Electro-optical device

Abstract

Provided is an electro-optical device including: a substrate; an optical waveguide composed of a film of electro-optical material formed in a ridge shape on the substrate; a buffer layer provided so as to cover the optical waveguide; and an upper electrode provided on the optical waveguide via the buffer layer, the buffer layer having a recess on the side of the upper electrode on the optical waveguide. According to the electro-optical device of the present application, propagation loss of light can be suppressed.

Description

Technical Field

[0001] This invention relates to an electro-optical device for use in optical communication and optical measurement. Background Technology

[0002] With the widespread adoption of the internet, communication volume has increased dramatically, making fiber optic communication extremely important. Fiber optic communication is a communication method that converts electrical signals into optical signals and transmits them through optical fibers. It features wide bandwidth, low loss, and strong noise immunity.

[0003] Due to these advantages, optical fiber communication is used in a wide variety of products, such as optical switching devices and optical modulators. Specifically, as a method of converting electrical signals into optical signals using optical modulators, direct modulation using semiconductor lasers and external modulation using optical modulators are known. While direct modulation does not require an optical modulator and is low-cost, it has limitations in high-speed modulation; therefore, external optical modulation is used for high-speed and long-distance applications.

[0004] As optical modulators, Mach-Zehnder type optical modulators, in which an optical waveguide is formed near the surface of a lithium niobate single-crystal substrate by Ti (titanium) diffusion, have been practically applied (see, for example, Patent Document 1). High-speed optical modulators of 40 Gb / s and above have been commercialized, but they suffer from the significant drawback of a total length of approximately 10 cm. Mach-Zehnder type optical modulators utilize optical waveguides with a Mach-Zehnder interferometer structure (Mach-Zehnder optical waveguide). A Mach-Zehnder interferometer is a device that splits light emitted from a light source into two beams, which then overlap again to produce interference after traveling along different paths. Mach-Zehnder type optical modulators employing Mach-Zehnder interferometers have been used to generate various modulated lights.

[0005] In contrast, Patent Document 2 discloses a Mach-Zehnder optical modulator using a lithium niobate film. Compared to optical modulators using a lithium niobate single-crystal substrate, the optical modulator using a lithium niobate film achieves significant miniaturization and lower drive voltage. Figure 5 The cross-sectional structure of a conventional optical modulator 300 described in Patent Document 2 is shown. A pair of optical waveguides 22a and 22b made of lithium niobate film are formed on a sapphire substrate 21, and a signal electrode 24a and a ground electrode 24b are respectively disposed on the upper part of the optical waveguides 22a and 22b via a buffer layer 23. This optical modulator 300 is a so-called single-drive type with one signal electrode 24a. The signal electrode 24a and the ground electrode 24b have a symmetrical structure, so the electric fields applied to the optical waveguides 22a and 22b are equal in magnitude but opposite in sign. However, the cross-sectional shape of the optical waveguides 22a and 22b is rectangular, resulting in relatively large light propagation loss.

[0006] Patent document 3 discloses a portion of the optical waveguide that needs to be bent in the case of a Mach-Zehnder optical modulator. In order to prevent losses in the bent portion, it is necessary to further enhance the light locking-in, and a technique for forming a ridge-shaped optical waveguide has been disclosed.

[0007] In order to reduce optical propagation loss by reducing the size of the optical modulator, it is necessary to reduce the absorption and reflection of light leaking from the optical waveguide in the ridged LN film optical waveguide and to effectively apply the electric field from the electrodes on the layer to the optical waveguide.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent No. 4485218

[0011] Patent Document 2: Japanese Patent Application Publication No. 2006-195383

[0012] Patent Document 3: Japanese Patent Application Publication No. 2007-328257 Summary of the Invention

[0013] The present invention was made in view of the above-mentioned problems, and its object is to provide an electro-optic device, characterized in that it comprises: a substrate; an optical waveguide composed of an electro-optic material film formed in a ridge shape on the substrate; a buffer layer disposed covering the optical waveguide; and an upper electrode disposed on the optical waveguide via the buffer layer, wherein the buffer layer has a recess on the upper electrode side of the optical waveguide.

