Optical modulator

Abstract

To provide an optical modulator that can suppress the optical loss while applying a material other than a metal material to an electrode.SOLUTION: An optical modulator (10) includes an optical waveguide (1), a first electrode (2), a second electrode (3), and a first low dielectric constant layer (4). The optical waveguide (1) is formed of a material with an electro-optic effect. The first electrode (2) is formed of a semiconductor material and is disposed apart from the optical waveguide (1). The second electrode (3) is formed to apply an electric field to the optical waveguide by forming a potential difference from the first electrode (2). The first low dielectric constant layer (4) has a lower refractive index than the optical waveguide (1) and is provided in a gap between the first electrode (2) and the optical waveguide (1).SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

[0003] A conventional optical modulator is described in, for example, Japanese Patent Application Laid-Open No. 2020-034610 (Patent Document 1). The optical modulator in Patent Document 1 includes a core portion having a slot waveguide structure. The core portion has an upper high-refractive index layer, a lower high-refractive index layer, and a low-refractive index layer provided in the gap (slot) between these high-refractive index layers. The refractive indices of the upper and lower high-refractive index layers are greater than the refractive index of the low-refractive index layer. The upper and lower high-refractive index layers each have a contact region. A metal electrode is connected to each of the contact regions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-034610 Summary of the Invention [Problem to be solved by the invention]

[0005] As described in Patent Document 1, conventional optical modulators use metal electrodes. Normally, materials other than metals are not selected as the material for the electrodes of optical modulators. When electrodes are made of materials other than metals, there is a possibility that light may be easily absorbed by the electrodes, depending on the material. When light is absorbed by the electrodes, light loss increases.

[0006] An object of the present disclosure is to provide an optical modulator that can suppress optical loss while realizing the application of materials other than metal materials to electrodes. [Means for solving the problem]

[0007] The optical modulator according to the present disclosure comprises an optical waveguide, a first electrode, a second electrode, and a first low-dielectric layer. The optical waveguide is made of a material having an electro-optic effect. The first electrode is made of a semiconductor material and is disposed with a gap between it and the optical waveguide. The second electrode is disposed so as to form a potential difference with the first electrode to apply an electric field to the optical waveguide. The first low-dielectric layer has a refractive index smaller than that of the optical waveguide and is disposed in the gap between the first electrode and the optical waveguide. [Effects of the Invention]

[0008] According to the optical modulator according to the present disclosure, it is possible to suppress optical loss while realizing the application of materials other than metal materials to the electrodes. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view showing a schematic configuration of an optical modulator according to the first embodiment. [Figure 2] FIG. 2 is a diagram showing the correlation between the ratio t2 / t4 of the thickness t2 of the first electrode to the thickness t4 of the first low dielectric constant layer and the ratio Z / Z0 of the resistance Z of the optical modulator to the termination resistance Z0 in the first embodiment. [Figure 3] FIG. 3 is a diagram showing a first modification of the optical modulator according to the first embodiment. [Figure 4] FIG. 4 is a diagram showing a second modification of the optical modulator according to the first embodiment. [Figure 5] FIG. 5 is a cross-sectional view showing a schematic configuration of an optical modulator according to the second embodiment. [Figure 6] FIG. 6 is a diagram showing the correlation between the ratio t2A / t4A of the thickness t2A of the first electrode to the thickness t4A of the first low dielectric constant layer and the ratio Z / Z0 of the resistance Z of the optical modulator to the termination resistance Z0 in the second embodiment. [Figure 7] FIG. 7 is a diagram showing the correlation between the ratio t1Ab / t4A of the thickness t1Ab of the optical waveguide to the thickness t4A of the first low-dielectric-constant layer and the effective refractive index n in the second embodiment. [Figure 8] FIG. 8 is a cross-sectional view showing a schematic configuration of an optical modulator according to the third embodiment. [Figure 9] FIG. 9 is a cross-sectional view showing a schematic configuration of an optical modulator according to the fourth embodiment. [Figure 10] FIG. 10 is a diagram showing a modified example of the optical modulator according to the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

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

[0011] An optical modulator according to an embodiment of the present disclosure includes an optical waveguide, a first electrode, a second electrode, and a first low-dielectric layer. The optical waveguide is made of a material having an electro-optic effect. The first electrode is made of a semiconductor material and is disposed with a gap between it and the optical waveguide. The second electrode is disposed so as to form a potential difference with the first electrode to apply an electric field to the optical waveguide. The first low-dielectric layer has a refractive index smaller than that of the optical waveguide and is disposed in the gap between the first electrode and the optical waveguide (first configuration).

[0012] In the first configuration, the first electrode, which is one of the first and second electrodes that apply an electric field to the optical waveguide, is made of a semiconductor material. Semiconductor materials are typically doped with impurities. To improve the function of the first electrode as an electrode, the amount of impurity doping must be increased. Increasing the amount of impurity doping increases the conductivity of the first electrode, but also increases the light absorption rate of the first electrode. Furthermore, since the refractive index of the first electrode made of a semiconductor material is greater than that of the optical waveguide, light is likely to leak from the optical waveguide to the first electrode when the first electrode is in contact with the optical waveguide. Therefore, in the first configuration, the first electrode is positioned with a gap between it and the optical waveguide, making it non-contact with the optical waveguide. A first low-dielectric layer with a refractive index smaller than that of the optical waveguide is placed in the gap between the first electrode and the optical waveguide. This reduces the leakage of light passing through the optical waveguide to the first electrode and reduces its absorption by the first electrode. Therefore, it is possible to suppress light loss while realizing the use of a semiconductor material other than a metal for the first electrode.

[0013] The optical modulator of the first configuration may further include a second low-dielectric layer having a refractive index smaller than that of the optical waveguide. In this case, the second electrode is disposed with a gap between it and the optical waveguide, and the second low-dielectric layer is provided in the gap between the second electrode and the optical waveguide (second configuration).

[0014] In the second configuration, of the first and second electrodes that apply an electric field to the optical waveguide, the second electrode is disposed with a gap between it and the optical waveguide. Therefore, the second electrode is not in contact with the optical waveguide. Furthermore, a second low-dielectric layer with a refractive index lower than that of the optical waveguide is disposed in the gap between the second electrode and the optical waveguide. This makes it less likely that light passing through the optical waveguide will leak to the second electrode side and be absorbed by the second electrode. Therefore, light loss can be further suppressed.

[0015] In the optical modulator of the first configuration, the first low dielectric layer may surround the optical waveguide when viewed in a cross section perpendicular to the extension direction of the optical waveguide, and may be provided between the optical waveguide and each of the first electrode and the second electrode (third configuration).

[0016] In the optical modulator of any one of the first to third configurations, preferably, the first electrode is laminated on the optical waveguide, and the second electrode is laminated on the optical waveguide on the opposite side of the first electrode (fourth configuration). In this case, the optical waveguide is present between the first electrode and the second electrode in the lamination direction of the first electrode, the optical waveguide, and the second electrode. Therefore, an electric field generated by the first electrode and the second electrode can be efficiently applied to the optical waveguide.

[0017] In the optical modulator of the fourth configuration, the ratio of the thickness of the first electrode to the thickness of the first low dielectric constant layer is preferably 20.0 or more and 44.0 or less (fifth configuration). In this case, the generation of reflected waves of the electrical signal can be suppressed.

