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

Abstract

The present invention provides an optical waveguide element that suppresses the occurrence of DC drift phenomena due to the diffusion of materials constituting the optical waveguide substrate, such as Li diffusion, even when an intermediate layer is present between the optical waveguide substrate and the support substrate. [Solution] The optical waveguide element comprises an optical waveguide substrate 1 having an optical waveguide OW, a support substrate 3 supporting the optical waveguide substrate, and an intermediate layer formed between the optical waveguide substrate and the support substrate, wherein the intermediate layer is composed of a high-density layer 21 disposed on the support substrate side and having a high density, and a low-density layer 20 disposed on the optical waveguide substrate side and having a lower density than the high-density layer.

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element, an optical modulation device and an optical transmitting device using the same, and more particularly to an optical waveguide element having an optical waveguide substrate, a support substrate for supporting the optical waveguide substrate, and an intermediate layer formed between the optical waveguide substrate and the support substrate, and to an optical modulation device and an optical transmitting device using the same. [Background technology]

[0002] In recent years, development has focused on high-bandwidth coherent driver modulators (HB-CDMs), and there is a growing demand for smaller optical waveguide elements, such as lithium niobate (LN) modulators, to be incorporated into HB-CDMs. To address this, optical waveguide elements are being designed with optical waveguide substrates (including films; also called functional layers) with optical waveguides formed on them, such as LN substrates, set to a thickness of approximately 1 μm. This approach aims to improve speed matching between the optical wave and the modulated signal, reduce the driving voltage of the modulated signal, and strengthen confinement to the optical waveguide.

[0003] As the thickness of the optical waveguide substrate decreases, its mechanical strength also decreases, requiring a support substrate to reinforce it. When a material with a higher refractive index than the optical waveguide substrate is used as the support substrate, propagation loss of light waves propagating through the waveguide occurs due to the support substrate, degrading the propagation characteristics. To suppress this, a composite substrate is used in which an intermediate layer (oxide film) is placed between the optical waveguide substrate and the support substrate.

[0004] As described in Patent Document 1, a schematic of a conventional optical waveguide element will be explained using Figure 1. The optical waveguide OW uses a dielectric material (optical waveguide substrate 1) with high dielectric constant and refractive index, such as LN or lithium tantalate (LT), which has been processed into a ridge or rib shape. The intermediate layer 2 uses a dielectric material with lower dielectric constant and refractive index compared to the optical waveguide substrate 1, such as SiO2, Al2O3, or Ta2O5. For the support substrate 3, a material with good processability, such as silicon, is preferably used, but glass or crystalline materials can also be used. The symbol EL is an electrode for applying an electric field to the optical waveguide OW. Conventionally, optical waveguides were formed in LN substrates by thermally diffusing Ti into the LN substrate, and optical waveguide substrates with a thickness of about 10 μm were used. In contrast, in HB-CDM, where ridge or rib-type optical waveguides are formed, the thickness of the LN substrate (film) is about 1 μm or less.

[0005] When an intermediate layer such as SiO2 is bonded to an LN substrate, which is an optical waveguide substrate, there is a possibility that Li in the LN substrate will diffuse into the intermediate layer, a phenomenon known as Li diffusion. In conventional LN substrate configurations with a thickness of approximately 10 μm, the distance between the thermal diffusion optical waveguide and the intermediate layer was large, so the effect of Li diffusion was not particularly observed. However, in HB-CDM, the thickness of the LN substrate (film) is about 1 μm or less, resulting in a close distance between the optical waveguide and the intermediate layer. Therefore, the effect of Li diffusion by the intermediate layer cannot be ignored. In particular, this effect becomes greater as the thickness of the LN substrate (thickness of the slab waveguide) decreases to 1 μm or less, 0.5 μm or less, and 0.2 μm or less. When the composition of the LN substrate changes, drift phenomena are more likely to occur when a DC voltage is applied. Therefore, in optical waveguide elements with thin LN substrates, it is necessary to suppress the Li diffusion phenomenon. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-131873 [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem that this invention aims to solve is to provide an optical waveguide element that solves the above-mentioned problems and suppresses the occurrence of DC drift phenomena due to the diffusion of materials constituting the optical waveguide substrate, such as Li diffusion, even when an intermediate layer is present between the optical waveguide substrate and the support substrate. Furthermore, the invention aims to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Means for solving the problem]

[0008] To solve the above problems, the optical waveguide element, optical modulation device and optical transmission device using the same of the present invention have the following technical features.

