Optical waveguide and waveguide mounting board

Abstract

To restrain deformation of a core of an optical waveguide.SOLUTION: An optical waveguide 1 includes a lower clad 21, a core 3 formed on the lower clad 21, and an upper clad 22 formed on the lower clad 21 and the core 3, and one portion of the core 3 in an incidence side or an emission side of a light signal is exposed. The optical waveguide 1 is composed of an upper clad non-formation area and an upper clad formation area. A core exposed section 3a in which an upper surface 31 of the core 3 is exposed is located in the upper clad non-formation area. A core non-exposed section in which the upper surface 31 of the core 3 is not exposed is located in the upper clad formation area. The sectional shape of the core 3 in the core exposed section 3a is a trapezoid, and an angle (θ) of an interior angle in the side of the lower clad 21 in the trapezoid satisfies a relationship of 80°<θ<90°.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide and a waveguide-mounted substrate. [Background technology]

[0002] Patent Document 1 discloses a polymer waveguide array formed on a polymer film and a silicon waveguide array formed on a silicon chip. The core of the polymer waveguide, which has a rectangular cross section, and the core of the silicon waveguide are arranged to overlap over a predetermined distance in the optical axis direction and are optically coupled by adiabatic coupling. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-81587 Summary of the Invention [Problem to be solved by the invention]

[0004] In the technology disclosed in Patent Document 1, a silicon chip on which a silicon waveguide is formed is adiabatically coupled to a polymer waveguide in a face-down configuration. The cores of the polymer waveguide and the silicon waveguide may be spaced apart at an extremely small distance, for example, on the order of a few micrometers. This may result in stress being transmitted to the core of the polymer waveguide, causing deformation of the polymer waveguide core. For example, thermal stress may be generated during use, and this stress may be transmitted to the core of the polymer waveguide, causing deformation of the polymer waveguide core. Furthermore, when the silicon waveguide is mounted, it may come into contact with the core of the polymer waveguide, and the force may be transmitted to the core of the polymer waveguide, causing deformation of the polymer waveguide core. It is believed that core deformation reduces the efficiency of the optical coupling between the polymer waveguide and the silicon waveguide. [Means for solving the problem]

[0005] The optical waveguide of the present invention includes a lower cladding, a core formed on the lower cladding, and an upper cladding formed on the lower cladding and the core, and a part of the core is exposed on the optical signal input side or output side. The optical waveguide includes an upper cladding non-formed region and an upper cladding formed region, the upper cladding non-formed region has a core exposed portion where the top surface of the core is exposed, the upper cladding formed region has a core unexposed portion where the top surface of the core is not exposed, the cross section of the core in the core exposed portion is trapezoidal, and the interior angle θ of the trapezoid on the lower cladding side satisfies the relationship of the following formula 1. 80°<θ<90° (Formula 1)

[0006] The waveguide-mounted substrate of the present invention includes a wiring board including an insulating layer and a conductor layer stacked on the insulating layer, and the optical waveguide described above disposed on the wiring board.

[0007] According to the embodiment of the present invention, even if stress generated by, for example, heat generation during use or contact with an external optical component propagates to the core of the optical waveguide, the stress is dispersed because the cross-sectional shape of the core is trapezoidal. Therefore, deformation of the core of the optical waveguide is suppressed. Therefore, it is presumed that the optical waveguide of the embodiment can be optically coupled to an optical component with good coupling efficiency. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a plan view showing an example of an optical waveguide according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the optical waveguide of FIG. 1 taken along line II-II. [Figure 3] FIG. 3 is a cross-sectional view of the optical waveguide of the example of FIG. 1 taken along line III-III. [Figure 4] 4 is a cross-sectional view of the optical waveguide of FIG. 1 taken along line IV-IV. [Figure 5] FIG. 4 is an enlarged view of a cross section of a core in a core exposed portion in the optical waveguide according to the embodiment. [Figure 6] 5A and 5B are diagrams showing a method for measuring the interior angle of the trapezoidal cross-sectional shape of the core of the optical waveguide according to the embodiment. [Figure 7] FIG. 10 is a cross-sectional view showing a first modified example of the shape of the core of the core exposed portion of the optical waveguide according to the embodiment. [Figure 8] 8 is a diagram showing a method for measuring the length of the rounded portion of the core in the first modified example of FIG. 7. [Figure 9] FIG. 10 is a cross-sectional view showing a second modified example of the shape of the core in the core exposed portion of the optical waveguide according to the embodiment. [Figure 10] FIG. 10 is a plan view showing a third modified example of the shape of the core of the core exposed portion of the optical waveguide according to the embodiment. [Figure 11] 1 is a cross-sectional view showing an example of a waveguide mounted substrate according to a first embodiment of the present invention. [Figure 12] FIG. 10 is a cross-sectional view showing an example of a waveguide mounted substrate according to a second embodiment of the present invention. [Figure 13] FIG. 10 is a plan view showing an example of a waveguide mounted substrate according to a second embodiment of the present invention. [Figure 14] FIG. 10 is a plan view showing a modified example of the waveguide mounting substrate according to the embodiment of the present invention. [Figure 15A] 5A to 5C are plan views showing an example of a manufacturing process of the optical waveguide according to the embodiment. [Figure 15B] 5A to 5C are front views showing an example of a manufacturing process of the optical waveguide according to the embodiment. [Figure 15C] 5A to 5C are front views showing an example of a manufacturing process of the optical waveguide according to the embodiment. [Figure 15D] 15D is a cross-sectional view of the optical waveguide of FIG. 15C taken along line XVD-XVD. [Figure 16] 5A to 5C are cross-sectional views showing an example of a manufacturing process of the waveguide mounted substrate according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] The optical waveguide and the waveguide mounting substrate according to the embodiments of the present invention will be described with reference to the drawings. In each of the drawings referred to in the following description, certain parts may be drawn enlarged to facilitate understanding of the disclosed embodiments. Therefore, the size and length of each component may not be drawn to exact proportions.

[0010] <Structure of optical waveguide according to embodiment> Fig. 1 shows a plan view of an optical waveguide 1, which is an example of an optical waveguide according to an embodiment. Fig. 2 shows a cross section of the optical waveguide 1 taken along line II-II in Fig. 1. Fig. 3 shows a cross section of the optical waveguide 1 taken along line III-III in Fig. 1, and Fig. 4 shows a cross section of the optical waveguide 1 taken along line IV-IV in Fig. 1. Note that the optical waveguide 1 illustrated in Fig. 1 and other figures is merely an example of an optical waveguide according to an embodiment. The structure of the optical waveguide according to the embodiment is not limited to the structure shown in each drawing, such as Fig. 1.

[0011] 1 to 4, an optical waveguide 1 according to an embodiment includes a core 3 that transmits an optical signal, and a clad 2 that surrounds the core 3. The clad 2 is composed of a lower clad 21 and an upper clad 22. The core 3 is formed on the lower clad 21. The upper clad 22 is formed on the lower clad 21 and the core 3. That is, in the optical waveguide 1, the lower clad 21, the upper clad 22, and the core 3 are formed in this order: lower clad 21, core 3, upper clad 22.

[0012] The core 3 has an upper surface 31 facing in the direction along which the cladding 2 and the core 3 are formed, and a lower surface 32 opposite the upper surface 31. The direction along which the cladding 2 and the core 3 are formed is also referred to as the "Z direction" hereinafter. The upper surface 31 may face in either of two directions along the Z direction. In the optical waveguide 1, as shown in FIG. 2, the upper surface 31 of the core 3 is a surface facing in the +Z direction, and the lower surface 32 is a surface facing in the -Z direction. Hereinafter, in the optical waveguide 1, the lower cladding 21 side is also referred to as the "lower side" or simply "bottom", and the upper cladding 22 side is also referred to as the "upper side" or simply "top".

[0013] The lower cladding 21 is located on the lower surface 32 side of the core 3 and is in contact with the lower surface 32. The upper cladding 22 is located on the upper surface 31 side of the core 3 and is in contact with the upper surface 31. The upper cladding 22 covers the upper surface 31 of the core 3. The upper cladding 22 also covers the upper surface 211 of the lower cladding 21. In the optical waveguide 1, as shown in FIG. 4, the upper cladding 22 also covers the side surface 35 of the core 3. The side surface 35 is a surface that connects the upper surface 31 and the lower surface 32 and extends along the direction in which the core 3 extends.

[0014] In the optical waveguide 1 in FIGS. 1 and 2, the core 3 extends along the +X direction and the −X direction. An optical signal propagating through the core 3 propagates in the +X direction or the −X direction. Hereinafter, the propagation direction of the optical signal, the +X direction and the −X direction, will be collectively referred to as simply the “X direction.” The optical waveguide 1 has two opposing ends, one end 11 and the other end 12. The one end 11 and the other end 12 face each other in the X direction. In the optical waveguide 1, an optical signal is incident on the one end 11 or the other end 12, and the optical signal is emitted from the other end 12 or the one end 11. When an optical signal is incident on the one end 11, the optical signal is emitted from the other end 12. When an optical signal is incident on the other end 12, the optical signal is emitted from the one end 11.

[0015] Therefore, in the optical waveguide 1, an optical signal may be incident on one end 11 and may be emitted from one end 11. Similarly, an optical signal may be incident on the other end 12 and may be emitted from the other end 12. In other words, both the one end 11 and the other end 12 are the incident and output sides of the optical signal in the optical waveguide 1. In the optical waveguide 1, a portion of the core 3 on the one end 11 side, which is the incident and output side of the optical signal, is exposed. Furthermore, a portion of the core 3 on the other end 12 side, which is the incident and output side of the optical signal, is exposed. In this way, in the optical waveguide 1, a portion of the core 3 is exposed on both the one end 11 side and the other end 12 side. As shown in FIGS. 2 and 3 , at one end 11, the top surface 31 of the core 3, one end face 33 of the core 3 in the X direction, and a side surface 35 are exposed from the cladding 2. 1 and 2, at the other end 12, an end face 34 of the core 3 in the X direction, which is opposite to the end face 33, is exposed from the cladding 2.

