JP2013064940A - Diffraction grating type optical coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction grating type optical coupler that can be easily formed at low cost.SOLUTION: A first diffraction grating 28 is formed by arranging a plurality of sub optical waveguides 24 in parallel on both sides of a part 30 of a silicon optical waveguide 26. A second diffraction grating 32 is formed at a part of the silicon optical waveguide 26. A diffraction grating part 14 is composed of the first diffraction grating and the second diffraction grating. A first optical waveguide loop 16 is connected to one end of the silicon optical waveguide, and a second optical waveguide loop 18 is connected to the other end. The first optical waveguide loop and the second optical waveguide loop are coupled together by an optical waveguide loop coupling part 20. An input/output silicon optical waveguide 22 is connected to the optical waveguide loop coupling part.

Description

  The present invention relates to a diffraction grating type optical coupler for inputting output light from an optical fiber or the like into a silicon fine wire type optical waveguide or inputting output light from a silicon fine wire type optical waveguide into an optical fiber or the like.

2. Description of the Related Art In recent years, a silicon fine wire type optical waveguide using silicon (Si), which is excellent in miniaturization and mass productivity, as a waveguide material has attracted attention. Furthermore, if a silicon fine wire type optical waveguide is used, it becomes possible to form an optical device and an electronic device by fusing them on the same chip. An optical waveguide having a configuration in which a silicon optical waveguide (core) is surrounded by a material having a refractive index smaller than that of silicon is referred to as a silicon fine wire type optical waveguide. The material having a refractive index smaller than that of silicon is, for example, silicon oxide (SiO ₂ ).

  In a silicon fine wire type optical waveguide, the refractive index difference between silicon forming the core and silicon oxide forming the cladding is very large. For this reason, most of the photoelectric field component of the guided light can be confined in the core, and the cross-sectional dimension of the core can be formed as thin as about submicron.

  Further, since the refractive index difference between silicon and silicon oxide is large, even if the bending radius of the silicon optical waveguide is made small, the waveguide light hardly leaks from the silicon optical waveguide to the clad side at this bent portion. In other words, even if the bending radius of the silicon optical waveguide is reduced, the waveguide loss is kept small. For example, even if the bending radius is reduced to about 1 μm, it is almost the same as the waveguide loss in the straight waveguide portion of the silicon optical waveguide.

  As described above, since the bending radius of the silicon optical waveguide can be reduced, the optical device formed by the silicon fine wire type optical waveguide can be formed in the same size as the silicon electronic device.

  However, in order to efficiently couple light input / output to / from an optical device formed using a thin silicon optical waveguide to an external element such as an optical fiber, a light emitting element, or a light receiving element, the silicon optical waveguide It is necessary to expand the size of the core to the size of the light flux input / output from the outside. That is, in order to efficiently capture light input from the outside into the silicon optical waveguide and to couple light output from the silicon optical waveguide to an external optical fiber or the like with low coupling loss, the dimensions of the core of the silicon optical waveguide The dimensions of the light flux input / output from the outside must be the same.

  As a measure for this, various optical couplers have been studied so far. For example, a spot size conversion optical waveguide that adjusts the width of the core of a silicon optical waveguide is disclosed. (See Patent Documents 1 to 3). Alternatively, a method of creating a spot size conversion optical waveguide in which the core thickness of the silicon optical waveguide is formed in a tapered shape is also disclosed (see Patent Document 4 or 5). In addition, a spot size converter is disclosed in which the clad of the silicon optical waveguide has a double clad structure (see Patent Document 6).

  Furthermore, an optical coupler is disclosed in which optical waveguides having different relative refractive index differences are arranged so that the relative refractive index difference that is the difference between the refractive index of the cladding and the refractive index of the core changes stepwise (patent). Reference 7). In addition, an optical coupler using a diffraction grating is disclosed (see Patent Documents 8 to 10).

JP 2002-162528 A JP 2000-235128 A US Pat. No. 6,684,011 Japanese Patent Laid-Open No. 09-15435 JP 2005-326876 A Japanese Patent Application Laid-Open No. 07-63935 JP 2003-207684 A JP-A-11-174270 JP-A-11-174271 JP 2009-288718 A

  In recent years, the configuration of optical devices has become complicated and precise, and high costs are generated in the manufacturing process. Therefore, there is a demand for a technique for manufacturing the above-described optical device at a low cost.

  The optical device formed by the above-mentioned silicon thin wire type optical waveguide is also formed at a low cost by a simple method with few manufacturing processes, including an optical coupler for inputting / outputting light to / from the optical device from the outside. It is requested to be done.

