Optical waveguide element and light modulation device using the same as well as light transmitter

Abstract

To provide an optical waveguide element that can achieving impedance matching of a modulation electrode and velocity matching between a light wave propagating through an optical waveguide and a modulation signal, and be easily manufactured.SOLUTION: The optical waveguide element has a rib type optical waveguide 10 formed on a substrate 1, and modulation electrodes (2S, 2G) arranged on the substrate so as to sandwich the rib type optical waveguide. The modulation electrode includes a plurality of segment electrodes SE arranged close to and along the rib type optical waveguide, and a signal transmission part SS electrically connected to the segment electrode and transmitting a modulation signal. Thickness of the signal transmission part is larger than that of the segment electrode. At least on a portion where the signal transmission part strides the rib type optical waveguide, a buffer layer BF is arranged between the signal transmission part SS and the rib type optical waveguide 10 while the buffer layer BF is not arranged between the substrate 1 and the segment electrode SE.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element and an optical modulation device and optical transmission device using the same, and in particular to an optical waveguide element having a rib-type optical waveguide formed on a substrate and modulation electrodes arranged on the substrate so as to sandwich the rib-type optical waveguide. [Background technology]

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

[0003] Furthermore, when a support substrate with a low dielectric constant is used for the optical waveguide element (chip) in order to broaden the bandwidth of the optical modulator, impedance matching and velocity matching (velocity matching between the light waves propagating through the optical waveguide and the microwaves that are the modulating signal) are difficult to achieve with coplanar electrodes.

[0004] As shown in Patent Document 1, for example, the use of a segment electrode SE as shown in FIG. 1 increases the number of adjustable parameters. However, it becomes difficult to adjust the electrode thickness to be thicker, which is effective for broadening the bandwidth. Rather, if the segment electrode SE is formed in a T-shape, the width of the electrode becomes narrower, resulting in a high aspect ratio (ratio of electrode thickness to electrode width), making manufacturing more difficult. Note that in Patent Document 1, a segment electrode is formed only on the ground electrode, which differs from the configuration in FIG. 1 in which segment electrodes are provided on both the signal electrode and ground electrode.

[0005] 1(a) is a plan view of an optical waveguide element, showing the shape of a modulation electrode arranged around an optical waveguide 10, specifically, a signal electrode 2S and a ground electrode 2G that constitute the modulation electrode. The signal electrode 2S and the ground electrode 2G have a plurality of segment electrodes SE ("T-shaped" portions) adjacent to the optical waveguide 10 and along the optical waveguide 10. Also, a signal transmission section SS is provided that is electrically connected to each segment electrode SE.

[0006] Fig. 1(b) is a cross-sectional view taken along dashed line A in Fig. 1(a). Fig. 1(c) is a cross-sectional view taken along dashed line B in Fig. 1(a). Furthermore, Fig. 1(d) shows a cross-sectional view taken along dashed line C in Fig. 1(a). Similarly, in the following Figs. 2 to 4, (a) of each figure is a plan view, and (b) to (d) of each figure are cross-sectional views.

[0007] The optical waveguide 10 is a rib-type optical waveguide formed on the substrate 1. In addition, since two ground electrodes 2G are arranged to sandwich the signal electrode 2S, the modulation electrode in Fig. 1 constitutes a coplanar electrode. Furthermore, in Fig. 1, the segment electrode SE and the signal transmission section SS are integrally formed in one pattern, so it is difficult to make the electrode thicker for the purpose of impedance matching of the signal transmission section SS, as shown in Figs. 1(b) to (c).

[0008] 1(b) to (d), the signal electrode 2S and the ground electrode 2G are formed directly on the substrate 1. This contributes to reducing the drive voltage of the optical modulator because the electric field formed by the segment electrodes SE is efficiently applied to the rib-type optical waveguide.

