Waveguide structure and planar optical wave circuit

Abstract

This waveguide structure is provided with: a first cladding layer (102A); a rib-shaped first core (103A) provided on the first cladding layer (102A); a second core (103B) provided on the first cladding layer (102A) so as to cover at least the first core (103A); and a second cladding layer (102B) provided so as to cover the second core (103B).

Description

Waveguide structures and planar lightwave circuits

[0001] The present disclosure relates to waveguide structures and planar lightwave circuits.

[0002] In recent years, circuit elements that combine three primary colors of visible light, such as RGB (red, green, and blue) coupler modules using PLCs, have been attracting attention for applications such as eyeglass-type terminals (also known as smart glasses) or projectors (see, for example, Non-Patent Document 1). PLC is an abbreviation for planar lightwave circuit.

[0003] On the other hand, existing silica-based PLCs generally use silicon dioxide (SiO) with germania (GeO) added to the core of the waveguide, in other words, Ge-doped SiO. Waveguides with this configuration are known to have an effective refractive index that changes when visible light is input (see, for example, Non-Patent Document 2).

[0004] A. Nakao, R. Morimoto, Y. Kato, Y. Kakinoki, K. Ogawa and T. Katsuyama, “Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays”, Optics Communications 330 pp.45-48 (2014)Y. Hibino, T. Kominato and Y. Ohmori, "Optical frequency tuning by laser-irradiation in silica-based March-Zehnder-type multi / demultiplexers," in IEEE Photonics Technology Letters, vol. 3, no. 7, pp. 640-642 (1991)Y. Fujiwara, J. Sakamoto, T. Hashimoto and K. Watanabe, “Zr-doped Silica-based Planar Lightwave Circuits with High Resistance against Blue Light,” Proc. IDW ’21, pp. 242-244 (2021)Y. Fujiwara, J. Sakamoto, S. Katayose, T. Hashimoto and K. Watanabe, ”Silica-based planar lightwave circuits with high resistance against blue light for visible-light application,” 2022 Jpn. J. Appl. Phys. 61 SK1021 (2022)J. Sakamoto, T. Goh, S. Katayose, K. Watanabe, M. Itoh and T.Hashimoto, "Compact and low-loss RGB coupler using mode-conversion waveguides," Optics Communications, Volume 420, pp. 46-51 (2018).

[0005] When the effective refractive index of a waveguide in a PLC changes, the characteristics of an optical circuit (e.g., a directional coupler or a multimode interferometer) formed by the waveguide may fluctuate. Therefore, one non-limiting objective of the present disclosure is to suppress fluctuations in the optical circuit characteristics.

[0006] A waveguide structure according to one embodiment of the present disclosure comprises a first cladding layer, a rib-shaped first core provided on the first cladding layer, a second core provided on the first cladding layer so as to cover at least the first core, and a second cladding layer provided so as to cover the second core.

[0007] Furthermore, a planar lightwave circuit according to one aspect of the present disclosure includes a plurality of waveguides, each of which guides at least one of a plurality of light beams having different wavelengths, and a coupling section in which optical power is at least partially transferred between any two of the plurality of waveguides, wherein any one or more of the plurality of waveguides and the coupling section have the above-described waveguide structure.

[0008] 1 is a schematic top view of an optical multiplexing circuit, which is an example of a planar lightwave circuit according to an embodiment. FIG. 2 is a cross-sectional view schematically showing the A-A' cross section of the optical multiplexing circuit illustrated in FIG. 1. FIG. 3 is a cross-sectional view schematically showing the B-B' cross section of the optical multiplexing circuit illustrated in FIG. 1. FIG. 4 is a diagram showing an example of a transmission spectrum of a first multiplexing unit according to an embodiment. FIG. 5 is a schematic cross-sectional view of a waveguide structure in a multiplexing unit of a comparative sample having no second core. FIG. 6 is a diagram showing an example of an arrangement of a plurality of optical multiplexing circuits on a wafer. FIG. 7(a) is a diagram showing an example of a wafer in-plane distribution of λ1 transmittance and λ2 transmittance in a waveguide structure according to an embodiment, and FIG. 7(b) is a diagram showing an example of a wafer in-plane distribution of λ1 transmittance and λ2 transmittance in a waveguide structure of a comparative sample. FIG. 8 is a diagram showing an example of a transmission spectrum of a directional coupler of a comparative sample. FIG. 9 is a diagram showing an example of a normalized distance dependency of optical power propagating through a cross port of a directional coupler. FIG. 10 is a diagram showing an example of a normalized distance dependency of optical power propagating through a through port of a directional coupler. 15A and 15B are diagrams showing an example of calculation results of mode field distributions for a waveguide cross-sectional structure having a second core and a waveguide cross-sectional structure not having a second core. A diagram showing an example of the relationship between the difference in core width and the λ1 transmittance of the cross port of a directional coupler. A diagram showing an example of the relationship between the λ1 transmittance of the cross port of a directional coupler and the relative refractive index difference of the second core. (a) is a diagram schematically showing the waveguide structure and relative refractive index difference of the present embodiment having a second core, (b) is a diagram schematically showing the waveguide structure and relative refractive index difference of a comparative sample not having a second core, and (c) is a diagram schematically showing the waveguide structure and relative refractive index difference with a reduced relative refractive index difference. A diagram showing an example of the relationship between the transmittance per bending angle of 90° and the bending radius of the bending waveguide in the waveguide structure shown in FIG. 14. (a) and (b) are diagrams showing an example of transmission spectra before and after blue light is input.

[0009] Hereinafter, embodiments will be described with reference to the drawings. Note that the present invention is not limited to these embodiments. In addition, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals. The drawings are schematic, and the dimensional relationships and ratios of each element may differ from the actual. Even between drawings, there may be parts in which the dimensional relationships and ratios differ. When numerical values ​​are described in the following description, they are merely examples, and other numerical values ​​may be used in addition or instead.

[0010] <Overview> PLCs can achieve various functions by creating optical waveguides on a planar substrate using patterning and etching processes such as photolithography, and combining multiple optical circuits (e.g., directional couplers, multimode interferometers, Mach-Zehnder interferometers, etc.).

[0011] As mentioned above, in a silica-based PLC that uses Ge-doped SiO2 for its waveguide (core), the effective refractive index can change when visible light is input. This change in the effective refractive index can vary depending on the wavelength, light intensity, or irradiation amount of the input visible light. Furthermore, the effective refractive index of a silica-based PLC can change over time as visible light is input, which can result in degradation of optical circuit characteristics over time.

