OPTICAL WAVEGUIDE DEVICE

An optical waveguide device includes a substrate, a first waveguide disposed on or in the substrate, and a second waveguide disposed on a first surface of the substrate, wherein the second waveguide includes a core and a cladding covering the core, wherein throughout an entirety of a predetermined region, a portion of the core overlaps the first waveguide when viewed from a direction normal to the first surface, in wherein the predetermined region, the core has a bottom surface in contact with the first surface and a convex surface connected to the bottom surface, and wherein the core includes a portion whose thickness gradually decreases from a widthwise center to widthwise ends in a transverse cross-sectional view.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application i based on and claims priority to Japanese Patent Application No. 2023-146474 filed on Sep. 8, 2023, with the Japanese Patent Office, the entire contents of which are incorporated herein by reference.

FIELD

The disclosures herein relate to optical waveguide devices.

BACKGROUND

Techniques for optically coupling a silicon waveguide and a polymer waveguide are known in the art. One example is a technique that forms a polymer waveguide so as to cover the silicon waveguide by the Mosquito method and realizes low-loss optical coupling between the silicon waveguide and the polymer waveguide by adiabatic coupling (see Patent Document 1, for example).

As illustrated in FIG. 1 of Patent document 1, in the above-described technique, a core having a cross-sectional is substantially circular shape fabricated and connected to a silicon waveguide on the lower side of the substantially circular core. Since the width of the substantially circular core becomes narrower toward the bottom, a shift of the position of the core in the width direction may cause the optical coupling efficiency with the silicon waveguide to be lowered.

There may be a need to provide an optical waveguide device in which two waveguides are coupled with high optical coupling efficiency.

PRIOR ART DOCUMENT

Patent Document

SUMMARY

According to an aspect of the embodiment, an optical waveguide device includes a substrate, a first waveguide disposed on or in the substrate, and a second waveguide disposed on a first surface of the substrate, wherein the second waveguide includes a core and a cladding covering the core, wherein throughout an entirety of a predetermined region, a portion of the core overlaps the first waveguide when viewed from a direction normal to the first surface, wherein in the predetermined region, the core has a bottom surface in contact with the first surface and a convex surface connected to the bottom surface, and wherein the core includes a portion whose thickness gradually decreases from a widthwise center to widthwise ends in a transverse cross-sectional view.

DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the invention will be described with reference to the accompanying drawings. In these drawings, the same components are referred to by the same reference numerals, and duplicate descriptions thereof may be omitted.

First Embodiment

FIGS.1A through1Care drawings illustrating an example of an optical waveguide device according to the first embodiment.FIG.1Ais an axonometric view, andFIG.1Bis a cross-sectional view taken along the line A-A inFIG.1A.FIG.1Cis a cross-sectional view taken along the line B-B inFIG.1A. To be more specific,FIG.1Billustrates a cross-sectional view of the optical waveguide device1cut in a plane parallel to the XZ plane through the center of a core31in the longitudinal direction.

InFIGS.1A through1C, as an example, the shape of the optical waveguide device1is a rectangular parallelepiped. The direction parallel to one edge of the bottom surface of the rectangular parallelepiped is the X direction, and the direction perpendicular to the X direction on the bottom surface of the rectangular parallelepiped is the Y direction. The direction perpendicular to the X direction and the Y direction (i.e., the thickness direction of the rectangular parallelepiped) are the Z direction (the same applies to the following drawings).

Referring toFIGS.1A through1C, the optical waveguide device1includes a substrate10, a first waveguide20, and a second waveguide30. The second waveguide30includes a core31and a cladding32surrounding the core31.

The substrate10may be formed of, for example, SiO2, SiOX, or the like. The substrate10may have a structure in which an upper layer made of SiO2, SiOX, or the like is formed on a lower layer made of Si. In this embodiment, the first waveguide20is provided on the first surface10aof the substrate10. That is, the lower surface of the first waveguide20is in contact with the first surface10a, and the upper surface and side surfaces of the first waveguide20are not covered by the substrate10. The first waveguide20is, for example, a silicon waveguide. Silicon waveguides are fine optical waveguides formed in silicon chips. The first waveguide20is not limited to a silicon waveguide, and may be made of silicon nitride, gallium arsenide, lithium niobate, or the like instead of silicon.

