Optical coupling element

Abstract

The present invention provides an optical coupling element that improves the optical coupling efficiency between an optical fiber and an optical waveguide on an integrated optical circuit. [Solution] The optical coupling element 100 comprises a substrate 140, a pair of first optical waveguides 110 arranged in a first direction D1 on the substrate 140, a second optical waveguide 120 arranged in the first direction D1 between the pair of first optical waveguides 110, and a cladding 130 covering the pair of first optical waveguides 110 and the second optical waveguide 120 on the substrate 140, wherein the second optical waveguide 120 is arranged to be optically coupled with the first optical waveguide 110, and each of the pair of first optical waveguides 110 has a first core 111 arranged to be optically coupled with the second optical waveguide 120, and a second core 112 which overlaps the first core 111 at a distance in a second direction D2 intersecting the first direction D1, and contributes to the light confinement structure when optically coupled with the first core 111.

Description

Technical Field

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

Background Art

[0002] Conventionally, in device development in the optical communication field and the like, technologies related to a spot size converter for realizing high-efficiency (low-loss) optical coupling between an optical fiber and an optical waveguide on an optical integrated circuit are known. For example, Patent Document 1 discloses an optical coupler having a three-way structure including a pair of first optical waveguides each having a discrete sub-wavelength periodic structure and a second optical waveguide having an inverse taper structure disposed between the pair of first optical waveguides.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the prior art described in Patent Document 1, there is room for improvement in the optical coupling efficiency between an optical fiber and an optical waveguide on an optical integrated circuit.

[0005] An object of the present disclosure is to provide an optical coupling element that improves the optical coupling efficiency between an optical fiber and an optical waveguide on an optical integrated circuit.

Means for Solving the Problems

[0006] Means for achieving the above object are as follows.

[0007] (1) A substrate, A pair of first optical waveguides arranged in a first direction on the substrate, A second optical waveguide is positioned in the first direction between the pair of first optical waveguides, On the substrate, a cladding covering the pair of first optical waveguides and the second optical waveguide, Equipped with, The second optical waveguide is arranged to be optically coupleable with the first optical waveguide. Each of the pair of first optical waveguides includes a first core that is optically coupled with the second optical waveguide, and a second core that overlaps the first core, spaced apart in a second direction intersecting the first direction, and contributes to the light confinement structure when optically coupled with the first core. Optical coupling element.

[0008] (2) The optical coupling element described in (1) above, The second core has a weaker light confinement effect than the first core, and a stronger light confinement effect than the cladding. Optical coupling element.

[0009] (3) The optical coupling element described in (2) above, The refractive index of the second core is smaller than the refractive index of the first core and larger than the refractive index of the cladding. Optical coupling element.

[0010] (4) An optical coupling element according to any one of (1) to (3) above, The second core has a continuous linear structure along the first direction, Optical coupling element.

[0011] (5) An optical coupling element according to any one of (1) to (4) above, In a front view of a cross section perpendicular to the first direction, the width of the second core in the third direction intersecting the first and second directions is greater than or equal to the width of the first core in the third direction, and less than the distance between a pair of first cores in the third direction. Optical coupling element.

[0012] (6) An optical coupling element according to any one of (1) to (5) above, In a front view of a cross-section perpendicular to the first direction, the height of the second core in the second direction is greater than 0.9 times the height of the first core in the second direction and less than 3 times the height of the first core in the second direction. Optical coupling element.

[0013] (7) The optical coupling element according to any one of (1) to (6) above, In a front view of a cross-section perpendicular to the first direction, the interval between the first core and the second core in the second direction is greater than the height of the first core in the second direction and less than 5 times the height of the first core in the second direction. Optical coupling element.

[0014] (8) The optical coupling element according to any one of (1) to (7) above, The first core has a discrete sub-wavelength periodic structure along the first direction. Optical coupling element.

[0015] (9) The optical coupling element according to any one of (1) to (8) above, The pair of first optical waveguides have the same structure as each other. Optical coupling element.

[0016] (10) The optical coupling element according to any one of (1) to (9) above, The second optical waveguide is arranged at the same height as the pair of first cores in the second direction. Optical coupling element. [Advantages of the Invention]

[0017] According to the present disclosure, it is possible to provide an optical coupling element capable of improving the optical coupling efficiency between an optical fiber and an optical waveguide on an optical integrated circuit. [Brief Description of the Drawings]

[0018] [Figure 1]This is a schematic diagram showing an example of the configuration of an optical system including an optical coupling element according to one embodiment of the present disclosure. [Figure 2] This is a schematic diagram showing the optical coupling element according to this embodiment in a top view. [Figure 3] Figure 2 is a schematic diagram showing a cross-section of the optical coupling element in section C. [Figure 4] This is a schematic diagram showing a side view of the optical coupling element according to this embodiment. [Figure 5] This graph shows the optical coupling efficiency of the optical coupling element according to the embodiment. [Figure 6] This is a diagram showing the electric field distribution of light for an optical coupling element according to the examples and comparative examples. [Modes for carrying out the invention]

[0019] In the following, one embodiment of this disclosure will be mainly described with reference to the attached drawings.

[0020] Figure 1 is a schematic diagram showing an example of the configuration of an optical system 1 including an optical coupling element 100 according to one embodiment of the present disclosure (hereinafter sometimes referred to as "this embodiment"). In addition to the optical coupling element 100, the optical system 1 includes an optical fiber 200 and an optical integrated circuit 300. Referring to Figure 1, the configuration and function of the optical system 1, in which light emitted from the optical fiber 200 is incident on the optical integrated circuit 300 via the optical coupling element 100, will be mainly described.

