Electro-optic waveguide device and optical module

An electro-optic waveguide device may include a slot waveguide including a lower high-refractive-index layer with a first refractive index and an upper high-refractive-index layer with a second refractive index, wherein the lower high-refractive-index layer and the upper high-refractive-index layer have conductivity and are disposed to face each other with a gap; and a slot part formed as a low-refractive-index layer, wherein the low-refractive-index layer is formed of a material producing an electro-optic effect and has a third refractive index lower than the first refractive index and the second refractive index, wherein the low-refractive-index layer is formed in the gap to come into contact with the lower high-refractive-index layer and the upper high-refractive-index layer, and wherein one of the lower high-refractive-index layer or the upper high-refractive-index layer includes a stretch stretching on both sides of a contact portion with the slot part in a width direction intersecting a transmission direction.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2018-158479 filed on Aug. 27, 2018, the content of which is hereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to an electro-optic waveguide device including a slot waveguide in which a material producing an electro-optic effect is arranged in a slot, and an optical module using the same.

BACKGROUND OF THE INVENTION

A slot waveguide is a waveguide that has a structure in which a narrow low-refractive-index region is interposed between high-refractive-index media and can strongly confine light in a slot part which is a sub-wavelength region.

Some techniques use a slot waveguide that has a laminated structure including two silicon layers as high-refractive-index media and a ferroelectric layer interposed as a low-refractive-index medium between the silicon layers. In the laminated structure, the two high-refractive-index layers and the low-refractive-index layer of the slot part interposed therebetween have substantially the same width and a side surface of each layer forms a common vertical surface as in a cross-sectional shape of a normal rectangular waveguide.

A slot waveguide may include two waveguides of high-refractive media that are formed of a semiconductor material doped to have conductivity respectively and are horizontally disposed in parallel on a substrate, and lithium niobate interposed as a low-refractive-index medium in a slot formed in the vertical direction between the two waveguides. In this structure, the upper surface of the waveguides of the high-refractive-index media and the upper surface of the low-refractive-index medium interposed in the slot form a common horizontal surface.

Some techniques use an electro-optic modulator that includes a body region formed of a first conductive silicon, a gate region formed of a second conductive silicon and overlapping the body region, and a dielectric layer inserted between the regions to come into contact with the regions. With regard to a waveguide used in connection with the electro-optic modulator, since the thickness of the middle dielectric layer is very thin, there is no slot mode and guided light is distributed in a high-refractive-index region. In addition, since the thickness of the middle dielectric layer is very thin in the waveguide, an operation voltage can be reduced. However, since the high-refractive-index region needs to be doped with high concentration to increase carrier density, an optical loss due to optical absorption by carriers may increase.

SUMMARY OF THE INVENTION

In an electro-optic waveguide device in which a material for producing an electro-optic effect is disposed as a slot part between high-refractive-index media to form a slot waveguide and a change in a refractive index is caused in the slot part by an electric field generated by applying a potential difference to the two high-refractive-index media, further low-voltage driving is preferable.

Regarding this point, the electric field of the guided light generated in the slot between the two high-refractive-index layers disposed in parallel is changed in accordance with a position in a direction parallel to an interface between a high-refractive-index layer and a low-refractive-index layer due to a light confinement edge effect. Specifically, the intensity of the electric field decreases at end portions, compared to the middle portion. Accordingly, in a slot waveguide in which two high-refractive-index layers and a low-refractive-index layer are laminated so that side surfaces are aligned, an effect of the slot waveguide such as intensification of an electric field in a low-refractive-index layer according to the Gauss law becomes weak. Therefore, in an electro-optic waveguide device that performs phase modulation of guided light using the slot waveguide with such a configuration, the confinement effect of guided light in the low-refractive-index layer configured as the slot part decreases, and further there is a problem that the effect of reducing a driving voltage by raising the phase modulation efficiency decreases. When the intensity of an electric field generated in the low-refractive-index layer by a voltage generated by a modulated electric signal or a direct-current electric bias applied to a high-refractive-index layer decreases at an end portion due to the edge effect of a parallel plate capacitor, it is also disadvantageous to reduce a driving voltage.

Here, a modulation device that includes an electro-optic waveguide retaining single lateral mode guided light as a phase modulator is a device that is indispensable for today's large-capacity transmission. In this device, in order to propagate only the single lateral mode guided light, the width of the slot waveguide can be reduced, for example, to 1 micrometer (μm) or less. In particular, when the width of the slot waveguide is small as described above, an influence of the above-described edge effect is relatively high, and therefore, the foregoing problem is serious.

The invention provides an electro-optic waveguide device and an optical module operating at a low-amplitude voltage with high phase modulation efficiency.

In some implementations, an electro-optic waveguide device may include a slot waveguide including a lower high-refractive-index layer with a first refractive index and an upper high-refractive-index layer with a second refractive index, wherein the lower high-refractive-index layer and the upper high-refractive-index layer have conductivity and are disposed to face each other with a gap; and a slot part formed as a low-refractive-index layer, wherein the low-refractive-index layer is formed of a material producing an electro-optic effect and has a third refractive index lower than the first refractive index and the second refractive index, wherein the low-refractive-index layer is formed in the gap to come into contact with the lower high-refractive-index layer and the upper high-refractive-index layer, wherein one of the lower high-refractive-index layer or the upper high-refractive-index layer includes a stretch stretching on both sides of a contact portion with the slot part in a width direction intersecting a transmission direction of the slot waveguide, and wherein the other one of the lower high-refractive-index layer or the upper high-refractive-index layer includes portions facing the stretches in a cross-sectional shape in the width direction.

In a first implementation, each of the lower high-refractive-index layer and the upper high-refractive-index layer includes a stretch, and the stretch of the lower high-refractive-index layer and the stretch of the upper high-refractive-index layer include portions facing each other.

In a second implementation, alone or in combination with the first implementation, the slot part has a strip shape extending in the transmission direction, and the gap is located at the contact portion and the stretch of the lower high-refractive-index layer and the stretch of the upper high-refractive-index layer are equal to each another.

In a third implementation, alone or in combination with one or more of the first through second implementations, the electro-optic waveguide device includes a clad layer that contacts side surfaces of the low-refractive-index layer, has a refractive index lower than the low-refractive-index layer, and is disposed in the gap.

In a fourth implementation, alone or in combination with one or more of the first through third implementations, the contact portion is formed in a rib shape and extends in the transmission direction.

In a fifth implementation, alone or in combination with one or more of the first through fourth implementations, the low-refractive-index layer has a width larger than the slot part, and clad layers with a refractive index lower than the low-refractive-index layer are disposed in the gap.

In a sixth implementation, alone or in combination with one or more of the first through fifth implementations, the electro-optic waveguide device includes a lower contact region configured to have an electric resistance lower than the lower high-refractive-index layer and to electrically connect the lower high-refractive-index layer to an electrode; and an upper contact region configured to have an electric resistance lower than the upper high-refractive-index layer and to electrically connect the upper high-refractive-index layer to an electrode.

In a seventh implementation, alone or in combination with one or more of the first through sixth implementations, the lower contact region and the upper contact region are disposed to come into contact with the lower high-refractive-index layer or the upper high-refractive-index layer at positions spaced with the slot part in the width direction.

