Optical waveguide element, method for manufacturing optical modulation element, optical modulator, optical modulation module, and optical transmitter

Abstract

To prevent over-etching of an underlayer of a working electrode to prevent overhang of a conductive layer above the underlayer in an optical waveguide element.SOLUTION: An optical waveguide element is provided, comprising a substrate, an optical waveguide formed on the substrate, and a working electrode for controlling light waves propagating through the optical waveguide, the working electrode being comprised of a first underlayer made of a first material, a first conductive layer provided on the first underlayer, a second underlayer provided on the first conductive layer and made of a second material different from the first material, and a second conductive layer provided on the second underlayer. An edge of the second underlayer is covered with the second conductive layer on a cross-section perpendicular to an extending direction of the optical waveguide.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element, a method for manufacturing an optical modulation element, an optical modulator, an optical modulation module, and an optical transmission device. [Background technology]

[0002] In commercial optical fiber communication systems, optical modulators incorporating an optical modulation element as an optical waveguide element, which is composed of an optical waveguide formed on a substrate and a working electrode that controls the light wave by applying an electric field to the optical waveguide, are widely used. Among them, optical modulation elements using LiNbO3 (hereinafter also referred to as LN) substrate with electro-optic effect can realize high-speed optical modulation characteristics with little optical loss, and are therefore widely used in optical fiber communication systems of high-speed and large-capacity trunk optical transmission networks and metro networks.

[0003] As one measure to make such optical modulation elements smaller, faster, and more energy-efficient, optical modulators in which the LN substrate is made thinner to strengthen the interaction between the signal electric field and the guided light in the substrate (i.e., to increase the electric field efficiency) and optical modulators using a rib-type optical waveguide or a ridge-type optical waveguide (hereinafter collectively referred to as a convex optical waveguide) formed by forming a strip-shaped convex portion on the surface of an LN substrate are beginning to be put into practical use (e.g., Patent Documents 1 and 2).

[0004] Also, in order to increase the operating speed, it has been known to configure the working electrode in two stages on a substrate in order to match the speed of the electrical signal propagating through the working electrode with the speed of the light wave propagating through the optical waveguide and set the impedance of the working electrode within a predetermined range (see, for example, Patent Document 3). Currently, in order to further increase the speed, studies are also underway to provide the first stage of the working electrode closer to the convex optical waveguide in order to further increase the electric field efficiency.

[0005] However, when the two-stage electrode is brought close to the convex optical waveguide in this way, a complex and fine uneven shape may be present on the substrate in the process of forming the electrode. Therefore, for example, when etching a metal film formed on a substrate during a patterning process, the etching rate of the metal film may differ within the substrate surface due to uneven distribution of the uneven structure on the substrate. As a result, as shown in FIG. 12, for example, when etching an unnecessary base layer 702 (black layer in the figure) formed in the fine uneven portion near the convex optical waveguides 700a and 700b (FIG. 12(a)), the second-stage base layer 702 of the working electrodes 704a, 704b, and 704c may be excessively etched (so-called over-etching) over a wide range, for example, up to the conductive layer 706 above it, causing an overhang of the conductive layer 706 (FIG. 12(b)). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2007-264548 A [Patent Document 2] International Publication No. 2018 / 1031916 [Patent Document 3] Japanese Patent Application Publication No. 9-185025 Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above background, it is desired to realize an electrode structure in an optical waveguide element that can prevent over-etching of the electrode underlayer and thus prevent the occurrence of overhanging of the upper conductive layer. [Means for solving the problem]

[0008] One aspect of the present invention is a method for controlling a light wave propagating through a substrate, an optical waveguide formed on the substrate, and a method for controlling a light wave propagating through the optical waveguide. Two optical waveguides are arranged opposite each other with the optical waveguide between them. An optical waveguide element comprising: Each The working electrode is composed of a first base layer made of a first material, a first conductive layer on the first base layer, a second base layer on the first conductive layer, the second base layer being made of a second material different from the first material, and a second conductive layer on the second base layer, and in a cross section perpendicular to the extension direction of the optical waveguide, On the side facing the optical waveguide An end portion of the second underlayer but , and is covered by the second conductive layer. According to another aspect of the invention, the device further comprises a wiring electrode connected to the working electrode, the wiring electrode including the second conductive layer extending from the working electrode. According to another aspect of the invention, the second material is comprised of titanium (Ti), and the first conductive layer and the second conductive layer are comprised of gold (Au). According to another aspect of the invention, the first material is comprised of niobium (Nb). According to another aspect of the present invention, the first underlayer has a thickness of 30 nm or less, and the second underlayer has a thickness of 100 nm or more. According to another aspect of the present invention, the optical waveguide is a convex optical waveguide constituted by a convex portion extending on the substrate. According to another aspect of the present invention, The two working electrodes are The optical waveguide is sandwiched within the plane of the substrate. The signal electrode and the ground electrode , The signal electrode and the ground electrode The interval is not less than 0.1 μm and not more than 5.0 μm. According to another aspect of the present invention, a conductor pattern is formed in an area on the substrate other than the path from the input end to the output end of the optical waveguide, the conductor pattern being composed of a third base layer made of a third material different from the first material, and a third conductive layer on the third base layer. According to another aspect of the present invention, the third material has a larger optical absorption coefficient at the wavelength of light propagating through the optical waveguide than the first material. Another aspect of the present invention is a method for producing an optical waveguide element, comprising the step of forming a part of the gold constituting the second conductive layer by plating. Another aspect of the present invention is an optical modulator comprising: any one of the optical waveguide elements described above which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light to the optical waveguide element; and an optical fiber for guiding light output by the optical waveguide element to the outside of the housing. Another aspect of the present invention is an optical modulation module including any one of the above optical waveguide elements which is an optical modulation element for modulating light, a housing for accommodating the optical waveguide element, an optical fiber for inputting light to the optical waveguide element, an optical fiber for guiding light output by the optical waveguide element to the outside of the housing, and a drive circuit for driving the optical waveguide element. Yet another aspect of the present invention is an optical transmission device comprising the optical modulator or the optical modulation module, and an electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation. Effect of the Invention

