Optical waveguide element, optical modulation device using the same, and optical transmission device

Abstract

To provide an optical waveguide element capable of removing air bubbles in adhesive for bonding a reinforcement member from the vicinity of optical waveguides, etc. and reducing optical losses, even when a diffusion waveguide such as Ti, a rib type optical waveguide, and further a convex portion such as a spot size conversion part are formed on an optical waveguide substrate.SOLUTION: The optical waveguide element comprises an optical waveguide substrate 1 including an optical waveguide 10 formed of the materials having electrooptical effect, a reinforcement member 3 arranged on the optical waveguide near an end of the optical waveguide, the optical waveguide substrate 1 and the reinforcement member 3 being bonded to one another via an adhesion layer 4. A groove 31 is formed on a surface of the reinforcement member 3 facing the optical waveguide substrate 1.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element, and an optical modulation device and an optical transmission device using the same, and in particular to an optical waveguide substrate having an optical waveguide formed of a material having an electro-optic effect, a reinforcing member disposed on the upper side of the optical waveguide near an end of the optical waveguide, and an optical waveguide element in which the optical waveguide substrate and the reinforcing member are bonded via an adhesive layer. [Background technology]

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements using a substrate on which an optical waveguide is formed, such as an optical modulator, are widely used. For example, an optical waveguide is formed by thermally diffusing Ti or the like into a substrate having an electro-optic effect, such as lithium niobate (LN). An input light is introduced from the outside into an optical waveguide substrate on which an optical waveguide is formed, or an output light is guided out to the outside. In order to introduce input light from the outside into an optical waveguide on a substrate, an input part of the optical waveguide is formed, and an optical block such as an optical fiber or a lens is connected to an end face of the substrate. In addition, an optical block such as an optical fiber, a lens, a polarization synthesis means, a reflection means, etc. is connected to an end face of the substrate on which an output part of the optical waveguide is formed, in order to properly guide out output light.

[0003] When connecting the optical block to the optical waveguide substrate, in order to fix the optical block more firmly, a reinforcing member is fixed to the substrate along the end face of the optical waveguide substrate with adhesive, so that the optical block is bonded to the two end faces of the optical waveguide substrate and the reinforcing member.

[0004] When an optical waveguide substrate and a reinforcing member are bonded with an adhesive, air bubbles contained in the adhesive may not escape even after bonding, and may remain on the optical waveguide substrate, particularly on or near the optical waveguide. In such cases, light waves propagating through the optical waveguide may be scattered when passing by the air bubbles, causing optical loss. Furthermore, in optical waveguides in which Ti or the like is thermally diffused, convex portions of about 50 nm are formed on the optical waveguide substrate, and these convex portions cause the phenomenon that air bubbles are difficult to escape.

[0005] On the other hand, with the recent increase in the volume of information and communication, there is a demand for faster and larger capacity optical communications between long distance cities and data centers. Furthermore, there is a space restriction for base stations, so there is a need to increase the speed and size of optical modulators.

[0006] In order to miniaturize optical modulators, the optical waveguide can be made narrower to increase the light confinement effect, which in turn reduces the bending radius of the optical waveguide and enables miniaturization. For example, LN is used as an optical modulator for long distances because it has little distortion and little optical loss when converting electrical signals into optical signals. Conventional optical waveguides in LN optical modulators, for example optical waveguides made by thermal diffusion of Ti, have a mode field diameter (MFD) of about 10 μm and a bending radius of the optical waveguide of several tens of mm, making miniaturization difficult.

[0007] As a method for strengthening the confinement of light waves in an optical waveguide, a rib-type optical waveguide 10 having a convex portion is formed on the surface of a substrate (1) as shown in Figs. 1 to 3. However, in an optical waveguide using thermal diffusion of Ti as described above, the height of the convex portion formed on the substrate surface is about 50 nm, whereas the convex portion of a rib-type optical waveguide is several µm. Fig. 1 is a plan view of the vicinity of an end of an optical waveguide element, Fig. 2 is a cross-sectional view taken along dashed line A-A' in Fig. 1, and Fig. 3 is a cross-sectional view taken along dashed line C-C' in Fig. 1. Reference numeral 1 denotes a substrate (thin plate or thin film) on which the optical waveguide 10 is formed, and reference numeral 2 denotes a holding substrate for holding the substrate.

