Waveguide substrate

Abstract

A waveguide substrate (1) according to the present disclosure comprises a substrate (10), a waveguide (20), and a mirror (30). The waveguide (20) is located on the substrate (10) and has a core (40) and a cladding (50) that covers at least a portion of the periphery of the core. The mirror (30) has a mirror surface (301) that faces an end of the core (40) in the longitudinal direction and that is inclined with respect to the plate surface of the substrate. The mirror reflects electromagnetic waves incident from the thickness direction of the substrate and introduces the electromagnetic waves into the core (40), or reflects electromagnetic waves incident from the core (40) in the thickness direction of the substrate. When an arbitrary position of the waveguide is defined as a first position (P1) and a position closer to the mirror than the first position is defined as a second position (P2), the thickness (T2) of the core at the second position is larger than the thickness (T1) of the core at the first position.

Description

Waveguide substrate

[0001] The present disclosure relates to a waveguide substrate.

[0002] Conventionally, optical communication modules have been known in which a light-emitting element and a light-receiving element are mounted on an optical waveguide substrate and which perform bidirectional communication using signal light. Patent Document 1 discloses a technique in which a mirror that reflects signal light is provided midway along an optical waveguide, and the signal light from the light-emitting element is reflected by the mirror and made to enter the optical waveguide. Patent Document 1 also discloses a technique in which the signal light output from the optical waveguide is reflected by the mirror and coupled to the light-receiving element.

[0003] Japanese Patent Application Laid-Open No. 2002-131586

[0004] A waveguide substrate according to one aspect of the present disclosure includes a substrate, a waveguide, and a mirror. The waveguide is located on the substrate and includes a core and a cladding covering at least a portion of the periphery of the core. The mirror faces the longitudinal end of the core and has a mirror surface inclined with respect to the surface of the substrate. The mirror reflects an electromagnetic wave incident from the thickness direction of the substrate and introduces it into the core, or reflects an electromagnetic wave incident from the core in the thickness direction of the substrate. When an arbitrary position of the waveguide is defined as a first position and a position closer to the mirror than the first position is defined as a second position, the thickness of the core at the second position is greater than the thickness of the core at the first position.

[0005] FIG. 1 is a schematic perspective view showing a state in which a waveguide chip is mounted on a waveguide substrate according to a first embodiment. FIG. 2 is a schematic cross-sectional view showing the configuration of a waveguide substrate according to the first embodiment. FIG. 3 is a schematic plan view showing the configuration of cores of a first waveguide and a second waveguide. FIG. 4 is a schematic view for explaining a method for manufacturing a waveguide substrate according to the first embodiment. FIG. 5 is a schematic view for explaining a method for manufacturing a waveguide substrate according to the first embodiment. FIG. 6 is a schematic view for explaining a method for manufacturing a waveguide substrate according to the first embodiment. FIG. 7 is a schematic view for explaining a method for manufacturing a waveguide substrate according to the first embodiment. FIG. 8 is a schematic cross-sectional view showing the configuration of a waveguide substrate according to a second embodiment.

[0006] Hereinafter, modes for implementing a waveguide substrate according to the present disclosure (hereinafter referred to as "embodiments") will be described in detail with reference to the drawings. Note that the present disclosure is not limited to these embodiments. Furthermore, the embodiments can be appropriately combined within the scope of not causing any contradiction in the processing content. Furthermore, the same components in the following embodiments will be assigned the same reference numerals, and redundant explanations will be omitted.

[0007] Furthermore, in the following embodiments, expressions such as "constant," "orthogonal," "perpendicular," or "parallel" may be used, but these expressions do not necessarily mean "constant," "orthogonal," "perpendicular," or "parallel" in the strict sense. In other words, the above expressions allow for deviations due to, for example, manufacturing precision or installation precision.

[0008] In the drawings referred to below, for ease of understanding, the X-axis, Y-axis, and Z-axis directions are defined as being orthogonal to each other, and an orthogonal coordinate system is shown in which the positive Z-axis direction is the vertically upward direction. The rotation direction around the vertical axis is also referred to as the θ direction.

[0009] Patent Document 1 discloses a technique in which a mirror that reflects signal light is provided midway along an optical waveguide, and the signal light from a light-emitting element is reflected by the mirror and made to enter the optical waveguide. Patent Document 1 also discloses a technique in which the signal light emitted from the optical waveguide is reflected by the mirror and coupled to a light-receiving element.

