Optical waveguide element and optical modulator

Abstract

To provide an optical waveguide element and an optical modulator capable of preventing damage to a substrate and deterioration of characteristics of the substrate due to stress generated by a buffer layer by reducing the effect of the stress on the substrate, an optical waveguide element (1) is characterized by including: a substrate (5) having an electro-optic effect; an optical waveguide (10) formed on the substrate (5); an upper surface buffer layer (first buffer layer) (9a) provided on the substrate (5); and a lower surface buffer layer (second buffer layer) (9b) provided below the substrate (5), the upper surface buffer layer (9a) and the lower surface buffer layer (9b) being composed of substantially the same material and having substantially the same thickness, the upper surface buffer layer (9a) being formed so as to be in contact with an upper surface of the substrate (1), and the lower surface buffer layer (9b) being formed so as to be in contact with a lower surface of the substrate (1).

Description

TECHNICAL FIELD

[0001] The present application relates to an optical waveguide element and an optical modulator used in the field of optical communication and the field of optical measurement. The present application particularly relates to an optical waveguide element provided with an optical waveguide and an electrode or the like on a substrate having an electro-optic effect, and an optical modulator in which the optical waveguide element is packaged. BACKGROUND

[0002] In recent years, in the field of optical communication and the field of optical measurement, an optical waveguide element in which an optical waveguide is formed on a substrate having an electro-optic effect, such as lithium niobate (LiNbO3: hereinafter referred to as LN), is used. Furthermore, an optical modulator in which the optical waveguide element is provided with an electrode or the like is used in order to modulate an optical wave propagating in the optical waveguide formed in the optical waveguide element.

[0003] In addition, it is important to achieve matching of the speeds of a microwave and an optical wave as a modulation signal in order to achieve broadbandization of the optical modulation frequency. Therefore, attempts have been made to achieve matching of the speeds of the microwave and the optical wave and to achieve reduction of the driving voltage by performing thinning of the substrate by reducing the thickness of the substrate.

[0004] Patent Document 1 described below discloses an optical element provided with a first substrate of a single crystal, a second substrate composed of an electro-optic medium in which a core of a ridge waveguide is formed, and an outer cladding layer that is bonded to the upper portion of the ridge waveguide by a direct bonding method. According to the optical element disclosed in Patent Document 1, the outer cladding layer is provided with a single crystal having a value of the coefficient of thermal expansion that is close to those of the first and second substrates that constitute the waveguide, and thus, generation of stress deformation accompanying formation of the outer cladding layer can be suppressed, and the symmetry of the shape of the waveguide mode can be improved.

[0005] Patent Documents 2 and 3 described below disclose a technology in which the DC drift characteristics can be improved by adding an oxide such as indium or titanium to a SiO2 buffer layer.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT DOCUMENTS

[0008] Patent Document 1: Japanese Patent Application Publication No. 2019-105808

[0009] Patent Document 2: Japanese Patent Application Publication No. 2013-25283

[0010] Patent Document 3: Japanese Patent No. 3001027 SUMMARY

[0011] PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] In optical modulators with thin-plate ribbed waveguide structures with a thickness of a few μm or less, a buffer layer with a thickness approximately the same as that of the thin plate is needed to suppress light absorption generated by the electrodes, formed by sputtering or vacuum evaporation. However, the wafer is thinner than before, making it sensitive to stress. Moreover, while materials with electro-optic effects, such as lithium nanotubes (LN), are used in the wafer, SiO2 or similar materials are used in the buffer layer.

[0013] The materials of the wafer (substrate) and the buffer layer have different coefficients of thermal expansion (linear expansion). Therefore, during wafer fabrication, when the buffer layer is deposited or the wafer or chip is heated, stress (internal stress or residual stress) is generated at the contact surface between the buffer layer and the wafer (substrate) due to the difference in their coefficients of thermal expansion. As a result, problems such as substrate damage and cracking can occur due to the stress exerted by the buffer layer on the substrate.

[0014] Furthermore, the substrate is made of an electro-optic material such as LN, and light modulation is achieved by changing the refractive index through the application of an electric current. However, when stress is generated on the substrate due to the buffer layer, the refractive index of the substrate changes due to the photoelastic effect, resulting in a change in the propagation speed of the light wave. As a result, for example, in an optical modulator with a Mach-Zehnder structure, a phase difference is generated during the combining of waves in the Mach-Zehnder structure, leading to problems such as bias voltage fluctuations and other characteristic degradation.

[0015] Reference Figure 8 This explains the problem of stress generated on the substrate due to the buffer layer. Figure 8 The schematic diagram illustrates the cross-sectional structure of the optical waveguide element used in next-generation optical modulators. Figure 8 The cross-sectional structure shown shows a state in which a substrate 102 composed of LN is formed on a reinforcing substrate 101, and a buffer layer 103 is formed on the substrate 102.

[0016] In the ribbed optical waveguide elements of the present time, the substrate is much thicker than the buffer layer (e.g., 0.5 to 1.0 μm). Therefore, even if there is a difference in the coefficient of thermal expansion between the materials of the buffer layer and the substrate, it is difficult to be affected by the stress caused by the difference in the coefficient of thermal expansion.

[0017] In contrast, in the optical waveguide elements used in next-generation optical modulators, such as Figure 8 As shown, the substrate 102 is thinned to the same film thickness as the buffer layer 103 (e.g., approximately 1.0 μm). The thinned substrate 102 is sensitive to stress generated by the buffer layer 103. As a result, during wafer fabrication or heating of the buffer layer 103, stress can occur... Figure 8In this way, the substrate 102 is subjected to stress caused by the difference in thermal expansion coefficients between the material of the buffer layer 103 and the material of the substrate 102, which causes the substrate 102 to deform (e.g., the substrate 102 warps), resulting in problems such as damage and cracking of the substrate 102 or deterioration of the properties of the substrate 102.