[0018] In the optical modulator of the first configuration, the optical waveguide includes a substrate portion and a ridge portion protruding from a surface of the substrate portion. A first low-dielectric layer may be laminated on the substrate portion and the ridge portion, and the first electrode and the second electrode may be laminated on the first low-dielectric layer and arranged in parallel with a gap between them (sixth configuration).

[0019] In the optical modulator of the sixth configuration, it is preferable that the ratio of the thickness of the first electrode to the thickness of the first low dielectric layer at the ridge portion is 0.1 or more and 4.0 or less (seventh configuration). In this case, the generation of reflected waves in the electrical signal can be suppressed.

[0020] In the optical modulator of any one of the first to seventh configurations, the size of the gap between the first electrode and the optical waveguide is preferably not less than 0.750 μm and not more than 1.675 μm (eighth configuration).

[0021] Light leaks from the optical waveguide, albeit weakly, into the first low-dielectric layer provided in the gap between the first electrode and the optical waveguide. This leaked light is called evanescent light. If the size of the gap between the first electrode and the optical waveguide is 0.750 μm or more, as in the eighth configuration, the evanescent light is less likely to come into contact with the first electrode, thereby further suppressing light loss.

[0022] In the eighth configuration, the gap between the first electrode and the optical waveguide is 1.675 μm or less, which prevents the distance between the first electrode and the optical waveguide from becoming too large, ensuring the strength of the electric field across the optical waveguide without increasing the voltage applied between the first electrode and the second electrode.

[0023] In the optical modulator of any one of the first to eighth configurations, the semiconductor material of the first electrode is preferably a silicon semiconductor material in which silicon is doped with impurities (ninth configuration).

[0024] In the optical modulator of the ninth configuration, the concentration of the impurity in the first electrode is preferably 1.0×10 17 cm -3 That's it, 1.0 x 10 22 cm -3 The following is the tenth configuration. In the first electrode, as the impurity concentration increases, the resistivity decreases and the conductivity increases. 17 cm -3 If the impurity concentration is 1.0×10 or more, the first electrode can function effectively as an electrode. 22 cm -3 If the temperature is below this, the precipitation of impurities can be prevented.

[0025] In the optical modulator of the ninth or tenth configuration, the first electrode is preferably a silicon single crystal substrate (eleventh configuration).

[0026] In the optical modulator of any one of the ninth to eleventh configurations, the first low-dielectric layer may be mainly composed of SiO2 (twelfth configuration). Because the semiconductor material used for the first electrode is a silicon semiconductor material, the first low-dielectric layer of SiO2 can be formed on the first electrode by thermal oxidation. When formed by thermal oxidation, the first low-dielectric layer has good adhesion to the first electrode, making it difficult for foreign matter to enter the interface between the first electrode and the first low-dielectric layer. This makes it possible to suppress electrical loss at the interface between the first electrode and the first low-dielectric layer. Furthermore, because the accumulation of foreign matter at the interface between the first electrode and the first low-dielectric layer is suppressed, the reliability and lifespan of the optical modulator can be improved.

[0027] In the optical modulator of any one of the first to twelfth configurations, the refractive index of the first electrode is smaller than 3 (thirteenth configuration). In this case, the refractive index of the first electrode becomes smaller than 3, for example, in accordance with the impurity concentration (doping amount) of the tenth configuration.

[0028] In the optical modulator of any one of the first to thirteenth configurations, the surface layer of the first electrode on the optical waveguide side is preferably doped with impurities at a higher concentration than other parts of the first electrode (fourteenth configuration). In this case, a region with high conductivity can be localized in the vicinity of the optical waveguide in the first electrode, and attenuation of high-frequency signals can be suppressed by the skin effect.

[0029] 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.

[0030] First Embodiment [Configuration of optical modulator] Fig. 1 is a cross-sectional view showing a schematic configuration of an optical modulator 10 according to a first embodiment. The optical modulator 10 includes an optical waveguide 1, a first electrode 2, a second electrode 3, a first low-dielectric layer 4, and a second low-dielectric layer 5. Fig. 1 shows a cross section perpendicular to the extension direction of the optical waveguide 1. In this specification, unless otherwise specified, the cross section refers to a cross section perpendicular to the extension direction of the optical waveguide 1.

[0031] As shown in Fig. 1, the optical waveguide 1 may have a substantially rectangular cross section. The optical waveguide 1 is made of a material having an electro-optic effect (electro-optic material). The optical waveguide 1 functions as an optical transmission path. Materials that can be used for the optical waveguide 1 include LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate), PLZT (lead lanthanum zirconate titanate), KTN (potassium tantalate niobate), and BaTiO3 (barium titanate). The optical waveguide 1 may also be made of an electro-optic polymer (EO polymer).

[0032] The first electrode 2 and the second electrode 3 function as control electrodes for controlling light passing through the optical waveguide 1. Each of the first electrode 2 and the second electrode 3 may have a substantially rectangular cross section. The first electrode 2 and the second electrode 3 are arranged to create a potential difference between them to apply an electric field to the optical waveguide 1. The optical waveguide 1 is arranged between the first electrode 2 and the second electrode 3.

[0033] In this embodiment, the first electrode 2 is laminated on the optical waveguide 1. The second electrode 3 is laminated on the optical waveguide 1 on the opposite side of the first electrode 2. From another perspective, the first electrode 2 and the second electrode 3 are arranged to sandwich the optical waveguide 1 therebetween.

[0034] The first electrode 2 is disposed with a gap between it and the optical waveguide 1. In this embodiment, the first electrode 2 is separated from the optical waveguide 1 in the stacking direction of the optical waveguide 1 and the electrodes 2 and 3. The first electrode 2 is not in contact with the optical waveguide 1. The size of the gap between the first electrode 2 and the optical waveguide 1 is, for example, 0.750 μm or more and 1.675 μm or less. In this specification, the size of the gap between the first electrode 2 and the optical waveguide 1 means the shortest distance from the first electrode 2 to the optical waveguide 1. In this embodiment, the distance from the first electrode 2 to the optical waveguide 1 in the stacking direction is the shortest distance from the first electrode 2 to the optical waveguide 1.

[0035] The first electrode 2 is made of a semiconductor material. That is, the first electrode 2 is a semiconductor electrode. The semiconductor material used for the first electrode 2 is typically a silicon semiconductor material in which impurities are doped into Si (silicon). As the semiconductor material, for example, other single-element semiconductors using Ge (germanium) or the like, or compound semiconductors such as GaAs (gallium arsenide) may also be used. The impurities may be either p-type impurities or n-type impurities. For example, when the semiconductor material is a silicon semiconductor material, a Group 3 element such as boron is used as the p-type impurity, and a Group 5 element such as phosphorus, arsenic, or antimony is used as the n-type impurity.

[0036] When the semiconductor material used for the first electrode 2 is a silicon semiconductor material, the impurity concentration (doping amount) in the first electrode 2 is preferably 1.0×10 17 cm -3 That's it, 1.0 x 10 22 cm -3 As the doping level of impurities increases, the resistivity of the semiconductor material decreases and the conductivity increases. 17 cm -3 If the doping amount is 1.0×10 or more, the first electrode 2 can function effectively as an electrode. 22 cm -3 If the doping amount is less than the solubility limit of the impurities in the silicon semiconductor material, precipitation of the impurities can be prevented. As the doping amount increases, the refractive index of the first electrode 2 decreases. For example, the refractive index of the first electrode 2 is less than 3.