  In the configuration of the optical coupler using the diffraction grating disclosed in the above-mentioned Patent Documents 8 to 10, the diffraction grating as the optical coupling functional part is formed in a layer different from the layer in which the silicon optical waveguide is formed. Yes. However, in order to form the silicon optical waveguide and the diffraction grating in different layers as described above, it is necessary to separately execute at least the step of forming the silicon optical waveguide and the step of forming the diffraction grating. For this reason, a manufacturing process increases and it cannot form easily.

  As a result of intensive research, the inventor of the present invention has devised the shape of the optical coupling functional part to form the silicon optical waveguide and the diffraction grating part to be the optical coupling functional part in the same layer, and has high efficiency. We were convinced that we could realize a diffraction grating type optical coupler capable of optical coupling. That is, a diffraction grating part is formed including a part of the silicon optical waveguide, or the diffraction grating part is incorporated in the middle of the silicon optical waveguide, and light can be input / output through the diffraction grating part. The configuration of the optical coupler was found.

  If the silicon optical waveguide and the diffraction grating are formed in the same layer, and the silicon optical waveguide formation and diffraction grating formation can be performed at once, the number of processes can be reduced, and optical coupling can be easily performed at low cost. A vessel can be formed.

  That is, an object of the present invention is to provide a diffraction grating type optical coupler that can be easily formed at low cost and has a coupling efficiency that is sufficient for practical use.

  According to the gist of the present invention, in order to achieve the above object, the diffraction grating type optical coupler has the following features.

  The first diffraction grating type optical coupler of the present invention includes a silicon optical waveguide having a portion where the waveguide width periodically changes, and a silicon optical waveguide parallel to both sides of the portion where the waveguide width periodically changes. And a plurality of sub optical waveguides provided periodically. The silicon optical waveguide and the plurality of sub optical waveguides constitute a first diffraction grating, and the second diffraction grating is constituted by a portion where the waveguide width of the silicon optical waveguide periodically changes.

  The light propagating through the silicon optical waveguide is excited as the first excitation light by the first diffraction grating, and then the first excitation light is excited as the second excitation light, and the second excitation light is output to the outside. On the other hand, light input from the outside is excited as second excitation light by the second diffraction grating, and then the second excitation light is excited as the first excitation light, and the first excitation light propagates through the silicon optical waveguide. .

  In the second diffraction grating type optical coupler of the present invention, a plurality of sub optical waveguides having portions where the waveguide width periodically changes are arranged in parallel and periodically. The first diffraction grating is formed by arranging the plurality of sub optical waveguides in parallel and periodically, and the second diffraction grating is formed by changing the waveguide width of the plurality of sub optical waveguides periodically. Is done. The first diffraction grating and the second diffraction grating constitute a diffraction grating part, and this diffraction grating part is built in the middle of the silicon optical waveguide.

  The light propagating through the silicon optical waveguide is excited as the first excitation light by the first diffraction grating, and then the first excitation light is excited as the second excitation light, and the second excitation light is output to the outside. On the other hand, light input from the outside is excited as second excitation light by the second diffraction grating, and then the second excitation light is excited as the first excitation light, and the first excitation light propagates through the silicon optical waveguide. .

  According to the first and second diffraction grating type optical couplers of the present invention, both the first and second diffraction gratings and the silicon optical waveguide for guiding the light input from the outside or the light output to the outside, Since they are configured to be formed in the same layer, it is possible to collectively form the silicon optical waveguide and the diffraction grating. Therefore, it is possible to provide a diffraction grating type optical coupler that can reduce the number of manufacturing steps and can be easily formed at low cost.

  Further, since the diffraction grating part is composed of the first diffraction grating and the second diffraction grating, the propagation direction of the light input from the outside and the propagation direction of the silicon optical waveguide connected to the diffraction grating part are A diffraction grating type optical coupler capable of high-efficiency optical coupling even in the orthogonal direction is realized.