The first waveguide20is arranged such that, for example, the propagation direction of the signal light is the same as that of the core31, and the distance (gap) between them in the Z-axis direction is as short (narrow) as possible. (The distance is zero in this embodiment.) The region R represents a region throughout an entirety of which the first waveguide20and the core31overlap when viewed from the direction (i.e., Z direction) normal to the first surface10a(hereinafter referred to as plan view). The width of the first waveguide20may or may not be constant, but it is preferable that the width gradually decreases toward one side in the X direction. In this embodiment, the first waveguide20in the region R includes a portion whose width gradually decreases toward one side in the longitudinal direction (i.e., toward the positive X direction) in plan view. That is, in this embodiment, a portion of the first waveguide20, which portion is toward the positive X direction, has a tapered shape in plan view. The length TL of the tapered portion in the X direction may be, for example, about 200 μm to 1000 μm. In the region R, the first waveguide20preferably includes a portion whose width gradually decreases toward one side in the longitudinal direction (positive X direction) in plan view.

With such a shape, the optical coupling efficiency between the first waveguide20and the core31can be improved. The width of the first waveguide20is, for example, about 200 nm to 500 nm except for the tapered portion. The width of the tip of the tapered portion is, for example, about ½ to ¼ of the constant width portion. The thickness of the first waveguide20is constant. The thickness of the first waveguide20is, for example, about 20 nm to 300 nm.

The second waveguide30is provided on the first surface10aof the substrate10. The second waveguide30is, for example, a polymer waveguide. The core31of the second waveguide30is provided on the first surface10aof the substrate10. The core31and the first waveguide20are adiabatically coupled. The core31may be formed of a material mainly composed of a silicone resin, for example. The core31may alternatively be formed of a material mainly composed of an acrylic resin, an epoxy resin, a polyimide resin, a polyolefin resin, a polynorbornene resin, or the like. Alternatively, the core31may be formed of a material obtained by mixing these resins. Alternatively, the core31may be formed of an inorganic glass such as quartz or borosilicate.

The core31may be an SI type having a uniform refractive index in the plane. In this case, the refractive index of the core31may be higher than that of the substrate10, and may be about 1.5 to 1.6, for example. The core31is preferably a GI type having a higher refractive index toward the center and a lower refractive index toward the periphery. In this case, the refractive index of the center portion of the core31may be higher than that of the substrate10, and may be about 1.5 to 1.6, for example.

The core31in the region R overlaps (is positioned directly above) the first waveguide20in plan view. In the region R, the width of the core31(i.e., the dimension in the Y direction in the example illustrated inFIG.1) is larger than the width of the first waveguide20(i.e., the dimension in the Y direction in the example illustrated inFIG.1) at any position in the longitudinal direction (i.e., the X direction in the example illustratedFIG.1) of the first waveguide20in plan view. The core31preferably covers the entire width of the first waveguide20. In other words, the core31preferably covers the upper surface and longitudinal lateral surfaces of the first waveguide20. In the vicinity of the end portion of the first waveguide20on the negative X side, there may be a region where the first waveguide20is exposed from the core31.

In the region R, the core31has a bottom surface31ain contact with the first surface10aand a convex surface31bconnected to the bottom surface31a. In the example illustrated inFIG.1, the convex surface31bis constituted by a curved surface. The convex surface31bmay be constituted by a curved surface and a flat surface. For example, as illustrated inFIG.2A, the top surface of the convex surface31bmay be a flat surface, and both side surfaces connecting the top surface and the bottom surface31amay be curved surfaces. Further, as illustrated inFIG.2B, the upper surface of the convex surface31bmay be a convex curved surface, and both side surfaces connecting the upper surface and the bottom surface31amay be flat surfaces. These side surfaces may or may not be perpendicular to the bottom surface31a.

