Optical waveguide element, optical modulator using optical waveguide element, and optical transmission device

Abstract

The utility model provides an optical waveguide element, an optical modulator using the optical waveguide element, and an optical transmitting device. According to the optical waveguide element, impedance matching in the modulation electrode can be achieved, the speed of light waves propagating in the optical waveguide is matched with the speed of modulation signals, and manufacturing is easy. A rib-type optical waveguide is formed on a substrate, and a modulation electrode disposed on the substrate so as to sandwich the rib-type optical waveguide is provided, characterized in that: the modulation electrode has a plurality of segmented electrodes disposed close to the rib-type optical waveguide and along the rib-type optical waveguide, and a signal transmission unit that transmits a signal transmitted from the segmented electrodes to the rib-type optical waveguide; the signal transmission part is electrically connected with the segmented electrode and transmits a modulation signal, the thickness of the signal transmission part is larger than that of the segmented electrode, and a buffer layer is arranged between the signal transmission part and the rib-type optical waveguide at least at the part, spanning the rib-type optical waveguide, of the signal transmission part. The buffer layer is not disposed between the substrate and the segmented electrode.

Description

Technical Field

[0001] The utility model relates to an optical waveguide element, an optical modulation device using the optical waveguide element, and an optical transmitting device, and in particular to an optical waveguide element having a rib-type optical waveguide formed on a substrate and having modulation electrodes arranged on the substrate in a manner of sandwiching the rib-type optical waveguide. Background Art

[0002] Optical waveguide components, such as optical modulators, are often used in the fields of optical communications and optical measurement. In recent years, the optical waveguide components that comprise the optical modulators built into transmitters of optical transceivers have been miniaturized to meet demands for miniaturization and reduced power consumption.

[0003] Furthermore, if a low-dielectric-constant supporting substrate is used in an optical waveguide element (chip) to achieve broadband optical modulators, impedance matching and velocity matching (velocity matching between the light waves propagating in the optical waveguide and the microwaves serving as the modulation signal) will become difficult to achieve with coplanar electrodes.

[0004] As shown in Patent Document 1, for example, by using a segmented electrode SE as shown in Figures 1(a) to 1(d), the number of adjustable parameters increases. However, adjusting the electrode thickness, which is effective for achieving wider bandwidth, is difficult. Conversely, when the segmented electrode SE is formed in a T-shape, the electrode width becomes thinner, resulting in a high aspect ratio (the ratio of electrode thickness to electrode width), making manufacturing more difficult. Furthermore, in Patent Document 1, the segmented electrode is formed only on the ground electrode, unlike the structure shown in Figures 1(a) to 1(d), which provides segmented electrodes on both the signal electrode and the ground electrode.

[0005] Figure 1(a) is a top view of an optical waveguide element, illustrating the shapes of the modulation electrodes arranged around the optical waveguide 10, specifically the signal electrode 2S and ground electrode 2G that constitute the modulation electrodes. The signal electrode 2S and ground electrode 2G have multiple segment electrodes SE ("T-shaped" portions) located proximate to and along the optical waveguide 10. Furthermore, a signal transmission portion SS is provided that is electrically connected to each segment electrode SE.

[0006] Figure 1(b) is a cross-sectional view taken along the dot-dashed line A in Figure 1(a). Figure 1(c) is a cross-sectional view taken along the dot-dashed line B in Figure 1(a). Furthermore, Figure 1(d) shows a cross-sectional view taken along the dot-dashed line C in Figure 1(a). Figure 4 In the same manner, (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. Furthermore, because two ground electrodes 2G are arranged to sandwich the signal electrode 2S, the modulation electrodes in Figures 1(a) to 1(d) constitute coplanar electrodes. Furthermore, in Figures 1(a) to 1(d), the segment electrodes SE and the signal transmission section SS are integrally formed from a single pattern. This makes it difficult to increase the thickness of the electrodes, as required for impedance matching of the signal transmission section SS, as shown in Figures 1(b) to 1(c).