[0008] In recent years, improvements in substrate polishing and bonding technologies have made it possible to make LN substrates thinner, and research and development is underway to reduce the MFD of optical waveguides to less than 3 μm, around 1 μm. As the MFD becomes smaller, the light confinement effect also increases, allowing the bending radius of the optical waveguide to be made smaller.

[0009] However, when using an optical waveguide having an MFD smaller than 10 μmφ, which is the MFD of an optical fiber (for example, 4 μm or less), a large coupling loss occurs when the end (element end face) of the optical waveguide provided in the optical waveguide element is directly joined to an optical fiber.

[0010] To solve such problems, it is possible to arrange a spot size converter (SSC) at the end of the optical waveguide. A typical SSC is configured to provide a tapered optical waveguide portion that expands the optical waveguide in two or three dimensions. For reference, Patent Documents 1 to 3 show examples of tapered waveguides.

[0011] Tapered optical waveguides, which increase the spot size by gradually expanding the core of the optical waveguide, have limitations on the designs that can be used as SSCs for optical waveguide elements, because it is difficult to adjust the refractive index of the core and cladding to suit the spot size and they are prone to inducing multimodes. Furthermore, a relatively long tapered portion must be formed to convert to the required spot size, which makes it difficult to miniaturize optical waveguide elements.

[0012] The present applicant has proposed an SSC as shown in Figs. 4 and 5, in which the tip of a rib-type optical waveguide 10 is tapered 10' with a narrower width, and a block portion 12 serving as a core portion is arranged to surround it. The refractive index of the block portion 12 is set lower than that of the optical waveguide 10, and the block portion 12 is surrounded by an organic material 4 having a refractive index lower than that of the block portion 12 by about 0.01 to 0.03. This organic material can be made of a cured adhesive or the like. In this SSC, the tip of the optical waveguide 10 is tapered 10' while covered with the block portion 12, so that the effective refractive index of the tapered portion 10' of the optical waveguide 10 is reduced, and the confinement of light is weakened. As a result, the optical mode is transferred to the block portion 12, and a larger MFD than that of the optical waveguide 10 is realized.

[0013] Furthermore, optical blocks such as optical fibers and lenses are connected to an end face of the optical waveguide substrate (the lower side of the drawing in FIG. 1 or 4, or the right side of the drawing in FIG. 3). A reinforcing member 3 is provided to reinforce the substrate side when these connections are made. The optical waveguide substrate 1 and the reinforcing member 3 are joined with an adhesive 4. In FIG. 1, the reference numeral 30 indicates the other end face of the reinforcing member 3 for reference, but the arrangement position of the end face is not limited to this.

[0014] As described above, in the case of the optical waveguide by thermal diffusion of Ti, there is a problem that bubbles contained in the adhesive are difficult to escape between the optical waveguide substrate and the reinforcing member 3. Similarly, when there is a high protruding convex part such as the rib type optical waveguide 10 of FIG. 2 or the SSC structure of FIG. 5 on the joint surface with the reinforcing member 3, the movement of the bubbles B contained in the adhesive is blocked by the protrusion, and the bubbles B tend to remain in the adhesive 4. In particular, compared with the conventional optical waveguide by thermal diffusion of Ti (height of about 50 nm), when the height H of the rib type optical waveguide of FIG. 3 is, for example, 2.5 μm, it is about 50 times higher. Furthermore, when the height H of the SSC of FIG. 5 is, for example, 3 μm or more, it is higher than the conventional rib type optical waveguide, so this problem becomes even more prominent. Moreover, since the refractive indexes of the adhesive 4 and the bubbles B are different, the presence of the bubbles B in the vicinity of the optical waveguide 10 causes the propagating light wave to be affected by scattering, etc., which may cause light loss. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] JP 2006-284961 A [Patent Document 2] JP 2007-264487 A [Patent Document 3] International Publication No. WO2012 / 042708 Summary of the Invention [Problem to be solved by the invention]

[0016] The problem to be solved by the present invention is to provide an optical waveguide element that solves the above-mentioned problems and reduces optical loss by removing air bubbles in the adhesive that bonds the reinforcing member from the vicinity of the optical waveguide, etc., even when a diffused waveguide such as Ti or the like, a rib-type optical waveguide, or a convex portion such as a spot size conversion portion is formed on the optical waveguide substrate, and further to provide an optical modulation device and an optical transmission device that use the optical waveguide element. [Means for solving the problem]

