Electro-optic modulator

Abstract

The electro-optical modulator includes an optical branching element, an optical coupling element, two waveguide arms, and a modulation electrode. The two waveguide arms are connected between the optical branching element and the optical coupling element, and each waveguide arm includes a first single-mode waveguide portion, a first coupling portion, a multimode waveguide portion, a second coupling portion, and a second single-mode waveguide portion. Here, the width of either the first single-mode waveguide portion or the second single-mode waveguide portion is smaller than the width of the multimode waveguide portion; the first coupling portion is configured to allow light to be coupled from the first single-mode waveguide portion to the multimode waveguide portion, and the second coupling portion is configured to allow light to be coupled from the multimode waveguide portion to the second single-mode waveguide portion; the modulation electrode is configured to apply a modulation voltage to the multimode waveguide portions of the two waveguide arms.

Description

【Technical Field】 【0001】 The present disclosure relates to the technical field of optical communication, and more particularly, to an electro-optical modulator. 【Background Art】 【0002】 In recent years, with the rapid development of newly emerging network application services such as the Internet of Things, autonomous driving, telemedicine, and distance education, the demand for high-speed and high-capacity communication technologies has been increasing. Optical communication has achieved rapid progress in the direction of high-speed and high-capacity communication due to its characteristics such as large bandwidth, high reliability, low cost, and high anti-interference ability. A method for loading high-speed electrical signals onto an optical carrier is one of the core research contents. 【0003】 An electro-optical modulator is a modulator made based on the electro-optical effect of an electro-optical material. The electro-optical effect means that when a voltage is applied to an electro-optical material such as a lithium niobate crystal, a gallium arsenide crystal, or a lithium tantalate crystal, the refractive index of the electro-optical material changes, thereby changing the characteristics of the light wave passing through the electro-optical material. By utilizing the electro-optical effect, it becomes possible to modulate parameters such as the phase, amplitude, intensity, and polarization state of an optical signal. 【0004】 Due to the increasing urgency of the demand for high-speed and high-capacity communication technologies, the requirements for the operating performance of electro-optical modulators are increasing. 【Summary of the Invention】 【0005】 This disclosure provides an electro-optic modulator comprising a light-splitting element, a light-combining element, two waveguide arms, and a modulation electrode. Two waveguide arms are connected between an optical branching element and an optical coupling element, and each waveguide arm includes a first single-mode waveguide portion, a first coupling portion, a multimode waveguide portion, a second coupling portion, and a second single-mode waveguide portion, wherein the width of either the first single-mode waveguide portion or the second single-mode waveguide portion is smaller than the width of the multimode waveguide portion; the first coupling portion is configured to allow light to be coupled from the first single-mode waveguide portion to the multimode waveguide portion, and the second coupling portion is configured to allow light to be coupled from the multimode waveguide portion to the second single-mode waveguide portion; and a modulation electrode is configured to apply a modulation voltage to the multimode waveguide portion of the two waveguide arms. 【0006】 In some embodiments, the first coupling portion includes a first portion connected to a first single-mode waveguide portion and a second portion connected to a multimode waveguide portion, wherein the first portion faces the second portion and the width of the first portion gradually decreases away from the first single-mode waveguide portion. 【0007】 In some embodiments, the second coupling portion includes a third portion connected to a second single-mode waveguide portion and a fourth portion connected to a multimode waveguide portion, wherein the third portion faces the fourth portion, and the width of the third portion gradually increases in the direction toward the second single-mode waveguide portion. 【0008】 In some embodiments, the first coupling portion includes a first portion connected to a first single-mode waveguide portion and a second portion connected to a multimode waveguide portion, wherein the second portion faces the first portion and the width of the second portion gradually increases away from the first single-mode waveguide portion. 