Optical waveguide devices

Abstract

The present invention provides an optical waveguide device capable of suppressing both temperature drift and DC drift. The optical waveguide device comprises an optical waveguide substrate (1) joined to a holding substrate (4), wherein the optical waveguide substrate (1) is a crystal having a photoelectric effect and anisotropic thermal expansion coefficient, has a thickness of 10 μm or less, and is provided with an optical waveguide (3), and the holding substrate (4) holds the optical waveguide substrate. The optical waveguide device is characterized in that the holding substrate is formed of a crystal having a lower dielectric constant than that of the optical waveguide substrate and anisotropic thermal expansion coefficient, and the optical waveguide substrate and the holding substrate are joined in such a manner that the difference in thermal expansion coefficient between the optical waveguide substrate and the holding substrate is reduced in different axial directions on the joining surface.

Description

Technical Field

[0001] The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device formed by bonding an optical waveguide substrate to a holding substrate that holds the optical waveguide substrate. The optical waveguide substrate is a crystal having a photoelectric effect and anisotropic thermal expansion coefficient, has a thickness of 10 μm or less, and includes an optical waveguide. Background Art

[0002] In the fields of optical communications and optical measurement, optical waveguide devices such as optical modulators, which feature optical waveguides formed on substrates with a photoelectric effect, such as lithium niobate (LN), are widely used. In order to achieve broadband optical modulators using LN, it is necessary to match the speed of the modulating electrical signal (microwaves) with the light waves propagating through the optical waveguide, and to reduce the driving voltage for the optical modulator. As a solution to this problem, a method has been adopted to thin the optical waveguide substrate and place a layer with a lower dielectric constant than the optical waveguide substrate immediately below the optical waveguide and modulation electrodes.

[0003] In this case, due to internal stress caused by the difference in linear expansion coefficient between the thinned optical waveguide substrate and the low-dielectric constant layer immediately below it, the thinned optical waveguide substrate may peel or crack, or the optical modulator characteristics may deteriorate due to temperature drift. To suppress this degradation, methods have been proposed, such as using an adhesive with a linear expansion coefficient close to that of the optical waveguide substrate and the low-dielectric constant layer (see Patent Document 1) and using a resin substrate as a holding substrate for holding the optical waveguide substrate (see Patent Documents 2 or 3).

[0004] Furthermore, LN crystal is a trigonal crystal, and its linear expansion coefficient differs between the direction perpendicular to the Z axis (the

[001] plane) and the horizontal direction (the

[100] plane). Specifically, the linear expansion coefficient of LN is 4 ppm / °C in the Z axis direction and 15 ppm / °C in the X and Y axis directions. Therefore, for X-cut substrates cut horizontally along the Z axis, the linear expansion coefficient is anisotropic on the substrate surface that serves as the bonding surface.

[0005] Conventional adhesives and resin substrates are isotropic, making it impossible to align the linear expansion coefficients for different directions on the bonding surface of an X-cut substrate. Therefore, Patent Document 4 proposes using a liquid crystal polymer with an anisotropic linear expansion coefficient as a substrate.

[0006] Meanwhile, optical waveguide devices such as optical modulators using Mach-Zehnder waveguides experience a so-called drift phenomenon, where the bias point shifts. This phenomenon can be divided into two types: temperature drift caused by temperature changes and DC drift caused by the application of a DC bias voltage.

[0007] One cause of temperature drift is a difference in linear expansion coefficient between the optical waveguide substrate and the material bonded to it. Temperature changes cause thermal stress between the materials, causing changes in the refractive index near the optical waveguide and shifting the bias point. DC drift, on the other hand, is thought to be caused by the polarization / depolarization of mobile carriers or polar groups in the optical waveguide substrate and other bonded components.

[0008] In the prior art disclosed in Patent Documents 1 to 4, it is difficult to simultaneously solve the problem of temperature drift caused by the difference in linear expansion coefficient between the optical waveguide substrate and the low dielectric constant layer formed immediately thereunder, and the problem of DC drift caused by movable carriers and polar groups contained in the resin and adhesive used as the low dielectric constant layer.

