Optical waveguide element, optical modulator, and optical transmission device

Abstract

Provided is an optical waveguide element that improves confinement of light waves in an optical waveguide in order to achieve low power consumption. An optical waveguide element 10A according to the present invention is configured to comprise: a substrate 120 made of an electro-optical crystal; a buffer layer 130 formed on the upper surface of the substrate 120; and a high refractive index part 140 that is made of a material having a refractive index higher than that of a material constituting the substrate 120 and that is loaded on the upper surface of the buffer layer 130. The optical waveguide element 10A is characterized by including a portion of the substrate 120 and having, in a cross section orthogonal to the light propagation direction of the optical waveguide 20, the width of the buffer layer 130 narrower than the width of the high refractive index part 140.

Description

Optical waveguide element, optical modulator, and optical transmitter

[0001] The present invention relates to an optical waveguide element in which an optical waveguide is formed, an optical modulator including the optical waveguide element, and an optical transmitter including the optical modulator.

[0002] In recent years, with the increase in the amount of information, there has been a demand for faster, larger capacity, and lower power consumption in optical communications used over long distances, between cities, and between data centers. Due to the space limitations of base stations, there is a need for faster, more energy-efficient, and more compact optical modulators.

[0003] Since there is little distortion and low optical loss when converting an electrical signal into an optical signal, for example, lithium niobate (LiNbO ３ Substrates made of materials with electro-optical effects, such as LN (Liquid Crystal No. 10001001: hereafter referred to as LN), are used. Advances in lamination and smart cut technology have made it possible to thin LN substrates, and research and development into miniaturized LN optical modulators is progressing.

[0004] Regarding low power consumption, it is possible to reduce the drive voltage of the optical modulator by tightly confining light in a thin LN substrate. For example, Non-Patent Document 1 below reports research into loading a high refractive index material such as silicon (Si) onto an LN substrate as a method of tightly confining light in a thin LN substrate.

[0005] However, as suggested in Patent Document 1 below, when Si comes into direct contact with an LN substrate, the Si takes in oxygen in the LN substrate, causing oxygen vacancies in the LN substrate, which leads to drift and deterioration in the quality of the LN crystal. To solve this problem, for example, silicon dioxide (SiO ２ This prevents direct contact of Si with the LN substrate, thereby preventing oxygen deficiency in the LN substrate.

[0006] In the following Patent Document 2, an LN layer, SiO ２ layer (upper layer), Si layer (light guide core segment), SiO ２The electro-optic modulator described in Patent Document 2 has a portion where a SiO layer (second cladding layer) is laminated between an LN layer and a Si layer as an optical waveguide core. ２ The LN layer is formed such that Si does not come into direct contact with the LN layer.

[0007] JP 2019-174733 A U.S. Patent Application Publication No. 2023 / 0055077

[0008] "Heterogeneous silicon-on-lithium niobate electro-optic modulator for 100-Gbaud modulation", APL Photonics 7(12), 126103 (2022)

[0009] However, as described in Patent Document 2, the SiO ２ When a layer is formed, the confinement of light waves in the LN layer becomes weaker, and there is a problem that the driving voltage must be increased to ensure a sufficient amount of light.

[0010] The present invention has been made in view of the above problems, and aims to provide an optical waveguide element in which the confinement of light waves in an optical waveguide is improved in order to realize low power consumption, an optical modulator including the optical waveguide element, and an optical transmitting device including the optical modulator.

[0011] In order to solve the above problems, the optical waveguide element, the optical modulator, and the optical transmitter according to the present invention have the following technical features.

[0012] In order to achieve the above-mentioned object, the optical waveguide element of the present invention is an optical waveguide element comprising a substrate made of an electro-optic crystal, a buffer layer formed on the upper surface of the substrate, and a high refractive index portion made of a material having a higher refractive index than the material constituting the substrate and loaded on the upper surface of the buffer layer, wherein the optical waveguide is formed to include a part of the substrate, and in a cross section perpendicular to the light propagation direction of the optical waveguide, the width of the buffer layer is narrower than the width of the high refractive index portion.

[0013] According to the above configuration, a refractive index difference can be created in the width direction between the substrate and the high refractive index portion, and oxygen deficiency in the substrate, which is a cause of drift, can be prevented by forming a buffer layer, while the high refractive index portion strengthens the confinement of light waves in the substrate, ensuring a sufficient amount of light and reducing the driving voltage.

[0014] In the above-described configuration, the optical waveguide element according to the present invention may further include a low-refractive-index portion disposed between the substrate and the high-refractive-index portion and in contact with a side surface of the buffer layer, and the refractive index of a material constituting the low-refractive-index portion may be lower than the refractive index of a material constituting the buffer layer.

[0015] According to the above configuration, by providing a refractive index difference between the substrate and the high refractive index portion along the width direction by arranging the low refractive index portion, it is possible to improve the effect of confining light waves in the substrate.

[0016] In the optical waveguide element according to the present invention, in the above configuration, the material constituting the buffer layer may have a lower refractive index than the material constituting the substrate, and the refractive index of the material constituting the low refractive index portion may be lower than the refractive index of the material constituting the buffer layer.

[0017] According to the above configuration, by making the refractive indexes of the components have a specific relationship, it is possible to improve the effect of confining light waves in the substrate.

[0018] In the optical waveguide element according to the present invention having the above configuration, the material forming the low refractive index portion may be air.

[0019] According to the above configuration, by making the low refractive index portion an air layer (air gap), the refractive index of the low refractive index portion can be easily reduced, thereby improving the effect of confining light waves to the substrate.

[0020] In the optical waveguide element according to the present invention, in the above configuration, the high refractive index portion may be made of a material that bonds with oxygen.

[0021] According to the above configuration, by using a high refractive index material that can form oxygen bonds with oxides, it is possible to make the buffer layer made of, for example, an oxide material adhere to the high refractive index portion through oxygen bonds.

[0022] In the optical waveguide element according to the present invention, in the above configuration, the material forming the high refractive index portion may contain an element of Groups 3 to 17 of the periodic table.

[0023] According to the above configuration, the selectivity of materials is increased, so that materials with various refractive indices can be selected, and the degree of freedom in design can be improved.

[0024] In the optical waveguide element according to the present invention having the above configuration, the buffer layer may be made of a material that is a metal oxide.

[0025] According to the above configuration, the buffer layer does not remove oxygen from the substrate, thereby preventing oxygen deficiency in the substrate, and the high refractive index portion, for example made of a material that bonds with oxygen, can be adhered to the buffer layer by oxygen bonding.

[0026] In the optical waveguide element according to the present invention, in the above configuration, a convex portion may be formed on the substrate, and the buffer layer may be formed on an upper surface of the convex portion.

[0027] According to the above configuration, the distribution of light waves onto the substrate can be strengthened.

[0028] In the optical waveguide element according to the present invention, in the above configuration, at least one of the upper surface and the lower surface of the high refractive index portion may be formed to have a curved surface.

[0029] According to the above configuration, it is possible to efficiently reflect light waves and suppress the occurrence of scattered light, thereby reducing the propagation loss of light waves distributed in the high refractive index portion.

[0030] In the optical waveguide element according to the present invention having the above configuration, the width of the buffer layer may be 10% to 99% of the width of the high refractive index portion.

[0031] According to the above-mentioned configuration, it is possible to effectively achieve both the effect of preventing oxygen deficiency in the substrate, which is a cause of drift, and the effect of strengthening the confinement of light waves in the substrate.

[0032] In the optical waveguide element according to the present invention having the above configuration, the thickness of the buffer layer may be 0.2% to 100% of the thickness of the high refractive index portion.

[0033] According to the above-mentioned configuration, it is possible to effectively achieve both the effect of preventing oxygen deficiency in the substrate, which is a cause of drift, and the effect of strengthening the confinement of light waves in the substrate.

[0034] In the optical waveguide element according to the present invention having the above configuration, the high refractive index portion may have a thickness of 1% to 40% of the thickness of the substrate.

[0035] According to the above configuration, excessive distribution of light waves in the high refractive index portion can be prevented, and the light waves can be more tightly confined in the substrate.

[0036] In the optical waveguide element according to the present invention, in the above configuration, the optical waveguide may be provided with an optical multiplexing / demultiplexing unit that branches or couples the optical waveguide, or a spot size conversion unit that changes the cross-sectional diameter of a light wave propagating through the optical waveguide.

[0037] According to the above configuration, it is possible to appropriately branch or couple the light waves propagating through the optical waveguide, and to appropriately change the cross-sectional diameter of the light waves.

[0038] In order to achieve the above object, an optical modulator according to the present invention is characterized by comprising the above optical waveguide element, a housing that houses the optical waveguide element, an input optical fiber connected to an optical input portion of the optical waveguide element, and an output optical fiber connected to an optical output portion of the optical waveguide element.

[0039] In the optical modulator according to the present invention, in the above configuration, a modulation electrode that modulates a light wave propagating through the optical waveguide is provided in the optical waveguide element, and a signal amplifier circuit that amplifies a modulation signal applied to the modulation electrode may be provided inside the housing.

[0040] In addition, in order to achieve the above object, an optical transmitting device according to the present invention is characterized by having the above optical modulator, a light source that inputs a light wave to the optical modulator, and a signal output circuit that outputs the modulated signal.