Alternatively, the convex surface31bmay be constituted only by flat surfaces. For example, as illustrated inFIG.3A, the upper surface of the convex surface31bmay be a flat surface, and both side surfaces connecting the upper surface and the bottom surface31amay be inclined flat surfaces that slope outwards toward the substrate10. Further, as illustrated inFIG.3B, substantially vertical flat surfaces may be formed on the substrate10side of the inclined flat surfaces in the shape illustrated inFIG.3A. Alternatively, the bottom surface31aand the convex surface31bmay form a substantially triangular shape in a transverse cross-sectional view.

As described above, the convex surface31bis preferably constituted by one or more curved surfaces and/or one or more flat surfaces, and the core31preferably includes a portion whose thickness gradually decreases from a the center side to the widthwise ends in a transverse cross-sectional view. With such a shape, the cross-sectional area of the core31can be reduced, allowing light to be more effectively confined to the central portion of the core31.

From the viewpoint of improving the optical coupling efficiency with the first waveguide20, the convex surface31bis preferably constituted by only a curved surface. The arrangement in which the convex surface31bis formed of only a curved surface can alleviate both the deterioration of the coupling efficiency occurring upon a change of the width W of the core31in the transverse direction and the requirement of the alignment accuracy of the core31with respect to the first waveguide20.

When the convex surface31bis formed of only a curved surface, it is preferable that the core31is oblong in a transverse cross-sectional view in which the thickness is smaller than the width and the thickness gradually decreases from the center side to the widthwise ends. In this case, the core31in the transverse cross-sectional view may be formed of, for example, a semi-circular shape, a semi-elliptical shape, a semi-oval shape, or the like.

In the transverse direction, as long as the bottom surface31ahas a sufficient width in the Y direction, the maximum width of the convex surface31bin the Y direction may be greater than the width of the bottom surface31ain the Y direction.

The width W of the core31in the transverse direction may be, for example, about 2 μm to 10 μm. The thickness T of the core31may be, for example, about 1 μm to 5 μm. It is preferable to determine the width W and the thickness T in consideration of the transverse cross-sectional area of the core31.

The cladding32is provided on the first surface10aof the substrate10and covers the core31. The cladding32may be formed of, for example, a material whose main component is a silicone resin. The cladding32may alternatively be formed of a material whose main component is an acrylic resin, an epoxy resin, a polyimide resin, a polyolefin resin, a polynorbornene resin, or the like. Alternatively, the cladding32may be formed of a material obtained by mixing these resins. Alternatively, the cladding32may be formed of inorganic glass such as quartz or borosilicate.

The cladding32is formed of a material having a lower refractive index than the central portion of the core31. If the refractive index of the central portion of the core31is, for example, about 1.52, the refractive index of the cladding32may be set to a lower value, for example, about 1.51. The refractive index of the cladding32is preferably equal to the refractive index of the substrate10. The cross-sectional shape of the cladding32may be, for example, rectangular. The thickness of the cladding32may be determined as appropriate depending on the width and thickness of the core31, manufacturing conditions, and the like, but is preferably about a few or several millimeters, more preferably about 50 to 1000 μm.

In this manner, the optical waveguide device1is such that in the region R, the core31has the bottom surface31ain contact with the first surface10a, and the convex surface31bconnected to the bottom surface31a. The convex surface31bis formed of one or more curved surfaces and/or one or more flat surfaces.

For example, if the cross-sectional shape of the core is substantially circular as in the related art, the contact area between the core and the first surface10abecomes extremely small. Since the width of the first waveguide is usually about 1 μm or less, even a slight positional displacement of the circular core along the width direction of the first waveguide causes an increase in the loss and a decrease in the optical coupling efficiency between the core and the first waveguide.

In the case of the optical waveguide device1, on the other hand, the core31has the bottom surface31ain contact with the first surface10a, so that the position of the core31is allowed to deviate in the width direction of the first waveguide20to some extent. With this arrangement, the alignment between the core31and the first waveguide20becomes easier than in the case of a substantially circular core as illustrated in, for example, Patent Document 1. This enables the realization of the optical waveguide device1in which the first waveguide20and the second waveguide30are coupled with high optical coupling efficiency. In addition, when a plurality of optical waveguide devices1are manufactured, variations in optical coupling efficiency among the optical waveguide devices1can be reduced. Details in this regard will be described later with simulation results.

