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

Abstract

Provided is an optical waveguide element, an optical modulation device, and an optical transmission apparatus. The optical waveguide element includes a spot size conversion unit that does not complicate the manufacturing process and that suppresses optical insertion loss. The optical waveguide element includes an optical waveguide substrate including a rib-type optical waveguide formed of a material having a photoelectric effect, and a spot size conversion unit that changes the mode field diameter of an optical wave propagating in the optical waveguide at a position of an input end or an output end of the rib-type optical waveguide, wherein the spot size conversion unit includes a first structure layer including a tapered portion connected to the rib-type optical waveguide and expanding the width of the optical waveguide, a second structure layer laminated to the first structure layer and having a width narrower than the width of the first structure layer, and a third structure layer disposed so as to cover the second structure layer except for a portion of the second structure layer close to the rib-type optical waveguide, the third structure layer having a width wider than the width of the second structure layer.

Description

Technical Field

[0001] This invention relates to an optical waveguide element and an optical modulation device and an optical transmission apparatus using the same, particularly to an optical waveguide element comprising an optical waveguide substrate and a spot size conversion unit, wherein the optical waveguide substrate comprises a ribbed optical waveguide formed of a material having a photoelectric effect, and the spot size conversion unit, at the input or output end of the optical waveguide in which the ribbed optical waveguide is formed, causes a change in the mode field diameter of the light wave propagating in the optical waveguide. Background Technology

[0002] In the fields of optical measurement and optical communication, optical waveguide components, such as optical modulators, which utilize materials with photoelectric effects within the waveguide, are frequently used. In recent years, there has been a demand for miniaturization, bandwidth reduction, and lower driving voltage in optical waveguide components. To achieve miniaturization, a structure has been proposed that folds the input light within the waveguide component, aligning the input and output directions. In this folded waveguide, reducing the bending loss of the waveguide light necessitates miniaturizing the mode field diameter (MFD) to approximately 1 μm. Furthermore, when using narrow-width optical waveguides, modulation electrodes that apply an electric field to the waveguide can be positioned close to it, further contributing to bandwidth reduction and lower driving voltage.

[0003] However, when the MFD within the optical waveguide element is set to approximately 1 μm, the MFD of the fiber coupled to the element is 10 μm. Therefore, there is a difference of up to 10 times in the MFD between the optical waveguide and the fiber within the optical waveguide element. Consequently, the coupling loss in the coupled portion becomes extremely high.

[0004] While methods exist such as installing lenses between the optical waveguide element and the optical fiber to amplify the MFD, lenses that can convert the MFD by approximately 10 times, such as from 1 μm to 10 μm, are not practically feasible in design. Assuming that a lens is used for conversion, the MFD of the optical waveguide at the end of the optical waveguide element must be at least 3 μm.

[0005] Furthermore, a spot size converter (SSC) is proposed to be placed near the incident / exit portion of the optical waveguide element to expand the MFD to approximately 3μm to 5μm within the optical waveguide element. Moreover, a lens is placed between the SSC and the optical fiber to optically couple the two.

[0006] As shown in Patent Documents 1 to 3, the SSC can use a ribbed optical waveguide whose width or thickness is expanded two-dimensionally or three-dimensionally at the end facing the optical waveguide. The advantages of this method include its simple design; however, because expanding the optical waveguide induces multimode, the available designs are limited. Furthermore, not only is the manufacturing process complex, but it is also affected by misalignment of the rib shapes in each layer and the roughness of the surfaces or sides of each layer, making it impossible to sufficiently reduce optical insertion loss.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. WO2012 / 042708

[0010] Patent Document 2: International Publication No. WO2013 / 146818

[0011] Patent Document 3: Japanese Patent No. 6369036 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] The present invention aims to solve the problems described above by providing an optical waveguide element that includes a spot size conversion unit that does not complicate the manufacturing process and suppresses optical insertion loss. Furthermore, it provides an optical modulation device and an optical transmission apparatus using the optical waveguide element.

[0014] Technical means to solve the problem

[0015] In order to solve the aforementioned problems, the optical waveguide element, the optical modulation device using the same, and the optical transmission device of the present invention have the following technical features.