[0041] According to the present invention, in an optical waveguide element in which a high refractive index portion having a refractive index higher than that of the substrate is loaded on a buffer layer formed on a substrate, by making the width of the buffer layer narrower than the width of the high refractive index portion, it is possible to improve the confinement of light waves in the optical waveguide and achieve low power consumption.

[0042] 1 is a plan view showing an example of the overall configuration of an optical waveguide element according to a first embodiment of the present invention. FIG. 1(a) is a view showing a cross section taken along line A-A in FIG. 1, and FIG. 1(b) is an enlarged view of the vicinity of the optical waveguide. FIG. 1(b) is a view showing the vicinity of the optical waveguide of the optical waveguide element according to the first embodiment of the present invention, and is a view for explaining the dimensions of each component. FIG. 1(c) is a graph showing a simulation result illustrating the relationship between the ratio of the height of a high refractive index portion to the height of a substrate and electrode absorption loss in the first embodiment of the present invention. FIG. 1(d) is a graph showing a simulation result illustrating the relationship between the ratio of the height of a buffer layer to the height of a high refractive index portion and electrode absorption loss in the first embodiment of the present invention. FIG. 1(a) to (d) are views for explaining first to fourth steps of a manufacturing method for an optical waveguide element according to the first embodiment of the present invention, respectively. FIG. 1(c) is a view showing a cross section of an optical waveguide element according to a second embodiment of the present invention. FIG. 1(d) is a view showing a cross section of an optical waveguide element according to a third embodiment of the present invention. FIG. 1(d) is a view showing a cross section of an optical waveguide element according to a fourth embodiment of the present invention. FIG. 1(d) is a view showing a cross section of a first example of an optical waveguide element according to a fifth embodiment of the present invention. FIG. 1 is a diagram showing a cross section of a second example of an optical waveguide element according to the fifth embodiment of the present invention. FIG. 2 is a diagram showing a cross section of a third example of an optical waveguide element according to the fifth embodiment of the present invention. FIG. 3 is a diagram showing a cross section of an optical waveguide element according to a sixth embodiment of the present invention. FIG. 4 is a diagram showing a cross section of an optical waveguide element according to a seventh embodiment of the present invention. FIG. 5 is a diagram showing a cross section of an optical waveguide element according to an eighth embodiment of the present invention. FIG. 6 is a diagram showing a cross section of an optical waveguide element according to a ninth embodiment of the present invention. FIG. 7 is a diagram showing a cross section of an optical waveguide element according to a tenth embodiment of the present invention. FIG. 8 is a diagram showing a cross section of an optical waveguide element according to an eleventh embodiment of the present invention. FIG. 9 is a diagram showing a cross section of an optical waveguide element according to a twelfth embodiment of the present invention. FIG. 10 is a diagram showing a cross section of an optical waveguide element according to a thirteenth embodiment of the present invention. FIG. 11 is a plan view showing an example of the overall configuration of an optical waveguide element according to a fourteenth embodiment of the present invention. FIG. 12 is a plan view showing an example of the overall configuration of an optical waveguide element according to a fifteenth embodiment of the present invention. FIG. 13 is a plan view showing an example of the overall configuration of an optical waveguide element according to a sixteenth embodiment of the present invention.Fig. 23 is a plan view showing an example of the overall configuration of an optical waveguide element according to a seventeenth embodiment of the present invention Fig. 24 is a plan view showing an optical modulator and an optical transmitter according to the present invention.

[0043] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The drawings referred to in this specification are not necessarily to scale with respect to actual dimensions, and some parts are exaggerated or simplified to schematically illustrate the configuration according to the present invention. Furthermore, the numerical ranges described in this specification are ranges that include upper and lower limits, meaning that any numerical value within the numerical range can be selected.

[0044] As exemplified in each embodiment, the optical waveguide element according to the present invention comprises a substrate made of electro-optic crystal, a buffer layer formed on the upper surface of the substrate, and a high-refractive-index portion made of a material having a higher refractive index than the material constituting the substrate and loaded on the upper surface of the buffer layer. The optical waveguide is formed to include a part of the substrate, and in a cross section perpendicular to the light propagation direction of the optical waveguide, the width of the buffer layer is set to be narrower than the width of the high-refractive-index portion.

[0045] First Embodiment An optical waveguide element 10A according to a first embodiment of the present invention will now be described.

[0046] First, the overall configuration of an optical waveguide element 10A according to this embodiment will be described with reference to Fig. 1. Fig. 1 is a plan view showing an example of the overall configuration of an optical waveguide element 10A according to this embodiment. Hereinafter, the left-right direction of the plan view shown in Fig. 1 may be referred to as the longitudinal direction of the optical waveguide element 10A, and the up-down direction of the plan view shown in Fig. 1 may be referred to as the width direction of the optical waveguide element 10A.

[0047] The optical waveguide element 10A shown in FIG. 1 is an optical waveguide element in which a Mach-Zehnder (MZ) optical waveguide is formed as the optical waveguide 20. In the optical waveguide element 10A, an X-cut LN substrate having a high electro-optic constant in the cross-sectional direction is used as the substrate 120 having an electro-optic effect. As shown in FIG. 1, a signal electrode S and a ground electrode G, which are modulation electrodes, are disposed on the substrate 120. Specifically, a single signal electrode S and two ground electrodes G are disposed on either side of the signal electrode S on the upper surface of the substrate 120. The optical waveguide element 10A shown in FIG. 1 is an optical waveguide element having an MZ traveling-wave single-ended electrode structure on an X-cut LN substrate.

[0048] However, the present invention is applicable to both X-cut and Z-cut optical crystals of the substrate 120. In the case of an X-cut LN substrate, the modulation electrode is disposed on the side of the optical waveguide 20, and in the case of a Z-cut LN substrate, the modulation electrode is disposed above or below the optical waveguide 20.

[0049] 1 is merely one example, and the present invention is applicable to optical waveguide elements 10B to 10E (see FIGS. 22 to 25) having various electrode structures, which will be described later. As will be described later, the present invention is capable of adjusting the group refractive index (ng) by arranging high refractive index portions 140 (see, for example, FIGS. 2(a) and 2(b)). This makes it possible to perform velocity matching regardless of whether the electrode structure is a traveling wave type or a capacitance-loaded type (segment electrode).

[0050] 1 has an MZ-type optical waveguide formed therein, as an example, but the optical waveguide 20 according to the present invention is not limited to the MZ-type optical waveguide.

[0051] An MZ-type optical waveguide is a waveguide having at least one branching section 30 and at least one multiplexing section 40 as basic components. The branching section 30 is a section that branches one optical waveguide 20 into two optical waveguides 20. The multiplexing section 40 is a section that connects two optical waveguides 20 to combine them into one optical waveguide 20. Optical multiplexing / demultiplexing sections such as the branching section 30 and the multiplexing section 40 may be fabricated by adjusting the shape, size, and refractive index of the sections that make up the optical waveguide 20, or an optical coupler or the like may be disposed. Note that an MZ-type optical waveguide may have two or more branching sections 30 or two or more multiplexing sections 40.

[0052] 1, an input optical waveguide 21, branching waveguides 22 and 23, and an output optical waveguide 24 are formed as an optical waveguide 20. The optical input section 11 is composed of an optical input end 20a and the input optical waveguide 21. A branching section 30 is provided downstream of the input optical waveguide 21, and the input optical waveguide 21 is branched into two branching waveguides 22 and 23. The two branching waveguides 22 and 23 extend approximately parallel to each other and merge at a multiplexing section 40. The output optical waveguide 24 and optical output end 20b that constitute the optical output section 12 are connected downstream of the multiplexing section 40.

[0053] An external light wave is input to the input optical waveguide 21 through the optical input end 20a and branched (demultiplexed) into two branch waveguides 22 and 23 at the branching section 30. The branch waveguides 22 and 23 extend between the signal electrode S and two ground electrodes G and are provided with a modulation section (operation section) 13 that modulates an optical signal using an electric field generated between the signal electrode S and the two ground electrodes G. The light waves propagating through each of the two branch waveguides 22 and 23 are appropriately modulated by the modulation section 13 and then combined (combined) at the combining section 40. Note that the modulation section 13 may be configured to modulate only one of the two branch waveguides 22 and 23. The light waves combined at the combining section 40 are output to the outside through the output optical waveguide 24 and the optical output end 20b.

[0054] As shown in FIG. 1 , the MZ-type optical waveguide may be provided with spot size converters 50 and 60 that change the mode field diameter of the light wave. The locations of the spot size converters 50 and 60 are not particularly limited. However, in the optical waveguide element 10A shown in FIG. 1 , the spot size converters 50 and 60 are provided at the optical input section 11 and the optical output section 12, which are the ends of the MZ-type optical waveguide. This allows the cross-sectional diameter of the light wave propagating through the MZ-type optical waveguide to be changed. The configuration of the spot size converters 50 and 60 is not particularly limited and can be realized using existing technology. For example, the shape, size, and refractive index of the components of the optical waveguide 20 may be adjusted to gradually change the cross-sectional diameter of the light wave along the light propagation direction, or an optical element or the like may be disposed therein.

