Optical modulator

Abstract

To provide an optical modulator capable of applying an electrical signal to an electrode while suppressing the occurrence of cracks and reducing power consumption.SOLUTION: An optical modulator (10) comprises an optical waveguide (1) made of a material having an electro-optic effect, a first electrode (2), and a second electrode (3). The first electrode (2) is made of a semiconductor material. The second electrode (3) is made of a metal material, and is arranged so as to create a potential difference with the first electrode (2) and apply an electric field to 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] Conventional optical modulators are described in, for example, Patent Document 1 and Non-Patent Document 1. The optical modulator in Patent Document 1 includes a core portion having a slot waveguide structure. The core portion includes 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 higher 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.

[0004] The optical modulator in Non-Patent Document 1 includes an optical waveguide and two metal electrodes. The optical waveguide is disposed between the two metal electrodes. For example, one metal electrode, the optical waveguide, and the other metal electrode are disposed in parallel. Alternatively, one metal electrode is stacked on the optical waveguide, and the other metal electrode is stacked on the optical waveguide on the opposite side. The two electrodes are disposed so as to apply an electric field to the optical waveguide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-034610 [Non-patent literature]

[0006] [Non-Patent Document 1] Integrated lithium niobate electro-optic modulators: when performance meets scalability, Mian Zhang et al., Optica Vol. 8, Issue 5, pp.652-667(2021) Summary of the Invention [Problem to be solved by the invention]

[0007] In order to apply a wideband, high-frequency signal to the electrode while suppressing power consumption, it is preferable for the electrode to be thick. However, when the electrode is formed using a metal material as in conventional optical modulators, increasing the electrode thickness generates large internal stress in the electrode. This internal stress makes the electrode prone to cracking. In particular, when two metal electrodes are stacked on both sides of an optical waveguide, internal stress is generated in each electrode that causes both electrodes to warp and deform in the same direction. As a result, the optical waveguide warps together with both electrodes, making the optical waveguide prone to cracking.

[0008] An object of the present disclosure is to provide an optical modulator that can apply an electrical signal to an electrode while suppressing the occurrence of cracks and reducing power consumption. [Means for solving the problem]

[0009] The optical modulator according to the present disclosure includes an optical waveguide, a first electrode, and a second electrode. The optical waveguide is made of a material having an electro-optic effect. The first electrode is made of a semiconductor material. The second electrode is made of a metal material and is arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide. [Effects of the Invention]

[0010] According to the optical modulator according to the present disclosure, it is possible to apply an electric signal to an electrode while suppressing the occurrence of cracks and reducing power consumption. [Brief explanation of the drawings]

[0011] [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 cross-sectional view showing a schematic configuration of an optical modulator according to the second embodiment. [Figure 3] FIG. 3 is a cross-sectional view showing a schematic configuration of an optical modulator according to the third embodiment. [Figure 4] FIG. 4 is a cross-sectional view showing a schematic configuration of an optical modulator according to the fourth embodiment. [Figure 5] FIG. 5 is a cross-sectional view showing a schematic configuration of an optical modulator according to the fifth embodiment. [Figure 6] FIG. 6 is a diagram showing a modified example of the optical modulator according to the fifth embodiment. [Figure 7] FIG. 7 is a cross-sectional view showing a schematic configuration of an optical modulator according to the sixth embodiment. [Figure 8] FIG. 8 is a cross-sectional view showing a schematic configuration of an optical modulator according to the seventh embodiment. [Figure 9] FIG. 9 is a cross-sectional view showing a schematic configuration of an optical modulator according to the eighth embodiment. [Figure 10] FIG. 10 is a cross-sectional view showing a schematic configuration of an optical modulator according to the ninth embodiment. [Figure 11] FIG. 11 is a cross-sectional view showing a schematic configuration of an optical modulator according to the tenth embodiment. [Figure 12] FIG. 12 is a diagram showing a modified example of the optical modulator according to the tenth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] An optical modulator according to an embodiment of the present disclosure includes an optical waveguide, a first electrode, and a second electrode. The optical waveguide is made of a material having an electro-optic effect. The first electrode is made of a semiconductor material. The second electrode is made of a metal material and is arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide (first configuration).

[0014] In the first configuration, of the first and second electrodes that apply an electric field to the optical waveguide, the first electrode is a semiconductor electrode made of a semiconductor material. Compared to metal electrodes, semiconductor electrodes can be formed thicker while suppressing internal stress. By forming the semiconductor electrode thicker, wideband and high-frequency signals can be applied to the semiconductor electrode while suppressing power consumption. Furthermore, because the semiconductor electrode can be formed while suppressing internal stress, the occurrence of cracks due to internal stress can be suppressed.

[0015] In the first configuration, the second electrode is a metal electrode made of a metal material, of the first and second electrodes that apply an electric field to the optical waveguide. The metal material that makes up the second electrode has a higher conductivity than the semiconductor material. Therefore, electrical loss can be reduced compared to when both the first and second electrodes are semiconductor electrodes.

[0016] The optical modulator of the first configuration may further include a low-dielectric layer having a refractive index smaller than that of the optical waveguide. In this case, at least the first electrode of the first and second electrodes may be disposed with a gap between it and the optical waveguide, and the low-dielectric layer may be provided in the gap (second configuration). The first electrode and the second electrode may each be disposed with a gap between them and the optical waveguide, and the low-dielectric layer may be provided in each gap (third configuration).

[0017] In the second configuration, at least the first electrode, which is a semiconductor electrode, is disposed with a gap between it and the optical waveguide. In this case, the first electrode is not in contact with the optical waveguide. Furthermore, a low-dielectric layer with a refractive index lower than that of the optical waveguide is provided in the gap between the first electrode and the optical waveguide. This makes it difficult for light passing through the optical waveguide to leak to the first electrode, thereby reducing optical loss.

