Optical coupling element

Abstract

An optical coupling element (1) according to the present invention comprises: a substrate (10); a cladding (20) that is superposed on the upper surface of the substrate; and an optical waveguide (30) that is embedded in the cladding and is disposed on one surface of the substrate. The optical waveguide (30) has two long-axis optical waveguides (31, 33) that are at a distance from each other and extend in parallel to each other, and a central optical waveguide (32) that is disposed between the two long-axis optical waveguides. The long-axis optical waveguides (31, 33) are each provided with a plurality of first regions (R1) and a plurality of second regions (R2), which are alternately connected in the extending direction of the central optical waveguide. The second regions (R2) each have a smaller area of the long-axis optical waveguide disposed with respect to the substrate than the first regions (R1) when viewed from the direction in which the cladding is superposed. The central optical waveguide (32) includes a tapered portion (32a) at least at one end in the extending direction of the central optical waveguide.

Description

Optical coupling element

[0001] The present invention relates to an optical coupling element.

[0002] In optical communications, optical coupling elements are required to improve the optical coupling efficiency between optical waveguides and optical fibers. Patent Document 1 discloses an optical coupling element in which three optical waveguides are arranged in a cladding layer formed on a semiconductor substrate, and two of the three optical waveguides have a plurality of spaced segments arranged along the longitudinal direction.

[0003] US Patent Application Publication No. 2019 / 0170936

[0004] Since there is a difference in mode size between the optical waveguide formed on the substrate and the optical waveguide of an external circuit, such as an optical fiber, loss at the coupling point is large and it is not easy to achieve optical coupling as is. Therefore, an optical coupling element to be placed on a substrate is required to reduce the difference in mode size between the optical waveguide formed on the substrate and the optical waveguide of the external circuit and reduce loss in optical coupling.

[0005] In view of the above problems, an object of the present invention is to provide an optical coupling element capable of reducing loss in optical coupling between an optical waveguide formed on a substrate and an optical waveguide in an external circuit such as an optical fiber.

[0006] The optical coupling element of the present invention is an optical coupling element comprising a substrate, a clad stacked on an upper surface of the substrate, and an optical waveguide embedded in the clad and arranged against one surface of the substrate, wherein the optical waveguide has two longitudinal optical waveguides extending in parallel and spaced apart from each other, and a central optical waveguide arranged between the two longitudinal optical waveguides, wherein the longitudinal optical waveguide is formed with a plurality of first regions and a plurality of second regions alternately connected in the extension direction of the central optical waveguide, and each of the second regions has a smaller area of ​​the longitudinal optical waveguide arranged against the substrate than each of the first regions when viewed from the direction in which the clad is stacked, and the central optical waveguide includes a tapered portion in at least one of the extension directions of the central optical waveguide.

[0007] The optical coupling element according to the present invention can reduce loss in optical coupling between an optical waveguide formed on a substrate and an optical waveguide in an external circuit such as an optical fiber.