[0018] The technology disclosed in Patent Document 1 addresses the problem of stress on the substrate caused by the buffer layer by employing a structure in which the outer cladding layer is bonded directly without forming a buffer layer. Buffer layers are layers formed to suppress light absorption caused by electrodes mounted on the substrate surface and have been widely used to effectively reduce propagation losses. However, when using a structure without a buffer layer, as disclosed in Patent Document 1, new problems arise, such as the need to find alternative creative solutions to compensate for the role of the buffer layer, and the need for significant changes to the design and manufacturing process of the optical waveguide element.

[0019] In order to solve the above-mentioned problems, the present invention aims to reduce the influence of stress on the substrate caused by the buffer layer, and prevent damage to the substrate and deterioration of substrate characteristics caused by such stress.

[0020] Solution for solving the problem

[0021] In order to solve the above-mentioned problems, the optical waveguide element and optical modulator of the present invention have the following technical features.

[0022] (1) In order to achieve the above-mentioned objective, the optical waveguide element of the present invention comprises: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; a first buffer layer disposed on the substrate; and a second buffer layer disposed under the substrate. The optical waveguide element is characterized in that the first buffer layer and the second buffer layer are made of substantially the same material and have substantially the same thickness, the first buffer layer is formed in contact with the upper surface of the substrate, and the second buffer layer is formed in contact with the lower surface of the substrate.

[0023] This structure allows stress of the same magnitude as that generated on the upper surface of the substrate through the first buffer layer to be generated on the lower surface of the substrate through the second buffer layer, thereby balancing and homogenizing the stress on the upper and lower surfaces of the substrate. As a result, it can mitigate the stress imbalance on the upper and lower surfaces of the substrate, preventing substrate deformation, damage to the substrate, and degradation of substrate properties.

[0024] (2) The optical waveguide element according to (1) above is characterized in that the first buffer layer and the second buffer layer are respectively composed of a transparent insulating film of a mixture or an oxide, wherein the mixture is a mixture formed by silicon oxide and an oxide of at least one of the elements selected from metal elements of groups III to VIII, IB and IIB of the periodic table and semiconductor elements other than silicon, wherein the oxide is an oxide formed by silicon and an oxide formed from one or more of the elements selected from the metal elements and semiconductor elements.

[0025] With this structure, by placing a first buffer layer and a second buffer layer, for example, containing metal oxides such as indium or titanium, on the upper and lower surfaces of the substrate, the added metal oxides can be used to flatten the increase in DC drift that occurs over time, thereby improving DC drift characteristics over a long period of time.

[0026] (3) The optical waveguide element according to (1) or (2) above is characterized in that the refractive index of the materials of the first buffer layer and the second buffer layer is lower than the refractive index of the material of the substrate having an electro-optic effect.

[0027] By using this structure, and by setting the top and bottom of the substrate to be made of materials with a lower refractive index than the substrate, the effect of confining the propagating light within the optical waveguide formed on the substrate can be increased, thereby effectively reducing propagation loss.

[0028] (4) The optical waveguide element according to any one of (1) to (3) above, characterized in that the resistivity of the materials of the first buffer layer and the second buffer layer is 10. 8 Ωcm or more and 10 16 Below Ωcm.

[0029] This structure allows for efficient light propagation in optical waveguides by controlling the resistivity of the materials in the first and second buffer layers, thereby achieving suitable DC drift characteristics. Specifically, by setting the resistance values ​​of the first and second buffer layers to 10... 8 Ωcm and above and 10 16 Within the range of Ωcm, a suitable DC drift can be stably obtained.

[0030] (5) The optical waveguide element according to any one of (1) to (4) above, characterized in that the thickness of the first buffer layer and the second buffer layer is 0.3 μm or more and 2.0 μm or less.

[0031] This structure allows for efficient light propagation in optical waveguides by controlling the thicknesses of the first and second buffer layers to achieve appropriate DC drift characteristics. Specifically, by setting the thicknesses of the first and second buffer layers to a range of 0.3 μm or more and 2.0 μm or less, a stable and appropriate DC drift can be obtained.

[0032] (6) The optical waveguide element according to any one of (1) to (5) above, characterized in that the optical waveguide element has a reinforcing substrate disposed below the second buffer layer, and the lower surface of the second buffer layer and the upper surface of the reinforcing substrate are directly bonded by a direct bonding method.

[0033] This structure allows for proper and reliable bonding of the second buffer layer to the reinforcing substrate.

[0034] (7) The optical waveguide element according to (6) above, characterized in that the second buffer layer is bonded to the reinforcing substrate via an adhesive layer.

[0035] This structure allows for proper and reliable bonding of the second buffer layer to the reinforcing substrate via an adhesive layer. Furthermore, by forming the second buffer layer on top of the adhesive layer, the second buffer layer can prevent light absorption in the optical waveguide caused by the adhesive layer.

[0036] (8) The optical waveguide element according to any one of (1) to (7) above, characterized in that a rib protruding on the substrate is used as the optical waveguide.

[0037] This structure enables the stress balance and homogenization of the upper and lower surfaces of the substrate to be achieved even when the substrate becomes thinner due to the ribbed waveguide structure. As a result, it can mitigate stress imbalances on the upper and lower surfaces of the substrate, prevent substrate deformation, and prevent damage and degradation of the substrate's properties.

[0038] (9) The optical waveguide element according to any one of (1) to (8) above, characterized in that the optical waveguide is formed by a plurality of Mach-Zehnder sections.

[0039] This structure enables the stress balance and homogenization of the upper and lower surfaces of the substrate in optical waveguide elements with multiple Mach-Zehnder waveguide structures capable of generating optical signals corresponding to various modulation methods. As a result, it can mitigate stress imbalances on the upper and lower surfaces of the substrate, preventing substrate deformation, damage, and degradation of substrate properties.

[0040] (10) In order to achieve the above-mentioned objective, the optical modulator of the present invention is characterized in that at least a portion of the optical modulator uses an optical waveguide constituting any one of the optical waveguide elements described in (1) to (9) above.

