Diffraction Grating Coupler

Abstract

A grating coupler 10 is provided with a diffraction grating 12 having a plate-shaped base material 121, and different refractive index regions 122 that are dot-shaped and two-dimensionally or one-dimensionally periodically disposed on the base material 121 or are line-shaped and one-dimensionally periodically disposed on the base material 121, and have a different refractive index from that of the base material 121. Each different refractive index region 122 has a planar shape in which the ratio |κ2| / |κ1| of the absolute value |κ2| of a second coupling coefficient that is an index indicating an intensity at which light traveling in a second direction different by 180° from a first direction parallel to the base material 121 is reflected in the first direction to the absolute value |κ1| of a first coupling coefficient that is an index indicating an intensity at which light traveling in the first direction is reflected in the second direction is 3 or more.

Description

[Technical Field]

[0001] The present invention relates to a grating coupler that optically couples optical elements such as optical integrated circuits and optical fibers together using a diffraction grating. [Background technology]

[0002] Conventionally, diffraction grating couplers equipped with a diffraction grating formed by periodically forming grooves, holes, etc. in a plate-shaped base material have been used to couple optical elements such as optical integrated circuits and optical fibers. In such diffraction grating couplers, light input (incident) from an end face (hereinafter simply referred to as the "end face") of the base material is diffracted, and light having a specific wavelength corresponding to the period of the grooves, holes, etc. is output (emitted) from the surface (hereinafter simply referred to as the "surface") of the base material. Conversely, light having a specific wavelength among light input from the surface can also be output from the end face. By arranging optical elements facing the surface and end face of such a diffraction grating coupler, respectively, light of a specific wavelength can be transmitted and received between the optical elements. In such a diffraction grating coupler, light can be transmitted and received from a larger area on the surface side than on the end face side. Therefore, by arranging optical elements with a relatively large area of ​​light transmitting and receiving portions on the surface side, such as at the end of an optical fiber, light can be transmitted and received with high efficiency.

[0003] Non-Patent Document 1 describes a diffraction grating coupler in which air holes are arranged in a square lattice pattern on a plate-shaped base material made of Si on a substrate made of SiO2. This document describes fabricating two types of diffraction grating couplers: one in which trapezoidal air hole types are arranged so that the upper and lower bases of the trapezoids are parallel to one of two directions in which the lattice points of the square lattice are aligned (orthogonal to each other), referred to as the "trapezoidal air hole type"; and one in which isosceles triangular air hole types are arranged so that the base of the isosceles triangle is parallel to one of the two directions (the "isosceles triangular air hole type"). This document claims that the trapezoidal air hole type has higher coupling efficiency with external optical elements than the isosceles triangular air hole type. In the isosceles triangular air hole type, light diffraction occurs mainly at the base of the isosceles triangle of each air hole, while the electric field of the light tends to concentrate near the apex angle of each air hole, resulting in reduced diffraction efficiency and therefore reduced coupling efficiency with external optical elements. In contrast, in the case of the trapezoidal hole type, electric field concentration is less likely to occur than in the case of the isosceles triangular hole type, and the electric field strength can be made relatively high near the base where light diffraction mainly occurs, thereby making it possible to increase the diffraction efficiency and the efficiency of coupling with external optical elements more than in the case of the isosceles triangular hole type. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Akio Mizutani et al., "A grating coupler with a trapezoidal hole array for perfectly vertical light coupling between optical fibers and waveguides", Applied Physics Express, Japan Society of Applied Physics, November 21, 2017, Vol. 10, No. 122501 [Non-patent document 2] Yong Liang and 4 others, "Three-dimensional coupled-wave model for square-lattice photonic crystal lasers with transverse electric polarization: A general approach", Physical Review B, (USA), American Physical Society, November 22, 2011, Vol. 84, No. 195119. Summary of the Invention [Problem to be solved by the invention]

[0005] In the diffraction grating couplers described in Non-Patent Document 1, when light is input from an end face, for both the trapezoidal hole type and the isosceles triangular hole type, a portion of the light input from the end face is reflected by the grooves or holes, generating reflected light whose direction of travel is changed by 180°, and this reflected light exits the end face. When light is reflected from the end face in this way, the efficiency of outputting light from the surface decreases. Furthermore, the exiting light may enter (reversely enter) an optical element on the input side, which may cause damage to the optical element on the input side.

[0006] Furthermore, when light is input from the surface, the light inside the base material mainly travels from the longer side of the lower or upper base of the trapezoidal air hole to the shorter side (in the case of a trapezoidal air hole type), or from the base side of the isosceles triangle to the apex side (in the case of an isosceles triangular air hole type), but it can also travel in a direction 180° different from this. Such light traveling in a direction 180° different from this will be lost.

[0007] The problem to be solved by the present invention is to provide a diffraction grating coupler that can efficiently output input light. [Means for solving the problem]

[0008] The diffraction grating coupler according to the present invention, which is made to solve the above problems, comprises: a diffraction grating having a plate-shaped base material and modified refractive index areas which are areas with a refractive index different from that of the base material, the modified refractive index areas being dot-like areas periodically arranged in a two-dimensional or one-dimensional manner on the base material, or line-like areas periodically arranged in a one-dimensional manner on the base material; The modified refractive index area has a planar shape in which a ratio |κ2| / |κ1| of an absolute value of a second coupling coefficient, |κ2|, which is an index indicating the intensity with which light traveling in a first direction parallel to the base material is reflected in a second direction that is 180° different from the first direction, to an absolute value of a first coupling coefficient, |κ1|, is 3 or more. It is characterized by:

[0009] As mentioned above, a modified refractive index area is an area with a different refractive index from the base material, and is typically made of air (holes in the case of point-like areas, or empty grooves in the case of line-like areas). Alternatively, a modified refractive index area may be formed by embedding an object made of a material different from the base material into the base material. When the modified refractive index areas are point-like, they are periodically arranged in a two-dimensional (square lattice, rectangular lattice, etc.) or one-dimensional pattern. When the modified refractive index areas are line-like, they are periodically arranged in a one-dimensional pattern. The modified refractive index area may be provided throughout the entire thickness of the base material (penetrating the base material), or may be provided only in a portion of the thickness of the base material. In the latter case, the modified refractive index area may be provided from one surface of the base material (appearing on that surface but not on the other surface), or may be provided only within the base material (not appearing on either surface of the base material).