In an eighth implementation, alone or in combination with one or more of the first through seventh implementations, a contact portion of the lower high-refractive-index layer and a contact portion of the upper high-refractive-index layer with the slot part face each other in parallel.

In a ninth implementation, alone or in combination with one or more of the first through eighth implementations, in regard to an electric field generated between the lower high-refractive-index layer and the upper high-refractive-index layer when a voltage is applied, dimensions of the stretch, in a horizontal direction, are determined so that a reducing rate of an electric field intensity at end portions compared to a middle portion of the slot part in the width direction is set to a predetermined value.

In some implementations, an optical module includes an electro-optic waveguide device comprising a slot waveguide including a lower high-refractive-index layer with a first refractive index and an upper high-refractive-index layer with a second refractive index, wherein the lower high-refractive-index layer and the upper high-refractive-index layer have conductivity and are disposed to face each other with a gap; and a slot part formed as a low-refractive-index layer, wherein the low-refractive-index layer is formed of a material producing an electro-optic effect and has a third refractive index lower than the first refractive index and the second refractive index, wherein the low-refractive-index layer is formed in the gap to come into contact with the lower high-refractive-index layer and the upper high-refractive-index layer, wherein one of the lower high-refractive-index layer or the upper high-refractive-index layer includes a stretch stretching on both sides of a contact portion with the slot part in a width direction intersecting a transmission direction of the slot waveguide, and wherein the other one of the lower high-refractive-index layer or the upper high-refractive-index layer includes portions facing the stretches in a cross-sectional shape in the width direction; a light source optically connected to the electro-optic waveguide device; and a medium for transmitting light passing through the electro-optic waveguide device.

In a tenth implementation, alone or in combination with one or more of the first through ninth implementations, each of the lower high-refractive-index layer and the upper high-refractive-index layer includes a stretch, and the stretch of the lower high-refractive-index layer and the stretch of the upper high-refractive-index layer include portions facing each other.

In an eleventh implementation, alone or in combination with one or more of the first through tenth implementations, the slot part has a strip shape extending in the transmission direction, and the gap is located at the contact portion and the stretch of the lower high-refractive-index layer and the stretch of the upper high-refractive-index layer are equal to each another.

In a twelfth implementation, alone or in combination with one or more of the first through eleventh implementations, the optical module includes a clad layer that contacts side surfaces of the low-refractive-index layer, has a refractive index lower than the low-refractive-index layer, and is disposed in the gap.

In a thirteenth implementation, alone or in combination with one or more of the first through twelfth implementations, the contact portion is formed in a rib shape and extends in the transmission direction.

In a fourteenth implementation, alone or in combination with one or more of the first through thirteenth implementations, the low-refractive-index layer has a width larger than the slot part, and clad layers with a refractive index lower than the low-refractive-index layer are disposed in the gap.

In a fifteenth implementation, alone or in combination with one or more of the first through fourteenth implementations, the optical module includes a lower contact region configured to have an electric resistance lower than the lower high-refractive-index layer and to electrically connect the lower high-refractive-index layer to an electrode; and an upper contact region configured to have an electric resistance lower than the upper high-refractive-index layer and to electrically connect the upper high-refractive-index layer to an electrode.

In a sixteenth implementation, alone or in combination with one or more of the first through fifteenth implementations, the lower contact region and the upper contact region are disposed to come into contact with the lower high-refractive-index layer or the upper high-refractive-index layer at positions spaced with the slot part in the width direction.

In a seventeenth implementation, alone or in combination with one or more of the first through sixteenth implementations, a contact portion of the lower high-refractive-index layer and a contact portion of the upper high-refractive-index layer with the slot part face each other in parallel.

In an eighteenth implementation, alone or in combination with one or more of the first through seventeenth implementations, in regard to an electric field generated between the lower high-refractive-index layer and the upper high-refractive-index layer when a voltage is applied, dimensions of the stretch, in a horizontal direction, are determined so that a reducing rate of an electric field intensity at end portions compared to a middle portion of the slot part in the width direction is set to a predetermined value.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention (hereinafter, referred to as embodiments) will be described with reference to the drawings.

The disclosure is merely exemplary and it is apparent to those skilled in the art that the appropriate changes easily made within the gist of the present invention are, of course, included in the scope of the present invention. In order to further facilitate the description, the width, thickness, shape, and the like of each portion are more schematically illustrated in the drawings than in the actual form, which is merely exemplary and does not limit interpretation of the present invention. In the present specification and each drawing, the same reference numerals are given to elements similar to those of the previously described drawings and the detailed description thereof will be appropriately omitted.

FIG. 1is a schematic vertical sectional view illustrating an electro-optic waveguide device2according to a first embodiment and illustrating a cross-section perpendicular to a transmission direction of light, that is, an extension direction of a waveguide.

First, the basic points of the electro-optic waveguide device2according to the present invention will be described. In the electro-optic waveguide device2according to the present invention, a waveguide is formed on a substrate4having a flat surface. The waveguide includes a structure serving as a core (a core part6) and a clad region8surrounding the core part6.FIG. 1is a schematic view. For example, a structure of other layers and the like between the substrate4and the clad region8is not illustrated. In the following description, a right-handed type of xyz Cartesian coordinate system is adopted, the x-axis is set to a direction orthogonal to the extension direction of the waveguide (the horizontal direction inFIG. 1), the y-axis is set to a direction orthogonal to the substrate4(the vertical direction inFIG. 1), and the z-axis is set to the extension direction of the waveguide. The positive direction of the x-axis inFIG. 1is the right direction and the positive direction of the y-axis is the upward direction.

The core part6has the structure of the above-described slot waveguide and includes two high-refractive-index layers10and12of thin films disposed to be laminated with a gap (slot) therebetween and a low-refractive-index layer14(slot part) disposed in a slot. In the slot waveguide, guided light tends to be strongly localized in the slot part. In case of two-dimensional slot waveguide, it is possible to achieve confinement of guided light in the core part by adjusting the film thicknesses of the high-refractive-index layers and the low-refractive-index layer (the dimension in the vertical direction inFIG. 1) and localizing the guided light in the slot part. The guided light is localized when an electric field of the guided light is orthogonal to a boundary surface between the high-reflective index layers and the low-refractive-index layer, that is, when the electric field of the guided light is in a TM polarization state in which the electric field of the guided light is linearly polarized in the vertical direction inFIG. 1.

An electro-optic modulation device that is used for large-capacity light transmission performs optical modulation with a high extinction ratio or Q value, using a 3-dimensional waveguide that propagates only single lateral mode guided light. Regarding this point, the electro-optic waveguide device according to the present invention has a characteristic form and structure on the xy cross-section of the core part. For example, when the high-refractive-index layers extend in the x-direction as in a slab type, the edge effect in the above-described slot part decreases.

Specifically, in the present invention, when the x-axis direction is the width direction, the high-refractive-index layers10and12have a larger width than the low-refractive-index layer14and have portions stretching to the laterally outer side from the portions contacting the low-refractive-index layer14. By applying a voltage to the high-refractive-index layers10and12from the outside, an electric field is formed also in the gap interposed between stretches20and22, and the electric field functions to suppress the edge effect with regard to the electric field of the guided light at the end portions of the low-refractive-index layer14.

