Optical waveguide structure and optical device

Abstract

An optical waveguide structure according to an embodiment of the present disclosure comprises a first optical waveguide (413a) and a second optical waveguide (43a). The first optical waveguide (413a) and the second optical waveguide (43a) each have a stepped tapered structure at an end thereof. The stepped tapered structure includes a wide-tapered structure and a narrow-tapered structure that are adjacent to each other. The upper surface of a wide-tapered structure (41311a) of the first optical waveguide (413a) and the lower surface of a narrow-tapered structure (4312a) of the second optical waveguide (43a) are adjacent or close to each other in a direction perpendicular to the longitudinal direction of the first optical waveguide (413a) and the second optical waveguide (43a). The tapered structure adiabatically decreases in width toward the end, and the narrowest width of the wide-tapered structure is wider than the widest width of the narrow-tapered structure. The length of the narrow-tapered structure of the first optical waveguide is equal to the length of the narrow-tapered structure of the second optical waveguide.

Description

Optical waveguide structure and optical device

[0001] The present disclosure relates to an optical waveguide structure and an optical device, and more particularly to an optical waveguide structure and an optical device for reducing connection loss between an optical waveguide and other optical waveguides.

[0002] Conventionally, various optical devices centered on a silica-based optical circuit composed of silica-based optical waveguides made mainly of silica glass have been researched and developed. Such optical devices are mainly used in the field of optical communication as key devices in a photonic network based on a wavelength division multiplexing (WDM) system. (See, for example, Non-Patent Documents 1-4)

[0003] In recent years, with the evolution of the fifth-generation mobile communication system (also referred to as 5G), Internet of Things (IoT), artificial intelligence (AI), etc., it has become an urgent task to respond to further increase in communication capacity. (See, for example, Non-Patent Document 5)

[0004] In addition, in combination with the research, development, and practical application of waveguide-type devices for optical transmission and reception, the research, development, and practical application of a compound semiconductor waveguide including an indium phosphide (InP) waveguide, or a complex integrated optical device of a silicon photonics device having a lithium niobate (LiNbO3, LN) waveguide and a silica-based optical waveguide or a silicon waveguide have been progressing. In such a complex integrated optical device, it is important to achieve low-loss optical coupling between different materials of an InP or LN waveguide and a silica-based optical waveguide or a silicon photonics waveguide. It is known that in the coupling between stacked waveguides or the optical coupling between waveguides adjacent in an upper and lower layer configuration, by using a waveguide having a tapered shape, it is possible to reduce the dissipation of optical energy and perform optical coupling. (See, for example, Non-Patent Document 6)

[0005] Figures 1 and 2 show a conventional optical waveguide structure that optically couples waveguides of different materials. Figure 1(a) is a top view of the optical waveguide structure, and Figure 1(b) is a cross-sectional view of the optical waveguide structure along the Ib-Ib line in Figure 1(a). Figure 2 is a perspective view of the optical waveguide structure in Figure 1. The optical waveguide structure 1 shown in Figures 1 and 2 includes a first optical waveguide 11 formed on a substrate 13, a second optical waveguide 12, and cladding 14 and 15 covering the first optical waveguide 11 and the second optical waveguide 12.

[0006] For example, substrate 13 is a Si substrate. Cladding 14 is an undercladding made of a silica-based material. The first optical waveguide 11 is a core made of a silica-based material. The second optical waveguide 12 is a core made of lithium niobate (LN). Cladding 15 is an overcladding made of a silica-based material.

[0007] The height (length in the Y-axis direction) of the first optical waveguide 11 is constant. The height (length in the Y-axis direction) of the second optical waveguide 12 is also constant. The height of the first optical waveguide 11 may be the same as or different from the height of the second optical waveguide 12.

[0008] The first optical waveguide 11 has a tapered structure 111 at one of its two ends in the optical axis direction. The second optical waveguide 12 has a tapered structure 121 at one of its two ends in the optical axis direction. Parts of the tapered structure 111 of the first optical waveguide 11 and part of the tapered structure 121 of the second optical waveguide 12 are arranged to overlap in a direction perpendicular to the substrate 13. In the optical waveguide structure 1, the upper surface of the tapered structure 111 and the lower surface of the tapered structure 121 are adjacent, but may also be close together.

