Optical circuit

Abstract

An optical circuit (20) is provided with a substrate (21), a first waveguide (22) and a second waveguide (23) formed on the substrate (21), a third waveguide (24) optically coupled to the first waveguide (22) and the second waveguide (23), and an optical functional layer (25) optically coupled to the third waveguide (24) so as to form a supermode.　The first waveguide (22) and the second waveguide (23) are made of a material having a refractive index n1, the third waveguide (24) is made of a material having a refractive index n2, the optical functional layer (25) is made of a material having a refractive index n3, and the absolute value of a difference between the refractive index n1 and the refractive index n3 is larger than the absolute value of a difference between the refractive index n2 and the refractive index n3.

Description

optical circuit

[0001] The present invention relates to a hybrid optical integrated circuit used in optical communications and the like.

[0002] In recent years, the amount of data communication has increased dramatically, and there is a demand for further miniaturization of optical devices to improve the processing power of optical devices in data centers that process that data, to enhance the functionality of optical transmission components, and to improve space efficiency. Silicon photonics (hereinafter referred to as SiPh) technology has attracted attention as a way to fabricate such high-performance and compact optical devices.

[0003] In SiPh, the optical waveguide has a core made of silicon (Si) and a cladding made of SiO2. The large difference in refractive index between silicon and SiO2 allows for strong optical confinement. Therefore, in SiPh, the bending radius of the optical waveguide can be reduced, and the size of the optical passive device can be reduced. Therefore, the use of the SiPh platform makes it possible to manufacture highly integrated, compact optical circuit chips.

[0004] At the same time, SiPh makes it possible to fabricate and integrate various optical active devices such as photodiodes (PDs), optical modulators, and optical attenuators by utilizing the semiconductor properties of silicon.

[0005] However, while SiPh can integrate a variety of optically active devices, its performance is limited by the optical and electrical properties of the silicon used as the material. Therefore, research and development is being conducted on the SiPh platform to manufacture higher performance optical circuit chips by using materials other than silicon that have excellent optical and electrical properties for some of the optically active devices.

[0006] For example, it is known that a PD formed by combining germanium and silicon has a faster response speed than a PD formed only from silicon.

[0007] In this way, by integrating germanium as a material other than silicon into a SiPh optical circuit, it is possible to manufacture a hybrid optical circuit chip that incorporates an ultrafast PD on the SiPh optical circuit. The complexity of the process is a challenge when fabricating a hybrid optical circuit. Hybrid optical circuits use multiple materials to take advantage of their excellent optical properties, and different processing steps are required for each material, increasing the complexity of the process.

[0008] Fig. 1 is a diagram showing the schematic configuration of one hybrid optical circuit structure that has been studied in the past. The optical circuit 10 in Fig. 1 includes a substrate 11, an input waveguide 12 formed on the substrate 11, a thin-wire waveguide 14, a tapered waveguide 13 connecting the input waveguide 12 and the thin-wire waveguide 14, an output waveguide 16, a tapered waveguide 15 connecting the thin-wire waveguide 14 and the output waveguide 16, and a second layer 17 disposed from the tapered waveguide 13 to above the tapered waveguide 15. In Fig. 1, a cladding layer formed of SiO2 is partially omitted to make it easier to understand the positional relationship between the optical waveguide and the second layer.

[0009] FIG. 2 is a diagram illustrating the arrangement of the waveguides and the second layer 17 of the optical circuit 10. FIG. 2(a) shows the arrangement of only the waveguides and the second layer 17 from the Z-axis direction of the optical circuit 10 of FIG. 1, and FIG. 2(b) shows the arrangement of only the waveguides and the second layer from the X-axis direction. The input waveguide 12, tapered waveguide 13, thin-wire waveguide 14, tapered waveguide 15, and output waveguide 16 of the optical circuit 10 are all formed of silicon, a material with the same refractive index. The second layer 17 is formed of lithium niobate. Each waveguide of the optical circuit 10 is formed by processing silicon on a silicon-on-insulator (SOI) substrate.

[0010] In this optical circuit 10, light incident on the input waveguide 12 made of silicon couples to a propagation mode (hereinafter referred to as a "supermode") formed by materials with multiple different refractive indices, consisting of the lithium niobate layer of the second layer 17 and the silicon of the thin-wire waveguide 14, while traveling through the tapered waveguide 13, and then propagates.

[0011] In the supermode, light exists in both the lithium niobate region and the silicon region, and the ratio of light present in both regions is determined by the cross-sectional shape (width, height) of the thin-wire waveguide 14, the film thickness of the second layer 17, and the gap in the height direction between the thin-wire waveguide 14 and the second layer.

