JP2017044780A - Polarization identification element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization independent type optical waveguide element with which it is possible to extend the width of an usable wavelength band.SOLUTION: The polarization independent type optical waveguide element comprises a polarization rotation unit 10 and an optical branch unit 22. When one of a TM wave basic propagation mode light and a TE wave basic propagation mode light or a mixed light of both is inputted to the polarization rotation unit 10, the input light is converted to the TE wave basic propagation mode when it is the TM wave basic propagation mode and not converted when it is a light of the TE wave basic propagation mode, before being inputted to the optical branch unit 22. The optical branch unit 22 is provided with a center waveguide 32c and two side installed waveguides 32a, 32b, both being a width tapered waveguide. An area is provided in which the propagation constant of the TE wave basic propagation mode or the TE wave basic propagation mode of the center waveguide 32c and the TE wave basic propagation mode of the two side installed waveguides 32a, 32b are equal, a propagation light of the TE wave basic propagation mode being outputted from the two side installed waveguides 32a, 32b.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical waveguide element that is formed using a silicon material and splits light independent of polarization, and to a wavelength separation element and a polarization identification element including the optical waveguide element.

  In recent years, an optical waveguide element using silicon as a core material of an optical waveguide has attracted attention from the viewpoint of miniaturization and mass productivity. A silicon waveguide core having an optical waveguide structure with a material having a refractive index lower than that of the core material (for example, silicon oxide) as a clad is called a silicon fine wire waveguide.

  The silicon thin wire waveguide can take a very large refractive index difference between the core and the clad, so that it is possible to efficiently confine light energy in the silicon waveguide core. Therefore, the inner diameter of the silicon waveguide core can be set to a submicrometer dimension, and the bending can be performed with a sufficiently small radius of curvature. For example, if a silicon thin wire waveguide is used, the radius of curvature can be reduced to about 1 μm, so that a compact element can be realized.

  A wavelength separation element using light interference is known as a silicon wire waveguide element (for example, see Non-Patent Documents 1 and 2 and Patent Document 1). However, these wavelength separation elements have polarization dependency in the separation operation.

  As an element that can operate independently of polarization, an element in a form that first performs polarization separation of input light and individually controls each polarization component to realize functions such as wavelength separation is known ( Non-Patent Document 3). If such a form is adopted, it is necessary to provide an independent functional part for realizing a function such as wavelength separation for each polarization component, and the shape of the element increases.

  An element using a polarization-independent waveguide is also known (see Non-Patent Document 4). In this case, it is necessary to design the polarization-independent waveguide so that the operation characteristics are uniform for each polarization, and the design is complicated and the device fabrication accuracy is also high.

  Therefore, a polarization rotation unit is provided, and the polarization direction of the input light is unified by this polarization rotation unit, and an optical branching device or a wavelength separator that operates in the unified polarization direction is connected to the polarization rotation unit. A structure provided in the subsequent stage is conceivable. Conventionally, an element using a high refractive index upper clad and a rib waveguide or an element using a low refractive index upper clad and a rib waveguide (for example, refer to Non-Patent Document 4) is disclosed as a polarization rotation unit. Has been. Further, the inventors of the present application have studied a polarization rotation element using a waveguide whose trapezoidal vertical cross-sectional shape is the core of the waveguide.

F. Horst, “Silicon integrated waveguide devices for filtering and wavelength demultiplexing” Technical digest OFC / NFOEC 2010, paper OWJ3, March 2010 S. Pathak, et al., “Compact SOI-based polarization diversity wavelength de-multiplexer circuit using two symmetric AWGs” OPTICS EXPRESS vol. 20, No. 26, pp. B493-B500 (2012). G. T. Reed, et al., “Waveguides and devices in silicon Photonics: Polarization independence” Proceedings of SPIE vol. 6476, 647602 (2007). Daoxin Dai, et al., “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires” Optics Express vol. 19, No. 11, pp. 10940-10949 (2011).

JP 2009-198914

  However, there is no optical branching element that branches the output light whose polarization direction is unified at the polarization rotating part over a wide wavelength band, and the usable wavelength band of this optical branching element provided at the subsequent stage of the polarization rotating part is expanded. Is an issue.

  The inventor of the present application has a configuration in which the optical branching element is configured such that the central waveguide and the side-installed waveguides are arranged symmetrically with respect to the central waveguide. It has been found that the operating wavelength band can be expanded by using a total of three waveguides as width tapered waveguides.

  Accordingly, an object of the present invention is a polarization-independent optical branching device including a polarization rotation unit and an optical branching unit, and can broaden the usable wavelength band (operating wavelength band). It is to realize an independent optical waveguide device.

  The optical waveguide device of the present invention includes a polarization rotation unit and an optical branching unit.