[0009] The tapered structures 111 and 121 are pointed or triangular in top view (XZ plane), and their width (length in the X-axis direction) changes adiabatically toward the ends, becoming zero at the ends. Here, adiabatic change means that the rate of change of the effective refractive index of the core is gradual (the change in core width or core height is approximately 1 / 100 or less with respect to the length in the propagation direction, i.e., for example, the change in core width is approximately 10 μm or less for a waveguide length of 1 mm), and is mathematically continuous or non-discrete (not discrete).

[0010] It is known that the effective refractive index of a core depends on the width and height of the core. In the optical waveguide structure 1 shown in Figure 1, the change in the effective refractive index in the portion where the tapered structure 111 and the tip of the tapered structure 121 overlap, and in the portion where the tip of the tapered structure 111 and the tapered structure 121 overlap, is continuous (adiabatic). As a result, the dissipation of optical energy when light propagating through the first optical waveguide 11 couples with the tapered structure 121 of the second optical waveguide 12 in the tapered structure 111 can be reduced.

[0011] However, attempting to create a pointed tapered structure at the end of a waveguide results in an excessively high aspect ratio (height / width), making fabrication difficult or impossible. Therefore, the tapered structure actually fabricated either has the pointed or triangular tip portion removed in a top view to create a trapezoidal shape, or the tip of the tapered structure becomes trapezoidal instead of pointed.

[0012] Figure 3 shows a conventional optical waveguide structure that optically couples waveguides of different materials as actually fabricated. Figure 3(a) is a top view of the optical waveguide structure, and Figure 3(b) is a cross-sectional view of the optical waveguide structure along the line IIIb-IIIb in Figure 3(a). The optical waveguide structure 3 shown in Figure 3 has a first optical waveguide 31 formed on a substrate 13, a second optical waveguide 32, and cladding 14, 15a, and 15b covering the first optical waveguide 31 and the second optical waveguide 32.

[0013] The first optical waveguide 31 has a tapered structure 311 at one of its two ends in the optical axis direction. The second optical waveguide 32 has a tapered structure 321 at one of its two ends in the optical axis direction. Parts of the tapered structure 311 of the first optical waveguide 31 and part of the tapered structure 321 of the second optical waveguide 32 are arranged to overlap in a direction perpendicular to the substrate 13. As described above, the tip portions of the tapered structures 311 and 321 have a trapezoidal shape, rather than a pointed or triangular shape, when viewed from above. Although Figure 3 shows the tip of the tapered structure as trapezoidal, in reality, the tip of the tapered structure does not strictly have corners and may have a rounded shape.

[0014] In the optical waveguide structure 3 shown in Figure 3, the change in cross-sectional area is steep in the areas where the tip of the tapered structure 311 and the tip of the tapered structure 321 overlap, and in the areas where the tip of the tapered structure 311 and the tapered structure 321 overlap, resulting in a steep (non-adiabatic) change in the effective refractive index. Consequently, higher-order modes are generated and / or light reflection occurs starting from the points where the effective refractive index changes steeply. As a result, low-loss coupling between waveguides is hindered in the optical waveguide structure 3.

[0015] Y. Hibino, An array of photonic filtering advantages: arrayed-waveguide-grating multi / demultiplexers for photonic networks, IEEE Circuits and Devices Magazine, Vol, 16, Issue 6, pp.21-27, Nov., 2000A. Himeno, et al., Silica-based planar lightwave circuits, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, Issue 6, pp.913-924, Nov. 1998M. Abe, Silica-Based Waveguide Devices for Photonic Networks, Journal of the Ceramic Society of Japan. Jan, 2008, pp.1063-1070M. Kawachi, Silica waveguides on silicon and their application to integrated-optic component, Optical and Quantum Electronics, Vol. 22, pp.391-416, 1990T. Sogawa, et al., All-Photonics Network and Photonics-electronics Convergence Technologies as a Vision of the Future, NTT Technical Review, Vol. 18, No. 10, pp.12-15, Oct. 2020Y. Shani, C.H. Henry, et al., Integrated Optic Adiabatic Devices on Silicon, IEEE Journal of Quantum Electronics, Vol. 27, Issue 3, pp.556-566, 1991

[0016] This disclosure has been made in view of such problems and aims to provide an optical waveguide structure that enables low-loss optical coupling between optical waveguides.