[0008] FIG. 1A is a schematic diagram showing the overall configuration of an optical coupling element according to one embodiment of the present invention. FIG. 1B is a schematic diagram showing the overall configuration of an optical coupling element according to one embodiment of the present invention. FIG. 2A is a schematic diagram showing the configurations of a longitudinal optical waveguide and a central optical waveguide of an optical coupling element according to a comparative example. FIG. 2B is a schematic diagram showing the configurations of a longitudinal optical waveguide and a central optical waveguide of an optical coupling element according to a comparative example. FIG. 3A is a diagram showing the results of calculating the loss during optical coupling relative to the shift in the light source position of an optical coupling element according to one embodiment of the present invention. FIG. 3B is a diagram showing the results of calculating the loss during optical coupling relative to the shift in the light source position of an optical coupling element according to one embodiment of the present invention. FIG. 3C is a diagram showing the results of calculating the loss during optical coupling relative to the shift in the light source position of an optical coupling element according to one embodiment of the present invention. FIG. 4A is a diagram showing the results of calculating the loss during optical coupling relative to the shift in the light source position when the width of the second region of the longitudinal optical waveguide of an optical coupling element according to one embodiment of the present invention is changed in a direction perpendicular to the extension direction of the central optical waveguide. Figure 4B is a diagram showing the results of calculations of optical coupling loss in relation to a shift in the light source position when the width of the second region of the longitudinal optical waveguide of an optical coupler according to an embodiment of the present invention is changed in a direction perpendicular to the extension direction of the central optical waveguide. Figure 4C is a diagram showing the results of calculations of optical coupling loss in relation to a shift in the light source position when the width of the second region of the longitudinal optical waveguide of an optical coupler according to an embodiment of the present invention is changed in a direction perpendicular to the extension direction of the central optical waveguide. Figure 4D is a diagram showing the results of calculations of optical coupling loss in relation to a shift in the light source position when the width of the second region of the longitudinal optical waveguide of an optical coupler according to an embodiment of the present invention is changed in a direction perpendicular to the extension direction of the central optical waveguide. Figure 4E is a diagram showing the results of calculations of optical coupling loss in relation to a shift in the light source position when the width of the second region of the longitudinal optical waveguide of an optical coupler according to an embodiment of the present invention is changed in a direction perpendicular to the extension direction of the central optical waveguide. 4A is a diagram showing a result of calculating the loss during optical coupling with respect to the shift in the light source position when the width of the second region of the longitudinal optical waveguide of the optical coupling element in the direction perpendicular to the extension direction of the central optical waveguide in the longitudinal optical waveguide of the optical coupling element according to the embodiment of the present invention is changed.Fig. 5B is a schematic diagram showing a modified configuration of a longitudinal optical waveguide of an optical coupling element according to an embodiment of the present invention. Fig. 5C is a schematic diagram showing a modified configuration of a longitudinal optical waveguide of an optical coupling element according to an embodiment of the present invention. Fig. 5D is a schematic diagram showing a modified configuration of a longitudinal optical waveguide of an optical coupling element according to an embodiment of the present invention. Fig. 5E is a schematic diagram showing a modified configuration of a longitudinal optical waveguide of an optical coupling element according to an embodiment of the present invention. Fig. 6A is a diagram showing the results of calculation of optical coupling loss relative to a shift in the light source position in the modified configuration of the longitudinal optical waveguide of the optical coupling element shown in Fig. 5A. Fig. 6B is a diagram showing the results of calculation of optical coupling loss relative to a shift in the light source position in the modified configuration of the longitudinal optical waveguide of the optical coupling element shown in Fig. 5B. Fig. 6C is a diagram showing the results of calculation of optical coupling loss relative to a shift in the light source position in the modified configuration of the longitudinal optical waveguide of the optical coupling element shown in Fig. 5C.

[0009] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in the drawings will be designated by the same reference numerals, and duplicate descriptions will be omitted as appropriate. Furthermore, the scale of each component in the drawings may differ from the actual scale.

[0010] In the drawings, directions may be indicated by the X-axis, Y-axis, and Z-axis. The X-axis, Y-axis, and Z-axis are mutually perpendicular directions. The direction in which an arrow points in the X-axis direction is referred to as the +X direction or +X side, and the direction opposite to the +X direction is referred to as the -X direction or -X side. The direction in which an arrow points in the Y-axis direction is referred to as the +Y direction or +Y side, and the direction opposite to the +Y direction is referred to as the -Y direction or -Y side. The direction in which an arrow points in the Z-axis direction is referred to as the +Z direction or +Z side, and the direction opposite to the +Z direction is referred to as the -Z direction or -Z side.

[0011] 1A and 1B are schematic diagrams showing the overall configuration of an optical coupling element 1 according to one embodiment of the present invention, with Fig. 1A being a plan view of the optical coupling element 1 and Fig. 1B being a cross-sectional view taken along line IB-IB of the optical coupling element 1 shown in Fig. 1A.

