Diffraction grating coupler

Abstract

A diffraction 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 regions having a refractive index different from that of the base material (121) and one-dimensionally or two-dimensionally arranged at regular intervals within a predetermined range of the base material (121); and a light reflection part (14) provided at an end on the opposite side from a light input part (13) side of the range. The different refractive index region (122) has a planar shape such that the absolute value of a direct coupling coefficient (κ1D), which is an index indicating the intensity at which light traveling in a first direction (positive x direction) parallel to the base material (121) from the light input unit (13) that is a part of an end of the range is reflected in a second direction (negative x direction) that is 180° different from the first direction (positive x direction) without being accompanied by radiation loss in the different refractive index region (122), is less than 1 / L that is the reciprocal of the span (L) of the range in the first direction (positive x direction).

Description

Diffraction Grating Coupler

[0001] The present invention relates to a grating coupler that uses a diffraction grating to optically couple optical elements such as optical integrated circuits and optical fibers, or optical elements and free space.

[0002] Conventionally, diffraction grating couplers have been used, each of which includes a diffraction grating formed by periodically arranging regions (modified refractive index regions) with refractive indices different from those of a plate-shaped base material. The modified refractive index regions typically consist of grooves or holes. In such diffraction grating couplers, light input (incident) from an end face of the base material is diffracted, and light having a specific wavelength corresponding to the period of the modified refractive index regions is output (emitted) from the surface of the diffraction grating coupler (more precisely, the surface of the base material within which the modified refractive index regions are arranged). 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, light of a specific wavelength input from one optical element can be input to the other optical element. Alternatively, an optical element can be arranged on the end face of the diffraction grating coupler, and light of a specific wavelength input from the optical element can be output from the surface of the diffraction grating coupler into free space, or light of a specific wavelength input from free space to the surface can be input to the optical element. Such output of light into free space or input of light from free space can be used for optical communication with other optical elements separated from the diffraction grating coupler through free space, sensing of objects separated from the diffraction grating coupler through free space, etc.

[0003] In conventional diffraction grating couplers, when light is input through an end face, a portion of the input light is reflected by the modified refractive index area, generating reflected light whose propagation direction changes by 180°. This reflected light exits the light input section (the end face). This reflected light exiting the light input section not only reduces the efficiency of light output from the surface of the diffraction grating, but also may cause light to enter (reversely propagate) into optical elements arranged near the light input section, potentially resulting in damage to the optical elements. Therefore, the diffraction grating coupler described in Patent Document 1 uses a modified refractive index area with a planar shape that does not have 180° rotational symmetry, thereby reducing the proportion of light intensity reflected by the modified refractive index area in a second direction 180° different from the first direction, of light traveling in a first direction parallel to the base material. In this diffraction grating coupler, the end face of the base material on the side where light traveling in the first direction enters serves as the light input section, thereby preventing light from entering (reversely propagating) into the light input section.

[0004] It is known that the larger the exit area of ​​a diffraction grating coupler, the smaller the divergence angle of light emitted from the surface. The smaller the divergence angle, the smaller the area over which the emitted light enters other optical elements or a sensing target object separated from the diffraction grating coupler via free space, thereby increasing the light intensity per unit area in the incident area. However, in conventional diffraction grating couplers, including those described in Patent Document 1, the intensity of light emitted from the surface exponentially attenuates with increasing distance from the light input section. This means that almost no light is emitted from areas far from the light input section, even within the area where the modified refractive index areas are located (i.e., the area where the diffraction grating is formed). As a result, the effective exit area is smaller than the entire surface of the diffraction grating (the area where the modified refractive index areas are located), resulting in a large divergence angle.

[0005] There is a need for a diffraction grating coupler that can emit light with nearly uniform intensity regardless of the distance from the light input section so that such a reduction in the effective emission area does not occur. Non-Patent Document 1 describes that in a diffraction grating coupler having a plurality of equally spaced grooves in a plate material, when the ratio (duty ratio) of the width of the grooves to the width of the portion other than the grooves is a specific value, a wavenumber region is formed in which two light energy bands in the diffraction grating overlap, and when light having a wavelength corresponding to the energy of the end (singular point) of the wavenumber region is input to the diffraction grating, light can be emitted from the entire surface of the diffraction grating regardless of the distance from the light input section.

[0006] International Publication No. WO2023 / 026712

[0007] Alexander Yulaev et al., "Exceptional points in lossy media lead to deep polynomial wave penetration with spatially uniform power loss," Nature Nanotechnology, (UK), Nature Research, April 21, 2022, Vol. 17, pp. 583-589; Yong Liang et al., "Three-dimensional coupled-wave model for square-lattice photonic crystal lasers with transverse electric polarization: A general approach," Physical Review B, (US), American Physical Society, November 22, 2011, Vol. 84, pp. 1951-199

[0008] In the diffraction grating coupler described in Non-Patent Document 1, more than half of the light input from the light input section is not output in a direction perpendicular to the surface of the diffraction grating and is lost, resulting in low light extraction efficiency.

[0009] The problem to be solved by the present invention is to provide a diffraction grating coupler that can emit light from the entire surface of the diffraction grating and has high light extraction efficiency.

[0010] The diffraction grating coupler according to the present invention, which has been made to solve the above problems, comprises a diffraction grating having a plate-shaped base material, modified refractive index areas which are areas having a refractive index different from that of the base material and which are periodically arranged one-dimensionally or two-dimensionally within a predetermined range of the base material, and a light reflecting section provided at an end of the range on the opposite side to the light input section side, and a direct coupling coefficient κ which is an index indicating the intensity at which light traveling in a first direction parallel to the base material from the light input section which is part of the end of the range is reflected in a second direction which is 180° different from the first direction without causing a radiation loss in the modified refractive index area. 1D The modified refractive index area has a planar shape in which the absolute value of is less than 1 / L, which is the reciprocal of the length L across the range in the first direction.

[0011] As mentioned above, a modified refractive index area is an area having a different refractive index from the base material, and is typically made of air (holes or grooves). Another example is forming a modified refractive index area by embedding an object made of a material different from the base material into the base material. The modified refractive index area may be provided throughout the entire thickness of the base material (so as to penetrate the base material), or it may be provided only in a portion of the base material in the thickness direction. 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 it may be provided only inside the base material (so as not to appear on either surface of the base material).

[0012] Direct coupling coefficient κ 1D is an index showing the intensity of light (hereinafter referred to as "directly reflected light"; the meaning of "directly" will be explained later) that is generated when the light traveling in a first direction within the diffraction grating is diffracted (i.e., reflected) in a 180° direction and the traveling direction of the light is directly changed by 180°. 1D is a complex number, and the coefficients of its real and imaginary parts have dimensions of the reciprocal of the length. 1D The complex conjugate κ of 1D * (|κ 1D |=|κ 1D * The direct coupling coefficient κ is an index that indicates the intensity of light traveling in the second direction that is directly reflected back in the first direction. 1Dis determined depending on the planar shape of the modified refractive index area, and can be found based on the method described in Non-Patent Document 2. As described in Patent Document 1, the light traveling within a diffraction grating that is reflected in the 180° direction includes, in addition to directly reflected light, light (referred to as "indirectly reflected light") that is generated when the traveling direction changes by 180° while accompanied by radiation loss due to interaction with light output in a direction perpendicular to the surface of the diffraction grating (in a direction that is 90° different from the traveling direction), but directly reflected light predominantly contributes to the reflection in the 180° direction.

[0013] If the intensity of the directly reflected light is too large, the light is locally confined in the direction parallel to the base material, and as a result, the intensity of the light in the diffraction grating decreases rapidly (exponentially) with increasing distance from the light input portion, and the intensity of the emitted light also decreases rapidly with increasing distance from the light input portion. 1D By reducing | to less than 1 / L to suppress the intensity of directly reflected light, the rate at which the light intensity decreases with distance from the light input section can be made more gradual (linear function) than before.

[0014] However, if the intensity of the directly reflected light is reduced, the proportion of light incident from the light input portion that passes through the diffraction grating without being emitted as output light in a direction perpendicular to the surface of the diffraction grating increases, resulting in a decrease in the extraction efficiency of the output light.In Non-Patent Document 1, it is believed that the decrease in the extraction efficiency of the output light is caused by a portion of the incident light passing through the diffraction grating without being emitted as output light in a direction perpendicular to the surface of the diffraction grating.

[0015] Therefore, in the present invention, a light reflecting section is further provided at the end of the range (the range in which the modified refractive index area is arranged) opposite to the light input section side. As a result, incident light that enters from the light input section and propagates in the first direction is reflected by the light reflecting section and returned to the diffraction grating side, preventing the incident light from passing through the diffraction grating and thereby increasing the extraction efficiency of the output light.

[0016] The light reflected by the light reflecting portion travels in the second direction, but like the light traveling in the first direction, the intensity decreases more gradually with increasing distance from the light reflecting portion. Furthermore, when the intensity of the light traveling in the second direction is viewed based on the distance from the light input portion, it increases more gradually with increasing distance from the light input portion. Therefore, when the light input from the light input portion traveling in the first direction and the light reflected by the light reflecting portion traveling in the second direction are combined, the change in light intensity with position within the diffraction grating becomes even more gradual, allowing light to be emitted from the entire surface of the diffraction grating.

[0017] |κ 1D The value of |κ is calculated by using L, which is the length across the first direction of the range in which the modified refractive index areas are arranged (in other words, the range in which the diffraction grating is formed), so as to weaken the effect of light being confined within the diffraction grating in the direction parallel to the base material. 1D |×L is less than 1, i.e., |κ 1D | is set to be less than 1 / L. 1D The value of |κ should be less than 1 / (2L) to minimize reflection loss due to direct reflection, and it is most desirable that it be 0. 1D When |κ|=0, the distribution of the light intensity in the diffraction grating is closest to uniform. For example, when the modified refractive index area is a combination of two subareas having the same shape and refractive index, and the two subareas are spaced apart in the first direction by a distance of ¼ times the wavelength of the light in the diffraction grating, |κ| 1D |=0.

[0018] As mentioned above, the direct reflection light is the dominant contributor to the light that travels through the diffraction grating and is reflected in the 180° direction, but the indirect reflection light also has a certain degree of influence. Therefore, it is advisable to set the position of the light reflecting part so that the indirect reflection light and the light reflected by the light reflecting part are in opposite phase or a phase close to each other. This cancels out the indirect reflection light, thereby reducing the reflection loss due to the indirect reflection light.

