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

Abstract

The objective of the present invention is to provide an optical waveguide element that suppresses temperature drift in a dense pattern area of an optical waveguide or an electrode.　An optical waveguide element according to the present invention is characterized by comprising: an optical waveguide substrate 1 forming an optical waveguide 10; a reinforcing substrate 11 disposed below the optical waveguide substrate 1; a lower buffer layer B1 which is disposed between the optical waveguide substrate 1 and the reinforcing substrate 11 and which bonds the two substrates together; and an upper buffer layer B2 disposed above the optical waveguide substrate 1 and in contact with the optical waveguide substrate 1, wherein the coefficient of linear expansion of the upper buffer layer B2 is set greater than the coefficient of linear expansion of the lower buffer layer B1.

Description

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

[0001] The present invention relates to an optical waveguide element, an optical modulation device using the same, and an optical transmission apparatus, and in particular to an optical waveguide element including an optical waveguide substrate having an optical waveguide formed therein, a reinforcing substrate disposed below the optical waveguide substrate, a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate and bonding them together, and an upper buffer layer disposed above the optical waveguide substrate and in contact with the optical waveguide substrate.

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements such as optical modulators using optical waveguide substrates on which optical waveguides are formed are widely used. In a typical optical waveguide element, an optical waveguide is formed on a substrate having an electro-optic effect, such as lithium niobate (LN), and electrodes for applying an electric field to the optical waveguide are formed on the substrate.

[0003] Patent Documents 1 to 3 propose a configuration in which buffer layers are arranged above and below an optical waveguide substrate so as to sandwich the optical waveguide substrate. In particular, when the optical waveguide substrate is a thin plate of a few micrometers or less, a reinforcing substrate is arranged to reinforce the mechanical strength of the optical waveguide substrate. The lower buffer layer serves to bond the optical waveguide substrate and the reinforcing substrate and to prevent the reinforcing substrate from absorbing light waves propagating through the optical waveguide.

[0004] The upper buffer layer is also a protective film that functions to prevent light waves propagating through the optical waveguide from being absorbed by electrodes arranged on the optical waveguide substrate, and to prevent light waves propagating through the optical waveguide from being scattered due to surface roughness of the optical waveguide.

[0005] Optical waveguide elements and optical modulation devices are required to be miniaturized, and as a means to achieve this, it has been proposed to set the height and width of the optical waveguide to approximately 1 μm or less, thereby strengthening the light confinement, and to arrange the optical waveguide in a bent manner.

[0006] Patent Document 1 discloses that when a thin optical waveguide substrate of several micrometers or less is used, the stress on the optical waveguide substrate caused by the buffer layer can damage the optical waveguide substrate and deteriorate its characteristics, and therefore discloses that the upper and lower buffer layers are made of the same material and have the same thickness.

[0007] However, when Si or SOI (a substrate in which a silicon layer is formed on silicon oxide, i.e., silicon on insulator) is used for the reinforcing substrate, the refractive index of the reinforcing substrate becomes higher than that of the optical waveguide substrate, which increases the optical absorption loss. For this reason, it is necessary to set the thickness of the lower buffer layer to about 2 to 3 μm.

[0008] Furthermore, as disclosed in Patent Document 1, a dense SiO 2 μm thick buffer layer is formed on the LN substrate 1. ２ When the film is formed, the LN substrate, which is the optical waveguide substrate, peels off due to internal stress caused by expansion and contraction of the buffer layer due to temperature changes.

[0009] In Patent Document 3, in order to suppress peeling of the optical waveguide substrate, it is proposed to form the thickness d2 of the upper buffer layer B2 to be thinner than the thickness d1 of the lower buffer layer B1 (d2<d1), as shown in Fig. 1. Note that reference numeral 1 denotes the optical waveguide substrate, reference numeral 10 denotes the optical waveguide, and reference numeral 11 denotes the reinforcing substrate.