[0016] (1) An optical waveguide element, comprising: an optical waveguide substrate including a ribbed optical waveguide formed of a material having a photoelectric effect; and a spot size conversion unit, which, at an input or output end of the optical waveguide on which the ribbed optical waveguide is formed, causes a change in the mode field diameter of a light wave propagating in the optical waveguide, wherein the optical waveguide element, the spot size conversion unit includes: a first structural layer including a tapered portion connected to the ribbed optical waveguide and widening the width of the optical waveguide; a second structural layer, stacked on the first structural layer, having a width narrower than the width of the first structural layer; and a third structural layer, configured to cover the second structural layer except for a portion of the second structural layer that is close to the ribbed optical waveguide, having a width wider than the width of the second structural layer.

[0017] (2) According to the optical waveguide element of (1), wherein the refractive index of the material constituting the second structural layer is higher than the refractive index of the material constituting the first structural layer or the material constituting the third structural layer.

[0018] (3) The optical waveguide element according to (1) or (2), wherein the front end portion of the rib-shaped optical waveguide side of the second structural layer is disposed on the rib-shaped optical waveguide.

[0019] (4) The optical waveguide element according to any one of (1) to (3), wherein the end face of the rib-shaped optical waveguide side of the third structural layer is configured with a slope other than 90 degrees relative to the direction of travel of the light wave propagating in the second structural layer.

[0020] (5) The optical waveguide element according to any one of (1) to (4), wherein, in the end face structure of the spot size conversion unit located at the end side of the optical waveguide substrate, the first structural layer and the third structural layer are arranged to surround the second structural layer.

[0021] (6) The optical waveguide element according to any one of (1) to (4), wherein, in the end face structure of the spot size conversion unit located at the end side of the optical waveguide substrate, the first structural layer and the third structural layer are arranged such that the second structural layer is not exposed.

[0022] (7) An optical waveguide element according to any one of (1) to (6), wherein the optical waveguide substrate comprises: a thin plate on which the optical waveguide is formed and a holding substrate for holding the thin plate. The refractive index of the material constituting the holding substrate is lower than the refractive index of the material constituting the thin plate.

[0023] (8) An optical waveguide element according to any one of (1) to (6), wherein the optical waveguide substrate comprises: a thin plate on which the optical waveguide is formed, a holding substrate for holding the thin plate, and an intermediate layer between the thin plate and the holding substrate. The refractive index of the material constituting the intermediate layer is lower than the refractive index of the material constituting the thin plate.

[0024] (9) An optical modulation device comprising: an optical waveguide element according to any one of (1) to (8), the optical waveguide element including a modulation electrode for modulating an optical wave propagating in the optical waveguide, wherein the optical modulation device houses the optical waveguide element and electronic circuitry for amplifying a modulation signal input to the modulation electrode of the optical waveguide element within a housing.

[0025] (10) An optical transmitting device, comprising: an optical modulator according to (9); and an electronic circuit for outputting a modulation signal that causes the optical modulator to perform a modulation operation.

[0026] The effects of the invention

[0027] This invention relates to an optical waveguide element, comprising: an optical waveguide substrate including a ribbed optical waveguide formed of a material having a photoelectric effect; and a spot size conversion unit that, at the input or output end of the optical waveguide on which the ribbed optical waveguide is formed, changes the mode field diameter of a light wave propagating in the optical waveguide. In the optical waveguide element, the spot size conversion unit comprises: a first structural layer including a tapered portion connected to the ribbed optical waveguide and widening the width of the optical waveguide; a second structural layer stacked on the first structural layer, having a width narrower than the width of the first structural layer; and a third structural layer configured to cover the second structural layer except for the portion of the second structural layer adjacent to the ribbed optical waveguide, having a width wider than the width of the second structural layer. Therefore, the positional precision in the configuration of each layer can be relatively relaxed, and the optical insertion loss caused by the surface roughness of each layer can be reduced. Attached Figure Description

[0028] Figure 1 This is a plan view showing a first embodiment of the optical waveguide element of the present invention.

[0029] Figure 2 yes Figure 1 Cross-sectional view of an optical waveguide component. Figure 2 (a) is a cross-sectional view of the single-dotted dashed line A-A'. Figure 2 (b) is a cross-sectional view of the dashed line B-B'. Figure 2 (c) is a cross-sectional view of the dashed line C-C'. Figure 2 (d) is a cross-sectional view of the single-dotted dashed line D-D'.

[0030] Figure 3 This is a diagram showing another example of an optical waveguide substrate.

[0031] Figure 4 This is a plan view showing a second embodiment of the optical waveguide element of the present invention.

[0032] Figure 5 This is a plan view showing a third embodiment of the optical waveguide element of the present invention.