  The polarization rotation unit receives light of TM (Transverse Magnetic) wave basic propagation mode, light of TE (Transverse Electric) wave basic propagation mode, or light in which both are mixed. The TM wave basic propagation mode light is polarized and rotated, converted to the TE wave primary propagation mode, and output. On the other hand, light in the TE wave basic propagation mode is output without conversion.

  The optical branching unit includes a central waveguide and two side-installed waveguides, both of which are width tapered waveguides. On the left and right of the central waveguide, symmetrical side-installed waveguides are arranged at symmetrical positions with respect to the central waveguide axis of the central waveguide. A first region, a second region, and a third region that connects the first region and the second region are provided.

  The first region is provided with a portion where the propagation constant (equivalent refractive index) of the TE wave primary propagation mode of the central waveguide is equal to the propagation constant of the TE wave basic propagation mode of the two side-installed waveguides. ing. The second region is provided with a portion where the propagation constant of the TE wave basic propagation mode of the central waveguide is equal to the propagation constant of the TE wave basic propagation mode of the two side-installed waveguides.

  The output light output from the polarization rotation unit is input to the central waveguide, and the propagation light in the TE wave basic propagation mode is output from the two side-installed waveguides.

  And if a wavelength separation part is provided in the back | latter stage of the optical branching part of this optical waveguide element, a wavelength separation element can be formed. The first and second interference arms that generate a phase difference of π / 2 between the two output lights output from the optical branching unit are provided at the subsequent stage of the optical branching unit of the optical waveguide element. A polarization discriminating element that selects and outputs an output port according to whether the input light is a TE wave or a TM wave if an optical coupler that combines and branches the output light output from the second interference arm is provided. Can be formed.

  According to the optical waveguide device of the present invention, since the polarization rotation unit and the optical branching unit are provided, the polarization direction of the input light is unified by the polarization rotation unit, and the unified polarization direction is In this structure, the optical branching unit that operates in this manner is provided after the polarization rotating unit.

  The optical branching unit includes a central waveguide and two side-installed waveguides, both of which are width tapered waveguides. Therefore, the location where the propagation constant of the TE wave primary propagation mode of the central waveguide is equal to the propagation constant of the TE wave fundamental propagation mode of the two side-installed waveguides, and the TE wave fundamental of the central waveguide A portion satisfying the condition that the propagation constant of the propagation mode and the propagation constant of the TE wave basic propagation mode of the two side-mounted waveguides are equal can be set in a wide range along the waveguide direction. That is, since it is a width-tapered waveguide, a location that satisfies these conditions can exist anywhere along the waveguide direction even if the wavelength changes. As a result, the operating wavelength band can be widened.

  The optical waveguide device of the present invention is a device in which the input light is first polarized and the wavelength splitting function is realized in the subsequent optical branching unit, but the operable wavelength bandwidth of the optical branching unit has been widened. Accordingly, it is not necessary to provide an independent functional part for realizing the wavelength separation function for each polarization component, and the shape of the element can be kept compact.

  If the optical waveguide element of the present invention is used, the polarization direction of the output light output from the optical waveguide element is unified. Therefore, the polarization-dependent wavelength separation unit is placed after the optical branching unit of the optical waveguide element. Even if connected, wavelength separation can be performed. That is, a wavelength separation element that operates in a polarization-independent manner can be formed. If this optical waveguide element is used, a polarization discriminating element that separates and outputs the TE wave fundamental propagation mode component and the TM wave fundamental propagation mode component can be formed regardless of the polarization state of the input light.

It is a figure where it uses for description of embodiment of the optical waveguide element of this invention. It is a figure where it uses for description of 1st Embodiment of a polarization rotation part. It is a figure where it uses for description of 2nd Embodiment of a polarization rotation part. It is a figure where it uses for description of embodiment of an optical branching part. It is a figure where it uses for description about the result of having simulated the operation | movement of the optical branching part by three-dimensional BPM. It is a figure where it uses for description of embodiment of the wavelength separation element formed using the optical waveguide element of this invention. It is a figure where it uses for description of embodiment of the polarization identification element formed using the optical waveguide element of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 to 4, 6, and 7 show one configuration example according to the embodiment of the present invention, and the present invention is not limited to the illustrated example. In the following description, specific constituent materials, design conditions, and the like may be used. However, these constituent materials, design conditions, and the like are only suitable examples, and are not limited to these. Moreover, in the figure which shows the example of 1 structure which concerns on embodiment of this invention, about the same component, it attaches and shows the same number, The duplication description may be abbreviate | omitted.

≪Optical waveguide element≫
An embodiment of the optical waveguide device of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a schematic configuration of a planar pattern of a waveguide core constituting an optical waveguide element. The optical waveguide device includes a polarization rotation unit 10 and an optical branching unit 22.