[0009] According to the present invention, in an optical waveguide element, over-etching of the underlayer of the working electrode can be prevented, thereby preventing the occurrence of overhanging of the upper conductive layer. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram showing a configuration of a light modulation element according to a first embodiment of the present invention. [Diagram 2] 2 is a cross-sectional view of the light modulation element shown in FIG. 1 taken along line II-II. [Diagram 3] 3 is a cross-sectional view of the light modulation element shown in FIG. 1 taken along line III-III. [Figure 4] FIG. 3 is a partial detailed view of part A shown in FIG. 2. [Diagram 5] 3 is a diagram for explaining an electrode formation step in part B shown in FIG. 2. FIG. [Figure 6] 13 shows the results of a simulation of the increase in optical absorption loss versus the thickness of the first underlayer when various metals are used as the first underlayer. [Figure 7] FIG. 4 is a diagram showing a configuration of a light modulation element according to a modified example of the first embodiment. [Figure 8] 8 is a cross-sectional view taken along the line VIII-VIII of the light modulation element shown in FIG. 7. [Figure 9] FIG. 4 is a diagram showing a configuration of an optical modulator according to a second embodiment of the present invention. [Figure 10] 13 is a diagram showing a configuration of an optical modulation module according to a third embodiment of the present invention. FIG. [Figure 11] FIG. 13 is a diagram illustrating a configuration of an optical transmitting device according to a fourth embodiment of the present invention. [Figure 12] 1A and 1B are diagrams for explaining electrode formation in a conventional optical waveguide element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [First embodiment] 1 is a diagram showing the configuration of an optical modulation element 100 which is an optical waveguide element according to a first embodiment of the present invention. The optical modulation element 100 operates at an optical wavelength in the near-infrared wavelength range including the 1.3 μm band and / or the 1.55 μm band which are generally used for optical communications.

[0012] The optical modulation element 100 is composed of an optical waveguide 104 formed on a substrate 102. The substrate 102 is, for example, an X-cut LN substrate having an electro-optic effect that has been processed and thinned to a thickness of 20 μm or less (e.g., 2 μm). The optical waveguide 104 is a convex optical waveguide (e.g., a rib-type optical waveguide or a ridge-type optical waveguide) that is composed of a convex portion extending in a strip shape formed on the surface of the thinned substrate 102.

[0013] The substrate 102 is, for example, rectangular and has two opposing left and right sides 140a, 140b extending in the vertical direction in the figure, and upper and lower sides 140c, 140d extending in the horizontal direction in the figure.

[0014] The optical modulation element 100 configures a DP-QPSK optical modulator using two nested Mach-Zehnder optical waveguides 108a and 108b. The nested Mach-Zehnder optical waveguide 108a includes two Mach-Zehnder optical waveguides 110a and 110b. The nested Mach-Zehnder optical waveguide 108b includes two Mach-Zehnder optical waveguides 110c and 110d.

[0015] The Mach-Zehnder optical waveguides 110a and 110b each have two parallel waveguides 112a, 112b and 112c, 112d, respectively. Also, the Mach-Zehnder optical waveguides 110c and 110d each have two parallel waveguides 112e, 112f and 112g, 112h, respectively.

[0016] An input light (arrow pointing to the right in the figure) that is incident on the input waveguide 106 of the optical waveguide 104 at the lower side of the left side 140a of the substrate 102 in the figure is turned back by 180 degrees, and then split into two lights, which are QPSK modulated by two nested Mach-Zehnder type optical waveguides 108a and 108b, respectively. The two QPSK modulated lights are output from the upper side of the left side 140a of the substrate 102 in the figure via output waveguides 126a and 126b, respectively (two arrows pointing to the left in the figure).

[0017] After these two output lights are emitted from the substrate 102, they are combined into one optical beam by, for example, a polarization combiner, and then sent to a transmission optical fiber as a DP-QPSK modulated optical signal.

[0018] For QPSK modulation in the nested Mach-Zehnder optical waveguide 108a, signal electrodes 114-1a and 114-1b to which high-frequency electrical signals for modulation are input are disposed between the two parallel waveguides 112a and 112b of the Mach-Zehnder optical waveguide 110a and between the two parallel waveguides 112c and 112d of the Mach-Zehnder optical waveguide 110b, respectively.

[0019] For QPSK modulation in the nested Mach-Zehnder optical waveguide 108b, signal electrodes 114-1c and 114-1d to which high-frequency electrical signals for modulation are input are disposed between the two parallel waveguides 112e and 112f of the Mach-Zehnder optical waveguide 110c and between the two parallel waveguides 112g and 112h of the Mach-Zehnder optical waveguide 110d, respectively.

[0020] The signal electrode 114-1a forms a coplanar transmission line together with the ground electrodes 114-2a and 114-2b facing each other across the parallel waveguides 112a and 112b, respectively, and the signal electrode 114-1b forms a coplanar transmission line together with the ground electrodes 114-2b and 114-2c facing each other across the parallel waveguides 112c and 112d, respectively.

[0021] The signal electrode 114-1c forms a coplanar transmission line together with the ground electrodes 114-2c and 114-2d that face each other across the parallel waveguides 112e and 112f, respectively, and the signal electrode 114-1d forms a coplanar transmission line together with the ground electrodes 114-2d and 114-2e that face each other across the parallel waveguides 112e and 112f, respectively.

[0022] Hereinafter, the nested Mach-Zehnder optical waveguides 108a and 108b are collectively referred to as the nested Mach-Zehnder optical waveguide 108. The Mach-Zehnder optical waveguides 110a, 110b, 110c, 110d, 110e, 110f, 110g, and 110h are collectively referred to as the Mach-Zehnder optical waveguide 110. The parallel waveguides 112a, 112b, 112c, 112d, 112e, 112f, 112g, and 112h are collectively referred to as the parallel waveguide 112. The signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d are collectively referred to as the signal electrode 114-1. Moreover, the ground electrodes 114-2a, 114-2b, 114-2c, 114-2d, and 114-2e are collectively referred to as ground electrodes 114-2.

[0023] Moreover, the signal electrode 114-1 and the ground electrode 114-2 are collectively referred to as the working electrodes 114. The signal electrode 114-1 and the ground electrode 114-2, which are the working electrodes 114, control the light waves propagating through the optical waveguide 104. Moreover, the signal electrode 114-1 and the ground electrode 114-2 are two working electrodes 114 that sandwich the parallel waveguide 112 of the optical waveguide 104 within the plane of the substrate 102.

[0024] The right ends of the signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d are connected to the signal wiring electrodes 118-1a, 118-1b, 118-1c, and 118-1d, respectively. The left ends of the signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d are connected to the signal wiring electrodes 118-1e, 118-1f, 118-1g, and 118-1h, respectively.