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

[0009] However, at the intersection of the signal electrode 2S and the ground electrode 2G with the optical waveguide 10 in Figure 1(a), the electrodes are arranged so as to contact the optical waveguide 10 and straddle the optical waveguide. This causes absorption and scattering of light waves propagating through the optical waveguide, increasing signal propagation loss. To address this problem, a buffer layer (not shown) is typically placed on the optical waveguide, and the electrodes are then placed thereon.

[0010] On the other hand, regarding the optical waveguide 10 and the segmented electrodes arranged near it, as described above, considering modulation efficiency, it is desirable to form the electrodes directly on the substrate (the layer on which the optical waveguide is formed, also referred to as the waveguide layer) 1, and in close proximity to the optical waveguide. However, there is a conflicting issue: in order to suppress scattering during light propagation in the optical waveguide 10, it is desirable to also form a buffer layer on the optical waveguide 10.

[0011] Furthermore, forming electrodes on the buffer layer can easily lead to DC drift. For example, in LN modulators using lithium niobate (LN) as substrate 1, DC drift caused by the buffer layer becomes a significant problem. DC drift is a phenomenon in which the bias point of a Mach-Zehnder modulator fluctuates due to the DC voltage applied between the electrodes.

[0012] Furthermore, to achieve broadband and miniaturization of optical modulators, structures that directly connect high-speed electronic components to the optical modulator are often used. For example, the drive circuit that drives the optical modulator and the optical modulator (optical waveguide element) that applies the electrical signal to the drive circuit are integrated into the same housing and directly connected to each other, or the digital-to-analog converter (DAC) output of a signal processor is directly connected to the optical modulator. In these cases, the output of the high-speed electronic component is often DC-coupled, making the optical modulator susceptible to DC drift due to the DC component output from the high-speed electronic component.

[0013] Prior art literature

[0014] Patent Literature

[0015] Patent Document 1: Japanese Patent No. 6075576 Utility Model Content

[0016] Issues to be solved by the utility model

[0017] The present invention aims to solve the aforementioned problems by providing an optical waveguide element that achieves impedance matching in the modulation electrode and velocity matching between the light waves propagating in the optical waveguide and the modulation signal, while also being easy to manufacture. Furthermore, an optical modulation device and an optical transmission apparatus using such an optical waveguide element are provided.

[0018] Means for solving problems

[0019] In order to solve the above-mentioned problems, the optical waveguide element, optical modulation device and optical transmission device of the present invention have the following technical features.

[0020] (1) An optical waveguide element comprising a rib-type optical waveguide formed on a substrate and a modulation electrode arranged on the substrate so as to sandwich the rib-type optical waveguide, wherein the modulation electrode comprises a segmented electrode and a signal transmission portion, the segmented electrode being arranged close to the rib-type optical waveguide and a plurality of segments being arranged along the rib-type optical waveguide, the signal transmission portion being electrically connected to the segmented electrode and transmitting a modulation signal, the thickness of the signal transmission portion being greater than the thickness of the segmented electrode, a buffer layer being arranged between the signal transmission portion and the rib-type optical waveguide at least in a portion where the signal transmission portion crosses the rib-type optical waveguide, and the buffer layer being not arranged between the substrate and the segmented electrode.

[0021] (2) In the optical waveguide element described in (1) above, the buffer layer covering the rib-type optical waveguide is arranged so as to cover the segment electrode.

[0022] (3) In the optical waveguide element described in (1) above, the buffer layer is arranged between the segment electrode and the signal transmission portion, and an opening for electrically connecting the segment electrode and the signal transmission portion is formed in a portion of the buffer layer.

[0023] (4) In the optical waveguide element described in (1) above, the segmented electrode is characterized in that it includes an electric field applying portion arranged along the rib-type optical waveguide and a power supply portion connecting the electric field applying portion and the signal transmission portion, the power supply portion includes a portion A arranged to overlap with the signal transmission portion and a portion B arranged not to overlap with the signal transmission portion, and the maximum width of the portion A in the direction in which the signal transmission portion extends is larger than the maximum width of the portion B.

