Optical modulator

Abstract

An optical modulator (100) comprises an optical waveguide (1A), a first electrode (2A), and a second electrode (3). The first electrode (2A) is disposed on one side of the optical modulator (100) in a height direction (HD) with respect to the optical waveguide (1A). The first electrode (2A) has one end (21Aa) and the other end (22Aa) overlapping the optical waveguide (1A) when viewed in the height direction (HD). The second electrode (3) is disposed on the other side in the height direction (HD) with respect to the optical waveguide (1A), and applies an electric field to the optical waveguide (1A) together with the first electrode (2A). When viewed in the height direction (HD), the length of the first electrode (2A) is different from the length, of the optical waveguide (1A), from the position of the one end (21Aa) to the position of the other end (22Aa) of the first electrode (2A).

Description

Optical Modulator

[0001] The present disclosure relates to optical modulators.

[0002] Optical communications require optical transceivers that convert optical signals to electrical signals and vice versa. An optical transceiver has an optical modulator as its main component, which converts electrical signals into optical signals.

[0003] For example, Patent Document 1 discloses a Mach-Zehnder optical modulator. This optical modulator includes first and second optical waveguides, a first ground electrode, a second ground electrode, and a hot electrode. In a plan view of the optical modulator, the hot electrode is disposed between the first optical waveguide and the second optical waveguide. The first ground electrode is disposed on the opposite side of the hot electrode with respect to the first optical waveguide and applies an electric field to the first optical waveguide together with the hot electrode. The second ground electrode is disposed on the opposite side of the hot electrode with respect to the second optical waveguide and applies an electric field to the second optical waveguide together with the hot electrode. The hot electrode, the first ground electrode, and the second ground electrode are disposed on the same plane.

[0004] In the optical modulator of Patent Document 1, each of the optical waveguides includes two straight waveguide sections and a curved waveguide section disposed between the straight waveguide sections. Each of the electrodes includes two straight electrode sections and a curved electrode section disposed between the straight electrode sections. In the optical modulator of Patent Document 1, a difference in length is generated between the curved waveguide section of each optical waveguide and the curved electrode section of the electrode corresponding to that optical waveguide in order to compensate for the mismatch between the speed of the electrical signal and the speed of the light wave.

[0005] US Patent Application Publication No. 2021 / 0311336

[0006] In the optical modulator of Patent Document 1, the first and second optical waveguides, the hot electrode, and the first and second ground electrodes have curved sections to compensate for the mismatch between the speed of an electrical signal and the speed of a light wave. In the optical modulator of Patent Document 1, the application of an unnecessary electric field and the absorption of light may occur in one of the first and second optical waveguides, particularly in the curved sections. For example, as shown in FIG. 1 of Patent Document 1, if the length of the curved section of the electrode is longer than the length of the curved section of the optical waveguide, in the curved section, the second optical waveguide located on the inner side of the bend in a plan view of the optical modulator does not overlap with any of the electrodes, while the first optical waveguide located on the outer side of the bend overlaps with the first ground electrode. Furthermore, for example, as shown in FIG. 2 of Patent Document 1, if the length of the curved section of the optical waveguide is longer than the length of the curved section of the electrode, in the curved section, the first optical waveguide located on the outer side of the bend in a plan view of the optical modulator does not overlap with any of the electrodes, while the second optical waveguide located on the inner side of the bend overlaps with the second ground electrode. Therefore, in the curved section, an electric field is unnecessarily applied to only one of the first and second optical waveguides, and there is a risk that light from either optical waveguide will be absorbed by the electrode.

[0007] The object of the present disclosure is to provide an optical modulator that can suppress the application of unnecessary electric fields in an optical waveguide and suppress the absorption of light, thereby compensating for the mismatch between the speed of an electrical signal and the speed of a light wave.

[0008] The optical modulator according to the present disclosure includes an optical waveguide, a first electrode, and a second electrode. The optical waveguide has an electro-optic effect. The first electrode is disposed on one side of the optical waveguide in the height direction of the optical modulator. The first electrode has one end and another end that overlap at least the optical waveguide when viewed along the height direction, and extends from the one end to the other end. The second electrode is disposed on the other side of the optical waveguide in the height direction. The second electrode forms a potential difference with the first electrode and applies an electric field to the optical waveguide together with the first electrode. When viewed along the height direction, the length of the first electrode is different from the length of the optical waveguide from the position of one end of the first electrode to the position of the other end.

[0009] According to the optical modulator of the present disclosure, it is possible to suppress the application of unnecessary electric fields in the optical waveguide and suppress the absorption of light, thereby compensating for the mismatch between the speed of the electrical signal and the speed of the light wave.

[0010] FIG. 1 is a plan view of an optical modulator according to a first embodiment. FIG. 2 is a cross-sectional view taken along line II-II in FIG. 1 . FIG. 3 is a cross-sectional view taken along line III-III in FIG. 1 . FIG. 4 is a schematic diagram showing the configuration of an optical modulator according to a second embodiment. FIG. 5 is a schematic diagram showing the configuration of an optical modulator according to a third embodiment. FIG. 6 is a schematic diagram showing the configuration of an optical modulator according to a fourth embodiment. FIG. 7 is a schematic diagram showing the configuration of an optical modulator according to the fourth embodiment. FIG. 8 is a schematic diagram showing the configuration of an optical modulator according to a fifth embodiment. FIG. 9 is a schematic diagram showing the configuration of an optical modulator according to the fifth embodiment. FIG. 10 is a schematic diagram showing the configuration of a modified example of the optical modulator according to the fifth embodiment. FIG. 11 is a schematic diagram showing the configuration of a modified example of the optical modulator according to the fifth embodiment. FIG. 12A is a schematic diagram showing the configuration of an optical modulator according to a sixth embodiment. FIG. 12B is a schematic diagram showing the configuration of an optical modulator according to the sixth embodiment. FIG. 12C is a schematic diagram showing the configuration of an optical modulator according to the sixth embodiment. Fig. 12D is a schematic diagram showing the configuration of an optical modulator according to a sixth embodiment. Fig. 13A is a schematic diagram showing the configuration of an optical modulator according to a sixth embodiment. Fig. 13B is a schematic diagram showing the configuration of an optical modulator according to a sixth embodiment. Fig. 13C is a schematic diagram showing the configuration of an optical modulator according to a sixth embodiment. Fig. 13D is a schematic diagram showing the configuration of an optical modulator according to a sixth embodiment. Fig. 14 is a schematic diagram showing the configuration of an optical modulator according to a seventh embodiment. Fig. 15 is a schematic diagram showing the configuration of an optical modulator according to an eighth embodiment.

[0011] 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.

[0012] An optical modulator according to an embodiment of the present disclosure includes an optical waveguide, a first electrode, and a second electrode. The optical waveguide has an electro-optic effect. The first electrode is disposed on one side of the optical waveguide in the height direction of the optical modulator. The first electrode has one end and another end that overlap at least the optical waveguide when viewed along the height direction, and extends from the one end to the other end. The second electrode is disposed on the other side of the optical waveguide in the height direction. The second electrode forms a potential difference between itself and the first electrode, and applies an electric field to the optical waveguide together with the first electrode. When viewed along the height direction, the length of the first electrode is different from the length of the optical waveguide from the position of one end of the first electrode to the position of the other end of the first electrode (first configuration).

[0013] In the first configuration, when the optical modulator is viewed along the height direction, the length of the first electrode is different from the length of the optical waveguide from one end position to the other end position of the first electrode, thereby compensating for the mismatch between the speed of the electrical signal and the speed of the light wave.

[0014] If at least one of the first electrode and the optical waveguide is bent to make the length of the first electrode different from the length of the portion of the optical waveguide corresponding to the first electrode, a portion of the optical waveguide may be significantly deviated from the first electrode when viewed along the height direction of the optical modulator, creating a portion of the optical waveguide to which an electric field is difficult to apply. However, if multiple electrodes including the first electrode and the second electrode are arranged on the same plane and the optical waveguide is arranged between the first electrode and the second electrode when viewed along the height direction, the portion of the optical waveguide that is significantly deviated from the first electrode may overlap with, for example, a second electrode arranged in parallel with the first electrode, and an electric field formed between the second electrode and another electrode may be unintentionally applied to that portion. Furthermore, even though the portion of the optical waveguide that is significantly deviated from the first electrode is not intended to be a portion to which an electric field is applied, its proximity to the second electrode may unnecessarily increase light absorption from the optical waveguide to the second electrode.

[0015] On the other hand, in the first configuration, the second electrode is disposed on the opposite side of the optical waveguide from the first electrode in the height direction of the optical modulator. Therefore, even if a portion of the optical waveguide is significantly deviated from the first electrode when viewed along the height direction of the optical modulator, the position of the second electrode can be adjusted relatively freely so that the portion of the optical waveguide significantly deviated from the first electrode does not overlap with, for example, the second electrode. Therefore, unnecessary electric fields are less likely to be applied to the optical waveguide, and light absorption can be reduced.

[0016] The optical modulator of the first configuration can include two optical waveguides connected to each other at both ends, a first electrode provided corresponding to each of the optical waveguides, and a second electrode provided corresponding to each of the optical waveguides (second configuration).

[0017] In the optical modulator of the second configuration, the optical waveguide may include two waveguide bodies and a waveguide bending portion connecting one ends of the two waveguide bodies to each other. In this case, the first electrode includes two electrode bodies overlapping the two waveguide bodies, respectively, as viewed along the height direction, and an electrode bending portion connecting one ends of the two electrode bodies to each other and disposed inside or outside the waveguide bending portion as viewed along the height direction, and the lengths of the electrode bending portions may be different from the lengths of the waveguide bending portions (third configuration).