[0017] In order to solve the above problems, the optical waveguide element and the optical modulation device and optical transmission device using the same according to the present invention have the following technical features. (1) An optical waveguide element comprising an optical waveguide substrate having an optical waveguide formed of a material having an electro-optic effect, a reinforcing member disposed on the upper side of the optical waveguide near an end of the optical waveguide, and the optical waveguide substrate and the reinforcing member bonded via an adhesive layer, the optical waveguide being a rib-type optical waveguide, the adhesive layer being configured to fill the space between the optical waveguide substrate and the reinforcing member with adhesive, the portion of the reinforcing member closest to the optical waveguide substrate being disposed at a distance greater than the height of the convex portion of the optical waveguide, and the surface of the reinforcing member facing the optical waveguide substrate having At a position that does not overlap the optical waveguide when viewed in a plan view and at a position that does not overlap with a mode field diameter of a light wave propagating through the optical waveguide, The groove is formed.

[0019] ( 2 ) Above (1 ) The optical waveguide element described herein is characterized in that a spot size conversion section that changes the mode field diameter of the light waves propagating through the optical waveguide is disposed at the end of the optical waveguide as part of the optical waveguide.

[0021] (3) (1) Above Or (2) In the optical waveguide element described in the above, a cross-sectional area of ​​the groove in a plane perpendicular to the extending direction of the optical waveguide is set to be larger than a cross-sectional area of ​​the convex portion of the optical waveguide in the plane.

[0022] (4) Above (1) to(3) In the optical waveguide element according to any one of claims 1 to 5, a volume of the groove is set to be larger than a volume of the convex portion of the optical waveguide located below the reinforcing member.

[0023] (5) Above (1) to (4) In the optical waveguide element according to any one of claims 1 to 5, a depth of the groove is set to be larger than a height of the convex portion of the optical waveguide, or a width of an opening of the groove is set to be larger than a width of the convex portion of the optical waveguide.

[0024] (6) Above (1) to (5) In the optical waveguide element according to any one of the above, the groove has a polygonal shape in a cross section perpendicular to the extending direction of the groove.

[0025] (7) Above (1) to (6) In the optical waveguide element according to any one of claims 1 to 5, at least a part of the inner surface of the groove is formed as a curved surface.

[0026] (8) Above (1) to (7) In the optical waveguide element according to any one of claims 1 to 5, the grooves are arranged so as to sandwich the optical waveguide when viewed in plan from above the reinforcing member.

[0027] (9) Above (1) to (8) Any of the optical waveguide elements described herein is an optical modulation device that is housed in a housing and includes an optical fiber that inputs or outputs a light wave to or from the optical waveguide.

[0028] (10) the above (9) In the optical modulation device described in the above, the optical waveguide element is provided with a modulation electrode for modulating a light wave propagating through the optical waveguide, and an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element is provided inside the housing.

[0029] (11) the above (9) or (10) and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation. Effect of the Invention

[0030] The present invention relates to an optical waveguide element including an optical waveguide substrate having an optical waveguide formed of a material having an electro-optic effect, a reinforcing member disposed on the upper side of the optical waveguide near an end of the optical waveguide, and the optical waveguide substrate and the reinforcing member are bonded via an adhesive layer, the optical waveguide being a rib-type optical waveguide, the adhesive layer being configured to fill the space between the optical waveguide substrate and the reinforcing member with adhesive, the portion of the reinforcing member closest to the optical waveguide substrate being disposed at a distance greater than the height of the convex portion of the optical waveguide, and the surface of the reinforcing member facing the optical waveguide substrate having: At a position that does not overlap the optical waveguide when viewed in a plan view and at a position that does not overlap with a mode field diameter of a light wave propagating through the optical waveguide, The grooves allow air bubbles in the adhesive to be collected in the grooves and removed from the vicinity of the rib-type optical waveguide, thereby reducing the optical loss of the optical waveguide element. [Brief description of the drawings]

