JP4354464B2 - Optical waveguide device - Google Patents

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device used for an optical modulator.

In an optical communication system that transmits an optical signal using an optical fiber as a transmission line, an external modulator that modulates light (laser light) emitted from a light source to generate an optical signal is used. As this external modulator, a light intensity modulator in which an optical waveguide is formed on a ferroelectric crystal substrate such as lithium niobate (LiNbO ₃ ; hereinafter abbreviated as LN) or lithium tantalate (LiTaO ₃ ; hereinafter abbreviated as LT). Is used.

FIG. 5 shows a configuration of a conventionally used light intensity modulator (however, the buffer layer formed on the substrate surface is omitted in the figure).
The light intensity modulator 20 includes a substrate 11 made of a ferroelectric crystal such as LN, a Mach-Zehnder type optical waveguide 12 formed on the main surface 21A of the substrate by thermal diffusion treatment of titanium, a signal electrode 13, First and second ground electrodes 14-1 and 14-2 are provided. The optical waveguide 12 includes an input waveguide 121, two branch optical waveguides 122-1 and 122-2, and an output waveguide 123.

  The light wave input to the input waveguide 121 is branched at a ratio equal to the two branched optical waveguides 122-1 and 122-2 in the branching section 124. A predetermined modulation signal voltage is applied from the external power supply 30 to the signal electrode 13 and the first ground electrode 14-1 and / or the signal electrode 13 and the second ground electrode 14-2. Then, the electric field due to the modulation voltage from the signal electrode 13 reaches the branch optical waveguides 122-1 and 122-2, and the light wave guided through the branch optical waveguide is modulated in phase by the refractive index change caused by the modulation electric field. receive. When these phase-modulated light waves are combined at the coupling unit 125, intensity modulation is given to the light wave output to the output waveguide 123 according to the phase of each light wave.

FIG. 6 shows a cross-sectional view of the light intensity modulator 20 shown in FIG. 5 taken along the line II of FIG. Between the substrate 11 and the signal electrode 13 and the ground electrodes 14-1 and 14-2, in order to prevent the evanescent component of the light wave guided through the optical waveguide from being absorbed by these electrodes, silicon oxide (SiO 2 ₂ ) or the like is provided.

  Here, lithium niobate that is a material of the substrate 11 constituting the light intensity modulator 20 is not only a ferroelectric material but also a piezoelectric material. Further, the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 in the light intensity modulator 20 as shown in FIGS. 5 and 6 are comb-shaped electrode pairs used for generating surface acoustic waves. This corresponds to a configuration in which Therefore, when a high-speed pulsed voltage as a modulation signal is applied between the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2, an acoustic wave is generated in a wide frequency band by electromechanical coupling. appear. Among such acoustic waves, one having a specific frequency resonates in the light intensity modulator 20 and causes a ripple in the response characteristic of the modulator (response of the optical output with respect to the frequency). Yes.

  The mechanism of acoustic wave generation will be described with reference to FIG. When the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 are formed on the substrate 11, the density of the substrate in the portions A <b> 2, A <b> 4, and A <b> 6 where each electrode is formed due to the load of these electrodes. There is a difference with respect to the density in the portions A1, A3, A5, and A7 where the electrodes are not formed. This difference in density means that the acoustic impedance is different between the portion where the electrode is formed and the portion where the electrode is not formed. Due to acoustic impedance mismatch, surface acoustic waves generated when a high-speed pulse voltage is applied are reflected, for example, at the boundary surface B1 between the portion A2 where the electrode is formed and the portion A1 where the electrode is not formed. It will be. Such acoustic wave reflection similarly occurs at the other boundary surfaces B2, B3, B4, B5, and B6 in the substrate 11. As a result, for example, an acoustic wave having a frequency corresponding to the distance between the boundary surfaces B2 and B3 increases due to resonance. Since the branch optical waveguides 122-1 and 122-2 exist between the boundary surfaces B2 and B3, the resonating acoustic wave interacts with the light wave guided through the branch optical waveguide. This causes a significant deterioration in the response characteristics of the optical waveguide element at the resonance frequency.

