Optical waveguide element

Abstract

To provide an optical waveguide element that can be manufactured easily, separate polarization, and be used in alignment applications.SOLUTION: An optical waveguide element is formed of a ridge part and a terrace part, which is thinner than the ridge part. End parts of the ridge part and the terrace part on the light input side are provided in the same surface that is orthogonal to a light propagation direction. The length of the ridge part is shorter than the length of the terrace part. The ridge part includes a wide part, which is formed to have a larger width or a uniform width along the input light propagation direction, and a narrow part, which is formed to have a narrowing width along the input light propagation direction, in this order along the input light propagation direction. At the end part where the wide part and the narrow part are connected, the width of the wide part and the width of the narrow part are equal. The end part of the narrow part on the narrow side has the width that is more than or equal to the minimum width that can be manufactured. The terrace part is formed to have the larger width along the input light propagation direction, and the end part on the light input side has the width that is more than or equal to the minimum width that can be manufactured.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element, and more particularly to an optical waveguide element that can be used in a system that requires separating and aligning polarized waves, such as an optical communication system. [Background technology]

[0002] In recent years, silicon (Si) photonics has attracted attention as a platform technology for optical waveguide devices. Si photonics features include the compactness and integration of optical waveguides and related optical devices, such as modulators and photodetectors, achieved by utilizing semiconductor manufacturing processes such as CMOS (Complementary Metal Oxide Semiconductor), and high productivity achieved through 200mm or 300mm wafer processes that utilize existing semiconductor manufacturing technologies. Furthermore, Si waveguides, which use Si as the waveguide core and Si oxide (SiO2) cladding, have a relative refractive index difference of up to 40%, providing a high optical confinement effect. In particular, Si nanowire waveguides allow the radius of curvature of bent waveguides and the parallel wiring pitch to be reduced to the order of several microns, enabling the miniaturization of optical circuit layouts.

[0003] One example of an application of Si waveguides is an optical communication system. Optical communication systems must handle optical signals transmitted over long-distance optical fibers, and polarization is often undefined. However, many Si optical waveguide elements that have Si waveguides operate only with specific polarizations. For this reason, an element that separates and aligns polarizations is required, and various elements have been proposed (see, for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Publication 2015-338577 [Patent Document 2] U.S. Patent Publication 2017-068048 [Patent Document 3] U.S. Patent Publication 2022-196813 Summary of the Invention [Problem to be solved by the invention]

[0005] However, all of the elements proposed to date have a rib tapered structure. This rib tapered structure has a portion where the terrace width expands from an infinitesimal width, making it difficult to actually fabricate. As a result, fabrication errors can significantly degrade characteristics such as the extinction ratio and loss.

[0006] However, no element was known that could provide satisfactory characteristics when the terrace width was set to a finite width that could tolerate manufacturing errors. Therefore, there was no element suitable for practical mass production that could separate and align polarization.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical waveguide element that is easy to fabricate and can be used to separate and align polarized waves. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the optical waveguide element of the present invention is an optical waveguide element in which a waveguide core is formed on a lower clad, and the waveguide core is embedded in a clad composed of the lower clad and an upper clad, and is configured with a polarization conversion section.

[0009] The polarization conversion section is composed of a ridge portion and a terrace portion, which is a portion whose thickness in a direction perpendicular to the upper surface of the lower cladding is thinner than that of the ridge portion. The ridge portion and the terrace portion have ends on the light input side that are arranged in the same plane perpendicular to the light propagation direction, and the length of the ridge portion along the light propagation direction is shorter than the length of the terrace portion along the light propagation direction. The ridge portion has a wide portion that is formed to increase in width or have a uniform width along the input light propagation direction, and a tapered portion that is configured to decrease in width along the input light propagation direction, arranged in this order along the input light propagation direction. At the end where the wide portion and the tapered portion are connected, the widths of the wide portion and the tapered portion are equal to each other, and the end where the tapered portion has a narrower width is configured to be equal to or greater than the minimum width that can be fabricated. The terrace portion is configured to increase in width along the input light propagation direction, and is formed with a width equal to or greater than the minimum width that can be fabricated at the end where the light is input.