[0009] (1) An optical waveguide element having an optical waveguide substrate, a support substrate for supporting the optical waveguide substrate, and an intermediate layer formed between the optical waveguide substrate and the support substrate, wherein the intermediate layer is characterized in that it is composed of a high-density layer disposed on the support substrate side and having a high density, and a low-density layer disposed on the optical waveguide substrate side and having a density lower than that of the high-density layer.

[0010] (2) The optical waveguide element described in (1) above is characterized in that the hydroxyl group concentration of the low-density layer is lower than the hydroxyl group concentration of the high-density layer.

[0011] (3) The optical waveguide element described in (1) above is characterized in that the thickness of the intermediate layer is 1 μm or more.

[0012] (4) In the optical waveguide element described in (1) above, the thickness of the low-density layer is characterized in that it is thinner than the thickness of the high-density layer.

[0013] (5) The optical waveguide element described in (4) above is characterized in that the thickness of the low-density layer is 10 nm or more.

[0014] (6) The optical waveguide element according to (4) above, characterized in that the thickness of the high-density layer is 1 µm or more.

[0015] (7) The optical waveguide element according to (1) above, characterized in that the high-density layer is formed of a plurality of layers having different densities, and the density of any one of the layers is higher than the density of the low-density layer.

[0016] (8) The optical waveguide element according to (1) above, characterized in that the thickness of the optical waveguide substrate is 1 µm or less.

[0017] (9) The optical waveguide element according to (1) above, characterized in that a buffer layer covering the optical waveguide is provided on the optical waveguide substrate, and the density of a portion of the buffer layer in contact with the optical waveguide substrate is equal to or less than the density of the layer having the highest density among the intermediate layers.

[0018] (10) The optical waveguide element according to (9) above, characterized in that the buffer layer is constituted by a low-density buffer layer having a low density disposed on the optical waveguide substrate side, and a high-density buffer layer disposed on the low-density buffer layer and having a higher density than the low-density buffer layer.

[0019] (11) The optical waveguide element according to (10) above, characterized in that the thickness of the buffer layer is set to be equal to or less than the thickness of the intermediate layer.

[0020] (12) The optical waveguide element according to (10) above, wherein the thickness (h LB ) and density (ρ LB ) of the low-density buffer layer, the thickness (h HB ) and density (ρ HB ) of the high-density buffer layer, the thickness (h LI ) and density (ρ LI ) of the low-density layer of the intermediate layer, and the thickness (h HI ) and density (ρ HI ) of the high-density layer of the intermediate layer satisfy the following formula. (Formula) h LB ×ρ LB +h HB ×ρ HB ≦h LI ×ρ LI +h HI ×ρ HI

[0021] (13) The optical waveguide element according to (1) above, characterized in that a void is formed in at least a part of the low-density layer of the intermediate layer, including directly below the optical waveguide.

[0022] (14) An optical modulation device, characterized in that the optical waveguide element according to any one of (1) to (13) above is housed in a housing, and the optical modulation device comprises an optical fiber that inputs or outputs a light wave to the optical waveguide.

[0023] (15) The optical modulation device according to (14) above, characterized in that the optical waveguide element has a modulation electrode for modulating a light wave propagating through the optical waveguide, and an electronic circuit that amplifies a modulation signal input to the modulation electrode is provided inside the housing.

