WO2024116447A1 - 光変調器 - Google Patents

光変調器 Download PDF

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Publication number
WO2024116447A1
WO2024116447A1 PCT/JP2023/022849 JP2023022849W WO2024116447A1 WO 2024116447 A1 WO2024116447 A1 WO 2024116447A1 JP 2023022849 W JP2023022849 W JP 2023022849W WO 2024116447 A1 WO2024116447 A1 WO 2024116447A1
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Prior art keywords
electrode
optical waveguide
optical modulator
optical
modulator according
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PCT/JP2023/022849
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English (en)
French (fr)
Japanese (ja)
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聡希 ▲浜▼村
康弘 會田
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Murata Manufacturing Co Ltd
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Murata Manufacturing Co Ltd
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Priority to JP2024518266A priority Critical patent/JP7729481B2/ja
Priority to CN202380070791.2A priority patent/CN119923588A/zh
Publication of WO2024116447A1 publication Critical patent/WO2024116447A1/ja
Priority to US18/768,269 priority patent/US20240361623A1/en
Anticipated expiration legal-status Critical
Priority to JP2025134633A priority patent/JP2025166144A/ja
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
    • G02F1/035—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction
    • G02F1/025—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
    • G02F2201/12—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 electrode
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2202/00—Materials and properties
    • G02F2202/16—Materials and properties conductive

Definitions

  • This disclosure relates to an optical modulator.
  • optical communications require optical transceivers to convert between optical and electrical signals.
  • the main component of an optical transceiver is an optical modulator.
  • the optical modulator is responsible for converting electrical signals into optical signals.
  • the optical modulator in Patent Document 1 has a core portion having a slot waveguide structure.
  • the core portion has an upper high refractive index layer, a lower high refractive index layer, and a low refractive index layer provided in the gap (slot) between these high refractive index layers.
  • the refractive indices of the upper and lower high refractive index layers are greater than the refractive index of the low refractive index layer.
  • the upper and lower high refractive index layers each have a contact region. A metal electrode is connected to each of the contact regions.
  • the optical modulator in Non-Patent Document 1 comprises an optical waveguide and two metal electrodes.
  • the optical waveguide is disposed between the two metal electrodes.
  • one metal electrode, the optical waveguide, and the other metal electrode are disposed in parallel.
  • one metal electrode is laminated on the optical waveguide, and the other metal electrode is laminated on the optical waveguide on the opposite side.
  • the two electrodes are disposed so as to apply an electric field to the optical waveguide.
  • Integrated lithium niobate electro-optic modulators when performance meets scalability, Mian Zhang et al., Optica Vol. 8, Issue 5, pp. 652-667 (2021)
  • the electrodes In order to apply a wideband, high-frequency signal to the electrodes while suppressing power consumption, it is preferable for the electrodes to be thick.
  • the electrodes are made of a metal material as in conventional optical modulators, making the electrodes thicker generates large internal stresses. This internal stress makes the electrodes susceptible to cracks.
  • two metal electrodes are stacked on either side of an optical waveguide, internal stresses are generated in each electrode that cause both electrodes to warp and deform in the same direction. As a result, the optical waveguide warps and deforms together with both electrodes, making the optical waveguide susceptible to cracks.
  • the objective of this 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.
  • the optical modulator according to the present disclosure comprises 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.
  • the optical modulator disclosed herein can suppress the occurrence of cracks and apply an electrical signal to the electrodes while reducing power consumption.
  • FIG. 1 is a cross-sectional view showing a schematic configuration of an optical modulator according to a first embodiment.
  • FIG. 2 is a cross-sectional view showing a schematic configuration of an optical modulator according to the second embodiment.
  • FIG. 3 is a cross-sectional view showing a schematic configuration of an optical modulator according to the third embodiment.
  • FIG. 4 is a cross-sectional view showing a schematic configuration of an optical modulator according to the fourth embodiment.
  • FIG. 5 is a cross-sectional view showing a schematic configuration of an optical modulator according to the fifth embodiment.
  • FIG. 6 is a diagram showing a modified example of the optical modulator according to the fifth embodiment.
  • FIG. 7 is a cross-sectional view showing a schematic configuration of an optical modulator according to the sixth embodiment.
  • FIG. 1 is a cross-sectional view showing a schematic configuration of an optical modulator according to a first embodiment.
  • FIG. 2 is a cross-sectional view showing a schematic configuration of an optical modulator according to the
  • FIG. 8 is a cross-sectional view showing a schematic configuration of an optical modulator according to the seventh embodiment.
