Optical waveguide element, optical modulation device using same, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element in which defects in the formation of a modulation electrode having a segment electrode are suppressed and which has a high product yield.　Provided is an optical waveguide element comprising a substrate (1) in which an optical waveguide (10) is formed and modulation electrodes (2S, 2G) which are disposed on the substrate (1) and which apply an electrical field to the optical waveguide (10) in order to modulate light waves that propagate through the optical waveguide (10), said optical waveguide element being characterized in that: the modulation electrodes (2S, 2G) have a plurality of segment electrodes (SE) which are disposed in proximity to the optical waveguide (10) and that are disposed along the optical waveguide (10), and a signal transmission unit (SS) which is electrically connected to the segment electrodes (SE) and which transmits a modulation signal (S); the segment electrodes (SE) have a T shape provided with a horizontal line (L1) and a vertical line (L2); and corner parts (K1, K2) of an end part of the horizontal line (L1) are formed by a circular arc or two or more corners.

Description

Optical waveguide element, optical modulation device using the same, and optical transmitter

[0001] The present invention relates to an optical waveguide element, an optical modulation device using the same, and an optical transmission device, and more particularly to an optical waveguide element including a substrate on which an optical waveguide is formed, and a modulation electrode disposed on the substrate for applying an electric field to the optical waveguide in order to modulate light waves propagating through the optical waveguide.

[0002] In the fields of optical communications and optical measurement, optical waveguide elements, such as optical modulators, in which an optical waveguide is formed on a substrate, are widely used. In recent years, it has become necessary to miniaturize the optical waveguide elements that make up optical modulators included in transmitters built into optical transceivers in order to meet the demands for miniaturization and low power consumption.

[0003] Furthermore, in order to broaden the bandwidth of optical modulators, structures using segment electrodes as modulating electrodes have been proposed, as shown in Patent Documents 1 and 2, in order to achieve speed and impedance matching between the light waves propagating through the optical waveguide and the microwaves serving as modulating signals propagating through the modulating electrodes. FIG. 1 shows an example of a modulating electrode using segment electrodes SE. The modulating electrode includes a plurality of segment electrodes SE arranged close to an optical waveguide 10 and arranged along the optical waveguide, and a signal transmission section SS electrically connected to the segment electrodes SE and transmitting a modulating signal. In Patent Document 1, the segment electrodes are formed only on the ground electrode, while in Patent Document 2, the segment electrodes and the RF electrode serving as the signal transmission path are arranged with a dielectric layer sandwiched between them.

[0004] In devices such as High Bandwidth-Coherent Driver Modulators (HB-CDMs), the optical waveguide elements (chips) themselves have become smaller, and compared to conventional optical modulators, the electrode width and thickness have become thinner. As a result, the electrodes have become thinner, which makes resist cracks more likely to occur in areas where stress is uneven within the resist pattern during resist patterning to form the segment electrode pattern. Resist cracks can result in poor electrode formation and a significant drop in product yield.

[0005] 1, the ends of the horizontal and vertical lines of the "T" constituting the segment electrode SE, as well as the intersections of the horizontal and vertical lines, form right angles. Internal stress tends to concentrate at such right-angled portions, which can cause resist cracks in the resist pattern.

[0006] Japanese Patent No. 6075576 ​​Japanese Patent Application Laid-Open No. 2016-194544

[0007] The object of the present invention is to provide an optical waveguide element that solves the above-mentioned problems, suppresses defective formation of modulation electrodes having segment electrodes, and achieves high product yield, and further to provide an optical modulation device and an optical transmission apparatus that use such an optical waveguide element.

[0008] In order to solve the above problems, the optical waveguide element, optical modulation device, and optical transmitter of the present invention have the following technical features: (1) In an optical waveguide element comprising a substrate on which an optical waveguide is formed, and a modulation electrode disposed on the substrate for applying an electric field to the optical waveguide in order to modulate light waves propagating through the optical waveguide, the modulation electrode is disposed close to the optical waveguide and has a plurality of segment electrodes disposed along the optical waveguide, and a signal transmission section electrically connected to the segment electrodes and transmitting a modulated signal, the segment electrodes having a T-shape with a horizontal line L1 and a vertical line L2, and the corners at the ends of the horizontal line L1 are formed as an arc or with two or more corners.