[0055] The cross-sectional structure of the optical waveguide element 10A in this embodiment will be described. FIGS. 2(a) and 2(b) are diagrams showing a cross section taken along line A-A in FIG. 1. FIG. 2(a) shows a cross section perpendicular to the longitudinal direction of the optical waveguide element 10A (a cross section perpendicular to the light propagation direction in the optical waveguide 20). FIG. 2(b) shows an enlarged view of the vicinity of the optical waveguide 20 (region R1). The left-right direction of the cross-sectional views shown in FIGS. 2(a) and 2(b) corresponds to the width direction of the optical waveguide element 10A. Hereinafter, the up-down direction of the cross-sectional views shown in FIGS. 2(a) and 2(b) may be referred to as the height direction of the optical waveguide element 10A.

[0056] As shown in the cross-sectional view of FIG. 2(a), the optical waveguide element 10A is made of SiO ２ A substrate 120 is provided on a support substrate (intermediate layer) 110 made of, for example, a metal layer, and a signal electrode S and a ground electrode G are provided on the substrate 120. The signal electrode S and the ground electrode G are arranged at a predetermined distance from each other. There are no particular restrictions on the distance between the signal electrode S and the ground electrode G, but it is preferably set to about 0.5 μm to 10 μm in order to accommodate low power consumption and miniaturization.

[0057] The substrate 120 is made of a material having an electro-optic effect. There are no particular limitations on the thickness (height H1) of the substrate 120, but it is preferably 0.024 μm to 3.0 μm, more preferably 0.050 μm to 2.0 μm, and even more preferably 0.100 μm to 1.0 μm. By making the substrate 120 thinner, it is possible to reduce the driving voltage.

[0058] Between the signal electrode S and the ground electrode G, a buffer layer 130 is formed on the substrate 120. Furthermore, a high refractive index portion 140 is loaded on the buffer layer 130. The width of the buffer layer 130 is set to be shorter than the width of the high refractive index portion 140, and a low refractive index portion 150 is disposed on the side of the buffer layer 130 (i.e., at a position adjacent to the buffer layer 130 in the width direction).

[0059] In this embodiment, the buffer layer 130 and the high refractive index portion 140 are arranged so that their widthwise central portions coincide with each other. The low refractive index portions 150, which are arranged symmetrically on both the left and right sides of the buffer layer 130, are set to have approximately the same width. Furthermore, the combined width of the buffer layer 130 and the low refractive index portions 150 arranged on both the left and right sides thereof is set to approximately coincide with the width of the high refractive index portion 140.

[0060] The high refractive index portion 140 loaded on the buffer layer 130 has the function of confining light waves within the substrate 120. The optical waveguide 20 is formed to include a portion of the substrate 120, and more specifically, as shown by the dotted lines in FIG. 2B , is formed in a region that includes a portion of the substrate 120, a portion of the buffer layer 130, and a portion of the high refractive index portion 140. When an electric field generated between the signal electrode S and the ground electrode G is applied to the optical waveguide 20, the light waves propagating through the optical waveguide 20 can be modulated.

[0061] Furthermore, an over-cladding layer 160 is provided so as to cover the upper and side surfaces of the high-refractive-index portions 140, the side surfaces of the low-refractive-index portions 150, and the upper surface of the substrate 120. By providing the over-cladding layer 160, the low-refractive-index portions 150 have their lower surfaces in contact with the substrate 120, their upper surfaces in contact with the high-refractive-index portions 140, and their side surfaces in contact with the buffer layer 130 and the over-cladding layer 160, and are thus surrounded by the substrate 120, the buffer layer 130, the high-refractive-index portions 140, and the over-cladding layer 160.

[0062] Thus, when viewed in a cross section perpendicular to the light propagation direction of the optical waveguide 20, the optical waveguide element 10A has a configuration in which the buffer layer 130 is formed on the substrate 120, the high refractive index portions 140 are loaded on the buffer layer 130, the low refractive index portions 150 are arranged on the sides of the buffer layer 130, and the upper surface side is covered with the overclad layer 160. The optical waveguide 20 is formed in the laminated structure in which the substrate 120, the buffer layer 130, and the high refractive index portions 140 are laminated, and includes at least a portion of the substrate 120.

[0063] 2(a) and 2(b) are cross sections taken along line A-A in FIG. 1, and illustrate a state in which the branched waveguide 22 appearing in this cross section is formed in a laminated structure of the substrate 120, the buffer layer 130, and the high-refractive-index portions 140. However, the laminated structure of the substrate 120, the buffer layer 130, and the high-refractive-index portions 140 may be provided at any location in the optical waveguide 20 of the optical waveguide element 10A. For example, the laminated structure of the substrate 120, the buffer layer 130, and the high-refractive-index portions 140 may be provided throughout the entire optical waveguide 20 of the optical waveguide element 10A, or the laminated structure of the substrate 120, the buffer layer 130, and the high-refractive-index portions 140 may be provided only at a portion of the optical waveguide 20.

[0064] The materials constituting the optical waveguide element 10A will be described below.

[0065] As described above, the substrate 120 can be made of a material having an electro-optic effect. For example, lithium niobate (LN) can be used as the material for the substrate 120, but lithium tantalate (LiTaO ３), lead lanthanum zirconate titanate (PLZT), etc. may also be used. Alternatively, the optical waveguide 20 may be formed by diffusing titanium (Ti) or the like onto the surface of the substrate 120 by thermal diffusion, proton exchange, or the like.

[0066] The buffer layer 130 can be made of a metal oxide, such as SiO 2 , which has excellent stability and processability. ２ or Al ２ O ３ By forming the buffer layer 130 in contact with the substrate 120 using a metal oxide, the buffer layer 130 can be prevented from taking oxygen from the substrate 120, and oxygen deficiency in the substrate 120 can be prevented.

[0067] The high refractive index portion 140 can be made of a material that bonds with oxygen. The material that makes up the high refractive index portion 140 can be selected from materials containing elements of groups 3 to 17 of the periodic table, and one example is Si. By making the high refractive index portion 140 in contact with the buffer layer 130 out of a material that bonds with oxygen, the buffer layer 130 made of a metal oxide can be closely attached to the high refractive index portion 140 by oxygen bonding.

[0068] The low refractive index portion 150 can be made of a material having a lower refractive index than the buffer layer 130. Since the low refractive index portion 150 is in contact with the substrate 120, it is preferable to use a material for the low refractive index portion 150 that does not remove oxygen from the substrate 120. As an example of the material for the low refractive index portion 150, air or resin can be used. Also, for example, Al can be used as the material for the buffer layer 130. ２ O ３ is used, the low refractive index portion 150 is made of SiO ２ can also be used.

[0069] When air is used as the material constituting the low refractive index section 150, the low refractive index section 150 becomes an air layer (air gap), and the low refractive index section 150 surrounded by the substrate 120, the buffer layer 130, the high refractive index section 140 and the overclad layer 160 becomes a cavity.

[0070] The buffer layer 130 and the low refractive index portion 150 are interposed between the substrate 120 and the high refractive index portion 140. By forming the low refractive index portion 150 from a material having a lower refractive index than the buffer layer 130, a refractive index difference can be provided in the width direction in which the buffer layer 130 and the low refractive index portion 150 are arranged.

[0071] The material for the overcladding layer 160 is not particularly limited, but may be, for example, SiO 2 , which has excellent stability and processability. ２ or Al ２ O ３ The overcladding layer 160 and the buffer layer 130 may be made of the same material.

[0072] For example, if the substrate 120 is made of LN and the high refractive index portion 140 is made of Si, when the high refractive index portion 140 comes into contact with the substrate 120, the Si constituting the high refractive index portion 140 may take in oxygen in the substrate 120, causing oxygen vacancies in the substrate 120. In contrast, the optical waveguide element 10A of this embodiment has a structure in which, for example, SiO ２ The buffer layer 130 is made of a material selected from the group consisting of silicon dioxide, silicon dioxide, and silicon dioxide itself, and the low refractive index portion 150 is made of air (i.e., an air gap). This prevents the substrate 120 from coming into contact with the high refractive index portion 140, thereby preventing oxygen deficiency in the substrate 120 that would occur due to contact with the high refractive index portion 140.

[0073] When selecting the materials constituting the substrate 120, buffer layer 130, high-refractive-index portion 140, and low-refractive-index portion 150, it is preferable to consider the relationship in the magnitude of the refractive index of each material. Specifically, by making the refractive index of the material constituting the high-refractive-index portion 140 (e.g., Si) higher than the refractive index of the material constituting the substrate 120 (e.g., LN), the distribution of light waves toward the upper surface side of the substrate 120 (the side where the high-refractive-index portion 140 is located) is suppressed, strengthening the confinement of light waves within the substrate 120, thereby reducing the driving voltage while ensuring a sufficient amount of light. The refractive index of LN is approximately 2.2, although this varies depending on the cut direction, temperature, wavelength of light, etc., while the refractive index of Si is approximately 3.5.

