Electro-optical device

Abstract

The present application relates to an electro-optical device. The electro-optical device has a substrate, an optical waveguide formed on the substrate, a buffer layer formed on the substrate so as to cover the optical waveguide, and an electrode formed on the buffer layer, and a side surface of the optical waveguide has at least one slope change point when viewed from a direction of propagation of light. According to the present application, an electro-optical device capable of reducing a driving voltage and reducing a propagation loss of light can be provided.

Description

Technical Field

[0001] This invention relates to an electro-optical device used in the fields of optical communication or optical measurement. Background Technology

[0002] With the widespread adoption of the internet and the dramatic increase in communication volume, fiber optic communication has become extremely important. Fiber optic communication converts electrical signals into optical signals and transmits them through optical fibers, featuring wide bandwidth, low loss, and noise immunity.

[0003] As methods for converting electrical signals into optical signals, direct modulation using semiconductor lasers and external modulation using optical modulators are known. Direct modulation does not require an optical modulator and is low in cost, but it has limitations for high-speed modulation. In high-speed and long-distance applications, external modulation is used.

[0004] As an optical modulator used in external modulation methods, optical modulators using optical waveguides formed of lithium niobate (LiNbO3, hereinafter referred to as "LN") have advantages such as high speed, low loss, and less distortion of the controlled optical waveform. However, compared with semiconductor optical devices, they have disadvantages such as high driving voltage.

[0005] To overcome the aforementioned drawbacks, optical waveguides utilizing LN films formed on sapphire substrates using thin-film technology are known, thereby enabling optical modulators with lower driving voltages compared to existing technologies (see Patent Documents 1 and 2). Furthermore, optical waveguide elements that reduce driving voltages by providing a high-refractive-index film at the bottom of the optical waveguide are known (see Patent Document 3).

[0006] However, in the aforementioned optical modulators utilizing thin-film technology and optical waveguides with high-refractive-index films, the propagation loss increases when the driving voltage decreases.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2006-195383

[0010] Patent Document 2: Japanese Patent Application Publication No. 2014-6348

[0011] Patent Document 3: Japanese Patent Application Publication No. 2012-078375 Summary of the Invention

[0012] The present invention is the result of careful research in view of the above-mentioned problems, and its purpose is to provide an electro-optic device that can reduce both the driving voltage and the light propagation loss.

[0013] The present invention has conducted in-depth research on the driving voltage of electro-optic devices and the mechanism of light propagation loss, and has discovered the following novel insights that are difficult for those skilled in the art to conceive of to date: (1) Ideally, the cross section of the optical waveguide when viewed from the propagation direction (i.e., the cross section perpendicular to the propagation direction of light) is circular; (2) In existing optical waveguides, the above-mentioned cross section of the optical waveguide is usually trapezoidal or quadrilateral, and light is prone to leak to the corners of these shapes, which causes an increase in propagation loss PL (dB / cm); (3) The narrower the width of the upper part of the optical waveguide, the more concentrated the electric field, the better the Vπ (V)@1550nm, which is an indicator of electric field efficiency, and the lower the driving voltage. However, considering that if the width of the upper part of the optical waveguide is too narrow, the light will be guided outside the optical waveguide formed by LN, the electro-optic effect will decrease, and the scattering loss caused by surface roughness will increase, thereby the driving voltage will increase. Therefore, as long as the driving voltage can be maintained, it is preferable that the width of the upper part of the optical waveguide is not too narrow. Therefore, based on the above novel insights, the inventors attempted to improve the cross-sectional shape of the optical waveguide. However, since optical waveguides are usually formed by dry etching, it is difficult to make the cross-section of the optical waveguide into an ideal circular shape due to process issues. Therefore, the inventors envisioned making the cross-section of the optical waveguide approximately circular by gradually narrowing the width of the optical waveguide towards the top, so that the side of the optical waveguide has at least one point of slope change when viewed from the direction of light propagation (i.e., the side of the optical waveguide is composed of multiple straight lines with different slopes when viewed from the direction of light propagation). This would reduce both the driving voltage and the light propagation loss, until the present invention was completed.

[0014] That is, an electro-optic device according to one aspect of the present invention includes a substrate, an optical waveguide formed on the substrate, a buffer layer formed on the substrate to cover the optical waveguide, and an electrode formed on the buffer layer, wherein the side of the optical waveguide has at least one slope change point when viewed from the direction of light propagation.