[0037] The refractive index of a semiconductor material is higher than the refractive index of the electro-optic material that constitutes the optical waveguide 1, regardless of whether it is doped with impurities or not. The refractive index of a semiconductor material without doping with impurities is, for example, 3.4 for Si, 5.5 for Ge, and 3.3 for GaAs. The refractive index of an electro-optic material is, for example, 2.3 for LiNbO3, 2.8 for LiTaO3, 2.5 for PLZT, 2.1 for KTN, and 2.6 for BaTiO3. When doped with impurities, the refractive index of the semiconductor material decreases, but the refractive index of the doped semiconductor material is still higher than the refractive index of the electro-optic material.

[0038] The above-mentioned range of the doping amount will be explained in more detail below. 22 cm -3 The reason why the doping amount is preferably 1.0×10 is based on the solubility limit of impurities in silicon semiconductor materials. 22 cm -3 If the doping amount exceeds 1.0×10, impurities will precipitate, reducing the reliability of the first electrode 2 and the optical modulator 10. 17 cm -3 The reason why it is preferable that the doping amount is 1.0×10 is as follows. The skin depth is an index used when designing the thickness and width of the electrode. If the thickness and width of the electrode are smaller than the skin depth, the resistance value increases. For this reason, it is preferable that the thickness and width of the electrode are equal to or larger than the skin depth. When considering the operation of an optical modulator that handles frequency signals of 1 GHz or more, it is preferable that the doping amount is 1.0×10 17 cm -3 When the conductivity is 1000 S / m, the skin depth is 500 μm. For example, when considering forming an electrode from a silicon semiconductor material by microfabrication, the thickness of the electrode is limited to about 500 μm. Therefore, from the viewpoint of ensuring the performance of the electrode, in order to obtain an electrode with a conductivity of 1000 S / m or more, the doping amount should be 1.0 × 10 17 cm -3 That's all there is to it.

[0039] The first electrode 2 is, for example, a silicon single crystal substrate. Impurities are doped in advance into, for example, a silicon single crystal base substrate, which is the material for the first electrode 2. The first electrode 2 can be formed by placing this base substrate on another substrate and patterning (etching, dicing, etc.). The first electrode 2 may be an active layer of an SOI (Silicon on Insulator) substrate. In this case, the first electrode 2 can be formed by patterning (etching, dicing, etc.) the active layer of the SOI substrate. Impurities may be further introduced into the first electrode 2 formed in this manner by thermal diffusion, ion implantation, or the like.

[0040] The first electrode 2 may be a semiconductor silicon layer formed on a substrate. For example, the silicon layer can be formed on the substrate by sputtering, vapor deposition, CVD, or the like. Impurities can be introduced into this silicon layer by thermal diffusion, ion implantation, or the like to form the semiconductor silicon layer as the first electrode 2.

[0041] The second electrode 3 is disposed with a gap between it and the optical waveguide 1. In this embodiment, the second electrode 3 is separated from the optical waveguide 1 in the stacking direction. The second electrode 3 is not in contact with the optical waveguide 1. The size of the gap between the second electrode 3 and the optical waveguide 1 is, for example, 0.750 μm or more and 1.675 μm or less. In this specification, the size of the gap between the second electrode 3 and the optical waveguide 1 means the shortest distance from the second electrode 3 to the optical waveguide 1. In this embodiment, the distance from the second electrode 3 to the optical waveguide 1 in the stacking direction is the shortest distance from the second electrode 3 to the optical waveguide 1.

[0042] The second electrode 3 is made of, for example, a metal material. That is, the second electrode 3 is a metal electrode. However, the second electrode 3 may also be made of, for example, a semiconductor material. That is, the second electrode 3 may be a semiconductor electrode. Examples of semiconductor materials include the same semiconductor materials as those used for the first electrode 2.

[0043] When the second electrode 3 is made of a metal material, the metal material is, for example, mainly composed of a noble metal. The noble metal is, for example, Au (gold). The noble metal may be Ag (silver), Pt (platinum), or the like. The metal material may contain trace amounts of other metal elements such as Cr and Ti. The metal material may be copper, aluminum, or an alloy thereof.

[0044] The second electrode 3 is used as a signal electrode and the first electrode 2 is used as a ground electrode, or conversely, the first electrode 2 may be used as a signal electrode and the second electrode 3 as a ground electrode.

[0045] The first low-dielectric layer 4 is provided in the gap between the first electrode 2 and the optical waveguide 1. In this embodiment, the first low-dielectric layer 4 is laminated on the first electrode 2, and the optical waveguide 1 is laminated on the first low-dielectric layer 4. In other words, the optical waveguide 1 is indirectly laminated on the first electrode 2 via the first low-dielectric layer 4, and the first electrode 2 is not in contact with the optical waveguide 1. It is preferable that the first low-dielectric layer 4 covers the entire surface of the optical waveguide 1 facing the first low-dielectric layer 4.

[0046] The first low-dielectric layer 4 has a refractive index smaller than that of the optical waveguide 1. For example, the refractive index of the first low-dielectric layer 4 is smaller than that of the optical waveguide 1 by 1% or more. The refractive index of the optical waveguide 1 is smaller than that of the first electrode 2. The ratio of the refractive index of the optical waveguide 1 to that of the first low-dielectric layer 4 is, for example, 1.8 or more and 2.5 or less. In this case, light can be sufficiently confined within the optical waveguide 1. Furthermore, the ratio of the refractive index of the first electrode 2 to that of the first low-dielectric layer 4 is, for example, 1.5 or more and 6.0 or less. In this case, when light enters the optical waveguide 1 from an optical fiber, it is possible to prevent light from entering the first electrode 2.

[0047] The main component of the first low-dielectric-constant layer 4 is typically SiO2. The main component of the first low-dielectric-constant layer 4 may be an oxide such as Al2O3, LaAlO3, LaYO3, ZnO, HfO2, MgO, or YO3, or a polymer such as BCB (benzocyclobutene) or PI (polyimide).

[0048] The second low-dielectric layer 5 is provided in the gap between the second electrode 3 and the optical waveguide 1. In this embodiment, the second low-dielectric layer 5 is laminated on the optical waveguide 1, and the second electrode 3 is laminated on the second low-dielectric layer 5. In other words, the optical waveguide 1 is indirectly laminated on the second electrode 3 via the second low-dielectric layer 5, and the second electrode 3 is not in contact with the optical waveguide 1. It is preferable that the second low-dielectric layer 5 covers the entire surface of the optical waveguide 1 facing the second low-dielectric layer 5.

[0049] The second low-dielectric layer 5 has a refractive index smaller than that of the optical waveguide 1. Examples of the main component of the second low-dielectric layer 5 include the same components as those of the first low-dielectric layer 4 described above. The main component of the second low-dielectric layer 5 may be the same as or different from the main component of the first low-dielectric layer 4.

[0050] In the optical modulator 10 configured as described above, the second electrode 3 can be laminated on the optical waveguide 1 and the first electrode 2, for example, as follows: First, a first low-dielectric-constant layer 4 is formed on the first electrode 2 by CVD, vapor deposition, sputtering, or the like. A material substrate having an electro-optic effect is placed on the first low-dielectric-constant layer 4 formed on the first electrode 2, and the material substrate is bonded to the first low-dielectric-constant layer 4. The material substrate is then subjected to lithography and etching to form the optical waveguide 1. Next, a second low-dielectric-constant layer 5 is formed on the optical waveguide 1 by CVD, vapor deposition, sputtering, or the like. Then, a metal layer is formed on the second low-dielectric-constant layer 5 by sputtering, vapor deposition, or the like. The formed metal layer is patterned by lithography, and the second electrode 3 is formed by etching.