[0010] However, in the optical communication module described in Patent Document 1, a space is located between the light-emitting element and the mirror and between the mirror and the optical waveguide. Therefore, the spot diameter of the optical signal expands from when it is emitted by the light-emitting element until it is reflected by the mirror, and further expands from when it is reflected by the mirror until it enters the optical waveguide. If the spot diameter of the optical signal is large, some of the light may not be incident upon reflection by the mirror or upon entering the optical waveguide, resulting in coupling loss. Similarly, coupling loss may occur when signal light from the optical waveguide is reflected by a mirror and coupled to a light-receiving element.

[0011] Therefore, there is a need for a technology that can reduce coupling loss.

[0012] First Embodiment First, the configuration of a waveguide substrate 1 according to the first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic perspective view showing a state in which a waveguide chip 2 is mounted on a waveguide substrate 1 according to the first embodiment. FIG. 2 is a schematic cross-sectional view showing the configuration of a waveguide substrate 1 according to the first embodiment. For convenience, FIG. 2 shows the configuration of a first waveguide 21, and omits the configuration of a second waveguide 22, but the second waveguide 22 has the same configuration as the first waveguide 21.

[0013] As shown in FIG. 1, the waveguide substrate 1 includes a substrate 10 , a plurality of waveguides 20 , and a mirror 30 .

[0014] The substrate 10 has, for example, a rectangular plate shape in a plan view. The substrate 10 has a mounting surface 101 on which the plurality of waveguides 20 and the mirror 30 are mounted. The substrate 10 may be an organic substrate. Alternatively, the substrate 10 may be a semiconductor substrate.

[0015] The multiple waveguides 20 are transmission paths for electromagnetic waves. The electromagnetic waves may be, for example, light, specifically near-infrared light or visible light. In the following description, the electromagnetic waves are assumed to be light. The multiple waveguides 20 are located on the substrate 10. Specifically, the waveguides 20 have multiple first waveguides 21 and multiple second waveguides 22. The first waveguides 21 are transmitting waveguides that propagate light transmitted from a light-emitting element 3 (described later) (see FIG. 3) toward the connector 60. The second waveguides 22 are receiving waveguides that propagate light input from the connector 60 to a light-receiving element 4 (described later) (see FIG. 3).

[0016] As shown in Fig. 2, the waveguide 20 has a core 40 and a cladding 50. The waveguide 20 has a curved portion (see Fig. 1), but extends in a generally fixed direction, in this case along the X-axis direction. An end of the core 40 in the longitudinal direction, in this case a first end 41 which is the end on the negative X-axis direction side, faces the mirror surface 301, and a second end (not shown) located opposite the first end 41 is connected to a transmission line via a connector 60 (see Fig. 1). The transmission line may be, for example, an optical fiber.

[0017] The cross-sectional shape of core 40 cut along a plane perpendicular to the longitudinal direction may be, for example, a rectangle. In this case, core 40 has a bottom surface, a top surface, and left and right side surfaces as its outer surfaces. Hereinafter, the bottom surface of core 40, i.e., the surface facing substrate 10, will be referred to as first surface 401, and the top surface of core 40, i.e., the surface opposite first surface 401, will be referred to as second surface 402.

[0018] The cladding 50 covers at least a portion of the periphery of the core 40. Specifically, the cladding 50 has a first cladding 51 and a second cladding 52. The first cladding 51 covers the first surface 401 of the core 40. The second cladding 52 covers the second surface 402 of the core 40. Note that the waveguide substrate 1 according to this embodiment does not necessarily need to cover the entire outer surface of the core 40. For example, the cladding 50 may be configured to cover the first surface 401 and both left and right side surfaces of the core 40, but not cover the second surface 402 of the core 40. In other words, the cladding 50 may not have the second cladding 52, but may have the first cladding 51 and a cladding (not shown) that covers the side surfaces of the core 40.