[0025] The right ends of the ground electrodes 114-2a, 114-2b, 114-2c, 114-2d, and 114-2e in the figure are connected to the ground wiring electrodes 118-2a, 118-2b, 118-2c, 118-2d, and 118-2e, respectively. As a result, the signal wiring electrodes 118-1a, 118-1b, 118-1c, and 118-1d, together with the ground wiring electrodes 118-2a, 118-2b, 118-2c, 118-2d, and 118-2e adjacent to these signal wiring electrodes, form a coplanar transmission line.

[0026] Similarly, the left ends of the ground electrodes 114-2a, 114-2b, 114-2c, 114-2d, and 114-2e in the figure are connected to the ground wiring electrodes 118-2f, 118-2g, 118-2h, 118-2i, and 118-2j, respectively. As a result, the signal wiring electrodes 118-1e, 118-1f, 118-1g, and 118-1h, together with the ground wiring electrodes 118-2f, 118-2g, 118-2h, 118-2i, and 118-2j adjacent to these signal wiring electrodes, form a coplanar transmission line.

[0027] Signal wiring electrodes 118-1e, 118-1f, 118-1g, and 118-1h extending to the side 140d of the substrate 102 on the lower side in the figure are terminated outside the substrate 102 by a terminating resistor having a predetermined impedance.

[0028] As a result, high-frequency electrical signals input from the signal wiring electrodes 118-1a, 118-1b, 118-1c, and 118-1d extending to the upper side 140c of the substrate 102 as shown in the figure propagate through the signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d as traveling waves, and modulate the light waves propagating through the Mach-Zehnder optical waveguides 110a, 110b, 110c, and 110d, respectively.

[0029] Hereinafter, the signal wiring electrodes 118-1a, 118-1b, 118-1c, 118-1d, 118-1e, 118-1f, 118-1g, and 118-1h are collectively referred to as signal wiring electrodes 118-1. The ground wiring electrodes 118-2a, 118-2b, 118-2c, 118-2d, 118-2e, 118-2f, 118-2g, 118-2h, 118-2i, and 118-2j are collectively referred to as ground wiring electrodes 118-2. The signal wiring electrodes 118-1 and the ground wiring electrodes 118-2 are collectively referred to as wiring electrodes 118. That is, the signal wiring electrodes 118-1 and the ground wiring electrodes 118-2 are wiring electrodes 118 connected to the working electrode 114.

[0030] The substrate 102 is also provided with a bias electrode 132a for adjusting the bias points of the Mach-Zehnder optical waveguides 110a and 110b, a bias electrode 132b for adjusting the bias points of the Mach-Zehnder optical waveguides 110c and 110d, and a bias electrode 132c for adjusting the bias points of the nested Mach-Zehnder optical waveguides 108a and 108b.

[0031] FIG. 2 is a cross-sectional view taken along the line II-II of the light modulation element 100 shown in FIG. 1. FIG. 3 is a cross-sectional view taken along the line III-III of the light modulation element 100 shown in FIG. 1. The back surface (the lower surface in the figure) of the substrate 102 is supported and reinforced by a support plate 142. The support plate 142 is made of, for example, glass. On the upper surface of the substrate 102 in the figure, parallel waveguides 112a, 112b, 112c, and 112d are configured as convex optical waveguides by convex portions 144a, 144b, 144c, and 144d formed on the substrate 102, respectively. The four dotted ellipses in the figure typically show light propagating through the parallel waveguides 112a, 112b, 112c, and 112d, which are convex optical waveguides. Hereinafter, the convex portions on the substrate 102 that constitute the optical waveguide 104, including the convex portions 144a, 144b, 144c, and 144d, will be collectively referred to as convex portion 144.

[0032] 2, on the substrate 102, a signal electrode 114-1b and ground electrodes 114-2b, 114-2c, which are the working electrode 114, are disposed at positions sandwiching the parallel waveguides 112c, 112d within the plane of the substrate 102. The signal electrode 114-1b and the ground electrodes 114-2b, 114-2c are two-stage electrodes each composed of first-stage electrodes 150a, 150b, 150c and second-stage electrodes 152a, 152b, 152c.

[0033] Fig. 4 is a partial detailed view of part A in Fig. 2. The signal electrode 114-1b is composed of a first underlayer 154a, a first conductive layer 156a on the first underlayer 154a, a second underlayer 158a on the first conductive layer 156a, and a second conductive layer 160a on the second underlayer 158a. The first underlayer 154a and the first conductive layer 156a constitute a first-stage electrode 150a, and the second underlayer 158a and the second conductive layer 160a constitute a second-stage electrode 152a.

[0034] The first underlayer 154a is made of a first material, and the second underlayer 158a is made of a second material different from the first material. In this embodiment, the first material is niobium (Nb), and the second material is titanium (Ti). The first conductive layer 156 and the second conductive layer 160 are both gold (Au).

[0035] In a cross section perpendicular to the extension direction of the parallel waveguide 112, an end 162a of the second underlayer 158a on the parallel waveguide 112c side is covered with the second conductive layer 160a. Similarly, in a cross section perpendicular to the extension direction of the parallel waveguide 112, an end 162b of the second underlayer 158a on the parallel waveguide 112d side is covered with the second conductive layer 160a.

[0036] The ground electrode 114-2b is composed of a first underlayer 154b, a first conductive layer 156b on the first underlayer 154b, a second underlayer 158b on the first conductive layer 156b, and a second conductive layer 160b on the second underlayer 158b. The first underlayer 154b and the first conductive layer 156b constitute a first-stage electrode 150b, and the second underlayer 158b and the second conductive layer 160b constitute a second-stage electrode 152b. The first underlayer 154b is made of a first material, and the second underlayer 158b is made of a second material different from the first material. In a cross section perpendicular to the extension direction of the parallel waveguide 112, an end 162c of the second underlayer 158b on the side of the parallel waveguide 112c is covered with the second conductive layer 160b.

[0037] Similarly, the ground electrode 114-2c is composed of a first underlayer 154c, a first conductive layer 156c on the first underlayer 154c, a second underlayer 158c on the first conductive layer 156c, and a second conductive layer 160c on the second underlayer 158c. The first underlayer 154c and the first conductive layer 156c constitute a first-stage electrode 150c, and the second underlayer 158c and the second conductive layer 160c constitute a second-stage electrode 152c. The first underlayer 154c is made of a first material, and the second underlayer 158c is made of a second material different from the first material. In a cross section perpendicular to the extension direction of the parallel waveguide 112, an end 162d of the second underlayer 158c on the side of the parallel waveguide 112d is covered with the second conductive layer 160c.