  Several solutions to the problem of characteristic deterioration due to acoustic waves have been proposed. For example, Patent Document 1 discloses a structure of an optical waveguide element in which the electrode width of a signal electrode and a ground electrode and the interval between the electrodes are changed along the propagation direction of the optical waveguide. By adopting such an electrode configuration, at each point in the propagation direction of the optical waveguide, the distance between the boundary surfaces described above is different, so the frequency of the acoustic wave that resonates also differs. Therefore, the acoustic wave having the same frequency is not enhanced over the propagation direction of the optical waveguide, and the influence of the resonance of the acoustic wave on the response characteristic of the modulator can be reduced.

According to Patent Document 2, the above-described problem is solved by forming a groove on the substrate surface near the electrode. The wall surface of the formed groove acts as an acoustic wave reflecting surface stronger than the boundary surface due to the difference in substrate density described above. By utilizing this, by providing a plurality of grooves and changing the distance between the grooves along the propagation direction of the optical waveguide, it is possible to reduce the influence of the acoustic wave as in Patent Document 1.
JP 2000-275455 A JP 2000-275587 A Japanese Patent Laid-Open No. 10-3065

However, the method proposed in the above-mentioned Patent Document 1 has the following drawbacks.
First, if the distance between the signal electrode and the ground electrode is changed along the propagation direction of the optical waveguide, the distance between the optical waveguide and the electrode varies depending on the location in the propagation direction, and the electric field applied to the optical waveguide is also different. It will be different. Therefore, it becomes impossible to optimize the degree of modulation with respect to the light wave over the entire length of the optical waveguide, and as a result, there is a problem that the modulation efficiency (EO (optical-electrical) conversion efficiency) as a whole deteriorates.

  Secondly, in order to prevent such deterioration of the modulation efficiency, it is conceivable to arrange the optical waveguide so that the electric field applied to the optical waveguide is constant. When the width and the distance between the electrodes change along the propagation direction, the electrical impedance with respect to the modulation signal defined by the electrode width and the electrode distance changes. As a result, reflection and loss occur in the high-frequency modulated electric signal, and the modulation efficiency is deteriorated.

Third, in general, in order to widen the modulation band (operating frequency band) of the optical intensity modulator, the speed of the microwave traveling through the signal electrode and the speed of the light wave guided through the optical waveguide are brought close to each other to achieve speed matching. However, in the electrode structure shown in Patent Document 1, the effective refractive index of the microwave and the effective refractive index of the light wave are greatly different from each other, so that there is a problem that speed matching cannot be realized.
Patent Document 3 listed above discloses an optical modulator in which a thin electric field adjustment electrode is provided under a signal electrode, which can achieve speed matching and at the same time can effectively apply an electric field to an optical waveguide. However, it is not a configuration capable of avoiding the above-described characteristic deterioration caused by acoustic waves.

  In addition, for the problem of acoustic waves, the method disclosed in Patent Document 2 requires an additional process for groove formation and introduction of a new processing apparatus, and thus is not necessarily effective in terms of manufacturing. It could not be said that it was a method.

  The present invention has been made in view of the above points, and an object of the present invention is to provide an optical waveguide element that has excellent response characteristics and can simultaneously realize good modulation efficiency by preventing resonance of an acoustic wave. Is to provide.

The present invention has been made to solve the above problems, the invention according to claim 1, a substrate having an electro-optic effect and the piezoelectric effect, an optical waveguide formed on the main surface of the substrate, A signal electrode and a ground electrode formed on a main surface of the substrate for controlling a light wave guided in the optical waveguide, and at least one of the signal electrode and the ground electrode between the substrate and the signal electrode An optical waveguide device comprising: a lower electrode formed; and changing a distance between at least one side surface of the signal electrode and at least one side surface of the ground electrode along a waveguide direction of the light wave, An optical waveguide element characterized in that a distance between the lower electrode and an electrode facing the lower electrode is constant along the waveguide direction of the light wave.

