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 an optical waveguide including at least one Mach-Zehnder type optical waveguide is formed on a substrate, and two modulation electrodes (E1, E2) for applying a differential modulation signal to each of two branch waveguides 10 constituting the 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. At least one of the two modulation electrodes (E1, E2) is provided with a capacitance adjustment mechanism (DE) for adjusting the phase speed of the modulation signal propagating through the modulation electrode.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 differential modulation signals 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 separately along the branch waveguide, a signal electrode for propagating the modulation signals, and a bypass electrode connecting the adjacent electrode and the signal electrode, characterized in that at least one of the two modulation electrodes is provided with a capacitance adjustment mechanism for adjusting 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 adjusting mechanism is a dummy electrode formed in a part of the modulation electrode and does not generate an electric field to be applied to the branching waveguide.

[0011] (3) In the optical waveguide element described in (2) above, the dummy electrode is a first dummy electrode extending from the adjacent electrode to the opposite side to the bypass electrode.

[0012] (4) In the optical waveguide element described in (2) above, the dummy electrode is a second dummy electrode extending from the bypass electrode.

[0013] (5) In the optical waveguide element described in (2) above, the dummy electrode is a third dummy electrode extending from the signal electrode.

[0014] (6) In the optical waveguide element described in (5) above, the third dummy electrode is made up of a plurality of fine electrodes arranged between the adjacent bypass electrodes.

[0015] (7) In the optical waveguide element described in (1) above, the capacitance adjusting mechanism is characterized by changing the electrode width of at least a part of the adjacent electrode, the bypass electrode, or the signal electrode.

[0016] (8) In the optical waveguide element described in (1) above, the capacitance adjusting mechanism is characterized by changing the electrode thickness of at least a part of the bypass electrode or the signal electrode.

[0017] (9) The optical waveguide element according to any one of (1) to (8) 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 part of the bypass electrode are disposed on the buffer layer.

[0018] (10) The optical waveguide element according to any one of (1) to (9) above is characterized in that a dummy optical waveguide that does not propagate light waves is disposed adjacent to the branching waveguide.

[0019] (11) In the optical waveguide element described in any one of (1) to (10) 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, for blocking the DC component of the modulation signal.

[0020] (12) The optical waveguide element according to any one of (1) to (11) 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.

[0021] (13) In the optical modulation device described in (12) 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.

[0022] (14) 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]

[0023] The present invention provides an optical waveguide element including an optical waveguide formed on a substrate, the optical waveguide including at least one Mach-Zehnder optical waveguide, 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 at least one of the two modulation electrodes is provided with a capacitance adjustment mechanism 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]

[0024] [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. 7 is a diagram illustrating an application example of the first embodiment of FIG. 6. [Figure 8]7A to 7C are diagrams illustrating another application example of the first embodiment of FIG. 6. [Figure 9] FIG. 2 is a plan view illustrating a second embodiment of the optical waveguide element of the present invention. [Figure 10] FIG. 10 is a plan view illustrating a third embodiment of the optical waveguide element of the present invention. [Figure 11] FIG. 2 is a plan view illustrating a configuration in which the symmetry of two electrodes (E1 and E2) is enhanced in the optical waveguide element of the present invention. [Figure 12] 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 13] FIG. 11 is a plan view illustrating an application example of the third embodiment of FIG. [Figure 14] FIG. 10 is a plan view illustrating a fourth embodiment of the optical waveguide element of the present invention. [Figure 15] 15 is a diagram showing an example of a cross-sectional view taken along dotted lines A to C in FIG. [Figure 16] FIG. 10 is a plan view illustrating a fifth embodiment of the optical waveguide element of the present invention. [Figure 17] FIG. 17 is a diagram showing an example of a cross-sectional view taken along dotted line A in FIG. [Figure 18] 1 is a diagram illustrating an optical transmitting device according to the present invention. [Figure 19] 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 20] 10A and 10B are diagrams illustrating an example in which a capacitor is disposed in a signal wiring connected to a modulation electrode in an optical waveguide element. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described in detail below using preferred examples. The present invention is characterized in that, as shown in FIG. 6, for example, in 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, each of the modulation electrodes is composed of a plurality of adjacent electrodes (PE11 to 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 at least one of the two modulation electrodes (E1, E2) is provided with a capacitance adjustment mechanism (DE) for adjusting the phase velocity of the modulation signal propagating through the modulation electrode.

