Optical modulator

Abstract

An optical modulator (10) is provided with an optical waveguide (1), a first electrode (2), a second electrode (3), and a first low dielectric constant layer (4). The optical waveguide (1) contains a material having an electro-optical effect. The first electrode (2) contains a semiconductor material and is disposed with a gap from the optical waveguide (1). The second electrode (3) is disposed so as to form a potential difference with the first electrode (2) and apply an electric field to the optical waveguide. The first low dielectric constant layer (4) has a refractive index smaller than the refractive index of the optical waveguide (1), and is provided in a gap between the first electrode (2) and the optical waveguide (1).

Description

Technical Field

[0001] The present disclosure relates to light modulators. Background Art

[0002] Due to the popularity of mobile terminals and cloud, the amount of Internet traffic has increased significantly. Therefore, the demand for optical communications has expanded. In optical communications, optical transceivers are required to convert optical signals and electrical signals. In optical transceivers, optical modulators are the main components. Optical modulators are responsible for converting electrical signals into optical signals.

[0003] Conventional optical modulators are described, for example, in Japanese Patent Publication No. 2020-034610 (Patent Document 1). The optical modulator of Patent Document 1 has a core having a slit waveguide structure. The core has an upper high refractive index layer, a lower high refractive index layer, and a low refractive index layer provided in a gap (slit) between these high refractive index layers. The refractive index of the upper and lower high refractive index layers is greater than the refractive index of the low refractive index layer. The upper and lower high refractive index layers each have a contact region. Metal electrodes are connected to the contact regions.

[0004] Prior Art Literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-034610 Summary of the invention

[0007] Problem that the invention aims to solve

[0008] As described in Patent Document 1, metal electrodes are used in conventional optical modulators. Generally, materials other than metal materials are not selected as the material of the electrodes of the optical modulator. When the electrodes are made of materials other than metal materials, light may be easily absorbed by the electrodes depending on the material. If light is absorbed by the electrodes, light loss increases.

[0009] An object of the present disclosure is to provide an optical modulator that can realize the use of materials other than metal materials for electrodes and can suppress light loss.

[0010] Technical solutions to solve problems

[0011] The optical modulator disclosed in the present invention comprises an optical waveguide, a first electrode, a second electrode, and a first low dielectric constant layer. The optical waveguide comprises a material having an electro-optical effect. The first electrode comprises a semiconductor material and is arranged with a gap between the optical waveguide and the second electrode. The second electrode is arranged to form a potential difference with the first electrode and apply an electric field to the optical waveguide. The first low dielectric constant layer has a refractive index smaller than that of the optical waveguide and is arranged in the gap between the first electrode and the optical waveguide.

[0012] Effects of the Invention

[0013] According to the optical modulator according to the present disclosure, it is possible to use materials other than metal materials for electrodes and suppress light loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a cross-sectional view showing a schematic structure of the optical modulator according to the first embodiment.

[0015] Figure 2 The diagram shows the relationship 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 terminal resistance Z0 in the first embodiment.

[0016] Figure 3 It is a diagram showing a first modification of the optical modulator according to the first embodiment.

[0017] Figure 4 It is a diagram showing a second modification of the optical modulator according to the first embodiment.

[0018] Figure 5 It is a cross-sectional view showing a schematic structure of an optical modulator according to the second embodiment.

[0019] Figure 6 The diagram shows the relationship 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 terminal resistance Z0 in the second embodiment.

[0020] Figure 7 1 is a diagram showing the relationship 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.

[0021] Figure 8 It is a cross-sectional view showing a schematic structure of an optical modulator according to a third embodiment.

[0022] Fig. 9 It is a cross-sectional view showing a schematic structure of an optical modulator according to a fourth embodiment.

[0023] Fig.10 It is a diagram showing a modified example of the optical modulator involved in the fourth embodiment. DETAILED DESCRIPTION

[0024] Hereinafter, embodiments of the present disclosure will be described. In addition, in the following description, embodiments of the present disclosure will be described by way of example, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and specific materials are sometimes exemplified, but the present disclosure is not limited to these exemplifications.

[0025] The optical modulator according to the embodiment of the present disclosure comprises an optical waveguide, a first electrode, a second electrode, and a first low dielectric constant layer. The optical waveguide comprises a material having an electro-optical effect. The first electrode comprises a semiconductor material and is arranged with a gap between the optical waveguide and the second electrode. The second electrode is arranged to form a potential difference with the first electrode and apply an electric field to the optical waveguide. The first low dielectric constant layer has a refractive index smaller than that of the optical waveguide and is arranged in a gap between the first electrode and the optical waveguide (first structure).

[0026] In the first structure, among the first electrode and the second electrode for applying an electric field to the optical waveguide, the first electrode includes a semiconductor material. Impurities are usually doped in the semiconductor material. In order to improve the function of the first electrode as an electrode, it is necessary to increase the doping amount of impurities. If the doping amount of impurities increases, the conductivity of the first electrode increases, but the light absorptivity of the first electrode increases. In addition, since the refractive index of the first electrode including the semiconductor material is greater than that of the optical waveguide, if the first electrode contacts the optical waveguide, light easily leaks from the optical waveguide to the first electrode. Therefore, in the first structure, the first electrode is configured with a gap between the first electrode and the optical waveguide so that it does not contact the optical waveguide, and on this basis, a first low dielectric constant layer having a refractive index smaller than that of the optical waveguide is configured in the gap between the first electrode and the optical waveguide. As a result, the light passing through the optical waveguide becomes less likely to leak to the first electrode side and becomes less likely to be absorbed by the first electrode. Therefore, it is possible to apply a semiconductor material other than a metal material to the first electrode, and it is possible to suppress the loss of light.

[0027] The optical modulator of the first structure may further include a second low dielectric constant layer. The second low dielectric constant layer has a refractive index smaller than that of the optical waveguide. In this case, the second electrode and the optical waveguide are arranged with a gap therebetween, and the second low dielectric constant layer is provided in the gap between the second electrode and the optical waveguide (second structure).

[0028] In the second structure, the second electrode and the optical waveguide are arranged with a gap between the first electrode and the second electrode for applying an electric field to the optical waveguide. Therefore, the second electrode and the optical waveguide are not in contact. Furthermore, a second low dielectric constant layer having a refractive index smaller than that of the optical waveguide is arranged in the gap between the second electrode and the optical waveguide. As a result, light passing through the optical waveguide becomes less likely to leak to the second electrode side and becomes less likely to be absorbed by the second electrode. Therefore, the loss of light can be further suppressed.

[0029] In the optical modulator of the first structure, the first low dielectric constant layer may surround the optical waveguide and be provided between the optical waveguide and the first electrode and between the optical waveguide and the second electrode, respectively, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends (third structure).