[0024] (5) An optical modulator, characterized in that the optical waveguide element described in any one of (1) to (4) above is housed in a housing, and the optical modulator includes an optical fiber for inputting or outputting light waves to or from the optical waveguide.

[0025] (6) In the optical modulation device described in (5) above, the optical waveguide element is provided with a modulation electrode for modulating the light wave propagating in the optical waveguide, and the housing includes an electronic circuit for amplifying the modulation signal to be input to the modulation electrode of the optical waveguide element.

[0026] (7) An optical transmitter, comprising the optical modulator described in (5) above and an electronic circuit that outputs a modulation signal for causing the optical modulator to perform a modulation operation.

[0027] Utility model effect

[0028] The present invention provides an optical waveguide component comprising a rib-type optical waveguide formed on a substrate and a modulation electrode disposed on the substrate so as to sandwich the rib-type optical waveguide. The modulation electrode comprises a segmented electrode and a signal transmission portion. The segmented electrode is disposed proximate to the rib-type optical waveguide and a plurality of segments are disposed along the rib-type optical waveguide. The signal transmission portion is electrically connected to the segmented electrode and transmits a modulation signal. The signal transmission portion is thicker than the segmented electrode. A buffer layer is disposed between the signal transmission portion and the rib-type optical waveguide, at least in the portion where the signal transmission portion straddles the rib-type optical waveguide. The buffer layer is not disposed between the substrate and the segmented electrode, resulting in different thicknesses between the segmented electrode and the signal transmission portion. This allows for easy impedance matching and velocity matching of the modulation electrode. Furthermore, since the segmented electrode and the signal transmission portion are formed separately, a manufacturing method suitable for each component can be employed, facilitating manufacture of the optical waveguide component. Furthermore, the use of this optical waveguide component enables the provision of an optical modulation device and an optical transmitter having similarly excellent characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figures 1(a) to 1(d) illustrate an example of providing segment electrodes on both the signal electrode and the ground electrode. Figure 1(a) is a top view, and Figures 1(b) to 1(d) are cross-sectional views taken along dashed lines A to C in Figure 1(a).

[0030] 2( a ) to 2 ( d ) are diagrams showing examples in which segment electrodes and signal transmission portions are formed by different electrode layers.

[0031] 3( a ) to 3 ( b ) are diagrams illustrating the dimensions of various parts such as segment electrodes and signal transmission paths.

[0032] Figure 4This is a diagram showing an application example of the electrical connection portion between the segment electrode and the signal transmission path.

[0033] 5( a ) to 5 ( d ) are diagrams showing an example in which a buffer layer is partially formed to cover an optical waveguide.

[0034] 6( a ) to 6 ( d ) are diagrams showing an example in which a buffer layer is formed to cover the entire surface of a substrate (a portion of which has an opening).

[0035] 7( a ) to 7 ( b ) are diagrams showing examples in which the arrangement pattern of the buffer layer arranged on the surface of the substrate is changed.

[0036] 8( a ) to 8 ( b ) are diagrams showing examples in which the shape pattern of the segment electrodes is changed.

[0037] Figure 9 It is a diagram for explaining the optical transmission device of the present invention.

[0038] Description of Reference Numerals

[0039] 1 Substrate (waveguide layer)

[0040] 3 Reinforcement components

[0041] 10 Rib-type optical waveguide

[0042] 2G ground electrode

[0043] 2S signal electrode

[0044] SE segmented electrode

[0045] SS Signal Transmission Department

[0046] SE1 segmented electrode (electric field application part)

[0047] SE2 segmented electrode (power supply part) DETAILED DESCRIPTION

[0048] Hereinafter, the present invention will be described in detail using preferred examples.

