Optical waveguide element

Abstract

The present invention provides an optical waveguide element that can perform mode conversion and polarization rotation with high efficiency between multiple optical waveguides composed of different materials. [Solution] An optical waveguide element according to one embodiment includes a polarization rotation section having a first tapered section formed in a first layer located above the substrate and connected to a first waveguide section, and a first inverse tapered section formed in a second layer located above the substrate and different from the first layer, and connected to a second waveguide section. The length in the Y-axis direction of the first tapered section intersects the X-axis direction and changes along the X-axis direction, and the length in the Y-axis direction of the first inverse tapered section changes along the X-axis direction. In a plan view of the substrate, the distance between a first center line extending along the center of the Y-axis direction of the first tapered section and a second center line extending along the center of the Y-axis direction of the first inverse tapered section changes along the X-axis direction.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 describes a polarization conversion element. The polarization conversion element is composed of an optical waveguide formed on a substrate. The optical waveguide has a lower cladding formed on the substrate, a core formed thereon, and an upper cladding formed on the lower cladding and the core. The optical waveguide structure of the polarization conversion element has a first waveguide section, a polarization rotation section, and a second waveguide section. The core of the polarization rotation section has a thick plate section and a thin plate section with a thickness lower than that of the thick plate section. The thin plate section exists below the thick plate section. The cross-section of the core of the polarization rotation section is asymmetric in the height direction. A tapered mode conversion section is formed in a portion of the second waveguide section opposite to the polarization rotation section. Thereby, the polarization conversion element simultaneously performs polarization rotation and mode conversion.

[0003] Patent Document 2 describes a mode converter that converts the mode of light between a silicon waveguide and a second waveguide. The mode converter has an adiabatic taper of the silicon waveguide and an adiabatic taper of the second waveguide. At least a part of the adiabatic taper of the silicon waveguide and at least a part of the adiabatic taper of the second waveguide are adjacent to each other and overlap each other on the substrate.

[0004] Non-Patent Document 1 describes a polarization rotation element including a Si element and a Si3N4 element. In the polarization rotation element, mode conversion from TE to TM is performed inside the Si element. The Si3N4 element is arranged asymmetrically around the Si element. Thereby, in the polarization rotation element, the mode is rotated.

[0005] Non-patent document 2 describes a structure comprising a Si waveguide and a metal cap. In this structure, the TE0 mode of light inside the Si waveguide is converted into the mode of a plasmonic waveguide made of Si and metal. The Si waveguide and the metal cap have asymmetric cross-sectional shapes. This makes polarization conversion possible. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2014 / 207949 [Patent Document 2] U.S. Patent Application Publication No. 2017 / 0269302 [Non-patent literature]

[0007] [Non-Patent Document 1] “Compact polarization rotator on silicon for polarization-diversified circuits”, OpticsLetters Vol.36, No.4 (2011) [Non-Patent Document 2] “Mode-evolution-based polarization rotation and coupling between silicon and hybrid plasmonic waveguides”Scientific Reports 5;18375 (2015) [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Optical waveguide elements may consist of multiple materials that are different from each other. In such cases, it is required that mode conversion and polarization rotation can be performed efficiently between multiple optical waveguides composed of different materials.

[0009] The present disclosure aims to provide an optical waveguide element that can perform mode conversion and polarization rotation with high efficiency between multiple optical waveguides composed of different materials. [Means for solving the problem]

[0010] The optical waveguide element according to this disclosure is connected between a first waveguide section and a second waveguide section formed on a substrate. The optical waveguide element comprises a polarization rotation section having a first tapered section formed on a first layer located above the substrate and connected to the first waveguide section, and a first inverse tapered section formed on a second layer located above the substrate and different from the first layer, and connected to the second waveguide section. The material of the first tapered section is different from the material of the first inverse tapered section. In a plan view of the substrate, a part of the first tapered section and a part of the first inverse tapered section overlap each other, and the first waveguide section, the polarization rotation section, and the second waveguide section are arranged in this order along the X-axis direction. The length in the Y-axis direction intersecting the X-axis direction in the first tapered section changes along the X-axis direction, and the length in the Y-axis direction in the first inverse tapered section changes along the X-axis direction. In a plan view of the substrate, the distance between the first center line extending along the center in the Y-axis direction of the first tapered portion and the second center line extending along the center in the Y-axis direction of the first inverse tapered portion changes along the X-axis direction. [Effects of the Invention]

[0011] According to this disclosure, mode conversion and polarization rotation can be performed with high efficiency between multiple optical waveguides composed of different materials. [Brief explanation of the drawing]

[0012] [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 taken along line AA in Figure 1. [Figure 3] Figure 3 is a plan view showing an optical waveguide element according to the first modified example. [Figure 4] Figure 4 is a cross-sectional view along line BB in Figure 3. [Figure 5] Figure 5 is a cross-sectional view along the CC line in Figure 3. [Figure 6] Figure 6 is a plan view showing an optical waveguide element according to the second modified example. [Figure 7] Figure 7 is a plan view showing an optical waveguide element according to the third modified example. [Modes for carrying out the invention]