[0074] Furthermore, the refractive index of the material constituting the buffer layer 130 may be lower than the refractive index of the material constituting the substrate 120, and the refractive index of the material constituting the low refractive index portion 150 may be lower than the refractive index of the material constituting the buffer layer 130. In other words, the relationship between the refractive indices of the respective components may be as follows:

[0075] Refractive index relationship: high refractive index portion 140 > substrate 120 > buffer layer 130 > low refractive index portion 150

[0076] For example, the substrate 120 is made of LN, and the buffer layer 130 is made of SiO. ２ When Si is used as the material of the high refractive index portion 140 and air is used as the material of the low refractive index portion 150, the refractive index of LN is about 2.2, and that of SiO ２ The refractive index of Al is about 1.45, the refractive index of Si is about 3.5, and the refractive index of air is about 1.0, which satisfy the above refractive index relationships. ２ O ３ When using Al ２ O ３ The refractive index of the low refractive index portion 150 is about 1.63. ２ Furthermore, the refractive index of the resin is 1.0 to 1.6, and a resin having a lower refractive index than the material of the buffer layer 130 may be selected as the material of the low refractive index portion 150.

[0077] Next, the dimensions of each component constituting the optical waveguide element 10A will be described with reference to Fig. 3. Fig. 3 is a diagram showing the vicinity of the optical waveguide 20 of the optical waveguide element 10A in this embodiment (region R1 in Fig. 2(a)), and is a diagram for explaining the dimensions of each component. The width W2 and height H2 indicate the width and thickness (layer thickness) of the high refractive index portion 140. The width W3 and height H3 indicate the width and thickness (layer thickness) of the buffer layer 130.

[0078] The width W2 of the high refractive index portion 140 can be appropriately selected depending on the cross-sectional diameter of the light wave propagating through the optical waveguide 20, and is, for example, 0.5 μm to 2.5 μm.

[0079] The height H2 of the high refractive index portion 140 is not particularly limited, but is, for example, 0.01 μm to 0.5 μm. The lower limit of 0.01 μm is the minimum film thickness required for the process when Si is used as the material for the high refractive index portion 140. On the other hand, the upper limit of 0.5 μm is the maximum value that takes into consideration the miniaturization of the optical waveguide element 10A.

[0080] The following simulation was further carried out to evaluate the appropriate range of values ​​for the height H2 of the high refractive index portion 140.

[0081] A simulation model having the structure shown in Figures 2(a) and 2(b) was fabricated. In the simulation model, the material of the support substrate (intermediate layer) 110 was SiO ２ The material of the substrate 120 is LN, and the material of the buffer layer 130 is SiO ２ The material of the high refractive index portion 140 is Si, the material of the low refractive index portion 150 is air (air gap), and the material of the overcladding layer 160 is SiO ２ The width W2 of the high refractive index portion 140 was set to 1.2 μm.

[0082] Using this simulation model, the coefficients a and b in the following equation (1) were determined by adjusting the distance between the signal electrode S and the ground electrode G while fixing the drive voltage Vπ, and the following equation (1) was obtained, which shows the relationship between the electrode absorption loss and the drive voltage Vπ.

[0083] y=a×exp(b×x)...(1) [y=Loss, x=VπL, L=constant (optical waveguide length), a, b=coefficient]

[0084] Under the condition of a specific driving voltage Vπ, the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 (100×W3 / W2 [%]) was further fixed to 67%. Under this condition, the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120 (100×H2 / H1 [%]) was changed, and the electrode absorption loss Loss was calculated from the above formula (1). Figure 4 shows the simulation results showing the relationship between the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120 and the electrode absorption loss Loss.

[0085] The vertical axis of the graph shown in Figure 4 is the standard value of electrode absorption loss (based on the value with the smallest electrode absorption loss), and the horizontal axis is the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120 (100 × H2 / H1 [%]).

[0086] On the vertical axis of the graph shown in FIG. 4, it is shown that the smaller the standard value of the electrode absorption loss, the smaller the electrode absorption loss, and that the larger the standard value of the electrode absorption loss, the larger the electrode absorption loss.

[0087] 4, the horizontal axis of the graph indicates that the smaller the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120, the smaller the height H2 of the high refractive index portion 140, and the larger the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120, the larger the height H2 of the high refractive index portion 140. The leftmost value of 0% indicates that the height H2 of the high refractive index portion 140 is zero, which corresponds to a state in which no high refractive index portion 140 is provided.

[0088] 4, it can be seen that the electrode absorption loss can be kept relatively small if the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120 is approximately 40% or less. However, here, only the ratio of the height H2 of the high refractive index portion 140 to the height H1 of the substrate 120 is changed, and it is estimated that if other conditions are changed, the range in which the electrode absorption loss is small may be expanded.

[0089] Considering the above simulation results comprehensively, the height H2 of the high refractive index portion 140 is preferably 1% to 40%, more preferably 2% to 35%, and even more preferably 4% to 35% of the height H1 of the substrate 120. If the height H2 of the high refractive index portion 140 is too small, the effect of confining the light wave to the substrate 120 will be weakened, and if the height H2 of the high refractive index portion 140 is too large, the light mode will be distributed in the high refractive index portion 140, so it is preferable to set the height H2 of the high refractive index portion 140 so that it falls within the above range.

[0090] A similar simulation was also performed to evaluate the appropriate range of values ​​for the width W3 of the buffer layer 130.

[0091] Under the condition of a specific driving voltage Vπ, the height H2 of the high refractive index portion 140 was set to 0.05 μm, 0.06 μm, and 0.07 μm, respectively, and the electrode absorption loss Loss for the width W3 of the buffer layer 130 at each height H2 was calculated using the above formula (1). Figure 5 shows the simulation results showing the relationship between the width W3 of the buffer layer 130 and the electrode absorption loss Loss.

[0092] The vertical axis of the graph shown in Figure 5 is the standard value of the electrode absorption loss (based on the case where the width W2 of the high refractive index portion 140 is equal to the width W3 of the buffer layer 130), and the horizontal axis is the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 (100 × W3 / W2 [%]).

[0093] 5, the smaller the normalized value of the electrode absorption loss (Loss), the smaller the electrode absorption loss, and the larger the normalized value of the electrode absorption loss (Loss), the larger the electrode absorption loss. Furthermore, when the normalized value of the electrode absorption loss (Loss) is smaller than 1, the electrode absorption loss is smaller than in a configuration in which the width W2 of the high-refractive-index portion 140 and the width W3 of the buffer layer 130 are equal (a configuration in which the low-refractive-index portion 150 is not provided). In other words, this configuration has strong light wave confinement and achieves a low driving voltage.

[0094] 5, the horizontal axis indicates that the smaller the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140, the smaller the width W3 of the buffer layer 130, and the larger the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140, the larger the width W3 of the buffer layer 130. The rightmost value of 100% corresponds to a state in which the width W2 of the high refractive index portion 140 and the width W3 of the buffer layer 130 are equal, i.e., a state in which the low refractive index portion 150 is not provided.

[0095] 5 shows that when the height H2 of the high refractive index portion 140 is 0.05 μm, the electrode absorption loss is smaller than 1 when the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 is greater than approximately 10%. Also, when the height H2 of the high refractive index portion 140 is 0.06 μm or 0.07 μm, the electrode absorption loss is smaller than 1 when the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 is greater than approximately 30% or approximately 35%. Furthermore, when the height H2 of the high refractive index portion 140 is 0.05 μm, 0.06 μm, or 0.07 μm, the electrode absorption loss is smaller than 1 up to the range where the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 is approximately 100%. However, here, only the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 is changed, and it is estimated that if other conditions are changed, the range in which the electrode absorption loss is small may be expanded.

[0096] Considering the above simulation results comprehensively, the width W3 of the buffer layer 130 is preferably 10% to 99% of the width W2 of the high refractive index portion 140, more preferably 30% to 95%, and even more preferably 50% to 90%. This makes it possible to strengthen the confinement of light waves in the substrate 120 while preventing oxygen deficiency in the substrate 120, which is a cause of drift.

[0097] A similar simulation was also performed to evaluate the appropriate range of values ​​for the height H3 of the buffer layer 130.

[0098] Under the condition of a specific driving voltage Vπ, the ratio of the width W3 of the buffer layer 130 to the width W2 of the high refractive index portion 140 (100×W3 / W2 [%]) was further fixed to 67%. Under this condition, the ratio of the height H3 of the buffer layer 130 to the height H2 of the high refractive index portion 140 (100×H3 / H2 [%]) was changed, and the electrode absorption loss Loss was calculated from the above formula (1). Figure 6 shows the simulation results showing the relationship between the ratio of the height H3 of the buffer layer 130 to the height H2 of the high refractive index portion 140 and the electrode absorption loss Loss.

[0099] The vertical axis of the graph shown in Figure 6 is the standard value of the electrode absorption loss (based on the case where the height H3 of the buffer layer 130 is zero), and the horizontal axis is the ratio of the height H3 of the buffer layer 130 to the height H2 of the high refractive index portion 140 (100 × H3 / H2 [%]).

[0100] On the vertical axis of the graph shown in FIG. 6, it is shown that the smaller the standard value of the electrode absorption loss, the smaller the electrode absorption loss, and that the larger the standard value of the electrode absorption loss, the larger the electrode absorption loss.

[0101] 6 indicates that the height H3 of the buffer layer 130 decreases as the ratio of the height H3 of the buffer layer 130 to the height H2 of the high refractive index portion 140 decreases, and the height H3 of the buffer layer 130 increases as the ratio of the height H3 of the buffer layer 130 to the height H2 of the high refractive index portion 140 increases. The leftmost value of 0% indicates that the height H3 of the buffer layer 130 is zero, which corresponds to a state in which the buffer layer 130 is not provided.

