Polarization conversion element

Abstract

The present invention provides a polarization conversion element that can suppress the impact of variations in the width of optical waveguides on polarization conversion efficiency. [Solution] The optical waveguide extends in a first direction and its width is in a second direction, and the low refractive index layer extends in the first direction and its width is in a second direction, and is a layer with a lower refractive index than the optical waveguide, spaced apart from the optical waveguide in a third direction. The low refractive index layer is wider than the optical waveguide. In the first region, the width of the optical waveguide is the width that selectively guides light of the fundamental mode. In the second region, the widths of the optical waveguide and the low refractive index layer expand toward the exit side. In the third region, the widths of the optical waveguide and the low refractive index layer are constant. In the fourth region, the width of the optical waveguide expands toward the exit side, and the width of the low refractive index layer is constant. In the fifth region, the width of the optical waveguide is constant, and the width of the low refractive index layer decreases toward the exit side.

Description

[Technical Field]

[0001] The present disclosure relates to a polarization conversion element. [Background Art]

[0002] In recent years, research and development on silicon photonics has been actively conducted. In silicon photonics, light can be efficiently and strongly confined by a waveguide made of silicon (Si) having a refractive index of approximately 3.5, so that optical circuits can be dramatically reduced in size. On the other hand, in Si waveguides, separation or conversion of guided polarized light is one of the important functions of optical circuits. A polarization conversion element is an element that outputs a polarized wave orthogonal to an input polarized wave. In order to cause polarization conversion in a waveguide-type polarization conversion element, it is necessary to provide asymmetry in the vertical direction of the cross-sectional structure of the waveguide. Further, a waveguide structure having greater asymmetry is characterized in that the design range of waveguide dimensions for polarization conversion is expanded. That is, this means that a waveguide structure with greater asymmetry provides a more robust polarization conversion element.

[0003] For example, Patent Document 1 and Non-Patent Document 1 propose polarization conversion elements using a Si waveguide structure. In Non-Patent Document 1, asymmetry of the waveguide cross-sectional structure is achieved by forming a tapered silicon nitride (SiN) layer in the upper region of the Si waveguide. In Non-Patent Document 1, the Si3N4 layer in the polarization rotator region is formed at a position away from directly above the Si waveguide. Further, since the Si3N4 layer in the structure of Non-Patent Document 1 has a shape that is asymmetric in the left-right direction, the position of the Si3N4 layer relative to the Si waveguide greatly affects polarization conversion. Therefore, in Non-Patent Document 1, polarization conversion efficiency with respect to an offset from the designed position of the Si3N4 layer is studied. Non-Patent Document 1 reports that a polarization conversion efficiency of 90% or more is achieved when the offset amount of the Si3N4 layer from the designed position is within a range of ±15 nm. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-169766 [Non-patent literature]

[0005] [Non-Patent Document 1] Long Chen, et al., “Compact polarization rotator on silicon for polarization-diversified circuits,” Optics Letters, Vol. 36, No. 4, pp. 469-471, 2011. [Overview of the project] [Problems that the invention aims to solve]

[0006] Non-patent document 1 investigated the effect of misalignment of the Si3N4 layer in a polarization conversion element on polarization conversion efficiency. On the other hand, the effect of manufacturing errors in the Si waveguide on polarization conversion efficiency was not investigated. However, since the Si waveguide is the structure that guides light, it is a factor that greatly affects polarization conversion efficiency. Therefore, polarization conversion elements require a robust design against width variations caused by manufacturing errors in the Si waveguide. [Means for solving the problem]

[0007] A polarization conversion element according to one aspect of the present disclosure comprises: an optical waveguide extending in a first direction and having a width direction in a cross section perpendicular to the first direction and perpendicular to the first direction; and a low refractive index layer having a lower refractive index than the optical waveguide, extending in the first direction and having a width direction in a cross section perpendicular to the first direction, spaced apart from the optical waveguide in a third direction perpendicular to the first and second directions, and overlapping with a part of the optical waveguide when viewed along the third direction; wherein the optical waveguide is divided into first to sixth regions along the first direction from the incident side to the exit side of the light, and the low refractive index layer extends between the incident side end of the second region and the exit side end of the sixth region with a width wider than the optical waveguide. The optical waveguide is provided in such a manner that, in the first region, the width of the optical waveguide is such that it selectively guides light in the fundamental mode; in the second region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer increases from the incident side to the exit side of the light; in the third region, the width of the optical waveguide and the low refractive index layer are the same as the width of the exit side of the second region, respectively; in the fourth region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer is the same as the width in the third region; and in the fifth region, the width of the optical waveguide is the same as the width of the exit side of the fourth region, and the width of the low refractive index layer decreases from the incident side to the exit side of the light. [Effects of the Invention]