[0013] [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 is connected between a first waveguide section and a second waveguide section formed on a substrate. The optical waveguide element includes a polarization rotation section having a first tapered section formed on a first layer located above the substrate and connected to the first waveguide section, and a first inverse tapered section formed on a second layer located above the substrate and different from the first layer, and connected to the second waveguide section. The material of the first tapered section is different from the material of the first inverse tapered section. In a plan view of the substrate, a part of the first tapered section and a part of the first inverse tapered section overlap each other, and the first waveguide section, the polarization rotation section, and the second waveguide section are arranged in this order along the X-axis direction. The length in the Y-axis direction intersecting the X-axis direction in the first tapered section changes along the X-axis direction, and the length in the Y-axis direction in the first inverse tapered section changes along the X-axis direction. In a plan view of the substrate, the distance between the first center line extending along the center in the Y-axis direction of the first tapered portion and the second center line extending along the center in the Y-axis direction of the first inverse tapered portion changes along the X-axis direction.

[0014] This optical waveguide device includes a polarization rotation section. The polarization rotation section has a first tapered section connected to the first waveguide section and a first inverse tapered section connected to the second waveguide section and made of a material different from that of the first tapered section. The length of the first tapered section in the Y-axis direction changes along the X-axis direction, and the length of the first inverse tapered section in the Y-axis direction changes along the X-axis direction. In a plan view of the substrate, a part of the first tapered section and a part of the first inverse tapered section overlap each other. Therefore, mode conversion can be performed with high efficiency in the first tapered section and the first inverse tapered section made of different materials. In a plan view of the substrate, the distance between a first center line extending along the center in the Y-axis direction of the first tapered section and a second center line extending along the center in the Y-axis direction of the first inverse tapered section changes along the X-axis direction. Since the polarization rotation section has an asymmetric shape due to the first tapered section and the first inverse tapered section, polarization rotation can be performed with high efficiency in the first tapered section and the first inverse tapered section.

[0015] (2) In (1) above, the first tapered section may be made of silicon nitride, and the first inverse tapered section may be made of silicon. The optical waveguide device may be formed in the second layer and include a second inverse tapered section that connects the first inverse tapered section and the second waveguide section to each other. The peripheries of the first tapered section, the first inverse tapered section, and the second inverse tapered section may be covered with silicon oxide. In this case, by interposing the second inverse tapered section between the first inverse tapered section and the second waveguide section, a rapid change in width from the first inverse tapered section to the second waveguide section can be suppressed. Therefore, the first inverse tapered section can be smoothly connected to the second waveguide section, and the refractive index of light passing through the first inverse tapered section made of silicon can be made constant.

[0016] (3) In the above (1) or (2), the first tapered portion may be made of silicon nitride, and the first reverse tapered portion may be made of silicon. The optical waveguide element may be formed on the first layer and include a second tapered portion connecting the first tapered portion and the first waveguide portion to each other. The periphery of each of the first tapered portion, the first reverse tapered portion, and the second tapered portion may be covered with silicon oxide. In this case, by interposing the second tapered portion between the first tapered portion and the first waveguide portion, a sudden change in width from the first tapered portion to the first waveguide portion can be suppressed. Therefore, the first tapered portion can be smoothly connected to the first waveguide portion.

[0017] (4) In any of the above (1) to (3), in the plan view of the substrate, the first center line and the second center line of the polarization rotation portion may intersect each other.

[0018] [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 is intended to include all modifications within the scope shown in the claims and equivalent to the claims. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant 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.

[0019] 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 A-A 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.

[0020] The individual parts covered by the cladding 3 are also called cores. The substrate 2 is, for example, a semiconductor substrate. The semiconductor substrate is made of, for example, silicon (Si). For example, the cladding 3 has thickness in the Z-axis direction, which intersects both the X-axis and Y-axis directions. Hereinafter, the X-axis direction will also be called the length direction, the Y-axis direction the width direction, and the Z-axis direction the height direction. As an example, the cladding 3 has a rectangular parallelepiped shape extending in the X-axis, Y-axis, and Z-axis directions. The cladding 3 has, for example, a first layer 3b and a second layer 3c located between the first layer 3b and the substrate 2. 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 within the cladding 3. Each of the first layer 3b and the second layer 3c extends in the X-axis and Y-axis directions and has thickness in the Z-axis direction. The first layer 3b is provided, for example, on the upper side of the cladding 3, and the second layer 3c is provided below the first layer 3b. For example, the cladding 3 is made of silicon dioxide (SiO2). The optical waveguide 4 is surrounded by the cladding 3.

