Optical modulator

Abstract

To provide an optical waveguide element with which the phase shift of a differential modulation signal propagating through electrodes is suppressed.SOLUTION: Provided is an optical waveguide element in which two modulation electrodes (E1, E2) for applying a differential modulation signal to each of two branch waveguides 10 constituting a Mach-Zehnder type optical waveguide are arranged. Each of the modulation electrodes is composed from a plurality of proximity electrodes (PE11-PE22) arranged separately along the branch waveguides, signal electrodes (LE1, LE2) for propagating the modulation signals, and bypass electrodes (BE1, BE2) for linking the proximity electrodes and the signal electrodes. Ground electrodes (G1, G2) are arranged so as to sandwich the two modulation electrodes (E1, E2), and capacitance adjustment mechanisms LET11-LET12 for adjusting the phase speed of the modulation signal propagating through the modulation electrodes are provided between the modulation electrode E1 and the ground electrode G1 that are adjacent to each other.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element, an optical modulation device using the same, and an optical transmission apparatus, and more particularly to an optical waveguide element in which an optical waveguide including at least one Mach-Zehnder optical waveguide is formed on a substrate, and two modulation electrodes for applying differential modulation signals are arranged on each of two branch waveguides constituting the Mach-Zehnder optical waveguide. [Background technology]

[0002] In the fields of optical communications and optical measurement, optical waveguide elements, such as optical modulators, in which optical waveguides are formed on a substrate, are widely used. In recent years, optical modulators included in transmitters built into optical transceivers have been required to be more compact, consume less power, and have wider bandwidths and lower drive voltages for their drive signals. To meet these demands for smaller size and wider bandwidths for drive signals, thin plates of less than a few micrometers are used for the substrates on which the optical waveguides are formed. Furthermore, to reduce the drive voltage, optical waveguide elements are driven by differential modulation signals.

[0003] Patent Document 1 discloses an optical waveguide element that uses such a thin plate and is driven by a differential modulation signal. Figures 1 and 2 are plan views showing a portion of the optical waveguide element disclosed in Patent Document 1. In Figure 1, an optical waveguide 10 is two branch waveguides that constitute a Mach-Zehnder optical waveguide, and each branch waveguide 10 is driven in a push-pull manner by electrodes E1 and E2 to which a differential modulation signal is applied. Electrodes E1 and E2 are configured such that a plurality of "T"- or "H"-shaped fine electrodes (segment electrodes) are connected to strip-shaped signal electrodes LE1 and LE2 through which the modulation signal propagates.

[0004] Also proposed is a configuration in which an "H"-shaped segment electrode is connected to both the signal electrodes LE1 and LE2, as shown in Fig. 2. Each segment electrode is arranged close to the optical waveguide (branch waveguide) 10 and is composed of a proximity electrode (PE1 to PE22) that applies an electric field to the optical waveguide, and a bypass electrode (BE1, BE2) that connects the proximity electrode and the signal electrode (LE1, LE2).

[0005] A differential modulation signal propagates through each of the electrodes E1 and E2. For this reason, modulation signals of opposite phases must always be applied to adjacent electrodes (for example, PE1 and PE21, or PE1 and PE22) in the same optical waveguide 10. However, because the segment electrodes connected to the electrodes E1 and E2 do not necessarily have the same shape, a phenomenon occurs in which the phases of the differential modulation signals propagating through the electrodes E1 and E2 gradually shift.

[0006] In Figure 1, the shape of the segment electrodes is different, being either "T" shaped or "H" shaped, but in Figure 2, each segment electrode has the same "H" shape, but the spacing between adjacent electrodes (PE11 and PE12, PE21 and PE22) is different from each other, which results in different propagation speeds of the differential modulation signal and a phase shift in the modulation signal. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] China patent publication CN115586663A Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to solve the above-mentioned problems by providing an optical waveguide element that suppresses the phase shift of a differential modulation signal propagating through electrodes, and further aims to provide an optical modulation device and an optical transmission apparatus that use such an optical waveguide element. [Means for solving the problem]

[0009] 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 in which an optical waveguide including at least one Mach-Zehnder optical waveguide is formed on a substrate, and two modulation electrodes for applying a differential modulation signal are arranged on each of two branch waveguides constituting the Mach-Zehnder optical waveguide, each of the modulation electrodes being composed of a plurality of adjacent electrodes arranged along the branch waveguide, a signal electrode for propagating the modulation signal, and a bypass electrode connecting the adjacent electrode and the signal electrode, characterized in that ground electrodes are arranged to sandwich the two modulation electrodes, and a capacitance adjustment mechanism is provided between the adjacent modulation electrode and the ground electrode to adjust the phase velocity of the modulation signal propagating through the modulation electrode.