[0012] The light propagating in the supermode along the thin-wire waveguide 14 reaches the tapered waveguide 15 and travels through the tapered waveguide 15, whereby the light is gradually optically coupled from the supermode to the propagation mode of the output waveguide 16. Therefore, the light propagates within the output waveguide 16 from the region where the lithium niobate layer, which is the second layer 17, ends.

[0013] When an electric field is applied to the second layer 17 of this optical circuit 10, the refractive index of the second layer, i.e., the lithium niobate layer, changes due to the Pockels effect. As described above, in this optical circuit 10, light propagating in the supermode is also present in the lithium niobate layer, and the light propagating in the supermode is affected by this change in refractive index and undergoes a phase change. Therefore, the above-described optical circuit 10 can be operated as an optical phase shifter. By incorporating this phase shifter into a Mach-Zehnder interferometer, a Mach-Zehnder modulator can be obtained.

[0014] A Mach-Zehnder modulator fabricated using this optical circuit 10 can operate at higher speeds than a Mach-Zehnder modulator fabricated using silicon alone, and is superior in terms of modulation bandwidth. By using the above optical circuit structure, it is possible to fabricate a hybrid optical circuit equipped with a high-speed optical modulator.

[0015] In this method, the propagation direction of light can be determined by the thin-wire waveguide 14 formed of silicon that exists under the second layer 17, and therefore there is no need to process the lithium niobate layer as in the conventional method of forming a waveguide using lithium niobate. Therefore, the complexity of the processing process can be reduced. In this way, it has been known that the optical circuit 10 method can utilize the high integration of silicon optical circuits and the excellent optical properties of lithium niobate while reducing the processing complexity that is an issue with hybrid optical circuits (for example, Non-Patent Document 1).

[0016] However, this method has a problem in that the abundance ratio of light in the supermode formed by the lithium niobate layer used in the second layer and the silicon wire waveguide 14 is sensitive to the width of the wire waveguide 14 and the height gap between the wire waveguide 14 and the lithium niobate layer.

[0017] This is because the refractive index of silicon used in the thin-wire waveguide 14 is about 3.46, while that of lithium niobate is about 2.2, resulting in a large difference in the refractive index between the two. Since light is attracted to materials with a large refractive index, if the width of the thin-wire waveguide is wide or if the gap in the height direction between the lithium niobate layer and the thin-wire waveguide is wide, less light will be present in the lithium niobate layer region.

[0018] Since the optical circuit 10 operates as a phase shifter by changing the refractive index of the lithium niobate layer due to the Pockels effect, controlling the ratio of light present in the lithium niobate layer is an important design issue. In other words, in the optical circuit 10, the amount of light present in the lithium niobate layer region is sensitive to the width of the thin-wire waveguide and the height gap between the lithium niobate layer and the thin-wire waveguide, which means that the optical circuit 10 has low tolerance to manufacturing errors.

[0019] As described above, when a high-density, high-performance optical circuit is manufactured using the conventional optical circuit structure as described in Non-Patent Document 1, there is a problem that tolerance to manufacturing errors is low.

[0020] To solve this problem of the conventional technology, it has been proposed to form each waveguide made of silicon from SiN in the structure of the optical circuit 10 (for example, Non-Patent Document 2). The refractive index of SiN is about 2.8, and by making the difference in refractive index between the SiN and the lithium niobate layer smaller than that of the optical circuit 10 described above, the tolerance to manufacturing errors is improved.

[0021] However, when each waveguide is made of SiN as in Non-Patent Document 2, the optical confinement is weaker than when it is made of silicon. As a result, restrictions are imposed on the bending rate of the optical waveguide, which causes a problem of a decrease in the degree of integration of the optical circuit.

[0022] The present disclosure has been made in consideration of these problems of the conventional technology, and one of its objectives is to improve tolerance to manufacturing errors while maintaining the advantage of not requiring processing of the second layer.

[0023] Another object of the present disclosure is to improve manufacturing error tolerance while maintaining the advantage of not requiring processing of the second layer, without compromising the integration density of the optical circuit.

[0024] Valdez, F., Mere, V., Wang, X. et al. 110 GHz, 110 mW hybrid silicon-lithium niobate Mach-Zehnder modulator. Sci Rep 12, 18611 (2022).Forrest Valdez, Viphretuo Mere, and Shayan Mookherjea, "100GHz bandwidth, 1 volt integrated electro-optic Mach-Zehnder modulator at near-IR wavelengths," Optica 10, 578-584 (2023).

