Optical circuit

Abstract

An optical circuit comprises a substrate 18 having a main surface, a first optical waveguide 10 provided on the main surface, and a second optical waveguide 20 optically coupled to the first optical waveguide. The first optical waveguide has a first cladding 12, a second cladding 16, and a first core 14 provided in the normal direction of the main surface. The second optical waveguide has a third cladding 22, a fourth cladding 26, and a second core 24 provided in the normal direction of the main surface. The material of the second core is different from the material of the first core. The refractive index of the substrate is greater than the refractive index of the third cladding with respect to the wavelength of light guided by the second optical waveguide. The second optical waveguide has a first region 31, a second region 32, and a third region 33. The first region includes an end portion on the side of the second core facing the first core. The thickness of the third cladding in the third region is greater than that of the first region. The thickness of the third cladding in the second region gradually increases from the first region side toward the third region side.

Description

optical circuit

[0001] The present disclosure relates to optical circuits.

[0002] Optical circuits, which integrate semiconductor optical elements and optical waveguides on a substrate, are in high demand in the fields of optical communications and optical sensors. In optical circuits, active optical waveguides, such as laser light sources, optical modulators, and light-receiving elements, are optically coupled to passive optical waveguides. Optical coupling between a photodetector and a passive optical waveguide, both of which have cores made of the same material, is known (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2005-182030

[0004] The most suitable core material varies depending on the application and function of the optical waveguide. When optical waveguides with different core materials are placed on the same substrate, light may leak from the core into the substrate.

[0005] The present disclosure aims to provide an optical circuit capable of reducing leakage of light from a core to a substrate or the like.

[0006] According to an embodiment of the present disclosure, an optical circuit includes a substrate having a principal surface, a first optical waveguide provided on the principal surface, and a second optical waveguide provided on the principal surface and optically coupled to the first optical waveguide, wherein the first optical waveguide has a first clad provided in a normal direction to the principal surface, a second clad, and a first core disposed between the first clad and the second clad, and the second optical waveguide has a third clad provided in a normal direction to the principal surface, a fourth clad, and a second core disposed between the third clad and the fourth clad, and the material of the second core is Unlike the material of the first core, the refractive index of the substrate is greater than the refractive index of the third cladding for the wavelength of light guided through the second optical waveguide, and the second optical waveguide has a first region, a second region, and a third region, the first region including the end of the second core facing the first core, the third region being a region where the thickness of the third cladding is greater than that of the first region, and the second region being a region connecting the first region and the third region so that the thickness of the third cladding increases from the first region toward the third region.

[0007] According to the present disclosure, leakage of light from the core to the substrate or the like can be reduced.

[0008] FIG. 1 is a plan view of the optical circuit according to the first embodiment. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. FIG. 3 is a diagram illustrating a simulation of the loss of the second optical waveguide versus the thickness of the third cladding. FIG. 4 is a cross-sectional view of another example of the optical circuit according to the first embodiment. FIG. 5A is a cross-sectional view illustrating manufacturing method 1 according to the first embodiment. FIG. 5B is a cross-sectional view illustrating manufacturing method 1 according to the first embodiment. FIG. 5C is a cross-sectional view illustrating manufacturing method 1 according to the first embodiment. FIG. 6A is a cross-sectional view illustrating manufacturing method 2 according to the first embodiment. FIG. 6B is a cross-sectional view illustrating manufacturing method 2 according to the first embodiment. FIG. 6C is a cross-sectional view illustrating manufacturing method 2 according to the first embodiment. FIG. 7A is a cross-sectional view illustrating manufacturing method 3 according to the first embodiment. FIG. 7B is a cross-sectional view illustrating manufacturing method 3 according to the first embodiment. FIG. 7C is a cross-sectional view illustrating manufacturing method 3 according to the first embodiment. FIG. 8 is an SEM image of a second optical waveguide formed using manufacturing method 3. FIG. 9 is a plan view of the optical circuit according to the second embodiment. FIG. 10 is a cross-sectional view taken along line X-X of FIG. 9. FIG. 11 is a plan view of an optical circuit according to a first modification of the second embodiment. FIG. 12 is a plan view of an optical circuit according to a third embodiment. FIG. 13 is a cross-sectional view taken along line XIII-XIII in FIG. 12. FIG. 14 is a cross-sectional view taken along line XIV-XIV in FIG. 12. FIG. 15 is a cross-sectional view taken along line XV-XV in FIG. 12. FIG. 16 is a cross-sectional view taken along line XVI-XVI in FIG. 12. FIG. 17 is a plan view of an optical circuit according to a fourth embodiment. FIG. 18 is a cross-sectional view taken along line XVIII-XVIII in FIG. 17. FIG. 19 is a cross-sectional view taken along line XIX-XIX in FIG. 17. FIG. 20 is a plan view of an optical circuit according to a fifth embodiment. FIG. 21 is a cross-sectional view taken along line XXI-XXI in FIG. 20. FIG. 22 is a plan view of an optical circuit according to a sixth embodiment. FIG. 23 is a cross-sectional view taken along line XXIII in FIG. 22. FIG. 24 is a plan view of an optical circuit according to a seventh embodiment. FIG. 25 is a cross-sectional view taken along line XXV-XXV in FIG. 24. Fig. 26 is a plan view of an optical circuit according to a first modified example of the seventh embodiment. Fig. 27 is a plan view of an optical circuit according to a second modified example of the seventh embodiment.

[0009] Hereinafter, embodiments for carrying out the present disclosure will be described in detail with reference to the drawings. The following embodiments are examples for embodying the technical ideas of the invention, and the present disclosure is not limited to the described configurations and numerical values. In each drawing, the same components are given the same reference numerals, and duplicate explanations may be omitted as appropriate. The size, positional relationship, etc. of each component shown in each drawing may be exaggerated to facilitate understanding of the invention.

[0010] First Embodiment First, the configuration of an optical circuit according to the first embodiment will be described. Fig. 1 is a plan view of the optical circuit according to the first embodiment. Fig. 2 is a cross-sectional view taken along line II-II of Fig. 1. The Z direction is the normal direction to the main surface 18A of the substrate 18 and the thickness direction of the substrate 18, the X direction is the direction in which the first optical waveguide 10 and the second optical waveguide 20 are optically coupled, and the Y direction is the direction orthogonal to the X direction and the Z direction.

[0011] As shown in FIGS. 1 and 2 , the optical circuit 100 includes a substrate 18, a first optical waveguide 10, and a second optical waveguide 20. The substrate 18 has a main surface 18A. The first optical waveguide 10 and the second optical waveguide 20 are provided on the main surface 18A, and the first optical waveguide 10 and the second optical waveguide 20 are optically coupled. The first optical waveguide 10 has a first cladding 12 and a second cladding 16 provided in the Z direction, and a first core 14 disposed between the first cladding 12 and the second cladding 16. The second optical waveguide 20 has a third cladding 22 and a fourth cladding 26 provided in the Z direction, and a second core 24 disposed between the third cladding 22 and the fourth cladding 26. A portion of the first cladding 12 is provided between the substrate 18 and the second optical waveguide 20.