In addition, an electric field generated in the gap between the high-refractive-index layers10and12in accordance with the applied voltage by a modulated electric signal or a direct-current electric bias from the outside is also weakened due to the edge effect in the end portions of the high-refractive-index layers10and12. However, by forming the stretches20and22and spacing the end portions of the high-refractive-index layers10and12from end portions of the low-refractive-index layer14, an influence of the edge effect can be avoided and the intensity difference between the electric fields applied to the middle and end portions of the low-refractive-index layer14can be reduced. That is, it is possible to apply an electric field with a uniform intensity distribution in the horizontal direction to the low-refractive-index layer14without receiving an influence of attenuation of the electric field at the end portions of the high-refractive-index layers.

When the guided light is localized with regard to the horizontal direction and only the single lateral mode guided light is propagated, the width of the low-refractive-index layer is preferably limited.

To perform electro-optic modulation of the guided light, the low-refractive-index layer14configured as the slot part is formed of a material that produces an electro-optic effect. In particular, for example, an effect of changing a refractive index in accordance with an external electric field, such as the Pockels effect or the Kerr effect, is used as the electro-optic effect. Specifically, the refractive index of the low-refractive-index layer14is modulated by applying a high-frequency electric signal to the high-refractive-index layers10and12.

The high-refractive-index layers10and12need to have conductivity. Specifically, the high-refractive-index layers10and12are formed of a semiconductor material doped with impurities to generate carriers. For example, metal electrodes are connected to the high-refractive-index layers10and12in order to apply an electric signal from the outside.FIG. 2is a schematic xy sectional view illustrating the electro-optic waveguide device2.FIG. 2illustrates a cross-section different fromFIG. 1in the position in the z-axis direction and an example of a connection structure of the high-refractive-index layers10and12and the metal electrodes. For ohmic contact of connection portions between the high-refractive-index layers10and12and plugs24and26forming parts of the metal electrodes, contact regions28and30that have an electric resistance lower than the high-refractive-index layers10and12are provided. Specifically, the contact regions28and30can be formed by doping parts of the high-refractive-index layers10and12with higher concentration.

Here, when a doped amount of the high-refractive-index layer increases, a carrier density increases and optical absorption by carriers increases. Accordingly, when the core part6that confines the guided light has a highly doped region, the problem that an optical loss increases occurs. From this viewpoint, as described above, the optical loss can be reduced by expanding the high-refractive-index layers10and12in the horizontal direction and disposing the contact regions28and30at positions spaced from the core part6.

In the electro-optic waveguide device2, the direction of an electric field of the guided light is also the vertical direction in correspondence with the alignment of the high-refractive-index layers10and12and the low-refractive-index layer14in the vertical direction. On the other hand, when two high-refractive-index regions and a low-refractive-index region interposed therebetween are arranged in the horizontal direction, as in the configuration of U.S. Pat. No. 7,970,241, the direction of the electric field of the guided light is the horizontal direction. In this case, when the high-refractive-index regions are expanded in the horizontal direction for connection or the like with the electrodes, the electric field of the guided light spreads in the horizontal direction inside the high-refractive-index regions, the electric field of the guided light localized in the low-refractive-index region of the slot part decreases, and thus phase modulation efficiency deteriorates. From this viewpoint, in the electro-optic waveguide device2according to the present invention, since the expansion direction of the high-refractive-index layer10is a direction perpendicular to the electric field of the guided light, the phase modulation efficiency can be ensured without damaging the confinement effect of the guided light in the slot waveguide in the core part6.

The case in which the guided light is a TM polarized wave has been described above as an example. However, since others are similar to the TM polarized wave except for the localization effect of the guided light mentioned at the end, despite a TE polarized wave, the present invention may be applied to a TE polarized wave.

FIGS. 1 and 2have been used regarding the electro-optic waveguide device2according to the above-described first embodiment in the foregoing description, but the content mentioned in the description is basically common to other embodiments to be described below.

Hereinafter, the description of the electro-optic waveguide device2according to the first embodiment will continue.

The clad region8includes a lower clad32, an upper clad34, and a side clad36, as illustrated inFIG. 1.

At a position in the horizontal direction at which the low-refractive-index layer14is present, the substrate4, the lower clad32, the lower high-refractive-index layer10, the low-refractive-index layer14, the upper high-refractive-index layer12, and the upper clad34are disposed in order from the lower side. The lower high-refractive-index layer10and the upper high-refractive-index layer12are of a slab type with a width greater than that of the low-refractive-index layer14. The surfaces of the lower high-refractive-index layer10and the upper high-refractive-index layer12closer to the low-refractive-index layer14are flat and are disposed to face each other in parallel with a gap therebetween. The low-refractive-index layer14is located in the middle of the gap in the x-direction. The low-refractive-index layer14is formed in a strip shape extending in the z-axis direction, the xy cross-section is basically rectangular, and the lower and upper surfaces are in contact with the upper surface of the lower high-refractive-index layer10and the lower surface of the upper high-refractive-index layer12, respectively.

On both sides of the low-refractive-index layer14in the horizontal direction, the side clad36is disposed to be in contact with the side surfaces of the low-refractive-index layer14, the upper surface of the stretch20of the lower high-refractive-index layer10, and the lower surface of the stretch22of the upper high-refractive-index layer12. The lower clad32is provided to be in contact with the lower surface and the side surfaces of the lower high-refractive-index layer10, and the upper surface of the lower high-refractive-index layer10and the upper surface of the lower clad32are formed in a common plane. The low-refractive-index layer14and the side clad36are laminated on this plane. The low-refractive-index layer14and the side clad36are formed with a common thickness and each upper surface forms a common plane. The upper high-refractive-index layer12and the upper clad34are laminated on the common plane. The upper clad34is provided to be in contact with the upper surface and the side surfaces of the upper high-refractive-index layer12.

As illustrated inFIG. 2, at a position in the z-axis direction at which the plug24is provided, the lower high-refractive-index layer10extends in the horizontal direction further than inFIG. 1and the lower contact region28is provided in the extension portion. Similarly, at a position in the z-axis direction at which the plug26is provided, the upper high-refractive-index layer12extends in the horizontal direction further than inFIG. 1and the upper contact region30is provided in the extension portion.FIG. 2illustrates an example in which the plug24connected to the lower high-refractive-index layer10and the plug26connected to the upper high-refractive-index layer12are disposed at the same position in the z-axis direction. In this case, the lower high-refractive-index layer10and the upper high-refractive-index layer12are extended in an opposite direction to each other in the horizontal direction. When the plug24and the plug26are disposed at different positions in the z-axis direction, both of the lower high-refractive-index layer10and the upper high-refractive-index layer12in the z coordinate can be extended in the same direction.

A refractive index (first refractive index) n1of the lower high-refractive-index layer10, a refractive index (second refractive index) n2of the upper high-refractive-index layer12, and a refractive index (third refractive index) n3of the low-refractive-index layer14are set to satisfy n1>n3and n2>n3. In addition, n1and n2are set to be basically the same.