[0012] Therefore, for example, the use of SiO2 without any additives (non-doped SiO2) or SiO2 doped with zirconia (ZrO2) (Zr-doped SiO2) for the waveguide (core) has been considered. A waveguide with this configuration can suppress changes in the effective refractive index of the waveguide due to visible light input (for example, Non-Patent Documents 3 and 4).

[0013] These silica-based materials have a smaller change in effective refractive index when visible light is input than Ge-doped SiO2, making them suitable as materials for the waveguides (cores) that make up silica-based PLCs for optical devices that handle visible light.

[0014] However, when undoped SiO or Zr-doped SiO is used for the waveguide of a silica-based PLC for optical devices that handle visible light, the optical circuit characteristics may vary. For example, variations in the core width or fluctuations in the refractive index of the core due to variations in the PLC fabrication process may cause variations in the characteristics of the optical circuit (e.g., directional coupler or multimode interferometer), which may result in a decrease in yield.

[0015] Therefore, in the embodiments described below, a waveguide structure capable of suppressing fluctuations in optical circuit characteristics and a PLC having this waveguide structure will be described.

[0016] An optical multiplexing circuit, which is an example of a planar lightwave circuit (PLC) according to an embodiment of the present disclosure, will be described with reference to FIGS. 1 to 16. FIG. 1 is a schematic top view of an exemplary optical multiplexing circuit 1 according to an embodiment. As shown in FIG. 1, the optical multiplexing circuit 1 includes, for example, on a substrate 101, first to third input terminals 10a, 20a, and 30a, first to third input waveguides 10, 20, and 30, first and second multiplexing sections 50 and 70, one output waveguide 90, and one output terminal 90a.

[0017] Each of the input waveguides 10, 20, and 30 may be a single waveguide including straight and curved portions. Therefore, in Fig. 1, "input waveguide 20" and "output waveguide 90" may be understood as convenient names for the "input section" and "output section," respectively, of a single continuous waveguide.

[0018] 1, the Z axis represents the light guide direction and corresponds to the length of the optical receiver 100 when viewed from above, the X axis represents the direction corresponding to the width of the optical multiplexing circuit 1 when viewed from above, and the Y axis represents the direction corresponding to the thickness (or height) of the optical multiplexing circuit 1 when viewed from the side or in a cross section. The directions represented by the X axis, Y axis, and Z axis are the same in the drawings used in the following description.

[0019] <Configuration Example of Input Waveguide> The first to third input ends 10a, 20a, and 30a are illustratively arranged on any end face of the optical multiplexing circuit 1 (for example, the left end face in the Z-axis direction in FIG. 1 ), and are respectively coupled to the first to third input waveguides 10, 20, and 30. In this embodiment, visible light of a first wavelength λ1 is input from the first input end 10a, coupled to the first input waveguide 10, and propagates therethrough.

[0020] Similarly, visible light of a second wavelength λ2 different from wavelength λ1 is input from second input end 20a, coupled to and propagating through second input waveguide 20. Visible light of a third wavelength λ3 different from wavelengths λ1 and λ2 is input from third input end 30a, coupled to and propagating through third input waveguide 30.

[0021] <Configuration Example of Multiplexing Section> The first multiplexing section 50 is coupled to, for example, the first input waveguide 10, the second input waveguide 20, and the second multiplexing section 70. The first multiplexing section 50 multiplexes, for example, visible light of wavelength λ1 and visible light of wavelength λ2 input from the first input waveguide 10 and the second input waveguide 20, respectively, and outputs the multiplexed light to the second multiplexing section 70.

[0022] In the multiplexing section 50, the path of light propagating through each of the two waveguides 10 and 20 that form the coupling section is sometimes referred to as a "through port," and the path of light propagating after optical power transfers from one of the waveguides 10 and 20 to the other is sometimes referred to as a "cross port."

[0023] The second multiplexing section 70 is coupled to the first multiplexing section 50, the third input waveguide 30, and the output waveguide 90. The second multiplexing section 70 multiplexes the visible light of wavelengths λ1 and λ2 that have been multiplexed in the first multiplexing section 50 and input through the waveguide 20, with the visible light of wavelength λ3 that has been input through the third input waveguide 30, and outputs the multiplexed light to the output waveguide 90.

[0024] For example, the second multiplexing section 70 may be a multiplexer (hereinafter referred to as a "mode coupler") that has a multimode waveguide (MW) 72 and involves mode conversion by the MW 72 (see, for example, Non-Patent Document 5). Note that, similar to the first multiplexing section 50, the concepts of "through port" and "cross port" can also be applied to the second multiplexing section 70.

[0025] <Configuration example of output waveguide> The output waveguide 90 is coupled to the second output port of the second multiplexing section 70 and the output end 90 a, and outputs the visible light of wavelengths λ1, λ2, and λ3 that has been multiplexed and input in the second multiplexing section 70 to the output end 90 a.

[0026] By inputting light from three different input terminals 10a, 20a, and 30a and outputting it from one output terminal 90a, the optical circuit 1 illustrated in FIG. 1 functions as an optical multiplexing circuit 1 that multiplexes light of multiple different wavelengths λ1, λ2, and λ3.

[0027] <Relationship Between Wavelength and Input Waveguide> In this embodiment, an example is shown in which light of wavelengths λ1, λ2, and λ3 is input to the first, second, and third input waveguides 10, 20, and 30, respectively, but this is not limiting. Depending on the design of the multiplexing section 50 or 70, the combination of the wavelength of the input light and the input waveguide may be changed as appropriate.

[0028] 1, the first multiplexing section 50 is a directional coupler, and the second multiplexing section 70 is a mode coupler with mode conversion as shown in Non-Patent Document 5. However, the present disclosure is not limited to this, and for example, one or both of the multiplexing sections 50 and 70 may be replaced with a directional coupler, a multimode interferometer, a mode coupler, a Y-branch circuit, a Mach-Zehnder interferometer, or the like. The configuration described in this embodiment can be effective in any multiplexing method.

[0029] <Wavelengths and Number of Wavelengths to be Combined> In this embodiment, the wavelengths λ1, λ2, and λ3 may be wavelengths that belong to different wavelength ranges that make up the three primary colors of light, namely, the wavelength ranges of blue (e.g., 400 to 495 nm), green (e.g., 495 to 570 nm), and red (e.g., 620 to 750 nm).