[0016] As shown in FIGS. 1 and 2, the optical waveguide 1 is composed of an upper cladding-free region 1a, which is a region where the upper cladding 22 is not formed in a planar view, and an upper cladding-formed region 1b, which is a region where the upper cladding 22 is formed in a planar view. That is, the optical waveguide 1 comprises the upper cladding-free region 1a and the upper cladding-formed region 1b. The optical waveguide 1 has the upper cladding-formed region 1b adjacent to the upper cladding-free region 1a, and the upper cladding region 1b is located on the other end 12 side of the upper cladding-free region 1a. As in the optical waveguide 1, the entire other end 12 side of the upper cladding-free region 1a may be the upper cladding-formed region 1b. In a planar view, the upper surface 31 of the core 3 and the upper surface 211 of the lower cladding 21 are exposed in the upper cladding-free region 1a. In the example of FIG. 1, the side surface 35 of the core 3 is also exposed in the upper cladding-free region 1a. Note that "planar view" means viewing an object from a line of sight along the Z direction.

[0017] In the optical waveguide 1, the upper cladding non-forming region 1a has an exposed core portion 3a where the upper surface 31 of the core 3 is exposed. In the optical waveguide 1, the upper cladding non-forming region 1a is provided on the one end 11 side. Therefore, as shown in Figures 1 and 2, the exposed core portion 3a is located on the one end 11 side of the optical waveguide 1. The exposed core portion 3a is a part of the core 3 on the one end 11 side of the optical waveguide 1. In the upper cladding forming region 1b, there is an unexposed core portion 3b where the upper surface 31 of the core 3 is not exposed. The unexposed core portion 3b is a part of the core 3 on the other end 12 side of the optical waveguide 1.

[0018] In the optical waveguide of the embodiment, the upper cladding non-forming region 1a may be provided at both ends of the optical waveguide. That is, the upper cladding non-forming region 1b may be disposed between two upper cladding non-forming regions 1a in the X direction. In the optical waveguide of the embodiment, the upper cladding non-forming region 1a is provided at at least one end of the optical waveguide. For example, the upper cladding non-forming region 1a is provided at one end 11 and / or the other end 12 in FIG. 1. Therefore, the core exposure portion 3a is also disposed at at least one end of the optical waveguide.

[0019] The core 3 and cladding 2 that form the optical waveguide 1 are made of any light-transmitting material. The optical waveguide 1 can be made of, for example, an organic material, an inorganic material, or a hybrid material containing an organic material and an inorganic material, such as an inorganic polymer. Examples of organic materials include acrylic resins such as polymethyl methacrylate (PMMA), polyimide resins, polyamide resins, polyether resins, and epoxy resins, while examples of inorganic materials include quartz glass and silicon. An optical waveguide 1 made of an organic material can be lightweight, highly tough, and flexible.

[0020] The core 3 and the clad 2 may be made of different materials or the same material. However, the core 3 is made of a material with a higher refractive index than the material used for the clad 2 so that the optical signal can be totally reflected at the interface between the core 3 and the clad 2. The core 3 and the clad 2 may be formed of materials with the same refractive index and then subjected to appropriate processing to make their refractive indices different. That is, the optical waveguide 1 may be formed using a forming method called photolithography or photobleaching, for example. The optical waveguide 1 may be formed on a support that supports the optical waveguide 1 during the manufacturing process and then separated from the support, or may be used together with the support. The optical waveguide 1 may be disposed on a wiring board. In this case, the optical waveguide 1 may be formed on the wiring board, or a pre-formed optical waveguide 1 may be placed on the wiring board.

[0021] The thickness of the core 3 is not particularly limited, but is 1 μm or more and 20 μm or less, and preferably 3 μm or more and 10 μm or less. The thickness of the core 3 is determined as the average value of values ​​measured at three points in the Z direction.

[0022] The thickness of the lower cladding 21 is not particularly limited, but is 5 μm to 100 μm, preferably 10 μm to 50 μm. The thickness of the upper cladding 22 is not particularly limited, but is 5 μm to 100 μm, preferably 10 μm to 50 μm. The thickness of the lower cladding 21 and the thickness of the upper cladding 22 are determined as the average value of values ​​measured at three points in the Z direction.

[0023] When the optical waveguide 1 is in use, the core 3 is optically coupled at one end 11 and the other end 12 to optical components such as a photoelectric conversion component, such as a semiconductor device including a photoelectric conversion element, and / or a connector member, such as an optical fiber or an optical connector, that connects the waveguide to the outside. In other words, the core 3 is positioned with respect to each optical component so that a positional relationship is established that allows transmission and reception of optical signals between the core 3 and these optical components. Note that "optically coupled" is also referred to as "optically coupled" hereinafter.

[0024] 1 to 3, a component E1 including a photoelectric conversion element (not shown) is indicated by a two-dot chain line as an example of an optical component optically coupled to the core 3 at one end 11. When the optical waveguide 1 is in use, the region between the component E1 and the portion of the optical waveguide 1 that overlaps with the component E1 in a plan view is preferably filled with any optically transparent transmissive resin TR. The transmissive resin TR adjusts the refractive index of the space between the component E1 and the optical waveguide 1 to a more appropriate refractive index than that of air.

[0025] The length L1 of the core 3 in the core exposed portion 3a is not particularly limited, but is preferably 100 μm or more and 3000 μm or less, and more preferably 100 μm or more and 2500 μm or less. If the core exposed portion 3a has a length in this range, it is considered that there is a high degree of freedom in selecting optical components such as the component E1 optically coupled to the core exposed portion 3a. It is also considered that a necessary and sufficient tolerance can be obtained for the alignment of the optical components and the optical waveguide 1 in the X direction.

[0026] The component E1 includes an optical terminal E1a, which is the portion where an optical signal enters the component E1 or the portion where an optical signal exits the component E1. The component E1 is optically coupled to the core 3 at the optical terminal E1a. An optical signal propagating through the core 3 from the other end 12 enters the component E1 via the optical terminal E1a at one end 11. Meanwhile, an optical signal exiting from the optical terminal E1a of the component E1 enters the core 3 at the one end 11, propagates through the core 3, and exits from the other end 12.

[0027] The exposed core portion 3a of the optical waveguide 1 is a portion that overlaps with the optical terminal E1a of the component E1 and transmits and receives optical signals. In the example of FIGS. 1 to 3, the core 3 is positioned at one end 11 of the optical waveguide 1 so that an upper surface 31 of the exposed core portion 3a faces the optical terminal E1a of the component E1 and is adiabatic coupled. That is, a portion of the optical signal that has propagated through the core 3 toward the one end 11 leaks out of the core 3 from the upper surface 31 as evanescent light and enters the optical terminal E1a of the component E1. Because the upper surface 31 faces the optical terminal E1a of the component E1 without the cladding 2 interposed therebetween, it is believed that highly efficient optical coupling is achieved.

[0028] The optical waveguide 1 has four parallel cores 3. The optical waveguide 1 of the embodiment is not limited to four and can have any number of cores 3, one or more. For example, the number of cores 3 is in the range of 2 to 64. When multiple cores 3 are provided, the arrangement pitch P1 of the cores 3 is not particularly limited, but is, for example, 10 μm to 300 μm, and preferably 20 μm to 250 μm. Note that the arrangement pitch P1 of the multiple cores 3 is not limited to these numerical examples. In the example of FIG. 1 , the arrangement pitch P1 of the multiple cores 3 is constant between one end 11 and the other end 12. The arrangement pitch P1 of the cores 3 may vary between the one end 11 and the other end 12. The arrangement pitch P1 of the cores 3 is the distance between the center of one core 3 and the center of the other core 3 of two adjacent cores 3.

[0029] The core 3 of the optical waveguide 1 has a width W1 at the core exposed portion 3a. The core 3 has a width W2 at the core unexposed portion 3b. In the optical waveguide 1 shown in FIG. 1 and other figures, the widths W1 and W2 are approximately equal. That is, the core 3 of the optical waveguide 1 has an approximately constant width from one end 11 to the other end 12 of the optical waveguide 1. The widths W1 and W2 may have a relationship of W1 = W2. However, in the optical waveguide of the embodiment, the width of the core at the core exposed portion 3a may be different from the width of the core at the core unexposed portion 3b, as in the optical waveguide 1a of FIG. 10, which will be referred to later.

[0030] The "width" of the core 3, such as width W1 and width W2, refers to the length of the core 3 at the bottom surface 32 in a direction perpendicular to the propagation direction of the optical signal propagating within the core 3, which is the X direction in FIG. 2, and the Z direction, which is the lamination direction of the optical waveguide 1. The direction along the width of the core 3 is also referred to as the +Y direction or -Y direction (see FIG. 1) below. The +Y direction and -Y direction are also collectively referred to simply as the "Y direction." If the core 3 is bent along its length, the propagation direction of the optical signal within the core 3 is not constant but changes depending on the position within the core 3. Therefore, the direction along the width of the core 3 may change from one end of the core 3 to the other.