[0010] According to a preferred embodiment of the optical waveguide element of the present invention, there is provided an input waveguide through which light input to the polarization conversion unit propagates, and the width of the end of the input waveguide connected to the polarization conversion unit is equal to or greater than the width of the ridge portion at the end where light from the polarization conversion unit is input, and is equal to or less than the combined width of the terrace portion and the ridge portion.

[0011] The minimum width that can be produced is, for example, 100 nm.

[0012] According to a further preferred embodiment of the optical waveguide element of the present invention, the optical waveguide element further comprises a polarized beam extraction section, which comprises a TE primary extraction section and a TE fundamental extraction section arranged in series in the longitudinal direction.

[0013] The TE primary extraction section and the TE basic extraction section each include a wide waveguide and a narrow waveguide, wherein the narrow waveguide in the TE primary extraction section and the TE basic extraction section includes an input side section, a tapered section, and an output side section connected in series in this order along the propagation direction, the tapered section is formed so that its width increases along the propagation direction of input light and is disposed close to the wide waveguide, the input side section is connected to an end of the tapered section where the width is narrow and is disposed close to the wide waveguide along the propagation direction, the output side section is connected to an end of the tapered section where the width is wide and is disposed away from the wide waveguide along the propagation direction, the wide waveguide is formed to have the same thickness as the terrace portion, and the narrow waveguide is formed to have the same thickness as the ridge portion.

[0014] In the TE primary extraction section, the tapered section is formed so that the equivalent refractive index differs from that of the TE fundamental mode and the TM fundamental mode, but matches that of the TE primary mode propagating through the wide waveguide near the center in the propagation direction. In the TE primary extraction section, the tapered section is formed so that the equivalent refractive index differs from that of the TM fundamental mode, but matches that of the TE fundamental mode propagating through the wide waveguide near the center in the propagation direction.

[0015] Furthermore, the terrace portion and the wide waveguide portion of the TE primary extraction portion may be connected by a tapered waveguide formed to the same thickness as the terrace portion, and the wide waveguide portion of the TE primary extraction portion and the wide waveguide portion of the TE basic extraction portion may be connected by a tapered waveguide formed to the same thickness as the terrace portion. [Effects of the Invention]

[0016] The optical waveguide element of the present invention is easy to fabricate and can be used to separate and align polarized waves. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram (1) for explaining an optical waveguide element. [Figure 2] FIG. 2 is a schematic diagram (2) for explaining an optical waveguide element. [Figure 3] 10 is a diagram for explaining the operation of the polarization conversion unit, and is a diagram for explaining the relationship between the propagation distance in the polarization conversion unit and the equivalent refractive index in a region where a ridge portion is provided. FIG. [Figure 4] FIG. 1 shows the characteristics of the polarization converter obtained using 3D FDTD. [Figure 5] FIG. 2 shows the characteristics of the polarization converter obtained using 3D FDTD. [Figure 6] FIG. 10 is a diagram showing the characteristics of the polarization extractor obtained using the three-dimensional BPM method. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and positional relationship of each component are merely shown in a schematic manner to enable understanding of the present invention. Furthermore, while preferred configuration examples of the present invention will be described below, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

[0019] (Optical waveguide element) An optical waveguide element according to an embodiment of the present invention will be described with reference to Figures 1 and 2. Figure 1 is a schematic diagram for explaining an optical waveguide element. Figure 1 is a schematic plan view of the optical waveguide element, omitting a support substrate and a lower cladding, which will be described later, and showing only the waveguide core. Figures 2(A) to 2(C) are diagrams showing cut end faces of the optical waveguide element taken along lines AA, BB, and CC, respectively. Note that although some hatching is applied to the plan view of Figure 1, it should be understood that this does not indicate a cross section, but is added to aid in understanding the invention.

