Optical waveguide device

Abstract

To provide an optical waveguide device in which two waveguides are coupled with high optical coupling efficiency.SOLUTION: An optical waveguide device includes a substrate, a first waveguide disposed on the substrate, and a second waveguide disposed on a first surface of the substrate. The second waveguide includes a core and a cladding covering the surroundings of the core. The core includes, a region overlapping the first waveguide when viewed from a direction normal to the first surface. In the region, the core has a bottom surface in contact with the first surface and a convex surface connected to the bottom surface. The core includes a portion whose thickness gradually decreases from the center to an outer periphery in a transverse cross-sectional view.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical waveguide device. [Background technology]

[0002] There are known techniques for optically coupling a silicon waveguide and a polymer waveguide. One example is a technique in which a polymer waveguide is formed so as to cover a silicon waveguide by the mosquito method, and low-loss optical coupling between the silicon waveguide and the polymer waveguide is realized by adiabatic coupling (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-97012 A Summary of the Invention [Problem to be solved by the invention]

[0004] As shown in Fig. 1 of Patent Document 1, in the above technology, a core part with a substantially circular cross-sectional shape is fabricated, and the core part is connected to a silicon waveguide below the substantially circular core part. Since the width of the substantially circular core part narrows toward the bottom, there was a risk of a decrease in the optical coupling efficiency with the silicon waveguide if the position of the core part is shifted in the width direction.

[0005] The present invention has been made in view of the above-mentioned points, and has an object to provide an optical waveguide device in which two waveguides are coupled with high optical coupling efficiency. [Means for solving the problem]

[0006] This optical waveguide device has a substrate, a first waveguide provided on the substrate, and a second waveguide provided on a first surface of the substrate, the second waveguide including a core portion and a cladding portion covering the periphery of the core portion, the core portion having a region overlapping with the first waveguide when viewed from the normal direction of the first surface, in the region, the core portion has a bottom surface in contact with the first surface and a convex surface connected to the bottom surface, and the core portion includes a portion whose thickness gradually decreases from the center to the outer periphery when viewed in a cross section in the short direction. Effect of the Invention

[0007] According to the disclosed technique, it is possible to provide an optical waveguide device in which two waveguides are coupled with high optical coupling efficiency. [Brief description of the drawings]

[0008] [Figure 1] 1A and 1B are diagrams illustrating an optical waveguide device according to a first embodiment. [Diagram 2] 13 is a diagram (part 1) showing another example of the cross-sectional shape of the core portion. FIG. [Diagram 3] FIG. 11 is a diagram (part 2) showing another example of the cross-sectional shape of the core portion. [Figure 4] 3A to 3C are diagrams illustrating manufacturing processes for the optical waveguide device according to the first embodiment. [Diagram 5] 4 is a photograph showing an example of a cross-sectional shape of a core portion. [Figure 6] FIG. 1 shows the results of a simulation. [Figure 7] FIG. 2 shows the results of the simulation. [Figure 8] FIG. 3 shows the results of the simulation. [Figure 9] 1A and 1B are diagrams illustrating an optical waveguide device according to a first modified example of the first embodiment. [Figure 10] 1 is a view of one end of an optical waveguide device 1A viewed from the positive side to the negative side in the X direction. [Figure 11] 10 is a cross-sectional view illustrating an optical waveguide device according to a second modified example of the first embodiment. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals, and duplicated explanations may be omitted.

[0010] First embodiment [Optical waveguide device] Fig. 1 is a diagram illustrating an optical waveguide device according to a first embodiment, in which Fig. 1(a) is a perspective view, Fig. 1(b) is a cross-sectional view taken along line AA in Fig. 1(a), and Fig. 1(c) is a cross-sectional view taken along line BB in Fig. 1(a). Fig. 1(b) shows a cross section of the optical waveguide device 1 cut along a plane that passes through the center of the core portion 31 in the longitudinal direction and is parallel to the XZ plane.