[0018] In a third configuration, both the first electrode and the second electrode are disposed with a gap between them and the optical waveguide. Furthermore, a low-dielectric-constant layer having a refractive index lower than that of the optical waveguide is provided in the gap between the first electrode and the optical waveguide and in the gap between the second electrode and the optical waveguide. This makes it difficult for light passing through the optical waveguide to leak to each of the first electrode and the second electrode, thereby further suppressing light loss.

[0019] In the optical modulator of the second or third configuration, the size of the gap is preferably 0.750 μm or more and 1.675 μm or less (fourth configuration).

[0020] Light seeps out from the optical waveguide into the low dielectric layer, albeit weakly. This seeped light is called evanescent light. If the size of the gap between the first electrode and / or the second electrode and the optical waveguide is 0.750 μm or more, as in the fourth configuration, the evanescent light is less likely to come into contact with the first electrode and / or the second electrode, and light loss can be further suppressed.

[0021] In the fourth configuration, the size of the gap between the first electrode and / or the second electrode and the optical waveguide is 1.675 μm or less. In this case, the distance between the first electrode and / or the second electrode and the optical waveguide is not too large, and the magnitude of the electric field across the optical waveguide can be ensured without increasing the voltage applied between the first electrode and the second electrode.

[0022] In the optical modulator of any one of the second to fourth configurations, the semiconductor material may be a silicon semiconductor material in which silicon is doped with impurities, and the main component of the low-dielectric layer may be SiO2 (fifth configuration). Since the semiconductor material is a silicon semiconductor material, when a low-dielectric layer is provided in the gap between the first electrode and the optical waveguide, the SiO2 low-dielectric layer can be formed on the first electrode of the silicon semiconductor material by thermal oxidation. Film formation by thermal oxidation provides good adhesion of the low-dielectric layer to the first electrode, making it difficult for foreign matter to enter the interface between the first electrode and the low-dielectric layer. This reduces electrical loss at the interface between the first electrode and the low-dielectric layer. Furthermore, because foreign matter is prevented from accumulating at the interface between the first electrode and the low-dielectric layer, the reliability and lifespan of the optical modulator can be improved.

[0023] In the optical modulator of any one of the first to fifth 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 (sixth 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.

[0024] In the optical modulator of the sixth configuration, the first electrode may include a convex portion. The convex portion is provided on the surface located on the optical waveguide side in the stacking direction of the first electrode, optical waveguide, and second electrode, and protrudes toward the optical waveguide (seventh configuration). In this case, the convex portion can concentrate the electric field in the optical waveguide. This allows the voltage applied between the first electrode and the second electrode to be reduced, further reducing power consumption.

[0025] In the optical modulator of the seventh configuration, when viewed in a cross section perpendicular to the extending direction of the optical waveguide, the length of the convex portion in a direction perpendicular to the stacking direction may be smaller the closer it is to the optical waveguide (eighth configuration). For example, if the convex portion has a rectangular shape in cross section, the side surface of the convex portion is continuous at a right angle with other parts of the surface of the first electrode facing the optical waveguide. On the other hand, as in the eighth configuration, if the length of the convex portion in a direction perpendicular to the stacking direction of the electrode and the optical waveguide becomes smaller the closer it is to the optical waveguide, the side surface of the convex portion can be made to continue relatively gently with other parts of the surface of the first electrode facing the optical waveguide. This makes it possible to reduce electrical loss at the boundary between the convex portion and other parts.

[0026] In the optical modulator of any one of the sixth to eighth configurations, preferably, when viewed in a cross section perpendicular to the extending direction of the optical waveguide, the length of the surface of the first electrode on the optical waveguide side in a direction perpendicular to the stacking direction of the first electrode, the optical waveguide, and the second electrode is longer than the length of the optical waveguide (ninth configuration). In this case, an electric field can be applied to the entire area of ​​the optical waveguide.

[0027] In the optical modulator of any one of the first to ninth configurations, the semiconductor material of the first electrode is preferably a silicon semiconductor material in which silicon is doped with impurities (tenth configuration).

[0028] In the optical modulator of the tenth 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 eleventh 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.

[0029] In the optical modulator of any one of the first to eleventh configurations, the refractive index of the first electrode is smaller than 3 (twelfth 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 eleventh configuration.

[0030] In the optical modulator of any one of the tenth to twelfth configurations, the first electrode is preferably a silicon single crystal substrate (thirteenth configuration).

[0031] In the optical modulator of any one of the tenth to twelfth configurations, the first electrode may be an active layer of an SOI substrate (fourteenth configuration).

[0032] In the optical modulator of any one of the first to fourteenth configurations, the metal material preferably contains a noble metal as its main component (fifteenth configuration). In this case, the metal material constituting the second electrode contains a noble metal, which is resistant to chemical reactions, as its main component, and therefore the modulation characteristics of the optical modulator are stable. Furthermore, if the metal material constituting the second electrode contains a noble metal as its main component, the resistance value of the second electrode can be reduced, and power consumption can be reduced.

[0033] In the optical modulator of any one of the first to fifteenth configurations, preferably, when viewed in a cross section perpendicular to the extension direction of the optical waveguide, the area of ​​the first electrode is larger than the area of ​​the second electrode (sixteenth configuration). Since the second electrode is made of a metal material, the resistance value is sufficiently small even without a large cross-sectional area. On the other hand, since the first electrode is made of a semiconductor material with lower conductivity than a metal material, the resistance value can be reduced by making the cross-sectional area larger than that of the second electrode. In this regard, in the sixteenth configuration, since the cross-sectional area of ​​the first electrode is larger than the cross-sectional area of ​​the second electrode, the resistance value of the first electrode is reduced, and power consumption can be reduced.

[0034] In the optical modulator of any one of the first to sixteenth configurations, preferably, the second electrode is a signal electrode and the first electrode is a ground electrode (seventeenth configuration). That is, the second electrode, which is a metal electrode, is used as the signal electrode, and the first electrode, which is a semiconductor electrode, is used as the ground electrode. The second electrode is made of a metal material that has high conductivity and low attenuation of high-frequency signals. By using this second electrode as the signal electrode, the drive voltage can be reduced.