[0012] Region 50 is a region where the first optical waveguide 10 is disposed. Regions 51 to 53 are regions where the second optical waveguide 20 is disposed. In region 51, the thickness T1 of the third cladding 22 is substantially constant, and the upper surface of the third cladding 22 is parallel to the major surface 18A of the substrate 18. In region 52, the thickness T2 of the third cladding 22 gradually increases from region 51 to 53. The upper surface of the third cladding 22 in region 52 is inclined at an inclination angle θ1. In region 53, the thickness T3 of the third cladding 22 is substantially constant, and the upper surface of the third cladding 22 is parallel to the major surface 18A of the substrate 18. In FIGS. 1 and 2 , the second optical waveguides 20 provided in regions 51 to 53 are disposed with their optical axes oriented in the X direction. A boundary 35A between regions 51 and 52 and a boundary 35B between regions 52 and 53 extend in the Y direction.

[0013] The second optical waveguide 20 has a first region 31, a second region 32, and a third region 33. The first region 31, the second region 32, and the third region 33 are provided in regions 51, 52, and 53, respectively. The first region 31 is a region that includes the end of the second core 24 that faces the first core 14. The third region 33 is a region where the thickness T3 of the third cladding 22 is larger than the thickness T1 of the first region 31. The second region 32 is a region that connects the first region 31 and the third region 33 so that the thickness T2 of the third cladding 22 increases from the first region 31 toward the third region 33.

[0014] Technologies for forming optical circuits by integrating optical waveguides composed of multiple semiconductor layers with various functions have been studied. When optical waveguides formed on different substrates are butted together to be optically coupled, the positional accuracy of the optical axis in the height direction becomes an issue due to factors such as voids or variations in the thickness of the adhesive. Furthermore, there is a risk that the process will become complicated. On the other hand, when forming two or more types of optical waveguides on the same substrate, the process is likely to be simplified compared to the above example, but it is difficult to prepare a substrate suitable for all optical waveguides.

[0015] When two or more types of optical waveguides are formed on the same substrate, the material for one of the optical waveguides is first placed on the substrate. Next, a portion of that material is removed by etching or other methods, and the material for the other optical waveguide is placed in the space where the material for the first optical waveguide was removed. If the etching depth is shallow, the cladding layer of the other optical waveguide placed on the etched surface will be thin. This will weaken the optical confinement of the other optical waveguide due to the influence of the refractive index of the substrate. If the etching depth is deep, the cladding layer can be formed thicker, reducing the influence of the substrate. However, variations in the etched surface will reduce the flatness of the etched surface. If the other optical waveguide is formed on this etched surface, unevenness will form at the interface between the cladding and the core, reflecting the variations in the etched surface, resulting in increased loss due to scattering.

[0016] A more specific description will be given. The first optical waveguide 10 and the second optical waveguide 20 according to the first embodiment have different materials for the first core 14 and the second core 24 depending on their applications or functions. For example, as described above, there are cases where the first optical waveguide 10 is a waveguide including a semiconductor element, and the second optical waveguide 20 is a passive optical waveguide. The first optical waveguide 10 can have various functions, such as a semiconductor laser element (i.e., a semiconductor laser oscillator or a semiconductor laser amplifier), an optical modulator, or a light-receiving element. On the other hand, the second optical waveguide 20 can have the function of propagating light to another optical waveguide. It is preferable that the second optical waveguide 20 has low loss during waveguiding. For this reason, the second core 24 of the second optical waveguide 20 is required to be translucent for the wavelength of the guided light. In this case, the refractive index of the material forming the second core 24 is often smaller than the refractive index of the material forming the first core 14 for the wavelength of the light guided through the second optical waveguide 20.

[0017] For example, when the first optical waveguide 10 includes a III-V semiconductor, at least a portion of the first optical waveguide 10 is formed by epitaxial growth on the substrate 18. Materials that can be epitaxially grown on the substrate 18 often have a similar crystal structure or material to that of the substrate 18. Therefore, when the first optical waveguide 10 is epitaxially grown on the substrate 18, the refractive index of the substrate 18 can be close to the refractive index of the first core 14 and the first cladding 12 of the first optical waveguide 10. When the second optical waveguide 20 is formed on the substrate 18, the refractive index of the second core 24 may be smaller than the refractive index of the first core 14, and the refractive index of the second core 24 may also be smaller than the refractive index of the substrate 18. In this case, the optical confinement of the second optical waveguide 20 is weakened by the influence of the refractive index of the substrate 18, causing light to leak into the substrate 18 and resulting in loss.

[0018] Such leakage of light into the substrate 18 can be reduced by increasing the thickness of the third cladding 22. This is because the light propagating through the second optical waveguide 20 is tightly confined within the second core 24, making it less sensitive to the refractive index of the substrate 18. This method does not require deep etching as described above.

[0019] A simulation was performed using the finite element method to estimate the optical loss. In the simulation, a structure was assumed in which the second optical waveguide 20 was disposed on the first cladding 12. The first cladding 12 was made of GaN with a refractive index of 2.45, and the third cladding 22 and the fourth cladding 26 were made of SiO with a refractive index of 1.45. ２ The second core 24 was made of SiON with a refractive index of 1.55. The width of the second core 24 in the Y direction and the width in the Z direction were set to 1 μm and 0.5 μm, respectively. The cross section of the second core 24 perpendicular to the optical axis was set to a rectangular shape. Figure 3 shows the results of this simulation, showing the loss of the second optical waveguide 20 relative to the thickness of the third cladding 22. The horizontal axis is the thickness of the third cladding 22, and the vertical axis is the TE of the second optical waveguide 20. ００ Mode and TE １０ The graph shows the loss per meter (dB / m) in the TE mode. As shown in FIG. 3, when the thickness of the third cladding 22 is reduced, the loss increases rapidly. For example, ００In this mode, when the thickness T1 is 1100 nm, the loss exceeds 1 dB / m. On the other hand, when the thickness of the third cladding 22 is 1200 nm, the loss is about 0.4 dB / m, which is less than half of the loss when the thickness of the third cladding 22 is 1100 nm. When the thickness of the third cladding 22 is 1500 nm, the loss is almost 0 dB / m. This simulation result shows that the loss of the second optical waveguide 20 can be reduced by increasing the thickness of the third cladding 22.

[0020] In the optical circuit 100 according to the first embodiment, in the second region 32, the thickness T2 of the third cladding 22 increases from the first region 31 toward the third region 33. This increases the thickness T3 in the third region 33, and therefore reduces leakage of light from the second core 24 to the first cladding 12 or the substrate 18 compared to a case where the thickness of the third cladding 22 of the second optical waveguide 20 is the same throughout the first region 31. This reduces loss in the second optical waveguide 20.

[0021] From the viewpoint of reducing loss in the second optical waveguide 20 due to leakage of light to the substrate 18, the difference D1 between the thickness T3 of the third cladding 22 in the third region 33 and the thickness T1 of the third cladding 22 in the first region 31 is, for example, preferably 500 nm or more, more preferably 1000 nm or more, and even more preferably 1500 nm or more. From the viewpoint of reducing loss due to an increase in the tilt angle θ1, the difference D1 is, for example, preferably 5000 nm or less, more preferably 4000 nm or less, and even more preferably 3000 nm or less. Therefore, the difference D1 between the thickness T3 of the third cladding 22 in the third region 33 and the thickness T1 of the third cladding 22 in the first region 31 is preferably 500 nm or more and 5000 nm or less, more preferably 1000 nm or more and 4000 nm or less, and even more preferably 1500 nm or more and 3000 nm or less.