[0011] In FIG. 1, as an example, the shape of the optical waveguide device 1 is a rectangular parallelepiped, the direction parallel to one side of the bottom surface of the rectangular parallelepiped is the X direction, the direction perpendicular to the X direction within the bottom surface of the rectangular parallelepiped is the Y direction, and the direction perpendicular to the X direction and the Y direction (thickness direction of the rectangular parallelepiped) is the Z direction (this also applies to the subsequent figures).

[0012] 1, the optical waveguide device 1 has a substrate 10, a first waveguide 20, and a second waveguide 30. The second waveguide 30 includes a core portion 31 and a cladding portion 32 that covers the periphery of the core portion 31.

[0013] The substrate 10 is made of, for example, SiO2 or SiO X The substrate 10 can be formed of a SiO2 or SiO Xor the like may be formed. In this embodiment, the first waveguide 20 is provided on the first surface 10a of the substrate 10. That is, the lower surface of the first waveguide 20 is in contact with the first surface 10a, and the upper surface and side surface of the first waveguide 20 are exposed from the substrate 10. The first waveguide 20 is, for example, a silicon waveguide. The silicon waveguide is a fine optical waveguide built into a silicon chip. Note that the first waveguide 20 is not limited to a silicon waveguide, and silicon nitride, gallium arsenide, lithium niobate, or the like may be used instead of silicon.

[0014] The first waveguide 20 is arranged, for example, in such a manner that the propagation direction of the signal light is the same as that of the core 31, and the distance (gap) between them in the Z-axis direction is as small as possible. The region R indicates a region where the first waveguide 20 and the core 31 overlap when viewed from the normal direction (Z direction) of the first surface 10a (hereinafter referred to as plan view). The width of the first waveguide 20 may be constant or may not be constant, but it is preferable that the width gradually decreases toward one side in the X direction. In this embodiment, in the region R, the first waveguide 20 includes a portion where the width gradually decreases toward one side in the longitudinal direction (the positive side in the X direction) in the plan view. That is, in this embodiment, the positive side in the X direction of the first waveguide 20 is tapered in the plan view. The length TL in the X direction of the tapered portion can be, for example, about 200 μm to 1000 μm. In region R, it is preferable that the first waveguide 20 includes a portion whose width gradually decreases toward one side in the longitudinal direction (the positive side in the X direction) in a plan view.

[0015] Such a shape can improve the optical coupling efficiency between the first waveguide 20 and the core portion 31. The width of the first waveguide 20 is, 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 1 / 2 to 1 / 4 of the portion of constant width. The thickness of the first waveguide 20 is constant. The thickness of the first waveguide 20 is, for example, about 20 nm to 300 nm.

[0016] The second waveguide 30 is provided on the first surface 10a of the substrate 10. The second waveguide 30 is, for example, a polymer waveguide. In the second waveguide 30, the core portion 31 is provided on the first surface 10a of the substrate 10. The core portion 31 and the first waveguide 20 are adiabatically coupled. The core portion 31 can be formed, for example, from a material mainly composed of silicone resin. The core portion 31 may be formed from a material mainly composed of acrylic resin, epoxy resin, polyimide resin, polyolefin resin, polynorbornene resin, or the like. Alternatively, the core portion 31 may be formed from a material in which these resins are mixed. Alternatively, the core portion 31 may be formed from inorganic glass such as quartz or borosilicate.

[0017] The core portion 31 may be of an SI type having a uniform refractive index within the plane. In this case, the refractive index of the core portion 31 is higher than that of the substrate 10, and can be, for example, about 1.5 to 1.6. The core portion 31 is preferably of 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 core portion 31 is higher than that of the substrate 10, and can be, for example, about 1.5 to 1.6.

[0018] The core portion 31 has a region R overlapping with the first waveguide 20 in plan view. In the region R, the width of the core portion 31 (length in the Y direction in the example of FIG. 1) is wider than the width of the first waveguide 20 (length in the Y direction in the example of FIG. 1) at any position in the longitudinal direction of the first waveguide 20 in plan view (X direction in the example of FIG. 1). The core portion 31 preferably covers the entire width of the first waveguide 20. That is, the core portion 31 preferably covers the top surface and both side surfaces in the longitudinal direction of the first waveguide 20. Note that there may be a region where the first waveguide 20 is exposed from the core portion 31 near the end of the first waveguide 20 on the negative side in the X direction.