The refractive index of the side clad36is set to be basically equal on both sides of the low-refractive-index layer14. When the refractive index is n4, n4<n3is set. A refractive index n5of the lower clad32is set to satisfy n5<n1and a refractive index n6of the upper clad34is set to satisfy n6<n2. In the embodiment, the refractive indexes n4, n5, and n6of each clad are set to be substantially equal, but the present invention is not limited thereto. To avoid mixing of polarization modes, it is preferable to maintain the symmetry of the polarization modes in the horizontal direction, but the present invention is not limited thereto.

As described above, the low-refractive-index layer14is formed of an electro-optic material. In the present invention, oriented lithium niobate (LiNbO3, LN: refractive index of 2.23) is used as the low-refractive-index layer14. The lower high-refractive-index layer10and the upper high-refractive-index layer12are formed of silicon (Si: refractive index of 3.45). For example, the lower high-refractive-index layer10has P-type polarity of conductivity and the upper high-refractive-index layer12has N-type polarity of conductivity. The polarities of the contact regions28and30are the same as the polarities of the high-refractive-index layers10and12in which the contact regions28and30are provided, respectively. Even when the polarities of the lower high-refractive-index layer10and the lower contact region28are reverse to each other and the polarities of the upper high-refractive-index layer12and the upper contact region30are reverse to each other, there is no influence on the operation principle of the electro-optic waveguide device2, and thus the selection may be made so that manufacturing is easy. The materials of the lower clad32, the upper clad34, and the side clad36are all silica (SiO2: refractive index of 1.45). The materials to be used are not limited to this material and other materials may be used. The refractive index depends on the wavelength of a light wave in a waveguide mode. The example of the case in which the value of the above-described refractive index is set when the guided light is set in a band of 1.3 to 1.5 μm has been described, but other values can be selected in other wavelengths.

Each of the lower high-refractive-index layer10and the upper high-refractive-index layer12according to the embodiment includes the stretch, and the stretch20of the lower high-refractive-index layer10and the stretch22of the upper high-refractive-index layer12include portions facing each other. The lengths of the gaps between the lower high-refractive-index layer10and the upper high-refractive-index layer12at the contact portions with the low-refractive-index layer14and the stretches20and22on both sides thereof are equal to one another. That is, when the thickness of the low-refractive-index layer14is expressed as tlow, the intervals of the gaps at the stretches20and22are also tlow. When a dielectric substance represented by lithium niobate (LN) that produces the Pockels effect is used for the low-refractive-index layer14, tlowis preferably in the range of 50 to 500 nm. When a material that has a nonlinear Kerr coefficient larger than the dielectric substance such as graphene is used for the low-refractive-index layer14, the low-refractive-index layer14may be a thin layer that has tlowof about 0.1 to 50 nm.

When the width of the low-refractive-index layer14in the horizontal direction is expressed as wlow, a confinement width of guided light in the horizontal direction is basically regulated to wlow. To propagate only a single lateral mode, wlowis preferably 1 μm or less. On the other hand, to reduce an optical loss by light scattering caused by the roughness of a sidewall of the low-refractive-index layer14, wlowis preferably 400 nm or more.

For a thickness thi1of the lower high-refractive-index layer10and a thickness thi2of the upper high-refractive-index layer12, suitable ranges can be set to improve confinement of guided light in the slot part (the low-refractive-index layer14). When the thicknesses thi1and thi2are small, a problem may arise in that the guided light effuses to the outside of the slot waveguide and the effusing light spreads to an area lower than the lower high-refractive-index layer10and higher than the upper high-refractive-index layer12. When the slot part is thickened in order to avoid this problem, a problem may arise in that a driving voltage for modulating the refractive index of the low-refractive-index layer14increases. Accordingly, the lower limits of the suitable ranges of thi1and thi2are determined in consideration of these problems. On the other hand, when the thicknesses thi1and thi2are large, most of the guided light distributes in the lower high-refractive-index layer10and the upper high-refractive-index layer12and a mode confinement coefficient of the low-refractive-index layer14decreases. As a result, the phase modulation efficiency by the electro-optic effect of the low-refractive-index layer14decreases and it is necessary to increase a driving voltage in order to supplement this decrease in the phase modulation efficiency. Accordingly, the upper limits of the suitable ranges of thi1and thi2are determined in consideration of this point. For example, the suitable ranges can be set to 100 to 250 nm.

The dimensions of the stretches20and22in the horizontal direction are expressed as wext. The stretches20and22mentioned herein basically relate to the cross-section illustrated inFIG. 1. By providing the stretches20and22, it is possible to suppress or reduce the edge effect, as described above, and it is possible to reduce the driving voltage for modulating the refractive index of the low-refractive-index layer14. Here, wextis set to obtain the effect of suppressing the driving voltage.

Here, the influence of the edge effect at the end portions in the gap between the lower high-refractive-index layer10and the upper high-refractive-index layer12on the low-refractive-index layer14is reduced to a negligible degree when wextis equal to or greater than a certain extent. In other words, the effect of reducing the influence of the edge effect is saturated with an increase in wext. On the other hand, as wextincreases, parasitic capacitance accompanying the lower high-refractive-index layer10and the upper high-refractive-index layer12increases, which interrupts high-speed driving of the electro-optic waveguide device2. Accordingly, wextcan be set in consideration of a trade-off between the parasitic capacitance and the influence of the edge effect. For example, in regard to the electric field generated between the lower high-refractive-index layer10and the upper high-refractive-index layer12in accordance with the voltage applied from the outside, wextis defined so that a reducing rate of the electric field intensity at the end portions compared to the middle portion of the low-refractive-index layer14in the width direction is set to a predetermined allowed value. That is, wextis enlarged to the degree of a level at which the edge effect is allowed, but wextcan be set not to exceed the degree of the level in order to avoid an unnecessary increase in the parasitic capacitance.

The widths wextof the stretches20and22may be about half of the width wlowof the low-refractive-index layer14from the viewpoint of the distribution of the guided light. In order to avoid mixing of polarization modes, it is important to maintain the symmetry of the distribution of the guided light. For this reason, it is preferable to extend the widths wextof the stretches20and22to 500 nm or more.

At the position corresponding to the cross-section ofFIG. 2in the z-axis direction, the stretches20and22are formed with a larger width than at the position corresponding to the cross-section ofFIG. 1and extend up to the positions at which the plugs24and26and the contact regions28and30are provided.

When the contact regions28and30are close to a distribution region of the guided light, power attenuation of the guided light due to optical absorption of carriers in the contact regions28and30may not be negligible. In contrast, when distances between the contact regions28and30and the contact portions of the high-refractive-index layers with the low-refractive-index layer14are long, a series electric resistance increases in the lower high-refractive-index layer10and the upper high-refractive-index layer12existing therebetween, which becomes a cause of interrupting a high-speed operation of the electro-optic waveguide device2. Horizontal distances between the contact regions28and30and the low-refractive-index layer14are set in a suitable range in consideration of these points. For example, the central point of the contact regions28and30in the horizontal direction is preferably disposed at a position of 1 to 10 μm from the central point of the low-refractive-index layer14in the horizontal direction.

The metal electrodes connected to the lower high-refractive-index layer10and the upper high-refractive-index layer12include electrodes40and42which are portions provided on the upper surface of the upper clad34; and the plugs24and26which are portions embedded in holes opened in the upper clad34. The plugs24and26are formed, for example, in a columnar shape within the upper clad34and electrically connect the electrodes40and42to the contact regions28and30.