[0018] In the third configuration, one ends of two waveguide main bodies are connected to each other by a waveguide bending portion, and one ends of two electrode main bodies are connected to each other by an electrode bending portion. The electrode bending portion is located inside or outside the waveguide bending portion when viewed along the height direction, and its length is different from that of the waveguide bending portion. In this case, because the electrode bending portion is separated from the waveguide bending portion when viewed along the height direction, it is difficult to apply an electric field to the optical waveguide at the position of the waveguide bending portion. Since the waveguide bending portion is a portion of the optical waveguide where the electric field is not intended to be applied by the first electrode, it is preferable to prevent unnecessary application of an electric field from other electrodes. In this regard, in the third configuration, as in the first configuration, the second electrode is located on the opposite side of the first electrode from the optical waveguide in the height direction of the optical modulator, allowing for relatively flexible adjustment of the position of the second electrode, and the second electrode can also be separated from the waveguide bending portion at the waveguide bending portion. Therefore, unnecessary application of an electric field to the optical waveguide can be suppressed, thereby suppressing light absorption.

[0019] In the optical modulator of the second or third configuration, it is preferable that the lengths between both ends of the two optical waveguides are equal when viewed along the height direction (fourth configuration). In this case, the time it takes for light to travel from one end to the other end of the two optical waveguides can be made the same. This simplifies the timing and design of applying the electrical signal from the first electrode.

[0020] In the optical modulator of any one of the first to fourth configurations, the effective refractive index of the electrical signal passing between the first and second electrodes may be smaller than the effective refractive index of the light wave passing through the optical waveguide (fifth configuration). In this case, since the effective refractive index of the electrical signal is smaller than the effective refractive index of the light wave, loss of the electrical signal can be reduced.

[0021] In the optical modulator of the fifth configuration, the length of the first electrode is preferably greater than the length of the optical waveguide (sixth configuration). When the effective refractive index of the electrical signal is smaller than the effective refractive index of the optical wave, the speed difference between the electrical signal and the optical wave can be matched by making the length of the first electrode greater than the length of the optical waveguide, thereby increasing the bandwidth of the optical modulator.

[0022] In any one of the optical modulators of the first to sixth configurations, the first electrode may include a meandering portion (seventh configuration). In this case, the path of the first electrode in the meandering portion has a periodic wave shape, which makes it easier for the strength of the electric field applied to the optical waveguide to be uniform along the optical waveguide, improving the signal quality of the optical modulation. Furthermore, it is possible to reduce the variation in the effective refractive index depending on the location, thereby suppressing narrowing of the bandwidth due to velocity mismatch. Furthermore, since the length of the first electrode can be made longer than the length of the optical waveguide, it is easy to obtain the effect of compensating for velocity mismatch.

[0023] In the optical modulator of any one of the first to fourth configurations, the effective refractive index of the electrical signal passing between the first electrode and the second electrode may be greater than the effective refractive index of the light wave passing through the optical waveguide (eighth configuration). In this case, the strength of the electric field applied to the optical waveguide becomes stronger, and therefore the driving voltage applied to the first electrode can be reduced.

[0024] In the optical modulator of the eighth configuration, the length of the optical waveguide is preferably greater than the length of the first electrode (ninth configuration). When the effective refractive index of the electrical signal is greater than the effective refractive index of the light wave, even if a low-dielectric-constant layer is interposed between the first electrode and the optical waveguide and / or between the second electrode and the optical waveguide, the strength of the electric field applied to the optical waveguide can be increased by reducing the amount of the low-dielectric-constant layer sandwiched between them. In this case, because the amount of the low-dielectric-constant layer sandwiched between them is smaller than the electro-optic material, the effective refractive index of the electrical signal is significantly dependent on the dielectric constant of the electro-optic material. As a result, the effective refractive index of the electrical signal is greater than the effective refractive index of the light wave, which tends to narrow the bandwidth of the optical modulator. Therefore, by making the length of the optical waveguide greater than the length of the first electrode, as in the ninth configuration, the velocity difference between the generated electrical signal and the light wave can be compensated for, enabling the optical modulator to be used over a wide bandwidth.

[0025] In the optical modulator of any one of the first to fourth, eighth, and ninth configurations, the optical waveguide may include a meandering section (tenth configuration). In this case, the path of the optical waveguide in the meandering section has a periodic wave shape, which makes it easier for the strength of the electric field applied to the optical waveguide to be uniform along the optical waveguide, improving the signal quality of the optical modulation. Furthermore, it is possible to reduce the variation in the effective refractive index depending on the location, thereby suppressing narrowing of the bandwidth due to velocity mismatch. Furthermore, since the length of the optical waveguide can be made longer than the length of the first electrode, it is easy to obtain the effect of compensating for velocity mismatch.

[0026] Any one of the optical modulators of the first to tenth configurations may further include an input port for inputting light into the optical waveguide and an output port for outputting light from the optical waveguide. In this case, it is preferable that the input port is located on an end face along the height direction of the optical modulator, and the output port is located on the end face (eleventh configuration). In this case, for example, processes for coupling an optical fiber or an optical coupler to the input port and the output port can be performed in one place. This reduces processing time and the number of components.

[0027] 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 redundant description will not be repeated.

[0028] First Embodiment [Configuration of Optical Modulator] FIG. 1 is a plan view of an optical modulator 100 according to a first embodiment. Referring to FIG. 1, in this embodiment, the optical modulator 100 is a so-called Mach-Zehnder type optical modulator. The optical modulator 100 includes two optical waveguides 1A and 1B, two first electrodes 2A and 2B, and a second electrode 3. FIG. 1 shows the optical waveguides 1A and 1B, the first electrodes 2A and 2B, and the second electrode 3 when projected onto a plane perpendicular to a height direction HD of the optical modulator 100. In FIG. 1, the optical waveguides 1A and 1B are indicated by dotted lines, and the second electrode 3 is indicated by a dashed line. In this specification, the height direction HD refers to the stacking direction of each element of the optical modulator 100 having a stacked structure.

[0029] The optical waveguides 1A and 1B function as optical transmission paths. The two optical waveguides 1A and 1B are connected to each other at both ends. Specifically, the optical waveguides 1A and 1B are each connected to an input optical waveguide 1I at a branch point P1, which is one end, and to an output optical waveguide 1O at a junction point P2, which is the other end. That is, the optical waveguides 1A and 1B branch from the input optical waveguide 1I at the branch point P1 and merge with the output optical waveguide 1O at the junction point P2.

[0030] In this embodiment, the optical modulator 100 includes an input port 1 in for inputting light into the optical waveguides 1A and 1B and an output port 1 out for outputting light from the optical waveguides 1A and 1B. The input port 1 in is provided at the tip of the input optical waveguide 1I and is connected to a branch point P1 of the optical waveguides 1A and 1B via the input optical waveguide 1I. The output port 1 out is provided at the tip of the output optical waveguide 1O and is connected to a junction point P2 of the optical waveguides 1A and 1B via the output optical waveguide 1O. In this embodiment, both the input port 1 in and the output port 1 out are disposed on an end face 101 along the height direction of the optical modulator 100. That is, the input port 1 in and the output port 1 out are disposed on the same end face 101 of the optical modulator 100.

[0031] In this embodiment, the two optical waveguides 1A, 1B are curved so as to be folded back. The optical waveguide 1A is disposed inside the optical waveguide 1B. The optical waveguide 1A includes two main body portions 11A, 12A and a bent portion 13A that connects one end of the two main body portions 11A, 12A to each other. The optical waveguide 1A further includes an inlet coupling portion 14A and an outlet coupling portion 15A. The inlet coupling portion 14A connects the main body portion 11A to the branch point P1. The outlet coupling portion 15A connects the main body portion 12A to the junction point P2.

[0032] The optical waveguide 1B includes two main body portions 11B and 12B and a bent portion 13B that connects one end of the two main body portions 11B and 12B. The optical waveguide 1B further includes an inlet coupling portion 14B and an outlet coupling portion 15B. The inlet coupling portion 14B connects the main body portion 11B to the branch point P1. The outlet coupling portion 15B connects the main body portion 12B to the junction point P2.

[0033] The length between both ends of optical waveguide 1A is the same as the length between both ends of optical waveguide 1B. The length between both ends of optical waveguide 1A being the same as the length between both ends of optical waveguide 1B means that the two lengths are substantially equal, for example, the difference between the two lengths is equal to or less than half the wavelength of the input light wave. However, the length between both ends of optical waveguide 1A may be different from the length between both ends of optical waveguide 1B. In this specification, the length between both ends of optical waveguide 1A means the entire length of optical waveguide 1A, i.e., the length from branching point P1 to junction point P2 of optical waveguide 1A. Similarly, the length of optical waveguide 1B means the entire length of optical waveguide 1B, i.e., the length from branching point P1 to junction point P2 of optical waveguide 1B.

[0034] In the optical waveguide 1A, the main body portion 11A and the main body portion 12A are arranged in parallel. In the example of this embodiment, the main body portions 11A and 12A have a linear shape. However, it is sufficient that the main body portions 11A and 12A extend substantially in a linear shape. The main body portions 11A and 12A are arranged in parallel, for example. The length of the main body portion 11A is the same as the length of the main body portion 12A. However, the length of the main body portion 11A may be different from the length of the main body portion 12A.

[0035] In the optical waveguide 1A, the bent portion 13A connects one end of the main body portions 11A and 12A to each other. The bent portion 13A is connected to an end of the main body portion 11A opposite the inlet coupling portion 14A and an end of the main body portion 12A opposite the outlet coupling portion 15A. In the example shown in Fig. 1 , the bent portion 13A has a convex curved shape extending from the main body portions 11A and 12A toward the opposite side of the branch point P1 and the junction point P2 when viewed along the height direction HD. The bent portion 13A causes the optical waveguide 1A to be folded back.

[0036] On the other hand, in the optical waveguide 1B, the main body portion 11B and the main body portion 12B are arranged in parallel. In the example of this embodiment, the main body portions 11B and 12B have a linear shape. However, it is sufficient that the main body portions 11B and 12B extend substantially in a linear shape. The main body portions 11B and 12B are arranged, for example, in parallel. The length of the main body portion 11B is the same as the length of the main body portion 12B. However, the length of the main body portion 11B may be different from the length of the main body portion 12B. The lengths of the main body portions 11B and 12B in the optical waveguide 1B are substantially the same as the lengths of the main body portions 11A and 12A in the optical waveguide 1A.