[0031] In the optical waveguide of the embodiment, as shown in FIG. 3 , the cross-sectional shape of the core 3 in the core exposed portion 3a is trapezoidal. That is, both of the two opposing side surfaces 35 in the cross-sectional shape of the core 3 are inclined so that the upper surface 31 is located closer to the inside of the core 3 than the lower surface 32. Because the cross-sectional shape of the core 3 in the core exposed portion 3a is trapezoidal, even if stress generated, for example, by heat generation during use or contact with an external optical component propagates to the core 3, it is thought that the stress is dispersed. As a result, deformation of the core 3 may be suppressed. Furthermore, stress within the core 3 caused by the placement of the core exposed portion 3a in close proximity to the optical terminal E1a of the component E1 may be alleviated. As a result, deformation of the core 3 may be suppressed. Note that, as shown in FIG. 4 , the cross-sectional shape of the core 3 in the core non-exposed portion 3b is a rectangle or square with two opposing side surfaces substantially parallel. However, in the optical waveguide of the embodiment, the cross-sectional shape of the core 3 in the core non-exposed portion 3b may be trapezoidal. In addition, in the cross-sectional shape of the core 3, one of the two opposing side surfaces 35 may be perpendicular to the lower surface 32.

[0032] <Cross-sectional shape of the core at the exposed core part> The trapezoidal cross-sectional shape of the core 3 in the core exposed portion 3a will be described with reference to Figures 5 and 6. Figure 5 shows an enlarged view of part V in Figure 3 as an example of the cross-sectional shape of the core in the core exposed portion of the optical waveguide of the embodiment. Figure 6 is a schematic diagram showing a method for measuring the interior angle of the trapezoidal cross-sectional shape of the core in the optical waveguide of the embodiment.

[0033] As shown in FIG. 5 , the cross-sectional shape of the core 3 at the exposed core portion 3a of the optical waveguide 1 is a trapezoid. That is, the cross-sectional shape of the core 3 at the exposed core portion 3a has four sides along the top surface 31, the bottom surface 32, and the two side surfaces 35, and the angle θ of the interior angle IA at both ends of the bottom surface 32 is larger than the angle of the interior angle at both ends of the top surface 31. Here, the “trapezoid” shape of the cross-section of the core 3 need only be a shape surrounded by a pair of substantially parallel opposite sides and another pair of opposite sides that narrow toward one of the pair of opposite sides, and does not need to have four vertices like a general rectangle. The trapezoidal cross-sectional shape of the core 3 is surrounded by an upper base that represents the top surface 31 of the core 3, a lower base that represents the bottom surface 32 of the core 3, and two legs that represent the two opposing side surfaces 35 of the core 3. The distance between the two opposing side surfaces 35 of the core 3 increases from the top surface 31 to the bottom surface 32 of the core 3. That is, the cross section of the core 3 has a tapered shape in which the width W decreases toward the top surface 31.

[0034] The advantages of the core 3 having a trapezoidal cross-sectional shape at the core exposed portion 3a are described below. As shown in FIGS. 1 to 3 and 5, when the optical waveguide 1 is in use, the core exposed portion 3a is positioned to face the optical terminal E1a of the component E1. For efficient optical coupling, the core exposed portion 3a is positioned very close to the optical terminal E1a of the component E1. For example, the core exposed portion 3a and the optical terminal E1a of the component E1 may be positioned with a design intent to leave a gap of only about 3 μm between them. The component E1 may include an element that generates heat during use. In this case, if the optical terminal E1a of the component E1 and the core 3 of the optical waveguide 1 are close to each other, stress caused by expansion or contraction of the component E1 due to heat generation during use may be transmitted to the core 3. Furthermore, when the component E1 and / or the optical waveguide 1 are positioned, the optical terminal E1a of the component E1 may come into contact with the core 3 of the optical waveguide 1. Therefore, it is conceivable that the optical waveguide 1 may be used while the optical terminal E1a of the component E1 remains in contact with the core 3 of the optical waveguide 1. Such contact between the optical terminal E1a of the component E1 and the core 3 of the optical waveguide 1 may cause stress at the contact point, and this stress may be transmitted to the core 3.

[0035] When stress is transmitted to the core, if the core has a rectangular cross-sectional shape like the conventional core described in Patent Document 1, deformation of the core may occur. In contrast, in the optical waveguide of the embodiment, the core 3 has a trapezoidal planar shape, thereby suppressing deformation of the core 3. Specifically, the cross-sectional shape of the core 3 at the core exposed portion 3a is a trapezoid whose width W increases toward the lower surface 32 of the core 3. Therefore, even if stress is transmitted to the core 3 due to contact with the component E1 or heat generation from the component E1, the stress is not concentrated near the upper surface 31 of the core 3 but is easily dispersed toward the lower surface 32. Therefore, deformation of the core 3 due to stress caused by contact with the component E1 or heat generation from the component E1 is suppressed. Because deformation of the core 3 is suppressed, the optical waveguide of the embodiment is considered to enable efficient optical coupling between the core 3 and the optical terminal E1a of the component E1.

[0036] Furthermore, in the optical waveguide 1, the core 3 has a trapezoidal cross-sectional shape, which may improve the resin filling efficiency of the transparent resin TR in the region between the optical waveguide 1 and the component E1. That is, filling of the region between the optical waveguide 1 and the component E1 with the transparent resin TR is usually performed after the optical waveguide 1 and the component E1 are positioned in predetermined positions with a small gap between them. The resin filling is usually performed by infiltrating the transparent resin TR into the region between the optical waveguide 1 and the component E1 in the direction indicated by arrow A1 in FIG. 5. In the optical waveguide 1, the cross-sectional shape of the core 3 at the core exposed portion 3a is a trapezoid whose width W decreases toward the top surface 31 of the core 3. This means that the transparent resin TR supplied near the side surface 35 of the core 3 easily penetrates into the gap between the core 3 and the optical terminal E1a of the component E1, for example, by capillary action. Therefore, it is considered that the region between the optical waveguide 1 and the component E1 can be sufficiently filled with the transparent resin TR without leaving any unfilled portions such as voids. Therefore, according to this embodiment, the optical waveguide 1 and the component E1 can face each other through a region filled with the transparent resin TR and having an appropriate refractive index. In other words, it is considered that the core 3 and the optical terminal E1a of the component E1 can be optically coupled well.

[0037] In the optical waveguide 1, the core 3 has a trapezoidal cross-sectional shape, and therefore the two interior angles IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of the core 3 have an angle θ that is smaller than 90°. The angle θ of the interior angles IA in the cross-sectional shape of the core 3 satisfies the relationship of the following formula 1. 80°<θ<90° (Formula 2)

[0038] When the angle θ of the interior angle IA on the lower cladding 21 side in the trapezoidal cross-sectional shape of the core 3 is smaller than 90°, the above-mentioned effect of dispersing the stress transmitted to the core 3 is obtained, and it is presumed that the intended effect of suppressing deformation of the core 3 is achieved. Furthermore, when the angle θ of the interior angle IA is larger than 80°, it is considered that the area on the upper surface 31 of the core 3 directly facing the optical terminal E1a of the component E1 is easily secured to a size required for good optical coupling. In the cross-sectional shape of the core 3, the angles θ of the two interior angles IA may be equal to or different from each other. Note that in the cross-sectional shape of the core 3, one of the interior angles IA at both ends of the lower surface 32 may be a right angle.

[0039] A method for measuring the angle θ of the interior angle IA of the cross-sectional shape of the core 3 in the core exposed portion 3a will be described with reference to Fig. 6. Fig. 6 schematically shows an image of the interior angle IA in the cross section of the core 3 in the core exposed portion of the optical waveguide according to the embodiment, observed with a scanning electron microscope (SEM × 5000). In measuring the angle θ of the interior angle IA, an imaginary line IS2 extending and overlapping the side surface 35 of the core 3 and an imaginary line IS1 extending and overlapping the bottom surface 32 of the core 3 are set on the observation image of the interior angle IA as shown in Fig. 6, which was taken with the SEM at a magnification of 1000 times. Then, the angle between the imaginary lines IS2 and IS1 is measured as the angle θ of the interior angle IA using the SEM photograph.

[0040] As an example of an embodiment, the width of core 3 is 5 μm, the thickness of core 3 is 5 μm, the angle of one interior angle IA of core 3 (θ1): 84.1°, the angle of the other interior angle IA of core 3 (θ2): 86.2°, the arrangement pitch of core 3: 50 μm, the thickness of lower cladding 21: 30 μm, the thickness of upper cladding 22: 20 μm, and the length L1 of core 3 of exposed core portion 3a: 2000 μm.

[0041] <First modified example of the core shape of the core exposed portion> FIG. 7 shows a first modified example of the cross-sectional shape of the core 3 at the core exposed portion 3a in the optical waveguide of the embodiment. In the first modified example shown in FIG. 7, the cross-sectional shape of the core 3 at the core exposed portion 3a is also trapezoidal. The corners 30 of the cross-sectional shape of the first modified example of the core 3 shown in FIG. 7 are rounded. The corners 30 are portions connecting the top surface 31 of the core 3 exposed at the core exposed portion 3a to the side surface 35 of the core 3. In other words, the corners 30 connecting the top surface 31 of the core 3 to the side surface 35 of the core 3 are rounded. The core 3 at the core exposed portion 3a does not have an angular boundary between the top surface 31 and the side surface 35. It can be said that the top surface 31 of the core 3 and the side surface 35 of the core 3 are connected via a belt-like curved surface that has a given width between the top surface 31 and the side surface 35 and extends along the propagation direction of the optical signal within the core 3. Therefore, even in the cross section of core 3 shown in Fig. 7, there is no angular boundary between the line indicating top surface 31 and the line indicating side surface 35. In other words, in the cross section shown in Fig. 7, the line indicating top surface 31 and the line indicating side surface 35 do not directly contact each other, but are connected via an arc-shaped curve.

[0042] In other words, in the core exposed portion 3a, the core 3 does not have a tangent line where the top surface 31 and the side surface 35 meet at a specific angle. Therefore, in Fig. 7, there is no intersection where a line representing the top surface 31 of the core 3 intersects with a line representing the side surface 35 at a specific angle, or no tangent point where a line representing the top surface 31 and a line representing the side surface 35 meet at a specific angle. In other words, in the cross section of the core 3 in the core exposed portion 3a, there is no vertex between the top surface 31 and the side surface 35. For example, the shape of the corner 30 of the core 3 in the core exposed portion 3a may be a shape obtained by R-chamfering with a specific curvature.