[0030] In any one of the optical modulators of the first to third structures, preferably, the first electrode is stacked on the optical waveguide, and the second electrode is stacked on the optical waveguide on the opposite side of the first electrode (fourth structure). In this case, in the stacking direction of the first electrode, the optical waveguide, and the second electrode, the optical waveguide exists between the first electrode and the second electrode. Therefore, the electric field generated by the first electrode and the second electrode can be efficiently applied to the optical waveguide.

[0031] In the optical modulator of the fourth configuration, preferably, the ratio of the thickness of the first electrode to the thickness of the first low dielectric constant layer is 20.0 to 44.0 (fifth configuration). In this case, generation of reflected waves of the electric signal can be suppressed.

[0032] In the optical modulator of the first structure, the optical waveguide includes a substrate portion and a protruding portion protruding from the surface of the substrate portion. Alternatively, the first low dielectric constant layer may be stacked on the substrate portion and the protruding portion, and the first electrode and the second electrode may be stacked on the first low dielectric constant layer and arranged in parallel with a gap therebetween (the sixth structure).

[0033] In the optical modulator of the sixth configuration, preferably, the ratio of the thickness of the first electrode to the thickness of the first low dielectric constant layer at the position of the protrusion is 0.1 to 4.0 (seventh configuration). In this case, generation of reflected waves of the electric signal can be suppressed.

[0034] In any one of the optical modulators of the first to seventh structures, preferably, a size of a gap between the first electrode and the optical waveguide is not less than 0.750 μm and not more than 1.675 μm (eighth structure).

[0035] Although it is weak, light still leaks out from the optical waveguide to the first low dielectric constant 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 greater than 0.750 μm as in the eighth structure, the evanescent light becomes less likely to contact the first electrode, and the loss of light can be further suppressed.

[0036] In the eighth structure, the gap between the first electrode and the optical waveguide is 1.675 μm or less. In this case, the distance between the first electrode and the optical waveguide does not become too large, and the electric field on the optical waveguide can be ensured without increasing the voltage applied between the first electrode and the second electrode.

[0037] In any one of the optical modulators of the first to eighth structures, preferably, the semiconductor material in the first electrode is a silicon semiconductor material obtained by doping silicon with impurities (ninth structure).

[0038] In the optical modulator of the ninth structure, preferably, the concentration of impurities in the first electrode is 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 Below (Structure 10). In the first electrode, as the impurity increases, the resistivity decreases and the conductivity increases. If the impurity concentration is 1.0×10 17 cm -3 If the concentration of impurities is 1.0×10 22 cm -3 The following can prevent the precipitation of impurities.

[0039] In the optical modulator of the ninth or tenth structure, preferably, the first electrode is a silicon single crystal substrate (the eleventh structure).

[0040] In any of the optical modulators of the 9th to 11th structures, the main component of the first low dielectric constant layer may also be SiO2 (the 12th structure). The semiconductor material used for the first electrode is a silicon semiconductor material, so the first low dielectric constant layer of SiO2 can be formed on the first electrode by a thermal oxidation method. In the film formation using the thermal oxidation method, the first low dielectric constant layer has good adhesion to the first electrode, and foreign matter is not easy to enter the interface between the first electrode and the first low dielectric constant layer. Therefore, at the interface between the first electrode and the first low dielectric constant layer, electrical loss can be suppressed. In addition, the aggregation of foreign matter can be suppressed at the interface between the first electrode and the first low dielectric constant layer, so the reliability and life of the optical modulator can be improved.

[0041] In any of the optical modulators of the first to twelfth structures, the refractive index of the first electrode is less than 3 (the thirteenth structure). In this case, for example, the refractive index of the first electrode becomes less than 3 according to the concentration (doping amount) of the impurity of the tenth structure.

[0042] In any one of the optical modulators of the first to thirteenth structures, preferably, the surface layer on the optical waveguide side of the first electrode is doped with impurities at a higher concentration than other parts of the first electrode (fourteenth structure). In this case, in the first electrode, a region with high conductivity can be locally present near the optical waveguide, and attenuation of high-frequency signals can be suppressed by the skin effect.

[0043] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In each of the drawings, the same or corresponding structures are denoted by the same reference numerals, and the description thereof will not be repeated.

[0044] <First Embodiment>

[0045] [Structure of light modulator]

[0046] Figure 1 2 is a cross-sectional view showing a schematic structure of an optical modulator 10 according to the first embodiment. The optical modulator 10 includes an optical waveguide 1, a first electrode 2, a second electrode 3, a first low dielectric constant layer 4, and a second low dielectric constant layer 5. Figure 1 , a cross section perpendicular to the direction in which the optical waveguide 1 extends is shown. In this specification, a cross section means a cross section perpendicular to the direction in which the optical waveguide 1 extends, unless otherwise specified.

[0047] like Figure 1 As shown, the optical waveguide 1 may have a substantially rectangular cross section. The optical waveguide 1 includes a material having an electro-optic effect (electro-optic material). The optical waveguide 1 functions as a light transmission path. As the optical waveguide 1, LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate), PLZT (lead lanthanum zirconate titanate), KTN (potassium tantalate niobate), or BaTiO3 (barium titanate) may be used. As the optical waveguide 1, an electro-optic polymer (EO polymer) may also be used.

[0048] The first electrode 2 and the second electrode 3 function as control electrodes for controlling light passing through the optical waveguide 1. The first electrode 2 and the second electrode 3 may each have a substantially rectangular cross section. The first electrode 2 and the second electrode 3 are arranged to form a potential difference between each other and 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.

[0049] In the example of this embodiment, the first electrode 2 is stacked on the optical waveguide 1. The second electrode 3 is stacked on the optical waveguide 1 on the opposite side of the first electrode 2. From another point of view, the first electrode 2 and the second electrode 3 are arranged so as to sandwich the optical waveguide 1.

[0050] The first electrode 2 is arranged with a gap between it and the optical waveguide 1. In the present 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, not less than 0.750 μm and not more than 1.675 μm. 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 the case of the present 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.

[0051] The first electrode 2 includes 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 in Si (silicon). As the semiconductor material, for example, other single element semiconductors using Ge (germanium) or compound semiconductors such as GaAs (gallium arsenide) can also be used. The impurity can be either a p-type impurity or an n-type impurity. For example, when the semiconductor material is a silicon semiconductor material, a Group 3 element such as boron can be used as a p-type impurity, and a Group 5 element such as phosphorus, arsenic, and antimony can be used as an n-type impurity.

[0052] When the semiconductor material used for the first electrode 2 is a silicon semiconductor material, the concentration (doping amount) of impurities in the first electrode 2 is preferably 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 As the doping amount of impurities increases, the resistivity of the semiconductor material decreases and the conductivity increases. If the doping amount is 1.0×10 17 cm -3 As described above, the first electrode 2 can effectively function as an electrode. 22 cm -3 Then, the precipitation of impurities can be prevented by the solid solubility limit of impurities in the silicon semiconductor material. 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.