  • FIG. 9 is a cross-sectional view showing a schematic configuration of an optical modulator according to the eighth embodiment.
  • FIG. 10 is a cross-sectional view showing a schematic configuration of an optical modulator according to the ninth embodiment.
  • FIG. 11 is a cross-sectional view showing a schematic configuration of an optical modulator according to the tenth embodiment.
  • FIG. 12 is a diagram showing a modification of the optical modulator according to the tenth embodiment.
  • An optical modulator 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).
  • the first electrode is a semiconductor electrode made of a semiconductor material.
  • the semiconductor electrode can be formed thicker while suppressing internal stress. By forming the semiconductor electrode thick, it is possible to apply a wideband, high-frequency signal to the semiconductor electrode while suppressing power consumption.
  • the semiconductor electrode can be formed while suppressing internal stress, it is possible to suppress the occurrence of cracks due to internal stress.
  • the second electrode is a metal electrode made of a metal material.
  • the metal material that makes up the second electrode has a higher conductivity than the semiconductor material. Therefore, electrical loss can be suppressed compared to when both the first and second electrodes are semiconductor electrodes.
  • 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.
  • at least the first electrode of the first and second electrodes may be disposed with a gap from 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 from the optical waveguide, and the low dielectric layer may be provided in each gap (third configuration).
  • the first electrode which is a semiconductor electrode
  • the first electrode is not in contact with the optical waveguide.
  • a low dielectric layer with a smaller refractive index than 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 suppressing optical loss.
  • 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 with a smaller refractive index than 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.
  • the size of the gap is preferably 0.750 ⁇ m or more and 1.675 ⁇ m or less (fourth configuration).
  • evanescent light Light seeps out from the optical waveguide into the low dielectric constant layer, albeit weakly. This seeping light is called evanescent light.
  • the evanescent light is less likely to come into contact with the first electrode and/or second electrode, and light loss can be further suppressed.
  • the size of the gap between the first electrode and/or the second electrode and the optical waveguide is 1.675 ⁇ m or less.
  • 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 for the optical waveguide can be ensured without increasing the voltage applied between the first electrode and the second electrode.
  • the semiconductor material may be a silicon semiconductor material in which impurities are doped into silicon, and the main component of the low dielectric constant layer may be SiO 2 (fifth configuration). Since the semiconductor material is a silicon semiconductor material, when a low dielectric constant layer is provided in the gap between the first electrode and the optical waveguide, a low dielectric constant layer of SiO 2 can be formed on the first electrode of the silicon semiconductor material by a thermal oxidation method. In the film formation by the thermal oxidation method, the adhesion of the low dielectric constant layer to the first electrode is good, and foreign matter is unlikely to enter the interface between the first electrode and the low dielectric constant layer.
  • 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 (sixth configuration).
  • the optical waveguide exists 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 from the first electrode and the second electrode can be efficiently applied to the optical waveguide.
  • 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).
  • the convex portion can concentrate the electric field on the optical waveguide. This allows the voltage applied between the first electrode and the second electrode to be reduced, further reducing power consumption.
  • the length of the convex portion in the direction perpendicular to the stacking direction may be smaller the closer it is to the optical waveguide (eighth configuration).
  • the convex portion has a rectangular shape in cross section, the side of the convex portion continues at a right angle to other parts of the surface of the first electrode on the optical waveguide side.
  • the side of the convex portion can be made to continue relatively gently to other parts of the surface of the first electrode on the optical waveguide side. This makes it possible to reduce electrical loss at the boundary between the convex portion and other parts.
  • any one of the optical modulators of the sixth to eighth configurations preferably, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends, 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 greater than the length of the optical waveguide (ninth configuration). In this case, an electric field can be applied to the entire optical waveguide.
  • the semiconductor material in the first electrode is preferably a silicon semiconductor material in which silicon is doped with impurities (tenth configuration).
  • the impurity concentration in the first electrode is preferably 1.0 ⁇ 10 17 cm ⁇ 3 or more and 1.0 ⁇ 10 22 cm ⁇ 3 or less (eleventh configuration).
  • the first electrode resistivity decreases and conductivity increases with an increase in impurity. If the impurity concentration is 1.0 ⁇ 10 17 cm ⁇ 3 or more, the first electrode can function effectively as an electrode. If the impurity concentration is 1.0 ⁇ 10 22 cm ⁇ 3 or less, precipitation of the impurity can be prevented.
