Optical devices

Abstract

To provide an optical device.SOLUTION: An optical device is provided, comprising a substrate and an optical waveguide formed on the substrate, and has a protruded portion formed adjacent to the optical waveguide on the substrate. The present invention provides a highly reliable optical device that enables further reduction of light transmission loss.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an optical device used in the field of optical communication or optical measurement.

Background Art

[0002] With the spread of the Internet, the communication volume has increased rapidly, and the importance of optical fiber communication has been extremely enhanced. Optical fiber communication converts an electrical signal into an optical signal and transmits the optical signal through an optical fiber, and has characteristics of wide bandwidth, low loss, and strong noise resistance.

[0003] As methods for converting an electrical signal into an optical signal, a direct modulation method using a semiconductor laser and an external modulation method using an optical modulator are known. Direct modulation does not require an optical modulator and has a low cost, but there is a limit in high-speed modulation, and in high-speed and long-distance applications, the external modulation method is adopted.

[0004] As an optical modulator, an optical modulator using an optical waveguide made of lithium niobate (LiNbO3, hereinafter referred to as "LN") has merits of high speed, low loss, and little distortion of a control optical waveform, but has demerits of high driving voltage and large size compared with semiconductor optical devices.

[0005] In order to overcome the above demerits, an optical device with significant miniaturization and low driving voltage has been realized by an optical waveguide using an LN film formed by applying a thin film technology on a sapphire substrate, as compared with the conventional one (see Patent Documents 1 and 2). In such an optical device, there is a problem that the propagation loss of light due to leakage of light transmitted in the optical waveguide to the side becomes large. In order to enhance the confinement of light transmitted in the optical waveguide in the lateral direction, it is disclosed in Patent Document 3 that a waveguide with strong confinement can be realized by forming a groove portion in the vicinity of the lateral direction of the optical waveguide.

[0006] However, there is a need for more reliable optical devices that can further suppress light propagation loss. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2006-195383 [Patent Document 2] Japanese Patent Publication No. 2014-6348 [Patent Document 3] Japanese Patent Publication No. 2005-292245 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] This invention has been made in view of the above-mentioned problems, and aims to provide a more reliable optical device that can further suppress the loss of light propagation.

[0009] As a result of repeated research into the mechanism of light propagation loss, the inventors have discovered a completely new problem that has not been previously discovered by those skilled in the art. Specifically, in optical devices utilizing the thin-film technology described above, the LN film is exposed to chemicals or pure water during the manufacturing process, or to gases or oxygen streams during the annealing process, and in the resist stripping process, the wafer is placed in a liquid and vibrated, or suspended in a liquid. Under these conditions, the LN film is subjected to external stress, and stress concentration in the LN film causes defects in the optical waveguide. Furthermore, defects in the optical waveguide lead to light propagation loss in the optical waveguide. In other words, the inventors have newly discovered that, in addition to the leakage of light to the sides as shown in Patent Document 3, stress concentration during the process described above is also a major factor causing light propagation loss. Based on the above-mentioned entirely new challenges, the inventors conducted further research on the structure of LN films and discovered a completely new structure, previously unthinkable in the field of LN films, in which a convex portion is formed adjacent to the optical waveguide, and stress is dispersed by utilizing this convex portion. This led to the completion of the present invention. [Means for solving the problem]

[0010] That is, an optical device according to one aspect of the present invention comprises a substrate and an optical waveguide formed on the substrate, characterized in that a protrusion is formed on the substrate adjacent to the optical waveguide.

[0011] In this optical device, the protrusions formed on the substrate adjacent to the optical waveguide allow for stress distribution using these protrusions, thereby preventing damage to the optical waveguide caused by stress concentration, and further reducing light propagation loss and improving reliability.

[0012] Furthermore, in the optical device relating to one aspect of the present invention described above, it is preferable that the slopes of the left and right bases of the protrusion are different. In this way, stress can be more effectively dispersed, preventing damage to the optical waveguide.

[0013] Furthermore, in the optical device relating to one aspect of the present invention described above, it is preferable that the convex portion is formed such that the first side and the second side intersect in a cross section perpendicular to the direction of light transmission.