[0033] Figure 6 yes Figure 5 Cross-sectional view of an optical waveguide component. Figure 6 (a) is a cross-sectional view of the dashed line E-E'. Figure 6 (b) is a cross-sectional view of the single-dotted line F-F'.

[0034] Figure 7 It means Figure 6 A diagram of another embodiment.

[0035] Figure 8 It means Figure 5 The diagram shows an application example.

[0036] Figure 9 This is a plan view illustrating the optical modulation device and optical transmission apparatus of the present invention.

[0037] [Explanation of Symbols]

[0038] 1: Optical waveguide substrate (first structural layer)

[0039] 2: Second structural layer

[0040] 3: Third structural layer

[0041] 4: Maintain substrate

[0042] 10: Rib-type optical waveguide

[0043] 11: Conical section

[0044] MD: Optical Modulation Device

[0045] OTA: Optical Transmission Device Detailed Implementation

[0046] Hereinafter, the optical waveguide element of the present invention will be described in detail using preferred examples.

[0047] Furthermore, in the following description, the structure of the optical waveguide spot size conversion unit is described with the output end as the center; of course, the input end can also be constructed in the same way.

[0048] like Figure 1 and Figure 2 As shown, the optical waveguide element of the present invention includes: an optical waveguide substrate (1, 4) comprising a ribbed optical waveguide 10 formed of a material having a photoelectric effect; and a spot size conversion unit, which, at the input or output end of the optical waveguide 10 on which the ribbed optical waveguide 10 is formed, causes a change in the mode field diameter of a light wave propagating in the optical waveguide. In the optical waveguide element, the spot size conversion unit comprises: a first structural layer (1) comprising a tapered portion 11 connected to the ribbed optical waveguide 10 and expanding the width of the optical waveguide; a second structural layer (2) stacked on the first structural layer and having a width narrower than the width of the first structural layer; and a third structural layer (3) configured to cover the second structural layer except for a portion of the second structural layer that is close to the ribbed optical waveguide, and having a width wider than the width of the second structural layer.

[0049] As the material used in the optical waveguide element constituting the present invention, ferroelectric materials with photoelectric effect can be utilized. Specifically, substrates such as lithium niobate (LN) or lithium tantalate (LT), lead lanthanum zirconate titanate (PLZT), or epitaxial films formed from these materials can be used. In addition, various materials such as semiconductor materials or organic materials can also be used as substrates for the optical waveguide element.

[0050] The optical waveguide 10 used in this invention has an extremely thin thickness H1 of less than 1 μm. Methods for thinning include mechanically grinding a crystalline substrate such as LN, or using an epitaxial film such as LN. In the case of an epitaxial film, the epitaxial film is formed, for example, based on the crystal orientation of a single-crystal substrate such as a SiO2 substrate, a sapphire single-crystal substrate, or a silicon single-crystal substrate, using methods such as sputtering, chemical vapor deposition (CVD), or sol-gel methods.

[0051] like Figure 3 As shown, the thickness of the substrate constituting the first structural layer 1 including the optical waveguide 10 is ( Figure 3 The thickness of H1(10) plus H1(12) is extremely thin, for example, about 2 μm to 3 μm. Therefore, in order to improve the mechanical strength of the optical waveguide element, a retaining substrate 4 (thickness H4) is disposed on the back side of the first structural layer 1. The retaining substrate 4 can be made of a material with a lower refractive index than the first structural layer 1 (optical waveguide 10), such as a SiO2 substrate. Alternatively, the first structural layer 1 and the retaining substrate 4 can be directly bonded or bonded using an adhesive. Furthermore, by using an intermediate layer between the first structural layer and the retaining substrate, the selection of the retaining substrate can be broadened. For example, if SiO2, which has a lower refractive index and lower dielectric constant than LN, is used as the intermediate layer and formed to a thickness of several micrometers, then Si or alumina, which have a higher refractive index, can also be used as the retaining substrate. On the other hand, such as Figure 2 As shown in (a), the substrate 4 can also be used as the basis for crystal growth to form the first structural layer 1 of the optical waveguide 10 in the form of an epitaxial film.

[0052] The rib-shaped protrusions constituting the optical waveguide 10 can be formed by dry or wet etching of the layer forming the optical waveguide (e.g., an LN layer). Alternatively, to increase the refractive index of the ribs, a method of thermally diffusing a high-refractive-index material such as Ti at the rib location can also be used.