[0030] Therefore, the optical multiplexing circuit 1 of this embodiment may be referred to as an RGB coupler 1. However, the present disclosure can be effective regardless of the input wavelength. Furthermore, although this embodiment illustrates an example in which light of three wavelengths, λ1, λ2, and λ3, is multiplexed, the number of wavelengths multiplexed in the optical multiplexing circuit 1 (multiplexing unit 50 or 70) may be two or more.

[0031] <When light of multiple wavelengths is input from the output terminal> When light of multiple wavelengths (e.g., λ1, λ3, and λ3) is input from the output terminal 90a of the optical multiplexing circuit 1 of this embodiment, the optical multiplexing circuit 1 also functions as an optical demultiplexing circuit that outputs light of multiple wavelengths from different input terminals 10a, 20a, and 30a.

[0032] This is because the above-mentioned directional coupler, mode coupler, Y-branching circuit, and Mach-Zehnder interferometer also function as a demultiplexer due to the reciprocity of light. Therefore, the optical multiplexing circuit 1 may also be called an optical multiplexing / demultiplexing circuit 1.

[0033] <Configuration example of a waveguide cross section constituting an RGB optical multiplexing circuit> Figure 2 is a cross-sectional view schematically showing a waveguide structure constituting an exemplary optical multiplexing circuit 1 according to one embodiment, illustrating the A-A' cross section shown in Figure 1. The waveguide structure shown in Figure 2 can be applied to any one or more of the waveguides 10, 20 (90), and 30.

[0034] 2 includes a substrate 101, a lower cladding layer 102A provided on the substrate 101, and a first core 103A provided on the lower cladding layer 102A. The waveguide structure further includes a second core 103B provided (e.g., deposited) on the lower cladding layer 102A so as to cover at least the first core 103A, and an upper cladding layer 102B provided so as to cover the second core 103B.

[0035] The shape of the second core 103B (e.g., the shape in a cross-sectional view) is not limited to the shape illustrated in Fig. 2, and may be any shape that covers the first core 103A. For example, the second core 103B may be formed as a layer whose thickness in the Y-axis direction is constant along the X-axis direction and that encloses the first core 103A.

[0036] 2, the entire upper surface of the lower cladding layer 102A is covered by the second core 103B, but the upper surface of the lower cladding layer 102A may be partially exposed. For example, as long as at least the first core 103A is covered, the width of the second core 103B in the X-axis direction may be smaller than the width of the lower cladding layer 102A. These points also apply to the example of FIG. 3, which will be described later.

[0037] The lower cladding layer 102A is an example of a first cladding layer, and the upper cladding layer 102B is an example of a second cladding layer. Note that the lower cladding layer 102A may be omitted depending on the material of the substrate 101. For example, this may be the case when the substrate 101 made of a quartz-based material also serves as the lower cladding layer 102A.

[0038] The first core 103A forms a convex portion (which may also be referred to as a “rib”) along the Y-axis direction relative to the lower cladding layer 102A in a cross-sectional view, and may be conveniently referred to as a “rib-type core 103A.” The second core 103B illustratively has a rib 1031 along the Y-axis direction in a cross-sectional view, and the first core 103A is enclosed within this rib 1031.

[0039] The second core 103B may be conveniently referred to as a "cap layer 103B" for the first core 103A. Also, a waveguide structure having the first core 103A and the second core 103B may be conveniently referred to as a "double-core waveguide."

[0040] It should be noted that three or more cores may be provided between the lower cladding layer 102A and the upper cladding layer 102B. For example, a third core covering the second core 103B may be provided on the second core 103B.

[0041] <Refractive Index Difference Between Cladding Layer and Core> In this embodiment, for example, the lower cladding layer 102A and the upper cladding layer 102B have the same or similar refractive index. Furthermore, the first core 103A has a refractive index that is, for example, about 1.0% higher than the lower cladding layer 102A and the upper cladding layer 102B in terms of relative refractive index difference.

[0042] The second core 103B has a refractive index that is, for example, about 0.3% higher in terms of relative refractive index difference than the lower cladding layer 102A and the upper cladding layer 102B. In the following, when there is no need to distinguish between the cladding layers 102A and 102B, they may be abbreviated as cladding layer 102.

[0043] The relative refractive index difference (Δ) is expressed by the following formula (1): core represents the refractive index of the first core 103A or the second core 103B, and n clad represents the refractive index of the cladding layer 102.

[0044] <Relationship between refractive index of core and cladding> The present disclosure is not limited to the refractive index of the first core 103A and the second core 103B being higher than that of the cladding layer 102 by approximately 1.0% and 0.3%, respectively, in terms of relative refractive index difference. The refractive index of the second core 103B needs to be lower than that of the first core 103A and higher than that of the cladding layer 102. When three or more cores are provided as described above, the refractive index of the core provided higher than the first core 103A may be set lower.

[0045] <Example of Core and Clad Materials> As a non-limiting example, the lower cladding layer 102A is undoped SiO 2 , and the upper cladding layer 102B is SiO 2 doped with boron and phosphorus (also called “borophosphosilicate glass” (BPSG)).

[0046] The first core 103A and the second core 103B are, by way of example and not limitation, Zr-doped SiO. Different refractive indices can be achieved for the first core 103A and the second core 103B by, for example, changing the concentration of a dopant.

[0047] However, the present disclosure is not limited to the above-mentioned additives. For example, the additives in one or both of the first core 103A and the second core 103B may be one or more of hafnium (Hf), titanium (Ti), germanium (Ge), aluminum (Al), phosphorus (P), boron (B), carbon (C), and fluorine (F).

[0048] The refractive index of the second core 103B may be increased by increasing the Si component in the SiO2 composition. The refractive index of the second core 103B may be increased by applying SiO2 to the second core 103B, which has been doped with hydrogen and contains hydroxyl groups (OH groups) in the glass network. Furthermore, the refractive index of the second core 103B may be increased by diffusing the dopant in the first core 103A into the second core 103B by heat treatment. Any of the above-mentioned methods may be combined as appropriate.

[0049] <Example of a method for fabricating a waveguide cross-sectional structure> The lower cladding layer 102A, first core 103A, and upper cladding layer 102B illustrated in Figure 2 can be realized by an existing PLC fabrication method. For example, after the lower cladding layer 102A and the first core 103A are formed by an existing method, the first core 103A is shaped by photolithography and dry etching. The cross-sectional shape of the first core 103A may be, for example, a rectangle such as a square, or may be a mesa shape in which both sidewalls located in the X-axis direction are inclined with respect to the Y-axis direction.