[0031] [Figure 1] FIG. 1 is a plan view of an optical waveguide element using a conventional rib-type optical waveguide. [Diagram 2] 2 is a cross-sectional view taken along dashed line AA' in FIG. [Diagram 3] 2 is a cross-sectional view taken along dashed line CC' in FIG. [Figure 4] FIG. 11 is a plan view of an optical waveguide element using a spot size conversion portion. [Diagram 5] 5 is a cross-sectional view taken along dashed line AA' in FIG. [Figure 6] 1 is a plan view showing an embodiment of an optical waveguide element according to the present invention. [Figure 7] 7 is a cross-sectional view taken along dashed line AA' in FIG. [Figure 8]FIG. 2 is a diagram illustrating an application example 1 of a groove used in an optical waveguide element of the present invention. [Figure 9] 10A and 10B are diagrams illustrating a second application example of a groove used in an optical waveguide element of the present invention. [Figure 10] 11A to 11C are diagrams illustrating an application example 3 of a groove used in an optical waveguide element of the present invention. [Figure 11] 1 is a plan view illustrating an optical modulation device and an optical transmission device using the optical waveguide element of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The optical waveguide element of the present invention will be described in detail below using preferred examples. As shown in Figures 6 and 7, the optical waveguide element of the present invention comprises an optical waveguide substrate 1 having an optical waveguide 10 formed of a material having an electro-optic effect, a reinforcing member 3 arranged on the upper side of the optical waveguide near an end of the optical waveguide, and the optical waveguide substrate 1 and the reinforcing member 3 are joined via an adhesive layer 4, and are characterized in that a groove 31 is formed on the surface of the reinforcing member 3 facing the optical waveguide substrate 1.

[0033] The material 1 having an electro-optic effect used in the optical waveguide element of the present invention can be a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a vapor-grown film made of these materials. Also, various materials such as semiconductor materials and organic materials can be used as optical waveguides.

[0034] The optical waveguide 10 can be formed by thermally diffusing Ti or the like into an LN substrate, or by etching the substrate 1 other than the optical waveguide, or by forming grooves on both sides of the optical waveguide, thereby making the portion of the substrate corresponding to the optical waveguide convex. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to increase the refractive index by diffusing Ti or the like onto the substrate surface by thermal diffusion or proton exchange. The following explanation focuses on rib-type optical waveguides and SSC, but these concepts also apply to other optical waveguides having convex portions, such as Ti-diffused waveguides.

[0035] The thickness of the substrate (thin plate) on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to achieve speed matching between the microwave and light waves of the modulated signal. The height of the rib-type optical waveguide is set to 4 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less or 0.4 μm or less. It is also possible to form a vapor-grown film on the reinforcing substrate 3 and process the film into the shape of the optical waveguide.

[0036] The substrate 1 on which the optical waveguide is formed is bonded to a holding substrate 2 by direct bonding or via an adhesive layer such as resin to increase mechanical strength. As the holding substrate 2 to be directly bonded, a material having a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed and a thermal expansion coefficient close to that of the optical waveguide, for example, a substrate including an oxide layer such as quartz, glass, or sapphire, is preferably used. Composite substrates, abbreviated as SOI or LNOI, in which a silicon oxide layer is formed on a silicon substrate or a silicon oxide layer is formed on an LN substrate, can also be used.

[0037] When the height of the optical waveguide 10 is 1 μm or less, a block body 12 of the spot size converter (SSC) similar to that of FIGS. 4 and 5 is formed, as shown in FIGS. 6 and 7. The present invention is not limited to these, and a tapered spot size converter as shown in Patent Documents 1 to 3 may be used. Note that, although the spot size converter has been described here as having a configuration in which the tip of a rib-type optical waveguide is tapered to narrow, and a block portion serving as a core portion is disposed to surround the tip, the present invention is not limited thereto, and a shape in which the width of the tip of a rib-type optical waveguide gradually increases without using a block portion may also be used.

[0038] As explained in Figs. 1 to 5, a reinforcing member 3 is disposed above the rib-type optical waveguide 10 and the spot size conversion section (SSC). A material having the same refractive index and linear expansion coefficient as the holding substrate 2 is used for the reinforcing member 3. If the linear expansion coefficients are the same, it is possible to reduce defects such as the reinforcing member (upper substrate) coming off due to thermal stress, and an optical waveguide element with excellent heat resistance is obtained. For the adhesive 4 that bonds the reinforcing member 3 to the optical waveguide substrate 1 or the holding substrate 2, an adhesive made of a UV-curable resin or an acrylic or epoxy resin can be used.

