JP6643437B1 - Optical waveguide device - Google Patents

Abstract

To output one polarization of a basic mode without mode conversion and to convert the other polarization of a basic mode to a desired mode order while suppressing conversion to a mode other than a desired mode order. An optical waveguide device is provided. A first optical waveguide core including a first coupling section and a second coupling section, and a second optical waveguide core including a third coupling section and a fourth coupling section are provided. The first joint 33 and the third joint 41 have the same width and the same thickness, and have different widths and thicknesses. The lengths of the first coupling portion and the third coupling portion are set to the coupling length of one polarization of the fundamental mode in which the oscillation direction of the optical electric field is along the smaller width or thickness. In the second coupling region in which the second coupling portion 35 and the fourth coupling portion 43 are spaced apart from each other and arranged side by side, the other polarization of the fundamental mode propagating in the second coupling portion and propagating in the fourth coupling portion. And the other polarization of the i-th mode. [Selection diagram] Fig. 1

Description

  The present invention relates to an optical waveguide element that gives a desired mode order to each of input TE (Transverse Electric) polarization and TM (Transverse Magnetic) polarization, and outputs the resulting signal.

  With an increase in the amount of information transmission, optical wiring technology has attracted attention. In the optical wiring technology, information transmission between devices in information processing equipment, between boards, between chips, and the like is performed by optical signals using an optical device using an optical fiber or an optical waveguide as a transmission medium. As a result, it is possible to improve the band limitation of the electric wiring, which is a bottleneck in information processing equipment requiring high-speed signal processing.

  The optical device includes optical elements such as an optical transmitter and an optical receiver. These optical elements can be spatially coupled to each other using, for example, a lens after performing complicated optical axis alignment for adjusting the center position (light receiving position or light emitting position) of each optical element to a design position.

  As a means for coupling each optical element, there is a technique using an optical waveguide element instead of a lens. When an optical waveguide element is used, since light is confined and propagated in the optical waveguide, complicated optical axis alignment is not required unlike the case where a lens is used. Accordingly, the assembly process of the optical device is simplified, which is advantageous as a mode suitable for mass production.

Here, the optical waveguide element can be made of, for example, silicon (Si) as a waveguide material. In an optical waveguide element (Si waveguide) using Si as a material, an optical waveguide core that substantially serves as a light transmission path is formed using Si as a material. Then, the periphery of the optical waveguide core is covered with a clad made of, for example, silicon oxide (SiO ₂ ) having a lower refractive index than Si. With such a configuration, the refractive index difference between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, a small-sized curved waveguide whose bending radius is reduced to, for example, about several μm can be realized. Therefore, it is possible to create an optical circuit having a size similar to that of an electronic circuit, which is advantageous in reducing the size of the entire optical device.

  In addition, in the case where a Si waveguide is used, mass production of optical devices in which elements having various functions are monolithically integrated on the same substrate by utilizing a manufacturing process of a complementary metal oxide semiconductor (CMOS). Is possible. Therefore, an optical device using a Si waveguide is advantageous for miniaturization and cost reduction (for example, refer to Patent Document 1 and Non-Patent Documents 1 and 2).

  On the other hand, in the Si waveguide, due to the large relative refractive index difference between the core and the cladding, the difference between the equivalent refractive index of the waveguide mode and the difference between the group refractive indices between the TE polarization and the TM polarization. Easy to grow. Therefore, the Si waveguide has a drawback that it has polarization dependence.

  Therefore, in a wavelength filter or an optical coupler such as a Mach-Zehnder interferometer, a grating, and an AWG (Arrayed Waveguide Grating) formed of a Si waveguide, the divergence of the wavelength response characteristics between both polarizations becomes large even at the same wavelength. Such a difference in characteristics causes crosstalk between channels to occur in, for example, a receiving device or the like in a fiber transmission system.

  In order to solve the problem of the polarization dependence in the Si waveguide, a polarization separation element is provided in front of an optical waveguide element such as a wavelength filter or an optical coupler to separate TE polarization and TM polarization. There is a way. However, in this case, it is necessary to form an optical waveguide element optimized for TE polarization and an optical waveguide element optimized for TM polarization after the polarization separation element. It becomes complicated and the size of the optical device increases. For this reason, as an essential solution to the polarization dependence, it is preferable to minimize the divergence of the characteristics between the polarizations in the wavelength response characteristics of the optical waveguide element itself such as a wavelength filter.

  For example, in the case of a wavelength filter using a Mach-Zehnder interferometer, it is desired that the polarization characteristics of the optical coupler and the arm waveguide, which are constituent elements, be uniform. More specifically, it is required that the polarization characteristics of the optical coupler match, and the phase characteristics of the arm waveguide match each other.

  As an optical coupler, a directional coupler is used because it is advantageous in terms of low loss and low reflection. The directional coupler is constituted by a pair of optical waveguide cores running in parallel. The design parameters of the directional coupler include the width and thickness of each optical waveguide core and the separation distance between each optical waveguide core (for example, see Patent Document 2).

  However, the thickness of the optical waveguide core depends on the thickness of the SOI layer of the SOI substrate used for manufacturing. Therefore, in eliminating the polarization dependence of the directional coupler, the design parameters that can be arbitrarily adjusted are only the width of each optical waveguide core and the separation distance between each optical waveguide core. Then, simply adjusting these two parameters may not be able to achieve the condition for eliminating the polarization dependence.

  For this reason, in order to reliably eliminate the polarization dependence, it is preferable to adjust the thickness of the optical waveguide core. However, as described above, since the thickness of the optical waveguide core depends on the thickness of the SOI layer, the available SOI substrate is limited.

  On the other hand, in an optical device, an optical waveguide element (for example, a spot size converter or a modulator) other than a wavelength filter may be formed collectively using a common SOI substrate. In order to determine the optimum thickness of the optical waveguide core for each of these optical waveguide elements, it is desirable that the thickness of the SOI layer can be flexibly selected. Therefore, it is not preferable that the available SOI substrate is limited.

  In order to solve such a problem, a directional coupler having a high degree of freedom in design with respect to the thickness of the optical waveguide core has been proposed (for example, see Patent Document 3).

  In the directional coupler according to Patent Literature 3, in addition to the width and thickness of each optical waveguide core and the separation distance between each optical waveguide core, a mode order involved in mode coupling between each optical waveguide core is designed. Parameters. As a result, when used in a polarization-independent manner, the degree of freedom in designing the thickness of the optical waveguide core increases.

JP 2011-77133 A JP 2011-43567 A JP 2016-24375 A

IEEE Journal of selected topics in quantum electronics, vol.11, No.1, January / February 2005 p.232-240 IEEE Journal of selected topics in quantum electronics, vol.12, No.6, November / December 2006 p.1371-1379

  In using the directional coupler according to Patent Literature 3, it is necessary to input a TE polarization and a TM polarization of a set mode order. When the mode order involved in the inter-mode coupling between the respective optical waveguide cores is set to, for example, the TE polarization of the first mode and the TM polarization of the fundamental mode, the TE polarization of the first mode and the fundamental mode are set. It is necessary to input the TM polarization of the mode to the directional coupler.

  Therefore, an optical waveguide device that converts the input fundamental mode TE polarization into the first-order mode and outputs the same and outputs the input fundamental mode TM polarization as it is in the directional coupler is provided. Must be provided in the preceding stage.

  In such an optical waveguide device, conversion into a mode other than the desired mode order not only results in loss but also leads to deterioration of branch characteristics in the directional coupler. Further, the deterioration of the branch characteristic in the directional coupler also leads to the deterioration of the isolation characteristic of the wavelength filter including the directional coupler.

  Therefore, an object of the present invention is to output one polarization of the fundamental mode without mode conversion while suppressing conversion to a mode other than the desired mode order, and to convert the other polarization of the fundamental mode to a desired mode order. An object of the present invention is to provide an optical waveguide device that converts and outputs the converted light.

In order to solve the above-described problems, an optical waveguide device according to the present invention includes a first optical waveguide core including a first coupling section and a second coupling section connected in series, a third coupling section connected in series, And a second optical waveguide core including a fourth coupling portion. In the optical waveguide device according to the present invention, the first coupling region is set in which the first coupling portion and the third coupling portion are spaced apart from each other and arranged side by side. The first coupling portion and the third coupling portion have the same width and the same thickness and the same thickness, and have different widths and thicknesses. The length of the first coupling portion and the third coupling portion is such that the width of the first coupling portion and the third coupling portion has a smaller dimension of the width or thickness, and the vibration direction of the optical electric field extends along the TE of the fundamental mode. The coupling length is set to the polarization length of one of the polarization and the TM polarization. In the first coupling region, one polarization of the fundamental mode propagating in the first coupling unit and one polarization of the fundamental mode propagating in the third coupling unit are coupled. Further, in the optical waveguide device according to the present invention, the second coupling region in which the second coupling portion and the fourth coupling portion are spaced apart from each other and arranged side by side is set. In the second coupling region, the other polarization of the fundamental mode propagating in the second coupling unit and the other polarization of the i-th mode (i is an integer of 1 ≦ i) propagating in the fourth coupling unit are coupled. You. The first and second coupling units receive the TE mode and TM polarization of the fundamental mode, and the fourth coupling unit supplies one of the polarizations of the fundamental mode coupled in the first coupling region and the second coupling mode. The other polarization of the i-th mode combined in the region is output.