[0010] In optical modulators such as high-bandwidth coherent driver modulators (HB-CDMs), not only are optical waveguide elements (chips) becoming smaller, but optical waveguides and electrodes are becoming finer and more densely packed. Therefore, if there is a difference in the linear expansion coefficients of the components that make up the optical waveguide, such as the optical waveguide substrate, electrodes, and buffer layer, internal stress is generated between the components due to temperature changes, and the DC bias point related to optical modulation fluctuates, which is known as temperature drift.

[0011] JP 2021-105650 A JP 2021-173792 A PCT / JP2022 / 16292 (International application date: March 30, 2022)

[0012] The problem to be solved by the present invention is to provide an optical waveguide element that solves the above-mentioned problems and suppresses temperature drift in areas where optical waveguides and electrodes are densely patterned, and further to provide an optical modulation device and an optical transmission device that use the optical waveguide element.

[0013] In order to solve the above problems, the optical waveguide element of the present invention, and the optical modulation device and optical transmission apparatus using the same have the following technical features: (1) In an optical waveguide element including an optical waveguide substrate having an optical waveguide formed thereon, a reinforcing substrate disposed below the optical waveguide substrate, a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate and bonding them together, and an upper buffer layer disposed above the optical waveguide substrate and in contact with the optical waveguide substrate, the linear expansion coefficient of the upper buffer layer is set to be larger than the linear expansion coefficient of the lower buffer layer.

[0014] (2) In the optical waveguide element described in (1) above, the upper buffer layer and the lower buffer layer are made of different materials.

[0015] (3) In the optical waveguide element described in (2) above, the lower buffer layer is made of one type of material, and the upper buffer layer is made of two or more types of materials, either layered or mixed.

[0016] (4) In the optical waveguide element described in (3) above, the upper buffer layer is formed in layers, and the layer closest to the optical waveguide substrate is set to have the largest linear expansion coefficient.

[0017] (5) The optical waveguide element according to (1) above, wherein the linear expansion coefficient of the upper buffer layer is smaller than the linear expansion coefficient of the optical waveguide substrate.

[0018] (6) The optical waveguide element according to (1) above, wherein an electrode is disposed on the upper side of the optical waveguide substrate, and the upper buffer layer is disposed so as to cover the optical waveguide and the electrode.

[0019] (7) The optical waveguide element according to (6) above, wherein the upper buffer layer has a thickness of 1 μm or more.

[0020] (8) In the optical waveguide element described in (1) above, the thickness of the lower buffer layer is set to 1 μm or more, and the thickness of the upper buffer layer is set to 1 μm or less.

[0021] (9) The optical waveguide element according to (1) above, wherein the density of the lower buffer layer is higher than the density of the upper buffer layer.

[0022] (10) In the optical waveguide element according to (1), the electrical resistivity of the upper buffer layer and the lower buffer layer is 10 ８ Ωcm or more, 10 １６ It is characterized by being Ωcm or less.

[0023] (11) The optical waveguide element according to any one of (1) to (9) above is an optical modulation device, characterized in that the optical waveguide element is housed in a housing and includes an optical fiber that inputs or outputs a light wave to or from the optical waveguide.

[0024] (12) In the optical modulation device described in (11) above, the optical waveguide element is provided with a modulation electrode for modulating a light wave propagating through the optical waveguide, and the housing has an electronic circuit therein for amplifying a modulation signal input to the modulation electrode of the optical waveguide element.

[0025] (13) An optical transmitter comprising the optical modulation device according to (12) above and an electronic circuit for outputting a modulation signal that causes the optical modulation device to perform a modulation operation.

[0026] The present invention provides an optical waveguide element comprising an optical waveguide substrate having an optical waveguide formed thereon, a reinforcing substrate disposed below the optical waveguide substrate, a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate and bonding them together, and an upper buffer layer disposed above the optical waveguide substrate and in contact with the optical waveguide substrate, wherein the linear expansion coefficient of the upper buffer layer is set to be greater than that of the lower buffer layer, thereby alleviating stress applied to the optical waveguide substrate by the upper buffer layer and suppressing temperature drift. Furthermore, by using an optical waveguide element having such excellent characteristics, it is possible to provide an optical modulation device or an optical transmission device that achieves similar effects.

