Optical waveguide elements, optical modulators, and optical transmitters

Abstract

This system suppresses thermal crosstalk between optical waveguides while appropriately adjusting the bias point using thermo-optical effects, and also achieves miniaturization and high density. [Solution] The optical waveguide element 10A comprises an electro-optical crystal, a rectangular substrate 101 having a first direction and a second direction in a plan view, an optical waveguide 110 formed on the substrate 101, and a bias control unit B11 having a heater electrode 171 for heating the optical waveguide 110. The optical waveguide 110 includes a Mach-Zehnder optical waveguide having a plurality of branched waveguides 130, 150 branched by at least one branching section 120. The plurality of branched waveguides 130, 150 each have curved waveguides C11, C12, and the curved waveguides C11, C12 of each branched waveguide 130, 150 are arranged at a distance of a predetermined distance or more along the first direction. The heater electrode 171 is disposed in at least one of the curved waveguides C11, C12.

Description

Technical Field

[0005] ,

[0004] , , , ,

[0003]

[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 transmission device including the optical modulator.

Background Art

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements including a substrate on which an optical waveguide is formed are used. For the optical waveguide element of an optical modulator, a substrate made of a material having an electro-optic effect such as lithium niobate (LiNbO3; hereinafter also referred to as LN) is used. In recent years, with the progress of substrate processing technology, the substrate can be thinned, and research and development for miniaturization and high density of optical waveguide elements have been advanced.

[0003] In an optical modulator using a material having an electro-optic effect, the bias point can be adjusted by applying a voltage to change the refractive index in the optical waveguide. However, the adjustment of the bias point by applying a voltage is likely to cause a DC drift due to the influence of charge accumulation or the like, and the bias point may move due to the DC drift. In order to suppress the occurrence of such a DC drift, adjustment of a plurality of process parameters is required, and it is not easy to control the bias point.

[0004] On the other hand, a method of adjusting the bias point using the thermo-optic effect has also been proposed. In this method, a heater electrode for raising the temperature of the optical waveguide is arranged. The temperature of the optical waveguide is raised by flowing a current through the heater electrode to heat it. By appropriately controlling the temperature of the optical waveguide to change the refractive index of the optical waveguide, the phase of the light wave propagating through the optical waveguide can be changed.

[0005] Patent Document 1 discloses a Mach-Zehnder type optical modulator utilizing the structure of a Mach-Zehnder interferometer. The optical modulator disclosed in Patent Document 1 includes a DC phase shifter composed of heater electrodes. By applying power to the heater electrodes and heating each optical waveguide after branching, the refractive index of each optical waveguide is appropriately changed by the thermo-optic effect, thereby shifting the phase of the signal light. The amount of phase shift in each optical waveguide is adjusted by setting the width of the heater electrodes and the effective length relative to the optical waveguide. Patent Document 1 also discloses setting the length of the waveguide to be heated by changing the number of times the optical waveguide is folded. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-156778 [Overview of the project] [Problems that the invention aims to solve]

[0007] Patent Document 1 describes a method for adjusting the phase shift amount of two branched optical waveguides by heating each of the two branched optical waveguides in different ways. However, Patent Document 1 does not consider the occurrence of thermal crosstalk in the two branched optical waveguides. Specifically, there is a problem that the heat from the heater electrode that heats a particular optical waveguide is transferred to another optical waveguide formed around it. For example, the heat used to heat one of the two branched optical waveguides is transferred to the other optical waveguide, resulting in a small temperature difference between the two optical waveguides, making it difficult to perform accurate phase control.

[0008] The present invention has been made in view of the above problems, and aims to provide an optical waveguide element that can appropriately adjust the bias point using the thermo-optic effect while suppressing thermal crosstalk between optical waveguides, and that can achieve miniaturization and high density, an optical modulator including the optical waveguide element, and an optical transmitting device including the optical modulator. [Means for solving the problem]

[0009] To solve the above problems, the optical waveguide element, optical modulator, and optical transmitting device according to the present invention have the following technical features.

[0010] The optical waveguide element according to the present invention comprises a rectangular substrate made of an electro-optic crystal having a first direction and a second direction in a plan view, an optical waveguide formed on the substrate, and a bias control unit having a heater electrode for heating the optical waveguide, wherein the optical waveguide includes a Mach-Zehnder optical waveguide having a plurality of branched waveguides branched by at least one branching section, each of the plurality of branched waveguides having a curved waveguide, and the curved waveguides of each branched waveguide are arranged at a distance of a predetermined distance or more along the first direction, and the heater electrode is arranged in at least one of the curved waveguides.

[0011] According to the above configuration, the phase of light can be controlled using the thermo-optic effect by heater electrodes placed in the curved waveguide. Furthermore, since the curved waveguides constituting each branch waveguide are arranged at a distance greater than a predetermined distance along the first direction, thermal crosstalk between branch waveguides can be suppressed, preventing deterioration of control accuracy due to thermal crosstalk. In addition, by arranging the curved waveguides on which the heater electrodes are placed in the first direction, the increase in dimensions in the second direction can be suppressed, enabling miniaturization and high density (high integration) of the optical waveguide element.

[0012] In the optical waveguide element according to the present invention, the curved waveguides of each branch waveguide may have the same shape.

[0013] According to the above configuration, by making the curved waveguides of each branch waveguide the same shape, variations in the optical path length of each branch waveguide can be suppressed, and uniformity of optical characteristics can be ensured.

[0014] In the optical waveguide element according to the present invention, the curved waveguide may be configured such that N (where N is a positive even number) curved folded sections and N+1 straight sections extending in a first direction are alternately connected.

[0015] According to the above configuration, a curved waveguide in which folded sections and straight sections are alternately connected can secure a predetermined area within the substrate surface. This allows the heater electrode for heating a specific branch waveguide to be positioned so that other branch waveguides are not included within its heat conduction range, thereby suppressing thermal crosstalk to other branch waveguides.

[0016] In the optical waveguide element according to the present invention, the heater electrode may be arranged near the linear portion in the above configuration.

[0017] According to the above configuration, by heating the linear section, the thermo-optical effect of the heater electrode can be applied uniformly and efficiently, thereby achieving highly accurate phase control.

[0018] In the optical waveguide element according to the present invention, the substrate may be covered with a buffer layer, and the heater electrode may be arranged to heat the curved waveguide via the buffer layer.

[0019] According to the above configuration, the buffer layer prevents the heater electrode from directly contacting the substrate, improving electrical and thermal stability, and also prevents the heater electrode from absorbing light propagating through the optical waveguide.

[0020] In the optical waveguide element according to the present invention, the curved waveguide may have a second direction extending portion that increases the separation distance in the second direction from other branch waveguides.

[0021] According to the above configuration, since the curved waveguide has the second-direction extending portion, the separation distance in the second direction between the branch waveguides can be appropriately ensured. Further, since the second-direction extending portions of the respective branch waveguides are arranged at a separation of a predetermined distance or more along the first direction, the separation distance in the first direction between the branch waveguides can also be appropriately ensured. Thereby, the thermal crosstalk from the heater electrode disposed in a specific curved waveguide to other branch waveguides can be suppressed.

[0022] In the optical waveguide device according to the present invention, in the above configuration, a groove portion formed by removing a part of the substrate may be formed between the heater electrode for heating a specific optical waveguide and another optical waveguide.

[0023] According to the above configuration, the amount of heat conducted from the heater electrode to other optical waveguides through the groove portion can be reduced, and the thermal crosstalk can be suppressed.

[0024] In the optical waveguide device according to the present invention, in the above configuration, a heat dissipation material made of metal may be disposed in part or all of the groove portion.

[0025] According to the above configuration, the heat energy accumulated during and after heating can be efficiently released to the outside.

[0026] In the optical waveguide device according to the present invention, in the above configuration, a plurality of Mach-Zehnder optical waveguides may be formed on the substrate.

[0027] According to the above configuration, thermal crosstalk can be suppressed for a large number of branch waveguides included in the plurality of Mach-Zehnder optical waveguides, and the optical waveguide device can be miniaturized and densified.

[0028] In the optical waveguide device according to the present invention, in the above configuration, the plurality of Mach-Zehnder optical waveguides may be arranged in a nested or parallel manner.

[0029] According to the above configuration, with respect to an optical waveguide element comprising multiple Mach-Zehnder optical waveguides arranged in a nested or parallel manner, thermal crosstalk can be suppressed, and the optical waveguide element can be miniaturized and its density increased.

[0030] Furthermore, in order to achieve the above objective, the optical modulator according to the present invention is characterized by comprising the above-mentioned optical waveguide element, a housing for housing the optical waveguide element, an input optical fiber connected to the optical input portion of the optical waveguide element, and an output optical fiber connected to the optical output portion of the optical waveguide element.

