WO2014208601A1 - 高次偏波変換素子、光導波路素子、及びdp-qpsk変調器 - Google Patents
高次偏波変換素子、光導波路素子、及びdp-qpsk変調器 Download PDFInfo
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- WO2014208601A1 WO2014208601A1 PCT/JP2014/066846 JP2014066846W WO2014208601A1 WO 2014208601 A1 WO2014208601 A1 WO 2014208601A1 JP 2014066846 W JP2014066846 W JP 2014066846W WO 2014208601 A1 WO2014208601 A1 WO 2014208601A1
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/126—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind using polarisation effects
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/105—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type having optical polarisation effects
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
- G02B6/1228—Tapered waveguides, e.g. integrated spot-size transformers
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/27—Optical coupling means with polarisation selective and adjusting means
- G02B6/2726—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/27—Optical coupling means with polarisation selective and adjusting means
- G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
- G02B6/2766—Manipulating the plane of polarisation from one input polarisation to another output polarisation, e.g. polarisation rotators, linear to circular polarisation converters
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B2006/12035—Materials
- G02B2006/12038—Glass (SiO2 based materials)
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B2006/12035—Materials
- G02B2006/12061—Silicon
Definitions
- the present invention relates to a substrate-type optical waveguide element used in, for example, optical fiber communication, and more particularly to a high-order polarization conversion element that performs polarization conversion, an optical waveguide element, and a DP-QPSK modulator.
- This application claims priority based on Japanese Patent Application No. 2013-135490 filed in Japan on June 27, 2013, the contents of which are incorporated herein by reference.
- optical circuit components such as a transceiver increases as the amount of information transmitted by optical communication increases. For this reason, in order to increase the number of optical circuit components in a limited space, it is necessary to reduce the size of optical elements constituting the optical circuit components and to achieve high density integration.
- Optical circuit components optical modulators
- silicon-based substrate-type optical waveguides silicon optical waveguides
- the silicon optical waveguide uses a silicon-based material (Si, Si 3 N 4 etc.) with a large refractive index for the core, and a material (SiO 2 , air, Si 3 N 4 etc.) with a large refractive index difference from the core for the clad.
- This is an optical waveguide having a large so-called relative refractive index difference.
- the relative refractive index difference is large, the confinement of light in the core becomes large, so that sharp bending is possible, which is suitable for downsizing and high-density integration of optical elements.
- a substrate-type optical waveguide has an asymmetric shape in the width direction parallel to the substrate and the height direction perpendicular to the substrate.
- TE mode a mode having substantially only an electric field component in the width direction
- TM mode a mode having substantially only an electric field component in the height direction
- the characteristics such as the effective refractive index are different from the polarization mode.
- the basic TE mode (TE 0 ) and the basic TM mode (TM 0 ) are often used.
- TE 0 is a mode having the largest effective refractive index among the TE modes.
- TM 0 is a mode having the largest effective refractive index among TM modes.
- the TE 0 is used as input light to optimized desired planar optical waveguide device with respect to TE 0, and a method of polarization converting the output into TM 0.
- polarization conversion refers to conversion from TE 0 to TM 0 or from TM 0 to TE 0 .
- a substrate type optical waveguide element that performs polarization conversion on the substrate is required.
- TE 1 represents a TE mode having the second highest effective refractive index. Since TE 1 has an electric field component in the same direction as TE 0 , it can be converted by using a directional coupler that can be manufactured only by a simple process such as paralleling rectangular optical waveguides. Therefore, if an element that converts TE 1 to TM 0 is realized, polarization conversion can be performed via TE 1 .
- a silicon optical waveguide has a large birefringence and thus has a strong polarization dependency. For example, when TE 0 and TM 0 are input to the optical element, the characteristics of the optical element are greatly different. In order to solve this problem, a polarization diversity system in which the same mode is input to the optical element using a polarization conversion element that converts TM 0 to TE 0 (or vice versa) is used. Therefore, a small polarization conversion element is essential for downsizing and high-density integration of optical elements. As a polarization conversion element technology using a silicon optical waveguide, a method of converting TE 0 into TE 1 and then converting TE 1 into TM 0 has been proposed.
- Non-Patent Document 1 is cited as a technique for performing polarization conversion using such conversion between TE 1 and TM 0 (hereinafter referred to as high-order polarization conversion) on a substrate-type optical waveguide.
- FIG. 2 (a) and FIG. An example is shown in FIG.
- the optical waveguide device disclosed in Non-Patent Document 1 includes a directional coupler portion (coupling portion) and a tapered optical waveguide portion (tapered portion), and has a structure in which the output end of the coupling portion is connected to the tapered portion.
- the coupling portion is a substrate type optical waveguide device that converts TE 0 into TE 1 and the taper portion converts TE 1 into TM 0 .
- FIG. 1 (a) and FIG. 1 (c) show a rectangular part called the core, a horizontal lower clad located at the bottom of the core and having a refractive index lower than that of the core, and an upper clad covering a core having a refractive index lower than that of the core and different from the lower clad. ing.
- FIG. 1 (a) and FIG. 1 (c) shows a graph of the effective refractive index with respect to the core width.
- FIG. 1 (b) shows a graph of the effective refractive index of an optical waveguide having a vertically symmetrical refractive index cross section in which the upper cladding and the lower cladding have the same refractive index.
- the graph showing the change in the effective refractive index of each mode with respect to the change in the width direction has a refractive index cross-sectional structure.
- the degenerated points TE 1 and TM 0 are separated.
- FIG. The graph of 1 (a) shows that the mode with the second highest effective refractive index is from TM 0 (basic TM mode) to TE 1 (higher order TE mode) while the waveguide width is increased near the waveguide width of 0.7 ⁇ m.
- TE 1 and TM 0 are continuously connected in the form of an effective refractive index curve, so that it is possible to perform high-order polarization conversion with a small loss by gently changing the waveguide width.
- This phenomenon is utilized in the taper portion of the above-described polarization conversion element, and by adopting a taper structure that gradually changes in a range in which the waveguide width is converted from TE 1 to TM 0 , a higher-order polarization is obtained. Wave conversion is performed.
- Non-Patent Document 2 discloses that high-order polarization conversion is performed by using the same material (SiO 2 ) for the upper and lower clads and making the cross-sectional structure of the core asymmetric in the vertical direction.
- Non-Patent Document 2 is shown in FIG. 11 etc. discloses a high-order polarization conversion element in which one end of the cross section of the input / output section has a cross section structure of a rib waveguide and the other end has a cross section structure of a rectangular waveguide.
- Non-Patent Document 1 discloses that materials having different refractive indexes in the upper clad and the lower clad are required in the tapered portion that performs higher-order polarization conversion. When such a new material is used, an extra process is required and a material that is not originally used in another optical waveguide portion is required. Therefore, it is disadvantageous in terms of efficiency and cost.
- the lower clad is a material used in the optical waveguide, and there is a method in which the upper clad material is air.
- the optical waveguide is exposed in the manufacturing process, the characteristics are deteriorated due to the adhesion of foreign substances, and the yield is lowered.
- Non-Patent Document 2 since the cladding region in the width direction of the rib waveguide is narrow, the light confinement in the width direction becomes weak, and a large loss is caused in the bending waveguide portion due to the steep bending radius. Sometimes. Therefore, when a rib waveguide is used, it is necessary to increase the bending radius (several tens to several hundreds ⁇ m), and it is difficult to achieve high-density integration with the structure disclosed in Non-Patent Document 2. That is, when aiming at high-density integration of the optical elements in the optical circuit component, it is necessary to connect the optical elements with a rectangular waveguide whose width direction is sufficiently covered with the clad.
- Non-Patent Document 2 Even if the conversion structure from the rib waveguide to the rectangular waveguide is combined with the end portion of the rib waveguide disclosed in Non-Patent Document 2, a rectangular-rib conversion section is required. Therefore, it is difficult to reduce the size of the optical element.
- the present invention has been made in view of the above circumstances, and is capable of performing polarization conversion between TE 1 and TM 0 without having different refractive indexes between the upper clad and the lower clad. It is an object of the present invention to provide a polarization conversion element and an optical waveguide element, and to provide a high-order polarization conversion element and an optical waveguide element that achieve both miniaturization and high density integration.
- a high-order polarization conversion device is provided with a substrate, a lower clad provided on the substrate, a lower clad provided on the lower clad, and having a constant rectangular cross section.
- the core can guide light from a start portion where the width of the lower core and the width of the upper core are the same to an end portion where the width of the lower core and the width of the upper core are the same. And at least one of the width of the upper core and the width of the lower core continuously decreases with respect to the light guiding direction between the start portion and the end portion.
- both the width of the upper core and the width of the lower core do not increase from the start portion to the end portion, and at the start portion, the effective refractive index of TE 0 is larger than the effective refractive index of TE 1 , and the TE 1 the effective refractive index is greater than the effective refractive index of the TM 0, at the end portion of the optical waveguide, greater than the effective refractive index of the TE 0 is the effective refractive index of the TM 0, the effective refractive index of the TM 0 greater than but effective refractive index of the TE 1
- the core has a vertically asymmetric structure in which the width of the upper core and the width of the lower core are different.
- the high-order polarization conversion element performs high-order polarization conversion between TE 1 at the start portion and TM 0 at the end portion.
- the width of the lower core is always larger than the width of the upper core, and the lower side of the upper core is the lower core in a cross section perpendicular to the direction in which the light is guided. It may always be included on the top side.
- both ends in the width direction of the upper core may not always overlap with both ends in the width direction of the lower core, respectively.
- One end in the width direction of the upper core may always overlap one end in the width direction of the lower core between the start portion and the end portion.
- the height of the core is 210 nm or more and 230 nm or less
- the width of the core is 700 nm or more
- the height of the core is 210 nm or more and 230 nm or less
- the width may be 620 nm or less.
- the lower core and the upper core may be made of Si, and the lower clad and the upper clad may be made of SiO2.
- the width of the upper core decreases from the start part to the intermediate part and is constant from the intermediate part to the end part
- the width of the lower core is constant from the start part to the intermediate part, and from the intermediate part to the end It may be reduced up to a part.
- An optical waveguide device includes the above-described high-order polarization conversion device, the first optical waveguide to which the high-order polarization conversion device is not connected, and the high-order polarization conversion.
- a directional coupler composed of a second optical waveguide connected to the start portion of the element. Further, TE 0 is guided in the first optical waveguide, TE 1 is guided in the second optical waveguide, and TE 0 of the first optical waveguide is TE of the second optical waveguide. 1 can be combined.
- the difference between the effective refractive index of TE 0 of the first optical waveguide and the effective refractive index of TE 0 of the second optical waveguide may be 0.2 or more.
- a DP-QPSK modulator includes the optical waveguide element.
- the core of the optical waveguide is composed of an upper core and a lower core having different widths, and has a core shape having asymmetry in the vertical direction, whereby different refractive indexes are obtained in the upper clad and the lower clad. Even if it is not provided, it is possible to perform high-order polarization conversion.
- FIG. 4B is a cross-sectional view taken along the line IVb-IVb in FIG. 4A.
- FIG. 4B is a sectional view taken along line IVc-IVc in FIG. 4A.
- FIG. 4B is a sectional view taken along line IVd-IVd in FIG. 4A.
- FIG. 6B is a sectional view taken along line VIb-VIb in FIG. 6A.
- FIG. 6B is a sectional view taken along line VIc-VIc in FIG. 6A. It is a top view of the core of the modification of the 1st or 2nd embodiment of the high-order polarization conversion element concerning the embodiment of the present invention.
- FIG. 9B is a cross-sectional view taken along line IXb-IXb in FIG. 9A.
- FIG. 9B is a cross-sectional view taken along line IXc-IXc in FIG. 9A.
- FIG. 11B is a cross-sectional view taken along line XIb-XIb in FIG. 11A.
- FIG. 11B is a sectional view taken along line XIc-XIc in FIG. 11A.
- FIG. 1 It is a top view of the core of the example of a modification of the high order polarization conversion element concerning a 5th embodiment of the present invention. It is a top view of the core of the example of a modification of the high order polarization conversion element concerning a 5th embodiment of the present invention. It is a top view of the core of the example of a modification of the high order polarization conversion element concerning a 5th embodiment of the present invention.
- the top view of the core of an example of the polarization conversion element which combined the high-order polarization conversion element which concerns on embodiment of this invention with the asymmetrical directional coupler, (b) is sectional drawing which follows the XIIIb-XIIIb line
- 13B is a sectional view taken along line XIIIb-XIIIb in FIG. 13A. It is a top view of the core of another example of the polarization conversion element which combined the high-order polarization conversion element which concerns on embodiment of this invention with the asymmetrical directional coupler. It is sectional drawing of the core of the asymmetrical directional coupler of another example of the polarization conversion element which combined the high-order polarization conversion element which concerns on embodiment of this invention with the asymmetrical directional coupler. It is a schematic diagram which shows an example of a DP-QPSK modulator. It is a schematic diagram which shows an example of a polarization diversity coherent receiver. It is a schematic diagram which shows an example of a polarization diversity system.
- FIG. 6 is a graph showing a change in effective refractive index with respect to the lower base in Calculation Example 1.
- (A) to (d) are calculation examples 1 and show electric field amplitudes when the lower base is 0.5 ⁇ m.
- (A) to (d) are calculation examples 1 and show electric field amplitudes when the lower base is 0.6 ⁇ m.
- (A) to (d) are calculation examples 1 and show electric field amplitudes when the lower base is 0.8 ⁇ m.
- (A) to (d) are calculation examples 1 and show electric field amplitudes when the lower base is 1.2 ⁇ m.
- 4 is a graph showing a change in effective refractive index with respect to a Z coordinate in Example 1.
- (A)-(d) is a figure which shows the electric field amplitude of a start part cross section in Example 1, respectively.
- 6 is a graph of wavelength dependence of conversion loss obtained in Example 1. It is the simulation result which calculated
- 10 is a graph showing changes in effective refractive index in the structure of Example 3.
- 6 is a graph showing changes in effective refractive index in the structure of Example 4;
- 10 is a graph showing changes in effective refractive index in the structure of Example 5.
- 10 is a graph showing the wavelength dependence of polarization conversion loss in the structure of Example 7.
- 10 is a graph showing the wavelength dependence of TE 0 transmission loss in the structure of Example 7.
- FIG. 33 is a plan view illustrating an example of a DP-QPSK modulator including the conversion multiplexing element in FIG. 32.
- FIG. 33 is a plan view showing another example of a DP-QPSK modulator including the conversion multiplexing element of FIG. 32.
- the conversion multiplexing element of Example 8 it is a figure which shows the simulation result of (a) mode distribution of the fundamental TE mode in the cross section of an input part, and (b) mode distribution of the high order TE mode in the cross section of a multiplexing part.
- FIG. 5 is an image diagram of embedding in a tapered waveguide when the configuration shown in FIGS. 4A to 4D is used. It is a figure which shows the example of a design of the high-order polarization conversion part using embodiment which concerns on this invention. It is a figure which shows the example of a design of the high-order polarization conversion part using embodiment which concerns on this invention. It is a figure which shows the example of a design of the high-order polarization conversion part using embodiment which concerns on this invention. It is a figure which shows the example of a design of the high-order polarization conversion part using embodiment which concerns on this invention.
- FIG. 10 It is a figure which shows the example of a design of the high-order polarization conversion part using embodiment which concerns on this invention. It is a figure which shows the conversion multiplexing element with which the side wall roughness produced. It is a figure which shows the conversion multiplexing element with which the side wall roughness produced. It is a figure which shows the structure of the high-order polarization conversion element of Example 10. FIG. It is a figure which shows the structure of the high-order polarization conversion element of Example 10. FIG. It is a graph which shows the effective refractive index of the mode which guides the high-order polarization conversion element of Example 10. R TE mode guided through higher polarization conversion element of Example 10 is a graph showing the R TM.