  In the optical waveguide device according to the present invention, in the first coupling region, only one polarization of the fundamental mode can be shifted from the first coupling portion to the third coupling portion while maintaining the fundamental mode. Further, in the second coupling region, only the other polarization of the fundamental mode can be shifted from the second coupling section to the fourth coupling section while being converted into the i-th mode (i is an integer of 1 ≦ i). Thereby, with respect to the input TE polarization and TM polarization, respectively, one polarization of the fundamental mode is output without mode conversion, and the other polarization of the fundamental mode is converted to a desired i-polarization. It can be converted to the next mode and output. In the optical waveguide device of the present invention, since undesired coupling does not occur in the first coupling region and the second coupling region, one polarization of the mode order other than the fundamental mode and the other polarization of the mode order other than the i-th mode are used. Conversion to waves can be suppressed.

FIG. 2 is a schematic plan view showing an optical waveguide device of the present invention. FIG. 2 is a schematic end view showing the optical waveguide device of the present invention. FIG. 4 is a diagram illustrating a relationship between an aspect ratio and a bond length. It is a figure showing the relation between the propagation constant of light which propagates the 2nd coupling part and the 4th coupling part, and the axis of propagation. FIG. 4 is a diagram for evaluating characteristics of the optical waveguide device of the present invention. FIG. 4 is a diagram for evaluating characteristics of the optical waveguide device of the present invention. 1 is a schematic plan view showing an optical coupler of the present invention. FIG. 2 is a schematic plan view showing a first wavelength filter of the present invention. FIG. 3 is a schematic end view showing a first wavelength filter of the present invention. FIG. 4 is a schematic plan view showing a second wavelength filter according to the present invention. FIG. 4 is a schematic end view showing a second wavelength filter of the present invention. It is a schematic plan view which shows the modification of a grating. It is a schematic plan view which shows the modification of a grating.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shapes, sizes, and arrangements of the components are only schematically shown to the extent that the present invention can be understood. Hereinafter, a preferred configuration example of the present invention will be described. However, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the present invention.

(Optical waveguide element)
The optical waveguide device of the present invention will be described with reference to FIGS. FIG. 1 is a schematic plan view showing an optical waveguide device. In FIG. 1, a support substrate and a clad, which will be described later, are omitted. FIG. 2 is a schematic end view of the structure shown in FIG. 1 taken along line II. In FIG. 2, hatching is omitted.

  In the following description, the direction along the light propagation direction is the length direction for each component. The direction along the thickness of the support substrate is defined as the thickness direction. The direction perpendicular to the length direction and the thickness direction is defined as the width direction.

  The optical waveguide device 100 includes a support substrate 10, a clad 20, a first optical waveguide core 30, and a second optical waveguide core 40. The first optical waveguide core 30 includes a first input / output port 31, an input / output tapered part 32, a first coupling part 33, a first coupling part 34, and a second coupling part 35 connected in series in this order. The second optical waveguide core 40 includes a third coupling section 41, a second coupling section 42, a fourth coupling section 43, and a second input / output port 44 connected in series in this order. In addition, a first coupling region 50 in which the first coupling portion 33 and the third coupling portion 41 are spaced apart from each other and arranged side by side is set. Further, a second coupling region 60 in which the second coupling portion 35 and the fourth coupling portion 43 are spaced apart from each other and arranged side by side is set.

  The optical waveguide element 100 is used as a mode converter that outputs one polarization of the fundamental mode without mode conversion and converts the other polarization of the fundamental mode into a desired mode order and outputs the same. Here, as an example, a configuration will be described in which the optical waveguide element 100 does not perform mode conversion on the TM polarization in the fundamental mode and converts the TE polarization in the fundamental mode into the first-order mode.

  In this example, light including TE polarization and TM polarization in the fundamental mode is input to the first input / output port 31 of the first optical waveguide core 30 and passes through the input / output taper section 32 to the first coupling region 50. To the first combining unit 33. Of the input light, the TM polarized light in the fundamental mode moves from the first coupling unit 33 to the third coupling unit 41 in the first coupling region 50 while maintaining the basic mode. The TM polarization of the fundamental mode transferred to the third coupling unit 41 is output from the second input / output port 44 via the second connection unit 42 and the fourth coupling unit 43. On the other hand, the TE-polarized light in the fundamental mode does not shift to the third coupling portion 41 in the first coupling region 50, but is sent to the second coupling portion 35 of the second coupling region 60 via the first coupling portion 34. The fundamental mode TE polarization is shifted from the second coupling unit 35 to the fourth coupling unit 43 while being converted into the first-order mode in the second coupling region 60. The TE polarization in the first mode transferred to the fourth coupling unit 43 is output from the second input / output port 44.

  The support substrate 10 is formed of, for example, a flat plate made of single crystal Si.

The clad 20 is provided on the support substrate 10. The clad 20 covers the upper surface of the support substrate 10 and is formed to include the first optical waveguide core 30 and the second optical waveguide core 40. The clad 20 is formed using, for example, silicon oxide (SiO ₂ ) as a material.

  The first optical waveguide core 30 is formed of, for example, Si having a higher refractive index than the cladding 20. As a result, the first optical waveguide core 30 functions as a light transmission path, and the light input to the first optical waveguide core 30 propagates in a propagation direction according to the planar shape of the first optical waveguide core 30.

  Like the first optical waveguide core 30, the second optical waveguide core 40 is formed of, for example, Si having a higher refractive index than the cladding 20. As a result, the second optical waveguide core 40 functions as a light transmission path, and the light input to the second optical waveguide core 40 propagates in a propagation direction according to the planar shape of the second optical waveguide core 40.

  In order to prevent the propagating light from escaping to the support substrate 10, the first optical waveguide core 30 and the second optical waveguide core 40 are preferably formed at least 3 μm or more apart from the support substrate 10. preferable.

  The first input / output port 31 is formed with a thickness and a width that achieve a single mode condition for both TE polarization and TM polarization. Accordingly, the first input / output port 31 propagates the fundamental mode TE polarization and TM polarization.

  The input / output tapered portion 32 extends from one end connected to the first input / output port 31 to the other end connected to the first coupling portion 33 from the width of the first input / output port 31 to the width of the first coupling portion 33. Up to a tapered shape whose width continuously increases.

  The first coupling section 33 is formed as a straight waveguide. The detailed design of the first coupling unit 33 will be described later.

  The first connecting portion 34 extends from one end connected to the first connecting portion 33 to the other end connected to the second connecting portion 35 from the width of the first connecting portion 33 to the width of the second connecting portion 35. It is formed in a tapered shape whose width changes continuously (here, it is reduced).

  The second coupling portion 35 is formed in a tapered shape whose width continuously decreases from one end 35a connected to the first connection portion 34 to the other end 35b opposite to the one end. The detailed design of the second coupling unit 35 will be described later.

  The third coupling section 41 is formed as a straight waveguide. The detailed design of the third coupling section 41 will be described later.

  The second connecting portion 42 extends from one end connected to the third connecting portion 41 to the other end connected to the fourth connecting portion 43 from the width of the third connecting portion 41 to the width of the fourth connecting portion 43. It is formed in a tapered shape whose width changes continuously (here, it is reduced).

  The fourth coupling portion 43 extends from the one end 43a connected to the second connection portion 42 to the other end 43b connected to the second input / output port 44 from the width of the second connection portion 42 to the second input / output port 44. Is formed in a tapered shape whose width continuously increases up to the width. The detailed design of the fourth coupling section 43 will be described later.

  The second input / output port 44 is formed to have a thickness and a width for transmitting at least the TE polarization in the first mode and the TM polarization in the fundamental mode.

  The first coupling region 50 will be described.

  The first joint 33 and the third joint 41 are included in the first joint region 50. The first input / output port 31 and the second input / output port 44 are arranged on opposite sides of the first coupling region 50. In the first coupling region 50, the first coupling section 33 and the third coupling section 41 constitute a directional coupler.