[0050] After the first core 103A is formed, the second core 103B is formed on the lower cladding layer 102A by, for example, flame deposition (FHD), sputtering, chemical vapor deposition (CVD), etc. As a result, the second core 103B is formed, which has a rib 1031 in a cross-sectional view and contains the first core 103A within the rib 1031, as shown in FIG.

[0051] Then, an upper clad layer 102B is formed on top of the second core 103B using, for example, an existing method, to form an upper clad layer 102B that covers the second core 103B, thereby realizing the cross-sectional structure illustrated in Figure 2.

[0052] <Example of Core Dimensions> In this embodiment, the thickness of the first core 103A in the Y-axis direction is, as a non-limiting example, 2 μm. The width of the first core 103A in the X-axis direction (core width) may be different between the first multiplexing section 50 and the second multiplexing section 70, as will be described later.

[0053] The thickness of the second core 103B is 0.5 μm as a non-limiting example. The present disclosure is not limited to the above thickness. The thickness of the second core 103B may be equal to or less than the thickness of the first core 103A.

[0054] However, even if the thickness of the second core 103B is thicker than the thickness of the first core 103A, it is possible to provide an optical module that can suppress fluctuations in optical circuit characteristics due to variations in manufacturing and achieve high yields. Furthermore, compared to existing PLCs that use Ge-doped SiO2 for the core, it is possible to obtain a PLC that can suppress deterioration of optical circuit characteristics caused by fluctuations in the effective refractive index of the waveguide due to visible light input.

[0055] If the thickness of the second core 103B is greater than that of the first core 103A, a secondary effect may occur in that the chip size tends to increase for reasons described below. Also, since the thickness of the second core 103B is made greater than that of the first core 103A, the film formation cost may increase.

[0056] <Optical circuit characteristics of the BG coupler constituting the RGB coupler> Figure 3 is a schematic cross-sectional view of the first multiplexing section 50 in the optical multiplexing circuit 1 shown in Figure 1, illustrating the B-B' cross section of the optical multiplexing circuit 1 shown in Figure 1. Figure 3 corresponds to a cross-sectional view focusing on the coupling section of the multiplexing section 50, which is a directional coupler, in other words, the coupling section of two waveguides constituting the directional coupler.

[0057] As illustrated in FIG. 3, the multiplexing section 50 has two first cores 103A arranged in the X-axis direction on the lower cladding layer 102A as cores that constitute the waveguides coupled to the two input waveguides 10 and 20, respectively.

[0058] Then, the second core 103B is formed so as to cover each of the two cores 103A. For example, two ribs 1031 are formed on the second core 103B corresponding to each of the two first cores 103A, and each of the ribs 1031 encloses the first core 103A.

[0059] 3, there is a "groove" filled with the upper cladding layer 102B between the two ribs 1031. However, this "groove" may be formed, for example, in the process of forming (e.g., depositing) the second core 103B, and may not be formed. Furthermore, the depth of the "groove" in the Y-axis direction, if formed, is not an issue. As long as each first core 103A is covered with the second core 103B, a "groove" deep enough to reach the top surface of the lower cladding layer 102A may be formed.

[0060] Here, as a non-limiting example, the width in the X-axis direction (core width) of each of the two cores 103A constituting the two waveguides of the directional coupler is 1 μm, the distance in the X-axis direction (gap width) between the two cores 103A is 1 μm, and the coupling length in the Z-axis direction of the directional coupler is 250 μm. The thicknesses of the first core 103A and the second core 103B and the relative refractive index difference of the first core 103A with respect to the cladding layer 102 may be the values ​​described above.

[0061] The waveguide structure having the second core 103B illustrated in FIG. 2 or FIG. 4 can be similarly applied to the multiplexing section 70.

[0062] 4 shows an example of the transmission spectrum of the first multiplexing section 50 in this embodiment. As shown in Fig. 4, the transmission spectrum 401 of the through port has a peak at 495 to 570 nm, which corresponds to the wavelength λ2, and the transmission spectrum 402 of the cross port has a peak at 400 to 495 nm, which corresponds to the wavelength λ1.

[0063] Therefore, it can be understood that the light input to each of the two waveguides constituting the multiplexing unit 50, which is a directional coupler, is multiplexed into one waveguide. In this embodiment, for convenience, the transmittance at a wavelength of 550 nm of the through port is referred to as the "λ2 transmittance," and the transmittance at a wavelength of 440 nm of the cross port is referred to as the "λ1 transmittance."

[0064] <Comparison with Comparative Sample Without Second Core> In order to explain the effects obtained in this embodiment, the optical circuit characteristics of multiplexer 50 fabricated using the waveguide structure having second core 103B as exemplified in Fig. 2 will be compared with the optical circuit characteristics of a comparative sample. The comparative sample is a multiplexer fabricated using a general waveguide structure not having second core 103B.

[0065] <Differences Between the Waveguide Structure of the Present Embodiment and the Comparative Sample> Figure 5 shows a schematic cross-sectional view of the waveguide structure in the multiplexing section of a comparative sample that does not have a second core. Figure 5 is a cross-sectional view corresponding to Figure 2, and as shown in Figure 5, the waveguide structure of the comparative sample has a substrate 101, a lower clad layer 102A, a rib-shaped first core 103A, and an upper clad layer 102B. The cross-sectional structure illustrated in Figure 5 may be understood to be equivalent to the cross-sectional structure of a waveguide in a typical silica-based PLC.

[0066] The width, thickness, and relative refractive index difference of core 103A in the waveguide of the comparative sample shown in Fig. 5 may be equivalent to the width, thickness, and relative refractive index difference of first core 103A illustrated in Fig. 2. Furthermore, the refractive indexes of upper cladding layer 102B and lower cladding layer 102A in the comparative sample of Fig. 5 may also be equivalent to the refractive indexes of upper cladding layer 102B and lower cladding layer 102A illustrated in Fig. 2. However, since the comparison conditions are adjusted depending on whether or not second core 103B is present, the coupling length is set to 250 μm in the example of Fig. 2 and 320 μm in the comparative sample of Fig. 5.