- FIGS. 1A to 2B schematically illustrate the structure of the high-order polarization conversion device of the present invention.
- these high-order polarization conversion elements are composed of a substrate type optical waveguide device including an optical waveguide 1 having a core 2 and a clad 5 on a substrate S.
- the shape of the core 2 is such that the core shape is formed by overlapping rectangles having two widths in a cross section perpendicular to the waveguide direction.
- cross section refers to a cross section perpendicular to the light guiding direction.
- the upper rectangular section is called the upper core 3
- the lower rectangular section is called the lower core 4
- the lower core 4 and the lower cladding 7 are in contact with the lower bottom 4 a, and the upper portion of the upper core 3.
- the upper core 3 is made of the same material as the lower core 4.
- the clad 5 has a lower clad 7 provided between the substrate S and the core 2, and an upper clad 6 provided on the core 2 and the lower clad 7. That is, the core 2 having the lower core 4 and the upper core 3 is provided on the lower clad 7 provided on the substrate S. Further, an upper clad 6 is provided on the core 2 and the lower clad 7.
- FIGS. 1B and 2B An example of the core shape of the optical waveguide having such a cross section is shown in FIGS. 1B and 2B.
- 1B is an example of the core shape of the optical waveguide having the cross section of FIG. 1A
- FIG. 2B is an example of the core shape of the optical waveguide having the cross section of FIG. 2A.
- TE 1 (higher order) has an effective refractive index of TE 0 (basic TE mode) at the start portion 8 of the optical waveguide.
- the core width at the end portion 9 is preferably smaller than the core width at the start portion 8.
- the width of the upper core 3 at the end portion 9 is smaller than the width of the upper core 3 at the start portion 8 and (2) the width of the lower core 4 at the end portion 9 is the width of the lower core 4 at the start portion 8. It is preferable to satisfy at least one of smaller.
- the higher-order polarization conversion element in order for the higher-order polarization conversion element to be an element that performs polarization conversion between the TE 1 of the start unit 8 and the TM 0 of the end unit 9, it is between the start unit 8 and the end unit 9.
- the core shape of the optical waveguide has a vertically asymmetric structure in which the width of the upper core and the width of the lower core are different, and the width of the upper core 3 or the width of the lower core 4 is continuous in the light guiding direction of the optical waveguide. It is necessary to change.
- the width of the upper core 3 continuously and sufficiently changes between the start portion 8 and the end portion 9, and (4) the start portion 8 and the end portion 9 It is preferable to satisfy at least one of having a structure in which the width of the lower core 4 changes continuously and sufficiently slowly.
- “continuously decreasing” is not limited to the case where the width continuously decreases from the start portion 8 to the end portion 9 like the width of the upper core 3 and the width of the lower core 4 in FIG. The case where there is a portion having a constant width such as the width of the upper core 3 and the width of the lower core 4 is also included.
- this high-order polarization conversion element functions as an element that performs polarization conversion (high-order polarization conversion) between TE 1 of the start unit 8 and TM 0 of the end unit 9.
- TM 0 is input to the end unit 9
- TE 1 is output from the start unit 8.
- FIGS. 1A and 1B one end in the width direction of the upper core 3 and one end in the width direction of the lower core 4 overlap each other, and a step is formed at the opposite end.
- both ends of the upper core 3 in the width direction do not overlap with both ends of the lower core 4 in the width direction, and there are steps on the left and right ends.
- the vertically asymmetric cross-sectional structure is not limited to FIGS. 1A to 2B, and may be a structure having one or two or more steps in total in arbitrary corners on the upper right, lower right, upper left, and lower left of the cross-sectional view. . As shown in FIGS.
- the vertically asymmetric structure is not limited to a structure in which the width of the upper core 3 is smaller than the width of the lower core 4, and a structure in which the width of the upper core 3 is larger than the width of the lower core 4.
- the polarization conversion efficiency is higher and the taper length, which is the length of the element, can be shortened when the upper core 3 is closer to the center position in the width direction of the lower core 4.
- the taper length which is the length of the element
- the upper core 3 and the lower core 4 are preferably made of the same material.
- both the upper core 3 and the lower core 4 can be made of Si.
- the core Si may contain intentional impurities (dopants) or inevitable impurities.
- high-order polarization conversion element of the present embodiment high-order polarization conversion can be performed even if the upper cladding 6 and the lower cladding 7 do not have different refractive indexes. For this reason, even if the materials of the upper clad and the lower clad are the same, high-order polarization conversion which is conversion from TE 1 to TM 0 and conversion from TM 0 to TE 1 is performed on the optical waveguide. It is possible.
- SiO 2 is used as the material of the lower cladding 7, it is preferable to use SiO 2 for the upper cladding 6.
- SiO 2 is deposited on the upper clad 6, intentional impurities (dopants) or inevitable impurities may be included.
- the upper clad and the lower clad are not the same material, the upper clad and the lower clad can be made of the same element species.
- the definition that “two materials are composed of the same element species” means that the elements constituting the two materials are all the same.
- the element species of silicon (Si) is only Si
- the element species of silica (SiO 2 ) are Si and O.
- a material composed of two element species of Si and O can be said to be the same element species as SiO 2 , but a material composed of only Si (such as Si) or a material including element species other than Si and O (Si 3 N 4 etc.) is not the same element type as SiO 2 .
- the effective refractive index of each mode changes according to the strength of light confinement in the optical waveguide.
- the strength of the confinement depends on the size of the core when the refractive indexes of the core and the clad are constant, and the confinement is larger as the core is larger. Therefore, the effective refractive index changes by changing the size of the core.
- change in the width direction is considered.
- the refractive index profile of the refractive index cross section is asymmetrical in the vertical direction.
- the points of TE 1 and TM 0 that have been separated are separated.
- TE 1 and TM 0 are continuously connected on the same effective refractive index curve, it is possible to perform high-order polarization conversion by gently changing the waveguide width.
- An optical waveguide having a core shape as in this embodiment can be manufactured by a process of manufacturing a rib waveguide.
- a rib waveguide having a smaller loss than a rectangular waveguide is often used together with a rectangular waveguide. Therefore, the two-stage core shape as shown in FIGS. 1A to 2B is often used. It can be made without the need for an extra process. It is possible to use the same material for the lower core and the upper core.
- an optical modulator with a rib-type phase modulator reference: K.
- a BOX (buried oxide) layer serving as a lower cladding is made of SiO 2 , and SiO 2 is frequently used for an upper cladding. Therefore, as in this embodiment, if the upper clad and the lower clad are made of the same material and can be subjected to high-order polarization conversion, it can be used for an optical waveguide based on an SOI substrate.
- Non-Patent Document 1 Even when the refractive indexes of the upper clad and the lower clad are different, if the refractive index difference between them is not large, the vertical asymmetry becomes small, and the effective refraction of TE 1 and TM 0 The rate approaches. Therefore, the length of the taper portion necessary for sufficient conversion becomes very long, which is disadvantageous from the viewpoint of miniaturization. In addition, even when different materials are used for the upper clad and lower clad in the optical waveguide and these materials can be used, high-order polarization conversion cannot be performed in a small size if the difference in refractive index is small. .
- the higher-order polarization conversion cannot be performed in a small size.
- a method for performing high-order polarization conversion with an element on a small substrate-type optical waveguide is an issue.
- the present invention even when there is a difference in the refractive index of the clad as described above, it is possible to increase the vertical asymmetry of the waveguide cross section by making the core shape asymmetric and to perform polarization conversion at a shorter distance.
- the lower core and the upper core may be displaced in the width and longitudinal directions.
- the waveguide is formed with a vertically asymmetric structure, and therefore, the influence on the high-order polarization conversion due to such manufacturing variation is small.
- the height of the lower core may vary during manufacturing, but for the same reason, the influence on the high-order polarization conversion is small.
- the effect on the loss is small if the section in which the upper core and the lower core are provided is short. Therefore, in the high-order polarization conversion device of this embodiment, the manufacturing variation may be the same as that of the conventional waveguide having a rib structure.
- the same (integrated) mask is used for the fabrication of the upper core of the higher-order polarization conversion element and the rib-structured rib, or the same as the fabrication of the lower-core of the higher-order polarization conversion element and the slab of the rib structure. Or use an (integral) mask.
- the rib process used in another place on the mask is used in combination, the height of the lower core and the upper core cannot be freely selected, but for the same reason, it is possible with high-order polarization conversion.
- the effective refractive index of TE 0 guided through the high-order polarization conversion element is significantly different from the effective refractive index of TM 0 and TE 1 , conversion to other waveguide modes of TE 0 is unlikely to occur.
- TE 0 and TE 1 are simultaneously input to such an effective refractive index, it also functions as an element in which TE 0 that is hardly converted and TE 1 that has been converted to TM 0 are simultaneously output. From this viewpoint, it is preferable that the difference between the effective refractive index of TE 0 and the effective refractive index of TE 1 is 0.2 or more.
- the difference between the effective refractive index of TE 0 and the effective refractive index of TM 0 is preferably 0.2 or more. These requirements regarding the effective refractive index difference are preferably satisfied over the entire length between the start portion and the end portion of the optical waveguide.
- the higher-order polarization conversion element of this embodiment may have a smaller conversion loss from TE 1 to TM 0 .
- Example 1 FIGS. 4A to 4D
- Comparative Example 1 FIGS. 4A to 4D
- Example 1 (0.004 dB) can be converted with a smaller loss compared to Comparative Example 1 (0.587 dB).
- FIGS. 4A to 4D A high-order polarization conversion device according to the first embodiment of the present invention is shown in FIGS. 4A to 4D.
- FIG. 4A shows a plan view of the core 2
- FIGS. 4B to 4D show sectional views of an end portion, an intermediate portion, and a start portion of the high-order polarization conversion element, respectively.
- a cladding 5 is provided around the core 2 as in FIG. 2A, but is not shown in FIGS. 4A to 4D. Details of this structure will be described later as a first embodiment.
- FIG. 4A the stepped portion where the lower core 4 comes out of the upper core 3 is shaded.
- FIG. 5A and the like to be described later the plan view may be shaded in the same manner.
- the upper core 3 is located at the center in the width direction of the lower core 4 from the start portion 8 to the end portion 9.
- higher-order polarization conversion is possible even in a structure in which the upper core is arranged in addition to the center of the lower core.
- the conversion efficiency is reduced, but the higher-order polarization conversion is reduced. Is possible.
- both ends in the width direction of the upper core 3 overlap with both ends in the width direction of the lower core 4, respectively, and the width of the lower core 4 and the width of the upper core 3 are the same.
- the cross section is rectangular.
- the core width W1 of the start portion 8 is larger than the core width W2 of the end portion 9.
- both ends in the width direction of the upper core 3 do not always overlap with both ends in the width direction of the lower core 4. It is similar to the structure. That is, as shown in FIG.
- the width of the lower core 4 is larger than the width of the upper core 3, and the lower side of the upper core 3 in the cross section perpendicular to the waveguide direction is included in the upper side of the lower core 4.
- the distance from the upper base to the lower base is equal to the core height H1 of the start portion 8 and the end portion 9, and the height H2 of the lower core 4 is constant.
- the structure in which the width of the upper core 3 is changed on the start portion 8 side and the width of the lower core 4 is reduced on the end portion 9 side after the difference in width between the upper core 3 and the lower core 4 is widened is the conversion efficiency.
- the taper length (the total length of L1 and L2), which is the length of the element, can be shortened.
- the method of changing the width in the longitudinal direction of the core 2 is changing the width linearly with respect to the distance in the longitudinal direction.
- This change in width can be changed to an arbitrary continuous curved change such as a quadratic function, but the reproducibility of the curved waveguide structure is lower than when the linear change is made. Therefore, the influence by a manufacturing process can be reduced by employ
- the degree of separation of the effective refractive index curves of TE 1 and TM 0 is the same as that of the upper core and the lower core, and the position of the upper core is other than the center and does not protrude from the lower core.
- FIG. 5A to 5D show a high-order polarization conversion device according to the second embodiment of the present invention.
- FIG. 5A shows a plan view of the core 2
- FIGS. 5B to 5D show sectional views of an end portion, an intermediate portion, and a start portion of the high-order polarization conversion element, respectively.
- a clad 5 is provided around the core 2 as in FIG. 1A, but is not shown in FIGS. 5A to 5D. Details of this structure will be described later as a second embodiment.
- one end in the width direction of the upper core 3 and the lower core 4 coincides from the start portion 8 to the end portion 9.
- the feature of the change in the cross-sectional shape in the longitudinal direction of the core 2 and the way of changing the width are the same as in the first embodiment of FIGS. 4A to 4D.
- this structure has a small effective refractive index difference and lowers polarization conversion efficiency, the lower core part not covered by the upper core is wider than the structure of the first embodiment, and the structure of the first embodiment is manufactured. The required accuracy is further reduced, and a highly reproducible structure can be manufactured.
- FIGS. 6A to 6C show a high-order polarization conversion device according to the third embodiment of the present invention.
- 6A shows a plan view of the core 2
- FIGS. 6B and 6C show sectional views of an end portion and a start portion of the high-order polarization conversion element, respectively.
- a cladding 5 is provided around the core 2 as in FIG. 1A, but is not shown in FIGS. 6A to 6C. Details of this structure will be described later as a third embodiment.
- the cross-sectional shape of the start portion 8 is a rib waveguide
- the cross-sectional shape of the end portion 9 is a rectangular waveguide
- the center in the width direction of the upper core 3 and the lower core 4 is the same.
- the width of the lower core 4 is larger than the width of the upper core 3 at the start portion 8
- the width of the upper core 3 and the width of the lower core 4 are the same at the end portion 9 as shown in FIG. 6B.
- the width is changed linearly with respect to the distance in the longitudinal direction.
- the width ratio of the lower core 4 and the upper core 3 differs between the start part 8 and the end part 9, it has the same cross-sectional shape as FIG. 6C.
- TE Conversion from 1 to TM 0 is possible.
- a characteristic of the change in the cross-sectional shape in the longitudinal direction of the core 2 of this structure is that the width of the upper core 3 and the width of the lower core 4 are linearly changed from the start portion 8 to the end portion 9.
- the start portion 8 side has a rib structure, and the difference in width between the upper core 3 and the lower core 4 is large, so that the vertical asymmetry is large. For this reason, a width
- the rib waveguide has a smaller loss due to side wall roughness caused by the manufacturing process than the rectangular waveguide.
- TE 1 having a widely spread electric field distribution has a particularly large loss of side wall roughness. Therefore, propagation with less loss is possible by guiding the rib waveguide.
- the above structure is characterized by a structure capable of directly converting TE 1 propagating through the rib waveguide into TM 0 . Since TE 1 can be converted to TM 0 without first converting it into a rectangular waveguide, there is no need to propagate an extra distance, and it is possible to eliminate side wall roughness and loss caused by waveguide conversion. is there.
- the high-order polarization conversion element of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention.
- a high-order polarization conversion element having a structure in which the input / output cross section (start and end portions) and changes in the longitudinal direction are as follows is also possible.
- 7A to 7D show modified examples of the high-order polarization conversion device according to the first or second embodiment of the present invention.
- 7A to 7D are plan views of the core. Similar to the first or second embodiment, these have a vertically symmetrical structure in which the cross section of the start portion 8 and the end portion 9 is rectangular. That is, the width of the lower core 4 and the width of the upper core 3 are the same in the cross section of the start portion 8, and the width of the lower core 4 and the width of the upper core 3 are the same in the cross section of the end portion 9. Further, as in the first or second embodiment, the width of the upper core 3 is always narrower than the width of the lower core 4 between the start portion 8 and the end portion 9.