[Method of Making an Optical Waveguide Device]

In the following, a method of making the optical waveguide device1will be described, with a focus on a manufacturing process of the second waveguide30.

FIGS.4A through4Dare drawings illustrating an example of the manufacturing process of the optical waveguide device according to the first embodiment. First, in the step illustrated inFIG.4A, a support70is prepared, and a frame80is detachably disposed on the upper surface of the support70. Then, a substrate10having the first waveguide20formed thereon is disposed on the upper surface of the support70exposed inside the frame80such that the first waveguide20is opposite from the upper surface of the support70across the substrate10.

The support for example, a70has, substantially rectangular shape in plan view. The frame80has, for example, a rectangular frame shape in plan view. As the materials of the support70and the frame80, for example, resin (e.g., acrylic), glass, silicon, ceramics, metals, and the like may be used. The support70and the frame80may be formed using the same material or different materials.

Next, in the step illustrated inFIG.4B, a predetermined material is applied to the first surface10aof the substrate10exposed inside the frame80, and a cladding32A having a substantially constant layer thickness is formed uniformly over the surface. The cladding32A is mainly composed of a resin precursor paste having viscosity (i.e., appropriate fluidity and formability), and is a portion that will be cured through polymerization in a later process to ultimately become the cladding32. The resin precursor is a precursor compound that can be polymerization-cured to form a resin.

The material of the cladding32A may be, for example, a material that is mainly composed of a resin precursor that can be polymerization-cured to form a silicone resin. The material of the cladding32A may be, for example, a material that is mainly composed of a resin precursor that can be polymerization-cured to form an acrylic resin, an epoxy resin, a polyimide resin, a polyolefin resin, a polynorbornene resin, or the like. Alternatively, the material of the cladding32A may be, for example, a material that is mainly composed of a plurality of resin precursors that can be polymerization-cured to form the above-noted resins. The material of the cladding32A can be selected as appropriate to provide photo-curable property, thermosetting property, thermoplastic property, and the like. The viscosity of the cladding32A may be, for example, about 10300 cPs. The cladding32A may be produced by using, for example, a coating apparatus (i.e., dispenser or the like), a printing apparatus, or the like.

In the step illustrated inFIG.4C, a coating apparatus (not shown) having a discharge unit90(including a discharge body91and a needle-shaped portion92) is prepared, and is then operated so that a part of the needle-shaped portion92at the tip of the discharge unit90is pierced into the cladding32A. The distance in the Z direction from the first surface10aof the substrate10to the tip of the needle-shaped portion92may be selected as appropriate, but may be, for example, about 1 to 8 μm, and preferably about 2 to 4 μm.

The coating apparatus includes a CPU, a memory, and the like, and has the function to move accurately the discharge unit90with respect to the cladding32A at predetermined moving speeds in the X, Y, and Z directions by programming. The needle-shaped portion92has, for example, an annular cross-sectional shape, and the coating apparatus has the function to discharge a predetermined material through the annular structure of the needle-shaped portion92at a predetermined discharge pressure. The inner diameter of the annular structure of the needle-shaped portion92may be selected as appropriate, and may be, for example, about 35 to 100 μm. The coating apparatus may include, for example, a desktop coating robot, a dispenser, and the like.

In the step illustrated inFIG.4D, the coating apparatus is operated to form the core31A by moving the needle-shaped portion92within the cladding32A while discharging the predetermined material from the needle-shaped portion92pierced into the cladding32A. The direction of movement of the needle-shaped portion92may be selected as appropriate, but in this example, the needle-shaped portion is moved along the X direction. The moving speed of the needle-shaped portion92may be selected as appropriate, and may be, for example, about 5 to 30 mm/s. The discharge pressure of the needle-shaped portion92may be selected as appropriate, and may be, for example, about 100 to 400 kPa.