[0049] As shown in Figures 2(a) to 6(d), the present invention is an optical waveguide element, wherein a rib-type optical waveguide 10 is formed on a substrate 1, and modulation electrodes (2S, 2G) are arranged on the substrate in a manner sandwiching the rib-type optical waveguide. The modulation electrode is characterized in that the modulation electrode has a segmented electrode SE and a signal transmission portion SS, wherein the segmented electrode SE is arranged close to the rib-type optical waveguide and a plurality of segments are arranged along the rib-type optical waveguide. The signal transmission portion SS is electrically connected to the segmented electrode and transmits a modulation signal. The thickness of the signal transmission portion is greater than that of the segmented electrode. At least in the portion where the signal transmission portion crosses the rib-type optical waveguide, a buffer layer BF is arranged between the signal transmission portion SS and the rib-type optical waveguide 10, and no buffer layer BF is arranged between the substrate 1 and the segmented electrode SE.

[0050] Any substrate capable of forming an optical waveguide, particularly a rib-type waveguide, can be used as a substrate for optical waveguide components. Specifically, substrates with an electro-optic effect can include materials such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), or substrates in which these materials are doped with materials such as MgO. Alternatively, a film can be formed by vapor-phase growing a material such as LN directly or through an intermediate layer onto a supporting substrate such as Si, glass, or sapphire. Alternatively, a substrate formed by bonding an electro-optical substrate to another substrate and then subjecting the electro-optic substrate to thin film processing can be used. Furthermore, semiconductor substrates, organic material substrates such as EO polymers, and quartz substrates used in PLCs can also be used. Different types of semiconductor films can also be grown on semiconductor substrates. Furthermore, because the substrate (waveguide layer) on which the optical waveguide is formed is very thin, for example, less than 1 μm, a supporting substrate with a lower dielectric constant than that of the waveguide layer is suitable.

[0051] Optical waveguides can be formed by thermally diffusing Ti or other materials onto an LN substrate, using a proton exchange method, or by creating a locally high-refractive-index portion. In the present invention, etching the substrate surface other than the waveguide or forming grooves on both sides of the waveguide allows for the formation of a rib-type waveguide, where the portion corresponding to the waveguide on the substrate is convex. The optical waveguide element of the present invention is particularly effective for step-index (SI) waveguides, which have discontinuities in the refractive index, such as rib-type waveguides, compared to graded-index (GI) waveguides like diffused waveguides.

[0052] The following description will focus on an example 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.

[0053] The optical waveguide element of the present invention has the following main features.

[0054] (1) The modulation electrode includes a segment electrode and a signal transmission unit. The segment electrode is arranged close to the rib-type optical waveguide and is arranged in plurality along the rib-type optical waveguide. The signal transmission unit is electrically connected to the segment electrode and transmits a modulation signal.

[0055] (2) The thickness of the signal transmission portion is greater than the thickness of the segmented electrode.

[0056] (3) At least in a portion where the signal transmission portion crosses the rib-type optical waveguide, a buffer layer is disposed between the signal transmission portion and the rib-type optical waveguide, and the buffer layer is not disposed between the substrate and the segment electrode.

[0057] In order to utilize the above-mentioned features (1) to (3), as shown in Figures 2(a) to 2(d), it is preferable that the segment electrode SE and the signal transmission portion SS be formed from different electrode layers. In the optical waveguide element of the present invention, the structure of the segment electrode SE and the signal transmission portion SS is adopted in both the signal electrode 2S and the ground electrode 2G constituting the modulation electrode.

[0058] Patent Document 1 shows an example of a structure with segmented electrodes only on the ground electrode. However, the signal electrode, like conventional electrodes without segmented electrodes, is a rectangular cross-section transmission portion, and its thickness is relatively thick, making it difficult to ensure sufficient accuracy in electrode position and size. Consequently, it is difficult to precisely position the signal electrode close to the optical waveguide, and it is difficult to reduce the driving voltage by narrowing the electrode spacing.

[0059] 3( a ) to 3 ( b ) are enlarged views of a portion of the optical waveguide element of FIG. 2 ( a ) to FIG. 2 ( d ).

[0060] The segment electrodes SE have a small patterning area, and their position and width relative to the optical waveguide require stringent accuracy. Therefore, processes that prioritize precision over film thickness are suitable, such as direct patterning (DI) with an electron beam. This allows for narrower electrode spacing (w3 in Figure 3(a)) than with existing products, down to less than 10μm, further improving electric field efficiency.