[0012] In the following description, the direction perpendicular to the extending direction of the central optical waveguide 32 when viewed from the direction in which the cladding 20 is stacked for each component is referred to as the X direction, the direction in which the cladding 20 is stacked is referred to as the Y direction, and the extending direction of the central optical waveguide 32 is referred to as the Z direction.

[0013] The optical coupling element 1 includes a substrate 10 , a clad 20 laminated on an upper surface 10 a of the substrate 10 , and an optical waveguide 30 embedded in the clad 20 and disposed against one surface of the substrate 10 .

[0014] 1A and 1B, w is the width in the X direction of the longitudinal optical waveguides 31 and 33 as viewed from the direction in which the cladding 20 is stacked. R1 is the first region, and R2 is the second region. Details of the first region R1 and the second region R2 will be explained later. Hereinafter, when it is not necessary to distinguish between the longitudinal optical waveguides 31 and 33 and the central optical waveguide 32, they will be referred to as the "optical waveguide 30."

[0015] The optical coupling element 1 is coupled to, for example, an optical fiber at one end surface 20 a. The optical coupling element 1 and the optical fiber can be coupled using, for example, an optically transparent adhesive, an index matching fluid, or the like, but is not limited to these.

[0016] The substrate 10 is preferably made of a material with a low dielectric constant, such as a flat plate made of single-crystal silicon, III-V compounds, polyimide resin, epoxy resin, phenolic resin, polyphenylene ether resin, fluororesin, cycloolefin polymer, liquid crystal polymer, quartz, ceramics, glass epoxy (glass fiber impregnated with epoxy resin), glass composite (glass cloth and nonwoven glass cloth impregnated with epoxy resin), paper epoxy (paper impregnated with epoxy resin), paper phenolic (paper impregnated with phenolic resin), LTCC (long-term thermal conductivity capacitor) containing alumina and glass, HTCC (high-temperature thermal conductivity capacitor) containing alumina and aluminum nitride, or composite materials combining ceramics with various resins. A substrate in which a single-crystal silicon layer is formed on the opposite side of an oxide film formed on the substrate is called SOI (Silicon-on-Insulator). The technology of high-density integration of multiple optical elements on SOI is called silicon photonics.

[0017] The optical waveguide 30 is a portion that transmits light and has a high refractive index. The optical waveguide 30 is made of, for example, a semiconductor containing silicon, SiO ２ The optical waveguide 30 is made of a compound semiconductor such as silicon or a variety of resin materials. The optical waveguide 30 is formed by, for example, a photolithography method, a photobleaching method, a direct exposure method, a stamping method, or the like. In particular, the optical waveguide 30 in Si photonics is made of Si and is made of SiO ２ is often formed as the cladding 20.

[0018] The optical waveguide 30 is not limited to Si, and various other materials may be used, such as InGaAs, InGaAsP, InAlAs, GaAs, Si, Ge, SiN, InP, and III-V group compound semiconductors. When the optical coupling element 1 is formed using a material other than Si, the same method as when the optical waveguide 30 is formed using Si is used.

[0019] The cladding 20 is laminated on the substrate 10 so as to cover the upper surface 10a of the substrate 10. The cladding 20 is a portion that physically protects the optical waveguide 30, and has a lower refractive index than the optical waveguide 30. The material of the cladding 20 is, for example, silicon dioxide (SiO ２ However, the present invention is not limited to these, and any material may be used as long as it protects the upper surface 10a of the substrate 10 and has insulating properties.

[0020] If Si is used as the material of the optical waveguide 30, the refractive index becomes 3.5. If SiO is used as the material of the cladding 20, the refractive index becomes 3.5. ２ The refractive index is 1.45 when the longitudinal optical waveguides 31, 33 and the central optical waveguide 32 are formed using a material having a higher refractive index than the material forming the cladding 20. ２ The large difference in refractive index between the cladding 20 and the Si photonics layer allows for total reflection of light, resulting in strong light confinement. This makes it possible to bend the optical waveguide 30 within a small range in Si photonics, enabling the optical circuit to be significantly miniaturized.