[0009] 1(a) where the signal electrode 2S or the ground electrode 2G intersects with the optical waveguide 10, the electrodes are arranged in contact with the optical waveguide 10 so as to straddle the optical waveguide, which causes absorption and scattering of the light waves propagating through the optical waveguide, resulting in large signal propagation loss. To solve this problem, a buffer layer (not shown) is usually arranged on the optical waveguide, and electrodes are arranged on top of that.

[0010] On the other hand, looking at the optical waveguide 10 and the segment electrodes disposed in its vicinity, as described above, in consideration of modulation efficiency, it is desirable to form the electrodes directly on the substrate (a layer on which the optical waveguide is formed, also called a waveguide layer) 1 and to form them in close proximity to the optical waveguide. However, in order to suppress scattering during light propagation through the optical waveguide 10, it is also desirable to form a buffer layer on the optical waveguide 10, which are mutually contradictory issues.

[0011] Furthermore, forming an electrode on the buffer layer increases the likelihood of the DC drift phenomenon. For example, in an LN modulator using lithium niobate (LN) for the substrate 1, the DC drift phenomenon caused by the buffer layer becomes a significant problem. The DC drift phenomenon is a phenomenon in which the bias point of a Mach-Zehnder type modulator fluctuates due to the DC voltage applied between the electrodes.

[0012] Moreover, in order to broaden the bandwidth and reduce the size of the optical modulator, it is becoming common to directly connect the high-speed electronic element to the optical modulator, for example, by integrating the driver circuit that drives the optical modulator and the optical modulation element (optical waveguide element) that applies the electric signal of the driver circuit in the same housing and directly connecting the two, or by directly connecting the digital-to-analog converter (DAC) output of the signal processor to the optical modulator. In such cases, the output of the high-speed electronic element is often DC-coupled, and in that case, the optical modulator is easily affected by the DC drift phenomenon due to the DC component output from the high-speed electronic element. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Patent No. 6075576 Summary of the Invention [Problem to be solved by the invention]

[0014] The object of the present invention is to provide an optical waveguide element that solves the above-mentioned problems, realizes impedance matching in a modulation electrode, and speed matching between a light wave propagating through an optical waveguide and a modulation signal, and is easy to manufacture, and further provides an optical modulation device and an optical transmission device using such an optical waveguide element. [Means for solving the problem]

[0015] In order to solve the above problems, the optical waveguide element, the optical modulation device, and the optical transmission device of the present invention have the following technical features. (1) An optical waveguide element comprising a substrate having a rib-type optical waveguide formed thereon and modulation electrodes arranged on the substrate to sandwich the rib-type optical waveguide, the modulation electrode being arranged close to the rib-type optical waveguide and having a plurality of segment electrodes arranged along the rib-type optical waveguide, and a signal transmission section electrically connected to the segment electrodes for transmitting a modulated signal, the signal transmission section being thicker than the segment electrodes, a buffer layer being arranged between the signal transmission section and the rib-type optical waveguide at least in the portion where the signal transmission section straddles the rib-type optical waveguide, and no buffer layer being arranged between the substrate and the segment electrodes.

[0016] (2) In the optical waveguide element described in (1) above, the buffer layer covering the rib-type optical waveguide is disposed so as to cover the segment electrodes.

[0017] (3) The optical waveguide element according to (1) above, further characterized in that the buffer layer is disposed between the segment electrode and the signal transmission section, and an opening is formed in a part of the buffer layer for electrically connecting the segment electrode and the signal transmission section.

[0018] (4) In the optical waveguide element described in (1) above, the segment electrode comprises an electric field application section arranged along the rib-type optical waveguide, and a power supply section connecting the electric field application section and the signal transmission section, the power supply section having a portion A where the signal transmission section is arranged to overlap and a portion B where the signal transmission section is not arranged to overlap, and the maximum width of portion A in the extension direction of the signal transmission section is larger than the maximum width of portion B.

[0019] (5) The optical waveguide element according to any one of (1) to (4) above is an optical modulation device, characterized in that it is housed in a housing and includes an optical fiber for inputting or outputting a light wave to the optical waveguide.