[0067] <In-Wafer Distribution of Optical Circuit Characteristics of BG Coupler> FIG. 6 is a diagram schematically illustrating an example in which a plurality of optical multiplexing circuits 1 are arranged on a wafer 600. FIG. 6 illustrates an example in which a plurality of optical multiplexing circuits 1 are arranged in a direction perpendicular to the orientation flat (O.F.) of the wafer 600 and on a line segment passing through the center of the wafer 600. In other words, FIG. 6 illustrates the in-wafer position of the multiplexing section 50 that constitutes the optical multiplexing circuit 1. As a non-limiting example, the diameter of the wafer 600 is 6 inches (approximately 15 cm). The wafer surface corresponds to the XZ plane shown in FIGS. 1 to 3, for example.

[0068] 7(a) and 7(b) are diagrams showing an example of the in-plane distribution of λ1 transmittance and λ2 transmittance on a wafer, with the horizontal axis representing the position (cm) on the wafer and the vertical axis representing the transmittance (dB). In FIGS. 7(a) and 7(b), the λ1 transmittance and the λ2 transmittance are plotted against the position of the multiplexing unit 50, with the center of the wafer 600 set as the reference (e.g., 0 cm). The square plots indicate the λ1 transmittance, and the diamond plots indicate the λ2 transmittance.

[0069] As shown in Fig. 7(a), in the directional coupler 50 having a waveguide structure according to this embodiment, which has the second core 103B, both the λ1 transmittance and the λ2 transmittance are stable within the plane of a 6-inch wafer. In contrast, as shown in Fig. 7(b), in the directional coupler having a waveguide structure without the second core 103B, which is the comparative sample, it can be seen that the λ1 transmittance varies greatly among the λ1 transmittance and the λ2 transmittance.

[0070] When the target value of λ1 transmittance in the first multiplexing section 50 is, for example, −1.0 dB or more, all of the multiplexing sections 50 measured within the 6-inch wafer surface satisfy the target value in FIG. 7A. In contrast, in the comparative sample in FIG. 7B, 8 of the 25 multiplexing sections fail to achieve the target value (−1.0 dB) of λ1 transmittance.

[0071] From these results, it was confirmed that the variation in the optical circuit characteristics of the multiplexing unit 50 can be suppressed by employing the second core 103B in the two waveguide structures that make up the multiplexing unit 50. Therefore, the yield of the multiplexing unit 50, and therefore the yield of the optical multiplexing circuit 1, is greatly improved.

[0072] <Reason for the decrease in transmittance of the comparative sample> Next, the reason for the decrease in transmittance of the comparative sample will be considered. Fig. 8 shows an example of the transmission spectrum of the directional coupler of the comparative sample. The transmission spectrum shown in Fig. 8 corresponds to the transmission spectrum of the directional coupler placed at a position of 0 cm, which corresponds to the center of the wafer within the surface of the 6-inch wafer shown in Fig. 6.

[0073] In Fig. 8, reference numeral 801 denotes the transmission spectrum of the through port, and reference numeral 802 denotes the transmission spectrum of the cross port. It can be seen from Fig. 8 that, compared to the transmission spectra 401 and 402 of the multiplexing unit (directional coupler) 50 configured as shown in Fig. 4, the λ1 transmittance at a wavelength of 440 nm of the cross port is reduced, and the transmittance at a wavelength of 440 nm of the through port is increased. This is thought to be because the optical coupling efficiency at wavelength λ1 of the comparative sample directional coupler is reduced due to, for example, manufacturing errors.

[0074] <Causes of Decrease in Optical Coupling Rate> One of the reasons for the decrease in optical coupling rate in the directional coupler of the comparison sample is the asymmetry of the effective refractive indexes of the two waveguides that make up the directional coupler. For example, the reason for the decrease in optical coupling rate in the directional coupler of the comparison sample can be explained as follows from the mode coupling theory.

[0075] The following formulas (2) and (3) show an example of the relationship between the coupling length and the intensity of light propagating through the through port and the cross port, respectively (for example, Non-Patent Document 6).

[0076] Equation (2) represents the normalized optical intensity propagating through the through port, and equation (3) represents the normalized optical intensity propagating through the cross port. These equations (2) and (3) are based on the assumption that light is input from the through port, and z represents the coupling length. At z=0 (input section), P Through Since (0)=1, the optical input to the through port can be confirmed.

[0077] F and q in formulas (2) and (3) are expressed by the following formulas (4) and (5), respectively.

[0078] In equations (4) and (5), χ (chi) represents the mode coupling constant of the directional coupler, and δ represents the difference between the propagation constants (≈effective refractive index) of the two waveguides that make up the directional coupler, and is expressed, for example, by the following equation (6). Through represents the propagation constant of the waveguide of the through port, and β Cross represents the propagation constant of the cross-port waveguide.

[0079] 9 shows an example of the normalized distance dependency of the optical power propagating through the cross port of a directional coupler, derived based on equation (3). The optical power is normalized by the input optical power, and its maximum value is 1. The normalized distance is the product of q and z.

[0080] As shown in Fig. 9, the optical power periodically reaches a maximum every time the normalized distance changes by, for example, π / 2. Fig. 9 also illustrates the optical coupling ratio for different F-numbers (e.g., F = 1.0, F = 0.6, F = 0.2). It can be seen that the maximum value of the optical power propagating through the crossport tends to decrease as the F-number decreases.

[0081] Figure 10 shows an example of the normalized distance dependence of the optical power propagating through the through port of a directional coupler, calculated in a similar manner based on equation (2). Figure 10 shows that the minimum optical power propagating through the through port tends to increase as the F value decreases. Therefore, F can be understood as a coefficient that determines the maximum optical coupling ratio.

[0082] When F=1.0, most of the optical power is transferred from the through port to the cross port, but when F<1, as the F value decreases, the optical coupling rate to the cross port decreases, and some light remains in the through port.

[0083] In the directional coupler of the comparative sample described above, the decrease in the λ1 transmittance of the cross port is thought to be due to the F-factor being significantly reduced to 1 or less. The F-factor is determined by the difference δ in the effective refractive index of the two waveguides that make up the directional coupler and the mode coupling constant χ, so it is thought that δ increased in the comparative sample due to variations in the processed shape of the first core. For example, it is suspected that variations occurred in one or both of the core widths and sidewall angles of the two waveguides that make up the directional coupler.

[0084] In the waveguide structure of this embodiment having the second core 103B, the first core 103A may be processed by the same method as the core 103B in the comparative sample, and similar variations in the processed shape may occur. However, in the waveguide structure of this embodiment, the presence of the second core 103B can suppress a decrease in the F-value and improve yield. The reason for this is explained below.

