Optical waveguides and optical waveguide modules

Abstract

This optical waveguide is optically coupled to another optical waveguide including a first core having a first refractive index, and comprises a core and a cladding. The core has a refractive index different from the first refractive index. The cladding is disposed so as to cover at least part of the periphery of the core. The core is provided with an end surface and a side surface, has an elongated shape, and has, in order, a coupling part and an extension part adjacent to the coupling part along the longitudinal direction. The coupling part has an optical coupling surface located on the side surface, exposed from the cladding, and optically coupled to the first core. In the extension part, when two cross sections located with a predetermined space in the longitudinal direction therebetween are defined as a first cross section and a second cross section, in order from the one closer to the coupling part, a second area that is the area of the second cross section is larger than a first area that is the area of the first cross section.

Description

Technical Field

[0001] The disclosed embodiments relate to an optical waveguide and an optical waveguide module.

Background Art

[0002] Conventionally, an optical waveguide module in which two optical waveguides having different refractive indices are optically coupled to each other is known. The two optically coupled optical waveguides are, for example, a silicon optical waveguide and a polymer optical waveguide having a lower refractive index than the silicon optical waveguide.

[0003] The polymer optical waveguide is composed of, for example, a core made of a polymer material and a cladding covering the periphery of the core. A part of the upper surface of the core of the polymer optical waveguide is exposed from the cladding, and forms an optical coupling surface that is optically coupled to the tapered core of the silicon optical waveguide. The core of the polymer optical waveguide extends inside the cladding in a direction away from the silicon optical waveguide to the end face of the cladding.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] An optical waveguide according to one embodiment is an optical waveguide that is optically coupled with another optical waveguide including a first core having a first refractive index, and comprises a core and a cladding. The core has a refractive index different from the first refractive index. The cladding is arranged to cover at least a portion of the periphery of the core. The core has end faces and sides, is elongated, and along its longitudinal direction, has, in order, a coupling portion and an extension portion adjacent to the coupling portion. The coupling portion has an optical coupling surface located on the side and exposed from the cladding to be optically coupled with the first core. In the extension portion, when two cross-sections located at a predetermined distance apart in the longitudinal direction are designated as the first cross-section and the second cross-section, in order from the one closest to the coupling portion, the area of ​​the second cross-section is larger than the area of ​​the first cross-section is larger. [Brief explanation of the drawing]

[0006] [Figure 1] Figure 1 is a schematic perspective view of an optical waveguide module according to the first embodiment. [Figure 2] Figure 2 is a plan view of the optical waveguide module according to the first embodiment, as seen from the positive Z-axis direction. [Figure 3] Figure 3 is a cross-sectional view taken along the line III-III in Figure 2. [Figure 4] Figure 4 is a side view of the optical waveguide module according to the first embodiment, viewed from the negative X-axis direction. [Figure 5] Figure 5 shows an example of the relationship between optical coupling loss and core cross-sectional size in the second optical waveguide. [Figure 6] Figure 6 shows an example of the relationship between optical propagation loss and core cross-sectional size in a second optical waveguide. [Figure 7] Figure 7 shows the effect of the optical waveguide module according to the first embodiment (simulation results of optical loss in response to changes in cross-sectional size). [Figure 8] Figure 8 is a plan view of the optical waveguide module according to the second embodiment, viewed from the positive Z-axis direction. [Figure 9] Figure 9 is a cross-sectional view of the IX-IX line in Figure 8. [Figure 10] Figure 10 is a plan view of the optical waveguide module according to the third embodiment, viewed from the positive Z-axis direction. [Figure 11] Figure 11 is a cross-sectional view along the line XI-XI in Figure 10. [Modes for carrying out the invention]

[0007] Embodiments of the optical waveguide and optical waveguide module disclosed herein will be described below with reference to the attached drawings. However, the embodiments described below do not limit this disclosure. Furthermore, each embodiment can be combined as appropriate, provided that the processing content is not inconsistent. Also, the same parts are denoted by the same reference numerals in each of the embodiments below, and redundant descriptions are omitted.

[0008] Furthermore, in the embodiments described below, expressions such as "constant," "orthogonal," "perpendicular," or "parallel" may be used, but these expressions do not require strict adherence to "constant," "orthogonal," "perpendicular," or "parallel" conditions. In other words, each of the above expressions allows for deviations such as manufacturing accuracy or installation accuracy.