[0020] (6) In the optical modulation device described in (5) above, the optical waveguide element is provided with a modulation electrode for modulating a light wave propagating through the optical waveguide, and an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element is provided inside the housing.

[0021] (7) An optical transmitter comprising the optical modulation device according to (5) above, and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation. Effect of the Invention

[0022] The present invention provides an optical waveguide element including a rib-type optical waveguide formed on a substrate and modulation electrodes arranged on the substrate to sandwich the rib-type optical waveguide, the modulation electrodes being arranged close to the rib-type optical waveguide and including a plurality of segment electrodes arranged along the rib-type optical waveguide, and a signal transmission section electrically connected to the segment electrodes for transmitting a modulated signal, the signal transmission section being thicker than the segment electrodes, and a buffer layer being arranged between the signal transmission section and the rib-type optical waveguide at least in the portion where the signal transmission section straddles the rib-type optical waveguide, and no buffer layer being arranged between the substrate and the segment electrodes, and therefore the thicknesses of the segment electrodes and the signal transmission section differ, making it possible to provide an optical waveguide element that easily realizes impedance matching and speed matching of the modulation electrodes. In addition, since the segment electrodes and the signal transmission section are separately constructed, a manufacturing method suitable for each member can be adopted, and the manufacturing of the optical waveguide element is also easy. Furthermore, by using this optical waveguide element, it is possible to provide an optical modulation device and an optical transmission device having similarly excellent characteristics. [Brief description of the drawings]

[0023] [Figure 1] 1A and 1B are diagrams illustrating an example in which segment electrodes are provided on both the signal electrode and the ground electrode, where (a) is a plan view, and (b) to (d) are cross-sectional views taken along dashed lines A to C in (a). [Diagram 2] 13 is a diagram showing an example in which a segment electrode and a signal transmission section are formed in different electrode layers. FIG. [Diagram 3] FIG. 4 is a diagram for explaining the sizes of each part related to the segment electrodes, signal transmission paths, etc. [Figure 4] 13A and 13B are diagrams showing application examples of electrical connection portions between segment electrodes and signal transmission lines. [Diagram 5] 13A and 13B are diagrams showing an example in which a buffer layer is partially formed to cover an optical waveguide. [Figure 6] FIG. 13 is a diagram showing an example in which a buffer layer is formed to cover the entire surface of a substrate (with openings in some parts). [Figure 7] 13A and 13B are diagrams showing an example in which the arrangement pattern of a buffer layer arranged on the surface of a substrate is changed. [Figure 8] 13A and 13B are diagrams showing examples in which the shape pattern of segment electrodes is changed. [Figure 9] FIG. 1 is a diagram illustrating an optical transmitting device according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The present invention will now be described in detail with reference to preferred embodiments. As shown in Figs. 2 to 6, the present invention provides an optical waveguide element having a rib type optical waveguide 10 formed on a substrate 1 and modulation electrodes (2S, 2G) arranged on the substrate so as to sandwich the rib type optical waveguide, the modulation electrode being arranged close to the rib type optical waveguide and having a plurality of segment electrodes SE arranged along the rib type optical waveguide and a signal transmission section SS electrically connected to the segment electrodes for transmitting a modulated signal, the signal transmission section being thicker than the segment electrodes, a buffer layer BF being arranged between the signal transmission section SS and the rib type optical waveguide 10 at least in the portion where the signal transmission section straddles the rib type optical waveguide, and no buffer layer BF being arranged between the substrate 1 and the segment electrode SE.