[0019] In the diffraction grating coupler of the present invention, the indirect coupling coefficient (non-Hermitian coupling coefficient) μ, which is an index showing the intensity of light (i.e., the indirect reflected light) whose propagation direction changes by 180° while incurring radiation loss due to the interaction of light traveling in the first direction or the second direction with outgoing light traveling in a direction perpendicular to the base material, can be configured to have the same value regardless of the position at which the modified refractive index areas are periodically arranged.

[0020] By making μ the same regardless of position, it is possible to emit light with nearly uniform intensity from the surface of the diffraction grating. Typically, the same μ value can be achieved regardless of position by making the modified refractive index areas arranged at each position have the same shape and material (refractive index). Note that the term "same value" here also includes cases where the μ value for each position varies within a range of ±20% due to manufacturing errors in the modified refractive index areas that inevitably occur during the manufacture of the diffraction grating coupler.

[0021] On the other hand, μ may be set to different values ​​at at least two locations among the plurality of locations where the modified refractive index areas are periodically arranged.

[0022] By varying the value of μ depending on the position, it is possible to intentionally create variations in the intensity of light emitted from the surface of the diffraction grating. To vary the value of μ depending on the position, the shape and / or material of the modified refractive index areas placed at each position can be varied. For example, if each modified refractive index area is a combination of a first subarea and a second subarea, the value of μ increases as the ratio of the area S1 of the first subarea to the area S2 of the second subarea becomes more distant from 1 (larger or smaller). Therefore, by arranging modified refractive index areas with different ratios S1 / S2 of the areas S1 and S2 at different positions, the value of μ can be varied depending on the position. The thickness of each modified refractive index area may be appropriately set in addition to or instead of the area.

[0023] In the diffraction grating coupler according to the present invention, the light reflecting portion can be formed, for example, by providing a photonic crystal outside the end of the range, in which the energy of light of a predetermined wavelength falls within the photonic band gap. A photonic crystal is a structure in which the refractive index changes periodically and is characterized by not passing light having a wavelength within a specific wavelength band (photonic band gap) corresponding to the period of the change in refractive index. The reason why a photonic band gap is formed for light in a photonic crystal is the same as the reason why an energy gap (band gap) is formed for electrons that have wave properties in a solid. Like the diffraction grating of the present invention, a photonic crystal is typically formed by periodically arranging modified refractive index areas in a plate-shaped base material. In other words, the diffraction grating of the present invention is also a type of photonic crystal. The difference between the diffraction grating of the present invention and the photonic crystal that forms the light reflecting portion is whether the energy of light of the predetermined wavelength is set to fall within the photonic band in which the light can propagate (diffraction grating) or within the photonic band gap (light reflecting portion), depending on the period in which the modified refractive index areas are arranged.

[0024] In the diffraction grating coupler according to the present invention, the indirect coupling coefficient μ(x) at each position x in the first direction and the second direction of the diffraction grating is expressed as the electric field intensity E RAD It is expressed using (x) It is preferable that the range is within the range of It is most preferable that:

[0025] The indirect coupling coefficient μ is determined depending on the planar shape of the modified refractive index area, and can be found based on the method described in Non-Patent Document 2. Since the magnitude of the interaction between the emitted light and the indirectly reflected light changes depending on the value of μ, by appropriately setting the value of μ, it is possible to reduce the proportion of light that returns to the light input side due to indirectly reflected light, and to increase the extraction efficiency of the emitted light that is emitted in a direction perpendicular to the base material. For example, the absolute value of the direct coupling coefficient |κ 1D | is 0, and the light reflecting portion is provided, and μ is set to the same value (E RADWhen (x) is a position-independent constant, the extraction efficiency of the emitted light exceeds 50% when the indirect coupling coefficient μ is in the range of 0.09 / L<μ<2.91 / L, exceeds 90% when it is in the range of 0.25 / L<μ<1 / L, which corresponds to the above-mentioned condition, and is 100% when μ=0.5 / L.

[0026] For convenience, up to this point, we have explained that a portion of the end of the range (diffraction grating) in which the modified refractive index areas are periodically arranged is the light input portion, and that light input from this light input portion is output from the surface of the diffraction grating. However, the diffraction grating coupler of the present invention can also be used so that light input from the surface of the diffraction grating is output from the light input portion (the portion named for convenience).

[0027] The diffraction grating coupler according to the present invention may be configured such that an edge-emitting laser is connected to the optical input section such that the optical output section of the edge-emitting laser faces the optical input section. This allows laser light oscillated by the edge-emitting laser to be output from the surface of the diffraction grating coupler. The edge-emitting laser may be formed integrally with the diffraction grating coupler, or may be separate from the diffraction grating coupler and connected to the diffraction grating coupler.

[0028] Two diffraction grating couplers according to the present invention (excluding those in which an edge-emitting laser is connected to the optical input section) can also be used as a single diffraction grating coupler by arranging them so that their optical input sections face each other and the first direction of one diffraction grating coupler coincides with the second direction of the other diffraction grating coupler. With this configuration, light input through the surface of the diffraction grating of one diffraction grating coupler can be output from the optical input section (a section conveniently named as such) of that one diffraction grating coupler, input to the optical input section of the other diffraction grating coupler, and output from the surface of the diffraction grating of the other diffraction grating coupler. In this case, by making the areas of the diffraction gratings (within the aforementioned range) of these two diffraction grating couplers different from each other, they can be used as a converter for converting the diameter of a light beam.

[0029] Although the above has described the case where light incident from the light input portion and traveling in a first direction is output from the surface of the diffraction grating in a direction perpendicular to the surface, depending on the shape of the modified refractive index area, it is also possible to change the direction of propagation of light incident from the light input portion and traveling in a first direction by 90° within the base material (to a direction parallel to the base material) and output from an end of the diffraction grating other than the light input portion. Such an end-emitting diffraction grating coupler comprises a diffraction grating having a plate-shaped base material and modified refractive index areas which are areas with a different refractive index from the base material and are periodically arranged one-dimensionally or two-dimensionally within a predetermined rectangular area of ​​the base material, one side of the rectangular area being the light input portion, and the other side of the rectangular area perpendicular to the first side being the light output portion, and a direct coupling coefficient κ is an index indicating the intensity at which light traveling from the light input portion in a first direction parallel to the base material is reflected in the modified refractive index area in a second direction parallel to the base material and 180° different from the first direction without radiation loss. 1D The modified refractive index area has a planar shape such that the absolute value of is less than 1 / L2, which is the reciprocal of the length L2 across the range in the first direction. This makes it possible to output light with nearly uniform intensity from each position of the linear light output section.

[0030] In such an end-emitting diffraction grating coupler, it is preferable that two sides of the rectangular region other than the light input and light output sections are light reflecting sections, which prevents the input light input from the light input section and the light diffracted by 90 degrees and traveling in the direction opposite to the light output section from passing through the diffraction grating and becoming a loss.

[0031] Such an edge-emitting diffraction grating coupler can also be configured such that an edge-emitting laser is connected to the optical input section so that the optical output section of the edge-emitting laser faces the optical input section.

[0032] A single diffraction grating coupler can be formed by combining the first diffraction grating coupler described above, which outputs input light from the optical input section in a direction perpendicular to the surface of the diffraction grating, with the end-emitting diffraction grating coupler, which is a second diffraction grating coupler. Specifically, the optical input section of the first diffraction grating coupler has a linear shape perpendicular to the first direction, and the optical input section of the first diffraction grating coupler and the optical output section of the second diffraction grating coupler are arranged parallel to each other.

[0033] With this configuration, the light input from the optical input portion of the second diffraction grating coupler is input to the optical input portion of the first diffraction grating coupler with nearly uniform intensity from each position on the linear light output portion of the second diffraction grating coupler, and further, the light input from each position on the optical input portion is output in a direction perpendicular to the diffraction grating of the first diffraction grating coupler with nearly uniform intensity in the first direction of the first diffraction grating coupler, thereby making it possible to make the output intensity of light nearly uniform across the planar range of the diffraction grating in the first diffraction grating coupler.

[0034] A diffraction grating coupler that combines a first diffraction grating coupler and a second emitting type diffraction grating coupler can also be configured such that an end-emitting laser is connected to the optical input portion of the second (end-emitting) diffraction grating coupler, with the optical output portion of the end-emitting laser facing the optical input portion.

[0035] According to the present invention, it is possible to obtain a diffraction grating coupler that can emit light from the entire surface of the diffraction grating and has high light extraction efficiency.

[0036] 6(a) and 6(b) are perspective and top views showing a diffraction grating coupler of the first embodiment; a top view showing a modified example of the diffraction grating coupler of the first embodiment; a top view showing another modified example of the diffraction grating coupler of the first embodiment; a longitudinal cross-sectional view and a top view showing the operation of the diffraction grating coupler of the first embodiment; a graph showing the results of calculation of the intensity distribution of output light for one modified example of the diffraction grating coupler of the first embodiment; top views showing two further modified examples of the diffraction grating coupler of the first embodiment; a photograph showing the output light when light is input to the optical input portion of the diffraction grating coupler shown in FIG. 6(b), and a photograph showing the output light when light is input to the optical input portion of a comparative example diffraction grating coupler; a top view showing a diffraction grating coupler of a second embodiment; a photograph showing the output light when light is input to the optical input portion of the diffraction grating coupler shown in FIG. 8; a top view showing a diffraction grating coupler of a third embodiment; 1 is an enlarged top view of a second modified refractive index area of ​​a second diffraction grating in a diffraction grating coupler of a third embodiment. 2 is a graph showing the calculated intensity distribution of output light for the diffraction grating coupler of the third embodiment. 3 is a top view of a diffraction grating coupler of a fourth embodiment. 4 is a diagram showing the calculated intensity distribution of output light for the diffraction grating coupler of the fourth embodiment. 5 is a diagram showing the calculated far-field pattern of output light for the diffraction grating coupler of the fourth embodiment. 6 is a top view of a diffraction grating coupler of a fifth embodiment (a), and longitudinal cross-sectional views of the same diffraction grating coupler (b) and (c) showing the operation of the same diffraction grating coupler. 7 is a top view of a diffraction grating coupler of a sixth embodiment. 8 is a top view of an input portion diffraction grating (a), an in-plane 90° diffraction portion diffraction grating (b), and an output portion diffraction grating (c) in a diffraction grating coupler of the sixth embodiment. 9 is a perspective view schematically showing the operation of the diffraction grating coupler of the sixth embodiment. Distribution of the design values ​​of the indirect coupling coefficient μ and the 90° coupling coefficient κ calculated for the diffraction grating coupler of the sixth embodiment. 2D 10A and 10B are diagrams showing the distribution of design values ​​(b) and the calculated intensity distribution of the emitted light (c). A conceptual diagram showing a diffraction grating coupler of the seventh embodiment. A top view showing the incident portion diffraction grating (a), the first and second in-plane 90° diffraction portion diffraction gratings (b), and the exit portion diffraction grating (c) of the diffraction grating coupler of the seventh embodiment. A top view ((a) to (c)) showing an example in which an end-emitting laser is connected to the light incident portion of each of the diffraction grating couplers of the first to fourth embodiments.