[0102] The graph shown in Figure 6 shows that as the ratio of the height H3 of the buffer layer 130 to the height H2 of the high-refractive-index portion 140 increases, the electrode absorption loss increases proportionally. As the height H3 of the buffer layer 130 increases, the separation distance between the substrate 120 and the high-refractive-index portion 140 also increases, which is expected to reduce the effect of confining light waves in the substrate 120. The graph shown in Figure 6 is consistent with this expectation. On the other hand, as the height H3 of the buffer layer 130 decreases, the high-refractive-index portion 140 may remove oxygen from the substrate 120, potentially causing oxygen vacancies in the substrate 120. Therefore, the height H3 of the buffer layer 130 needs to be set to a value that strengthens the confinement of light waves in the substrate 120 while preventing oxygen vacancies in the substrate 120, which can cause drift.

[0103] Considering the above simulation results comprehensively, the height H3 of the buffer layer 130 is preferably 0.2% to 100% of the height H2 of the high refractive index portion 140, more preferably 0.5% to 50%, and even more preferably 1.0% to 30%. This makes it possible to strengthen the confinement of light waves in the substrate 120 while preventing oxygen deficiency in the substrate 120, which is a cause of drift.

[0104] Next, a method for manufacturing the optical waveguide element 10A according to this embodiment will be described. Figures 7(a) to 7(d) are diagrams for explaining the method for manufacturing the optical waveguide element 10A according to this embodiment. Figures 7(a) to 7(d) show the first to fourth steps of the method for manufacturing the optical waveguide element 10A, respectively.

[0105] 7A, first, a substrate 120 is prepared, and a first layer 230 and a second layer 240 are formed on the substrate 120. A portion of the first layer 230 will eventually become the buffer layer 130, and a portion of the second layer 240 will eventually become the high refractive index portion 140. The material of the buffer layer 130 is used as the material for the first layer 230, and the material of the high refractive index portion 140 is used as the material for the second layer 240. At this time, the film thickness of the first layer 230 is set to the height H3 of the buffer layer 130, and the film thickness of the second layer 240 is set to the height H2 of the high refractive index portion 140.

[0106] 7B, a photomask is placed in close contact, master patterning is performed by photolithography or the like, and the widths of the first layer 230 and the second layer 240 are processed to the desired widths by dry etching or the like. Specifically, the widths of the first layer 230 and the second layer 240 disposed on the substrate 120 are processed to be the width W2 of the high refractive index portion 140.

[0107] 7( c), the side surfaces of the second layer 240 in the width direction are removed to form gaps 250 on the sides of the second layer 240. The processing method for removing the side surfaces of the second layer 240 in the width direction is not particularly limited, but for example, the side surfaces of the second layer 240 can be dissolved by etching using a corrosive chemical solution, taking advantage of the difference in etching rate between the first layer 230 and the second layer 240. In this process, the width of the second layer 240 is processed to be the width W3 of the buffer layer 130.

[0108] Voids 250 formed by scraping the side surfaces of the second layer 240 in the width direction are positions where the low refractive index portions 150 will be located. The voids 250 are filled with the material of the low refractive index portions 150. Note that when air is used as the material that constitutes the low refractive index portions 150, it is not necessary to fill the voids 250 with a specific material, and the voids 250 can simply be left as they are.

[0109] Next, as shown in FIG. 7D , the over-cladding layer 160 is formed. The over-cladding layer 160 is formed so as to cover the upper and side surfaces of the second layer 240, the side surfaces of the material filled in the voids 250, and the upper surface of the substrate 120. When air is used as the material forming the low-refractive-index portion 150, the sides of the voids 250 are blocked by the over-cladding layer 160 to form the low-refractive-index portion 150. The low-refractive-index portion 150 is a cavity surrounded by the upper surface of the substrate 120, the side surfaces of the buffer layer 130, the lower surface of the high-refractive-index portion 140, and the inner surface of the over-cladding layer 160.

[0110] As described above, optical waveguide element 10A in this embodiment is configured to include substrate 120 made of electro-optic crystal, buffer layer 130 formed on the upper surface of substrate 120, and high-refractive-index portion 140 made of a material having a higher refractive index than the material constituting substrate 120 and loaded on the upper surface of buffer layer 130. In this way, buffer layer 130 is interposed between substrate 120 and high-refractive-index portion 140, preventing high-refractive-index portion 140 from contacting substrate 120 and preventing oxygen deficiency in substrate 120, which is a cause of drift.

[0111] Furthermore, by making the width W3 of the buffer layer 130 narrower than the width W2 of the high-refractive-index portion 140 loaded on the buffer layer 130, a refractive index difference can be provided on the side of the buffer layer 130 interposed between the substrate 120 and the high-refractive-index portion 140. This can strengthen the confinement of light waves in the substrate 120. In particular, by providing the low-refractive-index portion 150 on the side of the buffer layer 130, the effect of confining light waves in the substrate 120 can be improved.

[0112] Second Embodiment A second embodiment of the present invention will be described. The optical waveguide element 10A in the second embodiment differs from the first embodiment in that a portion of the substrate 120 has a convex shape. Descriptions of the same components as those in the above-described embodiments will be simplified or omitted, and the same reference numerals will be used.

[0113] Fig. 8 is a cross-sectional view of the optical waveguide element 10A according to this embodiment. The cross-section shown in Fig. 8 corresponds to the cross-section taken along line A-A in Fig. 1, and Fig. 8 shows an enlarged view of the vicinity of the branching waveguide 22.

[0114] 8, the optical waveguide element 10A of this embodiment, like the first embodiment described above, is configured to include a substrate 120 made of an electro-optic crystal, a buffer layer 130 formed on the upper surface of the substrate 120, and a high-refractive-index portion 140 made of a material having a higher refractive index than the material constituting the substrate 120 and loaded on the upper surface of the buffer layer 130. Furthermore, the width W3 of the buffer layer 130 is narrower than the width W2 of the high-refractive-index portion 140 loaded on the buffer layer 130. A low-refractive-index portion 150 made of a material having a lower refractive index than the material constituting the buffer layer 130 is provided on the side of the buffer layer 130.

[0115] In this embodiment, as shown in Fig. 8, a convex portion 121 is formed on the upper surface of the substrate 120. The convex portion 121 is formed from a part of the substrate 120, and the substrate 120 has a convex shape that protrudes upward at the convex portion 121. The buffer layer 130 is formed on an upper surface portion 121a of the convex portion 121, which is a part of the substrate 120. The optical waveguide 20 is formed to include a part of the convex portion 121.

[0116] The width of the convex portion 121 is not particularly limited, but may be set to match the width W3 of the buffer layer 130 formed on the upper surface portion 121a of the convex portion 121. The width of the convex portion 121 may be larger or smaller than the width W3 of the buffer layer 130 formed on the upper surface portion 121a, or may be approximately the same as the width W3.

[0117] The height from the upper surface of the substrate 120 (the surface on which the convex portion 121 is not formed) to the upper surface portion 121a of the convex portion 121 (height H4 of the convex portion 121 shown in Figure 8) is, for example, preferably 1.0% to 70% of the height H1 of the substrate 120, more preferably 2.0% to 60%, and even more preferably 3.0% to 50%.

[0118] In this way, by forming the convex portions 121 on the substrate 120, the distribution of light waves propagating through the optical waveguide 20 is concentrated inside the convex portions 121. This makes it possible to further confine the light waves inside the convex portions 121 formed on the substrate 120, ensure the amount of light, and reduce the driving voltage.

[0119] Third Embodiment A third embodiment of the present invention will be described. An optical waveguide element 10A in the third embodiment is different from the second embodiment in the convex shape formed on the substrate 120. The same components as those in the above-described embodiments will be described briefly or omitted, and the same reference numerals will be used.

[0120] Fig. 9 is a cross-sectional view of the optical waveguide element 10A according to this embodiment. The cross-section shown in Fig. 9 corresponds to the cross-section taken along line A-A in Fig. 1, and Fig. 9 shows an enlarged view of the vicinity of the branching waveguide 22.

[0121] 9 , the optical waveguide element 10A of this embodiment, like the first embodiment described above, is configured to include a substrate 120 made of an electro-optic crystal, a buffer layer 130 formed on the upper surface of the substrate 120, and a high-refractive-index portion 140 made of a material having a higher refractive index than the material constituting the substrate 120 and loaded on the upper surface of the buffer layer 130. Furthermore, the width W3 of the buffer layer 130 is narrower than the width W2 of the high-refractive-index portion 140 loaded on the buffer layer 130. A low-refractive-index portion 150 made of a material having a lower refractive index than the material constituting the buffer layer 130 is provided on the side of the buffer layer 130.

[0122] Furthermore, the upper surface of the substrate 120 in this embodiment has a convex shape that protrudes upward, similar to the second embodiment described above. However, the substrate 120 in this embodiment has a curved convex portion 122 that protrudes upward, as shown in Fig. 9. The curved convex portion 122 is formed by a part of the substrate 120, and the substrate 120 has a convex shape that protrudes upward at the curved convex portion 122. The optical waveguide 20 is formed to include a part of the curved convex portion 122.