[0019] In the region R, the core portion 31 has a bottom surface 31a in contact with the first surface 10a and a convex surface 31b connected to the bottom surface 31a. In the example of FIG. 1, the convex surface 31b is formed by a curved surface. The convex surface 31b may be formed by a curved surface and a flat surface. For example, as shown in FIG. 2(a), the upper surface of the convex surface 31b may be a flat surface, and both side surfaces connecting the upper surface and the bottom surface 31a may be curved surfaces. Also, as shown in FIG. 2(b), the upper surface of the convex surface 31b may be a convex curved surface, and both side surfaces connecting the upper surface and the bottom surface 31a may be flat surfaces. Also, these both side surfaces may or may not be perpendicular to the bottom surface 31a.

[0020] Alternatively, the convex surface 31b may be formed only by a flat surface. For example, as shown in Fig. 3(a), the upper surface of the convex surface 31b may be a flat surface, and both side surfaces connecting the upper surface and the bottom surface 31a may be inclined planes so as to become wider toward the substrate 10 side. Furthermore, as shown in Fig. 3(b), in the shape of Fig. 3(a), a substantially perpendicular plane may be present on the substrate 10 side of the inclined plane. Alternatively, the bottom surface 31a and the convex surface 31b may form a substantially triangle in a cross section in the short direction.

[0021] In this way, it is preferable that the convex surface 31b is formed by a curved surface and / or a flat surface, and the core portion 31 includes a portion in which the thickness gradually decreases from the center to the outer periphery in a cross-sectional view in the short side direction. By using such a shape, the cross-sectional area of ​​the core portion 31 can be reduced, and light can be further confined in the center of the core portion 31.

[0022] From the viewpoint of improving the optical coupling efficiency with the first waveguide 20, it is preferable that the convex surface 31b is formed only from a curved surface. When the convex surface 31b is formed only from a curved surface, it is possible to alleviate the deterioration of the coupling efficiency when the width W of the core portion 31 in the short side direction varies, and the requirement for the alignment accuracy of the core portion 31 with respect to the first waveguide 20.

[0023] When the convex surface 31b is formed only by a curved surface, it is preferable that the core portion 31 is long in the short-side cross section, the thickness of which is smaller than the width, and the thickness of which gradually decreases from the center to the outer periphery. In this case, the core portion 31 may be, for example, semicircular, semielliptical, or semioval in the short-side cross section.

[0024] In addition, in the short direction, when the length of bottom surface 31a in the Y direction can be secured to a certain extent, the maximum length of convex surface 31b in the Y direction may be longer than the length of bottom surface 31a in the Y direction.

[0025] The width W of the core portion 31 in the short side direction can be, for example, about 2 μm to 10 μm. The thickness T of the core portion 31 can be, for example, about 1 μm to 5 μm. It is preferable to determine the width W and thickness T while taking into consideration the cross-sectional area of ​​the core portion 31 in the short side direction.

[0026] The cladding portion 32 is provided on the first surface 10a of the substrate 10, and covers the core portion 31. The cladding portion 32 can be formed of a material mainly composed of silicone resin, for example. The cladding portion 32 may also be formed of a material mainly composed of acrylic resin, epoxy resin, polyimide resin, polyolefin resin, polynorbornene resin, or the like. Alternatively, the cladding portion 32 may be formed of a material that is a mixture of these resins. Alternatively, the cladding portion 32 may be formed of inorganic glass such as quartz or borosilicate.

[0027] The cladding portion 32 is formed from a material having a lower refractive index than the central portion of the core portion 31. If the refractive index of the central portion of the core portion 31 is, for example, about 1.52, the refractive index of the cladding portion 32 can be lower than that, for example, about 1.51. The refractive index of the cladding portion 32 is preferably equal to the refractive index of the substrate 10. The cross-sectional shape of the cladding portion 32 can be, for example, rectangular. The thickness of the cladding portion 32 can be arbitrarily determined depending on the width and thickness of the core portion 31, manufacturing conditions, etc., but can be preferably about several mm, more preferably about 50 to 1000 μm.