[0037] 1 to 23, an embodiment of a diffraction grating coupler according to the present invention will be described.

[0038] (1) First Embodiment Figure 1 shows a diffraction grating coupler 10 of the first embodiment in (a) a perspective view and (b) a top view. This diffraction grating coupler 10 includes a rectangular plate-shaped base 11, a rectangular plate-shaped base material 121 formed on the surface of the base 11, a modified refractive index area 122 consisting of holes provided in the base material 121, an optical input section (input port) 13 provided on one of the short sides of the rectangle of the base material 121, and an optical reflector 14 provided within the base material 121. 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).

[0039] In this embodiment, silicon dioxide (SiO2) is used as the material for the base 11, and silicon (Si) is used as the material for the base 11 and the base material 121. However, the materials for the base 11 and the base material 121 are not limited to these. The base material 121 may be made of a material that transmits light of the wavelength used in the diffraction grating coupler, and the base 11 may be made of a material on which the base material 121 can be formed and that has a lower refractive index than the base material 121 (so that light traveling through the base material 121 is totally reflected when incident at a shallow angle on the interface with the base 11). In addition, in this embodiment, the base 11 supports the entire lower surface of the base material 121, but it is also possible to support only a portion of the lower surface of the base material 121 (for example, only near two opposing sides of the rectangular base material 121) with some kind of member. Instead of holes, the modified refractive index area 122 may be made of a member with a refractive index different from that of the base material 121.

[0040] 1B, in the first embodiment, a large number (N pieces) of modified refractive index areas 122 are periodically arranged one-dimensionally with a periodic length a in the direction in which the long side of the rectangle of the base material 121 extends (positive x direction, corresponding to the first direction). By arranging the modified refractive index areas 122 with a periodic length a in this way, a diffraction grating 12 is formed over a length L=N×a, and the wavelength in the diffraction grating 12 is a (the wavelength in a vacuum is a×n eff where n effis the effective refractive index of the diffraction grating 12.) is diffracted in a direction perpendicular to the surface of the base material 121. Note that the modified refractive index areas 122 may be arranged two-dimensionally as shown in FIG. 2 instead of being arranged one-dimensionally. In the example of FIG. 2, the modified refractive index areas 122 are arranged in a direction from the light input unit 13 toward the light reflecting unit 14 (positive x direction = first direction) with a periodic length a and N x , and N with periodic length a in the y direction perpendicular to the x direction. y They are arranged in a square lattice pattern.

[0041] Each modified refractive index area 122 is formed by arranging a first partial modified refractive index area 1221 and a second partial modified refractive index area 1222 having the same shape and size as the first partial modified refractive index area 1221, the first partial modified refractive index area 1221, at a distance d=a / 4 in the direction (x direction) from the light input section toward the light reflecting section 14. This results in a direct coupling coefficient κ 1D is 0 (details will be described later). Note that in this embodiment, the planar shapes of the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 are ellipses with the minor axis in the x direction, but are not limited to this example. For example, as shown in Fig. 3, the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 may have a rectangular planar shape with the minor axis in the x direction.

[0042] The light reflecting section 14 is formed by periodically arranging modified refractive index areas (hereinafter referred to as light reflecting modified refractive index areas 1411) composed of holes with a periodic length of a / 2 in the base material 121 outside the end of the diffraction grating 12 opposite the light input section 13 side. The area in which the light reflecting modified refractive index areas 1411 are arranged in this manner functions as a photonic crystal 141. In the photonic crystal 141, the periodic arrangement of the light reflecting modified refractive index areas 1411 creates a periodic change in the refractive index, preventing light having a wavelength within a specific wavelength band (photonic band gap) from passing through. In this embodiment, the periodic length of the arrangement of the light reflecting modified refractive index areas 1411 is a / 2, so that wavelength a in the photonic crystal 141 is included in the photonic band gap, and light having wavelength a cannot pass through the photonic crystal. As a result, light of wavelength a traveling through the diffraction grating 12 is reflected by the light reflecting section 14, which is the boundary between the diffraction grating 12 and the photonic crystal 141. The periodic length of the arrangement of the light reflecting modified refractive index areas 1411 is not limited to a / 2, but may be set so that the wavelength a is included within the photonic band gap.

[0043] The operation of the diffraction grating coupler 10 of this embodiment will be described. When the diffraction grating coupler 10 is used, a wavelength in a vacuum emitted from a light source (not shown) is a×n eff Input light having a wavelength of 1000 nm is input from the light input section 13 in a direction (first direction) parallel to the base material 121. The light input to the diffraction grating coupler 10 is diffracted by the periodically arranged modified refractive index areas 122 in the diffraction grating 12. At this time, because the wavelength in the diffraction grating 12 matches the period length a, the diffracted light diffracted in a direction perpendicular to the base material 121 is intensified by interference. As a result, the diffracted light diffracted in a direction perpendicular to the base material 121 is extracted as output light from the surface of the base material 121 (FIG. 4).

[0044] In conventional diffraction grating couplers, light input in a direction parallel to the base material is diffracted by the periodically arranged modified refractive index areas 122 in a direction perpendicular to the base material, as well as in a 180° direction (i.e., reflected). However, in the diffraction grating coupler 10 of this embodiment, the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 have the same shape and size and are spaced apart by approximately d=a / 4. This means that the light directly reflected in the 180° direction by the first partial modified refractive index area 1221 (without undergoing any interaction other than reflection) and the light directly reflected by the second partial modified refractive index area 1222 have the same intensity, and an optical path difference of approximately a / 2 is formed between them. As a result, these two directly reflected lights are canceled out by interference, and no directly reflected light is generated. That is, in this embodiment, the direct coupling coefficient κ 1D will be 0.

[0045] In the diffraction grating coupler 10 of this embodiment, no directly reflected light occurs, so that light in the direction parallel to the base material 121 is prevented from being confined within the range close to the light input section 13, and this prevents the intensity of light in the diffraction grating from decreasing with increasing distance from the light input section. As a result, the intensity of output light in the direction perpendicular to the base material 121 is also prevented from decreasing with increasing distance from the light input section, and can be made nearly uniform regardless of position.

[0046] In the diffraction grating coupler 10 of this embodiment, the direct coupling coefficient κ 1D was set to 0, but the absolute value of the direct coupling coefficient |κ 1D Even if | is slightly larger than 0, it is possible to prevent the light from being confined within the range close to the light input section 13 to some extent, and thereby to prevent the intensity of the emitted light from decreasing with increasing distance from the light input section to some extent. Specifically, using the length L of the diffraction grating 12 in the x direction, |κ 1D It is acceptable if | is less than 1 / L. 1D | becomes larger than 0 when the distance d between the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 deviates from a / 4 or when the sizes or shapes of the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 differ.1D If | can be kept below 1 / L, some deviation in the distance d and some difference in the size and shape of the first and second partial modified refractive index areas 1221 and 1222 are permissible.

[0047] On the other hand, since no direct reflection occurs, a higher proportion of the input light reaches the end of the diffraction grating 12 opposite the light input section 13. If the light that reaches this end were to pass through as is, the intensity of the output light in the direction perpendicular to the base material 121 would be weak (although the objective of making it nearly uniform regardless of position could be achieved). In contrast, in this embodiment, this end is provided with a light reflecting section 14 that reflects light with a wavelength a within the diffraction grating 12, so that input light with the same wavelength can be reflected by the light reflecting section 14 and returned to the diffraction grating 12 side. This prevents a decrease in the extraction efficiency of the output light.

[0048] In addition to the directly reflected light described above, light whose propagation direction changes by 180° from one direction parallel to the base material 121 also includes indirectly reflected light, which is light whose propagation direction changes by 180° while incurring radiation loss due to interaction with output light in a direction perpendicular to the base material 121. The intensity of the indirectly reflected light is determined by the indirect coupling coefficient μ (this coefficient can be determined based on the method described in Non-Patent Document 2), which is determined depending on the planar shape of the modified refractive index area 122. By finely adjusting the position of the light reflecting unit 14 in the one direction, the indirectly reflected light can be canceled out by the light reflected by the light reflecting unit 14, thereby reducing the proportion of light returning to the light input unit 13 and increasing the extraction efficiency of the output light.

[0049] Generally, the direct coupling coefficient is κ 1D , the indirect coupling coefficient μ (in this embodiment, κ 1D In a diffraction grating coupler in which μ and μ do not depend on the position x, i.e., μ are the same in all modified refractive index areas, the intensity R(x) of the forward wave traveling in the same direction as the direction of the input light input from the optical input section (positive x direction = first direction) and the intensity S(x) of the backward wave traveling in a direction 180° different from the forward wave (negative x direction = second direction) are expressed by the following propagation equation (1), and the intensity E of the output light is RAD(x) is expressed by the following equation (2).

[0050] In this embodiment, the direct coupling coefficient κ 1D (and its complex conjugate κ 1D ＊ ) is 0, the propagation equation (1) is expressed as the following equation (3).

[0051] The solution of equation (3) is as follows if there is no light reflecting portion 14: (E0 is a constant), and the intensity of the emitted light in this case is E RAD teeth On the other hand, when the light reflecting portion 14 is provided as in this embodiment and the reflection phase is adjusted so that the light reflected by the light reflecting portion 14 and the indirectly reflected light cancel each other out, the solution of equation (3) becomes In this case, the intensity of the emitted light is E RAD teeth This becomes:

[0052] Thus, the intensity of the emitted light E RAD (x) is a constant that does not depend on x (regardless of the presence or absence of the light reflecting portion 14). In other words, theoretically, the intensity of the emitted light is uniform regardless of the position.