  The first coupling portion 33 and the third coupling portion 41 are formed to have a common width and a common thickness. In addition, the first joint 33 and the third joint 41 are formed with different widths and thicknesses. As a result, in the first coupling region 50, the coupling length of one polarization along the vibration direction of the optical electric field is smaller in the smaller one of the width and thickness dimensions of the first coupling portion 33 and the third coupling portion 41. It becomes shorter than the coupling length of the other polarization. Therefore, when the thicknesses of the first coupling portion 33 and the third coupling portion 41 are set to be smaller than the widths, the coupling length of the TM polarization is shorter than the coupling length of the TE polarization. Also, when the width of the first coupling portion 33 and the third coupling portion 41 is set to be smaller than the thickness, the coupling length of the TE polarization is shorter than the coupling length of the TM polarization. Note that the coupling length refers to the parallel length of each waveguide required to completely transfer power from one waveguide (here, the first coupling portion 33) to the other waveguide (here, the third coupling portion 41). The mileage.

  By utilizing the dependency of the coupling length of each polarization on the aspect ratio (width / thickness ratio in the optical waveguide core), only one polarization in the first coupling region 50 is subjected to the first coupling. The transition from the part 33 to the third coupling part 41 is performed.

  In this embodiment, in the first coupling region 50, the TM polarization in the fundamental mode is shifted from the first coupling unit 33 to the third coupling unit 41 while maintaining the basic mode. On the other hand, the fundamental mode TE polarization is not transferred to the third coupling unit 41. Therefore, the thickness of the first joint 33 and the third joint 41 is designed to be smaller than the width. Thereby, the coupling length of the TM polarization between the first coupling unit 33 and the third coupling unit 41 is shorter than the coupling length of the TE polarization. The lengths of the first coupling unit 33 and the third coupling unit 41 are set to the coupling length of the TM-polarized light in the fundamental mode. As a result, only the TM-polarized light in the fundamental mode shifts from the first coupling unit 33 to the third coupling unit 41.

  Here, the relationship between the aspect ratio and the coupling length will be described with reference to FIG. FIG. 3 shows the relationship between the aspect ratio (width / thickness) of the two optical waveguide cores running in parallel (here, the first coupling portion 33 and the third coupling portion 41) and the coupling length of TM-polarized light in the fundamental mode. FIG. 4 is a diagram illustrating a relationship between a coupling length of TM polarization in the fundamental mode and a ratio (TE / TM) of coupling length of TE polarization in the fundamental mode. In FIG. 3, the aspect ratio is shown on the horizontal axis. Also, the vertical axis on the left side of the paper indicates the coupling length of TM polarization in the fundamental mode in units of μm. Further, the vertical axis on the right side of the drawing shows the ratio of the coupling length of the fundamental mode TE polarization to the coupling length of the fundamental mode TM polarization. In FIG. 3, the coupling length of the fundamental mode TM polarization (Lc / TM) and the ratio between the coupling length of the fundamental mode TM polarization and the coupling length of the fundamental mode TE polarization (LcTE / TM) are shown. Each is plotted.

  As shown in FIG. 3, as the aspect ratio increases, the ratio of the coupling length between the TE polarization and the TM polarization increases. Therefore, in order to suppress the coupling of the TE polarization while improving the coupling efficiency of the TM polarization in the first coupling unit 33 and the third coupling unit 41, the aspect ratio (width / thickness) is set to be large. Is preferred.

  However, by setting the aspect ratio to be large and increasing the coupling length of the TM polarization, the length of the first coupling region 50 including the first coupling portion 33 and the third coupling portion 41 becomes large. Since the wavelength dependence of the first coupling region increases, the operating wavelength range narrows. Therefore, it is preferable to set the aspect ratio (width / thickness) to, for example, about 2 from the viewpoint of miniaturization and broadband operation of the optical waveguide element 100.

  Next, the second coupling region 60 will be described.

  The second coupling portion 35 and the fourth coupling portion 43 are included in the second coupling region 60. The first input / output port 31 and the second input / output port 44 are arranged on opposite sides of the second coupling region 60. In the second coupling region 60, the second coupling section 35 and the fourth coupling section 43 constitute a directional coupler.

  As described above, the second coupling portion 35 is formed in a tapered shape whose width continuously decreases from one end 35a to the other end 35b. Further, the fourth coupling portion 43 is formed in a tapered shape whose width continuously increases from the width of the second connection portion 42 to the width of the second input / output port 44 from one end 43a to the other end 43b. I have.

  In the second coupling region 60, one end 35a of the second coupling portion 35 having the maximum width and one end 43a of the fourth coupling portion 43 having the minimum width are arranged on the same side. Further, the other end 35b of the second coupling portion 35 having the minimum width and the other end 43b of the fourth coupling portion 43 having the maximum width are arranged on the same side. Therefore, in the second coupling region 60, the width of the second coupling portion 35 continuously decreases and the width of the fourth coupling portion 43 continuously increases along the light propagation direction R shown in FIG. . In the configuration example of FIG. 1, the surface positions of the one end 35 a of the second joint 35 and the one end 43 a of the fourth joint 43 match, and the other end 35 b of the second joint 35 and the fourth joint 43 have the same surface position. The other end 43b is arranged so that the surface positions thereof match.

Here, the design of the second coupling portion 35 and the fourth coupling portion 43 will be described with reference to FIG. FIG. 4 shows the propagation constant of light propagating through the second coupling section 35, the propagation constant of light propagating through the fourth coupling section 43, the propagation axis coordinates (coordinates of the second coupling region 60 in the light propagation direction R) Z, and FIG. In FIG. 4, the vertical axis shows the propagation constant, and the horizontal axis shows the propagation axis coordinates in arbitrary units. Here, one end 60a of the second coupling region 60 on the one end 35a of the second coupling portion 35 and the one end 43a of the fourth coupling portion 43 is defined as the propagation axis coordinate Za. In addition, the other end 60b of the second coupling region 60 between the other end 35b of the second coupling portion 35 and the other end 43b of the fourth coupling portion 43 is set as the propagation axis coordinate Zb. In FIG. 4, a curve β _x0 indicates a propagation constant of the TE polarization of the fundamental mode propagating through the second coupling unit 35. Further, a curve β _yi-1 indicates the propagation constant of the TE polarization of the (i−1) -order mode propagating through the fourth coupling unit 43 (i is an integer of 1 ≦ i). Further, the curve β _yi indicates the propagation constant of the TE-polarized light of the i-th mode propagating through the fourth coupling section 43. Further, a curve β _{yi + 1} indicates the propagation constant of the TE polarization of the (i + 1) th mode propagating through the fourth coupling unit 43.

  In the second coupling section 35, the light propagation constant decreases as the width decreases from one end 60a to the other end 60b of the second coupling region 60. On the other hand, in the fourth coupling section 43, the light propagation constant increases as the width increases from one end 60a to the other end 60b of the second coupling region 60. Further, between the one end 60a and the other end 60b of the second coupling region 60, the second coupling portion 35 and the fourth coupling portion 43 have the propagation constant of the TE polarization of the fundamental mode propagating through the second coupling portion 35 and , And a point at which the propagation constant of the TE-polarization of the i-th mode propagating through the fourth coupling unit 43 matches. The second coupling portion 35 and the fourth coupling portion 43 include a width corresponding to the point where the propagation constants match, so that the TE polarization of the fundamental mode propagating through the second coupling portion 35 and the fourth coupling portion 43 And the TE-mode polarized wave propagating in the i-th mode.

Accordingly, the second coupling unit is configured such that β _yi <β _x0 <β _yy-1 at one end 60a of the second coupling region 60 and β _{yy + 1} <β _x0 <β _yy at the other end 60b of the second coupling region 60. The width of one end 35a and the other end 35b of 35 and the width of one end 43a and the other end 43b of the fourth coupling portion 43 are set. As a result, the fundamental mode TE polarization propagating in the second coupling unit 35 and the i-th mode TE polarization propagating in the fourth coupling unit 43 can be coupled.

  Further, by setting the widths of the one end 35a and the other end 35b of the second coupling portion 35 and the width of the one end 43a and the other end 43b of the fourth coupling portion 43 in this manner, the second coupling region 60 A point where the propagation constant of the TE polarization of the fundamental mode propagating through the coupling unit 35 and the propagation constant of the TE polarization other than the i-th mode propagating through the fourth coupling unit 43 match is not included. Therefore, coupling between the fundamental mode TE polarization propagating through the second coupling unit 35 and the TE polarization other than the i-th mode propagating through the fourth coupling unit 43 is suppressed.

