Optical waveguide components

Abstract

The present invention provides an optical waveguide element, which prevents leakage light generated in a branch portion from entering an optical waveguide located downstream of other branch portions, and has little degradation of optical characteristics. The optical waveguide element is characterized in that, in the optical waveguide, at least one of the two branch waveguides (20a, 20b) branching from a first branch portion (20) includes a second branch portion (21, 22), a slab waveguide (3c-1 to 3c-3) is formed between the two branch waveguides, a slit (41, 42) is formed between the first branch portion and the second branch portion, and the slit (41, 42) divides the slab waveguide into a first slab waveguide region (3c-1) close to the first branch portion and a second slab waveguide region (3c-2, 3c-3) close to the second branch portion.

Description

Technical Field

[0001] The present invention relates to an optical waveguide element, and in particular to an optical waveguide element including an optical waveguide having a branch portion formed on a substrate and a slab waveguide for guiding high-order mode light leaking from the branch portion. Background Art

[0002] In the field of optical communication or optical measurement, optical waveguide elements such as optical modulators formed on a substrate are often used. As methods for forming an optical waveguide, there are known methods of forming an optical waveguide by thermally diffusing a metal such as Ti on a substrate of quartz, lithium niobate, a semiconductor material, etc., or forming an optical waveguide by forming a ridge-shaped convex portion.

[0003] The optical waveguide includes a branching section and a combining section, such as a Mach-Zehnder type optical waveguide. In recent years, for example, a nested waveguide in which a plurality of Mach-Zehnder type optical waveguides are combined into a nested shape has been developed to correspond to multi-value modulation or polarization wave combining. Figure 1 ) have also been put to practical use, such as in the case of optical waveguide elements in which a plurality of Mach-Zehnder optical waveguides are arranged on a single substrate.

[0004] In order to achieve a high on / off extinction ratio in an optical modulator, it is necessary to suppress the slight deviation of the branching ratio in the branching section in the optical waveguide. Figure 1 As shown, when the optical waveguide 2 formed on the substrate 1 has a plurality of branching parts (20 to 22), light (high-order mode light) leaking from the branching part 20 upstream in the direction in which light is transmitted enters the branching parts (21, 22) downstream, thereby causing light interference. This light interference is particularly the cause of the deviation of the branching ratio.

[0005] Patent Document 1 discloses that a slab waveguide is provided around an optical waveguide to form an absorption region for absorbing high-order mode light. Figure 2 As shown in FIG. 1 , even if a slab waveguide (3a to 3e) is formed around the optical waveguide, the high-order mode light leaking from the branch portion 20 is also transmitted in the slab waveguide 3c, transferred to the optical waveguide located downstream of other branch portions (21, 22), and interferes with the waveguide light guided in the optical waveguide. Moreover, the above phenomenon becomes particularly significant when the thickness of the substrate on which the optical waveguide is formed is a thin plate of 20 μm or less.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2011-75906

[0009] Patent Document 2: Japanese Patent Application Publication No. 2014-35451 Summary of the invention

[0010] Problems to be solved by the invention

[0011] The problem to be solved by the present invention is to solve the problem mentioned above and provide an optical waveguide element, which controls the transmission path of leakage light such as high-order mode light generated in the branch part, prevents interference with the waveguide light transmitted in the optical waveguide located downstream of other branch parts, etc., and has less degradation of optical characteristics.

[0012] Technical means of solving problems

[0013] In order to solve the above-mentioned problems, the optical waveguide device of the present invention has the following technical features.

[0014] (1) An optical waveguide element having an optical waveguide formed on a substrate, wherein in the optical waveguide, at least one of two branch waveguides branching from a first branch portion includes a second branch portion, a slab waveguide is formed between the two branch waveguides, and a slit is formed between the first branch portion and the second branch portion, wherein the slit divides the slab waveguide into a first slab waveguide region close to the first branch portion and a second slab waveguide region close to the second branch portion.

[0015] (2) The optical waveguide element according to (1) above, wherein the refractive index of the slit is set lower than the refractive index of the slab waveguide region.