- FIG. 7A has a stepped portion in which the lower core 4 protrudes on one side of the upper core 3 as in FIG. 5A.
- 7B has a stepped portion in which the lower core 4 protrudes on both sides of the upper core 3 as in FIG. 4A.
- the upper core 3 does not have to be at the center in the width direction with respect to the lower core 4, and is asymmetric It is.
- FIG. 7C although the change of the width
- the height of the entire core is 220 nm
- the width of the entire core is 700 nm or more
- the end portion In the cross section the height of the entire core is preferably 220 nm
- the width of the entire core is preferably 620 nm or less.
- the height of the entire core in the cross section of the start part and the end part is preferably about 210 nm to 230 nm.
- the height of the lower core is preferably about 75 nm to 115 nm in consideration of manufacturing errors.
- the width of the upper core 3 is larger than the width of the lower core 4 in the portion between the start portion 8 and the end portion 9 excluding the start portion 8 and the end portion 9. Is always narrow. Therefore, it can be created by etching twice. Therefore, for example, it can be produced by etching the SI layer, which is the upper layer of the SOI substrate, and depositing SiO 2 thereon. Further, the change in the core width with respect to the light traveling direction may be stepwise as shown in FIG. 4A or may be continuous as shown in FIG. 7D.
- the design is easier if the change in the core width is gradual.
- the change in the core width is continuous, the waveguide structure can be changed more smoothly, and the loss can be further reduced.
- the portion of the lower core that protrudes from the upper core may protrude on both sides with respect to the light traveling direction as shown in FIG. 4A, or may protrude only on one side as shown in FIG. 5A. Good. Higher-order polarization conversion efficiency is obtained when projecting on both sides. When projecting to one side, the projecting of the lower core can be kept wider, so that the resolution required during manufacturing can be relaxed.
- rectangular waveguides having different waveguide widths can be connected to each other by an efficient tapered waveguide, and the width of the core 2 is narrowed from the start portion 8 to the end portion 9.
- a high-order polarization converter is provided (embedded) in the (tapered waveguide).
- FIG. 39B shows a tapered waveguide having a core 2 in which the cross section from the start portion to the end portion is always rectangular and the width becomes narrower from the start portion 8 to the end portion 9, and FIG. ) In which the high-order polarization conversion unit 74 is embedded.
- the core 2 between the start portion 8 and the end portion 9 has a structure in which waveguides are continuously connected, and two stages of an upper core 3 and a lower core 4 as shown in FIG. It is necessary to have a tapered waveguide structure.
- the portion having the two-step tapered waveguide structure is referred to as a two-step tapered portion 73.
- the effective refractive indexes of TE 1 and TM 0 are interchanged (degenerate points High-order polarization conversion is not performed.
- the widths of the upper core 3 and the lower core 4 are different as shown in FIG. In the vertically asymmetric refractive index distribution, the effective refractive indexes of TE 1 and TM 0 are not interchanged (no degeneracy point), and a waveguide mode called a hybrid mode in which TE 1 and TM 0 are mixed occurs. High-order polarization conversion is performed by using this hybrid mode.
- the portion where the hybrid mode occurs is referred to as a high-order polarization conversion unit 74.
- the high-order polarization conversion unit 74 needs to increase the taper length so that the electric field is continuously changed (adiabatic conversion).
- the width of the upper core 3 of the start portion 8 is always as long as the conditions of the effective refractive index order of the start portion 8 and the end portion 9 are satisfied.
- the end portion 9 becomes larger than the width of the upper core 3.
- the width of the lower core 4 of the start portion 8 is always larger than the width of the lower core 4 of the end portion 9.
- a wide rectangular waveguide is used. There is a method of using a tapered waveguide whose width is monotonously narrowed from the (starting portion 8) to the narrow rectangular waveguide (ending portion 9).
- a high-order polarization conversion unit 74 is provided in the waveguide.
- the high-order polarization conversion unit 74 can be realized by having a refractive index section that is asymmetrical in the vertical direction. Therefore, in the above-described embodiment, an upper core and a lower core are provided with respect to the above-described tapered waveguide, and a two-stage tapered waveguide structure in which the change in the width is different in the light traveling direction is defined as the start portion 8. Provided between the end portions 9.
- the width of the lower core 4 and the width of the upper core 3 in the cross section from the start portion 8 to the end portion 9 are always constant. Different. Further, the width of the upper core 3 or the lower core 4 is monotonously reduced and the widths of both the upper core and the lower core are not increased. Thereby, the rectangular waveguides having the start portion 8 and the end portion 9 having different widths can be efficiently connected, and the high-order polarization conversion portion 74 can be provided therebetween. As a result, a high-order polarization conversion element that is compact and capable of high-density integration can be realized.
- the high-order polarization conversion unit 74 can be arbitrarily designed because a high-order polarization conversion unit is provided in addition to a normal tapered waveguide. is there.
- the high-order polarization conversion unit 74 for the entire element length is used.
- the ratio can also be designed arbitrarily. This can be achieved by adjusting how the two-step taper changes with respect to the light guiding direction.
- the efficiency of high-order polarization conversion increases as the proportion of the high-order polarization conversion unit in the entire high-order polarization conversion element increases. Therefore, highly efficient conversion is possible at a shorter distance. However, it is necessary to set a portion of the waveguide other than the high-order polarization converter so that the loss is sufficiently small.
- the position of the high-order polarization conversion unit 74 with respect to the entire high-order polarization conversion device can be adjusted.
- the position of the high-order polarization conversion portion is shifted if the adjustment of the method of changing the two-step taper is the same.
- the position of the high-order polarization converter is preferably in the center of the element from the viewpoint of the wavelength band and manufacturing error. Therefore, as in FIG. 40, by adjusting the change of the two-step taper with respect to the traveling direction of light, as shown in FIGS.
- the start portion 8 and the end portion 9 having an arbitrary width are increased. It becomes possible to provide the secondary polarization conversion unit 74 near the center of the high-order polarization conversion element. In FIGS. 41A to 41C, the widths of the start portion 8 and the end portion 9 are reduced in the order of FIGS. 41A, 41B, and 41C. In all cases, the high-order polarization conversion portion 74 is disposed at the center.
- the above embodiment can operate over a wide wavelength band.
- the high-order polarization conversion unit 74 is in the center of the high-order polarization conversion element, the operation can be performed over a wider wavelength range. This is because the position shift of the high-order polarization conversion unit 74 increases as the change in wavelength increases.
- the high-order polarization converter can be easily designed at the center of the high-order polarization converter.
- the above-described implementation is possible in which the high-order polarization converter 74 can be designed at the center of the high-order polarization converter.
- higher-order polarization conversion is possible.
- the width of the upper core 3 or the lower core 4 deviates from the design value
- the height of the upper core 3 or the lower core 4 deviates from the design value
- the upper core 3 and the lower core 4 There are cases where the relative positions are shifted, and the side walls of the upper core or the lower core are inclined from the vertical direction with respect to the substrate.
- the width of the upper core 3 or the lower core 4 may randomly vary with respect to the design value (side wall roughness) as shown in FIG. 42A.
- the core width is slightly changed, the light confinement is changed, and the effective refractive index is also changed randomly. Therefore, the effective refractive index curve becomes substantially thick, and the degree of separation of the intersection of the effective refractive indexes of TE 1 and TM 0 is weakened. That is, the conversion efficiency of the high-order polarization conversion unit decreases due to the influence of the side wall roughness.
- the ratio of the high-order polarization conversion unit 74 occupying the entire element length can be increased, it is possible to suppress a decrease in conversion efficiency in the entire high-order polarization conversion element.
- the influence of the side wall roughness is smaller as the width between the upper core 3 and the lower core 4 is larger. This is because when the core width is large, light is confined inside the core, and as a result, the influence of the fluctuation of the core width can be reduced.
- the efficiency of the high-order polarization conversion is reduced by providing the high-order polarization conversion portion 74 at a position close to the start portion 8 as shown in FIG. 42B. Can be suppressed.
- the lower core 4 in the high-order polarization conversion element has a portion (wide portion) where the width of the lower core 4 of the start portion 8 is larger than the width of the lower core 4, the wide portion has a larger core than the start portion 8.
- a higher-order guided mode guided mode whose effective refractive index is smaller than TE 1 or MT 0
- perturbation occurs due to the fluctuation of the core width due to the manufacturing error, and the input TE 1 may be converted to this higher-order mode.
- Loss occurs due to the conversion to the higher-order mode, and the phase is shifted from the original TE 1 by performing reverse conversion from the higher-order mode to TE 1 after traveling a certain distance in the waveguide of the higher-order polarization conversion element. Further, TE 1 may overlap and propagate, which may lead to wavelength dependency of loss. On the other hand, in the said embodiment, the upper core 3 and the lower core 4 do not spread more than the width
- FIGS. 8A to 8C Modification examples of the high-order polarization conversion device according to the third embodiment of the present invention are shown in FIGS. 8A to 8C.
- 8A to 8C are plan views of the core, respectively. Similar to the third embodiment, these optical waveguides have a rectangular core in which the cross section of the start portion 8 is vertically asymmetric and the cross section of the end portion 9 is vertically symmetric.
- FIG. 8A has a stepped portion where the lower core 4 protrudes on one side of the upper core 3.
- FIG. 8B has the level
- FIG. 8A is plan views of the core, respectively. Similar to the third embodiment, these optical waveguides have a rectangular core in which the cross section of the start portion 8 is vertically asymmetric and the cross section of the end portion 9 is
- variety of the upper core 3 and the lower core 4 in a taper part is a continuous change, it is not linear (straight line) but curvilinear form.
- the height of the lower core that becomes the slab of the rib waveguide is 75 to 115 nm, and the height of the entire core is 210 to 230 nm.
- the width of the upper core is 600 nm or more, and in the cross section of the end portion, the height of the entire core is 210 to 230 nm, and the width of the entire core is 620 nm or less.
- the height of the entire core in the cross section of the start portion and the end portion is preferably about 210 nm to 230 nm, and the height of the lower core is preferably about 80 nm to 110 nm.
- the width of the upper core at the start portion is preferably wider than the width of the entire core at the end portion, and more preferably 700 nm or more. When the width of the upper core at the start portion is equal to or less than the width of the entire core at the end portion, the width of the lower core at the start portion is preferably wider than the width of the entire core at the end portion.
- FIGS. 9A to 9C A high-order polarization conversion device according to the fourth embodiment of the present invention is shown in FIGS. 9A to 9C.
- 9A shows a plan view of the core 2
- FIGS. 9B and 9C show sectional views of an end portion and a start portion of the high-order polarization conversion element, respectively.
- a cladding 5 is provided around the core 2 as in FIG. 2A, but is not shown in FIGS. 9A to 9C. Details of this structure will be described later as a fourth embodiment.
- 10A to 10C show modified examples of the fourth embodiment of the high-order polarization conversion device of the present invention.
- 10A to 10C are plan views of the core, respectively.
- the optical waveguide of the present embodiment has a rib-shaped core in which the cross section of the start portion 8 is a vertically symmetrical rectangle and the cross section of the end portion 9 is asymmetrical in the vertical direction.
- TE Conversion from 1 to TM 0 is possible.
- the left and right symmetry of the core shape is not necessarily required, and as shown in FIGS. 10A to 10C, the upper core 3 may not be centered with respect to the lower core 4, and the taper portion is continuously changed. If so, it may not be linear (straight line).
- the height of the entire core is equal to 220 nm and the width of the entire core is 700 nm or more in the cross section of the start portion.
- the height of the entire core is equal to 220 nm
- the height of the lower core is equal to 95 nm
- the width of the upper core is 620 nm or less.
- FIGS. 11A to 11C A high-order polarization conversion device according to a fifth embodiment of the present invention is shown in FIGS. 11A to 11C.
- FIG. 11A shows a plan view of the core 2
- FIGS. 11B and 11C show sectional views of an end portion and a start portion, respectively.
- a cladding 5 is provided around the core 2 as in FIG. 2A, but is not shown in FIGS. 11A to 11C. Details of this structure will be described later as a fifth embodiment.
- 12A to 12C show modified examples of the high-order polarization conversion device according to the fifth embodiment of the present invention.
- 12A to 12C are plan views of the core, respectively.
- the optical waveguide of the present embodiment has a core with a rib structure in which the cross sections of the start portion 8 and the end portion 9 are vertically asymmetric.
- TE Conversion from 1 to TM 0 is possible.
- the left and right symmetry of the core shape is not necessarily required, and the upper core 3 may not be centered with respect to the lower core 4 as shown in FIGS. 12A to 12C.
- the width of the lower core 4 may not be linear (straight line) as long as it changes continuously.
- the height of the lower core is 95 nm, the height of the entire core is 220 nm, and the upper core
- the width of the lower core is 600 nm or more
- the width of the lower core is 95 nm
- the height of the entire core is 220 nm
- the width of the upper core is 620 nm or less.
- the width of the upper core at the start portion is preferably wider than the width of the upper core at the end portion, and more preferably 700 nm or more.
- the width of the lower core at the start portion is preferably wider than the width of the upper core at the end portion.
- the high-order polarization conversion element of the present invention can be used in combination with other elements in an optical waveguide on the same substrate.
- a polarization conversion element can be realized by combining an asymmetric directional coupler and the high-order polarization conversion element of the present invention.
- This polarization conversion element converts TE 0 to TE 1 by an asymmetric directional coupler, and converts TE 1 to TM 0 by a high-order polarization conversion element.
- 13A and 13B show an example of a polarization conversion element in which the high-order polarization conversion element of the present invention is combined with an asymmetric directional coupler.
- FIG. 13A is a plan view of the core
- FIG. 13B is a cross-sectional view of the asymmetric directional coupler. Details of this structure will be described later as a sixth embodiment.
- the asymmetric directional coupler 13 includes a first optical waveguide 11 and a second optical waveguide 12 which are two rectangular waveguides. The periphery of these waveguides is covered with a clad 14 as shown in FIG. 13B.
- the high-order polarization conversion element 10 is connected only to the second optical waveguide 12.
- TE 0 is guided through the first optical waveguide 11.
- TE 1 is guided through the second optical waveguide 12.
- To have the TE 0 and TE 1 and is close to the effective refractive index of the second optical waveguide 12 of the first optical waveguide 11 can be coupled with the first optical waveguide 11 to the second optical waveguide 12.
- the input-side waveguide connected to the first optical waveguide 11 is referred to as a first port 11a
- the input-side waveguide connected to the second optical waveguide 12 is referred to as a second port 12a
- the third port 12 b on the output side of the second optical waveguide 12 is connected to the start portion 8 of the high-order polarization conversion element 10.
- the high-order polarization conversion element 10 shown in FIG. 13A has the same structure as that of FIGS. 4A to 4D as an example, it is not particularly limited to this.
- TE 0 input to the first port 11a is coupled to TE 1 of the second optical waveguide 12 by the asymmetric directional coupler 13, and is output as TE 1 from the third port 12b.
- TE 1 output from the third port 12b is input to the high-order polarization conversion element 10, it is ultimately converted to TM 0.
- the effective refractive index of TE 0 of the second optical waveguide 12 is equal to the effective refractive index of any mode of the first optical waveguide 11 in the asymmetric directional coupler 13. to differ greatly. Therefore, mode coupling and conversion do not occur.
- this structure can also operate as an element having both polarization conversion and polarization multiplexing functions.
- the modes of TE 0 and TM 0 multiplexed light input from the output unit are separated from each other, and TE 0 from the first port 11a and the second port 12a.
- the difference between the refractive index and the effective refractive index of TE 0 of the second optical waveguide 12 is preferably 0.2 or more.