[0021] By making the refractive index of the optical waveguide 30 higher than the refractive index of the cladding 20 , the light input to the optical waveguide 30 propagates in the Z direction according to the planar shape of the optical waveguide 30 .

[0022] To achieve single-mode conditions, the thickness of the optical waveguide 30 is preferably 200 to 400 nm in the Y direction. For example, when light in the 1550 nm wavelength band is used, the thickness of the optical waveguide 30 can be set to 300 nm. Note that, in order to prevent light propagating through the optical waveguide 30 from leaking into the substrate 10, the optical waveguide 30 is preferably formed at a distance of at least 1000 nm from the substrate 10.

[0023] The optical coupling element 1 according to this embodiment can be easily manufactured by using, for example, an SOI substrate as described above. An example of a method for manufacturing the optical coupling element 1 will now be described.

[0024] First, the substrate layer, SiO ２ The SOI substrate is formed by sequentially stacking a SiO 2 layer and a Si layer. Next, the Si layer is patterned by, for example, photolithography including an etching process. As a result, a SiO 2 layer is formed on the substrate layer as the substrate 10. ２ The layers are stacked, and SiO ２ The optical waveguide 30 is formed on the layer. The optical waveguide 30 has thicker and thinner portions. Therefore, photolithography including an etching step is performed in two stages, for example.

[0025] Next, for example, CVD (Chemical Vapor Deposition) is used to deposit SiO ２ The optical waveguide 30 is coated on the layer. ２ layer and SiO formed thereon ２ Thus, the cladding 20 is formed, and the optical coupling element 1 is obtained.

[0026] The optical coupling element 1 has two longitudinal optical waveguides 31, 33 extending in parallel and spaced apart from each other, and a central optical waveguide 32 disposed between the two longitudinal optical waveguides 31, 33. The two longitudinal optical waveguides 31, 33 and the central optical waveguide 32 extend into the cladding 20 at equal distances from the substrate 10. The two longitudinal optical waveguides 31, 33 and the central optical waveguide 32 are not limited to having rectangular cross sections as shown in the figure, but may also have a rib-like cross section. Specifically, the two longitudinal optical waveguides 31, 33 and the central optical waveguide 32 may be connected via a thin silicon layer on the substrate 10 side (Y direction side). In this case, the thin silicon layer does not function as the optical waveguide 30.

[0027] In the illustrated example, the longitudinal optical waveguides 31 and 33 extend from the end face 20 a, but may extend from the end face 20 a in the Z direction starting from within the clad 20. The longitudinal optical waveguides 31 and 33 are formed without including any part that separates them in the direction in which they extend.

[0028] The central optical waveguide 32 includes a tapered portion 32a in at least one of the extending directions of the central optical waveguide 32 when viewed from the direction in which the cladding 20 is laminated. The central optical waveguide 32 extends with a constant width in the extending direction of the longitudinal optical waveguides 31, 33 in the portions other than the tapered portion 32a.

[0029] The length in the Z direction of the tapered portion 32a of the central optical waveguide 32 is at least 10 nm, but this design can be changed as appropriate depending on the configuration of the optical circuit. The width in the X direction at the tip of the tapered portion 32a of the central optical waveguide 32 is 10 nm to 800 nm, preferably 50 nm to 500 nm, and more preferably 100 nm to 300 nm. When configured with these dimensions, optical coupling loss is easily reduced. In this embodiment, the width at the tip of the tapered portion 32a of the central optical waveguide 32 is 100 nm.