The plug24and the upper high-refractive-index layer12are preferably separated by the thickness thi2or more of the upper high-refractive-index layer12from the viewpoint of reducing the parasitic capacitance. From the viewpoint of reducing the parasitic capacitance between the upper contact region30(or the plug26) and the lower high-refractive-index layer10, the upper contact region30is preferably disposed at a horizontal position which does not overlap the lower high-refractive-index layer10.

Here, the laminated structure of the lower high-refractive-index layer10, the low-refractive-index layer14, and the upper high-refractive-index layer12, which constitutes the slot waveguide, is set to have a so-called PIN type in which the lower high-refractive-index layer10is of the P-type and the upper high-refractive-index layer12is of the N-type, but the present invention is not limited thereto. The laminated structure may have a PIP type or an NIN type.

Next, a method of manufacturing the electro-optic waveguide device2will be described.FIGS. 3 to 6are schematic vertical sectional views illustrating the electro-optic waveguide device2to describe processes in the manufacturing method and illustrate the cross-section corresponding toFIG. 2.

For example, the electro-optic waveguide device2can be manufactured using a silicon on insulator (SOI) wafer on which an oriented lithium niobate (LN) layer is laminated. Specifically, the wafer has a structure in which an embedded oxide film45is formed on the surface of a silicon substrate44, a silicon single-crystal layer is grown on the oxide film45to form an SOI layer46, and a thin film LN layer47is formed on the surface of the SOI layer46by wafer bonding or the like.

FIG. 3is a schematic vertical sectional view illustrating a state in which the low-refractive-index layer14is formed using the wafer. The silicon substrate44of the wafer forms the above-described substrate4and the embedded oxide film45forms the lower clad32. The lower high-refractive-index layer10is formed using the SOI layer46and the low-refractive-index layer14is formed using the thin film LN layer47. A region serving as the lower high-refractive-index layer10in the SOI layer46is doped with P-type impurities to have conductivity. A region on the outside of the lower high-refractive-index layer10in the SOI layer46is oxidized to become the lower clad32integrated with the embedded oxide film45. The low-refractive-index layer14is formed by processing the thin film LN layer47with a rectangular cross-sectional shape through the photolithographic technique. Specifically, the thin film LN layer47can be subjected to dry etching using a patterned photoresist as a mask to form the low-refractive-index layer14.

The carrier concentration of the lower high-refractive-index layer10is preferably in the range of 1017to 1019cm−3. This is because an increase in an optical loss is not negligible when the carrier concentration is higher than 1019cm−3whereas the series electric resistance increases and the operation speed is lowered when the carrier concentration is lower than 1017cm−3.

FIG. 4is a schematic vertical sectional view illustrating a state in which the side clad36is formed. The side clad36is formed of silica. In the state ofFIG. 3in which the thin film LN layer47is patterned to form the low-refractive-index layer14, silica is deposited on the surface of the substrate by chemical vapor deposition (CVD) and is flattened by chemical mechanical polishing (CMP). As a result, as illustrated inFIG. 4, the side clad36formed of the flattened silica region is formed on both sides of the low-refractive-index layer14.

FIG. 5is a schematic vertical sectional view illustrating a state in which the upper high-refractive-index layer12and the upper clad34are formed. The upper high-refractive-index layer12is formed of a silicon thin film and the upper clad34is formed of silica. First, in the state ofFIG. 4in which the side clad36and the low-refractive-index layer14are formed to be flat, a silicon thin film is formed on the surface of the substrate by CVD. The silicon thin film is doped with N-type impurities to have conductivity. Then, the silicon thin film is patterned by the photolithographic technique so that the region corresponding to the upper high-refractive-index layer12selectively remains. In this way, the silica is deposited by CVD on the surface of the substrate on which the upper high-refractive-index layer12is formed and is flattened by CMP. The flattened silica layer forms the upper clad34.

FIG. 6is a schematic vertical sectional view illustrating a process of forming electrodes connected to the lower high-refractive-index layer10and the upper high-refractive-index layer12. In the upper clad34formed to be flat inFIG. 5, contact holes50and52illustrated inFIG. 6are formed by dry etching. The contact hole50is provided at a position at which the plug24to be connected to the lower high-refractive-index layer10is formed and penetrates through the upper clad34and the side clad36to reach the surface of the lower high-refractive-index layer10. On the other hand, the contact hole52is provided at a position at which the plug26to be connected to the upper high-refractive-index layer12is formed and penetrates through the upper clad34to reach the surface of the upper high-refractive-index layer12.

Ions are injected to the lower high-refractive-index layer10and the upper high-refractive-index layer12via the contact holes50and52to form highly doped regions on the bottom surfaces of the contact holes50and52. The highly doped regions are the contact regions28and30, as illustrated inFIG. 6. Carrier concentrations of the contact regions28and30are set to, for example, 1020cm−3or more.

After the contact holes50and52and the contact regions28and30are formed, aluminum is deposited by sputtering or deposition to form the plugs24and26having the columnar shape in the contact holes50and52. Further, after aluminum is deposited on the flattened upper clad34, the electrodes40and42are formed from the aluminum by the photolithographic technique. Thus, the electro-optic waveguide device2having the cross-section structure illustrated inFIG. 2is formed.

In the above-described process, the structure illustrated inFIG. 1is also formed. The metal material used for the plugs24and26and the electrodes40and42is not limited to aluminum, and gold, copper, cobalt, or ruthenium which have a lower high-frequency electric resistance may be used.

The electro-optic waveguide device2according to a second embodiment of the present invention basically has a vertical cross-sectional structure similar to that of the electro-optic waveguide device2according to the first embodiment illustrated inFIGS. 1 and 2. However, the electro-optic waveguide device2according to the second embodiment of the present invention is created with a manufacturing method different from that of the first embodiment.

Hereinafter, a manufacturing method according to the second embodiment will be described with a structure corresponding toFIG. 2as an example. In the embodiment, two SOI wafers are bonded to form the electro-optic waveguide device2. A first SOI wafer is similar to the wafer used in the first embodiment and the wafer on which a thin film LN layer is laminated can be used. The SOI wafer is processed to have the structure illustrated inFIG. 3as in the first embodiment.

FIG. 7is a schematic vertical sectional view illustrating the first SOI wafer in a process subsequent toFIG. 3. InFIG. 7, ions are injected to the surface of the lower high-refractive-index layer10to form the lower contact region28. Thereafter, the side clad36is formed of silica, similarly to the process described inFIG. 4in the first embodiment.

FIG. 8is a schematic vertical sectional view illustrating a second SOI wafer. The second SOI wafer basically has a structure similar to that of the first SOI wafer except that the thin film LN layer is not included. That is, this wafer includes an embedded oxide film61formed on a silicon substrate60and an SOI layer62formed by growing a silicon single-crystal layer on the oxide film61. The process of forming the lower clad32and the lower high-refractive-index layer10from the embedded oxide film45and the SOI layer46has been described with reference toFIG. 3. Similarly to this, the structure ofFIG. 8can be obtained by forming the upper clad34and the upper high-refractive-index layer12from the embedded oxide film61and the SOI layer62of the second SOI wafer.