[0035] The optical modulator of any one of the first to seventeenth configurations may further include a metal thin film thinner than the first electrode. The metal thin film is provided on the surface of the first electrode facing the optical waveguide (eighteenth configuration). The metal thin film has high conductivity and low attenuation of high-frequency signals. If this metal thin layer is provided on the surface of the first electrode facing the optical waveguide, the driving voltage can be reduced. Furthermore, in the first electrode, high-frequency signals propagate through the surface layer due to the skin effect, so it is preferable that the conductivity near the surface layer is higher. In this regard, if the metal thin layer is provided on the surface of the first electrode facing the optical waveguide, the resistivity can be reduced and signal attenuation can be reduced.

[0036] In the optical modulator of any one of the first to eighteenth 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 (19th configuration). In this case, a region of 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.

[0037] 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.

[0038] 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, and a second electrode 3. Fig. 1 shows a cross section perpendicular to the direction in which the optical waveguide 1 extends. In this specification, unless otherwise specified, the cross section refers to a cross section perpendicular to the direction in which the optical waveguide 1 extends.

[0039] 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. 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).

[0040] 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.

[0041] 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. In this embodiment, the optical waveguide 1 is laminated directly on the first electrode 2, and the first electrode 2 is in contact with the optical waveguide 1. Furthermore, the second electrode 3 is laminated directly on the optical waveguide 1, and the second electrode 3 is in contact with the optical waveguide 1.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] The second electrode 3 is made of a metal material. That is, the second electrode 3 is a metal electrode. The metal material used for the second electrode 3 is, for example, mainly composed of a noble metal. The noble metal is, for example, Au (gold). The noble metal may also be Ag (silver), Pt (platinum), or the like. The metal material may also contain trace amounts of other metal elements such as Cr and Ti. The metal material may also be copper, aluminum, or an alloy thereof, or the like.

[0048] The second electrode 3 can be laminated on the optical waveguide 1 and the first electrode 2, for example, as follows: First, a material substrate having an electro-optic effect is placed on the first electrode 2, and the material substrate is bonded to the first electrode 2. Then, the material substrate is subjected to lithography and etching to form the optical waveguide 1. Next, a metal layer is formed on the optical waveguide 1 by sputtering, vapor deposition, or the like. The formed metal film is patterned by lithography, and the second electrode 3 is formed by etching.

[0049] 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.

[0050] 1, the optical waveguide 1 has a thickness t1 corresponding to the length of the optical waveguide 1 and electrodes 2 and 3 in the stacking direction, and a width w1 corresponding to the length in the direction perpendicular to the stacking direction. The first electrode 2 has a thickness t2 corresponding to the length in the stacking direction, and a width w2 corresponding to the length in the direction perpendicular to the stacking direction. The second electrode 3 has a thickness t3 corresponding to the length in the stacking direction, and a width w3 corresponding to the length in the direction perpendicular to the stacking direction.

[0051] When viewed in a cross section perpendicular to the extension direction of the optical waveguide 1, the area of ​​the first electrode 2 is larger than the area of ​​the second electrode 3. Specifically, the cross-sectional area of ​​the first electrode 2, which is a semiconductor electrode, is larger than the cross-sectional area 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. In other words, it is preferable that the performance of the first electrode 2 as an electrode is equivalent to that 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."

[0052] 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 is made to match the resistance value of the second electrode 3, so the product of the conductivity and the thickness t2 of the first electrode 2 should match the product of the conductivity and the 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.

[0053] 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 S / m, 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 is 4.3×10 times the thickness t3 of the second electrode 3. 3It can be doubled.

[0054] Therefore, 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 is 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 set to 8600 μm (8.6 mm) or less.

[0055] However, the thickness t2 of the first electrode 2 is determined taking into consideration, for example, processability, as well as the electrical conductivity of the electrodes 2 and 3 and the thickness t3 of the second electrode 3. For example, when the first electrode 2 is formed using a silicon semiconductor material, the thickness t2 of the first electrode 2 is preferably 500 μm or less from the standpoint of processability.

[0056] From another perspective, when 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 following equation (1) is used to calculate the skin depth of a conductor.

[0057]

number

[0058] 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 1×107 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.

[0059] The thickness t2 of the first electrode 2, which is a semiconductor electrode, 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 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 can be measured directly using interferometry. Either method produces substantially the same measurement results.

[0060] The thickness t3 of the second electrode 3, which is a metal electrode, 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 produce substantially the same measurement results.

[0061] 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.

[0062] 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 larger 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 that is closest to the optical waveguide 1. In this embodiment, the length of the surface of the first electrode 2 that is in contact with the optical waveguide 1 in the direction perpendicular to the stacking direction is the width w2.

[0063] [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 is made of a metal material. That is, of the first electrode 2 and second electrode 3 that apply an electric field to the optical waveguide 1, the first electrode 2 is a semiconductor electrode. A semiconductor electrode can be formed thicker while suppressing internal stress compared to a metal electrode. By forming the semiconductor electrode thick, it is possible to apply a wideband, high-frequency signal to the semiconductor electrode while suppressing power consumption. Furthermore, because the semiconductor electrode can be formed while suppressing internal stress, it is possible to suppress the occurrence of cracks due to internal stress.

[0064] 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 when a semiconductor silicon layer is formed on a substrate. Therefore, it is possible to form the first electrode 2 thick while further suppressing the internal stress of the first electrode 2.

[0065] In the optical modulator 10 according to this embodiment, the second electrode 3 is a metal electrode out of the first electrode 2 and second electrode 3 that apply an electric field to the optical waveguide 1. The metal material that constitutes the second electrode has higher conductivity than semiconductor materials. Therefore, electrical loss can be reduced compared to when both the first electrode and the second electrode are semiconductor electrodes.