[0030] The length in the Z direction of the central optical waveguide 32 varies depending on the configuration of the optical circuit. The width in the X direction of the central optical waveguide 32 other than the tapered portion 32a is 100 nm to 800 nm, preferably 200 nm to 600 nm, and more preferably 400 nm to 500 nm. When configured with such widths, loss in optical coupling is easily reduced. In this embodiment, the width of the central optical waveguide 32 other than the tapered portion 32a is 480 nm.

[0031] The longitudinal optical waveguide 31 may be configured to integrally include the protrusion 31a. In this case, along the extension direction of the central optical waveguide 32, the region where the protrusion 31a exists is the first region R1, and the region where the protrusion 31a does not exist is the second region R2. In the first region R1, the width w in the direction (X direction) perpendicular to the extension direction (Z direction) of the central optical waveguide 32, as seen from the direction in which the cladding 20 is stacked, is larger than the width w of the longitudinal optical waveguide 31 in the second region R2. In the second region R2, the longitudinal optical waveguide 31 extends in the Z direction with a constant width w.

[0032] The length in the Z direction of the first region of the longitudinal optical waveguides 31, 33 is at least 5 nm, but the design can be changed as appropriate depending on the configuration of the optical circuit. The width w in the X direction of the first region of the longitudinal optical waveguides 31, 33 is 10 nm to 1000 nm, preferably 50 nm to 500 nm, and more preferably 100 nm to 400 nm. In this embodiment, the length in the Z direction of the first region of the longitudinal optical waveguides 31, 33 is 200 nm, and the width w in the X direction is 300 nm.

[0033] The length in the Z direction of the second region of the longitudinal optical waveguides 31, 33 is at least 5 nm, but the design can be changed appropriately depending on the configuration of the optical circuit. The width w in the X direction of the second region of the longitudinal optical waveguides 31, 33 is 10 nm to 1000 nm, preferably 50 nm to 500 nm, and more preferably 100 nm to 400 nm. In this embodiment, the length in the Z direction of the second region of the longitudinal optical waveguides 31, 33 is 200 nm, and the width w in the X direction is 50, 100, 150, 200, 250, or 300 nm.

[0034] The longitudinal optical waveguide 33 may be configured to integrally include a protrusion 33a, similar to the longitudinal optical waveguide 31. In this case, in the Z direction, the region where the protrusion 33a exists is the first region R1, and the region where the protrusion 33a does not exist is the second region R2. In the first region R1, the width w of the longitudinal optical waveguide 33 in the X direction as seen from the direction in which the cladding 20 is stacked is larger than the width w of the longitudinal optical waveguide 33 in the second region R2. In the second region R2, the longitudinal optical waveguide 33 extends in the Z direction with a constant width w.

[0035] Of the light incident on the longitudinal optical waveguides 31 and 33, polarized light in a specific wavelength band leaks out of the longitudinal optical waveguides 31 and 33, is gradually input into the tapered portion 32a of the central optical waveguide 32, and propagates in the Z direction.

[0036] In this structure, the ratio of the volume of the optical waveguide 30 to the cladding is larger in the first region R1 than in the second region R2, and therefore the refractive index of the first region R1 is relatively higher than that of the second region R2. In the longitudinal optical waveguides 31 and 33, a plurality of first regions R1 and a plurality of second regions R2 are formed alternately and consecutively in the extension direction (Z direction) of the central optical waveguide 32. Furthermore, in each of the second regions R2, the area of ​​the longitudinal optical waveguides 31 and 33 arranged on the substrate 10 is smaller than that of the respective first regions R1 when viewed from the direction in which the cladding 20 is stacked.

[0037] 2A and 2B are schematic diagrams showing the configurations of longitudinal optical waveguides 91 and 93 and a central optical waveguide 92 of an optical coupling element 9 according to a comparative example. In Fig. 2A and Fig. 2B, components other than the optical waveguides are not shown.

