Optical waveguide element

Abstract

The present invention provides an optical waveguide element capable of performing mode conversion between a strip waveguide section and a slot waveguide section, which have different layers. [Solution] An optical waveguide element according to one embodiment comprises a substrate, a strip waveguide portion formed in a first layer located above the substrate, a slot waveguide portion formed in a second layer located above the substrate and different from the first layer, and a mode conversion portion connected between the strip waveguide portion and the slot waveguide portion. The mode conversion portion includes a first optical confinement portion formed in the first layer and connected to the strip waveguide portion, and a pair of second optical confinement portions formed in the second layer and connected to the slot waveguide portion. The width of the first optical confinement portion decreases as it approaches the slot waveguide portion from the strip waveguide portion. The pair of second optical confinement portions include the first optical confinement portion on the inside in a plan view of the substrate.

Description

Technical Field

[0001] The present disclosure relates to an optical waveguide device.

Background Art

[0002] Patent Document 1 describes a waveguide coupler configured to optically couple a strip waveguide to a first slot photonic crystal waveguide. The waveguide coupler is disposed between the first slot photonic crystal waveguide and the strip waveguide. The waveguide coupler includes a tapered region having a second slot photonic crystal waveguide aligned with the first slot photonic crystal waveguide and the strip waveguide.

[0003] Non-Patent Document 1 describes a structure in which a tapered section is provided between a slot waveguide and a strip waveguide. The strip waveguide has a strip portion with a high refractive index as a core, and the light confinement becomes stronger as the width of the strip portion increases. In the slot waveguide, a low refractive index portion sandwiched between two strip portions with a high refractive index is used as the core. When the distance between the two strip portions increases, the light confinement as a slot waveguide becomes weaker. In the tapered section, the light is converted between the two waveguides by gradually narrowing the width of the strip waveguide to weaken the light confinement and gradually narrowing the distance between the slot waveguides to strengthen the light confinement in the slot waveguide.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] Incidentally, optical waveguide elements sometimes consist of multiple materials that are different from each other. For example, if multiple layers can be stacked on a semiconductor substrate by a semiconductor process, manufacturing can be made more efficient by forming multiple different materials on different layers. Therefore, optical waveguide elements are required to be able to perform mode conversion of light propagating between strip waveguide sections and slot waveguide sections, each formed on different layers.

[0007] The present disclosure aims to provide an optical waveguide element that can efficiently perform mode conversion of light propagating between strip waveguide sections and slot waveguide sections on a semiconductor substrate, where the layers are different from each other. [Means for solving the problem]

[0008] The optical waveguide element according to this disclosure comprises a substrate, a strip waveguide portion formed in a first layer located above the substrate, a slot waveguide portion formed in a second layer located above the substrate and different from the first layer, and a mode conversion portion connected between the strip waveguide portion and the slot waveguide portion. The mode conversion portion includes a first optical confinement portion formed in the first layer and connected to the strip waveguide portion, and a pair of second optical confinement portions formed in the second layer and connected to the slot waveguide portion. The width of the first optical confinement portion decreases as it approaches the slot waveguide portion from the strip waveguide portion. The pair of second optical confinement portions include the first optical confinement portion on the inside in a plan view of the substrate. [Effects of the Invention]

[0009] According to this disclosure, mode conversion of light propagating between strip waveguides and slot waveguides on a semiconductor substrate, where the layers are different from each other, can be performed efficiently. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a plan view showing an optical waveguide element according to an embodiment. [Figure 2] Figure 2 is a cross-sectional view along line BB in Figure 1. [Figure 3] Figure 3 schematically shows the beam shape in the cross-sections along lines AA, BB, CC, and DD in Figure 1. [Figure 4] Figure 4 is a graph showing an example of the relationship between the length of the mode conversion section of an optical waveguide element and its conversion efficiency. [Modes for carrying out the invention]

[0011] [Description of Embodiments in this Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) An optical waveguide element according to one embodiment comprises a substrate, a strip waveguide portion formed in a first layer located above the substrate, a slot waveguide portion formed in a second layer located above the substrate and different from the first layer, and a mode conversion portion connected between the strip waveguide portion and the slot waveguide portion. The mode conversion portion includes a first optical confinement portion formed in the first layer and connected to the strip waveguide portion, and a pair of second optical confinement portions formed in the second layer and connected to the slot waveguide portion. The width of the first optical confinement portion decreases as it approaches the slot waveguide portion from the strip waveguide portion. The pair of second optical confinement portions include the first optical confinement portion on the inside in a plan view of the substrate.