[0010] (2) In the optical waveguide element described in (1) above, the capacitance adjustment mechanism is a dummy electrode formed on a part of the modulation electrode or the ground electrode, and does not generate an electric field to be applied to the branch waveguide.

[0011] (3) In the optical waveguide element described in (2) above, the dummy electrodes have a first dummy electrode provided on the modulation electrode and a second dummy electrode provided on the ground electrode, and the distance from the signal electrode to the portion of the first dummy electrode that is farthest from the signal electrode is longer than the distance from the signal electrode to the portion of the second dummy electrode that is closest to the signal electrode.

[0012] (4) In the optical waveguide element described in (2) above, the dummy electrodes have a first dummy electrode provided on the modulation electrode and a second dummy electrode provided on the ground electrode, and the distance from the signal electrode to the portion of the first dummy electrode that is farthest from the signal electrode is shorter than the distance from the signal electrode to the portion of the second dummy electrode that is closest to the signal electrode.

[0013] (5) In the optical waveguide element described in (2) above, the dummy electrode is a first dummy electrode provided on the modulation electrode, and the ground electrode has a shape that surrounds a part of the first dummy electrode.

[0014] (6) The optical waveguide element according to (2) above is characterized in that the length λ1 of the dummy electrode along the signal electrode and the interval λ0 between adjacent bypass electrodes are different from each other.

[0015] (7) In the optical waveguide element described in (1) above, the capacitance adjusting mechanism has a configuration for adjusting the distance between the adjacent modulation electrode and the ground electrode.

[0016] (8) The optical waveguide element according to (1) above, characterized in that a buffer layer is formed on the substrate, the adjacent electrode is disposed between the substrate and the buffer layer, and the signal electrode and a portion of the bypass electrode are disposed on the buffer layer.

[0017] (9) The optical waveguide element according to (1) above is characterized in that a dummy optical waveguide that does not propagate optical waves is disposed between the adjacent modulation electrode and ground electrode.

[0018] (10) In the optical waveguide element described in (1) above, a capacitor is formed in a part of the modulation electrode or in a part of the signal line electrically connected to the modulation electrode to block the DC component of the modulation signal.

[0019] (11) The optical waveguide element according to any one of (1) to (10) above is an optical modulation device that is housed in a housing and includes an optical fiber that inputs or outputs a light wave to or from the optical waveguide.

[0020] (12) In the optical modulation device described in (11) above, the optical waveguide element is provided with a modulation electrode for modulating the 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] (13) An optical transmitter comprising the optical modulation device according to (11) or (12) above, and an electronic circuit for outputting a modulation signal that causes the optical modulation device to perform a modulation operation. [Effects of the Invention]