[0008] According to this disclosure, it is possible to provide a polarization conversion element that can suppress the impact of variations in the width of the optical waveguide on the polarization conversion efficiency. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic perspective view showing the configuration of a polarization conversion element according to one embodiment. [Figure 2] This is a schematic top view showing the configuration of a polarization conversion element according to one embodiment. [Figure 3] This is a top view showing a modified example of a polarization conversion element according to one embodiment. [Figure 4] This figure shows the dependence of polarization conversion efficiency on variations in the width of the optical waveguide. [Figure 5] This figure shows the wavelength dependence of polarization conversion efficiency. [Figure 6] This figure shows the dependence of polarization conversion efficiency on variations in the width of the optical waveguide. [Figure 7] This figure shows the wavelength dependence of polarization conversion efficiency. [Figure 8] This figure shows the dependence of polarization conversion efficiency on variations in the width of the optical waveguide. [Figure 9] This figure shows the wavelength dependence of polarization conversion efficiency. [Figure 10] This figure shows the dependence of polarization conversion efficiency on variations in the width of the optical waveguide. [Figure 11] This figure shows the wavelength dependence of polarization conversion efficiency. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will now be described with reference to the drawings. In each drawing, the same elements are denoted by the same reference numerals, and redundant explanations are omitted where necessary.

[0011] When we refer to one embodiment below, it means that it is applicable to any of the embodiments described below, or to a combination of two or more embodiments, and that its application is not limited to a specific embodiment.

[0012] Embodiment 1 A polarization conversion element according to Embodiment 1 will now be described. The polarization conversion element according to Embodiment 1 is configured as, for example, a silicon photonic element. Figure 1 is a schematic perspective view showing the configuration of a polarization conversion element according to one embodiment. Figure 2 is a schematic top view showing the configuration of a polarization conversion element according to one embodiment. The polarization conversion element 100 is composed of three layers: an optical waveguide 1, a low refractive index layer 2, and a cladding layer 3.

[0013] Hereinafter, to facilitate understanding of the configuration of the polarization conversion element, an XYZ coordinate system defined by mutually orthogonal X-axis, Y-axis and Z-axis is introduced to the illustration in the drawings. The axis along the extending direction of the optical waveguide 1 and the low refractive index layer 2 is defined as the X-axis. The axis along the main surface direction of the optical waveguide 1 and the low refractive index layer 2 is defined as the Z-axis. The axis in the direction orthogonal to the X-axis direction on the main surfaces of the optical waveguide 1 and the low refractive index layer 2 is defined as the Y-axis. Hereinafter, references to the X, Y and Z directions mean directions along the X-axis, Y-axis and Z-axis respectively, regardless of whether they face the - direction or the + direction of each axis. The direction toward the + direction of each axis is expressed as the +X direction, +Y direction and +Z direction. The direction toward the - direction of each axis is expressed as the -X direction, -Y direction and -Z direction. Additionally, the X direction is also referred to as a first direction. The Y direction is also referred to as a second direction. The Z direction is also referred to as a third direction.

[0014] The optical waveguide 1 is, for example, an optical waveguide made of silicon (Si). The optical waveguide 1 is configured as an optical waveguide extending along the X direction. The width of the optical waveguide 1 in the Y direction increases stepwise from the end on the -X direction side toward the end on the +X direction side. Furthermore, when viewed along the Z direction, the optical waveguide 1 has a shape that is line-symmetric with respect to the central axis in the X direction. Hereinafter, the length of the optical waveguide 1 refers to the dimension of the optical waveguide 1 in the X direction, which is the extending direction of the optical waveguide 1. The width of the optical waveguide 1 refers to the dimension of the optical waveguide 1 in the Y direction. The thickness of the optical waveguide 1 refers to the dimension of the optical waveguide 1 in the Z direction.