[0024] (16) An optical transmitter, characterized by comprising the optical modulation device according to (15) above, and an electronic circuit that outputs a modulation signal to the optical modulation device. [Effects of the Invention]

[0025] In an optical waveguide element comprising an optical waveguide substrate having an optical waveguide, a supporting substrate that supports the optical waveguide substrate, and an intermediate layer formed between the optical waveguide substrate and the supporting substrate, the intermediate layer is composed of a high-density layer with high density disposed on the supporting substrate side, and a low-density layer disposed on the optical waveguide substrate side with lower density than the high-density layer. Therefore, diffusion of the material constituting the optical waveguide substrate, such as Li diffusion, into the intermediate layer is suppressed, making it possible to suppress the occurrence of the DC drift phenomenon. Furthermore, it becomes possible to provide an optical modulation device and an optical transmitter that use an optical waveguide element having such characteristics. [Brief Description of the Drawings]

[0026] [Figure 1] This is a schematic cross-sectional view of a conventional optical waveguide element. [Figure 2] This is a schematic cross-sectional view of the optical waveguide element of the present invention. [Figure 3] This diagram illustrates phenomena near the interface between the optical waveguide substrate and the intermediate layer. [Figure 4] This figure illustrates an example in which the intermediate layer of the optical waveguide element of the present invention is composed of three or more layers. [Figure 5] This figure illustrates an example in which an air gap is provided in the optical waveguide element of the present invention. [Figure 6] This figure illustrates examples of various air gap shapes in the optical waveguide element of the present invention. [Figure 7] This figure illustrates an example of a method for forming an air gap in the optical waveguide element of the present invention. [Figure 8] This figure illustrates an example in which a buffer layer is provided in the optical waveguide element of the present invention. [Figure 9] This is a plan view showing an example of the optical waveguide element of the present invention. [Figure 10] This figure illustrates an example of an application of an optical waveguide element of the present invention that includes an air gap. [Figure 11] This figure shows an example of the optical transmission device of the present invention. [Modes for carrying out the invention]

[0027] The optical waveguide element of the present invention will be described in detail below using preferred examples. As shown in Figure 2, the optical waveguide element of the present invention comprises an optical waveguide substrate 1 having an optical waveguide OW, a support substrate 3 supporting the optical waveguide substrate, and an intermediate layer formed between the optical waveguide substrate and the support substrate, wherein the intermediate layer is composed of a high-density layer 21 disposed on the support substrate side and having a high density, and a low-density layer 20 disposed on the optical waveguide substrate side and having a density lower than that of the high-density layer.

[0028] The material of the optical waveguide substrate 1 used in the optical waveguide element of the present invention is a ferroelectric substrate. Specifically, substrates such as lithium niobate (LN) and lithium tantalate (LT), or substrates doped with magnesium, can be used. Vapor-grown films made from these materials can also be used. In this invention, the term "optical waveguide substrate" refers not only to a thin, polished plate-like body such as an LN substrate, but also to a film formed by vapor phase growth. In some cases, the layer that forms the optical waveguide, as in the case of an optical waveguide substrate, is called a "functional layer."

[0029] On the surface of the optical waveguide substrate 1, protrusions are formed as optical waveguides, forming a rib-type optical waveguide OW. Electrodes EL are formed on either side of the rib-type optical waveguide OW to apply a modulation signal and an electric field based on a DC bias to the optical waveguide OW. For purposes such as speed matching between the microwave and optical waves of the modulation signal, the thickness of the substrate 1 on which the optical waveguide is formed is set to 1 μm or less. Furthermore, by setting the height and width of the optical waveguide OW provided on the optical waveguide substrate to 1 μm or less, the confinement intensity of light can be increased, and the curvature of the optical waveguide can be increased (the bending radius can be decreased), making it suitable for miniaturizing optical waveguide elements. Moreover, as the size of the optical waveguide (height and width of the optical waveguide OW) decreases, electrodes can be placed closer to the optical waveguide, which reduces the driving voltage of the modulated signal and makes it possible to shorten the length of the part of the optical waveguide that is involved in the modulation operation and DC bias, thus contributing even more to miniaturization.