  The polarization rotator 10 is input via the input waveguide 21 with either TM-wave basic propagation mode light or TE-wave basic propagation mode light, or a mixture of both. In FIG. 1, this input light is indicated as input light L1. The polarization rotation unit 10 rotates the polarization of the input TM wave basic propagation mode, converts it to the TE wave primary propagation mode, and outputs the TE wave. The polarization rotation unit 10 outputs the input TE wave basic propagation mode light without conversion.

  The optical branching unit 22 includes a central waveguide 32c, a side-installed waveguide 32a provided on the left side with respect to the central waveguide 32c in the waveguide direction, and a side-installed waveguide 32b provided on the right side. I have. In other words, the optical branching section 22 includes a central waveguide 32c and two side-installed waveguides (32a and 32b) disposed at symmetrical positions with respect to the central waveguide axis of the central waveguide 32c.

  The output light output from the polarization rotation unit 10 is input to the equal width waveguide 31b via the width taper waveguide 31a. The width tapered waveguide 31a changes the propagation mode (changes to the propagation mode corresponding to the waveguide width) so that the input light is input to the equal width waveguide 31b without radiation loss. The propagating light is input from the equal-width waveguide 31b to the center waveguide 32c.

  The center waveguide 32c is a width taper waveguide whose waveguide width gradually decreases along the waveguide direction, and the side-installed waveguides 32a and 32b are width tapers whose waveguide width gradually increases along the waveguide direction. It is a waveguide.

  The central waveguide 32c and the two side-installed waveguides (32a and 32b) constituting the optical branching section 22 are the two side-installed waveguides for propagating light in the TE wave primary propagation mode of the center waveguide 32c. A first region to be transferred to the waveguide, a second region to transfer the propagation light in the TE wave basic propagation mode of the central waveguide 32c to the two side-installed waveguides (32a and 32b), a first region, and a second region; A third region is provided to connect the two. Details of the first to third regions will be described later with reference to FIG.

  In the first region, there is provided a portion where the propagation constant of the TE wave primary propagation mode of the central waveguide 32c is equal to the propagation constant of the TE wave basic propagation mode of the two side installed waveguides (32a and 32b). ing. Further, in the second region, there is provided a portion where the propagation constant of the TE wave basic propagation mode of the central waveguide 32c is equal to the propagation constant of the TE wave basic propagation mode of the two side-installed waveguides (32a and 32b). It has been. Here, the central waveguide 32c and the two side-installed waveguides (32a and 32b) are width tapered waveguides. For this reason, the location satisfying the condition that the propagation constants are equal in the first region and the second region is not limited to one, and can be set at any position along the waveguide direction according to the wavelength. . That is, the optical branching unit 22 can operate over a wide wavelength band in the branching operation wavelength band.

  The output light from the polarization rotator 10 is input to the central waveguide 32c via the width taper waveguide 31a and the equal width waveguide 31b, and is connected to the two side-installed waveguides (32a and 32b). The propagation light in the TE wave basic propagation mode is output from the waveguide 23a and the waveguide 23b via the bent waveguide 34a and the bent waveguide 34b, respectively.

<Polarization rotator>
A first embodiment of the polarization rotation unit 10 of the optical waveguide device of the present invention will be described with reference to FIG. FIG. 2 (A) is a schematic perspective view showing the configuration of the first embodiment of the polarization rotation unit 10, and FIG. 2 (B) is the wave-guiding direction at the position indicated by AA in FIG. 2 (A). It is schematic sectional drawing cut | disconnected by the vertical cross section.

  As shown in FIG. 2A, the polarization rotation unit 10 includes an input waveguide unit 4, a width taper waveguide unit 5, and an output waveguide unit 6 connected in series in this order in the waveguide direction. It is a waveguide. In FIG. 2 (A), P, Q, and R represent regions constituting respective portions of the input waveguide section 4, the width taper waveguide section 5, and the output waveguide section 6. A waveguide core in a region indicated by P, Q, and R is formed of a silicon material, and a cladding layer 2 formed of a silicon oxide material is formed surrounding the waveguide core. A waveguide structure formed by the waveguide core and the cladding layer 2 is formed on the silicon substrate 1.

  As shown in FIG. 2B, the core of the width-tapered waveguide portion 5 has a trapezoidal cross-sectional shape cut perpendicular to the waveguide direction, and has a waveguide width that satisfies the conditions for causing polarization rotation. There is a certain place. Here, the waveguide width of the width-tapered waveguide portion 5 refers to the length of either the lower base or the upper base of the isosceles trapezoid. In any case, the condition that the polarization rotation occurs in any part of the region indicated by Q is satisfied, but in order to specify this part, either the length of the upper base or the length of the lower base is specified. Since it can be definitely specified even if it is used, the length of the selected upper or lower base is defined as the waveguide width of the width taper waveguide portion 5.