[0012] In this optical waveguide element, a strip waveguide section and a slot waveguide section are formed above the substrate, and a mode conversion section is located between the strip waveguide section and the slot waveguide section. The mode conversion section includes a first optical confinement section and a second optical confinement section. The first optical confinement section is connected to the strip waveguide section, and the second optical confinement section is connected to the slot waveguide section. The width of the first optical confinement section decreases as it approaches the slot waveguide section from the strip waveguide section, and the second optical confinement section includes the first optical confinement section inward in a plan view of the substrate. Therefore, the width of the first optical confinement section decreases as it approaches the slot waveguide section, and the distance between the second optical confinement section and the first optical confinement section in a plan view of the substrate decreases as it approaches the slot waveguide section. Consequently, optical conversion can be performed smoothly in the mode conversion section located between the strip waveguide section and the slot waveguide section.

[0013] (2) In (1) above, the strip waveguide, the mode conversion section, and the slot waveguide may be arranged in this order along the X-axis direction, and one of the pair of second optical confinement sections, the first optical confinement section, and the other of the pair of second optical confinement sections may be arranged in this order along the Y-axis direction intersecting the X-axis direction. Each of the pair of second optical confinement sections may be formed to be symmetric to each other with respect to a reference line that passes through the center of the first optical confinement section in the Y-axis direction and extends along the X-axis direction. In this case, the rotation of the mode, i.e., polarization, can be suppressed by arranging the two second optical confinement sections in positions symmetric to each other with respect to the reference line.

[0014] (3) In (2) above, the difference between the distance between a pair of second optical confinement sections aligned along the Y-axis and the width of the first optical confinement section may decrease monotonically as the strip waveguide section approaches the slot waveguide section. In this case, the optical conversion in the mode conversion section can be performed more smoothly.

[0015] (4) In any one of (1) to (3) above, the width of each of the pair of second optical confinement portions may monotonically decrease from the slot waveguide portion toward the strip waveguide portion. In this case, since the width of the second optical confinement portion in the strip waveguide portion is smaller than the width of the second optical confinement portion in the slot waveguide portion, reflection of light from the strip waveguide portion toward the slot waveguide portion can be suppressed.

[0016] (5) In any one of (1) to (4) above, in a cross-section orthogonal to the X-axis direction, which is the direction in which the strip waveguide portion, the mode conversion portion, and the slot waveguide portion are arranged, the first layer may be formed away from above the second layer.

[0017] (6) In any one of (1) to (5) above, the slot waveguide portion may have the same optical propagation mode as the optical propagation mode of the strip waveguide portion.

[0018] (7) In any one of (1) to (6) above, the optical waveguide element includes a cladding formed on a substrate, and the first layer and the second layer may be provided in the cladding.

[0019] [Details of Embodiments of the Present Disclosure] Hereinafter, a specific example of an optical waveguide element according to an embodiment will be described with reference to the drawings. The present disclosure is not limited to these examples, and it is intended to include all modifications within the scope shown in the claims and equivalent scopes. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and overlapping descriptions are omitted as appropriate. The drawings may be drawn with some parts simplified or exaggerated for ease of understanding, and dimensional ratios and the like are not limited to those described in the drawings.

[0020] FIG. 1 is a diagram showing an optical waveguide element 1 according to the present embodiment. FIG. 2 is a cross-sectional view taken along line B-B of FIG. 1. As shown in FIGS. 1 and 2, the optical waveguide element 1 includes a substrate 2, a cladding 3 formed on the substrate 2, and an optical waveguide portion 4 embedded in the cladding 3. Hereinafter, the direction from the substrate 2 to the cladding 3 may be referred to as upward, upper, or above, and the direction from the cladding 3 to the substrate 2 may be referred to as downward, lower, or below. However, these are directions for convenience of explanation and do not limit the arrangement position or direction of the components.