[0085] <The mode coupling constant χ is increased by the presence of the second core> In the configuration of this embodiment, the second core 103B can increase the mode coupling constant χ compared to when the second core 103B is not present. By increasing the mode coupling constant χ, a decrease in the F-number due to variations in the processed shape of the first core 103A is suppressed, and a decrease in the optical coupling rate is suppressed.

[0086] As described above with regard to the above formula (4), F is expressed by the mode coupling constant χ and the difference δ between the propagation constants (≈ effective refractive index) of the two waveguides that constitute the directional coupler. Therefore, it can be seen that the larger the mode coupling constant χ, the closer the F value becomes to 1 even if δ is the same.

[0087] Here, the mode coupling constant χ is qualitatively determined by the extent to which the electric field of light propagating through one of the two waveguides constituting the directional coupler reaches or affects the other waveguide (see, for example, Non-Patent Document 6). For example, the stronger the electric field of light propagating through one waveguide affects the other waveguide, the larger the mode coupling constant χ becomes.

[0088] In the waveguide structure of this embodiment, the mode field diameter of the light propagating through the first core 103A is expanded by using the second core 103B, which has a refractive index lower than that of the first core 103A and a refractive index higher than that of the cladding layer 102.

[0089] 11 shows examples of calculation results of the mode field distribution for a waveguide cross-sectional structure having the second core 103 B and a waveguide cross-sectional structure not having the second core 103 B. Fig. 11(a) shows an example of the relationship between the position (μm) in the X-axis direction and the normalized intensity (arbitrary unit (a.u.)), and Fig. 11(b) shows an example of the relationship between the position (μm) in the Y-axis direction and the normalized intensity (a.u.).

[0090] The width, thickness, and relative refractive index difference of the first core 103A are as described above. It can be seen from Figures 11(a) and 11(b) that the provision of the second core 103B increases the mode field diameter compared to the comparative sample. In this embodiment, the second core 103B increases the mode field diameter and increases the mode coupling constant, thereby improving the yield of directional couplers.

[0091] To confirm the effects of this embodiment, several directional couplers were fabricated in which the effective refractive indices of the two waveguides constituting the directional coupler were intentionally made different, and the transmittance was evaluated. For example, a difference in the effective refractive indices of the two waveguides was created by increasing the width of one of the cores 103A of the two waveguides by Δw. The width, thickness, relative refractive index difference of the first core 103A, the gap width in the directional coupler, and the coupling length may be understood to be equivalent to the values ​​described above.

[0092] Fig. 12 shows an example of the relationship between the core width difference Δw and the λ1 transmittance of the cross port of a directional coupler. Fig. 12 shows transmittance 1201 for a directional coupler having a waveguide structure including the second core 103B according to this embodiment, and transmittance 1202 for a directional coupler having a waveguide structure without the second core 103B as a comparative sample. In Fig. 12, the plots indicated by squares and circles represent the measured values ​​of the λ1 transmittance, and the dashed line represents the calculated values ​​of the λ1 transmittance.

[0093] In FIG. 12, it can be seen that when Δw is 0.1 μm, the transmittance of the comparative sample drops to −4.7 dB, whereas the drop in transmittance of the directional coupler having the waveguide structure of this embodiment is suppressed to −1.4 dB.

[0094] This result confirms that the directional coupler having the waveguide structure of this embodiment can suppress a decrease in transmittance due to variations in the processed shape of the first core 103 A. The relative refractive index difference of the second core 103 B with respect to the cladding layer 102 may be freely selected based on, for example, the degree of variations in the processed shape of the first core 103 A and the allowable loss for the multiplexing section 50.

[0095] <Refractive index of second core> In the above-described embodiment, the relative refractive index difference (Δ) of the second core 103B with respect to the cladding layer 102 is set to about 0.3%, but is not limited to this. If the refractive index of the second core 103B is higher than the refractive index of the cladding layer 102, it is possible to suppress a decrease in transmittance due to variations in the processed shape of the first core 103A.

[0096] Fig. 13 shows an example of the relationship between the λ1 transmittance of the cross port of the directional coupler and the relative refractive index difference of the second core 103B. In the example of Fig. 13, in a directional coupler having the cross-sectional structure shown in Fig. 3, the first core 103A has a core width of 1 μm and a thickness of 2 μm, and the gap width between the two cores 103A constituting the two waveguides of the directional coupler is 1 μm. Furthermore, the relative refractive index difference of the first core 103A is 1.0%, the difference Δw between the core widths of the cores 103A constituting the two waveguides is 0.06 μm, and the thickness of the second core 103B is 0.5 μm.

[0097] Fig. 13 plots the λ1 transmittance of the cross port when the relative refractive index difference of the second core 103B is changed. Fig. 13 plots the λ1 transmittance when the relative refractive index difference of the second core 103B is 0%, 0.1%, 0.3%, and 0.5%, and shows that the λ1 transmittance increases as the relative refractive index difference of the second core 103B increases.

[0098] This indicates that the larger the refractive index of the second core 103B, the more the influence of the difference Δw in core width can be reduced. Even when the relative refractive index difference of the second core 103B is 0.1%, the λ1 transmittance increases by about 0.5 dB compared to when the relative refractive index difference is the minimum of 0%.

[0099] Therefore, it can be confirmed that the effect of increasing the λ1 transmittance can be obtained if the relative refractive index difference of the second core 103B is greater than 0%. If the refractive index of the second core 103B is higher than that of the cladding layer 102, the mode field expands as described above, and the expected effect of this embodiment can be obtained.

[0100] <Comparison with other methods that can increase the mode coupling constant> In the above-described embodiment, it has been described that the decrease in optical coupling rate due to variations in the processed shape can be suppressed by increasing the mode coupling constant using the second core 103B. Below, the differences from other methods (or approaches) that can increase the mode coupling constant will be described.

[0101] <Comparison with the method of increasing the mode coupling constant by narrowing the gap width> Another example of an approach to increase the mode coupling constant is to narrow the gap width of a directional coupler. As the gap width is narrowed, the electric field of light propagating in one waveguide becomes more likely to reach the other waveguide, thereby increasing the mode coupling constant.

[0102] However, in practice, if the gap width is reduced beyond a certain point, the upper cladding layer 102B may not enter the gap of the directional coupler, resulting in the formation of a void. If a void is formed in the gap, the characteristics of the directional coupler may fluctuate significantly, and the circuit may not meet the desired performance or specifications (e.g., product specifications).