  In the example shown in FIG. 2B, the cross-sectional shape of the waveguide core constituting the width-tapered waveguide portion 5 is an isosceles trapezoid, and the inclination angle from the right angle direction at both ends of the bottom of the isosceles trapezoid is shown. θ. In the region indicated by Q, the position that satisfies the condition for causing the polarization rotation depends on the value of the angle θ.

  As shown in FIG. 2 (A), the input waveguide section 4 and the output waveguide section 6 also have a cross-sectional shape cut in a direction perpendicular to the waveguide direction of the waveguide core constituting this part. Although this is a leg trapezoid, this part is due to the convenience of the manufacturing process, and the cross-sectional shape does not necessarily have to be an isosceles trapezoid. The input waveguide section 4 and the output waveguide section 6 may be rectangular like a normal waveguide.

  In the polarization rotation unit 10, from the input end S1 of the input waveguide unit 4, either one of the TM wave basic propagation mode (TM0) light and the TE wave basic propagation mode (TE0) light, or an input in which both are mixed Light L1 is input. Then, the polarization rotation unit 10 rotates the polarization of the TM wave basic propagation mode light to convert it into the TE wave primary propagation mode (TE1), or the TE wave basic propagation mode (TE0) light. Is not converted and is output as output light L2 from the output end S2 of the output waveguide section 6.

  Next, a second embodiment of the polarization rotation unit 10 of the optical waveguide device of the present invention will be described with reference to FIG. FIG. 3A is a schematic perspective view showing the configuration of the second embodiment of the polarization rotating unit 10, and FIG. 3B is a view in the waveguide direction at a position indicated by AA in FIG. It is schematic sectional drawing cut | disconnected by the vertical cross section.

  As shown in FIG. 3 (A), the polarization rotation unit 10 is a waveguide in which an input waveguide unit 4, a width taper waveguide unit 5, and an output waveguide unit 6 are connected in series in this order in the waveguide direction. It is a waveguide. In FIG. 3 (A), P, Q, and R represent regions constituting respective portions of the input waveguide section 4, the width taper waveguide section 5, and the output waveguide section 6. Also in the second embodiment of the polarization rotator 10, the waveguide cores in the regions indicated by P, Q, and R are formed of silicon material, and are formed of silicon oxide material surrounding the waveguide core. A clad layer 2 is formed. A waveguide structure formed by the waveguide core and the cladding layer 2 is formed on the silicon substrate 1.

As shown in FIG. 3 (B), the core of the width taper waveguide portion 5 has a rectangular cross-sectional shape cut perpendicular to the waveguide direction. Terrace structures 14 are provided on both sides of the width tapered waveguide portion 5. Polarization rotation determined by the step width D, which is the difference between the waveguide width of the width-tapered waveguide portion 5 and the waveguide width of the terrace-like structure 14 and the height of the width-tapered waveguide portion 5 and the terrace-like structure 14 There are places that satisfy the conditions for expression of. Here, the waveguide width of the width-tapered waveguide portion 5 means a dimension indicated as W _{1 in} FIG. Also, the width of the combined width tapered waveguide portion 5 and terrace-like structure 14, shown as W ₂ in FIG. 3 (B). The waveguide width of the terrace structure 14 is given by (W ₂ −W ₁ ) / 2. Satisfy the condition that polarization rotation occurs in any part of the region indicated by Q. In the region indicated by Q, the position that satisfies the condition for causing the polarization rotation depends on the values of the dimensions W ₁ , W ₂ , and the step D.

  In the second embodiment of the polarization rotator 10, either the TM fundamental wave propagation mode (TM0) light or the TE fundamental wave propagation mode (TE0) light from the input end S1 of the input waveguide section 4 or Input light L1 in which both are mixed is input. Then, the TM wave fundamental propagation mode light from the output end S2 of the output waveguide section 6 is polarized and rotated to be converted into the TE wave primary propagation mode (TE1) or the TE wave fundamental propagation mode (TE0). ) Is not converted and is output as output light L2.

<Optical branching section>
An embodiment of the optical branching portion 22 of the optical waveguide device of the present invention will be described with reference to FIG. FIG. 4 is a diagram showing a schematic configuration of a planar pattern of the waveguide core that constitutes the optical branching section 22.

  In the optical branching section 22, the first region 41 for shifting the TE wave primary propagation mode propagating through the central waveguide 32c to the two side-installed waveguides (32a and 32b), and the TE wave basic propagation mode are set. A second region 42 for shifting from the central waveguide 32c to the two side-installed waveguides (32a and 32b) and a third region 43 for connecting the first region 41 and the second region 42 are provided. . Therefore, the center waveguide 32c is a multimode waveguide. In the first region 41, a design is made such that the propagation constant of the TE wave primary propagation mode of the central waveguide 32c matches the propagation constant of the TE wave fundamental propagation mode of the side-installed waveguides (32a and 32b). . Further, in the second region 42, a design is made such that the propagation constant of the TE wave basic propagation mode of the central waveguide 32c matches the propagation constant of the TE wave basic propagation mode of the side-installed waveguides (32a and 32b). To do. In order to enable the optical branching section 22 to operate in a wide wavelength band, in the first region 41 and the second region 42, the central waveguide 32c and the two side-installed waveguides (32a and 32b) have a width. A tapered waveguide is employed.