[0009] Furthermore, the diagrams referenced below are schematic representations for illustrative purposes. Therefore, details may be omitted, and the dimensional proportions may not necessarily match those of reality.

[0010] Furthermore, in the drawings referenced below, for the sake of clarity, mutually orthogonal X, Y, and Z axis directions are sometimes defined, and a Cartesian coordinate system is shown with the positive Z axis pointing vertically upward.

[0011] (First Embodiment) First, the configuration of the optical waveguide module 1 according to the first embodiment will be described with reference to Figure 1. Figure 1 is a schematic perspective view of the optical waveguide module 1 according to the first embodiment.

[0012] As shown in Figure 1, the optical waveguide module 1 includes a first optical waveguide 2 and a second optical waveguide 3. The first optical waveguide 2 and the second optical waveguide 3 are bonded together by a bonding material 4 made of, for example, a photocurable resin or the like.

[0013] The first optical waveguide 2 includes a substrate 21 and a first core 22.

[0014] The substrate 21 is a rectangular parallelepiped-shaped substrate having a thickness in the vertical direction (Z-axis direction). The substrate 21 is made of, for example, ceramics. The ceramics constituting the substrate 21 is, for example, a sintered body containing aluminum nitride (AlN), aluminum oxide (Al₂O₃, alumina), silicon carbide (SiC), silicon nitride (Si₃N₄) or the like as a main component.

[0015] The first core 22 is located on the lower surface of the substrate 21, and extends in one direction (X-axis direction) along the lower surface of the substrate 21. The first core 22 has a tapered shape (see Figure 2) with a width that narrows toward the tip. The first core 22 is a path for transmitting light, and is made of a material containing, for example, silicon.

[0016] The second optical waveguide 3 is an optical waveguide optically coupled to the first optical waveguide 2, and includes a second core 31 and a cladding 32.

[0017] The second core 31 is located at a position partially overlapping the first core 22 of the first optical waveguide 2 in a plan view as viewed from the vertical direction (Z-axis direction). One end face of the second core 31 is exposed at the end face of the cladding 32 on the side opposite to the first core 22 (the end face on the X-axis positive direction side), and forms a coupling surface that can be coupled to an unillustrated optical fiber. The second core 31 is a path for transmitting light, and is composed of a material having a refractive index different from that of the first core 22. The material constituting the second core 31 has, for example, a lower refractive index than the material constituting the first core 22. As the material constituting such a second core 31, for example, a polymer or the like having a lower refractive index than silicon can be used.

[0018] The cladding 32 is positioned to cover the periphery of the second core 31. The cladding 32 is made of a material with a refractive index different from that of the second core 31, and totally reflects light inside the second core 31 at the interface with the second core 31.

[0019] Next, details of the configuration of the second core 31 will be described with reference to Figure 2 to Figure 4. Figure 2 is a plan view of the optical waveguide module 1 according to the first embodiment as viewed from the positive Z-axis direction. Figure 3 is a cross-sectional view taken along line III-III in Figure 2. Figure 4 is a side view of the optical waveguide module 1 according to the first embodiment as viewed from the negative X-axis direction.

[0020] As shown in Figure 2 to Figure 4, the second core 31 has a coupling portion 311 and an extending portion 312.

[0021] The coupling portion 311 has an exposed surface 311a that is exposed from the cladding 32 at a notch 32a formed by cutting out a part of the upper surface of the cladding 32. The exposed surface 311a forms an optical coupling surface that is optically coupled to the first core 22 by being overlapped and joined to the first core 22 of the first optical waveguide 2.

[0022] The extending portion 312 extends inside the cladding 32 from the side surface adjacent to the exposed surface 311a of the coupling portion 311 to the end face (the end face on the positive X-axis direction side) located on the opposite side of the cladding 32 from the coupling portion 311, and propagates light between the coupling portion 311 and the end face of the cladding 32. A part of the extending portion 312 has a cross-sectional size perpendicular to the longitudinal direction (X-axis direction) larger than that of the coupling portion 311. Here, the cross-sectional size perpendicular to the longitudinal direction (X-axis direction) is a parameter defining the cross section, and is at least one parameter selected from a parameter group including thickness and width (diameter). In the examples of Figure 2 to Figure 4, a part of the extending portion 312 has a thickness and a width larger than those of the coupling portion 311.