[0025] As the substrate used for the optical waveguide element, any substrate can be used as long as it is made of a material that can form an optical waveguide, particularly a rib-type optical waveguide. Specifically, as the substrate having an electro-optic effect, substrates such as lithium niobate (LN), lithium tantalate (LT), PLZT (lead lanthanum zirconate titanate), and base materials obtained by doping these substrate materials with MgO or the like can be used. In addition, it is also possible to form a film by vapor-phase growth of a material such as LN on a supporting substrate such as Si, glass, or sapphire, either directly or with some intermediate layer sandwiched therebetween. In addition, a substrate obtained by thin-film processing of an electro-optic substrate after bonding a substrate having an electro-optic effect to another substrate can also be used. In addition, a semiconductor substrate, a substrate made of an organic material such as EO polymer, and a quartz substrate used in PLC can also be used. A different type of semiconductor film can be grown on a semiconductor substrate. Furthermore, since the substrate (waveguide layer) on which the optical waveguide is formed is very thin, for example, 1 μm or less, a supporting substrate having a dielectric constant lower than that of the waveguide layer is preferably used.

[0026] As a method for forming an optical waveguide, it is possible to form a part with a locally high refractive index by thermal diffusion of Ti or the like on an LN substrate, proton exchange, etc. In the present invention, it is possible to form a rib-type optical waveguide by etching the surface of the substrate other than the optical waveguide, or by forming grooves on both sides of the optical waveguide, so that the part of the substrate corresponding to the optical waveguide is convex. The optical waveguide element of the present invention is particularly effective for step-index (SI)-type optical waveguides in which discontinuous points of refractive index occur, such as rib-type optical waveguides, rather than graded-index (GI)-type optical waveguides such as diffusion waveguides. In the following, an example will be mainly described in which an X-cut LN substrate is used as the substrate 1 and the height of the rib-type optical waveguide (h0 in FIG. 3(b)) is set to 1 μm or less.

[0027] The optical waveguide element of the present invention has the following main features. (1) The modulation electrode is disposed in the vicinity of the rib-type optical waveguide and has a plurality of segment electrodes disposed along the rib-type optical waveguide, and a signal transmission section electrically connected to the segment electrodes and transmitting a modulation signal. (2) The thickness of the signal transmission section is greater than the thickness of the segment electrodes. (3) At least in the portion where the signal transmission section straddles the rib-type optical waveguide, a buffer layer is disposed between the signal transmission section and the rib-type optical waveguide, and the buffer layer is not disposed between the substrate and the segment electrode.

[0028] To employ the above features (1) to (3), it is preferable to configure the segment electrode SE and the signal transmission section SS in separate electrode layers, as shown in Fig. 2. In the optical waveguide element of the present invention, the configuration of the segment electrode SE and the signal transmission section SS is adopted for both the signal electrode 2S and the ground electrode 2G that configure the modulation electrode.

[0029] Patent Document 1 shows an example in which only the ground electrode has a segment electrode structure, but the signal electrode does not have a segment electrode, and like conventional electrodes, it is a transmission part with a rectangular cross section, and the electrode thickness is also thick, so the accuracy of the position and dimensions of the electrode formation cannot be fully guaranteed. This makes it difficult to precisely position the signal electrode close to the optical waveguide, and makes it difficult to reduce the driving voltage by narrowing the electrode spacing.

[0030] FIG. 3 is an enlarged view of a portion of the optical waveguide element of FIG. Since the segment electrode SE has a small drawing area and the position and width accuracy relative to the optical waveguide is critical, it is appropriate to use a process that prioritizes accuracy over film thickness, and it is considered to apply direct writing (DI) using an electron beam, etc. This makes it possible to form the electrode spacing (w3 in Figure 3(a)) narrower than conventional products at less than 10 μm, further improving the electric field efficiency.

[0031] On the other hand, by using a process that makes it easy to form a thick film for the signal transmission section SS, it becomes possible to form a high-frequency line with a large cross-sectional area and low loss. In addition, the increased freedom in the thickness of the signal transmission section makes it possible to design the gap between the two branching waveguides that make up the Mach-Zehnder type optical waveguide to be narrower, which is advantageous for reducing the mounting area and suppressing crosstalk of high-frequency signals.