[0025] In one aspect, the optical circuit of the present disclosure comprises a substrate, a first waveguide and a second waveguide formed on the substrate, a third waveguide optically connected to the first waveguide and the second waveguide, and an optical functional layer optically connected to the third waveguide so as to form a supermode, wherein the first waveguide and the second waveguide are formed from a material having a refractive index n1, the third waveguide is formed from a material having a refractive index n2, and the optical functional layer is formed from a material having a refractive index n3, and the absolute value of the difference between the refractive index n1 and the refractive index n3 is greater than the absolute value of the difference between the refractive index n2 and the refractive index n3.

[0026] According to the present disclosure, by inserting a waveguide made of a different material between a layer with excellent optical properties made of a material different from the waveguide and the waveguide that forms the optical circuit, it is possible to improve tolerance to manufacturing errors and prevent a decrease in the degree of integration of the optical circuit formed by the waveguide.

[0027] The engineering issue is whether it is more difficult to fabricate a waveguide using another material or to process an optically functional layer to use it as a waveguide while maintaining the advantages of the optical circuit structure of the prior art. For example, in the case of SiPh, inserting a waveguide made of SiN is easier in terms of the manufacturing process than processing lithium niobate as a waveguide.

[0028] FIG. 1 is a diagram showing a schematic configuration of an optical circuit according to a conventional technology. FIG. 2 is a schematic configuration diagram showing the arrangement of a waveguide and a second layer of an optical circuit according to a conventional technology. FIG. 3 is a diagram showing a schematic configuration of an optical circuit according to a first embodiment. FIG. 4 is a schematic configuration diagram showing the arrangement of a waveguide and an optical functional layer of an optical circuit according to the first embodiment. FIG. 5 is a diagram for explaining a manufacturing process of an optical circuit according to an example of the first embodiment. FIG. 6 is a flowchart of a manufacturing process of an optical circuit according to an example of the first embodiment. FIG. 7 is a diagram showing a schematic configuration of an optical circuit according to a second embodiment. FIG. 8 is a schematic configuration diagram showing the arrangement of a waveguide and an optical functional layer of an optical circuit according to the second embodiment. FIG. 9 is a diagram showing a schematic configuration of an optical circuit according to a third embodiment. FIG. 10 is a schematic configuration diagram showing the arrangement of a waveguide and an optical functional layer of an optical circuit according to the third embodiment. FIG. 11 is a schematic configuration diagram showing the arrangement of a waveguide and an optical functional layer of an optical circuit according to a fourth embodiment. Fig. 12 is a schematic diagram showing the arrangement of a waveguide and an optical functional layer of an optical circuit according to a fifth embodiment. Fig. 13 is a schematic diagram showing the arrangement of a waveguide and an optical functional layer of an optical circuit according to a sixth embodiment.

[0029] The optical circuit disclosed herein utilizes the behavior of light due to the relationship between the refractive indexes of multiple different materials, and by appropriately setting the relationship between the refractive indexes of the materials, improves tolerance to manufacturing errors without affecting the integration density of the optical circuit.

[0030] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the following description is an example, and some configurations may be changed without departing from the gist of the present disclosure. Furthermore, the materials used in the embodiments described below are not limited, and some or all of the materials may be replaced.

[0031] The dimensions shown in each embodiment are examples and are not limited to the described dimensions. In the following description, the input and output directions of light are illustrated as examples, but it should be noted that the input and output are interchangeable due to the reciprocity of light.

[0032] (First Embodiment) Fig. 3 is a diagram showing a schematic configuration of an optical circuit according to a first embodiment of the present disclosure, which is a basic structure. As shown in Fig. 3, an optical circuit 20 according to this embodiment includes a first waveguide 22 and a second waveguide 23 formed on a substrate 21, through which light is input or output, a third waveguide 24 optically connected to the first waveguide 22 and the second waveguide 23, and an optical functional layer 25 optically connected to the third waveguide 24 so as to form a supermode. In Fig. 3, a cladding layer formed of SiO2 or the like is partially omitted to make it easier to understand the relative positions of the waveguides and the optical functional layer.

[0033] 4A and 4B are schematic diagrams for explaining the arrangement of the waveguides and optical functional layers of the optical circuit 20. Fig. 4A shows the arrangement of only the waveguides and optical functional layers from the Z-axis direction of the optical circuit 20 in Fig. 3, and Fig. 4B shows the arrangement of only the waveguides and optical functional layers from the X-axis direction of the optical circuit 20 in Fig. 3.