[0051] When a high-frequency current is passed through the first electrode 2 during use of the optical modulator 10, the thickness t2 required for the first electrode 2 can be estimated based on the skin effect. The thickness t2 of the first electrode 2 corresponds to the length in the stacking direction. The following equation (1) is used to calculate the skin depth of a conductor.

[0052]

number

[0053] The thickness t2 required for the first electrode 2 can be determined from equation (1). More specifically, by making the thickness t2 of the first electrode 2 larger than the skin depth calculated using equation (1), the electrical resistance of the first electrode 2 can be reduced, and excess electrical loss can be suppressed. The higher the conductivity of the first electrode 2, the better. However, there is a solubility limit for the doping amount of impurities, and when the doping amount approaches the solubility limit, the impurities cluster and become inactive as carriers, so when the doping amount exceeds a certain amount, the conductivity of the first electrode 2 saturates. When the solubility limit for the doping amount is 1.0 × 10 22 cm -3 Then, the conductivity of the first electrode 2 is 1×10 7 S / m, and the skin depth of a 1 GHz electrical signal is 5 μm. However, the actual conductivity is an order of magnitude lower, at 1×10 6 For this reason, if the electrical signal is to have a bandwidth and is to be used to handle signals of 0.5 GHz or higher, the thickness t2 of the first electrode 2 is preferably 25 μm or greater.

[0054] When the optical modulator 10 is in use, the ratio Z / Z0 of the resistance Z of the optical modulator 10 to the termination resistance Z0 is preferably 0.8 or more and 1.2 or less. If the ratio Z / Z0 is outside the condition of 0.8 or more and 1.2 or less, an impedance mismatch will occur, causing a reflected wave of the electrical signal at the electrode termination. Therefore, it is preferable to set the various conditions of the optical modulator 10 so that the ratio Z / Z0 satisfies this condition. Specifically, it is preferable to set the ratio t2 / t4 of the thickness t2 of the first electrode 2 to the thickness t4 of the first low-dielectric layer 4 so that the ratio Z / Z0 of the resistance Z of the optical modulator 10 to the termination resistance Z0 is 0.8 or more and 1.2 or less. The thickness t4 of the first low-dielectric layer 4 corresponds to the size of the gap between the first electrode 2 and the optical waveguide 1.

[0055] 2 is a diagram showing the correlation between the ratio t2 / t4 of the thickness t2 of the first electrode 2 to the thickness t4 of the first low-dielectric-constant layer 4, and the ratio Z / Z0 of the resistance Z of the optical modulator 10 to the termination resistance Z0. As an example, Fig. 2 shows the relationship between the ratio t2 / t4 and the ratio Z / Z0 when an analysis is performed using a silicon semiconductor material for the first electrode 2, a metal material containing Au as the main component for the second electrode 3, SiO2 for the low-dielectric-constant layers 4 and 5, and LiNbO3 for the optical waveguide 1. In the analysis, the termination resistance Z0 was set to 50 Ω, the width w2 of the first electrode 2 was set to 50 μm, the width w3 of the second electrode 3 was set to 40 μm, the thickness t3 of the second electrode 3 was set to 5.0 μm, the thickness t4 of the first low dielectric constant layer 4 was set to 0.7 μm, the thickness t5 of the second low dielectric constant layer 5 was set to 0.7 μm, the width w1 of the optical waveguide 1 was set to 1.0 μm, and the thickness t1 of the optical waveguide 1 was set to 1.3 μm. Note that in this specification, thickness means the length in the stacking direction in the cross section of the optical modulator 10, and width means the length in the direction perpendicular to the stacking direction in the cross section of the optical modulator 10.

[0056] As shown in FIG. 2, for the ratio Z / Z0 to be 0.8 or more and 1.2 or less, the ratio t2 / t4 of the thickness t2 of the first electrode 2 to the thickness t4 of the first low dielectric constant layer 4 is 20.0 or more and 44.0 or less.

[0057] Referring again to FIG. 1 , in this embodiment, the first electrode 2 is formed of a semiconductor material. When a metal material is used as the material of the second electrode 3, it is preferable that the performance of the first electrode 2 as an electrode is equivalent to that of the second electrode 3, which is a metal electrode. The cross-sectional areas of the first electrode 2 and the second electrode 3 are preferably set so that the resistance value of the first electrode 2 substantially matches the resistance value of the second electrode 3. For example, the cross-sectional area of ​​the first electrode 2 can be calculated as "(conductivity of the second electrode 3 / conductivity of the first electrode 2) x cross-sectional area of ​​the second electrode 3."

[0058] Specifically, when viewed in a cross section perpendicular to the extension direction of the optical waveguide 1, the area of ​​the first electrode 2 is preferably larger than the area of ​​the second electrode 3. If the second electrode 3 is made of a metal material, the second electrode 3 has a relatively small resistance value even without a large cross-sectional area. On the other hand, since the first electrode 2 is made of a semiconductor material with lower conductivity than a metal material, the resistance value can be reduced to the same level as that of the second electrode 3 by making the cross-sectional area larger than that of the second electrode 3. This makes it possible to reduce power consumption.

[0059] Under the condition that the width w2 of the first electrode 2 is the same as the width w3 of the second electrode 3, the resistance value of the first electrode 2, which is a semiconductor electrode, is made to match the resistance value of the second electrode 3, which is a metal electrode, so the product of the conductivity and thickness t2 of the first electrode 2 should match the product of the conductivity and thickness t3 of the second electrode 3. For example, when the metal material of the second electrode 3 is Au, the thickness t3 of the second electrode 3 is usually set to be 0.1 μm or more and 2.0 μm or less, and the conductivity is 4.3 × 10 7 S / m. On the other hand, since the conductivity of the first electrode 2 is smaller than that of the second electrode 3, the thickness t2 of the first electrode 2 is larger than the thickness t3 of the second electrode 3. The conductivity of the first electrode 2 changes depending on the amount of impurity doping.

[0060] For example, when the first electrode 2 is made of a silicon semiconductor material, the doping amount of impurities is limited to the upper limit of 1.0 × 10 22 cm -3 When the conductivity of the first electrode 2 is 1×10 7 S / m. In this case, the value obtained by dividing the conductivity of the second electrode 3 by the conductivity of the first electrode 2 is 4.3, and the thickness t2 of the first electrode 2 can be set to 4.3 times the thickness t3 of the second electrode 3. On the other hand, when the doping amount of the impurity is set to the lower limit of 1.0 × 10 17 cm -3 When the conductivity of the first electrode 2 is 1000×10 4 In this case, the conductivity of the second electrode 3 divided by the conductivity of the first electrode 2 is 4.3 × 10 3 The thickness t2 of the first electrode 2 is 4.3×10 times the thickness t3 of the second electrode 3. 3 It can be doubled.