  • 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.
  • the first electrode is preferably a silicon single crystal substrate (thirteenth configuration).
  • the first electrode may be an active layer of an SOI substrate (14th configuration).
  • the main component of the metallic material is preferably a precious metal (fifteenth configuration).
  • the main component of the metallic material constituting the second electrode is a precious metal that is less susceptible to chemical reactions, so the modulation characteristics of the optical modulator are stable. Furthermore, if the main component of the metallic material constituting the second electrode is a precious metal, the resistance value of the second electrode can be reduced, and power consumption can be reduced.
  • the area of the first electrode is preferably larger than the area of the second electrode when viewed in a cross section perpendicular to the extension direction of the optical waveguide (sixteenth configuration). Since the second electrode is made of a metal material, the resistance value is sufficiently small even without increasing the cross-sectional area. On the other hand, since the first electrode is made of a semiconductor material with a 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.
  • 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.
  • the optical modulator of any one of the first to seventeenth configurations may further include a thin metal film thinner than the first electrode.
  • the thin metal film is provided on the surface of the first electrode facing the optical waveguide (configuration 18).
  • a thin metal layer has high conductivity and low attenuation of high-frequency signals. If this thin metal 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, because high-frequency signals propagate through the surface layer due to the skin effect, it is preferable for the conductivity near the surface to be higher. In this regard, if a thin metal 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.
  • 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).
  • impurities at a higher concentration than other parts of the first electrode (19th configuration).
  • a region of high conductivity can be localized in the first electrode near the optical waveguide, and attenuation of high-frequency signals can be suppressed by the skin effect.
  • 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 extension direction of the optical waveguide 1.
  • the cross section means a cross section perpendicular to the extension direction of the optical waveguide 1, unless otherwise specified.
  • 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 a light transmission path.
  • LiNbO3 lithium niobate
  • LiTaO3 lithium tantalate
  • PLZT lead lanthanum zirconate titanate
  • KTN potassium tantalate niobate
  • BaTiO3 barium titanate
  • an electro-optic polymer EO polymer
  • 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 form 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.
  • 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.
  • 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.
  • 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.
  • 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).
  • the semiconductor material for example, other single element semiconductors using Ge (germanium) or the like, or compound semiconductors such as GaAs (gallium arsenide) may be used.
  • the impurities may be either p-type impurities or n-type impurities.
  • a Group 3 element such as boron is used as the p-type impurity
  • a Group 5 element such as phosphorus, arsenic, or antimony is used as the n-type impurity.
  • the concentration (doping amount) of the impurity in the first electrode 2 is preferably 1.0 ⁇ 10 17 cm ⁇ 3 or more and 1.0 ⁇ 10 22 cm ⁇ 3 or less. As the doping amount of the impurity increases, the resistivity of the semiconductor material decreases and the conductivity increases. If the doping amount is 1.0 ⁇ 10 17 cm ⁇ 3 or more, the first electrode 2 can function effectively as an electrode. If the doping amount is 1.0 ⁇ 10 22 cm ⁇ 3 or less, the precipitation of the impurity can be prevented due to the solid solubility limit of the impurity 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 smaller than 3.
  • the upper limit of the doping amount is preferably 1.0 ⁇ 10 22 cm ⁇ 3 is based on the solid solubility limit of impurities in silicon semiconductor materials.
  • the reason why the lower limit of the doping amount is preferably 1.0 ⁇ 10 17 cm ⁇ 3 is as follows.
  • the skin depth is an index 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 will increase.
  • the thickness and width of the electrode is set to be equal to or greater than the skin depth.
  • the doping amount is 1.0 ⁇ 10 17 cm ⁇ 3
  • the conductivity is 1000 S/m
  • the skin depth is 500 ⁇ m.
  • the thickness of the electrode is limited to about 500 ⁇ m. From the viewpoint of ensuring the performance of the electrode, therefore, in order to obtain an electrode having a conductivity of 1000 S/m or more, the doping amount should be 1.0 ⁇ 10 17 cm ⁇ 3 or more.
  • the first electrode 2 is, for example, a silicon single crystal substrate.
  • impurities are doped in advance into a silicon single crystal base substrate that is the material of 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 thus formed by thermal diffusion, ion implantation, or the like.
  • the first electrode 2 may be a semiconductor silicon layer formed on a substrate.
  • a silicon layer can be formed on a 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 a semiconductor silicon layer as the first electrode 2.
  • 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 precious metal.