The second SOI wafer formed in the structure ofFIG. 8is bonded on the first SOI wafer formed in the structure ofFIG. 7to face the mutual surface (specifically, in the direction in which the xyz coordinate axes illustrated inFIGS. 7 and 8match each other). Thereafter, the silicon substrate60of the second SOI wafer is removed by polishing and wet etching.FIG. 9is a schematic vertical sectional view illustrating this state.

From the state ofFIG. 9, the contact holes50and52and the contact region30are further formed, basically similar to the process described with reference toFIG. 6, and the electrodes40and42are formed to obtain the structure illustrated inFIG. 2. In addition, the contact hole50is formed at a position corresponding to the lower contact region28.

In the manufacturing method according to the embodiment, since the upper high-refractive-index layer12can be formed using an SOI layer with good crystallinity as in the lower high-refractive-index layer10, reduction of a light scattering loss of the upper high-refractive-index layer12, and reduction of the series electric resistance, and the like are possible in addition to the effects of the first embodiment. That is, it is possible to achieve a reduction in an optical loss of the electro-optic waveguide device2and improvement in operation speed.

FIGS. 10 and 11are schematic vertical sectional views illustrating an electro-optic waveguide element2B according to a third embodiment and illustrate the xy cross-section as inFIGS. 1 and 2.FIGS. 10 and 11illustrate cross-sections different at the position in the z-axis direction.FIG. 10corresponds toFIG. 1of the first embodiment and illustrates a cross-section at a position at which the connection structure of the electrodes to the lower high-refractive-index layer and the upper high-refractive-index layer is not disposed. On the other hand,FIG. 11corresponds toFIG. 2of the first embodiment and illustrates a cross-section at a position at which the connection structure of the electrodes is disposed. Portions with common names (or numeral signs in the signs) in each part illustrated in the cross-section of the third embodiment and each part illustrated in the cross-section of the first embodiment can be formed of basically the same materials. For example, since a lower high-refractive-index layer10B, an upper high-refractive-index layer12B, and a low-refractive-index layer14B according to the embodiment are different in a cross-sectional shape from the lower high-refractive-index layer10, the upper high-refractive-index layer12, and the low-refractive-index layer14according to the first embodiment, “B” is given to the signs to distinguish from each other in the description of the specification but can be formed of the same materials corresponding to the layers.

In the electro-optic waveguide device2B, the lower high-refractive-index layer10B and the upper high-refractive-index layer12B include portions with a rib shape facing each other and extending in the transmission direction (rib portions70and72). The low-refractive-index layer14B is formed in a slab type.

In the first embodiment, the low-refractive-index layer14is formed with the relatively narrow width wlowcorresponding to the localization region of the guided light as the slot part. On the other hand, the lower high-refractive-index layer10and the upper high-refractive-index layer12are formed in the slab type with the uniform gap therebetween. In the structure of the first embodiment, the portions of the lower high-refractive-index layer10and the upper high-refractive-index layer12coming into contact with the low-refractive-index layer14are regulated in the planar shape of the low-refractive-index layer14. That is, the width of the contact portions of the lower high-refractive-index layer10and the upper high-refractive-index layer12with the low-refractive-index layer14in the xy cross-section is defined as the width wlowof the low-refractive-index layer14.

In the third embodiment, however, the rib portions70and72in the lower high-refractive-index layer10B and the upper high-refractive-index layer12B are contact portions with the low-refractive-index layer14B. That is, contact widths of the lower high-refractive-index layer10B and the upper high-refractive-index layer12B with the low-refractive-index layer14B of the slab type are regulated with a width wribof the rib portions70and72, a portion interposed between the rib portions70and72in the low-refractive-index layer14B and formed with the larger width than the rib portions70and72substantially serves as a slot part of a slot waveguide, and an electric field is applied from the lower high-refractive-index layer10B and the upper high-refractive-index layer12B, thereby contributing to electro-optic modulation of the guided light.

In the third embodiment, portions located on both sides of the rib portions70and72in the x-direction in the lower high-refractive-index layer10B and the upper high-refractive-index layer12B are equivalent to the stretches20and22mentioned in the first embodiment. By forming an electric field in the gap between the stretches20and22when a voltage is applied to the lower high-refractive-index layer10B and the upper high-refractive-index layer12B from the outside, it is possible to suppress or reduce the above-described edge effect in the slot part, and thus achieve the reduction in a driving voltage for refractive index modulation.

Clad layers74and76with a refractive index lower than the low-refractive-index layer14B are disposed in gaps in which the stretches20and22face the low-refractive-index layer14B, that is, gaps formed on the sides of the rib portions70and72.

The low-refractive-index layer14B is formed with a larger width than the slot part that localizes guided light, as described above. For example, when the low-refractive-index layer14B is patterned by dry etching, the roughness of the sidewall of the low-refractive-index layer14B occurs in some cases. The sidewall of the low-refractive-index layer14B scatters the guided light, and thus there is a possibility of an optical loss occurring. As a configuration for avoiding the optical loss, a configuration can be possible in which a thin film LN layer is used as the low-refractive-index layer14B in a slab shape without processing the thin film LN layer by dry etching. Alternatively, even when the sidewall is formed by dry etching, a configuration may be used in which the sidewall is formed outside the confinement region of guided light.

Accordingly, in the embodiment, the low-refractive-index layer14B is formed in the slab type with a larger width than guided light. On the other hand, the rib portions70and72are formed in the lower high-refractive-index layer10B and the upper high-refractive-index layer12B and only a part of the low-refractive-index layer14B is used as the slot part. In this structure, a confinement width in the horizontal direction in the guided light mode is determined as the width wribof the rib portions70and72. Since the width of the low-refractive-index layer14B is greater than the confinement width in the horizontal direction of the guided light mode, the optical loss due to the scattering on the sidewall is avoided. In addition, by setting wribto be 600 nm or less, only single lateral mode guided light propagates.

In the method of manufacturing the electro-optic waveguide device2B, differences from the first embodiment will be described. The manufacturing method according to the embodiment is basically different from that of the first embodiment in the forming of the rib portions70and72and the clad layers74and76.

The lower high-refractive-index layer10B can be formed using the SOI layer on the surface of the SOI wafer. For example, the surface of the SOI layer is partially subjected to dry etching to form the rib portion70in a non-etch portion. Thereafter, silica is deposited by CVD and is flattened by CMP to form the clad layers74on both sides of the rib portion70. The low-refractive-index layer14B formed of the thin film LN layer is formed on the surface of the substrate.

The upper high-refractive-index layer12B is formed to be partitioned into, for example, the rib portion72and a slab portion with a large width including the stretches22located above the rib portion72. Specifically, a silicon layer with a thickness equivalent to the height of the rib portion72is deposited on the surface of the substrate on which the low-refractive-index layer14B is formed and is processed in a strip shape by the photolithographic technique. The silicon layer patterned in the strip shape becomes the rib portion72. After the silicon layer is patterned, the silica is deposited by CVD and is flattened by CMP to form the clad layers76. A silicon layer is deposited on the surfaces of the clad layers76and the silicon layer serving as the rib portion72to form a slab portion of the upper high-refractive-index layer12B by the photolithographic technique. The slab portion formed of the silicon layer is integrated with the previously formed silicon layer having a strip shape to form the upper high-refractive-index layer12B. That is, the upper high-refractive-index layer12B that includes the rib portion72which is the contact portion with the low-refractive-index layer14B, and the slab portion located above the contact portion and including the stretches22on both sides thereof is formed.