[0023] To put it another way, the extended portion 312 has an elongated shape. Here, "elongated shape" does not refer only to thin, so-called strip-shaped shapes, but also includes any elongated shape with a predetermined thickness. This shape includes rectangular shapes, including those with a square cross-section. Furthermore, the shape may include portions with different cross-sectional areas at multiple locations along the longitudinal direction of the second core 31. In this case, the connecting portion 311 and the extended portion 312 constituting the second core 31 are adjacent to each other in the longitudinal direction of the second core 31. The surface of the extended portion 312 along the longitudinal direction is called the side surface 312cc.

[0024] Here, the extension portion 312 will be further explained using the reference numerals shown in Figures 2 and 3. Consider the two surfaces that intersect the longitudinal direction of the extension portion 312. In other words, the two intersecting surfaces are two cross-sections perpendicular to the longitudinal direction of the second core 31. The two cross-sections are surfaces in a direction that intersects the side surface 312cc. In particular, the two cross-sections are surfaces parallel to the end surface of the extension portion 312. Note that even when the shape of the extension portion 312 is such that the diameter (width) changes in the longitudinal direction relative to the side surface 312cc, as shown in Figures 2 and 3, the two cross-sections are surfaces parallel to the end surface of the extension portion 312.

[0025] The two cross-sections are located at a predetermined interval along the longitudinal direction of the extended portion 312. In Figures 2 and 3, the two cross-sections are the first cross-section 312aa and the second cross-section 312bb. In other words, with respect to the extended portion 312, when the cross-section located on the joint portion 311 side at a predetermined interval along its longitudinal direction is represented as the first cross-section 312aa, and the cross-section located further from the joint portion 311 than the first cross-section 312aa is represented as the second cross-section 312bb, the area of ​​the second cross-section 312bb is larger than the area of ​​the first cross-section 312aa.

[0026] The areas of the first cross-section 312aa and the second cross-section 312bb in the extended portion 312 can be determined, for example, by analyzing cross-sectional images taken with an electron microscope. The images used for image analysis may include general scanning electron microscope (SEM) images, or images of backscattered electrons obtained with the same scanning electron microscope. Furthermore, TOF-SIMS (time-of-flight secondary ion mass spectrometry) may be used to distinguish between the second core 31 and the cladding 32, including the first core 22.

[0027] Furthermore, the orientation along the plane of the first cross section 312aa and the second cross section 312bb (in other words, the direction in which they spread within the plane) is perpendicular to the side surface 312cc of the (second portion 312b).

[0028] By the way, in the second optical waveguide 3, which is optically coupled to the first optical waveguide 2, the optimal core cross-sectional size for reducing optical coupling loss is different from the optimal core cross-sectional size for reducing optical propagation loss.

[0029] Figure 5 shows an example of the relationship between optical coupling loss and core cross-sectional size in the second optical waveguide 3. In Figure 5, the "core cross-sectional size" on the horizontal axis refers to the size of the cross-section perpendicular to the longitudinal direction (X-axis direction) of the coupling portion 311 of the second core 31, and refers to the size of the cross-section when the thickness and width of the coupling portion 311 are equal.

[0030] As shown in Figure 5, the optical coupling loss in the second optical waveguide 3 decreases as the core cross-sectional size of the second optical waveguide 3 decreases. This is thought to be because as the core cross-sectional size of the second optical waveguide 3 decreases, the optical mode field diameter widens, and the movement of light between the coupling portion 311 of the first core 22 and the second core 31 becomes smoother. Therefore, from the viewpoint of coupling loss, it is desirable for the core cross-sectional size of the second optical waveguide 3 to be relatively small.

[0031] Figure 6 shows an example of the relationship between the optical propagation loss and the core cross-sectional size in the second optical waveguide 3. In Figure 6, the "core cross-sectional size" on the horizontal axis refers to the size of the cross-section perpendicular to the longitudinal direction (X-axis direction) of the extended portion 312 of the second core 31, and refers to the size of the cross-section when the thickness and width of the extended portion 312 are equal. In the example in Figure 6, the surface roughness Rq of the second core 31 is assumed to be 0.6 μm.

