulator according to the third modification. [Figure 12] FIG. 12 is a schematic diagram showing a cross section of an optical modulator according to the third modification. [Figure 13] FIG. 13 is a schematic diagram showing a cross section of an optical modulator according to the third modification. [Figure 14] FIG. 14 is a schematic diagram showing a cross section of an optical modulator according to the third modification. [Figure 15] FIG. 15 is a schematic diagram showing a cross section of the optical modulator according to the fourth embodiment. [Figure 16] FIG. 16 is a schematic diagram showing a cross section of the optical modulator according to the fifth embodiment. [Figure 17] FIG. 17 is a schematic diagram showing a cross section of the optical modulator according to the sixth embodiment. [Figure 18] FIG. 18 is a schematic diagram showing a cross section of the optical modulator according to the seventh embodiment. [Figure 19] FIG. 19 is a schematic diagram showing a plan view of the optical modulator according to the seventh embodiment. [Figure 20] FIG. 20 is a schematic diagram showing a cross section of the optical modulator according to the eighth embodiment. [Figure 21] FIG. 21 is a schematic diagram showing a plan view of the optical modulator according to the eighth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described. Note that in the following description, examples of embodiments of the present disclosure will be described, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and specific materials may be exemplified, but the present disclosure is not limited to these examples.

[0011] The optical modulator according to this embodiment includes an optical waveguide made of a material having an electro-optic effect and a control electrode for controlling light passing through the optical waveguide. The control electrode includes a first electrode, two second electrodes, and a third electrode that forms a potential difference with the group of the first and second electrodes. A voltage having the same phase as that of the first electrode is applied to each of the second electrodes. In a cross-sectional view perpendicular to the extension direction of the optical waveguide, the first electrode is provided on one side in the thickness direction of the optical waveguide. In this cross-sectional view, one of the two second electrodes is provided on one side of the first electrode in the width direction of the optical waveguide with a gap between it and the first electrode, and the other second electrode is provided on the other side of the width direction of the optical waveguide with a gap between it and the first electrode. In this cross-sectional view, the third electrode is provided on the other side of the thickness direction of the optical waveguide (first configuration).

[0012] In the optical modulator of the first configuration, the first electrode and the third electrode are arranged to sandwich the optical waveguide in the thickness direction. Furthermore, two second electrodes are arranged adjacent to the first electrode at a distance from the first electrode, sandwiching the first electrode in the width direction of the optical waveguide. When the optical modulator is in operation, voltages of the same phase are applied to the first electrode and the two second electrodes. As a result, electric fields act individually from the first electrode and the second electrode toward the third electrode, applying an electric field to the optical waveguide. In this case, compared to a case in which a single signal electrode is provided on one side of the optical waveguide in the thickness direction and only the electric field from this signal electrode is applied to the optical waveguide, as in Patent Document 1, not only the electric field from the first electrode but also the electric fields from the two second electrodes are applied to the optical waveguide. Therefore, the ratio of the electric field applied to the optical waveguide can be improved. In this specification, the ratio of the electric field applied to the optical waveguide is sometimes referred to as the electric field application efficiency of the optical waveguide.

[0013] The optical modulator of the first configuration preferably has the following configuration: In a cross-sectional view perpendicular to the extension direction of the optical waveguide, the center position in the width direction of the first electrode is located at the center part in the width direction of the optical waveguide, and the center position in the width direction of the third electrode is located at the center part in the width direction of the optical waveguide (second configuration). In this case, the electric field strength from the first electrode to the third electrode can be increased, and the electric field application efficiency for the optical waveguide can be improved.

[0014] The optical modulator preferably has the following configuration: In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the two second electrodes are arranged symmetrically with respect to the first electrode in the width direction of the optical waveguide (third configuration). In this case, it is possible to suppress bias in the effective refractive index and reduce optical loss.

[0015] The optical modulator preferably has the following configuration: In the width direction of the optical waveguide, one second electrode is arranged spaced apart from one end of the optical waveguide, and the other second electrode is arranged spaced apart from the other end of the optical waveguide. The optical modulator further includes a low-dielectric layer having a dielectric constant lower than that of the optical waveguide. The low-dielectric layer covers at least a portion of the surface of the second electrode so as to be interposed between the second electrode and the third electrode (fourth configuration).

[0016] The optical modulator of the fourth configuration preferably has the following configuration: The low dielectric layer covers at least a part of the surface of the first electrode so as to be interposed between the first electrode and the third electrode (fifth configuration).

[0017] In the optical modulator of the fourth configuration, the electric field from the second electrode to the optical waveguide passes through the low-dielectric layer. Furthermore, in the optical modulator of the fifth configuration, the electric field from the first electrode to the optical waveguide passes through the low-dielectric layer. This reduces the effective refractive index felt by the electric signal compared to when the low-dielectric layer is not provided. Typically, the effective refractive index felt by the electric signal is larger than the effective refractive index felt by the optical wave. Therefore, the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the optical wave becomes smaller. Therefore, the modulation frequency can be increased.

[0018] The optical modulator preferably includes an auxiliary low-dielectric layer having a dielectric constant lower than that of the optical waveguide, the auxiliary low-dielectric layer covering at least a part of the surface of the third electrode so as to be interposed between the second electrode and the third electrode (sixth configuration).

[0019] In the optical modulator of the sixth configuration, the electric field from the second electrode toward the optical waveguide passes through the auxiliary low-dielectric layer. This reduces the effective refractive index felt by the electrical signal compared to when the auxiliary low-dielectric layer is not provided. This reduces the difference between the effective refractive index felt by the electrical signal and the effective refractive index felt by the optical wave. This allows for an increase in the modulation frequency.

[0020] The optical modulator described above preferably has the following configuration. The material of the optical waveguide is LiNbO3 (seventh configuration). LiNbO3 (lithium niobate) has a particularly high electro-optic effect. In this specification, LiNbO3 may be referred to as LN. The material of the optical waveguide is not particularly limited as long as it has an electro-optic effect. For example, the material of the optical waveguide may be LiTaO3 (lithium tantalate), PLZT (lead lanthanum zirconate titanate), KTN (potassium tantalate niobate), BaTiO3 (barium titanate), or the like.

[0021] The optical modulator described above may further include a substrate on which an optical waveguide is provided (eighth configuration).

[0022] The optical modulator of the eighth configuration may have the following configuration. The substrate is made of the same material as the optical waveguide, and the optical waveguide is ridge-shaped (ninth configuration). In this case, light can be further confined within the optical waveguide. Furthermore, it becomes possible to cover the periphery of the optical waveguide, excluding the boundary with the substrate, with a low-dielectric layer. This makes it easy to adjust the effective refractive index.

[0023] However, optical waveguides can also be formed by diffusing titanium (Ti) into the substrate. Optical waveguides can also be formed by proton exchange.