  In FIG. 4, the input ends of the two side-arranged waveguides (32a and 32b) are shifted by E from the input end of the central waveguide 32c, but the input ends of both are aligned (region E). May be formed).

  The optical branching unit 22 includes first to third regions (a first region 41, a second region 42, and a third region 43) configured by a central waveguide 32c and two side-arranged waveguides (32a and 32b). A waveguide core is formed of a silicon material, and a cladding layer (not shown) is formed of a silicon oxide material so as to surround the waveguide core. A waveguide structure composed of a waveguide core and a cladding layer is formed on a silicon substrate.

  Here, the operation of the optical branching unit 22 will be described. In FIG. 4, the curve indicated by reference numeral 13 indicates the light intensity at the equiphase surface of the TE wave primary propagation mode propagating through the central waveguide 32c, and the curve indicated by reference numeral 44a indicates the TE wave propagating through the side-installed waveguide 32a. The light intensity on the equiphase surface of the fundamental propagation mode is shown, and the curve denoted by reference numeral 44b shows the light intensity on the equiphase surface of the TE wave fundamental propagation mode propagating through the side-installed waveguide 32b. The curve indicated by reference numeral 12 indicates the light intensity at the equiphase surface of the TE wave basic propagation mode propagating through the central waveguide 32c, and the curve indicated by reference numeral 45a indicates the TE wave basic propagation mode propagating through the side-installed waveguide 32a. The curve indicated by reference numeral 45b indicates the light intensity on the equiphase surface of the TE wave basic propagation mode propagating through the side-installed waveguide 32b.

  In the first region 41, the propagation constant of the TE wave primary propagation mode 13 propagating through the central waveguide 32c is the propagation constant of the TE wave basic propagation modes 44a and 44b propagating through the two side-installed waveguides (32a and 32b). Match (phase match). For this reason, the TE wave primary propagation mode 13 of the central waveguide 32c shifts to TE wave basic propagation modes 44a and 44b whose phases are reversed.

  The width of the central waveguide 32c in the second region 42 is different from the width in the first region 41, and the TE wave primary propagation mode does not propagate. For this reason, the TE wave basic propagation modes 44a and 44b input to the second region 42 are output as they are from the terminal ends of the side-installed waveguides (32a and 32b). Since the TE wave primary propagation mode 13 of the central waveguide 32c shifts to the TE wave fundamental propagation modes 44a and 44b whose phases are reversed, the TE wave fundamental propagation mode 44a and the TE wave are produced at the output end of the second region 42. The basic propagation modes 44b are in opposite phases.

  On the other hand, the TE wave basic propagation mode 12 of the central waveguide 32c is not phase-matched in the first region 41 and enters the second region 42 as it is. In the second region 42, there is a portion where the waveguide width of the central waveguide 32c is substantially equal to the waveguide width of the side-installed waveguides (32a and 32b), so the TE wave basic propagation mode 12 is the side-installed waveguide. (32a and 32b) are phase-matched with the TE wave fundamental propagation modes 45a and 45b and output from the ends of the two side-installed waveguides (32a and 32b). Therefore, when the TE wave basic propagation mode is input to the central waveguide 32c, the TE wave basic propagation mode 45a and the TE wave basic propagation mode 45b are in phase with each other at the output end of the second region 42. .

  The result of simulating the operation of the optical branching unit 22 using a three-dimensional BPM (Beam Propagation Method) will be described with reference to FIG. The thickness of the silicon waveguide core constituting the light branching portion 22 is 220 nm in all the first to third regions. As the input light L1, two wavelengths of TE waves having wavelengths of 1.55 μm and 1.31 μm were used. As input light L1, FIG. 5 (A) uses a fundamental propagation mode (TE0) with a wavelength of 1.55 μm, FIG. 5 (B) uses a primary propagation mode (TE1) with a wavelength of 1.55 μm, and FIG. FIG. 5D shows the light intensity distribution when the fundamental propagation mode (TE0) having a wavelength of 1.31 μm is used and the primary propagation mode (TE1) having a wavelength of 1.31 μm is used.