It is a diagram for explaining the configuration of the first diffraction grating type optical coupler of the embodiment of the present invention, (A) is a schematic perspective view showing the structure of the first diffraction grating type optical coupler, B) is a schematic plan view showing an enlarged waveguide pattern of the diffraction grating portion. FIG. 3 is a diagram showing optical coupling characteristics of the first diffraction grating optical coupler according to the embodiment of the present invention. FIG. 4 is an enlarged schematic plan view showing a waveguide pattern of a diffraction grating portion of a first diffraction grating type optical coupler in which the width of a sub optical waveguide and the width of a silicon optical waveguide are set equal. FIG. 6 is a diagram showing optical coupling characteristics of a first diffraction grating optical coupler in which the width of a sub optical waveguide and the width of a silicon optical waveguide are set equal. It is a diagram for explaining the configuration of a second diffraction grating type optical coupler according to an embodiment of the present invention, (A) is a schematic perspective view showing the structure of the second diffraction grating type optical coupler, B) is a schematic plan view showing an enlarged waveguide pattern of the diffraction grating portion. FIG. 6 is a diagram showing optical coupling characteristics of a second diffraction grating optical coupler according to an embodiment of the present invention.

  Embodiments of the present invention will be described below with reference to the drawings. Each drawing shows an example of the configuration according to this embodiment, and only schematically shows the arrangement relationship of each component to the extent that the embodiment of the present invention can be understood. Is not limited to the illustrated example. In the following description, specific constituent materials, design conditions, and the like may be used. However, these constituent materials, design conditions, and the like are only suitable examples, and are not limited to these. Moreover, in each figure, the same component is shown with the same number, and the overlapping description may be omitted.

<First diffraction grating type optical coupler>
The configuration of the first diffraction grating optical coupler according to the embodiment of the present invention will be described with reference to FIGS. 1 (A) and (B) to FIG. FIG. 1A is a schematic perspective view showing the structure of the first diffraction grating type optical coupler. FIG. 1B is an enlarged schematic plan view showing the waveguide pattern of the diffraction grating portion.

  As shown in FIGS. 1A and 1B, the first diffraction grating type optical coupler is configured as follows.

An optical waveguide pattern structure 12 is laminated on the silicon substrate 10. The optical waveguide pattern structure 12 includes an optical waveguide pattern made of Si and a SiO ₂ cladding layer surrounding the optical waveguide pattern as a core.

  Here, the optical waveguide pattern constituting the first diffraction grating type optical coupler will be described. The optical waveguide pattern includes the input / output silicon optical waveguide 22, the silicon optical waveguide 26 constituting the diffraction grating portion 14, the first optical waveguide loop 16, the second optical waveguide loop 18, and the optical waveguide loop coupling portion 20. ing. Since the structure is formed by surrounding silicon with a silicon oxide material using silicon as a core, it may be collectively referred to as a silicon optical waveguide within a range where no confusion occurs.

  A first diffraction grating 28 is formed by arranging a plurality of sub optical waveguides 24 in parallel on both sides of a part 30 of the silicon optical waveguide 26 (indicated by a rectangular broken line in FIG. 1B). A refractive index distribution in a direction perpendicular to the waveguide direction of the sub optical waveguide 24 is periodically changed, and a function as a diffraction grating is exhibited. This region is the first diffraction grating 28 (indicated by a rectangular broken line in FIG. 1B). A portion 30 of the silicon optical waveguide 26 guides light that is transmitted from the outside through the optical fiber 34 or light that is output to the outside so as to propagate through the optical fiber 34.

  The waveguide width of the portion 30 of the silicon optical waveguide 26 changes periodically. When the waveguide width of the portion 30 of the silicon optical waveguide 26 is periodically changed, the equivalent refractive index of the portion 30 of the silicon optical waveguide 26 is periodically changed in the waveguide direction of the silicon optical waveguide 26. The function as a diffraction grating appears. This region (indicated by a rectangular broken line in FIG. 1B) is the second diffraction grating 32.

  The diffraction grating portion 14 is constituted by a first diffraction grating 28 and a second diffraction grating 32. Since the first diffraction grating 28 and the second diffraction grating 32 are orthogonal to each other in the direction in which the period of the refractive index distribution is formed, the propagation direction of the first excitation light excited by the first diffraction grating 28 and the second The propagation direction of the second excitation light excited by the diffraction grating 32 is orthogonal. As a result, the propagation direction of the light input from the outside to the diffraction grating part 14 or the light output from the diffraction grating part 14 to the outside, and the light guided through the silicon optical waveguide 26 coupled by the diffraction grating part High coupling efficiency can be achieved even if the propagation direction of each is orthogonal.

  The first optical waveguide loop 16 is connected to one end of the silicon optical waveguide 26 (one end of the portion 30 of the silicon optical waveguide) constituting the first diffraction grating 28, and the second optical waveguide is connected to the other end. Loop 18 is connected. The first optical waveguide loop 16 and the second optical waveguide loop 18 are coupled by an optical waveguide loop coupling unit 20. An input / output silicon optical waveguide 22 is connected to the optical waveguide loop coupling portion 20.