[0053] Regardless of whether or not the impurity is doped, the refractive index of the semiconductor material is greater than the refractive index of the electro-optical material constituting the optical waveguide 1. The refractive index of the semiconductor material in the state of not being doped with impurities is, for example, 3.4 for Si, 5.5 for Ge, and 3.3 for GaAs. The refractive index of the electro-optical 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. If the semiconductor material is doped with impurities, the refractive index will decrease, but the refractive index of the semiconductor material after doping is also greater than the refractive index of the electro-optical material.

[0054] The above-mentioned doping amount range is described in more detail below. The upper limit of the doping amount is preferably 1.0×10 22 cm -3 The reason is based on the solid solubility limit of impurities in silicon semiconductor materials. In the first electrode 2, if the doping amount exceeds 1.0×10 22 cm -3, impurities are deposited, and the reliability of the first electrode 2 and the optical modulator 10 is reduced. On the other hand, the lower limit of the doping amount is preferably 1.0×10 17 cm -3 The reason is as follows. As an indicator for designing the thickness and width of the electrode, there is skin depth. If the thickness and width of the electrode are smaller than the skin depth, the resistance value increases. Therefore, it is preferable to set the thickness and width of the electrode to be greater than the skin depth. In the case of considering the use of an optical modulator that processes a frequency signal of 1 GHz or more, when the doping amount is 1.0×10 17 cm -3 When the conductivity is 1000 S / m, the skin depth becomes 500 μm. For example, if we consider forming an electrode using a silicon semiconductor material by microfabrication, the thickness of the electrode is limited to about 500 μm. In this way, from the perspective of ensuring the performance of the electrode, in order to obtain an electrode with a conductivity of more than 1000 S / m, it is sufficient to set the doping amount to 1.0×10 17 cm -3 That’s all.

[0055] The first electrode 2 is, for example, a silicon single crystal substrate. Impurities are, for example, pre-doped into a mother substrate of silicon single crystal that is a raw material of the first electrode 2. The mother substrate is arranged on another substrate and patterned (etched, cut, etc.), thereby forming the first electrode 2. The first electrode 2 may also be an active layer of an SOI (Silicon on Insulator) substrate. In this case, the first electrode 2 can be formed by patterning (etched, cut, etc.) the active layer of the SOI substrate. For the first electrode 2 formed in this way, impurities may be further introduced by a thermal diffusion method or an ion implantation method.

[0056] The first electrode 2 may also be a semiconductor silicon layer formed on a substrate. For example, a silicon layer may be formed on a substrate by sputtering, vapor deposition, or CVD. Impurities may be introduced into the silicon layer by thermal diffusion or ion implantation to form a semiconductor silicon layer serving as the first electrode 2.

[0057] The second electrode 3 is arranged with a gap between it and the optical waveguide 1. In the present embodiment, the second electrode 3 is separated from the optical waveguide 1 in the stacking direction. The second electrode 3 does not contact the optical waveguide 1. The size of the gap between the second electrode 3 and the optical waveguide 1 is, for example, not less than 0.750 μm and not more than 1.675 μm. 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 the case of the present 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.

[0058] The second electrode 3 includes, for example, a metal material. That is, the second electrode 3 is a metal electrode. However, the second electrode 3 may also include, for example, a semiconductor material. That is, the second electrode 3 may also be a semiconductor electrode. As an example of a semiconductor material, the same semiconductor material as that used for the first electrode 2 can be cited.

[0059] When the second electrode 3 includes a metal material, the main component of the metal material is, for example, a noble metal. The noble metal is, for example, Au (gold). As the noble metal, Ag (silver), Pt (platinum), etc. may also be used. The metal material may also contain trace amounts of other metal elements such as Cr and Ti. As the metal material, copper, aluminum, or alloys thereof may also be used.

[0060] The second electrode 3 is used as a signal electrode, and the first electrode 2 is used as a ground electrode. Conversely, the first electrode 2 may be used as a signal electrode, and the second electrode 3 may be used as a ground electrode.

[0061] The first low dielectric constant layer 4 is provided in a gap between the first electrode 2 and the optical waveguide 1. In the present embodiment, the first low dielectric constant layer 4 is stacked on the first electrode 2, and the optical waveguide 1 is stacked on the first low dielectric constant layer 4. That is, the optical waveguide 1 is indirectly stacked on the first electrode 2 via the first low dielectric constant layer 4, and the first electrode 2 is not in contact with the optical waveguide 1. The first low dielectric constant layer 4 preferably covers the entire surface of the optical waveguide 1 that faces the first low dielectric constant layer 4.

[0062] The first low dielectric constant layer 4 has a refractive index smaller than that of the optical waveguide 1. For example, the refractive index of the first low dielectric constant 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 the refractive index of the first low dielectric constant layer 4 is, for example, greater than 1.8 and less than 2.5. In this case, light can be sufficiently confined in the optical waveguide 1. In addition, the ratio of the refractive index of the first electrode 2 to the refractive index of the first low dielectric constant layer 4 is, for example, greater than 1.5 and less than 6.0. In this case, when light is incident from the optical fiber to the optical waveguide 1, the light can be prevented from being incident on the first electrode 2.

[0063] The main component of the first low dielectric constant layer 4 is typically SiO 2 . As the main component of the first low dielectric constant layer 4 , oxides such as Al 2 O 3 , LaAlO 3 , LaYO 3 , ZnO, HfO 2 , MgO, and Y 2 O 3 , or polymers such as BCB (benzocyclobutene) and PI (polyimide) may be used.

[0064] The second low dielectric constant layer 5 is provided in a gap between the second electrode 3 and the optical waveguide 1. In the present embodiment, the second low dielectric constant layer 5 is stacked on the optical waveguide 1, and the second electrode 3 is stacked on the second low dielectric constant layer 5. That is, the optical waveguide 1 is indirectly stacked on the second electrode 3 via the second low dielectric constant layer 5, and the second electrode 3 is not in contact with the optical waveguide 1. The second low dielectric constant layer 5 preferably covers the entire surface of the optical waveguide 1 that faces the second low dielectric constant layer 5.

[0065] The second low dielectric constant layer 5 has a refractive index smaller than that of the optical waveguide 1. As an example of the main component of the second low dielectric constant layer 5, the same main component as the first low dielectric constant layer 4 can be cited. The main component of the second low dielectric constant layer 5 may be the same as the main component of the first low dielectric constant layer 4, or may be different from the main component of the first low dielectric constant layer 4.