[0053] Furthermore, when the vertical diffraction efficiency η, which represents the extraction efficiency of the emitted light (the intensity of the emitted light relative to the intensity of the input light), was calculated, if the light reflecting portion 14 were not present, η=1−(R(L) / E0) 2 -(S(0) / E0) 2 ((R(L) / E0) 2 is the loss that occurs when the light travels in the first direction and passes through the end opposite to the light input section 13. (S(0) / E0) 2 is the loss that occurs when the light travels in the second direction and passes through the light input section 13. When the light reflecting portion 14 is provided as in this embodiment, η=1−(S(0) / E0) 2 (Because of the presence of the light reflecting portion 14, (R(L) / E0) 2 =0) by applying equation (6) Both equations (8) and (9) are functions with μL as a variable, but if you subtract the left side of equation (8) from the right side of equation (9), and is positive regardless of the value of μL (both μ and L are positive). That is, the value of η in equation (9) is larger across the entire range of μL. Therefore, by including the light reflecting portion 14, the diffraction grating coupler 10 of this embodiment can emit light with higher efficiency than when the light reflecting portion 14 is not included. For example, when the light reflecting portion 14 is not included, the vertical diffraction efficiency η is at most 0.5 (when μL = 1, i.e., μ = 1 / L). However, when the light reflecting portion 14 is included as in this embodiment, the vertical diffraction efficiency η exceeds 0.5 when 0.09 < μL < 2.91 (0.09 / L < μ < 2.91 / L), exceeds 0.9 when 0.25 < μL < 1 (0.25 / L < μ < 1 / L), and even becomes 1 when μL = 0.5, i.e., μ = 1 / 2L. In this way, the diffraction grating coupler 10 of this embodiment can emit light with an efficiency of up to 100% (vertical diffraction efficiency η is 1) by adjusting the value of μ (by setting the shape of the modified refractive index area 122).

[0054] The intensity distribution of output light from the diffraction grating coupler 10 of the first embodiment was calculated using the FDTD method. This calculation was performed on a diffraction grating coupler 10 having a modified refractive index area 122 consisting of a rectangular first partial modified refractive index area 1221 and a rectangular second partial modified refractive index area 1222, as shown in FIG. 3 . The periodic length a was set to 750 nm, and the distance d between the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 was set to 215 nm. The rectangular planar shape of the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 had a short side of 90 nm and a long side of 220 nm. Forty-two modified refractive index areas 122 consisting of the first partial modified refractive index area 1221 and the second partial modified refractive index area 1222 (N=42) were arranged in the x direction. In this example, the distance d was slightly larger than the aforementioned a / 4 (=187.5 nm), but the absolute value of the direct coupling coefficient |κ 1D| is nearly 0, and the indirect coupling coefficient μ is close to 0.5 / L. In the photonic crystal 141 for forming the light reflecting portion 14, the shape and size of the light reflecting portion modified refractive index area 1411 are the same as those of the first partial modified refractive index area 1221, and the periodic length is 375 nm (a / 2).

[0055] The calculation results are shown in Fig. 5. The design values ​​are reproduced over the entire diffraction grating 12, and it is shown that the emitted light has a nearly uniform intensity regardless of the position.

[0056] FIG. 6 shows a further modification of the diffraction grating coupler of the first embodiment.

[0057] The diffraction grating coupler 10A shown in Figure 6(a) has a diffraction grating 12A in which modified refractive index areas 122A, each consisting of a rectangle with a side length in the x direction of a / 2, are periodically arranged in the x direction with a periodic length of a. The light reflecting section 14A is formed by periodically arranging rectangular light reflecting section modified refractive index areas 1411A with a periodic length of a / 2 in the x direction. The other configurations are the same as those of the diffraction grating coupler 10 of the first embodiment. In this diffraction grating coupler 10A, the length of the side in the x direction of the modified refractive index area 122A is a / 2, which is equivalent to arranging two modified refractive index areas, each with a width of a / 4 in the same direction, at an interval of a / 4. Therefore, the absolute value of the direct coupling coefficient |κ 1D | can be made sufficiently smaller than 1 / L.

[0058] 6B further includes a diffraction grating 12B in which sub-modified refractive index areas 122B, which have a shape or size different from that of the modified refractive index area 122, are periodically arranged with a periodic length a and shifted by a / 2 in the x direction from the position of the modified refractive index area 122. In this modification, the sub-modified refractive index area 122B is formed by first and second partial sub-modified refractive index areas 1221B and 1222B, which have the same shape and size but different shapes or sizes from the first and second partial modified refractive index areas 1221 and 1222, and are arranged at a distance d=a / 4 apart. As a result, similarly to the first / second partial modified refractive index areas 1221 / 1222, the directly reflected light in the first partial sub-modified refractive index area 1221B and the directly reflected light in the second partial sub-modified refractive index area 1222B are canceled out by interference, and the directly reflected light is canceled out by interference over the entire diffraction grating 12B, so that the direct coupling coefficient κ 1D is 0. On the other hand, the indirect reflection coefficient μ can be adjusted appropriately by the differences in the shapes and sizes of the first / second partial modified refractive index areas 1221 / 1222 and the first / second partial sub-modified refractive index areas 1221B / 1222B.

[0059] Next, we fabricated the diffraction grating coupler 10B shown in Figure 6(b) and conducted an experiment in which we photographed the emitted light when light was input from the light input section 13. The diffraction grating coupler 10B fabricated here was constructed by arranging a modified refractive index area 122 and a sub-modified refractive index area 122B, both of which were made of holes, over a length of 175 µm with a periodic length a of 875 nm in a Si base material 121 having a thickness of 220 nm and a waveguide width of 520 nm, thereby forming a diffraction grating 12B. The area S1 of the modified refractive index area 122 was 11.0% of the area of ​​the unit cell in the diffraction grating 12B (the first partial sub-modified refractive index area 1221 and the second partial sub-modified refractive index area 1222 each accounted for 5.5%), and the area S2 of the sub-modified refractive index area 122B was 9.2% of the area of ​​the unit cell (the first partial sub-modified refractive index area 1221B and the second partial sub-modified refractive index area 1222B each accounted for 4.6%). For comparison, a conventional diffraction grating coupler was fabricated using a base material made of the same material and of the same size as the fabricated diffraction grating coupler 10B, in which a modified refractive index area (the area of ​​which is 10% of the area of ​​the unit grating) having a planar shape without 180° rotational symmetry as described in Patent Document 1 was arranged over a range of 175 μm in length with a periodic length of 750 nm, and an experiment similar to that for diffraction grating coupler 10B was conducted.

[0060] The experimental results for the diffraction grating coupler 10B are shown in Figure 7(a), and the experimental results for the conventional diffraction grating coupler are shown in Figure 7(b). It can be seen that in the conventional diffraction grating coupler, light is output only near the end of the diffraction grating where the light is input (the end on the left in Figure 7(b)). In contrast, in the diffraction grating coupler 10B, light is output from the entire diffraction grating 12B. Furthermore, when the efficiency of the light output from the diffraction grating coupler 10B (the ratio of the output light to the input light) was measured, a high value of approximately 85% was obtained.

[0061] (2) Second Embodiment In the first embodiment, the indirect coupling coefficient μ is a value that does not depend on the position, i.e., the same value in all modified refractive index areas, but the indirect coupling coefficient μ may be a value that varies depending on the position. In the second embodiment, such an example will be described.

[0062] For example, in a diffraction grating coupler having the same configuration as that of the first embodiment except for the shape of the modified refractive index area, the direct coupling coefficient κ1D When is 0, the indirect coupling coefficient μ is set to a value that varies depending on the position x so as to be expressed by a function μ(x) within a range that satisfies the inequality of the following equation (10), and the function E RAD It is possible to emit light with different electric field strengths depending on the position x, represented by (x). Furthermore, it is desirable that the function μ(x) be expressed by the following equation (10A).

[0063] The value of μ(x) can be different depending on the position by making the shape of the modified refractive index areas different for each position where they are arranged. For example, in the first embodiment, as in the diffraction grating coupler 10B shown in FIG. 6(b), when the modified refractive index areas 122 and the sub-modified refractive index areas 122B are spaced a / 2 apart from each other and arranged with a periodic length a, the value of μ increases as the ratio S1 / S2 of the area S1 of the modified refractive index area 122 to the area S2 of the sub-modified refractive index area 122B moves away from 1 (becomes larger or smaller). Therefore, by periodically arranging the modified refractive index areas 122 and the sub-modified refractive index areas 122B, in which S1 / S2 is set for each position so that the value of μ matches Equation (10) (preferably Equation (10A)), the function E RAD The light can be emitted with different electric field strength depending on the position x, which is represented by (x).

[0064] As an example of the second embodiment, a diffraction grating coupler 10C was fabricated by modifying the diffraction grating coupler 10B of the first embodiment, as shown in Fig. 8. Specifically, of a one-dimensional diffraction grating 12C, a modified refractive index area 122 with an area S1 and a sub-modified refractive index area 122B with an area S2 were arranged in a half area 12C1 on the light input section 13 side in the x direction, and a modified refractive index area 122C with an area S1C and a sub-modified refractive index area 122D with an area S2C were arranged in a half area 12C2 on the light reflector 14 side. The areas of the (sub) modified refractive index areas were set to satisfy 1 < (S1 / S2) < (S1C / S2C), and the value of μ in the area 12C2 was set to be twice the value of μ in the area 12C1. Specifically, S1 = 11.4%, S2 = 9.0% (S1 / S2 = 1.27), S1C = 11.8%, S2C = 8.6% (S1C / S2C = 1.37). The thickness (220 nm) and width (520 nm) of the base material, and the periodic length (875 nm) of the (secondary) modified refractive index areas were the same as those used in the experiment shown in Figure 7(a) in the first embodiment.

[0065] The experimental results for the fabricated diffraction grating coupler 10C are shown in Figure 9. A unique intensity distribution not found in conventional diffraction grating couplers was obtained, in which stronger light was emitted from area 12C2 farther from the optical input section 13 than from area 12C1 on the optical input section 13 side (light incident side).

[0066] (3) Third Embodiment Figure 10 shows a top view of a diffraction grating coupler 20 according to a third embodiment. This diffraction grating coupler 20 includes a rectangular, plate-shaped base (not shown; located on the back side of the paper in Figure 10 relative to a base material 221, which will be described later), a rectangular, plate-shaped base material 221 formed on the surface of the base, a modified refractive index area 222 consisting of holes that are periodically arranged within an area defined by the rectangle in the base material 221, a linear light input portion 231 provided on one of the four sides of the rectangle, a linear light output portion 232 provided on one of two sides (described later) perpendicular to the first side, a linear first reflecting portion 241 provided on the side opposite the side on which the light input portion 231 is provided, and a linear second reflecting portion 242 provided on the side opposite the side on which the light output portion 232 is provided. The surface of the base material 221 (the front side of the paper in FIG. 10 ) opposite the base (the back side of the paper in FIG. 10 ) is space (air). The diffraction grating 22 is formed within the rectangular area in which the modified refractive index areas 222 are arranged. Hereinafter, the direction perpendicular to the linear light input section 231 is defined as the positive x-direction, and the direction 90° counterclockwise from the positive x-direction is defined as the positive y-direction. The light output section 232 is provided on one of the two sides that is on the positive y-direction side.