[0039] The optical waveguide element of the present invention is characterized in that a groove 31 is provided in the reinforcing member 3 in order to collect air bubbles B contained in the adhesive 4 and remove them from the vicinity of the optical waveguide. The groove 31 is preferably provided at a position that does not overlap the mode field diameter (MFD) so as not to affect the profile of the mode field diameter (MFD) of the light wave propagating through the rib-type optical waveguide 10 shown in Figures 1 to 3 and the spot size converter (SSC) that is a part of the optical waveguide as shown in Figures 6 and 7. It is more preferable to provide the groove 31 at a position 15% or more away from the end of the MFD. The same applies when applied to the rib-type optical waveguide 10 in Figure 2. In the present invention, the term "optical waveguide" is intended to include the SSC as a part of the optical waveguide unless a specific configuration is specified.

[0040] The size of the groove 31 must be large enough to accommodate the air bubbles B near the optical waveguide. As an example, the cross-sectional area AR1 of the groove 31 in a plane (as shown in FIG. 7) perpendicular to the extension direction of the optical waveguide is set to be larger than the cross-sectional area AR0 of the rib portion of the optical waveguide in the plane (block body 12 of the SSC in the case of FIG. 7). More precisely, the minimum cross-sectional area of ​​the groove 31 is the cross-sectional area required to collect the air bubbles so that no air bubbles are present within the MFD range of the light wave propagating through the rib portion, more preferably within the range of a position 15% or more away from the end of the MFD, from the state in which the air bubbles are present around the rib portion.

[0041] The size of the groove 31 is set by the cross-sectional area AR1 of the groove, but it is not limited thereto. It is also possible to set the depth of the groove 31 to be larger than the height of the convex part of the optical waveguide, or to set the width of the opening of the groove 31 to be larger than the width of the convex part of the optical waveguide. Of course, these conditions can be used in combination. The convex part is a concept including not only the convex part of the Ti diffusion waveguide, the convex part of the rib type optical waveguide, and the convex part of the block body of the SSC, but also the convex shape change part of the optical waveguide substrate 1 or the holding substrate 2 in the adhesive layer 4, and a separate convex part provided on the upper part of the optical waveguide substrate 1 or the holding substrate 2.

[0042] Furthermore, as another method for setting the size of groove 31, it is also possible to set the volume of the groove to be larger than the volume of the rib portion (block body 12 in the case of SSC) of the optical waveguide located below reinforcing member 3. More precisely, the minimum volume of groove 31 can be set to the volume of a range of 1 μm around the rib portion (excluding the rib portion).

[0043] As shown in Fig. 7, two or more grooves 31 can be provided. In particular, it is preferable to arrange the grooves symmetrically so as to sandwich the optical waveguide when viewed in a plan view. This is also to configure the internal stress generated in the adhesive due to the presence of the grooves to be applied equally to the left and right of the optical waveguide. As the number of grooves arranged near the optical waveguide increases, the amount of air bubbles contained in each groove decreases accordingly, so the above-mentioned cross-sectional area and volume can be divided proportionally according to the number of grooves.

[0044] It is preferable that the surface of face 3D adjacent to the rib portion of the optical waveguide of the reinforcing member 3 (block body 12 of SSC in FIG. 7) is finished smoothly so that air bubbles can move easily, and the inner surface of the groove, especially the surface of face 3E at the deepest part, is finished roughly so that air bubbles can move less easily. This allows air bubbles to be contained in the groove more efficiently.

[0045] The cross-sectional shape of the groove formed in the reinforcing member 3 is not limited to a rectangular shape as shown in Fig. 7, but may also be a triangular shape 32 as shown in Fig. 8. When forming a polygonal shape as in Fig. 8, the internal stress of the adhesive may be concentrated at the corners R1 to R3, so it is preferable that the corners R1 to R3 have a curved shape that bulges outward from the groove. Also, the angle θ shown in Fig. 8 can be set in the range of 30 degrees to 90 degrees. Naturally, it is also possible to form the entire inner surface of the groove with a curved surface.

[0046] 9 and 10 show application examples of the groove shape, and in FIG. 9, the groove 31 is linear, but the groove may be wavy or non-linear. The groove 31 is arranged parallel to the direction in which the optical waveguide extends, but the arrangement angle of the groove may be inclined as necessary. For example, the distance between the grooves may be changed as the MFD expands.