[0030] To increase the mechanical strength of the optical waveguide substrate 1, a support substrate 3 is placed below the optical waveguide substrate 1, with an intermediate layer in between. Suitable materials for the support substrate 3 include silicon substrates and other materials with good processability; crystalline materials such as glass or SiO2 can also be used. Furthermore, the intermediate layers (20,21) utilize dielectric materials with low dielectric constant and refractive index, such as SiO2, Al2O3, and Ta2O5.

[0031] A key feature of the optical waveguide element of the present invention is that the intermediate layer is composed of a high-density layer located on the support substrate side and having a high density, and a low-density layer located on the optical waveguide substrate side and having a lower density than the high-density layer. Note that the density referred to here is the molecular density in each layer. The lower the density of the intermediate layer, the more effectively it is possible to suppress the diffusion of the material constituting the optical waveguide substrate 1 (especially Li) into the intermediate layer (oxide film). However, a low density of the intermediate layer not only reduces the mechanical strength supporting the optical waveguide substrate, but also degrades the high-frequency characteristics of the microwave, which is the modulation signal. For this reason, in this invention, the intermediate layer on the optical waveguide substrate side is set to a low density, while the intermediate layer on the support substrate side is configured to a high density.

[0032] One of the reasons why elements such as Li diffuse into the intermediate layer is related to the concentration of hydroxyl groups (OH) contained in the intermediate layer. Hereafter, the concentration of hydroxyl groups (OH) will be referred to as the hydroxyl group concentration. Figure 3 shows an example in which an LN substrate is used as the optical waveguide substrate 1 and an SiO2 layer is used as the intermediate layer. In Figure 3(a), a low-density layer 20 is used as the intermediate layer, and in Figure 3(b), a high-density layer 21 is used as the intermediate layer.

[0033] Li in the LN substrate undergoes a chemical reaction with hydroxyl groups (OH) in the intermediate layer. The SiO2 film forming the intermediate layer is created by methods such as sputtering and vapor deposition, as described later. Analysis of the film's composition revealed the presence of hydroxyl groups (OH) in the intermediate layer. The presence of these hydroxyl groups is thought to influence the diffusion of Li. In particular, as schematically shown in Figures 3(a) and (b), the higher the density of the oxide film forming the intermediate layer, the higher the hydroxyl group concentration, leading to a more rapid chemical reaction between Li and OH, and the diffusion of Li into the oxide film. As a result, the composition within the LN substrate changes, making drift more likely when a DC voltage is applied. Therefore, the hydroxyl group concentration of the low-density layer placed on the optical waveguide substrate is set lower than the hydroxyl group concentration of the high-density layer placed on the support substrate side. This suppresses the diffusion of Li from the LN substrate to the oxide film side, thereby suppressing the drift phenomenon when a DC voltage is applied.

[0034] The molecular density of the intermediate layer can be changed depending on the film deposition method. For example, vacuum deposition is suitable for forming relatively low-density films, followed by ion-assisted vacuum deposition, sputtering, and CVD (PVCVD) in order of suitability for forming high-density films. Also, during film formation, lower temperatures result in higher density than higher temperatures, and dry processing tends to result in higher density than wet processing. Therefore, to form layers of different densities within a single intermediate layer, the deposition method or deposition conditions for each layer must be changed.

[0035] Furthermore, various methods can be selected for integrally forming the optical waveguide substrate and the support substrate, such as sequentially forming a high-density layer and a low-density layer on the support substrate and joining the LN layer; forming a high-density layer on the support substrate and a low-density layer on the LN substrate and joining the two; or sequentially forming a low-density layer and a high-density layer on the LN substrate (on the support substrate side) and joining it to the support substrate.