[0038] The signal electrodes 114-1a, 114-1c, and 114-1d other than the signal electrode 114-1b, and the ground electrodes 114-2a, 114-2d, and 114-2e other than the ground electrodes 114-2b and 114-2c are configured similarly to the signal electrode 114-1a and the ground electrodes 114-2a and 114-2b. Hereinafter, the first foundation layer, the first conductive layer, the second foundation layer, and the second conductive layer of the signal electrode 114-1 and the ground electrode 114-2 are also collectively referred to as the first foundation layer 154, the first conductive layer 156, the second foundation layer 158, and the second conductive layer 160, respectively.

[0039] 4, the first stage electrodes constituted by the first base layer 154 and the first conductive layer 156 of the working electrode 114 are collectively referred to as the first stage electrode 150. The second stage electrodes constituted by the second base layer 158 and the second conductive layer 160 of the working electrode 114 are collectively referred to as the second stage electrode 152, including the second stage electrodes 152a, 152b, 152c. The end of the second base layer 158 on the side of the adjacent parallel waveguide 112 is collectively referred to as the end 162, including the ends 162a, 162b, 162c, 162d.

[0040] That is, in the light modulation element 100, the signal electrode 114-1 and the ground electrode 114-2, which are the working electrode 114, are respectively composed of a first underlayer 154 made of a first material, a first conductive layer 156 on the first underlayer 154, a second underlayer 158 on the first conductive layer 156c made of a second material different from the first material, and a second conductive layer 160 on the second underlayer 158. In a cross section perpendicular to the extension direction of the parallel waveguide 112, an end 162 of the second underlayer 158 of the working electrode 114 on the parallel waveguide 112 side is covered with the second conductive layer 160.

[0041] 2, a ground wiring electrode 118-2b connected to the ground electrode 114-2b is provided on the substrate 102. The ground wiring electrode 118-2b is configured such that the second stage electrode 152b of the ground electrode 114-2b extends leftward on the substrate 102 in the figure. That is, the ground wiring electrode 118-2b includes a second conductive layer 160b extending from the ground electrode 114-2b. In this embodiment, a resin layer 146 is provided between the ground wiring electrode 118-2b and the parallel waveguides 112a and 112b.

[0042] 3, a signal wiring electrode 118-1b connected to the signal electrode 114-1b is provided on the substrate 102. The signal wiring electrode 118-1b is configured such that the second stage electrode 152a of the signal electrode 114-1b extends leftward on the substrate 102 in the figure, similar to the ground wiring electrode 118-2b in FIG. 2. That is, the signal wiring electrode 118-1b includes a second conductive layer 160 extending from the signal electrode 114-1b. In this embodiment, a resin layer 146 is provided between the signal wiring electrode 118-1b and the parallel waveguides 112a, 112b, and 112c.

[0043] The ground wiring electrodes 118-2 other than the ground wiring electrode 118-2b are configured similarly to the ground wiring electrode 118-2b in Fig. 2. Moreover, the signal wiring electrodes 118-1 other than the signal wiring electrode 118-1b are configured similarly to the signal wiring electrode 118-1b.

[0044] That is, the light modulation element 100 includes a wiring electrode 118 connected to the working electrode 114, and the wiring electrode 118 includes a second conductive layer 160 extending from the working electrode 114. A resin layer 146 is formed between the wiring electrode 118 and the optical waveguide 104 located below the wiring electrode 118.

[0045] In the light modulation element 100 having the above configuration, the first material constituting the first underlayer 154 of the signal electrode 114-1 and the ground electrode 114-2, which are the working electrodes 114, is made of a material, such as niobium, different from the second material constituting the second underlayer 158. Niobium has a smaller optical absorption coefficient at the operating optical wavelength of the light modulation element 100 (for example, the 1.3 μm band and / or the 1.55 μm band, which are wavelengths for optical communication), compared to titanium, which has been conventionally and generally used as an underlayer material for electrodes. Therefore, the first-stage electrodes 150 of these working electrodes 114 can be disposed closer to the parallel waveguide 112 than in the past. For example, in this embodiment, the distance W (see FIG. 4) between the signal electrode 114-1 and the ground electrode 114-2, which are the two working electrodes 114 sandwiching the parallel waveguide 112, is 1.0 μm or more and 5.0 μm or less.

[0046] On the other hand, in the light modulation element 100, the second material of the second underlayer 158 can be a material different from the first material, for example titanium as in the past, so that the second-stage electrode 152 can be firmly fixed to the first-stage electrode 150. As a result, the light modulation element 100 can achieve high-speed light modulation operation with a smaller driving voltage and higher reliability than in the past.

[0047] In particular, in the optical modulation element 100, as shown in FIG. 4, in a cross section perpendicular to the extending direction of the parallel waveguide 112, the end 162 of the second underlayer 158 of the signal electrode 114-1 and the ground electrode 114-2, which are the working electrode 114, on the side of the parallel waveguide 112 is covered by the second conductive layer 160 on the second underlayer 158. This makes it possible to prevent the second underlayer 158 under the second conductive layer 160 from being over-etched when etching the film of the second material (the material of the second underlayer 158) remaining on the substrate 102 after the formation of the second conductive layer 160 of the working electrode 114. As a result, in the optical modulation element 100, it is possible to prevent the second underlayer 158 under the second conductive layer 160 from being over-etched, as shown in FIG. 12, in the second conductive layer 160.

[0048] Fig. 5 is a diagram showing an example of an electrode formation process of the light modulation element 100 in part A of Fig. 2. Fig. 5(a) shows a state in which a first layer 170 made of a first material and a second layer 172 made of Au are formed on the substrate 102 and a first patterning is performed, and then a third layer 176 made of a second material and a fourth layer 178 which is an Au thin film are formed. Here, the Au thin film of the fourth layer 178 is formed by, for example, electron beam evaporation.

[0049] In addition, in each of Figures 5(a), (b), and (c), the other layers shown with the same type of hatching as the first layer 170, second layer 172, third layer 176, and fourth layer 178 shown on the right side, and the resist 180 and fifth layer 182 described below, should be understood to be the first layer 170, second layer 172, third layer 176, fourth layer 178, resist 180, and fifth layer 182.

[0050] In (a) of Fig. 5, the first layer 170 and the second layer 172 are removed from above the parallel waveguides 112c and 112d by the first patterning. As a result, a third layer 176 and a fourth layer 178 are formed above and around the parallel waveguides 112c and 112d. In (a) of Fig. 5, a resist 180 for etching the third layer 176 and the fourth layer 178 is further formed on the fourth layer 178.