  • the precious metal is, for example, Au (gold).
  • As the precious metal Ag (silver), Pt (platinum), etc. may be used.
  • the metal material may contain trace amounts of other metal elements such as Cr and Ti.
  • As the metal material copper, aluminum, or an alloy thereof, etc. may be used.
  • 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. The material substrate is then subjected to lithography and etching to form the optical waveguide 1. Next, a metal layer is formed on the optical waveguide 1 by sputtering, deposition, or the like. The formed metal film is patterned by lithography, and the second electrode 3 is formed by etching.
  • 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.
  • the optical waveguide 1 has a thickness t1 corresponding to the length in the stacking direction of the optical waveguide 1 and the electrodes 2 and 3, 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.
  • the area of the first electrode 2 is larger than the area of the second electrode 3.
  • 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 is substantially equal to the resistance value of the second electrode 3.
  • the performance of the first electrode 2 as an electrode is equivalent to that of the second electrode 3.
  • 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".
  • the product of the conductivity and the thickness t2 of the first electrode 2 may be equal to the product of the conductivity and the thickness t3 of the second electrode 3.
  • the thickness t3 of the second electrode 3 is usually set to 0.1 ⁇ m or more and 2.0 ⁇ m or less, and the conductivity is 4.3 ⁇ 10 7 S/m.
  • 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 impurities doped.
  • the conductivity of the first electrode 2 is 1 ⁇ 10 7 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, and the thickness t2 of the first electrode 2 can be set to 4.3 times the thickness t3 of the second electrode 3.
  • the doping amount of the impurity is the lower limit of 1.0 ⁇ 10 17 cm ⁇ 3
  • 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 can be set to 4.3 ⁇ 10 3 times the thickness t3 of the second electrode 3.
  • 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 ⁇ m. That is, the thickness t2 of the first electrode 2 can be set to 0.43 ⁇ m or more.
  • the upper limit of the thickness t2 of the first electrode 2 can be set to 4.3 ⁇ 10 3 times the upper limit of the thickness t3 of the second electrode 3, which is 2.0 ⁇ m. That is, the thickness t2 of the first electrode 2 can be set to 8600 ⁇ m (8.6 mm) or less.
  • the thickness t2 of the first electrode 2 is determined taking into consideration, for example, processability, in addition to the electrical conductivity of the electrodes 2 and 3 and the thickness t3 of the second electrode 3.
  • the thickness t2 of the first electrode 2 is preferably 500 ⁇ m or less.
  • the thickness t2 required for the first electrode 2 can be estimated based on the skin effect.
  • the following formula (1) is used to calculate the skin depth of a conductor.
  • the thickness t2 required for 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 electrical resistance of the first electrode 2 can be reduced, and excess electrical loss can be suppressed. The higher the electrical conductivity of the first electrode 2, the more preferable it is. However, since there is a solubility limit for the doping amount of the impurity, and since the impurity clusters when the doping amount approaches the solubility limit and an inactive state occurs as a carrier, the electrical conductivity of the first electrode 2 is saturated when the doping amount exceeds a certain amount.
  • the solubility limit of the doping amount is 1.0 ⁇ 10 22 cm ⁇ 3
  • the electrical conductivity of the first electrode 2 is 1 ⁇ 10 7 S/m
  • the skin depth of the 1 GHz electrical signal is 5 ⁇ m.
  • the actual electrical conductivity will be about 1 ⁇ 10 6 S/m, which is one order of magnitude lower.
  • the thickness t2 of the first electrode 2 is 25 ⁇ m or more.
  • the thickness t2 of the first electrode 2, which is a semiconductor electrode, can be measured, for example, by the following methods.
  • the first method is a measurement method using SEM observation.
  • the optical modulator 10 is cut using a FIB (focused ion beam) 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.
  • the thickness t3 of the second electrode 3, which is a metal electrode, can be measured, for example, by 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 penetrate 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 amount of attenuation thus determined. Either method will produce substantially the same measurement results.
  • the amount of doping 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.
  • the width w2 of the first electrode 2 on the optical waveguide 1 side is greater than the width w1 of the optical waveguide 1.
  • the width w2 of the first electrode 2 on the optical waveguide 1 side refers to the width of the surface of the first electrode 2 that is closest to the optical waveguide 1.
  • 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 width w2.
  • the first electrode 2 is made of a semiconductor material
  • the second electrode 3 is made of a metal material. That is, of the first electrode 2 and the second electrode 3 that apply an electric field to the optical waveguide 1, the first electrode 2 is a semiconductor electrode.