[0032] Since the segment electrodes SE and the signal transmission section SS are formed separately, as mentioned above, separate manufacturing processes can be used for the segment electrodes and the signal transmission section, and for example, the thick film structure required for widening the bandwidth can be formed while keeping costs down by using plating methods only for the signal transmission section SS. Moreover, the segment electrodes can be formed with priority on precision, and it is also possible to reduce the driving voltage at the same time. For example, as a light source used in the photolithography process, an electron beam (EB) can be used in the segment electrodes, and ultraviolet light (UV) can be used in the signal transmission section.

[0033] The narrower the spacing w3 between the segment electrode SE of the signal electrode (2S) and the segment electrode SE of the ground electrode (2G), the lower the drive voltage will be, provided that the absorption loss of light propagating through the optical waveguide 10 by the segment electrodes is not an issue. Similarly, the wider the spacing w3 between the segment electrodes, the smaller the area in which an electric field is applied to the optical waveguide, so it is preferable to narrow it as much as possible. The narrower the widths (w1, w2) of the electrode patterns themselves, the more options there are for available structural parameters, and the greater the degree of freedom in design.

[0034] However, the photoresist used to form the electrode pattern becomes more difficult to form as the aspect ratio of the electrode thickness to the electrode line width or electrode spacing increases. This makes it difficult to make the ratio of the electrode thickness to the width or spacing of the pattern to be formed greater than or equal to 2. Therefore, it is desirable for the pattern width or spacing at the narrowest point of the segment portion to be at least 0.5 times the thickness of the electrode that forms the segment portion. This can also be expressed by the following relational equation. Segment electrode line width (w1, w2) / segment electrode thickness (h1)>0.5 Spacing between segment electrodes (w3, w4) / Thickness of segment electrodes (h1) > 0.5 Signal transmission line width (w5, w6) / signal transmission electrode thickness (h2) > 0.5 Distance between signal transmission parts (w7, w8) / electrode thickness of signal transmission part (h2) > 0.5

[0035] In the optical waveguide element to which the present invention is applied, the electric field efficiency of the modulation electrode is increased, and sufficient modulation operation can be realized even if the length of the action part (the part where the two branching waveguides of the Mach-Zehnder type optical waveguide in Fig. 2 are parallel) is set to 20 μm or less. This makes it possible to reduce the size of the optical waveguide element (chip).

[0036] As with signal wiring and signal transmission sections, the electrode thickness of high-frequency lines is larger when the surface area is larger, that is, when the width is the same, the thicker it is, the more the signal propagation loss can be suppressed. It is difficult to make the electrode thickness h2 of the signal transmission section 10 μm or more, or 10 times the thickness h3 of the waveguide layer, when forming the electrode in a conventional single layer, but with the present invention, this can be achieved by thickening only the signal transmission section that does not use fine patterns.

[0037] In addition, the thickness of the segment electrodes is not particularly limited in terms of transmission characteristics, but in order to provide a connection point with the signal transmission section, the thickness must be thick enough that it will not disappear even if it is etched to a certain extent when the electrode of the signal transmission section is formed. Specifically, it is desirable that the thickness h1 of the segment electrodes is 0.3 μm or more. In addition, the thickness h2 of the electrodes of the signal transmission section can be set to be five times or more the thickness h1 of the segment electrodes.

[0038] It is sufficient that the segment electrode SE and the signal transmission section SS are electrically connected at least partially. As shown in Fig. 3, a connection section may be provided at the boundary where the segment electrode SE and the signal transmission section SS overlap. When a buffer layer BF is provided above the segment electrode SE, as shown in Fig. 5, a configuration in which no buffer layer BF is provided between the signal transmission section SS and the segment electrode SE, or as shown in Fig. 6, an opening OP is provided in part of the buffer layer BF to electrically connect the segment electrode and the signal transmission section.

[0039] In addition, the connection portion between the segment electrode and the signal transmission portion must have a sufficient area (size) taking into consideration the alignment accuracy of the signal transmission portion, etc. Specifically, if the overlapping portion (w9, w10) between the segment electrode and the signal transmission portion is designed to overlap by 5 μm or more, a sufficient margin can be obtained for the alignment accuracy of lithography using a photomask, and problems such as poor contact can be eliminated.

