Optical waveguide and waveguide mounting substrate

Abstract

An optical waveguide (1) according to an embodiment includes a lower cladding (21), a core (3) formed on the lower cladding (21), and an upper cladding (22) formed on the lower cladding (21) and the core (3). A part of the core (3) on the incident side or the exit side of the optical signal is exposed. The optical waveguide (1) comprises a region without an upper cladding and a region with an upper cladding. The region without the upper cladding has a core-exposed portion in which an upper surface (31) of the core (3) is exposed. The region with the upper cladding has a core non-exposed portion in which the upper surface (31) of the core (3) is not exposed. A corner portion (30) of the cross-sectional shape of the core (3a) in the core exposed portion is rounded.

Description

Optical waveguide and waveguide mounting substrate

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

[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 cores of the polymer waveguide and the silicon waveguide, each having a rectangular cross section, are arranged to overlap over a predetermined distance in the optical axis direction and are optically coupled by adiabatic coupling.

[0003] JP 2014-81587 A

[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 core of the polymer waveguide and the core of the silicon waveguide may be arranged at an extremely small distance, for example, on the order of a few micrometers. As a result, stress may be transmitted to the core of the polymer waveguide, causing deformation of the core of the polymer waveguide. 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 core of the polymer waveguide. 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 core of the polymer waveguide. Deformation of the core is presumed to reduce the efficiency of the optical coupling between the polymer waveguide and the silicon waveguide.

[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. A portion of the core is exposed on the optical signal input side or output side. The optical waveguide comprises an upper cladding non-formed region and an upper cladding formed region, the upper cladding non-formed region having a core exposed portion where the top surface of the core is exposed, and the upper cladding formed region having a core unexposed portion where the top surface of the core is not exposed, and the corners of the cross-sectional shape of the core in the core exposed portion are rounded.

[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] 7 is a plan view showing an example of an optical waveguide according to an embodiment of the present invention. FIG. 1 is a cross-sectional view taken along line II-II of the optical waveguide according to the example of FIG. 1. FIG. 2 is a cross-sectional view taken along line III-III of the optical waveguide according to the example of FIG. 1. FIG. 3 is a cross-sectional view taken along line IV-IV of the optical waveguide according to the example of FIG. 1. FIG. 4 is an enlarged view of a cross section of a core at an exposed core portion in the optical waveguide according to the embodiment. FIG. 5 is a diagram showing a method for measuring the length of a rounded portion of a core in the optical waveguide according to the embodiment. FIG. 6 is a cross-sectional view showing a first modified example of a core shape at an exposed core portion of the optical waveguide according to the embodiment. FIG. 7 is a diagram showing a method for measuring the length of the rounded portion of the core and the interior angle of the first modified example of FIG. 7. FIG. 8 is a cross-sectional view showing a second modified example of a core shape at an exposed core portion of the optical waveguide according to the embodiment. FIG. 9 is a plan view showing a third modified example of a core shape at an exposed core portion of the optical waveguide according to the embodiment. FIG. 10 is a cross-sectional view showing an example of a waveguide-mounted substrate according to the first embodiment of the present invention. FIG. 11 is a cross-sectional view showing an example of a waveguide-mounted substrate according to the second embodiment of the present invention. FIG. 12 is a plan view showing a modified example of the waveguide-mounted substrate according to the embodiment of the present invention. FIG. 13 is a plan view showing an example of a waveguide-mounted substrate according to the second embodiment of the present invention. 15C is a cross-sectional view of the optical waveguide of the embodiment taken along line XVD-XVD of the optical waveguide of the embodiment shown in FIG. 15C. 15D is a cross-sectional view of the optical waveguide of the embodiment shown in FIG. 15C. 15E is a cross-sectional view of the optical waveguide of the embodiment shown in FIG. 15C. 15F is a cross-sectional view of the optical waveguide of the embodiment shown in FIG. 15C. 15G ...

[0008] 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.

[0009] <Structure of Optical Waveguide of Embodiment> Fig. 1 shows a plan view of an optical waveguide 1, which is an example of an optical waveguide of an embodiment. Fig. 2 shows a cross section of the optical waveguide 1 of Fig. 1 taken along line II-II. Fig. 3 shows a cross section of the optical waveguide 1 of Fig. 1 taken along line III-III, and Fig. 4 shows a cross section of the optical waveguide 1 of Fig. 1 taken along line IV-IV. Note that the optical waveguide 1 illustrated in Fig. 1 and other figures is merely an example of an optical waveguide of an embodiment. The structure of the optical waveguide of the embodiment is not limited to the structure shown in each drawing, such as Fig. 1.

[0010] 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.

[0011] The core 3 has an upper surface 31 facing in a direction along the formation direction of the cladding 2 and the core 3, and a lower surface 32 opposite to the upper surface 31. The formation direction of the cladding 2 and the core 3 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".

[0012] 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.

[0013] 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 an optical signal, collectively referred to as the +X direction and the −X direction, will also be simply referred to as 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 is emitted from the other end 12 or the one end 11. When an optical signal is incident on the one end 11 side, the optical signal is emitted from the other end 12 side. When an optical signal is incident on the other end 12 side, the optical signal is emitted from the one end 11 side.

[0014] Therefore, in the optical waveguide 1, an optical signal may be incident on one end 11 and may be emitted from the 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 the 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. At the other end 12 , as shown in FIGS. 1 and 2 , another 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 .

[0015] 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.

[0016] In the optical waveguide 1, the upper cladding non-forming region 1a is a core-exposed portion where the upper surface 31 of the core 3 is exposed, and the upper cladding forming region 1b is a core-non-exposed portion where the upper surface 31 of the core 3 is not exposed. The core-exposed portion of the optical waveguide 1 is provided on one end 11 side. The core-non-exposed portion of the optical waveguide 1 is provided on the other end 12 side. The core 3 is composed of core 3a and core 3b, with core 3a being arranged in the core-exposed portion and core 3b being arranged in the core-non-exposed portion. The lower cladding 21 is composed of lower cladding 21a and lower cladding 21b, with the lower cladding 21a being arranged in the core-exposed portion and the lower cladding 21b being arranged in the core-non-exposed portion. The core 3a in the core-exposed portion is a portion of the core 3 on the one end 11 side of the optical waveguide 1. The core 3b in the core-non-exposed portion is a portion of the core 3 on the other end 12 side of the optical waveguide 1.