[0024] The optical modulator of any one of the first to seventh configurations may include two optical modulator units arranged in parallel, each including an optical waveguide and a control electrode (tenth configuration).

[0025] The optical modulator of the tenth configuration is a Mach-Zehnder type optical modulator. In this case, intensity modulation is possible in addition to phase modulation. This allows multi-level modulation and increases the transmission capacity. Moreover, the optical modulator of the tenth configuration has the same effects as the first to seventh configurations.

[0026] An optical modulator of a tenth configuration may have the following configuration: Each of the optical modulator units further includes a substrate on which an optical waveguide is provided, and the substrate of one of the two optical modulator units is arranged in parallel with the substrate of the other optical modulator unit (eleventh configuration).

[0027] The optical modulator of the eleventh configuration may have the following configuration. In each of the optical modulator units, the substrate is made of the same material as the optical waveguide, and the optical waveguide is of a ridge type (twelfth configuration). The optical modulator of the twelfth configuration corresponds to the ninth configuration. Therefore, as with the ninth configuration, it is possible to better confine light within the optical waveguide, and further, it is easy to adjust the effective refractive index.

[0028] The optical modulator of the eleventh or twelfth configuration may have the following configuration: Of the two optical modulator units, the substrate of one optical modulator unit is integrated with the substrate of the other optical modulator unit. A voltage of opposite phase to that applied to the first electrode and second electrode of one optical modulator unit is applied to the first electrode and second electrode of the other optical modulator unit (thirteenth configuration).

[0029] In the optical modulator of the thirteenth configuration, the substrate of one optical modulator unit can be shared with the substrate of the other optical modulator unit. This allows the distance between the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit to be reduced. In this case, the overall width of the optical modulator can be narrowed.

[0030] The optical modulator of the eleventh or twelfth configuration may have the following configuration: Of the two optical modulator units, the substrate of one optical modulator unit is integrated with the substrate of the other optical modulator unit, and the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit have mutually opposite spontaneous polarization directions. One of the two second electrodes of one optical modulator unit is formed integrally with one of the two second electrodes of the other optical modulator unit. A voltage having the same phase as that of the first electrode and second electrode of the other optical modulator unit is applied to the first electrode and second electrode of one optical modulator unit (fourteenth configuration).

[0031] In the optical modulator of the fourteenth configuration, the second electrodes located close to each other are integrally formed and shared. This allows the distance between the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit to be made smaller. In this case, the overall width of the optical modulator can be made narrower.

[0032] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0033] First Embodiment [Configuration of optical modulator 100] FIG. 1 is a schematic diagram showing a cross section of an optical modulator 100 according to the first embodiment. FIG. 1 shows a cross section perpendicular to the extension direction of an optical waveguide 2. The extension direction of the optical waveguide 2 can also be referred to as the direction along the optical waveguide 2. In this specification, unless otherwise specified, a cross section refers to a cross section perpendicular to the extension direction of the optical waveguide 2 or optical waveguides 2A and 2B described below. In the cross section of the optical modulator 100, a support plate 7 that supports the entire device is at the bottom, the thickness direction of the optical modulator 100 corresponds to the up-down direction, and the width direction of the optical modulator 100 corresponds to the left-right direction. However, in this specification, the terms up, down, left, and right are defined for the convenience of explanation and do not limit the actual position of the optical modulator 100.

[0034] 1, the optical modulator 100 includes a substrate 1, an optical waveguide 2, a first electrode 31, two second electrodes 32, and a third electrode 4. The first electrode 31, the two second electrodes 32, and the third electrode 4 are included in control electrodes for controlling light passing through the optical waveguide 2.

[0035] The first electrode 31 and the two second electrodes 32 are each disposed on the substrate 1. A voltage of the same phase as that of the first electrode 31 is applied to each of the second electrodes 32. The third electrode 4 forms a potential difference with the group of the first electrodes 31 and the second electrodes 32. The first electrode 31 and the second electrodes 32 are, for example, signal electrodes. The third electrode 4 is, for example, a ground electrode. The third electrode 4 may be an inverse signal electrode that applies a voltage of the opposite phase to that of the voltages of the first electrodes 31 and the second electrodes 32.

[0036] The third electrode 4 is disposed below the substrate 1. The optical modulator 100 of this embodiment further includes an auxiliary low dielectric layer 6. The substrate 1, the optical waveguide 2, the first electrode 31, the second electrode 32, the third electrode 4, and the auxiliary low dielectric layer 6 are supported by a support plate 7. The support plate 7 is disposed at the bottom.

[0037] The optical waveguide 2 is made of a material that has an electro-optic effect. The material of the optical waveguide 2 is, for example, LN. The optical waveguide 2 is formed on the substrate 1. Specifically, the optical waveguide 2 is formed on the top of the substrate 1. This optical waveguide 2 is formed by diffusing Ti into the substrate 1. The portion of the substrate 1 where Ti is diffused has a high refractive index and can confine light, so it can be used as the optical waveguide 2.

[0038] The optical waveguide 2 may have a cross-sectional shape in which the width (horizontal dimension) is greater than the thickness (vertical dimension). In FIG. 1, the cross-sectional shape of the optical waveguide 2 is substantially a wide, roughly rectangular shape. In this case, the cross-sectional shape of the optical waveguide 2 includes a first side extending in the width direction and a second side arranged parallel to the first side and extending in the width direction. The cross-sectional shape of the optical waveguide 2 further includes a third side and a fourth side each extending in the thickness direction. In the example shown in FIG. 1, the first side and the second side are a pair of long sides, and the third side and the fourth side are a pair of short sides. When the cross-sectional shape of the optical waveguide 2 is a wide rectangle, one of the pair of long sides (the upper first side) is on the surface of the substrate 1, and the other long side (the lower second side) is inside the substrate 1.

[0039] In the cross section of the optical waveguide 2, the first and second long sides are connected by the third and fourth short sides. In the example shown in FIG. 1 , the third and fourth sides of the optical waveguide 2 are linear in the cross section of the optical modulator 100 and are parallel to the thickness direction of the optical waveguide 2. However, the third and fourth sides may be inclined with respect to the thickness direction of the optical waveguide 2 and are not necessarily linear. In the cross section of the optical modulator 100, the third and fourth sides of the optical waveguide 2 may be curved or may have a shape that combines straight and curved lines. Furthermore, the length of the third side may be the same as or different from the length of the fourth side. Similarly, the length of the first side may be the same as or different from the length of the second side.

[0040] The cross-sectional shape of the optical waveguide 2 may be a wide semi-ellipse. In this case, the cross-sectional shape of the optical waveguide 2 includes a base as a major axis extending in the width direction and elliptical arc-shaped sides extending in the width direction. When the cross-sectional shape of the optical waveguide 2 is a wide semi-ellipse, the base is on the surface of the substrate 1 and the elliptical arc-shaped sides are inside the substrate 1.