The core31A is composed mainly of a paste resin precursor having viscosity (i.e., appropriate fluidity and formability) and is polymerization-cured in a later process to ultimately become the core31. As the material of the core31A, for example, a material composed mainly of a resin precursor which can be polymerization-cured to form a silicone resin may be used. As the material of the core31A, for example, a material composed mainly of a resin precursor which can be polymerization-cured to form an acrylic resin, an epoxy resin, a polyimide resin, a polyolefin resin, a polynorbornene resin or the like may be used. Alternatively, as the material of the core31A, for example, a material composed mainly of a plurality of resin precursors which can be polymerization-cured to form the above-noted resins may be used. The material of the core31A may be selected as appropriate to provide photo-curable property, thermosetting property, thermoplastic property, or the like. The viscosity of the core31A may be, for example, about 70,000 cPs.

The moving speed of the discharge unit90, the discharge pressure of the needle-shaped portion92, and the inner diameter of the annular structure of the needle-shaped portion92are adjusted in accordance with the material of the core31A and the material of the cladding32A. Further, the tip of the needle-shaped portion92is brought to an appropriate proximity to the first surface10aof the substrate10. These arrangements effectively form, for example, the core31A having a semicircular cross-sectional shape or the like and having a refractive index higher at the center and lower at the periphery.

When there is a long distance in the Z direction between the first surface10aof the substrate10and the tip of the needle-shaped portion92, the cross-sectional shape of the created core31becomes nearly circular. In order to make the cross-sectional shape of the core31semicircular including the bottom surface31aand the convex surface31bas illustrated inFIG.1C, it is thus preferable to bring the tip of the needle-shaped portion92to an appropriate proximity to the first surface10aof the substrate10. When the tip of the needle-shaped portion92is at an appropriate distance from the first surface10aof the substrate10, the bottom surface31ais formed by the drag from the first surface10aof the substrate10, resulting in the cross-sectional shape of the core31becoming semicircular.

After the step illustrated inFIG.4D, the discharge unit90is removed, and the core31A and the cladding32A are polymerization-cured by a predetermined method. For example, the core31A and the cladding32A, when made of light-curable materials, are cured by irradiation with light (such as ultraviolet light). When the material used is such that it cannot be fully cured solely through light irradiation, heat may be applied after the light irradiation.

As a result, the core31A and the cladding32A, which are mainly composed of resin precursor paste, are both polymerization-cured to form the core31and the cladding32, respectively, which are mainly composed of a resin. Further, the core31and the cladding32are removed from the support70and the frame80, which completes the fabrication of the optical waveguide device1illustrated inFIGS.1Athrough This method is known as the Mosquito method.

FIG.5is a photographic image illustrating an example of a cross-sectional shape of the core, and, more specifically, illustrates a transverse cross-section of the core in the optical waveguide device manufactured by the Mosquito method. InFIG.5, a portion that appears bright is a cross-section of the core. It can be confirmed fromFIG.5that a core having a substantially semicircular transverse cross-sectional shape is formed. The width and thickness of the cross-sectional shape of the core are both 10 μm or less.

The method of forming the second waveguide30in the optical waveguide device1is not limited to the Mosquito method, and other methods may be used as appropriate. The second waveguide30may be formed by, for example, a photolithography method, an imprint method, a laser drawing method, a photo addressing method, or the like.

With respect to an optical waveguide device having the structure illustrated inFIGS.1A through1C, the optical coupling efficiency between the core and the first waveguide optically coupled to each other was derived by optical simulation (FIMMWAVE and FIMMPROP). FIMMWAVE and FIMMPROP are software for analyzing propagation modes of waveguides.

In the first simulation, the first waveguide was a silicon waveguide. The thickness of the silicon waveguide was fixed to 0.22 μm, the width of the non-tapered portion was fixed to 0.08 μm, and the length TL of the tapered portion was varied between 50 μm and 1000 μm. The second waveguide was a polymer waveguide. With respect to the second waveguide, the shape of the core was semicircular, the thickness T of the core was fixed at 1.5 μm, and the width W was varied between 2 μm and 6 μm. The core was assumed to be an organic-inorganic hybrid resin provided by Nissan Chemical. The cladding was assumed to have a refractive index that is the same as or close to that of the substrate.

The results are illustrated inFIG.6A. InFIG.6A, the horizontal axis represents length TL and the vertical axis represents loss. As illustrated inFIG.6A, it was found that, with the cross-sectional shape of the core being semicircular, the loss was 1 dB or less regardless of the length TL or the width W of the core, and the core and the first waveguide were optically coupled with high optical coupling efficiency. In particular, it was found that when the length TL was 150 μm or more, the loss was 0.2 dB or less, and the core and the first waveguide were optically coupled with extremely high optical coupling efficiency.