[0014] Another object of the present invention is to provide an electro-optic device with low light propagation loss, comprising: a substrate; first and second optical waveguides adjacent to each other, formed of an electro-optic material film formed in a ridge shape on the substrate; a buffer layer covering the first and second optical waveguides and embedded between the first and second optical waveguides; and first and second electrodes disposed above the buffer layer and opposite to the first and second optical waveguides, wherein the buffer layer has recesses on the first and second optical waveguides.

[0015] Another object of the present invention is to provide an electro-optic device comprising: a substrate; first and second optical waveguides adjacent to each other, formed of an electro-optic material film formed in a ridge shape on the substrate; a buffer layer covering the first and second optical waveguides and embedded between the first and second optical waveguides; and first and second electrodes disposed above the buffer layer and opposite to the first and second optical waveguides, wherein the distance from the surface of the substrate to the uppermost part of the buffer layer on the optical waveguides is less than the distance from the surface of the substrate to the uppermost part of the buffer layer on the portion where the optical waveguides are not formed.

[0016] Furthermore, in the electro-optic device of the present invention, preferably, the buffer layer disposed between the first and second optical waveguides and the first and second electrodes has a shape that protrudes downward at the first and second optical waveguides.

[0017] Furthermore, in the electro-optic device of the present invention, preferably, the buffer layer disposed between the first and second optical waveguides and the first and second electrodes has a shape that bends downward at the first and second optical waveguides.

[0018] Furthermore, in the electro-optic device of the present invention, preferably, the amount of recess in the buffer layer on the first and second optical waveguides is [missing information]. The above is preferred.

[0019] Furthermore, in the electro-optic device of the present invention, preferably, the first and second optical waveguides are Mach-Zehnder optical waveguides.

[0020] In addition, in the electro-optic device of the present invention, preferably, the substrate is a single crystal substrate and the electro-optic material film is a lithium niobate film.

[0021] The effects of the invention

[0022] According to the electro-optic device of the present invention, an electric field can be effectively applied to an optical waveguide, thereby locking light within the optical waveguide and suppressing light propagation loss. Attached Figure Description

[0023] Figures 1(a) and 1(b) are top views of an optical modulator 100 according to one embodiment of the present invention. Figure 1(a) shows only the optical waveguide, and Figure 1(b) shows the entire optical modulator 100 including the traveling wave electrode.

[0024] Figure 2 This is a schematic cross-sectional view of the optical modulator 100 along line A-A' in Figure 1(b).

[0025] Figure 3 This is a top view of an optical switching device 200 according to another embodiment of the present invention.

[0026] Figure 4 It is along Figure 3 A schematic cross-sectional view of the optical switching device 200 of the B-B' line.

[0027] Figure 5 This shows the cross-sectional structure of a prior art optical modulator 300.

[0028] Figure 6 The cross-sectional structure of a modified example of the optical modulator 100 is shown. Detailed Implementation

[0029] Hereinafter, with reference to the accompanying drawings, the manner in which the present invention is carried out will be described in detail.

[0030] Figures 1(a) and 1(b) are top views of an optical modulator (electro-optic device) 100 according to one embodiment of the present invention. Figure 1(a) shows only the optical waveguide, and Figure 1(b) shows the entire optical modulator 100 including the traveling wave electrode.

[0031] As shown in Figures 1(a) and 1(b), the optical modulator 100 includes: a Mach-Zehnder optical waveguide 10 having first and second optical waveguides 10a and 10b formed on a substrate 1 and arranged parallel to each other; a first electrode 7 disposed along the first optical waveguide 10a; and a second electrode 8 disposed along the second optical waveguide 10b.

[0032] The Mach-Zehnder waveguide 10 is an optical waveguide with a Mach-Zehnder interferometer structure. It has first and second optical waveguides 10a and 10b branching from an input waveguide 10i via a wave-splitting section 10c. The first and second optical waveguides 10a and 10b are combined via a combiner section 10d to form an output optical waveguide 10o. The input light Si is split by the wave-splitting section 10c and travels through the first and second optical waveguides 10a and 10b respectively. Then, it is combined at the combiner section 10d and output as modulated light So from the output optical waveguide 10.

[0033] The first electrode 7, when viewed from above, covers the first optical waveguide 10a, and similarly, the second electrode 8, when viewed from above, covers the second optical waveguide 10b. That is, the first electrode 7 is formed on the first optical waveguide 10a via a buffer layer (described later), and similarly, the second electrode 8 is formed on the second optical waveguide 10b via a buffer layer. The first electrode 7, for example, is connected to an AC signal and can be referred to as a signal electrode. The second electrode, for example, is grounded and can be referred to as a "ground" electrode.