  According to a second aspect of the present invention, in the optical waveguide device according to the first aspect, the width of the signal electrode is changed along the waveguide direction of the light wave.

  According to a third aspect of the present invention, there is provided the optical waveguide device according to the first or second aspect, wherein the distance between the at least one side surface of the signal electrode and the at least one side surface of the ground electrode is determined by the light wave. It is characterized by being continuously and smoothly changed along the waveguide direction.

  The invention according to claim 4 is the optical waveguide element according to any one of claims 1 to 3, wherein the ground electrode is formed on both sides of the signal electrode. The distance between the side surfaces of the signal electrode and each ground electrode is changed substantially symmetrically about the signal electrode.

  According to a fifth aspect of the present invention, in the optical waveguide device according to any one of the first to fourth aspects, the signal electrode and the ground electrode are arranged at intermediate points in the light wave guiding direction. It is characterized by a substantially symmetrical shape as the center.

  The invention according to claim 6 is the optical waveguide device according to any one of claims 1 to 5, wherein the lower layer electrode is made of Au, Cu, Al, Ti, Cr, ITO, ITiO, or the like. It is characterized by comprising a conductive material.

  According to the present invention, the acoustic wave resonance is effectively prevented by optimizing the shape of the signal electrode and the ground electrode, and the light wave is effectively modulated by providing the lower layer electrode. Thus, it is possible to obtain an excellent optical waveguide device that does not generate ripples in the frequency response characteristics and has good modulation efficiency.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
<< First Embodiment >>
FIG. 1 is a plan configuration diagram of an optical waveguide device according to the first embodiment of the present invention, and FIG. 2 is a sectional configuration diagram of the optical waveguide device of FIG. In the present embodiment, description will be made by taking up an optical waveguide element for a light intensity modulator. 2A is a cross-sectional view taken along the line AA ′ in the plan view of FIG. 1, and FIG. 2B is a cross-sectional view taken along the line BB ′. It is.

  In FIG. 1, an optical waveguide element 10 includes an LN substrate 11, a Mach-Zehnder optical waveguide 12, a signal electrode 13, a first ground electrode 14-1, and a second ground electrode as a substrate having a piezoelectric effect as well as an electro-optic effect. 14-2 and lower layer electrodes 15-1 to 15-3 formed on the lower layer of each of these electrodes.

The Mach-Zehnder optical waveguide 12 includes an input waveguide 121, two branch optical waveguides 122-1, 122-2, and an output waveguide 123.
The LN substrate 11 uses an X-cut crystal, and the Mach-Zehnder optical waveguide 12 is formed on the main surface of the substrate.

  The arrangement of each electrode is determined so that the electric field applied to the optical waveguide in the substrate is parallel to the substrate surface. That is, the signal electrode 13 is installed between the two branch optical waveguides 122-1 and 122-2, and the first ground electrode 14-1 and the second ground electrode 14-2 are connected to the branch optical waveguide 122-1, respectively. It is installed so as to face the signal electrode 13 across 122-2.

  Here, in the cross-sectional view of FIG. 2, the bottom surfaces of the signal electrode 13, the first and second ground electrodes 14-1, 14-2 are in contact with the lower layer electrodes 15-1 to 15-3, respectively. The lower layer electrodes 15-1 to 15-3 are formed on the SiO 2 buffer layer 16 provided on the surface of the LN substrate 11. The buffer layer 16 is used to prevent light wave absorption loss due to the electrodes.