[0021] The optical waveguide element 1 is connected between a first waveguide section 11 and a second waveguide section 12 formed on a substrate 2. The optical waveguide section 4 functions as the core of the optical waveguide element 1. In a typical optical waveguide, light is confined within the core due to the difference between the refractive index of the core and the refractive index of the cladding surrounding the core, and the light propagates in the direction extending from the core. The optical waveguide section 4 includes a polarization rotation section 10. When the substrate 2 is viewed from above (along the Z-axis), the first waveguide section 11, the polarization rotation section 10, and the second waveguide section 12 are arranged in this order along the X-axis. The polarization rotation section 10 has a first tapered section 5 connected to the first waveguide section 11 and a first inverse tapered section 6 connected to the second waveguide section 12. The optical waveguide section 4 further comprises a second inverse tapered section 7 connecting the first inverse tapered section 6 and the second waveguide section 12, and a second tapered section 8 connecting the first tapered section 5 and the first waveguide section 11. The first tapered section 5, the first inverse tapered section 6, the second inverse tapered section 7, and the second tapered section 8 are each covered by cladding 3. The second inverse tapered section 7 is provided to gradually bring the length of the first inverse tapered section 6 in the Y-axis direction closer to the length of the second waveguide section 12 in the Y-axis direction along the X-axis direction, and the second tapered section 8 is provided to gradually bring the length of the first tapered section 5 in the Y-axis direction closer to the length of the first waveguide section 11 in the Y-axis direction along the X-axis direction. The length Lt1 of the first tapered section 5 in the X-axis direction may be the same as the length Lrt1 of the first inverse tapered section 6 in the X-axis direction, or it may be different from the length Lrt1 of the first inverse tapered section 6 in the X-axis direction. Length Lt1 is, for example, 10 μm or more and 3000 μm or less. Length Lt1 may also be, for example, 100 μm or more and 2000 μm or less. Length Lt1 may also be, for example, 300 μm or more and 1000 μm or less. Length Lrt1 is, for example, 10 μm or more and 3000 μm or less. Length Lrt1 may also be, for example, 100 μm or more and 2000 μm or less. Length Lrt1 may also be, for example, 300 μm or more and 1000 μm or less.

[0022] The material of the first tapered section 5 is different from the material of the first inverse tapered section 6. For example, the first tapered section 5 is made of silicon nitride (Si3N4), and the first inverse tapered section 6 is made of silicon (Si). The first waveguide section 11, the second tapered section 8, and the first tapered section 5 are arranged in this order along the X-axis. The first inverse tapered section 6, the second inverse tapered section 7, and the second waveguide section 12 are arranged in this order along the X-axis. For example, the first waveguide section 11, the second tapered section 8, and the first tapered section 5 are made of Si3N4 as a single continuous unit, while the second waveguide section 12, the second inverse tapered section 7, and the first inverse tapered section 6 are made of Si as a single continuous unit. As described later, the integrally formed first waveguide section 11, second tapered section 8, and first tapered section 5, and the integrally formed second waveguide section 12, second inverse tapered section 7, and first inverse tapered section 6 are each formed at different distances (heights) from the upper surface of the substrate 2 in the Z-axis direction. When the optical waveguide element 1 is manufactured on the substrate 2 using a semiconductor process, the optical waveguide element 1 is formed by stacking multiple layers. For example, the first waveguide section 11, second tapered section 8, and first tapered section 5 are formed in a single layer, the first layer 3b, while the second waveguide section 12, second inverse tapered section 7, and first inverse tapered section 6 are formed in a second layer 3c, which is a different layer from the first layer 3b. The distance of the first layer 3b to the substrate 2 is different from the distance of the second layer 3c to the substrate 2. Figure 2 shows an example in which the first inverse tapered section 6 is formed in the second layer 3c, and the first tapered section 5 is formed in the first layer 3b above the second layer 3c. An intermediate layer 3d is provided between the first layer 3b and the second layer 3c. Figure 2 shows a cross-section viewed from the first waveguide section 11 toward the second waveguide section 12. The cross-sectional diagrams described below also show cross-sections viewed in the same direction as Figure 2. Note that the intermediate layer 3d may be omitted, and the first layer 3b may be formed directly above the second layer 3c.

[0023] In the optical waveguide element 1, light propagates from the first waveguide section 11 through the polarization rotation section 10 to the second waveguide section 12. However, light also propagates from the second waveguide section 12 through the polarization rotation section 10 to the first waveguide section 11. Therefore, the polarization rotation section 10 can propagate light bidirectionally in the X-axis direction. For example, the first waveguide section 11 transmits light in TE (Transverse Electric wave) mode, and the second waveguide section 12 transmits light in TM (Transverse Magnetic wave) mode. Alternatively, the first waveguide section 11 may transmit light in TM mode, and the second waveguide section 12 may transmit light in TE 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, the electric field of light oscillates horizontally in TE mode, and the electric field of light oscillates vertically in TM mode. The polarization direction of light is horizontal in TE mode and vertical in TM mode. The polarization rotation unit 10 converts TE mode light to TM mode, or TM mode light to TE mode. This conversion rotates the polarization direction of the light by 90° between the horizontal and vertical directions, and is therefore also called polarization rotation.