[0026] 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).

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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, and the dummy electrode is arranged so as to be connected to at least one of the adjacent electrode, the bypass electrode, and the signal electrode. (2) The electrode width or thickness of at least a part of the adjacent electrodes, bypass electrodes, or signal electrodes is changed. However, if the electrode thickness of the adjacent electrodes that apply an electric field to the optical waveguide is changed, the electric field distribution also changes. Therefore, for example, when arranging the adjacent electrodes (PE11 to PE22) as shown in Figure 5, it is preferable to configure the electrode thickness as it is. (3) Another possible location for the dummy electrode is between the adjacent modulation electrode and ground electrode, and the dummy electrode may be connected to either electrode. (4) Also, the distance between the adjacent modulation electrodes and the ground electrode is partially adjusted. The following will mainly describe the above (1) and (2) in detail. Of course, it is also possible to apply the above (1) and (2) in combination. It is also possible to use the above (1) to (4) in combination.

[0041] 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 device of the present invention. As a capacitance adjustment mechanism, a dummy electrode is connected to a part of the modulation electrode, more specifically, to the adjacent electrode. As shown in FIG. 6, a first dummy electrode DE is arranged extending from the adjacent electrode PE12 to the side opposite to the bypass electrode BE1.

[0042] The phase velocity of the modulation signal can be finely adjusted by adjusting the length α1 of the dummy electrode DE in the left-right direction of the drawing and the length α4 of the dummy electrode DE that protrudes downward from the adjacent electrode PE12. The phase velocity can also be changed by adjusting the thickness (width) α2 or α3 of each part that constitutes the dummy electrode. Furthermore, the phase velocity can also be adjusted by adjusting the distance α5 or α6 between the dummy electrode DE and the opposing electrode E2.

[0043] It goes without saying that the phase velocity can also be changed by changing the thickness of the dummy electrode DE. The shape of the first dummy electrode connected to the adjacent electrode is not limited to the T-shape shown in Fig. 6, but may be an I-shape (DE1) as shown in Fig. 7 or an I-shape (DE2) with a curved periphery as shown in Fig. 8.

[0044] The electrode width α2 when the dummy electrode DE in Fig. 6 straddles the lower optical waveguide 10 is important. This is because, in a configuration in which an electrode straddles an optical waveguide, the optical loss of the light wave propagating through the optical waveguide increases. Therefore, in order to set the optical loss due to the electrodes to be the same in the upper and lower optical waveguides 10, it is preferable to set the electrode width α2 to be the same width as the bypass electrode BE1. This also applies to Figs. 7 and 8.

[0045] 8, forming an electrode surrounded by curves eliminates corners around the electrode and prevents the electric field from concentrating in any area. This prevents the dummy electrode from affecting the nearby optical waveguide with the electric field, making this a more preferable shape for a dummy electrode.

[0046] FIG. 9 shows the second dummy electrode DE3 connected to the bias electrode BE1. FIG. 10 shows a third dummy electrode DE4 (or DE5) connected to the signal electrode LE1 (or LE2). In this way, a dummy electrode can be connected to any part of the electrodes constituting the electrode E1 or E2 to function as a capacitance adjustment mechanism. In addition, instead of providing a capacitance adjustment mechanism such as a dummy electrode only on one electrode (E1 or E2), it is also possible to arrange capacitance adjustment mechanisms on both of the two modulation electrodes (E1 and E2) as shown in FIG. 10, so that the phase velocity is the same across the two modulation electrodes.

[0047] It is also possible to add dummy electrodes DE6 and DE7 and configure the two modulation electrodes so that the electrode shape as a whole is more symmetrical, as shown in Figure 11. Improving the symmetry of the shapes of the two modulation electrodes makes the distribution of internal stress that the electrodes exert on the optical waveguide substrate due to thermal expansion, etc., more uniform, which also contributes to suppressing the occurrence of temperature drift.