[0123] The curved convex portion 122 may have any convex shape that is curved upward, but may have a configuration in which it has inclined surface portions 122a that are gently inclined upward at both ends in the width direction and a flat surface portion 122b in the center in the width direction, as shown in Fig. 9. This allows the buffer layer 130 to be formed on the flat surface portions 122b of the curved convex portions 122 so as to be approximately parallel to the upper surface of the substrate 120 (the surface on which the curved convex portions 122 are not formed).

[0124] Similar to the convex portion 121 in the second embodiment described above, the width of the curved convex portion 122 may be set to match the width W3 of the buffer layer 130 formed on the upper surface of the curved convex portion 122. In particular, when a flat surface portion 122b is provided on the curved convex portion 122, it is preferable to appropriately set the width of the flat surface portion 122b so that the buffer layer 130 is formed on the flat surface portion 122b. Furthermore, the height of the curved convex portion 122 may also be set to the same as the height H4 of the convex portion 121 in the second embodiment described above.

[0125] In this way, by forming the curved convex portion 122 on the substrate 120, the distribution of light waves propagating through the optical waveguide 20 is concentrated inside the curved convex portion 122. This makes it possible to further strengthen the confinement of light waves inside the curved convex portion 122 formed on the substrate 120, ensure the amount of light, and reduce the driving voltage. Furthermore, the curved convex shape of the curved convex portion 122 is a shape that matches the cross-sectional shape of the light waves propagating through the optical waveguide 20. This makes it possible to reduce the propagation loss of light waves on the upper surface of the curved convex portion 122, particularly on the inclined surface portion 122a.

[0126] (Fourth Embodiment) A fourth embodiment of the present invention will be described. The optical waveguide element 10A in the fourth embodiment differs from the first embodiment in that a portion of the substrate 120 has a concave shape. Descriptions of the same components as those in the above-described embodiments will be simplified or omitted, and the same reference numerals will be used.

[0127] Fig. 10 is a cross-sectional view of the optical waveguide element 10A according to this embodiment. The cross-section shown in Fig. 10 corresponds to the cross-section taken along line A-A in Fig. 1, and Fig. 10 shows an enlarged view of the vicinity of the branching waveguide 22.

[0128] 10 , similarly to the first embodiment described above, the optical waveguide element 10A of this embodiment is configured to include a substrate 120 made of an electro-optic crystal, a buffer layer 130 formed on the upper surface of the substrate 120, and a high-refractive-index portion 140 made of a material having a higher refractive index than the material constituting the substrate 120 and loaded on the upper surface of the buffer layer 130. Furthermore, the width W3 of the buffer layer 130 is narrower than the width W2 of the high-refractive-index portion 140 loaded on the buffer layer 130. A low-refractive-index portion 150 made of a material having a lower refractive index than the material constituting the buffer layer 130 is provided on the side of the buffer layer 130.

[0129] In this embodiment, as shown in Fig. 10, a recess 123 is formed on the upper surface of the substrate 120. The recess 123 is formed by excavating a part of the substrate 120, and the substrate 120 has a concave shape that is recessed downward at the recess 123. The buffer layer 130 is formed on a bottom surface 123a of the recess 123 that constitutes part of the upper surface of the substrate 120. The optical waveguide 20 is formed to include a part of the substrate 120 located below the bottom surface 123a of the recess 123.

[0130] 10 , when the recess 123 is formed so that the upper surface of the substrate 120 is recessed, a recess 161 may be formed in the over cladding layer 160 due to the influence of the recess 123 in the substrate 120. Therefore, if necessary, the over cladding layer 160 may be processed so that the upper surface is flat.

[0131] The width of the recess 123 is not particularly limited, but may be set to match the width W2 of the high refractive index portion 140 loaded on the buffer layer 130 so that at least a portion of the lower side of the high refractive index portion 140 is accommodated inside the depression of the recess 123. For example, the width of the recess 123 may be larger than the width W2 of the high refractive index portion 140 so that both ends in the width direction of the high refractive index portion 140 do not come into contact with the wall surface portion 123b of the recess 123 when the high refractive index portion 140 is accommodated inside the depression of the recess 123.

[0132] The depth of the recess 123 (height H5 shown in FIG. 10) from the upper surface of the substrate 120 (the surface on which the recess 123 is not formed) to the bottom surface 123a of the recess 123 is, for example, preferably 0.25% to 50% of the height H1 of the substrate 120, more preferably 0.5% to 45%, and even more preferably 1.0% to 40%.

[0133] In this way, by forming a recess 123 in the substrate 120 and arranging the buffer layer 130 and the high refractive index portion 140 so that they fit inside the depression of the recess 123, the light waves leaking from the optical waveguide 20 can be distributed to the substrate 120 located to the side of the wall portion 123b of the recess 123, thereby improving the modulation efficiency of the light waves.

[0134] Fifth Embodiment A fifth embodiment of the present invention will be described. An optical waveguide element 10A in the fifth embodiment is different from that in the first embodiment described above in the shape of the high refractive index portion 140. Descriptions of the same components as those in the above-described embodiments will be simplified or omitted, and the same reference numerals will be used.

[0135] 11 to 13 are cross-sectional views of first to third examples of the optical waveguide element 10A according to this embodiment. The cross sections shown in Fig. 11 to 13 correspond to the cross section taken along line A-A in Fig. 1, and Fig. 11 to 13 show enlarged views of the vicinity of the branching waveguide 22.

[0136] 11 to 13, similarly to the first embodiment, the optical waveguide element 10A of this embodiment is configured to include a substrate 120 made of an electro-optic crystal, a buffer layer 130 formed on the upper surface of the substrate 120, and a high-refractive-index portion 140 made of a material having a higher refractive index than the material constituting the substrate 120 and loaded on the upper surface of the buffer layer 130. Furthermore, the width W3 of the buffer layer 130 is narrower than the width W2 of the high-refractive-index portion 140 loaded on the buffer layer 130. A low-refractive-index portion 150 made of a material having a lower refractive index than the material constituting the buffer layer 130 is provided on the side of the buffer layer 130.

[0137] In the first embodiment described above, the cross-sectional shape of the high refractive index portion 140 is rectangular, and there are corners that are angled at approximately 90° at both ends in the width direction of each of the upper and lower sides. In contrast, in this embodiment, the corners of the high refractive index portion 140 are formed to have a rounded curved surface shape.

[0138] The high refractive index portion 140 shown in Fig. 11 has its upper surface portion 141 curved so as to have a convex shape toward the upper side. Specifically, the high refractive index portion 140 shown in Fig. 11 has curved surface portions 141a at both widthwise ends of the upper side, and the corners where both widthwise ends of the upper side are located are scraped off to form rounded curved surfaces. Note that the widthwise center portion of the upper surface portion 141 may be a flat surface, and only the widthwise ends of the upper surface portion 141 may have rounded curved surfaces.

[0139] 12 , the lower surface 142 of the high refractive index portion 140 is curved so that it is convex downward. Specifically, the high refractive index portion 140 shown in FIG. 12 has curved surface portions 142a at both widthwise ends of the lower side, and the corners where the widthwise ends of the lower side are located are scraped off to form rounded curved surfaces. Note that the widthwise center portion of the lower surface 142 may be a flat surface, and only the widthwise ends of the lower surface 142 may have rounded curved surfaces.

[0140] 13 has upper surface 141 curved to be convex upward, and lower surface 142 curved to be convex downward. Specifically, high refractive index portion 140 shown in Fig. 13 has curved surface portions 141a, 142a at both widthwise ends of both the upper and lower sides, and the corners where both widthwise ends of both the upper and lower sides are located, i.e., all four corners, are scraped off to form rounded curved surfaces.

[0141] By forming the high refractive index portion 140 to have a rounded curved surface in this way, the propagation loss of the light waves distributed in the high refractive index portion 140 can be reduced.

[0142] Furthermore, as shown in the sixth to twelfth embodiments below, for example, configurations that achieve a composite effect may be achieved by selecting and combining the configurations in the first to fifth embodiments described above. Hereinafter, the sixth to twelfth embodiments will be described with reference to FIGS. 14 to 20. FIGS. 14 to 20 are diagrams showing cross sections of optical waveguide elements 10A in the sixth to twelfth embodiments of the present invention, respectively. The cross sections shown in FIGS. 14 to 20 correspond to the cross section taken along line A-A in FIG. 1, and FIGS. 14 to 20 show enlarged views of the vicinity of branch waveguide 22.

[0143] Sixth Embodiment The sixth embodiment of the present invention is a combination of the second and fifth embodiments. As shown in FIG. 14 , convex portions 121 may be formed on the substrate 120, and high refractive index portions 140 may be formed to have rounded curved surfaces. This allows for stronger confinement of light waves inside the convex portions 121 formed on the substrate 120, ensuring a sufficient amount of light and reducing the driving voltage. Furthermore, the propagation loss of light waves distributed in the high refractive index portions 140 can be reduced.

[0144] Seventh Embodiment The seventh embodiment of the present invention is a combination of the third and fifth embodiments. As shown in FIG. 15 , curved convex portions 122 may be formed on the substrate 120, and high refractive index portions 140 may be formed to have a rounded curved surface. This allows for stronger confinement of light waves inside the convex portions 121 formed on the substrate 120, ensuring a sufficient amount of light and reducing the driving voltage. It also reduces the propagation loss of light waves on the upper surfaces of the curved convex portions 122, particularly the inclined surface portions 122a. It also reduces the propagation loss of light waves distributed in the high refractive index portions 140.