[0024] The second tapered section 8 is located between the first waveguide section 11 and the first tapered section 5 in the X-axis direction. The shape of the second tapered section 8 is trapezoidal in a plan view of the substrate 2. The length of the second tapered section 8 in the Y-axis direction decreases monotonically along the X-axis from the first waveguide section 11 to the first tapered section 5 (in this application, such a trapezoidal shape is called a taper or forward taper). The length of the second tapered section 8 in the Y-axis direction may decrease uniformly along the X-axis direction (at a constant rate per unit length in the X-axis direction). The first tapered section 5 extends from the second tapered section 8 in the opposite direction to the first waveguide section 11, that is, toward the second waveguide section 12. The shape of the first tapered section 5 is trapezoidal in a plan view of the substrate 2. The length of the first tapered section 5 in the Y-axis direction changes along the X-axis direction. For example, the length of the first tapered portion 5 in the Y-axis direction decreases monotonically as it moves away from the first waveguide portion 11 along the X-axis.

[0025] The second inverse tapered section 7 is located between the second waveguide section 12 and the first inverse tapered section 6 in the X-axis direction. The shape of the second inverse tapered section 7 is trapezoidal in a plan view of the substrate 2. The length of the second inverse tapered section 7 in the Y-axis direction decreases monotonically along the X-axis from the second waveguide section 12 to the first inverse tapered section 6 (in this application, such a trapezoidal shape is called an inverse taper). The length of the second inverse tapered section 7 in the Y-axis direction may decrease uniformly along the X-axis direction (at a constant rate per unit length in the X-axis direction). The change in the length in the Y-axis direction along the X-axis direction is opposite to that of a taper (forward taper) and an inverse taper. The first inverse tapered section 6 extends from the second inverse tapered section 7 in the opposite direction to the second waveguide section 12, that is, toward the first waveguide section 11. The shape of the first inverse tapered section 6 is trapezoidal in a plan view of the substrate 2. The length of the first inverse tapered section 6 in the Y-axis direction varies along the X-axis direction. For example, the length of the first inverse tapered section 6 in the Y-axis direction decreases monotonically as it moves away from the second waveguide section 12 along the X-axis.

[0026] In a plan view of the substrate 2, a portion of the first tapered portion 5 and a portion of the first inverse tapered portion 6 overlap each other. That is, in a plan view of the substrate 2, a portion of the first tapered portion 5 overlaps with the first inverse tapered portion 6. This overlap is possible because the first tapered portion 5 and the first inverse tapered portion 6 are formed at different distances from the substrate 2. In a plan view of the substrate 2, the first tapered portion 5 overlaps with the first inverse tapered portion 6 near the first waveguide portion 11 (second tapered portion 8), but does not overlap with the first inverse tapered portion 6 near the second waveguide portion 12 (second inverse tapered portion 7). As a result, highly efficient optical mode conversion (mode transition) can be performed in the first tapered portion 5 and the first inverse tapered portion 6. In a plan view of the substrate 2, the distance A1 between the first center line L1 extending along the center of the first tapered portion 5 in the Y-axis direction and the second center line L2 extending along the center of the first inverse tapered portion 6 in the Y-axis direction changes along the X-axis direction.

[0027] For example, the polarization rotation section 10 has a portion in which, in a plan view of the substrate 2, the distance A1 between the first center line L1 and the second center line L2 increases along the X-axis direction. In this embodiment, this portion is included in the region from the boundary between the first tapered section 5 and the second tapered section 8 in the polarization rotation section 10 to the boundary between the first inverse tapered section 6 and the second inverse tapered section 7, in the X-axis direction. Because the distance A1 changes along the X-axis direction, the first center line L1 and the second center line intersect each other, they do not coincide with each other, and they are not parallel to each other. Therefore, the first tapered section 5 has an asymmetric shape with respect to the second center line L2 of the first inverse tapered section 6. As used in this application, asymmetry means that it is not symmetric with respect to a certain axis of symmetry. Also, the first inverse tapered section 6 has an asymmetric shape with respect to the first center line L1 of the first tapered section 5.

[0028] In a plan view of the substrate 2, at least one of the shapes of the first tapered portion 5 and the first inverse tapered portion 6 is asymmetric with respect to the Y-axis. That is, at least one of the shapes of the first tapered portion 5 and the first inverse tapered portion 6 is asymmetric with respect to a center line that passes through the center of the optical waveguide element 1 in the Y-axis direction and extends in the X-axis direction. In this way, the first center line L1 and the second center line L2 intersect each other, and a part of the first tapered portion 5 and a part of the first inverse tapered portion 6 overlap each other, causing the polarization direction of the light to rotate as it propagates through the polarization rotation portion 10. Figure 1 shows an example where the center line coincides with the second center line L2. With respect to the center line as the axis of symmetry, the first tapered portion 5 is asymmetric, and the first inverse tapered portion 6 is symmetric (line symmetry). Incidentally, the first center line L1 may also coincide with the center line, in which case the first inverse tapered portion 6 becomes asymmetric, and the first tapered portion 5 becomes symmetric (line symmetry). Furthermore, both the first tapered portion 5 and the first inverse tapered portion 6 may be asymmetrical with respect to the center line.