[0015] In this electro-optic device, when viewed from the direction of light propagation, the side of the optical waveguide has at least one slope change point. In other words, when viewed from the direction of light propagation, the side of the optical waveguide is composed of multiple straight lines with different slopes. Therefore, it can be considered that the upper and lower slopes of the side of the optical waveguide can be made different to narrow the width of the upper part of the optical waveguide and reduce the driving voltage. Furthermore, the light propagation loss can be reduced by making the cross-section of the optical waveguide approximately circular.

[0016] In addition, in the electro-optic device according to one aspect of the present invention described above, the angle between the lower side of the optical waveguide and the upper surface of the substrate may be 60 degrees or more when viewed from the direction of light propagation.

[0017] In addition, in the electro-optic device according to one aspect of the present invention described above, the width of the uppermost part of the optical waveguide may be 40% to 75% of the width of the lowermost part.

[0018] In addition, in the electro-optic device according to one aspect of the present invention described above, when viewed from the direction of light propagation, the angle between the side of the uppermost segment of the optical waveguide and the upper surface of the substrate is greater than 0 degrees and less than 80 degrees.

[0019] In addition, the electro-optic device according to one aspect of the present invention described above may also have multiple slope change points.

[0020] Alternatively, in the electro-optic device according to one aspect of the present invention described above, the optical waveguide may also be a film composed of LiNbO3 or LiTaO3.

[0021] Alternatively, in the electro-optic device according to one aspect of the present invention described above, the optical waveguide may be a film formed by doping LiNbO3 with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb, and Lu.

[0022] In addition, in the electro-optic device according to one aspect of the present invention described above, the optical waveguide may also be an epitaxial film.

[0023] Alternatively, in the electro-optic device according to one aspect of the present invention described above, the epitaxial film may be oriented in a direction intersecting with the substrate.

[0024] According to one aspect of the present invention, an electro-optic device is provided that can reduce both the driving voltage and the light propagation loss. Attached Figure Description

[0025] Figures 1(a) and 1(b) are top views of an optical modulator according to a first embodiment of the present invention. Figure 1(a) shows only the optical waveguide, and Figure 1(b) shows the entire optical modulator including the traveling wave electrode.

[0026] Figure 2 This is a cross-sectional view of the optical modulator according to the first embodiment of the present invention, taken by a line A-A' passing through any position of the optical waveguide and perpendicular to the propagation direction of the light at that position.

[0027] Figure 3 The optical modulator involved in the modification of the first embodiment of the present invention corresponds to... Figure 2 Cross-sectional view.

[0028] Figure 4 The optical modulator according to the second embodiment of the present invention corresponds to Figure 2 Cross-sectional view.

[0029] Figure 5 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 1 of the present invention.

[0030] Figure 6 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 2 of the present invention.

[0031] Figure 7 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 3 of the present invention.

[0032] Figure 8 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 4 of the present invention.

[0033] Figure 9 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 5 of the present invention.

[0034] Figure 10 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 6 of the present invention.

[0035] Figure 11 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 7 of the present invention.

[0036] Figure 12 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 8 of the present invention.

[0037] Figure 13 This is a cross-sectional view showing the cross-sectional shape of the optical waveguide in Embodiment 9 of the present invention.

[0038] Figure 14 This is a cross-sectional view showing the cross-sectional shape of an optical waveguide in another variation of the present invention. Detailed Implementation

[0039] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or equivalent elements are marked with the same symbols, and repeated descriptions are omitted.

[0040] (First Implementation)

[0041] Figures 1(a) and 1(b) are top views of an optical modulator according to a first embodiment of the present invention. Figure 1(a) shows only the optical waveguide, and Figure 1(b) shows the entire optical modulator including the traveling-wave electrode. 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 signal electrode 20a disposed along the first optical waveguide 10a, a second signal electrode 20b disposed along the second optical waveguide 10b, a first bias electrode 30a disposed along the first optical waveguide 10a, and a second bias electrode 30b disposed along the second optical waveguide 10b. The first signal electrode 20a and the second signal electrode 20b together with the first optical waveguide 10a and the second optical waveguide 10b constitute the RF interaction section 40 of the Mach-Zehnder optical modulator. The first bias electrode 30a and the second bias electrode 30b together with the first optical waveguide 10a and the second optical waveguide 10b constitute the DC interaction section 50 of the Mach-Zehnder optical modulator.