[0034] An electrical signal (modulation signal) is input to the first electrode 7. Since the first and second optical waveguides 10a and 10b are made of materials with electro-optic effects, such as lithium niobate, the refractive indices of the first and second optical waveguides 10a and 10b change as +Δn and -Δn, respectively, by the electric field applied to them, thus changing the phase difference between the pair of waveguides. The signal light modulated by this phase difference is output from the output optical waveguide 10o.

[0035] Figure 2 This is a schematic cross-sectional view of the optical modulator 100 along line A-A' in Figure 1(b).

[0036] like Figure 2As shown, the optical modulator 100 of this embodiment has a multilayer structure in which a substrate 1, a waveguide layer 2, a buffer layer 3, and an electrode layer 4 are stacked in this order. The substrate 1 is, for example, a sapphire substrate, and a waveguide layer 2 made of a lithium niobate film is formed on the surface of the substrate 1. The waveguide layer 2 has first and second optical waveguides 10a and 10b formed by ridges 2r. The width of the first and second optical waveguides 10a and 10b can be, for example, 1 μm.

[0037] To prevent light propagating in the first and second optical waveguides 10a and 10b from being absorbed by the first electrode 7 or the second electrode 8, a buffer layer 3 is formed at least on the upper surface of the ridge 2r of the waveguide layer 2. Therefore, the buffer layer 3 only needs to function as an intermediate layer between the optical waveguide and the signal electrode, and the material of the buffer layer can be widely selected as long as it is non-metallic. For example, the buffer layer can be a ceramic layer made of insulating materials such as metal oxides, metal nitrides, and metal carbides. The buffer layer material can be a crystalline material or an amorphous material. As a more preferred embodiment, the buffer layer 3 can be a material with a refractive index lower than that of the waveguide layer 2, such as Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc. The thickness of the buffer layer formed on the optical waveguide can be approximately 0.2 to 1.2 μm. In this embodiment, the buffer layer 3 not only covers the upper surfaces of the first and second optical waveguides 10a and 10b, but is also embedded between the first and second optical waveguides 10a and 10b. In other words, the buffer layer 3 is also formed in the region that does not overlap with the first and second optical waveguides 10a and 10b when viewed from above. The buffer layer 3 covers the entire surface of the upper surface of the waveguide layer 2 in the region where the ridge 2r is not formed, and the side surfaces of the ridge 2r are also covered by the buffer layer 3.

[0038] In electrode layer 4, a first electrode 7 and a second electrode 8 are provided. The first electrode 7 is positioned opposite the first optical waveguide 10a via a buffer layer 3 and is arranged to overlap with the ridge 2r corresponding to the first optical waveguide 10a to modulate the light traveling within the first optical waveguide 10a. The second electrode 8 is positioned opposite the second optical waveguide 10b via a buffer layer 3 and is arranged to overlap with the ridge 2r corresponding to the second optical waveguide 10b to modulate the light traveling within the second optical waveguide 10b.

[0039] The waveguide layer 2 can be any electro-optic material without particular limitations, but it is preferably composed of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optical constant and is suitable as a constituent material for optical devices such as optical modulators. Hereinafter, the structure of the present invention when the waveguide layer 2 is a lithium niobate film will be described in detail.

[0040] As substrate 1, there is no particular limitation as long as the refractive index is lower than that of the lithium niobate film. Preferably, it is a substrate that allows the lithium niobate film to be formed into an epitaxial film, preferably a sapphire single-crystal substrate or a silicon single-crystal substrate. The crystal orientation of the single-crystal substrate is not particularly limited. The lithium niobate film has the property of being easily formed into a c-axis oriented epitaxial film with respect to single-crystal substrates of various crystal orientations. The c-axis oriented lithium niobate film has a three-dimensional symmetry. Therefore, it is preferable that the single-crystal substrate of the substrate also has the same symmetry. In the case of a sapphire single-crystal substrate, a substrate with the c-plane is preferred; in the case of a silicon single-crystal substrate, a substrate with the (111) plane is preferred.