[0053] like Figure 1 and Figure 1 The cross-sectional view shown at the single-dotted dashed line (A-A', etc.) is Figure 2As shown, the spot size conversion unit (SSC), which is a feature of the optical waveguide element of the present invention, includes: a first structural layer (1) including a tapered portion 11 connected to the rib-shaped optical waveguide 10 and widening the width of the optical waveguide; a second structural layer (2) stacked on the first structural layer and having a width narrower than the width of the first structural layer; and a third structural layer (3) configured to cover the second structural layer except for the portion of the second structural layer that is close to the rib-shaped optical waveguide, and having a width wider than the width of the second structural layer.

[0054] The refractive index of the material constituting the second structural layer (2) is higher than that of the materials constituting the first structural layer (1) and the third structural layer (3). Furthermore, the first structural layer (1) and the third structural layer (3) can also contain materials with the same refractive index, depending on the requirements. Specifically, as described above, the first structural layer can be a crystal such as lithium niobate or lithium tantalate, or a crystal doped with other substances. Additionally, the second structural layer can be a material containing either Si or Ge. Furthermore, the third structural layer can be formed from the same material as the first structural layer or from a material containing Ta, Nb, Ti, Zr, Ce, Zn, Sb, or Nd. The smaller the difference in refractive index between the material used in the third structural layer and the first structural layer, the more uniform the amount of light leakage from the second structural layer to the upper and lower layers. Therefore, in the MFD at the coupling end face, the higher the symmetry in the vertical direction sandwiching the second structural layer, the lower the coupling loss. From this perspective, the refractive index difference between the first and third structural layers can be less than 0.1.

[0055] In the spot size conversion unit used in this invention, for the optical waveguide 10 constituting the first structural layer (1), a tapered portion 11 is used in the transverse direction ( Figure 2 The light spot size is expanded along the width W1 direction. However, since a second structural layer (2) with a higher refractive index than the first structural layer is disposed on the upper side of the first structural layer (1), the light wave propagating in the first structural layer 1 (11) is also attracted to the second structural layer, and the light spot size expands. As a result, multimode is not easily generated, and there is no need to precisely design the angle θ1 of the cone portion. Even if it is slightly smaller than the desired angle (the expansion angle of the cone portion is large), there is no problem.

[0056] Effectively, before the optical waveguide 10 of the first structural layer changes into the tapered portion 11, it generates an effect that attracts the light waves to the second structural layer side. Therefore, it can be configured that the front end portion α of the optical waveguide 10 side of the second structural layer is located further towards the optical waveguide 10 side than the connection portion β between the optical waveguide 10 and the tapered portion 11.

[0057] like Figure 1 The cross-sectional view at the single-dotted dashed line B-B' is Figure 2As shown in (b), a second structural layer 2 with a higher refractive index than the first structural layer 1 appears on the optical waveguide 10 (with dimensions of height H1: 1 μm, width W1: 1 μm, and MFD: 1 μm). However, the thickness H2 of the second structural layer 2 is very thin, ranging from 50 nm to 100 nm. Therefore, the mode does not exist within the second structural layer. Instead, an MFD of approximately the same size as the optical waveguide of the first structural layer is observed, as if a portion of it has escaped to the outside of the first structural layer 1. Figure 2 (Represented by a circular or elliptical dashed line L).

[0058] Then, as Figure 1 As shown, the front end of the first structural layer thickens at a gentle angle, and is represented by a cross-sectional view with a single-dotted line C-C'. Figure 2 As shown in (c), the second structural layer extends the width W2 to 3 μm to 5 μm. The width W2 of the second structural layer can be adjusted within the range of 0.05 μm to 5 μm according to the relationship between the refractive indices of the first structural layer 1 and the second structural layer 2. When W2 is thinner than the range described above, although it has the advantage of easily maintaining a single mode, the formation of the second structural layer itself is difficult. Conversely, when W2 is thicker than the range described above, there is the disadvantage of easily forming a multimode, but the formation becomes easier, and therefore it can also be fabricated within the aforementioned range.