[0019] The mirror 30 reflects light output from the light-emitting element 3 or the core 40. As shown in FIG. 2 , the mirror 30 has a mirror surface 301 facing the first end 41 of the core 40. The mirror 30 is formed by vapor-depositing the mirror surface 301 on one surface of a prismatic base made of silicon, glass, resin, metal, or the like. The mirror surface 301 may be formed of aluminum, silver, or gold. The mirror surface 301 is inclined with respect to the surface of the substrate 10. The outer surfaces of the mirror 30 further include a third surface 302 (here, the bottom surface) that faces the substrate 10, and a fourth surface 303 (here, the top surface) located opposite the third surface 302. The third surface 302 and the fourth surface 303 may be surfaces of the prismatic base that are connected to the surface on which the mirror surface 301 is formed.

[0020] The mirror 30 reflects light incident in the thickness direction of the substrate 10, in this case, the Z-axis direction, and introduces the light into the core 40. Specifically, the mirror 30 reflects light transmitted from the light-emitting element 3 in the negative direction of the Z-axis and introduces the light into the core 40 of the first waveguide 21. The mirror 30 also reflects light incident from the core 40 in the thickness direction of the substrate 10. Specifically, the mirror 30 reflects light irradiated from the first end 41 of the core 40 included in the second waveguide 22 in the positive direction of the Z-axis and introduces the light into the light-receiving element 4.

[0021] The waveguide chip 2 is, for example, a silicon photonics chip including an optoelectric conversion circuit. The waveguide chip 2 as a silicon photonics chip is an optical semiconductor element including a light-emitting element 3 and a light-receiving element 4. The light-emitting element 3 is located directly above the mirror surface 301 and emits light toward the mirror surface 301 of the mirror 30. In other words, the waveguide chip 2 is attached at a position where the light incident from the light-emitting element 3 is reflected by the mirror surface 301 and introduced into the core 40. The light-receiving element 4 is located directly above the mirror surface 301 and receives light emitted from the waveguide 20 and reflected by the mirror surface 301. The light-emitting element 3 is an example of an electromagnetic wave transmitting element. The light-receiving element 4 is an example of an electromagnetic wave receiving element.

[0022] The number of light-emitting elements 3 may be the same as the number of first waveguides 21. Similarly, the number of light-receiving elements 4 may be the same as the number of second waveguides 22. That is, the light-emitting elements 3 may be located at positions corresponding to the first waveguides 21, and the light-receiving elements 4 may be located at positions corresponding to the second waveguides 22.

[0023] In addition to the waveguide chip 2, the waveguide substrate 1 according to the first embodiment may be equipped with electronic passive components such as capacitors or resistors, a heat sink, an electric socket, a microcontroller, or the like.

[0024] Furthermore, a solder resist may be positioned on the upper surface of the substrate 10 in a portion where the waveguide 20 and the mirror 30 are not disposed.

[0025] In the waveguide substrate 1 configured as described above, light projected from the light-emitting element 3 in the thickness direction of the substrate 10 (here, in the negative Z-axis direction) is reflected by the mirror surface 301 and introduced into the core 40 of the first waveguide 21. Here, because the light projected from the light-emitting element 3 is diffused, the spot diameter of the light expands between the time it is projected from the light-emitting element 3 and the time it is reflected by the mirror surface 301, and also expands between the time it is reflected by the mirror surface 301 and the time it is introduced into the core 40. If the spot diameter of the light is large, there is a risk that part of the light will not be reflected by the mirror surface 301 or part of the light will not be introduced into the core 40, resulting in a coupling loss.

[0026] On the other hand, if the core dimensions are sufficiently large, the above-mentioned problems do not occur. However, in conventional cores, the overall dimensions are specified, for example, according to the dimensions of the transmission line connected to the second end. That is, conventional cores are formed to have a constant thickness and width in the longitudinal direction based on the thickness and width of the second end. However, the first end of a core formed in this manner may not be large enough to guide all of the light transmitted from the light-emitting element 3 and reflected by the mirror 30 into the core. In this case, some of the light reflected by the mirror 30 may not be guided into the core, resulting in coupling loss.

[0027] Therefore, in the waveguide substrate 1 according to this embodiment, the thickness of the core 40 in the portion close to the mirror 30 is made larger than the thickness of the core 40 in other portions. Here, the "thickness of the core 40" refers to the dimension of the core 40 in the thickness direction of the substrate 10, which is in the Z-axis direction here. In other words, the "thickness of the core 40" refers to the dimension of the core 40 in the same direction as the stacking direction of the substrate 10, the mirror surface 301, and the light-emitting element 3 (or the light-receiving element 4).