[0015] Light L is incident on the optical waveguide 1 from the end on the -X direction side. While the light L incident into the optical waveguide 1 propagates through the optical waveguide 1, polarization conversion is performed on the light L according to the structure and arrangement of the optical waveguide 1 and the low refractive index layer 2. The light L after polarization conversion exits from the end on the +X direction side of the optical waveguide 1.

[0016] In the following description, for each configuration and each region, the -X direction side is also referred to as the incident side of light L, and the +X direction side is also referred to as the exit side of light L.

[0017] The low refractive index layer 2 is, for example, an optical waveguide made of silicon nitride. While Si3N4 is a known composition of silicon nitride, it is not limited to this. Furthermore, the low refractive index layer 2 only needs to be composed of a material with a lower refractive index than the optical waveguide 1 and a higher refractive index than the cladding layer 3; the material constituting the low refractive index layer 2 is not limited to silicon nitride.

[0018] The low refractive index layer 2 is configured as an optical waveguide extending along the X direction. The width of the low refractive index layer 2 in the Y direction increases gradually from the end on the -X side to the end on the +X side. When viewed from the Z direction, the low refractive index layer 2 has a shape that is symmetric with respect to the central axis in the X direction. Furthermore, the low refractive index layer 2 is positioned at a distance from the optical waveguide 1 in the +Z direction. That is, the optical waveguide 1 and the low refractive index layer 2 are positioned at a distance from each other along the Z direction. Note that when viewed along the Z direction, the central axis of the low refractive index layer 2 in the X direction coincides with the central axis of the optical waveguide 1 in the X direction.

[0019] In the following, the length of the low refractive index layer 2 refers to the dimension of the low refractive index layer 2 in the X direction, which is the direction in which the low refractive index layer 2 extends. The width of the low refractive index layer 2 refers to the dimension in the Y direction in the ZX cross-section of the low refractive index layer 2. The thickness of the low refractive index layer 2 refers to the dimension in the Z direction of the low refractive index layer 2.

[0020] The optical waveguide 1 and the low refractive index layer 2 are embedded in the cladding layer 3. The cladding layer 3 is made of a silica-based material such as SiO2. However, the cladding layer 3 is not limited to silica-based materials such as SiO2, as long as it is made of a material with a lower refractive index than the optical waveguide 1 and the low refractive index layer 2. As long as the polarization conversion element 100 can achieve the desired polarization conversion, the cladding layer 3 may be made of any material with a lower refractive index than the optical waveguide 1 and the low refractive index layer 2, such as air.

[0021] Next, the widths of the optical waveguide 1 and the low refractive index layer 2 will be described. The polarization conversion element 100 is divided into multiple regions along the X direction. The widths of the optical waveguide 1 and the low refractive index layer 2 are configured to vary depending on the region. In this embodiment, the element is divided into at least five first regions A1 to fifth regions A5 in order from the incident end to the exit end.

[0022] The first region A1 is a region where only the optical waveguide 1 extends, and the low refractive index layer 2 does not exist. In contrast, the second region A2 to the fifth region A5 are regions where both the optical waveguide 1 and the low refractive index layer 2 extend. Furthermore, between the second region A2 and the fifth region A5, the low refractive index layer 2 is configured to be wider than the optical waveguide 1. In other words, the low refractive index layer 2 is provided so as to overlap with the portion of the optical waveguide 1 that extends between the second region A2 and the fifth region A5.

[0023] The distance in the Z-direction between the optical waveguide 1 and the low refractive index layer 2 is preferably between 10 nm and 100 nm. In the following, the distance in the Z-direction between the optical waveguide 1 and the low refractive index layer 2 is the same between the second region A2 and the fifth region A5. The thickness of the low refractive index layer 2 in the Z-direction is preferably around 50 nm to 400 nm.

[0024] In the first region A1, the optical waveguide 1 is designed to selectively guide only light in fundamental modes (TE0 mode, TM0 mode). For example, if the height of the optical waveguide is 220 nm and the width is 400 nm, light L with a wavelength of 1550 nm will propagate through the optical waveguide 1 in single mode. In this embodiment, the light L propagating through the first region A1 is assumed to satisfy both the TE0 mode and the TM0 mode.