  • the 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 and high-frequency signal to the semiconductor electrode while suppressing power consumption. In addition, since the semiconductor electrode can be formed while suppressing internal stress, it is possible to suppress the occurrence of cracks caused by internal stress.
  • the semiconductor material used for the first electrode 2 is, for example, a silicon semiconductor material in which impurities are doped into Si.
  • the first electrode 2 may be a silicon single crystal substrate, or a semiconductor silicon layer formed on a 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.
  • the second electrode 3 is a metal electrode.
  • the metal material that constitutes the second electrode has a higher conductivity than the semiconductor material. Therefore, electrical loss can be suppressed compared to when both the first electrode and the second electrode are semiconductor electrodes.
  • 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 makes it possible to prevent light absorption by the adhesive layer. It also makes it possible to prevent the adhesive layer from diffusing into the optical waveguide 1, causing the refractive index of the optical waveguide 1 to change.
  • 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.
  • the optical waveguide 1 exists between the first electrode 2 and the second electrode 3 in the lamination direction. Therefore, an electric field from the first electrode 2 and the second electrode 3 can be efficiently applied to the optical waveguide 1.
  • the second electrode 3 is made of a metal material, and the main component of the metal material is, for example, a precious metal.
  • the main component of the metal material is, for example, a precious metal.
  • the length (width w2) of the surface of the first electrode 2 facing the optical waveguide 1 in the direction perpendicular to the stacking direction is greater than the length (width w1) of the optical waveguide 1.
  • an electric field can be applied to the entire optical waveguide 1.
  • stress can be alleviated when forming the optical waveguide 1 on the first electrode 2.
  • the area of the first electrode 2 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. Since the second electrode 3 is made of a metal material, the resistance value is sufficiently small even without increasing the cross-sectional area. On the other hand, since the first electrode 2 is made of a semiconductor material with a 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 3. In this embodiment, since the cross-sectional area of the first electrode 2 is larger than the cross-sectional area of the second electrode 3, the resistance value of the first electrode 2 is reduced and power consumption can be reduced.
  • 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, by using the second electrode 3 as a signal electrode, the driving voltage can be suppressed.
  • 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.
  • 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, etc. 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. For this reason, there is a risk of light leaking from the optical waveguide 1 to the first electrode 2.
  • 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 ⁇ m or more and 1.675 ⁇ m 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 in the stacking direction from the first electrode 2 to the optical waveguide 1 is the shortest distance from the first electrode 2 to the optical waveguide 1.
  • 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.
  • the low dielectric layer 4 is laminated on the first electrode 2, and the optical waveguide 1 is laminated on the low dielectric layer 4.
  • the optical waveguide 1 is indirectly laminated 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 covers the entire surface of the optical waveguide 1 facing the low dielectric layer 4.
  • the second electrode 3 is laminated directly on the optical waveguide 1 and is in contact with the optical waveguide 1, as in the first embodiment.
  • the main component of the low dielectric constant layer 4 is typically SiO 2.
  • the main component of the low dielectric constant layer 4 may be an oxide such as Al 2 O 3 , LaAlO 3 , LaYO 3 , ZnO, HfO 2 , MgO, or Y 2 O 3 , or a polymer such as BCB (benzocyclobutene) or PI (polyimide).
  • the low dielectric constant 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 constant layer 4, and the material substrate can be bonded to the low dielectric constant layer 4. Thereafter, the optical waveguide 1 and the second electrode 3 can be formed on the low dielectric constant layer 4, as described in the first embodiment.
  • the first electrode 2 is not in contact with the optical waveguide 1. Furthermore, a low dielectric layer 4 with a smaller refractive index than 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 is less likely to be absorbed by the first electrode 2. Therefore, light loss can be suppressed.
  • the semiconductor material used for the first electrode 2 is a silicon semiconductor material
  • a low dielectric constant layer 4 of SiO 2 can be formed on the first electrode 2 of the silicon semiconductor material by a thermal oxidation method.
  • the adhesion of the low dielectric constant layer 4 to the first electrode 2 is good, and foreign matter is unlikely to enter the interface between the first electrode 2 and the low dielectric constant layer 4. Therefore, electrical loss can be suppressed at the interface between the first electrode 2 and the low dielectric constant layer 4.
  • the reliability and life of the optical modulator 10A can be improved. 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.
  • 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, is equal to or greater than the wavelength of the light passing through the optical waveguide 1.