The light taken from the outside is excited as the first excitation light by the first diffraction grating 28 of the diffraction grating section 14, and the first excitation light is in the directions indicated by Q ₁ and Q ₂ as shown in FIG. It is generated as guided light propagating to. That is, the first excitation light propagates in the direction in which the period of the refractive index distribution of the diffraction grating portion 14 is formed.

First excitation light is generated as guided light propagating in the direction indicated with the excited P ₁ and P ₂ as the second excitation light by the second diffraction grating 32 of the diffraction grating portion 14. That is, the second excitation light propagates in the direction in which the period of the refractive index distribution of the second diffraction grating 32 is formed. Then, the second excitation light propagates through the silicon optical waveguide 26, the first optical waveguide loop 16, and the second optical waveguide loop 18, and is combined at the optical waveguide loop coupling portion 20 to the input / output silicon optical waveguide 22. It is captured.

In order to reduce the amount of the electric field component of the light propagating through the portion 30 of the silicon optical waveguide 26 to leak to the sub optical waveguides 24 provided on both sides of the silicon optical waveguide 26 by the action of the directional optical coupler, the width D ₁ of the width D ₂ and the silicon optical waveguide 26 of the waveguide 24 is set to different values. That is, the width of the sub optical waveguide 24 and the width of the silicon optical waveguide 26 are different so that the input light input from the optical fiber 34 to the diffraction grating portion 14 is concentrated and guided in the silicon optical waveguide 26. Set to value. This is based on the photonic crystal theory that when one diffraction grating component having a different width exists, light is concentrated on the diffraction grating component having a different width (here, a portion 30 of the silicon optical waveguide 26). .

  On the other hand, the light output to the outside propagates through the input / output silicon optical waveguide 22, branches at the optical waveguide loop coupling portion 20, propagates through the first optical waveguide loop 16 and the second optical waveguide loop 18, and is diffracted. Excited as second excitation light by the second diffraction grating 18 of the grating portion 14. Then, the second excitation light is excited as the first excitation light by the first diffraction grating 16 of the diffraction grating portion 14 and is output from the diffraction grating portion 14 to the outside.

  That is, according to the first diffraction grating type optical coupler, the light input from the outside or the light output to the outside is excited as the first excitation light by the first diffraction grating 28, and the first excitation light is the first excitation light. Excited as second excitation light by the two diffraction gratings 32. The second excitation light, the light input from the outside or the light output to the outside propagates through the silicon optical waveguide 26, so that light from the outside enters through the diffraction grating portion 14. Output is possible.

  Such an optical waveguide pattern structure 12 can be formed, for example, by obtaining an SOI (Silicon on Insulator) substrate and performing the following steps. The SOI substrate is widely available as a commercial product, and a silicon oxide layer is formed on the silicon substrate, and a silicon layer having a thickness equal to the thickness of the optical waveguide is formed on the silicon oxide layer.

The silicon layer formed on the silicon oxide layer of the SOI substrate is etched and removed by a technique such as dry etching, leaving only the optical waveguide pattern constituting the first diffraction grating type optical coupler. Subsequently, an SiO ₂ clad surrounding the optical waveguide pattern left by the etching process as a core of the waveguide structure is formed by a chemical vapor deposition (CVD) method or the like.

The SiO ₂ cladding may be formed at least as thick as the thickness of the silicon layer left as the optical waveguide pattern, and is not necessarily formed thick enough to completely cover the optical waveguide pattern. Further, the SiO ₂ cladding is not always necessary.

  In this way, the diffraction grating portion 14, the first optical waveguide loop 16, the second optical waveguide loop 18, the optical waveguide loop coupling portion 20, and the input / output silicon optical waveguide 22 are formed in the same layer (the optical waveguide formed on the silicon substrate 10). Formed in the waveguide pattern structure 12) as a form. That is, the diffraction grating portion 14, the first optical waveguide loop 16, the second optical waveguide loop 18, the optical waveguide loop coupling portion 20, and the input / output silicon optical waveguide 22 are collectively formed as the optical waveguide pattern structure 12.

  With reference to FIG. 2, a description will be given of the result of a simulation of the coupling efficiency with respect to the light input from the outside or the light output to the outside by the first diffraction grating type optical coupler. The horizontal axis of FIG. 2 shows the wavelength scaled in units of μm, and the vertical axis shows the intensity of the input light input to the diffraction grating portion 14 of the first diffraction grating type optical coupler on an arbitrary scale. .