[0022] The present invention provides an optical waveguide element including an optical waveguide including at least one Mach-Zehnder optical waveguide formed on a substrate, and two modulation electrodes for applying a differential modulation signal to each of two branch waveguides constituting the Mach-Zehnder optical waveguide, wherein each of the modulation electrodes comprises a plurality of adjacent electrodes arranged along the branch waveguide, a signal electrode for propagating the modulation signal, and a bypass electrode connecting the adjacent electrode and the signal electrode, and wherein ground electrodes are arranged to sandwich the two modulation electrodes, and a capacitance adjustment mechanism is provided between the adjacent modulation electrode and the ground electrode for adjusting the phase velocity of the modulation signal propagating through the modulation electrode, thereby making it possible to provide an optical waveguide element in which the phase shift of the differential modulation signal propagating through the electrodes is suppressed. Furthermore, by using this optical waveguide element, it is possible to provide an optical modulation device and an optical transmission device with similarly excellent characteristics. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a plan view showing an example of an optical waveguide element disclosed in Patent Document 1. [Figure 2] FIG. 10 is a plan view showing another example of the optical waveguide element disclosed in Patent Document 1. [Figure 3] FIG. 10 is a plan view illustrating an example of an optical waveguide element using segment electrodes. [Figure 4] FIG. 4 is a diagram showing an example of a cross-sectional view taken along dotted lines A to C in FIG. [Figure 5]3. FIG. 6 is a diagram showing another example of the cross-sectional view taken along dotted lines A to C in FIG. [Figure 6] FIG. 1 is a plan view illustrating a first embodiment of an optical waveguide element according to the present invention. [Figure 7] FIG. 2 is a plan view illustrating a second embodiment of the optical waveguide element of the present invention. [Figure 8] 10A and 10B are diagrams illustrating an example of adjusting the distance between a modulation electrode and a ground electrode. [Figure 9] 10A and 10B are diagrams illustrating an example in which the distance between a modulation electrode and a ground electrode is changed. [Figure 10] 10A and 10B are diagrams illustrating a "T"-shaped dummy electrode provided on the ground electrode. [Figure 11] 10 is a diagram illustrating the relationship between the length λ1 of a dummy electrode provided on a ground electrode and the spacing λ0 of bypass electrodes. FIG. [Figure 12] FIG. 10 is a diagram illustrating an example in which a first dummy electrode provided on a modulation electrode and a second dummy electrode provided on a ground electrode are arranged to face each other. [Figure 13] 10 is a diagram illustrating an example in which "T"-shaped dummy electrodes provided on both the modulation electrode and the ground electrode are arranged in an intertwined manner. FIG. [Figure 14] 10 is a diagram illustrating an example in which substantially "H"-shaped dummy electrodes provided on both the modulation electrode and the ground electrode are arranged in a mutually intertwined manner. FIG. [Figure 15] 10A and 10B are diagrams illustrating an example in which "L"-shaped dummy electrodes provided on both the modulation electrode and the ground electrode are arranged in an intertwined manner. [Figure 16] 10A and 10B are diagrams illustrating an example in which the shape of the ground electrode is set so as to surround a part of the dummy electrode of the modulation electrode. [Figure 17] FIG. 2 is a plan view illustrating a configuration in which the symmetry of two modulation electrodes (E1 and E2) and a ground electrode is enhanced in the optical waveguide element of the present invention. [Figure 18] 18A to 18C are diagrams illustrating an application example of the optical waveguide element shown in FIG. 17. [Figure 19] FIG. 10 is a plan view illustrating an example in which a dummy optical waveguide is used in the optical waveguide element of the present invention. [Figure 20]10 is a plan view illustrating an example in which dummy electrodes provided on both the modulation electrode and the ground electrode are spaced apart in the thickness direction of the substrate. FIG. [Figure 21] FIG. 21 is a diagram showing an example of a cross-sectional view taken along dotted line A in FIG. 20. [Figure 22] 1 is a diagram illustrating an optical transmitting device according to the present invention. [Figure 23] 10A and 10B are diagrams illustrating an example in which a capacitor is disposed in a part of a modulation electrode of an optical waveguide element. [Figure 24] 10A and 10B are diagrams illustrating an example in which a capacitor is disposed in a signal wiring connected to a modulation electrode of an optical waveguide element. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will be described in detail below using preferred examples. As shown in FIG. 6, for example, the present invention is an optical waveguide element in which an optical waveguide including at least one Mach-Zehnder optical waveguide is formed on a substrate, and two modulation electrodes (E1, E2) for applying a differential modulation signal are arranged for each of two branch waveguides 10 constituting the Mach-Zehnder optical waveguide, wherein each of the modulation electrodes is composed of a plurality of adjacent electrodes (PE11-PE22) divided and arranged along the branch waveguide, signal electrodes (LE1, LE2) for propagating the modulation signal, and bypass electrodes (BE1, BE2) connecting the adjacent electrodes and the signal electrodes, and wherein ground electrodes (G1, G2) are arranged to sandwich the two modulation electrodes (E1, E2), and capacitance adjustment mechanisms LET11-LET12 (LET21-LET22) for adjusting the phase velocity of the modulation signal propagating through the modulation electrodes are provided between the adjacent modulation electrode E1 (E2) and the ground electrode G1 (G2).

[0025] First, the structure of an optical waveguide element using segment electrodes will be explained. Figure 3 is a plan view showing a portion of the optical waveguide element using the segment electrodes of Figure 2. Two modulation electrodes (E1, E2) are arranged for the two branch waveguides 10 of the Mach-Zehnder optical waveguide. Proximal electrodes (PE11 to PE22) are arranged for each modulation electrode so that an electric field based on a modulation signal can always be applied to the two branch waveguides. In addition, ground electrodes (G1, G2) are arranged to sandwich the two modulation electrodes (E1, E2).

[0026] Parts of the cross section taken along dotted lines A to C in Fig. 3 are shown in Fig. 4 and Fig. 5. In Fig. 4 and Fig. 5, a lower layer UL made of a material with a lower refractive index than the optical waveguide substrate 1 is provided on the lower surface side of the optical waveguide substrate 1 provided with the rib-type optical waveguide 10. When a holding substrate is provided below the lower layer UL, the lower layer UL is sometimes called an intermediate layer. In addition, a buffer layer BL made of a material with a lower refractive index than the optical waveguide substrate 1 is disposed on the upper surface side of the optical waveguide substrate 1.