[0041] This structure enables the stress balance and homogenization of the upper and lower surfaces of the substrate. As a result, an optical modulator is realized that can mitigate stress imbalances on the upper and lower surfaces of the substrate, prevent substrate deformation, and prevent damage to the substrate and degradation of its properties.

[0042] Invention Effects

[0043] According to the present invention, in optical waveguide elements and optical modulators, by reducing the influence of stress on the substrate generated by the buffer layer, damage to the substrate and degradation of substrate characteristics caused by such stress can be prevented. Attached Figure Description

[0044] Figure 1 This is a top view illustrating an example of an optical waveguide formed on a substrate constituting an optical waveguide element, in an embodiment of the present invention.

[0045] Figure 2 This is a diagram showing a first example of the cross-sectional structure of an optical waveguide element according to an embodiment of the present invention. Figure 1 A sectional view of line segment PP.

[0046] Figure 3 This is a second example of a cross-sectional structure of an optical waveguide element according to an embodiment of the present invention, and is a diagram showing a state in which a modulation electrode is formed on a substrate.

[0047] Figure 4 This is a diagram showing a third example of the cross-sectional structure of an optical waveguide element according to an embodiment of the present invention, and a diagram showing a state in which a modulation electrode is formed on a substrate.

[0048] Figure 5A This is a diagram illustrating the manufacturing process of an optical waveguide element according to an embodiment of the present invention, and it shows the state after the first step.

[0049] Figure 5B This is a diagram illustrating the manufacturing process of an optical waveguide element according to an embodiment of the present invention, and it shows the state after the second step.

[0050] Figure 5C This is a diagram illustrating the manufacturing process of an optical waveguide element according to an embodiment of the present invention, and it shows the state after the third step.

[0051] Figure 5D This is a diagram illustrating the manufacturing process of an optical waveguide element according to an embodiment of the present invention, and it shows the state after the fourth step.

[0052] Figure 5E This is a diagram illustrating the manufacturing process of an optical waveguide element according to an embodiment of the present invention, and it shows the state after the fifth step.

[0053] Figure 5F This is a diagram illustrating the manufacturing process of an optical waveguide element according to an embodiment of the present invention, and it shows the state after the sixth step.

[0054] Figure 6 This is a fourth example of a cross-sectional structure of an optical waveguide element according to an embodiment of the present invention. Figure 1 A sectional view of line segment PP.

[0055] Figure 7 This is a top view illustrating an example of the structure of an optical modulator according to an embodiment of the present invention.

[0056] Figure 8 This is a diagram used to illustrate the problem that this invention aims to solve. Detailed Implementation

[0057] The optical waveguide element and optical modulator according to embodiments of the present invention will be described below.

[0058] Figure 1 This is a top view illustrating an example of an optical waveguide 10 formed on a substrate 5 constituting an optical waveguide element 1, as described in an embodiment of the present invention. It should be noted that... Figure 1 In the diagram, the optical waveguide element 1 is illustrated with its width direction as the vertical direction of the paper, its length direction as the horizontal direction of the paper, and its thickness direction as the direction perpendicular to the paper.

[0059] Figure 1 The optical waveguide element 1 shown is an optical waveguide element 1 integrating multiple Mach-Zehnder type optical waveguides. An optical waveguide combining multiple Mach-Zehnder type optical waveguides is also called a nested optical waveguide. The optical waveguide element 1 integrating multiple Mach-Zehnder type optical waveguides can generate optical signals corresponding to various modulation methods. Figure 1 As an example, the figure shows an optical waveguide element 1 that integrates multiple Mach-Zehnder type optical waveguides. However, the present invention is not limited to this structure. For example, it can be an optical waveguide element 1 with a single Mach-Zehnder type optical waveguide.

[0060] like Figure 1 As shown, the optical waveguide element 1 of the embodiment of the present invention includes an optical waveguide 10, which is formed on a substrate 5 formed of a material having an electro-optic effect. Figure 1The optical waveguide element 1 shown includes a first branch 2a that branches off an incident waveguide to introduce an external optical signal, a second branch 2b that further branches off the optical waveguide 10 obtained from the first branch 2a, and a third branch 2c that further branches off the optical waveguide 10 obtained from the second branch 2b. A total of eight parallel waveguides are formed through these three stages of branching. The first to third branches 2a to 2c are implemented using optical couplers or the like.

[0061] The phase of the light waves propagating in each parallel waveguide is adjusted, for example, in region D1. A metallic modulation electrode is formed in region D1. Figure 1 (Not shown) By changing the refractive index through an electric field applied from the modulation electrode to each parallel waveguide, the propagation speed of light waves can be adjusted.

[0062] The light waves propagating in each parallel waveguide are combined in the first to third combining sections 3a to 3c, which correspond to the first to third branches 2a to 2c mentioned above, and then output from the outgoing waveguide to the outside. Specifically, Figure 1 The optical waveguide element 1 shown includes a third combining section 3c, a second combining section 3b, and a first combining section 3a. It outputs an optical signal from the output waveguide through a three-stage combining process. The third combining section 3c combines parallel waveguides branched from the third branch section 2c, the second combining section 3b combines optical waveguides 10 branched from the second branch section 2b, and the first combining section 3a combines optical waveguides 10 branched from the first branch section 2a. Similar to the first to third branches 2a to 2c, the first to third combining sections 3a to 3c are also implemented using an optical coupler or the like.

[0063] It should be noted that, Figure 1 The optical waveguide 10 shown in the optical waveguide element 1 is an example, and the present invention is not limited thereto. For example, it can be as shown in the reference. Figure 7 Similar to the optical waveguide element 202 of the optical modulator 200 described later, two optical signals are output from the optical waveguide element 202 and polarization is combined by the polarization combining unit 228.

[0064] Additionally, a bias voltage is applied to the optical waveguide 10 to set the operating point. The bias voltage is applied to the phase-modulated light wave through, for example, a bias electrode formed in region D2.