[0021] In the optical system 1 according to this embodiment, the optical fiber 200, the optical coupling element 100, and the optical integrated circuit 300 are arranged coaxially with respect to each other along the optical axis Ax. The optical fiber 200, the optical coupling element 100, and the optical integrated circuit 300 are arranged sequentially along the direction of light propagation. In the optical system 1, the spot size of light at the exit surface of the optical fiber 200 is larger than the spot size of light at the incident surface of the optical waveguide 310 on the optical integrated circuit 300.

[0022] For example, the optical fiber 200 is a single-mode fiber. The spot size of light at the exit surface of the optical fiber 200 is approximately 10 μm in diameter. For example, the optical waveguide 310 on the optical integrated circuit 300 is a silicon nanowire waveguide. The spot size of light at the incident surface of the optical waveguide 310 on the optical integrated circuit 300 is, for example, approximately several hundred nm square.

[0023] Thus, if an optical fiber 200 and an optical waveguide 310 on an optical integrated circuit 300, which have significantly different light spot sizes, are directly connected without an optical coupling element 100, the optical coupling efficiency becomes very poor due to connection losses caused by mode mismatch. The optical coupling element 100 is placed between the optical fiber 200 and the optical integrated circuit 300 and improves the optical coupling efficiency by converting the light spot size between the optical fiber 200 and the optical waveguide 310 on the optical integrated circuit 300. In this disclosure, "optical coupling efficiency" means, for example, the proportion of light that propagates without loss from the time it leaves the optical fiber 200 until it enters the optical waveguide 310 on the optical integrated circuit 300.

[0024] Spot size conversion in optical system 1 is performed by optimizing the structure of the optical coupling element 100, for example, at least one of the shape and size of each component of the optical coupling element 100. Examples of structures used to optimize the optical coupling element 100 include a combination of a triple-pronged structure and a subwavelength structure. A "triple-pronged structure" refers to a structure that symmetrically includes a pair of first optical waveguides 110 and a second optical waveguide 120 positioned between the pair of first optical waveguides 110, as will be described later using Figure 2, for example. A "subwavelength structure" refers to a diffraction grating structure having a periodic structure of several hundred nanometers, for example.

[0025] In this disclosure, "optically coupled" means that light is transferred from one optical waveguide to another between two optical waveguides located at different spatial positions. "Optical confinement effect" means that light undergoes total internal reflection and propagates within a certain region. The stronger the optical confinement effect, the smaller the light propagation loss; the weaker the optical confinement effect, the larger the light propagation loss.

[0026] Figure 2 is a schematic diagram showing the optical coupling element 100 according to this embodiment in a top view. Figure 3 is a schematic diagram showing a cross-section of section C of the optical coupling element 100 in Figure 2. Figure 4 is a schematic diagram showing the optical coupling element 100 according to this embodiment in a side view. In Figure 2, for the purpose of simplifying the illustration, the substrate 140 and cladding 130, which will be described later, are omitted from the illustration.

[0027] The optical coupling element 100 includes a substrate 140, a pair of first optical waveguides 110 arranged on the substrate 140 in a first direction D1, and a second optical waveguide 120 arranged between the pair of first optical waveguides 110 in the first direction D1. The optical coupling element 100 has a cladding 130 on the substrate 140 that covers the pair of first optical waveguides 110 and the second optical waveguide 120. In this disclosure, "first direction D1" refers, for example, to the direction in which light propagates.

[0028] In the optical coupling element 100, a first optical waveguide 110 and a second optical waveguide 120 are arranged on a substrate 140. A cladding 130 is arranged to cover the periphery of the first optical waveguide 110 and the second optical waveguide 120. The pair of first optical waveguides 110 are symmetrical with respect to the second optical waveguide 120.

[0029] Referring to Figure 2, the pair of first optical waveguides 110 are arranged almost entirely along the first direction D1 in the optical coupling element 100, from the optical fiber 200 side to the optical integrated circuit 300 side. The pair of first optical waveguides 110 have identical structures. The pair of first optical waveguides 110 receive light incident from the optical fiber 200 into the optical coupling element 100 and propagate it internally along the first direction D1. The pair of first optical waveguides 110 optically couple the light that has propagated internally to the second optical waveguide 120. The light propagates from left to right in Figure 2.

[0030] Referring to Figures 3 and 4, each of the pair of first optical waveguides 110 has a first core 111 which is arranged to be optically coupled to a second optical waveguide 120. The first core 111 is optically coupled to the second optical waveguide 120. Each of the pair of first optical waveguides 110 has a second core 112 which overlaps the first core 111 at a distance in a second direction D2 intersecting a first direction D1, and which contributes to the light confinement structure when optically coupled to the first core 111. In this disclosure, "second direction D2" refers, for example, to the direction in which the second core 112 overlaps the first core 111 in a cross section perpendicular to the first direction D1 and in a cross section along the first direction D1.

[0031] As shown in Figure 2, the first core 111 has a discrete subwavelength periodic structure along the first direction D1. For example, the period Λ of the periodic structure of the first core 111 along the first direction D1 may be 0.2 μm to 1 μm, preferably 0.3 μm to 0.8 μm, and more preferably 0.4 μm to 0.6 μm. The width w1 of the first core 111 in the third direction D3 may be 0.01 μm to 1 μm, preferably 0.05 μm to 0.5 μm, and more preferably 0.1 μm to 0.4 μm. In this disclosure, "third direction D3" refers to, for example, a direction intersecting the first direction D1 and the second direction D2. For example, the first direction D1, the second direction D2, and the third direction D3 are orthogonal to each other.