[0061] Therefore, when the first electrode 2 is made of a semiconductor material and the second electrode 3 is made of a metal material, the lower limit of the thickness t2 of the first electrode 2 can be set to 4.3 times the lower limit of the thickness t3 of the second electrode 3, which is 0.1 μm. In other words, the thickness t2 of the first electrode 2 can be set to 0.43 μm or more. On the other hand, the upper limit of the thickness t2 of the first electrode 2 can be set to 4.3×10 times the upper limit of the thickness t3 of the second electrode 3, which is 2.0 μm. 3 That is, the thickness t2 of the first electrode 2 can be 8600 μm (8.6 mm) or less. However, when a silicon semiconductor material is used for the first electrode 2, the thickness t2 of the first electrode 2 is preferably 500 μm or less from the viewpoint of processability.

[0062] As described above, the second electrode 3 can also be made of a semiconductor material. When the first electrode 2 and the second electrode 3 are both made of a semiconductor material, it is preferable that the area of ​​the first electrode 2 is the same as the area of ​​the second electrode 3 when viewed in a cross section perpendicular to the extension direction of the optical waveguide 1.

[0063] The thickness t2 of the first electrode 2, which is a semiconductor electrode, and the thickness t4 of the low-dielectric-constant layer 4 can be measured by, for example, the following methods. The first method is a measurement method using SEM observation. In this method, the optical modulator 10 is cut using a focused ion beam (FIB) to obtain a sample. The cross section of the obtained sample is imaged using an SEM, and the thickness t2 of the first electrode 2 and the thickness t4 of the low-dielectric-constant layer 4 can be measured from the obtained image. The second method is an optical measurement method. In this method, the thickness t2 of the first electrode 2 and the thickness t4 of the low-dielectric-constant layer 4 can be directly measured using interferometry. Either method produces substantially the same measurement results.

[0064] When the second electrode 3 is a metal electrode, the thickness t3 of the second electrode 3 can be measured by, for example, the following methods. The first method is the measurement method using SEM observation described above. The second method is a measurement method using X-rays. In this method, X-rays are irradiated onto the second electrode 3, and the amount of X-rays that pass through is measured to determine the amount of attenuation by the second electrode 3. The thickness t3 of the second electrode 3 can be measured by back-calculating the determined amount of attenuation. Either method will yield substantially the same measurement results. When the second electrode 3 is a semiconductor electrode, the thickness t3 of the second electrode 3 can be measured by the method for measuring the thickness t2 of the first electrode 2 described above.

[0065] The doping amount in the first electrode 2 can be measured by epitaxial resistivity measurement, air gap CV measurement, mercury CV measurement, surface charge profiling, secondary ion mass spectrometry, spreading resistance measurement, etc. The measurement results are essentially the same regardless of the method.

[0066] When viewed in a cross section perpendicular to the extension direction of the optical waveguide 1, the width w2 of the first electrode 2 on the optical waveguide 1 side is preferably larger than the width w1 of the optical waveguide 1. The width w2 of the first electrode 2 on the optical waveguide 1 side refers to the width of the surface of the first electrode 2 closest to the optical waveguide 1. In this embodiment, the length in the direction perpendicular to the stacking direction of the surface of the first electrode 2 that is in contact with the first low dielectric layer 4 is the width w2. In this case, an electric field can be applied to the entire optical waveguide 1.

[0067] [effect] In the optical modulator 10 according to this embodiment, the first electrode 2 is made of a semiconductor material, and the second electrode 3 applies an electric field to the optical waveguide 1. Semiconductor materials are typically doped with impurities. To improve the function of the first electrode 2 as an electrode, the amount of impurity doping must be increased. Increasing the amount of impurity doping increases the conductivity of the first electrode 2, but also increases the light absorption rate of the first electrode 2. Furthermore, since the refractive index of the first electrode 2 made of a semiconductor material is greater than that of the optical waveguide 1, when the first electrode 2 is in contact with the optical waveguide 1, light is likely to leak from the optical waveguide 1 to the first electrode 2. Therefore, in this embodiment, the first electrode 2 is disposed with a gap between it and the optical waveguide 1, so that it is not in contact with the optical waveguide 1. Furthermore, a first low-dielectric-constant layer 4, whose refractive index is smaller than that of the optical waveguide 1, is disposed in the gap between the first electrode 2 and the optical waveguide 1. This makes it difficult for light passing through the optical waveguide 1 to leak to the first electrode 2 side and to be absorbed by the first electrode 2. Therefore, according to the optical modulator 10 of this embodiment, it is possible to suppress optical loss while realizing the application of a semiconductor material, which is a material other than a metal material, to the first electrode 2.

[0068] Typically, in an optical modulator, the effective refractive index of an electrical signal (modulation wave (GHz)) applied from an electrode to an optical waveguide is larger than the effective refractive index of a light wave (carrier wave (THz)) passing through the optical waveguide. If the effective refractive index of the electrical signal is significantly different from the effective refractive index of the light wave, the difference between the propagation velocity of the light wave and the propagation velocity of the electrical signal increases, resulting in a decrease in modulation speed. In the optical modulator 10 according to this embodiment, the first low-dielectric layer 4 is disposed at least between the first electrode 2 and the optical waveguide 1, making it possible to adjust the cross-sectional area ratio of the first low-dielectric layer 4 to the optical waveguide 1. Adjusting the cross-sectional area ratio of the first low-dielectric layer 4 to the optical waveguide 1 can reduce the difference between the effective refractive index of the electrical signal and the effective refractive index of the light wave, thereby reducing the difference between the propagation velocity of the light wave and the propagation velocity of the electrical signal. This can suppress a decrease in modulation speed.

[0069] In this embodiment, the second electrode 3 is disposed with a gap between it and the optical waveguide 1. In this case, the second electrode 3 is not in contact with the optical waveguide 1. Furthermore, a second low-dielectric layer 5 having a refractive index smaller than that of the optical waveguide 1 is provided in the gap between the second electrode 3 and the optical waveguide 1. This makes it difficult for light passing through the optical waveguide 1 to leak to the second electrode 3 side and also difficult for it to be absorbed by the second electrode 3. Therefore, it is possible to suppress light loss.

[0070] The semiconductor material used for the first electrode 2 is, for example, a silicon semiconductor material in which Si is doped with impurities. The first electrode 2 may be a silicon single crystal substrate or a semiconductor silicon layer formed on a substrate. For example, when the first electrode 2 is a silicon single crystal substrate, the internal stress of the first electrode 2 can be reduced compared to a metal electrode formed by sputtering, vapor deposition, or the like. Therefore, the first electrode 2 can be formed thick while suppressing the internal stress of the first electrode 2. By forming the first electrode 2 thick, the resistance value of the first electrode 2 is reduced, thereby reducing power consumption. Furthermore, by reducing the internal stress of the first electrode 2, the occurrence of cracks due to the internal stress can be suppressed. As a result, failure and damage to the optical modulator 10 can be suppressed.

[0071] Silicon semiconductor materials are cheaper than, for example, metallic materials using noble metals, and therefore, if the first electrode 2 is made of silicon semiconductor materials, the cost of the optical modulator 10 can be reduced.

[0072] In this embodiment, the first electrode 2 is stacked on the optical waveguide 1, and the second electrode 3 is stacked on the optical waveguide 1 on the opposite side of the first electrode 2. In this case, the optical waveguide 1 is present between the first electrode 2 and the second electrode 3 in the stacking direction. Therefore, an electric field generated by the first electrode 2 and the second electrode 3 can be efficiently applied to the optical waveguide 1.