[0066] The second electrode 3, which is a metal electrode, can be formed on the optical waveguide 1 by sputtering, vapor deposition, or the like without an adhesive layer. This prevents light absorption by the adhesive layer. It also prevents the adhesive layer from diffusing into the optical waveguide 1, which would otherwise change the refractive index of the optical waveguide 1.

[0067] 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.

[0068] In this embodiment, the second electrode 3 is made of a metal material, and the main component of the metal material is, for example, a noble metal. When the second electrode 3 is exposed to the atmosphere, impurities from materials in contact with the second electrode 3 or gas molecules released from other materials due to degassing may diffuse into the second electrode. When such diffusion occurs in the second electrode 3, the impedance changes and deviates from the original design value. When the impedance of the second electrode 3 deviates from the design value, the modulation characteristics of the optical modulator 10 are affected. When the metal material constituting the second electrode 3 is mainly composed of a noble metal that is resistant to chemical reactions, the modulation characteristics of the optical modulator 10 are stabilized. Furthermore, when the metal material constituting the second electrode 3 is mainly composed of a noble metal, the resistance value of the second electrode 3 can be reduced, thereby reducing power consumption.

[0069] In this embodiment, when viewed in a cross section perpendicular to the extending direction of the optical waveguide 1, the length (width w2) of the surface of the first electrode 2 on the optical waveguide 1 side in the direction perpendicular to the stacking direction is greater than the length (width w1) of the optical waveguide 1. In this case, an electric field can be applied to the entire optical waveguide 1. Furthermore, stress can be alleviated when forming the optical waveguide 1 on the first electrode 2.

[0070] In this embodiment, when viewed in a cross section perpendicular to the extension direction of the optical waveguide 1, the area of ​​the first electrode 2 is larger than the area of ​​the second electrode 3. Because the second electrode 3 is made of a metal material, its resistance value is sufficiently small even without a large cross-sectional area. On the other hand, because the first electrode 2 is made of a semiconductor material that has lower conductivity than a metal material, its resistance value can be reduced by making its cross-sectional area larger than that of the second electrode 3. In this embodiment, because the cross-sectional area of ​​the first electrode 2 is larger than that of the second electrode 3, the resistance value of the first electrode 2 is reduced, and power consumption can be reduced.

[0071] In this embodiment, the second electrode 3, which is a metal electrode, is used as a signal electrode, and the first electrode 2, which is a semiconductor electrode, is used as a ground electrode. The second electrode 3 is made of a metal material that has high conductivity and low attenuation of high-frequency signals. Therefore, using the second electrode 3 as a signal electrode can reduce the drive voltage.

[0072] Second Embodiment 2 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 that it includes a low dielectric constant layer 4.

[0073] In the first embodiment, the first electrode 2 is in contact with the optical waveguide 1. Even when the first electrode 2 is in contact with the optical waveguide 1, if the difference in refractive index between the first electrode 2 and the optical waveguide 1 is large, it is possible to confine light in the optical waveguide 1 by adjusting the thickness of the optical waveguide 1, for example. However, if the amount of impurity doping in the first electrode 2 is increased, the refractive index of the first electrode 2 decreases and approaches the refractive index of the optical waveguide 1. This may result in light leaking from the optical waveguide 1 to the first electrode 2.

[0074] In this embodiment, the first electrode 2 is disposed with a gap between it and the optical waveguide 1. That is, the first electrode 2 is separated from the optical waveguide 1 in the stacking direction. 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.

[0075] The low-dielectric layer 4 has a refractive index smaller than that of the optical waveguide 1. The low-dielectric layer 4 is provided in the gap between the first electrode 2 and the optical waveguide 1. In the optical modulator 10A, the low-dielectric layer 4 is stacked on the first electrode 2, and the optical waveguide 1 is stacked on the low-dielectric layer 4. In other words, the optical waveguide 1 is stacked indirectly on the first electrode 2 via the low-dielectric layer 4, and the first electrode 2 is not in contact with the optical waveguide 1. The low-dielectric layer 4 preferably covers the entire surface of the optical waveguide 1 facing the low-dielectric layer 4. On the other hand, the second electrode 3 is stacked directly on the optical waveguide 1 and is in contact with the optical waveguide 1, as in the first embodiment.

[0076] The main component of the low dielectric constant layer 4 is typically SiO2. The main component of the 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).

[0077] The low dielectric layer 4 is formed on the first electrode 2 by, for example, CVD, vapor deposition, sputtering, or the like. Then, a material substrate having an electro-optic effect can be placed on the low dielectric layer 4, and the material substrate can be bonded to the low dielectric layer 4. Thereafter, the optical waveguide 1 and the second electrode 3 can be formed on the low dielectric layer 4 as described in the first embodiment.

[0078] In this embodiment, the first electrode 2 is not in contact with the optical waveguide 1. Furthermore, a low-dielectric layer 4 having a refractive index smaller than that of the optical waveguide 1 is provided in the gap between the first electrode 2 and the optical waveguide 1. Therefore, compared to when the first electrode 2 is in contact with the optical waveguide 1, light passing through the optical waveguide 1 is less likely to leak to the first electrode 2 and be absorbed by the first electrode 2. Therefore, light loss can be suppressed.

[0079] For example, if the semiconductor material used for the first electrode 2 is a silicon semiconductor material, a low-dielectric-constant layer 4 of SiO2 can be formed on the first electrode 2 of the silicon semiconductor material by thermal oxidation. In this case, the 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 low-dielectric-constant layer 4. This reduces electrical loss at the interface between the first electrode 2 and the low-dielectric-constant layer 4. This also improves the reliability and lifespan of the optical modulator 10A. This is because if foreign matter accumulates at the interface between the first electrode 2 and the low-dielectric-constant layer 4 and an electric field concentrates on the accumulated foreign matter, the optical modulator 10A may be damaged.