[0037] In the optical waveguide 1B, the bent portion 13B connects one end of the main body portions 11B and 12B. The bent portion 13B is connected to the end of the main body portion 11B opposite the inlet coupling portion 14B and the end of the main body portion 12B opposite the outlet coupling portion 15B. In the example shown in FIG. 1 , the bent portion 13B has a convex curved shape from the main body portions 11B and 12B toward the opposite side of the branch point P1 and the junction point P2 when viewed along the height direction HD. The bent portion 13B bends the optical waveguide 1B back. The bent portion 13B of the optical waveguide 1B is located outside the bent portion 13A of the optical waveguide 1A. The length of the bent portion 13B in the optical waveguide 1B is substantially the same as the length of the bent portion 13A in the optical waveguide 1A.

[0038] The first electrodes 2A and 2B are disposed on one side of the optical waveguides 1A and 1B in the height direction HD of the optical modulator 100. The first electrodes 2A and 2B have one end 21Aa and 21Ba and the other end 22Aa and 22Ba that overlap at least the optical waveguides 1A and 1B when viewed along the height direction HD. The first electrodes 2A and 2B extend from the one end 21Aa and 21Ba to the other end 22Aa and 22Ba. The configuration of the first electrodes 2A and 2B will be described in detail below.

[0039] The first electrode 2A is disposed inside the first electrode 2B. The first electrode 2A includes two main body portions 21A and 22A and a bent portion 23A connecting one end of the two main body portions 21A and 22A. Wiring (not shown) is connected to the main body portions 21A and 22A to supply an electrical signal to the first electrode 2A or to extract an electrical signal from the first electrode 2A. For example, the wiring for supplying the electrical signal is connected to an end portion 21Aa of the main body portion 21A opposite the bent portion 23A. The wiring for extracting the electrical signal is connected to an end portion 22Aa of the main body portion 22A opposite the bent portion 23A.

[0040] In the first electrode 2A, the main body portion 21A and the main body portion 22A are arranged in parallel. The main body portion 21A of the first electrode 2A overlaps with the main body portion 11A of the optical waveguide 1A. The main body portion 21A is, for example, aligned with the main body portion 11A. The main body portion 22A of the first electrode 2A overlaps with the main body portion 12A of the optical waveguide 1A. The main body portion 21A is, for example, aligned with the main body portion 11A. In this embodiment, the main body portions 21A and 22A are linear. However, it is sufficient that the main body portions 21A and 22A overlap with the main body portions 11A and 12A of the optical waveguide 1A. The main body portion 21A and the main body portion 22A are, for example, arranged in parallel.

[0041] In the first electrode 2A, the bent portion 23A connects one end of the main body portions 21A and 22A to each other. The bent portion 23A is connected to the end of the main body portion 21A opposite the end portion 21Aa and the end of the main body portion 22A opposite the end portion 22Aa. In the example shown in FIG. 1 , the bent portion 23A has a convex curved shape from the main body portions 21A and 22A to the side opposite the one end 21Aa and the other end 22Aa when viewed along the height direction HD. The bent portion 23A folds the first electrode 2A back. The bent portion 23A of the first electrode 2A is disposed inside the bent portion 13A of the optical waveguide 1A and does not substantially overlap with the optical waveguide 1A. However, the bent portion 23A may overlap the main body portions 11A and 12A of the optical waveguide 1A near the main body portions 21A and 22A. In this case, the bent portion 13A of the optical waveguide 1A has a portion that is significantly deviated from the bent portion 23A of the first electrode 2A. The length of the bent portion 23A of the first electrode 2A is shorter than the length of the bent portion 13A of the optical waveguide 1A.

[0042] The length of the first electrode 2A, as viewed along the height direction HD, is shorter than the length of the optical waveguide 1A from the position of one end 21Aa of the first electrode 2A to the position of the other end 22Aa. The length of the first electrode 2A is the sum of the lengths of the main body portions 21A and 22A and the length of the bent portion 23A. The starting end of the overlap between the optical waveguide 1A and the first electrode 2A is the one end 21Aa of the first electrode 2A (main body portion 21A), and the ending end of the overlap is the other end 22Aa of the first electrode 2A (main body portion 22A). Therefore, the length of the optical waveguide 1A may be the sum of the lengths of the main body portions 11A and 12A and the length of the bent portion 13A.

[0043] On the other hand, the first electrode 2B includes two main body portions 21B, 22B and a bent portion 23B connecting one end of these two main body portions 21B, 22B. Wiring (not shown) is connected to the main body portions 21B, 22B to supply an electrical signal to the first electrode 2B or to extract an electrical signal from the first electrode 2B. For example, the wiring for supplying the electrical signal is connected to an end 21Ba of the main body portion 21B opposite the bent portion 23B. The wiring for extracting the electrical signal is connected to an end 22Ba of the main body portion 22B opposite the bent portion 23B.

[0044] In the first electrode 2B, the main body portion 21B and the main body portion 22B are arranged in parallel. The main body portion 21B of the first electrode 2B overlaps with the main body portion 11B of the optical waveguide 1B. The main body portion 21B is, for example, aligned with the main body portion 11B. The main body portion 22B of the first electrode 2B overlaps with the main body portion 12B of the optical waveguide 1B. The main body portion 21B is, for example, aligned with the main body portion 11B. In this embodiment, the main body portions 21B and 22B are linear. However, it is sufficient that the main body portions 21B and 22B overlap with the main body portions 11B and 12B of the optical waveguide 1B. The main body portion 21B and the main body portion 22B are, for example, arranged in parallel.

[0045] In the first electrode 2B, the bent portion 23B connects one end of the main body portions 21B and 22B. The bent portion 23B is connected to the end of the main body portion 21B opposite the end portion 21Ba and the end of the main body portion 22B opposite the end portion 22Ba. In the example shown in FIG. 1 , the bent portion 23B has a convex curved shape from the main body portions 21B and 22B toward the opposite side of the one end 21Ba and the other end 22Ba when viewed along the height direction HD. The bent portion 23B folds the first electrode 2B back. The bent portion 23B of the first electrode 2B is disposed outside the bent portion 13B of the optical waveguide 1B and does not substantially overlap with the optical waveguide 1B. However, the bent portion 23B may overlap the main body portions 11B and 12B of the optical waveguide 1B near the main body portions 21B and 22B. In this case, the bent portion 13B of the optical waveguide 1B has a portion that is significantly deviated from the bent portion 23B of the first electrode 2B. The length of the bent portion 23B of the first electrode 2B is greater than the length of the bent portion 13B of the optical waveguide 1B.

[0046] The length of the first electrode 2B, as viewed along the height direction HD, is greater than the length of the optical waveguide 1B from one end 21Ba of the first electrode 2B to the other end 22Ba. The length of the first electrode 2B is the sum of the lengths of the main body portions 21B and 22B and the length of the bent portion 23B. The starting end of the overlap between the optical waveguide 1B and the first electrode 2B is one end 21Ba of the first electrode 2B (main body portion 21B), and the ending end of the overlap is the other end 22Ba of the first electrode 2B (main body portion 22B). Therefore, the length of the optical waveguide 1B may be the sum of the lengths of the main body portions 11B and 12B and the length of the bent portion 13B.

[0047] In this embodiment, as described above, the length of the first electrode 2A is shorter than the length of the optical waveguide 1A. The length of the first electrode 2B is longer than the length of the optical waveguide 1B. In this manner, the length of the first electrode 2A is different from the length of the optical waveguide 1A from the position of one end 21Aa to the position of the other end 22Aa of the first electrode 2A. The length of the first electrode 2B is different from the length of the optical waveguide 1B from the position of one end 21Ba to the position of the other end 22Ba of the first electrode 2B.

[0048] The second electrode 3 is disposed so that at least a portion thereof overlaps with the optical waveguides 1A and 1B and the first electrodes 2A and 2B when viewed along the height direction HD. For example, when viewed along the height direction HD, the second electrode 3 overlaps with the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B, and with the main bodies 21A, 22A, 21B, and 22B and bent portions 23A and 23B of the first electrodes 2A and 2B. In this case, at least one end 21Aa and the other end 22Aa of the first electrode 2A overlap with the second electrode 3 when viewed along the height direction HD. At least one end 21Ba and the other end 22Ba of the first electrode 2B overlap with the second electrode 3 when viewed along the height direction HD.

[0049] The second electrode 3 forms a potential difference between itself and the main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B, respectively, and applies an electric field to the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B together with the first electrodes 2A and 2B. The first electrodes 2A and 2B and the second electrode 3 function, for example, as control electrodes for controlling light passing through the optical waveguides 1A and 1B. The main body 21A of the first electrode 2A and the second electrode 3 are arranged so as to apply an electric field to the main body 11A of the optical waveguide 1A. The main body 22A of the first electrode 2A and the second electrode 3 are arranged so as to apply an electric field to the main body 12A of the optical waveguide 1A. The main body 21B of the first electrode 2B and the second electrode 3 are arranged so as to apply an electric field to the main body 11B of the optical waveguide 1B. The main body 22B of the first electrode 2B and the second electrode 3 are arranged so that an electric field can be applied to the main body 12B of the optical waveguide 1B. The main bodies 11A, 12A, 11B, and 12B essentially form the optical modulation sections of the optical waveguides 1A and 1B. For example, the first electrodes 2A and 2B can be signal electrodes, and the second electrode 3 can be a ground electrode. Alternatively, the first electrodes 2A and 2B can be ground electrodes, and the second electrode 3 can be a signal electrode.

[0050] The configuration of the optical modulator 100 shown in Fig. 1 will be described in more detail with reference to Fig. 2 and Fig. 3. Fig. 2 is a cross-sectional view taken along line II-II in Fig. 1. In other words, Fig. 2 is a cross-sectional view of the optical modulator 100 cut along a plane in the height direction HD at the positions of the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B (optical modulation sections of the optical waveguides 1A and 1B).

[0051] 2, optical waveguides 1A and 1B have an electro-optic effect. As described above, optical waveguide 1A includes main body portions 11A and 12A and a bent portion 13A (FIGS. 1 and 3). Optical waveguide 1B includes main body portions 11B and 12B and a bent portion 13B (FIGS. 1 and 3).