  A curve a indicates the intensity of light that is input from the optical fiber 34 and optically coupled to the first optical waveguide loop 16. A curve b indicates the intensity of light reflected upward from the diffraction grating portion 14 (reflected from a layer in which an optical waveguide pattern such as the diffraction grating portion 14 is formed). Curve c shows the intensity of light transmitted downward from the diffraction grating portion 14 (transmitted toward the layer on which the waveguide pattern such as the diffraction grating portion 14 is formed and the silicon substrate 10). ing.

  The calculation simulation regarding the coupling efficiency shown in FIG. 2 was performed by a three-dimensional FDTD (Finite Difference Time Domain) method. The conditions used for the calculation are as follows.

The refractive index of the SiO ₂ cladding layer surrounding the optical waveguide pattern such as the diffraction grating portion 14, the first optical waveguide loop 16, the second optical waveguide loop 18, and the optical waveguide loop coupling portion 20 as the core of the waveguide structure was set to 1.46. The refractive index of Si constituting the optical waveguide pattern was set to 3.47, and the thickness of the Si layer constituting the optical waveguide pattern was set to 260 nm. It was assumed that the light beam incident on the diffraction grating portion 14 was a parallel light beam with a diameter of 5 μm having an intensity distribution given by a Gaussian function.

The diffraction grating period (Λ ₁ ) of the first diffraction grating 28 is 900 nm, the width (D ₁ ) of the silicon optical waveguide 26 included in the diffraction grating section 14 is 450 nm, and the width (D ₂ ) of the sub optical waveguide 24 is 500. nm, the diffraction grating period (Λ ₂ ) of the second diffraction grating 32 is 900 nm, the degree of modulation of the waveguide width of the second diffraction grating 32 (the portion of the silicon optical waveguide 26 where the waveguide width periodically changes) The difference between the maximum width and the minimum width of part 30: d) was set to 23 nm. That is, the first diffraction grating optical coupler shown in FIGS. 1A and 1B is set so that D ₁ ≠ D ₂ .

  When the wavelength of the light beam incident on the diffraction grating portion 14 is 1.52 μm, the light intensity input to the first optical waveguide loop 16 is maximum. In this case, since the same amount of light is also input to the second optical waveguide loop 18, about 80% of the light incident on the diffraction grating portion 14 is input / output silicon optical via the optical waveguide loop coupling portion 20. It is concluded that the signal is input to the waveguide 22. That is, it is concluded that about 80% of the light incident on the diffraction grating portion 14 is taken in by the first diffraction grating type optical coupler.

FIGS. 1 (A) and 1 first grating optical coupler shown in (B), the width D ₁ of the width D ₂ and the silicon optical waveguide 26 of the sub waveguides 24 are set to different values. As described above, this is set with the intention that the input light input from the optical fiber 34 to the diffraction grating section 14 is concentrated and guided in the silicon optical waveguide 26.

In FIGS. 1 (A) and 1 first grating optical coupler (B), the and the width of the sub optical waveguides 24 are all set equal to D _2. Here, without unifying the width of the sub-waveguide 24 to the D _2, although only the sub optical waveguides 24-1 adjacent to the silicon optical waveguide 26 and the width D ₁ of the silicon waveguide 26 is set to different values , check for how much effect is exerted on the optical coupling characteristics of the first grating optical coupler when the other of the width of the sub optical waveguides 24-2 was set equal to the width D ₁ of the silicon waveguide 26 In order to do this, the following studies are conducted.

As shown in FIG. 3, except for the sub optical waveguides 24-1 adjacent to the inner silicon optical waveguide 26 of the sub optical waveguides 24, the width D ₁ of the sub optical waveguides 24-2 otherwise the width of the silicon waveguide 26 Equally set to 450 nm. Although only two left and right sub optical waveguides 24-2 are shown in FIG. 3, in practice, a plurality of sub optical waveguides 24-2 are provided symmetrically adjacent to the silicon optical waveguide 26 on the left and right.

  A description will be given of the result of calculation simulation on the coupling efficiency of the first diffraction grating type optical coupler in which the widths of the sub optical waveguide 24-1 and the sub optical waveguide 24-2 are set as shown in FIG. Curves a to c shown in FIG. 4 indicate the intensity of light input to the first optical waveguide loop 16 as in FIG. 2, and the curve b is reflected upward from the diffraction grating portion 14. The intensity of light is shown, and the curve c shows the intensity of light transmitted downward from the diffraction grating portion 14.