Returning to FIG. 1, the signal electrode 13 has an electrode width W _sig (a direction perpendicular to P is defined as a width) along the propagation direction P of the light wave guided through the branched optical waveguides 122-1 and 122-2. It is formed into a shape that continuously changes and becomes thinner. Similarly, the first and second ground electrodes 14-1 and 14-2 are formed in such a shape that the electrode width W _gnd continuously changes along the propagation direction P and becomes gradually smaller. Then, the rate of change of the electrode width in the signal electrode 13 α _sig = (W _sig −W _sig ′) / L
And rate of change of electrode width in first and second ground electrodes α _gnd = (W _gnd −W _gnd ′) / L
Different values are selected from each other. Where L is the electrode length.

With such an electrode shape, for example, the distance d (S2, S3) between the side surface S3 of the signal electrode 13 and the side surface S2 of the first ground electrode 14-1 at a point in the propagation direction P is along the propagation direction P. It is changing continuously. However, the distance d (Sa, Sb) is defined as a distance between two side surfaces in the cross section when the optical waveguide element 10 is cut in a cross section perpendicular to the propagation direction P at the point (however, a and b are 1-6). Further, the distance d (S1, S3) also changes continuously along the propagation direction P because the change rates α _sig and α _gnd of the electrode width are different as described above. For other side surface combinations, the distance between the side surfaces continuously changes along the propagation direction P.

  Here, referring to FIG. 2A or 2B, for example, a boundary surface D2 between the portion C2 where the first ground electrode 14-1 is formed and the portion C3 where the electrode is not formed, Consider a case where acoustic waves are reflected between the boundary surface D3 and the portion C4 where the signal electrode 13 is formed. At this time, since the distance between the boundary surfaces D2 and D3, that is, the distance d (S2, S3) varies depending on each position in the propagation direction P as described above, the frequency of the acoustic wave that increases due to the resonance is increased one by one in the propagation direction P. Will be different. Thereby, the resonance of the acoustic wave between the boundary surfaces D2 and D3 is alleviated as a whole.

  The same applies to the reflection of acoustic waves generated in other combinations of boundary surfaces. Therefore, the shape of the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 as shown in FIG. 1 effectively prevents deterioration of the modulator response characteristics due to acoustic resonance.

  On the other hand, in FIG. 1, each of the lower layer electrodes 15-1 to 15-3 is formed so that the electrode width is constant along the propagation direction P. The electrode width is set so as to coincide with the electrode width of the signal electrode 13 and each of the ground electrodes 14-1 and 14-2 on the input side end face (A-A ′ line). Further, the gap (interval) between the lower layer electrode 15-1 below the signal electrode 13 and the lower layer electrode 15-2 below the first ground electrode 14-1, and below the lower layer electrode 15-1 and the second ground electrode 14-2. The gap between the lower electrode 15-3 is also kept constant along the propagation direction P. The thickness of each lower layer electrode 15-1 to 15-3 is sufficiently thinner than the signal electrode 13 and the ground electrodes 14-1 and 14-2. The value is set so that the density of the LN substrate 11 does not change.

  With such a configuration of the lower layer electrodes 15-1 to 15-3, the electric field applied to the branched optical waveguides 122-1 and 122-2 in the LN substrate 11 is generated by the signal electrode 13 and the first and second ground electrodes 14. -1 and 14-2 are equivalent to the electric field when the electrode widths of the lower layer electrodes 15-1 to 15-3 are equal to each other in the propagation direction P. Therefore, from the viewpoint of the EO conversion efficiency of the modulation signal with respect to the light wave guided through the branched optical waveguides 122-1, 122-2, the present optical waveguide device 10 has an optimum EO conversion efficiency over the entire waveguide length of the electrode portion. Thus, problems such as an increase in drive voltage during modulation and deterioration in modulation efficiency are prevented.