[0065] Figure 2 This is a diagram showing a first example of the cross-sectional structure of the optical waveguide element 1 according to an embodiment of the present invention. Figure 1 A sectional view of line segment PP. It should be noted that... Figure 2In the diagram, the optical waveguide element 1 is illustrated with its thickness direction as the vertical direction of the paper, its width direction as the horizontal direction of the paper, and its length direction as the direction perpendicular to the paper.

[0066] like Figure 2 As shown in the cross-sectional structure, the optical waveguide element 1 has a structure in which a lower surface buffer layer (second buffer layer) 9b is provided on the reinforcing substrate 7, a substrate 5 is provided on the lower surface buffer layer 9b, and an upper surface buffer layer (first buffer layer) 9a is provided on the substrate 5.

[0067] The substrate 5 is formed of a material with an electro-optic effect. Conventional substrates have a thickness of approximately 8 to 10 μm; in contrast, the substrate 5 in this embodiment can be an extremely thin plate, for example, with a thickness of 2.0 μm or less, preferably 1.0 μm or less. By making the substrate 5 extremely thin (for example, about 1 / 10 the thickness of conventional substrates), the driving voltage can be further reduced. The substrate 5 can use, for example, LN as the material with an electro-optic effect, such as lithium tantalate (LiTaO3) or lanthanum lead zirconate titanate (PLZT).

[0068] Ribs 6 are provided on the substrate 5. The ribs 6 protrude from the surface of the substrate 5 and serve to confine light waves within them, thus functioning as an optical waveguide 10. In conventional diffused optical waveguide structures, the light-confining effect is weak, and leakage of propagating light from the optical waveguide 10 sometimes occurs at curved sections, etc. In contrast, with the rib-type optical waveguide structure, the light-confining effect is strengthened, and the optical waveguide 10 can be bent to form a folded structure, enabling the optical waveguide element 1 to be narrower and longer. The height of the ribs 6 from the surface of the substrate 5 is, for example, 2.0 μm or less, preferably 1.0 μm or less.

[0069] The dimensions of the ribbed substrate will be described in more detail below. In the ribbed substrate of the embodiment of the present invention, for example, the maximum value of the thickness A of the substrate 5 including the rib 6 is 4.0 μm, the maximum value of the width B of the rib 6 is 4.0 μm, the maximum value of the height C of the rib 6 is 2.0 μm, and the ratio of thickness A to width B is 1:1. The smaller the rib 6, substrate 5, etc. are designed, the better. Therefore, the minimum values ​​of the thickness A, width B, and height C mentioned above become the limits for minimization in the manufacturing process. Moreover, from the viewpoint of confining light, as long as the dimensions are within the range of maintaining the single-mode condition of light, it is preferable to have smaller dimensions for both the thickness A and the width B in order to confine the light.

[0070] Figure 2As an example, the figure shows an optical waveguide element 1 having a ribbed substrate on which ribs 6 are formed. However, in the present invention, although it is preferred to have a structure having a ribbed substrate with ribs 6 formed as optical waveguides 10, it is not limited to this. For example, an optical waveguide element 1 with optical waveguides 10 formed in the substrate 5 by thermal diffusion of metal can be used.

[0071] The reinforcing substrate 7 compensates for the strength of the extremely thin substrate 5 and stably supports the lower surface buffer layer 9b, the substrate 5, the upper surface buffer layer 9a, and the electrodes formed on the substrate 5. The reinforcing substrate 7 is directly bonded to the lower surface buffer layer 9b by a direct bonding method, as described later. The material of the reinforcing substrate 7 can be, for example, a material with a lower dielectric constant than the material of the substrate 5 (e.g., LN), or the same material as the substrate 5 (e.g., LN).

[0072] Furthermore, an upper surface buffer layer 9a is provided on the substrate 5. In embodiments of the present invention, the upper surface buffer layer 9a has the same thickness as the substrate 5, for example, a thickness of 2.0 μm or less, preferably 1.0 μm or less. The material used for the upper surface buffer layer 9a is not particularly limited, but a material with a lower refractive index than LN and excellent light transmittance is preferred. The material used for the upper surface buffer layer 9a can be a material commonly used as a buffer layer, such as SiO2, Al2O3, MgF3, La2O3, ZnO, HfO2, MgO, CaF2, Y2O3, etc.

[0073] Conventional substrates have a thickness of 8.0 to 10.0 μm. In contrast, in the embodiments of the present invention, as described above, the thickness of the ribbed substrate can be made extremely thin, less than 2.0 μm, enabling speed matching between microwaves and light waves and further reduction of the driving voltage. However, such an extremely thin substrate 5 is particularly sensitive to stress.

[0074] Furthermore, as described above, the substrate 5 uses, for example, LN, while the upper surface buffer layer 9a disposed on the substrate uses, for example, SiO2. However, the thermal expansion rates of LN, the material of the substrate 5, and SiO2, the material of the upper surface buffer layer 9a, are different. As a result, especially in wafer processing accompanied by temperature changes, when the upper surface buffer layer 9a is deposited, or when the wafer (substrate 5) or chip is heated, stress (internal stress or residual stress) is generated on the surface where the upper surface buffer layer 9a contacts the substrate 5 due to the difference in thermal expansion rates between the substrate 5 and the upper surface buffer layer 9a.

[0075] As a result, the material of the upper surface buffer layer 9a and the material of the substrate 5 are affected by stress caused by the difference in thermal expansion coefficients, resulting in problems such as substrate 5 deformation and characteristic deterioration caused by bias voltage variation.

[0076] To address this problem, in the optical waveguide element 1 of the embodiments of the present invention, such as... Figure 2 As shown, a lower surface buffer layer 9b is provided between the reinforcing substrate 7 and the substrate 5. In embodiments of the present invention, the lower surface buffer layer 9b has a thickness approximately the same as that of the upper surface buffer layer 9a, for example, a thickness of 2.0 μm or less, preferably 1.0 μm or less. Furthermore, the lower surface buffer layer 9b uses approximately the same material as the upper surface buffer layer 9a.