[0022] From the viewpoint of reducing the loss of the second optical waveguide 20 in the first region 31, the thickness T1 of the third cladding 22 in the first region 31 is preferably, for example, 500 nm or more, and more preferably 1000 nm or more. From the viewpoint of reducing the amount of etching of the first optical waveguide to make the upper surface of the first cladding 12 relatively flat and making the third cladding 22 disposed thereon flat, the thickness T1 of the third cladding 22 in the first region 31 is preferably, for example, 2000 nm or less, and more preferably 1500 nm or less. Therefore, the thickness T1 of the third cladding 22 in the first region 31 is preferably, for example, 500 nm or more and 2000 nm or less, or 1000 nm or more and 2000 nm or less.

[0023] From the viewpoint of reducing loss in the second optical waveguide 20 due to leakage of light to the substrate 18 in the third region 33, the thickness T3 of the third cladding 22 in the third region 33 is preferably, for example, 1000 nm or more, and more preferably 2000 nm or more. From the viewpoint of reducing loss due to an increase in the tilt angle θ1, the thickness T3 in the third region 33 is preferably, for example, 7000 nm or less, and more preferably 5500 nm or less. Therefore, the thickness T3 of the third cladding 22 in the third region 33 is preferably, for example, 1000 nm or more and 7000 nm or less, or 2000 nm or more and 5500 nm or less.

[0024] As shown in FIG. 2 , the upper surface 40C of the third region 33 of the second core 24 is preferably parallel to the main surface 18A of the substrate 18. This makes it easier to route the second optical waveguide 20 within a plane. Furthermore, loss is reduced when light guided through the second optical waveguide 20 in the third region 33 is coupled to another optical waveguide having a core parallel to the main surface 18A of the substrate 18, thereby improving coupling efficiency. To make the upper surface 40C of the second core 24 parallel to the main surface 18A of the substrate 18, the thickness T3 of the third cladding 22 may be uniform in the third region 33. Here, in the first region 31 and the third region 33, the phrase "the upper surface of the second core 24 is parallel to the main surface 18A of the substrate 18" refers not only to a strictly parallel state but also to the following states: In other words, in this specification, the term also includes a state in which, in both a cross-sectional view parallel to the optical axis of the second optical waveguide 20 and a cross-sectional view perpendicular to the optical axis, the smaller angle between the upper surface of the substrate 18 and the upper surface of the second core 24 is tilted by 3° or less, preferably 1° or less, and more preferably 0.1° or less.

[0025] The length L3 of the second core 24 in the third region 33 in the optical axis direction is greater than the sum of the length L1 of the second core 24 in the first region 31 in the optical axis direction and the length L2 of the second core 24 in the second region 32 in the optical axis direction. This allows the length L3 of the third region 33, which has low loss, to be increased, thereby reducing the loss of the entire optical circuit 100. The length L3 is preferably 2 to 100 times, and more preferably 10 to 50 times, the sum of the lengths L1 and L2. The length L1 is, for example, 10 μm to 500 μm, and the length L2 is, for example, 1 μm to 100 μm. The length L3 is, for example, 100 μm to 10,000 μm, or 1,000 μm to 5,000 μm.

[0026] 2 , a portion of the material forming the first cladding 12 is disposed between the third cladding 22 and the substrate 18. If the refractive index of the material forming the first cladding 12 is greater than the refractive index of the material forming the second core 24, light is likely to leak from the second core 24 into a portion of the first cladding 12. Therefore, by providing the second region 32 and the third region 33 in the second optical waveguide 20, the thickness of the third cladding 22 can be increased to efficiently confine light in the second core 24, thereby reducing loss in the second optical waveguide 20.

[0027] The substrate 18 for forming the first optical waveguide 10 has a relatively high refractive index for the wavelength of light guided through the second optical waveguide 20. Therefore, the refractive index of the substrate 18 is greater than the refractive index of the third cladding 22 for the wavelength of light guided through the second optical waveguide 20. In this case, light is more likely to leak from the second core 24 to the substrate 18. Therefore, by providing the second region 32 and the third region 33 in the second optical waveguide 20, the thickness of the third cladding 22 can be increased to efficiently confine light in the second core 24, thereby reducing loss in the second optical waveguide 20.

[0028] The refractive index of the third cladding 22 is, for example, 0.98 times or less, or 0.95 times or less, the refractive index of the substrate 18. Also, the refractive index of the third cladding 22 is, for example, 0.98 times or less, or 0.95 times or less, the refractive index of a portion of the first cladding 12. Here, the refractive index is compared in terms of the refractive index at the wavelength of light guided through the second optical waveguide 20. The refractive index can be measured by spectroscopic ellipsometry.

[0029] Next, examples of materials for the substrate 18, the first optical waveguide 10 and the second optical waveguide 20 will be described.

[0030] The substrate 18 is a III-V group semiconductor substrate such as a GaN substrate, a GaAs substrate, an InP substrate, etc. The substrate 18 is a substrate for epitaxially growing the first optical waveguide 10.

[0031] The first optical waveguide 10 is, for example, a semiconductor element, and at least a portion of the first cladding 12, the first core 14, and the second cladding 16 includes a III-V semiconductor. Examples of III-V semiconductors include nitride semiconductors, arsenide semiconductors, and phosphide semiconductors. Examples of nitride semiconductors include GaN, AlN, InN, and mixed crystals thereof, such as AlGaN, InGaN, AlInN, and AlInGaN. Examples of arsenide semiconductors include GaAs, AlAs, InAs, and mixed crystals thereof, such as AlGaAs, InGaAs, AlInAs, and AlInGaAs. Examples of phosphide semiconductors include GaP, AlP, InP, and mixed crystals thereof, such as AlGaP, InGaP, AlInP, and AlInGaP. The refractive index of the first core 14 is higher than the refractive indexes of the first cladding 12 and the second cladding 16. This causes the light to be confined in the first core 14 in the Z direction.

[0032] The first optical waveguide 10 includes a nitride semiconductor. In this case, at least a portion of the first optical waveguide 10 is epitaxially grown on the substrate 18. Therefore, if the first optical waveguide 10 and the second optical waveguide 20 are to be disposed on the same substrate, the second optical waveguide 20 is disposed on the substrate 18. Furthermore, the light guided through the first core 14 is, for example, visible light. To increase the transmittance of the second core 24, the third cladding 22, and the fourth cladding 26 to visible light, the materials for the second core 24, the third cladding 22, and the fourth cladding 26 are limited. The refractive index of such materials is at most about 2.0. On the other hand, the refractive index of nitride semiconductors is 2.2 or higher. Therefore, it is preferable to provide a second region 32 and a third region 33 in the second optical waveguide 20 and to distance the second core 24 from the substrate 18.