[0010] The coupling coefficient is an index indicating the intensity of light that is diffracted (i.e., reflected) in a 180° direction when light traveling through a diffraction grating is diffracted. This 180° reflected light is the sum of light generated when the diffraction grating directly changes the traveling direction of the light by 180° and light generated when the light changes 180° while interacting with light output perpendicular to the base material (in a direction 90° different from the traveling direction). The first coupling coefficient κ1 is an index indicating the intensity of light that is reflected 180° (in a second direction) from light traveling in a predetermined first direction through the diffraction grating, and the second coupling coefficient κ2 is an index indicating the intensity of light that is reflected 180° (in the first direction) from light traveling in the second direction through the diffraction grating. These first coupling coefficient κ1 and second coupling coefficient κ2 can be determined based on the structure (shape, size, refractive index) of the modified refractive index area using the method described in Non-Patent Document 2.

[0011] According to the diffraction grating coupler of the present invention, by having a modified refractive index area having a planar shape such that |κ2| / |κ1| is 3 or greater, the ratio of the intensity of light reflected toward the second direction among light traveling in a first direction within the diffraction grating can be suppressed to approximately 10% or less. Therefore, when light is input from an end face of the diffraction grating coupler of the present invention, the efficiency of light output from the surface can be increased by inputting light traveling in the first direction. Furthermore, because the intensity of light entering (reversely entering) an optical element arranged on the input end face can be suppressed, the possibility of the optical element failing can be reduced.

[0012] If the planar shape of the modified refractive index area does not have 180° rotational symmetry, the absolute value |κ1| of the first coupling coefficient and the absolute value |κ2| of the second coupling coefficient will be different. In the present invention, since |κ2| / |κ1| is 3 or more, the planar shape of the modified refractive index area does not have 180° rotational symmetry.

[0013] For the diffraction grating couplers described in Non-Patent Document 1, which have holes with planar shapes of trapezoid and isosceles triangle, |κ2| / |κ1| was calculated using the method described in Non-Patent Document 2, based on parameters such as the side lengths of the planar shapes described in the same document. As a result, the value of |κ2| / |κ1| was 1.09 for the trapezoidal planar shape and 1.64 for the isosceles triangle planar shape, neither of which falls within the range of "3 or more" specified in the present invention.

[0014] In the present invention, it is preferable that each modified refractive index area consists of a pair of a first partial modified refractive index area and a second partial modified refractive index area that differs from the first partial modified refractive index area in either or both of shape and area.

[0015] By using such a modified refractive index area consisting of a pair of a first partial modified refractive index area and a second partial modified refractive index area, asymmetry in the planar shape can be easily introduced, and the value of |κ2| / |κ1| can be easily increased.

[0016] In the present invention, each of the modified refractive index areas may have, in addition to the first partial modified refractive index area and the second partial modified refractive index area, one or more partial modified refractive index areas that are different in either or both of shape and area from the first partial modified refractive index area or the second partial modified refractive index area.

[0017] In the present invention, light can be input from a third direction parallel to the base material and perpendicular to the first direction, simultaneously with the first direction. However, in this case, the input light from the first direction and the input light from the third direction must be input so that they have the same phase and amplitude within the diffraction grating. In this case, the first coupling coefficient κ1 is an index indicating the intensity of light that travels in the first and third directions with the same phase and amplitude and is reflected in the second and fourth directions, which are 180° different from the first and third directions, with the same phase and amplitude. Similarly, the second coupling coefficient κ2 is an index indicating the intensity of light that travels in the second and fourth directions with the same phase and amplitude and is reflected in the first and third directions with the same phase and amplitude. For such first coupling coefficient κ1 and second coupling coefficient κ2, if |κ2| / |κ1| is 3 or greater, the proportion of the intensity of light traveling in the first and third directions within the diffraction grating that is reflected in directions that are 180° different from each other (second and fourth directions) can be kept to approximately 10% or less. Therefore, it is possible to increase the efficiency with which light is output from the surface for both light traveling in the first direction and light traveling in the third direction, and to suppress the intensity of light that enters in reverse.

[0018] The diffraction grating coupler according to the present invention may further comprise a light amplifying layer for amplifying light of a predetermined wavelength within the base material of the diffraction grating or on the surface of the base material.

[0019] Here, the predetermined wavelength corresponds to the periodic length of the arrangement of the modified refractive index areas. Specifically, the wavelength in the diffraction grating (shorter than the wavelength in a vacuum at the same frequency) should be an integer multiple or an integer fraction of the periodic length. An active layer used in a laser element or the like can be used as the light amplification layer.

[0020] A diffraction grating coupler having such an optical amplification layer can be suitably used as an optical amplifier that amplifies input light by the optical amplification layer and then outputs the amplified light with high efficiency. [Effects of the Invention]

