electro-optical devices

Abstract

The present invention provides an electro-optical device having a substrate (1), optical waveguides (10a, 10b) formed on the substrate (1), and upper electrodes (7, 8) formed on the optical waveguides. The electro-optical device further includes buffer layers (31, 33) formed between the substrate and the upper electrodes, and an oxide layer (32) having a composition different from that of the optical waveguides and the buffer layer is formed at the boundary between the optical waveguides (10a, 10b) and the buffer layer (31). The electro-optical device provided by the present invention can suppress optical propagation loss.

Description

[Technical Field]

[0001] Technical Field The present invention relates to electro-optical devices for the fields of optical communication and optical measurement. [Background technology]

[0002] Background technology

[0002] With the widespread use of the Internet, communication traffic has increased significantly, and the importance of optical fiber communication has become extremely high. Optical fiber communication is a communication mode in which electrical signals are converted into optical signals and transmitted through optical fibers, and has the characteristics of wide bandwidth, low loss, and strong noise resistance.

[0003]

[0003] As modes for converting an electrical signal into an optical signal, there are known a direct modulation mode using a semiconductor laser and an external modulation mode using an optical modulator. Direct modulation does not require an optical modulator and is low cost, but has limitations in terms of high-speed modulation. As a result, the external optical modulation mode is used for high-speed and long-distance applications.

[0004]

[0004] Patent Document 1 discloses a Mach-Zehnder optical modulator using a lithium niobate film (LN film). Compared to optical modulators using lithium niobate single crystal substrates, optical modulators using lithium niobate films achieve significant miniaturization and lower drive voltages. Figure 9 shows the cross-sectional structure of an existing optical modulator 800 disclosed in Patent Document 1. A pair of optical waveguides 22a and 22b made of lithium niobate films is formed on a sapphire substrate 21, and a signal electrode 24a and a ground electrode 24b are respectively configured on top of the optical waveguides 22a and 22b via a buffer layer 23. The optical modulator 800 is a so-called single-drive optical modulator having one signal electrode 24a. The signal electrode 24a and the ground electrode 24b have a symmetrical structure. Therefore, the electric fields applied to the optical waveguides 22a and 22b are the same in magnitude but opposite in sign.

[0005] In Patent Document 1, the LN film is in direct contact with the buffer layer 23, and the lithium niobate film at the boundary position of the buffer layer 23 is prone to oxygen defects, which results in an increase in propagation loss. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2006-195383 Summary of the Invention [Means for solving the problem]

[0007] Summary of the Invention The present invention has been completed in consideration of the above-mentioned problems, and aims to provide an electro-optical device that can supply oxygen to an LN film at the boundary position of a buffer layer in order to suppress oxygen defects in the LN film. The electro-optical device includes a substrate, an optical waveguide formed on the substrate, and an upper electrode formed on the optical waveguide, and further includes a buffer layer formed between the substrate and the upper electrode, and an oxide layer having a composition different from that of the optical waveguide and the buffer layer is formed at the boundary between the optical waveguide and the buffer layer.

[0008]

[0008] Furthermore, in the electro-optical device provided by the present invention, preferably, the optical waveguide comprises a slab portion formed on the substrate and a ridge portion formed by protruding from the slab portion, and an oxide layer is formed on at least the upper surface of the ridge portion.

[0009] Furthermore, in the electro-optical device provided by the present invention, preferably, an oxide layer is further formed on the side surface of the ridge portion.

[0010]

[0010] Furthermore, in the electro-optical device provided by the present invention, preferably, an oxide layer is further formed on the upper surface of the slab portion.

[0011]

[0011] Furthermore, in the electro-optical device provided by the present invention, the oxide layer is preferably a single element oxide and is amorphous.

[0012]

[0012] Furthermore, in the electro-optical device provided by the present invention, the molar ratio of oxygen in the oxide layer is preferably 60% or more.

[0013]

[0013] Furthermore, in the electro-optical device provided by the present invention, the oxide layer is preferably an oxide of any one of Cr, Al, Si, Ti, Mn and Ta.

[0014]

[0014] Furthermore, in the electro-optical device provided by the present invention, preferably the buffer layer belongs to the M-Si-O system, where M is at least one of Al, Zr, Hf, La, Ba, Bai, Ti, Ca, Mo and In.

[0015] Furthermore, in the electro-optical device provided by the present invention, the oxide layer is preferably formed only on the side surfaces of the ridge portion and the top surface of the slab portion.