[0041] The first electrode 31, the second electrode 32, and the third electrode 4 are made of a metal material and each have a rectangular cross section. The first electrode 31 is provided on one side in the thickness direction of the optical waveguide 2. The third electrode 4 is provided on the other side in the thickness direction of the optical waveguide 2. For example, the first electrode 31 is stacked on top of the optical waveguide 2. In this case, the first electrode 31 is disposed approximately directly above the optical waveguide 2. The third electrode 4 is stacked below the optical waveguide 2. In this case, the third electrode 4 is disposed approximately directly below the optical waveguide 2. Of the two second electrodes 32, one second electrode 32 is disposed on one side of the first electrode 31 in the width direction of the optical waveguide 2 with a gap therebetween, and the other second electrode 32 is disposed on the other side of the first electrode 31 in the width direction of the optical waveguide 2 with a gap therebetween. Therefore, the first electrode 31 is disposed between the second electrodes 32. In addition, in the width direction of the optical waveguide 2, one of the second electrodes 32 is disposed spaced apart from one end of the optical waveguide 2, and the other second electrode 32 is disposed spaced apart from the other end of the optical waveguide 2.

[0042] In other words, the first electrode 31 and the third electrode 4 are arranged to sandwich the optical waveguide 2 in the vertical direction (thickness direction). Furthermore, two second electrodes 32 are arranged with respect to the optical waveguide 2 beside the first electrode 31 at a distance from the first electrode 31 so as to sandwich the first electrode 31 in the horizontal direction (width direction).

[0043] In this embodiment, the position of the center 31c in the width direction (left-right direction) of the first electrode 31 is located at the center of the width direction of the optical waveguide 2. The center 31c of the first electrode 31 is located, for example, within a range of an area that is located at the center when the optical waveguide 2 is divided into thirds in the width direction of the optical waveguide 2. For example, the position of the center 31c in the width direction of the first electrode 31 may coincide with the position of the center 2c in the width direction of the optical waveguide 2. The position of the center 4c in the width direction of the third electrode 4 is located at the center of the width direction of the optical waveguide 2. The center 4c of the third electrode 4 is located, for example, within a range of an area that is located at the center when the optical waveguide 2 is divided into thirds in the width direction of the optical waveguide 2. For example, the position of the center 4c in the width direction of the third electrode 4 may coincide with the position of the center 2c in the width direction of the optical waveguide 2. In this case, the positions of the center 31c of the first electrode 31, the center 4c of the third electrode 4, and the center 2c of the optical waveguide 2 are aligned in the width direction and are not shifted from one another. From another perspective, the first electrode 31 is disposed substantially or approximately directly above the third electrode 4, and the optical waveguide 2 is disposed between the first electrode 31 and the third electrode 4. Furthermore, the two second electrodes 32 are disposed symmetrically with respect to the first electrode 31 in the width direction of the optical waveguide 2.

[0044] In this embodiment, an auxiliary low dielectric constant layer 6 is laminated below the substrate 1. A support plate 7 is laminated below the auxiliary low dielectric constant layer 6. The third electrode 4 is disposed inside the auxiliary low dielectric constant layer 6 and is laminated below the substrate 1. The auxiliary low dielectric constant layer 6 covers at least a portion of the surface of the third electrode 4 so as to be interposed between the second electrode 32 and the third electrode 4. In this example of the present embodiment, the auxiliary low dielectric constant layer 6 directly covers the side and bottom surfaces of the third electrode 4. In this case, the portions of the auxiliary low dielectric constant layer 6 that cover the side surfaces of the third electrode 4 are interposed between the second electrode 32 and the third electrode 4.

[0045] The dielectric constant of the auxiliary low dielectric constant layer 6 is lower than that of the optical waveguide 2. The material of the auxiliary low dielectric constant layer 6 is not particularly limited as long as the dielectric constant is lower than that of the optical waveguide 2. The material of the auxiliary low dielectric constant layer 6 is, for example, SiO2. The auxiliary low dielectric constant layer 6 may be made of an oxide (e.g., Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, or Y2O3). The auxiliary low dielectric constant layer 6 may also be made of a polymer (e.g., BCB (benzocyclobutene), PI (polyimide)).

[0046] However, the auxiliary low dielectric constant layer 6 does not have to be provided. When the auxiliary low dielectric constant layer 6 is not provided, the third electrode 4 only needs to be disposed below the substrate 1. From another perspective, the third electrode 4 only needs to be buried below the substrate 1.

[0047] [effect] In this embodiment, the first electrode 31 and the third electrode 4 are arranged to sandwich the optical waveguide 2 in the thickness direction. Furthermore, two second electrodes 32 are arranged on the same side of the optical waveguide 2 in the thickness direction, beside the first electrode 31 and spaced apart from it, to sandwich the first electrode 31 in the width direction. In other words, the first electrode 31 and two second electrodes 32 that form a potential difference with the third electrode 4 are separately present. Furthermore, a space exists between the first electrode 31 and the second electrode 32.

[0048] According to the optical modulator 100 of this embodiment, when the optical modulator 100 is in operation, voltages of the same phase are applied to the first electrode 31 and the two second electrodes 32 arranged on the same side in the thickness direction of the optical waveguide 2. The third electrode 4 generates a potential difference with the group of the first electrode 31 and the second electrode 32. As a result, electric fields act from the first electrode 31 and the second electrode 32 individually toward the third electrode 4, and an electric field is applied to the optical waveguide 2. As a result, all of the electric field from the first electrode 31 passes through the optical waveguide 2. Most of the electric field from the second electrode 32 passes through the optical waveguide 2. In other words, since not only the electric field from the first electrode 31 but also the electric fields from the two second electrodes 32 are applied to the optical waveguide 2, the electric field application efficiency for the optical waveguide 2 can be improved compared to a case in which a single signal electrode and a ground electrode are arranged on the same side in the thickness direction of the optical waveguide 2.

[0049] Furthermore, in this embodiment, since the auxiliary low-dielectric-constant layer 6 is interposed between the second electrode 32 and the third electrode 4, the electric field from the second electrode 32 passes through the auxiliary low-dielectric-constant layer 6. This reduces the effective refractive index felt by the electric signal. This reduces the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the light wave. This allows the modulation frequency to be increased.

[0050] If the first electrode 31 and the two second electrodes 32 are arranged on the same side of the optical waveguide 2 in the thickness direction, and the first electrode 31 and the third electrode 4 are arranged to sandwich the optical waveguide 2 in the thickness direction, an electric field can be applied to the optical waveguide 2 from each of the first electrode 31 and the two second electrodes 32. Therefore, compared to a case where a single signal electrode and a ground electrode are arranged to sandwich the optical waveguide in the thickness direction, it is easier to apply an electric field to the optical waveguide 2 and adjust the effective refractive index, and as a result, the degree of freedom in the structural design of the optical modulator can be increased.