  5A to 5D, the vertical axis is taken in the z direction, which is the light propagation direction, and the horizontal axis is scaled in units of μm with respect to the x direction orthogonal to the light propagation direction. . The total length of the optical branching portion 22 is 10 μm (including the region E shown in FIG. 4), and the waveguide width of the central waveguide 32c is changed from 440 nm to 100 nm. The waveguide width of the side-installed waveguides (32a and 32b), which are width tapered waveguides, so that the distance (gap 33a, 33b) between the central waveguide 32c and the side-installed waveguides (32a and 32b) is 100 nm. Is set along the traveling direction of light. The central waveguide 32c is formed ahead of the side-mounted waveguides (32a and 32b) by E (= 1 μm), and the waveguide width at the input end (z = 0 μm) of the central waveguide 32c is 440 nm. It has become. The first region 41 of the central waveguide 32c is a multimode waveguide.

  As shown in FIGS. 5A to 5D, the input light input to the central waveguide 32c is separated into the side-installed waveguide 32a and the side-installed waveguide 32b and is guided. In FIGS. 5A to 5D, gray shades ranging from white to black are shown depending on the shade of light intensity, but if shown by color identification display ranging from red to blue depending on the shade of light intensity, Can be read clearly.

  Also, as can be seen by comparing FIG. 5 (A) and FIG. 5 (B), or FIG. 5 (C) and FIG. 5 (D), the primary propagation mode is better than the basic propagation mode for the same wavelength. First, they are separated in two directions. In addition, once the primary propagation mode is separated, it does not return to the central waveguide 32c again. Further, although not shown, the thickness of the silicon waveguide core constituting the optical branching section 22 is set to 300 nm, and the gaps 33a and 33b between the central waveguide 32c and the side-installed waveguides (32a and 32b) are set to 200 nm. It has been confirmed that a good operation can be obtained at a wavelength of 1.55 to 1.31 μm.

  As shown in the three-dimensional BPM simulation, in the second region 42, even if the two side-installed waveguides (32a and 32b) and the central waveguide 32c are in the phase matching condition, the second region 42 is centered in the first region 41. Light that has shifted from the TE wave primary propagation mode of the waveguide 32c to the side-installed waveguides (32a and 32b) does not return from the side-installed waveguides (32a and 32b) to the central waveguide 32c in the second region 42. . This fact is a knowledge obtained for the first time by the inventor of the present application, and is an important law for realizing the above-described operation in the optical branching unit 22 of the present invention.

≪Wavelength separation element≫
With reference to FIG. 6, an embodiment of a wavelength separation element formed using the above-described optical waveguide element described with reference to FIGS. 1 to 5 will be described. The wavelength separation element shown in FIG. 6 includes a wavelength separation unit 50 in the subsequent stage of the optical branching unit 22 of the above-described optical waveguide element composed of the polarization rotation unit 10 and the optical branching unit 22. The wavelength separation unit 50 includes, for example, two first branching devices 52a and 52b and four second branching devices 54a-1, 54a-2, 54b-1 and 54b-2. The first branching devices 52a and 52b and the second branching devices 54a-1, 54a-2, 54b-1 and 54b-2 are elements with one input and two outputs, and separate the input light into desired wavelengths. And output.

  Waveguides 23a and 23b are provided at the subsequent stage of the optical branching section 22. One first branching device 52a is connected to the waveguide 23a, and the second branching devices 54a-1 and 54a-2 are connected to the two outputs of the first branching device 52a, respectively. Similarly, the other first branching device 52b is connected to the waveguide 23b, and the second branching devices 54b-1 and 54b-2 are connected to the two outputs of the first branching device 52b, respectively.

  The input light L1 input from the input waveguide 21 is input to the polarization rotation unit 10, converted into a TE wave, and input to the optical branching unit 22. When input to the optical branching unit 22, as described above, the TE wave basic propagation modes are respectively transmitted from the waveguides 23a and 23b connected to the two side-installed waveguides (32a and 32b) of the optical branching unit 22. Are propagated respectively.

  The two outputs of the optical branching unit 22 are propagated to different waveguides 23a and 23b. The light propagated through the waveguide 23a is input to the first branching device 52a, and the light transmitted through the waveguide 23b is input to the first branching device 52b. One of the two output lights output from the first branching device 52a is input to the second branching device 54a-1, and the other is input to the second branching device 54a-2. One of the two output lights output from the first branching device 52b is input to the second branching device 54b-1, and the other is input to the second branching device 54b-2. Output light L2a-1 and L2a-2 are output from the second branching device 54a-1, output light L2a-3 and L2a-4 are output from the second branching device 54a-2, and the second branching device 54b- Output lights L2b-1 and L2b-2 are output from 1, and output lights L2b-3 and L2b-4 are output from the second splitter 54b-2.

  That is, the input light L1 input from the input waveguide 21 is wavelength-separated and divided into eight and output light L2a-1, L2a-2, L2a-3, L2a-4, L2b-1, L2b-2, L2b -3 and L2b-4 are output. The first branching devices 52a and 52b and the second branching devices 54a-1, 54a-2, 54b-1, and 54b-2 can be formed using, for example, a Mach-Zehnder interferometer. If the input light L1 input from the input waveguide 21 passes through the polarization rotator 10, all are the same polarization (TE polarization), so the optical branching unit 22 and the first and second branching units are It is only necessary to guarantee the operation with one polarization (here, TE polarization).