[0020] The optical waveguide element includes a lower clad 22 on a support substrate 10 and a waveguide core 30 on the lower clad 22. The first optical waveguide element can be easily manufactured by using, for example, a commercially available SOI (Silicon On Insulator) substrate. The SOI substrate is configured by sequentially stacking a support substrate layer, an SiO2 layer, and an Si layer, and the support substrate layer serves as the support substrate 10. The SiO2 layer serves as the lower clad 22.

[0021] The Si layer is dry-etched or the like to pattern the Si layer, thereby forming the waveguide core 30. As will be described later, the waveguide core 30 has thick and thin portions. For this reason, dry etching is performed, for example, in two stages. In this case, the Si layer is first dry-etched (first time) to pattern it until it reaches the lower cladding 22 so as to obtain the planar shape of the waveguide core 30, and then the patterned Si layer is dry-etched (second time) to form the thin portion. Alternatively, the waveguide core 30 may be formed by thinning the Si layer by dry-etching (first time) in areas other than the areas that will become the thick portions of the waveguide core 30, and then dry-etching (second time) the thinned portion of the Si layer until it reaches the lower cladding 22 so as to obtain the planar shape of the waveguide core 30.

[0022] Thereafter, SiO2 is deposited on the lower clad 22 by a chemical vapor deposition (CVD) method or the like to form the upper clad 24 that covers the waveguide core 30. As a result, the waveguide core 30 is embedded in the clad 20 consisting of the lower clad 22 and the upper clad 24.

[0023] To prevent light propagating through the waveguide core 30 from escaping to the support substrate 10, the distance between the support substrate 10 and the optical waveguide element 30, i.e., the thickness of the lower cladding 22, should be 1 μm or more. Furthermore, the thickness of the waveguide core 30 is preferably 200 to 400 nm, a value that can achieve single-mode conditions in the thickness direction. Typically, a waveguide core 30 with a thickness of 220 nm is used. In the example described here, the thickness of the thick portion is, for example, 220 nm. Furthermore, the difference in height between the top surfaces of the thick and thin portions is typically, for example, 70 nm in a rib waveguide. Therefore, in the example described here, the thickness of the thin portion is 150 nm.

[0024] Light propagates according to the planar shape of the waveguide core 30, achieving the desired function of the optical waveguide element. Light input to the first optical waveguide element propagates in the longitudinal direction of the waveguide core 30. In the following description, the longitudinal direction may also be referred to as the propagation direction. The direction perpendicular to the upper surface of the lower cladding 22 may also be referred to as the thickness direction, and the direction perpendicular to both the propagation direction and the thickness direction may also be referred to as the width direction. The dimension in the width direction may also be referred to as the width, and the dimension in the longitudinal direction may also be referred to as the length.

[0025] The optical waveguide element is configured to include an input waveguide 300, a polarization conversion section 100, and a polarization extraction section 200, which are obtained by the planar shape of a waveguide core 30.

[0026] The light propagating through the input waveguide 300 is input to the polarization conversion unit 100. The polarization conversion unit 100 transmits the TE fundamental mode light as is and converts the TM fundamental mode to the TE first-order mode. The light polarization-converted to a TE wave by the polarization conversion unit 100 is sent to the polarization extraction unit 200. The polarization extraction unit 200 extracts the TE fundamental mode light sent from the polarization conversion unit 100 and the light converted from the TM fundamental mode to the TE first-order mode.

[0027] (Polarization conversion section) The polarization conversion section 100 is configured as a so-called rib waveguide that includes a ridge portion 110 and a terrace portion 120. The ridge portion 110 is a portion that extends along the longitudinal direction (the direction of light propagation) and has a large thickness in a direction perpendicular to the upper surface of the lower cladding 22. The terrace portions 120 are provided on both sides of the ridge portion 110 and have a smaller thickness than the ridge portion 110 in a direction perpendicular to the upper surface of the lower cladding 22. The ends of the ridge portion 110 and the terrace portion 120 on the light input side are provided in the same plane perpendicular to the direction of light propagation.

[0028] The rib waveguide, particularly the terrace portion 120, is configured so that its width increases from the input waveguide 300 side toward the polarization extractor 200 along the longitudinal direction.