[0036] Furthermore, as shown in Figure 4, the intermediate layer can be composed of three or more layers of different densities. This divides the high-density layer into layers of different densities, with the lowest-density layer placed on the optical waveguide substrate side. As a result, the high-density layer is formed from multiple layers of different densities, but the density of each layer is higher than that of the low-density layer. As shown in Figure 4, for example, arranging the layers in the order of low-density layer 20, medium-density layer (a low-density layer within the high-density layer) 210, and high-density layer (a high-density layer within the high-density layer) 211 from the optical waveguide substrate side can reduce the internal stress (stress corresponding to temperature changes due to differences in linear expansion coefficients) generated at the boundaries of each layer. However, if the intermediate layer in contact with the optical waveguide substrate is a low-density layer, the arrangement of the layers other than the low-density layer can be reversed, with the high-density and medium-density layers being reversed. In this explanation, we have described a three-layer structure consisting of a low-density layer, a medium-density layer, and a high-density layer. However, the number of layers other than the low-density layer may be more than two. In this case, it is desirable to form the low-density layer on the optical waveguide substrate side and arrange the other high-density layers in order of increasing density, but the density of each high-density layer may not be arranged in this order.

[0037] Next, we will explain the thickness of the intermediate layer itself and each layer that makes up the intermediate layer. One reason for providing an intermediate layer is to suppress the absorption of light waves propagating through the optical waveguide by the support substrate. Therefore, the overall thickness of the intermediate layer is preferably 1 μm or more, more preferably 2 μm or more. However, since forming a thicker intermediate layer can easily cause stress (warping) in the optical waveguide substrate, the overall thickness of the intermediate layer is set to 10 μm or less, more preferably 5 μm or less.

[0038] Because low-density layers have weaker ion power that impacts the target during film deposition, they have the effect of reducing damage to the optical waveguide substrate and mitigating interface stress (warpage) compared to high-density layers. Furthermore, during or after bonding with the support substrate, the low-density layer acts as a buffer, and the effect of stress relief on the optical waveguide substrate is also obtained; therefore, a thicker low-density layer is preferable. On the other hand, from the viewpoint of the diffusion range of Li in the low-density layer, if the low-density layer is too thin, Li will diffuse to the high-density layer; therefore, it is desirable to set the thickness of the low-density layer to 10 nm or more, more preferably 50 nm or more, and even more preferably 100 nm or more. However, as will be described later, the high-density layer contributes to the improvement of high-frequency characteristics, so in order to obtain that improvement effect, the thickness of the low-density layer is set to 5 μm or less.

[0039] A higher density in the intermediate layer improves the high-frequency characteristics of the modulated signal propagating through the electrodes and also increases the bonding strength (adhesion) between the optical waveguide substrate and the support substrate. For this reason, a thicker high-density layer, such as 1 μm or more, 3 μm or more, or 5 μm or more, is desirable. However, considering the effect of stress (warpage) on the optical waveguide substrate, it is preferable to set it to less than 10 μm. Furthermore, from the viewpoint of further improving high-frequency characteristics and bonding strength, it is preferable to set the thickness of the low-density layer to be thinner than the thickness of the high-density layer.

[0040] Next, as shown in Figures 5 to 7, we will explain how to form a void OP in at least a portion of the low-density layer of the intermediate layer, including directly beneath the optical waveguide. By providing an air gap OP in the intermediate layer directly beneath the optical waveguide substrate 1, the effects of Li diffusion can be reduced.

[0041] Figure 5 shows an example of an optical waveguide element with a gap formed therein. Specifically, a gap OP is provided directly beneath the optical waveguide OW on the optical waveguide substrate 1, at the location of the low-density layer 20, with a gap approximately the same width as the optical waveguide. By providing a gap OP, damage to the optical waveguide substrate and stress (warpage) at the interface that occurs during the formation of the intermediate oxide film and bonding with the support substrate can be reduced. Furthermore, since the mode field diameter L radiates outward from the center of the optical waveguide, the effect is greatest when the gap is formed directly beneath the optical waveguide, and the effect decreases as the gap is formed further away from the optical waveguide.

[0042] While an air gap can be effective even if only a portion is formed directly beneath the optical waveguide, the width of the air gap should be equal to the width of the optical waveguide (the width of the lower base of the trapezoidal cross-section of the rib-type optical waveguide) (see Figure 5). Furthermore, the mode field diameter can expand to about 1.5 times the lower base of the trapezoidal optical waveguide. Therefore, a higher effect can be obtained by widening the air gap to 1.5 times the aforementioned width, and even to the gap width of the electrode EL (see Figure 6(d)).