[0016] (3) The optical waveguide element according to (1) or (2), wherein the first slab waveguide region is arranged along the optical waveguide formed on the downstream side of the second branch portion.

[0017] (4) An optical waveguide element according to any one of (1) to (3), characterized in that the shape of the boundary between the slit and the second slab waveguide area includes a plurality of protrusions protruding toward the second slab waveguide area, so that high-order mode light propagating in the second slab waveguide area travels to the outside of the first slab waveguide area or the entire optical waveguide.

[0018] (5) An optical waveguide element according to any one of (1) to (4), characterized in that the optical waveguide includes a combining section for combining two branch waveguides branched at the first branch section, and the shape of a portion of the first slab waveguide region close to the combining section is set so that high-order mode light emitted from the first slab waveguide region avoids the combining section and travels across the branch waveguides.

[0019] (6) The optical waveguide element according to any one of (1) to (5), wherein an absorption material for absorbing high-order mode light propagating in the slab waveguide is arranged in at least a portion of the first slab waveguide region.

[0020] Effects of the Invention

[0021] According to the present invention, an optical waveguide element can be provided, which has an optical waveguide formed on a substrate, and in the optical waveguide element, in the optical waveguide, at least one of the two branch waveguides branched from a first branch portion includes a second branch portion, a slab waveguide is formed between the two branch waveguides, and a slit is formed between the first branch portion and the second branch portion, the slit divides the slab waveguide into a first slab waveguide region close to the first branch portion and a second slab waveguide region close to the second branch portion, thereby controlling the transmission path of leakage light such as high-order mode light generated in the first branch portion, preventing interference with waveguide light transmitted in an optical waveguide located downstream of the second branch portion, and reducing degradation of optical characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] [ Figure 1 ] is a diagram showing a box-type optical waveguide used for an optical waveguide element.

[0023] [ Figure 2 ]yes Figure 1 1 is an enlarged view of a dotted line area A, and is a view showing a model in which a slab waveguide is arranged around the optical waveguide.

[0024] [ Figure 3 ] is a diagram showing a first embodiment of the optical waveguide element of the present invention.

[0025] [ Figure 4 ] is a diagram showing a second embodiment of the optical waveguide element of the present invention.

[0026] [ Figure 5 ]yes Figure 4 Magnified view of the dashed area B.

[0027] [ Figure 6 ] is a diagram showing another box-type optical waveguide used for optical waveguide elements.

[0028] [ Figure 7 ]yes Figure 6 1 is an enlarged view of a dotted line area C, and is a view showing a model in which a slab waveguide is arranged around the optical waveguide.

[0029] [ Figure 8 ] is a diagram showing a third embodiment of the optical waveguide element of the present invention.

[0030] [ Fig. 9 ] is a diagram showing a fourth embodiment of the optical waveguide element of the present invention.

[0031] [ Fig.10 ] is a diagram showing a fifth embodiment of the optical waveguide element of the present invention.

[0032] [ Fig.11 ]yes Fig.10 An enlarged view of the dashed area D.

[0033] [ Fig.12 ] is a diagram showing a sixth embodiment of the optical waveguide element of the present invention.

[0034] [ Fig.13 ] is a diagram showing the seventh embodiment of the optical waveguide element of the present invention.

[0035] [ Fig.14 ] is a diagram showing the eighth embodiment of the optical waveguide element of the present invention.

[0036] [ Fig.15 ] is a diagram showing a ninth embodiment of the optical waveguide element of the present invention.

[0037] [ Fig.16 ]yes Figure 3 Cross-sectional view at the dotted line XX' in FIG. DETAILED DESCRIPTION

[0038] Hereinafter, the present invention will be described in detail using preferred examples.