[0029] The ridge portion 110 is configured to include, in order from the input waveguide 300 side along the longitudinal direction, a wide portion 112 and a tapered portion 114. The length of the ridge portion 110 in the direction along the light propagation direction is shorter than the length of the polarization conversion unit 100, i.e., the length of the terrace portion 120 in the direction along the light propagation direction, and the ridge portion 110 ends midway along the longitudinal direction of the polarization conversion unit 100. Therefore, the output end does not have the ridge portion 110 and is composed only of the terrace portion 120.

[0030] The wide portion 112 is configured to have a constant width or to increase in width from the input waveguide 300 side toward the tapered portion 114. For example, the width of the end of the wide portion 112 connected to the input waveguide 300 is 300 nm, and the width of the end of the wide portion 112 connected to the tapered portion 114 is 350 nm.

[0031] The tapered portion 114 is configured so that its width becomes thinner (narrower) from the wide portion 112 side toward the polarization extracting section 200. The width of the end of the tapered portion 114 that is connected to the wide portion 112 is equal to the width of the end of the wide portion 112 that is connected to the tapered portion 114, and the width of the end (tip) of the tapered portion 114 on the polarization extracting section 200 side is equal to or greater than the minimum width that can be created. The width of this tip is, for example, 200 nm.

[0032] Here, the width of the rib waveguide (the combined width of the ridge portion 110 and the terrace portion 120) at the end (input end) connected to the input waveguide 300 of the polarization conversion unit 100 is, for example, 500 nm. Also, the width of the rib waveguide (the width of the terrace portion) at the end (output end) on the polarization extracting unit 200 side is 1100 nm.

[0033] As described above, the width of the rib waveguide at the input end is 500 nm, which is larger than the 300 nm width of the ridge portion 110. In this way, there are no areas where the width of the terrace portion 120 is zero, that is, no areas on either side of the ridge portion 110 where the terrace portion 120 is not formed, and the structure is such that the terrace portion 120 is provided on both sides of the ridge portion 110. In addition, the terrace portion 120 is formed with a width equal to or greater than the minimum width that can be created; for example, at the input end, the terrace portion 120 is formed with a width of 100 nm on both sides of the ridge portion 110.

[0034] A tapered structure that has been generally proposed is a structure that gradually decreases to an infinitesimal width that is smaller than the minimum width that can be produced, but it is difficult to actually produce it.

[0035] In contrast, the optical waveguide element of the present invention does not have a structure in which the width gradually decreases to an infinitesimal width in the tapered portion or the like, and even the narrowest portion is equal to or larger than the minimum width that can be produced, making it easy to produce.

[0036] This structure of the polarization conversion unit 100 ensures sufficient width for the terrace portion 120 and the tapered portion 114 for fabrication, and has been found to provide good characteristics through simulations.

[0037] The operation of the polarization conversion unit 100 will be described with reference to Fig. 3. Fig. 3 is a diagram for explaining the operation of the polarization conversion unit 100, and is a diagram for explaining the relationship between the propagation distance and the equivalent refractive index in the polarization conversion unit 100 for the region where the ridge portion 110 is provided. In Fig. 3, the horizontal axis represents the propagation distance, and the vertical axis represents the equivalent refractive index.

[0038] As the propagation distance increases, i.e., as the light propagates toward the polarization extractor 200, the equivalent refractive index of the TE1st-order mode (TE1) increases. On the other hand, as the propagation distance increases, i.e., as the light propagates toward the polarization extractor 200, the equivalent refractive index of the TM fundamental mode (TM0) decreases. If the ridge portion 110 is designed so that the equivalent refractive index of the TE1st-order mode and the equivalent refractive index of the TM fundamental mode are equal near the middle of the propagation direction, polarization conversion occurs between the TE fundamental mode and the TE1st-order mode.

[0039] The equivalent refractive indexes of the TM fundamental mode and the TE first-order mode are closest when the width of the ridge portion 110 is 350 nm and the width of the rib waveguide is 700 nm, near the region where the wide portion 112 and the tapered portion 114 are connected in the longitudinal direction.