[0061] Conversely, using a process that easily allows thick film formation in the signal transmission section SS allows for the formation of a high-frequency line with a large cross-sectional area and low loss. Furthermore, increasing the freedom in adjusting the thickness of the signal transmission section allows for a design that narrows the distance between the two branch waveguides that comprise the Mach-Zehnder optical waveguide, which helps minimize mounting area and high-frequency signal crosstalk.

[0062] Since the segment electrodes SE and signal transmission unit SS are formed separately, they can be manufactured using different processes, as described above. For example, plating can be used only for the signal transmission unit SS, thereby reducing costs and forming the thick film structure required for broadband operation. Furthermore, the segment electrodes can be formed with a higher degree of precision, while also achieving a lower driving voltage.

[0063] For example, as a light source used in a photolithography process, an electron beam (EB) is used in a segment electrode, and ultraviolet light (UV) can be used in a signal transmission portion.

[0064] Regarding the spacing w3 between the segment electrodes SE of the signal electrode (2S) and the segment electrodes SE of the ground electrode (2G), as long as absorption loss of light propagating through the optical waveguide 10 by the segment electrodes does not become a problem, the smaller the spacing w3, the lower the driving voltage. Similarly, as the spacing w3 between the segment electrodes increases, the area where the electric field is applied to the optical waveguide decreases, so it is preferably narrowed within a reasonable range. The smaller the widths (w1, w2) of the electrode patterns themselves, the more options for structural parameters can be used, increasing design freedom.

[0065] On the other hand, however, photoresist used to form electrode patterns becomes more difficult to form as the aspect ratio of the electrode thickness to the electrode line width or spacing increases. Therefore, it is difficult to achieve a ratio of electrode thickness to the width or spacing of the pattern to be formed of greater than 2. Therefore, it is preferable that the width or spacing of the thinnest pattern in a segment is at least 0.5 times the thickness of the electrode forming the segment. This can also be expressed by the following relationship.

[0066] Segment electrode line width (w1, w2) / segment electrode thickness (h1) > 0.5

[0067] · The interval between segment electrodes (w3, w4) / the thickness of the segment electrode (h1) > 0.5

[0068] · Signal transmission part line width (w5, w6) / signal transmission part electrode thickness (h2) > 0.5

[0069] · The distance between the signal transmission parts (w7, w8) / the thickness of the electrode of the signal transmission part (h2) > 0.5

[0070] The optical waveguide device employing the present invention improves the electric field efficiency of the modulation electrode, enabling sufficient modulation operation even when the length of the active portion (the portion where the two branch waveguides of the Mach-Zehnder optical waveguide in Figures 2(a) to 2(d) are parallel) is set to 20 μm or less. Consequently, the size of the optical waveguide device (chip) can be reduced.

[0071] Regarding the thickness of electrodes in high-frequency lines, such as signal wiring and signal transmission units, a larger surface area, i.e., a thicker thickness for the same width, can reduce signal propagation loss. While increasing the electrode thickness h2 of the signal transmission unit to 10 μm or greater, or to at least 10 times the thickness h3 of the waveguide layer, is difficult with conventional single-layer electrode formation, the present invention makes it possible to achieve this by simply increasing the thickness of the signal transmission unit, which does not use fine patterns.

[0072] The thickness of the segment electrodes is not particularly limited in terms of transmission characteristics. However, to provide connection points with the signal transmission unit, they must be thick enough to withstand some etching during the formation of the signal transmission unit's electrodes. Specifically, the segment electrode thickness h1 is preferably 0.3 μm or greater. Furthermore, the thickness h2 of the signal transmission unit's electrodes can be set to at least five times the segment electrode thickness h1.