[0027] In Figure 4, modulation electrodes (E1, E2) are arranged on the upper surface side of the buffer layer BL, and the signal electrodes (LE1, LE2), bypass electrodes (BE1, BE2), and proximity electrodes (PE11-22) that make up the modulation electrodes are all located on the upper side of the buffer layer BL.

[0028] 5, the proximal electrodes (PE11 to PE22) are arranged between the optical waveguide substrate 1 and the buffer layer BL, and by arranging the electrodes closer to the optical waveguide 10, an electric field can be efficiently applied to the optical waveguide. However, the signal electrodes (LE1, LE2) are arranged on the upper surface side of the buffer layer BL, and the bypass electrodes (BE1, BE2) penetrate the buffer layer BL to connect the signal electrodes and proximal electrodes. The signal electrodes are arranged on the upper side of the optical waveguide substrate, and some of them may intersect with the optical waveguide 10. Therefore, in order to suppress absorption or scattering of light waves propagating through the optical waveguide, the signal electrodes are arranged spaced apart from each other via the buffer layer BL.

[0029] Unless otherwise specified, the optical waveguide element of the present invention can selectively employ either the arrangement shown in Fig. 4 or the arrangement shown in Fig. 5. As described above, the arrangement shown in Fig. 5 is more efficient in terms of applying an electric field, but the manufacturing process becomes more complicated, so each arrangement should be selected appropriately taking these factors into consideration.

[0030] The optical waveguide substrate 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect. Specifically, single crystal materials such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), as well as materials obtained by doping these substrate materials with MgO or the like, can be used. These materials can also be used to form films using vapor phase growth methods such as sputtering, evaporation, or CVD. Alternatively, a substrate having an electro-optic effect can be bonded to another substrate and then thin-film processed to form a thin film. Furthermore, semiconductor substrates and substrates made of organic materials such as EO polymers can also be used.

[0031] The optical waveguide 10 can be an optical waveguide in which a high refractive index material such as Ti is thermally diffused into the optical waveguide substrate 1, an optical waveguide formed by proton exchange, or a rib-type optical waveguide 10 in which the portion of the substrate corresponding to the optical waveguide is convex, as shown in Figures 4 or 5, by etching the substrate 1 other than the optical waveguide or by forming grooves on both sides of the optical waveguide. Furthermore, in accordance with the rib-type optical waveguide, it is possible to further increase the refractive index by diffusing Ti or the like onto the substrate surface by thermal diffusion or proton exchange. The size of the rib-type optical waveguide is a finely structured optical waveguide with a width and height of approximately 1 μm or less to enhance light confinement.

[0032] The thickness (maximum thickness) of the optical waveguide substrate 1 on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to achieve speed matching between the microwave and light waves of the modulated signal. The height of the rib-type optical waveguide 10 (the height of the part protruding from the slab waveguide) is set to 80% or less of the maximum thickness of the optical waveguide substrate, specifically 4 μm or less, more preferably 3 μm or less, and even more preferably 0.8 μm or less or 0.4 μm or less.

[0033] A lower layer is provided on the underside of the optical waveguide substrate 1 on which the optical waveguide is formed. To increase the mechanical strength of the optical waveguide element, a holding substrate may be bonded to the underside of the optical waveguide substrate 1. The optical waveguide substrate 1 and the holding substrate are bonded directly or via an adhesive layer such as a resin. The holding substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but this is not limited to this. In the case of direct bonding, an intermediate layer such as a metal oxide or metal may be included in the bonding portion. Furthermore, the holding substrate is preferably made of a material with a thermal expansion coefficient similar to that of the optical waveguide substrate 1, such as glass, quartz, fused silica, synthetic quartz, Eagle glass, alkali glass, alkali-free glass, lead glass, Pyrex (registered trademark) glass, soda glass, sapphire, alumina, or a substrate containing an oxide layer such as a SiO2-based or Al2O3-based low-dielectric-constant substrate. Furthermore, it is also possible to use the same LN substrate as the optical waveguide substrate 1, a composite substrate in which a silicon oxide layer is formed on a silicon substrate, abbreviated as SOI or LNOI, or a composite substrate in which a silicon oxide layer is formed on an LN substrate. If the refractive index of the holding substrate is higher than that of the optical waveguide substrate 1, a layer (intermediate layer) with a refractive index lower than that of the optical waveguide substrate 1 is provided between the optical waveguide substrate 1 and the holding substrate.

