Optical waveguide element, optical modulator using optical waveguide element, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element that suppresses temperature drift in pattern-dense portions of an optical waveguide and electrodes. This optical waveguide element is provided with: an optical waveguide substrate (1) on which an optical waveguide (10) is formed; a reinforcing substrate (11) disposed on the lower side of the optical waveguide substrate (1); a lower buffer layer (B1) disposed between the optical waveguide substrate (1) and the reinforcing substrate (11) and joining the optical waveguide substrate (1) and the reinforcing substrate (11); and an upper buffer layer (B2) disposed on the upper side of the optical waveguide substrate (1) and disposed in contact with the optical waveguide substrate (1), 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).

Description

Technical Field

[0001] The present invention relates to an optical waveguide element, an optical modulation device using the optical waveguide element, and an optical transmission device, and particularly to an optical waveguide element including: an optical waveguide substrate on which an optical waveguide is formed; a reinforcing substrate disposed below the optical waveguide substrate; a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate to bond the two; and an upper buffer layer disposed above the optical waveguide substrate and in contact with the optical waveguide substrate. Background Art

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements such as optical modulators using an optical waveguide substrate on which an optical waveguide is 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 an electrode for applying an electric field to the optical waveguide is formed on the substrate.

[0003] Patent Documents 1 to 3 propose a structure in which buffer layers are disposed above and below the optical waveguide substrate so as to sandwich the optical waveguide substrate. Particularly, in the case where the optical waveguide substrate is a thin plate of several μm or less, a reinforcing substrate is disposed to enhance the mechanical strength of the optical waveguide substrate. The lower buffer layer functions to bond the optical waveguide substrate and the reinforcing substrate and to suppress absorption of light waves propagating in the optical waveguide by the reinforcing substrate.

[0004] In addition, the upper buffer layer functions as a protective film for suppressing the following situations: absorption of light waves propagating in the optical waveguide by the electrode disposed on the optical waveguide substrate, or scattering of light waves propagating in the optical waveguide due to the surface roughness of the optical waveguide.

[0005] In optical waveguide elements and optical modulation devices, miniaturization of the device itself is required. As a means for achieving this, the following scheme has been proposed: setting the height and width of the optical waveguide to about 1 μm or less, enhancing optical confinement, and arranging the optical waveguide in a bent manner.

[0006] In Patent Document 1, in the case of using a thin optical waveguide substrate of several μm or less, damage and characteristic deterioration of the optical waveguide substrate are caused due to the stress on the optical waveguide substrate generated by the buffer layer. Therefore, a technique in which the upper buffer layer and the lower buffer layer are made of the same material and the film thickness is also set to be the same is disclosed.

[0007] However, in the case where Si, SOI (a substrate having a silicon layer formed on silicon oxide, Silicon on Insulator), etc. are used for the reinforcing substrate, the refractive index of the reinforcing substrate is higher than that of the optical waveguide substrate, so that the optical absorption loss increases. Therefore, the thickness of the lower buffer layer needs to be set to about 2 to 3 μm.

[0008] In addition, as disclosed in Patent Document 1, when a dense SiO film with a thickness of 2 μm is formed on the substrate 1 of LN as a buffer layer, the LN substrate as an optical waveguide substrate peels off due to internal stress caused by the expansion and contraction of the buffer layer due to temperature changes. 2 When the thickness of the upper buffer layer B2 is formed to be thinner than the thickness of the lower buffer layer B1 (d2 < d1) as shown in Patent Document 3 in order to suppress the peeling of the optical waveguide substrate. Note that reference numeral 1 represents the optical waveguide substrate, reference numeral 10 represents the optical waveguide, and reference numeral 11 represents the reinforcing substrate.

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

[0010] In optical modulators such as high-bandwidth coherent modulators (HB-CDM: High Bandwidth-Coherent Driver Modulator), not only are the optical waveguide elements (chips) miniaturized, but also the optical waveguides and electrodes are miniaturized and densified. Therefore, if there is a difference in the linear expansion coefficient among the structural parts such as the optical waveguide substrate, electrodes, and buffer layer that make up the optical waveguide, internal stress is generated between the structural parts due to temperature changes, and so-called temperature drift in which the DC bias point of optical modulation changes easily occurs.