Except for the sub optical waveguides 24-1 adjacent to the inner silicon optical waveguide 26 of the sub optical waveguides 24, the width D ₁ of the sub optical waveguides 24-2 otherwise if equal to the width of the silicon waveguide 26, FIG. 4 As shown, when the wavelength of the light beam incident on the diffraction grating portion 14 is 1.52 μm, the light intensity input to the first optical waveguide loop 16 is maximum. However, as shown by the curve b, the light component reflected upward from the diffraction grating portion 14 is larger than that in FIG.

Therefore, if at least only the sub optical waveguide 24-1 adjacent to the silicon optical waveguide 26 is set to a value different from the width D ₁ of the silicon optical waveguide 26, a desired optical coupling characteristic can be obtained. the width of the waveguide 24 by setting all equal to D _2, it is seen that more excellent properties are obtained.

The width D ₁ and D ₂ the width of the sub optical waveguides 24 of the silicon waveguide 26 in FIG. 1 (A) and FIG. 3, <has been set as D _2, D _1> D ₁ and D ₂ It may be set.

<Second diffraction grating type optical coupler>
The configuration of the second diffraction grating optical coupler according to the embodiment of the present invention will be described with reference to FIGS. 5 (A), (B) and FIG. FIG. 5 (A) is a schematic perspective view showing the structure of the first diffraction grating type optical coupler. FIG. 5B is an enlarged schematic plan view showing the waveguide pattern of the diffraction grating portion.

  The second diffraction grating type optical coupler can also be formed in the same manner as the first diffraction grating type optical coupler. Here, the optical waveguide pattern constituting the second diffraction grating type optical coupler will be described.

  A first diffraction grating 38 is configured by arranging a plurality of sub optical waveguides 44 in parallel, and a second diffraction grating 52 is formed in each of the sub optical waveguides 44. That is, the region in which the plurality of sub optical waveguides 44 are arranged has a structure in which the refractive index distribution in the direction perpendicular to the propagation direction of the sub optical waveguides 44 changes periodically, and functions as a diffraction grating. . This region is the first diffraction grating 38.

  On the other hand, a second diffraction grating 52 is formed in the sub optical waveguide 44. The second diffraction grating 52 is formed by periodically changing the waveguide width of the sub optical waveguide 44. When the waveguide width of the sub optical waveguide 44 is periodically changed, the equivalent refractive index of the sub optical waveguide 44 is periodically changed, and a function as a diffraction grating is exhibited.

  The first diffraction grating 38 and the second diffraction grating 52 constitute a diffraction grating portion 40.

  A plurality of sub optical waveguides 44 are coupled by a first sub optical waveguide coupling portion 46 and a second sub optical waveguide coupling portion 48 provided at both ends of the sub optical waveguide 44, and are connected to the first sub optical waveguide coupling portion 46. The first optical waveguide loop 16 is connected, and the second optical waveguide loop 18 is connected to the second sub optical waveguide coupling portion 48.

  The first optical waveguide loop 16 and the second optical waveguide loop 18 are coupled by an optical waveguide loop coupling portion 20, and an input / output silicon optical waveguide 22 is connected to the optical waveguide loop coupling portion 20.

The light taken from the outside is excited as the first excitation light by the first diffraction grating 38 of the diffraction grating section 40, and the first excitation light is in the directions indicated by Q ₁ and Q ₂ as shown in FIG. It is generated as guided light propagating to. First excitation light is excited as the second excitation light by the second diffraction grating 52 of the diffraction grating portion _{40, P 1 (P 1 -1~P} 1 -4) and P ₂ (P ₂ -1~P ₂ -4 ) Is generated as guided light propagating in the direction indicated by), propagates through the first optical waveguide loop 16 and the second optical waveguide loop 18, and is combined at the optical waveguide loop coupling portion 20 to be input / output silicon optical It is taken into the waveguide 22.

  On the other hand, the light output to the outside propagates through the input / output silicon optical waveguide 22, branches at the optical waveguide loop coupling portion 20, propagates through the first optical waveguide loop 16 and the second optical waveguide loop 18, and passes through the first optical waveguide loop 18. The signal is input to the diffraction grating section 40 via the first sub optical waveguide coupling section 46 and the second sub optical waveguide coupling section 48. Then, it is excited as second excitation light by the second diffraction grating 52 of the diffraction grating section 40. Further, the second excitation light is excited as first excitation light by the first diffraction grating 38 of the diffraction grating section 40, and is output from the diffraction grating section 40 to the outside.