[0034] For example, it is possible to use a glass substrate as the holding substrate, provide a bonding layer (intermediate layer) such as SiO2 on the upper surface of the holding substrate via an adhesive layer such as Si, and then arrange the optical waveguide substrate 1. In addition, a buffer layer BL is arranged on the upper side of the optical waveguide substrate 1.

[0035] In the optical waveguide element of the present invention, the buffer layer BL and the lower layer UL sandwiching the optical waveguide substrate 1 function as cladding layers for the optical waveguide 10, and therefore use a dielectric material having a lower refractive index and higher transparency than the optical waveguide substrate 1. Specifically, oxides and fluorides of metal elements from groups 1 to 17 of the periodic table, such as SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, CaF2, and Y2O3, are used.

[0036] The electrodes (E1, E2) arranged on the upper side of the optical waveguide substrate 1 are made of metals such as Au and Cu. To increase the adhesive strength between the electrodes and the optical waveguide substrate or buffer layer on which they are arranged, the electrodes can be configured as a multilayer structure consisting of an upper electrode and an underlayer. The upper electrode is formed to cover the underlayer by electrolytic plating using an underlayer, electroless plating using a resist pattern, vapor phase methods such as vapor deposition and sputtering, or a combination of these. Materials such as Ti, Nb, Ni, Cr, or Al are used as the material for the underlayer, and it is formed on the upper surface of the optical waveguide substrate by sputtering, vapor deposition, or the like.

[0037] The optical waveguide element of the present invention is characterized in that at least one of the two modulation electrodes (E1, E2) is provided with a capacitance adjustment mechanism for adjusting the phase velocity of the modulation signal propagating through the modulation electrode. As mentioned above, the structure of the two modulation electrodes (E1, E2), which are the lines that transmit the differential modulation signal, is asymmetric, so there is a difference in the capacitance of each modulation electrode. The phase velocity of the high-frequency signal depends on the capacitance of the line (for example, the approximate solution for the propagation velocity v is v = 1 / (LC)). 1 / 2 where L is the inductance of the line and C is the capacitance of the line.) Therefore, by partially adjusting the capacitance of the modulation electrode, it is possible to match the phase velocities of the differential modulation signals propagating through the two lines.

[0038] When the capacitance of the modulation electrode changes, the characteristic impedance of the line also changes. Therefore, the capacitance adjustment mechanism of the present invention can also be used for characteristic impedance matching and propagation velocity matching between light waves and modulation signals. The characteristic impedance of the modulation electrode (signal electrode) is set to 80 to 120 Ω, preferably 90 to 110 Ω, and more preferably 95 to 105 Ω.

[0039] The specific configuration of the capacitance adjustment mechanism is not particularly limited as long as it is capable of adjusting the phase velocity of the modulation signal propagating through the electrodes, and the following configurations can be exemplified. (1) A dummy electrode that does not generate an electric field to be applied to the optical waveguide is provided. The dummy electrode is located between the adjacent modulation electrode and ground electrode, and may be connected to either electrode. (2) The distance between the adjacent modulation electrode and ground electrode is partially adjusted. (3) As another arrangement position of the dummy electrode, it is also possible to arrange it by connecting it to at least one of the adjacent electrode, the bypass electrode, and the signal electrode. (4) It is also possible to address this issue by changing the electrode width or electrode thickness of at least a part of the adjacent electrodes, bypass electrodes, or signal electrodes. The following will mainly focus on (1) and (2) above. It goes without saying that it is also possible to use a combination of (1) to (4) above.

[0040] Specific examples of the capacity adjusting mechanism will be described below with reference to FIGS. 6 is a plan view showing a first embodiment of the optical waveguide element of the present invention. As a capacitance adjustment mechanism, dummy electrodes LET11 to LET12 (LET21 to LET22) are connected between the modulation electrode E1 (E2) and the ground electrode G1 (G2), more specifically, on the ground electrode G1 (G2) side of the signal electrode LE1 (LE2) that constitutes the modulation electrode.

[0041] 6 shows a "T"-shaped dummy electrode as an example, but as will be described later, the shape of the dummy electrode is not limited to this. In addition, by changing the dimensions of each part of the "T"-shaped dummy electrode, it is possible to fine-tune the capacitance.

[0042] FIG. 7 shows a second embodiment of the optical waveguide element of the present invention, in which dummy electrodes GT11 to GT12 (GT21 to GT22) connected to the ground electrode G1 (G2) are provided between the modulation electrode E1 (E2) and the ground electrode G1 (G2).