[0017] In the optical waveguide of the embodiment, the upper cladding non-forming region 1a may be provided at both ends of the optical waveguide. In other words, the optical waveguide of the embodiment may have exposed core portions at both ends. 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 disposed at at least one end of the optical waveguide. For example, the upper cladding non-forming region 1a is disposed at one end 11 and / or the other end 12 in FIG. 1 . Therefore, the exposed core portion is also disposed at at least one end of the optical waveguide.

[0018] 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.

[0019] The core 3 and the clad 2 may be made of different materials or of the same material. However, the core 3 is made of a material having 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 having 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 used separately from the support, or it may be used together with the support. The optical waveguide 1 may also 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.

[0020] 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.

[0021] The thickness of the lower cladding 21 is not particularly limited, but is 5 μm to 100 μm, and preferably 10 μm to 50 μm. The thickness of the upper cladding 22 is not particularly limited, but is 5 μm to 100 μm, and preferably 10 μm to 50 μm. The thicknesses of the lower cladding 21 and the upper cladding 22 are determined by averaging values ​​measured at three points in the Z direction.

[0022] 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 as to have a positional relationship 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.

[0023] 1 to 3, a component E1 including a photoelectric conversion element (not shown) is shown 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 be more appropriate than that of air.

[0024] 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 component and the optical waveguide 1 in the X direction.

[0025] The component E1 includes an optical terminal E1a, which is a portion where an optical signal enters the component E1 or a 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 within the core 3, and exits from the other end 12.

[0026] 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 the 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 passing through, it is believed that highly efficient optical coupling is achieved.

[0027] 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 128. 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.

[0028] The core 3a in the core exposed portion of the optical waveguide 1 has a width W1. The core 3b in the core unexposed portion of the optical waveguide 1 has a width W2. 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 3a in the core exposed portion and the width of the core 3b in the core unexposed portion may be different, as in the optical waveguide 1a in FIG. 10, which will be referred to later.

[0029] The "width" of the core 3, such as width W1 and width W2, refers to the length of the core 3 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.

[0030] In the optical waveguide of the embodiment, as shown in FIG. 3 , the corners 30 of the cross-sectional shape of the core 3 a in the core exposed portion are rounded. The corners 30 are portions connecting the top surface 31 of the core 3 a exposed in the core exposed portion to the side surface 35 of the core 3 a. The corners 30 of the core 3 a are rounded in the core exposed portion. Because the corners 30 of the core 3 a are rounded, even if stress generated, for example, by heat generation during use or contact with an external optical component propagates to the core 3 a, it is believed that the stress is dispersed. As a result, deformation of the core 3 a may be suppressed. Furthermore, stress within the core 3 a caused by the placement of the core exposed portion in close proximity to the optical terminal E1 a of the component E1 may be alleviated. As a result, deformation of the core 3 a may be suppressed. Note that, as shown in FIG. 4 , the corners 30 of the core 3 b are not rounded in the core non-exposed portion. However, in the optical waveguide of the embodiment, the corners 30 of the core 3 b may be rounded in the core non-exposed portion.

[0031] <Shape of Core Corners at Core Exposed Portion> The shape of the corners 30 of the core 3a at the core exposed portion 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 at the core exposed portion of the optical waveguide of the embodiment. Figure 6 is a schematic diagram showing a method for measuring the length of the rounded portion of the core corner in the optical waveguide of the embodiment.

[0032] As shown in Figure 5, in the cross-sectional shape of the core 3a in the core exposed portion of the optical waveguide 1, a corner 30 connecting the top surface 31 of the core 3a to the side surface 35 of the core 3a is rounded. The core 3a in the core exposed portion does not have a sharp edge at the boundary between the top surface 31 and the side surface 35. The top surface 31 of the core 3a and the side surface 35 of the core 3a can be said to be 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 3a. Therefore, even in the cross-section of the core 3a shown in Figure 5, the boundary between the line representing the top surface 31 and the line representing the side surface 35 is not sharp. In other words, in the cross-section shown in Figure 5, the line representing the top surface 31 and the line representing the side surface 35 do not directly contact each other, but are connected via an arc-shaped curve.

[0033] In other words, the core 3a in the core exposed portion does not have a tangent line where the top surface 31 and the side surface 35 meet at a specific angle. Therefore, in Fig. 5, there is no intersection where a line representing the top surface 31 of the core 3a 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 3a in the core exposed portion, 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 3a in the core exposed portion may be a shape obtained by R-chamfering with a specific curvature.

[0034] The advantages of rounding the corners 30 of the core 3a in the core exposed portion are described below. As shown in FIGS. 1 to 3 and 5 , when the optical waveguide 1 is in use, the core 3a in the core exposed portion is positioned so as to face the optical terminal E1a of the component E1. For efficient optical coupling, the core 3a in the core exposed portion is positioned very close to the optical terminal E1a of the component E1. For example, the core 3a in the core exposed portion and the optical terminal E1a of the component E1 may be positioned with the intention of leaving a gap of only about 3 μm between them in design. The component E1 may include an element that generates heat during use. In such a case, if the optical terminal E1a of the component E1 is close to the core 3 of the optical waveguide 1, 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 3a 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 3a of the optical waveguide 1. Such contact between the optical terminal E1a of the component E1 and the core 3a of the optical waveguide 1 generates stress at the contact portion, and this stress may be transmitted to the core 3a.

[0035] When stress is transmitted to the core in this manner, if the corners of the core's cross section are angular, as in the rectangular cross section of the conventional core described in Patent Document 1, the transmitted stress may be concentrated at the corners, causing deformation of the core. In contrast, in the optical waveguide of the embodiment, the transmitted stress is less likely to be concentrated at the corners, and the corners 30 of the core 3a are rounded rather than angular, thereby suppressing deformation of the core 3a. That is, even if stress is transmitted to the core 3a due to contact with the component E1 or heat generation from the component E1, the corners 30 of the cross section of the core 3a at the exposed core portion are rounded, so the stress does not concentrate at the corners 30 but is easily dispersed to areas other than the corners 30. Therefore, it is believed that deformation of the core 3a due to stress caused by contact with the component E1 or heat generation from the component E1 is suppressed. Because deformation of the core 3a is suppressed, it is believed that the optical waveguide of the embodiment allows the core 3a and the optical terminal E1a of the component E1 to be optically coupled with high efficiency.