[0021] The diffraction grating coupler according to the present invention can efficiently output input light. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a perspective view showing a diffraction grating coupler according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a top view showing the diffraction grating coupler of the first embodiment. [Figure 3] FIG. 2 is a cross-sectional view A showing the diffraction grating coupler of the first embodiment. [Figure 4] FIG. 4 is a diagram showing parameters for determining a first coupling coefficient and a second coupling coefficient in the diffraction grating coupler of the first embodiment. [Figure 5] 1A and 1B are diagrams showing a mechanism by which reflected light occurs within a diffraction grating. [Figure 6] 4 is a graph showing an example of calculation of the real part R and the imaginary part I of the Hermite coupling coefficient in the diffraction grating coupler of the first embodiment. [Figure 7] FIG. 1(a) shows the calculated distribution of the intensity of output light and the intensity of reflected light when light is input from the input port for the case where d=0.265a1 and f1=3.80%, where |κ2| / |κ1| is 3 or more in the diffraction grating coupler of the first embodiment, and FIG. 1(b) shows the calculated distribution of the intensity of output light and the intensity of reflected light when light is input from the end opposite to the input port. [Figure 8] FIG. 10 is a top view showing a modification of the diffraction grating coupler of the first embodiment, in which point-like modified refractive index areas are arranged one-dimensionally. [Figure 9] Graph (a) shows the calculated results of the output light, reflected light, and transmitted light when light is input from the input port side (first partial modified refractive index area side) for the waveguide-type diffraction grating coupler of the example of Figure 8, where |κ2| / |κ1| is 3 or more, d = 0.286a, and the area S1 of the first partial modified refractive index area is 0.0440a2, and graph (b) shows the calculated results of the output light, reflected light, and transmitted light when light is input from the opposite side of the input port (second partial modified refractive index area side). [Figure 10]Graph (a) shows the calculated results of the output light, reflected light, and transmitted light when light is input from the input port side (first partial modified refractive index area side) for the diffraction grating coupler example of Figure 8, where d = 0.286a and S1 = 0.0475a2, where |κ1| / |κ2| is 3 or more, and graph (b) shows the calculated results of the output light, reflected light, and transmitted light when light is input from the opposite side of the input port (second partial modified refractive index area side). [Figure 11] 11 is a top view showing a structure that can be operated as a waveguide grating coupler when it has modified refractive index areas in which d and S1 have the values ​​shown in the example of FIG. 10. FIG. [Figure 12] FIG. 10 is a top view showing a modification of the diffraction grating coupler of the first embodiment, in which linear modified refractive index areas are arranged one-dimensionally. [Figure 13] 10A and 10B are cross-sectional views showing an example of an optical amplifier that is another modified example of the diffraction grating coupler of the first embodiment, in which an optical amplification layer is provided on the surface of the base material of the diffraction grating, and an example of an optical amplification layer that is provided within the base material. [Figure 14] FIG. 4 is a top view showing a diffraction grating coupler according to a second embodiment of the present invention. [Figure 15] FIG. 10 is a diagram showing parameters for determining a first coupling coefficient and a second coupling coefficient in the diffraction grating coupler of the second embodiment. [Figure 16] 10 is a graph showing an example of calculation of the real part R and the imaginary part I of the Hermite coupling coefficient in the diffraction grating coupler of the second embodiment. [Figure 17] FIG. 10 shows the calculated distribution of the intensity of output light and the intensity of reflected light when light is input from the first and second input ports in the diffraction grating coupler of the second embodiment, where d=0.278a and 2x=1.5 nm, in which |κ2| / |κ1| is 3 or greater. DETAILED DESCRIPTION OF THE INVENTION

[0023] 1 to 17, an embodiment of a diffraction grating coupler according to the present invention will be described.

[0024] (1) First embodiment 1 shows a perspective view of a diffraction grating coupler 10 of the first embodiment. This diffraction grating coupler 10 includes a base 11 made of silicon dioxide (SiO2) and having a rectangular planar shape, a rectangular plate-shaped base material 121 made of silicon (Si) formed on the surface of the base 11, a number of modified refractive index areas 122 made of holes provided in the base material 121, and an input port (light input section) 13 provided on one of the four rectangular sides of the base material 121 (the end on the starting point side in a first direction described below). The base material 121 and the modified refractive index area 122 together form a diffraction grating 12. The surface of the base material 121 opposite the base 11 is space (air).

[0025] In this embodiment, the entire lower surface of the base material 121 is supported by the base 11, but a portion of the lower surface of the base material 121 (for example, only the vicinity of two opposing sides of the rectangular base material 121) may be supported by some kind of member. Furthermore, the materials of the base material 11 and the base material 121 are not limited to the above examples, and other materials may be used. The modified refractive index areas 122 may be formed by embedding in the base material a material that has a refractive index different from that of the base material 121, instead of holes. Furthermore, although several tens of modified refractive index areas 122 are depicted in FIG. 1 and other figures, in reality, many more modified refractive index areas 122 are provided in the base material 121.

[0026] 2, each modified refractive index area 122 is formed by arranging, apart from each other, a first partial modified refractive index area 1221 and a second partial modified refractive index area 1222 having a different planar shape and area from the first partial modified refractive index area 1221. The first partial modified refractive index area 1221 has an elliptical planar shape, while the second partial modified refractive index area 1222 has a circular planar shape. Because the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 have different planar shapes and areas, the planar shape of the entire modified refractive index area 122 does not have 180-degree rotational symmetry.

[0027] The shape of each modified refractive index area 122 is designed so that the ratio |κ2| / |κ1| of the first coupling coefficient κ1 to the second coupling coefficient κ2 is 3 or greater, and details of this design will be described later. The first coupling coefficient κ1 in this embodiment is an index that indicates the intensity with which light traveling from input port 13 toward the side opposite one side of the rectangle of base material 121 in which input port 13 is provided (the direction in which this light travels is referred to as the "first direction," which is the direction from left to right in FIGS. 2 and 3) is reflected by modified refractive index area 122 in a direction 180° different from the first direction (referred to as the "second direction"). The second coupling coefficient κ2 is an index that indicates the intensity with which light traveling in the second direction is reflected by modified refractive index area 122 toward the first direction.

[0028] 2, the planar shape of the first partial modified refractive index area 1221 is elliptical and the planar shape of the second partial modified refractive index area 1222 is circular, but the planar shapes of these partial modified refractive index areas may be other shapes. Also, the second partial modified refractive index area 1222 may be different from the first partial modified refractive index area 1221 in only one of the planar shape and area.

[0029] The modified refractive index areas 122 are arranged on lattice points of a rectangular lattice within a range of the rectangular base material 121 excluding areas near the four sides of the rectangle. One of the two primitive translation vectors of the rectangular lattice is parallel to the first direction and the second direction, and the other is perpendicular to the first direction and the second direction. The lattice constants of the rectangular lattice are set so that the lattice constant a1 in the direction parallel to the first direction and the second direction is longer than the lattice constant a2 in the direction perpendicular to the first direction and the second direction. The minor axis in the planar shape of the first partial modified refractive index area 1221 is parallel to the first direction and the second direction, and the centers of gravity of the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 are spaced apart in a direction parallel to the first direction and the second direction.