[0040] When light in the TE fundamental mode and the TM fundamental mode is input from the input waveguide 300 to the polarization conversion unit 100, the TE fundamental mode passes through the polarization conversion unit 100 as is and is sent as TE polarization to the polarization extraction unit 200 while remaining in the TE fundamental mode. On the other hand, the TM fundamental mode is converted to the TE primary mode by the polarization conversion unit 100 and then sent as TE polarization to the polarization extraction unit 200.

[0041] The input waveguide 300 is provided with a width of, for example, 440 nm, which satisfies the single-mode condition, but the width changes toward the end connected to the polarization conversion unit 100. The input waveguide 300 is generated as a thick portion with the same thickness as the ridge portion.

[0042] FIG. 1 shows an example in which the width of the end of the input waveguide 300 connected to the polarization conversion unit 100 is 300 nm, which is the width of the end of the ridge portion 110 connected to the input waveguide 300. In this case, the width of the input waveguide 300 narrows toward the end connected to the polarization conversion unit 100. Note that the width of the end of the input waveguide 300 connected to the polarization conversion unit 100 may be smaller than the width of the end of the rib waveguide connected to the input waveguide 300 and larger than the width of the end of the ridge portion 110 connected to the input waveguide 300, for example, 450 nm. In this case, the width of the input waveguide 300 widens toward the end connected to the polarization conversion unit 100. Note that the input waveguide 300 may be connected to the polarization conversion unit 100 while maintaining its width of 440 nm, which satisfies the single-mode condition.

[0043] (Polarized wave extraction section) The polarized wave extraction section 200 is configured to include a TE primary extraction section 210 and a TE basic extraction section 240. In this configuration example, the TE primary extraction section 210 and the TE basic extraction section 240 are arranged in series in the longitudinal direction, and the TE primary extraction section 210 is arranged on the polarization conversion section 100 side.

[0044] The TE primary extraction section 210 is configured to include a wide waveguide 220 and a tapered section 232 of a narrow waveguide 230. The wide waveguide 220 is connected to the terrace section 120 of the polarization conversion section 100 and is formed as a thin section with the same thickness as the terrace section 120. When the width of the wide waveguide 220 and the width of the polarization conversion section 100 are different, the polarization conversion section 100 and the wide waveguide 220 are connected by a tapered waveguide 402, one of which is the width of the wide waveguide 220 and the other of which is the width of the polarization conversion section 100. The tapered waveguide 402 is formed to have the same thickness as the terrace section 120.

[0045] The narrow waveguide 230 is configured by connecting an input side portion 234, a tapered portion 232, and an output side portion 236 in series in this order along the propagation direction. The thickness of the narrow waveguide 230 is the same as that of the ridge portion 110, and is formed as a thick portion.

[0046] Tapered section 232 of thin-wire waveguide 230 is disposed close to wide waveguide 220. Tapered section 232 of thin-wire waveguide 230 has a structure in which its width increases along the propagation direction. Tapered section 232 of thin waveguide 230 is designed so that its equivalent refractive index does not match that of the TE fundamental mode and the TM fundamental mode, but matches that of the TE first-order mode propagating in wide waveguide 220 near the center of the propagation direction.

[0047] The output side 236 of the thin-wire waveguide 230 is connected to the wider end of the tapered section 232 and is positioned away from the wide waveguide 220 along the propagation direction.

[0048] The input side 234 of the thin-wire waveguide 230 is connected to the narrow end of the tapered section 232 and is positioned so as to approach the wide waveguide 220 along the propagation direction.

[0049] With this configuration, the TE primary mode can be extracted from the TE primary extraction section 210 via the output side section 236.

[0050] If the thin-wire waveguide 230 does not have the input side portion 234, the equivalent refractive index of the light propagating through the wide waveguide 220 of the TE primary extraction section 210 changes suddenly, which may cause loss. To suppress the sudden change in the equivalent refractive index, it is preferable to provide the thin-wire waveguide 230 with the input side portion 234.