[0077] Furthermore, the statement that the upper surface buffer layer 9a and the lower surface buffer layer 9b have substantially the same thickness means that the upper surface buffer layer 9a and the lower surface buffer layer 9b have the same or substantially the same film thickness. Specifically, in this invention, when the difference in thickness between the upper surface buffer layer 9a and the lower surface buffer layer 9b, including errors caused by uneven manufacturing processes, is within ±20% of the thickness of either the upper surface buffer layer 9a or the lower surface buffer layer 9b, it is defined as the upper surface buffer layer 9a and the lower surface buffer layer 9b having substantially the same thickness.

[0078] The statement that the upper surface buffer layer 9a and the lower surface buffer layer 9b are made of substantially the same material means that the upper surface buffer layer 9a and the lower surface buffer layer 9b are made of the same material or of substantially the same material. Specifically, in this invention, the upper surface buffer layer 9a and the lower surface buffer layer 9b are defined as being made of substantially the same material when the difference in resistivity between the upper surface buffer layer 9a and the lower surface buffer layer 9b, including errors caused by uneven manufacturing processes, is within ±20% of the resistivity of either the upper surface buffer layer 9a or the lower surface buffer layer 9b, and the difference in refractive index between the upper surface buffer layer 9a and the lower surface buffer layer 9b is within ±20% of the refractive index of either the upper surface buffer layer 9a or the lower surface buffer layer 9b.

[0079] It should be noted that there are no particular limitations on the methods for measuring film thickness, resistivity, and refractive index; these parameters can be measured using conventional methods. For example, regarding film thickness, for batches consisting of multiple wafers with dummy wafers inserted into them, the film thickness can be measured using a conventional stylus-type stepped system. Regarding resistivity, for the aforementioned batches containing dummy wafers, an IV measurement (current-voltage measurement) using the mercury probe method can be performed, and the resistivity can be calculated from the measurement results. Regarding refractive index, for the aforementioned batches containing dummy wafers, the refractive index can be measured using a prism coupler (e.g., measurement wavelength: 1550 nm).

[0080] In the optical waveguide element 1 of the embodiment of the present invention, an upper surface buffer layer 9a and a lower surface buffer layer 9b are formed, both made of substantially the same material and having substantially the same thickness. Furthermore, the upper surface buffer layer 9a is formed in contact with the upper surface of the substrate 5, and the lower surface buffer layer 9b is formed in contact with the lower surface of the substrate 5. This creates a structure in which the upper surface buffer layer 9a and the lower surface buffer layer 9b sandwich the substrate 5, thereby allowing stress of the same magnitude as the stress generated on the upper surface of the substrate 5 through the upper surface buffer layer 9a to also be generated on the lower surface of the substrate 5 through the lower surface buffer layer 9b. This achieves a uniform balance of stress on the upper and lower surfaces of the substrate 5. As a result, it can mitigate stress imbalance on the upper and lower surfaces of the substrate 5, preventing deformation of the substrate 5 and preventing damage to the substrate 5 and degradation of its properties.

[0081] Furthermore, it is known that by adding metal oxides such as indium or titanium to a buffer layer (upper surface buffer layer 9a) disposed on substrate 5, the added metal oxides can flatten the increase in DC drift that accompanies the passage of time, thereby improving DC drift characteristics over a long period of time (see Patent Documents 2 and 3). Using this technology, metal oxides can be added to both the upper surface buffer layer 9a and the lower surface buffer layer 9b to further improve drift characteristics.

[0082] More specifically, both the upper surface buffer layer 9a and the lower surface buffer layer 9b can be formed from a transparent insulating film of a mixture or an oxide, wherein the mixture is a mixture of silicon oxide and an oxide of at least one of the elements selected from Groups III to VIII, Group IB and Group IIB of the periodic table and semiconductor elements other than silicon, and the oxide is an oxide of silicon and an oxide of one or more of the elements selected from the metal elements and semiconductor elements. Specifically, the upper surface buffer layer 9a and the lower surface buffer layer 9b are made of materials in which metal oxides such as indium, titanium, zinc, tin, chromium, aluminum, germanium, etc., are added to SiO2 (doped).

[0083] It should be noted that the types of elements in the additives added to the upper surface buffer layer 9a and the lower surface buffer layer 9b can be the same or different in the upper surface buffer layer 9a and the lower surface buffer layer 9b. The upper surface buffer layer 9a and the lower surface buffer layer 9b of the present invention are composed of substantially the same material. As described above, the difference in resistivity between the upper surface buffer layer 9a and the lower surface buffer layer 9b, including errors caused by uneven manufacturing processes, is acceptable as long as it is within ±20% of the resistivity of either the upper surface buffer layer 9a or the lower surface buffer layer 9b, and the difference in refractive index between the upper surface buffer layer 9a and the lower surface buffer layer 9b is within ±20% of the refractive index of either the upper surface buffer layer 9a or the lower surface buffer layer 9b. If this condition is met, the additives added to the upper surface buffer layer 9a and the lower surface buffer layer 9b can be different.

[0084] Furthermore, it is known that by using a material with appropriate resistivity as the material of the buffer layer (upper surface buffer layer 9a) disposed on the substrate 5, it is possible to suppress light absorption generated by the buffer layer and suppress the generation of positive DC drift (for example, see Patent Document 3). By applying this technology, a material with appropriate resistivity can be used as the material for both the upper surface buffer layer 9a and the lower surface buffer layer 9b.

[0085] More specifically, the materials used for the upper surface buffer layer 9a and the lower surface buffer layer 9b can be those with a resistivity of 10. 8 Ωcm or more and 10 16 Materials with a resistivity of less than Ωcm. The resistivity of the materials used in the upper surface buffer layer 9a and the lower surface buffer layer 9b is set to 10 Ωcm. 8 With a resistivity of Ωcm or higher, light absorption caused by the upper surface buffer layer 9a and the lower surface buffer layer 9b can be prevented. Furthermore, the resistivity of the materials used in the upper surface buffer layer 9a and the lower surface buffer layer 9b is set to 10. 16 Below Ωcm, a stable negative DC drift can be obtained in the early stages of time.