[0038] 2A shows a structure in which the longitudinal optical waveguides 91 and 93 are absent in the region corresponding to the second region R2 of the optical coupling element 1 according to this embodiment, and only the cladding is disposed. In other words, in the region corresponding to the first region R1, the longitudinal optical waveguides 91 and 93 are each divided into multiple segments that are spaced apart from each other. In this structure, the difference in refractive index between the region corresponding to the first region R1 and the region corresponding to the second region R2 becomes large. Also, FIG. 2B shows an example in which the longitudinal optical waveguides 91 and 93 have a tapered structure, and the refractive index changes continuously.

[0039] <Calculation Results of Optical Coupling Loss Due to Misalignment of Light Source Position> Figures 3A to 3C are diagrams showing calculation results of optical coupling loss due to misalignment of the light source position of the optical coupling element 1 according to one embodiment of the present invention. The calculation results were obtained from electromagnetic field simulation using the finite difference time domain method (FDTD). Figure 3A shows the calculation results for the structure of the optical coupling element 1 according to this embodiment, Figure 3B shows the calculation results for the structure of the optical coupling element 9 according to the comparative example shown in Figure 2A, and Figure 3C shows the calculation results for the structure of the optical coupling element 9 according to the comparative example shown in Figure 2B.

[0040] The calculation results shown in the figure show the shift (nm) of the light source position in the X direction, the loss (dB) of the TE polarization component, and the loss (dB) of the TM polarization component for each wavelength between 1525 nm and 1600 nm.

[0041] 3A to 3C show calculation results for TE and TM polarizations, with the wavelength and the shift in the light source position in the X direction as variables. As shown in the figures, the maximum loss for the TE polarization component is approximately 1.4 dB, and the maximum loss for the TM polarization component is approximately 1.7 dB.

[0042] The loss (dB) of the TE polarization component and the loss (dB) of the TM polarization component are compared between the optical coupling device 1 according to this embodiment and the optical coupling device 9 according to the comparative example. In the structure of the optical coupling device 9 according to the comparative example shown in FIG. 2A, shown in FIG. 3B, the loss of the TM polarization component is particularly large when the light source position is misaligned. Furthermore, in the structure of the optical coupling device 9 according to the comparative example shown in FIG. 2B, shown in FIG. 3C, the loss of both the TE polarization component and the TM polarization component is large.

[0043] The refractive index difference between the first region R1 and the second region R2 in the optical coupling element 1 according to this embodiment is less than that in the optical coupling element 9 according to the comparative example shown in Fig. 2A, and the refractive index does not change continuously as in the optical coupling element 9 according to the comparative example shown in Fig. 2B. In other words, when the refractive index difference between the first region R1 and the second region R2 is moderately reduced as in the optical coupling element 1 according to this embodiment, it is thought that the loss during optical coupling will be small as shown in Figs. 3A to 3C.

[0044] These results show that the optical coupling element 1 according to this embodiment has smaller losses in the TE and TM polarized waves of light in any wavelength band and has lower polarization dependency than the optical coupling element 9 according to the comparative example. Furthermore, the optical coupling element 1 according to this embodiment exhibits high optical coupling efficiency even when the light source position is shifted, and has a wide alignment tolerance range.

[0045] 4A to 4F are diagrams showing calculation results of optical coupling loss in relation to a shift in the light source position when the width w of the longitudinal optical waveguides 31 and 33 in the second region R2 is changed. The width w in the second region R2 of the longitudinal optical waveguides 31 and 33 of the optical coupling element 1 according to one embodiment of the present invention is changed in a direction perpendicular to the extension direction of the central optical waveguide 32. The width w of the longitudinal optical waveguides 31 and 33 in the second region R2 was changed between 50 nm and 300 nm. In each case, the width w of the longitudinal optical waveguides 31 and 33 in the first region R1, including the protrusions 31 a and 33 a, is 300 nm.

[0046] The calculation results shown in the figures, similar to those in FIGS. 3A to 3C, show the shift (nm) of the light source position in the X direction, the loss (dB) of the TE polarization component, and the loss (dB) of the TM polarization component for wavelengths between 1525 nm and 1600 nm.