[0029] For example, the first tapered portion 5 is positioned above the first inverse tapered portion 6 in a cross-section perpendicular to the X-axis direction. However, the first tapered portion 5 may also be positioned below the first inverse tapered portion 6 in a cross-section perpendicular to the X-axis direction. That is, the relative positions of the first tapered portion 5 and the first inverse tapered portion 6 in a cross-section perpendicular to the X-axis direction may be reversed. The distance A2 between the first tapered portion 5 and the first inverse tapered portion 6 in the Z-axis direction is, for example, greater than 0 nm and less than or equal to 400 nm. As mentioned above, when the first tapered portion 5 is formed in the first layer 3b and the first inverse tapered portion 6 is formed in the second layer 3c, the distance A2 is equal to the distance between the first layer 3b and the second layer 3c. In the Z-axis direction, the portion between the first layer 3b and the second layer 3c is also formed as a single layer (intermediate layer 3d) by a semiconductor process. The intermediate layer 3d is composed of, for example, SiO2. The intermediate layer 3d may be omitted, in which case the spacing A2 may be zero.

[0030] The spacing A2 may be 1.0 μm or less. In this case, mode transitions in the polarization rotation section 10 can be performed appropriately. Hereinafter, in a cross section perpendicular to the X-axis direction, the direction from the substrate 2 as seen from the first inverse tapered section 6 and the first tapered section 5 may be referred to as up or upward, and the direction from the first tapered section 5 and the first inverse tapered section 6 as seen from the substrate 2 may be referred to as down or downward. However, these directions are for the convenience of describing the relative positions and directions of each other and do not limit the absolute placement position and direction that does not depend on the orientation of the substrate 2.

[0031] In the X-axis direction, the width A3 (length in the Y-axis direction) of the portion of the first inverse tapered section 6 furthest from the second inverse tapered section 7 (the portion closest to the first waveguide section 11) is, for example, greater than 0 nm and 300 nm or less. The width A3 is set according to the width A4 (length in the Y-axis direction) at the boundary portion of the first tapered section 5 with the second tapered section 8. The width A3 is smaller than the width A4. The width A4 is, for example, 400 nm or more and 1250 nm or less. The width A5 (length in the Y-axis direction) at the boundary portion of the first inverse tapered section 6 with the second inverse tapered section 7 is, for example, 200 nm or more and 600 nm or less. In the X-axis direction, the width A6 (length in the Y-axis direction) of the portion of the first tapered section 5 furthest from the second tapered section 8 (the portion closest to the second waveguide section 12) is, for example, greater than 0 nm and 400 nm or less. The width A6 is set smaller than the width A4 so that the first tapered section 5 has a forward taper. Also, the width A3 is set smaller than the width A5 so that the first inverse tapered section 6 has an inverse taper. The lower limits of width A3 and width A6 are the minimum values ​​that can be manufactured using the semiconductor process used to manufacture the optical waveguide element 1, and may be, for example, 0.05 μm. The length of the first tapered section 5 in the Y-axis direction may decrease uniformly (at a constant rate per unit length in the X-axis direction) from the boundary between the first tapered section 5 and the second tapered section 8 to the part of the first tapered section 5 furthest from the second tapered section 8 (the part closest to the second waveguide section 12), from the width A4 to the width A6 along the X-axis direction. The length of the first inverse tapered section 6 in the Y-axis direction may increase uniformly (at a constant rate per unit length in the X-axis direction) from the part of the first inverse tapered section 6 furthest from the second inverse tapered section 7 (the part closest to the first waveguide section 11) to the boundary between the first inverse tapered section 6 and the second inverse tapered section 7, so as to change from width A3 to width A5.

[0032] For example, the area of ​​the cross-section of the first tapered portion 5 when cut along a plane perpendicular to the X-axis direction, and the area of ​​the cross-section of the first inverse tapered portion 6 when cut along a plane perpendicular to the X-axis direction, change along the X-axis direction. The area of ​​the cross-section of the first tapered portion 5 when cut along a plane perpendicular to the X-axis direction decreases monotonically from the first waveguide portion 11 toward the second waveguide portion 12. The area of ​​the cross-section of the first inverse tapered portion 6 when cut along a plane perpendicular to the X-axis direction decreases monotonically from the second waveguide portion 12 toward the first waveguide portion 11.