≪Polarization identification element≫
With reference to FIG. 7, an embodiment of a polarization identifying element formed using the above-described optical waveguide element described with reference to FIGS. The polarization discriminating element shown in FIG. 7 is a polarization discriminating unit that outputs the separated light after the optical branching unit 22 after the optical branching unit 22 of the optical waveguide element composed of the polarization rotating unit 10 and the optical branching unit 22. Has 60.

  The input light L1 input from the input waveguide 21 is input to the polarization rotation unit 10, converted into a TE wave, and input to the optical branching unit 22. When the light is input to the optical branching section 22, as described above, each of the TE waves from the curved waveguide 34a and the curved waveguide 34b connected to the two side-installed waveguides (32a and 32b) of the optical branching section 22 Propagated light in the basic propagation mode is output.

  The output light output from the bent waveguide 34a passes through the first interference arm 61a constituting the Mach-Zehnder interferometer, is input to the optical coupler 62, and the output light output from the bent waveguide 34b is the same. The light passes through the second interference arm 61 b constituting the Mach-Zehnder interferometer and is input to the optical coupler 62. Here, the optical path length difference of the second interference arm 61b with respect to the first interference arm 61a is a phase difference of π / 2 between the light passing through the first interference arm 61a and the light passing through the second interference arm 61b. It is set to occur.

  As described above, when the TE wave primary propagation mode is input to the central waveguide 32c, the TE wave fundamental propagation mode 44a and the TE wave fundamental propagation mode 44b are in opposite phases at the output end of the second region 42. When the TE wave basic propagation mode is input to the central waveguide 32c, the output ends of the second region 42 are in phase with each other. The TE wave primary propagation mode is input to the central waveguide 32c when the TM wave basic propagation mode is input to the polarization rotation unit 10, and the TE wave basic propagation mode is input to the central waveguide 32c. This is the case where the TE wave basic propagation mode is input to the polarization rotation unit 10. As described above, when the TE wave primary propagation mode is input or the TE wave basic propagation mode is input, the TE wave basic propagation mode 44a and the TE wave basic propagation mode 44b at the output end of the second region 42 are input. These phases are opposite or in phase.

  For this reason, if the input light L1 input from the input waveguide 21 is a TE wave by the action of the optical coupler 62 in combination with the phase shift by the interference arm, the output from the first and second interference arms (61a and 61b). The output light is combined by the optical coupler 62 and then output from one output port 63a. If the input light L1 is a TM wave, the output light is output from the other output port 63b. In this way, a polarization discriminating element is formed in which the output port is selected and output depending on whether the input light L1 input from the input waveguide 21 is a TE wave or a TM wave.

≪Manufacturing method≫
The waveguide core pattern structure constituting the optical waveguide element, the wavelength separation element, and the polarization identification element described above can be formed, for example, by obtaining an SOI (Silicon on Insulator) substrate and performing the following steps. The SOI substrate is widely available as a commercial product, and a silicon oxide layer is formed on the silicon substrate, and a silicon layer having a thickness equal to the thickness of the optical waveguide pattern is formed on the silicon oxide layer. First, a step of preparing this SOI substrate is executed.

  Next, the following dry etching step is performed. That is, dry etching or the like is performed on the silicon layer formed on the silicon oxide layer of the SOI substrate, leaving a portion that becomes the waveguide core (waveguide core pattern structure), and the other portion of the silicon layer is removed. remove.

  Following the dry etching step, a silicon oxide layer surrounding the waveguide core pattern structure left by the etching process is formed by a chemical vapor deposition (CVD) method or the like (CVD step). And it grind | polishes so that the upper surface of a silicon oxide layer may become flat, and also a silicon oxide layer is formed on this. Thus, by performing the CVD step with the polishing step in between, the clad layer 2 shown in FIGS. 2 and 3 is formed. The same applies to the wavelength separation unit and the polarization identification unit other than the waveguide core part constituting the optical waveguide element in the wavelength separation element and the polarization identification element.

  As described above, the optical waveguide device, the wavelength separation device, and the polarization identification device of the present invention can be formed by a well-known etching process, a CVD method, or the like using an SOI substrate. It can be easily formed at a low cost.