[0011] Prior Art Documents

[0012] Patent Documents

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-105650

[0014] Patent Document 2: Japanese Patent Application Laid-Open No. 2021-173792

[0015] Patent Document 3: PCT / JP2022 / 16292 (International Filing Date: March 30, 2022) Summary of the Invention

[0016] Outline of the Invention

[0017] Problems to be Solved by the Invention

[0018] The problem to be solved by the present invention is to provide an optical waveguide element that solves the above problems and suppresses temperature drift at the pattern-dense portions of optical waveguides and electrodes. An optical modulation device and an optical transmission device using the optical waveguide element are also provided.

[0019] Solutions for Solving the Problems

[0020] In order to solve the above problems, the optical waveguide element, optical modulation device, and optical transmission device of the present invention have the following technical features.

[0021] (1) An optical waveguide element, comprising: an optical waveguide substrate on which an optical waveguide is formed; a reinforcing substrate disposed on the lower side of the optical waveguide substrate; a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate to bond the two; and an upper buffer layer disposed on the upper side of the optical waveguide substrate and in contact with the optical waveguide substrate, wherein the linear expansion rate of the upper buffer layer is set to be larger than that of the lower buffer layer.

[0022] (2) In the optical waveguide element described in (1) above, wherein the upper buffer layer and the lower buffer layer are made of different materials.

[0023] (3) In the optical waveguide element described in (2) above, wherein the lower buffer layer is made of one material, and the upper buffer layer uses two or more materials in a layered or mixed state.

[0024] (4) In the optical waveguide element described in (3) above, wherein the upper buffer layer is formed in a layered form, and the layer closest to the optical waveguide substrate is set to have the largest linear expansion rate.

[0025] (5) In the optical waveguide element described in (1) above, wherein the linear expansion rate of the upper buffer layer is smaller than that of the optical waveguide substrate.

[0026] (6) In the optical waveguide element described in (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.

[0027] (7) In the optical waveguide element described in (6) above, wherein the thickness of the upper buffer layer is 1 μm or more.

[0028] (8) In the optical waveguide element described in (1) above, wherein the thickness of the lower buffer layer is set to be 1 μm or more, and the thickness of the upper buffer layer is set to be 1 μm or less.

[0029] (9) In the optical waveguide element described in (1) above, wherein the density of the lower buffer layer is higher than that of the upper buffer layer.

[0030] (10) In the optical waveguide element described in (1) above, wherein the resistivity of the upper buffer layer and the lower buffer layer is 10 8 Ωcm or more and 10 16 Ωcm or less.

[0031] (11) An optical modulation device, characterized in that the optical waveguide element described in any one of the above (1) to (9) is accommodated in a housing, and the optical modulation device includes an optical fiber that inputs or outputs light waves with respect to the optical waveguide.

[0032] (12) In the optical modulation device described in the above (11), characterized in that the optical waveguide element includes a modulation electrode for modulating light waves propagating in the optical waveguide, and an electronic circuit is provided inside the housing, and the electronic circuit amplifies a modulation signal input to the modulation electrode of the optical waveguide element.

[0033] (13) An optical transmission device, characterized by comprising: the optical modulation device described in the above (12); and an electronic circuit that outputs a modulation signal for causing the optical modulation device to perform a modulation operation.

[0034] Advantages of the Invention

[0035] The optical waveguide element of the present invention includes: an optical waveguide substrate on which an optical waveguide is formed; a reinforcing substrate disposed on the lower side of the optical waveguide substrate; a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate to bond the two; and an upper buffer layer disposed on the upper side of the optical waveguide substrate and in contact with the optical waveguide substrate. Among them, the linear expansion rate of the upper buffer layer is set to be larger than that of the lower buffer layer. Therefore, the stress exerted by the upper buffer layer on the optical waveguide substrate can be alleviated, and temperature drift can be suppressed.