[0051] The impedance of the signal electrode that forms a potential difference with the third electrode 4 is ideally 50 Ω. In this embodiment, the first electrode 31 and two second electrodes 32 are separate signal electrodes that form a potential difference with the third electrode 4. From another perspective, the signal electrode that forms a potential difference with the third electrode 4 is divided into three. By increasing the number of signal electrodes from one to three, it is possible to apply an electric field to a wide range of the auxiliary low-dielectric-constant layer 6 or the low-dielectric-constant layer 5 described later, while bringing the impedance of the first electrode 31 and the second electrode 32 closer to the ideal 50 Ω. By increasing the number of signal electrodes to three, it is possible to set the applied voltage individually. This allows adjustment to achieve a suitable electric field intensity distribution according to the cross-sectional area of ​​the auxiliary low-dielectric-constant layer 6 or the low-dielectric-constant layer 5 described later and the shape of the optical waveguide.

[0052] In this embodiment, three signal electrodes, a first electrode 31 and two second electrodes 32, are provided as signal electrodes that form a potential difference with the third electrode 4. The first electrode 31 serves to apply an electric field to the optical waveguide 2, and the two second electrodes 32 serve to adjust the effective refractive index. The first electrode 31 is preferably placed near directly above the optical waveguide 2 in order to apply an electric field to the optical waveguide 2 efficiently and uniformly.

[0053] By arranging the second electrodes 32 beside the first electrodes 31, an electric field having a horizontal component can be applied from the second electrodes 32 to the third electrode 4. This increases the electric field component passing through the auxiliary low dielectric constant layer 6. If there were only one second electrode 32, the balance of the refractive index felt by the optical waveguide 2 in the horizontal direction would be poor. Therefore, to adjust the balance of the effective refractive index in the horizontal direction, two second electrodes 32 are arranged on the left and right of the first electrode 31 and work together.

[0054] In this way, it is preferable to provide two second electrodes 32 for one first electrode 31. This is because providing three or more second electrodes 32 does not provide any greater effect than providing two second electrodes 32 and leads to an increase in the size of the device (optical modulator 100). Therefore, a configuration with a first electrode 31 and two second electrodes 32 is most preferable.

[0055] Furthermore, in this embodiment, the position of the center 31c in the width direction of the first electrode 31 is located at the center in the width direction of the optical waveguide 2, and the position of the center 4c in the width direction of the third electrode 4 is located at the center in the width direction of the optical waveguide 2. In this case, the first electrode 31 is disposed substantially or approximately directly above the third electrode 4, and the optical waveguide 2 is disposed between the first electrode 31 and the third electrode 4. Therefore, the electric field strength from the first electrode 31 to the third electrode 4 can be increased, and the electric field application efficiency for the optical waveguide 2 can be improved.

[0056] Furthermore, in this embodiment, the two second electrodes 32 are disposed symmetrically with respect to the first electrode 31 in the width direction of the optical waveguide 2. Since the second electrodes 32 are disposed at symmetrical positions, a balanced electric field can be applied from the second electrodes 32. This makes it possible to suppress bias in the effective refractive index and reduce optical loss.

[0057] Furthermore, in this embodiment, a third electrode is not provided on the side of the optical waveguide 2 where the first electrode 31 and the second electrode 32 are provided. The third electrode is an electrode that forms a potential difference with the group of the first electrode 31 and the second electrode 32. If a third electrode were provided on the side of the optical waveguide 2 where the first electrode 31 and the second electrode 32 are provided, it would be inevitable that part of the electric field from the first electrode 31 or the second electrode 32 would leak to the third electrode. For this reason, it is difficult to say that the electric field application efficiency for the optical waveguide 2 is high. In this regard, in this embodiment, a third electrode is not provided on the side of the optical waveguide 2 where the first electrode 31 and the second electrode 32 are provided. Therefore, as described above, the electric field application efficiency for the optical waveguide 2 can be improved.

[0058] [Dimensions of the first electrode 31, the second electrode 32, and the third electrode 4] As the widthwise distance between the first electrode 31 and the second electrode 32 increases, the effective refractive index and impedance increase. As the thickness (film thickness) of the first electrode 31 and the second electrode 32 increases, the effective refractive index and impedance decrease. As the width of the first electrode 31 and the second electrode 32 increases, the effective refractive index increases and the impedance decreases. For example, assuming an ideal impedance of 50 Ω, the modulation speed of the optical signal can be improved by bringing the effective refractive index felt by the electrical signal and the effective refractive index felt by the optical wave closer together.

[0059] The width dimension of the third electrode 4 is preferably equal to or smaller than the width of the optical waveguide 2. This is because the efficiency of applying an electric field is improved. The width dimension of the first electrode 31 is preferably equal to or smaller than the width of the optical waveguide 2. This is because the efficiency of applying an electric field is improved. The width dimension of each second electrode 32 is preferably equal to or larger than the width dimension of the first electrode 31. However, the width dimension of each second electrode 32 may be smaller than the width dimension of the first electrode 31. The widthwise spacing (gap) between the first electrode 31 and the second electrode 32 is preferably equal to or larger than the widthwise dimension of the first electrode 31. The widthwise dimensions of the first electrode 31 and the widthwise dimensions of each second electrode 32 may be designed so that the impedances of the first electrode 31 and the second electrode 32 are approximately the same. From the viewpoint of suppressing a decrease in modulation speed, the width dimension may be set to be within a range of 50Ω±10Ω, for example, with a target of 50Ω. The distance between the first electrode 31 and the second electrode 32 may be set so that the effective refractive index felt by the electric signal does not fall below the refractive index of the optical waveguide 2 .

[0060] The width of the first electrode 31 may be larger than the width of the optical waveguide 2. In this case, the width of the third electrode 4 may be larger than the width of the optical waveguide 2, but is preferably smaller than the width of the first electrode 31. If the width of the first electrode 31 is larger than the width of the optical waveguide 2, part of the electric field from the first electrode 31 passes through the auxiliary low dielectric layer 6 on the way to the third electrode 4. This can further reduce the effective refractive index felt by the electric signal. Therefore, the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the light wave becomes smaller, and the modulation frequency can be increased.

[0061] [Method for manufacturing the optical modulator 100 of the first embodiment] An example of a manufacturing method for the optical modulator 100 of the first embodiment will be described below. A substrate 1 made of a material having an electro-optic effect is prepared. A third electrode 4 is formed on the substrate 1. For example, the third electrode 4 can be formed by patterning using photolithography, vapor deposition, lift-off, or the like. The third electrode 4 may also be formed by photolithography or plating. Alternatively, the third electrode 4 may be formed by depositing a film using vapor deposition, sputtering, CVD, or the like, patterning it using photolithography, and then etching it.