[0028] Thus, in the optical waveguide device 1, in the region R, the core portion 31 has a bottom surface 31a in contact with the first surface 10a and a convex surface 31b connected to the bottom surface 31a. The convex surface 31b is formed by a curved surface and / or a flat surface.

[0029] For example, if the cross-sectional shape of the core is generally circular as in the conventional case, the area where the core contacts the first surface 10a becomes extremely small. Since the width of the first waveguide is usually about 1 μm or less, even a slight deviation in the position of the circular core in the width direction of the first waveguide increases loss and reduces the efficiency of optical coupling between the core and the first waveguide.

[0030] In contrast, in the optical waveguide device 1, the core portion 31 has a bottom surface 31a that contacts the first surface 10a, so that it is possible to tolerate some degree of misalignment of the position of the core portion 31 in the width direction of the first waveguide 20. Therefore, it is easier to align the core portion 31 with the first waveguide 20 compared to the case of a substantially circular core portion as shown in, for example, Patent Document 1. This makes it possible to realize an optical waveguide device 1 in which the first waveguide 20 and the second waveguide 30 are coupled with high optical coupling efficiency. In addition, when a plurality of optical waveguide devices 1 are manufactured, it is possible to reduce the variation in optical coupling efficiency for each optical waveguide device 1. This will be described in detail later with the results of a simulation.

[0031] [Method of manufacturing optical waveguide device] Next, a method for manufacturing the optical waveguide device 1 will be described, focusing on the manufacturing process of the second waveguide 30.

[0032] 4A and 4B are diagrams illustrating the manufacturing process of the optical waveguide device according to the first embodiment. First, in the process shown in Fig. 4A, a support 70 is prepared, and a frame 80 is detachably disposed on the upper surface of the support 70. Then, a substrate 10 on which a first waveguide 20 is formed is disposed on the upper surface of the support 70 exposed inside the frame 80, so that the first waveguide 20 faces the upper surface of the support 70 with the substrate 10 in between.

[0033] The support 70 has, for example, a substantially rectangular planar shape. The frame 80 has, for example, a rectangular frame planar shape. The support 70 and the frame 80 may be made of, for example, resin (acrylic, etc.), glass, silicon, ceramics, metal, etc. The support 70 and the frame 80 may be made of the same material or different materials.

[0034] 4(b), a predetermined material is applied to the first surface 10a of the substrate 10 exposed within the frame 80 and spread evenly to produce the cladding portion 32A of a substantially uniform thickness. The cladding portion 32A is mainly composed of a paste-like resin precursor having viscosity (adequate fluidity and shapeability), and is polymerized and hardened in a later process to finally become the cladding portion 32. Here, the resin precursor is a precursor compound that can be polymerized and hardened to form a resin.

[0035] The material of the clad portion 32A may be, for example, a material mainly composed of a resin precursor that can be polymerized and cured to form a silicone resin. The material of the clad portion 32A may be, for example, a material mainly composed of a resin precursor that can be polymerized and 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 clad portion 32A may be, for example, a material mainly composed of a plurality of resin precursors that can be polymerized and cured to form these resins. The material of the clad portion 32A may be appropriately selected from photocurable, thermosetting, thermoplastic, and the like. The viscosity of the clad portion 32A may be, for example, about 10300 cPs. The clad portion 32A may be produced, for example, by using an applicator (dispenser, etc.) or a printer, etc.

[0036] 4(c), a coating device (not shown) having a discharge part 90 (including a discharge part main body 91 and a needle-like part 92) is prepared, and the prepared coating device is operated to insert a part of the needle-like part 92 at the tip of the discharge part 90 into the cladding part 32A. The distance in the Z direction from the first surface 10a of the substrate 10 to the tip of the needle-like part 92 can be appropriately selected, but can be, for example, about 1 to 8 μm, and preferably about 2 to 4 μm.

[0037] The coating device includes a CPU, a memory, and the like, and has a function of moving the discharge part 90 with precision in the X direction, the Y direction, and the Z direction at a predetermined moving speed relative to the cladding part 32A by programming. The needle part 92 has, for example, a circular cross-sectional shape, and the coating device has a function of discharging a predetermined material from the inside of the circular needle part 92 at a predetermined discharge pressure. The inner diameter of the circular needle part 92 can be appropriately selected, but can be, for example, about 35 to 100 μm. The coating device can be configured to include, for example, a tabletop coating robot, a dispenser, and the like.