[0017] To achieve this objective, one embodiment of the present invention is an optical waveguide structure. The optical waveguide structure comprises a first optical waveguide and a second optical waveguide. The first optical waveguide and the second optical waveguide each have a stepped tapered structure at their ends. The stepped tapered structures have adjacent wide tapered structures and narrow tapered structures. The upper surface of the wide tapered structure of the first optical waveguide and the lower surface of the narrow tapered structure of the second optical waveguide are adjacent or close in a direction perpendicular to the longitudinal direction of the first optical waveguide and the second optical waveguide.

[0018] As described above, the optical waveguide structure according to the present invention makes it possible to perform optical coupling between optical waveguides with low loss.

[0019] This figure shows the schematic configuration of a conventional optical waveguide structure that optically couples waveguides of dissimilar materials, where (a) is a top view and (b) is a cross-sectional view. This is a perspective view showing the schematic configuration of a conventional optical waveguide structure that optically couples waveguides of dissimilar materials. This figure shows the schematic configuration of a conventional optical waveguide structure that optically couples waveguides of dissimilar materials, where (a) is a top view and (b) is a cross-sectional view. This figure shows the schematic configuration of an optical device having an optical waveguide structure according to one embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view. This figure shows the optical waveguide structure indicated by Va in Figure 4, where (a) is a top view and (b) is a cross-sectional view. This figure illustrates a method for manufacturing an optical waveguide structure according to one embodiment of the present disclosure, where (a) is a perspective view showing the first and second optical waveguides before joining and (b) is a perspective view showing the first and second optical waveguides after joining. This figure shows a schematic configuration of an optical device having an optical waveguide structure according to one embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view. This figure shows the optical coupling loss of the optical waveguide structure according to one embodiment of the present disclosure.

[0020] Embodiments of the present disclosure will be described in detail below with reference to the drawings. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. Embodiments of the present disclosure can be implemented by omitting some components, adding other components, or substituting some components with other components. Numerical values ​​and materials in the present disclosure are illustrative, and embodiments of the present disclosure can be implemented with other numerical values ​​and material names without departing from the spirit of the disclosure.

[0021] (Optical Device) Figure 4 shows a schematic configuration of an optical device having an optical waveguide structure according to one embodiment of the present disclosure. Figure 4(a) is a perspective view of the optical device 4, and Figure 4(b) is a top view of the optical device 4. The optical device 4 in Figure 4 has a pair of optical input / output waveguides 41 and 42 formed on a substrate 13, and a pair of optical branch waveguides 412 and 422 connected to the optical input / output waveguides 41 and 42, respectively.

[0022] The optical branching waveguide 412 has a pair of first optical waveguides 413a and 413b on the side opposite to the optical input / output waveguide 41. The first optical waveguides 413a and 413b each have stepped tapered structures 4131a and 4131b at their ends.

[0023] Furthermore, the optical device 4 in Figure 4 has a pair of second optical waveguides 43a and 43b between the optical branching waveguide 412 and the optical branching waveguide 422. The second optical waveguides 43a and 43b have stepped tapered structures 431a and 431b on the optical branching waveguide 412 side and stepped tapered structures 432a and 432b on the optical branching waveguide 422 side, respectively.

[0024] Furthermore, the optical device 4 in Figure 4 has a pair of third optical waveguides 423a and 423b on the opposite side of the optical input / output waveguide 42 of the optical branch waveguide 422. The third optical waveguides 423a and 423b each have stepped tapered structures 4231a and 4231b at their ends.