[0031] Furthermore, in order to achieve the above objective, the optical transmitting device according to the present invention is characterized by having the above-mentioned optical modulator, a light source that inputs an optical wave to the optical modulator, and a signal output circuit that outputs the modulated signal. [Effects of the Invention]

[0032] According to the present invention, it is possible to provide an optical waveguide element that can appropriately adjust the bias point using the thermo-optic effect while suppressing thermal crosstalk between optical waveguides, and that can achieve miniaturization and high density, an optical modulator including the optical waveguide element, and an optical transmitting device including the optical modulator. [Brief explanation of the drawing]

[0033] [Figure 1] These figures illustrate the basic concepts of the present invention. (a) shows a state in which branched waveguides are formed in a straight line, (b) shows a state in which a curved waveguide is included in the branched waveguide, and (c) shows a part of a Mach-Zehnder optical waveguide that reflects the basic concepts of the present invention. [Figure 2] This figure shows an example of a curved waveguide that constitutes a part of a Mach-Zehnder optical waveguide, reflecting the basic concept of the present invention. (a) shows the case where the curved waveguide is a folded waveguide formed in an S shape, (b) shows the case where the curved waveguide is a folded waveguide formed in an inverted S shape, and (c) shows the case where the curved waveguide is a meandering waveguide. [Figure 3] This figure shows a partial example of a nested Mach-Zehnder optical waveguide that reflects the basic concepts of the present invention. [Figure 4] This is a plan view of an optical waveguide element in the first embodiment of the present invention. [Figure 5] This is a cross-sectional view along line AA in Figure 4. [Figure 6] This is a cross-sectional view showing a first example of the arrangement of heater electrodes near a branched waveguide in a first embodiment of the present invention. [Figure 7] This is a cross-sectional view showing a second arrangement example of heater electrodes near a branched waveguide in a first embodiment of the present invention. [Figure 8] This is a cross-sectional view showing a third arrangement example of heater electrodes near a branched waveguide in a first embodiment of the present invention. [Figure 9] This is a cross-sectional view showing a fourth arrangement example of heater electrodes near a branched waveguide in a first embodiment of the present invention. [Figure 10] This is a cross-sectional view showing a fifth arrangement example of heater electrodes near a branched waveguide in a first embodiment of the present invention. [Figure 11] This is a partial plan view of the vicinity of a curved waveguide, showing a first example of the arrangement of heater electrodes for a curved waveguide in a first embodiment of the present invention. [Figure 12] This is a partial plan view of the vicinity of a curved waveguide, showing a second example of the arrangement of heater electrodes for a curved waveguide in a first embodiment of the present invention. [Figure 13] This is a partial plan view of the vicinity of a curved waveguide, showing a third example of the arrangement of heater electrodes for a curved waveguide in a first embodiment of the present invention. [Figure 14] This is a plan view of an optical waveguide element in a second embodiment of the present invention. [Figure 15] This is a cross-sectional view along line BB in Figure 14. [Figure 16] This is a cross-sectional view along the CC line in Figure 14. [Figure 17] This is a plan view of an optical waveguide element in a derivative example of a second embodiment of the present invention. [Figure 18]This is a cross-sectional view along the DD line in Figure 17. [Figure 19] This is a cross-sectional view along the EE line in Figure 17. [Figure 20] This is a plan view of an optical waveguide element in a third embodiment of the present invention. [Figure 21] This is a partial plan view of the vicinity of a curved waveguide, showing a first example of the arrangement of heater electrodes for a curved waveguide in a third embodiment of the present invention. [Figure 22] This is a partial plan view of the vicinity of a curved waveguide, showing a second example of the arrangement of heater electrodes for a curved waveguide in a third embodiment of the present invention. [Figure 23] This is a partial plan view of the vicinity of a curved waveguide, showing a third example of the arrangement of heater electrodes for a curved waveguide in a third embodiment of the present invention. [Figure 24] This is a plan view of an optical waveguide element in a fourth embodiment of the present invention. [Figure 25] This is a partial plan view showing a first alternative example of a part of a Mach-Zehnder optical waveguide in a fourth embodiment of the present invention. [Figure 26] This is a partial plan view showing a second alternative example of a Mach-Zehnder optical waveguide in a fourth embodiment of the present invention. [Figure 27] This is a partial plan view showing a third alternative example of a Mach-Zehnder optical waveguide in the fourth embodiment of the present invention. [Figure 28] This is a plan view showing an optical modulator and optical transmitter according to the present invention. [Figure 29] This is the first figure illustrating the problem of the present invention. [Figure 30] This is a second figure illustrating the problem of the present invention. [Figure 31] This is a third figure illustrating the problem of the present invention. [Modes for carrying out the invention]

[0034] Embodiments of the present invention will be described below with reference to the drawings. The drawings referenced herein are not necessarily to the exact scale of the actual dimensions and are partially exaggerated or simplified in order to schematically illustrate the configuration of the present invention. The numerical ranges described herein include upper and lower limits, meaning that any numerical value within that range can be selected. The drawings will be described using an XYZ coordinate system. The optical waveguide element according to the present invention has a rectangular flat plate structure when viewed from above. The first direction when viewed from the top side is the X direction, the second direction is the Y direction, and the normal direction to the top surface is the Z direction. In the embodiments described herein, X-cut lithium niobate (LN) is used as the material having an electro-optic effect, in which case the first direction, the X direction, is the Y axis in the crystal axis, and the second direction, the Y direction, is the Z axis in the crystal axis.

[0035] <Basic Concepts of the Invention> First, I will explain the basic concepts of the present invention.

[0036] The optical waveguide element according to the present invention is configured to include a Mach-Zehnder optical waveguide having a plurality of branched waveguides branched by at least one branching section. The optical waveguide element according to the present invention may be provided with a plurality of Mach-Zehnder optical waveguides. The plurality of Mach-Zehnder optical waveguides may be provided in a nested structure.

[0037] The optical waveguide element according to the present invention has a heater electrode positioned near at least one branched waveguide. The heater electrode shifts the phase of light propagating through the branched waveguide using the thermo-optic effect. By heating the branched waveguide with the heater electrode and controlling the refractive index of the branched waveguide, the bias point is adjusted using the thermo-optic effect.

[0038] Figures 29 to 31 are the first to third figures illustrating the problems of the present invention.

[0039] Figure 29 schematically shows a portion of a conventional Mach-Zehnder optical waveguide 2100. In the Mach-Zehnder optical waveguide 2100 shown in Figure 29, one input optical waveguide 2101 branches into two branch waveguides 2103 and 2104 at a branching section 2102. The two branch waveguides 2103 and 2104 extend in parallel in the X direction. The direction of light propagation is the positive X direction (from left to right in the figure). The multiplexing section located at the end of the Mach-Zehnder optical waveguide is omitted from the figure.

[0040] A heater electrode 2501 is positioned near at least one of the branched waveguides 2103 and 2104. In Figure 29, the heater electrode 2501 is positioned near branched waveguide 2104. The heater electrode 2501 generates heat in proportion to the applied power, heating the branched waveguide 2104. The refractive index of the heated branched waveguide 2104 changes due to the thermo-optic effect. By controlling the power applied to the heater electrode 2501 to adjust the amount of heating and appropriately changing the refractive index of the branched waveguide 2104, the phase of light propagating through the branched waveguide 2104 can be appropriately shifted.

[0041] The heater electrode 2501 is positioned to heat a specific branch waveguide 2104. However, a thermal crosstalk problem can occur where the heat emitted by the heater electrode 2501 is transferred not only to the branch waveguide 2104 being heated, but also to other branch waveguides 2103. Therefore, it is desirable to set the formation positions of branch waveguides 2103 and 2104 and the placement position of the heater electrode 2501 so that other branch waveguides 2103 are not included within the heat conduction range R (heating range that can affect the refractive index) of the heater electrode 2501.

[0042] Figure 30 schematically shows a state in which the separation distance in the Y direction between the two branch waveguides 2103 and 2104 is increased.

[0043] One possible approach to ensure that other branch waveguides 2103 are not included within the heat conduction range R of the heater electrode 2501 is to extend the separation distance between branch waveguides 2103 and 2104 (separation distance in the Y direction in the figure), as shown in Figure 30. However, increasing the separation distance in the Y direction would increase the overall length of the optical waveguide element in the Y direction. In an optical waveguide element, it is necessary to extend the section to which a high-frequency voltage is applied in order to modulate light, and a certain length is secured, for example, along the X direction (first direction). Extending the separation distance between branch waveguides 2103 and 2104 in the Y direction would hinder the miniaturization and density reduction of the optical waveguide element.

[0044] This problem is particularly pronounced when an optical waveguide element has a large number of Mach-Zehnder optical waveguides. Figure 31 shows a portion of a nested Mach-Zehnder optical waveguide 2200 in which multiple Mach-Zehnder optical waveguides are nested together. In the nested Mach-Zehnder optical waveguide 2200 shown in Figure 31, one input optical waveguide 2201 branches into two branched waveguides 2203 and 2204 at the first branching section 2202. Branched waveguide 2203 branches into two branched waveguides 2251 and 2252 at the second branching section 2250. Branched waveguide 2204 branches into two branched waveguides 2261 and 2262 at the third branching section 2260. Note that the multiplexing sections located at the end of each Mach-Zehnder optical waveguide are not shown.