[0039] like Figure 3 As shown, the optical waveguide element of the present invention has an optical waveguide formed on a substrate, and the optical waveguide element is characterized in that, in the optical waveguide, at least one of the two branch waveguides (20a, 20b) branching from the first branch portion (20) includes a second branch portion (21, 22), a slab waveguide (3c-1 to 3c-3) is formed between the two branch waveguides, and a slit (41, 42) is formed between the first branch portion and the second branch portion, and the slit (41, 42) divides the slab waveguide into a first slab waveguide region (3c-1) close to the first branch portion and a second slab waveguide region (3c-2, 3c-3) close to the second branch portion.

[0040] The substrate constituting the optical waveguide element of the present invention is not particularly limited as long as it is a material such as quartz, lithium niobate, semiconductor material, etc. that can form an optical waveguide on the substrate. In the case of using the electric field formed by the electrodes of an optical modulator, etc. to modulate the light waves propagating in the optical waveguide, a substrate having an electro-optical effect such as lithium niobate or lithium tantalate can also be used. In addition, when a thin substrate of less than 20 μm is used, it is usually easy to guide leakage light (high-order mode light) in the substrate containing the planar waveguide. In contrast, by applying the present invention, it is easy to control the transmission path of the leakage light, thereby suppressing the interference of the leakage light with the waveguide light transmitted in the optical waveguide.

[0041] As a method for forming an optical waveguide, a method of thermally diffusing a metal such as Ti in a substrate to form a portion with a higher refractive index than the substrate material, or a method of forming projections and depressions on the substrate surface to form a ridge waveguide (rib waveguide) can be applied. Fig.16 Shows Figure 3 The cross section at the dotted line XX' of FIG. 1 shows that the branch waveguides (20a, 20b) as the optical waveguide are formed by a rib waveguide provided with a convex portion of the substrate 1. Alternatively, the slab waveguide can be formed by a Ti diffusion layer, or by a convex rib waveguide provided with a high refractive index medium having a higher refractive index than the substrate 1. Regarding the slits (41, 42) described later, as shown in FIG. Fig.16 As shown by the dotted line, a concave portion (ridge) can be formed in a part of the substrate 1. In addition, even if the method of implementing Ti diffusion or high refractive index medium configuration is not carried out in accordance with the shape of the slit, the slit can be formed. The refractive index of the slit portion must be set lower than the refractive index of the slab waveguide.

[0042] The present invention is characterized in that a slab waveguide located between a first branch portion (20) and a second branch portion (21, 22) is divided into a plurality of regions (3c-1 to 3c-3) by using slits (41, 42). Thus, it is possible to suppress high-order mode light leaking from the first branch portion and its vicinity (hereinafter referred to as "the first branch portion, etc.") from entering the optical waveguide of the second branch portion and its vicinity, and to confine the high-order mode light leaking from the first branch portion to the first slab waveguide region (3c-1) near the first branch portion (refer to Figure 3 The arrows for transmission in region 3c-1 in FIG.

[0043] However, part of the light leaking from the first branch portion etc. passes through the slits (41, 42) and reaches the second slab waveguide region (3c-2, 3c-3) near the second branch portion. Figure 3 As shown, this leakage light stays in the second slab waveguide region (eg, 3c-2) and eventually enters the second branch portion and the optical waveguide in its vicinity.

[0044] In the present invention, in order to further eliminate this problem, Figure 4 and Figure 5 As shown, for example, the shape of the boundary (411) between the slit (41) and the second flat waveguide area (3c-2) is formed by a plurality of triangular wave-shaped protrusions. With this structure, the restriction of the leakage light at the boundary is released, and the leakage light is guided from the second flat waveguide area (3c-2) to the first flat waveguide area (3c-1). Then, the leakage light can be restricted in the first flat waveguide area (3c-1), and the leakage light can be prevented from being incident on the second branch portion, etc. Figure 5 In the model, the shape of a portion (411a) of the boundary (411) is set to an angle such that the leakage light passes through. As described later, the shape of a portion of the boundary is not limited to a shape that allows the leakage light to pass through, but may also be a shape that allows the leakage light to be reflected toward the first planar waveguide region.

[0045] Figure 6 This is a modified example of a nested optical waveguide, and the positions of the two Mach-Zehnder optical waveguides provided in the branch waveguide are arranged so as to be offset in the lateral direction of the drawing. Even in this case, the present invention can be applied in the same manner as in the above-described embodiment.