The second simulation was substantially the same as the first simulation, except that the core thickness T was fixed at 1.0 μm, and the width W was varied between 2.5 μm and 8 μm.

The results are illustrated inFIG.6B. InFIG.6B, as inFIG.6A, the horizontal axis represents length TL and the vertical axis represents loss. As illustrated inFIG.6B, even when the core thickness T was set to 1.0 μm, the loss was less than 1 dB in all results, indicating that the core and the first waveguide were optically coupled with high optical coupling efficiency.

However, when the core width was 2.5 μm, the loss increased by a factor of about 5 as compared with when the core width was 4 μm or more, regardless of the length TL. When the core width was 3 μm, the loss increased by a factor of about 2 as compared with when the core width was 4 μm or more, regardless of the length TL. In contrast, when the core width was 4 μm or more, the loss was comparable to when the core thickness was 1.5 μm in the first simulation.

It is considered important at what slope the thickness of the convex surface31bdecreases from the center to the widthwise ends. Also, the cross-sectional area of the core31is important. When the area is too small, the loss tends to be high, and also when the area becomes large, the loss tends to increase. In other words, while ensuring that the core31has an appropriate cross-sectional area, it is important to increase the width of the convex surface31b. It is possible to determine an appropriate cross-sectional area of the core31by considering the effect of confining light into the center portion of the core31and whether the seeping light from the core31overlaps the first waveguide20.

In the third simulation, loss was compared between the semicircular core (of the embodiment) and a circular core (as a comparative example) for various core diameters and lengths TL. The diameter of the semicircular core is the width of the bottom surface of the core in contact with the first surface of the substrate in a transverse cross-sectional view. Other conditions were the same as those in the first simulation.

The results for the semicircular core are illustrated inFIG.7A. The results for the circular core are illustrated inFIG.7B. InFIG.7AandFIG.7B, the horizontal axis is the diameter of the core, and the vertical axis is the loss. FromFIG.7AandFIG.7B, it was found that when the diameter was 2 μm or more, the loss of the semicircular core is lower than that of the circular core regardless of the length TL.

In the fourth simulation, loss was compared between the semicircular or semi-elliptical core (of the embodiment) and a rectangular or square core (as a comparative example) for various thicknesses and widths. Other conditions were the same as those in the first embodiment.

The results for the semicircular or semi-elliptical core are illustrated inFIG.8A.FIG.8Billustrates the results for the rectangular or square core. InFIGS.8A and8B, the unit of loss is dB, and the shaded area indicates the area where the loss is 1 dB or less.

It was found, as illustrated inFIGS.8A and8B, that when the shape of the core was semi-elliptical or semi-circular, the shaded area greatly increased compared to when the shape of the core was rectangular or square. The wider the shaded area, the more choices of the thickness and width of the core, and the greater the tolerance of the dimensional accuracy at the time of manufacturing. That is, the semi-elliptical or semi-circular core offers more choices of the thickness and width of the core than the rectangular or square core, and also provides a greater tolerance of the dimensional accuracy at the time of manufacturing.

From the results of the first through fourth simulations, it can be seen that when the core having a semi-circular cross-sectional shape as illustrated inFIG.1Cis used, the tolerance of the width of the core can be increased from the viewpoint of optical coupling efficiency. That is, since the core having a properly wide width can be formed, the alignment accuracy requirements at the time of forming the core and the manufacturing accuracy requirements for the shape of the core can be relaxed in the manufacturing process of the optical waveguide device.

Variations of First Embodiment

In the first variation of the first embodiment, description of the same components as those of the already described embodiment may sometimes be omitted.

FIGS.9A through9Care drawings illustrating an example of an optical waveguide device according to the first variation of the first embodiment.FIG.9Ais an axonometric view, andFIG.9Bis a plan view.FIG.9Cis a cross-sectional view taken along the line C-C inFIG.9A.FIG.9Cis a cross-sectional view of an optical waveguide device1A cut in a plane parallel to the XZ plane passing through the longitudinal center of the core31.