[0016] Furthermore, in the electro-optical device provided by the present invention, the oxide layer is preferably formed only on the side surfaces of the ridge portion.

[0017] Furthermore, in the electro-optical device provided by the present invention, the oxide layer is preferably formed only on the upper surface of the slab portion.

[0018]

[0018] The electro-optical device provided by the present invention can suppress oxygen defects in the LN film at the boundary position of the buffer layer to reduce optical propagation loss. [Brief explanation of the drawings]

[0019] BRIEF DESCRIPTION OF THE DRAWINGS [Figure 1(a)]FIG. 1 is a top view of an optical modulator 100 according to a first embodiment of the present invention, showing only an optical waveguide. [Figure 1(b)] FIG. 1 is a top view of an optical modulator 100 according to a first embodiment of the present invention, showing the entire optical modulator 100 including traveling wave electrodes. [Figure 2]

[0019] FIG. 1(a) and FIG. 1(b) are schematic cross-sectional views of an optical modulator 100 taken along line AA'. [Figure 3] 2 is a schematic cross-sectional view of an optical modulator 200 according to another embodiment of the present invention. [Figure 4] 3 is a schematic cross-sectional view of an optical modulator 300 according to another embodiment of the present invention. [Figure 5] 4 is a schematic cross-sectional view of an optical modulator 400 according to another embodiment of the present invention. [Figure 6] 5 is a schematic cross-sectional view of an optical modulator 500 according to another embodiment of the present invention. [Figure 7] 6 is a schematic cross-sectional view of an optical modulator 600 according to another embodiment of the present invention. [Figure 8] 7 is a schematic cross-sectional view of an optical modulator 700 according to another embodiment of the present invention. [Figure 9] 8 is a schematic cross-sectional view of a prior art optical modulator 800. DETAILED DESCRIPTION OF THE INVENTION

[0020] Description of the embodiment

[0020] The mode of carrying out the present invention will be described in detail below with reference to the drawings.

[0021]

[0021] Figures 1(a) and 1(b) are top views of an optical modulator (optoelectronic device) 100 according to one embodiment of the present invention. Figure 1(a) shows only an optical waveguide, and Figure 1(b) shows the entire optical modulator 100 including traveling wave electrodes.

[0022]

[0022] As shown in Figures 1(a) and 1(b), the optical modulator 100 is formed on a substrate 1 and is provided with a Mach-Zehnder optical waveguide 10 having first and second optical waveguides 10a and 10b arranged parallel to each other, a first electrode 7 arranged along the first optical waveguide 10a, and a second electrode 8 arranged along the second optical waveguide 10b.

[0023]

[0023] The Mach-Zehnder optical waveguide 10 is an optical waveguide having a Mach-Zehnder interferometer structure. The Mach-Zehnder optical waveguide 10 includes first and second optical waveguides 10a and 10b branched from an input optical waveguide 10i via a branching section 10c, and the first and second optical waveguides 10a and 10b are coupled to an output optical waveguide 10o via a combining section 10d. The input light Si is branched via the branching section 10c, travels through the first and second optical waveguides 10a and 10b, respectively, and is then wave-coupled in the combining section 10d to be output from the output optical waveguide 10o as a modulated signal So.

[0024]

[0024] When viewed from above, the first electrode 7 covers the first optical waveguide 10a, and similarly, when viewed from above, the second electrode 8 covers the second optical waveguide 10b. In other words, 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. For example, the first electrode 7 can be connected to an alternating current (AC) signal and referred to as a "jump" electrode. For example, the second electrode can be grounded and referred to as a "ground" electrode.

[0025]

[0025] An electric signal (modulation signal) is input to the first electrode 7. The first and second optical waveguides 10a and 10b are formed of a material (e.g., lithium niobate) having an electro-optic effect. Therefore, an electric field applied to the first and second optical waveguides 10a and 10b changes the refractive indexes of the first and second optical waveguides 10a and 10b by +Δn and -Δn, respectively, and changes the phase difference between the pair of optical waveguides. Signal light modulated by the change in phase difference is output from the output optical waveguide 10o.

[0026] FIG. 2 is a schematic cross-sectional view of the optical modulator 100 taken along line AA' in FIGS. 1(a) and 1(b).

[0027] 2, the optical modulator 100 according to this embodiment has a multilayer structure formed by sequentially stacking at least a substrate 1, a waveguide layer 2, a first buffer layer 31, and an electrode layer 4. 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 includes first and second optical waveguides 10a and 10b formed by a slab portion 2s and a ridge portion 2r protruding from the slab portion 2s.