[0021] The optical waveguide portion 4 is also referred to as a core. The substrate 2 is, for example, a semiconductor substrate. The semiconductor substrate is, for example, composed of silicon (Si). In FIG. 1, for clarity of illustration, the optical waveguide portion 4 is shown by a solid line. For example, the cladding 3 has a thickness in the Z-axis direction that intersects both the X-axis direction and the Y-axis direction. Hereinafter, the X-axis direction is also referred to as the length direction, the Y-axis direction is also referred to as the width direction, and the Z-axis direction is also referred to as the height direction. As an example, the cladding 3 has a rectangular parallelepiped shape extending in the X-axis direction, the Y-axis direction, and the Z-axis direction. The cladding 3 includes, for example, a first layer 3b, a second layer 3c located between the first layer 3b and the substrate 2, and an intermediate layer 3d located between the first layer 3b and the second layer 3c. The cladding 3 further has an intermediate layer 3d located between the first layer 3b and the second layer 3c. The first layer 3b and the second layer 3c are provided in the cladding 3. Each of the first layer 3b and the second layer 3c extends in the X-axis direction and the Y-axis direction and has a thickness in the Z-axis direction. The first layer 3b is provided, for example, at the center of the cladding 3 in the Z-axis direction, and the second layer 3c is provided below the first layer 3b. For example, the cladding 3 is composed of silicon dioxide (SiO2). The optical waveguide portion 4 is surrounded by the cladding 3.

[0022] The optical waveguide section 4 functions as the core of the optical waveguide element 1. Due to the difference between the refractive index of the core and the refractive index of the cladding, for example, light is confined within the core, and this light propagates in the direction in which the core extends. The optical waveguide section 4 comprises a strip waveguide section 4b formed in a first layer 3b located above the substrate 2, a slot waveguide section 4c formed in a second layer 3c located above the substrate 2 and different from the first layer 3b, and a mode conversion section 4d located between the strip waveguide section 4b and the slot waveguide section 4c. When the substrate 2 is viewed in plan along the Z-axis direction, the strip waveguide section 4b, the mode conversion section 4d, and the slot waveguide section 4c are arranged in this order along the X-axis direction.

[0023] The slot waveguide section 4c is configured such that the light propagating through the slot waveguide section 4c has the same mode as the light propagating through the strip waveguide section 4b. For example, in the optical waveguide element 1, light is transmitted from the strip waveguide section 4b to the slot waveguide section 4c via the mode conversion section 4d. However, conversely, light may be transmitted from the slot waveguide section 4c to the strip waveguide section 4b via the mode conversion section 4d. For example, the strip waveguide section 4b transmits light in TE (Transverse Electric wave) mode, and the slot waveguide section 4c transmits light in TE mode. Alternatively, the strip waveguide section 4b may transmit light in TM (Transverse Magnetic wave) mode, and the slot waveguide section 4c may transmit light in TM mode. With the direction horizontal to the upper surface of the substrate 2 (X-axis and Y-axis directions) as the horizontal direction and the direction perpendicular to the upper surface of the substrate 2 (Z-axis direction) as the vertical direction, in TE mode the electric field of the propagating light oscillates horizontally, and in TM mode the electric field of the propagating light oscillates vertically. The polarization direction of light is horizontal in TE mode and vertical in TM mode.

[0024] The optical waveguide section 4 includes a SiN waveguide 5 and a Si waveguide 6, depending on the difference in the materials that constitute them. For example, light in TE0 mode input from the SiN waveguide 5 is transferred to the TE0 mode on the Si waveguide 6 side. The SiN waveguide 5 is formed to propagate light in TE mode, and the Si waveguide 6 is formed to propagate light in TE mode. Alternatively, the SiN waveguide 5 may be formed to propagate light in TM mode, and the Si waveguide 6 may be formed to propagate light in TM mode. The SiN waveguide 5 is made of silicon nitride (Si3N4), and the Si waveguide 6 is made of silicon (Si). For example, in a plan view of the substrate 2 (viewed along the Z-axis), the SiN waveguide 5 has a rectangular first portion 5b and a trapezoidal second portion 5c. The optical waveguide section 4 has a pair of Si waveguides 6. Each of the pair of Si waveguides 6 has a rectangular third portion 6b and a trapezoidal fourth portion 6c. Each of the pair of Si waveguides 6 is aligned with the Y-axis.

[0025] The strip waveguide section 4b includes the first portion 5b of the SiN waveguide 5, and the slot waveguide section 4c includes the third portion 6b of the Si waveguide 6. The mode conversion section 4d includes the second portion 5c of the SiN waveguide 5 and the fourth portion 6c of the Si waveguide 6. The mode conversion section 4d is a tapered region in which the width (length in the Y-axis direction) of the optical waveguide section 4 decreases as it moves toward the X-axis direction. The length LX in the X-axis direction of the mode conversion section 4d is, for example, 20 μm or more and 500 μm or less. The length LX in the X-axis direction of the mode conversion section 4d may also be, for example, 50 μm or more and 400 μm or less. The length LX in the X-axis direction of the mode conversion section 4d may also be, for example, 200 μm or more and 300 μm or less.