[0042] The Mach-Zehnder waveguide 10 is an optical waveguide with a Mach-Zehnder interferometer configuration. 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 waveguides 10a and 10b are converged into an output waveguide 10o via a combining section 10d. The input light Si is split at the wave-splitting section 10c and travels along the first and second waveguides 10a and 10b respectively before being combined at the combining section 10d and output as modulated light So from the output waveguide 10o.

[0043] The first and second signal electrodes 20a and 20b are linear electrode patterns overlapping the first and second waveguides 10a and 10b in the top view, and their ends extend to electrode pads disposed near the end face of the substrate 1. That is, one end 20a1 and 20b1 of the first signal electrode 20a and the second signal electrode 20b are led out to the electrode pads disposed near the end face of the substrate 1 to form a signal input port, and the drive circuit 60a is connected to the signal input port. Furthermore, the other ends 20a2 and 20b2 of the first signal electrode 20a and the second signal electrode 20b are led out to the electrode pads disposed near the end face of the substrate 1 and connected to each other via a terminating resistor 60b. Thus, the first and second signal electrodes 20a and 20b function as differential coplanar traveling wave electrodes.

[0044] The first and second bias electrodes 30a and 30b are independent of the first and second signal electrodes 20a and 20b, so as to apply a DC voltage (DC bias) to the first and second waveguides 10a and 10b. One end 30a1 and 30b1 of the first bias electrode 30a and the second bias electrode 30b are led out to an electrode pad disposed near the end face of the substrate 1 to form a DC bias input port, and the bias circuit 60c is connected to the DC bias port. In this embodiment, the formation regions of the first bias electrode 30a and the second bias electrode 30b are configured to be closer to the output side of the Mach-Zehnder waveguide 10 than the formation regions of the first signal electrode 20a and the second signal electrode 20b. However, they can also be disposed on the input side.

[0045] As described above, the first and second signal electrodes 20a and 20b apply RF signals to the first and second waveguides 10a and 10b, and the first and second bias electrodes 30a and 30b apply DC bias voltages to the first and second waveguides 10a and 10b.

[0046] Differential signals (modulated signals) with the same absolute value but different signs are input to one end of the first and second signal electrodes 20a and 20b. Since the first waveguide 10a and the second waveguide 10b are made of a material with an electro-optic effect, such as lithium niobate, their refractive indices change according to the electric fields applied to them, such as +Δn and -Δn, respectively, and the phase difference between the pair of optical waveguides changes. The signal light modulated by this phase difference is output from the output waveguide 10o.

[0047] In this embodiment, the first and second optical waveguides 10a and 10b are films made of LiNbO3, but are not limited thereto. The first and second optical waveguides 10a and 10b may also be films made of LiTaO3, or films formed by doping LiNbO3 with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb, and Lu.

[0048] In this embodiment, the first and second optical waveguides 10a and 10b are epitaxial films. The epitaxial films are preferably oriented along a direction intersecting with the substrate 1 described below.

[0049] Figure 2 This is a cross-sectional view of the optical modulator according to the first embodiment of the present invention, taken by a line A-A' passing through any position of the optical waveguide and perpendicular to the propagation direction of light at that position. Figure 2As shown, the optical modulator 100 has a multilayer structure consisting of a substrate 1, a waveguide layer 2, a buffer layer 3, and an electrode layer 4 stacked in this order. The substrate 1 is, for example, a sapphire substrate. A waveguide layer 2 made of an electro-optic material, such as lithium niobate (LiNbO3, hereinafter referred to as "LN"), 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.

[0050] The buffer layer 3 is formed on the substrate 1 to prevent light propagating in the first and second optical waveguides 10a and 10b from being absorbed by the electrodes 20a and 20b. It covers the entire surface of the upper surface S2 of the waveguide layer 2 where the ridge 2r is not formed, as well as the side surface S3 and upper surface S4 of the ridge 2r. The buffer layer 3 only needs to function as an intermediate layer between the optical waveguide and the electrodes, and its material can be widely selected as long as it is non-metallic. For example, the buffer layer 3 can be a ceramic layer made of insulating materials such as metal oxides, metal nitrides, and metal carbides. The material of the buffer layer 3 can be crystalline or amorphous. The buffer layer 3 is preferably made of a material with a lower refractive index than the waveguide layer 2 and higher transparency; for example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc., can be used. The thickness of the buffer layer 3 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 surface S4 of the first and second optical waveguides 10a and 10b, but also covers the entire surface of the region in the upper surface S2 of the waveguide layer 2 where the ridge 2r is not formed, as well as the side surface S3 of the ridge 2r. However, as long as the buffer layer 3 is formed at least on the upper surface S4 of the ridge 2r of the waveguide layer 2, the buffer layer 3 can also be patterned in a way that selectively covers only the area near the upper surface S4 of the first and second optical waveguides 10a and 10b.