- FIG. 14A and 14B show another example of a polarization conversion element in which the high-order polarization conversion element of this embodiment is combined with an asymmetric directional coupler.
- 14A is a plan view of the core
- FIG. 14B is a cross-sectional view of the core of the asymmetric directional coupler. Details of this structure will be described later as a seventh embodiment.
- the asymmetric directional coupler 13r is composed of a rib-type waveguide. Slabs 13s are formed between the ribs 11r and 12r of the two optical waveguides, and slabs 11s and 12s are also formed outside the ribs 11r and 12r, respectively.
- FIGS. 14A and 14B The functions of the polarization conversion elements in FIGS. 14A and 14B are the same as those of the polarization conversion elements in FIGS. 13A and 13B.
- TE 0 When TE 0 is simultaneously input to the first port 11a and the second port 12a, higher-order polarization conversion is performed.
- An output in which TE 0 and TM 0 are combined is obtained from the end portion 9 of the element 10.
- it can be used as an element for performing polarization multiplexing.
- FIG. 15 schematically shows an example of the DP-QPSK modulator.
- the DP-QPSK modulator 20 has a QPSK signal independent of both modes TE 0 / TM 0 by utilizing the fact that two modes of TE 0 and TM 0 can exist in a normal optical waveguide. Modulate. Specifically, light input from the input unit 21 at TE 0 is branched into two optical waveguides 22 and 22, modulated by QPSK modulators 23 and 23, respectively, and then one side of the optical waveguides 24 and 24. TE 0 is converted to TM 0 by the polarization conversion element 25, the two modes are combined on the same optical waveguide by the polarization beam combiner, and signals independent of TE 0 and TM 0 are output to the output unit 26. .
- One of the TE 0 is converted into TM 0, can be a part of the polarization conversion element 25 to be multiplexed with other TE 0, it utilizes the polarization conversion element of the embodiment shown in FIGS. 13A ⁇ 14B e.g. .
- the first port 11a and the second port 12a in FIGS. 13A to 14B correspond to the optical waveguide 24 in FIG.
- the end portion 9 of the high-order polarization conversion element 10 of FIGS. 13A to 14B corresponds to the output portion 26 of FIG.
- the method for modulating TE 0 and TM 0 is not limited to QPSK, and even a modulator having a complicated configuration can perform polarization multiplexing using the polarization conversion element of this embodiment. .
- ⁇ Polarization diversity coherent receiver> The polarization conversion element of this embodiment is described in a reference document (C. Doerr et al., “Packaged Monolithic Silicon 112-Gb / s Coherent Receiver,” IEEE Photonics Technology Letters, Vol. 23, pp. 762-764, 2011). Can be used for a coherent receiver on a Si optical waveguide of a polarization multiplexed signal in which TE 0 and TM 0 are simultaneously transmitted.
- FIG. 16 schematically shows an example of a polarization diversity coherent receiver.
- This coherent receiver 30 connects a polarization multiplexed signal optical waveguide 31 that transmits TE 0 and TM 0 simultaneously to a polarization conversion element 32 that can simultaneously perform polarization conversion and a polarization beam splitter.
- the signal of TE 0 is branched to one of these. Further, the signal of TE 0 converted from TM 0 is branched to the other side of the optical waveguide 33.
- a semiconductor laser light source generally used as the local light 34 uses only one polarized wave, for example, an output of TE 0 (local). When such a light source is used, it is necessary to perform polarization conversion of local light emission.
- the coupling part 36 has a polarization separation function for coupling light to the outside of the element, such as an inverted taper mode field converter coupled from the side of the substrate. It is possible to use a coupler without it.
- the reference Qing Fang, et al., “Suspended optical fiber-to-waveguide mode size converter for silicon photonics,” Optics Express, Vol. 18, Issue 8, pp. 7763-7769 (2010)
- the polarization conversion element of the embodiment shown in FIGS. 13A to 14B can be used for the portion of the polarization conversion element 32 that can simultaneously perform polarization conversion and polarization beam splitter.
- the end portion 9 of the high-order polarization conversion element 10 in FIGS. 13A to 14B corresponds to the optical waveguide 31 in FIG. 16, and the first port 11a and the second port 12a in FIGS. 13A to 14B are the optical waveguides in FIG. This corresponds to the waveguide 33.
- ⁇ Polarization diversity method The polarization conversion element of this embodiment is disclosed in a reference (Hiroshi Fukuda et al., “Silicon photonic circuit with polarization diversity,” Optics Express, Vol. 16, Issue 7, pp. 4872-4880 (2008)). I would like to use a polarization multiplexing transmission where TE 0 and TM 0 are transmitted at the same time, or an element that gives the same operation to both modes when one polarization is transmitted randomly In this case, it can be used to execute the polarization diversity scheme as shown in FIG. In the polarization diversity method 40 shown in FIG.
- an optical waveguide 41 of a polarization multiplexed signal in which TE 0 and TM 0 are transmitted simultaneously is connected to a polarization conversion element 42 that can simultaneously perform polarization conversion and polarization beam splitter.
- the TE 0 signal is branched to one of the optical waveguides 43. Further, the TE 0 signal converted from TM 0 is branched to the other of the optical waveguides 43.
- the TE 0 signal light operated by the element 44 is combined from the optical waveguide 45 by the polarization conversion element 46 and outputted to the polarization multiplexed signal optical waveguide 47 in which TE 0 and TM 0 are transmitted simultaneously.
- the polarization conversion element 42 As in the coherent receiver 30 shown in FIG. 16, the polarization conversion element of this embodiment capable of performing polarization conversion and polarization beam splitter at the same time can be used.
- the polarization conversion element 46 as in the DP-QPSK modulator 20 shown in FIG. 15, the polarization conversion element of the present invention that can simultaneously perform polarization conversion and polarization beam combiner can be used.
- Polarization conversion is possible by combining the branching unit and the high-order polarization conversion element of this embodiment.
- the branching unit include a 2 ⁇ 1 MMI (multimode interferometer) and a Y branch. These can generate TE 1 by adjusting the phase of the electric field input to the two input portions. Alternatively, TE 1 may be input only from one side. Therefore, the TE 1 can be converted to TM 0 by connecting the higher-order polarization conversion element of the present embodiment to the subsequent stage, and the function of polarization conversion can be provided.
- FIG. 32 shows an example of a conversion multiplexing element that generates TE 1 . Details of this structure will be described later as an eighth embodiment.
- the conversion multiplexing element 50 utilizes the property that the two TE 1 mode distributions in the planar rectangular combining unit 53 are similar to the TE 0 mode distributions of the two input portions 51 and 52.
- TE 0 is input from one or both of the two input parts 51 and 52, it is converted to TE 1 while passing through the taper part 54 from the multiplexing part 53 and output from the output part 55 as TE 1 .
- the phase difference between the two TE 0 to be input to each input portion 51, 52 and [pi.
- TE 0 having a symmetric mode distribution does not occur after multiplexing. This prevents the occurrence of TE 0 which becomes unnecessary after the multiplexing, it is possible to suppress the reduction of the polarization extinction ratio.
- a symmetric directional coupler 60 as shown in FIG. 33 can be used. Details of the structure of the symmetric directional coupler 60 will be described later in a ninth embodiment.
- the symmetric directional coupler 60 has a structure in which two waveguides 61 and 62 having the same core width W are arranged in parallel with a predetermined gap G between a predetermined coupling length L.
- the TE 0 from the waveguide 61 entering TM 0 from the waveguide 62, and proceeds TM 0 is the waveguide 61, the polarization multiplexed signal from the output side of the waveguide 61 (TE / TM ) Can be obtained.
- TE 0 basic TE mode
- TE 1 high-order TE mode
- TM 0 basic TM mode
- Each QPSK modulator 71, 72, 81, 82 includes two Mach-Zehnder interferometers.
- One Mach-Zehnder interferometer 71a, 72a, 81a, 82a is for the in-phase component (I), and the other Mach-Zehnder interferometer 71b, 72b, 81b, 82b is for the quadrature-phase component (Q).
- a conversion multiplexing element 50 is provided in the I and Q multiplexing sections of one of the two QPSK modulators 71 and 72, and TE 0 is changed to TE 1 .
- the high-order polarization conversion element 10 converts from TE 1 to TM 0 .
- This TM 0 is combined with TM 0 output from the other QPSK modulator 71 by the symmetric directional coupler 60.
- conversion multiplexing elements 50 are provided in the multiplexing sections of the two Mach-Zehnder interferometers 82a and 82b of one QPSK modulator 82 to convert TE 0 into TE 1.
- the high-order polarization conversion element 10 converts TE 1 to TM 0 .
- the TM mode MMI multiplexer 83 is provided in the I and Q multiplexer of the QPSK modulator 82, and the multiplexed TM 0 is output from the other QPSK modulator 81 by the symmetric directional coupler 60. to TM 0 and the combined was.
- the multiplexing of the TM 0 and TE 0 can be used with good symmetry directional coupler 60 in performance compared to the asymmetry of the directional coupler.
- the conversion multiplexing element 50 also serves as a multiplexing unit, polarization dependent loss can be reduced (excess loss of MMI is 0.3 dB or less).
- TE 2n + 1 refers to a mode having the highest (2n + 2) -th effective refractive index among TE modes (TE 0 , TE 1 , TE 2 ,).
- the reason why the odd-order mode of the TE mode is to be converted is as follows.
- TM 0 propagating through a rectangular core (a structure in which the width direction and height are symmetrical (refractive index distribution)) has an x component (Ex) of the electric field in an antisymmetric distribution in both the width direction and the height direction.
- Ex of odd order TE mode including TE 1 is antisymmetric with respect to the width direction, the symmetrical field distribution is the height direction.
- the TE 2n + 1 mode can be converted between TM 0 and the same structure as in the present invention.
- ⁇ Calculation Example 1> 18A and 18B are graphs showing changes in the effective refractive index of the four modes in descending order of the effective refractive index when the bottom of the optical waveguide having a convex core shape as shown in FIGS. 2A and 2B is changed.
- Wavelength is 1.55 ⁇ m
- core is Si and refractive index is 3.48
- upper cladding and lower cladding are SiO 2 and refractive index is 1.44
- optical waveguide height is 0.22 ⁇ m
- lower core height is 0 095 ⁇ m
- the width of the upper core is 0.5 ⁇ m
- the upper core is located at the center of the lower core.
- the mode with the highest effective refractive index is TE 0
- the mode with the second or third highest effective refractive index depends on (the width of) the bottom, and TE 1 , TM 0 or a mode changed from these.
- the mode having the fourth highest effective refractive index is not specified in the figure, but is a higher order mode.
- TE 1 and TM 0 always have an effective refractive index difference without degeneration due to the vertical asymmetry of the core cross-sectional shape.
- the approximate range of the mode conversion unit is shown as a dashed ellipse (near the bottom 0.7 ⁇ m).
- a high mode effective index in the second is a TE 1 when next TM 0 when the lower bottom is narrow, the lower base broad.
- the mode with the third highest effective refractive index is TE 1 when the lower base is narrow, and TM 0 when the lower base is wide.
- each mode can be converted by following the same effective refractive index curve (either the mode with the second highest or the third highest effective refractive index).
- FIGS. 19 to 22 show the modes with the second highest effective refractive index (# 1) and the third highest mode (# 2) with respect to the width of the bottom base.
- the electric field amplitude of the Ex component (component in the width direction) and the Ey component (component in the height direction) of the electric field is shown.
- each figure (a) shows the electric field amplitude of the Ex component of “# 1”
- each figure (b) shows the electric field amplitude of the Ey component of “# 1”.
- c) shows the electric field amplitude of the Ex component of “# 2”
- each figure (d) shows the electric field amplitude of the Ey component of “# 2”.
- Comparative Example 1 In Comparative Example 1, in the waveguide 101 shown in FIGS. 3A and 3B, the core 102 is made of Si, the lower clad 103 is made of SiO 2 , the upper clad 104 is made of air, and the height H0 of the core 102 is 0.22 ⁇ m.
- the width W1 of the start portion is 0.84 ⁇ m
- the width W2 of the end portion is 0.5 ⁇ m
- the length L0 in the longitudinal direction is 35 ⁇ m
- the core width is increased in the length direction from the rectangular waveguide of the start portion.
- the structure was changed linearly. Conversion loss of higher-order polarization conversion obtained by simulation using the Finite-Difference Time Domain (FDTD) method (ratio represented by the output power of TM 0 / input power of TE 1 ) was 0.587 dB.
- FDTD Finite-Difference Time Domain
- Example 1 Based on Calculation Example 1, the waveguide element of Example 1 having a structure in which the upper core is located at the center of the lower core (see the first embodiment described above) is manufactured.
- 4A to 4D are diagrams of the device of this example.
- a waveguide is manufactured based on an SOI (Silicon on insulator) substrate made of Si—SiO 2 —Si.
- the middle SiO 2 layer is used as the lower cladding, and the upper Si layer is used as the core.
- an SiO 2 layer is provided as an upper clad.
- the height H2 of the lower core 4 is 0.095 ⁇ m
- the height of the upper core 3 (difference between H1 ⁇ H2) is 0.125 ⁇ m
- the combined core height H1 of the lower core 4 and the upper core 3 is 0.22 ⁇ m.
- the width W1 of the upper core and the lower core is 0.84 ⁇ m
- the width W2 of the upper core 3 is 0.5 ⁇ m and is located at the center of the lower core 4.
- the width W1 of the lower core 4 is 0.84 ⁇ m
- the widths W2 of the upper core and the lower core are both 0.5 ⁇ m.
- parameters such as the width of the upper core of 0.5 ⁇ m are the same as those in the calculation example 1. Therefore, in the graph of FIG. 18 obtained above, the range where the width of the lower base is 0.5 to 0.84 ⁇ m can be applied to the first embodiment.
- the effective refractive index curve calculated in the cross section of Z regarding the full length of Example 1 is shown in FIG. Mode conversion is performed near the elliptical region in the figure.
- the second (# 1) and third (# 2) waveguide mode Ex having a large effective refractive index between the start portion and the end portion. Ey components are compared.
- the position of IVc (intermediate portion) in FIG. 4A 20 ⁇ m.
- the difference in effective refractive index between the two modes at the point where the second highest mode and the third highest mode are closest to each other is 0.16.
- the effective refractive index difference at the closest point is 0.10.
- FIG. 25 shows the wavelength dependence of the conversion loss. From this, it can be seen that the conversion loss is 0.05 dB or less over a wide band of 1530 to 1630 nm. The conversion loss at a wavelength of 1550 nm is 0.004 dB. Compared with the structure of Comparative Example 1 described above, Example 1 has the same input and end core shapes and the same length in the longitudinal direction. On the other hand, the conversion loss of Comparative Example 1 is 0.587 dB, the conversion loss of Example 1 is 0.004 dB, and Example 1 can be converted with a smaller loss than Comparative Example 1.
- FIG. 26 shows the result of calculating the electric field at a wavelength of 1550 nm by the FDTD method for the structure of Example 1.
- FIG. 26 (a) shows the Ex component
- FIG. 26 (b) shows the Ey component. It can be seen that the input TE 1 (Ex component is the main component) is finally changed to TM 0 (Ey component is the main component) by this structure.
- a high-order polarization conversion device having a structure in which the ends of the upper core and the lower core coincide with each other (see the second embodiment described above) is manufactured by the same method as in the first embodiment.
- the height H2 of the lower core 4 is 0.095 ⁇ m
- the height of the upper core 3 (difference between H1 ⁇ H2) is 0.125 ⁇ m
- the combined core height H1 of the lower core 4 and the upper core 3 is 0.22 ⁇ m.
- the width W1 of the upper core and the lower core is 0.84 ⁇ m
- the width W2 of the upper core 3 is 0.5 ⁇ m in the section L2 (length 15 ⁇ m).