[0028]

[0028] To prevent light propagating through the first and second optical waveguides 10a and 10b from being absorbed by the first electrode 7 or the second electrode 8, a first buffer layer 31 is formed at least on the upper surface of the ridge portion 2r of the waveguide layer 2. Therefore, the first buffer layer 31 only needs to function as an intermediate layer between the optical waveguides and the signal electrodes. As long as the material of the first buffer layer 31 is nonmetallic, a wide range of materials can be used for the first buffer layer 31. For example, the first buffer layer 31 can be a ceramic layer made of an insulating material (e.g., metal oxide, metal nitride, metal carbide, etc.). The material of the buffer layer can be a crystalline material or an amorphous material. In a more preferred embodiment, the first buffer layer 31 can be made of a material whose refractive index is lower than that of the waveguide layer 2 (for example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc.).

[0029] The first electrode 7 and the second electrode 8 are disposed on the electrode layer 4. The first electrode 7 faces the first optical waveguide 10 a via at least the first buffer layer 31, and is configured to overlap with the ridge portion 2 r corresponding to the first optical waveguide 10 a in order to modulate the light traveling in the first optical waveguide 10 a. The second electrode 8 faces the second optical waveguide 10 b via at least the first buffer layer 31, and is configured to overlap with the ridge portion 2 r corresponding to the second optical waveguide 10 b in order to modulate the light traveling in the second optical waveguide 10 b.

[0030]

[0030] The waveguide layer 2 is not particularly limited as long as it is made of an electro-optic material, but preferably, the waveguide layer 2 is made of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optic constant and is suitable for use as a constituent material of optical devices (e.g., optical modulators, etc.). The structure of the present invention when the waveguide layer 2 is a lithium niobate film will be described in detail below.

[0031]

[0031] As long as the refractive index of the substrate 1 is lower than that of the lithium niobate film, the substrate 1 is not particularly limited. Preferably, the substrate 1 is a substrate on which the lithium niobate film can be formed as an epitaxial film. Specifically, the substrate 1 is 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 can be easily formed as a c-axis oriented epitaxial film on a single crystal substrate with a different crystal orientation. Since the c-axis oriented lithium niobate film has three-fold symmetry, it is preferable that the underlying single crystal substrate have the same symmetry. Therefore, the single crystal sapphire substrate preferably has a c-plane, and the single crystal silicon substrate preferably has a (111) surface.

[0032]

[0032] As used herein, the term "epitaxial film" refers to a film whose crystalline orientation is aligned with respect to the underlying substrate or film. If the film surface is the XY plane and the film thickness direction is the Z axis, the crystals are aligned in the X-, Y-, and Z-axis directions. For example, the presence of an epitaxial film can be confirmed by first measuring the peak intensity at the orientation position by 2θ-θ X-ray diffraction and then identifying the poles.

[0033]

[0033] In detail, first, in 2θ-θ X-ray diffraction measurement, the total peak intensity excluding the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity on the target plane. For example, in a c-axis oriented epitaxial lithium niobate film, the peak intensity excluding the (00L) plane is 10% or less, preferably 5% or less, of the maximum peak intensity on the (00L) plane. (00L) is a general term that collectively refers to (001), (002), and other equivalent planes.

[0034] Next, poles must be observed in the measurement. Under the condition that peak intensity is confirmed at the first orientation position, only orientation in a single direction is indicated. Even if the first condition is met, if the in-plane crystal orientation is nonuniform, the X-ray intensity does not increase at a specific angle, and poles cannot be observed. Because LiNbO3 has a trigonal crystal system, single-crystal LiNbO3 (014) has three poles. It is known that lithium niobate films grow epitaxially in a so-called twin crystal state, in which crystals rotated 180 degrees around the c-axis are symmetrically bonded. In this case, three poles are symmetrically bonded to form six poles. When a lithium niobate film is formed on a single-crystal silicon substrate with a (100) plane, the substrate has four-fold symmetry, so 4 × 3 = 12 poles are observed. In the present invention, a lithium niobate film that is epitaxially grown in a twin crystal state is also considered to be an epitaxial film.

[0035]

[0035] The composition of the lithium niobate film is LixNbAyOz. A represents an element other than Li, Nb, and O. The number x is in the range of 0.5 to 1.2, preferably 0.9 to 1.05. The number y is in the range of 0 to 0.5. The number z is in the range of 1.5 to 4, preferably 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, either alone or in combination.