[0045] In Figure 31, as an example, a heater electrode 2511 is placed in one of the branched waveguides 2252 that branched off at the second branching section 2250. Also, a heater electrode 2512 is placed in one of the branched waveguides 2262 that branched off at the third branching section 2260. The heater electrode 2511 heats the branched waveguide 2252 and has a heat conduction range R1. The heater electrode 2512 heats the branched waveguide 2262 and has a heat conduction range R2.

[0046] To ensure that the other branch waveguides 2251, 2261, and 2262 are not included inside the heat conduction range R1, and that the other branch waveguides 2251, 2252, and 2261 are not included inside the heat conduction range R2, for example, one could consider expanding the separation distance between each branch waveguide 2251, 2252, 2261, and 2262 (separation distance in the Y direction in the figure), as shown in Figure 31. However, if the separation distance between each branch waveguide 2251, 2252, 2261, and 2262 is increased, the dimension in the Y direction (second direction) will increase dramatically.

[0047] Figure 1 is a diagram illustrating the basic concept of the present invention. Figure 1(a) schematically shows a state in which branched waveguides 1103 and 1104 are formed in a straight line. Figure 1(b) schematically shows a state in which curved waveguides C1 and C2 are included in branched waveguides 1103 and 1104. Figure 1(c) schematically shows a part of a Mach-Zehnder optical waveguide 1100 that reflects the basic concept of the present invention.

[0048] In the Mach-Zehnder optical waveguide 1100 shown in Figures 1(a) to 1(c), one input optical waveguide 1101 branches into two branch waveguides 1103 and 1104 at the branching section 1102. The multiplexing section located at the end of the Mach-Zehnder optical waveguide is not shown. Figure 1(a) shows the two branch waveguides 1103 and 1104 extending in parallel in the X direction. The distance D1 between the two parallel branch waveguides 1103 and 1104 is uniform.

[0049] In this invention, the separation distance in the X direction between the two branch waveguides 1103 and 1104 is increased. Specifically, as shown in Figure 1(b), curved waveguides C1 and C2 are formed in the branch waveguides 1103 and 1104, and then, as shown in Figure 1(c), the separation distance between the curved waveguides C1 and C2 is extended in the X direction.

[0050] The curved waveguide C1 constitutes a part of the branch waveguide 1103 and has a starting point S1 and an ending point T1. The starting point S1 and the ending point T1 are at different positions in the Y direction, and the curved waveguide C1 includes an extension component in the Y direction between the starting point S1 and the ending point T1. The vicinity of the starting point S1 of the curved waveguide C1 constitutes a second directional extension portion in which the separation distance in the Y direction from the branch waveguide 1104 is large.

[0051] The curved waveguide C2 constitutes a part of the branch waveguide 1104 and has a starting point S2 and an ending point T2. The curved waveguide C2 is formed to have the same shape as the curved waveguide C1. The starting point S2 and the ending point T2 are at different positions in the Y direction, and the curved waveguide C2 includes an extension component in the Y direction between the starting point S2 and the ending point T2. The vicinity of the ending point T2 of the curved waveguide C2 constitutes a second directional extension portion in which the separation distance in the Y direction from the branch waveguide 1103 is large.

[0052] As shown in Figure 1(a), when the two branch waveguides 1103 and 1104 extend linearly in the X direction (i.e., they do not include an extension component in the Y direction), it is not possible to separate the two branch waveguides 1103 and 1104 in the X direction. On the other hand, as shown in Figure 1(b), when curved waveguides C1 and C2 of the same shape are provided at the two branch waveguides 1103 and 1104, as shown in Figure 1(c), the two branch waveguides 1103 and 1104 can be separated in the X direction at the locations of the curved waveguides C1 and C2. In the region P formed between the curved waveguide C1 and the curved waveguide C2, the curved waveguides C1 and C2 have portions that face each other in the X direction.

[0053] The curved waveguides C1 and C2 are arranged in the X direction at a distance greater than a predetermined distance apart. The distance between curved waveguides C1 and C2 in the X direction is preferably set appropriately based on the heat conduction range of the heater electrodes. The heat conduction range of the heater electrodes is determined based on various factors, including heating performance such as size, material, and applied power, as well as the thermal conductivity of the materials constituting the optical waveguide element.

[0054] The optical waveguide element according to the present invention is designed so that a heater electrode is placed in at least one of the curved waveguides C1 and C2, and other branch waveguides are not formed within the heat conduction range of the heater electrode. As a result, other branch waveguides (e.g., branch waveguide 1103) are formed outside the heat conduction range of the heater electrode that heats a specific branch waveguide (e.g., branch waveguide 1104), thereby suppressing thermal crosstalk that may occur between branch waveguides.

[0055] Figure 2 shows an example of curved waveguides C1 and C2 that constitute a part of a Mach-Zehnder optical waveguide 1100, reflecting the basic concept of the present invention. Note that the multiplexing section located at the end of the Mach-Zehnder optical waveguide is omitted from the illustration. Figure 2(a) shows the case where the curved waveguides C1 and C2 are folded waveguides formed in an S shape. Figure 2(b) shows the case where the curved waveguides C1 and C2 are folded waveguides formed in an inverted S shape. Figure 2(c) shows the case where the curved waveguides C1 and C2 are meandering waveguides. Figures 2(a) to (c) also show the heater electrode 1501 and its heat conduction range R placed in the curved waveguide C1.

[0056] The curved waveguide C1 includes an extending component in the Y direction in addition to an extending component in the X direction. At least a portion of the curved waveguide C1 is separated from other branch waveguides by a predetermined distance or more in both the X and Y directions.

[0057] The shapes of the curved waveguides C1 and C2 are not particularly limited. For example, the S-shaped folded waveguide shown in Figure 2(a), the inverted S-shaped folded waveguide shown in Figure 2(b), and the meandering waveguide shown in Figure 2(c) can be used as curved waveguides C1 and C2. However, it is preferable to use an S-shaped or inverted S-shaped folded waveguide because it is easy to design and manufacture and is also advantageous in terms of optical propagation efficiency.

[0058] An S-shaped or inverted S-shaped folded waveguide refers to a structure in which N+1 straight sections extending in the X direction and N folded sections that reverse the direction of extension of the optical waveguide by 180° are alternately connected. Here, N is a positive even number. The S-shaped folded waveguide shown in Figure 2(a) and the inverted S-shaped folded waveguide shown in Figure 2(b) are examples where N=2.

[0059] In terms of suppressing thermal crosstalk, the shapes of the curved waveguides C1 and C2 do not necessarily have to be identical. For example, curved waveguide C1 may be a folded waveguide and curved waveguide C2 may be a meandering waveguide. However, if the shapes and optical path lengths of the branched waveguides 1103 and 1104 are different, differences in optical propagation loss may occur. For this reason, from the viewpoint of maintaining the symmetry of the branched waveguides 1103 and 1104, it is preferable that curved waveguides C1 and C2 have the same shape. Furthermore, by making the curved waveguides C1 and C2 have the same shape, it is possible to reliably face the curved waveguides C1 and C2 in the X direction.

[0060] Figure 3 shows an example of a nested Mach-Zehnder optical waveguide 1200 that reflects the basic concept of the present invention. In the nested Mach-Zehnder optical waveguide 1200 shown in Figure 3, one input optical waveguide 1201 branches into two branched waveguides 1203 and 1204 at the first branching section 1202. Branched waveguide 1203 branches into two branched waveguides 1251 and 1252 at the second branching section 1250. Branched waveguide 1204 branches into two branched waveguides 1261 and 1262 at the third branching section 1260. Note that the multiplexing sections located at the end of each Mach-Zehnder optical waveguide are not shown.

[0061] In Figure 3, as an example, a heater electrode 1501 is placed in one of the branched waveguides 1251 that branched off at the second branching section 1250. Also, a heater electrode 1501 is placed in one of the branched waveguides 1262 that branched off at the third branching section 1260. The heater electrode 1501 heats the branched waveguide 1251 and has a heat conduction range R1. The heater electrode 1502 heats the branched waveguide 1262 and has a heat conduction range R2.

[0062] As shown in Figure 3, a curved waveguide C3 is formed in branch waveguide 1251. A curved waveguide C4 is formed in branch waveguide 1252. A curved waveguide C5 is formed in branch waveguide 1261. A curved waveguide C6 is formed in branch waveguide 1262. Curved waveguides C3 to C6 are composed of S-shaped folded waveguides.

[0063] This makes it possible to ensure that, even in a nested Mach-Zehnder optical waveguide 1200, other branch waveguides 1252, 1261, and 1262 are not included inside the heat conduction range R1, and that other branch waveguides 1251, 1252, and 1261 are not included inside the heat conduction range R2, thereby suppressing thermal crosstalk.

[0064] <First Embodiment> A first embodiment of the present invention will now be described. Figure 4 is a plan view of the optical waveguide element 10A in the first embodiment of the present invention. Figure 5 is a cross-sectional view along line AA in Figure 4.

[0065] As shown in Figure 4, the optical waveguide element 10A includes a substrate 101 and an optical waveguide 110 formed on the substrate 101. As will be described later, the optical waveguide 110 is formed, for example, by a convex portion 101a provided on the surface of the substrate 101.