<< Second Embodiment >>
In the first embodiment, the electrode width W _gnd and the rate of change α _gnd are equal in the two ground electrodes 14-1 and 14-2. Therefore, in FIG. 1, side surface S1 of first ground electrode 14-1 and side surface S5 of second ground electrode 14-2, and side surface S2 of first ground electrode 14-1 and side surface S6 of second ground electrode 14-2. Are parallel to each other, and the distances d (S1, S5) and d (S2, S6) between these side surfaces do not change along the propagation direction P. Therefore, acoustic wave resonance may occur between the boundary surfaces in the LN substrate 11 corresponding to these side surfaces.

Therefore, in the second embodiment, in these two ground electrodes 14-1 and 14-2, the change rate α _gnd is set to a different value while the electrode width at the input side end face (AA ′ line) is kept the same. Set to. By doing so, the side surfaces S1 and S5 and the side surfaces S2 and S6 described above are not parallel, and as a result, the resonance of the acoustic wave can be further relaxed.

<< Third Embodiment >>
Next, FIG. 3 shows a plan view of an optical waveguide device according to the third embodiment of the present invention.
In the optical waveguide device 10 of the present embodiment, the signal electrode 13 is formed in a shape such that the electrode width W _sig continuously changes along the propagation direction P as in FIG. On the other hand, the first and second ground electrodes 14-1 and 14-2 are formed in such a shape that the electrode width W _gnd continuously changes along the propagation direction P and becomes thicker, contrary to FIG. Is done. The gaps between the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 are set to G1 and G2, respectively.
G1 = d (S2, S3), G2 = d (S4, S5)
, The electrode shape is set so that the ratio of the signal electrode 13 to the electrode width W _sig , G1 / W _sig and G2 / W _sig , becomes a constant value along the propagation direction P.

  Here, the characteristic impedance of the capacitor (equivalent circuit thereof) formed between the signal electrode 13 and the ground electrode 14-1 (or 14-2) is determined by using the ratio of the electrode width of the signal electrode 13 and the gap between the electrodes as a parameter. The value to be determined. Therefore, in the electrode configuration as shown in FIG. 3, the characteristic impedance with respect to the modulated electric signal has a constant value along the propagation direction P. As a result, no reflection or loss occurs in the high-frequency modulated electrical signal input to the signal electrode 13, and deterioration of the modulation efficiency is prevented.

  Also in this configuration, as in the case of FIG. 1, the distance d (Sa, Sb) between the side surfaces excluding the combination of the side surfaces S1 and S5 and the side surfaces S2 and S6 is continuous along the propagation direction P. Has changed. Therefore, as in the first embodiment, the frequency of the resonating acoustic wave differs depending on each position in the propagation direction P, and the influence of resonance is mitigated, thereby preventing the response characteristics of the modulator from deteriorating.

  In addition, about the shape of each lower layer electrode 15-1 to 15-3, it is set as a constant value along the propagation direction P with both electrode width and a gap similarly to FIG. Therefore, the optimum conditions for the EO conversion efficiency are realized over the entire length of the electrode.

<< Fourth Embodiment >>
Next, FIG. 4 shows a plan view of an optical waveguide device according to the fourth embodiment of the present invention.
In the optical waveguide device 10 of the present embodiment, the shape of the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 is set on the input side (AA ′ line with the central point in the propagation direction P as the center. ) And the output side (BB ′ line). That is, in the optical waveguide device 10 of FIG. 4A, the signal width of the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 are continuously reduced from the input side to the central point. From the center point to the output side, the electrode width is formed so as to increase continuously and gradually. 4B, the first and second ground electrodes 14-1 and 14-2 are the same as those in FIG. 4A, but the signal electrode 13 is opposite to the ground electrode. In addition, the electrode width is gradually increased from the input side to the center point, and the electrode width is continuously decreased from the center point to the output side. 4 (a) and 4 (b), the electrode width and the interelectrode gap of each electrode (signal electrode and ground electrode) are set to be equal on the input side end face and the output side end face.