[0033] The second optical waveguide 20 is, for example, a passive optical waveguide. The third cladding 22, the second core 24, and the fourth cladding 26 are made of amorphous oxide, nitride, or fluoride. The refractive index of the second core 24 is higher than the refractive indexes of the third cladding 22 and the fourth cladding 26. This allows light to be confined in the second core 24 in the Z direction. The material of the third cladding 22 and the fourth cladding 26 is, for example, SiO ２ , MgF ２ , CaF ２ , or Al ２O ３ The material of the second core 24 is, for example, SiN or Ta. ２ O ５ , Nb ２ O ５ , TiO ２ , HfO ２ , AlN or LiNbO ３ The third cladding 22 and the fourth cladding 26 may be made of a composite material of the above-mentioned materials in order to adjust the refractive index. For example, the material of the third cladding 22 and the fourth cladding 26 may be SiON instead of SiN. The third cladding 22 and the fourth cladding 26 may be doped with an additive to adjust the refractive index.

[0034] When the materials exemplified above are formed on the substrate 18 or the first cladding 12, at least the third cladding 22 contains an amorphous material. The second core 24 and the fourth cladding 26 may also contain amorphous materials. When the second optical waveguide 20 includes a curved waveguide in top view, it is preferable that both the second core 24 and the fourth cladding 26 contain amorphous materials. This reduces the roughness of the etching side surface that occurs when forming the second optical waveguide 20 compared to when the second optical waveguide 20 includes a single crystal or polycrystalline material, thereby reducing loss in the second optical waveguide 20. If the second optical waveguide 20 is single crystal or polycrystalline, the etching side surface is likely to become rough due to the influence of the crystal plane.

[0035] Another Example of the First Embodiment FIG. 4 is a cross-sectional view of another example of the optical circuit according to the first embodiment. As shown in FIG. 4 , in the optical circuit 101 according to the first embodiment, the thickness T2 of the third cladding 22 does not change linearly in the X direction in the second region 32. The inclination angle of the upper surface of the third cladding 22 gradually increases from the first region 31 toward the third region 33. After reaching the maximum inclination angle θ1A, the inclination angle gradually decreases from the first region 31 toward the third region 33. In this way, the inclination angle in the second region 32 smoothly changes from the first region 31 toward the third region 33. This reduces optical loss between the first region 31 and the second region 32 and between the second region 32 and the third region 33. In the following embodiments and their modifications, the inclination angle may also change smoothly.

[0036] Manufacturing methods 1 to 3 of the first embodiment are described below. These methods can simplify the process compared to manufacturing methods for integrated optical circuits in which optical waveguides formed on different substrates are butted together and optically coupled.

[0037] (Manufacturing Method 1 of First Embodiment) FIGS. 5A to 5C are cross-sectional views showing Manufacturing Method 1 of the first embodiment. As shown in FIG. 5A , a portion of the second cladding 16, the first core 14, and the first cladding 12 is removed in a region 57 where the second optical waveguide 20 is formed, forming a recess 58. The second cladding 16, the first core 14, and the first cladding 12 are left in a region 50 where the first optical waveguide 10 is formed. A photoresist mask 42 is formed to cover the region 50. The photoresist mask 42 has an opening 42A in a region 53 and an overhang 42B in a region 52. The overhang 42B is formed, for example, by reverse photolithography. Note that, although the corners of the cross-sectional shapes of the opening 42A and the overhang 42B are actually rounded, the corners of the cross sections of the opening 42A and the overhang 42B are illustrated as straight lines for convenience in FIG. 5A and subsequent figures. The same applies to Manufacturing Methods 2 and 3.

[0038] 5B, a layer 22A is formed to become a part of the third cladding 22. The layer 22A is formed by a method and conditions that result in poor coverage, such as physical vapor deposition (PVD), so that the upper surface of the layer 22A in the overhang 42B is inclined.

[0039] 5C , the photoresist mask 42 is then removed, thereby lifting off the layer 22A formed on the photoresist mask 42. A layer 22B that will become a part of the third cladding 22 is formed in the region 57. As a result, the third cladding 22 is formed from the layers 22A and 22B. A second core 24 is formed on the third cladding 22, and a fourth cladding 26 is formed on the second core 24.

[0040] 6A to 6C are cross-sectional views showing manufacturing method 2 of the first embodiment. As shown in Fig. 6A, the second cladding 16, the first core 14, and a portion of the first cladding 12 in a region 57 where the second optical waveguide 20 is formed are removed to form a recess 58. The second cladding 16, the first core 14, and the first cladding 12 in a region 50 where the first optical waveguide 10 is formed are left remaining. A layer 22A that will become a portion of the third cladding 22 is formed on the first cladding 12 in region 57.

[0041] 6B, a photoresist mask 43 is formed on the layer 22A. The photoresist mask 43 may also be formed on the second cladding 16 in a region 50. The photoresist mask 43 has an opening 43A in a region 51.

[0042] 6C , the layer 22A is then etched using the photoresist mask 43 as a mask. At this time, by using an isotropic etching method, etching proceeds first from the top of the layer 22A, so that the layer 22A is completely etched in the region 51, and the top surface of the layer 22A is inclined in the region 52. Thereafter, the layer 22B, the second core 24, and the fourth cladding 26 are formed in the same manner as in FIG. 5C of the manufacturing method 1.

[0043] 7A to 7C are cross-sectional views showing manufacturing method 3 of the first embodiment. As shown in Fig. 7A, after Fig. 6A of manufacturing method 2, a photoresist mask 44 is formed on layer 22A and region 50. Photoresist mask 44 has an opening 44A in a region that will become region 51.

[0044] As shown in FIG. 7B, the photoresist mask 44 is rounded by surface tension through the heat treatment, and the side surfaces of the openings 44A of the photoresist mask 44 are inclined.

[0045] 7C, the layer 22A is then etched using the photoresist mask 44 as a mask. At this time, the etching rates of the layer 22A and the photoresist mask 44 are appropriately controlled, and an anisotropic etching method is used, so that the layer 22A is also etched while the top surface of the photoresist mask 44 and the side surfaces of the opening 44A are etched. As a result, the layer 22A is completely etched in the region 51, and the top surface of the layer 22A is inclined in the region 52. Thereafter, the layer 22B, the second core 24, and the fourth cladding 26 are formed, similar to FIG. 5C of manufacturing method 1.

[0046] Fig. 8 is a scanning electron microscope (SEM) image of a second optical waveguide formed using manufacturing method 3. As shown in Fig. 8, the upper surface of the third cladding 22 is inclined. The upper surfaces of the second core 24 and the fourth cladding 26 are also inclined. In this manner, a second optical waveguide 20 can be formed in which the thickness T2 of the third cladding 22 gradually changes.

[0047] Second Embodiment FIG. 9 is a plan view of an optical circuit according to a second embodiment. FIG. 10 is a cross-sectional view taken along the line X-X in FIG. 9 . As shown in FIGS. 9 and 10 , in an optical circuit 102 according to the second embodiment, an optical axis direction 48C of a first core 14 of a first optical waveguide 10 and an optical axis direction 48B of a second core 24 of a second optical waveguide 20 are in the X direction. The extension directions of a boundary 35A between regions 51 and 52 and a boundary 35B between regions 52 and 53 are inclined with respect to the X direction. An angle θ2A between a direction 48A from region 51 to region 53 at the shortest distance and the optical axis direction 48B is greater than 0° and less than 90°. Therefore, the length D2 of the optical axis in the second region 32 of the second optical waveguide 20 is greater than the shortest distance D3 connecting regions 51 and 53. The shortest distance D3 connecting regions 51 and 53 is the shortest distance connecting a region including first region 31 where the upper surface of third cladding 22 is parallel to main surface 18A of substrate 18 and a region including third region 33 where the upper surface of third cladding 22 is parallel to main surface 18A of substrate 18. Assuming that the inclination angle of the upper surface of third cladding 22 in second region 32 is the same as inclination angle θ1 in Figure 2 on the cross section in direction 48A, inclination angle θ1B of the upper surface of third cladding 22 in second region 32 is smaller than inclination angle θ1 in Figure 2.