Here, when the thicknesses of the lower high-refractive-index layer10B and the upper high-refractive-index layer12B in the contact portions with the slot part are large, as described in the first embodiment, most of the guided light is distributed in the lower high-refractive-index layer10B and the upper high-refractive-index layer12B, a ratio of the guided light confined in the slot part decreases, and thus there is a possibility that the phase modulation efficiency by the electro-optic effect decreases. From this viewpoint, the thickness of each of the lower high-refractive-index layer10B and the upper high-refractive-index layer12B in the portions in which the rib portions70and72are formed can be set to, for example, 250 nm or less.

For the thicknesses of the clad layers74and76(or the heights of the rib portions70and72), when the clad layers74and76are thin, confinement of the guided light in the horizontal direction is not sufficient and there is a possibility that the guided light to be localized near the slot part spreads in the horizontal direction and a radiation loss occurs. From this viewpoint, the thicknesses of the clad layers74and76are preferably set to, for example, 50 nm or more.

On the other hand, when the thicknesses of the clad layers74and76are large, first, there is a possibility that the electric field formed between the stretches20and22is weakened and the effect of suppressing the edge effect deteriorates. Second, when the upper limit is set to the thicknesses of the lower high-refractive-index layer10B and the upper high-refractive-index layer12B, as described above, the thickness of the slab portion becomes smaller as the heights of the rib portions70and72increase. As a result, for example, there is a possibility that the series electric resistance between the rib portions70and72and the contact regions28and30increases and a high-frequency property of the electro-optic waveguide device2B deteriorates. In addition, there is a possibility that resistances increase in the stretches20and22and the function of the electric field between the stretches20and22suppressing the edge effect is thus weakened. Accordingly, from these viewpoints, the upper limit can be set to the thicknesses of the clad layers74and76. For example, the thicknesses of the clad layers74and76are preferably set to 100 nm or less.

FIG. 12is a schematic vertical sectional view illustrating an electro-optic waveguide device2C according to a fourth embodiment and illustrates the xy cross-section. As inFIGS. 1 and 9,FIG. 12illustrates a cross-section at a position at which a connection structure of the electrodes to the lower high-refractive-index layer and the upper high-refractive-index layer is not disposed. On the other hand, in the embodiment, the electrodes are connected to the lower high-refractive-index layer and the upper high-refractive-index layer in a similar structure to that of each of the foregoing embodiments, but the illustration is omitted.

Even when the stretches are formed in one of the lower high-refractive-index layer and the upper high-refractive-index layer, it is possible to obtain the effect of suppressing the above-described edge effect.FIG. 12illustrates this configuration. Specifically, the electro-optic waveguide device2C includes the low-refractive-index layer14B with the slab shape and the lower high-refractive-index layer10B including the rib portion70as in the third embodiment. On the other hand, the upper high-refractive-index layer12is of a slab type similar to that of the first embodiment and includes no rib portion.

That is, in the embodiment, the lower high-refractive-index layer10B includes the rib portion70which is a contact portion with the slot part and the stretches20stretching on both sides of the rib portion70in the x-direction. On the other hand, the upper high-refractive-index layer12is formed in a slab type with portions facing both stretches20of the lower high-refractive-index layer10B in the shape of the xy cross-section. Here, the low-refractive-index layer14B spreads in the x-direction, and therefore comes into contact with the entire lower surface of the upper high-refractive-index layer12. Therefore, unlike the first embodiment, the upper high-refractive-index layer12according to the embodiment does not include the stretches22.

By vertically reversing the structure ofFIG. 12, it is possible to realize a structure which includes the upper high-refractive-index layer12B including the rib portion72and the lower high-refractive-index layer10of the slab type including no rib portion.

FIG. 13is a schematic view illustrating a planar layout of a Mach-Zehnder (MZ) optical modulator80. The MZ optical modulator80includes two phase modulators81aand81b, a 1×2 splitter82, a 2×1 coupler83, and single-mode waveguides84,85a,85b,86a,86b, and87connecting the above, which can be integrated on a common substrate to configure a one-chip device, for example.

Input light is input to the 1×2 splitter82via the waveguide84and is split into two rays of guided light in the 1×2 splitter82. The split guided light is each input to the phase modulators81aand81b. The phase modulators81aand81bare of a push-pull type to perform phase modulation at mutually reverse phases to the input guided light. The guided light output from the phase modulators81aand81bis interfered and combined in the 2×1 coupler83, and the 2×1 coupler83outputs the coupled light.

The above-described electro-optic waveguide device according to each embodiment of the present invention can be applied to the phase modulators81aand81bof the MZ optical modulator80. The phase modulators81aand81binclude waveguides extending in the z-axis direction (the horizontal direction inFIG. 13) and are disposed side by side in the x-direction on the substrate in which the MZ optical modulator80is integrated. Here, a region in which the phase modulators81aand81bare formed on the substrate is referred to as a modulation part81for convenience.FIG. 14is a schematic vertical sectional view illustrating the modulation part81according to the embodiment and illustrates the xy cross-section in which a connection structure of electrodes and high-refractive-index layers of the slot waveguide is illustrated.

The basic structure of the slot waveguides of the phase modulators81aand81bcorresponds to that of the first embodiment described with reference toFIGS. 1 and 2. That is, the slot waveguides of the phase modulators81aand81binclude lower high-refractive-index layers10aand10bcorresponding to the lower high-refractive-index layer10of the electro-optic waveguide element2according to the first embodiment and similarly include upper high-refractive-index layers12aand12bcorresponding to the upper high-refractive-index layer12and low-refractive-index layers14aand14bcorresponding to the low-refractive-index layer14. Accordingly, in the phase modulator81a, the lower high-refractive-index layer10aand the upper high-refractive-index layer12aeach having a slab shape are disposed to face a flat surface and the low-refractive-index layer14ain a strip shape is disposed in the gap thereof. The lower high-refractive-index layer10b, the upper high-refractive-index layer12b, and the low-refractive-index layer14bof the phase modulator81bare similarly configured to form a slot waveguide. In addition, the lower high-refractive-index layer10aand the lower high-refractive-index layer10bare electrically separated and the upper high-refractive-index layer12aand the upper high-refractive-index layer12bare electrically separated.

Here, to realize the parallel push-pull connection of the phase modulators81aand81b, conductive types of the high-refractive-index layers are reverse between the phase modulators81aand81b. For example, the phase modulator81ais configured so that the lower high-refractive-index layer10ahas P-type polarity and the upper high-refractive-index layer12ahas N-type polarity. The phase modulator81bis configured so that the lower high-refractive-index layer10bhas N-type polarity and the upper high-refractive-index layer12bhas P-type polarity.

The lower high-refractive-index layers10aand10bare connected to electrodes40aand40bvia plugs24aand24b, respectively, and the upper high-refractive-index layers12aand12bare connected to the common electrode42via plugs26aand26b, respectively. The plugs24aand24bare configured basically similar to the plug24according to the first embodiment and the plugs26aand26bare configured basically similar to the plug26according to the first embodiment. The electrodes40a,40b, and42configure a coplanar type traveling-wave electrode.