[0030] 3, the modified refractive index area 122 is provided in the thickness direction from the upper surface side of the base material 121 to a predetermined depth without penetrating the base material 121. However, in the present invention, the modified refractive index area 122 may be formed so as to penetrate the base material 121, or the modified refractive index area 122 may be provided only inside the base material 121 by providing a lid made of the same material as the base material 121 on the upper surface of the modified refractive index area 122.

[0031] The input port 13 is provided in a portion of the rectangular base material 121 on one side of the rectangle where the modified refractive index area 122 is not arranged (where the diffraction grating 12 is not formed), and corresponds to the portion between two grooves 131 extending from the diffraction grating 12 toward one side of the rectangle.

[0032] An input-side optical element 91 is disposed at the end of the base material 121, which is outside the input port 13, and an output-side optical element 92 is disposed on the upper surface (the surface opposite to the base 11) of the diffraction grating 12. An optical IC or the like can be used for the input-side optical element 91, and an optical fiber or the like can be used for the output-side optical element 92.

[0033] The operation of the diffraction grating coupler 10 of this embodiment will be described. When using the diffraction grating coupler 10, the input optical element 91 is placed at the end of the input port 13 so that the propagation direction of the input light emitted by the input optical element 91 is parallel to the base material 121. The input light has a wavelength the same as a1, which is the lattice constant of the diffraction grating 12 in the propagation direction of the input light. If the wavelength of the input light is predetermined, the lattice constant a1 is set to match that wavelength. Note that the wavelength of the input light referred to here is the wavelength within the diffraction grating 12, and because the effective refractive index within the diffraction grating 12 is greater than 1, it is shorter than the wavelength in a vacuum.

[0034] Input light input through input port 13 travels in a first direction within diffraction grating 12. The input light is then diffracted by periodically arranged modified refractive index areas 122 within diffraction grating 12. At this time, because the wavelength of the input light within diffraction grating 12 matches the lattice constant a1 in the direction of propagation of the input light, the diffracted light diffracted in a direction perpendicular to base material 121 is intensified by interference. As a result, the diffracted light diffracted in a direction perpendicular to base material 121 is extracted as output light from the surface of base material 121 (FIG. 3). By placing output-side optical element 92 on this surface (for example, with one end of the optical fiber serving as output-side optical element 92 facing this surface as shown in FIG. 1), the output light is captured by output-side optical element 92. In this way, input-side optical element 91 and output-side optical element 92 are optically coupled by diffraction grating coupler 10.

[0035] Meanwhile, a portion of the input light traveling in the first direction within the diffraction grating 12 is reflected by the modified refractive index area 122 and becomes reflected light traveling in the second direction. However, in the diffraction grating coupler 10 of this embodiment, the shape and size of the modified refractive index area 122 are designed so that the ratio |κ2| / |κ1| of the first coupling coefficient κ1 to the second coupling coefficient κ2 is 3 or greater, and the intensity of the reflected light is therefore kept to approximately 10% or less of the intensity of the incident light. This makes it possible to suppress a decrease in the intensity of the output light, thereby increasing the output efficiency, and to prevent reflected light from entering back and damaging the input-side optical element 91.

[0036] Below, we will explain in detail an example in which the shape and size of the modified refractive index area 122 are designed so that |κ2| / |κ1| is 3 or greater, and we will also explain the results of simulating the output light and reflected light when using modified refractive index area 122 of the designed shape and size. Note that the design shown below is just one example, and even if the modified refractive index area has another shape, it can be appropriately designed so that |κ2| / |κ1| is 3 or greater after calculating the first coupling coefficient κ1 and the second coupling coefficient κ2 using the method described in Non-Patent Document 2.

[0037] The parameters used in this design example to determine κ1 and κ2 will be described with reference to FIG. 4. The refractive index of the base material 121 (Si) was 3.4, and the refractive index of the modified refractive index area 122 (air) was 1. The lattice constants were a1 = 470 nm and a2 = 316 nm. The thickness of the base material 121 was 330 nm, and the thickness of the modified refractive index area 122 was 220 nm. The wavelength of the input light and output light within the diffraction grating 12 is 470 nm, the same value as a1. Note that because the effective refractive index of the diffraction grating 12 depends on the size of the modified refractive index area 122, the wavelength of the input light and output light in a vacuum also differs depending on the size of the modified refractive index area 122.

[0038] Three examples of the distance d between the center of gravity of the first partial modified refractive index area 1221 and the center of gravity of the second partial modified refractive index area 1222 were prepared: 0.260a1, 0.265a1, and 0.270a1.

[0039] The planar shapes of the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 are determined so that the sum f=f1+f2 of the filling rate f1 of the first partial modified refractive index area 1221 and the filling rate f2 of the second partial modified refractive index area 1222 is 0.07 (7%). Here, the filling rate f1 (f2) of the first (second) partial modified refractive index area 1221 (1222) is defined as the value obtained by dividing the area of ​​the planar shape of the first (second) partial modified refractive index area 1221 (1222) by the area of ​​the unit lattice (=a1×a2). The length of the major axis of the ellipse of the first partial modified refractive index area 1221 was fixed at 137 nm, and for five examples in which the filling rate f1 of the first partial modified refractive index area 1221 was 3.75%, 3.80%, 3.85%, 3.90%, and 3.95%, the length of the minor axis of the ellipse and the circular diameter of the second partial modified refractive index area 1222 were determined so that the filling rate f of the modified refractive index area 122 was 7%.

[0040] For 15 examples combining the three examples of center-of-gravity distances d described above and the five examples of filling factors f1 of the first partial modified refractive index areas 1221, the Hermitian coupling coefficients R±iI and non-Hermitian coupling coefficients iμ (i is the imaginary unit) for calculating the first coupling coefficient κ1 and the second coupling coefficient κ2 were calculated using the method described in Non-Patent Document 2.