[0051] 5B shows a state in which the third layer 176 and the fourth layer 178 formed on the second layer 172 in the state shown in FIG. 5A are etched, and then a fifth layer 182 of an Au thick film is formed on the fourth layer 178 by plating and patterned. The fourth layer 178 and the fifth layer 182, both made of Au, form the second conductive layer 160 of the signal electrode 114-1b. The same is true for the ground electrodes 114-2c and 114-2d. That is, the method (manufacturing process) of manufacturing the optical modulation element 100 includes a process of forming a part of the gold constituting the second conductive layer by plating.

[0052] In addition, in FIG. 5(b), the third layer 176 and the fourth layer 178 located above and around the parallel waveguides 112c and 112d are left in order to reduce potential unevenness on the upper surface of the substrate 102 and ensure the homogeneity of the Au thick film when the Au thick film of the fifth layer 182 is formed by plating.

[0053] 5(c) shows a state in which the third layer 176 and the fourth layer 178 on and around the parallel waveguides 112c and 112d have been removed by etching. As shown in the figure, the ends 162a and 162b of the second underlayer 158 of the signal electrode 114-1b on the side of the parallel waveguides 112c and 112d are covered by the second conductive layer 160 on the second underlayer 158, so that over-etching of the second underlayer 158 does not occur. Therefore, the second-stage electrode 152a having good side surfaces without overhanging can be formed. The same is true for the ground electrodes 114-2c and 114-2d.

[0054] In the present embodiment, the first underlayer 154 constituting the first-stage electrode 150 of the signal electrode 114-1 is made of niobium (Nb). Niobium has a smaller optical absorption coefficient in the near-infrared wavelength range including the operating optical wavelength of the optical modulation element 100 than titanium (Ti), which has been conventionally and generally used as the material for the first underlayer 154. Therefore, in the optical modulation element 100, the working electrode 114 can be provided closer to the parallel waveguide 112 than in the past. Therefore, in the optical modulation element 100, the electric field efficiency can be improved to realize high-speed and low-voltage operation.

[0055] As described above, the material constituting the first underlayer 154 is not limited to niobium (Nb) and may be any material having a smaller optical absorption coefficient at the operating light wavelength than titanium, which has been conventionally used. Fig. 6 shows simulation results of the increase in optical absorption loss in the parallel waveguide 112c with respect to the thickness of the first underlayer 154 when the first underlayer 154 is made of various metals. Fig. 6 shows calculation results when the first underlayer 154 is not used in the first-stage electrode 150 (when only a single Au film (first conductive layer 156) is used), as well as when Ti, Al (aluminum), and Nb are used as the material for the first underlayer 154.

[0056] 6, the horizontal axis represents the thickness t of the first underlayer 154, and the vertical axis represents the amount of optical absorption loss per unit length (1 cm) of the parallel waveguide 112 adjacent to the first underlayer 154. In the simulation, Au was assumed for the first conductive layer 156, and 1.55 μm was assumed as the operating optical wavelength. In addition, when calculating the optical absorption loss, the amount of optical absorption at the operating wavelength of 1.55 μm of each metal obtained from the optical absorption spectra of Ti, Al, and Nb was used.

[0057] The optical absorption loss α0 in the case of a single Au film (no underlayer present) shown by line 200 is the background optical absorption loss in the above configuration, and the optical absorption loss in the first underlayer 154 can be evaluated as the increase in optical absorption loss from this reference line.

[0058] Lines 202, 204, and 206 represent the optical absorption loss when the metals constituting the first underlayer 154 are Ti, Al, and Nb, respectively. Among these metals, Ti, represented by line 202, has the largest optical absorption loss, and the optical absorption loss increases significantly with the thickness of the film.

[0059] On the other hand, lines 204 and 206, which show the cases where Al and Nb, which do not include the operating wavelength in their optical absorption range, are used as the first underlayer 154, show that the optical absorption loss is significantly reduced compared to the case where Ti is used (line 202). Within the range of the evaluation results shown in Fig. 6, the optical absorption loss is smallest when the first underlayer 154 is Al (line 204), and the optical absorption loss is almost constant with respect to the thickness of the underlayer. However, it is generally known that Al has low adhesion strength to a substrate.

[0060] 6, it is desirable that the thickness of the first underlayer 154 made of Nb is 30 nm or less. If the thickness of the Nb underlayer is within this range, the optical absorption loss can be suppressed to 2α0 or less, which is twice the value α0 of a single Au film, or to 1 / 3 of the value of about 6α0 in the case of the conventional first underlayer 154 using Ti.

[0061] [Second embodiment] Next, a second embodiment of the present invention will be described. This embodiment is an optical modulator using the optical modulation element 100 according to the first embodiment described above. Fig. 9 is a diagram showing the configuration of an optical modulator 400 according to the second embodiment. The optical modulator 400 has a housing 402, the optical modulation element 100 housed in the housing 402, and a relay substrate 406. A plate-shaped cover (not shown) is finally fixed to the opening of the housing 402, and the inside is hermetically sealed.

[0062] The optical modulator 400 also has a signal pin 408 for inputting a high-frequency electrical signal used to modulate the optical modulation element 100, and a signal pin 410 for inputting an electrical signal used to adjust the operating point of the optical modulation element 100.

[0063] Furthermore, the optical modulator 400 has an input optical fiber 414 for inputting light into the housing 402, and an output optical fiber 420 for guiding the light modulated by the optical modulation element 100 to the outside of the housing 402, on the same surface of the housing 402 (the left surface in the figure in this embodiment).

[0064] Here, the input optical fiber 414 and the output optical fiber 420 are fixed to the housing 402 via supports 422 and 424, which are fixing members. The light input from the input optical fiber 414 is collimated by a lens 430 arranged in the support 422, and then input to the light modulation element 100 via a lens 434. However, this is only an example, and the input of light to the light modulation element 100 can also be performed according to the conventional technology, for example, by introducing the input optical fiber 414 into the housing 402 via the support 422 and connecting the end face of the introduced input optical fiber 414 to the end face of the substrate 102 of the light modulation element 100.

[0065] The light output from the light modulation element 100 is coupled to an output optical fiber 420 via an optical unit 416 and a lens 418 disposed on a support 424. The optical unit 416 may include a polarization combiner that combines the two modulated lights output from the light modulation element 100 into one beam.