[0032] As shown in Figure 6, the propagation loss of light in the second optical waveguide 3 increases as the core cross-sectional size of the second optical waveguide 3 decreases. This is thought to be because the effect of the surface roughness Rq on the outer surface of the core becomes larger as the core cross-sectional size of the second optical waveguide 3 decreases, causing light scattering on the outer surface of the core. Therefore, from the viewpoint of propagation loss, it is desirable for the core cross-sectional size of the second optical waveguide 3 to be relatively large.

[0033] In this case, the polymer optical waveguide that is optically coupled to the silicon optical waveguide generally has a uniform core cross-sectional size along its longitudinal direction. Therefore, in a typical polymer optical waveguide, if the core cross-sectional size is set to a relatively small size from the standpoint of coupling loss, propagation loss may increase.

[0034] In contrast, the second optical waveguide 3 according to this embodiment has a second core 31 in which the cross-sectional size of the coupling portion 311 is relatively small and the cross-sectional size of the extension portion 312 is relatively large. By relatively reducing the cross-sectional size of the coupling portion 311 in the second core 31, coupling loss can be reduced compared to the case in which the core cross-sectional size of the second optical waveguide 3 is uniform in the longitudinal direction. Furthermore, by relatively increasing the cross-sectional size of the extension portion 312 in the second core 31, propagation loss can be reduced compared to the case in which the core cross-sectional size of the second optical waveguide 3 is uniform in the longitudinal direction.

[0035] Returning to the explanation of Figures 2 to 4, the extension 312 has a first portion 312a and a second portion 312b. The first portion 312a is connected to the joint 311. The size (area) of the cross-section of the first portion 312a increases as it moves away from the joint 311. Specifically, the size (area) of the cross-section of the first portion 312a gradually increases as it moves away from the joint 311. The second portion 312b is connected to the first portion 312a and has a constant cross-sectional size.

[0036] In this way, by providing the first portion 312a in the extended portion 312, light can be stably moved from the connecting portion 311 to the extended portion 312, and the scattering of light in the extended portion 312 can be reduced. Furthermore, by connecting the second portion 312b to the first portion 312a, the propagation of light in the extended portion 312 can be made smoother, and the scattering of light in the extended portion 312 can be reduced.

[0037] In this example, the size of the cross-section of the first part 312a gradually increases as it moves away from the joint 311. However, the example is not limited to this, and the size of the cross-section of the first part 312a may increase in stages as it moves away from the joint 311.

[0038] Furthermore, although an example is shown here in which the second part 312b is connected to the first part 312a, the example is not limited to this, and the second part 312b may be omitted. That is, the size (area) of the cross-section of the extended part 312 may increase along the entire length of the extended part 312 as it moves away from the joint part 311. This allows light to be transferred more stably from the joint part 311 to the extended part 312, and the scattering of light in the extended part 312 can be further reduced.

[0039] Next, the effects of the optical waveguide module 1 according to the first embodiment (simulation results of optical loss according to changes in cross-sectional size) will be explained with reference to Figure 7. Figure 7 is a diagram showing the effects of the optical waveguide module 1 according to the first embodiment (simulation results of optical loss according to changes in cross-sectional size).

[0040] In Figure 7, "Example" shows the simulation results of the optical loss measured when light is propagated from the first optical waveguide 2 to the second optical waveguide 3 in the optical waveguide module 1 according to this embodiment. Optical loss is the sum of coupling loss and propagation loss. The simulation conditions used in the example are as follows. Width of the starting end of the extended portion 312: 5 μm Width of the end of the extended portion 312: 9 μm Thickness at the starting end of the stretched portion 312: 5 μm Thickness of the end of the stretched portion 312: 9 μm Length of the second core 31 (= length of the joint 311 + length of the extended part 312): 6000 μm Length of the stretched portion 312: 3000 μm

[0041] Furthermore, in Figure 7, "Comparative Example" represents the simulation results of measuring the optical loss when light is propagated from the first optical waveguide 2 to the second optical waveguide 3 in the optical waveguide module 1 according to the comparative example. The optical waveguide module 1 according to the comparative example employs a structure in which the core cross-sectional size of the second optical waveguide 3 is uniform in the longitudinal direction, that is, a structure in which the core cross-sectional size of the coupling portion 311 and the extension portion 312 of the second core 31 are the same. The simulation conditions used in the comparative example are as follows. Width of the starting end of the extended portion 312: 5 μm Width of the end of the extended portion 312: 5 μm Thickness at the starting end of the stretched portion 312: 5 μm Thickness of the end of the stretched portion 312: 5 μm Length of the second core 31 (= length of the joint 311 + length of the extended part 312): 6000 μm Length of the stretched portion 312: 3000 μm