[0009] Patent Document 1: Japanese Patent No. 4375597

[0010] Patent Document 2: Japanese Patent No. 4961372

[0011] Patent Document 3: Japanese Patent No. 5262186

[0012] Patent Document 4: Japanese Patent No. 5691808 Summary of the Invention

[0013] An object of the present invention is to provide an optical waveguide device that solves the above-mentioned problems and can suppress both the temperature drift phenomenon and the DC drift phenomenon.

[0014] In order to solve the above-mentioned problems, the optical waveguide device of the present invention has the following technical features.

[0015] (1) An optical waveguide device formed by bonding an optical waveguide substrate to a holding substrate that holds the optical waveguide substrate, the optical waveguide substrate being a crystal having a photoelectric effect and anisotropic thermal expansion coefficient, the thickness of which is set to be 10 μm or less, and including an optical waveguide, the optical waveguide device being characterized in that the holding substrate is formed of a crystal having a lower dielectric constant than that of the optical waveguide substrate and anisotropic thermal expansion coefficient, and the optical waveguide substrate and the holding substrate are bonded so that the difference in thermal expansion coefficient between the optical waveguide substrate and the holding substrate is minimized in different axial directions on the bonding surface.

[0016] (2) In the optical waveguide device described in 1 above, the difference in thermal expansion coefficient in the anisotropic axial direction is set to 5 ppm / °C or less.

[0017] (3) In the optical waveguide device described in (1) or (2) above, the optical waveguide substrate is lithium niobate or lithium tantalate, and the holding substrate is α-quartz single crystal.

[0018] (4) In the optical waveguide device described in any one of (1) to (3) above, the bonding is direct bonding or bonding via an adhesive layer having a thickness of 100 nm or less.

[0019] Effects of the Invention

[0020] According to the present invention, an optical waveguide device is formed by bonding an optical waveguide substrate, which is a crystal exhibiting a photoelectric effect and having anisotropic thermal expansion coefficient, to a holding substrate that holds the optical waveguide substrate. The optical waveguide substrate has a thickness of 10 μm or less and includes an optical waveguide. The holding substrate is formed of a crystal having a lower dielectric constant than that of the optical waveguide substrate and having anisotropic thermal expansion coefficient. The optical waveguide substrate and the holding substrate are bonded to each other in such a manner that the difference in thermal expansion coefficient between the optical waveguide substrate and the holding substrate is minimized in different axial directions of the bonding surface. Therefore, an optical waveguide device capable of suppressing both temperature drift and DC drift can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 FIG1 is a cross-sectional view showing a first embodiment of the optical waveguide device according to the present invention.

[0022] Figure 2 It is a cross-sectional view showing a second embodiment of the optical waveguide device of the present invention.

[0023] Figure 3 FIG. 1 is a cross-sectional view showing a third embodiment of the optical waveguide device according to the present invention.

[0024] Description of labels

[0025] 1 Optical waveguide substrate

[0026] 2 signal electrodes (modulation electrodes, control electrodes)

[0027] 20 Ground electrode (modulation electrode, control electrode)

[0028] 3 Optical waveguide

[0029] 4. Hold the substrate

[0030] 5 Adhesive layer DETAILED DESCRIPTION

[0031] Hereinafter, the present invention will be described in detail using preferred examples.

[0032] like Figures 1 to 3As shown, the present invention relates to an optical waveguide device formed by bonding an optical waveguide substrate 1 to a holding substrate 4 that holds the optical waveguide substrate 1. The optical waveguide substrate 1 is a crystal having a photoelectric effect and anisotropic thermal expansion coefficient, has a thickness t set to 10 μm or less, and includes an optical waveguide 3. The optical waveguide device is characterized in that the holding substrate 4 is formed of a crystal having a lower dielectric constant than that of the optical waveguide substrate 1 and anisotropic thermal expansion coefficient, and the optical waveguide substrate and the holding substrate are bonded so that the difference in thermal expansion coefficient between the optical waveguide substrate and the holding substrate is minimized in different axial directions on the bonding surface.