[0025] In the second region A2, the optical waveguide 1 and the low refractive index layer 2 are configured to have their respective widths increased. As a result, the light L in the fundamental modes (TE0 mode and TM0 mode) incident from the first region A1 is converted into light L in a hybrid state of TM0 mode and TE1 mode. In this configuration, the second region A2 is divided to have at least a sixth region A6 located on the incident side of the light L and a seventh region A7 located on the incident side of the light L. In addition, in this configuration, a first connecting region C1 is inserted between the sixth region A6 and the seventh region A7, connecting the sixth region A6 and the seventh region A7.

[0026] In the sixth region A6, the optical waveguide 1 extends with the same width as the optical waveguide 1 in the first region A1. The low refractive index layer 2 is configured to have a tapered shape, with its width continuously increasing from the incident end to the outgoing end. As a result, the width of the incident end of the low refractive index layer 2 is narrower than the width of the optical waveguide 1, but the width of the outgoing end of the low refractive index layer 2 is wider than the width of the optical waveguide 1. It is desirable that the width of the incident end of the low refractive index layer 2 be as narrow as possible in order to suppress the reflection of the incident light L to a desired level or lower. For example, the width of the incident end of the low refractive index layer 2 is preferably 150 nm, more preferably 120 nm, and even more preferably 100 nm or less.

[0027] The optical waveguide 1 in the first connecting region C1 extends with the same width as the optical waveguide 1 in the first region A1 and the sixth region A6. The low refractive index layer 2 extends with the same width as the low refractive index layer 2 at the exit end of the sixth region A6. As a result, the mode state of the light L, which has been altered by the tapered low refractive index layer 2 in the sixth region A6, can be stabilized by the first connecting region C1, where the optical waveguide 1 and the low refractive index layer 2 extend with a constant width. Consequently, after the mode state of the light L is stabilized in the first connecting region C1, it is incident on the next seventh region A7.

[0028] In the seventh region A7, the optical waveguide 1 is configured to have a tapered shape, with its width continuously increasing from the incident end to the exit end. Therefore, the width of the incident end of the optical waveguide 1 is the same as the width of the exit end of the sixth region A6 of the optical waveguide 1. In contrast, the width of the exit end of the optical waveguide 1 is expanded to a width that allows polarization conversion of the optical L mode from TM0 mode to TE1 mode. Here, the width that allows polarization conversion is a width in which the equivalent refractive index for TM0 mode and TE1 mode is approximately the same, and hybridization of TM0 mode and TE1 mode can be realized. On the other hand, the low refractive index layer 2 extends with the same width as the optical waveguide 1 at the exit end of the first connecting region C1.

[0029] In the third region A3, optical waveguide 1 extends with the same width as optical waveguide 1 at the exit end of the seventh region A7. The low refractive index layer 2 also extends with the same width as the low refractive index layer 2 in the seventh region A7. As a result, in the third region A3, optical light L is guided in a hybrid state of TM0 mode and TE1 mode.

[0030] As described above, polarization conversion from TM0 mode to TE1 mode occurs through the seventh region A7 and the third region A3. The polarization conversion efficiency depends on the length and width of the seventh region A7 and the third region A3. Therefore, by appropriately designing the width and length of the optical waveguide 1 in the seventh region A7 and the third region A3, perfect polarization conversion (100%) can be achieved in principle.

[0031] In the fourth region A4, the optical waveguide 1 is configured to have a tapered shape, with its width continuously increasing from the incident end to the exit end. Therefore, the width of the incident end of the optical waveguide 1 is the same as the width of the exit end of the optical waveguide 1 in the third region A3. In contrast, the width of the exit end of the optical waveguide 1 is expanded to a width that creates a sufficient difference in the equivalent refractive indices of the TM0 mode and the TE1 mode. As a result, the hybrid state of the TM0 mode and the TE1 mode is eliminated, and the TE1 mode becomes dominant in light L. Consequently, light in the TE1 mode is emitted from the optical waveguide 1 in the fourth region A4.

[0032] In this configuration, a second connecting region C2 is inserted between the fourth region A4 and the sixth region A6, connecting the four regions A4 and A6. In the second connecting region C2, the optical waveguide 1 extends with the same width as the optical waveguide 1 in the fourth region A4. The low refractive index layer 2 also extends with the same width as the low refractive index layer 2 in the fourth region A4. As a result, the mode state of the light L, which has been changed by the tapered optical waveguide 1 in the fourth region A4, can be stabilized by the second connecting region C2, where the optical waveguide 1 and the low refractive index layer 2 extend with a constant width. Consequently, after the mode state of the light L is stabilized by the second connecting region C2, it is incident on the next fifth region A5.