[0032] Similarly, the width w2 of the first core 111 in the first direction D1 may be 0.1 μm to 0.9 μm, preferably 0.15 μm to 0.6 μm, and more preferably 0.2 μm to 0.5 μm. The height of the first core 111 in the second direction D2 may be 0.05 μm to 1 μm, preferably 0.1 μm to 0.5 μm, and more preferably 0.2 μm to 0.4 μm. Examples of materials for the first core 111 include Si.

[0033] As shown in Figure 4, the cross-sectional shape of the first core 111 in a cross-section along the first direction D1 is, for example, rectangular. Similarly, as shown in Figure 3, the cross-sectional shape of the first core 111 in a cross-section perpendicular to the first direction D1 is, for example, rectangular. The pair of first cores 111 are arranged at the same height as the second optical waveguide 120 in the second direction D2. The pair of first cores 111 are arranged symmetrically with respect to the second optical waveguide 120 along the third direction D3.

[0034] As shown in Figure 2, the second core 112 has a continuous linear structure along the first direction D1. For example, the total length of the second core 112 along the first direction D1 is preferably 50 μm to 200 μm. Examples of materials for the second core 112 include SiN. As shown in Figure 4, the cross-sectional shape of the second core 112 in a cross-section along the first direction D1 is, for example, rectangular.

[0035] Similarly, as shown in Figure 3, the cross-sectional shape of the second core 112 in a section perpendicular to the first direction D1 is, for example, rectangular. The pair of second cores 112 are positioned in the second direction D2 on the side opposite to the substrate 140 from the first core 111 and the second optical waveguide 120. The pair of second cores 112 are positioned symmetrically with respect to the second optical waveguide 120 along the third direction D3. The center of the third direction D3 of each of the pair of second cores 112 coincides, for example, with the center of the third direction D3 of the first core 111 which is located on the substrate 140 side relative to the second core 112.

[0036] The second core 112 has a weaker light confinement effect than the first core 111, but a stronger light confinement effect than the cladding 130. When the materials of the first core 111, the second core 112, and the cladding 130 are different, the refractive index of the second core 112 is smaller than that of the first core 111, and larger than that of the cladding 130.

[0037] For example, in a front view of a cross section perpendicular to the first direction D1, the width w of the second core 112 in the third direction D3 is at least 1 / 3 of the width w1 of the first core 111 in the third direction D3, and smaller than the spacing d between a pair of first cores 111 in the third direction D3. More preferably, in a front view of a cross section perpendicular to the first direction D1, the width w of the second core 112 in the third direction D3 is at least the width w1 of the first core 111 in the third direction D3, and smaller than the spacing d between a pair of first cores 111 in the third direction D3.

[0038] In this disclosure, the "d spacing d between the third direction D3 of a pair of first cores 111" is the distance from the center of the third direction D3 of one first core 111 to the center of the third direction D3 of the other first core 111, as shown in Figure 2, for example. For example, the d spacing d between the third direction D3 of a pair of first cores 111 is preferably 0.5 μm to 2 μm.

[0039] For example, in a front view of a cross section perpendicular to the first direction D1, the height h of the second core 112 in the second direction D2 is greater than 0.9 times the height of the first core 111 in the second direction D2 and less than 3 times the height of the first core 111 in the second direction D2. In a front view of a cross section perpendicular to the first direction D1, the distance z between the first core 111 and the second core 112 in the second direction D2 is greater than the height of the first core 111 in the second direction D2 and less than 5 times the height of the first core 111 in the second direction D2.

[0040] By setting the dimensions of width w, height h, and spacing z as described above, the light incident on the optical coupling element 100 is not confined solely to the second core 112, and can ultimately be coupled to the second optical waveguide 120. The optical coupling efficiency of the optical coupling element 100 is adjusted by adjusting the parameters of the first core 111, the second core 112, the second optical waveguide 120, and the cladding 130.

[0041] As shown in Figure 2, the second optical waveguide 120 has a continuous linear structure along the first direction D1. Examples of materials for the second optical waveguide 120 include Si. The cross-sectional shape of the second optical waveguide 120 along the first direction D1 is, for example, rectangular.

[0042] Similarly, as shown in Figure 3, the cross-sectional shape of the second optical waveguide 120 in a cross-section perpendicular to the first direction D1 is, for example, rectangular. The second optical waveguide 120 is positioned at the same height as the pair of first cores 111 in the second direction D2. In the third direction D3, the second optical waveguide 120 is positioned at the center between the pair of first optical waveguides 110. The second optical waveguide 120 is positioned to be optically coupled with the first optical waveguides 110. The second optical waveguide 120 propagates light coupled from the first optical waveguides 110. The second optical waveguide 120 couples the propagated light to the optical waveguide 310 of the optical integrated circuit 300.

[0043] As shown in Figure 2, the second optical waveguide 120 may have an inverse taper structure on the side where light enters the optical coupling element 100 from the optical fiber 200. In the optical coupling element 100, the section where only the pair of first optical waveguides 110 are located is called "Section A", the section where the second optical waveguide 120 has an inverse taper structure is called "Section B", and the section where the width of the third direction D3 of the second optical waveguide 120 is constant at w3 and the pair of first optical waveguides 110 are present is called "Section C".