[0066] In the optical modulator 10 of such a structure, the second electrode 3 can be stacked with respect to the optical waveguide 1 and the first electrode 2, for example, as follows. First, the first low dielectric constant layer 4 is formed on the first electrode 2 by CVD, evaporation, sputtering, etc. A material substrate having an electro-optical effect is arranged on the first low dielectric constant layer 4 formed on the first electrode 2, and the material substrate and the first low dielectric constant layer 4 are bonded. Then, photolithography and etching are performed on the material substrate to form the optical waveguide 1. Next, the second low dielectric constant layer 5 is formed on the optical waveguide 1 by CVD, evaporation, sputtering, etc. Then, a metal layer is formed on the second low dielectric constant layer 5 by sputtering, evaporation, etc. The formed metal layer is patterned by photolithography, and the second electrode 3 is formed by etching.

[0067] When the optical modulator 10 is used and a high frequency current is passed through the first electrode 2, 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 formula (1) is a formula for calculating the skin depth of a conductor.

[0068] [Mathematical formula 1]

[0069] (1)

[0070] d: Skin depth [m]

[0071] ρ: Resistivity of conductor [Ω·m] = 1 / σ

[0072] ω: angular frequency of alternating current [rds / s] = 2πf

[0073] μ: Magnetic permeability of conductor [H / m]

[0074] σ: Conductor conductivity [S / m]

[0075] f: frequency of alternating current [Hz]

[0076] According to formula (1), the required thickness t2 of the first electrode 2 can be determined. More specifically, by making the thickness t2 of the first electrode 2 larger than the skin depth calculated using formula (1), the resistance of the first electrode 2 can be reduced, and unnecessary electrical loss can be suppressed. The higher the conductivity of the first electrode 2, the better. However, there is a solid solubility limit for the doping amount of impurities. In addition, if the doping amount is close to the solid solubility limit, the impurities will cluster and become inert as carriers. Therefore, if the doping amount exceeds a certain amount, the conductivity of the first electrode 2 will be saturated. If the solid solubility limit of the doping amount is 1.0×10 22 cm -3 , then the conductivity of the first electrode 2 becomes 1×10 7 S / m, the skin depth of a 1 GHz electrical signal is 5 μm. However, it is conceivable that the actual conductivity decreases by one order of magnitude to 1×10 6 Therefore, if the electric signal is given a bandwidth and is used to process a signal of 0.5 GHz or higher, the thickness t2 of the first electrode 2 is preferably 25 μm or more.

[0077] When the optical modulator 10 is used, the ratio Z / Z0 of the resistance Z of the optical modulator 10 to the terminal resistance Z0 is preferably greater than 0.8 and less than 1.2. This is because if the ratio Z / Z0 deviates from the condition of being greater than 0.8 and less than 1.2, a reflected wave of the electrical signal is generated at the terminal of the electrode due to impedance mismatch. Therefore, it is preferable to set the various conditions of the optical modulator 10 so that the ratio Z / Z0 satisfies the 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 constant layer 4 so that the ratio Z / Z0 of the resistance Z of the optical modulator 10 to the terminal resistance Z0 is greater than 0.8 and less than 1.2. The thickness t4 of the first low dielectric constant layer 4 is equivalent to the size of the gap between the first electrode 2 and the optical waveguide 1.

[0078] Figure 2 2 is a diagram showing the relationship 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 terminal resistance Z0. Figure 2, as an example, the relationship between the ratio t2 / t4 and the ratio Z / Z0 when a silicon semiconductor material is used as the first electrode 2, a metal material mainly composed of Au is used as the second electrode 3, SiO2 is used as the low dielectric constant layers 4 and 5, and LiNbO3 is used as the optical waveguide 1 is analyzed. In the analysis, the terminal resistance Z0 is set to 50Ω, the width w2 of the first electrode 2 is set to 50μm, the width w3 of the second electrode 3 is set to 40μm, the thickness t3 of the second electrode 3 is set to 5.0μm, the thickness t4 of the first low dielectric constant layer 4 is set to 0.7μm, the thickness t5 of the second low dielectric constant layer 5 is set to 0.7μm, the width w1 of the optical waveguide 1 is set to 1.0μm, and the thickness t1 of the optical waveguide 1 is set to 1.3μm. In this specification, the thickness means the length in the stacking direction in the cross section of the light modulator 10 , and the width means the length in the direction perpendicular to the stacking direction in the cross section of the light modulator 10 .

[0079] like Figure 2 As shown, in order to make the ratio Z / Z0 equal to or greater than 0.8 and equal to or less than 1.2, 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 equal to or greater than 20.0 and equal to or less than 44.0.

[0080] Refer again Figure 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, the performance of the first electrode 2 as an electrode is preferably equivalent to that of the second electrode 3 as a metal electrode. It is preferable to set the cross-sectional area of ​​the first electrode 2 and the cross-sectional area of ​​the second electrode 3 so that the resistance value of the first electrode 2 is substantially the same as the resistance value of the second electrode 3. For example, the cross-sectional area of ​​the first electrode 2 can be set to "(electrical conductivity of the second electrode 3 / electrical conductivity of the first electrode 2)×cross-sectional area of ​​the second electrode 3".

[0081] Specifically, when viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the area of ​​the first electrode 2 is preferably larger than the area of ​​the second electrode 3. When the second electrode 3 is made of a metal material, the second electrode 3 has a relatively small resistance value even if the cross-sectional area is not increased. On the other hand, the first electrode 2 is made of a semiconductor material having a lower electrical conductivity than the metal material, and thus the resistance value can be reduced to the same level as the second electrode 3 by making the cross-sectional area larger than the second electrode 3. Thus, power consumption can be suppressed.

[0082] Under the condition that the width w2 of the first electrode 2 and the width w3 of the second electrode 3 are the same, in order to make the resistance value of the first electrode 2 as a semiconductor electrode consistent with the resistance value of the second electrode 3 as a metal electrode, the product of the conductivity and thickness t2 in the first electrode 2 and the product of the conductivity and thickness t3 in the second electrode 3 can be consistent. For example, when the metal material in the second electrode 3 is Au, the thickness t3 of the second electrode 3 is usually set to be greater than 0.1 μm and less than 2.0 μm, and its conductivity is 4.3×10 7 On the other hand, since the electrical 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 electrical conductivity of the first electrode 2 changes depending on the doping amount of the impurity.

[0083] For example, when the first electrode 2 is made of a silicon semiconductor material, when the doping amount of the impurity is 1.0×10 22 cm -3 When the conductivity of the first electrode 2 becomes 1×10 7 At this time, 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 impurities is 1.0×10 17 cm -3 When the conductivity of the first electrode 2 becomes 1000×10 4 At this time, the value obtained by dividing the conductivity of the second electrode 3 by the conductivity of the first electrode 2 is 4.3×10 3 The thickness t2 of the first electrode 2 can be set to 4.3×10 of the thickness t3 of the second electrode 3. 3 times.