[0014] Furthermore, in the optical device relating to one aspect of the present invention described above, it is preferable that at least one of the first side and the second side is inclined with respect to the upper surface of the substrate.

[0015] Furthermore, in the optical device relating to one aspect of the present invention described above, it is preferable that both the first side and the second side are inclined with respect to the upper surface of the substrate, and that the inclination of the first side and the inclination of the second side are different.

[0016] Furthermore, in the optical device relating to one aspect of the present invention described above, it is preferable that the second side is parallel to the substrate or has a smaller inclination than the first side which is located closer to the optical waveguide than the second side. In this way, stress can be more effectively distributed and the occurrence of damage to the optical waveguide can be prevented.

[0017] Furthermore, in the optical device relating to one aspect of the present invention described above, it is preferable that the peaks of the protrusions are spaced at a distance of 40 to 150% of the height of the optical waveguide from the side surface of the optical waveguide. In this way, stress can be more effectively distributed, preventing damage to the optical waveguide.

[0018] Furthermore, in the optical device relating to one aspect of the present invention described above, the height of the protrusion is preferably 5 to 100% of the height of the optical waveguide. In this way, stress can be more effectively distributed and damage to the optical waveguide can be prevented.

[0019] Furthermore, in the optical device relating to one aspect of the present invention described above, the optical waveguide is preferably a film made of LiNbO3 or LiTaO3.

[0020] Also, in the optical device according to one aspect of the present invention described above, it is preferable that the optical waveguide is a film in which LiNbO3 is doped with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb, and Lu.

[0021] Also, in the optical device according to one aspect of the present invention described above, it is preferable that the optical waveguide is an epitaxial film.

[0022] Also, in the optical device according to one aspect of the present invention described above, it is preferable that the epitaxial film is oriented in a direction intersecting the substrate.

[0023] The optical modulator according to another aspect of the present invention includes a substrate and an optical waveguide formed on the substrate, and is characterized in that a convex portion is formed on the substrate adjacent to the optical waveguide.

[0024] According to the present invention, it is possible to provide an optical device with higher reliability that can further suppress the propagation loss of light.

Brief Description of the Drawings

[0025] <( [Figure 1(a)] FIGS. 1(a) and 1(b) are top views of an optical modulator according to a first embodiment of the present invention. FIG. 1(a) shows only the optical waveguide, and FIG. 1(b) shows the entire optical modulator including the traveling wave electrode. [Figure 1(b)] FIGS. 1(a) and 1(b) are top views of an optical modulator according to a first embodiment of the present invention. FIG. 1(a) shows only the optical waveguide, and FIG. 1(b) shows the entire optical modulator including the traveling wave electrode. [Figure 2] FIG. 2 is a cross-sectional view taken along line A-A' of the optical modulator according to an embodiment of the present invention. [Figure 3] FIG. 3 is a cross-sectional view corresponding to FIG. 2 according to a modified example of the first embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0026] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. In the description of the drawings, the same or equivalent elements are assigned the same reference numerals, and redundant descriptions are omitted.

[0027] (First Embodiment) Figures 1(a) and 1(b) are top views of an optical modulator according to a first embodiment of the present invention, with Figure 1(a) showing only the optical waveguide and Figure 1(b) showing the entire optical modulator including the traveling wave electrode. As shown in Figures 1(a) and 1(b), the optical modulator 100 as an optical device includes a Mach-Zehnder optical waveguide 10 having first and second optical waveguides 10a and 10b formed on a substrate 1 and installed parallel to each other, a first signal electrode 20a installed along the first optical waveguide 10a, a second signal electrode 20b installed along the second optical waveguide 10b, a first bias electrode 30a installed along the first optical waveguide 10a, and a second bias electrode 30b installed 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.

[0028] The Mach-Zehnder optical waveguide 10 is an optical waveguide having the structure of a Mach-Zehnder interferometer. It has first and second optical waveguides 10a and 10b branched from a single input optical waveguide 10i by a demultiplexer 10c, and the first and second optical waveguides 10a and 10b are combined into a single output optical waveguide 10o via a multiplexer 10d. The input optical light Si is demultiplexed in the demultiplexer 10c and travels along the first and second optical waveguides 10a and 10b respectively, then is combined in the multiplexer 10d and output as modulated optical light So from the output optical waveguide 10o.