[0036] In addition, by using an optical waveguide element having such excellent characteristics, an optical modulation device and an optical transmission device that exhibit the same effects can also be provided. Brief Description of the Drawings

[0037] Figure 1 It is a cross-sectional view showing an example of the optical waveguide element of Patent Document 3.

[0038] Figure 2 It is a cross-sectional view showing an example of the optical waveguide element of the present invention.

[0039] Figure 3 It is a diagram for explaining the state of the upper buffer layer ((a) layer state, (b) mixed state).

[0040] Figure 4 It is a diagram for explaining the manufacturing process of the optical waveguide element of the present invention.

[0041] Figure 5 It is for explaining Figure 4 The consecutive diagram of the manufacturing process.

[0042] Figure 6 It is a diagram showing another shape (the first one) of the upper buffer layer of the optical waveguide element of the present invention.

[0043] Figure 7 It is a diagram showing another shape (the second one) of the upper buffer layer of the optical waveguide element of the present invention.

[0044] Figure 8 It is a diagram showing another shape (the third one) of the upper buffer layer of the optical waveguide element of the present invention.

[0045] Figure 9 It is a coordinate diagram showing the relationship between the driving voltage (Vπ) applied to the optical waveguide and the change in the thickness of the upper buffer layer.

[0046] Figure 10 It is a top view for explaining the optical modulation device and the optical transmission device of the present invention. Detailed Description of the Invention

[0047] Hereinafter, a preferred example will be used to describe the optical waveguide element of the present invention in detail.

[0048] Figure 2 A cross-sectional view showing an example of the optical waveguide element of the present invention is shown.

[0049] The optical waveguide element of the present invention includes: an optical waveguide substrate 1 on which an optical waveguide 10 is formed; 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 the two; and an upper buffer layer B2 disposed above the optical waveguide substrate 1 and in contact with the optical waveguide substrate 1, characterized in that the linear expansion rate of the upper buffer layer B2 is set to be larger than the linear expansion rate of the lower buffer layer B1.

[0050] It should be noted that in the wafer-shaped substrate (optical waveguide substrate or reinforcing substrate), a plurality of optical waveguide elements are formed, and then the wafer is cut to form each optical waveguide element (chip). The optical waveguide element of the present invention also refers to the cut chip itself.

[0051] As the optical waveguide substrate 1 used in the optical waveguide element of the present invention, a substrate having an electro-optic effect can be used. Specifically, a substrate of a single crystal material such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a substrate material doped with MgO or the like can be used. Moreover, these materials can be formed into films by vapor growth methods such as sputtering, evaporation, or CVD. In addition, a semiconductor substrate or the like can also be used.

[0052] As a method for forming the optical waveguide 10, a rib-type optical waveguide can be used as follows: etching the substrate 1 other than the optical waveguide, or forming grooves on both sides of the optical waveguide, etc., so that a portion corresponding to the optical waveguide in the substrate is formed into a convex shape. In addition, corresponding to the rib-type optical waveguide, Ti, etc. can be diffused onto the substrate surface by a thermal diffusion method, a proton exchange method, etc., so that the refractive index is higher. Moreover, an optical waveguide can be formed by forming a high refractive index region obtained by thermally diffusing Ti, etc. on the substrate 1. However, in terms of improving optical confinement with a fine optical waveguide having a width and height of about 1 μm, a rib-type optical waveguide is more preferable.

[0053] In order to achieve the speed matching between the microwave and the light wave of the modulation signal, 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 further preferably 1 μm or less. Moreover, the height of the rib-type optical waveguide is set to 4 μm or less, more preferably 3 μm or less, and further preferably 1 μm or less, 0.4 μm or less. Generally, the thinner the optical waveguide substrate 1 is, the more easily it is affected by the stress generated by thin films such as the lower buffer layer and the upper buffer layer formed in contact with the optical waveguide substrate 1.