[0027] FIG. 1 is a cross-sectional view showing an example of an optical waveguide element according to Patent Document 3. FIG. 2 is a cross-sectional view showing an example of an optical waveguide element of the present invention. FIG. 3 is a diagram explaining the state of the upper buffer layer ((a) layer state, (b) mixed state). FIG. 4 is a diagram explaining a manufacturing process for the optical waveguide element of the present invention. FIG. 5 is a diagram explaining a continuation of the manufacturing process of FIG. 4. FIG. 6 is a diagram showing another shape (part 1) of the upper buffer layer of the optical waveguide element of the present invention. FIG. 7 is a diagram showing another shape (part 2) of the upper buffer layer of the optical waveguide element of the present invention. FIG. 8 is a diagram showing another shape (part 3) of the upper buffer layer of the optical waveguide element of the present invention. FIG. 9 is a graph showing the relationship between the change in thickness of the upper buffer layer and the driving voltage (Vπ) applied to the optical waveguide. FIG. 10 is a plan view explaining an optical modulation device and an optical transmission device of the present invention.

[0028] The optical waveguide element of the present invention will be described in detail below using preferred embodiments. FIG. 2 shows a cross-sectional view of an example of an optical waveguide element of the present invention. The optical waveguide element of the present invention includes an optical waveguide substrate 1 having an optical waveguide 10 formed thereon, a reinforcing substrate 11 disposed below the optical waveguide substrate 1, a lower buffer layer B1 disposed between the optical waveguide substrate 1 and the reinforcing substrate 11 to bond them together, and an upper buffer layer B2 disposed above the optical waveguide substrate 1 and in contact with the optical waveguide substrate 1, wherein the linear expansion coefficient of the upper buffer layer B2 is set to be greater than the linear expansion coefficient of the lower buffer layer B1. The optical waveguide element is formed by forming a plurality of optical waveguide elements on a wafer-like substrate (optical waveguide substrate or reinforcing substrate), and then cutting the wafer to form individual optical waveguide elements (chips). The optical waveguide element of the present invention also refers to the cut chips themselves.

[0029] The optical waveguide substrate 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect. Specifically, substrates made of single crystal materials such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), or base materials made by doping these substrate materials with MgO or the like, can be used. Furthermore, these materials can also be used to form films using vapor phase growth methods such as sputtering, evaporation, or CVD. Furthermore, semiconductor substrates and the like can also be used.

[0030] The optical waveguide 10 can be formed by etching the substrate 1 other than the optical waveguide or by forming grooves on both sides of the optical waveguide, thereby forming a rib-type optical waveguide in which the portion of the substrate corresponding to the optical waveguide is 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 surface of the substrate by thermal diffusion or proton exchange. It is also possible to form an optical waveguide by forming a high refractive index region by thermally diffusing Ti or the like into the substrate 1, but a rib-type optical waveguide is more preferable for enhancing optical confinement in a fine optical waveguide with a width and height of about 1 μm.

[0031] The thickness of the substrate (thin plate) 1 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 velocity 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. Generally, the thinner the optical waveguide substrate 1, the more susceptible it is to stress caused by thin films such as the lower buffer layer and upper buffer layer formed in contact with the optical waveguide substrate 1.

[0032] To increase the mechanical strength of the optical waveguide substrate 1 having an optical waveguide formed thereon, a reinforcing substrate 11 is bonded to the underside of the optical waveguide substrate 1 via a lower buffer layer B1, as shown in FIG. 2 . The lower buffer layer B1 and the reinforcing substrate 11 are bonded directly or via an adhesive layer such as a resin. The reinforcing substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but is not limited thereto. Furthermore, the reinforcing substrate is preferably made of a material having a linear expansion coefficient close to that of the optical waveguide, such as a substrate containing an oxide layer of quartz or glass. Furthermore, it is also possible to use an LN substrate, like the optical waveguide substrate, or a composite substrate, abbreviated as SOI or LNOI, in which a silicon oxide layer is formed on a silicon substrate, or a composite substrate in which a silicon oxide layer is formed on an LN substrate.