[0044] For example, the width of the first direction D1 in section A is preferably 5 μm to 50 μm. The width of the first direction D1 in section B is preferably 10 μm to 50 μm. The width of the first direction D1 in section C is preferably 5 μm to 50 μm. The width w3 of the third direction D3 of the second optical waveguide 120 may be 0.1 μm to 0.8 μm, preferably 0.2 μm to 0.6 μm, and more preferably 0.4 μm to 0.5 μm. The width w4 of the third direction D3 at the tapered tip of the second optical waveguide 120 may be 0.01 μm to 0.8 μm, preferably 0.05 μm to 0.5 μm, and more preferably 0.1 μm to 0.3 μm.

[0045] The second optical waveguide 120 has an inverse taper structure in section B, which causes the average refractive index in a cross section perpendicular to the first direction D1 to continuously change along the first direction D1. For example, the average refractive index of the second optical waveguide 120 gradually increases from the incident side to the exit side of the light in the inverse taper structure. Therefore, the optical coupling element 100 avoids the discontinuous structure in the second optical waveguide 120 and reduces reflection due to abrupt changes in refractive index. This reduces the optical coupling loss in the optical coupling element 100.

[0046] As shown in Figures 3 and 4, the cladding 130 is placed on the substrate 140 and covers the pair of first optical waveguides 110 and second optical waveguides 120. Examples of materials for the cladding 130 include SiO2.

[0047] The overall function of the optical coupling element 100 according to this embodiment is to convert the spot size of incident light, which has a spot size of several micrometers or more and is incident on the optical coupling element 100 from the optical fiber 200, so that the spot size at the time of output is several hundred nanometers. Optical waveguides with different refractive indices have different light confinement effects from each other.

[0048] In the first core 111, which has a discrete subwavelength periodic structure along the first direction D1, one unit structure per period and other unit structures adjacent to that unit structure are optically coupled to each other by evanescent light. Evanescent light refers to the wave generated near the interface of the photomedium when light undergoes total internal reflection at the boundary between a material with a high refractive index and a material with a low refractive index. In optical coupling by evanescent light, light propagates without the optical waveguides directly touching each other. Most of the incident light entering the optical coupling element 100 from the optical fiber 200 propagates inside the first core 111.

[0049] The first core 111 and the second core 112 are optically coupled to each other by evanescent light. A portion of the incident light entering the optical coupling element 100 from the optical fiber 200 propagates inside the second core 112. For example, a portion of the incident light may be coupled to the second core 112 from the moment it enters the optical coupling element 100 from the optical fiber 200, or it may not propagate inside the first core 111 but leak out of the first core 111 to the side opposite the substrate 140 and then be coupled to the second core 112. A portion of the incident light may be coupled to the first core 111 as it propagates inside the second core 112 along the first direction D1 and then return to the first core 111 again.

[0050] The first core 111 and the second optical waveguide 120 are optically coupled to each other by evanescent light. Therefore, incident light propagates through the first core 111 along the first direction D1, and then finally couples with the second optical waveguide 120, where it propagates. The second optical waveguide 120 outputs the incident light that has propagated through it to the optical waveguide 310 of the optical integrated circuit 300.

[0051] As described above, the second core 112 of the first optical waveguide 110 is solely for enhancing the light confinement effect when coupling the incident light from the optical fiber 200 in the optical coupling element 100. It is preferable that the electric field of the incident light from the optical fiber 200 is mainly distributed near the first core 111 or the second optical waveguide 120. [Examples]

[0052] The optical coupling element 100 according to this embodiment will be described in more detail below with reference to the following examples, but this disclosure is not limited in any way to the following examples. The numerical values ​​described in the examples are merely examples and do not limit the scope of this disclosure. The scope of this disclosure should be determined based on the description in the claims. In the following, components similar to those in the embodiments will be denoted by the same reference numerals, and redundant explanations will be omitted.

[0053] As the optical coupling element 100 according to the embodiment, an optical coupling element 100 having the same configuration as the optical coupling element 100 of this embodiment described with reference to Figures 2 to 4 was created in simulation.

[0054] In the simulation, the spot size of light at the incident surface of the optical coupling element 100 was approximately 2.0 μm. The width w1 of the third direction D3 of the first core 111 in Figure 2 was 0.3 μm. The width w2 of the first direction D1 of the first core 111 was 0.2 μm. The height of the second direction D2 of the first core 111 was 0.22 μm. The period Λ along the first direction D1 of the first core 111 was 0.4 μm. The distance d between the pair of first cores 111 in the third direction D3 was 1.4 μm.

[0055] In section A, where only a pair of first optical waveguides 110 are located, the width of the first direction D1 was 30 μm. In section B, where the second optical waveguide 120 has an inverse taper structure, the width of the first direction D1 was 40 μm. In section C, where the width of the third direction D3 of the second optical waveguide 120 is constant at w3 and a pair of first optical waveguides 110 are present, the width of the first direction D1 was 20 μm. The width w3 of the third direction D3 of the second optical waveguide 120 was 0.48 μm. The width w4 of the third direction D3 at the tapered tip of the second optical waveguide 120 was 0.1 μm.

[0056] In the optical coupling element 100 according to the embodiment, each first core 111 of the pair of first optical waveguides 110 was made of Si. Each second core 112 of the pair of first optical waveguides 110 was made of SiN. The second optical waveguide 120 was made of Si. The cladding 130 was made of SiO2. The refractive index of Si was 3.5, the refractive index of SiN was 2.0, and the refractive index of SiO2 was 1.4.