[0043] In the first modified example shown in FIG. 7 , the corners 30 of the core 3 in the core exposed portion 3a are rounded rather than angular, thereby suppressing deformation of the core 3. That is, even if stress is transmitted to the core 3 due to contact with the component E1 or heat generation from the component E1 as described above, the corners 30 of the cross section of the core 3 in the core exposed portion 3a are rounded, so the stress is not concentrated at the corners 30 but is easily dispersed to portions other than the corners 30. Therefore, it is considered that deformation of the core 3 due to stress caused by contact with the component E1 or heat generation from the component E1 is suppressed. Because deformation of the core 3 is suppressed, it is considered that the optical waveguide 1 of the embodiment including the core 3 having corners 30 that are rounded in the cross section can optically couple the core 3 and the optical terminal E1a of the component E1 with good efficiency.

[0044] Furthermore, in the optical waveguide 1 of the embodiment including the core 3 having the cross-sectional shape of the first modification shown in FIG. 7 , the corners 30 of the core 3 are rounded in the core exposed portion 3a, which may improve the resin filling efficiency of the transparent resin TR in the region between the optical waveguide 1 and the component E1. That is, the resin filling of the region between the optical waveguide 1 and the component E1 with the transparent resin TR is typically performed by infiltrating the region between the optical waveguide 1 and the component E1 in the direction indicated by arrow A1 in FIG. 7 . When filling the resin, if the corners of the core cross-sectional shape are sharp, as in conventional optical waveguides, it is thought that the transparent resin TR will have difficulty penetrating the region between the optical waveguide and the component. In contrast, in the optical waveguide 1 including the core having the shape of the first modification shown in FIG. 7 , the corners 30 of the core 3 are rounded rather than sharp, so the core 3 is less likely to obstruct the penetration of the transparent resin TR. Therefore, it is considered that the region between the optical waveguide 1 and the component E1 can be sufficiently filled with the transparent resin TR without leaving any unfilled portions such as voids. Therefore, according to this embodiment, the optical waveguide 1 and the component E1 can face each other through a region filled with the transparent resin TR and having an appropriate refractive index. In other words, it is considered that the core 3 and the optical terminal E1a of the component E1 can be optically coupled well.

[0045] In the optical waveguide 1 of the embodiment, the length R of the rounded portion at the corner 30 of the cross-sectional shape of the core 3 is preferably 1.5 μm or more and 4.5 μm or less. That is, the length R of the rounded portion at the corner 30 may satisfy the relationship of the following formula 2. 1.5μm≦R≦4.5μm (Formula 2)

[0046] It is believed that when the rounded corners 30 of the core 3 have a length R of 1.5 μm or more, the above-mentioned effect of dispersing the stress transmitted to the core 3 is obtained, thereby achieving the intended effect of suppressing deformation of the core 3. Furthermore, when the rounded corners 30 of the core 3 have a length R of 4.5 μm or less, it is believed that the area on the top surface 31 of the core 3 directly facing the optical terminal E1a of the component E1 can be easily secured to a size required for good optical coupling. The lengths R of the rounded portions of the two corners 30 in the cross-sectional shape of the core 3 may be equal to or different from each other.

[0047] Referring to FIG. 8, a method for measuring the length R of the rounded portion of the corner 30 in the cross-sectional shape of the core 3 in the first modified example of FIG. 7 will be described. FIG. 8 schematically shows an SEM image of the corner 30 in the cross-section of the core 3 at the exposed core portion of the optical waveguide according to the embodiment. In measuring the length R of the rounded portion of the corner 30, an imaginary line IS5 extending and overlapping the top surface 31 of the core 3 and an imaginary line IS2 extending and overlapping the side surface 35 of the core 3 are set on the SEM image of the corner 30 as shown in FIG. 8 taken at 1000x magnification. Furthermore, an imaginary line segment IS3 is set between a point PS1 where the imaginary line IS5 and the top surface 31 begin to separate and a point PS2 where the imaginary line IS2 and the side surface 35 begin to separate. In addition, an imaginary line segment IS4 is assumed that is perpendicular to the imaginary line segment IS3 and connects the imaginary line segment IS3 to the vertex PS3 that is farthest from the imaginary line segment IS3 on the contour of the corner 30. Then, the length L of the imaginary line segment IS3 and the length H of the imaginary line segment IS4 are measured by SEM. The length R of the rounded portion of the corner 30 is calculated by the following formula A. R=((L / 2) 2 +H 2) / (2×H) (Formula A) As an example of the first modified example, the width of core 3 is 5 μm, the thickness of core 3 is 5 μm, the length (R1) of the rounded portion of one corner 30 of core 3 is 3.07 μm, the length (R2) of the rounded portion of the other corner 30 of core 3 is 3.01 μm, the angle (θ1) of one interior angle IA of core 3 is 83.2°, the angle (θ2) of the other interior angle IA of core 3 is 85.2°, the thickness of lower cladding 21 is 30 μm, the arrangement pitch of core 3 is 50 μm, the thickness of upper cladding 22 is 20 μm, and the length L1 of core 3 of the core exposed portion 3a is 2000 μm.

[0048] <Second modified example of the core shape of the core exposed portion> FIG. 9 shows a second modified example of the cross-sectional shape of the core 3 at the exposed core portion 3a in the optical waveguide of the embodiment. In the second modified example of FIG. 9, the cross-sectional shape of the core 3 is also trapezoidal. Therefore, in the second modified example of FIG. 9, the two interior angles IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of the core 3 are also smaller than 90°. Here, in the trapezoidal cross-sectional shape of the core 3, the angles of the two interior angles IA on the lower cladding 21 side are different from each other. That is, in a front view of the cross section of the core 3, the trapezoidal cross-sectional shape of the core 3 has a first interior angle IA1 at one end of the lower surface 32 and a second interior angle IA2 at the other end of the lower surface 32. The angle θ1 of the first interior angle IA1 and the angle θ2 of the second interior angle IA2 are different from each other. In the cross-sectional shape of the core 3 shown in FIG. 9, the angle θ1 of the first interior angle IA1 is smaller than the angle θ2 of the second interior angle IA2. That is, the side surface 35 of the core 3 that forms the first interior angle IA1 with the lower surface 32 is larger than the side surface 35 of the core 3 that forms the second interior angle IA2 with the lower surface 32, and is inclined toward the inside of the core 3. Note that "a front view of the cross section of the core 3" means that the cross section of the core 3 is viewed from a line of sight perpendicular to the cross section of the core 3. Note that in the cross-sectional shape of the core 3, one of the first interior angle IA1 and the second interior angle IA2 may be a right angle.

[0049] As in the second modification shown in FIG. 9 , even if the first interior angle IA1 and the second interior angle IA2 are different from each other, the stress transmitted to the core 3 is not concentrated at the corner 30 connected to the smaller interior angle IA, and is therefore likely to be dispersed to areas other than the corner 30. Furthermore, at the corner 30 connected to the larger interior angle IA, it is possible to ensure an area on the top surface 31 that can directly face the optical terminal E1a (see FIG. 3) of the component E1. Therefore, it may be possible to both suppress deformation of the core 3 and ensure a sufficient area that contributes to good optical coupling with the component 1. Furthermore, it is possible to effectively infiltrate the area between the core 3 and the component E1 by supplying the transparent resin TR (see FIG. 3) from the side of the smaller interior angle IA.

[0050] 9, both of the two corners 30 of the cross-sectional shape of the core 3 at the core exposed portion 3a are rounded, as in the first modified example of FIG. 7. Meanwhile, in the second modified example, the length R1 of the rounded portion of one corner 301 of the two corners 30 of the cross-sectional shape of the core 3 is different from the length R2 of the rounded portion of the other corner 302 of the two corners 30.

[0051] That is, in a front view of the cross section of the core 3, the corner 30 of the cross section of the core 3 at the core exposed portion 3a has one end of the upper surface 31 as a first corner 301 and the other end of the upper surface 31 as a second corner 302. The length R1 of the rounded portion of the first corner 301 and the length R2 of the rounded portion of the second corner 302 are different from each other. In the cross section of the core 3 shown in FIG. 9, the length R1 of the rounded portion of the first corner 301 is longer than the length R2 of the rounded portion of the second corner 302.

[0052] Even if the lengths of the rounded portions of the first corner 301 and the second corner 302 are different, as in the second modification, the stress transmitted to the core 3 is not concentrated at the corner 30 with the longer rounded portion, and is therefore likely to be dispersed to areas other than the corner 30. Furthermore, it is considered that the corner 30 with the shorter rounded portion can secure an area on the top surface 31 that can face the optical terminal E1a (see FIG. 3) of the component E1. Therefore, it may be possible to both suppress deformation of the core 3 and secure a sufficient area that contributes to good optical coupling with the component 1. It is also considered that supplying the transparent resin TR (see FIG. 3) from the corner 30 side with the longer rounded portion allows the transparent resin TR to penetrate well into the area between the core 3 and the component E1.

[0053] As an example of the second modified example, the width of core 3 is 5 μm, the thickness of core 3 is 5 μm, the length R1 of the rounded portion of first corner 301 of core 3 is 3.24 μm, the length R2 of the rounded portion of second corner 302 of core 3 is 3.11 μm, the angle θ1 of first interior angle IA1 of core 3 is 82.2°, the angle θ2 of second interior angle IA2 of core 3 is 88.2°, the thickness of lower cladding 21 is 30 μm, the arrangement pitch of core 3 is 50 μm, the thickness of upper cladding 22 is 20 μm, and the length L1 of core 3 of core exposed portion 3a is 2000 μm.