[0066] The relay board 406 relays, by a conductor pattern (not shown) formed on the relay board 406, a high-frequency electrical signal input from a signal pin 408 and an electrical signal for adjusting an operating point (bias point) or the like input from a signal pin 410 to the optical modulation element 100. The conductor patterns on the relay board 406 are connected to pads constituting one ends of the wiring electrodes 118 of the optical modulation element 100, for example, by wire bonding. The optical modulator 400 also includes a terminator 412 having a predetermined impedance in the housing 402.

[0067] The optical modulator 400 having the above configuration is constructed using the optical modulation element 100 according to the first embodiment described above, and therefore can achieve high-speed optical modulation operation with a smaller driving voltage and higher reliability than conventional techniques.

[0068] [Third embodiment] Next, a third embodiment of the present invention will be described. This embodiment is an optical modulation module 500 using an optical modulation element according to any of the above-mentioned embodiments or modifications. Fig. 10 is a diagram showing the configuration of the optical modulation module 500 according to this embodiment. In Fig. 10, the same components as those in the optical modulator 400 according to the second embodiment shown in Fig. 9 are indicated by the same reference numerals as those shown in Fig. 9, and the above description of Fig. 9 is used.

[0069] 9, the optical modulation module 500 differs from the optical modulator 400 in that it includes a circuit board 506 instead of the relay board 406. The circuit board 506 includes a driving circuit 508. The driving circuit 508 generates a high-frequency electrical signal for driving the optical modulation element 100 based on, for example, a modulation signal supplied from the outside via a signal pin 408, and outputs the generated high-frequency electrical signal to the optical modulation element 100.

[0070] The optical modulation module 500 having the above-mentioned configuration is constructed using the optical modulation element 100 according to the first embodiment described above, and therefore can achieve high-speed optical modulation operation with a smaller driving voltage and higher reliability than conventional techniques.

[0071] [Fourth embodiment] Next, a fourth embodiment of the present invention will be described. This embodiment is an optical transmission device 600 equipped with the optical modulator 400 according to the second embodiment. Fig. 11 is a diagram showing the configuration of the optical transmission device 600 according to this embodiment. This optical transmission device 600 has the optical modulator 400, a light source 604 that inputs light to the optical modulator 400, a modulator driving unit 606, and a modulation signal generating unit 608. Note that the optical modulation module 500 described above can also be used instead of the optical modulator 400 and the modulator driving unit 606.

[0072] The modulation signal generating unit 608 is an electronic circuit that generates an electrical signal for causing the optical modulator 400 to perform a modulation operation. Based on transmission data provided from outside, the modulation signal generating unit 608 generates a modulation signal, which is a high-frequency signal for causing the optical modulator 400 to perform an optical modulation operation in accordance with the modulation data, and outputs the modulation signal to the modulator driving unit 606.

[0073] The modulator driving unit 606 amplifies the modulation signal input from the modulation signal generating unit 608, and outputs a high-frequency electric signal for driving a signal electrode of the optical modulation element 100 included in the optical modulator 400. As described above, instead of the optical modulator 400 and the modulator driving unit 606, it is also possible to use an optical modulation module 500 including, for example, a driving circuit 508 including a circuit equivalent to the modulator driving unit 606 inside the housing 402.

[0074] The high-frequency electrical signal is input to the signal pin 408 of the optical modulator 400 to drive the optical modulation element 100. As a result, the light output from the light source 604 is modulated by the optical modulator 400, and is output from the optical transmitter 600 as modulated light.

[0075] The optical transmission device 600 having the above configuration uses the optical modulation element 100 and the like which are small and capable of modulation operation with little wavelength dependency, similar to the optical modulator 400 according to the second embodiment and the optical modulation module 500 according to the third embodiment described above, and therefore can realize high-speed optical modulation operation with a smaller driving voltage and high reliability compared to conventional techniques, thereby enabling good optical transmission.

[0076] The present invention is not limited to the configurations of the above-described embodiment and alternative configurations thereof, and can be embodied in various forms without departing from the spirit and scope of the present invention.

[0077] For example, in the light modulation element 100 shown in FIG. 1, the paths of the wiring electrode 118 and the wiring of the bias electrodes 132a, 132b, and 132c (hereinafter, bias electrode wiring) are not limited to the paths shown in FIG. 1. The paths of the wiring electrode 118 and the bias electrode wiring on the substrate 102 are arbitrary as long as they do not affect the optical waveguide 104, according to the conventional technology. For example, the wiring electrode 118 may extend linearly from the signal electrode 114-1 so as not to affect the optical waveguide 104. In addition, the bias electrode wiring may be formed along an arbitrary path so as to reach the side 140b opposite to the side 140a on which the input waveguide 106 and the output waveguides 126a and 126b are formed.

[0078] In addition, a conductor pattern composed of a third underlayer made of a third material different from the first material and a third conductive layer on the third underlayer may be formed in an area on the substrate 102 other than the path from the input end to the output end of the optical signal of the optical waveguide 104. Here, the input end is the end of the input waveguide 106 to which light is incident, and the output end is the end of the output waveguides 126a and 126b to which the light incident from the input end is output. In addition, the area other than the path from the input end to the output end of the optical signal of the optical waveguide 104 refers to an area where unnecessary light not used as an optical signal propagates. For example, a radiation light waveguide that is added to the Y-branch waveguide to propagate unnecessary radiation light, an area on the substrate 102 where stray light propagates, etc. are the above-mentioned "areas other than the path from the input end to the output end".

[0079] Fig. 7 is a diagram showing the configuration of an optical modulation element 100-1 in which a third base layer and a third conductive layer are formed in an area other than the path from the input end to the output end of an optical signal, according to a modified example of the optical modulation element 100 shown in Fig. 1. In Fig. 7, the same components as those shown in Fig. 1 are indicated by the same reference numerals as those in Fig. 1, and the above description of Fig. 1 is used.

[0080] In the optical modulation element 100-1 shown in Fig. 7, in addition to the components of the optical modulation element 100 shown in Fig. 1, radiated light waveguides 130a and 130b for propagating radiated light leaking without being multiplexed are provided in the multiplexing section 128, which is a Y-branch waveguide constituting the nested Mach-Zehnder optical waveguide 108b. A conductor pattern 194a is formed in the region on the substrate 102 where the radiated light waveguide 130a is formed, so as to cover at least a portion of the radiated light waveguide 130a.

[0081] Fig. 8 is a cross-sectional view taken along line VIII-VIII in Fig. 7, showing a cross-sectional view of a portion in which the radiated light waveguide 130a is formed. The conductor pattern 194a covering the radiated light waveguide 130a is composed of a third base layer 190 made of a third material different from the above-mentioned first material, and a third conductive layer 192 on the third base layer 190.