[0036] Furthermore, in the optical waveguide 1, the corners 30 of the core 3a in the core exposed portion are rounded, which may stabilize the resin filling of the region between the optical waveguide 1 and the component E1 with the transparent resin TR. 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 after the optical waveguide 1 and the component E1 are positioned in their predetermined positions with a small gap between them. The resin filling may be 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 . When filling the transparent resin TR, 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 into the region between the optical waveguide and the component. If the transparent resin TR does not penetrate, unfilled portions such as voids may be formed in the transparent resin TR. If there are unfilled portions such as voids in the transparent resin TR, the unfilled portions may cause thermal expansion or contraction, making it difficult to ensure the reliability of the wiring board.

[0037] In contrast, in the optical waveguide of the embodiment, the corners 30 of the core 3a are rounded rather than angular, so the penetration of the transparent resin TR is less likely to be hindered by the core 3a. Therefore, it is believed 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 believed that the core 3a and the optical terminal E1a of the component E1 can be optically coupled well. Furthermore, since no unfilled portions such as voids are formed in the transparent resin TR, it is believed that the reliability of the wiring board is ensured.

[0038] In the optical waveguide 1 of the embodiment, the radius R of the rounded portion at the corner 30 of the cross-sectional shape of the core 3a is preferably 1.5 μm or more and 4.5 μm or less. That is, the radius R of the rounded portion at the corner 30 may satisfy the relationship of the following formula 1: 1.5 μm≦R≦4.5 μm (Formula 1) Here, the radius R of the rounded portion at the corner 30 is the radius of curvature of the rounded portion at the corner 30 in the cross-sectional shape of the core 3a, as shown in FIG. 5 .

[0039] It is believed that when the rounded corners 30 of the core 3a have an R of 1.5 μm or more, the above-mentioned effect of dispersing the stress transmitted to the core 3a is achieved, thereby achieving the intended effect of suppressing deformation of the core 3a. Furthermore, when the rounded corners 30 of the core 3a have an R of 4.5 μm or less, it is believed that the area on the top surface 31 of the core 3a directly facing the optical terminal E1a of the component E1 is easily secured to the size required for good optical coupling. The R of the rounded portions of the two corners 30 in the cross-sectional shape of the core 3a may be equal to or different from each other.

[0040] Furthermore, in the optical waveguide 1 of the embodiment, the radius R of the rounded portion at the corner 30 of the cross-sectional shape of the core 3a is more preferably 2.0 μm or more and 3.5 μm or less. That is, the radius R of the rounded portion at the corner 30 may satisfy the relationship of the following formula 2: 2.0 μm≦R≦3.5 μm (formula 2).

[0041] It is believed that when the rounded corners 30 of the core 3a have an R of 2.0 μm or more, the aforementioned effect of dispersing stress transmitted to the core 3a is achieved, thereby achieving the intended effect of suppressing deformation of the core 3a. Furthermore, when the rounded corners 30 of the core 3a have an R of 3.5 μm or less, it is believed that the area on the top surface 31 of the core 3a directly facing the optical terminal E1a of the component E1 is easily secured to the size required for good optical coupling. The R of the rounded portions of the two corners 30 in the cross-sectional shape of the core 3a may be equal to or different from each other. Furthermore, when the R of the rounded portions of the corners 30 of the cross-sectional shape of the core 3a is within the range of Equation 2, the shape of the formed core 3a is more likely to be stable.

[0042] A method for measuring the radius of curvature of a rounded portion at a corner 30 in the cross-sectional shape of a core 3a will be described with reference to Fig. 6 . Fig. 6 schematically shows an image of a corner 30 in a cross-section of a core 3a at an exposed core portion of an optical waveguide according to an embodiment, observed with a scanning electron microscope (SEM × 5000). In measuring the radius of curvature of the rounded portion at the corner 30, an imaginary line IS1 extending and overlapping with the top surface 31 of the core 3a and an imaginary line IS2 extending and overlapping with the side surface 35 of the core 3a are set on the observation image of the corner 30 as shown in Fig. 6 , which was taken with the SEM at a magnification of 1000 times. Furthermore, an imaginary line segment IS3 is set between a point PS1 where the imaginary line IS1 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. Additionally, a virtual line segment IS4 is assumed that connects the virtual line segment IS3 to the vertex PS3 that is farthest from the virtual line segment IS3 on the contour of the corner 30, and is perpendicular to the virtual line segment IS3. The length L of the virtual line segment IS3 and the length H of the virtual line segment IS4 are then measured using an SEM. The radius R of the rounded portion of the corner 30 is calculated using the following formula A: R=((L / 2) 2 +H 2) / (2×H) ... (Formula A) As an example of an embodiment, the width of the core 3a is 5 μm, the thickness of the core 3a is 5 μm, the R (R1) of the rounded portion of one corner 30 of the core 3a is 2.31 μm, the R (R2) of the rounded portion of the other corner 30 of the core 3a is 2.21 μm, the arrangement pitch of the cores 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 L1 of the core 3a at the exposed core portion is 2000 μm.

[0043] <First Modification of Core Shape in Core Exposed Portion> Figure 7 shows a first modification of the cross-sectional shape of the core 3a in the core exposed portion of the optical waveguide according to the embodiment. In the first modification shown in Figure 7, the corners 30 of the cross-sectional shape of the core 3a in the core exposed portion are also rounded. The cross-sectional shape of the first modification of the core 3a shown in Figure 7 is a trapezoid. The "trapezoid" shape of the cross-section of the core 3a may be any shape surrounded by a pair of substantially parallel opposite sides and another pair of opposite sides that narrow toward one of the first pair of opposite sides, and does not have to have four vertices like a typical rectangle. The trapezoid cross-sectional shape of the core 3a shown in Figure 7 is surrounded by an upper base that represents the upper surface 31 of the core 3a, a lower base that represents the lower surface 32 of the core 3a, and two legs that represent the two opposing side surfaces 35 of the core 3a. The distance between the two opposing side surfaces 35 of the core 3a increases from the top surface 31 to the bottom surface 32 of the core 3a. That is, the cross section of the core 3a shown in Fig. 7 has a tapered shape in which the width W decreases toward the top surface 31.