[0009] (2) In the optical waveguide element described in (1) above, the end of the horizontal line L1 has a corner K1 closer to the optical waveguide and a corner K2 farther from the optical waveguide, and the corner K1 and the corner K2 are set to have a shape that is line-symmetrical with respect to a center line parallel to the longitudinal direction of the horizontal line L1.

[0010] (3) In the optical waveguide element described in (1) above, the end of the horizontal line L1 has a corner K1 closer to the optical waveguide and a corner K2 farther from the optical waveguide, and the bend formed by an arc or two or more corners at the corner K1 and the corner K2 is formed more gently at the corner K2 than at the corner K1.

[0011] (4) The optical waveguide element according to (1) above, wherein the radius of curvature of the arc is 1 μm or more.

[0012] (5) In the optical waveguide element described in (1) above, the thickness of the segment electrodes is 3 μm or less, the width of the horizontal lines L1 is 6 μm or less, and the width of the vertical lines L2 is 5 μm or more.

[0013] (6) In the optical waveguide element described in (5) above, the thickness of the signal transmission section is three times or more the thickness of the segment electrode, and the cross section of the signal transmission section perpendicular to the transmission direction of the modulated signal is formed in multiple steps.

[0014] (7) The optical waveguide element according to any one of (1) to (6) above is an optical modulation device that is housed in a housing and includes an optical fiber that inputs or outputs a light wave to or from the optical waveguide.

[0015] (8) The optical modulation device according to (7) above, further characterized in that the housing contains an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element.

[0016] (9) An optical transmitter comprising the optical modulation device according to (7) above and an electronic circuit for outputting a modulation signal that causes the optical modulation device to perform a modulation operation.

[0017] The present invention provides an optical waveguide element comprising a substrate on which an optical waveguide is formed and a modulation electrode disposed on the substrate for applying an electric field to the optical waveguide to modulate light waves propagating through the optical waveguide. The modulation electrode is disposed adjacent to the optical waveguide and includes a plurality of segment electrodes arranged along the optical waveguide, and a signal transmission section electrically connected to the segment electrodes for transmitting a modulation signal. The segment electrodes have a T-shape with a horizontal line L1 and a vertical line L2. The corners at the ends of the horizontal line L1 are formed as arcs or have two or more corners, so that the interior angle of the corners is 90 degrees or greater, thereby preventing resist cracks from occurring in the resist pattern during segment electrode formation. As a result, the production yield of the optical waveguide element can be increased. Furthermore, it is possible to provide optical modulation devices and optical transmission devices using this optical waveguide element.

[0018] 6 is a diagram showing an example in which segment electrodes are provided on both the signal electrode and the ground electrode. FIG. 6 is a plan view showing an example of an optical waveguide element of the present invention. FIG. 6 is a diagram explaining the shape of a segment electrode in an optical waveguide element of the present invention. FIG. 6 is a diagram explaining the dimensions of a segment electrode. FIG. 6 is a diagram explaining another shape of a segment electrode. FIG. 6 is a diagram explaining a state in which a segment electrode and a signal transmission section are combined. FIG. 6 is a diagram explaining the dimensions of a segment electrode and the positional relationship between a segment electrode and a signal transmission section. FIG. 6 is a cross-sectional view taken along the dashed dotted line D in FIG. 6. FIG. 6 is a cross-sectional view taken along the dashed dotted line E in FIG. 6. FIG. 6 is a diagram explaining an application example of FIG. 9. FIG. 9 is a diagram explaining an example (part 1) of application to a Z-cut substrate. FIG. 9 is a diagram explaining an example (part 2) of application to a Z-cut substrate. FIG. 9 is a diagram explaining an optical transmitting device of the present invention.