[0066] As shown in Figure 4, the optical waveguide element 10A is formed in a rectangular shape in a plan view. In this specification, the X direction in the plan view shown in Figure 4 is referred to as the first direction of the optical waveguide element 10A, and the Y direction in the plan view shown in Figure 4 is referred to as the second direction of the optical waveguide element 10A.

[0067] The optical waveguide element 10A shown in Figure 4 is implemented, for example, in an optical modulator. The optical waveguide element 10A is configured to modulate incident light L1 incident on the optical input terminal 111 and emit output light L2 from the optical output terminal 162. In Figure 4, the incident direction of the incident light L1 is the positive X direction (first direction) in the figure, and the output direction of the output light L2 is the negative X direction (first direction) in the figure.

[0068] In the optical waveguide element 10A shown in Figure 4, the optical input terminal 111 and the optical output terminal 162 are located on the same end face (end face 21), but the optical input terminal 111 and the optical output terminal 162 may be located on different end faces. End faces 21 and 22 are located opposite each other and constitute both ends of the optical waveguide element 10A in the first direction. End faces 23 and 24 are located opposite each other and constitute both ends of the optical waveguide element 10A in the short direction.

[0069] An optical waveguide element 10A has an optical waveguide 110 formed therein for propagating light. The optical waveguide 110 is composed of an optical input terminal 111, an input optical waveguide 112, a branching section 120, branching waveguides 130 and 150, a multiplexing section 160, an output optical waveguide 161, and an optical output terminal 162.

[0070] The optical input terminal 111 is the end of the optical waveguide 110 and functions as an optical input port that takes in incident light L1 into the optical waveguide 110. The optical input terminal 111 may be provided with a spot size conversion unit (SSC) or a grading unit that changes the cross-sectional diameter of the optical wave. The configuration of the spot size conversion unit or grading unit is not particularly limited and can be realized with existing technology.

[0071] The input optical waveguide 112 has one end connected to the optical input terminal 111 and the other end connected to the branching section 120. The incident light L1 that enters the optical input terminal 111 propagates through the input optical waveguide 112 to the branching section 120.

[0072] The branching section 120 is configured to branch the optical waveguide 110. For example, a Y-shaped waveguide or an optical coupler can be used in the branching section 120. The optical input side of the branching section 120 is connected to the input optical waveguide 112, and the optical output side is connected to the branched waveguides 130 and 150. The branching section 120 splits the light propagating within the input optical waveguide 112 into the branched waveguides 130 and 150.

[0073] As shown in Figure 4, a curved waveguide C11 is formed in the middle of the branched waveguide 130. In this embodiment, the curved waveguide C11 is a folded waveguide with an inverted S-shape in plan view. More specifically, the curved waveguide C11 has, in order from the branching section 120 side (optical input side), a straight section 131 extending in the first direction, a folded section 132 formed in an arc shape, a straight section 133 extending in the first direction, a folded section 134 formed in an arc shape, and a straight section 135 extending in the first direction. A folded section 136 formed in an arc shape and a straight section 137 extending in the first direction are connected to the straight section 135. The folded sections 132, 134, and 136 can have any curved shape, but it is preferable to have a smoothly connected arc shape in order to reduce optical loss. In this specification, an arc shape includes one that has transition curves at both ends of the circular curve. A transition curve is a curve designed to smoothly connect a straight line and a circular curve, with its curvature gradually changing.

[0074] Similarly, a curved waveguide C12 is formed in the middle of the branched waveguide 150. In this embodiment, the curved waveguide C12 is a folded waveguide with an inverted S-shape in plan view. More specifically, the curved waveguide C12 has, in order from the branching section 120 side (optical input side), a straight section 151 extending in the first direction, a folded section 152 formed in an arc shape, a straight section 153 extending in the first direction, a folded section 154 formed in an arc shape, and a straight section 155 extending in the first direction. A folded section 156 formed in an arc shape and a straight section 157 extending in the first direction are connected to the straight section 155. Although the folded sections 152, 154, and 156 can have any curved shape, it is preferable to have a smoothly connected arc shape in order to reduce optical loss.

[0075] The curved waveguides C11 and C12 include an extending component in a second direction in addition to an extending component in a first direction. Furthermore, the curved waveguides C11 and C12 are arranged at a distance greater than a predetermined distance in the first direction. The folded portion 132 of the curved waveguide C11 constitutes a second-direction extending portion where the separation distance in the second direction from the branch waveguide 150 is large. The folded portion 154 of the curved waveguide C12 constitutes a second-direction extending portion where the separation distance in the second direction from the branch waveguide 130 is large. The second-direction extending portion is an optical waveguide designed so that the heat from a heater electrode located in one of the Mach-Zehnder optical waveguides (a combination of a curved waveguide and a straight waveguide) does not affect the other Mach-Zehnder optical waveguide (a different branch waveguide) or other Mach-Zehnder optical waveguides.

[0076] Light that has traveled from the branching section 120 into the branched waveguide 130 propagates in the positive X direction (first direction) within the straight section 131. At the arc-shaped folding section 132, it is folded back while traveling in the negative Y direction (second direction) and propagates in the negative X direction (first direction) within the straight section 133. At the arc-shaped folding section 134, it is folded back again while traveling in the negative Y direction (second direction) and propagates in the positive X direction (first direction) within the straight section 135. Furthermore, when it is folded back at the arc-shaped folding section 136, it propagates in the negative X direction (first direction) within the straight section 137.

[0077] Light that has traveled from branch section 120 into branch waveguide 150 propagates in the positive X direction (first direction) within the straight section 151. At the arc-shaped folding section 152, it is folded back while traveling in the negative Y direction (second direction) and propagates in the negative X direction (first direction) within the straight section 153. At the arc-shaped folding section 154, it is folded back again while traveling in the negative Y direction (second direction) and propagates in the positive X direction (first direction) within the straight section 155. Furthermore, when it is folded back at the arc-shaped folding section 156, it propagates in the negative X direction (first direction) within the straight section 157.

[0078] Branch waveguides 130 and 150 are connected to a multiplexer 160. The multiplexer 160 is configured to merge the optical waveguide 110. The multiplexer 160 can use, for example, a Y-shaped waveguide or an optical coupler. The optical input side of the multiplexer 160 is connected to branch waveguides 130 and 150, and the optical output side is connected to the output optical waveguide 161. The light propagating through branch waveguides 130 and 150 is combined in the multiplexer 160.

[0079] The light waves combined in the combined section 160 propagate through the output optical waveguide 161 and are emitted as emitted light L2 from the optical output terminal 162 connected to the output optical waveguide 161.

[0080] The optical waveguide element 10A includes a bias control unit B11 that adjusts the bias point of the light propagating through the optical waveguide 110, and a modulation unit M that modulates the phase of the light propagating through the optical waveguide 110.

[0081] In this embodiment, the bias control unit B11 is provided only in the branch waveguide 130. That is, in this embodiment, only the branch waveguide 130 is heated, and the branch waveguide 150 is not heated. By adjusting the bias point of the light propagating through the branch waveguide 130 through heating in the bias control unit B11, the relative phase between the light propagating through the branch waveguide 130 and the light propagating through the branch waveguide 150 is adjusted.

[0082] The bias control unit B11 has a heater electrode 171 that heats the optical waveguide 110. A wiring electrode 172 and an electrode pad 173, which function as a power supply line to the heater electrode 171, are electrically connected to the heater electrode 171. A DC power supply 174 is connected to the electrode pad 173.

[0083] The heater electrode 171 generates heat in response to the applied voltage, heating the branched waveguide 130, which is the target of heating. In this embodiment, the heater electrode 171 is positioned near the straight section 133 of the branched waveguide 130. The refractive index of the branched waveguide 130 heated by the heater electrode 171 changes due to the thermo-optic effect. By controlling the voltage applied to the heater electrode 171, the amount of heating (temperature) of the branched waveguide 130 can be adjusted, and as a result, the amount of light phase shift can be appropriately adjusted. The material of the heater electrode 171 may be the same metal as that of the modulation section M, or a metal with excellent heat generation efficiency and heat resistance may be selected.

[0084] The optical waveguide 110 formed in the optical waveguide element 10A is a Mach-Zehnder optical waveguide, and at the branching section 120, it branches into two branch waveguides 130 and 150. A modulation section M is provided downstream of the bias control section B11. The modulation section M modulates the light that has been branched into the two branch waveguides 130 and 150 at the branching section 120 to control the phase of each light, thereby appropriately adjusting the intensity (signal intensity) of the light combined at the multiplexing section 160.

[0085] The electrode structure of the modulation section M is outside the scope of the present invention and is not particularly limited. Figure 4 schematically illustrates a state in which a control electrode 191 is arranged near the straight sections 137 and 157 of the branched waveguides 130 and 150, and a high-frequency signal source (high-frequency power supply) 192 and a termination resistor 193 are electrically connected to the control electrode 191. The control electrode 191 may include a signal electrode and a reference electrode (ground electrode). In the case of an X-cut substrate, it is preferable to arrange the signal electrode to the side of the straight sections 137 and 157. On the other hand, in the case of a Z-cut substrate, it is preferable to arrange the signal electrode directly above the straight sections 137 and 157. Furthermore, a capacitor 194 may be interposed between the control electrode 191 and the high-frequency signal source 192 to cut the DC component of the high-frequency signal source 192. However, if a DC component is applied to the control electrode 191, it is not necessary to interpose the capacitor 194. Furthermore, the termination resistor 193 and capacitor 194 may be mounted or formed on the optical waveguide element 10A.