[0048] In the first embodiment, when the inclination angle θ1 is large, the bending of the second core 24 near the boundaries 35A and 35B becomes large. This increases the loss in the second optical waveguide 20 near the boundaries 35A and 35B. Therefore, as in the optical circuit 102 of the second embodiment, the length D2 is made larger than the shortest distance D3. This allows the inclination angle θ1B in FIG. 10 to be smaller than the inclination angle θ1 in FIG. 2 of the first embodiment. This reduces the loss in the second optical waveguide 20 near the boundaries 35A and 35B. For example, when the inclination angle θ1 is 20° and the angle θ2A is 80°, the inclination angle θ1B can be set to 3.6°.

[0049] As described above, to effectively reduce the loss of the second optical waveguide 20 near the boundaries 35A and 35B, the length D2 is preferably 1.4 times or more, and more preferably 2 times or more, the shortest distance D3. The angle θ2A is preferably 45° or more, and more preferably 60° or more. The angle θ2A may be relatively large depending on the magnitude of the angle θ1 in FIG. 2. If the angle θ1 is sufficiently large, even if the angle θ2A is large, the angle θ1B does not become too small. Therefore, the length D2 required to reach the third region 33 can be relatively short, thereby reducing light leakage into the substrate 18. From this perspective, the length D2 is preferably 50 times or less, and more preferably 20 times or less, the shortest distance D3. The angle θ2A is preferably 89° or less, and more preferably 85° or less. Therefore, the length D2 is preferably 1.4 times or more and 50 times or less the shortest distance D3, and more preferably 2 times or more and 20 times or less the shortest distance D3. The angle θ2A is preferably 45° or greater and 89° or less, and more preferably 60° or greater and 85° or less.

[0050] Furthermore, when viewed from the Z direction, the optical axis direction 48B of the second core 24 in the second region 32 is the same as the optical axis direction 48C of the first core 14 of the first optical waveguide 10. As a result, compared to the first modified example of the second embodiment described later, there is no bending in the XY plane, and therefore loss due to bending of the optical axis can be reduced. Note that the optical axis directions do not have to be strictly the same; they only need to be small enough to cause negligible loss. For example, when viewed from the Z direction, the angle formed between the optical axis direction 48B of the second core 24 and the optical axis direction 48C of the first core 14 is 5° or less, 3° or less, 1° or less, or 0.1° or less. The other configurations are the same as those of the first embodiment, and therefore description thereof will be omitted.

[0051] (First Modification of Second Embodiment) Fig. 11 is a plan view of an optical circuit according to a first modification of the second embodiment. As shown in Fig. 11 , in an optical circuit 103 according to the first modification of the second embodiment, the extension direction of a boundary 35A between regions 51 and 52 and a boundary 35B between regions 52 and 53 is the Y direction, and a direction 48A extending from region 51 to region 53 at the shortest distance is the X direction. When viewed from the Z direction, in region 51, the optical axis of the second core 24 of the second optical waveguide 20 is inclined with respect to direction 48A, and in region 52, the optical axis direction 48B of the second core 24 of the second optical waveguide 20 is inclined with respect to direction 48A. As a result, when viewed from the Z direction, the optical axis direction 48B of the second core 24 in the second region 32 is inclined with respect to an optical axis direction 48C of the first core 14 of the first optical waveguide 10. Therefore, the length D2 of the optical axis in the second region 32 of the second optical waveguide 20 is greater than the shortest distance D3 connecting the regions 51 and 53. Therefore, assuming that the inclination angle of the upper surface of the third cladding 22 in the second region 32 in the cross section in the direction 48A is the same as the inclination angle θ1 in Fig. 2, the inclination angle θ1B of the upper surface of the third cladding 22 in the second region 32 is smaller than the inclination angle θ1 in Fig. 2.

[0052] As in the second embodiment, the length D2 is preferably 1.4 times or more, and preferably 2 times or more, the shortest distance D3. When viewed from the Z direction, the angle θ2B between the optical axis direction 48B and the optical axis direction 48C is preferably 45° or more, and more preferably 60° or more. The length D2 is preferably 50 times or less, and more preferably 20 times or less, the shortest distance D3. The angle θ2B is preferably 89° or less, and more preferably 85° or less. The other configurations are the same as those in the second embodiment, and therefore description thereof will be omitted.

[0053] (Third Embodiment) The third embodiment is an example of an optical circuit having a ring resonator. Fig. 12 is a plan view of the optical circuit according to the third embodiment. Fig. 13 is a cross-sectional view taken along line XIII-XIII in Fig. 12, Fig. 14 is a cross-sectional view taken along line XIV-XIV in Fig. 12, Fig. 15 is a cross-sectional view taken along line XV-XV in Fig. 12, and Fig. 16 is a cross-sectional view taken along line XVI-XVI in Fig. 12.

[0054] 12 and 13 , the optical circuit 104 of the third embodiment includes a first optical waveguide 10, a second optical waveguide 20, and a ring resonator 64. The first optical waveguide 10 and the second optical waveguide 20 are the same as those in the first embodiment. The ring resonator 64 is disposed in the region 53 and is formed by the optical waveguide 20A.

[0055] The first optical waveguide 10 is, for example, a semiconductor laser element 60. As shown in FIG. 14 , the first cladding 12 includes a layer 12A disposed on a substrate 18 and a layer 12B disposed on the layer 12A. The first cladding 12 may be, for example, a nitride semiconductor. The layers 12A and 12B are, for example, n-side AlGaN layers. The layer 12A is an n-side contact layer. The layer 12A or the layer 12B may include a crack prevention layer. The first core 14 may be, for example, a nitride semiconductor. The first core 14 is, for example, an InGaN layer and includes an active layer. The first core 14 may have an SQW (single quantum well) structure or an MQW (multiple quantum well) structure including InGaN or GaN. The first core 14 may further include an n-side optical guiding layer and a p-side optical guiding layer. The active layer is located between the n-side optical guiding layer and the p-side optical guiding layer. The second cladding 16 may be, for example, a nitride semiconductor. The second cladding 16 includes a layer 16A and a layer 16B disposed on the layer 16A. The layers 16A and 16B are, for example, p-side AlGaN layers. The layer 16B is a p-side contact layer. The upper part of the layer 16A and the layer 16B have a ridge structure. The first optical waveguide 10 is an optical waveguide with a ridge structure. In FIG. 14 , the lower end of the ridge structure is located in the second cladding 16. To further confine light, the lower end of the ridge structure may be located in the first cladding 12. The side surfaces of the ridge structure are protected by an insulating layer 28. An electrode 29 is disposed on the layer 16B.

[0056] 15 , in the first region 31 of the second optical waveguide 20 in the region 51, the third cladding 22 is disposed on the layer 12A of the first cladding 12. The second core 24 is disposed in a portion on the third cladding 22. The fourth cladding 26 is disposed on the third cladding 22 so as to cover the top and side surfaces of the second core 24. This allows for strong confinement of light in the second core 24. The second optical waveguide 20 may be a waveguide with a ridge structure. Note that the layer 12B of the first cladding 12 may be disposed between the third cladding 22 and the layer 12A of the first cladding 12.