[0080] The penetration depth of the evanescent light in the low dielectric layer 4 can be estimated using the wavelength of the light (carrier wave) passing through the optical waveguide 1 as a guide. If the first electrode 2 is separated from the optical waveguide 1 by a distance equal to or greater than the wavelength of the carrier wave, the evanescent light can be prevented from contacting the first electrode 2. Therefore, it is preferable that the size of the gap between the optical waveguide 1 and the first electrode 2, i.e., the thickness (length in the stacking direction) of the low dielectric layer 4, be equal to or greater than the wavelength of the light passing through the optical waveguide 1.

[0081] For example, as in this embodiment, if the size of the gap between the first electrode 2 and the optical waveguide 1 is 0.750 μm or more, the thickness of the low dielectric layer 4 becomes larger than the penetration depth of the evanescent light, and light passing through the optical waveguide 1 is less likely to leak to the first electrode 2. As described above, the size of the gap between the first electrode 2 and the optical waveguide 1 may be 1.675 μm or less. If the size of the gap between the first electrode 2 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 even if the voltage applied between the first electrode 2 and the second electrode 3 is not large.

[0082] <Third embodiment> 3 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 that it includes a low dielectric constant layer 5.

[0083] In this embodiment, the second electrode 3 is disposed with a gap between it and the optical waveguide 1. That is, 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.

[0084] The low-dielectric layer 5 has a refractive index smaller than that of the optical waveguide 1. The low-dielectric layer 5 is provided in the gap between the second electrode 3 and the optical waveguide 1. In the optical modulator 10B, the low-dielectric layer 5 is stacked on the optical waveguide 1, and the second electrode 3 is stacked on the low-dielectric layer 5. In other words, the optical waveguide 1 is stacked indirectly on the second electrode 3 via the low-dielectric layer 5, and the second electrode 3 is not in contact with the optical waveguide 1. It is preferable that the low-dielectric layer 5 covers the entire surface of the optical waveguide 1 facing the low-dielectric layer 5. On the other hand, the first electrode 2 is stacked directly on the optical waveguide 1 and is in contact with the optical waveguide 1, as in the first embodiment.

[0085] Examples of the main component of the low dielectric constant layer 5 include the same as that of the low dielectric constant layer 4 of the second embodiment. The main component of the low dielectric constant layer 5 may be the same as or different from that of the low dielectric constant layer 4.

[0086] The low dielectric constant layer 5 is formed on the optical waveguide 1 by, for example, CVD, vapor deposition, sputtering, etc. Before forming the low dielectric constant layer 5, the optical waveguide 1 may be formed on the first electrode 2 as described in the first embodiment. Then, a metal layer may be formed on the low dielectric constant layer 5 by sputtering, vapor deposition, etc., and the second electrode 3 may be formed on the formed metal film as described in the first embodiment.

[0087] In this embodiment, the second electrode 3 is not in contact with the optical waveguide 1. Furthermore, a 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. Therefore, compared to when the second electrode 3 is in contact with the optical waveguide 1, light passing through the optical waveguide 1 is less likely to leak to the second electrode 3 and be absorbed by the second electrode 3. Therefore, light loss can be suppressed.

[0088] In this embodiment, the size of the gap between the second electrode 3 and the optical waveguide 1 is 0.750 μm or more and 1.675 μm or less. Therefore, similar to the second embodiment, it is possible to suppress light loss and ensure the magnitude of the electric field to the optical waveguide 1.

[0089] <Fourth embodiment> 4 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 10 according to the first embodiment in that it includes a low-dielectric-constant layer 4 and a low-dielectric-constant layer 5. From another perspective, the optical modulator 10C is a combination of the configurations of the second and third embodiments.

[0090] In this embodiment, both the first electrode 2 and the second electrode 3 are disposed with a gap between them and the optical waveguide 1. Furthermore, low-dielectric-constant layers 4 and 5, each having a refractive index smaller than that of the optical waveguide 1, are provided in the gap between the first electrode 2 and the optical waveguide 1 and in the gap between the second electrode 3 and the optical waveguide 1, respectively. This makes it difficult for light passing through the optical waveguide 1 to leak to each of the first electrode 2 and the second electrode 3. Therefore, the optical modulator 10C can further reduce optical loss than the optical modulators 10A and 10B.

[0091] Fifth Embodiment 5 is a cross-sectional view showing a schematic configuration of an optical modulator 10D according to the fifth embodiment. The optical modulator 10D differs from the optical modulator 10C according to the fourth embodiment in the configuration of the first electrode 2A.

[0092] Referring to FIG. 5, the first electrode 2A includes a protrusion 2Aa. In the first electrode 2A, the protrusion 2Aa is provided on a surface located on the optical waveguide 1 side in the stacking direction, and protrudes toward the optical waveguide 1. In this example of the present embodiment, the first electrode 2A includes a protrusion 2Aa and a base 2Ab. The base 2Ab is disposed on the opposite side from the optical waveguide 1 in the stacking direction. The protrusion 2Aa protrudes toward the optical waveguide 1 from a surface 2Aba of the base 2Ab. Each of the protrusion 2Aa and the base 2Ab may have a substantially rectangular cross section. In the optical modulator 10D, a low-dielectric-constant layer 4 is stacked on the protrusion 2Aa of the first electrode 2A, and the protrusion 2Aa is in contact with the low-dielectric-constant layer 4.

[0093] When viewed in a cross section perpendicular to the extension direction of the optical waveguide 1, the width w2Ab of the base 2Ab is larger than the width w2Aa of the protruding portion 2Aa. The width w2Aa refers to the width of the surface 2Aaa of the protruding portion 2Aa that is closest to the optical waveguide 1. In this embodiment, the length in the direction perpendicular to the stacking direction of the surface 2Aaa of the protruding portion 2Aa that is in contact with the low dielectric layer 4 is the width w2Aa. The width w2Aa of the protruding portion 2Aa is larger than the width w1 of the optical waveguide 1.