[0029] The first and second signal electrodes 20a and 20b are linear electrode patterns that overlap with the first and second optical waveguides 10a and 10b in the top view, and both ends extend to electrode pads installed near the edge 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 to electrode pads installed near the edge face of the substrate 1 to form a signal input port, and the drive circuit 60a is connected to the signal input port. In addition, the other ends 20a2 and 20b2 of the first signal electrode 20a and the second signal electrode 20b are led to electrode pads installed near the edge face of the substrate 1 and are connected to each other via terminal resistors 60b. As a result, the first and second signal electrode signals 20a and 20b function as differential coplanar traveling wave electrodes.

[0030] The first and second bias electrodes 30a and 30b are independent of the first and second signal electrode signals 20a and 20b, respectively, so as to apply a DC voltage (DC bias) to the first and second optical waveguides 10a and 10b. One end 30a1 and 30b1 of the first bias electrode 30a and the second bias electrode 30b are extended to an electrode pad located 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 areas of the first bias electrode 30a and the second bias electrode 30b are located on the output end side of the Mach-Zehnder optical waveguide 10 than the formation areas of the first signal electrode 20a and the second signal electrode 20b, but they may also be located on the input end side.

[0031] As described above, the first and second signal electrode signals 20a and 20b apply RF signals to the first and second optical waveguides 10a and 10b, and the first and second bias electrodes 30a and 30b apply DC bias to the first optical waveguide 10a and the second optical waveguide 10b.

[0032] Difference signals (modulated signals) having 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 optical waveguide 10a and the second optical waveguide 10b are made of a material that exhibits an electro-optic effect, such as lithium niobate, the electric field applied to the first optical waveguide 10a and the second optical waveguide 10b changes the refractive indices of the first optical waveguide 10a and the second optical waveguide 10b, for example, by +Δn and -Δn respectively, and the phase difference between the pair of optical waveguides changes. The signal light modulated by this change in phase difference is output from the output waveguide 10o.

[0033] In this embodiment, the first and second optical waveguides 10a and 10b are films made of LiNbO3, but are not limited to this. The first and second optical waveguides 10a and 10b may also be films made of LiTaO3, or LiNbO3 that is doped with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb, and Lu.

[0034] In this embodiment, the first and second optical waveguides 10a and 10b are epitaxial films. The epitaxial films are preferably oriented in a direction intersecting the substrate 1 as described below.

[0035] Figure 2 is a cross-sectional view taken along line A-A' of an optical modulator according to a first embodiment of the present invention. As shown in Figure 2, the optical modulator 100 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. 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 an optical waveguide 10b consisting of a ridge portion 2r. In Figure 2, only the optical waveguide 10b is shown, but the optical waveguide 10a and structures such as protrusions formed adjacent to the optical waveguide 10a are the same as those of the optical waveguide 10b, so their illustration and description are omitted here.

[0036] Furthermore, a protrusion 5 is formed on the substrate 1 along the optical waveguide 10b. The protrusion 5 is formed on the substrate 1 adjacent to the optical waveguide 10b.

[0037] The convex portion 5 may be either higher than its surroundings to form a mountain shape, or it may be formed by excavating the surrounding area to create a mountain shape. Preferably, the final product, in its cross-section, is not too high in relation to the height of the optical waveguide, and the peak of the mountain is positioned not too far from the optical waveguide.

[0038] In this embodiment, the slopes of the left and right edges of the peak P in the convex portion 5 are different. Specifically, the convex portion 5 is formed by the intersection of a first edge L1 and a second edge L2, which are inclined at different angles with respect to the upper surface S of the substrate 1, in a cross section perpendicular to the direction of light transmission (i.e., the cross section shown in Figure 2). The first edge L1 and the second edge L2 intersect at the peak P of the convex portion 5, with the first edge L1 extending to the upper surface S of the substrate 1 and the second edge L2 extending to the upper surface S of the substrate 1.

[0039] In this embodiment, the convex portion 5 obtained by the intersection of the first side L1 and the second side L2 is a pointed peak, but it is not limited to this, and the peak may be rounded or flat.