[0028] 2 , when an arbitrary position of the waveguide 20 is defined as a first position P1 and a position closer to the mirror 30 than the first position P1 is defined as a second position P2, the thickness T2 of the core 40 at the second position P2 is greater than the thickness T1 at the first position P1. More specifically, when the position of the end of the core 40 that faces the mirror 30 of the first cladding 51 is defined as the second position P2, the thickness T2 of the core 40 at the second position P2 is greater than the thickness T1 at the first position P1.

[0029] With this configuration, the thickness of the core at the portion close to the mirror 30, for example, at the second position P2, is greater than the thickness of other portions, for example, at the first position P1, so that more light reflected by the mirror 30 can be introduced into the core 40 than when the core 40 has a constant thickness. Therefore, the coupling loss can be reduced.

[0030] Of the first waveguide 21 and the second waveguide 22, only the first waveguide 21, which is a transmitting waveguide, may have a configuration in which the thickness T2 at the second position P2 of the core 40 is larger than the thickness T1 at the first position P1. The second waveguide 22, which is a receiving waveguide, may have a configuration in which the thickness T2 at the second position P2 of the core 40 is smaller than the thickness T1 at the first position P1.

[0031] The thickness of the core 40 may gradually increase as it approaches the mirror 30. "Gradually increasing" means that the second surface 402 of the core 40 gradually moves away from the mounting surface 101 of the substrate 10, in other words, that there is no step on the second surface 402 of the core 40. If the thickness of the core 40 were to increase stepwise, the step would form an edge, which could cause propagation loss at the edge. In the waveguide substrate 1 according to this embodiment, the thickness of the core 40 gradually increases, making it less likely to form an edge, and therefore propagation loss is less likely to occur compared to when the thickness of the core 40 increases stepwise.

[0032] The second surface 402 in the portion where the thickness of the core 40 gradually increases may be curved concavely so as to move away from the substrate 10 as it approaches the mirror 30. If the second surface 402 of the core 40 is linear, that is, if the second surface 402 of the core 40 is composed of an inclined surface and a flat surface, the boundary between the inclined surface and the flat surface forms an edge, which may cause propagation loss at the edge. In the waveguide substrate 1 according to this embodiment, the second surface 402 of the core 40 is curved, which makes it less likely to form an edge, and therefore propagation loss is less likely to occur compared to when the second surface 402 is linear. If the second surface 402 of the core 40 is linear, the second surface 402 may be formed as an inclined surface that is inclined with respect to the first surface 401 of the core 40 or the mounting surface 101 of the substrate 10 from the first end 41 to the second end 42 of the core 40 so as to prevent an edge from forming on the second surface 402.

[0033] The first end 41 of the core 40 may be in contact with the mirror surface 301. That is, there may be no space between the core 40 and the mirror 30. If there is a space between the core 40 and the mirror 30, some of the light that expands after being reflected by the mirror 30 and before being introduced into the waveguide 20 may not be introduced into the waveguide 20, resulting in a coupling loss. In the waveguide substrate 1 according to this embodiment, there is no space between the core 40 and the mirror 30, and therefore the expansion of the light spot diameter can be reduced, and the coupling loss of the light can be reduced.

[0034] 2, the maximum height position of the first end 41 of the core 40 may be equal to or lower than the height position of the fourth surface 303 of the mirror 30. When the core 40 is located at a position higher than the fourth surface 303 of the mirror 30, it is possible to reduce the incidence of a portion of the light propagating through the core 40 being introduced into the upper end of the mirror surface 301, thereby reducing the coupling loss.

[0035] An end of the first cladding 51 may be in contact with the mirror surface 301. Specifically, the end of the first cladding 51 may be in contact with the mirror surface 301 on the mounting surface 101 of the substrate 10. Furthermore, as shown in Fig. 2, a core 40 may be formed between the end of the first cladding 51 and the mirror surface 301. With this configuration, more light reflected by the mirror 30 can be introduced into the core 40. Therefore, the coupling loss can be reduced.

[0036] The thickness T4 of the first cladding 51 at the second position P2 may be equal to or smaller than the thickness T3 of the first cladding 51 at the first position P1. Specifically, the thickness of the first cladding 51 may be constant along the longitudinal direction of the first cladding 51, or may gradually decrease toward the mirror 30. For example, the surface of the outer surface of the first cladding 51 facing the core 40 may be curved toward the mirror 30, so that the thickness of the first cladding 51 gradually decreases.