[0094] In the optical modulator 10D of this embodiment, the convex portion 2Aa can concentrate the electric field in the optical waveguide 1. This allows the voltage applied between the first electrode 2A and the second electrode 3 to be reduced, thereby reducing power consumption.

[0095] 5, the first electrode 2A including the protrusion 2Aa and the base 2Ab is, for example, a silicon single crystal substrate. The protrusion 2Aa and the base 2Ab can be formed by subjecting a silicon single crystal base substrate to lithography and etching.

[0096] FIG. 6 shows a modified example of the optical modulator 10D according to the fifth embodiment. In FIG. 6, an SOI substrate 20 is used as a substrate on which a first electrode 2A is provided. The SOI substrate 20 includes an oxide film 21 and active layers 22a and 22b arranged to sandwich the oxide film 21 from both surfaces thereof. In the SOI substrate 20, one of the active layers 22a serves as the first electrode 2A. Utilizing the active layer 22a of the SOI substrate 20 facilitates patterning for forming the first electrode 2A.

[0097] The first electrode 2A may be applied to the optical modulator 10 according to the first embodiment. In this case, the convex portion 2Aa is in contact with the optical waveguide 1. The first electrode 2A may be applied to the optical modulator 10A according to the second embodiment. In this case, the convex portion 2Aa is in contact with the low dielectric layer 4, but is not in contact with the optical waveguide 1. The first electrode 2A may be applied to the optical modulator 10B according to the third embodiment. In this case, the convex portion 2Aa is in contact with the optical waveguide 1.

[0098] Sixth Embodiment 7 is a cross-sectional view showing a schematic configuration of an optical modulator 10E according to the sixth embodiment. The optical modulator 10E differs from the optical modulator 10D according to the fifth embodiment in the configuration of the first electrode 2B.

[0099] Referring to FIG. 7, the first electrode 2B includes a protrusion 2Ba and a base 2Bb. The protrusion 2Ba protrudes from a surface 2Bba of the base 2Bb toward the optical waveguide 1. When viewed in a cross section perpendicular to the extension direction of the optical waveguide 1, the length (width) of the protrusion 2Ba in a direction perpendicular to the stacking direction decreases toward the optical waveguide 1. The protrusion 2Ba can have a minimum width w2Ba at a surface 2Baa on the optical waveguide 1 side. In this embodiment, the surface 2Baa of the protrusion 2Ba is in contact with the low-dielectric layer 4. The protrusion 2Ba may have a substantially trapezoidal cross section. In this case, the surface 2Baa of the protrusion 2Ba corresponds to the upper base of the trapezoid.

[0100] In the optical modulator 10E of this embodiment, the width of the protrusion 2Ba in its cross section decreases as it approaches the optical waveguide 1. In this case, the side surface 2Bab of the protrusion 2Ba can be made to connect relatively gently to the surface 2Bba of the base 2Bb. More specifically, the side surface 2Bab of the protrusion 2Ba can be smoothly connected to the surface 2Bba of the base 2Bb at an obtuse angle, that is, in a shape close to a curve. This makes it less likely that electrical loss will occur at the boundary between the protrusion 2Ba and the base 2Bb, compared to when the side surface 2Bab of the protrusion 2Ba is connected to the surface 2Bba of the base 2Bb at a right angle.

[0101] In this embodiment, the side surface 2Bab of the protrusion 2Ba is inclined at a constant gradient with respect to the surface 2Baa. However, the gradient of the side surface 2Bab with respect to the surface 2Baa may vary.

[0102] The first electrode 2B may be applied to each of the optical modulators 10, 10A, and 10B according to the first to third embodiments.

[0103] Seventh Embodiment 8 is a cross-sectional view showing a schematic configuration of an optical modulator 10F according to the seventh embodiment. The optical modulator 10F differs from the optical modulator 10D according to the fifth embodiment in that it includes a thin metal layer 6.

[0104] Referring to FIG. 8, the thin metal layer 6 is provided on the surface of the first electrode 2A facing the optical waveguide 1. Specifically, the thin metal layer 6 is provided on the surface 2Aaa of the protrusion 2Aa facing the optical waveguide 1. The thickness t6 of the thin metal layer 6 is thinner than the thickness t2 of the first electrode 2A. The cross-sectional area of ​​the thin metal layer 6 is smaller than the cross-sectional area of ​​the first electrode 2A. Furthermore, the thickness t6 of the thin metal layer 6 is thinner than the thickness t3 of the second electrode 3.

[0105] The thin metal layer 6 has high conductivity and low attenuation of high-frequency signals. If this thin metal layer 6 is provided on the surface 2Aaa of the protrusion 2Aa facing the optical waveguide 1, the driving voltage can be reduced. Furthermore, in the protrusion 2Aa of the first electrode 2A, high-frequency signals propagate more along the surface due to the skin effect, so it is preferable that the conductivity near the surface be higher. In this regard, if the thin metal layer 6 is provided on the surface of the first electrode 2A facing the optical waveguide 1, the resistance value can be reduced and signal attenuation can be reduced. In this case, the protrusion 2Aa is used for low-frequency signals.

[0106] When the first electrode 2A is made of a silicon semiconductor material, as described above, the thickness t2 of the first electrode 2A is, for example, 0.43 μm or more and 500 μm or less. The thickness t6 of the thin metal layer 6 is, for example, 10% or more and 50% or less of the thickness t2 of the first electrode 2A. The cross-sectional area of ​​the thin metal layer 6 is, for example, 10% or more and 50% or less of the cross-sectional area of ​​the first electrode 2A. The cross-sectional area of ​​the thin metal layer 6 is, for example, 0.043 μm 2 More than 2500μm 2 The thin metal layer 6 may be formed using the same metal material as the second electrode 3, for example.

[0107] The thin metal layer 6 may be applied to each of the optical modulators 10, 10A, 10B, and 10C according to the first to fourth embodiments. In this case, the thin metal layer 6 is provided on the surface of the first electrode 2 that does not have a convex portion. The thin metal layer 6 may be applied to the optical modulator 10E according to the sixth embodiment. In this case, the thin metal layer 6 is provided on the surface 2Baa of the convex portion 2Ba of the first electrode 2B.