[0041] Here, the epitaxial film is a film whose crystal orientation is consistent with that of the substrate or base film relative to the substrate. When the in-plane of the film is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals are aligned in the X, Y, and Z axis directions. For example, the epitaxial film can be verified by first confirming the peak intensity at the orientation position using 2θ-θ X-ray diffraction, and then confirming the poles.

[0042] Specifically, when performing measurements using 2θ-θ X-ray diffraction, the intensity of all peaks outside the target plane must be less than 10% of the maximum peak intensity of the target plane, preferably less than 5%. For example, in a c-axis oriented epitaxial film of lithium niobate, the peak intensity outside the (00L) plane is less than 10% of the maximum peak intensity of the (00L) plane, preferably less than 5%. (00L) is a general term for equivalent planes such as (001) or (002).

[0043] Secondly, poles need to be observed in pole determination. Under the condition of confirming the peak intensity at the first orientation position mentioned above, it only indicates orientation in one direction. Even if the first condition is met, the intensity of X-rays at a specific angle position is not high when the in-plane crystal orientation is inconsistent, and poles cannot be observed. Since LiNbO3 is a trigonal crystal structure, there are 3 poles in LiNbO3(014) in a single crystal. In the case of lithium niobate film, it is known that epitaxial growth is performed in a state of so-called bicrystalline state where the crystals are symmetrically combined after rotating 180° around the c-axis. In this case, it becomes a state where 2 of the 3 poles are symmetrically combined, so there are 6 poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate with (100) plane, since the substrate is four-fold symmetrical, 4×3=12 poles are observed. In addition, in this invention, the lithium niobate film epitaxially grown in a bicrystalline state is also included in the epitaxial film.

[0044] The lithium niobate film has the composition LixNbAyOz. A represents an element other than Li, Nb, and O. x is 0.5–1.2, preferably 0.9–1.05. y is 0–0.5. z is 1.5–4, preferably 2.5–3.5. Elements representing A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, etc., or combinations of two or more.

[0045] The thickness of the lithium niobate film is preferably 2 μm or less. This is because it is difficult to form a high-quality film when the thickness is increased to more than that. On the other hand, when the thickness of the lithium niobate film is too thin, the light cannot be completely confined in the lithium niobate film, and the light leaks to the substrate or buffer layer and is guided. Even if an electric field is applied to the lithium niobate film, the effective refractive index change of the optical waveguide (10a, 10b) may be reduced. Therefore, the thickness of the lithium niobate film is preferably more than 1 / 10 of the wavelength of the light used.

[0046] As a method for forming a lithium niobate film, sputtering, CVD, sol-gel methods, etc., are preferred. If the c-axis of lithium niobate is oriented perpendicular to the main surface of the substrate, the optical refractive index changes proportionally to the electric field by applying an electric field parallel to the c-axis. When sapphire is used as a single-crystal substrate, a lithium niobate film can be directly epitaxially grown on the sapphire single-crystal substrate. When silicon is used as a single-crystal substrate, a lithium niobate film is formed by epitaxial growth via a cladding layer (not shown). As the cladding layer (not shown), a cladding layer with a refractive index lower than that of the lithium niobate film and suitable for epitaxial growth is used. For example, if Y₂O₃ is used as the cladding layer (not shown), a high-quality lithium niobate film can be formed.

[0047] In addition, as a method for forming lithium niobate films, a method of thinly grinding and slicing a lithium niobate single-crystal substrate is also known. This method has the advantage of obtaining properties similar to those of a single crystal and can be applied to the present invention.

[0048] In this embodiment, the buffer layer 3 has a recess on the first and second optical waveguides 10a, 10b. Specifically, the buffer layer 3 disposed between the first and second optical waveguides 10a, 10b and the first and second electrodes 7, 8 has a shape that protrudes downward at the first and second optical waveguides 10a, 10b. In other words, the buffer layer 3 disposed between the first and second optical waveguides 10a, 10b and the first and second electrodes 7, 8 has a shape that bends downward at the first and second optical waveguides 10a, 10b. With the buffer layer 3 disposed in this way, the electrodes 7, 8 on the optical waveguides 10a, 10b can be brought closer to the optical waveguides 10a, 10b, an electric field can be effectively applied to the optical waveguides, thereby locking the light within the optical waveguides 10a, 10b and suppressing light propagation loss.