[0073] The segment electrode SE and the signal transmission unit SS only need to be locally electrically connected. As shown in Figures 3(a) and 3(b), a connection portion may be provided at the boundary where the segment electrode SE and the signal transmission unit SS overlap. When a buffer layer BF is disposed above the segment electrode SE, as shown in Figures 5(a) to 5(d), no buffer layer BF is provided between the signal transmission unit SS and the segment electrode SE. Alternatively, as shown in Figures 6(a) to 6(d), an opening OP is provided in a portion of the buffer layer BF to achieve electrical connection between the segment electrode and the signal transmission unit.

[0074] Furthermore, the connection between the segment electrode and the signal transmission unit must ensure a sufficient area (size) by taking into account the alignment accuracy of the signal transmission unit. Specifically, if the overlapping portions (w9, w10) of the segment electrode and the signal transmission unit are designed to overlap by at least 5μm, this provides a sufficient margin for the alignment accuracy of photolithography using a photomask, eliminating problems such as poor contact.

[0075] When a buffer layer BF is provided between the segment electrode and the signal transmission portion, the opening OP of the buffer layer also preferably has a width of 5 μm or more for the same reason.

[0076] As shown in FIG3 (a) and FIG3 (b), as an example, the segment electrode SE is composed of an electric field applying portion SE1 arranged along the optical waveguide and a power supply portion SE2 connecting the electric field applying portion SE1 to the signal transmission portion SS. In the case where the pattern width w2 of the power supply portion SE2 is desired to be thin, such as Figure 4As shown, the width (maximum width) w10 of the power supply portion SE2 (referred to as "portion A") located below the signal transmission portion SS can also be made larger than the width (maximum width) w2 of the power supply portion SE2 (referred to as "portion B") not located below the signal transmission portion.

[0077] Different conductive materials can also be used in the segmented electrodes and the signal transmission part. For example, regarding the relationship between electrical conductivity, the material can be set in such a way that the signal transmission part is larger than the segmented electrode. Specifically, a combination of using gold (Au) for the segmented electrode and copper (Cu) for the signal transmission part can be adopted. Cu is a low-loss material with high electrical conductivity, but there is a risk of short circuit due to ion migration. Therefore, if the segmented electrode where the electric field is concentrated is set to stable Au, migration can be avoided and low loss can be achieved. Alternatively, a combination of materials with different absorption rates of propagating light can be used. For example, the material can be set in such a way that the absorption rate of the segmented electrode is lower than the absorption rate of the signal transmission part. More specifically, by setting the segmented electrode to a conductor that is transparent to the propagating light (transparent electrode), the electrode interval w3 of the segmented electrode can be made narrower and the driving voltage can be reduced.

[0078] Next, a description will be given of a case where the buffer layer formation step is performed between the segment electrode formation step and the signal transmission portion formation step.

[0079] In Figures 2(a) to 2(d), a buffer layer (not shown) is placed between the rib-type optical waveguide 10 and the modulation electrodes (signal electrode 2S and ground electrode 2G) where they overlap. This prevents light propagating through the optical waveguide 10 from being absorbed or scattered by the modulation electrodes.

[0080] 5(a) to 5(d) show a case where a buffer layer BF is formed to cover the 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 in the optical waveguide and reduce propagation loss.

[0081] Furthermore, as shown in Figures 5(b) and 5(c), the buffer layer BF partially covers the segment electrodes SE, which also helps prevent segment electrode delamination. Specifically, the segment electrodes SE and substrate 1 have different linear expansion coefficients, making delamination more likely to occur due to temperature fluctuations. Therefore, the bonding strength between the buffer layer BF and substrate 1 can be utilized to prevent segment electrode delamination.

[0082] As shown in Figure 5(c), there is no buffer layer BF between the segment electrodes (SE) and substrate 1. Therefore, 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 concentrates. This significantly reduces the effects of DC drift compared to a structure with a buffer layer between the segment electrodes and substrate 1.

[0083] This is a more effective technology in driver-integrated optical modulators that directly connect a DC-coupled output drive circuit and an optical modulation element (optical waveguide element).

[0084] 6( a ) to 6 ( d ) illustrate a case where a buffer layer BF is formed on the entire surface of the substrate 1 , and openings OP of the buffer layer BF are formed at portions electrically connecting the segment electrodes SE and the signal transmission portions SS.