[0067] The modified refractive index area 222 is inclined at 45° with respect to the x-direction and the y-direction, and has a periodic length of 2 -1 / 2 The modified refractive index areas are arranged at lattice points of a square lattice, and are spaced apart at intervals a in the x and y directions. Generally, in a two-dimensional photonic crystal in which modified refractive index areas are arranged at lattice points of such a square lattice, the wavelength within the two-dimensional photonic crystal is a (the wavelength in a vacuum is n eff × a. where n eff is the effective refractive index of the diffraction grating 22.) It is known that light traveling in the x and y directions is not emitted in a direction perpendicular to the base material, but is diffracted in 180° and 90° directions in a plane parallel to the base material. This is also true for the diffraction grating 22 in this embodiment.

[0068] As shown in the enlarged view of Figure 11, each modified refractive index area 222 is formed by arranging four holes, each of which has a circular planar shape, at each vertex of a square (thin broken lines in Figure 11) with each side inclined at 45° to the x and y directions, and four holes at the midpoints of each side of the square. By adjusting the distance d2 in the x and y directions between the holes arranged at the midpoints of each side of the square within one modified refractive index area 222, the direct coupling coefficient κ 1D In this embodiment, the value of the direct coupling coefficient κ 1D The value of κ is set to be less than 1 / L2, which is the reciprocal of the x-direction length L2 of the diffraction grating 22. For example, when d2 is 0.254a and the radius r of each hole in the modified refractive index area 222 is 0.035a, 1D will be 0.

[0069] The direct 90° diffraction coefficient κ is an index that indicates the intensity of light diffracted in the 90° direction in a plane parallel to the base material. 2D The value of the direct 90° diffraction coefficient κ 2D The value of can be adjusted by the radius r. In this example, the radius r of the modified refractive index areas 222 is set to the same value throughout the entire diffraction grating 22.

[0070] Furthermore, in the diffraction grating coupler 20 of the third embodiment, in response to the indirectly reflected light in the diffraction grating coupler 10 of the first embodiment, light traveling in the positive or negative x direction interacts with light traveling in the y direction, resulting in indirectly reflected light whose direction of travel changes by 180° while causing loss. The indirect coupling coefficient μ 2D is the same as the indirect coupling coefficient μ in the first embodiment, 0.09 / L<μ 2D <2.91 / L2, and 0.25 / L2<μ 2D <1 / L2, and μ 2D It is most preferable that μ = 1 / 2L2. 2D The value of the direct 90° diffraction coefficient κ 2D and the y-direction cross length L1 of the diffraction grating 22, μ 2D =2(κ 2D ) 2 It is represented by L1.

[0071] The first reflecting portion 241 is formed by arranging a photonic crystal that does not allow light having a wavelength of a in the diffraction grating 22 to pass (the wavelength is included in the photonic band gap) further outside (on the positive x-direction side) of the diffraction grating 22. The second reflecting portion 242 is formed by arranging a photonic crystal similar to that of the first reflecting portion 241 further outside (on the negative y-direction side) of the diffraction grating 22.

[0072] Instead of holes, the modified refractive index area 222 may be made of a material having a refractive index different from that of the base material 221. The planar shape of the modified refractive index area 222 is not limited to the above example and may be any shape, but in order to achieve a uniform diffraction intensity, the shape needs to have four-fold rotational symmetry.

[0073] The operation of the diffraction grating coupler 20 of the third embodiment will be described. When light having a wavelength a in the diffraction grating coupler 20 is input from the light input unit 231 in the positive x direction, part of the light is diffracted in the diffraction grating 22 in the positive y direction or the negative y direction (as described above, it is not diffracted in the direction perpendicular to the base material 221). Furthermore, part of the light traveling in the positive x direction that is not diffracted in the y direction is reflected by the first reflecting unit 241, changing its traveling direction to the negative x direction. While traveling in the negative x direction, part or all of the light is diffracted in the positive y direction or the negative y direction. The light diffracted in the positive y direction in this way travels directly toward the light emitting unit 232. The light diffracted in the negative y direction is reflected by the second reflecting unit 242, changing its traveling direction to the positive y direction, and travels toward the light emitting unit 232.

[0074] In this way, light is emitted from the light emitting portion 232. At this time, the direct coupling coefficient κ 1D is sufficiently smaller than 1 / L2, light is prevented from being localized closer to the light input portion 231 (closer to the negative x direction) within the diffraction grating 22, and light can be emitted with nearly uniform intensity from the entire linear light emitting portion 232. Furthermore, by providing the first reflecting portion 241 and the second reflecting portion 242, the extraction efficiency of the emitted light can be increased.

[0075] The intensity distribution of the emitted light from the diffraction grating coupler 20 of the third embodiment was calculated using the FDTD method. In this calculation, the periodic length a was set to 460 nm, d2 was set to 0.254a (=117 nm), and r was set to 0.035a (=16.1 nm). 200 modified refractive index areas 222 were arranged in the x direction (L2=460 nm×200=92 μm), and 60 in the y direction. The direct coupling coefficient κ in this example was 1D is 0, and the direct 90° diffraction coefficient κ 2D The absolute value of is 95cm -1 , indirect coupling coefficient μ 2D The absolute value of μ 2D =2(κ 2D ) 2 L1=49cm -1 (=0.46 / L2).

[0076] The calculation results are shown in Figure 12. It can be seen that light is emitted with nearly uniform intensity over the entire linear light emitting portion 232. The extraction efficiency of the emitted light was calculated and a high value of 96%, close to 100%, was obtained.

[0077] (4) Fourth Embodiment Figure 13 shows a top view of a diffraction grating coupler 30 of the fourth embodiment. This diffraction grating coupler 30 is a combination of the diffraction grating coupler 10 of the first embodiment shown in Figure 2 (in which the modified refractive index areas 122 are arranged two-dimensionally) and the diffraction grating coupler 20 of the third embodiment shown in Figure 10. Specifically, the diffraction grating coupler 10 and the diffraction grating coupler 20 of the third embodiment are formed on a single (common) base material 321 so that the light output portion 232 of the diffraction grating coupler 20 of the third embodiment and the light input portion 13 of the diffraction grating coupler 10 of the first embodiment are aligned or arranged parallel to each other. In this diffraction grating coupler 30, the light input portion 231 of the diffraction grating coupler 20 of the third embodiment functions as the light input portion, and the entire top surface of the diffraction grating 12 of the first embodiment functions as the light output portion.

[0078] The operation of the diffraction grating coupler 30 of the fourth embodiment will be described. When light having a wavelength a in the diffraction grating coupler 30 is input from the light input section 231 in the positive x direction, the light is diffracted within the diffraction grating 22, as in the diffraction grating coupler 20 of the third embodiment, and the light is input from the light input section 13 into the diffraction grating 12 with nearly uniform intensity from the entire linear light output section 232. Within the diffraction grating 12, the light is diffracted in a direction perpendicular to the base material 321, as in the diffraction grating coupler 10 of the first embodiment, and is output with nearly uniform intensity from the entire planar range of the diffraction grating 12.

[0079] For the diffraction grating coupler 30 of the fourth embodiment, the intensity distribution of light emitted from the top surface of the diffraction grating 12 and the far-field pattern of the emitted light were calculated using the three-dimensional coupled wave theory described in Non-Patent Document 2. In this calculation, a = 280 nm (common to the diffraction grating couplers 10 and 20), L = 2.8 mm (10,000 modified refractive index areas 122 arranged in the y direction of the diffraction grating 12), and L2 = 2.8 mm (10,000 modified refractive index areas 222 arranged in the x direction of the diffraction grating 22). The planar shape of the modified refractive index area 122 is κ 1D The planar shape of the modified refractive index area 222 is set to κ 1D is 0, μ 2D was set to be 0.5 / L2.

[0080] Figure 14 shows the calculated intensity distribution of the emitted light on the top surface of diffraction grating 12, and Figure 15 shows the calculated far-field pattern of the emitted light. From Figure 14, it can be seen that the emitted light is emitted from the top surface of diffraction grating 12 with a substantially uniform intensity regardless of its position within diffraction grating 12. Furthermore, from Figure 15, it can be seen that the divergence angle of the emitted light is smaller than 0.05°. The reason the divergence angle of the emitted light is small is that the light is emitted with a substantially uniform intensity regardless of its position within diffraction grating 12, which effectively increases the emission area.

[0081] (5) Fifth Embodiment Figure 16(a) shows a top view of a diffraction grating coupler 40 of the fifth embodiment. This diffraction grating coupler 40 is formed by forming two diffraction grating couplers 10 (each of which is designated as a partial diffraction grating coupler 10α and 10β) of the first embodiment (in which the modified refractive index areas 122 are arranged two-dimensionally) shown in Figure 2 on a single (common) base material 421 so that the linear light input portions 13α and 13β of both couplers are aligned or parallel to each other. However, these light input portions 13α and 13β are each intended to receive light emitted from the other light input portions 13β and 13α, and are not intended to receive light from outside the diffraction grating coupler 40.

[0082] The diffraction grating 12α of the partial diffraction grating coupler 10α and the diffraction grating 12β of the partial diffraction grating coupler 10β are formed by modified refractive index areas 122 of the same shape and size arranged at the same periodic length a, but the areas of the diffraction gratings are different. Specifically, the diffraction gratings 12α and 12β are both rectangular and have the same size in the y direction (the direction parallel to the light input sections 13α and 13β), but their lengths L1 and L2 in the x direction are different. Accordingly, the numbers N1 and N2 of the modified refractive index areas 122 arranged in the x direction differ between the two.

[0083] The partial diffraction grating coupler 10α is provided with a light reflecting portion 14α at its end opposite the light input portion 13α, and the partial diffraction grating coupler 10β is provided with a light reflecting portion 14β at its end opposite the light input portion 13β. These light reflecting portions 14α and 14β have the same configuration as the light reflecting portion 14 in the diffraction grating coupler 10 of the first embodiment.