[0019] 2 and 3, the present invention provides an optical waveguide element comprising a substrate on which an optical waveguide 10 is formed, and modulation electrodes (2S, 2G) disposed on the substrate for applying an electric field to the optical waveguide 10 to modulate light waves propagating through the optical waveguide 10, wherein the modulation electrodes are disposed in proximity to the optical waveguide and have a plurality of segment electrodes SE disposed along the optical waveguide, and signal transmission sections SS electrically connected to the segment electrodes SE for transmitting modulation signals, the segment electrodes SE having a T-shape with a horizontal line L1 and a vertical line L2, and corners (K1, K2) at the end of the horizontal line L1 formed as a circular arc or with two or more corners.

[0020] Substrates used in optical waveguide elements include substrates with electro-optical effects, such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), as well as substrates made of these materials doped with MgO or other materials. Furthermore, a film can be formed by vapor deposition of a material such as LN directly or via an intermediate layer on a support substrate such as Si, glass, or sapphire. A substrate formed by bonding a substrate with electro-optical effects to another substrate and then thin-film processing the electro-optical substrate can also be used. Furthermore, semiconductor substrates, substrates made of organic materials such as EO polymers, and quartz substrates used in PLCs can also be used. A different type of semiconductor film can also be grown on a semiconductor substrate. Furthermore, since the substrate (waveguide layer) on which the optical waveguide is formed is very thin, e.g., less than 1 μm, a support substrate with a dielectric constant lower than that of the waveguide layer is preferably used.

[0021] Optical waveguides can be formed by thermally diffusing Ti or other elements into an LN substrate, forming a locally high refractive index portion, or by proton exchange. In the present invention, a rib-type optical waveguide can be formed by etching the substrate surface other than the optical waveguide or by forming grooves on both sides of the optical waveguide, thereby forming a convex portion corresponding to the optical waveguide. The optical waveguide element of the present invention is particularly effective for step-index (SI) optical waveguides, which have discontinuous refractive indexes, such as rib-type optical waveguides, rather than graded-index (GI) optical waveguides such as diffused waveguides. The LN substrate used in the present invention is not limited to X-cut or Z-cut, but the following description focuses on an example of an optical waveguide element (chip) using an X-cut LN substrate and forming a rib-type optical waveguide with a height of 1 μm or less.

[0022] The main features of the optical waveguide element of the present invention are: (1) as shown in FIG. 2, the modulation electrodes (2S, 2G) are arranged in the vicinity of the optical waveguide, and have a plurality of segment electrodes SE arranged along the optical waveguide, and a signal transmission section SS electrically connected to the segment electrodes and transmitting a modulation signal; and (2) as shown in FIG. 3, the segment electrode SE has a T-shape with a horizontal line L1 and a vertical line L2, and the corners (K1, K2) at the end of the horizontal line L1 are formed as an arc or with two or more corners.

[0023] In the optical waveguide element of the present invention, the segment electrodes SE of the modulation electrode and the signal transmission section SS are formed in different manufacturing processes. The segment electrodes SE have a thickness of 3 μm or less, preferably 1 μm or less, and are therefore formed by plating, vapor deposition, or sputtering. The signal transmission section SS is formed by plating because it is three times thicker than the segment electrodes. The thicker the resist pattern, the more likely resist cracks are to occur. Furthermore, the thinner the segment electrodes SE, the less high-frequency loss there is, and the less degradation of high-frequency characteristics can be reduced.

[0024] When forming the segment electrodes SE, in the regions A to C enclosed by dotted lines in FIG. 3 where the corners of the segment electrodes are formed, they are formed with an arc or two or more corners (the interior angle of a polygon is 90 degrees or more) to prevent resist cracks from occurring in the resist pattern. In particular, in the horizontal line L1 which plays a role in forming the electric field to be applied to the optical waveguide, the corners (K1, K2) at the ends of the horizontal line L1 are formed with an arc (see dotted line frame A') or two or more corners (see dotted line frame A") to prevent resist cracks from occurring. Dotted line frame A' and dotted line frame A" are enlarged views of the dotted line region A.

