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

Abstract

The invention provides an optical waveguide element, an optical modulator using the optical waveguide element, and an optical transmission apparatus. 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 present invention relates to an optical waveguide element, an optical modulation device and an optical transmitting apparatus using the optical waveguide element, 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 so as to sandwich the rib-type optical waveguide. Background Art

[0002] In the fields of optical communication and optical measurement, optical waveguide elements such as optical modulators, which are optical waveguides formed on a substrate, are often used. In recent years, in order to meet the requirements of miniaturization and low power consumption of optical modulators included in transmitters built into optical transceivers, it is necessary to miniaturize the optical waveguide elements constituting the optical modulators.

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

[0004] As shown in Patent Document 1, for example, by using a segmented electrode SE as shown in FIG. 1(a) to FIG. 1(d), the number of adjustable parameters increases. However, it is difficult to adjust the electrode thickness that is effective for broadbandization. On the contrary, when the segmented electrode SE is formed in a T-shape, the width of the electrode becomes thinner, so it becomes a high aspect ratio (ratio of electrode thickness to electrode width), and manufacturing becomes more difficult. In addition, in Patent Document 1, the segmented electrode is formed only on the ground electrode, which is different from the structure of providing the segmented electrode on both the signal electrode and the ground electrode in FIG. 1(a) to FIG. 1(d).

[0005] FIG1(a) is a top view of an optical waveguide element, showing the shapes of modulation electrodes arranged around an optical waveguide 10, specifically, signal electrodes 2S and ground electrodes 2G constituting the modulation electrodes. The signal electrode 2S and the ground electrode 2G have a plurality of segment electrodes SE ("T-shaped" portions) close to and along the optical waveguide 10. In addition, a signal transmission portion SS electrically connected to each segment electrode SE is provided.

[0006] FIG1(b) is a cross-sectional view taken along the dashed line A in FIG1(a). FIG1(c) is a cross-sectional view taken along the dashed line B in FIG1(a). Furthermore, FIG1(d) is a cross-sectional view taken along the dashed line C in FIG1(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. In addition, since two ground electrodes 2G are arranged so as to sandwich the signal electrode 2S, the modulation electrodes in FIG. 1 (a) to FIG. 1 (d) constitute coplanar electrodes. Furthermore, in FIG. 1 (a) to FIG. 1 (d), the segment electrode SE and the signal transmission part SS are formed integrally by one pattern, so as shown in FIG. 1 (b) to FIG. 1 (c), it is difficult to make the electrode thick in order to achieve impedance matching of the signal transmission part SS.

[0008] 1(b) to 1(d), the signal electrode 2S and the ground electrode 2G are directly formed 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 the reduction of the driving voltage of the optical modulator.

[0009] However, at the position where the signal electrode 2S and the ground electrode 2G of FIG. 1( a ) intersect the optical waveguide 10, the electrodes are arranged in contact with the optical waveguide 10 and across the optical waveguide, so that absorption and scattering of light waves propagating in the optical waveguide occur, and the propagation loss of the signal increases. In order to solve this problem, a buffer layer (not shown) is usually arranged on the optical waveguide, and the electrodes are arranged thereon.

[0010] On the other hand, regarding the optical waveguide 10 and the segmented electrodes arranged near it, 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 referred to as a waveguide layer) 1 and close to the optical waveguide. However, there is a conflicting issue that a buffer layer should also be formed on the optical waveguide 10 in order to suppress scattering during light propagation in the optical waveguide 10.

[0011] Furthermore, there is also a problem that a DC drift phenomenon is likely to occur if an electrode is formed on the buffer layer. For example, in an LN modulator using lithium niobate (LN) for 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 the Mach-Zehnder modulator changes due to the DC voltage applied between the electrodes.