[0086] The cross-sectional structure of the optical waveguide element 10A in this embodiment will be described with reference to Figure 5. Figure 5 shows a cross-section of the branched waveguide 130 near the straight section 133. In Figure 5, the left-right direction (Y direction) of the cross-sectional view corresponds to the second direction of the optical waveguide element 10A, and the up-down direction (Z direction) corresponds to the height direction of the optical waveguide element 10A.

[0087] The substrate 101 of the optical waveguide element 10A is made of a material that exhibits an electro-optic effect. Suitable materials for this electro-optic effect include lithium niobate (LN), lithium tantalate (LT), lead lanthanum zirconate titanate (PLZT), EO polymer, etc., and these materials may be doped with MgO or the like. Alternatively, vapor-grown films of these materials, or composite substrates formed by bonding these materials to different substrates, may be used.

[0088] The thickness of the substrate 101 is preferably, for example, 1.0 μm or less. The thickness of the substrate 101 refers to the height from the bottom surface of the substrate 101 to the flat top surface of the substrate where the convex portion 101a is not formed. By making the substrate 101 a thin film with a thickness of 1.0 μm or less, the driving voltage can be reduced and the size can be miniaturized.

[0089] An optical waveguide 110 is formed on the substrate 101 of the optical waveguide element 10A. A convex portion 101a is formed on the substrate 101, protruding from the flat upper surface of the substrate. The convex portion 101a is provided in the portion corresponding to the optical waveguide 110, and forms a convex optical waveguide, which is the path (optical path) through which light propagates.

[0090] The method for forming the convex optical waveguide is not particularly limited. For example, the substrate 101 may be etched to leave a convex portion 101a (rib portion), or a convex portion 101a (ridge portion) may be formed by cutting off parts of both sides of the optical waveguide 110 to form a concave cut portion 101b. Alternatively, instead of a convex optical waveguide, a thermal diffusion type waveguide may be formed by diffusing Ti or the like onto the surface of the substrate 101 using a thermal diffusion method, or a proton exchange type waveguide may be formed by diffusing Ti or the like onto the surface of the substrate 101 using a proton exchange method. Furthermore, after forming a convex optical waveguide, the refractive index may be further increased by diffusing Ti or the like onto the surface of the substrate 101 using a thermal diffusion method or a proton exchange method. The size of the convex portion 101a is not particularly limited, but like a normal convex optical waveguide, the width and height can be about 1.0 μm. Here, as shown in Figure 5, the convex portion 101a is formed by forming the cut portion 101b.

[0091] As shown in Figure 5, a reinforcing substrate (support substrate) 102 may be placed beneath the thinned substrate 101 to increase its mechanical strength. Note that the lower part of the reinforcing substrate 102 is omitted from the illustration in Figure 5. The thickness of the reinforcing substrate 102 is not particularly limited, but can be, for example, about 0.2 to 1.0 mm. The reinforcing substrate 102 may be bonded to the substrate 101 via a bonding layer (intermediate layer) 103, as shown in Figure 5, or it may be directly bonded to the substrate 101. The material of the reinforcing substrate 102 is not particularly limited, but for example, Si, glass, quartz, fused silica, synthetic silica, alkali glass, alkali-free glass, lead glass, borosilicate glass, soda glass, sapphire, alumina, etc. can be used.

[0092] As shown in Figure 5, a heater electrode 171 is placed on the substrate 101. In Figure 5, cut portions 101b are formed on both sides of the straight portion 133 of the branched waveguide 130, and heater electrodes 171a and 171b are arranged within the cut portions 101b. The heater electrodes 171a and 171b are electrically connected and constitute the heater electrode 171. In order to suppress light absorption by the heater electrode 171, it is preferable that the heater electrode 171 is arranged so as not to come into contact with the convex portion 101a that forms the optical waveguide 110.

[0093] The heating performance of the heater electrode 171 is determined by a combination of factors, including the size of the heater electrode 171 and the applied voltage. The size of the heater electrode 171 is preferably set to provide adequate heating performance for heating the branched waveguide 130. For example, the heater electrode 171 has a length of several micrometers along the branched waveguide 130 to be heated. The width and height of the heater electrode 171 can be appropriately determined to match the size of the convex portion 101a.

[0094] In Figure 5, the heater electrodes 171 are arranged on both sides of the straight section 133 of the branch waveguide 130, but the configuration is not limited to this.

[0095] The heater electrode 171 may be placed on only one side of the branched waveguide 130 that is to be heated. For example, as shown in Figure 6, the heater electrode 171 may be placed on one side of the straight section 133 of the branched waveguide 130 (on the positive Y-direction side with respect to the convex section 101a). Alternatively, as shown in Figure 7, the heater electrode 171 may be placed on the other side of the straight section 133 of the branched waveguide 130 (on the negative Y-direction side with respect to the convex section 101a).

[0096] The buffer layer 104 may be placed on part or the entire upper surface of the substrate 101. By placing the buffer layer 104 so as to cover the convex portion 101a, the light confinement effect at the convex portion 101a can be improved. Alternatively, by placing the buffer layer 104 so as to cover the entire upper surface of the substrate 101, the entire upper surface of the substrate 101 can be protected. In particular, it is preferable to interpose the buffer layer 104 at the intersection of the wiring electrode 172 and the optical waveguide 110 so that the wiring electrode 172 and the optical waveguide 110 do not come into direct contact. The buffer layer 104 is made of a material with a refractive index lower than that of the substrate 101, such as SiO2. Although the waveguide shape is shown as a convex shape, the optical waveguide 110 can also be rectangular or other shapes.

[0097] In the configuration shown in Figure 8, the buffer layer 104 is arranged on the upper surface of the substrate 101 so as to cover the convex portion 101a and the heater electrode 171. Figure 8 shows an example in which the buffer layer 104 is arranged in a configuration in which the heater electrode 171 is arranged on both sides of the straight portion 133 of the branched waveguide 130 (the configuration shown in Figure 5 above). However, even in a configuration in which the heater electrode 171 is arranged on one side of the straight portion 133 of the branched waveguide 130 (the configurations shown in Figures 6 and 7 above), the buffer layer 104 can be arranged to cover the convex portion 101a and the heater electrode 171 in the same way.

[0098] In the configurations shown in Figures 9 and 10, the buffer layer 104 is arranged to cover the entire upper surface of the substrate 101. As shown in Figure 9, the heater electrode 171 may be placed on the upper surface of the buffer layer 104. The heater electrode 171 is placed on the buffer layer 104, for example, so as to be positioned above the convex portion 101a. In this case, the upper side of the heater electrode 171 is exposed to the outside (air), which can improve the heat dissipation efficiency after heating.

[0099] As shown in Figure 10, the heater electrode 171 may be embedded in the buffer layer 104. For example, the heater electrode 171 is embedded in the buffer layer 104 so as to be located above the convex portion 101a. In this case, the distance between the heater electrode 171 and the convex portion 101a can be reduced to improve the heating efficiency of the optical waveguide 110.

[0100] Furthermore, although the heater electrode 171 is positioned near the straight section 133 that constitutes the curved waveguide C11 in Figure 4, the heater electrode 171 should be positioned at a distance from the other branch waveguides 150 so that the other branch waveguides 150 are not included within the heat conduction range.

[0101] For example, as shown in Figure 11, the heater electrode 171 may be positioned near the straight section 135 that constitutes the curved waveguide C11. When the straight section 135 is heated, the thermo-optic effect of the heater electrode 171 can be applied uniformly and efficiently. Alternatively, as shown in Figures 12 and 13, the heater electrode 171 may be positioned near the folded section 134 that constitutes the curved waveguide C11. In Figure 12, the heater electrode 171 is positioned near both the straight section 133 and the folded section 134. In Figure 13, the heater electrode 171 is positioned near both the folded section 134 and the straight section 135.

[0102] The extension direction and shape of the heater electrode 171 can be arbitrarily set. The heater electrode 171 may have a shape that extends in the X direction or the Y direction, or it may have a shape that extends at a predetermined angle (diagonally) with respect to the X and Y directions. Furthermore, the heater electrode 171 may have a linear shape, a curved shape, or a shape that is a combination of linear and curved. Moreover, the heater electrode 171 may be divided into multiple electrodes, such as linear and curved electrodes, and these electrodes may be electrically connected.

[0103] The bias control unit B11 will be described below.

[0104] The heat emitted by the heater electrode 171 is diffused within a predetermined heat conduction range (a heating range that can affect the refractive index), for example, centered around the heater electrode 171. Therefore, when a specific optical waveguide (branched waveguide 130) is to be heated, it is necessary to position the heater electrode 171 so that the heat emitted by the heater electrode 171 does not reach other optical waveguides (branched waveguide 150).