[0034] The first waveguide 22 and the second waveguide 23 are formed of a first material, and the third waveguide 24 is formed of a second material. The optical functional layer 25 is formed of a third material. Examples of materials that can be used in optical circuits include, but are not limited to, materials for forming the waveguide (waveguide core), such as silicon, SiN, indium phosphide, and polymer materials. Examples of materials for forming the optical functional layer include optical functional materials such as lithium niobate, tantalum niobate, lead lanthanum zirconate titanate, and barium titanate.

[0035] 3 and 4, in the optical circuit 20, the first waveguide 22 and the third waveguide 24 are optically connected. Similarly, the third waveguide 24 and the second waveguide 23 are also optically connected. The third waveguide 24 and the optical functional layer 25 are optically connected so as to form a supermode of the third waveguide 24 and the optical functional layer 25 through the propagation mode of the third waveguide 24.

[0036] The light incident from the first waveguide 22 propagates through the first waveguide 22 in the Y-axis direction, and is optically coupled from the propagation mode of the first waveguide 22 to the propagation mode of the third waveguide 24 in the overlapping region with the third waveguide 24, and then enters and propagates through the third waveguide 24. The light incident on the third waveguide 24 then further propagates through the third waveguide 24 in the Y-axis direction in the propagation mode of the third waveguide 24, and propagates in a supermode with the optical functional layer 25 in the overlapping region with the optical functional layer 25. Thereafter, the light reaches the end of the overlapping region between the optical functional layer 25 and the third waveguide 24, where it again changes to the propagation mode of the third waveguide 24 and propagates through the third waveguide 24, and then, in the overlapping region between the third waveguide 24 and the second waveguide 23, it is optically coupled from the propagation mode of the third waveguide 24 to the propagation mode of the second waveguide 23, and is input to and output from the second waveguide 23. Note that, due to the reciprocity of light, input and output are interchangeable, and light may be input to the optical circuit from the second waveguide 23 and output from the first waveguide 22.

[0037] In this embodiment, when the refractive index of the first material is n1, the refractive index of the second material is n2, and the refractive index of the third material is n3, materials are selected such that |n1-n3|>|n2-n3|.

[0038] By doing this, even if the first and second waveguides have large refractive indices, the difference between the refractive index of the optical functional layer and the refractive index of the third waveguide that forms a supermode between the optical functional layer can be reduced, thereby making it possible to improve tolerance to manufacturing errors.

[0039] Furthermore, in this embodiment, since the first and second waveguides are made of a material with a large refractive index, it is possible to prevent a decrease in the degree of integration of the optical circuit.

[0040] In the optical circuit 20 of this embodiment, the first waveguide, the second waveguide, the third waveguide, and the optical functional layer are arranged at different heights in the height direction (the Z-axis direction in FIGS. 2 and 3 ) from the viewpoint of suppressing reflections at the respective optical connection portions. However, the first waveguide, the second waveguide, and the third waveguide may also be arranged at the same height.

[0041] (Example) As an example of the first embodiment, the first waveguide 22 and the second waveguide 23 were formed from silicon, and the third waveguide 24 was formed from SiN. The optical function layer 25 was formed from lithium niobate. In this case, the refractive index of silicon was set to n Si , the refractive index of SiN is n SiN , the refractive index of lithium niobate is n LN Then, the relationship between their refractive indices is |n Si -n LN |>|n SiN -n LN |

[0042] Although not limiting, the dimensions of each element in the example are as follows: the first and second waveguides made of silicon have a height of 220 nm and a width of 500 nm, and the third waveguide made of SiN has a height of 300 nm and a width of 600 nm. The optically functional layer of lithium niobate is formed as a layer with a thickness of 600 nm.

[0043] In the optical circuit of this embodiment, when an electric field is applied to the optical functional layer of lithium niobate, the refractive index changes due to the Pockels effect, and the light propagating in the supermode is affected by this change in refractive index and changes in phase. Therefore, the optical circuit of this embodiment can be operated as an optical phase shifter. This embodiment can be manufactured using silicon on insulator (hereinafter referred to as SOI).

[0044] Next, the manufacturing process of the embodiment will be described with reference to Fig. 5. As shown in Fig. 5(a), first, an SOI substrate is prepared.

[0045] In the first step, as shown in FIG. 5B, a pattern is drawn on silicon (Si) on an SOI substrate by lithography, and then etching is performed to form a first waveguide and a second waveguide.

[0046] Thereafter, in the second step, as shown in FIG. 5(c), SiO2 is deposited as a cladding, and then in the third step, as shown in FIG. 5(d), the surface is polished by chemical mechanical polishing (hereinafter referred to as CMP) to ensure flatness.