[0054] To improve the mechanical strength, as Figure 2 shown, a reinforcing substrate 11 is bonded to the lower side of the optical waveguide substrate 1 on which the optical waveguide is formed via a lower buffer layer B1. The lower buffer layer B1 is directly bonded to the reinforcing substrate 11 or is bonded and fixed via an adhesive layer such as resin. As the reinforcing substrate for direct bonding, a substrate having a refractive index lower than that of the optical waveguide and the substrate on which the optical waveguide is formed is preferable, but it is not limited thereto. Moreover, the reinforcing substrate preferably uses a substrate containing a material having a linear expansion rate close to that of the optical waveguide, such as an oxide layer of crystal, glass, etc. In addition, an LN substrate identical to the optical waveguide substrate, or a composite substrate having a silicon oxide layer formed on a silicon substrate abbreviated as SOI or LNOI, or a composite substrate having a silicon oxide layer formed on an LN substrate can also be used.

[0055] The optical waveguide element (chip) of the present invention is characterized in that the linear expansion rate of the upper buffer layer B2 is set to be larger than that of the lower buffer layer B1.

[0056] The material used for the buffer layer is preferably a material having a refractive index lower and a transparency higher than that of the optical waveguide substrate 1 such as LN. For example, SiO 2 , Al 2 O 3 , MgF 2 , La 2 O 3 , ZnO, HfO 2 , MgO, CaF 2 , Y 2 O 3Oxides and fluorides of metal elements in Groups 1 to 17 of the equal periodic table.

[0057] The buffer layer is formed into a film by various methods such as CVD and other vapor growth methods, sputtering, and vacuum evaporation. When forming a film by vapor growth or sputtering, the density of the film body increases compared to the vacuum-evaporated film. As described later, the density of the lower buffer layer B1 is set higher than the density of the upper buffer layer B2.

[0058] For example, SiO 2 has a linear expansion rate of 0.5×10 -6 / °C, and Al 2 O 3 has a linear expansion rate of 7.0×10 -6 / °C. Therefore, by using SiO 2 for the lower buffer layer B1 and Al 2 O 3 for the upper buffer layer B2, the linear expansion rate of the upper buffer layer B2 can be set to be larger than the linear expansion rate of the lower buffer layer B1. From another perspective, the present invention reduces the difference in linear expansion rate between the upper buffer layer and the optical waveguide substrate compared to the difference in linear expansion rate between the lower buffer layer and the optical waveguide substrate. Thus, it can also be said that the stress applied by the upper buffer layer to the optical waveguide substrate is alleviated.

[0059] The linear expansion rate of the LN substrate constituting the optical waveguide substrate 1 varies depending on the crystal orientation, but is, for example, 15.4×10 -6 / °C. Moreover, the linear expansion rate of Au constituting the electrode is 14.2×10 -6 / °C. Therefore, the linear expansion rates of buffer layers such as SiO 2 , Al 2 O 3 are generally smaller than the linear expansion rates of the optical waveguide substrate and the electrode.

[0060] The lower buffer layer B1 needs to firmly bond the optical waveguide substrate 1 and the reinforcing substrate 11, so it is formed with a high density by a single material. Therefore, stress caused by the difference in linear expansion rate is generated between the lower buffer layer B1 and the optical waveguide substrate 1.

[0061] In contrast, the upper buffer layer B2 is set to have a linear expansion rate larger than that of the lower buffer layer B1. Thus, the stress applied by the upper buffer layer B2 to the optical waveguide substrate 1 can be alleviated, and damage to the optical waveguide substrate and temperature drift phenomena can be suppressed.

[0062] When the linear expansion rate of the upper buffer layer B2 is equal to or less than that of the lower buffer layer B1, the optical waveguide substrate is in close contact with the lower buffer layer B1. Therefore, not only is a strong stress applied from the lower buffer layer B1, but also a large stress is applied from the upper buffer layer B2 with a larger difference in linear expansion rate. As a result, the damage to the optical waveguide substrate and the temperature drift phenomenon become more obvious.

[0063] Regarding the thickness of the upper or lower buffer layer, as Figure 2 shown, the thickness d2 of the upper buffer layer B2 is adjusted to be in the relationship of d2 < d1 compared to the thickness d1 of the lower buffer layer B1. This has the following effect: by making the thickness of the upper buffer layer B2 thin, similar to Patent Document 3, for example, the stress applied from the upper buffer layer to the optical waveguide substrate 1 is reduced. However, as will be described later, the optical waveguide element of the present invention is not limited to the relationship of d2 < d1, and for example, depending on the situation, the condition of d2 > d1 may also be possible.