[0033] In the fifth region A5, the optical waveguide 1 extends with the same width as the optical waveguide 1 in the second connecting region C2. The low refractive index layer 2 is configured to have a tapered shape, with its width continuously decreasing from the incident end to the exit end. Therefore, the width of the incident end of the low refractive index layer 2 is the same as the width in the second connecting region C2. In contrast, the width of the exit end of the low refractive index layer 2 is preferably as narrow as possible in order to suppress the reflection of light L incident on the exit end to a desired level or lower. The width of the exit end of the low refractive index layer 2 is preferably 150 nm, more preferably 130 nm, and even more preferably 100 nm or less.

[0034] As mentioned above, the first connecting region C1 and the second connecting region C2 are inserted to stabilize the mode of the directed light. Therefore, it is desirable that the polarization conversion element 100 includes the first connecting region C1 and the second connecting region C2. On the other hand, if the mode of the directed light can be sufficiently stabilized to enable suitable polarization conversion, one or both of the first connecting region C1 and the second connecting region C2 may be omitted. Figure 3 is a top view showing a modified example of a polarization conversion element according to one embodiment. As shown in Figure 3, the polarization conversion element may be composed only of the first region A1 and the third to seventh regions A7, with the first connecting region C1 and the second connecting region C2 omitted.

[0035] When actually fabricating the polarization conversion element 100, the width and length of the optical waveguide 1 in the seventh region A7 and the third region A3 will vary due to manufacturing variations. Therefore, the polarization conversion efficiency will also vary in the actual device. In contrast, it is desirable that the polarization conversion efficiency of the polarization conversion element 100 does not fluctuate much even if the width ratio varies. Therefore, in this configuration, the width of the low refractive index layer 2 in the third region A3 is made wider than the width of the optical waveguide 1, thereby achieving a high and stable polarization conversion efficiency.

[0036] The following explains, based on simulations, the effect of making the width of the low refractive index layer 2 in the third region A3 wider than the width of the optical waveguide 1 on polarization conversion efficiency. In the following simulations, the ratio of the width of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3, W2 / W1, is referred to as the width ratio. The following four conditions were set for the design value of the width ratio. Then, under each condition, the effect of the variation from the design value of the width of the optical waveguide 1 in the third region A3 on the optical waveguide was simulated.

[0037] Furthermore, polarization conversion elements are expected to be used over a wide wavelength range. Therefore, in order to achieve high polarization conversion efficiency over a wide wavelength range, simulations were also performed to determine the wavelength dependence of polarization conversion efficiency in the C band under various conditions.

[0038] Condition 1 In the first condition, the design value for the width ratio W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was set to 1.0. The ratio of the thickness H2 of the low refractive index layer 2 to the thickness H1 of the optical waveguide 1 was set to 1:2. The lengths of the optical waveguide 1 and the low refractive index layer 2 were set to 46 μm.

[0039] Figure 4 shows the dependence of polarization conversion efficiency on the width variation of optical waveguide 1. Under condition 1, a polarization conversion efficiency of 90% or more was achieved when the variation in the width W1 of optical waveguide 1 in the third region A3 was within ±3.5 μm. On the other hand, when the variation in the width W1 of optical waveguide 1 in the third region A3 was within ±5.0 μm or more, the polarization conversion efficiency decreased sharply. Figure 5 shows the wavelength dependence of polarization conversion efficiency. In this case, the wavelength dependence of polarization conversion efficiency was large, and at a wavelength of 1525 nm, the polarization conversion efficiency decreased to approximately 60%.

[0040] In the third region A3, under the first condition where the design value of the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 is 1.0, it can be seen that the range of widths of the low refractive index layer 2 that can achieve high polarization conversion efficiency is narrow, and that the polarization conversion efficiency is highly wavelength-dependent.

[0041] Second condition In the second condition, the design value for the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was set to 1.2. Similarly, the ratio of the thickness H2 of the low refractive index layer 2 to the thickness H1 of the optical waveguide 1 was set to 1:2. The lengths of the optical waveguide 1 and the low refractive index layer 2 were set to 26 μm.