[0047] Then, in a fourth step, as shown in Fig. 5(e), SiN is deposited by sputtering or the like to form a SiN film. In a fifth step, as shown in Fig. 5(f), a pattern is drawn on the SiN by lithography, and the SiN is etched to form a third waveguide.

[0048] In the sixth step, as shown in Fig. 5(g), SiO2 is deposited again as a cladding, and in the seventh step, as shown in Fig. 5(h), the surface is polished by CMP to ensure flatness. Finally, in the eighth step, as shown in Fig. 5(i), an LN film (lithium niobate film) is bonded by direct bonding or the like to form an optical function layer. In this manner, the optical circuit structure of this example can be manufactured.

[0049] The LN film may be bonded using an adhesive or the like in the eighth step instead of directly bonding.

[0050] 6 is a flowchart of the manufacturing process of this embodiment. The contents of the first to eighth steps are as described with reference to FIG.

[0051] 7 and 8 are diagrams showing a schematic configuration of an optical circuit 30 according to a second embodiment of the present disclosure. As with Fig. 3, Fig. 7 omits some of the cladding layers formed of SiO2 or the like so as to make it easier to understand the relative positions of the waveguides and optical functional layers.

[0052] Figure 8 is a diagram for explaining the arrangement of the waveguides and optical functional layers of the optical circuit 30. Figure 8(a) shows the arrangement of only the waveguides and optical functional layers from the Z-axis direction of the optical circuit 30 in Figure 2, and Figure 8(b) shows the arrangement of only the waveguides and optical functional layers from the X-axis direction of the optical circuit 30 in Figure 7. In Figures 7 and 8, the same reference numerals as in Figures 3 and 4 are the same as in Figures 3 and 4, and therefore their explanation will be omitted here.

[0053] In this embodiment, the first waveguide 22 and the second waveguide 23 are also made of the same first material, the third waveguide 24 is made of a second material, and the optically functional layer 25 is made of a third material. When the refractive index of the first material is n1, the refractive index of the second material is n2, and the refractive index of the third material is n3, materials are selected such that |n1-n3|>|n2-n3|. Materials that can be used for the waveguides and the optically functional layer are the same as those exemplified in the first embodiment.

[0054] In the optical circuit 30 of this embodiment, the first waveguide 22, the second waveguide 23, and the third waveguide 24 are optically connected in an adiabatic manner. Specifically, a tapered structure 32 is formed in the first waveguide 22, and a tapered structure 33 is formed in the second waveguide 23. Also, in the third waveguide 24, tapered structures 34 and 35 are formed in the portions where the tapered structure 32 of the first waveguide 22 and the tapered structure 33 of the second waveguide 23 overlap, respectively.

[0055] The first waveguide 22 and the third waveguide 24 are optically connected adiabatically by the tapered structure 32 and the tapered structure 34. Similarly, the second waveguide 23 and the third waveguide 24 are optically connected adiabatically by the tapered structure 33 and the tapered structure 35. Note that "adiabatically" means that the mode change of the waveguide is gradual, and being optically connected adiabatically means that the mode change during optical connection is gradual.

[0056] The third waveguide 24 and the optically functional layer 25 are optically connected to each other so as to form a supermode between the third waveguide 24 and the optically functional layer 25 through the propagation mode of the third waveguide 24 .

[0057] According to this embodiment, the first and second waveguides are optically connected to the third waveguide in an adiabatic manner, so that loss at the connection can be reduced.

[0058] In the above embodiment, adiabatic connection is realized by the tapered structures of the first waveguide, the second waveguide, and the third waveguide shown in the figure, but adiabatic connection using, for example, a tapered structure in which the thickness of the waveguide is continuously reduced or a structure other than a tapered structure can be used.

[0059] In this embodiment, as in the example of the first embodiment, the first and second waveguides can be made of silicon, the third waveguide can be made of SiN, and the optical function layer can be made of lithium niobate.

[0060] Although not limiting, the dimensions in this case are exemplified as follows: the first and second waveguides formed of silicon have a height of 220 nm, a width of 500 nm at their widest point, and a width of 300 nm at their narrowest point. The third waveguide formed of SiN has a height of 300 nm, a width of 600 nm at their widest point, and a width of 300 nm at their narrowest point. The optically functional layer of lithium niobate has a thickness of 600 nm.