[0059] Furthermore, from Figure 1 The third structural layer 3 is formed by overlapping the second structural layer 2 with a dashed line C-C' to a dashed line D-D'. As mentioned above, the refractive index of the third structural layer 3 is lower than that of the second structural layer; for example, it can be set to the same refractive index as the first structural layer 1. Figure 1 The cross-sectional view of the single-dash line D-D' is... Figure 2 As shown in (d), in the end face structure of the spot size conversion unit located at the end side of the optical waveguide substrate (1, 4), the first structural layer 1 and the third structural layer 3 are arranged to surround the second structural layer 2. Regarding the dimensions of each component, as an example, the width W2 of the second structural layer 2 is set to 3μm to 5μm, and the thickness H1 of the first structural layer 1 is set to 1μm, and the thickness H3 of the third structural layer 3 is set to 1μm to 4μm. When set within this range, the MFD at the input or output end of the light wave L is as follows... Figure 2 As shown in (d), the lateral diameter expands to 3μm to 5μm and the longitudinal diameter expands to 2μm to 5μm. In this way, it can function as an SSC containing three materials from the first to the third structural layers.

[0060] Regarding the shape and configuration of the third structural layer 3, it can be configured such that it does not cause a sharp change in the refractive index of light waves propagating along the second structural layer 2. Specifically, as... Figure 1As shown, the angles (θ2, θ3) of the end faces (30, 31) on the ribbed waveguide 10 side of the third structural layer relative to the direction of light wave propagation (the same direction as arrow Lout) are set to be outside 90 degrees. For example, Figure 1 The angle θ2 can be set to 100 degrees to 170 degrees, and the angle θ3 can be set to 10 degrees to 80 degrees. θ2 and θ3 can also be opposite values. Furthermore, in Figure 1 In the middle, the end faces (30, 31) of the third structural layer are arranged in a straight line, but if Figure 4 As shown, end faces 30 and 31 can also be configured at an acute angle to form a gradually expanding width of the third structural layer. In this case, θ3 can be set to 100 degrees to 170 degrees.

[0061] In another embodiment, such as Figures 5 to 8 As shown, in the end face structure of the spot size conversion unit located at the end side of the optical waveguide substrate (1, 4), the first structural layer 1 and the third structural layer 3 are arranged such that the second structural layer 2 is not exposed. Figure 5 As shown, the width of the second structural layer 2 gradually narrows towards the end of the optical waveguide substrate (1, 4). Correspondingly, the third structural layer 3 undertakes the function of confining the light waves, therefore... Figure 5 As shown, the width W3 of the third structural layer is adjusted to, for example, 3μm to 5μm. This situation is illustrated in... Figure 5 The cross-sectional view at the single-dash line E-E' and the single-dash line F-F' is... Figure 6 (a) and Figure 6 In (b), the elliptical dashed line L shows the general outline of the MFD of the light wave.

[0062] A cross-sectional view at the dashed line F-F', which serves as the input or output of light. Figure 6 In (b)), the second structural layer 2 disappears, thus enabling it to function as an SSC containing two materials: the first structural layer 1 and the third structural layer 3 (thickness H3'). This structure is similar to... Figure 1 Compared to conventional SSCs, this method offers the following advantages: it can suppress the impact of the formation quality of the second structural layer 2, which serves as a micro-waveguide, on the characteristics of the SSC, thus expanding the manufacturing process margin. Furthermore, by omitting the second structural layer 2 on the substrate end face, the effective refractive index can be reduced, thereby effectively minimizing reflections. Moreover, by retaining the second structural layer 2 with a thinner tip near the light input or output end, the confinement effect of the second structural layer can be maintained, reducing the impact of the side roughness of the ridge of the third structural layer 3.

[0063] It can also be adopted Figure 7 The embodiment shown in the cross-sectional view is used instead of Figure 6 The cross-sectional view shown. Figure 7In this process, corresponding to the change in the width of the third structural layer 3, the width of the first structural layer 1 also changes. Therefore, the first and third structural layers can cooperate to perform optical confinement, and the shape of the MFD (Mean Displacement Factor) of the light wave can be stabilized. Furthermore, when propagating on the first and third structural layers, if the MFD increases to a predetermined level, such as... Figure 8 As shown, it is also possible to give a structure that gradually reduces the width of the third structural layer, or the third structural layer and the first structural layer, towards the light input or output end. Of course, it is also possible to gradually increase the width of the third structural layer, or the third structural layer and the first structural layer, to further expand the MFD of the light wave to be propagated.

[0064] Based on the above, the optical waveguide element of the present invention, by adjusting the shape and configuration of the first to third structural layers, not only functions as a spot size conversion unit, but also allows for a more relaxed positional accuracy in the configuration of each layer. In addition, by adjusting the shape / configuration of the second structural layer, the optical insertion loss caused by the surface roughness of the third structural layer and other layers can be reduced.