[0057] Figure 5 is a graph showing the optical coupling efficiency of the optical coupling element 100 according to the embodiment. For the optical coupling element 100 according to the embodiment, the dimensional conditions including the width w, height h, and spacing z of each second core 112 of the pair of first optical waveguides 110 were changed, and the optical coupling efficiency was calculated by simulation. For example, as shown in Table 1, the width w was set to one of 0.1 μm, 0.3 μm, 0.5 μm, 0.7 μm, and 1.0 μm. The spacing z was set to one of 0.6 μm, 0.8 μm, and 1.0 μm. The height h was set to one of 0.2 μm, 0.4 μm, and 0.6 μm.

[0058] [Table 1]

[0059] The graph in Figure 5 shows the results of simulations calculating the photocoupling efficiency for each combination of width w, height h, and spacing z, presented as bar graphs. Each bar graph corresponds to one condition. Below, we will use the condition numbers 1, 2, 3, 4, and 5 shown in Table 1 to indicate the combination conditions as wzh. For example, the numbers 3-2-1 mean that the width w is 0.5 μm (condition 3), the spacing z is 0.8 μm (condition 2), and the height h is 0.2 μm (condition 1).

[0060] In the graph shown in Figure 5, the values ​​of the photocoupling efficiency of the bar graphs, which are arranged along the horizontal axis for each condition, are shown on the vertical axis. This graph shows five bar graphs as a set, where only the width w is changed from condition 1 to condition 5, while the height h and spacing z are kept constant.

[0061] The five bar graphs in Group 1 C1 correspond to conditions 1-2-2, 2-2-2, 3-2-2, 4-2-2, and 5-2-2, respectively, from left to right. The five bar graphs in Group 2 C2 correspond to conditions 1-2-3, 2-2-3, 3-2-3, 4-2-3, and 5-2-3, respectively, from left to right. The five bar graphs in Group 3 C3 correspond to conditions 1-3-1, 2-3-1, 3-3-1, 4-3-1, and 5-3-1, respectively, from left to right.

[0062] The five bar graphs in Group 4, C4, correspond to conditions 1-3-2, 2-3-2, 3-3-2, 4-3-2, and 5-3-2, respectively, from left to right. The five bar graphs in Group 5, C5, correspond to conditions 1-4-1, 2-4-1, 3-4-1, 4-4-1, and 5-4-1, respectively, from left to right. The five bar graphs in Group 6, C6, correspond to conditions 1-4-2, 2-4-2, 3-4-2, 4-4-2, and 5-4-2, respectively, from left to right.

[0063] For all bar graphs from Group 1 C1 to Group 6 C6, the optical coupling efficiency of the optical coupling element according to the comparative example is shown by a dotted line. The optical coupling element according to the comparative example differs from the optical coupling element 100 according to the example in that it does not have a second core 112. The optical coupling element according to the comparative example has a structure similar to the conventional three-pronged structure, without a second core 112 that contributes to the light confinement structure. The optical coupling element according to the comparative example has the same structure as the optical coupling element 100 according to the example, except for the absence of a second core 112.

[0064] When using the optical coupling element according to the comparative example, the optical coupling efficiency was 0.761. On the other hand, the optical coupling efficiencies calculated by simulation under the conditions corresponding to all bar graphs from the first group C1 to the sixth group C6 shown in Figure 5 all exceeded the value of 0.761 in the comparative example. Therefore, it was shown that using the optical coupling element 100 according to the embodiment improves the optical coupling efficiency between the optical fiber 200 and the optical waveguide 310 on the optical integrated circuit 300.

[0065] Figure 6 shows the electric field distribution diagrams of light for optical coupling elements according to the examples and comparative examples. The upper part of Figure 6 shows the electric field distribution diagram of light for optical coupling element 100 according to the example. The upper left part of Figure 6 shows the electric field distribution diagram when the first optical waveguide 110 is viewed in a front view of a cross section perpendicular to the first direction D1 at position P1 in Figure 2, which corresponds to the incident surface of the first optical waveguide 110. The upper center part of Figure 6 shows the electric field distribution diagram when the first optical waveguide 110 is viewed in a front view of a cross section perpendicular to the first direction D1 at position P2, which is the boundary between section A and section B in Figure 2. The upper right part of Figure 6 shows the electric field distribution diagram when the first optical waveguide 110 and the second optical waveguide 120 are viewed in a front view of a cross section perpendicular to the first direction D1 at position P3, for example, which is the boundary between section B and section C in Figure 2.

[0066] The lower part of Figure 6 shows the electric field distribution diagrams for optical coupling elements related to comparative examples. The lower left diagram in Figure 6 corresponds to the electric field distribution diagram in the upper left part of Figure 6. The lower center diagram in Figure 6 corresponds to the electric field distribution diagram in the upper center part of Figure 6. The lower right diagram in Figure 6 corresponds to the electric field distribution diagram in the upper right part of Figure 6. In each diagram in Figure 6, the vertical axis z(m) indicates the position in the second direction D2. The horizontal axis x(m) indicates the position in the third direction D3. As an example, the origin of z(m) is set to the center of the first core 111 of the first optical waveguide 110 in the second direction D2.

[0067] Comparing the left-hand diagrams in Figure 6, it can be seen that in the comparative example, light is coupled only to the first optical waveguide having a sub-wavelength periodic structure at the incident surface, while in the embodiment, light is also slightly coupled to the second core 112 at the incident surface of the optical coupling element 100. Comparing the shapes of the upper part of the electric field distribution in the left-hand diagrams of Figure 6, it can be seen that, due to the coupling of light to the second core 112, the shape of the optical coupling element 100 in the upper part of Figure 6 is more spread along the third direction D3 than that of the comparative example.