[0054] <Third modified example of the core shape of the core exposed portion> A third modified example of the core shape of the core exposed portion of the optical waveguide of the embodiment will be described with reference to FIG. 10 . FIG. 10 shows a plan view of an optical waveguide 1α including a third modified example of the core shape of the core exposed portion of the optical waveguide of the embodiment. As shown in FIG. 10 , the optical waveguide 1α, like the optical waveguide 1 of FIG. 1 , is composed of an upper cladding non-forming region 1a and an upper cladding forming region 1b. The upper cladding non-forming region 1a has an exposed core portion 3a where the upper surface of the core 3 is exposed. The upper cladding forming region 1b has an unexposed core portion 3b where the core 3 is not exposed. In the optical waveguide 1α of FIG. 10 , components having the same functions as those of the optical waveguide 1 of FIG. 1 are denoted by the same reference numerals as those in FIG. 1 or are omitted as appropriate, and repeated descriptions of these similar components will be omitted.

[0055] In the optical waveguide 1α, the width W1 of the core 3 at the exposed core portion 3a is different from the width W2 of the core 3 at the non-exposed core portion 3b. In the optical waveguide 1α, the width W1 of the core 3 at one end 11 is larger than the width W2 of the core 3 at the other end 12. Therefore, the difference between the width W1 of the exposed core portion 3a and the width We of the optical terminal E1a of the component E1 is larger than the difference between the width W2 of the non-exposed core portion 3b and the width We of the optical terminal E1a. Therefore, in the optical waveguide 1α, the tolerance for misalignment along the width of the core 3 when aligning the core 3 with an optical signal input / output portion of an optical component, such as the optical terminal E1a, is greater than when the width W2 of the non-exposed core portion 3b is the same at the exposed core portion 3a. Therefore, in the optical waveguide 1α, it is considered that alignment for appropriate optical coupling between the optical component and the core 3 is easy. Therefore, it is considered that high coupling efficiency can be easily achieved when coupling with an optical component. It is also believed that poor coupling due to misalignment between the core 3 and optical components such as component E1 is suppressed even with the thermal history during use.

[0056] Additionally, in the optical waveguide 1α, the unexposed core portion 3b has a width W2 smaller than the width W1, which may facilitate realizing a desired transmission mode in the unexposed core portion 3b. For example, in a light-guiding member such as the optical waveguide 1α, the cross-sectional size of an optical path, such as the core 3, is selected based on the refractive index difference between the material constituting the optical path and the material constituting the covering body, such as the cladding 2, surrounding the optical path. That is, by realizing a core 3 having a cross-sectional size appropriate for the refractive index of each material, an optical signal can be propagated in a desired transmission mode. In this regard, in the optical waveguide 1α, the core 3 is not formed over its entire length with the width W1 of the exposed core portion 3a, which has a large width for high coupling efficiency with optical components, but the unexposed core portion 3b has a width W2 smaller than the width W1. This is believed to facilitate realizing optical transmission in a mode requiring a small core cross-section. For example, single-mode optical transmission, which requires a core diameter of 3 to 10 μm, may be possible.

[0057] The width W1 of the core 3 in the exposed core portion 3a may be constant or may vary depending on the position in the X direction within the exposed core portion 3a. In the optical waveguide 1α in Fig. 10, the maximum width (W1m) of the width W1 of the core 3 in the exposed core portion 3a is wider than the width W2 of the core 3 in the unexposed core portion 3b. That is, in the optical waveguide 1α, the maximum width W1m of the exposed core portion 3a and the width W2 of the unexposed core portion 3b satisfy the relationship of the following formula B. W1m>W2...(Formula B)

[0058] An example of the dimensions of each part of the optical waveguide 1α including the third modified example of the shape of the core 3 in the core exposed portion 3a is shown in Fig. 10. The width W1 of the core exposed portion 3a and the width W2 of the core non-exposed portion 3b are not particularly limited, but it is desirable that the width W1 of the core 3 in the core exposed portion 3a and the width W2 of the core 3 in the core non-exposed portion 3b satisfy the relationship of the following formula C. 1.0<(W1 / W2)≦3.0 (Formula C)

[0059] The width W1 of the core 3 in the core exposed portion 3a is not particularly limited, but is preferably 3 μm or more and 30 μm or less, and more preferably 5 μm or more and 20 μm or less. If the core 3 in the core exposed portion 3a has a width in this range, it is thought that the effect of facilitating alignment with optical components such as component E1 can be obtained, and an excessively large area for optical coupling with the optical component is not required. The width W2 of the core unexposed portion 3b may be, for example, 1 μm or more and 10 μm or less.

[0060] The width of the core 3 is determined by the average value of the length measurements at three points in the X direction. As an example, the core width of the core exposed portion 3a may be a width that gradually increases or decreases from one end to the other end. When the width of the core exposed portion 3a varies, the core width W1 of the core exposed portion 3a is determined by the average value of the length measurements at three points within the core exposed portion 3a in the X direction. The core width of the core unexposed portion 3b may be constant or may vary. As an example, the core width of the core unexposed portion 3b may be a width that gradually increases or decreases from one end to the other end. When the width of the core unexposed portion 3b varies, the core width W2 of the core unexposed portion 3b is determined by the average value of the length measurements at three points within the core unexposed portion 3b in the X direction.

[0061] The ratio of the width to the thickness of the core 3 in the core exposed portion 3a is not particularly limited, but is preferably 1.0:1.0 to 4.0:1.0. The ratio of the width to the thickness of the core 3 in the core unexposed portion 3b is not particularly limited, but is preferably 0.9:1.0 to 1.4:1.0.

[0062] An example of a combination of dimensions of each part of the optical waveguide 1α is as follows: The width of the core 3 in the core exposed portion 3a is 10 μm, and the thickness of the core 3 is 7 μm. The width of the core 3 in the core non-exposed portion 3b is 7 μm, and the thickness of the core 3 is 7 μm. The thickness of the lower clad 21 may be 20 μm, and the thickness of the upper clad 22 may be 25 μm. Note that the width of the core 3, the thickness of the core 3, the length of the core exposed portion 3a, the thickness of the lower clad 21, and the thickness of the upper clad 22 exemplified here are merely examples, and the dimensions of each part of the core 3 and the thickness of each clad are not limited to the numerical values ​​exemplified here.

[0063] In the optical waveguide 1α shown in FIG. 10 , the core 3 has a connecting portion 3c between the exposed core portion 3a and the non-exposed core portion 3b, connecting the exposed core portion 3a and the non-exposed core portion 3b. As shown in FIG. 10 , the connecting portion 3c refers to the core 3 formed in the upper cladding non-forming region 1a between the exposed core portion 3a and the non-exposed core portion 3b in a plan view. Here, the connecting portion 3c may be tapered in a plan view, may be a straight line of a constant width, or may be a combination of a straight line of a constant width and a tapered portion. The tapered shape of the core 3 at the connecting portion 3c may be such that the width gradually increases linearly or smoothly on both sides from the non-exposed core portion 3b to the exposed core portion 3a, or may be such that the width gradually increases linearly on only one side, or may be such that the width gradually increases stepwise on both sides or only one side.

[0064] The core width W3 of the connecting portion 3c is smaller than the core width W1 of the core exposed portion 3a and is equal to or larger than the core width W2 of the core unexposed portion 3b. In other words, the core widths W1, W2, and W3 satisfy the relationship of the following formula D. Core width W1 > Core width W3 ≥ Core width W2 (Equation D)

[0065] It is considered that the width W3 of the core of the connection part 3c suppresses the unintentional leakage of the optical signal from the core exposed part 3a toward the core non-exposed part 3b. That is, in the optical waveguide 1α, since the width W1 of the core exposed part 3a is larger than the width W2 of the core non-exposed part 3b, the connection part 3c connecting the core exposed part 3a and the core non-exposed part 3b is considered to suppress the unintentional leakage of the optical signal from the core exposed part 3a toward the core non-exposed part 3b by having a tapered shape. The width W3 of the core of the connection part 3c is not particularly limited, but is preferably 1 μm to 30 μm.

[0066] The taper ratio ((W1 - W2) / L2) of the tapered shape of the core 3 is, for example, 1 or more and 2 or less. Note that L2 is the length of the tapered shape part of the connection part 3c in the X direction. When the connection part 3c has a taper ratio within this range, it may be possible to efficiently suppress the leakage of the optical signal from the core 3 within a limited length. At this time, the formation ratio RT of the tapered shape in the core 3 is preferably 0.01 < RT < 0.2, and more preferably 0.03 < RT < 0.15. Note that the formation ratio R of the tapered shape is the ratio of the length of the tapered shape part to the total length of the core 3.

[0067] As an example of the third modification, the width of the core 3 of the core exposed part 3a is 10 μm, the width of the core 3 of the core non-exposed part 3b is 5 μm, the thickness of the core 3 is 5 μm, the length (R1) of the rounded part of one corner 30 of the core 3 is 3.10 μm, the length (R2) of the rounded part of the other corner 30 of the core 3 is 3.20 μm, the angle θ of one inner angle IA of the core 3 is 81.2°, the angle θ of the other inner angle IA of the core 3 is 82.4°, the arrangement pitch of the core 3 is 50 μm, the thickness of the lower cladding 21 is 30 μm, the thickness of the upper cladding 22 is 20 μm, and the length of the core 3 of the core exposed part 3a is 2000 μm.