  • the size of the gap between the first electrode 2 and the optical waveguide 1 may be 0.750 ⁇ m or more. If the size of the gap between the first electrode 2 and the optical waveguide 1 is 1.675 ⁇ m or less, the magnitude of the electric field for the optical waveguide 1 can be ensured without increasing the voltage applied between the first electrode 2 and the second electrode 3.
  • Third Embodiment 3 is a cross-sectional view showing a schematic configuration of an optical modulator 10B according to a 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.
  • 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 ⁇ m or more and 1.675 ⁇ m 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 in the stacking direction from the second electrode 3 to the optical waveguide 1 is the shortest distance from the second electrode 3 to the optical waveguide 1.
  • 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.
  • the low dielectric layer 5 is laminated on the optical waveguide 1, and the second electrode 3 is laminated on the low dielectric layer 5.
  • the optical waveguide 1 is indirectly laminated 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.
  • the low dielectric layer 5 covers the entire surface of the optical waveguide 1 facing the low dielectric layer 5.
  • the first electrode 2 is laminated directly on the optical waveguide 1 and is in contact with the optical waveguide 1, as in the first embodiment.
  • An example of the main component of the low dielectric constant layer 5 is 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 the main component of the low dielectric constant layer 4.
  • the low dielectric constant layer 5 is deposited on the optical waveguide 1 by, for example, CVD, vapor deposition, sputtering, etc.
  • the optical waveguide 1 may be formed on the first electrode 2 as described in the first embodiment.
  • a metal layer may be deposited on the low dielectric constant layer 5 by sputtering, vapor deposition, etc., and the second electrode 3 may be formed on the deposited metal film as described in the first embodiment.
  • the second electrode 3 is not in contact with the optical waveguide 1. Furthermore, a low dielectric layer 5 having a smaller refractive index than 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, the light passing through the optical waveguide 1 is less likely to leak to the second electrode 3 and is less likely to be absorbed by the second electrode 3. Therefore, the loss of light can be suppressed.
  • the size of the gap between the second electrode 3 and the optical waveguide 1 is 0.750 ⁇ m or more and 1.675 ⁇ m or less. Therefore, as in the second embodiment, it is possible to suppress light loss and ensure the magnitude of the electric field for the optical waveguide 1.
  • 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 point of view, the optical modulator 10C is a combination of the configurations of the second and third embodiments.
  • 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, which have a smaller refractive index than 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 suppress optical loss even more than the optical modulators 10A and 10B.
  • 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.
  • the first electrode 2A includes a convex portion 2Aa.
  • the convex portion 2Aa is provided on the surface located on the optical waveguide 1 side in the stacking direction, and protrudes toward the optical waveguide 1.
  • the first electrode 2A includes a convex portion 2Aa and a base portion 2Ab.
  • the base portion 2Ab is disposed on the opposite side to the optical waveguide 1 in the stacking direction.
  • the convex portion 2Aa protrudes toward the optical waveguide 1 from a surface 2Aba of the base portion 2Ab.
  • Each of the convex portion 2Aa and the base portion 2Ab may have a substantially rectangular cross section.
  • a low-dielectric layer 4 is stacked on the convex portion 2Aa of the first electrode 2A, and the convex portion 2Aa is in contact with the low-dielectric layer 4.
  • the width w2Ab of the base 2Ab is greater than the width w2Aa of the protruding portion 2Aa.
  • the width w2Aa means the width of the surface 2Aaa of the protruding portion 2Aa that is closest to the optical waveguide 1.
  • the length of the surface 2Aaa of the protruding portion 2Aa that is in contact with the low dielectric layer 4 in the direction perpendicular to the stacking direction is the width w2Aa.
  • the width w2Aa of the protruding portion 2Aa is greater than the width w1 of the optical waveguide 1.
  • the convex portion 2Aa allows the electric field to be concentrated 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.
  • the first electrode 2A including the convex portion 2Aa and the base portion 2Ab is, for example, a silicon single crystal substrate.
  • the convex portion 2Aa and the base portion 2Ab can be formed by performing lithography and etching on a silicon single crystal base substrate.
  • FIG. 6 shows a modified example of the optical modulator 10D according to the fifth embodiment.
  • an SOI substrate 20 is used as a substrate on which the 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.
  • one of the active layers 22a serves as the first electrode 2A.
  • patterning for forming the first electrode 2A can be easily performed.