- the width W1 of the lower core 4 is 0.84 ⁇ m, and is positioned so that the ends of the upper core and the lower core coincide with each other over the entire length of the sections L1 and L2.
- the width W2 of the upper core and the lower core is both 0.5 ⁇ m.
- the cross-sectional shape of the start portion 8 is a rib waveguide
- the cross-sectional shape of the end portion 9 is a rectangular waveguide
- the center in the width direction of the upper core 3 and the lower core 4 coincides.
- the high-order polarization conversion element (see the third embodiment described above) is manufactured by the same method as in Example 1.
- the height H2 of the lower core 4 is 0.095 ⁇ m
- the height of the upper core 3 (difference between H1 ⁇ H2) is 0.125 ⁇ m
- the combined core height H1 of the lower core 4 and the upper core 3 is 0.22 ⁇ m.
- the width W1a of the upper core 3 is 1 ⁇ m
- the width W1 of the lower core 4 is 4 ⁇ m
- the width W2 between the upper core and the lower core is both 0.5 ⁇ m.
- the maximum core width of the start portion 8 is 4 ⁇ m and is finite, the distribution of TE 1 passing through this cross section is sufficiently inside the both ends of the lower core 4 of the start portion 8. Therefore, the slab width is sufficiently large and can be regarded as a rib waveguide.
- FIG. 27 is a graph showing changes in the effective refractive index of the four modes in descending order of the effective refractive index in each cross section (denoted by #) in the structure of Example 3.
- the start part to the end part are equally divided into 10 parts in the longitudinal direction, and numbers are assigned in order from the start part (# 0) to the end part (# 10). It can be seen that the four modes are the same as those in FIG.
- the effective refractive indexes of TE 1 and TM 0 are separated by a broken line ellipse (mode conversion unit) shown in the vicinity of # 9, and high-order polarization conversion is possible between TE 1 and TM 0. I understand that.
- the minimum effective refractive index difference at this time is 0.15.
- the cross-sectional shape of the start portion 8 is a rectangular waveguide
- the cross-sectional shape of the end portion 9 is a convex shape
- the center in the width direction of the upper core 3 and the lower core 4 coincides (see above).
- the high-order polarization conversion element of the fourth embodiment is manufactured by the same method as in Example 1.
- the height H2 of the lower core 4 is 0.095 ⁇ m
- the height of the upper core 3 (difference between H1 ⁇ H2) is 0.125 ⁇ m
- the combined core height H1 is 0.22 ⁇ m.
- the width W1 of the upper core and the lower core is both 0.8 ⁇ m.
- the width W2a of the upper core 3 is 0.44 ⁇ m
- the width W2 of the lower core 4 is 0.5 ⁇ m.
- FIG. 28 is a graph showing changes in the effective refractive index of the four modes in descending order of the effective refractive index with respect to the size of the bottom base in the structure of Example 4. It can be seen that the four modes are the same as those in FIG.
- the effective refractive indexes of TE 1 and TM 0 are separated by a broken line ellipse (mode conversion unit) shown in the vicinity of the lower bottom 0.7 ⁇ m. Therefore, it can be seen between the TE 1 and TM 0 are possible higher polarization conversion.
- Example 5 As shown in FIGS. 11A to 11C, the cross-sectional shapes of the start portion 8 and the end portion 9 are convex, and the center in the width direction of the upper core 3 and the lower core 4 is coincident (see the fifth embodiment described above).
- the high-order polarization conversion element is manufactured by the same method as in the first embodiment.
- the height H2 of the lower core 4 is 0.095 ⁇ m
- the height of the upper core 3 (the difference between H1 ⁇ H2) is 0.125 ⁇ m
- the combined core height H1 is 0.22 ⁇ m.
- the width W1a of the upper core 3 is 0.7 ⁇ m
- the width W1 of the lower core 4 is 1.1 ⁇ m.
- the width W2a of the upper core 3 is 0.3 ⁇ m
- the width W2 of the lower core 4 is 0.7 ⁇ m.
- FIG. 29 is a graph showing changes in the effective refractive index of the four modes in descending order of the effective refractive index with respect to the size of the upper base in the structure of Example 5. It can be seen that the four modes are the same as those in FIG.
- the effective refractive indexes of TE 1 and TM 0 are separated by a broken-line ellipse (mode converter) shown in the vicinity of the upper base of 0.5 ⁇ m. Therefore, it can be seen between the TE 1 and TM 0 are possible higher polarization conversion.
- FIGS. 13A and 13B show an embodiment of a polarization conversion element obtained by combining the high-order polarization conversion element of Embodiment 1 with an asymmetric directional coupler.
- the core width E of the first optical waveguide 11 is 0.4 ⁇ m
- the core width F of the second optical waveguide 12 is 0.84 ⁇ m
- the heights H of the cores of the optical waveguide 11 and the second optical waveguide 12 are both 0.22 ⁇ m
- the gap G between the two waveguides is 0.35 ⁇ m.
- the straight portions of each waveguide are parallel to each other, and both end surfaces are on the same plane.
- the conversion loss was 0.105 dB when the wavelength was 1550 nm. This shows that conversion between TE 0 and TE 1 is possible with this structure. Since the conversion loss of the high-order polarization conversion element 10 is 0.004 dB as described in the first embodiment, the entire polarization conversion element including the asymmetric directional coupler 13 and the high-order polarization conversion element 10 is combined. The conversion loss is 0.109 dB, which indicates that polarization conversion is possible.
- a polarization conversion element in which the asymmetric directional coupler is a rib-type waveguide can also be manufactured.
- the length Lr of the rib waveguide is 20.8 ⁇ m.
- the width W1r of the rib 11r of the first optical waveguide is 0.4 ⁇ m
- the width W2r of the rib 12r of the second optical waveguide is 0.95 ⁇ m
- the height H1r of the ribs 11r and 12r is 0.22 ⁇ m.
- the width W1s of the slab 11s outside the first optical waveguide is 0.8 ⁇ m or more
- the width W2s of the slab 12s outside the second optical waveguide is 1.1 ⁇ m or more
- the width W3s of the slab 13s between the two waveguides Is 0.3 ⁇ m
- the slab height H1s is 0.095 ⁇ m.
- the high-order polarization conversion element 10 is manufactured in the same manner as in the first embodiment.
- the width W1 of the start portion 8 is 0.95 ⁇ m, which is the same as the width W2r of the rib 12r, and the width W2 of the end portion 9 is 0.6 ⁇ m.
- the length L1 of the section in which the width of the upper core 3 changes is 15 ⁇ m or more
- the length L2 of the section in which the width of the lower core 4 changes is 20 ⁇ m or more.
- FIG. 30 and FIG. 31 show the results of the wavelength dependence of the loss obtained by the simulation by the FDTD method for the polarization conversion element of Example 7.
- FIG. 30 is a graph showing the wavelength dependence of polarization conversion loss.
- the polarization conversion loss (loss in FIG. 30) is about 0.4 to 3.3 dB in the wavelength range of 1.53 to 1.63 ⁇ m.
- FIG. 31 is a graph showing the wavelength dependence of TE 0 transmission loss. This transmission loss (loss in FIG. 31) is about 0.14 to 0.16 dB in the wavelength range of 1.53 to 1.63 ⁇ m. From these graphs, it can be seen that conversion between TE 0 and TE 1 is possible, and TE 0 is transmitted with almost no loss, so that polarization multiplexing is also possible.
- the conversion / multiplexing element for generating TE 1 (TE 1 ) shown in FIG. 32 is manufactured based on an SOI (Silicon on insulator) substrate made of Si—SiO 2 —Si, as in Example 1.
- the middle SiO 2 layer is used as the lower cladding, and the upper Si layer is used as the core.
- an SiO 2 layer is provided as an upper clad.
- the width Wc of the input portions 51 and 52 is 600 nm
- the distance Wd between the input portion 51 and the input portion 52 is 350 nm
- the width Wa of the combining portion 53 is 1700 nm
- the width Wb of the output portion 55 is 840 nm.
- the length La of the combining portion 53 is 1000 nm
- the length Lb of the tapered portion 54 is 6000 nm.
- TE 0 has one elliptical mode distribution that is long in the width direction of the waveguide, and in the cross section of the multiplexing unit 53, two TE 1 are arranged in the width direction. Although it has a distribution, the mode distribution in the cross section of the input portions 51 and 52 is similar to the mode distribution in the cross section of the multiplexing unit 53.
- conversion multiplexing element 50 of this example TE 0 is attached to TE 1 multiplexers 53 of the input part 51 and 52, it is possible to convert a TE 0 to TE 1.
- FIG. 37 shows the result of the simulation of the excess loss of the conversion multiplexing element of this example by the finite element method (FEM). Even when the manufacturing error of the width of the waveguide core was ⁇ 25 nm, the excess loss was less than 0.35 dB over 1530 to 1630 nm (corresponding to C band and L band). From this, it can be seen that this structure has high manufacturing tolerance and small wavelength dependency.
- FEM finite element method
- FIG. 38 shows the result of electric field simulation of the conversion multiplexing device of this example by the FDTD method.
- the electric field value is normalized and displayed in the range of +1 to -1, where +1 is white and -1 is black.
- the nodes of TE 0 input from the two input portions are combined side by side at the multiplexing unit and output as the TE 1 mode.
- the TE 1 mode distribution has opposite signs in the waveguide width direction where Z is equal.
- TE 0 is not generated with the symmetrical mode distribution after multiplexing.
- the excess loss after TE 1 conversion can be 0.25 dB.
- the symmetric directional coupler shown in FIG. 33 is manufactured based on an SOI (Silicon on insulator) substrate made of Si—SiO 2 —Si, as in the first embodiment.
- the middle SiO 2 layer is used as the lower cladding, and the upper Si layer is used as the core.
- an SiO 2 layer is provided as an upper clad.
- the width W of each waveguide 61, 62 is 500 nm, and the gap G between the waveguides 61, 62 is 350 nm.
- the symmetrical directional coupler 60 of this embodiment uses the difference between the coupling length of TE 0 and the coupling length of TM 0 to set the length L at which the two waveguides 61 and 62 are arranged in parallel. by appropriately setting, from one waveguide to the other waveguide it can be transferred only TM 0. That is, a directional coupler is obtained in which the coupling loss of TM 0 is small and the coupling loss of TE 0 is large (TM 0 is easy to couple and TE 0 is difficult to couple to the other waveguide).
- the coordinates in FIG. 43A indicate normalized values with the total element length being 1.
- Core 2 is formed of Si
- the cladding 5 is formed of SiO 2.
- the start portion, the center portion, and the end portion are connected by a straight line.
- the high-order polarization conversion element shown in FIGS. 43A and 43B can be manufactured by etching the SI layer of the SOI substrate by etching and depositing SiO 2 thereon. This example shows that high-order polarization conversion according to the above embodiment is possible using simulation. First, the definition of the high-order polarization converter will be described.
- Ex (Ey) is dominant means that R TE (R TM ) is 0.7 or more, and 0.3 ⁇ R TE ⁇ 0.7 (0.3 ⁇ R TM When ⁇ 0.7), it is called a hybrid mode. Accordingly, the high-order polarization conversion unit is defined as a range having a waveguide mode of 0.3 ⁇ R TE ⁇ 0.7 (0.3 ⁇ R TM ⁇ 0.7) with respect to the light traveling direction. The In order to investigate the high-order polarization conversion unit in FIG.
- FIG. 44 shows the effective refractive index of this example
- FIG. 45 shows R TE and R TM of this example.
- # 0 is TE 0
- # 1 is TE 1 at the start unit 8
- # 2 is TM 0 at the start unit 8.
- the effective refractive index of # 1 monotonously decreases with respect to the traveling direction of light and is efficiently connected to the effective refractive index of the end portion.
- the Ex component is dominant at the start portion, but the hybrid mode occurs near the center of the element, and then the Ey component becomes dominant toward the end portion. That is, it is shown that high-order polarization conversion is possible by the two-stage tapered waveguide structure. Further, since the hybrid mode is in the central portion of the element, it is also shown that the high-order polarization conversion portion is located in the central portion.
- FIG. 46A and 46B it is the same as that of Example 10 except the width of the core 2.
- FIG. 48 shows the result of calculating R TE and R TM of # 1. 47 and 48, it can be seen that the higher-order polarization converter is generated on the end portion 9 side as compared with the tenth embodiment. Furthermore, it can be seen that the ratio of the high-order polarization conversion unit in the total element length is narrow, suggesting that the high-order polarization conversion efficiency is low.
- Example 10 shows the result of calculating the specific high-order polarization conversion efficiencies of Example 10 and Comparative Example 2 and the lengths of all element lengths at that time by simulation.
- the wavelength of the guided light was 1550 nm. From FIG. 49, it can be seen that when the comparison is made with the same conversion efficiency, Example 10 obtains a higher conversion efficiency at a shorter distance.
- the element length necessary to obtain a conversion efficiency of 90% is 7 ⁇ m in Example 10 and 22.5 ⁇ m in Comparative Example 2.
- the element length is shorter than 1/3 of Comparative Example 2. Enable. Therefore, in an optical circuit component in which optical elements are integrated on a plane, the area can be reduced by 1/9 or less.
- FIG. 50 shows the result of calculating the wavelength dependence of the conversion efficiency by simulation in Example 10.
- the element length was 10 ⁇ m.
- the conversion efficiency of high-order polarization conversion is 94% or more in the wavelength range of 1520 nm to 1640 nm, and the conversion efficiency is high in a wide wavelength range. Since this wavelength range includes C-band (1530-1565 nm) and L-band (1565-1625 nm) used in optical communication, this embodiment may be applicable to wavelength division multiplexing (WDM) communication. I understand. The reason why such a wide wavelength band can be obtained is that it is possible to design a high-order polarization converter at the center of the element as in this embodiment.
- WDM wavelength division multiplexing
- Example 11 A device based on Example 10 was prototyped and evaluated.
- the structure of the high-order polarization conversion element of this embodiment is the same as that of the tenth embodiment except for the width and height of the core 2.
- the width of the upper core 3 at the center portion (Z 0.5)
- FIG. 51 shows the measurement results. From FIG. 51, it was confirmed that a high conversion efficiency of 92% or more was obtained in the wavelength range of 1520 to 1640 nm.
- SYMBOLS 1 Optical waveguide, 2 ... Core, 3 ... Upper core, 4 ... Lower core, 5 ... Cladding, 6 ... Upper cladding, 7 ... Lower cladding, 8 ... Start part, 9 ... End part, 10 ... High-order polarization conversion Element, 11 ... first optical waveguide, 12 ... second optical waveguide, 13, 13r ... asymmetric directional coupler.