[0052] In this embodiment, the optical waveguides 1A and 1B are formed in the base layer 16. In this embodiment, the optical waveguides 1A and 1B protrude from the surface of the base layer 16. That is, the optical waveguides 1A and 1B are ridge-type optical waveguides. However, the optical waveguides 1A and 1B do not necessarily have to be ridge-type optical waveguides. The optical waveguides 1A and 1B may have a substantially trapezoidal cross section as shown in FIG. 2, but may also have a cross section of another shape, such as a rectangular shape.

[0053] The optical waveguides 1A and 1B have an electro-optic effect. That is, the optical waveguides 1A and 1B are made of an electro-optic material. The electro-optic material is LiNbO ３ (lithium niobate), LiTaO ３ (lithium tantalate), PLZT (lead lanthanum zirconate titanate), KTN (potassium tantalate niobate), or BaTiO ３ (barium titanate), etc. may be used. Electro-optical polymer (EO polymer) may also be used as the electro-optical material. The base layer 16 may be formed of the same electro-optical material as the optical waveguides 1A and 1B. However, the base layer 16 may be omitted in the optical modulator 100.

[0054] The first electrodes 2A and 2B are disposed on one side of the optical waveguides 1A and 1B in the height direction HD of the optical modulator 100. That is, the centers C2 of the first electrodes 2A and 2B in the height direction HD are shifted to one side of the height direction HD from the centers C1 of the optical waveguides 1A and 1B in the height direction HD of the optical modulator 100. In this embodiment, at the positions of the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B, the entire main body 21A of the first electrode 2A is disposed on one side of the main body 11A of the optical waveguide 1A in the height direction HD. The entire main body 22A of the first electrode 2A is disposed on one side of the main body 12A of the optical waveguide 1A in the height direction HD. The entire main body 21B of the first electrode 2B is disposed on one side of the main body 11B of the optical waveguide 1B in the height direction HD. The entire body 22B of the first electrode 2B is disposed on one side in the height direction HD of the body 12B of the optical waveguide 1B. In this embodiment, the first electrodes 2A and 2B have a substantially rectangular cross section. However, the cross-sectional shape of the first electrodes 2A and 2B is not limited to this.

[0055] In this embodiment, the width w2 of the first electrodes 2A and 2B is larger than the width w1 of the optical waveguides 1A and 1B. Specifically, as shown in FIG. 2 , the width w2 of the main body portion 21B of the first electrode 2B is larger than the width w1 of the main body portion 11B of the optical waveguide 1B. Similarly, the width w2 of the main bodies 21A, 22A, and 22B of the first electrodes 2A and 2B is larger than the width w1 of the main bodies 11A, 12A, and 12B of the optical waveguides 1A and 1B. When the width w2 of the first electrodes 2A and 2B varies along the height direction HD, it refers to its maximum width. When the width w1 of the optical waveguides 1A and 1B varies along the height direction HD, it refers to its maximum width.

[0056] 2, the same electrical signal flows through the main body portions 21A and 22A of the first electrode 2A, so a small separation distance between the main body portions 21A and 22A does not pose any particular problem. On the other hand, a different electrical signal flows through the main body portion 21A of the first electrode 2A than through the main body portion 21B of the first electrode 2B, so a small separation distance between the main body portions 21A and 21B increases the likelihood of crosstalk occurring. Similarly, a different electrical signal flows through the main body portion 22A of the first electrode 2A than through the main body portion 22B of the first electrode 2B, so a small separation distance between the main body portions 22A and 22B increases the likelihood of crosstalk occurring. Therefore, it is preferable that the separation distance between main body portions 21A and 22A on the first electrode 2A is greater than the separation distance between main body portion 21A and main body portion 21B on the first electrode 2B adjacent to this main body portion 21A, and greater than the separation distance between main body portion 22A and main body portion 22B on the first electrode 2B adjacent to this main body portion 22A.

[0057] The optical modulator 100 may further include a support substrate 4. However, the support substrate 4 may be omitted from the optical modulator 100. The support substrate 4 supports the optical waveguides 1A and 1B, the first electrodes 2A and 2B, and the second electrode 3. A semiconductor material may be used as the support substrate 4. As the semiconductor material, a single element semiconductor such as Si (silicon) or Ge (germanium), or a compound semiconductor such as GaAs (gallium arsenide) may be used. The support substrate 4 may be made of SiO ２ , Al ２ O ３ , LaAlO ３ , LaYO ３ , ZnO, HfO ２ , MgO, Y ２ O ３ Alternatively, the support substrate 4 may be made of an electro-optical material, specifically, LiNbO ３ , LiTaO ３ , PLZT, KTN, or BaTiO ３ etc. may also be used.

[0058] The second electrode 3 is disposed on the other side of the optical waveguides 1A and 1B in the height direction HD of the optical modulator 100 (the opposite side to the first electrodes 2A and 2B). That is, the center C3 of the second electrode 3 in the height direction HD of the optical modulator 100 is shifted to the other side in the height direction HD from the center C1 of the optical waveguides 1A and 1B. In the example of this embodiment, the second electrode 3 is stacked on the optical waveguides 1A and 1B on the opposite side in the height direction HD to the first electrodes 2A and 2B.

[0059] More specifically, when the optical modulator 100 is viewed along the height direction HD, as described above, the main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B overlap with the main bodies 11A, 12A, 11B, and 12B of the optical waveguide 1A. Referring to Fig. 2, at the positions of the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B, the second electrode 3 is stacked on the base layer 16 including the optical waveguides 1A and 1B on the opposite side of the main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B. The second electrode 3 is disposed on the opposite side of the first electrodes 2A and 2B with respect to the optical waveguides 1A and 1B so that the main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B are positioned. In this embodiment, the second electrode 3 is arranged to correspond to the entire main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B and the entire main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B. In this case, the main bodies 11A and 12A of the optical waveguide 1A are both sandwiched between the first electrode 2A and the second electrode 3 in the height direction HD. The main bodies 11B and 12B of the optical waveguide 1B are both sandwiched between the first electrode 2B and the second electrode 3 in the height direction HD.

[0060] Fig. 3 is a cross-sectional view taken along line III-III in Fig. 1. In other words, Fig. 3 is a cross-sectional view of the optical modulator 100 cut along a plane in the height direction HD at the positions of the bent portions 13A and 13B of the optical waveguides 1A and 1B.

[0061] When the optical modulator 100 is viewed along the height direction HD, as described above, the bent portions 23A, 23B of the first electrodes 2A, 2B do not overlap with the bent portions 13A, 13B of the optical waveguides 1A, 1B. Referring to FIG. 3 , at the positions of the bent portions 13A, 13B of the optical waveguides 1A, 1B, the bent portion 13A of the optical waveguide 1A is significantly offset in a direction perpendicular to the height direction HD from the bent portion 23A of the first electrode 2A. Similarly, the bent portion 13B of the optical waveguide 1B is significantly offset in a direction perpendicular to the height direction HD from the bent portion 23B of the first electrode 2B. For example, the portion of the bent portion 13A of the optical waveguide 1A that is most offset from the bent portion 23A of the first electrode 2A is separated from the bent portion 23A by a distance equal to or greater than half the width of the bent portion 23A. Similarly, the portion of the bent portion 13B of the optical waveguide 1B that is most deviated from the bent portion 23B of the first electrode 2B is separated from the bent portion 23B by a distance equal to or greater than half the width of the bent portion 23B.

[0062] The second electrode 3 is stacked on the base layer 16 including the optical waveguides 1A and 1B, on the side opposite the bent portions 23A and 23B of the first electrodes 2A and 2B. In this embodiment, the second electrode 3 is disposed to correspond to the bent portions 23A and 23B of the first electrodes 2A and 2B. The second electrode 3 is not provided at a position corresponding to the bent portions 13A and 13B of the optical waveguides 1A and 1B. That is, the second electrode 3 does not overlap portions of the bent portions 13A and 13B of the optical waveguides 1A and 1B that are far removed from the first electrodes 2A and 2B, as viewed along the height direction HD of the optical modulator 100. In this case, the bent portion 13A of the optical waveguide 1A is not sandwiched between the first electrode 2A and the second electrode 3 in the height direction HD. The bent portion 13B of the optical waveguide 1B is also not sandwiched between the first electrode 2B and the second electrode 3 in the height direction HD. There are no other electrodes in the vicinity of the bent portions 13A and 13B of the optical waveguides 1A and 1B.

[0063] In this embodiment, the second electrode 3 is provided in common to the two optical waveguides 1A and 1B. However, the second electrode 3 may be provided for each of the optical waveguides 1A and 1B.

[0064] [Effect] It is desirable for an optical modulator to be usable over a wide band, including high-frequency bands of 10 GHz or more and 1.5 THz or less, and it is preferable to match the speed of an electrical signal with the speed of a light wave. To achieve this, rather than making the length of the optical waveguide and the length of the control electrode the same, by making one length longer than the other, the difference in propagation distance caused by the speed difference between the electrical signal and the light wave can be reduced, thereby compensating for the mismatch between the speeds of the electrical signal and the light wave. In the optical modulator 100 according to this embodiment, the first electrodes 2A and 2B, which apply an electric field to the optical waveguides 1A and 1B together with the second electrode 3, have one end 21Aa, 21Ba and the other end 22Aa, 22Ba of the first electrodes 2A and 2B overlap the optical waveguides 1A and 1B when viewed along the height direction HD. The length of the first electrodes 2A and 2B is made different from the length of the optical waveguides 1A and 1B from the positions of the one ends 21Aa and 21Ba of the first electrodes 2A and 2B to the positions of the other ends 22Aa and 22Ba. This makes it possible to compensate for the mismatch between the speed of the electrical signal and the speed of the light wave. For example, if the effective refractive index of the electrical signal in the optical modulator is smaller than the effective refractive index of the light wave, it is preferable that the length of the first electrodes 2A and 2B be longer than the length of the optical waveguides 1A and 1B from the positions of the one ends 21Aa and 21Ba of the first electrodes 2A and 2B to the positions of the other ends 22Aa and 22Ba. For example, if the effective refractive index of the electrical signal in the optical modulator is larger than the effective refractive index of the light wave, the length of the optical waveguides 1A and 1B may be made longer than the length of the first electrodes 2A and 2B.