[0062] An auxiliary low dielectric constant layer 6 is formed on the surface of the substrate 1 on which the third electrode 4 is formed. The auxiliary low dielectric constant layer 6 has a lower dielectric constant than the substrate 1. The thickness of the auxiliary low dielectric constant layer 6 is greater than the thickness of the third electrode 4.

[0063] The substrate 1 is bonded to a support plate 7. The bonding surface of the substrate 1 is the surface on which the third electrode 4 and auxiliary low dielectric layer 6 are formed. The bonding method is, for example, surface activated bonding or atomic diffusion bonding.

[0064] The surface of the substrate 1 opposite to the bonding surface is processed to thin the substrate 1 to a desired thickness. Methods for thinning the substrate 1 include polishing by grinding or CMP. In addition to this method, the substrate 1 may be thinned by a method in which a release layer of a desired thickness is formed by implanting ions into the substrate 1 in advance, and then the substrate is peeled off after bonding and finished by grinding or CMP. The thickness of the thinned substrate 1 is 10 μm or less.

[0065] An optical waveguide 2 is formed on a substrate 1 by Ti diffusion, proton exchange, or the like.

[0066] A first electrode 31 and two second electrodes 32 are formed on the surface of the substrate 1 on which the optical waveguide 2 is formed. The thickness of each of the electrodes 31 and 32 is preferably thicker because the thicker the electrodes, the more reduced signal loss they are. The width and thickness of the second electrodes 32 arranged on the left and right sides should be equal to or greater than that of the first electrode 31 arranged in the center. For example, the electrodes 31 and 32 can be formed by patterning using photolithography, vapor deposition, lift-off, or the like. The electrodes 31 and 32 may also be formed by photolithography or plating. The electrodes 31 and 32 may also be formed by depositing a film using vapor deposition, sputtering, CVD, or the like, patterning it using photolithography, and then etching it.

[0067] [Modification 1 of the First Embodiment] 2 to 4 are schematic diagrams showing a first modification of the optical modulator 100 according to the first embodiment. 2 to 4 show a cross section of the optical modulator 100. In the first modification, the shapes of the first electrode 31, the second electrode 32, and the third electrode 4 relative to the optical waveguide 2 are changed from those of the optical modulator 100 shown in FIG.

[0068] In the example shown in FIG. 2, the first electrode 31 is shifted to one of the two sides in the width direction of the optical waveguide 2 (to the right in FIG. 2), and the third electrode 4 is shifted to the other side of the two sides in the width direction of the optical waveguide 2 (to the left in FIG. 2). In this case, the position of the center 31c in the width direction of the first electrode 31 does not coincide with the position of the center 2c in the width direction of the optical waveguide 2. The position of the center 4c in the width direction of the third electrode 4 does not coincide with the position of the center 2c in the width direction of the optical waveguide 2. In other words, the positions of the center 31c of the first electrode 31, the center 4c of the third electrode 4, and the center 2c of the optical waveguide 2 are not aligned in the width direction but are shifted from one another.

[0069] 2, the distance between the second electrode 32 on the right side and the first electrode 31 is smaller than the distance between the second electrode 32 on the left side and the first electrode 31. In other words, the two second electrodes 32 are arranged asymmetrically with respect to the first electrode 31 in the width direction of the optical waveguide 2.

[0070] 3, the position of the center 31c in the width direction of the first electrode 31 coincides with the position of the center 2c in the width direction of the optical waveguide 2. The position of the center 4c in the width direction of the third electrode 4 coincides with the position of the center 2c in the width direction of the optical waveguide 2. In other words, similar to the optical modulator 100 shown in FIG. 1, the position of the center 31c of the first electrode 31, the position of the center 4c of the third electrode 4, and the position of the center 2c of the optical waveguide 2 are aligned in the width direction and are not shifted from one another.

[0071] 3, the distance between the second electrode 32 on the right side and the first electrode 31 is smaller than the distance between the second electrode 32 on the left side and the first electrode 31. In other words, similar to the optical modulator 100 of the first modification shown in FIG. 2, the two second electrodes 32 are arranged asymmetrically with respect to the first electrode 31 in the width direction of the optical waveguide 2.

[0072] 4, the first electrode 31 is shifted to one of the two sides in the width direction of the optical waveguide 2 (to the right in FIG. 4) with respect to the optical waveguide 2, and the third electrode 4 is shifted to the other side of the two sides in the width direction of the optical waveguide 2 (to the left in FIG. 4). In this case, similar to the optical modulator 100 of the first modification shown in FIG. 2, the position of the center 31c in the width direction of the first electrode 31 does not coincide with the position of the center 2c in the width direction of the optical waveguide 2. The position of the center 4c in the width direction of the third electrode 4 does not coincide with the position of the center 2c in the width direction of the optical waveguide 2.

[0073] 4, the distance between the second electrode 32 on the right side and the first electrode 31 is the same as the distance between the second electrode 32 on the left side and the first electrode 31. In other words, similar to the optical modulator 100 shown in FIG. 1, the two second electrodes 32 are disposed symmetrically with respect to the first electrode 31 in the width direction of the optical waveguide 2.

[0074] However, in Modification 1, only one of the first electrode 31 and the third electrode 4 may be shifted in the width direction with respect to the optical waveguide 2. That is, the position of the center 31c in the width direction of the first electrode 31 may not coincide with the position of the center 2c in the width direction of the optical waveguide 2, and the position of the center 4c in the width direction of the third electrode 4 may coincide with the position of the center 2c in the width direction of the optical waveguide 2. The position of the center 4c in the width direction of the third electrode 4 may not coincide with the position of the center 2c in the width direction of the optical waveguide 2, and the position of the center 31c in the width direction of the first electrode 31 may coincide with the position of the center 2c in the width direction of the optical waveguide 2.

[0075] Second Embodiment 5 is a schematic diagram showing a cross section of an optical modulator 100 according to the second embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the first embodiment.

[0076] Referring to FIG. 5 , the optical modulator 100 further includes a low-dielectric layer 5. Specifically, the low-dielectric layer 5 is stacked on the substrate 1. The low-dielectric layer 5 covers at least a portion of the surface of each second electrode 32 so as to be interposed between the second electrode 32 and the third electrode 4. More specifically, the low-dielectric layer 5 is provided between the first electrode 31 and the second electrode 32. That is, the low-dielectric layer 5 directly covers each side surface of the first electrode 31 and the second electrode 32. As a result, a portion of the low-dielectric layer 5 is interposed between the second electrode 32 and the third electrode 4.