[0043] Figure 6 shows examples of various void patterns. Figure 6(a) shows an example in which a gap OP is formed to penetrate the intermediate layer, and it is possible to relieve the stress at the interface between the support substrate 3 and the intermediate layer 21.

[0044] Figure 6(b) shows that a void has formed by the material penetrating a portion of the high-density layer, but the void has not penetrated the high-density layer. By leaving the high-density layer intact, it is possible to suppress the decrease in the overall mechanical strength of the device.

[0045] Figure 6(c) shows a configuration in which shallow voids are formed only in the low-density layer, which allows for even higher mechanical strength than in Figure 6(b). From the viewpoint of the overall mechanical strength of the optical waveguide element, it is preferable to make the void depth as shallow as possible, ideally 10 nm or more.

[0046] Figure 6(d) shows a configuration where the gap is widened to the width of the electrode EL, suppressing Li diffusion over a wide area for light waves spreading from the optical waveguide. Furthermore, because the dielectric of the intermediate layer is not located beneath the optical waveguide substrate, it becomes possible to efficiently apply an electric field to the optical waveguide.

[0047] Figure 6(e) shows the case where the width of the air gap OP is set to less than half the width of the optical waveguide. Although the function of suppressing Li diffusion is reduced, the mechanical strength of the optical waveguide element is the highest among Figures 6(a) to (e).

[0048] Voids can be formed on the oxide film (intermediate layer) before bonding by methods such as mask deposition, lift-off, or dry etching. Furthermore, as shown in Figure 7, after joining the optical waveguide substrate and the support substrate, multiple through-holes HL are formed in the optical waveguide substrate on both sides of the optical waveguide OW. These through-holes HL may also be formed before joining with the support substrate. Then, using these through-holes HL, a chemical agent that is resistant to the optical waveguide substrate 1 and has an etching effect on the intermediate layer (oxide film) is injected. As a result, a portion of the intermediate layer can be removed by the chemical agent that enters through the through-holes HL, and a void can be formed. The left side of Figure 7 is a plan view of a part of the optical waveguide element, and the right side is a cross-sectional view along the dashed line AA in the plan view.

[0049] In optical waveguide elements, the most effective location for forming air gaps is the DC application section where a DC bias, prone to drift, is applied. The effect can also be obtained in a portion of the DC application section or in the RF section where the modulation signal is applied. Furthermore, even in areas where no air gaps are formed, drift suppression can be achieved by using a two-layer oxide film including a low-density layer. Additionally, considering mechanical strength, a thicker optical waveguide substrate allows for a larger air gap volume.

[0050] Next, we will explain the case where a buffer layer is provided, as shown in Figure 8. An oxide buffer layer can also be formed directly above or on the sides of the optical waveguide OW. For the buffer layer on the surface in contact with the optical waveguide OW, phenomena such as Li diffusion may occur, similar to the intermediate layer described above. Because this raises concerns about drift, the buffer layer is made up of two layers, a low-density buffer layer B1 and a high-density buffer layer B2, as shown in Figure 8. Naturally, it is also possible to construct it with three or more layers of different densities. Furthermore, it is effective for the buffer layer to cover the optical waveguide OW as shown in Figure 8, or it may cover the electrode EL as shown in Figure 8, or it may be formed extending below the electrode EL, or it may not extend above or below the electrode EL. This buffer layer configuration suppresses Li diffusion from the optical waveguide (optical waveguide substrate) and allows for good high-frequency characteristics. Furthermore, it reduces damage to the optical waveguide substrate during film deposition. In addition, the buffer layer can suppress light scattering loss caused by roughness on the sides of the optical waveguide that occurs when processing the optical waveguide into a rib shape.

[0051] The density of the low-density buffer layer (the portion of the buffer layer in contact with the optical waveguide substrate) is set to be the same as, or less than, the density of the densest layer in the intermediate layer. This is to ensure that the magnitude of the internal stress exerted by the buffer layer on the optical waveguide (optical waveguide substrate) is smaller than the magnitude of the internal stress exerted by the intermediate layer on the optical waveguide substrate.