[0086] Furthermore, materials used for the upper surface buffer layer 9a and the lower surface buffer layer 9b can be materials with a lower refractive index than the material of the substrate 5 exhibiting the electro-optic effect (e.g., LN). By using materials with a lower refractive index than the substrate 5 for the upper surface buffer layer 9a and the lower surface buffer layer 9b disposed on the upper and lower surfaces of the substrate 5, the effect of confining the propagating light within the optical waveguide 10 formed on the substrate 5 can be increased, and propagation loss can be reduced efficiently.

[0087] Furthermore, by controlling the thickness of the upper surface buffer layer 9a and the lower surface buffer layer 9b, appropriate DC drift characteristics can be obtained, enabling efficient propagation of light in the optical waveguide 10. Specifically, by setting the thickness of the upper surface buffer layer 9a and the lower surface buffer layer 9b to a range of 0.3 μm or more and 2.0 μm or less, an appropriate DC drift amount can be stably obtained.

[0088] Next, the cross-sectional structure of the phase modulation section that forms the modulation electrode will be described.

[0089] Figure 3 This is a second example of a cross-sectional structure of the optical waveguide element 1 in an embodiment of the present invention, and a diagram showing the state in which a modulation electrode is formed on the substrate 5. Figure 3 yes Figure 1 A sectional view of line segment QQ. It should be noted that... Figure 3 In the diagram, the optical waveguide element 1 is illustrated with its thickness direction as the vertical direction of the paper, its width direction as the horizontal direction of the paper, and its length direction as the direction perpendicular to the paper.

[0090] Figure 3 The cross-sectional structure of an optical waveguide element 1 is shown, in which modulation electrodes (signal electrodes S and ground electrodes G) are formed on a substrate 5 and the ribs 6 of the substrate 5 are used as optical waveguides 10. Figure 3 The substrate 5 shown has a structure in which a signal electrode S is disposed between it and the optical waveguide 10.

[0091] The signal electrode S and ground electrode G, serving as modulation electrodes, are formed, for example, by depositing Ti / Au onto the upper surface buffer layer 9a and then patterning the electrodes using a photolithography process. The modulation electrode can be any suitable metal, and the method for forming the modulation electrode on the upper surface buffer layer 9a is not particularly limited. The thickness of the modulation electrode is, for example, 20 μm or more. It should be noted that although descriptions and illustrations are omitted in this specification, when the modulation electrode is formed on the buffer layer 9a, a conductive film layer made of Si or the like can be formed between the upper surface buffer layer 9a and the modulation electrode to prevent current accumulation.

[0092] The signal electrode S is an electrode used to apply an electric field to the optical waveguide 10, and is configured, for example, to extend in parallel with the optical waveguide 10. Although not shown, the signal electrode S is connected to a signal source and a terminating resistor, from which a high-frequency electrical signal is supplied and terminated by the terminating resistor.

[0093] The ground electrode G is an electrode connected to a reference potential point, and is configured, for example, to extend parallel to the optical waveguide 10 in the same manner as the signal electrode S. The signal electrode S and the ground electrode G are separately disposed, and an electric field is formed between the signal electrode S and the ground electrode G. The signal electrode S and the ground electrode G constitute, for example, a coplanar circuit.

[0094] An electric field formed between the signal electrode S and the ground electrode G is applied to the optical waveguide 10 formed within the rib 6. By controlling the electric signal supplied from the signal source to adjust the electric field strength, the light wave propagating within the optical waveguide 10 can be appropriately modulated.

[0095] like Figure 3 As shown, by using an upper surface buffer layer 9a and a lower surface buffer layer 9b to sandwich the substrate 5, the stress balance on the upper and lower surfaces of the substrate 5 can be homogenized. As a result, the stress imbalance on the upper and lower surfaces of the substrate 5 can be mitigated, thus preventing deformation of the substrate 5 and preventing damage to the substrate 5 and deterioration of its properties.

[0096] Figure 4 This is a third example of the cross-sectional structure of the optical waveguide element 1 in an embodiment of the present invention, and a diagram showing the state in which a modulation electrode is formed on the substrate 5. Figure 4 yes Figure 1 A sectional view of line segment QQ. It should be noted that... Figure 4 In the diagram, the optical waveguide element 1 is illustrated with its thickness direction as the vertical direction of the paper, its width direction as the horizontal direction of the paper, and its length direction as the direction perpendicular to the paper.

[0097] Figure 4 The diagram shows a cross-sectional structure of an optical waveguide element 1 formed on a substrate 5, with modulation electrodes (signal electrode S and ground electrode G) formed on the substrate 5 and the ribs 6 of the substrate 5 used as optical waveguides 10. Figure 4 The substrate 5 shown has a structure in which signal electrodes S are disposed on the optical waveguide 10.

[0098] With the above Figure 3 Similarly, as Figure 4 As shown, by using an upper surface buffer layer 9a and a lower surface buffer layer 9b to sandwich the substrate 5, the stress balance on the upper and lower surfaces of the substrate 5 can be homogenized. As a result, the stress imbalance on the upper and lower surfaces of the substrate 5 can be mitigated, thus preventing deformation of the substrate 5 and preventing damage to the substrate 5 and deterioration of its properties.

[0099] As listed Figure 3 and Figure 4As illustrated by the cross-sectional structure, the present invention can mitigate stress skewing on the upper and lower surfaces of a substrate 5 having a structure in which signal electrodes S are disposed between optical waveguides 10, and a structure in which signal electrodes S are disposed on top of optical waveguides 10. Furthermore, the lower surface buffer layer 9b can be disposed across the entire lower surface of the substrate 5 regardless of the positions of the modulation electrodes (signal electrodes S and ground electrodes G) or the optical waveguides 10.