[0065] In the optical modulator 100 according to this embodiment, as viewed along the height direction HD, the bent portion 23A connecting the two main bodies 21A and 22A in the first electrode 2A is disposed inside the bent portion 13A connecting the two main bodies 11A and 12A in the optical waveguide 1A, and the length of the bent portion 23A in the first electrode 2A is shorter than the length of the bent portion 13A in the optical waveguide 1A. Also, the bent portion 23B connecting the two main bodies 21B and 22B in the first electrode 2B is disposed outside the bent portion 13B connecting the two main bodies 11B and 12B in the optical waveguide 1B, and the length of the bent portion 23B in the first electrode 2B is longer than the length of the bent portion 13B in the optical waveguide 1B. In this case, when viewed along the height direction HD, the bent portions 23A and 23B of the first electrodes 2A and 2B are spaced apart from the bent portions 13A and 13B of the optical waveguides 1A and 1B. Therefore, it is difficult to apply an electric field to the optical waveguides 1A and 1B at the positions of the bent portions 13A and 13B of the optical waveguides 1A and 1B. The bent portions 13A and 13B of the optical waveguides 1A and 1B are not intended to be subjected to application of an electric field by the first electrodes 2A and 2B. Therefore, it is preferable that an electric field not be applied to the bent portions 13A and 13B of the optical waveguides 1A and 1B from unrelated electrodes. For example, it is preferable that an electric field not be applied to the bent portion 13A of the optical waveguide 1A from the unrelated first electrode 2B.

[0066] In contrast, in the optical modulator 100 according to this embodiment, the second electrode 3 is disposed on the opposite side of the optical waveguides 1A and 1B from the first electrodes 2A and 2B in the height direction HD of the optical modulator 100. That is, the second electrode 3 is disposed on a different surface of the optical waveguides 1A and 1B from the first electrodes 2A and 2B. This allows for relatively flexible adjustment of the location of the second electrode 3, and allows other electrodes to be disposed at the bent portions 13A and 13B of the optical waveguides 1A and 1B so as to prevent unnecessary application of an electric field from the other electrodes. For example, an electric field is not applied to the bent portion 13A of the optical waveguide 1A from the unrelated first electrode 2B, and an electric field is not applied between the bent portion 13A and the second electrode 3. This prevents unnecessary application of an electric field to the optical waveguides 1A and 1B, thereby suppressing light absorption. Since unnecessary electric fields are less likely to be applied to the optical waveguides 1A and 1B, noise can be reduced.

[0067] Furthermore, in the optical modulator 100 according to this embodiment, the length between both ends of the optical waveguide 1A is the same as the length of the optical waveguide 1B when viewed along the height direction HD. That is, when the optical modulator 100 is viewed along the height direction HD, the lengths between both ends of the two optical waveguides 1A and 1B are equal to each other. In this case, the time it takes for light to pass from one end to the other end of the two optical waveguides 1A and 1B can be made the same. This simplifies the design and timing of application of electrical signals from the first electrodes 2A and 2B.

[0068] In the optical modulator 100 according to this embodiment, an input port 1 in for inputting light into the optical waveguides 1A and 1B and an output port 1 out for outputting light from the optical waveguides 1A and 1B are both arranged on the same end face 101 of the optical modulator 100. An optical fiber and an optical coupler are coupled to the input port 1 in and the output port 1 out. When the input port 1 in and the output port 1 out are provided on the same end face 101 of the optical modulator 100, the process of coupling an optical fiber and an optical coupler to the input port 1 in and the output port 1 out can be performed in one place. This allows for a reduction in processing time and the number of components.

[0069] In this embodiment, the width w2 of the first electrodes 2A and 2B is larger than the width w1 of the optical waveguides 1A and 1B. In this case, an electric field can be applied uniformly to the optical waveguides 1A and 1B, improving the quality of the modulated signal.

[0070] Second Embodiment The configuration of an optical modulator 100A according to the second embodiment will be described with reference to Fig. 4. Fig. 4 is a schematic diagram showing the configuration of the optical modulator 100A according to the second embodiment, and is a cross-sectional view corresponding to Fig. 3. In other words, Fig. 4 is a cross-sectional view of the optical modulator 100A cut along a plane in the height direction HD at the positions of the bent portions 13A and 13B of the optical waveguides 1A and 1B. The optical modulator 100A differs from the optical modulator 100 according to the first embodiment in the configuration of the second electrode 3.

[0071] 4 , in the optical modulator 100A, at the positions of the bent portions 13A and 13B of the optical waveguides 1A and 1B, the second electrode 3 is stacked on the base layer 16 including the optical waveguides 1A and 1B on the side opposite the bent portions 23A and 23B of the first electrodes 2A and 2B. In this embodiment, the second electrode 3 is disposed so as to correspond to the bent portions 23A and 23B of the first electrodes 2A and 2B and the entire bent portions 13A and 13B of the optical waveguides 1A and 1B. Therefore, the second electrode 3 is also provided at positions corresponding to the bent portions 13A and 13B of the optical waveguides 1A and 1B. In this case, the second electrode 3 overlaps the bent portions 13A and 13B of the optical waveguides 1A and 1B that are away from the first electrodes 2A and 2B when viewed along the height direction HD of the optical modulator 100. However, the bent portion 13A of the optical waveguide 1A is not sandwiched in the height direction HD between the first electrode 2A and the second electrode 3. The bent portion 13B of the optical waveguide 1B is also not sandwiched in the height direction HD between the first electrode 2B and the second electrode 3.

[0072] In the optical modulator 100A according to this embodiment, the bent portions 23A and 23B of the first electrodes 2A and 2B are separated from the bent portions 13A and 13B of the optical waveguide 1A, and therefore an electric field is less likely to be applied by the first electrodes 2A and 2B and the second electrode 3 to the portions of the optical waveguide 1A separated from the first electrodes 2A and 2B (parts of the bent portions 13A and 13B), and light is less likely to be absorbed. Therefore, as in the first embodiment, an unnecessary electric field is less likely to be applied to the optical waveguides 1A and 1B, and light absorption can be reduced.

[0073] Third Embodiment The configuration of an optical modulator 100B according to the third embodiment will be described with reference to Fig. 5. Fig. 5 is a schematic diagram showing the configuration of the optical modulator 100B according to the third embodiment, and is a cross-sectional view corresponding to Fig. 2. In other words, Fig. 5 is a cross-sectional view of the optical modulator 100B cut along a plane in the height direction HD at the positions of the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B. The optical modulator 100B differs from the optical modulator 100 according to the first embodiment in the configuration of the first electrodes 2A and 2B.

[0074] As shown in Figure 5, in the optical modulator 100B, the width w2 of the first electrodes 2A and 2B is smaller than the width w1 of the optical waveguides 1A and 1B. When the width w2 of the first electrodes 2A and 2B is smaller than the width w1 of the optical waveguides 1A and 1B, the electric field can be concentrated and applied to the optical waveguides 1A and 1B. This allows the drive voltage applied to the first electrodes to be reduced. The configuration of the first electrodes 2A and 2B in this embodiment can also be applied to the optical modulator 100A according to the second embodiment.

[0075] Fourth Embodiment The configuration of an optical modulator 100C according to a fourth embodiment will be described with reference to FIGS. 6 and 7. FIG. 6 is a schematic diagram showing the configuration of the optical modulator 100C according to the fourth embodiment, and is a plan view corresponding to FIG. 1. FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. 6. In other words, FIG. 7 is a cross-sectional view of the optical modulator 100C cut along a plane in the height direction HD at the positions of the bent portions 13A and 13B of the optical waveguides 1A and 1B. The optical modulator 100C differs from the optical modulator 100 according to the first embodiment in the configurations of the optical waveguide 1A and the first electrode 2A.

[0076] As shown in FIG. 6 , when the optical modulator 100C is viewed along the height direction HD, the bent portion 23A of the first electrode 2A has a curved shape that protrudes from the main body portions 21A and 22A. The bent portion 23A of the first electrode 2A is disposed outside the bent portion 13A of the optical waveguide 1A and does not substantially overlap with the optical waveguide 1A. However, the bent portion 23A of the first electrode 2A may overlap with the optical waveguide 1A near the main body portions 21A and 22A. In this case, as shown in FIG. 7 , the bent portion 13A of the optical waveguide 1A has a portion that is outside the bent portion 23A of the first electrode 2A.

[0077] The length of the first electrode 2A, as viewed along the height direction HD, is greater than the length of the optical waveguide 1A from the position of one end 21Aa of the first electrode 2A to the position of the other end 22Aa. That is, the length of the first electrode 2A is different from the length of the optical waveguide 1A. Therefore, the optical modulator 100C according to the fourth embodiment achieves the same effects as the optical modulator 100 according to the first embodiment. The configurations of the optical waveguide 1A and the first electrode 2A according to this embodiment can also be applied to the optical modulators 100A and 100B according to the second and third embodiments.

[0078] Fifth Embodiment The configuration of an optical modulator 100D according to a fifth embodiment will be described with reference to Fig. 8 and Fig. 9. Fig. 8 is a plan view of the optical modulator 100D according to the fifth embodiment. Referring to Fig. 8, in this embodiment, the optical modulator 100D is a so-called Mach-Zehnder type optical modulator. The basic configuration of the optical modulator 100D is the same as that of the optical modulator 100 according to the first embodiment.

[0079] The optical modulator 100D includes two optical waveguides 1AA and 1BA, two first electrodes 2AA and 2BA, and a second electrode 3A. Fig. 8 shows the optical waveguides 1AA and 1BA, the first electrodes 2AA and 2BA, and the second electrode 3A when projected onto a plane perpendicular to the height direction HD of the optical modulator 100D. In Fig. 8, the optical waveguides 1AA and 1BA are indicated by dotted lines, and the second electrode 3A is indicated by a dashed line.