[0041] Here, the Hermitian coupling coefficient R±iI is a coefficient indicating an index of how much the propagation direction of light changes by 180° without loss, as shown in the box labeled "Hermitian Coupling" in FIG. 5. The Hermitian coupling coefficient has a complex conjugate relationship between its value when changing from the second direction to the first direction (referred to as R+iI) and its value when changing from the first direction to the second direction (referred to as R-iI). The non-Hermitian coupling coefficient iμ is a coefficient indicating an index of how much the propagation direction of light changes by 180° from the initial direction while incurring a radiation loss in a direction 90° different from the initial direction (a direction perpendicular to the base material 121), as shown in the box labeled "Non-Hermitian Coupling" in FIG. 5. The non-Hermitian coupling coefficient iμ has the same value when changing from the second direction to the first direction as when changing from the first direction to the second direction. The sum of the light whose propagation direction changes by 180° without any loss and the light whose propagation direction changes by 180° with radiation loss becomes the reflected light. The first coupling coefficient κ1 and the second coupling coefficient κ2 are respectively calculated using the Hermitian coupling coefficient and the non-Hermitian coupling coefficient as follows: κ1=R-iI+iμ κ2=R+iI+iμ The relationship between the first coupling coefficient κ1 and the second coupling coefficient κ2, and the reflectance R1 at which light propagating in the first direction is reflected in the second direction, and the reflectance R2 at which light propagating in the second direction is reflected in the first direction, is expressed as follows: R1 / R2=|κ1 / κ2| 2 When |κ2| / |κ1| is 3 or more, R1 / R2 is 1 / 9 or less, and the reflectance of the input wave can be kept to approximately 10% or less (strictly speaking, 11.1% or less).

[0042] The calculation results of the values ​​of R and I in the Hermite coupling coefficient R±iI are shown in Figure 6. The calculation results of the value μ representing the imaginary part (the real part is 0) of the non-Hermite coupling coefficient are almost independent of the distance d between the centers of gravity and the filling factor f1 of the first partial modified refractive index area, and are almost the same value (approximately 70 cm -1) of the 17 data points shown in FIG. 6, 15 data points excluding the two indicated by dashed arrows indicate the values ​​of R and I for each of the 15 examples of combinations of the above-mentioned center-of-gravity distance d and the filling factor f1 of the first partial modified refractive index area 1221. The values ​​of the center-of-gravity distance d and the filling factor f1 of the first partial modified refractive index area 1221 at each data point correspond to the numerical values ​​written on the thin dashed curves that intersect near each data point. The two data points indicated by dashed arrows are the points where R = 0, I = +μ and R = 0, I = -μ, respectively. The former indicates R and I when the intensity of the reflected light is 0, and the latter indicates R and I when the intensity of the reflected light is 1. At the data points within the thick dashed circle in FIG. 6 (d = 0.265a1, f1 = 3.75%, and d = 0.265a1, f1 = 3.80%), |κ2| / |κ1| is 3 or greater.

[0043] 7 shows the calculated intensity distribution of the output light output in a direction perpendicular to the base material 121 and the reflected light whose propagation direction is changed by 180° after being reflected within the diffraction grating 12 when d=0.265a1 and f1=3.80% where |κ2| / |κ1| is 3 or more. Calculations were performed for two cases: (a) a normal case in which input light is input from input port 13 (input light propagates in a first direction), and (b) a reference case in which input light is input from the end of the base material 121 opposite to input port 13 (input light propagates in a second direction).

[0044] As a result, when light is input from input port 13, output light is output from the end of diffraction grating 12 on the input port 13 side (the point marked with a vertical arrow in the left diagram of Figure 7(a)) throughout the diffraction grating 12 in a direction perpendicular to the base material 121, while almost no reflected light is generated (right diagram of Figure 7(a); the dashed line in the diagram is the end of diffraction grating 12 on the input port 13 side). From this result, it can be seen that when d = 0.265a1 and f1 = 3.80%, the diffraction grating coupler 10 of the first embodiment can efficiently output light input from input port 13 in a direction perpendicular to the base material 121.

[0045] On the other hand, when light is input from the opposite side of input port 13, most of the input light is reflected at the end of diffraction grating 12 on the opposite side of input port 13 (the point marked with a vertical arrow in the right diagram of Figure 7(b)) and at a region slightly inside diffraction grating 12 from that end, and it can be seen that almost no light is output in a direction perpendicular to base material 121.

[0046] Up to this point, we have explained the example of using a modified refractive index area 122 consisting of a pair of a first partial modified refractive index area 1221 and a second partial modified refractive index area 1222, but as long as the planar shape satisfies the condition that |κ2| / |κ1| is 3 or greater, it is also possible to use a modified refractive index area consisting of only one area whose refractive index differs from that of the base material 121, or a modified refractive index area consisting of three or more partial modified refractive index areas.

[0047] Furthermore, although an example in which the dot-like modified refractive index areas 122 are arranged two-dimensionally has been shown up to this point, the dot-like modified refractive index areas 122 may also be arranged one-dimensionally as shown in Fig. 8. In this example, in order to prevent light from leaking in a direction perpendicular to the direction in which the modified refractive index areas 122 are arranged in the plane of the base material 121 (to the sides of the diffraction grating 13), two grooves 131 extending from the input port 13 are aligned in a direction in which the modified refractive index areas 122 are not arranged. (The diffraction grating 12 is not formed.) Not just a part ,times folded lattice 12 (in Figure 8, the diffraction grating 12 It extends to the top and bottom of the

[0048] As in the example shown in Figure 1, etc., the modified refractive index area 122 has a shape that combines an elliptical first partial modified refractive index area 1221 and a circular second partial modified refractive index area 1222, and in each modified refractive index area 122, the first partial modified refractive index area 1221 is positioned closer to the input port 13 than the second partial modified refractive index area 1222, and the minor axis of the ellipse of the first partial modified refractive index area 1221 is oriented in a direction parallel to the groove 131.