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Abstract
Description
本願は、2013年6月27日に、日本に出願された特願2013-135490号に基づき優先権を主張し、その内容をここに援用する。
また、光通信によって伝送される情報量の増加に応じて送受信器などの光回路部品の必要個数も増加する。そのため、限られたスペースの中で光回路部品を増やすために、光回路部品を構成する光素子の小型化と、高密度集積化が必要となる。
こうした課題に対して、加工が容易であり、集積化による小型化、大量生産による低コスト等のメリットを持つ、シリコンを用いた基板型光導波路(シリコン光導波路)による光回路部品(光変調器など)の研究及び開発が進められている。
シリコン光導波路は、コアに屈折率の大きなシリコン系材料(Si、Si3N4など)を、クラッドにコアとの屈折率差が大きい材料(SiO2、空気、Si3N4など)を利用した、いわゆる比屈折率差の大きな光導波路である。比屈折率差が大きいとコアへの光の閉じ込めが大きくなるため、急峻な曲げが可能になり、光素子の小型化、高密度集積化に適している。
特性が異なるこれらのモードに対して、光変調操作を行う場合、単一の基板型光導波路素子だけでは困難である。モードごとに最適化された基板型光導波路素子を必要とした場合、基板型光導波路素子の開発の面で大きな労力が必要となる。
シリコン光導波路による偏波変換素子の技術として、TE0をTE1に変換し、その後TE1をTM0に変換する方法が提案されている。
非特許文献1のFig.2(a)及びFig.2(b)に、その実施例が示されている。
非特許文献1の開示する光導波路素子は方向性結合器部分(結合部)とテーパ状光導波路部分(テーパ部)から成り、結合部の出射端がテーパ部に接続された構造となる。結合部はTE0をTE1に変換し、テーパ部はTE1をTM0に変換する基板型光導波路素子である。これらの2つの部分において使用される光導波路の屈折率の、導波方向に垂直な断面分布は非特許文献1のFig.1(a)及びFig.1(c)のグラフ中に示されている。これらの図では、コアと呼ばれる矩形部分とそのコアの下部に位置し屈折率がコアよりも低い水平な下部クラッドと、屈折率がコアより低く下部クラッドとは異なるコアを覆う上部クラッドが示されている。
非特許文献1のFig.1(a)及びFig.1(c)には、コア幅に対する実効屈折率のグラフが示されている。コアはSiで屈折率が3.455、下部クラッドはSiO2で屈折率が1.445、上部クラッドは空気(屈折率が1.0)又はSi3N4(屈折率が2.0)、コアの高さは220nmとしている。
また、非特許文献1のFig.1(b)には上部クラッドと下部クラッドとが等しい屈折率を持つ上下対称な屈折率断面形状の光導波路の実効屈折率のグラフが示されている。
これらの図からも分かるように屈折率断面が上下非対称な屈折率断面構造を持つ場合、幅方向の変化に対する各モードの実効屈折率の変化のグラフにおいて、上下対称な屈折率断面構造を持つ導波路では縮退していたTE1とTM0の点が分離している。
例えば、非特許文献1のFig.1(a)のグラフには、導波路幅0.7μm付近で、導波路幅が広がる間に、実効屈折率が2番目に高いモードはTM0(基本TMモード)からTE1(高次TEモード)へ、また、実効屈折率が3番目に高いモードはTE1(高次TEモード)からTM0(基本TMモード)へ変化することが示されている。このため、TE1とTM0は実効屈折率曲線状で連続的に繋がるので、導波路幅を緩やかに変化させることで損失の小さい高次偏波変換を行うことが可能となる。この現象を利用しているのが、前述の偏波変換素子におけるテーパ部で、導波路幅をTE1からTM0に変換する範囲で、緩やかに変化させるテーパ構造にすることで、高次偏波変換を行っている。
非特許文献2はFig.11などにおいて、入出力部の断面の一方の端部がリブ導波路の断面構造をもち、他方の端部は矩形導波路の断面構造をもつ高次偏波変換素子を開示している。
前記開始部と前記終了部との間において、前記上部コアの幅方向の両端がそれぞれ前記下部コアの前記幅方向の両端と常に重ならなくてもよい。
前記開始部と前記終了部との間において、前記上部コアの幅方向の一つの端が前記下部コアの前記幅方向の一つの端と常に重なってもよい。
前記開始部において、前記コアの高さが210nm以上230nm以下であり、前記コアの幅が700nm以上であり、かつ、前記終了部において、前記コアの高さが210nm以上230nm以下であり、前記コアの幅が620nm以下であってもよい。
前記下部コアと前記上部コアがSiからなり、前記下部クラッドと前記上部クラッドがSiO2からなってもよい。
前記上部コアの幅が前記開始部から中間部まで減少し前記中間部から前記終了部まで一定で、かつ前記下部コアの幅が前記開始部から前記中間部まで一定で、前記中間部から前記終了部までの間で減少してもよい。
前記第1の光導波路のTE0の実効屈折率と前記第2の光導波路のTE0の実効屈折率の差が0.2以上であってもよい。
また、本発明の第3態様に係るDP-QPSK変調器は、前記光導波路素子を備える。
図1A~図2Bに、本発明の高次偏波変換素子の構造を模式的に例示する。これらの高次偏波変換素子は、図1A及び図2Aに示すように、基板S上にコア2及びクラッド5を有する光導波路1を備える基板型光導波路素子から構成される。コア2の形状は、導波方向に垂直な断面において、コア形状が2つの幅を持つ矩形を重ねた形を成す。なお、本願では、単に「断面」と記載する場合は光の導波方向に垂直な断面を指す。
以下、上側の断面矩形状の部分を上部コア3、下側の断面矩形状の部分を下部コア4と呼び、下部コア4と下部クラッド7が接する部分を下底4a、上部コア3の上部を上底3aと呼ぶ。上部コア3は下部コア4と同じ材料からなる。クラッド5は、基板Sとコア2の間に設けられた下部クラッド7と、コア2及び下部クラッド7の上に設けられた上部クラッド6を有する。
つまり、基板S上に設けられる下部クラッド7上に、下部コア4と上部コア3とを有するコア2が設けられる。さらに、コア2及び下部クラッド7の上に上部クラッド6が設けられる。
このような断面を持つ光導波路のコア形状の一例を図1B及び図2Bに示す。図1Bは図1Aの断面を有する光導波路のコア形状の一例であり、図2Bは図2Aの断面を有する光導波路のコア形状の一例である。
すなわち、(1)終了部9における上部コア3の幅が開始部8における上部コア3の幅より小さいこと、(2)終了部9における下部コア4の幅が開始部8における下部コア4の幅より小さいこと、の少なくとも1つを満足することが好ましい。
なお、本願において「連続的に減少」とは、図2Aの上部コア3の幅及び下部コア4の幅のように開始部8から終了部9にかけて常に減少している場合だけでなく、図4Aの上部コア3の幅及び下部コア4の幅のように一定幅の部分がある場合も含まれる。
なお、以下においても開始部8からTE1を入力すると、終了部9からTM0として出力される構造であれば、終了部9にTM0を入力すると開始部8からTE1が出力される。
上下非対称な構造としては、図1A~図2Bに示すように、上部コア3の幅が下部コア4の幅より小さい構造に限られず、上部コア3の幅が下部コア4の幅より大きい構造とすることもできる。下部コア4の上により幅の小さい上部コア3を設ける場合、基板(及び下部クラッド)の上に設けたコア材料層の上部から一部をエッチング等で除去する方法により、同じ材料からなる上部コア3と下部コア4を容易に作製することができる。従って、下部コア4の上により幅の小さい上部コア3を設けることが好ましい。
この観点からは、図2A及び2Bに示すように、コアの幅方向の両端に段差を設けることが好ましい。さらに、下部コア4の幅が上部コア3の幅よりも大きく、上部コア3の下辺が下部コア4の上辺に含まれる場合は、図2Aの断面図に示すように、リブ導波路を作製するプロセスを利用することができる。
一方、図1A及び1Bに示すように、下部コア4と上部コア3の幅方向の一つの端が重なるようなコア形状では、段差が幅方向の反対側の一つの端のみに生じ、下部コア4の張り出しが大きくなる。従って、製造プロセスの要求精度が下がり、生産効率を高めることができる。
上部コア3と下部コア4は同じ材料からなることが好ましい。例えば、上部コア3と下部コア4をともにSiで構成することができる。コアのSiは、意図的な不純物(ドーパント)あるいは不可避の不純物を含んでもよい。
上部クラッドと下部クラッドとが同じ材料でないとしても、上部クラッドと下部クラッドとを同じ元素種から構成することもできる。ここで、「2つの材料が同じ元素種から構成される」ことの定義は、2つの材料を構成する元素がすべて同じであることをいう。例えば、シリコン(Si)の元素種はSiのみであり、シリカ(SiO2)の元素種はSi及びOである。Si及びOの2つの元素種から構成される材料は、SiO2と同じ元素種ということができるが、Siのみから構成される材料(Si等)や、Si及びO以外の元素種を含む材料(Si3N4等)は、SiO2と同じ元素種とはいえない。
上下対称な光導波路形状では、TE1の実効屈折率曲線とTM0の実効屈折率曲線とが交わり、TE1とTM0の実効屈折率は縮退する。この場合、この交点の前後でモードの変換は生じない。
一方で、上下非対称なコア形状をもつ光導波路の場合、非特許文献1で述べているように屈折率断面の屈折率分布が上下非対称になるため、上下対称なコア形状を持つ導波路では縮退していたTE1とTM0の点が分離している。このとき、TE1とTM0は同一の実効屈折率曲線上で連続的に繋がるので、導波路幅を緩やかに変化させることで高次偏波変換を行うことが可能となる。なお、詳細な具体例は、計算例1及び各実施例で述べる。
なお、図4Aでは、下部コア4が上部コア3の外側に出る階段状の部分に網かけを付けている。後述する図5A等でも、同様に平面図に網かけを付けた場合がある。
開始部8と終了部9との間のうち開始部8と終了部9とを除く部分では、上部コア3の幅方向の両端がそれぞれ下部コア4の幅方向の両端と常に重ならず、リブ構造と同様になっている。すなわち、図4Cに示すように、下部コア4の幅が上部コア3の幅よりも大きくなり、導波方向に垂直な断面における上部コア3の下辺が下部コア4の上辺に含まれている。上底から下底までの距離は、開始部8と終了部9のコア高さH1に等しく、下部コア4の高さH2は一定である。
この構造は、実効屈折率差が小さく偏波変換効率が低下するが、第1実施形態の構造に比べて上部コアに覆われていない下部コアの部分が広く、製造において第1実施形態のものより要求精度が下がり、再現性の高い構造の作製が可能である。
図7Aは、図5Aと同様に、上部コア3の片側に下部コア4がはみ出した段差部を有する。図7Bは、図4Aと同様に、上部コア3の両側に下部コア4がはみ出した段差部を有するが、上部コア3が下部コア4に対して幅方向の中心になくてもよく、左右非対称である。図7Cでは、上部コア3及び下部コア4のテーパ部分における幅の変化が連続的な変化であるが、線形(直線)でなくて曲線状である。つまり、開始部8から終了部9に向けて曲線状に上部コア3の幅が狭まっている。さらに、図7Dでは、開始部8から終了部9に向けて曲線状に上部コア3及び上部コア3の幅が狭まっている。
図4A,図5A,及び図7A~7Dでは、開始部8と終了部9との間のうち開始部8と前記終了部9とを除く部分で上部コア3の幅が下部コア4の幅よりも常に狭い。そのため、2回のエッチングによって、作成することが可能となる。そのため、例えば、SOI基板の上位層であるSI層をエッチングで削り、その上からSiO2を堆積させることで作製可能である。
また、光の進行方向に対して、図4Aのようにコア幅の変化は段階的でもよいし、図7Dのように連続的でもよい。コア幅の変化が段階的な方が設計が容易である。一方、コア幅の変化が連続的な場合はより滑らかに導波路構造を変化させることができ、さらなる低損失化が可能となる。
また、下部コアの上部コアに対してはみ出している部分は、図4A等のように光の進行方向に対して、両側の張り出していても良いし、図5Aのように片側だけ張り出していてもよい。両側に張り出している場合の方が高い高次偏波変換効率を持つ。片側に張り出している場合は、下部コアの張り出しをより広く保つことができるので、製造時に要求される解像度を緩和できる。
図39(b)は開始部から終了部までの断面が常に矩形で開始部8から終了部9にかけて幅が狭くなるコア2を有するテーパ導波路であり、図39(a)は図39(b)のテーパ導波路に高次偏波変換部74が埋め込まれた高次偏波変換素子である。
まず、高次偏波変換を行う場合、開始部8(入力断面)では、TE0の実効屈折率がTE1の実効屈折率よりも大きく、TE1の実効屈折率がTM0の実効屈折率よりも大きく、終了部9(出力断面)では、TE0の実効屈折率がTM0の実効屈折率よりも大きく、TM0の実効屈折率がTE1の実効屈折率より大きい必要がある。さらに、開始部8と終了部9との間のコア2が、連続的に導波路が接続された構造を有し、かつ図39(a)のように上部コア3と下部コア4の2段テーパ導波路構造を有する必要がある。なお、以下2段テーパ導波路構造を持つ部分を2段テーパ部73と呼ぶ。
一方、図39(b)で示すように上部コア3と下部コア4の幅が異なる場合、2段テーパ部73の断面は上下非対称な屈折率分布を持つ。上下非対称な屈折率分布では、TE1とTM0の実効屈折率は入れ替わらず(縮退点を持たず)、ハイブリッドモードと呼ばれるTE1とTM0が入り交じった導波モードが生じる。このハイブリッドモードを利用することで、高次偏波変換が行われる。なお、以下ではハイブリッドモードが生じる部分を高次偏波変換部74と呼ぶ。ただし、高次偏波変換を高い変換効率で行うには、高次偏波変換部74において、電界の連続的な変化(断熱変換)が行われるようにテーパ長を長くする必要がある。
この場合、上部コア3と下部コア4の高さを一定としたとき、開始部8と終了部9との実効屈折率の順番の条件を満たす場合、常に開始部8の上部コア3の幅は、終了部9の上部コア3の幅に比べて大きくなる。また、常に開始部8の下部コア4の幅は、終了部9の下部コア4の幅に比べて大きくなる。
これは次の理由による。
コア2の幅が狭いほど、幅方向の電界成分が支配的なTE1の光閉じ込めが弱くなる。光閉じ込めが弱いと、クラッド5に電界が広がり、コア2よりもクラッド5の屈折率の影響を受けるため、実効屈折率が低下する。それに対して、TM0は高さ方向の電界成分が支配的であるため、コア幅の狭窄化による実効屈折率の変化はTE1よりも小さい。そのため、前述の前提条件を満たすときは、常に開始部8のコアの幅は、終了部9のコア幅に比べて大きくなる。
例えば、図40に示すように、構造が定まった開始部8と終了部9に対して、上記実施形態の高次偏波変換素子を用いる際に全素子長に対する高次偏波変換部74の割合も任意に設計可能である。これは、光の導波方向に対する2段テーパの変化の仕方を調整することで可能となる。高次偏波変換素子全体に占める高次偏波変換部の割合が大きいほど、高次偏波変換の効率が高まる。そのため、より短い距離で高効率な変換が可能となる。ただし、損失が十分小さくなるように高次偏波変換部以外の部分の導波路を設定する必要がある。
また、側壁荒れの影響は、上部コア3と下部コア4との幅が大きいほど小さい。コア幅が大きいと、光がコア内部により閉じ込められ、その結果、コアの幅が変動した影響を小さくできるためである。その場合、開始部8に近いほどコア幅が大きいため、図42Bのように開始部8に近い位置に高次偏波変換部74を設けることで、側壁荒れによる高次偏波変換の効率低下を抑制することが可能となる。
一方、上記実施形態では、上部コア3及び下部コア4が開始部8の幅よりも広がることはない。従って、開始部で導波しない高次モードは、それ以降も導波モードとはならず、上記の問題は生じない。
図8Aは、上部コア3の片側に下部コア4がはみ出した段差部を有する。図8Bは、上部コア3の両側に下部コア4がはみ出した段差部を有するが、上部コア3が下部コア4に対して幅方向の中心にはなく、左右非対称である。図8Cでは、テーパ部分における上部コア3及び下部コア4の幅の変化が連続的な変化であるが、線形(直線)でなくて曲線状である。
第3実施形態の光導波路の場合、寸法の具体例として、開始部の断面において、リブ導波路のスラブとなる下部コアの高さが75~115nmであり、コア全体の高さが210~230nmであり、上部コアの幅が600nm以上であること、及び終了部の断面において、コア全体の高さが210~230nmであり、コア全体の幅が620nm以下であることが好ましい。このように、コア全体の高さ及び下部コアの高さを特定の値に統一して、コア幅のみを変更するようにすれば、SOI基板など特定の厚さのコア材料(Si)層を有する基板からエッチング等により下部コアを有する光導波路を作製する工程が容易になる。製造誤差を±10nmとした場合、開始部及び終了部の断面におけるコア全体の高さは、210nmから230nm程度が好ましく、下部コアの高さは、80nmから110nm程度が好ましい。