[0080] In the optical modulator 100D, two optical waveguides 1AA, 1BA are arranged in parallel when viewed along the height direction HD. Two first electrodes 2AA, 2BA are arranged in parallel when viewed along the height direction HD. The first electrode 2AA overlaps the optical waveguide 1AA in the range from one end 2AAa to the other end 2AAb. The first electrode 2BA overlaps the optical waveguide 1BA in the range from one end 2BAa to the other end 2BAb.

[0081] In this embodiment, the optical waveguide 1AA is linear, and the first electrode 2AA is curved. Because the optical waveguide 1AA is linear and the first electrode 2AA is curved, the length of the first electrode 2AA is greater than the length of the optical waveguide 1AA from one end 2AAa to the other end 2AAb of the first electrode 2AA. In other words, the length of the first electrode 2AA is different from the length of the optical waveguide 1AA. The degree of curvature of the first electrode 2AA is relatively small. However, the degree of curvature of the first electrode 2AA may be large, and the bending of the first electrode 2AA may cause a portion of the optical waveguide 1AA to deviate from the first electrode 2AA.

[0082] On the other hand, the first electrode 2BA is linear, while the optical waveguide 1BA is curved. Because the first electrode 2BA is linear and the optical waveguide 1BA is curved, the length of the first electrode 2BA is shorter than the length of the optical waveguide 1BA from the position of one end 2BAa of the first electrode 2BA to the position of the other end 2BAb. In other words, the length of the first electrode 2BA is different from the length of the optical waveguide 1BA. The degree of curvature of the optical waveguide 1BA is relatively small. However, the degree of curvature of the optical waveguide 1BA may be large, and the bending of the optical waveguide 1BA may cause a portion of the optical waveguide 1BA to deviate from the first electrode 2BA.

[0083] The second electrode 3A is disposed so that at least a portion of it overlaps with the optical waveguides 1AA and 1BA and the first electrodes 2AA and 2BA when viewed along the height direction HD. For example, the second electrode 3A overlaps with the optical waveguides 1AA and 1BA and the first electrodes 2AA and 2BA when viewed along the height direction HD.

[0084] Fig. 9 is a cross-sectional view taken along line IX-IX in Fig. 8. In other words, Fig. 9 is a cross-sectional view of the optical modulator 100D cut along a plane in the height direction HD at the positions of the optical waveguides 1AA and 1BA.

[0085] 9 , in this embodiment, the optical waveguides 1AA and 1BA are formed in the base layer 16A and protrude from the surface of the base layer 16A. The first electrodes 2AA and 2BA are disposed on one side of the optical waveguides 1AA and 1BA in the height direction HD of the optical modulator 100D. The second electrode 3A is disposed on the other side of the optical waveguides 1AA and 1BA in the height direction HD of the optical modulator 100D (the side opposite the first electrodes 2AA and 2BA). In other words, the second electrode 3A is disposed on a different surface of the optical waveguides 1AA and 1BA from the first electrodes 2AA and 2BA.

[0086] In the optical modulator 100D according to this embodiment, the length of the first electrodes 2AA, 2BA is made different from the length of the optical waveguide 1AA, 1BA from the position of one end 2AAa, 2BAa of the first electrodes 2AA, 2BA to the position of the other end 2AAb, 2BAb, which makes it possible to compensate for the mismatch between the speed of the electrical signal and the speed of the light wave, as in the first embodiment.

[0087] Furthermore, in the optical modulator 100D according to this embodiment, the second electrode 3A is disposed on the opposite side of the optical waveguides 1AA and 1BA from the first electrodes 2AA and 2BA in the height direction HD of the optical modulator 100D. That is, the second electrode 3A is disposed on a different surface of the optical waveguides 1AA and 1BA from the first electrodes 2AA and 2BA. As a result, as in the first embodiment, even if portions of the optical waveguides 1AA and 1BA are deviated from the first electrodes 2AA and 2BA when viewed along the height direction HD of the optical modulator 100D, unnecessary electric fields are less likely to be applied to the optical waveguides 1AA and 1BA, and light absorption can be reduced.

[0088] However, in the case of an optical modulator in which multiple electrodes including a first electrode and a second electrode are arranged on the same plane and an optical waveguide is arranged between the first electrode and the second electrode as viewed along the height direction, the distance between the first electrode and the optical waveguide and the distance between the second electrode and the optical waveguide are relatively narrow. In this case, bending the electrode relative to the optical waveguide or bending the optical waveguide relative to the electrode will result in contact between the electrode and the optical waveguide. Therefore, in such an optical modulator, it is difficult to bend the electrode or the optical waveguide.

[0089] The configurations of the optical waveguides 1AA, 1BA and the first electrodes 2AA, 2BA in this embodiment can be applied to the optical modulator 100 according to the first embodiment. For example, the configurations of the optical waveguides 1AA, 1BA in the fifth embodiment can be applied to the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B in the first embodiment. The configurations of the first electrodes 2AA, 2BA in the fifth embodiment can be applied to the main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B in the first embodiment. Furthermore, the configurations of the optical waveguides 1AA, 1BA and the first electrodes 2AA, 2BA in this embodiment may be applied to the optical modulators 100A, 100B, and 100C according to the second to fourth embodiments.

[0090] [Modification of Optical Modulator 100D According to Fifth Embodiment] Figures 10 and 11 are schematic plan views showing the configuration of a modification of the optical modulator 100D according to the fifth embodiment, corresponding to Figure 8. In the optical modulator 100D shown in Figure 10, when viewed along the height direction HD, both optical waveguides 1AA and 1BA are linear, and both first electrodes 2AA and 2BA are curved. In the optical modulator 100D shown in Figure 11, when viewed along the height direction HD, both first electrodes 2AA and 2BA are linear, and both optical waveguides 1AA and 1BA are curved to the same degree. In both of the optical modulators 100D shown in Figures 10 and 11, the length of the optical waveguide 1AA between the branch point P1 and the junction point P2, which correspond to both ends of the optical waveguide 1AA, is the same as the length of the optical waveguide 1BA between the branch point P1 and the junction point P2, which correspond to both ends of the optical waveguide 1BA. Therefore, the time it takes for light to pass from one end to the other end of the two optical waveguides 1AA and 1BA can be made the same.

[0091] Sixth Embodiment The configuration of an optical modulator 100E according to the sixth embodiment will be described with reference to FIGS. 12A to 12D and 13A to 13D. FIGS. 12A to 12D are schematic cross-sectional views showing the configuration of the optical modulator 100E according to the sixth embodiment, corresponding to FIG. 9. In other words, FIGS. 12A to 12D are cross-sectional views of a modified version of the optical modulator 100D according to the fifth embodiment, cut along a plane in the height direction HD at the positions of the optical waveguides 1AA and 1BA. The optical modulator 100E differs from the optical modulator 100D according to the fifth embodiment in that it includes low-dielectric-constant layers 5A, 5B, and 5C.

[0092] The optical modulator 100E shown in FIG. 12A includes optical waveguides 1AA and 1BA, first electrodes 2AA and 2BA, a second electrode 3A, a low-dielectric-constant layer 5A, and a support substrate 4A. The low-dielectric-constant layer 5A has a refractive index smaller than that of the optical waveguides 1AA and 1BA. The low-dielectric-constant layer 5A is uniformly disposed in the height direction HD between a base layer 16A including the optical waveguides 1AA and 1BA and the second electrode 3A. By providing the low-dielectric-constant layer 5A on the second electrode 3A side of the optical waveguides 1AA and 1BA in this manner, the effective refractive index of the electrical signal can be reduced compared to the optical modulator 100D ( FIG. 9 ) that does not include a low-dielectric-constant layer.

[0093] The optical modulator 100E shown in FIG. 12B includes optical waveguides 1AA and 1BA, first electrodes 2AA and 2BA, a second electrode 3A, a low-dielectric layer 5B, and a support substrate 4A. The low-dielectric layer 5B has a refractive index smaller than that of the optical waveguides 1AA and 1BA. The low-dielectric layer 5B is uniformly disposed in the height direction HD between the base layer 16A including the optical waveguides 1AA and 1BA and the first electrodes 2AA and 2BA. Typically, the first electrodes 2AA and 2BA are often used as signal electrodes. In this case, by arranging the low-dielectric layer 5B on the first electrode 2AA and 2BA side of the optical waveguides 1AA and 1BA, the effective refractive index of the electrical signal can be reduced compared to the optical modulator 100E shown in FIG. 12A. This is because filling the periphery of the signal electrode with the low dielectric layer 5B is more effective in reducing the effective refractive index of the electrical signal than providing the low dielectric layer 5A on the ground electrode side.

[0094] The optical modulator 100E shown in Figure 12C includes optical waveguides 1AA and 1BA, first electrodes 2AA and 2BA, a second electrode 3A, a low-dielectric-constant layer 5A, a low-dielectric-constant layer 5B, and a support substrate 4A. In the optical modulator 100E shown in Figure 12C, low-dielectric-constant layers 5A and 5B are provided on both the second electrode 3A side and the first electrode 2AA and 2BA side of the optical waveguides 1AA and 1BA. In this case, the effective refractive index of the electrical signal can be made smaller than in the optical modulator 100E shown in Figures 12A and 12B.

[0095] In the optical modulator 100E shown in Fig. 12D, the optical waveguides 1AA and 1BA are not formed in the base layer but are arranged independently. That is, in the optical modulator 100E shown in Fig. 12D, the optical waveguides 1AA and 1BA are surrounded by a low-dielectric-constant layer 5C. In this case, the effective refractive index of the electrical signal can be made smaller than in the optical modulator 100E shown in Figs. 12A to 12C.

[0096] In other words, the effective refractive index of the electric signal is largest in the optical modulator 100D shown in Fig. 9, and decreases in the order of the optical modulators 100E shown in Fig. 12A, 12B, 12C, and 12D. Furthermore, as the effective refractive index of the electric signal decreases, the efficiency of application of the electric field decreases.