[0012] Furthermore, in order to achieve broadband and miniaturization of optical modulators, the following structures are often used to directly connect high-speed electronic components to optical modulators: a drive circuit for driving the optical modulator and an optical modulator element (optical waveguide element) that applies an electrical signal to the drive circuit are integrated in the same housing and the two are directly connected, or a digital / analog converter (DAC) output of a signal processor is directly connected to the optical modulator, etc. In such a case, the output of the high-speed electronic component is often DC coupled, and in this case, the optical modulator is easily affected by the DC drift phenomenon 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 Summary of the invention

[0016] Problems to be solved by the invention

[0017] The present invention aims to solve the above-mentioned problems and provide an optical waveguide element that can achieve impedance matching in a modulation electrode and speed matching between a light wave propagating in an optical waveguide and a modulation signal and that can be easily manufactured. 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, the optical modulation device and the 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 being arranged in plurality along the rib-type optical waveguide, the signal transmission portion being electrically connected to the segmented electrode and transmitting a modulation signal, the signal transmission portion being thicker than 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 unit, and an opening for electrically connecting the segment electrode and the signal transmission unit is formed in a part of the buffer layer.

[0023] (4) In the optical waveguide element described in (1) above, 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 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 modulation device, characterized in that the optical waveguide element according to any one of (1) to (4) is accommodated in a housing, and the optical modulation device 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 has a modulation electrode for modulating the light wave propagating in the optical waveguide, and the housing has an electronic circuit inside for amplifying the modulation signal to be input to the modulation electrode of the optical waveguide element.

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

[0027] Effects of the Invention

[0028] The present invention provides an optical waveguide element, wherein a rib-type optical waveguide is formed on a substrate, and a modulation electrode is arranged on the substrate in a manner of sandwiching the rib-type optical waveguide, wherein the modulation electrode has a segmented electrode and a signal transmission part, the segmented 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 part is electrically connected to the segmented electrode and transmits a modulation signal, the thickness of the signal transmission part is greater than the thickness of the segmented electrode, at least in the portion where the signal transmission part crosses the rib-type optical waveguide, a buffer layer is arranged between the signal transmission part and the rib-type optical waveguide, and the buffer layer is not arranged between the substrate and the segmented electrode, so the thickness of the segmented electrode and the signal transmission part are different, thereby providing an optical waveguide element that can easily achieve impedance matching and speed matching of the modulation electrode. In addition, since the segmented electrode and the signal transmission part are separately formed, a manufacturing method suitable for each component can be adopted, and the manufacturing of the optical waveguide element also becomes easy. Furthermore, by using the optical waveguide element, an optical modulation device and an optical transmission device having similar excellent characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Fig. 1 (a) to Fig. 1 (d) are diagrams showing an example in which segment electrodes are provided on both the signal electrode and the ground electrode. Fig. 1 (a) is a plan view, and Fig. 1 (b) to Fig. 1 (d) are cross-sectional views taken along dashed lines A to C in Fig. 1 (a).

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

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

[0032] Figure 4It 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] Fig. 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 Strengthening 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) DETAILED DESCRIPTION

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

[0049] The present invention is an optical waveguide element as shown in FIGS. 2( a) to 6( d ), wherein a rib-type optical waveguide 10 is formed on a substrate 1 and a modulation electrode (2S, 2G) is arranged on the substrate in a manner of sandwiching the rib-type optical waveguide. 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 the thickness of the segmented electrode, and a buffer layer BF is arranged between the signal transmission portion SS and the rib-type optical waveguide 10 at least in a portion where the signal transmission portion crosses the rib-type optical waveguide, and the buffer layer BF is not arranged between the substrate 1 and the segmented electrode SE.