[0042] Figure 6 shows the dependence of polarization conversion efficiency on variations in the width of optical waveguide 1. Under the second condition, a polarization conversion efficiency of 90% or more was achieved when the variation in the width W1 of optical waveguide 1 in the third region A3 was within ±10 μm. Figure 7 shows the wavelength dependence of polarization conversion efficiency. In this case, the wavelength dependence was also suppressed to a small extent, and as a result, a polarization conversion efficiency of 90% or more was achieved across the entire C band.

[0043] In the second condition, compared to the first condition, the design value of the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was increased. As a result, the range of variation in the width of the optical waveguide 1 in which high polarization conversion efficiency can be achieved was expanded, and the wavelength dependence of the polarization conversion efficiency was improved.

[0044] Third condition In the third condition, the design value for the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was set to 1.6. Similarly, the ratio of the thickness H2 of the low refractive index layer 2 to the thickness H1 of the optical waveguide 1 was set to 1:2. The lengths of the optical waveguide 1 and the low refractive index layer 2 were set to 24XX μm.

[0045] Figure 8 shows the dependence of polarization conversion efficiency on variations in the width of the optical waveguide 1. Under the third condition, a polarization conversion efficiency of over 90% was achieved with a variation in the width W1 of the optical waveguide 1 in the third region A3 within a range of ±15 μm. Figure 9 shows the wavelength dependence of the polarization conversion efficiency. In this case, the wavelength dependence was further suppressed, and as a result, a polarization conversion efficiency of over 95% was achieved across the entire C band.

[0046] In the third condition, compared to the second condition, the design value of the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was increased. As a result, the range of variation in the width W1 of the optical waveguide 1 that can achieve high polarization conversion efficiency was expanded, and the wavelength dependence of the polarization conversion efficiency was further improved.

[0047] Fourth condition In the fourth condition, the design value for the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was set to 5.5. The ratio of the thickness H2 of the low refractive index layer 2 to the thickness H1 of the optical waveguide 1 was set to 2:1. The lengths of the optical waveguide 1 and the low refractive index layer 2 were set to 35 μm.

[0048] Figure 10 shows the dependence of polarization conversion efficiency on variations in the width of the optical waveguide 1. Under condition 4, a polarization conversion efficiency of 90% or more was achieved when the variation in the width W1 of the optical waveguide 1 in region A3 was within ±15 μm. Figure 11 shows the wavelength dependence of polarization conversion efficiency. In this case, the wavelength dependence was suppressed to a small extent, and as a result, a polarization conversion efficiency of 95% or more was achieved across the entire C band.

[0049] In the fourth condition, the design value for the ratio of the width W2 of the low refractive index layer 2 to the width W1 of the optical waveguide 1 in the third region A3 was significantly increased compared to the third condition. As a result, the fourth condition achieved a polarization conversion efficiency that was roughly the same as that of the third condition.

[0050] As described above, in the polarization conversion element 100, by making the width of the low refractive index layer 2 wider than the width of the optical waveguide 1 in the third region A3, it is possible to significantly suppress fluctuations in polarization conversion efficiency due to variations in the width of the optical waveguide 1. Therefore, the polarization conversion element 100 can suppress fluctuations in polarization conversion efficiency caused by manufacturing variations in the width of the optical waveguide 1.

[0051] Furthermore, as described above, the polarization conversion element 100 can also suppress the wavelength dependence of the polarization conversion efficiency.

[0052] Furthermore, unlike the Si3N4 layer provided at a distance from the optical waveguide as described in Non-Patent Document 1, it is not necessary to have an asymmetric shape with respect to the optical waveguide direction, and a symmetric shape with respect to the optical waveguide direction can be used. This makes it easier to fabricate the low refractive index layer 2. In addition, by making the low refractive index layer 2 a symmetric structure, even if the central axis in the X direction of the optical waveguide 1 and the low refractive index layer 2 is shifted in the Y direction, fluctuations in polarization conversion efficiency due to the shift can be suppressed. This makes it possible to prevent the problem of a decrease in polarization conversion efficiency due to left-right shifts that occurs when using a Si3N4 layer with a structure that is not symmetrical in the left-right (Y direction).

[0053] Other embodiments Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0054] In the embodiments described above, the optical waveguide 1 was described as having a tapered shape in the fourth region A4 and the seventh region A7, but this is merely an example. The optical waveguide 1 may have any shape in which its width changes continuously or in steps, as long as its width can be increased.