【0009】 In some embodiments, the second coupling portion includes a third portion connected to a second single-mode waveguide portion and a fourth portion connected to a multimode waveguide portion, wherein the fourth portion faces the third portion and the width of the fourth portion gradually decreases in the direction toward the second single-mode waveguide portion. 【0010】 In some embodiments, the first coupling portion is a first multimode interference coupling element, and the second coupling portion is a second multimode interference coupling element. 【0011】 In some embodiments, the first multimode interference coupling element includes a first rectangular interference portion, a first width-increasing portion, and / or a first width-decreasing portion. The first width-increasing portion is connected between a first single-mode waveguide portion and the first rectangular interference portion, and the width of the first width-increasing portion gradually increases in the direction toward the first rectangular interference portion. The first width-decreasing portion is connected between the first rectangular interference portion and the multimode waveguide portion, and the width of the first width-decreasing portion gradually decreases in the direction toward the first rectangular interference portion. 【0012】 In some embodiments, the second multimode interference coupling element includes a second rectangular interference portion, a second width-increasing portion, and / or a second width-decreasing portion. The second width-increasing portion is connected between the multimode waveguide portion and the second rectangular interference portion, and the width of the second width-increasing portion gradually increases in the direction toward the second rectangular interference portion. The second width-decreasing portion is connected between the second rectangular interference portion and the second single-mode waveguide portion, and the width of the second width-decreasing portion gradually decreases in the direction toward the second rectangular interference portion. 【0013】 In some embodiments, each waveguide arm includes a first single-mode waveguide portion, a first coupling portion, a multimode waveguide portion, a second coupling portion, and a second single-mode waveguide portion, arranged in sequence. The first single-mode waveguide portions of two waveguide arms are each connected to an optical branching element, and the second single-mode waveguide portions of two waveguide arms are each connected to an optical coupling element. 【0014】 In some embodiments, the first single-mode waveguide portion of the two waveguide arms is curved and symmetrically arranged. The second single-mode waveguide portion of the two waveguide arms is curved and symmetrically arranged. 【0015】 In some embodiments, the electro-optic modulator has a folding structure and includes at least one bending region. Each waveguide arm includes a plurality of unit segments, each unit segment comprising, in a sequential arrangement, a first single-mode waveguide portion, a first coupling portion, a multimode waveguide portion, a second coupling portion, and a second single-mode waveguide portion, wherein within any two adjacent unit segments, the second single-mode waveguide portion of one unit segment is integrally connected to the first single-mode waveguide portion of the other unit segment in the bending region. 【0016】 In some embodiments, the two waveguide arms intersect integrally in each bending region. 【0017】 In some embodiments, the two waveguide arms intersect perpendicularly in each bending region. 【0018】 These and other aspects of the present disclosure will become apparent from the embodiments described below and will be clarified by reference to the embodiments described below. 【0019】 Further details, features, and advantages of this disclosure are disclosed in the following description of exemplary embodiments with reference to the accompanying drawings. [Brief explanation of the drawing] 【0020】 [Figure 1] This is a schematic diagram showing a conventional electro-optic modulator. [Figure 2] This is a schematic top view showing an electro-optic modulator according to some embodiments of the present disclosure. [Figure 3]This is a schematic top view showing an electro-optic modulator according to some embodiments of the present disclosure. [Figure 4] This is a schematic top view showing an electro-optic modulator according to some embodiments of the present disclosure. [Modes for carrying out the invention] 【0021】 The following briefly describes only a few exemplary embodiments. As will be apparent to those skilled in the art, the embodiments described can be modified in various ways without departing from the spirit or scope of this disclosure. Accordingly, the accompanying drawings and this description are intended to be illustrative and not restrictive. 