[0049] 8, while light passes through the one-dimensional, in other words, waveguide-like portion between the two grooves 131, it is output in a direction perpendicular to the base material 121. Hereinafter, a diffraction grating coupler having such a waveguide-like structure will be referred to as a "waveguide-type diffraction grating coupler."

[0050] The following calculation was performed for the waveguide grating coupler shown in Figure 8. In this calculation, the periodic length (arrangement interval) a of the modified refractive index areas 122 was 0.720 µm, the material of the base material 121 was Si (refractive index 3.4), the width w of the base material 121 between the two grooves was 0.480 µm, and the thickness of the base material 121 was 0.22 µm. In addition, the sum S = S1 + S2 of the area S1 of the first partial modified refractive index area 1221 and the area S2 of the second partial modified refractive index area 1222 was 0.08a 2 As a result, the filling factor of the modified refractive index area 122 is f=S / (a×w)=0.08a / w=0.12. The ellipticity of the first partial modified refractive index area 1221 is 10S1 / a 2 The calculation was performed using an example in which 150 such modified refractive index areas 122 were arranged one-dimensionally.

[0051] First, the distance d between the center of gravity of the first partial modified refractive index area 1221 and the center of gravity of the second partial modified refractive index area 1222 (center-to-center distance) is set to 0.286a, and the area S1 of the first partial modified refractive index area 1221 is set to 0.0440a. 2 In the case of the waveguide grating coupler of the first example, the real part R and the imaginary part I of the Hermite coupling coefficient, and the imaginary part μ of the non-Hermite coupling coefficient (the real part is 0) were calculated, and the results were I ≒ μ ≒ 350 cm -1 The absolute value of R is sufficiently smaller than I and μ (|R|<50cm -1 ) Based on R, I, and μ, the absolute value |κ1| of the first coupling coefficient κ1 = R - iI + iμ is sufficiently smaller than the absolute value |κ2| of the second coupling coefficient κ2 = R + iI + iμ. Therefore, the waveguide grating coupler of the first example satisfies the requirement that |κ2| / |κ1| be 3 or greater.

[0052] For the first example of the waveguide grating coupler, we calculated the intensities of the emitted light emitted in a direction perpendicular to the base material 121, the reflected light reflected by the modified refractive index region 122 and returning to the input port 13, and the transmitted light that passes through the grating and exits the end of the waveguide on the opposite side of the input port 13. The results are shown in Figure 9(a). As shown in the figure, when light is input through the input port 13 as is typically done, most of the input light is output without reflection or transmission over the entire normalized frequency range (0.45 to 0.50) used for the calculations, demonstrating the function of the waveguide grating coupler. Here, the normalized frequency multiplied by c / a (c is the speed of light) represents the frequency of light in a waveguide grating coupler with a periodic length a.

[0053] For reference, Figure 9(b) shows the calculated intensities of emitted light, reflected light (light returning to the end of the waveguide), and transmitted light (light flowing out to input port 13) when light is input from the end of the waveguide opposite input port 13. In this case, strong reflected light occurs at a normalized frequency of around 0.475 (the region surrounded by the dashed line in Figure 9), and it is clear that this does not function satisfactorily as a diffraction grating coupler. In this case, the configuration shown in Figure 9(a) can be used as a reflector.

[0054] Next, as a comparative example of a waveguide type diffraction grating coupler, the area S1 of the first partial modified refractive index area 1221 is set to 0.0475a 2 When the other parameters were set to the same values ​​as in the first example, the real part R and imaginary part I of the Hermite coupling coefficient and the imaginary part μ of the non-Hermite coupling coefficient were calculated, and I ≒ -350 cm -1 , μ≒-350cm -1 , |R|<50cm -1 From these R, I, and μ, the absolute value |κ1| of the first coupling coefficient κ1 = R - iI + iμ is sufficiently larger than the absolute value |κ2| of the second coupling coefficient κ2 = R + iI + iμ. Therefore, the waveguide grating coupler of this comparative example does not satisfy the requirement that |κ2| / |κ1| be 3 or greater, and conversely, |κ1| / |κ2| is 3 or greater (|κ2| / |κ1| is less than 3).

[0055] For this comparative example of a waveguide grating coupler, the intensities of output light, reflected light, and transmitted light were calculated when light was incident from the input port 13 side (the first partial modified refractive index area 1221 side of the modified refractive index area 122) ( FIG. 10(a) ) and when light was incident from the opposite side of the input port 13 (the second partial modified refractive index area 1222 side) ( FIG. 10(b) ). As a result, as shown in FIG. 10(a) , when input light having a normalized frequency of approximately 0.475 was input from the input port 13 side, the intensity of the reflected light became too large, and it was found that the characteristics required for a waveguide grating coupler could not be obtained. Note that, since |κ1| / |κ2| is 3 or more, the requirement that the ratio of the coupling coefficients be 3 or more is satisfied when the first direction and the second direction are interchanged. Therefore, when input light is input from the opposite side of the input port 13, such reflection hardly occurs.

[0056] Therefore, as shown in Fig. 11, a second example of a waveguide grating coupler is introduced, in which the end of the waveguide opposite to the input port 13 in the example of Fig. 8 is a new input port 13A, and the input port 13 side in the example of Fig. 8 is the end of the waveguide opposite to the input port 13A, and the diffraction grating 12 has the same configuration as the waveguide grating coupler 12 of the comparative example. In this example, when light is incident from the input port 13A side (the second partial modified refractive index area 1222 side), it is output with almost no reflection or transmission, and it can be seen that it functions as a diffraction grating coupler (Fig. 10(b)).