[0051] To reduce light absorption by the electrodes, a thicker buffer layer 3 is preferable, while a thinner buffer layer 3 is preferable to apply a high electric field to the first and second optical waveguides 10a and 10b. Since the light absorption of the electrodes is a trade-off with the applied voltage, it is necessary to set an appropriate film thickness according to the objective. A higher dielectric constant of the buffer layer 3 reduces Vπ(V)@1550nm, an indicator of electric field efficiency, and is therefore preferred. Conversely, a lower refractive index of the buffer layer 3 allows for a thinner buffer layer, which is also preferable. Generally, since materials with high dielectric constants also have high refractive indices, it is important to select a material with a high dielectric constant and a relatively low refractive index, considering the balance between the two. For example, Al₂O₃, with a relative dielectric constant of approximately 9 and a refractive index of approximately 1.6, is a preferred material. LaAlO₃, with a relative dielectric constant of approximately 13 and a refractive index of approximately 1.7, and LaYO₃, with a relative dielectric constant of approximately 17 and a refractive index of approximately 1.7, are particularly preferred materials.

[0052] Electrode layer 4 is disposed on buffer layer 3. First and second signal electrodes 20a and 20b and first and second bias electrodes 30a and 30b are disposed on electrode layer 4. The first and second signal electrodes 20a and 20b and the first and second bias electrodes 30a and 30b are respectively disposed overlapping the ridges 2r corresponding to the first and second optical waveguides 10a and 10b, and are opposite to the first and second optical waveguides 10a and 10b via buffer layer 3.

[0053] The waveguide layer 2 is not particularly limited to any electro-optic material, but it is preferably made of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optic constant, making it suitable as a constituent material for optical devices such as optical modulators. The structure of this embodiment, in which the waveguide layer 2 is a lithium niobate film, will be described in detail below.

[0054] As for substrate 1, there are no particular limitations as long as it is a substrate with a refractive index lower than that of the lithium niobate film, but it is preferable to be a substrate that can be formed as an epitaxial film using the lithium niobate 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. Compared with single crystal substrates with various crystal orientations, the lithium niobate film has the property of being easily formed as an epitaxial film with c-axis orientation. Since the c-axis oriented lithium niobate film has triple symmetry, 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, the c-plane is preferred, and in the case of a silicon single crystal substrate, the (111) plane is preferred.

[0055] Here, an epitaxial film refers to a film in which the 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 crystal is consistently oriented along the X, Y, and Z axes. 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 second, confirming the poles.

[0056] Specifically, when performing: First, measurements using 2θ-θ X-ray diffraction, the intensities of all peaks outside the target plane need to be 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. For example, in a c-axis oriented epitaxial film of lithium niobate, the peak intensities outside the (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. (00L) is a general term for equivalent planes such as (001) or (002).

[0057] Second, in pole determination, it is necessary to be able to see the poles. In the conditions for confirming the peak intensity at the first orientation position mentioned above, only orientation in one direction is shown. Even if the first condition is met, if the in-plane crystal orientation is inconsistent, the intensity of X-rays at a specific angle position will not increase, and the poles will not be seen. Since LiNbO3 has a trigonal crystal structure, there are 3 poles in a single crystal of LiNbO3(014). In the case of lithium niobate film, it is known that epitaxial growth is performed in a state of twinning, where the crystals are symmetrically coupled about the c-axis. In this case, since the 3 poles are symmetrically coupled in two states, there are 6 poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate with a (100) plane, since the substrate is fourfold symmetric, 4×3=12 poles are observed. Furthermore, in this invention, the lithium niobate film epitaxially grown in a twinning state is also included in the epitaxial film.

[0058] The lithium niobate film is composed of Li x NbA y O z 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 that can be 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.