[0068] Comparing the central figures in Figure 6, it can be seen that in the comparative example, light is coupled only to the first optical waveguide having a sub-wavelength periodic structure, while in the embodiment, light is slightly coupled to the second core 112 as well. In the upper center figure of Figure 6, there is an electric field distribution of light coupled to the second core 112 above the first core 111. On the other hand, in the corresponding part of the lower center figure of Figure 6, there is no electric field distribution. Thus, in the embodiment, the second core 112 does not completely mode-couple the light, but it receives light leaking upward from the first core 111 and reduces propagation loss in the first optical waveguide 110.

[0069] Comparing the right-hand diagrams in Figure 6, it can be seen that, similar to the optical coupling element in the comparative example, the optical coupling element 100 in the embodiment also couples light only to the first optical waveguide 110, which has a sub-wavelength periodic structure. As the light propagates along the first direction D1, it couples to the second optical waveguide 120.

[0070] (effect) According to the optical coupling element 100 of the above embodiment, the optical coupling efficiency between the optical fiber 200 and the optical waveguide 310 on the optical integrated circuit 300 can be improved. For example, by using the optical coupling element 100, a higher optical coupling efficiency can be achieved in the optical coupling between the optical fiber 200 and the optical waveguide 310 compared to using a conventional optical coupling element that does not have a second core 112.

[0071] For example, in the optical coupling element 100, each of the pair of first optical waveguides 110 has a first core 111 that is arranged to be optically coupled with the second optical waveguide 120, and a second core 112 that overlaps with the first core 111 at a distance in a second direction D2 that intersects the first direction D1, and contributes to the light confinement structure when optically coupled with the first core 111.

[0072] As a result, the optical coupler 100 can enhance the light confinement effect compared to conventional optical couplers that do not have a second core 112. For example, the optical coupler 100 can receive light leaking from the first core 111 with the second core 112 and propagate it through the second core 112 without loss to the outside of the optical coupler 100. The optical coupler 100 may also recouple light from the second core 112 to the first core 111, and finally couple it to the second optical waveguide 120. Since the optical coupler 100 has a higher equivalent refractive index compared to conventional optical couplers, the overall light confinement efficiency can be increased. In other words, the coupling efficiency of the optical coupler 100 is increased.

[0073] In the optical coupling element 100, the second core 112 has a weaker light confinement effect than the first core 111, but a stronger light confinement effect than the cladding 130. As a result, light is strongly confined in the first core 111, and confined in the second core 112 with a weaker confinement efficiency compared to the first core 111. The optical coupling element 100 uses the first core 111 as the main component for optical mode coupling, while also using the second core 112 to increase the overall light confinement efficiency of the first optical waveguide 110. Therefore, the optical coupling element 100 improves the optical coupling efficiency.

[0074] In the optical coupling element 100, the refractive index of the second core 112 is smaller than that of the first core 111 and larger than that of the cladding 130. This allows the optical coupling element 100 to adjust the light confinement effect on the first core 111 and the second core 112 by adjusting the refractive index of the first core 111, the second core 112, and the cladding 130 using different materials. Therefore, the optical coupling element 100 can adjust the overall light confinement efficiency and thus the optical coupling efficiency by adjusting the refractive index of the materials used for the first core 111, the second core 112, and the cladding 130.

[0075] In the optical coupling element 100, the second core 112 has a continuous linear structure along the first direction D1. This simplifies the design parameters of the second core 112, making the design easier. It also facilitates the formation of the second core 112 during the manufacturing process of the optical coupling element 100.

[0076] In a front view of a cross-section perpendicular to the first direction D1, the width w of the third direction D3 of the second core 112 intersecting the first direction D1 and the second direction D2 is greater than or equal to the width w1 of the third direction D3 of the first core 111, and is smaller than the distance d between the third directions D3 of a pair of first cores 111. This facilitates the realization of a light confinement structure based on the second core 112 optically coupled with the first core 111.

[0077] In a front view of a cross-section perpendicular to the first direction D1, the height h of the second core 112 in the second direction D2 is greater than 0.9 times the height of the first core 111 in the second direction D2 and less than 3 times the height of the first core 111 in the second direction D2. This facilitates the realization of a light confinement structure based on the second core 112 optically coupled with the first core 111.

[0078] In a front view of a cross-section perpendicular to the first direction D1, the distance z between the first core 111 and the second core 112 in the second direction D2 is greater than the height of the first core 111 in the second direction D2 and less than five times the height of the first core 111 in the second direction D2. This allows for the easy realization of a light confinement structure based on the second core 112 which is optically coupled to the first core 111.

[0079] In the optical coupling element 100, the first core 111 has a discrete sub-wavelength periodic structure along the first direction D1. As a result, the first core 111 is divided by sub-wavelength periods, and the refractive index of one wavelength of light in the first core 111 can be reduced compared to the case where it has a continuous linear structure. Therefore, the optical coupling element 100 can couple the light incident from the optical fiber 200 to the first core 111 with higher efficiency.

[0080] In the optical coupling element 100, the pair of first optical waveguides 110 have the same structure. This simplifies the design of the pair of first optical waveguides 110. As a result, the design of the optical coupling element 100 becomes easier.

[0081] In the optical coupling element 100, the second optical waveguide 120 is positioned at the same height as the pair of first cores 111 in the second direction D2. This simplifies the design of the pair of first optical waveguides 110 and second optical waveguides 120. As a result, the design of the optical coupling element 100 is simplified.