[0033] The shape of the first tapered portion 5 in a cross-section perpendicular to the X-axis is, for example, rectangular. For example, when cut along a plane perpendicular to the X-axis, the length of the first tapered portion 5 in the Y-axis direction is greater than the length A7 of the first tapered portion 5 in the Z-axis direction. The length of the first tapered portion 5 in the Y-axis direction changes along the X-axis direction, while the length A7 of the first tapered portion 5 in the Z-axis direction remains constant. The length A7 is equal to the thickness of the first layer 3b. The shape of the first inverse tapered portion 6 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 of the first inverse tapered portion 6 in the Y-axis direction changes along the X-axis direction, while the length A8 of the first inverse tapered portion 6 in the Z-axis direction remains constant. Length A8 is equal to the thickness of the second layer 3c. For example, length A7 is 300 nm or more and 1000 nm or less, and length A8 is 100 nm or more and 400 nm or less. The shape of the first tapered portion 5 in a cross-section perpendicular to the X-axis direction, and the shape of the first inverse tapered portion 6 in a cross-section perpendicular to the X-axis direction, may be trapezoidal. In this case, the trapezoidal shape may be formed such that, for example, the length of the upper side along the Y-axis farther from the substrate 2 is greater than the length of the lower side along the Y-axis closer to the substrate 2.

[0035] As described above, the optical waveguide element 1 includes a polarization rotation section 10. The polarization rotation section 10 has a first tapered section 5 connected to a first waveguide section 11 and a first inverse tapered section 6 connected to a second waveguide section 12 and made of a different material than the first tapered section 5. The length of the first tapered section 5 in the Y-axis direction changes along the X-axis direction, and the length of the first inverse tapered section 6 in the Y-axis direction changes along the X-axis direction. The first tapered section 5 is formed in the first layer 3b in the Z-axis direction. The first inverse tapered section 6 is formed in the second layer 3c, which is different from the first layer 3b, in the Z-axis direction. In a plan view of the substrate 2, a part of the first tapered section 5 and a part of the first inverse tapered section 6 overlap each other. Therefore, the first tapered section 5, which is made of the first layer 3b and made of different materials, and the first inverse tapered section 6, which is made of the second layer 3c, can perform mode conversion of propagating light with high efficiency.

[0036] In a plan view of the substrate 2, the distance A1 between the first center line L1 extending along the center of the first tapered portion 5 in the Y-axis direction and the second center line L2 extending along the center of the first inverse tapered portion 6 in the Y-axis direction changes along the X-axis direction. Since the polarization rotation portion 10 has an asymmetric shape due to the first tapered portion 5 and the first inverse tapered portion 6, polarization rotation can be performed with high efficiency in the first tapered portion 5 and the first inverse tapered portion 6. Furthermore, the polarization rotation portion 10 may have a portion in a plan view of the substrate 2 where the distance A1 between the first center line L1 and the second center line L2 increases monotonically along the X-axis direction.

[0037] As mentioned above, the first tapered portion 5 formed in the first layer 3b may be made of Si3N4, and the first inverse tapered portion 6 formed in the second layer 3c may be made of Si. In the first layer 3b and the second layer 3c, portions other than the optical waveguide (core), such as the first tapered portion 5 and the first inverse tapered portion 6, are formed as cladding 3, for example, of SiO2. The optical waveguide element 1 may also include a second inverse tapered portion 7 that connects the first inverse tapered portion 6 and the second waveguide portion 12 to each other. In this case, the interposition of the second inverse tapered portion 7 between the first inverse tapered portion 6 and the second waveguide portion 12 can suppress abrupt changes in width (length in the Y-axis direction) from the first inverse tapered portion 6 to the second waveguide portion 12. Therefore, since the first inverse tapered section 6 can be gently connected to the second waveguide section 12, the refractive index of light passing through the first inverse tapered section 6, which is made of Si, can be stabilized.

[0038] As mentioned above, the optical waveguide element 1 may include a second tapered section 8 that connects the first tapered section 5 and the first waveguide section 11 to each other. In this case, the interposition of the second tapered section 8 between the first tapered section 5 and the first waveguide section 11 can suppress abrupt changes in width (length in the Y-axis direction) from the first tapered section 5 to the first waveguide section 11. Therefore, the first tapered section 5 can be connected to the first waveguide section 11 more gradually.

[0039] Next, various modified optical waveguide elements will be described. Some of the configurations of the optical waveguide elements in each of the modified elements described later are the same as some of the configurations of optical waveguide element 1 described above. Therefore, in the following description, explanations that overlap with the description of optical waveguide element 1 described above will be appropriately omitted and denoted by the same reference numerals.

[0040] Figure 3 shows an optical waveguide element 1A according to the first modified example. Figure 4 is a cross-sectional view taken along line BB of Figure 3. Figure 5 is a cross-sectional view taken along line CC of Figure 3. As shown in Figures 3, 4, and 5, the optical waveguide element 1A has an optical waveguide section 4A that functions as a core embedded in the cladding 3. The optical waveguide section 4A has a polarization rotation section 10A. The polarization rotation section 10A has a first tapered section 5A connected to the first waveguide section 11 and a first inverse tapered section 6A connected to the second waveguide section 12. The optical waveguide section 4A further has a second inverse tapered section 7A that connects the first inverse tapered section 6A and the second waveguide section 12 to each other. Unlike the optical waveguide section 4 described above, the optical waveguide section 4A does not have a second tapered section 8. The first tapered section 5A of the polarization rotation section 10A is directly connected to the first waveguide section 11.