[0025] Furthermore, the optical device 4 in Figure 4 includes an underclad 14 covering optical input / output waveguides 41 and 42, an optical branching waveguide 412 having first optical waveguides 413a and 413b, a second optical waveguide 43a and 43b, and an optical branching waveguide 422 having third optical waveguides 423a and 423b, and an overclad 15a and 15b.

[0026] The optical input / output waveguides 41 and 42, the optical branch waveguide 412 having the first optical waveguides 413a and 413b, and the optical branch waveguide 422 having the third optical waveguides 423a and 423b are cores formed of a silica-based material, having an upper and lower surface parallel to the substrate surface, and a constant height (corresponding to the distance between the upper and lower surfaces). The second optical waveguides 43a and 43b are cores formed of lithium niobate (LN), having an upper and lower surface parallel to the substrate surface, and a constant height. The height of the first optical waveguide 11 may be the same as or different from the height of the second optical waveguide 12. The cladding 15a and 15b are overcladding formed of a silica-based material. The second optical waveguides 43a and 43b may have cores formed from nonlinear optical material thin films such as barium titanate (BaTiO3) and lanthanum-doped lead zirconate titanate (PLZT), or compound semiconductors such as InP, instead of LN.

[0027] (Optical Waveguide Structure) Figure 5 shows an optical waveguide structure according to one embodiment of the present disclosure. The optical waveguide structure in Figure 5 is the optical waveguide structure shown as Va in Figure 4. Figure 5(a) is a top view, and Figure 5(b) is a cross-sectional view taken along the line Vb-Vb in Figure 5(a). The optical waveguide structure shown in Figure 5 has a first optical waveguide 413a having a stepped tapered structure 4131a, and a second optical waveguide 43a having a stepped tapered structure 431a.

[0028] The stepped tapered structure 4131a has a wide tapered structure 41311a and a narrow tapered structure 41312a in order from the optical branch waveguide 412 side toward the end. The wide tapered structure 41311a and the narrow tapered structure 41312a are adjacent to each other (continuous). In a top view (XZ plane), the wide tapered structure 41311a and the narrow tapered structure 41312a each have a trapezoidal shape in which the width decreases adiabatically toward the end. The widest width of the narrow tapered structure 41312a is wider than the narrowest width of the wide tapered structure 41311a. The side surfaces of the wide tapered structure 41311a and the narrow tapered structure 41312a do not have to be straight lines in a top view, but may be, for example, arcs with a large curvature. Furthermore, in the wide tapered structure 41311a and the narrow tapered structure 41312a, the side surface and the adjacent optical axis perpendicular to the side surface (the surface parallel to the XY plane) do not need to intersect in such a way that they form a sharp angle, and the intersection portion between the side surface and the adjacent optical axis perpendicular to the side surface may be rounded.

[0029] The stepped tapered structure 431a has a wide tapered structure 4311a and a narrow tapered structure 4312a in order from the optical branch waveguide 422 side toward the end. The wide tapered structure 4311a and the narrow tapered structure 4312a each have a trapezoidal shape in which the width changes adiabatically toward the end when viewed from above (XZ plane). The widest width of the narrow tapered structure 4312a and 4312a is wider than the narrowest width of the wide tapered structure 4311a. The side surfaces of the wide tapered structure 41311a and the narrow tapered structure 41312a do not have to be straight lines when viewed from above, but may be, for example, arcs with a large curvature. Furthermore, in the wide tapered structure 4311a and the narrow tapered structure 4312a, the side surface and the adjacent surface perpendicular to the optical axis do not need to intersect in a way that strictly forms an angle, and the intersection portion between the side surface and the adjacent surface perpendicular to the optical axis may be rounded.