[0057] As shown in Fig. 16, in the third region 33 of the second optical waveguide 20 in the region 53 and the ring resonator 64 of the optical waveguide 20A, the thickness T3 of the third cladding 22 is greater than the thickness T1 of the third cladding 22 in Fig. 15. The rest is the same as in Fig. 15.

[0058] The ring resonator 64 is optically coupled to the second optical waveguide 20. Laser light emitted by the semiconductor laser element 60 resonates in the ring resonator 64. A portion of the light circulating through the ring resonator 64 returns to the semiconductor laser element 60 by backscattering. This allows the laser light to have a narrower linewidth. The ring resonator 64 can also be used as a filter that passes laser light of a desired wavelength. By providing another optical waveguide optically coupled to the ring resonator 64 in a region other than the third region 33, the laser light filtered by the ring resonator 64 can be coupled to the other optical waveguide. The ring resonator 64 is optically coupled to the second optical waveguide 20 in the region 53. This reduces loss in the ring resonator 64 due to light leakage to the substrate 18, thereby improving the Q value of the ring resonator 64. Furthermore, by making the ring resonator 64 from the same material as the second optical waveguide 20, light confinement is strengthened and bending loss is reduced, thereby improving the Q value of the ring resonator 64. The other configurations are the same as those in the first embodiment, and therefore the description thereof will be omitted.

[0059] (Fourth Embodiment) The fourth embodiment is an example of an optical circuit having a third optical waveguide. Fig. 17 is a plan view of the optical circuit according to the fourth embodiment. Fig. 18 is a cross-sectional view taken along line XVIII-XVIII in Fig. 17, and Fig. 19 is a cross-sectional view taken along line XIX-XIX in Fig. 17. The cross-sectional views taken along lines XIV-XIV, XV-XV, and XVI-XVI in Fig. 17 are the same as Figs. 14, 15, and 16, respectively.

[0060] 17 and 18 , the optical circuit 105 of the fourth embodiment includes a first optical waveguide 10, a second optical waveguide 20, and a third optical waveguide 11. A region 54 is a region where the third optical waveguide 11 is disposed. The third optical waveguide 11 includes a fifth cladding 13, a third core 15, and a sixth cladding 17 disposed on the first cladding 12. The third core 15 is disposed between the fifth cladding 13 and the sixth cladding 17. In the third core 15, light is confined by the fifth cladding 13 and the sixth cladding 17 in the stacking direction. The third core 15 faces the second core 24 in the third region 33.

[0061] The third optical waveguide 11 is, for example, an optical modulator 62. As shown in FIG. 19 , the fifth cladding 13 has a layer 13A disposed on the first cladding 12 and a layer 13B disposed on the layer 13A. The fifth cladding 13 may be, for example, a nitride semiconductor. The layers 13A and 13B are, for example, n-side AlGaN layers. The layer 13A is an n-side contact layer. The third core 15 may be, for example, a nitride semiconductor. When the third optical waveguide 11 is an optical modulator 62, the third core 15 may include an undoped GaN layer or an undoped InGaN layer. Here, "undoped" refers to a semiconductor layer into which no impurities have been intentionally introduced during manufacturing. However, even undoped layers may contain unavoidable impurities. Whether the impurities are n-type impurities or p-type impurities, the impurity concentration is 1×10 －１７ cm －３The impurity concentration may be equal to or less than the detection limit of secondary ion mass spectrometry (SIMS). The third core 15 further includes an n-side optical guiding layer and a p-side optical guiding layer, and the active layer is located between the n-side optical guiding layer and the p-side optical guiding layer. The sixth cladding 17 may be, for example, a nitride semiconductor. The sixth cladding 17 has a layer 17A and a layer 17B disposed on the layer 17A. The layers 17A and 17B are, for example, p-side AlGaN layers. The layer 17B is a p-side contact layer. The layers 13A to 17B are layers regrown on the first cladding 12. The layer 13B, the third core 15, and the sixth cladding 17 have a ridge structure. The third optical waveguide 11 is an optical waveguide with a ridge structure. The third optical waveguide 11 may be an optical waveguide with a ridge structure including a part of the first cladding 12 , the fifth cladding 13 , the third core 15 and the sixth cladding 17 .

[0062] The height H2 of the third core 15 in the third optical waveguide 11 from the substrate 18 is greater than the height H1 of the first core 14 from the substrate 18 in the first optical waveguide 10 in Fig. 14 . In the fourth embodiment, the first optical waveguide 10 having different core heights can be coupled to the third optical waveguide 11 using the second optical waveguide 20. It does not matter which of the second optical waveguide 20 and the third optical waveguide 11 is formed first. That is, the height of the third core 15 may be matched to the height of the second core 24 in the third region 33, or the height of the second core 24 in the third region 33 may be matched to the height of the third core 15. The other configurations are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0063] Fifth Embodiment The fifth embodiment is an example of an optical circuit capable of beam steering. Fig. 20 is a plan view of the optical circuit according to the fifth embodiment. Fig. 21 is a cross-sectional view taken along XXI-XXI in Fig. 20. As shown in Figs. 20 and 21, an optical circuit 106 according to the fifth embodiment includes a first optical waveguide 10, a second optical waveguide 20, a third optical waveguide 11, and a fourth optical waveguide 21. A region 55 is a region in which the fourth optical waveguide 21 is provided.

[0064] The first optical waveguide 10 includes a semiconductor laser element 60. The second optical waveguide 20 includes waveguides 61A, 61B1, 61B2, and 61C1 to 61C4. The waveguide 61A is formed in the first region 31, the second region 32, and the third region 33, and the waveguides 61B1, 61B2, and 61C1 to 61C4 are formed in the third region 33. The third optical waveguide 11 includes optical modulators 62A to 62D. The fourth optical waveguide 21 includes waveguides 61D1 to 61D4 and grating couplers 63A to 63D.

[0065] The fourth optical waveguide 21 is provided on the opposite side of the third optical waveguide 11 from the second optical waveguide 20. The fourth optical waveguide 21 has a seventh cladding 23 and an eighth cladding 27 provided on the first cladding 12, and a fourth core 25 disposed between the seventh cladding 23 and the eighth cladding 27. The fourth core 25 faces the third core 15. The upper surface of the seventh cladding 23 in the region 55 is parallel to the main surface 18A of the substrate 18. A thickness T4 of the seventh cladding 23 is approximately the same as a thickness T3 of the third cladding 22 in the third region 33.

[0066] Laser light emitted from the semiconductor laser element 60 is branched into waveguides 61B1 and 61B2 via waveguide 61A, and further branched into waveguides 61C1 to 61C4. The light from waveguides 61C1 to 61C4 is incident on optical modulators 62A to 62D, respectively. The optical modulators 62A to 62D modulate the phase of the light emitted from waveguides 61C1 to 61C4 independently of one another by applying a voltage. The light output from optical modulators 62A to 62D is incident on grating couplers 63A to 63D via waveguides 61D1 to 61D4, respectively. The grating couplers 63A to 63D emit the light into space.