[0041] The first tapered section 5A and the first waveguide section 11 are made of Si3N4, while the first inverse tapered section 6A, the second inverse tapered section 7A, and the second waveguide section 12 are made of Si. The first tapered section 5A and the first waveguide section 11 are formed, for example, in a layer located above the substrate 2 (first layer 3b). The first inverse tapered section 6A, the second inverse tapered section 7A, and the second waveguide section 12 are formed, for example, in a layer different from the first layer 3b located above the substrate 2 (second layer 3c). In the first layer 3b, the parts other than the first tapered section 5A and the first waveguide section 11 are made of SiO2 as cladding 3. In the second layer 3c, the parts other than the first inverse tapered section 6A, the second inverse tapered section 7A, and the second waveguide section 12 are made of SiO2 as cladding 3. The first tapered section 5A is trapezoidal, and its length in the Y-axis direction gradually decreases as it moves away from the first waveguide section 11. The first inverse tapered section 6A is trapezoidal, and its length in the Y-axis direction gradually decreases as it moves away from the second waveguide section 12. The second inverse tapered section 7A is trapezoidal, and its length in the Y-axis direction decreases as it moves from the second waveguide section 12 towards the first inverse tapered section 6.

[0042] In a plan view of the substrate 2, a portion of the first tapered portion 5A and a portion of the first inverse tapered portion 6A overlap. In a plan view of the substrate 2, the distance B1 between the first center line L3 extending along the center of the first tapered portion 5A in the Y-axis direction and the second center line L4 extending along the center of the first inverse tapered portion 6A in the Y-axis direction changes along the X-axis direction. The polarization rotation portion 10A has a portion in a plan view of the substrate 2 where the distance B1 between the first center line L3 and the second center line L4 increases along the X-axis direction. This portion is the region on the X-axis from the intersection of the first center line L3 and the second center line L4 in a plan view of the substrate 2 to the portion furthest from the first waveguide portion 11 of the first tapered portion 5A. In a plan view of the substrate 2, the region on the X-axis from the boundary between the first tapered portion 5A and the first waveguide portion 11 to the intersection of the first centerline L3 and the second centerline L4 is the portion where the distance B1 between the first centerline L3 and the second centerline L4 decreases along the X-axis direction.

[0043] The width B2 of the first inverse tapered section 6A at the point furthest from the second waveguide section 12 (the point closest to the first waveguide section 11) is, for example, greater than 0 nm and 300 nm or less (100 nm as an example). The width B3 at the boundary between the first waveguide section 11 and the first tapered section 5A is, for example, 400 nm or more and 1250 nm or less (700 nm as an example). The width B4 at the boundary between the first inverse tapered section 6A and the second inverse tapered section 7A is, for example, 200 nm or more and 600 nm or less (300 nm as an example). The width B5 of the first tapered section 5A at the point furthest from the first waveguide section 11 (the point closest to the second waveguide section 12) is, for example, greater than 0 nm and 400 nm or less (200 nm as an example). The width B5 is set smaller than the width B3 so that the first tapered section 5A becomes a forward taper. Furthermore, the width B2 is set smaller than the width B4 so that the first inverse taper portion 6A has an inverse taper. The lower limit of width B2 and the lower limit of width B5 are the minimum values ​​that can be manufactured by the semiconductor process used to manufacture the optical waveguide element 1A, and may be, for example, 0.05 μm.

[0044] In a cross-section perpendicular to the X-axis, the first tapered portion 5A is positioned above the first inverse tapered portion 6. For example, as described above, the first tapered portion 5A is formed in the first layer 3b, and the first inverse tapered portion 6 is formed in the second layer 3c. The distance B6 between the first tapered portion 5A and the first inverse tapered portion 6A in the Z-axis direction is, for example, greater than 0 nm and less than or equal to 400 nm. An intermediate layer 3d is formed between the first layer 3b and the second layer 3c, and the distance B6 is equal to the thickness of the intermediate layer 3d. The intermediate layer is made of SiO2. Note that the intermediate layer 3d may be omitted, in which case the distance B6 may be zero. The length B7 of the first tapered portion 5A in the Z-axis direction is greater than the length B8 of the first inverse tapered portion 6A in the Z-axis direction. Length B7 is equal to the thickness of the first layer 3b, and length B8 is equal to the length of the second layer 3c. Therefore, the thickness of the first layer 3b is greater than the thickness of the second layer 3c. For example, length B7 is between 300 nm and 1000 nm, and length B8 is between 100 nm and 400 nm.

[0045] As described above, in the optical waveguide element 1A, the polarization rotation portion 10A has an asymmetrical shape due to the first tapered portion 5A and the first inverse tapered portion 6A. Therefore, the optical waveguide element 1A exhibits the same effects as the optical waveguide element 1 described above. Furthermore, the optical waveguide element 1A does not have a second tapered portion 8 interposed between the first tapered portion 5 and the first waveguide portion 11. Consequently, the optical waveguide element 1A can reduce the element length in the X-axis direction compared to the optical waveguide element 1 described above.