[0043] As another example of a capacitance adjustment mechanism, the capacitance of the line can also be changed by adjusting the distance d between the signal electrode LE constituting the modulation electrode and the adjacent ground electrode G, as shown in Fig. 8. The symbol BE denotes a bypass electrode constituting the segment electrode.

[0044] FIG. 9 illustrates an example in which the distance between the signal electrode LE and the ground electrode G is changed in stages. The distance (d1 to d3) between the signal electrode LE and each portion of the ground electrode (GS1 to GS3) is changed. As the distance (d1 to d3) increases, the capacitance generated decreases. By partially adjusting the distance between the ground electrode and the signal electrode, it is possible to fulfill the role of a capacitance adjustment mechanism. It is also possible to change the length along the signal electrode of each portion (GS1 to GS3) in FIG. 9, and furthermore, it is also possible to change the distance continuously rather than in stages.

[0045] Using Figure 10, we will explain the effect of changing the shape of the dummy electrodes (GS4 to GS6) provided on the ground electrode. All of the dummy electrodes (GS4 to GS6) have the same distance between the ground electrode G and the signal electrode LE. Therefore, the capacitance of the lines formed by each dummy electrode is generally similar. However, even changing the thickness of the horizontal line portion GT1 at the top of the "T" shape forming the dummy electrode can finely adjust the capacitance. This is because the electric field distribution formed between the signal electrode LE and the dummy electrode changes depending on the thickness of the horizontal line portion GT1. However, when the thickness (t1, t2) becomes larger than the distance between the signal electrode LE and the ground electrode G, there is almost no change in the electric field distribution, and the result is essentially the same as the dummy electrode GS6. Varying the thickness of the vertical line portion GT2 of the "T" shape also has some effect on capacitance adjustment, but this effect is greater for the dummy electrodes formed on the signal electrodes than for the ground electrodes.

[0046] Next, using Figure 11, we will explain the effect of changing the length λ1 of the dummy electrodes (GS7 ​​to GS8) along the signal electrode LE. In the optical waveguide element of the present invention, multiple segment electrodes are periodically arranged on the modulation electrode, so resonance is likely to occur when a modulation signal is applied. For this reason, the occurrence of this resonance can be suppressed by configuring the spacing λ0 between the bypass electrodes BE, which is the spacing between the segment electrodes, so that it does not match the length λ1 of the dummy electrodes (GS7 ​​to GS8). The length λ1 may be n times or 1 / n times the spacing λ0 (n is a natural number or a decimal).

[0047] Regarding the length λ1 of the dummy electrode, similar effects can be expected for the length λ1 of the horizontal line portions (LET11, GT11) of the "T"-shaped dummy electrode as shown in FIGS. 6 and 7 in addition to FIG.

[0048] FIG. 12 is a diagram illustrating an example in which dummy electrodes are provided on both the modulation electrode and the ground electrode. By arranging the dummy electrodes (LET1-LET2, GT1-GT2) facing each other, it becomes possible to adjust the line capacitance of the modulation electrodes more precisely. Specifically, the length α1 and width α3 of the horizontal line portion (LET1, GT1) of the "T" shape of each dummy electrode, as well as the width α2 of the vertical line portion (LET2, GT2), are adjusted. Note that the shapes of the opposing dummy electrodes do not need to be symmetrical, and their shapes are not limited to a "T" shape. Furthermore, the shape of one dummy electrode may be a "T" shape, and the shape of the other dummy electrode may be a different shape.

[0049] Figure 12 illustrates an example in which the dummy electrode has a dummy electrode (first dummy electrode) provided on the modulation electrode and a dummy electrode (second dummy electrode) provided on the ground electrode, and the distance from the signal electrode LE to the part of the first dummy electrode that is farthest from the signal electrode is shorter than the distance from the signal electrode to the part of the second dummy electrode that is closest to the signal electrode. In the following, we will explain the case where the distance from the signal electrode to the part of the first dummy electrode that is farthest from the signal electrode is longer than the distance from the signal electrode to the part of the second dummy electrode that is closest to the signal electrode (so-called a state in which the dummy electrodes are intertwined).