[0061] In the optical circuit 30, when light is incident on the first waveguide 22, the incident light is adiabatically optically coupled from the propagation mode of the first waveguide to the propagation mode of the third waveguide and then enters the third waveguide 24. The light incident on the third waveguide 24 passes through the propagation mode of the third waveguide and is optically coupled to the supermode of the third waveguide and the optical functional layer. Thereafter, the light is optically coupled from the supermode of the third waveguide and the optical functional layer to the propagation mode of the third waveguide, and is optically adiabatically optically coupled from the propagation mode of the third waveguide to the propagation mode of the second waveguide, and then enters the second waveguide 23 and is output. Note that, due to the reciprocity of light, input and output are interchangeable, and light may be input to the optical circuit from the second optical waveguide 23 and output from the first waveguide 22.

[0062] 9 and 10 are diagrams showing a schematic configuration of an optical circuit 40 according to a third embodiment of the present disclosure. As with Fig. 3, Fig. 9 omits some cladding layers made of SiO2 or the like to make it easier to understand the relative positions of the waveguides and optical functional layers.

[0063] 10A and 10B are schematic diagrams for explaining the arrangement of the waveguides and optical functional layers of the optical circuit 30. Fig. 10A shows the arrangement of only the waveguides and optical functional layers from the Z-axis direction of the optical circuit 40 in Fig. 9, and Fig. 10B shows the arrangement of only the waveguides and optical functional layers from the X-axis direction of the optical circuit 40 in Fig. 9.

[0064] In this embodiment, the third waveguide 24 includes, in addition to the tapered structures 34 and 35, tapered structures 41 and 42, and a thin line portion 43 connecting the tapered structures 41 and 42. In Figures 9 and 10, the same reference numerals as in Figures 7 and 8 are the same as those in Figures 7 and 8, and therefore description thereof will be omitted here.

[0065] In this embodiment, the first waveguide 22 and the second waveguide 23 are also made of the same first material, the third waveguide 24 is made of a second material, and the optically functional layer 25 is made of a third material. When the refractive index of the first material is n1, the refractive index of the second material is n2, and the refractive index of the third material is n3, materials are selected such that |n1-n3|>|n2-n3|. Materials that can be used for the waveguides and the optically functional layer are the same as those exemplified in the first embodiment.

[0066] The third waveguide 24 of the optical circuit 40 of this embodiment has a tapered structure 41 as an optical connection portion where light propagating through the third waveguide 24 transits through the propagation mode of the third waveguide 24 to a supermode formed by the third waveguide 24 and the optical functional layer 25. The third waveguide 24 also has a tapered structure 42 as an optical connection portion where light propagating in the supermode formed by the third waveguide 24 and the optical functional layer 25 transits to the propagation mode of the third waveguide 24. This makes it possible to reduce loss in the transition between the propagation mode of the third waveguide and the supermode of the third waveguide and the optical functional layer.

[0067] In this embodiment, as in the examples of the first embodiment, the first and second waveguides can be formed of silicon, the third waveguide can be formed of SiN, and the optical function layer can be formed of lithium niobate. Also in the optical circuit 40 of this embodiment, as in the optical circuit 30 of the second embodiment, the first waveguide 22 and the second waveguide 23 are optically connected to the third waveguide 24 in an adiabatic manner.

[0068] In the optical circuit 40, when light is incident on the first waveguide 22, the incident light is adiabatically optically coupled from the propagation mode of the first waveguide to the propagation mode of the third waveguide and then enters the third waveguide 24. The light incident on the third waveguide 24 passes through the propagation mode of the third waveguide and is optically coupled to the supermode of the third waveguide and the optical functional layer at the overlapping portion between the tapered structure 41 and the optical functional layer 25. Thereafter, the light propagating to the overlapping portion between the tapered structure 42 and the optical functional layer 25 is optically coupled from the supermode of the third waveguide and the optical functional layer to the propagation mode of the third waveguide, and is optically adiabatically optically coupled from the propagation mode of the third waveguide to the propagation mode of the second waveguide, and then enters the second waveguide 23 and is output. Due to the reciprocity of light, input and output are interchangeable, and light may be input to the optical circuit from the second waveguide 23 and output from the first waveguide 22 .

[0069] 11A and 11B are schematic diagrams illustrating the arrangement of waveguides and optical functional layers of an optical circuit 50 according to a fourth embodiment of the present disclosure. Fig. 11A shows the arrangement of only the waveguides and optical functional layers from the Z-axis direction of the optical circuit 50, and Fig. 11B shows the arrangement of only the waveguides and optical functional layers from the X-axis direction of the optical circuit 50. However, similar to the first to third embodiments, each waveguide and optical functional layer is formed on a substrate.

[0070] In the first to third embodiments described above, the optical functional layer 25 is disposed so as to cover only a portion of the third waveguide 24. However, as in the optical circuit 50 of this embodiment, the optical functional layer 51 may be disposed so as to completely cover the third waveguide 24.