[0108] Eighth Embodiment 9 is a cross-sectional view showing a schematic configuration of an optical modulator 10G according to the eighth embodiment. The optical modulator 10G differs from the optical modulator 10C according to the fourth embodiment in the configuration of an optical waveguide 1C and the arrangement of a first electrode 2C and a second electrode 3C.

[0109] Referring to FIG. 9, the optical modulator 10G includes an optical waveguide 1C, a first electrode 2C, a second electrode 3C, and a low-dielectric layer 4C. The optical waveguide 1C includes a substrate portion 1Ca and a ridge portion 1Cb. The ridge portion 1Cb protrudes from the surface of the substrate portion 1Ca. The ridge portion 1Cb essentially functions as an optical waveguide. A low-dielectric layer 4C is laminated on the optical waveguide 1C. More specifically, the low-dielectric layer 4C is laminated on the substrate portion 1Ca and the ridge portion 1Cb.

[0110] The first electrode 2C and the second electrode 3C are stacked on the low-dielectric layer 4C. The first electrode 2C and the second electrode 3C are arranged in parallel with a gap between them. Specifically, in a cross-sectional view of the optical modulator 10G, the first electrode 2C and the second electrode 3C are arranged side by side in a direction substantially perpendicular to the stacking direction of the optical waveguide 1C and the low-dielectric layer 4. In the direction substantially perpendicular to the stacking direction, the first electrode 2C is arranged on one side of the ridge portion 1Cb, and the second electrode 3C is arranged on the other side of the ridge portion 1Cb. The first electrode 2C and the second electrode 3C can generate a potential difference between them to apply an electric field to the ridge portion 1Cb of the optical waveguide 1C.

[0111] The optical modulator 10G according to this embodiment also achieves the same effects as the optical modulator 10C according to the fourth embodiment. The thin metal layer 6 according to the seventh embodiment may be applied to the optical modulator 10G according to this embodiment.

[0112] Ninth Embodiment 10 is a cross-sectional view showing a schematic configuration of an optical modulator 10H according to the ninth embodiment. The optical modulator 10H differs from the optical modulator 10 according to the first embodiment in the configuration of the first electrode 2D.

[0113] 10, the first electrode 2D has a surface layer 2Da on the optical waveguide 1 side and a remaining portion 2Db. In this embodiment, the surface layer 2Da is disposed adjacent to the optical waveguide 1 in the first electrode 2D. The surface layer 2Da is, for example, a portion of the first electrode 2D that is within a range of 10% of the length (thickness) of the first electrode 2D in the stacking direction of the first electrode 2D relative to the optical waveguide 1, from the surface of the first electrode 2D on the optical waveguide 1 side. The remaining portion 2Db refers to the portion of the first electrode 2D excluding the surface layer 2Da. In the first electrode 2D, the concentration of impurities doped into the semiconductor material is higher in the surface layer 2Da than in the remaining portion 2Db. That is, the first electrode 2D has different impurity concentrations, i.e., dopant amounts, between the surface layer 2Da and the remaining portion 2Db. For example, the impurity concentration in the surface layer 2Da is higher by 10% or more than the impurity concentration in the remaining portion 2Db. Such a concentration distribution of impurities in the first electrode 2D can be formed by a thermal diffusion method, an ion implantation method, or the like.

[0114] In the first electrode 2D, the impurity concentration may change sharply at the boundary between the surface layer 2Da and the remaining portion 2Db, or may gradually decrease with increasing distance from the surface layer 2Da in the stacking direction. The impurity concentration in the first electrode 2D 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. All methods produce substantially the same measurement results. The difference between the impurity concentration in the surface layer 2Da and the impurity concentration in the remaining portion 2Db 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 2D 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 2Da and the integral average of the impurity concentration in the remaining portion 2Db are calculated, and these are defined as the impurity concentration in the surface layer 2Da and the impurity concentration in the remaining portion 2Db, respectively. That is, the impurity concentration of the surface layer 2Da is the integral average of the impurity concentration within a range of 10% of the depth (thickness) of the first electrode 2D from the surface of the first electrode 2D on the optical waveguide 1 side, and the impurity concentration of the remaining portion 2Db is the integral average of the impurity concentration within the remaining range. The impurity concentration of the obtained surface layer 2Da is, for example, 10% or more higher than the impurity concentration of the obtained remaining portion 2Db.

[0115] In the optical modulator 10H according to this embodiment, a surface layer 2Da of the first electrode 2D on the optical waveguide 1 side is doped with impurities at a higher concentration than the remaining portion 2Db of the first electrode 2. In this case, a region with high conductivity can be localized in the vicinity of the optical waveguide 1 in the first electrode 2D, and attenuation of high-frequency signals can be suppressed by the skin effect.

[0116] The first electrode 2D may be applied to each of the optical modulators 10A, 10B, 10C, 10D, 10E, and 10F according to the second to seventh embodiments.

[0117] Tenth Embodiment 11 is a cross-sectional view showing a schematic configuration of an optical modulator 101 according to the tenth embodiment. The optical modulator 101 differs from the optical modulator 10G according to the eighth embodiment in the configuration of the first electrode 2E.

[0118] Referring to FIG. 11, the first electrode 2E has a surface layer 2Ea on the ridge portion 1Cb side of the optical waveguide 1C and a remaining portion 2Eb. The surface layer 2Ea on the ridge portion 1Cb side is a surface layer of the first electrode 2E through which an electric field applied to the ridge portion 1Cb passes together with the second electrode 3C. In the example of this embodiment, the surface layer 2Ea is a surface layer of the first electrode 2E located on the ridge portion 1Cb side that essentially functions as an optical waveguide in a direction (width direction) perpendicular to the stacking direction of the first electrode 2E relative to the optical waveguide 1C. The surface layer 2Ea is, for example, a portion within a range of 10% of the width direction length of the first electrode 2E from the surface located on the ridge portion 1Cb side in the width direction of the first electrode 2E. The remaining portion 2Eb refers to a portion of the first electrode 2E excluding the surface layer 2Ea. As with the first electrode 2D of the ninth embodiment described above, in the first electrode 2E, the concentration of impurities doped into the semiconductor material is higher in the surface layer 2Ea than in the remaining portion 2Eb. Therefore, the optical modulator 10I of this embodiment also achieves the same effects as the optical modulator 10H of the ninth embodiment.