[0067] The optical modulators 62A to 62D modulate the phase of the light, causing the light emitted from the grating couplers 63A to 63D to interfere with each other. This enables beam steering of the light emitted from the grating couplers 63A to 63D. Although an example has been described in which the number of grating couplers 63A to 63D and the number of optical modulators 62A to 62D are four, the number of grating couplers 63A to 63D and the number of optical modulators 62A to 62D may be any number as long as they are plural. The number of optical modulators 62A to 62D may be, for example, 10 to 10,000, 100 to 10,000, or 500 to 5,000.

[0068] According to the fifth embodiment, by providing the fourth optical waveguide 21 on the opposite side of the third optical waveguide 11 from the second optical waveguide 20, the thickness T4 of the seventh cladding 23 of the fourth optical waveguide 21 can be set to approximately the thickness T3. This makes it possible to reduce leakage of light from the fourth core 25 of the fourth optical waveguide 21 to the first cladding 12. Therefore, the loss of the fourth optical waveguide 21 can be reduced.

[0069] The fourth optical waveguide 21 includes grating couplers 63A to 63D, which reduces the loss in the grating couplers 63A to 63D. The other configurations are the same as those in the fourth embodiment, and therefore a description thereof will be omitted.

[0070] Sixth Embodiment The sixth embodiment is an example of an optical circuit having a ring laser. FIG. 22 is a plan view of the optical circuit according to the sixth embodiment. FIG. 23 is a cross-sectional view taken along the cross-sectional line XXIII in FIG. 22, in which the first optical waveguides 10 at the right and left ends are the same. As shown in FIGS. 22 and 23, in the optical circuit 107 according to the sixth embodiment, a region 51 is provided to surround the first optical waveguide 10 from the +X side, the −X side, and the +Y side, and a region 52 is provided to surround the region 51 from the +X side, the −X side, and the +Y side. The second optical waveguide 20 has a first region 31, a second region 32, a third region 33, a fourth region 31A, and a fifth region 32A. The first region 31 and the fourth region 31A are disposed in the region 51, and the second region 32 and the fifth region 32A are disposed in the region 52. The third region 33 is disposed in the region 53.

[0071] The fourth region 31A is located on the opposite side of the first region 31 with respect to the first optical waveguide 10, and includes a second core 24 facing the first core 14. The thickness T1A of the third cladding 22 in the fourth region 31A is smaller than the thickness T3 of the third cladding 22 in the third region 33 and is approximately the same as the thickness T1. In the fifth region 32A, the thickness T2A of the third cladding 22 increases from the fourth region 31A toward the third region 33. The fifth region 32A connects the fourth region 31A and the third region 33. In this manner, the second optical waveguide 20 is provided in a ring shape. The first optical waveguide 10 is a semiconductor laser element 60, and light emitted in the first optical waveguide 10 resonates due to the ring between the first optical waveguide 10 and the second optical waveguide 20. This causes stimulated emission in the first optical waveguide 10, resulting in laser oscillation. In this manner, the first optical waveguide 10 and the second optical waveguide 20 form a ring laser 65 .

[0072] The optical waveguide 61E disposed in region 53 is optically coupled to the second optical waveguide 20 in region 53. As a result, laser light oscillated in the ring laser 65 is guided through the optical waveguide 61E. By making the third region 33 at least half the length of the second optical waveguide 20 along the optical axis, the region of thickness T3 from the substrate 18 can be made relatively long. This reduces loss in the ring laser 65. It is more preferable that the third region 33 be at least two-thirds the length of the second optical waveguide 20 along the optical axis. By optically coupling the optical waveguide 61E and the second optical waveguide 20 in region 53, coupling loss can be reduced. The other configurations are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0073] Seventh Embodiment Fig. 24 is a plan view of an optical circuit according to the seventh embodiment. Fig. 25 is a cross-sectional view taken along the line XXV-XXV of Fig. 24. As shown in Figs. 24 and 25, in an optical circuit 108 according to the seventh embodiment, the third optical waveguide 11 is disposed on the +Y side of the first optical waveguide 10. The third region 33 of the second optical waveguide 20 is bent by 180°. The second core 24 of the third region 33 faces the third core 15 of the third optical waveguide 11. Region 52 is L-shaped and surrounds region 51. The area of ​​region 53 in a plan view is larger than the planar areas of regions 51 and 52. The other configurations are the same as those of the fourth embodiment, and therefore description thereof will be omitted.

[0074] (First Modification of Seventh Embodiment) Fig. 26 is a plan view of an optical circuit according to a first modification of the seventh embodiment. As shown in Fig. 26, in an optical circuit 109 according to a first modification of the seventh embodiment, regions 53 are provided along the optical axis on both sides of the third region 33. The area of ​​region 53 in a plan view is smaller than the area of ​​region 51 in a plan view. When viewed from the Z direction, region 52 is inclined with respect to the optical axis of the second optical waveguide 20. This allows the length of the second region 32 in the optical axis direction to be longer than the length of the shortest distance connecting regions 51 and 53, as in the second embodiment. The other configurations are the same as those in the seventh embodiment, and therefore description thereof will be omitted.

[0075] (Second Modification of Seventh Embodiment) Fig. 27 is a plan view of an optical circuit according to a second modification of the seventh embodiment. As shown in Fig. 27, in an optical circuit 110 according to the second modification of the seventh embodiment, the third optical waveguide 11 is provided on the +Y side of the first optical waveguide 10. The waveguide 61A of the second optical waveguide 20 is bent by 180° in the region 53. The other configurations are the same as those of the fifth embodiment, and therefore description thereof will be omitted.