[0077] The dielectric constant of the low dielectric layer 5 is lower than that of the optical waveguide 2, similar to that of the auxiliary low dielectric layer 6. The material of the low dielectric layer 5 is not particularly limited as long as the dielectric constant is lower than that of the optical waveguide 2. The material of the low dielectric layer 5 may be the same as or different from that of the auxiliary low dielectric layer 6.

[0078] In the optical modulator 100 of this embodiment, the low-dielectric layer 5 is interposed between the second electrode 32 and the third electrode 4, so the electric field from the second electrode 32 toward the optical waveguide 2 passes through the low-dielectric layer 5. This reduces the effective refractive index felt by the electric signal compared to when the low-dielectric layer is not provided. This reduces the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the light wave. This allows the modulation frequency to be increased.

[0079] In this embodiment, a low-dielectric layer 5 is provided between the first electrode 31 and the second electrode 32. If the second electrode 32 is assumed to be a ground electrode, a large potential difference will occur between the first electrode 31 and the ground electrode. In this case, if any component is provided between the first electrode 31 and the ground electrode, there is a high risk of a short circuit occurring between the first electrode 31 and the ground electrode. If a short circuit occurs between the first electrode 31 and the ground electrode, the electric field from the first electrode 31 toward the optical waveguide 2 will be weakened. Therefore, if the second electrode 32 is a ground electrode, it is unlikely that any component would be provided between the first electrode 31 and the ground electrode.

[0080] In this regard, in the present embodiment, voltages of the same phase are applied to the first electrode 31 and the second electrode 32, so that no potential difference occurs between the first electrode 31 and the second electrode 32. Therefore, even if the distance between the first electrode 31 and the second electrode 32 is small and the second electrode 32 is close to the first electrode 31, it is possible to provide the low dielectric layer 5 between the first electrode 31 and the second electrode 32. The distance between the first electrode 31 and the second electrode 32 may be set so that the effective refractive index experienced by the electrical signal is close to the effective refractive index experienced by the light wave passing through the optical waveguide 2.

[0081] <Third embodiment> 6 is a schematic diagram showing a cross section of an optical modulator 100 according to a third embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the second embodiment.

[0082] Referring to FIG. 6 , the first electrode 31 and the second electrode 32 are disposed above the substrate 1. The first electrode 31 and the second electrode 32 are disposed inside a low-dielectric layer 5 laminated on the substrate 1. In the example shown in FIG. 6 , the low-dielectric layer 5 directly covers the lower surface, side surfaces, and upper surface of the first electrode 31. Furthermore, the low-dielectric layer 5 directly covers the lower surface, side surfaces on the first electrode 31 side, and upper surface of each second electrode 32. That is, the low-dielectric layer 5 covers at least a portion of the surface of the second electrode 32 so as to be interposed between the second electrode 32 and the third electrode 4. Furthermore, the low-dielectric layer 5 covers at least a portion of the surface of the first electrode 31 so as to be interposed between the first electrode 31 and the third electrode 4. From another perspective, the low-dielectric layer 5 covers the entire upper surface of the optical waveguide 2 and the upper surface of the substrate 1 in the vicinity thereof.

[0083] In the optical modulator 100 of this embodiment, the low-dielectric layer 5 covers at least a portion of the surface of the second electrode 32 so as to be interposed between the second electrode 32 and the third electrode 4. Therefore, the same effect as in the second embodiment can be obtained. Furthermore, the low-dielectric layer 5 covers at least a portion of the surface of the first electrode 31 so as to be interposed between the first electrode 31 and the third electrode 4. In particular, in the example shown in FIG. 6 , the low-dielectric layer 5 covers the entire upper surface of the optical waveguide 2 and the surrounding upper surface of the substrate 1, and the low-dielectric layer 5 is interposed between the first electrode 31 and the optical waveguide 2. Furthermore, the low-dielectric layer 5 is interposed between the second electrode 32 and the optical waveguide 2. In this case, the electric field from the second electrode 32 toward the optical waveguide 2 passes through the low-dielectric layer 5, and the electric field from the first electrode 31 toward the optical waveguide 2 passes through the low-dielectric layer 5. This allows the effective refractive index perceived by an electrical signal to be further reduced. This reduces the difference between the effective refractive index felt by the electrical signal and the effective refractive index felt by the optical wave, thereby increasing the modulation frequency.

[0084] [Modification 2 of the Second and Third Embodiments] 7 to 9 are schematic diagrams showing a second modification of the optical modulator 100 according to the second and third embodiments. 7 to 9 show cross sections of the optical modulator 100. In the second modification, the form of the low dielectric layer 5 is changed from that of the optical modulator 100 shown in FIGS. 5 and 6.

[0085] 7, the low dielectric constant layer 5 directly covers the lower surface of the first electrode 31. The low dielectric constant layer 5 directly covers the lower surface of each second electrode 32.

[0086] 8, the low dielectric constant layer 5 directly covers the lower surface and side surfaces of the first electrode 31. The low dielectric constant layer 5 directly covers the lower surface and side surfaces of each second electrode 32.

[0087] 9, the low-dielectric layer 5 directly covers the side and top surfaces of the first electrode 31. The low-dielectric layer 5 directly covers the side and top surfaces of each second electrode 32.

[0088] [Modification 3 of the second and third embodiments] 10 to 14 are schematic diagrams showing a third modification of the optical modulator 100 according to the second and third embodiments. FIGS. 10 to 14 show a cross section of the optical modulator 100. In the third modification, the form of the auxiliary low-dielectric layer 6 is changed from that of the optical modulator 100 shown in FIGS. 5 to 9. The optical modulator 100 shown in FIG. 10 corresponds to the optical modulator 100 shown in FIG. 5. The optical modulator 100 shown in FIG. 11 corresponds to the optical modulator 100 shown in FIG. 6. The optical modulator 100 shown in FIG. 12 corresponds to the optical modulator 100 shown in FIG. 7. The optical modulator 100 shown in FIG. 13 corresponds to the optical modulator 100 shown in FIG. 8. The optical modulator 100 shown in FIG. 14 corresponds to the optical modulator 100 shown in FIG. 9.

[0089] 10 to 14, the third electrode 4 is disposed below the substrate 1. The third electrode 4 is disposed inside the auxiliary low dielectric layer 6 laminated below the substrate 1. Therefore, the auxiliary low dielectric layer 6 directly covers the lower surface, side surfaces, and upper surface of the third electrode 4. In other words, the auxiliary low dielectric layer 6 covers at least a part of the surface of the third electrode 4 so as to be interposed between the second electrode 32 and the third electrode 4, and also covers at least a part of the surface of the third electrode 4 so as to be interposed between the first electrode 31 and the third electrode 4.