開始部における上部コアの幅は、終了部におけるコア全体の幅より広いことが好ましく、700nm以上がより好ましい。開始部における上部コアの幅が終了部におけるコア全体の幅以下である場合、開始部における下部コアの幅が終了部におけるコア全体の幅より広いことが好ましい。
また、本発明の高次偏波変換素子の第4実施形態の改変例を図10A~10Cに示す。図10A~10Cはそれぞれコアの平面図である。
なお、コア形状の左右に関する対称性は必ずしも必要とせず、図10A~10Cに示すように、上部コア3が下部コア4に対して中心になくてもよく、また、テーパ部分は連続的な変化であれば線形(直線)でなくてもよい。
第4実施形態の光導波路の場合、第1~3実施形態の同様の趣旨に基づいて、開始部の断面において、コア全体の高さが220nmに等しく、コア全体の幅が700nm以上であり、終了部の断面において、コア全体の高さが220nmに等しく、下部コアの高さが95nmに等しく、上部コアの幅が620nm以下であることが好ましい。
また、本発明の第5実施形態に係る高次偏波変換素子改変例を図12A~12Cに示す。図12A~12Cはそれぞれコアの平面図である。
なお、コア形状の左右に関する対称性は必ずしも必要とせず、図12A~12Cに示すように、上部コア3が下部コア4に対して中心になくてもよく、また、テーパ部分(上部コア3が下部コア4の幅)は連続的な変化であれば線形(直線)でなくてもよい。
第5実施形態の光導波路の場合、第3実施形態の同様の趣旨に基づいて、開始部の断面において、下部コアの高さが95nmであり、コア全体の高さが220nmであり、上部コアの幅が600nm以上であること、及び終了部の断面において、下部コアの高さが95nmであり、コア全体の高さが220nmであり、上部コアの幅が620nm以下であることが好ましい。開始部における上部コアの幅は、終了部における上部コアの幅より広いことが好ましく、700nm以上がより好ましい。開始部における上部コアの幅が終了部における上部コアの幅以下である場合、開始部における下部コアの幅が終了部における上部コアの幅より広いことが好ましい。
本発明の高次偏波変換素子は、同一基板上の光導波路において、他の素子と組み合わせて用いることができる。例えば、非対称方向性結合器と本発明の高次偏波変換素子とを組み合わせることで偏波変換素子を実現することが可能である。この偏波変換素子は、TE0を非対称方向性結合器によってTE1に変換し、TE1を高次偏波変換素子によってTM0に変換する。
図13A及び13Bに、本発明の高次偏波変換素子を非対称方向性結合器と組み合わせた偏波変換素子の一例を示す。図13Aはコアの平面図であり、図13Bは非対称方向性結合器における断面図である。この構造の詳細は、実施例6として後述する。
第1の光導波路11にはTE0が導波する。また、第2の光導波路12にはTE1が導波する。第1の光導波路11のTE0と第2の光導波路12のTE1とが近い実効屈折率を持つため、第1の光導波路11から第2の光導波路12へと結合可能である。第1の光導波路11に接続される入力側の導波路を第1のポート11aとし、第2の光導波路12に接続される入力側の導波路を第2のポート12aとする。第2の光導波路12の出力側にある第3のポート12bは、高次偏波変換素子10の開始部8に接続される。図13Aに示す高次偏波変換素子10は、一例として図4A~4Dと同様な構造を示しているが、特にこれに限定されない。
一方、第2のポート12aにTE0を入力すると、非対称方向性結合器13では、第2の光導波路12のTE0の実効屈折率が第1の光導波路11のどのモードの実効屈折率とも大きく異なる。そのため、モード結合や変換が起きない。さらに高次偏波変換素子10においてもTE0はモード変換しないため、第3のポート12bから入力されるTE0は高次偏波変換素子10の終了部9までほとんど損失無く透過する。従って、第1のポート11aと第2のポート12aへ同時にTE0を入力すると、本構造の出力部である高次偏波変換素子10の終了部9では、TE0とTM0とが合波した出力が得られる。つまり、本構造は、偏波変換と偏波合波の機能を兼ね備えた素子として動作することも可能である。
第2の光導波路12のTE0の実効屈折率が第1の光導波路11のモードの実効屈折率とも異なる程度としては、非対称方向性結合器13の第1の光導波路11のTE0の実効屈折率と第2の光導波路12のTE0の実効屈折率の差が0.2以上であることが好ましい。
この偏波変換素子では、非対称方向性結合器13rがリブ型導波路から構成される。2つの光導波路のリブ11r,12rの間にスラブ13sが形成され、各リブ11r,12rの外側にもそれぞれスラブ11s,12sが形成されている。
本実施形態の偏波変換素子は、参考文献(P. Dong, C. Xie, L. Chen, L. L. Buhl, and Y.-K. Chen, “112-Gb/s Monolithic PDM-QPSK Modulator in Silicon,” European Conference and Exhibition on Optical Communication, Vol. 1, p. Th.3.B.1, June 16, 2012)に開示されているような偏波多重4値位相変調(DP-QPSK:Dual Polarization-Quadrature Phase Shift Keying)に使用することが可能である。図15にDP-QPSK変調器の一例を模式的に示す。このDP-QPSK変調器20は、通常の光導波路にTE0とTM0の2つのモードが存在できることを利用して、TE0/TM0の両モードに独立したQPSK信号を有する、DP-QPSK変調を行う。具体的には、入力部21からTE0で入力した光を2つの光導波路22,22に分岐し、QPSK変調器23,23により各々QPSK信号に変調した後、光導波路24,24の片側のTE0を偏波変換素子25によりTM0に変換させて、2つのモードを偏波ビームコンバイナで同一の光導波路上に合成し、TE0とTM0に独立した信号を出力部26に出力する。
なお、TE0とTM0を変調する方式はQPSKに限らず、複雑な構成を持つ変調器であっても、本実施形態の偏波変換素子を用いて偏波多重を行うことが可能である。
本実施形態の偏波変換素子は、参考文献(C. Doerr et al., “Packaged Monolithic Silicon 112-Gb/s Coherent Receiver,” IEEE Photonics Technology Letters, Vol. 23, pp. 762-764, 2011)で開示されているような、TE0とTM0を同時に伝送した偏波多重信号のSi光導波路上のコヒーレント受信機に使用することが可能である。図16に、偏波ダイバーシティ・コヒーレント受信機の一例を模式的に示す。このコヒーレント受信機30は、TE0とTM0とを同時に伝送した偏波多重信号の光導波路31を、偏波変換と偏波ビームスプリッタが同時に行える偏波変換素子32に接続し、光導波路33の一方にはTE0の信号を分岐させる。また、光導波路33の他方にはTM0から変換したTE0の信号を分岐させる。局発光34として、一般的に用いられる半導体レーザ光源は片偏波のみ、例えばTE0(local)の出力を用いる。このような光源を用いる場合、通常局発光の偏波変換が必要となる。しかし、図16のコヒーレント受信機30では、信号光は偏波分離後の光導波路33にいずれもTE0の信号(signal)が導波されるので、局発光の偏波変換が不要になる。信号光と局発光は、光合波部35を経て、結合部36から出力される。
偏波変換素子32に光導波路型の構造を用いる場合、結合部36における素子外部との光の結合には、基板側方より結合する逆テーパ型のモードフィールド変換器など、偏波分離機能を持たない結合器を利用することが可能である。結合器には、例えば参考文献(Qing Fang, et al., “Suspended optical fiber-to-waveguide mode size converter for silicon photonics,” Optics Express, Vol. 18, Issue 8, pp. 7763-7769 (2010))に開示されている、逆テーパ型の構造が開示できる。
本実施形態の偏波変換素子は、参考文献(Hiroshi Fukuda et al., “Silicon photonic circuit with polarization diversity,” Optics Express, Vol. 16, Issue 7, pp. 4872-4880 (2008))で開示されているような、TE0とTM0が同時に伝送される偏波多重伝送や、片方の偏波がランダムに伝送されるときに、両モードに対して同様の操作を与えるための素子を利用したい場合、図17に示すような偏波ダイバーシティ方式を実行するために用いることができる。図17に示す偏波ダイバーシティ方式40では、TE0とTM0が同時に伝送される偏波多重信号の光導波路41を、偏波変換と偏波ビームスプリッタが同時に行える偏波変換素子42に接続し、光導波路43の一方にはTE0の信号を分岐させる。また、光導波路43の他方にはTM0から変換したTE0の信号を分岐させる。素子44で操作されたTE0の信号光は、光導波路45から偏波変換素子46で合成して、TE0とTM0が同時に伝送される偏波多重信号の光導波路47に出力する。
偏波変換素子46には、図15に示すDP-QPSK変調器20と同様に、偏波変換と偏波ビームコンバイナが同時に行える本発明の偏波変換素子を用いることができる。
分岐部と本実施形態の高次偏波変換素子を組み合わせることで、偏波変換が可能である。分岐部としては、2×1のMMI(マルチモード干渉計)及びY分岐が挙げられる。これらは、2つの入力部分に入力する電界の位相を調整することで、TE1を発生させることが可能である。もしくは、片側からのみTE1を入力してもよい。そのため、この後段に本実施形態の高次偏波変換素子を接続することで、このTE1をTM0に変換することができ、偏波変換の機能を持たせることが可能となる。
また、参考文献(Wangqing Yuan, et al., “Mode-evolution-based polarization rotator-splitter design via simple fabrication process,” Optics Express, Vol. 20,Issue 9, pp. 10163-10169 (2012))では、非対称なY分岐と高次偏波変換素子とを接続することで偏波ビームスプリッタと偏波変換の機能を同時に実現しているが、非対称なY分岐と本実施形態を用いることでも同様の効果を実現することが可能である。
各入力部分51,52に入力する2つのTE0の位相差をπとすることが好ましい。入力が非対称(反対称)なモード分布を持つことにより、合波後は対称なモード分布を持つTE0は発生しなくなる。これにより、合波後に不必要となるTE0の発生を防ぎ、偏波消光比の低下を抑制することができる。
本発明と同様の原理で、実効屈折率曲線においてTE2n+1モード(nは0以上の整数)とTM0が交わる縮退点を本発明と同様の上下非対称構造によって分離することができ、その間をテーパ化することで変換を行うことができる。ここで、TE2n+1は、TEモード(TE0,TE1,TE2,・・・)の中で(2n+2)番目に実効屈折率が高いモードをいう。TE1モードは、n=0のTE2n+1モードである。
TEモードの奇数次のモードが変換対象になるのは以下の理由による。矩形状コア(幅方向と高さがともに対称な構造(屈折率分布))を伝搬するTM0は、その電界のx成分(Ex)は、幅方向と高さ方向でともに反対称な分布になる。一方、TE1を含む奇数次のTEモードのExは、幅方向に対して反対称、高さ方向に対しては対称な電界分布になる。そのため、屈折率分布を高さ方向に対して非対称にすることで、TE2n+1の高さ方向の対称性が崩れ、TM0と相互作用して縮退点付近でそれぞれのモードが混ざり、縮退点が分離する。そのため、本発明と同様の構造により、TE2n+1モードは、TM0との間で変換が可能である。
<計算例1>
図2A及び2Bに示すようなコア形状が凸型の光導波路の下底を変化させたとき、実効屈折率が高い順に4つのモードの実効屈折率の変化のグラフを図18に示す。波長は1.55μm、コアはSiで屈折率が3.48、上部クラッド及び下部クラッドはSiO2で屈折率が1.44、光導波路の高さが0.22μm、下部コアの高さが0.095μm、上部コアの幅が0.5μmとし、上部コアは下部コアの中心に位置している。
図18から分かるように、コア断面形状の上下非対称性のため、TE1とTM0は縮退することなく、実効屈折率差を常に持つ。図18において、破線の楕円(下底0.7μm付近)として、モード変換部の概略範囲を示した。このモード変換部では、実効屈折率が2番目に高いモードは、下底が狭いときTM0となり、下底が広いときTE1となる。逆に、実効屈折率が3番目に高いモードは、下底が狭いときTE1となり、下底が広いときTM0となる。よって、同一の実効屈折率曲線(実効屈折率が2番目に高いモードまたは3番目に高いモードのいずれか)をたどることで、それぞれのモードは変換可能である。
以上より、連続的にTE1とTM0とが変換していく様子を見ることができる。このことからも、TE1とTM0の間で相互に偏波を変換することが可能であることが分かる。
比較例1では、図3A及び3Bに示す導波路101において、コア102がSi、下部クラッド103の材料がSiO2であり、上部クラッド104が空気からなり、コア102の高さH0が0.22μm、開始部の幅W1が0.84μmであり、終了部の幅W2が0.5μmであり、長手方向の長さL0が35μmであり、開始部の矩形導波路からコア幅を長さ方向に対して線形に変化させた構造とした。有限差分時間領域(Finite-Difference Time Domain:FDTD)法によるシミュレーションにより求めた高次偏波変換の変換損失(出力されるTM0のパワー/入力されるTE1のパワー、で表される比)は0.587dBであった。
計算例1を踏まえて、上部コアが下部コアの中心にくる構造(上述した第1実施形態を参照)を有する実施例1の導波路素子を作製する。
図4A~4Dに本実施例の素子の図を示す。本実施例では、Si-SiO2-SiからなるSOI(Silicon on insulator)基板をもとに導波路を作製する。中間のSiO2層を下部クラッドとして、上部のSi層をコアとして用いる。コア形成後、上部クラッドとしてSiO2層を設ける。
図18及び図23によれば、実効屈折率が2番目に高いモードと3番目に高いモードとが最も接近する点における両モードの実効屈折率差は0.16である。
また、上部コアが幅方向に+60nmずれた場合でも(上部コアが下部コアの無い範囲にずれた場合は、その範囲に下部コアが新たに作られるとする)、最も接近する点の実効屈折率差は0.15であり、実効屈折率曲線は分離する(縮退しない)ので、高次偏波変換が可能である。
図5A~5Dに示すように、上部コアと下部コアの端が一致する構造(上述した第2実施形態を参照)の高次偏波変換素子を、実施例1と同様の方法で作製する。
下部コア4の高さH2が0.095μmであり、上部コア3の高さ(H1-H2の差)が0.125μmであり、下部コア4と上部コア3とを合わせたコア高さH1は0.22μmである。開始部8では上部コア及び下部コアの幅W1が0.84μmであり、区間L2(長さ15μm)で上部コア3の幅W2が0.5μmである。区間L1(長さ20μm)で下部コア4の幅W1は0.84μmであり、区間L1とL2の全長にわたって上部コアと下部コアの端が一致するように位置する。終了部9では上部コアと下部コアの幅W2がともに0.5μmである。
図6A~6Cに示すように、開始部8の断面形状がリブ導波路で、終了部9の断面形状が矩形導波路となり、上部コア3と下部コア4との幅方向の中心が一致する構造(上述した第3実施形態を参照)の高次偏波変換素子を、実施例1と同様の方法で作製する。
下部コア4の高さH2が0.095μmであり、上部コア3の高さ(H1-H2の差)が0.125μmであり、下部コア4と上部コア3とを合わせたコア高さH1は0.22μmである。開始部8では、上部コア3の幅W1aが1μmであり、下部コア4の幅W1が4μmである。終了部9では、上部コアと下部コアとの幅W2がともに0.5μmである。開始部8の最大のコア幅が4μmであり、有限ではあるが、この断面を通るTE1の分布は開始部8の下部コア4の両端よりも十分内側にある。そのため、スラブ幅が十分に大きく、リブ導波路とみなすことが可能である。