[0044] In the optical waveguide 1 of the embodiment including the core 3 having the cross-sectional shape of the first modification shown in Figure 7, the core 3a has a trapezoidal planar shape. Therefore, even if stress is transmitted to the core 3a due to contact with the component E1 or heat generation from the component E1, deformation of the core 3a is suppressed. That is, since the cross-sectional shape of the core 3a at the core exposed portion is a trapezoid whose width W increases toward the lower surface 32 of the core 3a, even if stress is transmitted to the core 3a, the stress is easily dispersed to the lower surface 32 without concentrating near the upper surface 31 of the core 3a. Therefore, deformation of the core 3a due to stress caused by contact with the component E1 or heat generation from the component E1 is suppressed. Because deformation of the core 3a is suppressed, the optical waveguide 1 of the embodiment including the core 3a having a trapezoidal cross-sectional shape is considered to be able to optically couple the core 3a and the optical terminal E1a of the component E1 with high efficiency.

[0045] Furthermore, in the optical waveguide 1 of the embodiment including the core 3 having the cross-sectional shape of the first modification shown in Figure 7, the core 3a 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, the resin filling of the region between the optical waveguide 1 and the component E1 with the transparent resin TR is usually performed by infiltrating the region between the optical waveguide 1 and the component E1 in the direction indicated by arrow A1 in Figure 7. In the optical waveguide of the embodiment shown in Figure 7, the cross-sectional shape of the core 3a at the core exposed portion is a trapezoid whose width W decreases toward the top surface 31 of the core 3a. Therefore, the transparent resin TR supplied near the side surface 35 of the core 3a easily penetrates into the gap between the core 3a and the optical terminal E1a of the component E1, for example, by capillary action. Therefore, it is believed that the region between the optical waveguide 1 and the component E1 is easily filled with the transparent resin TR without leaving any unfilled portions such as voids. 7, the optical waveguide 1 and the component E1 can be opposed to each other through a region having an appropriate refractive index filled with the transparent resin TR. In other words, the core 3a and the optical terminal E1a of the component E1 can be optically coupled satisfactorily.

[0046] 7 has a trapezoidal cross-sectional shape, the two interior angles IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of the core 3a 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 3a may satisfy the relationship of the following formula 3: 80°<θ<90° (Formula 3)

[0047] When the angle θ of the interior angle IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of the core 3a is smaller than 90°, the above-mentioned dispersion effect regarding the stress transmitted to the core 3a is obtained, and it is presumed that the intended effect of suppressing deformation of the core 3a 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 3a directly facing the optical terminal E1a of the component E1 is easily secured to a size required for good optical coupling. The angles θ of the two interior angles IA in the cross-sectional shape of the core 3a may be equal to or different from each other. Note that one of the interior angles IA in the cross-sectional shape of the core 3a may be a right angle.

[0048] With reference to Figure 8 , a method for measuring the angle θ of the interior angle IA of the cross-sectional shape of the core 3 of the first modified example shown in Figure 7 and a method for measuring the radius R of the rounded portion of the corner 30 will be described. Figure 8 schematically shows an SEM image of the corner 30 and the interior angle IA in the cross-section of the core 3a in the core exposed portion of the first modified example shown in Figure 7. Note that Figure 8 omits the portion between the corner 30 and the interior angle IA. In measuring the angle θ of the interior angle IA, an imaginary line IS2 extending and overlapping the side surface 35 of the core 3a and an imaginary line IS5 extending and overlapping the bottom surface 32 of the core 3a are set on the observation image of the interior angle IA as shown in Figure 8 taken with the SEM at 1000x magnification. Then, the angle between the imaginary lines IS2 and IS5 is measured as the angle θ of the interior angle IA using the SEM photograph.

[0049] In the first modified example of the cross-sectional shape of the core 3a shown in Fig. 7 , the radius of curvature of the rounded portion at the corner 30 of the core 3a is measured in the same manner as described with reference to Fig. 6 . That is, on an observation image of the corner 30 as shown in Fig. 8 taken by an SEM at 5000x magnification, an imaginary line IS1 extending and overlapping the top surface 31 of the core 3a and an imaginary line IS2 extending and overlapping the side surface 35 of the core 3a are set. Furthermore, an imaginary line segment IS3 is set connecting a point PS1 where the imaginary line IS1 and the top surface 31 begin to separate with a point PS2 where the imaginary line IS2 and the side surface 35 begin to separate. Furthermore, an imaginary line segment IS4 is set 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, and the radius 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 the core 3a: 5 μm, the thickness of the core 3a: 5 μm, the R (R1) of the rounded portion of one corner 30 of the core 3a: 3.07 μm, the R (R2) of the rounded portion of the other corner 30 of the core 3a: 3.01 μm, the angle θ of one interior angle IA of the core 3a: 83.2°, the angle θ of the other interior angle IA of the core 3a: 85.2°, the thickness of the lower clad 21: 30 μm, the arrangement pitch of the cores 3: 50 μm, the thickness of the upper clad 22: 20 μm, and the length of the core 3a at the exposed core portion: 2000 μm.

[0050] <Second Modification of Core Shape in Core Exposed Portion> Figure 9 shows a second modification of the cross-sectional shape of the core 3a in the core exposed portion of the optical waveguide of the embodiment. In the second modification shown in Figure 9, both of the two corners 30 of the cross-sectional shape of the core 3a in the core exposed portion are rounded. On the other hand, in the second modification, the rounded portion R (R1) of one corner 301 of the two corners 30 of the cross-sectional shape of the core 3a and the rounded portion R (R2) of the other corner 302 of the two corners 30 are different from each other.

[0051] That is, in a front view of the cross section of the core 3a, the corner 30 of the cross section of the core exposed portion has one end of the top surface 31 as a first corner 301 and the other end of the top surface 31 as a second corner 302. The radius R (R1) of the rounded portion of the first corner 301 and the radius R (R2) of the rounded portion of the second corner 302 are different from each other. In the cross section of the core 3a shown in FIG. 9 , the radius R (R1) of the rounded portion of the first corner 301 is longer than the radius R (R2) of the rounded portion of the second corner 302. Note that "a front view of the cross section of the core 3a" means that the cross section of the core 3a is viewed from a line of sight perpendicular to the cross section of the core 3a.

[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 3a 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 believed that the corner 30 with the shorter rounded portion can ensure 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 3a and ensure a sufficient area that contributes to good optical coupling with the component 1. Furthermore, it is believed that supplying the transparent resin TR (see FIG. 3) from the corner 30 with the longer rounded portion allows the transparent resin TR to effectively penetrate into the area between the core 3a and the component E1.