[0055] In the embodiments described above, the low refractive index layer 2 was described as having a tapered shape in the fifth region A5 and the sixth region A6, but this is merely illustrative. The optical waveguide 1 may have any shape in which the width changes continuously or stepwise, as long as the width of the low refractive index layer 2 can be increased or decreased.

[0056] Although the second region has been described as being divided into at least a sixth region A6 and a seventh region, this is merely an example. As long as the widths of the optical waveguide 1 and the low refractive index layer 2 increase from the incident side to the exit side, the optical waveguide 1 and the low refractive index layer 2 may have any shape and may be divided into any number of regions.

[0057] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments rather than with only one specific embodiment. As those skilled in the art will understand, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps shown in any of the drawings may be changed as appropriate.

[0058] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0059] (Note 1) An optical waveguide extending in a first direction, with a width direction perpendicular to the first direction in a cross section perpendicular to the first direction, A low refractive index layer having a refractive index lower than that of the optical waveguide is provided, extending in the first direction, with the second direction as the width direction in a cross section perpendicular to the first direction, spaced apart from the optical waveguide in a third direction perpendicular to the first and second directions, and overlapping with a portion of the optical waveguide when viewed along the third direction. The optical waveguide is divided into first to sixth regions along the first direction, from the light incident side to the light exit side. The low refractive index layer is provided so as to extend between the incident end of the second region and the outgoing end of the sixth region with a width wider than the optical waveguide. In the first region, the width of the optical waveguide is such that it selectively guides light in the fundamental mode. In the second region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer increases from the incident side to the exit side of the light. In the third region, the optical waveguide and the low refractive index layer are each the same width as the light-emitting end of the second region. In the fourth region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer is the same as the width in the fourth region. In the fifth region, the width of the optical waveguide is the same as the width at the exit end of the fifth region, and the width of the low refractive index layer decreases from the incident side to the exit side. Polarization conversion element.

[0060] (Note 2) The ratio of the width of the low refractive index layer to the width of the optical waveguide in the fourth region is 1.2 or greater. Polarization conversion element as described in Appendix 1.

[0061] (Note 3) The ratio of the width of the low refractive index layer to the width of the optical waveguide in the fourth region is 1.4 or greater. Polarization conversion element as described in Appendix 2.

[0062] (Note 4) The optical waveguide and the low refractive index layer are arranged such that their central axes in the first direction coincide when viewed along the third direction. A polarization conversion element as described in any one of the appendices 1 to 3.

[0063] (Note 5) The optical waveguide and the low refractive index layer each have a shape that is symmetrical with respect to the central axis of the first direction. Polarization conversion element as described in Appendix 4.

[0064] (Note 6) The width of the low refractive index layer at the incident light end of the second region is such that the reflection due to the incident light is suppressed to a desired value or less. The width of the low refractive index layer at the light-emitting end of the fifth region is such that the reflection due to the incident light is suppressed to a desired value or less. A polarization conversion element as described in any one of the appendices 1 to 5.

[0065] (Note 7) The second region includes a sixth region on the incident side of the light and a seventh region on the outgoing side of the light, In the sixth region, the width of the optical waveguide is the same as the width in the first region, and the width of the low refractive index layer increases from the incident side to the exit side of the light. In the seventh region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer is the same as the width at the exit end of the sixth region. A polarization conversion element as described in any one of the appendices 1 to 6.

[0066] (Note 8) The optical waveguide extends in the fourth and sixth regions such that its width changes continuously. The low refractive index layer extends in the fifth and seventh regions such that its width changes continuously. Polarization conversion element as described in Appendix 7.

[0067] (Note 9) The optical waveguide has a tapered shape in which the width changes continuously in the fourth and sixth regions. The low refractive index layer has a tapered shape in the fifth and seventh regions, in which the width changes continuously. Polarization conversion element as described in Appendix 8.

[0068] (Note 10) A first connecting region is inserted between the sixth region and the seventh region. A second connecting region is inserted between the fourth region and the fifth region. In the first and second connecting regions, the optical waveguide and the low refractive index layer extend with a constant width. A polarization conversion element as described in any one of the appendices 7 to 9.

[0069] (Note 11) A first connecting region is inserted between the sixth region and the seventh region. In the first connecting region, the optical waveguide and the low refractive index layer extend with a constant width. A polarization conversion element as described in any one of the appendices 7 to 9.