[0026] The mode conversion unit 4d includes a first optical confinement unit 4f connected to the strip waveguide unit 4b and a pair of second optical confinement units 4h connected to the slot waveguide unit 4c. The first optical confinement unit 4f is formed in the first layer 3b, and the second optical confinement units 4h are formed in the second layer 3c. The first optical confinement unit 4f is formed integrally (continuously) with the strip waveguide unit 4b. The second optical confinement units 4h are formed continuously with the third portion 6b of the Si waveguide 6. The width of the first optical confinement unit 4f decreases monotonically as it approaches the slot waveguide unit 4c from the strip waveguide unit 4b. In a plan view of the substrate 2, the pair of second optical confinement units 4h contain the first optical confinement unit 4f internally. For example, the mode conversion unit 4d has a pair of second optical confinement units 4h, with one first optical confinement unit 4f formed between the pair of second optical confinement units 4h. For example, the positions (heights) of the pair of second light confinement sections 4h in the Z-axis direction are identical. In a plan view of the substrate 2, one second light confinement section 4h, the first light confinement section 4f, and the other second light confinement section 4h are arranged in this order along the Y-axis direction.

[0027] For example, the first optical confinement section 4f is the second portion 5c of the SiN waveguide 5, and the second optical confinement section 4h is the fourth portion 6c of the Si waveguide 6. In a plan view of the substrate 2, the SiN waveguide 5 is positioned between the pair of Si waveguides 6. In a plan view of the substrate 2, the SiN waveguide 5 and the two Si waveguides 6 are formed in positions that do not overlap each other. In a plan view of the substrate 2, each of the pair of Si waveguides 6 is formed in a position that is symmetrical to each other with respect to a reference line L that passes through the center of the cladding 3 in the Y-axis direction and extends along the X-axis direction. That is, in a plan view of the substrate 2, a reference line L can be imagined as the axis of symmetry in which the shapes of the pair of Si waveguides 6 are symmetrical to each other. In this case, the reference line L does not have to pass through the center of the cladding 3 in the Y-axis direction. In a plan view of the substrate 2, the SiN waveguide 5 has a shape that is symmetrical with respect to the reference line L.

[0028] The pair of second optical confinement sections 4h are positioned symmetrically with respect to a reference line L. For example, the center line passing through the center of the first optical confinement section 4f in the Y-axis direction and extending along the X-axis direction (hereinafter referred to as the first center line), and the center lines passing through the centers of the pair of second optical confinement sections 4h in the Y-axis direction and extending along the X-axis direction (hereinafter referred to as the second center lines) coincide with each other. The second center line corresponds to the axis of symmetry related to the line symmetry described above. The first and second center lines may coincide with the reference line L. The difference between the distance A1 between the pair of second optical confinement sections 4h aligned along the Y-axis direction and the width A2 of the first optical confinement section 4f is constant along the X-axis direction. Furthermore, the difference between the distance A1 and the width A2 may decrease monotonically as one approaches the slot waveguide section 4c from the strip waveguide section 4b. The width A3 (length in the Y-axis direction) of each of the pair of second optical confinement sections 4h decreases monotonically as one approaches the strip waveguide section 4b from the strip waveguide section 4b. In the Z-axis direction, a cladding layer 3 is interposed between the first optical confinement section 4f and the pair of second optical confinement sections 4h. For example, along the Z-axis direction, an intermediate layer 3d is laminated on a second layer 3c containing the pair of second optical confinement sections 4h (Si waveguides 6), and a first layer 3b containing the first optical confinement section 4f (SiN waveguides 5) is laminated on the intermediate layer 3d. That is, the SiN waveguides 5 are formed in a layer higher than the layer of the Si waveguides 6.

[0029] For example, the first optical confinement section 4f (SiN waveguide 5) is positioned above the second optical confinement section 4h (Si waveguide 6) in a cross-section perpendicular to the X-axis direction. However, the first optical confinement section 4f may be positioned below the second optical confinement section 4h in a cross-section perpendicular to the X-axis direction. That is, the relative positions of the first optical confinement section 4f and the second optical confinement section 4h in a cross-section perpendicular to the X-axis direction may be reversed. Hereinafter, in a cross-section perpendicular to the X-axis direction, the direction from the pair of Si waveguides 6 to the SiN waveguide 5 may be referred to as "up," and the direction from the SiN waveguide 5 to the pair of Si waveguides 6 may be referred to as "down." In this case, the substrate 2 is located below the Si waveguides 6, and the pair of Si waveguides 6, SiN waveguides 5, and cladding 3 are formed on the substrate 2. However, these directions are for the convenience of explaining the relative positions and directions of each other and do not limit the absolute placement positions and directions that do not depend on the orientation of the substrate 2.