[0036]

[0036] For example, it is desirable to form the lithium niobate film by a film formation method such as a sputtering method, a CVD method, or a sol-gel method. When the c-axis of the lithium niobate film is oriented perpendicular to the main surface of the single crystal substrate, an electric field is applied parallel to the c-axis, thereby changing the optical refractive index in proportion to the strength of the electric field. When the single crystal substrate is sapphire, the lithium niobate film is formed by epitaxial growth directly on the sapphire single crystal substrate. When the single crystal substrate is silicon, the lithium niobate film is formed by epitaxial growth on a cladding layer (not shown) formed on the substrate. The cladding layer (not shown) is formed of a material that has a lower refractive index than the lithium niobate film and should be suitable for epitaxial growth.

[0037]

[0037] A method of thinly polishing or slicing a lithium niobate single crystal substrate is known as a method for forming a lithium niobate film. This method has the advantage of being able to obtain the same properties as a single crystal, and is applicable to the present invention.

[0038] In this embodiment, an oxide layer 32 having a composition different from that of the waveguide layer 2 and the first buffer layer 31 is formed at the boundary between the waveguide layer 2 and the first buffer layer 31. Specifically, as shown in FIG. 2 , the oxide layer 32 is formed below the first buffer layer 31 and covers the upper surface of the ridge portion 2r. Furthermore, the oxide layer 32 is also formed between the ridge portions 2r and on the slab portion 2s. By forming the oxide layer 32 on the upper surface of the ridge portion 2r, oxygen in the oxide layer 32 can be supplied to the LN film to suppress oxygen defects in the LN film.

[0039] When viewed from above, the second buffer layer 33 is formed in a position where the first and second optical waveguides 10a and 10b do not overlap with the second buffer layer 33. The second buffer layer 33 covers the entire region of the upper surface of the waveguide layer 2 where the ridge portion 2r is not formed, and the side surfaces of the ridge portion 2r are also covered with the second buffer layer 33. As a result, scattering loss caused by the rough side surfaces of the ridge portion 2r can be prevented. The thickness of the second buffer layer 33 is approximately the same as the height of the ridge portion 2r of the waveguide layer 2. The material of the second buffer layer 33 is not particularly limited, and silicon oxide (SiO2) can be used, for example. Here, the second buffer layer 33 and the oxide layer 32 are film layers formed in different processes. Furthermore, the material of the second buffer layer 33 may be the same as or different from the material of the first buffer layer 31.

[0040]

[0040] The oxide layer 32 is preferably a single element oxide and is amorphous. When the oxide layer 32 is amorphous, it is easier to supply oxygen from the oxide layer 32 to the LN film than when the oxide layer 32 is crystallized.

[0041]

[0041] The molar ratio of oxygen in the oxide layer 32 is preferably 60% or more. When the molar ratio of oxygen is high, oxygen can be added to the LN film by annealing. The oxide layer 32 can be an oxide of at least one selected from the group consisting of Cr, Al, Si, Ti, Mn, and Ta. It is preferable to use a metal oxide made of one metal selected from the above metals. Furthermore, the metal oxide can contain elements such as carbon, nitrogen, or hydrogen. Carbon is a particularly preferred element. The element content is preferably 5 atomic percent or less, more preferably 0.1 atomic percent to 3 atomic percent. The element content can be measured by EDS (energy dispersive X-ray spectroscopy).

[0042]

[0042] The first buffer layer 31 may preferably be of the M-Si-O system, where M is at least one of Al, Zr, Hf, La, Ba, Bai, Ti, Ca, Mo and In.

[0043]

[0043] Basically, the composition of the first buffer layer 31 is different from the composition of the oxide layer 32. More preferably, the first buffer layer 31 is composed of a composition doped with a metal heavier than the constituent metal of the oxide layer 32. For example, an M-Si-O based material is preferred. In particular, when an Si-O oxide is used for the oxide layer 32, it is more preferred that the first buffer layer 31 be an M-Si-O based material (the mass number of M is greater than the mass number of Si).

[0044]

[0044] Furthermore, the metal oxide may contain elements such as carbon, nitrogen, or hydrogen. Carbon is a particularly preferred element. The content of the element is preferably 5 atomic percent or less, more preferably 0.1 atomic percent to 3 atomic percent. The measurement value of the content of the element can be obtained by EDS (energy dispersive X-ray spectroscopy).