[0050] As a substrate used for an optical waveguide element, any substrate can be used as long as it is a material that can form an optical waveguide, especially a rib-type optical waveguide. Specifically, as a substrate with an electro-optical effect, a substrate such as lithium niobate (LN), lithium tantalate (LT), PLZT (lead lanthanum zirconate titanate), or a substrate in which these substrate materials are doped with MgO or the like can be used. In addition, a film can be formed by vapor-growing materials such as LN directly or through some intermediate layers on a supporting substrate such as Si, glass, sapphire, etc. In addition, a substrate formed by thin-film processing of an electro-optical substrate after bonding a substrate with an electro-optical effect to other substrates can also be used. Furthermore, a substrate of an organic material such as a semiconductor substrate or an EO polymer, or a quartz substrate used for PLC can also be used. Different types of semiconductor films can also be grown on a semiconductor substrate. In addition, since the substrate (waveguide layer) on which the optical waveguide is formed is very thin, for example, as thin as less than 1 μm, it is suitable to use a supporting substrate with a dielectric constant lower than that of the waveguide layer.

[0051] As a method for forming an optical waveguide, a portion with a locally high refractive index can be formed by thermally diffusing Ti or the like on an LN substrate, by a proton exchange method, etc., to form an optical waveguide. In the present invention, a rib-type optical waveguide in which a portion corresponding to the optical waveguide on the substrate is convex can be formed by etching the substrate surface other than the optical waveguide, or by forming grooves on both sides of the optical waveguide. The optical waveguide element of the present invention is particularly effective for a step-index (SI) type optical waveguide that generates a discontinuous point of refractive index separation like a rib-type optical waveguide, as compared to a gradient-index (GI) type optical waveguide like a diffuse waveguide.

[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 device of the present invention has the following main features.

[0054] (1) The modulation electrode includes a segment electrode disposed close to a rib-type optical waveguide and disposed in plurality along the rib-type optical waveguide, and a signal transmission unit electrically connected to the segment electrode and transmitting 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 adopt the above-mentioned features (1) to (3), as shown in Fig. 2 (a) to Fig. 2 (d), it is preferable that the segment electrode SE and the signal transmission part SS are formed of different electrode layers. In the optical waveguide element of the present invention, the structure of the segment electrode SE and the signal transmission part 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 in which only a segmented electrode is provided on a ground electrode, but the signal electrode is a transmission portion having a rectangular cross section, similar to the conventional electrode without a segmented electrode, and the electrode thickness is also relatively thick, so the position and size accuracy of the electrode cannot be sufficiently ensured. Therefore, it is difficult to arrange the signal electrode close to the optical waveguide with high accuracy, and it is difficult to reduce the electrode interval to reduce the driving voltage.

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

[0060] The segment electrode SE has a small drawing area, and the position and width accuracy relative to the optical waveguide are strict, so it is suitable to use a process that prioritizes accuracy over film thickness, and consider applying direct drawing (DI) based on electron beams, etc. As a result, the electrode interval (w3 in Figure 3 (a)) can be formed to be narrower than 10μm compared to existing products, which can further improve the electric field efficiency.

[0061] On the contrary, by using a process that makes it easy to form a thick film in the signal transmission part SS, a high-frequency line with a large cross-sectional area and low loss can be formed. In addition, by increasing the degree of freedom of the thickness of the signal transmission part, a design can be made to narrow the interval between the two branch waveguides constituting the Mach-Zehnder type optical waveguide, which is beneficial for suppressing the mounting area and crosstalk of high-frequency signals.

[0062] Since the segment electrode SE and the signal transmission unit SS are formed separately, as described above, the segment electrode and the signal transmission unit can be manufactured using different manufacturing processes. For example, a thick film structure required for wide bandwidth can be formed by plating only for the signal transmission unit SS while suppressing costs. In addition, the segment electrode can be formed with priority given to accuracy, and a reduction in driving voltage can also be achieved at the same time.

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

[0064] As for the interval w3 between the segment electrode SE of the signal electrode (2S) and the segment electrode SE of the ground electrode (2G), as long as it is within the range where the absorption loss of light propagating in the optical waveguide 10 by the segment electrode does not become a problem, the narrower the interval w3 is, the lower the driving voltage is. Similarly, the wider the interval w3 between the segment electrodes, the smaller the area where the electric field is applied to the optical waveguide, so it is preferably narrowed within the possible range. The smaller the width (w1, w2) of the electrode pattern itself, the more options of structural parameters that can be used, and the greater the degree of freedom in design.