For example, the modulation part81is push-pull driven by one modulator driver for single-signal output. For example, the electrodes40aand40bare grounded and apply alternating-current signals output from the modulator driver to the electrode42.

In the MZ optical modulator80, the modulation part81is configured using the phase modulators81aand81bof a common structure to the first embodiment, and thus, the driving voltage can be reduced and the optical loss can be reduced, as described in the first embodiment. Consequently, the MZ optical modulator80can obtain a high-intensity extinction ratio and phase modulation Q value.

FIG. 14illustrates the example in which the slot waveguides of the phase modulators81aand81bhave the structure of the first embodiment. Instead of this, for example, the structure of the slot waveguide according to another embodiment such as the third embodiment can also be adopted.

A sixth embodiment relates to the MZ optical modulator80in which the electro-optic waveguide device according to the present invention is applied to the phase modulators81aand81binFIG. 13as in the fifth embodiment.FIG. 15is a schematic vertical sectional view illustrating the modulation part81according to the sixth embodiment and illustrates the xy cross-section in which the connection structure of electrodes with high-refractive-index layers of the slot waveguide is illustrated as inFIG. 14. The basic structure of the slot waveguides of the phase modulators81aand81billustrated inFIG. 15corresponds to that of the first embodiment described with reference toFIGS. 1 and 2, as in the example illustrated inFIG. 14of the fifth embodiment.

In the embodiment, the MZ optical modulator80is push-pull driven using one modulator driver for differential signal output. The modulation part81illustrated inFIG. 15has a structure corresponding thereto.

Specifically, one of the differential signals is applied to the electrode40aand the other of the differential signals is applied to the electrode40b. The electrodes40aand40bconfigure, for example, a differential traveling-wave electrode such as a slot line. The electrode42is applied with a direct-current bias voltage. The electrode42may be grounded.

Corresponding to this connection, the lower high-refractive-index layer10aof the phase modulator81aand the lower high-refractive-index layer10bof the phase modulator81bare set to have the same conductive polarity. On the other hand, the upper high-refractive-index layers12aand12bof the respective phase modulators81aand81bboth have conductive polarity different from the conductive polarity of the lower high-refractive-index layers10aand10b. For example, the lower high-refractive-index layers10aand10bare set to have P-type polarity and the upper high-refractive-index layers12aand12bare set to have N-type polarity.

The upper high-refractive-index layers12aand12bof the respective phase modulators81aand81bhave common conductive polarity and may be mutually continuous since a direct-current bias voltage is commonly applied. In connection portions with the electrodes, as illustrated inFIG. 15, the upper high-refractive-index layers12aand12bare connected so that the plug26can be common to both the upper high-refractive-index layers. In the xy cross-sectional structure at a position at which the connection structure of the electrodes to the upper high-refractive-index layers is not disposed, each of the upper high-refractive-index layers12aand12bmay have a shape including the stretches22necessary to reduce the edge effect and both the upper high-refractive-index layers is not necessarily continuous.

In contrast to the configuration ofFIG. 15, differential signals can be applied to the upper high-refractive-index layers12aand12band a direct-current bias voltage can be applied to the lower high-refractive-index layers10aand10b.FIG. 16illustrates this configuration.FIG. 16is a schematic vertical sectional view illustrating the modulation part81and illustrates the xy cross-section in which the connection structure of electrodes with high-refractive-index layers of the slot waveguide is illustrated, as inFIG. 15.

Specifically, electrodes42aand42bconnected to the upper high-refractive-index layers12aand12bare separately provided and differential signals are applied to these electrodes. The lower high-refractive-index layers10aand10bare, for example, connected to be formed in the x-direction and a direct-current bias voltage is applied by using the common electrode40and plug24.

In the embodiment, the MZ optical modulator80is also configured using the phase modulators81aand81bwith a common structure to the first embodiment, thereby reducing the driving voltage and reducing the optical loss, as described in the first embodiment. Eventually, the MZ optical modulator80can obtain a high-intensity extinction ratio and phase modulation Q value.

FIGS. 15 and 16illustrate the examples in which the slot waveguides of the phase modulators81aand81bhave the structure of the first embodiment. Instead of this, for example, the structure of the slot waveguide according to another embodiment such as the third embodiment can also be adopted.

A seventh embodiment is an optical module that includes any electro-optic waveguide device according to each of the foregoing embodiment. The optical module includes the electro-optic waveguide device according to the present invention, a light source optically connected to the electro-optic waveguide device, and a medium that transmits light passing through the electro-optic waveguide device.

FIG. 17is a schematic block diagram illustrating an optical module90according to the embodiment. The optical module90is a transceiver that has a transmission function and a reception function and converts an electric signal to an optical signal, and vice versa. The optical module90includes a silicon photonics chip92, a continuous wave (CW) light source94, and a signal processing LSI96in the same casing.

The MZ optical modulator80according to the fifth or sixth embodiment is mounted on the silicon photonics chip92. As described above, in the MZ optical modulator80, the electro-optic waveguide device according to the present invention is used for the modulation part81.

The CW light source94generates light to be used as a carrier (a carrier wave) and inputs the light to the silicon photonics chip92. For example, the CW light source94is configured as a semiconductor laser such as a DFB laser.

The signal processing LSI96is an integrated circuit that includes a circuit processing an electric signal related to the transmission of an optical signal. For example, with regard to the transmission of the optical signal, the signal processing LSI96performs a process such as encoding from an electric transmission signal to generate an electric modulated signal of the optical signal and outputs the electric modulated signal to the silicon photonics chip92. With regard to the reception of the optical signal, an electric demodulated signal extracted from the optical signal is input from the silicon photonics chip92to the signal processing LSI96and is subjected to a process such as decoding, or error correction, to generate and output an electric transmission signal.

FIG. 18is a schematic block diagram illustrating the silicon photonics chip92. The silicon photonics chip92includes optical couplers100and101and the MZ optical modulator80with regard to the transmission function for the optical signal and includes an optical coupler110, a photodiode111, and a transimpedance amplifier112with regard to the reception function.

The optical coupler100causes light input from the CW light source94to be incident on a waveguide connected to an input end of the MZ optical modulator80. An electric signal is input from the signal processing LSI96to the MZ optical modulator80and a carrier from the CW light source94is modulated with the electric signal and is output. The optical coupler101couples a waveguide connected to an output end of the MZ optical modulator80and an optical transmission path such as an optical fiber. In this configuration, the silicon photonics chip92generates a modulated optical signal and outputs the modulated optical signal to the optical transmission path.

On the other hand, the optical coupler110couples the optical transmission path to a waveguide connected to the photodiode (PD)111. The photodiode111converts an optical signal received from the optical transmission path into a current. The transimpedance amplifier112performs impedance conversion and amplification on a current signal output from the photodiode111and outputs the current signal as a voltage signal. In this configuration, the silicon photonics chip92generates a demodulated electric signal from the optical signal and outputs the electric signal to the signal processing LSI96.

In the optical module90, the electro-optic waveguide device according to the present invention is used to configure the MZ optical modulator80, and thus, the driving voltage can be reduced and the optical loss can be reduced in the optic modulation.