[0046] Figures 7 to 10 yes Figure 6 The enlarged view of the dashed area C, especially Figures 8 to 10 An embodiment of the optical waveguide element of the present invention is shown.

[0047] Figure 7 A slab waveguide (3a to 3e) is provided around the optical waveguide to remove leakage light (high-order mode light). Figure 2 Similarly, the leakage light leaking from the first branch part (20) and its vicinity propagates inside the planar waveguide (3c) and reaches the second branch part (21, 22) and its vicinity. As a result, the waveguide light propagating in the optical waveguide interferes with the leakage light, causing degradation of optical characteristics.

[0048] To eliminate the above problem, Figure 3 Likewise, if Figure 8 As shown, the slits (41, 42) are used to divide the slab waveguide into a first slab waveguide region (3c-1) close to the first branch portion (20) and a second slab waveguide region (3c-2, 3c-3) close to the second branch portions (21, 22). The shape of the slit is not limited to Figure 3 It can be a straight line, or it can be a Figure 8 That kind of curve.

[0049] Furthermore, in order to guide the leakage light confined in the second slab waveguide region (for example, 3c-2) to the first slab waveguide region (3c-1), as shown in FIG. Fig. 9 As shown, for example, the shape of the boundary between the slit (41) and the second slab waveguide region (3c-2) can be formed as follows: Figure 5 That triangular wave shape.

[0050] In addition, if Fig.10 and Fig.11 As shown, the shape of the boundary (411) can also be composed of a combination of a portion (411a) through which the leakage light is transmitted and a portion (411b) through which the leakage light is reflected. Fig.11 yes Fig.10 In addition, the leakage light incident at an acute angle relative to the propagation direction of the leakage light in the first slab waveguide region is not limited to the first slab waveguide region (3c-1), for example, Fig.10 As shown, the optical waveguide travels along a path that extends to the outside of the entire optical waveguide.

[0051] Fig.12 and Fig.13 Four Mach-Zehnder waveguides (input side Y branch 211 to input side Y branch 222) are arranged in parallel, and each Mach-Zehnder waveguide is connected by a plurality of branches (20 to 22). In this way, even when the optical waveguide has a complex shape, such as Fig.12 As shown, by forming a slab waveguide in the area sandwiched between the branch waveguides and providing slits (41, 42) dividing the slab waveguide, most of the leakage light leaking from the first branch part (20) etc. can be suppressed from entering the second branch part (21, 22) etc.

[0052] Furthermore, if Fig.13 As shown, by using a portion of the slit Figure 5 The shape of the boundary (411) shown can guide the leakage light of the second slab waveguide region to the first slab waveguide region.

[0053] In addition, Fig.14 In the invention, two box-shaped optical waveguides are arranged in parallel, which is suitable for forming multiple independent optical waveguides on the same substrate. Slits (4a, 4b) that divide the planar waveguide are arranged between the independent optical waveguides (2a, 2b), and are divided into planar waveguide areas (30a-1 to 30a-3). In addition, a transmission path (30a-1) is provided to collect leakage light and guide the leakage light to the rear side of the substrate. The function of the planar waveguide area (30a-1) is basically the same as that of the first planar waveguide area, and a convex portion such as a triangular wave shape can also be provided as the shape of the boundary portion of the slit.

[0054] It is possible to release the leaked light near the input end of the optical waveguide to an independent optical waveguide ( Fig.14 2a) The entire external leakage light is guided into the planar waveguide area (30a-1).