  That is, according to the second diffraction grating type optical coupler, the light input from the outside or the light output to the outside is excited as the first excitation light by the first diffraction grating 38, and the first excitation light or the external The light input from or the light output to the outside is excited by the second diffraction grating 52 as the second excitation light. The second excitation light, the light input from the outside or the light output to the outside propagates through the sub optical waveguide 44, which is a silicon optical waveguide, etc. It is possible to input and output light.

  Similarly to the first diffraction grating type optical coupler, the second diffraction grating type optical coupler is also excited by the propagation direction of the first excitation light excited by the first diffraction grating 38 and the second diffraction grating 52. It is orthogonal to the propagation direction of the second excitation light. As a result, the propagation direction of the light input from the outside to the diffraction grating section 40 or the light output from the diffraction grating section 40 to the outside, and the sub optical waveguide 44 coupled by the diffraction grating section 40 are guided. Even if the light propagation direction is orthogonal, high coupling efficiency is realized.

  With reference to FIG. 6, a description will be given of the result of a calculation simulation of the coupling efficiency with respect to the light input from the outside or the light output to the outside by the second diffraction grating optical coupler. The horizontal axis in FIG. 6 shows the wavelength scaled in units of μm, and the vertical axis shows the light intensity on the scale of the input light to the diffraction grating section 40 of the second diffraction grating type optical coupler on an arbitrary scale. is there.

  A curve a indicates the intensity of light that is input from the optical fiber 34 and optically coupled to the first optical waveguide loop 16. A curve b indicates the intensity of light reflected upward from the diffraction grating section 40 (reflected from a layer in which a waveguide pattern such as the diffraction grating section 40 is formed). Curve c indicates the intensity of light transmitted downward from the diffraction grating section 40 (transmitted toward the layer on which the waveguide pattern such as the diffraction grating section 40 is formed and the silicon substrate 10). ing.

  The calculation simulation related to the coupling efficiency shown in FIG. 6 was also performed by the three-dimensional FDTD method which is the calculation simulation shown in FIGS. The conditions used for the calculation are as follows.

The refractive index of the SiO ₂ cladding layer surrounding the optical waveguide pattern such as the diffraction grating portion 40, the first optical waveguide loop 16, the second optical waveguide loop 18, and the optical waveguide loop coupling portion 20 as the core of the waveguide structure was set to 1.46. The refractive index of Si constituting the optical waveguide pattern was set to 3.47, and the thickness of the Si layer constituting the optical waveguide pattern was set to 260 nm. It was assumed that the light beam incident on the diffraction grating part 40 was a parallel light beam having a diameter of 5 μm having an intensity distribution given by a Gaussian function.

The diffraction grating period (Λ ₁ ) of the first diffraction grating 38 is 900 nm, the width (D ₁ ) of the sub optical waveguide 44 of the first diffraction grating 38 included in the diffraction grating section 40 is 450 nm, and the second diffraction grating 52 The diffraction grating period (Λ ₂ ) is 900 nm, and the degree of modulation of the waveguide width of the second diffraction grating 52 (the difference between the maximum width and the minimum width of the sub optical waveguide 44 in the portion where the waveguide width changes periodically: d) was set to 23 nm, respectively.

  As shown in FIG. 6, the light input to the first optical waveguide loop 16 is maximum when the wavelength of the light beam incident on the diffraction grating section 40 is around 1.5 μm. In this case, since the same amount of light is also input to the first optical waveguide loop 18, about 80% of the light input to the diffraction grating section 40, as in the first diffraction grating type optical coupler described above. Is input to the input / output silicon optical waveguide 22 via the optical waveguide loop coupling portion 20. That is, it is concluded that about 80% of the light input to the diffraction grating unit 40 is taken in by the second diffraction grating type optical coupler.

10: Silicon substrate
12: Optical waveguide pattern structure
14, 40: Diffraction grating part
16: First optical waveguide loop
18: Second optical waveguide loop
20: Optical waveguide loop coupling part
22: Input / output silicon optical waveguide
24, 44: Sub optical waveguide
26: Silicon optical waveguide
28, 38: First diffraction grating
30: Part of silicon optical waveguide
32, 52: Second diffraction grating
34: Optical fiber
46: 1st sub optical waveguide coupling part
48: Second sub optical waveguide coupling part

Claims (8)