[0105] The bias control unit B11 is set at the formation position of the curved waveguide C11, and the heater electrode 171 is placed in the curved waveguide C11. As shown in Figure 4, the curved waveguide C11 is composed of a straight section 131, a folded section 132, a straight section 133, a folded section 134, and a straight section 135, and is formed to be an inverted S-shaped folded waveguide in plan view.

[0106] The curved waveguide C11 includes extending components not only in the first direction but also in the second direction. Specifically, the inverted S-shaped folded waveguide constituting the curved waveguide C11 is curved in the XY plane and includes extending components in both the first and second directions. This allows a portion of the curved waveguide C11 to be separated from other branch waveguides 150 by a predetermined distance or more in both the first and second directions. By placing the heater electrode 171 in this separated portion, it is possible to ensure that other branch waveguides 150 are not included within the heat conduction range of the heater electrode 171. In this embodiment, at least a portion of the straight section 133 of the curved waveguide C11 is the separated portion, and the heater electrode 171 is placed in this portion.

[0107] Furthermore, the curved waveguides C11 and C12 have the same shape and are arranged in the X direction. Specifically, the folded portions 132 and 152 are curved in the same direction (negative Y direction) from the straight portions 131 and 151, respectively. If the folded portions 132 and 152 were formed to curve in opposite directions, the optical waveguide 110 would spread out in the second direction, hindering the miniaturization and density of the optical waveguide element 10A. In contrast, by curving the curved waveguides C11 and C12 in the same direction, miniaturization and density of the optical waveguide element 10A can be achieved. In addition, the curved waveguides C11 and C21 have portions facing each other in the X direction, and the curved waveguides C11 and C12 can be spaced apart by a predetermined distance or more while aligned in the X direction.

[0108] <Second Embodiment> The optical waveguide element 10B in the second embodiment will now be described. The optical waveguide element 10B differs from the optical waveguide element 10A in that grooves 210 and 220 are formed between the branched waveguides to shield the heat generated by the heater electrode 171. Components having the same function as in the above-described embodiment are denoted by the same reference numerals, and their descriptions are simplified or omitted.

[0109] Figure 14 is a plan view of the optical waveguide element 10B in a second embodiment of the present invention. Figure 15 is a cross-sectional view along line BB in Figure 14. Figure 16 is a cross-sectional view along line CC in Figure 14.

[0110] The heat generated by the heater electrode 171 may be conducted through the substrate 101 and diffused in the planar direction (X direction and Y direction). In the optical waveguide element 10B of the second embodiment, as shown in Figure 14, grooves 210 and 220 are formed in the substrate 101 to suppress heat diffusion through the substrate 101. Although the optical waveguide element 10B shown in Figure 14 has multiple grooves 210 and 220 formed thereon, only one of the grooves 210 and 220 may be formed.

[0111] The grooves 210 and 220 are formed by cutting and removing a portion of the substrate 101. The grooves 210 and 220 are formed between the heater electrode 171 that heats the branch waveguide 130 to be heated and another branch waveguide 150 that is different from the branch waveguide 130. The grooves 210 and 220 remove a portion of the heat conduction path (a portion of the substrate 101), thereby blocking heat conduction from the heater electrode 171. In other words, the grooves 210 and 220 have a heat shielding effect, which in turn narrows the heat conduction range of the heater electrode 171.

[0112] As shown in Figure 15, the groove 210 is formed between the heater electrode 171 that heats the straight section 133 of the branch waveguide and the straight section 151 of the other branch waveguide. By interposing the groove 210, the heat generated by the heater electrode 171 can be prevented from being transferred to the straight section 151 of the other branch waveguide.

[0113] As shown in Figure 16, the groove 220 is formed between the folded portion 132 of one branch waveguide and the folded portion 154 of the other branch waveguide. By interposing the groove 220, heat generated by the heater electrode installed in one branch waveguide can be prevented from being transferred to the folded portion 154 of the other branch waveguide.

[0114] In the configuration shown in Figure 16, a groove 220 is formed between the folded portion 132 of branch waveguide 130 and the folded portion 154 of branch waveguide 150. The groove 220 is located on a line approximately equidistant from the adjacent folded portions 132 and 154. The heater electrode 171 is provided near the straight portion 133 of branch waveguide 130, and as shown in Figure 16, by interposing the groove 220 between the heater electrode 171 that heats branch waveguide 130 and the folded portion 154 of the other branch waveguide 150, the heat generated by the heater electrode 171 can be prevented from being transferred to the other branch waveguide 150.

[0115] The size of the grooves 210 and 220 is not particularly limited. Larger grooves 210 and 220 can better prevent heat diffusion, but even small grooves 210 and 220 will exhibit a heat shielding effect. The grooves 210 and 220 are open to the outside and are filled with air. Air has high thermal insulation properties, so grooves 210 and 220 filled with air have excellent heat insulation properties. At the bottom of the grooves 210 and 220, a portion of the substrate 101 may remain, and the bonding layer 103 or reinforcing substrate 102 may be exposed. Furthermore, grooves may be formed in the reinforcing substrate 102.

[0116] Figure 17 is a plan view of the optical waveguide element 10B in a derivative example of a second embodiment of the present invention. Figure 18 is a cross-sectional view along the line DD in Figure 17. Figure 19 is a cross-sectional view along the line EE in Figure 17.

[0117] As shown in Figure 17, heat dissipation materials 215 and 225 may be placed in the grooves 210 and 220. The heat dissipation materials 215 and 225 have a function of dissipating heat to the outside and play a role in preventing performance degradation or failure due to overheating. For the material of the heat dissipation materials 215 and 225, for example, a metal with a low specific heat capacity can be used. By providing the heat dissipation materials 215 and 225, the heat energy accumulated during and after heating can be released to the outside. The exposed surfaces of the heat dissipation materials 215 and 225 are in contact with the outside air. By providing an uneven shape (for example, a rough surface) on these exposed surfaces, the surface area with the air can be increased and the heat dissipation effect can be improved.

[0118] The heat dissipation materials 215 and 225 may be placed in a portion of the grooves 210 and 220, or they may be placed throughout the entire grooves 210 and 220. Figures 16 to 18 show a state in which the heat dissipation materials 215 and 225 are placed in a portion of the grooves 210 and 220.

[0119] When heat dissipation materials 215 and 225 are placed in a portion of the grooves 210 and 220, the placement of the heat dissipation materials 215 and 225 within the grooves 210 and 220 is not particularly limited. In the groove 210 shown in Figure 18, the heat dissipation material 215 is placed near the heater electrode 171. In this case, the groove 210 located away from the heater electrode 171 is filled with air.

[0120] Figure 18 shows an example in which the groove 220 is formed in a configuration in which the heater electrode 171 is provided on one side of the straight section 133 of the branched waveguide 130 (see Figure 7). However, the groove 220 may also be formed in a configuration in which the heater electrode 171 is provided on one side of the straight section 133 of the branched waveguide 130 (see Figure 6) or on both sides (see Figure 8). In any case, heat diffusion can be suppressed by the groove 220, and heat dissipation can be improved by the arrangement of the heat dissipation materials 215 and 225.

[0121] Furthermore, in the groove 220 shown in Figure 19, the heat dissipation material 225 is positioned approximately in the center of the groove 220. In this case, air is filled into the grooves 210 located on both sides of the heater electrode 171, that is, on the side closer to the heater electrode 171 and the side further away from the heater electrode 171.

[0122] Figures 18 and 19 illustrate a configuration in which the buffer layer 104 is not provided on the upper surface of the substrate 101, but a configuration with the buffer layer 104 is also possible. When the buffer layer 104 is provided, it is preferable to leave the upper surfaces of the heat dissipation materials 215 and 225 exposed to the outside, rather than covering them with the buffer layer 104. This allows the heat dissipation effect of the heat dissipation materials 215 and 225 to be maintained.

[0123] In this example, the heat dissipation materials 215 and 225 are placed within the grooves 210 and 220, but the heat dissipation materials 215 and 225 may also be placed on the substrate 101 without providing the grooves 210 and 220.

[0124] <Third Embodiment> The optical waveguide element 10C in the third embodiment will now be described. The optical waveguide element 10C differs from the optical waveguide element 10A in that heater electrodes 171 and 181 are arranged in both the branch waveguide 130 and the branch waveguide 150. Components having the same function as in the embodiments described above are denoted by the same reference numerals, and their descriptions are simplified or omitted.

[0125] Figure 20 is a plan view of the optical waveguide element 10C in a third embodiment of the present invention. The optical waveguide element 10C shown in Figure 20 is provided with a bias control unit B11 that adjusts the bias point of light propagating through the branched waveguide 130, and a bias control unit B12 that adjusts the bias point of light propagating through the branched waveguide 150.

[0126] The bias control unit B12 has a heater electrode 181. A wiring electrode 182 and an electrode pad 183 are electrically connected to the heater electrode 181. A DC power supply 184 is connected to the electrode pad 183.