[0073] For example, if the semiconductor material used for the first electrode 2 is a silicon semiconductor material, the first low-dielectric-constant layer 4 of SiO2 can be formed on the first electrode 2 by thermal oxidation. In this case, the first low-dielectric-constant layer 4 has good adhesion to the first electrode 2, making it difficult for foreign matter to enter the interface between the first electrode 2 and the first low-dielectric-constant layer 4. This makes it possible to suppress electrical loss at the interface between the first electrode 2 and the first low-dielectric-constant layer 4. This also makes it possible to improve the reliability and lifespan of the optical modulator 10. This is because if foreign matter accumulates at the interface between the first electrode 2 and the first low-dielectric-constant layer 4 and an electric field concentrates on the accumulated foreign matter, the optical modulator 10 may be damaged.

[0074] The penetration depth of the evanescent light in each of the low-dielectric layers 4 and 5 can be estimated based on the wavelength of the light (carrier wave) passing through the optical waveguide 1. When each of the electrodes 2 and 3 is separated from the optical waveguide 1 by a distance equal to or greater than the wavelength of the carrier wave, it is possible to prevent the evanescent light from contacting each of the electrodes 2 and 3. Therefore, it is preferable that the size of the gap between the optical waveguide 1 and each of the electrodes 2 and 3, i.e., the thicknesses t4 and t5 (length in the stacking direction) of the low-dielectric layers 4 and 5, be equal to or greater than the wavelength of the light passing through the optical waveguide 1.

[0075] For example, as in this embodiment, if the size of the gap between each of the electrodes 2, 3 and the optical waveguide 1 is 0.750 μm or more, the thickness of each of the low dielectric layers 4, 5 becomes greater than the penetration depth of the evanescent light, and light passing through the optical waveguide 1 is less likely to leak to each of the electrodes 2, 3. As described above, the size of the gap between each of the electrodes 2, 3 and the optical waveguide 1 may be 1.675 μm or less. If the size of the gap between each of the electrodes 2, 3 and the optical waveguide 1 is 1.675 μm or less, the magnitude of the electric field across the optical waveguide 1 can be ensured without increasing the voltage applied between the first electrode 2 and the second electrode 3.

[0076] [Variation 1] FIG. 3 shows a first modification of the optical modulator 10 according to the first embodiment. As shown in FIG. 3, the optical modulator 10 does not necessarily have to include the second low-dielectric-constant layer 5 (FIG. 1). That is, the second electrode 3 may be directly stacked on the optical waveguide 1 and be in contact with the optical waveguide 1. In this case, too, the dimensional relationship between the first electrode 2, which is a semiconductor electrode, and the first low-dielectric-constant layer 4 can be determined as described above. For example, it is preferable to set the ratio t2 / t4 of the thickness t2 of the first electrode 2 to the thickness t4 of the first low-dielectric-constant layer 4 so that the ratio Z / Z0 of the resistance Z of the optical modulator 10 to the termination resistance Z0 is 0.8 or more and 1.2 or less.

[0077] [Variation 2] FIG. 4 shows a second modification of the optical modulator 10 according to the first embodiment. In this modification, similar to the example shown in FIG. 3, the optical modulator 10 does not include the second low-dielectric-constant layer 5 (FIG. 1). However, in this modification, the first low-dielectric-constant layer 4 is provided so as to surround the optical waveguide 1 in a cross-sectional view of the optical modulator 10. The first low-dielectric-constant layer 4 is provided not only between the first electrode 2 and the optical waveguide 1, but also between the second electrode 3 and the optical waveguide 1. That is, the first electrode 2 and the second electrode 3 are not in contact with the optical waveguide 1, and the first low-dielectric-constant layer 4 is interposed between each of the first electrode 2 and the second electrode 3 and the optical waveguide 1. In this case, the first low-dielectric-constant layer 4 can also serve as the second low-dielectric-constant layer 5 (FIG. 1).

[0078] Second Embodiment 5 is a cross-sectional view showing a schematic configuration of an optical modulator 10A according to the second embodiment. The optical modulator 10A differs from the optical modulator 10 according to the first embodiment in the configuration of the optical waveguide 1A and the arrangement of the first electrode 2A and the second electrode 3A.

[0079] Referring to FIG. 5, an optical modulator 10A includes an optical waveguide 1A, a first electrode 2A, a second electrode 3A, and a first low-dielectric layer 4A. The optical waveguide 1A includes a substrate portion 1Aa and a ridge portion 1Ab. The ridge portion 1Ab protrudes from the surface of the substrate portion 1Aa. The ridge portion 1Ab essentially functions as an optical waveguide. A first low-dielectric layer 4A is stacked on the optical waveguide 1A. More specifically, the first low-dielectric layer 4A is stacked on the substrate portion 1Aa and the ridge portion 1Ab.

[0080] The first electrode 2A and the second electrode 3A are stacked on the first low-dielectric layer 4A. The first electrode 2A and the second electrode 3A are arranged in parallel with a gap between them. Specifically, in a cross-sectional view of the optical modulator 10A, the first electrode 2A and the second electrode 3A are arranged side by side in a direction substantially perpendicular to the stacking direction of the optical waveguide 1A and the first low-dielectric layer 4A. In the direction substantially perpendicular to the stacking direction, the first electrode 2A is arranged on one side of the ridge portion 1Ab, and the second electrode 3A is arranged on the other side of the ridge portion 1Ab. The first electrode 2A and the second electrode 3A can generate a potential difference between them to apply an electric field to the ridge portion 1Ab of the optical waveguide 1A.

[0081] The optical modulator 10A according to this embodiment can also achieve the same effects as the optical modulator 10 according to the first embodiment.

[0082] As described above, when the optical modulator 10A is used, the ratio Z / Z0 of the resistance Z of the optical modulator 10A to the termination resistance Z0 is preferably 0.8 or more and 1.2 or less. Therefore, the ratio t2A / t4A of the thickness t2A of the first electrode 2A to the thickness t4A of the first low-dielectric-constant layer 4A is preferably set so that the ratio Z / Z0 of the resistance Z of the optical modulator 10A to the termination resistance Z0 is 0.8 or more and 1.2 or less. In this embodiment, the thickness t4A of the first low-dielectric-constant layer 4A is the thickness of the first low-dielectric-constant layer 4A at the position of the ridge portion 1Ab. The thickness t4A refers to the shortest distance from the interface between the ridge portion 1Ab and the first low-dielectric-constant layer 4A to the interface between the first low-dielectric-constant layer 4A and the first electrode 2A in the stacking direction of the ridge portion 1Ab, the first low-dielectric-constant layer 4A, and the electrodes 2A and 2B.

[0083] Fig. 6 is a diagram showing the correlation between the ratio t2A / t4A of the thickness t2A of the first electrode 2A to the thickness t4A of the first low-dielectric-constant layer 4A and the ratio Z / Z0 of the resistance Z of the optical modulator 10A to the termination resistance Z0 in the second embodiment. As an example, Fig. 6 shows the relationship between the ratio t2A / t4A and the ratio Z / Z0 when an analysis is performed using a silicon semiconductor material for the first electrode 2A, a metal material containing Au as the main component for the second electrode 3A, SiO2 for the first low-dielectric-constant layer 4A, and LiNbO3 for the optical waveguide 1A. In the analysis, the termination resistance Z0 was set to 50 Ω, the width w2A of the first electrode 2A was set to 50 μm, the width w3A of the second electrode 3A was set to 21.5 μm, the thickness t3A of the second electrode 3A was set to 16.6 μm, the thickness t4A of the first low dielectric constant layer 4A was set to 8.3 μm, the width w1Ab of the ridge portion 1Ab was set to 2.0 μm, the thickness t1Ab of the ridge portion 1Ab was set to 1.0 μm, and the gap g between the first electrode 2A and the second electrode 3A was set to 10 μm.