In this embodiment, in the second coupling region 60, the TE polarization in the fundamental mode is converted to the first-order mode, and is shifted from the second coupling section 35 to the fourth coupling section 43. Therefore, the second coupling unit 35 and the fourth coupling unit 43 are designed under the above-described conditions, where i in the propagation constant β _yi is 1. As a result, the TE polarization in the fundamental mode is converted to the first-order mode, and shifts from the second coupling unit 35 to the fourth coupling unit 43.

  Note that the relationship between the propagation constants shown in FIG. 4 also holds for TM polarization. Therefore, for the TM polarization, the width of the second coupling unit 35 is set to a width in which only the fundamental mode can propagate. By setting the width W1 of one end 35a of the second coupling portion 35 and the width W2 of one end 43a of the fourth coupling portion 43 to W1 <W2, the distance between the second coupling portion 35 and the fourth coupling portion 43 is set. TM polarization coupling can be suppressed.

  As described above, in the optical waveguide element 100, in the first coupling region 50, the TM polarization of the fundamental mode can be shifted from the first coupling unit 33 to the third coupling unit 41 while maintaining the fundamental mode. . Further, in the second coupling region 60, the TE polarization in the fundamental mode can be shifted from the second coupling section 35 to the fourth coupling section 43 while being converted into the first-order mode. As a result, the fundamental mode TM polarization is output without mode conversion with respect to the input fundamental mode TE polarization and TM polarization, and the fundamental mode TE polarization is set to a desired (here, It can be converted to (primary) mode and output. In the optical waveguide element 100, as described above, since undesired coupling does not occur in the first coupling region 50 and the second coupling region 60, TE polarization of a mode order other than the first-order mode and a mode other than the fundamental mode. Conversion of the mode order to TM polarization can be suppressed.

  Further, in the optical waveguide element 100, by providing the input / output taper portion 32, the first connection portion 34, and the second connection portion 42, each of which is formed in a tapered shape, low loss and low loss between the optical waveguide cores having different widths are provided. Can be connected with low reflection.

Here, a configuration example will be described in which the optical waveguide element 100 outputs the primary mode TE polarization and the fundamental mode TM polarization with respect to the input fundamental mode TE polarization and TM polarization, respectively. did. However, the optical waveguide element 100 is not limited to this configuration example. By arbitrarily setting i in the propagation constant β _yi described with reference to FIG. 4, in the second coupling region 60, the light that is transferred from the second coupling section 35 to the fourth coupling section 43 has an arbitrary i-th order. Mode can be converted.

  Conversely, in the optical waveguide element 100, in the first coupling region 50, the TE polarization of the fundamental mode is shifted from the first coupling portion 33 to the third coupling portion 41 while maintaining the fundamental mode, and In the region 60, the TM polarization of the fundamental mode can be shifted from the second coupling unit 35 to the fourth coupling unit 43 while being converted into the i-th mode. In this case, the coupling length of the TE polarization is made shorter than the coupling length of the TM polarization by setting the width of the first coupling portion 33 and the third coupling portion 41 to be smaller than the thickness. By setting the lengths of the first coupling section 33 and the third coupling section 41 to the coupling length of the fundamental mode TE polarization, only the fundamental mode TE polarization is transmitted from the first coupling section 33 to the third coupling section. It is possible to shift to the coupling section 41. In this case, the width is set to be smaller (preferably, the aspect ratio (width / thickness) is, for example, about １ ／) with respect to the thickness of the first joint 33 and the third joint 41.

  In this case, the width of the one end 35a and the other end 35b of the second coupling portion 35 and the width of the one end 43a and the other end 43b of the fourth coupling portion 43 under the above-described conditions with respect to the propagation constant of the TM polarization. Set the width of As a result, in the second coupling region 60, the TM polarization in the fundamental mode can be shifted from the second coupling section 35 to the fourth coupling section 43 while being converted into the i-th mode.

(Characteristic evaluation)
The inventor performed a simulation for evaluating the characteristics of the optical waveguide element 100 using FDTD (Finite Differential Time Domain).

  In this simulation, in the first coupling region 50, the TM polarization of the fundamental mode is shifted from the first coupling portion 33 to the third coupling portion 41 while maintaining the basic mode, and in the second coupling region 60, It is assumed that the TE polarization is shifted from the second coupling section 35 to the fourth coupling section 43 while being converted to the first-order mode. Then, with respect to the optical waveguide element 100, the fundamental mode TE polarization and the TM polarization are input from the first input / output port 31, and the primary mode TE polarization and the basic mode output from the second input / output port 44, respectively. , And the intensity of TM polarization in the fundamental mode.

  In this simulation, it is assumed that the wavelengths of the input TE polarization and TM polarization are 1550 nm, and the following design conditions are assumed.

That is, the material of the first optical waveguide core 30 and the second optical waveguide core 40 was Si. The material of the clad 20 was SiO ₂ . The thickness of the first optical waveguide core 30 and the second optical waveguide core 40 was set to 200 nm as a whole.

  The aspect ratio (width / thickness) of the first joint 33 and the third joint 41 was set to 2.4 by setting the width of each of the first joint 33 and the third joint 41 to 480 nm. Then, the length of the first coupling portion 33 and the third coupling portion 41 (that is, the length of the first coupling region 50) is set to 18 μm, which is the coupling length of the TM polarized light in the fundamental mode. In this case, the coupling length of the fundamental mode TM polarization is 18 μm, whereas the coupling length of the fundamental mode TE polarization is 300 μm, and the coupling length of the TM polarization and the coupling length of the TE polarization are both different. Is very different. The distance between the first coupling portion 33 and the third coupling portion 41 was 980 nm as the distance between the center lines along the length direction.

  The width of one end 35a of the second coupling portion 35 was 0.3 μm, and the width of the other end 35b was 0.12 μm. The width of one end 43a of the fourth coupling portion 43 was 0.5 μm, and the width of the other end 43b was 0.6 μm. In addition, the length of the second coupling portion 35 and the fourth coupling portion 43 (that is, the length of the second coupling region 60) was set to 100 μm. The distance between the second coupling portion 35 and the fourth coupling portion 43 was set to 1.0 μm as the distance between center lines along the length direction.

  In this simulation, the characteristics of a conventional structure disclosed in Patent Literature 3 and provided in a stage preceding the directional coupler were similarly evaluated. In the conventional structure, a tapered first conversion part whose width continuously decreases and a tapered second conversion part whose width continuously increases are spaced apart from each other and arranged side by side (Patent Document 1). 3 (see FIGS. 3 and 4). Here, the fundamental mode TE polarization and the TM polarization are input from the first conversion unit, respectively, and the primary mode TE polarization, the basic mode TE polarization, and the basic mode TM output from the second conversion unit. The polarization intensity was obtained respectively.

  The results of the simulation are shown in FIGS. FIG. 5A is a diagram showing the intensity of TM polarization in the fundamental mode output from the second input / output port 44 in the optical waveguide element 100. A line segment 101 in FIG. 5A indicates the intensity of TM polarization in the fundamental mode. FIG. 5B is a diagram showing the intensity of TM polarization in the fundamental mode output from the second converter in the conventional structure. A line segment 151 in FIG. 5B indicates the intensity of TM polarization in the fundamental mode. FIG. 6A is a diagram showing the intensity of the primary mode TE polarization and the fundamental mode TE polarization output from the second input / output port 44 in the optical waveguide element 100. In FIG. 6A, a line segment 102 indicates the intensity of the TE polarization in the first mode, and a line segment 103 indicates the intensity of the TE polarization in the fundamental mode. FIG. 6B is a diagram showing the intensity of the primary mode TE polarization and the fundamental mode TE polarization output from the second conversion unit in the conventional structure. A line segment 152 in FIG. 6B indicates the intensity of the TE polarization in the first mode, and a line segment 153 indicates the intensity of the TE polarization in the fundamental mode. 5 (A) and 5 (B) and FIGS. 6 (A) and 6 (B), the horizontal axis shows the wavelength in μm units, and the vertical axis shows the output light intensity in dB scale. .

  As shown in FIGS. 5A and 5B, it can be confirmed that the input TM polarization of the fundamental mode is output without mode conversion in both the optical waveguide element 100 and the conventional structure.

  Also, as shown in FIGS. 6A and 6B, it was confirmed that the input fundamental mode TE polarization was converted into the first-order mode and output in both the optical waveguide element 100 and the conventional structure. it can. 6A and 6B, in the optical waveguide element 100, the TE polarization of the fundamental mode output undesirably is improved by 7 dB or more at the wavelength of 1550 nm, as compared with the conventional structure. Can be confirmed.

(Optical coupler)
Referring to FIG. 7, an optical coupler including the above-described optical waveguide element 100 (see FIG. 1) will be described. FIG. 7 is a schematic plan view showing the optical coupler. In FIG. 7, the support substrate and the clad are omitted. Also, the same components as those of the above-described optical waveguide element 100 are denoted by the same reference numerals, and description thereof will be omitted.