[0127] The heater electrode 181 generates heat in response to the applied voltage, heating the branched waveguide 150, which is the target of heating. The refractive index of the branched waveguide 150, heated by the heater electrode 181, changes due to the thermo-optic effect. By controlling the voltage applied to the heater electrode 181, the amount of heating (temperature) of the branched waveguide 130 can be adjusted, and as a result, the amount of light phase shift can be appropriately adjusted.

[0128] As shown in Figure 20, the curved waveguides C11 and C12 are composed of folded waveguides having the same shape and are arranged in the X direction. The heater electrode 171 heats the branch waveguide 130. By positioning the heater electrode 171 near the straight section 133, which is part of the curved waveguide C11, the other branch waveguide 150 can be prevented from being included within the heat conduction range of the heater electrode 171. The heater electrode 181 also heats the branch waveguide 150. By positioning the heater electrode 181 near the straight section 153, which is part of the curved waveguide C12, the other branch waveguide 130 can be prevented from being included within the heat conduction range of the heater electrode 181.

[0129] In Figure 20, the heater electrode 171 is positioned near the straight section 133 that constitutes the curved waveguide C11, and the heater electrode 181 is positioned near the straight section 153 that constitutes the curved waveguide C12. The heater electrodes 171 and 181 may be positioned to extend in the same direction. Furthermore, the other heater electrode 181 may be positioned on a straight line extending in a first direction having the width of the short side of one heater electrode 171, that is, the short sides of the heater electrodes 171 and 181 may face each other. As a result, the heat emitted by the heater electrode 171 is received by the short side of the other heater electrode 181, so the effect of thermal crosstalk can be suppressed compared to, for example, a configuration in which the long sides of one heater electrode 171 and the other heater electrode 181 face each other, that is, a configuration in which the heat emitted by one heater electrode 171 is received by the long side of the other heater electrode 181. The term "a straight line extending in a first direction having the width of the short side of one heater electrode" as used herein includes a configuration in which at least a portion of the short side of the other heater electrode is arranged on a straight line having the width of the short side of one heater electrode. For example, the overlap ratio (Wop) of the widths of the short sides of the two heater electrodes can be Wop < 50%, 50 ≤ Wop < 100%, or Wop = 100%. In Figure 20, both heater electrodes 171 and 181 are arranged to extend in the X direction, but they may also be arranged to extend in the Y direction, or to extend at a predetermined angle (diagonally) with respect to the X and Y directions.

[0130] The placement of the heater electrodes 171 and 181 is not limited to the configuration shown in Figure 20. Figures 21 to 23 show first to third placement examples of the heater electrodes 171 and 181. Figures 21 to 23 are partial plan views showing the vicinity of the curved waveguides C11 and C12 in Figure 20. For example, as shown in Figure 21, the heater electrode 171 may be placed near the straight section 133 that constitutes the curved waveguide C11, and the heater electrode 181 may be placed near the straight section 151 that constitutes the curved waveguide C12. Alternatively, as shown in Figure 22, for example, the heater electrode 171 may be placed near the straight section 135 that constitutes the curved waveguide C11, and the heater electrode 181 may be placed near the straight section 151 that constitutes the curved waveguide C12. Furthermore, as shown in Figure 23, for example, the heater electrode 171 may be positioned near the straight section 135 that constitutes the curved waveguide C11, and the heater electrode 181 may be positioned near the straight section 153 that constitutes the curved waveguide C12. For example, in Figures 21 to 23, both the heater electrodes 171 and 181 are positioned to extend in the X direction, but they may also be positioned to extend in the Y direction, or to extend at a predetermined angle (diagonally) with respect to the X and Y directions. Moreover, the heater electrode 181 may be positioned near both the straight section 153 and the folded section 152, or near both the straight section 151 and the folded section 152.

[0131] <Fourth Embodiment> The optical waveguide element 10D in the fourth embodiment will now be described. Figure 24 is a plan view of the optical waveguide element 10D in the fourth embodiment of the present invention. The optical waveguide element 10D in the fourth embodiment is composed of a plurality of Mach-Zehnder optical waveguides. Components having the same function as those in the embodiments described above are denoted by the same reference numerals, and their descriptions are simplified or omitted.

[0132] The optical waveguide element 10D has an optical waveguide 310 formed therein for propagating light. The optical waveguide 310 is a nested Mach-Zehnder optical waveguide with a nested structure. The optical waveguide element 10D is composed of an optical input terminal 311, an input optical waveguide 312, branching sections 313, 321, 351, branching waveguides 320, 330, 340, 350, 360, 370, a multiplexing section 395, an output optical waveguide 396, and an optical output terminal 397.

[0133] The input optical waveguide 312 connected to the optical input terminal 311 branches into branch waveguide 320 and branch waveguide 350 at branch section 313. Branch waveguide 320 branches into branch waveguide 330 and branch waveguide 340 at branch section 321. Branch waveguide 350 branches into branch waveguide 360 ​​and branch waveguide 370 at branch section 351.

[0134] As shown in Figure 24, a curved waveguide C21 is formed in the middle of the branched waveguide 330. In this embodiment, the curved waveguide C21 is a folded waveguide with an inverted S-shape in plan view. More specifically, the curved waveguide C21 has, in order from the branching section 321 side (optical input side), a straight section 331 extending in the first direction, a folded section 332 formed in an arc shape, a straight section 333 extending in the first direction, a folded section 334 formed in an arc shape, and a straight section 335 extending in the first direction. A folded section 336 formed in an arc shape and a straight section 337 extending in the first direction are connected to the straight section 335. The folded sections 332, 334, and 336 can have any curved shape, but it is preferable to have a smoothly connected arc shape in order to reduce optical loss.

[0135] As shown in Figure 24, the heater electrode 381 of the bias control unit B21 is located near the straight section 333 that constitutes the curved waveguide C21. In addition, the control electrode 385 of the modulation unit M is located near the straight section 337.

[0136] Similarly, a curved waveguide C22 is formed in the middle of the branched waveguide 340. In this embodiment, the curved waveguide C22 has the same shape as the curved waveguide C21. The curved waveguide C21 has, in order from the branching section 321 side (optical input side), a straight section 341 extending in the first direction, an arc-shaped folded section 342, a straight section 343 extending in the first direction, an arc-shaped folded section 344, and a straight section 345 extending in the first direction. An arc-shaped folded section 346 and a straight section 347 extending in the first direction are connected to the straight section 345. The folded sections 342, 344, and 346 can have any curved shape, but it is preferable to have a smoothly connected arc shape in order to reduce optical loss.

[0137] As shown in Figure 24, the heater electrode 382 of the bias control unit B22 is located near the straight section 343 that constitutes the curved waveguide C22. Also, the control electrode 385 of the modulation unit M is located near the straight section 347.

[0138] Similarly, a curved waveguide C23 is formed in the middle of the branched waveguide 360. In this embodiment, the curved waveguide C23 has the same shape as the curved waveguide C21. The curved waveguide C23 has, in order from the branching section 351 side (optical input side), a straight section 361 extending in the first direction, an arc-shaped folded section 362, a straight section 363 extending in the first direction, an arc-shaped folded section 364, and a straight section 365 extending in the first direction. An arc-shaped folded section 366 and a straight section 367 extending in the first direction are connected to the downstream of the straight section 365. The folded sections 362, 364, and 366 can have any curved shape, but it is preferable to have a smoothly connected arc shape in order to reduce optical loss.

[0139] As shown in Figure 24, the heater electrode 383 of the bias control unit B23 is located near the straight section 363 that constitutes the curved waveguide C23. In addition, the control electrode 386 of the modulation unit M is located near the straight section 367.

[0140] Similarly, a curved waveguide C24 is formed in the middle of the branched waveguide 370. In this embodiment, the curved waveguide C24 has the same shape as the curved waveguide C21. The curved waveguide C24 has, in order from the branching section 351 side (optical input side), a straight section 371 extending in the first direction, an arc-shaped folded section 372, a straight section 373 extending in the first direction, an arc-shaped folded section 374, and a straight section 375 extending in the first direction. An arc-shaped folded section 376 and a straight section 377 extending in the first direction are connected to the downstream of the straight section 375. The folded sections 372, 374, and 376 can have any curved shape, but it is preferable to have a smoothly connected arc shape in order to reduce optical loss.

[0141] As shown in Figure 24, the heater electrode 384 of the bias control unit B24 is located near the straight section 373 that constitutes the curved waveguide C24. In addition, the control electrode 386 of the modulation unit M is located near the straight section 377.

[0142] Branch waveguides 330 and 340 merge at the multiplexer 391. The optical output side of the multiplexer 391 is connected to branch waveguide 392. Branch waveguides 360 and 370 merge at the multiplexer 393. The optical output side of the multiplexer 393 is connected to branch waveguide 394.

[0143] The branch waveguides 392 and 394 merge at the multiplexer 395. The multiplexer 395 is connected to the output optical waveguide 396 and the optical output terminal 397.