  With such an electrode shape, the characteristic impedance with respect to the modulated electric signal is matched between the input side end face and the output side end face. As a result, when a drive driver is connected to the signal electrode 13 and a modulation signal is input, the characteristic impedance on the drive driver side (usually 50Ω is selected) and the input impedance and output impedance on the optical waveguide element 10 side are simultaneously matched. Can be made.

Also in this configuration, as in the case of FIG. 1, the distance d (Sa, Sb) between the side surfaces excluding the combination of the side surfaces S1 and S5 and the side surfaces S2 and S6 is continuous along the propagation direction P. Has changed. Therefore, as in the first embodiment, the frequency of the resonating acoustic wave differs depending on each position in the propagation direction P, and the influence of resonance is mitigated, thereby preventing the response characteristics of the modulator from deteriorating.
Further, the shape of each of the lower layer electrodes 15-1 to 15-3 is also set to a constant value along the propagation direction P, as in the case of FIG. Therefore, the optimum conditions for the EO conversion efficiency are realized over the entire length of the electrode.

The embodiments of the present invention have been described in detail above with reference to the drawings. However, the specific configuration is not limited to the above-described one, and various design changes and the like can be made without departing from the scope of the present invention. Is possible.
For example, as for the side surfaces S1 to S6 of each electrode, if the distance between at least one pair of the side surfaces changes along the propagation direction P, it is possible to reduce the influence due to the resonance of the acoustic wave. As a special example, the electrode width of the signal electrode 13 may be constant along the propagation direction P, for example. This also makes it possible to achieve speed matching between the microwave and the light wave.

Further, the application destination of the optical waveguide device according to the present invention is not particularly limited to a light intensity modulator, an optical phase modulator, a polarization scrambler, and other light modulators.
Further, the substrate having the electro-optic effect and the piezoelectric effect can be used equally.

Hereinafter, an embodiment of an optical waveguide device according to the present invention will be described with reference to FIGS.
Using an X-cut LN substrate 11, a resist is applied to the surface of the main surface, and a Mach-Zehnder type optical waveguide pattern is formed by exposure and development. Then, 800 nm of titanium (Ti) is deposited on the entire surface by vapor deposition, and the titanium is removed by lift-off leaving the portion of the optical waveguide pattern. Thereafter, the entire substrate is heated at 1000 ° C. for 10 hours to thermally diffuse the optical waveguide pattern titanium into the substrate. Thereby, the Mach-Zehnder optical waveguide 12 is formed.

Further, a silicon oxide (SiO 2) film having a thickness of 1 μm is formed on the surface of the LN substrate 11 as a buffer layer 16 for preventing light waves from being absorbed by the electrodes.
Thereafter, gold (Au) is formed to a thickness of 0.1 μm by the same process as the above titanium pattern formation to form lower layer electrodes 15-1 to 15-3. Further thereon, gold is similarly formed to a thickness of 30 μm by electrolytic plating to form the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2. As a material for the electrode, a conductive material such as Cu, Al, Ti, Cr, ITO, and ITiO can be used in addition to gold.

In this embodiment, the shape (planar shape) of each electrode is set as follows. First, the electrode length L is kept constant at 30 mm for all electrodes. The electrode width W _sig of the signal electrode 13 is 60 μm at the input side end face (AA ′ line) and 40 μm at the output side end face (BB ′ line), and changes continuously and linearly along the propagation direction P. To do. The electrode width of the lower layer electrode 15-1 below the signal electrode 13 is constant at 60 μm. The two ground electrodes 14-1 and 14-2 have the same shape, and the electrode width W _gnd is 300 μm at the input side end face and 200 μm at the output side end face, and is continuous and linear in the propagation direction P similarly to the signal electrode 13. Change. The electrode widths of the lower layer electrodes 15-2 and 15-3 under the two ground electrodes are constant at 300 μm. The gap between the signal electrode 13 and each of the ground electrodes 14-1 and 14-2 is 25 μm on the input side end face, and the gap between the lower layer electrodes is 10 μm.