[0084] Therefore, when the first electrode 2 is formed of a semiconductor material and the second electrode 3 is formed 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, that is, 0.1 μm. That is, 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, that is, 2.0 μm. 3 That is, the thickness t2 of the first electrode 2 can be set to 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 set to 500 μm or less from the viewpoint of processability.

[0085] As described above, the second electrode 3 can also be formed of a semiconductor material. When both the first electrode 2 and the second electrode 3 include semiconductor materials, the area of ​​the first electrode 2 is preferably the same as the area of ​​the second electrode 3 when viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends.

[0086] The thickness t2 of the first electrode 2 as a semiconductor electrode and the thickness t4 of the low dielectric constant layer 4 can be measured, for example, by the following method. The first method is a measurement method based on SEM observation. In this method, the optical modulator 10 is cut by FIB (focused ion beam) to collect a sample. The cross section of the collected sample can be photographed by 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 by interference spectroscopy. Regardless of which method is used, the measurement results are substantially the same.

[0087] When the second electrode 3 is a metal electrode, the thickness t3 of the second electrode 3 can be measured, for example, by the following method. The first method is the above-mentioned measurement method based on SEM observation. The second method is a measurement method using X-rays. In this method, X-rays are irradiated to the second electrode 3, and the amount of transmitted X-rays is measured, thereby obtaining the attenuation caused by the second electrode 3. By performing an inverse operation on the obtained attenuation, the thickness t3 of the second electrode 3 can be measured. Regardless of which method is used, the measurement results are substantially the same. When the second electrode 3 is a semiconductor electrode, the thickness t3 of the second electrode 3 can be measured by the above-mentioned method for measuring the thickness t2 of the first electrode 2.

[0088] The doping amount in the first electrode 2 can be measured by epitaxial resistivity measurement, air gap CV measurement, mercury CV measurement, surface charge distribution, secondary ion mass spectrometry, or spreading resistance measurement, etc. The measurement results are substantially the same regardless of the method.

[0089] When viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the width w2 of the first electrode 2 on the optical waveguide 1 side is preferably greater than the width w1 of the optical waveguide 1. The width w2 of the first electrode 2 on the optical waveguide 1 side means the width of the surface of the first electrode 2 closest to the optical waveguide 1. In this embodiment, the length of the surface of the first electrode 2 in the direction perpendicular to the stacking direction that is in contact with the first low dielectric constant layer 4 is the width w2. In this case, an electric field can be applied to the entire region of the optical waveguide 1.

[0090] [Effect]

[0091] In the optical modulator 10 according to the present embodiment, of the first electrode 2 and the second electrode 3 for applying an electric field to the optical waveguide 1, the first electrode 2 includes a semiconductor material. Impurities are generally doped in the semiconductor material. In order to improve the function of the first electrode 2 as an electrode, it is necessary to increase the doping amount of the impurities. If the doping amount of the impurities increases, the conductivity of the first electrode 2 increases, but the light absorptivity of the first electrode 2 increases. In addition, since the refractive index of the first electrode 2 including the semiconductor material is larger than that of the optical waveguide 1, if 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 the present embodiment, the first electrode 2 is arranged with a gap between the optical waveguide 1 so as not to contact the optical waveguide 1, and on this basis, a first low dielectric constant layer 4 having a refractive index smaller than that of the optical waveguide 1 is arranged in the gap between the first electrode 2 and the optical waveguide 1. As a result, the light passing through the optical waveguide 1 becomes less likely to leak to the first electrode 2 side and becomes less likely to be absorbed by the first electrode 2. Therefore, according to the optical modulator 10 according to the present embodiment, it is possible to use a semiconductor material other than a metal material for the first electrode 2 and suppress light loss.

[0092] Typically, in an optical modulator, the effective refractive index of the electrical signal (modulation wave (GHz)) applied from the electrode to the optical waveguide is greater than the effective refractive index of the optical wave (carrier wave (THz)) passing through the optical waveguide. If the effective refractive index of the electrical signal and the effective refractive index of the optical wave are greatly different, the difference between the propagation speed of the optical wave and the propagation speed of the electrical signal becomes larger, and the modulation speed decreases. In the optical modulator 10 involved in the present embodiment, the first low dielectric constant layer 4 is configured at least between the first electrode 2 and the optical waveguide 1, so that the cross-sectional area ratio of the first low dielectric constant layer 4 relative to the optical waveguide 1 can be adjusted. By adjusting the cross-sectional area ratio of the first low dielectric constant layer 4 relative to the optical waveguide 1, the difference between the effective refractive index of the electrical signal and the effective refractive index of the optical wave can be reduced, and as a result, the difference between the propagation speed of the optical wave and the propagation speed of the electrical signal can be reduced. As a result, the decrease in the modulation speed can be suppressed.

[0093] In this embodiment, the second electrode 3 is arranged 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 constant layer 5 having a refractive index lower than that of the optical waveguide 1 is provided in the gap between the second electrode 3 and the optical waveguide 1. As a result, the light passing through the optical waveguide 1 becomes less likely to leak to the second electrode 3 side and becomes less likely to be absorbed by the second electrode 3. Therefore, the loss of light can be suppressed.

[0094] The semiconductor material used for the first electrode 2 is, for example, a silicon semiconductor material in which impurities are doped in Si. 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, evaporation, etc. Therefore, the internal stress of the first electrode 2 can be suppressed, and the first electrode 2 can be formed thick. By forming the first electrode 2 thick, the resistance value of the first electrode 2 becomes small, and power consumption can be suppressed. In addition, since the internal stress of the first electrode 2 is reduced, the generation of cracks caused by the internal stress can be suppressed. As a result, failure and damage of the optical modulator 10 can be suppressed.

[0095] For example, silicon semiconductor materials are cheaper than metal materials using noble metals. Therefore, if the first electrode 2 is formed of silicon semiconductor materials, the cost of the optical modulator 10 can be reduced.

[0096] 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, in the stacking direction, the optical waveguide 1 exists between the first electrode 2 and the second electrode 3. Therefore, the electric field generated by the first electrode 2 and the second electrode 3 can be efficiently applied to the optical waveguide 1.

[0097] 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 a thermal oxidation method. In this case, the first low dielectric constant layer 4 has good adhesion to the first electrode 2, and foreign matter is not easy to enter the interface between the first electrode 2 and the first low dielectric constant layer 4. Therefore, at the interface between the first electrode 2 and the first low dielectric constant layer 4, electrical loss can be suppressed. In addition, the reliability and life of the optical modulator 10 can be improved. This is because if foreign matter gathers at the interface between the first electrode 2 and the first low dielectric constant layer 4, and the electric field is concentrated on the gathered foreign matter, the optical modulator 10 may be damaged.