[0030] In the mode conversion section 4d, the width of the SiN waveguide 5 in a plan view of the substrate 2 (width A2 of the first optical confinement section 4f) decreases monotonically as it approaches the slot waveguide section 4c. As a result, the optical confinement of light propagating through the SiN waveguide 5 in the first optical confinement section 4f weakens as it approaches the slot waveguide section 4c, and the component spreading outside the interface between the core and cladding increases. Furthermore, in the mode conversion section 4d, the distance A1 between the pair of Si waveguides 6 in the Y-axis direction in a plan view of the substrate 2 decreases monotonically along with the width A2 of the first optical confinement section 4f. As a result, the optical confinement in the second optical confinement section 4h, which is adjacent to the first optical confinement section 4f, gradually strengthens as it approaches the slot waveguide section 4c, and ultimately all the light propagating through the SiN waveguide 5 transitions to the second optical confinement section 4h. This allows for a smooth conversion of light from the SiN waveguide 5 to the Si waveguide 6.

[0031] As described above, the width A3 of the second optical confinement section 4h decreases monotonically from the slot waveguide section 4c toward the strip waveguide section 4b. The width A4 of the second optical confinement section 4h at the point furthest from the slot waveguide section 4c is, for example, greater than 0 μm and less than or equal to 0.2 μm. The lower limit of width A4 is the minimum value that can be manufactured by the semiconductor process used to manufacture the optical waveguide element 1, and may be, for example, 0.05 μm. At the boundary between the strip waveguide section 4b and the mode conversion section 4d, the smaller the width A4, the more the reflection of light propagating through the first section 5b toward the negative side in the X-axis direction can be reduced.

[0032] The difference between distance A1 and width A2 at the negative end of the second optical confinement section 4h in the X-axis direction (i.e., at the X-axis boundary between the strip waveguide section 4b and the mode conversion section 4d) must be a value that allows for optical coupling so that the component spreading outward from the first optical confinement section 4f enters the second optical confinement section 4h, in order to cause the transition of light from the first optical confinement section 4f to the second optical confinement section 4h, as described above. The difference between distance A1 and width A2 is, for example, smaller than width A2 and smaller than width A3. The difference between distance A1 and width A2 is, for example, greater than 0 μm and 0.2 μm or less. Distance A1 is greater than width A2, and in a plan view of the substrate 2, the first optical confinement section 4f and the second optical confinement section 4h do not overlap with each other. The lower limit of the difference between distance A1 and width A2 is the minimum value that can be manufactured by the semiconductor process used to manufacture the optical waveguide element 1, and may be, for example, 0.05 μm. The width A3 increases monotonically from the strip waveguide section 4b toward the slot waveguide section 4c, and the width of the second optical confinement section 4h in the slot waveguide section 4c (for example, the maximum value of width A3) is, for example, 0.2 μm or more and 0.3 μm or less (0.24 μm as an example). The width A3 may also increase uniformly (at a constant rate) from the strip waveguide section 4b toward the slot waveguide section 4c. This constant rate can be determined, for example, by the amount of change in width A3 with respect to the length LX in the X-axis direction of the strip waveguide section 4b.

[0033] The gap A5 between the first optical confinement section 4f and the second optical confinement section 4h in the Z-axis direction is, for example, greater than 0 μm and less than or equal to 0.2 μm (for example, 0.1 μm). SiO2 is formed in the gap A5 as an intermediate layer 3d of the cladding 3. The lower limit of the gap A5 is the minimum value that can be manufactured by the semiconductor process used to manufacture the optical waveguide element 1, and may be, for example, 0.05 μm. The gap A5 may also be 1.0 μm or less. By setting the gap A5 to an appropriate value, the transition of light from the first optical confinement section 4f to the second optical confinement section 4h in the mode conversion section 4d can be appropriately performed. The shape of the first optical confinement section 4f in a cross-section perpendicular to the X-axis direction is, for example, rectangular.

[0034] For example, when cut along a plane perpendicular to the X-axis, the length (width) of the cross-section of the first light confinement portion 4f in the Y-axis direction is greater than the length (thickness) of the cross-section of the first light confinement portion 4f in the Z-axis direction. The shape of the second light confinement portion 4h in a cross-section perpendicular to the X-axis direction is, for example, rectangular. For example, when cut along a plane perpendicular to the X-axis direction, the length (width) of the second light confinement portion 4h in the Y-axis direction is greater than the length (thickness) of the second light confinement portion 4h in the Z-axis direction.