[0038] Next, in the step shown in Fig. 4(d), the coating device is operated to eject a predetermined material from the needle-like portion 92 inserted into the clad portion 32A while moving the needle-like portion 92 within the clad portion 32A to form the core portion 31A. The moving direction of the needle-like portion 92 can be selected as appropriate, but here, as an example, it is moved in the X direction. The moving speed of the needle-like portion 92 can be selected as appropriate, but can be, for example, about 5 to 30 mm / s. The ejection pressure of the needle-like portion 92 can be selected as appropriate, but can be, for example, about 100 to 400 kPa.

[0039] The core portion 31A is mainly composed of a paste-like resin precursor having viscosity (moderate fluidity and shapeability), and is polymerized and hardened in a later process to finally become the core portion 31. The material of the core portion 31A may be, for example, a material mainly composed of a resin precursor that can be polymerized and hardened to form a silicone resin. The material of the core portion 31A may also be, for example, a material mainly composed of a resin precursor that can be polymerized and hardened 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 core portion 31A may also be, for example, a material mainly composed of a plurality of resin precursors that can be polymerized and hardened to form these resins. The material of the core portion 31A may be appropriately selected from photocurable, thermosetting, thermoplastic, and the like. The viscosity of the core portion 31A may be, for example, about 70,000 cPs.

[0040] By adjusting the movement speed of the discharge portion 90, the discharge pressure of the needle-shaped portion 92, and the inner diameter of the ring of the needle-shaped portion 92 to match the material of the core portion 31A and the material of the cladding portion 32A, respectively, and by bringing the tip of the needle-shaped portion 92 appropriately close to the first surface 10a of the substrate 10, it is possible to form a core portion 31A having, for example, a semicircular cross-sectional shape, with a higher refractive index toward the center and a lower refractive index toward the periphery.

[0041] If the distance in the Z direction from the first surface 10a of the substrate 10 to the tip of the needle-like portion 92 is long, the cross-sectional shape of the core portion 31 that is formed becomes closer to a circle. Therefore, in order to make the cross-sectional shape of the core portion 31 a semicircular shape including the bottom surface 31a and the convex surface 31b as shown in Fig. 1(c), it is preferable to bring the tip of the needle-like portion 92 appropriately close to the first surface 10a of the substrate 10. When the tip of the needle-like portion 92 is brought appropriately close to the first surface 10a of the substrate 10, the bottom surface 31a is formed by the resistance from the first surface 10a of the substrate 10, and the cross-sectional shape of the core portion 31 becomes a semicircle or the like.

[0042] After the step shown in Fig. 4(d), the discharge portion 90 is removed, and the core portion 31A and the clad portion 32A are polymerized and cured by a predetermined method. For example, if the core portion 31A and the clad portion 32A are made of a photocurable material, they are irradiated with light (ultraviolet rays, etc.) to cure them. If a material that does not cure completely by only irradiating with light is used, it may be heated after irradiating with light.

[0043] As a result, the core portion 31A and the clad portion 32A, which are mainly composed of the paste-like resin precursor, are polymerized and hardened, respectively, to form the core portion 31 and the clad portion 32, which are mainly composed of resin. Furthermore, the core portion 31 and the clad portion 32 are removed from the support 70 and the frame portion 80, thereby completing the optical waveguide device 1 shown in Fig. 1. This method is a so-called mosquito method.

[0044] Figure 5 is a photograph showing an example of the cross-sectional shape of a core portion, showing a cross section in the short-side direction of a core portion in an optical waveguide device produced by the mosquito method. In Figure 5, the bright parts are the cross sections of the core portion. It can be seen from Figure 5 that a core portion having a roughly semicircular cross section in the short-side direction has been formed. The width and thickness of the cross-sectional shape of the core portion are both 10 μm or less.