[0049] like Figure 2 As shown, the recess amount R of the buffer layer 3 on the first and second optical waveguides 10a and 10b is... The above are preferred. More preferably Here, the indentation R is the distance from the uppermost point to the lowest point (the lowest point of the indentation) of the upper surface of the buffer layer 3. To verify the relationship between the indentation R of the buffer layer 3 and the light propagation loss, the inventors of this invention conducted the following experiment. Samples 1-3 and the comparative example are electro-optic devices with identical structures except for the difference in the indentation of the buffer layer on the ridge waveguide.

[0050]

[0051] As can be seen from the table, when the indentation amount R is The above, especially At this time, the propagation loss of light is relatively low. While the exact reason for this reduction in propagation loss is not fully understood, it can be speculated that the indentation allows for a more effective application of the electric field to the optical waveguide. Furthermore, the indentation prevents light leaking from the upper part of the waveguide from reflecting back and interacting with it, thus avoiding the adverse effects of scattering and reflection of the signal light. When the indentation amount R is... Without a depression, light propagation loss is significant. This may be due to: an increased likelihood of light leaking from the upper part of the light waveguide reflecting back and interacting with it, and light leaking from the signal light being scattered and reflected, thus adversely affecting the signal light. Furthermore, from a manufacturing ease perspective, the depression amount R is preferably... the following.

[0052] Furthermore, in this embodiment, the distance from the surface (upper surface) of the substrate 1 to the uppermost part of the buffer layer 3 on the optical waveguides 10a, 10b is less than the distance from the surface (upper surface) of the substrate 1 to the uppermost part of the buffer layer 3 on the portion where the optical waveguides are not formed (i.e., the portion between the optical waveguides 10a, 10b and the portion outside the optical waveguides 10a, 10b). With this arrangement, the electrodes 7, 8 on the optical waveguides 10a, 10b can be made closer to the optical waveguides 10a, 10b, effectively applying an electric field to the optical waveguides, thereby locking the light within the optical waveguides 10a, 10b and suppressing light propagation loss.

[0053] Figure 3 This is a top view of an optical switching device 200 according to another embodiment of the present invention. Figure 4 It is along Figure 3A schematic cross-sectional view of the optical switching device 200 with a B-B' line. The optical switching device 200 includes: first and second optical waveguides 310a and 310b formed on a substrate 1; a thin-film heater (thin-film electrode 34) 307 disposed along the first optical waveguide 310a; and wiring 309a and 309b for energizing the thin-film heater. The thin-film heater (thin-film electrode 34) 307 is disposed directly above the first optical waveguide 310a in a top view, overlapping with the first optical waveguide 310a. Figure 4 As shown, the optical switch device 200 of this embodiment has a multilayer structure in which a substrate 31, a waveguide layer 32, a buffer layer 33, and a thin-film electrode layer 34 are stacked in this order. The substrate 31 is, for example, a silicon substrate, and a waveguide layer 32 made of lithium niobate film is formed on the surface of the substrate 31. The waveguide layer 32 has first and second optical waveguides 310a and 310b formed by ridges 32r.

[0054] The optical switch device 200 uses the same ridged lithium niobate film as the optical modulator 100 of the first embodiment, and also has a structure in which the optical waveguide is embedded in a buffer layer. The optical switch device 200 does not have an electrode layer 4; instead, it has a thin-film heater (thin-film electrode 34) 307. That is, a thin-film heater 307 is formed on the upper layer of a portion of the optical waveguides 310a and 310b (in this example, optical waveguide 310a), and by allowing current to flow through the thin-film heater 307, the optical waveguide 310a can be heated.

[0055] In the optical switch device 200, when the thin-film heater 307 is turned on, the phase of the light passing through the optical waveguide 310a is shifted, and the light is switched by combining with another optical waveguide 310b. In the optical switch device 200, by having a recess in the buffer layer 33 on the side of the thin-film heater 307, the propagation loss of light can also be reduced, thus obtaining an optical switch device 200 with excellent performance.

[0056] In addition, the materials used in thin-film heaters can be thermistors such as MnNiCo oxides, or platinum heaters.