【0022】 Electro-optic modulation-related technologies have been widely developed and applied in fields such as optical communications, microwave photonics, laser beam deflection, and wavefront modulation. A Mach-Zehnder modulator is one type of electro-optic modulator in which an input optical signal is equally divided into two branched optical signals, each entering two waveguide arms. Each of the two waveguide arms is made of an electro-optic material and has a refractive index that changes with the applied modulation voltage. The change in the refractive index of the waveguide arms can cause a phase change in the branched optical signals. Therefore, the output resulting from the convergence of the two branched optical signals is an interference signal with an intensity that changes with the modulation voltage. In short, a Mach-Zehnder modulator can perform modulation of various sidebands by controlling the modulation voltage applied to the two waveguide arms. As a device for converting electrical signals to optical signals, the Mach-Zehnder modulator is one of the common core devices in optical interconnects, optical computing, and optical communication systems. 【0023】 FIG. 1 shows a schematic structural diagram of a conventional Mach-Zehnder modulator. In an ideal state, the two waveguide arms 02 of the Mach-Zehnder modulator 001 are identical to each other. When the Mach-Zehnder modulator 001 is not operating, neither of the two waveguide arms 02 undergoes the electro-optic effect. The input light passes through the optical branching element 01 and is then split into two branched optical signals. The two branched optical signals are in the same phase even after each of the two branched optical signals passes through one waveguide arm 02, and then a coherently enhanced signal of the two branched optical signals is output from the optical coupling element 05. When the Mach-Zehnder modulator 001 is operating, the modulation electrode 04 (for example, including the signal electrode 040, the first ground electrode 041, and the second ground electrode 042) applies a modulation voltage to the two waveguide arms 02, so that the phases of the two branched optical signals can be different by an odd multiple or an even multiple of π after each of the two branched optical signals passes through one waveguide arm 02. When the phases of the two branched optical signals are different by an even multiple of π, the optical coupling element 05 outputs a coherently enhanced signal of the two branched optical signals. When the phases of the two branched optical signals are different by an odd multiple of π, the optical coupling element 05 outputs a coherent cancellation signal of the two branched optical signals. 【0024】 Generally, after light is transmitted through an optical device, some degree of mixing of magnetic wave modes occurs. For example, when entering the optical device, the light is a TE-mode magnetic wave (having a magnetic field component in the propagation direction but no electric field component), and when emitted from the optical device, most of the light is a TE-mode magnetic wave, but a small portion of the TM-mode magnetic wave (having an electric field component in the propagation direction and no magnetic field component) is mixed into the light. In this case, the output ratio of the TM-mode electromagnetic wave is low, but in some optical devices that require a high purity of magnetic wave modes, a significant impact on its operating performance still exists, thereby resulting in some degree of optical loss. 【0025】 Based on this, embodiments of the present disclosure provide an electro-optic modulator that can improve the operating performance of the electro-optic modulator and reduce the transmission loss of the electro-optic modulator. 【0026】 As shown in FIG. 2, some embodiments of the present disclosure provide an electro-optic modulator 100 including an optical branching element 110, an optical coupling element 120, two waveguide arms 130, and a modulation electrode 140. The two electrode arms 130 are connected between the optical branching element 110 and the optical coupling element 120, and each waveguide arm 130 includes a first single-mode waveguide portion 131, a first coupling portion 132, a multimode waveguide portion 133, a second coupling portion 134, and a second single-mode waveguide portion 135. The width of either the first single-mode waveguide portion 131 or the second single-mode waveguide portion 135 is smaller than the width of the multimode waveguide portion 133. The first coupling portion 132 is configured to allow light to be coupled from the first single-mode waveguide portion 131 to the multimode waveguide portion 133, and the second coupling portion 134 is configured to allow light to be coupled from the multimode waveguide portion 133 to the second single-mode waveguide portion 135. The modulation electrode 140 is configured to apply a modulation voltage to the multimode waveguide portions 133 of the two waveguide arms 130. 【0027】 In an embodiment of the present disclosure, as shown in FIG. 2, referring to the straight extension portion of any waveguide arm 130, the extension direction thereof is defined as the longitudinal direction, and the direction orthogonal to the extension direction and parallel to a device substrate (not shown in the figure) is defined as the width direction. 