[0084] The operation of the diffraction grating coupler 40 of the fifth embodiment will be described using FIG. 16(b). Light having a wavelength a within the diffraction grating coupler 40 is input through the top surface of the diffraction grating 12α. The light is then diffracted by 90° within the diffraction grating 12α in a direction parallel to the base material 421. Of the diffracted light, the light directed toward the light input section 13α and the light directed toward the light reflecting section 14α and reflected by the light reflecting section 14α pass through the light input section 13α (in this case, the light is actually output from the light input section 13α, but this naming is used for convenience) and is input from the light input section 13β into the diffraction grating 12β of the partial diffraction grating coupler 10β. The light is then diffracted by 90° within the diffraction grating 12β in a direction perpendicular to the base material 421 and emitted with nearly uniform intensity from the entire planar range of the diffraction grating 12β. Here, since the areas of the diffraction gratings 12α and 12β are different, the light exits from the diffraction grating 12β with a diameter different from the diameter at which the light enters the diffraction grating 12α. That is, the diffraction grating coupler 40 of the fifth embodiment functions as a converter that converts the diameter of the light.

[0085] While Fig. 16(b) shows an example in which light is input from the top surface of diffraction grating 12α and output from the top surface of diffraction grating 12β, as shown in Fig. 16(c), light may be input from the top surface of diffraction grating 12β and output from the top surface of diffraction grating 12α. In the example of Fig. 16(b), the diameter of the light is expanded, while in the example of Fig. 16(c), the diameter of the light is contracted.

[0086] (6) Sixth Embodiment The diffraction grating coupler 30 of the fourth embodiment described above has a configuration that combines a diffraction grating 22 (the diffraction grating in the third embodiment) that diffracts light input from the outside in the x direction into the y direction (a direction that differs by 90° within the plane) with a diffraction grating 12 (the diffraction grating in the first embodiment) that diffracts light input from the diffraction grating 22 in the y direction in a direction perpendicular to the surface. In this configuration, a diffraction grating that diffracts light input from a direction perpendicular to the surface in the x direction may be further provided to input light in the x direction to the diffraction grating 22. The sixth embodiment describes such an example. In the following description, diffraction gratings having a different configuration from the diffraction gratings 22 and 12 used in the fourth embodiment are used. However, as described below, if it is not necessary to input light with different intensities depending on the input and output positions, these diffraction gratings 22 and 12 can also be used.

[0087] As shown in FIG. 17 , the diffraction grating coupler 60 of the sixth embodiment has an incident diffraction grating 62A, an in-plane 90° diffraction grating 62B, and an exit diffraction grating 62C. These three diffraction gratings are each provided within a rectangular region within a plate-shaped base material 621. One short side of the rectangular region in which the incident diffraction grating 62A is provided functions as a linear first optical output section 65A that outputs light from the incident diffraction grating 62A to the in-plane 90° diffraction grating 62B. One short side of the rectangular region in which the in-plane 90° diffraction grating 62B is provided functions as a linear second optical input section 63B that inputs light from the incident diffraction grating 62A to the in-plane 90° diffraction grating 62B, and one long side functions as a linear second optical output section 65B that outputs light from the in-plane 90° diffraction grating 62B to the exit diffraction grating 62C. One long side of the rectangular area in which the output diffraction grating 62C is provided functions as a linear third light input section 63C that inputs light from the in-plane 90° diffraction grating 62B to the output diffraction grating 62C. The first light output section 65A and the second light input section 63B face each other approximately parallel, and the second light output section 65B and the third light input section 63C face each other approximately parallel. A light reflecting section 64 is provided around these three diffraction gratings. The configuration of the light reflecting section 64 is similar to that of the light reflecting section 14 in each of the above embodiments.

[0088] FIG. 18 shows the configurations of the incident diffraction grating 62A, the in-plane 90° diffraction grating 62B, and the exit diffraction grating 62C. As shown in FIG. 18( a), the incident diffraction grating 62A is configured by arranging either a first incident modified refractive index area 6221A or a second incident modified refractive index area 6222A, each of which has a different area, at each lattice point of a square lattice. The first incident modified refractive index area 6221A and the second incident modified refractive index area 6222A are aligned in the y direction (a direction parallel to the first light output unit 65A) and alternately arranged in two rows in the x direction (a direction 90° in-plane relative to the y direction). The spacing between lattice points in the square lattice in the x direction is ¼ the wavelength of the light incident on the diffraction grating coupler 60 within the incident diffraction grating 62A. This configuration forms a periodic structure every four lattice points in the x direction, and the periodic length a of this periodic structure corresponds to the wavelength.

[0089] As shown in FIG. 18( b), the in-plane 90° diffraction grating 62B is formed by arranging either a first in-plane 90° diffraction modified refractive index area 6221B or a second in-plane 90° diffraction modified refractive index area 6222B, which have different areas, at each lattice point of a square lattice having the same periodicity as the incident portion diffraction grating 62A. In each of the rows in the x and y directions, two first in-plane 90° diffraction modified refractive index areas 6221B and two second in-plane 90° diffraction modified refractive index areas 6222B are arranged alternately. As a result, in the direction tilted 45° relative to the x and y directions, every four rows are formed, each containing only the first in-plane 90° diffraction modified refractive index areas 6221B (the rows indicated by the dashed line in FIG. 18( b)) and the second in-plane 90° diffraction modified refractive index areas 6222B (the rows indicated by the dashed line in FIG. 18( b)).

[0090] 18(c), the exit diffraction grating 62C has a configuration in which the entrance diffraction grating 62A is rotated 90° around an axis perpendicular to the base material 621 and then its range is expanded. That is, first exit modified refractive index areas 6221C and second exit modified refractive index areas 6222C, which have different areas, are arranged side by side in the x direction and alternately arranged in two rows in the y direction. For convenience, the entrance diffraction grating 62A is referred to as the "first light output section 65A" and the exit diffraction grating 62C is referred to as the "third light input section 63C," but it is also possible to make the first light output section 65A function as the "light input section" and the third light input section 63C function as the "light output section."

[0091] The diffraction grating coupler 60 of the sixth embodiment operates as shown in the schematic diagram of Fig. 19. That is, when incident light is made incident on the surface of the entrance section diffraction grating 62A, light having a wavelength corresponding to four lattice points among the light propagating in the x direction in the entrance section diffraction grating 62A is prevented from being reflected 180° in the x direction by two first entrance section modified refractive index areas 6221A that are adjacent in the x direction and arranged at an interval of ¼ of the wavelength (between adjacent lattice points) and have the same area, and by two second entrance section modified refractive index areas 6222A that are similarly adjacent in the x direction, and is then input to the in-plane 90° diffraction section diffraction grating 62B through the first light output section 65A and the second light input section 63B.

[0092] In the in-plane 90° diffraction grating 62B, 180° reflection of light propagating in the x direction is suppressed for the same reason as in the incident portion diffraction grating 62A. On the other hand, when light propagating in the x direction is reflected (diffracted) by 90° in the y direction by the first in-plane 90° diffraction modified refractive index area 6221B, the light has a phase difference of 180° with the light reflected by 90° in the y direction by the second in-plane 90° diffraction modified refractive index area 6222B located half a wavelength (two grating points) in the y direction from the first in-plane 90° diffraction modified refractive index area 6221B (due to the structure of the in-plane 90° diffraction grating 62B, the second in-plane 90° diffraction modified refractive index area 6222B is always located at this position), and therefore destructively interferes with the light. However, since the areas of the first in-plane 90° diffraction modified refractive index area 6221B and the second in-plane 90° diffraction modified refractive index area 6222B are different, even if such destructive interference occurs, the light propagating in the y direction is not lost, but is input to the exit diffraction grating 62C through the second light output section 65B and the third light input section 63C.

[0093] In the exit diffraction grating 62C, light input from the third light input section 63C is reflected in a direction perpendicular to the base material 621 in the first exit modified refractive index area 6221C or the second exit modified refractive index area 6222C and is emitted to the outside from the surface of the exit diffraction grating 62C.

[0094] In the incident diffraction grating 62A, the in-plane 90° diffraction diffraction grating 62B, and the exit diffraction grating 62C, light propagating in directions other than those described above remains within those diffraction gratings by being reflected by the light reflecting section 64. If the propagation direction changes to the above-described direction, for example, by being diffracted by 90° in the modified refractive index area, the light then propagates as described above.

[0095] Through the above operation, light incident on the surface of the incident diffraction grating 62A can be emitted from the surface of the emission diffraction grating 62C. In this case, by providing two types of modified refractive index areas with different areas in the incident diffraction grating 62A and the emission diffraction grating 62C, the intensity of the emitted light can be changed depending on the position in the emission diffraction grating 62C.

[0096] The intensity of the light emitted from the exit diffraction grating 62C can be set as follows. Here, the electric field intensity distribution E of the incident light to be incident on the entrance diffraction grating 62A is IN (X, Y) to E IN (x, y)=a(x)b(y) (11) and the electric field intensity distribution E of the light to be emitted from the emission diffraction grating 62C is RAD (X, Y) to E RAD (x, y)=c(x)d(y)...(12) Then, the direct coupling coefficient κ 1D are both 0, and the indirect coupling coefficient μ a (x), the direct 90° diffraction coefficient κ in the in-plane 90° diffraction section diffraction grating 62B 2D (x, y), and the indirect coupling coefficient μ in the y direction in the exit diffraction grating 62C c The shape of each modified refractive index area is determined so that (y) is a value calculated using the following equations (13) to (15). Here, L is the length of the incident diffraction grating 62A in the x direction, L1 and L2 are the lengths of the in-plane 90° diffraction grating 62B in the y direction and the x direction, respectively, and L3 is the length of the exit diffraction grating 62C in the y direction. When this condition is met, the electric field intensity distribution E that satisfies Equation (11) is IN By making the incident light incident on the incident portion diffraction grating 62A at (X, Y), the electric field intensity distribution E that satisfies the formula (12) is obtained. RAD The light can be emitted from the exit diffraction grating 62C at (X, Y).

[0097] The shape of each modified refractive index area is set so that a(x) is a function having one peak, b(y) is a function having one peak, c(x) is a function having three peaks, and d(y) is a function having three peaks. Then, the electric field intensity distribution E of the outgoing light emitted from the surface of the outgoing portion diffraction grating 62C is calculated. RAD (x, y) was calculated. The indirect coupling coefficient μ was calculated from a(x) and d(y) using equations (13) and (15), respectively. a (x) and μ c(y) is shown in Fig. 20(a), and the direct 90° diffraction coefficient κ calculated from b(y) and c(x) using Equation (14) is shown in Fig. 20(b). 2D (x, y) is shown in Figure (b), and the electric field intensity distribution E of the emitted light is shown. RAD (x, y) are shown in Fig. 1(c). As can be seen from Fig. 1(c), in this example, incident light having an intensity distribution with only one peak in the x direction is incident on the surface of incident diffraction grating 62A, and output light consisting of a total of nine beams, three aligned in the x direction and three aligned in the y direction, is output from the surface of output diffraction grating 62C.