[0047] As shown in Fig. 10, it is also possible to provide a locally isolated groove 34 in the reinforcing member 3, or a groove 35 extending in a direction at an angle, such as perpendicular to the extending direction of the rib portion (block body 12 of SSC) of the optical waveguide. By forming a groove exposed on the end face of the reinforcing member 3, such as grooves 31 and 33 in Fig. 9 and groove 35 in Fig. 10, air bubbles can be easily discharged to the outside of the reinforcing member 3, and the air bubble removal effect can be further improved. Although groove 35 is formed so as to be exposed on the end face of the reinforcing member 3, it may also be a locally isolated groove like groove 34.

[0048] The optical waveguide element of the present invention is provided with a modulation electrode for modulating the light wave propagating through the optical waveguide 10 on the optical waveguide substrate 1, and is housed in a housing CA as shown in FIG. 11. Furthermore, an optical modulation device MD can be configured by providing an optical fiber (F) for inputting and outputting light waves to the optical waveguide. In FIG. 11, the optical fiber is introduced into the housing through a through hole penetrating the side wall of the housing, and is directly joined to the optical waveguide element. The optical waveguide element and the optical fiber can also be optically connected via a spatial optical system.

[0049] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal S0 that causes the optical modulation device MD to perform a modulation operation to the optical modulation device MD. Since the modulation signal S to be applied to the optical waveguide element needs to be amplified, a driver circuit DRV is used. The driver circuit DRV and digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit. [Industrial Applicability]

[0050] As described above, according to the present invention, even when a diffused waveguide such as Ti, a rib-type optical waveguide, or a convex portion such as a spot size conversion portion is formed on an optical waveguide substrate, it is possible to provide an optical waveguide element with reduced optical loss by removing air bubbles in the adhesive that bonds the reinforcing member from the vicinity of the optical waveguide, etc. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using the optical waveguide element. [Explanation of symbols]

[0051] 1. Substrate (thin plate, film) on which the optical waveguide is formed 2. Holding substrate (part of the optical waveguide substrate) 3 Reinforcement members 4. Glue 10 Rib-type optical waveguide 12 Block letters (part of SSC) 31~35 groove

Claims

1. An optical waveguide element comprising an optical waveguide substrate having an optical waveguide formed of a material having an electro-optic effect, a reinforcing member disposed on an upper side of the optical waveguide near an end of the optical waveguide, and the optical waveguide substrate and the reinforcing member bonded together via an adhesive layer, the optical waveguide is a rib-type optical waveguide, the adhesive layer is configured to fill a space between the optical waveguide substrate and the reinforcing member with an adhesive; a portion of the reinforcing member closest to the optical waveguide substrate is disposed at a distance greater than the height of a convex portion of the optical waveguide; an optical waveguide element, characterized in that a groove is formed on a surface of the reinforcing member facing the optical waveguide substrate at a position that does not overlap with the optical waveguide when viewed in a plane and at a position that does not overlap with a mode field diameter of a light wave propagating through the optical waveguide.

2. 2. The optical waveguide element according to claim 1, wherein a spot size conversion portion for changing a mode field diameter of a light wave propagating through the optical waveguide is disposed at an end of the optical waveguide as a part of the optical waveguide.

3. 3. The optical waveguide element according to claim 1, wherein a cross-sectional area of ​​the groove in a plane perpendicular to an extension direction of the optical waveguide is set to be larger than a cross-sectional area of ​​the convex portion of the optical waveguide in said plane.

4. 4. The optical waveguide element according to claim 1, wherein the volume of the groove is set to be larger than the volume of the convex portion of the optical waveguide located below the reinforcing member.

5. 5. The optical waveguide element according to claim 1, wherein the depth of the groove is set to be greater than the height of the convex portion of the optical waveguide, or the width of the opening of the groove is set to be greater than the width of the convex portion of the optical waveguide.

6. 6. The optical waveguide element according to claim 1, wherein the groove has a polygonal shape in a cross section perpendicular to the extending direction of the groove.

7. 7. The optical waveguide element according to claim 1, wherein at least a part of the inner surface of the groove is curved.

8. 8. The optical waveguide element according to claim 1, wherein the grooves are disposed so as to sandwich the optical waveguide when viewed from above the reinforcing member in a plan view.

9. 9. An optical modulation device comprising an optical waveguide element according to claim 1, the optical waveguide element being accommodated in a housing and comprising an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

10. 10. The optical modulation device according to claim 9, wherein the optical waveguide element is provided with a modulation electrode for modulating an optical wave propagating through the optical waveguide, and an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element is provided inside the housing.

11. 11. An optical transmitter comprising: an optical modulation device according to claim 9; and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation.

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