[0053] In the second modified example of Fig. 9 , the cross-sectional shape of the core 3 a is trapezoidal, as in the first modified example of Fig. 7 . 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 a are smaller than 90°. Here, the angles of the two interior angles IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of the core 3 a are different from each other. That is, in a front view of the cross section of the core 3 a, the trapezoidal cross-sectional shape of the core 3 a 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 a 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 3a that forms the first interior angle IA1 with the lower surface 32 is larger than the side surface 35 of the core 3a that forms the second interior angle IA2 with the lower surface 32 and is inclined toward the inside of the core 3a.

[0054] As shown in the second modification of 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 3a 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, the corner 30 connected to the larger interior angle IA is likely to ensure 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 3a and ensure a sufficient area that contributes to good optical coupling with the component 1. Furthermore, by supplying the transparent resin TR (see FIG. 3 ) from the side of the smaller interior angle IA, the transparent resin TR can be effectively infiltrated into the area between the core 3a and the component E1.

[0055] <Third Modification of Core Shape of Core Exposed Portion> Referring to Figure 10, a third modification of the core shape of the core exposed portion of the optical waveguide of the embodiment will be described. Figure 10 shows a plan view of an optical waveguide 1α including a third modification of the core shape of the core exposed portion of the optical waveguide of the embodiment. As shown in Figure 10, the optical waveguide 1α, like the optical waveguide 1 of Figure 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 where the upper surface of the core 3a is exposed. The upper cladding forming region 1b also has an unexposed core portion where the core 3b is not exposed. In the optical waveguide 1α of Figure 10, components having the same functions as those of the optical waveguide 1 of Figure 1 are denoted by the same reference numerals as those in Figure 1 or are omitted as appropriate, and repeated descriptions of these similar components will be omitted.

[0056] In the optical waveguide 1α, the width W1 of the core 3a in the exposed core portion and the width W2 of the core 3b in the non-exposed core portion are different. In the optical waveguide 1α, the width W1 of the core 3 at one end 11 is greater than the width W2 of the core 3 at the other end 12. Therefore, the difference between the width W1 of the core 3a in the exposed core portion and the width We of the optical terminal E1a of the component E1 is greater than the difference between the width W2 of the core 3b in the non-exposed core portion 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 3a when aligning the optical signal input / output portion of an optical component, such as the optical terminal E1a, with the core 3a is greater than when the width W2 of the core 3b in the non-exposed core portion is the same in the exposed core portion. Therefore, in the optical waveguide 1α, it is considered easy to align the optical component and the core 3a to achieve appropriate optical coupling. As a result, it is considered easy to achieve high coupling efficiency when coupling with an optical component. It is also believed that poor coupling due to misalignment between the core 3a and optical components such as the component E1 is suppressed even with the thermal history during use.

[0057] Additionally, in the optical waveguide 1α, the core 3b in the non-exposed core portion has a width W2 smaller than the width W1, which may facilitate realizing a desired transmission mode in the non-exposed core portion. 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 core 3a in the exposed core portion, which is large for high coupling efficiency with optical components, but the core 3b in the non-exposed core portion has a width W2 smaller than the width W1. Therefore, it is believed that optical transmission in a mode requiring a small core cross-section is easily realized. For example, single-mode optical transmission, which requires a core diameter of 3 to 10 μm, may be possible.

[0058] The width W1 of the core 3a in the core exposed portion may be constant or may vary depending on the position in the core exposed portion in the X direction. In the optical waveguide 1α in Fig. 10, the maximum width (W1m) of the width W1 of the core 3a in the core exposed portion is wider than the width W2 of the core 3b in the core unexposed portion. That is, in the optical waveguide 1α, the maximum width W1m of the core 3a in the core exposed portion and the width W2 of the core 3b in the core unexposed portion satisfy the relationship of the following formula B: W1m>W2 (Formula B)

[0059] 10 illustrates an example of dimensions of each portion of the optical waveguide 1α including the third modified example of the shape of the core 3a in the core exposed portion. The width W1 of the core 3a in the core exposed portion and the width W2 of the core 3b in the core unexposed portion are not particularly limited, but it is desirable that the width W1 of the core 3a in the core exposed portion and the width W2 of the core 3b in the core unexposed portion satisfy the relationship of the following formula C: 1.0<(W1 / W2)≦3.0 (formula C)

[0060] The width W1 of the core 3a in the core exposed portion 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 3a in the core exposed portion has a width in this range, it is thought that the effect of facilitating alignment with an optical component such as the 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 3b in the core unexposed portion may be, for example, 1 μm or more and 10 μm or less.

[0061] 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 3 a in the core exposed portion may have a width that gradually increases or decreases from one end to the other end. When the width of the core 3 a in the core exposed portion varies, the core width W1 of the core 3 a in the core exposed portion is determined by the average value of the length measurements at three points in the core exposed portion in the X direction. The core width of the core 3 b in the core non-exposed portion may be constant or may vary. As an example, the core width of the core 3 b in the core non-exposed portion may have a width that gradually increases or decreases from one end to the other end. When the width of the core 3 b in the core non-exposed portion varies, the core width W2 of the core 3 b in the core non-exposed portion is determined by the average value of the length measurements at three points in the core non-exposed portion in the X direction.

[0062] The ratio of the width to the thickness of the core 3a in the core exposed portion is not particularly limited, but is preferably 1.0:1.0 to 4.0:1.0 (width of the core 3a:thickness of the core 3a). The ratio of the width to the thickness of the core 3b in the core unexposed portion is not particularly limited, but is preferably 0.9:1.0 to 1.4:1.0 (width of the core 3b:thickness of the core 3b).

[0063] An example of a combination of dimensions of each part of the optical waveguide 1α is as follows: The width of the core 3a in the core exposed portion is 10 μm, and the thickness of the core 3a is 7 μm. The width of the core 3b in the core non-exposed portion is 7 μm, and the thickness of the core 3b 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 3a in the core exposed portion, 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 exemplified values.

[0064] In the optical waveguide 1α shown in Fig. 10 , the core 3 has a connecting portion 3c connecting the core 3a and the core 3b between the core 3a in the core exposed portion and the core 3b in the core unexposed portion. As shown in Fig. 10 , the connecting portion 3c refers to a part of the core 3 formed in the upper cladding non-forming region 1a between the core 3a and the core 3b in a plan view. Here, the connecting portion 3c may have a taper in a plan view, may be arranged as a straight line of a constant width, or may be arranged as a combination of a straight line of a constant width and a taper. 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 core 3b in the core unexposed portion to the core 3a in the core exposed portion, 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.