[0035] For example, the area of ​​the cross-section of the first light confinement portion 4f perpendicular to the X-axis direction is larger than the area of ​​each of the cross-sections of the pair of second light confinement portions 4h perpendicular to the X-axis direction. The above describes an example in which the shape of the first light confinement portion 4f and the shape of the second light confinement portion 4h perpendicular to the X-axis direction are rectangular. However, the shape of the first light confinement portion 4f and the shape of the second light confinement portion 4h perpendicular to the X-axis direction may also be trapezoidal. For example, it may be a trapezoid in which the length of the side closer to the substrate 2 is smaller than the length of the side further away from the substrate 2.

[0036] The width A2 (length in the Y-axis direction) of the first optical confinement section 4f decreases monotonically from the strip waveguide section 4b toward the slot waveguide section 4c. The width A2 may decrease uniformly (at a constant rate) from the strip waveguide section 4b toward the slot waveguide section 4c. This constant rate can be determined, for example, by the amount of change in width A2 with respect to the length LX in the X-axis direction of the strip waveguide section 4b. The width A6 of the first optical confinement section 4f at the point furthest from the strip waveguide section 4b (i.e., the boundary between the mode conversion section 4d and the slot waveguide section 4c) is, for example, greater than 0 μm and 0.2 μm or less. The lower limit of width A6 is the minimum value that can be manufactured by the semiconductor process used to manufacture the optical waveguide element 1, and may be, for example, 0.05 μm. The width (length in the Y-axis direction) of the first optical confinement section 4f in the strip waveguide section 4b is, for example, 0.4 μm or more and 1.25 μm or less (0.7 μm as an example).

[0037] The distance A1 between the pair of second optical confinement sections 4h decreases monotonically as you move from the strip waveguide section 4b towards the slot waveguide section 4c. Alternatively, the distance A1 may decrease uniformly (at a constant rate) as you move from the strip waveguide section 4b towards the slot waveguide section 4c. This constant rate can be determined, for example, by the change in distance A1 with respect to the length LX in the X-axis direction of the strip waveguide section 4b. The distance A1 between the pair of second optical confinement sections 4h in the slot waveguide section 4c is, for example, 0.15 μm or more and 0.4 μm or less (0.28 μm as an example). The distance A1 between the two second optical confinement sections 4h at the point furthest from the slot waveguide section 4c (the boundary between the strip waveguide section 4b and the mode conversion section 4d) is, for example, 0.5 μm or more and 1.35 μm or less (0.8 μm as an example).

[0038] As mentioned above, the first center line of the first light confinement portion 4f may coincide with the reference line L. However, the first center line of the first light confinement portion 4f may be offset by a certain length from the reference line L. For example, in a plan view of the substrate 2, the first center line of the first light confinement portion 4f may be offset by a length such that the first light confinement portion 4f does not overlap with either of the pair of second light confinement portions 4h. The length such that the first light confinement portion 4f does not overlap with either of the pair of second light confinement portions 4h is, for example, 30 nm.

[0039] Figure 3 schematically shows the beam shape of light propagating along the X-axis within the optical waveguide element 1 in cross-sections AA, BB, CC, and DD of Figure 1. The TE0 mode light input from the SiN waveguide 5 gradually transitions to the pair of Si waveguides 6 as it passes through cross-sections AA, BB, CC, and DD along the X-axis. As shown in Figures 1, 2, and 3, as the light moves from cross-section AA to cross-section BB, a portion of the light that has propagated through the SiN waveguide 5 approaches the SiN waveguide 5 and moves to the Si waveguide 6 located below the SiN waveguide 5.

[0040] When moving from the BB cross section to the CC cross section, the change in light is adiabatic. That is, in the mode conversion section 4d, light smoothly transitions from the SiN waveguide 5 to each of the pair of Si waveguides 6 located below the SiN waveguide 5 with almost no dissipation of optical power to the outside. When moving from the CC cross section to the DD cross section, almost no change in light is observed. The light that has propagated through the SiN waveguide 5 propagates entirely through the pair of Si waveguides 6 in the CC cross section. Thus, the transmission of light from the strip waveguide section 4b to the slot waveguide section 4c via the mode conversion section 4d can reduce the loss of light in the optical waveguide element 1 because the waveguide transition is adiabatic. For example, the larger the length LX in the X-axis direction of the tapered region (mode conversion section 4d) and the smaller the degree of decrease (a certain percentage) in the length of the optical waveguide section 4 in the Y-axis direction along the X-axis direction, the smaller the loss of optical power can be. However, the smaller the length in the X-axis direction of the tapered region, the smaller the optical waveguide element 1 can be made.