[0025] In the dotted line frame A', the corner K1 is formed by an arc with a radius of curvature R1. The corner K2 is formed by an arc with a radius of curvature R2. To prevent the occurrence of resist cracks, the radius of curvature R1 (R2) of the arc is preferably 1 μm or more.

[0026] On the other hand, in the dotted line frame A", the corner K1 is formed by two corners. Also, the corner K2 is formed by four corners. The interior angles of all corners are set to be 90 degrees or more, more preferably 120 degrees or more, which suppresses the occurrence of resist cracks compared to conventional corners that are right angles. When one corner is formed by multiple corners, it is preferable to increase the number of corners and make the interior angles of each corner larger. However, if the distance between the first corner and the last corner is close, the effect of making the interior angles larger is reduced, so it is preferable to set the distance between the first corner and the last corner to be 0.2 μm or more, for example.

[0027] The shapes of corners K1 and K2 can be made symmetrical with respect to a center line CL (see dotted-line box A' in FIG. 3 ) that is parallel to the longitudinal direction of horizontal line L1. Specifically, the radii of curvature R1 and R2 of the arc-shaped corners are set to the same value. This makes the shape of the edge within dotted-line box A' in FIG. 3 symmetrical from top to bottom, which makes it possible to equalize the internal stress of the resist pattern located near the corner at the end of horizontal line L1 at the two corners (K1, K2). This not only suppresses the occurrence of resist cracks, but also suppresses distortion of the resist pattern and deformation of the segment electrodes.

[0028] As another example, even when the shape of corner K1 and corner K2 is formed with two or more corners, by forming them symmetrically with respect to the center line CL, the internal stress of the resist pattern arranged near the corners can be made equal between the two corners (K1, K2), thereby achieving the same effect as the corners formed with the above-mentioned circular arcs.

[0029] Furthermore, the "bend" formed by the arc or two or more corners at corners K1 and K2 can be formed more gently at corner K2 than at corner K1. Here, "bend" does not mean a curve formed only by a curve, but also includes a curve formed by a broken line with multiple corners. Specifically, by setting the radius of curvature of the corners to R1 < R2, stress relaxation and improved high-frequency characteristics can be achieved while maintaining a constant distance between the optical waveguide and the electrode. By setting the radius of curvature of R1 to 5 μm or less, the effect of widening the electrode spacing and reducing modulation efficiency can be suppressed. By setting the radius of curvature of R2 to 3 μm or more, the occurrence of resist cracks can be further suppressed. Furthermore, if necessary, the radius of curvature of the corners can be set to R1 > R2.

[0030] Furthermore, when a corner is formed by two or more corners, the sum of the interior angles of all the corners constituting the corner or the sum of the lengths of the lines connecting the multiple corners is set so that corner K2 is larger than corner K1. This makes it possible to maintain a constant distance between the optical waveguide and the segment electrode longer, which not only alleviates stress but also contributes to improving high-frequency characteristics.

[0031] Next, the dimensions of the segment electrodes will be described with reference to FIG. 4. Regarding the width of the segment electrodes, the width W1 of the horizontal line L1 is set to 6 μm or less, preferably 4 μm or less. Basically, a thinner horizontal line L1 is advantageous in terms of high-frequency characteristics. This is because, in the signal electrode, the wider the horizontal line L1, the more the modulated signal senses the segment electrode as unevenness protruding from the side of the signal electrode, degrading the high-frequency characteristics. For this reason, the narrower and thinner the horizontal line L1, the better. Furthermore, the width W2 of the vertical line L2 is set to 5 μm or more, preferably 10 μm or more. If the vertical line L2 is not thick enough, the electrical connection between the segment electrode SE and the signal transmission section SS becomes unstable.

[0032] Regarding the length of the segment electrodes, the length S1 of the horizontal line L1 depends on the frequency handled by the optical modulator, etc., and must be sufficiently short from the viewpoint of high frequencies. Specifically, the length should be about one-fourth to one-eighth of the wavelength of the microwave modulating signal, and may be even shorter. For example, when operating at a frequency of 100 GHz or less, the length S1 of the horizontal line L1 is preferably 400 μm or less.