With such an electrode configuration, the optical waveguide device 10 of this embodiment has the following characteristics.
First, the side surfaces S1 to S6 of the signal electrode 13 and the first and second ground electrodes 14-1 and 14-2 are parallel to each other in all combinations except the combinations of S1 and S5 and S2 and S6. It is formed so as not to become. Therefore, between the boundary surfaces D1 to D6 in the LN substrate 11 corresponding to these side surfaces, the resonance frequency of the acoustic wave generated by applying the modulation signal to the electrode differs for each position in the propagation direction P. As a result, deterioration of response characteristics during modulation of the optical waveguide element 10 (light intensity modulator) is prevented.
The film thickness of the lower layer electrodes 15-1 to 15-3 is as thin as 0.1 μm, and the influence of the load on the LN substrate 11 is small. Therefore, the resonance of the acoustic wave due to the presence of these lower layer electrodes can be ignored. .

  The lower layer electrodes 15-1 to 15-3 are formed so that the electrode width is constant over the entire propagation direction P. Since the modulated electric field applied to the branched optical waveguides 122-1 and 122-2 is generated by these lower layer electrodes, the EO conversion efficiency for the light wave is constant along the propagation direction P. Therefore, light wave modulation is realized without degrading the modulation efficiency.

  As described above, in the present optical waveguide device 10, it is possible to simultaneously achieve the prevention of the deterioration of the modulation response characteristic due to the resonance of the acoustic wave and the realization of the optimum modulation efficiency.

It is a plane lineblock diagram of an optical waveguide device by a 1st embodiment of the present invention. It is a cross-sectional block diagram of the optical waveguide element of FIG. It is a plane block diagram of the optical waveguide element by the 3rd Embodiment of this invention. It is a plane block diagram of the optical waveguide element by the 4th Embodiment of this invention. It is a plane block diagram of the light intensity modulator used conventionally. FIG. 6 is a cross-sectional configuration diagram of the light intensity modulator of FIG. 5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Optical waveguide element 11 ... LN board | substrate 12 ... Mach-Zehnder optical waveguide 13 ... Signal electrode 14-1 ... 1st ground electrode 14-2 ... 2nd ground electrode 15-1 to 15-3 ... Lower layer electrode 16 ... Buffer layer 20 ... Light intensity modulator 30 ... External power supply

Claims (6)

A substrate having an electro-optic effect and a piezoelectric effect;
An optical waveguide formed on the main surface of the substrate,
A signal electrode and a ground electrode for controlling a light wave guided in the optical waveguide , formed on the main surface of the substrate ;
A lower layer electrode formed between at least one of the signal electrode and the ground electrode and the substrate;
An optical waveguide device comprising:
Changing a distance between at least one side surface of the signal electrode and at least one side surface of the ground electrode along a waveguide direction of the light wave;
An optical waveguide device characterized in that a distance between the lower layer electrode and an electrode facing the lower layer electrode is constant along the waveguide direction of the light wave. The optical waveguide device according to claim 1, wherein a width of the signal electrode is changed along a waveguide direction of the light wave. The distance between at least one side surface of the signal electrode and at least one side surface of the ground electrode is continuously and smoothly changed along the waveguide direction of the light wave. 2. An optical waveguide device according to 1. In the optical waveguide device in which the ground electrode is formed on both sides of the signal electrode,
The light beam according to any one of claims 1 to 3, wherein the distance between the side surfaces of the signal electrode and each ground electrode is changed substantially symmetrically about the signal electrode. Waveguide element. 5. The optical waveguide device according to claim 1, wherein the signal electrode and the ground electrode have a substantially symmetric shape with an intermediate point in the waveguide direction of the light wave as a center. . The optical waveguide element according to any one of claims 1 to 5, wherein the lower layer electrode is made of a conductive material such as Au, Cu, Al, Ti, Cr, ITO, or ITiO.

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