[0065] The core width W3 of the connecting portion 3c is smaller than the core width W1 of the core 3a in the core exposed portion and is equal to or larger than the core width W2 of the core 3b in the core unexposed portion. 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 (Formula D)

[0066] It is believed that the core width W3 of the connecting portion 3c suppresses unintended leakage of an optical signal from the core 3a in the exposed core portion toward the core 3b in the unexposed core portion. That is, in the optical waveguide 1α, the width W1 of the core 3a in the exposed core portion is larger than the width W2 of the core 3b in the unexposed core portion, and therefore the connecting portion 3c connecting the core 3a and the core 3b has a tapered shape, which is believed to suppress unintended leakage of an optical signal from the core 3a in the exposed core portion toward the core 3b in the unexposed core portion. The core width W3 of the connecting portion 3c is not particularly limited, but is preferably 1 μm to 30 μm.

[0067] The taper ratio ((W1-W2) / L2) of the tapered shape of the core 3 is, for example, 1 or more and 2 or less. L2 is the length of the tapered portion of the connection portion 3c in the X direction. If the connection portion 3c has a taper ratio in this range, leakage of optical signals from the core 3 can be efficiently suppressed within a limited length. In this case, the tapered shape formation ratio RT of the core 3 is preferably 0.01<RT<0.2, and more preferably 0.03<RT<0.15. The tapered shape formation ratio RT is the ratio of the length of the tapered portion to the overall length of the core 3.

[0068] As an example of the third modified example, the width of the core 3a at the core exposed portion: 10 μm, the width of the core 3b at the core non-exposed portion: 5 μm, the thickness of the core 3: 5 μm, the R (R1) of the rounded portion of one corner 30 of the core 3a: 3.10 μm, the R (R2) of the rounded portion of the other corner 30 of the core 3a: 3.20 μm, the angle θ of one interior angle IA of the core 3a: 81.2°, the angle θ of the other interior angle IA of the core 3a: 82.4°, the arrangement pitch of the cores 3: 50 μm, the thickness of the lower cladding 21: 30 μm, the thickness of the upper cladding 22: 20 μm, and the length of the core 3a at the core exposed portion: 2000 μm.

[0069] <Structure of Waveguide Mounted Board of the Embodiment> Next, the waveguide mounted board of the embodiment will be described with reference to the drawings. FIG. 11 shows a cross-sectional view of a waveguide mounted board 100, which is an example of a waveguide mounted board of the first embodiment. Note that the waveguide mounted board 100 shown in FIG. 11 and the waveguide mounted boards 100α and 100β shown in FIGS. 12 to 13, which will be referred to later, are merely examples of the waveguide mounted board of the embodiment. The layered structure of the waveguide mounted board of the embodiment is not limited to the layered structure of the waveguide mounted board 100, the waveguide mounted board 100α, or the waveguide mounted board 100β shown in FIGS. 11 to 13. Furthermore, the number of conductor layers and the number of insulating layers included in the waveguide mounted board 100, the waveguide mounted board 100α, or the waveguide mounted board 100β are not limited to the number of conductor layers and the number of insulating layers included in the waveguide mounted board 100, the waveguide mounted board 100α, or the waveguide mounted board 100β.

[0070] As shown in FIG. 11 , the waveguide mount board 100 includes a wiring board 110 and an optical waveguide 101 disposed on the wiring board 110. The wiring board 110 includes an insulating layer and a conductor layer laminated on the insulating layer. The wiring board 110 includes conductor layers 41 to 43 as conductor layers and insulating layers 51 and 52 as insulating layers. The waveguide mount board 100 has a first surface 100a, which is a mounting surface for the component E1, and a second surface 100b, which is the surface opposite the first surface 100a. The optical waveguide 101 is disposed on the first surface 100a. The waveguide mount board 100 includes a component mounting pad 4, which is a conductor pad included in the conductor layer 41, on the first surface 100a.

[0071] The conductor layers 41 to 43 and the insulating layers 51 and 52 are stacked in the following order from the second surface 100b side toward the first surface 100a side of the waveguide mounting 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 layer 43 and the insulating layer 52, and a solder resist 61 that covers the conductor layer 41 and the insulating layer 51. The wiring board 110 also includes bumps 8 that are connected to each conductor pad 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 (e.g., a motherboard of an electrical device) located on the second surface 100b side. The waveguide-mounted substrate 100 may also be used as a motherboard without the bumps 8.

[0072] The insulating layers 51 and 52 may be formed using a thermosetting insulating resin such as epoxy resin, bismaleimide triazine resin (BT resin), or phenolic resin. The insulating layers 51 and 52 may also be formed using a thermoplastic insulating resin such as fluororesin, liquid crystal polymer (LCP), fluoroethylene (PTFE) resin, polyester (PE) resin, and modified polyimide (MPI) resin. The resins listed as insulating layer materials are merely examples of materials that can be used to form the insulating layers. Each insulating layer may 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 of silica (SiO), alumina, or mullite.

[0073] The solder resists 61 and 62 are formed of, for example, a photosensitive epoxy resin or polyimide resin.

[0074] 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 primarily made of copper. 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.

[0075] 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.

[0076] 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 may be the optical waveguide 1α shown in FIG. 10. FIG. 11 shows, as an example, an optical waveguide 101 that is 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 non-formed region 1a and an upper cladding formed region 1b. The core 3 has, at one end, a core 3a exposed in the upper cladding non-formed region 1a, and a core 3b in the upper cladding formed region 1b. The corners of the cross-sectional shape of the core 3a at the core exposed portion are rounded.

[0077] A component E1 is mounted on the waveguide mounting 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.

[0078] 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 core 3a in the core exposed portion of the core 3 of the optical waveguide 101 are positioned to face each other and are optically coupled. The corners of the cross section of the core 3a in the core exposed portion of the optical waveguide 101 are rounded. Therefore, even if stress is transmitted to the core 3a due to contact with the component E1 or heat generation by the component E1, the stress is likely to be dispersed to areas other than the corners rather than being concentrated at the corners.

[0079] Therefore, it is believed that deformation of the core 3a due to stress caused by contact with the component E1 or heat generation by the component E1 is suppressed. Because deformation of the core 3a is suppressed, the core 3a and the optical terminal E1a of the component E1 can be optically coupled with high efficiency. Furthermore, the core 3a is less likely to impede 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 with an appropriate refractive index. As a result, it is believed that good optical coupling can be achieved between the core 3a and the optical terminal E1a of the component E1.