[0045] The method for forming the second waveguide 30 in the optical waveguide device 1 is not limited to the mosquito method, and any appropriate method can be used. The second waveguide 30 can be formed by, for example, a photolithography method, an imprint method, a laser drawing method, a photoaddressing method, or the like.

[0046] <simulation> Assuming an optical waveguide device with the structure shown in Figure 1, the optical coupling efficiency when the core part and the first waveguide are optically coupled was calculated by optical simulation (FIMMWAVE and FIMMPROP). FIMMWAVE and FIMMPROP are software programs that perform propagation mode analysis of waveguides.

[0047] (Simulation 1) In Simulation 1, the first waveguide was a silicon waveguide. In addition, in the silicon waveguide, the thickness was fixed at 0.22 μm, the width of the non-tapered part was fixed at 0.08 μm, and the length TL of the tapered part was varied between 50 μm and 1000 μm. The second waveguide was a polymer waveguide. In addition, in the second waveguide, the shape of the core part was semicircular, the thickness T of the core part was fixed at 1.5 μm, and the width W was varied between 2 μm and 6 μm. For the core part, an organic-inorganic hybrid resin provided by Nissan Chemical was assumed. In addition, it was assumed that the cladding part had a refractive index that was the same as or close to that of the substrate.

[0048] The results are shown in Figure 6(a). In Figure 6(a), the horizontal axis is the length TL and the vertical axis is the loss. As shown in Figure 6(a), by making the cross-sectional shape of the core part semicircular, it was found that the loss was 1 dB or less regardless of the length TL or the width W of the core part, and the core part and the first waveguide could be optically coupled with high optical coupling efficiency. In particular, when the length TL is 150 μm or more, the loss was 0.2 dB or less, and it was found that the core part and the first waveguide could be optically coupled with extremely high optical coupling efficiency.

[0049] (Simulation 2) Simulation 2 was the same as Simulation 1, except that the thickness T of the core portion was fixed at 1.0 μm and the width W was varied between 2.5 μm and 8 μm.

[0050] The results are shown in Figure 6(b). In Figure 6(b), the horizontal axis is length TL and the vertical axis is loss, as in Figure 6(a). As shown in Figure 6(b), even when the core thickness T was set to 1.0 μm, the loss was 1 dB or less in all results, indicating that the core and the first waveguide could be optically coupled with high optical coupling efficiency.

[0051] However, when the core width is 2.5 μm, the loss is about five times higher than when the core width is 4 μm or more, regardless of the length TL. Also, when the core width is 3 μm, the loss is about two times higher than when the core width is 4 μm or more, regardless of the length TL. In contrast, when the core width is 4 μm or more, the loss is the same as when the core thickness in Simulation 1 is 1.5 μm.

[0052] It is considered important at what angle the thickness of the convex surface 31b decreases from the center to the periphery. The cross-sectional area of ​​the core portion 31 is also important; if it is too small, high loss occurs, and if it is too large, loss tends to increase. In other words, the core portion 31 needs to have an appropriate cross-sectional area, and it is important to increase the width of the convex surface 31b within that area. The appropriate cross-sectional area of ​​the core portion 31 can be determined while taking into consideration the effect of confining light to the center of the core portion 31 and whether the light leaking out of the core portion 31 overlaps with the first waveguide 20.

[0053] (Simulation 3) In Simulation 3, a comparison was made between the semicircular core (Example) and the circular core (Comparative Example) in terms of the loss when the diameter and length TL of the core were changed. The diameter of the semicircular core is the width of the bottom surface of the core that contacts the first surface of the substrate in a cross-sectional view in the short side direction. The other conditions were the same as in Simulation 1.

[0054] The results for the semicircular core portion are shown in Figure 7(a), and the results for the circular core portion are shown in Figure 7(b). In Figures 7(a) and 7(b), the horizontal axis is the core portion diameter, and the vertical axis is the loss. Figures 7(a) and 7(b) show that if the diameter is 2 μm or more, the semicircular core portion has lower loss than the circular core portion, regardless of the length TL.

[0055] (Simulation 4) In Simulation 4, a comparison was made of the loss when the thickness and width were changed for a semi-elliptical or semi-circular core portion (Example) and a rectangular or square core portion (Comparative Example). The other conditions were the same as in Simulation 1.