[0033] As with the width of the vertical line L2, the longer the length S2 of the vertical line L2, the greater the overlapping area between the segment electrode and the signal transmission section, resulting in a better electrical connection between them. However, as the length S2 increases, the spacing between the branching waveguides of the Mach-Zehnder optical waveguide also increases, resulting in a larger size of the optical waveguide element (chip) itself. Therefore, the length S2 is set to 100 μm or less. Furthermore, when segment electrodes SE are disposed on both sides of the signal transmission section SS of the signal electrode 2S in Fig. 6, the vertical lines L2 of the two segment electrodes can be connected in an "H" shape, as in the segment electrode in the center right of Fig. 6.

[0034] The radius of curvature (R1, R2) of the corners of the segment electrodes is preferably set to 1 μm or more to avoid resist cracks, as described above. However, since the magnitude of the radius of curvature also affects the shape of the end of the horizontal line L1, the radius of curvature should be set to be equal to or less than the width W1 of the horizontal line L1, preferably less than half the width W1, and more preferably equal to or less than one-third of the width W1.

[0035] Figure 5 explains various shapes of segment electrodes. It is possible to connect not only the end of the horizontal line L1 of the segment electrode but also the intersection of the horizontal line L1 and vertical line L2 (area B enclosed by a dotted line) with an arc, as shown in Figure 5(a). This makes it possible to prevent resist cracks from occurring at the intersection of the horizontal line L1 and vertical line L2.

[0036] 5(b), as shown in region B, a vertical line L2 forms an arc, which is connected to a straight horizontal line L1, and the intersection is constricted. Since the width of the resist film is narrow in this constricted portion, stress is weak and resist cracks are less likely to occur.

[0037] 5(c) shows a horizontal line L1 with a tapered shape that gradually narrows at both ends. The side (side) of the horizontal line L1 facing the optical waveguide needs to maintain a constant distance from the optical waveguide, so the shape of the side (side) away from the optical waveguide is changed. This shape also makes it possible to maintain a constant distance between the optical waveguide and the electrode, which is expected to further improve high-frequency characteristics.

[0038] 5(d) shows a conventional design in which the ends of the vertical lines L2 of the segment electrodes are formed at right angles. This increases the possibility of resist cracks occurring in the resist pattern at the ends of the vertical lines L2, but the area near these ends is covered by the signal transmission section SS, so the impact of resist cracks is small.

[0039] In FIG. 5( e ), the segment electrodes are shaped like a T, not a symmetrical T. If the propagation direction of light in the optical waveguide is to the right in the drawing, the modulation signal entering from the vertical line L2 propagates in the same direction as the propagating light (forward modulation) when propagating to the right of the horizontal line L1, thereby increasing modulation efficiency. Furthermore, when propagating to the left of the horizontal line L1, the signal propagates in the opposite direction to the propagating light (reverse modulation). Therefore, the configuration shown in FIG. 5( e ) makes it possible to lengthen the forward modulation section and shorten the reverse modulation section. In some cases, it is also possible to arrange the horizontal line L1 only in the direction of forward modulation, as in an L-shape. Thus, the shape of the segment electrodes can be modified in various ways depending on the characteristics desired for the optical waveguide element.

[0040] Fig. 6 is a diagram illustrating the arrangement of a signal transmission section SS superimposed on a segment electrode SE. To make the shape of the segment electrode easier to understand, the portion overlapping with the signal transmission section is also shown on the outside. Fig. 7 is a diagram illustrating the dimensions of a pair of segment electrodes sandwiching the optical waveguide 10 in Fig. 6 and explaining the positional relationship between the segment electrodes and the signal transmission section. The dimensions of the segment electrodes are the same as those in Fig. 4.