[0033] The optical waveguide element (chip) of the present invention is characterized in that the linear expansion coefficient of the upper buffer layer B2 is set to be greater than the linear expansion coefficient of the lower buffer layer B1.

[0034] The material used for the buffer layer is preferably a material having a lower refractive index than the optical waveguide substrate 1, such as LN, and high transparency. For example, SiO ２ , Al ２ O ３ , MgF ２ , La ２ O ３ , ZnO, HfO ２ , MgO, CaF ２ , Y ２ O ３ Oxides and fluorides of metal elements in groups 1 to 17 of the periodic table can be used.

[0035] The buffer layer is formed by various methods, such as vapor deposition (CVD), sputtering, and vacuum deposition. When a film is formed by vapor deposition or sputtering, the density of the film is higher than that of a vacuum deposition film. As will be described later, the density of the lower buffer layer B1 is set higher than that of the upper buffer layer B2.

[0036] For example, the linear expansion coefficient is SiO ２ is 0.5 x 10 －６ / °C, Al ２ O ３ is 7.0 x 10 －６ / °C, the lower buffer layer B1 is made of SiO ２ and Al for the upper buffer layer B2. ２ O ３ By using the above, the linear expansion coefficient of the upper buffer layer B2 can be set to be larger than the linear expansion coefficient of the lower buffer layer B1. From another perspective, the present invention can be said to alleviate the stress that the upper buffer layer applies to the optical waveguide substrate by making the difference in linear expansion coefficient between the upper buffer layer and the optical waveguide substrate smaller than the difference in linear expansion coefficient between the lower buffer layer and the optical waveguide substrate.

[0037] The linear expansion coefficient of the LN substrate constituting the optical waveguide substrate 1 varies depending on the crystal direction. For example, －６ / °C. The linear expansion coefficient of Au constituting the electrode is 14.2 × 10 －６ / °C. Therefore, SiO ２ and Al ２ O ３ The linear expansion coefficient of the buffer layer is generally smaller than that of the optical waveguide substrate and the electrodes.

[0038] The lower buffer layer B1 is formed with a high density from a single material because it is necessary to firmly bond the optical waveguide substrate 1 and the reinforcing substrate 11. As a result, stress occurs between the lower buffer layer B1 and the optical waveguide substrate 1 due to the difference in linear expansion coefficient. In contrast, by making the upper buffer layer B2 a layer with a linear expansion coefficient greater than that of the lower buffer layer B1, the stress that the upper buffer layer B2 imparts to the optical waveguide substrate 1 can be alleviated, making it possible to suppress damage to the optical waveguide substrate and the temperature drift phenomenon.

[0039] If the linear expansion coefficient of the upper buffer layer B2 were equal to or smaller than that of the lower buffer layer B1, the optical waveguide substrate would be subjected to not only strong stress from the lower buffer layer B1 but also from the upper buffer layer B2, which has a larger difference in linear expansion coefficient, since the optical waveguide substrate is in close contact with the lower buffer layer B1. As a result, damage to the optical waveguide substrate and the temperature drift phenomenon would become more pronounced.

[0040] 2, the thickness d2 of the upper buffer layer B2 is compared with the thickness d1 of the lower buffer layer B1, and is adjusted so that the relationship d2<d1 holds. This serves to reduce the stress applied from the upper buffer layer to the optical waveguide substrate 1, for example, by making the upper buffer layer B2 thinner, as in Patent Document 3. However, as will be described later, the optical waveguide element of the present invention is not limited to the relationship d2<d1, and a condition such as d2>d1 is also possible in some cases.

[0041] The thickness of the lower buffer layer B1 is set to 1 μm or more, and more preferably 2 μm or more in order to suppress light absorption by the reinforcing substrate 11. Furthermore, since the lower buffer layer B1 also serves as a bonding layer with the reinforcing substrate 11, it is preferable that the lower buffer layer B1 has a high density. If the density of the lower buffer layer B1 is low (a sparse film), the bonding strength between the optical waveguide substrate 1 and the reinforcing substrate 11 will decrease, and there is a risk that the reinforcing substrate 11 and the optical waveguide substrate (LN substrate) 1 will peel off when the surface of the optical waveguide substrate 1 is polished to make it thinner.