[0045]

[0045] Furthermore, the higher the film density of the oxide layer 32, the higher the oxygen composition ratio of the oxide layer 32. The oxide layer 32 is formed by, for example, CVD film formation, sputtering film formation, etc. After the oxide layer 32 is formed, an annealing process is performed.

[0046]

[0046] In the optical modulator 100, by forming an oxide layer 32 between the ridge portion 2r of the waveguide layer 2 and the first buffer layer 31, propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer can be suppressed.

[0047]

[0047] Fig. 3 is a schematic cross-sectional view of an optical modulator 200 according to another embodiment of the present invention. As shown in Fig. 3, an oxide layer 32 is formed under the first buffer layer 31 and covers the upper surfaces of the ridge portion 2r and the slab portion 2s. Furthermore, the oxide layer 32 is also formed between the ridge portions 2r. In other words, the side surfaces of the ridge portion 2r are also covered with the oxide layer 32.

[0048]

[0048] In the optical modulator 200, by forming the oxide layer 32 between the waveguide layer 2 and the first buffer layer 31, propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer can be suppressed.

[0049]

[0049] Figure 4 is a schematic cross-sectional view of an optical modulator 300 according to another embodiment of the present invention. As shown in Figure 4, the oxide layer 32 extends along the outer surface of the waveguide layer 2. In other words, the oxide layer 32 covers the top and side surfaces of the ridge portion 2r and the top surface of the slab portion 2s. The second buffer layer 33 covers the oxide layer 32, and the thickness of the second buffer layer 33 is approximately the same as the height of the top surface of the oxide layer 32. The first buffer layer 31 is formed to cover the oxide layer 32 and the second buffer layer 33. Here, the first buffer layer 31 and the second buffer layer 33 may be formed from the same buffer layer.

[0050] In the optical modulator 300, by disposing the oxide layer 32 along the boundary of the LN film, it is possible to suppress propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer.

[0051] FIG. 5 is a schematic cross-sectional view of an optical modulator 400 according to another embodiment of the present invention. As shown in FIG. 5, the oxide layer 32 covers only the upper surface of the ridge portion 2r. A second buffer layer 33 is formed in a position that does not overlap with the first and second optical waveguides 10a and 10b. The second buffer layer 33 covers the entire area of ​​the upper surface of the waveguide layer 2 where the ridge portion 2r is not formed, and the side surfaces of the ridge portion 2r are also covered with the second buffer layer 33. As a result, scattering loss caused by the rough side surfaces of the ridge portion 2r can be prevented. The thickness of the second buffer layer 33 is approximately the same as the height of the upper surface of the oxide layer 32. A first buffer layer 31 is formed to cover the oxide layer 32 and the second buffer layer 33. Here, the first buffer layer 31 and the second buffer layer 33 can be formed from the same buffer layer.

[0052]

[0052] In the optical modulator 400, by placing the oxide layer 32 between the first and second optical waveguides 10a and 10b and the buffer layer, propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer can be suppressed.

[0053]

[0053] Figure 6 is a schematic cross-sectional view of an optical modulator 500 according to another embodiment of the present invention. The optical modulator 500 shown in Figure 6 differs from the optical modulator 300 shown in Figure 4 only in that the oxide layer 32 does not cover the upper surface of the ridge portion 2r. In the optical modulator 500, by disposing the oxide layer 32 on the upper surface of the slab portion 2s of the waveguide layer 2 and on the side surface of the ridge portion 2r of the waveguide layer 2, it is possible to suppress propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer.

[0054]

[0054] Figure 7 is a schematic cross-sectional view of an optical modulator 600 according to another embodiment of the present invention. The only difference between the optical modulator 600 shown in Figure 7 and the optical modulator 500 shown in Figure 6 is that the oxide layer 32 covers only the side surfaces of the ridge portion 2r. In the optical modulator 600, by disposing the oxide layer 32 on the side surfaces of the ridge portion 2r of the waveguide layer 2, it is possible to suppress propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer.