[0057] Although the examples shown so far use point-like modified refractive index areas 122, linear modified refractive index areas may also be used. The diffraction grating coupler 10A shown in Fig. 12 includes a diffraction grating 12A configured by one-dimensionally arranging a large number of linear modified refractive index areas 122A, each of which is made up of a first partial modified refractive index area 1221A and a second partial modified refractive index area 1222A formed by grooves of different widths, with the linear modified refractive index areas 122A arranged one-dimensionally in the width direction of the grooves at a periodic length a. By adjusting the widths of the first partial modified refractive index area 1221A and the second partial modified refractive index area 1222A, it is possible to set |κ2| / |κ1| to 3 or more. The number of grooves (partial modified refractive index areas) may be three or more.

[0058] As a modification of the diffraction grating coupler of the first embodiment, as shown in FIG. 13, an optical amplification layer (active layer) 15 may be provided on the surface (FIG. 13(a)) or inside (FIG. 13(b)) of the base material 121 of the diffraction grating 12. The optical amplification layer 15 is a typical active layer used in laser elements and the like, and amplifies light whose wavelength in the diffraction grating 12 is an integer multiple or an integer fraction of the periodic length a1 of the modified refractive index region in the first direction. A diffraction grating coupler 10B equipped with such an optical amplification layer 15 functions as an optical amplifier that amplifies input light by the optical amplification layer and then outputs it with high efficiency.

[0059] In each of the examples shown so far, two grooves 131 are formed in the base material 11 and the region between them serves as the input port 13, but the groove 131 may be omitted.

[0060] (2) Second embodiment 14 shows a top view of a diffraction grating coupler 20 of the second embodiment. This diffraction grating coupler 20 includes a plate-shaped base material 221 made of Si provided on a base (not shown, similar to the base material 11 in the first embodiment) made of SiO2, and a diffraction grating 22 consisting of modified refractive index areas 222 arranged in a square lattice pattern with a periodic length a on the base material 221. One of the directions of two primitive translation vectors in this square lattice is the x-direction, and the other is the y-direction. The modified refractive index area 222 is composed of a pair of a first partial modified refractive index area 2221 having an elliptical planar shape and a second partial modified refractive index area 2222 having a circular planar shape. The minor axis of the ellipse of the first partial modified refractive index area 2221 is inclined at +45° with respect to the x-direction (the direction in which y increases as x increases), and the centers of gravity of the first partial modified refractive index area 2221 and the second partial modified refractive index area 2222 are spaced apart in the direction of the minor axis.

[0061] The shape of the modified refractive index area 222 is set so that, when the +x direction (direction from left to right in FIG. 14) is the first direction, the −x direction (direction from right to left in the same figure) is the second direction, the +y direction (direction from bottom to top in the same figure) is the third direction, and the −y direction (direction from top to bottom in the same figure) is the fourth direction, the ratio |κ2| / |κ1| of the second coupling coefficient κ2, which is an index indicating the intensity of light reflected in the first direction and the third direction with the same phase and amplitude, of light traveling in the second direction and the fourth direction with the same phase and amplitude, to the first coupling coefficient κ1, which is an index indicating the intensity of light reflected in the first direction and the third direction with the same phase and amplitude, of light traveling in the second direction and the fourth direction with the same phase and amplitude, is 3 or greater.

[0062] The detailed design of the shape of the modified refractive index area 222 will be described later.

[0063] The base material 221 is formed with two parallel grooves (a first outer edge groove 241 and a second outer edge groove 242) that are inclined at +45° with respect to the x direction, and the modified refractive index area 222 is provided between the first outer edge groove 241 and the second outer edge groove 242. The base material 221 is also provided with a first input port groove 2331 that extends in the −x direction from one end of the first outer edge groove 241 on the negative side in the x direction, and a second input port groove 2332 that extends in the −y direction from one end of the second outer edge groove 242 on the negative side in the x direction. The base material 221 is also provided with a third input port groove 2333 that is formed by connecting a groove that is parallel to the first input port groove 2331 and spaced apart in the −y direction with a groove that is parallel to the second input port groove 2332 and spaced apart in the −x direction. The area between the first input port groove 2331 and the third input port groove 2333 functions as the first input port 231 , and the area between the second input port groove 2332 and the third input port groove 2333 functions as the second input port 232 .

[0064] A first input-side optical element is disposed at the end of the base material 221 at the first input port 231, and a second input-side optical element is disposed at the end of the base material 221 at the second input port 232. In addition, an output-side optical element is disposed on the upper surface of the diffraction grating 22 (both not shown). Both of these optical elements input / output light whose wavelength in the diffraction grating 22 is a.

[0065] The operation of the diffraction grating coupler 20 of the second embodiment will be described. First input light traveling in the +x direction is input to the first input port 231 from the first input-side optical element. Second input light traveling in the +y direction is input to the second input port 232 from the second input-side optical element. Both the first input light and the second input light are introduced into the diffraction grating 22, diffracted by the diffraction grating 22 in a direction perpendicular to the base material 221, and extracted as output light from the surface of the diffraction grating 22. The extracted output light is introduced into the output-side optical element.

[0066] Note that, for the light 1 incident from the first input port 231 that enters near the end on the negative side of y, and the light 2 incident from the second input port 232 that enters near the end on the negative side of x, there is a risk that some of the light may pass through the diffraction grating 22 because a sufficient number of modified refractive index areas 222 are not present on the path that the light travels through the diffraction grating 22. In order to prevent such light from passing through the diffraction grating 22 by reflecting it, the first outer edge groove 241 and the second outer edge groove 242 are provided.

[0067] An example in which the shape and size of the modified refractive index area 222 are designed so that |κ2| / |κ1| is 3 or greater will be described in detail below.

[0068] The parameters used in this design example to determine κ1 and κ2 will be described with reference to FIG. 15. The refractive index of the base material 121 (Si) was 3.4, and the refractive index of the modified refractive index area 222 (air) was 1. The lattice constant was a = 278 nm. The center of gravity of the first partial modified refractive index area 2221 and the center of gravity of the second partial modified refractive index area 2222 were separated by a distance d in the x and y directions, respectively. Four examples of this distance d were prepared: 0.266a, 0.272a, 0.278a, and 0.284a. The length of the major axis of the ellipse of the first partial modified refractive index area 2221 was fixed at 125 nm, the length of the minor axis was (53 + 2x) nm, and the diameter of the circle of the second partial modified refractive index area 2222 was (67 - 2x) mm, with four examples of 2x = 1.5 nm, 3.0 nm, 4.5 nm, and 6.0 nm.