[0042] The thickness of the upper buffer layer B2 is set to 1 μm or less, and more preferably 0.5 μm or less, because a thinner upper buffer layer near the optical waveguide can further relieve stress on the optical waveguide. However, in the optical waveguide element of the present invention, the thickness of the upper buffer layer B2 is not limited to 1 μm or less, as will be described later, and may be greater than 1 μm depending on the conditions.

[0043] As described above, the lower buffer layer B1 is preferably made of one material from the viewpoint of forming a high-density bonding layer. ２ O３ Although it is possible to use a single material, as shown in Fig. 1, it is also possible to use a combination of two or more materials. Combining two materials allows for a higher linear expansion coefficient.

[0044] When the upper buffer layer is formed using two or more types of materials, different buffer layers may be stacked one on top of the other as shown in Fig. 3(a) or different materials (P1, P2) may be mixed as shown in Fig. 3(b). In addition, in order to further reduce the stress caused by the upper buffer layer B2 on the optical waveguide substrate 1, it is preferable to set the density of the upper buffer layer B2 lower than that of the lower buffer layer B1.

[0045] 3A, when the upper buffer layer is formed by stacking layers, it is preferable to form the layers thinner as they are closer to the optical waveguide substrate 1. This makes it possible to reduce stress caused by the buffer layer (part of the layer) in contact with the optical waveguide substrate 1. Furthermore, the linear expansion coefficient of each layer is set to be larger as it is closer to the optical waveguide substrate 1, which makes it possible to reduce the difference in linear expansion coefficient between the buffer layer (part of the layer) closest to the optical waveguide substrate 1 and the optical waveguide substrate 1. This makes it possible to alleviate the stress applied to the optical waveguide substrate 1 by the buffer layer closest to the optical waveguide substrate 1.

[0046] The electrical resistivity of the upper or lower buffer layer is 10 ８ Ωcm or more, 10 １６ It is preferable to set the resistivity at Ωcm or less. As described in Patent Documents 1 and 2, the buffer layer having such an electrical resistivity makes it possible to suppress the pyroelectric effect of an LN substrate, such as the DC drift phenomenon. In particular, since an electrode is disposed on the upper side of the optical waveguide substrate 1, it is preferable that the upper buffer layer B2 has the above-mentioned electrical resistivity.

[0047] Next, the manufacturing process of the optical waveguide element of the present invention will be described with reference to FIGS. 4 and 5. In the first step (STEP 1), a layer (e.g., an LN layer) that will become the optical waveguide substrate 1 is laminated with a layer (e.g., a SiO ２etc.) is formed by sputtering or the like.

[0048] In the second step (STEP 2), the lower surface of the layer that will become the lower buffer layer B1 and the upper surface of the reinforcing substrate 11 are directly bonded by a direct bonding method. The direct bonding method is suitable for bonding different materials. As shown in Patent Document 1, the direct bonding method can be appropriately performed using a plasma activated bonding method or a fast atom beam (FAB) method.

[0049] In the third step (STEP 3), the upper side of the optical waveguide substrate 1 is polished and processed to an appropriate thickness.

[0050] In the fourth step (STEP 4), the portions other than the rib portions 10 are removed by, for example, dry etching, to form the optical waveguide (10) on the optical waveguide substrate 1.

[0051] In a fifth step (STEP 5), an upper buffer layer B2 is formed on the optical waveguide substrate 1 by, for example, vacuum deposition.

[0052] In a sixth step (STEP 6), electrodes 2 (for example, signal electrodes, ground electrodes, DC bias electrodes, etc.) are formed on, for example, the upper buffer layer B2.