[0082] (modified version) It will be apparent to those skilled in the art that this disclosure can be implemented in other predetermined forms besides the embodiments described above without deviating from its spirit or essential features. Therefore, the prior description is illustrative and not limiting. The scope of the disclosure is defined not by the prior description but by the added claims. Any modifications within their equivalent scope are included therein.

[0083] For example, the shape, pattern, size, arrangement, orientation, type, and number of each component described above are not limited to those shown in the above description and drawings. The shape, pattern, size, arrangement, orientation, type, and number of each component may be configured arbitrarily as long as their function can be realized. Each component of the illustrated optical system 1 and optical coupling element 100 is a functional concept. The specific form of each component is not limited to those shown. For example, if it is possible to improve the optical coupling efficiency, the following modifications of the optical coupling element 100 can be considered.

[0084] In the above embodiment, the second core 112 was described as having a weaker light confinement effect than the first core 111 and a stronger light confinement effect than the cladding 130, but this is not limited to this. The second core 112 may have a stronger light confinement effect than the first core 111. The second core 112 may have a weaker light confinement effect than the cladding 130. The second core 112 may have the same light confinement effect as the first core 111 or the cladding 130.

[0085] In the above embodiment, the refractive index of the second core 112 was described as being smaller than the refractive index of the first core 111 and larger than the refractive index of the cladding 130, but this is not limited to this. The refractive index of the second core 112 may be larger than the refractive index of the first core 111. The refractive index of the second core 112 may be smaller than the refractive index of the cladding 130. The refractive index of the second core 112 may be the same as the refractive index of the first core 111 or the refractive index of the cladding 130.

[0086] In the above embodiment, the second core 112 was described as having a continuous linear structure along the first direction D1, but is not limited thereto. The second core 112 may, for example, have a continuous curved structure. The second core 112 is not limited to a continuous structure and may have a discrete subwavelength periodic structure, similar to the first core 111.

[0087] In the above embodiment, in a front view of a cross section perpendicular to the first direction D1, the width w of the second core 112 in the third direction D3 intersecting the first direction D1 and the second direction D2 is described as being greater than or equal to the width w1 of the third direction D3 of the first core 111 and smaller than the distance d between the third direction D3 of a pair of first cores 111, but this is not limited to this. The width w of the second core 112 in the third direction D3 may be smaller than the width w1 of the third direction D3 of the first core 111. The width w of the second core 112 in the third direction D3 may be greater than or equal to the distance d between the third direction D3 of a pair of first cores 111.

[0088] In the above embodiment, in a front view of a cross section perpendicular to the first direction D1, the height h of the second core 112 in the second direction D2 is described as being greater than 0.9 times the height of the first core 111 in the second direction D2 and less than 3 times the height of the first core 111 in the second direction D2, but this is not limited to this. The height h of the second core 112 in the second direction D2 may be 0.9 times or less the height of the first core 111 in the second direction D2. The height h of the second core 112 in the second direction D2 may be 3 times or more the height of the first core 111 in the second direction D2.

[0089] In the above embodiment, in a front view of a cross section perpendicular to the first direction D1, the distance z between the first core 111 and the second core 112 in the second direction D2 is described as being greater than the height of the first core 111 in the second direction D2 and less than five times the height of the first core 111 in the second direction D2, but this is not limited to this. The distance z between the first core 111 and the second core 112 in the second direction D2 may be less than or equal to the height of the first core 111 in the second direction D2. The distance z between the first core 111 and the second core 112 in the second direction D2 may be five times or more the height of the first core 111 in the second direction D2.

[0090] In the above embodiment, the first core 111 was described as having a discrete subwavelength periodic structure along the first direction D1, but is not limited thereto. The first core 111 may have a continuous linear structure along the first direction D1, for example, similar to the second core 112. In this case, the first core 111 may have an inverse taper structure on the side into which light enters from the optical fiber 200, similar to the second optical waveguide 120.

[0091] In the above embodiment, the pair of first optical waveguides 110 were described as having the same structure, but this is not limited to this. The pair of first optical waveguides 110 may have different structures.

[0092] In the above embodiment, the second optical waveguide 120 is described as being at the same height as the pair of first cores 111 in the second direction D2, but this is not limited to this. The second optical waveguide 120 may be at a different height from the pair of first cores 111 in the second direction D2.

[0093] In the above embodiment, it was explained that the center of the third direction D3 of each of the pair of second cores 112 coincides with the center of the third direction D3 of the first core 111 which is located on the substrate 140 side relative to the second core 112, but this is not limited to this. The center of the third direction D3 of each of the pair of second cores 112 does not have to coincide with the center of the third direction D3 of the first core 111.

[0094] In the above embodiment, the material of the first core 111 was described as, for example, Si, but is not limited to this. The material of the first core 111 may be other materials other than Si. For example, the material of the first core 111 may be InGaAs, InGaAsP, InAlAs, GaAs, Ge, SiN, or a compound semiconductor of InP and group III-V.

[0095] In the above embodiment, the material of the second core 112 was described as, for example, SiN, but is not limited to this. The material of the second core 112 may be other materials other than SiN. For example, the material of the second core 112 may be InGaAs, InGaAsP, InAlAs, GaAs, Si, Ge, or a compound semiconductor of InP and group III-V.

[0096] In the above embodiment, the material of the second optical waveguide 120 was described as, for example, Si, but is not limited to this. The material of the second optical waveguide 120 may be other materials other than Si. For example, the material of the second optical waveguide 120 may be InGaAs, InGaAsP, InAlAs, GaAs, Ge, SiN, or a compound semiconductor of InP and group III-V.