[0046] Figure 6 shows an optical waveguide element 1B according to the second modified example. Optical waveguide element 1B differs from the optical waveguide element 1 described above in that it does not have a second inverse tapered portion 7 and a second tapered portion 8. In optical waveguide element 1B, the first tapered portion 5 is directly connected to the first waveguide portion 11, and the first inverse tapered portion 6 is directly connected to the second waveguide portion 12. Optical waveguide element 1B can further reduce the element length in the X-axis direction compared to the optical waveguide element 1A described above. However, there is room for improvement in optical waveguide element 1B in terms of maintaining a constant refractive index of light.

[0047] Figure 7 shows an optical waveguide element 1C according to the third modified example. Optical waveguide element 1C differs from optical waveguide element 1B in that the first tapered portion 5C comprises a first trapezoidal portion 5b, a second trapezoidal portion 5c, and a third trapezoidal portion 5d, each having a different shape. In the first tapered portion 5C, the first trapezoidal portion 5b, the second trapezoidal portion 5c, and the third trapezoidal portion 5d are arranged in this order along the X-axis direction. Optical waveguide element 1C has a first region R1, a second region R2, and a third region R3, and the first region R1, the second region R2, and the third region R3 are arranged in this order along the X-axis direction.

[0048] In a plan view of substrate 2, in the first region R1, the first trapezoidal portion 5b overlaps the first inverse tapered portion 6. In a plan view of substrate 2, in the second region R2, the second trapezoidal portion 5c overlaps a part of the first inverse tapered portion 6, and the first inverse tapered portion 6 has a portion that does not overlap with the second trapezoidal portion 5c. In a plan view of substrate 2, in the third region R3, the third trapezoidal portion 5d does not overlap with the first inverse tapered portion 6. For example, the length in the X-axis direction of the second region R2 is greater than the length in the X-axis direction of the first region R1, and the length in the X-axis direction of the first region R1 is greater than the length in the X-axis direction of the third region R3.

[0049] As described above, in the optical waveguide element 1C, the first tapered portion 5C has multiple trapezoidal sections (for example, the first trapezoidal section 5b, the second trapezoidal section 5c, and the third trapezoidal section 5d). Therefore, the configuration of each trapezoidal section of the first tapered portion 5C that overlaps the first inverse tapered portion 6 in a plan view of the substrate 2, and the asymmetry of the first tapered portion 5C with respect to the Y-axis direction, can be adjusted. In the optical waveguide element 1C, because the first tapered portion 5C has multiple trapezoidal sections and various adjustments are possible, the refractive index of light can be kept constant while suppressing the element length of the optical waveguide element 1C in the X-axis direction.

[0050] 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]

[0051] 1,1A,1B,1C...Optical waveguide element 2… Circuit board 3…Clad 3b…1st layer 3c…Second layer 3d…Middle layer 4,4A…Optical waveguide section 5, 5A, 5C... First tapered section 5b...First trapezoidal section 5c...Second trapezoidal section 5d...Third trapezoidal section 6,6A...First reverse taper section 7,7A...Second reverse taper section 8…Second taper section 10,10A...Polarization rotation section 11…First Waveguide Section 12…Second Waveguide Section

Claims

1. An optical waveguide element connected between a first waveguide section and a second waveguide section formed on a substrate, A first tapered portion is formed in the first layer located above the substrate and connected to the first waveguide portion, A first inverse tapered portion is located above the substrate, formed in a second layer different from the first layer, and connected to the second waveguide portion, It is equipped with a polarization rotation section having The material of the first tapered section is different from the material of the first inverse tapered section. In a plan view of the substrate, a portion of the first tapered portion and a portion of the first inverse tapered portion overlap each other. The first waveguide section, the polarization rotation section, and the second waveguide section are arranged in this order along the X-axis direction. The length of the Y-axis direction intersecting the X-axis direction in the first tapered portion changes along the X-axis direction. The length of the first inverse tapered portion in the Y-axis direction changes along the X-axis direction. In a plan view of the substrate, the distance between the first center line extending along the center in the Y-axis direction of the first tapered portion and the second center line extending along the center in the Y-axis direction of the first inverse tapered portion changes along the X-axis direction. Optical waveguide element.

2. The first tapered portion is made of silicon nitride, The first inverse tapered portion is made of silicon, The second layer is formed and includes a second inverse tapered portion that connects the first inverse tapered portion and the second waveguide portion to each other, The periphery of the first tapered portion, the first inverse tapered portion, and the second inverse tapered portion is covered with silicon oxide. The optical waveguide element according to claim 1.

3. The first tapered portion is made of silicon nitride, The first inverse tapered portion is made of silicon, The first layer is formed and includes a second tapered portion that connects the first tapered portion and the first waveguide portion to each other. The periphery of the first tapered portion, the first inverse tapered portion, and the second tapered portion is covered with silicon oxide. The optical waveguide element according to claim 1 or claim 2.

4. In a plan view of the substrate, the polarization rotation section is such that the first center line and the second center line intersect each other. The optical waveguide element according to claim 1 or claim 2.

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