[0084] As shown in FIG. 6, for the ratio Z / Z0 to be 0.8 or more and 1.2 or less, the ratio t2A / t4A of the thickness t2A of the first electrode 2A to the thickness t4A of the first low dielectric constant layer 4A is 0.1 or more and 4.0 or less.

[0085] It is preferable that the effective refractive index n is a value that maximizes the modulation speed when the optical modulator 10 is used. Therefore, it is preferable to set the ratio t1Ab / t4A of the thickness t1Ab of the optical waveguide 1A (ridge portion 1Ab) to the thickness t4A of the first low-dielectric-constant layer 4A so that the effective refractive index n is a value that maximizes the modulation speed.

[0086] Fig. 7 is a diagram showing the correlation between the ratio t1Ab / t4A of the thickness t1Ab of the optical waveguide 1A to the thickness t4A of the first low-dielectric-constant layer 4A and the effective refractive index n in the second embodiment. Fig. 7 shows, as an example, the relationship between the ratio t1Ab / t4A and the effective refractive index n when an analysis is performed under the same conditions as in Fig. 6, except that the thickness t2A of the first electrode 2A is set to 16.6 µm.

[0087] When LiNbO3 is used for the optical waveguide 1A, the modulation speed can be maximized if the effective refractive index n is 2. As shown in Fig. 7, for the effective refractive index n to be 2, the ratio t1Ab / t4A of the thickness t1Ab of the optical waveguide 1A (ridge portion 1Ab) to the thickness t4A of the first low-dielectric-constant layer 4A is substantially 0.28.

[0088] Third Embodiment 8 is a cross-sectional view showing a schematic configuration of an optical modulator 10B according to the third embodiment. The optical modulator 10B differs from the optical modulator 10 according to the first embodiment in the configuration of the first electrode 2B.

[0089] 8, the first electrode 2B has a surface layer 2Ba on the optical waveguide 1 side and a remaining portion 2Bb. In this embodiment, the surface layer 2Ba is disposed adjacent to the first low-dielectric-constant layer 4 of the first electrode 2B. The surface layer 2Ba is, for example, a portion of the first electrode 2B that is within a range of 10% of the length (thickness) of the first electrode 2B in the stacking direction of the first electrode 2B relative to the optical waveguide 1, from the surface of the first electrode 2B on the optical waveguide 1 side. The remaining portion 2Bb refers to the portion of the first electrode 2B excluding the surface layer 2Ba. In the first electrode 2B, the concentration of impurities doped into the semiconductor material is higher in the surface layer 2Ba than in the remaining portion 2Bb. That is, the first electrode 2B has different impurity concentrations, i.e., dopant amounts, between the surface layer 2Ba and the remaining portion 2Bb. For example, the impurity concentration in the surface layer 2Ba is higher by 10% or more than the impurity concentration in the remaining portion 2Bb. Such a concentration distribution of impurities in the first electrode 2B can be formed by a thermal diffusion method, an ion implantation method, or the like.

[0090] In the first electrode 2B, the impurity concentration may change sharply at the boundary between the surface layer 2Ba and the remaining portion 2Bb, or may gradually decrease with increasing distance from the surface layer 2Ba in the stacking direction. The impurity concentration in the first electrode 2B can be measured by epitaxial resistivity measurement, air gap CV measurement, mercury CV measurement, surface charge profiling, secondary ion mass spectrometry, spreading resistance measurement, or the like. Measurement results are essentially the same regardless of the method. The difference between the impurity concentration in the surface layer 2Ba and the impurity concentration in the remaining portion 2Bb can be confirmed by any of the above measurement methods. Specifically, by performing the above measurement method, an impurity concentration profile in the depth direction from the surface of the first electrode 2B on the optical waveguide 1 side is obtained. From the obtained impurity concentration profile, the integral average of the impurity concentration in the surface layer 2Ba and the integral average of the impurity concentration in the remaining portion 2Bb are calculated, and these are defined as the impurity concentration in the surface layer 2Ba and the impurity concentration in the remaining portion 2Bb, respectively. That is, the impurity concentration of the surface layer 2Ba is the integral average of the impurity concentration within a range of 10% of the depth (thickness) of the first electrode 2B from the surface of the first electrode 2B on the optical waveguide 1 side, and the impurity concentration of the remaining portion 2Bb is the integral average of the impurity concentration within the remaining range. The impurity concentration of the obtained surface layer 2Ba is, for example, 10% or more higher than the impurity concentration of the obtained remaining portion 2Bb.

[0091] In the first electrode 2B, because the skin effect causes the high-frequency signal to propagate through the surface layer 2Ba, it is preferable that the conductivity near the surface layer 2Ba be higher. In the optical modulator 10B according to this embodiment, the surface layer 2Ba of the first electrode 2B on the optical waveguide 1 side is doped with impurities at a higher concentration than the remaining portion 2Bb of the first electrode 2. In this case, a region with high conductivity can be localized near the optical waveguide 1 in the first electrode 2B, and the attenuation of the high-frequency signal can be suppressed by the skin effect.

[0092] <Fourth embodiment> 9 is a cross-sectional view showing a schematic configuration of an optical modulator 10C according to the fourth embodiment. The optical modulator 10C differs from the optical modulator 10A according to the second embodiment in the configuration of the first electrode 2C.

[0093] 9, the first electrode 2C has a surface layer 2Ca on the ridge portion 1Ab side of the optical waveguide 1A and a remaining portion 2Cb. The surface layer 2Ca on the ridge portion 1Ab side is a surface layer of the first electrode 2C through which an electric field applied to the ridge portion 1Ab passes together with the second electrode 3A. In the example of this embodiment, the surface layer 2Ca is a surface layer of the first electrode 2C located on the ridge portion 1Ab side, which essentially functions as an optical waveguide, in a direction (width direction) perpendicular to the stacking direction of the first electrode 2C relative to the optical waveguide 1A. The surface layer 2Ca is, for example, a portion within a range of 10% of the width direction length of the first electrode 2C from the surface located on the ridge portion 1Ab side in the width direction of the first electrode 2C. The remaining portion 2Cb refers to the portion of the first electrode 2C excluding the surface layer 2Ca. As with the first electrode 2B of the third embodiment described above, in the first electrode 2C, the concentration of impurities doped into the semiconductor material is higher in the surface layer 2Ca than in the remaining portion 2Cb. Therefore, the optical modulator 10C of this embodiment can achieve the same effects as the optical modulator 10B of the third embodiment.

[0094] FIG. 10 shows a modified example of the optical modulator 10C according to the fourth embodiment. Referring to FIG. 10, the surface layer 2Ca may be a surface layer of the first electrode 2C that is located on the optical waveguide 1A side in the stacking direction of the first electrode 2C relative to the optical waveguide 1. In this case, the surface layer 2Ca is, for example, a portion that is within a range of 10% of the thickness of the first electrode 2C from the surface that is located on the optical waveguide 1A side in the stacking direction. Even with this configuration, it is possible to achieve the same effects as the optical modulator 10B according to the third embodiment.

[0095] 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.