[0048] 12, in addition to the optical waveguide (branch waveguide) 10 that propagates light waves, it is also possible to arrange a dummy optical waveguide DW that does not propagate light waves adjacent to the optical waveguide (branch waveguide) 10. The presence of this dummy optical waveguide DW makes the magnitude of the surrounding dielectric constant felt by the dummy electrode DE3 equal to the magnitude of the surrounding dielectric constant felt by the adjacent electrode PE22, making it possible to adjust the phase velocities to be more uniform. Naturally, it is also possible to eliminate bias in the internal stress that the electrodes exert on the optical waveguide.

[0049] 13, it is also possible to arrange a plurality of dummy electrodes (microelectrodes) DE41 and DE42 (or DE91 and DE92) between adjacent bypass electrodes BE1 (or BE2). Such a plurality of microelectrodes can suppress the occurrence of resonance phenomenon in the modulation electrode.

[0050] Another example of the capacity adjusting mechanism will be described with reference to FIGS. 14 and 15 show the segment electrodes arranged so as to overlap one another in order to make the shapes of the segment electrodes formed on the two modulation electrodes exactly the same. FIG. 14 is a plan view, and FIG. 15 shows a cross-sectional view taken along dotted lines A to C in FIG. 14. Focusing on the area indicated by dotted line A, dummy electrodes (DE10, DE12, DE13) are arranged above the adjacent electrodes (P21, PE11, PE12, PE22). Focusing on the area indicated by dotted line B, the shapes of the segment electrodes including the adjacent electrodes (PE11 and PE12) and the segment electrodes including the dummy electrodes (DE12 and DE13) are exactly the same (symmetrical). By making the shape of the segment electrodes of each modulation electrode the same in this way, it is possible to make the phase velocity of each modulation electrode the same.

[0051] 16 and 17 show a dummy electrode DE20 (DE21) connected below the signal electrode LE1 (LE2). In this way, the dummy electrodes can be arranged in a three-dimensional position. Naturally, dummy electrodes are not limited to signal electrodes, and they may also be arranged above and below bypass electrodes and, in some cases, adjacent electrodes.

[0052] 18 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 19 and 20, 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. 19, 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. 20, 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]

[0057] 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]

[0058] 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 DE, DE1~21 dummy electrodes DW dummy optical waveguide

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 at least one of the two modulation electrodes is provided with a capacitance adjustment mechanism for adjusting the phase velocity of a modulation signal propagating through the modulation electrode.

2. 2. The optical waveguide element according to claim 1, wherein the capacitance adjusting mechanism is a dummy electrode formed in a part of the modulation electrode and which 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 electrode is a first dummy electrode extending from the adjacent electrode to the opposite side to the bypass electrode.

4. 3. The optical waveguide element according to claim 2, wherein the dummy electrode is a second dummy electrode extending from the bypass electrode.

5. 3. The optical waveguide element according to claim 2, wherein the dummy electrode is a third dummy electrode extending from the signal electrode.

6. 6. The optical waveguide element according to claim 5, wherein the third dummy electrode is composed of a plurality of fine electrodes arranged between adjacent ones of the bypass electrodes.

7. 2. The optical waveguide element according to claim 1, wherein the capacitance adjusting mechanism changes the electrode width of at least a part of the adjacent electrode, the bypass electrode, or the signal electrode.

8. 2. The optical waveguide element according to claim 1, wherein the capacitance adjusting mechanism changes the electrode thickness of at least a part of the bypass electrode or the signal electrode.

9. 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.

10. 2. The optical waveguide element according to claim 1, further comprising a dummy optical waveguide that does not propagate light waves and is disposed adjacent to said branching waveguide.

11. 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.

12. 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.

13. 13. The optical modulation device according to claim 12, wherein the optical waveguide element comprises 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.

14. 13. An optical transmitter comprising: an optical modulation device according to claim 12; and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

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