[0053] In the optical waveguide element of the present invention, an example in which the upper buffer layer B2 is formed over the entire upper surface of the optical waveguide substrate 1 has been described as shown in Fig. 2, but the present invention is not limited to this. Fig. 6 shows an example in which the upper buffer layer B2 is formed only on the optical waveguide 10 and its vicinity. If the upper buffer layer covers the optical waveguide as shown in Fig. 6, it is possible to effectively suppress scattering of light waves due to the surface roughness of the optical waveguide, and it is desirable to form the upper buffer layer B2 with a width that is 1.5 times or more the mode field diameter (MFD) of the light propagating through the optical waveguide.

[0054] 7, it is also possible to form the upper buffer layer B2 so as to be disposed over a wide area including the optical waveguide 10, for example, between the electrodes 2 that sandwich the optical waveguide 10. In the case of Fig. 6, the upper buffer layer B2 needs to be precisely aligned with the position of the optical waveguide 10, but in the case of Fig. 7, it is possible to set the alignment accuracy relatively low, taking into account the fabrication position accuracy of the manufacturing process.

[0055] 6 and 7, by narrowing the area where the upper buffer layer B2 is formed, it is possible to suppress the stress that the upper buffer layer B2 applies to the optical waveguide 10 and the optical waveguide substrate 1. Naturally, the stress is smaller in FIG. 6 than in FIG. 7.

[0056] 8 shows a configuration in which the upper buffer layer B2 extends up to and covers the electrode 2. As described above, the linear expansion coefficient of the material used for the buffer layer is lower than that of the optical waveguide substrate (e.g., LN) and the electrode (e.g., Au). In fact, the difference between the linear expansion coefficient of the optical waveguide substrate and that of the electrode may be smaller than the difference between the linear expansion coefficient of the optical waveguide substrate and that of the upper buffer layer. Therefore, when the thickness d2 of the upper buffer layer is increased, changing the material in contact with the optical waveguide substrate 1 from the upper buffer layer B2 to the electrode 2 can better suppress the stress applied to the optical waveguide substrate.

[0057] 2, when the electrode 2 is disposed on the upper buffer layer B2, as the thickness d2 of the upper buffer layer B2 increases, it becomes more difficult for the electric field formed by the electrode 2 to be efficiently applied to the optical waveguide 10. FIG. 9 shows the change in the electric field efficiency per unit length with respect to the change in the thickness (μm) of the upper buffer layer B2 when the electrode spacing is variable and the optical loss is constant. The electric field efficiency is calculated by multiplying the value per unit length of the drive voltage Vπ of the optical modulator (Vπ Ｌ : unit V / m).

[0058] Graph A in FIG. 9 is a graph for the case where the upper buffer layer B2 is located below the electrode 2 (see FIG. 2). Graph B is a graph for the case where the upper buffer layer B2 is located above the electrode 2 (see FIG. 8). Looking at graphs A and B in FIG. 9, the thickness d2 of the buffer layer at the intersection of graphs A and B, where the drive voltage VπL of both graphs reverses, is 1 μm or more, and is a value between 2 and 5 μm, depending on the material used. For example, when the thickness d2 of the upper buffer layer is thin, such as 1 μm or less, it may cover either the bottom or top of the electrode 2. However, when the thickness of the upper buffer layer B2 exceeds 1 μm, it is preferable to adopt a configuration in which the upper buffer layer B2 is formed above the electrode 2, as shown in FIG. 8.

[0059] Next, we will explain examples in which the optical waveguide element of the present invention is applied to an optical modulation device or an optical transmission device. Below, we will explain an optical modulation device using the optical waveguide element shown in Figure 2, but the present invention is not limited to this and can also be applied to optical phase modulators, optical modulators with polarization combining functions, optical waveguide elements integrating more Mach-Zehnder type optical waveguides, junction devices with optical road elements made of other materials such as silicon, devices for sensor applications, etc. Furthermore, it goes without saying that it can also be applied to HB-CDM.