[0097] 13A to 13D are schematic diagrams showing another configuration of the optical modulator 100E according to the sixth embodiment, and are cross-sectional views corresponding to FIG. 5. In other words, FIGS. 13A to 13D are cross-sectional views of the optical modulator 100E according to the third embodiment, which is a modified version of the optical modulator 100B according to the third embodiment, cut along a plane in the height direction HD at the positions of the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B. The optical modulator 100E shown in FIGS. 13A to 13D, like the optical modulator 100E shown in FIGS. 12A to 12D, differs from the optical modulator 100B according to the third embodiment in that it includes low-dielectric-constant layers 5A, 5B, and 5C.

[0098] 13A includes optical waveguides 1A and 1B, first electrodes 2A and 2B, a second electrode 3, a low-dielectric layer 5A, and a support substrate 4. In the optical modulator 100E shown in Fig. 13A, the low-dielectric layer 5A is provided on the second electrode 3 side of the optical waveguides 1A and 1B. In this case, the effective refractive index of the electrical signal can be made smaller than in the optical modulator 100B (Fig. 5) that does not include a low-dielectric layer.

[0099] The optical modulator 100E shown in Fig. 13B includes optical waveguides 1A and 1B, first electrodes 2A and 2B, a second electrode 3, a low-dielectric layer 5B, and a support substrate 4. In the optical modulator 100E shown in Fig. 13B, the low-dielectric layer 5B is provided on the first electrode 2A and 2B side of the optical waveguides 1A and 1B. In this case, the effective refractive index of the electrical signal can be made smaller than in the optical modulator 100E shown in Fig. 13A.

[0100] The optical modulator 100E shown in Fig. 13C includes optical waveguides 1A and 1B, first electrodes 2A and 2B, a second electrode 3, a low-dielectric-constant layer 5A, a low-dielectric-constant layer 5B, and a support substrate 4. In the optical modulator 100E shown in Fig. 13C, the low-dielectric-constant layers 5A and 5B are provided on both the second electrode 3 side and the first electrode 2A and 2B side of the optical waveguides 1A and 1B. In this case, the effective refractive index of the electrical signal can be made smaller than in the optical modulator 100E shown in Figs. 13A and 13B.

[0101] In the optical modulator 100E shown in Fig. 13D, the optical waveguides 1A and 1B are not formed in the base layer but are arranged independently. That is, in the optical modulator 100E shown in Fig. 13D, a low-dielectric-constant layer 5C is filled around the optical waveguides 1A and 1B. In this case, the effective refractive index of the electrical signal can be made smaller than in the optical modulator 100E shown in Figs. 13A to 13C.

[0102] In other words, the effective refractive index of the electric signal is largest in the optical modulator 100B shown in Fig. 5 and decreases in the order of the optical modulators 100E shown in Fig. 13A, 13B, 13C, and 13D. Furthermore, as the effective refractive index of the electric signal decreases, the efficiency of application of the electric field decreases.

[0103] 13A, 13B, 13C, and 13D, the width of the first electrodes 2A and 2B is narrower than the width of the optical waveguides 1A and 1B. It is advantageous for the width of the first electrodes 2A and 2B to be wider than the width of the optical waveguides 1A and 1B, because the electric field is concentrated not only in the optical waveguides but also in the low dielectric layer.

[0104] In the optical modulator 100E of this embodiment, by adjusting the low-dielectric-constant layers 5A, 5B, and 5C, for example, the effective refractive index of the electrical signal can be made smaller than the effective refractive index of the light wave. In this specification, the effective refractive index of the electrical signal refers to the effective refractive index of the electrical signal passing between the first electrode and the second electrode. The refractive index of the light wave refers to the effective refractive index of the light wave passing through the optical waveguide. If the effective refractive index of the electrical signal is smaller than the effective refractive index of the light wave, the loss of the electrical signal can be reduced.

[0105] When the effective refractive index of the electrical signal is smaller than that of the optical wave, the length of the first electrode is preferably longer than that of the optical waveguide. By making the length of the first electrode longer than that of the optical waveguide, the speed difference between the electrical signal and the optical wave can be matched. As a result, the bandwidth of the optical modulator can be increased, enabling it to support high-frequency bands of 10 GHz or more and 1.5 THz or less.

[0106] More specifically, when the effective refractive index of the electrical signal is smaller than the effective refractive index of the light wave, the speed of the electrical signal is faster than the speed of the light wave. In this case, if the length of the first electrode is the same as the length of the optical waveguide, i.e., the length of the optical waveguide from one end of the first electrode to the other end, the time it takes for the electrical signal to pass through the first electrode is shorter than the time it takes for the light wave to pass through the optical waveguide. Therefore, if the length of the first electrode is made longer than the length of the optical waveguide so that the time it takes for the electrical signal to pass through the first electrode approaches the time it takes for the light wave to pass through the optical waveguide, the speed difference between the electrical signal and the light wave can be matched, enabling higher-speed optical modulation. For example, the optimal difference in length between the first electrode and the optical waveguide can be calculated using the following formula: (speed of the electrical signal - speed of the light wave) x (length of the optical waveguide from one end of the first electrode to the other end) / (speed of the electrical signal).

[0107] Furthermore, in the optical modulator 100E of this embodiment, by adjusting the low-dielectric-constant layers 5A, 5B, and 5C, it is possible to make the effective refractive index of the electrical signal greater than the effective refractive index of the optical wave. If the effective refractive index of the electrical signal is greater than the effective refractive index of the optical wave, the strength of the electric field applied to the optical waveguide becomes stronger. This allows the drive voltage applied to the first electrode to be reduced.

[0108] When the effective refractive index of the electrical signal is greater than that of the optical wave, even if low-dielectric-constant layers 5A, 5B, and 5C are interposed between the first electrode and the optical waveguide and / or between the second electrode and the optical waveguide, the strength of the electric field applied to the optical waveguide can be increased by reducing the sandwiching amount of the low-dielectric-constant layers 5A, 5B, and 5C. In this case, the sandwiching amount of the low-dielectric-constant layers 5A, 5B, and 5C is smaller than that of the electro-optic material, and the effective refractive index of the electrical signal depends mainly on the dielectric constant of the electro-optic material. As a result, the effective refractive index of the electrical signal is greater than that of the optical wave, making it difficult to widen the bandwidth of the optical modulator. Therefore, it is sufficient to make the length of the optical waveguide longer than the length of the first electrode relative to the velocity difference between the generated electrical signal and the optical wave. This allows the velocity difference to be compensated for, enabling the optical modulator to be used over a wide bandwidth, supporting high-frequency bands from 10 GHz to 1.5 THz.

[0109] More specifically, when the effective refractive index of the electrical signal is greater than the effective refractive index of the light wave, the speed of the electrical signal is slower than the speed of the light wave. In this case, if the length of the first electrode is the same as the length of the optical waveguide, i.e., the length of the optical waveguide from one end of the first electrode to the other end, the time it takes for the electrical signal to pass through the first electrode will be longer than the time it takes for the light wave to pass through the optical waveguide. Therefore, if the length of the first electrode is made shorter than the length of the optical waveguide so that the time it takes for the electrical signal to pass through the first electrode approaches the time it takes for the light wave to pass through the optical waveguide, the speed difference between the electrical signal and the light wave can be matched, enabling higher-speed optical modulation. For example, the optimal difference in length between the first electrode and the optical waveguide can be calculated using the following formula: (speed of light wave - speed of electrical signal) x (length of first electrode) / (speed of light wave).

[0110] In the optical modulator 100E, the effective refractive index of the electrical signal and the effective refractive index of the light wave can be determined by the following method. The optical modulator 100E is cut at a midpoint between both ends of the optical waveguide to which an electric field is applied by the first electrode. The cross section of the optical modulator 100E formed by the cutting is observed using an SEM, and the width and film thickness of each element, including the optical waveguide, the low-dielectric layer, the first electrode, and the second electrode, are measured. The refractive index of each element is measured based on the measurement results. Methods that can be used to measure the refractive index include the critical angle method, the V-block method, and the minimum deviation method. Then, the effective refractive index at 10 GHz is calculated using analysis software. The effective refractive index is analyzed using the finite element method, and COMSOL multiphysics® (manufactured by COMSOL) and its wave optics module can be used as analysis software.

[0111] Seventh Embodiment The configuration of an optical modulator 100F according to the seventh embodiment will be described with reference to Fig. 14. Fig. 14 is a schematic diagram showing the configuration of the optical modulator 100F according to the seventh embodiment, and is a plan view corresponding to Fig. 8. The optical modulator 100F differs from the optical modulator 100D according to the fifth embodiment in the configuration of the first electrodes 2AA, 2BA.

[0112] As shown in Fig. 14, in the optical modulator 100F, the first electrodes 2AA and 2BA overlap with the optical waveguides 1AA and 1BA when viewed along the height direction HD. The optical waveguides 1AA and 1BA have a linear shape. Meanwhile, the first electrodes 2AA and 2BA include a meandering portion. In the example shown in Fig. 14, the first electrodes 2AA and 2BA meander throughout the entire optical waveguides 1AA and 1BA.

[0113] The path of the first electrodes 2AA, 2BA in the meandering portion has a periodic wave shape. This makes it easier for the strength of the electric field applied to the optical waveguides 1AA, 1BA to be uniform along the optical waveguides 1AA, 1BA, improving the signal quality of the optical modulation. It also reduces the variation in the effective refractive index depending on the location, preventing the narrowing of the bandwidth due to velocity mismatch. Furthermore, the length of the first electrodes 2AA, 2BA can be made longer than the length of the optical waveguides 1AA, 1BA. This makes it easier to compensate for the mismatch between the speed of the electrical signal and the speed of the light wave.

[0114] The configuration of the first electrodes 2AA and 2BA in this embodiment can be applied to the optical modulator 100 according to the first embodiment. For example, the configuration of the first electrodes 2AA and 2BA in the seventh embodiment can be applied to the main bodies 21A, 22A, 21B, and 22B of the first electrodes 2A and 2B according to the first embodiment. Furthermore, the configuration of the first electrodes 2AA and 2BA in this embodiment may be applied to the optical modulators according to the above embodiments.

[0115] Eighth Embodiment The configuration of an optical modulator 100G according to the eighth embodiment will be described with reference to Fig. 15. Fig. 15 is a schematic diagram showing the configuration of the optical modulator 100G according to the eighth embodiment, and is a plan view corresponding to Fig. 8. The optical modulator 100G differs from the optical modulator 100D according to the fifth embodiment in the configuration of the optical waveguides 1AA and 1BA.