[0051] The TE basic extraction section 240 is configured to include a wide waveguide 250 and a tapered section 262 of a narrow waveguide 260. The wide waveguide 250 is connected to the wide waveguide 220 of the TE primary extraction section 210 and is formed as a thin section with the same thickness as the terrace portion 120. Note that when the width of the wide waveguide 220 of the TE primary extraction section 210 and the width of the wide waveguide 250 of the TE basic extraction section 240 are different, the wide waveguide 220 of the TE primary extraction section 210 and the wide waveguide 250 of the TE basic extraction section 240 are connected by a tapered waveguide 404, one of which has the width of the wide waveguide 220 of the TE primary extraction section 210 and the other has the width of the wide waveguide 250 of the TE basic extraction section 240. The tapered waveguide 404 is formed to have the same thickness as the terrace portion 120.

[0052] The narrow waveguide 260 of the TE basic extraction section 240 is configured by connecting an input side section 264, a tapered section 262, and an output side section 266 in series in this order along the propagation direction, similar to the narrow wire waveguide 230 of the TE primary extraction section 210. The thickness of the narrow waveguide 260 is the same as that of the ridge section 110, and is formed as a thick section.

[0053] The tapered portion 262 of the thin-wire waveguide 260 is disposed adjacent to the wide waveguide 250. The tapered portion 262 of the thin-wire waveguide 260 has a structure in which the width increases along the propagation direction.

[0054] The tapered portion 262 of the narrow waveguide 260 is designed so that its equivalent refractive index does not match that of the TM fundamental mode, but matches that of the TE fundamental mode propagating in the wide waveguide near the center of the propagation direction.

[0055] The output side 266 of the thin-wire waveguide 260 is connected to the wider end of the tapered section 262 and is positioned away from the wide waveguide 250 along the propagation direction.

[0056] The input side 264 of the thin-wire waveguide 260 is connected to the narrow end of the tapered section 262 and is positioned so as to approach the wide waveguide 250 along the propagation direction.

[0057] With this configuration, the TE fundamental mode can be extracted from the TE fundamental extraction section 240 via the output side section 226.

[0058] If the thin-wire waveguide 260 does not have the input side portion 264, the equivalent refractive index of the light propagating through the wide waveguide 250 of the TE basic extraction section 240 changes suddenly, which may cause loss. To suppress the sudden change in the equivalent refractive index, it is preferable to provide the thin-wire waveguide 260 with the input side portion 264.

[0059] The TM fundamental mode that is not converted to the TE primary mode in the polarization conversion section 100 propagates directly through the wide waveguides 220 and 250 of the TE primary extraction section 210 and the TE basic extraction section 240, and is output from the end of the wide waveguide 250 of the TE basic extraction section 240.

[0060] The simulation results will be explained with reference to FIGS.

[0061] Figures 4 and 5 are diagrams showing the characteristics of the polarization conversion unit obtained using three-dimensional FDTD (Finite Difference Time Domain). Figure 4 shows the characteristics when the width of the input waveguide connected to the polarization conversion unit is 300 nm, and Figure 5 shows the characteristics when the width of the input waveguide connected to the polarization conversion unit is 450 nm. In Figures 4(A) and (B) and Figures 5(A) and (B), the horizontal axis shows wavelength (unit: nm) and the vertical axis shows light intensity (unit: dB) output from the polarization conversion unit.

[0062] Figures 4(A) and 5(A) show the light intensity when the TE fundamental mode is transmitted as the TE fundamental mode (curve I). Figures 4(B) and 5(B) show the light intensity when the TM fundamental mode is transmitted as the TE first-order mode (curve II) and the light intensity when the TM fundamental mode is transmitted as the TM fundamental mode (curve III).

[0063] The conditions other than the width of the input waveguide are the same for Figures 4 and 5. At the end connected to the input waveguide, the width of the ridge portion is 300 nm, and the width of the terrace portion is 100 nm, i.e., the width of the rib waveguide is 500 nm. The thickness of the terrace portion is 150 nm, and the thickness of the ridge portion is 220 nm.