[0037] With this configuration, more light reflected by the mirror 30 can be introduced into the core 40. Therefore, the coupling loss can be reduced.

[0038] A thickness T6 of the second cladding 52 at the second position P2 may be smaller than a thickness T5 of the second cladding 52 at the first position P1. Specifically, the thickness of the second cladding 52 may gradually decrease as it approaches the mirror 30. For example, as shown in FIG. 2 , the surface of the outer surface of the second cladding 52 that faces the core 40 may be curved as it approaches the mirror 30, thereby gradually decreasing the thickness of the second cladding 52.

[0039] If the thickness T6 of the second cladding 52 at the second position P2 and the thickness T5 at the first position P1 were the same, the thickness of the entire first waveguide 21 would be large. In that case, the separation distance between the light-emitting element 3 and the mirror 30 would be large, and the distance between the light-emitting element 3 and the mirror 30 would be long, causing the light spot diameter to expand from when it is transmitted by the light-emitting element 3 to when it is reflected by the mirror 30. This could result in a coupling loss due to some of the light not being reflected when reflected by the mirror 30. In the waveguide substrate 1 according to this embodiment, the thickness T6 of the second cladding 52 at the second position P2 is smaller than the thickness T5 at the first position P1, and therefore the distance between the light-emitting element 3 and the mirror 30 is shortened, thereby reducing the expansion of the light spot diameter and reducing the coupling loss of the light.

[0040] The mirror surface 301 may have a concave cylindrical shape with its axis in the Y-axis direction. A mirror surface 301 having such a shape is less likely to spread reflected light compared to a flat or convex mirror surface. This reduces light coupling loss. The mirror surface 301 may also have a hemispherical shape that is concave toward the first end 41. A cylindrical or hemispherical mirror may have a vertex at a portion of the mirror surface that faces the central axis of the core 40.

[0041] Next, the configuration of the core 40 of the first waveguide 21 and the second waveguide 22 will be described with reference to Fig. 3. Fig. 3 is a schematic plan view showing the configuration of the core 40 of the first waveguide 21 and the second waveguide 22.

[0042] The width W2 of the core 40 at the second position P2 included in the first waveguide 21, which is a transmitting waveguide, may be larger than the width W1 at the first position P1. Here, when the cross-sectional shape of the core 40 is rectangular, the "width of the core 40" refers to the dimension in the direction perpendicular to the longitudinal direction of the core 40 and parallel to the mounting surface 101, here the Y-axis direction. When the cross-sectional shape of the core 40 is circular or elliptical, the "width of the core 40" refers to the longest part of the dimension in the Y-axis direction.

[0043] According to this configuration, the width W2 of the core 40 at the portion of the first waveguide 21 close to the mirror 30, for example, at the second position P2, is larger than the width W1 of other portions, for example, at the first position P1, so that more light reflected by the mirror 30 can be introduced into the core 40 compared to when the core 40 has a constant width. Therefore, the coupling loss can be reduced.

[0044] The width W4 of the core 40 of the second waveguide 22, which is a receiving waveguide, at the second position P2 may be smaller than the width W3 at the first position P1.

[0045] According to this configuration, the width W4 of the core 40 at the portion of the second waveguide 22 close to the mirror 30, for example, at the second position P2, is smaller than the width W3 at other portions, for example, at the first position P1. Therefore, compared to when the core 40 has a constant width, the spread of light at the second position P2 can be reduced, thereby reducing the optical coupling loss.

[0046] <Method for manufacturing waveguide substrate> Next, an example of a method for manufacturing the waveguide substrate 1 according to the first embodiment will be described with reference to Figures 4 to 7. Figures 4 to 7 are schematic views for explaining the method for manufacturing the waveguide substrate 1 according to the first embodiment.

[0047] 4, a first cladding 51 is formed on one surface of the substrate 10. The first cladding 51 is formed, for example, by applying a resin having a predetermined refractive index to a predetermined thickness on one surface of the substrate 10 and curing the resin with heat, light, or the like.