[0071] 11, components having the same reference numerals as those in FIGS. 9 and 10 are the same as those in FIGS. 9 and 10, and therefore description thereof will be omitted here. Furthermore, in a configuration in which the first waveguide, the second waveguide, and the third waveguide are arranged in the same manner as in the first or second embodiment, the optical functional layer 51 may be arranged so as to completely cover the third waveguide 24 as in this embodiment. Even when the optical functional layer 51 is arranged as in the optical circuit 50 of this embodiment, it is possible to operate the optical circuits in the same manner as the optical circuits of the first to third embodiments.

[0072] In this embodiment, the first waveguide 22 and the second waveguide 23 are also formed of the same first material, the third waveguide 24 is formed of a second material, and the optical function layer 25 is formed of a third material. When the refractive index of the first material is n1, the refractive index of the second material is n2, and the refractive index of the third material is n3, materials are selected such that |n1-n3|>|n2-n3|. Materials that can be used for the waveguides and the optical function layer are the same as those exemplified in the first embodiment.

[0073] In this embodiment, similarly to the example of the first embodiment, the first and second waveguides can be made of silicon, the third waveguide can be made of SiN, and the optical function layer can be made of lithium niobate.

[0074] 12A and 12B are schematic diagrams illustrating the arrangement of waveguides and optical functional layers of an optical circuit 60 according to a fifth embodiment of the present disclosure. Fig. 12A shows the arrangement of only the waveguides and optical functional layers from the Z-axis direction of the optical circuit 60, and Fig. 12B shows the arrangement of only the waveguides and optical functional layers from the X-axis direction of the optical circuit 60. However, similar to the first to third embodiments, the waveguides and optical functional layers are formed on a substrate.

[0075] In the optical circuit 60 of this embodiment, an optical functional layer 61 is provided with tapered structures 62 and 63 in portions corresponding to the tapered structures 41 and 42 of the third waveguide 24. In Fig. 12, the same reference numerals as in Figs. 9 and 10 are the same as those in Figs. 9 and 10, and therefore description thereof will be omitted here.

[0076] In this embodiment, the first waveguide 22 and the second waveguide 23 are also formed of the same first material, the third waveguide 24 is formed of a second material, and the optical function layer 61 is formed of a third material. When the refractive index of the first material is n1, the refractive index of the second material is n2, and the refractive index of the third material is n3, materials are selected such that |n1-n3|>|n2-n3|. Materials that can be used for the waveguides and the optical function layer are the same as those exemplified in the first embodiment.

[0077] In the optical circuit 60 of this embodiment, at an optical connection portion where light propagating through the third waveguide 24 transitions through the propagation mode of the third waveguide to a supermode formed by the third waveguide and the optical functional layer, a tapered structure 41 is formed in the third waveguide 24 and a tapered structure 62 is formed in the optical functional layer 61. Also, at an optical connection portion where light propagating in the supermode formed by the third waveguide and the optical functional layer transitions to the propagation mode of the third waveguide, a tapered structure 63 is formed in the optical functional layer 61 and a tapered structure 42 is formed in the third waveguide 24. Therefore, the optical circuit 60 of this embodiment can further reduce loss in the transition between the propagation mode of the third waveguide and the supermode of the third waveguide and the optical functional layer.

[0078] In this embodiment, as in the example of the first embodiment, the first and second waveguides can be formed from silicon, the third waveguide from SiN, and the optical function layer from lithium niobate.

[0079] (Sixth Embodiment) The optical circuits of the first to fifth embodiments described above all include the first waveguide 22 and the second waveguide 23 through which light is input or output, and the third waveguide 24 optically connected to the first waveguide 22 and the second waveguide 23.

[0080] However, when light is directly input to an input waveguide of the optical circuit from outside the optical device in which the optical circuit is implemented, or when light is directly output from an output waveguide of the optical circuit to outside the optical device in which the optical circuit is implemented, using a third waveguide instead of the first waveguide and the second waveguide as the waveguide through which light is input or output has little effect on the integration density of the optical circuit implemented in the optical device. The sixth and seventh embodiments shown in Figures 13 and 14 are aspects that can be used in such situations. The sixth embodiment will be described below with reference to Figure 13.

[0081] 13A and 13B are schematic diagrams illustrating the arrangement of the waveguides and optical functional layers of an optical circuit 70 according to a sixth embodiment of the present disclosure. Fig. 13A shows the arrangement of only the waveguides and optical functional layers from the Z-axis direction of the optical circuit 70, and Fig. 13B shows the arrangement of only the waveguides and optical functional layers from the X-axis direction of the optical circuit 70. However, similar to the third embodiment, the waveguides and optical functional layers are formed on a substrate.