[0064] The thickness of the lower buffer layer B1 is set to 1 μm or more, and in order to suppress the light absorption into the reinforcing substrate 11, it is more preferably set to 2 μm or more.

[0065] In addition, the lower buffer layer B1 also functions as a bonding layer for bonding to the reinforcing substrate 11, so a high density is preferred. Assuming that 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 decreases, and there is a risk of peeling between the reinforcing substrate 11 and the optical waveguide substrate (LN substrate) 1 when the surface of the optical waveguide substrate 1 is polished and made thin.

[0066] The thickness of the upper buffer layer B2 is set to 1 μm or less. If the upper buffer layer near the optical waveguide is thin, the stress on the optical waveguide can be more effectively alleviated, so it is more preferably set to 0.5 μm or less. However, in the optical waveguide element of the present invention, as will be described later, the thickness of the upper buffer layer B2 is not limited to 1 μm or less, and sometimes it may be greater than 1 μm depending on the conditions.

[0067] As described above, from the viewpoint of forming a high-density bonding layer, the lower buffer layer B1 is preferably composed of one material. In contrast, the upper buffer layer B2 can also be composed of one material such as the above-mentioned Al 2 O 3 as described above, but two or more materials can also be used in combination. By combining two materials, the linear expansion rate can be set higher.

[0068] When using two or more materials to form the upper buffer layer, methods such as Figure 3 (a) in which different buffer layers are laminated and overlapped, or as Figure 3(b), different materials (P1, P2) are used in a mixed state. Further, in order to further reduce the stress on the optical waveguide substrate 1 generated by the upper buffer layer B2, the density of the upper buffer layer B2 is preferably set lower than that of the lower buffer layer B1.

[0069] As Figure 3 (a), when the upper buffer layer is formed in a laminated state by overlapping, it is preferable that the layer closer to the optical waveguide substrate 1 is formed thinner. Thereby, the stress generated by the buffer layer (a part in the laminated state) in contact with the optical waveguide substrate 1 can be further reduced. Moreover, regarding the linear expansion coefficient of each layer, the closer to the optical waveguide substrate 1, the larger the linear expansion coefficient is set, and the difference in the linear expansion coefficient between the buffer layer (a part in the laminated state) closest to the optical waveguide substrate 1 and the optical waveguide substrate 1 can be further reduced. Therefore, the stress applied by the buffer layer closest to the optical waveguide substrate 1 to the optical waveguide substrate 1 can be alleviated.

[0070] Preferably, the resistivity of the upper or lower buffer layer is set to be 10 8 Ωcm or more and 10 16 Ωcm or less.

[0071] With the buffer layer having such resistivity, as described in Patent Document 1 or 2, the pyroelectric effect of the LN substrate etc., such as the DC drift phenomenon, can be suppressed. 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 resistivity.

[0072] Next, with reference to Figure 4 and 5 , the manufacturing process of the optical waveguide element of the present invention will be described.

[0073] In the first step (Step 1), with respect to the layer that becomes the optical waveguide substrate 1 (for example, the LN layer), the layer that becomes the lower buffer layer B1 (for example, SiO 2 etc.) is formed by sputtering or the like.

[0074] In the second step (Step 2), the lower surface of the layer that becomes the lower buffer layer B1 and the upper surface of the reinforcing substrate 11 are directly joined by the direct bonding method. It should be noted that the direct bonding method is a preferable method for bonding dissimilar materials. As shown in Patent Document 1, the plasma activation bonding method, the FAB (Fast Atom Beam) method, etc. can be appropriately used as the direct bonding method.

[0075] In the third step (Step 3), the upper side of the optical waveguide substrate 1 is polished to be processed into an appropriate thickness.

[0076] In the fourth step (Step 4), the portions other than the rib 10 are removed by, for example, dry etching, and an optical waveguide (10) is formed on the optical waveguide substrate 1.