[0090] In the optical modulator 100 of the third modification, the electric field from the second electrode 32 toward the optical waveguide 2 passes through the auxiliary low-dielectric layer 6, and further, the electric field from the first electrode 31 toward the optical waveguide 2 passes through the auxiliary low-dielectric layer 6. This allows the effective refractive index felt by the electric signal to be further reduced. As a result, the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the light wave becomes smaller. This effectively increases the modulation frequency.

[0091] <Fourth embodiment> 15 is a schematic diagram showing a cross section of an optical modulator 100 according to a fourth embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the first embodiment.

[0092] Referring to FIG. 15, the substrate 1 has a ridge-type optical waveguide 2. That is, the substrate 1 has a convex rib on the top, which functions as the optical waveguide 2. The convex rib is formed on the substrate 1 by processing a wafer, which is the material. The convex rib can confine light in its thickness direction and width direction. The cross-sectional shape of the ridge-type optical waveguide 2 is generally rectangular. Strictly speaking, the cross-sectional shape of the ridge-type optical waveguide 2 is often trapezoidal.

[0093] The substrate 1 is made of the same material as the optical waveguide 2. However, the material of the substrate 1 may be different from the material of the optical waveguide 2. In this case, the material of the substrate 1 is, for example, Si.

[0094] The optical modulator 100 of this embodiment has the same effects as the first embodiment. However, in this embodiment, since the optical waveguide 2 is ridge-shaped, light can be further confined within the optical waveguide 2. Furthermore, it becomes possible to cover the periphery of the optical waveguide 2 except for the boundary with the substrate 1 with the low-dielectric layer 5. In other words, the periphery of the optical waveguide 2 is widely covered with the low-dielectric layer 5. This makes it easy to adjust the effective refractive index.

[0095] The configuration of this embodiment may be applied to the optical modulator 100 of the second and third embodiments.

[0096] [Method for manufacturing the optical modulator 100 according to the fourth embodiment] An example of a method for manufacturing the optical modulator 100 of the fourth embodiment will be described below. In the optical modulator 100 of the fourth embodiment, the substrate 1 has a ridge-type optical waveguide 2. Therefore, the method for manufacturing the optical modulator 100 of the fourth embodiment differs from the method for manufacturing the optical modulator 100 of the first embodiment in terms of the method for forming the optical waveguide 2, but is common to the method for manufacturing the optical modulator 100 of the first embodiment in other respects. Only the differences will be described below. In the method for manufacturing the optical modulator 100 of the fourth embodiment, the substrate 1 is processed after being thinned using photolithography and etching to form ridges. These ridges become the optical waveguides 2.

[0097] Fifth Embodiment 16 is a schematic diagram showing a cross section of an optical modulator 100 according to a fifth embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulators 100 of the first to third embodiments.

[0098] 16, in this embodiment, the low dielectric layer 5 and the auxiliary low dielectric layer 6 are integrated. In a cross-sectional view of the optical modulator 100, the optical waveguide 2 is disposed inside the integrated low dielectric layer 5 and auxiliary low dielectric layer 6. In the example shown in FIG. 16, the low dielectric layers 5 and 6 directly cover the bottom and side surfaces of the optical waveguide 2. This increases the electric field passing through the low dielectric layers 5 and 6, making it easier to adjust the effective refractive index.

[0099] In the example shown in FIG. 16, the integrated low dielectric constant layers 5 and 6 may further cover the upper surface of the optical waveguide 2.

[0100] Sixth Embodiment 17 is a schematic diagram showing a cross section of an optical modulator 101 according to a sixth embodiment. The optical modulator 101 of this embodiment constitutes a Mach-Zehnder optical modulator. The optical modulator 101 of this embodiment is a modification of the optical modulator 100 of the first embodiment, and is obtained by arranging each element of the optical modulator 100 of the first embodiment in parallel.

[0101] Referring to FIG. 17, an optical modulator 101 of this embodiment includes two optical modulator units 100A and 100B.

[0102] One optical modulator unit 100A includes a substrate 1A, an optical waveguide 2A, a first electrode 31A, two second electrodes 32A, a third electrode 4A, and an auxiliary low-dielectric-constant layer 6A. The other optical modulator unit 100B includes a substrate 1B, an optical waveguide 2B, a first electrode 31B, two second electrodes 32B, a third electrode 4B, and an auxiliary low-dielectric-constant layer 6B. The optical modulator units 100A and 100B are supported by a support plate 7.

[0103] The substrates 1A and 1B correspond to the above-mentioned substrate 1. The optical waveguides 2A and 2B correspond to the above-mentioned optical waveguide 2. The first electrodes 31A and 31B correspond to the above-mentioned first electrode 31. The second electrodes 32A and 32B correspond to the above-mentioned second electrode 32. The third electrodes 4A and 4B correspond to the above-mentioned third electrode 4. The auxiliary low dielectric constant layers 6A and 6B correspond to the above-mentioned auxiliary low dielectric constant layer 6.

[0104] Substrate 1A on which optical waveguide 2A is provided is disposed in parallel with substrate 1B on which optical waveguide 2B is provided. That is, optical waveguide 2A and optical waveguide 2B are disposed side by side. Upstream of optical waveguide 2A and optical waveguide 2B, a single input optical waveguide branches into optical waveguide 2A and optical waveguide 2B. Downstream of optical waveguide 2A and optical waveguide 2B, optical waveguide 2A and optical waveguide 2B merge into a single output optical waveguide.

[0105] The optical modulator 101 of this embodiment can also achieve the same effects as those of the first embodiment. Furthermore, since the optical modulator 101 of this embodiment is a Mach-Zehnder type optical modulator, it is possible to perform intensity modulation in addition to phase modulation. This makes it possible to perform multi-level modulation and increase the transmission capacity.

[0106] The optical modulator 101 of this embodiment may not have auxiliary low dielectric layers 6A and 6B. Also, the optical modulator units 100A and 100B may be provided with a low dielectric layer equivalent to the low dielectric layer 5 of the second and third embodiments. Also, the optical modulator 101 of this embodiment may not have substrates 1A and 1B as in the fifth embodiment.

[0107] In the optical modulator 101 of this embodiment, the optical waveguides 2A and 2B are formed by Ti diffusion. However, the optical waveguides 2A and 2B may be ridge-shaped. In this case, the same effects as those of the fourth embodiment can be obtained.

[0108] Seventh Embodiment 18 and 19 are schematic diagrams showing an optical modulator 101 according to the seventh embodiment. Fig. 18 shows a cross section of the optical modulator 101. Fig. 19 shows a plan view of the optical modulator 101 as viewed from above. The optical modulator 101 of this embodiment is a modification of the optical modulator 101 of the sixth embodiment.