[0098] The penetration depth of the evanescent light in each of the low dielectric constant layers 4 and 5 can be estimated based on the wavelength of the light (carrier) passing through the optical waveguide 1. When the electrodes 2 and 3 are each separated from the optical waveguide 1 by more than the wavelength of the carrier, the evanescent light can be prevented from contacting the electrodes 2 and 3. Therefore, the size of the gap between the optical waveguide 1 and each of the electrodes 2 and 3, that is, the thickness t4 and t5 (the length in the stacking direction) of each of the low dielectric constant layers 4 and 5 is preferably greater than the wavelength of the light passing through the optical waveguide 1.

[0099] For example, if the size of the gap between each of the electrodes 2 and 3 and the optical waveguide 1 is 0.750 μm or more as in the present embodiment, the thickness of each of the low dielectric constant layers 4 and 5 becomes larger than the penetration depth of the evanescent light, and the light passing through the optical waveguide 1 is less likely to leak to each of the electrodes 2 and 3. As described above, the size of the gap between each of the electrodes 2 and 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 and 3 and the optical waveguide 1 is 1.675 μm or less, the size of the electric field to the optical waveguide 1 can be ensured even if the voltage applied between the first electrode 2 and the second electrode 3 is not increased.

[0100] [Modification 1]

[0101] exist Figure 3 , a modification 1 of the optical modulator 10 according to the first embodiment is shown. Figure 3 As shown, the optical modulator 10 may not include the second low dielectric constant layer 5 ( Figure 1 ). That is, the second electrode 3 may be directly stacked on the optical waveguide 1 and in contact with the optical waveguide 1. In this case, the dimensional relationship between the first electrode 2 as a semiconductor electrode and the first low dielectric constant layer 4 can also 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 terminal resistance Z0 is greater than 0.8 and less than 1.2.

[0102] [Modification 2]

[0103] exist Figure 4 , a second variation of the optical modulator 10 according to the first embodiment is shown. Figure 3 In the example shown in FIG. 1 , the second low dielectric constant layer 5 ( Figure 1 ). However, in this modification, the first low dielectric constant layer 4 is provided 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 the first electrode 2 and the second electrode 3, respectively, 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 ( Figure 1 )’s role.

[0104] <Second Embodiment>

[0105] Figure 51 is a cross-sectional view schematically showing a structure of an optical modulator 10A according to the second embodiment. The optical modulator 10A is different from the optical modulator 10 according to the first embodiment in the structure of an optical waveguide 1A and the arrangement of a first electrode 2A and a second electrode 3A.

[0106] Reference Figure 5 The optical modulator 10A includes an optical waveguide 1A, a first electrode 2A, a second electrode 3A, and a first low dielectric constant layer 4A. The optical waveguide 1A includes a substrate portion 1Aa and a protrusion 1Ab. The protrusion 1Ab protrudes from the surface of the substrate portion 1Aa. The protrusion 1Ab substantially functions as an optical waveguide. The first low dielectric constant layer 4A is stacked on the optical waveguide 1A. More specifically, the first low dielectric constant layer 4A is stacked on the substrate portion 1Aa and the protrusion 1Ab.

[0107] The first electrode 2A and the second electrode 3A are stacked on the first low dielectric constant layer 4A. The first electrode 2A and the second electrode 3A are arranged in parallel with a gap between them. Specifically, in the cross-sectional view of the optical modulator 10A, the first electrode 2A and the second electrode 3A are arranged in a direction substantially perpendicular to the stacking direction of the optical waveguide 1A and the first low dielectric constant layer 4A. In the direction substantially perpendicular to the stacking direction, the first electrode 2A is arranged on one side of the protrusion 1Ab, and the second electrode 3A is arranged on the other side of the protrusion 1Ab. The first electrode 2A and the second electrode 3A can form a potential difference between each other to apply an electric field to the protrusion 1Ab of the optical waveguide 1A.

[0108] The optical modulator 10A according to the present embodiment can also achieve the same effects as those of the optical modulator 10 according to the first embodiment.

[0109] When the optical modulator 10A is used, as described above, the ratio Z / Z0 of the resistance Z of the optical modulator 10A to the terminal resistance Z0 is preferably greater than 0.8 and less than 1.2. Therefore, it is preferable to set 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 so that the ratio Z / Z0 of the resistance Z of the optical modulator 10A to the terminal resistance Z0 is greater than 0.8 and less than 1.2. The thickness t4A of the first low dielectric constant layer 4A in the present embodiment is the thickness of the first low dielectric constant layer 4A at the position of the raised portion 1Ab. The so-called thickness t4A refers to the shortest distance from the interface between the raised 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 raised portion 1Ab, the first low dielectric constant layer 4A, and the electrodes 2A and 2B.

[0110] Figure 61 is a diagram showing the relationship 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 terminal resistance Z0 in the second embodiment. Figure 6 , as an example, the relationship between the ratio t2A / t4A and the ratio Z / Z0 when a silicon semiconductor material is used as the first electrode 2A, a metal material mainly composed of Au is used as the second electrode 3A, SiO2 is used as the first low dielectric constant layer 4A, and LiNbO3 is used as the optical waveguide 1A is analyzed. In the analysis, the terminal resistance Z0 is set to 50Ω, the width w2A of the first electrode 2A is set to 50μm, the width w3A of the second electrode 3A is set to 21.5μm, the thickness t3A of the second electrode 3A is set to 16.6μm, the thickness t4A of the first low dielectric constant layer 4A is set to 8.3μm, the width w1Ab of the raised portion 1Ab is set to 2.0μm, the thickness t1Ab of the raised portion 1Ab is set to 1.0μm, and the gap g between the first electrode 2A and the second electrode 3A is set to 10μm.

[0111] like Figure 6 As shown, in order to make the ratio Z / Z0 equal to or greater than 0.8 and equal to or less than 1.2, 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 set to be equal to or greater than 0.1 and equal to or less than 4.0.

[0112] When the optical modulator 10A is used, the effective refractive index n is preferably a value that maximizes the modulation speed. Therefore, it is preferable to set the ratio t1Ab / t4A of the thickness t1Ab of the optical waveguide 1A (protrusion 1Ab) to the thickness t4A of the first low dielectric constant layer 4A so that the effective refractive index n becomes the value that maximizes the modulation speed.

[0113] Figure 7 : is a graph showing the relationship 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. Figure 7 As an example, the Figure 6 The relationship between the ratio t1Ab / t4A and the effective refractive index n when the analysis was performed under the same conditions was as follows: However, the thickness t2A of the first electrode 2A was set to 16.6 μm.

[0114] When LiNbO3 is used as the optical waveguide 1A, if the effective refractive index n is 2, the modulation speed can be maximized. Figure 7As shown, in order to make the effective refractive index n 2, the ratio t1Ab / t4A of the thickness t1Ab of the optical waveguide 1A (the raised portion 1Ab) to the thickness t4A of the first low dielectric constant layer 4A is substantially 0.28.