[0068] <Structure of the waveguide-mounted substrate of the embodiment> Next, a waveguide mount board according to an embodiment will be described with reference to the drawings. FIG. 11 shows a cross-sectional view of a waveguide mount board 100, which is an example of a waveguide mount board according to the first embodiment. Note that the waveguide mount board 100 shown in FIG. 11 and the waveguide mount boards 100α and 100β shown in FIGS. 12 and 13, which will be referred to later, are merely examples of the waveguide mount board according to the embodiment. The layered structure of the waveguide mount board according to the embodiment is not limited to the layered structure of the waveguide mount board 100, the waveguide mount board 100α, or the waveguide mount board 100β shown in FIGS. 11 to 13. Furthermore, the number of conductor layers and the number of insulating layers included in the waveguide mount board 100, the waveguide mount board 100α, or the waveguide mount board 100β are not limited to the number of conductor layers and the number of insulating layers included in the waveguide mount board 100, the waveguide mount board 100α, or the waveguide mount board 100β.

[0069] As shown in FIG. 11 , the waveguide-mounted substrate 100 includes a wiring substrate 110 and an optical waveguide 101 disposed on the wiring substrate 110. The wiring substrate 110 includes an insulating layer and a conductor layer laminated on the insulating layer. The wiring substrate 110 includes conductor layers 41 to 43 as conductor layers and insulating layers 51 and 52 as insulating layers. The waveguide-mounted substrate 100 has a first surface 100a that is a mounting surface for a component E1 and a second surface 100b that is the surface opposite to the first surface 100a. The optical waveguide 101 is disposed on the first surface 100a. The waveguide-mounted substrate 100 includes, on the first surface 100a, a component mounting pad 4 that is a conductor pad included in the conductor layer 41.

[0070] The conductor layers 41 to 43 and the insulating layers 51 and 52 are laminated in the following order from the second surface 100b side toward the first surface 100a side of the waveguide-mounted substrate 100: conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, conductor layer 41. The conductor layers 41 and 42 are connected by via conductors 7 that penetrate the insulating layer 51. The conductor layers 42 and 43 are connected by via conductors 7 that penetrate the insulating layer 52. The wiring board 110 includes a solder resist 62 that covers the conductor layers 43 and the insulating layer 52, and a solder resist 61 that covers the conductor layers 41 and the insulating layer 51. The wiring board 110 also includes bumps 8 that are connected to the conductor pads of the conductor layer 43 and protrude from the solder resist 62. The bumps 8 are made of a conductor such as solder and are used for electrical and mechanical connection between the waveguide mounted substrate 100 and an external component (for example, a motherboard of any electrical device) located on the second surface 100b side. The waveguide mounted substrate 100 may also be used as a motherboard on which the bumps 8 are not arranged.

[0071] The insulating layers 51 and 52 can be formed using a thermosetting insulating resin such as an epoxy resin, a bismaleimide triazine resin (BT resin), or a phenolic resin. The insulating layers 51 and 52 may also be formed using a thermoplastic insulating resin such as a fluororesin, a liquid crystal polymer (LCP), a fluoroethylene (PTFE) resin, a polyester (PE) resin, or a modified polyimide (MPI) resin. Note that the resins listed as the insulating layer materials are merely examples of materials that can form the insulating layers. The insulating layers can be formed using any material that can provide insulation between conductor layers within the wiring substrate 110. Although not shown, each insulating layer may include a core made of a reinforcing material such as glass fiber or aramid fiber, or may include an inorganic filler made of fine particles such as silica (SiO2), alumina, or mullite.

[0072] The solder resists 61 and 62 are made of, for example, a photosensitive epoxy resin or polyimide resin.

[0073] Examples of conductors constituting the conductor layers 41-43 and via conductor 7 include copper, nickel, and silver, and it is preferable to use copper or an alloy containing copper as a main material. Although each of these conductors is depicted as a single layer in Fig. 11 for simplicity, they may have a multilayer structure including two or more films. For example, the conductor layers 41-43 and via conductor 7 may have a two-layer structure including an electroless plated film and an electrolytic plated film.

[0074] An opening 61a is provided in the solder resist 61, and the component mounting pad 4 is exposed in the opening 61a. An optical waveguide 101 is disposed on the solder resist 61. Although not shown, the optical waveguide 101 is fixed to the surface of the wiring board 110 by any fixing member such as an adhesive.

[0075] The optical waveguide 101 is the optical waveguide of the embodiment described above. The optical waveguide 101 may be the optical waveguide 1 shown in FIGS. 1 to 5 or the optical waveguide 1α shown in FIG. 10. FIG. 11 shows, as an example, an optical waveguide 101 identical to the optical waveguide 1 of FIG. 1. Thus, the optical waveguide 101 of FIG. 11 includes a laminated lower cladding 21, a core 3, and an upper cladding 22, and has an upper cladding-free region 1a and an upper cladding-formed region 1b. The core 3 has, at one end, an exposed core portion 3a exposed in the upper cladding-free region 1a, and an unexposed core portion 3b in the upper cladding-formed region 1b. The cross-sectional shape of the core 3 at the exposed core portion 3a is trapezoidal, and the interior angle θ on the lower cladding 21 side of the trapezoidal cross-sectional shape of the core 3 satisfies the relationship 80°<θ<90°.

[0076] A component E1 is mounted on the waveguide-mounted substrate 100. The component E1 is an optical component such as a semiconductor device including a photoelectric conversion element, as described in the description of the optical waveguide 1 in FIG. 1 and other figures. The component E1 includes an optical terminal E1a and a ball-shaped electrode E1b. Examples of the component E1 include a light-receiving element such as a photodiode, and a light-emitting element such as a light-emitting diode (LED), an organic light-emitting diode (OLED), a laser diode (LD), and a vertical-cavity surface-emitting laser (VCSEL). When the component E1 is a light-emitting element, the component E1 generates an optical signal based on an electrical signal input to the electrode E1b, and emits the optical signal from the optical terminal E1a, which functions as a light-emitting unit, toward the core 3. When the component E1 is a light-receiving element, an electrical signal based on an optical signal input from the optical terminal E1a, which functions as a light-receiving unit, is generated and output from the electrode E1b.

[0077] The component E1 is mounted on the waveguide-mounted substrate 100 by connecting the electrode E1b to the component mounting pad 4 using, for example, solder. In FIG. 11, the component E1 is flip-chip mounted. The optical terminal E1a and the exposed core portion 3a of the core 3 of the optical waveguide 101 are positioned to face each other and are optically coupled. The cross-sectional shape of the core 3 at the exposed core portion 3a of the optical waveguide 101 is trapezoidal. Therefore, even if stress is transmitted to the core 3 due to contact with the component E1 or heat generation by the component E1, the stress is likely to be effectively dispersed to the lower surface side as well, rather than being concentrated near the upper surface of the core 3.

[0078] Therefore, it is believed that deformation of the core 3 due to stress caused by contact with the component E1 or heat generation by the component E1 is suppressed. Because deformation of the core 3 is suppressed, the core 3 and the optical terminal E1a of the component E1 can be optically coupled with high efficiency. Furthermore, the core 3 is less likely to obstruct the penetration of the transparent resin TR that fills the region between the optical waveguide 101 and the component E1. Therefore, the region between the optical waveguide 101 and the component E1 is likely to be sufficiently filled without leaving unfilled portions such as voids. Therefore, the optical waveguide 101 and the component E1 can be opposed via a region having an appropriate refractive index. As a result, it is believed that good optical coupling can be achieved between the core 3 and the optical terminal E1a of the component E1.

[0079] 12 and 13 show a waveguide-mounted substrate 100α, which is an example of a waveguide-mounted substrate according to the second embodiment. FIG. 12 shows a cross-sectional view of the waveguide-mounted substrate 100α, and FIG. 13 shows a plan view of the waveguide-mounted substrate 100α. The waveguide-mounted substrate 100α includes a wiring substrate 110α and optical waveguides 101a and 101b disposed on the wiring substrate 110α. That is, the waveguide-mounted substrate 100α includes two optical waveguides. The optical waveguides 101a and 101b are optical waveguides according to the previously described embodiments, such as the optical waveguide 1 shown in FIGS. 1 to 4 or the optical waveguide 1α shown in FIG. 10. Therefore, the cross-sectional shapes of the cores 3 at the core-exposed portions 3a of the optical waveguides 101a and 101b are trapezoidal. 12 and 13, the components of the optical waveguide 101a and the optical waveguide 101b are assigned the same symbols as those assigned to the components of the optical waveguide 101 in FIG. 11, or are omitted as appropriate, and repeated explanations of the components are omitted.

[0080] The wiring board 110α has a similar structure to the wiring board 110 in the first embodiment of Fig. 11, except that the arrangement of the conductor patterns and via conductors 7 included in each of the conductor layers 41 to 43 is different from that of the wiring board 110 in the first embodiment of Fig. 11. In the wiring board 110α, components having the same functions as those of the wiring board 110 in the first embodiment of Fig. 11 are assigned the same reference numerals as those assigned in Fig. 11 or are omitted as appropriate, and repeated explanations thereof will be omitted.

[0081] When the waveguide mounting substrate 100α is used, the components E11 and E12 are mounted on the component mounting surface of the waveguide mounting substrate 100α. In the waveguide mounting substrate 100α, the components E11 and E12 are mounted on the first surface 100a of the wiring substrate 110α. The component E11 is optically coupled to the core 3 of the optical waveguide 101a, and the component E12 is optically coupled to the core portion 3 of the optical waveguide 101b. Like the component E1 shown in FIG. 11, the components E11 and E12 are optical components such as semiconductor devices including a photoelectric conversion element, and are provided with an optical terminal E1a and an electrode E1b.