[0100] Next, refer to Figures 5A-5F The manufacturing process of the optical waveguide element 1 in the embodiments of the present invention will be described. It should be noted that... Figures 5A-5F The figure shows having Figure 3 The manufacturing process of the optical waveguide element 1 with a cross-sectional structure is taken as an example.

[0101] In the first step, a layer (e.g., SiO2, etc.) is formed as the lower surface buffer layer 9b for the layer that becomes the substrate 5 (e.g., LN layer). Figure 5A This shows the state after the first step.

[0102] In the second step, the lower surface of the layer that will become the lower surface buffer layer 9b and the upper surface of the reinforcing substrate 7 are directly bonded by a direct bonding method. Figure 5B This shows the state after the second step.

[0103] In the third step, the layer of substrate 5, which is made of a material with an electro-optic effect, is processed to an appropriate thickness. Figure 5C This shows the state after the third step.

[0104] In the fourth step, for example by dry etching, the portion other than the rib 6 is removed to form a substrate 5 having the rib 6. Figure 5D This shows the state after the fourth step.

[0105] In the fifth step, for example, a buffer layer 9a is formed on the substrate 5 by sputtering or the like. Figure 5E The state after step five is shown.

[0106] In the sixth step, for example, electrodes (e.g., signal electrodes and ground electrodes) are formed on the upper surface buffer layer 9a. Figure 5F The state after step six is ​​shown.

[0107] It should be noted that the direct bonding method used in the second step described above is a preferred method for bonding dissimilar materials. The layer that forms the buffer layer 9b and the reinforcing substrate 7 are made of different materials, but by using the direct bonding method, they can be properly and reliably bonded together.

[0108] Direct bonding methods are broadly divided into two types: plasma-activated bonding and FAB (Fast Atom Beam) bonding.

[0109] Plasma-activated bonding is a method of direct bonding by directly overlapping two surfaces after hydrophilic treatment of the two surfaces, which improves their bonding properties through plasma or similar methods. In the case of plasma-activated bonding, an interface layer (bonding layer) is obtained by intertwining and dissolving the molecular chains of the buffer layer 9b and the reinforcing substrate 7 on their respective surfaces.

[0110] On the other hand, the FAB method involves forming thin Si layers and metal oxide layers on the two bonding surfaces respectively, activating the two surfaces by irradiating them with a neutral atom beam at room temperature, and then directly bonding the two surfaces together. In the FAB method, a thin adhesive layer, such as a Si layer and a metal oxide layer, is formed between the buffer layer 9b and the reinforcing substrate 7.

[0111] When the buffer layer 9b is directly bonded to the reinforcing substrate 7 using the FAB method, such as Figure 6 As shown, an extremely thin adhesive layer 20 of approximately 10–500 nm is formed between the buffer layer 9b and the reinforcing substrate 7. The adhesive layer 20 uses materials such as Si, Al2O3, Ta2O5, TiO2, Nb2O5, Si3N4, AlN, and SiO2.

[0112] Figure 6 This is a fourth example of a cross-sectional structure of an optical waveguide element according to an embodiment of the present invention. Figure 1 A sectional view of line segment PP. It should be noted that... Figure 6 The diagram illustrates the relationship between... Figure 2 The same viewpoint for cross-sectional structure. It should be noted that, in Figure 6 In the diagram, the optical waveguide element 1 is illustrated with its thickness direction as the vertical direction of the paper, its width direction as the horizontal direction of the paper, and its length direction as the direction perpendicular to the paper.

[0113] Materials that can be used as adhesive layer 20 include those with high light absorption. However, a lower surface buffer layer 9b exists between adhesive layer 20 and substrate 5, which suppresses light absorption generated by adhesive layer 20. In other words, when buffer layer 9b is directly bonded to reinforcing substrate 7 using the FAB method, lower surface buffer layer 9b mitigates stress skewing on the upper and lower surfaces of substrate 5, and also suppresses light absorption by adhesive layer 20.

[0114] In this embodiment, a ribbed substrate with ribs 6 formed on the substrate 5 is described as an example. However, as mentioned above, the present invention is not limited to ribbed substrates, and can also be applied to substrates in which the optical waveguide 10 is formed within the substrate 5 through thermal diffusion of metal, for example. In substrates with diffused optical waveguides, the structure in which the substrate 5 is sandwiched between an upper surface buffer layer 9a and a lower surface buffer layer 9b can similarly mitigate stress imbalance on the upper and lower surfaces of the substrate 5.

[0115] Furthermore, in this embodiment, a coplanar circuit structure with one ground electrode G arranged on each side of a signal electrode S is described as an example. However, the present invention is not limited to such a coplanar circuit structure; for example, a coplanar circuit structure having a differential circuit with one ground electrode G arranged on each side of two parallel signal electrodes S can be used.

[0116] The present invention can provide an optical modulator in which at least a portion of the optical modulator uses an optical waveguide constituting the optical waveguide element described in this embodiment.

[0117] Figure 7 This is a top view showing an example of the structure of the optical modulator 200 according to an embodiment of the present invention. Figure 7 The optical modulator 200 shown includes an optical waveguide element 202, a housing 204 housing the optical waveguide element 202, an input optical fiber 208 for directing light into the optical waveguide element 202, and an output optical fiber 210 for guiding light output from the optical waveguide element 202 to the outside of the housing 204. It should be noted that... Figure 7 The structure of the optical modulator 200 shown is merely an example, and the present invention is not limited to this structure. Optical waveguide elements with the features of the present invention can be incorporated into optical modulators with any structure.

[0118] Figure 7 The optical modulator 200 shown has an input optical fiber 208 at one end in the length direction (left side of the figure) and an output optical fiber 210 at the other end in the length direction (right side of the figure). However, the input and output positions of the light in the optical modulator 200 can be arbitrarily set.

[0119] The optical waveguide element 202, for example, has an optical waveguide 206 disposed on a substrate, and a plurality of electrodes 212a to 212d formed on the substrate for modulating light waves propagating within the optical waveguide 206. The optical waveguide element 202, for example... Figure 7 The optical waveguide 206 shown is a combination of multiple Mach-Zehnder type optical waveguides.