[0040] In this embodiment, both the first side L1 and the second side L2 are inclined with respect to the upper surface S of the substrate 1, but the embodiment is not limited to this. It is sufficient that at least one of the first side L1 and the second side L2 is inclined with respect to the upper surface of the substrate 1. For example, the first side L1 may be inclined with respect to the upper surface S of the substrate 1, and the second side L2 may be parallel to the substrate 1.

[0041] In this optical modulator, the protrusions formed on the substrate adjacent to the optical waveguide allow for stress distribution using these protrusions, thereby preventing damage to the optical waveguide caused by stress concentration, and further reducing light propagation loss and improving reliability.

[0042] Furthermore, in this embodiment, the inclination of the first side L1 and the inclination of the second side L2 are different, but the embodiment is not limited to this, and the inclination of the first side L1 and the inclination of the second side L2 may be the same.

[0043] Furthermore, if the inclination of the first side L1 and the inclination of the second side L2 are different, it is preferable that the inclination of the second side L2 is smaller than that of the first side L1, which is located closer to the optical waveguide than the second side L2. In this way, stress can be distributed more effectively, preventing damage to the optical waveguide.

[0044] Furthermore, in this embodiment, it is preferable that the peak P of the protrusion 5 is spaced at a distance t equal to 40-150% of the height h of the optical waveguide 10b from the side surface S1 of the optical waveguide 10b. In this way, stress can be more effectively distributed, preventing damage to the optical waveguide. The height h of the optical waveguide 10b is the rise height from the bottom. The bottom is defined as the point at the shortest distance measured from the side of the substrate 1 opposite to the side where the optical waveguide 10b is formed.

[0045] As an example, although not particularly limited, t can be designed within the range of 0.1 to 3 μm, and h can be designed within the range of 0.01 to 2 μm.

[0046] Furthermore, in this embodiment, the height h1 of the peak P of the protrusion 5 is preferably 5 to 100% of the height h of the optical waveguide 10b. In this way, stress can be more effectively distributed and damage to the optical waveguide can be prevented.

[0047] The buffer layer 3 is a layer formed on the substrate 1 to prevent light transmitted in the optical waveguides 10a and 10b from being absorbed by the electrodes 20a and 20b. It covers the entire area of ​​the upper surface of the waveguide layer 2 where the ridge portion 2r is not formed, as well as the sides and upper surface of the ridge portion 2r. The buffer layer 3 only needs to function as an intermediate layer between the optical waveguide and the electrodes, and can be broadly selected as long as the material of the buffer layer 3 is nonmetallic. For example, the buffer layer 3 may be a ceramic layer made of an insulating material such as a metal oxide, metal nitride, or metal carbide. The material of the buffer layer 3 may be crystalline or amorphous. Preferably, the buffer layer 3 is made of a material with a lower refractive index than the waveguide layer 2 and high transparency. For example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc., may be used. The thickness of the buffer layer 3 formed in the optical waveguide may be about 0.2 to 1.2 μm. In this embodiment, the buffer layer 3 covers not only the upper surfaces of the optical waveguides 10a and 10b, but also the entire area of ​​the upper surface of the waveguide layer 2 where the ridge portion 2r is not formed, and the sides of the ridge portion 2r. However, the buffer layer 3 only needs to be formed on the upper surface of the ridge portion 2r of the waveguide layer 2, and the buffer layer 3 may be patterned to selectively cover only the vicinity of the upper surfaces of the optical waveguides 10a and 10b.

[0048] To reduce light absorption by the electrodes, a thicker buffer layer 3 is preferable, while to apply a high electric field to the optical waveguides 10a and 10b, a thinner buffer layer 3 is preferable. Since there is a so-called "trade-off" relationship between the light absorption of the electrodes and the applied electrode, it is necessary to set an appropriate film thickness according to the purpose. A higher dielectric constant of the buffer layer 3 is preferable because it can reduce Vπ(V) (an index representing electric field efficiency), and a lower refractive index of the buffer layer 3 is preferable because it can make the buffer layer 3 thinner. Generally, materials with high dielectric constants also have high refractive indices, so it is important to select a material with a high dielectric constant and a relatively low refractive index, taking into account the balance between the two. As an example, Al2O3 has a relative permittivity of about 9 and a refractive index of about 1.6, making it a preferable material. LaAlO3 has a relative permittivity of about 13 and a refractive index of about 1.7, and LaYO3 has a relative permittivity of about 17 and a refractive index of about 1.7, making them particularly preferable materials.