  The optical coupler 200 includes three commonly designed optical waveguide elements 100 (first optical waveguide element 100-1 to third optical waveguide element 100-3), a first connection waveguide unit 210, and a second connection waveguide unit. 220, a third connection waveguide 230, and a directional coupler 240. The directional coupler 240 includes a first coupler waveguide section 250 and a second coupler waveguide section 260, which are separated from each other and arranged side by side. The first connection waveguide section 210, the second connection waveguide section 220, the third connection waveguide section 230, the first coupler waveguide section 250, and the second coupler waveguide section 260 form the first optical waveguide of the optical waveguide element 100. It is formed of the same material as the waveguide core 30 and the second optical waveguide core 40, and is formed on the common support substrate 10 (see FIG. 2) so as to be included in the common clad 20 (see FIG. 2).

  The optical coupler 200 couples each of the TE polarization and the TM polarization transmitted from the second input / output port 44 of the first optical waveguide element 100-1 via the first connection waveguide section 210 to the directional coupler 240. Branch into two. Then, the TE-polarized light and the TM-polarized light, each of which has been branched into two in the directional coupler 240, are transmitted to the second optical waveguide device 100-2 via the second connection waveguide 220 and to the third connection waveguide 230. The light is sent to the third optical waveguide element 100-3.

  In this example, light including TE polarization and TM polarization in the fundamental mode is input to the first input / output port 31 of the first optical waveguide core 30 of the first optical waveguide element 100-1. In the first optical waveguide element 100-1, the TM polarization of the fundamental mode is not mode-converted, and the TE polarization of the fundamental mode is converted to the first-order mode.

  Further, the second input / output port 44 of each of the second optical waveguide element 100-2 and the third optical waveguide element 100-3 has the TE polarization of the primary mode branched into two by the directional coupler 240, respectively. The fundamental mode TM polarization is input. In the second optical waveguide element 100-2 and the third optical waveguide element 100-3, the TE polarization of the first mode input from each second input / output port 44 is opposite to that of the first optical waveguide element 100-1. Is converted to the basic mode and output from the first input / output port 31. In addition, the TM-polarized light of the fundamental mode input from each second input / output port 44 passes through a path opposite to that of the first optical waveguide element 100-1 so that the first input / output port 31 is not converted. Output from

  The first connection waveguide section 210 connects between the second input / output port 44 of the first optical waveguide element 100-1 and the first coupler waveguide section 250. The first connection waveguide section 210 is formed with a thickness and a width that allow at least the first-order mode TE polarization and the fundamental mode TM polarization to propagate.

  As the directional coupler 240, a directional coupler disclosed in Patent Document 3 can be used. Here, in the directional coupler 240, the coupling length of the first-order mode TE polarization between the first coupler waveguide section 250 and the second coupler waveguide section 260, the first coupler waveguide section 250 and the second coupler waveguide section. The widths of the first coupler waveguide section 250 and the second coupler waveguide section 260 and the distance between them are set so that the coupling length of the TM polarization of the fundamental mode between the waveguide sections 260 matches. As a result, each of the primary mode TE polarization and the fundamental mode TM polarization can be split into two at a common split ratio according to the length of the directional coupler 240.

  The first coupler waveguide section 250 is connected to the first connection waveguide section 210 on one end 250a side, and is connected to the second connection waveguide section 220 on the other end 250b side. In addition, the second coupler waveguide section 260 has one end 260a side as a cutoff here, and is connected to the third connection waveguide section 230 at the other end 260b side. In the configuration example of FIG. 7, the first coupler waveguide section 250 and the second coupler waveguide section 260 are located at one end 250 a of the first coupler waveguide section 250 and the surface position of one end 260 a of the second coupler waveguide section 260. And the other end 250b of the first coupler waveguide unit 250 and the other end 260b of the second coupler waveguide unit 260 are arranged so as to coincide with each other.

  The second connection waveguide section 220 connects between the first coupler waveguide section 250 and the second input / output port 44 of the second optical waveguide element 100-2. The second connection waveguide portion 220 is formed with a thickness and a width that allow at least the first-order mode TE polarization and the fundamental mode TM polarization to propagate.

  The third connection waveguide unit 230 connects between the second coupler waveguide unit 260 and the second input / output port 44 of the third optical waveguide device 100-3. The third connection waveguide portion 230 is formed with a thickness and a width that allow at least the first-order mode TE polarization and the fundamental mode TM polarization to propagate.

  As described above, the optical coupler 200 combines the first optical waveguide element 100 and the directional coupler 240 to change each of the TE polarization and the TM polarization according to the length of the directional coupler 240. It is possible to branch into two at a common branching ratio. Therefore, the optical coupler 200 can be used independently of polarization.

  Here, a configuration example has been described in which the TE polarization in the first mode and the TM polarization in the fundamental mode are sent from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240. . However, the optical coupler 200 is not limited to this configuration example. A configuration for transmitting an i-th mode TE polarization and a fundamental mode TM polarization from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240, or a fundamental mode TE polarization and i A configuration in which the TM polarization of the next mode is transmitted may be adopted. In these cases, in the directional coupler 240, the coupling length of one polarization of the i-th mode between the first coupler waveguide section 250 and the second coupler waveguide section 260 and the first coupler waveguide section 250 The width of the first coupler waveguide section 250 and the second coupler waveguide section 260 and the distance between them so that the coupling length of the other polarization of the fundamental mode between the second coupler waveguide section 260 and the second coupler waveguide section 260 coincides with each other. Can be set.

(First wavelength filter)
A first wavelength filter including the above-described optical waveguide element 100 (see FIG. 1) will be described with reference to FIGS. FIG. 8 is a schematic plan view showing the first wavelength filter. In FIG. 8, the support substrate and the clad are omitted. FIG. 9 is a schematic end view of the structure shown in FIG. 8 taken along the line II-II. In FIG. 9, hatching is omitted. Components common to the optical waveguide device 100 and the optical coupler 200 described above are denoted by the same reference numerals in FIGS. 8 and 9 and description thereof is omitted.

  The first wavelength filter 300 includes three optical waveguide elements 100 (first optical waveguide element 100-1 to third optical waveguide element 100-3) designed in common, a first connection waveguide section 210, and a second connection. A waveguide section 220, a third connection waveguide section 230, n (n is an integer of 2 or more) directional couplers 240, and k (k is an integer of k = n−1) phase adjustment regions 350 are provided. It is composed. FIG. 8 illustrates a configuration example in which the first wavelength filter 300 includes three directional couplers 240-1 to 240-3 and two phase adjustment regions 350-1 and 350-2.

  The directional couplers 240-1 to 240-3 include a first coupler waveguide section 250 and a second coupler waveguide section 260, which are arranged separately from each other and arranged side by side. Further, the phase adjustment regions 350-1 and 350-2 include a first arm waveguide 310 and a second arm waveguide 320, which are arranged opposite to each other, respectively.

  These first connection waveguide section 210, second connection waveguide section 220, third connection waveguide section 230, first coupler waveguide section 250, second coupler waveguide section 260, first arm waveguide 310 and second connection waveguide section The arm waveguide 320 is formed of the same material as the first optical waveguide core 30 and the second optical waveguide core 40 of the optical waveguide element 100, and is formed on the common support substrate 10 and included in the common clad 20. Have been.

  The first wavelength filter 300 functions as a Mach-Zehnder interferometer type wavelength filter. In the first wavelength filter 300, the light including the TE polarization and the TM polarization transmitted from the second input / output port 44 of the first optical waveguide element 100-1 via the first connection waveguide unit 210 is transmitted in a directional manner. The signal is branched into two in the coupler 240-1. Then, the two branched lights are sent to the directional coupler 240-2 via the first arm waveguide 310 and the second arm waveguide 320 of the phase adjustment region 350-1. The light is sent to the second connection waveguide unit 220 and the third connection waveguide unit 230 through the directional coupler 240-2, the phase adjustment region 350-2, and the directional coupler 240-3 in this order. In the first wavelength filter 300, a phase difference occurs between the light propagating through the first arm waveguide 310 and the light propagating through the second arm waveguide 320 of each of the phase adjustment regions 350-1 and 350-2. The light having the phase difference interferes with the directional couplers 240-2 and 240-3. As a result, an optical signal having a wavelength corresponding to the phase difference between the first arm waveguide 310 and the second arm waveguide 320 is extracted from the directional coupler 240-3.