[0030] In this embodiment, the lengths of the wide tapered structure 41311a of the stepped tapered structure 4131a and the narrow tapered structure 4312a of the stepped tapered structure 431a in the optical axis direction (Z axis direction) are equal, but it is also possible to have different lengths as long as the adiabatic conditions are satisfied. The wide tapered structure 4311a and the narrow tapered structure 4312a are arranged adjacent to or close to each other so as to overlap in a direction perpendicular to the longitudinal direction of the first optical waveguide 413a and the second optical waveguide 43a (a direction perpendicular to the substrate 13). The position of the end of the narrow tapered structure 4312a of the stepped tapered structure 431a coincides with the position of the boundary between the wide tapered structure 41311a and the narrow tapered structure 41312a of the stepped tapered structure 4131a. The position of the end of the narrow tapered structure 41312a of the stepped tapered structure 4131a coincides with the position of the boundary between the wide tapered structure 4311a and the narrow tapered structure 4312a of the stepped tapered structure 4131a.

[0031] The optical waveguide structure between the stepped tapered structure 4131b of the first optical waveguide 413b and the stepped tapered structure 431b of the second optical waveguide 43b is the same as in Figure 5. The optical waveguide structure between the stepped tapered structures 432a and 432b of the second optical waveguides 43a and 43b and the stepped tapered structures 4231a and 4231b of the third optical waveguides 423a and 423b is also the same as in Figure 5.

[0032] In the optical waveguide structure shown in Figure 5, to prevent a steep change in the effective refractive index at the point where the stepped tapered structure 4131a and the tip of the narrow tapered structure 4312a of the tapered structure 431a overlap, the narrowest width of the wide tapered structure 41311a of the stepped tapered structure 4131a is made wider than the widest width of the narrow tapered structure 41312a. Similarly, the narrowest width of the wide tapered structure 4311a of the stepped tapered structure 431a is made wider than the widest width of the narrow tapered structure 4312a. As a result, the effective refractive index of the optical waveguide structure shown in Figure 5 changes adiabatically, and therefore low-loss coupling between waveguides is possible.

[0033] (Method 1 for Manufacturing Optical Waveguide Structure) Here, the method for manufacturing the optical device shown in Figure 4 will be described. First, an undercladding layer 14 of silica-based glass (glass mainly composed of SiO2) is formed on the main surface of the Si substrate 13. Next, a glass film (core layer) with a higher refractive index than the undercladding layer, to which GeO2 glass has been added as a dopant, is deposited on the undercladding layer 14 using flame hydrolysis deposition (FHD). Chemical vapor deposition (CVD) or sputtering may be used instead of FHD. Next, the glass layer is processed using photolithography and reactive ion etching (RIE) techniques to pattern the cores of the optical input / output waveguides 41 and 42, the optical branch waveguide 412 having the first optical waveguides 413a and 413b, and the optical branch waveguide 422 having the third optical waveguides 423a and 423b. Next, a quartz-based glass overcladding layer 15a is deposited to cover the sides and top surface of the patterned quartz-based glass core. Then, the overcladding layer 15a is removed and flattened up to directly above the quartz-based glass core.

[0034] Next, thin films of LN are bonded to the flattened upper surfaces of the first optical waveguides 413a and 413b and the third optical waveguides 423a and 423b using wafer bonding technology. Then, the thin films of LN are processed using photolithography and RIE technology to pattern the cores of the second optical waveguides 43a and 43b. Next, a silica-based glass overcladding layer 15b is deposited to cover the upper surface of the patterned glass core and the sides and upper surface of the patterned LN core. The method for manufacturing the optical device shown in Figure 4 is not limited to this. In addition, instead of the thin film of LN, thin films of nonlinear optical materials such as barium titanate (BaTiO3) and lanthanum-doped lead zirconate titanate (PLZT), or thin films of compound semiconductors such as InP may be used.