[0065] However, on the other hand, regarding the photoresist used to form the electrode pattern, the larger the aspect ratio of the electrode thickness and the electrode line width and the electrode spacing, the more difficult it is to form. Therefore, it is difficult to make the ratio of the electrode thickness to the width or spacing of the pattern to be formed be 2 or more. Therefore, it is preferred that the width or spacing of the thinnest pattern of the segmented portion is 0.5 times or more of the thickness of the electrode forming the segmented portion. 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 segment electrodes (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] In the optical waveguide element to which the present invention is applied, the electric field efficiency of the modulation electrode is improved, and even if the length of the action portion (the portion where the two branch waveguides of the Mach-Zehnder type optical waveguide of FIG. 2 (a) to FIG. 2 (d) are parallel) is set to 20 μm or less, sufficient modulation operation can be achieved. Therefore, the size of the optical waveguide element (chip) can be miniaturized.

[0071] As for the electrode thickness of the high-frequency line, such as the signal wiring and the signal transmission part, the one with a larger surface area, that is, the thicker one if the width is the same, can suppress the propagation loss of the signal. It is difficult to set the electrode thickness h2 of the signal transmission part to 10 μm or more, or to set it to 10 times or more relative to the thickness h3 of the waveguide layer when forming the electrode with a conventional single layer, but according to the present invention, it can be dealt with by only thickening the signal transmission part without using a fine pattern.

[0072] In addition, the thickness of the segmented electrode is not particularly limited in terms of transmission characteristics, but in order to set a connection point with the signal transmission unit, it is necessary to have a thickness that does not disappear even if it is etched to a certain extent when forming the electrode of the signal transmission unit. Specifically, it is preferred that the thickness h1 of the segmented electrode has a thickness of 0.3 μm or more. In addition, the thickness h2 of the electrode of the signal transmission unit can be set to be 5 times or more of the thickness h1 of the segmented electrode.

[0073] The segment electrode SE and the signal transmission part SS only need to be electrically connected locally at a certain place. As shown in Figures 3 (a) to 3 (b), a connection portion can also be provided at the boundary of the portion where the segment electrode SE and the signal transmission part SS overlap. In the case where a buffer layer BF is arranged on the upper side of the segment electrode SE, as shown in Figures 5 (a) to 5 (d), the buffer layer BF is not provided between the signal transmission part SS and the segment electrode SE, or as shown in Figures 6 (a) to 6 (d), an opening OP is provided in a part of the buffer layer BF to achieve electrical connection between the segment electrode and the signal transmission part.

[0074] In addition, regarding the connection portion between the segment electrode and the signal transmission unit, it is necessary to ensure a sufficient area (size) by taking into account the alignment accuracy of the signal transmission unit, etc. Specifically, if the overlapping portion (w9, w10) of the segment electrode and the signal transmission unit is designed to overlap by more than 5 μm, a sufficient margin can be obtained for the alignment accuracy of photolithography using a photomask, and defects such as poor contact can be eliminated.

[0075] When the 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] In addition, as shown in FIG. 3 (a) to FIG. 3 (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 electrode 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, materials with different absorptivity of propagating light can be combined. For example, the material can be set in such a way that the absorptivity of the segmented electrode is smaller than the absorptivity of the signal transmission part. More specifically, by setting the segmented electrode to a conductor (transparent electrode) that is transparent to the propagating light, 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 forming step is performed between the segment electrode forming step and the signal transmission portion forming step.