[0098] (7) Seventh Embodiment As shown in Figures 21 and 22, a diffraction grating coupler 70 of the seventh embodiment divides a base material 721 into four regions, two in the x direction and two in the y direction. An incident diffraction grating 72A is provided in one of the four regions, an exit diffraction grating 72C is provided in a region diagonally opposite the incident diffraction grating 72A, a first in-plane 90° diffraction grating 72B1 is provided in a region adjacent to the incident diffraction grating 72A in the x direction, and a second in-plane 90° diffraction grating 72B2 is provided in a region adjacent to the incident diffraction grating 72A in the y direction. Phase adjustment units 77 are provided between the incident diffraction grating 72A and the second in-plane 90° diffraction grating 72B2, and between the second in-plane 90° diffraction grating 72B2 and the exit diffraction grating 72C. Although the phase adjustment units 77 are provided in the two locations in this embodiment, the phase adjustment unit 77 may be provided in only one of these locations. In addition, although not shown, a light reflecting portion is provided so as to surround the combination of the above four regions.

[0099] As shown in FIG. 22A, the incident section diffraction grating 72A is configured by arranging one of a first incident section modified refractive index area 7221A, a second incident section modified refractive index area 7222A, and a third incident section modified refractive index area 7223A, each having a different area, at each lattice point of a square lattice. In the x direction, two adjacent first incident section modified refractive index areas 7221A and two adjacent second incident section modified refractive index areas 7222A are alternately arranged. In the y direction, two adjacent second incident section modified refractive index areas 7222A and two adjacent third incident section modified refractive index areas 7223A are alternately arranged. The spacing between the lattice points is ¼ of the wavelength of the light incident on the diffraction grating coupler 70 within the incident section diffraction grating 72A. With the above configuration, a periodic structure is formed every four lattice points in both the x and y directions, and the periodic length a of the periodic structure matches the wavelength.

[0100] As shown in Figure 22(b), the first in-plane 90° diffraction grating 72B1 and the second in-plane 90° diffraction grating 72B2 both have a structure similar to the in-plane 90° diffraction grating 62B in the sixth embodiment. In this embodiment, the two types of modified refractive index areas having different areas are referred to as the "first in-plane 90° diffraction modified refractive index area 7221B and the second in-plane 90° diffraction modified refractive index area 7222B."

[0101] 22(c), the exit diffraction grating 72C has a configuration in which the entrance diffraction grating 72A is rotated by 90° around an axis perpendicular to the base material 721. In the exit diffraction grating 72C, three types of modified refractive index areas with different areas are called a first exit modified refractive index area 7221C, a second exit modified refractive index area 7222C, and a third exit modified refractive index area 7223C.

[0102] The phase adjustment unit 77 has a phase adjustment region made of a part of the base material 721 and a heater for heating the phase adjustment region. By changing the temperature of the phase adjustment region by heating with the heater, the refractive index in the phase adjustment region changes, and therefore the phase of the light at the time of passing through the phase adjustment region can be changed.

[0103] The operation of the diffraction grating coupler 70 of the seventh embodiment will be described. Linearly polarized light having the above wavelength and tilted 45° with respect to both the x and y directions is incident on the surface of the incident diffraction grating 72A. As a result, within the incident diffraction grating 72A, linearly polarized light in the y direction (referred to as "y-polarized light") propagates in the x direction, and linearly polarized light in the x direction (referred to as "x-polarized light") propagates in the y direction. The y-polarized light propagating in the x direction within the incident diffraction grating 72A is input to the first in-plane 90° diffraction grating 72B1 while suppressing reflection in the 180° direction, as in the other embodiments. As in the sixth embodiment, the y-polarized light is reflected by 90° by the modified refractive index area within the first in-plane 90° diffraction grating 72B1, becoming x-polarized light whose polarization direction has been changed by 90°, and is input to the output diffraction grating 72C. On the other hand, the x-polarized light propagating in the y-direction within the incident diffraction grating 72A is suppressed from being reflected in the 180° direction, passes through the phase adjustment section 77, and is input to the second in-plane 90° diffraction section diffraction grating 72B2. There, it is reflected by 90° by the modified refractive index area within the second in-plane 90° diffraction section diffraction grating 72B2, becomes y-polarized light whose polarization direction has changed by 90°, passes through the phase adjustment section 77, and is input to the exit diffraction grating 72C.

[0104] In the output grating 72C, the x-polarized light that passed through the first in-plane 90° diffraction grating 72B1 and the y-polarized light that passed through the second in-plane 90° diffraction grating 72B2 and the two phase adjusters 77 are combined. If there is no phase difference between the two polarized lights, the combined light becomes linearly polarized light tilted at 45° with respect to the x and y directions. On the other hand, if a phase difference is created between the two polarized lights by the action of the phase adjuster 77, the combined light becomes polarized light that reflects the phase difference. For example, if a 90° phase difference is created between the x-polarized light and the y-polarized light, the combined light becomes circularly polarized light. Furthermore, if a 180° phase difference is created between the x-polarized light and the y-polarized light, the combined light becomes linearly polarized light tilted at 90° with respect to the polarization direction when there is no phase difference. As described above, the diffraction grating coupler 70 of the seventh embodiment functions as a polarization control element that controls the polarization of incident light.

[0105] Although several embodiments of the diffraction grating coupler according to the present invention have been described above, the present invention is not limited to these embodiments and various modifications are possible.

[0106] For example, in the diffraction grating coupler 10 of the first embodiment or the diffraction grating coupler 10C of the second embodiment, the end 511 of a known end-emitting laser 51 can be connected to the optical input section 13 (FIG. 23(a)). Similarly, the end 511 of the end-emitting laser 51 may be connected to the optical input section 231 of the diffraction grating coupler 20 of the third embodiment or the optical input section 231 of the diffraction grating coupler 30 of the fourth embodiment (FIGS. 23(b) and 23(c)). With this configuration, the laser light oscillated by the end-emitting laser 51 is emitted from the surface of the diffraction grating coupler 10 or 30 or from the end face of the diffraction grating coupler 20, making it possible to switch the emission direction (in all the examples given here) or to make the emission area larger than the end (end face) of the end-emitting laser 51 (in the case of the diffraction grating couplers 10 and 30).

[0107] The edge-emitting laser 51 may be formed in, for example, the base material 121 or 221, and thus may be integrated with the diffraction grating couplers 10, 20, and 30. Alternatively, the end 511 of the edge-emitting laser 51, which is separate from the diffraction grating couplers 10, 20, and 30, may be connected to the light input section 13 or 231. Note that, although one example of an edge-emitting laser 51 is provided for each embodiment here, an edge-emitting laser may also be connected to the light input section in the other examples of the first to fourth embodiments in the same manner.

[0108] Aspects It will be apparent to those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0109] (Item 1) A diffraction grating coupler according to one aspect of the present invention comprises: a diffraction grating having a plate-shaped base material; modified refractive index areas which are areas with a refractive index different from that of the base material and which are periodically arranged one-dimensionally or two-dimensionally within a predetermined range of the base material; and a light reflecting section provided at an end of the range on the opposite side to the light input section, wherein a direct coupling coefficient κ is an index indicating the intensity at which light traveling in a first direction parallel to the base material from the light input section which is part of the end of the range is reflected in a second direction which is 180° different from the first direction without causing a radiation loss in the modified refractive index area. 1DThe modified refractive index area has a planar shape in which the absolute value of is less than 1 / L, which is the reciprocal of the length L across the range in the first direction.

[0110] (Item 2) The diffraction grating coupler according to item 2 is the diffraction grating coupler according to item 1, wherein the indirect coupling coefficient μ, which is an index showing the intensity of light whose propagation direction changes by 180° while causing radiation loss due to the interaction of light traveling in the first direction or the second direction with outgoing light traveling in a direction perpendicular to the base material, is the same value regardless of the positions at which the modified refractive index areas are periodically arranged.

[0111] (Clause 3) The diffraction grating coupler according to clause 3 is the diffraction grating coupler according to clause 1, wherein an indirect coupling coefficient μ, which is an index showing the intensity of light whose propagation direction changes by 180° while causing radiation loss due to interaction of light traveling in the first direction or the second direction with outgoing light traveling in a direction perpendicular to the base material, is a value that differs from one another at least at two locations out of a plurality of positions at which the modified refractive index areas are periodically arranged.

[0112] (4) The diffraction grating coupler according to 4 is the diffraction grating coupler according to any one of 1 to 3, wherein the light reflecting portion is formed by providing a photonic crystal outside the end of the range, in which the energy of light of a predetermined wavelength is within the photonic band gap.

[0113] (Item 5) A diffraction grating coupler according to item 5 is the diffraction grating coupler according to any one of items 1 to 4, wherein an indirect coupling coefficient μ(x) is an index showing the intensity of light whose propagation direction changes by 180° while causing radiation loss when light traveling in the first direction or the second direction of the diffraction grating interacts with light output in a direction perpendicular to the base material at each position x in the first direction or the second direction of the diffraction grating, and the indirect coupling coefficient μ(x) is an index showing the intensity of light whose propagation direction changes by 180° while causing radiation loss when the indirect coupling coefficient μ(x) is RAD It is expressed using (x) is within the range.

[0114] (Item 6) The diffraction grating coupler according to item 6 is a diffraction grating coupler according to any one of items 1 to 5, in which an end-emitting laser is connected to the optical input section, and the optical output section of the end-emitting laser is connected to face the optical input section.

[0115] (Clause 7) The diffraction grating coupler according to clause 7 is configured by two diffraction grating couplers according to any one of clauses 1 to 5, arranged so that their optical input portions face each other and the first direction of one diffraction grating coupler coincides with the second direction of the other diffraction grating coupler.

[0116] (Item 8) A diffraction grating coupler according to another aspect of the present invention 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 and which are periodically arranged one-dimensionally or two-dimensionally within a predetermined rectangular range of the base material, one side of the rectangular area is a light input section, and the other side of the rectangular area perpendicular to the first side is a light output section, and a direct coupling coefficient κ is an index indicating the intensity at which light traveling from the light input section in a first direction parallel to the base material is reflected in the modified refractive index area in a second direction parallel to the base material and 180° different from the first direction without radiation loss. 1D The modified refractive index area has a planar shape in which the absolute value of is less than 1 / L2, which is the reciprocal of the length L2 across the range in the first direction.

[0117] (Item 9) The diffraction grating coupler according to Item 9 is the diffraction grating coupler according to Item 6, wherein an end-emitting laser is connected to the optical input section so that the optical output section of the end-emitting laser faces the optical input section.