[0082] 12 and 13, the optical waveguide 101a and the optical waveguide 101b are arranged so that the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b are close to each other. The optical waveguide 101a and the optical waveguide 101b are also arranged so that their ends 11 on the exposed core portions 3a side face each other. That is, the optical waveguide 101a and the optical waveguide 101b are arranged on the wiring substrate 110α such that the distance G1 between the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b is closer than the distance G2 between the non-exposed core portion 3b of the optical waveguide 101a and the non-exposed core portion 3b of the optical waveguide 101b. Since the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b are close to each other, the components E11 and E12 that are optically coupled to the cores 3 of the respective optical waveguides can be arranged so that their electrodes E1b are close to each other. Therefore, it is believed that electrical signals can be transmitted well between the components E11 and E12 while avoiding noise interference and the like.

[0083] In the waveguide-mounted substrate 100α, the optical waveguide 101a and the optical waveguide 101b are arranged along the same predetermined direction. That is, the core 3 of the optical waveguide 101a and the core 3 of the optical waveguide 101b are arranged along the same predetermined direction. Therefore, the propagation direction of the optical signal inside the optical waveguide 101a and the propagation direction of the optical signal inside the optical waveguide 101b are aligned on a straight line. Furthermore, since the exposed core portions 3a of the optical waveguides 101a and 101b face each other, the exposed core portion 3a of the optical waveguide 101a faces one of the two opposing ends of the wiring substrate 110α, and the exposed core portion 3a of the optical waveguide 101b faces the other of the two ends.

[0084] That is, the optical waveguide 101a and the optical waveguide 101b are arranged such that, in a plan view, the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b face each other at an angle φ of 180°. The phrase "the two exposed core portions 3a face each other at an angle φ" means that the extension direction of the core 3 of one exposed core portion 3a forms an angle φ with the extension direction of the core 3 of the other exposed core portion 3a. Therefore, it is considered possible to propagate a desired optical signal via the shortest path by applying appropriate electrical processing from one end of the waveguide-mounted substrate 100α to the other end. That is, when an optical signal is input from one end of the waveguide-mounted substrate 100α to, for example, the optical waveguide 101a, the optical signal is output as an electrical signal to the wiring substrate 110α via the component E11. The electrical signal is electrically processed as necessary on the wiring board 110α, and then converted into an optical signal by the component E12, and the optical signal is output from the optical waveguide 101b at the other end of the waveguide mounting board 100α.

[0085] The waveguide-mounted substrate of the embodiment may include not only one or two optical waveguides like the waveguide-mounted substrate 100 of FIG. 11 and the waveguide-mounted substrate 100α of FIG. 12, but also any number of optical waveguides, three or more. FIG. 14 shows a waveguide-mounted substrate 100β, which is a modification of the waveguide-mounted substrate of the second embodiment including multiple optical waveguides. The waveguide-mounted substrate 100β of the example of FIG. 14 includes a wiring substrate 110β having a layered structure similar to the wiring substrate 110 of the embodiment of FIG. 11 and the wiring substrate 110α of the embodiment of FIG. 12, and optical waveguides 101a, 101b, 101c, and 101d disposed on the wiring substrate 110β. The optical waveguides 101a to 101d are optical waveguides of the previously described embodiments, such as the optical waveguide 1 shown in FIGS. 1 to 5 or the optical waveguide 1α shown in FIG. 10. Although not shown, when the waveguide mounting substrate 100β is in use, an optical component similar to the component E1 shown in FIG. 11 is mounted on the waveguide mounting substrate 100β and optically coupled to the cores 3 of the optical waveguides 101a to 101d.

[0086] In the waveguide-mounted substrate 100β, the optical waveguides 101a to 101d are arranged on the surface of the wiring substrate 110β with the exposed core portions 3a of the optical waveguides close to and facing each other. The exposed core portions 3a of the optical waveguides 101a and 101c face each other on a straight line, and the exposed core portions 3a of the optical waveguides 101b and 101d face each other on a straight line. Meanwhile, the optical waveguides 101a and 101b are arranged such that the exposed core portions 3a of the optical waveguides 101a and 101b face each other at an angle φa of 90° in a plan view. Similarly, the optical waveguides 101b and 101c are arranged such that the exposed core portion 3a of the optical waveguide 101b and the exposed core portion 3a of the optical waveguide 101c face each other at an angle φb of 90° in a plan view. Similarly, the optical waveguides 101c and 101d are arranged such that the exposed core portion 3a of the optical waveguide 101c and the exposed core portion 3a of the optical waveguide 101d face each other at an angle φa of 90° in a plan view. Furthermore, the optical waveguides 101d and 101a are arranged such that the exposed core portion 3a of the optical waveguide 101d and the exposed core portion 3a of the optical waveguide 101a face each other at an angle φd of 90° in a plan view.

[0087] In the waveguide-mounted substrate of the embodiment, as in the waveguide-mounted substrate 100β of FIG. 14 , multiple optical waveguides may be arranged so that the cores 3 of each optical waveguide are aligned along different, unique directions. Furthermore, multiple optical waveguides may be arranged so that the exposed core portions 3 a of each optical waveguide face each other at any angle. For example, multiple optical waveguides may be arranged so that the exposed core portions 3 a of each optical waveguide face each other at any angle between 45° and 180°. An optical signal input to the waveguide-mounted substrate from any direction may be output as an optical signal from a desired end in a desired direction via a short path and after appropriate electrical processing.

[0088] <Method of Manufacturing Optical Waveguide of the Embodiment> A method for manufacturing an optical waveguide according to an embodiment will be described with reference to FIGS. 15A to 15D. Hereinafter, as an example, a method for directly forming the optical waveguide 1 shown in FIGS. 1 to 5 on the wiring board 110 of the waveguide-mounted substrate 100 of FIG. 11 will be described. As shown in FIG. 15A, the wiring board 110 is prepared, and the lower clad 21 is formed on the surface of a solder resist (not shown) of the wiring board 110. The lower clad 21 is formed, for example, by thermocompression bonding a film-shaped constituent material of the lower clad 21 to the wiring board 110 or by applying a resin composition by spin coating to form a film. For example, the constituent material of the lower clad 21, such as PMMA, is formed into a film and thermocompression bonded to the wiring board 110.

[0089] Then, the core 3 having a desired shape in plan view is formed. The core 3 is formed by any method. As an example, the core 3 is formed using photolithography. That is, a photosensitive layer made of a material for the core 3, such as PMMA, is formed on the entire surface of the lower clad 21. As an example, the material for the core 3 is applied by spin coating to form a film made of the material for the core 3. Alternatively, the material for the core 3 may be formed into a film and then thermocompressed onto the surface of the lower clad 21. Alternatively, a material for the core 3 having a higher refractive index than the lower clad 21 may be applied, or a film-like material having a higher refractive index than the lower clad 21 may be thermocompressed.

[0090] Then, by exposing to light through a mask corresponding to the shape of the core 3 to be formed and developing, the layer made of the material constituting the core 3 on the lower clad 21 is patterned to form a desired number of cores 3 having a desired shape. By making the width of the opening in the exposure mask different between the part corresponding to the core exposed portion 3a (see FIG. 10) and the part corresponding to the core unexposed portion 3b (see FIG. 10), it is possible to provide the core exposed portion 3a and the connection portion 3c having a width larger than the core unexposed portion 3b, as in the optical waveguide 1α in FIG. 10.

[0091] By appropriately selecting the exposure conditions during patterning of the core 3 and the material of the core 3, it is possible to form a core 3 having a trapezoidal cross-sectional shape. For example, by using an exposure mask in which the aperture ratio of the corresponding portion of the exposure mask gradually changes so that the amount of light reaching the portion corresponding to the side surface 35 of the core 3 gradually changes, it is possible to form a core 3 having a trapezoidal cross-sectional shape. Furthermore, by using an exposure mask in which the aperture ratio of the corresponding portion of the exposure mask gradually changes so that the amount of light reaching the portion corresponding to the corner 30 (see FIG. 7) gradually changes, it is possible to form a rounded corner 30. Furthermore, by selecting an appropriate photosensitivity for the material of the core 3 or by adjusting the degree of diffusion of light after passing through the aperture of the exposure mask using an appropriate optical system, it may also be possible to form a core 3 having a trapezoidal cross-sectional shape or rounded corner 30.

[0092] In addition to photolithography, photobleaching and core cutting are also examples of methods for forming the core 3 of a desired shape. In the photobleaching method, a layer having a higher refractive index than the lower cladding 21 and formed on the entire surface of the lower cladding 21 is irradiated with ultraviolet light or the like in the unmasked region using a mask that shields the region where the core 3 is to be formed. The core 3 is formed by reducing the refractive index of the irradiated region. In the core cutting method, a layer having a higher refractive index than the lower cladding 21 and formed on the entire surface of the lower cladding 21 is cut into the desired shape using laser light or the like, thereby forming the core 3 of a desired shape. The core 3 may be formed by such photobleaching or core cutting. In particular, cutting may be used to easily round the corners 30 of the core 3.

[0093] 15B, the upper clad 22 is formed on the core 3 and the lower clad 21. For example, similar to the formation of the lower clad 21, a material for the upper clad 22, such as PMMA, is spin-coated onto the lower clad 21 and the core 3 to form a film made of the material for the upper clad 22. Alternatively, the material for the upper clad 22, such as PMMA, may be formed into a film and then thermocompressed. The upper clad 22 is integrated with the lower clad 21 or at least closely adheres to it, thereby forming the clad 2 that surrounds the core 3.

[0094] 15C, a portion of the upper cladding 22 covering the portion of the core 3 corresponding to the exposed core portion 3a is removed. For example, the portion of the upper cladding 22 on the side of the exposed core portion 3a is removed over the entire width of the core 3. As a result, part of the core 3 is exposed from the cladding 2, thereby providing the exposed core portion 3a. Furthermore, part of the upper cladding 22 is removed, thereby providing an upper cladding non-formed region 1a and an upper cladding formed region 1b.