[0082] Here, the third material may have a larger optical absorption coefficient at the wavelength of light propagating through the optical waveguide 104 than the first material. The third material may be, for example, nickel. This allows the unwanted light propagating through the emitted light waveguide 130a to be efficiently absorbed by the nickel constituting the third underlayer 190. In this case as well, the first material may be niobium as described above. The third material may be titanium.

[0083] In the optical modulation element 100-1 shown in Fig. 7, conductor patterns 194b and 194c are formed on the substrate 102 so as to overlap a rectangular region 196 shown by a dotted line rectangle adjacent to the optical input end of the optical waveguide 104 (the portion of the left side 140a in the figure where the end of the input waveguide 106 is formed) and avoid the input waveguide 106. The conductor patterns 194b and 194c are composed of a third base layer and a third conductive layer on the third base layer, similar to the conductor pattern 194a shown in Fig. 8. This allows the conductor patterns 194b and 194c to effectively absorb unnecessary light that enters from the optical input end and propagates through the substrate 102 without being coupled to the input waveguide 106.

[0084] The position of the third underlayer formed on the substrate 102 is not limited to the positions of the conductor patterns 194a, 194b, and 194c, and may be any position capable of absorbing unnecessary light. In the example of FIG. 7, the conductor pattern 194a including the third underlayer 190 is formed on one of the radiated light waveguides 130a provided in the multiplexing section 128, but a similar conductor pattern may be formed on the radiated light waveguide 130b. A radiated light waveguide may also be provided on any other Y-branch portion included in the optical waveguide 104, and a similar conductor pattern including the third underlayer may be formed on each of the radiated light waveguides. For example, when a light receiving element is disposed at the end of the radiated light waveguide 130a to monitor the amount of light output from the radiated light waveguide 130a, the end of the radiated light waveguide 130a becomes the output end of the optical signal of the optical waveguide 104, and therefore a conductor pattern including the third underlayer is not formed.

[0085] According to these configurations, it is possible to suppress absorption of light waves by the working electrode that controls the light waves, and to effectively remove unnecessary light propagating through the substrate 102.

[0086] In the first embodiment, the optical modulation element 100 configured with the substrate 102 made of LN (LiNbO3) is shown as an example of the optical waveguide element according to the present invention, but the optical waveguide element according to the present invention is not limited to this. The optical waveguide element may be an element configured with a substrate made of any material (LN, InP, Si, etc.) and having any function (optical modulation, optical switch, optical directional coupler, etc.). Such an element may be, for example, a so-called silicon photonics waveguide device.

[0087] In the above-described embodiment, the substrate 102 is, as an example, an X-cut LN substrate (so-called X-plate) (the substrate normal direction is the X-axis of the crystal axis), but a Z-cut LN substrate can also be used as the substrate 102.

[0088] As described above, the optical modulation element 100, which is an optical waveguide element according to the above-mentioned embodiment, includes the substrate 102, the optical waveguide 104 formed on the substrate 102, and the working electrode 114 (i.e., the signal electrode 114-1 and the ground electrode 114-2) that controls the light wave propagating through the optical waveguide 104. The working electrode 114 is composed of a first underlayer 154 made of a first material (e.g., niobium), a first conductive layer 156 on the first underlayer 154, a second underlayer 158 on the first conductive layer 156, the second underlayer 158 made of a second material (e.g., titanium) different from the first material, and a second conductive layer 160 on the second underlayer 158. In a cross section perpendicular to the extension direction of the parallel waveguide 112, the end portion of the second base layer 158 on the optical waveguide 104 side, for example, the end portion 162 on the parallel waveguide 112 side which is part of the optical waveguide 104, is covered with a second conductive layer 160.

[0089] According to this configuration, over-etching of the second base layer 158 below the second conductive layer 160 can be prevented and the signal electrode 114-1 having a good side surface condition can be formed, thereby realizing an optical waveguide element with high operational stability and long-term reliability.

[0090] The light modulation element 100 also includes a wiring electrode 118 connected to the working electrode 114. The wiring electrode 118 includes a second conductive layer 160 extending from the working electrode 114. With this configuration, the wiring electrode 118 can be fabricated simultaneously with the formation of the second-stage electrode 152 of the working electrode 114.

[0091] Moreover, the second material constituting second base layer 158 is titanium (Ti), and first conductive layer 156 and second conductive layer 160 are made of gold (Au). According to this configuration, second base layer 158, first conductive layer 156, and second conductive layer 160 are formed using titanium and gold, which have been conventionally used as the material for the base layer and the conductive layer, respectively, and therefore these layers can be formed by the same process as conventional processes.

[0092] Moreover, the first material constituting the first underlayer 154 is niobium (Nb). With this configuration, compared to the case where the first underlayer is made of titanium, which has been conventionally used as a material for underlayers, the first-stage electrode 150 of the working electrode 114 can be provided closer to the parallel waveguide 112, so that the electric field efficiency can be improved to realize a high-speed, low-voltage driven optical waveguide element.

[0093] In the optical modulation element 100, the first underlayer 154 made of niobium has a thickness of 30 nm or less, and the second underlayer 158 has a thickness of 100 nm or more. With this configuration, the niobium constituting the first underlayer 154 suppresses an increase in optical absorption loss occurring in the parallel waveguide 112, while the second underlayer 158 having a sufficient thickness can firmly fix the second-stage electrode 152 onto the first-stage electrode 150.

[0094] Moreover, the optical waveguide 104 constituting the optical modulation element 100 is a convex optical waveguide constituted by a convex portion 144 extending on the substrate 102. With this configuration, it is possible to further increase the electric field efficiency and realize a high-speed, low-loss optical modulation element with a lower driving voltage.

[0095] Furthermore, the optical modulation element 100 has two working electrodes 114 (i.e., a signal electrode 114-1 and a ground electrode 114-2) sandwiching the parallel waveguide 112 of the optical waveguide 104 within the plane of the substrate 102. The distance between these two working electrodes 114 is 1.0 μm or more and 5.0 μm or less. With this configuration, even if minute irregularities are present on the substrate 102 at intervals of about 3 μm, over-etching of the second base layer 158 of the working electrode 114 can be prevented, and the working electrode 114 with a good side shape can be formed.