[0085] By placing the buffer layer BF between the signal transmission section SS and the high-dielectric-constant substrate 1, it is possible to achieve low loss for high-frequency signals, which serve as modulated signals. Furthermore, when designing on a high-dielectric-constant LN substrate, prioritizing drive voltage for formation, the characteristic impedance of conventional electrode structures is reduced. The present invention allows for adjustable electrode spacing in the signal transmission section SS, enabling a design that achieves higher impedance and further suppresses reflections.

[0086] When the buffer layer is disposed between the signal transmission parts, the buffer layer covers the optical waveguide, thereby having an effect of suppressing scattering of light propagating through the optical waveguide.

[0087] Furthermore, from the perspective of high-frequency signals, a structure in which a buffer layer is provided only near the optical waveguide (including a portion of the segment electrode), as shown in Figures 5(a) to 5(d), causes more signal attenuation than a structure in which no buffer layer is provided. However, as shown in Figures 6(a) to 6(d), separating the signal transmission portion from the high-dielectric-constant LN substrate actually helps reduce signal attenuation.

[0088] As shown in Figures 6(b) and 6(c), a buffer layer can be placed between the segment electrodes and the signal transmission unit. This allows for both the segment electrodes, which are in direct contact with the waveguide layer (substrate 1) without interrupting the buffer layer on the optical waveguide, and the signal transmission unit, which spans the optical waveguide via the buffer layer.

[0089] The buffer layer can be formed into a thin film, and materials that transmit infrared light can be used. A material with a low dielectric loss tangent can also suppress degradation of high-frequency signal transmission characteristics. Specifically, ceramics such as SiO2 and Al2O3, glass, and optical resins can be used.

[0090] When the buffer layer is formed on the entire surface of Figures 6(a) to 6(d), the position where the opening portion OP is formed is not limited to the examples in Figures 6(a) to 6(d), and the opening portion OP may be formed on the entire lower portion of the signal transmission portion SS (excluding the portion overlapping with the optical waveguide) as shown in Figure 7(a), or the opening portion OP may be formed on only a portion of the lower portion of the signal transmission portion SS as shown in Figure 7(b).

[0091] The shape of the segment electrode SE is not limited to the “T-shape” as shown in FIG. 2( a ) to FIG. 2 ( d ), and may be an “L-shape” as shown in FIG. 8( a ) or a rectangular shape as shown in FIG. 8( b ).

[0092] Furthermore, the present invention can also be applied to structures that fold back the optical waveguide. This folded structure allows the active portion to be lengthened without changing the length of the optical waveguide element (chip), thereby reducing the driving voltage. In this case, minimizing the bend in the folded portion can reduce the overall length of the line, which is advantageous from the perspectives of reducing signal loss and mounting area.

[0093] Furthermore, the present invention can of course be applied to an optical waveguide element in which a plurality of Mach-Zehnder optical waveguides are integrated in parallel, or an optical waveguide element in which Mach-Zehnder optical waveguides are assembled in a nested "nest" shape.

[0094] Next, we will describe examples of applying the optical waveguide element of the present invention to optical modulators or optical transmitters. While this description focuses on optical modulators utilizing these optical waveguide elements, the present invention is not limited to these devices and can also be applied to optical phase modulators, optical modulators with polarization combining capabilities, optical waveguide elements integrating multiple Mach-Zehnder optical waveguides, devices for splicing optical path elements made of other materials such as silicon, and devices for sensor applications. Furthermore, the present invention can also be applied to high-bandwidth coherent driver modulators (HB-CDMs).