[0048] 5, a mirror 30 is placed on one surface of the substrate 10. Specifically, the mirror 30 is placed at a position where the end of the first cladding 51 laminated on one surface of the substrate 10 and the mirror surface 301 come into contact.

[0049] Next, as shown in FIG. 6 , the core 40 is formed on the surface of the first cladding 51. The core 40 is formed, for example, by applying a resin having a predetermined refractive index to the surface of the first cladding 51 to a predetermined thickness, curing the resin with heat or light, and then patterning the cured resin into a predetermined planar shape using a known method. At this time, the resin is applied so that the thickness of the resin gradually increases toward the mirror 30 disposed on the substrate 10. The resin is also applied so as to cover the mirror surface 301. This makes it possible to form a core 40 in which the thickness of the portion close to the mirror 30 is greater than the thickness of the other portions. It is also possible to form a core 40 in which there is no space between the mirror surface 301 and the core 40.

[0050] Next, as shown in Fig. 7, the second cladding 52 is formed on the surface of the core 40. The second cladding 52 is formed, for example, by applying a resin having a predetermined refractive index to the surface of the core 40 to a predetermined thickness and then curing the resin with heat, light, or the like. At this time, the resin is applied so that the fourth surface 303 of the mirror 30 and the surface of the resin are flush with each other. This makes it possible to form the second cladding 52 such that the thickness of the portion close to the mirror 30 is smaller than the thickness of the other portions.

[0051] In this manner, the waveguide substrate 1 according to the first embodiment is fabricated.

[0052] As described above, in the waveguide substrate 1 according to this embodiment, the thickness of the core 40 in the portion close to the mirror 30 is made larger than the thickness of the core 40 in other portions. With this configuration, more light reflected by the mirror 30 can be introduced into the core 40 compared to when the core 40 has a constant thickness. Therefore, the coupling loss can be reduced.

[0053] Second Embodiment Fig. 8 is a schematic cross-sectional view showing the configuration of a waveguide substrate 1 according to a second embodiment. Fig. 8 also shows the configuration of a second waveguide 22. As shown in Fig. 8, the second cladding 52 may face at least a portion of the fourth surface 303 of the mirror 30. For example, the second cladding 52 may be located across the surface of the core 40 and the fourth surface 303 of the mirror 30. Alternatively, the second cladding 52 may cover only a portion of the surface of the core 40 and the fourth surface 303 of the mirror 30.

[0054] Incidentally, if an interface having a different refractive index exists between the light-emitting element 3 or the light-receiving element 4 and the second cladding 52, light emitted from the light-emitting element 3 or the core 40 may be reflected at the interface, resulting in reflection loss. To prevent such an interface from occurring, the light-emitting element 3 or the light-receiving element 4 may be in direct or indirect contact with the second cladding 52 to prevent the formation of an air gap. Therefore, as shown in FIG. 8 , the waveguide chip 2 may be mounted so that the light-emitting element 3 or the light-receiving element 4 is in contact with the second cladding 52. As another example, if the light-emitting element 3 or the light-receiving element 4 is provided with a cover (not shown), the waveguide chip 2 may be mounted so that the cover is in contact with the second cladding 52. Furthermore, a solid, gel, or liquid refractive index matching agent may be filled between the light-emitting element 3, the light-receiving element 4, or the cover and the second cladding 52 to indirectly contact the light-emitting element 3, the light-receiving element 4, or the cover and the second cladding 52.

[0055] According to this configuration, no air layer is provided between the second cladding 52 and the light-emitting element 3 or the light-receiving element 4, and it is possible to reduce the amount of light emitted from the light-emitting element 3 that leaks from the upper end of the mirror surface 301 without entering the core 40, thereby reducing coupling loss. It is also possible to reduce the amount of light that leaks from the upper end of the mirror surface 301 without entering the light-receiving element 4, thereby reducing coupling loss.