[0103] Therefore, although it depends on the thickness of the first core 103A, in the case where the thickness of the core 103A of this embodiment is 2 μm, the gap width is preferably 0.8 μm or more. In the configuration using the second core 103B of this embodiment, the mode coupling constant can be increased without reducing the gap width, which is also effective in terms of suppressing the formation of voids in the gap portion.

[0104] <Comparison with a method for increasing the mode coupling constant by reducing the refractive index difference between the core and the cladding> Another approach that can increase the mode coupling constant is to reduce the relative refractive index difference of the first core 103 A. When comparing cases where the core width, thickness, and gap width of the two waveguides that make up the directional coupler are the same, the mode coupling constant increases by reducing the relative refractive index difference of the core 103 A.

[0105] However, reducing the relative refractive index difference may increase the chip size of the optical circuit. For example, to reduce production costs, planar lightwave circuits are designed to have the smallest chip size possible that achieves the desired functionality. One of the factors that determines the chip size is the bending radius of the bending waveguide. The bending radius is generally set to the smallest possible value within the range of allowable radiation loss. While the approach of reducing the relative refractive index difference improves the yield of element circuits such as directional couplers, it may increase the bending radius and chip size.

[0106] In contrast, the configuration using the second core 103B of this embodiment can reduce the impact on the bending radius compared to the approach of reducing the relative refractive index difference. Figure 14(a) schematically shows the waveguide structure and relative refractive index difference (Δ) of this embodiment having the second core 103B. Figure 14(b) schematically shows the waveguide structure and relative refractive index difference (Δ) of a comparison sample not having the second core 103B. Figure 14(c) schematically shows the waveguide structure and relative refractive index difference (Δ) with a reduced relative refractive index difference.

[0107] The width, thickness, and relative refractive index difference of the core 103A in the waveguide structures illustrated in Figures 14(a) and 14(b) are as described above. In the waveguide structure illustrated in Figure 14(c), the relative refractive index difference of the core 103a with respect to the cladding layer 102 is reduced to 0.7%. This relative refractive index difference is the same as the relative refractive index difference of 0.7% between the first core 103A and the second core 103B in the waveguide structure of this embodiment illustrated in Figure 14(a).

[0108] Next, Fig. 15 shows an example of the relationship between the transmittance per 90° bending angle of the bent waveguide and the bending radius in the waveguide structures shown in Fig. 14(a) to Fig. 14(c). In Fig. 15, each plot shows the measured bending loss, and the dashed line shows the calculated value. The wavelength of the light input to the bent waveguide is 440 nm.

[0109] For example, in Fig. 15, square plots and dashed line 1501 respectively represent the measured and calculated values ​​for the waveguide structure of this embodiment illustrated in Fig. 14(a). Circle plots and dashed line 1502 respectively represent the measured and calculated values ​​for the waveguide structure of the comparative sample illustrated in Fig. 14(b). Diamond plots and dashed line 1503 respectively represent the measured and calculated values ​​for the waveguide structure with a reduced relative refractive index difference illustrated in Fig. 14(c).

[0110] If the minimum bending radius is defined as the bending radius at which the transmittance per 90° bending angle is 0.9 or more, it can be seen from Fig. 15 that the minimum bending radius of the comparative sample is about 450 µm, and the minimum bending radius of the waveguide structure of this embodiment is about 500 µm. On the other hand, the minimum bending radius of the waveguide structure employing the approach of reducing the relative refractive index difference to 0.7% is 1150 µm.

[0111] As these results show, the waveguide structure of this embodiment can adopt a minimum bending radius that is almost the same as that of the comparative sample, and can also reduce the minimum bending radius significantly more than a waveguide structure that adopts an approach that reduces the relative refractive index difference.

[0112] Furthermore, a waveguide structure that employs an approach to reduce the relative refractive index difference is essentially equivalent to a structure in which the thickness of the second core 103B is made much thicker than the thickness of the first core 103A in the waveguide structure of this embodiment. As a result, the bending radius is larger than that of the waveguide structure of this embodiment. This corresponds to the aforementioned secondary effect that "chip size tends to increase."

[0113] <Possibility of Improving Yield of Other Optical Circuits> In the above-described embodiment, it has been described that the waveguide structure of this embodiment having the second core 103B contributes to improving yield when the first multiplexing section 50 is a directional coupler. However, this embodiment is not limited to directional couplers, and may also be effective in other optical circuits including the second multiplexing section 70.

[0114] For example, even when an optical circuit such as a multimode interferometer (MMI), a mode coupler, a Y-branch circuit, or a Mach-Zehnder interferometer is applied to the multiplexing unit 50 or 70, the effect of stabilizing the optical circuit characteristics can be expected by increasing the mode coupling constant between the two waveguides.

[0115] For example, in an MMI, multiple input / output waveguides connected to a multimode waveguide portion form a directional coupler, so application of this embodiment is expected to improve yield. The same applies to a Y-branch circuit. A Mach-Zehnder interferometer is generally configured by combining two or more of an MMI, a directional coupler, and a Y-branch circuit, so application of this embodiment is also effective.

[0116] <Suppression of Deterioration of Optical Circuit Characteristics Caused by Fluctuations in the Effective Refractive Index Due to Visible Light Input> In the above-described embodiment, the improvement of yield of component circuits such as directional couplers has been described. Hereinafter, it will be described how the waveguide structure according to this embodiment can also be effective in suppressing deterioration of optical circuit characteristics caused by fluctuations in the effective refractive index of the waveguide due to visible light input. Visible light is an example of light in a wavelength range shorter than the wavelengths generally handled in existing PLCs. Handling light in a short wavelength range can require stricter product conditions or specifications than, for example, existing PLCs.

[0117] <Stabilization of Optical Circuit Characteristics for Blue Light Input> FIGS. 16( a) and 16(b) show an example of transmission spectra before and after inputting continuous wave (CW) blue light with a wavelength of 450 nm and an optical power of 60 mW for 10,000 hours.

[0118] FIG. 16( a) shows the transmission spectrum of a directional coupler configured with the waveguide structure of this embodiment having the second core 103B, where reference numeral 1601 indicates the transmission spectrum of the through port and reference numeral 1602 indicates the transmission spectrum of the cross port.

[0119] FIG. 16B shows the transmission spectrum of a directional coupler configured with the waveguide structure of the comparative sample, where reference numeral 1603 indicates the transmission spectrum of the through port and reference numeral 1604 indicates the transmission spectrum of the cross port.