[0077] In the fifth step (Step 5), an upper buffer layer B2 is formed on the optical waveguide substrate 1 by, for example, vacuum evaporation or the like.

[0078] In the sixth step (Step 6), electrodes 2 (for example, signal electrodes, ground electrodes, DC bias electrodes, etc.) are formed on the upper buffer layer B2, for example.

[0079] Regarding the optical waveguide element of the present invention, as Figure 2 shown, an example in which the upper buffer layer B2 is formed on the entire upper surface of the optical waveguide substrate 1 has been described, but the present invention is not limited thereto.

[0080] Figure 6 is a diagram in which the upper buffer layer B2 is formed only on the optical waveguide 10 and its vicinity. As Figure 6 such, if the upper buffer layer covers the optical waveguide, scattering of light waves caused by the surface roughness of the optical waveguide can be effectively suppressed. For example, it is preferable to form the upper buffer layer B2 with a width of 1.5 times or more of the mode field diameter (MFD) of the light wave propagating in the optical waveguide.

[0081] It may also be as Figure 7 such that the upper buffer layer B2 is formed so as to be disposed between electrodes 2 that sandwich the optical waveguide 10 in a wide range including the optical waveguide 10, for example. In Figure 6 this case, it is necessary to accurately align the upper buffer layer B2 with the position of the optical waveguide 10, but in Figure 7 this case, when considering the production position accuracy of the manufacturing process, the alignment accuracy can also be set relatively low.

[0082] As Figure 6 and Figure 7 shown, by narrowing the formation range of the upper buffer layer B2, the stress applied by the upper buffer layer B2 to the optical waveguide 10 and the optical waveguide substrate 1 can be suppressed. Of course, Figure 6 compared with Figure 7 the stress is smaller.

[0083] Figure 8 In, the upper buffer layer B2 covers the electrode 2.

[0084] As described above, the linear expansion rate of the material used for the buffer layer is lower than the linear expansion rates of the optical waveguide substrate (for example, LN) and the electrode (for example, Au). Preferably, the difference between the linear expansion rate of the optical waveguide substrate and the linear expansion rate of the electrode is smaller than the difference between the linear expansion rate of the optical waveguide substrate and the linear expansion rate of the upper buffer layer. Therefore, when the thickness d2 of the upper buffer layer becomes thicker, changing the material in contact with the optical waveguide substrate 1 from the upper buffer layer B2 to the electrode 2 can further suppress the stress applied to the optical waveguide substrate.

[0085] In addition, when the electrode 2 is disposed above the upper buffer layer B2 as Figure 2 shown, as the thickness d2 of the upper buffer layer B2 increases, it becomes difficult to efficiently apply the electric field formed by the electrode 2 to the optical waveguide 10.

[0086] Figure 9 The change in the electric field efficiency per unit length with respect to the thickness (μm) of the upper buffer layer B2 when the electrode spacing is variable and the optical loss is fixed is shown. The electric field efficiency is represented by the value per unit length of the driving voltage Vπ of the optical modulator (Vπ L : unit V / m).

[0087] Figure 9 Graph A is a graph when the upper buffer layer B2 is located below the electrode 2 (refer to Figure 2 ). Graph B is a graph when the upper buffer layer B2 is located above the electrode 2 (refer to Figure 8 ).

[0088] When observing Figure 9 graphs A and B, although the thickness d2 of the buffer layer at the intersection of graphs A and B where the driving voltage Vπ L is reversed also depends on the material used, it becomes a value of 1 μm or more, for example, 2 to 5 μm. For example, when the thickness d2 of the upper buffer layer is as thin as 1 μm or less, either the lower side or the upper side of the electrode 2 can be covered. However, when the thickness of the upper buffer layer B2 exceeds 1 μm, it is preferable to adopt a structure in which the upper buffer layer B2 is formed above the electrode 2 as Figure 8 shown.