[0041] In the optical waveguide element 1 described above, a strip waveguide section 4b and a slot waveguide section 4c are formed on a substrate 2 in different layers, and a mode conversion section 4d is located between the strip waveguide section 4b and the slot waveguide section 4c. The mode conversion section 4d includes a first optical confinement section 4f formed on the same first layer 3b as the strip waveguide section 4b, and a second optical confinement section 4h formed on the same second layer 3c as the slot waveguide section 4c. The first optical confinement section 4f is connected to the strip waveguide section 4b, and the second optical confinement section 4h is connected to the slot waveguide section 4c. The width A2 of the first optical confinement section 4f decreases monotonically as it approaches the slot waveguide section 4c from the strip waveguide section 4b, and the second optical confinement section 4h includes the first optical confinement section 4f in a plan view of the substrate 2. In a plan view of the substrate 2, the first optical confinement section 4f and the second optical confinement section 4h do not overlap with each other. Therefore, the width A2 of the first optical confinement section 4f decreases monotonically as it approaches the slot waveguide section 4c, and the distance A1 between the second optical confinement sections 4h that sandwich the first optical confinement section 4f in a plan view of the substrate 2 also decreases monotonically as it approaches the slot waveguide section. Consequently, light can be transmitted smoothly in the mode conversion section 4d located between the strip waveguide section 4b and the slot waveguide section 4c.

[0042] As described above, the strip waveguide 4b, the mode conversion section 4d, and the slot waveguide 4c may be arranged in this order along the X-axis direction, and the second optical confinement section 4h, the first optical confinement section 4f, and the second optical confinement section 4h may be arranged in this order along the Y-axis direction intersecting the X-axis direction. The pair of second optical confinement sections 4h may be positioned symmetrically with respect to a reference line L that passes through the center of the first optical confinement section 4f in the Y-axis direction and extends along the X-axis direction. Furthermore, the shape of each of the pair of second optical confinement sections 4h may be formed to be symmetrical with respect to the reference line L. In this case, the rotation of the polarization of the light transmitted from the first optical confinement section 4f to the pair of second optical confinement sections 4h can be suppressed because the pair of second optical confinement sections 4h are positioned symmetrically with respect to the reference line L and their shapes are formed to be symmetrical with respect to each other.

[0043] As mentioned above, the difference between the distance A1 between a pair of second optical confinement sections 4h aligned along the Y-axis and the width A2 of the first optical confinement section 4f may decrease monotonically as the strip waveguide section 4b approaches the slot waveguide section 4c. In this case, the optical coupling between the first optical confinement section 4f and the second optical confinement section 4h gradually strengthens as the strip waveguide section 4b approaches the slot waveguide section 4c, thereby reducing reflection at the boundary between the first optical confinement section 4f and the strip waveguide section 4b, and allowing for a smoother transition of light from the first optical confinement section 4f to the second optical confinement section 4h in the mode conversion section 4d.

[0044] As mentioned above, the width A3 of the second optical confinement section 4h may decrease monotonically as it moves from the slot waveguide section 4c towards the strip waveguide section 4b. In this case, since the width A3 of the second optical confinement section 4h in the strip waveguide section 4b is smaller than the width A3 of the second optical confinement section 4h in the slot waveguide section 4c, the reflection of light moving from the strip waveguide section 4b towards the slot waveguide section 4c can be reduced.

[0045] As described above, in a cross-section perpendicular to the X-axis direction, which is the direction in which the strip waveguide 4b, mode conversion section 4d, and slot waveguide 4c are aligned, the first optical confinement section 4f may be located at a position (layer) above the second optical confinement section 4h. The slot waveguide section 4c may be configured to have the same modes as the strip waveguide section 4b. The optical waveguide element 1 includes a cladding 3 formed on the substrate 2, and the strip waveguide section 4b and slot waveguide section 4c may be embedded in the cladding 3. The slot waveguide section 4c may be formed in a second layer 3c different from the first layer 3b on which the strip waveguide section 4b is formed.