[0041] The left side of Fig. 6 shows a normal segment electrode (SE), in which the individual segment electrodes are arranged separately. In contrast, as described above, the segment electrode SE on the right side of Fig. 6 adopts an "H" shape, connecting the vertical lines L2 of two segment electrodes on either side of the signal transmission path. Naturally, the "H" shape increases the area of ​​overlap between the vertical line L2 and the signal transmission section SS, making it possible to more reliably establish electrical connection.

[0042] 7, the electrode spacing G1 between the segment electrode SE of the signal electrode 2S and the segment electrode SE of the ground electrode 2G is preferably set close to the optical waveguide 10 within a range that does not absorb the light waves propagating through the optical waveguide 10. For example, the spacing G1 is set to at least twice the mode field diameter of the light waves propagating through the optical waveguide 10, i.e., 3 μm or more.

[0043] If the vertical line L1 of the segment electrode SE and the signal transmission section SS are too close to each other, the electric field generated by the signal transmission section SS will directly affect the optical waveguide 10. For this reason, although it depends on the width W1 of the horizontal line L1 of the segment electrode SE, it is preferable to set the gap G3 to 1 μm or more. Naturally, it is essential that the gap G3 be smaller than the length S2 of the vertical line L2 of the segment electrode SE (G3<S2). Conversely, the minimum value of the length S2 is set to a value that takes into account the alignment accuracy between the segment electrode and the signal transmission section plus some leeway.

[0044] The distance G2 between the two signal transmission sections (2S(SS), 2G(SS)) in Figure 7 is 10 μm or more, but it is preferable to set it to 100 μm or less in order to avoid increasing the size of the optical waveguide element (chip).

[0045] Fig. 8 is a cross-sectional view taken along dashed line D in Fig. 6, and Fig. 9 is a cross-sectional view taken along dashed line E in Fig. 6. As shown in Figs. 8 and 9, the optical waveguide 10 is a rib-type optical waveguide and is formed on a substrate 1. The convex portions of the substrate 1 below the segment electrodes SE and the signal transmission sections SS can be removed by etching or the like, as with the periphery of the rib-type optical waveguide, but may be left in place to ensure the mechanical strength of the substrate 1. Furthermore, when the substrate 1 is used by being bonded to a support substrate (not shown), it is also possible to completely remove unnecessary portions of the substrate 1.

[0046] Figure 10 is an application example of Figure 9, in which the signal transmission section SS is formed of multiple electrode layers (SS1, SS2). The cross section of the modulation electrode perpendicular to the transmission direction of the modulation signal is formed in multiple steps (step-like), and the spacing and height between the signal electrode and ground electrode in the signal transmission section SS can be changed in various ways, which is convenient for impedance adjustment.

[0047] While the above description has focused on X-cut LN substrates, it goes without saying that the technology relating to the optical waveguide element of the present invention can also be applied to Z-cut LN substrates, etc. Figures 11 and 12 illustrate electrode arrangements applied to Z-cut LN substrates, showing cross-sectional views passing through the vertical line L2 of the segment electrode SE as in Figure 9 . Figure 11 shows an example in which an electric field is applied to two optical waveguides 10 using one signal electrode 2S and one ground electrode 2G. Figure 12 also shows an example in which electrical signals applied to the electrodes are G, S+, S-, and G (G is ground potential, and S+ and S- are differential signals of opposite phases). In the drawings, the electrical signal S+ is represented by 2S+, and the electrical signal S- is represented by 2S-. The symbol BF denotes a buffer layer.

[0048] Next, we will explain examples in which the optical waveguide element of the present invention is applied to an optical modulation device or an optical transmission device. Below, we will explain an optical modulation device using the above-mentioned optical waveguide element, but the present invention is not limited to this and can also be applied to optical phase modulators, optical modulators with polarization synthesis functions, optical waveguide elements integrating more Mach-Zehnder type optical waveguides, junction devices with optical road elements made of other materials such as silicon, devices for sensor applications, etc. Furthermore, it goes without saying that it can be applied to HB-CDM.