[0059] The thickness of the lithium niobate film is preferably 2 μm or less. This is because if the film thickness is greater than 2 μm, it is difficult to form a high-quality film. On the other hand, if the lithium niobate film is too thin, the light confinement within the lithium niobate film weakens, and light leaks to the substrate 1 or the buffer layer 3. Even when an electric field is applied to the lithium niobate film, there is concern that the effective refractive index change of the first and second optical waveguides 10a and 10b will be reduced. Therefore, the lithium niobate film is preferably at least about 1 / 10 of the wavelength of the light used.

[0060] As a method for forming a lithium niobate film, sputtering, CVD, or sol-gel methods are preferred. The c-axis of the lithium niobate is oriented perpendicular to the main surface of the substrate 1, and an electric field is applied parallel to the c-axis, causing the optical refractive index to change proportionally to the electric field. When using sapphire as a single-crystal substrate, the lithium niobate film can be directly epitaxially grown on the sapphire single-crystal substrate. When using silicon as a single-crystal substrate, the lithium niobate film is formed by epitaxial growth via a cladding layer (not shown in the figure). As the cladding layer (not shown in the figure), a material with a lower refractive index than the lithium niobate film and suitable for epitaxial growth is used. For example, when Y₂O₃ is used as the cladding layer (not shown in the figure), a high-quality lithium niobate film can be formed.

[0061] Furthermore, as a method for forming lithium niobate films, methods for thinly polishing or slicing lithium niobate single-crystal substrates are also known. These methods have advantages such as being able to obtain properties similar to those of single crystals, and are applicable to the present invention.

[0062] In this embodiment, such as Figure 2 As shown, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by the line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide 10a (10b), the side surface S3 of the ridge 2r has a slope change point P1. In other words, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by the line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide 10a (10b), the side surface S3 of the ridge 2r is composed of two straight lines L1 and L2 with different slopes.

[0063] In this way, it can be assumed that the slopes of the upper and lower parts of the side of the ridge are different, so that the width of the upper part of the ridge is narrower than that of the lower part, thereby reducing the driving voltage, and the light propagation loss can be reduced by making the cross-section of the ridge approximately circular.

[0064] Furthermore, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide 10a (10b), the angle α1 formed between the lowermost side surface of the ridge 2r (also called the "lower side surface," as in the following description) and the upper surface S1 of the substrate 1 is preferably 60 degrees or more and 90 degrees or less. Also, the angle α1 refers to the angle on the inner side of the ridge side when the extension line of the lowermost side surface of the ridge intersects a straight line parallel to the upper surface of the substrate in the cross-section. The definition of angle α1 is the same in the following description.

[0065] Furthermore, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide 10a (10b), the angle α2 formed by the uppermost side surface S32 of the ridge 2r and the upper surface S1 of the substrate 1 is preferably greater than 0 degrees and less than 80 degrees. Also, the angle α2 refers to the angle on the inner side of the ridge side when the extension of the uppermost side surface of the ridge intersects a straight line parallel to the upper surface of the substrate in the cross-section. The definition of angle α2 is the same in the following description.

[0066] In this way, the optical waveguide 10a (10b) rises steeply from the side of the substrate 1 in a straight line or approximately vertically, and then the slope of the side with a slope change point changes. Thus, the upper part reaches the top of the optical waveguide smoothly with multiple slope change points.

[0067] Furthermore, the angle α1 between the lower side surface S31 of the ridge 2r and the upper surface S1 of the substrate 1 is greater than the angle α2 between the uppermost side surface S32 of the ridge 2r and the upper surface S1 of the substrate 1. In other words, as the ridge moves upward, the angles between each side surface of the lower side surface S31 and the upper side surface S32 and the upper surface S1 of the substrate 1 decrease.

[0068] Furthermore, when viewed from the direction of light propagation, i.e., in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide 10a (10b), the width T1 of the uppermost part of the ridge 2r is less than the width T2 of the lowermost part, preferably 40% to 75% of the width T2 of the lowermost part. Also, the width T1 of the uppermost part of the ridge 2r, when the upper part of the ridge 2r is trapezoidal, refers to the length of the upper side of the trapezoid. The definition of the width of the uppermost part of the ridge is the same in the following description.

[0069] (A variation of the first embodiment)

[0070] Figure 3The optical modulator involved in the modification of the first embodiment of the present invention corresponds to... Figure 2 A cross-sectional view. In the first embodiment, an example is given of using a buffer layer 3 made of the same material to cover the upper surface S4 and the side surface S3 of the ridge 2r; however, it is not limited to this, and other embodiments may also be used. Figure 3 The material of the buffer layer covering the upper surface S4 of the ridge 2r is different from the material of the buffer layer covering the side surface S3 of the ridge 2r.