[0115] <Third Embodiment>

[0116] Figure 8 2 is a cross-sectional view schematically showing a structure of an optical modulator 10B according to the third embodiment. The optical modulator 10B is different from the optical modulator 10 according to the first embodiment in the structure of a first electrode 2B.

[0117] Reference Figure 8 The first electrode 2B has a surface layer 2Ba on the optical waveguide 1 side and a remaining portion 2Bb. In the example of the present embodiment, the surface layer 2Ba is arranged adjacent to the first low dielectric constant layer 4 in the first electrode 2B. The surface layer 2Ba is, for example, a portion 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 means a portion of the first electrode 2B other than the surface layer 2Ba. In the first electrode 2B, the concentration of impurities doped in the semiconductor material is higher in the surface layer 2Ba than in the remaining portion 2Bb. That is, the first electrode 2B has different concentrations of impurities, i.e., doping amounts, in the surface layer 2Ba and the remaining portion 2Bb. For example, the concentration of impurities in the surface layer 2Ba is 10% or more higher than that in the remaining portion 2Bb. Such a concentration distribution of impurities in the first electrode 2B can be formed by a thermal diffusion method or an ion implantation method, etc.

[0118] In the first electrode 2B, the concentration of impurities may change sharply at the boundary between the surface layer 2Ba and the remaining part 2Bb, or may gradually become thinner as it moves away from the surface layer 2Ba in the stacking direction. The concentration of impurities in the first electrode 2B can be measured by epitaxial resistivity measurement, air gap CV measurement, mercury CV measurement, surface charge distribution, secondary ion mass spectrometry, or extended resistance measurement. Regardless of which method is used, the measurement results are substantially the same. The difference between the concentration of impurities in the surface layer 2Ba and the concentration of impurities in the remaining part 2Bb can be confirmed by any of the above-mentioned measurement methods. Specifically, by implementing the above-mentioned measurement method, a distribution diagram of the impurity concentration of the first electrode 2B in the depth direction from the surface of the optical waveguide 1 side is obtained. Based on the obtained distribution diagram of the impurity concentration, the integral average of the impurity concentration of the surface layer 2Ba and the integral average of the impurity concentration of the remaining part 2Bb are calculated as the concentration of impurities in the surface layer 2Ba and the concentration of impurities in the remaining part 2Bb, respectively. That is, the impurity concentration of the surface layer 2Ba is calculated by averaging the impurity concentration in the range from the surface of the first electrode 2B on the optical waveguide 1 side to 10% of the depth (thickness) of the first electrode 2B, and the impurity concentration of the remaining range is calculated by averaging the impurity concentration in the remaining portion 2Bb. The obtained impurity concentration of the surface layer 2Ba is, for example, 10% or more higher than the obtained impurity concentration of the remaining portion 2Bb.

[0119] In the first electrode 2B, the high-frequency signal propagates more through the surface layer 2Ba due to the skin effect, so it is preferable that the conductivity near the surface layer 2Ba is higher. In the optical modulator 10B according to the present embodiment, the surface layer 2Ba on the optical waveguide 1 side of the first electrode 2B is doped with impurities at a higher concentration than the remaining portion 2Bb of the first electrode 2. In this case, in the first electrode 2B, a region with high conductivity can be locally present near the optical waveguide 1, and the attenuation of the high-frequency signal can be suppressed by the skin effect.

[0120] <Fourth Implementation Method>

[0121] Fig. 9 1 is a cross-sectional view schematically showing a structure of an optical modulator 10C according to the fourth embodiment. The optical modulator 10C is different from the optical modulator 10A according to the second embodiment in the structure of a first electrode 2C.

[0122] Reference Fig. 9The first electrode 2C includes a surface layer 2Ca on the side of the raised portion 1Ab of the optical waveguide 1A and a remaining portion 2Cb. The surface layer 2Ca on the side of the raised portion 1Ab is a surface layer in the first electrode 2C where the electric field applied to the raised portion 1Ab together with the second electrode 3A passes. In the example of the present embodiment, the surface layer 2Ca is a surface layer in the first electrode 2C located on the side of the raised portion 1Ab that substantially 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 from a surface located on the side of the raised portion 1Ab in the width direction of the first electrode 2C to 10% of the length in the width direction of the first electrode 2C. The remaining portion 2Cb means a portion of the first electrode 2C other than the surface layer 2Ca. Similar to the first electrode 2B in the third embodiment described above, in the first electrode 2C, the concentration of impurities doped in the semiconductor material is higher in the surface layer 2Ca than in the remaining portion 2Cb. Therefore, even the optical modulator 10C according to the present embodiment can achieve the same effects as those of the optical modulator 10B according to the third embodiment.

[0123] exist Fig.10 , a modified example of the optical modulator 10C according to the fourth embodiment is shown. Fig.10 The surface layer 2Ca may be a surface layer of the first electrode 2C located on the optical waveguide 1A side in the stacking direction of the first electrode 2C with respect to the optical waveguide 1. In this case, the surface layer 2Ca is, for example, a portion ranging from the surface located on the optical waveguide 1A side in the stacking direction to 10% of the thickness of the first electrode 2C. Even with this structure, the same effects as those of the optical modulator 10B according to the third embodiment can be achieved.

[0124] As mentioned above, although the embodiment related to this disclosure was described, this disclosure is not limited to the said embodiment, Unless it deviates from the summary, various changes can be made.

[0125] 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 constant layer 4. In this case, the electric field can be concentrated on the optical waveguide by the convex portion. Therefore, the voltage applied between the first electrode 2 and the second electrode 3 can be reduced, and the power consumption can be further suppressed.

[0126] When the first electrode 2 includes a convex portion, the length of the convex portion in the direction perpendicular to the stacking direction may be smaller as the convex portion is closer to the optical waveguide 1 when viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends. In this case, the side surface of the convex portion can be made relatively smoothly continuous with respect to other portions of the surface of the first electrode 2 on the optical waveguide 1 side. Thus, it is possible to prevent electrical loss from occurring at the boundary between the convex portion and other portions. The side surface of the convex portion may be inclined at a fixed inclination relative to the surface, and the inclination of the side surface relative to the surface may be variable.

[0127] The optical modulator 10 involved in the first embodiment may also include a metal thin layer thinner than the first electrode 2. The metal thin layer is provided on the surface of the first electrode 2 on the optical waveguide 1 side. The metal thin layer has high conductivity and low attenuation of high-frequency signals. The metal thin layer can be formed using, for example, a metal material that can be applied to the second electrode 3. If the metal thin layer is provided on the surface of the first electrode 2 on the optical waveguide 1 side, the resistance value can be reduced and the attenuation of the signal can be suppressed. The metal thin layer can also be applied to each of the optical modulators 10A, 10B, and 10C involved in the second to fourth embodiments. In this case, the metal thin layer is provided on the surface of the first electrode 2A, 2B, and 2C where the electric field passes.