[0049] The electrode layer 4 is installed on the buffer layer 3. The electrode layer 4 contains the first and second signal electrodes 20a and 20b, and the first and second bias electrodes 30a and 30b. The first and second signal electrodes 20a and 20b, and the first and second bias electrodes 30a and 30b are installed overlapping with the ridge portions 2r corresponding to the first and second optical waveguides 10a and 10b, respectively, and face the first and second optical waveguides 10a and 10b via the buffer layer 3. In Figure 2, only the electrode 20b that is installed overlapping with the ridge portion 2r corresponding to the optical waveguide 10b and faces the optical waveguide 10b via the buffer layer 3 is shown.

[0050] The waveguide layer 2 is not particularly limited as long as it is an electro-optic material, but it is preferably made of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optic constant and is suitable as a constituent material for optical devices such as optical modulators. The structure of this embodiment when the waveguide layer 2 is a lithium niobate film will be described in detail below.

[0051] The substrate 1 is not particularly limited as long as it has a refractive index lower than that of the lithium niobate film, but a substrate on which the lithium niobate film can be formed as an epitaxial film is preferred, and a sapphire single crystal substrate or a silicon single crystal substrate is preferred. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film has the property of being easily formed as a c-axis oriented epitaxial film on single crystal substrates of various crystal orientations. Since the c-axis oriented lithium niobate film has triple symmetry, it is desirable that the underlying single crystal substrate also has the same symmetry, and a c-plane substrate is preferred in the case of a sapphire single crystal substrate, and a (111)-plane substrate is preferred in the case of a silicon single crystal substrate.

[0052] Here, an epitaxial film is a film whose crystal orientation is aligned with that of the underlying substrate or film. When the film plane is considered 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, an epitaxial film can be proven by firstly confirming the peak intensity at the orientation position using 2θ-θ X-ray diffraction, and secondly confirming the poles.

[0053] Specifically, firstly, when measurements are performed by 2θ-θ X-ray diffraction, the peak intensities of all planes other than the target plane must 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 of planes other than 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) and (002).

[0054] Secondly, in pole measurement, it is necessary for the poles to be visible. The conditions for confirming the peak intensity at the first orientation position described above only indicate orientation in one direction. Even if the first condition is met, if the crystal orientation is not aligned within the plane, the X-ray intensity will not increase at a specific angular position, and no poles will be observed. Since LiNbO3 has a trigonal crystal structure, there are three poles in LiNbO3(014) in a single crystal. In the case of lithium niobate films, it is known that epitaxial growth occurs in a so-called twinned state, where crystals rotated 180° around the c-axis are symmetrically bonded. In this case, since the three poles are symmetrically bonded in pairs, there are six poles. Furthermore, when a lithium niobate film is formed on a (100) plane silicon single crystal substrate, the substrate is symmetrical four times, so 4 × 3 = 12 poles are observed. In this invention, lithium niobate films grown epitaxially in a twinned state are also included in the definition of epitaxial films.

[0055] The composition of the lithium niobate film is LixNbAyOz. A represents an element other than Li, Nb, and O. x is 0.5 to 1.2, preferably 0.9 to 1.05. y is 0 to 0.5. z is 1.5 to 4, preferably 2.5 to 3.5. A can be any element such as K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, or a combination of two or more elements.

[0056] The thickness of the lithium niobate film should preferably be 2 μm or less. This is because it becomes difficult to form a high-quality film if the film thickness exceeds 2 μm. On the other hand, if the lithium niobate film is too thin, the light confinement in the lithium niobate film will weaken, and light will leak into the substrate 1 and buffer layer 4. Applying an electric field to the lithium niobate film may result in a small change in the effective refractive index of the first and second optical waveguides 10a and 10b. Therefore, the lithium niobate film should preferably have a thickness of at least 1 / 10 of the wavelength of the light used.