[0033] The optical waveguide substrate 1 used in the present invention is a trigonal crystal of lithium niobate (LN) or lithium tantalate (LT). The thickness t of the optical waveguide substrate is set to be less than 10 μm from the viewpoints of speed matching between microwaves and light waves and reducing the driving voltage. Figure 2 or Figure 3 When the ridge structure (ridge portion 10 ) is formed as shown, it is set to 2 μm or less. In order to reduce the curvature radius of the waveguide, it is more preferably set to less than 1 μm.

[0034] In thin optical waveguide substrates with a thickness t of 10 μm or less, toughness is compromised, making them extremely fragile. Furthermore, stress concentrates at the edges of the ridge waveguide, making cracking more likely due to physical properties. According to the present invention, cracking can be reduced by suppressing the generation of internal stress caused by thermal expansion.

[0035] As a method for forming the optical waveguide 3, the following method can be applied: Figure 1 As shown in FIG, a method of thermally diffusing a metal such as Ti into a substrate to form a portion having a higher refractive index than the substrate material, Figure 2 or Figure 3 As shown in the figure, a method for forming a ridge waveguide (rib waveguide) 10 by forming unevenness on the substrate surface is used. Furthermore, control electrodes such as modulation electrodes (signal electrode (S) 2, ground electrode (G) 20) and DC bias electrodes for applying an electric field to the optical waveguide are formed by laminating gold with a thickness of several to several tens of μm on a base electrode layer of Au / Ti, etc. formed on the substrate by plating or the like.

[0036] The holding substrate 4, which holds the optical waveguide substrate 1, is made of a crystal having a lower dielectric constant than the optical waveguide substrate and anisotropic thermal expansion coefficient. Specifically, a trigonal crystal such as α-quartz single crystal is preferably used. The relative dielectric constant of α-quartz single crystal is 4.6, which is much lower than that of single crystal substrates exhibiting the photoelectric effect, such as LN (relative dielectric constant of 29.5) and LT (relative dielectric constant of 44.5).

[0037] α-quartz single crystal is a trigonal crystal, unlike amorphous quartz glass (which has an isotropic linear expansion coefficient). The linear expansion coefficient of α-quartz single crystal is 8 ppm / °C in the Z-axis direction and 13 ppm / °C in the X and Y-axis directions, indicating anisotropy in its linear expansion coefficient. Furthermore, these linear expansion coefficients are close to those of LN (4 ppm / °C in the Z-axis direction, 15 ppm / °C in the X and Y-axis directions) and LT (5 ppm / °C in the Z-axis direction, 14 ppm / °C in the X and Y-axis directions).

[0038] Therefore, by bonding the optical waveguide substrate and the α-quartz single crystal serving as the support substrate (low-k dielectric constant layer) so that their Z-axis is parallel to each other, the linear expansion coefficient of the low-k dielectric constant layer can be brought close to that of the optical waveguide substrate. As a result, temperature drift caused by thermal stress between the optical waveguide substrate and the low-k dielectric constant layer can be effectively suppressed.

[0039] Furthermore, α-quartz single crystals contain fewer impurities and can suppress the occurrence of DC drift caused by mobile carriers. From this point of view, artificial crystals (low-temperature α-quartz) are more preferred because they can further reduce the amount of impurities in the crystal.

[0040] Furthermore, when using α-quartz single crystal, the linear expansion coefficient of the optical waveguide substrate and the holding substrate is extremely close to 5 ppm / °C or less. Even after a heating process close to 600°C, the substrates remain intact, allowing for direct bonding by covalent bonding. This is particularly true when performing annealing by heat treatment to suppress scattering from the side surfaces of the ridge waveguide, or when performing a sintering treatment on the resist material, which prevents substrate damage.

[0041] like Figure 1 As shown in FIG. 2 , the optical waveguide substrate 1 and the holding substrate 4 can be directly bonded by activating the surfaces of the respective substrates. Alternatively, Figure 3 As shown, the joining takes place via an adhesive layer 5 .