  Here, as an example, a configuration in which the primary mode TE polarization and the fundamental mode TM polarization are transmitted from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240-1 explain. In this example, light including TE polarization and TM polarization in the fundamental mode is input to the first input / output port 31 of the first optical waveguide core 30 of the first optical waveguide element 100-1. In the first optical waveguide element 100-1, the TM polarization of the fundamental mode is not mode-converted, and the TE polarization of the fundamental mode is converted to the first-order mode. Further, the second input / output port 44 of each of the second optical waveguide element 100-2 and the third optical waveguide element 100-3 is connected to the TE polarization of the first mode branched into two by the directional coupler 240-3. Wave and fundamental mode TM polarization are input. In the second optical waveguide element 100-2 and the third optical waveguide element 100-3, the TE polarization of the first mode input from each second input / output port 44 is opposite to that of the first optical waveguide element 100-1. Is converted to the basic mode and output from the first input / output port 31. In addition, the TM-polarized light of the fundamental mode input from each second input / output port 44 passes through a path opposite to that of the first optical waveguide element 100-1 so that the first input / output port 31 is not converted. Output from

  The first connection waveguide section 210 connects between the second input / output port 44 of the first optical waveguide element 100-1 and the first coupler waveguide section 250 of the directional coupler 240-1. Further, the second connection waveguide section 220 connects between the second input / output port 44 of the second optical waveguide element 100-2 and the first coupler waveguide section 250 of the directional coupler 240-3. Further, the third connection waveguide section 230 connects between the second input / output port 44 of the third optical waveguide element 100-3 and the second coupler waveguide section 260 of the directional coupler 240-3.

  The directional couplers 240-1 to 240-3 are the same as the directional coupler 240 in the optical coupler 200 described above. In each of the directional couplers 240, the coupling length of the first-order mode TE polarization between the first coupler waveguide unit 250 and the second coupler waveguide unit 260, the first coupler waveguide unit 250, and the second coupler waveguide The widths of the first coupler waveguide section 250 and the second coupler waveguide section 260 and the distance between them are set so that the coupling length of the TM polarization of the fundamental mode between the sections 260 matches. As a result, each of the primary mode TE polarization and the fundamental mode TM polarization can be split into two at a common split ratio according to the length of each of the directional couplers 240-1 to 240-3. .

  The first arm waveguides 310 of the phase adjustment regions 350-1 and 350-2 connect between the first coupler waveguide units 250 of the adjacent directional couplers 240. Further, the second arm waveguides 320 of each of the phase adjustment regions 350-1 and 350-2 connect between the second coupler waveguide units 260 of the adjacent directional couplers 240.

  In each of the phase adjustment regions 350-1 and 350-2, the first arm waveguide 310 and the second arm waveguide 320 are connected in series in this order to the first routing unit 301, the first phase adjustment unit 302, and the second phase adjustment unit 302. It includes a routing unit 303, a polarization conversion unit 304, a third routing unit 305, a second phase adjustment unit 306, and a fourth routing unit 307.

  The first routing section 301, the second routing section 303, the third routing section 305, and the fourth routing section 307 are formed as curved waveguides, and at least provide TE polarization in the first-order mode and TM polarization in the fundamental mode. It is formed with a thickness and width to propagate. The first to fourth routing sections 301, 303, 305, and 307 have a common design for the first arm waveguide 310 and the second arm waveguide 320. Therefore, the optical path lengths in the first to fourth routing sections 301, 303, 305, and 307 are set to be equal in the first arm waveguide 310 and the second arm waveguide 320.

  The first phase adjustment unit 302 and the second phase adjustment unit 306 are formed as linear waveguides, and are formed with a thickness and a width for transmitting at least the TE polarization in the first-order mode and the TM polarization in the fundamental mode. I have.

  In addition, the first phase adjustment unit 302 and the second phase adjustment unit 306 of the first arm waveguide 310 and the first phase adjustment unit 302 and the second phase adjustment unit 306 of the second arm waveguide 320 have different lengths. Formed by As a result, an optical path length difference occurs between the first arm waveguide 310 and the second arm waveguide 320. Then, a phase difference can be given to the light propagating through the first arm waveguide 310 and the second arm waveguide 320 according to the optical path length difference.

  Further, the first phase adjustment unit 302 and the second phase adjustment unit 306 of the first arm waveguide 310 and the first phase adjustment unit 302 and the second phase adjustment unit 306 of the second arm waveguide 320 have different widths. Is formed. Regarding the design of the width of each first phase adjustment unit 302 and each second phase adjustment unit 306 in the first arm waveguide 310 and the second arm waveguide 320, for example, the design described in Japanese Patent Application Laid-Open No. 2013-057847. Conditions can be adopted. Accordingly, the phase difference between the first-order mode TE polarization propagating in the first arm waveguide 310 and the first-order mode TE polarization propagating in the second arm waveguide 320 and the first-order waveguide 310 propagate. The first phase adjuster 302 and the second phase adjuster of the first arm waveguide 310 so that the phase difference between the fundamental mode TM polarization and the fundamental mode TM polarization propagating through the second arm waveguide 320 coincides with each other. The optical path length difference between the unit 306 and the first phase adjustment unit 302 and the second phase adjustment unit 306 of the second arm waveguide 320 is optimized. Thereby, in each of the phase adjustment regions 350-1 and 350-2, a common phase difference can be given to the TE polarization in the first-order mode and the TM polarization in the fundamental mode.

  As shown in FIG. 9, the polarization conversion unit 304 includes a lower optical waveguide core 304a having a different width from each other, and an upper optical waveguide core 304b integrally formed on the lower optical waveguide core 304a. FIG. 9 shows a configuration example in which the width of the lower optical waveguide core 304a is set to be larger than the width of the upper optical waveguide core 304b. Therefore, in the polarization conversion unit 304, the width of the lower surface and the width of the upper surface are set to different dimensions.

  As a result, in the polarization conversion unit 304, since the lower optical waveguide core 304a and the upper optical waveguide core 304b have different refractive indexes, the electromagnetic field distribution in the thickness direction of the propagating light is not symmetrical with respect to the core center. . That is, the electromagnetic field distribution of light is decentered downward or upward from the center of the core. In the polarization converter 304, the length and the width of the lower optical waveguide core 304a and the upper optical waveguide core 304b are designed so as to convert the TE polarization in the first mode and the TM polarization in the fundamental mode. . Therefore, the polarization converter 304 converts the primary mode TE polarization into the fundamental mode TM polarization, and converts the fundamental mode TM polarization into the primary mode TE polarization.

  The polarization converter 304 is disposed at the center in the length direction of the first arm waveguide 310 and at the center in the length direction of the second arm waveguide 320, respectively. As a result, in each of the first arm waveguide 310 and the second arm waveguide 320, the TM polarization in the fundamental mode and the TE polarization in the first mode propagate the same distance.

  Thus, by forming the first phase adjustment unit 302, the second phase adjustment unit 306, and the polarization conversion unit 304, in each phase adjustment region 350, the TE polarization in the first mode and the TM polarization in the fundamental mode , A common phase difference can be given independent of polarization.

  As described above, the first wavelength filter 300 combines the optical waveguide element 100 with the directional coupler 240 and the phase adjustment region 350 to form a directional coupler for the TE polarization and the TM polarization. A common split ratio can be provided at 240 and a common phase difference can be provided at each phase adjustment region 350. Therefore, the first wavelength filter 300 can be used as a polarization independent wavelength filter.

  Here, a configuration example in which the primary mode TE polarization and the fundamental mode TM polarization are sent from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240-1. explained. However, the first wavelength filter 300 is not limited to this configuration example. A configuration in which the i-th mode TE polarization and the fundamental mode TM polarization are transmitted from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240-1, or the fundamental mode TE polarization And the i-th mode TM polarization can be sent. In these cases, each of the above-described directional couplers 240 and each of the phase adjustment regions 350 can be designed to be optimized according to one polarization of the i-th mode and the other polarization of the fundamental mode.

(Second wavelength filter)
A second wavelength filter including the above-described optical waveguide element 100 (see FIG. 1) will be described with reference to FIGS. FIG. 10 is a schematic plan view showing the second wavelength filter. In FIG. 10, the support substrate and the clad are omitted. FIG. 11 is a schematic end view of the structure shown in FIG. 10 taken along the line III-III. In FIG. 11, hatching is omitted. Components common to the above-described optical waveguide element 100 and optical coupler 200 are denoted by the same reference numerals in FIGS. 10 and 11, and the description thereof will be omitted.

  The second wavelength filter 400 includes two optical waveguide elements 100 (a first optical waveguide element 100-1 and a second optical waveguide element 100-2) designed in common, a first connection waveguide section 210, and a second connection. Waveguide part 220, third connection waveguide part 230, fourth connection waveguide part 450, directional coupler 240, first connection taper part 410, first Bragg reflector 420, second connection taper part 430, and second It comprises a Bragg reflector 440.