A silicon optical waveguide having a portion where the waveguide width periodically changes;
A diffraction grating type optical coupler comprising a plurality of sub optical waveguides provided on both sides of the portion in parallel and periodically with the silicon optical waveguide. The silicon optical waveguide and the plurality of sub optical waveguides constitute a first diffraction grating,
A second diffraction grating is constituted by the portion of the silicon optical waveguide where the waveguide width periodically changes.
After the light propagating through the silicon optical waveguide is excited as the first excitation light by the first diffraction grating, the first excitation light is excited as the second excitation light, and the second excitation light is output to the outside. And
Light input from the outside is excited as the second excitation light by the second diffraction grating, and then the second excitation light is excited as the first excitation light. 2. The diffraction grating optical coupler according to claim 1, wherein the diffraction grating optical coupler propagates through a waveguide. The first optical waveguide loop is connected to one end of the portion where the waveguide width of the silicon optical waveguide periodically changes, and the second optical waveguide loop is connected to the other end,
The first optical waveguide loop and the second optical waveguide loop are coupled at an optical waveguide loop coupling portion,
An input / output silicon optical waveguide is connected to the optical waveguide loop coupling portion,
Light taken from the outside is excited as the first excitation light by the first diffraction grating of the diffraction grating portion, and the first excitation light is used as the second excitation light by the second diffraction grating of the diffraction grating portion. By being excited, the second excitation light propagates through the silicon optical waveguide, the first optical waveguide loop, and the second optical waveguide loop, and is combined at the optical waveguide loop coupling portion to be used for the input / output Taken into the silicon optical waveguide,
The light output to the outside propagates through the input / output silicon optical waveguide, branches at the optical waveguide loop coupling portion, propagates through the first optical waveguide loop and the second optical waveguide loop, and the diffraction grating. Excited as the second excitation light by the second diffraction grating of the portion, the second excitation light is excited as the first excitation light by the first diffraction grating of the diffraction grating portion, and the first excitation light is By propagating through the silicon optical waveguide,
3. The diffraction grating type optical coupler according to claim 2, wherein light can be input / output to / from the outside through the diffraction grating portion.   4. The diffraction grating type optical coupling according to claim 1, wherein the widths of the sub optical waveguides are all equal, and the width of the sub optical waveguide is different from the width of the silicon optical waveguide. vessel.   4. The diffraction grating mold according to claim 1, wherein the width of the silicon optical waveguide is different from at least the width of the sub optical waveguide disposed adjacent to the silicon optical waveguide. Optical coupler.   A diffraction grating type optical coupler, wherein a plurality of sub optical waveguides having portions where the waveguide width periodically changes are arranged in parallel and periodically. The first diffraction grating is formed by arranging the plurality of sub optical waveguides in parallel and periodically, and the second diffraction grating is formed by periodically changing the waveguide width of the plurality of sub optical waveguides. A diffraction grating portion is constituted by the first diffraction grating and the second diffraction grating,
The diffraction grating portion is configured to be incorporated in the middle of the silicon optical waveguide,
After the light propagating through the silicon optical waveguide is excited as the first excitation light by the first diffraction grating, the first excitation light is excited as the second excitation light, and the second excitation light is output to the outside. And
Light input from the outside is excited as the second excitation light by the second diffraction grating, and then the second excitation light is excited as the first excitation light. 7. The diffraction grating type optical coupler according to claim 6, wherein The plurality of sub optical waveguides are coupled by a first sub optical waveguide coupling portion and a second sub optical waveguide coupling portion provided at both ends of the plurality of sub optical waveguides,
A first optical waveguide loop is connected to the first sub optical waveguide coupling portion, and a second optical waveguide loop is connected to the second sub optical waveguide coupling portion, so that the diffraction grating portion is in the middle of the silicon optical waveguide. Built-in,
The first optical waveguide loop and the second optical waveguide loop are coupled by an optical waveguide loop coupling portion, and an input / output silicon optical waveguide is connected to the optical waveguide loop coupling portion,
Light taken from the outside is excited as the first excitation light by the first diffraction grating of the diffraction grating portion, and the first excitation light is used as the second excitation light by the second diffraction grating of the diffraction grating portion. By being excited, the second excitation light propagates through the silicon optical waveguide, the first optical waveguide loop, and the second optical waveguide loop, and is combined at the optical waveguide loop coupling portion to be used for the input / output Taken into the silicon optical waveguide,
The light output to the outside propagates through the input / output silicon optical waveguide, branches at the optical waveguide loop coupling portion, propagates through the first optical waveguide loop and the second optical waveguide loop, and the diffraction grating. Excited as the second excitation light by the second diffraction grating of the portion, the second excitation light is excited as the first excitation light by the first diffraction grating of the diffraction grating portion, and the first excitation light is By propagating through the silicon optical waveguide,
8. The diffraction grating type optical coupler according to claim 7, wherein light can be input / output to / from the outside through the diffraction grating portion.