[0120] 16(a) and 16(b), the solid line indicates the transmission spectrum before the blue light is input, and the dashed line indicates the transmission spectrum after the blue light is input. 16(a) and 16(b) show that the shape of the transmission spectrum of the directional coupler of the comparison sample changes significantly before and after the input of blue light, whereas the transmission spectrum of the directional coupler configured with the waveguide structure of this embodiment is stable.

[0121] <Changes in transmission spectrum> The change in the shape of the transmission spectrum observed for the comparison sample directional coupler is thought to be caused by a change in the effective refractive index of the waveguide due to the input of blue light. For example, as described in Non-Patent Documents 2 to 4, it is known that the refractive index changes when ultraviolet or visible light is input to or irradiated onto a quartz-based material.

[0122] Here, one of the reasons why the waveguide structure of this embodiment having the second core 103B suppresses such changes in refractive index is that the second core 103B suppresses changes in the refractive index of the upper cladding layer 102B.

[0123] It is known that Zr-doped SiO2 or non-doped SiO2 has a smaller change in refractive index when ultraviolet or visible light is input than Ge-doped SiO2, which is used as the core material of existing quartz-based PLCs (e.g., Non-Patent Documents 3 and 4).

[0124] The waveguide structure having the second core 103B of this embodiment is a structure in which the first core 103A made of Zr-doped SiO 2 is covered with the second core 103B also made of Zr-doped SiO 2 .

[0125] In such a waveguide structure, the light leaking from the first core 103A is covered by the second core 103B, and the intensity of blue light that can reach the upper cladding layer 102B (e.g., BPSG) is greatly attenuated.

[0126] Since the change in the effective refractive index caused by the input of blue light is caused by the change in the refractive index of the core material and the change in the refractive index of the clad material, the presence of the second core 103B reduces the intensity of the blue light that can reach the upper clad layer 102B, which is thought to suppress the change in the refractive index of the upper clad layer 102B and therefore the change in the optical circuit characteristics.

[0127] <Regarding the Material of the Second Core> In the embodiment described above, an example has been shown in which the second core 103B is made of Zr-doped SiO. However, this embodiment is not limited to this, and the second core 103B may be made of, for example, non-doped SiO or SiO containing the following bond species. By using these SiO as the material for the second core 103B, it is expected that the change in the effective refractive index when visible light is input can be further suppressed.

[0128] In this case, in order to make the refractive index of the second core 103B higher than that of the cladding layer 102, for example, the ratio of the Si component in the composition of SiO may be increased. In the sputtering method, the ratio of the Si component in the SiO film formed can be increased by reducing the ratio of oxygen contained in the atmosphere during film formation. The ratio of the Si component may be, for example, Si / O = 1 / 2 or more.

[0129] Additionally or alternatively, the refractive index of SiO2 can be increased by diffusing carbon (C) or hydroxyl groups (OH groups) into SiO2. For example, in plasma CVD, the concentration of Si-C bonds and Si-OH bonds in the film can be increased by lowering the wafer temperature during film formation.

[0130] Additionally or alternatively, Si—F bonds may be introduced into SiO by, for example, introducing a fluorine-based gas into the film-forming raw material. Since fluorine (F) has the effect of lowering the refractive index, adding fluorine (F) to SiO together with other additives makes it possible to adjust the refractive index of the second core 103B.

[0131] <Effects of the embodiment> According to the present embodiment, it is possible to suppress fluctuations in optical circuit characteristics due to variations during fabrication, and therefore it is possible to provide an optical circuit that can achieve a high yield. Furthermore, compared to existing PLCs that use Ge-doped SiO2 for the core, it is possible to obtain a PLC that can suppress deterioration of optical circuit characteristics due to fluctuations in the effective refractive index of the waveguide when visible light is input.

[0132] In the above-described embodiment, an example was described in which the input light to the optical multiplexing circuit 1 was visible light, but it is also possible to suppress fluctuations in the optical circuit characteristics when light in a wavelength range different from the wavelength range of visible light is used as the input light.

[0133] <Additional Terminology> When the term "connect" or "couple" is used in this disclosure, it may be understood to mean any direct or indirect "connection" or "coupling" between two or more elements. For example, it may be understood that the term also includes an indirect "connection" or "coupling" where one or more intermediate elements are interposed between two elements that are "connected" or "coupled" to each other.

[0134] Any reference to an element followed by a designation such as "first...," "second...," etc. does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

[0135] In the present disclosure, the terms "...circuit" and "...section" may be interchangeable, or may be appropriately interchangeable with other terms such as "structure," "...means," "...device," and "...module."

[0136] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0137] The present disclosure is useful for technologies relating to optical devices that handle visible light, such as smart glasses or projectors.

[0138] REFERENCE SIGNS LIST 1 Optical multiplexing circuit 10, 20, 30 Input waveguide 10a, 20a, 30a Input end 50, 70 Multiplexing section 90 Output waveguide 90a Output end 101 Substrate 102A Lower cladding layer 102B Upper cladding layer 103A First core 103B Second core 600 Wafer

Claims

1. The first cladding layer, A first core provided on the first cladding layer, A second core provided on the first cladding layer so as to cover at least the first core, A second cladding layer is provided to cover the second core, A waveguide structure equipped with [a specific feature].

2. The waveguide structure according to claim 1, wherein the refractive index of the second core is higher than the refractive index of the first cladding layer or the second cladding layer.

3. The waveguide structure according to claim 1, wherein the refractive index of the second core is lower than the refractive index of the first core.

4. The waveguide structure according to claim 1, wherein the thickness of the second core is smaller than the thickness of the first core.

5. The waveguide structure according to claim 1, wherein one or both of the first core and the second core are silicon dioxide, and the ratio of silicon to oxygen in the composition of the silicon dioxide is greater than 1 / 2.

6. The waveguide structure according to claim 1, wherein one or both of the first core and the second core are silicon dioxide to which hydrogen has been added and which contains hydroxyl groups in the glass network.

7. The waveguide structure according to claim 1, wherein the wavelength of light input to the waveguide structure belongs to one of the different wavelength ranges that constitute the three primary colors of light.

8. Multiple waveguides, each guiding at least one of several wavelengths of light, A coupling section is provided in which optical power is at least partially transferred between any two of the plurality of waveguides, A planar optical circuit in which one or more of the plurality of waveguides and the coupling portion have the waveguide structure described in any one of claims 1 to 7.