[0065] Furthermore, in the above description, the structure of each layer was described as a single structural layer. However, for example, at least one of the first to third structural layers can be constructed as a combination of two or more layers. In this case, the shape or material can be slightly modified to suitably cooperate with adjacent structural layers to form a suitable MFD.

[0066] Next, an optical modulation device and an optical transmission apparatus using the optical waveguide element of the present invention will be described.

[0067] The optical waveguide element further comprises modulation electrodes (not shown) on the optical waveguide substrate 1 (4) for modulating the light waves (L1, L2) propagating in the optical waveguide 10, and as shown in the figure. Figure 9 As shown, it is housed within a frame SC. Furthermore, by providing an optical fiber F that directs the input and output light waves relative to the optical waveguide, an optical modulation device MD can be constructed. Figure 9 In this design, the optical fiber is introduced into the frame through a through-hole in the side wall and directly connected to the optical waveguide element. The optical waveguide element and the optical fiber can also be optically connected via a space optical system. A spot size conversion unit (SSC1, SSC2) is provided at the input or output end of the optical waveguide 10.

[0068] An optical transmitting device (OTA) can be constructed by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal to the optical modulator MD. Since the modulation signal applied to the optical waveguide element needs to be amplified, a driver circuit (DRV) is used. The driver circuit (DRV) or the digital signal processor (DSP) can be configured either outside or inside the housing (SC). In particular, configuring the driver circuit (DRV) inside the housing further reduces the propagation loss of the modulation signals (So, S) from the driver circuit.

[0069] Industrial availability

[0070] As explained above, the present invention provides an optical waveguide element that includes a spot size conversion unit that does not complicate the manufacturing process and suppresses optical insertion loss. Furthermore, it enables the provision of optical modulation devices and optical transmission apparatuses using the optical waveguide element.

Claims

1. An optical waveguide element, characterized in that, include: Optical waveguide substrate, comprising a ribbed optical waveguide formed of a material having photoelectric effect; as well as The spot size conversion unit, located at the input or output end of the optical waveguide with the rib shape, changes the mode field diameter of the light wave propagating in the optical waveguide. in, The spot size conversion unit includes: The first structural layer includes a tapered portion that is connected to the ribbed optical waveguide and widens the optical waveguide. A second structural layer, stacked on top of the first structural layer, has a width narrower than that of the first structural layer; and The third structural layer is configured to cover the portion of the second structural layer that is close to the ribbed optical waveguide, and has a width wider than the second structural layer. The refractive index of the material constituting the second structural layer is higher than that of the material constituting the first structural layer or the material constituting the third structural layer. The refractive index difference between the first structural layer and the third structural layer is less than 0.

1.

2. The optical waveguide element according to claim 1, characterized in that, The front end portion of the rib-shaped optical waveguide side of the second structural layer is disposed on the rib-shaped optical waveguide.

3. The optical waveguide element according to claim 1 or 2, characterized in that, The end face of the rib-shaped optical waveguide side of the third structural layer is configured with a slope of more than 90 degrees relative to the direction of travel of the light wave propagating in the second structural layer.

4. The optical waveguide element according to claim 1 or 2, characterized in that, In the end face structure of the spot size conversion unit located at the end side of the optical waveguide substrate, the first structural layer and the third structural layer are arranged to surround the second structural layer.

5. The optical waveguide element according to claim 1 or 2, characterized in that, In the end face structure of the spot size conversion unit located at the end side of the optical waveguide substrate, the first structural layer and the third structural layer are configured such that the second structural layer is not exposed.

6. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide substrate includes: a thin plate on which the optical waveguide is formed and a holding substrate for holding the thin plate. The refractive index of the material constituting the substrate is lower than that of the material constituting the thin plate.

7. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide substrate includes: a thin plate on which the optical waveguide is formed, a holding substrate for holding the thin plate, and an intermediate layer between the thin plate and the holding substrate. The refractive index of the material constituting the intermediate layer is lower than that of the material constituting the thin plate.

8. An optical modulation device, characterized in that, include: The optical waveguide element as described in any one of claims 1 to 7, wherein the optical waveguide element comprises: a modulation electrode for modulating an optical wave propagating in the optical waveguide. The optical modulation device houses the optical waveguide element and an electronic circuit that amplifies the modulation signal input to the modulation electrode of the optical waveguide element within the frame.

9. An optical transmitting device, characterized in that, include: The optical modulation device as described in claim 8; as well as The electronic circuit outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

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