[0089] Next, an example of applying the optical waveguide element of the present invention to an optical modulator device and an optical transmission device will be described. Hereinafter, an optical modulator device using the Figure 2 optical waveguide element shown will be described, but the present invention is not limited thereto, and it can also be applied to an optical phase modulator, an optical modulator having a polarization wave synthesis function, an optical waveguide element in which more Mach-Zehnder type optical waveguides are integrated, a bonding device bonded to an optical waveguide element made of other materials such as silicon, a device for sensor applications, etc. In addition, of course, it can also be applied to HB-CDM.

[0090] As Figure 10 shown, the optical waveguide element has an optical waveguide 10 formed on an optical waveguide substrate 1 and a modulation electrode (not shown) for modulating the light wave propagating in the optical waveguide 10, and is housed in a housing CA. In addition, an optical modulator device MD can be configured by providing an optical fiber (F) for inputting and outputting light waves to the optical waveguide. The optical fiber F can be optically coupled to the optical waveguide in the optical waveguide element using an optical block having an optical lens, a lens barrel, etc. In Figure 10In this case, the optical fiber F is introduced into the housing CA through a through-hole penetrating the side wall of the housing, and directly joined to the optical waveguide substrate 1. Further, in order to stably join the optical fiber and the optical block, a reinforcing member 3 may be arranged overlapping the end face of the optical waveguide substrate 1.

[0091] An optical transmission device OTA can be constituted by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So for causing the optical modulation device MD to perform a modulation operation to the optical modulation device MD. In order to obtain the modulation signal S applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. Therefore, in Figure 10 this case, a drive circuit DRV is used to amplify the modulation signal. The drive circuit DRV and the digital signal processor DSP may be arranged outside the housing CA, but may also be arranged inside the housing CA. In particular, by arranging the drive circuit DRV inside the housing, the propagation loss of the modulation signal from the drive circuit can be further reduced.

[0092] Industrial Applicability

[0093] As described above, according to the present invention, an optical waveguide element that suppresses temperature drift at a pattern dense portion of an optical waveguide and an electrode can be provided. An optical modulation device and an optical transmission device using the optical waveguide element can also be provided.

[0094] Reference Numeral Explanation

[0095] 1 Substrate (thin plate, film) for forming an optical waveguide

[0096] 10 Optical waveguide

[0097] 11 Reinforcing substrate

[0098] B1 Lower buffer layer

[0099] B2 Upper buffer layer

[0100] 13F Optical fiber

[0101] CA Housing

[0102] MD Optical modulation device

[0103] DRV Drive circuit

[0104] DSP Digital signal processor

[0105] OTA Optical transmission device

Claims

1. An optical waveguide element, comprising: an optical waveguide substrate on which an optical waveguide is formed; a reinforcing substrate disposed on the lower side of the optical waveguide substrate; a lower buffer layer disposed between the optical waveguide substrate and the reinforcing substrate to bond the two; and an upper buffer layer disposed on the upper side of the optical waveguide substrate and in contact with the optical waveguide substrate, characterized in that, the linear expansion rate of the upper buffer layer is set to be greater than that of the lower buffer layer.

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

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

4. The optical waveguide element according to claim 3, characterized in that, the upper buffer layer is formed in a layered form, and the layer closest to the optical waveguide substrate is set to have the largest linear expansion rate.

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

6. The 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, characterized in that, the thickness of the upper buffer layer is 1 μm or more.

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

9. The optical waveguide element according to claim 1, characterized in that, the density of the lower buffer layer is higher than that of the upper buffer layer.

10. The optical waveguide element according to claim 1, characterized in that, The resistivity of the upper buffer layer and the lower buffer layer is 10 8 Ωcm or more and 10 16 Ωcm or less.

11. An optical modulation device, characterized in that, the optical waveguide element according to any one of claims 1 to 10 is housed in a housing, and the optical modulation device includes an optical fiber for inputting or outputting light waves to or from the optical waveguide.

12. The optical modulation device according to claim 11, characterized in that, the optical waveguide element includes a modulation electrode for modulating light waves propagating in the optical waveguide, and an electronic circuit is provided inside the housing for amplifying a modulation signal input to the modulation electrode of the optical waveguide element.

13. An optical transmission device, characterized in that, it has: the optical modulation device according to claim 12; and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation.

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