[0109] 18 and 19, the substrate 1A of the optical modulator unit 100A is integrated with the substrate 1B of the optical modulator unit 100B. In this case, when the optical modulator 101 is operating, a voltage of the opposite phase to that applied to the first electrode 31A and the two second electrodes 32A is applied to the first electrode 31B and the two second electrodes 32B.

[0110] In the optical modulator 101 of this embodiment, the substrate 1A and the substrate 1B can be shared. The optical waveguide 2A and the optical waveguide 2B are provided on the shared substrate 1A, 1B. This allows the distance between the optical waveguide 2A and the optical waveguide 2B to be reduced. In this case, the overall width of the optical modulator 101 can be narrowed, and the optical modulator 101 can be made more compact.

[0111] Eighth Embodiment 20 and 21 are schematic diagrams showing an optical modulator 101 according to the eighth embodiment. Fig. 20 shows a cross section of the optical modulator 101. Fig. 21 shows a plan view of the optical modulator 101 as viewed from above. The optical modulator 101 of this embodiment is a modification of the optical modulator 101 of the sixth embodiment.

[0112] 20 and 21, the substrate 1A of the optical modulator unit 100A is integrated with the substrate 1B of the optical modulator unit 100B. The spontaneous polarization directions of the optical waveguides 2A and 2B are opposite to each other. When the materials of the substrates 1A and 1B are ferroelectric crystals such as LN or LiTaO3, the spontaneous polarization direction can be reversed by applying a high voltage to the ferroelectric crystal material. The location of the reversed polarization can be recognized by observation with an atomic force microscope or an electron microscope. In this case, when the optical modulator 101 is operated, voltages of the same phase are applied to the first electrode 31A, the second electrode 32A, the first electrode 31B, and the second electrode 32B.

[0113] In the optical modulator 101 of this embodiment, the substrate 1A and the substrate 1B can be shared, as in the seventh embodiment. The optical waveguide 2A and the optical waveguide 2B are provided on the shared substrates 1A and 1B.

[0114] One of the two second electrodes 32B is formed integrally with one of the two second electrodes 32A. In other words, the second electrodes 32A and 32B, which are located close to each other, are electrically integrated. In this case, one of the two second electrodes 32B can be used in common with one of the two second electrodes 32A. This allows the distance between the optical waveguide 2A and the optical waveguide 2B to be further reduced. In this case, the overall width of the optical modulator 101 can be further narrowed, thereby further realizing miniaturization of the optical modulator 101.

[0115] Furthermore, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure. [Explanation of symbols]

[0116] 100, 101: Optical modulator 1: Circuit board 2: Optical waveguide 31: 1st electrode 32:Second electrode 4: Third electrode 5: Low dielectric constant layer 6: Auxiliary low dielectric constant layer 7: Support board

Claims

1. an optical waveguide made of a material having an electro-optic effect; a substrate on which the optical waveguide is provided; a control electrode for controlling light passing through the optical waveguide; the substrate is made of the same material as the optical waveguide; the optical waveguide is a ridge type, the control electrodes include a first electrode, two second electrodes to each of which a voltage of the same phase as that of the first electrode is applied, and a third electrode that forms a potential difference with the group of the first electrode and the second electrode; In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the first electrode is provided on one side in a thickness direction of the optical waveguide, one of the two second electrodes is provided on one side of the first electrode in the width direction of the optical waveguide with a gap therebetween, and the other second electrode is provided on the other side of the first electrode in the width direction of the optical waveguide with a gap therebetween, the third electrode is provided on the other side in the thickness direction of the optical waveguide, An optical modulator, wherein a center position in the width direction of the first electrode and a center position in the width direction of the optical waveguide are shifted from each other in the width direction.

2. an optical waveguide made of a material having an electro-optic effect; a substrate on which the optical waveguide is provided; a control electrode for controlling light passing through the optical waveguide; the control electrodes include a first electrode, two second electrodes to each of which a voltage of the same phase as that of the first electrode is applied, and a third electrode that forms a potential difference with the group of the first electrode and the second electrode; In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the first electrode is provided on one side in a thickness direction of the optical waveguide, one of the two second electrodes is provided on one side of the first electrode in the width direction of the optical waveguide with a gap therebetween, and the other second electrode is provided on the other side of the first electrode in the width direction of the optical waveguide with a gap therebetween, the third electrode is provided on the other side in the thickness direction of the optical waveguide, a dimension of the first electrode in a width direction is larger than a dimension of the optical waveguide in the width direction, and the optical waveguide is disposed within a region of the first electrode in the width direction; An optical modulator, wherein a dimension in a width direction of the third electrode is larger than a dimension in the width direction of the optical waveguide, and the optical waveguide is disposed within a region of the third electrode in the width direction.

3. 2. The optical modulator according to claim 1, In a cross-sectional view perpendicular to the extending direction of the optical waveguide, An optical modulator, wherein the two second electrodes are disposed asymmetrically with respect to the first electrode in a width direction of the optical waveguide.

4. 2. The optical modulator according to claim 1, one of the second electrodes is disposed spaced apart from one end of the optical waveguide in a width direction of the optical waveguide, and the other of the second electrodes is disposed spaced apart from the other end of the optical waveguide; The optical modulator further comprises: a low dielectric constant layer having a dielectric constant lower than that of the optical waveguide; The low dielectric layer covers at least a portion of a surface of the second electrode so as to be interposed between the second electrode and the third electrode.

5. 5. The optical modulator according to claim 4, The low dielectric layer covers at least a portion of a surface of the first electrode so as to be interposed between the first electrode and the third electrode.

6. 2. The optical modulator according to claim 1, further comprising: an auxiliary low dielectric layer having a dielectric constant lower than that of the optical waveguide; The auxiliary low dielectric layer covers at least a portion of a surface of the third electrode so as to be interposed between the second electrode and the third electrode.

7. 2. The optical modulator according to claim 1, The material of the optical waveguide is LiNbO ３ That is, an optical modulator.

8. The optical modulator according to any one of claims 1 to 7, An optical modulator comprising two optical modulator units arranged in parallel, each of which includes the optical waveguide, the substrate, and the control electrode.

9. 9. The optical modulator according to claim 8, the substrate of one of the two optical modulator units is integral with the substrate of the other optical modulator unit; an optical modulator, wherein a voltage having a phase opposite to that of the first electrode and the second electrode of the other optical modulator unit is applied to the first electrode and the second electrode of the one optical modulator unit;

10. 9. The optical modulator according to claim 8, the substrate of one of the two optical modulator units is integral with the substrate of the other optical modulator unit; the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit have mutually opposite directions of spontaneous polarization, one of the two second electrodes of the one optical modulator unit is formed integrally with one of the two second electrodes of the other optical modulator unit; an optical modulator, wherein a voltage having the same phase as that of the first electrode and the second electrode of the other optical modulator unit is applied to the first electrode and the second electrode of the one optical modulator unit;

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