[0095] like Figure 9As shown, the optical waveguide element comprises 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. The optical waveguide is housed within a housing CA. Furthermore, by providing an optical fiber (F) for inputting and outputting light waves within the optical waveguide, an optical modulator device MD can be constructed. In Figures 5(a) to 5(d), the optical fiber F is optically coupled to the optical waveguide 10 within the optical waveguide element using an optical module or lens barrel equipped with an optical lens. This is not limiting. Alternatively, the optical fiber can be introduced into the housing through a through-hole extending through the sidewall of the housing, directly joining the optical component or substrate to the optical fiber, or optically coupling the optical fiber with a lens at its end to the optical waveguide within the optical waveguide element. Furthermore, to ensure stable bonding with the optical fiber or optical module, a reinforcement member 3 can be arranged overlappingly along the end face of the optical waveguide substrate 1.

[0096] By connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So to the optical modulator MD, an optical transmitter OTA can be constructed. To obtain the modulation signal S applied to the optical waveguide element, the modulation signal So output from the digital signal processor DSP must be amplified. Therefore, in Figures 5(a) to 5(d), a drive circuit DRV is used to amplify the modulation signal. The drive circuit DRV and digital signal processor DSP can be located outside the housing CA, or they can be located inside the housing CA. In particular, locating the drive circuit DRV inside the housing further reduces transmission losses in the modulation signal from the drive circuit.

[0097] In addition, when the drive circuit DRV and the modulation electrode are connected by DC coupling, the DC offset output of the drive circuit can be applied to the modulation electrode.

[0098] Optical modulators with separate high-frequency electrodes and DC bias electrodes typically use a DC-cut drive circuit to input signals. However, in driver-integrated modulators, where the drive circuit is located within the housing, the demands for high-frequency performance and miniaturization are even greater. Consequently, a DC-coupled structure is adopted, where the DC bias electrode is directly connected to the modulation electrode without a DC bias electrode. In this case, the bias voltage is constantly input to the modulation electrode through the output of the drive circuit, increasing the impact of DC drift.

[0099] In the optical modulation device and optical transmission apparatus of the present invention, segmented electrodes that contribute to modulation can be formed directly on the substrate without passing through a buffer layer, thereby further reducing DC drift and being suitable for a driver-integrated structure.

[0100] Industrial availability

[0101] As described above, the present invention provides an optical waveguide element that achieves impedance matching in the modulation electrodes, velocity matching between the light waves propagating in the optical waveguide and the modulation signal, and is easily manufactured. Furthermore, an optical modulation device and an optical transmission apparatus using such an optical waveguide element can be provided.

Claims

1. An optical waveguide element comprising a rib-type optical waveguide formed on a substrate and modulating electrodes arranged on the substrate so as to sandwich the rib-type optical waveguide, characterized in that: The modulation electrode includes a segment electrode and a signal transmission unit. The segment electrode is arranged close to the rib-type optical waveguide and a plurality of segments are arranged along the rib-type optical waveguide. The signal transmission unit is electrically connected to the segment electrode and transmits a modulation signal. The thickness of the signal transmission portion is greater than the thickness of the segmented electrode. At least in the portion where the signal transmission portion crosses the rib-type optical waveguide, a buffer layer is disposed between the signal transmission portion and the rib-type optical waveguide. The buffer layer is not disposed between the substrate and the segmented electrode.

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

3. The optical waveguide element according to claim 1, wherein The buffer layer is disposed between the segment electrode and the signal transmission portion. An opening for electrically connecting the segment electrode and the signal transmission portion is formed in a portion of the buffer layer.

4. The optical waveguide element according to claim 1, wherein The segment electrode includes an electric field applying portion arranged along the rib-type optical waveguide and a power supply portion connecting the electric field applying portion and the signal transmission portion. The power supply portion includes a portion A overlapping with the signal transmission portion and a portion B not overlapping with the signal transmission portion. In the direction in which the signal transmission portion extends, the maximum width of the portion A is greater than the maximum width of the portion B.

5. An optical modulation device, characterized in that: The optical waveguide element according to any one of claims 1 to 4 is housed in a housing, and the optical modulation device includes an optical fiber for inputting or outputting light waves to or from the optical waveguide.

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

7. An optical transmitting device, characterized in that: This optical transmission device comprises the optical modulation device according to claim 5 and an electronic circuit that outputs a modulation signal for causing the optical modulation device to perform a modulation operation.

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