[0040] When a buffer layer BF is provided between the segment electrodes and the signal transmission section, it is desirable for the opening OP of the buffer layer to have a width of 5 μm or more for the same reason. As shown in Fig. 3, the segment electrode SE is, for example, composed of an electric field application section SE1 arranged along the optical waveguide, and a power supply section SE2 connecting the electric field application section SE1 and the signal transmission section SS. In cases where a narrower pattern width w2 of the power supply section SE2 is preferable in terms of characteristics, it is also possible to make the width (maximum width) w10 of the power supply section SE2 (referred to as "section A") located below the signal transmission section SS larger than the width (maximum width) w2 of the power supply section SE2 (referred to as "section B") not located below the signal transmission section, as shown in Fig. 4.

[0041] Different conductive materials may be used for the segment electrodes and the signal transmission section. For example, the materials can be set so that the electrical conductivity is greater in the signal transmission section than in the segment electrodes. Specifically, a combination of gold (Au) for the segment electrodes and copper (Cu) for the signal transmission section is possible. Cu is a material with high electrical conductivity and low loss, but there is a risk of short circuit due to ion migration. Therefore, by using stable Au for the segment electrodes where the electric field is concentrated, it is possible to avoid migration and achieve low loss. Alternatively, materials with different absorption rates of propagating light may be combined. For example, materials can be set so that the absorption rate of the segment electrodes is smaller than that of the signal transmission section. More specifically, by making the segment electrodes a conductor (transparent electrode) that is transparent to the propagating light, the electrode spacing w3 of the segment electrodes can be narrowed to reduce the driving voltage.

[0042] Next, a case where a buffer layer is formed between the process of forming the segment electrodes and the process of forming the signal transmission section will be described. 2, in the portion where the rib type optical waveguide 10 and the modulation electrodes (signal electrode 2S, ground electrode 2G) overlap, a buffer layer (not shown) is disposed between the rib type optical waveguide 10 and the modulation electrodes. This makes it possible to suppress the light propagating through the optical waveguide 10 from being absorbed or scattered by the modulation electrodes.

[0043] 5 shows a buffer layer BF formed to cover a rib type optical waveguide 10. When the surface of the rib type optical waveguide is rough, the buffer layer BF can suppress scattering of light propagating through the optical waveguide, thereby making it possible to reduce the propagation loss. 5(b) and (c), the buffer layer BF is configured to cover a portion of the segment electrode SE, which contributes to preventing peeling of the segment electrode. Specifically, the segment electrode SE and the substrate 1 have different linear expansion coefficients, which makes them susceptible to peeling due to temperature changes. Therefore, it is possible to prevent peeling of the segment electrode by utilizing the adhesive strength between the buffer layer BF and the substrate 1.

[0044] As shown in (c) of Figure 5, this structure has no buffer layer BF between the segment electrode (SE) and the substrate 1. Therefore, since there is no buffer layer on the substrate side between the electrodes where the electric field generated when a DC voltage is applied to the electrodes is concentrated, the effect of the DC drift phenomenon can be greatly reduced compared to a structure in which a buffer layer is disposed between the segment electrode and the substrate 1. This is a more effective technique for a driver-integrated optical modulator in which the driver circuit with a DC-coupled output and the optical modulation element (optical waveguide element) are directly connected.

[0045] In FIG. 6, a buffer layer BF is formed over the entire surface of a substrate 1, and openings OP are formed in the buffer layer BF at the portions where the segment electrodes SE and the signal transmission sections SS are electrically connected. By inserting the buffer layer BF between the signal transmission section SS and the substrate 1 with a high dielectric constant, it is possible to reduce the loss of the high-frequency signal, which is the modulated signal. Also, because it is formed on an LN substrate with a high dielectric constant, if the driving voltage is given priority in the design, the characteristic impedance is low with the conventional electrode structure. In the present invention, the electrode spacing of the signal transmission section SS can be adjusted, making it possible to design with higher impedance and less reflection.