  • the first electrode 2A may be applied to the optical modulator 10 according to the first embodiment. In this case, the convex portion 2Aa comes into 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 comes into contact with the low dielectric layer 4, but does not come into 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 comes into contact with the optical waveguide 1.
  • 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.
  • the first electrode 2B includes a convex portion 2Ba and a base portion 2Bb.
  • the convex portion 2Ba protrudes from a surface 2Bba of the base portion 2Bb toward the optical waveguide 1.
  • the length (width) of the convex portion 2Ba in a direction perpendicular to the stacking direction becomes smaller the closer it is to the optical waveguide 1.
  • the convex portion 2Ba can have a minimum width w2Ba at the surface 2Baa on the optical waveguide 1 side.
  • the surface 2Baa of the convex portion 2Ba is in contact with the low dielectric layer 4.
  • the convex portion 2Ba may have a substantially trapezoidal cross section. In this case, the surface 2Baa of the convex portion 2Ba corresponds to the upper base of the trapezoid.
  • the width of the convex portion 2Ba becomes smaller the closer it is to the optical waveguide 1.
  • the side surface 2Bab of the convex portion 2Ba can be made to continue relatively gently to the surface 2Bba of the base portion 2Bb. More specifically, the side surface 2Bab of the convex portion 2Ba can be smoothly connected to the surface 2Bba of the base portion 2Bb at an obtuse angle, that is, in a shape close to a curve.
  • the side surface 2Bab of the protrusion 2Ba is inclined at a constant gradient with respect to the surface 2Baa.
  • the gradient of the side surface 2Bab with respect to the surface 2Baa may vary.
  • the first electrode 2B may be applied to each of the optical modulators 10, 10A, and 10B according to the first to third embodiments.
  • 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.
  • 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 facing the optical waveguide 1 of the convex portion 2Aa.
  • 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.
  • the thickness t6 of the thin metal layer 6 is thinner than the thickness t3 of the second electrode 3.
  • 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 convex portion 2Aa on the optical waveguide 1 side, the drive voltage can be reduced. Furthermore, in the convex portion 2Aa of the first electrode 2A, high frequency signals propagate through the surface layer due to the skin effect, so it is preferable for the conductivity near the surface to be higher. In this regard, if the thin metal layer 6 is provided on the surface of the first electrode 2A on the optical waveguide 1 side, the resistance value can be reduced and signal attenuation can be reduced. In this case, the convex portion 2Aa is used for low frequency signals.
  • 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.
  • 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.
  • the thin metal layer 6 is provided on the surface 2Baa of the convex portion 2Ba of the first electrode 2B.
  • 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.
  • 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.
  • 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 a cross-sectional view of the optical modulator 10G.
  • the first electrode 2C is arranged on one side of the ridge portion 1Cb
  • 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 form a potential difference between each other to apply an electric field to the ridge portion 1Cb of the optical waveguide 1C.
  • the optical modulator 10G according to this embodiment has the same effect 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.
  • 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.
  • the first electrode 2D has a surface layer 2Da on the optical waveguide 1 side and a remaining portion 2Db.
  • the surface layer 2Da is disposed so as to be adjacent to the optical waveguide 1 in the first electrode 2D.
  • the surface layer 2Da is, for example, a portion 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 means a portion of the first electrode 2D excluding the surface layer 2Da.
  • the concentration of the impurity doped in 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.
  • the impurity concentration in the surface layer 2Da is 10% or more higher than the impurity concentration in the remaining portion 2Db.
  • Such an impurity concentration distribution in the first electrode 2D can be formed by a thermal diffusion method, an ion implantation method, or the like.
  • a profile of the impurity concentration in the depth direction from the surface of the first electrode 2D on the optical waveguide 1 side is obtained.
  • 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 are set as the impurity concentration in the surface layer 2Da and the impurity concentration in the remaining portion 2Db, respectively.
  • the integral average of the impurity concentration in the range from the surface of the first electrode 2D on the optical waveguide 1 side to 10% of the depth (thickness) of the first electrode 2D is the impurity concentration of the surface layer 2Da
  • the integral average of the impurity concentration in the remaining range is the impurity concentration of the remaining portion 2Db.
  • 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.
  • the 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.
  • a region of high conductivity can be localized in the first electrode 2D near the optical waveguide 1, and attenuation of high frequency signals can be suppressed by the skin effect.
  • 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.
  • Tenth Embodiment 11 is a cross-sectional view showing a schematic configuration of an optical modulator 10I according to the tenth embodiment.