[0042] Using an adhesive, resin, water glass, or glass frit as the adhesive layer 5 can cause temperature drift due to the difference in linear expansion coefficient between the adhesive layer and the optical waveguide substrate. Furthermore, the effects of polar groups and mobile carriers contained in the adhesive layer can also cause DC drift, making this undesirable. The thicker the adhesive layer, the more likely this drift will occur.

[0043] When the adhesive layer 5 is used to bond substrates together, it is preferable to form an oxide such as aluminum oxide, tantalum pentoxide, or niobium pentoxide extremely thinly (100 nm or less) from the viewpoint of suppressing the occurrence of temperature drift and DC drift even if the thickness is small.

[0044] To achieve a stable operating environment and prevent degradation over time, optical waveguide devices contain substrates (chips) within a housing made of metal or other materials. Using a cobalt alloy such as stainless steel (linear expansion coefficient of 10 ppm / °C) or Kovar (a registered trademark of Carpenter Technology Corporation, linear expansion coefficient of 5 ppm / °C) for the housing allows for a linear expansion coefficient close to that of the substrate, resulting in an optical waveguide device with high durability against temperature fluctuations.

[0045] Optical waveguide devices were fabricated using LN and LT as optical waveguide substrates and α-quartz single crystal as a retaining substrate, and the influence of the drift phenomenon was evaluated. The results are shown in Table 1. It should be noted that in Table 1, "X-LN" of the optical waveguide substrate refers to an X-cut LN substrate. The same applies to the LT substrate. "CTE" is the coefficient of linear expansion, and "horizontal" and "vertical" represent the different axial linear expansion coefficients within the bonding surface of the substrate. "None" of the adhesive refers to direct bonding, and "Al2O3 10nmt" refers to coating and bonding Al2O3 (aluminum oxide) with a thickness of 10nm. "Acrylic" and "Glass raw material" are the materials of the adhesive, and the numerical values indicate their thickness. The "X-α-quartz" of the retaining substrate is an X-cut α-quartz single crystal.

[0046] A modulator with a Mach-Zehnder (MZ) optical waveguide was fabricated and drift was evaluated. To evaluate temperature drift, an AC voltage was applied to the modulator while the temperature was varied from -5°C to 85°C. The change in peak intensity of light output from the modulator was measured as a voltage value.

[0047] On the other hand, the DC drift was evaluated at a measurement temperature of 85° C., with a constant applied voltage (3.5 V), and the 24-hour change in the peak intensity of light output from the modulator was measured as a voltage value.

[0048] In Table 1, the above voltage change amount is defined as a drift amount, and a case where the drift amount is 3 V or less is evaluated as "○", and a case where the drift amount is greater than 3 V is evaluated as "×".

[0049]

Table 1

[0050]

[0051] Offset evaluation: ○ = ≤ 3V, × = > 3V

[0052] The results in Table 1 confirm that when an α-quartz single crystal is used for the holding substrate and the difference in linear expansion coefficient in different axial directions in the bonding surface with the optical waveguide substrate is suppressed to 5 ppm / °C or less, both the temperature drift phenomenon and the DC drift phenomenon can be effectively suppressed.

[0053] Industrial Applicability

[0054] As described above, according to the optical waveguide device of the present invention, it is possible to provide an optical waveguide device capable of suppressing both the temperature drift phenomenon and the DC drift phenomenon.

Claims

1. An optical waveguide device comprising an optical waveguide substrate and a holding substrate for holding the optical waveguide substrate, wherein the optical waveguide substrate is a crystal having a photoelectric effect and anisotropic thermal expansion coefficient, has a thickness of 10 μm or less, and includes an optical waveguide, wherein: The optical waveguide substrate is lithium niobate or lithium tantalate, and the holding substrate is α quartz single crystal. The optical waveguide substrate and the holding substrate are bonded to each other so that a difference in thermal expansion coefficient between the optical waveguide substrate and the holding substrate in different axial directions on the bonding surface is set to 5 ppm / ° C. or less.

2. The optical waveguide device according to claim 1, wherein The bonding is direct bonding or bonding via an adhesive layer having a thickness of 100 nm or less.

Images