[0057] For forming lithium niobate films, it is desirable to use film formation methods such as sputtering, CVD, or sol-gel methods. The c-axis of the lithium niobate is oriented perpendicular to the main surface of the substrate 1, and by applying an electric field parallel to the c-axis, the optical refractive index changes in proportion to the electric field. When sapphire is used as the single crystal substrate, the lithium niobate film can be epitaxially grown directly on the sapphire single crystal substrate. When silicon is used as the single crystal substrate, the lithium niobate film is formed by epitaxial growth via a cladding layer (not shown). The cladding layer (not shown) should have a lower refractive index than the lithium niobate film and be suitable for epitaxial growth. For example, using Y2O3 as the cladding layer (not shown) allows for the formation of a high-quality lithium niobate film.

[0058] Furthermore, a method for forming lithium niobate films is also known, which involves thinly polishing or slicing a lithium niobate single crystal substrate. This method has the advantage of obtaining the same properties as a single crystal and can be applied to the present invention.

[0059] (Modified version of the first embodiment) Figure 3 is a cross-sectional view corresponding to Figure 2, showing a modified example of the first embodiment of the present invention. In the first embodiment, an example was shown in which the upper surface and side surface of the ridge portion 2r are covered with a buffer layer 3 made of the same material, but the invention is not limited to this, and as shown in Figure 3, the material of the buffer layer covering the upper surface of the ridge portion 2r and the material of the buffer layer covering the side surface of the ridge portion 2r may be different.

[0060] That is, as shown in Figure 3, the buffer layer 3' according to this modified example has a first buffer layer 31 and a second buffer layer 32. The first buffer layer 31 is formed between the ridge portions 2r and on the upper surface of the waveguide layer 2. The second buffer layer 32 is formed on the upper surface of the first buffer layer 31 and on the upper surface of the ridge portions 2r. The materials of the first buffer layer 31 and the second buffer layer 32 are different from each other. By making the material of the first buffer layer different from the material of the second buffer layer in this way, it is possible to more effectively suppress light propagation loss and further improve reliability.

[0061] Examples As described above, optical modulators with protrusions formed in Examples 1 to 11 and optical waveguides with protrusions, and optical modulators with a conventional optical waveguide without protrusions (Comparative Example 1) were created, and the failure rates of these optical waveguides were measured. The results are shown in Table 1. In Table 1, the ratio of the distance t from the side surface S1 of the peak P of the protrusion 5 to the height h of the optical waveguide 10b is denoted as "t / h", the ratio of the height h1 of the peak P of the protrusion 5 to the height h of the optical waveguide 10b is denoted as "h1 / h", and the ratio of the height h1 of the peak P of the protrusion 5 to the distance t from the side surface S1 of the peak P of the protrusion 5 to the height t of the optical waveguide 10b is denoted as "h1 / t". In Table 1, the ratios t / h and h1 / h are expressed as % (percentages), and the optical waveguide failure rates are also expressed as %.

[0062] [Table 1]

[0063] As can be seen from Table 1 above, by forming a protrusion adjacent to the optical waveguide on the substrate, stress can be distributed using this protrusion. This prevents damage to the optical waveguide due to stress concentration, reduces the failure rate of the optical waveguide, and more effectively further suppresses light propagation loss and improves reliability. In particular, from the results in Table 1, the ratio of h1 to t is preferably h1 / t = 0.05 to 1.17. More preferably 0.08 to 0.7, even more preferably 0.27 to 0.7, and most preferably 0.45 to 0.7.

[0064] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications may be made without departing from the spirit of the invention, and of course these are also included within the scope of the present invention.

[0065] For example, in the above embodiment, an optical modulator comprising a pair of optical waveguides 10a and 10b made of a lithium niobate film epitaxially grown on a substrate 1 was illustrated. However, the present invention is not limited to such a structure, and an optical modulator may also be made in which the optical waveguides are formed from electro-optic materials such as barium titanate or lead zirconite. Furthermore, semiconductor materials, polymer materials, etc., that have an electro-optic effect may be used as the waveguide layer 2.