  • the optical modulator 10I differs from the optical modulator 10G according to the eighth embodiment in the configuration of a first electrode 2E.
  • 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.
  • 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 with respect 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 positioned on the ridge portion 1Cb side in the width direction of the first electrode 2E.
  • the remaining portion 2Eb refers to the portion of the first electrode 2E excluding the surface layer 2Ea.
  • 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 achieves the same effects as the optical modulator 10H of the ninth embodiment.
  • FIG. 12 shows a modified example of the optical modulator 10I according to the tenth embodiment.
  • the surface layer 2Ea may be the 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.
  • 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, the same effect as the optical modulator 10H according to the ninth embodiment can be achieved.
  • 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 metallic material and arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide.
  • the optical modulator according to ⁇ 1> further comprising: a low dielectric constant layer having a refractive index smaller than a refractive index of the optical waveguide; At least the first electrode of the first electrode and the second electrode is disposed with a gap between the first electrode and the optical waveguide, The low dielectric layer is provided in the gap.
  • the optical modulator according to ⁇ 1> further comprising: a low dielectric constant layer having a refractive index smaller than a refractive index 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.
  • ⁇ 4> The optical modulator according to ⁇ 2> or ⁇ 3>, An optical modulator, wherein the size of the gap is not less than 0.750 ⁇ m and not more than 1.675 ⁇ m.
  • the semiconductor material is a silicon semiconductor material in which silicon is doped with an impurity
  • An optical modulator, wherein the main component of the low dielectric constant layer is SiO2 .
  • ⁇ 6> An optical modulator according to any one of ⁇ 1> to ⁇ 5>, the first electrode is laminated on the optical waveguide; The second electrode is laminated to the optical waveguide on an opposite side to the first electrode.
  • ⁇ 7> The optical modulator according to ⁇ 6>, 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.
  • ⁇ 8> The optical modulator according to ⁇ 7>, an optical modulator, wherein, when viewed in a cross section perpendicular to an extension direction of the optical waveguide, a length of the protrusion in a direction perpendicular to the stacking direction becomes smaller the closer to the optical waveguide.
  • An optical modulator according to any one of ⁇ 6> to ⁇ 8>, 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 a stacking direction of the first electrode, the optical waveguide, and the second electrode, a length of a surface of the first electrode on the optical waveguide side is greater than a length of the optical waveguide.
  • An optical modulator according to any one of ⁇ 1> to ⁇ 9>, An optical modulator, wherein the semiconductor material is a silicon semiconductor material in which silicon is doped with an impurity.
  • ⁇ 11> The optical modulator according to ⁇ 10>, An optical modulator, wherein a concentration of the impurity in the first electrode is not less than 1.0 ⁇ 10 17 cm ⁇ 3 and not more than 1.0 ⁇ 10 22 cm ⁇ 3 .
  • ⁇ 14> An optical modulator according to any one of ⁇ 10> to ⁇ 12>, The optical modulator, wherein the first electrode is an active layer of an SOI substrate.
  • ⁇ 16> An optical modulator according to any one of ⁇ 1> to ⁇ 15>, an area of the first electrode being larger than an area of the second electrode when viewed in a cross section perpendicular to an extension direction of the optical waveguide;
  • optical modulator according to any one of ⁇ 1> to ⁇ 17>, further comprising: an optical modulator comprising: a thin metal layer provided on a surface of the first electrode facing the optical waveguide, the thin metal layer having a thickness smaller than that of the first electrode;
  • An optical modulator according to any one of ⁇ 1> to ⁇ 18>, 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.
  • Optical modulator 1C Optical waveguide 1Ca: Substrate portion 1Cb: Ridge portion 2, 2A, 2B, 2C, 2D, 2E: First electrode 2Aa, 2Ba: Convex portion 2Ab, 2Bb: Base portion 2Da, 2Ea: Surface layer 2Db, 2Eb: Remaining portion 20: SOI substrate 21: Oxide film 22a, 22b Active layer 3, 3C: Second electrode 4, 5, 4C: Low dielectric constant layer 6: Thin metal layer

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US20040066250A1 (en) * 2000-08-25 2004-04-08 Hunt Andrew T Electronic and optical devices and methods of forming these devices
JP2005114868A (ja) * 2003-10-03 2005-04-28 Ntt Electornics Corp 半導体光変調導波路
JP2007101641A (ja) * 2005-09-30 2007-04-19 Sumitomo Osaka Cement Co Ltd 光変調器及びその製造方法

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