【0028】 The optical branching element 110 is not particularly limited in its type and includes at least one input port and two output ports, such as an optical branching element having one input portion and two output portions. The optical coupling element 120 is not particularly limited in its type and includes at least two input ports and one output port, such as an optical coupling element having two input portions and one output portion or an optical coupling element having two input portions and three output portions. The two waveguide arms 130 connect one of each of the two output ports of the optical branching element 110 and one of each of the two input ports of the optical coupling element 120. 【0029】 The material of the waveguide arm 130 includes electro-optical materials such as lithium niobate, lithium tantalate, or potassium titanium phosphate. The modulation electrode 140 is configured to apply a modulation voltage to the multimode waveguide portion 133 of the two waveguide arms 130. The form of the structure of the modulation electrode 140 is not limited. For example, in some embodiments, the modulation electrode 140 may include a first ground electrode 141, a signal electrode 143, and a second ground electrode 142, arranged in sequence. One of the waveguide arms 130 is configured to be positioned in the electric field formed by the first ground electrode 141 and the signal electrode 143, and the other waveguide arm 130 is configured to be positioned in the electric field formed by the second ground electrode 142 and the signal electrode 143. 【0030】 The first single-mode waveguide section 131 and the second single-mode waveguide section 135 function as single-mode waveguides and are characterized by a small width dimension, a small refractive index difference (meaning the difference between the refractive indices of the two mediating or low-level crystalline systems in various directions), and suitability for transmitting one mode of magnetic wave, such as TE-mode magnetic waves. The multimode waveguide section 133 functions as a multimode waveguide capable of transmitting multiple modes of magnetic waves and has a width dimension significantly larger than that of the first single-mode waveguide section 131 and the second single-mode waveguide section 135. Generally, single-mode waveguides have higher transmission loss than multimode waveguides, but single-mode waveguides are more suitable for design in a curved shape and have higher transmission stability than curved multimode waveguides. 【0031】 In embodiments of this disclosure, the two waveguide arms 130 employ multimode waveguides for optical transmission in the modulation region of the modulation electrode 140 (i.e., the region where the electric field of the modulation electrode 140 is applied), which can result in a lower overall transmission loss of the electro-optic modulator. The two waveguide arms 130 employ single-mode waveguides for optical transmission in the region outside the modulation region, which is suitable for various shapes (such as curved shapes) and has better transmission stability. By appropriately designing the first coupling portion 132 and the second coupling portion 134, it is possible to have various coupling effects for magnetic waves of various modes. For example, TE mode magnetic waves are allowed to pass through, while TM mode magnetic waves are blocked and filtered. In this way, the purity of the magnetic wave modes can be improved, making it particularly suitable for some situations where there is a greater demand for the purity of the magnetic wave modes. Thus, embodiments of this disclosure can improve the operating performance of the electro-optic modulator and reduce the transmission loss of the electro-optic modulator. 【0032】 The specific structural forms of the first coupling portion 132 and the second coupling portion 134 are not limited. As shown in Figure 2, in some embodiments, the first coupling portion 132 may include a first portion 1321 connected to a first single-mode waveguide portion 131 and a second portion 1322 connected to a multimode waveguide portion 133. The first portion 1321 faces the second portion 1322, and the width of the first portion 1321 gradually decreases in the direction away from the first single-mode waveguide portion 131. The second coupling portion 134 may include a third portion 1341 connected to a second single-mode waveguide portion 135 and a fourth portion 1342 connected to a multimode waveguide portion 133. The third portion 1341 faces the fourth portion 1342, and the width of the third portion 1341 gradually increases in the direction towards the second single-mode waveguide portion 135. 