[0056] The results for semi-elliptical and semi-circular cores are shown in Fig. 8(a), and the results for rectangular and square cores are shown in Fig. 8(b). In Fig. 8(a) and Fig. 8(b), the unit of loss is [dB], and the area where the loss is 1 dB or less is shaded.

[0057] 8(a) and 8(b), it can be seen that when the core part has a semi-elliptical or semi-circular shape, the shaded area is significantly larger than when the core part has a rectangular or square shape. The wider the shaded area, the more options there are for the thickness and width of the core part, and the greater the tolerance for dimensional accuracy during manufacturing. In other words, it can be said that a semi-elliptical or semi-circular core part has more options for the thickness and width of the core part and a greater tolerance for dimensional accuracy during manufacturing than a rectangular or square core part.

[0058] From the results of simulations 1 to 4, it can be seen that the tolerance for the width of the core part can be increased in terms of optical coupling efficiency when a core part having a cross-sectional shape such as a semicircle as shown in Fig. 1(c) is used. In other words, since a core part with a relatively wide width can be formed, it is possible to realize relaxation of the alignment precision required when forming the core part and relaxation of the manufacturing tolerance of the shape of the core part in the manufacturing process of the optical waveguide device.

[0059] Modification of the First Embodiment In the first modification of the first embodiment, the description of the same components as those in the embodiments already described may be omitted.

[0060] Fig. 9 illustrates an optical waveguide device according to Modification 1 of the first embodiment, where Fig. 9(a) is a perspective view, Fig. 9(b) is a plan view, and Fig. 9(c) is a cross-sectional view taken along line CC in Fig. 9(a). Fig. 9(c) shows a cross section of the optical waveguide device 1A taken along a plane that passes through the center of the core 31 in the longitudinal direction and is parallel to the XZ plane.

[0061] An optical waveguide device 1A shown in FIG. 9 differs from the optical waveguide device 1 in that a core portion 31 includes a tapered portion 31c and a non-tapered portion 31d.

[0062] 9(b), in the region R, the core portion 31 may include a tapered portion 31c whose width gradually decreases toward one side in the longitudinal direction (the positive side in the X direction) in a plan view. The non-tapered portion 31d is continuous with the tapered portion 31c on the negative side in the X direction. In the region R, the core portion 31 may include a tapered portion whose width gradually increases toward one side in the longitudinal direction (the positive side in the X direction) in a plan view.

[0063] In plan view, the core portion 31 may include the tapered portion 31c except in the region R. In plan view, the core portion 31 may be the tapered portion 31c throughout the region R and the entire region except for the region R. That is, the core portion 31 does not need to include the non-tapered portion 31d.

[0064] 9(c), the tapered portion 31c may have a thickness that gradually decreases toward one side in the longitudinal direction (the positive side in the X direction). Alternatively, the tapered portion 31c may have a thickness that gradually increases toward one side in the longitudinal direction (the positive side in the X direction).

[0065] FIG. 10 is a view of one end of the optical waveguide device 1A viewed from the positive side to the negative side in the X direction. As shown in FIG. 10, outside the region R, the core portion 31 may include a circular portion in a cross section in the short side direction. In the example of FIG. 10, one end of the core portion 31 in the longitudinal direction is circular. With such a shape, one end of the core portion 31 in the longitudinal direction can be easily optically coupled to an optical fiber. In this application, circular means approximately circular, and does not mean strictly a perfect circle. Therefore, it may deviate from a perfect circle within a range that does not substantially impair a predetermined effect as an optical waveguide.

[0066] To make the cross-sectional shape of the core portion 31 circular, for example, in the mosquito method, the distance in the Z direction from the first surface 10a of the substrate 10 to the tip of the needle-shaped portion 92 can be made longer than when the core portion 31 is made semicircular, etc.

[0067] Even if core portion 31 is composed of only non-tapered portion 31d, as long as it is outside region R, core portion 31 may include a circular portion when viewed in cross section in the short side direction.