[0042] As is clear from the simulation results in Figure 7, the embodiment was able to reduce optical loss compared to the comparative example. From these simulation results, it can be concluded that in the second core 31, by relatively reducing the cross-sectional size of the coupling portion 311 and relatively increasing the cross-sectional size of the extension portion 312, both coupling loss and propagation loss can be reduced.

[0043] (Second Embodiment) Figure 8 is a plan view of the optical waveguide module 1 according to the second embodiment, viewed from the positive Z-axis direction. Figure 9 is a cross-sectional view taken along the line IX-IX in Figure 8.

[0044] As shown in Figures 8 and 9, in the optical waveguide module 1 according to the second embodiment, the size of the cross-section of the coupling portion 311 of the second core 31 perpendicular to the longitudinal direction (X-axis direction) decreases as it moves away from the extended portion 312. Specifically, the size of the cross-section of the coupling portion 311 gradually decreases as it moves away from the extended portion 312.

[0045] Thus, the cross-sectional size of the joint portion 311 may decrease as it moves away from the extended portion 312. With this configuration, the cross-sectional size of the joint portion 311 can be made smaller in the second core 31, and the coupling loss can be further reduced.

[0046] (Third embodiment) Figure 10 is a plan view of the optical waveguide module 1 according to the third embodiment, viewed from the positive Z-axis direction. Figure 11 is a cross-sectional view taken along the line XI-XI in Figure 10.

[0047] As shown in Figures 10 and 11, in the optical waveguide module 1 according to the third embodiment, the extension portion 312 of the second core 31 has a cross-sectional size perpendicular to the longitudinal direction (X-axis direction) that is generally larger than that of the coupling portion 311. In the example of Figures 10 and 11, the width of the extension portion 312 is generally larger than the width of the coupling portion 311, and the thickness of the extension portion 312 is generally larger than the thickness of the coupling portion 311. That is, the extension portion 312 is discontinuously connected to the coupling portion 311 via a step in the width direction (Y-axis direction) and a step in the thickness direction (Z-axis direction).

[0048] Thus, the extended portion 312 may be discontinuously connected to the connecting portion 311 via a step. This configuration makes it possible to further reduce propagation loss in the second core 31.

[0049] As described above, the optical waveguide according to the embodiment (for example, the second optical waveguide 3) is an optical waveguide that is optically coupled with another optical waveguide (for example, the first optical waveguide 2) which includes a first core (for example, the first core 22) having a first refractive index, and comprises a core (for example, the second core 31) and a cladding (for example, cladding 32). The core has a refractive index different from the first refractive index. The cladding is arranged to cover at least a portion of the periphery of the core. The core has an end face and a side surface, is elongated, and along its longitudinal direction, sequentially has a coupling portion (for example, the coupling portion 311) and an extended portion adjacent to the coupling portion (for example, extended portion 312). The coupling portion has an optical coupling surface (for example, the exposed surface 311a) located on the side surface and exposed from the cladding to be optically coupled with the first core. In the extended portion, when two cross-sections located at a predetermined interval in the longitudinal direction are designated as the first cross-section (for example, the first cross-section 312aa) and the second cross-section (for example, the second cross-section 312bb) in order from the one closest to the coupling portion, the area of ​​the second cross-section, the second area, is larger than the area of ​​the first cross-section, the first area. As a result, according to the optical waveguide of this embodiment, both coupling loss and propagation loss can be reduced.

[0050] Furthermore, the extended portion may have a first portion connected to the joint and a second portion connected to the first portion. The first portion includes a first cross-section and a second cross-section, and the area of ​​the cross-section of the first portion may increase as it moves away from the joint. As a result, according to the optical waveguide of this embodiment, light scattering in the extended portion can be reduced.

[0051] Furthermore, the cross-sectional area of ​​the first portion may gradually increase as it moves away from the joint. This allows the optical waveguide according to the embodiment to reduce light scattering in the extended portion.