【0033】 Light is guided through the first single-mode waveguide section 131 into the first section 1321 of the first coupling section 132. As the width of the first section 1321 gradually decreases away from the first single-mode waveguide section 131, the light is forced out into the second section 1322 opposite the first section, and then into the multimode waveguide section 133. Thus, after the light has passed through the multimode waveguide section 133, it can then pass through the fourth section 1342 and the third section 1341 of the second coupling section 134 and enter the second single-mode waveguide section 135. 【0034】 In some embodiments, the width of the second portion 1322 of the multimode waveguide portion 133 can be gradually increased away from the first single-mode waveguide portion 131 (i.e., opposite to the trend of change in the width of the first portion 1321); and / or, the width of the fourth portion 1342 can be gradually decreased towards the second single-mode waveguide portion 135 (i.e., opposite to the trend of change in the width of the third portion 1341). Such designs can further improve the inductive effect on optical transmission and thus further reduce transmission loss. 【0035】 As shown in Figure 3, in some embodiments, the first coupling portion 132 may further become a first multimode interference coupling element, and the second coupling portion 134 may further become a second multimode interference coupling element. The operating principle of the multimode interference coupling element is based on multimode interference, which performs self-imaging at a specific location and periodically reproduces the input light field. The design of the multimode interference coupling element makes it possible to have diverse coupling effects for magnetic waves of various modes, thereby achieving the effect of improving the purity of the magnetic wave modes. 【0036】 The specific structural forms of the first multimode interference coupling element and the second multimode interference coupling element are not limited. 【0037】 In some embodiments, as shown in Figure 3, the first multimode interference coupling element includes a first rectangular interference portion 1324, a first width-increasing portion 1323, and / or a first width-decreasing portion (not shown in the figure). The first width-increasing portion 1323 is connected between a first single-mode waveguide portion 131 and the first rectangular interference portion 1324, and the width of the first width-increasing portion 1323 gradually increases in the direction toward the first rectangular interference portion 1324. The first width-decreasing portion is connected between the first rectangular interference portion 1324 and the multimode waveguide portion 133, and the width of the first width-decreasing portion gradually decreases in the direction toward the first rectangular interference portion 1324. 【0038】 Accordingly, the second multimode interference coupling element includes a second rectangular interference portion 1344, a second width-increasing portion (not shown in the figure), and / or a second width-decreasing portion 1345. The second width-increasing portion is connected between the multimode waveguide portion 133 and the second rectangular interference portion 1344, and the width of the second width-increasing portion gradually increases in the direction toward the second rectangular interference portion 1344. The second width-decreasing portion 1345 is connected between the second rectangular interference portion 1344 and the second single-mode waveguide portion 135, and the width of the second width-decreasing portion 1345 gradually decreases in the direction toward the second rectangular interference portion 1344. 【0039】 The designs described herein can be applied to both bar-type and folded-type electro-optic modulators. The electro-optic modulator shown in Figures 2 and 3 is a bar-type electro-optic modulator, in which each waveguide arm 130 of the bar-type electro-optic modulator includes, in order, a first single-mode waveguide portion 131, a first coupling portion 132, a multimode waveguide portion 133, a second coupling portion 134, and a second single-mode waveguide portion 135. The first single-mode waveguide portions 131 of each of the two waveguide arms 130 are connected to an optical branching element 110, and the second single-mode waveguide portions 135 of each of the two waveguide arms 130 are connected to an optical coupling element 120. 【0040】 As shown in Figures 2 and 3, the first single-mode waveguide portion 131 of the two waveguide arms 130 is curved and symmetrically arranged, and the second single-mode waveguide portion 135 of the two waveguide arms 130 is curved and symmetrically arranged. The first single-mode waveguide portion 131 and the second single-mode waveguide portion 135 have better transmission stability. 【0041】 In some embodiments of the present disclosure, the electro-optic modulator is a foldable type electro-optic modulator employing a foldable design, and the waveguide arm length may be designed to increase as desired, thereby reducing the size of the device in its longitudinal direction and even achieving better performance. 