[0055]

[0055] Figure 8 is a schematic cross-sectional view of an optical modulator 700 according to another embodiment of the present invention. The only difference between the optical modulator 700 shown in Figure 8 and the optical modulator 500 shown in Figure 6 is that the oxide layer 32 covers only the upper surface of the slab portion 2s. In the optical modulator 700, by disposing the oxide layer 32 on the upper surface of the slab portion 2s of the waveguide layer 2, it is possible to suppress propagation loss caused by oxygen defects in the LN film at the boundary position of the buffer layer. [Example]

[0056] Example 2 is used as an example, while an electro-optical device without the oxide layer 32 is used as a comparative example. The optical transmission attenuation in the example and the comparative example is compared.

[0057] Example 1 In Example 1, a 50 nm thick SiO2 layer 32 is formed on the second buffer layer 33 and the ridge portion 2r by CVD film formation, and the SiO2 layer 32 is amorphous. In Comparative Example 1, the structure is the same as that in Example 1 except that the SiO2 layer 32 is not formed. The evaluation results of Example 1 are as shown in Table 1.

[0058]

[0058] [Table 1]

[0059]

[0059] Table 1 shows that by forming a SiO2 oxide film having a thickness of 50 nm on the second buffer layer 33 and the ridge portion 2r, the transmission loss of light can be reduced and the yield rate can be improved.

[0060] Example 2 In Example 2, a 100 nm-thick SiO2 layer 32 is formed on the second buffer layer 33 and the ridge portion 2r by CVD film formation, and the SiO2 layer 32 is amorphous. In Comparative Example 2, the structure is the same as that in Example 2 except that the SiO2 layer 32 is not formed. The evaluation results of Example 2 are as shown in Table 2.

[0061]

[0061] [Table 2]

[0062]

[0062] Table 2 shows that by forming a SiO2 oxide film having a thickness of 100 nm on the second buffer layer 33 and the ridge portion 2r, the optical transmission loss can be reduced and the yield rate can be improved.

[0063] Example 3 In Example 3, an Al2O3 layer 32 having a thickness of 50 nm is formed on the second buffer layer 33 and the ridge portion 2r by sputtering film formation, and the Al2O3 layer 32 is amorphous. In Comparative Example 3, the structure is the same as that in Example 3 except that the Al2O3 layer 32 is not formed. The evaluation results of Example 3 are as shown in Table 3.

[0064]

[0064] [Table 3]

[0065]

[0065] Table 3 shows that by forming an Al2O3 oxide film having a thickness of 50 nm on the second buffer layer 33 and the ridge portion 2r, the optical transmission loss can be reduced and the finished product rate can be improved.

[0066]

[0066] Although the present invention has been described above in detail in conjunction with the drawings and embodiments, it can be understood that the above description does not limit the present invention in any way. For example, in the above description of the optical modulator 100, the first electrode is used as a signal electrode, and the second electrode is used as a ground electrode. However, the present invention is not limited to this example, and the first and second electrodes can be any electrodes that apply an electric field to the optical waveguide. 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 present invention, and these modifications and variations fall within the scope of the present invention. [Explanation of symbols]

[0067]

[0067] Reference sign: 1 board 2. Waveguide layer 31 First buffer layer 32 oxide layer 33 Second buffer layer 4 electrode layer 7 First electrode 8 Second electrode 10 Mach-Zehnder optical waveguide 10a First optical waveguide 10b Second optical waveguide 10c Branch 10d Multiplexing section 10i input optical waveguide 10o output optical waveguide

Claims

1. 1. An electro-optical device comprising: a substrate; an optical waveguide formed on the substrate, the optical waveguide being formed from lithium niobate; and an upper electrode formed on the optical waveguide, a buffer layer formed between the substrate and the upper electrode, the buffer layer being of an M-Si-O system, where M is at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, and In, and the buffer layer being formed from an oxide of multiple elements; an oxide layer, which is an amorphous oxide layer of a single element and has a different composition from the optical waveguide and the buffer layer, is formed at the boundary between the optical waveguide and the buffer layer;

2. the optical waveguide comprises a slab portion formed on the substrate and a ridge portion formed by protruding from the slab portion; The electro-optical device according to claim 1 , wherein the oxide layer is formed at least on the top surface of the ridge portion.

3. The electro-optical device according to claim 2 , wherein the oxide layer is further formed on the side surfaces of the ridge portion.

4. 4. The electro-optical device according to claim 2, wherein the oxide layer is further formed on the upper surface of the slab portion.

5. 5. The electro-optical device according to claim 1, wherein the molar ratio of oxygen in the oxide layer is 60% or more.

6. 6. The electro-optical device according to claim 1, wherein the oxide layer is an oxide of any one of Cr, Al, Si, Ti, Mn, and Ta.

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