[0052] Furthermore, the cross-sectional area of ​​the first portion may increase in stages as it moves away from the joint. This makes it possible to reduce light scattering in the extended portion according to the optical waveguide according to the embodiment.

[0053] Furthermore, the cross-sectional area of ​​the extended portion may increase along the entire length of the extended portion as it moves away from the joint. As a result, according to the optical waveguide of this embodiment, light scattering in the extended portion can be further reduced.

[0054] Furthermore, the cross-sectional area of ​​the coupling portion may decrease as it moves away from the extended portion. This allows for a further reduction in coupling loss according to the optical waveguide according to the embodiment.

[0055] Furthermore, the extended portion may be discontinuously connected to the joint portion via at least one of a step in the width direction and a step in the thickness direction. This allows for a further reduction in propagation loss in the core according to the optical waveguide of this embodiment.

[0056] Further effects and alternative embodiments can be readily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the specific details and representative embodiments expressed and described above. Accordingly, various modifications are possible without departing from the spirit or scope of the overall concept of the invention as defined by the appended claims and their equivalents. [Explanation of symbols]

[0057] 1. Optical waveguide module 2 First optical waveguide 3 Second optical waveguide 4 Bonding material 21 circuit boards 22 First Core 31 Second Core 32 Clad 311 Joint 311a Exposed surface 312 Extension section 312a Part 1 312aa 1st section 312bb 2nd cross section 312b Part 2 312cc side

Claims

1. An optical waveguide that is optically coupled with another optical waveguide, The core and A cladding arranged to cover at least a portion of the periphery of the core and It has, The core has an end face and a side surface, is elongated, and along its longitudinal direction, sequentially has a connecting portion and an extended portion adjacent to the connecting portion. The coupling portion has an optical coupling surface located on the side surface and exposed from the cladding, which is optically coupled to the core of the other optical waveguide. The extended portion has a portion in which the width and thickness perpendicular to the longitudinal direction increase in a direction away from the side closer to the joint along the longitudinal direction, In the extended portion, when two cross-sections located at a predetermined interval in the longitudinal direction are designated as the first cross-section and the second cross-section, respectively, starting from the one closest to the joint, the second area, which is the area of ​​the second cross-section, is larger than the first area, which is the area of ​​the first cross-section, in this optical waveguide.

2. The aforementioned extended portion is, The first portion connected to the aforementioned joint, A second part connected to the first part and It has, The first portion includes the first cross-section and the second cross-section, The optical waveguide according to claim 1, wherein the area of ​​the cross-section of the first portion increases as it moves away from the joint.

3. The optical waveguide according to claim 2, wherein the area of ​​the cross-section of the first portion gradually increases as it moves away from the joint.

4. The optical waveguide according to claim 2, wherein the area of ​​the cross-section of the first portion increases in stages as it moves away from the joint.

5. The optical waveguide according to claim 1, wherein the area of ​​the cross-section of the extended portion increases along the entire length of the extended portion as it moves away from the joint portion.

6. The optical waveguide according to claim 1, wherein the area of ​​the cross-section of the joint decreases as it moves away from the extended portion.

7. The optical waveguide according to claim 1, wherein the extended portion is discontinuously connected to the connecting portion via at least one of a step in the width direction and a step in the thickness direction.

8. The optical waveguide according to claim 1, wherein the extended portion is discontinuously connected to the connecting portion via a step in the width direction and a step in the thickness direction.

9. A first optical waveguide including a first core having a first refractive index, The first optical waveguide and the second optical waveguide which are optically coupled It has, The second optical waveguide is, A second core having a refractive index different from the first refractive index, A cladding arranged to cover at least a portion of the periphery of the second core and It has, The second core has an end face and a side surface, is elongated, and along its longitudinal direction, sequentially has a connecting portion and an extended portion adjacent to the connecting portion. The coupling portion is located on the side surface and has an optical coupling surface that is exposed from the cladding and optically coupled to the first core. The extended portion has a portion in which the width and thickness perpendicular to the longitudinal direction increase in a direction away from the side closer to the joint along the longitudinal direction, In the extended portion, when two cross-sections located at a predetermined interval in the longitudinal direction are designated as the first cross-section and the second cross-section, respectively, starting from the one closest to the joint, the area of ​​the second cross-section (second area) is larger than the area of ​​the first cross-section (first area), this is an optical waveguide module.

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