[0076] Although the preferred embodiments have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0077] The present disclosure includes the following configuration: (Item 1) A substrate having a principal surface, a first optical waveguide provided on the principal surface, and a second optical waveguide provided on the principal surface and optically coupled to the first optical waveguide, wherein the first optical waveguide has a first clad provided in a normal direction to the principal surface, a second clad, and a first core disposed between the first clad and the second clad, the second optical waveguide has a third clad provided in a normal direction to the principal surface, a fourth clad, and a second core disposed between the third clad and the fourth clad, wherein the material of the second core is different from the material of the first core, the refractive index of the substrate is higher than the refractive index of the third clad for a wavelength of light guided through the second optical waveguide, the second optical waveguide has a first region, a second region, and a third region, wherein the first region is a region including an end of the second core facing the first core, An optical circuit, wherein the third region is a region where the thickness of the third cladding is greater than that of the first region, and the second region is a region connecting the first region and the third region such that the thickness of the third cladding increases from the first region toward the third region. (Item 2) The optical circuit according to Item 1, wherein an upper surface of the second core in the third region is parallel to the primary surface. (Item 3) The optical circuit according to Item 1 or 2, wherein a part of the material forming the first cladding is disposed between the third cladding and the substrate. (Item 4) The optical circuit according to any one of Items 1 to 3, wherein the first optical waveguide includes a nitride semiconductor, and the third cladding includes an amorphous material. (Item 5) The optical circuit according to any one of Items 1 to 4, wherein the length of the second region in the optical axis direction is greater than the length of the shortest distance connecting a region including the first region and where the upper surface of the third cladding is parallel to the primary surface, and a region including the third region and where the upper surface of the third cladding is parallel to the primary surface. (Item 6) The optical circuit according to item 5, wherein the optical axis direction of the second core in the second region is the same as the optical axis direction of the first core when viewed from the thickness direction of the substrate. (Item 7) The optical circuit according to any one of items 1 to 5, wherein the optical axis direction of the second core in the second region is tilted from the optical axis direction of the first core when viewed from the thickness direction of the substrate.(Item 8) The optical circuit according to item 7, wherein, when viewed from the thickness direction of the substrate, the angle formed between the optical axis direction of the second core in the second region and the optical axis direction of the first core is 45° or more and 89° or less. (Item 9) The optical circuit according to any one of items 1 to 8, wherein a difference between a thickness of the third cladding in the third region and a thickness of the third cladding in the first region is 0.5 μm or more and 5 μm or less. (Item 10) The optical circuit according to any one of items 1 to 9, wherein a length of the second core in the third region in the optical axis direction is greater than a sum of a length of the second core in the first region in the optical axis direction and a length of the second core in the second region in the optical axis direction. (Item 11) The optical circuit according to any one of items 1 to 10, further comprising a ring resonator in the third region that is optically coupled to the second optical waveguide. (Item 12) The optical circuit according to any one of items 1 to 11, further comprising a third optical waveguide having a fifth cladding arranged on the first cladding, a sixth cladding, and a third core arranged between the fifth cladding and the sixth cladding and facing the second core in the third region. (Item 13) The optical circuit according to item 12, wherein the third optical waveguide is provided on the same side as the second optical waveguide with respect to the first optical waveguide. (Item 14) The optical circuit according to item 12 or 13, further comprising a fourth optical waveguide provided on the opposite side of the third optical waveguide to the second optical waveguide, and having a seventh cladding arranged on the first cladding, an eighth cladding, and a fourth core arranged between the seventh cladding and the eighth cladding and facing the third core. (Item 15) The optical circuit according to item 14, wherein the fourth optical waveguide includes a grating coupler.(Item 16) The optical circuit according to any one of items 1 to 15, wherein the second optical waveguide has a fourth region and a fifth region, the fourth region is located on the opposite side of the first region with respect to the first optical waveguide and includes the second core facing the first core, and the thickness of the third cladding is smaller than the thickness of the third cladding in the third region, and the fifth region is a region connecting the fourth region and the third region such that the thickness of the third cladding increases from the fourth region toward the third region, and the first optical waveguide and the second optical waveguide form a ring laser.

[0078] This application claims priority from basic patent application No. 2024-133983, filed with the Japan Patent Office on August 9, 2024, the entire contents of which are incorporated herein by reference.

[0079] 10: First optical waveguide 11: Third optical waveguide 12: First cladding 13: Fifth cladding 14: First core 15: Third core 16: Second cladding 17: Sixth cladding 18: Substrate 20: Second optical waveguide 20A: Optical waveguide 21: Fourth optical waveguide 22: Third cladding 23: Seventh cladding 24: Second core 25: Fourth core 26: Fourth cladding 27: Eighth cladding 28: Insulating layer 29: Electrode 31: First region 31A: Fourth region 32: Second region 32A: Fifth region 33: Third region 48A: Direction 48B: Optical axis direction 51, 52, 53, 54, 55: Regions 60: Semiconductor laser element 62A, 62B, 62C, 62D: Optical modulators 63A, 63B, 63C, 63D: Grating couplers 64: Ring resonator 65: Ring laser

Claims

1. A light emitting device comprising: a substrate having a principal surface; a first optical waveguide provided on the principal surface; and a second optical waveguide provided on the principal surface and optically coupled to the first optical waveguide; wherein the first optical waveguide has a first clad provided in a normal direction to the principal surface, a second clad, and a first core disposed between the first clad and the second clad; the second optical waveguide has a third clad provided in a normal direction to the principal surface, a fourth clad, and a second core disposed between the third clad and the fourth clad; the material of the second core is different from the material of the first core; the refractive index of the substrate is higher than the refractive index of the third clad for the wavelength of light guided through the second optical waveguide; the second optical waveguide has a first region, a second region, and a third region; the first region is a region including an end of the second core facing the first core; an optical circuit, wherein the third region is a region in which the thickness of the third cladding is greater than that of the first region, and the second region is a region connecting the first region and the third region such that the thickness of the third cladding increases from the first region toward the third region.

2. The optical circuit according to claim 1, wherein the top surface of said second core in said third region is parallel to said main surface.

3. An optical circuit according to claim 1 or 2, wherein a portion of the material constituting said first cladding is disposed between said third cladding and said substrate.

4. An optical circuit according to any one of claims 1 to 3, wherein the first optical waveguide includes a nitride semiconductor, and the third cladding includes an amorphous material.

5. An optical circuit described in any one of claims 1 to 4, wherein the length of the second region in the optical axis direction is greater than the length of the shortest distance connecting a region including the first region and in which the upper surface of the third cladding is parallel to the main surface and a region including the third region and in which the upper surface of the third cladding is parallel to the main surface.

6. An optical circuit according to claim 5, wherein the optical axis direction of said second core in said second region is the same as the optical axis direction of said first core when viewed in the thickness direction of said substrate.

7. An optical circuit according to any one of claims 1 to 5, wherein the optical axis direction of the second core in the second region is tilted from the optical axis direction of the first core when viewed in the thickness direction of the substrate.

8. An optical circuit as described in claim 7, wherein the angle formed between the optical axis direction of the second core and the optical axis direction of the first core in the second region is 45° or more and 89° or less when viewed from the thickness direction of the substrate.

9. An optical circuit according to any one of claims 1 to 8, wherein the difference between the thickness of the third cladding in the third region and the thickness of the third cladding in the first region is 0.5 μm or more and 5 μm or less.

10. An optical circuit described in any one of claims 1 to 9, wherein the length of the second core in the optical axis direction in the third region is greater than the sum of the length of the second core in the optical axis direction in the first region and the length of the second core in the optical axis direction in the second region.

11. The optical circuit according to any one of claims 1 to 10, further comprising a ring resonator optically coupled to the second optical waveguide in the third region.

12. An optical circuit according to any one of claims 1 to 11, further comprising a third optical waveguide having a fifth cladding arranged on the first cladding, a sixth cladding, and a third core arranged between the fifth cladding and the sixth cladding and facing the second core in the third region.

13. The optical circuit according to claim 12, wherein the third optical waveguide is provided on the same side of the first optical waveguide as the second optical waveguide.

14. An optical circuit as described in claim 12 or 13, further comprising a fourth optical waveguide provided on the opposite side of the third optical waveguide from the second optical waveguide, the fourth optical waveguide having a seventh clad provided on the first clad, an eighth clad, and a fourth core disposed between the seventh clad and the eighth clad and facing the third core.

15. The optical circuit of claim 14, wherein the fourth optical waveguide includes a grating coupler.

16. The optical circuit described in any one of claims 1 to 15, wherein the second optical waveguide has a fourth region and a fifth region, the fourth region is located on the opposite side of the first region with respect to the first optical waveguide and includes the second core facing the first core, and the thickness of the third cladding is smaller than the thickness of the third cladding in the third region, and the fifth region is a region connecting the fourth region and the third region so that the thickness of the third cladding increases from the fourth region toward the third region, and the first optical waveguide and the second optical waveguide form a ring laser.

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