[0096] For example, in the optical modulator 10 according to the first embodiment, the first electrode 2 may include a convex portion. The convex portion is provided on the surface located on the optical waveguide 1 side in the stacking direction, and protrudes toward the optical waveguide 1. The convex portion is in contact with the first low-dielectric layer 4. In this case, the convex portion can concentrate the electric field in the optical waveguide. This allows the voltage applied between the first electrode 2 and the second electrode 3 to be reduced, further reducing power consumption.

[0097] When the first electrode 2 includes a protrusion, the length of the protrusion in a direction perpendicular to the stacking direction may be smaller as it approaches the optical waveguide 1, as viewed in a cross section perpendicular to the extension direction of the optical waveguide 1. In this case, the side of the protrusion can be made to continue relatively gently with other parts of the surface of the first electrode 2 on the optical waveguide 1 side. This makes it possible to prevent electrical loss from occurring at the boundary between the protrusion and other parts. The side of the protrusion may be inclined at a constant gradient relative to the surface, or the gradient of the side with respect to the surface may vary.

[0098] The optical modulator 10 according to the first embodiment may further include a metal thin film thinner than the first electrode 2. The metal thin film is provided on the surface of the first electrode 2 facing the optical waveguide 1. The metal thin film has high conductivity and low attenuation of high-frequency signals. The metal thin film can be formed, for example, using a metal material that can be used for the second electrode 3. By providing the metal thin layer on the surface of the first electrode 2 facing the optical waveguide 1, the resistance value can be reduced and signal attenuation can be suppressed. The metal thin layer may be applied to each of the optical modulators 10A, 10B, and 10C according to the second to fourth embodiments. In this case, the metal thin layer is provided on the surface of the first electrodes 2A, 2B, and 2C where an electric field passes.

[0099] <1> an optical waveguide made of a material having an electro-optic effect; a first electrode made of a semiconductor material and disposed with a gap between it and the optical waveguide; a second electrode disposed to form a potential difference with the first electrode to apply an electric field to the optical waveguide; an optical modulator comprising: a first low dielectric layer having a refractive index smaller than a refractive index of the optical waveguide and provided in the gap between the first electrode and the optical waveguide;

[0100] <2> <1> The optical modulator according to claim 1, further comprising: a second low dielectric layer having a refractive index smaller than that of the optical waveguide; the second electrode is disposed with a gap between it and the optical waveguide, The optical modulator, wherein the second low dielectric layer is provided in the gap between the second electrode and the optical waveguide.

[0101] <3> <1> The optical modulator according to claim 1, an optical modulator, wherein the first low dielectric layer surrounds the optical waveguide when viewed in a cross section perpendicular to the direction in which the optical waveguide extends, and is provided between the optical waveguide and each of the first electrode and the second electrode.

[0102] <4> <1> ~ <3> 10. An optical modulator according to claim 9, the first electrode is laminated on the optical waveguide; The second electrode is laminated on the optical waveguide on the opposite side of the first electrode.

[0103] <5> <4> The optical modulator according to claim 1, An optical modulator, wherein a ratio of a thickness of the first electrode to a thickness of the first low dielectric constant layer is not less than 20.0 and not more than 44.0.

[0104] <6> <1> The optical modulator according to claim 1, the optical waveguide includes a substrate portion and a ridge portion protruding from a surface of the substrate portion; the first low dielectric constant layer is laminated on the substrate portion and the ridge portion, an optical modulator, wherein the first electrode and the second electrode are stacked on the first low dielectric constant layer and arranged in parallel with a gap between them;

[0105] <7> <6> The optical modulator according to claim 1, an optical modulator, wherein a ratio of a thickness of the first electrode to a thickness of the first low dielectric constant layer at the position of the ridge portion is not less than 0.1 and not more than 4.0;

[0106] <8> <1> ~ <7> 10. An optical modulator according to claim 9, An optical modulator, wherein the size of the gap between the first electrode and the optical waveguide is not less than 0.750 μm and not more than 1.675 μm.

[0107] <9> <1> ~ <8> 10. An optical modulator according to claim 9, An optical modulator, wherein the semiconductor material is a silicon semiconductor material in which silicon is doped with impurities.

[0108] <10> <9> The optical modulator according to claim 1, The concentration of the impurity in the first electrode is 1.0×10 17 cm -3 That's it, 1.0 x 10 22 cm -3 The following is an optical modulator.

[0109] <11> <9> or <10> The optical modulator according to claim 1, The optical modulator, wherein the first electrode is a silicon single crystal substrate.

[0110] <12> <9> ~ <11> 10. An optical modulator according to claim 9, An optical modulator, wherein the first low dielectric constant layer is mainly composed of SiO2.

[0111] <13> <1> ~ <12> 10. An optical modulator according to claim 9, an optical modulator, wherein the refractive index of the first electrode is less than 3;

[0112] <14> <1> ~ <13> 10. The optical modulator according to claim 9, An optical modulator, wherein a surface layer of the first electrode on the optical waveguide side is doped with an impurity at a higher concentration than other portions of the first electrode. [Explanation of symbols]

[0113] 10, 10A, 10B, 10C: Optical modulator 1,1A: Optical waveguide 1Aa: Substrate section 1Ab: Ridge 2,2A,2B,2C: 1st electrode 2Ba,2Ca: Surface layer 2Bb,2Cb:Remainder 3,3A: 2nd electrode 4,4A: 1st low dielectric constant layer 5: Second low dielectric constant layer

Claims

1. An optical waveguide made of a material having an electro-optic effect, A first electrode made of a semiconductor material is positioned with a gap between it and the optical waveguide, A second electrode is positioned to form a potential difference with the first electrode and apply an electric field to the optical waveguide, A first low dielectric constant layer having a refractive index smaller than that of the optical waveguide and provided in the gap between the first electrode and the optical waveguide, The aforementioned semiconductor material is a silicon semiconductor material in which silicon is doped with impurities. An optical modulator in which the surface layer of the first electrode on the optical waveguide side is doped with impurities at a higher concentration compared to other parts of the first electrode.

2. The optical modulator according to Claim 1, An optical modulator in which, in the first electrode, the concentration of the impurity changes abruptly at the boundary between the surface layer and the other part.

3. The optical modulator according to Claim 1, An optical modulator in which, in the first electrode, the concentration of the impurity gradually decreases as it moves away from the surface layer.

4. The optical modulator according to Claim 1, An optical modulator in which, in the first electrode, the concentration of the impurity in the surface layer is 10% or more higher than the concentration of the impurity in the other parts.

5. The optical modulator according to claim 1, The first electrode is stacked on the optical waveguide, The second electrode is stacked on the optical waveguide on the opposite side of the first electrode. The surface layer is a portion of the first electrode that extends from the optical waveguide side surface to a range of 10% of the thickness of the first electrode, in an optical modulator.

6. The optical modulator according to claim 1, The optical waveguide includes a substrate portion and a ridge portion protruding from the surface of the substrate portion. The first low dielectric constant layer is laminated on the substrate portion and the ridge portion. The first electrode and the second electrode are stacked on the first low dielectric constant layer and arranged in parallel with gaps between them. The optical modulator wherein the surface layer is a portion located within 10% of the width of the first electrode, from the surface positioned on the ridge side in the width direction of the first electrode.

7. The optical modulator according to claim 1, The concentration of the impurity in the first electrode is 1.0 × 10⁻⁶. １７ cm －３ The above is 1.0 x 10 ２２ cm －３ The following: The first electrode is a silicon single crystal substrate, which is an optical modulator.

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