[0064] The width of the terrace portion at the end on the polarized light extraction section side is 1100 nm. The width of the ridge portion increases to 350 nm at the wide portion and is 200 nm at the tapered portion at the end on the polarized light extraction section side.

[0065] The equivalent refractive indexes of the TM fundamental mode and the TE first-order mode are closest in the vicinity of the region where the wide portion and the tapered portion are connected, where the width of the ridge portion is 350 nm and the width of the rib waveguide is 700 nm.

[0066] When the width of the input waveguide connected to the polarization conversion section is 300 nm, as shown in Figure 4(A), the TE fundamental mode exhibits loss at long wavelengths, but the loss is within -0.5 dB and can be said to be transmitted as is. Also, as shown in Figure 4(B), the TM fundamental mode is converted to the TE first-order mode, and the residual amount is suppressed to approximately -15 dB or less in the wide range from 1300 to 1560 nm.

[0067] On the other hand, when the width of the input waveguide connected to the polarization converter is 450 nm, the loss of the TE fundamental mode is suppressed up to long wavelengths, as shown in Figure 5(A). Also, as shown in Figure 5(B), the residual amount of the TM fundamental mode is slightly increased compared to Figure 4(B), but good characteristics are obtained overall.

[0068] Figure 6 shows the characteristics of the polarization extraction unit obtained using the three-dimensional BPM (Beam Propagation Method). Figures 6(A) and (B) show the characteristics of the TE primary extraction unit, and Figures 6(C) and (D) show the characteristics of the TE basic extraction unit.

[0069] Figures 6(A) and (B) show the cross-sectional intensity distribution at the end of the TE primary extraction section. Figure 6(A) shows the TM fundamental mode, and Figure 6(B) shows the TE primary mode. The lengths of the wide and narrow waveguides in the TE primary extraction section are 100 μm, the width and thickness of the wide waveguide are 900 nm and 150 nm, respectively, and the thickness of the narrow waveguide is 220 nm. The planar shape of the narrow waveguide is tapered, with the width gradually increasing from 305 nm to 375 nm. The spacing between the wide and narrow waveguides is 300 nm.

[0070] 6A, which shows the TM fundamental mode, the intensity is high in a range of about 1 μm wide centered around 0 μm on the X axis. This shows that the TM fundamental mode remains in the wide waveguide 220.

[0071] On the other hand, in Figure 6(B) showing the TE1 mode, the intensity is high in a range of about 0.6 μm wide centered around -1.4 μm on the X axis. Therefore, it can be seen that the TE1 mode shifts to the narrow waveguide.

[0072] Figure 6(C) shows the cross-sectional intensity distribution at the end of the TE fundamental extraction section. Figure 6(C) shows the TM fundamental mode. Figure 6(D) is a top view of the TE fundamental extraction section, showing the TE fundamental mode. The lengths of the wide and narrow waveguides in the TE fundamental extraction section are 100 μm, the width and thickness of the wide waveguide are 400 nm and 150 nm, respectively, and the thickness of the narrow waveguide is 220 nm. The planar shape of the narrow waveguide is tapered, gradually widening from 305 nm to 375 nm. The spacing between the wide and narrow waveguides is 300 nm. Note that in Figures 6(C) and (D), for simulation purposes, the narrow waveguide is positioned to the left of the wide waveguide, unlike the structure shown in Figure 1. In other words, the left and right are reversed.

[0073] 6C, which shows the TM fundamental mode, the intensity is high in a range of about 0.8 μm wide centered around 0 μm on the X axis. This shows that the TM fundamental mode remains in the wide waveguide 250.

[0074] 6(D), which shows the fundamental TE mode, the intensity is high around 0 μm on the X axis up to about 20 μm on the Z axis, and the intensity is high around -0.7 μm on the X axis beyond about 20 μm on the Z axis. This shows that the fundamental TE mode shifts to the narrow waveguide 260.