[0046] When a buffer layer is disposed between the signal transmission sections, the buffer layer covers the optical waveguide, which has the effect of suppressing scattering of light propagating through the optical waveguide. Also, from the perspective of high-frequency signals, a configuration in which a buffer layer is provided only near the optical waveguide (including part of the segment electrode) as in Figure 5 is a cause of signal attenuation compared to a configuration in which no buffer layer is provided at all. However, locating the signal transmission section away from the LN substrate, which has a high dielectric constant, as in Figure 6, actually contributes to reducing signal attenuation. As shown in Figures 6(b) and (c), a buffer layer can be sandwiched between the segment electrode and the signal transmission section. This makes it possible to have both a segment electrode that is in direct contact with the waveguide layer (substrate 1) without creating a discontinuity in the buffer layer on the optical waveguide, and a signal transmission section that straddles the optical waveguide via the buffer layer.

[0047] The buffer layer can be made of a material that can be formed into a thin film and transmits infrared light. If the material has a small dielectric tangent, it can also suppress the deterioration of the transmission characteristics of high-frequency signals. Specifically, ceramics such as SiO2 and Al2O3, glass, and optical resins can be used.

[0048] When forming a buffer layer over the entire surface of Figure 6, the position at which the opening OP is formed is not limited to the example of Figure 6. It is also possible to form an opening OP over the entire lower part of the signal transmission section SS (excluding the part overlapping with the optical waveguide) as shown in Figure 7(a), or to form an opening OP only in a part of the lower part of the signal transmission section SS as shown in Figure 7(b).

[0049] Moreover, the shape of the segment electrodes SE is not limited to the "T-shape" as shown in FIG. 2, but may be an "L-shape" as shown in FIG. 8(a) or a rectangular shape as shown in FIG. 8(b).

[0050] Furthermore, the present invention can also be applied to a structure in which the optical waveguide is folded back. Such a folded back structure allows the length of the active part to be increased without changing the length of the optical waveguide element (chip), which has the effect of lowering the driving voltage. In this case, bending the folded part as little as possible is advantageous from the standpoint of minimizing the overall length of the line, reducing signal loss, and saving mounting area. Furthermore, it goes without saying that the present invention can be applied to an optical waveguide element in which a plurality of Mach-Zehnder type optical waveguides are integrated in parallel, or to a "nested" optical waveguide element in which Mach-Zehnder type optical waveguides are nested.

[0051] Next, an example of applying the optical waveguide element of the present invention to an optical modulation device or an optical transmission device will be described. An optical modulation device using the above-mentioned optical waveguide element will be described below, but the present invention is not limited to this, and can also be applied to an optical phase modulator, an optical modulator with a polarization synthesis function, an optical waveguide element integrating more Mach-Zehnder type optical waveguides, a junction device with an optical road element made of other materials such as silicon, a device for sensor use, etc. Furthermore, it goes without saying that the present invention can be applied to a high bandwidth coherent driver modulator (HB-CDM: High Bandwidth-Coherent Driver Modulator).

[0052] As shown in FIG. 9, the optical waveguide element has an optical waveguide 10 formed on a substrate 1 and a modulation electrode (not shown) that modulates the light wave propagating through the optical waveguide 10, and is housed in a housing CA. Furthermore, an optical fiber (F) that inputs and outputs light waves to and from the optical waveguide can be provided to configure an optical modulation device MD. In FIG. 5, the optical fiber F is optically coupled to the optical waveguide 10 in the optical waveguide element using an optical block with an optical lens or a lens barrel. Not limited to this, the optical fiber may be introduced into the housing through a through hole penetrating the side wall of the housing, and the optical fiber may be directly bonded to an optical component or a substrate, or an optical fiber having a lens function at the end of the optical fiber may be optically coupled to the optical waveguide in the optical waveguide element. In addition, in order to stably bond the optical fiber and the optical block, a reinforcing member 3 may be overlapped and arranged along the end face of the optical waveguide substrate 1.