[0066] Furthermore, although a pair of optical waveguides 10a and 10b were shown in the above embodiment, only one optical waveguide may be used, or three or more optical waveguides may be used, as long as they perform the function of optical communication or optical measurement.

[0067] Furthermore, although the above embodiments show an example of applying the present invention to an optical modulator having electrodes (first and second signal electrodes 20a, 20b, and first and second bias electrodes 30a, 30b), electrodes are not essential. The present invention can, of course, be applied to devices without electrodes as long as the substrate has a structure in which a protrusion is formed adjacent to the optical waveguide.

[0068] Furthermore, in the above embodiment, the ridge portion 2r is shown in the drawings as being formed vertically, but it is not limited to this, and the ridge portion 2r may have an overhanging shape or an inverted trapezoidal shape, and the side surface of the ridge portion 2r may have irregularities. It is preferable that the side surface of the ridge portion 2r has nanometer-order fine irregularities or streaky irregularities formed by thin film patterning.

[0069] In particular, when forming a buffer layer, having irregularities is preferable because it improves reliability due to improved adhesion.

[0070] Specific examples of applications of the present invention include any 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. [Explanation of Symbols]

[0071] 1 2 1 circuit board 2 Waveguides 2r Ridge section 3, 3' buffer layer 4 electrode layer 5. Convex part 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 20a First signal electrode 20a1 one end 20a2 Other end 20b Second signal electrode 20b1 One end 20b2 Other end 30a First bias electrode 30a1 one end 30b Second bias electrode 30b1 One end 31. First buffer layer 32. Second buffer layer 40 RF interaction part 50 DC interaction section 60a drive circuit 60b Terminal resistor 60c bias circuit 100 Optical Modulators

Claims

1. circuit board and An optical waveguide formed on the substrate, The substrate is provided with a protrusion formed adjacent to the optical waveguide, In a cross-section perpendicular to the direction of light transmission, The protrusion includes a first side facing the optical waveguide and a second side facing away from the optical waveguide, and the upper ends of the first side and the upper ends of the second side intersect at a peak. The first side slopes downward from the peak toward the optical waveguide, and the second side slopes downward in the direction away from the peak toward the optical waveguide. The inclination of the second edge with respect to the upper surface of the substrate is smaller than the inclination of the first edge with respect to the upper surface of the substrate. The peaks of the convex portion are spaced apart from the side surface of the optical waveguide by a distance of 40% to 60% of the height of the optical waveguide. The height of the protrusion is 5% to 35% of the height of the optical waveguide. An optical device characterized in that the ratio of the height of the protrusion to the distance at which the protrusion is separated from the side surface of the optical waveguide is 0.08 to 0.

70.

2. The optical device according to claim 1, characterized in that the slopes of the left and right bases of the protrusion are different.

3. The optical waveguide is LiNbO ３ Or LiTaO ３ The optical device according to claim 1 or 2, characterized in that it is a film made of the following.

4. The optical waveguide is LiNbO ３ The optical device according to any one of claims 1 to 3, characterized in that the film is doped with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb, and Lu.

5. The optical device according to any one of claims 1 to 4, characterized in that the optical waveguide is an epitaxial film.

6. The optical device according to claim 5, characterized in that the epitaxial film is oriented in a direction intersecting the substrate.

7. circuit board and An optical waveguide formed on the substrate, The substrate is provided with a protrusion formed adjacent to the optical waveguide, In a cross-section perpendicular to the direction of light transmission, The protrusion includes a first side facing the optical waveguide and a second side facing away from the optical waveguide, and the upper ends of the first side and the upper ends of the second side intersect at a peak. The first side slopes downward from the peak toward the optical waveguide, and the second side slopes downward in the direction away from the peak toward the optical waveguide. The inclination of the second edge with respect to the upper surface of the substrate is smaller than the inclination of the first edge with respect to the upper surface of the substrate. The peaks of the convex portion are spaced apart from the side surface of the optical waveguide by a distance of 40% to 60% of the height of the optical waveguide. The height of the protrusion is 5% to 35% of the height of the optical waveguide. An optical modulator characterized in that the ratio of the height of the protrusion to the distance from which the protrusion is separated from the side surface of the optical waveguide is 0.08 to 0.70.

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