[0046] Figure 4 is a graph showing an example of the relationship between the length LX (horizontal axis) of the mode conversion unit 4d and the optical conversion efficiency R (vertical axis) of the light from the optical waveguide element 1, when the distance A1 between the pair of second optical confinement units 4h at the boundary between the strip waveguide unit 4b and the mode conversion unit 4d is varied. The conversion efficiency R is expressed by the formula R = Pout / Pin, where Pin is the power of the TE mode light input from the strip waveguide unit 4b to the mode conversion unit 4d, and Pout is the power of the TE mode light output from the mode conversion unit 4d to the slot waveguide unit 4c. In Figure 4, the conversion efficiency R is shown in decibels (dB), and the conversion efficiency R is higher as it approaches 0 dB, and lower as it moves away from 0 dB (becomes smaller). The decrease in conversion efficiency R is thought to be due to an increase in loss in the mode conversion unit 4d. That is, the higher the conversion efficiency R, the smaller the loss in mode conversion, and the lower the conversion efficiency R, the larger the loss in mode conversion. As shown in Figure 4, the conversion efficiency R increases as the length LX increases. Also, a smaller distance A1 at the boundary between the strip waveguide section 4b and the mode conversion section 4d results in a higher conversion efficiency R. However, when the distance A1 is 0.71 μm or more, the difference in conversion efficiency R due to the difference in distance A1 is, for example, 0.03 dB or less when the length LX is 50 to 100 μm, and can be reduced to an even smaller value when the length LX is larger. The conversion efficiency R is preferably -0.15 dB (input / output power ratio 0.97) or higher. The conversion efficiency R may also be -0.1 dB (input / output power ratio 0.98) or higher, and even more preferably -0.05 dB (input / output power ratio 0.99) or higher.

[0047] Embodiments of the optical waveguide element according to this disclosure have been described above. However, the present invention is not limited to the embodiments described above and may be modified within the scope of the gist described in the claims. That is, the configuration, shape, size, material, number, and arrangement of each part of the optical waveguide element can be appropriately changed within the scope of the gist described above. [Explanation of Symbols]

[0048] 1… Optical waveguide element 2… Circuit board 3…Clad 3b…1st layer 3c…Second layer 3d…Middle layer 4...Optical waveguide 4b...Strip waveguide 4c... Slot waveguide 4D...Mode conversion section 4f...First light confinement section 4h...Second light containment section 5...SiN waveguide 5b…First part 5c…Second part 6...Si waveguide 6b...Third part 6c...4th part L…Reference line

Claims

1. circuit board and A strip waveguide portion formed on the first layer located above the substrate, A slot waveguide portion is located above the substrate and formed in a second layer different from the first layer, A mode conversion unit connected between the strip waveguide and the slot waveguide, Equipped with, The mode conversion unit includes a first optical confinement unit formed in the first layer and connected to the strip waveguide unit, and a pair of second optical confinement units formed in the second layer and connected to the slot waveguide unit. The width of the first optical confinement section decreases as it approaches the slot waveguide section from the strip waveguide section. The pair of second light confinement portions include the first light confinement portion on the inside when viewed in plan of the substrate. Optical waveguide element.

2. The strip waveguide, the mode conversion unit, and the slot waveguide are arranged in this order along the X-axis direction. Along the Y-axis direction intersecting the X-axis direction, one of the pair of second light confinement sections, the first light confinement section, and the other of the pair of second light confinement sections are arranged in this order. With respect to a reference line that passes through the center of the first light confinement portion in the Y-axis direction and extends along the X-axis direction, each of the pair of second light confinement portions is formed to be symmetrical with respect to the other. The optical waveguide element according to claim 1.

3. The difference between the distance between the pair of second optical confinement sections aligned along the Y-axis and the width of the first optical confinement section decreases monotonically as you move from the strip waveguide towards the slot waveguide. The optical waveguide element according to claim 2.

4. The width of each of the pair of second optical confinement sections decreases monotonically from the slot waveguide section toward the strip waveguide section. The optical waveguide element according to any one of claims 1 to 3.

5. In a cross-section perpendicular to the X-axis direction, which is the direction in which the strip waveguide, the mode conversion section, and the slot waveguide are aligned, the first layer is formed above the second layer. The optical waveguide element according to any one of claims 1 to 3.

6. The slot waveguide has the same mode of optical propagation as the strip waveguide. The optical waveguide element according to any one of claims 1 to 3.

7. The substrate comprises a cladding formed on the substrate, The first layer and the second layer are provided within the cladding. The optical waveguide element according to any one of claims 1 to 3.

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