[0075] In this way, the optical waveguide element described above is easy to fabricate by making the width of each component equal to or greater than the minimum width possible, and realizes both the polarization conversion function and the polarization extraction function. Combining the polarization conversion function and the polarization extraction function realizes an element that has the function of separating and aligning polarized waves. [Explanation of symbols]

[0076] 10 Support substrate 20 Clad 22 Lower Cladding 24 Upper Cladding 30 Waveguide core 100 Polarization conversion section 110 Ridge area 112 Wide section 114 Tapered part 120 Terrace area 200 Polarized wave extraction section 210 TE primary extraction section 220, 250 wide waveguide 230, 260 thin wire waveguide 232, 262 tapered section 234, 264 Input side 236, 266 Output side 240 TE basic extraction part 300 Input waveguide 402, 404 Tapered waveguide

Claims

1. An optical waveguide element in which a waveguide core is formed on a lower clad, and the waveguide core is embedded in a clad composed of the lower clad and an upper clad, A polarization conversion unit is provided, the polarization conversion section is composed of a ridge section and a terrace section, the terrace section being a section whose thickness in a direction perpendicular to the upper surface of the lower cladding is thinner than that of the ridge section, the ends of the ridge portion and the terrace portion on the light input side are provided in the same plane perpendicular to the light propagation direction, The length of the ridge portion along the light propagation direction is shorter than the length of the terrace portion along the light propagation direction, The ridge portion has a wide portion formed to widen or have a uniform width along the propagation direction of the input light, and a tapered portion configured to narrow along the propagation direction of the input light, arranged in that order along the propagation direction of the input light, and at the end where the wide portion and the tapered portion are connected, the widths of the wide portion and the tapered portion are equal to each other, and the end on the narrower side of the tapered portion is configured to have a width equal to or greater than the minimum width that can be produced, The terrace portion is configured so that its width increases along the propagation direction of the input light, and is formed with a width equal to or greater than the minimum width that can be created at the end portion on the light input side. Optical waveguide element.

2. an input waveguide through which light input to the polarization conversion unit propagates; The width of the end of the input waveguide connected to the polarization conversion unit is equal to or greater than the width of a ridge portion at the end where light from the polarization conversion unit is input, and is equal to or less than the combined width of the terrace portion and the ridge portion. The optical waveguide element according to claim 1 .

3. The minimum width that can be produced is 100 nm. The optical waveguide element according to claim 1 .

4. A polarization extractor is provided, the polarization extraction unit includes a TE primary extraction unit and a TE fundamental extraction unit arranged in series in a longitudinal direction, the TE primary extraction section and the TE basic extraction section each include a wide waveguide and a narrow waveguide; In the TE primary fetch unit and the TE basic fetch unit, the narrow waveguide is configured by connecting an input side portion, a tapered portion, and an output side portion in series in this order along a propagation direction; the tapered portion is formed so as to widen in a propagation direction of input light, and is disposed adjacent to the wide waveguide; the input side portion is connected to the narrow end of the tapered portion and is positioned so as to approach the wide waveguide along the propagation direction; the output section is connected to the wider end of the tapered section and is positioned away from the wide waveguide along the propagation direction; The wide waveguide is formed to the same thickness as the terrace portion, and the narrow waveguide is formed to have the same thickness as the ridge portion; In the TE primary extraction section, the tapered section is formed so that an equivalent refractive index does not match that of the TE fundamental mode and the TM fundamental mode, but matches that of the TE primary mode propagating in the wide waveguide near the center in the propagation direction, In the TE fundamental extraction section, the tapered section is formed so that the equivalent refractive index does not match that of the TM fundamental mode, but matches that of the TE fundamental mode propagating through the wide waveguide near the center in the propagation direction. The optical waveguide element according to any one of claims 1 to 3.

5. the terrace portion and the wide waveguide portion of the TE primary extraction portion are connected by a tapered waveguide formed to the same thickness as the terrace portion, The wide waveguide portion of the TE primary extraction portion and the wide waveguide portion of the TE basic extraction portion are connected by a tapered waveguide formed to the same thickness as the terrace portion.

5. The optical waveguide element according to claim 4.

Images