[0035] (Operation of the Optical Device) Next, the operation of the optical device 4 in Figure 4 will be explained. The light input to the optical input / output waveguide 41 is branched into the first optical waveguides 413a and 413b by the optical branching waveguide 412. The branched light transitions from the stepped tapered structures 4131a and 4131b in the optical waveguide structure shown in Figure 5 to the stepped tapered structures 431a and 431b of the second optical waveguides 43a and 43b, respectively. Furthermore, the light guided through the second optical waveguides 43a and 43b transitions from the stepped tapered structures 432a and 432b to the stepped tapered structures 4231a and 4231b of the third optical waveguides 423a and 423b, respectively. After that, the light that has transitioned to the third optical waveguides 423a and 423b is combined again by the optical branching waveguide 422 and output from the optical input / output waveguide 42. Electrodes (not shown) are provided near the second optical waveguides 43a and 43b of the LN, respectively, to constitute a phase modulator. The optical device 4 can modulate the phase and / or intensity of the light output from the optical input / output waveguide 42 by controlling the electrical signals applied to the second optical waveguides 43a and 43b.

[0036] (Method 2 for Manufacturing Optical Waveguide Structures) Another method for manufacturing optical devices will be described with reference to Figures 6 and 7. The above-described method for manufacturing optical devices involved bonding a thin film of LN to the flattened upper surfaces of the first optical waveguides 413a and 413b and the third optical waveguides 423a and 423b, and then patterning the cores of the second optical waveguides 43a and 43b. The manufacturing method described here involves bonding the second optical waveguides 43a and 43b of the patterned NL core to the first optical waveguides 413a and 413b and the third optical waveguides 423a and 423b of the patterned silica-based glass core.

[0037] For the sake of simplicity, an alternative manufacturing method for optical devices will be described, using the optical waveguide structure shown as Va in Figure 4 as a representative example. As shown in Figure 6(a), first, a silica-based glass undercladding layer 14 is formed on the substrate 13. Next, a glass film (core layer) with a higher refractive index than the undercladding layer, to which GeO2 glass has been added as a dopant, is deposited on the undercladding layer 14 using FHD. CVD or sputtering may be used instead of FHD. Next, the glass layer is processed using photolithography and RIE techniques to pattern it onto the core of the first optical waveguide 413a. Then, a silica-based glass overcladding layer 15a is deposited to cover the sides and top of the patterned silica-based glass core. Finally, the overcladding layer 15a is removed up to just above the silica-based glass core to flatten it. Subsequently, the substrate 13, which includes optical input / output waveguides 41 and 42, an optical branching waveguide 412 having first optical waveguides 413a and 413b, and an optical branching waveguide 422 having third optical waveguides 423a and 423b, is formed into a chip.

[0038] Furthermore, as shown in Figure 6(a), a thin film of LN is bonded to the main surface of the lithium tantalate (LT) substrate 16 using wafer bonding technology. Next, the LN thin film is processed using photolithography and RIE technology to pattern the cores of the second optical waveguides 43a and 43b. Then, a silica-based glass overcladding layer 15b is deposited to cover the sides and top surfaces of the LN cores. Subsequently, the overcladding layer 15b is removed up to just above the LN cores to flatten the substrate. After that, the substrate 16 including the second optical waveguides 43a and 43b is formed into a chip. Note that instead of the LN thin film, a thin film of a nonlinear optical material such as barium titanate (BaTiO3) and lanthanum-doped lead zirconate titanate (PLZT), or a thin film of a compound semiconductor such as InP may be used.

[0039] As shown in Figure 6(b), finally, the position between the first optical waveguide 413a and the second optical waveguide 43a is adjusted or aligned, and the chipped substrate 13 and the chipped substrate 16 are joined together with adhesive.

[0040] Figures 7(a) and 7(b) show the optical waveguide structure of an optical device fabricated using this manufacturing method. As shown in Figure 7(b), this optical waveguide structure differs from the optical waveguide structure shown in Figure 5 in that it contains the LT substrate 16.