[0052] The buffer layer thickness is preferably in the range of 50 to 1,000 nm. If the buffer layer is too thick, the mode field diameter (MFD) widens and approaches the electrode EL, which causes optical propagation loss (electrode absorption of light). Furthermore, it is preferable to make the high-density buffer layer B2, which is advantageous for high-frequency characteristics, thicker than the low-density buffer layer B1. However, the light scattering suppression effect can also be obtained by forming the low-density buffer layer alone. Furthermore, the buffer layer is significantly affected by internal stress on the optical waveguide, and it is preferable for it to be thinner than the intermediate layer. For this reason, the thickness of the buffer layer (total thickness of the buffer layer) is set to be less than or equal to the thickness of the intermediate layer (total thickness of the intermediate layer).

[0053] For reference, here is an example of the thickness of each layer that makes up the buffer layer and intermediate layer. Thickness of low-density buffer layer: 50nm~1,000nm Thickness of high-density buffer layer: 0nm~950nm Thickness of the intermediate layer (low-density layer): 10 nm or more, 5 μm or less Thickness of the intermediate layer (high-density layer): 1 μm or more, less than 10 μm

[0054] Since the magnitude of internal stress is affected by density and layer thickness, it is preferable to set the thickness (hLB) and density (ρLB) of the low-density buffer layer, the thickness (hHB) and density (ρHB) of the high-density buffer layer, the thickness (hLI) and density (ρLI) of the low-density intermediate layer, and the thickness (hHI) and density (ρHI) of the high-density intermediate layer to satisfy the following equations. (Formula) hLB×ρLB+hHB×ρHB≦hLI×ρLI+hHI×ρHI This makes it possible to set the magnitude of the internal stress caused by the buffer layer to be smaller than the internal stress caused by the intermediate layer.

[0055] Figure 9 is a plan view of an optical waveguide element, which includes a folded optical waveguide OW. Within this optical waveguide OW, the dotted line frame DCP represents the region where a DC bias is applied (DC application section), and the dotted line frame RFP represents the region where the microwave of the modulation signal is applied (RF section). When forming a void, the processing load can be reduced by limiting it to the DC application area (DCP) or a part of the DC application area where drift is likely to occur. Furthermore, the intermediate layer can be formed over the entire surface of the optical waveguide substrate, while the buffer layer can be formed on only a portion of the optical waveguide substrate (optical waveguide).

[0056] Figure 10 shows an example of the application of air gaps formed in optical waveguide elements. Not only can air gaps be provided for each optical waveguide, but as shown in Figure 10, it is also possible to provide countermeasures for multiple optical waveguides with a single air gap OP. This expands the degree of freedom in processing the air gaps.

[0057] Next, we will describe examples of applying the optical waveguide element of the present invention to optical modulation devices and optical transmitting devices. In the following description, we will use an example of HB-CDM, 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 or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide elements made of other materials such as silicon, and devices for sensor applications.

[0058] As shown in Figure 11, the optical waveguide element is housed within the housing CA. Furthermore, by providing an optical fiber (F) for inputting and outputting optical waves to the optical waveguide, an optical modulation device MD can be constructed.

[0059] In Figure 11, the optical fiber F is introduced into the housing CA through a through-hole penetrating the side wall of the housing CA, and the optical fiber is directly joined to the optical component or substrate. However, the optical fiber and the optical waveguide element may also be optically coupled via an optical block equipped with an optical lens optically coupled to the optical waveguide within the optical waveguide element, a lens barrel, a polarization multiplexer, etc. Furthermore, to ensure stable bonding with the optical fiber and optical block, reinforcing members can be stacked and arranged along the end face of the optical waveguide substrate.