The optical waveguide device1A illustrated inFIGS.9A through9Cdiffers from the optical waveguide device1in that the core31includes a tapered portion31cand a non-tapered portion31d.

As illustrated inFIG.9B, the core31may include, in the region R, a tapered portion31cwhose width gradually decreases toward one side in the longitudinal direction (toward the positive X direction) in plan view. The non-tapered portion31dcontinues from the tapered portion31ctoward the negative X direction. In the region R, the core31may include a tapered portion whose width gradually increases toward one side in the longitudinal direction (toward the positive X direction) in plan view.

In plan view, the core31may include a tapered portion31cin other regions than the region R. In plan view, the core31in other regions than the region R and in the region R may be entirely a tapered portion31c. That is, the core31may not include the non-tapered portion31d.

As illustrated inFIG.9C, the thickness of the tapered portion31cmay gradually decrease toward one side in the longitudinal direction (toward the positive X direction). Alternatively, the thickness of the tapered portion31cmay gradually increase toward one side in the longitudinal direction (toward the positive X direction).

FIG.10is a view illustrating one end of the optical waveguide device1A viewed from the positive X side toward the negative X side. As illustrated inFIG.10, outside the region R, the core31may include a circular portion in a transverse cross-sectional view. In the example illustrated inFIG.10, one longitudinal end of the core31is circular. With such a shape, the longitudinal end of the core31can be easily optically coupled with the optical fiber. In the present application, the term “circular” means approximately circular, and does not mean exactly a perfect circle. Therefore, the core may be deviated from a perfect circle within a range that does not substantially impair a predetermined effect as an optical waveguide.

In order to make the cross-sectional shape of the core31circular, in the Mosquito method, for example, the distance in the Z-direction from the first surface10aof the substrate10to the tip end of the needle-shaped portion92may be made longer than in the case where the core31is semicircular.

It may be noted that even if the core31is composed only of the non-tapered portion31d, the core31may include a circular portion in a transverse cross-sectional view as long as such a portion is situated outside the region R.

Further, even when the first waveguide20does not have a tapered portion, the core31of the second waveguide30may have a tapered portion31c. The tapered portion31cmay be disposed only outside the region R. Further, the core31may include a tapered portion whose width and thickness gradually increase toward one side in the longitudinal direction (toward the positive X direction).

FIG.11is a cross-sectional view illustrating an example of an optical waveguide device according to the second variation of the first embodiment, and illustrates a cross-sectional view corresponding toFIG.1C.

The optical waveguide device1B illustrated inFIG.11differs from the optical waveguide device1in that the first waveguide20is embedded in the substrate10. That is, the upper surface, the lower surface, and the side surfaces of the first waveguide20are covered with the substrate10. The upper surface of the first waveguide20may alternatively be exposed in the same plane as the first surface10aof the substrate10while the lower surface and the side surfaces may be covered with the substrate10. As described above, the first waveguide20

may not protrude above the first surface10aof the substrate10. In this with case as well, the configuration in which the core31has the bottom surface31ain contact with the first surface10aand the convex surface31bconnected to the bottom surface coupling efficiency31a, high optical can be maintained between the core31and the first waveguide20even when the position of the core31is shifted in the width direction of the first waveguide20. This allows for an increase in the yield of products, and enables the realization of the optical waveguide device1B that is stable with only small variations in optical coupling efficiency among products.

With the configuration in which the first waveguide20is embedded in the substrate10, the optical coupling efficiency between the first waveguide20and the core31is effectively improved when the cross-sectional shape of the core31is flatter than when the first waveguide20is not embedded in the substrate10. That is, by making the cross-sectional shape of the core31flatter, the confinement of light in the thickness direction of the core31is weakened, and the light in the propagation mode within the core31seeps deeper into the substrate10(evanescent light). The greater the overlap between the evanescent light and the first waveguide20, the more improved the optical coupling efficiency is due to an increased likelihood of light transition resulting from the mode coupling between the core31and the first waveguide20.

According to at least one embodiment, it is possible to provide an optical waveguide device in which two waveguides are coupled with high optical coupling efficiency.