[0080] 12 and 13 show a waveguide mount board 100α, which is an example of a waveguide mount board according to the second embodiment. FIG. 12 shows a cross-sectional view of the waveguide mount board 100α, and FIG. 13 shows a plan view of the waveguide mount board 100α. The waveguide mount board 100α includes a wiring board 110α and optical waveguides 101a and 101b disposed on the wiring board 110α. That is, the waveguide mount board 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 corners of the cross-sectional shapes of the cores 3a at the core-exposed portions of the optical waveguides 101a and 101b are rounded. 12 and 13, the components of the optical waveguide 101a and the optical waveguide 101b are given the same symbols as those given to the components of the optical waveguide 101 in FIG. 11, or are omitted as appropriate, and repeated explanations of the components are omitted.

[0081] The wiring board 110α has a similar structure to the wiring board 110 in the first embodiment shown in 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 shown in Fig. 11. In the wiring board 110α, components having the same functions as those of the wiring board 110 in the first embodiment shown in 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.

[0082] When the waveguide mounting substrate 100α is in use, 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 optical terminals E1a and electrodes E1b.

[0083] 12 and 13 , the optical waveguide 101a and the optical waveguide 101b are arranged so that the core 3a in the exposed core portion of the optical waveguide 101a and the core 3a in the exposed core portion of the optical waveguide 101b are close to each other. The optical waveguides 101a and 101b are also arranged so that their ends 11 on the exposed core side face each other. That is, the optical waveguides 101a and 101b are arranged on the wiring substrate 110α such that the distance G1 between the exposed core portion of the optical waveguide 101a and the exposed core portion of the optical waveguide 101b is closer than the distance G2 between the unexposed core portion of the optical waveguide 101a and the unexposed core portion of the optical waveguide 101b. Since the core 3a in the exposed core portion of the optical waveguide 101a and the core 3a in the exposed core portion of the optical waveguide 101b are brought close to each other, the components E11 and E12 that are optically coupled to the core 3 of each optical waveguide 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.

[0084] In the waveguide-mounted substrate 100α, the optical waveguides 101a and 101b are arranged along the same predetermined direction. That is, the cores 3 of the optical waveguides 101a and 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 cores 3a in the exposed core portions of the optical waveguides 101a and 101b face each other, the exposed core portion of the optical waveguide 101a faces one of the two opposing ends of the wiring substrate 110α, and the exposed core portion of the optical waveguide 101b faces the other of the two opposing ends.

[0085] That is, in a plan view, the optical waveguide 101a and the optical waveguide 101b are arranged such that the core 3a in the core exposed portion of the optical waveguide 101a and the core 3a in the core exposed portion of the optical waveguide 101b face each other at an angle φ of 180°. Note that the phrase "the cores 3a in the two core exposed portions face each other at an angle φ" means that the direction in which the core 3a in one core exposed portion extends forms an angle φ with the direction in which the core 3a in the other core exposed portion extends. 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 substrate 100α.

[0086] 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.

[0087] In the waveguide-mounted substrate 100β, the optical waveguides 101a to 101d are arranged on the surface of the wiring substrate 110β with the cores 3a at the exposed core portions of the optical waveguides being close to and facing each other. The cores 3a of the optical waveguides 101a and 101c face each other on a straight line, and the cores 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 cores 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 core 3a of the optical waveguide 101b faces the core 3a of the optical waveguide 101c at an angle φb of 90° in a plan view. Similarly, the optical waveguides 101c and 101d are arranged such that the core 3a of the optical waveguide 101c faces the core 3a of the optical waveguide 101d at an angle φc of 90° in a plan view. Furthermore, the optical waveguides 101d and 101a are arranged such that the core 3a of the optical waveguide 101d faces the core 3a of the optical waveguide 101a at an angle φd of 90° in a plan view.

[0088] 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 cores 3 a in the exposed core portions of each optical waveguide face each other at any angle. For example, multiple optical waveguides may be arranged so that the exposed core portions of each optical waveguide face each other at any angle between 45° and 180°. It may be possible to output an optical signal input to the waveguide-mounted substrate from any direction as an optical signal from a desired end toward a desired direction via a short path and after appropriate electrical processing.

[0089] <Method for Manufacturing Optical Waveguide of the Embodiment> A method for manufacturing an optical waveguide of the 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 substrate 110 of the waveguide-mounted substrate 100 of FIG. 11 will be described. As shown in FIG. 15A, the wiring substrate 110 is prepared, and the lower clad 21 is formed on the surface of a solder resist (not shown) of the wiring substrate 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 substrate 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 substrate 110.

[0090] 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 cladding 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 cladding 21. Alternatively, a material for the core 3 having a higher refractive index than the lower cladding 21 may be applied, or a film-like material having a higher refractive index than the lower cladding 21 may be thermocompressed.

[0091] 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 3a (see FIG. 10) in the core exposed portion and the part corresponding to the core 3b (see FIG. 10) in the core unexposed portion, it is possible to provide cores 3a having a width larger than core 3b, as well as connection portions 3c, as in the optical waveguide 1α in FIG. 10.

[0092] 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 rounded corners 30 (see FIG. 5 ). For example, the rounded corners 30 can be formed by using an exposure mask in which the aperture ratio of the corresponding portions of the exposure mask is gradually changed so that the amount of light reaching the portions corresponding to the corners 30 gradually changes. Similarly, by using an exposure mask in which the aperture ratio of the corresponding portions of the exposure mask is gradually changed so that the amount of light reaching the portions corresponding to the side surfaces 35 of the core 3 gradually changes, it is possible to form a core 3 having a trapezoidal cross-sectional shape as shown in the example of FIG. 7 . Furthermore, it may also be possible to form a core 3 having rounded corners 30 or a trapezoidal cross-sectional shape 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 apertures of the exposure mask using an appropriate optical system.

[0093] In addition to photolithography, examples of methods for forming the core 3 of a desired shape include photobleaching and core cutting. 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, using cutting processing may make it easier to round the corners 30 of the core 3.

[0094] 15B , the upper cladding 22 is formed on the core 3 and the lower cladding 21. For example, similar to the formation of the lower cladding 21, a material for the upper cladding 22, such as PMMA, is spin-coated onto the lower cladding 21 and the core 3 to form a film made of the material for the upper cladding 22. Alternatively, the material for the upper cladding 22, such as PMMA, may be formed into a film and then thermocompressed. The upper cladding 22 is integrated with or at least closely adheres to the lower cladding 21, thereby forming the cladding 2 surrounding the core 3.