  The directional coupler 240 includes a first coupler waveguide section 250 and a second coupler waveguide section 260, which are separated from each other and arranged side by side.

  These first connection waveguide section 210, second connection waveguide section 220, third connection waveguide section 230, fourth connection waveguide section 450, first coupler waveguide section 250, second coupler waveguide section 260, The first connection taper portion 410, the first Bragg reflection portion 420, the second connection taper portion 430, and the second Bragg reflection portion 440 are made of the same material as the first optical waveguide core 30 and the second optical waveguide core 40 of the optical waveguide device 100. And on the common support substrate 10 so as to be included in the common clad 20.

  In the second wavelength filter 400, the light including the TE polarization and the TM polarization transmitted from the second input / output port 44 of the first optical waveguide element 100-1 via the first connection waveguide unit 210 is directional. In the combiner 240, two branches are performed at an equal ratio. Then, the light branched into two in the directional coupler 240 passes through the second connection waveguide portion 220 and the first connection taper portion 410 to the first Bragg reflection portion 420 and the third connection waveguide portion 230 and the second connection waveguide portion 230. The light is sent to the second Bragg reflector 440 via the connection taper 430. In the first Bragg reflector 420 and the second Bragg reflector 440, the TE polarization of a specific wavelength is converted to a TM polarization, and the TM polarization of a specific wavelength is converted to a TE polarization and reflected. The light reflected by the first Bragg reflector 420 is transmitted to the directional coupler 240 again through the first connection taper 410 and the second connection waveguide 220. The light reflected by the second Bragg reflector 440 is sent to the directional coupler 240 again via the second connection taper 430 and the third connection waveguide 230. In the directional coupler 240, the lights transmitted from the first Bragg reflector 420 and the second Bragg reflector 440 are combined. The multiplexed light is sent to the second optical waveguide element 100-2 via the fourth connection waveguide section 450.

  Here, as an example, a configuration in which the TE polarization in the first mode and the TM polarization in the fundamental mode are transmitted from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240 will be described. . In this example, light including TE polarization and TM polarization in the fundamental mode is input to the first input / output port 31 of the first optical waveguide core 30 of the first optical waveguide element 100-1. In the first optical waveguide element 100-1, the TM polarization of the fundamental mode is not mode-converted, and the TE polarization of the fundamental mode is converted to the first-order mode. In addition, the first-order mode TE polarization and the fundamental mode TM polarization combined by the directional coupler 240 are input to the second input / output port 44 of the second optical waveguide element 100-2. In the second optical waveguide element 100-2, the first-order mode TE polarization input from the second input / output port 44 passes through a path opposite to that of the first optical waveguide element 100-1, and becomes the fundamental mode. The data is converted and output from the first input / output port 31. In addition, the TM-polarized light of the fundamental mode input from the second input / output port 44 passes through a path opposite to that of the first optical waveguide element 100-1 and is not converted into a mode, but is transmitted from the first input / output port 31. Is output.

  The first connection waveguide section 210 connects between the second input / output port 44 of the first optical waveguide element 100-1 and the first coupler waveguide section 250 of the directional coupler 240. Further, the second connection waveguide section 220 connects between the first coupler waveguide section 250 of the directional coupler 240 and the first connection taper section 410. The third connection waveguide 230 connects between the second coupler waveguide 260 of the directional coupler 240 and the second connection taper 430. Further, the fourth connection waveguide section 450 connects between the second input / output port 44 of the second optical waveguide element 100-2 and the second coupler waveguide section 260 of the directional coupler 240. The second connection waveguide 220 and the third connection waveguide 230 have the same optical path length. The first connection waveguide section 210, the second connection waveguide section 220, the third connection waveguide section 230, and the fourth connection waveguide section 450 include at least TE polarization in the first mode and TM polarization in the fundamental mode. It is formed with a thickness and a width that allow waves to propagate.

  The directional coupler 240 is similar to the directional coupler 240 in the optical coupler 200 described above. In the directional coupler 240, the coupling length of the first-order mode TE polarization between the first coupler waveguide section 250 and the second coupler waveguide section 260, the first coupler waveguide section 250, and the second coupler waveguide section The width of the first coupler waveguide section 250 and the second coupler waveguide section 260 and the distance between them are set so that the coupling length of the TM polarization of the fundamental mode between the first and second couplers 260 coincides with each other. As a result, each of the primary mode TE polarization and the fundamental mode TM polarization can be split into two at a common split ratio according to the length of the directional coupler 240. Here, the length of the directional coupler 240 (the length of the first coupler waveguide unit 250 and the length of the second coupler waveguide unit 260) is set such that the branching ratio becomes 3 dB.

  The first connection taper 410 connects between the second connection waveguide 220 and the first Bragg reflector 420. The width of the first connection taper portion 410 changes continuously along the light propagation direction from the width of the second connection waveguide portion 220 to the width of the rib waveguide portion 37 in the first Bragg reflector 420. Is set to By providing the first connection taper portion 410, reflection of light propagating between the second connection waveguide portion 220 and the first Bragg reflector 420 can be reduced.

  The first Bragg reflector 420 includes the rib waveguide 37 and the slab waveguides 38a and 38b.

  A grating is formed in the rib waveguide portion 37. The grating is configured to integrally include the base 51 and the protruding portions 53a and 53b. The base 51 is formed to have a constant width and extend along the light propagation direction. A plurality of protrusions 53a are periodically formed on one side surface of the base 51. A plurality of protrusions 53b are formed on the other side surface of the base 51 at the same period as the protrusions 53a. These protruding portions 53a and 53b are formed at positions symmetrical with respect to the base 51.

  In this embodiment, the grating converts the input fundamental mode TM polarization of a specific wavelength into a first-order mode TE polarization and performs Bragg reflection. The grating converts the input first-order mode TE polarization of a specific wavelength into the fundamental mode TM polarization and performs Bragg reflection.

At the wavelength λ, the conditions for converting the TM polarization of the fundamental mode and the TE polarization of the first-order mode and performing the Bragg reflection are as follows: the equivalent refractive index of the TM polarization of the fundamental mode is N _TM0 ; _Is expressed by the following equation (1), where N _TE1 is the equivalent refractive index of the above, and _{周期 is} the grating period (the formation period of the protrusions 53a and 53b).

(N _TM0 + N _TE1 ) Λ = λ (1)
In the grating, the wavelength λ satisfying the above equation (1), that is, the fundamental mode TM polarization of the Bragg wavelength is converted into the primary mode TE polarization, and the primary mode TE polarization is converted to the fundamental mode TM polarization. It is converted to polarized light and is Bragg reflected. Therefore, the grating period is designed so that the above equation (1) holds for the desired wavelength λ to be reflected. Further, other designs such as the width of the base 51 and the protrusion amounts of the protrusions 53a and 53b are also designed according to a desired wavelength λ to be reflected.

  Here, as a modification of the grating, the period で き る may be set to be constant, and the protrusion amount D of the protrusions 53a and 53b may be changed in each period. A modification of the grating will be described with reference to FIG. FIG. 12 is a schematic plan view for explaining a modification of the grating. In FIG. 12, only the rib waveguide portion of the Bragg reflector is shown, and the support substrate, the clad, and the slab waveguide portion are omitted.

In the configuration example shown in FIG. 12, the protruding amounts D of the protruding portions 53a and 53b (dimensions in the width direction of the protruding portions 53a and 53b) have their own protruding amounts, and the protruding amount D has at least two or more values. Here, with respect to the projecting amount D = D ₀ of the first cycle of protrusions 53a and 53b, the projecting amount D in each cycle is increased by ΔD at a constant variation. Accordingly, in the n periods (n is an integer of 1 or more), the protruding amount D of the protruding portions 53a and 53b is _{D 0} + [Delta] D (n-1).

  When the protrusion amount D of the protrusions 53a and 53b changes, the Bragg wavelength λ that satisfies the above equation (1) changes, and the equivalent refractive index changes accordingly. Therefore, by changing the protrusion amount D, the wavelength band (Bragg reflection band) that is subjected to Bragg reflection in the grating can be expanded.

The shift amount Δλ of the Bragg wavelength for each period can be approximately expressed by the following equation (2) using the change amount ΔD of the protrusion amount D. Note that N ₀ indicates an equivalent refractive index of the fundamental mode, and N ₁ indicates an equivalent refractive index of the first-order mode. In this embodiment, since the grating is a configuration example in which the fundamental mode TM polarization and the first-order mode TE polarization are converted and Bragg reflected, N ₀ is the equivalent refractive index of the TM polarized fundamental mode. N ₁ corresponds to the equivalent refractive index of the first-order mode of TE polarization.