[0118] (Item 10) A diffraction grating coupler according to item 10 comprises: a first diffraction grating coupler according to any one of items 1 to 5, wherein the optical input section is linear; and a second diffraction grating coupler according to item 8 or 9, wherein the optical input section of the first diffraction grating coupler and the optical output section of the second diffraction grating coupler face each other in parallel.

[0119] (Item 11) A diffraction grating coupler according to item 11 comprises: a first diffraction grating coupler according to any one of items 1 to 5, wherein the optical input section is linear; a second diffraction grating coupler according to item 8 or 9; and a third diffraction grating coupler according to any one of items 1 to 5, wherein the optical input section is linear; wherein the optical input section of the first diffraction grating coupler and the optical input section of the second diffraction grating coupler face each other in parallel; and the optical input section of the third diffraction grating coupler and the optical output section of the second diffraction grating coupler face each other in parallel.

[0120] In the diffraction grating coupler according to paragraph 11, the "optical input section of the first diffraction grating coupler" is structurally the same as the optical input section defined in any one of paragraphs 1 to 5, but actually functions as an optical output section that outputs light toward the optical input section of the second diffraction grating coupler (see the first optical output section 65A in the sixth embodiment). In the sixth embodiment, the region where the entrance diffraction grating 62A is formed corresponds to the first diffraction grating coupler, the region where the in-plane 90° diffraction grating 62B is formed corresponds to the second diffraction grating coupler, and the region where the exit diffraction grating 62C is formed corresponds to the third diffraction grating coupler.

[0121] (Item 12) A diffraction grating coupler according to item 12 is a first diffraction grating coupler according to any one of items 1 to 5, in which the modified refractive index areas are arranged two-dimensionally, the optical input section is linear, and a second linear optical input section is provided in a direction that is 90° different from the optical input section; a second diffraction grating coupler according to item 8 or 9, in which the optical input section is arranged opposite the optical input section of the first diffraction grating coupler; or a third diffraction grating coupler according to item 8 or 9, in which the optical input section is arranged opposite the second optical input section of the first diffraction grating coupler; A fourth diffraction grating coupler according to any one of paragraphs 1 to 5, wherein the modified refractive index areas are arranged two-dimensionally, the optical input section is linear, and a second linear optical input section is provided in a direction that is 90° different from the optical input section, the optical input section being arranged opposite the optical output section of the second diffraction grating coupler, and the second optical input section being arranged opposite the optical output section of the diffraction grating coupler of the third diffraction grating coupler; and a phase adjustment section that changes the phase of light passing through, which is provided either or both between the second optical input section of the first diffraction grating coupler and the optical input section of the third diffraction grating coupler, and between the optical output section of the third diffraction grating coupler and the optical input section of the fourth diffraction grating coupler.

[0122] The optical input portion and the second optical input portion of the first diffraction grating coupler in the diffraction grating coupler according to paragraph 12 are structurally similar to the optical input portion defined in any one of paragraphs 1 to 5, but actually function as optical output portions that output light toward the optical input portions of the second and third diffraction grating couplers (in the seventh embodiment, these correspond to the ends of the entrance diffraction grating 72A that face the first in-plane 90° diffraction grating 72B1 and the second in-plane 90° diffraction grating 72B2). In the seventh embodiment, the region where the entrance diffraction grating 72A is formed corresponds to the first diffraction grating coupler, the region where the first in-plane 90° diffraction grating 72B1 is formed corresponds to the second diffraction grating coupler, the region where the second in-plane 90° diffraction grating 72B2 is formed corresponds to the third diffraction grating coupler, and the region where the exit diffraction grating 72C is formed corresponds to the fourth diffraction grating coupler.

[0123] 10, 10A, 10B, 20, 30, 40, 60, 70... Diffraction grating coupler 10α, 10β... Partial diffraction grating coupler 11... Base 12, 12A, 12B, 12α, 12β, 22... Diffraction grating 121, 221, 321, 421, 621, 721... Base material 122, 122A, 222... Modified refractive index area 1221... First partial modified refractive index area 1221B... First partial sub-modified refractive index area 1222... Second partial sub-modified refractive index area 1222B... Second partial sub-modified refractive index area 122B... Sub-modified refractive index area 13, 13α, 13β, 231... Light input section 14, 14A, 14α, 14β... Light reflecting section 141... Photonic crystal 1411, 1411A... Light reflecting section modified refractive index area 232...light emitting portion 241...first reflecting portion 242...second reflecting portion 51...end (end surface) emitting laser 511...end of end (end surface) emitting laser 6221A, 7221A...first incident portion modified refractive index area 6221B, 7221B...first in-plane 90° diffraction portion modified refractive index area 6221C, 7221C...first exit portion modified refractive index area 6222A...second incident portion modified refractive index area 6222B, 7222B...second in-plane 90° diffraction portion modified refractive index area 6222C, 7222C...second exit portion modified refractive index area 62A, 72A...incident portion diffraction grating 62B...in-plane 90° diffraction portion diffraction grating 62C, 72C...exit portion diffraction grating 63B...second light input portion 63C...third light input portion 64... Light reflecting portion 65A... First light output portion 65B... Second light output portion 72B1... First in-plane 90° diffraction portion diffraction grating 72B2... Second in-plane 90° diffraction portion diffraction grating 7223A... Third incident portion modified refractive index area 7223C... Third exit portion modified refractive index area 77... Phase adjustment portion

Claims

1. A diffraction grating comprising a plate-shaped base material, modified refractive index areas that are areas with a refractive index different from that of the base material and that are periodically arranged in one or two dimensions within a predetermined range of the base material, and a light reflecting section provided at an end of the range on the opposite side to the light input section side, and a direct coupling coefficient κ that is an index showing the intensity at which light traveling in a first direction parallel to the base material from the light input section, which is a part of the end of the range, is reflected in a second direction 180° different from the first direction without radiation loss in the modified refractive index area. 1D the modified refractive index area has a planar shape in which the absolute value of is less than 1 / L, which is the reciprocal of the diameter L of the range in the first direction.

2. The diffraction grating coupler according to claim 1, characterized in that the indirect coupling coefficient μ, which is an index showing the intensity of light whose propagation direction changes by 180° while causing radiation loss due to interaction of light traveling in the first direction or the second direction with outgoing light in a direction perpendicular to the base material, has the same value regardless of the position at which the modified refractive index areas are periodically arranged.

3. The diffraction grating coupler of claim 1, characterized in that an indirect coupling coefficient μ, which is an index showing the intensity of light whose propagation direction changes by 180° while causing radiation loss due to interaction of light traveling in the first direction or the second direction with outgoing light in a direction perpendicular to the base material, has different values ​​at at least two of the multiple positions where the modified refractive index areas are periodically arranged.

4. A diffraction grating coupler as described in claim 1, characterized in that the light reflecting portion is formed by providing a photonic crystal outside the end of the range in which the energy of light of a specified wavelength is within the photonic band gap.

5. The indirect coupling coefficient μ(x), which is an index showing the intensity of light whose propagation direction changes by 180° while accompanied by radiation loss due to interaction between light traveling in the first direction or the second direction at each position x in the first direction and the second direction of the diffraction grating and light output in a direction perpendicular to the base material, is calculated by the following equation: RAD Expressed using (x) 2. The grating coupler of claim 1, wherein the grating coupler has a width in the range of .mu.m.

6. A diffraction grating coupler as claimed in claim 1, wherein an end-emitting laser is connected to said optical input section such that the optical output section of said end-emitting laser faces said optical input section.

7. A diffraction grating coupler comprising two of the diffraction grating couplers according to claim 1, the optical input portions of which face each other and are arranged so that the first direction of one diffraction grating coupler coincides with the second direction of the other diffraction grating coupler.

8. A diffraction grating having a plate-shaped base material and modified refractive index areas that are areas with a refractive index different from that of the base material and are arranged one-dimensionally or two-dimensionally periodically within a predetermined rectangular range of the base material, one side of the rectangular area is a light input section, and one side of the rectangular area perpendicular to the one side is a light output section, and a direct coupling coefficient κ is an index indicating the intensity at which light traveling from the light input section in a first direction parallel to the base material is reflected in the modified refractive index area in a second direction that is parallel to the base material and 180° different from the first direction without radiation loss. 1D a diffraction grating coupler, characterized in that the modified refractive index area has a planar shape in which the absolute value of is less than 1 / L2, which is the reciprocal of the across length L2 of the range in the first direction.

9. A diffraction grating coupler as claimed in claim 8, wherein an end-emitting laser is connected to said optical input section such that the optical output section of said end-emitting laser faces said optical input section.

10. A diffraction grating coupler comprising: a first diffraction grating coupler as defined in any one of claims 1 to 5, the optical input section of which is linear; and a second diffraction grating coupler as defined in claim 8 or 9, wherein the optical input section of the first diffraction grating coupler and the optical output section of the second diffraction grating coupler are opposed to each other in parallel.

11. A diffraction grating coupler comprising: a first diffraction grating coupler as defined in any one of claims 1 to 5, the optical input portion of which is linear; a second diffraction grating coupler as defined in claims 8 or 9; and a third diffraction grating coupler as defined in any one of claims 1 to 5, the optical input portion of which is linear, wherein the optical input portion of the first diffraction grating coupler and the optical input portion of the second diffraction grating coupler face each other in parallel, and the optical input portion of the third diffraction grating coupler and the optical output portion of the second diffraction grating coupler face each other in parallel.

12. A first diffraction grating coupler as defined in any one of claims 1 to 5, in which the modified refractive index areas are arranged two-dimensionally, the optical input section is linear, and a second linear optical input section is provided in a direction different by 90° from the optical input section; a second diffraction grating coupler as defined in claim 8 or 9, in which the optical input section is arranged opposite the optical input section of the first diffraction grating coupler; and a third diffraction grating coupler as defined in claim 8 or 9, in which the optical input section is arranged opposite the second optical input section of the first diffraction grating coupler.

6. The diffraction grating coupler according to claim 1, further comprising: a fourth diffraction grating coupler, in which the modified refractive index areas are arranged two-dimensionally, the optical input portion is linear, and a second linear optical input portion is provided in a direction different by 90° from the optical input portion, the optical input portion being arranged facing the optical output portion of the second diffraction grating coupler, and the second optical input portion being arranged facing the optical output portion of the diffraction grating coupler of the third diffraction grating coupler; and a phase adjustment portion for changing a phase of light passing through, the phase adjustment portion being provided either or both between the second optical input portion of the first diffraction grating coupler and the optical input portion of the third diffraction grating coupler, and between the optical output portion of the third diffraction grating coupler and the optical input portion of the fourth diffraction grating coupler.

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