[0070] (Note 12) A second connecting region is inserted between the fourth region and the fifth region. In the second connecting region, the optical waveguide and the low refractive index layer extend with a constant width. A polarization conversion element as described in any one of the appendices 1 to 8.

[0071] (Note 13) The light in the aforementioned basic mode is light in both TE0 mode and TM0 mode. A polarization conversion element as described in either Appendix 1 or 2.

[0072] (Note 14) The polarization conversion element is configured as a silicon photonics element. A polarization conversion element as described in any one of the appendices 1 to 13.

[0073] (Note 15) The optical waveguide is made of silicon. The low refractive index layer is made of silicon nitride. Polarization conversion element as described in Appendix 14.

[0074] (Note 16) The optical waveguide and the low refractive index layer are embedded in the cladding layer. Polarization conversion element as described in Appendix 15.

[0075] (Note 17) The cladding layer is made of silicon oxide. Polarization conversion element as described in Appendix 16. [Explanation of symbols]

[0076] 1 Optical waveguide 2 Low refractive index layer 3. Cladding layer 100 Polarization conversion elements A1 First area A2 Second area A3 Third area A4, Fourth Domain A5 Fifth Domain A6 The sixth area A7 The seventh area C1 First connecting region C2 Second connection area L light

Claims

1. An optical waveguide extending in a first direction, with a width direction perpendicular to the first direction in a cross section perpendicular to the first direction, A low refractive index layer having a refractive index lower than that of the optical waveguide is provided, extending in the first direction, with the second direction as the width direction in a cross section perpendicular to the first direction, spaced apart from the optical waveguide in a third direction perpendicular to the first and second directions, and overlapping with a part of the optical waveguide when viewed along the third direction. The optical waveguide is divided into first to fifth regions along the first direction, from the incident side to the exit side of the light. The low refractive index layer is provided so as to extend with a width wider than the optical waveguide between the incident end of the second region and the outgoing end of the fifth region. In the first region, the width of the optical waveguide is such that it selectively guides light in the fundamental mode. In the second region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer increases from the incident side to the exit side of the light. In the third region, the optical waveguide and the low refractive index layer are each the same width as the light-emitting end of the second region. In the fourth region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer is the same as the width in the third region. In the fifth region, the width of the optical waveguide is the same as the width of the fourth region at the exit end of the light, and the width of the low refractive index layer decreases from the incident side to the exit side of the light. Polarization conversion element.

2. The ratio of the width of the low refractive index layer to the width of the optical waveguide in the third region is 1.2 or more. Polarization conversion element according to claim 1.

3. The ratio of the width of the low refractive index layer to the width of the optical waveguide in the third region is 1.4 or more. Polarization conversion element according to claim 2.

4. The optical waveguide and the low refractive index layer are arranged such that their central axes in the first direction coincide when viewed along the third direction. Polarization conversion element according to claim 1 or 2.

5. The optical waveguide and the low refractive index layer each have a shape that is symmetrical with respect to the central axis of the first direction. Polarization conversion element according to claim 4.

6. The width of the low refractive index layer at the incident light end of the second region is such that the reflection due to the incident light is suppressed to a desired value or less. The width of the low refractive index layer at the light-emitting end of the fifth region is such that the reflection due to the incident light is suppressed to a desired value or less. Polarization conversion element according to claim 1 or 2.

7. The second region includes a sixth region on the incident side of the light and a seventh region on the outgoing side of the light. In the sixth region, the width of the optical waveguide is the same as the width in the first region, and the width of the low refractive index layer increases from the incident side to the exit side of the light. In the seventh region, the width of the optical waveguide increases from the incident side to the exit side of the light, and the width of the low refractive index layer is the same as the width at the exit end of the sixth region. Polarization conversion element according to claim 1 or 2.

8. The optical waveguide extends in the fourth and sixth regions such that its width changes continuously. The low refractive index layer extends in the fifth and seventh regions such that its width changes continuously. Polarization conversion element according to claim 7.

9. The optical waveguide has a tapered shape in which the width changes continuously in the fourth and sixth regions. The low refractive index layer has a tapered shape in which the width changes continuously in the fifth and seventh regions. Polarization conversion element according to claim 8.

10. A first connecting region is inserted between the sixth region and the seventh region. A second connecting region is inserted between the fourth region and the fifth region. In the first and second connecting regions, the optical waveguide and the low refractive index layer extend with a constant width. Polarization conversion element according to claim 7.

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