[0097] In the above embodiment, the material of the cladding 130 was described as, for example, SiO2, but it is not limited to this. The material of the cladding 130 may be other materials other than SiO2. For example, the material of the cladding 130 may be an insulating material such as epoxy resin.

[0098] In the above embodiment, the cross-sectional shape of the first core 111 in a cross-section along the first direction D1 was described as rectangular, but is not limited thereto. The cross-sectional shape of the first core 111 in a cross-section along the first direction D1 may be other shapes such as other polygons and circles. The cross-sectional shape of the first core 111 in a cross-section perpendicular to the first direction D1 was described as rectangular, but is not limited thereto. The cross-sectional shape of the first core 111 in a cross-section perpendicular to the first direction D1 may be other shapes such as other polygons and circles. For example, the first core 111 may be rib-shaped.

[0099] In the above embodiment, the cross-sectional shape of the second core 112 in a cross-section along the first direction D1 was described as rectangular, but is not limited thereto. The cross-sectional shape of the second core 112 in a cross-section along the first direction D1 may be other shapes such as other polygons and circles. The cross-sectional shape of the second core 112 in a cross-section perpendicular to the first direction D1 was described as rectangular, but is not limited thereto. The cross-sectional shape of the second core 112 in a cross-section perpendicular to the first direction D1 may be other shapes such as other polygons and circles.

[0100] In the above embodiment, the cross-sectional shape of the second optical waveguide 120 in a cross-section along the first direction D1 was described as rectangular, but is not limited to this. The cross-sectional shape of the second optical waveguide 120 in a cross-section along the first direction D1 may be other shapes such as other polygons and circles. The cross-sectional shape of the second optical waveguide 120 in a cross-section perpendicular to the first direction D1 was described as rectangular, but is not limited to this. The cross-sectional shape of the second optical waveguide 120 in a cross-section perpendicular to the first direction D1 may be other shapes such as other polygons and circles. For example, the second optical waveguide 120 may be rib-shaped.

[0101] In the above embodiment, the optical coupling element 100 was described as being located between the optical fiber 200 and the optical integrated circuit 300, but it is not limited to this. The optical coupling element 100 may be integrally configured with the optical waveguide 310. That is, the second optical waveguide 120 of the optical coupling element 100 may be a structure at the end of the optical waveguide 310.

[0102] In the above embodiment, the optical fiber 200, the optical coupling element 100, and the optical integrated circuit 300 were described as being arranged in order along the direction of light propagation, but this is not limited to this. Instead of light propagating from the optical fiber 200 to the optical integrated circuit 300 via the optical coupling element 100, light may propagate from the optical integrated circuit 300 to the optical fiber 200 via the optical coupling element 100. Accordingly, although it was described that light propagates from left to right in Figure 2, this is not limited to this. Light may propagate from right to left in Figure 2. [Explanation of Symbols]

[0103] 1 Optical System 100 Optical coupling elements 110 First optical waveguide 111 First Core 112 Second Core 120 Second optical waveguide 130 Clad 140 circuit boards 200 optical fibers 300 Optical Integrated Circuits 310 Optical waveguide Ax optical axis Section A Section B Section C C1 1st group C2 2nd group C3 3rd group C4 4th group C5 Group 5 C6 Group 6 D1 1st direction D2 2nd direction D3 Third direction d interval h height P1 position P2 position P3 position w width w1 width w2 width w3 width w4 width z interval Λ period

Claims

1. circuit board and A pair of first optical waveguides arranged in a first direction on the substrate, A second optical waveguide is positioned in the first direction between the pair of first optical waveguides, On the substrate, a cladding covering the pair of first optical waveguides and the second optical waveguide, Equipped with, The second optical waveguide is arranged to be optically coupleable with the first optical waveguide. Each of the pair of first optical waveguides includes a first core that is optically coupled with the second optical waveguide, and a second core that overlaps the first core, spaced apart in a second direction intersecting the first direction, and contributes to the light confinement structure when optically coupled with the first core. Optical coupling element.

2. The optical coupling element according to claim 1, The second core has a weaker light confinement effect than the first core, and a stronger light confinement effect than the cladding. Optical coupling element.

3. The optical coupling element according to claim 2, The refractive index of the second core is smaller than the refractive index of the first core and larger than the refractive index of the cladding. Optical coupling element.

4. An optical coupling element according to any one of claims 1 to 3, The second core has a continuous linear structure along the first direction, Optical coupling element.

5. An optical coupling element according to any one of claims 1 to 3, In a front view of a cross section perpendicular to the first direction, the width of the second core in the third direction intersecting the first and second directions is greater than or equal to the width of the first core in the third direction, and less than the distance between the pair of first cores in the third direction. Optical coupling element.

6. An optical coupling element according to any one of claims 1 to 3, In a front view of a cross section perpendicular to the first direction, the height of the second core in the second direction is greater than 0.9 times the height of the first core in the second direction and less than 3 times the height of the first core in the second direction. Optical coupling element.

7. An optical coupling element according to any one of claims 1 to 3, In a front view of a cross-section perpendicular to the first direction, the distance between the first core and the second core in the second direction is greater than the height of the first core in the second direction and less than five times the height of the first core in the second direction. Optical coupling element.

8. An optical coupling element according to any one of claims 1 to 3, The first core has a periodic structure of discrete subwavelengths along the first direction, Optical coupling element.

9. An optical coupling element according to any one of claims 1 to 3, The pair of first optical waveguides have the same structure as each other. Optical coupling element.

10. An optical coupling element according to any one of claims 1 to 3, The second optical waveguide is positioned at the same height as the pair of first cores in the second direction. Optical coupling element.

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