[0119] Fig. 12 shows a modified example of the optical modulator 10I according to the tenth embodiment. Referring to Fig. 12, the surface layer 2Ea may be a surface layer of the first electrode 2E that is located on the optical waveguide 1C side in the stacking direction of the first electrode 2E relative to the optical waveguide 1. In this case, the surface layer 2Ea is, for example, a portion that is within a range of 10% of the thickness of the first electrode 2E from the surface that is located on the optical waveguide 1C side in the stacking direction. Even with this configuration, it is possible to achieve the same effects as the optical modulator 10H according to the ninth embodiment.

[0120] 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.

[0121] The configuration of the optical modulator according to the present disclosure is summarized below.

[0122] <1> an optical waveguide made of a material having an electro-optic effect; a first electrode made of a semiconductor material; an optical modulator comprising: a second electrode made of a metal material and arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide;

[0123] <2> <1> The optical modulator according to claim 1, further comprising: a low dielectric layer having a refractive index smaller than that of the optical waveguide; of the first electrode and the second electrode, at least the first electrode is disposed with a gap between it and the optical waveguide, The low dielectric layer is provided in the gap.

[0124] <3> <1> The optical modulator according to claim 1, further comprising: a low dielectric layer having a refractive index smaller than that of the optical waveguide; the first electrode and the second electrode are each disposed with a gap between them and the optical waveguide; The low dielectric layer is provided in the gap.

[0125] <4> <2> or <3> The optical modulator according to claim 1, An optical modulator, wherein the size of the gap is not less than 0.750 μm and not more than 1.675 μm.

[0126] <5> <2> ~ <4> 10. An optical modulator according to claim 9, the semiconductor material is a silicon semiconductor material in which silicon is doped with impurities, An optical modulator, wherein the low dielectric constant layer is mainly composed of SiO2.

[0127] <6> <1> ~ <5> 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.

[0128] <7> <6> The optical modulator according to claim 1, an optical modulator, wherein the first electrode is provided on a surface located on the optical waveguide side in a stacking direction of the first electrode, the optical waveguide, and the second electrode, and includes a convex portion protruding toward the optical waveguide.

[0129] <8> <7> The optical modulator according to claim 1, an optical modulator, wherein, when viewed in a cross section perpendicular to the extending direction of the optical waveguide, the length of the convex portion in a direction perpendicular to the stacking direction decreases as the convex portion approaches the optical waveguide.

[0130] <9> <6> ~ <8> 10. The optical modulator according to claim 9, An optical modulator, wherein, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends, in a direction perpendicular to the stacking direction of the first electrode, the optical waveguide, and the second electrode, the length of the surface of the first electrode on the optical waveguide side is longer than the length of the optical waveguide.

[0131] <10> <1> ~ <9> 10. The 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.

[0132] <11> <10> 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.

[0133] <12> <1> ~ <11> 10. The optical modulator according to claim 9, an optical modulator, wherein the refractive index of the first electrode is less than 3;

[0134] <13> <10> ~ <12> 10. An optical modulator according to claim 9, The optical modulator, wherein the first electrode is a silicon single crystal substrate.

[0135] <14> <10> ~ <12> 10. An optical modulator according to claim 9, The optical modulator, wherein the first electrode is an active layer of an SOI substrate.

[0136] <15> <1> ~ <14> 10. An optical modulator according to claim 9, The optical modulator, wherein the metallic material is mainly composed of a noble metal.

[0137] <16> <1> ~ <15> 10. An optical modulator according to claim 9, an optical modulator, wherein the area of ​​the first electrode is larger than the area of ​​the second electrode when viewed in a cross section perpendicular to the extending direction of the optical waveguide;

[0138] <17> <1> ~ <16> 10. An optical modulator according to claim 9, an optical modulator, wherein the second electrode is a signal electrode and the first electrode is a ground electrode;

[0139] <18> <1> ~ <17> The optical modulator according to any one of the above items, further comprising: an optical modulator comprising a thin metal layer provided on a surface of the first electrode on the side of the optical waveguide, the thin metal layer being thinner than the first electrode;

[0140] <19> <1> ~ <18> 10. An 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]

[0141] 10, 10A, 10B, 10C, 10D, 10E, 10F, 10G, 10I: Optical modulators 1,1C: Optical waveguide path 1Ca:Substrate part 1Cb: Lipubu 2,2A,2B,2C,2D,2E: Electrode 1 2Aa, 2Ba: convex part 2Ab,2Bb: Base 2Da,2Ea: Surface layer 2Db,2Eb: Remnants 20:SOI substrate 21:Acidified film 22a, 22b active layers 3,3C: Second electrode 4,5,4C: Low induced electrophoresis layer 6: Thin metal layer

Claims

1. an optical waveguide made of a material having an electro-optic effect; a first electrode made of a semiconductor material; a second electrode made of a metal material and arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide; An optical modulator, wherein a surface layer of the first electrode on the side of the optical waveguide is doped with an impurity at a higher concentration than other portions of the first electrode.

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

3. 2. The optical modulator according to claim 1, An optical modulator, wherein the concentration of the impurity in the first electrode gradually decreases with increasing distance from the surface layer.

4. The optical modulator according to any one of claims 1 to 3, An optical modulator, wherein the concentration of the impurity in the surface layer of the first electrode is 10% or more higher than the concentration of the impurity in the other portion.

Images