[0049] As shown in FIG. 13 , the optical waveguide element includes an optical waveguide 10 formed on a substrate 1 and a modulation electrode (not shown) that modulates the light waves propagating through the optical waveguide 10, and is housed within a housing CA. Furthermore, an optical fiber (F) that inputs and outputs light waves to and from the optical waveguide can be provided to form an optical modulation device MD. In FIG. 13 , the optical fiber F is introduced into the housing through a through-hole penetrating the side wall of the housing and directly bonded to the optical waveguide element (chip). Alternatively, an optical block or lens barrel equipped with an optical lens may be used to optically couple to the optical waveguide 10 within the optical waveguide element. Furthermore, to ensure stable bonding with the optical fiber or optical block, a reinforcing member 3 may be placed along the end face of the optical waveguide substrate 1.

[0050] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So that causes the optical modulation device MD to perform a modulation operation to the optical modulation device MD. To obtain a modulation signal S to be applied to the optical waveguide element, the modulation signal So output from the digital signal processor DSP must be amplified. For this reason, in FIG. 13 , a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and digital signal processor DSP can be located outside the housing CA, but they can also be located inside the housing CA. In particular, by locating the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.

[0051] As described above, according to the present invention, it is possible to provide an optical waveguide element that can suppress defective formation of modulation electrodes having segment electrodes and that can achieve a high product yield. Furthermore, it is possible to provide an optical modulation device and an optical transmission device that use such an optical waveguide element.

[0052] REFERENCE SIGNS LIST 1 Substrate (waveguide layer) 3 Reinforcement member 10 Rib-type optical waveguide 2G Ground electrode 2S Signal electrode SE Segment electrode SS Signal transmission section L1 Horizontal line L2 Vertical line K1, K2 Corner portions R1, R2 Radius of curvature

Claims

1. An optical waveguide element comprising a substrate on which an optical waveguide is formed, and a modulation electrode disposed on the substrate for applying an electric field to the optical waveguide in order to modulate the light waves propagating through the optical waveguide, wherein the modulation electrode is disposed adjacent to the optical waveguide and has a plurality of segment electrodes disposed along the optical waveguide, and a signal transmission section electrically connected to the segment electrodes and for transmitting a modulation signal, wherein the segment electrode has a T-shape with a horizontal line L1 and a vertical line L2, and the corner of the end of the horizontal line L1 is formed into a circular arc or two or more corners.

2. An optical waveguide element as claimed in claim 1, characterized in that the end of the horizontal line L1 has a corner K1 closer to the optical waveguide and a corner K2 further away from the optical waveguide, and the corner K1 and the corner K2 are set to have a shape symmetrical with respect to a center line parallel to the longitudinal direction of the horizontal line L1.

3. An optical waveguide element as claimed in claim 1, wherein the end of the horizontal line L1 has a corner K1 closer to the optical waveguide and a corner K2 further away from the optical waveguide, and the bend formed by an arc or two or more angles at the corner K1 and the corner K2 is formed more gently at the corner K2 than at the corner K1.

4. The optical waveguide element according to claim 1, wherein the radius of curvature of the circular arc is 1 μm or more.

5. An optical waveguide element according to claim 1, wherein the thickness of said segment electrodes is 3 μm or less, the width of said horizontal lines L1 is 6 μm or less, and the width of said vertical lines L2 is 5 μm or more.

6. An optical waveguide element according to claim 5, wherein the thickness of said signal transmission section is three times or more the thickness of said segment electrodes, and said signal transmission section has a cross section perpendicular to the transmission direction of the modulated signal formed in multiple steps.

7. An optical modulation device comprising the optical waveguide element according to any one of claims 1 to 6, housed in a housing and equipped with an optical fiber for inputting or outputting light waves to the optical waveguide.

8. An optical modulation device according to claim 7, further comprising an electronic circuit disposed within said housing for amplifying a modulation signal input to said modulation electrode of said optical waveguide element.

9. An optical transmitter comprising: an optical modulation device according to claim 7; and an electronic circuit for outputting a modulation signal for causing said optical modulation device to perform a modulation operation.

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