[0145] Eighth Embodiment The eighth embodiment of the present invention is a combination of the fourth and fifth embodiments. As shown in FIG. 16 , a recess 123 may be formed in a substrate 120, and the buffer layer 130 and the high refractive index portion 140 may be disposed so as to fit inside the depression of the recess 123, with the high refractive index portion 140 being formed to have a rounded curved surface. This allows light waves leaking from the optical waveguide 20 to be distributed to the substrate 120 located to the side of the wall portion 123b of the recess 123, thereby improving the modulation efficiency of the light waves. Furthermore, the propagation loss of the light waves distributed in the high refractive index portion 140 may be reduced.

[0146] Ninth Embodiment The ninth embodiment of the present invention is a combination of the second and fourth embodiments. As shown in FIG. 17 , a convex portion 121 may be formed on a substrate 120, and a concave portion 123 may be formed on the substrate 120, with the buffer layer 130 and the high refractive index portion 140 disposed so as to extend into the recess of the concave portion 123. This allows for stronger confinement of light waves inside the convex portion 121 formed on the substrate 120, ensuring a sufficient amount of light and reducing the driving voltage. Furthermore, light waves leaking from the optical waveguide 20 can be distributed to the substrate 120 located to the side of the wall portion 123 b of the concave portion 123, thereby improving the modulation efficiency of the light waves.

[0147] Tenth Embodiment The tenth embodiment of the present invention is a combination of the second, fourth, and fifth embodiments. As shown in FIG. 18 , a convex portion 121 may be formed on a substrate 120, a concave portion 123 may be formed on the substrate 120, and the buffer layer 130 and the high-refractive-index portion 140 may be disposed so as to fit inside the depression of the concave portion 123. Furthermore, the high-refractive-index portion 140 may be formed to have a rounded curved surface. This may further enhance the confinement of light waves inside the convex portion 121 formed on the substrate 120, thereby ensuring sufficient light intensity and reducing the driving voltage. Furthermore, light waves leaking from the optical waveguide 20 may be distributed to the substrate 120 located to the side of the wall portion 123b of the concave portion 123, thereby improving the modulation efficiency of the light waves. Furthermore, the propagation loss of the light waves distributed in the high-refractive-index portion 140 may be reduced.

[0148] Eleventh Embodiment The eleventh embodiment of the present invention is a combination of the third and fourth embodiments. As shown in FIG. 19 , a curved convex portion 122 may be formed on a substrate 120, and a concave portion 123 may be formed on the substrate 120, with the buffer layer 130 and the high-refractive-index portion 140 disposed within the recess of the concave portion 123. This configuration further enhances the confinement of light waves within the convex portion 121 formed on the substrate 120, ensuring sufficient light intensity and reducing the driving voltage. It also reduces the propagation loss of light waves on the upper surface of the curved convex portion 122, particularly the inclined surface portion 122a. Furthermore, light waves leaking from the optical waveguide 20 can be distributed to the substrate 120 located on the side of the wall portion 123b of the concave portion 123, thereby improving the modulation efficiency of the light waves.

[0149] (Twelfth Embodiment) The twelfth embodiment of the present invention is a combination of the third, fourth, and fifth embodiments. As shown in FIG. 20 , a curved convex portion 122 may be formed on the substrate 120, a recess 123 may be formed on the substrate 120, and the buffer layer 130 and the high-refractive-index portion 140 may be disposed so as to fit inside the depression of the recess 123. Furthermore, the high-refractive-index portion 140 may be formed to have a rounded curved surface. This may further enhance the confinement of light waves inside the protrusion 121 formed on the substrate 120, thereby ensuring sufficient light intensity and reducing the driving voltage. Furthermore, it may also be possible to reduce the propagation loss of light waves on the upper surface of the curved convex portion 122, particularly the inclined surface 122a. Furthermore, light waves leaking from the optical waveguide 20 may be distributed to the substrate 120 located to the side of the wall portion 123b of the recess 123, thereby improving the modulation efficiency of the light waves. Furthermore, it may also be possible to reduce the propagation loss of light waves distributed in the high-refractive-index portion 140.

[0150] Thirteenth Embodiment A thirteenth embodiment of the present invention will be described below. The same components as those in the above-described embodiments will be denoted by the same reference numerals and descriptions thereof will be simplified or omitted.

[0151] Fig. 21 is a cross-sectional view of the optical waveguide element 10A according to this embodiment. The cross-section shown in Fig. 21 corresponds to the cross-section taken along line A-A in Fig. 1, and Fig. 21 shows an enlarged view of the vicinity of the branching waveguide 22.

[0152] 21 , similarly to the first embodiment described above, the optical waveguide element 10A of this embodiment is configured to include a substrate 120 made of an electro-optic crystal, a buffer layer 130 formed on the upper surface of the substrate 120, and a high-refractive-index portion 140 made of a material having a higher refractive index than the material constituting the substrate 120 and loaded on the upper surface of the buffer layer 130. Furthermore, the width W3 of the buffer layer 130 is narrower than the width W2 of the high-refractive-index portion 140 loaded on the buffer layer 130. A low-refractive-index portion 150 made of a material having a lower refractive index than the material constituting the buffer layer 130 is provided on the side of the buffer layer 130.

[0153] In the first to twelfth embodiments described above, the widthwise center of the buffer layer 130 and the widthwise center of the high refractive index portion 140 are arranged so as to coincide with each other, and the low refractive index portions 150 are arranged so as to be symmetrical on both the left and right sides of the buffer layer 130. As a result, the buffer layer 130 on both the left and right sides is set to have approximately the same width. However, instead of arranging the low refractive index portions 150 so as to be symmetrical on both the left and right sides of the buffer layer 130, the low refractive index portions 150 may be arranged so as to be asymmetrical on both the left and right sides of the buffer layer 130, as in this embodiment.

[0154] 21 , the buffer layer 130 is disposed so as to be shifted from directly below the center in the width direction of the high refractive index portion 140. Specifically, the width W4 of the low refractive index portion 150 (low refractive index portion 150 a) disposed on the left side of the buffer layer 130 is set to be different from the width W5 of the low refractive index portion 150 (low refractive index portion 150 b) disposed on the right side of the buffer layer 130. Note that either the width W4 of the low refractive index portion 150 a or the width W5 of the low refractive index portion 150 b may be set to zero, and the low refractive index portion 150 may be disposed on only one of the left and right sides of the buffer layer 130.

[0155] In this way, by forming the low-refractive index portions 150 asymmetrically on both the left and right sides of the buffer layer 130, even if the cross-sectional shape of the light waves propagating through the optical waveguide 20 is distorted asymmetrically, the light waves can be appropriately confined within the substrate 120, ensuring a sufficient amount of light and reducing the driving voltage. For example, in curved portions where the optical waveguide 20 is bent, the cross-sectional shape of the light waves tends to be distorted, and the light wave distribution tends to spread toward the outer periphery of the curved portion. In such curved portions, by making the width of the low-refractive index portions 150 located on the outer periphery larger than the width of the low-refractive index portions 150 located on the inner periphery, the light wave distribution can be made symmetrical, and the light waves can be appropriately confined within the substrate 120.

[0156] In the above-described first to thirteenth embodiments, the optical waveguide element 10A is described as an optical waveguide element having an MZ traveling-wave type single-ended electrode structure on an X-cut LN substrate, but the electrode structure is not particularly limited in the present invention. For example, the electrode structure may be as in the following fourteenth to seventeenth embodiments.

[0157] Fourteenth Embodiment A fourteenth embodiment of the present invention will be described below. Fig. 22 is a plan view showing an example of the overall configuration of an optical waveguide element 10B according to this embodiment.

[0158] In the optical waveguide element 10B shown in FIG. 22, an MZ-type optical waveguide is formed on an X-cut LN substrate, similar to the optical waveguide element 10A shown in FIG.

[0159] As shown in Fig. 22, a signal electrode S and a ground electrode G are arranged on a substrate 120. Specifically, two ground electrodes G are arranged on both sides of a single signal electrode S. The signal electrode S has a plurality of segments Ss, and the ground electrode G has a plurality of segments Gs. In other words, the optical waveguide element 10A shown in Fig. 22 is an optical waveguide element having an MZ segment type single-ended electrode structure on an X-cut LN substrate.

[0160] Each segment Ss of the signal electrode S and each segment Gs of the ground electrode G are spaced apart so as to face each other in the width direction, and an optical waveguide 20 is disposed between each segment Ss and each segment Gs. This reduces the electric field area and allows an electric field to be applied locally to the optical waveguide 20, thereby achieving miniaturization and high density of the electrodes.

[0161] Fifteenth Embodiment A fifteenth embodiment of the present invention will be described below. Fig. 23 is a plan view showing an example of the overall configuration of an optical waveguide element 10C according to this embodiment.

[0162] The optical waveguide element 10C shown in FIG. 23 has an MZ-type optical waveguide formed on an X-cut LN substrate, similar to the optical waveguide element 10A shown in FIG.