  By adjusting ΔD and setting Δλ such that the Bragg wavelengths of each period overlap, the Bragg reflection band can be expanded. For example, when a grating having n periods is formed, the Bragg reflection band can be expanded by about Δλ × n compared to the case where the protrusion amount D is constant. As a result, the wavelength dependence of Bragg reflection can be reduced.

  Further, as another modified example of enlarging the Bragg reflection band in the grating, the period Λ changes every period while the protrusion amount D and the duty ratio of the protrusions 53a and 53b are constant (that is, along the light propagation direction). The distance between the adjacent protruding portions 53a and the distance between the adjacent protruding portions 53b change with a constant change amount along the light propagation direction). This modification is shown in FIG. FIG. 13 is a schematic plan view for explaining a modification of the grating. In FIG. 13, only the rib waveguide portion of the Bragg reflector is shown, and the support substrate, the clad, and the slab waveguide portion are omitted.

In the configuration example shown in FIG. 13, with respect to the period lambda = lambda ₀ of the first period, the period lambda is increased by ΔΛ with a constant amount of change in each cycle. Therefore, in the n-th cycle, the cycle Λ becomes _{０ 0} + ΔΛ (n−1).

  When the period Λ changes, the Bragg wavelength λ satisfying the above equation (1) changes, and the equivalent refractive index changes accordingly. Therefore, the Bragg reflection band can be expanded in the grating by changing the period 拡 大. The shift amount Δλ of the Bragg wavelength for each period can be approximately expressed by the following equation (3) using the change amount ΔΛ of the period Λ.

  By adjusting ΔΛ and setting Δλ such that the Bragg wavelengths of each period overlap, the Bragg reflection band can be expanded. As in the case where the protrusion amount D is changed, for example, when a grating having n periods is formed, the Bragg reflection band can be expanded by about Δλ × n compared to the case where the period Λ is constant. As a result, the wavelength dependence of Bragg reflection can be reduced.

  The slab waveguide portions 38a and 38b are formed integrally with the rib waveguide portion 37 on both side surfaces of the rib waveguide portion 37 along the light propagation direction with a thickness smaller than that of the rib waveguide portion 37. ing.

  The second connection taper part 430 connects between the third connection waveguide part 230 and the second Bragg reflector 440. The second connection taper portion 430 is formed with the same design as the first connection taper portion 410. By providing the second connection taper portion 430, reflection of light propagating between the third connection waveguide portion 230 and the second Bragg reflector 440 can be reduced.

  The second Bragg reflector 440 has the same design as the first Bragg reflector 420.

  As described above, the second wavelength filter 400 combines the optical waveguide element 100 with the directional coupler 240, the first Bragg reflector 420, and the second Bragg reflector 440, thereby forming the TE polarization and the TM polarization. Waves can be provided with a common branching ratio at the directional coupler 240 and reflected at a common wavelength at the first Bragg reflector 420 and the second Bragg reflector 440. Therefore, the second wavelength filter 400 can be used as a polarization-independent wavelength filter that extracts a wavelength corresponding to the reflection wavelength of the first Bragg reflector 420 and the second Bragg reflector 440.

Here, a configuration example has been described in which the TE polarization in the first mode and the TM polarization in the fundamental mode are sent from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240. . However, the second wavelength filter 400 is not limited to this configuration example. A configuration for transmitting the i-th mode TE polarization and the fundamental mode TM polarization from the second input / output port 44 of the first optical waveguide element 100-1 to the directional coupler 240, or the fundamental mode TE polarization and i A configuration in which the TM polarization of the next mode is transmitted may be adopted. In these cases, the above-described directional coupler 240 and the first Bragg reflector 420 and the second Bragg reflector 440 are optimized according to one polarization of the i-th mode and the other polarization of the fundamental mode. Can be designed. In this case, the equivalent refractive index of the fundamental mode TM polarization is N _TM0 , the equivalent refractive index of the i-order mode TE polarization is N _TEi , the equivalent refractive index of the i-th mode TM polarization is N _TMi , and The following equation (4) or (5) is satisfied as the Bragg reflection condition in the first Bragg reflector 420 and the second Bragg reflector 440 with respect to the equivalent refractive index of TE polarization in the fundamental mode to N _TE0 . The first Bragg reflector 420 and the second Bragg reflector 440 can be designed.

(N _TM0 + N _TEi ) Λ = λ (4)
(N _TMi + N _TE0 ) Λ = λ (5)
(Production method)
The above-described optical waveguide element 100, optical coupler 200, first wavelength filter 300, and second wavelength filter 400 can be easily manufactured by using, for example, an SOI (Silicon On Insulator) substrate. Hereinafter, a method for manufacturing the optical waveguide device 100 will be described as an example.

That is, first, an SOI substrate is prepared in which a support substrate layer, a SiO ₂ layer, and a Si layer are sequentially laminated. Next, the first optical waveguide core 30 and the second optical waveguide core 40 are formed by patterning the Si layer using, for example, an etching technique. As a result, it is possible to obtain a structure in which the SiO ₂ layer is stacked on the support substrate layer as the support substrate 10 and the first optical waveguide core 30 and the second optical waveguide core 40 are formed on the SiO ₂ layer. . Then, for example, by CVD on the SiO ₂ layer, the SiO _2, is formed to cover the first optical waveguide core 30 and the second optical waveguide core 40. As a result, the first optical waveguide core 30 and the second optical waveguide core 40 are included by the cladding 20 of SiO ₂ , and the optical waveguide element 100 can be manufactured. In the case where the optical coupler 200, the first wavelength filter 300, and the second wavelength filter 400 are manufactured, in the step of patterning the Si layer, together with the first optical waveguide core 30 and the second optical waveguide core 40, Additional components, such as sex couplers, can be formed.

  The optical waveguide element 100, the optical coupler 200, the first wavelength filter 300, and the second wavelength filter 400 can be manufactured without using an SOI substrate. In this case, for example, after the cladding 20 and the Si layer are sequentially deposited on the support substrate 10, the Si layer is removed by polishing or the like until the Si layer has an arbitrary thickness. Then, the first optical waveguide core 30 and the second optical waveguide core 40 are formed by patterning the Si layer. The steps after the step of patterning the Si layer are the same as the above-described manufacturing method using the SOI substrate. In this manufacturing method, the thicknesses of the first optical waveguide core 30 and the second optical waveguide core 40 are not limited to the thickness of the Si layer as the SOI layer in the SOI substrate. For this reason, in the above-described first coupling region 50, when obtaining a configuration in which the TE polarization in the fundamental mode is shifted from the first coupling unit 33 to the third coupling unit 41 while maintaining the basic mode, the first coupling unit 33 It is suitable when the width is set to be smaller than the thickness of the third coupling portion 41 (that is, the thickness is set to be larger than the width).

10: Support substrate 20: Cladding 30: First optical waveguide core 31: First input / output port 32: Input / output taper portion 33: First connection portion 34: First connection portion 35: Second connection portion 40: Second light guide Waveguide core 41: third coupling portion 42: second coupling portion 43: fourth coupling portion 44: second input / output port 50: first coupling region 60: second coupling region 100: optical waveguide element 200: optical coupler 300: First wavelength filter 400: Second wavelength filter

Claims (1)

A first optical waveguide core including a first coupling unit and a second coupling unit connected in series;
A second optical waveguide core including a third coupling portion and a fourth coupling portion connected in series,
A first coupling region in which the first coupling portion and the third coupling portion are spaced apart from each other and arranged side by side;
The first coupling portion and the third coupling portion are formed to have a width having a common dimension and a thickness having a common dimension, and have a width and a thickness different from each other,
The length of the first coupling portion and the third coupling portion is such that the oscillation direction of the optical electric field is along the smaller of the width and thickness dimensions of the first coupling portion and the third coupling portion. It is set to the coupling length of one polarization of TE polarization or TM polarization,
In the first coupling region, one polarization of the fundamental mode propagating in the first coupling unit and one polarization of the fundamental mode propagating in the third coupling unit are coupled,
A second coupling region in which the second coupling portion and the fourth coupling portion are arranged apart from each other and arranged side by side,
In the second coupling region, the other polarization of the fundamental mode propagating in the second coupling portion and the other polarization of the i-th mode (i is an integer of 1 ≦ i) propagating in the fourth coupling portion. Are combined ,
The first coupling unit receives the fundamental mode TE polarization and the TM polarization, and receives, from the fourth coupling unit, one polarization of the fundamental mode coupled in the first coupling region; The optical waveguide device, wherein the other polarization of the i-th mode coupled by the two coupling regions is output .

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