[0082] In the optical circuit 70 of this embodiment, the second waveguide 23 of the optical circuit according to the third embodiment described above is omitted, and light is output from the third waveguide 24. In Fig. 13, the same reference numerals as in Figs. 9 and 10 are the same as those in Figs. 9 and 10, and therefore their description will be omitted here. In addition, the second waveguide may be omitted in the optical circuits of the other embodiments described above, as in this embodiment.

[0083] The optical circuit 70 of this embodiment includes a first waveguide 22 formed on a substrate 21 into which light is input, a second waveguide 71 optically connected to the first waveguide 22, and an optical functional layer 25 optically connected to the second waveguide 71 to form a supermode.

[0084] The first waveguide 22 is made of a first material, the second waveguide 71 is made of a second material, and the optical function layer 25 is made of a third material. When the refractive index of the first material is n1, the refractive index of the second material is n2, and the refractive index of the third material is n3, materials are selected such that |n1-n3|>|n2-n3|. Materials that can be used for the waveguide and the optical function layer are the same as those exemplified in the first embodiment.

[0085] In this embodiment, similarly to the example of the first embodiment, the first waveguide can be made of silicon, the second waveguide can be made of SiN, and the optical function layer can be made of lithium niobate.

[0086] In the optical circuit 70, when light is incident on the first waveguide 22, the incident light is adiabatically optically coupled from the propagation mode of the first waveguide to the propagation mode of the second waveguide and then enters the second waveguide 71. The light that has entered the second waveguide 71 passes through the propagation mode of the second waveguide and is optically coupled to a supermode of the second waveguide and the optical functional layer at the optically overlapping portion where the tapered structure 41 and the optically functional layer 25 overlap. Thereafter, the light that has propagated to the optically overlapping portion where the tapered structure 42 and the optically functional layer 25 overlap is optically coupled from the supermode of the second waveguide and the optically functional layer to the propagation mode of the second waveguide and is output from the second waveguide 71. Due to the reciprocity of light, input and output are interchangeable, and light may be input to the optical circuit from the second waveguide 71 and output from the first waveguide 22 .

[0087] As described above, according to the present disclosure, in an optical circuit that utilizes the optical properties of an optical functional layer, it is possible to improve manufacturing error tolerance without affecting the integration level, while maintaining the elimination of the need to process the optical functional layer.

[0088] 10, 20, 30, 40, 50, 60, 70... Optical circuit 11, 21... Substrate 22... First waveguide 23, 71... Second waveguide 24... Third waveguide 25, 51, 61... Optical function layer 32, 33, 34, 35, 41, 42, 62, 63... Tapered structure 43... Thin wire portion

Claims

1. An optical circuit comprising: a substrate; a first waveguide and a second waveguide formed on said substrate; a third waveguide optically connected to said first waveguide and said second waveguide; and an optical functional layer optically connected to said third waveguide so as to form a supermode; wherein said first waveguide and said second waveguide are formed from a material with a refractive index n1, said third waveguide is formed from a material with a refractive index n2, and said optical functional layer is formed from a material with a refractive index n3; and wherein the absolute value of the difference between said refractive index n1 and said refractive index n3 is greater than the absolute value of the difference between said refractive index n2 and said refractive index n3.

2. The optical circuit according to claim 1, wherein at least one of said first waveguide and said second waveguide has a tapered structure.

3. The optical circuit according to claim 1, wherein the third waveguide has a tapered structure at the optical connection portion between the propagation mode of the third waveguide and the supermode between the third waveguide and the optical functional layer.

4. The optical circuit according to claim 1, wherein said optically functional layer is disposed so as to completely cover said third waveguide.

5. The optical circuit according to claim 1, wherein the optical functional layer has a tapered structure at the optical connection portion between the propagation mode of the third waveguide and the supermode between the third waveguide and the optical functional layer.

6. An optical circuit comprising: a substrate; a first waveguide formed on said substrate; a second waveguide optically connected to said first waveguide; and an optical functional layer optically connected to said second waveguide so as to form a supermode; wherein said first waveguide is formed from a material with a refractive index n1, said second waveguide is formed from a material with a refractive index n2, and said optical functional layer is formed from a material with a refractive index n3; and the absolute value of the difference between said refractive index n1 and said refractive index n3 is greater than the absolute value of the difference between said refractive index n2 and said refractive index n3.

Images