[0120] Figure 7The optical modulator 200 shown is an example in which two beams of light output from the optical waveguide element 202 are polarized and combined by the polarization combining unit 228, and the resulting light is output to the outside of the housing 204 via the output optical fiber 210. However, the optical modulator 200 of the present invention is not limited to such a structure. For example, it can be the structure described above. Figure 1 The optical waveguide element 1 shown has a first combining section 3a and outputs an optical signal from the output waveguide.

[0121] Furthermore, the optical waveguide element 202, like the optical waveguide element 1 described above, has the following structure: an upper surface buffer layer is formed in contact with the upper surface of the substrate, and a lower surface buffer layer is formed in contact with the lower surface of the substrate. Both the upper and lower surface buffer layers are made of approximately the same material and have approximately the same thickness. This structure sandwiches the substrate between the upper and lower surface buffer layers, achieving uniform stress balance on the upper and lower surfaces of the substrate.

[0122] The housing 204 consists of a shell and a cover that secure the optical waveguide element 202. The cover is configured to cover the entire shell, thereby sealing the interior of the housing 204 airtight. It should be noted that electronic components such as drivers and photodetectors (PD) can be housed inside the housing 204.

[0123] The housing 204 has multiple pins 240a to 240d, which are conductors for inputting high-frequency signals. Pins 240a to 240d are connected via a relay substrate 218 to one end of each of multiple electrodes 212a to 212d disposed in the Mach-Zehnder type optical waveguide of the optical waveguide element 202. Furthermore, the other end of each of the multiple electrodes 212a to 212d is terminated via a terminating substrate 250, which serves as an impedance element. It should be noted that although in Figure 7 The detailed structure is omitted from the diagram, but the multiple electrodes 212a to 212d, including the signal electrode S and the ground electrode G, are capable of modulating the light waves propagating in the optical waveguide 206.

[0124] As described above, according to the present invention, it is possible to provide an optical modulator including an optical waveguide element having the following structure: an upper surface buffer layer is formed in contact with the upper surface of a substrate, and a lower surface buffer layer is formed in contact with the lower surface of a substrate, wherein the upper surface buffer layer and the lower surface buffer layer are made of substantially the same material and have substantially the same thickness.

[0125] This invention is not limited to the above-described embodiments and variations. Various variations and design changes that do not depart from the technical concept of this invention are included within its technical scope.

[0126] Industrial applicability

[0127] This invention provides an optical waveguide element and an optical modulator that can prevent damage to the substrate and degradation of substrate characteristics caused by stress by reducing the influence of stress on the substrate generated by the buffer layer, and can be applied to the fields of optical communication, optical measurement, etc.

[0128] Label Explanation

[0129] 1. 202 Optical Waveguide Component

[0130] Branches 2a to 2c

[0131] 3a~3c Synthesis Section

[0132] 5. 102 substrate

[0133] 6. Ribs

[0134] 7. 101 reinforced substrate

[0135] 9a Upper surface buffer layer (first buffer layer)

[0136] 9b Lower surface buffer layer (second buffer layer)

[0137] 10, 206 Optical Waveguide

[0138] 20 Adhesive Layer

[0139] 103 Buffer Layer

[0140] 200 optical modulator

[0141] 204 Stainless Steel Casing

[0142] 208 Input Fiber Optics

[0143] 210 Output Fiber

[0144] Electrodes 212a, 212b, 212c, and 212d

[0145] 218 relay substrate

[0146] 228 Polarization Combining Section

[0147] 240a, 240b, 240c, 240d pins

[0148] 250 terminal substrate

[0149] G Grounding electrode

[0150] S signal electrode

Claims

1. An optical waveguide element comprising: The substrate exhibits an electro-optic effect; An optical waveguide is formed on the substrate; A first buffer layer is disposed on the substrate; A second buffer layer is disposed under the substrate; and A reinforcing substrate is disposed beneath the second buffer layer. The optical waveguide element is characterized in that... The first buffer layer and the second buffer layer are made of substantially the same material and have substantially the same thickness. The first buffer layer is formed in contact with the upper surface of the substrate, and the second buffer layer is formed in contact with the lower surface of the substrate. The thickness of the substrate is less than 2.0 μm. The lower surface of the second buffer layer and the upper surface of the reinforcing substrate are directly bonded together by a direct bonding method. An interface layer or adhesive layer is formed between the second buffer layer and the reinforcing substrate. The interface layer is a layer obtained by the molecular chains of the respective faces of the second buffer layer and the reinforcing substrate intertwining and dissolving with each other. The thickness of the adhesive layer is 10 to 500 nm.

2. The optical waveguide element according to claim 1, characterized in that, The first buffer layer and the second buffer layer are respectively composed of a transparent insulating film of a mixture or an oxide, wherein the mixture is a mixture formed of silicon oxide and an oxide of at least one of the elements selected from groups III to VIII, IB and IIB of the periodic table and semiconductor elements other than silicon, and the oxide is an oxide formed of silicon and an oxide formed from one or more of the elements selected from the metal elements and semiconductor elements.

3. The optical waveguide element according to claim 1 or 2, characterized in that, The refractive index of the materials of the first buffer layer and the second buffer layer is lower than that of the material of the substrate having an electro-optic effect.

4. The optical waveguide element according to claim 1 or 2, characterized in that, The resistivity of the materials of the first buffer layer and the second buffer layer is 10. 8 Ωcm or more and 10 16 Below Ωcm.

5. The optical waveguide element according to claim 1 or 2, characterized in that, The thickness of the first buffer layer and the second buffer layer is 0.3 μm or more and 2.0 μm or less.

6. The optical waveguide element according to claim 1 or 2, characterized in that, The ribs protruding from the substrate are used as the optical waveguide.

7. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide is formed by multiple Mach-Zehnder sections.

8. An optical modulator, characterized in that, At least a portion of the optical modulator uses an optical waveguide that constitutes an optical waveguide element according to any one of claims 1 to 7.

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