[0116] As shown in Fig. 15 , in the optical modulator 100G, the first electrodes 2AA and 2BA overlap with the optical waveguides 1AA and 1BA when viewed along the height direction HD. The first electrodes 2AA and 2BA have a linear shape. Meanwhile, the optical waveguides 1AA and 1BA include a meandering portion. In the example shown in Fig. 15 , the optical waveguides 1AA and 1BA meander over the entire first electrodes 2AA and 2BA.

[0117] The paths of the optical waveguides 1AA and 1BA in the meandering sections have a periodic wave shape. This makes it easier for the strength of the electric field applied to the optical waveguides 1AA and 1BA to be uniform along the optical waveguides 1AA and 1BA, improving the signal quality of optical modulation. This also reduces variations in the effective refractive index depending on the location, preventing narrowing of the bandwidth due to velocity mismatch. Furthermore, the length of the optical waveguides 1AA and 1BA can be made longer than the length of the first electrodes 2AA and 2BA. This makes it easier to compensate for mismatches between the speed of the electrical signal and the speed of the light wave.

[0118] The configuration of the optical waveguides 1AA and 1BA in this embodiment can be applied to the optical modulator 100 according to the first embodiment. For example, the configuration of the optical waveguides 1AA and 1BA in the eighth embodiment can be applied to the main bodies 11A, 12A, 11B, and 12B of the optical waveguides 1A and 1B according to the first embodiment. Furthermore, the configuration of the optical waveguides 1AA and 1BA in this embodiment may be applied to the optical modulators according to the above embodiments.

[0119] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0120] The configuration of the optical modulator according to the present disclosure is summarized below.

[0121] <1> An optical modulator comprising: an optical waveguide having an electro-optic effect; a first electrode disposed on one side of the optical waveguide in a height direction of the optical modulator, the first electrode having one end and the other end overlapping at least the optical waveguide when viewed along the height direction, and extending from the one end to the other end; and a second electrode disposed on the other side of the optical waveguide in the height direction, the second electrode forming a potential difference between itself and the first electrode and applying an electric field to the optical waveguide together with the first electrode, wherein a length of the first electrode when viewed along the height direction is different from a length of the optical waveguide from a position of the one end to a position of the other end.

[0122] <2> The optical modulator according to <1>, comprising: two optical waveguides connected to each other at both ends; the first electrodes provided corresponding to the optical waveguides, respectively; and the second electrodes provided corresponding to the optical waveguides, respectively.

[0123] <3> The optical modulator according to <2>, wherein the optical waveguide includes two waveguide main bodies and a waveguide bending portion connecting one ends of the two waveguide main bodies to each other, the first electrode includes two electrode main bodies respectively overlapping the two waveguide main bodies when viewed along the height direction, and an electrode bending portion connecting one ends of the two electrode main bodies to each other and disposed inside or outside the waveguide bending portion when viewed along the height direction, and a length of the electrode bending portion is different from a length of the waveguide bending portion.

[0124] <4> The optical modulator according to <2> or <3>, wherein, when viewed along the height direction, the lengths between the two ends of the two optical waveguides are equal to each other.

[0125] <5> The optical modulator according to any one of <1> to <4>, wherein an effective refractive index of an electrical signal passing between the first electrode and the second electrode is smaller than an effective refractive index of a light wave passing through the optical waveguide.

[0126] <6> The optical modulator according to <5>, wherein the length of the first electrode is greater than the length of the optical waveguide.

[0127] <7> The optical modulator according to any one of <1> to <6>, wherein the first electrode includes a meandering portion.

[0128] <8> The optical modulator according to any one of <1> to <4>, wherein an effective refractive index of an electrical signal passing between the first electrode and the second electrode is greater than an effective refractive index of a light wave passing through the optical waveguide.

[0129] <9> The optical modulator according to <8>, wherein the length of the optical waveguide is greater than the length of the first electrode.

[0130] <10> The optical modulator according to any one of <1> to <4>, <8> and <9>, wherein the optical waveguide includes a meandering portion.

[0131] <11> The optical modulator according to any one of <1> to <10>, further comprising: an input port arranged on an end face of the optical modulator along the height direction, for inputting light into the optical waveguide; and an output port arranged on the end face, for outputting light from the optical waveguide.

[0132] 100, 100A, 100B, 100C, 100D, 100E, 100F, 100G: optical modulator 1A, 1B: optical waveguide 11A, 12A, 11B, 12B: main body portion 13A, 13B: bent portion 2A, 2B: first electrode 21A, 22A, 21B, 22B: main body portion 23A, 23B: bent portion 21Aa: one end portion 22Aa: other end portion 21Ba: one end portion 22Ba: other end portion 3: second electrode 5A, 5B, 5C: low dielectric layer HD: height direction

Claims

1. 1. An optical modulator comprising: Two optical waveguides having an electro-optic effect and connected to each other at both ends; a first electrode provided corresponding to each of the optical waveguides, the first electrode being disposed on one side of the optical waveguide in a height direction of the optical modulator, the first electrode having one end and another end overlapping at least the optical waveguide when viewed along the height direction, and extending from the one end to the other end; a second electrode provided corresponding to each of the optical waveguides, the second electrode being disposed on the other side of the optical waveguide in the height direction, forming a potential difference between the second electrode and the first electrode and applying an electric field to the optical waveguide together with the first electrode; When viewed along the height direction, a length of the first electrode is different from a length of the optical waveguide from a position of the one end to a position of the other end, the optical waveguide includes two waveguide main bodies and a waveguide bend portion connecting one ends of the two waveguide main bodies to each other, the first electrode includes two electrode body portions overlapping the two waveguide body portions, respectively, when viewed along the height direction, and an electrode bending portion connecting one ends of the two electrode body portions to each other and disposed inside or outside the waveguide bending portion when viewed along the height direction, An optical modulator, wherein the length of the electrode bend is different from the length of the waveguide bend.

2. 2. The optical modulator according to claim 1, An optical modulator, wherein the lengths between the two ends of the two optical waveguides are equal to each other when viewed along the height direction.

3. 2. The optical modulator according to claim 1, An optical modulator, wherein an effective refractive index of an electrical signal passing between the first electrode and the second electrode is smaller than an effective refractive index of an optical wave passing through the optical waveguide.

4. 4. The optical modulator according to claim 3, An optical modulator, wherein the length of the first electrode is greater than the length of the optical waveguide.

5. 2. The optical modulator according to claim 1, The first electrode includes a meandering portion.

6. 2. The optical modulator according to claim 1, An optical modulator, wherein an effective refractive index of an electrical signal passing between the first electrode and the second electrode is greater than an effective refractive index of an optical wave passing through the optical waveguide.

7. 7. The optical modulator according to claim 6, An optical modulator, wherein the length of the optical waveguide is greater than the length of the first electrode.

8. 2. The optical modulator according to claim 1, The optical waveguide includes a meandering portion.

9. The optical modulator according to any one of claims 1 to 8, further comprising: an input port disposed on an end surface of the optical modulator along the height direction, for inputting light into the optical waveguide; an output port disposed on the end face for outputting light from the optical waveguide.

10. An optical modulator comprising: A first optical waveguide having an electro-optic effect, the first optical waveguide including two first waveguide bodies and a first waveguide bend portion connecting one ends of the two first waveguide bodies; a second optical waveguide having an electro-optic effect, the second optical waveguide including two second waveguide bodies disposed between the two first waveguide bodies, and a second waveguide bending portion connecting one ends of the two second waveguide bodies and disposed between the two second waveguide bodies and the first waveguide bending portion; a first electrode for a first optical waveguide, which is disposed on one side of the optical modulator in a height direction with respect to the first optical waveguide, has one end and another end overlapping at least the first optical waveguide when viewed along the height direction, and extends from the one end to the other end; a first electrode for a second optical waveguide, disposed on the one side in the height direction with respect to the second optical waveguide, having one end and another end overlapping at least with the second optical waveguide when viewed along the height direction, and extending from the one end to the other end; a second electrode that is disposed on the other side in the height direction with respect to the first optical waveguide and the second optical waveguide, forms a potential difference between itself and the first electrode for the first optical waveguide to apply an electric field to the first optical waveguide together with the first electrode for the first optical waveguide, and forms a potential difference between itself and the first electrode for the second optical waveguide to apply an electric field to the second optical waveguide together with the first electrode for the second optical waveguide, When viewed along the height direction, The first electrode for the first optical waveguide includes: two first waveguide side electrode bodies respectively overlapping the two first waveguide bodies; a first waveguide side electrode bending portion that connects one ends of the two first waveguide side electrode main body portions to each other and is disposed outside the first waveguide bending portion or between the first waveguide bending portion and the second waveguide bending portion, The second optical waveguide first electrode is two second waveguide side electrode bodies respectively overlapping the two second waveguide bodies; a second waveguide side electrode bending portion connecting one ends of the two second waveguide side electrode main portions to each other and disposed inside the second waveguide bending portion or between the first waveguide bending portion and the second waveguide bending portion.

11. The optical modulator of claim 10, the first waveguide side electrode bent portion is provided outside the first waveguide bent portion, The second waveguide side electrode bent portion is provided inside the second waveguide bent portion.

12. The optical modulator according to claim 10, an optical modulator, wherein, when viewed along the height direction, the second electrode overlaps with the two first waveguide body portions and the two second waveguide body portions, and does not overlap with the first waveguide bending portion and the second waveguide bending portion.

13. The optical modulator according to claim 10, the other end of one of the two first waveguide bodies is connected to the other end of one of the two second waveguide bodies that is closer to the one of the first waveguide bodies, an optical modulator, wherein the other end of the other of the two first waveguide bodies is connected to the other end of the other of the two second waveguide bodies.

14. The optical modulator of claim 13, an optical modulator, wherein a length of both ends of the first optical waveguide connected to the second optical waveguide is the same as a length of both ends of the second optical waveguide connected to the first optical waveguide.