[0071] That is, such as Figure 3 As shown, the buffer layer 3' involved in this modified example includes a first buffer layer 31 and a second buffer layer 32. The first buffer layer 31 is formed between the ridges 2r and on the upper surface S2 of the waveguide layer 2. The second buffer layer 32 is formed on the upper surface S5 of the first buffer layer 31 and the upper surface S4 of the ridges 2r. The materials of the first buffer layer 31 and the second buffer layer 32 are different from each other. In this way, by making the materials of the first buffer layer and the second buffer layer different from each other, the light propagation loss can be reduced more effectively.

[0072] (Second Implementation)

[0073] Figure 4 The optical modulator according to the second embodiment of the present invention corresponds to Figure 2 A cross-sectional view. The only difference between the optical modulator of the second embodiment and the optical modulator of the first embodiment is the shape of the ridge 2r'. The other structures are the same as those of the optical modulator of the first embodiment, and therefore their description is omitted.

[0074] Specifically, such as Figure 4 As shown, when viewed from the direction of light propagation, i.e., in a cross-sectional view taken by line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide, the side surface S3' of the ridge 2r' has multiple (in this case, two) points of varying slope, P1' and P2'. In other words, when viewed from the direction of light propagation, i.e., in a cross-sectional view taken by line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide, the side surface S3' of the ridge 2r' is composed of three straight lines L1', L2', and L3' with different slopes.

[0075] Furthermore, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position in the Y-axis direction of the optical waveguide, the angle α1 formed by the lower side surface S31' of the ridge 2r' and the upper surface S1 of the substrate 1 is preferably 60 degrees or more and 90 degrees or less.

[0076] Furthermore, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position along the Y-axis of the optical waveguide, the angle β formed by the side surface S32' of the second segment of the ridge 2r' and the upper surface S1 of the substrate 1 is preferably greater than 0 degrees and less than 80 degrees. Also, the angle β refers to the angle on the inner side of the ridge side when the extension of the side surface of the second segment of the ridge intersects a straight line parallel to the upper surface of the substrate in the cross-section.

[0077] Furthermore, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position in the Y-axis direction of the optical waveguide, the angle α2 formed by the side surface S33' of the uppermost segment of the ridge 2r' and the upper surface S1 of the substrate 1 is preferably greater than 0 degrees and less than 80 degrees.

[0078] Furthermore, the angle α1 formed by the lower side surface S31' of the ridge 2r' and the upper surface S1 of the substrate 1 is greater than the angle β formed by the side surface S32' of the second segment of the ridge 2r' and the upper surface S1 of the substrate 1, and the angle β formed by the side surface S32' of the second segment of the ridge 2r' and the upper surface S1 of the substrate 1 is greater than the angle α2 formed by the side surface S33' of the uppermost segment of the ridge 2r' and the upper surface S1 of the substrate 1. In other words, as the orientation increases upwards, the angles formed by each of the sides—the lower side surface S31', the second side surface S32', and the uppermost side surface S33'—with the upper surface S1 of the substrate 1 decrease.

[0079] Furthermore, when viewed from the direction of light propagation, that is, in a cross-sectional view taken by a line A-A' perpendicular to the direction of light propagation at any position in the Y-axis direction of the optical waveguide, the width T1 of the uppermost part of the ridge 2r' is less than the width T2 of the lowermost part, preferably 40% to 75% of the width T2 of the lowermost part.

[0080] (Example)