[0068] Furthermore, even if the first waveguide 20 does not have a tapered portion, the core 31 of the second waveguide 30 may have a tapered portion 31c. Furthermore, the tapered portion 31c may be disposed only outside the region R. Furthermore, the core 31 may include a tapered portion whose width and thickness gradually increase toward one side in the longitudinal direction (the positive side in the X direction).

[0069] FIG. 11 is a cross-sectional view illustrating an optical waveguide device according to Modification 2 of the first embodiment, showing a cross section corresponding to FIG. 1(c).

[0070] 11 differs from the optical waveguide device 1 in that the first waveguide 20 is embedded in the substrate 10. That is, the upper surface, lower surface, and side surfaces of the first waveguide 20 are covered by the substrate 10. The upper surface of the first waveguide 20 may be exposed in the same plane as the first surface 10a of the substrate 10, and the lower surface and side surfaces may be covered by the substrate 10.

[0071] In this way, the first waveguide 20 does not need to protrude from the first surface 10a of the substrate 10. Even in this case, by providing the core portion 31 with the bottom surface 31a that contacts the first surface 10a and the convex surface 31b that connects to the bottom surface 31a, the core portion 31 and the first waveguide 20 can maintain high optical coupling efficiency even if the position of the core portion 31 is shifted in the width direction of the first waveguide 20. As a result, it is possible to improve the product yield and realize a stable optical waveguide device 1B with small variation in optical coupling efficiency between products.

[0072] In addition, when the first waveguide 20 is embedded in the substrate 10, the optical coupling efficiency between the first waveguide 20 and the core 31 can be improved by making the cross-sectional shape of the core 31 flatter than when the first waveguide 20 is not embedded in the substrate 10. That is, by making the cross-sectional shape of the core 31 flatter, the confinement of light in the thickness direction of the core 31 is weakened, and the optical intensity of the propagation mode in the core 31 seeps deeper into the substrate 10 (evanescent light). The greater the overlap between the evanescent light and the first waveguide 20, the easier it is for optical transition to occur between the core 31 and the first waveguide 20 due to mode coupling, and the optical coupling efficiency is improved.

[0073] Although the preferred embodiments have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims. [Explanation of symbols]

[0074] 1,1A,1B Optical waveguide device 10 Substrate 10a 1st page 20 First Waveguide 30 Second Waveguide 31 Core 31a Bottom 31b Convex 31c Tapered section 31d Non-tapered section 32 Clad section 70 Support 80 Frame 90 Discharge part 91 Discharge unit body 92 Needle

Claims

1. A substrate; A first waveguide provided on the substrate; a second waveguide provided on the first surface of the substrate; the second waveguide includes a core portion and a clad portion covering the periphery of the core portion, the core portion includes a region overlapping with the first waveguide when viewed from a normal direction of the first surface, In the region, the core portion includes a bottom surface in contact with the first surface and a convex surface connected to the bottom surface, The optical waveguide device, wherein the core portion includes a portion whose thickness gradually decreases from the center to the outer periphery in a cross section in the short side direction.

2. The optical waveguide device according to claim 1 , wherein the core portion is laterally elongated and has a thickness smaller than a width in a cross section taken along a short side direction.

3. The optical waveguide device according to claim 1 , wherein the convex surface is a curved surface.

4. 4. The optical waveguide device according to claim 1, wherein in the region, when viewed from a direction normal to the first surface, the first waveguide includes a portion whose width gradually decreases toward one side in the longitudinal direction.

5. The optical waveguide device according to claim 1 , wherein the core portion includes a portion whose width gradually decreases or increases toward one side in a longitudinal direction when viewed from a normal direction of the first surface.

6. The optical waveguide device according to claim 5 , wherein the thickness of the portion gradually decreases or increases toward one side in the longitudinal direction.

7. The optical waveguide device according to claim 1 , wherein the core portion includes a circular portion outside the region when viewed in cross section in the short side direction.

8. The optical waveguide device according to claim 1 , wherein the first waveguide is embedded in the substrate.

9. 4. The optical waveguide device according to claim 1, wherein the first waveguide is a waveguide using silicon, silicon nitride, gallium arsenide, or lithium niobate.

10. The optical waveguide device according to claim 1 , wherein the second waveguide is a polymer waveguide.

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