[0053] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulating signal So that causes the optical modulation device MD to perform a modulation operation to the optical modulation device MD. In order to obtain a modulating signal S to be applied to the optical waveguide element, it is necessary to amplify the modulating signal So output from the digital signal processor DSP. For this reason, in FIG. 5, a driver circuit DRV is used to amplify the modulating signal. The driver circuit DRV and the digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulating signal from the driver circuit.

[0054] Furthermore, when the driver circuit DRV and the modulation electrode are connected by DC coupling, it becomes possible for the DC offset output of the driver circuit to be applied to the modulation electrode. Optical modulators with a structure in which the high-frequency electrode and DC bias electrode are separated usually use a DC-cut driver circuit to input signals. However, in the case of driver-integrated modulators with the driver circuit located inside the housing, there is a stronger demand for high-frequency performance and small element size, so a DC-coupled structure is used in which the DC bias electrode is not located and it is directly connected to the modulation electrode. In this case, the bias voltage is always input to the modulation electrode due to the output of the driver circuit, which increases the impact of the DC drift phenomenon. In the optical modulation device and optical transmission device of the present invention, the segment electrodes that contribute to modulation can be formed directly on the substrate without a buffer layer, which makes it possible to further reduce the DC drift phenomenon and makes the device suitable for a driver-integrated structure. [Industrial Applicability]

[0055] As described above, according to the present invention, it is possible to provide an optical waveguide element that realizes impedance matching in a modulation electrode and speed matching between a light wave propagating through an optical waveguide and a modulation signal, and that is easy to manufacture. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Explanation of symbols]

[0056] 1. Substrate (waveguide layer) 3 Reinforcement members 10 Rib-type optical waveguide 2G ground electrode 2S signal electrode SE segment electrode SS signal transmission section SE1 Segment electrode (electric field application part) SE2 Segment electrode (power supply part)

Claims

1. An optical waveguide element including a substrate having a rib-type optical waveguide formed thereon, and modulation electrodes disposed on the substrate so as to sandwich the rib-type optical waveguide, the modulation electrode is disposed in the vicinity of the rib-type optical waveguide and has a plurality of segment electrodes disposed along the rib-type optical waveguide, and a signal transmission section electrically connected to the segment electrodes and transmitting a modulation signal; The thickness of the signal transmission portion is greater than the thickness of the segment electrode, a buffer layer is disposed between the signal transmission section and the rib type optical waveguide at least in a portion where the signal transmission section straddles the rib type optical waveguide; an optical waveguide element, wherein the buffer layer is not disposed between the substrate and the segment electrodes;

2. 2. The optical waveguide element according to claim 1, The buffer layer covering the rib-type optical waveguide is disposed so as to cover the segment electrodes.

3. 2. The optical waveguide element according to claim 1, The buffer layer is disposed between the segment electrode and the signal transmission section; an opening for electrically connecting the segment electrode and the signal transmission section is formed in a part of the buffer layer;

4. 2. The optical waveguide element according to claim 1, the segment electrode includes an electric field application section disposed along the rib-type optical waveguide, and a power supply section connecting the electric field application section and the signal transmission section, an optical waveguide element, characterized in that the power supply section has a portion A where the signal transmission section is arranged to overlap and a portion B where the signal transmission section is not arranged to overlap, and in a direction in which the signal transmission section extends, a maximum width of the portion A is larger than a maximum width of the portion B.

5. 5. An optical modulation device comprising an optical waveguide element according to claim 1, the optical waveguide element being accommodated in a housing and comprising an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

6. 6. The optical modulation device according to claim 5, wherein the optical waveguide element is provided with a modulation electrode for modulating an optical wave propagating through the optical waveguide, and an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element is provided inside the housing.

7. 6. An optical transmitter comprising: an optical modulation device according to claim 5; and an electronic circuit for outputting a modulation signal for causing said optical modulation device to perform a modulation operation.

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