[0096] The manufacturing process of the light modulation element 100 also includes a process of forming a part of the second conductive layer 160 with gold by plating. With this configuration, even if a layer of the same second material as the second base layer 158 must be maintained in a minute gap on the substrate 102 until after the formation of the second conductive layer 160, over-etching of the second base layer 158 of the working electrode 114 can be prevented during the subsequent etching of the second material, and the working electrode 114 with a good side shape can be formed.

[0097] In addition, the optical modulator 400 according to the second embodiment described above includes an optical modulation element 100 that modulates light, a housing 402 that accommodates the optical modulation element 100, an input optical fiber 414 that inputs light to the optical modulation element 100, and an output optical fiber 420 that guides the light output by the optical modulation element 100 to the outside of the housing 402.

[0098] Moreover, the optical modulation module 500 according to the third embodiment described above includes an optical modulation element 100, a housing 402 that houses the optical modulation element 100, an input optical fiber 414 that inputs light to the optical modulation element 100, an output optical fiber 420 that guides the light output by the optical modulation element 100 to the outside of the housing 402, and a driving circuit 508 that drives the optical modulation element.

[0099] In addition, the optical transmitting device 600 according to the fourth embodiment described above includes the optical modulator 400 according to the second embodiment or the optical modulation module 500 according to the third embodiment, and a modulation signal generating unit 608 which is an electronic circuit that generates an electrical signal for causing the optical modulation element 100 to perform a modulation operation.

[0100] According to these configurations, it is possible to realize the optical modulator 400, the optical modulation module 500, or the optical transmission device 600 that consumes little power, is high speed, and has low loss. [Explanation of symbols]

[0101] 100, 100-1... Optical modulation element, 102... Substrate, 104... Optical waveguide, 106... Input waveguide, 108a, 108b... Nested Mach-Zehnder type optical waveguide, 110, 110a, 110b, 110c, 110d... Mach-Zehnder type optical waveguide, 112, 112a, 112b, 112c , 112d, 112e, 112f, 112g, 112h...Parallel waveguide, 114, 704a, 704b, 704c...Working electrode, 114-1, 114-1a, 114-1b, 114-1c, 114-1d, 704...Signal electrode, 114-2, 114-2a, 114-2b, 114-2c , 114-2d, 114-2e...Ground electrode, 118...Wiring electrode, 118-1, 118-1a, 118-1b, 118-1c, 118-1d, 118-1e, 118-1f, 118-1g, 118-1h...Signal wiring electrode, 118-2, 118-2a, 118-2b, 118 -2c, 118-2d, 118-2e, 118-2f, 118-2g, 118-2h, 118-2i, 118-2j...ground wiring electrode, 126a, 126b...output waveguide, 128...combining section, 130a, 130b...waveguide for synchrotron radiation, 132a, 132b, 132c...bias voltage Electrode, 140a, 140b, 140c, 140d...side, 142...support plate, 144, 144a, 144b, 144c, 144d...projection, 146...resin layer, 150a, 150b, 150c...first stage electrode, 152, 152a, 152b, 152c...second stage electrode, 154, 154a, 154b, 154c...first base layer, 156, 156a, 156b, 156c...first conductive layer, 158, 158a, 158b, 158c...second base layer, 160, 160a, 160b, 160c...second conductive layer, 162, 162a, 162b...end, 170...first layer, 172...second layer, 1 76...third layer, 178...fourth layer, 180...resist, 182...fifth layer, 190...third base layer, 192...third conductive layer, 194a, 194b, 194c...conductor pattern, 196...rectangular area, 200, 202, 204, 206...line, 402...housing, 406...relay board, 408, 410...signal pin, 412...terminator, 414...input optical fiber, 416...optical unit, 418, 430, 434...lens, 420...output optical fiber, 422, 424...support, 506...circuit board, 508...drive circuit, 600...optical transmitter, 604...light source, 606...modulator driver,608: modulation signal generating unit, 700a, 700b: convex optical waveguide, 702: base layer, 706: conductive layer.

Claims

1. An optical waveguide element including a substrate, an optical waveguide formed on the substrate, and two working electrodes disposed opposite to each other across the optical waveguide for controlling a light wave propagating through the optical waveguide, Each of the working electrodes comprises: a first underlayer made of a first material; a first conductive layer on the first underlayer; a second underlayer on the first conductive layer, the second underlayer being made of a second material different from the first material; a second conductive layer on the second underlayer; an end portion of the second underlayer facing the optical waveguide in a cross section perpendicular to an extension direction of the optical waveguide is covered with the second conductive layer; Optical waveguide element.

2. A wiring electrode connected to the working electrode is provided, The wiring electrode includes the second conductive layer extending from the working electrode.

2. The optical waveguide element according to claim 1.

3. the second material is titanium (Ti); The first conductive layer and the second conductive layer are made of gold (Au).

3. The optical waveguide element according to claim 1.

4. The first material is composed of niobium (Nb). The optical waveguide element according to claim 3 .

5. The first underlayer has a thickness of 30 nm or less, The thickness of the second underlayer is 100 nm or more.

5. The optical waveguide element according to claim 4.

6. The optical waveguide is a convex optical waveguide formed by a convex portion extending on the substrate.

6. The optical waveguide element according to claim 1.

7. The two working electrodes are a signal electrode and a ground electrode that sandwich the optical waveguide within a plane of the substrate, The distance between the signal electrode and the ground electrode is 1.0 μm or more and 5.0 μm or less. The optical waveguide element according to claim 6 .

8. a conductor pattern including a third base layer made of a third material different from the first material and a third conductive layer on the third base layer is formed in a region on the substrate other than a path from the input end to the output end of the optical waveguide; 8. The optical waveguide element according to claim 1.

9. The third material has a larger optical absorption coefficient at a wavelength of light propagating through the optical waveguide than the first material. The optical waveguide element according to claim 8 .

10. A method for producing the optical waveguide element according to claim 3, comprising the steps of: forming a part of the gold constituting the second conductive layer by plating; A method for fabricating an optical waveguide element.

11. The optical waveguide element according to claim 1 , which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light into the optical waveguide element; an optical fiber for guiding light output from the optical waveguide element to the outside of the housing; An optical modulator comprising:

12. The optical waveguide element according to claim 1 , which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light into the optical waveguide element; an optical fiber for guiding light output from the optical waveguide element to the outside of the housing; A drive circuit for driving the optical waveguide element; An optical modulation module comprising:

13. An optical modulator according to claim 11 or an optical modulation module according to claim 12; an electronic circuit for generating an electrical signal for causing the optical waveguide element to perform a modulation operation; An optical transmitting device comprising:

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