[0047] The width w of the second region R2 of the longitudinal optical waveguides 31 and 33 is 50 nm in FIG. 4A, 100 nm in FIG. 4B, 150 nm in FIG. 4C, 200 nm in FIG. 4D, 250 nm in FIG. 4E, and 300 nm in FIG. 4F.

[0048] As shown in the figure, the loss (dB) of the TE polarization component and the loss (dB) of the TM polarization component tend to be closer to 1 dB when the width w of the second region R2 of the longitudinal optical waveguides 31 and 33 is smaller.

[0049] 5A to 5E are schematic diagrams showing modified configurations of the longitudinal optical waveguides 31, 33 of the optical coupling element 1 according to one embodiment of the present invention. In each of the cases shown in FIGS. 5A to 5E, the areas of the longitudinal optical waveguides 31, 33 in the first region R1 and the second region R2 are different when viewed from the direction in which the cladding 20 is stacked. Also, in each of the cases shown in FIGS. 5A to 5C, the width w in the second region R2 in the direction (X direction) perpendicular to the extension direction (Z direction) of the central optical waveguide 32 when viewed from the direction in which the cladding 20 is stacked varies in the extension direction of the central optical waveguide 32.

[0050] 5A, the width w of the longitudinal optical waveguides 31, 33 is the same in the first region R1 and the second region R2, and multiple circular portions without optical waveguides are formed within the longitudinal optical waveguides 31, 33 in the second region R2. In the illustrated example, there are three portions without optical waveguides, but this is not limited to this. Also, for example, the width w of the longitudinal optical waveguides 31, 33 may be 300 nm, and the diameter r1 of the circular portions without optical waveguides may be 100 nm, but this is not limited to this.

[0051] 5B , the width w of the longitudinal optical waveguides 31, 33 is the same in the first region R1 and the second region R2, and in the second region R2, one circular portion where no optical waveguide is formed is formed within the longitudinal optical waveguides 31, 33. For example, the width w of the longitudinal optical waveguides 31, 33 may be 300 nm, and the diameter r2 of the circular portion where no optical waveguide is formed may be 200 nm, but this is not limitative.

[0052] 5C shows a structure in which the width w of the longitudinal optical waveguides 31, 33 in the second region R2 decreases toward the center of the second region R2. For example, the width w of the longitudinal optical waveguides 31, 33 in the first region R1 may be 300 nm, and the minimum width r3 in the second region R2 may be 50 nm, but this is not limitative.

[0053] The example of Fig. 5D is a structure in which the long-axis shaped optical waveguides 31 and 33 are provided in the second region R2, biased to one side in the X direction with respect to the first region R1. Furthermore, the example of Fig. 5E is a structure in which the long-axis shaped optical waveguides 31 and 33 are provided in the second region R2, tilted with respect to the Z direction.

[0054] 6A to 6C are diagrams showing the results of calculations of losses during optical coupling relative to the shift in the light source position in modified configurations of the longitudinal optical waveguides 31 and 33 of the optical coupling element 1 shown in Figs. 5A to 5C. Fig. 6A shows the calculation results for the structure shown in Fig. 5A, Fig. 6B shows the calculation results for the structure shown in Fig. 5B, and Fig. 6C shows the calculation results for the structure shown in Fig. 5C.

[0055] The calculation results shown in the figures, similar to those in Figures 3A to 3C and Figures 4A to 4F, show the light source position shift (nm), the loss of the TE polarization component (dB), and the loss of the TM polarization component (dB) for wavelengths between 1525 nm and 1600 nm.

[0056] The results shown in Figures 6A to 6C show that the loss of the TE polarization component and the TM polarization component is better than the calculated results for the structure of the optical coupling element 9 according to the comparative example shown in Figure 3C, such as that shown in Figure 2B.