[0056] The present technology may also be configured as follows. (1) The waveguide substrate includes a substrate (for example, substrate 10), a waveguide (for example, waveguide 20), and a mirror (for example, mirror 30). The waveguide is located on the substrate and includes a core (for example, core 40) and a clad (for example, clad 50) that covers at least a portion of the periphery of the core. The mirror faces an end (for example, first end 41) of the core in the longitudinal direction and has a mirror surface (for example, mirror surface 301) that is inclined with respect to the surface of the substrate. The mirror reflects electromagnetic waves (for example, light) incident from the thickness direction of the substrate and introduces them into the core, or reflects electromagnetic waves incident from the core in the thickness direction of the substrate. When an arbitrary position of the waveguide is defined as a first position (for example, first position P1) and a position closer to the mirror than the first position is defined as a second position (for example, second position P2), the thickness of the core at the second position is greater than the thickness of the core at the first position. (2) The waveguide substrate described in (1) above may have a waveguide chip (for example, waveguide chip 2) mounted on the substrate and including a photoelectric conversion circuit and a light-emitting element, and the waveguide chip may be attached at a position where the electromagnetic wave incident from the light-emitting element is reflected by the mirror surface and introduced into the core. (3) In the waveguide substrate described in (1) or (2) above, the thickness of the core may gradually increase as it approaches the mirror. (4) In the waveguide substrate described in (3) above, when the surface of the outer surface of the core facing the substrate is defined as a first surface and the surface opposite to the first surface (for example, first surface 401) is defined as a second surface (for example, second surface 402), the second surface of the portion of the core where the thickness gradually increases may be curved in a concave shape so as to move away from the substrate as it approaches the mirror. (5) In the waveguide substrate described in any one of (1) to (4) above, an end of the core may be in contact with the mirror surface.(6) In the waveguide substrate according to any one of (1) to (5) above, when the surface of the core's outer surface facing the substrate is designated as a first surface, the surface opposite the first surface is designated as a second surface, and the surface of the mirror's outer surface facing the substrate is designated as a third surface (e.g., third surface 302), and the surface opposite the third surface is designated as a fourth surface (e.g., fourth surface 303), the clad may have a first clad (e.g., first clad 51) covering the first surface of the core and a second clad (e.g., second clad 52) covering the second surface of the core, and the second clad may face at least a part of the fourth surface of the mirror. (7) The waveguide substrate according to (6) above has a waveguide chip mounted on the substrate and including a light-emitting element, and the waveguide chip is attached at a position where an electromagnetic wave incident from the light-emitting element is reflected by the mirror surface and introduced into the core, and the light-emitting element and the second clad may be in direct or indirect contact with each other. (8) In the waveguide substrate described in (6) above, the second position may be a position of an end of the first cladding that faces the fourth surface of the mirror. (9) In the waveguide substrate described in (6) above, the thickness of the first cladding at the second position may be equal to or less than the thickness of the first cladding at the first position. (10) In the waveguide substrate described in (6) above, the thickness of the second cladding at the second position may be smaller than the thickness of the second cladding at the first position. (11) In the waveguide substrate described in (6) above, the end of the first cladding may be in contact with the mirror surface. (12) In the waveguide substrate described in (6) above, the substrate may have a mounting surface on which the waveguide and the mirror are mounted, and the end of the first cladding may be in contact with the mirror surface on the mounting surface. (13) In the waveguide substrate described in (6) above, a core may be formed between the end of the first cladding and the mirror surface. (14) In the waveguide substrate according to any one of (1) to (13) above, the mirror surface may have a concave cylindrical shape.(15) The waveguide substrate according to any one of (1) to (14) above includes a plurality of waveguides, and the plurality of waveguides include a first waveguide (for example, the first waveguide 21) for propagating an electromagnetic wave transmitted from an electromagnetic wave transmitting element (for example, the light-emitting element 3) and a second waveguide (for example, the second waveguide 22) for propagating the electromagnetic wave to an electromagnetic wave receiving element (for example, the light-receiving element 4), and the width of the core included in the first waveguide at the second position may be larger than the width of the core included in the first waveguide at the first position. (16) The waveguide substrate according to any one of (1) to (14) above may include a plurality of waveguides, the plurality of waveguides including a first waveguide for propagating an electromagnetic wave transmitted from an electromagnetic wave transmitting element and a second waveguide for propagating the electromagnetic wave to an electromagnetic wave receiving element, and the width of a core included in the second waveguide at a second position may be smaller than the width of the core included in the second waveguide at the first position.

[0057] The disclosed embodiments should be considered in all respects as illustrative and not restrictive. Indeed, the above-described embodiments may be embodied in various forms. Furthermore, the above-described embodiments may be omitted, substituted, or modified in various ways without departing from the scope and spirit of the appended claims.