図9A~9Cに示すように、開始部8の断面形状が矩形導波路で、終了部9の断面形状が凸型となり、上部コア3と下部コア4の幅方向の中心が一致する構造(上述した第4実施形態を参照)の高次偏波変換素子を、実施例1と同様の方法で作製する。
下部コア4の高さH2が0.095μm、上部コア3の高さ(H1-H2の差)が0.125μm、合わせたコア高さH1は0.22μmである。開始部8では、上部コアと下部コアの幅W1がともに0.8μmである。終了部9では、上部コア3の幅W2aが0.44μmであり、下部コア4の幅W2が0.5μmである。この場合、下底と上底(単位μm)の寸法関係は、「上底=0.8-1.2×(0.8-下底)」である。
図11A~11Cに示すように、開始部8と終了部9の断面形状が凸型となり、上部コア3と下部コア4の幅方向の中心が一致する構造(上述した第5実施形態を参照)の高次偏波変換素子を、実施例1と同様の方法で作製する。
下部コア4の高さH2が0.095μm、上部コア3の高さ(H1-H2の差)が0.125μmであり、合わせたコア高さH1は0.22μmである。開始部8では、上部コア3の幅W1aが0.7μmであり、下部コア4の幅W1が1.1μmである。終了部9では、上部コア3の幅W2aが0.3μmであり、下部コア4の幅W2が0.7μmである。この場合、上底と下底(単位μm)の寸法関係は、「下底=上底+0.4」である。
図13A及び13Bに、実施例1の高次偏波変換素子を非対称方向性結合器と組み合わせた偏波変換素子の1つの実施例を示す。
図13Bに示す非対称方向性結合器13の断面において、第1の光導波路11のコア幅Eは0.4μmであり、第2の光導波路12のコア幅Fは0.84μmであり、第1の光導波路11及び第2の光導波路12のコアの高さHはともに0.22μmであり、2つの導波路の間隔Gは0.35μmとする。
第1の光導波路11は、長さL=54μmの直線部とそれに接続される半径40μmの曲げ半径を持つ曲げ導波路からなる。第2の光導波路12は、長さL=54μmの直線部とそれに接続される半径40μmの曲げ半径を持つ曲げ導波路からなる。各導波路の直線部は互いに平行で、両端面がそれぞれ同一平面上にある。
第1のポート11aに入力したTE0が、非対称方向性結合器13で第2の光導波路12のTE1に結合し、第3のポート12bよりTE1として出力されるとき、TE0からTE1に変換される変換損失を、FDTD法によるシミュレーションで求めた。本構造によれば、変換損失は、波長が1550nmのとき、0.105dBとなった。このことより、本構造によってTE0とTE1との間の変換が可能であることが分かる。
高次偏波変換素子10の変換損失が実施例1で述べたように0.004dBであるので、非対称方向性結合器13と高次偏波変換素子10を合わせた偏波変換素子として全体の変換損失は、0.109dBとなり、偏波変換が可能であることが分かる。
図14A及び14Bに示すように非対称方向性結合器がリブ型導波路である偏波変換素子も作製可能である。図14A及び図14Bに示す非対称方向性結合器13rにおいて、リブ型導波路の長さLrは20.8μmである。第1の光導波路のリブ11rの幅W1rは0.4μmであり、第2の光導波路のリブ12rの幅W2rは0.95μmであり、リブ11r,12rの高さH1rは0.22μmである。第1の光導波路の外側のスラブ11sの幅W1sは0.8μm以上、第2の光導波路の外側のスラブ12sの幅W2sは1.1μm以上、2つの導波路の間のスラブ13sの幅W3sは0.3μm、スラブの高さH1sは0.095μmである。
図32に示す、TE1(TE1)を発生させる変換合波素子を、実施例1と同様に、Si-SiO2-SiからなるSOI(Silicon on insulator)基板をもとに作製する。中間のSiO2層を下部クラッドとして、上部のSi層をコアとして用いる。コア形成後、上部クラッドとしてSiO2層を設ける。
入力部分51,52の幅Wcは600nmであり、入力部分51と入力部分52との間隔Wdは350nmであり、合波部53の幅Waは1700nmであり、出力部分55の幅Wbは840nmである。合波部53の長さLaは1000nmであり、テーパ部分54の長さLbは6000nmである。
図33に示す、対称方向性結合器を、実施例1と同様に、Si-SiO2-SiからなるSOI(Silicon on insulator)基板をもとに作製する。中間のSiO2層を下部クラッドとして、上部のSi層をコアとして用いる。コア形成後、上部クラッドとしてSiO2層を設ける。各導波路61,62の幅Wは500nmであり、導波路61,62の間隔Gは350nmである。
FDTD法によるシミュレーションの結果、導波路コアの製造誤差が±25nmでも、過剰損失は小さく、1530~1630nm(Cバンド及びLバンドに相当)という広い波長範囲で、TM0の結合損失は0.6dB未満、TE0の結合損失は12dB以上となった。このことから、本構造が高い製造トレランスと小さな波長依存性を有することが分かる。
第1実施形態に基づく構造を有する実施例10の高次偏波変換素子を図43Aに示し、その中央部(Z=0.5)における断面図を43Bに示す。ここで、図43Aの座標は、素子全長を1とした規格化された値を示している。コア2はSiで形成され、クラッド5はSiO2で形成される。開始部8(Z=0)における幅W1=850nm、終了部9(Z=1)の幅W2=500nm、中央部(Z=0.5)における上部コア3の幅及び下部コア4の幅をそれぞれW3=500nm、W4=850nmに設定した。上部コア3の幅及び下部コア4ともに開始部、中央部、終了部は直線で結ばれている。また、コア2の高さ及び下部コア4の高さはそれぞれH1=220nm、H2=90nmで一定とした。
図43A及び43Bに示す高次偏波変換素子は、SOI基板のSI層をエッチングで削り、その上からSiO2を堆積させることで作製可能である。本実施例で上記実施形態に係る高次偏波変換が可能であることをシミュレーションを用いて示す。
まず、高次偏波変換部の定義について述べる。導波モードの支配的な電界成分がExかEyかを定量化に示すために、以下の式(1)及び式(2)のようにそれぞれの割合を示す量RTE、RTMを定義する。
ここで、Hx、Hyはそれぞれ幅方向と高さ方向の磁界成分であり、積分は幅方向、高さ方向を含む平面全体で行う場合を想定する。Ex(Ey)が支配的な導波モードでは、Hy(Hx)が支配的であり、電界成分と磁界成分の積の積分値は、電力の次元を持つことから、RTE(RTM)は、Ex(Ey)の電力割合を示している。本明細書では、Ex(Ey)が支配的であるとは、RTE(RTM)が0.7以上の場合をいい、0.3<RTE<0.7(0.3<RTM<0.7)のとき、ハイブリッドモードと呼ぶ。したがって、高次偏波変換部は、光の進行方向に対して、0.3<RTE<0.7(0.3<RTM<0.7)の導波モードを持つ範囲と定義される。
図43Aの高次偏波変換部を調べるため、光の進行方向の座標(z)に対して、実施例10の高次偏波変換素子を導波するモードの実効屈折率のグラフ(実効屈折率の大きい順に、#0、#1、#2と呼称している)と、#1のRTE、RTMを計算した。それぞれの結果を図44及び45に示す。図44は本実施例の実効屈折率を示し、図45は本実施例のRTE、RTMを示す。図44において、#0はTE0であり、#1は開始部8でTE1、#2は開始部8でTM0となる。図44より、#1の実効屈折率は、光の進行方向に対して単調に減少し、効率的に終了部の実効屈折率へと接続されていることが分かる。図45を見ると、開始部ではEx成分が支配的であるが、素子の中央付近でハイブリッドモードが生じ、その後終了部にかけてEy成分が支配的となる。即ち、2段テーパ導波路構造により、高次偏波変換が可能であることが示されている。また、ハイブリッドモードが素子の中央部にあることから、高次偏波変換部は、その中央部に位置することも示される。
続いて、実施例10に対する比較例2の高次偏波変換素子の構造を図46Aに示し、その中央部(Z=0.5)における断面図を46Bに示す。ここで、図46A及びBにおいてコア2の幅以外は実施例10と同様である。比較例2において開始部8(Z=0)における幅W1=850nm、終了部9(Z=1)の幅W2=500nm、中央部(Z=0.5)における上部コア3の幅及び下部コアの幅をそれぞれW3=550nm、W4=1550nmに設定した。
図47に、比較例2の構造に対して、光の進行方向の座標(z)に対して、実施例の構造中を導波するモードの実効屈折率(実効屈折率の大きい順に、#0、#1、#2と呼称している)を計算した結果を示す。また、図48に#1のRTE、RTMを計算した結果を示す。図47及び48より、高次偏波変換部は実施例10に比べ終了部9側で生じていることが分かる。さらに、全素子長に占める高次偏波変換部の割合が狭いことが分かり、高次偏波変換効率が低いことが示唆される。
実施例10と比較例2の具体的な高次偏波変換効率と、そのときの全素子長の長さをシミュレーションで計算した結果を図49に示す。導波光の波長は1550nmとした。図49より、同じ変換効率で比較した場合、実施例10の方がより短い距離で高い変換効率を得ていることが分かる。例えば、90%の変換効率を得るのに必要な素子長は、実施例10では7um、比較例2では22.5umとなり、本実施例10では比較例2の1/3以下の素子長の短尺化を可能とする。
したがって、平面上に光素子を集積する光回路部品においては、1/9以下の面積削減が可能である。
上記実施例10に基づくデバイスを試作し、評価を行った。本実施例の高次偏波変換素子の構造はコア2の幅及び高さ以外は実施例10と同様である。本実施例において開始部8(Z=0)における幅W1=860nm、終了部9(Z=1)の幅W2=500nm、中央部(Z=0.5)における上部コア3の幅及び下部コアの幅をそれぞれW3=500nm、W4=860nmであった。また、コア2の高さ及び下部コア4の高さはそれぞれH1=220nm、H2=95nmであった。
図51にその測定結果を示す。図51より、1520-1640nmの波長範囲で、92%以上の高い変換効率が得られることが確かめられた。
Claims (10)
- 基板型光導波路を構成する高次偏波変換素子であって、
基板と、
前記基板上に設けられる下部クラッドと、
前記下部クラッド上に設けられ、断面矩形状で一定の高さを有する下部コアと、前記下部コアと同じ材料で形成されかつ前記下部コアの上に連続して配置される断面矩形状で一定の高さを有する上部コアとを有するコアと、
前記コア及び前記下部クラッドの上に設けられ、前記下部クラッドと同じ材料で形成される上部クラッドと、を備え、
前記コアは、前記下部コアの幅と前記上部コアの幅とが同じである開始部から、前記下部コアの幅と前記上部コアの幅とが同じである終了部まで光が導波可能な光導波路を構成し、
少なくとも前記上部コアの幅及び前記下部コアの幅のうちの一方は、前記開始部と前記終了部との間で前記光の導波方向に対して連続的に減少しかつ前記上部コアの幅及び前記下部コアの幅の両方が前記開始部から前記終了部まで増加せず、
前記開始部において、TE0の実効屈折率がTE1の実効屈折率よりも大きく、前記TE1の実効屈折率がTM0の実効屈折率よりも大きく、
前記光導波路の終了部において、前記TE0の実効屈折率が前記TM0の実効屈折率よりも大きく、前記TM0の実効屈折率が前記TE1の実効屈折率よりも大きく、
前記開始部と前記終了部との間の前記光導波路のうち前記開始部と前記終了部とを除く部分において、前記コアは前記上部コアの幅と前記下部コアの幅とが異なる上下非対称な構造を有し、
前記高次偏波変換素子は、前記開始部のTE1と前記終了部のTM0との間で高次偏波変換をする高次偏波変換素子。 - 前記開始部と前記終了部との間において、前記下部コアの幅が前記上部コアの幅よりも常に大きくなり、前記光が導波する方向に垂直な断面において前記上部コアの下辺が前記下部コアの上辺に常に含まれる請求項1に記載の高次偏波変換素子。
- 前記開始部と前記終了部との間において、前記上部コアの幅方向の両端がそれぞれ前記下部コアの前記幅方向の両端と常に重ならない請求項2に記載の高次偏波変換素子。
- 前記開始部と前記終了部との間において、前記上部コアの幅方向の一つの端が前記下部コアの前記幅方向の一つの端と常に重なる請求項2に記載の高次偏波変換素子。
- 前記開始部において、前記コアの高さが210nm以上230nm以下であり、前記コアの幅が700nm以上であり、かつ、前記終了部において、前記コアの高さが210nm以上230nm以下であり、前記コアの幅が620nm以下である請求項1に記載の高次偏波変換素子。
- 前記下部コアと前記上部コアがSiからなり、前記下部クラッドと前記上部クラッドがSiO2からなる請求項1~5のいずれか1項に記載の高次偏波変換素子。
- 前記上部コアの幅が前記開始部から中間部まで減少し前記中間部から前記終了部まで一定で、かつ前記下部コアの幅が前記開始部から前記中間部まで一定で、前記中間部から前記終了部までの間で減少する請求項1~6のいずれか1項に記載の高次偏波変換素子。
- 光導波路素子であって、
請求項1~7のいずれか1項に記載の高次偏波変換素子と、
前記高次偏波変換素子が接続されていない第1の光導波路と、前記高次偏波変換素子の前記開始部と接続された第2の光導波路とで構成される方向性結合器と、を備え、
前記第1の光導波路にはTE0が導波し、前記第2の光導波路にはTE1が導波し、前記第1の光導波路のTE0が前記第2の光導波路のTE1と結合可能である光導波路素子。 - 前記第1の光導波路のTE0の実効屈折率と前記第2の光導波路のTE0の実効屈折率の差が0.2以上である請求項8に記載の光導波路素子。
- 請求項8又は9に記載の光導波路素子を備えたDP-QPSK変調器。
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EP3287821A4 (en) * | 2015-05-08 | 2018-05-16 | Huawei Technologies Co. Ltd. | Tapered waveguide and silicon-based chip |
JP2017044713A (ja) * | 2015-08-24 | 2017-03-02 | 日本電信電話株式会社 | モード変換器及びモード変換方法並びにモード合分波器及びモード合分波方法 |
JP2017090575A (ja) * | 2015-11-05 | 2017-05-25 | 株式会社フジクラ | 光合分波素子及び光変調器 |
JP2021124578A (ja) * | 2020-02-04 | 2021-08-30 | 富士通株式会社 | 光回路素子、これを用いた光送受信器、及び光回路素子の製造方法 |
US11726259B2 (en) | 2020-02-04 | 2023-08-15 | Fujitsu Optical Components Limited | Optical circuit element, optical communication apparatus, and method for manufacturing optical circuit element |
WO2021161747A1 (ja) * | 2020-02-14 | 2021-08-19 | Tdk株式会社 | 光導波路素子及び光変調素子 |
JP7477761B2 (ja) | 2020-06-09 | 2024-05-02 | 富士通オプティカルコンポーネンツ株式会社 | モード変換素子 |
Also Published As
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CN105408786B (zh) | 2017-05-10 |
US9557482B2 (en) | 2017-01-31 |
CN105408786A (zh) | 2016-03-16 |
JPWO2014208601A1 (ja) | 2017-02-23 |
JP5728140B1 (ja) | 2015-06-03 |
US20160178841A1 (en) | 2016-06-23 |
SG11201510495TA (en) | 2016-01-28 |
EP3002616A1 (en) | 2016-04-06 |
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