[0055] Fig.15 Shows Figure 1 The structure of the rear section of the optical waveguide is shown on the right side of the dotted line area A. Figure 3 and Fig.15 As shown in FIG. 1 , the first slab waveguide region (30a-1) is arranged along the optical waveguide formed on the downstream side of the second branching portion (21, 22). Furthermore, in order to guide the leakage light guided by the first slab waveguide region (3c-1) to the outside of the substrate, the following method can also be adopted: Fig.15 The structure shown in Fig.15 The optical waveguide includes Figure 3 A combining section (24) is provided for combining two branch waveguides branched from the first branch section (20), and the shape of a portion of the first planar waveguide region (3c-1) close to the combining section is set so that high-order mode light emitted from the first planar waveguide region avoids the combining section (24) and travels in a manner that crosses the branch waveguides (between symbols 23 and 24).

[0056] Through Fig.15 Such a structure can suppress leakage light (high-order mode light) from entering the optical waveguide in the combining section and its vicinity.

[0057] Furthermore, in order to absorb the leakage light transmitted in the first slab waveguide region, an absorbing material that absorbs the high-order mode light transmitted in the slab waveguide may be arranged at least in a part of the first slab waveguide region as disclosed in Patent Document 1 or Patent Document 2. As the absorbing material, a conductive metal material is suitable, and it can also be used as a part of a control electrode such as a ground electrode arranged on a substrate. In addition, by arranging the metal material via a high refractive index film having a higher refractive index than the slab waveguide, the absorption efficiency of the leakage light can also be improved.

[0058] Industrial Applicability

[0059] As described above, according to the optical waveguide element of the present invention, it is possible to provide an optical waveguide element which suppresses the transmission path of leakage light such as high-order mode light generated in a branch portion, prevents interference with waveguide light transmitted in an optical waveguide located downstream of other branch portions, etc., and has less degradation of optical characteristics.

[0060] Explanation of symbols

[0061] 1: Substrate

[0062] 2: Optical waveguide

[0063] 3c-1: First slab waveguide region

[0064] 3c-2, 3c-3: Second slab waveguide area

[0065] 41, 42: Slit

Claims

1. An optical waveguide element, characterized in that An optical waveguide is formed on the substrate. In the optical waveguide, at least one of the two branch waveguides branched from the first branch portion includes a second branch portion, Three slab waveguides are formed between the two branch waveguides. A slit is formed between the first branch portion and the second branch portion, the slit dividing the slab waveguide into a first slab waveguide region near the first branch portion and a second slab waveguide region near the second branch portion, the first slab waveguide region being arranged along the optical waveguide formed on the downstream side of the second branch portion, The refractive index of the slit is set to be lower than the refractive indexes of the first slab waveguide region and the second slab waveguide region.

2. The optical waveguide element according to claim 1, wherein: The shape of the boundary between the slit and the second slab waveguide region includes a plurality of protrusions protruding toward the second slab waveguide region, so that high-order mode light propagating in the second slab waveguide region travels to the outside of the first slab waveguide region or the entire optical waveguide.

3. The optical waveguide element according to claim 1 or 2, characterized in that: The optical waveguide includes a combining section for combining two branch waveguides branched at the first branch section, and the shape of a portion of the first slab waveguide region close to the combining section is set so that high-order mode light emitted from the first slab waveguide region avoids the combining section and travels across the branch waveguides.

4. The optical waveguide element according to claim 1 or 2, characterized in that: An absorbing material that absorbs high-order mode light propagating in the slab waveguide is disposed in at least a portion of the first slab waveguide region.

5. The optical waveguide element according to claim 1 or 2, wherein: The optical waveguide element includes two optical waveguides arranged in parallel. A slab waveguide is formed between the two optical waveguides, A slit for dividing the slab waveguide is arranged between the optical waveguides, and the slit divides the slab waveguide into three slab waveguide regions.

6. The optical waveguide element according to claim 1 or 2, characterized in that: The optical waveguide includes a combining section for combining two branch waveguides branched from the first branch section, and the shape of a portion of the first slab waveguide region close to the combining section is set so that high-order mode light emitted from the first slab waveguide region avoids the combining section and travels in a manner that crosses one of the two branch waveguides.

7. The optical waveguide element according to claim 1 or 2, characterized in that: The branch waveguide of the optical waveguide is formed by a rib-type waveguide provided with a convex portion of a substrate, and the slit forms a concave portion in a part of the substrate.

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