[0058] REFERENCE SIGNS LIST 1 Waveguide substrate 10 Substrate 20 Waveguide 30 Mirror 40 Core 50 Cladding 51 First cladding 52 Second cladding P1 First position P2 Second position

Claims

1. A waveguide substrate comprising: a substrate; a waveguide located on the substrate, the waveguide having a core and a cladding covering at least a portion of the periphery of the core; and a mirror facing an end of the core in the longitudinal direction, the mirror having a mirror surface inclined with respect to the surface of the substrate, the mirror reflecting an electromagnetic wave incident from the thickness direction of the substrate and introducing it into the core, or reflecting an electromagnetic wave incident from the core in the thickness direction of the substrate, wherein when an arbitrary position of the waveguide is defined as a first position and a position closer to the mirror than the first position is defined as a second position, the thickness of the core at the second position is greater than the thickness of the core at the first position.

2. The waveguide substrate according to claim 1, further comprising a waveguide chip mounted on said substrate and including a photoelectric conversion circuit and a light-emitting element, said waveguide chip being attached at a position where electromagnetic waves incident from said light-emitting element are reflected by said mirror surface and introduced into said core.

3. The waveguide substrate according to claim 1 or 2, wherein the thickness of said core gradually increases with increasing distance from said mirror.

4. A waveguide substrate as described in claim 3, wherein, among the outer surfaces of said core, a surface facing said substrate is defined as a first surface, and a surface opposite said first surface is defined as a second surface, and said second surface in a portion of said core whose thickness gradually increases is concavely curved so as to move away from said substrate as it approaches said mirror.

5. The waveguide substrate according to any one of claims 1 to 4, wherein an end of the core is in contact with the mirror surface.

6. A waveguide substrate according to any one of claims 1 to 5, wherein, when the surface of the outer surface of the core that faces the substrate is defined as a first surface, the surface opposite the first surface is defined as a second surface, and the surface of the outer surface of the mirror that faces the substrate is defined as a third surface, and the surface opposite the third surface is defined as a fourth surface, the cladding has a first cladding covering the first surface of the core and a second cladding covering the second surface of the core, and the second cladding faces at least a portion of the fourth surface of the mirror.

7. A waveguide substrate according to claim 6, further comprising a waveguide chip mounted on said substrate and including a light-emitting element, said waveguide chip being attached at a position where an electromagnetic wave incident from said light-emitting element is reflected by said mirror surface and introduced into said core, and said light-emitting element and said second clad are in direct or indirect contact with each other.

8. The waveguide substrate according to claim 6, wherein the second position is a position of an end of the first cladding opposite to a fourth surface of the mirror.

9. The waveguide substrate according to claim 6, wherein the thickness of said first cladding at said second position is equal to or less than the thickness of said first cladding at said first position.

10. The waveguide substrate according to claim 6, wherein the thickness of said second cladding at said second location is smaller than the thickness of said second cladding at said first location.

11. The waveguide substrate of claim 6, wherein an end of said first cladding is in contact with said mirror surface.

12. The waveguide substrate according to claim 6, wherein the substrate has a mounting surface on which the waveguide and the mirror are mounted, and an end of the first cladding contacts the mirror surface at the mounting surface.

13. The waveguide substrate according to claim 6, wherein said core is formed between an end of said first cladding and said mirror surface.

14. The waveguide substrate according to any one of claims 1 to 13, wherein the mirror surface has a concave cylindrical shape.

15. A waveguide substrate as claimed in any one of claims 1 to 14, comprising a plurality of said waveguides, the plurality of said waveguides comprising: a first waveguide for propagating an electromagnetic wave transmitted from an electromagnetic wave transmitting element; and a second waveguide for propagating an electromagnetic wave to an electromagnetic wave receiving element, wherein the width of the core of the first waveguide at the second position is greater than the width of the core of the first waveguide at the first position.

16. A waveguide substrate according to any one of claims 1 to 14, comprising a plurality of said waveguides, the plurality of said waveguides comprising: a first waveguide for propagating an electromagnetic wave transmitted from an electromagnetic wave transmitting element; and a second waveguide for propagating an electromagnetic wave to an electromagnetic wave receiving element, wherein the width of the core of the second waveguide at the second position is smaller than the width of the core of the second waveguide at the first position.

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