【0042】 As shown in Figure 4, the folding electro-optic modulator includes at least one bending region 150, and each waveguide arm 130 includes multiple unit segments (shown as two unit segments in the figure), each unit segment including a first single-mode waveguide portion 131, a first coupling portion 132, a multimode waveguide portion 133, a second coupling portion 134, and a second single-mode waveguide portion 135, arranged in order. Within any two adjacent unit segments of the waveguide arm 130, the second single-mode waveguide portion 135 of one unit segment is integrally connected to the first single-mode waveguide portion 131 of the other unit segment in the bending region 150. As mentioned above, single-mode waveguides are better suited to being designed in a curved shape and have higher transmission stability. 【0043】 In this embodiment, within each bending region 150, the second single-mode waveguide portion 135 and the first single-mode waveguide portion 131 of one waveguide arm 130, which is integrally connected to one another, intersect integrally with the second single-mode waveguide portion 135 and the first single-mode waveguide portion 131 of the other waveguide arm 130, which is similarly integrally connected to one another. The two waveguide arms 130 are designed as an intersecting structure within the bending region 150, thereby ensuring that the direction of the electric fields of the two waveguide arms 130 in the modulation region is the same. In some embodiments, the two waveguide arms 130 intersect perpendicularly within each bending region 150. In this way, transmission interference between waveguide branches at the intersection can be further reduced. 【0044】 In this description, orientations, positional relationships, or dimensions indicated by terms such as “center,” “longitudinal,” “lateral,” “length,” “width,” “thickness,” “top,” “bottom,” “front,” “rear,” “left,” “right,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” “outer,” “clockwise,” “counterclockwise,” “axial,” “radial,” and “circumferential” are orientations, positional relationships, or dimensions shown in the accompanying drawings, and it should be understood that these terms are used solely for ease of explanation and should not be construed as limiting the scope of protection of this disclosure, and do not imply or suggest that the device or element referred to must have a particular orientation and must be constructed and operated in that particular orientation. 【0045】 In addition, terms such as “first,” “second,” and “third” are for descriptive purposes only and should not be interpreted as indicating or implicitly meaning relative importance, or as implicitly indicating the number of technical features being described. Accordingly, features defined using “first,” “second,” and “third” may include one or more features, whether explicitly or implicitly. In this disclosure, the term “a plurality of” means two or more unless explicitly or specifically defined otherwise. 【0046】 In this disclosure, unless otherwise specified or defined, terms such as “install,” “connect,” “connected,” and “fixed” should be interpreted in a broad sense, for example, whether they refer to a fixed connection, a detachable connection, or an integral connection; whether mechanical or electrical; or communication, whether direct or indirect through an intermediate medium; or internal communication between two elements or interaction between two elements. Those skilled in the art will be able to understand the specific meaning of the above terms in this disclosure according to their particular context. 【0047】 In this disclosure, unless otherwise specified or defined, the expression that a first feature is “above” or “below” a second feature may include cases where the first feature directly touches the second feature, or cases where the first and second features do not directly touch but are touched through another feature between them. Furthermore, the expression that a first feature is “above,” “above,” or “touching above” a second feature includes cases where the first feature is directly above or diagonally above the second feature, that is, simply indicating that the first feature is at a higher level than the second feature. The expression that a first feature is “below,” “below,” or “just below” a second feature includes cases where the first feature is directly below or diagonally below the second feature, that is, simply indicating that the first feature is at a lower level than the second feature. 【0048】 This description provides many different implementations or embodiments that can be used to implement the Disclosure. It should be understood that these diverse implementations or embodiments are merely illustrative and are not intended to limit the scope of protection of the Disclosure. Based on the disclosures in this description, a person skilled in the art will be able to conceive of many variations or substitutions. All such variations or substitutions will fall within the scope of protection of the Disclosure. Therefore, the scope of protection of the Disclosure will be governed by the scope of protection of the claims.