[0095] The portion of the upper cladding 22 to be removed is removed using, for example, photolithography. The portion of the upper cladding 22 to be removed may also be removed by laser processing, but the removal method is not limited to these methods. In this process of removing a portion of the upper cladding 22, the cross-sectional shape of the core 3 exposed in response to the removal of the target portion of the upper cladding 22 may be shaped into a trapezoid. For example, the cross-sectional shape of the core 3 in the exposed core portion 3a may be shaped into a trapezoid by appropriately selecting exposure conditions using photolithography or by mechanical processing after removing the corresponding portion of the upper cladding 22. It is considered that the cross-sectional shape of the core 3 in the unexposed core portion 3b can be kept rectangular, such as a rectangle, by shaping the cross-sectional shape of the core 3 exposed by partial removal of the upper cladding 22 into a trapezoid, rather than by the core 3 patterning process described with reference to FIG. 15A .

[0096] 15B, the film constituting the upper clad 22 may be thermocompressed so as not to cover the exposed core portion 3a. Through the above steps, an optical waveguide 1 including a core 3 having a trapezoidal cross-sectional shape in which two opposing side surfaces 35 are inclined inward toward the upper surface in the exposed core portion 3a is completed on the wiring substrate 110 as shown in FIG. 15D. The interior angle IA on the lower surface side of the trapezoidal cross-sectional shape of the core 3 has an angle θ in the range of 80°<θ<90°. FIG. 15D shows a cross section taken along line XVD-XVD in FIG. 15C.

[0097] Alternatively, a support plate (not shown) may be prepared instead of the wiring substrate 110, and the optical waveguide may be formed on the support plate. The support plate is preferably formed of a material having a higher rigidity than the optical waveguide to be manufactured. The support plate may also be formed of a material having a lower thermal expansion coefficient than the optical waveguide to be manufactured. This may suppress displacement of the optical waveguide due to temperature changes. Examples of materials for the support plate include glasses such as soda-lime glass, borosilicate glass, and quartz glass; various metals such as tungsten, titanium, and molybdenum; and various ceramics such as alumina, silicon nitride, and silicon oxide. The support plate supports the optical waveguide during manufacturing and provides appropriate rigidity to the optical waveguide during manufacturing.

[0098] When the manufactured optical waveguide 1 is to be used without a support plate, the support plate is removed after defining the upper cladding-free region 1a as shown in FIG. 15C. Removal of the support plate completes the production of a single optical waveguide 1. The support plate can be removed by any method. For example, when the support plate and the lower cladding 21 are bonded with an adhesive layer (not shown) made of a thermoplastic adhesive, the optical waveguide with the support plate is heated until the adhesive strength of the adhesive is reduced, and then the support plate is peeled off from the lower cladding 21. Note that the manufactured optical waveguide may be used with the support plate still attached after completion.

[0099] <Method for manufacturing a waveguide-mounted substrate according to an embodiment> An example of a method for manufacturing a waveguide-mounted substrate according to an embodiment will be described using the waveguide-mounted substrate 100α shown in Fig. 12 as an example. The waveguide-mounted substrate 100α is manufactured by forming a wiring substrate 110α by sequentially forming a conductor layer 43, an insulating layer 52, a conductor layer 42, an insulating layer 51, and a conductor layer 41, and then forming an optical waveguide 101a and an optical waveguide 101b on the wiring substrate 110α. Each optical waveguide may be separately manufactured on a support plate (not shown) and then disposed on the wiring substrate 110α after removing the support plate or with the support plate still attached.

[0100] The conductor layer 43 is formed on a support substrate (not shown), such as a double-sided copper-clad laminate, by a semi-additive method or the like. After the conductor layer 43 is formed, an insulating layer 52 covering the conductor layer 43 is formed, for example, by laminating an insulating resin, such as a film-like epoxy resin, and thermocompression bonding. Through holes are formed in the insulating layer 52 by, for example, irradiation with a carbon dioxide laser beam. After the through holes are formed, the conductor layer 42 is formed on the insulating layer 52 by, for example, a semi-additive method, and via conductors 7 are formed in the through holes in the insulating layer 52. Furthermore, an insulating layer 51 is formed on the conductor layer 42 and the insulating layer 52 in the same manner as the insulating layer 52. The conductor layer 41 is formed on the insulating layer 51 in the same manner as the conductor layer 42, and the via conductors 7 are formed in the insulating layer 51 in the same manner as the via conductors 7 in the insulating layer 52. After the support substrate is removed by peeling or the like, solder resists 61 and 62 are formed by coating or spraying a photosensitive epoxy resin. By exposure and development, openings are formed in the solder resists 61 and 62, exposing parts of the conductor layer 41 and parts of the conductor layer 43. As a result, the wiring board 110α is completed.

[0101] Then, the optical waveguides 101a and 101b are formed on the surface of the wiring board 110α by the method described with reference to FIGS. 15A to 15D. Alternatively, two optical waveguides may be separately manufactured on an arbitrary support plate (not shown). The manufactured two optical waveguides may then be arranged on the wiring board 110α as the optical waveguides 101a and 101b, as shown in FIG. 16. For example, an arbitrary adhesive AH, such as a thermosetting, room temperature curing, or photocuring adhesive, is applied to the surface of the solder resist 61, and the optical waveguides 101a and 101b are mounted thereon. If necessary, the adhesive AH is cured by heating or the like, and each optical waveguide is fixed on the wiring board 110α. After the manufacturing or arrangement of each optical waveguide, the bumps 8 are formed by mounting conductive balls using solder or the like and performing a reflow process. Through the above steps, the waveguide mounting substrate 100α of the example shown in FIG. 12 is completed.

[0102] The optical waveguides 101a and 101b may be manufactured not on the solder resist 61 but on an area on the surface of the insulating layer 51 where the solder resist 61 and the conductor layer 41 are not provided, or two optical waveguides manufactured separately may be disposed in such an area on the surface of the insulating layer 51. Furthermore, the optical waveguides 101a and 101b may be disposed on the wiring board 110α in a state where they are still provided with a support plate (not shown) used in manufacturing the optical waveguides.

[0103] The optical waveguide and waveguide-mounted substrate of the embodiments are not limited to those having the structures illustrated in the drawings and the structures, shapes, and materials illustrated in this specification. The waveguide-mounted substrate of the embodiments may have any laminated structure and may include any number of conductor layers and insulating layers. For example, the wiring board constituting the waveguide-mounted substrate of the embodiments may be a build-up wiring board including a core substrate, a multilayer wiring board without a build-up layer, or a double-sided or single-sided wiring board. Bumps and / or via conductors are not necessarily provided. Furthermore, the thickness of the core of the optical waveguide of the embodiments may vary between the core-exposed portion and the core-unexposed portion. The core of the core-unexposed portion may have a trapezoidal cross-sectional shape and / or rounded corners. [Explanation of symbols]

[0104] 1, 1α, 101, 101a~101d Optical waveguide 100, 100α, 100β waveguide mounting board 110, 110α, 110β wiring board 1a Upper cladding-free region 1b Upper cladding formation region 2. Clad 21 Lower Cladding 22 Upper Cladding 3 cores 30 Corner 301 1st corner 302 Second corner 3a Exposed core 3b Unexposed core 31 Top of the core 35 Core Aspects 41~43 Conductor layers 51~52 Insulation layer R Length of the rounded corner of the core R1 Length of the rounded part of the first corner of the core R2 Length of the rounded part of the second corner of the core θ is the inner angle of the trapezoidal core cross section on the lower cladding side W1 Core width at exposed core W2 Width of unexposed core φ Angle between exposed core parts

Claims

1. An optical waveguide including a lower clad, a core formed on the lower clad, and an upper clad formed on the lower clad and the core, wherein a part of the core is exposed on an input side or an output side of an optical signal, the optical waveguide comprises an upper cladding-free region and an upper cladding region; the upper cladding-free region has a core exposed portion where an upper surface of the core is exposed; the upper cladding forming region has a core non-exposed portion where the upper surface of the core is not exposed; a cross-sectional shape of the core at the core exposed portion is trapezoidal, The interior angle θ of the trapezoid on the lower cladding side satisfies the relationship of the following formula 1. 80°<θ<90°...(Formula 1)

2. 2. The optical waveguide according to claim 1, In the cross-sectional shape, the two interior angles on the lower cladding side are different from each other.

3. 2. The optical waveguide according to claim 1, wherein corners of the cross-sectional shape of said core are rounded.

4. 4. The optical waveguide according to claim 3, wherein the corner portion is a portion connecting the top surface of the core exposed in the core exposed portion with a side surface of the core.

5. In the optical waveguide according to claim 4, the length (R) of the rounded portion at the corner of the cross-sectional shape satisfies the relationship of the following formula 2. 1.5 μm≦R≦4.5 μm (Formula 2)

6. 6. The optical waveguide according to claim 5, the corners of the cross-sectional shape have one end as a first corner and the other end as a second corner when viewed from the front of the cross section of the core, The length of the rounded portion of the first corner is different from the length of the rounded portion of the second corner.

7. 2. The optical waveguide of claim 1, wherein the maximum width of the core in the core exposed portion is greater than the width of the core in the core unexposed portion.

8. a wiring substrate including an insulating layer and a conductor layer laminated on the insulating layer; the optical waveguide according to claim 1 disposed on the wiring substrate; a waveguide mounting substrate including:

9. 9. The waveguide mounting substrate according to claim 8, comprising two or more of the optical waveguides, The two or more optical waveguides are arranged on the wiring substrate such that the exposed core portions of the two or more optical waveguides are closer to each other than the unexposed core portions of the two or more optical waveguides.

10. 10. The waveguide mounting substrate according to claim 9, the two or more optical waveguides are formed along a predetermined direction, The optical waveguides are arranged such that the exposed core portions face each other at an angle between 45° and 180°.

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