[0069] For 16 examples combining the above-mentioned four examples of distance d and four examples of 2x, the Hermite coupling coefficient R±iI and the non-Hermite coupling coefficient iμ for determining the first coupling coefficient κ1 and the second coupling coefficient κ2 were calculated using the method described in Non-Patent Document 2. The calculation results are shown in FIG. 16. The calculation result of the value μ representing the imaginary part (the real part is 0) of the non-Hermite coupling coefficient is almost independent of the distance d between the centers of gravity and 2x, which is the value that determines the minor axis of the ellipse of the first partial modified refractive index area 2221 and the diameter of the second partial modified refractive index area 2222, and is approximately the same value (approximately 87 cm -1 ) of the 18 data points shown in Figure 16, 16 data points excluding the two indicated by dashed arrows indicate the calculation results for the above 16 examples. The two data points indicated by dashed arrows are the points where R = 0, I = +μ and R = 0, I = -μ, respectively. The former indicates R and I when the intensity of the reflected light is 0, and the latter indicates R and I when the intensity of the reflected light is 1. At the data points within the circle indicated by the thick dashed line in Figure 16 (d = 0.272a, 2x = 1.5nm and d = 0.278a, 2x = 1.5nm), |κ2| / |κ1| is 3 or more.

[0070] 17 shows the calculated intensity distribution of the output light output in a direction perpendicular to the base material 221 and the reflected light whose propagation direction is changed by 180° after being reflected within the diffraction grating 22 when d=0.278a and 2x=1.5 nm. When the first input light is input through the first input port 231 and the second input light is input through the second input port 232, it can be seen that the output light is output from the diffraction grating 22 in a direction perpendicular to the base material 221 (left diagram in FIG. 17), while almost no reflected light is generated (right diagram in FIG. 17). From this result, it can be seen that when d=0.278a and 2x=1.5 nm, the diffraction grating coupler 20 of the second embodiment can efficiently output the light input through the first input port 231 and the second input port 232 in a direction perpendicular to the base material 221.

[0071] In the second embodiment, as in the first embodiment, by providing an optical amplification layer (active layer) on the surface or inside of the diffraction grating 22, it can be used as an optical amplifier.

[0072] Two embodiments of the diffraction grating coupler according to the present invention and their modifications have been described above, but the present invention is not limited to these examples, and various modifications are possible within the scope of the gist of the present invention.

[0073] [Aspect] It will be apparent to those skilled in the art that the above-described exemplary embodiments are examples of the following aspects.

[0074] (Item 1) The diffraction grating coupler according to item 1 is a diffraction grating having a plate-shaped base material and modified refractive index areas which are areas with a refractive index different from that of the base material, the modified refractive index areas being dot-like areas periodically arranged in a two-dimensional or one-dimensional manner on the base material, or line-like areas periodically arranged in a one-dimensional manner on the base material; The modified refractive index area has a planar shape in which the ratio |κ2| / |κ1| of the absolute value of a second coupling coefficient, |κ2|, which is an index indicating the intensity with which light traveling in a first direction parallel to the base material is reflected in a second direction that is 180° different from the first direction, to the absolute value |κ1| of a first coupling coefficient, which is an index indicating the intensity with which light traveling in the second direction is reflected in the first direction, is 3 or greater.

[0075] (Item 2) The diffraction grating coupler according to item 2 is a diffraction grating coupler according to item 1, wherein each of the modified refractive index areas is a pair of a first partial modified refractive index area and a second partial modified refractive index area that is different from the first partial modified refractive index area in either or both of shape and area.

[0076] (Item 3) The diffraction grating coupler according to item 3 is the diffraction grating coupler according to item 1 or 2, and further comprises an optical input section for inputting input light at an end of the base material on the starting point side in the first direction.

[0077] (Item 4) The diffraction grating coupler according to item 4 is the diffraction grating coupler according to any one of items 1 to 3, further comprising an optical amplification layer that amplifies light of a predetermined wavelength within the base material in the diffraction grating or on the surface of the base material. [Explanation of symbols]

[0078] 10, 10A, 10B, 20... Diffraction grating coupler 11...Foundation 12, 12A, 22...diffraction grating 121, 221...Base material 122, 122A, 222...Modified refractive index areas 1221, 1221A, 2221...first partial modified refractive index region 1222, 1222A, 2222...Second partial modified refractive index region 13...Input port 131...Input port groove 15...Active layer (light amplification layer) 231...First input port 232...Second input port 2331...First input port groove 2332...Second input port groove 2333...Third input port groove 241...First outer groove 242...Second outer groove 91...input side optical element 92...Output side optical element

Claims

1. a diffraction grating having a plate-shaped base material and modified refractive index areas which are areas having a refractive index different from that of the base material, the modified refractive index areas being dot-like areas periodically arranged in a two-dimensional or one-dimensional manner on the base material, or line-like areas periodically arranged in a one-dimensional manner on the base material; The modified refractive index area has a planar shape in which a ratio |κ2| / |κ1| of an absolute value of a second coupling coefficient, |κ2|, which is an index indicating the intensity with which light traveling in a first direction parallel to the base material is reflected in a second direction that is 180° different from the first direction, to an absolute value of a first coupling coefficient, |κ1|, is 3 or more.

1. A diffraction grating coupler comprising:

2. The diffraction grating coupler of claim 1, characterized in that each of the modified refractive index areas consists of a pair of a first partial modified refractive index area and a second partial modified refractive index area that differs from the first partial modified refractive index area in either shape or area or both.

3. 3. The diffraction grating coupler according to claim 1, further comprising an optical input section for inputting input light at an end of the base material on a starting point side in the first direction.

4. 3. The diffraction grating coupler according to claim 1, further comprising an optical amplification layer for amplifying light of a predetermined wavelength, within the base material of the diffraction grating or on the surface of the base material.

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