1: Silicon substrate
2: Clad layer
4: Input waveguide section
5: Width taper waveguide part
6: Output waveguide section
10: Polarization rotating part
12, 44a, 44b, 45a, 45b: TE wave basic propagation mode
13: TE wave primary propagation mode
14: Terrace structure
22: Optical branch
23a, 23b: Waveguide
31a: Width taper waveguide
31b: Equal width waveguide
32a, 32b: Side-mounted waveguide
32c: Central waveguide
33a, 33b: Distance (gap) between the center waveguide and side waveguides
34a, 34b: Curved waveguide
41: First area
42: Second area
43: Third area
50: Wavelength separation part
52a, 52b: First turnout
54a-1, 54a-2, 54b-1, 54b-2: Second turnout
60: Polarization identifier
61a: First interference arm
61b: Second interference arm
62: Optical coupler
63a, 63b: Output port
L1: Input light
L2, L2a-1, L2a-2, L2a-3, L2a-4, L2b-1, L2b-2, L2b-3, L2b-4: Output light
S1: Input terminal
S2: Output terminal
P: Waveguide core region that forms part of the input waveguide
Q: Waveguide core region that forms part of the width taper waveguide
R: Waveguide core region that forms part of the output waveguide section

Claims (8)

Either TM (Transverse Magnetic) wave basic propagation mode light or TE (Transverse Electric) wave basic propagation mode light, or a mixture of both is input.
Polarization rotation unit that rotates the polarization for light in the TM wave basic propagation mode, converts it to the TE wave primary propagation mode and outputs it, and outputs the light in the TE wave basic propagation mode without conversion,
Each includes a center waveguide and two side-mounted waveguides, each of which is a width taper waveguide,
On the left and right sides of the central waveguide, the symmetrically installed side-mounted waveguides are arranged at symmetrical positions with respect to the central waveguide axis of the central waveguide,
A first region provided with a portion where the propagation constant of the TE wave primary propagation mode of the central waveguide is equal to the propagation constant of the TE wave basic propagation mode of the two side-installed waveguides;
A second region provided with a portion where the propagation constant of the TE wave basic propagation mode of the central waveguide is equal to the propagation constant of the TE wave basic propagation mode of the two side-installed waveguides;
A third region connecting the first region and the second region is provided;
Output light from the polarization rotation unit is input to the central waveguide, and includes an optical branching unit that outputs propagation light of a TE wave basic propagation mode from the two side-installed waveguides. Optical waveguide element. The polarization rotation unit is a waveguide in which an input waveguide unit, a width taper waveguide unit, and an output waveguide unit are connected in series in the waveguide direction in this order,
The core of the width-tapered waveguide portion has a trapezoidal cross-sectional shape cut perpendicular to the waveguide direction, and there is a portion where the waveguide width satisfies the condition for causing polarization rotation,
One of TM wave basic propagation mode light and TE wave basic propagation mode light or a mixture of both is input from the input waveguide section, and it is biased against TM wave basic propagation mode light. 2. The optical waveguide according to claim 1, wherein the optical waveguide is rotated and converted into a TE wave primary propagation mode, and is output from the output waveguide section without being converted with respect to light in a TE wave basic propagation mode. element. The polarization rotation unit is a waveguide in which an input waveguide unit, a width taper waveguide unit, and an output waveguide unit are connected in series in the waveguide direction in this order,
The width taper waveguide section has a rectangular cross-sectional shape cut perpendicular to the waveguide direction, and both sides of the width taper waveguide section have a terrace structure, and the waveguide width of the width taper waveguide section is And there is a place that satisfies the conditions for expressing polarization rotation, which is determined by the waveguide width of the terrace-shaped structure,
One of TM wave basic propagation mode light and TE wave basic propagation mode light or a mixture of both is input from the input waveguide section, and it is biased against TM wave basic propagation mode light. 2. The optical waveguide according to claim 1, wherein the optical waveguide is rotated and converted into a TE wave primary propagation mode, and is output from the output waveguide section without being converted with respect to light in a TE wave basic propagation mode. element.   The wavelength separation element provided with the wavelength separation part in the back | latter stage of the said optical branching part of the optical waveguide element as described in any one of Claims 1-3. In the subsequent stage of the optical branching portion of the optical waveguide device according to any one of claims 1 to 3,
A Mach-Zehnder interferometer including first and second interference arms for generating a phase difference of π / 2 between output lights output from the two side-installed waveguides of the optical branching unit;
An optical coupler for branching output light output from each of the first and second interference arms;
A polarization discriminating element characterized in that an output port is selected and output by the optical coupler depending on whether the input light to the polarization rotation unit is a TE wave or a TM wave.   The waveguide core constituting the polarization rotation part and the optical branching part is made of silicon, and the cladding layer surrounding the waveguide core is made of silicon oxide. The optical waveguide element as described in any one of Claims.   The waveguide core constituting the polarization rotation unit, the optical branching unit, and the wavelength separation unit is made of silicon, and the cladding layer surrounding the waveguide core is made of silicon oxide. Item 5. A wavelength separation element according to Item 4.   A waveguide core constituting the polarization rotation unit, the optical branching unit, the first and second interference arms, and the optical coupler is made of silicon, and a cladding layer surrounding the waveguide core is made of silicon oxide. The polarization identifying element according to claim 5, wherein

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