[0128] <1>

[0129] An optical modulator comprising:

[0130] Optical waveguides, comprising materials having an electro-optic effect;

[0131] A first electrode, comprising a semiconductor material, and arranged with a gap between the first electrode and the optical waveguide;

[0132] a second electrode configured to form a potential difference with the first electrode to apply an electric field to the optical waveguide; and

[0133] The first low dielectric constant layer has a refractive index smaller than that of the optical waveguide and is provided in the gap between the first electrode and the optical waveguide.

[0134] <2>

[0135] according to <1> The optical modulator, wherein

[0136] The invention further comprises: a second low dielectric constant layer having a refractive index smaller than that of the optical waveguide;

[0137] The second electrode is arranged with a gap between it and the optical waveguide.

[0138] The second low dielectric constant layer is provided in the gap between the second electrode and the optical waveguide.

[0139] <3>

[0140] according to <1> The optical modulator, wherein

[0141] The first low dielectric constant layer surrounds the optical waveguide and is provided between the optical waveguide and the first electrode and between the optical waveguide and the second electrode, respectively, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends.

[0142] <4>

[0143] according to <1> ~ <3> An optical modulator as described in any one of claims , wherein

[0144] The first electrode is stacked on the optical waveguide,

[0145] The second electrode is stacked on the optical waveguide on the opposite side of the first electrode.

[0146] <5>

[0147] according to <4> The optical modulator, wherein

[0148] A ratio of a thickness of the first electrode to a thickness of the first low dielectric constant layer is greater than or equal to 20.0 and less than or equal to 44.0.

[0149] <6>

[0150] according to <1> The optical modulator, wherein

[0151] The optical waveguide includes a substrate portion and a protrusion protruding from a surface of the substrate portion.

[0152] The first low dielectric constant layer is stacked on the substrate portion and the protruding portion.

[0153] The first electrode and the second electrode are stacked on the first low dielectric constant layer and are arranged in parallel with a gap therebetween.

[0154] <7>

[0155] according to <6> The optical modulator, wherein

[0156] A ratio of a thickness of the first electrode to a thickness of the first low dielectric constant layer at a position of the protruding portion is greater than or equal to 0.1 and less than or equal to 4.0.

[0157] <8>

[0158] according to <1> ~ <7> An optical modulator as described in any one of claims , wherein

[0159] A size of the gap between the first electrode and the optical waveguide is greater than or equal to 0.750 μm and less than or equal to 1.675 μm.

[0160] <9>

[0161] according to <1> ~ <8> An optical modulator as described in any one of claims , wherein

[0162] The semiconductor material is a silicon semiconductor material obtained by doping impurities in silicon.

[0163] <10>

[0164] according to <9> The optical modulator, wherein

[0165] The concentration of the impurity in the first electrode is 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 the following.

[0166] <11>

[0167] according to <9> or <10> The optical modulator, wherein

[0168] The first electrode is a silicon single crystal substrate.

[0169] <12>

[0170] according to <9> ~ <11> An optical modulator as described in any one of claims , wherein

[0171] The main component of the first low dielectric constant layer is SiO2.

[0172] <13>

[0173] according to <1> ~ <12> An optical modulator as described in any one of claims , wherein

[0174] The refractive index of the first electrode is less than 3.

[0175] <14>

[0176] according to <1> ~ <13> An optical modulator as described in any one of claims , wherein

[0177] The surface layer on the optical waveguide side of the first electrode is doped with impurities at a higher concentration than other portions of the first electrode.

[0178] Description of Reference Numerals

[0179] 10, 10A, 10B, 10C: optical modulator;

[0180] 1. 1A: optical waveguide;

[0181] 1Aa: substrate part;

[0182] 1Ab: raised part;

[0183] 2. 2A, 2B, 2C: first electrode;

[0184] 2Ba, 2Ca: surface layer;

[0185] 2Bb, 2Cb: remaining part;

[0186] 3. 3A: second electrode;

[0187] 4, 4A: first low dielectric constant layer;

[0188] 5: Second low dielectric constant layer.

Claims

1. An optical modulator comprising: Optical waveguides, comprising materials having an electro-optic effect; A first electrode, comprising a semiconductor material, and arranged with a gap between the first electrode and the optical waveguide; a second electrode configured to form a potential difference with the first electrode to apply an electric field to the optical waveguide; and The first low dielectric constant layer has a refractive index smaller than that of the optical waveguide and is provided in the gap between the first electrode and the optical waveguide.

2. The optical modulator according to claim 1, wherein The invention further comprises: a second low dielectric constant layer having a refractive index smaller than that of the optical waveguide; The second electrode is arranged with a gap between it and the optical waveguide. The second low dielectric constant layer is provided in the gap between the second electrode and the optical waveguide.

3. The optical modulator according to claim 1, wherein: The first low dielectric constant layer surrounds the optical waveguide and is provided between the optical waveguide and the first electrode and between the optical waveguide and the second electrode, respectively, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends.

4. The optical modulator according to any one of claims 1 to 3, wherein: 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.

5. The optical modulator according to claim 4, wherein: A ratio of a thickness of the first electrode to a thickness of the first low dielectric constant layer is greater than or equal to 20.0 and less than or equal to 44.

0.

6. The optical modulator according to claim 1, wherein: The optical waveguide includes a substrate portion and a protrusion protruding from a surface of the substrate portion. The first low dielectric constant layer is stacked on the substrate portion and the protruding portion. The first electrode and the second electrode are stacked on the first low dielectric constant layer and are arranged in parallel with a gap therebetween.

7. The optical modulator according to claim 6, wherein: A ratio of a thickness of the first electrode to a thickness of the first low dielectric constant layer at a position of the protruding portion is greater than or equal to 0.1 and less than or equal to 4.

0.

8. The optical modulator according to any one of claims 1 to 7, wherein: A size of the gap between the first electrode and the optical waveguide is greater than or equal to 0.750 μm and less than or equal to 1.675 μm.

9. The optical modulator according to any one of claims 1 to 8, wherein: The semiconductor material is a silicon semiconductor material obtained by doping impurities in silicon.

10. The optical modulator according to claim 9, wherein: The concentration of the impurity in the first electrode is 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 the following.

11. The optical modulator according to claim 9 or 10, wherein: The first electrode is a silicon single crystal substrate.

12. The optical modulator according to any one of claims 9 to 11, wherein: The main component of the first low dielectric constant layer is SiO2.

13. The optical modulator according to any one of claims 1 to 12, wherein: The refractive index of the first electrode is less than 3.

14. The optical modulator according to any one of claims 1 to 13, wherein: The surface layer on the optical waveguide side of the first electrode is doped with impurities at a higher concentration than other portions of the first electrode.

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