[0163] As shown in Fig. 23, a signal electrode S and a ground electrode G are arranged on a substrate 120. Specifically, ground electrodes G are arranged on both sides of two signal electrodes S(+) and S(-), resulting in a GSGSG-type electrode structure that performs optical modulation using differential signals. Furthermore, the signal electrode S has multiple segments Ss, and the ground electrode G has multiple segments Gs. The optical waveguide element 10C shown in Fig. 23 is an optical waveguide element having an MZ segment-type differential electrode structure on an X-cut LN substrate.

[0164] Each segment Ss of the signal electrode S and each segment Gs of the ground electrode G are spaced apart so as to face each other in the width direction, and an optical waveguide 20 is disposed between each segment Ss and each segment Gs. This reduces the electric field area and allows an electric field to be applied locally to the optical waveguide 20, thereby achieving miniaturization and high density of the electrodes.

[0165] Sixteenth Embodiment A sixteenth embodiment of the present invention will be described below. Fig. 24 is a plan view showing an example of the overall configuration of an optical waveguide element 10D according to this embodiment.

[0166] In the optical waveguide element 10D shown in FIG. 24, an MZ type optical waveguide is formed on a Z-cut LN substrate having a high electro-optic constant in the optical axis direction.

[0167] As shown in Fig. 24, a signal electrode S and a ground electrode G are arranged on a substrate 120. Specifically, two ground electrodes G are arranged on both sides of a single signal electrode S. The signal electrode S has a plurality of segments Ss, and the ground electrode G has a plurality of segments Gs. The optical waveguide element 10A shown in Fig. 24 is an optical waveguide element having an MZ segment type single-ended electrode structure on a Z-cut LN substrate.

[0168] Each segment Ss of the signal electrode S and each segment Gs of the ground electrode G are spaced apart so as to face each other in the width direction, and each segment Ss of the signal electrode S is disposed above the optical waveguide 20. This reduces the electric field area and allows the electric field to be applied locally to the optical waveguide 20, thereby realizing miniaturization and high density of the electrodes.

[0169] Seventeenth Embodiment A seventeenth embodiment of the present invention will be described below. Fig. 25 is a plan view showing an example of the overall configuration of an optical waveguide element 10E according to this embodiment.

[0170] In the optical waveguide element 10E shown in FIG. 25, an MZ-type optical waveguide is formed on a Z-cut LN substrate, similar to the optical waveguide element 10D shown in FIG.

[0171] As shown in Fig. 25, a signal electrode S and a ground electrode G are arranged on a substrate 120. Specifically, two signal electrodes S(+) and S(-) are arranged, and a ground electrode G is arranged on the outer side of each signal electrode S in the width direction. The signal electrode S also has a plurality of segments Ss. The optical waveguide element 10A shown in Fig. 25 is an optical waveguide element having an MZ segment type differential electrode structure on a Z-cut LN substrate.

[0172] Each segment Ss of the signal electrode S is disposed above the optical waveguide 20. This reduces the electric field area and allows the electric field to be applied locally to the optical waveguide 20, thereby realizing miniaturization and high density of the electrodes.

[0173] The present invention can provide an optical modulator and an optical transmitter that utilize the optical waveguide elements 10A to 10E according to the above-described embodiments.

[0174] Fig. 26 is a plan view showing an optical modulator 300 and an optical transmitter 400 according to the present invention. As an example, Fig. 26 shows the optical modulator 300 and the optical transmitter 400 having the optical waveguide element 10A shown in Fig. 1.

[0175] The optical modulator 300 shown in FIG. 26 includes an optical waveguide element 10A, a housing 301, an input optical fiber 302, and an output optical fiber 303.

[0176] In the optical modulator 300, an optical waveguide element 10A is housed in a housing 301. An input optical fiber 302 is connected to the optical input section 11 of the optical waveguide element 10A, and an output optical fiber 303 is connected to the optical output section 12. In this way, by connecting the optical waveguide element 10A in the housing 301 to the outside of the housing 301 with optical fibers, a compact optical modulator 300 can be provided. Note that a spatial optical system may be interposed between the optical input section 11 and the input optical fiber 302, and between the optical output section 12 and the output optical fiber 303.

[0177] 26 , an optical transmitting device 400 can be configured by connecting a signal output circuit 401 that generates a modulated signal (electrical signal), which is a high-frequency signal for performing a modulation operation, and a signal amplifier circuit 402 that amplifies the modulated signal to an optical modulator 300. The signal output circuit 401 and the signal amplifier circuit 402 may be disposed outside the housing 301 of the optical modulator 300, but disposing them inside the housing 301 can achieve efficient transmission of the modulated signal and miniaturization of the optical transmitting device 400.

[0178] Furthermore, the optical transmitter 400 may be configured to include a light source (not shown) so that light emitted from the light source is input to the optical input section 11 of the optical waveguide element 10A. This allows the light output from the light source to be modulated by the optical modulator 300, and the modulated light to be output from the optical transmitter 400.

[0179] The above-described embodiments are provided to facilitate understanding of the present invention and are not intended to limit the present invention. The components disclosed in the above-described embodiments are intended to encompass all design modifications and equivalents that fall within the technical scope of the present invention. Furthermore, the present invention also encompasses technical ideas obtained by appropriately combining the concepts exemplified in the respective embodiments.

[0180] 10A, 10B, 10C, 10D, 10E Optical waveguide element 11 Optical input section 12 Optical output section 13 Modulation section (action section) 20 Optical waveguide 20a Optical input end 20b Optical output end 21 Input optical waveguide 22, 23 Branching waveguide 24 Output optical waveguide 30 Branching section 40 Multiplexing section 50, 60 Spot size conversion section 110 Support substrate (intermediate layer) 120 Substrate 121 Convex section 121a, 141 Upper surface section 122 Curved convex section 122a Inclined surface section 122b Flat surface section 123, 161 Concave section 123a Bottom surface section 123b Wall surface section 130 Buffer layer 140 High refractive index section 141a, 142a Curved surface section 142 Lower surface portion 150, 150a, 150b Low refractive index portion 160 Overcladding layer 230 First layer 240 Second layer 250 Air gap 300 Optical modulator 301 Housing 302 Input optical fiber 303 Output optical fiber 400 Optical transmitter 401 Signal output circuit 402 Signal amplifier circuit S, S(+), S(-) Signal electrode G Ground electrode Ss, Gs Segment

Claims

1. An optical waveguide element comprising: a substrate made of electro-optic crystal; a buffer layer formed on the upper surface of said substrate; and a high refractive index portion made of a material having a higher refractive index than the material constituting said substrate and loaded on the upper surface of said buffer layer, wherein an optical waveguide is formed including a part of said substrate, and in a cross section perpendicular to the light propagation direction of said optical waveguide, the width of said buffer layer is narrower than the width of said high refractive index portion.

2. An optical waveguide element according to claim 1, characterized in that it comprises a low refractive index portion disposed between the substrate and the high refractive index portion and in contact with a side surface of the buffer layer, and the refractive index of the material constituting the low refractive index portion is lower than the refractive index of the material constituting the buffer layer.

3. The optical waveguide element according to claim 2, characterized in that the material constituting the buffer layer has a lower refractive index than the material constituting the substrate, and the material constituting the low refractive index portion has a lower refractive index than the material constituting the buffer layer.

4. The optical waveguide element according to claim 2 or 3, wherein the material constituting the low refractive index portion is air.

5. The optical waveguide element according to claim 1 or 2, wherein the high refractive index portion is made of a material that bonds with oxygen.

6. The optical waveguide element according to claim 1 or 2, wherein the material constituting the high refractive index portion contains an element of Groups 3 to 17 of the periodic table.

7. The optical waveguide element according to claim 1 or 2, wherein the material constituting the buffer layer is a metal oxide.

8. The optical waveguide element according to claim 1 or 2, wherein a convex portion is formed on the substrate, and the buffer layer is formed on the upper surface of the convex portion.

9. The optical waveguide element according to claim 1 or 2, wherein at least one of the upper and lower surfaces of said high refractive index portion is formed to have a curved surface.

10. The optical waveguide element according to claim 1 or 2, wherein the width of said buffer layer is 10% to 99% of the width of said high refractive index portion.

11. The optical waveguide element according to claim 1 or 2, wherein the thickness of said buffer layer is 0.2% to 100% of the thickness of said high refractive index portion.

12. The optical waveguide element according to claim 1 or 2, wherein the thickness of the high refractive index portion is 1% to 40% of the thickness of the substrate.

13. An optical waveguide element according to claim 1 or 2, characterized in that the optical waveguide is provided with an optical multiplexing / demultiplexing section that branches or couples the optical waveguide, or a spot size conversion section that changes the cross-sectional diameter of the light wave propagating through the optical waveguide.

14. An optical modulator comprising: an optical waveguide element according to claim 1 or 2; a housing for accommodating said optical waveguide element; an input optical fiber connected to the optical input section of said optical waveguide element; and an output optical fiber connected to the optical output section of said optical waveguide element.

15. An optical modulator according to claim 14, characterized in that the optical waveguide element is provided with a modulation electrode that modulates the light wave propagating through the optical waveguide, and the housing has a signal amplifier circuit inside that amplifies the modulation signal applied to the modulation electrode.

16. An optical modulation device comprising: the optical modulator according to claim 15; a light source for inputting a light wave to said optical modulator; and a signal output circuit for outputting said modulated signal.

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