[0060] An optical transmitter (OTA) can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So to the optical modulation device MD to perform modulation operations on the optical modulation device MD. In order to obtain the modulation signal S to be applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. For this reason, in Figure 11, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and the digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit. [Industrial applicability]

[0061] As described above, according to the present invention, even when an intermediate layer is present between the optical waveguide substrate and the support substrate, it is possible to provide an optical waveguide element that suppresses the occurrence of DC drift phenomena due to the diffusion of materials constituting the optical waveguide substrate, such as Li diffusion. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Explanation of Symbols]

[0062] 1 Optical waveguide substrate 2. Middle class 20 Low density layer (middle layer) 21 High density layer (middle layer) B1 Low-density buffer layer B2 High-Density Buffer Layer OW Optical Waveguide F Optical Fiber CA cabinet MD Optical Modulation Device DRV driver circuit DSP (Digital Signal Processor) OTA Optical Transmitter

Claims

1. An optical waveguide element having an optical waveguide substrate having an optical waveguide, a support substrate for supporting the optical waveguide substrate, and an intermediate layer formed between the optical waveguide substrate and the support substrate, The optical waveguide element is characterized in that the intermediate layer is disposed on the support substrate side and consists of a high-density layer with a high density and a low-density layer disposed on the optical waveguide substrate side and having a lower density than the high-density layer.

2. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that the hydroxyl group concentration in the low-density layer is lower than the hydroxyl group concentration in the high-density layer.

3. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that the thickness of the intermediate layer is 1 μm or more.

4. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that the thickness of the low-density layer is thinner than the thickness of the high-density layer.

5. In the optical waveguide element according to claim 4, An optical waveguide element characterized in that the thickness of the low-density layer is 10 nm or more.

6. In the optical waveguide element according to claim 4, An optical waveguide element characterized in that the thickness of the high-density layer is 1 μm or more.

7. In the optical waveguide element according to claim 1, The optical waveguide element is characterized in that the high-density layer is formed of multiple layers with different densities, and the density of each layer is higher than the density of the low-density layer.

8. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that the thickness of the optical waveguide substrate is 1 μm or less.

9. In the optical waveguide element according to claim 1, A buffer layer covering the optical waveguide is provided on the optical waveguide substrate. An optical waveguide element characterized in that the density of the portion of the buffer layer in contact with the optical waveguide substrate is the same as or less than the density of the densest layer in the intermediate layer.

10. In the optical waveguide element according to claim 9, The optical waveguide element is characterized in that the buffer layer is composed of a low-density buffer layer with a low density disposed on the optical waveguide substrate side and a high-density buffer layer disposed on the low-density buffer layer with a higher density than the low-density buffer layer.

11. An optical waveguide element according to claim 10, characterized in that the thickness of the buffer layer is set to be less than or equal to the thickness of the intermediate layer.

12. In the optical waveguide element according to claim 10, the thickness (h) of the low-density buffer layer ＬＢ ) and density (ρ ＬＢ ), the thickness of the high-density buffer layer (h ＨＢ ) and density (ρ ＨＢ ), the thickness of the low-density layer in the intermediate layer (h ＬＩ ) and density (ρ ＬＩ ), and the thickness of the high-density layer of the intermediate layer (h ＨＩ ) and density (ρ ＨＩ The optical waveguide element is characterized by satisfying the following equation. (Formula) h ＬＢ ×ρ ＬＢ +h ＨＢ ×ρ ＨＢ ≦h ＬＩ ×ρ ＬＩ +h ＨＩ ×ρ ＨＩ

13. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that a gap is formed in at least a portion of the low-density layer of the intermediate layer, including directly beneath the optical waveguide.

14. The optical waveguide element according to any one of claims 1 to 13 is housed in a housing, An optical modulation device characterized by comprising an optical fiber that inputs or outputs an optical wave to the optical waveguide.

15. In the optical modulation device according to claim 14, The optical waveguide element has a modulation electrode for modulating the light wave propagating through the optical waveguide. An optical modulation device characterized by having an electronic circuit inside the housing that amplifies the modulation signal input to the modulation electrode.

16. The optical modulation device according to claim 15, An optical transmitting device characterized by having an electronic circuit that outputs a modulation signal to the optical modulation device.

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