[0079] In Fig. 2(a) to Fig. 2(d), a buffer layer (not shown) is disposed between the rib-type optical waveguide 10 and the modulation electrode (signal electrode 2S, ground electrode 2G) at the portion where the rib-type optical waveguide 10 overlaps with the modulation electrode. This can prevent the light propagating in the optical waveguide 10 from being absorbed or scattered by the modulation electrode.

[0080] 5(a) to 5(d) show the 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] Moreover, as shown in FIG. 5( b ) and FIG. 5( c ), by forming the buffer layer BF to cover a portion of the segment electrode SE, it is also helpful to prevent the segment electrode from peeling off. Specifically, the linear expansion coefficients of the segment electrode SE and the substrate 1 are different, so peeling off is easily caused by temperature changes. Therefore, the bonding strength between the buffer layer BF and the substrate 1 can be used to prevent the segment electrode from peeling off.

[0082] As shown in FIG5(c), there is no buffer layer BF between the segment electrode (SE) and the 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 is concentrated, so compared with a structure in which a buffer layer is arranged between the segment electrode and the substrate 1, the influence of the DC drift phenomenon can be greatly reduced.

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

[0084] 6( a ) to 6 ( d ) show a case where a buffer layer BF is formed on the entire surface of the substrate 1 , and an opening OP of the buffer layer BF is formed at a portion where the segment electrode SE and the signal transmission portion SS are electrically connected.

[0085] By inserting the buffer layer BF between the signal transmission part SS and the substrate 1 with a high dielectric constant, it is possible to achieve low loss of high-frequency signals as modulation signals. In addition, in order to form on a high dielectric constant LN substrate, when the design is given priority to the driving voltage, the characteristic impedance becomes low in the conventional electrode structure. In the present invention, since the electrode spacing of the signal transmission part SS can be adjusted, it is possible to achieve a design that achieves high impedance and further suppresses reflection.

[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 propagation light of the optical waveguide.

[0087] In addition, when viewed from the perspective of high-frequency signals, the configuration in which the buffer layer is provided only near the optical waveguide (including a portion of the segment electrode) as shown in FIG5 (a) to FIG5 (d) is a cause of signal attenuation compared to the case in which no buffer layer is provided. However, as shown in FIG6 (a) to FIG6 (d), the configuration in which the signal transmission portion is separated from the LN substrate having a high dielectric constant contributes to the reduction of signal attenuation.

[0088] As shown in Fig. 6(b) and Fig. 6(c), a buffer layer can be sandwiched between the segment electrode and the signal transmission unit. In this way, it is possible to arrange the segment electrode directly in contact with the waveguide layer (substrate 1) without interrupting the buffer layer on the optical waveguide, and the signal transmission unit crossing the optical waveguide through the buffer layer.

[0089] The material of the buffer layer can be formed into a thin film, and a material that transmits infrared light can be used. If the material has a small dielectric loss tangent, the transmission characteristics of high-frequency signals can be suppressed from deteriorating. Specifically, ceramics such as SiO2 and Al2O3, or glass, optical resins, etc. can be used.

[0090] When a 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 of 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 (except for the portion overlapping with the optical waveguide) as shown in Figure 7(a), or the opening portion OP may be formed on the lower portion of the signal transmission portion SS and only on a portion thereof as shown in Figure 7(b).

[0091] The shape of the segment electrode SE is not limited to the “T-shape” as shown in FIGS. 2( a ) to 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 a structure in which the optical waveguide is folded back. Such a folded back structure can lengthen the length of the active portion without changing the length of the optical waveguide element (chip), and can also bring about the effect of reducing the driving voltage. In this case, the folded back portion can be bent as small as possible to suppress the total length of the line, which is advantageous from the viewpoint of signal loss and saving of mounting area.

[0093] Furthermore, it is of course possible to apply the present invention to an optical waveguide element in which a plurality of Mach-Zehnder optical waveguides are integrated in parallel, or to an optical waveguide element in which Mach-Zehnder optical waveguides are assembled in a nested manner.