[0095] 15C , a portion of the upper cladding 22 covering the exposed core portion of the core 3 is removed. For example, the portion of the upper cladding 22 on the exposed core portion side is removed over the entire width of the core 3. As a result, a portion of the core 3 is exposed from the cladding 2, thereby providing a core 3a in the exposed core portion. Furthermore, a portion of the upper cladding 22 is removed, thereby providing an upper cladding non-formed region 1a and an upper cladding formed region 1b.

[0096] 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, corners 30 (see FIG. 15D ) of the core 3 a exposed in response to the removal of the portion of the upper cladding 22 to be removed may be rounded. For example, the corners 30 of the core 3 a in the exposed core portion may be rounded by appropriately selecting exposure conditions using photolithography or by mechanical processing after the removal of the corresponding portion of the upper cladding 22. It is considered that the corners of the core 3 b in the unexposed core portion can be kept sharp by rounding the corners 30 of the core 3 a exposed by partial removal of the upper cladding 22, rather than by the core 3 patterning process described with reference to FIG. 15A .

[0097] In the step shown in Fig. 15B, the film that is the constituent material of the upper clad 22 may be thermocompression bonded so that the core 3a in the core exposed portion is not covered. By going through the above steps, an optical waveguide 1 in which the corners 30 of the core 3a in the core exposed portion are rounded is completed on the wiring substrate 110, as shown in Fig. 15D. Fig. 15D shows a cross section taken along line XVD-XVD in Fig. 15C.

[0098] 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.

[0099] 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 to reduce the adhesive strength of the adhesive, 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.

[0100] <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 Figure 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 the optical waveguides 101a and 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.

[0101] 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, for example, by irradiating it with a carbon dioxide laser beam. After the through holes are formed, the conductor layer 42 is formed on the insulating layer 52 by a semi-additive method or the like, 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 applying or spraying a photosensitive epoxy resin. The solder resists 61 and 62 are exposed and developed to form openings that expose parts of the conductor layer 41 and the conductor layer 43. As a result, the wiring board 110α is completed.

[0102] Then, optical waveguides 101a and 101b are formed on the surface of wiring board 110α by the method described with reference to FIGS. 15A to 15D. Alternatively, two optical waveguides may be separately manufactured on an optional support plate (not shown). The two manufactured optical waveguides may then be arranged on wiring board 110α as optical waveguides 101a and 101b, as shown in FIG. 16. For example, any adhesive AH, such as a thermosetting, room temperature curing, or photocuring adhesive, is applied to the surface of solder resist 61, and optical waveguides 101a and 101b are mounted thereon. If necessary, the adhesive AH is cured by heating or other means, and each optical waveguide is fixed to wiring board 110α. After manufacturing or arranging each optical waveguide, bumps 8 are formed by mounting conductive balls using solder or other means and performing a reflow process. Through the above steps, the waveguide mounting substrate 100α of the example shown in FIG. 12 is completed.

[0103] 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 while still including a support plate (not shown) used in manufacturing the optical waveguides.

[0104] 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 rounded corners and / or a trapezoidal cross-sectional shape.

[0105] 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 rounded corners of the core are believed to disperse the stress. Therefore, deformation of the core of the optical waveguide is believed to be suppressed. Therefore, it is believed that the optical waveguide of the embodiment can be optically coupled to an optical component with good coupling efficiency.

[0106] 1, 1α, 101, 101a to 101d Optical waveguide 100, 100α, 100β Waveguide mounting substrate 110, 110α, 110β Wiring substrate 1a Upper clad non-forming region 1b Upper clad forming region 2 Clad 21 Lower clad 21a Lower clad of core exposed portion 21b Lower clad of core non-exposed portion 22 Upper clad 3 Core 30 Corner portion 301 First corner portion 302 Second corner portion 3a Core of core exposed portion 3b Core of core non-exposed portion 31 Upper surface of core 35 Side surface of core 41 to 43 Conductive layers 51 to 52 Insulating layers R Radius of curvature of rounded portion of corner of core R1 Radius of curvature of rounded portion of first corner of core R2 Radius of curvature of rounded portion of second corner of core θ The angle of the inner angle of the trapezoidal cross section of the core on the lower cladding side W1 The width of the core at the exposed core part W2 The width of the core at the unexposed core part φ The angle between the exposed core parts

Claims

1. An optical waveguide comprising a lower cladding, a core formed on the lower cladding, and an upper cladding formed on the lower cladding and the core, wherein a portion of the core is exposed on the input or output side of an optical signal, the optical waveguide comprising an upper cladding non-forming region and an upper cladding formed region, the upper cladding non-forming region having a core exposed portion where the top surface of the core is exposed, the upper cladding formed region having a core non-exposed portion where the top surface of the core is not exposed, and the corners of the cross-sectional shape of the core in the core exposed portion are rounded.

2. An optical waveguide according to claim 1, wherein the corner portion is a portion connecting the top surface of the core exposed in the core exposure portion with the side surface of the core.

3. An optical waveguide according to claim 1, wherein the radius R of the rounded corners of the cross-sectional shape satisfies the relationship of the following formula 1: 1.5 μm≦R≦4.5 μm (Formula 1) 4. An optical waveguide as claimed in claim 1, wherein the corners of the cross-sectional shape, when viewed from the front of the cross section of the core, have one end as a first corner and the other end as a second corner, and the radius of the rounded portion of the first corner is different from the radius of the rounded portion of the second corner.

5. An optical waveguide according to claim 1, wherein the cross-sectional shape of said core is trapezoidal.

6. An optical waveguide according to claim 5, wherein the interior angle θ of the trapezoid on the side of the lower cladding satisfies the relationship of the following formula 3: 80°<θ<90° (formula 3) 7. An optical waveguide according to 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 waveguide-mounted substrate comprising: a wiring board including an insulating layer and a conductor layer laminated on said insulating layer; and the optical waveguide according to claim 1 disposed on said wiring board.

9. A waveguide-mounted substrate according to claim 8, comprising two or more of the optical waveguides, the two or more optical waveguides being arranged on the wiring board with the exposed core portions of the two or more optical waveguides closer to each other than the non-exposed core portions of the two or more optical waveguides.

10. A waveguide-mounted substrate as set forth in claim 9, wherein the two or more optical waveguides are formed along a predetermined direction, and 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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