[0060] As shown in FIG. 10 , the optical waveguide element includes an optical waveguide 10 formed in an optical waveguide substrate 1 and a modulation electrode (not shown) that modulates the light waves propagating through the optical waveguide 10, and is housed within a housing CA. Furthermore, an optical fiber (F) that inputs and outputs light waves to and from the optical waveguide can be provided to form an optical modulation device MD. The optical fiber F can be optically coupled to the optical waveguide in the optical waveguide element using an optical block or lens barrel equipped with an optical lens. In FIG. 10 , the optical fiber F is introduced into the housing CA through a through-hole penetrating the sidewall of the housing CA and directly bonded to the optical waveguide substrate 1. Furthermore, to ensure stable bonding with the optical fiber or optical block, a reinforcing member 3 can be placed along the end face of the optical waveguide substrate 1.

[0061] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So that causes the optical modulation device MD to perform a modulation operation to the optical modulation device MD. To obtain a modulation signal S to be applied to the optical waveguide element, the modulation signal So output from the digital signal processor DSP must be amplified. For this reason, in FIG. 10 , a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and digital signal processor DSP can be located outside the housing CA, but they can also be located inside the housing CA. In particular, by locating the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.

[0062] As described above, according to the present invention, it is possible to provide an optical waveguide element that suppresses temperature drift in a pattern-dense area of ​​an optical waveguide or an electrode, and further to provide an optical modulation device and an optical transmission device that use the optical waveguide element.

[0063] REFERENCE SIGNS LIST 1 Substrate (thin plate, film) forming optical waveguide 10 Optical waveguide 11 Reinforcement substrate B1 Lower buffer layer B2 Upper buffer layer F Optical fiber CA Housing MD Optical modulation device DRV Driver circuit DSP Digital signal processor OTA Optical transmitter

Claims

1. An optical waveguide element comprising an optical waveguide substrate having an optical waveguide formed therein, a reinforcing substrate disposed below the optical waveguide substrate, a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate and bonding them together, and an upper buffer layer disposed above the optical waveguide substrate and in contact with the optical waveguide substrate, wherein the linear expansion coefficient of the upper buffer layer is set to be greater than the linear expansion coefficient of the lower buffer layer.

2. The optical waveguide element according to claim 1, wherein the upper buffer layer and the lower buffer layer are made of different materials.

3. An optical waveguide element according to claim 2, characterized in that the lower buffer layer is made of one type of material, and the upper buffer layer uses two or more types of materials in a layered or mixed state.

4. An optical waveguide element according to claim 3, characterized in that the upper buffer layer is formed in layers, and the layer closest to the optical waveguide substrate is set to have the largest linear expansion coefficient.

5. An optical waveguide element according to claim 1, characterized in that the linear expansion coefficient of said upper buffer layer is smaller than the linear expansion coefficient of said optical waveguide substrate.

6. An optical waveguide element according to claim 1, characterized in that an electrode is disposed on the upper side of the optical waveguide substrate, and the upper buffer layer is disposed so as to cover the optical waveguide and the electrode.

7. The optical waveguide element according to claim 6, wherein the thickness of the upper buffer layer is 1 μm or more.

8. An optical waveguide element according to claim 1, characterized in that the thickness of said lower buffer layer is set to 1 μm or more, and the thickness of said upper buffer layer is set to 1 μm or less.

9. An optical waveguide element according to claim 1, wherein the density of said lower buffer layer is higher than the density of said upper buffer layer.

10. In the optical waveguide element according to claim 1, the electrical resistivity of the upper buffer layer and the lower buffer layer is 10 ８ Ωcm or more, 10 １６ An optical waveguide element having a resistivity of Ωcm or less.

11. An optical waveguide element according to any one of claims 1 to 10, characterized in that the optical waveguide element is housed in a housing and is provided with an optical fiber for inputting or outputting a light wave to the optical waveguide.

12. An optical modulation device according to claim 11, wherein the optical waveguide element is provided with a modulation electrode for modulating a light wave propagating through the optical waveguide, and the housing includes an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element.

13. An optical transmitter comprising: an optical modulation device according to claim 12; and an electronic circuit for outputting a modulation signal for causing said optical modulation device to perform a modulation operation.

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