[0041] (Optical Coupling Loss) Figure 8 shows the results of measuring the optical coupling loss in the optical waveguide structure according to the embodiment of the present disclosure shown in Figure 5. The height of the underclad 14 was set to 20 μm, the difference in refractive index between the clad and the core was set to approximately 1.5%, and the cross-sectional size of the first optical waveguide 413a and the second optical waveguide 43a, excluding the stepped tapered structure, was set to approximately 4.5 μm × 4.5 μm, and the optical waveguide structure was fabricated by the manufacturing method 1 described above. The average optical coupling loss between the first optical waveguide 413a and the second optical waveguide 43a was 0.27 dB. The average optical coupling loss of the conventional optical waveguide structure shown in Figure 3 was 0.68 dB. It was confirmed that the optical waveguide structure according to the embodiment of the present disclosure can perform optical coupling between optical waveguides with low loss.

[0042] Furthermore, the average optical coupling loss between the first optical waveguide 413a and the second optical waveguide 43a in the optical waveguide structure according to the embodiment of the present disclosure shown in Figure 7 was 0.3 dB. The optical waveguide structure fabricated by the manufacturing method 2 described above was also able to perform optical coupling between optical waveguides with low loss.

[0043] According to the optical waveguide structure of one embodiment of the present invention, optical coupling between optical waveguides can be performed with low loss.

[0044] 1 Optical waveguide structure 11 First optical waveguide 111 Tapered structure 12 Second optical waveguide 121 Tapered structure 13 Substrate 14 Underclad 15, 15a, 15b Overclad 3 Optical waveguide structure 31 First optical waveguide 311 Tapered structure 32 Second optical waveguide 321 Tapered structure 4 Optical device 41 Optical input / output waveguide 412 Optical branch waveguide 413a, 413b First optical waveguide 4131a, 4131b Stepped tapered structure 42 Optical input / output waveguide 422 Optical branch waveguide 423a, 423b Third optical waveguide 4231a, 4231b Stepped tapered structure 43a, 43b Second optical waveguide 431a, 431b Stepped tapered structure 432a, 432b Stepped tapered structure 16 substrate

Claims

1. An optical waveguide structure comprising a first optical waveguide and a second optical waveguide, wherein the first optical waveguide and the second optical waveguide each have a stepped tapered structure at their ends, the stepped tapered structure has adjacent wide tapered structures and narrow tapered structures, and the upper surface of the wide tapered structure of the first optical waveguide and the lower surface of the narrow tapered structure of the second optical waveguide are adjacent or close in a direction perpendicular to the longitudinal direction of the first optical waveguide and the second optical waveguide.

2. The optical waveguide structure according to claim 1, wherein the position of the end of the narrow tapered structure of the second optical waveguide coincides with the position of the boundary between the wide tapered structure and the narrow tapered structure of the first optical waveguide, and the position of the end of the narrow tapered structure of the first optical waveguide coincides with the position of the boundary between the wide tapered structure and the narrow tapered structure of the second optical waveguide.

3. The optical waveguide structure according to claim 1, wherein the stepped tapered structure has adiabatically decreasing width toward the end, and the narrowest width of the wide tapered structure is wider than the widest width of the narrow tapered structure.

4. The optical waveguide structure according to claim 3, wherein the rate of change of the effective refractive index of the first optical waveguide and the second optical waveguide in the direction of light propagation is continuous or non-discrete.

5. The optical waveguide structure according to claim 1, wherein the wide tapered structure and the narrow tapered structure have a trapezoidal shape when viewed from above.

6. The optical waveguide structure according to claim 1, wherein the second optical waveguide is stacked on the upper surface of the first optical waveguide.

7. The optical waveguide structure according to claim 1, wherein the second optical waveguide is joined to the upper surface of the first optical waveguide.

8. An optical device comprising: a set of optical input / output waveguides formed on a substrate; a set of optical branch waveguides connected to the set of optical input / output waveguides, each of which has a set of first optical waveguides; and a set of second optical waveguides, wherein each of the first optical waveguides and the second optical waveguides has a stepped tapered structure at its end, each of the stepped tapered structures has an adjacent wide tapered structure and a narrow tapered structure, and the upper surface of the wide tapered structure of the first optical waveguide and the lower surface of the narrow tapered structure of the second optical waveguide are adjacent or close in a direction perpendicular to the longitudinal direction of the first optical waveguide and the second optical waveguide.

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