[0057] <Effects> The optical coupling element 1 according to this embodiment exhibits high optical coupling efficiency even when the light source position is shifted, and has a wide alignment tolerance range, thereby reducing loss during optical coupling.

[0058] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as set forth in the claims.

[0059] Note that aspects of the present invention are as follows, for example: <1> An optical coupling element comprising: a substrate; a cladding layered on an upper surface of the substrate; and an optical waveguide embedded in the cladding and arranged on one surface of the substrate, wherein the optical waveguide has two longitudinal optical waveguides extending in parallel and spaced apart from each other, and a central optical waveguide arranged between the two longitudinal optical waveguides, the longitudinal optical waveguide having a plurality of first regions and a plurality of second regions formed in such a way that they are alternately connected in the extending direction of the central optical waveguide, each of the second regions having a smaller area of ​​the longitudinal optical waveguide arranged on the substrate than each of the first regions when viewed from the direction in which the cladding is layered, and the central optical waveguide including a tapered portion in at least one of the extending directions of the central optical waveguide. <2> The optical coupling element according to <1>, wherein the widths of the two long-axis shaped optical waveguides in the first region, as viewed from the direction in which the cladding is stacked, in a direction perpendicular to the extending direction of the central optical waveguide are greater than the width of the long-axis shaped optical waveguide in the second region. <3> The optical coupling element according to <1> or <2>, wherein the refractive index of materials forming the long-axis shaped optical waveguides and the central optical waveguide is higher than the refractive index of a material forming the cladding. <4> The optical coupling element according to any one of <1> to <3>, wherein the long-axis shaped optical waveguides and the central optical waveguide are each spaced the same distance from the substrate and extend into the cladding. <5> The optical coupling element according to any one of <1> to <4>, wherein widths of the two longitudinal optical waveguides in the second region, as viewed from the direction in which the cladding is stacked, in a direction perpendicular to the extension direction of the central optical waveguide, vary in the extension direction of the central optical waveguide.

[0060] This application claims priority from Japanese Patent Application No. 2024-112341, filed on July 12, 2024 with the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0061] 1. Optical bonding element 10. Substrate 20. Cladding 31. Long-axis optical waveguide 32. Central optical waveguide 33. Long-axis optical waveguide R1. First field R2. Second field

Claims

1. An optical coupling element comprising: a substrate; a clad laminated on an upper surface of the substrate; and an optical waveguide embedded in the clad and arranged against one surface of the substrate, wherein the optical waveguide has two longitudinal optical waveguides extending in parallel and spaced apart from each other, and a central optical waveguide arranged between the two longitudinal optical waveguides, wherein the longitudinal optical waveguide is formed with a plurality of first regions and a plurality of second regions alternately connected in the extending direction of the central optical waveguide, wherein each of the second regions has a smaller area of ​​the longitudinal optical waveguide arranged against the substrate than each of the first regions when viewed from the direction in which the clad is laminated, and the central optical waveguide includes a tapered portion in at least one of the extending directions of the central optical waveguide.

2. An optical coupling element as described in claim 1, wherein the width of the two longitudinal optical waveguides in the first region in a direction perpendicular to the extension direction of the central optical waveguide, as viewed from the direction in which the cladding is stacked, is greater than the width of the longitudinal optical waveguide in the second region.

3. An optical coupling element according to claim 1 or 2, wherein the refractive index of the material forming said longitudinal optical waveguide and said central optical waveguide is higher than the refractive index of the material forming said cladding.

4. An optical coupling element according to any one of claims 1 to 3, wherein the longitudinal optical waveguide and the central optical waveguide extend into the cladding at the same distance from the substrate.

5. An optical coupling element according to any one of claims 1 to 4, wherein the width of the two longitudinal optical waveguides in the second region, as viewed from the direction in which the cladding is stacked, in a direction perpendicular to the extension direction of the central optical waveguide, varies in the extension direction of the central optical waveguide.

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