[0081] Fabricate optical waveguides with the cross-sectional shapes shown in Table 1 respectively. Figures 5-13 Examples 1-11 of the optical waveguides shown, and Comparative Example 1 (an optical waveguide with a rectangular cross-section without a slope change point), were used to measure their electric field efficiency Vπ(V)@1550nm and propagation loss PL (dB / cm), and the results are shown in Table 1. Furthermore, as... Figures 5-6 As shown in Figures 12-13, the shapes of the optical waveguides in Embodiments 1-2 and 8-9 correspond to the first embodiment. The shapes of the optical waveguides in Embodiments 10-11 are the same as those in Embodiments 1-2 and 8-9, corresponding to the first embodiment. Figures 7-9 As shown, the shapes of the optical waveguides in Examples 3-5 correspond to those in the second embodiment. Additionally, as... Figures 10-11As shown, in Examples 6 and 7, the optical waveguide has a stepped shape and the upper corners are cut off. Table 1 shows the positions of the initial slope change points (the points where α1 or β is generated) counting from the substrate, with the waveguide height set to 100%, and the positions of the initial slope change points (the points where α2 is generated) counting from the top of the waveguide, with the waveguide height set to 100%. Furthermore, (1.98-PL) / Vπ is shown. This data shows that a smaller value for Vπ is preferred as the denominator, and a larger value (representing the propagation amount compared to the characteristic of Comparative Example 1, 1.98 minus the loss) is preferred as the numerator. Therefore, the larger the value of (1.98-PL) / Vπ, the better the balance between Vπ and PL, resulting in superior characteristics.

[0082] [Table 1]

[0083]

[0084]

[0085] As shown in Table 1 above, by making the angle α1 between the lower side of the optical waveguide and the upper surface of the substrate greater than 60 degrees and the angle α2 between the uppermost side of the optical waveguide and the upper surface of the substrate greater than 0 degrees and less than 80 degrees, it is possible to reduce the light propagation loss more effectively while basically maintaining a low driving voltage.

[0086] The height of the initial slope change point counting from the substrate is preferably 50% or more of the optical waveguide height, more preferably 59 to 85%.

[0087] The height of the initial slope change point, counting from the top of the optical waveguide, is preferably 60% or more of the optical waveguide height, more preferably 80-90%.

[0088] The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Various modifications can be made without departing from the spirit of the present invention, and these modifications are also included within the scope of the present invention.

[0089] For example, in the above embodiment, an optical modulator having a pair of optical waveguides 10a and 10b formed by an epitaxially grown lithium niobate film on substrate 1 is described. However, the present invention is not limited to this structure, and an optical modulator with optical waveguides formed by electro-optic materials such as barium titanate or lead zirconate titanate can also be used. In addition, as the waveguide layer 2, semiconductor materials, polymer materials, etc., that have electro-optic effects can also be used.

[0090] Furthermore, in the above embodiments, examples were given of the sides of the spine having one slope change point and two slope change points, respectively. However, the sides of the spine may also have three or more slope change points.

[0091] In addition, in the above embodiments, a pair of optical waveguides 10a and 10b are listed. However, as long as the function of optical communication or optical measurement can be realized, there may be only one optical waveguide, or there may be more than three optical waveguides.

[0092] Furthermore, in the above embodiment, the side of the ridge is composed of multiple straight lines; however, it is not limited to this, such as... Figure 14 As shown, there are also cases where one or more lines forming the side of the ridge become curves.

[0093] Furthermore, while the above embodiments illustrate the application of the present invention to optical modulators, the present invention can also be applied to any electro-optical device capable of realizing optical communication or optical measurement functions, such as optical switches, optical resonators, optical branching circuits, sensor elements, and millimeter-wave generators.

Claims

1. An electro-optical device, characterized in that, The device includes a substrate, an optical waveguide formed on the substrate, a buffer layer formed on the substrate to cover the optical waveguide, and electrodes formed on the buffer layer. The optical waveguide is composed of a ridge. When viewed from the direction of light propagation, the side of the optical waveguide has multiple points where the slope changes. When viewed from the direction of light propagation, the angle between the lower side of the lowest segment of the optical waveguide and the upper surface of the substrate is 60 degrees or more.

2. The electro-optic device as described in claim 1, characterized in that, The width of the uppermost part of the optical waveguide is 40% to 75% of the width of the lowermost part.

3. The electro-optic device as described in claim 1 or 2, characterized in that, When viewed from the direction of light propagation, the angle between the uppermost side of the optical waveguide and the upper surface of the substrate is greater than 0 degrees and less than 80 degrees.

4. The electro-optic device as described in claim 1 or 2, characterized in that, The optical waveguide is a film composed of LiNbO3 or LiTaO3.

5. The electro-optic device as described in claim 1 or 2, characterized in that, The optical waveguide is a film formed by doping LiNbO3 with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb, and Lu.

6. The electro-optic device as described in claim 1 or 2, characterized in that, The optical waveguide is an epitaxial film.

7. The electro-optic device as described in claim 6, characterized in that, The epitaxial film is oriented in a direction that intersects with the substrate.

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