[0050] 13 to 15 show a state in which the dummy electrodes are intertwined, and FIG. 16 illustrates a state in which the dummy electrodes are embedded within the ground electrodes. The shapes and arrangements of the dummy electrodes in FIGS. 13 to 16 contribute to further enhancing the capacitance adjustment function of the dummy electrodes. By surrounding a portion of the dummy electrode formed on the signal electrode with the ground electrode itself or with a dummy electrode formed on the ground electrode, the capacitance can be adjusted more precisely. Specifically, in FIG. 12, only a portion of the dummy electrode, for example, the periphery of the horizontal line portion of the "T" shape close to the ground electrode, contributes to capacitance adjustment. In contrast, in FIGS. 13 to 16, the entire portion surrounded by the ground electrode can contribute to capacitance adjustment. Therefore, the shape of the dummy electrode and the distance from the surrounding ground electrode (the dummy electrode connected to the ground electrode) affect the capacitance adjustment, enabling more precise capacitance adjustment.

[0051] In FIG. 13, a "T" shape is selected as the shape of the dummy electrodes, and the dummy electrodes (LET1 to LET2) serving as modulation electrodes and the dummy electrodes (GT1 to GT2) serving as ground electrodes are arranged in a mutually intertwined manner. In FIG. 14, a substantially "H" shape is used as the dummy electrodes, and similar to FIG. 13, the dummy electrodes (LEH1 to LEH4, GH1 to GH4) are intertwined with one another. In addition, in FIG. 15, "L"-shaped dummy electrodes (LEL1 to LEL2, GL1 to GL2) are arranged in a complicated manner.

[0052] 16 shows an example in which the dummy electrode LEI of the modulation electrode is surrounded by the ground electrode body. The dummy electrode is shown to be "I" shaped, but is not limited to this and may be "T" shaped, approximately "H" shaped, or "L" shaped.

[0053] As shown in Figure 17, by adding multiple dummy electrodes (LEH1-LEH2, GH1-GH2) between the modulation electrode E1 (E2) and the ground electrode G1 (G2), and configuring the two modulation electrodes so that the electrode shapes seen as a whole are almost the same, the capacitance of each line is made the same and the phase velocity is adjusted. Moreover, by improving the symmetry of the shapes of the two modulation electrodes, the distribution of internal stress that the electrodes exert on the optical waveguide substrate due to thermal expansion, etc. is made uniform, which also contributes to suppressing the occurrence of temperature drift. 18, the position of the dummy electrode LEH14 (LEH24) is set to match the position of the bypass electrode BE1 (BE2), thereby further improving the symmetry of the two modulation electrodes.

[0054] 19, in addition to the optical waveguide (branching waveguide) 10 that propagates optical waves, it is also possible to arrange dummy optical waveguides DW1 to DW2 that do not propagate optical waves in the vicinity of the optical waveguide (branching waveguide) 10. The presence of these dummy optical waveguides DW1 to DW2 further improves the symmetry of the shapes of the two modulation electrodes, and the distribution of internal stress that the electrodes exert on the optical waveguide substrate due to thermal expansion, etc. becomes more uniform.

[0055] 20 and 21 illustrate a method of stacking two dummy electrodes with a buffer layer sandwiched between them. This method provides a higher capacitance adjustment function than arranging two dummy electrodes side by side on the same plane. FIG. 21 shows a cross-sectional view taken along dotted line A in FIG. 20, in which the horizontal line portions LET1 and GT1 of the "T"-shaped dummy electrodes are arranged to overlap each other in the thickness direction of the substrate 1.

[0056] 22 is a diagram showing an example of an optical transmission device. In recent years, optical modulation devices such as a High Bandwidth-Coherent Driver Modulator (HB-CDM) in which a driver IC, an optical waveguide element, and the like are integrated in the same housing have been attracting attention, and there is a growing need for a configuration suitable for miniaturization, such as the optical waveguide element of the present invention.

[0057] In the optical modulation device of the present invention, an optical waveguide element is disposed in a housing CA made of metal or the like. Input light L1 is input to an optical waveguide 10 formed in the optical waveguide element in the housing via an optical fiber FB or other optical components such as a lens. On the other hand, light waves output from the optical waveguide element are input to another optical fiber F to become output light L2. When outputting light, optical components such as a polarization combining means and a lens are used as needed. A modulation electrode (not shown) is formed on a substrate 1 of the optical waveguide element. Furthermore, reinforcing members RI for increasing mechanical strength are arranged on the substrate of the input / output sections of the optical waveguide element as needed.

[0058] In the optical modulation device, a driver circuit element DRV that generates an electrical signal S to be applied to a modulation electrode of the optical waveguide element is arranged adjacent to the optical waveguide element, and the optical waveguide element and the driver circuit element DRV are housed in the same housing CA.

[0059] Furthermore, it is possible to provide a signal generator DSP (digital signal processing device) that generates a modulated signal So to be input to the driver circuit element DRV, and configure it as an optical transmitter. The housing CA and the signal generator DSP can also be incorporated into a single chassis.