Claims

[Claim 1] An electro-optic modulator comprising a light-splitting element, a light-combining element, two waveguide arms, and a modulation electrode, The two waveguide arms are connected between the optical branching element and the optical coupling element, and each waveguide arm comprises a first single-mode waveguide portion, a first coupling portion, a multimode waveguide portion, a second coupling portion, and a second single-mode waveguide portion. The width of either the first single-mode waveguide portion or the second single-mode waveguide portion is smaller than the width of the multimode waveguide portion. The first coupling portion is configured to allow light to be coupled from the first single-mode waveguide portion to the multi-mode waveguide portion, and the second coupling portion is configured to allow light to be coupled from the multi-mode waveguide portion to the second single-mode waveguide portion. An electro-optic modulator in which the modulation electrode is configured to apply a modulation voltage to the multimode waveguide portion of the two waveguide arms. [Claim 2] The first coupling portion comprises a first portion connected to the first single-mode waveguide portion and a second portion connected to the multi-mode waveguide portion, wherein the first portion faces the second portion, and the width of the first portion gradually decreases in the direction away from the first single-mode waveguide portion. The electro-optic modulator according to claim 1. [Claim 3] The second coupling portion comprises a third portion connected to the second single-mode waveguide portion and a fourth portion connected to the multi-mode waveguide portion, wherein the third portion faces the fourth portion and the width of the third portion increases toward the second single-mode waveguide portion. The electro-optic modulator according to claim 1. [Claim 4] The first coupling portion comprises a first portion connected to the first single-mode waveguide portion and a second portion connected to the multi-mode waveguide portion, wherein the second portion faces the first portion and the width of the second portion gradually increases in the direction away from the first single-mode waveguide portion. The electro-optic modulator according to claim 1. [Claim 5] The second coupling portion comprises a third portion connected to the second single-mode waveguide portion and a fourth portion connected to the multi-mode waveguide portion, wherein the fourth portion faces the third portion and the width of the fourth portion gradually decreases in the direction approaching the second single-mode waveguide portion. The electro-optic modulator according to claim 1. [Claim 6] The first coupling portion is a first multimode interference coupling element, and the second coupling portion is a second multimode interference coupling element. The first multimode interference coupling element comprises a first rectangular interference portion, a first width-increasing portion, and / or a first width-decreasing portion, The first width-increasing portion is connected between the first single-mode waveguide portion and the first rectangular interference portion, and the width of the first width-increasing portion gradually increases in the direction approaching the first rectangular interference portion. The first width reduction portion is connected between the first rectangular interference portion and the multimode waveguide portion, and the width of the first width reduction portion gradually decreases in the direction away from the first rectangular interference portion. The electro-optic modulator according to claim 1. [Claim 7] The second multimode interference coupling element comprises a second rectangular interference portion, a second width-increasing portion, and / or a second width-decreasing portion, The second width-increasing portion is connected between the multimode waveguide portion and the second rectangular interference portion, and the width of the second width-increasing portion gradually increases in the direction of approaching the second rectangular interference portion. The second width reduction portion is connected between the second rectangular interference portion and the second single-mode waveguide portion, and the width of the second width reduction portion gradually decreases in the direction away from the second rectangular interference portion. The electro-optic modulator according to claim 6. [Claim 8] Each waveguide arm comprises a first single-mode waveguide section, a first coupling section, a multimode waveguide section, a second coupling section, and a second single-mode waveguide section, arranged in sequence. The first single-mode waveguide portion of each of the two waveguide arms is connected to the optical branching element, and the second single-mode waveguide portion of each of the two waveguide arms is connected to the optical coupling element. An electro-optic modulator according to any one of claims 1 to 7. [Claim 9] The first single-mode waveguide portion of the two waveguide arms is curved and arranged symmetrically. The second single-mode waveguide portion of the two waveguide arms is curved and arranged symmetrically. The electro-optic modulator according to claim 8. [Claim 10] The electro-optic modulator has a foldable structure that includes at least one bending region, Each waveguide arm includes a plurality of unit segments, each unit segment comprising, in order, a first single-mode waveguide portion, a first coupling portion, a multimode waveguide portion, a second coupling portion, and a second single-mode waveguide portion, wherein within any two adjacent unit segments, the second single-mode waveguide portion of one unit segment is integrally connected to the first single-mode waveguide portion of the other unit segment in the bending region. An electro-optic modulator according to any one of claims 1 to 7. [Claim 11] The two waveguide arms are arranged in a cross structure in each bending region. The electro-optic modulator according to claim 10. [Claim 12] The two waveguide arms intersect perpendicularly in each bending region. The electro-optic modulator according to claim 10.

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