[0094] Next, an example of applying the optical waveguide element of the present invention to an optical modulator or an optical transmitter will be described. The optical modulator using the optical waveguide element will be described below, but the present invention is not limited thereto 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 optical waveguides, a junction device with an optical path element made of other materials such as silicon, a device for sensor use, etc. Moreover, it can of course be applied to a high bandwidth coherent driver modulator (HB-CDM: High Bandwidth-Coherent Driver Modulator).

[0095] like Fig. 9As shown, the optical waveguide element has an optical waveguide 10 formed on a substrate 1 and a modulation electrode (not shown) for modulating the light wave propagating in the optical waveguide 10, and is housed in a housing CA. Furthermore, by providing an optical fiber (F) for inputting and outputting light waves in the optical waveguide, an optical modulation device MD can be formed. In FIG. 5 (a) to FIG. 5 (d), the optical fiber F is optically coupled to the optical waveguide 10 in the optical waveguide element using an optical module, a lens barrel, etc. having an optical lens. Not limited to this, the optical fiber can also be introduced into the housing through a through hole penetrating the side wall of the housing, the optical component or the substrate can be directly connected to the optical fiber, or the optical fiber having a lens function at the end of the optical fiber can be optically coupled to the optical waveguide in the optical waveguide element. In addition, in order to stably perform the connection with the optical fiber and the optical module, the reinforcing component 3 can also be overlapped and arranged 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 for performing a modulation operation to the optical modulation device MD to the optical modulation device MD, an optical transmission device OTA can be formed. In order to obtain the modulation signal S applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. Therefore, in Figures 5 (a) to 5 (d), a drive circuit DRV is used to amplify the modulation signal. The drive circuit DRV and the digital signal processor DSP can also be arranged outside the housing CA, but can also be arranged inside the housing CA. In particular, by arranging the drive circuit DRV inside the housing, the transmission loss of the modulation signal from the drive circuit can be further reduced.

[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] In an optical modulator with a structure where the high-frequency electrode and the DC bias electrode are separated, a DC-cut driving circuit is usually used to input a signal. However, in the case of a driver-integrated modulator in which the driving circuit is arranged in a housing, the requirements for high-frequency performance and miniaturization of the element size are stronger, so a DC coupling structure is adopted in which a DC bias electrode is not arranged but directly connected to the modulation electrode. In this case, the bias voltage is always input to the modulation electrode through the output of the driving circuit, so the influence of the DC drift phenomenon becomes greater.

[0099] In the optical modulation device and optical transmission apparatus of the present invention, segment electrodes that contribute to modulation can be formed directly on the substrate without passing through a buffer layer, so that the DC drift phenomenon can be further reduced, and it can be said that it is suitable for a driver integrated structure.

[0100] Industrial availability

[0101] As described above, according to the present invention, it is possible to provide an optical waveguide element that can achieve impedance matching in modulation electrodes, speed matching of light waves propagating in an optical waveguide and a modulation signal, and can be easily manufactured. Furthermore, it is possible to provide an optical modulation device and an optical transmission apparatus using such an optical waveguide element.

Claims

1. An optical waveguide element, comprising a rib-type optical waveguide formed on a substrate, and comprising modulation electrodes arranged on the substrate so as to sandwich the rib-type optical waveguide, characterized in that: The modulation electrode comprises a segment electrode and a signal transmission part, wherein 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, and the signal transmission part 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 at 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. The buffer layer is not disposed between the substrate and the segment electrode.

2. The optical waveguide element according to claim 1, wherein The buffer layer covering the rib-type optical waveguide is arranged in a manner covering 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 portion 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 that overlaps with the signal transmission portion and a portion B that does not overlap 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 accommodated in a housing, and the optical modulation device includes an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

6. The optical modulation device according to claim 5, characterized in that: The optical waveguide element includes a modulation electrode for modulating a light wave propagating in 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. An optical transmitting device, characterized in that: The 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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