[0057] Although the present invention has been specifically described above in conjunction with the accompanying drawings and embodiments, it is to be understood that the above description does not limit the invention in any way. For example, in the description of the optical modulator 100 above, the first electrode is described as a signal electrode and the second electrode as a ground electrode. However, it is not limited thereto, and the first and second electrodes can be any electrodes that apply an electric field to the optical waveguide. Furthermore, as... Figure 6As shown, the recesses of the buffer layer 3 on adjacent first and second optical waveguides 10a and 10b can be formed integrally. That is, a recess can be formed spanning the optical waveguides 10a and 10b. Furthermore, the buffer layer 3 may include a buffer layer 3a and a buffer layer 3b, wherein the buffer layer 3b is formed on the same layer as the optical waveguides 10a and 10b, and the buffer layer 3a is formed on both the optical waveguides 10a and 10b. Both the buffer layer 3a and the buffer layer 3b may have a recess spanning the optical waveguides 10a and 10b. Those skilled in the art can make modifications and variations to the present invention as needed without departing from the essential spirit and scope of the invention, and all such modifications and variations fall within the scope of the present invention.

[0058] Symbol Explanation

[0059] 100… Optical modulator; 1… Substrate; 2… Waveguide layer; 2r… Ridge; 3… Buffer layer; 4… Electrode layer; 7… First electrode; 8… Second electrode; 10… Mach-Zehnder waveguide; 10a… First optical waveguide; 10b… Second optical waveguide; 10c… Wavelength splitter; 10d… Wavelength combiner; 10i… Input optical waveguide; 10o… Output optical waveguide; 200… Optical switch; 31… Substrate; 32… Waveguide layer; 32r… Ridge; 33… Buffer layer; 34… Thin film electrode layer; 307… Thin film heater; 309a, 309b… Wiring; 310a, 310b… First and second optical waveguides.

Claims

1. An electro-optical device characterized by comprising: a substrate; an optical waveguide constituted by a film of electro-optical material formed in a ridge shape on the substrate; a buffer layer provided so as to cover the optical waveguide; and an upper electrode provided on the optical waveguide via the buffer layer, wherein the buffer layer has a recess on the upper electrode side on the optical waveguide, and wherein the direction of protrusion of the ridge shape of the optical waveguide is opposite to the direction of the recess of the buffer layer.

2. An electro-optical device characterized by comprising: a substrate; first and second optical waveguides constituted by films of electro-optical material formed in a ridge shape on the substrate, the first and second optical waveguides being adjacent to each other; a buffer layer provided so as to cover the first and second optical waveguides and to be buried between the first and second optical waveguides; and first and second electrodes provided so as to face the first and second optical waveguides above the buffer layer, wherein the buffer layer has a recess on the first and second optical waveguides, and wherein the direction of protrusion of the ridge shape of the first and second optical waveguides is opposite to the direction of the recess of the buffer layer.

3. An electro-optical device characterized by comprising: a substrate; first and second optical waveguides constituted by films of electro-optical material formed in a ridge shape on the substrate, the first and second optical waveguides being adjacent to each other; a buffer layer provided so as to cover the first and second optical waveguides and to be buried between the first and second optical waveguides; and first and second electrodes provided so as to face the first and second optical waveguides above the buffer layer, wherein the distance from the surface of the substrate to the uppermost part of the buffer layer on the optical waveguide is smaller than the distance from the surface of the substrate to the uppermost part of the buffer layer on a portion where the optical waveguide is not formed, and wherein the distance from the surface of the substrate to the lowermost part of the buffer layer on the optical waveguide is larger than the distance from the surface of the substrate to the lowermost part of the buffer layer on a portion where the optical waveguide is not formed.

4. The electro-optical device according to claim 2 or 3, wherein the buffer layer provided between the first and second optical waveguides and the first and second electrodes has a shape that protrudes downward at the first and second optical waveguides.

5. The electro-optical device according to claim 2 or 3, wherein the buffer layer provided between the first and second optical waveguides and the first and second electrodes has a shape that curves downward at the first and second optical waveguides.

6. The electro-optical device according to any one of claims 1 to 5, wherein the amount of recess of the buffer layer is 500 A or more.

7. The electro-optical device according to any one of claims 1 to 5, wherein the amount of recess of the buffer layer is 1000 A to 10000 A.

8. The electro-optical device according to any one of claims 2 to 5, wherein the first and second optical waveguides are Mach-Zehnder optical waveguides.

9. The electro-optical device according to any one of claims 2 to 5, wherein the substrate is a single crystal substrate, and wherein the film of electro-optical material is a lithium niobate film. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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