[0060] 23 and 24, it is also possible to block DC components contained in the modulation signal by providing a capacitor in part of the signal electrode of the modulation electrode or in part of the signal line that supplies the modulation signal to the modulation electrode. In FIG. 23, a capacitor CS1 (CS2) is provided on the input side of the modulation signal (differential modulation signals S+, S-) of the signal electrode LE1 (LE2). In FIG. 24, capacitors (CS3, CS4) are provided on signal lines (LE11, LE21) provided on the relay board RS to supply the modulation signal to the signal electrode. Symbols G11 and G21 are lines for ground electrodes provided on the relay board. [Industrial Applicability]

[0061] As described above, the present invention provides an optical waveguide element that suppresses the phase shift of a differential modulation signal propagating through electrodes, and further provides an optical modulation device and an optical transmission device that use such an optical waveguide element. [Explanation of symbols]

[0062] 1 Optical waveguide substrate 10 Optical waveguide (branching waveguide) E1, E2 modulation electrodes LE1,LE2 Signal electrode BE1, BE2 bypass electrodes PE11~PE22 adjacent electrodes LET11~LET12, GT11~GT12 dummy electrodes DW1, DW2 dummy optical waveguides

Claims

1. an optical waveguide including at least one Mach-Zehnder optical waveguide is formed on a substrate; In an optical waveguide element in which two modulation electrodes for applying differential modulation signals are arranged on two branch waveguides constituting the Mach-Zehnder optical waveguide, respectively, each of the modulation electrodes is composed of a plurality of adjacent electrodes arranged along the branch waveguide, a signal electrode for propagating the modulation signal, and a bypass electrode connecting the adjacent electrodes and the signal electrodes; An optical waveguide element characterized in that ground electrodes are arranged to sandwich the two modulation electrodes, and a capacitance adjustment mechanism is provided between the adjacent modulation electrodes and the ground electrodes to adjust the phase velocity of the modulation signal propagating through the modulation electrodes.

2. 2. The optical waveguide element according to claim 1, wherein the capacitance adjustment mechanism is a dummy electrode formed in a part of the modulation electrode or the ground electrode and does not generate an electric field to be applied to the branch waveguide.

3. 3. The optical waveguide element according to claim 2, wherein the dummy electrodes include a first dummy electrode provided on the modulation electrode and a second dummy electrode provided on the ground electrode, and the distance from the signal electrode to the portion of the first dummy electrode that is farthest from the signal electrode is longer than the distance from the signal electrode to the portion of the second dummy electrode that is closest to the signal electrode.

4. 3. The optical waveguide element according to claim 2, wherein the dummy electrodes include a first dummy electrode provided on the modulation electrode and a second dummy electrode provided on the ground electrode, and the distance from the signal electrode to the portion of the first dummy electrode that is farthest from the signal electrode is shorter than the distance from the signal electrode to the portion of the second dummy electrode that is closest to the signal electrode.

5. 3. The optical waveguide element according to claim 2, wherein the dummy electrode is a first dummy electrode provided on the modulation electrode, and the ground electrode has a shape surrounding a portion of the first dummy electrode.

6. 3. The optical waveguide element according to claim 2, wherein the length .lambda.1 of the dummy electrode along the signal electrode and the interval .lambda.0 between adjacent bypass electrodes are different in length.

7. 2. The optical waveguide element according to claim 1, wherein the capacitance adjusting mechanism has a structure for adjusting the distance between the adjacent modulation electrode and the adjacent ground electrode.

8. 2. The optical waveguide element according to claim 1, wherein a buffer layer is formed on the substrate, the proximity electrode is disposed between the substrate and the buffer layer, and the signal electrode and a portion of the bypass electrode are disposed on the buffer layer.

9. 2. The optical waveguide element according to claim 1, wherein a dummy optical waveguide that does not propagate optical waves is disposed between the adjacent modulation electrode and ground electrode.

10. 2. The optical waveguide element according to claim 1, wherein a capacitor for blocking a DC component of the modulation signal is formed in a part of the modulation electrode or in a part of the signal line electrically connected to the modulation electrode.

11. 2. An optical modulation device according to claim 1, wherein the optical waveguide element is accommodated in a housing and comprises an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

12. 12. The optical modulation device according to claim 11, wherein the optical waveguide element comprises a modulation electrode for modulating an optical 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.

13. 12. An optical transmitter comprising: an optical modulation device according to claim 11; and an electronic circuit for outputting a modulation signal for causing said optical modulation device to perform a modulation operation.

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