[0144] As shown in Figure 24, even in an optical waveguide element 10D in which multiple Mach-Zehnder optical waveguides are formed, the curved waveguides C21, C22, C23, and C24 can be arranged in the X direction. This makes it possible to prevent other branched waveguides from being included in the heat conduction range of the heater electrodes 381, 382, ​​383, and 384, thereby enabling miniaturization and high density of the optical waveguide element 10D while suppressing thermal crosstalk.

[0145] Figure 25 is a partial plan view showing a first alternative example of a part of a Mach-Zehnder optical waveguide in a fourth embodiment of the present invention. Note that the multiplexing section located at the end of the Mach-Zehnder optical waveguide is omitted from the illustration. In the Mach-Zehnder optical waveguide shown in Figure 24, the bias control units B21, B22, B23, and B24 are arranged in a single row in the X direction, but as shown in Figure 25, the bias control units B21, B22, B23, and B24 may be arranged in two rows in the X direction.

[0146] Figures 26 and 27 are partial plan views showing a second and third alternative example of a Mach-Zehnder optical waveguide in a fourth embodiment of the present invention. The Mach-Zehnder optical waveguide shown in Figures 24 and 25 is a nested Mach-Zehnder optical waveguide in which multiple Mach-Zehnder optical waveguides used in optical modulators that perform QPSK modulation (QPSK optical modulators) and optical modulators that perform DP-QPSK modulation (DP-QPSK optical modulators) are nested in a nested structure. However, bias control units B21, B22, B23, and B24 may be arranged in the X direction in parallel, with each independent Mach-Zehnder optical waveguide being formed in parallel.

[0147] The configurations shown in Figures 26 and 27 differ from those shown in Figures 24 and 25 in that two Mach-Zehnder optical waveguides are formed independently. Specifically, the optical waveguides 310 shown in Figures 24 and 25 are connected at a branching section 313 and a multiplexing section 395 to form a nested Mach-Zehnder optical waveguide.

[0148] In the configurations shown in Figures 24 and 25, the optical waveguide 310 emits output light L2 when incident light L1 is incident on it. In contrast, in the configurations shown in Figures 26 and 27, the optical waveguide 310a emits output light L21 when incident light L11 is incident on it, and the optical waveguide 310b emits output light L22 when incident light L12 is incident on it. The optical waveguides 310a and 310b are not connected to each other and each constitutes an independent optical path. Thus, the present invention can also be applied when multiple independent Mach-Zehnder optical waveguides are formed in the optical waveguide element 10D.

[0149] The optical modulator and optical transmitter according to the present invention will be described below.

[0150] Figure 28 is a plan view showing the optical modulator 600 and optical transmitter 700 according to the present invention. The optical modulator 600 shown in Figure 28 includes an optical waveguide element 500, a housing 601, an input optical fiber 602, and an output optical fiber 603. Here, as an example, we will describe the case in which the optical waveguide element 500 according to the present invention is applied to a broadband coherent driver modulator (HB-CDM).

[0151] In the optical modulator 600, the optical waveguide element 500 is housed in the housing 601. The optical waveguide elements 10A, 10B, 10C, and 10D from the above-described embodiments can be used as the optical waveguide element 500. An input optical fiber 602 is connected to the input section of the optical waveguide element 500, and an output optical fiber 603 is connected to the optical output section. By connecting the optical waveguide element 500 inside the housing 601 to the outside of the housing 601 with optical fibers, a compact optical modulator 600 can be provided. Note that a spatial optical system may be interposed between the optical input section and the input optical fiber 602, and between the optical output section and the output optical fiber 603.

[0152] As shown in Figure 28, the optical transmitter 700 can be configured by connecting a signal output circuit 701, which generates an electrical signal So (modulation signal) that is a high-frequency signal for performing modulation operation, and a signal amplification circuit 702, which amplifies the electrical signal So to generate an amplified signal S (modulation signal). The signal output circuit 701 and the signal amplification circuit 702 may be located outside the housing 601 of the optical modulator 600, but by placing them inside the housing 601, efficient transmission of the modulation signal and miniaturization of the optical transmitter 700 can be achieved.

[0153] Alternatively, the optical transmitter 700 may be equipped with a light source 703, and the light emitted from the light source 703 (incident light L1) may be input to the optical waveguide element 500. This allows the light output from the light source 703 to be modulated by the optical modulator 600, and the modulated light (emitted light L2) to be output from the optical transmitter 700. Alternatively, the optical modulator 600 may not be housed in the housing 601, but may be combined with other transmitting and receiving elements to form a subassembly.

[0154] The embodiments described above are provided to facilitate understanding of the present invention and do not limit it. The components disclosed in the embodiments described above are intended to include all design modifications and equivalents that fall within the technical scope of the present invention. Furthermore, technical ideas obtained by appropriately combining the concepts illustrated in each embodiment are also encompassed within the present invention. [Explanation of Symbols]

[0155] 10A, 10B, 10C, 10D, 500 optical waveguide device 21, 22, 23, 24 end face 101 circuit board 101a Convex part 101b Cutting part 102 Reinforcement board 103 Bonding layer 104 Buffer Layer 110, 310, 310a, 310b optical waveguide 111, 311 Optical input terminal 112, 312, 1101, 1201, 2101, 2201 Input Optical Waveguides Branching points: 120, 313, 321, 351, 1102, 2102 130, 150, 320, 330, 340, 350, 360, 370, 392, 394, 1103, 1104, 1203, 1204, 1251, 1252, 1261, 1262, 2103, 2104, 2203, 2204, 2251, 2252, 2261, 2262 Branch Waveguides 131, 133, 135, 137, 151, 153, 155, 157, 331, 333, 335, 337, 341, 343, 345, 347, 361, 363, 365, 367, 371, 373, 375, 377 Straight section 132, 134, 136, 152, 154, 156, 332, 334, 336, 342, 344, 346, 362, 364, 366, 372, 374, 376 Folded section 160, 391, 393, 395 Wave section 161, 396 output optical waveguide 162, 397 Optical output terminal 171, 171a, 171b, 181, 381, 382, ​​383, 384, 1501, 1502, 1511, 1512, 2501, 2511, 2512 Heater electrodes 172, 182, 385, 386 wiring electrode 173, 183 electrode pads 174, 184 DC power supply 191, 385, 386 control electrodes 192 High-frequency signal source 193 Termination resistor 194 Capacitors 210, 220 grooves 215, 225 Heat dissipation material 600 Optical Modulators 601 cabinet 602 Input optical fiber 603 Output optical fiber 700 Optical Transmitter 701 Signal Output Circuit 702 Signal Amplifier Circuit 703 Light source 1100, 2100 Mach-Zehnder optical waveguide 1200, 2200 nested Mach-Zehnder optical waveguide 1202, 2202 First branch 1250, 2250 Second branch 1260, 2260 Third branch B11, B12, B21, B22, B23, B24 Bias Control Unit C1, C2, C3, C4, C5, C6, C11, C12, C21, C22, C23, C24 curved waveguide L1, L11, L12 incident light L2, L21, L22 output light M Modulation Section R, R1, R2 Thermal Conductivity Range S1, S2 starting point T1, T2 Terminus

Claims

1. A rectangular substrate made of electro-optic crystal, having a first direction and a second direction in a plan view, An optical waveguide formed on the substrate, A bias control unit having a heater electrode for heating the optical waveguide, An optical waveguide element comprising, The optical waveguide includes a Mach-Zehnder optical waveguide having a plurality of branched waveguides branched by at least one branching section, Each of the aforementioned branch waveguides has a curved waveguide, and the curved waveguides of each branch waveguide are arranged at a distance of a predetermined distance or more along the first direction. An optical waveguide element characterized in that the heater electrode is arranged in at least one of the curved waveguides.

2. The optical waveguide element according to claim 1, characterized in that the curved waveguides of each branched waveguide have the same shape.

3. The optical waveguide element according to claim 1 or 2, characterized in that the curved waveguide is configured by alternately connecting N curved (N is a positive even number) folded sections and N+1 straight sections extending in a first direction.

4. The optical waveguide element according to claim 3, characterized in that the heater electrode is arranged in the vicinity of the straight portion.

5. The optical waveguide element according to claim 1 or 2, characterized in that the substrate is covered with a buffer layer, and the heater electrode is arranged to heat the curved waveguide via the buffer layer.

6. The optical waveguide element according to claim 1 or 2, characterized in that the curved waveguide has a second direction extending portion which increases the separation distance in the second direction from other branch waveguides.

7. The optical waveguide element according to claim 1 or 2, characterized in that a groove is formed between the heater electrode that heats a specific optical waveguide and another optical waveguide, by cutting out a portion of the substrate.

8. The optical waveguide element according to claim 7, characterized in that a heat dissipation material made of metal is arranged in part or all of the groove.

9. The optical waveguide element according to claim 1 or 2, characterized in that a plurality of the Mach-Zehnder optical waveguides are formed on the substrate.

10. The optical waveguide element according to claim 9, characterized in that a plurality of Mach-Zehnder optical waveguides are arranged in a nested or parallel manner.

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

12. An optical transmitting device comprising: an optical modulator according to claim 11; a light source for inputting an optical wave to the optical modulator; and a signal output circuit for outputting a modulation signal.

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